Scott M. Auerbach

Handbook of Zeolite Science and Technology

The Handbook of Zeolite Science and Technology offers effective analyses ofsalient cases selected expressly for their relevance to current and prospective research. Presenting the principal theoretical and experimental underpinnings of zeolites, this international effort is at once complete and forward-looking, combining fundamental concepts with the most sophisticated data for each scientific subtopic and budding technology. Supplying over 750 figures, and 350 display equations, this impressive achievement in zeolite science observes synthesis through the lens of MFI (ZSM-5 and silicalite). Chapters progress from conceptual building blocks to complex research presentations.

Handbook of Layered Materials

CLAY MINERALS Introduction: Clay Structure, Surface Acidity, and Catalysis, K.A. Carrado Molecular Modeling of Clay Mineral Structure and Surface Chemistry, S.-H. Park and G. Sposito Clay-Organic Interactions: Organoclay Complexes and Polymer-Clay Nanocomposites, E. Ruiz-Hitzky, P. Aranda, and J.M. Serratosa Sorption of Nitroaromatic Compounds on Clay Surfaces, C.T. Johnston, S.A. Boyd, B.J. Teppen, and G. Sheng Photoprocesses in Clay-Organic Complexes, M. Ogawa Pillared Clays and Porous Clay Heterostructures, P. Cool and E.F. Vansant OTHER LAYERED MATERIALS Layered alpha-Zirconium Phosphates and Phosphonates, C.V. Kumar, A. Bhambhani, and N. Hnatiuk Layered Double Hydroxides (LDHs), P.S. Braterman, Z.P. Xu, and F. Yarberry Layered Manganese Oxides: Synthesis, Properties, and Applications, J. Liu, J.P. Durand, L. Espinal, L.-J. Garces, S. Gomez, Y.-C. Son, J. Villegas, and S.L. Suib Layered Metal Chalcogenides, C.O. Oriakhi and M.M. Lerner Alkali Silicates and Crystalline Silicic Acids, W. Schweiger and G. Lagaly

Theory and simulation of jump dynamics, diffusion and phase equilibrium in nanopores

We review theory and simulation of rare event dynamics, diffusion and phase equilibrium in nanopores, focusing on benzene in Na-X and Na-Y zeolites because of persistent experimental discrepancies. We discuss transition state theory and its application to zeolite-guest systems, suggesting that calculations on flexible lattices and at finite guest loadings are important areas for future research. We consider many-body adsorption and diffusion in zeolites, focusing on the coupling between rare event dynamics and strong guest-guest interactions. We explore the possibility that benzene can undergo phase transitions from low to high sorbate density in Na-X, and find that this type of phase transition might explain intriguing loading dependencies of water and ammonia diffusion in terms of a subcritical droplet picture of adsorption in zeolites. We discuss various formulations of non-equilibrium diffusion through finite lattices, and describe a tracer counter-permeation simulation technique. We find that transport in finite single-file systems is characterized by a diffusivity that decreases monotonically with file length, but that this transport is otherwise completely described by Ficks laws. We conclude by speculating on the prospect for cross-fertilization between zeolite science and other fields.

Self-diffusion in single-file zeolite membranes is Fickian at long times

We have developed a theory for self-diffusion in single-file Langmuirian zeolites of finite extent, which has been validated by open system kinetic Monte Carlo simulations. Our theory is based on a two-stage, Fickian diffusion mechanism, wherein a vacancy must traverse the entire file length to produce particle displacements of one lattice spacing. For times shorter than the vacancy diffusion time, tc, particle transport proceeds via the nonFickian, single-file diffusion mode, with mean-square displacements increasing with the square-root of time. For times longer than tc, however, we find that self-diffusion in single-file systems is completely described by Fick’s laws. We find that the fraction of time in the single-file diffusion mode scales inversely with file length for long files, suggesting that Fickian self-diffusion dominates transport in longer single-file zeolites. Through correlations among the particle movements, the single-file self-diffusivity is sensitive to sorption limitations for short ...

Ab initio dynamics of cellulose pyrolysis: nascent decomposition pathways at 327 and 600 °C.

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 < pre-HMF < formic acid, explaining why LGA is the kinetically favored product of fast cellulose pyrolysis.

Modeling proton mobility in acidic zeolite clusters. II. Room temperature tunneling effects from semiclassical rate theory

We have developed a novel semiclassical transition state theory (SC-TST) for truncated parabolic barriers, based on the formulation of Hernandez and Miller [Chem. Phys. Lett. 214, 129 (1993)]. Our SC-TST rate coefficient has the form kSC-TST=kTST⋅Γ, where Γ depends on the zero point corrected barrier, ΔE0, and the barrier curvature, |ωF‡|. Our rate expression is stable to arbitrarily low temperatures, as opposed to purely harmonic SC-TST, because we identify the maximum possible semiclassical action in the reaction coordinate. For low temperatures, we derive an analytical approximation for Γ that is proportional to eβ ΔE0. We develop a theory for the tunneling crossover temperature, Tx, yielding kBTx≅ℏ|ωF‡|ΔE0/(2π ΔE0−ℏ|ωF‡|ln 2), which generalizes the harmonic theory for systems with large but finite barriers. We have calculated rate coefficients and crossover temperatures for the O(1)→O(4) jump in H–Y and D–Y zeolites, yielding Tx=368 K and 264 K, respectively. These results suggest that tunneling domin...

