A silver-based catalyst with a bifunctional carrier, its preparation method and application

Abstract

The present invention discloses a silver-based catalyst with a dual-functional support, its preparation method and application. The structure of the silver-based catalyst is as follows: cerium oxide is dispersed on the surface of silica nanoparticles in the form of nano-islands, and the silver species are located on the cerium oxide nano-islands in the form of nanoparticles. The silver-based catalyst is prepared by a two-step Coulomb force attachment method: first, a dual-functional support is prepared by the Coulomb force attachment method, and then the silver-based catalyst is prepared using the dual-functional support as a raw material. In the present invention, a dual-functional support is first prepared by the Coulomb force attachment method to improve the dispersion of cerium oxide. When the silver species are further loaded by the Coulomb force attachment method, a coordination reagent is added to enable the silver species to selectively locate on the cerium oxide nano-islands. When the silver-based catalyst of the present invention is used in the reaction of hydrogenating dimethyl oxalate to prepare methyl glycolate, it has the characteristics of high product orientation selectivity, good activity and excellent stability, and is suitable for industrial application.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and relates to a catalyst for hydrogenating dimethyl oxalate to prepare methyl glycolate (MG), in particular to a catalyst with a bifunctional support, its preparation method and application. Background Art

[0002] Methyl glycolate (MG) is an important chemical raw material and can be used to prepare a variety of high-value-added products. For example, glycolic acid prepared by hydrolysis can be used as a monomer for synthesizing biodegradable plastic polyglycolic acid (PGA); glycine prepared by amination reaction can be used in the synthesis of food, medicine, feed, etc.; dimethyl malonate prepared by carbonylation can be used in the synthesis of pesticides, medicine, dyes, etc.; methyl glyoxylate prepared by oxidative dehydrogenation and hydrolysis has both aldehyde and acid properties and can be used to synthesize pharmaceutical intermediates, etc. The most important industrial process for producing methyl glycolate is to prepare glycolic acid by hydrolyzing chloroacetic acid in an alkaline environment and then carrying out an esterification reaction with methanol using concentrated sulfuric acid as a catalyst. Although the reaction conditions of this method are mild, the yield of methyl glycolate is not high, and strong acids and bases are used in the preparation process, which is easy to corrode equipment and cause serious pollution. At present, the technology for preparing dimethyl oxalate from syngas is relatively mature, and the technology for hydrogenating dimethyl oxalate to prepare ethylene glycol has also been industrialized. The process conditions are mild, green and environmentally friendly, and the raw materials are cheap and easy to obtain. Therefore, relying on the existing technology, developing a process for producing methyl glycolate, an intermediate product of hydrogenation of dimethyl oxalate from syngas, is the best choice for producing methyl glycolate at present.

[0003] At present, silver-based catalysts show excellent catalytic activity in the hydrogenation of dimethyl oxalate to prepare methyl glycolate and have broad industrial application prospects. Compared with copper-based catalysts, silver has weaker hydrogenation ability, so that the hydrogenation reaction of dimethyl oxalate can stop at the first step. In addition, the adsorption of methyl glycolate by silver species is relatively weak compared with copper-based catalysts, which prevents the further hydrogenation of the product methyl glycolate to form by-product ethylene glycol. Therefore, the selectivity of methyl glycolate using silver-based catalysts can reach more than 85%. However, at present, silver-based catalysts have the following problems: 1. In order to ensure that silver species exist as metallic silver and show excellent catalytic activity, the silver loading in silver-based catalysts is generally high, which is expensive and has a high cost; 2. The Tamman temperature of silver is only 345 °C. High temperature causes silver particles to easily migrate, and silver species catalysts are prone to agglomerate into large particles during use, resulting in poor catalyst stability and easy deactivation, which hinders their industrial application.

[0004] The present invention aims to solve the above problems. Summary of the Invention

[0005] The present invention aims to overcome the deficiencies of the prior art. Aiming at the problems of low stability of silver-based catalysts and low selectivity of methyl glycolate in the hydrogenation of dimethyl oxalate to methyl glycolate, the present invention first prepares a bifunctional support by the Coulomb force attachment method to improve the dispersion of cerium oxide. When further loading silver species by the Coulomb force attachment method, a coordination reagent is added to enable the silver species to selectively locate on the cerium oxide nanodomains. When the silver-based catalyst of the present invention is used in the reaction of hydrogenating dimethyl oxalate to methyl glycolate, it has the characteristics of high product orientation selectivity, good activity, and excellent stability, and is suitable for industrial applications.

[0006] The technical solutions adopted by the present invention are as follows:

[0007] In the first aspect, the present invention provides a silver-based catalyst with a bifunctional support. The silver-based catalyst is prepared by a two-step Coulomb force attachment method. The silver-based catalyst includes a support and an active component. The active component is silver species, and the support is a bifunctional support. The bifunctional support includes cerium oxide and silica. The structure of the silver-based catalyst is that cerium oxide is dispersed on the surface of silica nanoparticles in the form of nanodomains, and the silver species are located on the cerium oxide nanodomains in the form of nanoparticles.

