EP0000460A1 - Silver-based catalysts and their use in the preparation of oxide of olefins - Google Patents

Abstract

1. Catalysts based on silver supported on porous artificial graphite or on porous graphite-containing materials, the catalysts being intended for the vapour phase synthesis of epoxides by reaction of an ethylenic hydrocarbon with oxygen or with gaseous mixtures containing oxygen, characterised in that the graphite-type supports have : a specific surface area of less than 10 m**2 /g, a particle size of 50 mu m to 10 mm, a total porosity volume of between 0.1 and 0.4 cm**3 /g, and a porosity distribution which is such that the pores of diameter less than 6.6 mu m represent at most 60% of the total porosity volume if the particle size of the support is greater than 0.7 mm, and at most 90% of the total porosity volume if the particle size of the support is less than or equal to 0.7 mm.

Description

The present invention relates to silver-based catalysts on a graphite support for the vapor phase epoxidation of olefins and more particularly for the production, by this technique, of ethylene oxide from ethylene and oxygen. molecular.

Generally the production of ethylene oxide is carried out in the vapor phase, in tubular reactors with fixed bed, by reaction of oxygen and ethylene on dss catalytic phases based on silver deposited on refractory supports. and inert mainly formed from alwaiian, silica-alumina, magnesia, pumice, zirconia, clay, ceramic, asbestos, natural or artificial zeolite or silicon carbide. Prior art generally prefers supports based on alumina a having a specific surface of less than a few m2 / g. This specific surface is determined by the nitrogen adsorption method, the so-called BET method described by BRUNAUER, EMMET and TELLER in "The journal of the amarican chamical Society" vol. 60, page 309, 1938. The second important characteristic of these supports is the porosity: it is generally high: 30 to 60% by volume, and made up of large pores of rayeas. This is thus found in French patents n ° 2,006,849 and 2,253,747 of pore radii from 1 to 15 microns, in French patent n ° 2,117,189 radii of pearls from 2 to 40 microns, in French patent R ⁸ 2,243,193 radii of pores from 10 to 300 microns and finally in French patent n ° 2,029,751 radii of pores from 200 to 1,500 microns.

The Applicant has discovered that the catalytic epoxidation of olefins and in particular that of ethylene could be carried out with good selectivities and with higher productivities than those of the catalysts described in the prior art by the use as carriers of certain graphites, such as those recently developed by the company. Carbone Lorraine and making the ^t subject of French Patent 2,315,462 of June 24, 1975 and the French patent application 77 14581 of May 12, 1977.

• . une parfaite résistance à l'oxydation
• . une granulométrie suffisamment élevée, 50 microns à 10 mm permettant leur emploi dans les réacteurs industriels.
• . des propriétés mécaniques satisfaisantes, évitant la formation de poussières au cours de la préparation du catalyseur, de ses manipulations ou de son fonctionnement.
• . une surface spécifique faible, inférieure à 10m2/g de préférence comprise entre 0,1 et 2 m2/g.
• . un volume de porosité de 0,1 à 0,4 cm3/g, réparti différemment suivant la granulométrie dans les domaines da rayons de pores inférieurs ou supérieurs à 6,6 microns. Pour les faibles grabwlométries, inférieures à 0,7 mm, la macropo- rosité, constituée de pores de rayons supérieure à 6,6 microns peut être faible et une partie importants de la porosité, pouvant atteindrs 90 % de la porosité totals peut être comcentrée dans is domains de rayons de pores de 50 A' à 6,6 microns. Pour les granulométries plus élevées il doit exister, à cêté du domaine de microporosité précédemment décrit, une macroporesité fermés de rayons de pores ponvant atteindra 200 microns. Cetts macroporesité peut représenter 50 à 90 % de la porosité totale.

