One-Pot Defunctionalization of Lignin-Derived Compounds by Dual

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One-pot defunctionalization of lignin-derived compounds by dual functional 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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01319 • Publication Date (Web): 13 Oct 2015 Downloaded from http://pubs.acs.org on October 17, 2015

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One-pot defunctionalization of lignin-derived

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compounds by dual functional Pd50Ag50/Fe3O4/N-rGO

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catalyst

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Ajay K. Singh,† Seungwook Jang,† Jae Yul Kim,‡ Siddharth Sharma,†,§ K.C Basavaraju,† Min-Gyu Kim,∥

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Kyung-Rok Kim†, Jae Sung Lee,‡ Hong H. Lee#,* and Dong-Pyo Kim†,*

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†National

Center of Applied Microfluidic Chemistry, Department of Chemical Engineering, POSTECH

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(Pohang University of Science and Technology), Pohang 790-784, Korea. ‡School

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of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology

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(UNIST), 50 UNIST-gil, Ulsan 689-798, Korea §Department

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University, Amritsar 143005, India ∥

Beamline Research Division, Pohang Accelerator Laboratory (PAL), POSTECH (Pohang University

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of Chemistry, U.G.C. Centre of Advance Studies in Chemistry, Guru Nanak Dev

of Science and Technology), Pohang 790-784, Korea #School

of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Korea.

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16 17 18 19 20 ACS Paragon Plus Environment

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ABSTRACT

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Generation of hydrogen from renewable source and its safe utilization for efficient one-pot upgrading of

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renewable biofuels are a challenge. Bimetallic PdAg catalyst supported on Fe3O4/nitrogen-doped

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reduced graphene oxide (N-rGO) were synthesized for hydrogen generation from formic acid with high

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TOF (497 h-1 at 50 oC), and subsequently utilized the hydrogen in-situ for selective defunctionalization

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of lignin-derived chemicals with preserved aromatic nature at ambient pressure. Hydrodeoxygenation of

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aromatic aldehydes and ketones showed excellent yield (99% at 130 oC) with no use of additives.

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Furthermore, hydrogenolysis of -O-4 and -O-4 C-O model compounds produced only two products

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with high selectivity at 120 oC, which is an efficient and versatile one-pot platform for valorization of

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lignin biomass.

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KEYWORDS Formic acid, bimetallic catalyst, magnetic separable, N-doped reduced graphene oxide,

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selective defunctionalization

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MANUSCRIPT TEXT

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■ Introduction

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The production of fuel and fine chemicals from biomass has received much attention due to world’s

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major oil fields have been in production decline for decades.1-3 Non-edible liganocellulosic biomass is

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the one of the promise source for renewable carbon feedstocks to make fuels and/or useful synthetic

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building blocks.4 But liganocellulosic-derived molecules pose the problem of being overfunctionalized

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by virtue of having an abundance of oxygen atoms with variety of functionality including hydroxyl and

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carbonyl

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Defunctionalization of lignin biomass into tailor-made fuels and chemicals has been identified as one of

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the key enabling technologies ultimately essential for the recognition of the “ideal biorefinery”.

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However, a mixture of various phenolic monomers was mostly produced from bimetallic hydrogenolysis

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of organosolv lignin in water.7 Recently, the selective phenolic monomer synthesis from lignin was

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achieved only by a time-consuming three step process.8 Thus, the development of selective catalysts

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which are capable of dehydrating, hydrodeoxygenating and hydrogenolysis with preserved aromatic

groups.5

Lignin

is

only

biorenewable

source

to


make

aromatic

compounds.6

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nature represents a promising yet challenging approach. In general, the catalytic dehydrating,

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hydrogenating and hydrodeoxygenating reactions are carried out using hydrogen molecule (H2) for

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upgrading the biomass and the derived chemicals. However, molecular H2 is dangerous to handle and

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extreme care should be taken to avoid disasters caused by even small leakage during production, storage,

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and transportation.9 Besides, stoichiometry of H2 is difficult to control, often leading to over-reduction.

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Therefore, facile generation of H2 from a liquid source of formic acid (FA) or methanol,10-14 which is

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derivable from biomass, has drawn much interest. It would be highly desirable for safety that the

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hydrogen is consumed in-situ while it is being produced in the same reactor.

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Many homogeneous catalysts have been used for decomposition of FA, the first half of the tandem

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reaction system at ambient temperatures.12,13 But the homogeneous ligand complexes are difficult to

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synthesize, handle, separate, and recycle, which makes them less suitable for industrial applications.15

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On the other hand, heterogeneous catalysts of metal nanoparticles (NPs) are much less active compared

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to homogeneous catalyst. For formic acid decomposition, bimetallic catalysts have been shown to be

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better than their single metallic catalyst due to charge redistribution between bimetals which strengthens

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the adsorption of formate through strong back donation.16 In fact, the bimetallic catalyst of 2.2 nm PdAg

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alloy nanoparticles turned out to be a better choice for the first part of the tandem reaction.14 But the

