MXPA98003931A - Catalysts for hidrogenac - Google Patents

Abstract

The present invention relates to catalysts for hydrogenation comprising metallic palladium supported on activated carbon, wherein less than 50% by weight of the palladium is comprised within a surface layer with a depth of up to 50 microns, the rest is comprised in a layer between 50 and 400 microns deep. The catalysts can be used in particular in the purification of the terephthalic acid obtained by the oxidation of p-xile

Description

CATALYSTS FOR HYDROGENATION DESCRIPTION OF THE INVENTION The present invention relates to catalysts for hydrogenation which comprise palladium supported on activated carbon, with its method of preparation and with its use in hydrogenation reactions. In particular, it relates to the use of catalysts in the purification of terephthalic acid from impurities comprising 4-carboxybenzaldehyde (4-CBA). Catalysts for hydrogenation comprising palladium supported on activated carbon are known in the literature. They are prepared by adsorbing a palladium compound on activated carbon and then reducing the ^ compound adsorbed to palladium metal. It is known from USP 3,138,560 that if palladium is adsorbed to a solution of sodium tetrachloropaladalate or palladium chloride, most palladium is immediately reduced and deposited on the surface in the form of a thin film of palladium metal. The resulting catalysts are poorly active. The reduction of palladium to palladium metal is attributed to functional groups, such as aldehyde groups REF: 27191 jÉque are present on the activated carbon surface. To avoid this reduction, solutions of palladium compound containing an oxidizing agent, such as hydrogen peroxide, have been used. In order to deposit on the activated carbon surface crystallites of palladium having a relatively small size (less than 35 A if possible), solutions of palladium compound in organic solvents have been used (USP 4,476,242). In this case, all the palladium is concentrated in a layer of less than 70-80 microns thick. Results similar to those mentioned above are obtained if the palladium is deposited from aqueous solutions of sodium tetranitropaladate (USP 4,421,676). According to conventional methods, the catalyst is prepared by washing the activated carbon with water to remove the fine particles; the carbon thus treated is dispersed in water, optionally correcting its basicity, and the aqueous palladium salt solution is then added dropwise thereto. The impurities contained in the terephthalic acid contained by oxidation of p-xylene are essentially constituted by paratoluic acid and 4-CBA.

Itfj; While paratoluic acid can be removed by cooling and crystallization of the terephthalic acid solutions containing it, the removal of 4-CBA requires transformation by reduction to compounds that can be separated by crystallization. The transformation is effected by hydrogenation over palladium catalysts on an activated carbon support. The catalysts must have high activity and selectivity to reduce the concentration of 4-CBA to levels acceptable to terephthalic acid users and to convert 4-CBA into compounds that can be separated by crystallization. It has unexpectedly been found that the activity and selectivity of catalysts comprising palladium supported on activated carbon can be considerably improved if the palladium compound is dry impregnated on the activated carbon, adsorbing the palladium compound preferably from solutions Aqueous concentrates which are used in an amount equal to, or less than, the pore volume of the activated carbon. Preferably, the support is impregnated by spraying the solution of the palladium compound onto the activated carbon, preferably using solution amounts equal to 1/2 to 1/5 of the volume of the pores of the carbon (the pore size). it is determined by multiplying the weight of the activated carbon used as support by the porosity of the carbon). Operating according to this method, less than 50% of the metallic palladium is comprised in a layer up to 50 microns deep; the rest is comprised in a layer of 50 to 400 microns deep. The distribution of palladium determined by. X-ray electronic microprobe analysis (EMPA). The palladium compounds that can be used are of various types. The examples are halides, diacetate, nitrate, alkaline salts of chloroplatinic acid, Pd complexes with amines and salts such as sodium tetranitropaladate (Na2Pd (N02) 4) • Preferably, compounds which are highly soluble in water are used, such as sodium tetrachloropaladate. In this case, the palladium salt was used at concentrations equal to 1-10% by weight. In other cases, the concentration of the palladium compound is preferably equal to 50-100% of its saturation concentration measured at room temperature. The solution of the palladium compound is sprayed, using devices of known type, at a rate of 0.1-1 ml of solution per minute per 100 g of support.

The spraying is preferably carried out at room temperature. If the temperature increases, the speed of drying increases, possibly forming a very fine dispersed phase; however, the rate of spontaneous reduction of palladium also increases, possibly forming an undesirable thin film of metallic palladium on the surface of the support. After spraying, the support impregnated with the palladium compound solution, support in which the palladium may be partially ready in the metallic palladium state, is treated with reducing agents of a known type, used to reduce palladium to metallic palladium, to complete the palladium reduction. It is possible to use compounds such as formaldehyde, sodium hypophosphite, glucose and others. It is also possible to use hydrogen, provided that the reduction is carried out at room temperature or slightly above it (the use of high temperatures leads to an undesirable increase in the size of the cpstalites). Conveniently, a hypophosphite solution at a concentration of 8-10% is used, using a volume of solution that is smaller than the pore volume. The catalyst is then drained of the solution, washed and dried.