The Rouveaux supports graphitized or grapkite materials are characterized by:

• . perfect resistance to oxidation
• . a sufficiently high particle size, 50 microns to 10 mm allowing their use in industrial reactors.
• . satisfactory mechanical properties, avoiding the formation of dust during the preparation of the catalyst, its handling or its operation.
• . a low specific surface, less than 10 m 2 / g preferably between 0.1 and 2 m 2 / g.
• . a porosity volume of 0.1 to 0.4 cm3 / g, distributed differently according to the particle size in the areas of pore radii less than or greater than 6.6 microns. For low grabwlometries, less than 0.7 mm, the macroporosity, consisting of pores with radii greater than 6.6 microns may be low and a significant part of the porosity, up to 90% of the total porosity may be concentrated. in is domains of pore radii from 50 A 'to 6.6 microns. For higher particle sizes there must exist, close to the microporosity range previously described, a closed macroporesity of rays of pore ponvant will reach 200 microns. This macroporesity can represent 50 to 90% of the total porosity.

These porous artificial graphites can be obtained in very different forms such as spheres, pellets, rings, cassettes or extruded shapes.

The improvement brought by this new type of support is twofold, it makes it possible to obtain, with better selectivities, higher productivity in ethylene oxide. This improvement can be attributed to the texture and structure of the graphite but also to the good thermal conductivity of these solids which allows rapid elimination of the calories formed in the reaction zone. This rapid elimination of calories limits the thermal degradation reaction of ethylene oxide to carbon dioxide and therefore allows high productivity. In the prior art, there are some attempts to use supports which are good thermal conductors, but these are solids which it is difficult to obtain with a determined texture. We can note metals (English patent n ° 1,133,484), ferro-silicon (German patent n ° 1,093,344) magnetite (American patent n ° 2,593,156) silicon carbide (English patent n ° 1,133. 484).

The use of graphite as a silver support is also mentioned in the synthesis of ethylene oxide in French patents No. 1,079,601 and English No. 775,218 as well as in German patent No. 1,066 .569.

French patent n ° 1.079.601 does not specify any textural characteristic: particle size, porosity, specific surface of the graphite used and in addition to a good absorption capacity the only quality required is limited to perfect oil removal from the solid. The catalysts of this patent were tested on loads of 1.100 to 1.200 kg and under these conditions the productivities obtained are low, less than 15 g of ethylene oxide per liter of catalyst per hour for tests at atmospheric pressure and of the order of 115 g for tests under 10 bars. The selectivities announced in the latter case are poor: 60 to 68%.

German Patent No. 1,066,559 very briefly describes a single test in a reactor 3 m long and 25 mm in diameter. The artificial graphite used is not defined and the authors report a low productivity accompanying poor selectivity not exceeding 55%.

Although tested only on 30 ml fillers, therefore under more unfavorable conditions than those described above, the new catalysts proposed have proved to be clearly superior. With silver contents much lower than those used in the previous patents: 100 to 250 g / liter instead of 75 to 500 g / 1, selectivities of 70 to 76% are obtained in a series of tests at atmospheric pressure, while under 20 bars the selectivities reach 71% for productivities of 140 g of ethylene oxide per liter of catalyst per hour.

The preparation of these new catalysts poses no problem and can be carried out by any conventional process. One can in particular operate in a known manner in two stages by impregnating or coating a silver compound in solution or suspension in a volatile solvent, followed by a treatment allowing the passage through the support to the metal.

The silver compounds used can be either salts: nitrate, formate, lactate, citrate, carbonate, oxalate, silicylate, acetate, sulfate, propionate, maleate, malate, malonate, phthalate, tartrate, glycolate, succinate, oxide, hydroxide, acetylide or cetaure, either complexes of silver salts with nitrogen molecules, ammonia, acrylonitrile, pyridine, ethanolamine _f ethylene diamine, or with β-diketones. The main solvents or suspension liquids used are water, acetone, light alcohols, ether, pyridine, ethylene glycol, diethylene glycol or chlorinated solvents.

All the techniques which allow the passage of these compounds to the metal or to the oxide can be applied in the presence of graphite or graphite materials, for example precipitation, thermal decomposition in atmosphere inert, oxidizing or reducing and chemical reduction. A particularly suitable treatment is thermal decomposition on the supports of silver acetate under the following conditions: thermal rise from 20 to 280 ^° C. at a rate of 20 ° C. / hour followed by a plateau of 10 to 30 hours at 280-300 ° C, the entire operation being carried out with nitrogen sweeping with the possible addition of oxygen or hydrogen.

It is possible to add to these catalysts, in the usual contents of 0 to 2% by weight, all of the conventional solid silver promoters:

These elements may be present in the finished catalysts, in metallic form or in the form of oxide or compound.

