CN111393629B

CN111393629B - Method for continuous production of polypropylene carbonate by preheating liquid phase method in pipeline manner

Info

Publication number: CN111393629B
Application number: CN202010241105.1A
Authority: CN
Inventors: 罗铭; 王慧君; 王自修
Original assignee: Hangzhou Puli Material Technology Co ltd
Current assignee: Hefei Puli Advanced Materials Technology Co.,Ltd.
Priority date: 2020-03-30
Filing date: 2020-03-30
Publication date: 2021-06-01
Anticipated expiration: 2040-03-30
Also published as: CN111393629A

Abstract

The invention discloses a method for continuously producing polypropylene carbonate in a pipeline manner by a preheating liquid phase method, which comprises the following steps: (1) raw materials comprising propylene oxide and a catalyst are put into a premixing tank according to a certain proportion and are uniformly mixed, and then the mixture is pumped into a pipeline reactor; (2) preheating carbon dioxide to 40-150 ℃, pumping into a pipeline reactor, and pressurizing to 1-15 MPa; (3) heating to 70-150 ℃, so that propylene oxide and carbon dioxide are contacted in a pipeline reactor in the presence of a catalyst to carry out polymerization reaction; (4) after part or all of the polymerization reaction product flow passes through the cooling section group, separating part of the polypropylene carbonate from the polymerization reaction product flow, and recycling the rest polymerization reaction product flow to the step (1); the catalyst is modified by mixed acid, and the zinc-cobalt double metal cyanide complex catalyst is obtained by the reaction of water-soluble metal salts of zinc and cobalt in a water-soluble solvent.

Description

Method for continuous production of polypropylene carbonate by preheating liquid phase method in pipeline manner

Technical Field

The invention relates to a method for continuously producing polypropylene carbonate in a pipeline manner by a preheating liquid phase method.

Background

Carbon dioxide, one of the main greenhouse gases, has attracted attention as a carbon resource that can replace the conventional fossil fuels to synthesize a variety of important chemicals due to the advantages of low price, easy availability, no toxicity and incombustibility. Polycarbonate materials have significant performance advantages including high strength, high light transmission, high durability, high light transmission, heat resistance, and electrical insulation, and in addition, the materials are easier to process and dye. Polycarbonate (abbreviated as PC) is a high molecular polymer containing a carbonate group (-OC (═ O) O-) in its molecular chain. Carbon dioxide (CO)₂) Can be subjected to alternate copolymerization with alkylene oxide to obtain the polypropylene green material with biodegradability.

At present, the production of polypropylene carbonate mainly has two aspects of problems to be solved urgently, firstly, the content of polycarbonate units is improved, secondly, the molecular weight distribution of the polymerization product polypropylene carbonate is optimized, and thirdly, the amplification effect of the polypropylene carbonate is solved.

In increasing the content of polycarbonate units, some aliphatic alkylene oxides are reacted with CO₂When the alternating copolymerization process is carried out, the alkylene oxide may be continuously inserted, at this time, the obtained polycarbonate product has polyether units, and the generation of excessive polyether segments reduces the content of the polycarbonate units in the polycarbonate product and reduces the content of the polycarbonate units in the polycarbonate productBiodegradability of the ester material. Theoretically, CO in the system is increased₂The pressure of (2) can inhibit the homopolymerization of the alkylene oxide to a certain extent, thereby improving the content of polycarbonate units in a polymerization product, but along with the progress of the polymerization reaction, the molecular weight of the polypropylene carbonate gradually increases, the viscosity of a polymerization reaction system rises, and CO is prevented₂Polymerizing into polypropylene carbonate molecules; from the perspective of the catalyst, as the polymerization reaction proceeds, the molecular weight of the polypropylene carbonate gradually increases, the viscosity of the polymerization reaction system increases, and as the polymerization reaction monomer is continuously converted, the content of the polymer in the system increases, the viscosity of the reaction medium increases, which hinders the coordination of the monomer in the system and the central metal of the catalyst, and is not beneficial to CO₂And (4) converting the monomer. Generally, the dissolution of the monomer is facilitated by increasing the amount of the polymerization solvent, but the distance between the polymerized monomer and the active site of the catalyst is also increased, and the catalytic activity is remarkably decreased.

From the viewpoint of the catalyst used, the zinc-cobalt DMC catalyst used in the production of polypropylene carbonate in the prior art has the problem that, in the industry, when the mass ratio of the added catalyst to the added epoxy monomer is at least equal to or less than 1/1000 (i.e., the catalyst charge is equal to or less than 1000ppm, i.e., 0.1 wt%), the overall production cost and the cost of catalyst residues are relatively low, and thus, the catalyst has economic value. Otherwise, the catalyst has no application value because the addition amount of the catalyst is too much, which leads to the increase of the production cost. Or very few still remain weakly active, but the reaction time is greatly prolonged (more than 12 hours) to obtain the polymer.

Secondly, optimizing the molecular weight distribution of the polypropylene carbonate, wherein the side reaction and the viscosity rise of a polypropylene carbonate system during the production of the propylene carbonate are main limiting factors for optimizing the molecular weight distribution of the polypropylene carbonate as a polymerization product; if intramolecular ring elimination of the growing polymer segment occurs, a cyclic polycarbonate byproduct is obtained, typically the catalyst structure, additives, CO₂Both the pressure and the state of aggregation of the alkylene oxide influence the molecular weight of the product polypropylene carbonate. Cyclic carbonate and polycarbonate material phaseMore thermodynamically stable than, and therefore, more cyclic carbonate by-product is produced if the polymerization temperature is higher; while if CO in the reaction system₂The pressure is low, and the generation of cyclic carbonate is also facilitated. Intramolecular ring elimination reactions can occur at the end of the chain of the carbonate anion or alkoxide anion. Since the viscosity of the polypropylene carbonate varies with the proportion of carbonate units (F)_CO2) And an increase in molecular weight (Mn); this in turn creates two problematic problems, one of which is F_CO2In contrast to Mn, the high molecular weight polypropylene hinders CO, since the viscosity of the polypropylene carbonate increases as the number average molecular weight increases₂Such that an increase in the molecular weight of the polypropylene carbonate generally means a lower F_CO2. Second is with F_CO2The viscosity of the polypropylene ester is increased, so that the molecular weight distribution range of the polypropylene ester product is widened, and the molecular weight distribution (PDI) and high F of the polypropylene ester polymer also exist_CO2In contradiction, the polypropylene carbonate with low PDI value often cannot have high F at the same time_CO2More difficult to aggregate into a polymer with a low PDI value and a high F_CO2And high molecular weight polypropylene carbonate.