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initiation turn over frequency (TOF) was in the range of 382 h-1 at 50 oC. Improvement of TOF with

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earth abundant metal is still demanded for efficient FA dehydrogenation reaction. Another hand,

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heterogeneous catalysts based on metal NPs have to contend with deactivation. The deactivation via

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cluster agglomeration or Oswald ripening is a serious problem.17 In addition, the metal catalysts on

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supporters such as metal oxides and activated carbons could be often leached out to reaction mixture due

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to weak metal-support interaction. 18,19

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Catalytic deoxygenation of lignin or lignin-model compounds with formic acid has been reported by

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several groups.20-23 Lignin has a complex chemical structure that is highly stable and branched (due to

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the > 90% of β-O-4, α-O-4 aryl ether bonds). Therefore, decomposition of lignin to small molecules is

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quite difficult. In particular, the selective hydrodeoxygenation of lignin-derived pyrolysis oil is more ACS Paragon Plus Environment

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challenging because similar bond strength of C-O and C=C leads to competition between

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hydrodeoxygenation and aromatic hydrogenation reaction.24 Consider the limitation seen from the

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previous biomass and chemical feedstocks upgrading, synthesis in controlling NPs size and composition

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we decided to make novel dual functionalized catalyst to achieve the defunctionalization of lignin-based

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biomass with no use of H2 at mild conditions. Here we report an efficient dual functional PdAg alloy

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metal catalyst with high TOF supported on N-doped reduced graphene oxide (N-rGO) that generates H2

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from formic acid and subsequently utilizes the generated hydrogen to hydrodeoxygenate selectively the

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lignin biomass-derived chemicals at mild conditions without any additives in one-pot. We also loaded

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magnetite (Fe3O4) particles for easy separation and enhanced mixing efficiency. Furthermore, the ~4

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at% basic N hetero-atom on the N-rGO facilitates binding of nanoparticles to make well distributed on

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the supporter as well as reaction promotor, which prevents metal agglomeration and leaching even after

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catalyst recycling process. The selective upgrading of lignin biomass-derived compounds was

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successfully demonstrated by employing several model molecules including vanillin, isatin and two

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types of benzyl phenyl ethers, well-known natural compounds, in an environment-friendly one-pot

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manner. It enables to meet the critical needs in the production of upgraded biofuels and chemical

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feedstocks from biomass resources.

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■ Results and discussion

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To evaluate efficiency and selectivity of a series of catalysts for selective hydrodeoxygenation with H2

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formed in-situ from formic acid, vanillin was chosen as a model substrate for upgrading bio-fuel.

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Vanillin is a common component of pyrolysis oil derived from non-edible lignin, which constitutes ~30

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wt% of woody biomass. It is more challenging to deoxygenate lignin-derived pyrolysis oil than

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cellulose-derived one.24 For the purpose, a series of mono-, bi-, and tri-, metallic catalysts were

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synthesized on Fe3O4/N-rGO support (see supplementary information S2 and Table S1-S4).

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It is apparent from Table 1 that no reaction takes place in 10 h at 130 oC (entries 1-4) when a single

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component is used, as in GO, N-rGO and Fe3O4 particles. The Fe3O4 was initially added for easy

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separation (supporting movie S1) and enhancement of mixing properties (supporting movie S2). When ACS Paragon Plus Environment

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we introduced metallic component, i.e., Pd/N-rGO (Table 1, entry 5), we observed that the aldehyde

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group in vanillin was selectively hydrodeoxygenated to methyl group with a low turnover frequency

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(TOF based on the number of active metal sites, see supporting information, S4, Table S5) of 8 h-1. We

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further found that TOF increased significantly with Fe3O4 loading (Table 1, entry 6-10), indicating that

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Fe3O4 does not simply play the physical roles of helping easy separation and enhanced mixing. As

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indicated in the Table 1, it promotes TOF of the reaction presumably by aiding the catalytic cycle of the

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tandem reactions as discussed later. Among many transition metals tested, only Pd showed nearly

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complete conversion. Similar results were reported with gaseous hydrogen supplied externally for Pd

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nanoparticles supported on mesoporous N-doped carbon.24 Hence, one may surmise that Pd could be the

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main active component of the catalyst responsible for the selective hydrodeoxygenation of vanillin. Of

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the bimetallic catalysts tested (entry 11-15), yield of Pd50Co50 and Pd50Ag50 pairs were better than 99%

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but the TOF was much greater for Pd50Ag50 (521 h-1) than for Pd50Co50 (50 h-1). Other catalysts

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including a trimetallic catalyst (entry 16) did not offer any advantage over the Pd50Ag50 bimetallic

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catalyst.

