One-Pot Defunctionalization of Lignin-Derived Compounds by Dual

Oct 13, 2015 - Department of Chemistry, U.G.C. Centre of Advance Studies in Chemistry, Guru Nanak Dev University, Amritsar 143005, India. ∥ Beamline...
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One-Pot Defunctionalization of Lignin-Derived Compounds by DualFunctional Pd50Ag50/Fe3O4/N-rGO Catalyst Ajay K. Singh,† Seungwook Jang,† Jae Yul Kim,‡ Siddharth Sharma,†,§ K.C Basavaraju,† Min-Gyu Kim,∥ Kyung-Rok Kim,† Jae Sung Lee,‡ Hong H. Lee,*,⊥ and Dong-Pyo Kim*,† †

National Center of Applied Microfluidic Chemistry, Department of Chemical Engineering, POSTECH (Pohang University of Science and Technology), Pohang 790-784, Korea ‡ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan 689-798, Korea § Department of Chemistry, U.G.C. Centre of Advance Studies in Chemistry, Guru Nanak Dev University, Amritsar 143005, India ∥ Beamline Research Division, Pohang Accelerator Laboratory (PAL), POSTECH (Pohang University of Science and Technology), Pohang 790-784, Korea ⊥ School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea S Supporting Information *

ABSTRACT: Generation of hydrogen from renewable sources and its safe utilization for efficient one-pot upgrading of renewable biofuels are a challenge. Bimetallic PdAg catalyst supported on Fe3O4/nitrogen-doped reduced graphene oxide (N-rGO) were synthesized for hydrogen generation from formic acid with high TOF (497 h−1 at 50 °C), and the hydrogen was subsequently utilized in situ for selective defunctionalization of lignin-derived chemicals with preserved aromatic nature at ambient pressure. Hydrodeoxygenation of aromatic aldehydes and ketones gave excellent yields (99% at 130 °C) with no use of additives. Furthermore, hydrogenolysis of β-O-4 and α-O-4 C−O model compounds produced only two products with high selectivity at 120 °C, which is an efficient and versatile one-pot platform for valorization of lignin biomass. KEYWORDS: formic acid, bimetallic catalyst, magnetically separable, N-doped reduced graphene oxide, selective defunctionalization

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INTRODUCTION The production of fuel and fine chemicals from biomass has received much attention due to the fact that the world’s major oil fields have been in production decline for decades.1−3 Nonedible liganocellulosic biomass is the one of the promising sources for renewable carbon feedstocks to make fuels and/or useful synthetic building blocks.4 However, liganocellulosicderived molecules pose the problem of being overfunctionalized by virtue of having an abundance of oxygen atoms with a variety of functionalities, includin g hydroxyl and carbonyl groups.5 Lignin is the only biorenewable source for the production of aromatic compounds.6 Defunctionalization of lignin biomass into tailor-made fuels and chemicals has been identified as one of the key enabling technologies ultimately essential for the recognition of the “ideal biorefinery”. However, a mixture of various phenolic monomers was mostly produced from bimetallic hydrogenolysis of organosolv lignin in water.7 Recently, the selective phenolic monomer synthesis from lignin was achieved only by a time-consuming three-step process.8 Thus, the development of selective catalysts which are capable of dehydration, hydrodeoxygenation, and hydrogenolyzation © 2015 American Chemical Society

with preserved aromatic nature represents a promising yet challenging approach. In general, the catalytic dehydration, hydrogenation, and hydrodeoxygenation reactions are carried out using the hydrogen molecule (H2) for upgrading the biomass and the derived chemicals. However, molecular H2 is dangerous to handle and extreme care should be taken to avoid disasters caused by even small leakage during production, storage, and transportation.9 In addition, the stoichiometry of H2 is difficult to control, often leading to over-reduction. Therefore, facile generation of H2 from a liquid source of formic acid (FA) or methanol,10−14 which is derivable from biomass, has drawn much interest. It would be highly desirable for safety that the hydrogen be consumed in situ while it is being produced in the same reactor. Many homogeneous catalysts have been used for decomposition of FA, the first half of the tandem reaction system at ambient temperatures.12,13 However, the homogeneous ligand Received: June 23, 2015 Revised: September 18, 2015 Published: October 13, 2015 6964

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catalysts were synthesized on Fe3O4/N-rGO support (see section S2 and Tables S1−S4 in the Supporting Information). It is apparent from Table 1 that no reaction takes place in 10 h at 130 °C (entries 1−4) when a single component is used, as

