Porous Organic Polymer-Driven Evolution of High

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24140−24153

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Porous Organic Polymer-Driven Evolution of High-Performance Cobalt Phosphide Hybrid Nanosheets as Vanillin Hydrodeoxygenation Catalyst Subhash Chandra Shit,†,⊥ Paramita Koley,†,∥,⊥ Boby Joseph,‡ Carlo Marini,§ Lingaiah Nakka,† James Tardio,∥ and John Mondal*,† †

Catalysis & Fine Chemicals Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 007, India GdR IISc-ICTP, Elettra-Sincrotrone Trieste, S.S. 14, Km 163.5 in Area Science Park, Basovizza 34149, Italy § Alba Synchrotron Ctra. BP 1413 km. 3,3, erdanyola del Vallès, Barcelona 08290, Spain ∥ Centre for Advanced Materials & Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne 3001, Australia

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‡

S Supporting Information *

ABSTRACT: Hydrodeoxygenation (HDO) is a promising route for the upgrading of bio-oils to eco-friendly biofuel produced from lignocellulose. Herein, we report the sequential synthesis of a hybrid nanocatalyst CoxP@POP, where substoichiometric CoxP nanoparticles are distributed in a porous organic polymer (POP) via solid-state phosphidation of the Co3O4@POP nanohybrid system. We also explored the catalytic activity of the above two nanohybrids toward the HDO of vanillin, a typical compound of lignin-derived bio-oil to 2-methoxy-4-methylphenol, which is a promising future biofuel. The CoxP@POP exhibited superior catalytic activity and selectivity toward desired product with improved stability compared to the Co3O4@POP. Based on advanced sample characterization results, the extraordinary selectivity of CoxP@POP is attributed to the strong interaction of the cation of the CoxP nanoparticle with the POP matrix and the consequent modifications of the electronic states. Through attenuated total reflectance-infrared spectroscopy, we have also observed different interaction strengths between vanillin and the two catalysts. The decreased catalytic activity of Co3O4@POP compared to CoxP@POP catalyst could be attributed to the stronger adsorption of vanillin over the Co3O4@POP catalyst. Also from kinetic investigation, it is clearly demonstrated that the Co3O4@POP has higher activation energy barrier than the CoxP@POP, which also reflects to the reduction of the overall efficiency of the Co3O4@POP catalyst. To the best of our knowledge, this is the first approach in POP-encapsulated cobalt phosphide catalyst synthesis and comprehensive study in establishing the structure− activity relationship in significant step-forwarding in promoting biomass refining. KEYWORDS: cobalt phosphide, porous organic polymer, vanillin, hydrodeoxygenation (HDO), biofuel, ATR-IR

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INTRODUCTION In the past few decades, biofuels and high-value-added chemicals manufactured from renewable biomass feedstocks have received a staggering degree of attention as an alternative energy source owing to the constant depletion of fossil fuels.1−3 The effective biofuel upgrading with deoxygenation step is highly recommended to reduce the oxygen content (30−50 wt %) and improve its stability along with the miscibility with conventional fuels.4−6 The general problem associated with the effective utilization of most abundant lignin-derived pyrolysis oil (30 wt % of woody biomass) is the presence of oxygen-rich subunits derived from phenol, pcoumaryl, coniferyl, and sinapyl alcohols.7,8 Over the past decades, various methods like hydrogenolysis, dehydration, decarbonylation, hydrogenation, decarboxylation, and hydrodeoxygenation (HDO) have been employed to remove oxygen functionalities from these molecules. Among various ap© 2019 American Chemical Society

proaches, hydrodeoxygenation (HDO), involving reactions of bio-oil with hydrogen, is a highly attractive route for upgrading bio-oil because it can enhance the H/C ratio and eventually can produce hydrocarbons.9−11 Vanillin (4-hydroxy-3-methoxybenzaldehyde), a typical model component derived from lignin fraction, partially soluble in both organic and aqueous phases, like a bio-oil, is considered as a promising platform molecule to bridge the gap between biomass resources and biochemicals, since they can be converted into a variety of high-value-added chemicals and fuels. Metal sulfide-based catalytic systems were employed for its HDO, but substitution of sulfur atoms of the catalysts by oxygen atoms in the course of the reaction causes their Received: April 18, 2019 Accepted: June 14, 2019 Published: June 14, 2019 24140

DOI: 10.1021/acsami.9b06789 ACS Appl. Mater. Interfaces 2019, 11, 24140−24153

ACS Applied Materials & Interfaces

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deactivation.12 Recently, several research groups have brought into focus the development of noble-metal-based catalysts, including Pd,13−15 Pt,16 Rh,17 Ru,18 and Au,19 for HDO reactions, but higher cost and limited abundance of these materials in the earth’s crust are the big hurdles for practical applications. As alternatives to the unstable sulfides and the expensive noble-metal catalysts, metal carbides, nitrides, and oxides have also been employed for HDO reactions.20,21 Recently, transition-metal phosphides have drawn great attention as functional catalysts because of their high thermal and chemical stabilities.22 Among them, cobalt-phosphorousderived films (Co−P)23 and nanostructured Ni5P4 films24 were reported as promising electrocatalysts in electrochemical water splitting. These materials also exhibit catalytic activity in HDO reactions. Lin et al. reported that Al2O3, ZrO2, and SiO2supported nickel phosphide catalysts show excellent activity for HDO of guaiacol,25 while Serrano et al. studied HDO of phenol using Ni, Co, and Mo phosphides supported on Al2O3-, Al2O3/SiO2-, and C-based ordered mesoporous materials.26 The presence of phosphorous atoms in metal phosphide catalysts is reported to induce “ligand” effects at the metal sites, thereby altering the electron density of the metal cation and possibly facilitating facile dissociation of hydrogen molecule.27 In addition, suitable choice of support material is essential to offer the resulting catalyst good resistance to acid and base media, low cost by high active site dispersion, and high thermal stability. Carbon materials meet these criteria28 with high resistance against coking compared to acidic supports.29 However, the weak interaction of catalytically active elements such as metal phosphides with rather inert carbon support can lead to their aggregation or leaching, consequently leading to loss of catalytic activity.30 Herein, we report the use of porous organic polymers (POPs) as a promising support of cobalt phosphide and an outstanding catalytic performance of the composite material in HDO of vanillin. The advantageous nature of POPs, besides their expected stronger interaction with the active element (CoxP), is their mechanically stable rigid covalent framework with designable pore structure, low skeleton density, large surface area, and controllable compositions.31 A sequential synthesis method was developed to prepare cobalt phosphide hybrid nanosheets (CoxP@POP) via solid-state phosphidation of spinel-type cobalt oxide deposited on POP (Co3O4@POP). CoxP@POP showed superior catalytic activity for the HDO of vanillin to the targeted product 2-methoxy-4-methylphenol (MMP), with higher selectivity compared to Co3O4@POP at full conversion. Several characterization techniques, including wide-angle powder X-ray diffraction (XRD); N2 physisorption; 13 C cross-polarization (CP) solid-state magic angle spinning (MAS); NMR, Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS); high-resolution transmission electron microscopy (HR-TEM); and field emission scanning electron microscopy (FE-SEM), along with energydispersive X-ray (EDX) mapping, extended X-ray absorption fine structure (EXAFS), attenuated total reflectance-infrared spectroscopy (ATR-IR) were employed to obtain insights into the links between the physicochemical properties and the possible origin of the improved product selectivity. In the case of CoxP@POP, the strong interaction of the nanosized CoxP with the host matrix (POP) enables the stabilization of several closely related metastable phases with metallic nature, which may be the key to the enhanced catalytic properties of this system.

