Preparation method of novel catalyst of NaY molecular sieve loaded palladium and metal additive for co-production of dimethyl carbonate

Abstract

The invention belongs to the technical field of catalyst preparation, and particularly relates to a preparation method of a novel NaY molecular sieve supported palladium and metal additive catalyst for co-production of dimethyl carbonate, the catalyst mainly comprises 0.1-3.0% by mass of Pd active component, 0.1-10.0% by mass of metal additive and 0.1-10.0% by mass of NaY molecular sieve carrier. The metal auxiliary agent not only effectively eliminates acid in the catalyst, but also promotes the generation of high-valence Pd active species, and the Pd active species are stabilized by utilizing the electron withdrawing effect of the auxiliary agent on the Pd active species. The catalyst is applied to a reaction for synthesizing dimethyl carbonate by carbonylation of methyl nitrite in a process of co-producing ethylene glycol from coal and co-producing dimethyl carbonate, shows excellent catalytic performance, and has certain industrial application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation, and in particular relates to a method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal additive for the novel co-production of dimethyl carbonate. Background Art

[0002] Dimethyl carbonate (DMC), with the molecular formula C3H6O3 and the structural formula CH3OCOOCH3, is a biodegradable and environmentally friendly chemical product. It can be used as a methylation and carbonylation reagent, replacing highly toxic or carcinogenic phosgene, sulfates, and halogenated alkanes in organic synthesis reactions and is considered a "new cornerstone" of organic synthesis. In 2020, lithium battery electrolyte solvents and polycarbonate raw materials accounted for 29% and 27% of DMC's downstream demand, respectively. Coatings, adhesives, and solid phosgene accounted for a combined 41%, with exports accounting for 3%. With the growing market demand for lithium batteries and polycarbonate, it is predicted that by 2027, demand for DMC for lithium battery electrolyte solvents and polycarbonate raw materials will reach 2.25 million tons / year, demonstrating significant market potential and application prospects.

[0003] The main routes for dimethyl carbonate synthesis are direct methanol synthesis and indirect methanol synthesis via methyl nitrite (MN). Direct synthesis methods include phosgene, transesterification, urea alcoholysis, and direct synthesis from methanol and CO2. Phosgene, however, uses highly toxic and corrosive raw materials, and requires stringent catalyst conditions, and has been gradually replaced by other processes. Transesterification, compared to phosgene, is more in line with green chemical environmental requirements, consuming greenhouse gases and converting them into valuable chemicals. However, ethylene oxide and propylene oxide are expensive raw materials and homogeneous catalyst separation is difficult. Urea alcoholysis, while characterized by readily available raw materials and a short process, still faces challenges such as low yield and difficulty separating the product from the catalyst. Direct synthesis from methanol and CO2 is a route to reduce carbon emissions and increase product value, but due to the inertness of CO2, the raw material conversion rate is low, and this process is still in the laboratory research stage.

[0004] Methyl nitrite, with the chemical formula CH3ONO, is an important organic chemical intermediate. Research on the carbonylation of methyl nitrite to dimethyl carbonate was initiated by obtaining dimethyl carbonate as a by-product during the production of dimethyl oxalate by Ube Corporation of Japan. This method features readily available and inexpensive raw materials, mild reaction conditions, and no harmful emissions. Furthermore, the reaction is heterogeneously catalyzed and can be carried out in a fixed-bed reactor, making it easy to implement large-scale continuous production. However, the Pd-based catalyst currently used in this method has poor catalytic performance, which greatly limits its industrial application.

[0005] The catalysts used in the process of synthesizing dimethyl carbonate from methyl nitrite can be mainly divided into two categories: one is the Wacker-type chlorine-containing catalyst with PdCl2 and CuCl2 as active components, and the other is the Pd-based catalyst that does not contain chlorine. Chlorine-containing catalysts will be deactivated as chlorine is lost during the reaction, and chlorine needs to be regularly added to the reaction system. In addition, chlorine loss will also be accompanied by equipment corrosion, so subsequent research has gradually focused on the development of high-performance chlorine-free catalysts. The chlorine-free catalysts used in this reaction are mainly porous materials loaded with Pd, such as Pd / NaY, Pd / MOFs, Pd / MeO x Based on the characteristics of NaY molecular sieve, such as high specific surface area, which is conducive to the dispersion of precious metal Pd, low acidity, and relatively stable structure, researchers have conducted extensive research on Pd / NaY catalysts supported by NaY. However, the selectivity and stability of Pd / NaY catalysts are still poor.

[0006] The dispersion and valence of Pd species in the catalyst play a key role in the reaction of methyl nitrite to dimethyl carbonate. Highly dispersed Pd species, with a higher content of high-valence Pd species can significantly improve the selectivity of DMC, while agglomerated Pd species promote the formation of by-products (Atomic-level insights into the steric hindrance effect of single-atom Pd catalyst to boost the synthesis of dimethylcarbonate. Ji S, et al. Applied Catalysis B: Environmental. 2022, Vol. 304, p. 120922). As the reaction proceeds, the reactant CO converts the active species Pd 2+ Gradually reduced to Pd 0 , and Pd 0 The generation of NH4 in the dichlorotetraammine palladium precursor will reduce the selectivity of DMC. + It will react with Na on the NaY molecular sieve framework + Exchange occurs, and after subsequent calcination, NH3 molecules are removed to form bridged hydroxyl groups. acid sites, and during the reaction, the Acid sites can lead to inefficient decomposition of the methyl nitrite reactant, thereby reducing the selectivity of the main product DMC (Catalytic Decomposition of Methyl nitrite over Supported Palladium Catalysts in Vapor phase. Zhuo G, et al. Reaction Kinetics and Catalysis Letters. 2002, Vol. 77, pp. 219-226).

