Alkyl t-alkyl ether synthesis using modified beta-zeolite catalyst

Abstract: A method is disclosed for producing alkyl tertiary alkyl ethers which comprises reacting a tertiary alcohol with an alkanol in the presence of a catalyst which exhibits extended life. The catalyst is a β-zeolite modified with one of the following: (a) on or more metals selected from the group consisting of Group IA and Group IIIB of the Periodic Table, or (b) phosphorus-containing compounds, or (c) silicon-containing compounds. The alkanol and the tertiary alcohol are continuously contacted over said zeolite catalyst at a temperature from 20°C to 250°C and a pressure from 0.1 to 7 MPa. Preferred reactants are tertiary butyl alcohol and methanol, these reacting together to form methyl tertiary butyl ether.

Description: This invention concerns a process for preparing alkyl tertiary-alkyl ethers and particularly methyl tertiary butyl ether (MTBE), along with isobutylene, in one step by the reaction of tertiary alcohol and an alkanol in the presence of a catalyst comprising β-zeolite modified with one or more metals selected from Groups IA and IIIB of the Periodic Table, as defined in the Condensed Chemical Dictionary, 10 ed., p. 789. Preferred elements include lithium and the rare earth metals. The invention is especially advantageous in that lithium or rare earth metal-modified β-zeolite gives conversion levels of around 70% for over 2000 hours of service, without loss of tertiary alcohol conversion levels and with improved ether productivity in comparison to the prior art. Preferred reactants are tertiary butyl alcohol (tBA) and methanol which are reacted together to form methyl tertiary butyl ether (MTBE). Generally, it is known that asymmetrical C 4 -C 7 alkyl tertiary alkyl ethers are particularly useful as octane improvers for liquid fuels, especially gasoline. Methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), isopropyl t-butyl ether (IPTBE) and tertiary amyl methyl ether (TAME) are known to exhibit high octane properties. Much attention has been focused on production of these ethers due to the rapidly increasing demand for lead-free octane boosters for gasoline. It is known in the art to produce MTBE or ETBE by reacting isobutylene with either methanol or ethanol, resulting in the formation of MTBE or ETBE, respectively. The reaction is normally conducted in the liquid phase using relatively mild conditions. The isobutylene can be obtained from various sources, such as naphtha cracking, catalytic cracking, etc. The resulting reaction product stream contains the desired MTBE or ETBE, as well as unreacted isobutene and other C 4 hydrocarbons and methanol or ethanol. Since oxygenates are used as gasoline blending components, extenders, octane boosters and as key ingredients for reducing the emissions of CO and VOCs (Volatile Organic Compounds), it is expected that the demand for oxygenates will increase enormously in the coming years. Of all oxygenates, the tertiary ethers, such as methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), and tertiary amyl methyl ether (TAME) are preferred by refineries to lighter alcohols. They have lower blending Reid vapour pressure (BRvp), lower vaporization latent heats and low solubilities in water. The most common ether in use today is MTBE with a projected production of about 25 million tonnes. With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene. A number of recent patents to Texaco Chemical Co., noted below, use t-butanol, rather than isobutylene, in a one-step reaction. The main drawback of tertiary ethers is that they substantially increase aldehyde emissions. These are subject to regulations imposed by the Environmental Protection Agency in the United States, stipulating that they must decrease 15% by 1995. It is believed this drawback could be largely circumvented by mixing the tertiary ethers with tertiary alcohols. Tertiary butyl alcohol (tBA) has a very low atmospheric reactivity and low aldehyde emissions, since no hydrogen atoms are contained in the carbon link to the oxygen. U.S. Patent Application Serial No. 08/096,873 (abandoned) concerns an improved process for preparing methyl tertiary butyl ether (MTBE) along with isobutylene and, optionally diisobutylene in one step by the reaction of tertiary butanol and methanol in the presence of a catalyst comprising β-zeolite or β-zeolite modified with a metal selected from the group consisting of Groups VB, VIIB, and VIII of the Periodic Table. U.S. Patent Application Serial No. 08/057,373 (abandoned, continuation application granted as U.S. 5,449,839) concerns an improved process for preparing ethyl t-butyl ether (ETBE) in one step by reaction of tertiary butanol and ethanol in the presence of β-zeolite modified with a metal selected from the group consisting of Groups IB, VB, VIB, VIIB