Synthesis of methyl t-butyl ether using Group IV oxides treated with sulfates or sulfuric acid

Abstract: A process for the preparation of an alkyl tertiary alkyl ether from a tertiary alcohol and a primary alcohol in the presence of a catalyst, characterised in that the catalyst is a Group IV oxide that has been treated with a sulfating agent. The process is particularly suited for the preparation of methyl t-butyl ether from t-butanol and methanol.

Description: This invention concerns an improved process for preparing methyl tertiary butyl ether (MTBE) by the reaction of tertiary butanol and methanol in the presence of a catalyst comprising a Group IV oxide treated with ammonium sulfate or sulfuric acid. Good performance was demonstrated for an extended period and total MTBE plus isobutylene selectivity remains close to quantitative. Where the levels of t-butanol conversion are high, typically >80%, phase separation of the crude product into an isobutylene-MTBE rich phase and a heavier aqueous methanol phase may be observed. It is known to those skilled in the art that ethers, including unsymmetrical ethers, may be prepared by reacting an alcohol with another alcohol to form the desired product. The reaction mixture, containing catalyst and/or condensing agent may be separated and further treated to permit attainment of the desired product. Such further treatment commonly includes one or more distillation operations. Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as gasoline additives based on lead and manganese have been phased out. Currently all commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol (Eq. 1), catalyzed by a cationic ion-exchange resin (see, for example: Hydrocarbon Processing, Oct. 1984, p. 63; Oil and Gas J., Jan. 1, 1979, p. 76; Chem. Economics Handbook-SRI, Sept. 1986, p. 543-7051P). The cationic ion-exchange resins used in MTBE synthesis normally have the sulfonic acid functionality (see: J. Tejero, J. Mol. Catal., 42 (1987) 257; C. Subramamam et al., Can. J. Chem. Eng., 65 (1987) 613). With the expanding use of MTBE as an acceptable gasoline additive, however, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene (Oil and Gas J., June 8, 1987, p. 55). It would be advantageous, therefore, to have a process to make MTBE that does not require isobutylene as a building block. It would be advantageous to have an efficient process for making MTBE by reaction of methanol with tertiary butyl alcohol, since t-butanol (TBA) is readily available commercially through isobutane oxidation. The preparation of methyl tert-butyl ether from methyl and tert-butyl alcohols is discussed in S. V. Rozhkov et al., Prevrashch Uglevodorodov, Kislotno-Osnovn. Geterogennykh Katal. Tezisy Dokl. Vses Konf., 1977, 150 (C. A. 92:58165y). Here the TBA and methanol undergo etherification over KU-2 strongly acidic sulfopolystyrene cation-exchangers under mild conditions. This reference contains data on basic parameters of such a process. It is also pointed out that, although a plant for etherification over cation exchangers does not present any problems, considerations include the fact that recycling large amounts of tert-butyl alcohol and methanol, as well as isobutylene, causes the scheme to be somewhat more expensive. Also, the progress of the reaction over cation exchangers is usually complicated by various adsorption and diffusion factors, by swelling phenomena, and by the variable distribution of the components between the solution and ion-exchanger phase. Furthermore, said acidic cation-exchangers with an organic (polystyrene or polymethacrylate) backbone generally have a very limited stability range with regard to operating temperatures, with temperatures above 120°C normally leading to irreversible destruction of the resin and loss of catalytic activity. In U. S.