[0007] In summary, chlorine-free catalysts have a significant application prospect in the carbonylation of methyl nitrite to dimethyl carbonate in the process of coal-to-ethylene glycol co-production of dimethyl carbonate. However, during the reaction, the strongly reducing reactant CO will convert Pd 2+ The active species are gradually reduced, resulting in catalyst deactivation. In addition, the Therefore, it is necessary to develop a novel DMC with simple preparation process, low price, high stability and high selectivity. The acidic Pd / NaY catalyst is of great significance for the carbonylation of methyl nitrite to dimethyl carbonate. Summary of the Invention

[0008] In view of the above problems existing in the prior art, the purpose of the present invention is to provide a novel non-toxic and novel non-toxic product with simple preparation process, low price, high stability and high selectivity. The invention discloses a preparation method and application of a Pd / NaY catalyst having a high catalytic performance, in particular for the carbonylation reaction of methyl nitrite to synthesize dimethyl carbonate.

[0009] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0010] 1. A method for preparing a catalyst of NaY molecular sieve loaded with palladium and metal promoter for the novel co-production of dimethyl carbonate, characterized in that the preparation method adopts an efficient and simple solid phase grinding method, wherein the catalyst is composed of a Pd active component, a metal promoter and a NaY molecular sieve carrier, wherein the mass fraction of the Pd active component is 0.1% to 3.0%, and the mass fraction of the metal promoter is 0.01% to 10.0%. The metal promoter not only effectively eliminates the The acid is formed and the generation of high-valent Pd active species is promoted. The electron-withdrawing effect of the metal promoter on the Pd active species stabilizes the Pd active species, thereby preparing a high-catalytic performance NaY molecular sieve supported palladium and metal promoter catalyst, namely Me-Pd / NaY catalyst, wherein Me is the metal promoter. The catalyst is applied to the reaction of methyl nitrite carbonylation to synthesize dimethyl carbonate in the process of coal-to-ethylene glycol co-production of dimethyl carbonate. The specific surface area of ​​the catalyst is 100-800m 2 / g, the preparation method of the catalyst comprises the following steps:

[0011] 1) Synthesis of NaY molecular sieve support: 20-50 g of sodium hydroxide, 30-120 g of a silicon source, 5-80 g of an aluminum source, and 50-310 g of water were weighed and mixed in a container. The mixture was stirred at 15-80° C. for 1-12 hours to form a gel. The gel was transferred to a stainless steel reactor and crystallized in an oven at a certain temperature for a certain time. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0012] 2) Loading of the Pd active component: Pd is loaded by an ion exchange method. First, a certain concentration of a noble metal Pd precursor solution is prepared. The NaY molecular sieve support synthesized in step 1) is mixed with water to form a suspension. Then, a certain amount of the Pd precursor solution is added to the suspension. Ion exchange is performed at a certain temperature for a certain time. The suspension is then filtered, washed, and dried. The resulting solid powder is then transferred to a muffle furnace for calcination. The temperature is increased to 150-600°C at a heating rate of 0.5-10°C / min and maintained for 0.5-12 hours to obtain a Pd / NaY powder.

[0013] 3) Introducing a metal additive by solid phase grinding: Weigh a certain amount of a metal additive and a certain amount of the Pd / NaY powder obtained in step 2) and mix them uniformly, then solid phase grind them for a certain period of time to obtain a metal additive-modified Me-Pd / NaY catalyst, wherein Me is the metal additive.

[0014] As a preferred embodiment, the silicon source used in step 1) is one or a combination of silica sol, tetraethyl orthosilicate, nano-silica, sodium silicate, and ITQ-1.

[0015] As a preferred embodiment, the aluminum source used in step 1) is one or a combination of sodium metaaluminate, aluminum sol, aluminum sulfate, boehmite, aluminum nitrate, aluminum isopropoxide, and aluminum hydroxide.

[0016] As a preferred embodiment, the crystallization temperature used in step 1) is 30-190° C., and the crystallization time is 2-72 hours.

[0017] As a preferred embodiment, the noble metal precursor used in step 2) is one or a combination of palladium nitrate, palladium acetate, palladium chloride, ammonium chloropalladate, potassium chloropalladate, tetraamminepalladium chloride, tetraamminepalladium nitrate, dichlorotetraamminepalladium, and sodium chloropalladate.

[0018] As a preferred solution, the ion exchange temperature in step 2) is 30-120° C., and the ion exchange time is 0.5-15 hours.

[0019] As a preferred embodiment, the metal auxiliary agent used in the step 3) is one or a combination of LiCl, SrCl2, KCl, NaCl, MgCl2, CaCl2, CoCl2, MoCl2, Li2CO3, LiHCO3, LiNO3, Li2SO4, K2CO3, KHCO3, KNO3, K2SO4, Na2CO3, NaHCO3, NaNO3, Na2SO4, SrCO3, Sr(HCO3)2, Sr(NO3)2, SrSO4, CaCO3, Ca(HCO3)2, Ca(NO3)2, CaSO4, MgCO3, Mg(HCO3)2, Mg(NO3)2, and MgSO4.

