(514g) Spectroscopic Analysis of Coke Precursors on Acid Catalysts

Spectroscopic Analysis of Coke Precursors on Acid Catalysts

Matthew J.
Wulfers¹, Carine Villa,² Genka Tzolova-Müller,² Friederike
C. Jentoft^{1 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

*fcjentoft@umass.edu

Much understanding of
hydrocarbon chemistry on surfaces has been gained by drawing analogies to
Olah's superacid chemistry [1], 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 [4]. 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 [5]. 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.

[1]   G.A. Olah,
Angew. Chem. Int. Ed. 34 (1995) 1393-1405.

[2]   Z.N. Ma, Y.
Zou, W.M. Hua, H.Y. He, Z. Gao, Top. Catal. 35 (2005) 141-153.

[3]   S. Ilias,
A. Bhan, ACS Catalysis 3 (2013) 18-31.

[4]   M. Hino, S.
Kobayashi, K. Arata, J. Am. Chem. Soc. 101 (1979) 6439-6441.

[5]   M.J.
Wulfers, F.C. Jentoft, J. Catal. 307 (2013) 204-213.