Conversion of Synthesis Gas With Cobalt-Containing Fluid Catalyst

This application is related to application Ser. No. 970,301, filed on even date herewith.

This invention is concerned with an improved process for converting synthesis gas, i.e. mixtures of gaseous carbon oxides with hydrogen or hydrogen donors, to hydrocarbon mixtures.

Processes for the conversion of coal and other hydrocarbons, such as natural gas, to a gaseous mixture consisting essentially of hydrogen and carbon monoxide and/or dioxide are well known. Those of major importance depend either on the partial combustion of the fuel with an oxygen-containing gas or on the high temperature reaction of the fuel with steam, or on a combination of these two reactions. An excellent summary of the art of gas manufacture, including synthesis gas, from solid and liquid fuels is given in Encyclopedia of Chemical Technology, Edited by Kirk-Othmer, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York, New York.

It is also well known that synthesis gas will undergo conversion to reduction products of carbon monoxide, such as hydrocarbons, at from about 300.degree. F. to about 850.degree. F., under from about one to one thousand atmospheres pressure, over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a range of liquid hydrocarbons, a portion of which have been used as low octane gasoline. Catalysts that have been studied for this and related processes include those based on iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium, or their oxides.

Recently, it has been discovered that the conversion of synthesis gas into valuable products can be greatly enhanced by employing a special type of crystalline aluminosilicate zeolite exemplified by ZSM-5 in admixture with a carbon monoxide reduction catalyst. Thus, for example, in U.S. Pat. No. 4,086,262, there is disclosed a process for the conversion of syngas by passing the same at elevated temperature over a catalyst which comprises an intimate mixture of a Fischer-Tropsch component and a special type of zeolite such as ZSM-5. Said patent points out that the products produced are hydrocarbon mixtures which are useful in the manufacture of heating oil, high octane gasoline, aromatic compounds, and chemical intermediates.

Although U.S. Pat. No. 4,086,262 is primarily directed to multi-particle composite catalysts, i.e. the crystalline aluminosilicate component (one particle) is physically admixed with the Fischer-Tropsch component (another particle), nevertheless, Example 5 of said patent does disclose a single particle iron-containing catalyst (an alumina bound zeolite catalyst impregnated with iron).

As can well be appreciated, the patent and technical literature relating to the Fischer-Tropsch process is, indeed, extensive and the various catalysts reported in the prior art have been used by themselves as well as in admixture with catalytically inactive supports such as kieselguhr. Although the reasons for using catalytically inactive supports have varied, nevertheless, it would appear that one reason for using the same was that it resulted in increases surface area of the Fischer-Tropsch component upon which it was deposited or admixed and that it also aided in controlling the heat requirements of the overall exothermic reactions.

It is also known in the art of admix a Fischer-Tropsch component with a material, such as silica-alumina which is known to be catalytically active for the conversion of hydrocarbons.

The novel fluid bed process of this invention is directed towards an improvement in the process of converting syngas to a very specific product. The product with which the instant invention is concerned is a predominantly olefinic naphtha having a boiling range of less than 400.degree. C. at a 90% overhead which is defined as an olefinic C.sub.5 + naphtha in about 60% yield with an aromatic content of less than 10 weight percent wherein said fraction is at least 50 weight percent of the total hydrocarbons produced. The instant invention is also concerned with obtaining the above-defined product in good yields and good selectivities from the starting syngas material.

The novel process of this invention is carried out by contacting said synthesis gas with a fluid catalyst which comprises at least three separate components which are present in a single particle as opposed to a mixture of three separate particles. The fluid catalyst of this invention comprises cobalt, an acidic crystalline aluminosilicate zeolite having a pore size of about 5 Angstrom units, a silica alumina ratio of at least 12, and a constraint index of about 1-12 (preferably ZSM-5) and a siliceous matrix material. The crystalline aluminosilicates employed in the novel process of this invention are fully set forth in aforementioned U.S. Pat. No. 4,086,262 which is herein incorporated by reference. The preferred class of zeolites used is exemplified by ZSM-5, ZSM-11, ZSM-12, etc.

