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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 5591−5598

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Characterization of Oxidation States in Metal/Metal Oxide Catalysts in Liquid-Phase Hydrodeoxygenation Reactions with a Trickle Bed Reactor Matthew J. Gilkey, Casper Brady, Dionisios G. Vlachos, and Bingjun Xu*

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Catalysis Center for Energy Innovation, Chemical & Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ABSTRACT: Bifunctional hydrodeoxygenation catalysts containing both metal and metal oxide phases are widely employed in biomass upgrading reactions. Determining the oxidation state of metals in such complex reaction media has been challenging. In this work, we developed a high-pressure trickle-bed reactor capable of conducting temperatureprogramed reduction of catalysts after liquid-phase reactions without exposing the catalyst bed to ambient conditions. Two case studies on metal/metal oxide catalysts employed in key biomass upgrading processes were investigated. The reduction of the RuOx phase in Ru/RuOx/SiO2 occurs at temperatures as low as 115 °C via catalytic transfer hydrogenation reactions using liquid 2-propanol as a hydrogen source. Pretreatment of Ir-ReOx/SiO2 catalyst with H2 in the presence of either liquid cyclohexane or liquid water reduces Re to an oxidation state of +2.6, while the residual ReOx phase cannot be reduced in H2 at up to 900 °C.

1. INTRODUCTION Bifunctional catalysts containing both metal and metal oxide phases have been shown to be effective in the hydrodeoxygenation (HDO) of biomass-derived furanics, e.g., furfural and tetrahydrofurfuryl alcohol, to value-added products.1−13 As a result, recent efforts have been focused on understanding the structure−activity relationships of HDO reactions on metal/metal oxide catalysts.11,14−18 Catalysts in HDO reactions typically operate in reducing conditions, e.g., high H2 pressures and high temperatures, often in the presence of a solvent due to the low volatility of biomass-derived substrates. The exact oxidation state of the metal oxide phase of metal/metal oxide catalysts in these complex, 3-phase reaction systems is often ambiguous because ex situ characterization techniques, e.g., X-ray photoelectron spectroscopy,16,19,20 could lead to misleading conclusions due to the exposure of spent catalysts to the ambient conditions, while in situ or operando characterization techniques are challenging. Operando X-ray absorption spectroscopy (XAS) with custom designed spectroscopic cells can provide accurate assessment of the oxidation state of various elements and the bonding configuration of catalysts in different gas atmospheres and at a wide range of temperatures.21 Operando XAS cells capable of operating at high pressures and high temperatures, either to mimic the liquid phase conditions or to work in the presence of liquids, have been reported.22−26 However, limited access to synchrotron sources creates a demand to develop lab-scale techniques which are capable of determining the oxidation state of spent metal/metal oxide catalysts without air exposure. © 2018 American Chemical Society

In this work, we describe the development of a high-pressure trickle-bed reactor capable of conducting temperature-programmed reduction (TPR) of solid catalysts after pretreatments under conditions at or close to HDO reactions without exposing the catalyst to ambient conditions. This setup allows “freezing” the oxidation state of the HDO catalyst under working conditions by quickly lowering its temperature and switching to an inert atmosphere before conducting TPR. As a result, it could provide quantitative information regarding the oxidation state of the catalyst. The accuracy of the setup was first verified using NiO with and without treatment of a highpressure liquid/gas mixture. Then, two case studies on the characterization of HDO catalysts in biomass upgrading reactions, i.e., Ru/RuOx/SiO2 and Ir-ReOx/SiO2, were conducted. In the former case, we verified quantitatively that the reduction of RuOx phases is facile in the presence of 2propanol at temperatures as low as 115 °C, with liquid 2propanol as an organic hydrogen donor. In the latter case, we determined the oxidation state of Ir and Re in Ir-ReOx after its pretreatment with a high pressure of H2 and a solvent, which is commonly employed to activate the catalyst. Through the comparison with other in situ/operando and ex situ techniques, we show that the setup developed in this work is capable of accurately determining oxidation states of elements in metal/ Received: Revised: Accepted: Published: 5591

February 19, 2018 March 26, 2018 April 3, 2018 April 3, 2018 DOI: 10.1021/acs.iecr.8b00797 Ind. Eng. Chem. Res. 2018, 57, 5591−5598