[0008] The preparation of the silver-based catalyst by the two-step Coulomb force attachment method means that a bifunctional support is first prepared by the Coulomb force attachment method, and then the silver-based catalyst is prepared using the bifunctional support as a raw material.

[0009] Preferably, the silver species account for 1-10 wt.% of the total mass of the silver-based catalyst, and the bifunctional support accounts for 90-99 wt.% of the total mass of the silver-based catalyst. Cerium oxide accounts for 1-12 wt.% of the total mass of the bifunctional support, and silica accounts for 88-99 wt.% of the total mass of the bifunctional support.

[0010] Preferably, the active species silver exists in the form of metal or metal oxide, the cerium exists in the form of cerium oxide, and the silica exists in the form of amorphous.

[0011] The specific surface area of the silver-based catalyst is 50-200 m 2 / g; the average pore volume is 0.7-1.4 cm 3 / g, and the average pore diameter is 10-30 nm.

[0012] Preferably, the particle size of the silver species is 1-20 nm, and the size of the cerium oxide nanodomains is 1-25 nm. Preferably, the particle size of the silver species is 1-5 nm, and the size of the cerium oxide nanodomains is 2-6 nm. More preferably, the particle size of the silver species is 4-5 nm, and the size of the cerium oxide nanodomains is 4-6 nm.

[0013] The second aspect of the present invention provides a method for preparing the silver-based catalyst described in the first aspect of the present invention. The silver-based catalyst is prepared by the Coulomb force attachment method, which includes the following steps:

[0014] (1) Dissolve the silver precursor salt in deionized water, add a complexing agent, adjust the pH of the solution to 6.5 - 7.5, and stir to obtain solution A;

[0015] (2) Dissolve the bifunctional support in deionized water, adjust the pH of the solution to 6.5 - 7.5, and keep stirring vigorously to obtain solution B;

[0016] (3) Keep in a light-shielded environment, pump solution A into the vigorously stirred solution B, and then continue stirring to obtain an opaque solution C with solid particles;

[0017] (4) Filter solution C by vacuum filtration, wash it with deionized water, obtain a filter cake solid, place it in a vacuum oven for drying to obtain solid D;

[0018] (5) Grind and calcine solid D to obtain the silver-based catalyst.

[0019] Preferably, the silver precursor salt is one or more of silver nitrate, silver phosphate, silver lactate, or silver acetate; the complexing agent is any one of sodium thiosulfate, ammonium thiosulfate, potassium thiocyanate, ammonium thiocyanate, disodium ethylenediaminetetraacetate, or potassium iodide.

[0020] Preferably, the molar ratio of the silver precursor salt to the bifunctional support is 1:15 - 20; the stirring time in step (1) is 5 - 10 min; in step (3), solution A is slowly pumped into the vigorously stirred solution B over 8 - 12 h, and then continue stirring for 8 - 12 h; in step (4), wash with deionized water 3 - 6 times, the drying temperature is 60 - 120 °C, and the drying time is 2 - 24 h; in step (5), the calcination temperature is 300 - 600 °C, and the calcination time is 2 - 8 h.

[0021] Preferably, the bifunctional support is prepared by the Coulomb force attachment method, which includes the following steps:

[0022] (a) Add the silicon source to deionized water, ultrasonicate and then keep stirring vigorously to obtain a solid-containing solution A;

[0023] (b) Add the cerium precursor salt to the above solution A and stir to obtain a solid-containing solution B;

[0024] (c) Add an alkali solution to solution B to adjust the pH of the solution to 8.5 - 9.0, and stir to obtain a solid-containing solution C;

[0025] (d) Immediately filter solution C by suction filtration to obtain a filter cake solid, place it in a vacuum oven for drying to obtain solid D;

[0026] (e) Grind the solid D and then calcine it in a muffle furnace to obtain the bifunctional support.

[0027] Preferably, the cerium precursor salt is any one of cerium nitrate, cerium sulfate or ammonium cerium nitrate; the silicon source is any one of hydrophilic fumed silica, lipophilic fumed silica or nano-silica powder; the alkali solution is any one of ammonia water, ammonium carbonate solution, ammonium bicarbonate solution, sodium carbonate solution, sodium bicarbonate solution, dilute sodium hydroxide solution.

[0028] Preferably, the ultrasonic time in step (a) is 10 - 15 min; the stirring time in step (a) is 5 - 10 min; the stirring time in step (c) is 5 - 10 min; the drying temperature in step (d) is 60 - 120 °C and the drying time is 2 - 24 h; the calcination temperature in step (e) is 300 - 600 °C and the calcination time is 2 - 8 h.

