- Email: [email protected]
Fe: A mechanistic study
Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Liquid-phase hydrodeoxygenation of lignin-derived phenolics on Pd/Fe: A mechanistic study ⁎
Jianghao Zhanga, Junming Suna, , Berlin Suddutha, Xavier Pereira Hernandeza, Yong Wanga,b, a b
⁎
The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, United States Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA, 99352, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Tautomerization Reaction mechanism Liquid-phase hydrodeoxygenation Pd/Fe Lignin-derived phenolics
Although Pd/Fe bimetallic catalysts have been extensively studied in vapor-phase hydrodeoxygenation of phenolics (i.e., guaiacol, cresol), little is yet known about their performance in liquid-phase reactions. In this work, we present a mechanistic study on the Pd/Fe bimetallic catalysts in liquid-phase hydrodeoxygenation of phenolics. The role of tautomerization in hydrodeoxygenation of the lignin-derived phenolics, particularly for ring saturation, is systematically studied by employing two representative modeling compounds: phenol (a molecule that is keto-enol tautomeric) and diphenyl ether (a molecule that does not allow ketol-enol tautomerization). It was found that although the addition of oxyphilic Fe inhibits the direct aromatic ring saturation reaction typically occurring on Pd, tautomerization opens another reaction pathway toward ring saturation products (i.e. cyclohexanone, cyclohexanol, cyclohexane et al.), where both tautomerization and the hydrogenation of keto isomers are significantly enhanced to produce cyclohexanol followed by direct hydrogenolysis of the cyclohexanol to cyclohexane.
1. Introduction The diminishing fossil resources initiated great efforts seeking for the sustainable energy [1]. Among those, one promising solution is to produce the renewable fuels and chemicals from the earth-abundant and inexpensive lignocellulosic biomass [2–4], which mainly consists of cellulose, hemicellulose and up to 30 wt.% lignin [5]. Pyrolysis of biomass is a feasible process that produces the bio-oil containing hundreds of oxygenates subjecting to further upgrading [6–8]. One of the most critical processes for this upgrade is catalytic hydrodeoxygenation (HDO) [9]. A series of HDO catalysts have been extensively studied for the past decades [9–13]. Traditional HDO catalysts were based on metal sulfides [7,14], which yield high arene selectivity in the conversion of phenolics [15,16]. A common issue with sulfide catalysts is the contamination of sulfur in products [17]. To this end, more and more researches have been focusing on the development of other alternative catalysts including supported noble metal catalysts such as Pt-Mo/ MWCNT [5], Pd [17–19], Pt [20,21], Pd/Nb2O5/SiO2 [22], Ru/H-Beta [23], Pt-Zn [24], base metal catalysts such as FeMoP [25], Ni [26,27], Fe [28,29], Ni-Fe [30], MoO3 [31,32], and metal carbides such as WCx [33], MoCx [12,34]. Depending on the catalysts employed, several reaction pathways
have been proposed in the HDO of phenolic compounds: direct CeO bond cleavage/hydrogenolysis, direct aromatic ring hydrogenation, and keto-enol tautomerization [35], as summarized in Scheme 1 for the case of phenol. While direct CeO bond cleavage produces benzene (black color highlighted), direct aromatic ring hydrogenation will lead to the formation of cyclohexane (red color highlighted). In the former pathway, the adsorbed phenol on catalyst is activated by the dissociation on the surface via cleavage of CeO bond, being followed by hydrogenation of phenyl and hydroxyl radical to produce benzene and water. In the latter case, adding the first hydrogen atom to the phenyl ring was calculated having the highest barrier in the direct aromatic ring hydrogenation over Ni(111) and Pt(111), [35,36] leading to the primary formation of cyclohexanol intermediate rather than enols such as cyclohexadienol and cyclohexenol [37]. Then, cyclohexanol can be converted to cyclohexane by: 1) direct CeO bond cleavage or hydrogenolysis or 2) dehydration to form cyclohexene followed by hydrogenation of cyclohexane. It should be noted, although dehydrogenation of cyclohexene to benzene has been shown feasible in the presence of phenol with 2-propanol as H-transfer initiator and H donor [38], this reaction is thermodynamically unfavorable in the presence of high pressure H2. It is thus believed the contribution for benzene production from cyclohexene dehydrogenation is minimum. Tautomerization (blue
⁎ Corresponding authors at: The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, United States. E-mail addresses: [email protected] (J. Sun), [email protected] (Y. Wang).
https://doi.org/10.1016/j.cattod.2018.12.027 Received 5 August 2018; Received in revised form 16 November 2018; Accepted 12 December 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Zhang, J., Catalysis Today, https://doi.org/10.1016/j.cattod.2018.12.027
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Scheme 1. The proposed reaction pathways for HDO of phenol.
phase [46], while only ring-saturation products were detected in the HDO of phenol at 250 °C in liquid phase [17]. Recently, our group reported the vapor-phase HDO of guaiacol over carbon-supported Pd-Fe catalyst [54], which exhibited enhanced activity and arene selectivity without ring saturation. The synergic effect between Pd and Fe was further verified using Pd-promoted Fe nanoparticles for vapor-phase HDO of m-cresol, which displayed > 90% of arene selectivity [46]. Herein, we further studied the HDO of phenolics on the same Pd/Fe catalyst in liquid-phase reactions, and attempted to understand the reaction mechanisms involved.
color highlighted) is another important reaction pathway since it has the lowest calculated reaction barrier (e.g. 0.78 eV for cresol [39] on PtMo catalyst and 0.52 eV for catechol [40] on Pt catalyst) among the three proposed reaction pathways shown in Scheme 1. After the ketoenol tautomerization, the subsequent hydrogenation of the tautomerized enone could have two competitive secondary reactions: one is hydrogenation of C]O bond of the enone to cyclohexadienol, followed by an acid catalyzed dehydration of cyclohexadienol to form benzene as proposed on Pt/SiO2 and Ni-Fe by Nie et al [21,30]. The other is a consecutive hydrogenation of C]C and then C]O bond in enone to produce cyclohexanone and cyclohexanol. The obtained cyclohexanol could be further converted into cyclohexane by either dehydration/ hydrogenation as proposed on the Pd/Al2O3 with metal-acid bi-functionality by Souza et al, [41] or direct hydrogenolysis of cyclohexanol although it has rarely been accepted yet. It was found that the reaction pathways and the consequent products were affected by the adsorption configuration of phenols [42], i.e. horizontal and vertical adsorption. The vertical position (taking place on MoS2 et al. catalysts) is beneficial to direct CeO bond cleavage, while the horizontal position favors hydrogenation of the aromatic ring [43]. In the case of noble metal catalysts where horizontal adsorption is the most favorable configuration [35,40,44], the direct CeO bond cleavage of p-cresol over Pt(111) has a higher reaction barrier (2.45 eV) than that of direct hydrogenation (1.11 eV) [35]. Interestingly, addition of an oxophilic metal on a noble metal (i.e., PtZn) was also reported to change adsorption of aromatic ring by tilting it away from Pt surface, leading to a favorable CeO bond cleavage [24]. In addition to the adsorption configuration, the reaction pathway may also be influenced by the interaction between the functional group and catalyst surface. For example, it has been reported that guaiacol has a stronger adsorption on Pd(111) and Fe(110) which distorts the CeO bond in guaiacol to a greater degree [45], and is likely the reason that Fe favors the CeO bond cleavage while Pd favors the ring hydrogenation [46]. Other previous studies also indicate that the dominant reaction pathway also depends on many parameters including catalysts [47,48] and reacting conditions such as pressure [5,49,50], temperature [25] and solvent/ additive [51,52]. For example, Nie et al. [30] reported a 47.5% selectivity to ring saturation products in vapor-phase HDO of m-cresol over 5 wt.% Ni/SiO2 at 0.1 MPa and 300 °C. With the same 5 wt.% Ni/ SiO2 catalyst, another study conducted by Mortensen et al. [53] reported 100% selectivity to ring-saturation products in the liquid-phase HDO of phenol at 10 MPa and 275 °C. Similarly, Pd/C catalyst displayed around 75% selectivity to arene in the HDO of cresol at 300 °C in vapor-
2. Experimental 2.1. Catalyst preparation 2.1.1. Preparation of support The Fe2O3 support was prepared with precipitation method as previously reported [46]. Typically, the (NH4)2CO3 (Sigma-Aldrich, 99.999%, 1.5 M) solution was added dropwise to a solution of Fe(NO3)3 (Sigma-Aldrich, > 99.99%, 3 M) under stirring, forming dark crimson slurry. The precipitate was then washed with Milli-Q water by filtration until the pH reached to 8. After being dried at 80 °C overnight and sieved to < 100 mesh, the sample was calcined at 400 °C for 5 h to achieve the Fe2O3 support. 2.1.2. Preparation of supported 1 wt.% Pd catalysts The 1 wt.% Pd/Fe2O3 was prepared using incipient wetness impregnation method. Pd(NH3)4(NO3)2 solution (Sigma-Aldrich, 10 wt.% solution with 99.99% metal based purity) was diluted by certain amount of Milli-Q water such that the concentration and volume of the precursor solution matched with pore volume of the support as well as the metal loading of the obtained catalysts. The diluted solution was then added dropwise to Fe2O3 powder, followed by drying at 80 °C overnight in air and calcination at 350 °C for 2 h in flowing N2 (50 mL/ min). 2.2. Characterization In situ X-ray powder diffraction (XRD) experiments were done in a Rigaku SmartLab X-ray equipped with an XRK900 in situ reactor and a DTex high-speed detector that allows for higher signal-to-noise ratio at high scan rates. The diffraction spectra were collected between 10° and 80°, at 5°/min at a step of 0.01°. The radiation source was Cu K-α with a 2
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
were sampled at 25 °C and compared with those sampled at the reaction temperatures. No significant deviation (< 10%) in terms of both conversion and selectivity was found after this correction. The products were analyzed on a gas chromatograph (GC, Agilent 7890 A) equipped with a DB-FFAP column (30 m, 0.32 mm, 0.25 μm) and flame ionization detector (FID), as well as a GC–MS (Shimadzu, GCMS-QP2020) equipped with a HP-5 column (30 m, 0.32 mm, 0.25 μm) connected to a FID. The conversion, selectivity and yield were defined as follows: conversion [%] = (moles of carbon in the reacted substrate/moles of carbon in the substrate fed) × 100; selectivity [%] = (moles of carbon in the specific product/moles of carbon in all product) × 100; yield [%] = (moles of carbon in the specific product/ moles of carbon in the substrate fed) ×100. The carbon balance was usually in the range of 92%–100%.
