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Competition and Cooperation of Hydrogenation and Deoxygenation Reactions During Hydrodeoxygenation of Phenol on Pt(111) Dan Liu, Gaofeng Li, Feifei Yang, Hua Wang, Jinyu Han, Xinli Zhu, and Qingfeng Ge J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 20, 2017

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The Journal of Physical Chemistry

Competition and Cooperation of Hydrogenation and Deoxygenation Reactions during Hydrodeoxygenation of Phenol on Pt(111)

Dan Liu, † Gaofeng Li,† Feifei Yang†, Hua Wang, † Jinyu Han, † Xinli Zhu,*,† and Qingfeng Ge*,†,‡ †

Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 300072, China ‡

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901,

United States

Corresponding Authors * Email: [email protected] (X.Z.)

* Email: [email protected] (Q.G.)

1

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ABSTRACT: A combined experimental and density functional theory computational study was performed to understand the reaction mechanism of hydrodeoxygenation of phenol on the Pt(111) surface. Partial hydrogenation of phenyl ring reduces the barrier of deoxygenation. The intermediate formed by adding 5 H atoms to the phenyl ring and with α-C adsorption on Pt is identified as the key intermediate responsible for the formation of different products with mild barriers: deprotonating the hydroxyl to cyclohexanone at 0.38 eV, hydrogenation at α-C to cyclohexanol at 0.56 eV, and deoxygenation at 0.76 eV and followed by dehydrogenation to benzene. Microkinetic parameter analysis indicates that the hydrogenation steps are fast and reversible while deoxygenation steps are slow and almost irreversible, which is consistent with the experimental observation that hydrogenation products are the major primary products at low conversions while deoxygenation product dominates at high conversions, at 523 K and ambient H2 pressure. H2 pressure plays an essential role on surface coverage of H and available adsorption sites, modulating the competition between hydrogenation and deoxygenation reactions and, thereby, the product distributions.

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1.


INTRODUCTION Conversion of renewable biomass to fuels and chemicals attracts more and more attention.1-4

Catalytic hydrodeoxygenation (HDO) of bio-oil derived from biomass is an important approach to reduce the oxygen content and refine these oxygenates to desired fuels and chemicals. As a major component of biomass, phenolic compounds derived from lignin represents a major fraction of bio-oil.5-6 Selective deoxygenation of phenolics to aromatics is particularly interesting since the reaction consumes few hydrogens and aromatics are important chemicals as well as fuel components with high octane numbers. During HDO of phenolics, a number of reactions, including hydrogenation, deoxygenation, C-C hydrogenolysis, may take place simultaneously on transition metal catalysts.7-11 Understanding the reaction pathways at the molecular level is therefore of great importance to develop an efficient catalytic process for selective deoxygenation of phenolics to aromatics. Among different types of catalysts,11-19 supported Pt catalysts have been widely investigated.20-24 For Pt on a strong acidic support at low temperature (< 250 °C) and higher pressures (several MPa), it is generally accepted that HDO of phenolics follows the hydrogenation-deoxygenation (HYD) pathway: complete saturation of the phenolic ring to cyclohexanols on metal, dehydration to cyclochexenols on acid site and finally hydrogenation to cyclohexanes.25-26 At medium and higher temperatures (> 250 °C), aromatics are usually observed as a major primary product over supported Pt catalysts, which suggests that the reaction follow direct deoxygenation (DDO) path.27-29 However, it has been argued that the production of aromatics may follow a different surface mechanism, because the delocalization effect strengthened the Caromatic-O bond significantly as compared with the C-O bond of aliphatic alcohols. Lobo et al. studied HDO of m-cresol on Pt/Al2O3 at 260 °C and 0.5 atm H2.12 They attributed the 3



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formation of toluene to complete or partial hydrogenation on Pt followed by dehydration on the acid sites of Al2O3. Nie and Resasco studied HDO of m-cresol on the Pt/SiO2 catalyst at 300 °C and 1 atm H2, and proposed that the formation of toluene follows a tautomerization mechanism, i.e., tautomerization of m-cresol

to

3-methyl-3,5-cyclohexadienone,

followed

by

hydrogenation

to

3-methyl-3,5-cyclohexadienol, and finally dehydration to toluene.30 On the TiO2 supported metal catalysts, Nelson et al. suggested that the metal-support interface catalyzes the DDO path to aromatics on Ru/TiO231 whereas others argued that the reducible TiO2 support promotes the tautomerization path to aromatics on Pt/TiO2.30,

32

In our recent work, HDO of m-cresol was conducted at relatively low

temperature of 250 °C and 1 atm H2 on Pt/SiO2 to follow the major products evolution.28 The hydrogenation products of methylcyclohexanone and methylcyclohexanol, and deoxygenation product of toluene are found to be the primary products. Interestingly, increasing space-time results in the hydrogenation products eventually converting to toluene at high conversions. Kinetic analysis indicated that hydrogenation and apparent DDO reactions were in competition and excluded the HYD pathway in the formation of toluene. Importantly, there exists an apparent DDO path even when Pt was supported on acidic HBeta.33-34 Increasing reaction temperature inhibits the hydrogenation reaction but promotes the apparent DDO path. An increasing number of density functional theory (DFT) calculations have been reported to understand the mechanism of HDO of phenolics on metal surfaces and supported metals.17-18, 31, 35-36 For HDO of guaiacol on Ru(0001) and Pt(111) surfaces, the pathways to phenol have been identified, while further DDO of phenol to benzene was found to be unlikely under mild conditions.9, 13 Tan et al. calculated the tautomerization path for conversion of m-cresol to toluene on Pt(111) and determined the 4


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barrier for deoxygenation (dehydration) step to be 1.58 eV and the overall barrier to be as high as 2.10 eV.10 Gu et al. performed DFT calculation of HDO of p-cresol on Pt(111).37 By combining Brønsted-Evans-Polanyi (BEP) estimation and statistical microkinetic modeling, they proposed that the deoxygenation toward toluene takes place through dehydration of the partially hydrogenated phenol with 3-5 H atoms. They also suggested that desorption of methylcyclohexanol and cyclohexanone determines the kinetics of their respective formation at low conversions. Li et al. explored the reaction pathways of forming the hydrogenation products from o-cresol on Pt(111).38 Despite of the efforts, the apparent DDO mechanism for the formation of aromatics is not well-understood. Furthermore, a direct comparison between the hydrogenation and deoxygenation reactions, both being observed experimentally on noble metal surfaces, is not available. In this work, we performed a combined experimental and DFT computational study on HDO of m-cresol and phenol. Our results indicate that the hydrogenation intermediate formed by adding 5 H atoms to phenol plays a key role: it can be hydrogenated to cyclohexanol, dehydrogenated to cyclohexanone, and deoxygenated and followed by dehydrogenation to benzene. All the steps have comparable low barriers. We also showed that these reactions could be modulated by surface coverage of hydrogen.