[0029] The third aspect of the present invention provides an application of the silver-based catalyst described in the first aspect of the present invention, using the silver-based catalyst to catalytically hydrogenate dimethyl oxalate selectively to produce methyl glycolate;

[0030] Specifically: Pass the mixed vaporized dimethyl oxalate and hydrogen into a reactor encapsulated with the silver-based catalyst for reaction;

[0031] The reaction pressure is 0.5 - 3.5 MPa; the reaction temperature is 200 - 240 °C; the mass space velocity of dimethyl oxalate is 0.1 - 4.0 h -1 ; the molar ratio of hydrogen to ester is 60 - 150.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention prepares a silver-based bifunctional support catalyst by a two-step Coulomb force attachment method. By regulating the silver source, cerium source, silicon source, coordination reagent and cerium loading amount, the sizes of silver particles and cerium oxide nanodomains are controlled, so that the size of nano-ceria is within 4 - 6 nm and the size of silver particles is within 4 - 5 nm. The sizes of nano-ceria and silver particles are appropriate, and the regulation method is simple and easy to implement.

[0034] 2. When the silver-based catalyst of the present invention is used in the heterogeneous hydrogenation reaction of dimethyl oxalate, it has high stability. The reason is that cerium oxide is uniformly dispersed on silica in the form of nano-islands. Silica provides a large specific surface area, which is conducive to the dispersion of cerium oxide nano-islands. And Ag species are located on the cerium oxide nano-islands. Cerium oxide is a strong interaction carrier for silver, providing the function of anchoring silver species and preventing the sintering of silver particles. Silica is a weak interaction carrier and is not easily aged through single-atom migration, thus preventing the Ostwald ripening of metallic silver. This structure of the silver-based catalyst ensures that there is no problem of silver particle growth during long-term use, thereby improving its stability and greatly reducing the catalyst replacement cost in industry.

[0035] 3. In the present invention, when loading silver species using the Coulomb force attachment method, the best coordination reagent - disodium ethylenediaminetetraacetate is screened out, and silver coordination anions are successfully constructed, enabling the silver species to selectively locate on the cerium oxide nano-islands, thereby improving the distribution of silver species and further enhancing the stability of the catalyst.

[0036] 4. The silver-based catalyst in the present invention has good effects when applied to the hydrogenation reaction of dimethyl oxalate for the selective hydrogenation to produce the intermediate methyl glycolate. Among them, when the reaction temperature is 225 °C, the reaction pressure is 2.5 MPa, and the feed hydrogen-to-ester ratio is 80, the yield of the intermediate methyl glycolate reaches 94%, and the space-time yield per unit mass of silver is as high as 86.9 g·g Ag -1 ·h -1 . The space-time yield per unit mass of silver of the reported silver-silica catalysts is less than 10 g·g Ag -1 ·h -1 , and the space-time yield per unit mass of silver of the catalysts with cerium promoter added is less than 1 g·g Ag -1 ·h -1 . Therefore, the catalyst described in the present invention has excellent catalytic activity and a high yield of methyl glycolate. At the same time, the raw materials for catalyst preparation are easily available, the reaction conditions are mild, the structure is stable, the operability is strong, and it has good industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is the high-resolution TEM image of the bifunctional support in Example 1;

[0038] Figure 2 is the high-resolution TEM image of the bifunctional support in Example 2;

[0039] Figure 3 is the high-resolution TEM image of the catalyst in Example 3;

[0040] Figure 4High-resolution TEM images of the catalyst in Example 4;

[0041] Figure 5 High-resolution TEM images of the catalyst in Example 8;

[0042] Figure 6 High-resolution TEM images of the bifunctional support in Example 16;

[0043] Figure 7 Stability evaluation results of the catalysts in Comparative Example 1 and Examples 1-4 of the present invention for the hydrogenation of dimethyl oxalate to methyl glycolate (switch to nitrogen at 120 h and rapidly heat up to 450 °C for thermal shock for 12 h, and then resume the evaluation conditions). Detailed implementation mode

[0044] The present invention will be further described below through examples, not limited to this example. For the experimental methods without specific conditions in the examples, they are usually carried out according to the conventional conditions and the conditions described in the manual, or the general equipment, materials, reagents, etc. recommended by the manufacturer. If there is no special description, they can all be obtained from commercial channels.

