Epoxide-assisted alumina aerogels by rapid supercritical extraction

Abstract

Aerogels offer a potential alternative to noble metals that could reduce both the cost and environmental impact associated with catalytic converter production. The environmental impact of the production of aerogel catalysts could be further reduced by using a rapid supercritical extraction (RSCE) technique, which reduces the time and solvent waste associated with aerogel preparation. Alumina aerogels, which have shown activity in catalyzing exhaust processing reactions, were prepared using an epoxide-assisted gelation technique with RSCE processing in a contained mold in a hydraulic hot press. Samples were characterized by FTIR, XRD, SEM, EDX, nitrogen adsorption porosimetry and pycnometry. Solvent characterization by GC-MS headspace analysis shows that excess propylene oxide and chloropropanol products of an irreversible epoxide ring-opening reaction are present in the alumina gel following gelation, but can be removed via solvent exchange. Alumina aerogels with surface areas as high as 790 m 2 /g and bulk densities as low as 0.05 g/mL were prepared. Preliminary characterization of these aerogels, utilizing a catalytic test bed and a simulated emissions gas blend, demonstrates that they have moderate ability for removal of hydrocarbons, carbon monoxide and nitrogen oxide.

Cobalt-alumina sol gels: Effects of heat treatment on structure and catalytic ability

“…The formation and development of γ-alumina likely contributed to the improved catalytic performance for aerogels heat-treated to 750˚C prior to analysis. In previous work, we demonstrated that alumina aerogels exhibited modest catalytic activity [14]; however, the catalytic performance of the Co-Al aerogels described in the present work was significantly higher. We therefore conclude that the cobalt species in the aerogel are a significant source of catalytic activity.…”

“…Cobalt-alumina (Co-Al) RSCE gels were fabricated using a modified version of an epoxide-based precursor recipe approach adapted from Baumann et al [21] for preparation of alumina RSCE aerogels [14]. First, 2.9 g of aluminum nitrate hexahydrate (Fluka Analytical, 99%) was dissolved in 20 mL of reagent-grade ethanol (denatured ethyl alcohol, Fisher Scientific).…”

“…Uncertainty estimates for each variable were determined by repeat measurements or based on instrument specifications, as detailed in our previous work describing the preparation of epoxide-assisted alumina aerogels by RSCE [14].…”

“…In prior work, we have developed processes for fabricating alumina and alumina-based aerogels using the RSCE method [12][13][14]. Incorporation of transition metal species such as nickel, copper, cobalt and vanadium in the alumina aerogel structure have resulted in catalytically active aerogels that are structurally capable of withstanding the high temperatures in a catalytic converter's environment [14][15][16][17].…”

“…Thus, the purpose of this study is to determine ways to optimize the catalytic potential of Co-Al based aerogels, particularly through gaining an understanding of the role of heat treatment on the phase transformations of the aerogel support matrix and metal catalyst. Co-Al aerogels were synthesized using the RSCE method [10,14], heat treated at temperatures ranging from 400 to 1100°C, and characterized using techniques such as scanning electron microscopy (SEM) to determine sample morphology, Brunauer-Emmett-Teller (BET) gas adsorption, and thermogravimetric analysis (TGA) to measure their structural stability after heating, and powder x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS) to determine the composition and distribution of the catalyst and aerogel support phases. As a control, Co-Al xerogels, materials similar in structure to the aerogels but with a collapsed matrix, were also synthesized and characterized in order to better understand the interactions between the catalyst and aerogel support and the influence of the stability of the matrix.…”