KR20120102080A - Preparation of palladium-gold catalyst - Google Patents

Abstract

A process for preparing a palladium-gold catalyst comprising a titania extrudate is disclosed. Titania extrudate is prepared using carboxyalkyl cellulose and hydroxyalkyl cellulose as extrusion aids. Titania extrudate improved improved processability and / or mechanical properties. After calcination, the extrudate is used as a carrier for the palladium-gold catalyst. The catalyst is useful for producing vinyl acetate by oxidizing ethylene with oxygen in the presence of acetic acid.

Description

Preparation of Palladium-Gold Catalysts {PREPARATION OF PALLADIUM-GOLD CATALYST}

The present invention relates to a process for preparing a palladium-gold catalyst comprising a titania extrudate.

Palladium-gold catalysts are useful for the oxidation of ethylene or propylene in the presence of acetic acid to produce vinyl acetate or allyl acetate. Acetoxylation is typically carried out in the gas phase using a supported palladium-gold catalyst. Palladium-gold catalysts comprising titania carriers are known (US Pat. Nos. 6,022,823; US Patent Application Publication Nos. 2008/0146721 and 2008/0281122; Application Serial No. 11 / 801,935, filed May 11, 2007, co-pending) ). Acetoxylation is preferably carried out in a fixed-bed reactor.

Commercially, titania is made as a fine powder. In order to produce a catalyst suitable for fixed bed reactions, it is essential to form titania powder into particles such as spheres, tablets, extrudates and the like. Although much effort has been made in the past to develop methods for making titania extrudate, the poor processability makes the majority unsuitable for commercial production. Thus, there is a continuing need to develop new methods for producing titania extrudates that can be used as carriers for palladium-gold catalysts (e.g., application Docket No. 01, filed December 16, 2009, co-pending). -2768A (serial number not yet assigned).

Summary of the Invention

The present invention is a process for preparing a palladium-gold catalyst. The method comprises the steps of (a) mixing titania, carboxyalkyl cellulose and hydroxyalkyl cellulose to form a dough; (b) extruding the dough to produce an extrudate; (c) calcining the extrudate to produce a calcined extrudate; (d) impregnating the calcined extrudate with a palladium compound and a gold compound to produce an impregnated extrudate; And (e) calcining the impregnated extrudate to produce a palladium-gold catalyst. The invention also includes a process for the production of vinyl acetate, which comprises reacting ethylene, oxygen and acetic acid in the presence of a palladium-gold catalyst.

The present invention is a process for preparing a palladium-gold catalyst comprising a titania carrier. The method includes mixing titania, carboxyalkyl cellulose and hydroxyalkyl cellulose to form a dough. Suitable titania may be rutile, anatase, brookite or mixtures thereof. Preferably, titania is anatase. Titania can be prepared by a chloride process, a sulfate process, a hydrothermal process or flame hydrolysis of titanium tetrachloride. Examples of suitable titania are the TiONA? Millennium inorganic chemicals? DT-51, DT-52, DT-51D, DT-40 and DT-20.

Cellulose is an organic compound of formula (C ₆ H ₁₀ O ₅ ) _n , ie a polysaccharide consisting of linear chains of β-1,4-linkages as shown in Scheme I with n = 50 to 20,000. Cellulose is a structural component of the primary cell wall of green plants. Cellulose can be converted into a number of derivatives.

The method uses carboxyalkyl cellulose. The carboxyalkyl cellulose is a cellulose having a carboxyalkyl group bonded to a portion of the hydroxyl group of the glucopyranose monomer forming the cellulose backbone as shown in Scheme I (where R = H, carboxyalkyl and m = 50 to 20,000). Derivatives. It is often used as its sodium salt, sodium carboxyalkyl cellulose. The functionality of the carboxyalkyl cellulose depends not only on the degree of substitution of the cellulose structure (ie, how many hydroxyl groups are substituted), but also on the chain length of the cellulose backbone and the degree of colonization of the substituents. The average number of substituted hydroxyl groups per glucose unit in the cellulose derivative is referred to as degree of substitution (DS). Full substitution will give 3 DS. Preferably, carboxymethyl cellulose is used. Preferred degrees of substitution of carboxymethyl cellulose are from 0.5 to 0.9 (DB Braun and MR Rosen, Rheology Modifiers Handbook: Practical Use and Applications (2000) William Andrew Publishing, pp. 109-131). Carboxymethyl cellulose is known as extrusion aid (US Pat. Nos. 5,884,138 and 6,709,570; US Patent Application Publication No. 2008/0146721).

