FR2495958A1 - Supported silver catalysts with high surface area - for alkylene oxide prodn. by olefin oxidn. - Google Patents

Abstract

New Ag-contg. catalysts have a wt. ratio of Ag. to refractory support of 20-60:100 and contain Ag. crystallites with an av. dia. of 80-600 Angstroms. The catalysts can be prepd. by impregnating the support with a soln. of an Ag. salt (esp. a carboxylate salt) and an alkali(ne earth) metal salt in an amine solvent contg. less than 20 wt. % H20. The wt. ratio of alkaline earth metal to Ag. is pref. 5-20:100 and the wt. ratio of alkali metal to Ag. is pref. 0.01-0.5:100. The support is pref. calcined silica or an alumina contg. more than 95% alpha-Al2O3; its specific surface is pref. 5-400 (esp 10-100) square m/g and its particle dia. is pref. less than 300 microns. Prodn. of E0 or P0 is pref. effected at a temp. below 210 deg.C and an 02/olefin vol. ratio of less than 1. The process is pref. carried put in a multi-tube pneumatic transport reactor with a reaction zone and a regeneration zone. The catalyst residence time in the reaction zone is pref. 5-60 sec. Regeneration can be effected with H2 or O2. The catalysts are esp. useful for prodn. of ethylene oxide (EO) or propylene oxide (PO) by vapour-phase reaction of ethylene or propylene with 02. Due to their high Ag. surface area, the catalysts have high activity, which can be maintained for long periods with frequent regeneration. Acceptable selectivity can be achieved by using moderate reaction temps. (120-210 deg.C).

Description

 The present invention relates to silver catalysts which can be used for the vapor phase production of ethylene oxide or propylene oxide from ethylene or propylene and molecular oxygen. It also relates to the use of these catalysts.

 The production of ethylene oxide is generally carried out in vapor phase in tubular reactors with fixed bed and consequently with supported catalysts whose diameter of the grains of the support is relatively large, of the order of 4mm (4000 microns) . The prior art generally prefers supports of low specific surface, less than 5 m / g, because the presence of a large specific surface and of micropores promotes, at high temperature, the decomposition of ethylene oxide on the support and, by diffusion braking, - the combustion of ethylene oxide produced in the micropores.

 The silver is deposited in the form of crystallites on these supports t the preferred support is generally a support type "Alundum" containing approximately 99% of α-alumina, of particle size 4 to 8 mm in diameter, of specific surface less than 0, 5 m2 / g and whose average pore diameter is of the order of 5 microns. Thanks to the characteristics of this support and the characteristics of the impregnation solutions used, it is possible to obtain silver crystallites of uniform diameter and of the order of 0.3 micron (3000 A).

The importance of the characteristics of the silver crystallites thus obtained and mainly of their size was highlighted in the theses of
J. Wu (1973): "The effect of crystallite size on the activity and selectivity of silver catalysts" at CORNELL University (USA) and by MM Jarjoui (1976) "Study of the partial oxidation of ethylene on contact of supported catalysts based on silver "at the University of LYON (France). It has thus been demonstrated that crystallites with a diameter of less than 1000 A are unfavorable for obtaining high selectivity.

 The prior art therefore prefers catalysts with a low specific surface area for crystallites, which leads to operating the process at a relatively high temperature for obvious reasons of reactor productivity.

 ~ However, it is well known that, both for the epoxidation of methylene and for the epoxidation of propylene, the high temperatures contribute to the decrease in selectivity which, under these conditions, rarely exceeds 75% for the oxide d ethylene and is close to zero for propylene oxide.

 The applicant has found that it is possible, under certain conditions, to make a better compromise between the low selectivity but high reactivity of the small crystallites and the high selectivity but low reactivity of the large crystallites and that the use of catalysts with crystallites of small diameter silver (80 to 600) in a moderate temperature process (120 to 2100C) gave increased selectivity for the two processes considered.

