Process for preparing Ti-Beta zeolite through steam-assisted dealumination Beta zeolite 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 by a steam-assisted dealumination Beta zeolite local structure reconstruction method. The method is mainly and technically characterized in that water vapor is used for assisting a tetraethylammonium hydroxide template agent, local desilicication etching and repairing reconstruction are carried out on hydroxyl nests of dealuminated Beta zeolite, and Ti-Beta zeolite is prepared without assistance of fluoride or / and an aluminum source. The method has the advantages that the dosage of the strongly alkaline TEAOH template agent is reduced, and by means of limited dissolution of the dealumination Beta zeolite pore channel condensation liquid and the zeolite solid surface liquid film on silicate in saturated steam, controllable etching desiliconization of the dealumination Beta zeolite hydroxyl nest is achieved, and the admitting ability of the hydroxyl nest to large-size titanium ions is improved. Local structure defects generated by controllable etching desiliconization can be repaired by directly using a TEAOH template agent solution, so that fluoride or / and an aluminum source are / is not used in the method.

Description

Technical Field

[0001] The present invention belongs to the technical field of petrochemical catalysis, and relates to a process for preparing Ti-Beta zeolite by local structure reconstruction of steam-assisted dealuminated Beta zeolite. Background Art

[0002] Ti-Beta zeolite is an important member of the titanium silicate zeolite family because it has a three-dimensional cross-linked twelve-membered ring pore system. Compared with TS-1 zeolite (which has a three-dimensional cross-linked ten-membered ring pore system), Ti-Beta zeolite is more suitable for the low-temperature selective oxidation reaction of larger organic molecules and hydrogen peroxide, including the oxidation reaction of alkanes and the epoxidation reaction of alkenes. The excellent catalytic performance of Ti-Beta zeolite in these reactions is recorded in the following published literature and patent literature: Inorg. Chem. 2022, 61, 4887-4894; ACS Catal. 2020, 10, 10169-10184; Microporous and Mesoporous Materials 278 (2019) 30–34; Applied Catalysis A: General 515 (2016) 51–59; Phys. Chem. Chem. Phys., 2011, 13, 16282–16294; Pol. J. Chem. Tech., Vol. 9, No. 2, 2007; J. Phys. Chem. B 2004, 108, 4242-4244; Chem. Commun. 1998, 2609–2610; Chemical Engineering Journal 2021, 410, 128234. In addition, the excellent catalytic performance of Ti-Beta in photocatalysis and sugar isomerization is also mentioned in European Patent 16162989.4, European Patent EP11832883A; J. Am. Chem. Soc. 2018, 140, 14244-14266; ACS Catal. 2013, 3, 1469-1476 and Chemical Engineering Journal 2021, 410, 128234.

[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 features of the traditional hydrothermal method are 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 fluoride-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. The following invention patents and published literatures are all related to the synthesis of Ti-Beta zeolite by the traditional hydrothermal method: Chinese Invention Patent (Application No. 201510278969.x); Chinese Invention Patent (Application No. 201510278966.6); Chinese Invention Patent (Application No. 201310250820.1); Chinese Invention Patent (Application No. 200710119550.5); Inorg. Chem. 2022, 61, 4887-4894; ACS Catal. 2020, 10, 10169-10184; Microporous and Mesoporous Materials 278 (2019) 30–34; J. Am. Chem. Soc. 2018, 140, 14244-14266; Phys. Chem. Chem. Phys., 2011, 13, 16282–16294; Catalysis Today 126 (2007) 375–381; Journal of Catalysis 217 (2003) 160–171; Phys. Chem. Chem. Phys., 2011, 13, 16282–16294; Journal of Catalysis 203, 201–212 (2001); J. Phys. Chem. B 2001, 105, 8350-8355; J. Phys. Chem. B 1998, 102, 7126-7131.

[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 by 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 open 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 specific steps are as follows: First, the Ti-MWW zeolite and the fully dealuminated Beta seed crystals are added to an alkaline solution for degradation to become 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 mineralizing agent; the open literature (Research on the Synthesis and Catalytic Properties of Heteroatom Zeolites with BEA and FAU Topological Structures [D]. East China Normal University, 2018) has a more detailed description of the transformation of Ti-MWW to Ti-Beta. Another example is that the open literature Microporous and Mesoporous Materials 311 (2021) 110702 also reported a transformation method for preparing Ti-Beta zeolite. In this method, a ultrastable 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 mixed 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 mineralizing agents.

[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 introducing 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] In the gas-solid isomorphous substitution method, volatile TiCl4 is used as the titanium source. TiCl4 is carried by an inert gas (such as N2) to contact with dealuminated Beta zeolite, and the isomorphous substitution reaction occurs at high temperature. The main problems of this method are the high operation difficulty, the easy generation of non-framework titanium, and the 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, the Chinese invention patent (application number 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 published 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 number 202011450442.8); Chinese invention patent (application number 202010855082.3); published literature Chinese Journal of Catalysis 42 (2021) 1176–1184; published literature Microporous and Mesoporous Materials 330 (2022) 111625; published 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 detailed records of this method, and the main points are as follows: (1) Contact Al-β zeolite with Si / Al = 11 with an aqueous solution of 13 mol / L HNO3 and react at 100 °C for 24 h to obtain dealuminated Beta zeolite; (2) The dealuminated Beta zeolite is pretreated 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 in these public literatures, the step of acid dealumination of the zeolite is omitted, and Ti-Beta zeolite is directly prepared by carrying out an isomorphous substitution reaction between Al-Beta zeolite and Ti(SO4)2 solution. This approach is based on the property that the strong acidity of the Ti(SO4)2 solution enables dealumination and titanium supplementation to occur simultaneously in the liquid-solid reaction. However, when using Ti(SO4)2 as the titanium source to carry out the liquid-solid isomorphous substitution reaction on Al-Beta zeolite, effective removal of the framework aluminum of Beta zeolite cannot be achieved, and thus the degree of isomorphous substitution reaction of Ti 4+ ions is limited.

[0012] In fact, in the above-mentioned gas-solid, solid-solid, and liquid-solid isomorphous substitution methods, there are all problems of limited degree of isomorphous substitution reaction of Ti 4+ ions. This is because the radius of tetravalent titanium ions (Ti 4+ ) is greater than the radius of trivalent aluminum ions (Al 3+ ) Therefore, the hydroxyl nest vacancies generated after removing Al 3+ from the Al-Beta zeolite framework are 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, with the assembly of the structure building units, it is easier to enter the Beta zeolite framework.

[0014] The open literature Catal. Sci. Technol., 2019, DOI: 10.1039 / C9CY00071B. reported the method for rapidly synthesizing 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 subjected to post-treatment of drying (120 °C) and calcination (550 °C, 6 h) 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 subjected to a dissolution treatment 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 prepared within 1 h.

[0015] The open literature 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 open literature also relates 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 requires the assistance of the mineralization of a large amount of fluorides for crystallization. The discharge of a large amount of high-fluoride 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 process for preparing Ti-Beta zeolite by a steam-assisted dealumination Beta zeolite 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 the structure through steam-assisted tetraethylammonium hydroxide (TEAOH) templating agent without involving the use of fluoride or / and aluminum source. This process includes the following three most prominent technical features: (1) When formulating the hydrogel precursor for local structure reconstruction with dealuminated Beta zeolite and titanium source, it is required that the dosage of tetraethylammonium hydroxide (TEAOH) templating agent should be low enough to avoid complete dissolution and structural fragmentation of dealuminated Beta zeolite; (2) Before local structure reconstruction of the hydrogel precursor, the free water in it should be removed by low-temperature drying to convert it into a xerogel intermediate; (3) The local structure reconstruction process of the xerogel intermediate is carried out in the 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 is that the starting point of its structure reconstruction is not the fragmented structural units generated after complete dissolution of the crystals of dealuminated Beta zeolite (long-range disorder, no characteristic diffraction peaks of Beta zeolite in the XRD pattern), but the whole dealuminated Beta zeolite crystal with good long-range order of the Beta zeolite structure although the hydroxyl nests are slightly etched by the limited alkalinity of TEAOH with a low enough dosage (with complete characteristic diffraction peaks in the XRD pattern). Therefore, the structure reconstruction of the present invention belongs to local structure reconstruction.