The method also uses hydroxyalkyl cellulose. Hydroxyalkyl cellulose is a derivative of cellulose in which some of the hydroxyl groups in the repeat glucose unit are hydroxyalkylated. Some of the hydroxyl groups in the hydroxyalkyl celluloses may also be alkylated. Typical structures of hydroxyalkyl celluloses are shown in Scheme II, where R = H, alkyl, hydroxyalkyl and m = 50 to 20,000.

Preferably, the hydroxylalkyl group is selected from the group consisting of 2-hydroxyethyl, 2-hydroxypropyl and mixtures thereof. More preferably, hydroxyalkyl cellulose is alkylated. Most preferably, hydroxyalkyl cellulose is selected from the group consisting of methyl 2-hydroxyethyl cellulose, methyl 2-hydroxypropyl cellulose and mixtures thereof. Preferably, the methyl substitution degree is 1 to 2, more preferably 1.5 to 1.8; 2-hydroxyethyl or 2-hydroxypropyl molar substitution is from 0.1 to 0.3. Preference is given to using METHOCEL ^™ K4M cellulose derivatives from Dow Chemical Company with a degree of methyl substitution of 1.4 and a degree of hydroxypropyl molar substitution of 0.21. Hydroxyalkyl celluloses are known as extrusion aids (US Pat. Nos. 5,884,138, 6,316,383 and 6,709,570).

The weight ratio of carboxyalkyl cellulose to titania is preferably 0.2: 100 to 5: 100, more preferably 0.5: 100 to 4: 100 and most preferably 1: 100 to 3: 100. The weight ratio of hydroxyalkyl cellulose to titania is preferably from 0.1: 100 to 2.5: 100, more preferably from 0.2: 100 to 2: 100 and most preferably from 0.5: 100 to 1: 100. The weight ratio of carboxyalkyl cellulose to hydroxyalkyl cellulose is preferably 5: 1 to 1: 2, more preferably 3: 1 to 1: 1.

The method includes mixing titania, carboxyalkyl cellulose and hydroxyalkyl cellulose to form a dough. If necessary, a solvent may be used. Suitable solvents include water, alcohols, ethers, esters, amides, aromatic compounds, halogenated compounds and the like and mixtures thereof. Preferred solvents are water and alcohols.

Titania sol can be used as the titania source. Titania sol is a colloidal suspension of titania particles in a liquid. Titania sol can be prepared by hydrolyzing a titania precursor. Suitable titania precursors include titanium salts, titanium halides, titanium alkoxides, titanium oxyhalides and the like.

The method includes extruding a dough by an operation called extrusion to produce an extrudate. An extrudate is a method of pushing a dough through a die or orifice to produce a long object of fixed cross-sectional area. Extrusion is commonly used to process plastics or foods and to form adsorbents, catalysts or catalyst carriers. Any conventional extruder can be used. Suitable screw extruders are described in "Particle Size Enlargement", Handbook of Powder Technology , vol. 1 (1980) pp. 112-22.

Carboxyalkyl celluloses and hydroxyalkyl celluloses are used as extrusion aids. Extrusion aids may aid in mixing, kneading and extrusion operations and may improve the mechanical and / or physical properties of the extrudate such as fracture strength, surface area, pore size or pore volume. The extrudate comprising titania, carboxyalkyl cellulose and hydroxyalkyl cellulose has a smooth outer surface. There is no tendency for them to stick together during formation, drying and calcining, which is suitable for large scale manufacturing. In addition, the combination of carboxyalkyl cellulose and hydroxyalkyl cellulose minimizes "feathering". The term "feathering" means that instead of having a smooth outer surface, the extrudate exhibits cracks on its surface, where small pieces or "feathers" of the extrudate are separated from the surface. "Feathering" not only causes loss of valuable materials but also tends to damage the physical strength of the extrudate.

