PROCESS FOR THE CONVERSION OF LIGHT OLEFINS INTO HEAVIER HYDROCARBONS

Abstract: Olefins containing 2 to 5 carbon atoms are oligomerized over crystalline silicate or a crystalline aluminosilicate which contains gallium, preferably framework gallium, at 177 to 316°C (350-600°F) to produce product containing a high ratio of heavy hydrocarbon liquid to C₁-C₅ aliphatics.

Description: The invention relates to converting C₂-C₅ olefins to gasoline, distillate and lube range hydrocarbons. Zeolites, which are crystalline aluminosilicates, have long been used for cracking, isomerization, hydrocracking, and the like. The invention is not concerned with cracking, but rather with the preparation of higher molecular weight products, with decreased light gas production. The invention resides in the discovery that certain crystalline silicates, having a characteristic X-ray diffraction pattern of known zeolites, convert C₂-C₅ olefins to gasoline, distillate and lube range hydrocarbons with less light gas production. The alpha-value of the crystalline silicate is not critical. Thus, the invention differs from U.S. 3,960,978, where C₂-C₅ oligomerization requires a zeolite with an alpha of 0.1 to 120 at a WHSV from 0.1 to 25 and temperature of 260 to 482°C (500 to 900°F). Accordingly, the present invention provides a process for converting an olefin containing two to five carbon atoms to higher molecular weight hydrocarbons by contacting the olefin with a catalyst characterized by use of a catalyst comprising crystalline silicate, containing 0.1 to 15% by weight gallium contacting the olefins with the catalyst at 177 to 316°C (350 to 600°F). The method produces less light gas and more distillate and lube boiling range hydrocarbons than methods which employ [Al] ZSM-5 of comparable alpha value which is free of gallium. The gallium is preferably framework gallium. The crystal lattice framework of zeolites can be described as a rigid three-­ dimensional framework of SiO₄ and AlO₄ in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminium and silicon atoms to oxygen atoms is one:two. Framework gallium is a substitute for the Si of SiO₄ or for Al of the AlO₄ tetrahedra. The present invention is directed to the catalytic conversion of a C₂-C₅ olefin, mixtures thereof, and mixtures thereof with paraffins to higher molecular weight products. The feed may also contain other light hydrocarbons, e.g., C₂-C₅ aliphatics. The products include gasoline distillate and lube range hydrocarbons. The amount of light gases produced, including C₁-C₅ aliphatics, is substantially reduced, The products are characterized by a high ratio of 343°C+ (650°F+) to C₁-C₅ aliphatics, relative to prior art processes. Reaction conditions include temperatures from 177°C to 316°C (350 to 600°F), preferably 232°C to 271°C (450 to 520°F) and most preferably temperatures high enough to achieve conversion of most of the olefins, but below 255-260°C, to minimize formation of aromatics and enhance the production of heavy hyrocarbons suitable for use as lubricant stock. Pressures may vary widely, and preferably range from 3500 to 14,000 kPa (500 to 2,000 psig). The weight hourly space velocity (WHSV) may range from 0.1 to 25. The relatively low temperatures used in the process of the present invention are important in maximizing production of lube stocks and minimizing formation of aromatics and olefins. The catalyst is a gallium containing crystalline silicate or a gallium containing crystalline alumino silicate, a zeolite, having a characteristic X-ray diffraction pattern. The gallium content can be expressed in terms of wt% Ga, and can range from 0.1 to 15%. In a preferred embodiment the X-ray diffraction corresponds to that of ZSM-5, as disclosed in U.S. No. 3,702,886. In general, ZSM-5 can be prepared from a solution containing water, tetrapropyl ammonium hydroxide and the elements of sodium oxide, an oxide of aluminum or gallium, an oxide of silica, and having a composition in terms of mole ratios of oxides, falling within the following ranges: wherein R is propyl, W is aluminum and Y is silicon. This mixture is maintained at reaction conditions until the crystals of the zeolite are formed. Thereafter the crystals are separated from the liquid and recovered. Typical reaction conditions consist of a temperature of 75°C to 175°C for six hours to 60 days. A preferred temperature range is 90° to 150°C, for 12 hours to 20 days. In the examples below the preparation of ZSM-5 was modified to replace aluminum with gallium. Various sources of gallium may be used, Ga₂(SO₄)₃ was used in the examples below. The solid product is separated from the reaction mixture by cooling it to room temperature, filtering and water washing. Any other method for preparing a gallosilicate may also