Hydrodealkylation Process With Catalyst Of Group Vib Metals Promoted By Tin Oxide Or Lead Oxide

Abstract

A process for the hydrodealkylation of alkyl-substituted aromatic hydrocarbons, including contacting the alkyl-substituted aromatic hydrocarbons with a catalyst comprising a metal of Group VIB of the Periodic System, such as chromium, molybdenum and tungsten, in an amount from about 5 to 15 percent by weight based on the finished catalyst and a promoter comprising a metal of Group IVA of the Periodic System, such as tin and lead, in an amount between about 1 to 15 percent by weight of the final catalyst, both deposited on an inert oxide support, such as gamma aluminas, silica-alumina, magnesia-alumina, etc., at a temperature of about 1,050.degree. to 1,200.degree. F, a pressure of about 100 to 1,000 psig., a liquid hourly space velocity of about 0.1 to 5 and a hydrogen-to-hydrocarbon mole ratio between about 3 and 15 to 1. A second promoting agent selected from the Group consisting of alkali metals, alkaline earth metals, and rare earth metals, such as potassium, rubidium, cesium, calcium, strontium, barium, cerium, thorium, etc., may also be deposited on the carrier.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a process for the hydrodealkylation of alkyl aromatics to the parent aromatic hydrocarbons. More specifically, the present invention relates to a process for the hydrodealkylation of alkyl aromatic hydrocarbons to the parent aromatic hydrocarbons, utilizing a unique catalyst system.

The hydrodealkylation of alkyl aromatics has been practiced for many years. The principal processes involve the conversion of toluene and like alkyl-substituted benzenes to benzene, and coal tar light oils and coal tar methyl naphthalene to benzene and naphthalene, respectively. These processes may be catalytic or non-catalytic in nature. The non-catalytic system which involves thermal dealkylation, in the presence of hydrogen, requires high temperatures and pressures. While the catalytic processes require lower temperatures and pressures, these temperatures and pressures are still quite high and therefore result in short catalyst life. Most commercial catalytic processes employ chromia-magnesia deposited on an alumina base as a catalyst. Since the development of this catalyst, there has really been no improvement in catalysts for this reaction.

It is therefore an object of the present invention to provide a new process for the hydrodealkylation of alkyl aromatics employing a novel catalyst system. In a more specific aspect, the present invention relates to the process for the hydrodealkylation of alkyl aromatics wherein catalysts which improve conversion are employed. Another and further object of the present invention is to provide a process for the hydrodealkylation of aromatics wherein catalysts of higher selectivity are utilized. A still further object of the present invention is to provide an improved process for the hydrodealkylation of alkyl aromatics wherein catalysts which reduce carbon lay-down on the catalyst are employed. A further object of the present invention is to provide an improved hydrodealkylation process for the hydrodealkylation of alkyl aromatics wherein novel catalysts are employed which permit operation at lower than conventional temperatures. Another and further object of the present invention is to provide an improved system for the hydrodealkylation of alkyl aromatics wherein catalysts are employed which permit the use of lower hydrogen partial pressures.

SUMMARY OF THE INVENTION

Briefly, in accordance with the present invention, alkyl aromatic hydrocarbons are hydrodealkylated by contacting the alkyl aromatics with a catalyst comprising a metal of Group VIB of the Periodic System and a promoter of Group IVA of the Periodic System and these materials in combination with additional promoters selected from the group consisting of alkali metals, alkaline earth metals, and rare earth metals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A suitable feedstocks for use in accordance with the present invention include toluene, polymethyl benzenes, coal tar light oils, coal tar methylnaphthalene concentrates, and bicyclic concentrates from light cycle oils and heavy reformates. Feedstock preparation includes fractionation to remove front ends or bottoms to thereby remove undesired fractions such as unsaturates, indanes and resinous materials. For example, it has been found that coal tar methylnaphthalene concentrates, as received from the coke oven, contain a large amount of contaminants, such as polymers, resins and free carbon. Distillation of such raw materials to yield a 90 percent overhead leaves these materials as a bottoms. Hydrogenation and hydrotreating of the overhead fraction removes sulfur, nitrogen and oxygen contaminants, but, due to the thermal instability of the feedstocks, a heavy resinous material is produced through thermal polymerization. Distillation of the hydrotreated product is required to remove these resins and thereby reduce carbon lay-down on the hydrodealkylation catalyst and reduce hydrogen consumption due to hydrocracking of the resins and polymers.

