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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Ethanol Conversion to Ethylene and Acetaldehyde over Rhodium(I) Exchanged Faujasite Zeolite. A QM/MM and Microkinetic Study Velina Markova,† Graham Rugg,† Agalya Govindasamy,† Alexander Genest,† and Notker Rösch*,†,‡ †

Institute of High Performance Computing, Agency for Science, Technology and Research, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore ‡ Department Chemie and Catalysis Research Center, Technische Universität München, 85747 Garching, Germany S Supporting Information *

ABSTRACT: In a computational study, we examined the conversion of ethanol to ethylene or acetaldehyde over Rh(I)exchanged zeolite (faujasite) using QM/MM DFT calculations and microkinetic modeling. To elucidate how the composition of the active site affects the reactivity in this model system, the dehydration and dehydrogenation reaction mechanisms at the Rh center were modeled by varying the number n (n = 1−3) of preadsorbed ethanol molecules. For a coverage of one ethanol, ethylene formation was determined to proceed via a two-step mechanism, whereas concerted C(β)-H and C−O bond cleavage occurs for higher coverage at the metal center. In contrast, the dehydrogenation mechanism of ethanol does not vary with coverage. A crucial step in both transformations is the expulsion of the products which was found to be more facile at higher ethanol coverage, n = 2, 3. However, at n = 1, we calculated much lower barriers for the corresponding bond scission steps. Microkinetic modeling, based on the QM/MM results, revealed a strong temperature dependence of the activity of the catalytic system. Little to no reactivity is predicted at lower temperatures, whereas both types of reactions appear likely at temperatures above 500 K. Up to 700 K, the accumulated amount of acetaldehyde is higher than that of ethylene. At even higher temperatures ethylene is predicted as the preferred product.

1. INTRODUCTION For supplementing in part the use of fossil carbon, be it petroleum or natural gas, there is a growing drive to increase the production of fuels and hydrocarbons from biomass for environmental, but also for economic reasons.1−3 Ethanol is readily available from biomass,4 and can be used as a fuel, but also as a precursor for valuable chemicals such as diethyl ether, ethylene, acetaldehyde, higher olefins, aromatics, etc.5−9 In recent years, the dehydration of bioethanol has been widely explored, in experiments9−14 and by computational approaches,15−20 as a “green” method for producing ethylene, which is a chemical of major industrial importance. The conversion of alcohols to olefins typically is an acidcatalyzed reaction. Acidic zeolites are particularly active in the transformation of ethanol to olefins and other hydrocarbons, owing to the presence of Brønsted acid sites in their crystalline framework.5,13,21−25 Zeolites with supported transition metal species have also been observed in a number of experiments to show catalytic activity in the transformation of alcohols to light olefins.6,26−29 Inaba et al. 6 reported a comprehensive experimental study of ethanol dehydration in H-ZSM5, exchanged with 14 transition metals and magnesium. Copper was found to be particularly active in the conversion of ethanol (90%), with the highest yield of ethylene, 92%. These results supported the findings of Shultz et al., who, in a study of 10 © XXXX American Chemical Society

transition metals supported on ZSM-5, observed Cu-ZSM-5 to yield the highest selectivity for ethylene, followed by Fe-ZSM5.28 A particularly relevant feature of ethanol transformations is their temperature-dependence. A considerably reduced rate of conversion was observed in nonloaded zeolites at room temperature, with the reactivity substantially increasing at 673 K.6 Furthermore, the selectivity is affected with increasing temperature, from predominantly diethyl ether as product to higher hydrocarbons. Similarly, ethanol conversion by supported noble-metal catalysts shows a strong temperature dependence, for both the product selectivity and the total ethanol conversion.30 The active sites in metal catalysts supported on zeolites often vary with the type of the framework, synthesis procedure, catalyst pretreatment etc., hence yielding a very diverse range of products.6,26,31−33 A computational study using a density functional theory (DFT) method on the dehydrogenation of ethanol over gold complexes, supported on ZSM-5,16 found the activation barrier of the rate-determining step to be reduced from 180 kJ mol−1 to 38 kJ mol−1 when the active site interacts Received: November 20, 2017 Revised: January 2, 2018

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DOI: 10.1021/acs.jpcc.7b11416 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C with a nearby surface oxygen center. In a combined experimental and theoretical investigation of methanol dehydrogenation on V-ZSM-5, Tranca et al. determined different product distributions for two active-site structures.19 Supported metal centers as active sites are an important feature of such catalysts and their adequate representation is essential when modeling this type of catalysis. Therefore, in the present study we use an already well understood catalytic site− a zeolite-supported Rh+ center. Inspired by a [Rh(C2H4)2]+ complex, anchored on faujasite, that was successfully synthesized and its structure was thoroughly examined by spectroscopic techniques 34,35 and computational approaches,36,37 we modeled analogous zeolite-supported Rh(I) complexes with up to three preadsorbed ethanol molecules. We examined computationally such complexes for their potential catalytic activity in the conversion of ethanol. Thus far, the theoretical studies of alcohol conversion using metal-exchanged zeolites did not take into account the effect of ethanol coverage at the metal center, but were restricted to models with a single alcohol molecule, coordinated at the metal center.16,17,33,38 This simpler modeling strategy neglects the potential effect of the (partial) pressure of reactant molecules. Given the mechanistic variations described above, regarding the details of the active site, it seems appropriate to explore such coverage effects. During ethanol dehydration various side reactions can occur, depending on the temperature and the type of the active site. Apart from diethyl ether that has been observed at low temperatures,6 acetaldehyde can also be produced via dehydrogenation.28,39 The dehydrogenation process is generally associated with the presence of basic sites. 40,41 Such dehydrogenation processes as side reactions should not be overlooked because Rh(I) can easily be involved in oxidative addition, essentially acting as a Lewis base. Therefore, in this study we used a QM/MM approach to model both dehydration and dehydrogenation reactions of ethanol at supported Rh(I) centers, with one to three associated reactant molecules in the initial complexes, addressing the reactivity and the selectivity of this potential catalyst. With electronic structure calculations, we first produced a thermodynamic database, which we subsequently used to develop microkinetic models for examining in detail the kinetic properties of the catalytic system. The present computational study attempts to connect in a quantitative fashion the structure of the active site to the selectivity of the ethanol-to-olefin conversion, benefiting from the highly symmetric faujasite framework with an active site that mimics a well characterized zeolite-supported metal complex. Note, this work is intended as a study of concepts, but does not aim at suggesting a novel efficient catalyst.

