Computational Insights into the Role of Metal and Acid Sites in

Research Article pubs.acs.org/acscatalysis

Computational Insights into the Role of Metal and Acid Sites in Bifunctional Metal/Zeolite Catalysts: A Case Study of Acetone Hydrogenation to 2‑Propanol and Subsequent Dehydration to Propene Sai Sriharsha M. Konda, Stavros Caratzoulas,* and Dionisios G. Vlachos Catalysis Center for Energy Innovation and Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: We employ electronic structure calculations to elucidate the catalytic pathways on bifunctional metal/zeolite catalysts by modeling a HZSM-5-supported nickel tetramer cluster (Ni4-ZSM-5). Hydrogenation of acetone to 2-propanol followed by dehydration to propene have been investigated as model reactions. In Ni4-ZSM-5, we observe reverse hydrogen spillover, whereby the Brønsted hydrogen migrates from the zeolite active site to the metal cluster. Consequently, the zeolitesupported metal cluster becomes electron-deficient, facilitating the hydrogenation reaction. In contrast, studies conducted on the dehydration reaction pathways indicate that the Brønsted acid catalysis in HZSM-5 is preferred over the metal catalyzed pathway in the Ni4-ZSM-5 system, again as a result of the electrondeficient nature of the metal species. KEYWORDS: bifunctional catalysts, zeolites, hydrodeoxygenation, density functional theory, reaction mechanisms

1. INTRODUCTION Bifunctional catalysts, featuring both metal and acid sites, have been extensively investigated for their unique properties toward the application of various reactions. The term “bifunctional” not only encompasses catalysts in solution or on oxide surfaces such as Pd/H3PO41 and Pt/Al2O3,2 but also includes metals supported on solid acid catalysts such as Nafion,3 zeolites,4−8 and polyoxometalates.9 Boudart et al.10 first pointed to the notion of zeolite-supported metal particles being electrondeficient on various supports due to their interaction with the acid sites of the support. Sachtler and co-workers coined the term “metal-proton adducts”,11−13 where the positive charge is delocalized over the metal atoms. They reported enhanced catalytic activity toward hydrogenation, hydrogenolysis, and improved selectivity in hydrocarbon conversion over different metals, such as Pd, Rh, and Ru supported on zeolite Y.13,14 The Brønsted hydrogen atoms in the zeolites were also believed to anchor and stabilize the metal particles, thereby causing high dispersion.12,15 Pioneering work by Gates and co-workers expanded the field by introducing novel experimental techniques to investigate faujasite-supported metal tetramers and hexamers.16,17 Further application of techniques such as infrared (IR), X-ray photoelectron (XPS), and extended X-ray absorption fine structure (EXAFS) spectroscopy yielded remarkable geometric and electronic insight into these systems.18−21 Computational models employing density functional theory (DFT) followed suit to ascertain irregularities in © 2015 American Chemical Society

the intermetal distances reported in the aforementioned EXAFS experiments.22 Rösch and co-workers noted that the supported metal clusters interact almost instantaneously with the Brønsted hydrogen, resulting in its transfer to the metal in a process termed reverse hydrogen spillover (RHS).23−25 The accompanying charge balance yielded surface adsorbed hydride species, resulting in the oxidation of the metal cluster. Similar reports have underscored the importance of the spillover process in understanding the interactions of the metal cluster with the zeolite framework.26,27 Evidently, a common theme in the work reviewed above is that the enhanced catalytic activity of the zeolite-supported metal clusters follows from the electrondeficient nature of the metal species, a direct consequence of the spillover process. In recent years, there has been an increased interest in bifunctional metal/zeolite catalysts for bio-oil upgrade via hydrodeoxygenation (HDO), a process involving high temperature (500−700 K) under hydrogen pressures of about 5 MPa to remove oxygen as water.28,29 Specifically, Lercher and coworkers have reported the conversion of lignin-derived aromatics to medium-chain hydrocarbons using a HZSM-5supported nickel catalyst.6,30−32 For phenol conversion, they reported a sequence of steps: hydrogenation to cyclohexanol Received: August 3, 2015 Revised: October 18, 2015 Published: November 18, 2015 123

