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Nanoporous Cu-Al-Co alloys for selective furfural hydrodeoxygenation to 2-methylfuran Gregory S Hutchings, Wesley W Luc, Qi Lu, Yang Zhou, Dionisios G. Vlachos, and Feng Jiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00316 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 19, 2017

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Industrial & Engineering Chemistry Research

Nanoporous Cu-Al-Co alloys for selective furfural hydrodeoxygenation to 2-methylfuran Gregory S. Hutchings,1 Wesley Luc,1 Qi Lu,1 Yang Zhou,2 Dionisios G. Vlachos,1,3 and Feng Jiao*,1

[1] Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA. [2] Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA. [3] Catalysis Center for Energy Innovation, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, USA.

KEYWORDS: nanoporous, catalysis, furfural, hydrodeoxygenation, operando XAS

ABSTRACT

By finding new catalysts for selective and efficient conversion of biomass-derived products to industrially-relevant chemicals and fuels, a transition from fossil fuel feedstocks may be achieved. Furfural (C5H4O2) is a platform chemical which may be converted to multiple 1 ACS Paragon Plus Environment


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heterocyclic and ring-opening products, but to date there have been few catalysts which enable selective hydrodeoxygenation to 2-methylfuran (2-MF, C5H6O). In this work, we present a selfsupported nanoporous Cu-Al-Co ternary alloy catalyst with high furfural HDO activity towards 2-MF, achieving up to 66.0% selectivity and 98.2% overall conversion at 513K with only a ~5 atomic % Co composition. Further analysis over multiple temperature conditions and nominal Co concentrations was performed to examine optimal conditions and tune catalyst performance, and operando X-ray absorption spectroscopy experiments were conducted to elucidate the structure of the catalyst in the reaction environment.

1. INTRODUCTION The production of fuels and useful chemicals from lignocellulosic biomass is important for the transition from fossil fuel dependency. Upgrading of the biomass-derived platform chemical furfural is an attractive target due to the variety of value-added compounds that may be synthesized.1 Depending on the type of catalyst, furfural can be converted into 2-methylfuran (2MF), furfuryl alcohol, and tetrahydrofuran through selective hydrogenolysis or hydrogenation (Scheme 1). Over the past decade, many efforts have been devoted into hunting for a suitable hydrodeoxygenation (HDO) catalyst that can selectively break C−O bonds in furfural followed by hydrogenation of the carbon. The most widely studied HDO catalysts for converting furfural are metallic Pt, Ru, Ni, Pd, and Cu,2-9 as well as carbides such as Mo2C.10, 11 Since the group VIIIB elements, such as Pt, Ru, Pd, and Ni, tend to interact strongly with the furan ring, they attack furfural through decarbonylation to furan, ring hydrogenation to tetrahydrofuran, and ringopening products.2 In contrast, Cu-based catalysts are less oxophilic, and interact weakly with 2 ACS Paragon Plus Environment

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the furan ring. As a result, they primarily enable selective hydrogenation of the carbonyl of furfural to form furfuryl alcohol (FOL), followed to a lesser extent by HDO to 2-MF.12 While impressive conversion to 2-MF on Cu-containing catalysts has been achieved,13-18 none of the primarily Cu-based catalysts to date are able to produce 2-MF with comparable selectivity. A challenge with Cu is that it is moderately oxophilic, and as a result it is not a good deoxygenation catalyst, which is a prerequisite for HDO. A potential strategy to tune the selectivity of Cu catalysts is through minor component alloying with a more oxophilic element, as has been utilized for other combinations in heterogeneous catalysis.19-28 This alloying results in the formation of Cu-M bimetallic sites that can enhance the interaction between the active site and furan ring without triggering undesired ring-opening reactions.

Scheme 1. Reaction pathways for furfural transformations.