-A-.2,282,469 to Frolich there is disclosed a process for preparing methyl tertiary butyl ether over a catalyst comprising Kieselguhr impregnated with phosphoric acid at a temperature of 80° to 176°C (175°F to 350°F). EP-A-0 333 077 (Texaco) describes a process for the production of MTBE by reacting butanol and methanol in the presence of a supported phosphoric acid catalyst comprising a titanic support. EP-A-0 333 078 (Texaco) describes a similar reaction to the above wherein the catalyst is an acidic alumina or acidic silica alone or combined with a zeolite or with an acidic clay material. There is a need in the art for a stable catalyst for producing MTBE. It would be especially desirable if the catalyst allowed the reaction to be accomplished in one step under relatively mild conditions, but was thermally and chemically stable at higher temperatures. Although some of the work discussed above suggests the isomerization of alkanes or dehydration of alcohols, there seems to be nothing in the art which suggests that reacting a primary and tertiary alcohol such as methanol and t-butanol over a solid superacid would produce MTBE and isobutylene. Further the related art would seem to indicate catalysts such as TiO₂/SO₄ would be poor candidates for industrial use. It has now been discovered that a catalyst composition comprising sulfuric acid on a Group IV metal oxide or a Group IV oxide having ammonium sulfate calcinated thereon provides these desirable characteristics and good yields of a valuable product. The catalysts have performed well over a 10-day period in the manufacture of MTBE. They exhibit good stability and show promise for suitability for commercial use which the art suggests is unfeasible. The present invention accordingly provides a process for the preparation of an alkyl tertiary alkyl ether from a tertiary alcohol and a primary alcohol in the presence of a catalyst, characterised in that the catalyst is a Group IV oxide that has been treated with a sulfating agent and calcined. The invention particularly provides a process for the preparation of methyl t-butyl ether from t-butanol and methanol in the presence of a catalyst, characterised in that the catalyst is a Group IV oxide that has been treated with a sulfating agent. Fig. 1 represents the concentration of isobutylene, methyl tertiary butyl ether and t-butanol over a period of days where the catalyst is sulfuric acid-on-zirconia. Preparation of the product of this invention may be carried out typically by the etherification in one step of tertiary and primary alcohols in the presence of a catalyst which preferably comprises a Group IV oxide having ammonium sulfate deposited thereon or sulfuric acid on a Group IV oxide. The reaction can be represented by the following: The reactants comprise mixtures of primary alcohols and tertiary alcohols which can be reacted over the catalyst to produce alkyl tertiary alkyl ethers. For example, methanol and t-butanol (tBA) coreactants are reacted to form MTBE and may be mixed in any proportion in order to generate the desired methyl t-butyl ether, but preferably the molar ratio of methanol 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 methanol in the liquid feed is desirable. The most preferred methanol-to-tertiary butanol molar ratio is from 1:1 to 5:1. In certain circumstances, it may be particularly desirable that the t-butanol conversion be high enough (e.g. >80% per pass), such that the crude product mix phase separates into an isobutylene-MTBE product-rich phase and a heavier aqueous methanol phase. Preferably such a product phase separation would be achieved at as low an etherification temperature as possible, but particularly in the range 160-200°C. The same process may also be applied to the preparation of other alkyl tertiary alkyl ethers, such as, for example ethyl t-butyl ether (ETBE) and TAME. Said process may be applied to the- reaction of a C₁-C₆ primary alcohol such as methanol, ethanol, n-propanol and n-hexanol with a C₄-C₁₀ tertiary alcohol such as, for example, tertiary butanol and tertiary amyl alcohol. Reaction of methanol with tertiary amyl alcohol (2-methyl-2-butanol) would then yield methyl tertiary amyl ether (TAME). Alternatively a mixture of alcohols, e.g., a mixture of C₁-C₅ alcohols, could be reacted to give a mixture of alkyl tert-alkyl ethers. The catalyst of this invention consists of Group IV oxides treated with certain sulfur-containing compounds. In the primary embodiment, Group IV oxides are sulfated by treatment with a sulfate-containing compound. The Group IV oxides can be sulfated by the use of ammonium sulfate, ammonium hydrogen sulfate, or sulfuric acid as the sulfur-containing compound, as well as sulfur trioxide, sulfur dioxide and hydrogen sulfide. The preferred sulfating agents are ammonium sulfate and sulfuric acid. Said agents may be employed neat, or as an aqueous, ketonic, alcoholic, or ether solution, but preferably as an aqueous solution. Said sulfating agents may also be employed as mixtures thereof. Excess sulfating agent may be removed by a number of procedures including filtration