Figure 1. 11/83T cluster model used in the QM/MM calculations. The zeolite-supported Rh center with two ethanol molecules adsorbed is shown as an example. Color coding: Si, light gray; Al, pink; Rh, blue; C, dark gray; O, red; H, white.

positions of these capping H−Si moieties were kept fixed, to prevent the structure of the zeolite cage from distorting, thus inducing structure artifacts. All calculations were carried out using the software Gaussian09.44 We adopted a two-layer ONIOM45 scheme, in which the high-level partition of 11 T atoms is treated at a quantum mechanical (QM) level, using the B3LYP46−49 density functional method. Besides the Rh complex and the 11T cluster model, the QM region always contained any molecules that participate in the reaction (Figure 1). The remaining 83 − 11 = 72 T atoms of the zeolite cluster, including any bond-capping H centers at the overall surface of the 83 T cluster, were assigned as low-level partition (MM = molecular mechanics), described by the UFF force field.43 The QM calculations were carried out with the 6-311G(d,p) basis set,50,51 utilizing an SDD pseudopotential for Rh together with the corresponding basis set.52 Transition states were located with the Berny algorithm,53 and confirmed by a normal-mode analysis. Energies reported are Gibbs free energies, calculated at 673 K unless stated otherwise. The temperature was chosen to mimic experimental conditions where a higher conversion of ethanol was observed.6 Note that at this temperature the formation of diethyl ether is not expected.11,13 In a previous work on [Rh(C2H4)2]+ species in faujasite, carried out in our group,36,37 the complex was found to absorb preferentially on two oxygen centers located at the 12-member ring window of the supercage, adjacent to the Al center. In the present study, we used the same adsorption site in faujasite for the species [Rh(C2H5OH)n]+, n = 1−3. We label the three initial structures of the supported complexes [Rh(C2H5OH)n]+ as 1, 2 and 3, for n = 1−3, respectively (Figure 2). The intermediates along the two pathways (concerted and stepwise) to forming ethylene, e-path for short, are labeled nem, where n = 1−3 denotes the number of ethanol molecules of the initial complex, e indicates the e-path, and m is a consecutive number, identifying the intermediates. In the same spirit, we used the

2. METHODS AND MODELS We employed an embedded-cluster model for the metal complexes anchored in the supercage of a faujasite zeolite. This modeling strategy has previously been validated.36 We used a “11/83T model”, composed of 278 atoms in total, where 83 T atoms are included from a segment of a supercage unit with nearby sodalite cages and hexagonal prisms, cut from the crystal structure of all-silica faujasite (Figure 1).42 The dangling bonds of Si atoms at the boundary of the 83T cluster were saturated by hydrogen atoms, with the new Si−H bonds oriented to the nearest missing oxygen centers. The new Si−H bonds were first optimized at the force-field level (UFF)43 with the rest of the zeolite structure kept “frozen”; subsequently the B


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The Journal of Physical Chemistry C

temperatures, e.g., at 673 K, where ethanol conversion usually is carried out in experiment.6,29 The loss of entropy of a molecule due to adsorption in the zeolite pores roughly corresponds to removing two degrees of translational freedom,58 which for ethanol is ∼104 J mol −1 K −1 . In consequence, the corresponding adsorption steps from the gas phase could be quite endothermic; see Table S3 of the Supporting Information. Therefore, we used species in an otherwise “empty” (no metal complex) zeolite cavity, Zeo-H, as reference. Here the label refers to the protonated O center near an Al moiety. The molecules were located far from the Brønsted acid site to account only for their interaction with the zeolite wall. Accordingly, the reference free energy of a molecule X was taken as its physisorbed state, in the fashion of ref 57, which we calculated as G[X]ref = G[Zeo-H + X] − G[Zeo-H], with a single species X = H2, H2O, C2H4, CH3CHO, or C2H5OH in an empty cavity. This approach implies that the molecules interacting with the Rh center originate from neighboring zeolite cavities, Zeo-H, due to diffusion. After desorption from the metal center a molecule will be released into the pore system of the zeolite. However, for the sake of brevity, we will address all gas phase species in the pores of the zeolite, also referred as interstitial gas, simply as “gas phase”. The initial complexes with ethanol loadings n = 1−3 were formed via successive addition of one ethanol molecule at a time to the supported Rh(I) center. Table 1 shows the

Figure 2. Sketches of the initial Rh complexes 1, 2, and 3, with increasing loading of ethanol. The remaining zeolite framework is omitted for clarity. The dashed line in complex 3 represents the H bond of the third ethanol molecule to one of the two ethanol molecules already adsorbed on the Rh center.

labels nam for the intermediates on the path to forming acetaldehyde, a-path for short. The thermodynamic data obtained from the QM/MM calculations were used to develop microkinetic models for examining the formation of the two products, ethylene and acetaldehyde, on the zeolite-supported Rh(I) center. Transition state theory was invoked for describing the elementary steps of the transformations of ethanol on the metal center. All adsorption steps, assumed to be nonactivated and reversible, were described by collision theory.54 As collision theory is mainly used for metal surfaces, we also considered transition state theory for these nonactivated adsorption reactions as this approach was previously shown to yield results comparable to experiment.20,55 Both strategies gave very similar results; see section S2 of the Supporting Information, especially Table S5. For thermodynamic consistency, all rates of reverse reactions were calculated from the equilibrium constants of the corresponding elementary step and the calculated rate of the forward reaction. To quantify the concentrations of the various intermediates, we set up a system of rate equations which were solved with the help of the BzzMath library.56 We will refer to the resulting probabilities, for the system to occupy certain discrete states, as fractions σ of adsorbed species. For details of the microkinetic models, see section S2 of the Supporting Information.

