CN114471644B - Porous heteropolyacid catalyst and preparation method and application thereof - Google Patents

Porous heteropolyacid catalyst and preparation method and application thereof Download PDF

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CN114471644B
CN114471644B CN202011163252.8A CN202011163252A CN114471644B CN 114471644 B CN114471644 B CN 114471644B CN 202011163252 A CN202011163252 A CN 202011163252A CN 114471644 B CN114471644 B CN 114471644B
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porous
heteropolyacid
catalyst
roasting
heteropolyacid catalyst
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CN114471644A (en
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万里
李静霞
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China Petroleum and Chemical Corp
Sinopec Shanghai Research Institute of Petrochemical Technology
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Sinopec Shanghai Research Institute of Petrochemical Technology
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    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J27/00Catalysts comprising the elements or compounds of halogens, sulfur, selenium, tellurium, phosphorus or nitrogen; Catalysts comprising carbon compounds
    • B01J27/14Phosphorus; Compounds thereof
    • B01J27/186Phosphorus; Compounds thereof with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
    • B01J27/195Phosphorus; Compounds thereof with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium with vanadium, niobium or tantalum
    • B01J27/198Vanadium
    • B01J27/199Vanadium with chromium, molybdenum, tungsten or polonium
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00Catalysts, in general, characterised by their form or physical properties
    • B01J35/60Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61Surface area
    • B01J35/61310-100 m2/g
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00Catalysts, in general, characterised by their form or physical properties
    • B01J35/60Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61Surface area
    • B01J35/615100-500 m2/g
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00Catalysts, in general, characterised by their form or physical properties
    • B01J35/60Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/64Pore diameter
    • B01J35/6472-50 nm
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08Heat treatment
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08Heat treatment
    • B01J37/082Decomposition and pyrolysis
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B01PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01JCHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/08Heat treatment
    • B01J37/082Decomposition and pyrolysis
    • B01J37/084Decomposition of carbon-containing compounds into carbon
    • CCHEMISTRY; METALLURGY
    • C07ORGANIC CHEMISTRY
    • C07CACYCLIC OR CARBOCYCLIC COMPOUNDS
    • C07C51/00Preparation of carboxylic acids or their salts, halides or anhydrides
    • C07C51/16Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation
    • C07C51/21Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen
    • C07C51/23Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen of oxygen-containing groups to carboxyl groups
    • C07C51/235Preparation of carboxylic acids or their salts, halides or anhydrides by oxidation with molecular oxygen of oxygen-containing groups to carboxyl groups of —CHO groups or primary alcohol groups

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Abstract

The invention relates to a porous heteropolyacid catalyst and a preparation method thereof, wherein the porous heteropolyacid catalyst comprises heteropolyacid and a carrier, and is obtained by assembling an organic template agent and the carrier under an alkaline condition, further mixing and drying the organic template agent and the heteropolyacid, and then roasting the organic template agent and the carrier in an inert gas atmosphere and then roasting the organic template agent and the carrier in an oxygen-containing atmosphere. The porous heteropoly acid catalyst can be used for preparing methacrylic acid by selectively oxidizing methacrolein, so that the methacrolein is subjected to oxidation reaction in the presence of mixed gas of oxygen and nitrogen and water to generate the methacrylic acid. The catalyst of the invention maintains higher single pass yield of methacrylic acid for a long time under higher load, the conversion rate of the methacrylic aldehyde can reach more than 70%, and the selectivity of the methacrylic acid can reach more than 84%.

Description

Porous heteropolyacid catalyst and preparation method and application thereof
Technical Field
The invention belongs to the field of catalysts, and particularly relates to a porous heteropolyacid catalyst for methacrylic acid synthesis and a preparation method thereof.
