A promising route to produce olefins, the building blocks for plastics and chemicals, is the nonoxidative dehydrogenation of alkanes on metal oxides, taking advantage of the Lewis acid−base surface functionalities of the oxides. However, how alkane dehydrogenation activity depends on the strength of surface acid−base site pairs is still elusive. In this work, we provide fundamental insights into the reaction mechanisms of propane dehydrogenation on different facets of γ-Al 2 O 3 and develop structure−activity relationships, using density functional theory calculations and first-principles molecular dynamics simulations. We identified the binding energy of dissociated H 2 as an activity descriptor for alkane dehydrogenation. Interestingly, a volcano relationship between catalytic activity and dissociative H 2 binding energy was discovered for propane dehydrogenation, unraveling a sitedependent catalytic behavior on γ-Al 2 O 3 , with a concerted surface mechanism being energetically preferred to a sequential one on the most active sites. We demonstrated that although surface hydration, in general, blocks strong Lewis acid−base pairs on the (110) γ-Al 2 O 3 surface, the presence of hydroxyl groups (on neighboring to strong Lewis sites) can enhance the propane dehydrogenation activity of a "defect site pair" (Al III −O III ) of the metastable surface. Moreover, we performed ab initio metadynamics simulations of the most active site on γ-Al 2 O 3 to examine the hydrogen formation and surface dynamics under dehydrogenation reaction conditions. Metadynamics simulations demonstrated that the poisoning of active sites by hydrogen adsorption is unlikely under experimental conditions. The developed relationships can be utilized to screen metal oxide surfaces and accelerate the discovery of active catalysts for alkane conversion to olefins.

We report a novel catalytic conversion of biomass-derived furans and alcohols to aromatics over zeolite catalysts. Aromatics are formed via Diels-Alder cycloaddition with ethylene, which is produced in situ from ethanol dehydration. The use of liquid ethanol instead of gaseous ethylene, as the source of dienophile in this one-pot synthesis, makes the aromatics production much simpler and renewable, circumventing the use of ethylene at high pressure. More importantly, both our experiments and theoretical studies demonstrate that the use of ethanol instead of ethylene, results in significantly higher rates and higher selectivity to aromatics, due to lower activation barriers over the solid acid sites. Synchrotron-diffraction experiments and proton-affinity calculations clearly suggest that a preferred protonation of ethanol over the furan is a key step facilitating the Diels-Alder and dehydration reactions in the acid sites of the zeolite.

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Fast pyrolysis of biomass is an important technology in the conversion of lignocellulosic feedstocks to value-added fuels and chemicals. Significant efforts have been dedicated to modeling of these processes to improve the viability of large-scale operation through reactor design, feedstock selection and processing, and optimization of operating conditions, among others. This work is a review of the current progress in the field of modeling of biomass fast pyrolysis processes across multiple length and time scales. Enclosed are summaries of the current state of the art in atomistic and kinetic modeling of biomass fast pyrolysis toward production of fuels and chemicals. Decomposition of aggregate biomass and its individual components was reviewed for models at various scales, highlighting important considerations. Recent applications of machine learning methods to couple multiscale phenomena with the goal of reducing computational complexity were also included. Historical context was provided for existing models and correlations, highlighting some of those most widely applied. Some of the shortcomings and bottlenecks in existing models were identified as areas for further study. Finally, potential future directions for the field are suggested with the goal of improving the viability and sustainability of pyrolysis processes and the applications of multiscale modeling toward this goal.

Herein, we report a synthetic strategy to convert biomass-derived unsubstituted furan to aromatics at high selectivity, especially to ethylbenzene via alkylation/Diels-Alder cycloaddition using ethanol, while greatly reducing the formation of the main side product, benzofuran, over zeolite catalysts. Using synchrotron X-ray powder diffraction and first principles calculations, it is shown that the above methodology favors the formation of aromatic products due to readily alkylation of furan by the first ethanol molecule, followed by Diels-Alder cycloaddition with 2 derived ethylene from the second ethanol molecule on a Brønsted acid site in a one pot synthesis. This gives a double promoting effect: alkyl substituent(s) on furan creates steric hindrance to inhibit self-coupling to benzofuran while alkylated furan (diene) undergoes Diels-Alder reaction more favorably due to higher HOMO energy.

Olefin formation pathways on Lewis acid (LA) sites of Al 2 O 3 , Ga 2 O 3 and In 2 O 3 and Gallium-and Indiumdoped alumina were investigated using Density Functional Theory (DFT) calculations. We considered two different olefin formation pathways, through alcohol dehydration and ether decomposition via both E1 and E2 type of elimination mechanisms. Both pathways evolve through a concerted E2 type of mechanism. Our results indicate that alumina is most active in alcohol dehydration reactions and the presence of dopants does not improve the catalytic activity. Correlations between the calculated energy barriers and the physicochemical properties of the oxides were identified and a dehydration model was constructed. The model accounts for surface acidity, base-site strength and the alcohol carbenium ion stability (CIS). We demonstrate a strong correlation between the ether decomposition and the alcohol dehydration barriers allowing us to extend the dehydration model predictions to ether decomposition chemistry.

Reaction conditions:In a typical experiment, 15 mL of the model compound 2,5-Dimethylfuran (99% Sigma-Aldrich), 8.2 mL of ethanol (99.5% Sigma-Aldrich) and 0.4 g of zeolite were used over a temperature range of 200 to 300 °C and 1.0 mL of tridecane as an internal standard. The reaction vessel was flushed with inert gas (N2). Liquid products were identified and quantified using a gas chromatograph/mass spectrometer (Agilent 6890 and Agilent MSD 5973 (N)) calibrated with pure standards. After reaction, the autoclave was cooled down to −60°C by dry ice/acetone bath. The gas products (CO, CO 2 , CH 4 ) analyzed by a Perkin Elmer Autosystem XL Arnel Gas phase GC-FID-Methanator. C 2 H 4 gas product was assumed to be mainly produced from ethanol. The solid residue was filtrated and washed with acetone and dried overnight (80 °C). Then it was analyzed by thermogravimetric analyses (TA instruments Q50), the samples were heated at 10 °C min −1 from room temperature to 900 °C under air to quantify the coke on zeolite to complete the carbon balance. The conversion was expressed in term of the molar conversion of 2,5-dimethylfuran and carbon balance was achieved at the minimum of 90-95%. Assessment of a possible diffusional controlled (mass limitations) reaction:The reaction conditions for ethylene gas or ethanol liquid placed with DMF liquid (generated pressurized vapors at 300 o C) were employed following the conditions established in reference [1]

Alcohol dehydration mechanisms identified through the slopes of activation energies vs. carbenium ion stability of alcohols.