Article

Industrial & Engineering Chemistry Research Scheme 1. Trickle-Bed Reactor with an on-Line TCD for Catalyst Characterizationa

a

Lines in orange are heat-traced.

produced from reduction of oxides by hydrogen or residual solvent built up in the system, from reaching the TCD and interfering with the quantification. Calibrations of the H2 signal with TCD for quantification were carried out by passing H2 through the TCD at several flow rates while keeping the total gas flow rate (50 mL/min) constant with balancing N2. The TCD signal was then plotted versus H2 composition (from 0 to 5 mL/min H2 at 45 mL/min N2), which resulted in a linear calibration curve. This calibration procedure was done prior to each TPR measurement in this work. To prevent the catalyst bed from being flushed out during treatment or TPR, the catalyst was first pelletized into 20−40 mesh and packed between several layers of inert materials, i.e., one layer of quartz wool, a layer of 1 mm quartz beads, and another layer of quartz wool at the top and bottom of the catalyst bed (Scheme 1). Furthermore, to eliminate dead volume and provide additional security to the catalyst bed, a 1/ 8″ thermocouple was installed in the upper portion of the 1/4″ (outer diameter) reactor tube such that the tip of the thermocouple is in contact with the top layer of quartz wool. The dimensions of the reactor were 19.25 in. (length) by 0.18 in. (inner diameter), where each quartz wool/bead layer was ∼0.5 in. in length. After pelletization and packing, the length of the catalyst bed was often dependent upon the density of the material used but was often around ∼0.3 to 0.4 in. 2.2. Operating Procedures. TPR measurements were conducted with a gas mixture consisting of 10% H2 in N2 (50 mL/min) using a ramp rate of 5 °C/min up to a predetermined temperature (up to 900 °C), followed by a temperature hold for 30 min. The catalyst can be pretreated with a liquid/gas mixture at a predetermined temperature and pressure prior to TPR without being exposed to the ambient environment between the pretreatment and the TPR measurement. After loading the catalyst bed following the aforementioned procedure, sections S1 and S2 (Scheme 1, sealed by V1, V2, VF and V2, VL, VP,

metal oxide catalysts after pretreatment under conditions at or close to those used in the HDO of biomass feedstocks.

2. EXPERIMENTAL SECTION 2.1. Instrument Design. The instrument is based on an up-flow fixed-bed reactor with gas and liquid feeds controlled by mass-flow controllers (MFCs) and an HPLC pump, respectively (Scheme 1). Gas and liquid feeds are combined before the mixture flows upward through a stainless-steel reactor tube containing the catalyst bed. A pressure gauge (PIi) is placed on the front-side of the reactor to monitor pressure buildup at the reactor inlet. In addition, a series of valves, including a purge valve Vp, were installed at the inlet of the reactor to ensure precise control over feed composition. After the reactor tube, a 3-way valve is used to direct the reactor effluent to (1) a condenser and back-pressure regulator (BPR) or (2) a thermal conductivity detector (TCD), which correspond to the two operational modes of the system, i.e., the trickle-bed mode and the TPR mode. In the trickle-bed mode, the instrument functions as a highpressure trickle-bed flow reactor with the catalyst bed in contact with a liquid−gas mixture at a predetermined pressure and temperature. The effluent from the reactor in this mode is directed to a 250 mL condenser to allow for liquid collection and analysis, while the gas stream passes through the condenser and exits the reactor through the BPR (rated to 1000 psi) to be analyzed or vented. A pressure gauge at the back-end of the reactor (PIb) is installed to monitor the pressure drop (ΔP) across the system during reaction. ΔP is less than 20 psi in all experiments reported in this work. The configuration of the condenser followed by the BPR is designed to prevent liquid from reaching the BPR. In the TPR mode, TPR profiles can be collected by directing the reactor effluent through a cold-trap followed by an online TCD. The cold trap (acetone and dry ice mixture) is installed to prevent any condensable compounds, e.g., H2O vapor 5592

