Subnanometric Pt-Ga alloy clusters confined within Silicalite-1 zeolite for n-hexane dehydrogenation

Dehydrogenation of light naphtha followed by catalytic cracking into high-value end products can meet the high demand in the world's petrochemical market. In this work, at attempting synthesis of novel catalysts for hexane dehydrogenation, a series of bimetallic Pt-Ga clusters encapsulated within Silicalite-1 (S-1) catalysts were synthesized using the ligand-protected direct H-2 reduction method. The reduced Pt-Ga@S-1-H catalysts comprised restricted ultra-fine Pt-Ga alloys with an average size 0.86-0.95 nm, mainly corresponding to the Pt-3 clusters with coordinate number 1.5-2.1. Ga doping significantly improved the catalytic stability, depending on the Ga content, which ranged from 0.02 to 0.27 wt%. As confirmed by the XPS and EXAFS analysis, after the Ga addition, electron transfer occurred between the Pt-Ga alloys and the skeleton oxygen in zeolite, stabilizing the Pt clusters on the zeolite, thereby inhibiting the growth of Pt crystals. During the n-hexane dehydrogenation, the 0.40Pt0.04 Ga@S-1-H catalyst exhibited a hexene selectivity of 87.1 %, achieving a hexene formation rate of 101.4 mol(hexene)center dot g(Pt)(-1)center dot h(-1) at 550 degrees C with a WHSV of 90 h(-1). After 1000 min of reaction, this best catalyst experienced only slight deactivation with a deactivation constant of 0.066 h(-1). The catalytic activity remained almost unchanged after consecutive five cycles of H-2 regeneration. DFT calculation using the synchronous structural models showed that the Gibbs free energy changes (-Delta G) for the first three steps of hexane dehydrogenation over the 0.4Pt0.04 Ga@S-1-H catalyst were much lower than those obtained on the 0.4Pt/S-1-H catalyst, suggesting that the introduction of Ga energetically favored the hexane dehydrogenation.