CA2018991C

CA2018991C - Process for the preparation of vinyl acetate

Info

Publication number: CA2018991C
Application number: CA002018991A
Authority: CA
Inventors: Friedrich Wunder; Peter Wirtz; Klaus Eichler; Gunter Roscher
Original assignee: Celanese GmbH
Current assignee: Celanese GmbH
Priority date: 1989-06-15
Filing date: 1990-06-14
Publication date: 2001-04-24
Anticipated expiration: 2010-06-14
Also published as: EP0403950A1; JPH0324034A; DE3919524A1; CA2018991A1; ES2070957T3; EP0403950B1; MX173275B; JP2911547B2; BR9002802A; AU633102B2; DE59008459D1; AU5704990A

Abstract

The invention relates to a process fox the preparation of vinyl acetate in the gas phase from ethylene, acetic acid and oxygen and/or oxygen-containing gases on a catalyst of palladium and/or compounds thereof, and optionally.
in additions gold and/or gold compounds, and, as activators, alkali metal compounds and optionally in addition, cadmium compounds on a support. The support comprises SiO2 or an SiO2-Al2O3 mixture having a surface area of 50-250 m2/g and a pore volume of 0.4-1.2 ml/g, and has a grain size of from 4 to 9 mm. 5 to 20% of the pore volume of the support is farmed by pores having radii of from 200 to 3,000 angstrom and 50 to 90% of the pore volume is formed by pores having radii of from 70 to 100 .ANG.ngstrom.

