Aditya Bhan

Synthesis of self-pillared zeolite nanosheets by repetitive branching

Go with the Flow Effective absorption or filtration can be achieved by having a material with multiple levels of porosity, so that the main flow can occur in the larger channels, while smaller passageways can be used to sequester a secondary material. It can be difficult to make these materials because the pores need to be different sizes, but still fully connected to each other. Zhang et al. (p. 1684) show that a hierarchical zeolite can be made through a simple process using a single structure-directing agent that causes repetitive branching. This leads to a material with improved transport and catalytic properties. Single-step synthesis of pillared zeolite nanosheets is achieved with a common structure-directing agent. Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.

A Link between Reactivity and Local Structure in Acid Catalysis on Zeolites

The extent to which spatial constraints influence rates and pathways in catalysis depends on the structure of intermediates, transition states, and active sites involved. We aim to answer, as we seek insights into catalytic mechanisms and site requirements, persistent questions about the potential for controlling rates and selectivities by rational design of spatial constraints around active sites within inorganic structures useful as catalysts. This Account addresses these matters for the specific case of reactions on zeolites that contain Brønsted acid sites encapsulated within subnanometer channels. We compare and contrast here the effects of local zeolite structure on the dynamics of the carbonylation of surface methyl groups and of the isotopic exchange of CD4 with surface OH groups on zeolites. Methyl and hydroxyl groups are the smallest monovalent cations relevant in catalysis by zeolites. Their small size, taken together with their inability to desorb except via reactions with other species, allowed us to discriminate between stabilization of cationic transition states and stabilization of adsorbed reactants and products by spatial constraints. We show that apparent effects of proton density and of zeolite channel structure on dimethyl ether carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant steps that form *COCH3 species via CO insertion into methyl groups. This specificity reflects the selective stabilization of cationic transition states via interactions with framework oxygen anions. These findings for carbonylation catalysts contrast sharply the weak effects of channel structure on the rate of exchange of CD4 with OH groups. This latter reaction involves concerted symmetric transition states with much lower charge than that required for CH3 carbonylation. Our Account extends the scope of shape selectivity concepts beyond those reflecting size exclusion and preferential adsorption. Our ability to discriminate among various effects of spatial constraints depends critically on dissecting chemical conversions into elementary steps of kinetic relevance and on eliminating secondary reactions and accounting for the concomitant effects of zeolite structure on the stability of adsorbed reactants and intermediates.

A hybrid genetic algorithm for efficient parameter estimation of large kinetic models

Abstract The development of predictive models is a time consuming, knowledge intensive, iterative process where an approximate model is proposed to explain experimental data, the model parameters that best fit the data are determined and the model is subsequently refined to improve its predictive capabilities. Ascertaining the validity of the proposed model is based upon how thoroughly the parameter search has been conducted in the allowable range. The determination of the optimal model parameters is complicated by the complexity/non-linearity of the model, potentially large number of equations and parameters, poor quality of the data, and lack of tight bounds for the parameter ranges. In this paper, we will critically evaluate a hybrid search procedure that employs a genetic algorithm for identifying promising regions of the solution space followed by the use of an optimizer to search locally in the identified regions. It has been found that this procedure is capable of identifying solutions that are essentially equivalent to the global optimum reported by a state-of-the-art global optimizer but much faster. A 13 parameter model that results in 60 differential-algebraic equations for propane aromatization on a zeolite catalyst is proposed as a more challenging test case to validate this algorithm. This hybrid technique has been able to locate multiple solutions that are nearly as good with respect to the “sum of squares” error criterion, but imply significantly different physical situations.

Propane Aromatization over HZSM-5 and Ga/HZSM-5 Catalysts

The selective transformation of light alkanes to aromatics that are more valuable and versatile feedstocks for the chemical industry is one of the major challenges of catalytic chemistry. The complexity of the aromatization chemistry makes it difficult to unravel reaction mechanisms and, mechanistic information is largely developed from observed product distributions. This article reviews the current mechanistic understanding for the conversion of propane to aromatic compounds over HZSM‐5 and Ga/HZSM‐5 catalysts based on experimental as well as theoretical studies. Following a general discussion of acidity and confinement effects in these systems, this review focuses on understanding specific reactions occurring on Brønsted acid sites in HZSM‐5. Mechanistic details available from Density Functional Theory (DFT) calculations, as well as kinetic modeling efforts for various complex hydrocarbon systems are critically reviewed. A detailed, tabulated review of the literature compares the catalytic performance of gallium modified ZSM‐5 catalysts and subsequently the promotional effect of gallium as an additive is critically discussed in terms of the nature of the active sites, as well as the new reaction pathways introduced by gallium addition.

Catalyst design: knowledge extraction from high-throughput experimentation

We present a new framework for catalyst design that integrates computer-aided extraction of knowledge with high-throughput experimentation (HTE) and expert knowledge to realize the full benefit of HTE. We describe the current state of HTE and illustrate its speed and accuracy using an FTIR imaging system for oxidation of CO over metals. However, data is just information and not knowledge. In order to more effectively extract knowledge from HTE data, we propose a framework that, through advanced models and novel software architectures, strives to approximate the thought processes of the human expert. In the forward model the underlying chemistry is described as rules and the data or predictions as features. We discuss how our modeling framework—via a knowledge extraction (KE) engine— transparently maps rules-to-equations-to-parameters-to-features as part of the forward model. We show that our KE engine is capable of robust, automated model refinement, when modeled features do not match the experimental features. Further, when multiple models exist that can describe experimental data, new sets of HTE can be suggested. Thus, the KE engine improves (i) selection of chemistry rules and (ii) the completeness of the HTE data set as the model and data converge. We demonstrate the validity of the KE engine and model refinement capabilities using the production of aromatics from propane on H-ZSM-5. We also discuss how the framework applies to the inverse model, in order to meet the design challenge of predicting catalyst compositions for desired performance.  2003 Elsevier Science (USA). All rights reserved.