[0020] It is found in the research that because the process provided by the present invention does not involve establishing 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 the hydroxyl nests being slightly etched accept titanium ions, the process provided by the present invention can prepare high-quality Ti-Beta zeolite with high framework titanium content and less non-framework titanium by steam-assisted local structure reconstruction starting from dealuminated Beta zeolite 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 crucial to first perform low-temperature drying on the hydrogel precursor to remove the free water therein and convert it into a dry gel intermediate in order to prepare the Ti-Beta zeolite when the amount of TEAOH used is low enough. Moreover, before performing local structure reconstruction, first performing low-temperature drying on the hydrogel precursor to remove the free water therein and convert it into a dry gel intermediate, and then subjecting the dry gel intermediate to local structure reconstruction assisted by steam is also a necessary practice to avoid the excessive etching of the hydroxyl nests of dealuminated Beta zeolite by the alkaline solution of TEAOH, which may cause the disintegration of part of the crystal structure and the generation of excessive fragmented substances. 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 solids are consumed to reach the equilibrium state. This means that when the feed amount of silicate solids (such as dealuminated Beta zeolite) is fixed, the larger the volume of the alkaline solution, the greater the dissolution degree of the silicate solids (such as dealuminated Beta zeolite). However, for the present invention, since the silicate solids are in contact with steam and the silicate fragments are non-volatile, the dissolution equilibrium of silicate is limited to the liquid film with a limited surface thickness and the liquid water filled in its pores due to capillary condensation. And 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 on the hydrogel precursor to remove the free water therein and convert it into a dry gel intermediate, and then subjecting the dry gel intermediate to structure reconstruction assisted by steam is also beneficial to maximizing the local structure reconstruction rate of dealuminated Beta zeolite when the amount of TEAOH used is very low, 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 dry gel intermediate, the limited TEAOH template agent molecules and the soluble Beta zeolite structure units generated by mild etching of the hydroxyl nests are concentrated in the liquid water condensed 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 dry gel conversion method for preparing Ti-Beta zeolite. Although seemingly both involve steam assistance on the surface, the latter has to go through the crystallization process of amorphous dry gel. The dry gel crystallization process assisted by steam, although sometimes can be carried out without adding fluoride additives, requires the introduction of an aluminum source to assist the crystallization of amorphous dry gel without adding fluoride mineralizer. Therefore, it is difficult to obtain high-performance Ti-Beta zeolite by the existing dry gel conversion method.

[0023] It should be emphasized that the process for preparing Ti-Beta zeolite by the local structure reconstruction method of steam-assisted dealuminated Beta zeolite provided by the present invention is not a simple combination of the existing dry gel conversion method and structure reconstruction method. The innovation and advancement of the present invention lie in that by reducing the dosage of the strong alkaline TEAOH template agent and simultaneously restricting the dissolution amount of silicate by means of the pore condensation liquid generated by dealuminating Beta zeolite in saturated steam and the liquid film on the zeolite solid surface, the controllable etching and desilication of the hydroxyl nests of dealuminated Beta zeolite are realized, the acceptance ability of the hydroxyl nests for larger titanium ions is improved, which is beneficial to the preparation of high-performance Ti-Beta zeolite with a high framework titanium content and a low non-framework titanium content. The local structure defects generated by controllable etching and desilication can be quickly repaired directly with the TEAOH template agent solution, completely getting rid of the troubles brought by the need to start from amorphous silicate fragments for the structure reconstruction of Beta zeolite in the existing conventional hydrothermal method, and thus the need to add fluoride or / and aluminum source for auxiliary structure reconstruction.

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

[0025] A process for preparing Ti-Beta zeolite by the local structure reconstruction method of steam-assisted dealuminated Beta zeolite comprises the following steps:

[0026] The first step: preparing dealuminated Beta zeolite from Al-Beta zeolite as raw material

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

[0028] (1) Selecting the Al-Beta zeolite raw material

[0029] The Al-Beta zeolite mentioned refers to silicon-aluminum Beta zeolite. The present invention has no limitation on the crystal grain size of Al-Beta zeolite and no limitation on the production process of Al-Beta zeolite. However, in order to facilitate the implementation effect of the present invention, the following limitations are imposed on 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 silicon-aluminum oxide (the 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 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 the Al-Beta zeolite products with a lower molar ratio of SiO2 to Al2O3, there may generally be impurity crystals of mordenite (MOR), while in the Al-Beta zeolite with a higher molar ratio of SiO2 to Al2O3, there may generally be impurity crystals of ZSM-5 zeolite. 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 can be determined whether there are characteristic peaks of MOR zeolite and ZSM-5 zeolite impurity crystals in the XRD pattern of the sample, so as to know whether the Al-Beta zeolite is a pure Beta zeolite phase.

[0031] Theoretically, the crystallization of the 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 the 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 the 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, it has poor universality to use the XRD relative crystallinity index to determine whether the crystallization of the purchased or synthesized Al-Beta zeolite is good. For this reason, the present invention recommends using the specific surface area index of the 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 literature reported values of the specific surface area data of the Al-Beta zeolite, the BET specific surface area value of the 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 the 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 indicator 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 difficult to synthesize pure-phase Al-Beta zeolite with a very low molar ratio of SiO2 to Al2O3 by hydrothermal method. Therefore, the suitable range of the molar ratio of SiO2 to Al2O3 of the Al-Beta zeolite required by the present invention is between 10 and 200, preferably between 20 and 100, and more preferably between 25 and 60.

[0033] The molar ratio analysis of 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 this field 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, on the basis of Al-Beta zeolite, the dealuminated Beta zeolite can be prepared by using conventional acid dealumination methods. 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 the 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] Since the molar ratio of SiO2 to Al2O3 of the dealuminated Beta zeolite is very high and the aluminum content is very low, the inductively coupled plasma emission spectrometry (ICP) method or atomic absorption (AA) method is required to accurately determine its molar ratio of SiO2 to Al2O3. The ICP method is recommended in the present invention.

[0038] When the Al-Beta zeolite is subjected to acid dealumination treatment, 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 the Ti-Beta zeolite (more precisely, the Ti-Al-Beta zeolite) as an oxidation reaction catalyst and weaken the advantages of the structure reconstruction method in the preparation of high-performance Ti-Beta zeolite.

[0039] Although the framework aluminum of the Al-Beta zeolite is easily removed, so that the dealuminated Beta zeolite meeting the requirements of the present invention can be prepared on the basis of the 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 a dealumination method formed by any combination of the above different methods, considering the production cost, process complexity of the dealuminated Beta zeolite and the treatment difficulty of the waste liquid generated by dealumination, the present invention recommends using the concentrated nitric acid aqueous solution dealumination method to prepare the dealuminated Beta zeolite meeting the requirements of the present invention.

[0040] Engineers familiar with this field 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 is ultimately reflected 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-time 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, and the dealumination reaction time is 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 dealumination of Al-Beta zeolite, 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 later use.

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

[0042] In addition to dealuminated Beta zeolite, other materials required for preparing a hydrogel precursor for local structure reconstruction include an aqueous solution of tetraethylammonium hydroxide (TEAOH) template agent, a 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 TEAOH to SiO2: The suitable range is 0.01 - 0.3; the preferred range is 0.03 - 0.2; the more preferred range is 0.05 - 0.15.

[0045] 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.

[0046] 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.

