Propylene epoxidation by molecular oxygen using supported silver catalysts: Effect of support type, preparation method and promotion with alkali chloride and/or steam

Abstract

Abstract The direct gas-phase epoxidation of propylene using molecular oxygen remains a challenging method to produce propylene oxide (PO) with minimum environmental impact. The epoxidation of propylene using Ag catalysts supported on non-porous or low surface area supports, such as α-Al3O2, CaCO3, TiO2 rutile, SiO2 and ZrO2, as well as on meso/macroporous ZrO2 and TiO2 anatase, was thoroughly investigated in the present work. The catalysts (Ag loading 10–50 wt. %) were prepared by wet impregnation followed by in-situ reduction with amines or by ex situ treatment with H2 at 350 °C. The combination of moderate surface area (ca. 60 m2/g) of the meso/macroporous ZrO2 and TiO2 anatase with the in situ reduction with amines method offered a relatively high Ag dispersion and small crystallite size of 25–35 nm, even at the high loading of 50 wt.%. Propylene conversion was as high as ca. 40% in the temperature range of 200–320 °C, depending on the type of the support and the preparation method, with the mesoporous ZrO2 and the amines reduction method affording the most active catalysts. The selectivity of PO was maximized (ca. 6%) at ∼220–260 °C, i.e. 6.1% at 33.0% conversion for the TiO2 rutile amine reduced catalyst. Promotion of the catalysts with NaCl (2–5 wt.%), led to significantly lower reactivity (conversion ≤ 15%) even at high temperatures, but the selectivity to PO increased up to ca. 14%, with the low surface area ZrO2 and the TiO2 rutile based catalysts being the most selective. However, propylene tends to react also with the chloride ions, being on the promoted catalyst surface, leading to allyl chloride co-production, thus leading to gradual decrease of Cl− ions and loss of their beneficial effect on PO formation. Co-feeding of the reactant gas mixture with steam offered an alternative method of promoting PO production, even in the absence of NaCl, with the TiO2 rutile and CaCO3 based catalysts being the most effective via this route.