We present a theoretical investigation of the c-Al2O3-catalyzed conversion of ethanol into ethylene and diethyl ether. Reported with a fully first-principles parameterization of multi-site surface kinetics for a c-Al2O3-catalyzed reaction. To our knowledge, there has been no microkinetic model
It is known that for pure metal oxides frequently both metal and oxygen surface sites play an important catalytic role. There has recently been some progress in applying such techniques to the metal/metal oxide interface as well as pure metal oxides. First-principles-based microkinetic modeling of heterogeneous catalysis is most frequently applied to transition metals. In this way, microkinetic models help to bridge the gap between site- or molecular-level observations and mesoscopic quantities such as reaction rates. The development of detailed, first-principles kinetic models to account for such fundamental insights offers a systematic approach to exploration and quantification of reaction networks. Complementary to these insights, we have previously performed quantum mechanical calculations to explore the mechanisms of ethanol dehydration and etherification on various c-Al2O3 active sites, using in particular the non-spinel model of Raybaud, Sautet, and co-workers in our more recent work (we note that there is ongoing debate in the literature regarding the bulk structure of c-Al2O3 see Ref. Measurements with deuterium-labeled ethanol demonstrate a primary isotope effect for dehydration (likely involving Hb elimination), but not etherification. In addition, steady state reaction kinetics ⇑ Corresponding author. For example, recent microscopy studies of c-Al2O3 nanoparticles have highlighted the abundance and importance of (1 1 1) faceting on particle surfaces. Fundamental knowledge of the catalyst surface and of alcohol reaction mechanisms provides insights into the properties of cAl2O3 that promote dehydration and etherification.
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Rationalizing the activity of this material can form a foundation for understanding how to promote and control acid-catalyzed deoxygenation reactions. c-Al2O3 is one such solid acid employed as a catalyst in, among other reactions, alcohol dehydration, which is the focus of this work. Substantial effort is being invested in developing catalytic materials, such as solid acids, to remove excess functionality from oxygen-rich biomass. One of the most significant challenges in improving its utilization is the development of methods to efficiently process it into useful products. Introduction Biomass constitutes a vast and underutilized supply of renewable carbon for producing chemicals, fuels, and energy. The success of the models demonstrates the applicability of the DFT-computed mechanisms to powdered c-Al2O3 catalysts. The outcome of the fitting suggests that an additional active site may be responsible for some ethanol conversion.
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Analytical rate expressions are derived from the full microkinetic model and fit to experimental data, capturing reaction order trends with similar success as the full model.
Microkinetic analysis indicates that the SN2 and E2 elementary steps control the overall rate. The DFT-parameterized two-site mean-field microkinetic model successfully captures trends in experimentally measured reaction orders. DFT calculations on the cAl2O3(1 1 1) surface facet demonstrate that the energetically favorable pathway for ethylene formation is an E2 mechanism (Ea = 28 kcal/mol), while ether formation takes place via a bimolecular (SN2) mechanism (Ea = 32 kcal/mol). Keywords: c-Al2O3 Ethanol Dehydration Etherification Ethylene Diethyl ether Microkinetic model Lewis acid DFTĪ b s t r a c t We investigate the c-Al2O3-catalyzed production of ethylene and diethyl ether from ethanol in a combined density functional theory (DFT) and microkinetic modeling study. Vlachos a,⇑ a Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation and Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716-3110, USA b Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USAĪrticle history: Received Revised 5 December 2014 Accepted 21 December 2014 Christiansen a, Giannis Mpourmpakis b, Dionisios G. Journal of Catalysis journal homepage: DFT-driven multi-site microkinetic modeling of ethanol conversion to ethylene and diethyl ether on c-Al2O3(1 1 1) Matthew A.
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