Table 1. Complexation Energies, Er, and Free Energies, Gr, in kJ mol−1 of Ethanol with the Faujasite-Supported Rh(I) Center in Complexes 1, 2 and 3, i.e., for Ethanol Coverage n = 1−3, Respectivelya,b,c complex

Er

Gr(298 K)

Gr(673 K)

1 2 3 [Zeo−OH + EtOH]

−122 −75 −46 −38

−101 −55 −31 2

−73 −36 −13 40

The corresponding values for the adsorption of ethanol in an “empty” zeolite cavity, Zeo−OH, are also shown. bZeolite cavity without a Rh(I) center. The negative charge of Al is compensated by a proton. c The reaction free energy is calculated using the equation, Gr = G[Rh(C2H5OH)n] − G[Rh(C2H5OH)n‑1] − Gref(C2H5OH). The latter reference free energy of ethanol in a zeolite cavity is calculated as Gref(C2H5OH) = G[Zeo−OH + C2H5OH] − G[Zeo−OH]. a

3. RESULTS FROM DFT CALCULATIONS In this section, we will discuss the QM/MM results. We will address the energy characteristics of the formation of ethylene and acetaldehyde, starting from the complexes with different initial amounts n of ethanol ligands, n = 1−3, adsorbed at the zeolite-supported Rh(I) metal center, Figure 2. The calculated free reaction and activation energies are collected in Tables S1 and S2 of the Supporting Information. The optimized structures of the reactants, intermediates, and products of the various transformations are shown in Figures S1 and S2 of the Supporting Information for the e-pathways and the a-pathways, respectively. The structures of the corresponding transition states are shown in Figures S3 and S4 of the Supporting Information for the e-pathways and the a-pathways, respectively. 3.1. Adsorption of Ethanol in the Initial Complexes. Experimental reaction or adsorption free energies at the supported metal complex reflect processes inside the zeolite, hence do not refer to the reacting molecules in the gas phase.57,58 Molecules entering zeolite cavities from the gas phase lose rotational and translational degrees of freedom. This implies a severe loss of entropy, in particular at higher

corresponding complexation energies Er of ethanol, the free energies of adsorption at temperatures 298 and 673 K, as well as the values for the adsorption of an ethanol molecule in an “empty” zeolite cavity, Zeo-H. The complexation energy of one ethanol molecule on Rh(I) is calculated at −122 kJ mol−1. Ethanol interacts with the anchored Rh(I) species through its oxygen center, Rh−O = 214 pm, and additionally exhibits an agostic interaction via β−H. Two ethanol molecules as ligands of Rh(I) interact with the metal center exclusively through their O centers, Rh−O = 215 pm on average. The third ethanol ligand (n = 3) undergoes hydrogen bonding, Hl···Oethanol = 183 pm, with one of the previously adsorbed ethanol molecules; the resulting Rh−O distance is rather long, 331 pm (Figure S1 of the Supporting Information). The calculated complexation energies Er of the second and third ethanol molecules at Rh(I) are −75 and −46 kJ mol−1, respectively, indicating a saturation of this interaction. The calculated free energy values for the adsorption of the first C


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Figure 3. Free energy profile (T = 673 K) for the formation of ethylene, along the e-path, starting from complex 1, i.e., with one ethanol ligand at the Rh center. Relative free energies of reactions (values in black) and the corresponding activation free energies (values in red) are given in kJ mol−1. The profile implies an overall reaction free energy Gr(1 → 1′) = 5.5 kJ mol−1 for the formation of ethylene from ethanol.

Figure 4. Free energy profile (T = 673 K) for the formation of ethylene, along the e-path, starting from complexes 2 and 3, i.e., with two (black) or three (green) ethanol ligands at the Rh center, respectively. Relative free energies of reactions (values in black) and the corresponding activation free energies (values in red) are given in kJ mol−1. Both profiles arrive at the overall free reaction energy for the formation of ethylene from ethanol: Gr(2 → 2′) = Gr(3 → 3′) = 5.5 kJ mol−1. For clarity, only sketches of the complexes on the e-path, starting from 2, are shown; the structures of the complexes starting from 3 are analogous.

ethanol are −101 kJ mol−1 at 298 K and −73 kJ mol−1 at 673 K (Table 1). In experiment, ethanol conversion usually is carried out at temperatures around 673 K;6,29 therefore, we will discuss free energy values only at this high temperature. At the free energy level, the strength of interaction of the ethanol

molecules with the Rh(I) center decreases with increasing temperature due to increased entropy contributions (Table 1). 3.2. Ethanol Conversion to Ethylene at Complex 1. Starting from complex 1, with a single ethanol ligand, ethylene is generated via dehydration in two consecutive steps: see the D


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Figure 5. Free energy profile (T = 673 K) for the formation of acetaldehyde, along the a-path, starting from complex 1, i.e., with one ethanol ligand at the Rh center. Relative free energies of reactions (values in black) and the corresponding activation free energies (values in red) are given in kJ mol−1. The profile implies an overall reaction free energy Gr(1 → 1′) = 10 kJ mol−1 for the formation of acetaldehyde from ethanol.

