Late-Transition-Metal-Modified β-Mo2

Research Article pubs.acs.org/journal/ascecg

Late-Transition-Metal-Modified β‑Mo2C Catalysts for Enhanced Hydrogenation during Guaiacol Deoxygenation Frederick G. Baddour, Vanessa A. Witte, Connor P. Nash, Michael B. Griffin, Daniel A. Ruddy,* and Joshua A. Schaidle* National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Molybdenum carbide has been identified as a promising bifunctional catalyst in the deoxygenation of a variety of pyrolysis vapor model compounds. Although high deoxygenation activity has been demonstrated, complementary hydrogenation activity has been limited, especially for lignin-derived, aromatic model compounds. The ability to control the relative site densities of acidic and hydrogenation functionalities represents a catalyst design challenge for these materials with the goal to improve hydrogenation activity under ex situ catalytic fast pyrolysis (CFP) conditions. Here we demonstrate that the addition of Pt and Ni to β-Mo2C resulted in an increase in the H*-site density with only a minor decrease in the acid-site density. In contrast, the addition of Pd did not significantly alter the H*- or acid-site densities. High conversions (>94%) and high selectivities to 0-oxygen products (>80%) were observed in guaiacol deoxygenation under ex situ CFP conditions (350 °C and 0.44 MPa H2) for all catalysts. Pt addition resulted in the greatest deoxygenation, and site-time yields to hydrogenated products over the Pt/Mo2C catalyst were increased to 0.048 s−1 compared to 0.015−0.019 s−1 for all other catalysts. The Pt/Mo2C catalyst demonstrated the highest hydrogenation performance, but modification with Ni also significantly enhanced hydrogenation performance, representing a promising lower-cost alternative. KEYWORDS: Catalytic fast pyrolysis, Vapor phase upgrading, Deoxygenation, Hydrogenation, Molybdenum carbide, Guaiacol

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The effectiveness of carbide catalysts is exemplified by βMo2C, which has been demonstrated to deoxygenate ligninderived phenolic compound mixtures (anisole, m-cresol, guaiacol, and 1,2-dimethoxybenzene) under mild conditions (260 °C, 0.1 MPa) resulting in high selectivity for benzene.6 However, complementary hydrogenation activity was limited, and low selectivity (94%) and high selectivities to 0-oxygen products (>80%) were observed for guaiacol over all catalysts. Interestingly, site-time yields to hydrogenated products (STYHYD) over the Pt/Mo2C catalyst were 3-fold higher (0.048 s−1) than those observed for all other catalysts (0.015−0.019 s−1). While the Pt/Mo2C catalyst demonstrated the highest hydrogenation performance with a cyclohexane selectivity of 23.5% compared to 1.0% over the parent β-Mo2C catalyst, modification with Ni also significantly enhanced hydrogenation performance with a cyclohexane selectivity of 7.5%, representing a promising lower-cost alternative.

Figure 1. XRD patterns of Ni/Mo2C, Pd/Mo2C, Pt/Mo2C, and βMo2C catalysts. Symbols ∗ and † denote peaks associated with metallic Pd and Pt, respectively.

Ni/Mo2C and Pd/Mo2C materials, a small peak at 26.0° 2θ and shoulder at 53.5° 2θ indicate the presence of a small amount of MoO2, which likely formed during the metal deposition reaction because of the lack of a controlled passivation prior to metal deposition and the use of aqueous nitrate salt solutions.17 Diffraction peaks associated with metallic Pd and Pt were also detected by XRD (Figure 1). Line-broadening analysis of the peaks associated with the Pd(200) and Pt(200) families of crystal planes resulted in approximate crystallite sizes of 26.3 and 11.2 nm, respectively. No peaks related to Ni species were observed in the diffraction pattern of Ni/Mo2C (Figure 1). The absence of peaks for metallic Ni or any of its oxides in the XRD pattern suggests the formation of small, polycrystalline particles of Ni that exhibit broad, low-intensity diffraction features that are obscured by the high crystallinity of the β-Mo2C support at the low Ni loading (0.95 wt %), which is consistent with previous reports.18 It is well-known that Mo−C catalysts are bifunctional,9,19,20 possessing both H*-site and acid-site functionalities, and that this bifunctionality is essential to hydrodeoxygenation.1 The ability of Mo−C catalysts to activate hydrogen has been attributed to H-adsorption to exposed surface C and Mo sites, and the acidic character of these catalysts has been attributed to 11434