Molybdenum carbide as a highly selective deoxygenation catalyst for converting furfural to 2-methylfuran.

Selectively cleaving the C=O bond outside the furan ring of furfural is crucial for converting this important biomass-derived molecule to value-added fuels such as 2-methylfuran. In this work, a combination of density functional theory (DFT) calculations, surface science studies, and reactor evaluation identified molybdenum carbide (Mo2 C) as a highly selective deoxygenation catalyst for converting furfural to 2-methylfuran. These results indicate the potential application of Mo2 C as an efficient catalyst for the selective deoxygenation of biomass-derived oxygenates including furanics and aromatics.

Catalytic deoxygenation on transition metal carbide catalysts

We discuss the evolution of catalytic function of interstitial transition metal formulations as a result of bulk and surface structure modifications via alteration of synthesis and reaction conditions, specifically, in the context of catalytic deoxygenation reactions. We compare and contrast synthesis techniques of molybdenum and tungsten carbides, including temperature programmed reaction and ultra-high vacuum methods, and note that stoichiometric reactions may occur on phase-pure materials and that in situ surface modification during deoxygenation likely results in the formation of oxycarbides. We surmise that catalytic metal–acid bifunctionality of transition metal carbides can be tuned via oxygen modification due to the inherent oxophilicity of these materials, and we demonstrate the use of in situ chemical titration methods to assess catalytic site requirements on these formulations.

Driving forces for adsorption of polyols onto zeolites from aqueous solutions.

Ambient temperature adsorption isotherms have been developed for C(2)-C(6) diols and triols on small (FER), medium (MWW, MFI, BEA), and large (MOR, FAU) pore zeolites as well as on ordered mesoporous materials (MCM-36, 3DOm-MFI, and SBA-15) using gravimetry. Henrys constants for diol and triol adsorption on silicalite-1 increase exponentially with carbon number demonstrating that confinement of the adsorbate in the zeolite pores is the primary driving force for adsorption. This conclusion is supported by results for propylene glycol adsorption at low coverages on materials differing in topology and chemical composition. It is shown that adsorption decreases with an increase in the adsorbent pore size, and aluminum content only has a marginal effect. Comparison of diol and triol adsorption on silicalite-1 shows that increasing the number of hydroxyl groups causes a decrease in the Henrys constant possibly due to a change of the configuration of the adsorbate in the zeolite pores, while the location of the hydroxyl groups does not have a significant effect. Overall, this study provides evidence that polyol adsorption is primarily a function of dispersion forces that are derived from the fit of the adsorbate in the adsorbent pores. These findings could have an impact on the separation and catalytic conversion of oxygenates in the processing of biomass to chemicals and fuels.

Probing the Relationship between Silicalite-1 Defects and Polyol Adsorption Properties

The relationship between polyol adsorption affinity and silanol defect density was investigated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials which vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes were confirmed as crystalline mordenite framework inverted (MFI) by SEM and XRD and were shown to contain ~8.5-0 silanol defects per unit cell by (29)Si MAS, (1)H MAS, and (1)H-(29)Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10(-3)P/P0 evolved with a decrease in silanol defects, and, through features in the XRD and (29)Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic-monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption at ~10(-3) g/mL with an increase from ~0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption isotherms on defect-free silicalite-1 at 333 K, both of which exhibit the Type-V character, indicates that water enhances adsorption by a factor of ~2 in the Henrys Law regime. Henrys constants for aqueous C2-C4 polyol adsorption (concentrations below 0.004 g/mL) at 298 K are shown to have a linear dependence on the silanol defect density, demonstrating that these molecules preferentially adsorb at silanol defects at dilute concentrations. This systematic study of polyol adsorption on silicalite-1 materials highlights the critical role of defects on adsorption of hydrophilic molecules and clearly details the effects of coadsorption of water, which can guide the selection of zeolites for separation of biomass-derived oxygenates.

Ethanol dehydration to ethylene in a stratified autothermal millisecond reactor.

The concurrent decomposition and deoxygenation of ethanol was accomplished in a stratified reactor with 50-80 ms contact times. The stratified reactor comprised an upstream oxidation zone that contained Pt-coated Al(2)O(3) beads and a downstream dehydration zone consisting of H-ZSM-5 zeolite films deposited on Al(2)O(3) monoliths. Ethanol conversion, product selectivity, and reactor temperature profiles were measured for a range of fuel:oxygen ratios for two autothermal reactor configurations using two different sacrificial fuel mixtures: a parallel hydrogen-ethanol feed system and a series methane-ethanol feed system. Increasing the amount of oxygen relative to the fuel resulted in a monotonic increase in ethanol conversion in both reaction zones. The majority of the converted carbon was in the form of ethylene, where the ethanol carbon-carbon bonds stayed intact while the oxygen was removed. Over 90% yield of ethylene was achieved by using methane as a sacrificial fuel. These results demonstrate that noble metals can be successfully paired with zeolites to create a stratified autothermal reactor capable of removing oxygen from biomass model compounds in a compact, continuous flow system that can be configured to have multiple feed inputs, depending on process restrictions.