Description

F
HOECHST ARTIENGESELLSCHAFT HOE 89/F' 183 Dr. MA/rh Description Process for the preparation of vinyl acetate It is known that ethylene can be reacted in the gas phase with acetic acid and oxygen or oxygen--containing gases on fixed-bed catalysts to form vinyl acetate. Suitable catalysts contain a noble-metal component and an activator component. The noble-metal component preferably comprises palladium and/or compounds thereof; in addi-20 tion, gold and/or compounds thereof may be presewt (US Patent 3,939,199, German Offenlegungsschrift 2,100,778, US Patent 4,668,81.9). The activator component here comprises compounds of elements of main group 1 and/or main group 2 and/or cadmium. Potassium is Z5 preferred as the element of main group 1. These active components are applied, in finely divided form, to supports, the support material used generally being silica or alumina.
The specific surface area of the supports is generally 20 40-350 m~/g. According to tJS Patent 3,939,199, the total pore volume should be 0.4-1.2 ml/g, and of this, less than 10~ should be formed by "micropores°° having a pore diameter of less than 30 angstrom. Examples of suitable supports having these properties are aerogenic Si02 or an 25 aerogenic SiOa-A1Z03 mixture. The support particles iwthe preparation of vinyl acetate generally have a spherical shape. However, tablets and cylinders have also already been employed.
The invention relates to a process for the preparation of 30 vinyl acetate in 'the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases on a catalyst which contains palladium and/or compounds thereof, and optionally in addition, gold and/or gold compounds, and, as activators, alkali metal compounds and 35 optionally in addition, cadmium compounds on a support .. v 29381-4 ca o2ois99i 2ooo-oi-3i which comprises Si02 or an Si02-A1203 mixture having a surface area of 50-250 m2/g and a pore volume of 0.4-1.2 ml/g, and has a grain size of from 4 to 9 mm, wherein 5 to 20% of the pore volume of the support is formed by pores having radii of from 200 to 3,000 Angstrom and 50 to 900 of the pore volume is formed by pores having radii of from 70 to 100 Angstrom.
Preferably, catalysts which comprise palladium and gold as metals and in which more than 90o by weight of each of these two metals is in the outer part of the support particles, extending to a depth of 300 of the support particles are excluded.
Preferably, 8 to 150 of the pore volume of the support is formed by pores having radii of from 200 to 3,000 Angstrom, and 55-750 of the pore volume is formed by pores having radii of from 70 to 100 Angstrom.
Supports of this type are obtained as follows:
first, glassy microspheres are produced, for example by flame hydrolysis of silicon tetrachloride or a silicon tetrachloride/aluminum trichloride mixture in an oxyhydrogen flame (US Patent 3,939,199). The microspheres can also be produced by melting very fine Si02 dust in a sufficiently hot flame and subsequently rapidly cooling the melt. The microspheres produced by one of the two methods have a surface area of 100-300 m2/g. Particularly suitable microspheres are those having a surface area of 150-250 m2/g which comprise at least 95o by weight of Si02 and at most 5o by weight of A1203, in particular comprise at least 99o by weight of Si02 and at most to by weight of A1203. Microspheres having the surface area mentioned are commercially available, for example under _ ~_ 29381-4 CA o2ois99i Zooo-oi-3i 2a the name (R)Aerosil or (R)Cabosil or as "highly disperse silica" .
The microspheres are then pressed, for example by tableting (after precompaction) or extrusion, using organic fillers (such as sugars, urea, higher fatty acids, longer-chain paraffins, microcrystalline cellulose) and lubricants (such as kaolin, graphite, metal soaps) to form moldings. Ignition of these moldings in 02-containing gases subsequently removes these auxiliaries again. The surface area of the finished support, its pore volume and the proportion of the pore volume formed by pores of a 3 _ certain radius ("pore-radius distribution") are deter-mined by the type of deformation (tablets, extrudates, etc.), the temperature and duration of ignition, 'the relative amounts of fillers, lubricants and microspheres, and the surface area of the microspheres. The most suit-able values for these determining parameters can be determined by sample preliminary experiments.
It is also possible to omit lubricants and fillers and instead to add a silica sol to the microspheres, and then to dry and ignite the material. This method is parti-cularly suitable for extrudates. In this method, the surface area, pore volume and "pore-radius distribution°' (see above) are determined by the following parameterse type of silica sol used (size of tine primary particles, measurable from the Tyndall effect), the 'type of micro-spheres used, the drying rate and temperature, and the ignition duration and temperature. Again, the most suitable values for these parameters can be determined by simple preliminary experiments.
The finished supp~rt obtained by one of the two methods then has a surface area of from 50 to 250 m2/g and a pore v~lume of from 0.4 to 1.2 ml/g, and a grain size of from 4 to 9 mm (adjustable by tableting or extrusion).
By using supports having the specific pore-radius dis-tribution mentioned, it is possible to significantly increase the space-time yield of the catalysts compared with conventional supports under otherwise identical conditions (same content of active substances on the support and same reaction conditions) and simultaneously to reduce the principal side reaction, the combustion of ethylene to form CO2, by more than 30~. Likewise, the formation of ethyl acetate, which proceeds as a further side reaction, is substantially reduced. The advantages of the process according to the invention are that this increase in selectivity from about 92~ to about J5&
allows significant savings to be achieved, and that, in ~o:~.~~~~.
-~_ addition, the increase in performance with significantly increased selectivity in new plants means that the amount of catalyst and the reactor volume can be reduced, which results in considerable reductions in plant costs, or that, in existing plants, the capacity can be substan-tially increased witho~.rt rebuilding and the investment costs for plant expansion can thus be saved.
The surface area of the supports mentioned is in all cases the so-called BET surface area, measured by the method of Brunauer, Emmett and Teller. It gives the total surface area of ~. g of support material, i.e. the sum of the external surface area of the support and the internal surface area of all the open pores. The total pore volume and the proportion thereof provided by pores of a cextain size (for example those having a diameter of from 70 to Z00 l~ngstrom) can be measured using mercury porosimetry.
Suitable measurement equipment is produced, for example, by the Carlo Erba or Micromeritics companies.
The catalytically active substances axe applied to the support in the customary manner, for example by impreg-na~ting the support with a solution of the active substances, subsequently drying and, if necessary, reducing the material. However, it is also possible to apply the active substances to the support by, for example, precipitation, spraying, vapor-deposition ar dipping.
Suitable solvents for the catalytically active substances are, in particular, unsubstituted carboxylic acids having 2 to 10 carbon atoms in the molecule, such as acetic acid, propionic acid, n- and iso-butyric acid and ~.he various valeric acids. Due to their physical properties and also for reasons of economy, acetic acid is prefer-ably used as the solvent. The additional use of an inert solvent is expedient if the subs~tancos are not suffi-ciently soluble in the carboxylic acid. Thus, for ex-ample, palladium chloride is substantially more soluble ~o~.~oo~
-in aqueous acetic acid than in glacial acetic acid.
Suitable additional solvents are those which are inert and miscible with the carboxylic acid. Examples which may be mentioned, besides water, are ketones, such as acetone 5 and acetylacetone, furthermore ethers, such as tetra-hydrofuran or dioxane, but also hydrocarbons, such as benzene.
The catalyst contains palladium and/or compounds thereof as the noble-metal component and alkali metal compounds ~.0 as the activator component. It may contain gold and/or compounds thereof as an additional noble-metal component and it may contain cadmium compounds as an additional activator component.
Suitable palladium compounds are alI salts and complexes which axe soluble (and also, where appropriate, reducible) and do not leave any deactivating substances, such as halogen or sulfur, in the finished catalyst. The carboxylates, preferably the salts of aliphatic moncar-boxylic acids having ~. to 5 carbon atoms, for example the acetate, the propionate or the butyrate, are particularly suitable. The nitrate, nitrite, oxide hydrate, oxalate, acetylacetonate or acetoacetate, for example, are also suitable. However, compounds such as the sulfate and the halides can also be used if care is taken that the sulfate radical is removed, for example by precipitation using barium acetate, or the halogen is removed, for example by precipitation using silver nitrate, before impregnation, so that the sulfate or halogen anion does not enter the support. Due to its solubility and acces-sibility, palladium acetate i.s the particularly preferred palladium compound.
In general, the cowtent of palladium in the catalyst is 1.0 to 3~ by weight, preferably 1.5 to 2.5~ by weight, in particular 2 to 2.5~ by weight, the proportion of metal being related to the total weight of the supported catalyst.

