CN110882686A

CN110882686A - Monolithic catalyst for preparing dimethyl carbonate by direct synthesis method, preparation method and direct synthesis method of dimethyl carbonate

Info

Publication number: CN110882686A
Application number: CN201911306595.2A
Authority: CN
Inventors: 陈永东; 李月; 唐强; 王健礼; 刘锐; 王俊杰; 袁亮; 朱金权; 许正伟
Original assignee: Southwest Petroleum University
Current assignee: Southwest Petroleum University
Priority date: 2019-12-18
Filing date: 2019-12-18
Publication date: 2020-03-17
Anticipated expiration: 2039-12-18
Also published as: CN110882686B

Abstract

The invention discloses an integral catalyst for preparing dimethyl carbonate by a direct synthesis method, a preparation method and the direct synthesis method of the dimethyl carbonate, wherein the catalyst consists of a cerium-based composite oxide coating and a cordierite honeycomb ceramic matrix, wherein cerium ammonium nitrate and lanthanum nitrate are used as raw materials, urea is used as a precipitator, a composite material is prepared by a coprecipitation method, then the obtained composite material is mixed with an alumina ball mill and roasted to form a composite oxide powder catalyst, and finally the prepared composite oxide powder catalyst is subjected to post-treatmentCoating the catalyst on a cordierite honeycomb ceramic substrate to form a required monolithic catalyst; the invention places the monolithic catalyst in the center of the steel tube of the continuous fixed bed reactor, and then liquid CH is put in the heating environment₃OH is pumped to a preheater to be gasified and is mixed with CO by a high-pressure constant-flow pump₂Premixing; CH in the reaction gas₃OH and CO₂The molar ratio of (A) to (B) is 1-3, and the dimethyl carbonate is obtained after the reaction is carried out for 2-6 hours. The invention provides a novel catalyst and a preparation method thereof, which can improve the production efficiency.

Description

Monolithic catalyst for preparing dimethyl carbonate by direct synthesis method, preparation method and direct synthesis method of dimethyl carbonate

Technical Field

The invention relates to the technical field of preparation of nano metal catalysts, in particular to a catalyst for preparing dimethyl carbonate by a direct synthesis method, a preparation method and the direct synthesis method of the dimethyl carbonate.

Background

With the development of society, the emission of carbon dioxide is also gradually increased, and carbon dioxide is a main gas causing greenhouse effect and is also a potential carbon source. Thus converting the greenhouse effect gas CO₂The carbon resource is effectively fixed and recycled, and has important significance for relieving the crisis of the carbon resource, protecting the environment and the like.

Dimethyl Carbonate (CH)₃OCOOCH₃Abbreviated as DMC) is a widely used green chemical raw material, has very active chemical properties, and can participate in various chemical reactions such as methylation, carboxylation, methoxylation and the like. DMC can also be used as a carbonylation agent in place of phosgene in the synthesis of carbonic acid derivatives, as a methylation agent in place of dimethyl sulfate in the methylation reaction, and as a low toxicity agent in organic synthesis and clean productionA green solvent. DMC is widely used in many fields, has very wide market prospect. There are many kinds of production processes of dimethyl carbonate, and the main method for synthesizing dimethyl carbonate comprises the following steps: phosgene method, methanol oxidation carbonylation method, ester exchange method (such as the methods described in Chinese patents CN201410300187.7, CN201210226548.9 and CN 201110415385.4), urea alcoholysis method (such as the methods described in Chinese patent CN201410585479. X), electrochemical synthesis method, direct synthesis method of methanol and carbon dioxide, and the like.

