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Bell, Alexis T.

Publications and source records attributed to Bell, Alexis T..

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Catalytic Upgrading of Biomass-Derived Methyl Ketones to Liquid Transportation Fuel Precursors by an Organocatalytic Approach

A highly efficient water-tolerant, solid-base catalyst for the self-condensation of biomass-derived methyl ketones to jet-diesel fuel precursors was developed by grafting site-isolated secondary amines on silica-alumina supports. Here, it is shown that apart from the nature and density of amine groups and the spatial separation of the acidic and basic sites, the acidity of the support material plays a critical role in defining the catalytic activity. It is also found that a combination of weakly acidic silanol/aluminol with secondary amine groups can mimic proline catalysts and are more effective in catalyzing the selective dimerization reaction than the combination of amines with organic acids. In situ FTIR measurements demonstrate that acidic groups activate methyl ketones through their carbonyl groups leading to a favorable C-C bond formation step involving an enamine intermediate. DFT analysis of the reaction pathway confirms that C-C bond formation is the rate-limiting step.

aldol condensation↗
DOE OSTI2015

Pretreatment of Miscanthus giganteus with Lime and Oxidants for Biofuels

To make biomass more accessible for enzymatic hydrolysis, lime pretreatment of Miscanthus giganteus with oxidants was explored from 100 to 150 °C. Composition data for the recovered solid were obtained to determine the effects of the reaction time, lime dosage, oxidant loading, and temperature on sugar production efficiency. Under selected conditions (0.2 g of lime/g of biomass, 200 psig O 2 , and 150 °C for 1 h), delignification was 64.7%. The pretreated biomass was then followed by enzymatic hydrolysis. The yield of cellulose in the recovered solid to glucose was 91.7% and hemicellulose to xylose was 67.3%, 7.1 and 18.2 times larger than those obtained from raw biomass, respectively. Finally, pretreatment with oxidants substantially raised delignification of raw M. giganteus , thereby enhancing enzymatic hydrolysis to sugars, while results were not improved when pretreatment included ammonium molybdate.

09 BIOMASS FUELS↗
DOE OSTI2015

Synthesis of biomass-derived methylcyclopentane as a gasoline additive via aldol condensation/hydrodeoxygenation of 2,5-hexanedione

A novel approach to produce biomass-derived gasoline is the hydrolysis of 2,5-dimethylfuran (DMF) to produce 2,5-hexanedione followed by base-catalyzed intramolecular aldol condensation of this product to form 3-methylcyclopent-2-enone (MCP). By proper choice of catalysts and conditions, MCP yields of 98% can be achieved. Here, we further show that hydrogenation of MCP over Pt/NbOPO 4 gives methylcyclopentane with virtually quantitative yields. Methylcyclopentane is an attractive gasoline substitute for ethanol, since its octane number is similar to ethanol and its gravimetric energy density is 58% higher.

09 BIOMASS FUELS↗
DOE OSTI2015

The Role of Metal Halides in Enhancing the Dehydration of Xylose to Furfural

The dehydration of xylose yields furfural, a product of considerable value as both a commodity chemical and a platform for producing a variety of fuels. When xylose is dehydrated in aqueous solution in the presence of a Brønsted acid catalyst, humins are formed via complex side processes that ultimately result in a loss in the yield of furfural. Such degradative processes can be minimized via the insitu extraction of furfural into an organic solvent. The partitioning of furfural from water into a given extracting solvent can be enhanced by the addition of salt to the aqueous phase, a process that increases the thermodynamic activity of furfural in water. Although the thermodynamics of using salts to improve liquid-liquid extraction are well studied, their impact on the kinetics of xylose dehydration catalyzed by a Brønsted acid are not. The aim of the present study was to understand how metal halide salts affect the mechanism and kinetics of xylose dehydration in aqueous solution. We found that the rate of xylose consumption is affected by both the nature of the salt cation and anion, increasing in the order no salt < K + < Na + < Li + and no salt < Cl - < Br - < I - . Furfural selectivity increases similarly with respect to metal cations, but in the order no salt < I - < Br - < Cl - for halide anions. Multinuclear NMR was used to identify the interactions of cations and anions with xylose and to develop a model for explaining xylose-metal halide and water-metal halide interactions. The results of these experiments coupled with 18 O-labeling experiments indicate that xylose dehydration is initiated by protonation at the C 1 OH and C 2 OH sites, with halide anions acting to stabilize critical intermediates. Finally, the means by which metal halides affect the formation of humins was also investigated, and the role of cations and anions in affecting the selectivity to humins is discussed.

