Fluoride-free and aluminum-free process for preparing Ti-Beta zeolite based on local structure reconstruction method

Abstract

The invention belongs to the technical field of petrochemical catalysis, and relates to a process for preparing Ti-Beta zeolite based on a local structure reconstruction method. The main technical characteristics are as follows; the Ti-Beta zeolite is prepared by using water vapor to assist a tetraethylammonium bromide template agent and alcohol amine weak organic alkali to carry out local desilicication etching and repair reconstruction on hydroxyl nests of the dealuminated Beta zeolite. The method provided by the invention has the beneficial effects that the dissolution amount of silicate is limited by the combination of the tetraethylammonium bromide template agent and the alcohol amine weak organic base, and the pore channel condensation liquid and the zeolite solid surface liquid film generated by dealuminating the Beta zeolite in saturated steam; the controllable etching desiliconization of the dealuminated Beta zeolite hydroxyl nest is easier; and the admitting ability of the hydroxyl nest to titanium ions with larger volume is improved. Local structure defects generated by controllable etching desilicication can be repaired by directly using quaternary ammonium cations provided by a TEABr template agent as a guiding agent, so that the method does not need the assistance of fluoride or / and an aluminum source.

Description

Technical Field

[0001] The present invention belongs to the technical field of petrochemical catalysis, and relates to a fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method. Background Art

[0002] Ti-Beta zeolite is an important member of the titanium silicate zeolite family because it has a three-dimensional intersecting twelve-membered ring pore system. Compared with TS-1 zeolite (which has a three-dimensional intersecting ten-membered ring pore system), Ti-Beta zeolite is more suitable for the low-temperature selective oxidation reaction of larger organic molecules with hydrogen peroxide, including the oxidation reaction of alkanes and the epoxidation reaction of alkenes.

[0003] So far, Ti-Beta zeolite has been prepared by various methods, including traditional hydrothermal method, dry gel conversion method (steam-assisted), transformation method, isomorphous substitution method and structure reconstruction method.

[0004] The main technical feature of the traditional hydrothermal method is that Ti-Beta zeolite is produced by the hydrothermal crystallization of a hydrogel. A large amount of fluoride mineralizer usually needs to be added to the hydrogel, and even a small amount of aluminum source needs to be introduced for auxiliary synthesis. The crystallization process of the hydrogel is slow and time-consuming. Therefore, when preparing Ti-Beta zeolite by the traditional hydrothermal method, there are not only problems in the treatment of fluorine-containing wastewater, but also problems of high energy consumption and low efficiency. In addition, the introduction of a small amount of aluminum source will inevitably result in framework aluminum in the Ti-Beta zeolite framework, and a post dealumination step needs to be added.

[0005] The main technical feature of the dry-gel conversion method (steam-assisted) is that instead of directly synthesizing Ti-Beta zeolite from hydrogel, the hydrogel is first dried at low temperature to form a dry gel, and then the dry-gel conversion method is used to synthesize Ti-Beta zeolite. Since the conversion process of the dry gel is not carried out in the aqueous phase but in the vapor generated from the vaporization of water, it is called the steam-assisted dry-gel conversion method. For example, the public literature J. Phys. Chem. B 1998, 102, 7126-7131 discloses a method for synthesizing Ti-Beta zeolite by dry-gel conversion. The procedure is as follows: Using aerosol silica as the silicon source, a titanium source is prepared with tetrabutyl titanate, deionized water, and hydrogen peroxide solution, and an aluminum source is prepared with sodium aluminate, sodium hydroxide, and tetraethylammonium hydroxide solution. First, the aluminum source solution is added to the titanium source solution under vigorous stirring, and then the silicon source is added to the mixture, and stirring is continued until a homogeneous colloidal solution is formed. Then, the colloidal solution is evaporated to dryness at 80 °C to obtain a dry gel. The composition of the dry gel is SiO2:TiO2:Al2O3:Na2O:TEAOH = 304:10:0.46:1.55:132.5. Crystallization is carried out in a special autoclave. The dry gel is placed on a tray, and deionized water is placed under the tray. The conversion of the dry gel is carried out at 175 °C for a duration of 18 h. During this period, the dry gel is converted into Ti-Beta zeolite with the assistance of water vapor. Compared with the conventional hydrothermal method, the dry-gel conversion method can be carried out without adding a fluoride mineralizer, and the crystallization time is shorter, but a small amount of aluminum source still needs to be introduced for auxiliary synthesis.

[0006] The main technical feature of the transformation method is that Ti-Beta zeolite is transformed from a zeolite with a completely different topological structure. For example, the public literature Chem. Commun., 2019, 55, 14279 reported a transformation method for preparing Ti-Beta zeolite. In this method, a layered Ti-MWW zeolite was used as the raw material. The Ti-MWW zeolite provides both the silicon source and the titanium source for the preparation of Ti-Beta. The procedure is as follows: First, the Ti-MWW zeolite and the fully dealuminated Beta seed crystals are added to an alkaline solution for degradation to form the structural building units for synthesizing Ti-Beta zeolite. Then, the degradation solution is crystallized into Ti-Beta zeolite with the assistance of a templating agent (tetraethyl cation) and a fluoride mineralizer; there is a more detailed description of the transformation of Ti-MWW to Ti-Beta in the public literature (Research on the Synthesis and Catalytic Performance of Heteroatom Zeolites with BEA and FAU Topological Structures [D]. East China Normal University, 2018). Another example is that the public literature Microporous and Mesoporous Materials 311 (2021) 110702 also reported a transformation method for preparing Ti-Beta zeolite. In this method, a dealuminated Y zeolite (USY, Si / Al = 6.0) was used as the starting material. First, the USY was dealuminated with 6 mol / L nitric acid solution at reflux temperature (liquid-solid ratio 30, 12 h), filtered, washed, dried, and calcined. The above acid dealumination operation was repeated at least 2 - 3 times to make the obtained dealuminated USY (USY-DA) have a Si / Al molar ratio of more than 500 (analyzed by ICP). In this method, USY-DA is only used as the silicon source. Then, a mixture solution with the composition of 1.0SiO2:0.02TiO2:0.4TEAOH:0.2CTAB:0.5NH4F:7.5H2O was prepared with USY-DA, dealuminated Beta zeolite seed crystals (Beta-DA, Si / Al molar ratio ≥ 1900, dosage 10 wt% based on USY-DA), titanium source (TBOT), pore-forming agent (CTAB), templating agent TEAOH, and NH4F, and crystallized at 140 °C for 3 days to obtain Ti-Beta zeolite. Generally speaking, the transformation method for preparing Ti-Beta zeolite takes less time and has high product quality. However, the transformation method cannot dispense with the use of fluoride mineralizers.

[0007] The main technical feature of the isomorphous substitution method is to prepare Ti-Beta zeolite with dealuminated Beta zeolite. Specifically, on the premise of not destroying the crystal structure of dealuminated Beta zeolite, by using tetravalent titanium ions (Ti 4+)Ti-Beta zeolite is prepared by implanting hydroxyl nests of dealuminated Beta zeolite. The isomorphous substitution method can be divided into three technical routes: gas-solid, solid-solid, and liquid-solid isomorphous substitution methods.

[0008] The gas-solid isomorphous substitution method uses volatile TiCl4 as the titanium source. Through an inert gas (such as N2) carrying TiCl4 to contact with the dealuminated Beta zeolite, the isomorphous substitution reaction is carried out at high temperature. The main problems of this method are the high operation difficulty, easy generation of non-framework titanium, and poor reproducibility of catalyst preparation. Therefore, later, the solid-solid isomorphous substitution method and the liquid-solid isomorphous substitution method were proposed as improved technical routes.

[0009] The solid-solid isomorphous substitution method generally uses titanium dichloride bis(cyclopentadienyl) solid powder as the titanium source for the isomorphous substitution reaction of dealuminated Beta zeolite. For example, Chinese Invention Patent (Application No. 202111439797.1) discloses a method for preparing Ti-Beta zeolite by isomorphous substitution. Its technical features are as follows: First, the Beta zeolite is completely dealuminated to obtain all-silica Si-Beta zeolite. Then, the Si-Beta zeolite is pretreated by dehydration and removal of miscellaneous gases. After that, the pretreated Si-Beta zeolite and titanium dichloride bis(cyclopentadienyl) solid powder are uniformly mixed in an inert gas environment. The mixed powder is transferred to a covered flat-bottom crucible and evenly spread as thinly as possible in the flat-bottom crucible. The covered crucible containing the mixed powder is transferred to a vacuum environment for heating, so that the Ti metal in the precursor in the crucible is uniformly impregnated and loaded into the framework of the Si-Beta zeolite to obtain Ti-Beta zeolite. In order to make the dealuminated Beta zeolite and titanium dichloride bis(cyclopentadienyl) solid powder mix evenly, it is generally necessary to use a ball mill to fully process the mixture of dealuminated Beta zeolite and titanium dichloride bis(cyclopentadienyl). For example, the use of a ball mill is involved in the open literature Catal. Sci. Technol., 2019, DOI: 10.1039 / C9CY00957D, Green Chem., 2014, 16, 2281 and Chinese Journal of Catalysis 36 (2015) 906–912. In addition, the following literatures also involve the preparation of Ti-Beta zeolite by the solid-solid isomorphous substitution method: Chinese Invention Patent (Application No. 202011450442.8); Chinese Invention Patent (Application No. 202010855082.3); open literature Chinese Journal of Catalysis 42 (2021) 1176–1184; open literature Microporous and Mesoporous Materials 330 (2022) 111625; open literature Microporousand Mesoporous Materials 288 (2019) 109588. The Ti-Beta zeolite prepared by the solid-solid isomorphous substitution method has good quality, but the titanium source is relatively expensive, and the operation process needs to be carried out under the protection of inert gas, and generally requires the use of a glove box.

[0010] The liquid-solid isomorphous substitution method preferably uses an ethanol solution of TiCl4 as the titanium source to carry out an isomorphous substitution reaction with dealuminated Beta zeolite. For example, the public literature Ind. Eng. Chem. Res. 2021, 60, 1219-1230 has a detailed record of this method, and the main points are as follows: (1) Contact Al-β zeolite with Si / Al = 11 with 13 mol / L HNO3 aqueous solution and react at 100 °C for 24 h to obtain dealuminated Beta zeolite; (2) The dealuminated Beta zeolite is pre-treated by drying (110 °C, 2 h) and calcination (550 °C, 3 h) before use. Prepare a TiCl4 solution as the titanium source with ethanol dried over 3A zeolite (dehydrated for more than 24 h). Then, contact the ethanol solution of TiCl4 with the dealuminated Beta zeolite (grind) and carry out an isomorphous substitution reaction at room temperature to obtain Ti-Beta zeolite. The post-treatment of the Ti-Beta zeolite prepared by the above liquid-solid isomorphous substitution method includes drying (120 °C, 3 h) and calcination steps (550 °C, 3 h).

[0011] In addition to using an ethanol solution of TiCl4 as the titanium source, there are also reports in the public literature (Wang Yanan. Preparation, Characterization and Application Research of Ionic Liquids in Catalytic Fuel Desulfurization [D]. Tianjin University, 2010, Zhang Juan. Preparation, Characterization and Performance Research of Photocatalytic Oxidation Diesel Desulfurization Catalysts [D]. Tianjin University, 2008, and Chemical Engineering, Vol. 40, No. 10, 2012) on the preparation of Ti-Beta zeolite by the liquid-solid isomorphous substitution method using titanium sulfate (Ti(SO4)2) as the titanium source. When using Ti(SO4)2 as the titanium source, these public literatures omit the step of acid dealumination of the zeolite and directly carry out an isomorphous substitution reaction between Al-Beta zeolite and Ti(SO4)2 solution to prepare Ti-Beta zeolite. This approach is based on the strong acidity of the Ti(SO4)2 solution, which enables the simultaneous occurrence of dealumination and titanium supplementation in the liquid-solid reaction. However, when using Ti(SO4)2 as the titanium source for the liquid-solid isomorphous substitution reaction of Al-Beta zeolite, effective removal of the framework aluminum of Beta zeolite cannot be achieved, and thus the extent of the isomorphous substitution reaction of Ti 4+ ions is limited.

[0012] In fact, for the gas-solid, solid-solid, and liquid-solid isomorphous substitution methods described above, there are all problems with the limited extent of the isomorphous substitution reaction of Ti 4+ ions. This is because the radius of the tetravalent titanium ion (Ti 4+ ) is greater than that of the trivalent aluminum ion (Al 3+ ) Therefore, the hydroxyl vacancy generated after removing Al 3+ from the Al-Beta zeolite framework is relatively small, which is not conducive to Ti4+ Implantation.

[0013] Theoretically, the structure reconstruction method can avoid the problems of the isomorphous substitution method. Although the structure reconstruction method also uses dealuminated Beta zeolite to prepare Ti-Beta zeolite, the structure reconstruction process occurs in the presence of a strongly alkaline TEAOH templating agent solution. Existing studies have shown that the strongly alkaline TEAOH templating agent solution can completely dissolve (fragment) and recrystallize (structure reconstruction) the dealuminated Beta zeolite framework, which helps larger Ti 4+ ions combine with the fragmented Beta zeolite structure building units, so that when the framework recrystallizes, they can more easily enter the Beta zeolite framework by means of the assembly of the structure building units.

[0014] The open literature Catal. Sci. Technol., 2019, DOI: 10.1039 / C9CY00071B. reported the method for the rapid synthesis of Ti-Beta by the structure reconstruction method. The technical key points are as follows: (1) First, commercially available Al-Beta zeolite is fully dealuminated with 13 mol / L -1 HNO3 solution (dealumination conditions: liquid-solid ratio 50 ml / g -1 , 140 °C, 24 h); (2) The dealuminated Beta zeolite is dried (120 °C) and calcined (550 °C, 6 h) for post-treatment to prepare dealuminated Beta zeolite (Beta-DA) with a Si / Al molar ratio > 1900; (3) Using Beta-DA as the silicon source, TEAOH solution as the templating agent, and TBOT as the Ti source, a colloidal solution is prepared; (4) The colloidal solution is dissolved at 140 °C for 1 h; (5) The colloidal solution after the dissolution treatment is quenched, and then NH4F is added to it at room temperature to assist the recrystallization process. The chemical composition of the colloid before recrystallization is 1SiO2: 1 / x TBOT: (0.3 - 0.5)TEAOH: (4 - 7.5)H2O: 0.1NH4F; (6) Recrystallization is carried out at 140 °C, and highly crystalline Ti-Beta zeolite can be obtained within 1 h.