Among the above methods, the phosgene process is the most developed classical method for preparing dimethyl carbonate, but the conventional production route of the phosgene process is gradually eliminated due to the high toxicity and corrosiveness of phosgene and the environmental problems of the emission of various other pollutants. Methanol liquid phase oxidative carbonylation, methanol gas phase oxidative carbonylation, and transesterification processes have all been commercialized, but industrial production has found that these processes all have some inherent drawbacks. The liquid phase methanol oxidation and carbonylation process has CuCl as catalyst, certain corrosion to the production apparatus and CH in the whole system₃OH、CO、O₂The proportion of the mixed gas of the three gases is changed along with the reaction, and when the mixed gas reaches an explosion value, great safety threat is generated; the methanol gas-phase carbonylation method has higher danger coefficient of the reaction because the reaction raw material relates to toxic gas CO, and is not suitable for large-scale industrial popularization; most of the reaction processes related to the ester exchange method are reversible, so that the reaction yield is low under thermodynamic limitation conditions, and meanwhile, a large number of byproducts are generated in the reaction, so that the later separation and purification are difficult. Therefore, synthetic methods developed in laboratories, such as electrochemical synthesis and direct synthesis, have been receiving much attention. Compared with the electrochemical synthesis method, the direct synthesis method has simpler equipment requirement, no complex process flow requirement, obvious economic advantage and easy industrial popularization, and attracts the wide attention of a large number of researchers at home and abroad. However, CO₂As one of the main raw materials of the reaction, the chemical property is extremely stable, the activation is difficult in the reaction, and a proper catalyst needs to be added to solve the problem of CO₂The problem of difficulty in activation; however, the reaction usually produces a large amount of by-productsWater, which is liable to cause deactivation of the catalyst due to water poisoning, and thus proper catalyst and reaction equipment are selected to realize CO₂Rapid activation and timely removal of byproduct water is central to the conduct of this study.

For the direct synthesis of dimethyl carbonate catalyst, CeO₂Due to the abundant acid-base sites and oxygen vacancies, the method has attracted the attention of researchers. In 1999, Tomishige et al (K Tomishige, et al, Catal, Lett.,1999,58,225-₂Application as catalyst to CO₂And CH₃The reaction of OH directly to DMC, although not at that moment with high yield, finds that the acid-base position on the catalyst surface is related to the performance of the catalyst. In 2006, Yoshida et al (Yoshida, et al, Catal Total, 2006,115,95-101.) for CO₂And CH₃CeO in reaction for directly synthesizing DMC by OH₂And ZrO₂By comparison of the catalytic effects of (A) and (B), CeO was found₂Has a catalytic activity higher than that of ZrO₂This is because of CeO₂Simultaneously has an acid-base center, and the acid position can activate CH₃OH, basic site can activate CO₂Therefore, good catalytic performance is exhibited. However, CeO₂Poor thermal stability, easy sintering at high temperature, greatly shortened catalyst life and reduced catalyst performance, on the other hand, pure CeO₂The acid-base active sites that can be provided are limited. Studies have shown that (G.A. Turko, et. Kinet. Catal.,2005,46, 884-containing 890.), at CeO₂Adding some other metal ions such as Zr⁴⁺、La³⁺、Ti⁴⁺、Y³⁺Etc. to make it into CeO₂Forms a solid solution in the crystal lattice of (A), can greatly improve CeO₂Thermal stability, redox properties and oxygen storage properties. Therefore, researchers began to resort to working with CeO₂Doping