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In order to investigate the relative efficiency of the bimetallic Pd50Ag50/Fe3O4/N-rGO catalyst with

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BET surface area 251 m2/g25, control experiments of Pd/C, Pd on N doped activated carbon (Pd/N-AC),

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Pd/N-rGO, core-shell Pd@Ag/C, Pd/Fe3O4/N-rGO and Pd50Ag50/N-rGO were carried out by comparing

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the conversion and

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hydrodeoxygenation in Table 2. Commercial Pd/C with high surface area produced 63% conversion

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only with 32% yield of MMP by forming considerable by-products (entry 1, Figure S1). The Pd/N-AC

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produced 64% conversion but slightly improved selectivity (40% of MMP, entry 2).26 Upon

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immobilizing the Pd nanoparticles with identical size on N-rGO (as measured by TEM and XRD), the

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similar conversion (66%) but highly improved yield (51%) of MMP were achieved (entry 3),

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presumably due to reaction promoting effect of well distributed Pd nanoparticles on basic nitrogen sites

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as well as - interaction between graphene and aromatic reactant to activate the -carbon position.27

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And the core-shell Ag@Pd/C catalyst produced 69% MMP yield with conversion 100% (entry 4, Figure

yield

of 2-methoxy-4-methylphenol (MMP) as a main product of


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S2). However, Pd/Fe3O4/N-rGO significantly improved the selectivity to 70% yield with conversion

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80% (entry 5, Figure S3). Both Pd50Ag50/N-rGO and Pd50Ag50/Fe3O4/N-rGO catalysts showed excellent

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conversion (100%) and yield (~99%) in water solvent under mild conditions (entry 6&7, Figure S4&S5).

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Note that the decreased Ag composition lowered the efficiency (entry 8&9, Figure S6&S7). Furthermore,

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the excellent catalytic efficiency was not affected by the loaded amount of formic acid (entry 10) and a

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gram scale of MMP production (entry 11). In addition, catalytic superiority of the Pd50Ag50/Fe3O4/N-

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rGO catalyst in the selective hydrodeoxygenation of vanillin was clearly illustrated in comparison to

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various reported catalysts in the literature (Table 3). The time profile of the reaction revealed that

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vanillin was converted to MMP by two pathway; (i) two step-wise hydrogenation/hydrogenolysis via 4-

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(hydroxymethyl)-2-methoxyphenol intermediate and (ii) direct hydrogenolysis, similar to the previous

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report.24

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To understand the behaviour of Pd50Ag50/Fe3O4/N-rGO catalyst in the tandem reaction of formic acid

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decomposition-hydrodeoxygenation sequence, decomposition of formic acid alone28 was first studied at

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50 °C (supplementary section S5, TOF calculation for formic acid decomposition). The total volume of

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CO2 and H2 collected for 1 h was 20.3 ml with the ratio of 1.06 (Figure S9). The corresponding TOF for

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formic acid dehydrogenation was found to be in the range of 497 h-1 (see Table S6), using H2

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chemisorption measurement methods for active surface Pd atoms, which comparable to the reported

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values for active formic acid dehydrogenation catalysts.11,29,30 Possible by-product of CO by formic acid

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dehydroxylation was not detected even at a higher temperature (90 oC) and the catalyst showed

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prolonged stability. This finding together with the hydrodeoxygenation of vanillin shows that our

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Pd50Ag50/Fe3O4/N-rGO catalyzes the sequential tandem formic acid decomposition hydrodeoxygenation

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reactions, without any additives and additional pressure.

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To look into the structure and nature of the catalyst, we resorted to various instrumental methods.

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First, X-ray photoelectron spectroscopy (XPS) was utilized to find surface compositions (Table S4,

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Figure S10&S11) for C, O, and N in N-rGO and Pd50Ag50/Fe3O4/N-rGO catalyst. Binding energies C1s,

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O1s, and N1s of Pd50Ag50/Fe3O4/N-rGO (Figure S11a-c) were identical with those of the as-prepared NACS Paragon Plus Environment

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rGO, indicating that the structure was unaffected by the loading of catalyst NPs.27,31 The absence of the

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satellite peak at 719 eV characteristic of Fe3+ in Fe2O3 (Figure S11d) clearly indicates the absence of

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Fe2O3.32 As given in Table S4, N-rGO has large amounts of oxygen (~10 at %) and nitrogen (~4 at %).

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These hetero-atoms in graphene structure can coordinate metals, resulting in highly distributed and

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stable metal NPs and preventing re-oxidation of noble metal M0 (M=Ag, Pd), as similarly reported at N-

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doped carbon.24 And the metallic nature of Pd0 and Ag0 in the Pd50Ag50/Fe3O4/N-rGO was confirmed by

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doublet XPS peaks at 368 and 374 eV designated as Ag 3d, 336.1 and 341.2 eV as Pd 3d, respectively

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(Figure S11e, f), which is consistent with the literature.33

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Detailed SEM results of our bifunctional Pd50Ag50/Fe3O4/N-rGO catalyst shown in Figure 1a&b

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reveal that the catalyst takes on a shape of nanosheets resembling tiny rose petals. High resolution

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transmission electron microscopy (HR-TEM) shows that the catalytic nanoparticles are reasonably

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dispersed with no agglomeration (Figure 1c&d). Ordered crystalline structure of N-rGO supporter was

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demonstrated by selected area electron diffraction pattern (SAED) with two bright rings corresponding

