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Influence of co-adsorbed water and alcohol molecules on isopropanol dehydration on #-alumina: Multiscale modeling of experimental kinetic profiles Kim Larmier, Andre Nicolle, Céline Chizallet, Nicolas Cadran, Sylvie Maury, Anne-Félicie Lamic-Humblot, Eric Marceau, and Hélène Lauron-Pernot ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00080 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 5, 2016
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ACS Catalysis
Influence of co-adsorbed water and alcohol molecules on isopropanol dehydration on γ-alumina: Multi-scale modeling of experimental kinetic profiles
Kim Larmier,1,2,3,* André Nicolle,4 Céline Chizallet,3,* Nicolas Cadran, 3 Sylvie Maury, 3 AnneFélicie Lamic-Humblot,1,2 Eric Marceau,1,2,# Hélène Lauron-Pernot1,2,* 1
Sorbonne Universités, UPMC Univ Paris 06, UMR 7197 CNRS, Laboratoire de Réactivité de
Surface, F-75005, Paris, France. 2
CNRS, UMR 7197 CNRS, Laboratoire de Réactivité de Surface, F-75005, Paris, France.
3
IFP Energies nouvelles, Catalysis and Separation Division, Rond-Point de l’échangeur de Solaize,
BP3, 69360 Solaize. 4
IFP Energies nouvelles, Powertrain and Vehicle Division, 1-4 avenue de Bois-Préau, 92852 Rueil-
Malmaison Cedex - France
* Corresponding authors:
- Dr. Kim Larmier,
[email protected], current address : Department of Chemistry and Applied Biosciences, HCI H 229, ETH Zürich, Vladimir-Prelog-Weg 2, CH-8093 Zürich, Switzerland - Dr. Céline Chizallet,
[email protected] - Prof. Hélène Lauron-Pernot,
[email protected] # Current address : Unité de Catalyse et Chimie du Solide, UMR 8181 CNRS, Université Lille 1, Cité Scientifique, Bâtiment C3, 59650 Villeneuve d'Ascq Cedex, France
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ABSTRACT: Successfully modeling the behavior of catalytic systems at different scales is a matter of importance not only for fundamental understanding, but also for a more rational design of catalysts and a more precise definition of the kinetic laws used as input in chemical engineering. We have developed here a multi-scale modeling of the dehydration of isopropanol to propene and diisopropylether on γ-alumina catalysts, that clearly evidences and explains the central character of cooperative effects between co-adsorbates in the kinetic network. The evolution of partial pressures with contact time was simulated using an original DFT-based micro-kinetic model based on a “macro-site” centered on the main active site located on the (100) planes of alumina, and comprising several neighboring adsorption sites. The formation of isopropanol-isopropanol or water-isopropanol dimers on the surface was required to correctly simulate the production of the minor product, diisopropylether, and the evolution of the products partial pressures at high conversion. DFT calculations were used to identify the structure of these dimers. In addition to entropic effects, the selectivity to ether is ruled by (i) stabilizing interactions between co-adsorbed isopropanol or water molecules, and the nucleophilic alcohol molecule reacting with the alcoholate intermediate; (ii) the formation of alcoholate-water dimers that selectively inhibit the formation of propene and increase the selectivity to ether at low conversion; (iii) the reverse transformation of diisopropylether into propene and isopropanol that consumes ether at high conversion. The analytical expression of the reaction rate derived from this model and based on the existence of ensembles of interacting isopropanol and water molecules leads to a satisfactory modeling of the experimental kinetic measurements at all conversions.
Keywords.
