Catalyst for direct oxidation of methanol to polymethoxydimethyl ether, its preparation method and application

Abstract

The application discloses a catalyst for preparing polyoxymethylene dimethyl ethers by directly oxidizing methanol, a preparation method and application thereof, and belongs to the field of catalyst preparation and application. The metal oxide and the acidic molecular sieve are effectively coupled to prepare a bifunctional composite catalyst with a core-shell structure, the composition and proportion of the metal oxide and the acidic molecular sieve are regulated, and high-selectivity synthesis of polyoxymethylene dimethyl ethers by directly oxidizing methanol is realized. The unique core-shell structure can effectively regulate the distance and distribution between the two types of active sites, significantly promote the gradual chain growth of methanol oxidation, and obviously inhibit the occurrence of side reactions such as methanol dehydration to dimethyl ether and deep methanol oxidation, so that the high-selectivity synthesis of polyoxymethylene dimethyl ethers by directly oxidizing methanol is realized. The catalyst preparation process is simple and easy to operate, and the one-step oxidation of methanol to prepare polyoxymethylene dimethyl ethers has the advantages of short flow, good atom economy and green process.

Description

TECHNICAL FIELD

[0001] The present application relates to a catalyst for directly oxidizing methanol to polyoxymethylene dimethyl ethers and a preparation method and application thereof, and belongs to the field of catalyst preparation and application. BACKGROUND

[0002] Polyoxymethylene dimethyl ethers (DMM x ) is a general term for a class of substances. Its simple formula can be represented as: CH3O(CH2O) x CH3 (wherein x≥1, generally≤8), DMM x has a high oxygen content and a high cetane number. It is reported that adding 10-20% of medium-chain DMMx by mass fraction to diesel can improve the combustion quality of diesel in the engine, increase the thermal efficiency, and reduce pollutant emissions, and therefore is considered to be a clean diesel additive with good application prospect.

[0003] The traditional synthesis method of polyoxymethylene dimethyl ethers usually needs two types of raw materials: one type of raw material provides the end-capping group CH3O- and CH3-, such as methanol (MeOH), dimethyl ether (DME) and dimethyl formal (DMM); the other type of raw material provides the methylene oxide polymerization monomer -CH2O-, such as formaldehyde (FA), trioxymethylene (TOX) and polyoxymethylene (PFn). The two types of raw materials are reacted under acid catalysis to generate DMM x , and the types of acid catalysts used include metal oxide sulfate, ion exchange resin, molecular sieve and other solid acid, and ionic liquid and other homogeneous acid catalysts. The alcohol aldehyde condensation method for preparing DMM x includes processes such as methanol oxidation to formaldehyde, methanol and formaldehyde condensation to dimethyl formal, alcohol aldehyde condensation of formaldehyde and dimethyl formal to DMM x , etc. The process has high energy consumption and involves formaldehyde or polyoxymethylene as a reactant in the reaction process, which is not conducive to environmental protection. Current research work focuses more on simplifying the existing process by using fewer operation steps, simpler reactants and simple reaction paths. Direct oxidation of methanol to DMM x not only has the advantages of easy availability of raw materials, low price, short process, good atom economy and low carbon emission, but also can partially replace petroleum, reduce dependence on petroleum, effectively alleviate the problem of excess production of methanol in China, and meet the development direction of new coal chemical industry in China. It is a very competitive and good application prospect of clean fuel additive green synthesis route.

[0004] At present, the direct oxidation of methanol to DMM xThe research mainly focuses on the first CO chain product, dimethyl acetal (DMM). The catalysts are primarily vanadium-titanium catalysts (Chinese Patent CN107008249A). In addition, the low-temperature oxidation of methanol on a molybdenum-tin oxide catalyst can also selectively prepare dimethyl acetal (Chinese Patent CN111632592A). Although the above reports all involve direct oxidation of methanol, the reaction products are limited to low-polymerization degree DMM, and the large molecular weight DMM product is not generated. x This is mainly because it enables the direct oxidation of methanol to produce DMM, a product with a longer carbon-oxygen chain. x The efficient matching between the required acidic sites and redox sites is quite challenging, and methanol readily undergoes dehydration to form dimethyl ether and undergoes deep oxidation, which significantly competes with the methanol oxidation reaction. Therefore, the direct oxidation of methanol to synthesize large-molecule DMM is not feasible. x This is subject to significant limitations. Chinese patent CN102030624A discloses a gas-phase method for producing DMM from methanol. x The process primarily uses metal-impregnated rhenium (Re)-modified Y molecular sieves as catalysts, and is characterized by effective control of the reactor temperature range. This process employs a circulating feed, with the product separated and recycled back to the reactor. Although methanol can be oxidized in one step to produce larger molecular weight DMM... x However, byproducts such as dimethyl ether and carbon oxides (CO) still exist. x Many problems. Summary of the Invention

