ally a dimer of the carbene, the Illinois chemist says. The Illinois chemists postulate that the nitrene forms first, then hybridizes before it can react with anything. The carbenelike hybrid probably adds across the CE=C bond of the acetylenic azide to give a cyclopropenyl azide. This compound loses N 2 and ringopens to give the stilbene. The phenylethynylazide represents the first recorded example of an acetylenic azide, Dr. Boyer says. He believes, however, that this sensitive compound was probably produced a few years ago during an unpublished study by Illinois Institute of Technology's Dr. S. I. Miller and his coworker Dr. H. Taniguchi (now at Kyushu University, Japan). While investigating the nucleophilic attack of azide ion on phenylethynylbromide, these workers also obtained dicyanostilbene.
Reforming n-butylbenzene over supported platinum catalysts results in many products
Catalyst type affects reforming reactions 1 RA
ACS NAT,0NAL MEET,NG
1 0T*TH
Petroleum Chemistry
Reactions over platinum catalysts in the process of hydrocarbon reforming are more complex and varied than has been thought. In work with reforming catalysts of platinum on various supports, Dr. Sigmund M. Csicsery of Chevron Research Co. (Richmond, Calif.) finds that platinum helps to dehydrocyclize and isomerize hydrocarbons more than expected during reforming. A basic objective of Dr. Csicsery's work is to clarify the roles of metal and acid components of reforming catalysts. Some of the catalysts which he has used are commercially available. Others he prepared by depositing platinum on silica gel or on silicaalumina, he told the Symposium on New Chemistry of the Hydrogen Processing of Petroleum. A silicaalumina catalyst was used for comparison with platinum catalysts. Dr. Csicsery uses n-butylbenzene as the reactant because it has proved a good model compound to help understand reaction mechanisms. With data from the reactions, Dr. Csicsery predicts that specific catalysts can be selected for specific uses—for example, dehydrogenation or hydrogenolysis. The studies bring out several specific features of the overall reaction. The contribution of each reaction can be determined from the patterns of n-butylbenzene conversion. All the reactions, however, proceed simultaneously to some degree.
Information on the reaction products suggests some of the processes which occur. At least two different dehydrocyclization processes are found. Cyclization catalyzed by acidic catalysts alone is a self-alkylation process involving carbonium ion intermediates. Whether five- or sixmembered rings form next to the benzene ring depends on the stability of the carbonium ion intermediate. With n-butylbenzene, acid-catalyzed cyclization produces five-membered rings. A different mechanism is followed for cyclization catalyzed by platinum. Both five- (methylindan) and sixmembered (naphthalene) rings are formed as products in this reaction. Between 371° and 427° C , at atmospheric pressure, and a hydrogento-hydrocarbon ratio of three, metalcatalyzed rates of cyclization to fiveand six-membered rings are about the same. Two mechanisms are involved in isomerization. One, with a neutral platinum catalyst, goes first to cyclic intermediates. Another involves a C1 to C-2 shift of methyl group, with platinum on silica-alumina acting by an acidic mechanism. n-Butylbenzene is dehydrogenated to phenylbutenes, which—after accepting protons from the silica-alumina—form carbonium ions. Rearrangement of the carbo-
nium ions results in isomerized products. Fragmentation of n-butylbenzene goes either by cracking or by hydrogenolysis. Without platinum, the reaction is exclusively cracking. Cracking also is selective, Dr. Csicsery says. The bond between the ring and side chain breaks 100 times more frequently than all other bonds combined. Hydrogenolysis occurs with platinum on neutral supports and breaks the different bonds in the side chains with about equal probability. Over platinum on acidic supports, both cracking and hydrogenolysis occur. Practically all olefins hydrogenate. Dehydrogenation is equilibrium limited over platinum on silica or on alumina supports. The rate for the dehydrogenation reaction is substantially lower over platinum on silicaalumina than it is over the other supported platinum catalysts. Without platinum no dehydrogenation appears to occur. Dr. Csicsery's data also reveal that specific activities of platinum and silica-alumina decrease when platinum is impregnated on silica-alumina. This mutual activity loss suggests an interaction between platinum and acid sites of the support. Similar interactions have been noted for nickel and silica-alumina by others. SEPT. 25, 1967 C&EN
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