It has also been observed that certain halogenated derivatives of hydrocarbons, added in small amounts to the reactants, increase the selectivity of the catalysts developed by the applicant. The use of 1,2-dichloroethane, at a maximum concentration of 1 ppm relative to the total gas volume, proving to be particularly advantageous.

• - taux de transformation globale de l'éthylène (T.T.G.) :
  

Examples 1 to 12 below illustrate, without limitation, the preparation and use of the catalysts according to the invention. The results obtained in these examples are expressed in terms of overall ethylene conversion rate, selectivity and productivity.

• - overall ethylene conversion rate (TTG):
  

- Example 1

42.5 g of an artificial graphite prepared by Le Carbone Lorraine are placed in a solid flask mounted on a rotary evaporator, the textural characteristics of which are given in Table 1.

The temperature of the oil bath of the evaporator is brought to 120 ° C. and the support is left to degas for one hour under a partial pressure of 100 mm of mercury. Under the same conditions of temperature and pressure, 200 ml of a pyridine solution at 5% by weight of silver acetate are then introduced dropwise over 3 hours. Under these conditions the evaporation of the solvent is instantaneous. After introduction of all of the solution, the impregnated and dried support is transferred to a tubular reactor to decompose, in a stream of nitrogen, the silver acetate according to the known reaction:

In order to thermally control the reaction, this treatment is carried out with a temperature rise of 20 ° C / h up to a plateau of 18 hours at 270 ° C. Analysis indicates that the catalyst thus prepared contains 10% by weight of silver.

- Example 2

We place in a solids balloon mounted on an eva 84 g rotary porator of an artificial graphite prepared by the company Le Carbone Lorraine and the characteristics of which are collated in Table No. ⁸ 2. 200 ml of a 10% by weight solution of silver acetate are added thereto. pyridine and the solvent is distilled under a partial pressure of 5 mm of mercury. The impregnated support is dried, then loaded into a tubular reactor for decomposition of the silver acetate under the same conditions as those of Example No. 1. A product containing 10% silver by weight is thus obtained.

- Example 3

30 ml of catalyst prepared in Example 1 are loaded into a laboratory reactor working at atmospheric pressure. The reactor consists of a stainless steel tube 600 mm long and 16 mm in internal diameter. The reagents are admitted from below and are preheated on a bed of 200 mm high porcelain rings, which also supports the catalyst charge. The whole is heated by an oil circulation in a double jacket. The gases entering and leaving the reactor are analyzed online using a double detection chromatograph: a flame ionization detector for ethylene oxide, methane, formaldehyde, propylene, propane, methanol and acetaldehyde, and a thermal conductivity detector for oxygen-nitrogen, carbon dioxide, ethy lene and the ed. The two columns, 1/8 inch in diameter and 2.5 meters long, mounted in series, are filled one with chromenorb 101, the other with porapak Q.

A 14 liter / hour gas stream is passed over the catalyst, consisting of a mixture of 12% ethylene, 4.7% oxygen, 83.3% nitrogen and 600 ppb of 1,2-dichloro ethane. After 100 hours, the results of Table 3 are obtained.

- Example 4

Les sélectivités sont plus faibles et les activités inférieures sont mises en évidence par des températures de marche plus élevées pour des taux de transformation voisins.By way of comparison with the results of test 3 above, an industrialized catalyst containing 15% silver, on a silica-alumina support, was tested on the same apparatus and under strictly identical conditions. The results obtained are shown in Table 4.

The selectivities are lower and the lower activities are highlighted by higher running temperatures for similar transformation rates.

- Example 5

Into the reactor described in Example No. 3, a charge of 30 ml of catalyst prepared in Example No. 2 is introduced. After an activation treatment of 30 hours with sistant to pass over the catalyst an air-ethylene mixture 50% -50% at a temperature below 200 ° C, is introduced into the reactor a gas stream of 14 liters / hour consisting of a mixture of 14% ethylene , 4.6% oxygen, 81.4% nitrogen and 200 ppb of 1,2-dichloroethane. After 60 hours under reagents, a conversion rate of ethylene of 7% is obtained at 230 ° C. with a selectivity to ethylene oxide of 70%.