Finally, what is needed to be solved is the amplification effect of polypropylene carbonate, and the processes for synthesizing polypropylene carbonate mostly adopt batch processes, and although a small amount of continuous flow processes are developed, the amplification effect inevitably exists due to certain differences of temperature and concentration of each point of a reaction system in a reaction device. The Scaling up Effect (Scaling up Effect) refers to the research result obtained from the chemical process (i.e. small scale) experiment (e.g. laboratory scale) performed by small equipment, and the result obtained from the same operation condition is often very different from that obtained from the large scale production apparatus (e.g. industrial scale). The effect on these differences is called the amplification effect. The reason for this is mainly that the temperature, concentration, material residence time distribution in small-scale experimental facilities are different from those in large-scale facilities. That is, the results of the small scale experiments cannot be completely repeated on an industrial scale under the same operating conditions; to achieve the same or similar results on an industrial scale as in small scale experiments, process parameters and operating conditions need to be changed by optimal adjustment. For chemical processes, the amplification effect is a difficult and urgent problem to solve. If not solved, the production process and the product quality have great uncertainty, and firstly, the quality of downstream products is directly unstable and is difficult to control; secondly, the uncertainty can bring about the fluctuation of the technological parameters in the production process, so that the production process cannot be effectively controlled, the production safety cannot be ensured, and a plurality of potential safety hazards are buried in the production process. Amplification effect is ubiquitous, and the general theory of chemical process development (zhong constitutional writing, chinese petrochemical press, published in 2010) gives the concept of amplification effect, and the core problem of chemical reaction process development is considered to be the solution of amplification effect. The last paragraph of 42-the first paragraph of 43 describing the so-called amplification effect refers to the difference in process results between the chemical plant or process plant after amplification and the small plant of raw materials. For example, if, after the scale-up of a chemical reactor, a reduction in the indices of reaction conversion, selectivity, yield and product quality is observed under the same operating conditions, if the cause of this reduction is not readily ascertained, this phenomenon is attributed to the scale-up effect, as compared with a pilot plant before the scale-up. As the core problem of chemical process development is amplification effect, the chemical process research is to a great extent to find the reason for generating amplification effect and a compensation method. Pages 115, the last 2 to 3 paragraphs in the foundation of chemical process engineering and process design (Zheng Dong, Wuhan university Press, 2000 published) describe that since the chemical process is often a complex interweaving of chemical and physical processes, there are many unknown problems, and therefore, not only the quantitative change but also the qualitative change is reflected in the amplification process. Before people can not know the essence of the chemical process, indexes can not be repeated by simply using the operation conditions of the original test without adopting the adjustment of acquisition measures in the amplification process. This phenomenon that the scale of the process becomes large and the index cannot be repeated until the scale of the amplification is not sufficiently known is called "amplification effect". Generally, the amplification effect is a phenomenon in which the reaction condition is deteriorated, the conversion rate is decreased, the selectivity is decreased, the yield is decreased, or the quality of the product is deteriorated after the amplification. The reason for the amplification effect is explained at page 133-135 of "typical fine chemistry optimization and amplification techniques" (edited by Zhang Showa, Zhejiang university Press, published in 2015): (1) reasons for temperature, concentration gradients; (2) the heat exchange specific surface area and the reaction period are different; (3) the dead zone is different from equipment cleaning; (4) the deviation of the temperature indication is different. The publication reveals a great difference between laboratory devices and industrial plants. Macroscopically, the two look identical on the surface, but the two have obvious difference due to the difference of the mixed state on the microscopic level, so the amplification effect must be eliminated through the research of the amplification test. It can be seen from the above three textbooks that the amplification effect is a ubiquitous technical problem which plagues the development of the chemical process in the development of the chemical process. From the pilot scale, the pilot scale to the mass production, any process must have enough data and experience accumulation, and repeated experimental verification is carried out, rather than being completed by simply adjusting some parameters, and the process can be completed by the inventor with great creative labor. If the amplification effect is eliminated without carrying out amplification test, the performance indexes such as conversion rate, selectivity, quality and the like may be greatly reduced, which is fatal to chemical development. Therefore, the amplification effect is an unavoidable technical problem for a general chemical development process.

At present, the process for synthesizing the polypropylene carbonate mostly adopts a batch process. There are mainly the following problems:

1. batch operation is inefficient and reaction times are long.

2. The reaction of the polypropylene carbonate is exothermic, and the reactor is required to have good heat exchange performance so as to ensure that the reaction does not fly. Too high a temperature results in a low content of carbonate chain units, a broad molecular weight distribution, and a high proportion of cyclic carbonate as a by-product, which reduces the quality of the product.

Although a small number of continuous flow processes have been developed, there are problems: the amplification effect inevitably exists, which brings many uncertainties for further industrial application; some continuous flow processes have incomplete reaction in a short time, and increase of the reaction time by delaying the pipeline is required to improve the conversion rate, which results in reduction of the production efficiency. For example, in the Chinese patent "preparation of polyether carbonate polyol" (application No: CN201180060482.4), a tubular reactor is used for temperature uniformity at each point of the reaction system, and a cooling jacket is arranged outside the tubular reactor, so that the temperature control purpose can be realized, but the reaction system becomes viscous due to the formation of the product polypropylene carbonate, and therefore, the invention adopts the technical means that "the first 20-60 percent of the length of the tubular reactor has the inner diameter of the tubular reactor of 1.1mm to less than 100mm, and the last 80-40 percent of the length of the tubular reactor has the inner diameter of the tubular reactor of 100mm to 500 mm. ", and therefore, the inner diameters of the pipes before and after the tubular reactor were not uniform, and therefore, the heat transfer efficiency was not uniform, and the compositions of the reaction systems in the pipes before and after the tubular reactor were not uniform.

Another disadvantage of semi-batch or batch processes is that the process must be stopped in order to remove the product, thus resulting in a loss of time.

