Catalytic conversion of glycerol to acrolein over modified molecular sieves: Activity and deactivation studies

The catalytic conversion of glycerol to acrolein by liquid-phase dehydration over molecular sieves catalysts was studied. In order to understand the role of the acidity, structure and porosity, five structures were synthesized and evaluated. The lower the Si/Al ratio, the higher the activity; large pore molecular sieves, as HY showed high performance (conversion = 89.0%; selectivity to acrolein = 99.5%), while siliceous molecular sieves such as SBA-15 possessing weak acidity, exhibited low conversion (40.6%) and decreased selectivity to acrolein (84.0%). The structure and the acidity govern the selectivity to acrolein and hydroxyacetone, the latter being a by-product mainly dependent on the amount of weak and medium strengths acidic sites, as in the case of Hβ. The activity has almost the same order of the acidity: HY > Hβ > Mor > SBA-15 > ZSM-23. Modifications of the most acidic and active catalysts by silanation (CVD) or Pt incorporation did not result in an enhanced conversion. The studies of reusability and the nature of the coke deposits indicated that heavy polycondensed and cyclic C6 compounds resulting from the reaction between glycerol and acrolein block the pores and the acidic sites. The blockage was the main cause of deactivation, whereas acidic site poisoning led to a less extensive deactivation of the surface active sites. From these results, it was possible to establish acidity–activity–deactivation relationships that allow to explain the behaviour of the catalysts.