Background
The heteropoly compound has good acid site and oxidation-reduction performance, is a widely used catalyst, and has been industrially applied in the fields of olefin hydration, bisphenol A synthesis, methacrylic acid preparation by methacrolein oxidation and the like. Most of heteropoly compounds synthesized by the traditional hydrothermal method have no uniform mesoscopic pore structure, and the specific surface area is very low, so that the contact between the active site of the heteropoly compound and a substrate is insufficient, and the activity of the catalyst is reduced. The specific surface area of the porous heteropoly compound prepared by the template agent can be improved, but the removal of the template agent is a relatively troublesome problem. The conventional method for removing the template agent comprises a solvent extraction method and a roasting method, wherein the organic template agent is dissolved and removed by heating and refluxing in an organic solvent, the method is carried out at a lower temperature, so that the stability of a heteropolyacid structure can be ensured, but a large amount of organic solvent is required, the production process cost is increased, and meanwhile, the heteropolyacid catalyst maintains the stability of the structure in the preparation process, but the structure is easy to collapse when the heteropolyacid catalyst is used in catalytic reactions with higher temperatures; the latter is to remove the template agent by high-temperature roasting in an oxygen-containing atmosphere, and because the Keggin structure heteropolyanion of the heteropolyacid has poor thermal stability at a higher temperature, the method can directly lead to collapse of the heteropolyanion structure to form a compact oxide, and the compact oxide does not have a porous structure any more.
Therefore, how to ensure the high specific surface area and the porous structure of the heteropoly compound catalyst and the thermal stability of the catalyst is an important research direction in the field. US4621155A proposes adding an organic base during precipitation to increase the specific surface area of the catalyst and to regulate the pore size distribution, and the catalyst prepared by this method can improve the yield of methacrylic acid to some extent during the preparation of methacrylic acid, but has limited effect. The heteropoly acid compound is loaded on the porous carrier, so that the specific surface area of the catalyst can be increased, and the utilization rate of the active components can be improved. US3939096a reports a catalyst using porous silica as a carrier, which has good catalytic performance for the reaction of preparing acrylic acid by oxidizing acrolein, but has poor catalytic effect for the oxidation reaction of methacrolein. The anion unit size of the heteropoly acid compound is larger, the microporous material is used as the carrier, the loading is difficult to carry out by using the surface of the inner hole, and particularly, the blocking of the carrier pore canal can be serious under high loading. CN110694687A proposes a supported nano heteropoly acid catalyst with large pore size silica of 50-500 nm as carrier, and the supported active component heteropoly acid has particle size of 4-10 nm and high selectivity and stability to catalyze the oxidation reaction of methacrolein. However, the large-aperture silica carrier in the method is prepared by a polymer gel crystal template method, has very complicated steps, and is not suitable for large-scale industrial production.
Disclosure of Invention
The invention provides a porous heteropolyacid catalyst with higher specific surface area and thermal stability, a preparation method and application thereof, and aims to solve the problems of low specific surface area, poor thermal stability and the like of the heteropolyacid catalyst. Namely, a first object of the present invention is to provide a porous heteropolyacid catalyst comprising a heteropolyacid and a support, wherein the active elements of the heteropolyacid comprise one or more of Mo, V and P, and the support comprises a silicon-based porous support;
The porous heteropolyacid catalyst is obtained by assembling an organic template agent and a carrier under alkaline conditions, mixing the organic template agent with the heteropolyacid, drying, and roasting twice in an inert gas atmosphere and an oxygen-containing atmosphere.
According to some embodiments of the invention, the precursor of elemental Mo comprises H 3PMo12O40 or H 4PMo11VO40.
According to some embodiments of the invention, the precursor of element V comprises one or more of vanadyl sulfate, vanadyl oxalate, vanadyl acetylacetonate, V 2O5, or vanadyl nitrate.
According to some embodiments of the invention, the support comprises silica; preferably, the precursor of the silicon dioxide comprises one or more of silicate, silicate ester and organosilane. According to some embodiments of the invention, the precursor of silica comprises ethyl orthosilicate, methyl orthosilicate.
According to some embodiments of the invention, the organic templating agent comprises amphiphilic alkyl chain quaternary ammonium salts or polyether amphiphilic block copolymers. According to some embodiments of the invention, the organic template comprises one or more of cetyltrimethylammonium bromide, P123 (polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer), F127 (polyoxyethylene-polyoxypropylene ether block copolymer).
According to some embodiments of the invention, the organic templating agent comprises cetyltrimethylammonium bromide.
According to some embodiments of the invention, the mass of the organic template and the carrier is 0.005-0.05, and according to some embodiments of the invention, the mass of the organic template and the carrier is 0.01-0.03.
According to some embodiments of the invention, the mass ratio of the heteropoly acid to the carrier is 0.1-1.0. According to some embodiments of the invention, the mass ratio of the heteropoly acid to the carrier is 0.5-1.0.