anions↗
DOE OSTI2014

Improved Force-Field Parameters for QM/MM Simulations of the Energies of Adsorption for Molecules in Zeolites and a Free Rotor Correction to the Rigid Rotor Harmonic Oscillator Model for Adsorption Enthalpies

Quantum mechanics/molecular mechanics (QM/MM) simulations provide an efficient avenue for studying reactions catalyzed in zeolite systems; however, the accuracy of such calculations is highly dependent on the zeolite MM parameters used. Previously reported parameters (P1), which were chosen to minimize the root mean square (RMS) deviations of adsorption energies compared with full QM ωB97X-D/6-31+G** adsorption energies, are shown to overestimate binding energies compared with experimental values, particularly for larger substrates. To address this issue, a new parameter set (P2) is derived by rescaling the previously reported characteristic energies of the Lennard-Jones potential in P1. The accuracy of the thermal correction for adsorption enthalpies determined by the rigid rotor-harmonic oscillator approximation (RRHO) is examined and shown to be improved by treating low-lying vibrational modes as free translational and rotational modes via a quasi-RRHO model. Here, with P2 and quasi-RRHO, adsorption energies calculated with QM/MM agree with experimental values with an RMS error of 1.8 kcal/mol for both nonpolar and polar molecules adsorbed in MFI, H-MFI, and H-BEA. By contrast, the RMS error for the same test sets obtained using parameter set P1 is 8.3 kcal/mol. Glucose-fructose isomerization catalyzed by Sn-BEA is taken as an example to demonstrate that improved values for apparent activation energies can be obtained using the methodology reported here. With parameter set P2, the apparent activation energy calculated with QM/MM reproduces the experimental value to within 1 kcal/mol. By contrast, using parameter set P1, the error is -12.9 kcal/mol. (Graph Presented).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

How to Chemically Tailor Metal-Porphyrin-Like Active Sites on Carbon Nanotubes and Graphene for Minimal Overpotential in the Electrochemical Oxygen Evolution and Oxygen Reduction Reactions

Density functional theory calculations are used to study the energetics of the electrochemical oxygen evolution reaction (OER) of water and the reverse oxygen reduction reaction (ORR) on metal-porphyrin-like centers incorporated into graphene layers or single-walled carbon nanotubes (SWCNTs). The objective is to explore the reductions in computational thermodynamic overpotential that can be achieved relative to catalysis on metal oxide surfaces (OER) or platinum (ORR) by varying the metal center and axial ligand. This permits a degree of simultaneous control over the free energy gap between the lowest energy OH and highest energy OOH intermediates, and the position of the oxo (O) intermediate in this gap. Optimal choice of metal toward the right of the first transition series largely controls the gap. Given a suitable metal such as Fe, the overpotential for OER can be tuned over a range greater than 0.35 V by choice of the axial ligand. For OER occurring within the SWCNTs, a minimum predicted overpotential of 0.35 V is found, very close to the gap-imposed limit of 0.30 V for this system. Similarly, the overpotential of ORR can be tuned over a range more than 0.30 V by selection of the axial ligand. While the calculations necessarily have limited accuracy, the principles should provide a transferable path toward overpotential optimization for the OER and ORR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

The Role of Hydroxyl Group Acidity on the Activity of Silica-Supported Secondary Amines for the Self-Condensation of n -Butanal