The activated carbon used to prepare the catalysts may be of animal origin or plant origin. Coconut charcoal is preferred. The surface area of the carbon is preferably greater than 600 m2 / g (BET) and can reach 1000 m2 / g or more; the porosity (BET) is from 0.3 to 0.9 cm3 / g. The carbon is preferably used in granular form, with a particle size generally between 2 and 15 mesh; however, it is possible to use activated carbons in the form of flakes, granules or other granular forms. The fine fraction of the coal (fine particles) is removed in a stream of dry air and / or by sieving. The activated carbon is preferably pretreated, before spraying the solution of the Pd compound, with an aqueous acidic solution which contains for example 1-5% by weight of concentrated HCl. The solution is sprayed onto the activated carbon in an amount that is equal to, or smaller than, the pore volume. The treatment with the acid solution improves the activity of the catalyst. The amount of fixed Pd on the activated carbon is generally from 0.1 to 5% by weight; Preferably, in the case of catalysts for purifying terephthalic acid, it is from 0.2 to 0.6% by weight; more preferably, from 0.3 to 0.5% by weight. In addition to palladium, other metals, such as rhodium or copper, may be present. The purification of crude terephthalic acid by using the catalysts of the ntion is carried out by heating the acid suspension in water, working at temperatures between about 100 ° and 300 ° and with a hydrogen pressure of approximately 15 to 100 bar. The catalysts can also be used to purify drinking water of nitrate and / or nitrite ions and organic chlorides (eg, trichlorethylene). In this case, the catalysts, in addition to the palladium, also preferably comprise copper with a Pd / Cu ratio of 2: 1 to 20: 1. In the case of nitrates, the process is carried out at room temperature, causing the water and a stream of hydrogen to flow through a fixed catalyst bed, while in the case of chlorides the process is it drives between 15 ° and 150 ° C. The palladium catalysts obtained with the method according to the ntion are characterized by a distribution of the metallic palladium crystallites on the surface of the activated carbon granules, so that less than 50% of the palladium is contained within a layer 50 microns and the rest is distributed from 50 to 400 microns deep. The surface area of the palladium crystallites, referred to as palladium metal, is greater than 150 m2 / g of j? Palladium and can be as high as 300 m2 / g or more; with reference to the catalyst, the surface area is greater than 0.8 m2 / g and can reach 1.5-2 m2 / g. The surface area referred to palladium is obtained from the chemosorption data of CO; the surface area in relation to the catalyst is obtained by determination of BET. The longitudinal size < numerical average of the JJ crystallites is generally less than 10 A °: in some cases 0 can decrease (particularly in the catalysts obtained by spraying) to values of less than 50 A ° or even less than 35 A ° (35 A ° is the limit below the which it is not possible to determine the dimensions of the crystallites by X-ray measurements). The average size of the 5 crystallites is determined from the X-ray diffraction data). The percentage of crystallites with average size ^ of less than 35 A ° is determined by calculating the difference between the concentration of Pd on the catalyst (which is determined by chemical methods) and the concentration that 0 results from the percentage of crystallites with a size greater than 35.

TO. The catalysts of the invention are further characterized by the chemosorption values of CO, referred to the catalyst, of more than 0.2 ml of CO / g of the catalyst and which can reach up to 0.5 ml of CO / g of catalyst; The jusjimimorbration referred to palladium is greater than 50 ml / g of Pd and can reach 100 ml / g of Pd. The chemisorption of CO was measured by feeding known volumes of CO into the sample. These feeds were made until the sample, saturated with CO, is no longer able to chemisorb and release constant volumes of CO corresponding to the volumes fed. The measurements were made at room temperature: approximately 6 g of dry catalyst was introduced into a U-shaped support provided with an inlet valve and an outlet valve. Before the analysis, the sample was treated with hydrogen at room temperature to reduce the Pd that could have been oxidized during the drying of the sample and also, to eliminate any trace of oxygen, which could impose the conversion of CO to C02. The sample is then washed with helium to remove any traces of hydrogen. Next, known volumes of CO (measured with a set of tubes at a known volume) in a stream of inert gas (helium): the adsorbed volumes are measured with a thermal conductivity cell.