The matrix portion of the single particle fluid catalyst is not narrowly critical and suitable matrices include silica, silica-zirconia, silica-magnesia, etc. As is well known in the art, these matrices are synthetic materials as opposed to being naturally occurring. It has been found that one important criticality with regard to the synthetic matrix is that it would be relatively free of alumina. For reasons which are not completely understood, it has been found that the presence of alumina has an adverse effect on the activity of the cobalt and that therefore the total amount of alumina which is present in the synthetic matrix in the novel process of this invention should not exceed 10% by weight. More preferably, it should not exceed around 7% by weight.

One surprising feature of the novel process of this invention is that the catalysts which are employed are unpromoted and yet they still exhibit high activity with little evident aging, and, in fact, are capable of converting syngas to the olefinic naphtha product previously described while producing no more than 30 weight percent of methane plus ethane, based on total hydrocarbons. In fact, the use of promoters, which the prior art found necessary in previous cobalt-containing catalysts, is definitely not preferred due to the fact that most promoters are alkaline in nature and they have a tendency to migrate to the acidic crystalline aluminosilicate zeolite component and to decrease the activity of the same. Therefore, it would appear that the single particle fluid catalyst of the instant invention represents a significant departure from the teachings of the prior art in that not only are promoters not necessary for sustained operation but, in fact, are detrimental to the activity of the zeolitic component.

The single particle cobalt-containing fluid catalysts which are used in the novel process of this invention are prepared by adding the appropriate acidic crystalline aluminosilicate previously defined to a siliceous hydrogel before drying, homogenizing the same, thereafter spray-drying in order to obtain particles in the fluidized particle region, and impregnating with a water soluble cobalt salt such as cobalt nitrate.

It is to be understood that methods of making fluidized catalysts containing crystalline aluminosilicate zeolites and siliceous matrices are well known in the art and that no novelty is claimed in this step per se. Thus, for example, a composite of the crystalline aluminosilicate zeolite and a siliceous matrix can be made by admixing an aqueous alkali metal silicate with or without a particulate weight agent, such as kaolin clay, desirably as a dispersion in water so as to intimately mix the clay particles with the alkali metal silicate. This admixing is conveniently done at room temperature, although, of course, higher or lower temperatures may be employed if desired. The mixture is then heated, generally to a temperature of from 100.degree.-160.degree. F. and acid is added to adjust the pH to from about 8-10. The temperature is maintained for a time of about 1-6 hours or longer. At this point, if a silica-zirconia weighting agent (e.g. clay) matrix is desired, a zirconium salt is added, desirably as an aqueous solution thereof. Acid is then added to reduce the pH to about 4-7 and form a silica gel weighting agent or a silica gel-zirconia gel weighting agent slurry, which is then admixed with a slurry of the acidic crystalline aluminosilicate zeolite previously described. The resulting composite is then homogenized and spray dried to produce fluid size particles which are then treated with a source of ammonium ions or hydrogen ions in order to reduce the sodium content to a low level which is desirably less than about 0.1% sodium.

The fluid particles are therafter impregnated with a soluble aqueous cobalt salt solution, which is preferably cobalt nitrate in order to form the finished single particle catalyst of this invention. As is generally known in fluid catalysts for catalytic cracking, the catalyst additionally includes a weighting agent. The most preferred weighting agent is kaolin clay. Other weighting agents may be substituted in whole or in part for the kaolin clay so long as the weighting agents are not detrimental to the finished catalyst.

The relative proportion of crystalline aluminosilicate zeolite to matrix is not narrowly critical and the weight ratio between crystalline aluminosilicate and matrix can range from 1:5 to 5:1 with ratios of 1:2 to 1:3 being preferred.