[0045] The methods for in-situ reduction and catalytic effect evaluation of the catalysts in this example and the comparative example are as follows:

[0046] The hydrogenation reaction of dimethyl oxalate in the present invention is carried out in a fixed-bed reactor. Load 1.0 g of the catalyst and reduce it at 250 °C in a hydrogen atmosphere of 2.5 MPa with a gas flow rate of 100 mL / min for 4 h. Cool down to the reaction temperature, vaporize the dimethyl oxalate solution and mix it with hydrogen and then enter the reaction system. The hydrogen-to-ester ratio is 80, and the hydrogenation reaction is carried out at 2.5 MPa, 225 °C and a space velocity at which the conversion rate is not 100%. Analyze the reaction products by gas chromatography and calculate the conversion rate of dimethyl oxalate, the selectivity and yield of methyl glycolate. Since the hydrogenation reaction of dimethyl oxalate is a consecutive reaction and methyl glycolate is an intermediate hydrogenation product, only when the conversion rate is not 100% can the difference in selectivity to the intermediate product be reflected. Therefore, the reaction space velocity is adjusted so that the reaction conversion rate is 98.5%; during the stability evaluation process, in order to make full use of all active sites to participate in the reaction and prevent the situation where the catalyst is deactivated but the conversion rate is still 100%, it is necessary to carry out the stability evaluation under the condition that the conversion rate is not 100%. The thermal shock experiment is used to investigate the stability of the catalyst. After the catalyst is evaluated for 120 h at a conversion rate of 98.5%, it is rapidly heated to 450 °C for thermal shock treatment for 12 h, and then the previous reaction conditions are restored and evaluated for another 380 h.

[0047] The specific experimental conditions of the catalysts in the comparative example and each example are shown in Table 1 below, and the activity data are shown in Table 2.

[0048] Comparative Example 1

[0049] Preparation of Silver-Silicon Catalyst

[0050] This comparative example is the preparation of the comparative sample 1 of the single-carrier silver-silicon catalyst, which is as follows:

[0051] Dissolve 0.2624 g of silver nitrate in 300 ml of deionized water, stir evenly for 10 min, add 4 g of hydrophilic fumed silica powder purchased commercially to the silver nitrate aqueous solution, and stir in the dark for 20 h. Remove water by rotary evaporation, place the obtained catalyst precursor in an oven at 110 °C overnight to dry and remove residual moisture. Finally, heat the catalyst precursor to 500 °C in an air atmosphere and calcine for 4 h to obtain the catalyst.

[0052] The catalyst of this comparative example was reacted under the above online reduction conditions and catalyst evaluation method for 1.2 h -1 The yield decreased by 38.4% after the thermal shock experiment.

[0053] Example 1

[0054] This example uses a two-step loading method to prepare a silver-cerium-silicon catalyst, which is as follows:

[0055] Preparation of bifunctional carrier:

[0056] Dissolve 1.4157 g of ammonium cerium nitrate in 300 ml of deionized water, stir evenly for 10 min, add 4 g of hydrophilic fumed silica powder purchased commercially to the ammonium cerium nitrate aqueous solution, and stir for 20 h. Remove water by rotary evaporation, place the obtained bifunctional carrier precursor in an oven at 110 °C overnight to dry and remove residual moisture. Finally, heat the catalyst precursor to 500 °C in an air atmosphere and calcine for 4 h to obtain the bifunctional carrier.

[0057] Preparation of silver-loaded bifunctional carrier catalyst:

[0058] Dissolve 0.2578 g of silver acetate in 300 ml of deionized water, stir evenly for 10 min, add 4.0 g of the above-prepared cerium oxide-silica bifunctional carrier powder to the silver acetate aqueous solution, keep it in the dark and stir for 20 h. Remove water by rotary evaporation, place the obtained catalyst precursor in an oven at 110 °C overnight to dry and remove residual moisture. Finally, heat the catalyst precursor to 400 °C in an air atmosphere and calcine for 4 h to obtain the catalyst.

[0059] The catalyst of this example was reacted under the above online reduction conditions and catalyst evaluation method for 0.6 h -1 The yield decreased by 65.1% after the thermal shock experiment.

[0060] Example 2

[0061] In this example, a bifunctional carrier was first prepared by the Coulomb force attachment method, and then a silver-cerium-silicon catalyst was prepared by the loading method, specifically as follows:

[0062] Preparation of bifunctional carrier:

[0063] Dissolve 4 g of hydrophilic fumed silica in 300 mL of deionized water. After ultrasonic dispersion for 15 min, continuously stir. Weigh 1.4157 g of ammonium cerium nitrate powder and add it to the above solution, and monitor the real-time pH. Dropwise add the pre-prepared 0.25 wt.% ammonia water to the solution, adjust the pH to 8.7, and stir for 3 min while timing. Immediately perform suction filtration after completion, place the obtained filter cake in a vacuum oven and dry at 120 °C for 12 h. Take out the solid, grind it, spread it flat in a quartz boat, place it in a muffle furnace, heat up to 500 °C, and calcine for 4 h to obtain the bifunctional carrier.

[0064] Preparation of the silver-loaded bifunctional carrier catalyst: The same as Example 1.

[0065] The catalyst of this example was reacted under the above online reduction conditions and catalyst evaluation method for 2.4 h -1 The yield decreased by 25.4% after the thermal shock experiment.

