Spectroscopic Analysis of Coke Precursors on Acid Catalysts
Matthew J. Wulfers1, Carine Villa,2 Genka Tzolova-Müller,2 Friederike C. Jentoft1 3 *
1School of Chemical, Biological & Materials Engineering, University of Oklahoma
100 East Boyd Street, Norman, OK 73019-1004, USA
2Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin, Germany
3present address: Department of Chemical Engineering, University of Massachusetts
686 North Pleasant Street, 159 Goessmann Laboratory, Amherst, MA 01003-9303, USA
Much understanding of hydrocarbon chemistry on surfaces has been gained by drawing analogies to Olah's superacid chemistry , but the mechanisms of many heterogeneously catalyzed reactions are still being debated, for example those of C4 skeletal isomerization or methanol-to-olefins conversion [2,3]. A recurring question concerns the existence and stability of carbenium ions and other surface species, which play a pivotal role as reaction intermediates or transition states, for desired as well as for undesired reactions. By using in situ IR and UV-vis spectroscopy, and solvent extractions (of spent catalysts), we compare the reactions of paraffins and olefins on the surfaces of sulfated zirconia and zeolite catalysts.
Both H-mordenite and sulfated zirconia are capable of catalyzing the conversion of n-butane to isobutane; however, the zeolite requires a reaction temperature of 250 °C or higher, whereas sulfated zirconia is active even at room temperature . This difference can, in part, be ascribed to the ability of the catalysts to dehydrogenate butane to butenes, whose presence triggers the isomerization. Significant olefin concentrations, which are associated with good dehydrogenation capability of the paraffin isomerization catalyst and low H2 partial pressures, almost inevitably result in oligomerization reactions and deactivation by coke formation. Time-resolved in situ UV-vis spectra show the multi-step conversion of olefins into cyclic hydrocarbons with extended π-systems . Their electronic spectra are surprisingly unaffected by the nature of the solid acid and are similar to those obtained in organic or acid solutions. Depending on their proton affinity and the presence or absence of a competing base, cyclic unsaturated species are neutral or protonated. A charged state implies that thermal desorption of these species is difficult. Low-molecular weight species on H-mordenite and sulfated zirconia are similar in nature. The growth of such species is confined by the pore size and shape of the zeolite, whereas on the more open surface of sulfated zirconia, dimerization via Diels-Alder chemistry appears possible.
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