The processing conditions for the hydrodealkylation reaction of the present invention include a temperature between about 1,050.degree. and 1,200.degree. F, a pressure between about 100 and 1,000 psig., a liquid hourly space velocity between about 0.1 and 5, and a hydrogen-to-hydrocarbon mole ratio of about 3 to 15/1.

The catalysts to be employed in accordance with the present invention include metal oxides from Group VIB of the Periodic System, particularly chromium, molybdenum and tungsten. Primary promoters include Group IVA metal oxides, such as tin and lead. The additional promoters include alkali metal oxides of Group I of the Periodic System, and alkaline earth metal oxides of Group II of the Periodic System, and rare earth metals. Examples of materials of this nature which may be employed include potassium, rubidium, and cesium; magnesium, calcium and strontium, and cerium and thorium, etc. The active metal and the promoter are deposited on an inert oxide support, which preferably includes a high area alumina having a boehmite, bayerite, beta, or eta crystalline form, or other aluminas, silica-alumina, silica, silica-magnesia, silica-zirconia, alumina-magnesia, etc.

The optimum active metal content of the catalyst is about 5 to 15 percent by weight based on the final catalyst. The primary and secondary metal oxide promoters should be present in amounts of about 1 to 15 percent by weight.

The catalysts of the present invention may be prepared by well-known impregnation techniques. One may employ extrudates or pellets for impregnation or powders followed by pelletization or extrusion to yield the finished catalyst. The active metal and the promoter may be added through the use of water-soluble salts, such as their halides, nitrates, sulfates, acetates, etc. Easily hydrolyzed salts can be kept in solution without decomposition by employing appropriate inorganic acids.

The following examples illustrate method of preparing the composite catalysts of the present invention.

EXAMPLE I

To 900 ml. of distilled water was added 81 g. of stannous sulfate and 30 ml. of concentrated sulfuric acid. The sulfuric acid was required to bring the insolubles from the stannous sulfate into solution. This was believed to be tin hydroxide. This solution was added to 900 ml. of a boehmite alumina as pellets and after contact for fifteen minutes, the unadsorbed liquid was decanted from the catalyst pellets. The resulting impregnated catalyst was dried at 250.degree. F for one hour and calcined in air at 950.degree. F for 16 hours in a muffle furnace. This yielded a catalyst of the following composition:

4% SnO--Al.sub.2 O.sub.3

A solution containing 150 ml. of distilled water, 45 g. of chromic acid, and 9.5 g. of potassium nitrate was added to 150 ml. of 4% SnO--Al.sub.2 O.sub.3 pellets from above. Catalyst and solution was in contact for 15 minutes and the unadsorbed liquid was decanted. The resulting catalyst was dried at 250.degree. F for 1 hour and calcined in air at 950.degree.F in a muffle furnace for 16 hours. This yielded a catalyst of the following composition:

15% Cr.sub.2 O.sub.3 --2% K.sub.2 O--4% SnO--Al.sub.2 O.sub.3

EXAMPLE II

To 600 ml. of distilled water was added 20 g. of lead nitrate. This solution was added to 600 ml. of a boehmite alumina as pellets and after contact for 15 minutes, the unadsorbed liquid was decanted from the catalyst pellets. The resulting impregnated catalyst was dried at 250.degree.F for 1 hour and calcined in air at 950.degree.F in a muffle furnace for 16 hours. This yielded a catalyst of the following composition:

2% PbO--Al.sub.2 O.sub.3

A solution containing 150 ml. of distilled water, 36 g. of chromic acid, and 5.5 g. of cesium nitrate was added to 150 ml. of 2% PbO--Al.sub.2 O.sub.3 pellets from above. Catalyst and solution was in contact for 15 minutes and the unadsorbed liquid was decanted. The resulting catalyst was dried at 250.degree.F for one hour and calcined in air at 950.degree.F in a muffle furnace for 16 hours. This yielded a catalyst of the following composition:

12% Cr.sub.2 O.sub.3 --2% Cs.sub.2 O--2% PbO--Al.sub.2 O.sub.3

EXAMPLE III

To 200 ml. of distilled water was added 15 g. of cesium nitrate and 40 g. of chromic acid. This solution was added to 200 ml. of a boehmite alumina and after contact for 15 minutes, the unadsorbed liquid was decanted from the catalyst pellets. The resulting impregnated catalyst was dried at 250.degree.F for 1 hour and calcined in air at 950.degree.F in a muffle furnace for 16 hours. This yielded a catalyst of the following composition:

10% Cr.sub.2 O.sub.3 --4% Cs.sub.2 O--Al.sub.2 O.sub.3

EXAMPLE IV

To 500 ml. of distilled water as added 41 g. of cerous nitrate hexahydrate and 100 g. of chromic nitrate. This solution was added to 500 ml. of a bayerite alumina and after contact for fifteen minutes, the unadsorbed liquid was decanted from the catalyst pellets. The resulting impregnated catalyst was dried at 250.degree.F for 1 hour and calcined in air at 950.degree.F in a muffle furnace for 16 hours. This yielded a catalyst of the following composition:

10% Cr.sub.2 O.sub.3 --2% Ce.sub.2 O.sub.3 --Al.sub.2 O.sub.3

EXAMPLE V

To 600 ml. of distilled water was added 54 g. of stannous sulfate and 20 ml. of concentrated sulfuric acid. The tin sulfate was partially insoluble and the sulfuric acid brought it into solution. This insolubility was probably due to the presence of tin hydroxide. This solution was added to 600 ml. of a boehmite alumina and after contact for 15 minutes, the unadsorbed liquid was decanted from the catalyst pellets. The resulting impregnated catalyst was dried at 250.degree.F for 1 hour and calcined at 950.degree.F for 16 hours to yield a catalyst of the following composition:

4% SnO--Al.sub.2 O.sub.3

A solution containing 150 ml. of distilled water and 30 g. of chromic acid was added to 150 ml. of 4% SnO--Al.sub.2 O.sub.3 pellets (prepared as above) and allowed to remain in contact for 15 minutes before decanting the unadsorbed liquid. The impregnated catalyst was dried at 250.degree.F for 1 hour and calcined in air at 950.degree.F for 16 hours in a muffle furnace. This yielded a catalyst of the following composition:

10% Cr.sub.2 O.sub.3 -4% SnO--Al.sub.2 O.sub.3

EXAMPLE VI

A 4% SnO--Al.sub.2 O.sub.3 catalyst was prepared according to the procedure described in Example V. To 150 ml. of 4% SnO--Al.sub.2 O.sub.3 pellets was added a solution containing 150 ml. of distilled water and 1 g. of rhodium trichloride. The unadsorbed liquid was decanted and the catalyst dried and calcined according to the procedure outlined in Example I. To the rhodium oxide-tin oxide-alumina catalyst was added a solution containing 150 ml. of distilled water and 45 g. of chromic acid. The catalyst was dried and calcined (see Example I) to yield the following composition:

15% Cr.sub.2 O.sub.3 -0.5% Rh--4% SnO--Al.sub.2 O.sub.3

EXAMPLE VII

By employing the techniques and procedures outlined previously, other catalytic compositions were prepared. A solution containing 600 ml. of distilled water, 54 g. of stannous sulfate and 20 ml. of concentrated sulfuric acid was added to 600 ml. of a boehmite alumina. Drying and calcination yielded to the following composition:

3% SnO--Al.sub.2 O.sub.3

A solution containing 150 ml. of distilled water, 29 g. of ammonium molybdate, 10 g. of potassium nitrate, and 5 ml. of concentrated ammonium hydroxide was added to 150 ml. of 3% SnO--Al.sub.2 O.sub.3 pellets. Drying and calcination yielded the following composition:

12% MoO.sub.3 --2% K.sub.2 O--3% SnO--Al.sub.2 O.sub.3

It has also been found advantageous to add trace amounts of a Group VIII metal, such as platinum, rhodium, ruthenium, palladium, nickel, etc. as a promoter. The amount of this metal should be about 0.05 to 0.5 percent by weight and the metal is preferably in its oxide form.

