Process for the Production of Synthesis Gas

1. Field of the Invention

The present invention relates to a catalyst for the production of synthesis gas in the form of CO and H.sub.2 from CO.sub.2 and CH.sub.4 and/or other light hydrocarbons.

2. Description of the Related Art

The combustion of fossil fuels in the form of coal, petroleum or natural gas or of secondary fuels produced therefrom which is effected predominantly in the industrialized countries is leading to an increased concentration of CO.sub.2 in the atmosphere. Since CO.sub.2 is one of the so-called greenhouse gases, even relatively slight increases in concentration lead to traumatic changes which have their effect throughout the world. An increase of the average temperature in the atmosphere has already been observed and a further increase is expected in the coming years, resulting in more frequent and more violent natural catastrophes (for instance periods of drought, flood, storm). Therefore, increased efforts are required to at least reduce the tempo of the further increase in the concentration of CO.sub.2. It is being attempted to achieve this primarily by decreasing the consumption of energy. To achieve this, some countries have thought, for instance, of introducing an energy tax, the amount of which is based on the amount of CO.sub.2 liberated upon the consumption of energy. Another possible manner of reducing the liberation of CO.sub.2 is to reuse this gas for instance, for the production of other products.

Synthesis gas (CO+H.sub.2) is used in a number of chemical processes. Such synthesis gas can be produced in different ways. The method most used consists of the reforming of methane. This leads to a product having an H.sub.2 /CO ratio of 3 in accordance with the reaction:

The H.sub.2 /CO ratio can be shifted further to even higher values by increasing the amount of H.sub.2 O relative to the amount of CH.sub.4, followed by a CO-shift reaction:

A synthesis gas produced in this manner is particularly suitable for the synthesis of methanol, which in its turn can be further converted into other petrochemical products.

The partial oxidation of methane with oxygen represents another method for the production of synthesis gas and is characterized by a H.sub.2 /CO ratio of 2:

Such a synthesis gas is particularly suitable for the Fischer-Tropsch synthesis, which requires an H.sub.2 /CO ratio of about 1.7-2.5.

The production of oxoalcohols is another important field of use of synthesis gas. Oxoalcohols are produced by hydroforming .alpha.-olefins in accordance with the reaction:

The aldehyde is then hydrogenated to form the desired alcohol. In this production process an H.sub.2 /CO ratio of about 1 is required. One possibility for producing such a synthesis gas is afforded by the gasification of coal.

Another possibility of producing such a synthesis gas having an H.sub.2 /CO ratio of 1 is the reaction of CO.sub.2 with CH.sub.2 :

It is known to effect this reaction at, for instance, 500.degree. C. under increased pressure in the presence of a catalyst. Since such processes have not acquired any practical importance up to now, there are relatively few publications on this subject (for instance, Chem. Eng. Science 1988, No. 11, pages 3049-3062, and Chem. Eng. Science 1989, No. 12, pages 2825-2829). As suitable catalysts the metals Ni, Pt, Rh and Pd are mentioned, these elements being applied in each case to a support material which consists of Al.sub.2 O.sub.3 or SiO.sub.2. The effectiveness in principle of such catalysts can easily be shown in a laboratory test. However, they are unsuitable for commercial use. This is due to the fact that undesired side reactions are also promoted by the action of the catalyst, namely:

These side reactions are undesired, since they lead to the formation of free carbon which deposits on the catalyst (coking) and reduces the activity thereof more and more (deactivation). The coking can, to be sure, be reduced by a drastic increase in the amount of CO.sub.2 beyond the amount stoichiometrically necessary for the amount of CH.sub.4 used, but this, of course, results in the need to separate large amounts of excess CO.sub.2 from the synthesis gas produced and return it to the process. The economy of such a process is placed in question by the additional expense necessary for this.

In order to reduce the tendency towards coking, it is known to permit the reaction between CO.sub.2 and CH.sub.4 to take place in the presence of steam. This also impairs the economy of the process, without giving results which are truly technically satisfactory.