DOI: 10.1021/acs.iecr.8b00797 Ind. Eng. Chem. Res. 2018, 57, 5591−5598

Article

Industrial & Engineering Chemistry Research respectively) were first thoroughly purged with N2, and V2 was then closed to allow N2 to purge the reaction system of ambient air. The catalyst bed was purged with N2 (30 mL/min) for ∼15 min in the trickle bed mode with the BPR fully open (VF, V1 open, V2 closed). Meanwhile, a liquid stream was delivered through the purge valve at a flow rate of 0.2 mL/min by the HPLC pump (with VF and VP open) to thoroughly purge section S2 with the solvent of choice. After ∼15 min, the BPR was closed, the liquid stream was introduced to the reactor by opening V2 and closing VP, and the trickle-bed system was allowed to pressurize. After the system pressure reached ∼50 psi greater than the desired operating value, the BPR was then opened slowly until the desired pressure was reached, and the reactor temperature was raised to the desired temperature for catalyst pretreatment and held for 3 h. After the pretreatment, the HPLC pump was stopped under continuous N2 flow, and the system was allowed to cool to room temperature. At this time, pressure was slowly released by slowly adjusting the BPR to the fully open position. To remove the majority of the liquid, the system was purged with N2 overnight (∼12 h) at 30 mL/ min with sections S1 and S2 heated to 80 °C. To further remove residual or adsorbed water, the reactor was heated to ∼100 °C for ∼15 min prior to TPR. The system was then switched to the TPR mode, and TPR profiles were obtained as described above. Prior to packing the catalyst in the stainless-steel reactor tube as described above, the catalyst was pelletized and sieved (20− 40 mesh). NiO (Alfa Aesar, 99%) was mixed with SiO2 to make a NiO-SiO2 mixture (75 wt % NiO) to facilitate pelletization. NiO was pretreated with a water/N2 mixture at 150 °C for 3 h. Ru/RuOx/SiO2 was pretreated with 2-propanol as the liquid feed. Ir/ReOx catalysts were pretreated at 200 °C for 1 h with either cyclohexane or double-deionoized (DDI) water as the liquid feed, and the system was pressurized to 500 psi with H2 rather than N2. No additional thermal treatment was needed to remove residual 2-propanol and cyclohexane after overnight purging with N2 due to their high volatility. 2.3. Sample Preparation. Chemicals used for catalyst preparation in this work were purchased from Alfa-Aesar (NiO (99%), Ru(NO)(NO3)x(OH)y (1.5% in nitric acid), NH4ReO4· xH2O (99%), H2IrCl4 (99%)) and Fuji Silysia (SiO2 G-6) and used without further treatment. Solvents, i.e., 2-propanol (99%) and cyclohexane (98%), were purchased from Sigma-Aldrich. In-house DDI water was also used. All catalysts were prepared by incipient wetness impregnation. M/SiO2 catalysts were prepared by impregnating a measured amount of metal precursor (Ru = Ru(NO)(NO3)x(OH)y, Ir = H2IrCl4, Re = NH4ReO4·xH2O) dissolved in DDI water onto SiO2 with nominal loadings of 4 wt % for Ru and Ir catalysts and 7.8 wt % for Re catalysts (based on the optimal Re loading for Ir-ReOx/SiO2 in ref 20). These samples were dried at 110 °C overnight and calcined at 500 °C in air for 3 h. Ir-ReOx/SiO2 catalysts were prepared by sequential incipient wetness impregnations. SiO2 was first impregnated with a measured amount of aqueous H2IrCl4 and then dried at 110 °C for 12 h. A measured amount of aqueous NH4ReO4· xH2O was then impregnated onto the dried uncalcined Ir/SiO2 catalyst until the point of incipient wetness. The Ir-ReOx/SiO2 was allowed to dry at 110 °C for 12 h followed by calcination at 500 °C for 3 h to generate the fresh catalyst with a nominal loading of 4 wt % Ir and a molar ratio of Re/Ir of 2. This preparation method closely resembles that in ref 20, where the optimal Re/Ir ratio was 2.

