Vishal Agarwal | University of California, Santa Barbara (original) (raw)

Papers by Vishal Agarwal

Journal of Physical Chemistry C, 2011

ABSTRACT We have studied base strengths of nitrogen-substituted (nitrided) zeolites with faujasit... more ABSTRACT We have studied base strengths of nitrogen-substituted (nitrided) zeolites with faujasite (FAU) structure by calculating sorption energies of probe molecules (BF3 and BH3) using density functional theory with mixed basis sets applied to embedded clusters. BH3 was found to be a better probe of base strength because it does not introduce competing metal−fluorine interactions that obfuscate trends. In all cases, the base strengths of nitrided zeolites (denoted M−N−Y) were found to exceed those of the corresponding standard M−Y zeolites, where M = Li, Na, K, Rb, or Cs charge-compensating cations. We have found that for a particular Si:Al ratio, BH3 sorption energies vary in the order Li < Na < K Rb Cs. Sorption energy and hence base strength was found to decrease with increasing Si:Al ratio from 1 to 3 beyond which the base strength was found to increase again. The initial regime (1 < Si:Al < 3) is consistent with the prevailing understanding that the base strength increases with Al content, while the latter regime (Si:Al > 3) involves the surprising prediction that the base strength can be relatively high for the more stable, high-silica zeolites. In particular, we found the sorption energy in Na−N−Y (Si:Al = 11) to be nearly equal to that in (Si:Al = 1). Taken together, these results suggest that K−N−Y (Si:Al = 11) optimizes the balance of activity, stability, and cost.

Nitrogen-substituted zeolites, those with Si-O-Si/Si-O-Al- linkages replaced with Si-NH-Si/Si-NH-... more Nitrogen-substituted zeolites, those with Si-O-Si/Si-O-Al- linkages replaced with Si-NH-Si/Si-NH-Al- groups, can act as next generation strongly basic, shape-selective catalysts, with likely application in biomass processing for fuels and products. Nitrided zeolites have been successfully synthesized, and have shown strong catalytic activity for reactions that rely on basic sites. Despite empirical reports demonstrating use of nitrogen-substituted zeolites as base catalysts, there has been no systematic study revealing the variation of base strength versus aluminum content or alkali cation in these systems. We have studied the base strengths of nitrogen-substituted X and Y zeolites by calculating adsorption energies of probe molecules BF3 and BH3, using DFT with mixed basis sets via the ONIOM embedded-cluster approach. Although BF3 is a stronger lewis acid, BH3 is found to be a better probe of base strength as it eliminates metal-fluorine interaction. For a particular Si:Al ratio, w...

Abstract Zeolites in protonic forms have traditionally been used as acid catalyst due to large su... more Abstract Zeolites in protonic forms have traditionally been used as acid catalyst due to large surface area and shape selective properties. Substituting oxygen with isoelectronic -NH- group inside zeolite, one can incorporate high basicity into the framework. Amine functionalized zeolites have been synthesized in other laboratories, and have shown strong catalytic activity for reactions that rely on basic sites. Despite these advances, the synthesis mechanism and atomic nature of basic and catalytic sites remain poorly understood. Also stability of these materials in presence of water is in debate. We have performed Density Functional Theory calculations (DFT) to understand the synthesis mechanism in HY and Silicalite zeolites. The mechanism found is a two step process with overall barriers of about 350 KJ/mol. The formation process occurs at temperatures >600 OC which is in qualitative agreement with the experiments. The yield of substituted sites depends on water content, espec...

The Journal of Physical Chemistry C, 2011

We have studied base strengths of nitrogen-substituted (nitrided) zeolites with faujasite (FAU) s... more We have studied base strengths of nitrogen-substituted (nitrided) zeolites with faujasite (FAU) structure by calculating sorption energies of probe molecules (BF 3 and BH 3 ) using density functional theory with mixed basis sets applied to embedded clusters. BH 3 was found to ...

Journal of the American Chemical Society, 2008

Journal of the American Chemical Society, Nov 1, 2008

Journal of the American Chemical Society, 2012

We modeled nascent decomposition processes in cellulose pyrolysis at 327 and 600 °C using Car-Par... more We modeled nascent decomposition processes in cellulose pyrolysis at 327 and 600 °C using Car-Parrinello molecular dynamics (CPMD) simulations with rare events accelerated with the metadynamics method. We used a simulation cell comprised of two unit cells of cellulose Iβ periodically repeated in three dimensions to mimic the solid cellulose. To obtain initial conditions at reasonable densities, we extracted coordinates from larger classical NPT simulations at the target temperatures. CPMD-metadynamics implemented with various sets of collective variables, such as coordination numbers of the glycosidic oxygen, yielded a variety of chemical reactions such as depolymerization, fragmentation, ring opening, and ring contraction. These reactions yielded precursors to levoglucosan (LGA)-the major product of pyrolysis-and also to minor products such as 5-hydroxy-methylfurfural (HMF) and formic acid. At 327 °C, we found that depolymerization via ring contraction of the glucopyranose ring to the glucofuranose ring occurs with the lowest free-energy barrier (20 kcal/mol). We suggest that this process is key for formation of liquid intermediate cellulose, observed experimentally above 260 °C. At 600 °C, we found that a precursor to LGA (pre-LGA) forms with a free-energy barrier of 36 kcal/mol via an intermediate/transition state stabilized by anchimeric assistance and hydrogen bonding. Conformational freedom provided by expansion of the cellulose matrix at 600 °C was found to be crucial for formation of pre-LGA. We performed several comparison calculations to gauge the accuracy of CPMD-metadynamics barriers with respect to basis set and level of theory. We found that free-energy barriers at 600 °C are in the order pre-LGA &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; pre-HMF &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; formic acid, explaining why LGA is the kinetically favored product of fast cellulose pyrolysis.

aiche.confex.com

Acidic zeolite catalysts have long been back-bone of petroleum industry because of their high sur... more Acidic zeolite catalysts have long been back-bone of petroleum industry because of their high surface area, large adsorption capacities, and shape selective properties. Substituting oxygen with isoelectronic '-NH-' group inside zeolite can incorporate high basicity, which could ...