Compared with the conventional copolymerization reaction, the particularity of the copolymerization reaction of the polypropylene carbonate is that the content of carbonate units in the polypropylene carbonate is increased, the molecular weight distribution of a product of the polypropylene carbonate is optimized, the viscosity is increased along with the increase of the molecular weight of the polypropylene carbonate, the high-viscosity copolymerization reaction system limits the carbon dioxide to be polymerized into the molecules of the polypropylene carbonate, meanwhile, the heat and mass transfer efficiency of the high-viscosity copolymerization reaction system is reduced, high temperature and epoxide aggregation are easily generated locally, the epoxide is subjected to violent polymerization, a large amount of polyether chain links are generated in a short time, a tailing phenomenon is generated, the viscosity of the copolymerization reaction system is further increased, therefore, under the prior art condition, the high-molecular weight polypropylene carbonate usually means that the content of carbonate units in the polypropylene carbonate is low, correspondingly, the high-carbonate unit content usually means low molecular weight, or the molecular weight distribution range of the product of the polypropylene carbonate is wide, the molecular weight distribution (PDI) of the polymer has higher value, and even has obvious trailing phenomenon; the amplification effect for producing the polypropylene ester is not isolated, and is also related to the improvement of carbonate unit content in the polypropylene ester and the optimization of molecular weight distribution of a product of the polypropylene ester, the viscosity is increased along with the increase of the molecular weight of the polypropylene ester, the heat transfer and mass transfer efficiency of a copolymerization reaction system with high viscosity is reduced, a dead zone is easily formed in a reaction device, the amplification effect is enlarged, the molecular weight distribution range of the product of the polypropylene ester is wide, and the product of the polypropylene ester has a larger PDI value.

In summary, how to increase the content of carbonate units, reduce the proportion of cyclic carbonate as a byproduct in the crude product, increase the molecular weight of the polypropylene carbonate as a polymerization product, reduce the relative molecular mass distribution coefficient, and solve the scale-up effect of mass production is a problem that needs to be solved urgently by those skilled in the art, and those skilled in the art especially need a technical scheme capable of solving the above three problems at the same time, but the technical scheme is not disclosed at present.

Disclosure of Invention

The invention aims to solve the technical problem of providing a method for continuously producing polypropylene carbonate by a preheating liquid phase method in a pipeline way. The method has the characteristics of small online reaction amount, small potential safety hazard, convenient control of reaction, continuous production and low production cost. The method of the invention can simultaneously solve the problems of increasing the content of carbonate units, optimizing the molecular weight distribution of the product of the polypropylene carbonate and solving the amplification effect of large-scale production when the polypropylene carbonate is produced by organically combining and using a novel catalyst, improving reaction equipment and optimizing a production process.

The purpose of the invention is realized as follows:

a method for continuously producing polypropylene carbonate in a pipeline way by a preheating liquid phase method,

the method comprises the following steps:

(1) the method comprises the following steps of (1) enabling raw materials comprising propylene oxide and a catalyst to enter a premixing tank in a certain proportion to be uniformly mixed, and then pumping the mixture into a pipeline reactor, wherein the pipeline reactor comprises a reaction section group and a cooling section group, the reaction section group is arranged at the inlet end of the pipeline reactor, and the cooling section group is arranged at the outlet end of the pipeline reactor;

(2) preheating carbon dioxide to 40-150 ℃, and pumping from an inlet of a channelization reactor to pressurize the channelization reactor to 1-15 MPa;

(3) mixing the raw materials in a pipeline reactor to form reaction liquid, heating to 70-150 ℃, and allowing propylene oxide and carbon dioxide to contact in the pipeline reactor in the presence of the catalyst so as to perform polymerization reaction to obtain a polymerization reaction product material flow containing polypropylene carbonate;

(4) after part or all of the polymerization reaction product material flow passes through the cooling section group, separating the polypropylene carbonate from the polymerization reaction product material flow to form a polypropylene carbonate product material flow, and then recycling the residual polymerization reaction product material flow to the step (1);

the catalyst is a zinc-cobalt double metal cyanide complex catalyst obtained by reacting water-soluble metal salts of zinc and cobalt in a water-soluble solvent, the catalyst is modified by a mixed acid during synthesis, the mixed acid comprises at least one organic acid and at least one water-soluble inorganic acid, the organic acid is selected from one or more of succinic acid, glutaric acid, phthalic acid, iminodiacetic acid, pyromellitic acid and butane tetracarboxylic acid, the molar ratio of the water-soluble inorganic acid to the organic acid is 1: 10-10: 1, and the molar ratio of zinc and cobalt in the catalyst is 1: 5-5: 1, preferably 1: 4-4: 1, more preferably 1: 3-3: 1, and more preferably 1: 2-2: 1.

Preferably, after the propylene oxide is preheated to 40-120 ℃, the propylene oxide is supplemented into the pipeline reactor from the front end of the reaction section group.

Preferably, the remaining polymerization product stream is recycled to step (1) after preheating to 70-120 ℃.

Preferably, the raw materials are mixed by a static mixer and then pumped into the pipeline reactor.

Preferably, before the raw materials enter the premixing tank, carbon dioxide is introduced to ensure that the air pressure in the premixing tank is 0.1-2 MPa, and the raw materials are added into the premixing tank in the carbon dioxide atmosphere.

Preferably, after the carbon dioxide is preheated to 70-120 ℃, the carbon dioxide is supplemented into the pipeline reactor from the reaction section group.

Preferably, the reaction liquid flows through the reaction section group at a flow rate of 0.01-3m/s in a pipeline reactor, is heated to 70-120 ℃, and is subjected to polymerization reaction in the pipeline reactor.

Further, the reactant liquid flows through the pipeline reactor and becomes a polymerization product material flow after polymerization reaction, and is pumped to a gas-liquid separation device from an outlet of the pipeline reactor, and is pre-separated and separated into a gas-phase material and a liquid-phase material, wherein the gas-phase material contains carbon dioxide, the liquid-phase material contains polypropylene carbonate, and after the catalyst is filtered out by a filtering device, the liquid-phase material is pumped into a purification device to be subjected to a separation procedure, and the liquid-phase material is separated to obtain the polypropylene carbonate product material flow and flows to a polypropylene carbonate storage tank to be stored.

Preferably, the catalyst is a zinc-cobalt double metal cyanide complex, the concentration of the catalyst in the feed or the polymerization reaction product stream being from 0.01 to 0.5 wt%.

Preferably, the catalyst is modified by mixed acid during synthesis, the mixed acid comprises at least one organic acid and at least one water-soluble inorganic acid, and the molar ratio of the water-soluble inorganic acid to the organic acid is 1: 10-10: 1, wherein: the water-soluble inorganic acid is selected from dilute sulfuric acid and dilute hydrochloric acid, and the pH value is 0-5; preferably 0 to 4; more preferably 1 to 3; more preferably 1 to 2; the dilute sulfuric acid is H₂SO₄The pH value of the aqueous solution of (1) can be obtained by adding concentrated sulfuric acid into deionized water for dilution, and is between 0 and 5; the dilute hydrochloric acid is an HCl aqueous solution, and can be diluted by adding concentrated hydrochloric acid into deionized water to obtain an organic acid with a pH value of 0-5, wherein the organic acid is selected from any one or more of succinic acid, glutaric acid, phthalic acid, iminodiacetic acid, pyromellitic acid and butane tetracarboxylic acid.

