(731e) 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.