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Nanoporous metals with highly porous structures are of great interest as potential heterogeneous catalysts.29 We recently reported a nanoporous silver catalyst, which is able to convert carbon dioxide electrochemically into carbon monoxide in a very selective and efficient way.30,

31

The porous structure creates high surface area for catalytic reaction and the curved

internal surface generates a large number of highly active step sites for CO2 conversion, resulting in an exceptional activity that is over three orders of magnitude higher than that of the polycrystalline counterpart at a moderate overpotential. Using a similar synthetic strategy, a hierarchical nanoporous Cu-Ti bimetallic alloy was successfully synthesized, which is able to electrocatalytically produce hydrogen from water under a mild overpotential at a rate more than two times higher than that of the current state-of-the-art carbon-supported Pt catalyst.32 The ability to generate highly porous Cu-M alloys offers us an opportunity to design and fabricate highly active catalysts for selective HDO of furfural to 2-MF. Here, we report the successful synthesis of a series of nanoporous Cu-Al-Co alloys with enhanced catalytic properties for furfural HDO to 2-MF. By incorporating oxophilic elements, Al and Co, into Cu, the ternary alloy catalysts show much higher selectivities of 2-MF production over FOL compared to pure Cu. The nanoporosity provides an excellent support for the unique Cu-Al-Co active sites with high material utilization, leading to highly improved activities, with consistently higher total conversions compared to pure Cu counterparts. In particular, the nanoporous Cu-AlCo alloy with ~5 atomic % Co content showed an exceptional catalytic performance, 98.2% overall conversion at 513K with 66.0 % 2-MF selectivity, representing the most effective production of 2-MF reported to date for a primarily Cu-based catalyst. Following a thorough structural analysis, the origin of the enhanced catalytic properties is explored.


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2. EXPERIMENTAL SECTION 2.1 Synthesis of nanoporous Cu-Al-Co alloys The nanoporous Cu-Al-Co alloys were synthesized by first forming alloys with Al at Al80(Cu20-xCox) (x = 0, 0.2, 1, and 2) through a repeated arc melting process under an Ar atmosphere (Cu and Al: 99.999 % purity, Alfa Aesar; Co: 99.99 % purity, Alfa Aesar). After obtaining uniform high-purity ingots, ribbons were formed through a melt spinning process over a metal roller spinning at a speed of 50 m s-1, followed by a rapid quenching. The resulting products were thin, silver-colored ribbons. Nanoporous Cu-Al-Co was freshly prepared before each experiment. Before removal of the Al from the alloys, the ribbons were ground in a mortar and pestle to form a rough powder. The grinding procedure aids in equalizing the required dealloying times for each material and facilitates more uniform dispersion during catalytic reaction studies. Formation of the nanoporous structure was achieved through dealloying of ribbon precursor in a 3M KOH aqueous solution, which effectively removed the majority of Al. For each catalyst, approximately 55 mg of the precursor alloy was added to 25 mL of the 3M KOH solution. Dealloying time was set at 1.5 h. After dealloying, the catalysts were recovered through centrifugation at 5000 RPM (VWR Clinical 200, using 50 mL centrifuge tubes). The remaining KOH and Al-containing salts were removed through repeated washing and centrifugation with deionized water. Between each washing step, the H2O-catalyst mixture was sonicated for 2 min in a sonicator bath (FS30D, Fisher Scientific) to assist the dealloying process. The as-synthesized materials were used immediately without drying to prevent severe surface oxidation.

2.2 Structural characterizations 5 ACS Paragon Plus Environment


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Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 DISCOVER diffractometer using a Cu Kα radiation source. To prevent potential oxidation, the nanoporous samples were loaded in slurry form into boron-rich thin-walled capillaries (Charles Supper Co., Inc.), then dried under vacuum and sealed in an Ar glove box (MBraun) for measurement. Amorphous SiO2 (Sigma-Aldrich) was used as a packing material for correct positioning during measurement. Scanning electron microscopy (SEM) images were recorded on a Zeiss Auriga-60 instrument, at an accelerating voltage of 1.5 kV. Energy-dispersive spectroscopy (EDS) mapping and quantification was performed with an on-board Oxford Synergy X- MAX80 Si drift detector, with an accelerating voltage of 15 kV. During EDS mapping, the sample position was locked insoftware to mitigate drift effects and maximize spatial accuracy. Quantification of the elemental composition of the nanoporous materials was performed using microwave plasma atomic emission spectroscopy (MP-AES) on an Agilent HP 4100 capable of ppm resolution of trace metal composition. Surface area measurements were conducted using N2 adsorption-desorption analysis, performed on a Micromeritics 3Flex surface characterization analyzer. The surface stoichiometry measurements were performed using a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer (XPS) System. The XPS data were calibrated using the binding energy of adventitious carbon at 285 eV and analyzed using CasaXPS software. All peaks were fitted using a Gaussian/Lorentzian product line shape and a Shirley background. Operando X-ray absorption spectroscopy (XAS) characterization was performed in fluorescence mode on line 5-BM-D at the Advanced Photon Source (Argonne National Laboratory). Reaction conditions were achieved with a Clausen-type flow-through reactor,33 with