complexes are difficult to synthesize, handle, separate, and recycle, which makes them less suitable for industrial applications.15 On the other hand, heterogeneous catalysts of metal nanoparticles (NPs) are much less active in comparison to homogeneous catalyst. For formic acid decomposition, bimetallic catalysts have been shown to be better than their single-metal catalysts due to charge redistribution between bimetals which strengthens the adsorption of formate through strong back-donation.16 In fact, the bimetallic catalyst of 2.2 nm PdAg alloy nanoparticles turned out to be a better choice for the first part of the tandem reaction.14 However, the initiation turnover frequency (TOF) was in the range of 382 h−1 at 50 °C. Improvement of TOF with earth-abundant metal is still demanded for efficient FA dehydrogenation reactions. On the other hand, heterogeneous catalysts based on metal NPs have to contend with deactivation. The deactivation via cluster agglomeration or Oswald ripening is a serious problem.17 In addition, metal catalysts on supports such as metal oxides and activated carbons can often leach out into the reaction mixture due to weak metal−support interaction.18,19 Catalytic deoxygenation of lignin or lignin-model compounds with formic acid has been reported by several groups.20−23 Lignin has a complex chemical structure that is highly stable and branched (due to the >90% of β-O-4, α-O-4 aryl ether bonds). Therefore, decomposition of lignin to small molecules is quite difficult. In particular, the selective hydrodeoxygenation of lignin-derived pyrolysis oil is more challenging because the similar bond strengths of C−O and CC leads to competition between hydrodeoxygenation and aromatic hydrogenation reaction.24 Considering the limitation seen from the previous biomass and chemical feedstock upgrading, synthesis in controlling the NP size, and composition, we decided to make a novel dual-functionalized catalyst to achieve the defunctionalization of lignin-based biomass with no use of H2 under mild conditions. Here we report an efficient dualfunctional PdAg alloy metal catalyst with high TOF supported on N-doped reduced graphene oxide (N-rGO) that generates H2 from formic acid and subsequently utilizes the generated hydrogen to hydrodeoxygenate selectively the lignin biomass derived chemicals under mild conditions without any additives in one pot. We also loaded magnetite (Fe3O4) particles for easy separation and enhanced mixing efficiency. Furthermore, the ∼4 atom % basic N heteroatom on the N-rGO facilitates binding of nanoparticles to make them well distributed on the support as well as reaction promotor, which prevents metal agglomeration and leaching even after the catalyst recycling process. The selective upgrading of lignin biomass derived compounds was successfully demonstrated by employing several model molecules, including vanillin, isatin, and two types of benzyl phenyl ethers, well-known natural compounds, in an environmentally friendly one-pot manner. This enables meeting critical needs in the production of upgraded biofuels and chemical feedstocks from biomass resources.

Table 1. Comparative TOFs of Vanillin Hydrodeoxygenation Reaction with Various Synthesized Catalystsa

entry

catalyst

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

GO N-rGO Fe3O4 Fe3O4/N-rGO Pd/N-rGO Pd/Fe3O4/N-rGO Ag/Fe3O4/N-rGO Au/Fe3O4/N-rGO Pt/Fe3O4/N-rGO Co/Fe3O4/N-rGO Pd50Co50/Fe3O4/N-rGO Pd50Au50/Fe3O4/N-rGO Pd50Pt50/Fe3O4/N-rGO Pd50Ag50/Fe3O4/N-rGO Pd50Ag50/Fe3O4/N-rGO Pd33Co33Au33/Fe3O4/ N-rGO Pd50Ag50/N-rGO Pd50Ag50/Fe3O4/N-rGO Pd50Ag50/N-rGO + Fe3O4 Pd50Ag50/Fe3O4/N-rGO

17 18b 19c 20d

temp (°C)

BET area (m2/ g)

TOF (h‑1)

130 130 130 130 130 130 130 130 130 130 130 130 130 130 80 130

558 230

N/A N/A N/A N/A 8 32 N/A N/A N/A N/A 50 15 N/A 521 5 63

130 150 130 130

220 202 202 214 222 226 210 152 260 201 251 251 206 222 251 N/A

99 2937 100 482

a

Reaction conditions unless specified otherwise: vanillin (2 mmol), formic acid (2.5 equiv), catalyst (20 mg), water (1 mL), specified reaction time (2 h). Conversion was based on GC analysis. The turnover frequency (TOF; average of three runs, error limit 1%) is given in units of (mol of 2-methoxy-4-methylphenol production)/(h (mol of active site)). bWith hydrogen gas, water 5 mL, pressure 1.0 MPa, reaction time 2 h, catalyst 20 mg, temperature 150 °C. cAddition of 20 mg of Fe3O4 powder. dCatalyst reused 11 times.

in GO, N-rGO, and Fe3O4 particles. The Fe3O4 was initially added for easy separation (Movie S1 in the Supporting Information) and enhancement of mixing properties (Movie S2 in the Supporting Information). When we introduced a metallic component, i.e., Pd/N-rGO (Table 1, entry 5), we observed that the aldehyde group in vanillin was selectively hydrodeoxygenated to a methyl group with a low turnover frequency (TOF based on the number of active metal sites, see section S4 and Table S5 in the Supporting Information) of 8 h−1. We further found that the TOF increased significantly with Fe3O4 loading (Table 1, entries 6−10), indicating that Fe3O4 does not simply play the physical roles of helping easy separation and enhanced mixing. As indicated in Table 1, Fe3O4 promotes the TOF of the reaction presumably by aiding the catalytic cycle of the tandem reactions as discussed later. Among the many transition metals tested, only Pd showed nearly complete conversion. Similar results were reported with gaseous hydrogen supplied externally for Pd nanoparticles supported on mesoporous N-doped carbon.24 Hence, one may surmise

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RESULTS AND DISCUSSION To evaluate the efficiency and selectivity of a series of catalysts for selective hydrodeoxygenation with H2 formed in situ from formic acid, vanillin was chosen as a model substrate for upgrading biofuel. Vanillin is a common component of pyrolysis oil derived from nonedible lignin, which constitutes ∼30 wt % of woody biomass. It is more challenging to deoxygenate lignin-derived pyrolysis oil than cellulose-derived oil.24 For this purpose, a series of mono-, bi-, and trimetallic 6965

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Table 2. Control Selectivity Experiments of Hydrodeoxygenation of Vanillin over Various Pd-Based Catalystsa

yield (%)b entry

catalyst

S/C

conversn (%)b

2-methoxy-4-methylphenol

2-methoxyphenol

other

c

Pd/C Pd/N-AC Pd/N-rGO Pd@Ag/C Pd/Fe3O4/N-rGO Pd50Ag50/N-rGO Pd50Ag50/Fe3O4/N-rGO Pd70Ag30/Fe3O4/N-rGO Pd80Ag20/Fe3O4/N-rGO Pd50Ag50/Fe3O4/N-rGO Pd50Ag50/Fe3O4/N-rGO