other metal ions to CeO₂The catalyst undergoes a series of modifications. At present, CeMO is concerned_δStudies on (M ═ Zr, Ti, Ca, etc.) solid solutions have been reported in large numbers. Liu et al (B.Liu, equivalent. ACS Catal.,2018,8:10446-₂The nano-rod researches the doping amount of Zr to the lattice structure,The microstructure, the amount of oxygen vacancies, and the catalytic activity of the DMC synthesis. The research result shows that the Zr providing the highest oxygen vacancy amount_0.1CeO₂The nanorods showed the highest DMC synthesis activity (DMC yield ═ 14.2 mmol. g)_cat ^-1，CH₃OH conversion 0.65%). Z.W.Fu et al (Z.W.Fu, et al. ACS omega.,2018,3,198-207.) investigated TiO in cerium dioxide nanorods₂The effect of the doping ratio and various reaction conditions on the catalytic performance was found to be TiO₂The introduction of the cerium dioxide nano-rod can greatly improve the catalytic performance of the cerium dioxide nano-rod because the cerium dioxide nano-rod has more surface acidic basic sites. Ti_0.04Ce_0.96O₂The nanorod catalyst has the advantage of being superior to pure CeO₂And other Ce_1-xTi_xO₂The catalytic performance of the nano-rod is improved under the optimal reaction conditions (140 ℃,1.0MPa and 360 h)^-1) Lower, Ti_0.04Ce0_.96O₂The nanorod catalyst has the highest catalytic performance, and CH is contained in a fixed bed reactor₃The OH conversion was 5.38% and the DMC selectivity was 83.1%. Liu et al (B.Liu, et al. New Jchem.,2017,41:12231-12240) prepared a series of CeO containing varying amounts of CaO₂Based catalyst, research results show that CaO and CeO are used as catalyst₂The interaction of the catalyst improves the acid-base property of the surface of the catalyst and the surface oxygen vacancy amount, and the increase of the oxygen vacancy enhances the CO pairing of the catalyst₂Adsorption of (3). Ca under the reaction conditions of 3MPa and 140 DEG C_1.5The DMC yield of the Ce sample was highest (2.47 mmol. multidot.g)^-1). The catalysts used in these documents are all particulate catalysts, and the use of these particulate catalysts in the reaction has the same problem-CH₃The OH conversion rate is low (generally 2-6%), which seriously hinders the application and popularization of the particle cerium-based catalyst for preparing DMC by direct synthesis. After analyzing the reason, we conclude that the water is mainly caused by the accumulation of the byproduct water generated by the reaction, and the byproduct water cannot be removed in time in the catalytic process of the reaction kettle or the fixed bed reactor, so that the reaction equilibrium moves to the left (Le Chatelier principle). Thus, dehydrating agents (e.g. 2-cyanopyridines) are those which achieve high CH₃OH conversion is necessary. Wang et al (s.p.wang,et al Chinese ChemLett. 2015,26,1096-1100.) 2-cyanopyridine and its derivatives in CeO₂In situ hydrolysis on CO₂And CH₃Influence of OH on the Synthesis of dimethyl carbonate (DMC). The results show that the DMC yield after addition of 2-cyanopyridine had exceeded 12.8mmol g_cat ^-1Greatly increased to 378.5 mmol/g_cat ^-1. However, dehydrating agents are generally expensive and cause great harm to the environment. Therefore, it would be very valuable to develop cost-effective and less toxic accelerators under the concept of "green chemistry". The monolithic catalyst has the advantages of large specific surface area, low pressure drop, excellent mechanical property and good thermal stability when being compressed into corresponding granular catalyst, and is introduced into the reaction system and Ce_1-xLa_xO_δ-Al₂O₃Formation of composite oxide Ce_1-xLa_xO_δ-Al₂O₃The monolithic catalyst is expected to greatly improve CH₃OH conversion and DMC yield. The applicant has therefore filed a prior application with application number 201910863440.2, but this is only an attempt and needs to explore further possibilities to find a more efficient solution.