Modeling Proton Transfer in Zeolites: Convergence Behavior of Embedded and Constrained Cluster Calculations.

We have studied the convergence properties of embedded and constrained cluster models of proton transfer in zeolites. We applied density functional theory to describe clusters and ONIOM to perform the embedding. We focused on converging the reaction energy and barrier of the O(1) to O(4) jump in H-Y zeolite as well as vibrational and structural aspects of this jump. We found that using successively larger clusters in vacuo gives convergence of this reaction energy to 14 ± 2 kJ mol(-)(1) and the barrier to 135 ± 5 kJ mol(-)(1) at a cluster size of 5 Å, which contains 11 tetrahedral (Si or Al) atoms. We embedded quantum clusters of various sizes in larger clusters with total radii in the range 7-20 Å, using the universal force field as the lower level of theory in ONIOM. We found convergence to the same values as the constrained clusters, without the use of reactive force fields or periodic boundary conditions in the embedding procedure. For the reaction energy, embedded cluster calculations required smaller clusters than in vacuo calculations, reaching converged reaction energies for quantum systems containing at least 8 tetrahedral atoms. In addition, optimizations on embedded clusters required many fewer cycles, and hence much less CPU time, than did optimizations on comparable constrained clusters.

Simulating infrared spectra and hydrogen bonding in cellulose Iβ at elevated temperatures

We have modeled the transformation of cellulose Iβ to a high temperature (550 K) structure, which is considered to be the first step in cellulose pyrolysis. We have performed molecular dynamics simulations at constant pressure using the GROMOS 45a4 united atom forcefield. To test the forcefield, we computed the density, thermal expansion coefficient, total dipole moment, and dielectric constant of cellulose Iβ, finding broad agreement with experimental results. We computed infrared (IR) spectra of cellulose Iβ over the range 300-550 K as a probe of hydrogen bonding. Computed IR spectra were found to agree semi-quantitatively with experiment, especially in the O-H stretching region. We assigned O-H stretches using a novel synthesis of normal mode analysis and power spectrum methods. Simulated IR spectra at elevated temperatures suggest a structural transformation above 450 K, a result in agreement with experimental IR results. The low-temperature (300-400 K) structure of cellulose Iβ is dominated by intrachain hydrogen bonds, whereas in the high-temperature structure (450-550 K), many of these transform to longer, weaker interchain hydrogen bonds. A three-dimensional hydrogen bonding network emerges at high temperatures due to formation of new interchain hydrogen bonds, which may explain the stability of the cellulose structure at such high temperatures.

Activated diffusion of benzene in NaY zeolite: Rate constants from transition state theory with dynamical corrections

We calculated transition state theory and exact rate coefficients for benzene jumps in Na-Y zeolite between 150 and 500 K. This is the first exact flux correlation function rate calculation for a non-spherical molecule inside a zeolite. We calculated rates for jumps between SII and W sites, located near Na ions in 6-rings and in 12-rings windows, respectively. Partition function ratios were calculated using Voter’s displacement vector method. A general Arrhenius behavior is observed over the whole temperature range for all processes. The activation energies are close to the difference between the minimum energies in the sites, and between the sites and the transition states. The calculated prefactors present reasonable values around 1012–1013 s−1, in good agreement with nuclear magnetic resonance relaxation experiments. We were not able to decompose the prefactors into simple vibrational and entropic components, and therefore a complete calculation of the rate constant seems necessary to obtain reliable v...

Modeling the concentration dependence of diffusion in zeolites. II. Kinetic Monte Carlo simulations of benzene in Na-Y

We have performed kinetic Monte Carlo simulations of benzene diffusion in Na-Y at finite loadings for various temperatures to test the analytical theory presented in Paper I, immediately preceding this paper. Our theory and simulations assume that benzene molecules jump among SII and W sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. The theory exploits the fact that supercages are identical on average, yielding Dθ=16kθaθ2=κaθ2/6〈τ1〉[1+Keq(1→2)], where kθ is the cage-to-cage rate coefficient, Keq(1→2) is the W→SII equilibrium coefficient, 〈τ1〉 is the mean W site residence time, and κ is the transmission coefficient for cage-to-cage motion. The simulations use fundamental rate coefficients calculated at infinite dilution for consistency with the theory in Paper I. Our theory for kθ, Keq(1→2) and 〈τ1〉 agrees quantitatively with simulation for various temperatures and loadings. The simulated transmission coefficient is nearly 12 for all but the highest loadings, qualitatively val...