As has been indicated earlier, the crystalline alumino-silicate zeolite and a matrix are then thoroughly mixed in a form of an aqueous slurry in order to homogenize the same and thereafter subdivided and dried to form the desired particles. A particularly good method of making microspherical particles (e.g. of particle size of about 1-200 microns) especially suitable for use in the fluidized process of this invention is spray-drying.

The temperature of the air (or other gas) entering the spray drier is ordinarily within the range of 500.degree.-1,000.degree. F. The temperature used will depend on such factors as the quantity of material to be dried and the quality of air used in the drying. The evaporization rate will vary depending on the quantity of air used in the drying. The temperature of the particles which are being dried is generally within the range of 150.degree.-400.degree. F. at the completion of the drying, but preferably 200.degree.-350.degree. F.

The drying may be affected by a process in which the particles to be dried and a hot air stream are moving in the same direction for the entire drying period (concurrent drying) or where the hot stream flows in the opposite direction (countercurrent drying), or by semi-concurrent drying. It is to be understood that spray-drying to form fluidized catalysts is well known in the art and a representative procedure is described in U.S. Pat. No. 3,553,104, the entire contents of which are incorporated by reference.

In order to complete the preparation of the single particle fluid catalyst of this invention, the spray-dried material above-described is thereafter impregnated with an aqueous solution of a cobalt salt. The particular cobalt salt employed is not narrowly critical and can include the nitrate, the chloride, the sulfate, the acetate, etc. It is to be understood, however, that the nitrate is preferred due to the fact that other salts such as the sulfate are sometimes harmful to the overall catalyst unless great care is taken to remove all the residual sulfate ion by washing.

The amount of cobalt which is contained in the finished catalyst can range from 1-20 weight percent, based on the weight of the total catalyst, with 3-10% being preferred.

The cobalt impregnated catalyst is thereafter usually calcined in air at elevated temperatures for periods of time ranging from 1-24 hours.

The acidic crystalline aluminosilicate component of the catalyst is characterized by a pore dimension greater than about 5 Angstroms, i.e. it is capable of sorbing paraffins, and it has a silica-to-alumina ratio of at least 12 and a constraint index within the range of 1 to 12. Zeolite A, for example, with a silica-to-alumina ratio of 2.0 is not useful in this invention, and it has no pore dimension greater than about 5 Angstroms.

The crystalline aluminosilicates herein referred to, also known as zeolites, constitute an unusual class of natural and synthetic materials. They are characterized by having a rigid crystalline framework structure composed of an assembly of silicon and aluminum atoms, each surrounded by a tetrahedron of shared oxygen atoms, and a precisely defined pore structure. Exchangeable cations are present in the pores.

The acidic crystalline aluminosilicate component preferably is in the hydrogen form.

The catalysts referred to herein utilize members of a special class of zeolites exhibiting some unusual properties. They are very active even with silica-to-alumina ratios exceeding 30. This activity is surprising since catalytic activity of zeolites is generally attributed to framework aluminum atoms and cations associated with these aluminum atoms. These zeolites retain their crystallinity for long periods in spite of the presence of steam even at high temperatures which induce irreversible collapse of the crystal framework of other zeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits, when formed, may be removed by burning at higher than usual temperatures to restore activity. In many environments the zeolites of this class exhibit very low coke forming capability, conducive to very long times on stream between burning regenerations.

An important characteristic of the crystal structure of this class of zeolites is that it provides constrained access to, and egress from, the intra-crystalline free space by virtue of having a pore dimension greater than about 5 Angstroms and pore windows of about a size such as would be provided by 10-membered rings of oxygen atoms. It is to be understood, of course, that these rings are those formed by the regular disposition of the tetrahedra making up the anionic framework of the crystalline aluminosilicate, the oxygen atoms themselves being bonded to the silicon or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred zeolites useful in this invention comprise, in combination: a silica-to-alumina ratio of at least about 12; and a structure providing constrained access to the crystalline free space.