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to (1100) plane and (1100) reflections patterns (Figure 1e), in addition to interlayer distance 0.37 nm of

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restored graphitic layers (Figure 1f), which is consistent with the reported.27 The atomic scale image by

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STEM with a probe correction and ~0.1 nm point resolution provided the evidence for close proximity

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between the PdAg and the Fe3O4 particles, as indicated by the line scan (upper part of Figure 1g). The

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brighter part (red circle) corresponds to PdAg atoms (Figure 1i) and the less bright part (yellow

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rectangle) to Fe atoms (Figure 1h) because the intensity is directly proportional to the square of the

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atomic number of the elements.34 Importantly, selected area EDX line scans across the PdAg/Fe3O4

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nanoparticles indicated that three metals were co-located in the identical NP with different sizes of 5-8

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nm for PdAg and 8-15 nm for Fe3O4. Conversely, the PdAg alloy NP display uniform lattice patterns

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with no intraparticular boundaries, indicative of a single alloyed phase with a (111) lattice fringe

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distance of 0.23 nm (Figure 1i) between the (111) lattice of face-centered cubic (fcc) Ag (0.24 nm) and

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fcc Pd (0.22 nm) NPs. These findings indicate that PdAg is formed as an alloy and not as a core/shell

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structure.14 Alternatively, it is additional evidence of alloy structure that the ratio of Pd/Ag (2.47 ACS Paragon Plus Environment

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wt%/2.53 wt% = 0.97, Table S3) measured by inductively coupled plasma atomic emission spectroscopy

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(ICP-AES) was nearly similar to the surface compositional Pd/Ag ratio (0.32 at%/0.34 at% = 0.94)

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measured by XPS. Fe3O4 nanoparticles displayed 0.25 nm (311) lattice fringe distance indicating that co-

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presence with PdAg did not affect crystallization of Fe3O4 (Figure 1h)35 and its superparamagnetic

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property (see supplementary S6, Figure S12, Table S7).36 The XRD patterns of the PdAg NP showed a

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very weak peak intensity because of their small sizes and the distance of (111) planes of 2.3 Å (Figure

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S13&S14), further indicating alloy type of the NP. From the (111) diffraction peak and Scherrer’s

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formula, the average size of the crystallites was calculated to be 6.2±0.5 nm, in agreement with the value

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observed by TEM. Based on these results of physical characterization, a structural model of the

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Pd50Ag50/Fe3O4/N-rGO catalyst could be proposed as shown in Figure 1j-m. Over a 2D surface of N-

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rGO sheet, Fe3O4 (8-15 nm in size) particles are uniformly distributed with PdAg (5-8 nm in size) alloy

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NP in close proximity.

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In order to understand the evolution of catalyst structure during the reaction, X-ray absorption near edge

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structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses have been carried

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out in-situ under reaction conditions. (Figure 2, Figure S15, see the supplementary S7). From XANES

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data of the pristine Pd50Ag50/Fe3O4/N-rGO catalyst, it is clear that both Pd and Ag show typical feature

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of metallic phases (Figure S15a&b). In the radial distribution functions (RDF) of Fourier-transformed

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(FT) Pd and Ag K-edge EXAFS spectra, the pristine catalyst also shows intense singlet peaks at around

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2.5 and 2.6 Å, corresponding to the Pd-Pd/Ag and Ag-Ag/Pd nearest metal-metal bonding, respectively

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(Figure 2a&b). The data indicate that PdAg exists in a metallic phase in the pristine Pd50Ag50/Fe3O4/N-

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rGO catalyst. In addition, Pd atom in the pristine catalyst shows a slight peak shift to higher energy in

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XANES and an additional weak FT peak in the lower r region at ~2.0 Å. The result could be interpreted

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as the chemical bonding between surface Pd atoms on the metallic phase and hetero-atoms (N or O) on

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N-rGO layer. These spectral features indicate partial oxidation of Pd atom by a metal-to-ligand charge

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transfer, and anchoring of PdAg nanoparticles onto N-rGO layer. Under reaction conditions the first

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metallic FT peak in Pd K-edge EXAFS is slightly shifted to a longer interatomic distance, while the Ag ACS Paragon Plus Environment

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K-edge spectral behaviour is clearly invariant before and after the catalytic reaction (Figure 2b). These

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results indicate that the Ag does not actively participate in the reaction. An earlier report for

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decomposition of formic acid with an Ag-Pd core/shell nanoparticles suggested that Ag electronically

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promotes Pd metal for formate ion adsorption by a strong back donation.16 In this work, both Fourier-

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transformed (FT) magnitudes of Pd K-edge and Ag K-edge EXAFS decreased at similar ratio, compared

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to those of each bulk metallic phase (Figure S15d). It indicated that the averaged coordination numbers

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around each atom are similar to each other under the face-centered cubic structure, as additional

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evidence of alloy PdAg NPs.