3. Results 3.1. Characterization of the catalysts The characterization of fresh and reduced/passivated 1 wt% Pd/ Fe2O3 samples show general consistency with our previous study [46]. For the fresh sample, Pd or PdOx is highly dispersed on the Fe2O3 support that has a uniform size distribution at ∼20 nm (Supplementary material Fig. S1). The in situ XRD pattern of the fresh sample shows a hematite (JCPDS: 33-0664) crystalline structure, while metallic Fe (JCPDS: 06-0696) and magnetite (JCPDS: 19-0629) dominate on the reduced catalyst [46]. No diffractions characteristic of Pd were observed, suggesting the highly dispersed nature of Pd. In situ Raman spectra were also studied and compared on the 1 wt% Pd/Fe2O3 catalysts before and after the reduction at 350 °C, as shown in Fig. 1. Spectrum of fresh Pd/Fe2O3 (Fig. 1b) showed the typical peaks of hematite [55,56], consistent with the XRD analysis. After reduction, only a small hump at 655 cm−1, characteristic of FeO was detected, due to dominant formation of Raman silent metallic Fe species (Fig. 1b). These characterization results suggest that a FeOx core (incomplete reduction) and metallic Fe shell structure was formed on the reduced catalysts. In consistent with this deduction, our previous XPS study showed only surface metallic Fe [46]. It is thus concluded that, the catalysts shown in this work is representative of our previous observations that the surface structure of the catalyst is Pd modified metallic Fe [46].
Fig. 1. In situ-XRD patterns for fresh and reduced 1 wt% Pd/Fe2O3 (a), and in situ Raman spectra for fresh and reduced 1 wt% Pd/Fe2O3 (b). Reduction condition: 350 °C with 50% H2 (40 mL/min, balanced by N2) for 2 h.
wavelength of 0.154056 nm. Approximately 0.1 g of catalyst was loaded into the sample holder. A scan was then collected at RT under flowing N2 (40 ml/min) which was set as fresh sample. After that, the sample temperature was increased to 350 °C at a ramping rate of 10 °C/ min in flowing 50%H2/He (40 ml/min), and then held at the temperature for another 2 h. Another scan was then taken before the sample was cooled down to ambient temperature. Raman spectra were collected on a Horiba LabRAM HR Raman/ FTIR microscope equipped with a 532 nm (Ventus LP 532) laser source and Synapse Charge Coupled Device detector (CCD). After calibration using the silica reference, the sample was loaded in an Linkam CCR1000 in situ reactor, and a spectrum of the sample was then taken at room temperature. The sample was then reduced by heating to 350 °C (ramp rate of 10 °C/min) and holding the temperature for 2 h under flowing H2 (10 ml/min), after which it was cooled to room temperature before collecting the spectra of the reduced samples.
3.2. Comparison of HDO of m-cresol on Pd/Fe: vapor phase vs liquid phase
2.3. Activity test
We have shown in our previous studies that the Pd/Fe catalyst exhibited > 90% selectivity to arene in the vapor-phase m-cresol HDO, and Pd played essential role in enhancing the reducibility of Fe and maintaining the active metallic iron phase [46]. Since the liquid-phase reaction has the advantage in minimizing the energy consumption, for comparison, the performance of the Pd/Fe catalyst was also investigated and compared in liquid-phase reaction conditions. As such, the N2 diluent employed in vapor phase was replaced with hexadecane in liquid-phase reactions. Fig. 2 shows a comparison of the catalytic performances of Pd/Fe at comparable cresol conversion in the two phases. The high arene selectivity in vapor phase dramatically decreased to ∼2% in the liquid phase. This observation is not surprising since similar observations were also reported on other catalysts reported by different research groups [17,30,31,46,53,57](Supplementary material Table S1). It is should be noted, however, no evidence has been shown to unambiguously interpret why the reaction pathway shifted toward aromatic ring saturation in liquid phase, although it has been reported that hydrogen pressure and temperature strongly affect the proportion of arene in the products [5,25,49]. This motivated our further studies on the reaction pathways aiming to understand the factors that control the product distribution.
The HDO reaction of the modeling substrates were carried out in a stainless steel Parr reactor (300 mL) equipped with a glass liner. Typically, the catalyst was first reduced ex-situ with 40 ml/min 50% H2/N2 at 350 °C for 2 h followed by a passivation with 40 ml/min 0.2% O2/N2 at room temperature for 1 h. The passivated catalyst was then loaded into the reactor. After 3 times purging with 4 MPa H2, the temperature was ramped to 300 °C (15 °C/min) where it was held for another 30 min in H2 (3 MPa) for in situ reduction. After the reduction, the temperature was cooled down to ambient temperature where the pressure was decreased to ambient, and 6.35 mmol modeling compound (Sigma-Aldrich, ≥ 99%) in 50 mL hexadecane (Sigma-Aldrich, 99%) were then injected with syringe into the reactor. After one time purging H2 and pressurized with H2 to 4 MPa, the mixed catalyst and substrate solution were heated up to the target temperature at a stirring rate of 800 rpm for the reaction. The products were sampled periodically for analysis at the reaction temperature. Given the variable composition with temperature in the liquid phase, a linear correction assuming a constant coefficient in the Henry’s law was made in the concentration range studied. After the reaction, some of the analysis 3
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Fig. 2. Comparison of catalytic performances of Pd/Fe in vapor-phase and liquid-phase HDO of m-cresol. Reaction condition: 300 °C, 0.04 MPa H2 and 0.06 MPa N2 for vapor-phase reaction (data adapted from the previous study [46]); 300 °C, hexadecane as solvent, 4 MPa H2 for liquid-phase reaction.
3.3. Reaction pathway in liquid-phase hydrodeoxygenation of phenol on Pd/Fe To better understand the enhanced hydrogenation of aromatic ring of phenolics in liquid-phase reactions, the simplest phenolic, phenol, was further studied in the HDO reaction. Fig. 3a shows the evolution of phenol conversion and product yields as a function of time at 300 °C and 4 MPa (at 25 °C) H2 pressure, and the product selectivity versus phenol conversion is replotted in Fig. 3b. Similar to the m-cresol reactions, ring saturation is dominant in liquid phase, leading to the formation of cyclohexane as a major product along with small amounts of cyclohexanol, cyclohexene, and cyclohexanone. With time-on-stream (TOS), cyclohexanol first increases and then decreases with its highest yield being observed at around 30 min. The yield to cyclohexene exhibited a similar trend as that of cyclohexanol albeit at a lower yield. This observation implies that the cyclohexanol and cyclohexene are the intermediates to cyclohexane during hydrodeoxygenation of phenol. Indeed, the selectivity to cyclohexane increases at the expenses of cyclohexanol and cyclohexene. At extrapolated zero conversion analyzed using SPSS 22.0 software [58], cyclohexanol selectivity surpasses the cyclohexane and reaches up to above 50%, an indication of primary product in HDO of phenol under liquid-phase reaction conditions. The total selectivity to ring saturation products reaches up to 98.4%, leaving only 1.6% selectivity to benzene via direct CeO bond cleavage. Two reaction pathways could lead to cyclohexanol formation: one is direct hydrogenation of aromatic ring of phenol, and another is keto-enol tautomerization followed by hydrogenation [35], as shown in Scheme 1. Given the much higher activation barrier for direct aromatic ring saturation than that for tautomerization calculated on phenolics [35,40], it is believed that the primary formation of cyclohexanol should be from the latter (i.e., tautomerization). Note that, no other intermediate (i.e. 2, 4-cyclohexadienone) but only trace amount of cyclohexanone was detected, and its selectivity barely shows any dependence on the phenol conversion. This could be due to a high hydrogenation reactivity of the intermediate at a relatively high temperature (i.e., 300 °C). Indeed, when the reaction temperature decreased from 300 °C to 200 °C (Fig. 4), appreciable amount of cyclohexanone was produced, and its selectivity increased rapidly as phenol conversion decreased close to zero. It should be noted that the secondary cyclohexanol reactions are also very limited at low reaction temperature (i.e., 200 °C), evidenced by the high selectivity above 90% regardless of phenol conversion. To further confirm the relatively higher reactivity of cyclohexanone than that of cyclohexanol, cyclohexanone and cyclohexanol were used as substrates and tested on the
Fig. 3. (a) Phenol conversion and product yields over Pd/Fe catalyst as a function of time-on-stream. (b) Product distribution as a function of conversion. Reaction condition: 300 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.15 g catalyst, 4 MPa H2.
Fig. 4. Production distribution as a function of conversion over Pd/Fe catalyst. Reaction condition: 200 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.3 g catalyst, 4 MPa H2.