The invention has the following beneficial effects:

1. the method of the invention organically combines three aspects of using a novel catalyst, improving reaction equipment and optimizing a production process, improves the carbonate unit content of the polypropylene carbonate product, and reduces the proportion of a byproduct cyclic carbonate in a crude product; in the aspect of the catalyst, the catalytic copolymerization reaction is realized at lower temperature and under the condition of lower catalyst feeding amount by improving the activity and selectivity of the catalyst, the catalyst selected by the invention can reduce the synthetic energy barrier of the copolymer of carbon dioxide and epoxide under the same condition, and the catalytic activity is realized at lower temperature, so that the problems that the selectivity and the activity of the catalyst in the prior art are poor, the catalytic activity can only be realized at higher temperature, and the generation reaction of polyether and cyclic carbonate is inevitably caused are avoided; in the aspect of production process, after part or all of a polymerization reaction product material flow flows through the cooling section group, polypropylene carbonate is separated from the polymerization reaction product material flow, and after the polypropylene carbonate product material flow is formed, the residual polymerization reaction product material flow is circulated to the step (1), so that the mass transfer and heat transfer capacity and effect of reaction equipment are improved, local overheating is avoided, and the mass production of polyether and cyclic carbonate caused by the local overheating of the reaction equipment in the prior art is avoided; because the reaction equipment of the invention is a pipeline reactor, the mass transfer effect of the reaction kettle is obviously higher than that of the prior art, secondly, the catalyst selected by the invention can realize the catalytic copolymerization reaction at lower temperature and lower catalyst feeding amount, and thirdly, in the method of the invention, by separating the polypropylene carbonate from the polymerization product stream, the viscosity of the copolymerization reaction system can be maintained in a suitable range, thereby ensuring the mass transfer and heat transfer effects of a copolymerization reaction system, therefore, by organically combining three aspects of using a novel catalyst, improving reaction equipment and optimizing a production process, in the invention, the carbon dioxide in the copolymerization reaction system can be distributed more uniformly, so that the higher proportion of polycarbonate chain links in the product polypropylene carbonate can be realized, and the generation of polyether and cyclic carbonate can be inhibited.

2. The molecular weight distribution of the polypropylene carbonate product is optimized by organically combining and using a novel catalyst, improving reaction equipment and optimizing a production process; the applicant has also found, surprisingly, that the molecular weight distribution of the products of the polypropylene carbonate of the invention is in the right range, the polymer molecular weight distribution (PDI) has a low value, and there is no significant tailing in the polymerization product; the principle of the invention is that due to the reaction equipment or production process adopted in the prior art, when the polypropylene carbonate is produced by copolymerization, the heat transfer capacity is insufficient, high temperature is easily generated locally, so that the epoxide is subjected to violent polymerization, a large amount of polyether chain links are generated in a short time, and the tailing phenomenon is reflected macroscopically. The applicant of the present invention has also found that the catalyst used in the present invention has synergistic effect with the reaction equipment and production process selected in the present invention, and can realize the preparation of polypropylene carbonate with narrow molecular weight distribution and high carbon dioxide fixing rate (i.e. the molar ratio of carbonate chain link structure on the main chain of the polymer is not less than 56.5%, i.e. the carbon dioxide insertion mass fraction is not less than 30%) while still maintaining high catalytic activity under the condition of high reaction temperature (reaction time is 1 hour, and the monomer conversion rate is more than 60%). The method of the invention realizes higher carbon dioxide fixation rate, so that the material cost is greatly reduced (the cost is very low compared with that of epoxide due to carbon dioxide), and the higher the proportion of carbonate chain links on the main chain of the polymer is, the higher the glass transition temperature of the polymer is, and the higher the glass transition temperature is, the better the requirements of the use strength, heat resistance.

3. The invention organically combines and uses a novel catalyst, improves reaction equipment and optimizes a production process, so that the method for producing the polypropylene carbonate has no obvious amplification effect, and the principle is that the catalyst with high activity is selected, and simultaneously, the reaction equipment and the optimized production process are improved; furthermore, the invention supplements sufficient carbon dioxide to the copolymerization reaction system by supplementing carbon dioxide from the reaction section group to the pipeline reactor, and remixes the materials of the copolymerization reaction system by supplementing carbon dioxide, thereby overcoming the local enrichment of epoxy compounds and effectively avoiding the homopolymerization reaction between the epoxy compounds.

Detailed Description

The present invention is further illustrated by the following examples, which are not intended to limit the invention to these embodiments. It will be appreciated by those skilled in the art that the present invention encompasses all alternatives, modifications and equivalents as may be included within the scope of the claims.

In the present invention, the raw materials and equipment used are commercially available or commonly used in the art, unless otherwise specified. The methods in the following examples are conventional in the art unless otherwise specified.

Example 1

the method comprises the following steps:

(4) after part or all of the polymerization reaction product material flow passes through the cooling section group, separating part of the polypropylene carbonate from the polymerization reaction product material flow to form a polypropylene carbonate product material flow, and recycling the residual polymerization reaction product material flow to the step (1);

Preferably, the catalyst is a zinc-cobalt double metal cyanide complex, the concentration of the catalyst in the feed or the polymerization reaction product stream being from 0.01 to 0.5 wt%; specifically, it is 0.15 wt%. In embodiment 1, the used apparatus includes a raw material purification system, a premixing tank, a pipeline reactor, a gas-liquid separator, a catalyst filter, a rectification device, a recovery device, etc., first, the raw material enters the premixing tank after the water content of the purification system reaches the standard, after being uniformly mixed, the raw material is pumped into the pipeline reactor, the pipeline reactor includes a reaction section group and a cooling section group, the reaction section group is heated to 70-150 ℃ for polymerization, carbon dioxide is always introduced during the reaction process to ensure that the pressure is within the range of 1-15MPa, specifically, about 5MPa, after the reaction is finished, the raw material flows out from the cooling section group, the raw material is separated from the catalyst through gas-liquid separation, the excess carbon dioxide gas is separated in the gas-liquid separation, a small amount of PO is taken out when the gas volatilizes, the PO is respectively recovered through a condenser, and the catalyst nanoparticles are recovered in the catalyst filter, the filtrate contains reaction products of polypropylene carbonate, cyclic carbonate and unreacted PO, the three can be effectively separated after passing through a falling film tower and a scraper evaporator in a rectifying device, the separated reaction products reach the industrial application standard and are respectively canned for standby, and the unreacted PO is recycled through a buffer tank. The premixing tank has the function of uniformly mixing the catalyst and the PO in advance, and the temperature is controlled to be 0-60 ℃ in the premixing time to ensure that the reaction cannot occur in advance, so that the catalyst is activated, and the rapid reaction can be realized after the catalyst enters the reactor.

