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Core Basic Energy Sciences Catalysis Program

Thrust 1: Catalytic Conversion of Polar Molecules

Catalysis with polar biomass-derived molecules addresses individual reactions of the multistep conversion of polar molecules on surfaces and in the pores of metal oxides as well as on metal particles and coordination complexes. The aim is to develop a better understanding of the role of the metal (linking homogeneous and heterogeneous catalysts) as well as the acid-base and redox functions as they interact with polar bio-derived molecules. The goal is further to achieve relative stabilization of reactants, intermediates, and products as well as the impact of the local environment. We will characterize the nature of the catalytic sites as well as the steric environments around those sites. Our goal is to link insight into reactions by molecular catalysts and on surfaces, as well as in pores. We will make quantitative comparisons of the reactions in the condensed phase to analogous reactions at gas-solid interfaces. Hydrodeoxygenation and hydroalkylation will be the reactions we use to exemplify potential routes from lignocellulosic biomass to alkane energy carriers requiring several catalytic functions acting sequentially. These reactions start in an aqueous environment, but the products form a nonpolar phase as the conversion progresses. Thus, the effect of the polarity of the reaction environment on the catalytic site has to be explored. We will compare single-site solid catalysts with molecular catalysts.

Multifunctional Solid Catalysts for Lignin to Hydrocarbons – Understanding and Controlling Scalable Catalytic Routes in Aqueous Phase

Donald M. Camaioni, Nigel Browning, Mirek Derewinski, John Fulton, Jian Zhi Hu, Andreas Jentys, Johannes Lercher, Donghai Mei, Yong Wang

The catalyzed-conversion of lignin to alkane energy carriers requires a cascade of reactions for deconstructing and reducing the polymeric, highly oxofunctionalized material. While lignin is the most intractable component of lignocellulose, its conversion to useful products is particularly important, because the carbon in lignin is the most reduced fraction of lignocellulose. Our recent work has been structured to investigate the catalysis of steps important for the deconstruction of lignin, for hydrogenation and hydrode-functionalization of oxygenated intermediates, and for C-C bond coupling reactions to adjust the size of the product molecules. This has been complemented by focusing on understanding the state and stability of catalysts in the reaction media. Mechanistic and kinetic studies have emphasized cooperative effects between acid and metal functionalities and the influence of the reaction environment (confinement and solvent) on reaction rates and transformation pathways. This has been complemented with advanced characterization of the catalyst and an understanding of the implications of catalyst structure on stability in diverse reaction media. The work emphasizes gaining fundamental insight into the principal chemistry of representative functionalities and their transformation at the active sites. This knowledge provides directions to synthesize a new generation of catalysts.

Dehydration of Alcohols within Zeolite Pores in Condensed Phase

Donald M. Camaioni, Nigel Browning, Mirek Derewinski, John Fulton, Jian Zhi Hu, Andreas Jentys, Johannes Lercher, Donghai Mei, Yong Wang

Figure 2. Measured turnover frequencies of cyclohexene formation in aqueous phase dehydration of cyclohexanol on different acid catalysts. For zeolites, the Si/Al ratio is denoted as the number following the framework type code. H₄SiW and H₃PW stand for tungstosilicic and phosphotungstic acids.

The dehydration of cyclohexanol, a model compound for hydrogenated oxygenated intermediates in the transformation of bio-oil to fuel, has been explored to understand the mechanism of acid-catalyzed C-O bond cleavage. In the aqueous phase, inside or outside the zeolite pores, cyclohexanol dehydration proceeds via an E1 mechanism with the C-H bond cleavage as the rate-determining step. The reaction, however, proceeds with significantly higher rates with hydronium ions confined in zeolite pores. This is due in part to the two to three times higher intrinsic rate constant for dehydration in zeolite (BEA) than in water. Where the enthalpies of activation are similar in the zeolite and in water, the associated activation entropy is higher for hydronium ions in the zeolite pores than in water. The stark effect of confinement on rates, however, is mainly due to lower entropy losses when the association complex (alcohol-hydronium ion) is formed in the zeolite. This is attributed to the low entropy of molecules diffusing within the pores, which increases the extent of association between substrate and acid site.

