FR2720956A1 - Hydrogenation catalyst for selective hydrogenation of e.g. acetylene - Google Patents

Abstract

A new catalyst for the selective hydrogenation of acetylenic hydrocarbons in the gaseous phase comprises an Al2O3 support in the form of balls or extrudates, Pd, at least one gp. IB metal and at least one alkali or alkaline earth metal is claimed. At least 80% of the Pd and at least 80% of the gp. IB metal are present in a vol. at the periphery of the catalyst defined by a spherical or cylindrical surface (radius r1) corresp. to the mean radius of the balls or extrudates, and a spherical or cylindrical surface (radius r2) equal to at least 0.8r. Also claimed is a selective hydrogenation process using the catalyst.

Description

 The invention relates to a novel supported catalyst that can be used in particular in the selective hydrogenation in the gas phase of acetylenic hydrocarbons of 2 or 3 carbon atoms in the corresponding ethylenic hydrocarbons. It relates more particularly to a regenerable catalyst for the selective hydrogenation of acetylenic compounds such as acetylene or propyne, respectively to ethylene or propylene in the gas phase.

 Ethylene is a monomer used for the preparation of a large number of polymers. It is generally obtained by pyrolysis or steam cracking processes. The ethylene thus produced contains small amounts of acetylene (generally less than 3%) which must be removed before use. The ethylene acetylene contents generally tolerated for use in the manufacture of polymers are generally less than 10 ppm and most often less than 5 ppm.

 One of the techniques used to remove acetylene from ethylene is to selectively hydrogenate it to ethylene in the presence of a palladium catalyst supported on a refractory support such as alumina. The problem generally encountered with monometallic catalysts (consisting solely of palladium supported on alumina) is that, when the operating conditions are brought to allow the total elimination of acetylene, a portion of the ethylene is also converted to ethane. In addition, these monometallic catalysts generally have relatively low stabilities due to the large formation of oligomers which progressively overlap the catalyst surface under the reaction conditions. This hydrocarbon deposit can certainly be removed by controlled oxidation processes, but it is advantageous, in an industrial process, to have a catalyst operating time between two regenerations the largest possible.

To improve the properties of the catalysts, the palladium addition of promoters has long been described. These additions may be, for example, silver (patent US-A-2 802 889), iron and silver (patent
US-A-3,243,387)
These promoters may also be chosen from alkali or alkaline earth metals, such as lithium (US Pat. No. 3,325,556), potassium (application EP-A-124,744) or calcium (US Pat. 4,329,530).

 Whether for monometallic catalysts (palladium catalysts only) or catalysts promoted (catalysts comprising palladium and at least one other element), it is known to those skilled in the art that when palladium is concentrated on the surface bead of said catalyst, its catalytic performance is significantly higher than that of a catalyst of identical formula for which the palladium is distributed homogeneously in the catalyst beads. For example, in the case of the use of the palladium silver bimetallic formulas, it has been found that when the palladium was located at the periphery of the catalyst beads and the silver was homogeneously distributed, this gave the catalyst better properties. (US Pat. No. 4,404,124), in particular the lower formation of ethane and oligomerization products.

We also know the Japanese patent application
JP-A-04 108 540 which describes selective liquid phase hydrogenation catalysts for butadiene-1 3, in which silver is precipitated and supported on the surface of palladium. In these catalysts, the carrier consists of relatively high surface area alumina and the Ag / Pd weight ratio is 0.3 to 5.0, preferably 0.5 to 3.0.

 It has now surprisingly been found that it is possible advantageously to perform the selective hydrogenation in the gas phase of acetylenic hydrocarbons of 2 or 3 carbon atoms (acetylene or propyne) to the corresponding ethylenic hydrocarbons (ethylene or propylene) using a catalyst in the form of beads or extrudates containing palladium; at least one Group IB metal of the Periodic Table, at least one alkali or alkaline earth metal and alumina, in which at least 80% of the palladium content and at least 80% of the metal content of group IB are present in a volume at the periphery of the catalyst delimited by a spherical or cylindrical surface of radius r1 corresponding to the mean radius of the balls or catalyst extrudates and a spherical or cylindrical surface of radius r2 at least equal to 0.8 Ri.

In the case of catalysts in the form of beads or extrusions, r1 and r2 can be represented as follows:

 More particularly, the palladium content is between 0.01 and 0.5% by weight of the catalyst, preferably between 0.025 and 0.05% by weight. The element of group IB is most often silver, in a content of between 0.001 and 0.02% by weight. Advantageously, the silver / palladium weight ratio is between 0.05 and 0.4 and preferably between 0.05 and 0.25.

 The alkali or alkaline earth metal content of the catalyst is chosen to have an alkali metal or alkaline earth metal to palladium ratio of between 2 and 20 and preferably between 4 and 15. This content is preferably between 0.05 and 0.2% by weight of the catalyst.