[0047] The titanium source suitable for the present invention is mainly organic compounds of titanium, such as tetrabutyl titanate (TBOT), tetraethyl titanate (TEOT), tetraisopropyl 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 beneficial for titanium ions to enter the framework of titanium silicalite and to avoid the generation of non-framework titanium in titanium silicalite to the greatest extent. Based on this principle, the preferred titanium sources for the present invention are tetrabutyl titanate (TBOT) and tetraethyl titanate (TEOT), and more preferably tetrabutyl titanate. The titanium source suitable for the present 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.

[0048] 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, Si to Ti, and TEAOH to SiO2, measure deionized water, the titanium source, and the TEAOH templating agent solution; then, mix the titanium source and the TEAOH templating agent solution under stirring, and then add the required deionized water to prepare a homogeneous alkaline solution containing the titanium source and the TEAOH templating agent; finally, pour all the dealuminated Beta zeolite into the alkaline solution containing the titanium source and the TEAOH templating agent under stirring, and stir and react at room temperature (mild desilication reaction) for a period of time. 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.

[0049] The TEAOH templating agent can ionize to produce TEA + cations and OH - anions in aqueous solution. We found in the research that TEA + cations play a role 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 being mildly etched accept titanium ions. And OH -Anions play the role of an etchant during the early stage of preparing Ti-Beta zeolite, including the preparation process of the hydrogel precursor, the process of converting the hydrogel precursor into a xerogel intermediate by removing free water through low-temperature drying, and the initial stage of the steam-assisted local structure reconstruction process (during the autoclave heating process), and are responsible for slightly desiliconizing the hydroxyl nests of dealuminated Beta zeolite. In order to achieve the objective of the present invention without the assistance of fluoride, that is, to prepare high-performance Ti-Beta zeolite through steam-assisted local structure reconstruction, the dosage of TEAOH template agent should be low enough when preparing the hydrogel precursor to avoid the complete dissolution and structural fragmentation of dealuminated Beta zeolite in the early stage of Ti-Beta zeolite preparation. This is the main reason for the present invention to select the above-mentioned TEAOH / SiO2 molar ratio range.

[0050] The silicon-titanium ratio of the hydrogel precursor, that is, 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 during the preparation of the hydrogel precursor will release titanium ions during steam-assisted local structure reconstruction, and the latter will enter the slightly etched hydroxyl nests of dealuminated Beta zeolite, thereby converting dealuminated Beta zeolite into Ti-Beta zeolite. Therefore, the maximum dosage of the titanium source during the preparation of the hydrogel precursor can be estimated by the number of hydroxyl nests of 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 from the dealuminated Beta zeolite obtained from the Al-Beta zeolite raw material with a silicon-aluminum ratio (SiO2 / Al2O3) of 30 is 15. Therefore, when preparing the hydrogel precursor, the maximum dosage of the titanium source should make the Si / Ti ratio of the precursor ≮15, otherwise too large a dosage of the titanium source will lead to an increase in non-framework titanium in the Ti-Beta zeolite product. Generally speaking, the framework titanium content in titanium silicate zeolite within an appropriate range is beneficial to catalytic reactions. On the one hand, too low a framework titanium content in titanium silicate zeolite (that is, too high a Si / Ti molar ratio) is not conducive to catalyst activity, and the reason need not be elaborated. On the other hand, too high a framework titanium content in titanium silicate zeolite (that is, too low a Si / Ti ratio) is also not beneficial to increasing catalytic activity. This is because, in this case, diffusion control within the zeolite micropores will become the rate-determining step of the catalytic reaction. The above is the main basis for the present invention to determine the Si / Ti molar ratio range.

[0051] In the stage of preparing the hydrogel precursor, the role of adding deionized water is to ensure that the prepared hydrogel precursor has an appropriate H2O / SiO2 ratio range. It can be easily seen from the raw materials required for preparing the hydrogel precursor that the externally added deionized water volume and the water volume carried by the TEAOH template agent (the concentrations of commercially available products are usually 25 wt.% and 40 wt.%) are the sources of the water volume in the hydrogel precursor. However, when the dosage of the TEAOH template agent is low enough, if deionized water is not supplemented additionally, relying solely on the water volume brought in by the TEAOH template agent with a low enough dosage, it is bound to be difficult to ensure the formation of a uniform hydrogel from the three components of dealuminated Beta zeolite, TEAOH, and titanium source. In addition, if the H2O / SiO2 ratio of the hydrogel precursor is too low, it is also not conducive to the limited alkalinity of the TEAOH template agent with a low enough dosage to carry out the necessary degree of desilication etching on the hydroxyl nests of the dealuminated Beta zeolite. The present invention requires that the minimum H2O / SiO2 ratio of the hydrogel precursor (the sum of the externally added deionized water and the water volume carried by TEAOH, divided by the feed amount of dealuminated Beta zeolite (dry basis, calculated as SiO2)) should be greater than the saturated water absorption rate of the dealuminated Beta zeolite (the saturated water absorption rate of a typical dealuminated Beta zeolite is generally 1.2 ml H2O / g of 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 the samples are uniformly wet but no free water appears). The maximum H2O / SiO2 ratio of the hydrogel precursor, that is, the maximum dosage of deionized water under the premise of a certain feed amount of the TEAOH template agent, is mainly restricted by the aging time and energy consumption of removing the free water in the hydrogel precursor by low-temperature drying in the next step and converting it into a dry gel intermediate. Undoubtedly, on the premise of ensuring that the three components of dealuminated Beta zeolite, TEAOH, and titanium source can form a uniform hydrogel, and the limited alkalinity of the TEAOH template agent with a low enough dosage can carry out the necessary degree of desilication etching on the hydroxyl nests of the dealuminated Beta zeolite, the dosage of deionized water should not be too much, so as not to increase the drying and dehydration burden of the subsequent step of preparing the dry gel intermediate from the hydrogel precursor due to too high an H2O / SiO2 ratio of the hydrogel precursor. The above is the main basis for the present invention to determine the H2O / SiO2 molar ratio range.

[0052] Step 3: Remove the free water in the hydrogel precursor by low-temperature drying and convert it into a dry gel intermediate

[0053] The low-temperature drying 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 substances (especially the TEAOH templating agent) contained therein. In addition to removing most of the free water in the hydrogel precursor, during the low-temperature drying process, the limited alkalinity generated by the TEAOH templating agent with a sufficiently low dosage in the hydrogel precursor (provided by the OH - anion generated by the ionization of TEAOH in water) will simultaneously etch the hydroxyl nests of dealuminated Beta zeolite and cause a mild desilication reaction. Therefore, the dry gel intermediate described in the present invention refers to the product obtained after the hydrogel precursor is dehydrated by low-temperature drying. Compared with the hydrogel precursor, the dry gel intermediate not only contains much less free water, but also the hydroxyl nests of its dealuminated Beta zeolite have been further etched by the limited alkalinity of the TEAOH templating agent with a sufficiently low dosage, and a small amount of silicon tetrahedra (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 physicochemical properties of the organic substances (especially the TEAOH templating agent) therein can be ignored.

[0054] To enable engineers in the art to have a more specific understanding of the low-temperature drying 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: placing the hydrogel precursor in an ordinary electric heating oven at 80°C and drying it to a constant weight state to obtain the dry gel intermediate that meets the requirements of the present invention. Analysis data shows that the TEAOH templating agent will not decompose when the hydrogel precursor is dried and dehydrated 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 20 wt.% of water, mainly in the form of bound water and very little free water. We found by thermogravimetric (TG) analysis that the weight loss caused by the removal of free water from the hydrogel precursor occurs in the low-temperature range of 30-130°C; while the weight loss caused by the removal of the bound water (bound by pores and hydroxyl nests) from the dry gel occurs in the medium-temperature range of 130-166°C, and the weight loss caused by the decomposition and removal of the TEAOH templating agent therein appears in the high-temperature range above 166°C.

[0055] Therefore, for the low-temperature drying described in the present invention, the suitable temperature range is between room temperature (30°C) and 130°C, preferably between 50-100°C, and more preferably between 70-90°C. By means of controlling the drying pressure, drying time, and atmosphere conditions, the total water content in the dry gel intermediate is controlled between 15-25 wt%, and the decomposition of TEAOH during the drying process is prevented.