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Table 1. Metal Loading, Metal Particle Size, H*-Site Densities, and Acid-Site Densities for β-Mo2C and the M/Mo2C Catalysts catalyst

metal loading (wt %)

β-Mo2C Ni/Mo2C Pd/Mo2C Pt/Mo2C a

0.95 1.19 2.40

metal particle size (XRD, nm)

H*-site density (μmol/gcat)

acid-site density (μmol/gcat)

acid-/H*-site ratio (mol/mol)

n.d.a 26.3 11.2

57.2 132 52.1 127

615 605 625 600

11 4.6 12 4.7

n.d., not detected.

(4.7) materials from the parent β-Mo2C (11). The minor increase in acidity and slight decrease in strong H* sites upon modification with Pd resulted in a slightly increased acid-/H*site ratio for Pd/Mo2C (12). These data highlight that, for Pt and Ni, this metal deposition method can successfully reduce the acid-/H*-site ratio, in contrast to changing the crystal phase and particle size that was only observed to increase the ratio on the basis of our previous report.4 Guaiacol Deoxygenation. The deoxygenation and hydrogenation performance of β-Mo2C, Ni/Mo2C, Pd/Mo2C, and Pt/Mo2C were assessed using guaiacol as a substrate in a continuous-flow fixed-bed reactor operated at a weight hourly space velocity (WHSV) of 10 h−1 under ex situ CFP conditions (350 °C, 0.44 MPa H2). Guaiacol is an informative model compound to study over multifunctional catalysts because its multiple functional groups and stepwise deoxygenation enable investigation into a variety of reactions, such as demethylation (DME), demethoxylation (DMO), direct deoxygenation (DDO), and hydrogenation (HYD).1,6 High conversion was targeted in these reactions to investigate deoxygenation and hydrogenation activity of these catalysts in secondary reactions, such as phenol deoxygenation to benzene and benzene hydrogenation to cyclohexane (Scheme 1). All catalysts were highly active and exhibited greater than 94% guaiacol conversion throughout the reaction time investigated (Figure S2).

surface hydroxyls or Lewis acidic Mo sites.4,5,7 H2-chemisorption and NH3-desorption experiments were conducted to probe the impact of metal-modification on the abundance of the acid and H* sites of the prepared catalysts. The parent β-Mo2C material had an acid-site density of 615 μmol/gcat, which is higher than in our previous reports,4,5 but lies within the range of 450−650 μmol/gcat that we typically observe in our laboratory, on the basis of particle size, extent of passivation, and the reactivation procedure (e.g., 450 versus 400 °C). In all cases, deposition of the late-transition metal onto the surface of the β-Mo2C resulted in a negligible change in the acid-site densities of the M/Mo2C catalysts, with a ±2% maximum deviation from the parent β-Mo2C value, as shown in Table 1, and had no discernible impact on the observed broad NH3desorption profiles that are typical of Mo−C catalysts (Figure S1).4,5 It is evident from the NH3-TPD traces for all catalysts that there are multiple desorption events characterized by two broad convoluted peaks. The breadth of these desorption peaks is consistent with what we have typically observed over Mo−C catalysts in our laboratory and previous literature that report NH3-TPD profiles.5,21 Conversely, the addition of latetransition metals to β-Mo2 C significantly affected the abundance of H* sites (Table 1). The H*-site densities of Ni/Mo2C (132 μmolH*/gcat) and Pt/Mo2C (127 μmolH*/gcat) were more than 2-fold greater than that of the parent β-Mo2C material (57.2 μmolH*/gcat). Consistent with the lack of observed XRD peaks for metallic Ni, the H*-site density for Ni/Mo2C corresponds to a Ni crystallite size of 1−2 nm, which is below the detection limit of the XRD analysis. Interestingly, the high H*-site density observed for Pt/Mo2C also suggests the presence of a fraction of small Pt particles of 2−3 nm in addition to the larger 11.2 nm particles that dominate the XRD analysis. In contrast, modification of β-Mo2C with Pd resulted in a similar, slightly lower H*-site density. It is worth noting that, in general, H2 chemisorption of Pd catalysts can be complicated by uptake of hydrogen into the lattice.22 However, considering the large average crystallite size determined from XRD (26.3 nm) and the low Pd loading (1.19 wt %), the expected Pd contribution to the H*-site density is low (ca. 5 μmol/gcat) and lies within the error of the analysis (±10% relative standard deviation for the Pt/Al2O3 standard). These data suggest that the synthetic method used for modification of the β-Mo2C with Pd favors the formation of large particles over small particles, resulting in minimal changes to the H*-site density. We hypothesize that some combination of a more positive reduction potential,18 a different degree of surface oxidation of the unpassivated β-Mo2C,8 or differences in the coordination environments of the Pt and Pd precursors in solution may result in the observed larger Pd particles sizes compared to Ni or Pt; however, a more thorough synthetic and structural analysis is required to quantify this effect. The increase in H*-site density coupled with the negligible change in acid-site density resulted in a significant decrease in the acid-/H*-site ratio of the Ni/Mo2C (4.6) and Pt/Mo2C