Other extrusion aids may be used to form the dough. Other suitable extrusion aids include alkyl amines, carboxylic acids, alkyl ammonium compounds, amino alcohols, starches, polyacrylates, polymethacrylates, poly (vinyl alcohol), poly (vinylpyrrolidone), poly (amino acids), poly Ethers, poly (tetrahydrofuran), metal carboxylates, and the like and mixtures thereof. Preferred poly (alkylene oxides) are poly (ethylene oxide), poly (propylene oxide) or copolymers of ethylene oxide and propylene oxide. The organic extrusion aid is usually removed by calcination.

The extrudate is optionally dried after formation. The drying operation removes at least a portion of the solvent from the extrudate. The drying operation can be carried out at 30 to 200 ° C., at atmospheric pressure or under vacuum. Drying can occur in air or in an inert atmosphere. Sometimes, it is desirable to slowly increase the drying temperature so that the extrudate does not crack or weaken.

The method includes calcining the extrudate to produce the calcined extrudate. Preferably, calcination is carried out in an oxygen-comprising gas to burn off the organic materials (eg, residual solvents and extrusion aids) contained in the extrudate. Calcination may be carried out at 400 to 1000 ° C, more preferably at 450 to 800 ° C, most preferably at 650 to 750 ° C. Sometimes, it is advantageous to calcinate the extrudate in an inert atmosphere (eg nitrogen, helium) to thermally decompose the organic compound contained in the extrudate, and then burn off the organic material in the oxygen-comprising gas. Generally, the calcined extrudate after calcination comprises less than 0.5 weight percent carbon. Preferably, it contains less than 0.1 weight percent carbon.

The method includes impregnating the calcined extrudate with a palladium compound and a gold compound to produce an impregnated extrudate. Suitable palladium compounds include palladium chloride, sodium chloropalladate, palladium nitrate, palladium sulfate and the like and mixtures thereof. Suitable gold compounds include gold chloride, tetrachlorosuccinic acid, sodium tetrachlorosuccinate, and the like and mixtures thereof.

The impregnated extrudate may comprise 0.1% to 3% by weight of palladium and 0.1% to 3% by weight of gold, with a weight ratio of palladium to gold in the range of 5: 1 to 1: 3. More preferably, the impregnated extrudate comprises 0.5% to 1.5% by weight of palladium and 0.25% to 0.75% by weight of gold.

A fixing agent can be added, preferably, while impregnating an extrudate. The fixer helps to bind the palladium compound and the gold compound to the extrudate. Suitable fixing agents include alkali metal, alkaline earth metal or ammonium compounds, for example their hydroxides, carbonates, bicarbonates, metasilicates and the like and mixtures thereof.

Any suitable impregnation method can be used to prepare the impregnated extrudate. The calcined extrudate can be impregnated simultaneously or sequentially with a palladium compound, a gold compound and optionally a fixative. Other impregnation solvents may be used but impregnation with an aqueous solution is preferred.

The impregnated extrudate is preferably washed with water or other solvent to remove any halides (eg chloride) from the extrudate.

The method optionally includes drying the impregnated extrudate. Typically, the impregnated extrudate may be dried at 50 to 150 ° C. at atmospheric pressure or under vacuum to first remove at least some of the solvent. Drying can occur in air or inert gas, at atmospheric pressure or under vacuum. Sometimes it is important to slowly increase the drying temperature so that the extrudate does not lose its mechanical strength.

