Use of Hydrogen Sulfide to Improve Benzene Production Over Zeolites

The present invention relates to improved benzene production by converting C.sub.2 -C.sub.8 non-aromatic hydrocarbons over zeolites such as ZSM-5 in the presence of hydrogen sulfide.

The Chester et al U.S. Pat. No. 4,350,835 describes a process for converting ethane to liquid aromatics by contacting the ethane with a zeolite catalyst such as ZSM-5 having incorporated therein a minor amount of gallium.

The Davies et al U.S. Pat. No. 4,175,057 describes a process for producing aromatics by contacting a C.sub.3 -C.sub.8 hydrocarbon with a gallium catalyst supported on an aluminosilicate in which the ratio of silica to alumina is between 20:1 and 70:1.

The Bulford et al U.S. Pat. No. 4,157,356 describes a process for producing aromatic hydrocarbons by contacting a C.sub.3 -C.sub.8 hydrocarbon with a gallium catalyst on a silica support which has a surface area greater than 500 m.sup.2 /g and a pore volume less than 0.8 ml/g.

The entire disclosures of the above-mentioned U.S. Patents are expressly incorporated herein by reference.

According to one aspect of the present invention, there is provided a process for producing aromatic hydrocarbons, said process comprising contacting a feedstock comprising one or more C.sub.2 -C.sub.8 non-aromatic hydrocarbons with a catalyst comprising a crystalline zeolite material having a constraint index within the approximate range of 1 to 12, said contacting taking place under sufficient aromatization conditions, said feedstock further comprising hydrogen sulfide in an amount sufficient to increase the proportion of benzene in the aromatic hydrocarbons produced.

Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various type of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.

Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.

Although zeolites may contain silica and alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. More particularly, GeO.sub.2 is an art recognized substitute for SiO.sub.2 and B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, and Ga.sub.2 O.sub.3 are art recognized replacements for Al.sub.2 O.sub.3. Accordingly, the term zeolite as used herein shall connote not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and/or aluminum. On the other hand, the term aluminosilicate zeolite as used herein shall define zeolite materials consisting essentially of silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, as opposed to materials which contain substantial amounts of suitable replacement atoms for such silicon and/or aluminum.

An important characteristic of the crystal structure of the particular class of zeolites suitable for use in accordance with the present invention is that it provides a selective constrained access to and egress from the intracrystalline free space by virtue of having an effective pore size intermediate between the small pore Linde A and the large pore Linde X, i.e. the pore windows of the structure are of about a size such as would be provided by 10-membered rings of silicon atoms interconnected by 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 zeolite, the oxygen atoms themselves of the aluminosilicate zeolites being bonded to the silicon or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred type of aluminosilicate zeolites useful in this invention possess, in combination: a silica to alumina mole ratio of at least about 12; and a structure providing constrained access to the intracrystalline free space.

The silica to alumina mole 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 aluminosilicate zeolites with a silica to alumina mole ratio of at least 12 are useful, it is preferred in some instances to use aluminosilicate zeolites having substantially higher silica/alumina ratios, e.g. 70 and above, 200 and above or even 1600 and above. In addition, aluminosilicate zeolites as otherwise characterized herein but which are substantially free of aluminum, that is zeolites having silica to alumina mole ratios of up to infinity, are found to be useful and even preferable in some instances. Such "high silica" or "highly siliceous" aluminosilicate zeolites are intended to be included within this description. Thus also to be included within the aluminosilicate zeolite definition are substantially pure silica forms of the useful zeolites described herein, that is to say those aluminosilicate zeolites having no measurable amount of aluminum (silica to alumina mole ratio of infinity) but which otherwise embody the characteristics disclosed.

Members of this particular class of zeolites, after activation, acquire an intracrystalline sorption capacity for normal hexane which is greater than that for water, i.e. they exhibit "hydrophobic" properties. This hydrophobic character can be used to advantage in some applications.

The zeolites of the particular class useful herein have an effective pore size such as to freely sorb normal hexane. In addition, their structure must provide constrained access to 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, then access by molecules of larger cross-section than normal hexane is excluded and the zeolite is not of the desired type. Windows of 10-membered rings are preferred, although in some instances excessive puckering of the rings or pore blockage may render these zeolites ineffective. Twelve-membered rings usually do not offer sufficient constraint to produce the advantageous conversions, although the puckered 12-ring structure of TMA offretite shows constrained access. Other 12-ring structures may exist which may be operative.

