Structure-Property Relationships Between Fractal Description of Pore Structure and Catalyst Effectiveness In V-SBA-15

Structure-property relationships between fractal description of pore structure and catalyst effectiveness in V-SBA-15

Alexander Zoelle, Thomas Baldassare, Michael A. Smith^*,

 Department of Chemical Engineering, Villanova University Villanova, PA, 19085  ^*michael.a.smith@villanova.edu

                SBA-15 is a template-synthesized mesoporous silicate that has found extensive use as a model support for studies in supported catalysis. Thorough structural analyses clearly describe the dual micropore-mesopore structure with a broad distribution of micropore sizes; alternatively, it has been shown that the microporosity may alternatively be understood as a manifestation of the fractal nature of the mesopore surface.  Moreover, one can use calcination temperature from 300–900 ºC to adjust the degree of microporosity or fractal character in the SBA materials, where calcination to higher temperatures effectively anneals out the micropores without destroying the mesopore topology of the SBA-15. We believe the effect of surface roughness or fractal character has an underappreciated effect on catalyst performance. In this work, we hypothesize that catalytic effectiveness of vanadia supported on SBA-15 should depend upon the fractal character of the original SBA-15.

                SBA-15 was prepared following a protocol adapted from Zhao et al. We use calcination temperature of 500º, 700º and 850 ºC to adjust the degree of fractal character in the SBA materials. Nitrogen sorption isotherms were collected to assess fractal character. VO_x -SBA-15 was prepared using a modified grafting technique to yield loadings corresponding to ¹/₈, ¹/₄, ¹/₂ and full monolayer coverage. Samples are evaluated using partial oxidation of methanol in a microreactor under differential conditions between 300º and 400º C. Additional characterization includes SAXS and XRD.

Preliminary results for two catalysts, each with the same nominal vanadia loading but with much different parent SBA-15 fractal character indicate that as fractal dimension or surface roughness of the parent decreases, catalytic activity increases.  We will show results of ongoing efforts to support these findings.