Neutron scattering studies of the structure and hydrothermal stability of the mesoporous catalyst support SBA-15
Rachel A. Pollock, Gennady Yu. Gor, Brenna R. Walsh, Jason Fry, I. Tyrone Ghampson, Yuri B. Melnichenko, William J. DeSisto, and Brian G. Frederick
Forest biomass is an abundant, renewable energy source and recent research has focused on converting this biomass into transportation fuels. Fast pyrolysis is one process that can be used to convert woody biomass into fuels and chemicals, however the liquid product created is an unsuitable fuel because of its low pH and high oxygen content. In order to have a fungible fuel the pyrolysis oil must be catalytically upgraded.
Mesoporous materials have the potential to be good catalyst supports because molecules can move efficiently in and out of the pore network, but they must also be stable in water if the are to be used for the production of biofuels. This research focuses on using SBA-15, a hexagonally ordered mesoporous silica material that contains a broad distribution of pore sizes in the walls of the silica matrix, as a catalyst support for the upgrading of biofuel. Before investigating hydrothermal stability and transport properties, the pore structure of SBA-15 was characterized using small angle neutron scattering (SANS) and non-local density functional theory (NLDFT) analysis of nitrogen sorption isotherms.
Contrast Matching SANS experiments, conducted using a range of probe molecules to directly probe the micropore size, gave a pore size distribution onset of 6 ± 0.2 Å, consistent with cylindrical pores formed from polymer template strands that unravel into the silica matrix. Diffraction intensity analysis of SANS measurements, combined with pore size distributions calculated from NLDFT, showed that the secondary pores are distributed relatively uniformly throughout the silica framework.
The hydrothermal stability of SBA-15 was evaluated using a post-calcination hydrothermal treatment in both liquid and vapor phase water. The results were consistent with a degradation mechanism in which silica dissolves from regions of small positive curvature, e.g. near the entrance to the secondary pores, and is re-deposited deeper into the framework. Under water treatment at 115 °C, the mesopore diameter increases and the intra-wall void fraction decreases significantly. The behavior is similar for steam treatment, but occurs more slowly, suggesting that transport is faster when condensation occurs in the pores.