Research Article

EXPERIMENTAL SECTION

All chemicals and reagents were purchased from Sigma-Aldrich and used as received unless noted otherwise. Deionized water used was purified by a Thermo Scientific Barnstead Nanopure system (≥18 MΩ cm), and the quality of ethanol used was 200 proof.

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SYNTHESIS OF POROUS ORGANIC POLYMER (POP) POP was synthesized with triphenylamine (TPA) as the building block by a one-step oxidative polymerization process.32 A mixture of TPA (98% pure, 2 mmol, 491 mg) and anhydrous FeCl3 (≥99.99% pure, 6 mmol, 973 mg) was first taken in a 250 mL round-bottom flask, then dry dichloroethane (≥99.8% pure, 30 mL) was poured into that flask and the mixture was stirred for 20 h under N2 atmosphere with constant heating at 80 °C. Then, the as-obtained solution was poured into acetone and the resulting polymer product was filtered and washed successively with acetone, tetrahydrofuran, and methanol. To remove iron from the polymer, the product was further washed with methanol for 3 days in a Soxhlet apparatus. We have analyzed iron content in our assynthesized POP material after thorough washing with methanol using Soxhlet apparatus for 3 days continuously. After that, we have isolated POP and conducted atomic absorption spectroscopy (AAS) analysis to check any iron is present or not. AAS analysis result revealed that iron content in the POP is below the detection limit of AAS, which is less than 0.2 ppm level. So, from this result, we can surely prove that all of the iron contents have been removed from the POP surface. Synthesis of Co3O4@POP Material. POP (50 mg) was thoroughly dispersed in 30 mL of ethanol with sonication for 1 h. Then, 9.6 mL of an aqueous 0.2 M Co(OAc)2·4H2O (99.99% pure, 0.478 g) solution was added to the previous solution, followed by subsequent addition of 7.2 mL of NH4OH aqueous solution (30 wt %) and stirred at room temperature for 1 h. The resulting mixture was heated to 80°C in an oil bath and maintained at that temperature under stirring for 12 h. After cooling to room temperature, the product was isolated from the mixture by centrifugation, followed by washing with ethanol and water. The resulting black solid was dried at 60°C in an oven and is designated as Co3O4@POP. Synthesis of CoxP@POP Material. The as-synthesized Co3O4@POP (100 mg) sample was mixed together with 1 g of sodium hypophosphite monohydrate (NaPO2H2·H2O) as a solid phosphorous source and ground in a mortar and pestle. Subsequently, the resulting mixture was calcined at 300 °C in a tubular furnace under N2 atmosphere for 2 h to obtain a brownish white powder (CoxP@POP). Catalytic Hydrodeoxygenation (HDO) of Vanillin. Vanillin (300 mg, 2 mmol), catalyst (50 mg), and isopropanol (30 mL) were charged into the reactor (Parr Instrument Company, 50 mL autoclave). The reactor was evacuated by using a vacuum pump for 15 min at room temperature to remove dissolved O2 or air and then pressurized with H2 to 40 bar. The temperature of the autoclave was fixed at 150 °C for the desired reaction time with continuous stirring at a speed of 800 rpm. After the reaction, the autoclave was cooled to room temperature and catalyst was separated from the reaction mixture by centrifugation prior to quantitative analysis of the reaction mixture. The reaction solutions were analyzed by a gas chromatograph (Shimadzu 2010) equipped with a flame ionization detector using INNOWax capillary column 24141


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Figure 1. Schematic illustration of the sequential synthesis of the CoxP@POP hybrid nanosheets as HDO catalyst.

(diameter: 0.25 mm; length: 30 m). The products were also identified by gas chromatography−mass spectrometry (Shimadzu, GCMS-QP2010S).

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RESULTS AND DISCUSSION

Synthesis of Porous Organic Polymer and Co3O4@ POP, Co xP@POP Catalysts. The POP material was synthesized in one step using anhydrous FeCl3-assisted oxidative polymerization of triphenylamine to produce a light green solid powder, as previously reported by Bhaumik et al.32 The CoxP@POP nanohybrid structure was synthesized by two subsequent steps, as illustrated in Figure 1a, where in the first step, Co3O4 nanoparticles were homogeneously incorporated in the N-rich POP framework (designated as Co3O4@POP) by a hydrolysis reaction of cobalt acetate at 80 °C in ethanol/ water mixed solvent. Selective nucleation and growth of Co3O4 nanoparticles on POP surface is persuaded in this mixed solvent, which has been employed to slow down the hydrolysis reaction, thereby inducing interactions between the Co2+ and the N-functional groups of POP. The hydrolysis rate was further decreased with the addition of ammonia solution into the reaction medium, which coordinates with the Co2+ thereby controlling the size and homogeneous distribution of the resulting Co3O4 on the POPs.33 In the second step, the assynthesized Co3O4@POP material was transformed to CoxP@ POP nanohybrids via a simple solid/gas-phase low-temperature phosphidation process using sodium hypophosphite monohydrate (NaH2PO2·H2O), where PH3 gas was generated during thermal treatment at 300 °C from NaH2PO2·H2O precursor and reacts with the Co3O4@POP to deliver the final compound CoxP@POP. Nanostructure. Two broad peaks located at 2θ = 16.3 and 43.2° in the wide-angle powder X-ray diffraction pattern of the as-synthesized POP (Figure S1, SI) could be ascribed to the (002) and (101) carbon planes, respectively, suggesting the amorphous framework structure. To understand the structural modifications, the phosphidation reaction brings out on Co3O4@POP, both the hybrids were analyzed by synchrotron X-ray powder diffraction (XRPD) (Figure 2) (details in Structural Characterizations in the SI). In the case of Co3O4@ POP, XRPD presents Bragg reflections, which are well described by a spinel structure phase of Co3O4 (JCPDS no. 42-1467; see vertical red lines in Figure 2).34 Compared to Co3O4@POP, the XRPD spectra of CoxP@POP present a large number of Bragg reflections underlining a complex