Confirming the correlation between confinement and enhanced rates, the systematic study of MFI, BEA, and FAU zeolites for cyclohexanol dehydration showed increasing rates with decreasing pore size (Figure 2). Hence, the association between hydronium ions and alcohol is enhanced by molecularly sized pores, which pose steric environments similar to those around active site pockets in enzymes.

Effect of Liquid Water and Framework Sites on Zeolite Lattice Stability and Catalytic Activity

Donald M. Camaioni, Nigel Browning, Mirek Derewinski, John Fulton, Jian Zhi Hu, Andreas Jentys, Johannes Lercher, Donghai Mei, Yong Wang

Figure 3. Comparison between the zeolite stability in pure hot liquid water (retained crystallinity) and for aqueous phase catalysis (i.e., turnover number).

The key to the application of zeolites in the transformation of polar moleculesin aqueous phase is their stability. Thus, strategies to understand and counteract the corrosive effects of hot water are being developed. The states of Al and Si atoms are characterized by cross-polarization enhanced ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), ²⁷Al MAS NMR, and infrared (IR) spectroscopy. The stability of the BEA zeolites has been found to be primarily determined by the concentration of hydronium ions in the pores. Thus, reducing the water concentration slows down the hydrolysis of the zeolite. The concentration of framework defects, on the other hand, is important only at low concentrations of framework Al, which correlate with hydronium ion concentrations in the presence of water. Notably, the concentration of hydronium ions does not affect the rate of zeolite hydrolysis. Thus, the hydrolysis of the framework is a self-limiting process because the availability of mobile water molecules (needed for continuous hydrolysis) is limited by the presence of cyclohexanol as well as hydrolyzed silica species in the pores. The effect of cyclohexanol is illustrated in Figure 3, which shows opposing trends for the structural stability in liquid water and activity for cyclohexanol dehydration. The high affinity of BEA40 and BEA75 for cyclohexanol stabilizes the zeolite and compensates for the destabilizing effect of structural defects.

Hydroalkylation to Adjust the Size of Alkanes

Donald M. Camaioni, Nigel Browning, Mirek Derewinski, John Fulton, Jian Zhi Hu, Andreas Jentys, Johannes Lercher, Donghai Mei, Yong Wang

Figure 4. Concentration profiles, monitored by in situ NMR, during the alkylation of phenol with cyclohexene only (a) and with equimolar concentrations of cyclohexene and cyclohexanol (b) at 127 °C.

Acid-catalyzed C-C coupling yields alkylated phenolics with carbon-numbers in the fuel range through the reactions of phenolics with alcohols. In zeolites, the catalytic activity, mechanism, and reaction pathways (C- or O-alkylation) depend on the concentration and strength of acid sites, space constraints for the reaction (whether or not the pore size allows the transition state) and alkylating agent. Detailed kinetic analyses and in situ ¹³C MAS NMR spectroscopy in nonpolar media show that phenol alkylation with cyclohexanol does not appreciably occur before a majority of the cyclohexanol has been dehydrated to cyclohexene. Figure 4 shows that alkylation reactions are slowed when cyclohexanol and cyclohexene are present. In contrast, alkylation products are readily formed when the solution initially contains just phenol and cyclohexene. The strict reaction sequence is not posed by competitive adsorption (phenol and cyclohexanol show similar adsorption strengths) but by the absence of a reactive electrophile. This is due to the preferential formation of protonated dimers of cyclohexanol at Brønsted acid sites, which hinders the adsorption of cyclohexene. At low coverage of the acid sites by protonated dimers, cyclohexene adsorption and protonation yields cyclohexyl carbenium ions, which perform an electrophilic attack on phenol to produce alkylated products. This further implies that protonated cyclohexanol dimers dehydrate without the formation of carbenium ions.

Cleavage of Ether Bonds

Donald M. Camaioni, Nigel Browning, Mirek Derewinski, John Fulton, Jian Zhi Hu, Andreas Jentys, Johannes Lercher, Donghai Mei, Yong Wang

Figure 5. Mechanistic pathways for reductive hydrolysis of aromatic ethers on Pd catalysts (R denoted phenyl, cyclohexyl and n-butyl).