[0066] Example 3

[0067] In this example, a silver-cerium-silicon catalyst was prepared by the two-step Coulomb force attachment method, specifically as follows:

[0068] Preparation of bifunctional carrier:

[0069] Dissolve 4 g of hydrophilic fumed silica in 300 mL of deionized water. After ultrasonic dispersion for 15 min, continuously stir. Weigh 1.4157 g of ammonium cerium nitrate powder and add it to the above solution, and monitor the real-time pH. Dropwise add the pre-prepared 0.25 wt.% ammonia water to the solution, adjust the pH to 8.7, and stir for 3 min while timing. Immediately perform suction filtration after completion, place the obtained filter cake in a vacuum oven and dry at 120 °C for 12 h. Take out the solid, grind it, spread it flat in a quartz boat, place it in a muffle furnace, heat up to 500 °C, and calcine for 4 h to obtain the bifunctional carrier.

[0070] Preparation of the silver-loaded bifunctional carrier catalyst:

[0071] Weigh 0.2578 g of silver acetate, add it to 100 mL of deionized water, stir in the dark until the solution is clear, add a small amount of dilute nitric acid to adjust the pH to 7.0 to obtain a precursor solution; weigh 4 g of the bifunctional support, add it to 300 mL of deionized water, add a small amount of dilute sodium hydroxide solution to adjust the pH to 7.0; under the overall dark condition, slowly drip the silver precursor solution through a peristaltic pump at a rate of 10 ml / h, and stir for another 10 h after the dripping is completed; filter the above solid-containing solution, wash it 5 times with deionized water to obtain a filter cake solid, place it in a vacuum oven and dry it at 80 °C for 12 h, grind the obtained solid into powder with an agate mortar and spread it flat in a quartz boat, place it in a muffle furnace, heat it to 400 °C, and calcine it for 4 h to obtain the silver-based catalyst.

[0072] The catalyst of this example was reacted under the above online reduction conditions and catalyst evaluation method for 1.8 h -1 The yield decreased by 46.0% after the thermal shock experiment.

[0073] Example 4

[0074] In this example, a silver-cerium-silicon catalyst was prepared by a two-step Coulomb force attachment method. The difference from Example 3 is that when loading silver species by the Coulomb force attachment method, a silver anion complex (coordination reagent) was selected to make the silver species better dispersed on the cerium oxide nanodots. Specifically as follows:

[0075] Preparation of the bifunctional support: The same as in Example 3.

[0076] Preparation of the bifunctional support catalyst loaded with silver:

[0077] Weigh 0.2578 g of silver acetate and 0.5750 g of disodium ethylenediaminetetraacetate dihydrate, add them to 100 mL of deionized water, stir in the dark until the solution is clear, add a small amount of dilute nitric acid to adjust the pH to 7.0 to obtain a precursor solution; weigh 4 g of the bifunctional support, add it to 300 mL of deionized water, add a small amount of dilute sodium hydroxide solution to adjust the pH to 7.0; under the overall dark condition, slowly drip the silver precursor solution through a peristaltic pump at a rate of 10 ml / h, and stir for another 10 h after the dripping is completed; filter the above solid-containing solution, wash it 5 times with deionized water to obtain a filter cake solid, place it in a vacuum oven and dry it at 80 °C for 12 h, grind the obtained solid into powder with an agate mortar and spread it flat in a quartz boat, place it in a muffle furnace, heat it to 400 °C, and calcine it for 4 h to obtain the silver-based catalyst.

[0078] The catalyst of this example was reacted under the above online reduction conditions and catalyst evaluation method for 3.0 h -1 The yield decreased by 7.82% after the thermal shock experiment.

[0079] Example 5

[0080] This example is the preparation of the silver-cerium-silica catalyst sample 5 of the bifunctional support, which has no difference from the preparation of the silver-cerium-silica catalyst sample 4 in Example 4, except that the space velocity of the reaction evaluation is different, as follows:

[0081] The catalyst of this example was reacted under the above online reduction conditions and catalyst evaluation method at 2.7 h -1 below.

[0082] From Comparative Example 1 and Example 1, it can be seen that the construction of the bifunctional support helps to improve the stability. In Comparative Example 1, a single support silica was used, which is a weak interaction support for silver. Therefore, the valence state of silver is low, the hydrogenation activity is good, and the yield of methyl glycolate is high. However, the silver particle size is large and the silver specific surface area is small. As Figure 7 shown, a thermal shock experiment was used to investigate the stability of the catalyst. After purging with nitrogen at 450 °C for 12 h and then evaluating, the catalyst was rapidly deactivated after being thermally shocked. In Example 1, cerium element was introduced into the silver-silica catalyst. Since cerium oxide has a strong interaction with silver, the valence state of silver is relatively high and the hydrogenation activity decreases, but the directional selectivity of methyl glycolate is improved. The space-time yield per unit mass of silver of methyl glycolate decreased significantly, and the performance of the catalyst was sacrificed. However, the catalyst successfully constructed a bifunctional support structure, large-sized cerium oxide nanoislands were dispersed on silica, and silver was partially dispersed on the cerium oxide nanoislands and partially dispersed on the silica support. Cerium oxide anchors silver species through strong interaction forces and prevents their further sintering and agglomeration. Therefore, as Figure 7 shown, in the thermal shock experiment, the catalyst in Example 1 was significantly better than that in Comparative Example 1. Therefore, although the construction of the bifunctional support sacrificed some performance of the catalyst, the stability was greatly improved.

