Synergy effect between Zr and Cr active sites in binary CrZrOx or supported CrOx/LaZrOx: Consequences for catalyst activity, selectivity and durability in non-oxidative propane dehydrogenation

Abstract

Bulk CrZrOx, supported CrOx/LaZrOx, and CrOx/Al2O3 catalysts were prepared with Cr surface site density varying between 0.04 and 2.3 nm−2. They were characterized by BET, XRD, temperature-programmed reduction with H2, and NH3 temperature-programmed desorption. Their activity, selectivity, and durability in non-oxidative propane dehydrogenation (PDH) were determined under industrially relevant conditions. The catalysts on the basis of ZrO2 outperform their Al2O3-based counterparts in terms of PDH activity. This was explained by the presence of two catalytically active sites, i.e. CrOx and coordinatively unsaturated Zr4+ (Zrcus) sites. The former species promote formation of Zrcus under reaction conditions or upon reductive catalyst treatment.Practical relevance of CrZrOx and CrOx/LaZrOx catalysts was checked in a series of 10 PDH/regeneration cycles performed at 550 °C. The catalysts possessing up to 40 times lower chromium content performed superior to an analogue of commercial K-CrOx/Al2O3 with respect to space time yield of propene and the amount of propene formed within one PDH cycle. In terms of durability, bulk promoting of ZrO2 with Cr was found to be advantageous over simple deposition of CrOx on the surface of LaZrOx.