In Federal Republic of Germany 41 02 185 A1, a catalytic system for the production of synthesis gas by the reforming of light hydrocarbons with CO.sub.2 is described in which a metal or a compound of metals of the platinum group is provided as catalytically active coating substance. The preferred metals are rhodium, ruthenium and iridium. These metals are applied to an oxidic support material which is selected from the group consisting of Al, Mg, Zr, Si, Ce and/or La. The examples of embodiments are limited to two support materials MgO.sub.2 and Al.sub.2 O.sub.3. Before the application of the coating material to the support material, the support material is silicified in a prior treatment. Upon the application of the coating material itself, a chemical reaction of the materials used takes place in the form of a heterogeneous solid-liquid reaction, in connection with which a CO atmosphere or an inert atmosphere must be maintained. Thereupon, the material which is obtained in this manner is dried and calcined in a known manner. By the special manner of production of this catalytic system, the tendency towards coking is to be substantially reduced.

The object of the invention is to find a catalyst which is not only sufficiently active to obtain a high yield of CO and H.sub.2 but which, in particular, avoids impermissibly strong coking for sufficiently long periods of time and thus remains sufficiently active for a long period of time, even if the amounts of CO.sub.2 and CH.sub.4 used are relatively close to the stoichiometric values and the CO.sub.2 /CH.sub.4 ratio (molecular weights) is therefore equal to about 1. It should be possible to dispense, as far as possible, with the use of steam during the reaction.

A catalyst of Ni (5% by weight) which was applied to a .gamma.-Al.sub.2 O.sub.3 support material showed in experiments an excellent catalytic action which led to conversion rates in the vicinity of the thermodynamic equilibrium values at the corresponding temperature. At a reaction temperature of, for instance, 500.degree. C., the catalyst, however, was deactivated by coking within a very short period of time. On the other hand, a corresponding Ni catalyst having a base of .gamma.-Al.sub.2 O.sub.3 while it did not exhibit any substantial coking, was, however, substantially less active, since it permitted only conversion rates with the range of 20% of the amounts corresponding to the thermodynamic equilibrium at the corresponding temperature. Good results with respect to its activity were again produced by a catalyst of Pt (2% by weight) which was also applied to an Al.sub.2 O.sub.3 matrix. The conversion rate for CH.sub.4 was, to be sure, very high, namely 90% with a reaction temperature of 650.degree. C. (in the literature, a conversion rate of 100% is indicated for a temperature of 850.degree. C. (see A. T. Ashcroft, A. K. Cheetham, M. L. H. Green, P. D. F. Vernon: "Partial oxidation of methane to synthesis gas using carbon dioxide," Nature 352, Jul. 18, 1991), but after only 8 hours of use a deactivation of the catalyst was noted. The same applies also to catalysts having a base of an SiO.sub.2 matrix.

It was therefore entirely surprising that, in experiments, the excellent suitability in particular of Ni and Pt catalysts on the base of thermally stabilized ZrO.sub.2 matrix material could be noted. These catalysts not only gave satisfactory activity values but at the same time they were characterized also by long-term resistance to deactivation by coking, without the use of steam being necessary. Thus, a 5% Ni/ZrO.sub.2 catalyst permitted a CH.sub.4 conversion rate of about 50% of the thermodynamic weight equilibrium values at the corresponding temperature and did not exhibit any substantial coking for a long period of time at 500.degree. C. Even clearly better CH.sub.4 conversion rates, lying close to the thermodynamic equilibrium values (for instance 90% at 650.degree. C.) were obtained for a 2% Pt/ZrO.sub.2 fixed-bed catalyst, the CO.sub.2 conversion rate was 55% and the selectivity for CO was 100%. No visible coking could be noted even after 100 hours of operation.

