DEHYDRATION OF PRIMARY ALCOHOLS

Abstract: Primary alcohols are dehydrated into alpha-unsaturated hydrocarbons by contacting them with a catalyst consisting of niobic acid and/or tantalic acid. The catalyst can be activated by heating before the reaction.

Description: DEHYDRATION OF PRIMARY ALCOHOLS The invention relates to a process for dehydrating primary alcohols into alpha-unsaturated hydrocarbons in the presence of a dehydration catalyst. More particu¬ larly, the invention relates to a process for dehydrating isobutanol selectively to isobutene. In addition, the invention relates to a process for preparing methyl tert¬ butyl ether (hereinafter MTBE) from methanol and isobu¬ tanol. According to section 7-1 of the standard textbook, "Advanced Organic Chemistry", third ed. by J. March, dehydration of alcohols can be accomplished in several ways. According to this textbook, the Broensted acids H2SO 4 and H 3 PO 4 are common reagents, as well as a score of Lewis acids such as AI2O 3 . Besides, also solid Broensted acids like Siθ2*Al2θ3 are typically used. The ease of dehydration of alcohols increases with α- branching. It follows the sequence tertiary > secondary > primary. When protic acids catalyse alcohol dehydration, the mechanism is El. It involves the conversion of ROH into the protonated alcohol ROH 2 + and cleavage of the latter into the carbonium ion R + and H2O. The dehydration rate in the presence of a protic acid is quite high. Ter¬ tiary alcohols are dehydrated easily with even a trace of acid. However, in many cases the use of protic acids lead to rearrangement products or to ether formation. Thus, less stable carbonium ions will rearrange whenever possible to form a more stable carbonium ion. n-Butanol, for,example, yields the n-butyl cation that rearranges to the sec-butyl cation. According to sections 5.19 to 5.23 of "Organic Chemistry", third ed. , by Morrison and Boyd, this cation loses a hydrogen ion to give 2-butene, in particular the trans isomer. Only the methyl cation and, of the primary alcohols, only the ethyl cation cannot rearrange to form a more stable carbonium ion. In case side reactions involving shift of the double bond or skeletal rearrangements are to be avoided, va¬ pour-phase dehydration over AI2O 3 is an excellent method for dehydrating volatile alcohols ("concerted, E2 mecha¬ nism") . Drawback of AI2O 3 and similar solid Lewis acids, is that the excellent selectivity to the (optionally substituted) alpha-olefin is provided at the cost of activity. The inventors set out to provide: (i) a catalyst for dehydration of primary alcohols, in particular isobu- tanol, that is as selective as but more active than the aforementioned AI2O3; and (ii) a process for dehydrating primary alcohols in the presence of this catalyst. Surprisingly, it was found that this objective could be achieved with Group Vb metal oxide hydrates, i.e., niobic acid and tantalic acid (respectively Nb2θ5-nH2θ and Ta2θ5-nH2θ) . Of these, niobic acid is preferred. Ac¬ cordingly, the invention provides a process for dehydra¬ ting primary alcohols into alpha-unsaturated hydrocarbons in the presence of a dehydration catalyst, wherein the dehydration catalyst is niobic acid and/or tantalic acid. It is to be understood, that methanol is excluded from the definition of primary alcohols, as dehydration thereof (into dimethylether) proceeds via a different mechanism. In order for the dehydration to the corre- sponding alpha-olefin to occur, the primary alcohol must have a vicinal hydrogen atom. Methanol does not have an adjacent carbon atom, let alone a hydrogen atom thereon, and is hence not dehydrated into an alpha-unsaturated hydrocarbon. Suitably, the primary alcohol has at least 3 carbon atoms. Although ethanol may also be dehydrated using the present dehydration catalyst, selectivity of the catalyst is usually not a problem. The primary alcohol typically has less than 20, say less than 10 carbon atoms, to allow for gas-phase dehydration. It is, however, possible to carry out the process with larger primary alcohols, dis¬ solved in an inert solvent. The primary alcohol may contain further functional groups and unsaturated carbon-carbon bonds. Preferably, it is an alkanol. Use of a functionalised primary alcohol such as ethylene glycol may lead to the isomer (tautomer) of the alpha-unsaturated hydrocarbon. Primary alcohols that may be dehydrated advanta- geously into