Scheme 1. Common Transformationsa during Guaiacol Deoxygenation under ex situ CFP Conditions1,6

a

The methylated analogs of all products (e.g, cresol, toluene, methylcyclohexane) are also observed because of methyl transfer reactions, but are omitted here for clarity.

Because of the high levels of conversion, deoxygenation performance is compared here as the liquid phase product selectivity (i.e., mol % of C6+ products) at 240 min time-onstream, and the products are categorized as containing 0 oxygen atoms (e.g., benzene, toluene, cyclohexane) or 1 oxygen atom (e.g., phenol, cresol, anisole) (Table 2). Selectivity to 2-oxygen 11435

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product cyclohexane accounting for just 1.0% of the C6+ product composition. Cyclohexene was also observed with a selectivity of 3.2%. As described in more detail below, hydrogen incorporation is more challenging for Mo−C catalysts under these conditions than oxygen removal, which motivated catalyst-modification with hydrogenation metals. The limited ring hydrogenation and high selectivity to benzene and toluene observed here are consistent with previous reports in which lignin-derived model compounds and mixtures (e.g., anisole, mcresol, 1,2-dimethoxybenzene, guaiacol) were deoxygenated over Mo2C at 150−280 °C and atmospheric pressure.6−8 Compared to the parent β-Mo2C, Ni/Mo2C exhibited a minor enhancement in 0-oxygen product selectivity from 87.8% to 89.2% with a corresponding minor decrease in 1-oxygen product selectivity (10.8%). Within the 0-oxygen products, decreased benzene (66.3%) and toluene (11.3%) selectivities were observed, with increased selectivity to the ring-saturated cyclohexane product (7.5%). The highest deoxygenation performance was observed over Pt/Mo2C, which exhibited 99.6% selectivity to 0-oxygen products. Further decreased benzene (65.2%) and toluene (8.5%) selectivities were observed, with a drastic increase in cyclohexane selectivity to 23.5%. It has previously been described that Pt can facilitate the removal of surface oxygen on Pt/Mo2C materials, possibly through H* spillover from Pt to β-Mo2C.14 We hypothesize that a similar phenomenon may be responsible for the high guaiacol deoxygenation selectivity observed in these experiments, where deposited surface oxygen is more rapidly removed through efficient H2 activation at Pt sites, therefore opening sites for oxygen removal, potentially via a reverse Mars−van Krevelen mechanism.5 In contrast, the Pd/Mo2C catalyst exhibited a decreased 0-oxygen product selectivity of 81.2% compared to the parent β-Mo2C with a corresponding increase in 1-oxygen products to 18.8%, and a minor increase in cyclohexane selectivity from 1.0% to 2.0%. This is somewhat surprising since Pd is often a highly active hydrogenation catalyst. This result is attributed to the large Pd particle size, and underscores the challenge of designing catalysts that achieve high hydrogen incorporation under ex situ CFP conditions, where traditional catalyst materials do not always perform as expected. Further efforts to control Pd particle sizes, possibly through the use of different precursors with controlled molecular structures and reduction potentials, are required to better understand the Pd-modification of β-Mo2C catalysts. The extent of reaction for a sequential step process, as suggested by Scheme 1 to produce cyclohexane, is dependent on the reaction conditions and conversion. As noted above, these catalysts were evaluated under the same conditions of temperature, pressure, and WHSV and at similarly high guaiacol conversions (>94%); thus the specific activity of the catalysts is expected to drive the observed differences in selectivity. Saturated oxygenate compounds (e.g., cyclohexanol, cyclohexanone, methoxycyclohexane, cyclohexane-1,2-diol) were not observed in the final liquid product from any of the catalysts (Table S1). Here, we observe that the Pt/Mo2C catalyst exhibited the highest selectivity to deoxygenated and hydrogenated products. Considering the reaction network, this could be due to (1) the reaction proceeding as described in Scheme 1, where ring hydrogenation does not occur until complete deoxygenation, or (2) ring hydrogenation occurring first, but the saturated oxygenate products are short-lived intermediates that quickly undergo deoxygenation. For the first case, the enhanced deoxygenation activity of the Pt/Mo2C