Pd/Fe catalyst under the same reaction conditions (Table 1). It is clear that the conversion rate for cyclohexanone is much faster than that of cyclohexanol. It is thus hypothesized that, over the Pd/Fe bimetallic catalysts, majority of the aromatic ring saturation should be from 4
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Table 1 Catalytic performances of Pd/Fe catalyst in converting possible intermediates. Reaction condition: 200 °C, 6.35 mmol substrates, 50 mL hexadecane, 0.3 g catalyst, 4 MPa H2. Substrate
Benzene Cyclohexanol Cyclohexanone
Time
4h 4h 10min
conversion
10.0 9.3 67.0
Selectivity C6H12
C6H10
Benzene
C6H12O
Phenol
C6H10O
99.4 64.2 0.2
0.6 31.5 trace
– 0.0 0.0
0.0 – 99.8
0.0 0.0 0.0
0.0 0.0 –
tautomerization reaction pathway. Diphenyl ether (DPE) is a lignin-derived compound with 4-O-5 linkage, which is the strongest ether bond in lignin [59]. As one of the lignin-derived model compounds, DPE has also been widely studied in the HDO reactions [60]. Note that the structure of DPE with two symmetric Caromatic-O bonds is unique, making the keto-enol tautomerization impossible within this molecule [61]. Based on the proposed reaction pathways [62], it is anticipated that, unlike phenol, DPE should only proceed with two primary reactions on the Pd/Fe catalysts in organic phase: direct ring hydrogenation and direct CaromaticeO bond cleavage [63]. A comparison of HDO of phenol and DPE could help differentiate the contribution of tautomerization to the ring-saturation products. It should be mentioned that although anisole could be considered as an alternative model compounds for this purpose, the presence and easy hydrogenolysis of Caliphatic-O [11,54,64] could lead to the facile formation of phenol and thus complicate this study. To confirm our hypothesis that tautomerization rather than direct aromatic ring hydrogenation is dominating toward ring saturation products on Pd/Fe catalyst, we further studied the HDO of DPE (Supplementary material Fig. S2), which was performed under the same reaction conditions as that of phenol. In this particular case, if direct ring hydrogenation occurs, cyclohexyl phenyl ether (CPE) and dicyclohexyl ether (DCE) primary products are expected. Whereas, direct CeO bond cleavage (i.e., hydrogenolysis) will result in benzene and phenol primary products. Fig. 5 depicts the product distribution as a function of conversions. Benzene and cyclohexane/cyclohexanol (secondary products from phenol) from direct CeO bond cleavage are always dominant in the products. At extrapolated zero conversion where secondary reaction is minimized, ∼94% selectivity to direct CeO bond cleavage was observed. In contrast, selectivity to CPE is only ∼6% and no DCE is observed, suggesting the low aromatic ring hydrogenation activity on Pd/Fe. This result confirms that aromatic ring saturation products in the phenol conversion over Pd/Fe catalyst were mainly from tautomerization/hydrogenation rather than direct aromatic ring hydrogenation. It is worth mentioning that, as one of the primary CeO cleavage product, phenol was not detected in the diphenyl ether conversion, indicating that the CeO bond cleavage during the DPE conversion is the kinetic relevant step, matching well with the high activation energy of the CaromaticeO bond [39]. Moreover, our results (Fig. 3) showed that cyclohexane is always dominant in the final products for the HDO of phenol, especially at high phenol conversion. It is thus expected that cyclohexane is the other major products during the HDO of DPE (Fig. 5 and Fig. S2) for the relatively facile secondary phenol conversion. In addition, as DPE conversion increases (longer reaction time), cyclohexane increases at the expense of benzene. It suggests secondary benzene hydrogenation occurs on the Pd/Fe catalysts, which is also confirmed at low reaction temperature (Table 1). However, it does not affect our conclusion that during the HDO of phenol, the facile keto-enol tautomerization opened a pathway to readily form enone from phenol and played the major role in the aromatic ring saturation reaction under liquid-phase reaction conditions.
Fig. 5. (a) Product distribution as a function of conversion over Pd/Fe. (b) Distribution of classified products as a function of conversion. Reaction condition: 300 °C, 6.35 mmol diphenyl ether (1 mL), 50 mL hexadecane, 0.15 g catalyst, 4 MPa H2.
liquid-phase HDO reaction, particularly for inhibiting the direct aromatic ring hydrogenation reactions, the reaction mechanism is significantly different from that in vapor-phase reaction. In the vaporphase reaction at atmospheric pressure (H2 partial pressure 0.04 MPa), majority of the final products are arene whereas ring-saturation products (mainly cyclohexane for phenol) dominate in the liquid-phase reaction at ∼6 MPa H2 (at 300 °C). More importantly, our experimental results explicitly revealed ring-saturation products in the HDO of phenol on PdFe catalysts were mainly from tautomerization pathway. The enhancement of tautomerization and thus ring saturation in the liquid-phase HDO of phenol likely results from two factors: hydrogen partial pressure and solvent effect. In the former case, the H supply and thus further hydrogenation of tautomerized enone could be limited by the low hydrogen pressure in the vapor-phase conditions. In liquid phase, although H2 solubility could be low in hexadecane, the high
4. Discussions Although the synergistic effect on Pd/Fe still plays key roles in the 5
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Fig. 6. The influence of H2 pressure to the reaction rate of benzene production and ring saturation products. Reaction condition: 300 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.15 g monometallic Fe as catalyst. Note that this experiment was done on monometallic Fe to minimize the direct ring hydrogenation observed on Pd/Fe albeit low, which can complicate this verification.
Fig. 7. Product distribution as a function of conversion over Pd/Fe. Reaction condition: 300 °C, 6.35 mmol cyclohexanol, 50 mL hexadecane, 0.15 g catalyst, 4 MPa H2.
on the less or non-acidic Pd/Fe catalyst, which is different with the acid catalysts showing dehydration as the main pathway [17]. Based on the above results, the reaction pathway for liquid-phase HDO of phenolics can be proposed over Pd/Fe catalysts. For the case of phenol, as shown in Scheme 2, two parallel routes exist: direct CeO bond cleavage and tautomerization followed by ring saturation. Different from the vapor-phase HDO of phenolics where direct hydrogenolysis of CaromaticeO bond prevails, ring saturation becomes the major reaction pathway in liquid phase. The fact that ring saturation pathway is dominant in liquid phase is mainly due to the enhanced tautomerization reaction pathway toward cylcohexanol, followed by a direct hydrogenolysis of the cyclohexanol to cyclohexane.
hydrogen pressure could offset this [66] and thus ascertain an efficient H supply on the catalyst surface [50]. Therefore, the enone hydrogenation could be enhanced in liquid-phase reaction conditions. Indeed, Yohe et al. [5] reported a significant influence of H2 partial pressure on the product distribution. The aromatic hydrocarbon reaches up to 93.2% at 0.1 MPa H2, whereas this selectivity dramatically decreased to be less than 3% at 2.3 MPa H2. Note that the Caromatic-O bond cleavage has been identified as the rate-determining step in the HDO of phenol to produce benzene [67], which means, if this is the case, the improved H supply should have negligible effect on the formation rate of benzene. In other words, given the unaltered rate for benzene formation, the selectivity to ring saturation products in liquid phase reaction conditions should be improved by an enhanced kinetics for the parallel tautomerization pathway. To verify this, the HDO of phenol was tested at different H2 pressures. For a better comparison of kinetics, the conversion was controlled below 15%. As shown in Fig. 6, the rate to produce benzene remains almost constant (∼0.13 mmol/g/h) when the H2 pressure increases from 1 MPa to 4 MPa, whereas the rate to ringsaturated products increases dramatically from 0.38 mmol/g/h to 1.11 mmol/g/h, leading to decreased benzene selectivity from 26.6% to 9.8%. This result confirms that the enhanced tautomerization pathway is indeed facilitated by hydrogen supply in liquid-phase conditions. In addition, a recent DFT study also showed that, although the barrier for tautomerization in vacuum condition is higher than direct Ce–O bond cleavage, this barrier significantly decreases in the presence of water solvent due to the nearly barrierless proton transfer [65]. In the current case with batch reaction, the generated water molecule may adsorb/readsorb on the catalyst surface, which in turn facilitated the tautomerization. It can be concluded that, under liquid-phase conditions, the improved formation of ring-saturation products is mainly due to a combination of enhanced tautomerization by water solvent and hydrogenation of tautomerized enone by H supply. To complete the whole pathway, conversion of cyclohexanol was also studied under the same reaction condition as that of phenol (Fig. 7). As expected, both cyclohexane and cyclohexene are observed. As cyclohexanol conversion decreases, the cyclohexene selectivity increases slowly at the expense of cyclohexane. Regardless, selectivity to cyclohexane are always above > 90% selectivity. Even at extrapolated zero conversion, the cyclohexane selectivity is still much higher than cyclohexene. This result indicates that two parallel reactions occur in the cyclohexanol reaction on the Pd/Fe catalysts, namely the direct C–O bond cleavage (hydrogenolysis) to produce cyclohexane and the dehydration of cyclohexanol to form cyclohexene followed by hydrogenation. The former should be the major pathway (> 90% selectivity)
5. Conclusions By using phenol and DPE as the model compounds, HDO of phenolics has been carefully studied on Pd/Fe catalyst in liquid-phase, in an attempt to elucidate the distinct product distribution from those in vapor-phase conditions. Our results suggest that, although the direct aromatic ring saturation observed on Pd is inhibited by Fe (i.e., on Pd/ Fe bimetallic catalyst), the tautomerization of phenol opened another pathway toward ring saturation products under liquid-phase reaction conditions. The favorable tautomerization reaction pathway in liquid phase against vapor phase is likely due to a combination of enhanced tautomerization and hydrogenation of tautomerized enone. Due to the lack of acidity, direct hydrogenolysis of cyclohexanol rather than the dehydration followed hydrogenation proposed on the catalyst with acidity, is dominant pathway for the formation of cyclohexane on the Pd/Fe catalysts.
Conflict of interest The authors declare no competing financial interest. Acknowledgements This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) and Division of Chemical Sciences (grant DE-FG02-05ER15712). The authors thank the Franceschi Microscopy and Imaging Center (FMIC) in Washington State University for the access to TEM. We would also like to acknowledge Prof. Hongfei Lin and Dr. Teng Li in Washington State University for their GC–MS analysis of the products. 6
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Scheme 2. Proposed reaction pathway for the conversion of phenol over Pd/Fe catalyst.