 Thus, in a preferred embodiment, quantities of silver representing 40% of the weight of the support are used; Since the crystallites obtained have an average diameter of 300 A, the surface area per unit volume of the catalyst is 40 times greater than that of a 10% silver catalyst and the diameter of the crystallites is 3,000 A.

 The catalyst thus obtained, like the catalysts of the interior art, can give high selectivities only in the presence of an inhibitor; dichloro-i, 2 ethane, has a maximum concentration of 5 ppm in the gas volume, is of particularly advantageous use. The applicant has also verified, in agreement with the prior art, that the use of a low oxygen / hydrocarbon ratio in the reaction mixture is essential to obtain increased selectivities.

Surprisingly, it has been found that the catalysts with a large silver surface, partially covered with chlorine ions, operating at a temperature below 210 ° C., with a gas with an oxygen / hydrocarbon ratio lower than α, rapidly recover from organic deposits which harm the obtaining of a satisfactory activity and selectivity. In the moderate conditions made possible by the gain in surface area and which should have made it possible to find a very high selectivity, even with crystallites of small diameter, the catalyst is therefore not self-cleaning, which reduces the gain due to the large surface area. This phenomenon is particularly marked when the hydrocarbon is propylene; at 1800C only carbon dioxide and water vapor are produced and in relatively small quantities
However, it has been found that the catalyst can recover a large part of its initial activity and selectivity by periodic treatment either with hydrogen or with oxygen, that is to say by partial or total elimination of organic residues. the efficiency of this reactivation is all the greater the higher its frequency, this reactivation having to be carried out at least once per minute, that is to say after a period of operation of the catalyst, variable according to the operating conditions, between 5 seconds and 1 minute. Frequent reactivation of the catalyst cannot be carried out efficiently in a fixed-bed reactor made up of support grains of large diameter (4 mm or more) and of large internal specific surface; diffusional braking would indeed harm the rapid propagation of the concentration fronts and would cause significant mixing of the gases. It is therefore considered essential to have recourse to gas-solid fluidization techniques, that is to say techniques similar to those used for the combustion of carbonaceous residues of the catalysts in certain processes of catalytic cracking of heavy petroleum products.

 Another very important reason leads to choosing the techniques of fluidization: the catalyst with a large silver surface can only be prepared, as will be indicated below, by using supports with large internal specific surface. Even at moderate temperature a drop in selectivity is inevitable, as a result of diffusion braking, in micropores with a grain of catalyst with a diameter greater than 0.5 mm. The applicant therefore considers that the invention which consists in reactivating periodically, by elimination of organic deposits, a catalyst with a large surface area of silver operating under low self-cleaning conditions - low temperature, catalyst partially covered with chlorine, low O 2 / hydrocarbon ratio in the reaction mixture - must be used in one or more fluidized reactors with circulation of the catalyst from the reaction zone to the regeneration zone and vice versa.

 Many materials, with a diameter of less than 0.5 mm, can be chosen as a support for the catalysts of the invention. These materials must be resistant to attrition and have good fluidization properties. Preferably, as is known in the art of fluidization, spherical particles are used, the diameter distribution of which varies continuously from 20 microns to 120 microns.

 The material chosen must have a large internal specific surface so that it is possible to disperse relatively large quantities of silver in the form of crystallites of small diameter; this surface must be between 5 m2 / g and 400 m2 / g of support and preferably between 10 m2 / g and 100 m2 / g. It is also important that its apparent porosity is high, equal to or greater than 50% by volume, that is to say that a network of macropores promotes both the impregnation of the catalyst during its manufacture and the diffusion of the reagents. gaseous during the reaction.

A third condition which the material must satisfy is that it is compatible with the actual epoxidation reaction 1 thus the following three materials
- Silica-alumina (catalytic cracking catalyst) with an average diameter of 60 microns and a specific surface of 400 m / g,
- Alumina y (Sphéralite SAP from the Rhône-Poulenc Company) with a mean diameter of 2 microns and a specific surface of 350 m
- Silica gel (Rhone-Poulenc company) ground and sieved, average diameter 70 microns, specific surface 475 m / g
have not given satisfactory results; the catalysts prepared with these supports did not make it possible to obtain selectivities for ethylene oxide greater than 30%, under the best operating conditions.