The method preferably comprises drying first and then calcining the impregnated extrudate to produce a palladium-gold catalyst. Generally, calcination of the impregnated extrudate occurs in a non-reducing atmosphere at elevated temperature. Preferably, the calcination of the impregnated extrudate is carried out at a temperature of 150 to 600 ° C. Suitable non-reducing gases for use in calcination include inert or oxidizing gases such as helium, nitrogen, argon, neon, oxygen, air, carbon dioxide and the like and mixtures thereof. Preferably, calcination is carried out in an atmosphere of nitrogen, oxygen or air, or mixtures thereof.

The palladium-gold catalyst obtained from the calcination step is preferably chemically reduced to produce a reduced palladium-gold catalyst. The reduction is usually carried out by contacting the palladium-gold catalyst with a reducing agent. Suitable reducing agents include hydrogen, carbon monoxide, hydrocarbons, olefins, aldehydes, alcohols, hydrazines and the like and mixtures thereof. Hydrogen, ethylene, propylene, alkali hydrazine and alkali formaldehyde are preferred reducing agents, with ethylene and hydrogen being particularly preferred. The temperature utilized for the reduction may range from 20 to 700 ° C. When hydrogen is a reducing agent, gas mixtures comprising hydrogen and another gas such as argon, helium, nitrogen and the like are commonly used. The reduction temperature with hydrogen is preferably in the range from 300 to 700 ° C, more preferably in the range from 450 to 550 ° C.

Palladium-gold catalysts prepared according to the invention can be used for the acetoxylation of olefins such as ethylene or propylene to produce acetoxylated olefins such as vinyl acetate or allyl acetate. Preferably, an accelerated palladium-gold catalyst, which can be prepared by adding an activator to the reduced palladium-gold catalyst, is used in the acetoxylation reaction. Activators are alkali or alkaline earth metal compounds, examples being hydroxides such as potassium, sodium, cesium, magnesium, barium, acetates, nitrates, carbonates and bicarbonates. Potassium salts are preferred activators. The activator content may range from 0 to 15%, preferably 1.5 to 10% by weight of the catalyst.

The present invention also reacts a feed comprising ethylene, oxygen and acetic acid in the presence of a palladium-gold catalyst, preferably in the presence of a reduced palladium-gold catalyst, more preferably in the presence of a promoted palladium-gold catalyst. Methods of making vinyl acetate, including

The feed typically comprises 20 to 70 mol% ethylene, 2 to 8 mol% oxygen and 2 to 20 mol% acetic acid. The feed may comprise a diluent. Examples of suitable diluents include propane, nitrogen, helium, argon, carbon dioxide and the like and mixtures thereof.

The reaction is generally carried out at a temperature in the range from 100 to 250 ° C, preferably 125 to 200 ° C and under a pressure of 15 to 500 psig.

Example 1

D-T51 Titania (2500 g), high-purity WALOCEL ™ C sodium carboxymethyl cellulose (The Dow Chemical Company, 52.5 g), poly (ethylene oxide) (MW = 100,000, 35 g) and cellulose derivative (METHOCEL ™ K4M , 25 g) are mixed for 5 minutes with an Eirich mixer. Water (1005 g), aqueous ammonium hydroxide (14.8 M, 100 g) and benzyl alcohol (17.5 g) are added to the mixer. Mix them for 5 minutes at the "low" speed setting and then 10 minutes at the "high" speed setting. The resulting dough is placed in a hopper of a Bonnot 2-inch extruder (The Bonnot Company) equipped with 25 holes of die face 1/8 inch in diameter. Extrusion is carried out at a rate of about 0.25 kg / minute. The resulting extrudate has a smooth outer surface and minimal sticking to each other occurs. Almost no feathering is observed.

The extrudate is stacked 1 inch deep in the collection tray and dried in air at 80 ° C. for 12 hours. They are then calcined in air. The calcination temperature is raised from room temperature to 500 ° C. at a rate of 2 ° C./min, maintained at 500 ° C. for 2 hours, raised from 500 ° C. to 700 ° C. at a rate of 10 ° C./min, and held at 700 ° C. for 3 hours. After cooling down to room temperature.

