Carbocations on surfaces

Carbocations on Surfaces Formation of Bicyclobutonium Cation via Ionization of Cyclopropylcarbinyl Chloride over NaY Zeolite... 

A book containing authoritative reviews of many aspects of carbocation chemistry has been published.1 These reviews are up-to-date to the end of 1991, as the book is the result of a symposium held in honour of George Olah in 1992 it features many of the world s leading carbocation chemists. At least one book review of it has also appeared.2 Olah has an introductory chapter concerning his decades-long search for stable long-lived carbocations in superacid media,3 and other review chapters include ones on carbocations at surfaces and interfaces,4 on the X-ray structural analyses that have been performed on many carbocation salts and related compounds in recent years,5 and on natural product chemistry in superacids.6 Other review chapters will be referred to below, as appropriate. 

On the contrary, similar EHT calculations of possible intermediate structures in the isomerization reaction (also exemplified by propylene) (91) predicted the stabilization of the carbocation on a surface that contradicted the experimental data (92). This was likely due to the limitations of EHT as applied to the total energy computations, especially of the charged forms. Essentially the same conclusion, that EHT overestimated the stability of the cationic form of propylene, was drawn by Schliebs et al. (93), who compared different mechanisms of the double-bond migration in zeolites using the EHT calculations for the cluster composed of four Si(Al)04 tetrahedra. 

When the reaction was halted at a fixed TOS of 3.0 h, the integrated product yields were higher compared to those obtained from the fresh catalyst, the maximum product yields declined only slightly per run, and the overall product quality inproved. Over 24 reaction cycles (see Table V), the TMP selectivity was relatively stable while dimethylhexane (DMH) selectivity decreased slightly. This provided an improvement in the TMP/DMH ratio from ca. 3.8 to ca. 5.0. This behavior, which was consistently observed, was mostly the result of higher product yields in the first hour of reaction likely due to the increased abundance of tert-butyl carbocations on the catalyst surface after regeneration. 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

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