CZ282053B6 - Coated impregnated catalyst used in the preparation of vinyl acetate and process for preparing vinyl acetate - Google Patents

Abstract

Catalyst for vinyl acetate prodn. from ethylene, acetic acid and O2-contg. gas has a productivity of above 661g/hr. per litre of catalyst at 150 deg.C and consists of (i) a support with particle dia. 3-7mm and pore vol. 0.2-1.5 ml/g; (ii) Pd amd Au distributed in the outermost 1.0mm layer of the support particles, with the Au:Pd wt. ratio being 0.60-1.25, rather than the value of below 0.5 as per US4048096 or the apparently non-critical value of ca. 0.05-1.5 as per US4087622; and (iii) 3.5-9.5 wt% of K acetate.

Description

(57)

Coated impregnated catalyst for use in the preparation of vinyl acetate from ethylene, acetic acid and oxygen-containing gas

(a) a catalyst support having a particle diameter of 3 to 7 mm and a pore volume of 0.2 to 1.5 ml per gram;

(b) palladium and gold distributed in the outer 1,0 mm thick layer of catalyst support particles; and

c) from 3.5 to 9.5 wt. potassium acetate, wherein the weight ratio of gold to palladium in said catalyst is in the range of 0.60 to 1.25 and

(d) a palladium content of more than 3.0 grams per liter of catalyst.

Catalysts are highly selective with respect to vinyl acetate production by suppressing by-products such as carbon dioxide.

Coated impregnated catalyst for use in the preparation of vinyl acetate and a process for the preparation of vinyl acetate

Field of the invention

The present invention relates to improved palladium / gold catalysts suitable for the production of vinyl acetate from ethylene, acetic acid and oxygen-containing gas.

BACKGROUND OF THE INVENTION

The preparation of vinyl acetate by the reaction of ethylene, acetic acid and oxygen in the gas phase in the presence of a catalyst containing palladium, gold and an alkali metal acetate as a promoter is a known process. The catalyst components are typically placed on a porous support material such as silica or alumina.

In earlier examples of these catalysts, palladium and gold are distributed more or less homogeneously by the support (see, eg, US 3725680, US 3743607 and GB 1333449). This was later found to be disadvantageous since it was found that the material in the inner part of the support did not participate in the reaction, as the reactants could not diffuse significantly into the support before the reaction. In other words, a significant amount of gold will never come into contact with the reactants.

To overcome this problem, new catalyst production methods have been developed in which the active ingredients have been concentrated in the outer shell of the support (coated impregnated catalysts). For example, GB 1500167 claims catalysts in which at least 90% of the palladium and gold are distributed in this part of the carrier particle that is not more than 30% of the particle radius from the surface, while GB 1283737 mentions that the degree of penetration into the pores of the carrier can be controlled. by pretreating the carrier pores with an alkaline solution, for example sodium carbonate or sodium hydroxide.

Another approach found for the production of particularly active catalysts is described in US 4048096. In this patent, coated impregnated catalysts are produced by a process comprising the steps of impregnating a carrier with aqueous solutions of water-soluble palladium and gold compounds, the total volume of these impregnated catalysts. (2) precipitation of the water-insoluble palladium and gold compounds on the carrier by soaking the impregnated carrier in an alkali metal silicate solution, wherein the amount of alkali metal silicate is such that upon contact of the alkali metal silicate with the support, 12 to 24 hours, the pH of the solution is from 6.5 to 9.5, (3) converting the water-soluble palladium and gold compounds to metal palladium and gold by treatment with a reducing agent, (4) washing with water, (5) introducing the catalyst into contacting the alkali metal acetate and (6) drying the catalyst. Using these methods, catalysts having a specific activity of at least 83 grams of vinyl acetate per gram of noble metal measured at 150 ° C are obtained.