In example 1, the feed was mixed in a static mixer and pumped into the pipeline reactor.

In embodiment 1, before the raw material enters the premix tank, carbon dioxide is introduced so that the air pressure in the premix tank is 0.1 to 2MPa, and the raw material is added into the premix tank in the carbon dioxide atmosphere.

In example 1, the pipelined reactor of the present invention is formed by two or more tubular reactors connected in series or in parallel, the length-to-diameter ratio L/dR of the reactors is not less than 50, a sample introduction mixer or a packing is arranged inside the tubular reactor, the mixer is of an SX type, an SK type, an SV type, an SL type, an SH type, an SN type, etc., the packing is a regular packing, the geometry is a grid, a pulse, a ripple, etc., and the presence of the mixer or the packing facilitates the mixture to be mixed rapidly and uniformly, accelerates the reaction process, and improves the reaction rate.

In example 1, the catalyst was a zinc-cobalt double metal cyanide complex catalyst obtained by reacting a water-soluble metal salt of zinc and cobalt in a water-soluble solvent; the catalyst is modified during synthesis by a mixed acid comprising at least one organic acid and at least one water-soluble inorganic acid, wherein: the water-soluble inorganic acid is selected from dilute sulfuric acid and dilute hydrochloric acid, and the pH value is 0-5; preferably 0 to 4; more preferably 1 to 3; more preferably 1 to 2; the dilute sulfuric acid is H₂SO₄Aqueous solution of (A)Adding concentrated sulfuric acid into deionized water for dilution to obtain a solution with a pH value of 0-5; the dilute hydrochloric acid is an HCl aqueous solution, and can be diluted by adding concentrated hydrochloric acid into deionized water to obtain a pH value of 0-5.

In example 1, a zinc-cobalt double metal cyanide complex catalyst was obtained from the reaction of water-soluble metal salts of zinc and cobalt in a water-soluble solvent using a DMC catalyst; the preparation method comprises the following steps: weighing a certain mass of cobalt salt and zinc salt, specifically sodium thiocyanatocobaltate and zinc sulfate, wherein the molar ratio is about 1:2, dissolving in an aqueous solvent, and continuously stirring, wherein the aqueous solvent comprises water and propanol, and the mass ratio of the total mass of the metal salt (namely cobalt salt and zinc salt) to the aqueous solvent is about 1: 20. Adding inorganic acid and organic acid, wherein the inorganic acid is dilute hydrochloric acid, the pH value is about 4, the organic acid is iminodiacetic acid, and the molar ratio of the inorganic acid to the organic acid is about 3:1, the ratio of the total mole number of the metal salt to the mole number of the acid is about 5:1, and the mixture is stirred for a plurality of hours at the temperature of 10-100 ℃, and precipitates are continuously generated. And carrying out suction filtration on the turbid liquid, and drying to obtain a filter cake. And (2) reslurrying and washing the filter cake at the temperature of 10-100 ℃ by using an aqueous solvent, specifically, washing for 1 minute at the temperature of 60 ℃, stirring for several hours, carrying out suction filtration and drying to obtain the filter cake, and repeating the steps of reslurrying, washing and drying at the temperature of 20 ℃ for multiple times until the pH of the system liquid is 6-7, specifically, the temperature is 60 ℃ and each time is 5 minutes. And further drying the solid product at 80-100 ℃ under a vacuum condition to obtain a final catalyst, and processing the catalyst into powder particles by mechanical grinding under an anhydrous drying condition before use.

In example 1, the reaction solution was passed through the reaction zone group at a flow rate of 0.1 to 3m/s, specifically about 0.1m/s, heated to 70 to 120 ℃, specifically to about 120 ℃, in a pipelined reactor, and the polymerization was carried out in the pipelined reactor, with an average residence time of each component in the pipelined reactor of 2 hours.

In example 1, the reaction liquid stream is subjected to a polymerization reaction in the pipeline reactor to form the polymerization reaction product stream, and the polymerization reaction product stream is pumped out from an outlet of the pipeline reactor to a gas-liquid separation device for pre-separation and separation into a gas phase material and a liquid phase material, wherein the gas phase material contains carbon dioxide, the liquid phase material contains polypropylene carbonate, the liquid phase material passes through a filtering device to filter out a catalyst, the liquid phase material is pumped into a purification device for a separation procedure, and the separation is performed to obtain the polypropylene carbonate product stream, and the polypropylene carbonate product stream flows to a polypropylene carbonate storage tank for storage.

Sampling and analyzing a polymerization reaction product material flow, specifically, sampling and collecting the polymerization reaction product material flow (polypropylene carbonate, cyclic propylene carbonate and unreacted propylene oxide) in a container, performing nuclear magnetic hydrogen spectrum characterization on a polymerization reaction product material flow sample to calculate the proportion of a polymer and a cyclic micromolecule in a crude product, performing nuclear magnetic hydrogen spectrum test after purifying the polymer, and calculating to obtain the proportion of a polycarbonate chain link and a polyether chain link on a polymer main chain, wherein only two structures of the polycarbonate chain link and the polyether chain link are arranged on the polymer main chain, and the sum of the percentages of the two structures is 100%. The number average molecular weight and the molecular weight distribution were determined by gel permeation chromatography on the polymer.