10 10 10 106 106 106 106 106 106 106 303

63 64 66 100 80 100 100 98 88 100 100

32 40 51 69 70 98 99 43 24 99 99

14 10 4 17 N/A N/A N/A N/A N/A N/A N/A

17 13 11 14 10 1 N/A 54 63 N/A N/A

1 2d 3 4e 5 6 7 8 9 10f 11g a

Reaction conditions unless specified otherwise: vanillin (0.5 mmol), formic acid (2.5 equiv), S/C = 106 (substrate mmol/Pd mmol), water (1 mL), specified reaction time (6 h), temperature 130 °C. bConversion and yield based on GC analysis and anisole as a standard, average of two measurements, data errors 99 63 98 98 88 95 85 100 100 100

81 32 100 74 14 69 47 3 >99 100

6 this study 24 24 24 24 47 20 this study this study

Reaction conditions unless specified otherwise: aData taken from ref 6. bFormic acid (2.5 equiv), S/C = 106 (substrate mmol/Pd mmol (wt %), water (1 mL). cS/C = 1000, water (2 mL). dS/C = 1000, water (1 mL), decalin (1 mL). eS/C = 70, formic acid (10.9 equiv). fS/C = 303, water (1 mL), formic acid (2.5 equiv).

that Pd could be the main active component of the catalyst responsible for the selective hydrodeoxygenation of vanillin. Of the bimetallic catalysts tested (entries 11−15), the yields of Pd50Co50 and Pd50Ag50 pairs were better than 99% but the TOF was much greater for Pd50Ag50 (521 h−1) than for Pd50Co50 (50 h−1). Other catalysts, including a trimetallic catalyst (entry 16), did not offer any advantage over the Pd50Ag50 bimetallic catalyst. In order to investigate the relative efficiency of the bimetallic Pd50Ag50/Fe3O4/N-rGO catalyst with BET surface area 251 m2/g,25 control experiments of Pd/C, Pd on N-doped activated carbon (Pd/N-AC), Pd/N-rGO, core−shell Pd@Ag/C, Pd/ Fe3O4/N-rGO, and Pd50Ag50/N-rGO were carried out by comparing the conversion and yield of 2-methoxy-4-methylphenol (MMP) as a main product of hydrodeoxygenation in Table 2. Commercial Pd/C with high surface area produced

63% conversion with only a 32% yield of MMP by forming considerable byproducts (entry 1, Figure S1 in the Supporting Information). The Pd/N-AC produced 64% conversion but slightly improved selectivity (40% of MMP, entry 2).26 Upon immobilization of the Pd nanoparticles with identical size on NrGO (as measured by TEM and XRD), a similar conversion (66%) but highly improved yield (51%) of MMP was achieved (entry 3), presumably due to the reaction-promoting effect of well distributed Pd nanoparticles on basic nitrogen sites as well as π−π interaction between graphene and aromatic reactant to activate the α-carbon position.27 In addition, the core−shell Ag@Pd/C catalyst produced 69% MMP yield with 100% conversion (entry 4, Figure S2 in the Supporting Information). However, Pd/Fe3O4/N-rGO significantly improved the selectivity to 70% yield with 80% conversion (entry 5, Figure S3 in the Supporting Information). Both Pd50Ag50/N-rGO and 6966

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Figure 1. Microscopic analysis and proposed structure of Pd50Ag50/Fe3O4/N-rGO catalyst: (a, b) SEM images; (c) low-resolution TEM analysis; (d) high-resolution TEM analysis; (e) selected area electron diffraction pattern of N-rGO layer; (f) d spacing from stacked N-rGO layers; (g) dark field HAADF images with elemental line profile (yellow line, Fe; green line, Pd; red line, Ag); (h, i) CS-corrected TEM images of magnetite and PdAg nanoalloy; (j, k) proposed models for Fe3O4 and PdAg nanoalloy; (l) 2D scratch art model (yellow, magnetite; blue background, N-rGO; and green/ red cluster, PdAg alloy); (m) proposed 3D structural model.

Fe3O4/N-rGO catalyzes the sequential tandem formic acid decomposition−hydrodeoxygenation reactions, without any additives or additional pressure. To look into the structure and nature of the catalyst, we resorted to various instrumental methods. First, X-ray photoelectron spectroscopy (XPS) was utilized to find surface compositions (Table S4 and Figures S10 and S11 in the Supporting Information) for C, O, and N in N-rGO and Pd50Ag50/Fe3O4/N-rGO catalyst. C 1s, O 1s, and N 1s binding energies of Pd50Ag50/Fe3O4/N-rGO (Figure S11a−c) were identical with those of the as-prepared N-rGO, indicating that the structure was unaffected by the loading of catalyst NPs.27,31 The absence of the satellite peak at 719 eV characteristic of Fe3+ in Fe2O3 (Figure S11d) clearly indicates the absence of Fe2O3.32 As given in Table S4, N-rGO has large amounts of oxygen (∼10 atom %) and nitrogen (∼4 atom %). These heteroatoms in a graphene structure can coordinate metals, resulting in highly distributed and stable metal NPs and preventing reoxidation of noble metal M0 (M = Ag, Pd), as similarly reported for N-doped carbon.24 In addition, the metallic nature of Pd0 and Ag0 in the Pd50Ag50/Fe3O4/N-rGO was confirmed by doublet XPS peaks at 368 and 374 eV designated as Ag 3d and 336.1 and 341.2 eV as Pd 3d, respectively (Figure S11e,f), which is consistent with the literature.33 Detailed SEM results of our bifunctional Pd50Ag50/Fe3O4/NrGO catalyst shown in Figure 1a,b reveal that the catalyst takes on a shape of nanosheets resembling tiny rose petals. Highresolution transmission electron microscopy (HR-TEM) shows that the catalytic nanoparticles are reasonably dispersed with no agglomeration (Figure 1c,d). The ordered crystalline structure of N-rGO support was demonstrated by a selected area electron diffraction pattern (SAED) with two bright rings corresponding to (1100) plane and (1100) reflection patterns (Figure 1e), in