[0015] The public document Mater. Chem. Front., 2021, 5, 6101 also reported a similar method. The Si / Al molar ratio of the dealuminated Beta zeolite used was >1500 (dealumination conditions of Al-Beta zeolite: 65 wt% HNO3 solution, liquid-solid ratio 30, dealumination at reflux temperature for 12 h, repeated once). The specific procedure for preparing Ti-Beta zeolite using dealuminated Beta zeolite (Beta-DA) as the silicon source is as follows: First, Beta-DA was added to a strongly basic aqueous solution of TEAOH under stirring and stirred for 10 min. Then, TBOT was added dropwise thereto. Finally, NH4F was added to the mixture to obtain a colloidal solution with a composition of 1.0SiO2:xTEAOH:yTiO2:0.5NH4F:7.0H2O. The above gel solution was recrystallized at 140 °C for 1 day to obtain the Ti-Beta zeolite product. The post-treatment of Ti-Beta zeolite includes filtration, washing, drying (80 °C, overnight) and calcination (550 °C, 6 h).

[0016] The following public documents are also related to the preparation of Ti-Beta zeolite by the structure reconstruction method: "Wang Bowen. Research on the Preparation and Catalytic Performance of MFI and BEA-Type Zeolites [D]. East China Normal University, 2021"; "You Qing. Preparation, Pore Multistaging and Catalytic Performance of Heteroatom Beta Zeolites [D]. Northwest University, 2021"; Microporous and Mesoporous Materials 330 (2022) 111625; "Ma Haikuo. Design and Synthesis of Heteroatom Zeolites with BEA Topology and Their Catalytic Performance [D] Yantai University, 2022."; "Pan Huang. Synthesis, Post-treatment Modification and Epoxidation Performance of Titanium Silicate Zeolites [D]. East China Normal University, 2022"; New J. Chem., 2021, 45, 10303; Chemical Industry Progress https: / / doi.org / 10.16085 / j.issn.1000-6613.2021-2097 (first online published on January 27, 2022). Generally speaking, the structure reconstruction method is a Ti-Beta zeolite preparation method with relatively high industrial application value, and its main advantages are high framework titanium content, short preparation time and good preparation repeatability. However, the existing structure reconstruction method still needs to rely on the mineralization of a large amount of fluorides for auxiliary crystallization. Discharging a large amount of highly fluorinated wastewater is the main challenge for the industrial application of the existing structure reconstruction method. Summary of the Invention

[0017] The present invention provides a fluoride-free and aluminum-free process for preparing Ti-Beta zeolite based on the local structure reconstruction method.

[0018] In other words, the present invention provides a process for preparing Ti-Beta zeolite by locally etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and reconstructing its structure using a composition of steam-assisted tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine, without involving the use of fluoride or / and aluminum source. This process has the following three most prominent technical features: (1) When preparing the hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite and titanium source, tetraethylammonium hydrobromide (TEABr) is required to be added as a templating agent, and at the same time, alkanolamine organic compounds with high boiling point and good thermal stability are added as the weak base source. The composition of the two is used to replace the strongly basic tetraethylammonium hydroxide (TEAOH) templating agent used in the existing preparation process, thus making it easier to avoid the complete dissolution and structural fragmentation of dealuminated Beta zeolite; (2) Before the local structure reconstruction of the hydrogel precursor, the free water in it should be removed by low-temperature drying treatment to convert it into a xerogel intermediate; (3) The local structure reconstruction process of the xerogel intermediate is carried out in a vapor atmosphere in an autoclave, that is, under steam assistance. And the vapor is generated by the liquid water at the bottom of the autoclave that does not directly contact the xerogel intermediate.

[0019] The fundamental difference between the present invention and the existing process for preparing Ti-Beta zeolite by structure reconstruction method lies in that the starting point of its structure reconstruction is not the fragmented structural units generated after the complete dissolution of the crystals of dealuminated Beta zeolite (long-range disorder, without characteristic diffraction peaks of Beta zeolite in the XRD pattern), but the whole dealuminated Beta zeolite crystal in which the hydroxyl nests are mildly etched and desilicated by the weak organic base of alkanolamine but the long-range order of the Beta zeolite structure is still well maintained (with complete characteristic diffraction peaks in the XRD pattern). Therefore, the structure reconstruction of the present invention belongs to local structure reconstruction. In addition, the present invention uses a combination of tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine to replace the strongly basic tetraethylammonium hydroxide (TEAOH) templating agent used in the existing preparation process. This not only helps to reduce the manufacturing cost of Ti-Beta zeolite, but also enables the flexible combination of the templating agent and the weak organic base to unlock the TEA + cation part that acts as a structure-directing agent (SDA) and the OH - anion part in a 1:1 ratio relationship. It is easy to mildly etch and desilicate the hydroxyl nests of dealuminated Beta zeolite under weak alkalinity and accelerate the structure reconstruction reaction with sufficient dosage of the structure-directing agent. In particular, both the TEABr templating agent and the weak organic base of alkanolamine used in the present invention are organic compounds with high thermal stability and high boiling point, which are more heat-resistant than TEAOH and allow the preparation of the xerogel intermediate to be carried out at a higher temperature, so it has higher practical value.

[0020] It was found in the research that because the process provided by the present invention does not involve the establishment of the crystal structure of Beta zeolite from scratch (i.e., does not involve the crystal growth of Beta zeolite), but only involves the repair of local structural defects left after titanium ions are accepted by the mildly etched hydroxyl nests, the process provided by the present invention can prepare high-quality Ti-Beta zeolite with a high framework titanium content and less non-framework titanium starting from dealuminated Beta zeolite through steam-assisted local structure reconstruction without the assistance of fluoride or / and aluminum source.

[0021] It was also found in the research that for the present invention, before performing local structure reconstruction, it is necessary to first perform low-temperature drying treatment on the hydrogel precursor to remove the free water therein and convert it into a xerogel intermediate, and then perform local structure reconstruction on the xerogel intermediate under steam assistance to avoid excessive etching of the hydroxyl nests of dealuminated Beta zeolite by the weakly basic solution of organic amine compounds, thereby avoiding partial disintegration of the crystal structure and excessive fragmentation. Those skilled in the art know that there is a chemical equilibrium in the dissolution of silicate solids in alkaline solutions. However, the larger the solution volume, the more silicate solid is consumed to reach the equilibrium state. This means that when the feed amount of the silicate solid (such as dealuminated Beta zeolite) is fixed, the larger the volume of the alkaline solution, the greater the dissolution degree of the silicate solid (such as dealuminated Beta zeolite). However, for the present invention, since it is steam that contacts the silicate solid and the silicate fragments are non-volatile, the dissolution equilibrium of silicate only involves the liquid film with a limited surface thickness and the liquid water filled in its pores due to capillary condensation. Due to the limited thickness of the liquid film on the solid surface and the pore volume of the zeolite, that is, the total amount of liquid is limited, the silicate fragments that can be dissolved by the liquid water condensed in the liquid film and pores are also limited. In addition, before performing local structure reconstruction, first performing low-temperature drying treatment on the hydrogel precursor to remove the free water therein and convert it into a xerogel intermediate, and then performing structure reconstruction on the xerogel intermediate under steam assistance is also beneficial to maximizing the local structure reconstruction rate of dealuminated Beta zeolite in the presence of TEABr template agent and the composition of weakly organic bases of organic amine compounds, thereby facilitating the maximum shortening of the time for preparing Ti-Beta zeolite by the structure reconstruction method. This is mainly because, under the condition of steam-assisted local structure reconstruction of the xerogel intermediate, TEABr template agent molecules, weakly organic base molecules of organic amine compounds, and soluble Beta zeolite structure units generated by mild etching of hydroxyl nests are all concentrated in the condensed liquid in the surface liquid film and pores, which is beneficial to accelerating the recrystallization reaction.

[0022] The present invention also has an essential difference from the existing process for preparing Ti-Beta zeolite by the dry gel conversion method. Although both processes involve steam assistance on the surface, the latter has to go through the crystallization process of amorphous dry gel. The dry gel crystallization process under steam assistance can sometimes be carried out without the addition of fluoride additives, but in the absence of fluoride mineralizer, an aluminum source needs to be introduced to assist the crystallization of amorphous dry gel. Therefore, it is difficult to obtain high-performance Ti-Beta zeolite by the existing dry gel conversion method.

[0023] It should be particularly emphasized that the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite based on the local structure reconstruction method provided by the present invention is not a simple combination of the existing dry gel conversion method and the structure reconstruction method. The innovation and advancement of the present invention lie in: First, the present invention uses a combination of tetraethylammonium bromide (TEABr) template agent and weak organic base of alkanolamine to replace the strong basic tetraethylammonium hydroxide (TEAOH) template agent used in the existing preparation process. At the same time, by means of the pore condensate generated by dealuminating Beta zeolite in saturated steam and the liquid film on the surface of zeolite solid to limit the dissolution amount of silicate, it is easier to realize the controllable etching and desilication of the hydroxyl nests of dealuminated Beta zeolite, improve the acceptance ability of the hydroxyl nests to larger titanium ions, and thus is more conducive to the preparation of high-performance Ti-Beta zeolite with high framework titanium content and low non-framework titanium content. Second, the controllable etching and desilication generate local structural defects, and the TEA + cationic structure-directing agent (SDA) ionized from the TEABr template agent in the solution can be used for rapid repair in the weakly basic environment generated by alkanolamine organic matter, completely getting rid of the trouble brought by the need to add fluoride or / and aluminum source for auxiliary structure reconstruction in the existing process due to the need to start from amorphous silicate fragments for the structure reconstruction of Beta zeolite. In addition, since both the TEABr template agent and the weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, they are more heat-resistant than TEAOH, and are inexpensive and widely available, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature. Therefore, the process provided by the present invention has higher practical value.

[0024] The technical solution of the present invention is as follows:

[0025] A fluoride-free and aluminum-free process for preparing Ti-Beta zeolite based on the local structure reconstruction method, the steps are as follows:

[0026] The first step: Prepare dealuminated Beta zeolite using Al-Beta zeolite as the raw material

[0027] Engineers familiar with the art can, according to the requirements of the present invention, combine their work experience and refer to the conventional acid dealumination methods in relevant literature to prepare dealuminated Beta zeolite using Al-Beta zeolite as the raw material. The requirements of the present invention are as follows:

[0028] (1) Select an Al-Beta zeolite raw material

[0029] The Al-Beta zeolite mentioned refers to a silica-alumina Beta zeolite. There are no restrictions on the crystal grain size of the Al-Beta zeolite in the present invention, nor are there any restrictions on the production process of the Al-Beta zeolite. However, in order to facilitate the implementation effect of the present invention, the following restrictions are imposed on the Al-Beta zeolite: 1) There are no impurity crystals in the Al-Beta zeolite; 2) The crystallization of the Al-Beta zeolite is good; 3) The molar ratio of silica-alumina oxides (molar ratio of SiO2 to Al2O3) of the Al-Beta zeolite is appropriate.

[0030] Among them, whether there are impurity crystals in the Al-Beta zeolite can be checked and confirmed by the X-ray polycrystalline powder diffraction (XRD) method. Those familiar with the art know that the molar ratio of SiO2 to Al2O3 of the Al-Beta zeolite produced by the hydrothermal synthesis method is usually between 10 and 200 (US3 308 069 (1967)). In Al-Beta zeolite products with a relatively low molar ratio of SiO2 to Al2O3, there may generally be mordenite (MOR) impurity crystals, while in Al-Beta zeolites with a relatively high molar ratio of SiO2 to Al2O3, there may generally be ZSM-5 zeolite impurity crystals. By sampling the Al-Beta zeolite and performing XRD analysis, and comparing the XRD pattern of the sample with the standard diffraction cards of Beta zeolite, MOR zeolite, and ZSM-5 zeolite, it is possible to determine whether there are characteristic peaks of MOR zeolite and ZSM-5 zeolite impurity crystals in the XRD pattern of the sample, and thus know whether the Al-Beta zeolite is a pure Beta zeolite phase.

[0031] In theory, the crystallization of Al-Beta zeolite can also be analyzed by XRD and measured by the relative crystallinity index. However, considering that the XRD relative crystallinity index needs to be obtained by comparing the sum of the intensities of the medium-intensity characteristic diffraction peaks of Al-Beta zeolite between 2θ = 7.6 - 8° and the highest-intensity characteristic diffraction peaks between 2θ = 22 - 23° with the sum of the intensities of the corresponding diffraction peaks of the reference sample (standard Beta zeolite with a crystallinity of 100%), and the reference sample is not uniformly defined; and considering that the intensities of the medium-intensity characteristic diffraction peaks of Al-Beta zeolite between 2θ = 7.6 - 8° and the highest-intensity characteristic diffraction peaks between 2θ = 22 - 23° are greatly affected by the processes and conditions of post-processing such as calcination. Therefore, using the XRD relative crystallinity index to determine whether the crystallization of the purchased or synthesized Al-Beta zeolite is good has poor universality. For this reason, the present invention recommends using the specific surface area index of Al-Beta zeolite to measure whether the crystallization of the purchased or synthesized Al-Beta zeolite meets the requirements. According to the statistical results of the reported values of the specific surface area data of Al-Beta zeolite in the literature, the BET specific surface area value of well-crystallized Al-Beta zeolite produced by the hydrothermal synthesis method is generally not less than 450m 2 / g. Engineers familiar with the art can use the conventional nitrogen physical adsorption method to first measure the nitrogen adsorption isotherm data of Al-Beta zeolite, and then calculate its BET specific surface area value according to the BET model. In short, the present invention requires that the BET specific surface area value of the used Al-Beta zeolite ≧ 450m 2 / g, indicating that its crystallization is good.