 The reasons for explaining the poor performance of these materials under the operating conditions of the invention (moderate temperatures, absence of diffusion braking, periodic reactivation of the catalyst) are unknown.

 Surprisingly, it has been found that two materials (silica and alumina treated at high temperature) are very good supports for these catalysts for the epoxidation of ethylene and propylene with a large silver surface.

 The first is the "SPHEROSIL" from the Rhône-Poulenc Company, whose silica beads are spherical and the particle size between 20 microns and 120 microns. This material has a pore volume of 1 cm / g (apparent porosity 70%) and is sold under six names corresponding to areas ranging from 10 m2 / g to 400 m2 / g. Whatever its internal surface, this material can be used as it is without undergoing preliminary heat treatment.

 The second is an alumina with a large specific surface area prepared according to the technique described in US Pat. between 20 microns and 120 microns and the internal surface equal to 350 m / g is impregnated with a concentrated sucrose solution, brought to 11500C under a reducing atmosphere and then oxidized to 6800C in the presence of air. The specific surface of the alumina obtained is 33 m / g (B.E.T.).

 The method of impregnating these supports is also important for obtaining silver crystallites of small diameter and uniform diameter.

In a preferred embodiment, an impregnation solution is used which contains:
a) a silver salt of a carboxylic acid
b) an organic amine
c) a salt of an alkaline earth metal (barium or calcium) or a salt of an alkali metal (sodium, potassium or cesium)
d) a small amount of water which makes it possible to dissolve the preceding salt in the amine.

 The silver salt can be formate, lactate, oxalate, acetate, silver benzoate, or any other carboxylic salt. The organic amine can be pyridine, quinoline, N, N, N ', N', tetramethylethylene-diamine, N, N, N ', N ", N", pentamethyldiethylene-triamine, triethylamine, or any another aliphatic, aromatic, heterocyclic amine having sufficient dissolving power for the silver salt and for the aqueous solution of the alkaline or alkaline earth salt.

 The alkali or alkaline earth metal salt may be acetate, oxalate, cyclohexane butyrate, that is to say preferably a salt of carboxylic acid which is easier to dissolve in the organic amine. When an alkaline earth metal salt is chosen, the amount used corresponds to a weight percentage of this metal, relative to silver, of between 5 and 20%. This percentage is between 0.01 and 0.5% when using an alkali metal salt.

 Water can be used to dissolve this salt before mixing in the solution of the silver salt in the organic amine. After mixing the final solution must be clear and homogeneous.

 The choice of the silver salt, the organic amine and the anion of the alkali or alkaline earth salt are not very critical; the preferred combination is that which provides a clear solution with the minimum of water.

 The support can be impregnated with the solution on a rotary evaporator under vacuum, after drying the support. A preconditioning at 2800C under nitrogen is then carried out and the catalyst is ready for its introduction into the reactor.

The reactor which makes it possible to carry out the reaction and to rapidly transfer this very fine catalyst through several zones - regeneration, purging, reductive or oxidizing reactivation - can be of one of the types described in US Pat. Nos. 3,669,877 and US , 4,152,293. The Multitubular Pneumatic Transport Reactor, described for the first time at the Congress
International de Fluidisation de Toulouse (October 1973) organized by the
Society of Industrial Chemistry, is much preferred. Its operation can be simulated with precision using the two-tube study reactor described by H. DE LASA and G. GAU (Chemical Engineering Science 28, p. 1875, 1973) and the attached drawing of which makes it possible to understand the operation.

 The reaction tube 1 is supplied with reactis through the tubing 2 s a tubing 3 and a fluidization plate make it possible to fluidize the catalyst using the regeneration gas - hydrogen or oxygen - or an inert gas.

