5-(Hydroxymethyl)furfural (HMF) and levulinic acid production from glucose in a cascade of reactions using a Lewis acid (CrCl3) catalyst together with a Brønsted acid (HCl) catalyst in aqueous media is investigated. It is shown that CrCl3 is an active Lewis acid catalyst in glucose isomerization to fructose, and the combined Lewis and Brønsted acid catalysts perform the isomerization and dehydration/rehydration reactions. A CrCl3 speciation model in conjunction with kinetics results indicates that the hydrolyzed Cr(III) complex [Cr(H2O)5OH](2+) is the most active Cr species in glucose isomerization and probably acts as a Lewis acid-Brønsted base bifunctional site. Extended X-ray absorption fine structure spectroscopy and Car-Parrinello molecular dynamics simulations indicate a strong interaction between the Cr cation and the glucose molecule whereby some water molecules are displaced from the first coordination sphere of Cr by the glucose to enable ring-opening and isomerization of glucose. Additionally, complex interactions between the two catalysts are revealed: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of [Cr(H2O)5OH](2+). In contrast, Lewis acidity increases the overall rate of consumption of fructose and HMF compared to Brønsted acid catalysis by promoting side reactions. Even in the absence of HCl, hydrolysis of Cr(III) decreases the solution pH, and this intrinsic Brønsted acidity drives the dehydration and rehydration reactions. Yields of 46% levulinic acid in a single phase and 59% HMF in a biphasic system have been achieved at moderate temperatures by combining CrCl3 and HCl.

A renewable route to p-xylene from biomass-derived dimethylfuran and ethylene is investigated with zeolite catalysts. Cycloaddition of ethylene and 2,5-dimethylfuran and subsequent dehydration to p-xylene has been achieved with 75% selectivity using H–Y zeolite and an aliphatic solvent at 300 °C. Competitive side reactions include hydrolysis of dimethylfuran to 2,5-hexanedione, alkylation of p-xylene, and polymerization of 2,5-hexanedione. The observed reaction rates and computed energy barriers are consistent with a two-step reaction that proceeds through a bicyclic adduct prior to dehydration to p-xylene. Cycloaddition of ethylene and dimethylfuran occurs without a catalytic active site, but the reaction is promoted by confinement within microporous materials. The presence of Brønsted acid sites catalyzes dehydration of the Diels–Alder cycloadduct (to produce p-xylene and water), and this ultimately causes the rate-determining step to be the initial cycloaddition.

Electroreduction of CO2 in a highly selective and efficient manner is a crucial step towards CO2 utilization. Nanostructured Ag catalysts have been found to be effective candidates for CO2 to CO conversion. In this report, we combine experimental and computational efforts to explore the electrocatalytic reaction mechanism of CO2 reduction on nanostructured Ag catalyst surfaces in an aqueous electrolyte. In contrast to bulk Ag catalysts, both nanoparticle and nanoporous Ag catalysts show enhanced ability to reduce the activation energy of the CO2 to intermediate step through the low coordinated Ag surface atoms, resulting in a reaction mechanism involving a fast first electron and proton transfer followed by a slow second proton transfer as the rate limiting step. Experimental SectionComputational Modeling

A seeded growth method for the fabrication of high-permeance, high-separation-factor zeolite (siliceous ZSM-5, [Si96O192]-MFI) membranes is reported. The method consists of growing the crystals of an oriented seed layer to a well-intergrown film by avoiding events that lead to a loss of preferred orientation, such as twin overgrowths and random nucleation. Organic polycations are used as zeolite crystal shape modifiers to enhance relative growth rates along the desirable out-of-plane direction. The polycrystalline films are thin (approximately 1 micrometer) with single grains extending along the film thickness and with large in-plane grain size (approximately 1 micrometer). The preferred orientation is such that straight channels with an open diameter of approximately 5.5 angstroms run down the membrane thickness. Comparison with previously reported membranes shows that these microstructurally optimized films have superior performance for the separation of organic mixtures with components that have small differences in size and shape, such as xylene isomers.