The catalytic activity of secondary amines supported on meso-porous silica for the self-condensation of n -butanal to 2-ethylhexenal can be altered significantly by controlling the Brønsted acidity of M - OH species present on the surface of the support. In this study, M - OH (M = Sn, Zr, Ti, and Al) groups were doped onto the surface of SBA-15, a mesoporous silica, prior to grafting secondary propyl amine groups on to the support surface. The catalytic activity was found to depend critically on the synthesis procedure, the nature and amount of metal species in-troduced and the spatial separation between the acidic sites and amine groups. DFT analysis of the reaction pathway indicates that, for weak Brønsted acid groups, such as Si - OH, the rate-limiting step is C - C bond formation, whereas for stronger Brønsted acid groups, such as Ti and Al, hydrolysis of iminium species produced upon C - C bond formation is the rate-limiting step. Finally, theoretical analysis shows further that the apparent activation energy decreases with increasing Brønsted acidity of the M - OH groups, consistent with experimental observation.

aldol reaction↗
DOE OSTI2014

Selective Hydrogenation of Furan-Containing Condensation Products as a Source of Biomass-Derived Diesel Additives

Here, we demonstrate that while the energy density and lubricity of the C 15 and C 16 products of furan condensation of biomass-derived aldehydes with 2-methylfuran are consistent with requirements for diesel, these products do not meet specifications for cetane number and pour point due to their aromatic furan rings. However, a novel class of products that fully meet or exceed most specifications for diesel can be produced by converting the furan rings in these compounds to cyclic ether moieties. Full hydrodeoxygenation of furan condensation products to alkanes would require 55-60% higher hydrogen demand, starting from biomass, compared to the products of furan ring saturation, providing an additional incentive to support the saturated products. We also report here on a tunable class of catalysts that contain Pd nanoparticles supported on ionic liquid-modified SiO 2 that can achieve complete saturation of the furan rings in yields of 95% without opening these rings. Furans to fuels: Production of diesel fuels from lignocellulosic feedstocks has largely relied on hydrodeoxygenation of oxygenates to alkanes. However, the products of selective ring saturation have been demonstrated herein to provide diesel additives of exceptional quality with minimal H 2 input. Further, a new class of heterogeneous palladium catalysts with tunable surface properties achieves excellent yields to cyclic ethers without hydrogenolysis.

biomass↗
DOE OSTI2014

Computational Study of p -Xylene Synthesis from Ethylene and 2,5-Dimethylfuran Catalyzed by H-BEA

Detailed mechanisms for the synthesis of p -xylene as well as the primary byproducts observed experimentally, 2,5-hexadione and 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan, from ethylene and 2,5-dimethylfuran (DMF) mediated by H-BEA are obtained using an extended QM/MM model containing 208 tetrahedral atoms. The formation of p -xylene proceeds via Diels-Alder cycloaddition of ethylene and DMF, which is rate-limiting, followed by Brønsted acid-catalyzed dehydration. Secondary addition of DMF to the substrate following the Diels-Alder reaction leads to 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan. The analysis of the free energies associated with the mechanisms suggests that the secondary addition can be eliminated by introducing n -heptane as an inert solvent to decrease the loading of DMF in the zeolite or by using a weak Brønsted acid site to facilitate the dehydration of the Diels-Alder product, for which the rate is determined by the deprotonation via the conjugate base of the active site. Water formed in the dehydration process can react directly with DMF to form 2,5-hexadione, thereby decreasing the yield of p -xylene. However, the free-energy barriers for the formation of 2,5-heaxdione compared to the Diels-Alder reaction indicate that DMF and 2,5-hexadione will be equilibrated. Finally, the 2,5-hexadione yield can be minimized by operating at a high conversion of DMF.