Figure 2. Synchrotron X-ray powder diffraction (XRPD) patterns of Co3O4@POP and CoxP@POP together with five possible structural phase contributions (labeled as (1)−(3)) present in the system.

mixture of phases. We could be able to identify three possible crystalline reflections assigning to orthorhombic CoP (JCPDS file no. 65-2593), orthorhombic Co2P, and a monoclinic CoP2 phases.35,36 Measured synchrotron XRPD data compared with contribution from the above-mentioned theoretical three phases are shown in Figure 2 and Table S1 in the SI. Although XRPD data support the presence of coexisting phases in CoxP@POP, it also indicates the main phase present to be CoP phase (JCPDS file no. 65-2593), which is in good agreement with the uniform distribution of the Co and P in the elemental mapping results (Figure 3), as well as the selected area (electron) diffraction (SAED) pattern, and this phase is mainly due to lower-temperature phosphidation process (at around 300 °C).37 During phosphidation at 300 °C, first, the orthorhombic Co2P phase is formed, which is gradually transformed to the orthorhombic CoP phase.38 The reason behind the formation of monoclinic CoP2 phase was attributed to higher amount of sodium hypophosphite monohydrate (NaH2PO2·H2O) taken compared to metal salt during the phosphidation process.39 We underline that the synthesis process we follow is highly reproducible as it yields identical results upon repeating synthesis procedure. To elucidate the chemical bonding information, we have also performed FT-IR analysis of three materials (POP, Co3O4@POP, and CoxP@ POP), which are shown in Figure S2. For POP, the peaks at approximately 1273 and 815 cm−1 could be assigned to the C− N stretching of the tertiary amines and C−H out-of plane bending of the para-disubstituted benzene rings, respectively, 24142


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Figure 3. TEM images of Co3O4@POP (a) and CoxP@POP (b, c). SAED pattern from CoP nanoparticles of CoxP@POP (d). FE-SEM images (e, f) and the corresponding elemental mapping analysis of CoxP@POP with merge elements, P (green), Co (red), and N (purple). The total area of the representative FE-SEM image (h) for mapping is considered. Energy-dispersive X-ray (EDX) spectrum of CoxP@POP material (g).

nanofibers. The low-magnification TEM images (Figures S5 and S6, SI) demonstrated that the black Co3O4 and CoP particles are homogeneously incorporated in the POP matrix. After the phosphidation of Co3O4@POP at 300°C, the nanofibers were changed into nanosheets (Figure 3b,c), where the POP nanosheets act as building blocks that are interconnected with each other forming a three-dimensional (3D) agaric-like porous structure. The morphological change may be due to the PH3 generation by the decomposition of NaH2PO2·H2O during the solid/gas-phase phosphidation reaction and structural reorganization of nanofibers at high reaction temperature, which is in accordance with the previous observation by Qu et al. on ternary Ni2−xCoxP as hydrogen evolution reaction catalyst and their hybrids with graphene as water splitting catalyst.44 Gradual aggregation of these primary Co2P particles took place in chains that deposit on the surface of the POP nanofibers. With the progress of the reaction, the Co2P primary particles continue to transform followed by crystallization along the c-axis to construct nanosheets.45 This could also be attributed to another possible reason, as demonstrated by Li and co-workers in their hematite nanorod−nanoflake morphological transformation.46 We believe that the morphological transformation is thermodynamically favorable. The growth of inorganic crystals is principally assisted toward the minimum surface energy with most exposed densely packed surface in the absence of capping ligands. Co3O4 comprises hexagonal close-packed structure, with the most densely packed facets as the basal plane (111). A larger driving force for the structural transformation is vigorous PH3 gas evolution during postannealing treatment, which could assist the nanostructure growth in perpendicular to the (111) direction, resulting in the formation of flakelike morphology. Since proper mechanistic investigation with experimental evidence for this morphological transformation is under investigation, some additional studies will be carried out in the near future. The corresponding elemental mapping

which confirms the coupling between triphenylamine monomers. The peaks around 2900, 1596, and 1487 cm−1 indicate the presence of phenylic C−H bond and aromatic CC stretching vibrations of the benzene ring.32 For Co3O4@POP, the peaks are almost similar to those of parent POP, which implies that the POP framework remains intact after composite formation with Co3O4. CoxP@POP also carries similar characteristic peaks of POP framework but with less intensity. The extra peaks at 558.7 and 1430.2 cm−1 could be attributed to Co−P and P−C vibrations, respectively.40 The FT-IR spectra show an intense band around ∼2356 cm−1 (Figure S2, SI), which could be assigned to the vibrations of double bonds between carbon and nitrogen resulting from conjugation in the polymeric framework, and this similar observation has been identified by Pieczyńska et al.41 The corresponding peak intensity is decreased for Co3O4@POP and CoxP@POP, which may be due to the interaction between POP support and Co3O4 and CoxP, respectively, in accordance with the previous report by Li et al.42 In another way, this peak could also be attributed to the adsorbed atmospheric CO2 (or an artifact), which is not under consideration in our present investigation.43 Electron Microscopy Analysis. To evaluate the morphological information of the as-synthesized POP, Co3O4@POP, and CoxP@POP, FE-SEM and TEM studies were carried out. TEM images of the as-synthesized POP (Figure S3, SI) reveal that the material exhibits inherent fibrous morphology, which is corroborated with the FE-SEM images analysis (Figure S4a,b, SI). We could not speculate any major morphological difference from the FE-SEM images (Figure S4c,d, SI) after selective growth of Co3O4-NPs of the as-synthesized POP to produce Co3O4@POP analogue. The nanofibers got interconnected or self-assembled with each other to grow a large number of densely packed bundlelike morphology in Co3O4@ POP analogue. TEM image of the Co3O4@POP (Figure 3a) shows that the dark contrast spherical particles with high density are uniformly distributed on the surface of the POP 24143