[0020] As a preferred solution, the grinding method adopted in step 3) is one or a combination of manual grinding, ball milling, and vibration milling, and the grinding time is 0.5 to 200 minutes.

[0021] As a preferred embodiment, the reaction temperature for synthesizing dimethyl carbonate by carbonylation of methyl nitrite in the process of producing ethylene glycol from coal and co-producing dimethyl carbonate is 100-250° C., the pressure is 0.1-5.0 MPa, and the molar ratio of reactants CO:CH 3 ONO is 1:2-7.

[0022] The present invention discloses a method for preparing a novel catalyst comprising NaY molecular sieves loaded with palladium and metal promoters for the co-production of dimethyl carbonate. Compared with the prior art, the catalyst comprising NaY molecular sieves loaded with palladium and metal promoters has the following significant features:

[0023] (1) None acid sites

[0024] During the solid phase grinding process, the metal additives react with H in Si-O(H)-Al in Pd / NaY. + Solid phase ion exchange effectively eliminates the acid sites, reducing the ineffective decomposition of the reactant methyl nitrite and significantly improving the selectivity of the main product DMC. However, traditional Pd / NaY catalysts often contain Acid sites are difficult to eliminate with conventional treatment methods while maintaining high dispersion of Pd species. Acid, these acid sites will lead to ineffective decomposition of the reactant methyl nitrite.

[0025] (2) Metal additives stabilize Pd active sites

[0026] During the reaction, the strongly reducing reactant CO will convert the traditional catalyst Pd 2+ The active species are gradually reduced, resulting in catalyst deactivation. In the present invention, the metal promoter promotes the generation of high-valent Pd active species and stabilizes the high-valent Pd active species by utilizing the electron-withdrawing effect of the metal promoter on the Pd active species.

[0027] (3) The present invention adopts the solid phase grinding method to introduce the metal additive, and the preparation process is very simple. It only requires mixing and grinding the metal additive and the catalyst to be modified. There is no waste liquid discharge, the equipment requirements are simple, and it is suitable for industrial production.

[0028] The Pd-supported catalyst prepared in the present invention is an environmentally friendly catalyst with high selectivity, high conversion rate, high stability, low cost and simple preparation process, and is particularly suitable for the reaction of carbonylation of methyl nitrite to synthesize dimethyl carbonate in the process of coal-to-ethylene glycol co-production of dimethyl carbonate. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the XRD pattern of the NaY molecular sieve support prepared in Example 1;

[0030] Figure 2 The XRD pattern of the Na-Pd / NaY catalyst prepared in Example 1 is shown;

[0031] Figure 3 This is a SEM image of the Na-Pd / NaY catalyst prepared in Example 1;

[0032] Figure 4 TEM image of the Na-Pd / NaY catalyst prepared in Example 1;

[0033] Figure 5 This is the infrared image of the hydroxyl group of the Na-Pd / NaY catalyst prepared in Example 1;

[0034] Figure 6 This is the pyridine infrared image of the Na-Pd / NaY catalyst prepared in Example 1;

[0035] Figure 7 TEM image of the K-Pd / NaY catalyst prepared in Example 2;

[0036] Figure 8 This is the infrared image of the hydroxyl group of K-Pd / NaY prepared in Example 2;

[0037] Figure 9 TEM image of Li-Pd / NaY prepared in Example 3;

[0038] Figure 10 This is the infrared image of the hydroxyl group of Li-Pd / NaY prepared in Example 3;

[0039] Figure 11 This is the XRD pattern of Pd / NaY prepared in Comparative Example 1;

[0040] Figure 12 TEM image of Pd / NaY prepared in Comparative Example 1;

[0041] Figure 13 This is the infrared image of the hydroxyl group of Pd / NaY prepared in Comparative Example 1;

[0042] Figure 14 This is the pyridine infrared image of Pd / NaY prepared in Comparative Example 1;

[0043] Figure 15 TEM image of Na-Pd / NaY prepared in Comparative Example 2;

[0044] Figure 16 This is the infrared image of the hydroxyl group of Na-Pd / NaY prepared in Comparative Example 2;

[0045] Figure 17 Graph showing the CO conversion rates of the catalysts prepared in Example 1 and Comparative Example 1;

[0046] Figure 18 This is a TEM image of Na-Pd / NaY prepared in Example 1 after reaction for 60 hours. DETAILED DESCRIPTION

[0047] To facilitate understanding of the present invention, the present invention is provided below with examples. These examples are provided solely to aid understanding of the present invention and should not be construed as limiting the present invention. Since the present invention may be described and explained using other alternatives that do not depart from the technical features of the present invention, all modifications within the scope of the present invention or equivalents thereof are intended to fall within the scope of protection of the present invention.

[0048] The present invention is further described below with reference to embodiments, comparative examples and application examples.

[0049] Example 1

[0050] 1) Synthesis of NaY molecular sieve support: 20 g of sodium hydroxide, 50 g of water, 5 g of sodium metaaluminate, and 30 g of silica sol were mixed and added to a container. The mixture was stirred at 15°C for 1 hour to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 30°C for 72 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0051] 2) Loading of Pd active component: Prepare 1 mol / L potassium chloropalladate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L potassium chloropalladate solution to the molecular sieve suspension, heat to 80°C in a water bath and stir for 6 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 400°C at a heating rate of 0.5°C / min and calcined for 2 hours to obtain Pd / NaY powder.