Li, Yi-Pei↗
DOE OSTI2014

Synthesis and Characterization of Supported Cobalt-Manganese Nanoparticles as Model Catalysts for Fischer-Tropsch Synthesis

Supported Co is an effective catalyst for the Fischer-Tropsch synthesis of various hydrocarbon products that can be converted to diesel. Recent studies have shown that the formation of methane can be suppressed and the formation of C 5+ products enhanced by promoting Co with Mn. Because the activity and product selectivity of Co-based catalysts are dependent on the size of Co nanoparticles and the extent of Co promotion by Mn, it is desirable to understand these effects by investigating the performance of Co nanoparticles with well-defined size and elemental composition. The present study was undertaken with the aim of producing well-defined nanoparticles of Co and Co-Mn and then supporting them on silica. Co and Co-Mn particles were synthesized through the polyol reduction of Co and Mn acetylacetonates. By controlling synthesis conditions, Co particles with diameters of 7-10 nm and similarly sized Co-Mn (Mn/Co=0.1) particles were prepared. XRD and elemental mapping with scanning TEM-energy-dispersive X-ray spectroscopy and scanning TEM-electron energy loss spectroscopy studies suggested that most of the Mn species was associated with the Co particles. Ex situ prepared Co and Co-Mn nanoparticles were first supported on silica and then investigated for the catalytic activity for the Fischer-Tropsch synthesis. The turnover frequencies and product distributions obtained with silica-supported Co and Co-Mn nanoparticles were similar to those obtained with catalysts prepared by using the conventional incipient wetness impregnation method. However, the rate of CO consumption per mass of Co was much lower for the catalysts produced by supporting ex situ prepared nanoparticles. This effect was attributed to the sintering of the nanoparticles during their calcination and reduction. Magnetic interactions among nanoparticles during their immobilization and thermal pretreatment were identified as the primary cause of sintering. Balancing yin and yang energy: The immobilization of well-defined cobalt-manganese nanoparticles on porous supports leads to the formation of Fischer-Tropsch catalysts with turnover frequencies and product selectivities comparable to those of conventionally prepared catalysts. The catalytic activity per cobalt mass is found to be significantly lower owing to sintering. Finally, magnetic interactions among nanoparticles during immobilization and thermal pretreatment are identified as the primary cause of this phenomenon.

cobalt↗
DOE OSTI2014

Reaction Dynamics of Zeolite-Catalyzed Alkene Methylation by Methanol

A hybrid quantum mechanics/molecular mechanics (QM/MM) model and the quasiclassical trajectory (QCT) method have been combined to study the reaction of alkene methylation by methanol catalyzed by the zeolite H-MFI. The rate-limiting step of this reaction is the methylation of the alkene, and the apparent activation energy calculated at the ωB97X-D/6-31G(d,p)//ωB97X-D/6-311++G(3df,3pd) level of theory for this step agrees well with experiment and previous full QM studies. Following the ethene methylation transition state toward the products along the intrinsic reaction coordinate reveals the existence of a protonated cyclopropane (PCP + ) carbocation intermediate. A similar protonated methylcyclopropane (mPCP + ) carbocation intermediate is found for propene methylation. The intermediates produced during the alkene methylation reaction are metastable with a lifetime of O (1 ps) obtained from QCTs. Because of the short lifetime of these intermediates, the available energy in the carbocation is not in thermal equilibrium distribution with the zeolite lattice before subsequent reaction occurs. The qualitative difference between product distributions obtained by static and dynamic reaction pathways suggests the pathways of zeolite-catalyzed reactions proceed through high-temperature pathways that differ from the 0 K potential energy surface. The transformation of the m-PCP + intermediate to the longer-lived secondary 2-butyl carbocation observed during QCTs suggests that more stable carbocations can properly thermalize and exist as reaction intermediates for longer than 1 ps.

Gomes, Joseph↗
DOE OSTI2014

Erratum: “A finite difference Davidson procedure to sidestep full ab initio hessian calculation: Application to characterization of stationary points and transition state searches”

Certain key contributions to the field were not presented in the Introduction section of our recent paper. In addition to the variational method described in Ref. 14 and iterative diagonalization methods in Refs. 15–17, we wish to direct the attention of the reader to other significant efforts that have utilized similar principles for various applications. Reiher and Neugebauer developed a mode-tracking algorithm using the Davidson method for calculating only the modes relevant for vibrational analysis. Similarly, a block-Davidson approach via finite differentiation of the gradient was implemented for calculating hessian-vector products for normal mode analysis. Deglmann and Furche developed a method for stationary point characterization with density functional theory where full solution of all coupled-perturbed Kohn-Sham equations, the bottleneck in hessian calculation, could be avoided through iterative calculation of hessian-vector products to determine only the lowest eigenvalues.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2014