3. RESULTS AND DISCUSSION 3.1. Proof of Concept: NiO Reduction in Solvent. The objective of this work is to develop a lab-scale setup to determine the oxidation state of various elements in metal/ metal oxide catalysts employed in liquid phase reactions, and thus it is imperative to demonstrate (1) the quantitative accuracy in the measured H2 consumption from TPR experiments, (2) the complete removal of solvent after the pretreatment/reaction in the presence of liquid solvents prior to TPR measurements, and (3) minimal loss of catalyst during the pretreatment/reaction in the flow of a liquid/gas mixture. NiO, a bulk metal oxide with well-defined oxidation state of the metal, was chosen to verify the accuracy and reliability of the instrument with and without the pretreatment with a liquid solvent. Cyclohexane was employed as the solvent in this test because it is not expected to alter the oxidation state of Ni in NiO at 150 °C, and thus any reduction in the hydrogen consumption after water treatment could be attributed to the loss of catalyst during pretreatment. TPR profiles of NiO obtained from our system agree well with the literature,27−29 with the amount of H2 consumption within 5% of the theoretical value. Fresh NiO (75 wt % in SiO2) exhibits a reduction band centered at 270 °C, which can be attributed to the reduction of Ni2+ to Ni0 (Figure 1, red

Figure 1. TPR profiles of NiO (red) and cyclohexane-treated NiO (blue). Conditions: 100 mg of 75% NiO-SiO2, 5 °C/min with 10% H2/N2 (50 mL/min total), pretreatment conducted at 150 °C under 300 psi H2 under gas and liquid flow (0.2 mL/min H2O and 30 mL/ min N2).

trace). The integrated area of the H2 consumption peak corresponds to a value of 13.2 ± 0.4 mmol H2/gNiO, which is in excellent agreement with the theoretical value of 13.4 mmol/ gNiO. After pretreating NiO under cyclohexane and N2 flow (0.2 mL/min and 30 mL/min, respectively) at 300 psi and 150 °C for 3 h, the TPR profile remains similar compared to the fresh NiO (Figure 1, blue trace), confirming that minimal catalyst was lost during the pretreatment and complete solvent removal can be achieved prior to TPR measurements. Moreover, H2 consumption also remains similar (at 13.0 mmol H2/gNiO), suggesting that the packed catalyst bed remained intact after the pretreatment with a liquid solvent. These control experiments confirm the accuracy and reproducibility of our system. The following sections discuss two case studies in which the method 5593

DOI: 10.1021/acs.iecr.8b00797 Ind. Eng. Chem. Res. 2018, 57, 5591−5598

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Industrial & Engineering Chemistry Research

Figure 2. (a) TPR profiles for various Ru/SiO2 catalysts, showing the effect of 2-propanol pretreatment on the reduction band, as well as (b) the H2 consumption after pretreatment at various temperatures, and (c) the effect of cofeeding furfural during the pretreatment step at 120 °C. Conditions: 100 mg Ru/SiO2, 5 °C/min with 10% H2/N2 (50 mL/min total), pretreatment conducted at temperatures up to 130 °C under 300 psi N2 under gas and liquid flow (0.2 mL/min 2-propanol and 30 mL/min N2).

conducted on the treated catalyst to quantify the oxidation state of Ru after 2-propanol was removed from the reactor system by purging the system with N2 overnight. TPR measurements of fresh Ru/RuOx/SiO2 (without pretreatment with 2-propanol) show a single reduction band centered at ∼142 ± 5 °C (Figure 2), which is attributable to the reduction of Ru2+ to Ru0 and in agreement with previous reports.16,31 A shoulder at ∼160 °C is also visible, suggesting a slightly more stable RuOx phase. A total of 0.65 mmol H2/gcat was consumed on 100 mg of 4 wt % Ru/RuOx/SiO2, which corresponds to an average oxidation state of +1.6 for Ru, consistent with our previous report where Ru-based catalyst was found to contain both metallic and oxide phases.16 TPR measurements show that pretreatment of Ru/RuOx/ SiO2 with 2-propanol drives appreciable reduction of RuOx at above ∼115 °C (Figure 2A). The Ru/RuOx/SiO2 catalyst was pretreated with liquid 2-propanol for 3 h at temperatures up to 130 °C followed by solvent removal and TPR measurements. Pretreatments of Ru/RuOx/SiO2 at 25 and 100 °C have a negligible impact on the TPR profile of the catalyst, in terms of both the peak temperature and the amount of consumed H2, as compared with that without liquid pretreatment (0.59 to 0.65 mmol H2/gcat, Figure 2A and B and Table 1). This indicates that 2-propanol is an ineffective hydrogen donor to reduce the RuOx phase below 100 °C. When the pretreatment temperature reached 115 °C, H2 consumption decreases to 0.46 mmol/gcat, corresponding to a ∼31% reduction of RuOx during the solvent pretreatment. Interestingly, no detectable change in