DOI: 10.1021/acscatal.5b01686 ACS Catal. 2016, 6, 123−133

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ACS Catalysis

ring zeolite channel. The mechanistic aspects discussed in this work are based on the electron-deficient nature of the metal cluster as a consequence of the spillover process. Since the RHS process was shown to occur in tetramers and hexamers,25 the results discussed herein are expected to hold true for the hexamer as well. For the same reason, we choose to model a single Brønsted acid site, as compared to multiple sites used in the investigation of the RHS process.23,25 In those studies, multiple hydrogen transfers have been proposed as part of the spillover process in the zeolite-supported metal clusters. However, this only leads to the metal cluster being more electron-deficient as compared to the single hydrogen transfer.23,25 Consequently, the expected chemical reactivity governing the hydrogenation and dehydration reactions would be similar in the case of single or multiple Brønsted acid sites. 2.2. Computational Methods. All electronic structure calculations were performed with the Gaussian 09 package.42 The optimization of the open-shell systems was carried out using the unrestricted DFT formalism. Careful consideration was given to the possible occurrence of different spin states by checking the stability of the wave function and accounting for spin contamination (refer to Tables S1 and S2 in the Supporting Information for the expectation value of the total spin, ⟨S2⟩). The metal-zeolite systems were modeled by the M06-L meta-GGA functional43,44 along with the 6-31g(d,p) basis45 for the Al, C, H, and O atoms and the Lanl2DZ basis set and effective core potential for the Ni and Si atoms (for computational efficiency).46,47 Effective core potential offers significant computational savings and has been reported in the literature for understanding chemical reactivity in zeolitesupported metal clusters.38,48−51 The M06-L functional has been parametrized for transition metal bonding43,44 and employed to model metal clusters in zeolites.33,44 In benchmarking calculations, Truhlar and co-workers have demonstrated that, for binding in zeolite clusters, the M06-L functional gives a mean unsigned error of only 0.87 kcal/mol.52 Furthermore, the M06-L functional was parametrized to include medium-range dispersion interactions.43,44 All the atoms in the zeolite cluster, including the metal species, were allowed to relax, except for the capping H atoms that were frozen in Cartesian space (see the Supporting Information for details regarding the optimized structures). Transition states (TS) were optimized using the Berny algorithm,53 following which a steepest-descent protocol using the intrinsic reaction coordinate (IRC) method was employed to confirm that the TS were connected with the desired minima. The TS were further characterized using frequency calculations, yielding a single imaginary frequency corresponding to the normal mode along the reaction coordinate. The population analysis was performed using the Natural Bond Orbital (NBO) method as implemented in the NBO 6.0 program.54 All the free energy profiles are reported at 298.15 K and 1.00 bar. The binding energies (BSSE uncorrected) were calculated in the standard fashion as

via a cyclohexanone intermediate; dehydration to cyclohexene; and finally hydrogenation to cyclohexane.31 The authors reported enhanced hydrogenation activity over the metal sites and that dehydration happens exclusively on acid sites. Despite the abundance of experimental evidence suggesting the utility of bifunctional catalysts in the lignin HDO chemistry, there is still very limited understanding of the mechanistic pathways over the metal and acid sites. Simulations have been mostly limited to supporting EXAFS and IR data. Only a few articles have investigated reaction pathways on the active sites on bifunctional catalysts.33,34 In this article, we model a similar process: the conversion of acetone to propene. First, we investigate the hydrogenation of acetone to 2-propanol on a HZSM-5-supported metal cluster, modeled as a tetramer, and compare to the unsupported nickel cluster. We then elucidate the dehydration reaction of 2propanol to propene on both the metal and acid sites, represented by the zeolite-supported nickel tetramer and bare HZSM-5, respectively. We further discuss the consequences of the electron-deficient nature of the zeolite-supported metal cluster arising from the reverse hydrogen spillover on both the hydrogenation and dehydration reactions.

2. METHODS 2.1. Zeolite Model. A 34T cluster model, shown in Figure 1, was chosen so as to feature the active site at the intersection

Figure 1. Optimized 34T cluster model of HZSM-5: (a) straight channel view and (b) sinusoidal channel view. Atoms are color-coded as Si (yellow), O (red), H (white), and Al (pink). For clarity, the active site is shown as spheres, and the rest of the framework is shown as a tube frame.

of the 10-ring straight and sinusoidal channels.35 The initial structure was built from the periodic zeolite framework of MFI (using the X-ray crystal structure),36 which was cropped to obtain the appropriate cluster size. Within this cavity, the active site was generated at the T12 position by substituting an aluminum atom for a silicon atom and adding a proton to maintain charge neutrality. The dangling Si- bonds obtained from cropping the cluster were capped with H atoms with a bond length of 1.47 Å (corresponding to the Si−H distance in silane) oriented in the direction of the crystallographic Si−O bonds. This model provides a reasonably sized cluster to capture the steric environment provided by the 10-ring windows in HZSM-5. Similar clusters have been used before.37−39 We model the nickel cluster inside the zeolite as a tetramer, drawing a parallel with the experimental and computational work done on zeolite-supported Ir4, Rh4, and Pt4 clusters by Gates,17 Rösch,40,41 and co-workers. Furthermore, the tetramer provides sufficient sites to coordinate the various species involved in the hydrogenation and dehydration reaction mechanisms while being small enough to fit inside the 10-

EBE = Eads + cat − Eads − Ecat

where Eads+cat is the total energy of the adsorbate on the respective catalyst, Eads is the gas phase adsorbate energy, and Ecat is the energy of the bare catalyst. Based on this convention, a negative value would indicate a favorable binding interaction and vice versa. 124


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3. RESULTS AND DISCUSSION In the following, we report the hydrogenation and dehydration mechanisms on the metal-zeolite systems. We first describe the nickel tetramer in the gas phase, which serves as a baseline; the geometric and electronic changes resulting from the interaction between the metal cluster and the zeolite are also analyzed. Subsequently, we investigate the hydrogenation of acetone to 2propanol on the bare nickel cluster and on the zeolite support. We conclude with a comparative analysis of the dehydration mechanisms on bare HZSM-5 and the zeolite-supported metal cluster. 3.1. Ni4 Cluster in the Gas Phase. In order to define a reference structure for the calculations, we started by optimizing the ground spin state for the bare nickel cluster. Different planar and nonplanar geometries such as square planar, rhombic, tetrahedral, and pyramidal shapes along with the resulting point group symmetries, Cs, C2v, C3v, and D2h, were considered in the search for the ground state configuration (see Figure S1 for details). In agreement with earlier work, the optimization yielded a ground quintet state with C 2v symmetry. 55 The stability of the cluster was further characterized by evaluating the binding energy per Ni atom given as