Single iron sites for catalytic, nonoxidative conversion of methane

In this paper we discuss how the availability of inexpensive natural gas from the fracturing of shale and as a byproduct of oil production has stimulated the investigation of catalytic processes for the direct conversion of methane to products. Efforts in this direction are further driven by the desire to avoid the generation of CO 2 and its emission to the atmosphere, as occurs when methane is steam-reformed to produce synthesis gas, a mixture of CO and H 2 that can be used for the production of fuels and chemicals via Fischer-Tropsch synthesis. During the past forty years, numerous investigators have attempted to convert methane to higher value products by either thermal dehydrogenation or by oxidative dehydrogenation. While methane can be converted to benzene and hydrogen via thermal dehydrogenation, this process is accompanied by the deposition of large amounts of coke, leading to rapid catalyst deactivation. The oxidative coupling of methane has focused on the formation of ethylene, but this chemistry is not selective and also results in a substantial combustion of methane to CO 2 and water.

Bell, Alexis T.↗
DOE OSTI2014

A finite difference Davidson procedure to sidestep full ab initio hessian calculation: Application to characterization of stationary points and transition state searches

The cost of calculating nuclear hessians, either analytically or by finite difference methods, during the course of quantum chemical analyses can be prohibitive for systems containing hundreds of atoms. In many applications, though, only a few eigenvalues and eigenvectors, and not the full hessian, are required. For instance, the lowest one or two eigenvalues of the full hessian are sufficient to characterize a stationary point as a minimum or a transition state (TS), respectively. We describe here a method that can eliminate the need for hessian calculations for both the characterization of stationary points as well as searches for saddle points. A finite differences implementation of the Davidson method that uses only first derivatives of the energy to calculate the lowest eigenvalues and eigenvectors of the hessian is discussed. This method can be implemented in conjunction with geometry optimization methods such as partitioned-rational function optimization (P-RFO) to characterize stationary points on the potential energy surface. With equal ease, it can be combined with interpolation methods that determine TS guess structures, such as the freezing string method, to generate approximate hessian matrices in lieu of full hessians as input to P-RFO for TS optimization. This approach is shown to achieve significant cost savings relative to exact hessian calculation when applied to both stationary point characterization as well as TS optimization. The basic reason is that the present approach scales one power of system size lower since the rate of convergence is approximately independent of the size of the system. Therefore, the finite-difference Davidson method is a viable alternative to full hessian calculation for stationary point characterization and TS search particularly when analytical hessians are not available or require substantial computational effort.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Analysis of the Reaction Mechanism and Catalytic Activity of Metal-Substituted Beta Zeolite for the Isomerization of Glucose to Fructose

Glucose-fructose isomerization mediated by Sn-BEA is investigated using an extended QM/MM model containing 208 tetrahedral atoms. The isomerization mechanism consists of a sequence of ring-opening, isomerization, and ring-closing processes, consistent with the previously reported experimental observations. In agreement with the experimentally observed kinetic isotope effect, the rate-determining step is found to involve a hydride shift from the C 2 carbon to the C 1 carbon. The apparent activation energy for the rate-limiting step is 22.3 kcal/mol at 343 K. The difference in the reaction barriers for the partially hydrolyzed and the fully coordinated Sn sites was investigated using energy decomposition analysis. It is found that the higher activity of the partially hydrolyzed site comes from the extra flexibility provided by the defect in the lattice. The effect of substituting Sn in the active site by Ti, Zr, V, Nb, Si, and Ge was examined, and it was found that Sn and Zr are metals that result in the lowest reaction barrier for glucose isomerization. By using energy decomposition analysis, two physical properties are shown to contribute to the magnitude of the reaction barrier: the polarizability of the metal atom in the active site and the Brønsted basicity of the oxygen atom bound to the metal atom.

anions↗