Disclosure of Invention

Aiming at the defects of the existing granular catalyst for preparing the dimethyl carbonate by the direct synthesis method, the invention provides an integral catalyst for preparing the dimethyl carbonate by the direct synthesis method, which is used for directly synthesizing the dimethyl carbonate in a continuous fixed bed reactor.

The purpose of the invention is realized by the following technical scheme:

an integrated catalyst for preparing dimethyl carbonate by direct synthesis is prepared from Ce_1-xLa_xO_δ-Al₂O₃The composite oxide coating and the cordierite honeycomb ceramic base body. The Ce_1-xLa_xO_δ-Al₂O₃The composite oxide is coated on a cordierite honeycomb ceramic matrix after ball milling and pulping to form Ce_1-xLa_xO_δ-Al₂O₃A monolithic catalyst. The catalysisIn the agent, the molar ratio of cerium dioxide to lanthanum oxide is 0.99-0.80: 0.01-0.20, and the content of aluminum oxide is Ce_1-xLa_xO_δ1 wt% of the mass of the composite material.

Ce for preparing dimethyl carbonate by direct synthesis method_1-xLa_xO_δ-Al₂O₃The preparation method of the monolithic catalyst comprises the following steps:

s1, weighing a certain mass of (NH) according to a specific molar ratio₄)₂Ce(NO₃)₆、La(NO₃)₃·6H₂O and urea are respectively and completely dissolved by 500mL of deionized water to form the solution containing Ce⁴⁺、La³⁺And urea solution of (2), containing Ce⁴⁺、La³⁺Respectively transferring the solution and the urea solution into a 1000mL three-neck flask, uniformly mixing, carrying out coprecipitation reaction for 4-6 h under the conditions of mechanical stirring and water bath heating at 80-100 ℃, and collecting precipitates generated by the reaction;

s2, filtering and washing the reaction product, mashing the obtained filter cake, adding a proper amount of deionized water to prepare emulsion, adding a polyethylene glycol solution accounting for 10-50 wt% of the total mass of the obtained reaction product, mixing, spray drying, controlling the average particle size of the powder to be 5-10 microns, and finally vacuum drying the catalyst powder at 60-100 ℃ for more than 12 hours to obtain Ce_1-xLa_xO_δA composite material;

s3, Ce prepared in step S2_1-xLa_xO_δComposite material and Al with certain mass₂O₃Ball milling and mixing the powder, roasting the mixture for 1 to 2 hours at the temperature of between 150 and 200 ℃ in the air or oxygen atmosphere, and roasting the mixture for 3 to 5 hours at the temperature of between 400 and 500 ℃ in the air or oxygen atmosphere to obtain the formed Ce_1-xLa_xO_δ-Al₂O₃A composite oxide powder catalyst;

s4, Ce prepared in step S3_1-xLa_xO_δ-Al₂O₃Carrying out ball milling on the composite oxide powder catalyst, 2-5 wt% of glacial acetic acid and a proper amount of deionized water according to a ball mill to prepare slurry, and then soaking the cordierite honeycomb ceramic substrate in the slurryBlowing off redundant catalyst slurry by using compressed air, finally drying the cordierite honeycomb ceramic substrate coated with the slurry for 3-4 hours at the temperature of 70-80 ℃, then roasting for 1-2 hours at the temperature of 150-200 ℃ in the air or oxygen atmosphere, and then roasting for 2-5 hours at the temperature of 400-500 ℃ in the air or oxygen atmosphere to obtain Ce_1-xLa_xO_δ-Al₂O₃A monolithic catalyst.

By using Ce_1-xLa_xO_δ-Al₂O₃Monolithic catalyst for catalyzing CO₂And CH₃The method for directly synthesizing dimethyl carbonate by OH comprises the following steps: adding Ce_1-xLa_xO_δ-Al₂O₃Putting the monolithic catalyst into a steel pipe of a continuous fixed bed reactor, and introducing CO for 5min₂Removing other gases in the reaction system, then heating up, and when the temperature of the system reaches 100 ℃ and 180 ℃, adding liquid CH₃OH is transported to a preheater for gasification treatment through a high-pressure constant flow pump, the reaction pressure is controlled to be 1.2-3.0 Mpa, and CH is contained in the reaction gas₃OH and CO₂The molar ratio of (A) to (B) is 1 to 3, and the gas flow rate is 2880g_cat ^-1h^-1And after reacting for 2-6 h, the reaction gas mixture passes through a gas chromatography to realize the online detection of the contents of reactants and products.

Compared with the prior art, the invention has the advantages that:

1. the invention adopts Ce with excellent oxygen storage performance, high specific surface area, abundant acid-base sites and other characteristics_1-xLa_xO_δ-Al₂O₃Preparing composite oxide powder catalyst, preparing the powder catalyst into catalyst slurry by adopting a special coating technology, and coating the catalyst slurry on cordierite honeycomb ceramic to prepare high-performance Ce_1-xLa_xO_δ-Al₂O₃Monolithic catalyst suitable for direct synthesis of dimethyl carbonate in a fixed bed reactor. La₂O₃Doping into CeO₂Formation of Ce in the crystal lattice_1-xLa_xO_δCan increase the specific surface area of the catalyst and simultaneously increase the concentration of oxygen vacancies and acid-base sites of the catalyst to provide for catalytic reactionMore active sites and more frequent contact of reactants with the active sites. The Ce_1-xLa_xO_δ-Al₂O₃Compared with the traditional particle catalyst, the monolithic catalyst has better catalytic performance, because the monolithic catalyst can realize the quick removal of reactants, promote the forward movement of reaction, realize the process reinforcement and further improve the catalytic performance. Experimental results show that the catalyst has excellent catalytic performance and stability. Ce prepared by the invention_1-xLa_xO_δ-Al₂O₃Compared with the particle catalyst, the monolithic catalyst has obviously improved reactant conversion rate and product yield. CH (CH)₃The highest OH conversion is up to 22% and DMC yield is up to 18%, which is mainly benefited by Ce_1-xLa_xO_δ-Al₂O₃The monolithic catalyst is more beneficial to fully contacting active particles with reactants, and can discharge generated byproduct water to a reaction system in time so as to be beneficial to forward reaction, thereby achieving the purposes of high activity and high stability.