[0083] From Example 1 and Example 2, it can be seen that the use of the Coulomb force attachment method to prepare the bifunctional support can effectively improve the dispersion of cerium oxide nanoislands and silver particles. As Figure 1-2 shown, the cerium oxide size in Example 1 was 6.9 nm, and the cerium oxide size in Example 2 was 4.6 nm. Example 2 used the Coulomb force attachment method, and the size of the prepared cerium oxide nanoislands decreased. Therefore, the silver particle size loaded on the nanoislands decreased, the dispersion increased, the exposed silver specific surface area increased, the number of active sites increased, and the catalytic activity increased. At the same time, more silver particles were loaded on the cerium oxide nanoislands, and the stability of the catalyst was further improved, as Figure 7 shown. However, the silver particle size on the silica support was still relatively large, and these silver particles would still sinter and grow during the thermal shock experiment. Therefore, the performance of the catalyst still decreased partially.

[0084] It can be seen from Examples 2-4 that when the silver species are loaded by the Coulomb force attachment method, selecting silver anion complexes enables the silver species to be better dispersed on the ceria nanodots. In Example 3, no coordination reagent was added during the preparation of the catalyst, and silver participated in the reaction as hydrated silver ions. As Figure 3 shown, large-sized silver species are dispersed on silica rather than ceria. Since the isoelectric point of ceria is higher than that of silica, within the isoelectric point range of the two, cations are preferentially attached to silica under the action of the Coulomb force. After calcination, silver particles are mainly distributed on silica, so the particle size of silver particles is relatively large, and the space-time yield per unit mass of silver for methyl glycolate decreases, and the catalyst is significantly deactivated during the evaluation process. In Example 4, disodium ethylenediaminetetraacetate with a molar ratio of 1:1 was added during the preparation of the catalyst to form silver anion complexes. As Figure 4 shown, the silver species are all loaded on ceria. The silver species are preferentially attached to ceria and are restricted by the size of the ceria nanodots, so the size of silver particles is further reduced. Therefore, the activity of the catalyst is improved, and the space-time yield per unit mass of silver for methyl glycolate further increases. As Figure 7 shown, compared with Examples 2-3, this catalyst has excellent stability, and the catalyst still maintains its initial activity after thermal shock. Therefore, selecting silver anion complexes enables the silver species to be better dispersed on the ceria nanodots, thereby improving the performance and stability of the catalyst.

[0085] It can be seen from Example 4 and Example 5 that in Example 5, the catalyst was evaluated under the condition of a conversion rate of 100%, the selectivity of the catalyst was 93.7%, and the space-time yield per unit mass of silver was 79.1 g·g Ag -1 ·h -1 , slightly lower than the space-time yield per unit mass of silver of 86.9 g·g Ag -1 ·h -1 of the catalyst in Example 4. This indicates that appropriately reducing the space velocity can enable the complete conversion of dimethyl oxalate, thereby reducing the separation cost of the product.

[0086] Examples 6-9

[0087] The effects of different coordination reagents on the particle size and distribution of silver-ceria particles and on the activity of the catalyst were investigated.

[0088] Compared with Example 4, only the coordination reagent and the space velocity are different. The specific experimental conditions are shown in Table 1 below, and the activity data are shown in Table 2.

[0089] Examples 10-11

[0090] The effects of different cerium sources on the particle size of silver-ceria particles and on the activity of the catalyst were investigated.

[0091] It is only different from Example 4 in terms of the cerium source and the space velocity. The specific experimental conditions are shown in Table 1 below, and the activity data are shown in Table 2.

[0092] Examples 12 - 14

[0093] The effects of different silver sources on the particle size of silver - cerium particles and on the catalyst activity were investigated.

[0094] It is only different from Example 4 in terms of the silver source and the space velocity. The specific experimental conditions are shown in Table 1 below, and the activity data are shown in Table 2.

[0095] Example 15

[0096] The effects of different silicon sources on the particle size of silver - cerium particles and on the catalyst activity were investigated.

[0097] It is only different from Example 4 in terms of the silver - cerium loading and the space velocity. The specific experimental conditions are shown in Table 1 below, and the activity data are shown in Table 2.

[0098] Example 16

[0099] The effects of different cerium loadings on the particle size of silver - cerium particles and on the catalyst activity were investigated.

[0100] It is only different from Example 4 in terms of the silver - cerium loading and the space velocity. The specific experimental conditions are shown in Table 1 below, and the activity data are shown in Table 2.

