WO1990008758A1

WO1990008758A1 - Catalyst induced yield enhancement of reversible chemical processes

Info

Publication number: WO1990008758A1
Application number: PCT/US1990/000463
Authority: WO
Inventors: Michael R. Ladisch
Original assignee: Purdue Research Foundation
Priority date: 1989-01-26
Filing date: 1990-01-26
Publication date: 1990-08-09
Also published as: AU5043390A

Abstract

The equilibrium constant of a reversible chemical reaction occurring between first and second reactant species in the presence of an ion-exchange catalyst is effected to enhance reaction product yield by modifying or selecting a catalyst to increase catalyst sorption of at least one of the reactant species. Methyl tert-butyl ether can be produced in yields of about 98 % in a single stage reactor at equilibrium conditions utilizing sulfonic acid substituted aromatic resin catalyst having at least 1.2 sulfonyl substituents for each aromatic ring system in the resin structure.

Description

CATALYST INDUCED YIELD ENHANCEMENT OF REVERSIBLE CHEMICAL PROCESSES

Background and Summary of Invention This invention relates to a method for shifting the thermodynamic equilibrium of reversible chemical reactions. More particularly, this invention is directed to a method for enhancing product yield in a reversible chemical process using an ion exchange resin catalyst that is modified or selected to preferentially sorb one or more reactant species. In accordance with this invention, product yields can be enhanced in both continuous and batch chemical processes.

Heterogenous organic catalysts enjoy wide use in the chemical industry. A commonly used class of heterogenous organic catalysts are the acid cation exchange resins or base anion exchange resins. Such resin catalysts are produced commercially by the polymerization of aromatic vinyl compounds to which catalytically active functional groups are covalently bonded before, or more typically, after polymerization. Many species of polymers have been utilized as the chemical "backbone" or framework for the active ion exchangeable/catalytically active functional groups. Sytrene, cross-linked with a divinyl benzene copolymer, forms a particularly suitable resin or polymer having a macroreticular pore structure. The aromatic ring systems of such polymer structures are electrophilically substituted, for example, to provide covalently bound carboxylic acid, sulfonic acid or phosphonic acid groups which serve as the active functional groups in acid cation exchange resins for acid catalysis. Amberlyst® brand of acid cation exchange resins, manufactured by Rohm and Haas Company of Philadelphia, Pennsylvania, is one example of commercially available sulfonic acid substituted resins widely used in commercial chemical processes. The Amberlyst resins are the subject of U.S. Patents 4,224,415; 4,256,840; 4,382,124 and 4,486,313, the disclosures of which are incorporated herein by reference.

The present invention is directed to modification or identification of ion exchange resin catalysts to facilitate and/or complement their function as a catalyst in selected reversible chemical reactions. It is based on Applicant's discovery that the thermodynamic equilibrium of a reversible chemical reaction catalyzed by the presence of an ion exchange resin can be shifted to favor higher yields of reaction products by modifying the catalyst or selecting a alternate catalyst to increase catalyst sorption of at least one of the reactant species. Thus, for example, ^■ the relative hydrophilicity or hydrophobicity of an ion exchange catalyst can be adjusted by chemical modifications, for example by partial neutralization of oxyacids to form alkali metal salts or quaternary ammonium salts, by met ylation of amine functions on base anion exchange resins or by covalent bonding of neutral substituents to the aromatic ring systems of the polymer backbones, can be effective to modify resin/reactant species affinity. Preliminary studies have involved an evaluation of the effect of the level of sulfonic acid substitution in sulfonated resin catalysts on the acid catalyzed ether-forming reaction between methanol and isobutene. Increasing the level of sulfonic acid substitution on the catalyst to at least about 1.2 sulfonyl substituents per aromatic ring system in the resin structure unexpectedly increased ether yield in a single stage reactor operated at near equilibrium conditions. Yield enhancement has been attributable, at least in part, to selective adsorption of the more polar alcohol reactant on the more hydrophilic catalyst surface. Further, increase in sulfonyl substitution level in such acid cation exchange catalyst resulted' in an unexpectedly high increase in the rate at which the etherification reaction approached the shifted thermodynamic equilibrium point favoring higher yields of ether product.