- Example 6

30 ml of the catalyst prepared in Example 2 are loaded into a laboratory reactor operating under pressure and consisting essentially of a stainless steel tube 355 mm long and 16 mm in internal diameter, heated by a bath of molten nitrates . The reagents admitted from the bottom of the reactor are preheated on a 42 mm high porcelain filling. The analytical device is identical to that described in Example 3.

A gas stream containing 13% ethylene, 5% oxygen and 82% nitrogen is passed over the catalyst, at a pressure of 20 bars, at a specific hourly flow rate of 9000 liters / hour normal per liter of catalyst. With an ethylene conversion rate of 8.5% and a selectivity to ethylene oxide of 71%, an productivity in ethylene oxide of 139 g per hour and per liter of catalyst is obtained at 221 ° C.

- Example 7

By way of comparison with the results of test No. 6 above, the commercialized catalyst which has already been the subject of test No. ⁸ was tested on the same apparatus and under strictly identical conditions. It was not possible with this catalyst to work under the same temperature conditions. Above 200 ° C, a runaway reaction is observed, resulting in a rapid rise in temperature to 300 ° C and a total consumption of the incoming oxygen. At a temperature of 195 ° C, which constitutes the li mite for thermal control of the reaction, one obtains, for an ethylene conversion rate of 3.5% and a selectivity to ethylene oxide of 69%, an ethylene oxide productivity of 59 g per hour and per liter of catalyst.

- Example 8

L'analyse indiqua que la catalyseur ainsi préparé contient 13% en poids d'argent.According to the operating method described in Example No. 1, a catalyst is prepared with an artificial graphite of Le Carbone Lorraine origin, the characteristics of which are collated in Table No. 5.

Analysis indicated that the catalyst thus prepared contains 13% by weight of silver.

- Example 9

30 ml of the catalyst prepared in example n ° 8 are charged to the reactor described in example n ° 3. After an activation treatment identical to that of example n ° 5 is introduced at atmospheric pressure a gas flow of 14 liters / hour formed from a mixture of 14% ethylene, 4.6% oxygen, 81.4% nitrogen and 120 ppb of 1,2-dichloroethane. After about twenty hours of walking, we obtain the results of table n ° 6.

- Example 10

L'analyse indique une teneur en argent du catalyseur de 11 % en poids. On charge 30 ml de ce catalyseur dans le réacteur décrit dans l'exemple n° 3. Après un traitement d'activation identique à celui de l'exemple n° 5, on introduit à la pression atmosphérique un débit gazeux de 14 litres/heure formé d'un mélange de 13 % d'éthylène, 4,7 % d'oxygène, 82,3 % d'azote et 600 ppb de dichloro-1,2 éthane. Dans le domaine de température de 200 à 240°C l'activité de ce catalyseur exprimée par le taux de transformation de l'éthylène est faible. En poussant la température à 245°C on obtient un taux de transformation de l'éthylène de 2 % avec une sélectivité en oxyde d'éthylène de 65 %. Les performances de ce catalyseur sont nettement inférieures à celles obtenues dans les essais des exemples 3, 5, 6 et 9 avec les catalyseurs selon l'invention.A comparative example was carried out on a catalyst prepared according to the procedure described in Example 1, with an artificial graphite with a particle size of 4 to 5 mm and a total volume of low porosity formed only of pores with radii less than 6.6 microns. The characteristics of the graphite used are collated in Table 7.

Analysis indicates a silver content of the catalyst of 11% by weight. 30 ml of this catalyst are loaded into the reactor described in Example No. 3. After an activation treatment identical to that of Example No. 5, a gas flow rate of 14 liters / hour is introduced at atmospheric pressure. formed from a mixture of 13% ethylene, 4.7% oxygen, 82.3% nitrogen and 600 ppb of 1,2-dichloroethane. In the temperature range from 200 to 240 ° C the activity of this catalyst expressed by the ethylene conversion rate is low. By pushing the temperature to 245 ° C., a 2% ethylene conversion rate is obtained with a selectivity for ethylene oxide of 65%. The performance of this catalyst is much lower than that obtained in the tests of Examples 3, 5, 6 and 9 with the catalysts according to the invention.