The following Tables illustrate the effectiveness of the present catalysts compared with a commercial chromia-magnesia on alumina hydrodealkylation catalyst. --------------------------------------------------------------------------- TABLE I

Feed: Coal Tar Methylnaphthalene

Standard Conditions: 1100.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2 /H'C

Run 1 2 3 Catalyst 12Cr-1K-3Sn-Al.sub.2 O.sub.3 15Cr-4Sn- Al.sub.2 O.sub.3 Feed Special processing none topped to topped to 500.degree.F 500.degree.F product distribution <Naphthalene* 37.2 2.50 40.4 Naphthalene 56.8 71.4 56.4 Methylnaphthalene 2.0 1.0 2.2 Dimethylnaphthalene 3.5 2.6 3.0 Wt. % Feed Me Naph. Conversion 87 94 86 Carbon on Catalyst Wt. % Feed 0.91 1.1 0.73 __________________________________________________________________________

Standard Conditions: 1100.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2

/H'C Run 4 5 6 7 8 Catalyst Cr.sub.2 O.sub.3 -K.sub.2 O Cr.sub.2 O.sub.3 Cr.sub.2 O.sub.3 -SnO-Al.sub.2 O.sub.3 -K.sub.2 O -SnO-Al.sub .2 O.sub.3 SnO-Al.sub.2 O.sub.3 Feed Toluene 47.7 50.6 51.6 51.6 75% Toluene Naphthalene 33.4 32.6 48.3 48.3 25% Tetralin Me Naphthalene 13.0 16.0 -- -- -- Di Me Naphthalene 5.8 -- -- -- -- product distribution <Naphthalene 42.6 47.4 52.3 44.0 80 Naphthalene 56.6 52.4 47.7 56.0 20 Me Naphthalene 0.6 0.2 -- -- -- Di Me Naphthalene 0.2 -- -- -- -- Carbon on Catalyst Wt. % Feed 1.5 0.96 1.60 0.20 0.05 __________________________________________________________________________ --------------------------------------------------------------------------- TABLE III

Standard Conditions: 1100.degree.F, 600 PSIG, 0.5 LHSV, 8/1 H.sub.2

/H'C Run 9 10 11 Catalyst 12Cr.sub.2 O.sub.3 5Cr.sub.2 O.sub.3 -10SnO 5Cr.sub.2 -2MgO-Al.sub.2 O.sub.3 O.sub.3 -Al.sub.2 O.sub.3 O.sub.3 Feed (a) (a) (b) product distribution <Naphthalene 34.7 28.7 43.6 Naphthalene 61.7 66.2 55.9 Me Naphthalene 0.7 2.1 0.4 Di Me Naphthalene 2.9 3.0 -- Carbon on Catalyst Wt. % Feed 0.88 0.21 0.11 (a) (b) Toluene 50.4 Toluene 40.0 Naphthalene 30.4 Decalin 5.0 Me Naphthalene 13.2 Tetralin 40.0 Di Me Naphthalene 6.0 Methyldecalin 1.5 Methyltetralin 13.5 __________________________________________________________________________ --------------------------------------------------------------------------- TABLE IV

Feed: Toluene

Standard Conditions: 1150.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2 /H'C

Run 12 13 14 15 16 catalyst 12Cr-2Mg 15Cr-2K 15Cr-2Cs-2Sn 12Cr 12Mo -Al.sub.2 O.sub.3 -4Sn-Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -2 Cs-2Pb -2K-3Sn -Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 liquid recovery vol. % Feed 84 80 81 82 82 product distribution <benzene 0.8 0.6 0.9 0.8 0.6 benzene 66.8 82.3 88.6 82.4 81.0 toluene 32.4 17.1 10.5 16.7 18.4 wt. % feed toluene conversion 72.8 86.3 91.5 86.3 84.9 select- ivity to 92 92 94 94 94 benzene carbon on catalyst wt. %. feed 0.26 0.014 0.02 0.04 0.08 __________________________________________________________________________

The following Table illustrates the effect of sulfur in the feed. --------------------------------------------------------------------------- TABLE V

