AVS 53rd International Symposium
    Surface Science Monday Sessions
       Session SS1-MoM

Paper SS1-MoM3
Double-Bond Isomerization in Cyclic Olefins Adsorbed on Platinum Surfaces.

Monday, November 13, 2006, 8:40 am, Room 2002

Session: Catalytic Chemistry of Hydrocarbons
Presenter: R. Morales, University of California, Riverside
Authors: R. Morales, University of California, Riverside
F. Zaera, University of California, Riverside
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The migration & cis-trans isomerization of C=C in olefins is central to many processes in oil refining and food processing.It has been well established that those reactions follow simple half hydrog. and dehydrog. steps according to the Horiuti-Polanyi mech.,but that still leaves a number of subtle issues associated with the stereo- & regiospecificity of the reactions involved.For instance,the branched alkyls that may form as intermediates after half hydrog. of substituted olefins may have beta-H in diff. environments,and elimination of those in the step that regenerates the olefin may lead to diff. products.Here we show that this is certainly the case with methyl cyclopentenes & methyl cyclohexenes,where the C=C may be in either exo and endo positions.We have studied the chemistry of those compounds on Pt(111) single-crystal surfaces under UHV by means of TPD & RAIRS.Those molecules were chosen because of the particular lability of their allylic H,and because they may offer significance hindrance towards normal olefin adsorp. and alkyl-alkene interconversion reactions.It was found that both the C5- and C6-cyclic molecules undergo dehydrog. in a stepwise manner,but with different kinetics depending on the position of the C=C.For instance,while methylene cycloalkanes decompose in four dehydrogenation stages,1-methyl-1-cycloalkenes display only three distinct decomposition stages.Also,the molecules with internal C=C proved more stable than those with the unsaturation in the external position,a fact that facilitates the hydrog. of methylene-C5 and methylene-C6 to the corresponding cycloalkanes and their isomerization to 1MC5= and 1MC6=,resp.In addition,it was shown that aromatization reactions,to form benzene or toluene,can be driven at higher temps.It was determined by using RAIRS that ring expansion for the 1MC5= and methylene cyclopentane does not take place via an allylic intermediate,but by the formation of a methylcyclopentadiene species instead.