Cleavage of C-O bonds in aromatic ethers, a crucial step for the conversion of lignocellulosic biomass to fuels, is challenging due to the stability of these linkages. Hydrogenolytic cleavage requires high temperatures and pressures. Thus, alternative reaction pathways have been explored to optimize the transformations of aromatic ethers. Pd catalysts are active and highly selective toward hydrolysis of diaryl and aryl alkyl ethers in aqueous phase under mild conditions. This is a reductive transformation, which is initiated by partial Hydrogenation of the arene ring yielding an enol ether intermediate that is highly reactive with water. This novel pathway (illustrated in Figure 5) contrasts with the acid-catalyzed mechanism for ether cleavage. The mechanism can be generalized (solvolysis) as the analogous transformation and has been observed in methanol. In this case, the enol ether undergoes methanolysis to a ketal, which eliminates phenol/alkanol to generate methoxycyclohexene. The attack of water, however, is faster than the attack of methanol, which leads to more hydrolyzed products, compared to methanolysis products, in aqueous/alcohol media.

Synergetic Effect of Support Acidity on the Metal-Catalyzed Hydrogenation of Phenol

Donald M. Camaioni, Nigel Browning, Mirek Derewinski, John Fulton, Jian Zhi Hu, Andreas Jentys, Johannes Lercher, Donghai Mei, Yong Wang

Figure 6. Phenol hydrogenation pathway on Pt sites in the absence (a) or in the presence (b) of hydronium ions.

The enhancement of the activity of supported metal catalysts by the acidity of the support has been well documented in the gas phase albeit without a widely accepted interpretation. In contrast, this phenomenon is hardly documented for reactions in condensed phase, which poses particular challenges for fundamental studies. The mechanistic aspects of the synergetic effect of Brønsted acid sites on the metal functionality were studied with Pt clusters encapsulated in a MFI zeolite with varying acid site concentrations. Pt clusters in MFI zeolites with increasing hydronium ion concentrations exhibit increasing hydrogenation activity despite decreased affinities to adsorb phenol and H₂. These observations and changes in the measured reaction orders support a shift of the rate-determining step from the first H addition at the ortho-position (in the absence of hydronium ions) to the second one at the ortho- and para-position (if hydronium ions are present). The decreased activation barrier for the first H addition (by 30 kJ·mol^-1) is attributed to a mechanism, where a hydronium ion protonates the adsorbed phenol ring followed by an electron transfer from an adsorbed H. The latter replaces the hydronium ion upon being oxidized (Figure 6). The enhancing effect of hydronium ions on the activity of supported metals is of special importance for biomass conversion as it is a potential strategy to boost rates at mild conditions.

Fundamentals of Acid-Base and Redox Reactions on Metal Oxide Catalysts

David Dixon (University of Alabama), Zdenek Dohnálek, Feng Gao, Jian Zhi Hu, Enrique Iglesia (University of California, Berkeley), Bruce Kay, Jun Liu, Roger Rousseau, Huamin Wang, Yong Wang (Washington State University)

A primary goal of these activities is to advance our fundamental understanding of metal oxide-based catalysts and to design new and improved acid and redoxactive catalysts. In particular, we seek atomic-level geometric and electronic structure descriptions of active sites and precise determinations of reaction mechanisms at the level of elementary steps. Where fundamental studies of oxide catalysts are limited by current methods and by materials that can be precisely characterized, we are developing new experimental and computational approaches, and synthesizing model catalysts. Our research team contains broad expertise in catalysis and surface science, materials synthesis, and computational chemistry. This breath allows an integrated experimental/theoretical approach involving state-of-the-art characterization and mechanistic kinetics studies of high surface area model metal oxide catalysts, precise and detailed ultrahigh vacuum studies of single crystals, and the use of cluster and extended surface computational approaches that take advantage of the tractable and known structures of the catalysts used.

Site-Specific Measurement of Acid-Base Properties on TiO₂(110)

Zdenek Dohnálek, Vassiliki-Alexandra Glezakou, Igor Lyubinetsky, Roger Rousseau, Gregory Schenter

Figure 7. Potential energy surface for H₂O deprotonation on TiO₂(110) determined in this study.