[0005] This invention aims to provide a catalyst for the direct oxidation of methanol to prepare polyoxymethylene dimethyl ether, its preparation method, and its application. This invention effectively couples metal oxide and acidic molecular sieve into a core-shell bifunctional composite catalyst. The main features of this catalyst are: (1) the metal oxide provides redox active sites, and the acidic molecular sieve provides acidic sites of different intensities. By controlling the composition and ratio of the metal oxide and the acidic molecular sieve, the high-selectivity synthesis of direct oxidation of methanol to polyoxymethylene dimethyl ether is achieved; (2) the unique core-shell structure of this catalyst can effectively control the distance and distribution between redox active sites and acidic sites of different intensities, achieving efficient matching between the two types of active sites, promoting the gradual chain growth of methanol oxidation, and inhibiting the occurrence of side reactions such as methanol dehydration and deep oxidation; (3) the degree of methanol dehydration through the molecular sieve shell can be precisely controlled by adjusting the acidity and coating thickness of the molecular sieve shell, thereby controlling the concentration of methanol in contact with the oxidation core and the intermediate products generated by methanol oxidation, achieving effective growth of the CO chain.

[0006] This invention provides a core-shell catalyst for the direct oxidation of methanol to produce polyoxymethylene dimethyl ether. The catalyst is composed of three parts: a metal oxide, an acidic molecular sieve, and a silica sol binder. The metal oxide accounts for 17-86% by weight; the molecular sieve accounts for 9-78% by weight; and the binder accounts for 5-11% by weight.

[0007] The metal oxide includes two or three of the following: MoO3, Fe2O3, SnO2, CeO2, V2O5, or TiO2; the acidic molecular sieve is one of the following: HZSM-5, Hβ, HY, or HMOR molecular sieve; and the binder is 30 wt.% silica sol.

[0008] Furthermore, the metal oxide accounts for 17-86% by weight, and the molar ratio of each component is M1:M2:M3=1:(0.2-0.8):(0-0.5). When M1 is V2O5, M2 and M3 are two of MoO3, Fe2O3, SnO2, CeO2 or TiO2; when M1 is MoO3, M2 and M3 are two of Fe2O3, SnO2, CeO2, V2O5 or TiO2.

[0009] This invention provides a method for preparing the above-mentioned catalyst, comprising the following steps:

[0010] (1) Preparation of metal oxides:

[0011] Two or three soluble metal salts from molybdenum, iron, tin, vanadium, cerium, and titanium are dissolved separately in deionized water or ethanol to prepare solutions with a mass fraction of 5-15%. These solutions are then mixed at a rate of 300 r / min, and the pH of the co-precipitation system is adjusted with nitric acid or ammonia. o After stirring at a constant temperature of C for 0.5–6 h, transfer to a beaker. o Dry at C until the clear liquid at the top disappears, stir thoroughly, and then transfer to a petri dish at 120°C. o Dry the product until it shrinks into a block shape, then place it in a muffle furnace and heat it to 250-500°C. o Metal oxides were obtained by calcining at C for 6-12 h.

[0012] (2) Preparation of core-shell structured catalysts:

[0013] A core-shell bifunctional composite catalyst was prepared using a coating method with a metal oxide core and a molecular sieve shell. The specific preparation method is as follows: First, 20-40 mesh metal oxide particles were impregnated with silica sol (30 wt.%); molecular sieve was added at a mass ratio of 0.125-4, and the mixture was homogeneous in a round glass flask; finally, the mixture was subjected to an 80-120°C... oC dry 2~12 h, 300~600 o C calcine to prepare the catalyst.

[0014] In the above preparation method, the soluble metal salt includes: the soluble molybdenum salt is ammonium molybdate, the soluble iron salt is ferric nitrate, the soluble cerium salt is cerium nitrate, the soluble tin salt is tin tetrachloride, the soluble vanadium salt is ammonium metavanadate, and the soluble titanium salt is titanium tetrachloride.

[0015] The application provides application of the core-shell structure bifunctional catalyst in a reaction of preparing polyoxymethylene dimethyl ethers from direct oxidation of methanol.

[0016] The application provides the application of the catalyst for preparing polyoxymethylene dimethyl ethers from direct oxidation of methanol, and comprises the following steps:

[0017] (1) catalyst pretreatment: the catalyst is heated to 300~350 o C under an oxidizing atmosphere in a fixed bed, and activated for 2~5 h.

[0018] (2) catalytic reaction: the methanol oxidation reaction is carried out on a fixed bed reaction device, the molar ratio of methanol to oxidant (oxygen or air) in the reaction gas is 1:4~1:20, the space velocity of the reaction gas is 2600~12000 h -1 , the reaction temperature is 120~240 o C, and the reaction pressure is 0.1~1 MPa. The final product is subjected to gas-liquid separation through a condenser and then subjected to chromatographic analysis.