Table 2. Conversion, Product Selectivities Grouped by Oxygen Content during Guaiacol Deoxygenation, and SiteTime Yield of Hydrogenated Products under ex Situ CFP Conditions (350 °C, 0.44 MPa H2) at 240 min Time-onStream catalyst

conversion (%)

0-oxygen products (%)

1-oxygen products (%)

STYHYD (s−1)

β-Mo2C Ni/Mo2C Pd/Mo2C Pt/Mo2C

99.8 99.8 99.7 98.8

87.8 89.2 81.2 99.6

12.2 10.8 18.8 0.40

0.017 0.015 0.019 0.048

products was less than 0.1% for all catalysts. The selectivities to all products are provided in Table S1. The parent β-Mo2C catalyst exhibited exceptional deoxygenation performance, achieving 87.8% selectivity to 0-oxygen products, with benzene and toluene as the dominant products at 69.4% and 11.8%, respectively. This high selectivity to completely deoxygenated products over a β-Mo2C catalyst is consistent with previous reports that demonstrated similar selectivities to benzene from anisole (90% selectivity, 150 °C, ca. 0.110 MPa) and high selectivity to 0-oxygen products from a mixture of biomassrelevant oxygenates (89−98%, 280 °C, 0.114 MPa).6,23 The combined specific activity of benzene and toluene exhibited by β-Mo2C in our experiments was 18.9 μmol/(gcat s), which is an order of magnitude greater than the specific activity previously demonstrated from the mixture of phenolics with a β-Mo2C catalyst6 and from guaiacol over a MoO3 catalyst.24 It is worth noting that these previous reports employed lower space velocities and lower temperatures. Here we demonstrate high deoxygenation performance by β-Mo2C at 350 °C, which is more representative of ex situ CFP conditions, and the production rate of benzene and toluene is 5-fold higher than for the MoO3 catalyst at 350 °C [3.8 μmol/(gcat s)].24 The major 1-oxygen products were anisole (5.0%), phenol (2.0%), and cresol isomers (3.7%). The selectivities for the major liquid phase products are presented in Figure 2. In addition to

Figure 2. Selectivity to the major 0-oxygen and 1-oxygen products during guaiacol deoxygenation under ex situ CFP conditions (350 °C, 0.44 MPa H2) at 240 min time-on-stream.

toluene, transmethylation products of 0-oxygen (e.g., xylenes, trimethylbenzenes) and 1-oxygen (e.g., cresol isomers) products were observed with low selectivities and are attributed to reactions at the acidic sites on the carbide surface, consistent with previous reports.6 Limited hydrogenation was observed over the parent β-Mo2C catalyst, with the ring-saturated 11436