[0032] The molar ratio of SiO2 to Al2O3 is a key index of Al-Beta zeolite. This is because, on the one hand, the lower the molar ratio of SiO2 to Al2O3 in Al-Beta zeolite, that is, the higher the content of framework aluminum, the more the number of hydroxyl nests in dealuminated Beta zeolite, and the more the number of titanium ions that may be introduced into the framework during the steam-assisted local structure reconstruction process, which is beneficial to the preparation of Ti-Beta zeolite with a low Si / Ti ratio; on the other hand, it is more difficult to synthesize pure-phase Al-Beta zeolite with a very low molar ratio of SiO2 to Al2O3 by the hydrothermal method. Therefore, the suitable range of the molar ratio of SiO2 to Al2O3 of Al-Beta zeolite required by the present invention is between 10 - 200, the preferred range is between 20 - 100, and the more preferred range is between 25 - 60.

[0033] The molar ratio analysis of SiO2 to Al2O3 in Al-Beta zeolite can be carried out by traditional chemical analysis methods (titration method), or by X-ray fluorescence spectroscopy (XRF) method or inductively coupled plasma emission spectroscopy (ICP) method. The present invention recommends using the simple and fast XRF method.

[0034] The Al-Beta zeolite meeting the requirements of the present invention can be obtained through commercial channels or synthesized by oneself. Engineers familiar with the art can also synthesize the Al-Beta zeolite meeting the requirements of the present invention according to their own experience and other literature reports.If synthesizing Al-Beta zeolite by oneself, the methods reported in the following invention patents and published documents can be selected: US3 308 069 (1967), EP187 522A2 (1986), US4 847 055 (1989), CN1 086 792A (filing date: September 20, 1993), CN1 108 213A (filing date: March 11, 1994), CN1 108 214A (filing date: March 11, 1994), CN1154 341A (filing date: January 11, 1996), CN1 154 242A (filing date: January 9, 1996), CN1 154 342A (filing date: January 11, 1996), CN1 268 545A (filing date: March 30, 1999), CN1 133 497C (filing date: March 30, 1999), CN1108 275C (filing date: September 10, 1999), CN1 100 004C (filing date: May 19, 2000), CN1 335 258A (filing date: February 28, 2001), CN1 116 227C (filing date: March 12, 2001), CN101 205 072B (filing date: December 18, 2006), Chem.Comm., 1996, 625; J.Mater.Chem., 1998, 8(9), 2137-2145; Microporous and Mesoporous Materials 21(1998)305-313; Applied Catalysis A-GENERAL, 166(1998), 97–103; Microporous and Mesoporous Materials 48(2001)23-29; Microporous and Mesoporous Materials 56(2002)1–10.; Journal of Molecular Catalysis A: Chemical 252(2006)76–84; Microporous and Mesoporous Materials 94(2006)1–8; J.Mater.Sci.41(2006)1861-1864; Cryst.Res.Technol.44, No.4, 379-385(2009) DOI10.1002 / crat.200800474; Microporous and Mesoporous Materials 143(2011)97-103; RSC Adv.2019, 9, 3653-3660..

[0035] (2) Preparation of dealuminated Beta zeolite

[0036] As described above, to prepare dealuminated Beta zeolite based on Al-Beta zeolite, conventional acid dealumination methods can be used. The present invention requires that the molar ratio of SiO2 to Al2O3 of the dealuminated Beta zeolite be as high as possible, that is, the framework aluminum of Al-Beta zeolite should be removed as completely as possible. For the dealuminated Beta zeolite meeting the requirements of the present invention, the suitable range of the molar ratio of SiO2 to Al2O3 is ≥700, the preferred range is ≥800, and the more preferred range is ≥900.

[0037] Because the molar ratio of SiO2 to Al2O3 of the dealuminated Beta zeolite is very high and the aluminum content is very low, accurate determination of its molar ratio of SiO2 to Al2O3 requires the use of inductively coupled plasma emission spectrometry (ICP) method or atomic absorption (AA) method. The present invention recommends using the ICP method.

[0038] When performing acid dealumination treatment on Al-Beta zeolite, efforts should be made to remove all of its framework aluminum. The harm of excessive residual framework aluminum on the dealuminated Beta zeolite is that the strong acidity of the framework aluminum will significantly reduce the selectivity of Ti-Beta zeolite (more precisely, Ti-Al-Beta zeolite) as an oxidation reaction catalyst and weaken the advantages of the structure reconstruction method in preparing high-performance Ti-Beta zeolite.

[0039] Although the framework aluminum of Al-Beta zeolite is easy to remove, so that dealuminated Beta zeolite meeting the requirements of the present invention can be prepared based on Al-Beta zeolite by using high-temperature steam dealumination method, EDTA and other complexing agent dealumination methods, organic acid solution dealumination method, inorganic acid (concentrated hydrochloric acid, concentrated nitric acid) solution dealumination method, or dealumination methods formed by any combination of the above different methods, considering the production cost, process complexity of dealuminated Beta zeolite and the treatment difficulty of the waste liquid generated by dealumination, the present invention recommends using concentrated nitric acid aqueous solution dealumination method to prepare dealuminated Beta zeolite meeting the requirements of the present invention.

[0040] Engineers familiar with the art can, based on their own experience or by referring to the specific methods disclosed in the following literature, perform acid dealumination treatment on Al-Beta zeolite with an aqueous solution of concentrated nitric acid to prepare dealuminated Beta zeolite that meets the requirements of the present invention: Chemical Communications, 1998, 1: 87-88; Micropor. Mesopor. Mater., 1999, 31: 163-173; Micropor. Mesopor. Mater., 2001, 49: 103–109; Micropor. Mesopor. Mater., 2008, 110: 480–487; Micropor. Mesopor. Mater., 2012, 163: 122-130; ACS Catalysis, 2014, 4(8): 2801-2810. When performing acid dealumination on Al-Beta zeolite with an aqueous solution of concentrated nitric acid to prepare dealuminated Beta zeolite, the concentration of the nitric acid aqueous solution, the ratio of the acid solution to the zeolite (liquid-solid ratio), and the temperature and time of the acid treatment are all important factors affecting the degree of acid dealumination of Al-Beta zeolite. The influence of the above factors on the dealumination of Al-Beta zeolite ultimately reflects in the residual aluminum content of the dealuminated Beta zeolite. However, if dealuminated Beta zeolite with a molar ratio of SiO2 to Al2O3 that meets the requirements cannot be obtained after one-step dealumination, it is entirely possible to make the molar ratio of SiO2 to Al2O3 of the dealuminated Beta zeolite meet the requirements of the present invention through secondary or even multiple supplementary dealuminations. The present invention recommends using 13M concentrated nitric acid as the dealumination acid solution and using the acid solution amount according to a liquid-solid ratio of 20:1 (ml / g). Under this premise, the dealumination reaction is carried out at 95 °C for 20 h; after the dealumination reaction is completed, the solid product is first recovered by solid-liquid separation, then the solid product is washed with water until the pH value is neutral, and then dried at a temperature of 80-200 °C for 3-24 h and calcined at a temperature of 500 °C - 600 °C for 3-8 h to obtain dealuminated Beta zeolite. After the Al-Beta zeolite is dealuminated, due to the formation of a large number of hydroxyl nest lattice defect sites, its water absorption and moisture absorption capacity is stronger, and it should be sealed and stored for standby.

[0041] Step 2: Prepare a hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite

[0042] In addition to dealuminated Beta zeolite, other raw materials required for preparing a hydrogel precursor for local structure reconstruction include: tetraethylammonium bromide (TEABr) template agent, alcoholamine-based weak organic base, titanium source, and deionized water.

[0043] The dosage of dealuminated Beta zeolite is calculated based on SiO2, and the dosages of other raw materials are as follows:

[0044] The molar ratio of TEABr to SiO2: The suitable range is 0.05 - 0.5; the preferred range is 0.1 - 0.4; the more preferred range is 0.2 - 0.3.

[0045] The molar ratio of alkanolamine to SiO2: The suitable range is 0.1 - 0.5; the preferred range is 0.15 - 0.4; the more preferred range is 0.2 - 0.3.

[0046] The molar ratio of H2O to SiO2: The suitable range is 2 - 10; the preferred range is 3 - 8; the more preferred range is 4 - 6.

[0047] The molar ratio of Si to Ti: The suitable range is 5 - 100; the preferred range is 10 - 80; the more preferred range is 15 - 50.

[0048] The alkanolamine - type weak organic base mentioned above mainly refers to monoethanolamine, mainly because it is widely sourced and inexpensive, especially its boiling point (170 °C) meets the requirements of low - temperature drying.

[0049] Engineers familiar with this field know that, in addition to tetraethylammonium bromide, other tetraethylammonium halides, such as tetraethylammonium fluoride, tetraethylammonium chloride, and tetraethylammonium iodide, can also release the structure - directing TEA + cation in aqueous solution. However, the problem is that tetraethylammonium fluoride will result in fluoride - containing wastewater; tetraethylammonium iodide is relatively expensive and the I - ion has poor stability; tetraethylammonium chloride is not as inexpensive and easily available as tetraethylammonium bromide. Therefore, based on a comprehensive consideration of the above factors, tetraethylammonium bromide is selected as the template agent in this invention.

[0050] The titanium source suitable for this invention is mainly organic compounds of titanium, such as tetrabutyl titanate (TBOT), tetraethyl titanate (TEOT), tetra - isopropyl titanate (TPOT), and titanium acetylacetonate. Since generally the cost of the titanium source has little impact on the production cost of titanium - silicalite, the selection of the titanium source should mainly consider whether it is conducive to the entry of titanium ions into the framework of titanium - silicalite and to maximize the avoidance of non - framework titanium in titanium - silicalite. Based on this principle, the preferred titanium sources for this invention are tetrabutyl titanate (TBOT) and tetraethyl titanate (TEOT), and more preferably tetrabutyl titanate. The titanium source suitable for this invention can be stabilized with common complexing agents before use. For example, tetrabutyl titanate can be stabilized with an appropriate amount of isopropanol before use.

[0051] The operation of preparing the hydrogel precursor can be carried out at room temperature. The basic procedure is as follows: First, according to the dosage of dealuminated Beta zeolite and the selected molar ratios of H2O to SiO2, alkanolamine to SiO2, Si to Ti, and TEABr to SiO2, deionized water, alkanolamine, titanium source, and TEABr template agent are measured; then, under stirring, the TEABr template agent is dissolved in water, and the titanium source is dissolved in alkanolamine. Finally, under stirring, the TEABr template agent solution and dealuminated Beta zeolite are successively added to the weakly basic solution containing alkanolamine and titanium source, and stirred at room temperature for a period of time (mild desilication reaction). The suitable range of the stirring reaction time is 0.5 - 12 h; the preferred range is 1 - 6 h; the more preferred range is 2 - 4 h.

[0052] The TEABr template agent can ionize in aqueous solution to produce TEA + cations. We found in the study that the TEA + cations generated by the ionization of the TEABr template agent in aqueous solution can, like the TEA + cations ionized from the TEAOH template agent solution, act as a structure-directing agent (SDA) in the final stage of preparing Ti-Beta zeolite, that is, in the later stage of the steam-assisted local structure reconstruction process, which is the process of repairing the local structural defects left after the hydroxyl nests that have been mildly etched accept titanium ions. The weakly basic OH - anions hydrolyzed from alkanolamine weak organic bases in aqueous solution (R-NH2 + H2O → R-NH3 + + OH - ) act as an etching agent in the early stage of preparing Ti-Beta zeolite - including the preparation process of the hydrogel precursor and the process of removing free water from the hydrogel precursor by low-temperature drying treatment to be converted into a xerogel intermediate, and the initial stage of the steam-assisted local structure reconstruction process (during the autoclave heating process), and are responsible for mildly desilicating the hydroxyl nests of dealuminated Beta zeolite. Within the range of the TEABr / SiO2 molar ratio and alkanolamine / SiO2 molar ratio recommended in the present invention, the etching and desilication effect of the weakly basic aqueous solution provided by alkanolamine organic matter can cooperate with the structure-directing effect of the TEA + cations released by the TEABr template agent in aqueous solution to promote two local structure reconstruction reactions for preparing Ti-Beta zeolite from dealuminated Beta zeolite, including the etching and desilication reaction of hydroxyl nests and the recrystallization reaction of silicate fragments.

[0053] The silicon-titanium ratio of the hydrogel precursor, i.e., the Si / Ti molar ratio, determines the silicon-titanium ratio of the Ti-Beta zeolite prepared by the method of the present invention. According to the principle of the present invention, the titanium source introduced when preparing the hydrogel precursor will release titanium ions during the steam-assisted local structural reconstruction, and the latter will enter the slightly etched dealuminated Beta zeolite hydroxyl pits, thereby converting the dealuminated Beta zeolite into Ti-Beta zeolite. Therefore, the maximum amount of titanium source used when preparing the hydrogel precursor can be estimated by the number of hydroxyl pits of the dealuminated Beta zeolite, or directly estimated by the silicon-aluminum ratio (SiO2 / Al2O3) of the Al-Beta zeolite raw material. For example, the lowest Si / Ti ratio of the Ti-Beta zeolite that can be prepared by using the dealuminated Beta zeolite prepared from the Al-Beta zeolite raw material with a SiO2 / Al2O3 ratio of 30 is 15. Therefore, when preparing a hydrogel precursor, the maximum amount of titanium source used should make the Si / Ti ratio of the precursor ≮15, otherwise excessive titanium source use will lead to an increase in non-framework titanium in the Ti-Beta zeolite product. Generally speaking, the framework titanium content in titanium silicalite is beneficial to the catalytic reaction within an appropriate range. On the one hand, a too low framework titanium content of titanium silicalite (i.e., the Si / Ti molar ratio is too high) is not conducive to catalyst activity, and the reason is needless to elaborate. On the other hand, a too high framework titanium content of titanium silicalite (i.e., the Si / Ti ratio is too low) is also not beneficial to increasing catalytic activity. This is because, in this case, diffusion control within the zeolite micropores will become the rate-controlling step of the catalytic reaction. The above is the main basis for determining the range of the Si / Ti molar ratio in the present invention.

