KINETIC STUDY OF THE ISOMERIZATION OF n-BUTENES ON CHROMIA–ALUMINA

Abstract

hZIutnal i~lterconversions of the three ti-butenes on a chromia-alumina catalyst have bccn st~~diccl in a t e m p c r a t~~r e range between 210 and 2G0° C and in a pressure rangc from about 10 to about 100 mnl. Depenclence of initial rates on the initial pressures of the reactants was clctermi~~ed expcrimei~tallp. The initial rates of isolnerization could be, in each case, expressed ernpiricall); b?. a rate ecluation conforming to the Langmuir-I-linsl~el\\~ood form~rla. It was possible to explain thc experimental results by assunling the existence of three din'erent aclsorbecl species for the thrcc 71-butene iso~~lers and their surface intercor~versions as the rate-cleterrninir~g step.Catalytic isomerization of butenes has been extensively investigated in attempts to contribute to the understanding of the mechanism of the catalytic isomerization of olefins. I t is well known that the presence of hydrogen is necessary for the olefin isomerization on some ~netallic catalysts (1-G), while the reaction occurs in the absence of hydrogen on metal oxide, alkali, and acid catalysts (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Several kinetic studies have been carried out on inetallic catalysts and the mechanism of olefill isomerization has been discussed in conjunction with the mechanism of hydrogenation (2-3). On the other hand, relatively few kinetic studies of olefin iso~nerizations on oxide catalysts are avaiIable (12,13,18,21).Carbonium ions and carbanions have been postulated by many investigators as intermediates in the catalytic reactions of olefins ancl have been also recently invoked to explain the stereoselectivity in the catalytic isomerization of butenes (6, 15). I t is felt, ]lowever, that further lcinetic studies of these reactions, on catalytic materials of various types, are necessary, and the present work has, therefore, been carried out with the object of obtaining kinetic information on butene isomerization on a chromia-alumina catalyst. This catalyst is used commercially as a dehydl-o-isomerization catalyst. CatalystChrornia-alumina catalyst was prepared by mixing and grinding a l u m i n~~m hydroxide and chrorni~~m hydroxide, both of which were spearately precipitated by adding ammonium hydroxide to the solutions of the respective nitrates. The composition of the cataIyst was about 20% Cr?Os and 80% A1?03 by weight, as calculated from the loss on ignition of the two hydroxides. The catalyst was heated a t 500' C for 1 hour, a;lcl crushecl into pieces of convenient size. The sarne catalyst sample, weighing 0.0038 g, was used in all kinetic measurements. The surface area of the catalyst, determined by the B.E.T. method with nitrogen, -was 100.5 m2/g. Research grade butenes of Phillips Petroleum Co. were used as reactants. They \\.ere twice condensed in liouid nitrocen tram follo\ved bv evacuation, and were kept in a 3-1. reservoir. The irnpuritics in 1-butene -and cis-butene were negligibly small, but tra7bs-butene contained a small amount of cis-butenc, which \\.as subtracted ...

Isomerization of butene over a chromia–alumina catalyst

“…2 and 3). (2) The maximum found in the rate of disappearance of 1-butene over the same range of partial pressures (Fig. 4).…”

“…[ l ] gave an adequate fit of the 2-butene data with both A and B positive. Equations [I] and [2] failed to give an adequate fit of the 1-butene data as might be expected; eq. [3], although allowing for the shape of Fig.…”

“…Catalytic isomerization of butenes exhibits a high degree of stereoselectivity mhich is dependent upon the catalyst (2)(3)(4)(5)(6)(7)(8)(9), temperature, and conversiois (6,11). Foster and CvetanoviC (6) show that alkaline catalysts favor double bond migration, whereas typically acidic catalysts favor cistrans isomerization.…”