2. EXPERIMENTAL AND COMPUTATIONAL METHOD 2.1. Experimental Study of Hydrodeoxygenation of m-Cresol on Pt/SiO2. The preparation and characterization of 1 wt.% Pt/SiO2 catalyst and the experimental details of hydrodeoxygenation (HDO) of m-cresol have been reported in previous work.28 Briefly, to study the effect of hydrogen partial pressure, the reaction was carried out at 523 K and atmospheric pressure with a space-time (W/F, 5



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gcatgreactant-1h) of 0.4 h. m-Cresol pressure was kept at 1.64 kPa, while hydrogen partial pressure was adjusted by mixing with helium. The conversion and selectivity were reported in molcarbon%. Note that characterizations showed that Pt has an average particle size of ~5 nm,28 which mainly exposes the Pt(111) surface. 2.2. Density Functional Theory Calculations. Density functional theory periodic slab calculations were carried out using the Vienna Ab Initio Simulation Package (VASP).39-41 The periodic DFT code uses the projector-augmented wave (PAW) method to create effective core potentials and plane waves for valence electrons.41-42 The exchange-correlation energy of interacting electrons was evaluated by the Perdew-Burke-Ernzerhof (PBE) functional.43 A 4-layer 4×4 Pt (111) slab was built using the computed lattice constant of 3.978 Å with a 15 Å vacuum gap between slabs. The bottom two layers were fixed, while the top two layers were allowed to relax. Our previous study44 of hydrogenation of phenol on Pt (111) showed that a cutoff energy of 400 eV and a (2×2×1) k-point grid generated with the Monkhorst-Pack scheme45 provide converged results. The atomic structures were relaxed using either the quasi-Newton scheme or conjugate gradient algorithm implemented in the VASP code. Van der Waals interactions were not considered in this work, because the PBE functional with van der Waals (vdW) corrections overestimates the adsorption energies of phenolics on the transition metals and different vdW functions gave different adsorption energies.46 On the other hand, using vdW functionals has little effect on the adsorption structures, reaction energies and activation barriers.9, 46 The adsorption energy (∆Eads) was defined by:

∆Eads = E(adsorbate/surface) - E(adsorbate) - E(bare surface)

(1)

where E(adsorbate/surface) is the total energy of an adsorbate bound to Pt slab, E(adsorbate) is the total energy of 6


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an isolated molecule or intermediate, E(bare surface) is the total energy of the bare slab. The reaction energy (∆Erxn) and the activation energy (Ea) were calculated using following equations:

∆Erxn = Eproduct - Ereactant

(2)

Ea = Etransition - Ereactant

(3)

where Eproduct, Ereactant and Etransition state are the total energies of reactant, transition state and product, respectively, bound to the Pt slab. Stationary and transition states were confirmed by a normal-mode frequency analysis. No imaginary mode was found for the optimized stable structures, and only one imaginary mode for the transition state. All energies reported in this paper had been corrected by Zero-Point-Energy (ZPE). To reduce the computation costs, phenol instead of m-cresol was used in the calculation. The forward rate constant (kf) was estimated using transition state theory: (4)

where kB, T, h, and Ea are Boltzmann constant, reaction temperature, Planck’s constant, ZPE corrected activation barrier for the forward reaction obtained from DFT calculations, respectively. And qTS,vib and qIS,vib are the (harmonic) vibrational partition functions for the transition state and the initial state, respectively. The vibrational partition functions are based on:

qvib = ∏ i

1 1− e

− hvi / kBT

(5)

where νi is the vibrational frequency for each vibrational mode of the adsorbed intermediate, i runs to N-1 and N for the transition state and initial state, respectively. The reverse rate constant (kr) is estimated similarly. And the thermodynamic equilibrium constant (K) is calculated according to: 7



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K=

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kf kr

(6)

3. RESULTS AND DISCUSSION 3.1. Experimental Study on Hydrodeoxygenation of m-Cresol on Pt/SiO2. Figure 1 shows the conversion of m-cresol and the product selectivities as a function of H2 partial pressure. It is clear that the hydrogenation products of methylcyclohexanone and methylcyclohexanol and the deoxygenation product of toluene are the primary products, regardless of the H2 partial pressure.

However, their

yields vary significantly as a function of H2 partial pressure. At 10 kPa H2, toluene is the major product and methylcyclohexanone is the minor one, while selectivity to methylcyclohexanol is almost zero. Increasing H2 partial pressure leads to increased selectivities to methylcyclohexanone and methylcyclohexanol but decreased selectivity to toluene. The selectivity to methylcyclohexanone reaches a maximum at 50 kPa H2 and then decreases at a similar rate to that of toluene with further increasing H2 pressure. When H2 partial pressure is increased to 98 kPa, the selectivities to the three products are at a similar level, following an order of methylcyclohexanone > toluene > methylcyclohexanol. The results clearly indicate that the hydrogen partial pressure, and therefore, the surface coverage of H could be used to regulate the product distribution. It is interesting to note that increasing H2 partial pressure also leads to increase in the conversion of m-cresol and the yield of toluene, indicating that there exists a cooperation of hydrogenation and deoxygenation reactions. 3.2. Hydrogenation of Phenol to Cyclohexanol and Cyclohexanone on Pt(111). Figure 2 shows the most stable adsorption configurations of phenol, deoxygenation product of benzene, and hydrogenation products of cyclohexanol and cyclohexanone on Pt(111). The C atoms were labeled

8


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clockwise with consecutive numbers of 1-6, starting with the α-C atom. Consistent with our previous work44, phenol prefers adsorption in a Bri30 configuration (Figure 2A): C1-C2 and C4-C5 are π-bonded to Pt1 and Pt3, respectively, while C3 and C6 are σ-bonded to Pt2 and Pt4, respectively. The O atom of hydroxyl is pushed away from the surface. The adsorption energy (∆Eads) of phenol is -1.16 eV. Similarly, benzene also adsorbs in the Bri30 configuration (Figure 2B), and the adsorption is slightly stronger, with ∆Eads = -1.21 eV.