According to some embodiments of the invention, the porous heteropolyacid catalyst has a specific surface area of 80 to 120m 2/g and an average pore diameter of 3 to 8nm.
A second object of the present invention is to provide a method for producing the above porous heteropolyacid catalyst, comprising mixing and drying a heteropolyacid and a support in the presence of an organic template agent, followed by performing a first calcination in an inert gas atmosphere and a second calcination in an oxygen-containing atmosphere.
According to some embodiments of the invention, the method of mixing and drying the heteropolyacid with the support in the presence of the organic template further comprises the steps of:
(1) Heating and dissolving the heteropoly acid precursor;
(2) Dissolving an organic template agent, slowly adding a silicon dioxide precursor and an alkali solution at one time under the condition of low-speed stirring, and adjusting the pH to be slightly alkaline;
(3) Raising the stirring speed, adding the heteropoly acid precursor solution obtained in the step (1) into the mixed solution obtained in the step (2), and continuously heating, refluxing and stirring;
(4) Evaporating the slurry obtained in the step (3) by a rotary evaporation method to obtain powder; and roasting the powder twice to obtain the catalyst.
According to some embodiments of the invention, the stirring speed of the low-speed stirring in the step (2) is 60-220 rpm; according to some embodiments of the invention, the low speed stirring is at a stirring speed of 120 to 180rpm.
According to some embodiments of the invention, the rate of addition of the silica precursor in step (2) is 0.5-10 mL/min; according to some embodiments of the invention, the rate of addition of the silica precursor is 2 to 4mL/min.
According to some embodiments of the invention, the alkaline solution of step (2) is selected from the group consisting of ammonia, sodium hydroxide solution, potassium hydroxide solution; according to some embodiments of the invention, the alkaline solution of step (2) is preferably aqueous ammonia.
According to some embodiments of the invention, the stirring speed of step (3) is increased to 220-800 rpm.
According to some embodiments of the invention, the heating reflux temperature in step (3) is 60-120 ℃ and the heating time is 0.5-2 h.
According to some embodiments of the invention, the rotary evaporation heating temperature in the step (4) is 80-150 ℃, the heating time is 0.5-10 h, and the stirring speed is 220-800 rpm.
According to some embodiments of the invention, the inert gas used for the first calcination comprises nitrogen, argon or helium.
According to some embodiments of the invention, the inert gas used in the first calcination comprises nitrogen, argon or helium having a purity of 99.99% or more.
According to some embodiments of the invention, the first firing is at a temperature of 250-450 ℃, at a rate of 5 ℃/min, and for a firing time of 1-5 hours.
According to some embodiments of the invention, the second calcination is performed rapidly under an oxygen-containing atmosphere, the calcination temperature is 180-600 ℃, the heating rate is 10 ℃/min, and the calcination time is 20-60 min.
The invention also aims to provide the application of the porous heteropoly acid catalyst in the preparation of methacrylic acid by the selective oxidation of methacrolein.
According to some embodiments of the invention, the use comprises oxidizing methacrolein in the presence of a mixed gas of oxygen and nitrogen and water to produce methacrylic acid using the catalyst.
Compared with the prior art, the invention has the following beneficial effects:
1) The invention is based on the following findings, and the inventor provides a method for performing heat treatment by two-step roasting to solve the problems that the heteropolyanion structure of a porous heteropolyacid catalyst and the pore channel structure of the catalyst are damaged in the heat treatment process. After the dry powder is obtained through rotary evaporation, the first step is roasting in an inert gas atmosphere, and the purpose of the first step is to carbonize the template agent, so that the obtained carbon and the silica carrier together play a good role in stabilizing and protecting the structure and the porous framework of the heteropoly acid, and the direct structural collapse of the heteropoly acid is avoided to form metal oxides. The second step is quick roasting in an oxygen-containing atmosphere, the purpose of which is to remove the carbon obtained in the last step and avoid the reduction of the catalyst activity caused by covering the surface of the active component, in the step, the residual carbon in the last step is favorable for maintaining the stable structure of heteropolyacid, meanwhile, the quick heating and short-time roasting can avoid or reduce the decomposition and inactivation of heteropoly compounds, and the FT-IR and XRD spectrogram analysis of the catalyst obtained after the final roasting shows that the catalyst can maintain a very stable heteropoly structure, and meanwhile, the BET nitrogen adsorption and desorption analysis shows that the catalyst has a good pore channel structure and a higher specific surface area.