Appendix A. Supplementary data
[31] T. Prasomsri, M. Shetty, K. Murugappan, Y. Román-Leshkov, Energ. Environ. Sci. 7 (2014) 2660–2669. [32] M. Shetty, K. Murugappan, T. Prasomsri, W.H. Green, Y. Román-Leshkov, J. Catal. 331 (2015) 86–97. [33] Q. Lu, C.-J. Chen, W. Luc, J.G. Chen, A. Bhan, F. Jiao, ACS Catal. 6 (2016) 3506–3514. [34] C.-J. Chen, W.-S. Lee, A. Bhan, Appl. Catal. A 510 (2016) 42–48. [35] G.H. Gu, C.A. Mullen, A.A. Boateng, D.G. Vlachos, ACS Catal. 6 (2016) 3047–3055. [36] Y. Yoon, R. Rousseau, R.S. Weber, D. Mei, J.A. Lercher, J. Am. Chem. Soc. 136 (2014) 10287–10298. [37] G. Li, J. Han, H. Wang, X. Zhu, Q. Ge, ACS Catal. 5 (2015) 2009–2016. [38] X. Wang, R. Rinaldi, Angew. Chem. Int. Ed. 52 (2013) 11499–11503. [39] A. Robinson, G.A. Ferguson, J.R. Gallagher, S. Cheah, G.T. Beckham, J.A. Schaidle, J.E. Hensley, J.W. Medlin, ACS Catal. 6 (2016) 4356–4368. [40] K. Lee, G.H. Gu, C.A. Mullen, A.A. Boateng, D.G. Vlachos, ChemSusChem 8 (2015) 315–322. [41] P.M. de Souza, R.C. Rabelo-Neto, L.E.P. Borges, G. Jacobs, B.H. Davis, T. Sooknoi, D.E. Resasco, F.B. Noronha, ACS Catal. 5 (2015) 1318–1329. [42] E. Furimsky, J.A. Mikhlin, D.Q. Jones, T. Adley, H. Baikowitz, Can. J. Chem. Eng. 64 (1986) 982–985. [43] M. Badawi, J.-F. Paul, S. Cristol, E. Payen, Catal. Commun. 12 (2011) 901–905. [44] Q. Tan, G. Wang, A. Long, A. Dinse, C. Buda, J. Shabaker, D.E. Resasco, J. Catal. 347 (2017) 102–115. [45] A.J.R. Hensley, Y. Wang, J.-S. McEwen, Surf. Sci. 648 (2016) 227–235. [46] Y. Hong, H. Zhang, J. Sun, K.M. Ayman, A.J.R. Hensley, M. Gu, M.H. Engelhard, J.S. McEwen, Y. Wang, ACS Catal. 4 (2014) 3335–3345. [47] M.V. Bykova, D.Y. Ermakov, V.V. Kaichev, O.A. Bulavchenko, A.A. Saraev, M.Y. Lebedev, V.А. Yakovlev, Appl. Catal. B 113 (114) (2012) 296–307. [48] Y. Shao, Q. Xia, L. Dong, X. Liu, X. Han, S.F. Parker, Y. Cheng, L.L. Daemen, A.J. Ramirez-Cuesta, S. Yang, Y. Wang, Nat. Commun. 8 (2017) 16104. [49] S. Jin, Z. Xiao, C. Li, X. Chen, L. Wang, J. Xing, W. Li, C. Liang, Catal. Today 234 (2014) 125–132. [50] Z. Luo, Y. Wang, M. He, C. Zhao, Green Chem. 18 (2016) 433–441. [51] X. Wang, R. Rinaldi, ChemSusChem 5 (2012) 1455–1466. [52] R.C. Nelson, B. Baek, P. Ruiz, B. Goundie, A. Brooks, M.C. Wheeler, B.G. Frederick, L.C. Grabow, R.N. Austin, ACS Catal. 5 (2015) 6509–6523. [53] P.M. Mortensen, J.-D. Grunwaldt, P.A. Jensen, A.D. Jensen, ACS Catal. 3 (2013) 1774–1785. [54] J. Sun, A.M. Karim, H. Zhang, L. Kovarik, X.S. Li, A.J. Hensley, J.-S. McEwen, Y. Wang, J. Catal. 306 (2013) 47–57. [55] M. Hanesch, Geophys. J. Int. 177 (2009) 941–948. [56] D.L.A. de Faria, S. Venâncio Silva, M.T. de Oliveira, J. Raman Spectrosc. 28 (1997) 873–878. [57] V.M.L. Whiffen, K.J. Smith, Energ. Fuel 24 (2010) 4728–4737. [58] L.F. Feitosa, G. Berhault, D. Laurenti, T.E. Davies, V. Teixeira da Silva, J. Catal. 340 (2016) 154–165. [59] M. Chatterjee, A. Chatterjee, T. Ishizaka, H. Kawanami, Catal. Sci. Technol. 5 (2015) 1532–1539. [60] J. He, C. Zhao, D. Mei, J.A. Lercher, J. Catal. 309 (2014) 280–290. [61] L. Zhu, J.W. Bozzelli, J. Phys. Chem. A 107 (2003) 3696–3703. [62] J. He, C. Zhao, J.A. Lercher, J. Am. Chem. Soc. 134 (2012) 20768–20775. [63] S. Jin, X. Chen, C. Li, C.-W. Tsang, G. Lafaye, C. Liang, ChemistrySelect 1 (2016) 4949–4956. [64] C.-c. Chiu, A. Genest, A. Borgna, N. Rösch, ACS Catal. 4 (2014) 4178–4188. [65] A.J.R. Hensley, Y. Wang, D. Mei, J.-S. McEwen, ACS Catal. 8 (2018) 2200–2208. [66] H.-M. Lin, H.M. Sebastian, K.-C. Chao, J. Chem. Eng. Data 25 (1980) 252–254. [67] A.J.R. Hensley, Y. Wang, J.-S. McEwen, ACS Catal. 5 (2015) 523–536.
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cattod.2018.12.027. References [1] Y. Roman-Leshkov, J.N. Chheda, J.A. Dumesic, Science 312 (2006) 1933–1937. [2] Z. Jiang, C. Hu, J. Energy Chem. 25 (2016) 947–956. [3] J. Zakzeski, P.C. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, Chem. Rev. 110 (2010) 3552–3599. [4] Y. Nakagawa, S. Liu, M. Tamura, K. Tomishige, ChemSusChem 8 (2015) 1114–1132. [5] S.L. Yohe, H.J. Choudhari, D.D. Mehta, P.J. Dietrich, M.D. Detwiler, C.M. Akatay, E.A. Stach, J.T. Miller, W.N. Delgass, R. Agrawal, F.H. Ribeiro, J. Catal. 344 (2016) 535–552. [6] C. Liu, H. Wang, A.M. Karim, J. Sun, Y. Wang, Chem. Soc. Rev. 43 (2014) 7594–7623. [7] D.C. Elliott, Energ. Fuel 21 (2007) 1792–1815. [8] H.S. Choi, Y.S. Choi, H.C. Park, Renew. Energ. 42 (2012) 131–135. [9] T.-S. Nguyen, D. Laurenti, P. Afanasiev, Z. Konuspayeva, L. Piccolo, J. Catal. 344 (2016) 136–140. [10] L. Chen, J. Xin, L. Ni, H. Dong, D. Yan, X. Lu, S. Zhang, Green Chem. 18 (2016) 2341–2352. [11] J.E. Peters, J.R. Carpenter, D.C. Dayton, Energ. Fuel 29 (2015) 909–916. [12] W.-S. Lee, Z. Wang, R.J. Wu, A. Bhan, J. Catal. 319 (2014) 44–53. [13] X. Diao, N. Ji, M. Zheng, Q. Liu, C. Song, Y. Huang, Q. Zhang, A. Alemayehu, L. Zhang, C. Liang, J. Energy Chem. 27 (2018) 600–610. [14] B. Yoosuk, D. Tumnantong, P. Prasassarakich, Fuel 91 (2012) 246–252. [15] N. Ji, X. Wang, C. Weidenthaler, B. Spliethoff, R. Rinaldi, ChemCatChem 7 (2015) 960–966. [16] G. Liu, A.W. Robertson, M.M.-J. Li, W.C.H. Kuo, M.T. Darby, M.H. Muhieddine, Y.C. Lin, K. Suenaga, M. Stamatakis, J.H. Warner, S.C.E. Tsang, Nat. Chem. 9 (2017) 810–816. [17] C. Zhao, J. He, A.A. Lemonidou, X. Li, J.A. Lercher, J. Catal. 280 (2011) 8–16. [18] P.M. de Souza, R.C. Rabelo-Neto, L.E.P. Borges, G. Jacobs, B.H. Davis, D.E. Resasco, F.B. Noronha, ACS Catal. 7 (2017) 2058–2073. [19] P.M. de Souza, R.C. Rabelo-Neto, L.E.P. Borges, G. Jacobs, B.H. Davis, U.M. Graham, D.E. Resasco, F.B. Noronha, ACS Catal. 5 (2015) 7385–7398. [20] B. Güvenatam, O. Kurşun, E.H.J. Heeres, E.A. Pidko, E.J.M. Hensen, Catal. Today 233 (2014) 83–91. [21] L. Nie, D.E. Resasco, J. Catal. 317 (2014) 22–29. [22] Y. Shao, Q. Xia, X. Liu, G. Lu, Y. Wang, ChemSusChem 8 (2015) 1761–1767. [23] G. Yao, G. Wu, W. Dai, N. Guan, L. Li, Fuel 150 (2015) 175–183. [24] D. Shi, L. Arroyo-Ramírez, J.M. Vohs, J. Catal. 340 (2016) 219–226. [25] D.J. Rensel, S. Rouvimov, M.E. Gin, J.C. Hicks, J. Catal. 305 (2013) 256–263. [26] Y. Yang, C. Ochoa-Hernández, V.A. de la Peña O’Shea, P. Pizarro, J.M. Coronado, D.P. Serrano, Appl. Catal. B 145 (2014) 91–100. [27] Y. Li, J. Fu, B. Chen, RSC Adv. 7 (2017) 15272–15277. [28] J.K. Kim, J.K. Lee, K.H. Kang, J.C. Song, I.K. Song, Appl. Catal. A 498 (2015) 142–149. [29] R.N. Olcese, M. Bettahar, D. Petitjean, B. Malaman, F. Giovanella, A. Dufour, Appl. Catal. B 115 (116) (2012) 63–73. [30] L. Nie, P.M. de Souza, F.B. Noronha, W. An, T. Sooknoi, D.E. Resasco, J. Mol. Catal. A 388 (389) (2014) 47–55.