However, very different adsorption structures were obtained for

cyclohexanol and methylcyclohexanone (Figure 2C and D). Cyclohexanol adsorbs on top of a Pt atom through the hydroxyl O atom, with the ring being tilted to the surface. As expected, the adsorption is rather weak, with ∆Eads = -0.48 eV.

It should be noted that the H atom on C1 is pointing away from the

Pt surface. Cyclohexanone adsorbs on top of a Pt atom through the carbonyl O atom, with the ring being almost vertical to the surface (∠C1-O-Pt = 133.2°). The adsorption is even weaker, with ∆Eads = -0.24 eV. We started by determining which of the six C atoms being preferably hydrogenated first. To start hydrogenation, the H adatom needs to be first activated from the most stable hollow site. Figure 3 shows the structures of adding the first H atom to C2. In initial state (IS), H adsorbs on a bridge site with a H-C2 distance of 2.38 Å. Along the reaction coordinate, the H adatom moves to the top of Pt6, with a shortened H-C2 distance of 1.65 Å. At the same time, the C2-Pt1 bond elongates from 2.20 Å in IS to 2.28 Å in transition sate (TS). The 4-centered Pt6-H-C2-Pt1 structure stabilizes TS. In final state (FS), the C2 atom becomes detached from the Pt surface (C2-Pt1 distance is 2.91 Å) with a newly formed C-H bond. This elementary step is mildly endothermic (∆Erxn = 0.26 eV) with a relatively high barrier of Ea = 0.94 eV.

Similar Pt-H-C-Pt TS structure has been found for hydrogenating C1, C2, C4, and C5. A 9



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common feature of these carbon atoms is that they are π-bonded to the surface Pt atoms. Hydrogenating the σ-bonded C atoms (C3 and C6) forms a different type of TS, i.e., a 3-centered H-C-Pt triangle structure (not shown) as the σ-bonded Pt has enough space to accommodate H. The reaction energy and activation energy for hydrogenating different C atoms are summarized in Table 1. Considering both thermodynamic and kinetic aspects, hydrogenating C2 first is most favorable and C1 is least favorable. However, we noted that the differences in activation barriers are small among C2-C6 and there is no strong preference as to which C atom being hydrogenated first under typical HDO conditions. Two hydrogenation pathways toward cyclohexanol formation are presented here in detail. The first hydrogenation pathway follows an order of 264351 of carbon atoms, which has been reported as the most favorable pathway for benzene and phenol hydrogenation.37, 47 Another pathway follows the order of 435261. The IS, TS and FS structures along both pathways are summarized in Figure S1 and S2, respectively. It is evident that the ring is gradually lifted from the surface with an increasing degree of hydrogenation. Figure 4A shows the structures of last hydrogenation step by adding H to C1 to form cyclohexanol. In IS, the C2-6 hydrogenated intermediate (denoted as H43526Ph, where the superscripted numbers stand for sequence of H atoms added to C atoms of phenol) adsorbs on top of Pt1 through a σ-bond with C1, and H atom is on an adjacent hollow site. In TS, the C1-Pt1 bond elongates while the H atom migrates onto Pt1, forming a 3-centered H-C1-Pt1 structure. In FS, the formation of new C-H bond lifts cyclohexanol from the surface. Note that the hydroxyl is pointing away from the surface while H is pointing to the surface with a Pt-H distance of 1.99 Å. This configuration adsorbs weakly, ∆Eads = - 0.25

10


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eV, which is readily desorbing from the surface. This elementary step is almost thermo-neutral (∆Erxn = 0.04 eV) and has a moderate barrier of 0.56 eV. Figure 5 compares the potential energy profiles of the two different hydrogenation pathways to cyclohexanol. Even though these two pathways have apparently different reaction barriers in some elementary steps, both show a downhill profile and have the highest barrier in adding the first H atom to the ring. The results imply that once first H atom is added, the reaction would take place spontaneously toward cyclohexanol. Overall, the differences in the two pathways are relatively small, which predict the absence of a strong preference of hydrogenation order. The formation of cyclohexanone has been suggested to go through dissociation of phenol to phenoxy followed by ring hydrogenation on Pt.38 However, our previous work indicates that the surface coverage of phenoxy is low and hydrogenation of phenoxy is less favorable than hydrogenation of phenol on Pt(111).44 On the other hand, the formation of cyclohexanone from cyclohexenol through surface mediated tautomerization has a barrier of 1.17 eV44, which is higher than further hydrogenation of cyclohexenol to cyclohexanol (Figure 5). We then explored the formation of cyclohexanone from deprotonating the hydroxyl of the H43526Ph intermediate. As shown in Figure 4B, along the reaction coordinate, the C1-Pt1 bond elongates from 2.18 Å in IS to 3.40 Å in TS, while the O-H bond of hydroxyl increases from 0.98 Å to 1.36 Å and the H atom of hydroxyl moves toward surface Pt7. In FS, the H atom moves to atop Pt7 with a H-O distance of 1.57 Å and cyclohexanone desorbs from the surface with increased O-Pt7 distance of 4.17 Å. It is noted that the configuration of cyclohexanone is almost lying flat on the surface and has a weak adsorption energy of -0.06 eV, indicating that cyclohexanone readily desorbs from the surface to form gaseous product. This elementary step is 11



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slightly endothermic (∆Erxn = 0.05 eV) with a low barrier of 0.38 eV, even lower than that for H43526Ph hydrogenation to cyclohexanol (0.56 eV).