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ACS Sustainable Chemistry & Engineering catalyst would enable higher total hydrogenation selectivity. In the second case, the enhanced hydrogenation activity of the Pt/ Mo2C catalyst would enable greater deoxygenation selectivity. Further investigations into the reaction network are necessary to resolve the mechanism of hydrogenation−deoxygenation over this highly active catalyst. The hydrogen incorporation exhibited by the Pt/Mo2C and Ni/Mo2C, highlighted by the cyclohexane selectivities of 23.5% and 7.5%, respectively, represents a significant increase versus the parent β-Mo2C reported here and in previous reports of lignin-derived model compound deoxygenation.6 Further insight into this increased hydrogenation activity was obtained by normalizing the production rate of hydrogenated products (predominantly cyclohexane and methylcyclohexane, with contributions from methylcyclopentane and cyclohexene) to the H*-site density to give site-time yield (STYHYD) values taken at 240 min time-on-stream (Table 2). As noted above, all catalysts exhibited greater than 94% guaiacol conversion throughout the reaction time, and therefore, these STYHYD values do not necessarily reflect the intrinsic rates of each catalyst. Rather, they allow for a comparative analysis of the catalysts at a single reaction condition relevant to ex situ CFP and at similar high conversions. The parent β-Mo2C exhibits an STYHYD value of 0.017 s−1. The Pd/Mo2C catalyst, that exhibited a similarly low hydrogenated product selectivity to βMo2C, exhibited a similar STYHYD of 0.019 s−1. Interestingly, despite the approximately 2.3-fold increase in H*-site density for Ni/Mo2C to 132 from 57.2 μmol/gcat and increased cyclohexane selectivity highlighted above, this catalyst demonstrated a similar STYHYD value of 0.015 s−1 when the production rate was normalized to H*-site density. This suggests that although more hydrogenation sites are present on the Ni/Mo2C catalyst, they are not more active on a per site basis than those on the parent β-Mo2C. Finally, the Pt/Mo2C catalyst clearly demonstrates a significantly increased STYHYD value of 0.048 s−1. This catalyst possesses a similar H*-site density to Ni/Mo2C, but this STYHYD analysis suggests that the hydrogenation sites on this catalyst are ca. 3-fold more active on a per site basis (0.048 s−1) than those on any of the other catalysts tested (0.015−0.019 s−1). Oxide-supported Ni and Pt catalysts have been extensively studied in benzene hydrogenation, and Pt catalysts have demonstrated turnover frequency (TOF) values that are approximately two orders of magnitude more active than Ni catalysts, depending on the metal particle size and shape for this structure-sensitive reaction.25−27 Therefore, although we cannot rule out synergistic effects that lead to higher per site activity for Pt on β-Mo2C, as suggested in previous reports of Pt deposition on β-Mo2C where increased activity of Pt was attributed to metal−support interactions17 or interfacial effects,13 we attribute the observed difference in per site activity in these guaiacol deoxygenation experiments to the innate activities of Pt and Ni in arene hydrogenation. Further experiments to explore synergistic effects between the supported metals and the β-Mo2C surface are under development. Under the high temperatures and low H2 pressures employed during ex situ CFP, hydrogenation is constrained by thermodynamic limitations, therefore setting a maximum anticipated selectivity for hydrogenated products.1,10 This is exemplified by calculating the equilibrium constant as a function of temperature for the hydrogenation of benzene to cyclohexane under the ex situ CFP conditions employed here (Figure 3). The equilibrium constant decreases with increasing

Figure 3. Equilibrium constant for the hydrogenation of benzene to cyclohexane as a function of temperature, and measured reaction quotient values over the four catalysts taken at 240 min time-onstream. A log10 Keq value of 0 corresponds to a Keq value of 1. The Keq values were determined by integrating the van’t Hoff equation from the standard state to each temperature. NIST values were used for the standard state enthalpies, standard molar entropies, and the heat capacities, which were fit to third-order polynomials to account for temperature dependence.

temperature, with cyclohexane being favored below 280 °C, and benzene being favored above that temperature. At 350 °C, the equilibrium constant is 0.0057. From this analysis, the approach to equilibrium for the four catalysts was calculated using the observed reaction quotient (Table 3; eq 3 in the SI). Table 3. Experimentally Observed Reaction Quotient and Approach to Equilibriuma Based on the Product Distribution Observed at 240 min Time-on-Stream catalyst

reaction quotient

approach to equilibrium

β-Mo2C Ni/Mo2C Pd/Mo2C Pt/Mo2C

0.000 19 0.0015 0.000 44 0.0048

0.033 0.26 0.077 0.84

a

The equilibrium constant under the conditions reported here is 0.0057.

An approach to equilibrium value of 1.0 would indicate that the catalyst is operating at the thermodynamic limit of benzene hydrogenation. The parent β-Mo2C catalyst exhibited the lowest reaction quotient value (0.000 19) and corresponding approach to equilibrium of 0.033, which is far below equilibrium and highlights the low hydrogenation and low selectivity to hydrogenated products described above. The Pd/ Mo2C catalyst demonstrated an increased reaction quotient value (0.000 44), but this still corresponds to a relatively low approach to equilibrium of 0.077. In contrast, the Ni/Mo2C and Pt/Mo2C catalysts demonstrated significantly greater hydrogenation toward equilibrium. The Ni/Mo2C reaction quotient value (0.0015) corresponds to an approach to equilibrium of 0.26, and the Pt/Mo2C value (0.0048) corresponds to an approach to equilibrium of 0.84, highlighting the observed high levels of ring hydrogenation at nearequilibrium performance for this Pt/Mo2C catalyst. Recent research from our laboratory described the relationship between acid-site strength and product selectivity in acetic acid deoxygenation over bulk β-Mo2C, bulk α-MoC1−x, and supported α-MoC1−x nanoparticles.4 Increased acid-site density 11437