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a 3 mm outer diameter sample chamber consisting of a polyimide tube. For each nanoporous material studied, new samples were freshly-prepared from the high-Al ribbons and loaded in slurry form and dried under 10 cm3 min-1 He at a temperature of 120 °C until dried, then brought to reaction temperature and subjected to 10 cm-3 min-1 H2 and 0.001 mol h-1 furfural using a NE1000 syringe pump (New Era). Additionally, full extended X-ray absorption fine structure (EXAFS) spectra were recorded ex situ under identical conditions and used for calibration and scan alignment. The energy values for all measurements were calibrated to the known K-edge energy of a pure Co foil (7709 eV). The Demeter software package was used for EXAFS fitting, alignment, and data processing.34 For EXAFS fitting, the Co state was modeled as a single Co atom isolated in a 399 atom FCC cluster of Cu. Both Co oxides and Co-Al alloys were considered in the fit, but were found to contribute extremely small signals and were excluded from final consideration. To determine coordination numbers for the alloy materials, the amplitude reduction factor (ܵ଴ଶ ) of the EXAFS equation was determined from fitting a pure Co foil and fixed at 0.653. Scattering path lengths (ܴ௝ ) were parameterized as a function of the Cu FCC lattice constant, which in turn was treated as a variable and optimized in the fit. Additionally, the mean square displacement factor (ߪ ଶ ) for each path and the overall energy deviation (Δ‫ܧ‬଴ ) were parameterized and optimized.

2.3 Vapor-phase catalytic hydrodeoxygenation Nanoporous Cu-Al-Co alloys were tested for vapor-phase hydrodeoxygenation of furfural or furfuryl alcohol feed in a stainless steel packed bed tubular flow reactor operated at atmospheric pressure. Catalyst loading was fixed at 20 mg, and dried in situ in a thin layer in the center of the reactor tube between packed quartz wool. To prepare the samples, a heating rate of 4K min-1 was



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used with a constant 10 cm3 min-1 H2 flow (Keen Gas). During each reaction, the temperature along the reactor length and on effluent tubing was kept at a constant 483K, 513K, or 543K. Molar flow rate of H2 gas during reaction was set at 60 cm3 min-1, and furfural or furfuryl alcohol was injected at a rate of 0.006 mol h-1 using a NE-1000 syringe pump (New Era). Product distributions were determined through direct measurement of the effluent gas with an inline Shimadzu GC-2010 gas chromatograph equipped with an Agilent HP-INNOWAX column (30 m×0.250 mm, with 0.50 µm film thickness), using He as the carrier gas and a H2-fueled flame ionization detector (FID). Calibration curves relating FID peak areas to molar flow rates of products were determined from injecting mixed standards at set molar flow rates through an empty reactor system (after first verifying that no products could be detected when only H2 was flowing). Effluent streams were also checked periodically with an external mass spectrometer to verify peak assignments, and no tetrahydrofuran or other hydrogenated furan ring products were detected. Product yields were determined by dividing the molar flow rate of each product by the molar flow rate of the reactant stream, while selectivity was calculated by dividing the molar flow of each product by the total molar flow of detected products (excluding the reactant stream). Catalyst W/F loading was based on the total catalyst weight. In all cases, the carbon balance (all detected products and reactants) was found to be greater than 90%.