and evaporation. The preformed, sulfated Group IV oxide may then, optionally, be calcined prior to use as an etherification catalyst. Calcination in air or in an inert gas environment, such as nitrogen, may be conducted at a temperature of at least 100°C , but below the temperature at which thermal destruction leads to catalyst deactivation. This can be determined by routine experimentation for a particular catalyst. The sulfated catalyst is typically calcined for from 1 to 24 hours at a temperature of from 500° to 800°C. Good results were achieved, for example, for the ammonium sulfate on zirconia catalyst by calcining at 625°-750°C, for 16-20 hours, in a stream of nitrogen. Temperatures above 900°C should be avoided. Suitable Group IV oxides used in conjunction with said sulfur-containing compounds include the oxides of silicon, titanium, zirconium, hafnium, germanium, tin and lead, as well as combinations thereof. Particularly preferred are oxides of titanium and zirconium, such as the anatase or rutile forms of titania, or zirconia. In a more specific embodiment, the Group IV oxide is treated with sulfuric acid by adding said acid neat, or diluted with distilled water, to the oxide extrudates, mixing for 1 to 24 hours, filtering, washing and calcining in a stream of air for 1 to 24 hours. The prepared sulfuric acid-treated oxide should then have a titratable acidity of at least 0.1 meq/g. The weight percent of sulfuric acid to Group IV support is preferably such that the concentration of the sulfur in the formulated catalyst is in the range of 0.1 wt% to 30 wt%, although concentrations outside this range may also be employed. Where sulfuric acid, for example, is supported on titania, a suitable quantity of sulfur is >0.5 wt%. Generally, the catalyst composition is prepared by impregnating a preformed pellet. A suitable procedure to be used is to immerse titania pellets, for example, in an aqueous or polar organic solvent (such as acetone) solution of the acid, preferably at ambient temperature. Higher temperatures of 100° to 150°C can be used, if desired. This treatment should be continued, preferably with agitation, for 0.1 to 5 hours sufficient to permit the solution to penetrate the pores of the titania pellet. Suitably, the amount of solution of the acid that is used should be adequate to permit full immersion of the titania pellets. Larger amounts of the solution can be used, if desired, but there is no particular advantage in doing so. At the end of the immersion step, the excess solution can be evaporated from the treated pellets, or the pellets can be removed from the solution and permitted to dry (e.g., in a drying oven). The Group IV oxide may be in the form of powders, pellets, spheres, shapes and extrudates. The examples described herein demonstrate the advantages of using pellets and extrudates. Titania pellets can be prepared by extrusion or by compaction in conventional pelleting apparatus using a pelleting aid such as graphite. Extrudates which work well include HSA titania carrier extrudate from Norton Company, with a surface area of 51 m²/g, and zirconia extrudates from Norton having a surface area of 77 m²/g. As will be demonstrated by the examples, the Group IV oxides are preferably of high purity and high surface area. It has been found in the process of this invention that greater conversion of tertiary butanol and methanol is achieved where the surface area of the support is generally >10 m²/g. Cylindrically-shaped catalyst pellets having a diameter essentially equal to the length thereof can be employed. Diameters ranging from about 0.794 mm (1/32 inch) to about 9.525 mm (3/8 inch) possess desirable dimensions. It will be understood that the shape and dimensions of the pellets are not critical to the present invention and that pellets of any suitable shape and dimensions may be used as desired by one wishing to practice the process of the present invention. When cylindrical pellets of catalyst of the type described above are used, the liquid hourly space velocity may be varied within wide limits (e.g., 0.1 to 10) in order to obtain a desired rate of conversion, as explained above. Normally, space velocities of about 0.5 to 2 LHSV will be employed. The pelleted catalyst compositions of the present invention are preferably employed as a fixed bed of catalyst in a continuous reaction system. In a continuous process of this nature, the time of contact of the reactants with the catalyst