acid site at the intersection of the straight and sinusoidal channels. Similar to the bare Ni4 in the gas phase, the cluster supported on the zeolite retained its quintet spin state. Upon geometry optimization, the Brønsted hydrogen migrated to the metal cluster, consistent with the RHS process.24,25,49 The hydrogen atom is positioned along the edge of the metal cluster, which is coordinated to the active site O atoms (Figure 2c); we will be referring to this system as Ni4H-ZSM-5, to explicitly indicate that RHS has taken place. In order to quantify the spillover energy, ERHS, a constraint simulation was carried out in which the Brønsted hydrogen was not allowed to migrate from the oxygen atom at the active site; we will be referring to this system as c-Ni4-HZSM-5. In another calculation, the metal cluster was placed inside a purely siliceous zeolite, where RHS cannot happen; we will be referring to this system as Ni4-SILIC (see Figures 2b and 2d). The interaction energy between the metal cluster and the zeolite framework was determined by two parameters: (a) ERHS, the stabilization energy from the RHS, and (b) EBE, the binding energy of the bare nickel cluster to the zeolite, defined as ERHS = E[Ni4H‐ZSM‐5] − E[c‐Ni4‐HZSM‐5] HZSM EBE = E[c‐Ni4‐HZSM‐5] − E[HZSM‐5] − E[Ni4]

BE Ni4 = [(Total energy of Ni4cluster) − 4*(Energy of Ni atom)]/4

SILIC EBE = E[Ni4‐SILIC] − E[SILIC] − E[Ni4]

The calculated value of −47.7 kcal/mol was in agreement with the literature values of −46.3 kcal/mol from Arvizu et al.56 and −50.1 kcal/mol from Petkov et al.,57 employing the PW86/ TZVP and the PBE/LCGTO-FF theory levels, respectively. Given the geometry dictated by the C2v point group, the symmetry unique Ni−Ni bond lengths are 2.39 and 2.24 Å. From an electronic viewpoint, NPA (Natural Population Analysis) suggests that the metal unpaired spin orbitals are dominated by the 3d orbitals with a natural electron configuration of 4s0.883d9.024p0.08. 3.2. Zeolite-Supported Ni4 Cluster. As shown in Figure 2a, the nickel metal cluster was initialized near the Brønsted

The calculations suggest that the RHS process is highly exergonic, by −49.2 kcal/mol. The binding energy also indicates a favorable interaction between the metal cluster and the zeolite (EHZSM‑5 = −67.0 kcal/mol and ESILIC = −47.9 BE BE kcal/mol). There are no reports on nickel tetramers in HZSM5 for a direct comparison. The values by Rösch and coworkers24 (−40.4 kcal/mol for the binding and −38.0 kcal/mol for the spillover stabilization) on a faujasite-supported nickel hexamer are of similar magnitude to ours. Moving on to the geometric changes, selected metal clusterframework and average ⟨Ni−Ni⟩ bond lengths are presented in Table 1. The metal cluster is coordinated more closely to the Table 1. Key Optimized Bond Distances (Å) in the Different Zeolite Systemsa

a

system

Of1−Hbr

Of1−Ni

Of2−Ni

⟨Ni−NI⟩

HZSM-5 Ni4-SILIC Ni4H-ZSM-5

0.96 -na-na-

-na2.41 2.25

-na2.19 2.07

-na2.34 2.38

Refer to Figure 2 for labels.

zeolite framework in the Ni4-HZSM-5 system than in the Ni4SILIC system, as indicated by the smaller Of1−Ni1 and Of2−Ni1 bond distances (see Figure 2c for labels). This finding is consistent with previous experimental EXAFS and computational studies for several zeolite-supported transition metals.25 Weakening of the metal−metal bonds is also indicated via their elongation, whereas the ⟨Ni−Ni⟩ distance in the Ni4-SILIC system is identical to the average bond length in the bare Ni4 cluster (Table 1). The most interesting feature is manifested in electronic changes of the metal. The charge varies from −0.03 in Ni4SILIC to +1.18 in Ni4H-ZSM-5. Indeed, Gates and co-workers have experimentally demonstrated partial oxidation of Rh atoms supported on zeolite Y.58 It is now widely accepted in the literature that the RHS process is a direct consequence of

Figure 2. Initial (a,b) and optimized final (c,d) structures of nickel tetramer supported on HZSM-5 (left) and siliceous zeolite (right). Color code for active site shown as spheres: Brønsted hydrogen, Hbr (white), nickel (blue), framework oxygen, Of1/f2 (red) and aluminum (pink). 125


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ACS Catalysis this phenomenon.23,25 The balance in the electron flow dictates that the Brønsted proton that migrated to the cluster be reduced by gaining electron density from the oxidized metal, formally obtaining a hydride character (NPA charge of −0.42). Modification of the acid sites has also been suggested in zeolitesupported Pt clusters to the extent of reduced Brønsted acidity or even its complete suppression.26,27 In summary, our conclusions about the physical properties of the catalysts concur with previous findings. 3.3. Hydrogenation of Acetone to 2-Propanol. The gas-phase conversion of acetone to 2-propanol is almost thermoneutral with a reaction energy of −0.5 kcal/mol. The hydrogenation reaction proceeds through a sequence of steps, starting with the dissociative adsorption of H2 and the adsorption of the reactant from the gas phase. We discuss these steps next. 3.3.1. H2 Adsorption and Activation. We begin with the activation of hydrogen on the bare Ni4 cluster. As shown in Figure 3a, the process is initiated with the hydrogen molecule