[0056] For the present invention, the drying temperature is a key control parameter for the low-temperature drying step 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 matter (especially the TEAOH template agent) therein 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 TEAOH has not decomposed and its structure has 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 TEAOH has decomposed and its structure has changed during the drying process, not meeting the first characteristic; (2) Take a sample of the dry gel intermediate for thermogravimetric analysis. 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 wt%, indicating 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. The dry gel intermediate meeting the requirements of the present invention is sealed for standby after simple grinding or crushing.

[0057] Generally speaking, when drying the hydrogel precursor in the specified low-temperature region above, the dehydration process is gentle, capable of selectively removing free water, and the degree of dehydration is easy to control. It is beneficial for the limited alkalinity generated by the TEAOH template agent with a sufficiently low dosage in the hydrogel precursor to mildly etch and desilicate the hydroxyl nests of dealuminated Beta zeolite during the drying process. Additionally, low-temperature heat sources in various industrial processes can be utilized, saving energy consumption costs and reducing carbon emissions. Moreover, the dry gel intermediate prepared in the above low-temperature region also has the characteristics of relatively loose texture and easy crushing.

[0058] 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 convenient to use an ordinary electric heating oven as the low-temperature drying equipment to perform 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 low-temperature drying 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 of the hydrogel precursor and the indicators of the dry gel intermediate, and other aspects, including drying equipment, drying pressure, atmosphere, and time and other parameters, are all selected by engineers familiar with the art according to needs.

[0059] Step 4: Preparation of Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate

[0060] The reaction process of preparing Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate is carried out in an autoclave. The inside of the autoclave is provided with a bracket and a tray. The dry gel 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 below the tray.

[0061] In this step, the addition amount of the liquid water at the bottom of the kettle, as well as the reaction temperature and time of the structure reconstruction are the main influencing factors. For their value ranges, the present invention requires the following:

[0062] The addition amount of the liquid water at the bottom of the kettle is based on the mass ratio of H2O to the dry gel intermediate (W H2O / W 干胶中间体 ratio), and 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.

[0063] The suitable range of the reaction temperature for the structure reconstruction is: 120 - 170 °C; the preferred range is: 130 - 160 °C; the more preferred range is: 135 - 150 °C.

[0064] The suitable range of the reaction time for the structure reconstruction is: 0.5 - 48 h; the preferred range is: 4 - 24 h; the more preferred range is: 6 - 18 h.

[0065] The selection of the above condition ranges is to meet the needs of the reaction of preparing Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate. To enable engineers in the art to better understand the selection of the above condition ranges made by the present invention, the reaction process of preparing Ti-Beta zeolite by steam-assisted local structure reconstruction of the dry gel intermediate is described as follows:

[0066] 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 partial pressure inside the autoclave continuously rises accordingly. As a result, the dry gel intermediate powder above the tray obtains a correspondingly higher water content due to the infiltration of more and more water vapor. During this process, dealuminated Beta zeolite, TEAOH template agent in the dry gel, and silicate fragments (soluble Beta zeolite structure units) generated by the mild etching of the limited alkalinity of TEAOH with a sufficiently low dosage during the preparation of the hydrogel precursor and the subsequent low-temperature drying process will also undergo corresponding changes: First, for 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 zeolite surface will develop from monolayer to multilayer water molecule adsorption, ultimately forming a liquid film with a certain thickness. Second, for the TEAOH template agent molecules originally in the dehydrated state in the dry gel intermediate, their main change is to redissolve in water and ionize into TEA + cations and OH - anions, thus turning the capillary condensed liquid in the zeolite pores and the water film on the zeolite surface into alkaline solutions. The alkaline solutions throughout the inside and outside of the dealuminated Beta zeolite pores will inevitably undergo further etching and desilication reactions with the hydroxyl nests of the dealuminated Beta zeolite. In addition, for the silicate fragments (soluble Beta zeolite structure units) that have been generated in the dry gel intermediate due to the mild etching of the limited alkalinity of TEAOH with a sufficiently low dosage during the preparation of the hydrogel precursor and the subsequent low-temperature drying process, their main change is to dissolve in the alkaline solutions throughout the inside and outside of the dealuminated Beta zeolite pores, playing a role in delaying the further etching and desilication reaction of the hydroxyl nests of the dealuminated Beta zeolite in the alkaline solution.

[0067] When the temperature of the autoclave rises to a level sufficient to carry out the structural reconstruction reaction (recrystallization reaction), the concentration of silicate fragments (soluble Beta zeolite structure units) dissolved in the alkaline solution both inside and outside the channels of dealuminated Beta zeolite also reaches its peak - the chemical equilibrium concentration. At this time, the etching and desilication reaction of the hydroxyl nests of dealuminated Beta zeolite by the limited alkalinity of TEAOH with a sufficiently low dosage in the dry gel intermediate is inhibited, and the entire closed system enters the later stage of the steam-assisted local structural reconstruction process. The main characteristics of this stage are as follows: 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 increase in volume due to mild desilication, enhancing their ability to accommodate larger titanium ions. For this reason, the process for preparing Ti-Beta zeolite by the steam-assisted partial structural reconstruction method provided by the present invention features a high framework titanium content. Then, the silicate fragments (soluble Beta zeolite structure units) dissolved and concentrated in the alkaline solution with a limited total liquid volume both inside and outside the channels of dealuminated Beta zeolite, under the action of TEA + cations with a structure-directing effect (SDA), rapidly carry out a local structural reconstruction reaction (recrystallization reaction) at the local structural defect sites remaining after the hydroxyl nests of dealuminated Beta zeolite accept titanium ions to repair the structural defects.

[0068] In the present invention, in order to achieve the reaction purpose and effect of steam-assisted local structural reconstruction of the dry gel intermediate to prepare Ti-Beta zeolite, 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 structural reconstruction. Because only under the condition of coexistence of the vapor-liquid two-phase, the channels 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 a monolayer to a multilayer due to water molecule adsorption, ultimately forming a liquid film with a certain thickness. It can be easily seen from the above that the liquid water condensed in the channels 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 processes of each stage of steam-assisted local structural reconstruction of the dry gel intermediate, thereby realizing the preparation of Ti-Beta zeolite.

[0069] 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, then at 120 °C, 130 °C, 140 °C, 150 °C, 160 °C, and 170 °C, with a volume of V (unit: m 3) When the autoclave is filled with saturated water vapor, the reduced mass of water vapor (unit: Kg) is approximately 1.1V, 1.3V, 1.6V, 1.8V, 2.2V, and 2.5V in sequence.

[0070] Taking the preparation of Ti-Beta zeolite by steam-assisted local structure reconstruction reaction of dry gel intermediate at 140 °C as an example, in order to form 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 into the bottom of the autoclave in advance 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 form a vapor-liquid two-phase inside the small autoclave at the structure reconstruction reaction temperature of 140 °C, the estimated value of the minimum amount of water to be introduced into the bottom of the autoclave in advance is 12.16 g (1.6V = 1.6×1×10 -4 Kg = 0.16 g; 1.2W = 1.2×10×10 -3 Kg = 12 g). Therefore, theoretically, as long as more than 12.16 g of liquid water is added to the bottom of the small autoclave in advance, and 10 g of dry gel intermediate is added to the tray on the bracket, the preparation of Ti-Beta zeolite can be realized by steam-assisted local structure reconstruction reaction of dry gel intermediate at 140 °C. Of course, in actual work, the threshold value of the amount of liquid water to 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 (bracket height and tray size), 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 as the appropriate range of water addition amount in the stage of preparing Ti-Beta zeolite by steam-assisted local structure reconstruction reaction of dry gel intermediate.