Pd50Ag50/Fe3O4/N-rGO catalysts showed excellent conversion (100%) and yield (∼99%) in water solvent under mild conditions (entries 6 and 7, Figures S4 and S5 in the Supporting Information). Note that the decreased Ag composition lowered the efficiency (entries 8 and 9, Figures S6 and S7 in the Supporting Information). Furthermore, the excellent catalytic efficiency was not affected by the loaded amount of formic acid (entry 10) and a gram scale of MMP production (entry 11). In addition, catalytic superiority of the Pd50Ag50/Fe3O4/N-rGO catalyst in the selective hydrodeoxygenation of vanillin was clearly illustrated in comparison to various reported catalysts in the literature (Table 3). The time profile of the reaction revealed that vanillin was converted to MMP by two pathways: (i) two-step hydrogenation/hydrogenolysis via a 4-(hydroxymethyl)-2-methoxyphenol intermediate and (ii) direct hydrogenolysis, similar to that in the previous report.24 To understand the behavior of Pd50Ag50/Fe3O4/N-rGO catalyst in the tandem reaction of the formic acid decomposition−hydrodeoxygenation sequence, decomposition of formic acid alone28 was first studied at 50 °C (section S5 in the Supporting Information, TOF calculation for formic acid decomposition). The total volume of CO2 and H2 collected for 1 h was 20.3 mL with a ratio of 1.06 (Figure S9 in the Supporting Information). The corresponding TOF for formic acid dehydrogenation was found to be in the range of 497 h−1 (see Table S6 in the Supporting Information), using H2 chemisorption measurement methods for active surface Pd atoms, which is comparable to the reported values for active formic acid dehydrogenation catalysts.11,29,30 The possible byproduct of CO by formic acid dehydroxylation was not detected even at higher temperature (90 °C), and the catalyst showed prolonged stability. This finding together with the hydrodeoxygenation of vanillin shows that our Pd50Ag50/ 6967

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Figure 2. X-ray absorption spectroscopy: (a) Pd, (b) Ag, and (c) Fe radial distribution functions of Fourier-transformed EXAFS spectra in different catalytic reactions.

(EXAFS) analyses have been carried out in situ under reaction conditions (Figure 2 and Figure S15 and section S7 in the Supporting Information). From XANES data of the pristine Pd50Ag50/Fe3O4/N-rGO catalyst, it is clear that both Pd and Ag show typical features of metallic phases (Figure S15a,b). In the radial distribution functions (RDF) of Fourier-transformed (FT) Pd and Ag K-edge EXAFS spectra, the pristine catalyst also shows intense singlet peaks at around 2.5 and 2.6 Å, corresponding to the Pd−Pd/Ag and Ag−Ag/Pd nearest metal−metal bonding, respectively (Figure 2a,b). The data indicate that PdAg exists in a metallic phase in the pristine Pd50Ag50/Fe3O4/N-rGO catalyst. In addition, Pd atom in the pristine catalyst shows a slight peak shift to higher energy in XANES and an additional weak FT peak in the lower r region at ∼2.0 Å. The result could be interpreted as the chemical bonding between surface Pd atoms on the metallic phase and heteroatoms (N or O) on the N-rGO layer. These spectral features indicate partial oxidation of Pd atom by a metal to ligand charge transfer and anchoring of PdAg nanoparticles onto the N-rGO layer. Under the reaction conditions the first metallic FT peak in Pd K-edge EXAFS is slightly shifted to a longer interatomic distance, while the Ag K-edge spectral behavior is clearly invariant before and after the catalytic reaction (Figure 2b). These results indicate that the Ag does not actively participate in the reaction. An earlier report of decomposition of formic acid with Ag−Pd core/shell nanoparticles suggested that Ag electronically promotes Pd metal for formate ion adsorption by a strong back-donation.16 In this work, both Fourier-transformed (FT) magnitudes of Pd K-edge and Ag K-edge EXAFS decreased in a similar ratio, in comparison to those of each bulk metallic phase (Figure S15d). This indicated that the averaged coordination numbers around each atom are similar to each other in the face-centered cubic structure, as additional evidence of alloy PdAg NPs. Finally, Fe K-edge XANES spectra of pristine catalyst show peak shifting to higher energy due to binding heteroatoms (O/ N) of graphene (Figure S15c in the Supporting Information, black line). In the RDF of Figure 2c, a clear peak at ∼2 Å denoted by an arrow can be assigned to a weak chemical bond between Fe elements and heteroatoms (N or O) in the N-rGO layer, which can be expected to drive the Fe3O4 nanoparticles uniformly distributed through the N-rGO layer. Note that a detailed structural arrangement (Pd−Ag−Fe3O4 or Ag−Pd− Fe3O4) of the catalyst is still difficult due to the similar atomic numbers of Ag and Pd. The in situ decomposition of formic