provides key information regarding the state of catalyst in two important biomass upgrading processes. 3.2. Case Study 1: Reduction of Ru/RuOx/SiO2 in Liquid 2-Propanol. Based on previous reports, mildly oxidized Ru catalysts, e.g., Ru/RuOx/C, are susceptible to irreversible reduction at low temperatures (120 to 160 °C) during catalytic transfer hydrogenation (CTH) reactions using 2-propanol as the hydrogen donor to convert furfural to 2methylfuran (2-MF),9,16 a valuable potential fuel additive. It has been concluded that the coexistence of Ru and RuOx phases is critical to obtaining high yields to 2-MF based on detailed mechanistic assessments from both computational and experimental investigations.14,16,30 Pre- and postreaction characterization as well as catalyst recycling tests indicate that catalyst deactivation is caused at least in part by the reduction of RuOx into metallic Ru (Ru0) under HDO conditions.9,16 The spent Ru-based catalyst after the HDO of furfural is partially reoxidized upon removal from the reactor and exposure to air, and thus, results from ex-situ characterizations, e.g., via XPS, may not provide a truthful representation of the state of the catalyst under working conditions. The trickle-bed reactor developed in this work could pretreat the catalyst at or close to working conditions and characterize the catalyst with TPR without exposing it to air. Thus, the oxidation state of the spent Ru-based catalyst could be obtained. This was accomplished by pretreating the Ru/RuOx/SiO2 catalyst with flowing 2-propanol (0.2 mL/min) under 300 psi N2 (30 mL/ min) for 3 h at reaction temperature. A TPR run was 5594

DOI: 10.1021/acs.iecr.8b00797 Ind. Eng. Chem. Res. 2018, 57, 5591−5598

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Industrial & Engineering Chemistry Research

furfural in 2-propanol are indistinguishable within experimental error (Figure 2c), suggesting that low concentrations of furfural at this temperature are unlikely to affect the stability of the catalyst to any significant degree. Furthermore, the slow reoxidation and fast reduction of the catalyst indicate that at low temperatures over the Ru/RuOx catalyst, the C−O bond scission is likely the rate-determining step in the catalytic cycle. 3.3. Case Study 2: Effect of Pretreating of Ir-ReOx/SiO2 in the Presence of a Solvent. SiO2-supported metal/metal oxide catalysts, e.g., Ir-ReOx, Rh-ReOx, Ir-MoOx, and Pt-WOx, are a class of catalysts with unique activity and selectivity for the ring opening of furanic compounds to produce linear diols.32−36 In the case of Ir-ReOx catalysts, ReOx is reported to be stable under reaction conditions and key to the ring opening reaction.15,37 However, prior to use in catalytic evaluations, the Ir-ReOx/SiO2 catalyst is treated with high pressure H2 in the presence of a solvent, e.g., cyclohexane20 or water,36 at elevated temperatures to achieve optimal activity and selectivity. The reactant is then introduced to the reactor with the pretreated catalyst shielded from air with the solvent. Thus, there is ambiguity in the oxidation state of the Ir-ReOx/ SiO2 catalyst after the pretreatment, which could be clarified by the trickle-bed setup developed in this work. To understand this class of catalysts, Ir/SiO2 (4% Ir by weight), Re/SiO2 (7.8% Re by weight), and Ir-ReOx/SiO2 (4% Ir by weight with Re:Ir = 2) were prepared (see Experimental Section) and systematically evaluated. TPR profiles of freshly calcined Ir/SiO2 and Re/SiO2 catalysts without solvent pretreatment agree well with those observed in the literature (Figure 3a and b).15,37 Although referred to as Ir-ReOx/SiO2, Ir/SiO2, and Re/SiO2 in this work, all metals in these samples are in their oxide form after calcination at 500 °C in air. The TPR profile of Ir/SiO2 shows a broad reduction band at around 225 °C, which can be assigned to the reduction of IrO2 to metallic Ir.2,37 The measured H2 consumption (0.37 mmol H2/gcat) is largely consistent with the amount required to reduce Ir4+ to Ir based on Ir’s nominal loading (0.42 mmol H2/gcat). Re/SiO2 exhibits a sharp reduction feature at around 255 °C, which is consistent with ReOx reduction in the literature.15,37 Previous reports suggest that freshly calcined Re/SiO2 catalysts are largely Re2O7,15,37 corresponding to 1.46 mmol H2/gcat for the complete reduction of Re to its metallic state. During TPR to 900 °C, an H2 consumption of 0.61 mmol H2/gcat was measured, which deviates significantly from the theoretical H2 consumption value based on the complete reduction of Re7+ to its metallic form. This suggests that Re7+ is not fully reduced during the TPR, leaving highly stable, recalcitrant ReOx on the surface with an average Re oxidation state of +2.9 up to 900 °C. Several authors reported similar hypotheses regarding stable ReOx species on the surface;15,37 for example, Deng et al. measured an average Re oxidation state of +4.7 after TPR up to 800 °C.37 This is significantly higher than the oxidation state calculated in this work, which could be due to the nearly 2-fold increase in nominal Re loading (3.8 wt % in Deng et al.37 vs 7.8 wt % in this work). Reduction features in the TPR profile of Ir-ReOx/SiO2 appear at a significantly lower temperature (120−145 °C) than the case for either Ir/SiO2 or Re/SiO2 (Figure 3c), consistent with the observations by Amada et al. and Deng et al.2,37 This reflects the strong interaction between the Ir and Re phases. During reduction, ∼0.92 mmol H2/gcat was consumed, which is significantly lower than the expected H2 consumption