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ACS Sustainable Chemistry & Engineering for bulk α-MoC1−x over β-Mo2C with similar acid strengths and H*-site densities (i.e., increased acid-/H*-site ratio to 14 from 8.1) resulted in moderate changes to the observed hydrodeoxygenation and decarbonylation selectivity. However, the generation of stronger acid sites on the α-MoC1−x nanoparticles resulted in significantly increased ketonization selectivity of 15−18% at high temperatures where the bulk materials exhibited less than 3% selectivity. Considering the guaiacol deoxygenation data, STYHYD data, and the approach to equilibrium analysis presented here, a structure−function relationship between the H*-site density and the observed hydrogenation performance can be postulated for the M/Mo2C catalysts prepared herein. The changes in performance were found not to simply be due to differences in the acid-/H*-site ratio, where only a moderate increase in selectivity to hydrogenated products resulted from increasing the H*-site density with sites of the same activity (i.e., decreasing the acid-/ H*-site ratio), as exemplified here by the Ni/Mo2C catalyst. The drastic improvement in hydrogenation performance demonstrated by the Pt/Mo2C, which approached thermodynamic equilibrium, is attributed to more active H* sites, not only an increased site density or reduced acid-/H*-site ratio.

stream upgrading in biomass catalytic fast pyrolysis processes, for example, in a dual-stage fixed-bed reactor design.2

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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/acssuschemeng.7b02544. Experimental details, NH3-TPD profiles, plot of guaiacol conversion versus time-on-stream, and table of complete product selectivity (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Daniel A. Ruddy: 0000-0003-2654-3778 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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CONCLUSIONS We have demonstrated that the synthetic modification of βMo2C with H2-activating metals enables tuning of the selectivity over these catalysts toward hydrogenated products in guaiacol deoxygenation. Specifically, we have determined not only that modification of β-Mo2C with Ni and Pt increases the density of H* sites on the catalyst and shifts the product slate to hydrogenated products, but also that the synthetic method investigated to deposit Pt on the catalyst surface resulted in additional H* sites that were found to be 3-fold more active (STYHYD of 0.048 s−1) than the parent β-Mo2C material (STYHYD of 0.017 s−1). The Pt/Mo2C catalyst exhibited a 99.2% selectivity to 0-oxygen products, possibly due to enhanced surface oxygen removal facilitated by H* spillover from Pt, and exemplifies the concept to increase hydrogenation activity to the thermodynamic equilibrium limit. The promising performance of Ni makes Ni/Mo2C materials of interest for further development as a lower-cost alternative. From a catalyst design and synthesis perspective, the deposition of Ni presents a challenge with respect to the previously developed β-Mo2C surface-mediated reduction method, and the Ni speciation on Ni/Mo2C remains difficult to determine. However, a variety of Ni complexes, including Ni(0) complexes (e.g., biscyclooctadiene nickel), are attractive precursors to develop a similar deposition method for low-valent Ni, and future research will seek to develop methods to control Ni speciation and particle size on Ni/Mo2C materials. Similarly, preformed Ni nanoparticles have demonstrated promise in guaiacol deoxygenation,28,29 and incorporating these active phases onto β-Mo2C may enable further control over the bifunctional nature of these catalysts. Finally, alloyed Ni species may offer an opportunity to increase hydrogenation in these catalysts. For example, alloying a low mol % of Pt into Ni may be a cost-effective method to further increase the hydrogenation activity toward the equilibrium limit. The results reported here, combined with the growing number of synthetic modifications targeted at preparing Mo−C catalysts with a highly tailored acid-/H*-site ratio, suggest that deoxygenation and hydrogenation could be balanced to generate an optimal product mixture for down-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy Bioenergy Technologies Office, Contract DE-AC36-08GO28308 at the National Renewable Energy Laboratory, and in collaboration with the Chemical Catalysis for Bioenergy Consortium (ChemCatBio), a member of the Energy Materials Network (EMN). The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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REFERENCES