and VIII. U.S. Patent No. 5,364,981 concerns the use of a palladium/platinum modified β-zeolite for the one-step synthesis of methyl t-butyl ether (MTBE). European Patent Application No. 0 546 695 A1 concerns a process for preparing alkyl t-alkyl ether in one step by reaction of tertiary alkyl alcohol and primary alcohol in the presence of a catalyst comprising zeolite modified with a metal selected from the group consisting of groups IB, VB, VIB, VIIB or VIII of the Periodic Table. The process is particularly useful for preparing methyl t-butyl ether from tertiary butanol and methanol. Some of the limitations present in the MTBE catalyst systems generally available in the art include loss of activity at temperatures above 120°C, deactivation due to the presence of peroxides and other organic components in the feedstock, lower than desirable selectivity and the need for multiple steps to accomplish the synthesis and separation of the product. From the known art relating to the synthesis of alkyl tertiary alkyl ethers, it is apparent that methods have become available in the recent past for one step synthesis from alcohols and butanol using various catalysts. However, there is still a need for identifying catalysts which demonstrate improved catalyst life without sacrificing long-term productivity and good conversion levels. It would represent a significant advance in the art if tertiary butanol, instead of isobutylene and methanol, could be reacted to form MTBE in one step over a modified β-zeolite catalyst which exhibits high levels of productivity and conversion for over 2000 hours of service. According to the invention, there is provided a method for synthesizing alkyl tertiary alkyl ethers, particularly methyl t-butyl ether (MTBE), which comprises reacting t-alcohol with an alkanol in the presence of a catalyst comprising β-zeolite modified with one of the following: (a) one or more metals selected from Group IA and Group IIIB of the Periodic Table, or (b) phosphorous pentoxide, or (c) a silicon-containing reagent from the group consisting of organosiloxanes such as hexamethyldisiloxane, and alkylsilyl halides such as trimethylsilyl chloride at an elevated temperature and pressure. The specific examples herein demonstrate the particular effectiveness of lithium and rare earth metal-modified β-zeolite. The production of substantial amounts of isobutylene is also documented. Performance of this invention may be carried out typically by reacting t-butyl alcohol and methanol in the presence of other organic oxygenates and in the presence of an etherification catalyst. The etherification is carried out in one step and the catalyst comprises β-zeolite modified with one or more metals selected from the group consisting of Groups IA and IIIB of the Periodic Table. The reaction can be represented by the following: Generally the methanol and t-butanol co-reactants may be mixed in any proportion in order to generate the desired methyl t-butyl ether (MTBE) and isobutylene but, preferably, the molar ratio of primary alcohol to t-butanol in the feed mixture should be between 10:1 and 1:10, if the yield of desired MTBE is to be maximized. In order to achieve maximum selectivity to MTBE, and optimum conversion per pass, an excess of primary alcohol in the liquid feed is desirable. The most preferred methanol or ethanol-to-tertiary butanol molar ratio is from 1:1 to 5:1. In certain circumstances, it may be particularly desirable that the tBA conversion is high enough (e.g. 70% or greater), to cause the crude product mix to phase separate into an isobutylene-MTBE product-rich phase and a heavier aqueous methanol phase. Preferably, such product phase separation would be achieved at as low an etherification temperature as possible, but it is particularly observed in the range 140°-200°C. The synthesis of Eq. 1 can be conducted where the t-butanol and methanol reactants are mixed with certain other components including water, ketones such as acetone (Me 2 CO) and methyl ethyl ketone (MEK), peroxides and hydroperoxides such as di-t-butyl peroxide (DTBP), allyl t-butyl peroxide (ATBP), and t-butyl hydroperoxide (TBHP), as well as esters such as t-butyl formate (TBF). Typically each of said classes of components makes up less than 10% of the total feed mixture. It has been discovered in the present invention that β-zeolite modified with lithium and/or rare earth metal as herein disclosed gives conversion levels of around 70% for over 2000 hours of service, without a loss of tBA conversion levels and with improved MTBE productivity in comparison to the prior art. This constitutes an important commercial advantage. The present one-step process may also be applied to the preparation of other alkyl tertiary alkyl ethers. For example, said process may be applied to the reaction of a C 1 -C 6 primary