Some physical properties of the calcined titania extrudate are outlined in Table 1. The crush strength of the calcined titania extrudate is measured with a Chatillon crush strength analyzer (Model DPP 50). For 25 measurements, the average of the forces required for failure is reported. Bulk density is measured by placing 40 g of calcined extrudate in a 100-mL graduated cylinder (nominal outer diameter 1 "). Tapping the graduated cylinder until there is no apparent volume change The value is then divided by mass to calculate the bulk density: The voidage is measured by adding pellets to 50 mL water in a second graduated cylinder and then tapping until all voids are filled. The water level obtained is subtracted from the total volume of pellets and water taken separately to obtain the void volume occupied by water The total pore volume is poured through a sieve basket and shaken to remove excess water and then wet The extrudate is measured by weighing The increase in mass for the initial 40 g of extrudate divided by the density of water is taken as a measure of the pore volume.

Comparative Example 2

The procedure of Example 1 is repeated with the formulation shown in Table 1. The extrudate tends to droop when exiting the die face of the extruder and tend to stick together when in the pool tray.

Comparative Example 3

The procedure of Example 1 is repeated with the formulation shown in Table 1. The extrudate tends to droop when exiting the die face of the extruder and tend to stick together when in the pool tray.

Comparative Example 4

The procedure of Example 1 is repeated except that the formulation is as follows: DT-51 (2000 g), TAMOL ™ 1124 dispersant (Hydrophilic polyelectrolyte copolymer from The Dow Chemical Company, 32.6 g), METHOCEL ™ K4M Cellulose derivative (54.6 g), lactic acid (6 g), water (950 g), aqueous ammonium hydroxide (14.8 M, 70 g).

Comparative Example 5

And to repeat the procedure in Example 4 except that alumina ^(DISPERAL? P2, available from Sasol, 20 g). The extrudate tends to droop when exiting the die face of the extruder and to stick together when in the metal tray. The calcined extrudate comprises 1 wt% alumina and 99 wt% titania.

Example 6

The procedure of Example 1 is repeated except that the formulation is as follows: DT51 (300 g), TAMOL ™ 1124 dispersant (5 g), WALOCEL ™ C cellulose (6 g), METHOCEL ™ K4M cellulose derivative (6 g ), Lactic acid (4.5 g), water (155 g) and aqueous ammonium hydroxide (14.8 M, 11 g). The extrudate has a smooth outer surface. Minimal feathering is observed. Almost no extrudate is observed to stick with others.

Comparative Example 7

The procedure of Example 6 is repeated except that the formulation is shown in Table 3. The extrudate falls off as it exits the die. They stick to each other in the collection tray.

Comparative Example 8

The procedure of Example 6 is repeated except that the formulation is shown in Table 3. There is no tendency for the extrudate to stick to each other after they have been placed in the harvesting tray. However, they appear to have feathering.

Example 9

NaHCO ₃ powder (27 g) is added slowly to an aqueous solution comprising Na ₂ PdCl ₄ .3H ₂ O (31.4 g), NaAuCl ₄ .2H ₂ O (11.3 g) and water (235.4 g). The mixture is stirred at room temperature for 10 minutes. The solution is sprayed onto a calcined titania extrudate (1000 g) prepared in Example 1 using a pipette while tumbling it in a rotating flask. Once impregnation is complete, the rotating flask is heated to about 100 ° C. with a heat gun. The impregnated extrudate is tumbled at 100 ° C. for another 30 minutes, then placed in an oven at 80 ° C. for 2 hours and then cooled to room temperature.

The dried extrudate is washed with warm water (50-80 ° C.) until no chloride is detected by mixing the wash filtration solution with 1 wt% silver nitrate solution to observe precipitation. After washing is complete, the catalyst is dried at 80-100 ° C. to remove water. They are then heated in air at 230 ° C. for 3 hours and under nitrogen flow at 230 ° C. for 30 minutes. The temperature is raised to 500 ° C. under a flow of 10 mol% hydrogen in nitrogen gas, held for 3 hours, and then cooled to room temperature.

