MODIFICATION OF SUPPORTED PT CATALYSTS BY PREADSORBED PHOSPHINES - ENHANCED SELECTIVITY IN THE OXIDATION OF L-SORBOSE

The influence of phosphine and aminophosphine modifiers on the perform ance of 5 wt.-% Pt/C and Pt/alumina catalysts was studied in the parti al oxidation of L-sorbose with molecular oxygen. The molar ratio of mo difier to surface Pt atoms (Pt,) was varied between 0.1 and 1.0. In ge neral, higher modifier/Pt-s ratios resulted in higher selectivity to 2 -keto-L-gulonic acid and lower reaction rate. The activity and selecti vity of Pt strongly depended on the chemical structure of phosphine. T ributhylphosphine in trace amounts (modifier: Pt-s=0.1 mol/mol; modifi er: sorbose = 600 ppm) proved to be an outstanding modifier, which imp roved the selectivity of Pt/C by 28-37% in the whole investigated conv ersion range. Monitoring the catalyst potential during the reaction re vealed that the oxidation state of Pt decreased by promotion, which is an indication that some side reactions leading to catalyst poisoning are suppressed.