Density Functional Theory-Computed Mechanisms of Ethylene and Diethyl Ether Formation from Ethanol on γ‑Al₂O₃(100)

Multiple potential active sites on the surface of γ-Al₂O₃ have led to debate about the role of Lewis and/or Brønsted acidity in reactions of ethanol, while mechanistic insights into competitive production of ethylene and diethyl ether are scarce. In this study, elementary adsorption and reaction mechanisms for ethanol dehydration and etherification are studied on the γ-Al₂O₃(100) surface using density functional theory calculations. The O atom of adsorbed ethanol interacts strongly with surface Al (Lewis acid) sites, while adsorption is weak on Brønsted (surface H) and surface O sites. Water, a byproduct of both ethylene and diethyl ether formation, competes with ethanol for adsorption sites. Multiple pathways for ethylene formation from ethanol are explored, and a concerted Lewis-catalyzed elimination (E2) mechanism is found to be the energetically preferred pathway, with a barrier of <i>E</i>_a = 37 kcal/mol at the most stable site. Diethyl ether formation mechanisms presented for the first time on γ-Al₂O₃ indicate that the most favorable pathways involve Lewis-catalyzed S_N2 reactions (<i>E</i>_a = 35 kcal/mol). Additional novel mechanisms for diethyl ether decomposition to ethylene are reported. Brønsted-catalyzed mechanisms for ethylene and ether formation are not favorable on the (100) facet because of weak adsorption on Brønsted sites. These results explain multiple experimental observations, including the competition between ethylene and diethyl ether formation on alumina surfaces

Understanding the Gas Phase Chemistry of Alkanes with First-Principles Calculations

Alkyl radicals are key intermediates in multiple industrially important reactions, including the dehydrogenation of alkanes. Because of their diverse chemistry, alkyl radicals form various products via a number of competing reactions in the gas phase. Using Density Functional Theory (DFT) and accurate ab initio electronic structure calculations (CBS-QB3), we investigated the thermodynamics and kinetics of gas phase alkyl radical reactions. Specifically, we investigated the hydrogen abstraction, radical recombination, and alkene formation reactions of light alkyl radicals (C₁–C₈). We show that the hydrogen abstraction Gibbs energies are correlated with the relative Gibbs energies of the corresponding radicals. On the basis of the reaction energy calculations, we identified that the competition between radical recombination reactions and alkene formation reactions is governed by the stability of the alkene products, with the alkene formation being preferred when more substituted alkenes are formed. It was found that the radical recombination is preferred over alkene formation at 298 K, but at high temperatures (773 K) alkene formation becomes highly preferred. Importantly, owing to the competition of different reactions, we demonstrate a systematic methodology to select a computational method to investigate the complex chemistry of alkyl radicals. Overall, this study provides a rich database of reaction energies involving alkyl radicals and identifies their thermodynamic preference that can aid in the design of more efficient processes for the chemical conversion of alkanes

Au₁₃: CO Adsorbs, Nanoparticle Responds

Nanoparticle properties are strongly correlated with their morphologies, such as shape and size. By combining density functional theory calculations with ab initio molecular dynamics simulations, we investigated the CO adsorption behavior on Au₁₃ nanoparticles of I_h, O_h, and planar symmetries. Our results revealed a shape-specific adsorption response of the nanoparticles. Contrary to the behavior in bulk, we observe a symmetry-dependent d-band center shift on the nanoparticles with CO coverage, which affects the overall electronic stability of the nanoparticles. As a result, we observe 2D to 3D (planar to I_h) transition at high CO coverage. Because of the interactions with the adsorbed CO molecules, the 3D nanoparticles can accommodate more charge in their core than the 2D. All of these effects result in observing an unconventional, stronger CO adsorption on I_h Au₁₃ nanoparticles that expose higher surface coordination number (CN = 6) than the peripheral atoms of the planar Au₁₃ (peripheral CN = 3.4). This work highlights the shape effect on the adsorption behavior of small-sized Au nanoparticles (∼1 nm diameter)