The extrudate is washed with an aqueous solution (10 L) comprising 10 wt% potassium acetate and 1 wt% potassium hydroxide. The washed extrudate is dried under nitrogen at 125 ° C. for 2 hours. Palladium-gold catalyst is obtained. It comprises 0.93 wt% Pd, 0.54 wt% Au and 1.5 wt% K.

Example 10

The palladium-gold catalyst prepared in Example 9 is tested for vinyl acetate production in a fixed bed reactor (stainless steel, 1 inch OD). The reactor is charged with a mixture of catalyst (10 g) and inert alpha alumina cylindrical pellets (1/8 "diameter, surface area 4 m ² / g, pore volume 0.25 mL / g, 25 g). The feed is 46.1 mol%. Helium, 33.9 mol% ethylene, 11.48 mol% acetic acid, 4.2 mol% oxygen and 4.2 mol% nitrogen The reactor pressure is 80 psig, and the space velocity for the volume of catalyst is 3050 h ^{- 1} at standard temperature and pressure The reactor is cooled using a fluidized sand bath with the temperature set at 130 ° C. The resulting stream is analyzed by gas chromatography (GC) for oxygen conversion between 75 to 100 hours, for vinyl acetate. Oxygen selectivity, oxygen yield and ethylene selectivity for vinyl acetate are calculated from the GC results, which are outlined in Table 4. Oxygen conversion is calculated by dividing the amount of oxygen consumed by the total amount of oxygen supplied to the reactor. Oxygen selectivity for vinyl acetate is the amount of oxygen consumed in vinyl acetate production divided by the total amount of oxygen consumed Oxygen yield for vinyl acetate is the result of oxygen conversion multiplied by oxygen selectivity Ethylene selectivity for vinyl acetate Is the amount of ethylene consumed in the production of vinyl acetate divided by the total amount of ethylene consumed The catalyst productivity is grams of vinyl acetate produced per liter of catalyst per hour.

Claims (15)

(a) mixing titania, carboxyalkyl cellulose and hydroxyalkyl cellulose to form a dough; (b) extruding the dough to produce an extrudate; And (c) calcining the extrudate to produce a calcined extrudate; (d) impregnating the calcined extrudate with a palladium compound and a gold compound to produce an impregnated extrudate; And (e) calcining the impregnated extrudate to produce a palladium-gold catalyst. The method of claim 1, further comprising reducing the palladium-gold catalyst to produce a reduced catalyst. The method of claim 2, wherein the palladium-gold catalyst is reduced with hydrogen. 4. The process of claim 3 further comprising adding an activator to the reduced catalyst to produce a promoted palladium-gold catalyst. The method of claim 1 wherein the titania is anatase. The method of claim 1 wherein the weight ratio of carboxyalkyl cellulose to titania is from 1: 100 to 3: 100. The process of claim 1 wherein the weight ratio of hydroxyalkyl cellulose to titania is from 0.5: 100 to 1: 100. The method of claim 1 wherein the weight ratio of carboxyalkyl cellulose to hydroxyalkyl cellulose is 3: 1 to 1: 1. The method of claim 1 wherein the hydroxyalkyl cellulose is selected from the group consisting of methyl 2-hydroxypropyl cellulose, methyl 2-hydroxyethyl cellulose and mixtures thereof. The method of claim 1 wherein the extrudate is calcined at a temperature of 650 to 750 ° C. (a) mixing titania, carboxyalkyl cellulose and hydroxyalkyl cellulose to form a dough; (b) extruding the dough to produce an extrudate; And (c) calcining the extrudate to produce a calcined extrudate; (d) impregnating the calcined extrudate with a palladium compound and a gold compound to produce an impregnated extrudate; (e) calcination of the impregnated extrudate to produce a palladium-gold catalyst, comprising reacting ethylene, oxygen and acetic acid in the presence of a palladium-gold catalyst prepared according to the method. . 12. The process of claim 11, wherein the palladium-gold catalyst is further reduced. 13. The process of claim 12 wherein the palladium-gold catalyst is reduced with hydrogen. The method of claim 13, wherein the reduced catalyst is further promoted by an activator. The method of claim 11, wherein titania is anatase.