By means of¹H-NMR (Bruker, DPX400, 400 MHz; pulse program zg30, waiting time d1:10s, 64 scans) determination of the incorporated CO in the resulting polypropylene carbonate₂The amount of (carbonate chain unit content) and the ratio of propylene carbonate (cyclic carbonate) to polypropylene carbonate. The samples were dissolved in deuterated chloroform in each case.¹The relevant resonances in H-NMR (based on TMS ═ 0ppm) are as follows:

wherein 5.0ppm and 4.2ppm are proton peaks on the last methyl and methylene of polycarbonate chain, 4.9ppm,4.5ppm and 4.1ppm are proton peaks on the methylene and methylene of five-membered cyclic carbonate, and 3.5-3.8ppm are proton peaks on ether chain. The integrated Area of a peak at a certain ppm in the nuclear magnetic hydrogen spectrum is represented by capital letter A plus a numerical subscript, A being an abbreviation for the English writing Area of the Area, e.g. A_5.0Represents the integrated area of the peak at 5.0 ppm. According to the copolymerisation of the crude products¹H NMR spectrum and integral area of its associated proton peak, we define the proportion (molar ratio) of carbonate units in the copolymerization (F)_CO2) And cyclic carbonate content mass fraction (W)_PCwt), amount (by mass) of carbon dioxide inserted (M)_CO2) Is/are as followsThe calculation method comprises the following steps:

wherein the content of the first and second substances,

F_CO2＝(A_5.0+A_4.2-2×A_4.6)/[(A_5.0+A_4.2-2×A_4.6)+A_3.5]×100％；

W_PC＝102×A_1.5/[102×(A_5.0+A_4.2-2×A_4.6+A_1.5)+58×A_3.5]×100％；

M_CO2＝44×F_CO2/[102×F_CO2+58×(1-F_CO2)]×100％；

coefficient 102 is formed by CO₂The sum of the molar mass of (2) (molar mass 44g/mol) and the molar mass of PO (molar mass 58g/mol), the factor 58 being derived from the molar mass of PO.

Illustrative of the proportion of carbonate units (F)_CO2) And amount of carbon dioxide incorporation (M)_CO2) The calculation of (2):

when F is present_CO2When the content is 50%, that is, when the polymer contains 50% carbonate linkages, the amount of carbon dioxide incorporated M_CO2＝27.5％。

Conversely, when M_CO2When 30%, F_CO2If the mass fraction of carbon dioxide is required to be added to 56.5%, that is, 30% or more, the carbonate chain unit ratio is 56.5% or more.

Comparative example 1

Referring to chinese patent CN103403060B technical solution, polypropylene carbonate is continuously produced using the same raw materials of example 1, under the same reaction temperature conditions and within the same time to complete the reaction, which is different from example 1 in that comparative example 1 employs a DMC catalyst (double metal cyanide catalyst) according to WO0180994a1, comparative example 1 employs a tubular reactor in one stage, which is externally provided with a cooling jacket, and the reaction temperature is controlled by the cooling jacket.

In particular, a ground and dried DMC catalyst (double metal cyanide catalyst) prepared according to example 6 of WO0180994A1 was suspended in propylene oxide.

0.15 wt.% of DMC catalyst in propylene oxide with ground and dried DMC catalyst% of the suspension is transferred at 80g/h from the stirring supply first container to the first mixer (cascade mixer 2S from Ehrfeld Mikrotechnik BTS GmbH, minimum gap between cascades 0.6mm) by means of a diaphragm pump. Propylene oxide from the second feed vessel was transported to the mixer by means of an HPLC pump (97 g/h). In the mixer, mixing is carried out at a temperature of 20 ℃, wherein the resulting mixture remains unreacted. This mixed stream was conveyed with carbon dioxide (32 g/h from a cylinder with dip tube by means of an HPLC pump) into a second mixer (cascade mixer 2S of Ehrfeld Mikrotechnik BTS GmbH, with a minimum gap of 0.6mm between cascades), in which the components were mixed at a temperature of 20 ℃. Here too, no reaction has taken place. The reaction mixture was sent from the second mixer to the tubular reactor. The tubular reactor had an outer diameter of 2.2mm and was controlled at a reaction temperature of about 120 ℃. The volume of the tubular reactor was 45cm³. The average residence time of the components in the tubular reactor was in each case 2 hours. The pressure was regulated by means of a pressure regulating valve to maintain a constant pressure of about 5MPa in the tubular reactor. The resulting product (mainly polypropylene carbonate) was collected in a vessel.

The polypropylene carbonates produced in example 1 and comparative example 1 were examined and analyzed, and the results are shown in Table 1.

Examination and analysis of the polypropylene carbonates produced in Table 1, example 1 and comparative example 1

Parameter(s)	Example 1	Comparative example 1
			Reaction temperature (. degree.C.)	120	120
Epoxide conversion¹(％)	>99	52
			Cyclic carbonate mass fraction W_PC ²	20	35
Proportion of carbonate chain units (%)³	65	21
			M_n ⁴(g/mol)	15800	3310
PDI⁵	1.25	2.78

Note: 1 epoxide conversion: conversion of epoxide feedstock in the system after a given reaction time according to the crude product nuclear magnetic hydrogen spectrum (¹H NMR) is calculated; 2 molar ratio of cyclic carbonate, i.e. the molar percentage of cyclic small molecules (propylene carbonate) in the product stream of the polymerization reaction, according to the product nuclear magnetic hydrogen spectrum (¹H NMR) is calculated; 3 molar ratio of polycarbonate structure to polyether structure in the polymer chain unit according to the product nuclear magnetic hydrogen spectrum (¹H NMR) is calculated; 4 polymer number average molecular weight (Mn), as determined by Gel Permeation Chromatography (GPC); 5 Polymer molecular weight distribution (PDI), determined by Gel Permeation Chromatography (GPC). The measurement error of the above parameters is within ± 5%.

From the results in Table 1, we can obtain that the method of the present invention can still maintain high catalytic activity at a higher reaction temperature (120 ℃), and prepare polypropylene carbonate with narrower molecular weight distribution and higher proportion of carbonate chain units. The ratio of carbonate units and the molecular weight of the polymer in example 1 are much higher than those in comparative example 1. The principle is that on one hand, a mixed acid modified zinc-cobalt double metal cyanide complex catalyst obtained by reacting water-soluble metal salts of zinc and cobalt in a water-soluble solvent is used as a reaction for polymerizing polypropylene carbonate to reduce the activation energy of generated polymers, on the other hand, a pipeline reactor is adopted to comprise a heating section group and a cooling section group, the cooling section group is arranged before materials enter a gas-liquid separation device to cool the materials, and the step (1) of separating the residual materials after the product polypropylene carbonate is obtained, so that the mass and heat transfer effect is better, the proportion of carbonate chain links is improved, the molecular weight distribution is more concentrated, and the PDI is smaller.