Rather than attempt to judge from crystal structure whether or not a zeolite possesses the necessary constrained access to molecules of larger cross-section than normal paraffins, a simple determination of the "Constraint Index" as herein defined may be made by passing continuously a mixture of an equal weight of normal hexane and 3-methylpentane over a sample 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 540.degree. C. for at least 15 minutes. The zeolite is then flushed with helium and the temperature is adjusted between 290.degree. C. and 510.degree. C. to give an overall conversion of 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 zeolite 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. Zeolites suitable for the present invention are those having a Constraint Index of about 3 to 12. Constraint Index (CI) values for some typical materials are:

The above-described Constraint Index is an important and even critical definition of those zeolites which are useful in the instant invention. 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 exhibit different Constraint Indices. Constraint Index seems to vary somewhat with severity of operation (conversion) and the presence or absence of binders. Likewise, other variables such as crystal size of the zeolite, the presence of occluded contaminants, etc., may affect the constraint index. Therefore, it will be appreciated that it may be possible to so select test conditions as to establish more than one value in the range of 3 to 12 for the Constraint Index of a particular zeolite. Such a zeolite exhibits the constrained access as herein defined and is to be regarded as having a Constraint Index in the range of 3 to 12. Also contemplated herein as having a Constraint Index in the range of 3 to 12 and therefore within the scope of the defined class of zeolites are those zeolites which, when tested under two or more sets of conditions within the above-specified ranges of temperature and conversion, produce a value of the Constraint Index slightly less than 3, e.g. 2.9, or somewhat greater than 12, e.g. 14 or 15, with at least one other value within the range of 3 to 12. Thus, it should be understood that the Constraint Index value as used herein is an inclusive rather than a exclusive value. That is, a crystalline zeolite when identified by any combination of conditions within the testing definition set forth herein as having a Constraint Index in the range of 3 to 12 is intended to be included in the instant zeolite definition whether or not the same identical zeolite, when tested under other of the defined conditions, may give a Constraint Index value outside of the range of 3 to 12.

The particular class of zeolites defined herein is exemplified by ZSM-5, ZSM-11, ZSM-23, ZSM-35, ZSM-48 and other similar materials.

ZSM-5 is described in greater detail in U.S. Pat. Nos. 3,702,886 and Re. 29,948. The descriptions contained within those patents include the X-ray diffraction pattern of therein disclosed ZSM-5.

ZSM-11 is described in U.S. Pat. No. 3,709,979. The description in that patent includes the X-ray diffraction pattern of said ZSM-11.

ZSM-23 is described in U.S. Pat. No. 4,076,842 along with a specification of the X-ray diffraction pattern of the disclosed ZSM-23 zeolite.

ZSM-35 is described in U.S. Pat. No. 4,016,245 along with a description of the X-ray diffraction pattern of the zeolite.

ZSM-48 is more particularly described in U.S. Pat. No. 4,375,573. Such a description includes the X-ray diffraction pattern for ZSM-48.

It is to be understood that by incorporating by reference the foregoing patent documents to describe examples of specific members of the specified zeolite class with greater particularity, it is intended that identification of the therein disclosed crystalline zeolites be resolved on the basis of their respective X-ray diffraction patterns. As discussed above, the present invention contemplates utilization of aluminosilicate zeolites wherein the mole ratio of silica to alumina is essentially unbounded. The incorporation of the identified patent documents should therefore not be construed as limiting the disclosed crystalline zeolites to those having the specific compositions, e.g., silica-alumina mole ratios discussed therein, it now being known that such aluminosilicate zeolites may be substantially aluminum-free and yet, having the same crystal structure as the disclosed materials, may be useful or even preferred in some applications. It is the crystal structure, as identified by the X-ray diffraction "fingerprint", which establishes the identity of the specific crystalline zeolite material.

The specific zeolites described, when prepared in the presence of organic cations, are substantially catalytically inactive, possibly because the intra-crystalline free space is occupied by organic cations from the forming solution. They may be activated by heating in an inert atmosphere at 540.degree. C. for one hour, for example, followed by base exchange with ammonium salts followed by calcination at 540.degree. C. in air. The presence of organic cations in the forming solution may not be absolutely essential to the formation of this type zeolite; however, the presence of these cations does appear to favor the formation of this special class of zeolite. More generally, it is desirable to activate this type catalyst by base exchange with ammonium salts followed by calcination in air at about 540.degree. C. for from about 15 minutes to about 24 hours.