addition to an interlayer distance of 0.37 nm of restored graphitic layers (Figure 1f), which is consistent with that reported.27 The atomic scale image by STEM with a probe correction and ∼0.1 nm point resolution provided the evidence for close proximity between the PdAg and the Fe3O4 particles, as indicated by the line scan (upper part of Figure 1g). The brighter part (red circle) corresponds to PdAg atoms (Figure 1i) and the less bright part (yellow rectangle) to Fe atoms (Figure 1h) because the intensity is directly proportional to the square of the atomic number of the elements.34 Importantly, selected area EDX line scans across the PdAg/Fe 3 O 4 nanoparticles indicated that three metals were colocated in the identical NP with different sizes of 5−8 nm for PdAg and 8−15 nm for Fe3O4. Conversely, the PdAg alloy NP displays uniform lattice patterns with no intraparticular boundaries, indicative of a single alloyed phase with a (111) lattice fringe distance of 0.23 nm (Figure 1i) between the (111) lattice of face-centered cubic (fcc) Ag (0.24 nm) and fcc Pd (0.22 nm) NPs. These findings indicate that PdAg is formed as an alloy and not as a core/shell structure.14 Alternatively, additional evidence of alloy structure is that the Pd/Ag ratio (2.47 wt %/2.53 wt % = 0.97, Table S3 in the Supporting Information) measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) was nearly similar to the surface compositional Pd/Ag ratio (0.32 atom %/0.34 atom % = 0.94) measured by XPS. Fe3O4 nanoparticles displayed a 0.25 nm (311) lattice fringe distance, indicating that copresence with PdAg did not affect crystallization of Fe3O4 (Figure 1h)35 and its superparamagnetic properties (see section S6, Figure S12, and Table S7 in the Supporting Information).36 The XRD patterns of the PdAg NP showed a very weak peak intensity because of their small sizes and the distance of (111) planes of 2.3 Å (Figures S13 and S14 in the Supporting Information), further indicating the alloy type of the NP. From the (111) diffraction peak and Scherrer’s formula, the average size of the crystallites was calculated to be 6.2 ± 0.5 nm, in agreement with the value observed by TEM. On the basis of these results of physical characterization, a structural model of the Pd50Ag50/ Fe3O4/N-rGO catalyst could be proposed as shown in Figure 1j−m. Over a 2D surface of N-rGO sheet, Fe3O4 (8−15 nm in size) particles are uniformly distributed with PdAg (5−8 nm in size) alloy NP in close proximity. In order to understand the evolution of the catalyst structure during the reaction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure 6968

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Figure 3. Proposed pathways for tandem hydrodeoxygenation of vanillin with formic acid.

reduction. Magnetic Fe3O4 particles promote the reaction by chemisorbing formic acid and transferring the resulting formate and hydrogen to the PdAg surface. On the basis of these mechanistic ideas derived from the literature and our own experimental results for our bifunctional Pd50Ag50/Fe3O4/N-rGO catalyst, the reaction pathways and the related hypothetical structures are proposed as shown in Figure 3 and Figure S8 in the Supporting Information. The promoting role of Fe3O4 is further supported by the importance of the close proximity of magnetite particles to the PdAg alloy particles, which is necessary for the facile migration of the formate species. Furthermore, in situ formic acid decomposition in the absence of vanillin revealed the collapse of longrange Fe−O−Fe order, as observed in XANES spectra. Thus, rapid consumption of the hydrogen by the vanillin preserves the structural integrity of the magnetite phase. It is important for practical applications that the catalyst is highly stable, easily separable, and readily recyclable without loss of activity. With the model reaction involving vanillin (Table S8 in the Supporting Information), the catalyst was recycled up to 11 times with negligible loss of activity. The yield of 2-methoxy-4methylphenol remained constant all throughout the 11 cycles. Moreover, the catalyst could easily be separated from the reaction solution by simply using either a magnet (Movie S1 in the Supporting Information) or filtration/centrifugation (in the filtration process 100% catalyst recovery was not possible). After each reaction cycle the composite catalyst was checked by XRD (Figures S17 and S18 in the Supporting Information), which showed negligible change in PdAg but slight oxidation of Fe3O4 due to the phase-changing behavior of iron catalyst during biomass upgradation (details in section S9 in the Supporting Information). To check the possible leaching of metal NPs during the reaction, the reaction mixture was analyzed by inductive coupled plasma emission spectroscopy (ICP-ES). Less than 0.1 ppm of Ag and Pd was detected, indicating negligible leaching of Ag and Pd into the reaction mixture. This fact clearly shows that the heteroatoms (N, O) of N-rGO strongly bind metal NPs, making them well distributed throughout the surface. The surface of the catalyst recycled 11 times was analyzed in detail by XPS. It was found that the

acid leads to peak broadening of the XANES and abrupt collapse of Fe−O−Fe long-range ordering around 2 Å in the RDF. On the other hand, the phase change of iron oxide is not observed in the in situ vanillin hydrodeoxygenation reaction. Hence, hydrogen formed by formic acid decomposition on the Pd50Ag50/Fe3O4/N-rGO catalyst can effectively reduce Fe3O4 in the N-rGO layer. When the vanillin is present in the medium, however, the hydrogen preferentially reacts with vanillin rather than Fe3O4. From these in situ XANES and EXAFS spectral analyses, we could confirm that the Pd atom actively participates in both formic acid decomposition and vanillin hydrodeoxygenation reactions. Well-defined PdAg nanoalloys or the core−shell nanopaticles have been investigated in the past for formic acid decomposition, which suggested that Ag could play the role of electronic promoter.14 It has also been reported that vanillin is hydrodeoxygenated by Pd nanoparticle catalysts with hydrogen gas.24 We found that the magnetite Fe3O4 particle itself or Fe3O4/N-rGO has no catalytic activity for vanillin hydrodeoxygenation with formic acid (Table 1, entries 3 and 4). Yet, it is interesting that the absence or separate addition of the magnetite particles showed only low TOFs of 99 and 100 (Table 1, entries 17 and 19), while Pd50Ag50/Fe3O4/N-rGO catalyst had a much higher TOF of 521 (Table 1, entry 14). To elucidate the role of Fe3O4 in the notable synergistic effect, formic acid adsorption over Pd50Ag50/Fe3O4/N-rGO catalyst was investigated by ATR-IR spectroscopy. We observed characteristic peaks of monodentate formate at 1620 cm−1 arising from νas(OCO) stretching and bridging formate at 1362 and 1585 cm−1 from symmetrical and asymmetrical modes of O−C−O stretching (Figure S16c in the Supporting Information) on both Pd50Ag50/Fe3O4/N-rGO and Fe3O4 alone. The peak positions are consistent with the literature reports on formate species binding to magnetite center in a variety of ways through formic acid adsorption,37,38 consistent with the molecular scale investigation.39 It could be assumed here that the formate species could migrate to a nearby PdAg particle and is decomposed to facilitate hydrodeoxygenation of vanillin on the PdAg surface. Hence, PdAg alloy surface is the main active site catalyzing both formic acid decomposition and vanillin 6969