[0019] The application has the following beneficial effects:

[0020] (1) the application develops a preparation method of a catalyst for preparing polyoxymethylene dimethyl ethers from direct oxidation of methanol with high selectivity, a metal oxide is used as a core, an acid molecular sieve is used as a shell, and a core-shell structure bifunctional composite catalyst is prepared through a physical adhesion method. The core-shell structure can effectively adjust the distance and distribution between the redox active sites and the acid sites with different strengths, enhance the contact between the intermediate products generated on the oxide core and the shell layer of the molecular sieve, and at the same time, the generated reaction products must diffuse through the molecular sieve channels, thereby playing a high-efficiency shape-selective role on the product molecules; on the other hand, the composition of the metal oxide, the type of the molecular sieve shell, the silicon-aluminum ratio, the shell thickness and the like can be adjusted to further adjust the oxidation-reduction property and the acidity of the core-shell catalyst, inhibit the occurrence of side reactions such as methanol dehydration and deep oxidation, achieve high-efficiency matching between the redox active sites and the acid sites with different strengths, and ultimately realize the preparation of DMM xHighly selective synthesis.

[0021] (2) Compared with the current direct oxidation of methanol, which mainly focuses on low-polymerization degree DMM, this invention successfully realizes large-molecule DMM. x The synthesis of DMM x The selectivity reaches 45-85%, of which DMM 2-6 The selectivity ranges from 15% to 65%, and the single-pass methanol conversion rate can be as high as 55% to 95%.

[0022] (3) The process of this invention is simple, the reaction temperature is low, and the catalyst is easy to prepare, has good reproducibility, and DMM. x It has high selectivity and is a green synthesis route with good prospects for industrial application. Attached Figure Description

[0023] Figure 1 This is a scanning electron microscope image of the metal oxide core in Example 1.

[0024] Figure 2 This is a scanning electron microscope image of the molecular sieve shell in Example 1.

[0025] Figure 3 This is a scanning electron microscope image of the cross-section of the core-shell catalyst in Example 1. Detailed Implementation

[0026] The present invention will be further illustrated by the following embodiments, but is not limited to the following embodiments. Example 1

[0027] Weigh 11.3 g of ferric nitrate nonahydrate into a beaker and prepare a 7% (w / w) aqueous solution. Weigh 12.5 g of ammonium molybdate tetrahydrate into an Erlenmeyer flask and prepare a 5% (w / w) aqueous solution. Mix the two completely dissolved solutions and stir at 300 rpm for 2.5 h. Adjust the pH to 2.5 with nitric acid. In a constant temperature water bath at 60°C... o After stirring the reaction mixture, transfer it to a beaker and heat it at 90°C. o Dry in an oven at C until the supernatant disappears, stir thoroughly, and then place in a petri dish at 120°C. o The iron-molybdenum precursor was obtained by drying at C for 12 h.

[0028] Take 5 g of iron-molybdenum precursor and heat it at 350 °C. o The iron-molybdenum oxide particles were calcined in a muffle furnace at C for 7 hours, then crushed and sieved to obtain 20-40 mesh particles. The scanning electron microscope image is shown below. Figure 1 As shown.

[0029] Take 5 g of HZSM-5 molecular sieve (SiO2 / Al2O3=300) and heat it at 550 °C. oC in a muffle furnace for 5 h, and its scanning electron microscope image is shown in Figure 2

[0030] After the preparation of the two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a Fe-Mo and molecular sieve core-shell catalyst was prepared by coating. First, the prepared Fe-Mo particles were soaked with the diluted silica sol; then, HZSM-5 molecular sieve was added at a mass ratio of 1:1, and the mixture was stirred until uniform; finally, the prepared mixture was dried at 100 o C for 2 h and calcined at 350 o C in a muffle furnace for 7 h to obtain the core-shell catalyst, and its cross-sectional scanning electron microscope image is shown in Figure 3 It can be seen that the core-shell catalyst has two phases of oxides and molecular sieve, and there is a clear phase interface between the two phases, which fully determines the core-shell structure.

[0031] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, it was heated to 320 o C under an oxidizing atmosphere and activated for 2 h; then, liquid methanol was injected into the pipeline by a micro-injection pump and mixed with air, and the molar ratio of methanol to air was 1:8, the space velocity of the reaction gas was 2600 h -1 , the reaction temperature was 220 o C, and the reaction pressure was 0.6 MPa. The reaction results were as follows: the methanol conversion rate was 77.8%, the DMM x selectivity was 63.1%, and the DMM 2-6 selectivity reached 50.3%. Example 2

[0032] 14.4 g of crystalline tin tetrachloride was weighed into a beaker and prepared into an aqueous solution with a mass fraction of 15%, and 12.5 g of molybdate ammonium tetrahydrate was weighed into an Erlenmeyer flask and prepared into an aqueous solution with a mass fraction of 7%. The two completely dissolved solutions were mixed, stirred at 300 r / min for 0.5 h, and adjusted to pH 2 with nitric acid. After stirring in a constant-temperature water bath at 60 o C, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared. After sufficient stirring, it was placed in a watch glass and dried at 120 o C for 12 h to obtain a molybdenum tin oxide precursor.