The invention has shown that, in particular, Pt and Ni catalysts on a thermally stabilized support material consisting predominantly of ZrO.sub.2 (at least 80 wt. % and preferably 90 wt. %) not only give good activity values but, at the same time, also show excellent resistance to deactivation by coking. The percentage by weight of the coating can be limited to maximum 7%. In the case of Pt catalysts, the coating should constitute a percentage by weight of 0.1 to 5%, and preferably 0.1 to 2.0%. For Ni catalysts, the percentage by weight is preferably 0.5-5.0%. Several catalyst materials (also other elements of groups VIII of the periodic system, for instance Pd, Co) can also be applied onto the support material. Further, 0.1-2.0 wt. % Pt in combination with 2-5 wt. % Ni are particularly suitable. A combination of Pt and Pd is also advantageous.

The use of a support material which consists predominantly of ZrO.sub.2 is of decisive importance for the invention. Chemically pure ZrO.sub.2, however, exhibits an undesired strong inclination towards sintering at temperatures above 600.degree. C. The support material is therefore thermally stabilized by admixture of 0.5-10 mol % of accompanying substances in the form of one or more oxides of the elements Y, La, Ce, Si, Ca or Al, i.e. its tendency to sinter is reduced at the intended temperatures of use. Surprisingly, due to the presence of Y, La, or Ce in the support material, the activity of the catalyst is furthermore improved.

FIG. 1 is a graph showing the conversion rates of CO.sub.2, CH.sub.4 and CO over a specific period of time using one of the catalysts embodied in the present invention.

FIG. 2 is a graph showing the conversion rates of CO.sub.2, CH.sub.4 and CO over a specific period of time using another one of the catalysts.

FIG. 3 is a graph showing the conversion rate of CO as a function of temperature using a traditional catalyst.

FIG. 4 is a graph showing the conversion rate of CO.sub.2 as a function of temperature using a traditional catalyst.

FIG. 5 is a graph showing the conversion rate of CH.sub.4 as a function of temperature using a traditional catalyst.

FIG. 6 is a graph showing the results for CO.sub.2 and CH.sub.4 conversion rates and the CO yield for a first temperature cycle.

The catalyst of the invention is produced by calcinating the ZrO.sub.2 at a suitable temperature up to about 670.degree. C. and then mixing it with the thermal stabilizer (for instance Y.sub.2 O.sub.3) in order to obtain the support material. The application of the catalytically active coating substance is effected purely physically by the known dry-impregnation method or the wet-impregnation method. In this connection, adsorption of the coating substance, present as a complex compound in a solvent, takes place on the support material. The solvent is then evaporated using any suitable means (for instance by thermal drying under a vacuum). The catalyst material thus obtained is then again calcined at a temperature not great than about 800.degree. C.

The production of CO/H.sub.2 -synthesis gas with the use of a catalyst in accordance with the invention is effected at temperatures of between about 400 and about 900.degree. C., and preferably between about 700 and 800.degree. C. The pressure upon the reaction can be between about 1 and about 30 bar and is preferably between about 10 and 20 bar. The amounts of CO.sub.2 and CH.sub.4 used should be so adapted to each other that the molar weight ratio of CO.sub.2 to CH.sub.4 is generally between about 0.5 and 4, more specifically between 0.5 and 1.5, and particularly preferred the value 1. The use of steam for reducing the tendency to coking is not necessary.

The invention will be further explained on basis of the following examples. Reference is had therein, as criteria of activity, to the conversion rates for CO.sub.2 and CH.sub.4, as well as the CO yield and CO selectivity. These values are defined as follows: ##EQU1##

First of all, it may be pointed out that, in the following measurement results, the conversion rates for CO.sub.2 and CH.sub.4 under the different conditions are not in a ratio of 1:1 as would be expected purely mathematically on the basis of the CO.sub.2 /CH.sub.4 reaction equation. This is due to the fact that, in addition, the following side reaction also takes place, which, to be sure, does not lead to coking but increases the amount of CO.sub.2 converted in favor of the CO:

FIGS. 1 to 6 graphically show conversion rates for CO.sub.2 and CH.sub.4 as well as the CO yield under different experimental conditions.