interacting in a side-on configuration with a single metal atom. The accompanying dissociation goes downhill to the energetically most favorable binding mode with the H atoms adsorbed 4 on two neighboring edge sites (ENi rxn = −19.0 kcal/mol). The barrier for the activation process was not calculated and is estimated to be small based on the exothermicity of the reaction and the nature of the transition state, which should resemble the reactant state, as per Hammond’s postulate.59 The quintet spin state for the cluster is retained in the process, and a consistency check for the first excited triplet state yielded an energy profile higher than the ground state. From here on, unless stated explicitly, we work with the ground quintet spin state for both the bare and the zeolite-supported cluster (see Figure S2 in the Supporting Information for details regarding first excited spin state). The origin of the activation process was deduced via NBO analysis of the reactant geometry in Figure 3a. Evidence for the apparent weakening of the H5−H6 bond is given from the strong, second-order perturbation theory donor−acceptor interactions, E(2), of 25.9 kcal/mol between (BD)H5−H6 → (LP*)Ni1 and 7.0 kcal/mol between (LP)Ni1 → (BD*)H5−H6 (the arrow indicates the electron flow from the donor to the acceptor). The simultaneous charge depletion from the bonding hybrid orbital (BD) and accumulation in the antibonding orbital (BD*) leads to the activation of the adsorbed hydrogen molecule. The bond weakening is manifested in the lengthening of the H5−H6 bond to 0.84 Å from the equilibrium value of 0.74 Å in the gas phase. On breaking of the bond, both hydrogen atoms acquire a hydride character, with charges of −0.44 each. Consequently, the nickel cluster becomes partially oxidized and carries a positive charge of +0.90. A similar situation arises in the metal-zeolite systems of Ni4H-ZSM-5 (Figure 3b) and Ni4-SILIC (Figure 3c), wherein the thermodynamically favorable activation process results in the hydrogen atoms being adsorbed on the edges of the cluster 4H‑ZSM‑5 4‑SILIC (ENi = −12.7 kcal/mol and ENi = −18.7 kcal/mol). rxn rxn As in the bare Ni4 cluster case, the lengthening of the H5−H6 bond was a prelude to the activation process. Furthermore, the

Figure 3. Hydrogen activation on the nickel cluster (a) bare tetramer; (b) supported on the acid zeolite, Ni4H-ZSM-5; and (c) supported on the siliceous zeolite, Ni4-SILIC.

Figure 4. Reaction pathways for the hydrogenation of acetone to 2-propanol on Ni4 via the alkoxy and hydroxy mechanisms. Highlighted atoms undergo hydrogenation in the respective reaction mechanisms. Asterisk indicates the atom/s coordinated to the metal surface. 126


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ACS Catalysis positive charge on the cluster increases in the presence of additional hydrogen atoms in the following order: Ni4-SILIC (+0.83) < Ni4 (+0.90) < Ni4H-ZSM-5 (+2.01). The inherently high formal charge in Ni4H-ZSM-5, resulting from the RHS process, makes a significant contribution to the net charge on the cluster with adsorbed hydrogen atoms. Depending on reaction conditions and cluster size, both the bare and the supported metal clusters are capable of coordinating large amounts of hydrogen.60 Rösch and coworkers have investigated the saturation of Rh, Ir, and Pt tetramers with hydrogen and observed a hydrogen/metal ratio of either 2 or 3, depending on the metal.41 Given the extreme temperatures involved in HDO reactions (500−700 K), the choice of two hydrogen atoms in the present work is reasonable. 3.3.2. Alkoxy vs Hydroxy Mechanisms. We next look at the configurations of acetone on Ni4. Experimental and theoretical studies provide insight into the binding modes of carbonyl compounds, which are categorized as (i) the η(O) mode, with the carbonyl O bound end-on to a single Ni atom, and (ii) the μ(C,O) mode, with the C and O atoms either bound to the same or different metal atoms. Bonding in the η(O) mode takes place through the donation of the oxygen lone pair to the metal d-orbitals, whereas the μ(C,O) binding takes place through the carbonyl π orbital. Both binding modes have been suggested for acetone adsorption on metal surfaces. It has been argued that the η(O) mode is favorable on account of minimized steric interactions of the methyl groups with the surface.61,62 However, binding on the bare cluster is devoid of such steric effects, and, consequently, the μ1(C,O) configuration is the energetically most favored binding mode (the superscript indicates the number of metal atoms coordinated to acetone). The hydrogenation of the carbonyl species has been shown to proceed via the Horiuti−Polanyi mechanism.63 As depicted in Figure 4, the reaction can occur along two pathways that involve the sequential addition of hydrogen atoms to the carbonyl functional group in acetone. In the first path, referred to as the alkoxy mechanism, the surface adsorbed hydrogen attacks the carbonyl carbon of acetone bound in the μ1(Ca,Oa) mode (EBE = −18.5 kcal/mol) to generate an alkoxy intermediate, which further undergoes a hydrogen addition to the oxygen atom, Oa, yielding 2-propanol. Alternately, in the hydroxy mechanism, the first hydrogen addition occurs at the carbonyl oxygen bound in the μ2(Ca,Oa) mode (EBE = −17.6 kcal/mol) to form a hydroxyalkyl intermediate followed by hydrogen addition to the carbonyl carbon resulting in 2propanol. The energy profile of the individual pathways plotted in Figure 5 indicates that the alkoxy mechanism is favored, because it is easier to hydrogenate the carbonyl carbon over the oxygen atom. This is evident from the lowest activation energy of 19.3 kcal/mol (TS1A) involved in the first hydrogen addition in the alkoxy pathway compared to the rest of the steps. The second hydrogen addition to the alkoxy intermediate is the rate-determining step with a barrier of 34.8 kcal/mol (TS2A). The results are rationalized given that surface adsorbed hydrogen atoms carry a partial negative charge of −0.40. Not only does the addition of hydrogen to the nucleophilic Oa atom (NPA charge of −0.66) raise the energy profile of the hydroxy mechanism, but also it increases the barrier for the second hydrogen addition in the alkoxy mechanism, resulting in an overall endergonic process (22.6 kcal/mol). There is also a marked difference in the respective intermediate states. The