It is stated in column US 4048096 that the process described above is particularly suitable for preparing catalysts containing 1.65-3.3 grams of palladium and 0.75-1.5 grams of gold per liter of final catalyst. For a typical silica carrier having a density of 600 grams per liter, these ranges are in% by weight. 0.25 to 0.54% and 0.11 to 0.25%; % Au: Pd wt. from 0.23 to 0.91. The amounts of alkali metal acetate which are said to be effective are 5 to 60 grams per liter, preferably 25 to 35 grams per liter, corresponding to 0.75 to 9.2% by weight, in the case of potassium acetate, preferably 3.8 to 5 , 4%.

Six examples of the invention are described in US 4048096. These all disclose the use of catalysts having a gold to palladium weight ratio in the range of 0.42 to 0.45 and a gold content of 2.1 grams per liter

Or less. The most active catalyst described herein is shown in Example 3 and has a gold content of 2.1 grams per liter, a gold to palladium weight ratio of 0.42, and a productivity of 610 grams of vinyl acetate per hour per liter of catalyst at 150 ° C. Computer predictions of this catalyst activity based on data obtained during our studies indicate a higher activity at 661.

It has now been found that coated impregnated catalysts having productivity above 661 grams vinyl acetate per hour per liter catalyst at 150 ° C can be obtained by providing a gold to palladium weight ratio in the range of 0.60 to 1.25. It has further been found that for these metals, productivity can be further improved by providing a potassium acetate content in the catalyst in the range of 3.5 to 9.5% by weight.

This finding is quite unexpected and contrary to the findings of US 4048096, which indicates that the gold to palladium weight ratio used should be less than 0.5.

Coated catalysts (impregnated) are also disclosed in US 4087622. Column 1, lines 47-50, states that the percentage of metallic gold to the weight of palladium and gold in the form of metals is preferably from about 5 to 60 weight percent. Such values correspond to a wide range of gold to palladium weight ratios of 9.05 to 1.5, indicating that nothing significant has been said to criticize these variables. Furthermore, a wide range of alkali metal contents such as acetates (1 to 30 wt%) is shown, and only catalysts having a potassium acetate content of 3% are given in the examples. Examples 4-1 and 4-7 show the effect of changing the gold content of the catalyst relative to solid palladium loading, but there is nothing to confirm the criticality of the gold to palladium ratio over a wide palladium loading range.

SUMMARY OF THE INVENTION

According to the present invention there is provided a coated impregnated catalyst for use in the preparation of vinyl acetate from ethylene, acetic acid and an oxygen-containing gas comprising

(a) a catalyst support having a particle diameter of 3 to 7 mm and a pore volume of 0.2 to 1.5 ml per gram;

(b) palladium and gold distributed in the outer 1,0 mm thick layer of catalyst support particles; and

c) from 3.5 to 9.5 wt. potassium acetate, wherein the weight ratio of gold to palladium in said catalyst is in the range of 0.60 to 1.25, and

(d) a palladium content of more than 3.0 grams per liter of catalyst.

The catalysts of the present invention have the further advantage that they are highly selective with respect to the production of vinyl acetate by suppressing by-products such as carbon dioxide.

With respect to the catalyst components, it is preferred that the catalyst support is either a porous silica, alumina, silica / alumina, or titanium dioxide. The support should have a specific surface area in the range of 100 to 800 m ² per g. With respect to the palladium content of the catalyst, this is preferably in excess of 2.5 grams per liter of catalyst in order to obtain the high productivity mentioned above. For typical catalysts having a sodium content of about 0.5 wt%, the palladium content should be greater than 3.0 grams per liter of catalyst, more preferably greater than 3.9 grams per liter of catalyst, and most preferably in the range of 3.9 to 6, 1 grams per liter of catalyst. On the other hand, the gold content should preferably be higher than 1.5 grams per liter of catalyst and for catalysts having the above sodium content preferably higher than 1.8 grams per liter of catalyst, more preferably higher than

2.3 grams per liter of catalyst, and most preferably in the range of 2.3 to 7.6 grams per liter of catalyst. Finally, it is preferred that the weight ratio of gold to palladium is in the range of 0.7 to 1.05. most preferably 0.85 to 1.05.