be used. The gallosilicate zeolite can also be prepared by treatment of high silica zeolite with a source of Ga in aqueous solution with pH 7 at reflux or lower temperatures; the composite is then ammonium exchanged and calcined to produce a catalyst with increased Bronsted acidity due to the inserted framework elements. This alternative method produces a more crystalline gallosilicate than is possible by crystallization from gels containing hydroxygallium constituents. The reaction mixture can be prepared with materials which supply the elements of the appropriate oxide. Such materials include an aluminosilicate, sodium aluminate, alumina, sodium silicate, silica hydrosil, silica gel, silicic acid, sodium hydroxide and tetrapropylammonium hydroxide. Each oxide component of the reaction mixture may be supplied by one or more initial reactants. The reaction mixture can be prepared batchwise or continuously. Crystal size and crystallization time will vary with the nature of the reaction mixture employed. The as-synthesized crystalline silicates or crystalline aluminosilicates may contain cations, which are undesirable. Replacement techniques are well known in the art. Typical replacing cations include hydrogen, ammonium and one or more metal cations, or mixtures thereof. Specific replacing cations include hydrogen, zinc, nickel, platinum, palladium, rhenium, and chromium. Representative ion exchange techniques are disclosed in U.S. 3,140,249; 3,140,251 and 3,140,253. Following cation replacement the crystalline material may be washed with water and dried at 66°C to 316°C (150°F to 600°F) and then heated in air or other gas at 260°C to 816°C (500° F to 1500°F) for 1 to 48 hours or more. The crystalline material may then be treated with steam at 427°C to 871°C (800°F to 1600°F), preferably 538°C to 816°C (l000°F to 1500°F), if desired. The treatment may be accomplished in atmospheres consisting partially or entirely of steam. Similar treatments can be accomplished at lower temperatures at elevated pressures, for example, at 177 to 371°C (350 to 700°F) at 10 to about 200 atmospheres. The alpha-value of the crystalline material containing framework gallium is not critical; it preferably ranges from 5 to 300. The Alpha Value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst. It gives the relative rate of normal hexane conversion per volume of catalyst per unit time. It is based on the activity of the highly active silica alumina cracking catalyst taken as an Alpha of 1 (Rate Constant = 0.16 sec⁻¹). The Alpha Test is described in U.S. Patent 3,354,078 and in The Journal of Catalysis , Vol. IV. pp. 522-529 (August 1965). The silicate tested in this Alpha Test must be at least partially in the hydrogen form. Conversion to the hydrogen form may be accomplished by contact with an ammonium salt or acid solution followed by thermal treatment to eliminate ammonia and water. To achieve high olefin conversion to 343°C (650°F) plus hydrocarbons, crystalline aluminosilicates, e.g. ZSM-5, free of gallium, must have high alpha-values. By comparison, crystalline materials containing framework gallium, of comparable alpha-­ value, produce less light gas and more 343°C (650°F+) hydrocarbons. Generally, the crystalline silicates containing framework gallium may contain up to 0.1 wt.% Al₂O₃. Although the crystalline silicates containing framework gallium, used in the examples are used neat, without a porous matrix, the crystalline materials may be used in combination, dispersed or otherwise intimately admixed with a porous binder or matrix, so that the resulting product contains from 1% to 95% by weight, and preferrably from 20 to 80% by weight, of the gallium silicate (or gallium aluminosilicate)in the final composite. The terms "binder" and "matrix" refer to inorganic compositions with which the silicate can be combined, disbursed or otherwise intimately admixed. Such binders impart strength and attrition resistance to the relatively severe conditions under which the silicate is used. Representative binder or matrix materials include metals and alloys thereof, sintered metals and sintered glass, asbestos, silicon carbide aggregates, pumice, firebrick,diatomaceous earths and inorganic oxides. Inorganic compositions especially those of a siliceous nature are preferred. Of these matrices, inorganic oxides such as clay, chemically treated clay, silica, alumina, and silica-alumina are particularly preferred. The following examples show conversion of C₂ - C₅ olefins. EXAMPLE 1 (a) A standard procedure for producing ZSM-5 composition was followed except the source of aluminum, Al₂(SO₄)₃, was replaced by Ga₂(SO₄)₃ as a source of gallium. . The silica source was 50 grams Q brand silica in 50 grams of water. The gallium source was a solution of 