7
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Liquid-phase hydrodeoxygenation of lignin-derived phenolics on Pd/Fe: A mechanistic study ⁎
Jianghao Zhanga, Junming Suna, , Berlin Suddutha, Xavier Pereira Hernandeza, Yong Wanga,b, a b
⁎
The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, United States Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA, 99352, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Tautomerization Reaction mechanism Liquid-phase hydrodeoxygenation Pd/Fe Lignin-derived phenolics
Although Pd/Fe bimetallic catalysts have been extensively studied in vapor-phase hydrodeoxygenation of phenolics (i.e., guaiacol, cresol), little is yet known about their performance in liquid-phase reactions. In this work, we present a mechanistic study on the Pd/Fe bimetallic catalysts in liquid-phase hydrodeoxygenation of phenolics. The role of tautomerization in hydrodeoxygenation of the lignin-derived phenolics, particularly for ring saturation, is systematically studied by employing two representative modeling compounds: phenol (a molecule that is keto-enol tautomeric) and diphenyl ether (a molecule that does not allow ketol-enol tautomerization). It was found that although the addition of oxyphilic Fe inhibits the direct aromatic ring saturation reaction typically occurring on Pd, tautomerization opens another reaction pathway toward ring saturation products (i.e. cyclohexanone, cyclohexanol, cyclohexane et al.), where both tautomerization and the hydrogenation of keto isomers are significantly enhanced to produce cyclohexanol followed by direct hydrogenolysis of the cyclohexanol to cyclohexane.
1. Introduction The diminishing fossil resources initiated great efforts seeking for the sustainable energy [1]. Among those, one promising solution is to produce the renewable fuels and chemicals from the earth-abundant and inexpensive lignocellulosic biomass [2–4], which mainly consists of cellulose, hemicellulose and up to 30 wt.% lignin [5]. Pyrolysis of biomass is a feasible process that produces the bio-oil containing hundreds of oxygenates subjecting to further upgrading [6–8]. One of the most critical processes for this upgrade is catalytic hydrodeoxygenation (HDO) [9]. A series of HDO catalysts have been extensively studied for the past decades [9–13]. Traditional HDO catalysts were based on metal sulfides [7,14], which yield high arene selectivity in the conversion of phenolics [15,16]. A common issue with sulfide catalysts is the contamination of sulfur in products [17]. To this end, more and more researches have been focusing on the development of other alternative catalysts including supported noble metal catalysts such as Pt-Mo/ MWCNT [5], Pd [17–19], Pt [20,21], Pd/Nb2O5/SiO2 [22], Ru/H-Beta [23], Pt-Zn [24], base metal catalysts such as FeMoP [25], Ni [26,27], Fe [28,29], Ni-Fe [30], MoO3 [31,32], and metal carbides such as WCx [33], MoCx [12,34]. Depending on the catalysts employed, several reaction pathways
have been proposed in the HDO of phenolic compounds: direct CeO bond cleavage/hydrogenolysis, direct aromatic ring hydrogenation, and keto-enol tautomerization [35], as summarized in Scheme 1 for the case of phenol. While direct CeO bond cleavage produces benzene (black color highlighted), direct aromatic ring hydrogenation will lead to the formation of cyclohexane (red color highlighted). In the former pathway, the adsorbed phenol on catalyst is activated by the dissociation on the surface via cleavage of CeO bond, being followed by hydrogenation of phenyl and hydroxyl radical to produce benzene and water. In the latter case, adding the first hydrogen atom to the phenyl ring was calculated having the highest barrier in the direct aromatic ring hydrogenation over Ni(111) and Pt(111), [35,36] leading to the primary formation of cyclohexanol intermediate rather than enols such as cyclohexadienol and cyclohexenol [37]. Then, cyclohexanol can be converted to cyclohexane by: 1) direct CeO bond cleavage or hydrogenolysis or 2) dehydration to form cyclohexene followed by hydrogenation of cyclohexane. It should be noted, although dehydrogenation of cyclohexene to benzene has been shown feasible in the presence of phenol with 2-propanol as H-transfer initiator and H donor [38], this reaction is thermodynamically unfavorable in the presence of high pressure H2. It is thus believed the contribution for benzene production from cyclohexene dehydrogenation is minimum. Tautomerization (blue
⁎ Corresponding authors at: The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA, 99164, United States. E-mail addresses: [email protected] (J. Sun), [email protected] (Y. Wang).
https://doi.org/10.1016/j.cattod.2018.12.027 Received 5 August 2018; Received in revised form 16 November 2018; Accepted 12 December 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Zhang, J., Catalysis Today, https://doi.org/10.1016/j.cattod.2018.12.027
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Scheme 1. The proposed reaction pathways for HDO of phenol.
phase [46], while only ring-saturation products were detected in the HDO of phenol at 250 °C in liquid phase [17]. Recently, our group reported the vapor-phase HDO of guaiacol over carbon-supported Pd-Fe catalyst [54], which exhibited enhanced activity and arene selectivity without ring saturation. The synergic effect between Pd and Fe was further verified using Pd-promoted Fe nanoparticles for vapor-phase HDO of m-cresol, which displayed > 90% of arene selectivity [46]. Herein, we further studied the HDO of phenolics on the same Pd/Fe catalyst in liquid-phase reactions, and attempted to understand the reaction mechanisms involved.
color highlighted) is another important reaction pathway since it has the lowest calculated reaction barrier (e.g. 0.78 eV for cresol [39] on PtMo catalyst and 0.52 eV for catechol [40] on Pt catalyst) among the three proposed reaction pathways shown in Scheme 1. After the ketoenol tautomerization, the subsequent hydrogenation of the tautomerized enone could have two competitive secondary reactions: one is hydrogenation of C]O bond of the enone to cyclohexadienol, followed by an acid catalyzed dehydration of cyclohexadienol to form benzene as proposed on Pt/SiO2 and Ni-Fe by Nie et al [21,30]. The other is a consecutive hydrogenation of C]C and then C]O bond in enone to produce cyclohexanone and cyclohexanol. The obtained cyclohexanol could be further converted into cyclohexane by either dehydration/ hydrogenation as proposed on the Pd/Al2O3 with metal-acid bi-functionality by Souza et al, [41] or direct hydrogenolysis of cyclohexanol although it has rarely been accepted yet. It was found that the reaction pathways and the consequent products were affected by the adsorption configuration of phenols [42], i.e. horizontal and vertical adsorption. The vertical position (taking place on MoS2 et al. catalysts) is beneficial to direct CeO bond cleavage, while the horizontal position favors hydrogenation of the aromatic ring [43]. In the case of noble metal catalysts where horizontal adsorption is the most favorable configuration [35,40,44], the direct CeO bond cleavage of p-cresol over Pt(111) has a higher reaction barrier (2.45 eV) than that of direct hydrogenation (1.11 eV) [35]. Interestingly, addition of an oxophilic metal on a noble metal (i.e., PtZn) was also reported to change adsorption of aromatic ring by tilting it away from Pt surface, leading to a favorable CeO bond cleavage [24]. In addition to the adsorption configuration, the reaction pathway may also be influenced by the interaction between the functional group and catalyst surface. For example, it has been reported that guaiacol has a stronger adsorption on Pd(111) and Fe(110) which distorts the CeO bond in guaiacol to a greater degree [45], and is likely the reason that Fe favors the CeO bond cleavage while Pd favors the ring hydrogenation [46]. Other previous studies also indicate that the dominant reaction pathway also depends on many parameters including catalysts [47,48] and reacting conditions such as pressure [5,49,50], temperature [25] and solvent/ additive [51,52]. For example, Nie et al. [30] reported a 47.5% selectivity to ring saturation products in vapor-phase HDO of m-cresol over 5 wt.% Ni/SiO2 at 0.1 MPa and 300 °C. With the same 5 wt.% Ni/ SiO2 catalyst, another study conducted by Mortensen et al. [53] reported 100% selectivity to ring-saturation products in the liquid-phase HDO of phenol at 10 MPa and 275 °C. Similarly, Pd/C catalyst displayed around 75% selectivity to arene in the HDO of cresol at 300 °C in vapor-
2. Experimental 2.1. Catalyst preparation 2.1.1. Preparation of support The Fe2O3 support was prepared with precipitation method as previously reported [46]. Typically, the (NH4)2CO3 (Sigma-Aldrich, 99.999%, 1.5 M) solution was added dropwise to a solution of Fe(NO3)3 (Sigma-Aldrich, > 99.99%, 3 M) under stirring, forming dark crimson slurry. The precipitate was then washed with Milli-Q water by filtration until the pH reached to 8. After being dried at 80 °C overnight and sieved to < 100 mesh, the sample was calcined at 400 °C for 5 h to achieve the Fe2O3 support. 2.1.2. Preparation of supported 1 wt.% Pd catalysts The 1 wt.% Pd/Fe2O3 was prepared using incipient wetness impregnation method. Pd(NH3)4(NO3)2 solution (Sigma-Aldrich, 10 wt.% solution with 99.99% metal based purity) was diluted by certain amount of Milli-Q water such that the concentration and volume of the precursor solution matched with pore volume of the support as well as the metal loading of the obtained catalysts. The diluted solution was then added dropwise to Fe2O3 powder, followed by drying at 80 °C overnight in air and calcination at 350 °C for 2 h in flowing N2 (50 mL/ min). 2.2. Characterization In situ X-ray powder diffraction (XRD) experiments were done in a Rigaku SmartLab X-ray equipped with an XRK900 in situ reactor and a DTex high-speed detector that allows for higher signal-to-noise ratio at high scan rates. The diffraction spectra were collected between 10° and 80°, at 5°/min at a step of 0.01°. The radiation source was Cu K-α with a 2
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
were sampled at 25 °C and compared with those sampled at the reaction temperatures. No significant deviation (< 10%) in terms of both conversion and selectivity was found after this correction. The products were analyzed on a gas chromatograph (GC, Agilent 7890 A) equipped with a DB-FFAP column (30 m, 0.32 mm, 0.25 μm) and flame ionization detector (FID), as well as a GC–MS (Shimadzu, GCMS-QP2020) equipped with a HP-5 column (30 m, 0.32 mm, 0.25 μm) connected to a FID. The conversion, selectivity and yield were defined as follows: conversion [%] = (moles of carbon in the reacted substrate/moles of carbon in the substrate fed) × 100; selectivity [%] = (moles of carbon in the specific product/moles of carbon in all product) × 100; yield [%] = (moles of carbon in the specific product/ moles of carbon in the substrate fed) ×100. The carbon balance was usually in the range of 92%–100%.
