Suppressing Pt sintering for designing alkane dehydrogenation catalyst with exceptionally high selectivity and regenerability

Abstract

Suppressing irreversible catalyst deactivation is critical in heterogeneous catalysis. In particular, deactivation via sintering of active sites is a significant issue for reactions involving harsh reaction/regeneration conditions. In this thesis, two different metal-encapsulated catalyst systems were presented as light alkane dehydrogenation catalysts with exceptionally high activity, selectivity, and long-term stability by markedly suppressing Pt sintering under harsh conditions (reaction/regeneration at >823 K). First catalyst system is spillover-based model catalyst where Pt is encapsulated in a dense aluminosilicate matrix with controlled diffusional properties and surface hydroxyl concentrations. The catalytic investigation and theoretical modeling showed that surface hydroxyls, presumably $Br \phi nsted$ acids, are crucial for utilizing the catalytic functions of hydrogen spillover on the aluminosilicate surface. The catalyst showed remarkable activities in hydro-/dehydrogenation, but virtually no activity for hydrogenolysis, which resulted in high propylene selectivity in propane dehydrogenation reaction because the undesired hydrogenolysis pathway producing light hydrocarbons of low value (methane and ethane) is greatly suppressed. Secondly, $PtGa/ \gamma -Al_2O_3$ was investigated as selective and stable light alkane dehydrogenation catalyst. To stabilize Pt, physical and chemical stabilization strategies were synergistically combined. For the former, Pt was introduced during the synthesis of $Al_2O_3$ via sol-gel chemistry, which can increase the interfacial contact between Pt and $\gamma -Al_2O_3$ due to the partial entrapment of Pt in $\gamma -Al_2O_3$ . For the latter, atomically dispersed Ce was doped on $\gamma Al_2O_3$ , which can stabilize Pt via strong Pt-O-Ce interaction. Because of effective Pt stabilization, the catalyst showed remarkably steady activity and selectivity behaviors over the repeated reaction cycles although the catalyst is regenerated via simple oxidation rather than industrially used oxychlorination. The Pt stabilization strategies reported in this work can be applied to other metal-catalyzed reactions that involve severe reaction/regeneration conditions.