1.11 grams Ga₂(SO₄)₃, 84.2 grams water and 5.3 grams of H₂SO₄; 6.16 grams of TPA Br (tetrapropylammonium bromide) were used. The Si/Ga₂ ratio of the reaction mixture was about 90. The reaction mixture was stirred for several days at 160°C, until product crystallized. (b) The silica source in the 1(a) preparation was replaced by 13.9 grams of extracted SiO₂ in 86 grams of water. The amounts of other components remained the same. Stirring of the components was for seven days with addition of 20cc of 10N NaOH to get a pH of about 10. (c) The 1(b) preparation was repeated, but the amount of extracted silica was doubled. The catalyst compositions are set forth in Table 1. In the Table, Meq/g ash means the milliequivalents of ammonia per gram of catalyst, based on weight of catalyst after calcining at 1000°C. Both calculated and experimental values are reported. The experimental value was determined using the well known TPD technique, or Temperature Programmed Desorption of ammonia. The ratio of the experimental to calculated values, expressed as %, is a measure of how much gallium is in the crystal framework. Each catalyst above exhibited high crystallinity ZSM-5 X-ray diffraction patterns. (d) In this preparation a 6g sample of HZSM-5 (SiO₂/Al₂O₃ = 26,000/1) was mixed with 300cc 0.2N NaOH solution containing 1.5g of Ga₂(SO₄)₃. The mixture was refluxed for 2 hours and then washed, converted into the ammonium form by exchange with NH₄NO₃. The resultant zeolite has an exchange capacity of 0.5446 meq/g ash and n-hexane cracking activity of 803. EXAMPLE 2 ZSM-5 containing gallium in the framework, prepared as in preparation (c) of Example 1, converted propylene to gasoline distillate and lube range hydrocarbons. ZSM-5 containing no gallium and having an alpha value of 55, was also tested for propylene conversion. All tests were run at 230°C and pressure of (1500 psig) 10,400 kPa. The results are tabulated in Table 2: ZSM-5 containing framework gallium produces less light gas and more 343°C+ (650°F+) hydrocarbons than ZSM-5 free of gallium with comparable alpha value. Table 3 below presents comparisons of the 343°C (650°F) plus product from runs at 230°C reactor temperature in which catalyst compositions employing ZSM-5, free of gallium, of differing alpha values are employed: ZSM-5, free of gallium, must have a much higher alpha value than [Ga]ZSM-5 to achieve comparable conversion to 343°C+ (650°F+) product. The [Ga]ZSM-5 makes less C₁-C₅. The gallium is preferably framework gallium, at least 80% framework gallium giving very good results. The relatively low temperatures used aid in producing a heavy liquid fraction of 343°C⁺ material (650°F⁺) which is a good lubricating oil base stock, one which can be characterized as containing less than 10 wt. % aromatics, preferably less than 5 wt. % aromatics.

Claims: 1. A process for converting an olefin containing two to five carbon atoms to higher molecular weight hydrocarbons by contacting the olefin with a catalyst characterized by use of a catalyst comprising crystalline silicate, containing 0.1 to 15% by weight gallium and contacting the olefins with the catalyst at 177 to 316°C (350 to 600°F). 2. The process of claim 1 further characterized in that the crystalline silicate contains 0 to 0.1 wt. % aluminum, expressed as Al₂O₃, and wherein the crystalline silicate is a three-dimentional network of SiO₄ tetrahedra linked to each other by sharing of the oxygen atoms thereof, wherein at least a portion of the gallium is a substitute for Si in the SiO₄ tetrahedra. 3. The process of claim 1 or 2 further characterized in that the temperature is 232 to 271 (450 to 520°F). 4. The process of any preceding claim further characterized in that the reactor pressure is 3,500 to 14,000 kPa (500 to 2000 PSIG). 5. The process of any preceding claim further characterized in that the crystalline silicate is ZSM-5. 6. The process of any preceding claim further characterized in that the weight hourly space velocity (WHSV) of olefin to catalyst is 0.1 to 25. 7. The process of any preceding claim wherein the silicate results in reduced amounts of aliphatics of 1 to 5 carbon atoms, compared to the same zeolite free of gallium. 8. The process of any preceding claim wherein at least 80% of the gallium on the catalyst is framework gallium. 9. The process of any preceding claim further characterized in that a 343°C⁺ (650°F⁺) liquid fraction is produced which contains less than 10 wt. % aromatics. 10. The process of claim 9 further characterized in that the 343°C⁺ (650°F⁺) liquid fraction contains less than 5 wt. % aromatics.

Inventors: CHANG, CLARENCE DAYTON, CHU, CYNTHIA TING-WAH
Applicants: MOBIL OIL CORPORATION
Application year: 1986
Main classification: C07C2
IPC codes: C07C2, C10G50, C10G57, B01J29