Titania-Silica Catalysts for Lactide Production from Renewable Alkyl Lactates: Structure-Activity Relations

Different Ti-Si catalysts, viz. TiO2 supported on amorphous SiO2 or Si-MCM-41, TiO2-SiO2 xerogels, and Ti zeolites (TS-1 and Ti-beta), were compared in terms of activity and selectivity for the direct conversion of methyl lactate to lactide in the gas phase. Except for Ti-beta, all catalysts exhibit a high lactide selectivity of 88-92% at conversions below 50%. From DR UV-vis spectroscopy, it is evidenced that the catalytic activity of tetrahedral TiO4 sites is higher than those of polymerized TiO5 or the octahedral TiO6 counterparts, irrespective of the catalyst structure, an analysis supported by ToF-SIMS measurements. A kinetic analysis shows that the catalytic activity is proportional to the number of vacant sites on the catalyst surface. Thus, the activity increase observed for tetrahedral TiO4 sites may be attributed to an increased number of vacant sites (e.g., two for TiO4, zero for TiO6). Lactide productivity thus highly benefits from an increased dispersion of Ti sites on the catalyst surface and could be increased by a factor of 2.5 (up to 10 g(LD) g(cat)(-1) h(-1)) when TiO2 is dispersed on a Si-MCM-41 support, with higher surface areas in comparison to amorphous SiO2 gels.