6 _ Besides palladium and/or compounds thereof, it is also possible for gold and/or compounds thereof to be addi-tionally present. A particularly suitable gold compound is barium acetoaurate. In general, gold or one of its compounds, if it is employed, is added in a proportion of 0.2 to 0.7~ by weighty the proportion of metal being related to the total weight of the supported catalyst.
The catalyst contains, as activators, alkali metal Compounds arid, o~p$iQSl~lly ir1 addi$iori Cadml.um Com-pounds. Examples of suitable compounds are alkali metal carboxylates, such as, for example, potassium acetate, sodium acetate, lithium acetate and sodium propionate.
Those alkali metal compounds which are converted into the carboxylates under the reaction conditions, such as, for example, hydroxides, oxides and carbonates, are also suitable. Suitable cadmium compounds are those which contain no halogen or sulfur, for example earboxylate (preferred), oxide, hydroxide, carbonate, citrate, tartrate, nitrate, acetacetonate, benzoylacetonate and.
acetylacetate. Cadium acetate is particularly suitable.
It is also possible to employ mixtures of different ac tivators. Each individual activator is generally added in a proportion of 0.5-4~ by weight, the proportion of metal in the activator being related to the total weight of the supporting catalyst.
The following catalysts are preferred:
Palladium/alkali metal element/cadmium and palladium) gold/alkali metal element, it being possible for pal-ladium or gold to be in the form of metals or compounds in the finished catalyst and the preferred alkali metal element being potaso:ium (in the form of a carboxyJ.ate).
The K:Pd or K:(Pd+Au) ratio here is preferably 0.?:1 to 201. The Cd:Pd or Cd:(Pd+Au) ratio is preferably 0.6:1 to 2:Z, in par~tioular 0.6x1 to 0.9a~.. Pd, Au, Cd and K are 95 always calculated as elements here, i.e>, for example, only the metal proportions of Pd acetate, Cd acetate and K acetate on the support are compared with one another.
The catalysts palladium acetate/potassium acetate/cadmium acetate and palladium acetatelbarium acetoaurate/potas-sium acetate are particularly preferred.
The impregnation of the catalyst support with the solution of the active components is preferably carried out by coating the support material with the solution and then pouring off or filtering off 'the excess solution.
With consideration for solution losses, it is ~.0 advantageous only to employ the solution corresponding to the integral pore volume of the catalyst support and to carry out mixing carefully so that the particles of the support material are uniformly wetted. This mixing can be achieved, for example, by stirring. Tt is expedient to carry out the impregnation process and the mixing s3.mu1-taneously, far example in a rotary drum or a drum drier, it being possible for the drying to be parried out immediately thereafter. 3t is furthermore expedient to ad just the amount and composition of the solution used.
for impregnation of the catalyst support in a manner such that it corresponds to the pare volume of the support material and that the desired amouwt of active substaxaces is applied by a single impregnation.
The patalyst support impregnated with the solution of the active substances is preferably dried under reduced pressure. The temperature during drying should be below 120°C, preferably below 90°C. In addition, it is generally advisable 'to carry out the drying in a stream of inert gas, far examgle in a stream of nitrogen or carbon dioxide. The residual solvent content after drying should preferably be less than 8~ by weight, in par-ticular less than 6~ by weight.
Tf reduction of the palladium cornpounds (and, where appropriate, the gold compounds ) is parried out, which may sometimes be useful, this may be carried out in vacuo, at atmospherip pressure or at elevated pressure of up to 10 bar. It is advisable here to dilute the reducing agent with an inert gas the greater the higher 'the pressure. The reduction temperature is between 40 and 260°C, preferably between 70 and 200°C. It is generally expedient for the reduction to use an inert gas/reducing agent mixture which contains 0.01 to 505 by volume, preferably 0.5 to 20~k by volume, of xeducing agent.
Examples of inert gases which may be used are nitrogen, carbon dioxide or a noble gas. Examples of suitable reducing agents are hydrogen, methanol, formaldehyde, ethylene, propylene, isobutylene, butylene and other olefins. The amount of reducing agent depends on the amount of palladium and, where appropriate, on the amount of gold employed; the reduction equivalent should be a~t least 1- to 1.5-times the oxidation equivalent, but larger amounts of reducing agent have no adverse effect.
For example, at least 1 mole of hydrogen should be used for 1 mole of palladium. The reduction can be carried out after the drying in the same plant.
The vinyl acetate is generally prepared by passing acetic acid, ethylene and oxgyen or oxygen-containing gases at temperatures of from 100 to 220°C, preferably 120 to 200°C, and at pressures of from 1 to 25 bar, preferably 1 to 20 bar, over the finished catalyst, it being pos-sible to circulate unreacted components. The oxygen concentration is expediently kept below 10~ by volume (relative to the acetic acid-free gas mixture). However, dilution with inert gases, such as nitrogen or carbon dioxide, may also be advantageous under certain cir-cumstances. C(32, in particular, is suitable for the dilution in circulation processes, since it is formed in small amounts during 'the reaction.
The examples below are intended to illustrate the invention.