alcohol such as methanol, ethanol, n-propanol and nhexanol with a C 4 -C 10 tertiary alcohol such as tertiary butanol and tertiary amyl alcohol. In particular, reaction of ethanol with tertiary butanol would yield ethyl t-butyl ether (ETBE). Alternatively, a mixture of alcohols, e.g., a mixture of C 1 -C 5 alcohols, could be reacted to give a mixture of alkyl tertalkyl ethers. In the modified catalyst of the present invention, good results were obtained using β-zeolites as catalysts for the reaction represented in Eq. 1. β-zeolite is a crystalline aluminosilicate having a pore size greater than 5 Angstroms and a composition in its as-synthesized form which may be expressed as shown below: [XNa(1.0±0.1-X)TEA]AlO 2 ·YSiO 2 ·WH 2 O where X is less than 1, preferably less than 0.7; TEA represents the tetraethylammonium ion; Y is greater than 5 but less than 100, and W is up to about 60. In the fully base-exchanged form, β-zeolite has the composition: [(X/n)M(1±0.1-X)H]AlO 2 ·YSiO 2 ·WH 2 O where X, Y and W have the values listed above and n is the valence of the metal M. This form of the zeolite may be converted partly to the hydrogen form by calcination, e.g. at 200°C to 900°C or higher. The completely hydrogenated form may be made by ammonium exchange followed by calcination in air or an inert atmosphere such as nitrogen. The preferred forms of β-zeolite are the highly acidic, high silica forms, having silica-to-alumina mole ratio of at least 10:1, and preferably in the range of 10:1 to 50:1 in the as-synthesized form. A narrower range of 20:1 to 30:1 is preferred. It has been found, in fact, that β-zeolite may be prepared with silica-to-alumina mole ratios above the 200:1 maximum specified in U.S. Patent Nos. 3,308,069 and these forms of the zeolite may perform well in the method according to the present invention. Ratios of 50:1, or even higher, may be used where available. Illustrative of suitable β-zeolites for the practice of this invention include Valfor C806β, Valfor CP815β and Valfor C861. Valfor is the registered trademark of the PQ Corporation. Valfor C806β zeolite is β-zeolite powder in template cation form. It is a high silica shape selective zeolite which contains the organic template used in the crystallization step, having been isolated after filtration and washing of the synthesis product. C806β has a SiO 2 /Al 2 O 3 molar ratio of 23-26; the crystal size is 0.1-0.7 µm; the surface area after calcination is about 700-750 m 2 /g; the cyclohexane adsorption capacity after calcination is 19-24g/100g; Na 2 O content is about 0.5-1.0% by weight anhydrous; and the organic content is about 11-13% by weight, on a water-free basis. Valfor C815β zeolite is a calcined β-zeolite powder in hydrogen, sodium form. It is similar to C806β except the product has been calcined to decompose the organic template. C815β is a high silica, shape selective aluminosilicate with a large pore diameter. C815β also has a SiO 2 /Al 2 O 3 molar ratio of about 23-26; the crystal size, surface area, cyclohexane adsorption capacity and Na 2 O content are all within the same ranges as given for C806β. Said β-zeolites may be formed with a Group III or Group IV oxide binder, such as alumina or silica. The β-zeolite content of the formed catalyst may be 5% to 95%, but preferably the β-zeolite content is 30% to 80%. Valfor C861β is an extrudate made of 80% C815β powder and 20% alumina powder. As indicated above, the metals used for modifying the zeolite in the present invention comprise those from Group IA and IIIB of the Periodic Table. Group IA includes the alkali metals from the group consisting of lithium, sodium, potassium, rubidium and cesium. Zeolites modified with each of these alkali metals, as well as combinations thereof, exhibit improved catalyst life. The preferred alkali metal is lithium. Members of Group IIIB metals which are useful for modifying zeolites in the present invention include each of the lanthanide elements, as well as lanthanum, scandium and yttrium. Suitable lanthanides include cerium, praseodymium, terbium, neodymium, dysprosium, europium, samarium, ytterbium, thulium, promethium, gadolinium, holmium, erbium, and lutetium, as well as mixtures thereof, plus lanthanum itself. The preferred Group IIIB metals are the mixed rare earth metals (the lanthanides plus lanthanum), and lanthanum alone. Said β-zeolites are preferably impregnated with said specified metals as their salts, particularly their metal nitrate or chloride salts, in an aqueous, alcoholic, or ketonic medium over a period of 1-24 hours. Then the solids are filtered off and treated with 1 litre of 0.2M rare earth chloride solution. The mix is again slurried at 40°-100°C for 1 to 24 hours. The exchanged zeolite was then recovered by filtration, washed with distilled water about 20 times, dried at an elevated