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Finally, Fe-K edge XANES spectra of pristine catalyst show peak shifting into higher energy due to

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binding hetero-atoms (O/N) of graphene (Figure S15c, black line). In RDF of Figure 2c, a clear peak at

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~2 Å denoted by arrow would be assigned to weak chemical bond between Fe elements and hetero-

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atoms (N or O) in the N-rGO layer, which can be expected to drive the Fe3O4 nanoparticles uniformly

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distributed through N-rGO layer. Note that detailed structural arrangement (Pd-Ag-Fe3O4 or Ag-Pd-

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Fe3O4) of catalyst is still difficult due to the similar atomic number of Ag and Pd. The in-situ

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decomposition of formic acid leads to peak broadening of the XANES and abrupt collapse of Fe-O-Fe

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long-range ordering around 2 Å in the RDF. On the other hand, the phase change of iron oxide is not

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observed in the in-situ vanillin hydrodeoxygenation reaction. Hence, hydrogen formed by formic acid

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decomposition on the Pd50Ag50/Fe3O4/N-rGO catalyst can effectively reduce Fe3O4 in N-rGO layer.

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When the vanillin is present in the medium, however, the hydrogen preferentially reacts with vanillin

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rather than Fe3O4. From these in-situ XANES and EXAFS spectral analyses, we could confirm that the

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Pd atom actively participates in both formic acid decomposition and vanillin hydrodeoxygenation

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reactions.

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Well defined PdAg nanoalloys or the core-shell nanopaticles have been investigated in the past for

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formic acid decomposition, which suggested that Ag could play the role of electronic promoters.14 It has

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also been reported that vanillin is hydrodeoxygenated by Pd nanoparticle catalysts with hydrogen gas.24

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We found that magnetite Fe3O4 particle itself or Fe3O4/N-rGO have no catalytic activity for vanillin ACS Paragon Plus Environment

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hydrodeoxygenation with formic acid (Table 1, entry 3, 4). Yet, it is interesting that absence or separate

2

addition of the magnetite particles showed only a low TOF of 99 and 100 (Table 1, entry 17, 19), while

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Pd50Ag50/Fe3O4/N-rGO catalyst had a much higher TOF of 521 (Table 1, entry 14). To elucidate the role

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of Fe3O4 for the notable synergistic effect, formic acid adsorption over Pd50Ag50/Fe3O4/N-rGO catalyst

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was investigated by ATR-IR spectroscopy. We observed characteristic peaks of monodentate formate at

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1620 cm-1 arising from ν as(OCO) stretching, bridging formate at 1362 cm-1 and 1585 cm-1 from

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symmetrical and asymmetrical modes of O-C-O stretching (Figure S16c) on both Pd50Ag50/Fe3O4/N-

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rGO and Fe3O4 alone. The peak positions are consistent with the literature reports on formate species

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binding to magnetite center in a variety of ways through formic acid adsorption,37,38 consistent with the

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molecular scale investigation.39 It could be assumed here that the formate species could migrate to

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nearby PdAg particle and is decomposed to facilitate hydrodeoxygenation of vanillin on the PdAg

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surface. Hence, PdAg alloy surface is the main active sites catalyzing both formic acid decomposition

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and vanillin reduction. Magnetic Fe3O4 particles promote the reaction by chemisorbing formic acid and

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transferring the resulting formate and hydrogen to PdAg surface.

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Based on these mechanistic ideas derived from literature and our own experimental results for our

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bifunctional Pd50Ag50/Fe3O4/N-rGO catalyst, the reaction pathways and the related hypothetical

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structures are proposed as shown in Figure 3 and Figure S8. The promoting role of Fe3O4 is further

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supported by the importance of the close proximity of magnetite particle to the PdAg alloy particles,

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which is necessary for the facile migration of the formate species. Furthermore, in-situ formic acid

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decomposition in the absence of vanillin revealed collapse of long range Fe-O-Fe order as observed in

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XANES spectra. Thus, rapid consumption of the hydrogen by the vanillin preserves structural integrity

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of the magnetite phase. It is important for practical applications that the catalyst is highly stable, easily

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separable, and readily recyclable without loss of activity. With the model reaction involving vanillin

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(Table S8), the catalyst was recycled up to eleven times with negligible loss of activity. Yield to 2-

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methoxy-4-methylphenol remained constant all throughout the eleven cycles. Moreover, the catalyst

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could easily be separated from the reaction solution by simply using either magnet (supporting movies ACS Paragon Plus Environment

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Page 12 of 26

1

S1) or filtration/centrifugation (in filtration process 100% catalyst recovery was not possible). After each

2

reaction cycle the composite catalyst was checked by XRD (Figure S17&S18), which showed negligible

3

change in PdAg but slight oxidation of Fe3O4 due to phase changing behaviour of iron catalyst during

4

biomass upgradation (details in supplementary S9). To check the possible leaching of metal NPs during

5

reaction, the reaction mixture was analyzed by inductive coupled plasma emission spectroscopy (ICP-

6

ES). Less than 0.1 ppm of Ag and Pd was detected, indicating negligible leaching of Ag and Pd into the

7

reaction mixture. This fact clearly shows that the hetero-atoms (N, O) of N-rGO bind strongly metal

8

NPs, making them well distributed throughout the surface. The surface of the catalyst recycled eleven

9

times was analyzed in detail by XPS. It was found that the catalyst Pd and Ag surface was almost the

10

same with little alteration in elemental atomic percentage (Table S4, Figure S19) and that the catalyst

11

showed only slightly decreased TOF (Table 1, Entry 20). On the other hand, securing mass balance is

12

the most important consideration for industrial scale biofuel upgrading. During mass balance study, we

13

observed that catalyst was fully recovered without any coke deposition over catalyst surface and also

14

total mass balance during the reaction was almost completely closed (>99 %, Table S6).