corresponding free energy profile, Figure 3. In the first step, the metal hydride complex 1e1 is formed and the − CH2CH2OH species attaches to the metal center via oxidative addition of the bond C(β)−H of ethanol, Figure S1 of the Supporting Information. The free energy of activation of this exergonic reaction, Gr(1 → 1e1) = −34 kJ mol−1, is very small, 14 kJ mol−1. Subsequently, the C−O bond of −CH2CH2OH is cleaved, yielding the product complex 1e2 with the ligands ethylene and − OH. This thermoneutral reaction, Gr(1e1 → 1e2) = 2 kJ mol−1, exhibits a relatively high free activation barrier, Ga(1e1 → 1e2) = 74 kJ mol−1. Next, a water molecule is formed via reductive elimination of the ligands −H and −OH, Gr(1e2 → 1e3) = −69 kJ mol−1. This exergonic step proceeds via a moderate activation barrier, Ga(1e2 → 1e3) = 48 kJ mol−1. The following desorption of the product ethylene is endergonic by 114 kJ mol−1, leaving behind a highly unsaturated Rh center. To circumvent this problematic situation, an additional ethanol molecule is introduced at the metal center. Then the desorption of π-C2H4, with H2O still bound to Rh(I), is 63 kJ mol−1 more endergonic than the desorption of H2O with C2H4 still bound to the Rh(I), Gr(1e3 → 1e4) = 3 kJ mol−1. Therefore, the water molecule is expelled first, resulting in complex 1e5 which thereby is slightly stabilized, by 4 kJ mol−1 (Figure 3). The highly endergonic detachments of the product, Gr(1e5 → 1e6) = 105 kJ mol−1, yields complex 1e6, where the released ethylene molecule no longer interacts with the Rh center. The cycle of the e-path is closed after regenerating the initial complex 1 by a final, almost thermoneutral migration of the product to a neighboring empty zeolite cage. 3.3. Ethanol Conversion to Ethylene at Higher Ethanol Coverage. At higher ethanol coverage, i.e., starting from complexes 2 and 3, ethylene is formed via another mechanism where the bonds C(β)−H and C−O of ethanol break in a concerted fashion, forming the ligands −H and −OH, respectively, at the metal (Figure S5 of the Supporting

Information). The reductive elimination of H2O follows; for the corresponding free energy profiles see Figure 4. The free energy of activation of the concerted ethylene formation in complex 2, Ga(2 → 2e1) = 128 kJ mol−1, is 17 kJ mol−1 lower than the corresponding step starting from complex 3, Ga(3 → 3e1) = 145 kJ mol−1. The associated reactions are almost thermoneutral, Gr(2 → 2e1) = 6 kJ mol−1 and Gr(3 → 3e1) = −2 kJ mol−1. The subsequent formation of a water ligand exhibits a free energy of activation, Ga(3e1 → 3e2) = 91 kJ mol−1, which is by 24 kJ mol−1 higher than the corresponding activation barrier for n = 2. The elimination of ethylene after its concerted formation is barely exergonic for both ethanol coverages, Gr(2e2 → 2e3) = −6 kJ mol−1, Gr(3e2 → 3e3) = −1 kJ mol−1. Two ligands, water and ethanol, remain adsorbed at the Rh center in a faujasite cavity, yielding the structures 2e3 and 3e3. In the latter complex the third ethanol ligand is loosely bound, via an H bond to the H2O ligand at the metal center, Oethanol···H = 172 pm, Figure S1 of the Supporting Information. The expulsion of water is preceded by the slightly exergonic adsorption of an additional ethanol molecule: Gr(2e3 → 2e4) = −14 kJ mol−1, Gr(3e3 → 3e4) = −13 kJ mol−1. Both catalytic cycles for the e-path starting from 2 and 3 are closed with the slightly endergonic desorption of the water molecule; the corresponding reaction free energies are Gr(2e4 → 2) = 18 kJ mol−1 and Gr(3e4 → 3) = 24 kJ mol−1. 3.4. Ethanol Conversion to Acetaldehyde at Complex 1. Dehydrogenation, as alternative reaction pathway starting from complex 1, yields acetaldehyde in two steps. The free energy profile of this a-path at a coverage of one ethanol is shown in Figure 5. The first step is the oxidative addition of the C(α)−H bond to Rh(I), yielding the metal hydride complex 1a1 and the CH3CHOH species bonded to the metal center via the centers O and C(α), with Rh−C = 199 pm and Rh−O = 215 pm, Figure S2 of the Supporting Information. The activation barrier of this exergonic addition, Gr(1 → 1a1) = −40 kJ mol−1, is calculated at 27 kJ mol−1. Subsequently the O−H bond of CH3CHOH is cleaved, resulting in complex 1a2 E


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Figure 6. Free energy profile (T = 673 K) for the formation of acetaldehyde, along the a-path, starting from complexes 2 and 3, i.e., with two (black) or three (green) ethanol ligands at the Rh center, respectively. Relative free energies of reactions (values in black) and the corresponding activation free energies (values in red) are given in kJ mol−1. Both profiles arrive at the overall free reaction energy for the formation of acetaldehyde from ethanol: Gr(2 → 2′) = Gr(3 → 3′) = 10 kJ mol−1. Sketches of the complexes are omitted as there are no significant differences in mechanism on the a-path for varying loading of ethanol.

for the scission of the C(α)−H bond when starting from the complexes 2 and 3, Ga(2 → 2a1) = 70 kJ mol−1 and Ga(3 → 3a1) = 112 kJ mol−1. Moreover, on the a-path starting from complex 3 the latter reaction is rather endergonic, Gr(3 → 3a1) = 67 kJ mol−1. The activation barriers of the subsequent O−H activation steps are calculated at Ga(2a1 → 2a2) = 99 kJ mol−1 and Ga(3a1 → 3a2) = 58 kJ mol−1. Complex 2a2 is analogues to 1a3, but the mode of ligand coordination at the Rh center differs. The barrier toward forming the H2 ligand, H···H = 90 pm, is small, Ga(2a2 → 2a3) = 14 kJ mol−1, and the expulsion of this ligand from the metal center is moderately endergonic, Gr(2a3 → 2a4) = 42 kJ mol−1. H2 migration to a neighboring empty zeolite cage is almost thermoneutral, Gr(2a4 → 2a5) = 8 kJ mol−1. From 2a5, the catalytic cycle ends with the adsorption, exergonic by −27 kJ mol−1, for regenerating the reactant. The product acetaldehyde desorbs without any change in free energy (Figure 6). Complex 3a2 is very stable, Gr(3a1 → 3a2) = −130 kJ mol−1 (Figure 6). This Rh complex features an approximately octahedral ligand configuration with the equatorial plane defined by the two H atoms and the two oxygen centers of the zeolite framework, leaving the ethanol molecule and the product acetaldehyde in axial positions. A further ethanol molecule interacts with the equatorial ethanol molecule via an H bond, Figure S2 of the Supporting Information. We were unable to locate a transition state for the formation of a hydrogen molecule starting from complex 3a2. Nevertheless, we scanned the Rh−H distance to discover that the energy only increases with the formation of the diatomic molecule and its separation from the Rh center, up to complex 3a3, when H2