However, as shown in Figure 5, the hydrogenation to

cyclohexanol is slightly more favorable than dehydrogenation to cyclohexanone by 0.08 eV with respect to the adsorbed H43526Ph. This difference is a result of adsorption of an additional H atom. Therefore, this result indicates that a high surface coverage of H, a likely situation under high H2 pressure and low reaction temperature conditions, favors the formation of cyclohexanol. Conversely, a low H coverage would favor the formation of cyclohexanone. This prediction is in good agreement with the current (Figure 1) and previous experimental results.28, 48 3.3. Deoxygenation of Phenol and Its Hydrogenation Intermediates.

Since benzene has been

observed as one of the primary products, direct deoxygenation (dehydroxylation) of phenol was first explored. As shown in Figure 6, the O and C1 atoms in IS move toward the surface Pt5 and Pt1 atoms, respectively, accompanied by an elongation of the O-C1 distance to 2.39 Å. In FS, the –OH stabilizes on a bridge site with the O-C1 distance being further increased to 3.81 Å. The ring rotated slightly counter-clockwise and C1 adsorbs on bridge site of Pt1-Pt5. This step is strongly endothermic (∆Erxn = 1.81 eV) with an activation barrier of 2.61 eV. Consistent with previous studies,10, 13,

37

the result

indicates that direct deoxygenation of phenol is unlikely under the typical HDO conditions. The strengthened C-O bond in phenolics has been suggested to be a result of delocalization, and hydrogenation of the C atoms next to the C-O bond may reduce delocalization and facilitate deoxygenation.33, 49 Consequently, hydrogenating C1 of phenol results in a structure (denoted as H1Ph) with the H atom pointing to the surface while –OH almost perpendicular to the ring (Figure S3). To proceed with deoxygenation, –OH rotates toward the surface while H rotates away from the surface. 12


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This step is strongly endothermic with a reaction energy of 1.0 eV. Further progressing along the reaction coordinate, the O atom begin to bind Pt5 and the C1-O bond is stretched further. Breaking the C-O bond results in a surface bound benzene and –OH. This step is thermo-neutral (∆Erxn = 0.06 eV) with a high barrier of 1.01 eV. As the barrier of –OH rotation was not determined, we estimate the overall barrier for this deoxygenation path is at least 2.48 eV with respect to adsorbed phenol. Although this pathway has a slightly lower barrier compared to the direct deoxygenation, the high barrier makes this path unlikely on Pt(111). When H is added to the C atoms next to C1, two configurations may be formed: H2Ph and H6Ph. As shown in Figure 7, adding H to either C atom would lift the corresponding C atom from the surface, resulting in H2Ph adsorption on bridge site of four Pt atoms (Figure 7A) while H6Ph adsorption on the hollow site of three Pt atoms (Figure 7B). In TS, both O and C1 atoms move closer to the surface with increased C1-O bond. In FS, C1-O bond is broken, with –OH on top of a Pt atom. The difference between H6Ph and H2Ph deoxygenation is the ring rotation. H6Ph deoxygenation (Figure 7B) involves a slight clockwise rotation, making C2 coadsorb with C1 on Pt1 in IS toward the Pt1-Pt2 bridge site in TS and finally to co-adsorb with C3 on Pt2 in FS.

In contrast, the deoxgenation of H2Ph involves almost no

ring rotation. Both H2Ph and H6Ph deoxygenation steps are endothermic, by 1.62 and 1.64 eV, respectively. The activation barrier for H6Ph (2.14 eV) is slightly higher than that for H2Ph (2.04 eV). Hydrogenating the ring C atom may reduce the barrier for deoxygenation. Therefore, deoxygenation after adding more H atoms to the ring was further examined. As shown in Figure S4, the H26Ph intermediate with two H atoms added to both C2 and C6 lifts these C atoms from the surface and results in the intermediate adsorbing in the hollow site. The structure evolution along the reaction coordinate of 13



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deoxygenation from IS to FS (Figure S4) is similar to that observed for deoxygenation of H2Ph. The deoxygenation step becomes even less endothermic, with a reaction energy of 1.47 eV and a reduced barrier of 1.70 eV. H264Ph and H435Ph form after three C atoms being hydrogenated. As shown in Figure 8A, H264Ph adsorbs through C1-, C3-, and C5- surface σ-bond in the hollow site, with the O atom of hydroxyl away from the surface (nearest O-Pt distance, 3.41 Å). The ring is almost parallel to the surface. Similar to H2Ph and H26Ph, deoxygenation of H264Ph involves the move of O and C1 toward the surface Pt7 and Pt1 atoms, respectively, and the elongation and finally cleavage of the C1-O bond. The reaction is endothermic by 1.34 eV with a barrier of 1.55 eV. In contrast, H435Ph adsorbs at Pt1-Pt4 bridge site with half of the ring being tilted to the surface due to hydrogenation, which results in the O atom being closer to the surface, with the nearest O-Pt distance of 2.78 Å (Figure 8B). In TS, the O adsorbs on Pt5 and C1 on hollow site with the elongated C1-O bond of 2.03 Å. In FS, C1-O bond is broken, with –OH stabilized on the adjacent bridge site and C1 in the hollow site next to Pt5. This initial adsorption structure makes this deoxygenation step significant less endothermic (0.72 eV) with a much reduced barrier of 1.31 eV. As the product of hydrogenating four C atoms, the surface adsorbed cyclohexenol (H4352Ph) deoxygenation was studied. The structural evolution along the deoxygenation reaction pathway is shown in Figure S5. This step is endothermic by 1.06 eV with a barrier of 1.45 eV. Note that the deoxygenation barrier is higher than the tautomerization barrier to cyclohexanone (1.17 eV). When five C atoms are hydrogenated (H43526Ph), only C1 is left to adsorb on top of Pt while the other carbon atoms of the ring are lifted from the surface (Figure 9). The distance of O to the nearest surface Pt atom is 3.0 Å. Along the reaction coordinate, the O and C1 moves toward the surface, with C1-O bond being elongated and 14


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cleaved, resulting in –OH adsorption on Pt7 and the ring drifting away from the surface. This step is mildly endothermic by 0.61 eV with a significantly reduced barrier of 0.76 eV. Note that this barrier is the lowest among the all the deoxygenation barriers of the partially hydrogenated intermediates, and even lower than the typical barriers for ring hydrogenation. Figure 10 compares the potential energy profiles of deoxygenation of phenol and its hydrogenated intermediates.