Feed: Toluene

Standard Conditions: 1150.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2

/H'C Run 17 18 Catalyst 15Cr-2K-4Sn-Al.sub.2 O.sub.3 Sulfur, ppm 0 400 Liquid Recovery Vol. % Feed 80 84 Product Distribution <Benzene 0.6 0.5 Benzene 82.3 78.2 Toluene 17.1 21.3 Wt. % Feed Toluene Conversion 86.3 82.2 Selectivity to Benzene 92 96 Carbon on Catalyst Wt. % Feed 0.014 0.014 __________________________________________________________________________

Further studies yielded the following results: --------------------------------------------------------------------------- TABLE VI

Feed: Coal Tar Methylnaphthalene.sup.(a)

Standard Conditions: 1100.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2 /H'C

Run 19 20 Catalyst 12Cr-2Mg-Al.sub.2 O.sub.3 12Cr-1K-3Sn-Al.s ub.2 O.sub.3 Product Distri- bution <Naphthalene 37.8 38.3 Naphthalene 59.0 55.3 Methylnaphthalene 1.4 1.6 Dimethylnaphalene 2.9 5.1 Wt. % Feed Me Naphthalene Conversion 90 90 Carbon on Catalyst Wt. % Feed 1.32 0.33

(a) Wt.% <Naphthalene 50.4 Naphthalene 30.4 Methylnaphthanele 13.4 Dimethylnaphthalene 5.8 >DMN -- __________________________________________________________________________ --------------------------------------------------------------------------- TABLE VII

Feed: Toluene

Standard Conditions: 1150.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2

/H'C Run 21 22 23 Catalyst 12Cr-2Mg 10Cr-4K 10Cr-4Cs -Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 Liquid Recovery Vol. % Feed 84 83 80 product distribution <Benzene 0.8 0.7 0.9 Benzene 66.8 75.6 81.8 Toluene 32.4 23.7 17.3 Wt. % Feed Toluene Conversion 72.8 80.2 86.2 Selectivity to 92 93 92 Benzene Carbon on Catalyst Wt. % Feed 0.26 0.19 0.07 __________________________________________________________________________ --------------------------------------------------------------------------- TABLE VIII

Conditions: 1150.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2 /H'C

Feed:

Toluene Run 24 25 Catalyst 12 Cr--1Ce 12 Cr--1Sr 3 Pb--Al.sub.2 O.sub.3 3 Pb--Al.sub.2 O.sub.3 Liquid Recovery Vol. % Feed 82.0 83.3 Product Distribution <Benzene 0.6 0.8 Benzene 74.3 71.8 Toluene 25.1 27.4 Wt. % Feed Toluene Conversion 79.4 77.3 Selectivity to Benzene 92 92 Carbon on Catalyst Wt. % Feed 0.005 0.005 __________________________________________________________________________ --------------------------------------------------------------------------- TABLE

IX Run 26 27 351-64 351-71 Catalyst 10Cr-2Pb-Al 12Mo-4Sn-Al Feed Operating Conditions Temperature, .degree.F 1100 1100 Pressure, PSIG 500 500 LHSV 0.5 0.5 H.sub.2 /H'C, m/m 5/1 5/1 Liquid Recovery Vol. % Feed 81.0 80.5 Product Distribution, Vol.% <Naphthalene 41.0 41.4 Naphthalene 54.9 55.3 Methylnaphthalene 2.0 1.3 Dimethylnaphthanele 2.1 2.0 Wt. % Feed Methylnaphthalene Conversion 87 90 Carbon on Catalyst Wt. % Catalyst 6.95 5.90 Wt. % Feed 1.88 1.95 __________________________________________________________________________

The following illustrate the value of adding a noble metal as a promoter. --------------------------------------------------------------------------- TABLE X

Feed: Toluene

Standard Conditions: 1150.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2

/H'C run 28 29 30 31 32 catalyst 12Cr-2Mg 10Cr-4Sn 15Cr-4Sn 15Cr-4Sn 15Cr-4Sn -Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 0.5Pt-Al.sub.2 O.sub.3 -0.5Rh -Al.sub.2 O.sub.3 liquid recovery vol. % feed 84 85 81.3 80 81.7 product distribution <benzene 0.8 0.7 1.0 0.6 0.8 benzene 66.8 74.0 79.7 75.7 81.0 toluene 32.4 25.3 19.3 23.7 18.2 wt. % feed toluene conversion 72.8 78.5 84.3 81.0 85.1 select- ivity to benzene 92 96 92 90 93 carbon on catalyst wt. % feed 0.26 0.07 0.03 0.04 0.04 __________________________________________________________________________