[0071] After the steam-assisted local structure reconstruction reaction of the dry gel intermediate is completed, in accordance with the consistent practice of conventional hydrothermal synthesis of zeolite molecular sieves, the autoclave is cooled and depressurized. When the temperature and pressure inside the autoclave approach the ambient temperature and pressure, the autoclave is opened and the Ti-Beta zeolite product is taken out.

[0072] Step 5: Post-treatment of Ti-Beta zeolite product

[0073] 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, and only needs to be subjected to conventional drying and calcination treatments to obtain the Ti-Beta zeolite product. That is, after the steam-assisted local structure reconstruction reaction of the dry gel intermediate ends, the Ti-Beta zeolite product is collected from the tray in the autoclave, and then dried and calcined. Among them, the purpose of calcination is to remove a small amount of organic matter and TEAOH template agent in the product. Engineers familiar with the art can perform the post-treatment operations according to common sense.

[0074] Advantages of the present invention:

[0075] The present invention provides a new process for preparing Ti-Beta zeolite mainly from dealuminated Beta zeolite as the main raw material, without the need to add fluoride or / and aluminum source for assistance. Mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, 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 reducing the dosage of the strongly alkaline TEAOH template agent, and by means of the limited dissolution of silicate in the condensed liquid in the pores of dealuminated Beta zeolite and the liquid film on the surface of the zeolite solid in saturated steam, controllable desilication etching of the hydroxyl nests of dealuminated Beta zeolite is realized, and the acceptance ability of the hydroxyl nests to larger titanium ions is improved. The local structural defects generated by controllable desilication etching can be directly repaired with the TEAOH template agent solution, completely getting rid of the troubles brought by the need to start from amorphous silicate fragments for the structure reconstruction of Beta zeolite in the existing conventional hydrothermal method, and thus the need to add fluoride or / and aluminum source for assistance. The process provided by the present invention is conducive to the preparation of high-quality Ti-Beta zeolite with high crystallinity, high purity, high framework titanium content and less non-framework titanium. It not only has no corrosion and pollution problems of fluoride, but also has a short preparation time, simple post-treatment, less template agent dosage and low preparation cost. Therefore, it has strong practicability. Description of the drawings

[0076] Figure 1 It is the XRD pattern of the dry gel intermediate prepared in Example 1.

[0077] Figure 2 It is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-1 sample prepared in Example 1.

[0078] Figure 3 It is the X-ray powder diffraction (XRD) pattern of the Ti-Beta-1 sample prepared in Example 1.

[0079] Figure 4 It is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-1-F-Com sample prepared in Comparative Example 1.

[0080] Figure 5 It is the Fourier transform infrared spectrum of the framework vibration of the Ti-Beta-2-Cl-Com sample prepared in Comparative Example 2.

[0081] 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

[0082] The following further illustrates the detailed implementation manners of the present invention in combination with the accompanying drawings and technical solutions.

[0083] The implementation effects 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.

[0084] In terms of the characterization of 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.

[0085] 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.

[0086] 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, 5 mmol of hydrogen peroxide (35%), and 50 mg of Ti-Beta zeolite catalyst are added to the flask, and then the reaction is vigorously stirred at 353 K for 1 hour. Finally, samples are taken from the reaction product, and the composition analysis is carried out 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 rate of cyclohexene and the selectivity of cyclohexene oxide are calculated by the external standard method.

[0087] In addition, in the process of preparing Ti-Beta zeolite by the method of local structure reconstruction of steam-assisted dealuminated Beta zeolite, 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 are also involved; 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 content, and the degree of retention of the crystal structure of the dealuminated Beta zeolite in the dry gel intermediate.

[0088] 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 silica-alumina 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).

[0089] The present invention will be further described below through examples, but the present invention is not limited by these examples.

[0090] Example 1: This example is used to illustrate the process of preparing Ti-Beta zeolite by the steam-assisted local structure reconstruction method of dealuminated Beta zeolite provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, first mildly desiliconizing and etching the hydroxyl nests of dealuminated Beta zeolite and then repairing and reconstructing to form Ti-Beta zeolite. By reducing the dosage of strongly alkaline TEAOH template agent and relying on the limited dissolution of silicate by the condensed liquid in the pores of dealuminated Beta zeolite and the liquid film on the zeolite solid surface in saturated steam, the purpose of mildly etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions can be achieved, and the purpose of preventing the dealuminated Beta zeolite crystals from being completely dissolved by the strongly alkaline TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. 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). 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.

[0091] The first step is to prepare dealuminated Beta zeolite using Al-Beta zeolite as the raw material

[0092] (1) According to the hydrothermal crystallization method provided by US Patent US3 308 069 (1967), synthesize Al-Beta zeolite with a molar ratio of silicon-aluminum oxide (molar ratio of SiO2 to Al2O3) of 25 by itself as the raw material for preparing dealuminated Beta zeolite. After the synthesized Al-Beta zeolite is subjected to 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 by TEM to be less than 100 nanometers, belonging to nano-Beta zeolite; no any impurity crystals are found by XRD method, and its BET specific surface area is calculated from its nitrogen physical adsorption data to be about 540 m 2 / g, and its molar ratio of silicon-aluminum oxide (molar ratio of SiO2 to Al2O3) is measured by XRF method to be about 24, meeting the technical requirements of the Beta zeolite raw material for this invention.

[0093] (2) Prepare dealuminated Beta zeolite.

[0094] First, prepare a concentrated nitric acid solution with a molar concentration of 13M. Then, according to the liquid-solid ratio of 20:1 (ml / g), while stirring, add 20 g of the Beta zeolite matrix after the above-mentioned drying and calcination treatment into a three-necked flask containing 400 ml of 13M 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 is completed, 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-aluminum oxide (molar ratio of SiO2 to Al2O3) of the dealuminated Beta zeolite is measured by ICP method to be 990. It meets the requirements of this invention. Seal and store for later use, avoiding moisture absorption.

[0095] Second step, prepare a hydrogel precursor for local structure reconstruction using dealuminated Beta zeolite

[0096] Use a 25 wt.% aqueous solution of tetrapropylammonium hydroxide (TEAOH) as the template agent source, use tetrabutyl titanate (TBOT) as the titanium source, and determine the amount of the template agent solution according to the TEAOH / SiO2 molar ratio (using the number of moles of SiO2 to represent the number of moles of dealuminated Beta zeolite); determine the amount of tetrabutyl titanate (TBOT) according to the Si / Ti molar ratio of 50; determine the amount of additional deionized water added according to the H2O / SiO2 molar ratio of 5.

[0097] The specific procedure is as follows: First, based on the dosage of 10 g dealuminated Beta zeolite, 10 g of 25 wt.% tetrapropylammonium hydroxide (TEAOH) aqueous solution, 1.16 g of tetrabutyl titanate (TBOT), and 7.8 g of added deionized water were weighed respectively. Then, under stirring, the titanium source and the TEAOH templating agent solution were mixed, and the required deionized water was added to prepare a uniform alkaline solution containing the titanium source and the TEAOH templating agent. Finally, under stirring, all the dealuminated Beta zeolite was poured into the alkaline solution containing the titanium source and the TEAOH templating agent, and stirred at room temperature for reaction (mild desilication reaction) for 3 h to obtain a hydrogel precursor for the preparation of Ti-Beta zeolite by local structure reconstruction.

[0098] In the third step, the free water in the hydrogel precursor was removed by low-temperature drying to convert it into a xerogel intermediate.

[0099] 25 g of the hydrogel precursor prepared in the second step was added to a crucible, and then the crucible was placed in a common electric oven at 80 °C. It was slowly dried and dehydrated under normal pressure and still air. After 12 h, it reached a constant weight state, and about 14.5 g of the xerogel intermediate was obtained. The XRD pattern of the xerogel intermediate is shown in Figure 1 . From Figure 1 It can be seen 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 from the xerogel intermediate on the thermogravimetric analyzer occurred in the low-temperature region of 30 - 130 °C, the weight loss caused by the removal of bound water occurred in the medium-temperature region of 130 - 166 °C, and the weight loss caused by the decomposition and removal of the TEAOH templating agent in it appeared in the high-temperature region above 166 °C. The results show that the prepared xerogel intermediate still contains about 20 wt.% 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 physicochemical properties of the organic matter (mainly the TEAOH templating agent) 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.