 As an alkali metal, sodium or potassium is preferably used.

 The support used is an alumina and preferably an alpha alumina. In a common way, it is used in the form of balls with diameters generally between 2 and 4 mm. The characteristics of the alumina used are generally as follows: a specific surface area of between 5 and 150 m 2 / g; a pore volume of 0.3 to 0.95 cm3 / g and a pore diameter greater than 100. These different characteristics are determined by the analysis techniques known to those skilled in the art.

 The palladium may be introduced according to techniques known to those skilled in the art to obtain a distribution of palladium on the surface of the support beads, which corresponds to the criteria described above.

The good distribution of palladium can be verified by conventional techniques such as for example the microprobe of Castaing. Palladium may for example be introduced by impregnation techniques of aqueous or organic solution of a palladium precursor. This precursor may for example be a mineral compound such as palladium chloride, palladium nitrate, palladium tetrammonium dihydroxide, palladium tetrammine chloride, or an organometallic compound, such as for example palladium bis xallyl, or palladium. acetylacetonate.

 The element of group IB, in particular silver, is introduced in such a way that it remains concentrated at the periphery of the balls of the support. Analysis of the silver content after controlled abrasion of the catalyst beads makes it possible to ensure the good distribution of the silver in the catalyst beads. The precursor generally used is silver nitrate.

For example, silver acetate, silver citrate, silver chloride and silver carbonate may also be used.

 The alkali or alkaline earth metal is introduced according to the techniques known to those skilled in the art. The precursors generally used are nitrates, acetates, chlorides, carbonates and hydroxides.

 The three elements can be introduced from a common solution of three precursors or from separate solutions containing one or two elements. In the latter case, drying, calcination or reduction treatments at temperatures between 120 ° C. and 900 ° C. may possibly be carried out between two consecutive impregnation steps.

When the palladium and the group IB element are introduced from different solutions, the preparation techniques that can be used are for example those described in the patent.
US-A-4,533,779, which uses silver chloride as a precursor or in US-A-4,504,593, which uses silver citrate as a precursor.

 The catalyst thus obtained is generally dried at temperatures between room temperature and 150 ° C. The catalyst thus dried can be used as or, most often, it is preferably calcined in order to decompose the metal precursors and / or reduced calcination is generally carried out by treating said catalyst under air flow at a temperature of between 400 ° C. and 900 ° C. The reduction may be carried out by treating the catalyst with a gas containing hydrogen at a temperature between room temperature and 500 C.

 The invention also relates to a hydrogenation process which can be defined in a general manner in that it comprises the passage in the gas phase of a feedstock containing in particular at least one acetylenic hydrocarbon of 2 or 3 carbon atoms ( acetylene or propyne) in the presence of hydrogen over a catalyst as described above.

 This process is more particularly applicable to the hydrogenation of acetylene present in a gas containing in addition ethylene. In order to approach the reaction conditions which make it possible to completely eliminate acetylene, the molar ratio of hydrogen to acetylene is generally between 1 and 2, the temperature of the reaction is generally between 25 ° C. and 100 ° C., the pressure is generally between 1 and 5 MPa The charge rate expressed in liters of gaseous feedstock per liter of catalyst per hour is generally between 1000 and 10,000 h -1.

 During use, the catalyst deactivates due to a deposit of hydrocarbon compounds gradually covering the active phase.

When the catalyst performance is judged to be insufficient, the catalyst can be regenerated. The regeneration of the catalyst is carried out by controlled combustion of the hydrocarbon species present thereon. This combustion is carried out under the conditions known to those skilled in the art, generally by gradually heating the catalyst in the presence of an oxygen-containing gas at a temperature of between 350 and 500 ° C.

 The nonlimiting examples which follow illustrate the invention. Examples 3 and 4 are given for comparison.

EXAMPLE 1 Preparation of catalyst A (according to the invention)
A catalyst according to the invention (Catalyst A) is prepared by impregnating 100 g of an alumina-based support with 60 ml of a solution of nitric acid, palladium nitrate and sodium nitrate. The support used is in the form of beads 2 to 4 mm in diameter having a specific surface area of 10 m 2 / g and a pore volume of 0.6 cm 3 / g. After impregnation, the catalyst is dried at 120 ° C. and calcined under air at 750 ° C. Catalyst A thus obtained contains, by weight, 0.05% of palladium, 0.005% of silver and 0.05% of sodium. metallic elements in the catalyst grains are shown in FIG.

 On the diagram, the radii are plotted on the abscissa in micrometers and on the ordinate, on the left are the palladium local weight concentrations represented by (I) and on the right, the local weight concentration in silver, in the form of a histogram.