DFT Study of Furfural Conversion to Furan, Furfuryl Alcohol, and 2‑Methylfuran on Pd(111)

Dispersion-corrected density functional theory calculations were performed to investigate the adsorption of furan, furfural, furfuryl alcohol, and 2-methylfuran as well as the reaction barriers for their interconversion. The most stable configuration for furan, furfural, furfuryl alcohol, and 2-methylfuran entails the furan ring lying flat on the surface, centered over a hollow site. We performed an elementary step analysis for the reaction of furfural to furan, furfuryl alcohol, and 2-methylfuran. Thermodynamics favors the production of furan and CO. The activation energy for furfural reduction to furfuryl alcohol is lower than that for its decarbonylation to furan. The formation of 2-methylfuran occurs via dehydration of furfuryl alcohol or a dehydrogenation pathway through a methoxy intermediate. Our findings are in agreement with recently reported experimental results

Understanding the Importance of Carbenium Ions in the Conversion of Biomass-Derived Alcohols with First-Principles Calculations

Dehydration reactions play a key role in the conversion of biomass derivatives to valuable chemicals, such as alcohols to alkenes. Both Lewis and Brønsted acid-catalyzed dehydration reactions of biomass-derived alcohols involve transition states with carbenium ion characteristics. In this work, we employed high-level ab initio theoretical methods to investigate the effect of molecular structure on the physicochemical properties of a set of alcohols that appear to control dehydration chemistry. Specifically, we calculated the carbenium ion stability (CIS, alkene-binding H⁺) and proton affinity (PA, alcohol-binding H⁺) of various C2–C8 alcohols to show the effect of alcohol size and degree of primary heteroatom substitution on the properties of the reactive species. Our results show a strong linear correlation between CIS and PA, following the substitution order of the reacting alcohols (i.e., primary < secondary < tertiary). Additionally, the calculated binding free energy (BE) of water on the formed carbenium ions was found to be exothermic and to decrease in magnitude with increasing alcohol substitution level. We demonstrate that the CIS and/or the PA are excellent structural descriptors for the alcohols and, most importantly, they can serve as reactivity descriptors to screen a large number of alcohols in the conversion of biomass-based alcohols involving the formation of carbenium ions. We demonstrate this concept in both Lewis and Brønsted acid-catalyzed dehydration reactions

Size‑, Shape‑, and Composition-Dependent Model for Metal Nanoparticle Stability Prediction

Although tremendous applications for metal nanoparticles have been found in modern technologies, the understanding of their stability as related to morphology (size and shape) and chemical ordering (e.g., in bimetallics) remains limited. First-principles methods such as density functional theory (DFT) are capable of capturing accurate nanoalloy energetics; however, they are limited to very small nanoparticle sizes (<2 nm in diameter) due to their computational cost. Herein, we propose a bond-centric (BC) model able to capture cohesive energy trends over a range of monometallic and bimetallic nanoparticles and mixing behavior (excess energy) of nanoalloys, in great agreement with DFT calculations. We apply the BC model to screen the energetics of a recently reported 23 196-atom FePt nanoalloys (Yang et al. Nature 2017, 542, 75−79), offering insights into both segregation and bulk-chemical ordering behavior. Because the BC model utilizes tabulated data (diatomic bond energies and bulk cohesive energies) and structural information on nanoparticles (coordination numbers), it can be applied to calculate the energetics of any nanoparticle morphology and chemical composition, thus significantly accelerating nanoalloy design

Catalyst Design Based on Morphology- and Environment-Dependent Adsorption on Metal Nanoparticles