[0052] 3) Solid-phase grinding method to introduce metal additives: 0.01 g of NaCl was mixed with 1 g of the Pd / NaY catalyst prepared in step 2), and ground manually in a mortar for 2 minutes to obtain a Na-Pd / NaY catalyst modified with a metal additive.

[0053] Figure 1 This is the XRD pattern of the NaY molecular sieve carrier prepared in Example 1. Figure 2 This is the XRD pattern of Na-Pd / NaY modified with the metal additive prepared in Example 1. It has the same XRD pattern as the NaY molecular sieve support prepared in Example 1, and no Pd-related diffraction peak appears, indicating that the Pd species is highly dispersed in the NaY molecular sieve. Figure 3 This is the SEM image of the metal additive-modified Na-Pd / NaY prepared in Example 1. Figure 4 This is a TEM image of the modified catalyst prepared in Example 1 after the reaction. No agglomerated Pd particles were seen under high-magnification TEM, indicating that the Pd species in the catalyst are highly dispersed. Figure 5 This is the infrared image of the hydroxyl group of the modified catalyst prepared in Example 1. It can be observed that the sample has a hydroxyl group at 3628 cm -1 The band at the end of the spectrum disappears, indicating that the sample has no Si-O(H)-Al bridge hydroxyl group. acid. Figure 6 This is the pyridine infrared image of the modified catalyst prepared in Example 1. It can be observed that the sample has a 1542 cm -1 The band at disappeared and was not detected in the sample. According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example is 2.7%, and the mass fraction of the metal additive is 0.4%.

[0054] Example 2

[0055] 1) Synthesis of NaY molecular sieve support: 25 g of sodium hydroxide, 100 g of water, 15 g of sodium metaaluminate, and 7 g of nano-silica were mixed and added to a container. The mixture was stirred at 30°C for 2 hours to form a gel. The gel was transferred to a stainless steel reactor and crystallized in an oven at 100°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0056] 2) Loading of Pd active component: Prepare 0.1 mol / L tetraamminepalladium chloride solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 5 mL of 0.1 mol / L tetraamminepalladium chloride solution to the molecular sieve suspension, heat to 70°C in a water bath and stir for 0.5 hour, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 400°C at a heating rate of 10°C / min and calcined for 2 hours to obtain Pd / NaY powder.

[0057] 3) Solid-phase grinding method for introducing metal additives: 0.01 g of KCl was mixed with 1 g of the Pd / NaY catalyst prepared in step 2) and ground manually in a mortar for 0.5 minutes to obtain a K-Pd / NaY catalyst modified with a metal additive.

[0058] Figure 7 This is a TEM image of the K-Pd / NaY modified with a metal additive prepared in Example 2 after reaction. No agglomerated Pd particles were observed under high-magnification TEM, indicating that the Pd species in the catalyst were highly dispersed. Figure 8 This is the infrared image of the hydroxyl group of K-Pd / NaY prepared in Example 2. It can be observed that the sample has a -1 The band at the end of the spectrum disappears, indicating that the sample has no Si-O(H)-Al bridged hydroxyl group and the catalyst has no According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example is 1.4%, and the mass fraction of the metal additive is 0.5%.

[0059] Example 3

[0060] 1) Synthesis of NaY molecular sieve support: 30 g of sodium hydroxide, 150 g of water, 25 g of aluminum sol, and 50 g of silica sol were mixed and added to a container. The mixture was stirred at 80°C for 8 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0061] 2) Loading of Pd active component: Prepare 1 mol / L tetraammine palladium nitrate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L tetraammine palladium nitrate solution to the molecular sieve suspension, heat to 50°C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 400°C at a heating rate of 2°C / min and calcined for 2 hours to obtain Pd / NaY powder.

[0062] 3) Introduction of metal promoter by solid phase grinding method: 0.015 g of LiCl was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground in a ball mill for 10 minutes to obtain a Li-Pd / NaY catalyst modified with a metal promoter.

[0063] Figure 9 This is a TEM image of the modified catalyst prepared in Example 3 after the reaction. No agglomerated Pd particles were seen under high-magnification TEM, indicating that the Pd species in the catalyst are highly dispersed. Figure 10 This is the infrared image of the hydroxyl group of the modified catalyst prepared in Example 3. It can be observed that the sample has a hydroxyl group at 3628 cm -1 The band at the end of the spectrum disappears, indicating that the sample has no Si-O(H)-Al bridged hydroxyl group and the catalyst has no According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example is 2.5%, and the mass fraction of the metal additive is 0.1%.

[0064] Example 4

[0065] 1) Synthesis of NaY molecular sieve support: 35 g of sodium hydroxide, 225 g of water, 45 g of boehmite, and 80 g of silica sol were mixed and added to a container. The mixture was stirred at 50°C for 8 hours to form a gel. The gel was transferred to a stainless steel reactor and crystallized in an oven at 190°C for 12 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0066] 2) Loading of Pd active component: A 1 mol / L dichlorotetraamminepalladium solution was prepared, 20 g of water and 4 g of the molecular sieve support obtained in step 1) were mixed to form a molecular sieve suspension, 0.5 mL of 1 mol / L dichlorotetraamminepalladium solution was added to the molecular sieve suspension, and the mixture was heated to 90°C in a water bath and stirred for 15 hours. The mixture was then filtered, washed, and dried. The resulting solid powder was then transferred to a muffle furnace, heated to 500°C at a heating rate of 5°C / min, and calcined for 1 hour to obtain Pd / NaY powder.