The chromatogram related to the chemisorption has several "peaks: each peak is related to a CO feed to the sample." The catalytic activity (measured according to the hydrogenation test of standard 4-CBA and expressed as the slope of the line obtained gratifying the logarithm of the concentration of 4-CBA as a function of the reaction time in minutes) is greater than 0.05 and can reach 0.08. The activity, expressed as the ratio between the slope and the weight of Pd in grams, is greater than 10 and can reach 20. The conversion of the 4-CBA after 45 minutes of reaction is greater than 90% and can reach 99 %. The hydrogenation reaction is carried out in a 2 liter autoclave equipped with a paddle stirrer and with a wire mesh catalyst support. * An automatic valve allows the fastener to be immersed in the reaction solution only when the reaction conditions (200 ° C) are reached. The autoclave has an elevator from which the samples can be 0 taken at regular intervals. 1.5 1 of aqueous solution with a content of 500 ppm of 4-CBA are introduced into the autoclave and 0.750 g of catalyst is loaded into the catalyst holder in an elevated position.

The autoclave is closed and washed first with argon, to remove all traces of oxygen, and then with nitrogen. Then 18 bar of hydrogen are charged before starting stirring and heating. Once the operating temperature (200 ° C) has been reached, before submerging the clamp in the solution, a sample is taken which constitutes the sample at time zero. Next, successive samples are taken at pre-established times (5, 10, 15, 20, 30, 45, 60 and 90 minutes).

All samples were analyzed by CLAP to determine the content of residual 4-CBA and hydroxymethylbenzoic acid and paratoluic acid that was formed. In addition to having a high activity, the catalysts according to the invention are also characterized by their considerable selectivity in the reduction of 4-CBA to paratoluic acid and hydroxymethylbenzoic acid (by way of example, after operating for 45 minutes according to the test described above, they are formed from 200 to 300 ppm of the acids mentioned above). By using a catalyst of known type, obtained by the example according to USP 4,476,242, and operating according to the test described above, one obtains slopes of less than 0.04 for the lines plotting the logarithm of the concentration of 4 -CBA as the function of time in minutes. The concentration of paratoluic acid and p-hydroxymethylbenzoic acid is less than 100 ppm. The following examples are given to illustrate but not to limit the invention.

Example 1 JB ^ 50 g of coconut charcoal in flakes, with a surface area 0 (BET) of 1100 m2 / g, a porosity (BET) of 0.68 cm3 / g and a particle size between 4 and 8 mesh, and in which the Fine particles have been removed by moderate selection and rinsing with nitrogen, loaded into a vessel equipped with a variable speed motor which allowed the vessel to rotate (24 rpm). 40 ml of an aqueous solution containing 7.1 g of Na 2 PdCl 4, brought to a pH between 0 and 1 with HCl and with the addition of 6 ml of H202 in an amount equal to % by weight, for 20 minutes on the carbon, which was continuously rotated. The impregnation was carried out at room temperature while the container was rotating. At the end of the spraying process, activated carbon, which retained its original appearance, was treated with 300 ml of an aqueous solution containing 25 g of Na2H2P02 »H20. The catalyst was drained, washed and dried at 120 ° C. The palladium content was 0.411% by weight. The other characteristics of the catalyst and its activity in the reduction of 4-CBA according to the standard test described above are presented in the table. ffc Example 2 The preparation of Example 1 was repeated, except that the impregnation of the carbon was carried out by adding 300 ml of sodium chloropaladate solution to the vessel. The content of Pd in the finished catalyst was 5 of 0.464% by weight. The characteristics of the catalyst and its activity * are presented in the table.

COMPARATIVE EXAMPLE 1 The test of Example 2 was repeated, except that the carbon was flushed with water to remove the fine particles and then impregnated with 600 ml of Pd salt solution. 5 minutes after the addition, the decolorized and the support had an appearance. The reduction was completed by adding 50 ml of aqueous solution containing 25 g of sodium hypophosphite. The Pd content in the finished catalyst was 0.474% by weight. Other characteristics of the catalyst and its Jh activity according to the standard test are presented in the table.

Example 3 The preparation of Example 1 was repeated, the only difference being that the activated carbon from which the fine particles had been removed, as in Example 1, was ^ T pretreatment with an aqueous solution of HCl before impregnation with the palladium salt solution. The pretreatment with HCl solution was carried out with the same method used for the impregnation, that is, by spraying the HCl solution (100 ml with a content of 3.5% by weight of HCl) on the carbon contained in the container which rotated continuously. The characteristics of the catalyst are presented in the table.

