Web Release Date: April 5,
Theoretical Modeling of Zeolite Nanoparticle Surface Acidity for Heavy Oil Upgrading
and
National Institute for Nanotechnology, National Research Council of Canada, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada, Department of Mechanical Engineering, University of Alberta, Edmonton T6G 2M9, Canada, and Department of Chemical and Materials Engineering, University of Alberta, Edmonton T6G 2M9, Canada
Received: July 19, 2007
In Final Form: December 7, 2007
Abstract:
We performed periodic density functional theory (DFT) and quantum-mechanics/molecular mechanics (QM/MM) investigations of the surface acidity of zeolite nanoparticles derived from natural minerals that can be
used for bitumen upgrading; in particular, in the process combining bitumen precracking with impurities
removal that we recently reported. Bitumen molecules are large and cannot enter zeolite pores. These mainly
adsorb on the outer surface of zeolite nanoparticles, which can be optimized for efficient bitumen upgrading
and impurities removal. Two chabazite slab models obtained by (003) and (00
) cuts that have four and two
surface OH groups per unit cell, respectively, are used for the periodic DFT modeling of nanoparticle surfaces.
The first model is also treated by using the QM/MM method. Bitumen molecules are represented by probing
bases such as ammonia, pyridine, and 2,6-dimethylpyridine that are commonly used for experimental acidity
characterization. Analysis of the model acidity characteristics, such as deprotonation energies, aluminum
substitution energies, OH stretching frequencies. and Fukui functions produces very good correlations. For
the deprotonated chabazite, the electrophilic Fukui functions predict the most stable Br
nsted site. Our results
suggest that the most reactive chabazite sites are O1 and O3. The three bases investigated become fully
protonated upon adsorption to the chabazite Brnsted sites. The molecular orbital spatial distributions obtained
by using the periodic and QM/MM methods are very similar, which indicates good correlations between the
two modeling methods. The results of our zeolite acidity calculations are in good agreement with experimental
data and other computational studies available. Our findings can be useful for the further modeling and rational
design of catalytic zeolite nanoparticles for heavy oil upgrading.
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