[0033] 10 g of the precursor was calcined in a muffle furnace at 450 o C for 7 h, then crushed and sieved to obtain 20-40 mesh molybdenum tin oxide particles.

[0034] 2.5 g of HZSM-5 molecular sieve (SiO2 / Al2O3=200) was calcined in a muffle furnace at 550​o C in a muffle furnace for 5 h.

[0035] After the preparation of two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a molybdenum-tin oxide and molecular sieve core-shell catalyst was prepared by coating. First, the prepared molybdenum-tin oxide particles were soaked with the diluted silica sol; then, HZSM-5 molecular sieves were added at a mass ratio of molecular sieve / metal oxide = 0.25, and stirred and mixed uniformly; finally, the prepared mixture was dried at 100 o C for 2 h, and calcined in a muffle furnace at 450 o C for 7 h to obtain a core-shell catalyst.

[0036] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, it was heated to 300 o C under an oxidizing atmosphere, and activated for 2 h; then, liquid methanol was injected into the pipeline by a micro-injection pump and mixed with oxygen, the molar ratio of methanol to oxygen was 1:8, the space velocity of the reaction gas was 7200 h -1 , the reaction temperature was 230 o C, and the reaction pressure was 1 MPa. The reaction results were as follows: the methanol conversion rate was 85.2%, the DMM x selectivity was 70.8%, and the DMM 2-6 selectivity reached 57.2%. Example 3

[0037] 11.7 g of ammonium metavanadate was weighed into a beaker and prepared into an ethanol solution with a mass fraction of 7%, and 15.2 g of titanium tetrachloride was weighed into an Erlenmeyer flask and prepared into an ethanol solution with a mass fraction of 10%. The two completely dissolved solutions were mixed, and after stirring at a rate of 300 r / min for 3.5 h, they were co-precipitated with ammonia water. After stirring in a constant-temperature water bath at 60 o C, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and after stirring thoroughly, it was placed in a watch glass and dried at 120 o C for 12 h to obtain a vanadium-titanium precursor.

[0038] 15 g of the precursor was calcined in a muffle furnace at 500 o C for 7 h, and then crushed and sieved to obtain vanadium-titanium oxide particles of 20-40 mesh.

[0039] 7 g of Hβ molecular sieves (SiO2 / Al2O3=30) were calcined in a muffle furnace at 550 o C for 5 h.

[0040] After the preparation of the two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a vanadium-titanium and molecular sieve core-shell catalyst was prepared by a coating method. First, the prepared metal oxide particles were soaked with the diluted silica sol; then Hβ molecular sieve was added at a mass ratio of molecular sieve / metal oxide = 0.5, and the mixture was stirred and mixed uniformly; finally, the prepared mixture was dried at 100 o C for 2 h, and calcined in a muffle furnace at 550 o C for 7 h to obtain the core-shell catalyst.

[0041] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, the reactor was heated to 320 o C under an oxidizing atmosphere, and activated at 320 o C for 5 h; then liquid methanol was injected into the pipeline by a micro-injection pump and mixed with oxygen, the molar ratio of methanol to oxygen was 1:4, the space velocity of the reaction gas was 9000 h -1 , the reaction temperature was 200 o C, and the reaction pressure was 0.4 MPa. The reaction results were as follows: the methanol conversion rate was 64.1%, the DMM x selectivity was 62.2%, and the DMM 2-6 selectivity reached 44.6%. Example 4

[0042] 9.3 g of iron nitrate nonahydrate was weighed into a beaker and prepared into an aqueous solution with a mass fraction of 9%, and 14.6 g of ammonium molybdate tetrahydrate was weighed into an Erlenmeyer flask and prepared into an aqueous solution with a mass fraction of 11%. The two completely dissolved solutions were mixed, stirred at a speed of 300 r / min for 0.5 h, and the pH of the coprecipitation system was adjusted to 2.5 with nitric acid. After stirring in a constant-temperature water bath at 60 o C, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and then placed in a watch glass and dried at 120 o C for a certain period of time to obtain the iron-molybdenum precursor.

[0043] 5 g of the precursor was taken and calcined in a muffle furnace at 500 o C for 7 h, and then crushed and sieved to obtain 20-40 mesh iron-molybdenum oxide particles.

[0044] 20 g of HY molecular sieve (SiO2 / Al2O3 = 5) was calcined in a muffle furnace at 550 o C for 5 h.