Detailed Description of the Invention Heterogenous sulfonated organic resin catalysts, like other catalysts, are effective in changing the rate of reaction. This occurs essentially by lowering the activation energy of the rate determining step of the catalyzed chemical reaction. Thus chemical processes that would ordinarily be unduly slow and inefficient for commercial production can, with the addition of a functional catalyst, become industrially practicable. Catalysts find particular utility in chemical reactions that are exothermic or endothermic, and in those reactions in which the yields cannot be readily enhanced by higher pressures or temperatures because of concomitant production of unwanted chemical by-products.

In one embodiment of the present invention heterogenous sulfonic acid substituted organic resin catalysts are selected to shift the thermόdynamic equilibrium position of an acid catalyzed reaction between an olefin and an alcohol to favor higher product yields. Applicant has found that such can be accomplished by increasing the level of sulfonic acid substitution in the resin catalyst to a level of at least 1.2 sulfonyl substituents per aromatic ring system in the resin structure. Further, while the rate of a catalyzed chemical reaction is generally proportional to the number of active catalytic sites, it has also been found in accordance with this invention that increases in sulfonic acid substitution level of the catalyst results in a seemingly disproportionately high increase in reaction rate. The higher reaction rate permits the use of shorter columns in continuous flow chemical reactors and would especially benefit those reactions in which it is desirable to minimize the residence time of reactant species in the catalyst environment.

Sulfonic acid substituted ion exchange resins have been used commercially in many hydrocarbon conversions including isomerizations, polymerizations, aromatic alkylations, etherifications, esterifications and epoxidations. The efficacy of methyl tert-butyl ether (MTBE) as a gasoline additive has focused much attention on new methodologies for efficient acid catalyzed ether-forming reactions between olefins and alcohols, particularly methanol and isobutene. Art-recognized limitations on conversion efficiency in single stage reactors has focused research and development efforts on the more complex, capital-intensive multi-stage etherification reactions, typically involving intermediate distillation steps to separate ether product from reactant species and thereby shift the equilibrium and the conversion efficiency to favor the ether product. In contrast to that approach, the present invention allows use of the less capital-cost intensive single stage chemical reactor. Yield enhancement in accordance with the present invention is achieved not by physical removal of the product ether from the bulk-phase reaction mixture, but instead by modifying the heterogenous catalyst presumably to enhance its affinity for, and thus its sorption of at least one of the reactant species (i.e., relative to reaction product) . This approach to yield enhancement of reversible chemical reactions conducted in the presence of a heterogenous catalyst is without precedent in the art. Traditional thinking is that the major factor contributing to the thermodynamic equilibrium of a reversible chemical reaction are pressure, temperature, reactant vs. product concentrations, and reaction medium, if any, other than the reactants themselves. It is accepted dogma as well that while catalysts typically do affect the rate of reaction, all other conditions being the same, a catalyst would not affect the reaction equilibrium constant. Yet Applicant has found that if the catalyst can be selected or modified to increase its affinity toward, i.e., propensity for sorption of, at least one of the reactant species, a measurable shift in equilibrium toward increased product yield can be realized, particularly where the catalyzed reaction is conducted at near equilibrium conditions and where the reactants are utilized in near stoichiometric proportions in a single stage chemical reactor. lⁿ accordance with the present invention, therefore, the equilibrium constant of a reversible chemical reaction occurring between a first chemical species and a second chemical species in the presence of an ion exchange resin catalyst can be shifted so that increased product yields can be realized even in a single stage chemical reactor when operated at near equilibrium conditions. The yield enhancement is achieved by selecting a catalyst that exhibits increased affinity for and sorption of at least one of the reactant species.

Thus, for example, in the reversible acid catalyzed reaction between an olefin and an alcohol to provide an ether, the catalyst can be chemically modified, for example, to increase its hydrophilicity and therefore its affinity for the more hydrophilic alcohol reactant species. The effect is to passively shift the equilibrium to favor the production of the less hydrophilic ether product. Again, in contrast to the principles underlying the operation of the more capital-intensive multiple stage chemical reactors, or distilling column chemical reactors in which the reaction product(s) is(are) continuously removed by distillation to drive the reaction to completion, the present invention relies instead on the reactant affinity/sorption of the catalyst resulting in a localized concentration excess of the sorbed reactant relative to the bulk-phase concentration of that reactant. The increase in concentration of the adsorbed reactant localized on the catalyst has the effect of shifting the equilibrium constant to favor the reaction product. Thus, the reaction equilibrium constant is shifted in accordance with the present invention without any substantial change in the concentration of the reactants in the bulk-phase. Instead there is a reaction-driving elevated, localized concentration of the reactant species in the immediate vicinity of the more hydrophilic organic resin catalyst. The result is a higher yield of the conversion product than would normally be expected given the bulk-phase concentrations of the reactant species.