[0100] In the fourth step, steam-assisted local structure reconstruction of the xerogel intermediate to prepare Ti-Beta zeolite

[0101] The reaction process of steam-assisted local structure reconstruction of the xerogel intermediate to prepare Ti-Beta zeolite was carried out in a 100 ml laboratory-scale autoclave. The inside of the autoclave was equipped with a bracket and a tray. 10 g of the xerogel intermediate powder was placed on the tray, and the auxiliary steam was provided by the vaporization of the liquid water at the bottom of the autoclave under the tray. The local structure reconstruction reaction was carried out in a common electric oven with the temperature set at 140 °C for 12 h.

[0102] According to calculations, when performing local structure reconstruction of steam-assisted dry gel intermediates 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 pores 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.

[0103] 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 are close to the ambient temperature and pressure, the autoclave is opened and the Ti-Beta zeolite product is taken out.

[0104] Step 5, Post-treatment of the Ti-Beta zeolite product

[0105] Since the Ti-Beta zeolite product prepared by the method of the present invention does not require filtration and washing treatments, and even less requires defluorination and dealumination post-treatments, it is only subjected to conventional drying and calcination treatments. The drying is carried out in an electric oven at a drying temperature of 110 °C for a drying time of 12 h; the calcination is carried out in a muffle furnace at a calcination temperature of 540 °C for a calcination time of 6 h. The obtained white powder is the Ti-Beta zeolite product, coded as Ti-Beta-1.

[0106] The Si / Ti molar ratio of the Ti-Beta-1 sample measured by the XRF method is about 50. The infrared spectrum of the framework vibration 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 framework titanium near the wave number of 960 cm -1 , and its framework titanium content index value (I 960 / I 800 ) is 1.12. In addition, it can be seen from Figure 3 that this sample has a high crystallinity and no impurity crystals. In addition, through ultraviolet Raman spectroscopy characterization (excitation light source wavelength 325 nm), it is confirmed that this sample contains a very small amount of anatase-phase titanium dioxide (there are weak resonance peaks of anatase-structured titanium dioxide at 144, 390, and 635 cm -1 ). The above results show that according to the steam-assisted dealumination Beta zeolite local structure reconstruction process 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.

[0107] Comparative Example 1: This example is used to illustrate the method of 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 make a hydrogel from dealuminated Beta zeolite, a large amount of tetraethylammonium hydroxide (TEAOH) templating agent solution, a titanium source, and deionized water. Then, perform hydrothermal pretreatment on the hydrogel at a higher temperature so that the dealuminated Beta zeolite in it is completely dissolved by the large amount of strongly basic TEAOH solution and becomes fragmented structural units with amorphous properties. Then, cool the hydrogel belonging to the amorphous substance and add fluoride to it. Finally, reheat the amorphous hydrogel to start 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.

[0108] The first step is to prepare dealuminated Beta zeolite using Al-Beta zeolite as the raw material.

[0109] 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 a molar ratio of silicon-aluminum oxide (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.

[0110] The second step is to prepare a hydrogel precursor for structure reconstruction using dealuminated Beta zeolite.

[0111] Use a 25 wt.% aqueous solution of tetrapropylammonium hydroxide (TEAOH) as the source of the templating agent, use tetrabutyl titanate (TBOT) as the titanium source, and determine the amount of the templating agent solution 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; determine the amount of tetrabutyl titanate (TBOT) 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.

[0112] The specific procedure is 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) were weighed respectively. Then, under stirring, the titanium source and the TEAOH templating agent solution were mixed to prepare a uniform alkaline solution containing the titanium source and the TEAOH templating agent. Finally, under stirring, all the dealuminated Beta zeolite was poured into the alkaline solution containing the titanium source and the TEAOH templating agent, and stirred at room temperature for reaction (mild desilication reaction) for 3 h to obtain a hydrogel (with a H2O / SiO2 molar ratio of about 7.4) for the preparation of Ti-Beta zeolite by structure reconstruction.

[0113] Step 3, hydrothermal pretreatment of the hydrogel

[0114] All the hydrogel prepared in the second step was loaded into a small laboratory autoclave with a polytetrafluoroethylene lining for hydrothermal pretreatment. The pretreatment was carried out in a common electric heating oven with a set temperature of 140 °C, and the pretreatment time was 1 h. After the pretreatment, the autoclave was taken out of the oven and quenched to room temperature with water. A small amount of the sample was taken out after opening the autoclave for detecting the retention degree of the Beta zeolite crystal structure (XRD method). The detection results showed 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 had 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 amorphous properties.

[0115] Step 4, using fluoride to assist the structure reconstruction of the hydrothermally treated hydrogel to prepare Ti-Beta zeolite

[0116] 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 (in which the crystal structure of the dealuminated Beta zeolite has been completely dissolved and destroyed, becoming fragmented structural units with amorphous properties (in a state of long-range disorder)). Finally, reseal the autoclave for hydrothermal crystallization treatment, and the hydrothermal crystallization treatment is at 140 °C and the crystallization time is 12 h. After the crystallization, the autoclave was taken out of the oven and quenched to room temperature with water.

[0117] Step 5, post-treatment of the Ti-Beta zeolite product

[0118] In this example, the crystallization product obtained in the fourth step needs to undergo four post-treatment steps of filtration, repeated water washing, drying, and calcination in sequence to obtain the Ti-Beta zeolite product, designated as Ti-Beta-1-F-Com (Com represents the comparative example).

[0119] 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.

[0120] The Si / Ti molar ratio of the Ti-Beta-1-F-Com sample measured by 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, fluoride is required for assistance, so there are practical problems such as fluoride-containing wastewater that are not conducive to industrial application.

[0121] 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 accept the relatively large 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 content of framework titanium is low.

[0122] The first step is to prepare dealuminated Beta zeolite using Al-Beta zeolite as the raw material

[0123] Repeat the steps for preparing dealuminated Beta zeolite in Example 1, using a zeolite without hetero-crystalline phase, a crystal grain size less than 100 nm, and a BET specific surface area of about 540 m 2Al-Beta zeolite raw materials with a molar ratio of silicon-aluminum oxide (molar ratio of SiO2 to Al2O3) of about 24 were used to prepare dealuminated Beta zeolite with a molar ratio of SiO2 to Al2O3 of 990 through acid dealumination, and it was sealed and stored for later use.

[0124] In the second step, an anhydrous ethanol solution of titanium tetrachloride was prepared.

[0125] 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.

[0126] In the third step, a liquid-solid isomorphous substitution reaction was carried out at room temperature.

[0127] 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.

[0128] 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, and an ultrasonic-assisted liquid-solid isomorphous substitution reaction was carried out at room temperature. The reaction time was 20 min.

[0129] After the reaction, the ethanol solvent was recovered by reduced pressure evaporation at 80 °C, and the solid product was collected.

[0130] In the fourth step, the Ti-Beta zeolite product was obtained by post-treating the liquid-solid isomorphous substitution reaction product.

[0131] The post-treatment steps mainly included conventional drying (120 °C, overnight) and calcination (540 °C, 3 h). The Ti-Beta zeolite prepared in this example was 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 infrared spectrum of the framework vibration of the Ti-Beta-2-Cl-Com sample is shown in Figure 5 . From Figure 5 it can be seen that the sample also had a characteristic absorption of framework titanium near the wave number 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-2-Cl-Com sample prepared by this method was weaker, indicating that its framework titanium content was lower.

[0132] Comparative Example 3: This example is used to illustrate the practice of the existing solid-solid isomorphic substitution method 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 an 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 migrating out a 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.