-2GB 282053 B6

In view of the potassium acetate content, it is preferred that it is in the range of 5 to 9% by weight for optimum activity.

In an embodiment of the present invention, it has been found that the level of unwanted ethyl acetate by-product can be substantially reduced by using catalysts in which the gold to palladium weight ratio is greater than or equal to about 0.9.

The catalysts of the present invention may conveniently be prepared by the method detailed in GB 1559540. In the first step of this process, the support is impregnated with a solution containing the required amounts of palladium and gold in the form of soluble salts. Examples of salts include soluble halide derivatives. The impregnating solution is preferably an aqueous solution and the volume of solution used is such that it corresponds to 95 to 100% of the pore volume of the carrier, preferably 98-99%.

After impregnation, the wet carrier is treated with an aqueous alkali metal salt solution selected from the group consisting of alkali metal silicates, carbonates or hydroxides. The amount of alkali metal salt used is such that after the solution has been in contact with the impregnated carrier for 12 to 24 hours, the pH of the solution is preferably in the range of 6.5 to 9.5, preferably 7.5 to 8, measured at 25 ° C. Deň: 32 ° C. Preferred alkali metal salts are sodium silicate, sodium carbonate and sodium hydroxide.

In the above-described treatment, it is expected that the palladium and gold hydroxides will precipitate or be incorporated into the carrier. To convert these materials to metal, the impregnated carrier is treated with a reducing agent such as ethylene, hydrazine, formaldehyde or hydrogen. When hydrogen is used, it is usually necessary to heat the catalyst to 100-300 ° C to complete the reaction.

After carrying out the steps described above, the reduced catalyst is washed with water, impregnated with the required amount of alkali metal acetate and then dried.

The catalysts of the present invention, when prepared as described above, typically contain 0.5 wt. sodium, predominantly derived from a precipitating agent. As will be described in more detail in other separate applications, it is desirable that the sodium content of such catalysts be less than this in order to achieve optimum catalyst productivity. The sodium content of the catalyst can be reduced, for example, by washing with a potassium acetate solution or by using non-sodium reagents.

The preparation of vinyl acetate using the catalysts of the invention is typically carried out by contacting ethylene, acetic acid and oxygen or air with a catalyst sample at a temperature in the range of 100 to 200 ° C, preferably in the range of 140 to 180 ° C and pressure at atmospheric pressure to 20 bar. Typically, the process is carried out heterogeneously such that the reactants are present in the gas phase and with oxygen levels below the flammability limits. The reaction is preferably carried out with an excess of ethylene, while the amount of acetic acid is determined by the dew point. After the reaction, the vinyl acetate is separated and purified by conventional methods.

The present invention will now be illustrated by the following non-limiting examples.

DETAILED DESCRIPTION OF THE INVENTION

General method for the preparation of catalyst samples

Only deionized water was used in this method.

-3 CZ 282053 B6

Step (1) Impregnation of the gram carrier of a high specific surface area silica spherical carrier (KA160-ex Sud Chemistry) was added to 8.7 ml of an aqueous solution of Na ₂ PdCl ₄ and HAuCl ₄ . The amount of palladium-gold complexes used is such that the desired loading of the palladium-gold carrier is achieved. The addition was done in portions and the mixture was gently swirled until the solution was fully absorbed. After impregnation, the impregnated support was allowed to stand at room temperature for two hours.

Step / 2 / Precipitation

A solution of 18 ml of sodium metasilicate in water was quickly added to the impregnated carrier. The concentration of 18 ml sodium metasilicate (SMS) was determined using the following formula:

1.8 x [/ 1 mol SMS / mol Na ₂ PdCl ₄ / + / 2 mol SMS / mol HAuCl ₄ + / 0.02 millimoles SMS / gram carrier /]. The mixture was vortexed continuously for 15 minutes and allowed to stand overnight.

