Propylene oligomerization over silica modified zeolites

Abstract: Oligomerizing propylene by contacting propylene with silica modified zeolites. The modification of the zeolite with silica results in a product which is enriched in hydrocarbons falling within the boiling range of jet fuel and diesel fuel. The zeolite may be ZSM-5, which may be modified by sorbing methylhydrogen-silicone into the pores of the zeolite followed by calcination to convert the sorbed methylhydrogen-silicone into amorphous silica.

Description: This invention relates to a process for oligomerizing propylene. Propylene oligomerization over acidic catalyst is known. For example, GB 2 156 380 A discloses a process for converting a light olefin feedstock into gasoline by contact with a zeolite catalyst containing at least 0.5 wt.% of a Group 1A metal oxide and specific X-ray powder diffraction pattern. WO 85/05102 teaches a process for converting an ethylene feedstock to heavier hydrocarbons in the gasoline or distillate boiling range by contact with siliceous crystalline molecular sieves. Over HZSM-5 in a fixed bed, propylene oligomerizes to a nearly Gaussian distribution of products from C₆ to C₃₆. These products correspond to gasoline, jet fuel, diesel fuel, and a heavy fraction. For the purposes of the present disclosure these fractions are defined by the following boiling ranges: gasoline = less than 330°F (166°C) jet fuel = 330 - 480°F (166 - 249°C) diesel fuel = 480 - 650°F (249 - 343°C) heavy fraction = greater than 650°F (343°C) The heavy fraction is also referred to herein as the 343+°C fraction or the 343+°C range product. Silica modified zeolites, such as ZSM-5, can be prepared by impregnation followed by air calcination. US-A 4,145,315, 4,100,219 and 4,060,568 describe methods for introducing amorphous silica into the interior pore space of zeolites. We have now found that propylene oligomerization over silica-modified zeolites gives a product distribution different from that of the unmodified zeolite. The 343+°C fraction is selectively reduced with a corresponding increase in the lighter fractions. Recycle of the gasoline fraction can give ultimate yields of jet and diesel fuels significantly higher than the corresponding yields using unmodified HZSM-5. According to the present invention a process for oligomerizing propylene comprises contacting propylene under oligomerization conditions with a catalyst comprising an aluminosilicate zeolite having a silica to alumina molar ratio of at least 12 and a Constraint Index within the range of 1 to 12 and containing within its interior crystalline structure added amorphous silica in an amount of at least 0.1 weight percent. The 343+°C fraction so obtained is selectively reduced, with a corresponding increase in the lighter fractions. The invention thus offers a means of selectively producing jet and diesel fuels from inexpensive propylene. Oligomerization to 343+°C product is suppressed by the catalyst, and gasoline range product can be recycled to give high ultimate yields of jet and diesel fuels. The product contains no aromatics which are deleterious to the combustion of jet and diesel fuels. Additionally, the jet fuel product can be extended to include some diesel range material to lower average fuel volatility and increase post crash survivability for airline passengers. Propylene oligomerisation conditions which may be adopted include a temperature of 100 to 400°C, a pressure of 1 to 139 bar (0 psig to 2000 psig), and a weight hourly space velocity of 0.1 to 20. Preferred conditions comprise a temperature of 160 to 270°C, pressure of 20 to 80 bar and WHSV of 0.1 to 5. The oligomerization product typically comprises less than 25 wt%, often less than 20 wt% of hydrocarbons having a boiling point of greater than 343°C (650°F). The significance and manner of determination of Constraint Index is described fully in US-A-4,016,218. Constraint Index (CI) Values for some representative materials are: CI (at test temperature °C) ZSM-4 0.5 (316) ZSM-5 6-8.3 (371-316) ZSM-11 5-8.7 (371-316) ZSM-12 2.3 (316) ZSM-20 0.5 (371) ZSM-22 7.3 (427) ZSM-23 9.1 (427) ZSM-35 4.5 (454) ZSM-38 2 (510) ZSM-48 3.5 (538) ZSM-50 2.1 (427) TMA Offretite 3.7 (316) TEA Mordenite 0.4 (316) Clinoptilolite 3.4 (510) Mordenite 0.5 (316) REY 0.4 (316) Amorphous Silica-alumina 0.6 (538) Dealuminized Y 0.5 (510) Erionite 38 (316) Zeolite Beta 0.6-2.0 (316-399) The nature of Constraint Index and the technique by which it is determined admit the possibility that a given zeolite, tested under different conditions but prescribed, may exhibit different Constraint Indices. Thus, Constraint Index seems to vary somewhat with severity of operation (conversion), presence or absence of binder, crystal size of the zeolite, the presence of occluded contaminants, etc. For the purposes of this specification zeolites which exhibit different Constraint Indices according to test conditions are considered to have a CI of 1 to 12 provided that one of those indices is within that range. Zeolites ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38 and ZSM-48 are defined by the x-ray data set forth in US-A-3,702,886, 3,709,979, 3,832,449, 4,556,477, 4,076,842, 4,016,245, 4,406,859 and 4,234,231 respectively. Catalysts useful in the present oligomerization process may be prepared as described in the aforementioned US-A-4,145,315, 4,100,219 and 4,060,568. Thus introduction of amorphous silica into the interior pore space of zeolites may be effected by contacting the zeolite, in a form substantially free of alkali metal, i.e. containing less than about 1.5 weight percent alkali metal and preferably having at least a portion of the original cations associated therewith replaced by ammonium and/or