TABLE XI

Feed: Coal Tar Methylnaphthalene.sup.(a)

Standard Conditions: 1100.degree.F, 600 PSIG, 0.5 LHSV, 8/1 H.sub.2

/H'C run 33 34 35 36 37 catalyst 12Cr-2Mg 5Cr-6Sn 8Cr-6Sn 8Cr-6Sn 5Cr-10Sn -Al.sub.2 O.sub.3 Al.sub.2 O.sub.3 -Al -0.1Pt -Al.sub.2 O.sub.3 -Al.sub.2 O.sub.3 product dist- ribution <naph- thalene 34.7 29.5 41.2 29.0 28.7 naph- thalene 61.7 66.0 52.5 67.2 66.2 methylnaph- thalene 0.7 -- 0.8 0.8 2.1 dimethylnaph- thalene 2.9 4.5 5.5 3.0 3.0 >DMN wt. % feed methylnaph. conversion 95 100 95 95 87 carbon on catalyst wt. % feed 0.88 0.23 0.18 0.41 0.21

(a) Wt.% <Naphthalene 50.4 Naphthalene 30.4 Methylnaphthalene 13.4 Dimethylnaphthalene 5.8 >DMN -- __________________________________________________________________________

the value of the Group IVA metals over metals of Group IVB is illustrated by the following run which should be compared with runs 29 and 30. --------------------------------------------------------------------------- TABLE XII

Conditions: 1150.degree.F, 500 PSIG, 0.5 LHSV, 5/1 H.sub.2 /H'C

Feed:

Toluene Run 38 Catalyst 10Cr-4Ti-Al.su b.2 O.sub.3 Liquid Recovery Vol. % Feed 72.7 Product Distribution <Benzene 4.7 Benzene 68.9 Toluene 26.4 Wt. % Feed Toluene Conversion 82.1 Selectivity to Benzene 75 Carbon on Catalyst Wt. % Feed 0.09 __________________________________________________________________________

An effort was also made to prepare a catalyst having the following composition:

5Cr -- 10Ti -- Al.sub.2 O.sub.3

The catalyst disintegrated completely when the titanium solution was added.

When reference is made herein to the Periodic System of the elements, the particular groupings referred to are as set forth in the Periodic Chart of the Elements in "The Merck Index," Seventh Edition, Merck & Co., Inc., 1960.

Claims

We claim:

1. A process for hydrodealkylating alkyl aromatic hydrocarbon materials, comprising; contacting the hydrocarbon materials with a catalyst comprising about 5 to 15 percent by weight of an active metal of Group VIB of the Periodic System and a promoting amount of about 1 to 15 percent by weight of a metal selected from the group consisting of tin oxide and lead oxide, both impregnated on a carrier consisting essentially of at least one solid, pellet-form inert oxide, under conditions sufficient to effect said hydrodealkylation reaction, including, a temperature of about 1,050.degree. to 1,200.degree.F, a pressure of about 100 to 1,000 psig, a liquid hourly space velocity of about 0.1 to 5, and a hydrogen to hydrocarbon mole ratio between about 3 and 15 to 1.

2. A process in accordance with claim 1 wherein about 0.05 to 0.5 percent by weight of a secondary promoting metal from Group VIII of the Periodic System is impregnated on the carrier.

3. A process in accordance with claim 1 wherein about 1 to 15 percent by weight of a secondary promoter selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals is impregnated on the carrier.

4. A process in accordance with claim 3 wherein the secondary promoting metal is in its oxide form.

5. A process in accordance with claim 4 wherein about 0.05 to 0.5 percent by weight of a tertiary promoting metal from Group VIII of the Periodic System is impregnated on the carrier.

6. A process in accordance with claim 3 wherein the secondary promoting metal is an alkali metal.

7. A process in accordance with claim 3 wherein the secondary promoting metal is an alkaline earth metal.

8. A process in accordance with claim 3 wherein the secondary promoting metal is a rare earth metal.

9. A process in accordance with claim 1 wherein the inert oxide carrier is a gamma alumina.