In an continuous flow chemical reactor, in which a packed bed of ion exchange resin is used to catalyze reversible chemical reactions such as the etherification mentioned above, the selection of a catalyst with the desired affinities toward at least one of the reactant species to change the catalyst surface localized concentration of such reactant species compared to its concentration in the bulk-phase can have significant effects on product yield. Similarly, in a batch process, the addition of large amounts of a catalyst can increase the product yield to levels greater than expected for the bulk solution concentrations of reactant species. Plug flow reactors, which are equivalent to packed catalytic bed continuous flow reactors with an indefinite residence time for the reaction mixture, and recycle reactors are exemplary of circulating flow batch-type process chemical reactors suitable for carrying out the present methodology.

The present invention involves enhancing the differential sorptive properties of a heterogenous resin catalyst with respect to a reacting chemical species in a bulk fluid phase. The heterogenous catalyst is designed or modified to exhibit a preferential affinity for one or more reactant species in the bulk-phase to provide selective sorption of said reactant species resulting in an increase in the concentration of that reactant species localized at the reactive catalyst surface. Specifically with respect to sulfonic acid substituted ion exchange resin catalysts, it has been found that increasing the number of sulfonyl groups bonded to the aromatic ring systems in the polymer support to a level of at least 1.2 sulfonic acid groups per aromatic ring system in the resin structure is effective to promote the desired differential sorption by enhancing the affinity of the catalyst surface for the more polar hydrophilic reactant species in the bulk-phase. Thus, where sulfonic acid substituted ion exchange resins having elevated levels of sulfonic acid substituents, preferably about 1.2 to about 1.8 sulfonyl groups for each aromatic ring system in the polymer structure, are used as the catalysts in the reaction of an olefin, for example, a C-2 to a C-4 olefin and a lower alkanol such as methanol or ethanol, the catalyst exhibits a preferential affinity for the more polar reactant alcohol species. It is believed that the hydrophilic nature of the surface and the presence of a hydrophilic alcohol species in the bulk-phase facilitates release or displacement of the ether product from the catalyst surface (where it is formed) into the bulk-phase, thereby effectively lowering the concentration of the reaction product at the reactive surface of the resin catalyst. The overall effect is the shift of the equilibrium to favor product formation. A slight excess of the more hydrophilic reactant species, for example 1.05 moles of alcohol for each 1.0 mole of olefin, has been found to provide the highest product yields.

For economic reasons, the more highly sulfonated ion exchange resins utilized in accordance with a preferred embodiment of this invention is best used to catalyze those reversible chemical reactions which have an equilibrium constant that highly favors product formation. Thus, if a reversible reaction has an equilibrium constant such that there is provided a 10% conversion yield, a 13% conversion yield, using a more highly sulfonated resin catalyst will not necessarily allow the savings in production costs contemplated by this invention, principally because multiple reactors and intermediate distillation or separation steps will still be necessary to produce a product of adequate purity. On the other hand, if a reversible acid catalyzed chemical reaction has a normal conversion yield of 97%, a 3% shift in the equilibrium favoring product formation could allow a one-step chemical reaction to produce a product substantially free from reactant species, without costly intermediate separation/distillation steps.

The production of methyl tert-butyl ether from reactant species isobutene and methanol is one example of a high yielding reversible chemical reaction; typical isobutene conversion under industrial reaction conditions is about 90-95% in a single stage chemical reactor. The calculated theoretical limit of isobutene conversion for that reaction is about 96%. See Columbo, et al., Ind. Enσ. Chem. Fundam. 1983, 22., 219-223. That calculation has been corroborated by others in the ^• literature. However, if methyl tert-butyl ether is to be economically produced on the scale contemplated by its use as an octane enhancer for automotive gasolines, even very small increases in yields in the commercial process can provide significant commercial advantages. Of course, higher yields are achievable in the more capital intensive multi-stage reactors having intermediate distillation steps. For example, a first reactor operating at about 70°C and having about a 90% isobutene conversion, can be used to feed a second polishing reactor operating at 40°C to achieve a final conversion level of about 98%. Alternative processes in which a reactor is divided into several regions having different pressures and reaction temperatures has also been used to increase the conversion yield by simultaneously reacting isobutene and methanol and distilling the methyl tert-butyl ether from the bulk-phase. The efficiency of such methods is based on the continuous removal of the product ether from the reaction environment, resulting in a shift in the equilibrium to favor product formation.