For brevity, the hydrogenation steps shown in Figure 5 are not included in this figure.

Clearly, the deoxygenation barrier and the overall activation barrier with respect to the adsorbed phenol are reduced with increasing degree of hydrogenation of the ring.

Although the deoxygenation steps of

H264Ph and H435Ph have different barriers, the overall deoxygenation barriers with respect to the adsorbed phenol are similar. We further compare the barriers for hydrogenation and deoxygenation of each intermediate involved in HDO of phenol.

As shown in Figure 11, both hydrogenation and

deoxygenation barriers for the same intermediate are reduced with increased degree of hydrogenation of the phenolic ring.

For the same intermediate, the deoxygenation barrier is always higher than the

hydrogenation barrier. However, the decrease in the deoxygenation barrier is much more prounced than that of hydrogenation.

For intermediate with five C atoms being hydrogenated, the barriers for

hydrogenation and deoxygenation are comparable (0.76 vs 0.56 eV).

The barriers for the formation of

cyclohexanone from both tautomerization of cyclohexenol and dehydrogenation of H43526Ph were also included. Clearly, deprontonating the hydroxyl of H43526Ph is more favorable for cyclohexanone formation. We note that the difference among deoxygenation, hydrogenation, and dehydrogenation of H43526Ph are quit small, and predict that preference to each reaction pathway may be controllable by varying operating conditions such as surface H coverage. 15



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3.4. Formation of Benzene from Deoxygenated Intermediates. The deoxygenation of phenol or intermediates from partially hydrogenating phenol results in -OH and radicals of benzene or partially hydrogenated intermediates adsorption through C1 (which is absence of H) on the surface. Figure S6 shows the energy profile of benzene formation from deoxygenated intermediates, using C6H10● radical (no hydrogen on C1) as an example. The hydrogenation of –OH to water is expected to take place prior to other steps due to the exothermicity of -0.55 eV and a rather low barrier of 0.20 eV13. Adding H to C1 of the C6H10● radical to the surface adsorbed C6H11 occurs next, which is slightly endothermic by -0.41 eV with a mild barrier of 0.58 eV. The surface C6H11 prefers dehydrogenation to benzene due to the lower barrier of dehydrogenation at low hydrogen coverages.47 The step with the highest barrier for the formation of benzene from phenol is the first hydrogenation step with a barrier of 0.94 eV, and the overall HDO barrier relative to the adsorbed phenol at 0.67 eV. Apparently, the overall HDO barrier is significantly lower than the direct deoxygenation barrier (2.34 eV)13 or the tautomerization path barrier (2.10 eV) toward formation of aromatics32. As these hydrogenated intermediates are not resolved in products, this pathway for aromatics formation appears as direct deoxygenation (DDO). 3.5. Deoxygenation of Cyclohexanol and Cyclohexanone. Although the results in previous sections show that hydrogenation of the ring facilitates deoxygenation, experimental results showed that toluene produced from deoxygenation of either methylcyclohexanol, the product of complete hydrogenation, or methylcyclohexanone, the product of partial hydrogenation, is much less than that from methylphenol under the identical reaction conditions.28, cyclohexanone are therefore examined.

30

The direct deoxygenation of cyclohexanol and

As shown in Figure 12, the reaction starts with the most stable

configuration of cyclohexanol with O to the nearest Pt atom distance of 2.49 Å. Along the reaction 16


Page 17 of 40

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coordinate, the O and C1 adsorb on two adjacent Pt atoms followed by cleavage of the C1-O bond. This reaction is highly endothermic by 1.17 eV with a high barrier of 1.81 eV. Similarly, the direct deoxygenation of cyclohexanone from its most stable adsorption configuration (Figure 2D) is even more endothermic, 1.44 eV, and would have an even higher barrier. We point out that these barriers are lower than that of direct deoxygenation of phenol but higher than most of the deoxgenation barriers of the partially hydrogenated intermediates. More importantly, these barriers are significantly higher than those of cyclohexanol dehydrogenation (0.3-0.7 eV) and phenol hydrogenation (0.6-1.1 eV). This comparison indicates that the dehydrogenation reactions are more favorable over direct deoxygenation reactions for cyclohexanol, resulting in more phenol formed over benzene. One may also expect that the formation of benzene undergo through the reverse reaction of the formation cyclohexanol and cyclohexanone to surface adsorbed H43526Ph intermediate (see Figure 4), followed by deoxygenation (Figure 9). This would suggest that more benzene could be produced when cyclohexanol or cyclohexanone were used as the feed at low conversions, in contradiction with the experimental results.28, 30 This is because FSs from the adsorbed cyclohexanol and cyclohexanone (Figure 4) are less stable than the most stable configuration of surface cyclohexanol and cyclohexanone (Figure 2) by ~0.2 eV. The difference would results in two orders of magnitude of less surface coverages of the configuration readily forming H43526Ph followed by deoxygenation, resulting in a less favorable pathway.