Comparison Example 1 (Spherical support particles comprising conventional silica gel) 200 g of a silica support comprising conditioned (800°C) silica gel spheres 5 - 8 mm in diameter were employed.
The (commercially available] support comprising theca spherical particles had a BET surface area of 169 mz/g and a pore volume of 0. X48 ml/g, formed by 8~s of pores 70-100 ~ngs~trom in diameter and 29~ of pores 200-3,000 angstrom in diameter. The support was impregnated with a solution (corresponding to this pore volume) of 11.5 g of Pd acetate, 10.0 g of Cd acetate and 10.8 g of K acetate in: 66 mi of glacial acetic acid and was dried at 60°C under nitrogen at a pressure of 200 mbar to a residual solvent content of 2~ by weight. This gave a doping of 2.3~a by weight of Pd, 1.8~ by weight of Cd and 2.0~ by weight of K (Cd:Pd = 0.78:1, K:Pd = 0.87:1).
50 m1 of the finished catalyst were introduced into a reaction tube of internal diameter 8 mm and length 1.5 m.
The gas to be reacted was then pried over the catalyst at a pressure of 8 bar (reactor inlet) and a catalyst temperature of 150°C. This gas comprised, at the reactor input, 27~ by volume of ethylene, 55~ by volume of file, 12~
by volume of acetic acid and 6~ by volume of C2. The results are shown in the table.
Comparison Example 2 (Spherical support particles comprising conventional SiO~) 200 g of a silica support which had been pressed from bentonite which had been roasted and then washed with HCl (Si02 content after this washing at 96$ by weight] to form spheres 5--6 mm in diametex were employed. The support comprising these spherical particles had a BET surface area of 121 m~/g and a pare volume of 0.66 ml/g, formed from 21~ of pores 70-100 .~xgstrom in diameter and ~k2~ of pores 200-3,000 angstrom in diameter. The support -particles were impregnated as in Comparison Example 1 (the only difference being that 114 ml of glacial acetic acid were used instead of G6 ml) dried, so that the same doping was present. The catalyst was subsequently tested 5 as in Comparison Example 1. The results are shown in the table.
Exaromple 1 Ta. support was first prepared from Ei02 mi.crospheres having a surface area of 200 mz/g and a filler and a lubricant.
10 The finished support had a pore volume of 0,80 ml/g formed from G2~ of pores 70-100 .$~ngstrom in diameter and 9~ of pores 200-3,000 gstrom in diameter. The support particles had a cylindrical shape with curved faces (G mm in diameter and 6 mm in height; the shape is similar to the shape of knbwn medicament capsules). The surface area of the support particles was 185 m2/g.
The support particles (200 g) were impregnated as in Comparison Example 1 (the only difference being that 141 ml of glacial acetic acid were used instead of 6G ml) and dried, so that the same doping was present. The catalyst was subsequently tested as in Comparison Example 1. The results are shown in the table.
Example 2 A support was first prepared from Sx.O2/A12O3 microspheres ( 97~ by weight of SiClz, 3~ by weight Of .~12Q3) having a surface area of 170 m2lg, and a filler and a lubricant.
The finished support had a pore volume of 0.75 m:L/g, formed from 58~ of pores 70-100 angstrom in diameter and 12~ of pores 200-3,000 angstrom in diameter. The support particles had the same shape and size as in Example 1., ' but they now had a surface area of 132 m2/g. The support particles (200 g) were impregnated as in Comparison Example 1 (the only difference being that 131 m1 of glacial acetic acid were used instead of 66 ml) and dried, so that the same doping was present. The catalyst ~o:~.~o~:~