Stage / 3 / Reduction

The aqueous phase over the black pellets was treated with 85% hydrazine hydrate solution. The amount of hydrazine hydrate used was determined using the following formula:

22.5 x [/ 1 mol N ₂ H ₄ / mol Na ₂ PdCl ₄ / + / 1.5 mol N ₂ H ₄ / mol HAuCl ₄ ]]. The mixture was gently swirled and then allowed to stand undisturbed overnight.

Step (4) Washing

The aqueous phase, which contains a small amount of suspended black solids, is decanted and the beads are washed four times with about 50 ml of water, decanting after each wash. The catalyst is transferred to a glass column equipped with a stopcock and then washed further with water at an approximate rate of one liter every 12 hours until the wash water is negative for the chloride test with a silver nitrate solution.

Steps (5) - / 7 / Drying, loading with potassium acetate and final drying

The catalyst is dried overnight at 60 ° C in a forced-flow air oven, cooled, then impregnated with a solution of the required amount of potassium acetate in 8.7 ml of water. The mixture is vortexed until all the liquid has been absorbed, then the catalyst is dried again at 60 ° C on a stainless steel sieve in a forced flow air dryer.

Catalyst testing methods and results

The tests are carried out at 7.8 bar of ^{5 bar} overpressure (7.8 barg) and 150 ° C on 2.5 g samples of 5-6 mm catalyst pellets, diluted with 30 ml of 1 mm glass beads and filled into a stainless steel inner tube. diameter 10-11 mm. The catalyst was activated at a pressure of 7,8.10 ⁵ Pa (7.8 barg) by heating at 160 ° C for 3 hours in a stream of nitrogen or helium and then at 150 ° C for 10 minutes in stream of ethylene. The acetic acid vapor is then mixed with ethylene and passed over the catalyst for at least one hour. A mixture of 21% oxygen in helium is added sequentially to the feed gas while maintaining a maximum catalyst bed temperature of 150 ° C. The hot spot of the catalyst is maintained at 150 ° C for 6 hours and then allowed to sink as the catalyst

-4GB 282053 B6 deactivates. The final composition of the reactant mixture was ethylene: acetic acid: oxygen: helium = 53.1: 10.4: 7.7: 28.6 and the total hourly space velocity was 3850 h ^-1 . The product stream was analyzed in the vapor phase at hourly intervals using an on-line gas-liquid chromatograph.

Activity was calculated as grams of vinyl acetate produced per liter of catalyst per hour (space-time yield, STY) and selectivity was calculated as a percentage of the converted ethylene present in the product. All calculated values are based on activities and selectivity measured 20 hours after reaching full oxygen content.

Alternative method for the preparation of catalyst samples

The following is an alternative, improved method for producing the catalyst.

Stage (1) Impregnation of carrier

The same method as above applies.

Step / 2 / Precipitation

The same method as above was used except that sodium hydroxide was used as the precipitating agent. The amount of sodium hydroxide was determined using the formula:

1.8 x [/ 2 mol NaOH / mol Na ₂ PdCl ₄ / + / 4 mol NaOH / mol HAuCl ₄ / + / 0.04 millimol NaOH / gram carrier /]

Stage / 3 / Reduction

The same method as above applies.

Step (4) Washing

The product of step (3) was washed with water as above. Potassium acetate (5%) was used instead of water in the column wash.

Step (5) Drying

The product of step (4) is dried overnight at 60 ° C on a stainless steel sieve in an air circulation oven with forced circulation.

Example 1

Catalyst tests to demonstrate the effect of noble metal content on catalyst activity and selectivity

The catalyst is prepared according to the first general method described above. The precious metal contents vary to prepare a statistically significant set of experiments. The composition of the catalysts tested was

The target potassium acetate content in each case was 7.0% by weight, corresponding to 2.8% by weight. potassium. The carrier obtained contained 0.4 wt. % of potassium, corresponding to a total potassium content of 3.2 wt. In each experiment, the catalyst was used in an amount varying from 0.5 to 2.5 g to allow measurement of catalyst activity and selectivity over a range of concerts. The sodium content of the catalysts was approximately 0.5 wt.