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Figure 4. Selective hydrodeoxygenation of aromatic aldehyde/ketone and functionalized aromatic aldehyde/ketone to aromatic methylene compounds catalyzed by Pd50Ag50/Fe3O4/N-rGO. Reaction conditions: reactant (0.5 mmol), formic acid (2.5 equiv), 30 mg of catalyst (∼0.32 atom % Pd), water (1 mL). Conversion and yield are based on gas chromatographic (GC) analysis using anisole (0.5 mmol) as an internal standard. At least three measurements were taken to obtain an average yield. Yields in parentheses refer to isolated yields.

catalyst Pd and Ag surface was almost the same with little alteration in elemental atomic percentage (Table S4 and Figure S19 in the Supporting Information) and that the catalyst showed only slightly decreased TOF (Table 1, entry 20). On the other hand, securing mass balance is the most important consideration for industrial scale biofuel upgrading. During the mass balance study, we observed that the catalyst was fully recovered without any coke deposition over the catalyst surface and also the total mass balance during the reaction was almost completely closed (>99%, Table S6 in the Supporting Information). Now that the suitability of the catalyst for practical applications is assured, we undertook an examination of the efficiency of our Pd50Ag50/Fe3O4/N-rGO catalyst in defunctionalizing a number of compounds derived from carbonyl groups, including vanillin and isatin as well-known natural compounds. Of the hydrodeoxygenation reactions, which are considered the most effective method for upgrading oxygenrich biofuels,40 the most challenging reaction involves direct hydrodeoxygenation of carbon−oxygen bonds in aromatic aldehydes.41 The usual direct hydrodeoxygenation leads to significant amounts of undesired products such as aromatic alcohol, methylcyclohexane, and benzene.42 The usual approach, therefore, is to reduce aldehyde/ketones to alkanes in the presence of aniline41 or H2S gas43 at high temperature and pressure. Direct hydrodeoxygenation of aromatic aldehydes and ketones with hydrogen produced in situ from formic acid was attempted with our catalyst to produce a variety of industrially applicable synthetic feedstocks, and the results are given in Figure 4. Aldehydes with various functionalities were

converted selectively to methyl-substituted aromatic compounds (1−9) at 130 °C, and the yield was better than 90% for all molecules tested whenever the conversion took place. Aromatic ketones, such as acetophenone, benzophenone, and fluorenone, were also selectively hydrodeoxygenated to aromatic alkanes (10−12) without arene hydrogenation. To our delight, even substrates containing amide and aliphatic ester substituted aldehyde or ketone groups were also converted smoothly and selectively to the corresponding alkanes (13−16). The Pd50Ag50/Fe3O4/N-rGO catalyst, however, was only reactive with aromatic carbonyl groups; it did not have any activity toward the CO bonds in amides, carboxylic acids, esters, and aliphatic carbonyls (Figure S20 in the Supporting Information). Selective catalytic hydrogenolysis of C−O bonds with its preserved aromatic nature is a challenging chemistry for lignin biomass utilization. In particular, hydrogenolysis of β-O-4 and α-O-4 C−O model compounds, taking up to 90% monomeric units of the alkyl aryl ether in the lignin heterogeneous structure, led to unsatisfactory levels of selective hydrogenolysis with various byproducts.7,44 Recently selective depolymerization of lignin model compounds with a prevented benzene ring was achieved by a complex multiple-step process where aerobic oxidation of the β-O-4 linkage model under high O2 gas pressure was followed by zinc metal mediated cleavage of the aryl C−O bond and a subsequent NaBH3CN mediated reduction.8 However, a simple one-pot single step of selective hydrogenolysis at atmospheric pressure was demonstrated by guaiacylglycerol-β-guaiacyl ether with a β-O-4 C−O bond as a model compound, which was selectively hydrogenated and 6970

DOI: 10.1021/acscatal.5b01319 ACS Catal. 2015, 5, 6964−6972

Research Article

ACS Catalysis deoxygenated with no use of additives except formic acid over Pd50Ag50/Fe3O4/N-rGO bifunctional catalyst at 120 °C (Figure 5). The reaction resulted in complete conversion, yielding two

Figure 5. Selective hydrogenolysis reactions of β-O-4 linkage model aryl C−O group with formic acid over Pd50Ag50/Fe3O4/N-rGO catalyst. Conditions: reactant (0.1 mmol), formic acid (3.5 equiv), catalyst (50 mg), time 3 h, temperature 120 °C. Conversion and yield are based on NMR analysis using decane (0.1 mmol) as an internal standard. At least two measurements were taken for an average yield. In all cases complete conversion of the β-O-4 linkage aryl C−O group was achieved.

Figure 6. Selective hydrogenolysis reactions of α-O-4 linkage model aryl C−O group with formic acid over Pd50Ag50/Fe3O4/N-rGO catalyst. Conditions: reactant (0.5 mmol), formic acid (1.5 equiv), catalyst (50 mg), temperature 120 °C. Conversion and yield are based on GC analysis using decane (0.5 mmol) as an internal standard. At least two measurements were taken for an average yield. In all cases complete conversion of the α-O-4 linkage model aryl C−O group was achieved.