In accordance with the present invention, a sulfonic acid substituted ion exchange resin having at least 1.2 sulfonic acid groups, more preferably about

1.2 to about 1.8 sulfonic acid groups, for each aromatic ring system in the resin structure can be used in single stage chemical reactors for the reaction between isobutene and methanol to achieve an isobutene conversion yield of greater than 98%. No secondary chemical reactors or complex distillation reactors are necessary for the production of commercially usable methyl tert-butyl ether. The reactants isobutene and methanol are mixed in about stoichiometric quantities, with methanol most preferably in a slight excess (1.05 moles of methanol to each 1.0 mole of isobutene) and allowed to contact a partially disulfonated ion exchange resin catalyst having at least about 1.2 sulfonyl groups per aromatic ring system (in the resin structure), in a single stage chemical reactor, operated at near equilibrium conditions. If the initial temperature of the reactants is between about 55°C and 65°C and the maximum temperature of the chemical reactor is not allowed to exceed about 75°C, an isobutene to methyl tert-butyl ether conversion of at least 98% can be obtained. Comparable results are anticipated when the present methodology is applied to reactions of isobutene or other olefins with alcohols such as methanol, ethanol, isopropanol, butanol, isobutynol, 2-butanol, isopentanol, hexanols, heptanols, glycerin, ethylene glycol, or propylene glycol.

Most preferred embodiments of this invention comprise reacting isobutene with methanol or ethanol in the presence of highly sulfonated resin catalysts to form respectively methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE) .

Pure isobutene or isobutene as a component of a stream of non-reacting hydrocarbon species can be used in the present invention. A particularly useful mixture having an isobutene component that is widely available for industrial purposes is a C-4 hydrocarbon stream composed mainly of isobutene, isobutane, n-butane, and minor amounts of propane and n-pentane. Under typical reaction conditions for an acid catalyzed etherification, isobutene is the only reactive species; the other constituents of the C-4 stream are inert under such conditions.

Isobutene is present in amounts which can range from about 1% to about 100%, more typically from about 10% to about 50% by weight of the C-4 hydrocarbon stream with the balance of the C-4 stream being isobutane, n-butane and possibly other alkanes. A typical reaction feed mixture used in the testing procedure for the present invention contained an unreacted 10.7% by weight methanol in a C-4 hydrocarbon mixture of 17.78% isobutene by weight, 34.92% isobutane by weight, 35.8% n-butane by weight, and 0.4% by weight of both n-pentane and propane. The molar ratio of isobutene to methanol is about 1:1.05. The heterogenous organic catalyst used in the preliminary evaluation of the present invention was a macroreticular, partially disulfonated copolymer of styrene and divinylbenzene, designated Amberlyst® 35 (Rohm and Haas Co.). Each aromatic ring system in the resin structure has an average of between about 1.2 and 1.8 sulfonyl functional groups, and an ion exchange capacity of 2.1 meq/ml of catalyst. Comparative tests were made to Amberlyst® 15, a commonly used macroreticular copolymer of styrene and divinylbenzene in which each aromatic nucleus is essentially mono-substituted, i.e, having a single sulfonyl group, and having an ion exchange capacity of about 1.8 meq/ml of material.

Tests were conducted to determine and compare the percentage of isobutene conversion in a reaction feed mixture as described above reacted in the presence of Amberlyst 15 resin and Amberlyst 35 resin, respectively. The tests were carried out in a 5-foot long jacketed reactor operated at near equilibrium conditions at varying temperatures, with approximately the same flow rate and maximal temperature. The jacket temperature was the same as the feed temperature unless otherwise noted. As indicated by the following data, use of the partially disulfonated Amberlyst 35 resin increases the isobutene conversion efficiency as compared to that obtained with Amberlyst 15 resin, where the respective reactions are run under near equilibrium conditions.

n nv rsi n ff

*Feed Temp. = 65°C but Wall Temp. = 70°C.