[0067] 3) Introduction of metal additives by solid phase grinding method: 0.01 g of SrCl2 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground in a ball mill for 200 minutes to obtain a Sr-Pd / NaY catalyst modified with a metal additive.

[0068] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 1.2%, and the mass fraction of the metal additive was 0.2%.

[0069] Example 5

[0070] 1) Synthesis of NaY molecular sieve support: 45 g of sodium hydroxide, 280 g of water, 65 g of aluminum sulfate, and 100 g of silica sol were mixed and added to a container. The mixture was stirred at 65° C. for 5 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 55° C. for 62 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0071] 2) Loading of Pd active component: Prepare 0.1 mol / L sodium chloropalladate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 0.1 mol / L sodium chloropalladate solution to the molecular sieve suspension, heat to 40°C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 600°C at a heating rate of 2°C / min and calcined for 1 hour to obtain Pd / NaY powder.

[0072] 3) Solid-phase grinding method to introduce metal additives: 0.03g MgCl2 was mixed with 3g Pd / NaY catalyst prepared in step 2) and ground in a ball mill for 0.5min to obtain a Mg-Pd / NaY catalyst modified with a metal additive.

[0073] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 0.3%, and the mass fraction of the metal additive was 0.3%.

[0074] Example 6

[0075] 1) Synthesis of NaY molecular sieve support: 26 g of sodium hydroxide, 300 g of water, 50 g of aluminum sulfate, and 50 g of sodium silicate were mixed and added to a container. The mixture was stirred at 80°C for 12 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0076] 2) Loading of Pd active component: Prepare 1 mol / L palladium chloride solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L palladium chloride solution to the molecular sieve suspension, heat to 120°C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 300°C at a heating rate of 3°C / min and calcined for 2 hours to obtain Pd / NaY powder.

[0077] 3) Introduction of metal additives by solid phase grinding method: 0.3 g of CaCl2, 0.1 g of Li2SO4 and 3 g of the Pd / NaY catalyst prepared in step 2) were mixed and ground in a ball mill for 80 minutes to obtain a Ca-Li-Pd / NaY catalyst modified with a metal additive.

[0078] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 2.4%, the mass fraction of the metal promoter Ca was 3.4%, and the mass fraction of Li was 1.9%.

[0079] Example 7

[0080] 1) Synthesis of NaY molecular sieve support: 23 g of sodium hydroxide, 310 g of water, 10 g of sodium metaaluminate, and 70 g of nano-silica were mixed and added to a container. The mixture was stirred at 80°C for 1 hour to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0081] 2) Loading of Pd active component: Prepare 0.5 mol / L palladium acetate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 0.5 mol / L palladium acetate solution to the molecular sieve suspension, heat to 120°C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 150°C at a heating rate of 5°C / min and calcined for 8 hours to obtain Pd / NaY powder.

[0082] 3) Solid phase grinding method to introduce metal additives: 0.6g Na2CO3 was mixed with 3g Pd / NaY catalyst prepared in step 2), and ground with an oscillating mill for 10 minutes to obtain a Na-Pd / NaY catalyst modified with a metal additive.

[0083] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 1.1%, and the mass fraction of the metal additive was 8.4%.

[0084] Example 8

[0085] 1) Synthesis of NaY molecular sieve support: 18 g of sodium hydroxide, 200 g of water, 20 g of boehmite, 3 g of aluminum nitrate, and 80 g of ITQ-1 were mixed and added to a container. The mixture was stirred at 80°C for 1 hour to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0086] 2) Loading of Pd active component: Prepare 1 mol / L palladium nitrate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L palladium nitrate solution to the molecular sieve suspension, heat to 120°C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 600°C at a heating rate of 2°C / min and calcined for 0.5 hour to obtain a Pd / NaY catalyst.

[0087] 3) Introduction of metal additives by solid phase grinding: 0.2 g of Na2SO4 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground for 200 minutes using an oscillating mill to obtain a Na-Pd / NaY catalyst modified with a metal additive.

[0088] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 2.2%, and the mass fraction of the metal additive was 2.1%.

[0089] Example 9

[0090] 1) Synthesis of NaY molecular sieve support: 22 g of sodium hydroxide, 55 g of water, 30 g of aluminum sulfate, 5 g of aluminum hydroxide, and 90 g of ethyl orthosilicate were mixed and added to a container. The mixture was stirred at 80°C for 6 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 190°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0091] 2) Loading of Pd active component: Prepare 1 mol / L palladium chloride solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L palladium chloride solution to the molecular sieve suspension, heat to 30°C in a water bath and stir for 15 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 550°C at a heating rate of 0.5°C / min and calcined for 4 hours to obtain Pd / NaY powder.

[0092] 3) Solid phase grinding method to introduce metal additives: 0.9g NaNO3 was mixed with 3g Pd / NaY catalyst prepared in step 2) and ground in a ball mill for 8 minutes to obtain a Na-Pd / NaY catalyst modified with a metal additive.

