Fine Tuning of the Acid/Base Properties of Pt/Alumina for Enantioselective Reactions

The acid/base properties of Pt/Al₂O₃ were modified by doping the support with SiO₂ or Cs₂O and the catalysts were prepared by flame spray pyrolysis in a rapid, single step process with excellent reproducibility. The catalysts contained 4.7 wt.% Pt and the support composition varied in the range 0-100 wt.% SiO₂ or 0-10 wt.% Cs₂O. Up to 50 wt.% SiO₂ the BET surface areas could be kept in a narrow range (141±15 m²g^-1), while higher SiO₂ content increased the surface area and diminished the crystallinity of the support. CO chemisorption and XRD showed a positive correlation between the SiO₂ content and the size of the dominantly spherical Pt particles. The catalysts containing 22-30 wt.% SiO₂ were the most acidic as evidenced by TPD of NH₃. In the hydrogenation of acetophenone the SiO₂-doped catalysts were highly active and the selectivity to the hydrogenolysis byproducts increased with increasing SiO₂-content by more than 20-fold. This is a confirmation of the effect of support acidity on the properties of the Pt-H system. On the other hand, addition of Cs₂O barely affected the Pt particle size and TPD of CO₂ confirmed the increase of surface basicity with increasing Cs₂O content. Cs₂O was probably enriched at the Al₂O₃ surface and strongly influenced the acid/base properties and thus the catalytic performance already at very low concentrations. Only 0.25 wt.% Cs₂O blocked the hydrogenolysis of 1-phenylethanol to ethylbenzene.

Additonally the influence of support acidity and basicity was investigated in the enantioselective hydrogenation of methyl benzoylformate and ketopantolactone on cinchonidine–modified catalysts. Addition of SiO₂ improved the enantioselectivity with a maximum at 30 wt.%. Enantioselectivity correlated well with the acidity of the catalysts characterized by TPD of NH3 and with the selectivity to hydrogenolysis of methyl cyclohexyl ketone to ethylcyclohexane. On the contrary, doping with Cs was detrimental to the formation of the (R)-alcohols and the drop in enantioselectivity could unambiguously be attributed to the basicity of the support characterized by TPD of CO₂. The practical importance of our findings is demonstrated by the best ee (94 ± 0.5%) achieved so far in the industrially relevant hydrogenation of ketopantolactone to (R)-pantolactone.