temperature (e.g. 120°C) overnight and calcined at 300°-800°C (e.g. 540°C) for 1 to 24 hours. Optionally, said impregnated β-zeolite may be steamed in the temperature range 300° to 800°C, and/or reduced in a stream of hydrogen at 100° to 500°C. Example 1 demonstrates the preparation of the lithium/ rare earth-exchanged β-zeolite catalysts. Salts of lithium and lanthanum such as their chloride salts, in anhydrous or hydrated forms, were dissolved in water, alcohol or acetone and the β-zeolites were added, in most cases, in the form of extrudates. The catalysts were then calcined by heating to 300° to 800°C, optionally steamed in the temperature range 300° to 800°C, and/or optionally reduced in a stream of hydrogen at 200°C. In a second embodiment, demonstrated in Examples 3-6, the zeolite is modified with phosphorus pentoxide or a silicon-containing reagent selected from the group consisting of organosiloxanes such as hexamethyldisiloxane, and alkylsilyl halides such as trimethylsilyl chloride. The amount of the various metals deposited on the zeolite can vary. The amount of each individual metal, lithium or lanthanum, can vary from 0.01 to 10.0%. Where lithium and lanthanum are deposited on C861β, the preferred weight proportion is from 0.1% to 5.0%. Said catalysts may be in the form of powders, pellets, granules, spheres, shapes and extrudates. The examples described herein demonstrate some advantages in the use of extrudates. The reaction may be carried out in either a stirred slurry reactor or in a fixed bed continuous flow reactor. The catalyst concentration should be sufficient to provide the desired catalytic effect. Said catalysts may be formed in the presence of a binder, such as a Group III or Group IV oxide, including alumina or silica. Said binders may comprise 5% to 95% of the formed catalyst. Etherification can generally be conducted at temperatures from 20° to 250°C, the preferred range being 80° to 200°C. Good results are observed throughout this temperature range. The total operating pressure may be from 0 to 1000 psig (0.1 to 7 MPa), or higher. The preferred pressure range is 50 to 500 psig (0.35 to 3.5 MPa). Typically, MTBE is generated continuously in up to 40 wt% concentration or greater in the crude liquid product at total liquid hourly space velocities (LHSV) of up to 6 or higher and relatively mild conditions, where: LHSV = Volume of Total Liquid Feed Run Through The Reactor Per Hour Volume of Catalyst In Reactor Conversions of t-butanol (tBA, wt%) are estimated in the following examples using the equation: (Mole% of tBA in Feed - Mole% of tBA in Product) Mole% of tBA in Feed x 100 The accompanying examples illustrate: a) The production of MTBE/isobutylene from crude t-butanol/methanol using a β-zeolite catalyst over an extended period, where tBA conversion levels drop from 71% to 62%-63% over the 80+ day run (see Comparative Example A, Table 1) and the tBA concentration in the effluent rises from 13% to around 19%; b) The production of MTBE/isobutylene from crude tBA/MeOH using a lithium/rare earth-modified β-zeolite catalyst where, over an extended period (80+ days), the t-butanol conversion remained at roughly 70% (see Example 2 and Table 2), while the tBA effluent concentration held steady at 13% c) The production of MTBE/isobutylene from t-butanol/methanol feedstock using a series of β-zeolite catalysts where the zeolite is modified with phosphorus pentoxide (Table 3), hexamethyldisiloxane (Table 4) and trimethylsilyl chloride (Tables 5 and 6). Product phase separation occurs in these examples at higher operating temperatures (140°C-180°C). COMPARATIVE EXAMPLE A This example illustrates the production of methyl t-butyl ether from crude t-butanol/methanol feedstock using an unmodified β-zeolite catalyst. Synthesis was conducted in a tubular reactor (12.5 mm i.d., 300 mm long) constructed of 316 stainless steel, operated upflow, and mounted in a furnace, controllable to ±1.0°C and fitted with pumps allowing flow control to ±1 cc/hr. The reactor was also fitted with a pressure regulating device and equipment for monitoring temperature, pressure and flow rate. The reactor was charged at the beginning of the experiment with 25cc of β-zeolite (CP861-DL from PQ Corp., 1/16" (1.6 mm) diameter extrudates). A screen of glass wool was placed at the top and bottom of the reactor to ensure the catalyst would remain in the middle portion. The catalyst bed was treated with crude t-butanol/ methanol (1:2 molar mix) also containing significant quantities of water, acetone (Me 2 CO), isopropanol (2-PrOH), methyl ethyl ketone (MEK), t-butyl formate (TBF) and di-t-butyl peroxide (DTBP). This mix was fed upflow at a rate of 50 cc/hr, while the reactor was held at 120°C, using a total pressure of 300 psi (2.1 MPa). Samples of crude product effluent were collected periodically