A catalyst was prepared in accordance with the wet impregnation (incipient wetness) method. For this purpose, 5 g of monoclinic ZrO.sub.2 were calcined at 650.degree. C. for 15 hours in air, thermally stabilized, compacted to form pellets, and then comminuted into grains having a particle size of 0.3-0.6 mm. This material had a BET surface of 33 m.sup.2 /g and a pore volume of 0.17 cm.sup.3 /g. The material was then treated with 5 cc of an aqueous H.sub.2 PtCl.sub.4 .times.H.sub.2 O solution (0.02 g Pt/cm.sup.3) in a rotary evaporator at 50.degree. C. Thereupon the catalyst was dried for 4 hours at 110.degree. C. and then calcined for 15 hours at 650.degree. C.

In a series of tests, 300 mg (0.8 cm.sup.3) of the granular catalyst were examined using a flow of gas of 170 cm.sup.3 /min in a temperature range of about 400-620.degree. C. The gas had a CH.sub.4 /CO.sub.2 ratio of 1:3.9 and flowed through the fixed-bed catalyst in an upward direction. The composition of the resultant product was examined in the same way as in the following examples in a gas chromatograph with activated carbon column. This gave the measurement values set forth in Table 1. Above 560.degree. C., excellent conversion rates are obtained for CH.sub.4. It is to be noted that the selectivity for CO was practically 100%. This effect can be noted indirectly also from FIG. 1 which shows, for the temperature of 511.degree. C., the change of the conversion rates of CO.sub.2 and CH.sub.4 as well as the CO yield over a long period of time. It can be clearly noted that the effectiveness of the catalyst itself is still at a very high value even after 500 hours of operation. For example, the conversion rate for CH.sub.4 decreases merely from about 62% at the start to about 57%. This means that coking has remained extremely slight.

A Pt catalyst which had ZrO.sub.2 --Y.sub.2 O.sub.3 as support material, the ZrO.sub.2 containing 3 mol % Y.sub.2 O.sub.3, was produced in the same manner as in Example 1. This catalyst was again tested under conditions similar to those employed in Example 1 and it gave the results set forth in Table 2. In particular, at temperatures above 560.degree. C., even somewhat better values than in the first example were obtained here with respect to the CO.sub.2 conversion rate and the CO yield.

In the same way as in Example 1, a Pt catalyst was prepared which had ZrO.sub.2 --La.sub.2 O.sub.3 as the support material, the ZrO.sub.2 containing 3 mol % La.sub.2 O.sub.3. This catalyst was again tested under conditions similar to those in Example 1 and gave the results set forth in Table 3. This catalyst gave values which were even clearly better than the catalyst in the first example in all temperature ranges, with respect both to the CO.sub.2 conversion rate and to the CH.sub.4 conversion rate and the CO yield.

In a further test, the conditions of the first example were modified so that the material used had a CH.sub.4 /CO.sub.2 ratio of 1:2.16. Since in this connection the CH.sub.4 percentage in the material used was greater, this necessarily had to lead to a corresponding reduction of the CH.sub.4 conversion rate. The decisive factor is however that the CO.sub.2 conversion rate and also the CO yield lie clearly above the corresponding values of Example 1, as the measurement values set forth in Table 4 show.

In Table 5 there can be noted the experimental results which were ascertained for the catalyst of Example 1 for a feed material having a CH.sub.4 /CO.sub.2 ratio of 1:1.09, which corresponds practically to the stoichiometric ratio. The values of the CH.sub.4 conversion rate and of the CO yield are again better than in Example 4.

Another catalyst was prepared by the dry impregnation method. For this purpose, 4 g of monoclinic ZrO.sub.2 were calcined in air at 650.degree. C. for 15 hours, thermally stabilized, compacted to form pellets, and then comminuted to form grains having a particle size of 0.3-0.6 mm. This material had a BET surface of 33 m.sup.2 /g and a pore volume of 0.17 cm.sup.3 /g. The material was then treated with 1.74 cc of an aqueous Ni(NO.sub.3).sub.2 .times.H.sub.2 O solution (2 mol/dm.sup.3) in a rotary evaporator at 60.degree. C. The catalyst was then dried for 4 hours at 120.degree. C. and thereupon calcined for a further 19 hours at 650.degree. C.