15

Now that the suitability of the catalyst for practical applications is assured, we went into examination

16

of the efficiency of our Pd50Ag50/Fe3O4/N-rGO catalyst in defunctionalizing a number of compounds

17

derived from carbonyl groups, including vanillin and isatin as a well-known natural compounds. Of the

18

hydrodeoxygenation reactions, which are considered the most effective method for upgrading oxygen-

19

rich bio-fuels,40 the most challenging reaction involves direct hydrodeoxygenation of carbon-oxygen

20

bonds in aromatic aldehydes.41 The usual direct hydrodeoxygenation leads to significant amounts of

21

undesired products such as aromatic alcohol, methyl cyclohexane and benzene.42 The usual approach,

22

therefore, is to reduce aldehyde/ketones to alkanes in the presence of aniline,41 or H2S gas43 at high

23

temperature and pressure. Direct hydrodeoxygenation of aromatic aldehydes and ketones with hydrogen

24

produced in-situ from formic acid was attempted with our catalyst to produce a variety of industrially

25

applicable synthetic feedstocks, and the results are given in Figure 4. Aldehydes with various

26

functionalities were converted selectively to methyl substituted aromatic compounds (1-9) at 130 oC, ACS Paragon Plus Environment

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ACS Catalysis

1

and the yield was better than 90% for all the molecules tested whenever the conversion took place.

2

Aromatic ketones, such as acetophenone, benzophenone, and fluorenone, were also selectively

3

hydrodeoxygenated to aromatic alkanes (10-12) without arene hydrogenation.

4

To our delight, even substrates containing amide and aliphatic ester substituted aldehyde or ketone

5

group were also converted smoothly and selectively to the corresponding alkanes (13-16). The

6

Pd50Ag50/Fe3O4/N-rGO catalyst, however, was only reactive to aromatic carbonyl groups; it did not have

7

any activity toward the C=O bonds in amides, carboxylic acids, esters and aliphatic carbonyls (Figure

8

S20).

9

Selective catalytic hydrogenolysis of C-O bonds with its preserved aromatic nature is a challenging

10

chemistry for lignin biomass utilization. In particular, hydrogenolysis of -O-4 and -O-4 C-O model

11

compounds, taking up to 90% monomeric units of the alkyl aryl ether in lignin heterogeneous structure,

12

led to unsatisfactory level of selective hydrogenolysis with various by-products.7,44 Recently selective

13

depolymerisation of lignin model compounds with prevented benzene ring was achieved by a complex

14

multiple step process that aerobic oxidation of the -O-4 linkage model under high O2 gas pressure was

15

followed by zinc metal mediated cleavage of aryl C-O bond, and subsequent NaBH3CN mediated

16

reduction.8 However, a simple one-pot single step of selective hydrogenolysis at atmospheric pressure

17

was demonstrated by guaiacylglycerol--guaiacyl ether with -O-4 C-O bond as a model compound that

18

was selectively hydrogenated and deoxygenated with no use of additives except formic acid, over

19

Pd50Ag50/Fe3O4/N-rGO bifunctional catalyst at 120 oC (Figure 5). The reaction resulted in complete

20

conversion, yielding two products: 3-(3-methoxy-4-hydroxyphenyl)-1-propanol (17a) (till to date the

21

production of 17a as a major product has rarely been reported)6 and guaiacol (17b) with excellent yields

22

(Figure S21&S22). Another reaction of interest is hydrogenolysis reaction of 2-phenoxy-1-

23

phenylethanol. It has been reported that the reaction with bimetallic catalyst under high hydrogen

24

pressure led to more than 12 products with poorly preserved aromatic nature of product.44,45 In contrast,

25

the use of Pd50Ag50/Fe3O4/N-rGO catalyst for hydrogenolysis of 2-phenoxy-1-phenylethanol in the

26

presence of formic acid at comparatively low temperature of 120 oC produced only two products (18a, ACS Paragon Plus Environment

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Page 14 of 26

1

18b) with high selectivity via -O-4 C-O model bond cleavage. It is noted in this regard that the

2

absence/presence of substituted functional groups did not affect hydrogenolysis of benzyl phenyl ether,

3

and excellent conversion and yield were generally obtained at 120 oC (Figure 6, 19-21). Comparison to

4

previously reported heterogeneous catalysts for hydrogenolysis of -O-4 and -O-4 C-O bonds reveals

5

superior performance of Pd50Ag50/Fe3O4/N-rGO catalyst with excellent selectivity under much milder

6

conditions (Table S9).