[0045] After preparing the two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and iron-molybdenum and molecular sieve core-shell catalysts were prepared using a coating method. First, the prepared metal oxide particles were soaked in the diluted silica sol; then, HY molecular sieve was added at a mass ratio of molecular sieve / metal oxide = 4, and the mixture was stirred until homogeneous; finally, the prepared mixture was heated to 100... o Dry at 350°C for 2 hours, and then at 350°C. o The core-shell catalyst was prepared by calcination in a muffle furnace at C for 7 h.

[0046] The prepared core-shell catalyst was loaded into a fixed-bed reactor for methanol oxidation. First, the reactor was heated to 300°C under an oxidizing atmosphere. o C, and activated for 5 h; then liquid methanol was injected into the pipeline using a micro-injection pump and mixed with air, with a methanol to air molar ratio of 1:20 and a reaction gas space velocity of 3600 h⁻¹. -1 The reaction temperature is 180°C. o C, the reaction pressure is 0.3 MPa. The reaction result is a methanol conversion rate of 55%, DMM. x The selectivity rate was 56.0%, of which DMM 2-6 The selectivity rate reached 35.5%. Example 5

[0047] Weigh 12.4 g of ammonium molybdate tetrahydrate and add it to a beaker to prepare an 11% aqueous solution. Weigh 12.6 g of cerium nitrate hexahydrate and add it to an Erlenmeyer flask to prepare a 6% aqueous solution. Weigh 0.7 g of ammonium metavanadate and add it to a beaker to prepare a 7% aqueous solution. Mix the three completely dissolved solutions and stir at 300 r / min for 3 h. Adjust the pH of the coprecipitation system to 1.8 with nitric acid. Heat the mixture in a constant temperature water bath at 60°C. o After stirring the reaction mixture, transfer it to a beaker and heat it at 90°C. o Dry in an oven at C until the supernatant disappears, stir thoroughly, and then place in a petri dish at 120°C. o The metal oxide precursor was obtained by drying at C for 12 h.

[0048] Take 16 g of precursor and salvage it at 500°C. o The metal oxide particles were calcined in a muffle furnace at C for 7 hours, then crushed and sieved to obtain 20-40 mesh metal oxide particles.

[0049] Take 2 g of HZSM-5 molecular sieve (SiO2 / Al2O3=300) and calcine it in a muffle furnace at 550°C for 5 h.

[0050] After the preparation of the two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a metal oxide and molecular sieve core-shell catalyst was prepared by a coating method. First, the prepared metal oxide particles were soaked with the diluted silica sol; then HZSM-5 molecular sieves were added at a mass ratio of molecular sieve / metal oxide = 0.125, and stirred and mixed uniformly; finally, the prepared mixture was dried at 100 o C for 2 h, and calcined at 350 C in a muffle furnace for 7 h to obtain the core-shell catalyst.

[0051] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, it was heated to 320 o C under an oxidizing atmosphere and activated for 5 h; then liquid methanol was injected into the pipeline by a micro-injection pump and mixed with air, the molar ratio of methanol to air was 1:8, the space velocity of the reaction gas was 5400 h -1 , the reaction temperature was 230 o C, and the reaction pressure was 0.6 MPa. The reaction results were as follows: the methanol conversion rate was 84.3%, the DMM x selectivity was 71.4%, and the DMM 2-6 selectivity reached 48.6%. Example 6

[0052] 8.1 g of crystalline tin tetrachloride was weighed into a beaker and prepared into an aqueous solution with a mass fraction of 13%, and 14.4 g of molybdate ammonium tetrahydrate was weighed into an Erlenmeyer flask and prepared into an aqueous solution with a mass fraction of 9%. The two completely dissolved solutions were mixed, stirred at 300 r / min for 0.5 h, and the pH of the co-precipitation system was adjusted to 2 with nitric acid. After stirring at 60 o C in a constant temperature water bath, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and after sufficient stirring, it was placed in a watch glass and dried at 120 o C for 12 h to obtain a molybdenum-tin precursor.

[0053] 10 g of the precursor was calcined at 450 o C in a muffle furnace for 7 h, and then crushed and sieved to obtain 20-40 mesh molybdenum-tin oxide particles.

[0054] 2.5 g of HZSM-5 molecular sieves (SiO2 / Al2O3 = 470) were calcined at 550 o C in a muffle furnace for 5 h.

[0055] After the preparation of two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a molybdenum tin oxide and molecular sieve core-shell catalyst was prepared by a coating method. First, the prepared molybdenum tin oxide particles were soaked with the diluted silica sol; then HZSM-5 molecular sieves were added at a mass ratio of molecular sieve / metal oxide = 0.25, and stirred and mixed uniformly; finally, the prepared mixture was dried at 100 o C for 2 h, and calcined in a muffle furnace at 350 o C for 7 h to obtain a core-shell catalyst.