Figure 5. Energy profile diagram (ΔG(298 K)) of the alkoxy (blue) and the hydroxy (red) pathways for the hydrogenation of acetone (R) to 2-propanol (P) on Ni4. The labels correspond to structures in Figure S3. The energy is relative to the most stable reactant state. alkoxy alkoxy intermediate (INTA, EBE = −69.0 kcal/mol) is stabilized more than the hydroxyalkyl species (INTH, Ehydroxy BE = −43.0 kcal/mol) by forming stronger bonds to the Ni4 cluster (the binding energies reported above are in the absence of surface hydrogen atoms). As seen in Figure 4, in contrast to the hydroxyalkyl intermediate retaining the same binding mode as in the reactant state, the intermediate in the alkoxy pathway preferentially binds through its oxygen atom to the oxophilic nickel cluster, thus forming stronger bonds indicated by the higher binding energies. While there has been no literature on the hydrogenation mechanism of acetone on clusters, similar predictions have been proposed for C1−C4 aldehydes and ketones over Ru(0001).64 Henceforth, only the alkoxy route will be considered for investigation of the hydrogenation reaction on the zeolite-supported metal clusters. Upon adsorption either on the zeolite-supported species or on the bare Ni4 cluster, acetone retains the same μ1(Ca,Oa) configuration. Presented in Figure 6 are the free energy profiles for the alkoxy route on all three catalysts, Ni4, Ni4H-ZSM-5, and Ni4-SILIC (see the Supporting Information for structures and coordinate files). Comparing Ni4 with Ni4-SILIC, we can deduce that the effect of the zeolite is rather minor, as the activation energy barriers and reaction energies are quite similar. However, on account of the RHS, the hydrogenation

Figure 6. Calculated energy profile (ΔG(298 K)) for the hydrogenation of acetone (R) to 2-propanol (P) along the alkoxy route on Ni4 (blue), Ni4H-ZSM-5 (red), and Ni4-SILIC (black). The energy is relative to the respective reactant state on the catalytic system. The corresponding structures are provided in Figure S4. 127


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ACS Catalysis reaction is thermodynamically more favorable on Ni4H-ZSM-5, by about 20.0 kcal/mol. In the absence of the acid site and, consequently, of hydrogen spillover, the binding of acetone Ni4‑SILIC 4 (ENi Acetone = −18.5 kcal/mol, EAcetone = −20.1 kcal/mol) and 2Ni4 Ni4‑SILIC = −15.2 kcal/ propanol (EPropanol = −14.3 kcal/mol, EPropanol mol) to Ni4 and Ni4-SILIC are comparable. However, both are 4H‑ZSM‑5 bound stronger to Ni4H-ZSM-5, where RHS occurs (ENi Acetone Ni4H‑ZSM‑5 = −25.4 kcal/mol, EPropanol = −26.6 kcal/mol). Additionally, the intrinsic barrier for the rate-determining step involving the hydrogen addition to the carbonyl oxygen is lower in Ni4HZSM-5 (32.2 kcal/mol) compared to Ni4 (34.8 kcal/mol) and Ni4-SILIC (36.1 kcal/mol). Earlier, we presented the electrondeficient nature of the HZSM-5-supported metal cluster on account of the spillover process. Likewise, when 2-propanol is bound to the Ni4H-ZSM-5 catalyst, we observe a net positive of +1.16 on the metal cluster as opposed to −0.04 when bound to unsupported Ni4. Similar conclusions were derived from the study of cyclopropane hydrogenolysis on supported Pt catalysts, wherein the enhanced activity was explained by the stronger binding of the adsorbate to the electron-deficient Pt species.15 Our findings are also in qualitative agreement with the conclusions drawn by Lercher and co-workers,31 in that the acid support enhances the adsorption of phenol, thereby increasing the rate of hydrogenation on the HZSM-5-supported nickel catalyst. 3.4. Dehydration of 2-Propanol to Propene. As the term “bifunctional catalyst” implies, these materials are not only capable of hydrogenation but also of carrying out dehydration reactions. The mechanistic picture in dehydration is complicated by the presence of both metal and Brønsted acid sites in the zeolite and the hydride nature of H on the metal. In order to understand the dehydration mechanism, we model two systems for the dehydration of 2-propanol to propene and water: (a) HZSM-5, the bare zeolite containing a Brønsted hydrogen as a model for the acid site, and (b) Ni4H-ZSM-5. In the absence of a catalyst, the dehydration of 2-propanol requires 63.0 kcal/mol of activation (see Figure S7). 3.4.1. Brønsted Acid Catalyzed Elimination Reactions. Acid catalyzed dehydration falls under the broader class of elimination reactions, which usually involve deprotonation in conjunction with the expulsion of a leaving group. The reaction mechanisms can be classified either as stepwise (E1) or concerted (E2), as sketched in Scheme 1. Following the adsorption of the alcohol on the Brønsted acid site, the first step in the E1 mechanism is the formation of an alkoxy intermediate by the elimination of water. The subsequent step involves deprotonation of the intermediate by the oxygen atom from the Brønsted site to form propene. In contrast, the E2 mechanism involves a concerted process of water elimination and β-hydrogen removal by the neighboring framework oxygen atom. The active site is regenerated in the same position as in the bare zeolite in the E1 mechanism, whereas the adjacent framework oxygen picks up the hydrogen in the E2 mechanism. As shown in Figure 7, 2-propanol can adsorb in two ways depending on whether the alcohol group is on the same or opposite side as the dihedral plane formed by Hbr−Of1−Al−Of2 atoms, giving the syn- (RE2) and anticonfiguration (RE1), respectively. The binding energies are almost identical, with the former being slightly more stabilized (ERE2 BE = −20.7 kcal/mol RE1 = −20.2 kcal/mol). The RE2 configuration is and EBE characterized by additional hydrogen bonding interactions, indicated in Figure 7a and given as Ho···Of2 (1.46 Å) and Oa··· Hb (1.00 Å). In fact, the extended bond length between the