2) The catalyst of the invention is adopted, and the methacrolein is subjected to oxidation reaction in the presence of mixed gas of oxygen and nitrogen and water to generate methacrylic acid. The catalyst of the invention maintains higher single pass yield of methacrylic acid for a long time under higher load, the conversion rate of the methacrylic aldehyde can reach more than 70%, and the selectivity of the methacrylic acid can reach more than 84%.
3) The preparation method of the catalyst is simple and easy to carry out, and is suitable for large-scale production and application.
Drawings
FIG. 1 is a FT-IR spectrum of a calcined catalyst according to an embodiment of the invention;
figure 2 is an XRD pattern for a calcined catalyst in accordance with one embodiment of the invention.
Detailed Description
The present invention is described in detail below with reference to specific embodiments, and it should be noted that the following embodiments are only for further description of the present invention and should not be construed as limiting the scope of the present invention, and some insubstantial modifications and adjustments of the present invention by those skilled in the art from the present disclosure are still within the scope of the present invention.
The raw materials used in examples and comparative examples, if not particularly limited, are all as disclosed in the prior art, and are, for example, available directly or prepared according to the preparation methods disclosed in the prior art.
The performance of the invention is measured according to the following method:
The method for measuring the specific surface area and the aperture adopts a Tristar physical adsorption instrument for measurement. Before the sample is tested, heating, vacuumizing and degassing are needed. The porosity was measured on the samples at 77K, the specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution and pore volume were calculated from isothermal adsorption branches using the Barrettner-Joyner-Halenda (BJH) model.
The X-ray diffraction measurement in the invention uses an XRD powder tester produced by Bruker D4 Germany, and the test condition is 40kV and 40mV.
Fourier transform infrared (FT-IR) spectroscopy in the present invention uses Nicolet Fourier spectrophotometry manufactured in the united states.
In the invention, gas chromatography is adopted to carry out on-line analysis on the product gas, and the conversion rate of the methacrolein, the selectivity of the methacrylic acid and the single pass yield are used as evaluation catalyst performance indexes, and are defined as follows:
Methacrolein conversion (%) = (moles of methacrolein reacted/moles of methacrolein fed) ×100%
Methacrylic acid selectivity (%) = (moles of methacrylic acid produced/moles of methacrolein reacted) ×100%
Methacrylic acid single pass yield (%) = (moles of methacrylic acid produced/moles of methacrolein feed) ×100%.
The percentages and concentrations in the examples and comparative examples are by weight, unless otherwise specified.
[ Example 1]
(1) Preparation of the catalyst
9.1G H 3PMo12O40 of the solution is dissolved in deionized water, heated, refluxed and stirred at 80 ℃, then 0.25g of VOSO 4 is added after the dissolution, and the heating, refluxing and stirring are continued for 30min to obtain a solution I. 0.12g of cetyltrimethylammonium bromide was dissolved in deionized water, stirred slowly at 180rpm, 6mL of ethyl orthosilicate was added slowly dropwise at a rate of 2mL/min, and then 0.5mol/L of aqueous ammonia was added to adjust the pH to about 8 to obtain solution II. Solution I was added to solution II and heating was continued at 100 ℃ for 3h, and the solvent was evaporated by rotary evaporation. Roasting the obtained powder for 2 hours at 400 ℃ in a high-purity nitrogen atmosphere, and roasting the obtained powder for 30 minutes at a temperature rising rate of 10 ℃/min to 300 ℃ in an air atmosphere to obtain the final porous heteropolyacid catalyst, wherein the specific surface area of the catalyst can reach 108m 2/g and the aperture is about 6 nm.
The analysis of the catalyst obtained by the method through infrared spectrum shows that four main characteristic peaks exist in the catalyst within the interval of 500-1200 cm -1, which correspond to the following steps: 1066cm -1 is a central P-O bond, 966cm -1 is a terminal Mo=O t bond, 867cm -1 and 804cm -1 are intra-group Mo-O b -Mo bridge oxygen bonds and inter-group Mo-O c -Mo bridge oxygen bonds respectively, which indicates that the catalyst still maintains a stable Keggin heteropolyacid structure after the roasting method of the catalyst is disclosed in the patent (see figure 1).