is one of the interrelated factors that those skilled in the art will adjust, along with temperature, pressure, bed geometry, pellet size, etc. in order to obtain a desired rate of reaction and, hence, a desired percentage of conversion of the reactants. Thus, in a continuous process, it is not necessary to drive the reaction to completion because unreacted feedstock components can be recycled to the reactor. Catalyst life is an important factor in conducting a continuous reaction. For example, if a catalyst is easily poisoned, or if catalyst pellets do not have good structural properties, the economics of the process will be adversely affected. The catalysts of the present invention are relatively insensitive to poisoning, so this should not present a problem. As a consequence, the catalyst compositions of the present invention are advantageously used in a continuous process for the continuous production of methyl t-butyl ether reaction products from tertiary butanol and methanol. Such catalyst compositions can be used for prolonged periods without the need for regeneration. Nevertheless, with the passage of time deactivation will tend to slowly occur. Deactivation can be measured qualitatively by the loss of butanol conversion, or as the increase of temperature required to maintain an essentially constant conversion rate for the t-butanol and methanol. The fact that this method can be achieved under relatively mild operating conditions is an attractive feature of this invention. Etherification can generally be conducted at temperatures from 20° to 250°C. The preferred range is 100° to 200°C. The total operating pressure may be from 0 to 6895 kPa gauge, or higher. The preferred pressure range is 345 to 3450 kPa gauge. Typically, MTBE is generated continuously in up to ca. 40 wt% concentration in the crude liquid product at total liquid hourly space velocities (LHSV) of at least 0.1 and up to ten, and relatively mild conditions, where: The examples which follow illustrate the one-step synthesis of MTBE from TBA and MeOH (Eq. 2) using sulfuric acid-treated and sulfated Group IV oxide extrudates. Conversion of t-butanol (TBA, wt%) is estimated in the following examples using the equation: Selectivities to methyl t-butyl ether (MTBE, mole %) are estimated from: EXAMPLES Examples 1 through 6 demonstrate the first embodiment employing ammonium sulfate treated Group IV oxides. It may be noted that: a) In Example 4, the ammonium sulfate treated zirconium extrudates, after calcination at 625°C, gave MTBE in up to ca. 38% concentration when run at LHSV of 2 using a MeOH/tBA molar feed ratio of 1.1:1. Under these conditions the tBA conversion is typically 71% at 160°C and 80% at 180°C, while the isobutylene and MTBE molar selectivities are 51 and 46% respectively at 160°C (Sample #7). b) MTBE/isobutylene cogeneration has also been demonstrated with an ammonium sulfate-treated titania catalyst, calcined at 625°C, and with an ammonium sulfate-treated zirconia catalyst, calcined at 750°C (see Examples 5 and 6). c) In Example 6, using the ammonium sulfate-treated zirconia catalyst, tBA conversion at 160°C is typically 70%, while the isobutylene and MTBE molar selectivities are 55% and 46% respectively (Sample #5). At 180°C, tBA conversion is >80% and product phase separation into an isobutylene-MTBE product-rich phase and a heavier aqueous methanol-rich phase is observed. d) Essentially no MTBE formation is detected with the zirconia extrudates that have not been (NH₄)SO₄ treated and calcined (see Comparative Example A). Examples 7 through 13 demonstrate the embodiment using sulfuric acid-treated Group IV oxides. It may be noted that: a) In Example 8, a sulfuric acid-treated extrudate catalyst performed very well under the fairly forcing conditions used in this test, i.e.: At high operating temperature - 160°C. Low methanol to t-butanol molar feed Ratio of 1.1:1. High throughput - LHSV = 2. Tert-butanol conversion levels remained satisfactory at 61% to 71% throughout the test using a fixed-bed, plug-flow, reactor design. While MTBE selectivity increases modestly over this 10 day period, MTBE plus isobutylene remain the major products and in fact MTBE + C₄H₈ selectivity is close to quantitative over the life of the run. b) In Examples 11-13 three catalysts, namely sulfuric acid-on-zirconia, sulfuric acid-on-titania and sulfuric acid-on-silica, are demonstrated to be effective catalysts for MTBE production from tBA plus