[0101] Table 1 Preparation conditions of each catalyst and particle sizes of silver and cerium oxide

[0102]

[0103]

[0104] Table 2 Activity data of each catalyst

[0105]

[0106]

[0107] As can be seen from Examples 4 and 6-9 in Table 1-2, disodium ethylenediaminetetraacetate has the best effect as a complexing reagent. The complexing reagents used in Examples 6-9 all contain sulfur elements. Examples 6 and 8 contain thiosulfate, which decomposes upon heating to form sulfur dioxide and sulfide ions. Sulfide ions and silver elements are likely to form silver sulfide, which has a stable structure and is not easily removed after calcination at 400 °C. Therefore, the valence state of silver particles in the catalyst increases, the hydrogenation ability decreases, and the activity decreases. The complexing reagents in Examples 7 and 9 contain thiocyanate, and a small amount of sulfur will remain after calcination, so the activity will also decrease. Examples 8-9 contain ammonium ions, which are prone to hydrolysis in aqueous solution to form ammonia water. Ammonia can coordinate with silver ions to form cationic complexes, and the stability constants are both 10 -7 orders of magnitude. Taking Example 8 as an example, as Figure 5 shown, small-sized silver species are loaded on cerium oxide, and large-sized silver species are loaded on silica. Since silica is a weakly interacting support and cannot anchor silver species, some agglomeration occurs during the reaction process, and its stability cannot reach the expected level. In Example 4, disodium ethylenediaminetetraacetate is used as a complexing reagent, which can be completely removed during the calcination process, and all silver species are loaded on cerium oxide. The silver particle size is appropriate, and its activity and stability are the best. Therefore, disodium ethylenediaminetetraacetate has the best effect as a complexing reagent.

[0108] As can be seen from Examples 4 and 10-11 in Table 1-2, ammonium cerium nitrate has a better effect as a cerium source. Compared with cerium nitrate, ammonium cerium nitrate has a higher nitrogen content. Therefore, more gases are generated during calcination in a muffle furnace, which promotes the dispersion of cerium oxide, and the size of cerium oxide nanoislands is smaller. Cerium sulfate is not easily decomposed during calcination, and the residual sulfate ions form silver sulfate with silver, which increases the silver valence state, decreases the hydrogenation ability, and the catalyst activity is low. Therefore, ammonium cerium nitrate has a better effect as a cerium source.

[0109] As can be seen from Examples 4 and 12-14 in Table 1-2, silver acetate has the best effect as a silver source. Silver acetate and silver lactate can be completely decomposed after calcination at 400 °C and generate a large amount of gas, which is beneficial to the dispersion of silver species. Silver phosphate cannot be decomposed at this temperature, and silver nitrate decomposes incompletely, so the silver valence is relatively high and the catalyst activity is low. Silver acetate has a higher solubility and good stirring and dispersion effect. Therefore, silver acetate has the best effect as a silver source.

[0110] As can be seen from Examples 4 and 15 in Table 1-2, hydrophilic fumed silica has the best effect as a silicon source. Compared with lipophilic fumed silica, hydrophilic fumed silica has a better dispersion in water, and cerium species can better adhere to the surface of silica. Therefore, in Example 4, the size of cerium oxide nanoislands is smaller, and the silver dispersion is also improved, and the catalyst has excellent activity. Therefore, hydrophilic fumed silica has the best effect as a silicon source.

[0111] As can be seen from Examples 4 and 16 in Table 1-2, the best effect is achieved when the cerium loading is 10 wt.%. In Example 15, the cerium oxide loading is 2 wt.%. As Figure 6 shown, the size of the prepared cerium oxide nanoislands is only 2.4 nm, and the particle size of silver species is reduced to 2.1 nm. The too small particle size of silver particles leads to an increase in the overall valence state and a decrease in the catalyst activity. Therefore, the best effect is achieved when the cerium loading is 10 wt.%.

[0112] The preparation method of the bifunctional support catalyst disclosed in the present invention has the characteristics of mild conditions, simple preparation process, easy availability of raw materials, low cost, excellent catalytic performance, etc. When actually applied to industry, it can be used alone for the production of methyl glycolate with high economic value and high demand, and can still maintain a yield of more than 90% at a relatively high space velocity. The space-time yield per unit mass of silver is as high as 86.9 g·g Ag -1 ·h -1 . In addition, the silver species are located on the cerium oxide nanoislands by the Coulomb force attachment method, giving full play to the characteristics of the bifunctional support structure, greatly improving the stability of the catalyst, significantly increasing the service life of the catalyst, reducing the losses caused by replacing the catalyst in industry, and reducing its industrialization cost.