These analyzes show that 84% of the silver is concentrated in a volume delimited by a sphere of radius r1 of 1.5 mm and a sphere of radius r2 of 1.39 mm. The ratio r2irl is therefore equal to 0.93 and therefore much greater than 0.8. In the case of palladium, 94% of the palladium is concentrated in a volume delimited by a sphere of radius r1 of 1.5 mm and a sphere of radius r2 of 1.2 mm. The ratio r2 / r1 is here equal to 0.8. The distribution of the elements in the catalyst grain is therefore in accordance with the invention
EXAMPLE 2 Preparation of catalyst B (according to the invention)
A catalyst according to the invention (Catalyst B) is prepared by impregnating 100 g of an alumina-based support with 60 ml of a solution of nitric acid, palladium nitrate, silver nitrate and of sodium nitrate. The support used is in the form of beads 2 to 4 mm in diameter having a specific surface area of 10 m 2 / g and a pore volume of 0.6 cm 3 / g. After impregnation, the catalyst is dried at 1200 ° C. and calcined under air at 750 ° C. The catalyst B thus obtained contains, by weight, 0.05% of palladium, 0.010% of silver and 0.05% of sodium. The average distribution of the elements in the catalyst grains is in accordance with the invention.

EXAMPLE 3 Preparation of catalyst C (comparative)
Catalyst C is prepared according to the same procedure as in Example 1, but using an impregnating solution containing nitric acid, palladium nitrate and sodium nitrate. Catalyst C thus obtained contains 0.05% by weight of palladium and 0.05% of sodium. The Castaing microprobe analysis of the catalysts A and C does not reveal any significant differences in the distribution of palladium between these two samples.

EXAMPLE 4 Preparation of Catalyst D (Comparative)
A catalyst D is prepared by immersing at room temperature 100 g of carrier in 120 ml of an aqueous solution of silver nitrate containing 8 mg of silver. The catalyst is left stirring for a few minutes. The supernatant solution is then removed. The catalyst is then dried at 1200 ° C. and calcined at 500 ° C. 60 ml of a solution of nitric acid, palladium nitrate and sodium nitrate are then impregnated on the catalyst and, after impregnation, the catalyst is dried. 1200 C. and calcined under air at 750 ° C. The catalyst D thus obtained contains, by weight, 0.05% of palladium, 0.005% of silver and 0.005% of sodium.The Castaing microprobe analysis of the catalysts A and D does not make it possible to highlighting significant differences in the distribution of palladium in these two samples, and the analysis of the silver content after controlled abrasion of the catalyst beads does not make it possible to identify a difference in the concentration of silver in the catalyst.

EXAMPLE 5 Preparation of catalyst E (according to the invention)
A catalyst according to the invention (Catalyst E) is prepared by impregnating 100 g of an alumina-based support with 60 ml of a solution of nitric acid, palladium nitrate, silver nitrate and of sodium nitrate. The support used is in the form of beads 2 to 4 mm in diameter having a specific surface area of 10 m 2 / g and a pore volume of 0.6 cm 3 / g. After impregnation, the catalyst is dried at 1200 ° C. and calcined under air at 750 ° C. The catalyst E thus obtained contains, by weight, 0.05% of palladium, 0.020% of silver and 0.05% of sodium. The average distribution of the elements in the catalyst grains is in accordance with the invention.

EXAMPLE 6 Preparation of catalyst F (according to the invention)
A catalyst according to the invention (Catalyst F) is prepared by impregnating 100 g of an alumina-based support with 60 ml of a solution of nitric acid, palladium nitrate, silver nitrate and of sodium nitrate. The support used is in the form of beads 2 to 4 mm in diameter having a specific surface area of 10 m 2 / g and a pore volume of 0.6 cm 3 / g. After impregnation, the catalyst is dried at 1200 ° C. and calcined under air at 750 ° C. The catalyst F thus obtained contains, by weight, 0.05% of palladium, 0.010% of silver and 0.1% of sodium. The average distribution of the elements in the catalyst grains is in accordance with the invention.

EXAMPLE 7 Comparison of the Hydrogenating Properties of the Different Catalysts
Catalytic tests are carried out on catalysts A, B, C, D, E and
F to determine their selectivity and stability during the hydrogenation of acetylene contained in a feed containing 98% by weight of ethylene and 2% by weight of acetylene.

 15 ml of the catalyst to be tested are first placed in a vertical steel reactor. This reactor is then placed in an oven to control the temperature. In a first step, the catalyst is reduced under a current of hydrogen at 1500C for 2 hours at atmospheric pressure. The temperature is then raised to 50 ° C, the hydrogen flow rate to 1.5 Ih-1 and the pressure to 2.5 MPa The feedstock, composed of 98% by weight of ethylene and 2% by weight of acetylene is then injected with a volume flow corresponding to a space velocity of 3300 h -1.