with a further hydride ligand at Rh(I) and the product acetaldehyde, Figure S2 of the Supporting Information. The latter, slightly exergonic step, Gr(1a1 → 1a2) = −9 kJ mol−1, requires overcoming a high free energy barrier, Ga(1a1 → 1a2) = 91 kJ mol−1. The two H ligands are adsorbed at the metal in an elongated η2-H2 manner, H−H = 162 pm. Desorption of either product, acetaldehyde or molecular hydrogen, from this structure, is endergonic, with reaction energies of 58 kJ mol−1 and 42 kJ mol−1, respectively. Therefore, as in the case of ethylene, an additional ethanol molecule is introduced, to stabilize the resulting unsaturated Rh complex; this step is slightly exergonic by −8 kJ mol−1. After ethanol adsorption at the metal center, the free energy barrier for H2 formation is calculated at Ga(1a3 → 1a4) = 28 kJ mol−1. That reaction is slightly endergonic, Gr(1a3 → 1a4) = 7 kJ mol−1, and the thus formed hydrogen molecule, H···H = 88 pm, remains bound to the Rh center. The detachment of H2, endergonic by 51 kJ mol−1, leads to complex 1a5 where the two ligands ethanol and acetaldehyde interact with the metal center, with the hydrogen molecule rather far away (Figure 5). The catalytic cycle is easily closed by regenerating the initial complex 1 via two desorption steps, involving rather minor free energy changes of 3 kJ mol −1 (H 2 ) and 6 kJ mol −1 (acetaldehyde). 3.5. Ethanol Conversion to Acetaldehyde at Higher Ethanol Coverage. In contrast to the formation of ethylene, the a-path of dehydrogenation follows the same mechanism for all initial ethanol coverages examined. The calculated free energy profiles starting from complexes 2 and 3 are shown in Figure 6. Much higher activation free energies were calculated F


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Figure 7. Reaction network used in the microkinetic modeling of scenario 1 for the formation of ethylene at ethanol loadings n = 1−3 of the supported Rh center. The preferred pathway for the formation of ethylene, as obtained from the simulations, is highlighted by blue arrows.

ethylene from the three initial complexes 1, 2, or 3, Figure 7. This network traces the steps described by the energy profiles shown in Figures 3 and 4. The corresponding microkinetic simulations will be addressed as scenario 1. A corresponding network describing the a-path for ethanol loading n = 1−3 is shown in Figure S6 of the Supporting Information. Next, to address the issue of selectivity, we coupled the formation of ethylene and acetaldehyde for the dominant complexes, namely those with one ethanol ligand, in a joint reaction network, Figure 8, representing the steps of the energy profiles shown in Figures 3 and 5. We will refer to the simulations for this network as scenario 2. A coupled network that includes pathways starting from complexes with ethanol coverage n = 2 and 3 was not studied because they have a low impact on the examined reactions, as will be shown below. The elementary steps of both scenarios 1 and 2 are summarized in Table S6 of the Supporting Information. Throughout this section we will address the elementary reactions via their labels, see Figures 7 and 8. Note that reactions R4, R8, R14−R17, R21, R24, R28, and R33 are adsorption reactions, while the products can desorb via reactions R5, R6, R11, R13, R20, R22, R23, R25, R30, R31, R32, and R34. The simulations were intended to describe batch-type processes, starting from various species initially available. The simulations were stopped after 3 × 104 s (∼500 min) to mimic the longest reaction time used in the experiments.6 Two

leaves the ligand sphere, Figure S2 of the Supporting Information. The desorption requires a free energy expense of only 4 kJ mol−1. The catalytic cycle closes with the adsorption of a further ethanol molecule, regenerating the initial complex. Acetaldehyde desorbs easily, requiring only 3 kJ mol−1 (Figure 6) In summary, for both dehydration and dehydrogenation processes at coverage n = 1, the C−H scission step is thermodynamically favorable and exhibits low activation barriers, also lower than the subsequent C−O/O−H cleavage (Figures 3, 5). The values calculated for the energy span of the e-path increase slightly with ethanol loading, from 144, 147, and 152 kJ mol−1 for n = 1−3, respectively. The same trend is found for the a-path, but with larger energy differences; the corresponding values are 118, 139, and 198 kJ mol−1. Yet, from a purely thermodynamic point of view, it may be difficult to argue which of the reactions will preferentially occur at the supported Rh center.

4. RESULTS FROM MICROKINETIC MODELING AND DISCUSSION The thermodynamic data from the QM/MM calculations, just described, were used to develop microkinetic models for the ethanol conversion to the two products ethylene and acetaldehyde. To elucidate the role of ethanol loading we employed a network describing the pathways to forming G


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Figure 8. Reaction network used in the microkinetic modeling of scenario 2 for the coupled formation of ethylene and acetaldehyde, starting from complex 1 with one ethanol molecule on the supported Rh center. The preferred pathways for the formation of ethylene and acetaldehyde as obtained in the simulations are highlighted by blue arrows.

