Tuning oxidative propane dehydrogenation while co-converting CO2 over vanadium containing CHA zeolites

Abstract

Propane dehydrogenation (PDH) is an essential industrial process for on-purpose propene production, but current technologies face equilibrium limitations and severe coking. Using CO₂ as a soft oxidant can reduce coking, enhance propene yield, and convert the greenhouse gas into CO, a platform chemical. An exclusive CO₂-based oxidative catalyst has not been reported because when feeding soft oxidants, concomitant routes - oxidative and non-oxidative - are present. We prepared vanadium supported on CHA zeolites (and alumina as a benchmark) by modulating the vanadium loading but also the zeolite particle size and their aluminum content (Si/Al ratio) and the samples were characterized by X-ray Diffraction (XRD), temperature-programmed reduction with hydrogen (H₂-TPR) and X-ray photoelectron spectroscopy (XPS), among other techniques. The role of the acidity in side-reactions is that it encourages coke formation rather than propane cracking. Catalyst with small crystals showing better metal dispersion, and likely from that high activity and stability. Small zeolite particle size and low acidity seem essential attributes for better V/small pore zeolite catalyst design. Interestingly, the V/CHAs activity are favorably impacted by working at higher pressure compared to the impact on the benchmark γ-Al₂O₃ support. The optimal metal loading range was investigated between 0.6 mmol g⁻¹_support and 2.6 mmol g⁻¹_support, with an eye on both propylene production and co-production of CO as well as deactivation and its relation to coke formation. The catalytic performance evaluation covers a set of experiments with different conditions and next to classic conversion, selectivity and yield numbers, we display molar rates in the form of a unique box and whisker analysis. This provides a comprehensive view of the catalytic performance and allows comparing multiple time-on-stream (TOS) profiles of multiple products and reagents all in one plot. The best material, V/CHA70, combines a small particle size and the highest Si/Al ratio tested here. With a vanadium load of 1.3 mmol g⁻¹_support this material achieves a conversion of 33.9% for propane simultaneous co-conversion of CO₂ of 21.3% with a propylene rate of 8.4 mol h⁻¹ kg⁻¹ at initial TOS. Moreover, the material demonstrates excellent cyclability and the highest stability within the set of samples, showing a decrease of only 0.6% per regeneration cycle, ensuring good activity even after 10 regenerations and 25 hours of reaction in near-industrial conditions.