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binding energy peaks centered at ∼399.3, ∼400.5, and ∼401.7 eV, which are assignable to Co−N, pyrrolic N, and graphitic N type of species, respectively. This is in accordance with the previous work reported by Zhang et al. on Co nanoparticles encapsulated in porous N-doped carbon nanofibers as hydrogen evolution catalyst.55 In fact, the P 2p XPS signal is different from that of the CoP56 and Co2P57 systems, although the features ∼129.2 and ∼133.0 eV (Figure 4c) are related to the P species in CoP and oxidized P species, respectively.58 This fact correlates well with the observation from diffraction that there exists more than one binary phase involving Co and P. The extra peak seen around ∼134 eV could be due to the P−C bonding.59 The strong interaction between Co3O4 NPs and nitrogen-enriched as-synthesized POP has been evaluated by conducting XPS analysis in Co 2p core region of neat Co3O4 NPs. Figure S7, SI, demonstrated a negative binding energy shift of about ∼0.5 eV in Co3O4@POP in comparison to the neat or bare Co3O4-NPs, which obviously signify a strong coordination interaction between Co3O4 and N-POP. The N 1s XPS image of CoxP@POP showed a positive binding energy displacement of about ∼0.54 eV compared to the as-synthesized POP (Figure S8, SI), revealing to the strong interaction between CoP and N-doped POP via intrinsic synergistic effect between nitrogen and CoP, making the CoP surface more electron-rich in nature. Molecular connectivity and chemical environment of carbon nuclei of CoxP@POP has been accessed by 13C CP MAS solid-state NMR spectrum analysis (Figure 4d), which shows a chemical shift at 126.3 ppm corresponding to the ortho (o-) and meta (m-) carbon atoms of aromatic subunits attached with the N atoms within the polymers. Interestingly, we have also observed a strong signal at 146.2 ppm, which could be attributed to the carbon atoms directly attached with the N atoms of the TPA, as previously reported for the nitrogen-enriched porous polymeric nanofibers.32 N2 adsorption/desorption isotherms of the as-synthesized POP (Figure S9) exhibited high N2 uptake at very low relative pressure region with a broad hysteresis loop corresponding to the presence of mesopores. The large hysteresis loop could be defined by the swelling of polymeric network during gas uptake when the polymer−solvent interaction is much greater than the polymer−polymer interaction. The Brunauer−Emmett−Teller (BET) surface area of the POP is 1356 m2 g−1 with the respective pore volume of 0.881 cm3 g−1. The irreversible N2 sorption isotherms of Co3O4@POP (Figure S10a) show a typical type I curve with a rapid uptake at low relative pressures, suggesting the existence of micropores. Co3O4@POP exhibits a high BET (Brunauer−Emmett−Teller) surface area of 585 m2 g−1 than CoxP@POP (SBET = 450 m2 g−1). In strong contrast, CoxP@ POP yielded a typical type II isotherm (Figure S10b) with the gradual N2 uptake in the high-pressure region with spanning hysteresis loop in the wide range, characteristic of the existence of interparticle void spaces, for which irreversible gas uptake can take place. The pore volumes for Co3O4@POP and CoxP@POP are 0.23 and 0.32 cm3 g−1, respectively. Pore size distributions as measured by nonlocal density functional theory method correspond to the narrow pore size distributions appearing predominately at the micropores region having sizes 1.54, 1.61, and 1.49 nm of POP, Co3O4@POP, and CoxP@ POP, respectively (Figure S11, SI). X-ray Absorption (XAS) Spectra. To gain insights into the local structural information around cobalt and its influence on the Co bulk valence state, X-ray absorption near-edge

(Figure 3) and energy-dispersive X-ray spectroscopy (Figure 3g) show that the Co and P elements appear at nearly the same places, verifying successful introduction of P into the Co3O4@ POP structure. Apart from HR-TEM, the crystalline nature of these nanoparticles is evident also from the SAED pattern (Figure 3d), where most of the spots can be readily indexed to the CoP phase. X-ray Photoelectron Spectra, 13C CP MAS NMR Spectra, and N2 Sorption Analyses. To shed light on the electronic and local structure, we have undertaken the X-ray photoemission (XPS) and X-ray absorption (XAFS) studies of the CoxP@POP and Co3O4@POP. Similarities and differences between CoxP@POP and Co3O4@POP can be readily noted in the Co 2p3/2 core-level XPS images shown in Figure 4a. The

Figure 4. X-ray photoelectron spectra for Co 2p3/2 (a), N 1s (b), and P 2p (c) regions. (d) 13C CP solid-state MAS NMR spectra.

Co 2p3/2 core-level spectrum of bulk Co3O4 is typically deconvoluted into five components, three low-binding-energy components related to the Co2+ and Co3+ states and two satellites.47 Nanohybrids show similar spectral deconvolution, but with different spectral weights, due to surface offstoichiometry, interaction with the host matrix, etc.48 Compared to the case of Co3O4@POP, the Co 2p3/2 corelevel spectrum of CoxP@POP presents a slight shift in the binding energy components related to the Co2+ and Co3+, for example, the Co2+-related feature shifts from 780.3 to 782.0 eV and the Co3+-related feature shifts from 779.8 to 781.3 eV. However, more importantly, there is a significant change in the spectral weights, in particular at ∼1.1 eV, the two high-bindingenergy components related to the shakeup satellites underline noteworthy variation of the Co electronic configurations in the two systems.49−53 Such changes also can be due to the significantly different interaction of the matrix (POP) and the nanoparticles (Co3O4 or CoxP). The N 1s spectrum for Co3O4@POP could be deconvoluted into three peaks with binding energies of ∼397.6, ∼398.7, and ∼400.4 eV, which correspond to the pyridinic, pyrrolic, and graphitic type of nitrogen atoms doped in the N-rich polymer matrix, in accordance with the previous observation by Cao et al.54 In the deconvoluted N 1s XPS image of CoxP@POP, three different 24144