In this way a granular 5 wt. % Ni/ZrO.sub.2 catalyst was obtained, 300 mg (0.8 cm.sup.3) of which were examined with a flow of feed gas of 170 cm.sup.3 /min in a temperature range of about 400-620.degree. C. The feed gas had a CH.sub.4 /CO.sub.2 ratio of 1:3.9 and flowed through the fixed-bed catalyst in an upward direction. The composition of the resultant product was examined in a

gas chromatograph with an activated carbon column. This gave the measurement values set forth in Table 6. Up to temperatures within the range of about 540-550.degree. C., this catalyst, as shown by a comparison with Table 4, is slightly superior in its activity to that of the first example. At higher temperatures, the conditions, however, reverse. A useful life under the above-indicated conditions with a test temperature of 601.degree. C. gave the results shown in FIG. 2. The conversion rates for CO.sub.2 and CH.sub.4 obtained, as well as the CO yield, decreased, to be sure, with increasing time of use, but still had very high values even after 50 hours of operation. This means that, also in this case, no substantial coking could be noted and the selectivity for CO was almost 100%.

The catalyst of Example 6 was examined under the condition that the CH.sub.4 /CO.sub.2 ratio in the feed gas was 1:1.09. In this case, as can be noted from Table 7, the catalyst showed, in comparison with Table 6, up to about 460.degree. C., better results than previously shown. To be sure, with even higher temperatures, disturbing coking phenomena occurred. This effect could be avoided almost completely in the case of the Pt--ZrO.sub.2 catalyst. It is also to be noted here that, despite this coking which could be noted,

a Ni--ZrO.sub.2 catalyst still shows considerably better stability behavior at higher temperatures than, for instance, a Ni catalyst on an AlO.sub.3 support material, which is clearly deactivated more rapidly.

A traditional Pt catalyst was prepared, in a manner similar to that described in Example 1. Instead of the ZrO.sub.2, 5 g of --Al.sub.2 O.sub.3 were used and worked, in the same manner, into a granular support material of a particle size of 0.3-0.6 mm. This material, with a BET surface of 103 m.sup.2 /g, had a clearly larger surface and with 0.04 cm.sup.3 /g a substantially smaller pore volume. The catalyst coating was produced precisely in the same manner as in the first example and led to a 1 wt. % Pt/--Al.sub.2 O.sub.3 catalyst. Again under the same experimental conditions as in Example 1, the measured values listed in Table 8 were obtained. They show an activity which, it is true, is higher than that appearing in Table 1 of Example 1 of the invention. The decisive factor, however, is that this traditional catalyst was inactivated within a very short period of time by coking. This can be noted from FIGS. 3, 4 and 5, which indicate the conversion rates for CO.sub.2 and CH.sub.4 and the CO yield as a function of the temperature for a first and only one second temperature cycle. It results therefrom that this catalyst is practically deactivated, and therefore

has become unusable, already after passing through a single temperature cycle up into the temperature range of about 600.degree. C.

A support material of 5 g .gamma.-Al.sub.2 O.sub.3 having a particle size of 0.3-0.5 mm was first of all prepared in the same manner as in Comparative Example 1. This material was then treated with 10 cc of an aqueous Ni(NO.sub.3).times.6H.sub.2 O solution at 60.degree. C. in a rotary evaporator in order to obtain a 10 wt. % Ni/.gamma.-Al.sub.2 O.sub.3 catalyst. After the impregnation, the catalyst was dried for 14 hours at 110.degree. C. as in Example 1 and then calcined for 15 hours at 850.degree. C. The results for the CO.sub.2 conversion rate, the CH.sub.4 conversion rate and the CO yield for the first temperature cycle can be noted from FIG. 6. At temperatures of about 500.degree. C., a drastic deactivation appeared already in this first cycle, so that such a Ni catalyst cannot be used in practical operation.