The silica-to-alumina ratio referred to may be determined by conventional analysis. This ratio is meant to represent as closely as possible, the ratio in the rigid anionic framework of the zeolite crystal and to exclude aluminum in the binder or in cationic or other form within the channels. Although zeolites with a silica-to-alumina ratio of at least 12 are useful, it is preferred to use zeolites having higher ratios of at least about 30. Such zeolites, after activation, acquire an intracrystalline sorption capacity for normal hexane which is greater than that for water, i.e. they exhibit "hydrophobic" properties. It is believed that this hydrophobic character is advantageous in the present invention.

The zeolites useful as catalysts in this invention freely sorb normal hexane and have a pore dimension greater than about 5 Angstroms. In addition, their structure must provide constrained access to some larger molecules. It is sometimes possible to judge from a known crystal structure whether such constrained access exists. For example, if the only pore windows in a crystal are formed by 8-membered rings of oxygen atoms, then access by molecules of larger cross-section than normal hexane is substantially excluded and the zeolite is not of the desired type. Zeolites with windows of 10-membered rings are preferred, although excessive puckering or pore blockage may render these zeolites substantially ineffective. Zeolites with windows of 12-membered rings do not generally appear to offer sufficient constraint to produce the advantageous conversions desired in the instant invention, although structures can be conceived, due to pore blockage or other cause, that may be operative.

Rather than attempt to judge from crystal structure whether or not a zeolite possesses the necessary constrained access, a simple determination of the "constraint index" may be made by continuously passing a mixture of equal weight of normal hexane and 3-methylpentane over a small sample, approximately 1 gram or less, of zeolite at atmospheric pressure according to the following procedure. A sample of the zeolite, in the form of pellets or extrudate, is crushed to a particle size about that of coarse sand and mounted in a glass tube. Prior to testing, the zeolite is treated with a stream of air at 1,000.degree. F. for at least 15 minutes. The zeolite is then flushed with helium and the temperature adjusted between 550.degree. F. and 950.degree. F. to give an overall conversion between 10% and 60%. The mixture of hydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1 volume of liquid hydrocarbon per volume of catalyst per hour) over the zeolite with a helium dilution to give a helium to total hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a sample of the effluent is taken and analyzed, most conveniently by gas chromatography, to determine the fraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rate constants for the two hydrocarbons. Catalysts suitable for the present invention are those which employ a zeolite having a constraint index from 1.0 to 12.0. Constraint Index (CI) values for some typical zeolites including some not within the scope of this invention are:

The above-described Constraint Index is an important and even critical, definition of those zeolites which are useful to catalyze the instant process. The very nature of this parameter and the recited technique by which it is determined, however, admit of the possibility that a given zeolite can be tested under somewhat different conditions and thereby have different constraint indexes. Constraint index seems to vary somewhat with severity of operation (conversion). Therefore, it will be appreciated that it may be possible to so select test conditions to establish multiple constraint indexes for a particular given zeolite which may be both inside and outside the above-defined range of 1 to 12.

Thus, it should be understood that the parameter and property "Constraint Index" as such value is used herein as an inclusive rather than an exclusive value. That is, a zeolite when tested by any combination of conditions within the testing definition set forth hereinabove to have a constraint index of 1 to 12 is intended to be included in the instant catalyst definition regardless that the same identical zeolite tested under other defined conditions may give a constraint index value outside of 1 to 12.

The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-35 and ZSM-38, and other similar materials. Recently issued U.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporated herein by reference.

ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, the entire contents of which are incorporated herein by reference.

ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449, the entire contents of which are incorporated herein by reference. ZSM-35 is described in U.S. Pat. No. 4,016,245 and ZSM-38 is described in U.S. Pat. No. 4,046,859, both of which are incorporated herein by reference.