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ACS Applied Materials & Interfaces structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra are collected from the two nanohybrid systems and a metallic cobalt foil. There is a significant difference in the Co K-edge XANES images of the two hybrids (Figure 5a). In particular, there is a 4.65 eV shift in

modeling providing quantitative information on the nearneighbor coordination around Co in the Co metallic foil and the two nanohybrids are also included in Figure 5b as solid red solid lines. For the first-shell EXAFS analysis of the Co3O4@ POP, considering the inputs from the XRPD data, we assumed a six-oxygen-atom coordination shell at ∼2 Å, which is found to describe well the local structure. A group of six phosphorus atoms at a distance of ∼2.32 Å seems to describe the first peak in the FT magnitude of the CoxP@POP, however with a huge Debye−Waller factor (see in Table 1 comparing the first-shell EXAFS analysis), implying a large atomic disorder in this system. Such a large local disorder reconciles well with the XRPD data, indicating the presence of more than one phase involving Co and P as well as the emergence of a long-range order involving the host matrix mostly stabilized due to the inclusion of a part of the Co atoms in the porous organic polymer. Catalytic Activity for HDO of Vanillin. We have evaluated the catalytic activities of our newly designed Co3O4@POP and CoxP@POP nanosheets in biofuel upgrading by considering the liquid-phase vanillin HDO into 2methoxy-4-methylphenol (MMP). Evolution of the reactant and product distributions with the progress of the reaction as a function of reaction time over the Co3O4@POP and CoxP@ POP catalysts is provided in Figure 6a,b, respectively. From the profile at 150 °C, it is easily interpreted that CoxP@POP exhibited satisfactorily higher catalytic activity providing 100% conversion of vanillin; however, we have achieved 70% vanillin conversion with Co3O4@POP (Figure 6a,b). The complete vanillin conversion took place at 5 h for the CoxP@POP catalyst, although selectivity toward MMP was 65%. Furthermore, the maximum yield of MMP for CoxP@POP was obtained at 7 h, exhibiting a turnover frequency (TOF) of about 0.0004 s−1. In sharp contrast, the Co3O4@POP catalyst displayed a substantially lower catalytic activity (∼2-fold lower) under identical conditions affording a TOF value of 0.0002 s−1 compared to the CoxP@POP. After 5 h, we have not noted any peak of 4-hydroxymethyl-2-methoxy phenol (HMP), which demonstrated the transformation of vanillin into MMP proceeded by hydrogenation to HMP followed by hydrogenolysis. Marginal drop in MMP selectivity was observed in the extended reaction time (8 h) due to the transformation of MMP to p-cresol (PC). The decisive role of reaction temperature has been thoroughly investigated, where CoxP@POP appeared to be more active as well as more selective than Co3O4@POP. We have achieved 70 and 41% vanillin conversion for CoxP@POP and Co3O4@POP, respectively, at 90 °C (Figure 6c,d).

Figure 5. (a) Co K-edge XANES images of Co3O4@POP, CoxP@ POP, and Co foil. (b) Co K-edge EXAFS image of Co−P/NC and its first-shell fitting. (c) EXAFS oscillations of Co3O4@POP and CoxP@ POP systems in comparison to that from a Co metallic foil.

CoxP@POP evidencing the reduction of the Co3O4 system due to the low-temperature phosphidation involved in the synthesis process.56,60 Absorption edge positions of CoxP@ POP and Co metallic foil are closer, albeit with a significant reduction in the prepeak (∼7713.2 eV) feature intensity. Notable differences are seen also in the XANES features of the two hybrids, indicating a large coordination environment changes in the two systems. Compared to a bulk metallic system, in the nanohybrids, the Co local coordination is characterized by short bond lengths involving Co−O or Co−P, which is readily observed in the Fourier transform magnitudes of the EXAFS images (Figure 5b). The EXAFS oscillations of the two hybrid systems are characterized by damping (weaker oscillation strength), which in turn reflects in the reduced amplitude of the first-shell peak in the FT magnitudes. EXAFS oscillations of the two systems are compared to those of the bulk Co metallic foil in Figure 5c. Results of a first-shell

Table 1. Local Structural Results from the First-Shell EXAFS Analysis of the Co3O4@POP and CoxP@POP Systems in Comparison to that from a Co Metallic Foil system Co metal

Co3O4@POP

CoxP@POP

structure

first shell

P63/mmc (194) a = b = 2.507 Å c = 4.068 Å α = β = γ = 90 Fm3̅c (227) a = b = c = 8.0837 Å α = β = γ = 90

Co−Co

R (Å)

DWF (Å2)

6

2.48

0.0022

Co−O

6

1.99

0.009

Co−P

5.94

2.32

0.015

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Figure 6. Evolution of reactant and product distributions as a function of time over Co3O4@POP (a), CoxP@POP (b), temperature effect of Co3O4@POP (c), and CoxP@POP (d). Reaction conditions: vanillin (300 mg, 2 mmol), catalyst (50 mg), isopropanol (30 mL), H2 pressure (40 bar), temperature (150 °C), and time (7 h).

observed from XRPD and XPS studies, which is the main active component for hydrogenation reaction, and polymer support stabilizes the oxide phase. This is in accordance with the work on heterogenized cobalt oxide catalysts for nitroarene reduction by Beller and co-workers.64 To gain insights into the factors explaining the increased catalytic activity of the CoxP@POP catalyst over Co3O4@ POP, we investigated the interaction of vanillin with the surface of the two catalysts by ATR-IR spectroscopy. The IR spectra acquired for the two catalysts appear very different, indicating dissimilar interactions of vanillin with the catalysts (Figure 7). Three distinct IR band regions characteristic of vanillin at 1676, 1589, 1512, and 1292 cm−1 can be observed. The band at 1676 cm−1 can be assigned to the CO stretching vibration, and the bands at 1589, 1512, and 1292 cm−1 are assigned to aromatic ring stretch along with C−H bending vibrations in the vanillin molecule. These bands are in good agreement with those of the simulated vanillin spectrum, affirming the assignments. A strong adsorption of vanillin on the surface of Co3O4@POP catalyst is indicated by the higher intensity of IR absorbance compared to the absorbance measured for the vanillin solution in the absence of catalyst. The vanillin at the enhanced concentration on/near Co3O4@ POP interacts strongly with the material surface, as evidenced by the red shifts of the three bands at 1500−1700 cm−1 (e.g., 1691−1673, 1514−1505 cm−1). Based on the absorbance values, the local vanillin concentration at the surface is roughly 3 times higher than the vanillin concentration in the bulk. After washing the surface with the neat solvent, these bands, although lower in intensity, can still be observed with further shift of the CO stretching vibration. These results suggest that there are two types of adsorbed vanillin molecules