[0056] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, it was heated to 320 o C under an oxidizing atmosphere, and activated for 5 h; then liquid methanol was injected into the pipeline by a micro-injection pump and mixed with oxygen, the molar ratio of methanol to oxygen was 1:14, the space velocity of the reaction gas was 7200 h -1 , the reaction temperature was 230 o C, and the reaction pressure was 1 MPa. The reaction results were as follows: the methanol conversion rate was 85.3%, the DMM x selectivity was 84.7%, and the DMM 2-6 selectivity reached 65%. Example 7

[0057] 13.7 g of cerium nitrate hexahydrate was weighed into a beaker and prepared into an aqueous solution with a mass fraction of 14%, and 11.3 g of ammonium metavanadate was weighed into an Erlenmeyer flask and prepared into an aqueous solution with a mass fraction of 10%. The two completely dissolved solutions were mixed, stirred at 300 r / min for 2 h, and the pH of the coprecipitation system was adjusted to 2 with nitric acid. After stirring at 60 o C in a constant temperature water bath, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and then placed in a watch glass at 120 o C for 12 h to obtain a vanadium cerium precursor.

[0058] 4 g of the precursor was taken and calcined in a muffle furnace at 350 o C for 7 h to obtain vanadium cerium oxide.

[0059] 2 g of HY molecular sieves (SiO2 / Al2O3=5) were taken and calcined in a muffle furnace at 550 o C for 5 h.

[0060] After the preparation of the two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a vanadium cerium oxide and molecular sieve core-shell catalyst was prepared by a coating method. First, the prepared vanadium cerium oxide particles were soaked with the diluted silica sol; then, HY molecular sieves were added at a mass ratio of molecular sieve / metal oxide = 0.5, and the mixture was stirred and mixed uniformly; finally, the prepared mixture was dried at 100 o C for 2 h, and calcined in a muffle furnace at 450 o C for 7 h to obtain the core-shell catalyst.

[0061] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, the reactor was heated to 300 o C under an oxidizing atmosphere, and activated for 2 h; then, liquid methanol was injected into the pipeline by a micro-injection pump and mixed with air, the molar ratio of methanol to air was 1:14, the space velocity of the reaction gas was 5400 h -1 , the reaction temperature was 240 o C, and the reaction pressure was 0.6 MPa. The reaction results were as follows: the methanol conversion rate was 90.7%, the DMM x selectivity was 65.7%, and the DMM 2-6 selectivity reached 44.6%. Example 8

[0062] 11.5 g of cerium nitrate hexahydrate was weighed into a beaker and prepared into an ethanol solution with a mass fraction of 13%, 9.3 g of ammonium metavanadate was weighed into an Erlenmeyer flask and prepared into an aqueous solution with a mass fraction of 8%, and 2.3 g of tin tetrachloride was weighed into a beaker and prepared into an aqueous solution with a mass fraction of 9%. The three completely dissolved solutions were mixed, stirred at a speed of 300 r / min for 0.5 h, and the pH of the coprecipitation system was adjusted to 2 with nitric acid. After stirring in a constant-temperature water bath at 60 o C, the reaction was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and after sufficient stirring, the mixture was placed in a watch glass and dried at 120 o C for 12 h to obtain the metal precursor.

[0063] 10 g of the precursor was taken, calcined in a muffle furnace at 350 o C for 7 h, then crushed and sieved to obtain metal oxide particles with a size of 20-40 mesh.

[0064] 2.5 g of HMOR molecular sieves (SiO2 / Al2O3=20) were calcined in a muffle furnace at 550 o C for 5 h.

[0065] After the preparation of the two components, 5 g of silica sol was diluted with deionized water at a mass ratio of 1:1, and a metal oxide and molecular sieve core-shell catalyst was prepared by a coating method. First, the prepared metal oxide particles were soaked with the diluted silica sol; then, HMOR molecular sieves were added at a mass ratio of molecular sieve / metal oxide = 0.25, and the mixture was stirred and mixed uniformly; finally, the prepared mixture was dried at 100 o C for 2 h, and calcined in a muffle furnace at 450 o C for 7 h to obtain the core-shell catalyst.

[0066] The prepared core-shell catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, the reactor was heated to 300 o C under an oxidizing atmosphere, and activated for 2 h; then, liquid methanol was injected into the pipeline by a micro-injection pump and mixed with air, the molar ratio of methanol to air was 1:14, the space velocity of the reaction gas was 5400 h -1 , the reaction temperature was 200 o C, and the reaction pressure was 0.3 MPa. The reaction results were as follows: the methanol conversion rate was 72.1%, the DMM x selectivity was 69.1%, and the DMM 2-6 selectivity reached 36.5%.