methanol over a range of operating temperatures (80-180°C). At 160°C, the order of activity in terms of tBA conversion per pass at LHSV 2 is found to be: H₂SO₄/TiO₂ > H₂SO₄/ZrO₂ >> H₂SO₄/SiO₂ c) In Example 12, product phase separation into an isobutylene-MTBE product-rich phase and a heavier aqueous methanol phase is evident at 180°C. tBA conversion is >80%. EXAMPLE 1 This example illustrates the preparation of a typical ammonium sulfate-treated zirconia-catalyst. Zirconia, high purity, extrudates (200g,0.35cm diameter, 77 m²/g surface area, from the Norton Company) were covered with about 250g of an aqueous, 10% solution of ammonium sulfate and the mixture let stand for about 5 minutes. The mixture was then placed in an oven and heated to 625°C under a nitrogen flow. The oven was held at 625°C for 20 hours and then cooled to ambient temperature under nitrogen. The formed catalyst was found to have a titratable acidity of 0.21 meq/g and a sulfate content of 2.9%. EXAMPLE 2 This example illustrates the preparation of a typical ammonium sulfate-treated titania catalyst. Employing the procedures of Example 1, a sample of titania extrudates (200g, 0.35cm diameter, 51 m²/g surface area from Norton Company) was treated with aqueous (NH₄)₂)SO₄ and calcined at 625°C under nitrogen. The formed catalyst was found to have a titratable acidity of 0.07 meq/g and a sulfate content of 0.5%. EXAMPLE 3 Employing the procedures of Example 1, a sample of ammonium sulfate-treated zirconia was prepared where said extrudates were calcined at 750°C for 16 hours. EXAMPLE 4 This example illustrates the production of methyl t-butyl ether from t-butanol and methanol using the ammonium sulfate-treated zirconia catalyst of Example 1. Synthesis was conducted in a tubular reactor (1.25 cm id., 12" 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 ammonium sulfate-treated zirconia prepared by the method of Example 1. 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 methanol/t-butanol (1.1:1 molar mix) upflow, at a flow rate of 50 cc/hr., while the reactor was held at 100°C, with a total pressure of 2070 kPa. Samples of crude product effluent were collected periodically on stream, in 316ss bombs, and analyzed by glc and gc-ir. Typical analyses data for samples taken under these conditions are summarized in Table I. Performance at a series of other temperatures (120°, 140°, 160° and 180°C) and flow rates (200 cc/hr.) was measured using the same procedures. These results are also given in Table I. Of note, the conversion levels and isobutylene/MTBE selectivities at 160° and 180°C are as follows: The high overall isobutylene plus MTBE selectivity at 180°C operating temperature indicates the onset of phase separation in the crude product effluent. EXAMPLES 5-6 These examples illustrate the production of methyl t-butyl ether from t-butanol and methanol using the ammonium sulfate-treated Group IV oxide catalysts of Examples 2 and 3. Synthesis procedures and operating conditions were as described in Example 4, the results are tabulated in Tables II and III. For Example 6, Sample 5, taken at 160°C, it may be noted that: t-Butanol conversion is 70% isobutylene selectivity is 55 mole% MTBE selectivity is 46 mole% COMPARATIVE EXAMPLE A This example illustrates that methyl t-butyl ether is not generated from t-butanol and methanol when the catalyst is a Group IV oxide alone, e.g. zirconia extrudates as above. Synthesis procedures and operating conditions were as described in Example 4. The results are tabulated in Table IV. EXAMPLE 7 The example illustrates the preparation of a sulfuric acid-treated zirconia catalyst. To a flask containing 250cc of zirconia 0.35 cm diameter extrudates (77 m₂/g, from the Norton Company) was added a solution of sulfuric acid (96.8%, 400g) in 1 litre of distilled water. The mixture was stirred for 1-2 hours and the solids filtered then washed with distilled water until the washings were pH neutral. The white extrudates were calcined at 600°C in a stream of air for 3 hours. The titratable acid content of the sulfur acid-treated zirconia was determined to be 0.15 meq/g. The percentage sulfate, by ion chromatography, was 0.73%. EXAMPLE 8 The example illustrates the production of methyl t-butyl ether from t-butanol and methanol using the sulfuric acid-on-zirconia catalyst of Example 7. Synthesis was conducted, in a tubular reactor as above (Example 4). The reactor was charged at the beginning of the experiment with 25cc of