Figure 6c demonstrates that with increasing temperature, the rate of the vanillin HDO enhanced up to 150 °C, as well as HMP formed by the hydrogenation of CO bond of vanillin was gradually deoxidized to the targeted product MMP. When the temperature was increased from 90 to 150 °C, the yield of MMP enhanced from 30 to 83.4%. However, at the elevated temperature (above 150 °C), the selectivity toward MMP declines due to the formation of PC by the elimination of the −OCH3 group.61 Therefore, optimum temperature for the conversion of vanillin to MMP is 150 °C. The CoxP@POP exhibited higher catalytic activity and selectivity to the desired reaction product than Co3O4@POP. The hydrodeoxygenation (HDO) of vanillin to 2-methoxy-4-methylphenol (MMP) is mainly a two-step reaction: the first step is the hydrogenation of vanillin to 4-hydroxymethyl-2-methoxy phenol (HMP), precisely aldehyde (−CHO) transformed into alcohol (−CH2OH) and followed by hydrogenolysis of HMP to MMP. Hydrogenolysis is the breakage of carbon and heteroatom bond (C−O bond) in the presence of breakdown of molecular hydrogen.62 The conversion of vanillin increased with increasing hydrogen pressure, and this dependence undoubtedly interpreted that hydrogenation predominated the overall reaction. From XRPD data and SAED pattern of CoxP@POP, it can be clearly observed that the orthorhombic CoP phase is mainly present in catalyst, where the P site creates a ligand effect on the Co metal sites changing the electron density of the metal cation, which helps in hydrogen dissociation,63 and N-rich polymer stabilizes and changes the electronic state of the cobalt phosphide phase through synergistic effect. Therefore, it can be concluded that CoP phase is the true active species in HDO activity. In the case of Co3O4@POP catalyst, mainly spinel cobalt oxide (Co3O4) was 24146


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observed in the catalytic tests (Figure 6) following the Sabatier principle or simply site blocking. It is also worth mentioning that these two materials may have different abilities to activate H2, although no difference was observed with the nature of adsorbed vanillin when the identical sorption experiments were performed with the solutions saturated with H2 instead of N2 (Figure 8). The ATR-IR data suggest that vanillin was strongly adsorbed on the surface of the catalyst Co3O4@POP compared to CoxP@POP, and it reflects in the superior catalytic activity of CoxP@POP compared to Co3O4@POP (Figure 6a,b). This phenomenon could be explained based on the Sabatier principle, which states that the interactions between the catalyst and the substrate should be “just right” that defines neither too strong nor too weak. The substrate will never bind on the catalyst surface if the interaction appeared too weak, and as a result, no reaction will occur. On the other hand, if the interaction becomes so strong, desorption of the product will become more difficult with sluggish nature and the key active sites of the surface will be blocked, which is the main reason of less catalytic activity of the catalyst. On the basis of the wellestablished principle, we can unambiguously conclude that due to strong interaction between vanillin and respective Co3O4@ POP catalyst, the product elution will be slow, thereby resulting in less selectivity toward deeper hydrogenation product.65,66 To get deeper insights into the reduction phenomenon and reactivity difference, we have carried out temperature-programmed reduction with hydrogen (H2-TPR) experiment. The multiple reduction peaks appearing in the TPR profile of Co3O4@POP (Figure S14, SI) suggest that reduction occurred through some subsequent steps and Co comprises with different oxidation. This profile consists of three major reduction peaks, where the low-temperature peak corresponds to the reduction of Co+3 ions to Co+2 ions, and the higher reduction peak could be attributed to the reduction of CoO to metallic Co. As the bulk cobalt oxide completely reduced at 500 °C, the peak appearing above 500 °C (583 °C) indicates that Co interacts with the support used in the

Figure 7. (a, b) ATR-IR spectra of 10 mM vanillin in isopropanol in comparison to a computed IR spectrum (B3PW91/6-311++G(2d,2p); the frequency was scaled by 0.978) of vanillin taking into account the solvent effects with the polarizable continuum model and assuming the dielectric constant of isopropanol to be 17.9. (c−f) ATR-IR spectra after adsorption/desorption of vanillin over CoxP@ POP and Co3O4@POP by passing N2-saturated 10 mM vanillin in isopropanol (adsorption) and neat isopropanol (desorption) at 50 °C.

interacting with Co3O4@POP: weakly bound ones and strongly bound ones, both interacting with the catalyst through the formyl group in different adsorption configurations judging from the broad nature of the CO stretching band and the most notable red shifts. On the other hand, almost no difference in the spectral features was noted in the presence or absence of CoxP@POP. Similar band intensity and position indicate that vanillin remains in solution without strongly interacting with the surface. This negligible interaction was further confirmed by the disappearance of the bands upon switching to pure isopropanol. These IR studies of vanillin interaction with the catalysts suggest that a weaker interaction of vanillin with CoxP@POP surface is beneficial for the reaction performance, whereas in the case of Co3O4@POP, its adsorption is too strong and slows down the reaction as

Figure 8. (a) IR absorbance spectra of vanillin on Co3O4@POP (a) and on CoxP@POP (b) in N2-purged and H2-purged solutions. Simulated IR absorbance spectra of MMP and HMP. Vanillin IR absorption bands assignment. 24147


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ACS Applied Materials & Interfaces catalyst.67,68 On the other hand, CoxP@POP exhibits a broad peak around 785−1120 °C, attributing an overlap in the reduction of both Co and phosphorus with highly dispersed Co phosphide species. To elucidate the reason for activity difference, we have also calculated H2 consumption values from H2 TPR study for both oxide and phosphide catalyst. We have clearly observed different H2 consumption values (Co3O4@POP 5.18 mmol g−1 and CoxP@POP 5.47 mmol g−1), which means a greater amount of hydrogen spill over species on cobalt phosphide phases than cobalt oxide phase. This result also confirmed that the active cobalt oxide phase is generated during the standard reduction treatment, but is highly unstable in the presence of air, being quickly reoxidized, corroborating the low catalytic activity compared to the cobalt phosphide analogue. Serrano and co-workers reported that the catalytic activity of MoP/Al-SBA-15 for HDO reaction is strongly influenced by H2 chemisorption results, revealing that the MoP phase was probably formed but it underwent a fast reoxidation in air.63 The screening results of different H2 pressures (1−6 MPa) show that an increase in H2 pressure led to an enhancement in vanillin conversion and selectivity to MMP (Figures S12a and S13, SI) because the concentration of the dissolved hydrogen gas is increased.69 The increase of the H2 pressure from 1.0 to 4.0 MPa enhances MMP yield up to 83.4%. However, enrichment in undesired side products (e.g., guaiacol and anisole) through overhydrogenation and decarbonylation pathways was observed with further increase of H2 pressure above 4 MPa (Figure 9).61 So, the optimum pressure for the production of MMP is 4 MPa.