products: 3-(3-methoxy-4-hydroxyphenyl)-1-propanol (17a) (to date the production of 17a as a major product has rarely been reported)6 and guaiacol (17b) with excellent yields (Figures S21 and S22 in the Supporting Information). Another reaction of interest is the hydrogenolysis reaction of 2-phenoxy1-phenylethanol. It has been reported that the reaction with bimetallic catalyst under high hydrogen pressure led to more than 12 products with a poorly preserved aromatic nature of the product.44,45 In contrast, the use of Pd50Ag50/Fe3O4/NrGO catalyst for hydrogenolysis of 2-phenoxy-1-phenylethanol in the presence of formic acid at a comparatively low temperature of 120 °C produced only two products (18a,b) with high selectivity via β-O-4 C−O model bond cleavage. It is noted in this regard that the absence/presence of substituted functional groups did not affect the hydrogenolysis of benzyl phenyl ether, and excellent conversion and yield were generally obtained at 120 °C (Figure 6, 19−21). A comparison to previously reported heterogeneous catalysts for hydrogenolysis of β-O-4 and α-O-4 C−O bonds reveals the superior performance of Pd50Ag50/Fe3O4/N-rGO catalyst with excellent selectivity under much milder conditions (Table S9 in the Supporting Information). The PdAg bimetallic catalyst promoted by Fe3O4 introduced here produces hydrogen from biomass-derived formic acid and then utilizes the generated hydrogen in situ to upgrade biofuels and chemical feedstocks. The efficiency of this dual-functional catalyst is exemplified by more than 1 order of magnitude increase in TOF and yield in comparison with only Pd supported on N-rGO (Table 1). The versatility and effectiveness of the catalyst advanced here are demonstrated by 99% yields in almost all cases involving selective hydrodeoxygenation of aromatic aldehydes and ketones with various functional groups and selective hydrogenolysis of β-O-4 and α-

O-4 C−O model compounds. This new catalyst could be an efficient and versatile platform for the valorization of biomass.

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CONCLUSIONS We have introduced a novel strategy for the preparation of the multicomponent and multifunctional transition-metal solid catalyst Pd50Ag50/Fe3O4/N-rGO for the selective upgrading of biomass-derived vanillin, isatin, and two types of benzyl phenyl ethers as lignin biomass-derived model compounds, and synthetic feedstocks derived from aromatic carbonyl by a simple one-pot reaction under mild conditions. This represents a significant advance toward the efficient valorization of biomass by production of bio-oil and useful chemicals. Our novel catalyst is effective in dehydrogenating formic acid to form H2 and subsequently utilizing the hydrogen generated in situ for upgrading bio-oil and chemical feedstocks. The catalyst is easy to synthesize, stable, easily separable, and readily recyclable without loss of activity, and the reactions can be carried out in water, making the process environmentally friendly. It could play a critical role in upgrading biomassderived chemicals to fill future gaps in the supply of transportation fuels and chemical feedstocks.

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ASSOCIATED CONTENT

* Supporting Information S

The following file is available free of charge on the ACS Publications Web site at DOI: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.5b01319. 6971

DOI: 10.1021/acscatal.5b01319 ACS Catal. 2015, 5, 6964−6972

Research Article

ACS Catalysis

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(19) Huang, Y. J.; Zhou, X. C.; Yin, M.; Liu, C. P.; Xing, W. Chem. Mater. 2010, 22, 5122−5128. (20) Liguori, L.; Barth, T. J. Anal. Appl. Pyrolysis 2011, 92, 477−484. (21) Oregui Bengoechea, M.; Hertzberg, A.; Miletić, N.; Arias, P. L.; Barth, T. J. Anal. Appl. Pyrolysis 2015, 113, 713−722. (22) Kloekhorst, A.; Shen, Y.; Yie, Y.; Fang, M.; Heeres, H. J. Biomass Bioenergy 2015, 80, 147−161. (23) Toledano, A.; Serrano, L.; Pineda, A.; Romero, A. A.; Luque, R.; Labidi, J. Appl. Catal., B 2014, 145, 43−55. (24) Xu, X.; Li, Y.; Gong, Y.; Zhang, P.; Li, H.; Wang, Y. J. Am. Chem. Soc. 2012, 134, 16987−16990. (25) Scheuermann, G. M.; Rumi, L.; Steurer, P.; Bannwarth, W.; Mülhaupt, R. J. Am. Chem. Soc. 2009, 131, 8262−8270. (26) Konnerth, H.; Zhang, J.; Ma, D.; Prechtl, M. H. G.; Yan, N. Chem. Eng. Sci. 2015, 123, 155−163. (27) Singh, A. K.; Basavaraju, K. C.; Sharma, S.; Jang, S.; Park, C. P.; Kim, D.-P. Green Chem. 2014, 16, 3024−3030. (28) Broggi, J.; Jurčík, V.; Songis, O.; Poater, A.; Cavallo, L.; Slawin, A. M. Z.; Cazin, C. S. J. J. Am. Chem. Soc. 2013, 135, 4588−4591. (29) Zhou, X.; Huang, Y.; Xing, W.; Liu, C.; Liao, J.; Lu, T. Chem. Commun. 2008, 3540−3542. (30) Gu, X.; Lu, Z.-H.; Jiang, H.-L.; Akita, T.; Xu, Q. J. Am. Chem. Soc. 2011, 133, 11822−11825. (31) Park, S.; Hu, Y.; Hwang, J. O.; Lee, E.-S.; Casabianca, L. B.; Cai, W.; Potts, J. R.; Ha, H.-W.; Chen, S.; Oh, J.; Kim, S. O.; Kim, Y.-H.; Ishii, Y.; Ruoff, R. S. Nat. Commun. 2012, 3, 638. (32) Zhang, D.; Liu, Z.; Han, S.; Li, C.; Lei, B.; Stewart, M. P.; Tour, J. M.; Zhou, C. Nano Lett. 2004, 4, 2151−2155. (33) Sanyal, U.; Davis, D. T.; Jagirdar, B. R. Dalton Trans. 2013, 42, 7147−7157. (34) Gross, E.; Liu, J. H.-C.; Toste, F. D.; Somorjai, G. A. Nat. Chem. 2012, 4, 947−952. (35) Li, J.; Zeng, H.; Sun, S.; Liu, J. P.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 14005−14008. (36) Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I.-C.; Kim, K. S. ACS Nano 2010, 4, 3979−3986. (37) Cutting, R. S.; Muryn, C. A.; Thornton, G.; Vaughan, D. J. Geochim. Cosmochim. Acta 2006, 70, 3593−3612. (38) Busca, G.; Lamotte, J.; Lavalley, J. C.; Lorenzelli, V. J. Am. Chem. Soc. 1987, 109, 5197−5202. (39) Tedsree, K.; Kong, A. T. S.; Tsang, S. C. Angew. Chem., Int. Ed. 2009, 48, 1443−1446. (40) Sutton, A. D.; Waldie, F. D.; Wu, R.; Schlaf, M.; Silks, L. A. P.; Gordon, J. C. Nat. Chem. 2013, 5, 428−432. (41) Liu, S.; Fan, X.; Yan, X.; Du, X.; Chen, L. Appl. Catal., A 2011, 400, 99−103. (42) Procházková, D.; Zámostný, P.; Bejblová, M.; Č ervený, L.; Č ejka, J. Appl. Catal., A 2007, 332, 56−64. (43) Ferrari, M.; Maggi, R.; Delmon, B.; Grange, P. J. Catal. 2001, 198, 47−55. (44) Zhang, J. G.; Asakura, H.; van Rijn, J.; Yang, J.; Duchesne, P.; Zhang, B.; Chen, X.; Zhang, P.; Saeys, M.; Yan, N. Green Chem. 2014, 16, 2432−2437. (45) He, J. Y.; Zhao, C.; Lercher, J. A. J. Am. Chem. Soc. 2012, 134, 20768−20775. (46) Zhang, B.; Wen, Z. H.; Ci, S. Q.; Mao, S.; Chen, J. H.; He, Z. ACS Appl. Mater. Interfaces 2014, 6, 7464−7470. (47) Crossley, S.; Faria, J.; Shen, M.; Resasco, D. E. Science 2010, 327, 68−72.