γ-Al2O3,
isopropanol,
alcohol
dehydration,
propene,
diisopropylether
decomposition, Density Functional Theory, kinetics, multi-scale modeling, kinetic modeling 2 ACS Paragon Plus Environment
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Introduction The concept of active site has been thoroughly used in heterogeneous catalysis since
Taylor proposed that ‘the surface of a granule may be regarded as composed of atoms in varied degrees of saturation’1 on which molecules react.2–7 The recent development of ab initio calculations allowed making great progress in unraveling the nature and specificities of the active sites and of the reaction intermediates for a variety of catalytic processes.8–13 However, the rate of the reaction may also be influenced by phenomena external to the active site, such as interactions between intermediates and co-adsorbed spectator molecules, through cooperative or inhibiting effects.14–27 Identifying precisely which interactions contribute to the reaction mechanism would require to consider an extended portion of the surface, as well as a large set of possible configurations depending on the surface coverage and on the partial pressures of the reactants, all of which goes beyond the classical ab initio methodology.9 In the particular case of the dehydration of alcohols on aluminic materials, a domain that has been widely investigated both experimentally28–47 and theoretically,48–52 the issue of the interactions between adsorbed species has been raised in a number of studies. Water formed during the reaction and re-adsorbed on the catalyst is either postulated as an inhibitor of the reaction20,21,28,41–43,52 or as a potential anchoring site for incoming alcohol.31,53 Interactions between co-adsorbed alcohol molecules are obviously required for the bimolecular formation of ether29,35,47,50,51 but have sometimes been proposed to have inhibiting effects on the dehydration reaction.20,21 Determining where and how these interactions establish on the surface, and to which extent they influence the catalyst activity and the alkene/ether selectivity as conversion increases (i. e., as less and less alcohol, and more and more water competing for adsorption, are present in the gas-phase) is a challenging question that has not been addressed yet satisfactorily. 3 ACS Paragon Plus Environment
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As a matter of fact, top-down kinetic modeling approaches based on the fitting of experimental data have not provided clear answers to this question. Knoezinger et al.35 first tried to rationalize the dehydration of alcohols on alumina through kinetic models, but concluded that the discrimination between the different models was not possible due to complex multi-parameter equations. More recently, DeWilde et al.20 have built an analytical model of ethanol conversion on γ-alumina by extracting rate and adsorption constants from a fitting of initial reaction rates. They have hypothesized that inhibiting water-ethanol dimers or trimers form at the surface of the catalyst. However their model, tested at low conversions only, does not provide a molecular description of these intermediates, and does not consider the competing decomposition of diethylether, that becomes significant in high conversion conditions. 20,21 A possible way to solve this intricate problem consist in bringing together the benefits of ab initio calculations (evaluation of plausible kinetic and thermodynamic constants from the electronic structure of the catalyst and of the adsorbates) and kinetic modeling (selection and adaptation of a set of elementary steps consistent with the experimental measurements), through a multi-scale modeling approach. This approach has been used for gas-phase reactions,54–57 and seminal works provide a firm basis for its extension to surface reactions,27,58–61 including the influence of coverage effects and lateral interactions between co-adsorbates.23–27 Very recently, Christiansen et al.62 have proposed a Density Functional Theory (DFT)-driven kinetic modeling of ethanol dehydration on γ-alumina. They compared predicted rates to the measurements published by DeWilde et al.,20 with the same restrictions to the low conversion domain. In the present paper, we will aim at constructing a global reaction scheme of isopropanol dehydration on γ-alumina over the whole conversion range, which is of great importance for the prediction of catalytic behaviors under working conditions, through a bottom-up approach 4 ACS Paragon Plus Environment
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based on DFT calculations. In a previous study,63 we have shown that a specific Lewis acidic site located on the (100) facets of γ-alumina is central in the formation of a surface alcoholate intermediate leading to the production of both propene and diisopropylether. Starting from that basis, we herein examine the relevance of a reaction model involving this active site and neighboring adsorption sites. We seek to refine this model through successive feedback between microkinetic modeling and DFT calculations in order to better match experimental measurements carried out at different conversions, at various temperatures and involving secondary reactions such as diisopropylether decomposition. The trends extracted from multiscale modeling will help us to precise why and to what extent lateral interactions between coadsorbed isopropanol and water molecules should be invoked to explain the selectivities of propene and diisopropylether on alumina at any conversion of isopropanol.
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Methods
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Steady-state kinetic measurements
Isopropanol dehydration and diisopropylether decomposition were carried out in a quartz tubular fixed-bed reactor. γ-alumina (Sasol, Puralox TH100/150) was pressed into a wafer and crushed to get a particle size between 125 and 200 µm in order to avoid diffusional limitations. A given amount of the catalyst sample (between 4 and 80 mg) was diluted in SiC (same particle size, inert toward isopropanol dehydration below 350 °C) and loaded into the reactor to form a catalytic bed of 0.1 mL (diameter: 1.00 cm, length : 0.13 cm). γ-alumina was activated for 3 h at 450 °C (7.5 °C.min-1) under nitrogen flow (Azote U Air Liquide, 20 mL.min-1) and cooled down under N2 to the reaction temperature (T = 180 to 220 °C). Isopropanol (Sigma-Aldrich, 99 %) or diisopropylether (Sigma-Aldrich, 98 %) was 5 ACS Paragon Plus Environment
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stored in a saturator whose temperature was set to 5 °C in order to deliver a partial pressure of 1.5 and 4.5 kPa respectively, in the nitrogen flow (6.0 to 60.0 mL.min-1). It was checked, by varying the amount of catalyst loaded and the reactant flow rate, that diffusion limitations were not observed in these conditions. Only propene and diisopropylether were detected from isopropanol conversion, and only propene and isopropanol from diisopropylether decomposition. Based on the very low axial Peclet numbers computed (Pe