the corresponding alpha-unsaturated hydro¬ carbons are for instance l-propanol, 1-butanol, 2-methyl- 1-propanol (isobutanol) , 2-methyl-1-butanol, etc. The process of the invention is particularly suitable for the selective conversion of isobutanol into isobutene in high yields. The dehydration catalyst may be prepared either as a bulk oxide or supported on an acidic or amphotheric car¬ rier, like gamma-Al2θ 3 or the more neutral Siθ2 supports. The synthesis of niobic acid and the use thereof as dehydration catalyst is known. For instance, in JP 1290636, the preparation of isobutene is disclosed, comprising gaseous phase dehydration of tert-butanol over niobic acid. In an article entitled "Acidic and Catalytic Properties of Niobium Penta-oxide" by T. Izuka et al, published in 1983 in Bull. Chem. Soc. Jpn., 56, 2927- 2931, the dehydration of 2-butanol is disclosed. That this catalyst and its analogue tantalic acid could also be used for the selective preparation of isobutene from isobutanol is neither disclosed nor hinted at. Rather, the aforementioned article discloses that Nb2θ5-nH2θ showed a remarkable isomerization activity. The dehydration catalyst is suitably prepared by pre¬ cipitating niobium hydroxide and/or tantalum hydroxide from e.g., a solution of the Group Vb metal oxalate, fol- lowed by washing the niobium hydroxide and/or tantalum hydroxide with water and then heat-treating the Group Vb metal oxide hydrates thus obtained at a low temperature. Expediently, the hydrates may be treated with e.g., sul¬ phuric acid, hydrofluoric acid or phosphoric acid before the low- emperature heat treatment. Typically (cf. EP-A-0,135,145) the hydrates are pre-treated with heat at a moderate temperature in the range of 100 to 400 °C. At temperatures in excess of 400 °C the hydrates become fully dehydrated, resultling in a change of properties. Preferably, the dehydration catalyst is pre-treated at a temperature in the range of 100 to 300 °C, more prefer¬ ably at about 300 °C. The dehydration process is carried out at conditions typically found for dehydration of alcohols by heteroge- neous catalysts. Thus, the dehydration catalyst is packed into a reactor to prepare a fixed catalyst bed. The cata¬ lyst is then activated at 100 to 400 °C, and the reactor is set to 150 to 350 °C, preferably 250 to 300 °C. The primary alcohol is fed to the catalyst bed at a weight hourly space velocity (WHSV) of 1 to 20 kg/kg cat -hr, preferably 3 to 10 kg/kg cat •hr. The reaction pressure is ordinarily in the range of 0 and 20 bar g, preferably in the range of 1 to 10 bar g. The production of fuel oxygenates entirely from synthesis gas is one of petrochemical industries' most interesting challenges. Indeed, Air Products (sponsored by the US Department of Energy) is focusing on developing a "Ci" route to MTBE that involves three reaction steps: (1) synthesis of mixtures of methanol and isobutanol from synthesis gas, (2) dehydration of isobutanol to isobutene, and (3) reaction of isobutene with methanol from step (1) , using known technology (OIL GAS -European Magazine 1 (1994) , pp. 39-42) . The selective dehydration of isobutanol to isobutene is a crucial step in this process, since the direct etherification of the inter¬ mediates isobutanol and methanol using conventional catalysts leads to methyl isobutyl ether, a lower octane isomer of MTBE. Accordingly, the present invention also provides a process for preparing MTBE from methanol and isobutanol, comprising the dehydration of the isobutanol in the presence of a dehydration catalyst into isobutene, followed by the etherification of the isobutene with the methanol, wherein the dehydration catalyst is niobic acid and/or tantalic acid. The first step of the C^ route to MTBE comprises the conversion of the synthesis gas into methanol and iso¬ butanol (e.g., using a caesium-promoted Cu ZnO/Al2θ3 methanol synthesis catalyst as described in the OIL GAS reference, one of the catalysts described in Uhlmann, 5th ed., A16, pp. 469-471 or according to the experiments reported by Klier and co-workers, e.g., J.G. Nunan, K. Klier and R.G. Herman, J. Catal., 139, (1993), 406-420) . The (indirect) etherification of isobutene and metha¬ nol is ordinarily performed in the presence of an acidic catalyst. Typically, the catalyst is a sulphonic acid substituted ion exchange resin or an acidic natural or synthetic silicate (e.g., amorphous silica-alumina