[0067] Preparation and performance of a powder mixed catalyst

[0068] 12.5 g of ammonium molybdate tetrahydrate was weighed into a beaker and prepared into an aqueous solution with a mass fraction of 7%, and 11.3 g of iron nitrate nonahydrate was weighed into an Erlenmeyer flask and prepared into an aqueous solution with a mass fraction of 5%. The two completely dissolved solutions were mixed, stirred at a rate of 300 r / min for 2 h, and the pH of the coprecipitation system was adjusted to 2 with nitric acid. After stirring in a constant-temperature water bath at 60 o C, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and then placed in a watch glass at 120 o C for 12 h to obtain the iron-molybdenum precursor.

[0069] 4 g of the precursor was calcined in a muffle furnace at 350 o C for 7 h, and then crushed and sieved to obtain 20-40 mesh iron-molybdenum oxide particles.

[0070] 2 g of HZSM-5 molecular sieves (SiO2 / Al2O3=300) were calcined in a muffle furnace at 550 o C for 5 h.

[0071] After the preparation of two components, the iron-molybdenum catalyst and the molecular sieve were mixed in a mass ratio of 1:1, ground in a marver, re-compressed, broken and sieved into 20-40 mesh particles to obtain a powder mixed catalyst.

[0072] The prepared powder mixed catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, it was heated to 320 o C under an oxidizing atmosphere and activated for 2 h; then liquid methanol was injected into the pipeline by a micro-injection pump and mixed with air, the molar ratio of methanol to air was 1:8, the space velocity of the reaction gas was 2600 h -1 , the reaction temperature was 220 o C, and the reaction pressure was 0.6 MPa. The reaction results were as follows: the methanol conversion rate was 78.9%, the DMM x selectivity was 38.6%, and the DMM 2-6 selectivity reached 7.6%.

[0073] Preparation and performance of a granular mixed catalyst

[0074] 14.4 g of crystalline tin tetrachloride was weighed into a beaker and prepared into a 15% mass fraction aqueous solution, and 12.5 g of ammonium molybdate tetrahydrate was weighed into an Erlenmeyer flask and prepared into a 7% mass fraction aqueous solution. The two completely dissolved solutions were mixed, stirred at a rate of 300 r / min for 3 h, and the pH of the co-precipitation system was adjusted to 1.8 with nitric acid. After stirring in a constant temperature water bath at 60 o C, it was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared. After sufficient stirring, it was placed in a watch glass and dried at 120 o C for 12 h to obtain a molybdenum-tin precursor.

[0075] 8 g of the precursor was calcined in a muffle furnace at 500 o C for 7 h, then broken and sieved to obtain 20-40 mesh molybdenum-tin oxide particles.

[0076] 2 g of HZSM-5 molecular sieve (SiO2 / Al2O3=200) was calcined in a muffle furnace at 550 o C for 5 h.

[0077] After the preparation of two components, the molybdenum-tin oxide and the molecular sieve were respectively compressed, broken, sieved into 20-40 mesh particles, and uniformly mixed in a mass ratio of 0.25:1 to obtain a granular mixed catalyst.

[0078] The prepared granular mixed catalyst was loaded into a fixed bed reactor for methanol oxidation reaction. First, it was heated to 320 oC, and activated for 2 h; then liquid methanol was injected into the pipeline with air by a micro-injection pump, the molar ratio of methanol to air was 1:8, the reaction gas space velocity was 7200 h -1 , the reaction temperature was 230 o C, and the reaction pressure was 1 MPa. The reaction results were that the methanol conversion rate was 79.1%, the DMM x selectivity was 42.2%, and the DMM 2-6 selectivity reached 14.6%.

[0079] Preparation and performance of catalyst in a divided bed

[0080] 15.2 g of titanium tetrachloride was weighed into a beaker and prepared into an ethanol solution with a mass fraction of 7%, and 11.7 g of ammonium metavanadate was weighed into an Erlenmeyer flask and prepared into an ethanol solution with a mass fraction of 10%. The two completely dissolved solutions were mixed, and after stirring at a speed of 300 r / min for 0.5 h, they were co-precipitated with ammonia water. After stirring at 60 o C in a constant-temperature water bath, the reaction was transferred to a beaker and dried in an oven at 90 o C until the supernatant disappeared, and after sufficient stirring, it was placed in a watch glass and dried at 120 o C for 12 h to obtain a vanadium-titanium precursor.

[0081] 10 g of the precursor was calcined in a muffle furnace at 400 o C for 7 h, and then crushed and sieved to obtain vanadium-titanium oxide particles with a size of 20-40 mesh.

[0082] 5 g of Hβ molecular sieve (SiO2 / Al2O3=30) was calcined in a muffle furnace at 550 o C for 5 h.

[0083] After the preparation of the two components was completed, the metal oxide and the molecular sieve were respectively pressed into tablets, crushed, and sieved into particles with a size of 20-40 mesh for use.