7

The PdAg bimetallic catalyst promoted by Fe3O4 introduced here produces hydrogen from biomass-

8

derived formic acid and then utilizes the generated hydrogen in situ to upgrade biofuels and chemical

9

feedstocks. The efficiency of this dual functional catalyst is exemplified by more than an order of

10

magnitude increase in TOF and yield when compared with only Pd supported on N-rGO (Table 1). The

11

versatility and effectiveness of the catalyst advanced here is demonstrated by 99% yield in almost all

12

cases involving selective hydrodeoxygenation of aromatic aldehydes and ketones with various functional

13

groups, and selective hydrogenolysis of -O-4 and -O-4 C-O model compounds. This new catalyst

14

could be an efficient and versatile platform for valorisation of biomass.

15

■ Conclusions

16

We have introduced a novel strategy for the preparation of a multicomponent and multi-functional

17

transition metal solid catalyst Pd50Ag50/Fe3O4/N-rGO for the selective upgrading of biomass-derived

18

vanillin, isatin and two types of benzyl phenyl ethers as lignin biomass-derived model compounds, and

19

synthetic feedstocks derived from aromatic carbonyl by simple one-pot reaction under mild conditions.

20

This represents a significant advance toward the efficient valorisation of biomass by production of bio-

21

oil and useful chemicals. Our novel catalyst is effective in dehydrogenating formic acid to form H2 and

22

subsequently utilizing the hydrogen generated in-situ for upgrading bio-oil and chemical feedstocks. The

23

catalyst is easy to synthesize, stable, easily separable, and readily recyclable without loss of activity, and

24

the reactions can be carried out in water, making the process environment-friendly. It could play a

25

critical role in upgrading biomass-derived chemicals to fill future gap in the supply of transportation

26

fuels and chemical feedstocks.


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1

ASSOCIATED CONTENT

2

Supporting Information

ACS Catalysis

3

The following file is available free of charge on the ACS Publications website at DOI: Experimental

4

details, magnetic separable catalyst video, catalyst characterization data, formic acid dehydrogenation

5

graph.

6

AUTHOR INFORMATION

7

Corresponding Author

8

*E-mail: [email protected]

9

*E-mail: [email protected]

10

Author Contributions

11

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

12

Notes

13

The authors declare no competing financial interest.

14

ACKNOWLEDGMENT

15

We gratefully acknowledge the support from the National Research Foundation (NRF) of Korea grant

16

funded by the Korean government (MEST) (No.2008-0061983). AKS thanks the National Institute of

17

Nanomaterial Technology (NINT), Material Science Departments, Chemistry Departments, PAL,

18

POSTECH, Pohang, Korea Basic Science Institute (KBSI) Daejeon, for the analytical support. The

19

Chemical Engineering, POSTECH, has filed a patent on the catalysts reported herein. 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, 258262. (9) Dormán, G.; Kocsis, L.; Jones, R.; Darvas, F. J. Chem. Health Safety 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. ACS Paragon Plus Environment

ACS Catalysis

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(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. 2013, 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. (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. Pyrol. 2011, 92, 477-484. (21) Oregui Bengoechea, M.; Hertzberg, A.; Miletić, N.; Arias, P. L.; Barth, T. J. Anal. Appl. Pyrol. 2015, 113, 713-722. (22) Kloekhorst, A.; Shen, Y.; Yie, Y.; Fang, M.; Heeres, H. J. Biomass Bioenergy 2015, 80, 147161. (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, 1698716990. (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.; LiuJack, 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, 39793986. (37) Cutting, R. S.; Muryn, C. A.; Thornton, G.; Vaughan, D. J. Geochim. Cosmochim. Ac. 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.; ‘Pete’ Silks, L. A.; Gordon, J. C. Nat. Chem. 2013, 5, 428-432. (41) Liu, S.; Fan, X.; Yan, X.; Du, X.; Chen, L. Appl. Catal. A-Gen. 2011, 400, 99-103. (42) Procházková, D.; Zámostný, P.; Bejblová, M.; Červený, L.; Čejka, J. Appl. Catal. A-Gen. 2007, 332, 56-64. (43) Ferrari, M.; Maggi, R.; Delmon, B.; Grange, P. J. Catal. 2001, 198, 47-55. 15 ACS Paragon Plus Environment

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ACS Catalysis

(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. Inter. 2014, 6, 7464-7470. (47) Crossley, S.; Faria, J.; Shen, M.; Resasco, D. E. Science 2010, 327, 68-72.


16

ACS Catalysis

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Page 18 of 26

Table 1. Comparative TOF of vanillin hydrodeoxygenation reaction with various synthesized catalysts.