We have constructed a new combined supersonic molecular beam, scanning tunneling microscopy (STM) instrument and carried out novel measurements that in combination with ab initio molecular dynamics (AIMD) yield a detailed kinetic and dynamic description of dissociation and recombination of surface species. We have utilized this approach for the first time and determined the relative stability of molecularly and dissociatively bound H₂O on rutile TiO₂(110) (Figure 8), which leads to the formation of pairs of terminal and bridging hydroxyl species, H₂O ↔ HO_t + HO_b. The energetics of this process has been debated for decades, but it has never been successfully measured. The results of our measurements show the onset of H₂O dissociation at 0.3 eV of H₂O incident energy, independent of whether the molecules impinge along or across the Ti rows at an incident angle of 60° relative to surface normal. Following the onset, the dissociation probability increases linearly with increasing incident energy. Ensembles of AIMD simulations at several incident energies reproduce the product distribution seen in the STM. Additionally, the theoretical studies show that the dissociation occurs only for the impacts near surface Ti ions with an activation energy of ~ 0.3 eV and that the O-H bond cleavage is accomplished within the time of a single vibration. The AIMD simulations were further used to construct a classical potential energy surface for H₂O/TiO₂(110) interactions and execute classical molecular dynamics simulations that closely reproduce the onset and linear energy dependence of the dissociation probabilities. We quantify the deprotonation/protonation barriers of 0.36 eV and find that molecularly bound H₂O is preferred over the surface-bound hydroxyls by only 0.035 eV. We demonstrate that long-range electrostatic fields emanating from the oxide lead to steering and reorientation of molecules approaching the surface, activating the O-H bonds and inducing deprotonation. The developed methodology for studying metastable reaction intermediates prepared with a high-energy molecular beam in the STM can be readily extended to other systems to clarify a wide range of important bond activation processes.

Surface Species Following Water Adsorption on Oxidized RuO₂(110)

Zdenek Dohnálek, Vassiliki-Alexandra Glezakou, Igor Lyubinetsky, Roger Rousseau

Figure 8. Consecutive STM images of the same area of the RuO₂(110) dosed with H₂O at 295 K (left) and subsequently with O₂ (right). The image sequence illustrates the following surface reaction: HO_t-H₂O + HO_b + ½O₂ → 2HO_t-HO_t + O_b.

Identifying and understanding how the extent of surface oxidation affect the adsorption and dissociation of H₂O on metal oxides is crucial for the understanding the acid-base properties of different surface sites on oxides. Here, by means of high-resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that excess oxygen atoms on the stoichiometric RuO₂(110) significantly change the clustering, conformation, and deprotonation equilibrium of adsorbed H₂O. To elucidate effects of oxygen on H₂O adsorption, we considered two reaction sequences at room temperature: (1) the stoichiometric surface is first exposed to H₂O followed by reaction with oxygen (Figure 8) and (2) the surface is first in part oxidized then H₂O is dosed. In both cases, the [OH-OH] complex on Ru rows is the dominant species, showing a significant difference from H₂O only adsorption on the stoichiometric surface in which the [OH-H2O] species is found to be prevalent. We have also studied surface reactivity at almost full O coverage; there we show that site selectivity of the surface for H adsorption and dissociation of H₂O is hindered, notwithstanding the increase of the dynamical motion of both species.

Ceria and Titania Nanoclusters on Supported Graphene as New Model Systems for Catalytic Studies

Zdenek Dohnálek, Bruce Kay

Figure 9. Ceria nanoclusters deposited on Gr on Ru(0001) via reactive deposition of Ce in 1 x 10^-7 Torr of O₂ at 500 K.