As indicated by the data, the isobutene conversion efficiency is very sensitive to temperature, with maximal yields occurring when the initial temperature of the reactants is in the range between about 55°C and about 65°C. Because the formation of methyl tert butyl ether is an exothermic reaction, the excess heat generated is preferably removed by a cooling system. A water or oil cooled jacketed chemical reactor capable of keeping the maximal temperature in the reactor below about 75°C is preferred.

An enhancement of the reaction rate is also realized by the use of partially disulfonated Amberlyst 35 catalyst. As the data below demonstrates, the actual increase in isobutene conversion rate at low temperature conditions is greatly enhanced.

Low Temperature Isobutene Conversion Efficiency

Feed Jacket Inlet T_max Flow IB ID τ°C T°C °C fα/min) conv(%)

Amberlyst 15 CT2286 40 40 40 50.9 12.9

Amberlyst 35 CG9399 40 40 44 50.9 23.4

The isobutene conversion efficiency is increased 82%, while the corresponding increase in catalytic sites is only 23%.

Claims

I claim:

1. A method for increasing the yield of an ether produced by reaction of approximately stoichiometric proportions of an olefin and an alcohol in the presence of a sulfonic acid substituted ion-exchange resin in a single stage chemical reactor operated at near equilibrium conditions, said method comprising increasing the affinity of the catalyst for the alcohol by increasing the level of sulfonic acid substitution on the resin to a level of at least 1.2 sulfonyl substituents per aromatic ring system in the resin structure.

2. The method of claim 1 wherein the olefin is isobutene.

3. The method of claim 2 wherein the alcohol is methanol.

4. The method of claim 3 wherein the molar ratio of isobutene to methanol is about 1 mole of isobutene to about 1.05 moles of methanol.

5. The method of claim 4 wherein the reaction is conducted at a temperature between about 55°C and about 75°C.

6. The method of claim 5 wherein the single stage chemical reactor is a packed bed continuous flow chemical reactor.

7. The method of claim 1 wherein the chemical reactor is a plug flow chemical reactor.

8. The method of claim 5 wherein the isobutene is a component of a mixed C-4 hydrocarbon stream.

9. A method for- the production of methyl tert-butyl ether by the reaction of isobutene and methanol at about 98% or greater conversion efficiency in a single stage chemical reactor, said method consisting essentially of conducting the reaction utilizing near stoichiometric proportions of isobutene and methanol at a temperature between about 55 and about 75°C in the presence of a sulfonic acid substituted aromatic resin catalyst having at least 1.2 sulfonyl 0 substituents for each aromatic ring system in said resin,

10. The method of claim 9 wherein the molar ratio of isobutene to methanol is about 1:1.05 and the sulfonic acid substituted resin catalyst has about 1.2 to about 1.8 sulfonyl substituents for each aromatic ring structure in the resin.

11. A method for shifting the equilibrium position of a reversible chemical reaction occurring between a first chemical species and a second chemical species in the presence of an ion-exchange resin o catalyst in a single stage chemical reactor operated at near equilibrium conditions to enhance reaction product yield, said method comprising the step of increasing sorption of at least one of the chemical species on the surface of the catalyst.

12. The method of claim 11 wherein the first chemical species is a hydrophilic alcohol, the ion-exchange resin catalyst is a sulfonic acid substituted resin, and the resin catalyst is further sulfonated to enhance its sorption of the hydrophilic 0 alcohol species.

13. The method of claim 12 wherein the chemical reaction is an acid-catalyzed esterification reaction, .

14. The method of claim 12 wherein the c chemical reaction is an acid-catalyzed ether-forming reaction.

15. The method of claim 14 wherein the sulfonic acid substituted resin is selected to have about 1.2 to about 1.8 sulfonyl groups per aromatic ring ₀ system in the resin.

16. A method for increasing the reaction rate of an ether-forming chemical reaction between an olefin and an alcohol catalyzed by a sulfonic acid substituted aromatic ion-exchange resin, said method comprising tlie ₅ step of increasing the level of sulfonic acid substitution on the resin to a level between about 1.2 to about 1.8 sulfonyl groups per aromatic ring system.

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