3. RESULTS AND DISCUSSION 3.1 Synthesis of nanoporous Cu-Al-Co alloys A range of nanoporous Cu-Al-Co alloys with various concentrations of Co were synthesized from arc-melted Al80(Cu20-xCox) (x = 0, 0.2, 1, and 2) ingots, which had been cast into thin


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ribbons through a melt-spinning technique. Due to the very rapid quenching in the melt-spinning process, the resulting alloys are quenched primarily into Al2Cu and pure Al phases, as shown in the wide-angle PXRD patterns (Figure S1). Patterns for Al2Cu and Al standards are also shown for comparison. For the materials with low Co concentration, an additional Al-Cu solid solution phase is observed at ~39°, which is consistent with a shift of the Al(111) peak due to accommodation of the smaller Cu atoms (approximate metallic radii: 143 pm for Al and 128 pm for Cu).35 Partial dealloying in 3M KOH solution removed the majority of Al from the Al80(Cu20xCox)

(x = 0, 0.2, 1, and 2) phases, resulting in highly nanoporous Cu-Al-Co alloys (denoted as

np-Cu-Al, np-Cu-Al-Co1, np-Cu-Al-Co5, and np-Cu-Al-Co10, respectively). The nanoporosities of the as-synthesized Cu-Al-Co alloys were examined by SEM analysis. The SEM images clearly show porous networks (a pore diameter of 10-200 nm) in all four samples (Figures 1a-d), confirming the successful formation of nanoporous Cu-Al-Co alloys. N2 adsorption-desorption measurements were also performed to examine the porosity of as-synthesized nanoporous CuAl-Co alloys. Based on the adsorption-desorption isotherms in Figure S2, both Brunauer– Emmett–Teller (BET) surface areas and average pore sizes were calculated and summarized in Table 1. The values are very similar to the ones recently reported for nanoporous Cu-Ti materials,32 though all of the Cu-Al-Co alloys show a pore size distribution shifted towards large pores and comparatively lower surface areas.



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Figure 1. Typical SEM images for (a) np-Cu-Al, (b) np-Cu-Al-Co1, (c) np-Cu-Al-Co5, and (d) np-Cu-Al-Co10. (e) PXRD patterns for nanoporous Cu-Al and ternary catalysts. Data for bulk Cu is also included for comparison.

Table 1. Structural characterization results for as-synthesized nanoporous catalysts.

Samples

BET surface area 2

(m /g)

BJH average pore size (nm)

Crystallite size

MP-AES metal compositions

(nm)

(atomic %) Cu

Co

Al

np-Cu-Al

39.0

20.4

14.6

99.6

0.0

0.4

np-Cu-Al-Co1

18.7

39.1

10.5

98.1

1.0

0.9

np-Cu-Al-Co5

29.6

29.7

9.4

94.5

4.0

1.5

np-Cu-Al-Co10

22.4

31.2

10.9

90.9

7.5

1.6

The metal compositions of the as-synthesized samples were checked with MP-AES and the results (Table 1) show the Co contents are consistent with the expected nominal values, i.e., 0,


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1%, 5%, 10%. The Al contents in nanoporous Cu-Al-Co alloys increase proportionally with the Co concentration, spanning between 0.4 to 1.6 %. This paired increase is likely due to strong binding between Al and Co atoms in the alloys, which prevents Al atoms from fully leaching during dealloying. The PXRD results for nanoporous Cu-Al-Co alloys confirmed that all the nanoporous alloys share a similar crystalline fcc Cu structure (space group: Fm-3m) and no phase impurity was found (Figure 1e). The average crystallite sizes of the alloy phases were estimated using the Scherrer formula for the nanoporous Cu-Al-Co and the results range from 9.4 nm to 10.9 nm with an exception of 14.6 nm for the np-Cu-Al (Table 1).

3.2 Surface structures of nanoporous Cu-Al-Co alloys The surface properties of as-synthesized nanoporous Cu-Al-Co alloys were characterized using XPS. Despite the fact that the samples were handled very carefully during the synthesis to avoid any exposure to air, the XPS results (Figure 2) still show a significant amount of oxidized Cu, Co, and Al species. The formation of oxidized species is likely due to the presence of dissolved oxygen in the KOH solution during dealloying as well as the highly oxophilic nature of these metals, especially at a nanoscale. The surface compositions as well as the oxidation states of each element are estimated through curve fitting, and the total atomic compositions are summarized in Table 2. For all the alloys, the surface Co composition matches well with the bulk concentration (Table 1), suggesting that the dealloying process did not affect the homogeneous dispersion of Co across the whole alloy.