Analysis of the catalyst powder by X-ray diffraction shows that diffraction peaks of the catalyst at a plurality of positions such as 2θ=10.5°, 14.9 °, 21.2 ° and 26.0 ° completely correspond to heteropolyacid H 3PMo12O14 (XRD database number 43-0314) with a Keggin structure, and further shows that the catalyst maintains a stable heteropolyacid structure after the roasting method is disclosed in the patent (see figure 2).
(2) Oxidation of methacrolein
The reactor is a phi 38 mm fluidized bed reactor: the reaction tube was filled with 3.0g of the catalyst obtained in the above step, and oxidation reaction conditions for producing methacrylic acid were: methacrolein, oxygen and nitrogen are mixed together in a ratio of water vapor=1:2.5:12:8 (molar ratio), the space velocity is 800h -1, the reaction temperature is 295 ℃, the conversion rate of methacrolein can reach more than 70%, and the selectivity of methacrylic acid can reach more than 84%. Wherein, the reaction time is 3 hours, the conversion rate of the methacrolein is 75.6%, the selectivity of the methacrylic acid is 86.8%, the conversion rate is 75.0% after the reaction time is 800 hours, and the selectivity is 86.2% after the analysis of sampling again.
Example 2
(1) Preparation of the catalyst
9.1G H 3PMo12O40 of the solution is dissolved in deionized water, heated, refluxed and stirred at 80 ℃, then 0.50g of VOSO 4 is added after the dissolution, and the heating, refluxing and stirring are continued for 30min to obtain a solution I. 0.24g of cetyltrimethylammonium bromide was dissolved in deionized water, stirred slowly at 180rpm, 10mL of ethyl orthosilicate was added slowly dropwise at a rate of 2mL/min, and then 0.5mol/L of aqueous ammonia was added to adjust the pH to about 8 to obtain solution II. Solution I was added to solution II and heating was continued at 100 ℃ for 3h, and the solvent was evaporated by rotary evaporation. The finally obtained powder is roasted for 2 hours at 400 ℃ in a high-purity nitrogen atmosphere, then is roasted for 40 minutes at a heating rate of 10 ℃/min to 300 ℃ in an air atmosphere, and the final porous heteropolyacid catalyst is obtained, and the specific surface area of the catalyst can reach 95m 2/g and the aperture is about 7.5nm through measurement.
The analysis of the catalyst obtained by the method through infrared spectrum shows that four main characteristic peaks exist in the catalyst within the interval of 500-1200 cm -1, which correspond to the following steps: 1067cm -1 is a central P-O bond, 964cm -1 is a terminal Mo=O t bond, 866cm -1 and 802cm -1 are intra-group Mo-O b -Mo bridge oxygen bonds and inter-group Mo-O c -Mo bridge oxygen bonds respectively, which indicates that the catalyst still maintains a stable Keggin heteropolyacid structure after the roasting method of the catalyst.
Analysis of the catalyst powder by X-ray diffraction shows that diffraction peaks of the catalyst at a plurality of positions such as 2θ=10.6°, 14.8 °, 21.4 ° and 26.0 ° completely correspond to heteropolyacid H 3PMo12O14 (XRD database numbering) with a Keggin structure, and further shows that the catalyst maintains a stable heteropolyacid structure after the roasting method is adopted in the patent.
(2) Oxidation of methacrolein
The reactor is a phi 38 mm fluidized bed reactor: the reaction tube was filled with 3.0g of the catalyst obtained in the above step, and oxidation reaction conditions for producing methacrylic acid were: methacrolein-oxygen-nitrogen-steam=1:2.5:12:8, space velocity 800h -1, reaction temperature 295 ℃, reaction time of 3h, sampling after reaction, methacrolein conversion of 75.6%, methacrylic acid selectivity of 86.8%, sampling again after 800h reaction for analysis, conversion of 75.0%, and selectivity of 86.2%.