(1) Ruddy, D. A.; Schaidle, J. A.; Ferrell, J. R., III; Wang, J.; Moens, L.; Hensley, J. E. Recent Advances in Heterogeneous Catalysts for BioOil Upgrading via ex situ Catalytic Fast Pyrolysis: Catalyst Development through the Study of Model Compounds. Green Chem. 2014, 16, 454−490. (2) Dutta, A.; Schaidle, J. A.; Humbird, D.; Baddour, F. G.; Sahir, A. Conceptual Process Design and Techno-Economic Assessment of ex situ Catalytic Fast Pyrolysis of Biomass: A Fixed Bed Reactor Implementation Scenario for Future Feasibility. Top. Catal. 2016, 59 (1), 2−18. (3) Habas, S. E.; Baddour, F. G.; Ruddy, D. A.; Nash, C. P.; Wang, J.; Pan, M.; Hensley, J. E.; Schaidle, J. A. A Facile Molecular Precursor Route to Metal Phosphide Nanoparticles and Their Evaluation as Hydrodeoxygenation Catalysts. Chem. Mater. 2015, 27 (22), 7580− 7592. (4) Baddour, F. G.; Nash, C. P.; Schaidle, J. A.; Ruddy, D. A. Synthesis of α-MoC1‑x Nanoparticles with a Surface-Modified SBA-15 Hard Template: Determination of Structure-Function Relationships in Acetic Acid Deoxygenation. Angew. Chem., Int. Ed. 2016, 55, 9026− 9029. (5) Schaidle, J. A.; Blackburn, J.; Farberow, C. A.; Nash, C. P.; Steirer, K. X.; Clark, J.; Robichaud, D. J.; Ruddy, D. A. Experimental and Computational Investigation of Acetic Acid Deoxygenation over 11438

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Research Article

ACS Sustainable Chemistry & Engineering

into Aromatic Hydrocarbons under Low Hydrogen Pressures. Energy Environ. Sci. 2014, 7, 2660−2669. (25) Keane, M. A.; Patterson, P. M. Compensation Behaviour in the Hydrogenation of Benzene, Toluene, and o-Xylene over Ni/SiO2: Determination of True Activation Energies. J. Chem. Soc., Faraday Trans. 1996, 92, 1413−1421. (26) Molina, R.; Poncelet, G. Hydrogenation of Benzene over Alumina-Supported Nickel Catalysts Prepared from Ni(II) Acetylacetonate. J. Catal. 2001, 199, 162−170. (27) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. Platinum Nanoparticle Shape Effects on Benzene Hydrogenation Selectivity. Nano Lett. 2007, 7 (10), 3097−3101. (28) Griffin, M. B.; Baddour, F. G.; Habas, S. E.; Ruddy, D. A.; Schaidle, J. A. Evaluation of Silica-Supported Metal and Metal Phosphide Nanoparticle Catalysts for the Hydrodeoxygenation of Guaiacol under ex situ Catalytic Fast Pyrolysis Conditions. Top. Catal. 2016, 59, 124−137. (29) Roberts, E. J.; Habas, S. E.; Wang, L.; Ruddy, D. A.; White, E. A.; Baddour, F. G.; Griffin, M. B.; Schaidle, J. A.; Malmstadt, N.; Brutchey, R. L. High-Throughput Continuous Flow Synthesis of Nickel Nanoparticles for the Catalytic Hydrodeoxygenation of Guaiacol. ACS Sustainable Chem. Eng. 2017, 5 (1), 632−639.