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Figure 2. (a) Cu 3p and (b) Co 2p XPS spectra for nanoporous np-Cu-Al, np-Cu-Al-Co1, np-CuAl-Co5, and np-Cu-Al-Co10. Table 2. Surface metal composition for each as-synthesized nanoporous catalyst based on XPS analysis. Samples

XPS surface metal compositions (atomic %) Cu

Co

Al

np-Cu-Al

91.0

0.0

9.0

np-Cu-Al-Co1

82.5

1.5

16.0

np-Cu-Al-Co5

76.0

7.0

17.0

np-Cu-Al-Co10

66.0

12.0

22.0

In sharp contrast, the surface Al concentrations (~16-20%) are significantly higher than the bulk Al content (~0.4-1.6%), which indicates the surface segregation of Al during KOH dealloying processes in all the nanoporous alloy cases. In our previous studies of nanoporous Ag 12 ACS Paragon Plus Environment

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and Cu-Ti materials, almost no Al residue was observed in XPS analysis,30, 32 indicating that HCl is more efficient than KOH in removing Al from the alloys. However, the acid dealloying is not possible in current study, as it causes leaching of Co at the same time. It should be noted that the oxidation states of Cu, Co, and Al in the as-synthesized nanoporous alloys are likely different from the states in realistic furfural HDO conditions (i.e., a strong reducing environment). Therefore, operando structural characterization provides important insights, and the results obtained from operando XAS experiments are discussed in the operando characterization section.

3.3 Vapor-phase Furfural hydrodeoxygenation The as-synthesized nanoporous Cu-Al-Co alloys were treated in hydrogen before being subjected to a furfural stream. At a lower reaction temperature of 483K, there is a clear improvement in 2-MF selectivity from 11.7% to 41.5% when Co concentration increases (Figure 3). Remarkably, the high 2-MF selectivities of nanoporous cobalt-containing alloys were achieved together with high total conversions of furfural HDO at 483K, compared to np-Cu-Al catalyst. Considering the surface area of np-Cu-Al is much higher than the nanoporous cobaltcontaining alloys, the results represent more than 200% of enhancement in total conversion by incorporating as less as 1% of Co into the alloy. The decarbonylation product furan was also observed in nanoporous cobalt-containing alloys, while it is undetectable on np-Cu-Al at 483K.



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Figure 3. Vapor-phase furfural hydrodeoxygenation results obtained at 483 K, 513 K, and 543 K. Catalysts were pretreated under H2. Molar flow rate of H2 gas during reaction was set at 60 cm3 min-1 and furfural was injected at a rate of 0.006 mol h-1.

Increasing reaction temperature to 513K significantly enhanced furfural HDO activity preferentially towards 2-MF, whereas the activity towards aldehyde hydrogenation decreased (Figure 3). The np-Cu95Co5-Al catalyst exhibited a 64.9% yield of 2-MF for a remarkable 66.0% selectivity while maintaining a total conversion of 98%. In contrast, the yield of feed furfural to 2-MF over np-Cu-Al was only 28.9%, with a maximum selectivity of 41.8%. The pronounced temperature effect is very likely due to the higher reaction barrier for HDO compared to aldehyde hydrogenation, and similar effects have been observed for other 2-MF selective


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catalysts (such as supported Cu-Co catalysts in liquid-phase furfural HDO).18 It should be noted that although np-Cu-Al exhibited lower overall conversion and 2-MF selectivity than those of the cobalt-containing nanoporous alloys, the performance of np-Cu-Al is already much better than typical supported Cu catalysts under identical reaction conditions,2, 12 in line with previous reports.28 More importantly, no ring-opening product was observed for any of the nanoporous Cu-Al-Co alloy catalysts. Further increasing reaction temperature to 543K resulted in a significant decrease in overall conversion and a high FOL selectivity for most catalysts, although high selectivity towards 2-MF over np-Cu-Al-Co5 is retained. At this elevated reaction temperature, the nanoporous catalysts were significantly destabilized, which can be seen in the 4hour reaction data (Figures S3 and S4).