Natural zeolites may sometimes be converted to zeolite structures of the class herein identified by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, alone or in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite, and clinoptilolite.

In a preferred aspect of this invention, the zeolites hereof are selected as those providing among other things a crystal framework density, in the dry hydrogen form, of not less than about 1.6 grams per cubic centimeter. It has been found that zeolites which satisfy all three of the discussed criteria are most desired.

The dry density for known aluminosilicate zeolites may be calculated from the number of silicon plus aluminum atoms per 1000 cubic Angstroms, as given, e.g., on Page 19 of the article Zeolite Structure by W. M. Meier. This paper, the entire contents of which are incorporated herein by reference, is included in Proceedings of the Conference on Molecular Sieves, (London, April 1967) published by the Society of Chemical Industry, London, 1968.

When the crystal structure is unknown, the crystal framework density may be determined by classical pycnometer techniques. For example, it may be determined by immersing the dry hydrogen form of the zeolite in an organic solvent which is not sorbed by the crystal. Or, the crystal density may be determined by mercury porosimetry, since mercury will fill the interstices between crystals but will not penetrate the intracrystalline free space.

It is possible that the unusual sustained activity and stability of this special class of zeolites is associated with its high crystal anionic framework density of not less than about 1.6 grams per cubic centimeter. This high density must necessarily be associated with a relatively small amount of free space within the crystal, which might be expected to result in more stable structures. This free space, however, is important as the locus of catalytic activity.

Crystal framework densities of some typical zeolites, including some which are not within the purview of this invention, are:

When synthesized in the alkali metal form, the zeolite is conveniently converted to the hydrogen form, generally by intermediate formation of the ammonium form as a result of ammonium ion exchange and calcination of the ammonium form to yield the hydrogen form. In addition to the hydrogen form, other forms of the zeolite can be employed. Such other forms of the zeolite are those wherein the original alkali metal content has been reduced to less than about 50 percent by weight of the original alkali metal contained in the zeolite as synthesized, usually 0.5 percent by weight or less. Thus, the original alkali metal of the zeolite may be replaced by ion exchange with other suitable metal cations of Groups I through VIII of the Periodic Table, including, by way of example, nickel, copper, zinc, palladium, calcium or rare earth metals.

In practicing conversion processes using the catalyst of the present invention, it may be useful to incorporate the above-described crystalline zeolites with a matrix comprising another material resistant to the temperature and other conditions employed in such processes. Such matrix materials include synthetic or naturally occurring substances as well as inorganic materials such as clay, slica and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the zeolite include those of the montmorillonite and kaolin families, which families include the sub-bentonites and the kaolins commonly known as Dixie, McNamee-Georgia and Florida clays or others in which the main mineral constitutent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification.

In addition to the foregoing materials, the zeolites employed herein may be composited with a porous matrix material, such as alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, and silica-titania, as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may be in the form of a cogel. The relative proportions of zeolite component and inorganic oxide gel matrix, on an anhydrous basis, may vary widely with the zeolite content ranging from between about 1 to about 99 percent by weight and more usually in the range of about 5 to about 80 percent by weight of the dry composite.

An optional component of the conversion catalysts in accordance with the present invention comprises a minor proportion, e.g., from about 0.05% to 50% by weight of the catalyst composite, of a difficultly reducible oxide, incorporated into the zeolite. Oxides of this type can include oxides of phosphorus as well as those oxides of the metals of Groups IA, IIA, IIIA, IVA, VA, VIA, VIIA, VIIIA, IB, IIB, IIIB, IVB, or VB of the Periodic Chart of the Elements (Fisher Scientic Company, Catalog No. 5-702-10) which serve to enhance the para-selectivity properties of the catalysts modified therewith. The difficultly reducible oxides most commonly employed to modify the selectivity properties of the zeolite-based catalysts herein are oxides of phosphorus and magnesium. Thus, the catalysts herein can be treated with phosphorus and/or magnesium compounds in the manner described in U.S. Pat. Nos. 3,894,104; 4,049,573; 4,086,287; and 4,128,592, the disclosures of which are incorporated herein by reference.