Scheme 1. E1 and E2 Proposed Mechanism for the 2Propanol Dehydration to Propene on HZSM-5

Brønsted proton and the framework oxygen, Of1···Hbr (1.65 Å) in RE2 compared to that in RE1 (1.06 Å) indicates that 2propanol is in a protonated state in RE2, thereby creating a good leaving group in the form of water. Moving on to the mechanistic details, interatomic distances pertinent to the E1 and E2 mechanisms are presented in Table 2, and the intermediate and TS structures involved in the respective reaction energy profiles are presented in Figure 7. In the E2 mechanism, the protonated state of 2-propanol in RE2 activates the concerted scission of Ca−Oa and the β-hydrogen removal by the framework Of2 atom (Figure 7a). The overall reaction is endergonic (13.5 kcal/mol) with an activation energy barrier of 21.1 kcal/mol (Figure 8). The TS E2 configuration is a late-transition state with a significant double-bond character indicated by the elongated Cb···Hb bond and the shortened Ca···Cb distance compared to the reactant (Table 2). In the product state, PE2, the Brønsted acid site is regenerated at the Of2 atom with propene and water physisorbed inside the HZSM-5 cavity. In contrast, the chemistry in the E1 mechanism begins with 2-propanol being bound in the RE1 configuration (Figure 7b). Following adsorption, the Ca−Oa scission takes place in the first step leading to the generation of a propyl alkoxide that is bound to the framework Of2 atom. We observe a late-transition state for this step (Erxn = 12.7 kcal/mol) with a barrier of 25.3 kcal/ mol. More importantly, TS1E1 resembles a secondary carbocation with sp2 hybridization, suggested by the almost in-plane arrangement of substituents around the Ca atom (dihedral angle of 174.5°). NBO analysis confirms this: the positive charge is concentrated on Ca, and the net charge on the hydrocarbon fragment is +0.74. The following deprotonation step is also endergonic (7.4 kcal/mol) resulting in a latetransition state with a barrier of 16.5 kcal/mol (TS2E1). As in 128


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Figure 7. Reactant (R), transition state (TS), intermediate (INT), and products (P) involved in the (a) E2 and (b) E1 dehydration reaction mechanisms in HZSM-5. Bond lengths involving important atoms are given in Table 2. Color code for the active species shown as spheres: oxygen (red), hydrogen (white), and carbon (gray).

Table 2. Selected Bond Lengths (Å) within the Reactant (R), Transition State (TS), Intermediate (INT), and Products (P) Involved in the Dehydration Reaction of 2-Propanol to Propene on HZSM-5a E2 Mechanism states

Oa−Ca

R TS P

1.50 3.10 3.27

states

Oa−Ca

R TS1 INT TS2 P

1.46 2.45 4.53 4.71 3.72

Cb−Hb

Hb−Of2

Ca−Cb

2.92 1.44 1.00

1.50 1.38 1.34

Cb−Hb

Ca−Of2

Hb−Of1

1.09 1.10 1.09 1.28 2.12

3.11 2.36 1.54 3.19 3.06

4.90 4.23 3.00 1.38 1.00

1.09 1.25 2.02 E1 Mechanism

Structures with the corresponding labels are illustrated in Figure 7.

Figure 8. Reaction energy diagram (ΔG(298 K)) of the E1 (red) and E2 (blue) dehydration mechanisms for the conversion of 2-propanol (R) to propene and water (P) in HZSM-5. The labels correspond to structures in Figure 7.