Temperature BET area (m2/g)

-1

Entry

Catalyst

1

GO

130

558

N/A

2

N-rGO

130

230

N/A

3

Fe3O4

130

-

N/A

4

Fe3O4/N-rGO

130

220

N/A

5

Pd/N-rGO

130

202

8

6

Pd/Fe3O4/N-rGO

130

202

32

7

Ag/Fe3O4/N-rGO

130

214

N/A

8

Au/Fe3O4/N-rGO

130

222

N/A

9

Pt/Fe3O4/N-rGO

130

226

N/A

10

Co/Fe3O4/N-rGO

130

210

N/A

11

Pd50Co50/Fe3O4/N-rGO

130

152

50

12

Pd50Au50/Fe3O4/N-rGO

130

260

15

13

Pd50Pt50/Fe3O4/N-rGO

130

201

N/A

14

Pd50Ag50/Fe3O4/N-rGO

130

251

521

15


80

251

5

16

Pd33Co33Au33/Fe3O4/N-rGO

130

206

63

17

Pd50Ag50/N-rGO

130

222

99

18a


150

251

2937

19b

Pd50Ag50/N-rGO+Fe3O4

130

NA

100

20c


130

-

482

TOF (h )

Reaction condition: vanillin (2 mmol), formic acid (2.5 equiv.), catalyst (20 mg), water (1 ml), specified reaction time (2 h), conversion based on GC analysis, Turnover frequency (TOF, average of three run, error limit 1%) = mole of 2-methoxy-4-methylphenol production per hour/mole of active site, (a) with hydrogen gas, water 5 ml, pressure 1.0 MPa, reaction time 2 h, catalyst 20 mg, temperature 150 oC, (b) addition 20 mg Fe3O4 powder, (c) eleven times reused catalyst.


17

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ACS Catalysis

Table 2. Control selectivity experiments of hydrodeoxygenation of vanillin over various Pd-based catalysts.

Entry

Catalyst

S/C

Conversion (%)a

1b

Pd/C

10

2c

Pd/N-AC

3

Yield (%)a 2-methoxy-4methylphenol

2-methoxy phenol

Other

63

32

14

17

10

64

40

10

13

Pd/N-rGO

10

66

51

4

11

4d

Pd@Ag/C

106

100

69

17

14

5

Pd/Fe3O4/N-rGO

106

80

70

NA

10

6

Pd50Ag50/N-rGO

106

100

98

NA

1

7


106

100

99

NA

NA

8


106

98

43

NA

54

9


106

88

24

NA

63

10e


106

100

99

NA

NA

11f


303

100

99

NA

NA

Reaction condition: 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 oC; (a) conversion and yield based on GC analysis and anisole as a standard, average of two measurements data errors 99

81

6

Pd/Ca

130

No

HCOOH/Water

6

63

32

This study

[email protected]

150

10 bar

Water

6

98

100

24

Pd@C

150

10 bar

Water

6

98

74

24

Pd@CeO2

120

10 bar

Water

6

88

14

24

Pd@γ-Al2O3

150

10 bar

Water

6

95

69

24

Pd@SWNT-SiO2

150

10 bar

Water/decalin

6

85

47

47

Pd-Nafion SAC-13b

300

No

Water

2

100

3

20

Pd50Ag50/Fe3O4/N-rGOa

130

No

HCOOH/Water

6

100

>99

This study

Pd50Ag50/Fe3O4/N-rGOa

150

10 bar

Water

2

100

100

This study

Reaction condition: (a) vanillin (0.5 mmol), formic acid (2.5 equiv.), S/C = 106 (substrate mmol/Pd mmol (wt%), water (1 ml).; (b) vanillin (1.31 mmol), S/C = 70, formic acid (10.9 equiv.)


19

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ACS Catalysis

FIGURE CAPTIONS

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, and red line Ag); (h&i) CS-corrected TEM image of magnetite and PdAg nanoalloy; (j&k) proposed model 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. 20 ACS Paragon Plus Environment

ACS Catalysis

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(a)

(b)

Page 22 of 26

(c)

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

21 ACS Paragon Plus Environment

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ACS Catalysis

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

22 ACS Paragon Plus Environment

ACS Catalysis

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Page 24 of 26

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 catalyst (~0.32 at% Pd), water (1 mL). Conversion and yield are based on gas chromatography (GC) analysis using anisole (0.5 mmol) as an internal standard. At least three measurements were taken to obtain an average yield. Yield in parenthesis refers to an isolated yield.


23

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ACS Catalysis

Figure 5. Selective hydrogenolysis reactions of -O-4 linkage model aryl C-O group with formic acid over Pd50Ag50/Fe3O4/N-rGO catalyst. Reactant (0.1 mmol), formic acid (3.5 equiv), catalyst (50 mg), time 3h, temperature 120 oC; 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 -O-4 linkage aryl C-O group was achieved.


24

ACS Catalysis

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Figure 6. Selective hydrogenolysis reactions of -O-4 linkage model aryl C-O group with formic acid over Pd50Ag50/Fe3O4/N-rGO catalyst. Reactant (0.5 mmol), formic acid (1.5 equiv), catalyst (50 mg), temperature 120 oC; 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 -O-4 linkage model aryl C-O group was achieved.


25

One-Pot Defunctionalization of Lignin-Derived Compounds by Dual

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