satisfactory catalytic activity for vanillin conversion, which is still lower than that for POP-supported catalyst. This superior activity of the catalyst compared to other supports may be explained by the special properties of nitrogen-enriched POP, as reported previously for the catalytic hydrogenation reaction that includes electron donation offered by the nitrogen group of the POP framework, making the CoP surface more electronrich and active with homogeneous dispersion, easy adsorption of organic substrates onto the porous framework by hydrophobic and π−π interactions between aromatic aldehyde and aromatic POP framework, and huge surface area favoring hydrogenolysis of the intermediate to the desired product with fast and easy diffusion, thereby facilitating the effective reaction.71 Nitrogen atoms present in the as-synthesized POP framework play a decisive role in the enhancement in catalytic activity. An intrinsic synergistic interaction between N atoms and CoP is generated, which could easily modulate the electronic states of CoP, as experimentally evidenced by the N 1s XPS study (Figure S8, SI). The modulation of the electronic states of CoP may contribute to the lowering of d-band center, which has a significant impact in weakening of chemisorption energy of H2 on CoP surface, promoting the facile hydrogen transfer for catalytic hydrogenation. Our statement is strongly supported by H2-TPR analysis (Figure S14, SI), where a greater amount of hydrogen spill over cobalt phosphide phases than cobalt oxide phase is observed. It could also be emphasized that the polarization effect between N and adjacent carbon atoms significantly alters the charge distribution with the modification of interfaces by electronic effect, which might favor interactions with electron-rich functional groups in the vanillin molecule and, as a result, exhibiting a superior catalytic performance, consistent with the report by Jiang and co-workers.72 Nitrogen atoms can also enormously enhance the hydrophilic nature of the catalyst, serving homogeneous dispersion with stabilization of CoP NPs, along with the maximum exposure of catalytic active sites to the substrates (e.g., vanillin), thereby increasing the catalytic performance considerably compared to the conventional catalysts without nitrogen dopant. Our explanation is in accordance with the improved catalytic activity of [email protected] compared to the conventional catalysts in the HDO of vanillin, as reported by Wang et al.73 Only 5% vanillin conversion along with no MMP selectivity has been achieved employing bare CoxP nanoparticles. Negligible conversion has been attained with bare POP material. Table S2 demonstrates the effect of different solvents on the HDO of vanillin, and excellent activity as well as selectivity toward MMP was obtained when isopropanol was used as solvent. However, the selectivity of MMP decreased by using isopropanol and water mixture as a solvent due to the high solubility of vanillyl alcohol in water. However, the good selectivity toward MMP but poor conversion of vanillin was obtained in water solvent, and oil−water solvent follows the same trend, which was clearly explained in the literature. The polymer framework support contains N atoms belonging to the −NH2 group containing lone pair of electron, which does not exhibit convenient interaction with water molecule, and because of this reason, the activity of the catalyst decreases. Kinetic analysis was carried out over the two catalysts Co3O4@POP and CoxP@POP to find the reaction rate as well as the reactivity differences in vanillin conversion. We have investigated the overall reaction order for this vanillin reaction for both the catalysts by plotting vanillin concentration vs time

Figure 9. Possible hydrodeoxygenation (HDO) pathway of vanillin and the various reactions are involved to produce different products fragments.

To evaluate the effect of support on the catalyst, we have compared POP-supported catalyst with the Al2O3, TiO2, carbon, and N-doped carbon (Figure S12b) under the optimized reaction condition. The poor catalytic activities of alumina and titania are attributed to the fact that alumina is regarded to be in metastable state under hydrothermal conditions and titania is acidic in nature, which is not suitable for HDO reaction.70 Carbon and N-doped carbon displayed 24148


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Figure 10. Reaction order determination employing 1/vanillin concentration vs time plots for two catalysts Co3O4@POP (a) and CoxP@POP (b) at reaction condition temperature (150 °C) and H2 pressure (40 bar). Activation energy determination using ln k vs 1/T plots for two catalysts Co3O4@POP (c) and CoxP@POP (d).

Figure 11. Recyclability potentials of Co3O4@POP (a), CoxP@POP (b), and CoxP@C (c). TEM images of used CoxP@POP catalyst (d, e).

activation energy for Co3O4@POP than for CoxP@POP reduces the overall efficiency of the Co3O4@POP catalyst. The reusability of all catalysts during vanillin HDO was investigated up to five cycles (Figure 11a−c). After completion of the reaction, the catalyst was filtered and washed with methanol three to four times to remove the adsorbed reactant from the surface and pores of the catalyst. Furthermore, it was dried and used for the next run under identical conditions. Although a drop in MMP selectivity from 83.4 to 71.3% has

(Figure 10a,b) and it was found to be of second order, which is in accordance with vanillin hydrogenation in aqueous solutions using a Ru/C catalyst conducted by Vaidya and co-workers.74 We have also calculated activation energies separately for the reaction over the two catalysts from temperature dependence of the second-order rate constants following the Arrhenius equation, as shown in Figure 10c,d. The activation energy for Co3O4@POP was found to be 114.398 kJ, whereas for CoxP@ POP, it was found to be 81.59 kJ. As a consequence, the higher 24149


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ACS Applied Materials & Interfaces Table 2. Reaction Scope of Biomass-Based Various Feedstock Compounds with CoxP@POP Catalysta

a Reaction conditions: Substrate (300 mg); CoxP@POP (50 mg); solvent (30 mL); 40 bar H2 pressure; 150 °C; time, 7 h. *Here, the reaction was conducted with CoxP@Bn-POP.

image (Figure S17b, SI), corresponding to the P species in CoP and oxidized P species. This experimental result strongly confirmed that the oxidation state of P in CoP species remains unaltered after the catalytic runs. To examine the catalyst stability, hot-filtration test was conducted, which predicted that no additional increase in MMP selectivity was achieved after catalyst separation on further continuation of reaction. Only a small amount of a light blue solution was observed, which could be due to the leaching of Co species, which was below the detection limit of AAS analysis, but this blue solution did not catalyze this reaction up to any significant vanillin conversion. To address the issue raised for the leaching of cobalt from catalyst during reaction, we have conducted inductively coupled plasma-mass spectrometry (ICP-MS) technique for both fresh and used catalysts. The Co contents in the fresh and used CoxP@POP catalysts are 19.47 and 18.89 wt %, respectively, as measured by the ICP-MS technique (Table S4, SI). This result clearly signifies that a negligible leaching of cobalt took place during the reaction, which may take place by the collision of the stirrer and the autoclave wall. From this experimental result, we could conclude that our catalyst is indeed heterogeneous in nature and CoP nanoparticles are entrapped inside the porous cage in such a way that the leaching of active metal sites becomes restricted. TEM images (Figure 11d,e) of the reused CoxP@POP catalyst revealed that morphology with the agaric-like porous structure remains intact. The catalytic performance of our newly generated catalyst was compared to that of the reported catalysts for HDO of vanillin, where our catalyst exhibits