We have investigated the growth of CeO₂ and TiO₂ nanoclusters on single-layer graphene/Ru(0001) with a view toward fabricating stable systems for model catalysis studies. Both oxides were deposited by evaporating Ce and Ti metals as a function of oxygen pressure and substrate temperature. Quartz crystal microbalance measurements and AES were used to determine the oxygen pressure for growth of stoichiometric oxides. Depositions performed at 300 K resulted in the formation of nano-sized clusters nucleating on intrinsic defects in the graphene layer, with an average separation between clusters of ~11 nm. The saturation cluster density decreased with increasing substrate growth temperatures from 300 to 650 K. While, the CeO₂ clusters are of (111) type already at 500 K (Figure 9), only a fraction of TiO₂ clusters shows crystallographic order even at 650 K. Thermal stability of the clusters was evaluated with AES after annealing the clusters prepared at 300 K. No chemical reduction of clusters or etching of the graphene film was observed up to ~900 K, although AES indicated significant sintering of clusters. STM studies of annealed clusters prepared at 300 K demonstrated that Smoluchowski ripening is the dominant sintering mechanism up to 800 K. Above 900 K, the clusters undergo reduction accompanied by concomitant oxidation and etching of the graphene. Our studies demonstrate that highly thermally stable CeO₂ and TiO₂ nanoclusters with varying morphologies can be successfully prepared on graphene supports.

Acid-Base Reactions on Shape-Selected Anatase TiO₂ Nanocrystals

Zdenek Dohnálek, Feng Gao, Jian Zhi Hu, Enrique Iglesia, Bruce Kay, Donghai Mei, Roger Rousseau, Huamin Wang, Yong Wang

Figure 10. Arrhenius plots of isopropanol dehydration on (■) TiO₂(101) and (▲) TiO₂(001) catalysts. 1 bar, 2 kPa isopropanol, 220-260 °C, and GHSV = 2 L·min^-1·g^-1cat.

We continued to study single facet anatase TiO₂(101) and (001) nano-materials as model catalysts for alcohol dehydration to better bridge the single crystal and practical high surface area studies. We synthesized and applied two anatase titania model catalysts, with preferential exposure of (101) and (001) facets for isopropanol dehydration, a representative probe reaction for bio-derived alcohol dehydration. A series of microscopic and spectroscopic techniques, including X-ray diffraction, scanning electron microscopy, TEM, NH₃-TPD, and pyridine-IR, were employed to correlate the structure properties of the model catalysts to their catalytic performance. The Lewis site was found to be the active site, based on active site poisoning titrations using 2,6-di-tert-butyl pyridine titrants. The higher activity for TiO₂(101) catalyst was ascribed to its higher acid strength and density as compared to TiO₂(001) (Figure 10). Reaction rate profiles as a function of partial pressure showed a Langmuir-Hinshelwood mechanism for both model catalysts, where the surface dehydration was the rate-limiting step. The kinetic isotope effect measurement indicated that the β C-H bond cleavage governed the reaction rate and the dehydration appeared to follow a concerted E2 elimination pathway. This notion was fully supported by theoretical calculations using density functional theory. This work clearly revealed the nature of active sites, mechanisms, and faceting effects for model anatase catalysts in alcohol dehydration. The results shed lights on designing highly efficient metal oxide catalysts for dehydration process during the conversion of biomass-derived molecules.

Formation of Gas Phase Radicals in Surface Reactions: Deoxygenation of Phenylmethanol on Rutile TiO₂(110)

Zdenek Dohnálek, Vassiliki-Alexandra Glezakou, Bruce Kay, Roger Rousseau

Figure 11. Proposed reaction pathways for the conversion of phenolmethanol on TiO₂(110).

The role of radicals in the reaction mechanisms leading to functionalized aromatics has been extensively argued. The involvement of radical species has been firmly established for a small number of simple reactions on high surface area oxide catalysts, such as oxidative coupling of CH₄ and selective oxidation of propylene. As a part of our ongoing studies of alcohol and diol deoxygenation, we have focused on the reaction pathways of simple lignin-derived aromatic alcohols, i.e., phenol, phenylmethanol, and 2-phenylethanol, on rutile TiO₂(110), using a combination of molecular beam dosing and TPD. For phenylmethanol, the coverage dependent TPD data show that about 40 percent of molecules adsorbed on the surface at a saturation coverage are converted to reaction products indicating that the reactions proceed on regular five-fold coordinated Ti sites. This is in contrast to aliphatic alcohols where the reactions are shown to proceed exclusively on bridging oxygen vacancy defect sites. The studies of OD-labelled phenyl-methanol demonstrate that a fraction of OD hydrogen is transferred to the benzyl group to form toluene that desorbs between 300 K and 480 K (Figure 11). In the competing reaction, the OD hydrogen is converted to H₂O at ~350 K. Once the OD hydrogen is depleted above 480 K, the remaining plenyl-methoxy surface species dissociate yielding benzyl radicals in the gas phase (Figure 11). Combined, these results show that the conversion of phenyl-methanol on TiO₂(110) proceeds via a unique chemistry. In contrast, both phenol and 2-phenylethanol exhibit expected surface chemistry analogous to that of aliphatic alcohols. These findings reveal for the first time the formation of free radical species from the interaction of phenylmethanol with TiO₂(110) and demonstrate a new direct mechanism for deoxygenation of lignin-derived benzylic alcohols to aromatics on TiO₂.

