PROCESS FOR CONVERSION OF LOWER ALIPHATIC OXYGENATES TO OLEFINS AND AROMATICS WITH GALLIUM CONTAINING ZSM-5 CATALYST

Abstract: C₂-C₅ olefins and aromatics are prepared by catalytic con­ version of lower aliphatic C₁-C₄ oxygenates in the presence of a medium pore gallium containing zeolite catalyst having a ZSM-5 framework at elevated conversion temperatures. The process favors production of valuable unsaturated hydro­ carbons with low paraffin output.

Description: Recently, efforts to prepare olefins from methanol have become increasingly important. Methanol can be readily produced from coal, via coal gasification and the production of synthesis gas, with the aid of well-tried technology. If it were possible to convert methanol to lower olefins in an economical manner, the further processing methods which are conventional in the chemical industry today, and employ coal as a raw material, could also be preserved. In the past few years, processes have, therefore, been developed with the object of preparing olefins from methanol, dimethyl ether or mixtures thereof. US-A-3,894,106, 3,894,107, and 3,907,915, respectively, disclose the conversion of alcohols, ethers, carbonyls, or mixtures thereof to olefinic and aromatic hydrocarbons by contact with a catalyst comprising a crystalline aluminosilicate zeolite having a silica-to-alumina ratio of at least about 12 and a Constraint Index of about 1 to 12. The conversion of methanol and dimethyl ether to olefinic and aromatic hydrocarbons is described in US-A-3,911,041. DE-A-2,615,150 relates to a process for the preparation of olefins from a methanol feed using a ZSM-5 aluminosilicate zeolite. US-A-4,361,715 relates to the catalytic conversion of lower molecular weight oxygenates to olefins over a bed of aluminosilicate zeolite catalyst. Gallium can be incorporated into the framework of the zeolite, but the zeolite does not have the ZSM-5 framework structure. US-A-4,401,637 discloses a process for preparing olefins from methanol over a catalyst bed of a modified crystalline isotactic zeolite comprising framework gallium and an organic amine of low basicity, but the catalysts differ from ZSM-5 type catalysts. UA-A-4,582,949 discloses a two-stage conversion process for the preparation of aromatic hydrocarbons from olefins over metallosilicate catalysts. A crystalline gallium silicate catalyst having a silicon dioxide to gallium trioxide ratio of between 25 and 250 and a specific x-ray powder diffraction pattern is employed in the second stage. EP-A 74652 discloses that offretite containing gallium or indium introduced during synthesis is superior to the metal-free material as catalyst for conversion of methanol to hydrocarbons. A new catalytic process has now been discovered for the conversion of lower aliphatic C₁-C₄ oxygenates by a series of dehydration (or decarbonylation) and dehydrogenation reactions employing a medium pore acidic crystalline gallium-containing siliceous zeolite of ZSM-5 framework. It has been found that gallium containing ZSM-5 zeolites have superior catalytic activity in the production of C₂-C₅ olefins and aromatics, useful as intermediates in the preparation of gasoline/distillate or as valuable chemical feedstock. The process gives much lower yield of less valuable C₁-C₅ paraffins than a similar process employing a standard aluminosilicate H-ZSM-5 catalyst. According to the present invention a process for the conversion of a C₁ to C₄ aliphatic oxygenate feed to C₂-C₅ olefins, with augmented yield of aromatics and diminished yield of C1-C5 alkanes, comprises contacting said feed, at a temperature between 350 and 575°C and a pressure of 10 to 3500 kPa, with crystalline acidic zeolite ZSM-5 having a silica/alumina mole ratio of at least 500 and containing gallium of which a portion is in tetrahedral coordination in the framework of the zeolite, and withdrawing a product containing light olefins, aromatics, and less than 10% wt. of said alkanes. Preferably the contacting is performed at 370 to 500°C and the ratio of product light olefins to product aromatics is at least 3:1. Suitably the contacting is effected at WHSV of 0.1 to 10. A particularly contemplated feed is methanol and/or dimethyl ether. The upgrading of lower molecular weight C₁-C₄ oxygenates, such as alcohols, ethers, ketones and their acetals, and carboxylates is achieved in an efficient manner by contacting the oxygenates under reaction conditions with a medium pore acidic crystalline gallium containing ZSM-5 type catalyst having at least a portion of zeolitic gallium in tetrahedral coordination therein to provide Bronsted acid sites. In a preferred embodiment, the process comprises: maintaining a reaction zone containing a crystalline acidic gallium containing ZSM-5 catalyst; passing a feedstock comprising methanol, dimethylether or mixtures thereof through the reaction zone at conditions of elevated temperature; and withdrawing a product rich in light olefins and aromatics. Preferably, the ratio of light olefins (C₂-C₅) to aromatic hydrocarbons is at least about 3:1. The amount of less desirable paraffinic hydrocarbons is reduced to a minimum by the process, producing less than 10 wt.