[0093] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 2.4%, and the mass fraction of the metal additive was 8.2%.

[0094] Example 10

[0095] 1) Synthesis of NaY molecular sieve support: 40 g of sodium hydroxide, 65 g of water, 40 g of aluminum isopropoxide, and 100 g of ethyl orthosilicate were mixed and added to a container. The mixture was stirred at 80°C for 5 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0096] 2) Loading of Pd active component: Prepare 1 mol / L tetraammine palladium nitrate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L tetraammine palladium nitrate solution to the molecular sieve suspension, heat to 120°C in a water bath and stir for 0.5 hour, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 580°C at a heating rate of 0.5°C / min and calcined for 6 hours to obtain Pd / NaY powder.

[0097] 3) Solid-phase grinding method to introduce metal additives: 0.9 g of NaHCO3 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground in a ball mill for 0.5 min to obtain a Na-Pd / NaY catalyst modified with a metal additive.

[0098] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 2.1%, and the mass fraction of the metal additive was 7.9%.

[0099] Example 11

[0100] 1) Synthesis of NaY molecular sieve support: 50 g of sodium hydroxide, 75 g of water, 50 g of aluminum hydroxide, and 110 g of sodium silicate were mixed and added to a container. The mixture was stirred at 80° C. for 4 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120° C. for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0101] 2) Loading of Pd active component: A 1 mol / L sodium chloropalladate solution was prepared, 20 g of water and 4 g of the molecular sieve support obtained in step 1) were mixed to form a molecular sieve suspension, 1 mL of 1 mol / L sodium chloropalladate solution was added to the molecular sieve suspension, heated to 120°C in a water bath and stirred for 12 hours, then filtered, washed, and dried. The resulting solid powder was transferred to a muffle furnace, heated to 150°C at a heating rate of 5°C / min, and calcined for 6 hours to obtain Pd / NaY powder.

[0102] 3) Introduction of metal promoter by solid phase grinding method: 0.1 g of MoCl2 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground manually in a mortar for 100 minutes to obtain a Mo-Pd / NaY catalyst modified with a metal promoter.

[0103] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 1.9%, and the mass fraction of the metal additive was 1.7%.

[0104] Example 12

[0105] 1) Synthesis of NaY molecular sieve support: 40 g of sodium hydroxide, 110 g of water, 60 g of aluminum isopropoxide, and 120 g of silica sol were mixed and added to a container. The mixture was stirred at 80° C. for 4 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120° C. for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0106] 2) Loading of Pd active component: Prepare 1 mol / L potassium chloropalladate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 1 mL of 1 mol / L potassium chloropalladate solution to the molecular sieve suspension, heat to 120°C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 380°C at a heating rate of 0.5°C / min and calcined for 10 hours to obtain Pd / NaY powder.

[0107] 3) Introduction of metal promoter by solid phase grinding method: 0.5 g of MoCl2 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground manually in a mortar for 150 minutes to obtain a Mo-Pd / NaY catalyst modified with a metal promoter.

[0108] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 2.1%, and the mass fraction of the metal additive was 9.1%.

[0109] Example 13

[0110] 1) Synthesis of NaY molecular sieve support: 23 g of sodium hydroxide, 160 g of water, 70 g of aluminum nitrate, and 115 g of silica sol were mixed and added to a container. The mixture was stirred at 80° C. for 4 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 150° C. for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0111] 2) Loading of Pd active component: Prepare 0.1 mol / L ammonium chloropalladate solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 5 mL of 0.1 mol / L ammonium chloropalladate solution to the molecular sieve suspension, heat to 120 ° C in a water bath and stir for 6 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 500 ° C at a heating rate of 0.5 ° C / min and calcined for 0.5 hour to obtain Pd / NaY powder.

[0112] 3) Introduction of metal additives by solid phase grinding method: 0.5 g of MoCl2 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground in a ball mill for 8 minutes to obtain a Mo-Pd / NaY catalyst modified with a metal additive.

[0113] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 1.1%, and the mass fraction of the metal additive was 9.4%.

[0114] Example 14

[0115] 1) Synthesis of NaY molecular sieve support: 10 g of sodium hydroxide, 230 g of water, 80 g of boehmite, and 84 g of ITQ-1 were mixed and added to a container. The mixture was stirred at 80°C for 4 hours to form a gel. The gel was then transferred to a stainless steel reactor and allowed to crystallize in an oven at 120°C for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0116] 2) Loading of Pd active component: Prepare 0.5 mol / L palladium chloride solution, mix 20 g of water with 4 g of the molecular sieve support obtained in step 1) to form a molecular sieve suspension, add 2 mL of 0.5 mol / L palladium chloride solution to the molecular sieve suspension, heat to 120 ° C in a water bath and stir for 3 hours, then filter, wash, and dry. After that, the obtained solid powder is transferred to a muffle furnace, heated to 600 ° C at a heating rate of 10 ° C / min and calcined for 12 hours to obtain Pd / NaY powder.

[0117] 3) Introduction of metal promoter by solid phase grinding method: 0.3 g of MoCl2 was mixed with 3 g of the Pd / NaY catalyst prepared in step 2) and ground in a mortar for 200 minutes using a manual grinding method to obtain a Mo-Pd / NaY catalyst modified with a metal promoter.