[0054] In the stage of preparing the hydrogel precursor, the role of adding deionized water is to ensure that the four components of dealuminated Beta zeolite, TEABr, weak organic base of alkanolamine, and titanium source form a uniform hydrogel precursor. Therefore, in the preparation stage of the hydrogel precursor, if the amount of water introduced is too low (i.e., the H2O / SiO2 ratio is too low), on the one hand, it is difficult to ensure the uniformity of the hydrogel precursor, and on the other hand, it is not conducive to the formation of a weak alkaline solution by alkanolamine organic compounds to carry out desilication etching to a necessary extent on the hydroxyl nests of dealuminated Beta zeolite. The present invention requires that the minimum H2O / SiO2 ratio of the hydrogel precursor (the ratio of the added deionized water to the feed amount of dealuminated Beta zeolite (dry basis, calculated as SiO2)) should be greater than the saturated water absorption rate of dealuminated Beta zeolite (the saturated water absorption rate of typical dealuminated Beta zeolite is generally 1.2 ml H2O / g dehydrated zeolite. The dehydrated zeolite is defined as the zeolite sample that is first dried overnight at 110 °C and then calcined at 550 °C for 3 h. The saturated water absorption amount of a certain amount of dehydrated zeolite is the amount of deionized water consumed when the dehydrated zeolite is titrated with deionized water until all samples are uniformly wet but no free water appears). The maximum H2O / SiO2 ratio of the hydrogel precursor, that is, the maximum amount of deionized water used, is mainly restricted by the aging time and energy consumption of removing the free water in the hydrogel precursor through low-temperature drying treatment in the next step and converting it into a dry gel intermediate. Undoubtedly, on the premise of ensuring the uniformity of the hydrogel precursor and that the weak alkaline solution formed by alkanolamine organic compounds can carry out desilication etching to a necessary extent on the hydroxyl nests of dealuminated Beta zeolite, the amount of deionized water used should not be too much, so as not to increase the low-temperature drying and dehydration burden in the subsequent step of preparing the dry gel intermediate with the hydrogel precursor due to too high H2O / SiO2 ratio of the hydrogel precursor. The above is the main basis for the present invention to determine the range of H2O / SiO2 molar ratio.

[0055] Step 3: Remove the free water in the hydrogel precursor through low-temperature drying treatment and convert it into a dry gel intermediate

[0056] The low-temperature drying treatment described in the present invention generally refers to various applicable low-temperature drying methods that can remove most of the free water in the hydrogel precursor by heating and evaporation at a certain temperature, but will not decompose, transform, or remove the organic compounds contained therein (mainly TEABr template agent and alkanolamine organic compounds). In addition to removing most of the free water in the hydrogel precursor, during the low-temperature drying treatment, due to the effect of temperature, the weak alkaline solution environment generated by alkanolamine organic compounds in the hydrogel precursor (R-NH2 + H2O → R-NH3 + + OH -) It will also undergo a mild etching and desilication reaction with the hydroxyl nests of dealuminated Beta zeolite. Therefore, the dry gel intermediate described in the present invention refers to the product obtained after the hydrogel precursor is subjected to low-temperature drying treatment to remove most of the free water. Compared with the hydrogel precursor, the dry gel intermediate not only lacks most of the free water, but also the hydroxyl nests of its dealuminated Beta zeolite have been further etched by the weak alkaline solution environment generated by the alkanolamine organic matter in the hydrogel precursor, and a small amount of silicon oxygen tetrahedrons (SiO4) have fallen off from the edge of the hydroxyl nests and become soluble monomeric silicate or oligomeric silicate species. However, the changes in the content, structure, and physical and chemical properties of the organic matter therein (mainly the TEABr template agent and alkanolamine organic matter) can be ignored.

[0057] To enable engineers in the art to have a more specific understanding of the low-temperature drying treatment and dry gel intermediate described in the present invention, the present invention hereby provides a typical method for preparing a small amount of dry gel intermediate in the laboratory: Place the hydrogel precursor in an ordinary electric heating oven at 80 °C and dry it until it reaches a constant weight state, and the dry gel intermediate meeting the requirements of the present invention can be obtained. Analysis data shows that the hydrogel precursor will not decompose, transform, or remove the TEABr template agent and alkanolamine organic matter during drying and dehydration in an ordinary electric heating oven at 80 °C, and the dry gel intermediate dried to a constant weight state at 80 °C still contains about 25 wt.% of water, mainly in the form of bound water and very little free water. We found through thermogravimetric (TG) analysis that the weight loss caused by the removal of free water from the hydrogel precursor occurs in the low-temperature region of 30-130 °C; while the weight loss caused by the dry gel removing its bound water (constrained by pores and hydroxyl nests) occurs in the medium-temperature region of 130-166 °C, and the weight loss caused by decomposing and removing the TEABr template agent and alkanolamine organic matter therein appears in the high-temperature region above 166 °C.

[0058] Therefore, for the low-temperature drying treatment described in the present invention, the suitable temperature range is between room temperature (30 °C) and 160 °C, preferably between 50-150 °C, and more preferably between 80-120 °C. The total water content in the dry gel intermediate can be controlled between 15-25 wt% by means of drying pressure, drying time, and atmosphere conditions, and the decomposition or removal of TEABr and the weak organic base of alkanolamine during the drying process can be prevented.

[0059] For the present invention, the low-temperature drying temperature is a key control parameter in the drying process of the hydrogel precursor. At the selected temperature, a dry gel intermediate meeting the requirements of the present invention can be prepared through the coordination of pressure and time parameters. For the present invention, a qualified dry gel intermediate should have two characteristics: First, the changes in the content, structure, and physical and chemical properties of the organic substances therein (mainly the TEABr template agent and organic substances of the alkanolamine type) can be ignored; Second, most of the free water in the hydrogel has been removed. To facilitate engineers in the art to judge whether the dry gel intermediate meets the requirements of the present invention, the present invention provides the following simplest judgment method: (1) Judge whether the first characteristic is met from the appearance color of the dry gel intermediate. If the color of the dry gel intermediate is white, it means that TEABr and alkanolamine organic substances have not decomposed and their structures have not changed during the drying process, meeting the first characteristic. If the color of the dry gel intermediate is light yellow or egg yolk, it means that TEABr or / and alkanolamine organic substances have decomposed and their structures have changed during the drying process, not meeting the first characteristic; (2) Take a sample of the dry gel intermediate for thermogravimetric analysis. If the sample has weight loss due to dehydration in both the low-temperature region (free water removal region) of 30-130 °C and the medium-temperature region (bound water removal region) of 130-166 °C, and the total weight loss rate is between 15-25%, it indicates that the degree of dehydration during low-temperature drying is appropriate. If the total weight loss rate > 25%, it indicates that the degree of dehydration during low-temperature drying is insufficient. If the total weight loss rate < 15%, it indicates that the degree of dehydration during low-temperature drying is excessive. For a dry gel intermediate meeting the requirements of the present invention, if it is severely caked, it needs to be post-treated by conventional grinding or crushing methods and then sealed for standby.

[0060] Engineers familiar with the art can flexibly select a specific low-temperature drying method according to the preparation scale of the dry gel intermediate and the convenient conditions of the experimental and production sites. For example, when preparing the dry gel intermediate on a laboratory scale, it is very convenient to select an ordinary electric heating oven as the low-temperature drying equipment to conduct low-temperature drying on the hydrogel precursor. When preparing the dry gel intermediate on an industrial scale, either a box furnace or a tunnel kiln can be conveniently used as the low-temperature drying equipment. The drying process can be carried out in a flowing air atmosphere, a static air atmosphere, or under a slightly negative pressure condition. The present invention only limits the temperature range of the low-temperature drying process of the hydrogel precursor and the indicators of the dry gel intermediate, and other aspects including drying equipment, drying pressure, atmosphere, and time parameters are all selected by engineers familiar with the art according to needs.

[0061] Step 4: Preparation of Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate

[0062] The reaction process for preparing Ti-Beta zeolite by local structure reconstruction of a steam-assisted dry gel intermediate is carried out in an autoclave. A bracket and a tray are provided inside the autoclave. The dry gel intermediate powder is placed on the tray, and the auxiliary steam is provided by vaporizing the liquid water at the bottom of the autoclave below the tray.

[0063] In this step, the addition amount of the liquid water at the bottom of the autoclave, as well as the reaction temperature and time for structure reconstruction, are the main influencing factors. For their value ranges, the present invention requires the following:

[0064] The addition amount of the liquid water at the bottom of the autoclave is calculated based on the mass ratio of H2O to the dry gel intermediate (W H2O / W 干胶中间体 ratio). The suitable range of the mass ratio of H2O to the dry gel intermediate is: 1.5 - 3; the preferred range is: 1.8 - 2.8; the more preferred range is: 2.0 - 2.5.

[0065] The suitable range of the reaction temperature for structure reconstruction is: 120 - 170 °C; the preferred range is: 130 - 160 °C; the more preferred range is: 135 - 150 °C.

[0066] The suitable range of the reaction time for structure reconstruction is: 0.5 - 48 h; the preferred range is: 4 - 24 h; the more preferred range is: 6 - 18 h.

[0067] The selection of the above condition ranges is to meet the requirements of the reaction for preparing Ti-Beta zeolite by local structure reconstruction of a steam-assisted dry gel intermediate. To enable engineers in the field to better understand the selection of the above condition ranges made by the present invention, the reaction process for preparing Ti-Beta zeolite by the method of local structure reconstruction of a steam-assisted dry gel intermediate is described as follows:

[0068] In the initial stage of the local structure reconstruction process, that is, during the heating stage of the autoclave, as the temperature inside the autoclave gradually rises, more and more liquid water at the bottom of the autoclave turns into water vapor, and the vapor pressure inside the autoclave continuously rises accordingly. As a result, the dry gel intermediate powder on the tray obtains a correspondingly higher water content due to the infiltration of more and more water vapor. During this process, the dealuminated Beta zeolite, TEABr template agent, alcohol amine-based weak organic base, and silicate fragments (soluble Beta zeolite structure units) generated by the mild etching of the weak alkaline solution formed by alcohol amine-based organic substances in the hydrogel precursor during the preparation of the hydrogel precursor and the subsequent low-temperature drying process in the dry gel will also undergo corresponding changes: First, for the dealuminated Beta zeolite, on the one hand, its pores will be filled with liquid water due to capillary condensation, and on the other hand, the surface of the zeolite will develop from a monolayer to a multilayer of water molecule adsorption, eventually forming a liquid film with a certain thickness. Second, for the TEABr template agent and alcohol amine molecules originally in a dehydrated state in the dry gel intermediate, their main change is to redissolve in water and ionize TEA+ Cations and OH - anions, thus turning the capillary condensed liquid in the zeolite channels and the water film on the zeolite surface into an alkaline solution containing TEA + ions. The alkaline solution that permeates inside and outside the channels of dealuminated Beta zeolite will inevitably undergo a further etching and desilication reaction with the hydroxyl nests of dealuminated Beta zeolite. In addition, for the mild etching of the weak alkaline solution generated by the alcoholamine organic matter in the hydrogel precursor during the preparation of the hydrogel precursor and the subsequent low-temperature drying process, and for the silicate fragments (soluble Beta zeolite structural units) that have been generated in the dry gel intermediate, their main change is to dissolve in the alkaline solution that permeates inside and outside the channels of dealuminated Beta zeolite, which plays a role in delaying the further etching and desilication reaction of the hydroxyl nests of dealuminated Beta zeolite in the alkaline solution.

[0069] When the temperature of the autoclave rises to a level sufficient to carry out the structure reconstruction reaction (recrystallization reaction), the concentration of the silicate fragments (soluble Beta zeolite structural units) dissolved in the alkaline solution that permeates inside and outside the channels of dealuminated Beta zeolite will also reach the highest point - the chemical equilibrium concentration. At this time, the etching and desilication reaction of the hydroxyl nests of dealuminated Beta zeolite by the weak alkaline solution generated by the alcoholamine organic matter in the dry gel intermediate is inhibited, and the entire closed system enters the later stage of the steam-assisted local structure reconstruction process. The main characteristics of this stage are: First, the titanium ions released from the titanium source in the dry gel intermediate begin to enter the etched hydroxyl nests of dealuminated Beta zeolite. The etched hydroxyl nests have increased in volume due to mild desilication, improving their ability to accept larger titanium ions. Because of this, the process for preparing Ti-Beta zeolite by the steam-assisted partial structure reconstruction method provided by the present invention has the characteristic of a high framework titanium content. Then, the silicate fragments (soluble Beta zeolite structural units) dissolved and concentrated in the alkaline solution with a limited total liquid volume that permeates inside and outside the channels of dealuminated Beta zeolite, under the action of the TEA + cations with a structure-directing effect (SDA), quickly carry out a local structure reconstruction reaction (recrystallization reaction) at the local structural defect sites left after the hydroxyl nests of dealuminated Beta zeolite accept titanium ions to repair the structural defects.

[0070] In the present invention, in order to achieve the reaction purpose and effect of preparing Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate, the amount of water introduced in advance to the bottom of the autoclave should at least enable the formation of a vapor-liquid two-phase inside the autoclave at the reaction temperature of the structure reconstruction. Because only under the condition of coexistence of the vapor-liquid two-phase, the pores of dealuminated Beta zeolite will be filled with liquid water due to capillary condensation, and at the same time, the surface of dealuminated Beta zeolite will develop from monolayer to multilayer water molecule adsorption, and finally form a liquid film with a certain thickness. It is not difficult to see from the above that the liquid water condensed in the pores of dealuminated Beta zeolite and the liquid film with a certain thickness formed on the surface of the zeolite are the phase interface micro-hydrothermal systems that support the reaction process of each stage of steam-assisted local structure reconstruction of the dry gel intermediate, thereby realizing the preparation of Ti-Beta zeolite.

[0071] According to the saturated vapor pressure of water vapor, when the temperatures inside the autoclave are 120 °C, 130 °C, 140 °C, 150 °C, 160 °C and 170 °C respectively, the saturated water vapor pressures generated by the liquid water added to the bottom of the autoclave are 198.5 kPa, 247.5 kPa, 295.0 kPa, 355.1 kPa, 431.3 kPa and 517.8 kPa in sequence. Regarding the saturated water vapor inside the autoclave as an ideal gas approximately, when the volume of the autoclave is V (unit: m 3 ) and it is filled with saturated water vapor at 120 °C, 130 °C, 140 °C, 150 °C, 160 °C and 170 °C, the reduced masses of water vapor (unit: Kg) are approximately 1.1V, 1.3V, 1.6V, 1.8V, 2.2V and 2.5V in sequence.