2. Ce of the invention_1-xLa_xO_δ-Al₂O₃The composite oxide powder catalyst is prepared by adopting a coprecipitation method, Ce_1- _xLa_xO_δ-Al₂O₃The monolithic catalyst is prepared by adopting an immersion method, is simple and easy to operate, is convenient for carrying out comparison experiments, and systematically studies the influence of the preparation conditions of the catalyst on the catalytic performance of the catalyst.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are required to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained based on these drawings without inventive efforts.

FIG. 1 shows CeO₂-Al₂O₃Powder catalyst and inventive example 2And 4 obtaining Ce_0.90La_0.10O_δ-Al₂O₃、Ce_0.80La_0.20O_δ-Al₂O₃XRD pattern of the powder catalyst.

FIG. 2 shows CeO₂-Al₂O₃Powder catalyst and Ce obtained in examples 2 and 4 of the invention_0.90La_0.10O_δ-Al₂O₃、Ce_0.80La_0.20O_δ-Al₂O₃TEM pictures of the powder catalyst.

FIG. 3 shows Ce obtained in example 1 of the present invention_0.95La_0.05O_δ-Al₂O₃Monolithic catalyst and Ce obtained in example 5_0.95La_0.05O_δ-Al₂O₃Activity comparison of the granular catalyst.

Detailed Description

The preferred embodiments of the present invention will be described below with reference to the accompanying drawings, and it should be understood that the preferred embodiments described herein are merely for the purpose of illustrating and explaining the present invention and are not intended to limit the present invention. Advantages and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.

Example 1

Preparation of 5% lanthanum oxide doped Ce_0.95La_0.05O_δ-Al₂O₃Monolithic catalyst (ceria to lanthanum oxide molar ratio 0.95: 0.05). The method comprises the following steps:

weighing 15.000g (NH) according to the molar ratio of 0.95:0.05₄)₂Ce(NO₃)₆And 0.624g La (NO)₃)·6H₂And O, stirring and dissolving the mixture in a proper amount of deionized water to obtain a nitrate precursor mixed solution, and stirring and dissolving 70g of urea in a proper amount of deionized water, wherein the total amount of the deionized water used in the two dissolving processes is 500 mL. Sequentially transferring the nitrate precursor mixed solution and the urea solution into a 1000mL three-necked flask, heating the mixed solution and the urea solution to 80-100 ℃ in a water bath under the condition of mechanical stirring for reaction, and keeping the temperature for 4-6 h(preferably 5h) until the pH value reaches 8-9, naturally cooling to room temperature after complete reaction, filtering and washing a reaction product, mashing the obtained filter cake, adding deionized water to prepare an emulsion, adding a polyethylene glycol solution containing 1.7097g of polyethylene glycol to prepare slurry, spray-drying, controlling the average particle size of the powder to be 5 microns, and vacuum-drying the prepared powder at 80 ℃ for more than 12h to obtain Ce_0.95La_0.05O_δA composite material.

Ce prepared by the above experiment_0.95La_0.05O_δThe composite material is ball-milled and mixed with 0.0570g of alumina powder, then is roasted for 1h at 150 ℃ in the atmosphere of air or oxygen (the roasting effect of oxygen is better, and the economic cost of air is lower), and then is roasted for 2-5 h (preferably 4h) at 400-500 ℃ (preferably 400 ℃), so that Ce with the diameter of 5-10 nm is obtained_0.95La_0.05O_δ-Al₂O₃A powder catalyst. Is named as Pow-Ce_0.95La_0.05O_δ-Al₂O₃。

4g of Pow-Ce prepared in the above experiment was taken_0.95La_0.05O_δ-Al₂O₃0.32mL of 25 wt% glacial acetic acid and a proper amount of deionized water are ball-milled to prepare slurry, cordierite honeycomb ceramic is soaked in the slurry, then the redundant slurry is blown out by compressed air, finally the substrate coated with the slurry is dried for 3h at 80 ℃, then is roasted for 1h at 150 ℃ in air atmosphere, and finally is roasted for 4h at 400 ℃ to obtain Ce_0.95La_0.05O_δ-Al₂O₃Monolithic catalyst, named Mon-Ce_0.95La_0.05O_δ-Al₂O₃And is marked as Cat 1.