[0133] First step, prepare dealuminated Beta zeolite using Al-Beta zeolite as raw material

[0134] Repeat the dealuminated Beta zeolite preparation steps of 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 silicon-aluminum oxide molar ratio (molar ratio of SiO2 to Al2O3) of about 24. Through acid dealumination, a dealuminated Beta zeolite with a SiO2 to Al2O3 molar ratio of 990 is prepared and stored sealed for later use.

[0135] Second step, prepare the precursor mixture by grinding the solid powders of dealuminated Beta zeolite and titanium dichloride

[0136] First, calculate according to preparing Ti-Beta zeolite with a Si / Ti molar ratio of 50 using 10 g of dealuminated Beta zeolite, and accurately weigh 0.85 g of titanium dichloride solid powder in the glove box.

[0137] 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.

[0138] Third step, prepare Ti-Beta zeolite by calcining the precursor mixture

[0139] 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 resulting white solid powder was the Ti-Beta zeolite product, designated 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 . As can be seen from Figure 6 this, the sample also has a characteristic absorption of framework titanium near a 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-3-Cp-Com sample prepared by this method is weaker, indicating that its framework titanium content is lower.

[0140] Example 2: This example is used to illustrate the process of preparing Ti-Beta zeolite by the steam-assisted dealumination Beta zeolite local structure reconstruction method provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, first mildly desilicate and etch the hydroxyl nests of dealuminated Beta zeolite and then repair and reconstruct them into Ti-Beta zeolite. By reducing the amount of strongly basic TEAOH template agent and relying on the limited dissolution of silicate by the condensed liquid in the pores of dealuminated Beta zeolite and the liquid film on the surface of the zeolite solid in saturated steam, the purpose of mildly etching and desilicating the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions can be achieved, and the purpose of preventing the dealuminated Beta zeolite crystals from being completely dissolved by the strongly basic TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. Thus, synthesizing Ti-Beta zeolite crystals (traditional structure reconstruction method) is transformed into 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, during the repair and reconstruction, that is, when repairing the local structural defects left after the mildly 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 autoclave 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.

[0141] 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 amount of liquid water (W H2O / W 干胶中间体If the ratios are changed to 1.5, 2.5, and 3.0 in sequence, then the 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 wave number, and the index values (I 960 / I 800 ) of their framework titanium contents are 1.06, 1.10, and 1.15 in sequence.

[0142] Example 3: This example is used to illustrate the process of preparing Ti-Beta zeolite according to the method for local structure reconstruction of steam-assisted dealuminated Beta zeolite provided by the present invention, that is, using dealuminated Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, first mildly desiliconizing and etching the hydroxyl nests of dealuminated Beta zeolite and then repairing and reconstructing to make Ti-Beta zeolite. By reducing the dosage of strongly alkaline TEAOH template agent and relying on the limited dissolution of silicate by the condensate in the pores of dealuminated Beta zeolite and the liquid film on the surface of the zeolite solid in saturated steam, the purposes of mildly etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions can be achieved, and the purpose of avoiding the complete dissolution of dealuminated Beta zeolite crystals by strongly alkaline TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. Thus, synthesizing 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 mild etching accept titanium ions (the present invention). Among them, during the repair and reconstruction, that is, when repairing the local structural defects left after the hydroxyl nests with mild etching accept titanium ions assisted by water vapor, the temperature and time of the reconstruction reaction are 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.

[0143] Repeat Example 1, but when preparing Ti-Beta zeolite by local structure reconstruction of steam-assisted dry gel intermediate 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 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 wave number, and the index values (I 960 / I 800 ) of their framework titanium contents are 1.03, 1.10, 1.08, 1.07, and 1.06 in sequence.

[0144] Example 4: 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, that is, using dealuminated Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) templating agent, first mildly desilicating and etching the hydroxyl nests of dealuminated Beta zeolite and then repairing and reconstructing them to form Ti-Beta zeolite. By reducing the dosage of strongly alkaline TEAOH templating agent and relying on the limited dissolution of silicate by the condensed liquid in the pores of dealuminated Beta zeolite and the liquid film on the surface of the zeolite solid in saturated steam, the purpose of mildly etching and desilicating the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions can be achieved, and the purpose of preventing the dealuminated Beta zeolite crystals from being completely dissolved by the strongly alkaline TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. Thus, synthesizing Ti-Beta zeolite crystals (traditional structure reconstruction method) is transformed into 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 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.

[0145] 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 (drying air is introduced into the oven) until a constant weight state, 40 °C (drying air is introduced into the oven) until a constant weight state, 50 °C (drying under slightly negative pressure) until a constant weight state, 70 °C until a constant weight state, 90 °C × 8 h, 100 °C × 6 h, and 130 °C × 4 h. Then, the water content of the prepared dry gel intermediate is about 20 wt.%, and it is mainly bound water, with a relatively small proportion of free water. All dry gel intermediate samples are white in color and soft in texture. The changes in the content, structure, and physicochemical properties of the organic matter (mainly TEAOH templating agent) among them 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.

[0146] For the Ti-Beta zeolite samples prepared from the above dry gel intermediate, the infrared spectrum of the framework vibration has obvious characteristic absorption of framework titanium near the wave number of 960 cm -1 The index value (I 960 / I 800 ) of the framework titanium content fluctuates between 1 and 1.15.

[0147] Example 5: 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, that is, using dealuminated Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, first mildly desiliconizing and etching the hydroxyl nests of dealuminated Beta zeolite and then repairing and reconstructing to make Ti-Beta zeolite. By reducing the dosage of strongly basic TEAOH template agent and relying on the limited dissolution of silicate by the condensate in the pores of dealuminated Beta zeolite and the liquid film on the zeolite solid surface in saturated steam, the purpose of mildly etching and desiliconizing the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions can be achieved, and the purpose of preventing the dealuminated Beta zeolite crystals from being completely dissolved by the strongly basic TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. Thus, synthesizing 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 dosage of tetraethylammonium hydroxide (TEAOH) template agent and the H2O / SiO2 molar ratio are 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.

[0148] 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 TEAOH / SiO2 molar ratio (using the number of moles of SiO2 to represent the number of moles of dealuminated Beta zeolite) to 0.01, 0.05, 0.15, 0.2, 0.25, and 0.3, then the dosages of 25 wt.% tetrapropylammonium hydroxide (TEAOH) aqueous solution are successively 1 g, 5 g, 15 g, 20 g, 25 g, and 30 g; at the same time, successively change the H2O / SiO2 molar ratio to 2, 3, 4, 10, 8, 6.13, and 7.35, then the supplementary amounts of deionized water are successively 5.34 g, 5.43 g, 4.74 g, 19.35 g, 9.48 g, 0 g, and 0 g. Based on the above hydrogel precursor, remove the free water in the hydrogel precursor by low-temperature drying to convert it into a xerogel intermediate, and further prepare Ti-Beta zeolite 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 wave number of 960 cm -1 The index value (I 960 / I 800 ) of the framework titanium content fluctuates between 1 and 1.2.

[0149] Example 6: 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, that is, using dealuminated Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, first mildly desilicating and etching the hydroxyl nests of dealuminated Beta zeolite and then repairing and reconstructing to form Ti-Beta zeolite. By reducing the dosage of strongly alkaline TEAOH template agent and relying on the limited dissolution of silicate by the condensed liquid in the pores of dealuminated Beta zeolite and the liquid film on the surface of zeolite solid in saturated steam, the purpose of mildly etching and desilicating the hydroxyl nests of dealuminated Beta zeolite and improving the acceptance ability of hydroxyl nests for larger titanium ions can be achieved, and the purpose of avoiding the complete dissolution of dealuminated Beta zeolite crystals by strongly alkaline TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. Thus, synthesizing 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, different titanium sources are allowed to be used, the Si / Ti molar ratio and the 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.