3.4 Operando X-ray absorption spectroscopic studies To investigate the true structure of the nanoporous Cu-Al-Co alloy catalysts in the reducing conditions of furfural HDO, Co K-edge XAS measurements in a Clausen-type flow polyimide tube flow-through reactor were performed operando at 513 K in a furfural/H2 stream. The spectra recorded during the 7-hour reaction are shown in Figure 4. Prior to reaction, the freshlydealloyed catalyst was loaded in slurry form, dried under He, then reduced under a H2 stream at elevated temperature in order to closely match the conditions described for the vapor-phase furfural HDO described above. Prior to this in situ reduction, the X-ray absorption near-edge spectroscopy (XANES) is a linear combination of metal and oxide Co states, in agreement with XPS data (Figure S5). As demonstrated in the in operando time series of XANES data (Figure 4a), no additional oxidation state changes are observed over the course of reaction.



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Figure 4. Operando XAS results for np-Cu-Al-Co5 in furfural HDO conditions. a) Sequential XANES analysis over the course of 7-hour reaction at 513 K in a furfural/H2 stream, and b) k3weighted EXAFS fitting results for the np-Cu-Al-Co5 before and after reaction. Data: black line, fit: red line.

Furthermore, the EXAFS spectra appear qualitatively similar both before and after reaction (Figure 4b). The spectra are dominated by Co-Cu scattering at a phase-uncorrected radial distance of 2.15 Å. Fitting results of the EXAFS spectra (Table S1 and Figure S6) reveal that the Co-Cu coordination is 11.4±2.5 for the as-prepared catalyst and 12.2±2.6 post-reaction with nearly identical path lengths, indicating that there is no clear deviation from the results expected for isolated Co sites in a Cu FCC lattice. As there are no major structural changes to the catalyst during the full 7-hour reaction considered in this experiment, it is reasonable to assume that the observed degradation of catalytic activity at extended reaction times is likely due to coking and blocking of the Co active sites, rather than widespread structural changes. Interestingly, both Co-Al and Co-O scattering paths are not prevalent in the EXAFS spectra, and do not contribute to the overall fit. For the lack of Co-O, this is explained by the relatively


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large size domains, which drown out surface contributions to the XAS signal, and the reducing environment that is maintained in the operando reaction vessel. For Co-Al, the lack of contributing signal suggests that Co atoms are relatively isolated from Al atoms within a few nearest neighbors. Combined with the MP-AES and XPS results, it is likely that the Co and Al atoms do not preferentially aggregate within the structure, further implying that a three-atom CuCo-Al site is unlikely to be the active site. Further insights into the role of these Cu-Co active sights and impact on the overall mechanism, including reactor studies with a FOL feedstream, are presented in Section SI.1 in the Supporting Information. 363738

4. CONCLUSIONS A series of nanoporous Cu-Al-Co alloys have been successfully synthesized using a dealloying method and their catalytic properties for vapor-phase furfural HDO have been investigated in detail. By incorporating Co into Cu-Al alloy, the nanoporous Co-Cu-Al catalysts showed a significant improvement in both activity and selectivity for furfural HDO to 2-MF. In particular, a 98.2% overall conversion at 513K with a 64.9% yield and a 66% selectivity of 2-MF was achieved using a nanoporous Cu-Al-Co catalyst with approximately 5% Co content. This result represents a significant improvement to the efficiency of non-precious primarily Cu-based furfural HDO catalyst for 2-MF production. Operando XAS studies confirmed the stability of nanoporous catalysts under realistic reaction conditions, and possible explanations for the improved activity of these catalysts over supported Cu have been explored.



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ASSOCIATED CONTENT Supporting Information. PXRD of the precursor materials, N2 adsorption data, selectivity and conversion of furfural for longer TOS, XANES of as-made np-Cu-Al-Co5, EXAFS fitting parameters, q-space EXAFS data, and mechanistic discussion with FOL reactor studies.

AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the US Dept. of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001004.

ACKNOWLEDGMENT G.S.H. and F. J. would like to thank the financial support from the University of Delaware Research Foundation Strategic Initiative (UDRF-SI). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility


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operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors would like to thank Dr. Jingguang Chen at Columbia University and Dr. Nebojsa Marinkovic at Brookhaven National Laboratory for use of the operando reactor.

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