Comparative example 1
(1) Preparation of the catalyst
9.1G H 3PMo12O40 of the solution is dissolved in deionized water, heated, refluxed and stirred at 80 ℃, then 0.25g of VOSO 4 is added after the dissolution, and the heating, refluxing and stirring are continued for 30min to obtain a solution I. 0.12g of cetyltrimethylammonium bromide was dissolved in deionized water, stirred slowly at 180rpm, 6mL of ethyl orthosilicate was added slowly dropwise at a rate of 2mL/min, and then 0.5mol/L of aqueous ammonia was added to adjust the pH to about 8 to obtain solution II. Solution I was added to solution II and heating was continued at 100 ℃ for 3h, and the solvent was evaporated by rotary evaporation. And finally, roasting the obtained powder to 400 ℃ at a heating rate of 5 ℃/min under the direct air atmosphere for 30min to obtain the final porous heteropolyacid catalyst, wherein the specific surface area of the catalyst is measured to be 32m 2/g, the pore diameter is about 23nm, and the distribution is uneven. The catalyst is characterized by infrared spectrum and X-ray diffraction, no obvious characteristic peak of heteropolyacid is found, which indicates that the catalyst is directly roasted in air, which can lead to the destruction of the heteropolyacid structure.
(2) Oxidation of methacrolein
The reactor is a phi 38 mm fluidized bed reactor: the reaction tube was filled with 3.0g of the catalyst obtained in the above step, and oxidation reaction conditions for producing methacrylic acid were: methacrolein-oxygen-nitrogen-steam=1:2.5:12:8, space velocity 800h -1, reaction temperature 295 ℃, sampling after 3h reaction, methacrolein conversion rate only 50.2%, methacrylic acid selectivity 60.8%, sampling again after 800h reaction analysis, conversion rate 42.6%, selectivity 49.2%.

Claims (9)

1. A porous heteropolyacid catalyst comprising a heteropolyacid and a support, the active elements of the heteropolyacid comprising Mo, V and P, the support comprising a silica-based porous support; the porous heteropolyacid catalyst is obtained by assembling an organic template agent and a precursor of silicon dioxide under an alkaline condition, mixing and drying the porous heteropolyacid catalyst with the precursor of the heteropolyacid, and then performing first roasting in an inert gas atmosphere and second roasting in an oxygen-containing atmosphere;
The specific surface area of the porous heteropolyacid catalyst is 80-120 m 2/g, and the average pore diameter is 3-8 nm;
The organic template agent comprises amphiphilic alkyl chain quaternary ammonium salt or polyether amphiphilic block copolymer;
the mass ratio of the organic template agent to the carrier is 0.005-0.05, and the mass ratio of the heteropoly acid to the carrier is 0.1-1.0;
the temperature of the first roasting is 250-450 ℃;
The second roasting temperature is 180-600 ℃;
The second calcination is performed rapidly in an oxygen-containing atmosphere.
2. The porous heteropolyacid catalyst according to claim 1, wherein the precursor of the element Mo comprises H 3PMo12O40 or H 4PMo11VO40 and/or the precursor of the element V comprises one or several of vanadyl sulfate, vanadyl oxalate, vanadyl acetylacetonate, V 2O5 or vanadyl nitrate.
3. The porous heteropolyacid catalyst according to claim 1 or 2, wherein the precursor of silica comprises one or more of silicate, silicate ester, organosilane.
4. A process for the preparation of a porous heteropolyacid catalyst according to any one of claims 1 to 3, which comprises assembling an organic template with a precursor of silica under alkaline conditions and further mixing and drying with a precursor of a heteropolyacid, followed by a first calcination in an inert gas atmosphere and a second calcination in an oxygen-containing atmosphere.
5. The method for preparing a porous heteropolyacid catalyst according to claim 4, wherein the inert gas for the first calcination comprises nitrogen, argon or helium; and/or
The temperature rising rate of the first roasting is 5 ℃/min, and the roasting time is 1-5 h.
6. The method for preparing a porous heteropolyacid catalyst according to claim 5, wherein the inert gas for the first calcination comprises nitrogen, argon or helium having a purity of 99.99% or more.
7. The method for preparing a porous heteropolyacid catalyst according to any one of claims 4 to 6, wherein the second firing temperature rise rate is 10 ℃/min and the firing time is 20 to 60min.
8. Use of a porous heteropolyacid catalyst according to any one of claims 1 to 3 in the selective oxidation of methacrolein to produce methacrylic acid.
9. The use according to claim 8, comprising the oxidation of methacrolein in the presence of a mixture of oxygen and nitrogen and water in the presence of the porous heteropoly acid catalyst to form methacrylic acid.
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