[0072] Taking the preparation of Ti-Beta zeolite by steam-assisted local structure reconstruction reaction at 140 °C as an example, in order to enable the formation of a vapor-liquid two-phase inside the autoclave at the structure reconstruction reaction temperature of 140 °C, the estimated minimum amount of water to be introduced in advance to the bottom of the autoclave is: 1.6V + 1.2W. Among them, 1.2 is the saturated water absorption rate of typical dealuminated Beta zeolite, and W is the weight of the dry gel intermediate. For a small laboratory autoclave with a volume of 100 ml, assuming the addition amount of the dry gel intermediate is 10 g (0.01 Kg), in order to enable the formation of a vapor-liquid two-phase inside the small autoclave at the structure reconstruction reaction temperature of 140 °C, the estimated minimum amount of water to be introduced in advance to the bottom of the autoclave is 12.16 g (1.6V = 1.6 × 1 × 10 -4 Kg = 0.16 g; 1.2W = 1.2 × 10 × 10 -3Kg = 12 g). Therefore, theoretically, as long as more than 12.16 g of liquid water is added in advance to the bottom of the small autoclave, and 10 g of dry gel intermediate is added to the tray above the support, the preparation of Ti-Beta zeolite can be achieved through the steam-assisted local structure reconstruction reaction of the dry gel intermediate at 140 °C. Of course, in actual work, the threshold value of the amount of liquid water that should be introduced into the bottom of the autoclave in advance can be appropriately increased according to the volume of the autoclave, the size of the internal components (the height of the support and the size of the tray), and the single autoclave production requirement of Ti-Beta zeolite. Therefore, for the convenience of engineers in this field, the present invention finally recommends using W H2O / W 干胶中间体 The ratio of 1.5 - 3 is used as the appropriate range of water addition amount in the stage of preparing Ti-Beta zeolite through the steam-assisted local structure reconstruction reaction of the dry gel intermediate.

[0073] After the steam-assisted local structure reconstruction reaction of the dry gel intermediate is completed, the autoclave is cooled and depressurized in accordance with the usual practice of hydrothermal synthesis of zeolite molecular sieves. When the temperature and pressure in the autoclave are close to the ambient temperature and pressure, the autoclave is opened and the Ti-Beta zeolite product is taken out.

[0074] Step 5: Post-treatment of the Ti-Beta zeolite product

[0075] The Ti-Beta zeolite product prepared by the method of the present invention does not need to be filtered, washed, especially post-treated for defluorination and dealumination. Only conventional drying and calcination treatments are required to obtain the Ti-Beta zeolite product. That is, after the steam-assisted local structure reconstruction reaction of the dry gel intermediate is completed, the Ti-Beta zeolite product is collected from the tray in the autoclave, and then subjected to drying and calcination treatments. Among them, the purpose of calcination is to remove the TEABr template agent and alcohol amine organic substances in the product. Engineers familiar with this field can perform the post-treatment operations according to common sense.

[0076] Advantages of the present invention:

[0077] The present invention provides a new process for preparing Ti-Beta zeolite. The main raw material is dealuminated Beta zeolite, and there is no need to add fluoride or / and aluminum source for assistance. Mainly through the combination of steam-assisted tetraethylammonium bromide (TEABr) template agent and weak organic base of alkanolamine, local desilication etching and repair and reconstruction of the hydroxyl nests of dealuminated Beta zeolite are carried out, and then Ti-Beta zeolite is prepared. The main technical innovation of the present invention lies in that, firstly, the present invention uses the combination of tetraethylammonium bromide (TEABr) template agent and weak organic base of alkanolamine to replace the strong basic tetraethylammonium hydroxide (TEAOH) template agent used in the existing preparation process. At the same time, by means of the pore condensation liquid generated by dealuminated Beta zeolite in saturated steam and the liquid film on the zeolite solid surface to limit the dissolution amount of silicate, it is easier to realize the controllable etching desilication of the hydroxyl nests of dealuminated Beta zeolite, improve the acceptance ability of the hydroxyl nests for larger titanium ions, and thus is more conducive to the preparation of high-performance Ti-Beta zeolite with high framework titanium content and low non-framework titanium content. Secondly, the local structural defects generated by controllable etching desilication can be directly repaired quickly in the weak alkaline environment generated by alkanolamine with the TEA + cationic structure-directing agent (SDA) ionized by the TEABr template agent in the solution, completely getting rid of the trouble brought by the need to start from amorphous silicate fragments for the structure reconstruction of Beta zeolite in the existing process, and thus the need to add fluoride or / and aluminum source for assisting structure reconstruction. In addition, since both the TEABr template agent and the weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, they are more heat-resistant than TEAOH, and are inexpensive and widely available, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature. Therefore, the process provided by the present invention has higher practical value. Description of the Drawings

[0078] Figure 1 is the XRD pattern of the dry gel intermediate prepared in Example 1.

[0079] Figure 2 is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-1 sample prepared in Example 1.

[0080] Figure 3 is the X-ray powder diffraction (XRD) pattern of the Ti-Beta-1 sample prepared in Example 1.

[0081] Figure 4 is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-1-F-Com sample prepared in Comparative Example 1.

[0082] Figure 5 is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-2-Cl-Com sample prepared in Comparative Example 2.

[0083] Figure 6 It is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-3-Cp-Com sample prepared in Comparative Example 3. Detailed implementation manners

[0084] The following further illustrates the detailed implementation manners of the present invention in combination with the attached drawings and technical solutions.

[0085] The implementation effect of the present invention can be evaluated from two aspects: characterizing the physical and chemical properties of the prepared Ti-Beta zeolite product and detecting its catalytic performance in the cyclohexene epoxidation reaction.

[0086] In terms of characterizing the physical and chemical properties of the Ti-Beta zeolite product, the titanium content, framework titanium content, non-framework titanium content, and relative crystallinity can be mainly characterized.

[0087] Among them, the titanium content is detected by X-ray fluorescence spectrometry (XRF); the framework titanium content can be characterized by Fourier transform infrared spectroscopy of framework vibration (FT-IR); the non-framework titanium content can be characterized by ultraviolet resonance Raman spectroscopy (UV-Raman) with an excitation light source wavelength of 325 nm.

[0088] The cyclohexene epoxidation reaction is carried out in a 25 mL round-bottom flask equipped with a condenser and a magnetic stirrer. The procedure is as follows: First, 10 mmol of cyclohexene, 10 mL of acetonitrile, 10 mmol of hydrogen peroxide (35%), and 100 mg of Ti-Beta zeolite catalyst are added to the flask, and then vigorous stirring is started at 323 K for 2 hours. Finally, a sample is taken from the reaction product, and the composition analysis is carried out with a gas chromatograph (Shimadzu GC-2014C) equipped with a flame ionization detector (FID) and a DB-WAX capillary column (30 m × 0.32 mm, 30 μm). The conversion rate of cyclohexene and the selectivity of cyclohexene oxide are calculated by the external standard method.

[0089] In addition, in the process of preparing Ti-Beta zeolite by the method of local structure reconstruction of steam-assisted dealuminated Beta zeolite, it also involves the detection of impurity crystals, relative crystallinity analysis, and analysis of the silicon-aluminum ratio (molar ratio of SiO2 to Al2O3) of the Al-Beta zeolite raw material; the detection of the residual aluminum content (expressed as the molar ratio of SiO2 to Al2O3) in the dealuminated Beta zeolite, and the analysis and characterization of the water content, template agent content, and the degree of retention of the crystal structure of the dealuminated Beta zeolite in the dry gel intermediate.

[0090] Among them, the detection of hetero-crystals in the Al-Beta zeolite raw material and the characterization of the degree of retention of the dealuminated Beta zeolite crystal structure in the dry gel intermediate can be carried out by the X-ray powder diffraction (XRD) method; the relative crystallinity detection of the Al-Beta zeolite raw material can be achieved by measuring the nitrogen physical adsorption data of the sample, calculating the BET total surface area value of the sample on this basis, and drawing a conclusion; the silicon-aluminum ratio (molar ratio of SiO2 to Al2O3) of the Al-Beta zeolite raw material can be detected by X-ray fluorescence spectrometry (XRF); the residual aluminum content of the dealuminated Beta zeolite (expressed as the molar ratio of SiO2 to Al2O3) can be analyzed by inductively coupled plasma emission spectrometry (ICP); the water content and template agent content of the dry gel intermediate are analyzed by thermogravimetry (TG).

[0091] The present invention will be further described below by way of examples, but the present invention is not limited by these examples.

[0092] Example 1: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source, mainly through the combination of steam-assisted tetraethylammonium bromide (TEABr) template agent and weak organic amine bases, locally etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and reconstructing the structure to prepare Ti-Beta zeolite. The process can replace the strong basic tetraethylammonium hydroxide (TEAOH) template agent used in the existing preparation process with the combination of tetraethylammonium bromide (TEABr) template agent and weak organic amine bases. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the zeolite solid surface to limit the dissolution amount of silicate, it is more convenient to achieve the controllable etching and desiliconization of the hydroxyl nests of dealuminated Beta zeolite, so as to achieve the purpose of slightly etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and avoiding the complete dissolution of dealuminated Beta zeolite crystals by strong basic TEAOH solution into fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed into repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the slightly etched hydroxyl nests accept titanium ions (the present invention). Since the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both the TEABr template agent and weak organic amine bases are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH and allow the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0093] Step 1: Prepare dealuminated Beta zeolite using Al-Beta zeolite as raw material

[0094] (1) According to the hydrothermal crystallization method provided by US Patent US3 308 069 (1967), self-synthesize Al-Beta zeolite with a molar ratio of silicon to aluminum oxide (molar ratio of SiO2 to Al2O3) of 25 as the raw material for preparing dealuminated Beta zeolite. After the synthesized Al-Beta zeolite undergoes conventional filtration, washing, drying (110 °C, 12 h), and calcination to remove the template agent (540 °C, 6 h), its crystal grain size is observed to be less than 100 nm by TEM, belonging to nano-Beta zeolite; no any impurity crystals are detected by XRD method, and its BET specific surface area is calculated to be approximately 540 m 2 / g using its nitrogen physical adsorption data, and the molar ratio of silicon to aluminum oxide (molar ratio of SiO2 to Al2O3) is measured to be approximately 24 by XRF method, meeting the technical requirements of the Beta zeolite raw material for this invention.

[0095] (2) Prepare dealuminated Beta zeolite.

[0096] Firstly, prepare a concentrated nitric acid solution with a molar concentration of 13 M. Then, according to the liquid-solid ratio of 20:1 (ml / g), under stirring, add 20 g of the Beta zeolite matrix after the above drying and calcination treatments to a three-necked flask containing 400 ml of 13 M concentrated nitric acid solution for dealumination treatment. The dealumination temperature is 95 °C and the dealumination time is 20 h. During the dealumination reaction, the three-necked flask is kept in a reflux state. After the dealumination reaction ends, cool the feed liquid to room temperature and then filter to recover the solid product. Then, perform conventional water washing, drying (overnight at 110 °C), and calcination treatment (550 °C, 3 h) to obtain dealuminated Beta zeolite. The molar ratio of silicon to aluminum oxide (molar ratio of SiO2 to Al2O3) of the dealuminated Beta zeolite is measured to be 990 by ICP method, meeting the requirements of this invention. Seal and store for later use, avoiding moisture absorption.

[0097] Step 2: Prepare a hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite

[0098] Using tetraethylammonium bromide (TEABr) solid as the template agent source, monoethanolamine (abbreviated as ethanolamine) as the weak organic base, and tetrabutyl titanate (TBOT) as the titanium source, determine the amount of template agent according to the TEABr / SiO2 molar ratio (using the number of moles of SiO2 to represent the number of moles of dealuminated Beta zeolite) of 0.3; determine the amount of ethanolamine according to the ethanolamine / SiO2 molar ratio of 0.3; determine the amount of tetrabutyl titanate (TBOT) according to the Si / Ti molar ratio of 45; determine the amount of deionized water according to the H2O / SiO2 molar ratio of 6.

[0099] The specific method is as follows: First, taking 10 g of dealuminated Beta zeolite as the dosage, weigh 10.72 g of tetraethylammonium bromide (TEABr), 3.12 g of ethanolamine, 1.29 g of tetrabutyl titanate (TBOT), and 18.36 g of deionized water respectively. Then, under stirring, mix the titanium source with ethanolamine first, and then add the aqueous solution prepared from TEABr and deionized water to the mixture of the titanium source and ethanolamine, and stir evenly; finally, under stirring, pour all the dealuminated Beta zeolite into the weakly alkaline solution containing the titanium source, TEABr template agent and ethanolamine, and stir and react at room temperature (mild desilication reaction) for 3 h to obtain the hydrogel precursor for the preparation of Ti-Beta zeolite by local structure reconstruction.

[0100] The third step is to remove the free water in the hydrogel precursor by low-temperature drying to convert it into a xerogel intermediate.

[0101] Add 35 g of the hydrogel precursor prepared in the second step to a crucible, and then place the crucible in an ordinary electric heating oven at 80 °C. Slowly dry and dehydrate it under normal pressure and still air. After 12 h, it reaches a constant weight state, and about 25 g of the xerogel intermediate is obtained. The XRD pattern of the xerogel intermediate is shown in Figure 1 . It can be seen from Figure 1 that the xerogel intermediate prepared according to the requirements of the present invention well retains the crystal structure of the dealuminated Beta zeolite. The weight loss analysis of the xerogel intermediate was carried out with a thermogravimetric analyzer (TG). It was found that the weight loss caused by the removal of free water of the xerogel intermediate on the thermogravimetric analyzer occurred in the low-temperature region of 30 - 130 °C, and the weight loss caused by the removal of bound water occurred in the medium-temperature region of 130 - 166 °C. The weight loss caused by the decomposition and removal of the TEABr template agent and ethanolamine in it appeared in the high-temperature region above 166 °C. The results show that the prepared xerogel intermediate still contains about 20 wt.% of water, of which the free water accounts for about one-third and the bound water accounts for about two-thirds. The xerogel intermediate is white in color and soft in texture. The changes in the content, structure and physical and chemical properties of the organic matter (mainly the TEABr template agent and ethanolamine weak organic base) in it can be ignored. After gently grinding with a mortar, it becomes a powder, which meets the requirements of the present invention, and is sealed for standby.

[0102] The fourth step is to prepare Ti-Beta zeolite by steam-assisted local structure reconstruction of the xerogel intermediate.