sulfuric acid-on-zirconia catalyst prepared by the procedures of Example 7. The procedure of Example 4 was followed. Typical of analyses data for samples taken under these conditions are summarized in Table 5. Concentrations of isobutylene, MTBE and t-butanol in the crude products are plotted in Fig. 1. For Samples 1 and 8 taken after one and 10 days operating time, respectively, the t-butanol conversion levels and MTBE and isobutylene selectivities are as follows: EXAMPLE 9 This example illustrates the preparation of a sulfuric acid-on-titania solid super acid. To a flask containing 250cc of titania 0.35 cm tablets (158 m²/g surface area, from the Calsicat Company) was added 150cc of 96.8% concentrated sulfuric acid. The mixture was stirred for 1-2 hours and the solids filtered then washed with distilled water until the washings were pH neutral. The grey tablets were dried at 40°C overnight in vacuo, then at 150°C for 3 hours. The titratable acid content of the sulfuric acid-treated titania was determined to be 0.20 meq/g. The percentage sulfate, by ion chromatography, was 1.02%. EXAMPLE 10 This example illustrates the preparation of a sulfuric acid-on-silica catalyst. To a flask containing 250cc of silica 5mm spheres (130 m²/g surface area, from United Catalyst Company) was added 150cc of 96.8% concentrated sulfuric acid. The mixture was stirred for 1-2 hours and the solids filtered then washed with distilled water until the washings were pH neutral. The white spheres were dried at 40°C overnight in vacuo, then at 150°C for 3 hours. The acid content of the sulfuric acid-treated silica was determined to be 0.19 meq/g. The percentage sulfate, by ion chromatography, was 0.66%. EXAMPLES 11-13 Following the procedures of Example 8, the production of methyl t-butyl ether from t-butanol plus methanol was demonstrated using three sulfuric acid-on-Group IV metal oxide supported catalysts, namely: Sulfuric acid-on-zirconia, prepared according to the method of Example 7. Sulfuric acid-on-titania, prepared according to the method of Example 9. Sulfuric acid-on-silica, prepared according to the method of Example 10. In each case, the MTBE synthesis was demonstrated over a range of operating temperatures (typically from 80° to 180°C) using the same plug reactor design of Example 4. Typical analyses data for samples taken during the screening of each of these three catalysts are summarized in Tables VI through VIII. At the operating temperatures of 160°C, the t-butanol conversion levels and MTBE and isobutylene selectivities are as follows: COMPARATIVE EXAMPLE A This example illustrates that methyl t-butyl ether is not generated from t-butanol and methanol when the catalyst is untreated silica. Synthesis procedures and operating conditions were as described in Example 8. The results are tabulated in Table IX.

Claims: A process for the preparation of an alkyl tertiary alkyl ether from a tertiary alcohol and a primary alcohol in the presence of a catalyst, characterised in that the catalyst is a Group IV oxide that has been treated with a sulfating agent and calcined. A process as claimed in claim 1 for the preparation of methyl t-butyl ether from t-butanol and methanol. A process as claimed in Claim 2 wherein said t-butanol and methanol are present in a molar amount of 0.1-10 moles of methanol per mole of t-butanol. A process as claimed in Claim 2 or Claim 3 wherein said t-butanol and methanol are contacted with said catalyst at a temperature of 20°C to 250°C and a pressure of atmospheric to 6895 kPa gauge pressure. A process as claimed in any one of the preceding claims wherein the Group IV oxide is in the form of a pellet or extrudate. A process as claimed in any one of the preceding claims wherein the Group IV oxide is selected from zirconia, titania and silica. A process as claimed in any one of the preceding claims wherein the sulfating compound is ammonium sulfate or sulfuric acid. A process as claimed in any one of Claims 2 to 7 wherein the product comprises a two-phase mix of an isobutylene-MTBE product rich phase and a heavier aqueous methanol-rich phase. The use of a sulfated Group IV oxide in a method of preparing t-butyl ether from t-butanol and methanol.

Inventors: KNIFTON JOHN FREDERICK, SANDERSON JOHN RONALD, KNIFTON, JOHN FREDERICK, SANDERSON, JOHN RONALD
Applicants: TEXACO CHEMICAL, TEXACO CHEMICAL COMPANY
Application year: 1992
Main classification: C07C41
IPC codes: C07C43, B01J27, C07C41, B01J21, C07B61