The novel process of this invention is carried out at temperatures ranging from about 400.degree. to 550.degree. F. and more preferably from 475.degree. to about 525.degree. F. The novel process of this invention is carried out at gas hourly space velocities (GHSV), ranging from 400 to 20,000 and more desirably from 500 to 6,000, based on fresh feed and total catalyst volume. Hydrogen to carbon oxides ratios can vary from 1:1 to 5:1 and more preferably are about 2:1, pressures ranging from 50 to 1,000 psig and more preferably from 150 to 400 psig.

The following examples will illustrate the novel process of this invention.

A silica-clay hydrogel was prepared by adding 153.2 cc of 97.1% sulfuric acid to a slurry of 698 grams of WP kaolin clay in 32.4 lbs of demineralized water and 2989 grams of Q-Brand sodium silicate (containing 8.9% Wt. Na.sub.2 O and 28.7% Wt. SiO.sub.2) at 120.degree. F. After heating the mixture to 140.degree. F. for two hours a solution containing 52.3 grams of aluminum sulfate in 209 cc of water was added, followed by 66.8 grams of sodium zirconium silicate in 648 cc water and the pH adjusted to 4.7 by adding sulfuric acid. The composition of the hydrogel (on a solids basis) was 54.5% SiO.sub.2, 0.5% Al.sub.2 O.sub.3, 1% ZrO.sub.2, and 44% clay. After standing overnight the equivalent of 40% HZSM-5 was added and the resulting gel was homogenized and spray dried, NH.sub.4 + exchanged and washed until the sodium level was <0.01 %.

The catalyst in this example was prepared by impregnating 600 gms of the matrix prepared in Example 1 with an aqueous solution containing 185.5 gms Co(NO.sub.3).sub.2.6H.sub.2 O, drying the catalyst and calcining in air at 1,000.degree. F. for three hours. The finished catalyst contained 5.9% cobalt.

Q-Brand sodium silicate solution (7972.9 grams) was added to a slurry of 1866.4 grams Georgia kaolin (WP) clay in 86.4 lbs of water. The mixture was heated to 120.degree. F. and 408.4 cc of 97.1% sulfuric acid was added, followed by heating to 140.degree. F. for two hours. After cooling, 139.5 grams of aluminum sulfate in 558.1 cc H.sub.2 O was mixed in, followed by 178.0 g sodium zirconium silicate in 1730 cc H.sub.2 O. After adjusting to pH 4 with H.sub.2 SO.sub.4, 1333 grams of HZSM-5 zeolite was added to one-half of the gel mass. The mixture was then homogenized and spray dried. After exchange with ammonium sulfate solution (6 lbs (NH.sub.4).sub.2 SO.sub.4 in 40 lbs H.sub.2 O), the gel was washed with water and dried at 250.degree. F. until the sodium level was less than 0.01%. The composition was approximately 33% SiO.sub.2, 0.3% Al.sub.2 O.sub.3, 26.7% Clay, 38% zeolite, and 2% ZrO.sub.2.

The catalyst matrix prepared in Example 3 was elutriated to remove most of 20 micron and smaller particles, and 1214 grams was impregnated with solution containing 376 grams of cobaltous nitrate in 709 cc H.sub.2 O. After drying and calcining in air at 1,000.degree. F. for three hours, the finished catalyst contained 5.5% cobalt.

The procedure of Example 2 was repeated with the exception that 1.1 weight percent thoria was added as a promoter.

The catalysts of Examples 2, 4 and 5 were evaluated for the conversion of syngas to gasoline at 200 psig and a 1:1 CO/H.sub.2 ratio. The following table gives the results obtained during the second day of the evaluation after the catalysts had been conditioned in hydrogen for 14 hours at 950.degree. F.

As can be seen from the above data, the promoted catalyst of Example 5 resulted in a significant loss of activity with regard to CO conversion, i.e. 24.1% as opposed to 36.8% for Example 2--even though the catalysts of Examples 2 and 5 contained the same amount of cobalt.