The microscopic spatial kinetic Monte Carlo (KMC) method has been employed extensively in materials modeling. In this review paper, we focus on different traditional and multiscale KMC algorithms, challenges associated with their implementation, and methods developed to overcome these challenges. In the first part of the paper, we compare the implementation and computational cost of the null-event and rejection-free microscopic KMC algorithms. A firmer and more general foundation of the null-event KMC algorithm is presented. Statistical equivalence between the null-event and rejection-free KMC algorithms is also demonstrated. Implementation and efficiency of various search and update algorithms, which are at the heart of all spatial KMC simulations, are outlined and compared via numerical examples. In the second half of the paper, we review various spatial and temporal multiscale KMC methods, namely, the coarse-grained Monte Carlo (CGMC), the stochastic singular perturbation approximation, and the τ -leap methods, introduced recently to overcome the disparity of length and time scales and the one-at-a time execution of events. The concepts of the CGMC and the τ -leap methods, stochastic closures, multigrid methods, error associated with coarse-graining, a posteriori error estimates for generating spatially adaptive coarse-grained lattices, and computational speed-up upon coarse-graining are illustrated through simple examples from crystal growth, defect dynamics, adsorption-desorption, surface diffusion, and phase transitions.

Pyrolytic biofuels have technical advantages over conventional biological conversion processes since the entire plant can be used as the feedstock (rather than only simple sugars) and the conversion process occurs in only a few seconds (rather than hours or days). Despite decades of study, the fundamental science of biomass pyrolysis is still lacking and detailed models capable of describing the chemistry and transport in real-world reactors is unavailable. Developing these descriptions is a challenge because of the complexity of feedstocks and the multiphase nature of the conversion process. Here, we identify ten fundamental research challenges that, if overcome, would facilitate commercialization of pyrolytic biofuels. In particular, developing fundamental descriptions for condensed-phase pyrolysis chemistry (i.e., elementary reaction mechanisms) are needed since they would allow for accurate process optimization as well as feedstock flexibility, both of which are critical to any modern high-throughput process. Despite the benefits to pyrolysis commercialization, detailed chemical mechanisms are not available today, even for major products such as levoglucosan and hydroxymethylfurfural (HMF). Additionally, accurate estimates for heat and mass transfer parameters (e.g., thermal conductivity, diffusivity) are lacking despite the fact that biomass conversion in commercial pyrolysis reactors is controlled by transport. Finally, we examine methods for improving pyrolysis particle models, which connect fundamental chemical and transport descriptions to real-world pyrolysis reactors. Each of the ten challenges is presented with a brief review of relevant literature followed by future directions which can ultimately lead to technological breakthroughs that would facilitate commercialization of pyrolytic biofuels. Paper bodyAs the world population grows, there is a need for new energy technologies that are domestic and sustainable. Achieving both objectives requires improving existing energy systems as well as utilizing renewable feedstocks, such as biomass. In addition to supporting agricultural economies, biomass is the only renewable source for liquid fuels and chemicals. 1,2 For this reason, the U.S. Department of Energy has made it a goal to replace 30% of all transportation fuels with biofuels. 3 The 2005 'Billion-Ton Study' (BTS) sponsored by the U.S. Department of Energy employed conservative assumptions to determine that more than a billion tons of biomass (unrestricted by price) is available annually for biofuels. This amount of biomass is capable of displacing 30% of U.S. petroleum consumption, as put forth in the government targets. 3 In 2011, an update to the BTS revisited the resource availability and confirmed the findings of the 2005 study. 4 Both government-

We have found that the spontaneous formation of silica nanoparticles is a general phenomenon in basic solutions of small tetraalkylammonium (TAA) cations. The nanoparticle formation and structure have been investigated using conductivity, pH, and small-angle scattering methods. The particles have a core−shell structure with silica at the core and the TAA cations at the shell. The particle core size is nearly independent of the size of the TAA cation but decreases with pH, suggesting electrostatic forces are a key element controlling their size and stability. The nanoparticle formation is a reversible process at low temperatures, in several ways similar to surfactant aggregation into micelles. These silica nanparticles may be a connection between the synthesis of zeolites and ordered mesoporous silicas such as MCM-41.

Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify a-cyclodextrin as an appropriate smallmolecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.