Ether Synthesis

This invention relates to processes for producing aromatic ethers. It more specifically relates to the vapor phase production of aromatic ethers from phenols. Aromatic ethers are useful reactants and solvents, heat-transfer media and herbicides.

Williamson's ether synthesis is a well-known process for the manufacture of ethers. Williamson's ether synthesis can be the liquid phase reaction of a sodium phenoxide and an alkyl halide to form an alkyl, phenyl ether and sodium halide salt. See Morrison, R. T., et al., Organic Chemistry, 3rd ed, p. 556 (1975). This process disadvantageously consumes stoichiometric amounts of NaOH (to generate the sodium phenoxide) and disadvantageously produces a stoichiometric amount of a sodium halide salt by-product (which converts the high value halide reactant to a low value product).

Alternate processes which do not form unacceptable by-products are also known. Direct alkylation of phenols by treatment with diazomethane, tetramethoxymethane or pentamethoxyphosphorane, have been previously disclosed in the art. Other processes involve treating phenol with methanol in the presence of dicyclohexylcarbodiimide or a mixture of diethylazodicarboxylate and triphenylphosphine. These processes have proven to be rather inefficient, uneconomical or ecologically unsuitable.

Soviet Pat. No. 276,014, issued to I. I. Yukel'son et al. teaches that a catalyst containing the mixture of 45-55 percent Cr.sub.2 O.sub.3, 25-30 percent Fe.sub.2 O.sub.3 and 20-25 percent ZnO catalyzes the liquid phase alkylation of phenol and t-butyl chloride to alkyl phenols at over 98 percent yield. Most of the product is dialkylated. This reaction does not produce ethers.

It would be desirable to have a process that converted aromatic alcohols to ethers without consuming a stoichiometric amount of a catalyst and without generating a stoichiometric amount of a halide salt.

The present invention is such a process for producing aromatic ethers. This process comprises contacting a phenolic compound, in the vapor phase, and an organic halide (which has a haloaliphatic moiety), in the vapor phase with a catalytic amount of a solid catalyst containing a metal oxide or a metal under conditions such that the corresponding ether and hydrogen halide are formed.

It is surprising that an ether is formed, particularly in view of Soviet Pat. No. 276,014 which teaches alkylation instead of the etherification observed in the process of this invention.

This process produces ethers without consuming a stoichiometric amount of a catalyst. It further does not produce stoichiometric amounts of a salt. The process produces a hydrogen halide as a by-product which is of more utility and value than are the by-products produced by previous processes.

The phenolic compounds for use in the invented process are compounds of formula Ar(OH).sub.x, where Ar is any aromatic carbocyclic group, unsubstituted or substituted with one or more groups, which are unreactive under the reaction conditions, and x is an integer from 1 up to and including the number corresponding to total available substitution for Ar, preferably x is 1 or 2.

In the above formula, a monovalent Ar can be phenyl, naphthyl, biphenylyl, or the similar monovalent aromatic moieties which are optionally substituted with unreactive groups. Such unreactive groups may be either electron-donating or electron-withdrawing substituents and include lower alkyl, lower alkoxy, halo, cyano, nitro, ester groups, trifluoromethyl, aralkyl and phenoxy. If x is greater than one, Ar represents the multivalent aromatic moieties, e.g., phenylene, biphenylene, naphthylylene, alkylidene diphenylene, alkylene triphenylene, oxydiphenylene, and similar aromatic diyls, aromatic triyls and aromatic polyyls, etc., which are also optionally substituted as described above.

Preferably, the phenolic compound is phenol itself or a phenol substituted with one or more unreactive groups. Most preferably, the phenolic compound is phenol itself.

The organic halides suitable for the purposes of this invention are those wherein at least one of the halo atoms is bound to an aliphatic carbon and is capable of undergoing reaction with a phenolic moiety to form an ether. Preferable organic halides are compounds of up to about 20 carbons of the formula RX.sub.n where X is chloro, bromo or iodo, provided that X is bound to an aliphatic carbon atom; n is one or two. If n is one, R is alkyl, aralkyl or an alkyl or aralkyl group further substituted with alkoxy, alkoxy(polyalkyleneoxy), aryloxy, cyano, nitro or ester groups. If n is 2, R is the corresponding alkylene, aralkylene group or such group further substituted with alkoxy, alkoxy(poly)alkyleneoxy, aryloxy, cyano, nitro or ester groups as above described. Most preferred are those compounds wherein n is 1 and X is chloro.

Examples of these organic halides include: chloroethane, 1-bromohexane, 1-chlorododecane, chloroacetonitrile, 1-chloro-2-methoxyethane, chloromethoxyethoxymethane, 1-chloro-3-nitropropane, methyl chloromethylacetate, ethyl chloromethylacetate, phenyl chloromethylacetate, benzyl chloride, benzyl bromide, 1-chloro-2-phenoxypropane, 2-chloro-2-methylpropane, 2-chloropropane, .beta.-chlorovinylbenzene and the like.