Experimental details, catalyst characterization data, and formic acid dehydrogenation graph (PDF) Magnetic separation of catalyst (AVI) Magnetic field movement of catalyst particle and decomposition of formic acid at 50 °C (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for H.H.L.: [email protected]. *E-mail for D.-P.K.: [email protected]. Author Contributions

A.K.S. and S.J. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the National Research Foundation (NRF) of Korea grant funded by the Korean government (MEST) (No.2008-0061983). A.K.S. thanks the National Institute of Nanomaterial Technology (NINT), Material Science Departments, Chemistry Departments, PAL, POSTECH, Pohang, and Korea Basic Science Institute (KBSI) Daejeon, for the analytical support. Chemical Engineering, POSTECH, has filed a patent on the catalysts reported herein.

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REFERENCES

(1) Kerr, R. A. Science 2012, 335, 522−523. (2) Murray, J.; King, D. Nature 2012, 481, 433−435. (3) Tverberg, G. E. Energy 2012, 37, 27−34. (4) Zhao, C.; Lercher, J. A. Angew. Chem., Int. Ed. 2012, 51, 5935− 5940. (5) Ma, R.; Hao, W.; Ma, X.; Tian, Y.; Li, Y. Angew. Chem., Int. Ed. 2014, 53, 7310−7315. (6) Parsell, T. H.; Owen, B. C.; Klein, I.; Jarrell, T. M.; Marcum, C. L.; Haupert, L. J.; Amundson, L. M.; Kenttamaa, H. I.; Ribeiro, F.; Miller, J. T.; Abu-Omar, M. M. Chem. Sci. 2013, 4, 806−813. (7) Zhang, J. G.; Teo, J.; Chen, X.; Asakura, H.; Tanaka, T.; Teramura, K.; Yan, N. ACS Catal. 2014, 4, 1574−1583. (8) Lancefield, C. S.; Ojo, O. S.; Tran, F.; Westwood, N. J. Angew. Chem., Int. Ed. 2015, 54, 258−262. (9) Dormán, G.; Kocsis, L.; Jones, R.; Darvas, F. J. Chem. Health Saf. 2013, 20, 3−8. (10) Nielsen, M.; Alberico, E.; Baumann, W.; Drexler, H.-J.; Junge, H.; Gladiali, S.; Beller, M. Nature 2013, 495, 85−89. (11) Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Nat. Nanotechnol. 2011, 6, 302−307. (12) Boddien, A.; Mellmann, D.; Gärtner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Science 2011, 333, 1733−1736. (13) Fellay, C.; Dyson, P. J.; Laurenczy, G. Angew. Chem., Int. Ed. 2008, 47, 3966−3968. (14) Zhang, S.; Metin, Ö .; Su, D.; Sun, S. Angew. Chem., Int. Ed. 2013, 52, 3681−3684. (15) Jagadeesh, R. V.; Surkus, A.-E.; Junge, H.; Pohl, M.-M.; Radnik, J.; Rabeah, J.; Huan, H.; Schünemann, V.; Brückner, A.; Beller, M. Science 2013, 342, 1073−1076. (16) Tedsree, K.; Li, T.; Jones, S.; Chan, C. W. A.; Yu, K. M. K.; Bagot, P. A. J.; Marquis, E. A.; Smith, G. D. W.; Tsang, S. C. E. Nat. Nanotechnol. 2011, 6, 302−307. (17) Prieto, G.; Zečević, J.; Friedrich, H.; de Jong, K. P.; de Jongh, P. E. Nat. Mater. 2012, 12, 34−39. (18) Zhou, X. C.; Huang, Y. J.; Xing, W.; Liu, C. P.; Liao, J. H.; Lu, T. H. Chem. Commun. 2008, 3540−3542. 6972

DOI: 10.1021/acscatal.5b01319 ACS Catal. 2015, 5, 6964−6972