Table 1. TPR Summary of 2-Propanol Treatment of Ru/ RuOx/SiO2 Catalystsa Pretreatment Temp (°C)

Peak Temp (°C)

H2 Consumption (mmol H2/ gcat)

N/A 25 100 115 120 125 130

142 142 143 141 143 144 N/A

0.65 0.63 0.59 0.46 0.47 0.33 0

Conditions: 100 mg Ru/SiO2, 5 °C/min with 10% H2/N2 (50 mL/ min total), pretreatment conducted at temperatures up to 130 °C under 300 psi N2 under gas and liquid flow (0.2 mL/min 2-propanol and 30 mL/min N2). a

the temperature of the main reduction peak of the TPR profile was observed, suggesting that RuOx was reduced to metallic Ru rather than a more reduced form of Ru oxide. Similar observation was made when the catalyst was pretreated at 120 °C, only with a higher fraction (∼49%) of RuOx being reduced during the pretreatment. However, the TPR profile appeared significantly different when Ru/RuOx/SiO2 was pretreated at 125 °C, with a more prominent shoulder at ∼165 °C in addition to the main peak at 144 °C. While the shoulder is visible in all other pretreated Ru/RuOx/SiO2 TPR profiles, it is more prominent after pretreatment at 125 °C, which is likely due to the reduction of more reducible oxide phases during the solvent treatment. After pretreatment at 130 °C, no reduction peak was observed, indicating that RuOx can be completely reduced with 2-propanol as the hydrogen donor at this temperature. These results are consistent with those reported elsewhere, where significant catalyst deactivation was observed when the HDO of furfural was conducted between 120 and 160 °C.14 In light of the multifunctional mechanism for this reaction,11,14 the catalyst deactivation could be attributed to the loss of the Lewis acid sites on RuOx that convert furfural to furfuryl alcohol and the lack of vacancies on the surface of the reduced catalyst that effectively deoxygenate the furfuryl alcohol to 2-methylfuran. To verify that no reoxidation of Ru occurs during the solvent-treating and removal process, a control experiment was performed with cyclohexane, an inert solvent. After Ru/RuOx/ SiO2 was loaded into the reactor, a TPR was performed to 300 °C to completely reduce RuOx. Then, a cyclohexane (0.2 mL/ min) and N2 (30 mL/min) mixture was introduced to the reactor and treated at 150 °C and 300 psi for 3 h. After removal of the solvent, a new TPR profile was acquired and no reduction features were observed at around 145 °C (Figure 2a, gray trace). This demonstrates that no air was introduced into the reaction system during the introduction of a liquid solvent and pretreatment period to oxidize Ru0. The presence of oxygenates, e.g., furfural and furfuryl alcohol, could suppress or even stop the reduction of RuOx by oxidizing the reduced oxide phase during the HDO reaction proceeding via a vacancy-mediated mechanism.14 To test this possibility, a pretreatment was conducted with 1 wt % furfural in 2-propanol at 120 °C for 3 h. 120 °C was chosen because partial reduction of RuOx was observed when pretreated with pure 2-propanol at this temperature, and any change in the H2 consumption or the line shape of the TPR profile could be attributed to furfural. TPR profiles of Ru/RuOx/SiO2 pretreated with and without 5595