Understanding metal–adsorbate interactions is key to controlling and improving the functionality of metal nanoparticles (NPs) in energy and biomedical application areas. However, adsorption is dependent on the morphological characteristics of the NPs, such as their size and shape, and in turn, the NP morphology is dependent on the chemical environment (presence of adsorbates). In this work, we introduce a novel and computationally tractable framework that is able to capture adsorption trends as a function of NP size and shape, including the impact of chemical environment on NP morphology. Our methodology is tested in the area of catalysis and specifically on the CO oxidation behavior on gold (Au), a highly structure-sensitive reaction. Our results reveal a strong correlation between the experimentally observed CO oxidation activity and the average CO adsorption (binding) energy on Au NPs enabling catalytic behavior prediction as a function of NP morphology. We demonstrate that the Au NP size plays a pivotal role on CO adsorption, whereas the Au NP shape appears to be less significant. Most importantly, the developed methodology introduces NP morphology effects on adsorption that are key for the rational design of materials with fine-tuned properties in applications ranging from catalysis to targeted medical imaging to drug delivery

Multiscale Modeling Reveals Poisoning Mechanisms of MgO-Supported Au Clusters in CO Oxidation

Catalyst deactivation mechanisms on MgO-supported Au₆ clusters are studied for the CO oxidation reaction via first-principle kinetic Monte Carlo simulations and shown to depend on support vacancies. In defect-poor MgO or in the presence of a Mg vacancy, O₂ does not bind to the clusters and the catalyst is poisoned by CO. On Au clusters interacting with O vacancies of the support, O₂ can be chemisorbed and transient activity is observed. In this case, an unexpected catalyst “breathing” mechanism (restructuring) leads to carbonate formation and catalyst deactivation, rationalizing several experimental observations. Our study underscores the importance of the cluster’s charge state and dynamics on catalytic activity

Understanding and Optimizing the Behavior of Al- and Ru-Based Catalysts for the Synthesis of Polyisobutenyl Succinic Anhydrides

Polyisobutenyl succinic anhydrides (PIBSAs) are an important class of chemicals in the automotive industry due to their wide use in lubricant and fuel formulations. However, the synthesis of these molecules takes place at elevated temperatures through the ene reaction between maleic anhydride (MAA) and polyisobutylene (PIB). Lewis acid catalysts (e.g., AlCl3) have been shown to facilitate PIBSA synthesis by lowering the activation energy of the reaction; however, the desorption of the final product (PIBSA) from the catalyst can be highly endergonic. Herein, we demonstrate ligand engineering strategies to optimize the performance of Al- and Ru-based catalysts by combining first-principles calculations with kinetic modeling. We discover that alkyl chlorides such as the EtAlCl2 retain relatively low activation barriers like AlCl3, while lowering the desorption energy of the final product (PIBSA). In addition, we address metal oxidation state and ligand effects on the ene reaction performance of Ru-based catalysts. We demonstrate that depending on the metal oxidation state and type of ligands there is a competition between concerted and stepwise mechanisms. We uncover a Ru(II) catalyst, RuCl2·2H2O, exhibiting enhanced activity but suffering from low stability. Overall, our work identifies catalysts of industrial importance that can reduce the energy input required for intensified processes and highlights challenges associated with catalyst performance

Mechanistic Studies on the Michael Addition of Amines and Hydrazines To Nitrostyrenes: Nitroalkane Elimination via a Retro-aza-Henry-Type Process

In this article we report on the mechanistic studies of the Michael addition of amines and hydrazines to nitrostyrenes. Under the present conditions, the corresponding <i>N</i>-alkyl/aryl substituted benzyl imines and <i>N</i>-methyl/phenyl substituted benzyl hydrazones were observed via a retro-aza-Henry-type process. By combining organic synthesis and characterization experiments with computational chemistry calculations, we reveal that this reaction proceeds via a protic solvent-mediated mechanism. Experiments in deuterated methanol CD₃OD reveal the synthesis and isolation of the corresponding deuterated intermediated Michael adduct, results that support the proposed slovent-mediated pathway. From the synthetic point of view, the reaction occurs under mild, noncatalytic conditions and can be used as a useful platform to yield the biologically important <i>N</i>-methyl pyrazoles in a one-pot manner, simple starting with the corresponding nitrostyrenes and the methylhydrazine