In summary, the formation

of benzene should be a minor pathway from cyclohexanol or cyclohexanone at low conversions, under which conditions dehydrogenation (or hydrogenation) of cyclohexanol (or cyclohexanone) should be the dominant pathway due to their low reaction barriers. In addition, the low adsorption energies of the

17



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Page 18 of 40

formed cyclohexanone/cyclohexanol make them easier to desorb from the surface and appear as the dominant products at low conversions of cyclohexanol/cyclohexanone. 3.6. Microkinetic Parameter Analysis. To understand the reaction mechanism during HDO of phenol on Pt(111), we estimated the microkinetic parameters (kf and K) for key elementary steps at different temperatures, as reported in Table 2. We note that the overall reaction is complex and consists of many elementary steps. Only the kinetic data for one type of intermediate was reported for the intermediates with same degree of the hydrogenation. This should not change the overall trend since different intermediates with the same hydrogenation degree show similar overall barriers in the potential energy profiles for both hydrogenation and deoxygenation (Figure 5 and 10). From Table 2, several important features can be identified. First, for the same intermediate at the 523 K, kf of hydrogenation (step 1-6) is several orders of magnitude higher than that of deoxygenation (step 7-12). Importantly, the difference becomes small with increasing degree of hydrogenation. In addition, kf of H43526Ph dehydrogenation to cyclohexanone (step 14) is even higher than that of hydrogenation reactions. Second, all hydrogenation (step 1-6), deoxygenation (step 7-12), and cyclohexanone formation (tautomerization step 13 and dehydrogenation step 14) steps are reversible, with the backward reactions much more favorable. Third, kf for hydroxyl hydrogenation to H2O (step 16) is the highest among all steps and is at least 4 and 6 orders of magnitude higher than those for hydrogenation and deoxygenation steps 1-12 at 523 K. And this step is almost irreversible (very high K). These features indicate that the hydrogenation/dehydrogenation reactions for the formation of cyclohexanol and cyclohexanone are fast but reversible while the deoxygenation reactions are slow but almost irreversible due to quick consumption of surface –OH by irreversible hydrogenation to H2O. 18


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Taking the low adsorption energies of cyclohexanol and cyclohexanone into account, it predicts that more cyclohexanol and cyclohexanone than benzene would be produced at low conversions of phenol, due to the faster hydrogenation reactions and quick desorption of cyclohexanol and cyclohexanone, making the overall reaction run toward the oxygenated products over benzene. This prediction is in good agreement the experimental results of cresol conversion at 523 K and 1 atm H2.28, 30 Recall that direct deoxygenation of cyclohexanol and cyclohexanone are not favored as compared with the (de)hydrogenation reactions. At high conversions, the accumulation cyclohexanol and cyclohexanone would increase their vapor pressure in gas phase and reach quasi-equilibrium with the partially hydrogenated phenol intermediates on the surface. Under this condition, the reaction proceeds through the slow but irreversible deoxygenation reactions to benzene, until all gaseous cyclohexanol and cyclohexanone are consumed. This is also in good agreement with the decreasing yields of methylcyclohexanol and methylcyclohexanone and increasing yield of toluene with increasing m-cresol conversion.28 The high kf and K indicate that deoxygenated radicals are quickly hydrogenated (for example, step 15) and then dehydrogenated to benzene (see the estimated kf and K for these steps in Table S1, which is calculated according to the reference47). Through the above analysis, the major reaction path of HDO of phenol was proposed for the reaction at 523 K and atmospheric H2 pressure. As shown in Figure 13, hydrogenation is more favorable than deoxygenation, resulting in the formation of the intermediate with 5 C atoms of the phenyl ring being hydrogenated. This intermediate may undergo dehydrogenation to cyclohexanone, hydrogenation to cyclohexanol and deoxygenation and dehydrogenation to benzene. The hydrogenation/dehydrogenation steps are fast and reversible, resulting in these steps in quasi-equilibrium under steady state.

In 19



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Page 20 of 40

contrast, the deoxygenation is slow but almost irreversible. The deoxygenation channels of other partially hydrogenated phenol intermediates may be accessible at higher temperatures.

As shown in

Table 2, kf of the deoxygenation steps increase by several orders of magnitude when the temperature is increased to 623 K, while kf for cyclohexanol and cyclohexanone formations increased less. Increasing reaction temperature also leads to an increase in the fraction of empty sites but reduce the H coverage (see Figure S7A). Furthermore, increasing reaction temperature makes deoxygenation and dehydrogenation to aromatics thermodynamically more favorable than hydrogenation. Consequently, deoxygenation reaction will become the dominant reaction channel, which is consistent with the experimental result that toluene is the dominant product at high temperatures higher than 623 K.27-28, 34 As has been shown in Figure 1, the hydrogen pressure has a significant effect on the product distributions. The surface coverages are estimated at 523 K as a function of hydrogen partial pressure, by assuming phenol and H occupies 4 and 1 Pt surface sites, respectively. The adsorption constant was estimated from the calculated enthalpy and entropy. As shown in Figure S7B, the coverage of H (θH) increases from 10% to 30% as the H2 pressure increases from 10 to 98 kPa; while coverage of empty site (θ*) decreases from 16% to 13%. Since the same intermediate with 5 C atoms being hydrogenated is responsible for various products, the rate ratio of hydrogenation to cyclohexanol and deoxygenation to benzene (rHY/rDO) is largely dependent on the coverage ratio of θH/θ*. Apparently, increasing H2 pressure will increase θH/θ*, resulting in improved hydrogenation reactions over deoxygenation reactions. Furthermore, the reverse dehydrogenation of cyclohexanol and cyclohexanone became more favorable with an increased θ* at low H2 pressures. As a result, toluene would be the major product and methylcyclohexanone would be the minor one while cyclohexanol is even less at H2 pressure of 10 kPa. 20


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We note that the most favorable pathway for deoxygenation requires a cooperation of hydrogenation, i.e., hydrogenating the ring with 5 H atoms. Therefore, the yield of toluene is rather low at 0.1 atm H2 but increases with increasing H2 partial pressure. We also note that increasing H2 pressure promotes hydrogenation to methylcyclohexanone and methylcyclohexanol over deoxygenation to toluene, suggesting that increasing θH favors hydrogenation over deoxygenation. One would expect that further increasing H2 pressure > 100 kPa would leads to methylcyclohexanol as the dominant primary product.