hydrogen, with a silicon-containing compound of molecular dimensions such that it is readily sorbed into the pores of the zeolite. Representative and preferred silicon-containing compounds include silicones characterized by the general formula: (Si(R₁)(R₂)-O) n where R₁ and R₂ are, independently, hydrogen, methyl, fluorine, chlorine or hydroxy and n is at least 3 and generally in the range of 4 to 1000. The molecular weight of the silicone compound employed is generally between about 250 and about 60,000 and preferably within the approximate range of 300 to 20,000. Representative silicone compounds include methylhydrogensilicone, dihydrogen silicone, dimethyl silicone, dichlorosilicone, fluorohydrogen silicone and difluorosilicone. Silicone compounds are preferred since the zeolites modified by them have been found to provide selective conversion of low molecular weight alcohols to light olefins with extremely low production of C₉+ hydrocarbon, e.g., durene. Another silicon-containing compound which may be employed is a silane having the formula: SiR₁R₂R₃R₄ where R₁ and R₂ are hydrogen, fluorine, chlorine, methyl, ethyl, amino, methoxy or ethoxy; R₃ is hydrogen, fluorine, chlorine, methyl, amino or methoxy; and R₄ is hydrogen or fluorine. Other suitable silanes include polysilanes, such as disilanes, trisilanes and higher silanes, up to decasilanes. It is also contemplated to use derivatives of the aforenoted polysilanes having methyl, chloro or fluoro substituents. The silicon compound employed may be in the form of a solution, a liquid or a gas under the conditions of contact with the zeolite. The pores of the latter are preferably, but not necessarily, saturated with the contacting medium containing the silicon compound. When a silicone compound is employed it may be dissolved in a suitable solvent, e.g. n-hexane, pentane, heptane, benzene, toluene, xylenes, trimethyl benzene, chloroform, carbon tetrachloride, dichloro benzene or trimethyl benzene. Contact between the silicon compound and the zeolite is maintained at a temperature between about 10°C and about 200°C for a period of time sufficient to sorb the desired amount of silicone. Time of contact will generally be within the range of 0.2 to 5 hours, during which time the mixture is desirably subjected to evaporation of the solvent fluid. The resulting residue is then calcined in an oxygen-containing atmosphere, preferably air, at a rate of 0.2°C to 5°C/minute to a temperature greater than 300°C but below a temperature at which the crystallinity of the zeolite is adversely affected. Generally, such temperature will be below 700°C. Preferably, the temperature of calcination is within the approximate range of 350 to 550°C. The product is maintained at the calcination temperature usually for 1 to 24 hours to yield a zeolite having silica contained in its interior porous structure. When the employed silicon compound is a silane, the latter desirably undergoes hydrolysis, catalyzed either by base, e.g. by contacting the zeolite containing the sorbed silane with a solution of aqueous ammonia or by acid in the presence of Lewis or Bronsted acids, such as for example, by contact with an aqueous solution of hydrochloric acid. Contact of the zeolite containing sorbed silane with a suitable acid or base is maintained for a period of time sufficient to effect the desired hydrolysis with evolution of hydrogen. The resulting product is then calcined as above to yield a zeolite having silica contained within its interior structure. The amount of silica incorporated with the zeolite will depend on several factors. One of these is the time that the zeolite and the silicon-containing source are maintained in contact with each other. With greater contact times, all other factors being equal, a greater amount of silica is incorporated with the zeolite. Other factors upon which the amount of silica incorporated with the zeolite is dependent include temperature, concentration of the treating compound in the contacting media, the degree to which the zeolite has been dried prior to contact with the silicon-containing compound, the conditions of hydrolysis, when practised, and calcination of the zeolite after contact of the same with the treating compound and the amount and type of binder incorporated with the zeolite. Generally, the amount of silica contained in the interior porous structure of the zeolite will be between about 0.3 and about 40 and preferably between about 0.5 and about 30 weight percent. The amount of silica incorporated into the zeolite crystal can be assessed from a reduction in the zeolitic sorption capacity. The latter is determined from the amount of n-hexane sorbed at a temperature of 90°C and a n-hexane partial pressure of 83 mm mercury (11.07 kPa) and is determined from the increase in zeolite weight upon sorption. The decrease in sorption capacity of the zeolite under the above conditions attributable to the presence of added amorphous silica is at least 1 percent and generally in the range of 5 to 60 percent. Representative sorption data for a binder-free zeolite and an alumina-zeolite extrudate are shown below: The oligomerization catalyst can be shaped into a wide variety of particle sizes. The particles can be in the form of a powder, a granule, or a molded product such as an extrudate having a particle size sufficient to pass through a 2 mesh (Tyler) screen and be retained on a 400 mesh (Tyler) screen. In cases where the catalyst is molded, such as by extrusion, it can be extruded before drying or partially dried and then extruded. It may be desired to incorporate the catalyst with another material which is resistant to the temperatures and other conditions employed in the oligomerization process described herein. Such