ALUMINA-SUPPORTED BIMETALLICS OF PALLADIUM ALLOYED WITH GERMANIUM, TIN, LEAD, OR ANTIMONY FROM ORGANOMETALLIC PRECURSORS .2. GAS-PHASE HYDROGENATION OF 2-METHYL-1-BUTEN-3-YNE (VALYLENE) AND 2-METHYL-1,3-BUTADIENE (ISOPRENE)

A series of PdGe, PdSb, PdSn, and PdPb/α-Al2O3 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 (TONV) and isoprene (TONI) hydrogenation were not modified by addition to the 0.09 wt% Pd/α-Al2O3 base catalyst of Ge, Sb, Sn, or Pb, up to 0.1 wt% (at 293 K: TONV = 20 s-1 and TONI = 33 s-1). Pd/α-Al2O3 reduced at 773 K was severely sintered (dTEM increased from 2.8 run to 12.4 nm) and TONV and TONI 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 TONV (by a factor of 2) and TONI (by a factor of 2-5) was observed. At high conversion, both the selectivity to isoprene (SI) in valylene hydrogenation, and to olefins (SO) in isoprene hydrogenation on Pd/α-Al2O3 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-Al2O3. 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. © 1991.