Phosphorus, for example, can be incorporated into such catalysts at least in part in the form of phosphorus oxide in an amount of from about 0.25% to about 25% by weight of the catalyst composition, preferably from about 0.7% to about 15% by weight. Such incorporation can be readily effected by contacting the zeolite composite with a solution of an appropriate phosphorus compound, followed by drying and calcining to convert phosphorus in the zeolite to its oxide form. Preferred phosphorus-containing compounds include diphenyl phosphine chloride, trimethylphosphite and phosphorus trichloride, phosphoric acid, phenyl phosphine oxychloride, trimethylphosphate, diphenyl phosphinous acid, diphenyl phosphinic acid, diethylchlorothiophosphate, methyl acid phosphate and other alcohol-P.sub.2 O.sub.5 reaction products. Particularly preferred are ammonium phosphates, including ammonium hydrogen phosphate, (NH.sub.4).sub.2 HPO.sub.4, and ammonium dihydrogen phosphate, NH.sub.4 H.sub.2 PO.sub.4. Calcination is generally conducted in the presence of oxygen at a temperature of at least about 150.degree. C. However, higher temperatures, i.e., up to about 500.degree. C. or higher are preferred. Such heating is generally carried out for 3-5 hours but may be extended to 24 hours or longer.

In addition to treatment of the zeolite composites to incorporate phosphorus and/or magnesium oxides as hereinbefore described in detail, such zeolites may also be modified in a substantially similar manner to incorporate thereinto a variety of other oxide materials to enhance para-selectivity. Such oxide materials include oxides of boron (U.S. Pat. No. 4,067,920); antimony (U.S. Pat. No. 3,979,472); beryllium (U.S. Pat. No. 4,260,843); Group VIIA metals (U.S. Pat. No. 4,275,256); alkaline earth metals (U.S. Pat. No. 4,288,647); Group IB metals (U.S. Pat. No. 4,276,438); Group IVB metals (U.S. Pat. No. 4,278,827); Group VIA metals (U.S. Pat. No. 4,259,537); Group IA elements (U.S. Pat. No. 4,329,533); cadmium (U.S. Ser. No. 139,611, filed Apr. 11, 1980); iron and/or cobalt (U.S. Ser. No. 150,868, filed May 19, 1980); Group IIIB metals (U.S. Pat. No. 4,276,437); Group IVA metals (U.S. Pat. No. 4,302,620); Group VA metals (U.S. Pat. No. 4,302,621); and Group IIIA elements (U.S. Pat. No. 4,302,622).

Additional catalyst modifying procedures which may also optionally be employed to modify catalyst activity or selectivity include precoking and presteaming (e.g., before oxide incorporation), or combinations thereof.

The aromatization conditions suitable for use in accordance with the present invention may include, e.g., a temperature of from about 100.degree. C. to about 700.degree. C., a pressure of from about 0.1 atmosphere to about 60 atmospheres, a weight hourly space velocity (WHSV) of from about 0.1 to about 400 and a hydrogen/hydrocarbon mole ratio of from about 0 to about 20.

The feedstock to be aromatized may comprise ethylene, propylene and/or propane, e.g., from about 1 to about 30 weight percent of ethylene, from about 1 to about 30 weight percent of propylene and from about 1 to about 30 weight percent of propane. This feedstock may be, e.g., a refinery off gas.

The feedstock may comprise from about 2 to about 50 volume percent of hydrogen sulfide (i.e. H.sub.2 S).

To 5.0 g of the hydrogen form of ZSM-5 (i.e., HZSM-5) heated at 600.degree. C. was passed through propylene at 40 cc/min., Ex. 1. In the second run, propylene was passed under the same conditions except that H.sub.2 S co-feed was added at 4 cc min. (or 10% by volume of propylene). Runs were repeated with H.sub.2 S co-feed increased by 10%. Results are shown in Table 1.

The yield of BTX (i.e. benzene, toluene, xylene and ethylbenzene) were 50.+-.2% in all runs. In the presence of H.sub.2 S the benzene yield increased from 23.8% to the maximum of 39.5%, an increase of 66% over the run in the absence of H.sub.2 S.

This Example demonstrates that the process of the present invention is also applicable for diluted propylene or a mixture of hydrocarbons such as a refinery off gas containing, by wt, 2% H.sub.2, 37.6% CH.sub.4, 22.6% C.sub.2 H.sub.6, 15.1% C.sub.2 H.sub.4 and 6.7% C.sub.3 H.sub.6. Runs were made as in Examples 1-6 except propylene feed was replaced with the mixed gas. In the absence of H.sub.2 S, benzene yield was 12.6%. The yield increased to 13.6% with 10% H.sub.2 S and 14.3% with 20% H.sub.2 S co-feed at 600.degree. C.

The foregoing Examples point out that higher benzene yields are obtained at the expense of toluene and C8+ aromatics production.