Mariana Stoyanova

One-pot aerosol route to MoO3-SiO2-Al2O3 catalysts with ordered super microporosity and high olefin metathesis activity

Aerosol processing coupled with surfactant-templated sol–gel synthesis is used to produce MoO3-SiO2-Al2O3 catalysts. By quenching the sol–gel kinetics by fast drying of the aerosol, molecular-scale dispersion of each component is achieved. The structuring agent generates an organized porosity at the nanoscale. The catalysts have a high specific surface area and outstanding olefin metathesis activity.

ZrO2-Based Alternatives to Conventional Propane Dehydrogenation Catalysts: Active Sites, Design, and Performance

Non-oxidative dehydrogenation of propane to propene is an established large-scale process that, however, faces challenges, particularly in catalyst development; these are the toxicity of chromium compounds, high cost of platinum, and catalyst durability. Herein, we describe the design of unconventional catalysts based on bulk materials with a certain defect structure, for example, ZrO2 promoted with other metal oxides. Comprehensive characterization supports the hypothesis that coordinatively unsaturated Zr cations are the active sites for propane dehydrogenation. Their concentration can be adjusted by varying the kind of ZrO2 promoter and/or supporting tiny amounts of hydrogenation-active metal. Accordingly designed Cu(0.05 wt %)/ZrO2 -La2 O3 showed industrially relevant activity and durability over ca. 240 h on stream in a series of 60 dehydrogenation and oxidative regeneration cycles between 550 and 625 °C.

A non-hydrolytic sol–gel route to highly active MoO3–SiO2–Al2O3 metathesis catalysts

MoO3-based materials are known to be appropriate catalysts for the heterogeneous metathesis of light olefins. Classical preparation methods involve the deposition of a Mo oxide phase on the surface of preformed support via impregnation, grafting or thermal spreading. An alternative sol–gel approach for the elaboration of Mo-based catalysts is presented in this article. Mesoporous ternary Si–Al–Mo mixed oxides are prepared in one step, via non-hydrolytic condensation of chloride precursors in non-aqueous media. After calcination, effective catalysts with very good textures and highly dispersed surface molybdenum species are obtained. The Si/Al ratio influences both the texture and the acidity of the materials, which significantly affects the propene self-metathesis activity. The activity also increases with the MoO3 content. The best catalysts with optimized composition significantly outperform the catalysts prepared by other methods.

Effect of support on selectivity and on-stream stability of surface VOx species in non-oxidative propane dehydrogenation

Al2O3, SiO2(MCM-41), and Al2O3–SiO2 (Siral®) with a SiO2 content varying from 1 to 70 wt.% were used to prepare supported catalysts with a V loading below one monolayer. Their activity, selectivity and on-stream stability were tested in non-oxidative propane dehydrogenation (DH) at 550 °C. The highest space–time yield of propene was only 25% lower than that over industrially relevant Pt–Sn/Al2O3 under the same reaction conditions. All catalysts deactivated with time on stream, but restored their initial performance after oxidative regeneration as proven in a sequence of 10 DH/regeneration cycles lasting in total over 60 h. The deactivation was related to propene-derived carbon deposits covering active VOx sites. However, depending on the catalyst, such deposits formed on bare support sites can also participate in propane dehydrogenation. Their DH activity is, however, significantly lower compared to VOx species. Acidic properties of the support were found to be crucial for the generation of such catalytically active carbon species.

Facile preparation of MoO3/SiO2-Al2O3 olefin metathesis catalyst by thermal spreading

This paper reports a very straightforward preparation method producing active metathesis catalysts. The simple physical mixing of molybdenum oxide with a silica-alumina support followed by an adapted thermal treatment leads to the spreading of Mo oxide species at the surface of the silica-alumina. The catalysts are tested in the self-metathesis of propene to butene and ethene and compared with samples prepared by classical wet impregnation. Characterization (XRD, in-situ XRD, Raman spectroscopy, XPS) shows that the spreading is particularly efficient at low loading and confirms the superior activity of well-spread Mo oxide species as compared to MoO3 crystallites.

Effect of VOx Species and Support on Coke Formation and Catalyst Stability in Nonoxidative Propane Dehydrogenation

VOx/SiO2–Al2O3 catalysts were prepared by grafting vanadyl acetylacetonate onto the supports with a SiO2 content between 0 and 100 wt. %. The degree of polymerization of VOx species and acidity both of pristine supports and the catalysts were evaluated. To determine their on‐stream stability and carbon deposition activity in nonoxidative propane dehydrogenation, continuous‐flow tests and in situ thermogravimetric measurements were performed. The rate constants of catalyst deactivation and carbon deposition were derived from kinetic evaluation of these experiments. Gathered experimental evidence pointed out that VOx species were significantly more active for coke formation than acid sites of the supports. The rate constant of carbon formation was found to increase with the degree of polymerization of VOx species, whereas no correlation between catalyst acidity and the rate constants of coking or deactivation could be drawn.