on stream, in 3.6 ss bombs, and analyzed by glc. Typical results for samples taken over the 88 day duration of this run are summarized in Table 1. Typical tBA conversion and MTBE selectivity data are summarized below: Sample Time on Stream (Hrs) Operating Temp (°C) t BA Conv (%) Molar Selec (%) MTBE C 4 H 8 1 24 120 71 67 14 4 504 120 73 80 19 10 1560 120 62 79 20 13 2064 120 63 84 23 EXAMPLE 1 This example illustrates the preparation of lithium, rare earth-exchanged β-zeolite. To 100g of β-zeolite (CP861-DL Beta, 80% β-zeolite, 20% alumina binder, from PQ Corporation, as 1/16" (1.6 mm) diameter extrudates) was added 1 litre of 1M lithium chloride solution and the mix slurried at 60°C for 5 hours. Solids were filtered off and treated with 1 litre of a 0.2M solution of mixed rare earth chlorides. Again the mix was slurried at 60°C for 5 hours. The exchanged zeolite was then recovered by filtration, washed with distilled water (100cc) about 20 times, dried at 120°C overnight, and calcined at 540°C for 5 hours. EXAMPLE 2 This example illustrates the production of methyl t-butyl ether from crude t-butanol/methanol feedstock using a lithium, rare earth-treated β-zeolite catalyst. Following the procedures of comparative Example A, the lithium, rare earth-modified β-zeolite of Example 1 was treated with the same crude 2:1 molar mix of methanol/ t-butanol of comparative Example A. Etherification was conducted at 120°C, 300 psi (2.1 MPa), using a LHSV of 2, for a period of 84 days. Concentrations of each component were followed by glc analyses of the product effluents. Typical data are given in Table 2 for Samples 1-17, taken over the 84 day duration of this run. Typical tBA conversion and MTBE selectivity data are summarized below: Sample Time on Stream (Hrs) Operating Temps (°C) t BA Conv(%) Molar Selec(%) MTBE C 4 H 8 1 24 120 72 83 12 5 336 120 68 82 16 11 1104 120 73 89 15 15 1752 120 72 87 15 17 2016 120 71 86 15 EXAMPLES 3-6 These examples illustrate the production of methyl t-butyl ether from t-butanol/methanol feedstocks using a series of β-zeolite catalysts modified with silicon, phosphorus and halogen-containing reagents. Following the procedures of Example 2, a series of modified β-zeolite catalysts was tested for etherification activity using a 1.1:1 methanol/t-butanol feed mix over a range of operating temperatures (120°-180°C). Concentrations of the t-butanol, methanol, isobutylene and MTBE in the crude effluent products were followed by glc analyses. Typical data are given in Tables 3-6 for: a) β-zeolite, treated with phosphorus pentoxide (Table 3). b) β-zeolite, treated with hexamethyldisiloxane (HMDS) (Table 4). c) Two β-zeolite samples, with different alumina binder levels, treated with trimethylsilyl chloride (TSMCl) (Tables 5 and 6).

Claims: A method for synthesizing alkyl tertiary alkyl ethers which comprises reacting t-alcohol with an alkanol in the presence of a catalyst comprising β-zeolite modified with one of the following: (a) one or more metals selected from Group IA and Group IIIB of the Periodic Table, or (b) phosphorous pentoxide, or (c) a silicon-containing reagent selected from the group consisting of organosiloxanes such as hexamethyldisiloxane, and alkylsilyl halides such as trimethylsilyl chloride and continuously contacting said alkanol and t-alcohol over said modified β-zeolite catalyst at a temperature from 20°C to 250°C and a pressure from 0.1 to 7 MPa. A method as claimed in claim 1 wherein the molar proportions of said alkanol and said t-alcohol are from 10:1 to 1:10. A method as claimed in claim 1 or claim 2 wherein the alkanol is methanol and the product is methyl tertiary butyl ether (MTBE). A method as claimed in any preceding claim wherein the β-zeolite has a silica-to-alumina molar ratio of at least 10:1. A method as claimed in any preceding claim wherein the β-zeolite is bound to an oxide binder selected from Group III or IV of the Periodic Table. A method as claimed in claim 5 wherein the Group III oxide is alumina. A method as claimed in any preceding claim wherein the Group IA metal is lithium. A method as claimed in any preceding claim wherein the Group IIIB metal is selected from the group consisting of scandium, yttrium, lanthanum, and the lanthanide elements, as well as mixtures thereof. A method as claimed in any preceding claim wherein the β-zeolite is modified with lithium and the rare earth metals. A method as claimed in any preceding claim wherein the concentrations of metals deposited on said β-zeolite vary from 0.01 % to 10.0 % by weight for each metal.

Inventors: DAI PEI-SHING EUGENE, KNIFTON JOHN FREDERICK, DAI, PEI-SHING EUGENE, KNIFTON, JOHN FREDERICK
Applicants: HUNTSMAN SPEC CHEM CORP, HUNTSMAN SPECIALTY CHEMICALS CORPORATION
Application year: 1995
Main classification: C07C43
IPC codes: C07C43, C07C41, B01J29, C07B61