An Efficient Alternative Reductant for O2 Methanol

Besides hydrogen, other reductants for O2 in the liquid-phase epoxidation of propene include carbon monoxide, aldehydes, alcohols and other organic compounds. The reaction proceeds very efficiently with methanol as the reductant, in the presence of Pd and the Ti-Al-MCM-22 catalyst or Pd and peroxo-polyoxometalate catalysts the latter have been intercalated inside layered double hydroxides to make them heterogeneous [29e,h]. A propene conversion of 47%, with 91.5% selectivity for PO, was obtained at 80°C [29h]. 

The high activity of this heterogeneous system stemmed from the highly active Pd particles for effectively generating peroxy intermediates in situ, the good epoxidation 

It has been proposed that the methanol molecule reacts with the oxygen molecule rather than with Pd, to form a peroxy intermediate HOCH2OOH, which regenerates the peroxoheteropoly-compound species that is active in propene epoxidation [29i]. 

As the peroxy intermediate is not stable and eventually decomposes to COx and H2O, a part of the methanol is co-oxidized. Moreover, HP is probably also formed in situ in the catalytic system during the reaction and plays an important role in regenerating the active species. 

---