Example 2

Example 2 referring to the experimental example 1, the difference is that in example 2, after preheating carbon dioxide to 70-120 ℃, specifically preheating to about 100 ℃, the carbon dioxide is supplemented into the pipeline reactor from the reaction section group, so that the reaction section group has enough carbon dioxide by adding the carbon dioxide, raw materials in the reaction section group are uniformly mixed, dead zones are avoided, local enrichment of propylene oxide is avoided, and the polymerization reaction temperature of the reaction section group is controlled by adding the carbon dioxide.

Comparative example 2

Comparative example 2 referring to example 2 and comparative example 1, comparative example 2 employs the reaction apparatus and process of comparative example 1, unlike comparative example 1 in that comparative example 2 employs a mixed acid-modified zinc-cobalt double metal cyanide complex catalyst obtained by reacting water-soluble metal salts of zinc and cobalt in a water-soluble solvent according to the present invention.

Examination and analysis of the polypropylene carbonates produced in Table 2, example 2 and comparative example 2

Parameter(s)	Example 2	Comparative example 2
			Reaction temperature (. degree.C.)	100	100
Epoxide conversion¹(％)	>99	56
			Cyclic carbonate mass fraction W_PC ²	15	19
Proportion of carbonate chain units (%)³	83	51
			M_n ⁴(g/mol)	27500	5890
PDI⁵	1.19	2.55

Note: 1 epoxide conversion: conversion of epoxide feedstock in the system after a given reaction time according to the crude product nuclear magnetic hydrogen spectrum (¹H NMR) is calculated; 2 Cyclic carbonate molThe molar ratio, i.e.the molar percentage of cyclic small molecules (propylene carbonate) in the product stream of the polymerization reaction, is based on the product nuclear magnetic hydrogen spectrum (¹H NMR) is calculated; 3 molar ratio of polycarbonate structure to polyether structure in the polymer chain unit according to the product nuclear magnetic hydrogen spectrum (¹H NMR) is calculated; 4 polymer number average molecular weight (Mn), as determined by Gel Permeation Chromatography (GPC); 5 Polymer molecular weight distribution (PDI), determined by Gel Permeation Chromatography (GPC). The measurement error of the above parameters is within ± 5%.

From the results shown in table 2, it can be seen that in example 2, carbon dioxide is preheated to about 100 ℃, and is supplemented into the pipelined reactor from the reaction zone group, so that on one hand, the temperature of the copolymerization reaction is controlled more precisely, so that the copolymerization reaction is performed under the desired temperature condition, and on the other hand, the reaction zone group has enough carbon dioxide, and the raw materials in the reaction zone group are uniformly mixed, thereby avoiding the occurrence of dead zones, and avoiding the local enrichment of propylene oxide, thereby not only accelerating the reaction rate, shortening the reaction time, but also improving the proportion of polycarbonate chain links in the product.

From the results of tables 1 and 2, we can obtain that both examples 1 and 2 use the mixed acid modified zinc-cobalt double metal cyanide complex catalyst obtained by reacting water-soluble metal salts of zinc and cobalt in a water-soluble solvent according to the present invention, but since example 2 adds carbon dioxide from the reaction zone group to the piping reactor by preheating the carbon dioxide to about 100 ℃, the temperature of the copolymerization reaction is controlled more precisely, so that the copolymerization reaction is carried out under the desired temperature conditions, and dead zones are also avoided, so that example 2 can be reacted under the desired reaction temperature conditions more than example 1, and since local enrichment of propylene oxide can be avoided, the carbonate chain segment ratio is also increased from 65% of example 1 to 83% of example 2. Since comparative example 2 employs the mixed acid-modified zinc-cobalt double metal cyanide complex catalyst obtained by reacting water-soluble metal salts of zinc and cobalt in a water-soluble solvent according to the present invention, it can be seen in tables 1 and 2 that the proportion of carbonate units in the product increased from 21% in comparative example 1 to 51% in comparative example 2.

However, since comparative example 1 and comparative example 2 both use the one-stage tubular reactor of the CN103403060B technical scheme, the obtained polypropylene carbonate polymer has the number average molecular weight M_nSignificantly lower than in examples 1 and 2.

Example 3

Example 3 referring to experimental example 1, except that in example 3, after propylene oxide was preheated to 40 to 120 ℃, specifically to about 90 ℃, propylene oxide was additionally added to the pipelined reactor from the reaction zone group, in order to control the concentration of propylene oxide in the reaction zone group within a target range by adding propylene oxide, to uniformly mix the raw materials in the reaction zone group, and to control the polymerization temperature of the reaction zone group by adding propylene oxide.

Comparative example 3

Comparative example 3 referring to example 3 and comparative example 1, comparative example 3 used the catalyst of comparative example 1, differing from comparative example 1 in that comparative example 3 used the reaction equipment and process of example 3, except that the tubular reactor was not supplemented with preheated propylene oxide.

Inspection analysis of the polypropylene carbonates produced in Table 3, example 3 and comparative example 3

Parameter(s)	Example 3	Comparative example 3
			Reaction temperature (. degree.C.)	90	90
Epoxide conversion¹(％)	>99	75
			Cyclic carbonate mass fraction W_PC ²	13	17
Proportion of carbonate chain units (%)³	90	65
			M_n ⁴(g/mol)	36800	7890
PDI⁵	1.13	2.47

From the results in table 3, propylene oxide preheated to 90 ℃ is added into the pipeline reactor from the reaction section group, unexpectedly, the polycarbonate chain link ratio in the product is improved in the embodiment 3 compared with the comparative example 3, and the principle is that the temperature of the copolymerization reaction is accurately controlled in the embodiment 3, so that the copolymerization reaction is carried out under the expected temperature condition, and the raw materials of the reaction section group are uniformly mixed, so that dead zones are avoided, and the local enrichment of the added propylene oxide can be avoided due to the sufficient amount of carbon dioxide in the pipeline reactor of the embodiment 3, so that the polycarbonate chain link ratio in the product is improved.

Example 4

Example 4 referring to example 1, example 4 used the feedstock and catalyst and reaction process of example 1, except that the pipelined reactor of example 4 had two sets, one set having a 10mm internal diameter of the pipeline and being a laboratory set-up, the other set having an internal diameter of the pipeline of 100mm and being a pilot plant set-up, with the objective of examining whether the process of the present invention had an amplification effect by enlarging the size of the pipeline.

Comparative example 4

Comparative example 4 referring to comparative example 1, comparative example 4 using the raw material and catalyst and reaction process of comparative example 1, different from comparative example 1 in that the piping reactor of comparative example 4 has two sets, one set having a pipe inner diameter of 10mm as a laboratory apparatus and the other set having a pipe inner diameter of 100mm as a pilot plant, in order to see whether the method of comparative example 4 has an amplification effect by amplifying the size of the pipe.