The test conditions that were used are described above. The total flow rate of the reactants was such that, for the test, 2.5 g of GHSV catalyst was 3850 h ^-1 . For tests on smaller amounts of catalyst, the total flow rate was maintained, providing compensation for changes in GHSV. Activity, selectivity and oxygen conversion measurements were performed after 20 hours of introduction and are also listed in Table 2.

Changes in activity, selectivity and conversion with catalyst composition and weight are best expressed by the following expressions:

Equation 1. STY = e ^y , where y = 6.76 + 0.40 / Pd-0.82 / + 0.13 / Au / Pd-0.81 / - 0.57 / Au / Pd-0.81 / ² - 0.16 / Cat.Wt-1.5 /

R ² = 0.95

Equation 2. Selectivity = 100 - e ^z , where:

z = 1.41 - 0.29 / Au / Pd-0.81 / + 0.49 / Au / Pd-0.81 / ² - 0.39 (K-3.25) + 0.10 / Cat. Wt-1.5 / -0.10 / Cat.Wt 1.5 / ² R ² = 0.74

Equation 3. Oxygen Conversion:

= 24.5 + 10.0 / Pd-0.82 / + 7.74 / Pd-0.82 // Cat.Wt-1.5 / + 1.97 / Au / Pd-0.81 / - 14 , l / Au / Pd-0.81 / ² + 1 l, 6 / Cat.Wt-1.5 / R ² = 0.96

Abbreviations used in the equations mean Pd = weight percent palladium in the catalyst, Au / Pd = weight ratio gold to palladium, Cat.Wt = weight catalyst in grams, K = weight percent potassium, STY = space-time yield in grams of vinyl acetate per liter of catalyst per hour . Correlation coefficients R ‘indicate a good match with the data.

In order to determine the effect of the catalyst composition at constant conversion, Equation 3 was adjusted to express the catalyst weight as a function of palladium content, gold / palladium ratio and conversion. A representative conversion of 30% was introduced into this function. The catalyst weights expressed in Equations 1 and 2 were then replaced by the mass function obtained from the modified Equation 3 to provide expressions describing variations in catalyst activity and selectivity with metal content at constant conversion.

The effect of the gold / palladium ratio is shown in Figure 1, while the effect on selectivity is shown in Figure 2. The curves show that improved catalyst activity can be achieved by increasing the catalyst loading with palladium and increasing the gold / platinum ratio to an optimum of 0.9 to 0.9. 0.95. A further increase in the gold / palladium ratio above that reduces the activity of the catalyst.

An increase in selectivity can also be achieved by increasing the gold / palladium ratio. Finally, as shown in Fig. 3, catalysts having a gold to palladium ratio of 0.9 or higher produce little or no by-product.

-6GB 282053 B6

Table 1

Values used for modeling STY, selectivity and conversion containing palladium and Au / Pd ratio