[0103] The reaction process of preparing Ti-Beta zeolite by steam-assisted local structure reconstruction of the xerogel intermediate is carried out in a laboratory small autoclave with a volume of 100 ml. There are brackets and trays inside the autoclave. 10 g of the xerogel intermediate powder is placed on the tray, and the auxiliary steam is provided by the vaporization of the liquid water at the bottom of the kettle under the tray. The local structure reconstruction reaction is carried out in an ordinary electric heating oven with the temperature set at 140 °C for 12 h.

[0104] According to calculations, when performing local structure reconstruction of the steam-assisted dry gel intermediate in this small autoclave, more than 12.16 g of liquid water is added to the bottom of the autoclave (at this time, the W H2O / W 干胶中间体 ratio is about 1.2), and a gas-liquid two-phase can appear at 140 °C. However, in this case, the liquid phase mainly refers to the capillary condensed liquid in the channels of dealuminated Beta zeolite and the liquid film on the zeolite surface. To enable the local structure reconstruction reaction to proceed under suitable conditions with liquid water at the bottom of the autoclave, the W H2O / W 干胶中间体 ratio is increased to 2.0. Therefore, 20 g of deionized water is added to the bottom of the autoclave.

[0105] After the local structure reconstruction reaction is completed, in accordance with the usual practice of conventional hydrothermal synthesis of zeolite molecular sieves, the autoclave is cooled and depressurized. When the temperature and pressure in the autoclave approach the ambient temperature and pressure, the autoclave is opened and the Ti-Beta zeolite product is taken out.

[0106] Step 5, Post-treatment of the Ti-Beta zeolite product

[0107] Since the Ti-Beta zeolite product prepared by the method of the present invention does not require filtration and washing treatment, and even less requires defluorination and dealumination post-treatment, only conventional drying and calcination treatments are performed on it. The drying is carried out in an electric oven, the drying temperature is 110 °C, and the drying time is 12 h; the calcination is carried out in a muffle furnace, the calcination temperature is 540 °C, and the calcination time is 6 h. The obtained white powder is the Ti-Beta zeolite product, coded as Ti-Beta-1.

[0108] The Si / Ti molar ratio of the Ti-Beta-1 sample measured by the XRF method is about 45. The skeletal vibration infrared spectrum of the Ti-Beta-1 sample is shown in Figure 2 , and the XRD pattern is shown in Figure 3 . It can be seen from Figure 2 that this sample has an obvious characteristic absorption of skeletal titanium near the wavenumber of 960 cm -1 , and its skeletal titanium content index value (I 960 / I 800 ) is 1.15. In addition, it can be seen from Figure 3 that this sample has a high crystallinity and no impurity crystals. In addition, after ultraviolet Raman spectroscopy characterization (excitation light source wavelength 325 nm), it is confirmed that this sample only contains a very small amount of anatase-phase titanium dioxide (at 144, 390, 635 cm -1There is a resonance peak of weakly crystalline anatase titanium dioxide). The above results show that, according to the process of local structure reconstruction of steam-assisted dealuminated Beta zeolite provided by the present invention, high-quality Ti-Beta zeolite products are prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source.

[0109] Comparative Example 1: This example is used to illustrate that, according to the existing structure reconstruction process (reference: "Wang Bowen. Preparation and Catalytic Performance of MFI and BEA Zeolites [D]. East China Normal University, 2021"), that is, first, dealuminated Beta zeolite, a large amount of tetraethylammonium hydroxide (TEAOH) templating agent solution, a titanium source, and deionized water are made into a hydrogel. Then, the hydrogel is hydrothermally pretreated at a higher temperature so that the dealuminated Beta zeolite in it is completely dissolved by the large amount of strongly alkaline TEAOH solution and becomes fragmented structural units with amorphous properties. Then, the hydrogel belonging to the amorphous substance is cooled, and fluoride is added to it. Finally, the hydrogel amorphous substance is reheated to carry out structure reconstruction from the amorphous substance under hydrothermal conditions to prepare Ti-Beta zeolite (synthesize Ti-Beta zeolite crystals), which is also an effective way to prepare Ti-Beta zeolite. However, when preparing Ti-Beta zeolite according to the existing structure reconstruction process, the assistance of fluoride is required, so there are practical problems such as fluoride-containing wastewater that are not conducive to industrial applications.

[0110] The first step is to prepare dealuminated Beta zeolite using Al-Beta zeolite as the raw material.

[0111] Repeat the steps for preparing dealuminated Beta zeolite in Example 1, using an Al-Beta zeolite raw material without hetero-crystalline phase, crystal grain size less than 100 nm, BET specific surface area of about 540 m 2 / g, and a silica-alumina oxide molar ratio (molar ratio of SiO2 to Al2O3) of about 24. Through acid dealumination, dealuminated Beta zeolite with a molar ratio of SiO2 to Al2O3 of 990 is prepared and stored sealed for later use.

[0112] The second step is to prepare a hydrogel precursor for structure reconstruction using dealuminated Beta zeolite.

[0113] Using 25 wt.% aqueous solution of tetrapropylammonium hydroxide (TEAOH) as the source of the templating agent, and tetrabutyl titanate (TBOT) as the titanium source, the amount of the templating agent solution is determined according to the molar ratio of TEAOH / SiO2 (using the number of moles of SiO2 to represent the number of moles of dealuminated Beta zeolite) of 0.3; the amount of tetrabutyl titanate (TBOT) is determined according to the Si / Ti molar ratio of 50; the molar ratio of H2O / SiO2 is determined by the amount of water brought in by the TEAOH templating agent.

[0114] The specific procedures are as follows: First, based on the dosage of 10 g dealuminated Beta zeolite, 30 g of 25 wt.% tetrapropylammonium hydroxide (TEAOH) aqueous solution and 1.16 g of tetrabutyl titanate (TBOT) are weighed respectively. Then, under stirring, the titanium source and the TEAOH template agent solution are mixed to prepare a uniform alkaline solution containing the titanium source and the TEAOH template agent. Finally, under stirring, all the dealuminated Beta zeolite is poured into the alkaline solution containing the titanium source and the TEAOH template agent, and stirred at room temperature for reaction (mild desilication reaction) for 3 h to obtain a hydrogel (with an H2O / SiO2 molar ratio of about 7.4) for the preparation of Ti-Beta zeolite by structure reconstruction.

[0115] Step 3, hydrothermal pretreatment of the hydrogel

[0116] All the hydrogel prepared in the second step is loaded into a small laboratory autoclave with a polytetrafluoroethylene liner for hydrothermal pretreatment. The pretreatment is carried out in a common electric heating oven with a set temperature of 140 °C, and the pretreatment time is 1 h. After the pretreatment, the autoclave is taken out of the oven and quenched with water to room temperature. A small amount of the sample is taken out after opening the autoclave for the detection of the retention degree of the Beta zeolite crystal structure (XRD method). The detection results show that after hydrothermal pretreatment at a relatively high temperature of 140 °C, the two main characteristic diffraction peaks of Beta zeolite at 2θ = 7.7° and 22.5° on the XRD pattern of the hydrogel sample have basically disappeared. This indicates that the crystal structure of the dealuminated Beta zeolite has been basically completely dissolved by the strongly alkaline TEAOH solution with a large dosage and has become fragmented structural units with an amorphous nature.

[0117] Step 4, use fluoride to assist the hydrothermally treated hydrogel for structure reconstruction to prepare Ti-Beta zeolite

[0118] The fluoride used in this example is ammonium fluoride (NH4F). The addition amount of ammonium fluoride is calculated according to F - / SiO2 = 0.2. The specific procedure is as follows: Weigh the ammonium fluoride solid accurately, and then add all the ammonium fluoride solid powder into the autoclave, and manually stir to dissolve it and mix it evenly with the hydrogel after hydrothermal pretreatment (the crystal structure of the dealuminated Beta zeolite in which has been completely dissolved and destroyed and has become fragmented structural units with an amorphous nature (in a state of long-range disorder)). Finally, reseal the autoclave for hydrothermal crystallization treatment, and the hydrothermal crystallization treatment is at 140 °C for a crystallization time of 12 h. After the crystallization is completed, the autoclave is taken out of the oven and quenched with water to room temperature.

[0119] Step 5, post-treatment of the Ti-Beta zeolite product

[0120] In this example, the crystallization product obtained in the fourth step needs to go through four post-treatment steps of filtration, repeated water washing, drying, and calcination in sequence to obtain the Ti-Beta zeolite product, coded as Ti-Beta-1-F-Com (Com represents the comparative example).

[0121] In this example, the reason for repeatedly washing the crystallization product with water is to remove the F-ion impurities in the Ti-Beta zeolite product. Obviously, in this example, the fluorides in the crystallization mother liquor and a large amount of washing liquid will cause problems in the treatment of wastewater.

[0122] The Si / Ti molar ratio of the Ti-Beta-1-F-Com sample measured by the XRF method is about 50. The infrared spectrum of the framework vibration of the Ti-Beta-1-F-Com sample is shown in Figure 4 . From Figure 4 it can be seen that the sample also has obvious characteristic absorption of framework titanium near the wave number of 960 cm -1 . Comparing with Example 1, it can be seen that the existing structure reconstruction process is also an effective way to prepare Ti-Beta zeolite. However, when preparing Ti-Beta zeolite by the existing structure reconstruction process, the assistance of fluoride is required, so there are practical problems such as fluoride-containing wastewater that are not conducive to industrial applications.

[0123] Comparative Example 2: This example is used to illustrate that according to the method of the existing liquid-solid isomorphous substitution process (Ind. Eng. Chem. Res. 2021, 60, 1219 - 1230), that is, first contacting dealuminated Beta zeolite with an anhydrous ethanol solution of titanium tetrachloride at room temperature, and performing a short ultrasonic oscillation treatment, then evaporating the ethanol solvent in the feed liquid, and then drying and calcining the solid product to obtain Ti-Beta zeolite. However, due to the small size of the hydroxyl nests in dealuminated Beta zeolite (the space left by migrating out one framework Al 3+ ion), it is not easy to accommodate the larger Ti 4+ ion (the ionic radius of Ti 4+ ion is while the ionic radius of Al 3+ ion is ), so when preparing Ti-Beta zeolite by this method, the framework titanium content is low.

[0124] First step, preparing dealuminated Beta zeolite using Al-Beta zeolite as raw material

[0125] Repeat the steps for preparing dealuminated Beta zeolite in Example 1, using a zeolite without hetero-crystalline phase, crystal grain size less than 100 nanometers, and BET specific surface area of about 540 m 2Al-Beta zeolite raw material with a molar ratio of silicon-aluminum oxide (molar ratio of SiO2 to Al2O3) of about 24 was used to prepare dealuminated Beta zeolite with a molar ratio of SiO2 to Al2O3 of 990 through acid dealumination, which was sealed and stored for later use.

[0126] The second step is to prepare an anhydrous ethanol solution of titanium tetrachloride.

[0127] In the glove box, 6 mL of TiCl4 was extracted with a pipette and quickly transferred to 100 mL of anhydrous ethanol, and then an ethanol solution of TiCl4 with a concentration of 0.5 mol / L was prepared for later use.

[0128] The third step is to carry out a liquid-solid isomorphous substitution reaction at room temperature.

[0129] First, calculated according to the preparation of Ti-Beta zeolite with a Si / Ti molar ratio of 50 using 10 g of dealuminated Beta zeolite, 6.8 mL of the solution was extracted from the 0.5 mol / L TiCl4 ethanol solution prepared in the second step and diluted to 30 mL with anhydrous ethanol, and the resulting solution became the impregnation solution.

[0130] Then, 10 g of dry dealuminated Beta zeolite was poured into 30 mL of the impregnation solution and stirred evenly to obtain a reactant slurry. Then, the reactant slurry was transferred to an ultrasonic oscillator for ultrasonic-assisted liquid-solid isomorphous substitution reaction at room temperature. The reaction time was 20 min.

[0131] After the reaction, the ethanol solvent was recovered by reduced pressure evaporation at 80 °C, and the solid product was collected.

[0132] The fourth step is to obtain the Ti-Beta zeolite product through post-treatment of the liquid-solid isomorphous substitution reaction product.

[0133] The post-treatment steps mainly include conventional drying (120 °C, overnight) and calcination (540 °C, 3 h). The Ti-Beta zeolite prepared in this example is designated as Ti-Beta-2-Cl-Com. The Si / Ti molar ratio of the Ti-Beta-2-Cl-Com sample was measured to be about 50 by the XRF method. The skeletal vibration infrared spectrum of the Ti-Beta-2-Cl-Com sample is shown in Figure 5 . From Figure 5 it can be seen that the sample also has a characteristic absorption of framework titanium near the wavenumber of 960 cm -1 . However, by comparing with Example 1, it can be seen that the intensity of the characteristic absorption peak of framework titanium in the Ti-Beta-2-Cl-Com sample prepared by this method is weaker, indicating that its framework titanium content is lower.

[0134] Comparative Example 3: This example is used to illustrate the method of the existing solid-solid isomorphic substitution process (Microporous and Mesoporous Materials 288 (2019) 109588), that is, first, dealuminated Beta zeolite and titanium dichloride solid powder are fully ground and uniformly mixed under the protection of inert gas. Then, the solid mixture is calcined to obtain Ti-Beta zeolite. Similarly, due to the small size of the hydroxyl nests in dealuminated Beta zeolite (the space left by removing one framework Al 3+ ion), it is not easy to accommodate larger Ti 4+ ions (Ti 4+ ion radius is while the Al 3+ ion radius is ), so when preparing Ti-Beta zeolite by this method, the framework titanium content is also low.

[0135] First step, prepare dealuminated Beta zeolite using Al-Beta zeolite as raw material

[0136] Repeat the preparation steps of dealuminated Beta zeolite in Example 1, using an Al-Beta zeolite raw material without hetero-crystalline phase, crystal grain size less than 100 nm, BET specific surface area of about 540 m 2 / g, and silicon-aluminum oxide molar ratio (molar ratio of SiO2 to Al2O3) of about 24. Through acid dealumination, dealuminated Beta zeolite with a molar ratio of SiO2 to Al2O3 of 990 is prepared and stored sealed for later use.

[0137] Second step, prepare the precursor mixture by grinding the solid powders of dealuminated Beta zeolite and titanium dichloride

[0138] First, calculate according to preparing Ti-Beta zeolite with a Si / Ti molar ratio of 50 using 10 g of dealuminated Beta zeolite. In the glove box, accurately weigh 0.85 g of titanium dichloride solid powder.

