Alumina-supported bimetallics of palladium alloyed with germanium, tin, lead, or antimony from organometallic precursors II. gas-phase hydrogenation of 2-methyl-1-buten-3-yne (valylene) and 2-methyl-1,3-butadiene (isoprene)

Abstract

A series of PdGe, PdSb, PdSn, and PdPb/α-Al 2O 3 catalysts, prepared by the controlled surface reaction (CSR) technique from organometallic precursors have been tested in the gas-phase hydrogenation of 2-methyl-1-buten-3-yne (valylene) and 2-methyl-1,3-butadiene (isoprene). On catalysts reduced at 573 K, the turnover frequencies for valylene (TON V) and isoprene (TON I) hydrogenation were not modified by addition to the 0.09 wt% Pd/α-Al 2O 3 base catalyst of Ge, Sb, Sn, or Pb, up to 0.1 wt% (at 293 K: TON V = 20 s −1 and TON I = 33 s −1). Pd/α-Al 2O 3 reduced at 773 K was severely sintered ( d TEM increased from 2.8 run to 12.4 nm) and TON V and TON I at 293 K increased to 190 and 283 s −1, respectively, as a result of an apparent crystal size effect: the reactants adsorb more strongly on the smaller Pd particles. Upon alloying with Ge, Sb, Sn, or Pb and a subsequent reduction at 773 K, a modest decrease of both TON V (by a factor of 2) and TON I (by a factor of 2–5) was observed. At high conversion, both the selectivity to isoprene (S I) in valylene hydrogenation, and to olefins ( S O ) in isoprene hydrogenation on Pd/α-Al 2O 3 were improved upon alloying with Sb, Sn, or Pb (Ge had no effect). In addition, the isomerization of 2-methyl-1-butene and 3-methyl-1-butene (double bond migration reaction) during isoprene hydrogenation was partially suppressed on PdSn and PdPb/α-Al 2O 3. The improvement in selectivities was interpreted in terms of a change in the relative adsorption strength of the reactants and intermediate products over the new bimetallic sites.