Table 2. Fraction, σ, of Adsorbed Species of Available Adsorbed Intermediates (Normalized to 1) at the End of the Kinetic Simulations Performed for Scenario 1b at Two Temperatures, Starting from Various Initial Statesa

temperatures were used for either scenario, 298 and 673 K, to explore the influence of the temperature on the kinetics of the reactions studied. As discussed in section 2, we refer free energy changes associated with adsorption/desorption processes to the situation in the cavities Zeo-H without an extra-framework metal center. Furthermore, we assume desorbed product molecules to migrate easily through the zeolite channels, thus ruling out any diffusion limitation in our models. This latter assumption is justified because in experiment the reactant gas feed is pumped through the catalyst’s pores at a higher pressure than the self-diffusivity of ethylene, calculated via molecular dynamics,59 therefore facilitating the evacuation of the products.11 4.1. Scenario 1: Effect of Ethanol Loading. We examined scenario 1 for four initial conditions (Table 2): the batch-type variants with initially (i) only complex 1, (ii) only complex 2, and (iii) only complex 3, as well as (iv) a variant with complex 1 with gas phase ethanol at the partial pressure p = 1 bar. The latter case corresponds to less than one ethanol molecule per Rh center if we assume a perfectly mixed batch reactor, with an average distribution of Rh complexes and the available ethanol molecules at 1 bar (of an ideal gas) in the zeolite probe (2 wt % Rh in 0.2 g faujasite zeolite).6 Therefore, initial conditions in case iv represent a case of low ethanol loading. Table 2 shows the resulting fractions, σ, of adsorbed species of the various intermediates at the end of the simulations, at room temperature, 298 K, and the temperature in the experiments,6 673 K. First we discuss the results of the simulations at room temperature. Starting from complex 1, variant i, intermediate 1e3 is formed after the system proceeds through the three activated steps R1, R2, and R3 (Figure 7). Complex 1e3 is the only species found at the end of the simulation (Table 2). In

initial state final fractions

T, K

1

2

1e3 1e4 1e5 3 3e4 EtOH(g) 1e3 1e4 1e5 EtOH(g) H2O(g) C2H4(g)

298

1.00

0.32 0.04 0.32 0.32

673

1.00

0.67 0.14 0.19 0.36 0.50 0.31

3

0.07 0.86 0.07 0.07 0.49 0.27 0.24 0.96 0.77 0.53

1 + EtOH 0.32 0.04 0.32 0.32 0.67 0.14 0.19 0.36 0.50 0.31

a

The obtained amount of products in the gas phase is expressed as a partial pressure, p (in bar), of the total pressure of 1 bar. bOnly ethylene formation pathways at different ethanol loadings are included in the reaction network; see Figure 7.

that complex both ligands, ethylene and water, obviously are sufficiently strongly bound to the Rh center so that no product molecule is able to desorb at such a relatively low temperature. When starting with complex 2, variant ii, several complexes on the e-path with one ethanol are formed as well as complex 3e4 with σ = 0.32 (Table 2). The presence of complexes 1e4 and 1e5 shows that ethanol is able to desorb from complex 2 via the reverse of reaction R8, resulting in complex 1 and ethanol in the gas phase (Figure 7). Complex 1 easily H


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ethylene product. After desorption of ethylene, the Rh center is unsaturated and another ligand is required for filling the ligand shell of the metal. In our scenario, we assumed this to be ethanol (Figures 3, 4, and 7). The calculated endergonic elimination of the product and the regeneration of the catalyst may be due to the formation of an unsaturated 14-electron Rh(I) center whereas a metal center with d8 configuration typically forms a stable 16-electron complex.60 Our microkinetic modeling further suggests that complex 1, with one ethanol molecule initially adsorbed on the supported Rh center, can hardly be recovered. If such a catalytic system with ethanol ligands is experimentally prepared, it is plausible that the catalytically active site present will have the structure of complex 1e3a square planar Rh complex with one water species and one ethylene molecule as ligands; see Figure 7 and Figure S1 of the Supporting Information. 4.2. Scenario 2: Product Selectivity. In scenario 2 with a coupling of e-path and a-path, Figure 8, we carried out simulations for two initial mixtures: only complex 1, and complex 1 with ethanol in the gas phase at a partial pressure p(C2H5OH) = 1 bar. Table 3 provides the resulting fractions σ

transforms into intermediate 1e3, via the above-mentioned activated steps R1 to R3. The reactant in the gas phase, to some extent, is involved in the adsorption reaction R4, resulting in complex 1e4 (Figure 7). From 1e4 water desorbs (R5), yielding complex 1e5. Furthermore, part of the ethanol formed via desorption from 2 is consumed in reaction R17, accumulating complex 3 with three ethanol ligands around the Rh center. The latter complex reacts with the water released in step R5 and complex 3e4 is accumulated through the reverse of reaction R22 (Figure 7). Nevertheless, the ethylene formed is unable to desorb at this low temperature and thus remains bound in complexes 1e3 and 1e5; see Figure S1 of the Supporting Information. Simulations starting from complex 3 show a similar outcome (Table 2). Only a very small amount of ethanol desorbs from complex 3 to form complexes 2 and 1 via the reverse of the steps R17 and R8, respectively (Figure 7). Complex 1e3 and 1e4 are fully converted to 1e5 and the desorbed water interacts with complex 3 to form complex 3e4 as just discussed for simulations starting from complex 2. When starting with complex 3, also a small amount of the desorbed ethanol remains in the gas phase (Table 2). Even at the higher temperature of 673 K, simulations starting from complex 1 complete after formation of complex 1e3 with σ = 1; water and ethylene remain bound to the metal center. Simulations starting from complex 2 progress with the desorption of ethanol via the reverse of step R8 as discussed above. The products of dehydration, ethylene and water, are formed mainly via the e-path starting from 1, ethanol loading n = 1. At the end of the simulation one finds final partial pressure values of p(C2H4) = 0.31 bar and p(H2O) = 0.50 bar (Table 2). Ethanol is not fully converted even at this high temperature, with a final partial pressure p(C2H5OH) = 0.36 bar. Simulations starting from complex 3 have a similar outcome, except for higher σ values for complexes 1e4 and 1e5 and a higher yield of ethylene, with the final partial pressure p(C2H4) = 0.53 bar (Table 2). Simulations starting with only complex 2, and those starting from complex 1 and additional ethanol molecules in the gas phase led to the same σ values of the resulting adsorbed intermediates (Table 2). A detailed analysis of the model simulations showed that for any temperature considered, one of the ethanol molecules in the starting complex 2 desorbs during the simulation and therefore acts as a reactant in the gas phase. The evolution over time of the fractions of adsorbed species, σ, of the various intermediates in the reaction network are identical from 1 × 10−3 s onward, regardless of the initial distribution of reactants; see Figure S7 of the Supporting Information. Moreover, the product ethylene is mainly formed along the e-path starting from complex 1, i.e., with only one ethanol ligand at the metal center. Ultimately, ethylene desorbs from complex 1e5 via reaction R6. This production process is indicated by the blue arrows in Figure 7. As already mentioned, in the light of this observation, we disregarded the pathways starting from complexes 2 and 3, i.e., with higher ethanol coverage, when examining the selectivity of the system; see the following section. The kinetic simulations revealed that the initial ethanol coverage at the metal center is largely irrelevant for outcome of the ethylene production. In situations with several ethanol ligands at the metal center, these additional ethanol species easily desorb and act as reactants in the gas phase. Nevertheless, ethanol in the gas phase is crucial for the expulsion of the