Oxophilic Molybdenum Carbide: Surface Chemistry and active Site Identity. ACS Catal. 2016, 6 (2), 1181−1197. (6) Chen, C.-J.; Lee, W.-S.; Bhan, A. Mo2C Catalyzed Vapor Phase Hydrodeoxygenation of Lignin-Derived Phenolic Compound Mixtures to Aromatics under Ambient Pressure. Appl. Catal., A 2016, 510, 42− 48. (7) Lee, W.-S.; Wang, Z.; Wu, R. J.; Bahn, A. Selective Vapor-Phase Hydrodeoxygenation of Anisole to Benzene on Molybdenum Carbide Catalysts. J. Catal. 2014, 319, 44−53. (8) Lee, W.-S.; Kumar, A.; Wang, Z.; Bhan, A. Chemical Titration and Transient Kinetic Studies of Site Requirements in Mo2CCatalyzed Vapor Phase Anisole Hydrodeoxygenation. ACS Catal. 2015, 5 (7), 4104−4114. (9) Sullivan, M. M.; Held, J. T.; Bhan, A. Structure and Site Evolution of Molybdenum Carbide Catalysts upon Exposure to Oxygen. J. Catal. 2015, 326, 82−91. (10) Griffin, M. B.; Ferguson, G. A.; Ruddy, D. A.; Biddy, M. J.; Beckham, G. T.; Schaidle, J. A. Role of the Support and Reaction Conditions on the Vapor-Phase Deoxygenation of m-Cresol over Pt/C and Pt/TiO2 Catalysts. ACS Catal. 2016, 6 (4), 2715−2727. (11) Griboval-Constant, A.; Giraudon, J.-M.; Leclercq, G.; Leclercq, L. Catalytic Behaviour of Cobalt or Ruthenium Supported Molybdenum Carbide Catalysts for FT Reaction. Appl. Catal., A 2004, 260 (1), 35−45. (12) Lewandowski, M.; Szymańska-Kolasa, A.; Da Costa, P.; Sayag, C. Catalytic Performances of Platinum Doped Molybdenum Carbide for Simultaneous Hydrodenitrogenation and Hydrodesulfurization. Catal. Today 2007, 119 (1), 31−34. (13) Schweitzer, N. M.; Schaidle, J. A.; Ezekoye, O. K.; Pan, X.; Linic, S.; Thompson, L. T. High Activity Carbide Supported Catalysts for Water Gas Shift. J. Am. Chem. Soc. 2011, 133 (8), 2378−2381. (14) Schaidle, J. A.; Lausche, A. C.; Thompson, L. T. Effects of Sulfur on Mo2C and Pt/Mo2C Catalysts: Water Gas Shift Reaction. J. Catal. 2010, 272 (2), 235−245. (15) Perret, N.; Wang, X.; Delannoy, L.; Potvin, C.; Louis, C.; Keane, M. A. Enhanced Selective Nitroarene Hydrogenation over Au Supported on β-Mo2C and β-Mo2C/Al2O3. J. Catal. 2012, 286, 172−183. (16) Xiang, M.; Li, D.; Xiao, H.; Zhang, J.; Li, W.; Zhong, B.; Sun, Y. K/Ni/β-Mo2C: A Highly Active and Selective Catalyst for Higher Alcohols Synthesis from CO Hydrogenation. Catal. Today 2008, 131 (1), 489−495. (17) Wyvratt, B. M.; Gaudet, J. R.; Thompson, L. T. Effects of Passivation on Synthesis, Structure and Composition of Molybdenum Carbide Supported Platinum Water-Gas Shift Catalysts. J. Catal. 2015, 330, 280−287. (18) Schaidle, J. A.; Schweitzer, N. M.; Ajenifujah, O. T.; Thompson, L. T. On the Preparation of Molybdenum Carbide-Supported Metal Catalysts. J. Catal. 2012, 289, 210−217. (19) Bej, S. K.; Thompson, L. T. Acetone Condensation over Molybdenum Nitride and Carbide Catalysts. Appl. Catal., A 2004, 264, 141−150. (20) Zhang, W.; Zhang, Y.; Zhao, L.; Wei, W. Catalytic Activities of NiMo Carbide Supported on SiO2 for the Hydrodeoxygenation of Ethyl Benzoate, Acetone, and Acetaldehyde. Energy Fuels 2010, 24, 2052−2059. (21) Bej, S. K.; Bennett, C. A.; Thompson, L. T. Acid and base characteristics of molybdenum carbide catalysts. Appl. Catal., A 2003, 250, 197−208. (22) Amorim, C.; Keane, M. A. Catalyst Support Effects: Gas-Phase Hydrogenation of Phenol over Palladium. J. Colloid Interface Sci. 2008, 322, 196−208. (23) Lu, Q.; Chen, C.-J.; Luc, W.; Chen, J.; Bhan, A.; Jiao, F. Ordered Mesoporous Metal Carbides with Enhanced Anisole Hydrodeoxygenation Selectivity. ACS Catal. 2016, 6, 3506−3514. (24) Prasomsri, T.; Shetty, M.; Murugappan, K.; Roman-Leshkov, Y. Insights into the Catalytic Activity and Surface Modification of MoO3 during the Hydrodeoxygenation of Lignin-Derived Model Compounds 11439

DOI: 10.1021/acssuschemeng.7b02544 ACS Sustainable Chem. Eng. 2017, 5, 11433−11439