or acid zeolites) . Preferably, the catalyst is an ion exchange resin, e.g., produced by polymerization of aro¬ matic vinyl compounds to which catalytically active func¬ tional groups are covalently bonded. Suitable sulphonic acid substituted ion exchange resins and their proper use are disclosed in SRI Report PEP No. 158A, in European patent application No. 102,840 and in International application No. 90/08758. A very suitable catalyst is a sulphonic acid substituted ion exchange resin having at least 1.2, more preferably about 1.2 to 1.8 sulphonic acid groups for each aromatic ring system. Typical exam¬ ples of very suitable catalysts are sulphonic acid sub¬ stituted divinylbenzene-styrene resins sold under the trademarks "DUOLITE" C20, "DUOLITE" C26, "AMBERLYST" 15, "AMBERLITE" IR-120, "AMBERLITE" 200 and "DOWEX" 50. The source of raw material for the synthesis gas may be coal, coke, natural gas, associated gas or (fractions of) petroleum. The raw material may be converted into synthesis gas by steam reforming and/or by partial oxida¬ tion (e.g., as described in Uhlmann, 5th ed. , A16, pp. 472-473) . The invention will be illustrated by the following examples. Example 1 Niobic acid is pre-calcined at 300 °C for 2 hours. Isobutanol ("IBA") was dehydrated in a reactor operating at an isobutanol partial pressure of 1.2 bar g and a WHSV of 4,5 kg/kg ca -hr. The yield of isobutene ("i-C4" in %mol/mol IBA) was monitored as a function of the temperature as well as of residence time. It was compared at alike pressures, temperatures and conversions (the latter by varying the residence time) with that using either a commercial gamma-Al2θ3 (ex. Engelhard) or Siθ2*Al2θ3 ("ASA-13", i.e., amorphous alumina-silica containing 13% alumina, ex. Grace-Davison) as dehydration catalyst. Note that at these reaction conditions, the maximum yield attainable is about 75 % mol/mol. The activity of the Si0 2 -Al 2 θ3, tne gamma-Al2θ 3 , and the niobic acid for the conversion of isobutanol is shown in Fig. 1. The activity follows the order Siθ2'Al2θ3 > niobic acid > gamma-Al2θ3. From Fig. 2 it is clear that - i - -within the margins of experimental error- the overall selectivity towards isobutene follows the order niobic acid « gamma-Al2θ 3 > Siθ 2 'Al2θ 3 . By-products of the dehy¬ dration comprise the isomers of isobutene; 1-butene, cis- 2-butene and trans-2-butene, as well as some oligomeric hydrocarbons.

Claims: C L A T M fi 1. A process for dehydrating primary alcohols into alpha-unsaturated hydrocarbons in the presence of a dehydration catalyst, wherein the dehydration catalyst is niobic acid and/or tantalic acid, 2. A process as claimed in claim 1, wherein the dehydra¬ tion catalyst is niobic acid. 3. A process as claimed in claim 2, wherein the dehydra¬ tion catalyst is niobic acid, pre-treated at a tempera¬ ture in the range of 100 to 400 °C. 4. A process as claimed in any one of claims 1 to 3, wherein the primary alcohol has at least 3 carbon atoms. 5. A process as claimed in claim 4, wherein the primary alcohol has less than 20 carbon atoms. 6. A process as claimed in claim 4, wherein the primary alcohol is selected from 1-propanol, 1-butanol, 2-methyl- 1-propanol (isobutanol), 2-methyl-1-butanol. 7. A process as claimed in claim 4, wherein the primary alcohol is isobutanol. 8. A process as claimed in any one of claims 1 to 7, wherein the dehydration catalyst is packed into a reactor to prepare a fixed catalyst bed, is then activated at 100 to 400 °C, and wherein the reactor is set to 150 to 350 °C, preferably 250 to 300 °C. 9. A process as claimed in claim 8, wherein the primary alcohol is fed to the catalyst bed at a weight hourly space velocity (WHSV) of 1 to 20 kg/kg ca t-hr, preferably 3 to 10 kg/kg cat -hr. 10. A process as claimed in claim 8 or 9, wherein the re¬ action pressure is between 0 and 20 bar g, preferably in the range of 1 to 6 bar g. 11. A process for preparing methyl tert-butyl ether from methanol and isobutanol, comprising the dehydration of the isobutanol in the presence of a dehydration catalyst into isobutene, followed by the etherification of the isobutene with the methanol, wherein the dehydration catalyst is niobic acid and/or tantalic acid.

Inventors: MERCERA PATRICK DENNIS LAWRENC, MERCERA, PATRICK, DENNIS, LAWRENCE
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1996
Main classification: C07C1
IPC codes: B01J23, B01J27, C07B61, C07C1, C07C11