was subsequently tested as in Comparison Example 1. The results are shown in the table.
Example 3 A silica sol was first mixed with glassy Si02 microspheres (surface area 200 mz/g), dried and roasted, to produce a support. The finished support had a pore volume of 0.64 ml/g, formed from 65~ of pores 70-100 angstrom in diameter and 9~ of pores 200--3,000 angstrom in diameter.
The support particles were extrudate offcuts (diameter and height each 6 mm) obtained by extrusion and having a surface area of 1?0 m2/g.
The support particles (200 g) were impregnated as in Comparison Example 1 (the only difference being that 110 ml of glacial acetic acid were used in place of 66 ml) and dried, so that the same doping was prese;at.
The catalyst was subsequently tested as in Comparison Example 1. The results are shown in the table.
Example 4 A support was first prepared from Si02 microspheres having a surface area of 300 m2/g, and a filler and a lubricant.
The finished support had a pore volume of 0.91 ml/g, formed from 56~k of pores 70-100 ~rn.gstrom in diameter and 10~ of pores 200-3,000 l~ngstrom in diameter. The support particles had the same shape and size as in Example 1.
However, the surface area of the support particles was now 184 mz/g.
The support particles (200 g) were impregnated as in Comparison Example 1 (the only difference being that 163 ml of glacial acetic acid were used instead of 66 ml) and dried, so that the same doping was present. The catalyst was subseduently tested as in Comparison Example 1. The results are shown in the table.

In the table, °'contribution (~)" denotes the percentage contribution to the pore volume provided by the pores of diameter 70-100 .d~ngstrom or 200-3,000 angstrom.
"STY" denotes the space-tame yield;
"combustion (~j°' denotes the percentage of reacted ethylene converted into C02, and "ethyl acetate content°' refers to the cowtent of ethyl acetate produced as a by-product in the condensed part of the reaction product.

- m e a,yr tn l0 M

h N

w M

Q) e-1 M O1N O O

i'~ N r-It'~

t. ri W

N

O

r-i OD N M

tC1 rit0 GO

ri W

O

r-i N OYM tO O

r. r~1 x F~

O N

N

..1N

S-dr-I e-1 N N In O

W N ~ ~Q'.~-1c~

N

O '~' U W

O

rlO

i.lr4 tdg1, o~ vP

00 0~~ ~w o N ~!'r-1tp.

O DC d N

U W

fw fn Pn U7 H 1-f f~"
~~~ O O O
dP W pa U
.... .-.
.p O O ~ '1.,' .W
r-I O O N
li ,~."'1 a r-1 ~rl U
M ~ ~ ct~
S-1 1 1 ~ ".1 r-i ~.
o~~.~G~la C.1 N C!1 CJ W

Claims

1. A process for the preparation of vinyl acetate in the gas phase from ethylene, acetic acid and oxygen or oxygen-containing gases on a catalyst which contains palladium and/or compounds thereof, and optionally in addition, gold and/or gold compounds, and, as activators, alkali metal compounds, and optionally in addition, cadmium compounds on a support which comprises SiO2 or an SiO2-Al2O3 mixture having a surface area of 50-250 m2/g and a pore volume of 0.4-1.2 ml/g, and has a grain size of from 4 to 9 mm, wherein 5 to 20% of the pore volume of the support is formed by pores having radii of from 200 to 3,000 .ANG.ngstrom and 50 to 90% of the pore volume is formed by pores having radii of from 70 to 100 .ANG.ngstrom, excluding catalysts which comprise palladium and gold as metals and in which more than 90% by weight of each of these two metals is in the outer part of the support particles, extending to a depth of 300 of the support particles.

2. The process as claimed in claim 1, wherein 8 to 15%
of the pore volume is formed by pores having radii of from 200 to 3,000 .ANG.ngstrom and 55 to 75% of the pore volume is formed by pores having radii of from 70 to 100 .ANG.ngstrom.