Example 2

10 wt% of Ce doped with lanthanum oxide_0.90La_0.10O_δ-Al₂O₃The monolithic catalyst (designated Cat2) was prepared by the same procedure as in example 1, except that: the amounts of each precursor and polyethylene glycol were varied, and the specific amounts are shown in table 1.

Example 3

Lanthanum oxideDoping amount of 15 wt% Ce_0.85La_0.15O_δ-Al₂O₃The monolithic catalyst (designated Cat3) was prepared by the same procedure as in example 1, except that: the amounts of each precursor and polyethylene glycol were varied, and the specific amounts are shown in table 1.

Example 4

The doping amount of lanthanum oxide is 20 wt% of Ce_0.80La_0.20O_δ-Al₂O₃The monolithic catalyst (designated Cat4) was prepared by the same procedure as in example 1, except that: the amounts of each precursor and polyethylene glycol were varied, and the specific amounts are shown in table 1.

Example 5

5 wt% of Ce doped with lanthanum oxide_0.95La_0.05O_δ-Al₂O₃The preparation of the particulate catalyst was the same as the preparation of the monolithic catalyst of example 1, except that: synthesized Ce_0.95La_0.05O_δ-Al₂O₃The composite oxide powder catalyst is directly tabletted by a tablet machine and sieved (40-60 meshes) to obtain Ce_0.95La_0.05O_δ-Al₂O₃A particulate catalyst, noted Cat 5.

TABLE 1 preparation of different doping amounts of Ce_1-xLa_xO_δDosage parameter of each raw material component of catalyst

Examples

Ammonium cerium nitrate

Lanthanum nitrate

Urea

Alumina oxide

Polyethylene glycol

Glacial acetic acid

Example 2(Cat2)

15.0000g

1.3160g

70.0000g

0.0570g

1.7097g

0.32mL

Example 3(Cat3)

15.0000g

2.0910g

70.0000g

0.0628g

1.8846g

0.32mL

Example 4(Cat4)

15.0000g

2.9620g

70.0000g

0.0694g

2.0812g

0.32mL

Ce_1-xLa_xO_δ-Al₂O₃Monolithic catalyst for catalyzing CO₂And CH₃The method for directly synthesizing dimethyl carbonate by OH comprises the following steps:

CH₃OH and CO₂The reaction for direct DMC synthesis is carried out in a continuous mannerCarried out on a fixed bed reactor. Ce prepared in examples 1-4_1-xLa_xO_δ-Al₂O₃(x is 0.05,0.10,0.15,0.20) monolithic catalysts are respectively arranged in a steel tube reactor, the temperature is 140 ℃, the pressure is 2.4MPa, and the space velocity is 2880g_cat ^-1h^-1(nCH₃OH:nCO₂2:1), and the composition of the product was analyzed and detected on-line using an Agilent GC7890B gas chromatograph. The test results are shown in Table 2. (in the table, C_CH3OHRepresents CH₃OH conversion, S_DMCDenotes the selectivity to dimethyl carbonate, Y_DMCRepresents the yield of dimethyl carbonate. )

TABLE 2 CeO₂-Al₂O₃Comparison of the Activity of the monolithic catalysts with the monolithic catalysts prepared in examples 1 to 4

Ce prepared in example 1_0.95La_0.05O_δ-Al₂O₃Monolithic catalyst and Ce prepared in example 5_0.95La_0.05O_δ-Al₂O₃The granular catalysts are respectively arranged in a steel tube reactor, the temperature is 140 ℃, the pressure is 2.4MPa, and the space velocity is 2880g_cat ^-1h^-1(nCH₃OH：nCO₂The reaction is carried out under the reaction condition of 2:1), and the product is detected on line. The composition of the product was analyzed using an Agilent GC7890B gas chromatograph. In addition, the activity of the monolithic catalyst prepared by the present invention was compared with the particulate catalyst prepared by the present invention and the particulate catalyst commonly used in the literature, and the results are shown in table 3.