% C₁-C₅ alkanes. At higher conversion temperatures, the ratio of aromatics to saturates is at least 2:1, and preferably at least 3:1. The gallium zeolite catalyst can be prepared by various methods; for example: by a cocrystallization procedure, vapor phase treatment, or by employing an alkaline medium. ZSM-5 zeolite having a silica-to-alumina ratio of at least 500 can be directly synthesized by known methods, or prepared from a zeolite having a lower silica-to-alumina ratio by steaming, dealuminizing, or framework exchange (lattice substitution). The prepared gallium containing zeolite, in a preferred embodiment, has the framework structure of ZSM-5 with Bronsted acid sites provided by tetrahedrally coordinated gallium in the framework structure. The amount of alumina in the zeolite is less than 0.5 wt.%. The tetrahedral gallium of the framework may be indicated by ammonium exchange capacity and Nuclear Magnetic Resonance (NMR) signal. Part of the gallium associated with the zeolite may, however, not be in tetrahedral coordination in the framework, since it has been observed that such non-framework gallium assists in the minimisation of light alkane production. The Examples which follow illustrate the process according to the invention. PREPARATION OF CATALYST A gallium containing zeolite was synthesized by lattice substitution from ZSM-5 having a silica-to-alumina ratio of 26,000, which has been calcined at 538°C (1000°F). A teflon bottle containing 2g of the calcined ZSM-5 and 4g of GaCl₃ in 30 cc of H₂O was sealed and heated at 150°C for 18 hours. The product was washed and ion exchanged with IN ammonium nitrate, followed by calcination in air. It contained 2.2% gallium. EXAMPLE 1 A feedstock of methanol was reacted over a catalyst bed of gallium containing ZSM-5 zeolite prepared according to Example 1, at 500°C (932°F), a pressure of 101 kPa (1 atmosphere) and a weight hourly space velocity (WHSV) of the methanol feed of 1. The results are given in Column A of Table 1. At 98% conversion, the process produces a mixture of hydrocarbons consisting of 62 wt.% C₂-C₅ olefins, 20 wt.% aromatics, and only 6 wt.% C₂-C₅ light paraffins. EXAMPLE 2 Example 1 was repeated at a temperature of 370°C. The results are given in Column B of Table 1. At 7% conversion, the process produces a mixture of hydrocarbons consisting of 81 wt.% C₂-C₅ olefins and 20 wt.% C₁-C₅ paraffins. EXAMPLE 3 (Comparative) A feedstock of methanol was reacted over a catalyst bed of aluminosilicate ZSM-5 zeolite having a silica-to-alumina ratio of 1,670 and an acid-cracking (alpha) value of 5. The reaction was conducted under the conditions of Example 1. The results of the experiment are given in Column C of Table 1. EXAMPLE 4 (Comparative) A feedstock of methanol was reacted over a catalyst bed of aluminosilicate ZSM-5 zeolite having a silica-to-alumina ratio of 500 and an acid-cracking (alpha) value of 12. The reaction was conducted under the conditions of Example 1. The results of the experiment are given in Column D of Table 1. EXAMPLE 5 (Comparative) A feedstock of methanol was reacted over a catalyst bed of aluminosilicate ZSM-5 zeolite having a silica-to-alumina ratio of 70 and an acid-cracking (alpha) value of 150. The reaction was conducted under the conditions of Example 1. The results of the experiment are given in Column E of Table 1. The results indicated in Table 1 illustrate the effectiveness of the inventive process for producing olefinic and aromatic hydrocarbons from oxygenate feed with a minimum formation of light paraffins.

Claims: 1. A process for the conversion of a C1 to C4 aliphatic oxygenate feed to C2-C5 olefins, with augmented yield of aromatics and diminished yield of C1-C5 alkanes, which comprises contacting said feed, at a temperature between 350 and 575°C and a pressure of 10 to 3500 kPa, with crystalline acidic zeolite ZSM-5 having a silica/alumina mole ratio of at least 500 and containing gallium of which a portion is in tetrahedral coordination in the framework of the zeolite. 2. A process according to claim 1 wherein the contacting is performed at 370 to 500°C. 3. A process according to claim 1 or claim 2 wherein the contacting is effected at WHSV of 0.1 to 10. 4. A process according to any preceding claim wherein said feed is methanol and/or dimethyl ether.

Inventors: CHU, CYNTHIA TING-WAH
Applicants: MOBIL OIL CORPORATION
Application year: 1987
Main classification: C07C1
IPC codes: C07C11, C07C67, C07B61, B01J29, C10G3, C07C1