[0118] According to ICP testing, the mass fraction of the Pd active component of the catalyst prepared in this example was 2.5%, and the mass fraction of the metal additive was 5.6%.

[0119] Comparative Example 1

[0120] 1) Synthesis of NaY molecular sieve support: 33.4 g of sodium hydroxide, 220 g of water, 20 g of sodium metaaluminate, and 28 g of silica sol were mixed and added to a container. The mixture was stirred at 20° C. for 1 hour to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120° C. for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0121] 2) Loading of Pd active component: A 1 mol / L potassium chloropalladate solution was prepared, 20 g of water and 4 g of the molecular sieve support obtained in step 1) were mixed to form a molecular sieve suspension, 1 mL of 1 mol / L potassium chloropalladate solution was added to the molecular sieve suspension, heated to 80°C in a water bath and stirred for 6 hours, then filtered, washed, and dried. The resulting solid powder was transferred to a muffle furnace, heated to 400°C at a heating rate of 0.5°C / min, and calcined for 2 hours to obtain a Pd / NaY catalyst.

[0122] Figure 11 This is the XRD pattern of the catalyst prepared in Comparative Example 1. Figure 12 This is a TEM image of the catalyst prepared in Comparative Example 1. Pd particles can be observed in the sample, with an average size of about 6.8 nm. Figure 13 The IR image of the hydroxyl group of the catalyst prepared in Comparative Example 1 shows that the sample has a hydroxyl group at 3628 cm -1 There is an obvious band at the end, indicating that there is a certain amount of Si-O(H)-Al bridged hydroxyl in the sample, which is part of the ion exchange process. acid. Figure 14 This is the pyridine infrared image of the catalyst prepared in Comparative Example 1, which is attributed to According to ICP test, the mass fraction of the Pd active component of the catalyst prepared in this comparative example is 2.6%.

[0123] Comparative Example 2

[0124] 1) Synthesis of NaY molecular sieve support: 33.4 g of sodium hydroxide, 220 g of water, 20 g of sodium metaaluminate, and 28 g of silica sol were mixed and added to a container. The mixture was stirred at 20° C. for 4 hours to form a gel. The gel was transferred to a stainless steel reactor and allowed to crystallize in an oven at 120° C. for 23 hours. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support.

[0125] 2) Loading of Pd active component: A 1 mol / L dichlorotetraammine palladium solution was prepared, 20 g of water and 4 g of the molecular sieve support obtained in step 1) were mixed to form a molecular sieve suspension, 1 mL of 1 mol / L dichlorotetraammine palladium solution was added to the molecular sieve suspension, heated to 80°C in a water bath and stirred for 6 hours, then filtered, washed, and dried. The resulting solid powder was then transferred to a muffle furnace, heated to 400°C at a heating rate of 0.5°C / min, and calcined for 3 hours to obtain Pd / NaY powder.

[0126] 3) Introduction of the metal promoter by saturation impregnation: First, the saturated adsorption capacity of the Pd / NaY catalyst prepared in step 2) was determined, and then 0.01 g of NaCl was dissolved in deionized water of the same mass; the resulting NaCl solution was mixed with 1 g of the Pd / NaY catalyst prepared in step 1), stirred for 2 hours, and filtered, washed, and dried to obtain a modified Na-Pd / NaY catalyst.

[0127] Figure 15 This is a TEM image of the catalyst prepared in Comparative Example 2. Obvious Pd particles can be observed with an average particle size of 10 nm, indicating that the Pd species in the catalyst have agglomerated. Figure 16 The IR image of the hydroxyl group of the catalyst prepared in Comparative Example 2 shows that the sample has a hydroxyl group at 3628 cm -1 The presence of a distinct band at indicates the presence of a certain amount of Si-O(H)-Al bridged hydroxyl groups in the sample, demonstrating that the catalyst prepared by the NaCl saturation impregnation method cannot eliminate these Si-O(H)-Al bridged hydroxyl groups. ICP testing revealed that the mass fraction of the Pd active component in the catalyst prepared in this comparative example was 2.4%, and the mass fraction of the metal promoter was 0.4%.

[0128] Application Example 1

[0129] The catalysts prepared in Examples 1 to 14 and Comparative Examples 1 and 2 were evaluated for catalytic activity in a fixed-bed reactor. A catalyst loading of 0.1 g was used. CO and methyl nitrite were used as raw materials, nitrogen was used as the diluent, and the gas flow volume ratio was CO:CH3ONO:N2 = 1:6:33. The reaction temperature was 110°C, the reaction pressure was 0.1 to 5.0 MPa, and the gas hourly space velocity (GHSV) was 8000 mL gcat. -1 h -1The products include the main product dimethyl carbonate and the by-products dimethoxymethane (DMM) and methyl formate (MF). The CO conversion rate X is calculated based on the online gas chromatography peak area and relative correction coefficient. CO , selectivity of dimethyl carbonate S DMC and the selectivity of each by-product S DMM and S MF .

[0130] Figure 17 Graph showing the CO conversion rates of Example 1 and Comparative Example 1. It can be observed that as the reaction time increases, the CO conversion rate of the modified catalyst obtained in Example 1 remains almost unchanged, while that of Comparative Example 1 decreases significantly, dropping to approximately 70% after 60 hours. Figure 18 This is a TEM image of the Me-Pd / NaY prepared in Example 1 after 60 hours of reaction. After 60 hours of reaction, no agglomeration of Pd metal was observed in the sample, indicating that Pd was highly dispersed.