1,2-Ethanediol and 1,3-Propanediol Conversions over (MO₃)₃ (M=Mo, W) Nanoclusters

David Dixon, Zdenek Dohnálek, Bruce Kay, Roger Rousseau

Figure 12. Lowest maximum CCSD(T) barriers in kcal/mol for the conversion of ethanol, 1-propanol, ethylene glycol, and trimethylene glycol on M₃O₉ (M = Mo, W) clusters at 298 K.

The dehydration and dehydrogenation reactions of one and two 1,2-ethanediol and 1,3-propanediol molecules on (MO₃)₃ (M=Mo, W) nanoclusters have been studied computationally using density functional and coupled cluster (CCSD(T)) theory (Figure 12). The reactions are initiated by formation of a Lewis acid-base complex with an additional hydrogen bond. Dehydration is the dominant reaction proceeding via a metal bisdiolate. Acetaldehyde, the major product for 1,2-ethanediol, is produced by a hydrogen transfer from one CH₂ group to the other. For 1,3-propanediol, the C-C bond breaking pathways to produce C₂H₄ and HCH=O simultaneously and proton transfer to generate propylene oxide have comparable barrier energies. The barrier to produce propanal from the propylene oxide complex is less than that for epoxide release from the cluster. On the M₃O₉ cluster, a redox reaction channel for 1,2-ethanediol to break the C-C bond to form two formaldehyde molecules and then to produce C₂H₄ is slightly more favorable than the formation of acetaldehyde. For WVI, the energy barrier for the reduction pathway is larger due to the lower reducibility of W₃O₉. Similar reduction on Mo^VI for 1,3-propanediol to form propene is not a favorable pathway compared to the other pathways as additional C-H bond breaking is required in addition to breaking a C-C bond. The dehydrogenation and dehydration activation energies for the selected glycols are larger than the reactions of ethanol and 1-propanol on the same clusters. The CCSD(T) method is required as density functional theory (DFT) with the M06 and B3LYP functionals does not predict quantitative energies on the potential energy surface. The M06 functional performs better than does the B3LYP functional.

Benchmark Calculations of the Energetic Properties of Transition Metal Compounds

David Dixon

Figure 13. Errors in the best functionals for each property in comparison to the Feller-Peterson-Dixon values.

The heats of formation and the normalized clustering energies (NCEs) for the group 4 and group 6 transition metal oxide trimers and tetramers have been calculated by the Feller-Peterson-Dixon method. New and improved heats of formation for (CrO₃)_n clusters were obtained using PW91 orbitals instead of Hartree-Fock orbitals. Diffuse functions are necessary to predict accurate heats of formation. The fluoride affinities (FAs) are calculated with the CCSD(T) method. The relative energies (REs) of different isomers, NCEs, electron affinities (EAs), and FAs of (MO₂)_n (M = Ti, Zr, Hf, η = 1-4 ) and (MO₃)_n (M = Cr, Mo, W, η = 1-3) clusters have been benchmarked with 55 exchange-correlation density functional theory (DFT) functionals including both pure and hybrid types (Figure 13). The absolute errors of the DFT results are mostly less than ±10 kcal/mol for the NCEs and the EAs, and less than ±15 kcal/mol for the FAs. Hybrid functionals usually perform better than the pure functionals for the REs and NCEs. The performance of the two types of functionals in predicting EAs and FAs is comparable. The B1B95 and PBE1PBE functionals provide reliable energetic properties for most isomers. Long-range corrected pure functionals usually give poor FAs. The standard deviation of the absolute error is always close to the mean errors and the probability distributions of the DFT errors are often not Gaussian (normal). The breadth of the distribution of errors and the maximum probability are dependent on the energy property and the isomer. In addition, the heats of formation and bond dissociation energies for a benchmark set of first row transition metal diatomics have been predicted using the Feller-Peterson-Dixon approach. A number of the experimental values were re-evaluated and found to have issues. This work showed that the Feller-Peterson-Dixon approach based on CCSD(T) provides good predictions for the heats of formation of transition metal compounds in contrast to previous reports.