[0113] The above is an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification or equivalent substitution that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

Claims

1. A silver-based catalyst with a dual-functional carrier, characterized in that, The silver-based catalyst includes a support and an active component. The active component is a silver species, and the support is a bifunctional support. The bifunctional support includes cerium oxide and silica. The structure of the silver-based catalyst is that cerium oxide is dispersed on the surface of silica nanoparticles in the form of nano-islands, and the silver species are located on the cerium oxide nano-islands in the form of nanoparticles. The silver-based catalyst is prepared by a two-step Coulomb force attachment method: first, a bifunctional support is prepared by the Coulomb force attachment method, and then the silver-based catalyst is prepared using the bifunctional support as a raw material. The preparation of the bifunctional support by the Coulomb force attachment method includes the following steps: (a) Add a silicon source to deionized water, ultrasonicate, and keep stirring to obtain solution A. The silicon source is hydrophilic fumed silica. (b) Add a cerium precursor salt to the above solution A and stir to obtain solution B. The cerium precursor salt is any one of cerium nitrate or ammonium cerium nitrate. (c) Add an alkali solution to solution B to adjust the solution pH to 8.5 - 9.0, and stir to obtain solution C. (d) Filter solution C by suction to obtain a filter cake solid, dry it to obtain solid D. (e) Grind solid D and place it in a muffle furnace for calcination to obtain the bifunctional support. The preparation of the silver-based catalyst using the bifunctional support as a raw material includes the following steps: (1) Dissolve a silver precursor salt in deionized water, add a complexing agent, adjust the solution pH to 6.5 - 7.5, and stir to obtain solution E. The silver precursor salt is one or several of silver nitrate, silver lactate, or silver acetate. The complexing agent is disodium ethylenediaminetetraacetate. (2) Dissolve the bifunctional support in deionized water, adjust the solution pH to 6.5 - 7.5, and keep stirring to obtain solution F. (3) Keep in a light-proof environment, pump solution E into the stirred solution F, and continue stirring to obtain an opaque solution G with solid particles. (4) Filter solution G by vacuum suction, wash it with deionized water, obtain a filter cake solid, dry it to obtain solid H. (5) Grind solid H and calcine it to obtain the silver-based catalyst.

2. The silver-based catalyst according to claim 1, wherein The silver species exist in the form of metal or metal oxide, and the silica exists in an amorphous form. The specific surface area of the silver-based catalyst is 50-200 m 2 / g; the average pore volume is 0.7-1.4 cm 3 / g, and the average pore diameter is 10-30 nm; The silver species account for 1 - 10 wt% of the total mass of the silver-based catalyst, and the bifunctional support accounts for 90 - 99 wt% of the total mass of the silver-based catalyst. Cerium oxide accounts for 1 - 12 wt% of the total mass of the bifunctional support, and silica accounts for 88 - 99 wt% of the total mass of the bifunctional support.

3. The silver-based catalyst according to claim 1, wherein The particle size of the silver species is 1 - 20 nm, and the size of the cerium oxide nano-islands is 1 - 25 nm.

4. The silver-based catalyst according to claim 1, characterized in that, The particle size of the silver species is 1 - 5 nm, and the size of the cerium oxide nano-islands is 2 - 6 nm.

5. The silver-based catalyst according to claim 1, wherein The particle size of the silver species is 4 - 5 nm, and the size of the cerium oxide nano-islands is 4 - 6 nm.

6. The silver-based catalyst according to claim 1, wherein The mass ratio of the silver precursor salt to the bifunctional support is 1:15 to 20; the stirring time in step (1) is 5 - 10 min; in step (3), solution E is slowly pumped into the vigorously stirred solution F over 8 - 12 h, and then stirring is continued for 8 - 12 h; in step (4), washing is carried out 3 - 6 times with deionized water, the drying temperature is 60 - 120 °C, and the drying time is 2 - 24 h; in step (5), the calcination temperature is 300 - 600 °C, and the calcination time is 2 - 8 h.

7. The silver-based catalyst according to claim 1, characterized in that, The lye described in step (c) is any one of ammonia water, ammonium carbonate solution, ammonium bicarbonate solution, sodium carbonate solution, sodium bicarbonate solution, and dilute sodium hydroxide solution.

8. The silver-based catalyst according to claim 1, wherein, In the steps of preparing the bifunctional support by the Coulomb force attachment method, the ultrasonic time in step (a) is 10 - 15 min; the stirring time in step (a) is 5 - 10 min; the stirring time in step (c) is 5 - 10 min; the drying temperature in step (d) is 60 - 120 °C, and the drying time is 2 - 24 h; the calcination temperature in step (e) is 300 - 600 °C, and the calcination time is 2 - 8 h.

9. Use of the silver-based catalyst according to any one of claims 1-5, characterized in that, The silver-based catalyst is used for catalytic selective hydrogenation of dimethyl oxalate to methyl glycolate; Specifically: In a reactor encapsulated with the silver-based catalyst, the mixed vaporized dimethyl oxalate and hydrogen are introduced for reaction; The reaction pressure is 0.5 - 3.5 MPa; the reaction temperature is 200 - 240 °C; the mass hourly space velocity of dimethyl oxalate is 0.1 - 4.0 h -1 ; the molar ratio of hydrogen to ester is 60 - 150.

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