A catalyst for the carbonylation of methanol to acetic acid, its preparation method and application

Abstract

A catalyst for the carbonylation of methanol to acetic acid, its preparation method, and its application are disclosed. The method comprises: 1) mixing a silicon source, an aluminum source, and an alkaline source to form a sol; 2) mixing a quaternary ammonium salt and an organic base, adding the mixture to the sol prepared in step 1), followed by aging, crystallization, filtration, washing, drying, and calcination; 3) dispersing the molecular sieve prepared in step 2) in an NH4Cl aqueous solution for ammonium exchange, followed by drying and calcination; 4) subjecting the molecular sieve prepared in step 3) to pyridine adsorption to obtain the catalyst. The quaternary ammonium salt in step 2) is one or more of octaalkyltrimethylammonium bromide, decaalkyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, and octadecyltrimethylammonium bromide. The nano-sized mordenite zeolite prepared by this invention can improve the diffusion of methanol and carbon monoxide, thereby enhancing the activity and selectivity of methanol carbonylation to acetic acid.

Description

Technical Field

[0001] This invention relates to the field of catalysis for improving the reactivity of methanol carbonylation to acetic acid, and particularly to a catalyst for methanol carbonylation to acetic acid, its preparation method, and its application. Background Technology

[0002] Acetic acid (AA) is an important organic chemical raw material that can be used to derive a series of high-value-added chemicals, such as vinyl acetate monomer (Vac or VAM), acetic anhydride, chloroacetic acid (MCA), polyvinyl alcohol, and acetate esters. Its downstream applications are very broad, including synthetic fibers, pharmaceuticals, printing and dyeing, textiles, food processing, pesticides, and fragrances. In recent years, the development prospects of acetic acid derivatives have been promising, entering a period of rapid capacity growth, and the market demand for acetic acid has also increased year by year.

[0003] Acetic acid synthesis processes include anaerobic fermentation, light hydrocarbon liquid-phase oxidation, acetaldehyde oxidation, and methanol carbonylation. Among these, methanol carbonylation for acetic acid synthesis offers high atom economy, mild reaction conditions, and meets the requirements of green chemistry. With the introduction of molecular sieve catalysts, the advantages of this process become even more pronounced: it avoids the use of halides, the catalyst is inexpensive, readily available, and easily recyclable, and the selectivity for acetic acid is high, making it a promising candidate for industrial application.

[0004] The reaction in this process is as follows: CH3OH + CO → CH3COOH. Currently, research on catalysts and processes for this reaction is increasing year by year, mainly focusing on two catalyst systems: noble metal-supported heteropolyacids and molecular sieves. However, considering the cost of catalytic reactions, most scholars concentrate on molecular sieve research, particularly on how to obtain catalysts with superior reactivity from mordenite (MOR) and ZSM-5, which have eight-membered ring structures. Summary of the Invention

[0005] The purpose of this invention is to solve the above-mentioned problems in the prior art, and to provide a catalyst for the carbonylation of methanol to produce acetic acid, its preparation method and application. The catalyst prepared by this invention has high activity, good selectivity, simple preparation process and is easy to scale up industrially.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for preparing a catalyst for the carbonylation of methanol to acetic acid includes the following steps:

[0008] 1) Mix the silicon source, aluminum source, and alkali source evenly to form a sol;

[0009] 2) After mixing the quaternary ammonium salt and the organic base, add it to the sol prepared in step 1), and then age, crystallize, filter, wash, dry and calcinate.

[0010] 3) Disperse the molecular sieve prepared in step 2) in an NH4Cl aqueous solution, perform ammonium exchange, and then dry and calcine;

[0011] 4) The molecular sieve prepared in step 3) is subjected to pyridine adsorption to obtain the catalyst;

[0012] Wherein, the quaternary ammonium salt in step 2) is one or more of octaalkyltrimethylammonium bromide, decaalkyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, and octadecyltrimethylammonium bromide.

[0013] The preferred quaternary ammonium salt is decaalkyltrimethylammonium bromide.

[0014] The organic base is at least one of tetraethylammonium hydroxide and tetrapropylammonium hydroxide.

[0015] The molar ratio of the organic base to the quaternary ammonium salt is (1-10):1.

[0016] In step 1), the silicon source is silica sol, tetraethyl orthosilicate, water glass or silicon dioxide, the aluminum source is sodium aluminate, aluminum nitrate, aluminum sulfate, etc., and the alkali source is sodium hydroxide. Water is added and stirred when forming the sol to ensure thorough and uniform mixing.

[0017] In step 2), the crystallization temperature is 140–200°C, preferably 150–180°C, and the crystallization time is 24–120 h, preferably 48–72 h.