3. Results 3.1. Characterization of the catalysts The characterization of fresh and reduced/passivated 1 wt% Pd/ Fe2O3 samples show general consistency with our previous study [46]. For the fresh sample, Pd or PdOx is highly dispersed on the Fe2O3 support that has a uniform size distribution at ∼20 nm (Supplementary material Fig. S1). The in situ XRD pattern of the fresh sample shows a hematite (JCPDS: 33-0664) crystalline structure, while metallic Fe (JCPDS: 06-0696) and magnetite (JCPDS: 19-0629) dominate on the reduced catalyst [46]. No diffractions characteristic of Pd were observed, suggesting the highly dispersed nature of Pd. In situ Raman spectra were also studied and compared on the 1 wt% Pd/Fe2O3 catalysts before and after the reduction at 350 °C, as shown in Fig. 1. Spectrum of fresh Pd/Fe2O3 (Fig. 1b) showed the typical peaks of hematite [55,56], consistent with the XRD analysis. After reduction, only a small hump at 655 cm−1, characteristic of FeO was detected, due to dominant formation of Raman silent metallic Fe species (Fig. 1b). These characterization results suggest that a FeOx core (incomplete reduction) and metallic Fe shell structure was formed on the reduced catalysts. In consistent with this deduction, our previous XPS study showed only surface metallic Fe [46]. It is thus concluded that, the catalysts shown in this work is representative of our previous observations that the surface structure of the catalyst is Pd modified metallic Fe [46].
Fig. 1. In situ-XRD patterns for fresh and reduced 1 wt% Pd/Fe2O3 (a), and in situ Raman spectra for fresh and reduced 1 wt% Pd/Fe2O3 (b). Reduction condition: 350 °C with 50% H2 (40 mL/min, balanced by N2) for 2 h.
wavelength of 0.154056 nm. Approximately 0.1 g of catalyst was loaded into the sample holder. A scan was then collected at RT under flowing N2 (40 ml/min) which was set as fresh sample. After that, the sample temperature was increased to 350 °C at a ramping rate of 10 °C/ min in flowing 50%H2/He (40 ml/min), and then held at the temperature for another 2 h. Another scan was then taken before the sample was cooled down to ambient temperature. Raman spectra were collected on a Horiba LabRAM HR Raman/ FTIR microscope equipped with a 532 nm (Ventus LP 532) laser source and Synapse Charge Coupled Device detector (CCD). After calibration using the silica reference, the sample was loaded in an Linkam CCR1000 in situ reactor, and a spectrum of the sample was then taken at room temperature. The sample was then reduced by heating to 350 °C (ramp rate of 10 °C/min) and holding the temperature for 2 h under flowing H2 (10 ml/min), after which it was cooled to room temperature before collecting the spectra of the reduced samples.
3.2. Comparison of HDO of m-cresol on Pd/Fe: vapor phase vs liquid phase
2.3. Activity test
We have shown in our previous studies that the Pd/Fe catalyst exhibited > 90% selectivity to arene in the vapor-phase m-cresol HDO, and Pd played essential role in enhancing the reducibility of Fe and maintaining the active metallic iron phase [46]. Since the liquid-phase reaction has the advantage in minimizing the energy consumption, for comparison, the performance of the Pd/Fe catalyst was also investigated and compared in liquid-phase reaction conditions. As such, the N2 diluent employed in vapor phase was replaced with hexadecane in liquid-phase reactions. Fig. 2 shows a comparison of the catalytic performances of Pd/Fe at comparable cresol conversion in the two phases. The high arene selectivity in vapor phase dramatically decreased to ∼2% in the liquid phase. This observation is not surprising since similar observations were also reported on other catalysts reported by different research groups [17,30,31,46,53,57](Supplementary material Table S1). It is should be noted, however, no evidence has been shown to unambiguously interpret why the reaction pathway shifted toward aromatic ring saturation in liquid phase, although it has been reported that hydrogen pressure and temperature strongly affect the proportion of arene in the products [5,25,49]. This motivated our further studies on the reaction pathways aiming to understand the factors that control the product distribution.
The HDO reaction of the modeling substrates were carried out in a stainless steel Parr reactor (300 mL) equipped with a glass liner. Typically, the catalyst was first reduced ex-situ with 40 ml/min 50% H2/N2 at 350 °C for 2 h followed by a passivation with 40 ml/min 0.2% O2/N2 at room temperature for 1 h. The passivated catalyst was then loaded into the reactor. After 3 times purging with 4 MPa H2, the temperature was ramped to 300 °C (15 °C/min) where it was held for another 30 min in H2 (3 MPa) for in situ reduction. After the reduction, the temperature was cooled down to ambient temperature where the pressure was decreased to ambient, and 6.35 mmol modeling compound (Sigma-Aldrich, ≥ 99%) in 50 mL hexadecane (Sigma-Aldrich, 99%) were then injected with syringe into the reactor. After one time purging H2 and pressurized with H2 to 4 MPa, the mixed catalyst and substrate solution were heated up to the target temperature at a stirring rate of 800 rpm for the reaction. The products were sampled periodically for analysis at the reaction temperature. Given the variable composition with temperature in the liquid phase, a linear correction assuming a constant coefficient in the Henry’s law was made in the concentration range studied. After the reaction, some of the analysis 3
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Fig. 2. Comparison of catalytic performances of Pd/Fe in vapor-phase and liquid-phase HDO of m-cresol. Reaction condition: 300 °C, 0.04 MPa H2 and 0.06 MPa N2 for vapor-phase reaction (data adapted from the previous study [46]); 300 °C, hexadecane as solvent, 4 MPa H2 for liquid-phase reaction.
3.3. Reaction pathway in liquid-phase hydrodeoxygenation of phenol on Pd/Fe To better understand the enhanced hydrogenation of aromatic ring of phenolics in liquid-phase reactions, the simplest phenolic, phenol, was further studied in the HDO reaction. Fig. 3a shows the evolution of phenol conversion and product yields as a function of time at 300 °C and 4 MPa (at 25 °C) H2 pressure, and the product selectivity versus phenol conversion is replotted in Fig. 3b. Similar to the m-cresol reactions, ring saturation is dominant in liquid phase, leading to the formation of cyclohexane as a major product along with small amounts of cyclohexanol, cyclohexene, and cyclohexanone. With time-on-stream (TOS), cyclohexanol first increases and then decreases with its highest yield being observed at around 30 min. The yield to cyclohexene exhibited a similar trend as that of cyclohexanol albeit at a lower yield. This observation implies that the cyclohexanol and cyclohexene are the intermediates to cyclohexane during hydrodeoxygenation of phenol. Indeed, the selectivity to cyclohexane increases at the expenses of cyclohexanol and cyclohexene. At extrapolated zero conversion analyzed using SPSS 22.0 software [58], cyclohexanol selectivity surpasses the cyclohexane and reaches up to above 50%, an indication of primary product in HDO of phenol under liquid-phase reaction conditions. The total selectivity to ring saturation products reaches up to 98.4%, leaving only 1.6% selectivity to benzene via direct CeO bond cleavage. Two reaction pathways could lead to cyclohexanol formation: one is direct hydrogenation of aromatic ring of phenol, and another is keto-enol tautomerization followed by hydrogenation [35], as shown in Scheme 1. Given the much higher activation barrier for direct aromatic ring saturation than that for tautomerization calculated on phenolics [35,40], it is believed that the primary formation of cyclohexanol should be from the latter (i.e., tautomerization). Note that, no other intermediate (i.e. 2, 4-cyclohexadienone) but only trace amount of cyclohexanone was detected, and its selectivity barely shows any dependence on the phenol conversion. This could be due to a high hydrogenation reactivity of the intermediate at a relatively high temperature (i.e., 300 °C). Indeed, when the reaction temperature decreased from 300 °C to 200 °C (Fig. 4), appreciable amount of cyclohexanone was produced, and its selectivity increased rapidly as phenol conversion decreased close to zero. It should be noted that the secondary cyclohexanol reactions are also very limited at low reaction temperature (i.e., 200 °C), evidenced by the high selectivity above 90% regardless of phenol conversion. To further confirm the relatively higher reactivity of cyclohexanone than that of cyclohexanol, cyclohexanone and cyclohexanol were used as substrates and tested on the
Fig. 3. (a) Phenol conversion and product yields over Pd/Fe catalyst as a function of time-on-stream. (b) Product distribution as a function of conversion. Reaction condition: 300 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.15 g catalyst, 4 MPa H2.
Fig. 4. Production distribution as a function of conversion over Pd/Fe catalyst. Reaction condition: 200 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.3 g catalyst, 4 MPa H2.