materials include active and inactive materials and synthetic or naturally occurring zeolites as well as inorganic materials such as clays, silica and/or metal oxides such as alumina. The latter may be either naturally occurring or in the form of gelatinous precipates or gels including mixtures of silica and metal oxides. Use of a material in conjunction with zeolite, i.e. combined therewith or present during its synthesis, which itself is catalytically active may change the conversion and/or selectivity of the catalyst. Inactive materials suitably serve as diluents to control the amount of conversion so that oligomerization products can be obtained economically and orderly without employing other means for controlling the rate of reaction. These materials may be incorporated into naturally occurring clays, e.g. bentonite and kaolin, to improve the crush strength of the catalyst under commercial operating conditions. Said materials, i.e. clays, oxides, function as binders for the catalyst. It is desirable to provide a catalyst having good crush strength because in commercial use it is desirable to prevent the catalyst from breaking down into powder-like materials. These clay binders have been employed normally only for the purpose of improving the crush strength of the catalyst. Naturally occurring clays which can be composited with catalysts include the montmorillonite and kaolin family, which families include the subbentonites, the kaolins commonly known as Dixie, McNamee, Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, macrite, or anauxite. Such clays can be used in the raw state as originally mined or initially subject to calcination, acid treatment or chemical modification. Binders useful for compositing with zeolites also include inorganic oxides, notably alumina. In addition to the foregoing materials, the catalysts can be composited with a porous matrix material such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The relative proportions of zeolite and matrix vary widely, with the zeolite content ranging from 1 to 90 percent by weight and more usually, particularly when the composite is prepared in the form of beads, in the range of 2 to 80 weight percent of the composite. EXAMPLES 12.00g HZSM-5 was added to 0.63g methylhydrogensilicone dissolved in 50cc hexane. The hexane was distilled off and the residue calcined at 2°C/min to 538°C, then 7 hours at 538°C. The resultant catalyst contained 5% added silica. Propylene oligomerization was carried out at 200°C and 42.4 bar (600 psig) in a fixed bed reactor using 2.00g catalyst. The propylene weight hourly space velocity was 0.25 or 0.125. Propylene oligomerization was carried out as above over HZSM-5 and silica modified HZSM-5. The following table compares the results. Relative to unmodified ZSM-5, the SiO₂-ZSM-5 catalyst produces significantly less 343+°C range product (6% and 11% vs 27% and 38% for HZSM-5). At approximately the same diesel yield (34% vs 27%), the 343+°C fraction yield is selectively reduced from 38% to 11% by the silica-modified catalyst. Similarly, for diesel yields of 35% and 24%, the silica-modified catalyst lowers the 343+°C fraction yield from 27% to only 6%. As a consequence of the lower 343+°C fraction yield, increased lighter product is formed. Recycle of the gasoline fraction to extinction gives the quoted ultimate yields for the other products. The ultimate yields of jet fuel + diesel fuel are significantly higher for the silica-modified catalyst (91% and 84% vs 69% and 57% for HZSM-5).

Claims: A process for oligomerizing propylene comprising contacting propylene under oligomerization conditions with a catalyst comprising an aluminosilicate zeolite having a silica to alumina molar ratio of at least 12 and a Constraint Index within the range 1 to 12 and containing within its interior crystalline structure added amorphous silica in an amount of at least about 0.1 weight percent based on the weight of the zeolite. A process according to claim 1 wherein the amount of added amorphous silica is at least 0.3 weight percent. A process according to claim 1 or claim 2 wherein the zeolite has an n-hexane sorption capacity at 90°C and n-hexane partial pressure of 11.07 kPa (83mm of mercury) which is at least 1 percent less than the corresponding sorption capacity under identical conditions for the unmodified zeolite. A process according to any preceding claim wherein the amount of added amorphous silica is between 0.5 and 30 weight percent. A process according to any preceding claim wherein said oligomerization conditions include a temperature of from about 100°C to about 400°C, a pressure of 1 to 139 bar (0 to 2000 psig) and a weight hourly space velocity of from about 0.1 to about 20. A process according to claim 5 wherein the temperature is 160 to 270°C, the pressure is 20 to 80 bar and the WHSV is 0.1 to 5. A process according to claim 5 wherein less than 25 wt% of the product has a boiling point of greater than 343°C. A process according to claim 6 wherein less than 20 wt% of the product has a boiling point of greater than 343°C. A process according to any preceding claim wherein gasoline boiling range product is recycled. A process according to any preceding claim wherein the zeolite is ZSM-5. A process according to any preceding claim wherein the catalyst is prepared by sorbing a silicone into the pores of the zeolite and calcining to convert sorbed silicone to amorphous silica. A process according to claim 11 wherein the silicone is methylhydrogen-silicone.

Inventors: CHANG CLARENCE DAYTON, RODEWALD PAUL GERHARD, CHANG, CLARENCE DAYTON, RODEWALD, PAUL GERHARD
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1991
Main classification: C07C2
IPC codes: B01J29, C07C2, C07B61, C07C11