[0084] The two prepared components were loaded into a fixed bed reactor in a layer separated by quartz wool according to a mass ratio of molecular sieve / metal oxide=0.5 for a methanol oxidation reaction. First, it was heated to 350 o C under an oxidizing atmosphere, and activated for 2 h; then liquid methanol was injected into the pipeline with oxygen by a micro-injection pump, the molar ratio of methanol to oxygen was 1:4, the reaction gas space velocity was 9000 h -1 , the reaction temperature was 200 o C, and the reaction pressure was 0.4 MPa. The reaction results were that the methanol conversion rate was 51.5%, the DMM x selectivity was 49.2%, and the DMM 2-6 selectivity reached 11.7%.

[0085] Comparative Examples 1-3 and the inventive Examples 1-3, the data show that, under the same catalyst composition and reaction conditions, the selectivity of the target product DMM 2-6 on the catalysts with other combination modes are all significantly lower than the selectivity of DMM 2-6 on the core-shell catalyst prepared by the inventive method, which is mainly attributed to the fact that the unique core-shell structure of the core-shell catalyst can effectively regulate the distance and distribution between the redox active sites and the acid sites with different strengths, and achieve high-efficiency matching between the two types of active sites; moreover, the occurrence of side reactions such as deep oxidation and dehydration of methanol can be inhibited, the step-by-step growth of the C-O chain through intermediates from methanol oxidation can be promoted, and finally the high-selectivity synthesis of macromolecular DMM 2-6 is realized.

Claims

1. Use of a catalyst in the reaction of direct oxidation of methanol to polymethoxydimethyl ether, characterized in that: The catalyst is composed of metal oxide, acid molecular sieve and silica sol binder, wherein the weight percentage of metal oxide is 17-86%; the weight percentage of molecular sieve is 9-78%; the weight percentage of binder is 5-11%; the metal oxide comprises two or three of MoO3, Fe2O3, SnO2, CeO2, V2O5 or TiO2; the acid molecular sieve is one of HZSM-5, Hβ, HY and HMOR molecular sieve; the catalyst takes metal oxide as core and molecular sieve as shell, and a core-shell structure catalyst is prepared by coating method. The catalyst is loaded into a fixed bed reactor, activated at high temperature for 2-5 h in an oxidizing atmosphere, cooled to reaction temperature, and then methanol and oxidant are introduced for reaction to prepare polymethoxy dimethyl ether.

2. Use according to claim 1, characterized in that: The binder is 30 wt.% silica sol.

3. Use according to claim 1, characterized in that: The molar ratio of each component in the metal oxide is M1:M2:M3=1:(0.2-0.8):(0-0.5), when M1 is V2O5, M2 and M3 are two of MoO3, Fe2O3, SnO2, CeO2 or TiO2; when M1 is MoO3, M2 and M3 are two of Fe2O3, SnO2, CeO2, V2O5 or TiO2.

4. Use according to claim 1, characterized in that: The preparation method of the catalyst comprises the following steps: (1) Preparation of metal oxide: Two or three kinds of soluble metal salts of molybdenum, iron, tin, vanadium, cerium and titanium are dissolved in deionized water or ethanol respectively to prepare a solution with a mass fraction of 5-15%, and then the above solutions are mixed at a speed of 300 r / min, and the pH of the co-precipitation system is adjusted with nitric acid or ammonia water, 60 o C constant temperature stirring for 0.5-6 h, then transferred to a beaker; 90 o C dried to the disappearance of the upper clear liquid, and after sufficient stirring, 120 o C dried to shrink into a block, put into a muffle furnace, heated to 250-500 o C calcined for 6-12 h to obtain metal oxides; (2) Preparation method of core-shell structure catalyst as follows: First, the prepared metal oxide particles of 20-40 mesh are impregnated with 30 wt.% silica sol; molecular sieves are added at a mass ratio of molecular sieve / metal oxide = 0.125-4, and mixed uniformly in a round glass flask; finally, the mixture is dried at 80-120 o C dried for 2-12 h at 300-600 o C calcined to produce a catalyst.

5. Use according to claim 4, characterized in that: The soluble metal salt comprises: soluble molybdenum salt is ammonium molybdate, soluble iron salt is ferric nitrate, soluble cerium salt is cerium nitrate, soluble tin salt is tin tetrachloride, soluble vanadium salt is ammonium metavanadate, and soluble titanium salt is titanium tetrachloride.

6. Use according to claim 1, characterized in that Comprising the following steps: (1) Catalyst pretreatment: The catalyst is heated to 300-350°C under an oxidizing atmosphere in a fixed bed and activated for 2-5 h. o C, and activated for 2-5 h. (2) Catalytic reaction: methanol oxidation reaction is carried out in a fixed bed, the molar ratio of methanol to oxidant in the reaction gas is 1:4~1:20, the reaction gas space velocity is 2600~12000 h -1 , the reaction temperature is 120~240 o C, and the reaction pressure is 0.1~1MPa.

7. Use according to claim 6, characterized in that: The oxidant is oxygen or air.

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