Table 3. Fraction, σ, of Adsorbed Species of Available Adsorbed Intermediates (Normalized to 1) at the End of the Kinetic Simulations Performed for Scenario 2b at Two Temperatures, Starting from Various Initial Statesa initial state final fractions

T, K

1

1 + EtOH

1e3 1e4 1e5 EtOH(g) H2O(g) 1e3 1e4 1e5 1a8 EtOH(g) H2O(g) C2H4(g) H2(g) CH3CHO(g)

298

1.00

673

1.00

0.10 0.89 0.01 0.10 0.01 0.65 0.09 0.16 0.10 0.25 0.40 0.24 0.17 0.26

a

The obtained amount of products in the gas phase is expressed as a partial pressure, p (in bar), of the total pressure of 1 bar. bCoupled formation of ethylene and acetaldehyde for the reaction network shown in Figure 8.

of adsorbed species of the various intermediates available at the end of the simulations, performed at the same two temperatures as for scenario 1, 298, and 673 K. As in scenario 1, for simulations starting only with complex 1, the chain of reactions again terminates at both temperatures with intermediate 1e3. Therefore, although dehydration and dehydrogenation of ethanol now are competing, the e-path is preferred over the a-path in the absence of (extra) ethanol. At the lower temperature of 298 K, simulations starting from complex 1 and additional ethanol in the gas phase resulted in a distribution of available intermediates close to that found in scenario 1, cf. Tables 2 and 3, thus demonstrating once again the prevalence of the e-path. Intermediate 1e4 has the highest σ value, 0.89; it is engaged in reaction R5 resulting in a very small I


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The Journal of Physical Chemistry C amount of 1e5, σ = 0.01, and water in the gas phase, p(H2O) = 0.01 bar. At the higher temperature, the presence of ethanol in the gas phase in the initial stage of the simulation once again boosts both transformation processes of ethanol, leading to the successful desorption of dehydrogenation and dehydration products (Table 3). Yet, the conversion of ethanol is not complete even at the relatively high temperature of 673 K, with the final partial pressure p(C2H5OH) = 0.25 bar. As in scenario 1, the presence of complexes 1e3, 1e4, and 1e5 of the e-path with n = 1 at the end of the simulation confirms that the ethylene product desorbs mainly from complex 1e5 via reaction R6; see the blue arrows in Figure 8. Moreover, test simulations including reactions of the pathways with initial ethanol loadings of n = 2 and 3 showed a slight increase (by 0.01 bar), equivalent to 6% of the final total amount of ethylene. From a sensitivity analysis (section S2 of the Supporting Information) one concludes that the reactions along the chain R2 to R6 have a strong positive effect on the ethylene yield, whereas the path via reaction R23 does not play a significant role; see Figure S8 and Table S7 of the Supporting Information. The a-path goes through reactions R26 and R27 to form complex 1a2 with the dehydrogenation products, acetaldehyde and hydrogen, bound at the Rh center (Figure 8). From 1a2 two directions exist for the product release−via initial adsorption of ethanol (R28) or direct desorption of the product acetaldehyde via R32, Figure 8. The presence of intermediate 1a8 with σ = 0.1 at the end of the simulation suggests a direct desorption of acetaldehyde from 1a2 via 1a7. The latter step leads to the formation of complex 1a7, a zeolitesupported Rh(I) bis(hydride) species, Figure S2 of the Supporting Information. After attaching an ethanol molecule (R33) to this unsaturated metal center, complex 1a8 is formed (Figure 8). Test simulations without reactions R28 to R31 in the network lead to essentially the same final fraction of acetaldehyde, reduced by 7 × 10−5 only. Therefore, the latter result and a sensitivity analysis (Figure S8, Table S7 of the Supporting Information) confirm that acetaldehyde will be produced mainly via direct desorption. Our simulations suggest that once the products of dehydration and dehydrogenation are accumulated at the metal center, forming complexes 1e3 and 1a2, respectively, acetaldehyde desorbs more easily than ethylene. This result corresponds to the calculated free energies of adsorption of ethylene and acetaldehyde in the latter complexes, −114 and −42 kJ mol−1, respectively. In contrast to the dehydration of ethanol, where the ethanol loading is playing a crucial role (see above), the discussed path to the formation of acetaldehyde suggests that ethanol loading will not have a direct effect on the dehydrogenation reaction. However, ethanol in the gas phase is still required for producing acetaldehyde. The reactant ethanol in the gas phase is essential for pushing the very stable complex 1e3 out of equilibrium, which represents a deep minimum on the potential energy surface; see Figure 3. In scenario 2, with extra ethanol initially in the gas phase, one obtains the final partial pressure values p(C2H4) = 0.24 bar and p(CH3CHO) = 0.26 bar. Thus, the ratio of ethylene to acetaldehyde at the end of the simulation is close to 1, showing that both reactions are proceeding simultaneously in competition and at comparable rates. Zhang and co-workers27 experimentally studying the conversion of ethanol on several materials, including H-ZSM-5 and Ni exchanged SAPO, found that the selectivity to ethylene decreases due to the formation