Suitable catalysts are solid materials containing a metal oxide or metal which cause the reaction of the phenols and a suitable organic halide to form hydrogen halides and corresponding aromatic ethers. Preferred are transition metals and their oxides, rare earth oxides, silica and carbon catalysts which are optionally supported. Most preferred are catalysts selected from the group of: Pd supported on Al.sub.2 O.sub.3, ZnO, Al.sub.2 O.sub.3, TiO.sub.2 and Y.sub.2 O.sub.3. These catalysts are commercially available. The supported catalysts can be prepared by standard impregnation methods such as those set forth in chapter 1 of the fourth volume of Catalysis published by the Royal Society of Chemistry in London (1981). The surface area of the catalysts is not critical, but typically ranges from about 10 m.sup.2 /g to about 250 m.sup.2 /g, preferably from about 100 m.sup.2 /g to about 250 m.sup.2 /g.

The reaction can be carried out at any operable temperature and pressure. Preferred is atmospheric pressure. Preferably, reaction temperatures are below about 400.degree. C., more preferably below about 300.degree. C. and most preferably below about 270.degree. C. Preferably, reaction temperatures are above about 100.degree. C., more preferably above about 150.degree. C., even more preferably above about 200.degree. C. and most preferably above about 230.degree. C.

The reaction can occur in any container which can contain the reaction mixture and allow the reaction to proceed. Preferred are normal stainless steel reactors.

The reactants are preferably mixed together, pumped into a heated inert carrier gas stream which vaporizes the reactants and carries them to the catalyst where they can react. The heated inert carrier gas can be any operable non-reactive gas. Examples include nitrogen, argon, and other inert gases.

The reactants can be combined in any operable ratio. Preferably, the more expensive reactant is exceeded by the less expensive reactant by less than about 2:1, more preferably less than about 1.5:1, most preferably less than about 1.2:1.

Preferably, the selectivity is greater than about 50 percent, more preferably more than about 75 percent, most preferably more than about 80 percent. In especially preferred embodiments, the selectivity is more than about 90 percent. Selectivity is defined, for the purposes of this patent, as the amount of the reacted phenolic compound which is converted into the ether product.

The reactants in gaseous state are passed over or through the catalyst at a rate sufficient to achieve the desired conversion of reactants and yield of products with minimum formation of undesired by-products. A continuous flow of reactants is preferably employed wherein the reactants are in contact with the catalyst for uniform periods of time, i.e., no part of the reactants remain in contact with catalyst for prolonged periods of time.

The reactants are thus in contact with the catalyst for a sufficient time for the reaction to proceed, preferably to at least about 50 percent completion, more preferably to at least about 75 percent completion. A typical residence time of reactants in the reactor is between about 20 seconds and about 30 seconds. Flow rates of from about 10 standard cubic centimeters per minute (scc/m) through about 200 scc/m are preferred, more preferred are those between about 30 scc/m and about 70 scc/m.

The reaction produces hydrogen halide as one of the reaction products. The hydrogen halide can be reacted as generated with a halogen acceptor. Suitable halogen acceptors are those that react with the generated hydrogen halide and do not prevent the reaction from proceeding. Preferred halogen acceptors are alcohols in which the hydroxy moiety is bound to an aliphatic carbon atom, tertiary amines and dialkyl carbonates. Most preferred are methanol and diethyl carbonate.

The invention is further illustrated and not limited by the following illustrative embodiments in which unspecified percentages are weight percentages.

A 20-ml portion of 24 percent ZnO on .alpha.-Al.sub.2 O.sub.3 is placed in a stainless steel tube (0.25-inch inside diameter, 12-inch length) and heated by a nitrogen gas stream. A mixture of 36 g of phenol, 30 g of methanol and 6 g of toluene (as a gas chromatograph standard) is passed over the prepared catalyst at a rate of 5 grams/hour as the temperature is raised from 116.degree. C. to 203.degree. C. The products are periodically sampled by a gas chromatograph. No products are detected.

To the 33 g of mixture remaining from Illustrative Embodiment 1 is added 4 g of methyl chloride. This new mixture is passed over the same catalyst at a rate of 5 grams/hour while the temperature is raised from 90.degree. C. to 255.degree. C. Periodic gas chromatograph analysis shows anisole formation over the tested temperature range.

Illustrative Embodiment 1 is repeated with a mixture of 32 g of phenol, 56 g of methyl chloroacetate, 13 g of methanol and 8 g of dimethoxy benzene (as a gas chromatograph standard). Methyl phenoxy acetate is formed.

Illustrative Embodiment 2 is repeated using TiO.sub.2 pellets as the catalyst. At a temperature of 230.degree. C., the selectivity to methyl phenoxy acetate based on phenol is 100 percent while the conversion of phenol is 3 percent.

Illustrative Embodiment 1 is repeated with a feed mixture of 94 g of methyl chloroacetate, 68 g of phenol and 5 g of dimethoxybenzene (as a gas chromatograph standard). A catalyst of 22 percent Y.sub.2 O.sub.3 on .alpha.-Al.sub.2 O.sub.3 produces methyl phenoxy acetate.

Illustrative Embodiment 4 is repeated with a .gamma.-Al.sub.2 O.sub.3 catalyst. At a temperature of 230.degree. C., the phenol conversion is 7 percent and the selectivity is 84 percent to methyl phenoxy acetate. Phenyl chloroacetate and transesterification between phenol and dimethoxybenzene are observed.

Illustrative Embodiments 1-5 illustrate the formation of ethers with a variety of reactants.