E2, the final products are propene and water; contrastingly, the energy of the final state in E1 is different owing to the regeneration of the active site at Of1 rather than at Of2. It is clear by the comparison of the intrinsic barriers in the free energy profiles in Figure 8 that the E2 mechanism is the favored pathway for the dehydration reaction. Similar concerted mechanisms have also been proposed for alcohol dehydration on γ-Al2O3 surfaces.65,66 Dixon and co-workers have also provided experimental and computational evidence for the concerted C−O and C−H scission process in the dehydration

reaction of deuterated alcohols on cyclic molybdenum and tungsten oxides.67,68 There are however contradictory reports made by Duca and co-workers,69 who proposed a different E2 mechanism for the dehydration of 2-propanol in HZSM-5: a concerted intramolecular β-hydrogen abstraction by the protonated alcohol group, with reported barriers of 40.3 kcal/ mol. We have examined a similar mechanism for the uncatalyzed reaction in the gas phase (Figure S7), but a TS could not be located in the zeolite system. Neurock and coworkers have proposed an E1 mechanism for the dehydration of 2-butanol on polyoxometalates70 on the basis of similarities

a

129


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ACS Catalysis between the E1 first transition state and the E2 one. The E2 transition state reported in the current work is indeed unique to the specific pathway and is geometrically and electronically different from the first TS found in the E1 route (see Table 2 for details). Apparently, quantitative differences arise from the different frameworks involved in the polyoxometalates and the zeolite pore, and, as such, the preference for the concerted process may be governed strongly by the environment around the adsorption site of the alcohol, which in our case seems to favor the E2 pathway. 3.4.2. Metal Catalyzed Dehydration Reactions. While in HZSM-5 we were able to identify both concerted and stepwise bond breaking processes, only stepwise mechanisms were found in the case of Ni4H-ZSM-5 catalyzed dehydration reactions. Accordingly, we categorized the reaction mechanisms based on the bond breaking sequence (Scheme 2), namely the

resulting from the recombination between the surface adsorbed hydroxyl and hydrogen species. In addition, we identified a third mechanism, labeled D2′, where the C−O bond scission is followed by water formation via the deprotonation of the intermediate by the surface adsorbed hydroxyl group. D1 and D2 merge at the water formation step, whereas the D2 and D2′ pathways bifurcate at the intermediate generated by breaking the C−O bond (Scheme 2). The case where the Cb−Hb scission occurs first is exemplified by the D1 pathway (Figure 9). 2-Propanol is bound in a η(Oa) mode via its oxygen atom in a configuration similar to that seen in the hydrogenation reaction. In section 3.2, we described the electron-deficient nature of the metal cluster that arises as a direct consequence of the reverse hydrogen spillover process in the acid zeolite. Consequently, such an electrophilic species would find it harder to remove the β-hydrogen, Hb, when compared to an electron-rich base or a nucleophile. Indeed, the hydrogen abstraction step is an energy demanding endergonic process (12.3 kcal/mol) with a high activation energy barrier (TS1D1, 29.8 kcal/mol). The resulting intermediate, INTD1, is destabilized with respect to the reactant due to a carbanion-like character at the β-carbon, Cb. Geometrically, the intermediate is bound in a μ1(Cb,Oa) configuration with Ni−Cb and Ni−Oa bond lengths of 1.96 and 2.10 Å, respectively. Since we have a destabilized intermediate in INTD1, the subsequent downhill process (−14.7 kcal/mol) involving the Ca−Oa scission has a low energy barrier (8.9 kcal/mol) to form the most stable state, INT2D1. Because of the sp2-type hybridization of the alphacarbon in TS2D1, the bonds surrounding Ca assume a coplanar configuration leading to the formation of propene, which is bound in the μ1(Ca,Cb) mode. The stabilization of INT2D1 can also be attributed to the stronger binding of the more nucleophilic hydroxyl group to the electron-deficient metal H2O cluster (EOH BE = −65.5 kcal/mol) as compared to water (EBE = −14.3 kcal/mol) in the product state. As a result, we observe the highest activation energy barrier (TS3D1, 32.4 kcal/mol) for the water formation step that takes place via an intermolecular hydrogen abstraction by the surface adsorbed hydroxyl group.

Scheme 2. Proposed D1, D2, and D2′ Pathways for the Metal-Catalyzed Dehydration of 2-Propanol in Ni4H-ZSM5a

a

For the sake of clarity the nickel tetramer is represented as a triangle with the fourth atom being out of plane of the paper.

following: (i) D1 mechanism, where the C−H bond breaks first, followed by the C−O bond; and (ii) D2, with the order being reversed. Both pathways end with water formation

Figure 9. Metal-catalyzed D1 reaction pathway in Ni4H-ZSM-5 (ΔG(298 K) in kcal/mol). The zeolite framework has been cropped to a few atoms for better structural representation. 130


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Figure 10. Metal-catalyzed D2 reaction pathway in Ni4H-ZSM-5 (ΔG(298 K) in kcal/mol). Note that the D1 and D2 pathways merge along the path INT2D1 → TS3D1 → P indicated by identical labels.

Figure 11. Metal-catalyzed D2′ reaction pathway in Ni4H-ZSM-5 (ΔG(298 K) in kcal/mol). Note that the D2 and D2′ pathways bifurcate after the R → TS1D2 → INTD2 step.

→ TS3D1 → P is identical in both D1 and D2 (Figures 9 and 10). While the formation of propene and water requires multiple steps in D1 and D2, the reaction happens in a single step in the D2′ mechanism. The intermolecular reaction takes place via the hydrogen abstraction from the propyl-fragment in INTD2 by the surface adsorbed hydroxyl group (Figure 11). This interaction is exergonic (−18.9 kcal/mol) with a significantly lower barrier (TS2D2′, 12.5 kcal/mol) compared to the first step (TS1D2, 33.0 kcal/mol). The chemistry can be traced to the acidity of the β-hydrogen in the hydrocarbon fragment (NPA charge of +0.27) that is in the vicinity of the nucleophilic hydroxyl group (−0.99) leading to a favorable interaction. A similar reaction to keep in mind would be the intramolecular hydrogen abstraction reaction in the uncatalyzed conversion of 2-propanol to