4. CONCLUSIONS Through a combined experimental and DFT computational study on hydrodeoxygenation of phenol on Pt(111), the following major conclusions could be reached: (1) Partial hydrogenation of phenol significantly reduces the barrier for deoxygenation. The higher degree of hydrogenation, the lower is the barrier of deoxygenation. (2) For each partially hydrogenated phenol intermediate, hydrogenation barrier is always lower than that of deoxygenation. However, the difference becomes small as the degree of hydrogenation increases. (3) The partially hydrogenated intermediate with 5 ring C atoms being hydrogenated is identified as the key intermediate for the formations of different products with mild reaction barriers: deprotonation of hydroxyl to cyclohexanone at 0.38 eV, hydrogenation to cyclohexanol at 0.56 eV and deoxygenation toward benzene at 0.76 eV. (4) The microkinetic parameter analysis indicates that hydrogenation/dehydrogenation steps are fast and reversible while deoxygenation is slow but almost irreversible, due to fast consumption of –OH from deoxygenation by hydrogenation to H2O. This

analysis

successfully

explains

the

previous

experimental

observation

that

methylcyclohexanone/methylcyclohexanol are the major primary products at low conversion of m-cresol, 21



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Page 22 of 40

while toluene becomes the major final product at high conversion of m-cresol at 423 K and atmospheric pressure of hydrogen. (5) H2 pressure plays an important role to control the surface coverages of the H adatom and the empty sites, and can be used to regulate the competition between the hydrogenation and deoxygenation reactions. At low H2 partial pressure of 0.1 atm, toluene is observed as the major primary products over oxygenated products as a result of reduced H coverage. ASSOCIATED CONTENT Supporting Information Structures of initial and transition states following different hydrogenation pathways, structures of initial, transition, and final states of deoxygenation step of partially hydrogenated intermediate of phenol as well as kinetics based surface coverage analysis are provided as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors thank the support from the National Natural Sciences Foundation of China (#21676194, #21576204 and #21373148) and the Ministry of Education of China for Program of New Century Excellent Talents in University (NCET-12-0407). The High-Performance Computing Center of Tianjin University is acknowledged for providing services to our computing cluster and, in part, computing resources.

22


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25



Figures and Tables

Methylcyclohexanol Toluene Methylcyclohexanone Conversion

80

Conversion or Selectivity (%)

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Page 26 of 40

60

40

20

0 20

40

60

80

100

Hydrogen Partial Pressure (kPa)

Figure 1. Effect of hydrogen partial pressure on m-cresol conversion and product distributions on Pt/SiO2. Reaction condition: P = 100 kPa, (He+H2)/m-Cresol = 60, W/F=0.4 h, T=523 K, fresh catalyst was used for each pressure.

26


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(A)

(B)

(C)

(D)

Figure 2. The most stable adsorption configuration of phenol (A), benzene (B), cyclohexanol (C) and cyclohexanone (D) on Pt(111). Carbon atoms were labeled with number clockwise. The Pt, O, H, and C atoms are in blue, red, white, and gray, respectively, and the unit of distance is Å.

27



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Page 28 of 40

Figure 3. Structures of initial state (IS), transition state (TS) and final state (FS) in adding the first H atom to C2 of phenol on Pt(111). Side view (upper) and top view (lower). The H atom for hydrogenating is highlighted in yellow.

28


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(A)

(B)

Figure 4. Structures of initial state (IS), transition state (TS) and final state (FS) in H43526Ph intermediate hydrogenation to cyclohexanol (A) and dehydrogenation to cyclohexanone (B) on Pt(111).

29



Ph(g)+3H2(g)

0.0

Hydrogenation to CHol via 264351

Ph(g) +3H2(g)

Hydrogenation to CHol via 435261

1

Hydrogenation to CHone

1.17

1.02

-1.16

-0.5

Energy (eV)

0.38

0.27

0.88

0.94

0.28

0.26

0.85

0.82

0.34

0.22

0.84 0.25

-1.0

0.5H2

0.5H2 0.5H2

0.5H2

0.94

0.68

0.05

0.62 CHone*

0.33 -0.15

0.31

0.5H2

-1.5

0.38

0.39

0.23

1

CHone(g)

0.57 0.04

0.5H2

Diff

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

Ph*+H*

+H2(g)

CHone*+H* CHol(g)

HPh*+H* 2HPh*+H*

CHol* 3HPh*+H* 4HPh*+H*

5HPh*+H*

-2.0

Reaction Coordinate Figure 5. Potential energy profiles of hydrogenation of phenol to cyclohexanol and cyclohexanone on Pt(111). Blue and black lines indicate hydrogenation to cyclohexanol via 264351 and 435261, respectively. Magenta lines correspond to the formation of cyclohexanone. The numbers in horizontal direction represent activation barrier (in bold) and reaction energy (in italics) for each elementary step. Diff is short for diffusion of H atom from hollow site to the site that is ready for reaction. Ph, CHol, and CHone stand for phenol, cyclohexanol, and cyclohexanone, respectively. A* represents the adsorption state of species A on the surface. The number preceding HPh indicates how many H atoms added to phenol.

30


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Figure 6. Structures of initial state (IS), transition state (TS) and final state (FS) in direct deoxygenation of phenol on Pt(111).

31



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Page 32 of 40

(A)

(B)

Figure 7. Structures of initial state (IS), transition state (TS) and final state (FS) in deoxygenation of H2Ph (A) and H6Ph (B) on Pt(111).

32


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(A)

(B)

Figure 8. Structures of initial state (IS), transition state (TS) and final state (FS) in deoxygenation of H264Ph (A) and H435Ph (B) on Pt(111).

33



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Page 34 of 40

Figure 9. Structures of initial state (IS), transition state (TS) and final state (FS) in deoxygenation of H43526Ph on Pt(111).

34


Page 35 of 40

2.0

Phenol DO H2Ph DO H26Ph DO H264Ph DO

TS-DO

1.5

2.61

1.81

1.0

TS-DO

H435Ph DO H4352Ph DO H43526Ph DO 0.55

Energy (eV)

2.04 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C6H5●* +OH*

0.5 0.0

1.64

TS-DO 12

TS-DO

C6H6●* 1.70 1.55 12 1.34 +OH* 1.47 C H ●* 6 7 12 +OH* 1.31 C6H8●* 0.72 +OH* TS-DO 1.24 C6H9●* 12

-0.5

12 1.06 +OH* C6H8●* TS-DO +OH*

1

0.76

-1.0

Ph* -1.5

12

H2Ph*

26 H Ph*

H435Ph* 264 H Ph*

12

0.61 C6H10●* +OH*

H4352Ph*

43526

H

Ph*

Reaction Coordinate

Figure 10. Potential energy profiles of deoxygenation of phenol (Ph) and its hydrogenated intermediates (HPh) on Pt(111). The numbers represent activation energy (in bold) and reaction energy (in italics) for each elementary step. * reprensent adsorption on surface, ● represent radical which is H deficient on C1 atom.