of acetaldehyde via dehydrogenation. Their result supports the conclusion from our simulations on the conversion of ethanol that both dehydration and dehydrogenation are competing transformations. 4.3. Effect of the Temperature. Both conversion processes of ethanol, to ethylene or acetaldehyde, are endothermic, with enthalpies of formation of ΔH298= 45 kJ mol−1 and ΔH298= 69 kJ mol−1, respectively. Therefore, these transformations are usually carried out at high temperatures. In the current study, the calculated overall reaction energy of the dehydration of ethanol to C2H4 and H2O is 71 kJ mol−1 when starting from complex 1. This value is close to that calculated for ethanol dehydration in H-MOR, ΔEr= 85 kJ mol−1, using an embedded cluster model with an ONIOM approach at the B3LYP/6-31G(d,p):UFF level.15 With increasing temperature, the process becomes thermodynamically more favorable. The calculated reaction free energy decreases to 34 kJ mol−1 at 298 K and 5.5 kJ mol−1 at 673 K. The calculated reaction energy for the dehydrogenation of ethanol to acetaldehyde at 0 K is 84 kJ mol−1. It decreases to 35 kJ and 10 kJ mol−1 at temperatures of 298 and 673 K, respectively. The positive effect of increased temperature values on the catalytic conversion of ethanol to olefins has also been found in experiment.10 Higher reaction temperatures were observed to favor the dehydration of ethanol to ethylene on La-modified ZSM-5 zeolite, with highest yield at 533 K.14 Increased temperature resulted in shifting the selectivity from diethyl ether to ethylene on P-modified ZSM-5, reaching 99% at 673 K.61 Therefore, to further examine the performance of the system under study, we carried out microkinetic simulations for a wide temperature range. For these simulations, free energy values were evaluated for the appropriate temperatures. Figure 9 depicts the change in the product yield and the selectivity with increasing temperature, as obtained from simulations of scenarios 1 and 2. Addressing only scenario 1, the formation of ethylene only becomes feasible at temperatures above 500 K (Figure 9a), and the ethylene yield increases with temperature, in qualitative agreement with experiment. At the highest simulated temperature, 900 K, the accumulated amount of ethylene reaches 0.77 bar. According to our microkinetic modeling, the product selectivity depends strongly on the temperature (Figure 9b). As seen from the final product distribution, both products start to be formed at zeolite-supported Rh centers at temperatures around 500 K. Our microkinetic models predict a complicated interplay between the two reactions. Acetaldehyde is predicted to form preferentially for temperatures almost up to 700 K. Above this threshold, the selectivity shifts to ethylene, at increasing higher yield (Figure 9b). The ratio between the two main products C2H4:CH3CHO vary with temperature from 0.24 at 450 K to 0.98 at 700 K and reaches 1.24 at 900 K. The formation of both products, ethylene and acetaldehyde, is facilitated with increasing temperature (Figure 9b). As shown by a sensitivity analysis (Figure S8, Table S7 of the Supporting Information) the desorption of the products strongly affects both final yields. Increasing the temperature has a major impact on these desorption steps which appear to be rate-limiting for both pathways. As the temperature gets higher, the entropy contributes more markedly, and it becomes more and more thermodynamically as well as kinetically favorable for the product species to detach form the metal center, due to the prevailing contributions of the rotational and translational J


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kinetic modeling. We modeled the active zeolite-supported metal sites with initial coverages of n = 1−3 ethanol molecules. The initial ethanol loading at the metal center affects the mechanism of ethylene formation, whereas no such effect was determined for the formation of acetaldehyde. The calculated free activation barriers increase with ethanol loading, for both dehydration and dehydrogenation reactions. Nevertheless, microkinetic modeling revealed that the role of ethanol is mostly related to the proper saturation of the ligand shell of the active metal site, thus facilitating the product desorption. Both types of conversions of ethanol, dehydration and dehydrogenation, were found to be feasible at the supported Rh centers, and they appear to occur as parallel reactions, with the final ethylene-to-acetaldehyde ratio varying with temperature. In agreement with experiment, we find that temperature plays a key role in the conversion of ethanol. The microkinetic models predict a higher yield of both products at higher temperatures. These results from microkinetic simulations provide notably expanded insight beyond the underlying QM/MM calculations. Indeed, in this way, one gains additional understanding of the catalytic system that cannot simply be derived from thermodynamics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11416. Energy aspects of dehydration/dehydrogenation reactions, free energies of adsorption, 3D models of all intermediates and TS structures, and details of the microkinetic modeling as well as a complete ref 44 (PDF) Cartesian coordinates of all stationary states in xyz format (ZIP)

Figure 9. Change in the final fractions of adsorbed species, σ, and partial pressure, p, of gas phase products of selected species obtained from the simulations of (a) scenario 1 and (b) scenario 2 at increasing temperature T. At the beginning of both simulations, complex 1 is available with additional ethanol in the gas phase at partial pressure, p = 1 bar. The lines are drawn to guide the eye.

degrees of freedom of the leaving molecules, Table S4 of the Supporting Information. The present microkinetic results agree qualitatively with experiments on the steam reforming of ethanol over aluminasupported Rh where a strong temperature dependence of the concentrations of ethylene and acetaldehyde was observed.30 In addition, over the 2% Rh/Al2O3 catalyst, both products show very similar behavior and concentration dependence in the temperature range studied, 873−1073 K. The largest amounts of acetaldehyde and ethylene in the final product mixture are somewhat lower (

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