DOI: 10.1021/acs.iecr.8b00797 Ind. Eng. Chem. Res. 2018, 57, 5591−5598

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H2O, 30 mL/min H2) was passed over a freshly calcined IrReOx/SiO2 catalyst bed for 1 h at 500 psi and 200 °C. After removing the solvent from the system, a TPR profile was collected. No reduction peaks were observed in the temperature range tested (25 to 900 °C, Figure 3d). This suggests that prereducing Ir-ReOx/SiO2 in the presence of solvent is sufficient to reduce the ReOx species formed during calcination, except for the recalcitrant ReOx phase. In our recent work, a similar pretreatment step employed cyclohexane as the solvent.20 To test whether the solvent plays a significant role, an identical experiment was conducted, replacing DDI water with cyclohexane. After treatment at 200 °C, cyclohexane was removed from the system, and the TPR was measured. Similar to the case where water was used, no TPR peaks were observed, suggesting the catalyst was similarly reduced during the pretreatment step independent of the solvent choice (Figure 3e).

4. CONCLUSIONS We have developed a system capable of assessing the oxidation state of metal/metal oxide catalysts after high pressure liquidphase reactions under conditions relevant to biomass upgrading. We first demonstrated the system reliability in conducting quantitative TPR by employing a metal oxide (NiO) with a well-established oxidation state, which generated TPR profiles consistent with the literature with and without a pretreatment with an inert solvent (cyclohexane). We then demonstrated the ability in evaluating complex metal/metal oxide catalysts, such as Ru/RuOx/SiO2 and Ir-ReOx/SiO2. In the former case, we employed 2-propanol as a solvent, which is known to participate in catalytic transfer hydrogenation (CTH) to reduce the catalyst over time at high temperature and pressure. The onset of reduction of RuOx phases is ∼115 °C, with complete reduction occurring at 130 °C. In the latter case, we confirmed that pretreatment of the catalyst in the presence of solvent completely reduces Ir and partially reduces the ReOx phase in the Ir-ReOx/SiO2 catalyst below 200 °C.

Figure 3. Temperature-programmed reduction profiles for (a) Ir/ SiO2, (b) Re/SiO2, (c) Ir-ReOx/SiO2, (d) Ir-ReOx/SiO2 after prereduction in water and (e) Ir-ReOx/SiO2 after prereduction in cyclohexane. Conditions: TPR measurements conducted with ∼100 mg catalyst in a 10% H2/N2 mixture with a heating rate of 5 °C min−1. Pretreatment for traces (d) and (e) conducted at 200 °C for 1 h under flow conditions (0.2 mL/min liquid, 30 mL/min H2) at 500 psi.

(1.88 mmol H2/gcat) needed to completely reduce Ir and Re species to their metallic form. In this case, it is likely that Ir4+ is completely reduced to Ir0 while ReOx is partially reduced to form the stable, recalcitrant ReOx species as in the TPR of Re/ SiO2 discussed above. The assumption of complete reduction of IrO2 is consistent with reports found in the literature, where only metallic Ir is observed by XPS after reduction at 200 °C,20 and Ir−O shells are not observed by in situ EXAFS.15 Assuming IrO2 is completely reduced during TPR, the oxidation state of Re in Ir-ReOx/SiO2 after reduction is estimated to be +2.6 based on the H2 consumption, which is consistent with the residual phase after TPR of Re/SiO2. This is reasonable given that the recalcitrant ReOx species is stable under H2 at 900 °C, and the presence of Ir is unlikely to significantly change its redox stability. Our computed Re oxidation state is consistent with literature observations that show the existence of both Re2+ and Re4+ via XPS after reduction at 200 °C.18 Similarly, in situ EXAFS data suggests that Re maintains a +1.7 valence during reduction up to 625 °C,21 which is in qualitative agreement with our data. The stability of the recalcitrant ReOx phase suggests that it is highly unlikely to participate in the HDO reaction of biomass feedstocks (typically