Pd Ouch Au / Pd TO Cat. HUNDREDTH Sel. / measured after Cbkonv. 20 h / 0.58 0.26 0.45 3.3 0.77 801 95.4 12.3 0.58 0.26 0.45 3.3 2.49 602 94.5 30.0 0.88 0.42 0.48 3.3 2.50 702 94.9 36.3 0.91 0.34 0.37 3.2 2.50 654 93.8 33.9 0.91 0.34 0.37 3.2 2.50 624 94.2 32.2 0.91 0.34 0.37 3.2 0.76 835 95.2 12.7 1.01 0.44 0.44 3.1 2.50 727 94.7 35.9 0.91 0.34 0.37 3.2 1,25 747 94.7 19.0 1.06 1,21 1.14 3.1 0.85 993 96.2 16.0 0.58 0.83 1.43 3.2 2.50 592 95.4 28.6 0.58 0.83 1.43 3.2 0.80 771 96.0 H.4 0.92 0.83 0.90 3.1 2.46 760 95.4 35.1 1.00 0.87 0.87 3.1 1.55 939 95.3 28.0 1.01 0.44 0.44 3.1 0.80 991 95.1 15.7 0.91 0.34 0.37 3.2 0.50 879 95.8 9.2 0.91 0.34 0.37 3.2 2.09 709 94.0 30.7 0.74 0.49 0.66 3.3 2.50 693 94.8 35.7 0.70 0.29 0.41 3.4 2.50 644 95.2 32.2 0.88 0.42 0.48 3.3 2.50 691 94.9 36.3 0.87 1.16 1.33 3.2 0.85 916 96.0 14.5 0.58 0.26 0.45 3.3 2.48 555 95.5 25.3 0.92 0.83 0.90 3.1 1.48 937 06.2 26.6 0.87 1.16 1.33 3.2 0.89 857 96.4 14.0

3.2 wt. K is approximately equal to 7.0 wt. KoAc

Example 2

Catalyst tests to demonstrate the effect of potassium acetate content on catalyst activity and selectivity

Catalysts containing potassium acetate to some extent were prepared by the general method described above as part of a statistically significant set of experiments. The catalyst feed was synthesized with a target palladium loading of 0.9 wt. and a gold / palladium ratio of 0.44. Potassium acetate target levels of 0, 3, 5, 7, 9 wt. were impregnated on the same catalyst batches. Metal analyzes were determined using XRF and are shown in Table 2.

Catalysts were evaluated using the conditions described above. The activities (STY, g VA / h / L catalyst) and selectivity (% based on ethylene) measured after 20 h flow are also shown in Table 2.

Variations in activity are best expressed by the equation:

STY = 160.5 KoAc - 12.4 KoAc ² + 147

R ² = 0.998 where KoAc = actual KoAc mass%

The square of the correlation coefficients (R2 ⁾ indicates a good match of the data. The effect of potassium loading on activity (STY) is shown in Figure 4. The figure clearly demonstrates the extent of optimal potassium acetate loading that provides improved catalyst activity. It has also been found that selectivity increases above the range of potassium levels studied.

Claims (8)

PATENT CLAIMS A coated impregnated catalyst for use in the preparation of vinyl acetate from ethylene, acetic acid and an oxygen-containing gas, characterized in that the catalyst comprises: (a) a catalyst support having a particle diameter of 3 to 7 mm and a pore volume of 0.2 to 1.5 ml per gram; (b) palladium and gold distributed in the outer 1,0 mm thick layer of catalyst support particles; and c) from 3.5 to 9.5 wt. potassium acetate, wherein the weight ratio of gold to palladium in said catalyst is in the range of 0.60 to 1.25, and (d) a palladium content of more than 3.0 grams per liter of catalyst. Catalyst according to claim 1, characterized in that the weight ratio of gold to palladium is in the range of 0.7 to 1.05. 3. The catalyst of claim 2 wherein the weight ratio of gold to palladium is in the range of 0.85 to 1.05. The catalyst of claim 1 or claim 3, wherein the ratio of gold to palladium is greater than 0.9. by weight Catalyst according to any one of claims 1 to 4, characterized in that the potassium acetate content is 5-9% by weight. Catalyst according to any one of claims 1 to 5, characterized in that the palladium content is greater than 3.9 grams per liter of catalyst. -8EN 282053 B6 Catalyst according to claim 6, characterized in that the palladium content is in the range 3.9 to 6.1 grams per liter of catalyst. 8. A process for the preparation of vinyl acetate comprising reacting ethylene 5 with acetic acid in the presence of an oxygen-containing gas at a temperature in the range of from 100 to 100. 200 ° C in the presence of a coated impregnated catalyst according to any one of claims 1 to 7.