[0139] Then, in the glove box, put the dry dealuminated Beta zeolite powder and titanium dichloride solid powder into a mortar and grind them evenly to obtain the precursor mixture.

[0140] Third step, prepare Ti-Beta zeolite by calcining the precursor mixture

[0141] The precursor mixture was quickly placed in a crucible and spread thinly. After covering the crucible, it was placed in a muffle furnace for calcination treatment (540 °C, 3 h). The obtained white solid powder was the Ti-Beta zeolite product, coded as Ti-Beta-3-Cp-Com. The Si / Ti molar ratio of the Ti-Beta-2-Cp-Com sample was measured to be approximately 50 by the XRF method. The skeletal vibration infrared spectrum of the Ti-Beta-3-Cp-Com sample is shown in Figure 6 . It can be seen from Figure 6 that this sample also has a characteristic absorption of framework titanium near the wavenumber of 960 cm -1 . However, compared with Example 1, it can be seen that the intensity of the characteristic absorption peak of framework titanium in the Ti-Beta-3-Cp-Com sample prepared by this method is weaker, indicating that its framework titanium content is lower.

[0142] Example 2: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. It mainly uses a combination of steam-assisted tetraethylammonium bromide (TEABr) templating agent and weak organic amine bases to locally etch silicon and reconstruct the structure of the hydroxyl nests of dealuminated Beta zeolite, and prepare Ti-Beta zeolite. The process can replace the strong basic tetraethylammonium hydroxide (TEAOH) templating agent used in the existing preparation process with a combination of tetraethylammonium bromide (TEABr) templating agent and weak organic amine bases. At the same time, by means of the pore condensation liquid generated by dealuminated Beta zeolite in saturated steam and the liquid film on the surface of the zeolite solid to limit the dissolution amount of silicate, it is more convenient to achieve controllable etching and desilication of the hydroxyl nests of dealuminated Beta zeolite, so as to achieve the purpose of slightly etching and desilication of the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and avoiding the complete dissolution of dealuminated Beta zeolite crystals by strong basic TEAOH solution into fragmented structural units (amorphous substances). Thus, synthesizing Ti-Beta zeolite crystals (traditional structure reconstruction method) becomes repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the slightly etched hydroxyl nests accept titanium ions (the present invention). Among them, during the repair and reconstruction, that is, when repairing the local structural defects left after the slightly etched hydroxyl nests accept titanium ions with the assistance of water vapor, it is allowed to change the amount of liquid water added to the bottom of the kettle within a certain range without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both the TEABr templating agent and weak organic amine bases are organic substances with high thermal stability and high boiling points, and are more heat-resistant than TEAOH, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0143] Repeat Example 1, but when preparing Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate in the fourth step, the ratio of the amount of liquid water (W H2O / W 干胶中间体 ratio) added to the bottom of the kettle is changed to 1.5, 2.5, and 3.0 in sequence. Then, the Fourier transform infrared spectra of the framework vibrations of the prepared Ti-Beta zeolite samples all have obvious characteristic absorptions of framework titanium near the wave number of 960 cm -1 . The framework titanium content index values (I 960 / I 800 ) are 1.10, 1.14, and 1.16 in sequence.

[0144] Example 3: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. Mainly through the combination of steam-assisted tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine, local etching of silicon and structure reconstruction of the hydroxyl nests of dealuminated Beta zeolite are carried out to prepare Ti-Beta zeolite. The process can replace the strong basic tetraethylammonium hydroxide (TEAOH) templating agent used in the existing preparation process with the combination of tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine. At the same time, by means of the pore condensation liquid generated by dealuminated Beta zeolite in saturated steam and the liquid film on the zeolite solid surface to limit the dissolution amount of silicate, it is more convenient to achieve controllable etching of silicon from the hydroxyl nests of dealuminated Beta zeolite, so as to achieve the purpose of slightly etching silicon from the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and to avoid the complete dissolution of dealuminated Beta zeolite crystals by strong basic TEAOH solution into fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed to repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the slightly etched hydroxyl nests accept titanium ions (the present invention). Among them, during the repair and reconstruction, that is, when repairing the local structural defects left after the slightly etched hydroxyl nests accept titanium ions with the assistance of water vapor, it is allowed to change the temperature and time of the reconstruction reaction within a certain range without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both the TEABr templating agent and the weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0145] Repeat Example 1, but when performing steam-assisted local structure reconstruction of the dry gel intermediate to prepare Ti-Beta zeolite in the fourth step, change the temperature and time of the reconstruction reaction to: 120 °C × 48 h, 130 °C × 24 h, 150 °C × 6 h, 160 °C × 2 h, and 170 °C × 0.5 h in sequence. Then, the Fourier transform infrared spectra of the framework vibrations of the prepared Ti-Beta zeolite samples all have obvious characteristic absorptions of framework titanium near 960 cm -1 Wavenumber, and the framework titanium content index values (I 960 / I 800 ) are 1.12, 1.16, 1.18, 1.14, and 1.17 in sequence.

[0146] Example 4: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. Mainly through the composition of steam-assisted tetraethylammonium bromide (TEABr) template agent and weak organic base of alkanolamine, local etching of silicon and structure reconstruction of the hydroxyl nests of dealuminated Beta zeolite are carried out to prepare Ti-Beta zeolite. The process can replace the strong basic tetraethylammonium hydroxide (TEAOH) template agent used in the existing preparation process with the combination of tetraethylammonium bromide (TEABr) template agent and weak organic base of alkanolamine. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the surface of zeolite solid to limit the dissolution amount of silicate, it is more convenient to achieve the controllable etching of silicon from the hydroxyl nests of dealuminated Beta zeolite, so as to achieve the purpose of slightly etching silicon from the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and to avoid the complete dissolution of dealuminated Beta zeolite crystals by strong basic TEAOH solution and becoming fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed to repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the hydroxyl nests with slight etching accept titanium ions (the present invention). Among them, when preparing the dry gel intermediate, the temperature of low-temperature drying is allowed to be changed within a certain range without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both the TEABr template agent and the weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0147] Repeat Example 1, but in the third step, when removing the free water in the hydrogel precursor by low-temperature drying and converting it into a dry gel intermediate, the low-temperature drying temperature is successively changed to: 30 °C (dry air is introduced into the oven) until a constant weight state, 45 °C (dry air is introduced into the oven) until a constant weight state, 55 °C (drying under slightly negative pressure) until a constant weight state, 70 °C until a constant weight state, 100 °C × 8 h, 130 °C × 5 h, and 160 °C × 3 h. Then the water content of the prepared dry gel intermediate is about 20 wt.%, and mainly consists of bound water, with a relatively small proportion of free water. All the dry gel intermediate samples are white in color and soft in texture. The changes in the content, structure, and physical and chemical properties of the organic substances (mainly TEABr template agent and ethanolamine weak organic base) can be ignored. After gently grinding with a mortar, it becomes a powder, meeting the requirements of the present invention, and is sealed for standby.

[0148] The Ti-Beta zeolite sample prepared from the above-mentioned dry gel intermediate has obvious characteristic absorption of framework titanium in the infrared spectrum of framework vibration near 960 cm -1 wave number, and the index value (I 960 / I 800 ) of its framework titanium content fluctuates between 1.1 and 1.2.

[0149] Example 5: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. mainly through the combination of steam-assisted tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine, locally etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and reconstructing the structure to prepare Ti-Beta zeolite. The process can replace the strongly basic tetraethylammonium hydroxide (TEAOH) templating agent used in the existing preparation process with the combination of tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the surface of zeolite solid to limit the dissolution amount of silicate, it is more convenient to realize the controllable etching and desiliconization of the hydroxyl nests of dealuminated Beta zeolite, so as to achieve the purpose of slightly etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests to larger titanium ions, and the purpose of avoiding the complete dissolution of dealuminated Beta zeolite crystals by strongly basic TEAOH solution and becoming fragmented structural units (amorphous substances), thereby changing the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) into repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the slightly etched hydroxyl nests accept titanium ions (the present invention). Among them, when preparing the hydrogel precursor, the amount of tetraethylammonium bromide (TEABr) templating agent is allowed to be changed within a certain range without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both the TEABr templating agent and the weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0150] Repeat Example 1, but when preparing the hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite in the second step, successively change the TEABr / SiO2 molar ratio (expressing the molar number of dealuminated Beta zeolite in terms of the molar number of SiO2) to 0.05, 0.1, 0.2, 0.25, 0.4, and 0.5. Then the amounts of tetraethylammonium bromide (TEABr) are successively 1.79 g, 3.57 g, 7.14 g, 8.93 g, 14.29 g, and 17.86 g. Based on the above hydrogel precursor, free water in the hydrogel precursor is removed by low-temperature drying to convert it into a xerogel intermediate, and further Ti-Beta zeolite is prepared by steam-assisted local structure reconstruction of the xerogel intermediate. For the prepared Ti-Beta zeolite samples, the infrared spectra of the framework vibrations all have obvious characteristic absorptions of framework titanium near the wavenumber of 960 cm -1 The wavenumber, and the index value (I 960 / I 800 ) of the framework titanium content fluctuates between 1.1 and 1.2.

[0151] Example 6: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. The process mainly uses a composition of steam-assisted tetraethylammonium bromide (TEABr) template and weak organic base of alkanolamine to locally etch silicon and reconstruct the structure of the hydroxyl nests of dealuminated Beta zeolite to prepare Ti-Beta zeolite. The strong basic tetraethylammonium hydroxide (TEAOH) template used in the existing preparation process can be replaced by the combination of tetraethylammonium bromide (TEABr) template and weak organic base of alkanolamine. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the surface of the zeolite solid to limit the dissolution amount of silicate, the controllable etching and desilication of the hydroxyl nests of dealuminated Beta zeolite can be more conveniently realized, achieving the purpose of mildly etching and desilication of the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and avoiding the complete dissolution of dealuminated Beta zeolite crystals by strong basic TEAOH solution into fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed to repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the mildly etched hydroxyl nests accept titanium ions (the present invention). Among them, when preparing the hydrogel precursor, the amount of weak organic base of ethanolamine is allowed to be changed within a certain range without changing the beneficial effects of the present invention. In short, since the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both TEABr template and weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH and allow the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0152] Repeat Example 1, but when using dealuminated Beta zeolite to prepare the hydrogel precursor for local structure reconstruction in the second step, successively change the ethanolamine / SiO2 molar ratio (using the molar number of SiO2 to represent the molar number of dealuminated Beta zeolite) to 0.1, 0.2, 0.25, 0.4, and 0.5, then the amounts of weak organic base of ethanolamine are 1.04 g, 2.08 g, 2.60 g, 4.15 g, and 5.19 g in sequence. Based on the above hydrogel precursor, remove the free water in the hydrogel precursor by low-temperature drying to convert it into a dry gel intermediate, and further prepare Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate. For the prepared Ti-Beta zeolite samples, the infrared spectrum of the framework vibration has obvious characteristic absorption of framework titanium near the wave number of 960 cm -1 The wavenumber has obvious characteristic absorption of framework titanium, and its framework titanium content index value (I960 / I 800 Fluctuates between 1.1 and 1.2.

[0153] Example 7: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. It mainly uses a combination of steam-assisted tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine to locally etch and desilicate the hydroxyl nests of dealuminated Beta zeolite and reconstruct the structure. The process for preparing Ti-Beta zeolite can replace the strong basic tetraethylammonium hydroxide (TEAOH) templating agent used in the existing preparation process with a combination of tetraethylammonium bromide (TEABr) templating agent and weak organic base of alkanolamine. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the zeolite solid surface to limit the dissolution amount of silicate, it is more convenient to achieve controllable etching and desilication of the hydroxyl nests of dealuminated Beta zeolite, so as to achieve the purpose of mildly etching and desilicating the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests to larger titanium ions, and to avoid the complete dissolution of dealuminated Beta zeolite crystals by the strong basic TEAOH solution and becoming fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed to repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the mildly etched hydroxyl nests accept titanium ions (the present invention). Among them, when preparing the hydrogel precursor, the amount of deionized water is allowed to be changed within a certain range without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both the TEABr templating agent and the weak organic base of alkanolamine are organic substances with high thermal stability and high boiling points, which are more heat-resistant than TEAOH and allow the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0154] Repeat Example 1, but when preparing the hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite in the second step, change the H2O / SiO2 molar ratio (expressing the molar number of dealuminated Beta zeolite in terms of the molar number of SiO2) to 2, 3, 4, 8, and 10 in sequence. Then the amounts of deionized water used are 6.12 g, 9.18 g, 12.24 g, 24.48 g, and 30.6 g in sequence. Based on the above hydrogel precursor, free water in the hydrogel precursor is removed by low-temperature drying to convert it into a xerogel intermediate. Further, Ti-Beta zeolite is prepared by steam-assisted local structure reconstruction of the xerogel intermediate. For the prepared Ti-Beta zeolite samples, the infrared spectrum of the framework vibration has obvious characteristic absorption of framework titanium near the wavenumber of 960 cm -1 and the index value (I 960 / I 800 ) of the framework titanium content fluctuates between 1.1 and 1.2.