TABLE 3 comparison of monolithic catalyst to granular catalyst Activity

As can be seen from Table 2, with CeO₂-Al₂O₃Ce doped with lanthanum oxide as compared with monolithic catalyst_1-xLa_xO_δ-Al₂O₃Monolithic catalyst catalyzed CH₃OH and CO₂The activity of direct synthesis of DMC is obviously improved. Wherein, when the doping amount of lanthanum oxide is 5 percent, the corresponding Ce_0.95La_0.05O_δ-Al₂O₃The catalytic activity of the monolithic catalyst is the best. As can be seen from Table 3, the monolithic catalyst prepared by the present invention (No. 1) catalyzes CO in comparison with the particulate catalyst prepared by the present invention (No. 2) and the particulate catalysts commonly used in the literature (No. 3-5)₂And CH₃The catalytic activity of OH for direct DMC synthesis is clearly the best. Under the experimental conditions, the CH of the catalytic reaction₃The OH conversion rate is as high as 22 percent, and the DMC yield is as high as 18 percent. The catalyst slurry in the monolithic catalyst is uniformly distributed on the cordierite honeycomb ceramic matrix, so that the catalyst slurry is more beneficial to fully contacting active particles with reactants. Meanwhile, by using the continuous fixed bed reactor in a matching way, byproduct water generated by the reaction can be discharged out of the reaction system in time, and the forward reaction is facilitated, so that the effects of high activity and high stability of the catalytic reaction are achieved.

FIG. 1 and FIG. 2 show CeO, respectively₂-Al₂O₃Powder catalyst and Ce prepared in examples 2 and 4 of the invention_1-xLa_xO_δ-Al₂O₃XRD pattern and TEM picture of the powder catalyst. As can be derived from FIG. 1, La₂O₃Successfully dope into CeO₂Form Ce in the crystal lattice of_1-xLa_xO_δ-Al₂O₃A composite oxide; as can be seen from FIG. 2, CeO₂-Al₂O₃(FIG. 2(a)) and Ce_1-xLa_xO_δ-Al₂O₃All the (fig. 2(b, c)) are spherical nanoparticles with uniform morphology. However, after further particle size analysis we found that La is accompanied by La₂O₃Doping and increase of doping amount of (3), formed Ce_1-xLa_xO_δ-Al₂O₃Particle size of (D) is smaller than that of CeO₂-Al₂O₃Is reduced, mainly due to La₂O₃Is doped into CeO₂Causing lattice contraction in the crystal lattice. FIG. 3 shows Ce prepared in example 1 of the present invention_0.95La_0.05O_δ-Al₂O₃Monolithic catalyst and Ce prepared according to the invention in example 5_0.95La_0.05O_δ-Al₂O₃Activity comparison of the granular catalyst. As can be seen from FIG. 3, Ce_1-xLa_xO_δ-Al₂O₃The catalytic activity of the monolithic catalyst is obviously higher than that of Ce_1-xLa_xO_δ-Al₂O₃A particulate catalyst.

Although the present invention has been described with reference to a preferred embodiment, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. The monolithic catalyst for preparing dimethyl carbonate by direct synthesis is characterized in that the catalyst is prepared from Ce_1-xLa_xO_δ-Al₂O₃Composite oxide coating and cordierite honeycomb ceramic base body.

2. The monolithic catalyst for the direct synthesis of dimethyl carbonate according to claim 1, wherein lanthanum oxide is doped into the crystal lattice of cerium oxide to form Ce_1-xLa_xO_δComposite material, Ce_1-xLa_xO_δThe Ce is formed by ball milling and mixing the composite material and alumina powder and then roasting and molding_1-xLa_xO_δ-Al₂O₃Composite oxide powder catalyst, last Ce_1- _xLa_xO_δ-Al₂O₃The composite oxide powder catalyst is coated on a cordierite honeycomb ceramic matrix after ball milling and pulping to form Ce_1-xLa_xO_δ-Al₂O₃A monolithic catalyst.