[0018] In step 3), the concentration of NH4Cl is 0.1–1 mol / L, preferably 0.1–0.5 mol / L; the calcination temperature is 450–650℃, and the temperature is maintained for 2–6 hours.

[0019] In step 4), the conditions for pyridine adsorption are: introducing N2, heating to 300-450℃ for dehydration for 4-6 hours, and then cooling to 200-350℃ for pyridine adsorption for 1-15 hours.

[0020] A catalyst for the carbonylation of methanol to produce acetic acid, said catalyst having a mordenite zeolite crystal phase.

[0021] Application of a catalyst for the carbonylation of methanol to produce acetic acid, used in the methanol carbonylation reaction of methanol and carbon monoxide to produce acetic acid. The catalyst is added to a fixed-bed reactor, the feed molar ratio of methanol to carbon monoxide is 1:1 to 100, the temperature is 150 to 350°C, and the reaction pressure is 0.5 to 5.0 MPa, preferably 2 to 3 MPa.

[0022] The catalyst for the carbonylation of methanol to acetic acid prepared in this invention has a significant impact on the performance and structure of the catalyst through a carefully designed preparation process, especially the variation of the quaternary ammonium salt chain length.

[0023] Compared with the prior art, the beneficial effects achieved by the technical solution of this invention are:

[0024] 1) The molecular sieve catalyst prepared by this invention has a mordenite zeolite crystal phase and high crystallinity. During the preparation process, organic base and quaternary ammonium salts with different chain lengths are used as dual template agents to improve the crystallinity of the molecular sieve and reduce the negative impact of non-framework Al and amorphous SiO2. Moreover, the catalyst has low cost, simple preparation process, simple post-processing, and is easy to repeat.

[0025] 2) Significantly improved methanol conversion and acetic acid selectivity: When the chain length of the quaternary ammonium salt is appropriate, especially when decaalkyltrimethylammonium bromide is used, the prepared catalyst exhibits optimal methanol conversion and acetic acid selectivity. As shown in Example 2, the methanol conversion can reach 99.9%, and the acetic acid selectivity can reach 94.7%, which is much higher than the catalyst prepared without the addition of quaternary ammonium salt. This indicates that a suitable quaternary ammonium salt chain length can significantly improve the activity and selectivity of the catalyst.

[0026] 3) Increased reaction rate: A suitable pore size distribution promotes rapid diffusion of reactants and products, thereby increasing the reaction rate. As shown in Table 2, the space-time yield of acetic acid changes with the quaternary ammonium salt chain length, and decaalkyltrimethylammonium bromide exhibits excellent space-time yield of acetic acid when used as a template agent.

[0027] 4) Changes in catalyst morphology and crystallinity: The chain length of the quaternary ammonium salt directly affects the morphology and crystallinity of the catalyst. With changes in chain length, the catalyst morphology changes from nano-sized flower-like particles to micro-sized rod-like particles. This change in morphology not only affects the surface area and pore structure of the catalyst, but also further affects its catalytic performance.

[0028] 5) Optimization of pore size distribution: The chain length of the quaternary ammonium salt determines the occupancy of the template agent within the pores, thus affecting the pore size distribution of the catalyst. Example data shows that when decaalkyltrimethylammonium bromide is used as the template agent, the catalyst exhibits larger mesoporous and microporous volumes, and this pore size distribution is more conducive to the rapid diffusion of reactants and products.

[0029] 6) Regulation of acid sites: With changes in the chain length of the quaternary ammonium salt, the silicon-to-aluminum ratio (Si / Al) and the distribution of acid sites in the catalyst also change. In particular, when decaalkyltrimethylammonium bromide is used as a template agent, the proportion of 8-membered ring B acid in the catalyst is the highest, which is beneficial to the methanol carbonylation reaction.

[0030] In summary, variations in the quaternary ammonium salt chain length have a profound impact on the performance and structure of catalysts. By selecting an appropriate quaternary ammonium salt chain length, highly active and selective catalysts for the methanol carbonylation to acetic acid production can be prepared, thereby improving reaction efficiency and product quality. These catalysts are not only easy to prepare but also inexpensive, possessing immense potential for industrial application. Attached Figure Description

[0031] Figure 1 The XRD patterns are of the mordenite molecular sieves prepared in Example 2 and Comparative Example 1.

[0032] Figure 2 The image shows a scanning electron microscope (SEM) image of the mordenite molecular sieve prepared in Example 1.

[0033] Figure 3 The image shows a scanning electron microscope (SEM) image of the mordenite molecular sieve prepared in Example 2.

[0034] Figure 4 This is a scanning electron microscope image of the mordenite molecular sieve prepared in Example 3.

[0035] Figure 5 The attached figure shows the physical adsorption and desorption of the mordenite molecular sieve prepared in Example 2.