 Lightened by the reaction gas, the gas-solid suspension rapidly circulates through the tube 1; the gas is separated from the catalyst in the chamber 4, using the filters 5. The catalyst then descends into the tube 6 after having been purged of the gases contained in its pores or adsorbed on its surface by a flow of water vapor supplied by the tubing 7. The reaction heat is removed and the temperature controlled using a heat transfer fluid circulating in the double jacket 8. A sample taking 9 allows ç by assaying the carbon dioxide content, determine the efficiency of catalyst regeneration.

 The industrial multitubular reactor would consist of tubes whose length could reach i0m and the diameter 0.2 m. The study reactor consists of two tubes 4m long and 0.02m in diameter. The residence time of the gas in the reaction tube was chosen to be 4 seconds, that of the catalyst equal to 10 seconds in the reaction tube and 5 seconds in the regeneration chamber.

The preparation of the catalysts according to the present invention, as well as the production of ethylene oxide and propylene oxide, in a pneumatic transport reactor with two tubes will be illustrated by the following nonlimiting examples
Example I
A catalyst A according to the present invention is prepared as follows. As support for the catalyst, the "SPHEROSIL" XOB 030 from the Rhône-Poulenc company is used, with a particle size of 20 microns - 120 microns and a specific surface of 50 m / g. It is impregnated with a rotary evaporator under vacuum with a solution of silver formate in pyridine of molarity 1, obtained by adding a slight excess of formic acid to the suspension of silver oxide, the barium is added in the form of a solution of barium acetate in 11 water. The impregnation solution thus contains 10% water by weight. The catalyst obtained after drying under vacuum and conditioning for 5 hours at 2800C under a nitrogen atmosphere contains 38% silver and 4.1% barium.

 Examination with an electron microscope makes it possible to verify that the particle size of the silver particles is relatively homogeneous and that their average diameter is 300 A.

 This catalyst is loaded into the study reactor which operates under the residence time conditions defined above, at a pressure close to atmospheric pressure, with a flow rate of reagents of 650 l / h and a flow rate of regeneration gas of 15 l / h.

The results obtained for the production of methylene oxide and propylene oxide, depending on the nature of the gas used in the regeneration chamber are shown in Table I
Table I - Reagents Selectivity conversion gas temperature in
Dioxide hydrocarbon regeneration
Ethylene 85% 180 C H2 3.8% 91% + Oxygen 15%
id. id. N2 0.4% 78%
Propylene 85% 1600C 2 + Oxygen 15% 2 2.2% 59%
id. id. 02 2.7% 46%
id. id. N2 0.2% 'negligible
The regeneration gas contained in the six experiments indicated 2 ppm of 1,2-dichloro-ethane. Analysis of the gases withdrawn from sample 9 shows that the hydrogen as well as the oxygen used as regeneration gas produced carbon dioxide which therefore comes from the decomposition of organic residues which cannot be absorbed by water vapor; the comparison test with nitrogen only makes it possible to detect carbon dioxide in the trace state.

 It is found that this decomposition of organic residues - regeneration makes it possible to improve the activity of the catalyst and the selectivity of the reaction.

The selectivity indicated in Table I represents the percentage of moles of hydrocarbon having reacted which are transformed into epoxide.

Example II
Catalyst B according to the present invention is prepared as follows. As support for the catalyst, the "SPHERALITE" SAP from the company Rhêne Poulenc is used, treated at high temperature after impregnation with a sucrose solution as indicated above. The silver salt is replaced by silver acetate and the barium salt by cesium nitrate. The preparation method remaining identical to that of catalyst A, the preconditioned catalyst contains 35% silver by weight and 0.05% cesium by weight, its silver crystallites have an average diameter of 220 A.