[0131] As can be seen from Table 1, the CO conversion rate and DMC selectivity of the metal-modified molecular sieve catalysts prepared in Examples 1 to 14 are significantly higher than those of Comparative Examples 1 to 2. Since the Pd dispersion of the catalysts prepared in Examples 1 to 14 is higher, the CO conversion rate is higher. The Me-Pd / NaY catalyst prepared in the present invention has high selectivity and conversion rate.

[0132] Table 1 Catalytic performance of catalysts of Examples 1 to 14 and Comparative Examples 1 to 2 after 60 hours of reaction

[0133]

Claims

1. A method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate, characterized in that: The preparation method adopts an efficient and simple solid phase grinding method. The catalyst is composed of a Pd active component, a metal auxiliary and a NaY molecular sieve carrier, wherein the mass fraction of the Pd active component is 0.1% to 3.0%, and the mass fraction of the metal auxiliary is 0.01% to 10.0%. The metal auxiliary not only effectively eliminates the The acid is formed and the generation of high-valent Pd active species is promoted. The electron-withdrawing effect of the metal promoter on the Pd active species stabilizes the Pd active species, thereby preparing a high-catalytic performance NaY molecular sieve supported palladium and metal promoter catalyst, namely Me-Pd / NaY catalyst, wherein Me is the metal promoter. The catalyst is applied to the reaction of methyl nitrite carbonylation to synthesize dimethyl carbonate in the process of coal-to-ethylene glycol co-production of dimethyl carbonate. The specific surface area of ​​the catalyst is 100-800m 2 / g, the preparation method of the catalyst comprises the following steps: 1) Synthesis of NaY molecular sieve support: 20-50 g of sodium hydroxide, 30-120 g of a silicon source, 5-80 g of an aluminum source, and 50-310 g of water were weighed and mixed in a container. The mixture was stirred at 15-80° C. for 1-12 hours to form a gel. The gel was transferred to a stainless steel reactor and crystallized in an oven at a certain temperature for a certain time. The crystallized product was filtered, washed, and dried to obtain a NaY molecular sieve support. 2) Loading of the Pd active component: Pd is loaded by an ion exchange method. First, a certain concentration of a noble metal Pd precursor solution is prepared. The NaY molecular sieve support synthesized in step 1) is mixed with water to form a suspension. Then, a certain amount of the Pd precursor solution is added to the suspension. Ion exchange is performed at a certain temperature for a certain time. The suspension is then filtered, washed, and dried. The resulting solid powder is then transferred to a muffle furnace for calcination. The temperature is increased to 150-600°C at a heating rate of 0.5-10°C / min and maintained for 0.5-12 hours to obtain a Pd / NaY powder. 3) Introducing a metal additive by solid phase grinding: Weigh a certain amount of a metal additive and a certain amount of the Pd / NaY powder obtained in step 2) and mix them uniformly, then solid phase grind them for a certain period of time to obtain a metal additive-modified Me-Pd / NaY catalyst, wherein Me is the metal additive.

2. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The silicon source used in step 1) is one or a combination of silica sol, tetraethyl orthosilicate, nano-silicon dioxide, sodium silicate, and ITQ-1.

3. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The aluminum source used in step 1) is one or a combination of sodium metaaluminate, aluminum sol, aluminum sulfate, boehmite, aluminum nitrate, aluminum isopropoxide, and aluminum hydroxide.

4. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The crystallization temperature used in step 1) is 30-190° C., and the crystallization time is 2-72 hours.

5. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The noble metal precursor used in step 2) is one or a combination of palladium nitrate, palladium acetate, palladium chloride, ammonium chloropalladate, potassium chloropalladate, tetraamminepalladium chloride, tetraamminepalladium nitrate, dichlorotetraamminepalladium, and sodium chloropalladate.

6. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The ion exchange temperature in step 2) is 30 to 120° C., and the ion exchange time is 0.5 to 15 hours.

7. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The metal additive used in the step 3) is one or a combination of LiCl, SrCl2, KCl, NaCl, MgCl2, CaCl2, CoCl2, MoCl2, Li2CO3, LiHCO3, LiNO3, Li2SO4, K2CO3, KHCO3, KNO3, K2SO4, Na2CO3, NaHCO3, NaNO3, Na2SO4, SrCO3, Sr(HCO3)2, Sr(NO3)2, SrSO4, CaCO3, Ca(HCO3)2, Ca(NO3)2, CaSO4, MgCO3, Mg(HCO3)2, Mg(NO3)2, and MgSO4.

8. The method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate according to claim 1, wherein: The grinding method used in step 3) is one or a combination of manual grinding, ball milling, and vibration milling, and the grinding time is 0.5 to 200 minutes.

9. A method for preparing a catalyst comprising a NaY molecular sieve loaded with palladium and a metal promoter for the novel co-production of dimethyl carbonate as claimed in claim 1, wherein the reaction temperature for carbonylation of methyl nitrite to synthesize dimethyl carbonate in the process of co-production of dimethyl carbonate from coal to ethylene glycol is 100-250° C., the pressure is 0.1-5.0 MPa, and the molar ratio of reactants CO:CH 3 ONO is 1:2-7.

Images