Larger Scale Molecular Simulations Accessing Operando Morphology and Reactivity of Catalytic Systems

Vassiliki-Alexandra Glezakou, Roger Rousseau

Figure 14. Redox effects on the structure and morphology of supported AuPd nanoalloys.

Modern electron structure theory coupled with molecular dynamics allows one to examine the structure and dynamics of complex reactive systems at elevated temperatures. These simulations can both rationalize the way catalysts work, but also allow for unique discoveries that may arise spontaneously out of the simulations. Using advances simulations consisting of hundreds of atoms for millions of configurations, several new transient phenomena have been discovered. For instance, simulations have that Au₂₀ on a rutile TiO₂ exhibits liquid-like morphology upon CO adsorption, due to charge transfer from the support during catalytic conversion of CO to CO₂. Likewise, the reactivity and dynamics of Au nanoparticles on both titania and ceria reveal the dynamic formation of single Au atoms forming Au-CO+ intermediates before CO₂ is formed. The Au atom is reintegrated with the Au nanoparticle to fully complete the catalytic cycle. Similar studies have shown that modulations of the redox state of both the support and nanoparticles can have a dramatic impact on the accessibility of Pd atoms in mixed AuPd nanoalloys (Figure 14).

Conversion of Ethanol to 1,3-butadiene over Na Doped Zn_xZr_yO_z Mixed Metal Oxides

Yong Wang

Figure 15. Proposed acetone-to-isobutene reaction mechanism over Zn_xZr_yO_z catalyst.

In this effort, we have been focusing on the effects of surface acidity on the cascade ethanol-to-isobutene conversion using Zn_xZr_yO_z catalysts. The ethanol-to-isobutene reaction was found to be limited by the secondary reaction of the key intermediate, acetone, namely the acetone-to-isobutene reaction. Although catalysts with coexisting Brønsted acidity could catalyze the rate-limiting acetone-to-isobutene reaction, the presence of Brønsted acidity is also detrimental. First, secondary isobutene isomerization is favored, producing a mixture of butene isomers. Second, undesired polymerization and coke formation prevail, leading to rapid catalyst deactivation. Most importantly, both steady-state and kinetic reaction studies as well as Fourier transform infrared spectroscopy (FTIR) analysis of adsorbed acetone-d₆ and D₂O unambiguously showed that a highly active and selective nature of balanced Lewis acid-base pairs was masked by the coexisting Brønsted acidity in the aldolization and self-deoxygenation of acetone to isobutene. As a result, Zn_xZr_yO_z catalysts with only Lewis acid-base pairs were discovered, on which nearly a theoretical selectivity to isobutene (~88.9 percent) was successfully achieved, which has never been reported before. Moreover, the absence of Brønsted acidity in such Zn_xZr_yO_z catalysts also eliminates the side isobutene isomerization and undesired polymerization/coke reactions, resulting in the production of high purity isobutene with significantly improved catalyst stability (< 2 percent activity loss after 200 h time-on-stream). Our integrated experimental and theoretical work suggested that Lewis acid-base pairs on a ZrO₂ catalyst can catalyze the acetone aldolization reaction to form mesityl oxide. The formed mesityl oxide strongly adsorbs and blocks the Lewis acid-base active site, resulting in the dominant acetone decomposition (Figure 15, red highlighted pathway), as well as other possible side reactions, such as polymerization. However, adding ZnO significantly modifies the properties of surface Lewis acid-base pairs. As a result, the cascade acetone aldolization and self-deoxygenation reactions are significantly accelerated, leading to the highly active and stable Zn_xZr_yO_z catalyst for both acetone-to-isobutene and ethanol-to-isobutene reactions.