[0036] Figure 6 The image shows the NH3-TPD of the mordenite molecular sieve prepared in Example 2. Detailed Implementation

[0037] To make the technical problems, technical solutions and beneficial effects of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] Example 1

[0039] Weigh 10g of silica sol (30wt% silica), 1g of sodium aluminate, and 2g of sodium hydroxide into a plastic beaker, mix them thoroughly, and stir at room temperature for 4 hours to obtain a homogeneous sol.

[0040] Octyltrimethylammonium bromide and an organic base were weighed out as dual template agents, such that the molar ratio of octaalkyltrimethylammonium bromide to silica was 0.1, and the molar ratio of organic base to silica was 0.1. After thorough mixing, the mixture was added to the above sol, and the mixture was aged at room temperature for 2 hours. The mixture was then loaded into a reactor and crystallized at 150℃ for 72 hours. After crystallization, the mixture was filtered, washed with water until pH < 10, calcined at 600℃ in air for 5 hours to remove the template agent, repeatedly exchanged twice in a 0.1M ammonium chloride solution, and calcined at 500℃ in air for 4 hours to remove ammonium, yielding hydrogen-form mordenite. Pyridine was adsorbed at 320℃ under a nitrogen atmosphere, resulting in pyridine-adsorbed mordenite.

[0041] 0.3 g of the catalyst sample after tableting and sieving (40-60 mesh) was reacted in a fixed-bed reactor at 3 MPa and 310 °C with a raw material molar ratio of CO / methanol = 12:1. The product tail gas was kept warm and then analyzed by online gas chromatography.

[0042] Examples 2-6

[0043] Under the same conditions as in Example 1, only the type of quaternary ammonium salt was changed. Octyltrimethylammonium bromide was replaced with decaalkyltrimethylammonium bromide (Example 2), dodecyltrimethylammonium bromide (Example 3), tetradecyltrimethylammonium bromide (Example 4), hexadecyldimethylbenzylammonium bromide (Example 5), and octadecyltrimethylammonium bromide (Example 6). After thorough mixing, the mixture was added to the above sol, stirred for 2 hours, and then loaded into a reactor to begin crystallization. After crystallization, the mixture was filtered, washed with water, calcined, subjected to ammonium exchange, calcined again, and pyridine adsorption was performed.

[0044] Comparative Example 1

[0045] Comparative Example 1 is Example 2 without the addition of quaternary ammonium salt, and all other conditions remain unchanged.

[0046] Using the mordenite zeolite prepared in Example 2 as a relative standard, assuming its crystallinity is 100%, the sum of the intensities of several crystallization peaks was selected to calculate the diffraction intensity value of the crystalline phase. The selection principle for the diffraction peaks of the crystalline phase is: good peak shape, no overlap, and the distance between the two peaks is more than 2° (2θ).

[0047] See Figure 1 XRD analysis results show that the crystallinity of the sample prepared with quaternary ammonium salt is significantly lower than that of the sample without quaternary ammonium salt, as shown in the relative crystallinity values ​​in Table 1. Table 1 also shows that the sample prepared with quaternary ammonium salt has higher crystallinity, better crystallization, and better performance compared to the sample without quaternary ammonium salt, indicating that quaternary ammonium salt plays an important role as a template agent in the synthesis of MOR.

[0048] Table 1

[0049] sample Relative crystallinity <![CDATA[CH3OH conversion rate (%)]]> AA selectivity (%) MA selectivity (%) Example 2 100 99.9 94.7 4.2 Comparative Example 1 73 70.4 32.4 57.6

[0050] Table 2 shows the results of MOR-catalyzed methanol carbonylation to acetic acid synthesized from different quaternary ammonium salts. The reactivity indicates that the selectivity for acetic acid first increases and then decreases with decreasing quaternary ammonium salt chain length, with decaalkyltrimethylammonium bromide exhibiting the highest selectivity as the second template agent.

[0051] In addition, see Figures 2-4 As shown in Table 3, the morphology and crystallinity of the catalyst also change with the change of quaternary ammonium salt chain length.

[0052] Table 2

[0053]

[0054] Table 3

[0055] sample Relative crystallinity (%) Example 1 80 Example 2 100 Example 3 130 Example 4 121 Example 5 102 Example 6 104

[0056] To clarify the reasons for different reaction performances, the silica-alumina ratio of the molecular sieve was characterized by XRF; the total acidity of the molecular sieve was characterized using NH3-TPD (2920). The specific procedure was as follows: heating to 500℃ at 10℃ / min and maintaining this temperature for 60 min for pretreatment to remove moisture and other impurities from the sample; then cooling to 100℃, pulse-injecting ammonia gas until adsorption saturation, purging under an Ar atmosphere for 30 min to remove physically adsorbed ammonia, followed by cooling to 50℃. After baseline stabilization, the temperature was increased to 800℃ at 10℃ / min, and the peak signal was recorded using a thermal conductivity detector. (See attached diagram). Figure 6 .