- Example 11

L'analyse indique une teneur en argent du catalyseur de 11 % en poids. On charge 30 ml de ce catalyseur dans le réacteur décrit dans l'exemple n° 3. Après un traitement d'activation de 50 heures par un mélange air-éthylène 50%-50% à une température de 175 à 215°C, on introduit dans le réacteur un courant gazeux de 14 litres/heure formé d'un mélange de 14 % d'éthylène, 4,8 % d'oxygène, 81,2 % d'azote et 200 ppb de dichloro-1,2 éthane. Les meilleurs résultats obtenus figurent sur le tableau n° 9.

A comparative example was carried out on a prepared catalyst, according to the procedure described in the example. ple 1, with an artificial graphite with a particle size of 200 to 700 microns and a low porosity volume. The characteristics of the graphite used are collated in Table ⁸ 8.

Analysis indicates a silver content of the catalyst of 11% by weight. 30 ml of this catalyst are loaded into the reactor described in Example No. 3. After an activation treatment of 50 hours with a 50% -50% air-ethylene mixture at a temperature of 175 to 215 ° C., introduced into the reactor a gas stream of 14 liters / hour formed of a mixture of 14% ethylene, 4.8% oxygen, 81.2% nitrogen and 200 ppb of 1,2-dichloroethane. The best results obtained are shown in Table 9.

The performance of this catalyst is significantly lower than that obtained in Examples 3, 5, 6 and 9 with the catalysts according to the invention.

- Example 12

OR loads 30 ml of the catalyst prepared in the example ple 8, in the reactor described in example 3. A gas flow rate of 13.5 l / hour is passed over the catalyst, at atmospheric pressure, consisting of a mixture of 50% propylene, 10% d and 40% nitrogen. At 260 ° C., a selectivity to propylene oxide of 25.7% is obtained for an overall conversion of propylene of 1.1%.

- Example 13

A comparative example was carried out on a catalyst with a silver content of 12.4%, prepared according to the procedure described in Example 1, with an artificial graphite in grains of diameter 400 to 500 millimicrons, with a specific surface of 10 m2 / g and total volume of porosity 0.085 cm3 / g. The specific surface of this catalyst is 7 m2 / g.

A gas stream containing 5% ethylene, 5% oxygen and 90% nitrogen is passed over the catalyst at a pressure of 20 bars and at a temperature of 276 ° C. at a specific hourly flow rate of 9,000 1 / h / normal per liter of catalyst. The ethylene conversion rate is 4.7% and the selectivity for ethylene oxide is 36%, which shows that a specific surface area of the catalyst of less than 10 m2 / g is not a sufficient characteristic for that graphite is a good support for silver.

Claims (9)

1.- Silver-based catalysts for the vapor phase synthesis of epoxides by the reaction of oxygen, or of a gaseous mixture containing them, on an ethylenic hydrocarbon characterized by the use as a support for porous artificial graphites, or graphite materials porous, having a specific surface of less than 10 m2 / g, a particle size of 50 microns to 10 mm, a total volume of porosity between 0.1 and 0.4 cm3 / g. 2. Catalysts according to claim 1 with a particle size greater than 0.7 mm and whose porosity is constituted for at least 40% of the total volume of pores with rays greater than 6.6 micrors. 3. Catalysts according to claim 1 with a particle size less than or equal to 0.7 mm and whose porosity consisting of pores of radius less than 6.6 microns can reach 90% of the total porosity. 4. Catalysts according to one of claims 1 to 3 containing 5 to 20% by weight of silver and prepared by impregnating the graphite support with a solution of a silver salt followed by a decomposition of this liberating salt metal. 5. Catalysts according to one of claims 1 to 4 for the synthesis of ethylene oxide from ethylene. 6. A process for preparing an epoxide by reacting oxygen or a gaseous mixture containing oxygen, on an ethylene hydrocarbon, characterized by the use as catalyst of a catalyst according to one of claims 1 to 5. 7. A process for the preparation of ethylene oxide from ethylene according to claim 6. 8.- Method according to one of claims 6 and 7 wherein one operates in the presence of a halogenated organic compound. 9.- The method of claim 8 wherein the halogenated derivative used is 1,2-dichloroethane.