[0150] Repeat Example 1, but when using dealuminated Beta zeolite to prepare the hydrogel precursor for local structure reconstruction in the second step, the Si / Ti molar ratio (expressing the molar number of dealuminated Beta zeolite with the molar number of SiO2) is changed to 60, 70, 80, 90, and 100 in sequence, and the stirring time is changed to 12 h, 6 h, 4 h, 2 h, and 0.5 h in sequence. Moreover, the titanium sources used when preparing the above hydrogel precursor are tetraethyl titanate, tetraisopropyl titanate, titanium acetylacetonate, the complex of tetrabutyl titanate and isopropanol with a molar ratio of 1:3, and the complex of tetraethyl titanate and isopropanol with a molar ratio of 1:4 in sequence. Based on the above hydrogel precursor, the free water in the hydrogel precursor is removed by low-temperature drying to convert it into a xerogel intermediate, and Ti-Beta zeolite is further prepared by steam-assisted local structure reconstruction of the xerogel intermediate. For the prepared Ti-Beta zeolite samples, the infrared spectrum of the skeletal vibration has obvious characteristic absorption of skeletal titanium near the wave number of 960 cm -1 The wavenumber, and the index value of its skeletal titanium content (I 960 / I 800) varies between 0.8 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 sample shows a decreasing trend.

[0151] Example 7: This example is used to illustrate the process of preparing Ti-Beta zeolite by the steam-assisted dealumination Beta zeolite local structure reconstruction method provided by the present invention, that is, using dealumination Beta zeolite as the main raw material, mainly through steam-assisted tetraethylammonium hydroxide (TEAOH) template agent, first mildly desilicate and etch the hydroxyl nests of dealumination Beta zeolite and then repair and reconstruct them into Ti-Beta zeolite. By reducing the dosage of strongly alkaline TEAOH template agent and relying on the limited dissolution of silicate by the condensed liquid in the pores of dealumination Beta zeolite and the liquid film on the zeolite solid surface in saturated steam, the purpose of mildly etching and desilicating the hydroxyl nests of dealumination Beta zeolite and improving the acceptance ability of the hydroxyl nests for larger titanium ions can be achieved, and the purpose of preventing the dealumination Beta zeolite crystals from being completely dissolved by the strongly alkaline TEAOH solution and becoming fragmented structural units (amorphous substances) can be achieved. 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 mildly etched hydroxyl nests 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 dealumination Beta zeolite, and the Si / Ti molar ratio of the hydrogel precursor can be correspondingly changed according to the number of hydroxyl nests of dealumination 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 dealumination Beta zeolite without the assistance of fluoride or / and aluminum source.

[0152] Example 1 was repeated, but when preparing dealuminated Beta zeolite using Al-Beta zeolite as the raw material in the first step, first, Al-Beta zeolites with a molar ratio of silicon to aluminum oxide (molar ratio of SiO2 to Al2O3) of 10, 20, 40, 60, 80, 100, 150, and 200 were synthesized by the hydrothermal crystallization method provided in US Patent US3 308 069 (1967) as the raw materials for preparing dealuminated Beta zeolite. After the synthesized Al-Beta zeolites were subjected to conventional filtration, washing, drying (110 °C, 12 h), and calcination to remove the template agent (540 °C, 6 h), TEM was used to observe that their average crystal grain size belonged to the nanometer level and the small crystal grain (less than 1 μm) level. As the molar ratio of silicon to aluminum oxide (molar ratio of SiO2 to Al2O3) increased, the crystal grain size increased; XRD examination showed no any impurity crystals, and their BET specific surface areas calculated from their nitrogen physical adsorption data were all higher than 500 m 2 / g; the molar ratio of silicon to aluminum oxide (molar ratio of SiO2 to Al2O3) measured by XRF was 10, 20, 38, 57, 72, 94, 136, and 189 in turn, meeting the technical requirements of the Beta zeolite raw materials of the present invention. The above 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 turn, meeting the technical requirements of the dealuminated Beta zeolites of the present invention.

[0153] The above dealuminated Beta zeolites were used to prepare hydrogel precursors, and then the free water in the hydrogel precursors was removed by low-temperature drying to convert them into dry gel intermediates. Finally, Ti-Beta zeolite was further prepared by steam-assisted local structure reconstruction of the dry gel intermediates. Among them, when preparing hydrogel precursors using dealuminated Beta zeolites with different numbers of hydroxyl nests, in the order of decreasing number of hydroxyl nests of the dealuminated Beta zeolites, that is, in the order of increasing silicon-aluminum ratio of the Al-Beta zeolite raw materials (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 turn. For the prepared Ti-Beta zeolite samples, their skeletal vibration infrared spectra had obvious characteristic absorptions of skeletal titanium near the wave number of 960 cm -1 The content index value of their skeletal titanium (I 960 / I 800 ) varied between 0.8 - 1.5. Generally, as the Si / Ti molar ratio increased, the content index value of the skeletal titanium (I 960 / I 800 ) of the Ti-Beta zeolite samples showed a decreasing trend.

[0154] Example 8: This example is used to illustrate the process of preparing Ti-Beta zeolite by the local structure reconstruction method of steam-assisted dealumination 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.

[0155] The above-mentioned cyclohexene epoxidation reaction 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, 5 mmol of hydrogen peroxide (35%), and 50 mg of Ti-Beta zeolite catalyst were added to the flask, and then the reaction was vigorously stirred at 353 K for 1 hour. Finally, samples were taken from the reaction product, and the composition analysis was carried out by 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 were calculated by the external standard method.

[0156] 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 conversion rates of cyclohexene of the above Ti-Beta zeolite catalysts were 30%, 23%, 43%, and 45% in sequence, and the selectivities of cyclohexene oxide were 91%, 86%, 87%, and 87% in sequence.

Claims

1. A process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite 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 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 needed to prepare the hydrogel precursor include tetraethylammonium hydroxide TEAOH template aqueous solution, 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: TEAOH to SiO2 molar ratio: 0.01-0.3; Molar ratio of H2O to SiO2: 2-10; Si to Ti molar ratio: 5-100; 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 molar ratio of H2O to SiO2, the molar ratio of Si to Ti and the molar ratio of TEAOH to SiO2, deionized water, titanium source and TEAOH template solution are calculated; then, the titanium source and TEAOH template solution are mixed under stirring, and deionized water is added to prepare a uniform alkaline solution containing the titanium source and TEAOH template; finally, all the dealuminated Beta zeolite is poured into the alkaline solution containing the titanium source and TEAOH template under stirring, and the reaction is stirred at room temperature; the stirring reaction time is 0.5-12h; Step 3: Remove the free water in the hydrogel precursor by low-temperature drying and convert it into a dry gel intermediate The temperature range of the low-temperature drying is between room temperature and 130° C. The total water content in the dry rubber intermediate is controlled between 15-25wt% by means of drying pressure, drying time and atmosphere conditions, and TEAOH is prevented from decomposing 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. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite 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 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. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite 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 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 process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite local structure reconstruction method according to claim 1, characterized in that: In the second step, TEAOH to SiO2 molar ratio: 0.03-0.2; Molar ratio of H2O to SiO2: 3-8; Si to Ti molar ratio: 10-80; The titanium sources are tetrabutyl titanate and tetraethyl titanate; The stirring reaction time is 1-6h.

5. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite local structure reconstruction method according to claim 4, characterized in that: In the second step, TEAOH to SiO2 molar ratio: 0.05-0.15; 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 is 2-4h.

6. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite local structure reconstruction method according to claim 1, characterized in that: In the third step, the temperature range of the low temperature drying is between 50-100°C.

7. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite local structure reconstruction method according to claim 6, characterized in that: In the third step, the temperature range of the low temperature drying is between 70-90°C.

8. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite local structure reconstruction method according to claim 1, characterized in that: In the fourth step, 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 1.8-2.

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

9. The process for preparing Ti-Beta zeolite by steam-assisted dealuminated Beta zeolite local structure reconstruction method according to claim 8, characterized in that: In the fourth step, 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 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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