Elucidating the Nature of Active Sites and Fundamentals for their Creation in Zn-Containing ZrO₂–Based Catalysts for Nonoxidative Propane Dehydrogenation

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Abstract

Environmentally friendly and low-cost catalysts are required for large-scale nonoxidative dehydrogenation of propane to propene (PDH) to replace currently used CrOx- or Pt-based catalysts. This work introduces ZnO-containing ZrO2- or MZrOx-supported (M = Ce, La, Ti or Y) catalysts. The most active materials outperformed the state-of-the-art catalysts with supported CrOx, GaOx, ZnOx, or VOx species as well as bulk ZrO2-based catalysts without ZnO. The space–time yield of propene of 1.25 kgC3H6·kg–1cat·h–1 at a propane conversion of about 30% with a propene selectivity of 95% was obtained over Zn(4 wt %)/TiZrOx at 550 °C. For deriving key insights into the structure of active sites, reactivity, selectivity, and on-stream stability, the catalysts were characterized by XRD, HRTEM, EDX mapping, XPS, X-ray absorption, CO-TPR, CO2-TPD, NH3-TPD, pyridine-FTIR, operando UV–vis spectroscopy, Raman spectroscopy, TPO, and temporal analysis of products. In contrast with previous reports that used bulk ZrO2-based catalysts without ZnO, coordinatively unsaturated Zr cations are not the main active sites in the ZnO-containing catalysts. Supported ZnOx species were concluded to participate in the PDH reaction. The current X-ray absorption analysis proved that their structure is affected by the type of metal oxide used as a dopant for ZrO2 and by the crystallinity of ZrO2. Isolated tricoordinated Zn2+ species were concluded to show high activity and on-stream stability. Their intrinsic activity is enhanced when TiO2 and ZrO2 coexist in the support or when ZrO2 is promoted by TiO2. This is probably due to accelerating hydrogen formation in the course of the PDH reaction as concluded from temporal analysis of products with sub-millisecond resolution. The results of temperature-programmed oxidation of spent catalysts as well as ex situ Raman and operando UV–vis studies enabled us to conclude that the high on-stream stability of isolated tricoordinated Zn2+ species in the PDH reaction is related to their low ability to form coke. In general, the tendency for coke formation seems to increase with an increase in the degree of ZnOx agglomeration.

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