[0155] Example 8: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. The process mainly uses a combination of steam-assisted tetraethylammonium bromide (TEABr) template and weak organic base of alkanolamine to locally etch silicon and reconstruct the structure of the hydroxyl nests of dealuminated Beta zeolite to prepare Ti-Beta zeolite. The strong basic tetraethylammonium hydroxide (TEAOH) template used in the existing preparation process can be replaced by the combination of tetraethylammonium bromide (TEABr) template and weak organic base of alkanolamine. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the surface of the zeolite solid to limit the dissolution amount of silicate, the controllable etching of silicon from the hydroxyl nests of dealuminated Beta zeolite can be more conveniently realized, so as to achieve the purpose of slightly etching silicon from the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and to avoid the complete dissolution of dealuminated Beta zeolite crystals by the strong basic TEAOH solution into fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed to repairing the local defects of Ti-Beta zeolite crystals, that is, repairing the local structural defects left after the hydroxyl nests with slight etching accept titanium ions (the present invention). Among them, when preparing the hydrogel precursor, different titanium sources are allowed to be used, and the Si / Ti molar ratio and stirring time are changed within a certain range without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both TEABr template and weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH, allowing the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0156] Repeat Example 1, but when preparing the hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite in the second step, sequentially change the Si / Ti molar ratio (expressing the molar number of dealuminated Beta zeolite in terms of the molar number of SiO2) to 60, 70, 80, 90, and 100, and simultaneously change the stirring time to 12 h, 6 h, 4 h, 2 h, and 0.5 h in sequence. Moreover, the titanium sources used in preparing the above hydrogel precursor are tetraethyl titanate, tetraisopropyl titanate, titanium acetylacetonate, a complex of tetrabutyl titanate and isopropanol in a 1:5 molar ratio, and a complex of tetraethyl titanate and isopropanol in a 1:8 molar ratio in sequence. Based on the above hydrogel precursor, free water in the hydrogel precursor is removed by low-temperature drying to convert it into a xerogel intermediate, and further Ti-Beta zeolite is prepared by steam-assisted local structure reconstruction of the xerogel intermediate. For the prepared Ti-Beta zeolite samples, their framework vibration infrared spectra all have obvious characteristic absorptions of framework titanium near 960 cm -1 wave number, and the index value (I 960 / I 800 ) of their framework titanium content varies between 0.85 and 1.2. Generally, with the increase of the Si / Ti molar ratio, the index value (I 960 / I 800 ) of the framework titanium content of the Ti-Beta zeolite samples shows a decreasing trend.

[0157] Example 9: This example is used to illustrate the fluoride-free and aluminum-free process for preparing Ti-Beta zeolite according to the local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, without involving the use of fluoride or / and aluminum source. The process mainly uses a combination of steam-assisted tetraethylammonium bromide (TEABr) template and weak organic base of alkanolamine to locally etch and desilicate the hydroxyl nests of dealuminated Beta zeolite and reconstruct the structure to prepare Ti-Beta zeolite. The strong basic tetraethylammonium hydroxide (TEAOH) template used in the existing preparation process can be replaced by the combination of tetraethylammonium bromide (TEABr) template and weak organic base of alkanolamine. At the same time, by means of the pore condensate generated by dealuminated Beta zeolite in saturated steam and the liquid film on the surface of zeolite solid to limit the dissolution amount of silicate, the controllable etching and desilication of the hydroxyl nests of dealuminated Beta zeolite can be more conveniently realized, so as to achieve the purpose of slightly etching and desilicating the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions, and avoid the complete dissolution of dealuminated Beta zeolite crystals by strong basic TEAOH solution to become fragmented structural units (amorphous substances). Thus, the synthesis of Ti-Beta zeolite crystals (traditional structure reconstruction method) is changed to repair the local defects of Ti-Beta zeolite crystals, that is, to repair the local structural defects left after the hydroxyl nests with slight etching accept titanium ions (the present invention). Among them, when preparing the hydrogel precursor, Al-Beta zeolite raw materials with different silicon-aluminum ratios are allowed to be used to prepare dealuminated Beta zeolite, and the Si / Ti molar ratio of the hydrogel precursor can be correspondingly changed according to the number of hydroxyl nests of dealuminated Beta zeolite without changing the beneficial effects of the present invention. In short, because the present invention avoids reconstructing the Ti-Beta zeolite structure starting from amorphous fragmented structural units, high-quality Ti-Beta zeolite can be prepared from dealuminated Beta zeolite without the assistance of fluoride or / and aluminum source. In addition, both TEABr template and weak organic base of alkanolamine are organic substances with high thermal stability and high boiling point, which are more heat-resistant than TEAOH and allow the preparation of the dry gel intermediate to be carried out at a higher temperature.

[0158] Example 1 was repeated, but when preparing dealuminated Beta zeolite using Al-Beta zeolite as the raw material in the first step, first, according to the hydrothermal crystallization method provided by US Patent US3 308 069 (1967), Al-Beta zeolites with silica-alumina oxide molar ratios (molar ratio of SiO2 to Al2O3) of 10, 20, 40, 60, 80, 100, 150, and 200 were synthesized by themselves as the raw materials for preparing dealuminated Beta zeolite. After the synthesized above-mentioned Al-Beta zeolites were subjected to conventional filtration, washing, drying (110 °C, 12 h), and calcination to remove the template agent treatment (540 °C, 6 h), their average crystal grain size was observed by TEM to be in the nanometer level and small crystal grain (less than 1 μm) level. As the silica-alumina oxide molar ratio (molar ratio of SiO2 to Al2O3) increased, the crystal grain size increased; no impurity crystals were found by XRD inspection, and their BET specific surface areas calculated from their nitrogen physical adsorption data were all higher than 500 m 2 / g; the silica-alumina oxide molar ratios (molar ratio of SiO2 to Al2O3) measured by XRF were 10, 20, 38, 57, 72, 94, 136, and 189 in sequence, meeting the technical requirements of the present invention for the Beta zeolite raw material. The above-mentioned Al-Beta zeolites were subjected to acid dealumination to prepare dealuminated Beta zeolites, and the molar ratios of SiO2 to Al2O3 of the obtained dealuminated Beta zeolites were 751, 770, 870, 861, 855, 932, 1088, and 960 in sequence, meeting the technical requirements of the present invention for the dealuminated Beta zeolite.

[0159] The above-mentioned dealuminated Beta zeolite was used to prepare a hydrogel precursor, and then the free water in the hydrogel precursor was removed by low-temperature drying to convert it into a xerogel intermediate. Finally, Ti-Beta zeolite was further prepared by steam-assisted local structure reconstruction of the xerogel intermediate. Among them, when preparing the hydrogel precursor using dealuminated Beta zeolites with different numbers of hydroxyl nests, in the order of decreasing number of hydroxyl nests of the dealuminated Beta zeolite, that is, in the order of increasing silica-alumina ratio of the Al-Beta zeolite raw material (in the order from 10, 20, 38, 57, 72, 94, 136 to 189), the Si / Ti molar ratio of the hydrogel was changed to 5, 10, 15, 25, 60, 70, 80, and 110 in sequence. For the prepared Ti-Beta zeolite samples, the infrared spectrum of the framework vibration has obvious characteristic absorption of framework titanium near the wave number of 960 cm -1 The content index value (I 960 / I 800 ) of the framework titanium in them varies between 0.8 and 1.5. Generally, as the Si / Ti molar ratio increases, the content index value (I 960 / I 800 ) of the framework titanium in the Ti-Beta zeolite samples shows a decreasing trend.

[0160] Example 10: This example is used to illustrate the process of preparing Ti-Beta zeolite by the method of local structure reconstruction of steam-assisted dealuminated Beta zeolite provided by the present invention. The prepared Ti-Beta zeolite has excellent catalytic performance for the epoxidation reaction of cyclohexene and hydrogen peroxide.

[0161] The above-mentioned epoxidation reaction of cyclohexene was carried out in a 25 mL round-bottom flask equipped with a condenser and a magnetic stirrer. The procedure was as follows: First, 10 mmol of cyclohexene, 10 mL of acetonitrile, 10 mmol of hydrogen peroxide (35%) and 100 mg of Ti-Beta zeolite catalyst were added to the flask, and then the reaction was vigorously stirred at 323 K for 2 hours. Finally, samples were taken from the reaction product and analyzed for composition using a gas chromatograph (Shimadzu GC-2014C) equipped with a flame ionization detector (FID) and a DB-WAX capillary column (30 m × 0.32 mm, 30 μm). The conversion of cyclohexene and the selectivity of cyclohexene oxide were calculated by the external standard method.

[0162] In this example, four Ti-Beta zeolites (Ti-Beta-1, Ti-Beta-1-F-Com, Ti-Beta-2-Cl-Com, and Ti-Beta-3-Cp-Com) prepared in Example 1 and Comparative Examples 1-3 were selected as catalysts to catalyze the epoxidation reaction of cyclohexene and hydrogen peroxide. The results showed that under the same reaction conditions, the cyclohexene conversions of the above Ti-Beta zeolite catalysts were 32%, 24%, 42%, and 43% in sequence, and the selectivities of cyclohexene oxide were 92%, 85%, 88%, and 87% in sequence.

Claims

1. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method, characterized in that: Here are the steps: Step 1: Prepare dealuminated Beta zeolite using Al-Beta zeolite as raw material (1) Selection of Al-Beta zeolite raw materials The Al-Beta zeolite is a silicon-aluminum Beta zeolite. The Al-Beta zeolite has the following restrictions: 1) there is no impurity crystal in the Al-Beta zeolite; 2) the crystallization of the Al-Beta zeolite is good, that is, the BET specific surface area value of the Al-Beta zeolite is ≥ 450m 2 / g; 3) the molar ratio of silicon aluminum oxide of Al-Beta zeolite, that is, the molar ratio of SiO2 to Al2O3 is in the range of 10-200; (2) Preparation of dealuminated Beta zeolite Aluminum Beta zeolite is prepared by using an acid dealumination method based on Al-Beta zeolite, and the molar ratio of SiO2 to Al2O3 in the dealumination Beta zeolite is required to be ≥700; Step 2: Preparation of hydrogel precursors for local structural reconstruction using dealuminated Beta zeolite Other raw materials required for preparing the hydrogel precursor include: tetraethylammonium bromide TEABr template, alcohol amine weak organic base, titanium source and deionized water; The amount of dealuminated Beta zeolite is calculated based on SiO2, and the amounts of other raw materials are as follows: TEABr to SiO2 molar ratio: 0.05-0.5; Molar ratio of alcohol amine to SiO2: 0.1-0.5; Molar ratio of H2O to SiO2: 2-10; Si to Ti molar ratio: 5-100; The alcoholamine weak organic base is ethanolamine; The titanium source is tetrabutyl titanate, tetraethyl titanate, tetraisopropyl titanate or titanium acetylacetonate; The operation of preparing the hydrogel precursor can be carried out at room temperature; the specific steps are as follows: first, according to the amount of dealuminated Beta zeolite and the selected H2O to SiO2 ratio, alcohol amine to SiO2 molar ratio, Si to Ti molar ratio and TEABr to SiO2 molar ratio, deionized water, alcohol amine, titanium source and TEABr template are taken; then, the TEABr template is dissolved in water and the titanium source is dissolved in alcohol amine under stirring; finally, the TEABr template solution and dealuminated Beta zeolite are added to the weak alkaline solution containing alcohol amine and titanium source in sequence under stirring, and the reaction is stirred at room temperature; the stirring reaction time ranges from 0.5 to 12 hours; Step 3: Remove the free water in the hydrogel precursor through low-temperature drying and convert it into a dry gel intermediate The temperature range of the low-temperature drying treatment is between room temperature and 160° C. The total water content in the dry rubber intermediate is controlled between 15 and 25 wt % by means of drying pressure, drying time and atmosphere conditions, and TEABr and alcohol amine weak organic bases are prevented from being decomposed or removed during the drying process; Step 4: Steam-assisted local structural reconstruction of dry gel intermediate to prepare Ti-Beta zeolite The reaction process of preparing Ti-Beta zeolite by steam-assisted local structural reconstruction of dry colloid intermediate is carried out in an autoclave; a support and a tray are provided inside the autoclave; the dry colloid intermediate powder is placed on the tray, and the auxiliary steam is provided by vaporizing liquid water at the bottom of the autoclave under the tray; The parameter requirements are as follows: The amount of liquid water added to the bottom of the kettle is calculated based on the mass ratio of H2O to the dry gel intermediate. The mass ratio of H2O to the dry gel intermediate is in the range of 1.5-3. The reaction temperature range for structural reconstruction is: 120-170°C; The reaction time range of structural reconstruction is: 0.5-48h; Step 5: Post-treatment of Ti-Beta zeolite product After the steam-assisted local structural reconstruction reaction of the dry colloid intermediate is completed, the Ti-Beta zeolite product is collected from the tray in the autoclave, and then dried and calcined to obtain the Ti-Beta zeolite product.

2. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 1, characterized in that: In the first step (1), the molar ratio of silicon to aluminum oxide of Al-Beta zeolite, that is, the molar ratio of SiO2 to Al2O3, is in the range of 20-100; in the first step (2), the molar ratio of SiO2 to Al2O3 of dealuminated Beta zeolite is in the range of ≧800.

3. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 2, characterized in that: In the first step (1), the molar ratio of silicon aluminum oxide of Al-Beta zeolite, that is, the molar ratio of SiO2 to Al2O3, is in the range of 25-60; in the first step (2), the molar ratio of SiO2 to Al2O3 of dealuminated Beta zeolite is in the range of ≧900.

4. The fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 1, characterized in that: In the second step, TEABr to SiO2 molar ratio: 0.1-0.4; Molar ratio of alcohol amine to SiO2: 0.15-0.4; Molar ratio of H2O to SiO2: 3-8; Si to Ti molar ratio: 10-80; The titanium source is tetrabutyl titanate or tetraethyl titanate; The stirring reaction time ranges from 1 to 6 hours.

5. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 4, characterized in that: In the second step, TEABr to SiO2 molar ratio: 0.2-0.3; Molar ratio of alcohol amine to SiO2: 0.2-0.3; Molar ratio of H2O to SiO2: 4-6; Si to Ti molar ratio: 15-50; The titanium source is tetrabutyl titanate; The stirring reaction time ranges from 2 to 4 hours.

6. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 1, characterized in that: In the third step, the temperature range of the low temperature drying treatment is 50-150°C.

7. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 6, characterized in that: In the third step, the temperature range of the low temperature drying treatment is 80-120°C.

8. The fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 1, characterized in that: In the fourth step, The amount of liquid water added to the bottom of the kettle is calculated based on the mass ratio of H2O to the dry gel intermediate. The mass ratio of H2O to the dry gel intermediate is in the range of 1.8-2.

8. The reaction temperature range for structural reconstruction is: 130-160°C; The reaction time range of structural reconstruction is: 4-24h.

9. A fluorine-free and aluminum-free process for preparing Ti-Beta zeolite based on a local structure reconstruction method according to claim 8, characterized in that: In the fourth step, The amount of liquid water added to the bottom of the kettle is calculated based on the mass ratio of H2O to the dry gel intermediate. The mass ratio of H2O to the dry gel intermediate is in the range of 2.0-2.

5. The reaction temperature range for structural reconstruction is: 135-150°C; The reaction time range of structural reconstruction is: 6-18h.

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