Overall, the reaction energy on going from adsorbed 2propanol to propene and water is endergonic by 7.6 kcal/mol. In both the D2 and D2′ mechanisms, the R → TS1D2 → INTD2 step is an endergonic reaction (26.5 kcal/mol) involving the cleavage of the Ca−Oa bond in 2-propanol (Figures 10 and 11). The transition state involved in the process has a resultant carbocation character with a charge of +0.27 on the hydrocarbon fragment. A bifurcation in the reaction pathway via INT1D2 → TS2D2 → INT2D1 leads to the Cb−Hb scission in the D2 mechanism. The reaction is exergonic (−9.7 kcal/mol) with a very low energy barrier (1.1 kcal/mol) to the same intermediate seen in the D1 mechanism, INT2D1, at which point the D1 and D2 pathways merge (Scheme 2). Indeed, the energetics for the water formation step in going form INT2D1 131


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different note, the high barriers involved in the metal-catalyzed pathways indicate that dehydration can be effectively shut down in the absence of acid sites. This finding can have major implications toward the design and synthesis of the bifunctional catalyst by striking a balance between the Si/Al ratio dictating the number of Brønsted acid sites and metal loading. To sum up, the results provided by our computational model present novel insights into the behavior of the different active sites that can be used to design better and efficient bifunctional catalysts with application toward HDO.

propene (Figure S7). One of the reasons for the high barrier in the uncatalyzed case is the relatively lower nucleophilicity and acidity of the hydroxyl group (−0.78) and the β-hydrogen (+0.24) in 2-propanol compared to the metal adsorbed species in the D2′ mechanism. Upon inspection of the energy profiles in all the reaction mechanisms proposed so far we see lower activation energy barriers in the Brønsted acid catalyzed E2 pathway over the metal-mediated dehydration routes. It means that the acid site in the bifunctional catalyst does indeed carry out dehydration more efficiently than the metal site as proposed in the experiments by Lercher and co-workers.31 Moreover, these observations align with the conceptual thinking about the zeolite-supported metal clusters being electron-deficient and consequently find it harder to break the C−O and C−H bonds required for dehydration.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01686. Plots of absolute binding energy of the nickel tetramer under different symmetry constraints, a comparison of the alkoxy pathway in the quintet and triplet spin states for the nickel tetramer and HZSM-5-supported cluster, structures of the various species involved in the hydrogenation reaction, and the uncatalyzed 2-propanol dehydration pathway and tabulated data of the expectation value of the total spin for all the open shell species involved in the hydrogenation and dehydration reactions (PDF) Geometries, enthalpies, and free energies for all reported structures and imaginary frequencies for the transition states and sample Gaussian 09 input files (ZIP)

4. SUMMARY AND CONCLUSIONS We have carried out DFT calculations to elucidate the hydrogenation of acetone to 2-propanol followed by the dehydration of the alcohol to propene on the metal and acid sites of a HZSM-5-supported nickel bifunctional catalyst. Computational characterization of the catalyst showed reverse hydrogen spillover from the Brønsted acid site to the metal cluster, which becomes electrophilic. These findings are consistent with experimental reports which suggest the formation of metal-proton adducts11,15,16 in zeolites and with recent computational studies which correlate geometric changes in the metal cluster with the hydrogen spillover.23,25 In studying the hydrogenation reaction, we highlighted the possibility of the alkoxy and hydroxy mechanisms over the reference bare nickel tetramer and showcased the preference for the former pathway. The alkoxy route was characterized as a two-step mechanism, wherein the carbonyl carbon of acetone is hydrogenated first, followed by a second hydrogen addition to the carbonyl oxygen atom, to make 2-propanol. In comparing the different catalysts, we showed that the hydrogenation is thermodynamically most favored over the zeolite-supported nickel cluster, on account of the spillover process. The enhanced activity of the catalyst was attributed to the stronger binding of the stable species as a direct result of the electrondeficient nature of the metal cluster in the zeolite and to a slightly lower intrinsic barrier. We further investigated the 2-propanol dehydration pathways on the HZSM-5 Brønsted acid site, as well as on the metal site in the zeolite-supported nickel cluster. The studies on Brønsted acid catalysis revealed both a concerted and a stepwise mechanism. In the dominant pathway, the C−O and C−H bonds in 2-propanol undergo simultaneous scission to propene and water. On the other hand, the stepwise mechanism involves the generation of an alkoxy intermediate via the C−O bond breakage, followed by the deprotonation of the intermediate in the second step. Furthermore, we identified three stepwise mechanisms in the metal-mediated dehydration reaction based on the sequential scission of the C−O and C−H bonds. We find that the barriers involved in these mechanisms are higher than the acid catalyzed pathway, thus implying that the main route for dehydration of 2-propanol to propene is via Brønsted acid catalysis on sites without metal clusters. These results are in qualitative agreement with the experimental work of Lercher and co-workers.31 The electrophilic nature of the zeolitesupported metal catalyst was again shown to be the reason behind the sluggish nature of metal-catalyzed dehydration. On a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the US Department of Energy, Energy Efficiency and Renewable Energy’s Bioenergy Technologies Office through the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the US Department of Energy. This research was supported in part through the use of Information Technologies (IT) resources at the University of Delaware, specifically the high-performance computing resources. The authors also acknowledge the resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We are grateful to Dr. Jeffrey Christianson and Dr. Glen Jenness for comments and discussions.

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REFERENCES

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