Pd/Fe catalyst under the same reaction conditions (Table 1). It is clear that the conversion rate for cyclohexanone is much faster than that of cyclohexanol. It is thus hypothesized that, over the Pd/Fe bimetallic catalysts, majority of the aromatic ring saturation should be from 4
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Table 1 Catalytic performances of Pd/Fe catalyst in converting possible intermediates. Reaction condition: 200 °C, 6.35 mmol substrates, 50 mL hexadecane, 0.3 g catalyst, 4 MPa H2. Substrate
Benzene Cyclohexanol Cyclohexanone
Time
4h 4h 10min
conversion
10.0 9.3 67.0
Selectivity C6H12
C6H10
Benzene
C6H12O
Phenol
C6H10O
99.4 64.2 0.2
0.6 31.5 trace
– 0.0 0.0
0.0 – 99.8
0.0 0.0 0.0
0.0 0.0 –
tautomerization reaction pathway. Diphenyl ether (DPE) is a lignin-derived compound with 4-O-5 linkage, which is the strongest ether bond in lignin [59]. As one of the lignin-derived model compounds, DPE has also been widely studied in the HDO reactions [60]. Note that the structure of DPE with two symmetric Caromatic-O bonds is unique, making the keto-enol tautomerization impossible within this molecule [61]. Based on the proposed reaction pathways [62], it is anticipated that, unlike phenol, DPE should only proceed with two primary reactions on the Pd/Fe catalysts in organic phase: direct ring hydrogenation and direct CaromaticeO bond cleavage [63]. A comparison of HDO of phenol and DPE could help differentiate the contribution of tautomerization to the ring-saturation products. It should be mentioned that although anisole could be considered as an alternative model compounds for this purpose, the presence and easy hydrogenolysis of Caliphatic-O [11,54,64] could lead to the facile formation of phenol and thus complicate this study. To confirm our hypothesis that tautomerization rather than direct aromatic ring hydrogenation is dominating toward ring saturation products on Pd/Fe catalyst, we further studied the HDO of DPE (Supplementary material Fig. S2), which was performed under the same reaction conditions as that of phenol. In this particular case, if direct ring hydrogenation occurs, cyclohexyl phenyl ether (CPE) and dicyclohexyl ether (DCE) primary products are expected. Whereas, direct CeO bond cleavage (i.e., hydrogenolysis) will result in benzene and phenol primary products. Fig. 5 depicts the product distribution as a function of conversions. Benzene and cyclohexane/cyclohexanol (secondary products from phenol) from direct CeO bond cleavage are always dominant in the products. At extrapolated zero conversion where secondary reaction is minimized, ∼94% selectivity to direct CeO bond cleavage was observed. In contrast, selectivity to CPE is only ∼6% and no DCE is observed, suggesting the low aromatic ring hydrogenation activity on Pd/Fe. This result confirms that aromatic ring saturation products in the phenol conversion over Pd/Fe catalyst were mainly from tautomerization/hydrogenation rather than direct aromatic ring hydrogenation. It is worth mentioning that, as one of the primary CeO cleavage product, phenol was not detected in the diphenyl ether conversion, indicating that the CeO bond cleavage during the DPE conversion is the kinetic relevant step, matching well with the high activation energy of the CaromaticeO bond [39]. Moreover, our results (Fig. 3) showed that cyclohexane is always dominant in the final products for the HDO of phenol, especially at high phenol conversion. It is thus expected that cyclohexane is the other major products during the HDO of DPE (Fig. 5 and Fig. S2) for the relatively facile secondary phenol conversion. In addition, as DPE conversion increases (longer reaction time), cyclohexane increases at the expense of benzene. It suggests secondary benzene hydrogenation occurs on the Pd/Fe catalysts, which is also confirmed at low reaction temperature (Table 1). However, it does not affect our conclusion that during the HDO of phenol, the facile keto-enol tautomerization opened a pathway to readily form enone from phenol and played the major role in the aromatic ring saturation reaction under liquid-phase reaction conditions.
Fig. 5. (a) Product distribution as a function of conversion over Pd/Fe. (b) Distribution of classified products as a function of conversion. Reaction condition: 300 °C, 6.35 mmol diphenyl ether (1 mL), 50 mL hexadecane, 0.15 g catalyst, 4 MPa H2.
liquid-phase HDO reaction, particularly for inhibiting the direct aromatic ring hydrogenation reactions, the reaction mechanism is significantly different from that in vapor-phase reaction. In the vaporphase reaction at atmospheric pressure (H2 partial pressure 0.04 MPa), majority of the final products are arene whereas ring-saturation products (mainly cyclohexane for phenol) dominate in the liquid-phase reaction at ∼6 MPa H2 (at 300 °C). More importantly, our experimental results explicitly revealed ring-saturation products in the HDO of phenol on PdFe catalysts were mainly from tautomerization pathway. The enhancement of tautomerization and thus ring saturation in the liquid-phase HDO of phenol likely results from two factors: hydrogen partial pressure and solvent effect. In the former case, the H supply and thus further hydrogenation of tautomerized enone could be limited by the low hydrogen pressure in the vapor-phase conditions. In liquid phase, although H2 solubility could be low in hexadecane, the high
4. Discussions Although the synergistic effect on Pd/Fe still plays key roles in the 5
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Fig. 6. The influence of H2 pressure to the reaction rate of benzene production and ring saturation products. Reaction condition: 300 °C, 6.35 mmol phenol, 50 mL hexadecane, 0.15 g monometallic Fe as catalyst. Note that this experiment was done on monometallic Fe to minimize the direct ring hydrogenation observed on Pd/Fe albeit low, which can complicate this verification.
Fig. 7. Product distribution as a function of conversion over Pd/Fe. Reaction condition: 300 °C, 6.35 mmol cyclohexanol, 50 mL hexadecane, 0.15 g catalyst, 4 MPa H2.
on the less or non-acidic Pd/Fe catalyst, which is different with the acid catalysts showing dehydration as the main pathway [17]. Based on the above results, the reaction pathway for liquid-phase HDO of phenolics can be proposed over Pd/Fe catalysts. For the case of phenol, as shown in Scheme 2, two parallel routes exist: direct CeO bond cleavage and tautomerization followed by ring saturation. Different from the vapor-phase HDO of phenolics where direct hydrogenolysis of CaromaticeO bond prevails, ring saturation becomes the major reaction pathway in liquid phase. The fact that ring saturation pathway is dominant in liquid phase is mainly due to the enhanced tautomerization reaction pathway toward cylcohexanol, followed by a direct hydrogenolysis of the cyclohexanol to cyclohexane.
hydrogen pressure could offset this [66] and thus ascertain an efficient H supply on the catalyst surface [50]. Therefore, the enone hydrogenation could be enhanced in liquid-phase reaction conditions. Indeed, Yohe et al. [5] reported a significant influence of H2 partial pressure on the product distribution. The aromatic hydrocarbon reaches up to 93.2% at 0.1 MPa H2, whereas this selectivity dramatically decreased to be less than 3% at 2.3 MPa H2. Note that the Caromatic-O bond cleavage has been identified as the rate-determining step in the HDO of phenol to produce benzene [67], which means, if this is the case, the improved H supply should have negligible effect on the formation rate of benzene. In other words, given the unaltered rate for benzene formation, the selectivity to ring saturation products in liquid phase reaction conditions should be improved by an enhanced kinetics for the parallel tautomerization pathway. To verify this, the HDO of phenol was tested at different H2 pressures. For a better comparison of kinetics, the conversion was controlled below 15%. As shown in Fig. 6, the rate to produce benzene remains almost constant (∼0.13 mmol/g/h) when the H2 pressure increases from 1 MPa to 4 MPa, whereas the rate to ringsaturated products increases dramatically from 0.38 mmol/g/h to 1.11 mmol/g/h, leading to decreased benzene selectivity from 26.6% to 9.8%. This result confirms that the enhanced tautomerization pathway is indeed facilitated by hydrogen supply in liquid-phase conditions. In addition, a recent DFT study also showed that, although the barrier for tautomerization in vacuum condition is higher than direct Ce–O bond cleavage, this barrier significantly decreases in the presence of water solvent due to the nearly barrierless proton transfer [65]. In the current case with batch reaction, the generated water molecule may adsorb/readsorb on the catalyst surface, which in turn facilitated the tautomerization. It can be concluded that, under liquid-phase conditions, the improved formation of ring-saturation products is mainly due to a combination of enhanced tautomerization by water solvent and hydrogenation of tautomerized enone by H supply. To complete the whole pathway, conversion of cyclohexanol was also studied under the same reaction condition as that of phenol (Fig. 7). As expected, both cyclohexane and cyclohexene are observed. As cyclohexanol conversion decreases, the cyclohexene selectivity increases slowly at the expense of cyclohexane. Regardless, selectivity to cyclohexane are always above > 90% selectivity. Even at extrapolated zero conversion, the cyclohexane selectivity is still much higher than cyclohexene. This result indicates that two parallel reactions occur in the cyclohexanol reaction on the Pd/Fe catalysts, namely the direct C–O bond cleavage (hydrogenolysis) to produce cyclohexane and the dehydration of cyclohexanol to form cyclohexene followed by hydrogenation. The former should be the major pathway (> 90% selectivity)
5. Conclusions By using phenol and DPE as the model compounds, HDO of phenolics has been carefully studied on Pd/Fe catalyst in liquid-phase, in an attempt to elucidate the distinct product distribution from those in vapor-phase conditions. Our results suggest that, although the direct aromatic ring saturation observed on Pd is inhibited by Fe (i.e., on Pd/ Fe bimetallic catalyst), the tautomerization of phenol opened another pathway toward ring saturation products under liquid-phase reaction conditions. The favorable tautomerization reaction pathway in liquid phase against vapor phase is likely due to a combination of enhanced tautomerization and hydrogenation of tautomerized enone. Due to the lack of acidity, direct hydrogenolysis of cyclohexanol rather than the dehydration followed hydrogenation proposed on the catalyst with acidity, is dominant pathway for the formation of cyclohexane on the Pd/Fe catalysts.
Conflict of interest The authors declare no competing financial interest. Acknowledgements This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) and Division of Chemical Sciences (grant DE-FG02-05ER15712). The authors thank the Franceschi Microscopy and Imaging Center (FMIC) in Washington State University for the access to TEM. We would also like to acknowledge Prof. Hongfei Lin and Dr. Teng Li in Washington State University for their GC–MS analysis of the products. 6
Catalysis Today xxx (xxxx) xxx–xxx
J. Zhang et al.
Scheme 2. Proposed reaction pathway for the conversion of phenol over Pd/Fe catalyst.