The X-X electron microprobe analysis was conducted on the fractures of some catalyst granules. This method (EMPA) consists of detecting the primary X-ray radiation, which is typical of an element, in a sample subjected to electronic bombardment. Using the EDX method (or EDS, energy dispersion spectrometer), it is possible to quickly evaluate the elements that make up the sample being examined and obtain composition maps. However, the resolution of this system is very low (150 ev) to obtain reliable information about the concentration of the elements under discussion. To obtain quantitative information it is necessary to have higher resolutions, which are obtained using the WDX method (or WDS, wavelength dispersion spectrometer). In this case, they are obtained in a distinctive way The highest resolutions (10 evaluation) have been obtained and quantitative charts have been obtained that plot the percentage of Pd as a function of the position of the beam on the surface of the fracture with this system. In this case, the complete quantitative analysis along an axis of the exploration area is obtained point by point, by deviating the coordinates of the electron beam after each acquisition, with the help of a computer.

The profile of the palladium Lalfa radiation along the line that includes the entire thickness of the granule shows that the palladium is comprised within a layer that is approximately 350 microns deep and that less than 50% of the palladium is present. in a layer that is 50 microns deep, Figure 1 graphs the percentage of Pd as a function of the position of the beam on the surface of the fracture.

Examples 4 and 5 The preparation of Example 3 was repeated, the only difference being that in Example 4 a solution of 100 ml of 7% by weight HCl was sprayed and that in Example 5 200 ml of a 1.75% HCl solution was sprayed on the weight. The characteristics of the catalyst are presented in the table.

Comparative example 2 The catalyst prepared according to Example C of USP 4,476,241. 12 g of activated charcoal in flakes used in Example 1 were washed with methanol to remove the fine particles. The methanol was decanted and the charcoal was transferred into a 300 ml three-neck jjiatraz and then suspended in 40 ml of methanol. The dispersion system was agitated by immersing the agitator blades in the upper layer of the liquid (without contact with the solid). 5 Cooling was carried out at 5 ° C and a solution of 0.072 g of Pd (N03) 2 in 50 ml of methanol drop by drop was introduced. Stirring was continued for 21 hours. Water was then added and the catalyst was washed and dried at 60 ° C. The content of Pd in the catalyst was 0.17% by weight. The catalyst had a metallic appearance (indicating poor palladium dispersion). The characteristics of the catalyst are presented in the table. 5 Electron microprobe analysis, as described in Example 3, indicated that all palladium was # comprised in a layer with a depth of 60 to 70 microns.

^ TABLE t-1 oo It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (16)

1. CLAIMS * 1. Catalysts for hydrogenation, characterized in that they comprise palladium metal supported on activated carbon, where less than 50% by weight of palladium is comprised within a surface layer up to 50 microns deep, the rest is located in a layer comprised of 50 to 400 microns of depth.
2. # ^ 2. The catalysts according to claim 1, characterized in that they are capable of chemoadding carbon monoxide in an amount greater than 0.2 ce of CO / g of catalyst.
3. 3. The catalysts in accordance with . claims 1 and 2, characterized in that the values of carbon monoxide absorption are greater than 10 ce of CO / g of fixed Pd on carbon.
4. 4. The catalysts according to claim 1, characterized in that the content of palladium is between 0.05 and 5"by weight.
5. 5. The catalysts according to Claims 2 and 3, characterized in that the palladium content is from 0.1 to 0.6% by weight.
6. 6. The catalysts according to claims 2 and 3, characterized in that the palladium has a surface area of between 150 and 300 m2 / g.
7. 7. The catalysts according to claim 1, characterized in that they comprise, in addition to palladium, metals chosen from rhodium and copper.
8. 8. The catalysts according to claims 2 and 3, characterized in that the average size of the palladium crystallites is less than 100 A °.
9. JÉ 9. The catalysts according to any of claims 2 and 3, characterized in that the average size of the crystallites is less than 50 0 A °.
10. 10. The catalysts according to the preceding claims, characterized in that the activated carbon is of vegetable origin and has a surface area of more than 600 m '/ g. he
11. ll. A method for preparing the catalyst according to the preceding claims, characterized in that an aqueous solution of a palladium compound is impregnated dry on activated carbon, using an amount of solution that is equal to, or less than, the pore volume of the carbon.
12. 12. The method according to claim 1, characterized in that the impregnation is carried out by spraying.
13. 13. The method according to claims 1 and 2, characterized in that the adsorption is carried out from solutions of the palladium compound at concentrations equal to 70-90% of the saturation concentration at room temperature.
14. 14. The method according to claims 1 to 3, characterized in that the activated carbon is pretreated with acid solutions having a pH between 0 and 1, using amounts of solution that are equal to, or less than, the pore volume of the carbon .
15. 15. The use of the catalysts according to claims 2 to 10 in the purification of the crude 4-terephthalic acid obtained by the oxidation of p-xylene and containing 4-carboxybenzaldehyde as an impurity.
16. 16. The use of the catalysts according to claims 1 to 10 for purifying water containing nitrate and / or nitrite ions.