3. The monolithic catalyst for preparing dimethyl carbonate by a direct synthesis method as claimed in claim 2, wherein the molar ratio of cerium oxide to lanthanum oxide in the monolithic catalyst is 0.99-0.80: 0.01-0.20, and the content of aluminum oxide is Ce_1-xLa_xO_δ1 wt% of the mass of the composite material.

4. A method for preparing the monolithic catalyst for preparing the dimethyl carbonate by the direct synthesis method according to claim 3, which is characterized by comprising the following steps:

s1, weighing a certain mass of (NH) according to a specific molar ratio₄)₂Ce(NO₃)₆、La(NO₃)₃·6H₂Dissolving O and urea completely in 500mL of deionized water, and adding Ce⁴⁺、La³⁺The solution is mixed with urea solution, the coprecipitation reaction is carried out by mechanical stirring under the heating of water bath, and after the reaction is finished, the precipitate generated by the reaction is collected;

s2, filtering and washing the reaction product, adding a polyethylene glycol solution which accounts for 10-50 wt% of the total mass of the obtained reaction product, mixing, spray drying, controlling the average particle size of the powder to be 5-10 microns, and finally, carrying out vacuum drying on the catalyst powder to obtain Ce_1-xLa_xO_δA composite material;

s3, Ce prepared in step S2_1-xLa_xO_δComposite powder anda certain mass of Al₂O₃Ball milling and mixing the powder, and then continuously roasting for two times at a given temperature and a given atmosphere to obtain the formed Ce_1-xLa_xO_δ-Al₂O₃A composite oxide powder catalyst;

s4, Ce prepared in step S3_1-xLa_xO_δ-Al₂O₃Ball-milling composite oxide powder catalyst, glacial acetic acid and deionized water in a certain proportion by a ball mill to prepare slurry, soaking the cordierite honeycomb ceramic substrate in the slurry, blowing off redundant catalyst slurry by compressed air, and finally drying and roasting the cordierite honeycomb ceramic substrate coated with the slurry to obtain Ce_1-xLa_xO_δ-Al₂O₃A monolithic catalyst.

5. The method for preparing the monolithic catalyst for preparing dimethyl carbonate by the direct synthesis method according to claim 4, wherein in the step S1, the water bath heating temperature is 80-100 ℃, and the water bath heating time is 4-6 h.

6. The method of claim 4, wherein in step S2, the temperature of vacuum drying is 60-100 ℃, and the time of vacuum drying is more than 12 h.

7. The method for preparing the monolithic catalyst for preparing dimethyl carbonate by the direct synthesis method according to claim 4, wherein the roasting atmosphere in the step S3 is air or oxygen, and the conditions of the two times of roasting are that roasting is performed at 100-150 ℃ for 1-2 h and roasting is performed at 400-500 ℃ for 3-5 h in sequence.

8. The method of claim 4, wherein in step S4, cordierite honeycomb ceramic substrate is soaked in the slurry, and then the substrate is removed and compressed air is used to blow off the majority of the catalystThe residual slurry is dried and roasted to obtain Ce_1-xLa_xO_δ-Al₂O₃A monolithic catalyst. The drying and roasting processes are as follows: drying the substrate coated with the slurry at 70-80 ℃ for 3-4 h, roasting at 150-200 ℃ for 1-2 h, and roasting at 400-500 ℃ for 2-5 h.

9. A process for the direct synthesis of dimethyl carbonate, characterized in that the Ce of claim 3 is used_1-xLa_xO_δ-Al₂O₃Monolithic catalyst for catalyzing CO₂And CH₃OH is prepared by reacting Ce_1-xLa_xO_δ-Al₂O₃Putting the monolithic catalyst into a steel pipe of a continuous fixed bed reactor, and introducing CO firstly₂Removing other gases in the reaction system, starting heating the system after 5min, and heating the liquid CH when the temperature of the system rises to 100-180 DEG C₃OH is conveyed to a preheater for gasification treatment through a high-pressure constant flow pump, the reaction pressure is controlled to be 1.2-3.0 MPa, and CH is contained in the reaction gas₃OH and CO₂The molar ratio of (A) to (B) is 1 to 3, and the flow rate of the gas is controlled to be 2880g_cat ^-1h^-1(ii) a And after reacting for 2-6 h, the reaction gas mixture passes through a gas chromatograph to realize the online detection of the components and the content of each component of the product.