The tests carried out under the conditions of the previous example give:
Reagents Regeneration gas Conversion Selectivity
C2H4 2 H2 2.5% 88%
O 3.0% 87%
CH + O2 H2 2.5% 53%
36
"O2 1.8% 37%
Example III (Comparative)
The support of Example I is replaced by alumina SA 5151 from the Company
Norton and whose specific surface is 0.20 m / g. Before impregnation this material is ground and sieved to a diameter of less than 150 microns. The composition of the solution is identical to that of Example I but, in accordance with the prior art, the amounts of salts are reduced. The preconditioned catalyst contains 11% silver and 0.98 barium; its crystallites have an average diameter of 3000. The results obtained in the study reactor are shown in Table II
This catalyst therefore has too low a reactivity at low temperature and, at high temperature, regeneration does not improve its performance, either because it is sufficiently self-cleaning at this temperature, or because under these conditions the frequency of cleaning is insufficient .

Table ~~~~ Table II
Selectivite conversion gas
Reagents Temperature Regeneration of epoxy hydrocarbon C2H4 85% + O2 15% 180 C O2 negligible
id 230 C H2 1% 78%
id 2300C 02 1.1% 76%
id 230 C N2 1% 76% CH 85% + O2 15 * 2300C O2 negligible 36 2 2
Example IV (comparative)
Catalyst A is used under the same conditions of Example I, but with a mixture of reactants rich in oxygen and a higher temperature.

Table III
Selectivity conversion gas
Reagents Temperature Regeneration of epoxy hydrocarbon C2H4 2% + Air 98% 1900C H2 1.4% 65%
id id O2 1.5% 64%
id id N2 1.5% 65% C3H6 2% + Air 98% 190 C H2 3.8% negligible
id id O2 4.2% negligible
id id N2 1.2% negligible
The measurement of the carbon dioxide content when taking sample 9 shows that under these conditions the catalyst is self-cleaning for the production of ethylene oxide but only partially self-cleaning for the production of propylene oxide.

Claims (13)

 - CLAIMS  i - Silver-based catalysts characterized in that they have a large silver surface area per unit weight of the catalyst as a result of the use of a silver / refractory support weight ratio of between 20% and 60% and silver crystallites with an average diameter between 80 and 600 A.  2 - Catalysts according to claim i prepared by impregnation of porous refractory supports whose internal specific surface is between 5 m / g and 400 m / g and preferably between 10 m / g and 100 m / g.  3 - Catalysts according to claim 2, characterized in that the support is a silica treated at high temperature or an alumina containing more than 95% of alumina.  4 - Catalysts according to claims 2 and 3 characterized in that the support has a diameter less than 300 microns and good fluidization properties.  5 - Catalysts according to claims 1 and 4 prepared with an impregnation solution containing silver salts and alkali or alkaline earth metal salts dissolved in an organic amine solvent + water containing less than 20% water in weight.  6 - Catalysts according to claim 5 characterized in that the silver salt is a carboxylic acid salt.  7 - Catalysts according to claim 5 characterized in that the weight ratio of the alkaline earth metal to silver is between 5% and 20%.  8 - Catalysts according to claim 5 characterized in that the weight ratio of the alkali metal to silver is between 0.01 and 0.5%.  9 - The use of these catalysts in processes for the synthesis of ethylene oxide or for the synthesis of propylene oxide at temperatures below 21ope and with a reaction mixture characterized by an oxygen / hydrocarbon volume ratio below a i.  10 - Methods for synthesizing ethylene oxide and propylene oxide according to claim 9, characterized in that the reactor used allows the circulation of the catalyst in several zones and in particular a reaction zone and a zone of regeneration.  11 - Processes according to claims 9 and 10 characterized in that the regeneration is carried out by bringing the catalyst into contact with hydrogen or oxygen.  12 - Processes according to claims 9 to 11 characterized in that the average residence time of the catalyst in the reaction zone is between 5 seconds and 60 seconds.  13 - Processes according to the preceding claims characterized by the use of the Multibubular Reactor with Pneumatic Transport.