Appendix A. Supplementary data
[31] T. Prasomsri, M. Shetty, K. Murugappan, Y. Román-Leshkov, Energ. Environ. Sci. 7 (2014) 2660–2669. [32] M. Shetty, K. Murugappan, T. Prasomsri, W.H. Green, Y. Román-Leshkov, J. Catal. 331 (2015) 86–97. [33] Q. Lu, C.-J. Chen, W. Luc, J.G. Chen, A. Bhan, F. Jiao, ACS Catal. 6 (2016) 3506–3514. [34] C.-J. Chen, W.-S. Lee, A. Bhan, Appl. Catal. A 510 (2016) 42–48. [35] G.H. Gu, C.A. Mullen, A.A. Boateng, D.G. Vlachos, ACS Catal. 6 (2016) 3047–3055. [36] Y. Yoon, R. Rousseau, R.S. Weber, D. Mei, J.A. Lercher, J. Am. Chem. Soc. 136 (2014) 10287–10298. [37] G. Li, J. Han, H. Wang, X. Zhu, Q. Ge, ACS Catal. 5 (2015) 2009–2016. [38] X. Wang, R. Rinaldi, Angew. Chem. Int. Ed. 52 (2013) 11499–11503. [39] A. Robinson, G.A. Ferguson, J.R. Gallagher, S. Cheah, G.T. Beckham, J.A. Schaidle, J.E. Hensley, J.W. Medlin, ACS Catal. 6 (2016) 4356–4368. [40] K. Lee, G.H. Gu, C.A. Mullen, A.A. Boateng, D.G. Vlachos, ChemSusChem 8 (2015) 315–322. [41] P.M. de Souza, R.C. Rabelo-Neto, L.E.P. Borges, G. Jacobs, B.H. Davis, T. Sooknoi, D.E. Resasco, F.B. Noronha, ACS Catal. 5 (2015) 1318–1329. [42] E. Furimsky, J.A. Mikhlin, D.Q. Jones, T. Adley, H. Baikowitz, Can. J. Chem. Eng. 64 (1986) 982–985. [43] M. Badawi, J.-F. Paul, S. Cristol, E. Payen, Catal. Commun. 12 (2011) 901–905. [44] Q. Tan, G. Wang, A. Long, A. Dinse, C. Buda, J. Shabaker, D.E. Resasco, J. Catal. 347 (2017) 102–115. [45] A.J.R. Hensley, Y. Wang, J.-S. McEwen, Surf. Sci. 648 (2016) 227–235. [46] Y. Hong, H. Zhang, J. Sun, K.M. Ayman, A.J.R. Hensley, M. Gu, M.H. Engelhard, J.S. McEwen, Y. Wang, ACS Catal. 4 (2014) 3335–3345. [47] M.V. Bykova, D.Y. Ermakov, V.V. Kaichev, O.A. Bulavchenko, A.A. Saraev, M.Y. Lebedev, V.А. Yakovlev, Appl. Catal. B 113 (114) (2012) 296–307. [48] Y. Shao, Q. Xia, L. Dong, X. Liu, X. Han, S.F. Parker, Y. Cheng, L.L. Daemen, A.J. Ramirez-Cuesta, S. Yang, Y. Wang, Nat. Commun. 8 (2017) 16104. [49] S. Jin, Z. Xiao, C. Li, X. Chen, L. Wang, J. Xing, W. Li, C. Liang, Catal. Today 234 (2014) 125–132. [50] Z. Luo, Y. Wang, M. He, C. Zhao, Green Chem. 18 (2016) 433–441. [51] X. Wang, R. Rinaldi, ChemSusChem 5 (2012) 1455–1466. [52] R.C. Nelson, B. Baek, P. Ruiz, B. Goundie, A. Brooks, M.C. Wheeler, B.G. Frederick, L.C. Grabow, R.N. Austin, ACS Catal. 5 (2015) 6509–6523. [53] P.M. Mortensen, J.-D. Grunwaldt, P.A. Jensen, A.D. Jensen, ACS Catal. 3 (2013) 1774–1785. [54] J. Sun, A.M. Karim, H. Zhang, L. Kovarik, X.S. Li, A.J. Hensley, J.-S. McEwen, Y. Wang, J. Catal. 306 (2013) 47–57. [55] M. Hanesch, Geophys. J. Int. 177 (2009) 941–948. [56] D.L.A. de Faria, S. Venâncio Silva, M.T. de Oliveira, J. Raman Spectrosc. 28 (1997) 873–878. [57] V.M.L. Whiffen, K.J. Smith, Energ. Fuel 24 (2010) 4728–4737. [58] L.F. Feitosa, G. Berhault, D. Laurenti, T.E. Davies, V. Teixeira da Silva, J. Catal. 340 (2016) 154–165. [59] M. Chatterjee, A. Chatterjee, T. Ishizaka, H. Kawanami, Catal. Sci. Technol. 5 (2015) 1532–1539. [60] J. He, C. Zhao, D. Mei, J.A. Lercher, J. Catal. 309 (2014) 280–290. [61] L. Zhu, J.W. Bozzelli, J. Phys. Chem. A 107 (2003) 3696–3703. [62] J. He, C. Zhao, J.A. Lercher, J. Am. Chem. Soc. 134 (2012) 20768–20775. [63] S. Jin, X. Chen, C. Li, C.-W. Tsang, G. Lafaye, C. Liang, ChemistrySelect 1 (2016) 4949–4956. [64] C.-c. Chiu, A. Genest, A. Borgna, N. Rösch, ACS Catal. 4 (2014) 4178–4188. [65] A.J.R. Hensley, Y. Wang, D. Mei, J.-S. McEwen, ACS Catal. 8 (2018) 2200–2208. [66] H.-M. Lin, H.M. Sebastian, K.-C. Chao, J. Chem. Eng. Data 25 (1980) 252–254. [67] A.J.R. Hensley, Y. Wang, J.-S. McEwen, ACS Catal. 5 (2015) 523–536.
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cattod.2018.12.027. References [1] Y. Roman-Leshkov, J.N. Chheda, J.A. Dumesic, Science 312 (2006) 1933–1937. [2] Z. Jiang, C. Hu, J. Energy Chem. 25 (2016) 947–956. [3] J. Zakzeski, P.C. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, Chem. Rev. 110 (2010) 3552–3599. [4] Y. Nakagawa, S. Liu, M. Tamura, K. Tomishige, ChemSusChem 8 (2015) 1114–1132. [5] S.L. Yohe, H.J. Choudhari, D.D. Mehta, P.J. Dietrich, M.D. Detwiler, C.M. Akatay, E.A. Stach, J.T. Miller, W.N. Delgass, R. Agrawal, F.H. Ribeiro, J. Catal. 344 (2016) 535–552. [6] C. Liu, H. Wang, A.M. Karim, J. Sun, Y. Wang, Chem. Soc. Rev. 43 (2014) 7594–7623. [7] D.C. Elliott, Energ. Fuel 21 (2007) 1792–1815. [8] H.S. Choi, Y.S. Choi, H.C. Park, Renew. Energ. 42 (2012) 131–135. [9] T.-S. Nguyen, D. Laurenti, P. Afanasiev, Z. Konuspayeva, L. Piccolo, J. Catal. 344 (2016) 136–140. [10] L. Chen, J. Xin, L. Ni, H. Dong, D. Yan, X. Lu, S. Zhang, Green Chem. 18 (2016) 2341–2352. [11] J.E. Peters, J.R. Carpenter, D.C. Dayton, Energ. Fuel 29 (2015) 909–916. [12] W.-S. Lee, Z. Wang, R.J. Wu, A. Bhan, J. Catal. 319 (2014) 44–53. [13] X. Diao, N. Ji, M. Zheng, Q. Liu, C. Song, Y. Huang, Q. Zhang, A. Alemayehu, L. Zhang, C. Liang, J. Energy Chem. 27 (2018) 600–610. [14] B. Yoosuk, D. Tumnantong, P. Prasassarakich, Fuel 91 (2012) 246–252. [15] N. Ji, X. Wang, C. Weidenthaler, B. Spliethoff, R. Rinaldi, ChemCatChem 7 (2015) 960–966. [16] G. Liu, A.W. Robertson, M.M.-J. Li, W.C.H. Kuo, M.T. Darby, M.H. Muhieddine, Y.C. Lin, K. Suenaga, M. Stamatakis, J.H. Warner, S.C.E. Tsang, Nat. Chem. 9 (2017) 810–816. [17] C. Zhao, J. He, A.A. Lemonidou, X. Li, J.A. Lercher, J. Catal. 280 (2011) 8–16. [18] P.M. de Souza, R.C. Rabelo-Neto, L.E.P. Borges, G. Jacobs, B.H. Davis, D.E. Resasco, F.B. Noronha, ACS Catal. 7 (2017) 2058–2073. [19] P.M. de Souza, R.C. Rabelo-Neto, L.E.P. Borges, G. Jacobs, B.H. Davis, U.M. Graham, D.E. Resasco, F.B. Noronha, ACS Catal. 5 (2015) 7385–7398. [20] B. Güvenatam, O. Kurşun, E.H.J. Heeres, E.A. Pidko, E.J.M. Hensen, Catal. Today 233 (2014) 83–91. [21] L. Nie, D.E. Resasco, J. Catal. 317 (2014) 22–29. [22] Y. Shao, Q. Xia, X. Liu, G. Lu, Y. Wang, ChemSusChem 8 (2015) 1761–1767. [23] G. Yao, G. Wu, W. Dai, N. Guan, L. Li, Fuel 150 (2015) 175–183. [24] D. Shi, L. Arroyo-Ramírez, J.M. Vohs, J. Catal. 340 (2016) 219–226. [25] D.J. Rensel, S. Rouvimov, M.E. Gin, J.C. Hicks, J. Catal. 305 (2013) 256–263. [26] Y. Yang, C. Ochoa-Hernández, V.A. de la Peña O’Shea, P. Pizarro, J.M. Coronado, D.P. Serrano, Appl. Catal. B 145 (2014) 91–100. [27] Y. Li, J. Fu, B. Chen, RSC Adv. 7 (2017) 15272–15277. [28] J.K. Kim, J.K. Lee, K.H. Kang, J.C. Song, I.K. Song, Appl. Catal. A 498 (2015) 142–149. [29] R.N. Olcese, M. Bettahar, D. Petitjean, B. Malaman, F. Giovanella, A. Dufour, Appl. Catal. B 115 (116) (2012) 63–73. [30] L. Nie, P.M. de Souza, F.B. Noronha, W. An, T. Sooknoi, D.E. Resasco, J. Mol. Catal. A 388 (389) (2014) 47–55.
7