The analysis of the gaseous effluent at the outlet of the reactor is carried out by gas chromatography. Under these conditions, the stability of the catalyst is defined as the time from which acetylene is detected at the reactor outlet. The selectivity of the catalyst corresponds to the ethylene content of the filler after complete elimination of the acetylene.

The results obtained are reported in Table I.

Table 1. Comparison of catalyst performance
BCD E and F for the hydrogenation of acetylene.

<Tb>

<SEP> Catalysts <SEP> Stability <SEP> of <SEP> catalysts <SEP> Seiocity <SEP> of
<tb><SEP> (hours) <SEP> catalysts <SEP> (%)
<tb><SEP> Catalyst <SEP> A <SEP> 105 <SEP> 98.2
<tb> (according to <SEP> the invention)
<tb><SEP> Catalyst <SEP> B <SEP> 120 <SEP> 98.3
<tb> (according to <SEP> the invention
<tb><SEP> Catalyst <SEP> C <SEP> 65 <SEP> 98.3
<tb><SEP> (comparative)
<tb><SEP> Catalyst <SEP> D <SEP> 66 <SEP> 98.2
<tb><SEP> (comparative)
<tb><SEP> Catalyst <SEP> E <SEP> 101 <SEP> 98.3
<tb> (according to <SEP> the invention
<tb><SEP> Catalyst <SEP> F <SEP> 121 <SEP> 98.7
<tb> (according to <SEP> the invention
<Tb>
These results clearly show that the catalysts according to the invention (catalysts A, B, E or F) have catalytic performances (stability and selectivity) higher than those of monometallic catalysts (catalyst C) or those of catalysts where the silver is distributed. uniformly in the catalyst beads (catalyst D).

EXAMPLE 6 Regeneration of a Catalyst According to the Invention
After using Catalyst A for 120 hours under the conditions of Example 7, Catalyst A is regenerated. In this regeneration procedure, the catalyst is heated to 200 ° C. under nitrogen and then treated under dilute air at a temperature of between 200 and 500 ° C. to burn the hydrocarbon compounds present on the catalyst.

After regeneration, the performances of the regenerated catalyst A are evaluated under the conditions of example 7. The performance of such a regenerated system is reported in table 2 below.
Table 2

<tb><SEP> Catalysts <SEP> Stability <SEP> of <SEP> catalysts <SEP> Selectivity <SEP> of
<tb><SEP> (hours) <SEP> catalysts <SEP> (%)
<tb><SEP> Catalyst <SEP> A <SEP> 105 <SEP> 98.5
<tb> (according to <SEP> the invention
<tb><SEP> Catalyst <SEP> A <SEP> 104 <SEP> 98.7
<tb><SEP> Regenerated
<Tb>
These results show that, to the experimental errors, the regenerated catalyst A has the same performance in hydrogenation of acetylene as the new catalyst.

Claims (10)

1-Catalyst, especially for the selective hydrogenation of acetylenic hydrocarbons in the gas phase, comprising a support of alumina in the form of beads or extrudates, palladium, at least one metal of group IB of the periodic table and at least one alkali or alkaline earth metal, characterized in that a proportion of at least 80% of the palladium and at least 80% of the group IB metal are present in a volume at the periphery of the catalyst delimited by a spherical surface or cylindrical of radius r1 corresponding to the average radius of the balls or catalyst extrudates and a spherical or cylindrical surface of radius r2 at least equal to 0.8 μm. 2 - Catalyst according to claim 1, characterized in that the alumina has a specific surface area of 5 to 150 m2 / g. 3- Catalyst according to claim 1 or 2 characterized in that the alumina has a specific surface area of 5 to 60 m 2 / g. 4 - Catalyst according to one of claims 1 to 3, characterized in that its palladium content is from 0.01 to 0.5% by weight. 5. Catalyst according to one of claims 1 to 4 characterized in that the element content of group IB is from 0.001 to 0.02% by weight. 6. Catalyst according to one of claims 1 to 4 characterized in that the element of group IB is silver. 7 - Catalyst according to one of claims 1 to 6 characterized in that the content of alkali metal or alkaline earth is 0.05% to 0.2% by weight. 8 - Catalyst according to one of claims 1 to 7 characterized in that the weight ratio of metal group IB palladium is 0.05 to 0.4. 9 - Catalyst according to one of claims 1 to 7 characterized in that the atomic ratio alkali metal or alkaline-earth on palladium is 2 to 20. 10- Process for the selective hydrogenation in gas phase of at least one acetylenic hydrocarbon of 2 or 3 carbon atoms in corresponding ethylenic hydrocarbon, characterized in that, in the presence of hydrogen, a gas phase is passed through charge comprising at least one acetylenic hydrocarbon of 2 or 3 carbon atoms on a catalyst according to one of claims 1 to 5.