[0057] Peak fitting correction was performed on NH3-TPD, and the peak area of ​​the high-temperature peak was calculated. The content of Brønsted acid (B acid) was determined. Subsequently, the content of B acid within the twelve-membered and eight-membered rings of the MOR molecular sieve was determined by hydroxyl infrared spectroscopy. The specific procedure was as follows: the sample was pretreated under vacuum at 400℃ for 4 hours in an in-situ transmission cell, cooled to 50℃, and then scanned from 4000 nm to 600 nm. -1 Using a resolution of 4cm -1 The FT-IR spectra were recorded 64 times.

[0058] Table 4

[0059]

[0060] Table 4 shows the Brønsted acid content of the eight-membered and twelve-membered rings in the synthesized MOR molecular sieve. As can be seen from the table, the Brønsted acid content of the eight-membered ring first increases and then decreases as the chain length shortens. The Brønsted acid content is relatively high when decaalkyltrimethylammonium bromide is used as the second template agent.

[0061] Table 5 shows the pore structure characterization results of the samples using low-temperature physical adsorption. All adsorption-desorption isotherms exhibit significant hysteresis loops, as shown in the typical nitrogen adsorption-desorption isotherm. Figure 5 As shown. Samples in Examples 2-5 all have large mesopore and micropore volumes, meaning the samples contain both mesopores and micropores. Samples in Examples 1 and 6, on the other hand, are predominantly micropores and contain almost no mesopores.

[0062] Table 5

[0063]

[0064] The mordenite molecular sieve catalyst prepared in this invention has better stability and reproducibility compared with traditional MOR due to the use of a dual template agent in the synthesis process. It can also effectively improve the activity of methanol carbonylation to acetic acid synthesis reaction, with methanol conversion rate reaching 99% and acetic acid selectivity reaching 90%.

[0065] In this invention, the crystallinity and morphology of the catalyst change with the variation of the quaternary ammonium salt chain length. When decaalkyltrimethylammonium bromide is used, the prepared molecular sieve consists of nano-sized flower-like particles, while with the increase or decrease of the chain length, the morphology changes to micron-sized rod-like particles. The chain length of the quaternary ammonium salt directly affects the occupancy of the template agent within the pores during the synthesis process, thus affecting the pore size distribution of the final catalyst. A suitable pore size facilitates the rapid diffusion of reactants and products, thereby improving reaction performance and reaction rate.

Claims

1. A method for preparing a catalyst for the carbonylation of methanol to acetic acid, characterized in that, Includes the following steps: 1) Mix silicon source, aluminum source and alkali source to form a sol; 2) After mixing the quaternary ammonium salt and the organic base, add it to the sol prepared in step 1), and then age, crystallize, filter, wash, dry, and calcine. 3) Disperse the molecular sieve prepared in step 2) in an NH4Cl aqueous solution, perform ammonium exchange, and then dry and calcine; 4) The molecular sieve prepared in step 3) is subjected to pyridine adsorption to obtain the catalyst; Wherein, the quaternary ammonium salt in step 2) is one or more of octaalkyltrimethylammonium bromide, decaalkyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, and octadecyltrimethylammonium bromide; and the organic base is at least one of tetraethylammonium hydroxide and tetrapropylammonium hydroxide.

2. The method for preparing a catalyst for the carbonylation of methanol to acetic acid as described in claim 1, characterized in that: The quaternary ammonium salt is decaalkyltrimethylammonium bromide.

3. The method for preparing a catalyst for the carbonylation of methanol to acetic acid as described in claim 1, characterized in that: The molar ratio of the organic base to the quaternary ammonium salt is (1-10):

1.

4. A catalyst for the carbonylation of methanol to acetic acid, characterized in that: Prepared by any one of the preparation methods of claims 1 to 3.

5. The catalyst for the carbonylation of methanol to acetic acid as described in claim 4, characterized in that: The catalyst has a mordenite zeolite crystal phase.

6. The application of the catalyst for the methanol carbonylation to acetic acid as described in claim 4 or 5, characterized in that: It is used to prepare acetic acid by methanol carbonylation reaction with methanol and carbon monoxide.

7. The application as described in claim 6, characterized in that: The catalyst is added to a fixed-bed reactor, the feed molar ratio of methanol and carbon monoxide is 1:1 to 100, the temperature is 150 to 350°C, and the reaction pressure is 0.5 to 5.0 MPa.

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