Silica-alumina inorganic catalysts are employed ubiquitously in the modern chemical industry. They can be found in applications including crude oil processing, downstream petrochemistry, and fine/specialty chemical production. Since the late 1940’s microporous crystalline silica-aluminas, zeolites, have dominated. However, there are many challenges to synthesizing larger pore zeolites that are better adapted to more difficult/bulky feedstocks (including tar sands, biomass, and fine chemicals), which opens the door to alternative synthesis method development. Ideally these alternatives will allow for a priori acid active site identity and surrounding environment (i.e. pore structure) control. Following these requirements, a three step procedure has been used here to synthesize novel catalysts we call “Nanobowls”: (i) the bulky template molecule, 4-tert-butylcalix[4]arene, is grafted to aluminum oxide nanospheres at 0.25 groups/nm2 maximum under toluene reflux, (ii) the surrounding surface is passivated using a sol-gel, base-catalyzed SiO2 overcoating technique that deposits ~2-3 nm of the oxide, and (iii) the templates are removed by high temperature calcination at 650 oC to re-expose the underlying core alumina and create interfacial sites between the core and overlayer. These materials have been characterized by TGA, TEM, BET, solid state NMR, TPD, and DRIFTS. BET demonstrates that the templated route imparts surface substructure as an over 30% increase in external surface area is observed. In a Brønsted acidity probe reaction, triisopropylbenzene catalytic cracking, the overcoated materials demonstrate high selectivity towards the cracking pathway and negligible isomerization/coking byproducts. Enhanced coking stability is shown for these overcoated materials by their time on stream data and low coke loadings (as measured by TGA). Furthermore, both TPD and DRIFTS suggest that the templating procedure selectively preserves only the strongest Brønsted acidic features on the core alumina. Future work includes developing analogous materials with enhanced silanol acidity and using more strongly Brønsted acidic core particles.

Nanobowls: Creating silica-alumina interfaces to tune metal oxide behavior

251st National Meeting (2016)

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