35



3.0

Deoxygenation Hydrogenation Cyclohexanone formation

2.61

2.5

2.0

Ea (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

1.5

1.17

0.94

1.0

0.76 0.56

0.5

0.38 0

1

2

3

4

5

Hydrogenation Degree

Figure 11. Comparison of the activation energies for hydrogenation, deoxygenation and cyclohexanone formation from phenol or its hydrogenation intermediates.

Figure 12. Structures of initial state (IS), transition state (TS) and final state (FS) in direct deoxygenation of cyclohexanol on Pt(111).

36


Page 37 of 40

O

O

CHone*

OH

OH

DH

HO

2

H

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HY HY

HO

CHol*

HO 1HPh*

DO

4HPh*

Ph*

HY

5HPh*

DH, Des DO, HY, DH, Des

C6H10 *

C6H11*

Figure 13. Schematic representation of the major reaction pathways for the formation of cyclohexanone (CHone), cyclohexanol (CHol) and benzene from hydrodeoxygenation of phenol (Ph) on Pt(111) at 523 K and atmospheric H2 pressure. HY, DO, DH, Des, * and ● stand for hydrogenation, deoxygenation, dehydrogenation, desorption, adsorbed species, and radical, respectively. Note that the reverse reactions of the formation of CHone* and CHol* may form isomers of 5HPh other than that shown in this figure.

37



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Page 38 of 40

Table 1. Reaction Energy (∆Erxn) and Activation Barrier (Ea) for Adding the First H Atom to Phenol.

a

Name a

∆Erxn ( eV )

Ea ( eV )

H1Ph

0.47

1.07

H2Ph

0.26

0.94

3

H Ph

0.42

1.06

4

H Ph

0.38

1.02

H5Ph

0.40

1.04

H6Ph

0.55

0.98

The superscripted number stands for the C atom of phenol (Ph) to which the H atom is added.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 2. Activation Energy (Ea), Reaction Energy (∆Erxn), Forward Rate Constant (kf) and Equilibrium Coefficient (K) for Elementary Steps in Hydrodeoxygenation of Phenol on Pt (111) Surface. Step

C atoms a Ea (eV)

Reaction

∆Erxn (eV)

523 K -1

kf (s )

573 K K

-1

kf (s )

623 K K

-1

kf (s )

K

Hydrogenation 1 2

C6H5OH**** +H * → C6H6OH****+ * C6H6OH**** +H* → C6H7OH***+* *

2 26

0.94 0.88

0.26 0.28

1.21E+04 4.54E+04

1.39E-03 3.71E-03

7.81E+04 2.67E+05

2.14E-03 6.12E-03

3.75E+05 1.18E+06

3.05E-03 9.25E-03

3

C6H7OH*** +H* → C6H8OH*** + *

264

0.84

0.25

6.67E+04

2.39E-03

3.52E+05

3.60E-03

1.42E+06

5.04E-03

4

C6H8OH*** +H* → C6H9OH** + **

2643

0.68

0.23

1.10E+05

9.08E-03

3.71E+05

3.24E-02

1.01E+06

9.30E-02

5 6

**

*

* **

0.62 0.57

0.31 0.04

1.50E+07 7.90E+07

7.62E-03 3.34E-02

5.55E+07 1.50E+08

1.38E-02 6.70E-02

1.67E+08 2.51E+08

2.25E-02 1.19E-01

7 8

C6H9OH +H → C6H10OH + 26435 * * * * C6H10OH +H → C6H11OH + 264351 Deoxygenation C6H5OH**** + * → C6H5●**** +OH* C6H6OH**** + * → C6H6●**** + OH* 2

2.61 2.04

1.81 1.64

8.00E-13 1.41E-07

2.54E-19 1.55E-15

1.37E-10 8.07E-06

8.11E-18 4.05E-14

1.03E-08 2.42E-04

1.48E-16 6.28E-13

9

C6H7OH*** + * → C6H7● *** + OH*

26

1.70

1.47

2.51E-04

2.73E-14

7.26E-03

5.15E-13

1.23E-01

6.10E-12

435 4352

1.31 1.24

0.72 1.06

1.11E+00 5.26E+00

4.16E-06 2.49E-09

1.55E+01 6.39E+01

1.92E-05 3.29E-08

1.42E+02 5.24E+02

6.95E-05 2.91E-07

43526

0.76

0.61

9.53E+05

7.02E-05

5.59E+06

9.84E-04

2.48E+07

2.87E-03

4352

1.17

0.27

4.17E+03

4.41E-01

2.38E+04

7.80E-01

1.78E+05

1.26E+00

43526

0.38

0.05

1.32E+08

1.96E-01

2.34E+08

4.78E-01

3.78E+08

9.36E-01

435261

0.58

-0.41

3.75E+07

1.12E+03

1.24E+08

4.75E+02

3.40E+08

2.29E+02

0.20

-0.55

1.35E+11

1.97E+06

2.10E+11

7.16E+05

3.04E+11

3.06E+05

***

*

***

10 11

C6H8OH + → C6H8● + OH C6H9OH** + * → C6H9●** + OH*

12

C6H10OH* + * → C6H10● * + OH*

*

Tautomerization to cyclohexanone 13

C6H9OH* +* → C6H10O*+ * Dehydrogenation to cyclohexanone

14 15 16 b a

C6H10OH* + * → C6H10O*+H * Radical Hydrogenation C6H10● * +H* → C6H11*+ * Water Formation OH* +H* → H2O*+ *

“C atoms” stands for the C atoms of phenol that have been hydrogenated; b Ea and ∆Erxn for this step are taken from reference.13 * reprensent adsorption site, ● represent radical which is H deficient on C1 atom. 39



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TOC Graphic

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