Inspection analysis of polypropylene carbonate produced in Table 4, example 4 and comparative example 4

As can be seen from Table 4, the process for producing polypropylene carbonate according to the present invention has no amplification effect, while comparative example 4 has a certain amplification effect, and the amplification effect in comparative example 4 is remarkable, the carbonate chain ratio abruptly decreases from 45% to 14%, the laboratory apparatus is close to 4 times that of the pilot apparatus, whereas the experimental apparatus of example 4 according to the present invention has a carbonate chain ratio of 93%, the pilot apparatus is 92%, the cyclic carbonate ratio is 10%, and the pilot apparatus is also 10%, with variations of less than 5%. The experimental set-up for the molecular weight distribution of the polymer was 37600g/mol, whereas the pilot set-up was 37200g/mol, which is quite close, so the amplification effect was less pronounced. The method provided by the patent has no influence on the conversion rate of epoxide, the content of carbonate chain links of the polypropylene carbonate, the proportion of cyclic byproducts and other product indexes by amplifying the reaction scale, namely, the process has no obvious amplification effect. The proportion of carbonate chain links in the polypropylene carbonate produced by the process is higher than 90%, which shows that the process is beneficial to improving the carbon dioxide fixation amount of the product, accelerating the reaction speed and being easy for large-scale production. The principle of the method is that the method continuously separates the polypropylene carbonate from the reaction system, so that the concentration of the polypropylene carbonate serving as a product in the reaction system is maintained at a low content, the viscosity of the reaction system is proper in a variation range, the variation range of the material ratio of the reaction system is also proper, the pipelines of the pipeline reactor can have the same inner diameter from front to back, and the mass transfer and heat transfer capacities of the reaction equipment have better consistency from front to back; according to the invention, the gas pressure-supplementing feeding device is arranged between the heating sections of the pipeline reactor, so that on one hand, carbon dioxide is supplemented, on the other hand, materials in a reaction system are remixed by supplementing carbon dioxide, and the local enrichment of epoxy compounds is overcome, thereby effectively avoiding homopolymerization reaction between the epoxy compounds.

The above-mentioned embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims

1. A method for continuously producing polypropylene carbonate in a pipeline way by a preheating liquid phase method is characterized in that,

the method comprises the following steps:

the catalyst is a zinc-cobalt double metal cyanide complex catalyst obtained by reacting water-soluble metal salts of zinc and cobalt in a water-soluble solvent, the catalyst is modified by mixed acid during synthesis, the mixed acid comprises at least one organic acid and at least one water-soluble inorganic acid, the organic acid is selected from any one or more of succinic acid, glutaric acid, phthalic acid, iminodiacetic acid, pyromellitic acid and butane tetracarboxylic acid, the molar ratio of the water-soluble inorganic acid to the organic acid is 1: 10-10: 1, and the molar ratio of zinc and cobalt in the catalyst is 1: 5-5: 1.

2. The method of claim 1, wherein the molar ratio of zinc to cobalt in the catalyst is 1:4 to 4: 1.

3. The method of claim 1, wherein the molar ratio of zinc to cobalt in the catalyst is 1:3 to 3: 1.

4. The method of claim 1, wherein the molar ratio of zinc and cobalt elements in the catalyst is 1:2 to 2: 1.

5. The process according to any one of claims 1 to 4, wherein propylene oxide is added to the pipelining reactor from the front end of the series of reaction sections after the propylene oxide has been preheated to 40 to 120 ℃.

6. The process according to any one of claims 1 to 4, wherein the remaining polymerization product stream is recycled to step (1) after preheating to 70 to 120 ℃.

7. The method according to any one of claims 1 to 4, wherein the feedstock is mixed in a static mixer before being pumped into the pipeline reactor.

8. The method according to any one of claims 1 to 4, wherein carbon dioxide is introduced into the premix tank before the raw material enters the premix tank so that the pressure in the premix tank is 0.1 to 2MPa, and the raw material is added to the premix tank in a carbon dioxide atmosphere.

9. The method according to any one of claims 1 to 4, wherein the carbon dioxide is supplemented from the reaction section group into the pipelining reactor after preheating the carbon dioxide to 70 to 120 ℃.

10. The process according to any one of claims 1 to 4, wherein the reaction liquid flows through the set of reaction sections at a flow rate of 0.01 to 3m/s in a pipeline reactor, and is heated to 70 to 120 ℃ to carry out the polymerization reaction in the pipeline reactor.

11. The method according to claim 10, wherein the reaction liquid after flowing through the pipeline reactor for polymerization is a product stream of the polymerization reaction, and is pumped out from an outlet of the pipeline reactor to a gas-liquid separation device for pre-separation to separate into a gas phase material and a liquid phase material, the gas phase material contains carbon dioxide, the liquid phase material contains polypropylene carbonate, after passing through a filtering device to filter out the catalyst, the liquid phase material is pumped into a purification device for separation procedure, and the product stream of the polypropylene carbonate is separated and flows to a polypropylene carbonate storage tank for storage.

12. The process of any one of claims 1 to 4, wherein the catalyst is a zinc-cobalt double metal cyanide complex and the concentration of the catalyst in the feed or the polymerization product stream is from 0.01 to 0.5 wt.%.

13. The method according to any one of claims 1 to 4, wherein the catalyst is modified during synthesis by a mixed acid comprising at least one organic acid and at least one water-soluble inorganic acid, wherein the molar ratio of the water-soluble inorganic acid to the organic acid is 1:10 to 10:1, and wherein: the water-soluble inorganic acid is selected from dilute sulfuric acid and dilute hydrochloric acid, and the pH value is 0-5; the dilute sulfuric acid is H₂SO₄The aqueous solution of (A) can be prepared by adding concentrated sulfuric acidAdding deionized water for dilution to obtain a pH value of 0-5; the dilute hydrochloric acid is an HCl aqueous solution, and can be diluted by adding concentrated hydrochloric acid into deionized water to obtain an organic acid with a pH value of 0-5, wherein the organic acid is selected from any one or more of succinic acid, glutaric acid, phthalic acid, iminodiacetic acid, pyromellitic acid and butane tetracarboxylic acid.

14. The method of claim 13, wherein the pH is between 0 and 4.

15. The method according to claim 13, wherein the pH value is between 1 and 3.

16. The method according to claim 13, wherein the pH value is between 1 and 2.