been observed for CoxP@POP, significant decreases in MMP selectivity from 62 to 27% and from 45.6 to 25% were attributed for Co3O4@POP and CoxP@C, respectively. This decrease may be due to deposition of carbonaceous layer on catalyst surface or entrapment of residual reactants or products inside the pores, thus blocking the active sites, which is in accordance with the report by Mondal and co-workers.31 We have conducted XPS analysis at Co 2p core region of the used Co3O4@POP catalyst after the catalysis (Figure S16, SI). The deconvoluted Co 2p XPS image consists of characteristic peaks appearing at 779.9 and 781.6 eV along with shakeup satellite features attributable to the typical spinel Co3O4 system, which demonstrated that the chemical states and crystalline phase of Co3O4 remain unaltered. 13C CP solid-state MAS NMR spectrum of the reused CoxP@POP catalyst (Figure S18, SI) exhibited two major signals at δ = 126.5 and 146 ppm, respectively, attributed to the aromatic carbon atoms and carbon atoms directly attached with the N atoms of triphenylamine, suggesting that the structural integrity of the POP framework unit has been preserved with no associated degradation during the course of catalytic cycles. To get insight into the oxidation state of the catalytic active species (CoP NPs), we have also performed XPS analysis of the reused CoxP@POP catalyst. The peaks appearing at 778.4 and 781.1 eV binding energies in the XPS image of the used catalyst in Co 2p core level (Figure 17a, SI) demonstrated that no severe change in the oxidation state of Co species in CoP occurred after the catalytic runs. We have also found the signals located at ∼125.7 and 134.7 eV binding energies in the P 2p XPS 24150


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activation energy barrier than the CoxP@POP, which also renders the reaction on the former more sluggish. This concept in robust active catalyst design of producing value-added chemicals provides an attractive avenue in biomass upgrading with added economic and environmental benefits. It could be anticipated that metal phosphides will open new opportunities as a significant step forward in promoting biomass refining in the future.

considerable vanillin conversion and MMP selectivity (Table S3). In addition, to highlight the superiority of the catalyst, we have investigated the activity of the catalyst CoxP@POP toward other useful substrates like furfural and guaiacol, which can produce value-added products under similar reaction conditions (Table 2, Supporting Information). Interestingly, for reaction with furfural, we have achieved 96.4% conversion with a valuable product yield of 2-methylfuran (74.3%; Table 2, entry 1), whereas with guaiacol, we have achieved 72.6% conversion with a valuable product yield of phenol (67%; Table 2, entry 2). We have also checked the scope of the Cox P@POP catalyst taking other monophenol having aldehyde/methyl group. For p-cresol, we have achieved 76.2% conversion with a methylcyclohexane yield of 72%, whereas for 2-hydroxybenzaldehyde and 4-hydroxybenzaldehyde having conversions of 47.6 and 68.7% with phenol, the yields were 47.6 and 68.7%, respectively (Table 2, entries 3− 5). We have prepared another one benzene organic backbone unit POP (Bn-POP) using divinyl benzene as monomer (Scheme S1, SI). The detailed synthesis procedure of Bn-POP is provided in the Experimental section (SI). After that, CoP NPs was impregnated in the Bn-POP following a similar solidstate phosphidation procedure to furnish CoP@Bn-POP. Then, we have conducted vanillin HDO reaction to elucidate the specific role of N atoms in the POP support. We have observed in this case 78% conversion with 59 and 19% selectivities of 2-methoxy-4-methylphenol (MMP) and 4hydroxymethyl-2-methoxy phenol (HMP), respectively (Table 2, entry 6). From this experimental result, we can conclude that the N atoms present in the POP support play a pivotal role to generate an intrinsic synergistic effect to modulate the electronic state of CoP NPs caused by an obvious electronic interaction between N and CoP, resulting in an increase in the catalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06789. Wide-angle powder XRD patterns of POP, FT-IR, FESEM, and TEM images of POP; FE-SEM image of Co3O4@POP; N2 adsorption/desorption isotherms of POP; Co3O4@POP, CoxP@POP, catalysis data (H2 pressure effect of HDO for Co3O4@POP, possible hydrodeoxygenation (HDO) pathway of vanillin, effect of catalyst dose for HDO with CoxP@POP); IR absorbance spectra of vanillin on Co3O4@POP and CoxP@POP in N2-purged and H2-purged solutions; crystalline phase information of CoxP@POP; effect of various solvents on reaction using CoxP@POP; and comparison study over different types of catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

John Mondal: 0000-0001-7813-2108 Author Contributions ⊥

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S.C.S. and P.K. have equally contributed.

Notes

The authors declare no competing financial interest.

CONCLUSIONS In summary, we have successfully developed agaric-like porous CoxP@POP hybrid nanosheets via solid-state phosphidation of Co3O4@POP and investigated the catalytic activity toward HDO of vanillin, a typical compound of lignin-derived bio-oil, in promoting biomass refining to furnish 2-methoxy-4methylphenol (MMP), which is a promising future biofuel. We have achieved an impressive catalytic activity of CoxP@ POP, with the selectivity toward 2-methoxy-4-methylphenol (MMP) higher than that of Co3O4@POP at full conversion. Various characterization techniques, including X-ray absorption spectroscopy (XAS), revealed that the superior activity, improved stability, and selectivity of CoxP@POP originated from its 3D interconnected porous structure, synergistic effect of CoxP encapsulated within N-doped polymer matrices, and modulation of the electronic properties of Co with the introduction of P. Our thorough investigation with attenuated total reflectance-infrared (ATR-IR) spectroscopy unambiguously revealed that weaker interaction of vanillin with CoxP@ POP surface is beneficial for the reaction performance, whereas in the case of Co3O4@POP, its adsorption is too strong and the reaction becomes sluggish. Through H2-TPR study, we have clearly observed different H2 consumption values (Co3O4@POP, 5.18 mmol g−1; CoxP@POP, 5.47 mmol g−1), implying greater H2 activation over phosphide catalyst than oxide analogue. Also from kinetic investigation, it is clearly demonstrated that the Co3O4@POP has higher

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ACKNOWLEDGMENTS S.C.S. and P.K. acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, and IICT-RMIT PhD Program, for their respective junior research fellowships. J.M. acknowledges the Department of Science and Technology, India, for the DST-INSPIRE Faculty Research project grant (GAP-0522) at CSIR-IICT, Hyderabad. B.J. acknowledges IISc Bangalore and ICTP Trieste for the IISc-ICTP fellowship. The authors thank Sorin Bunea and Prof. Atsushi Urakawa of Institute of Chemical Research of Catalonia (ICIQ), Spain, for interpretation of the results of ATR-IR spectroscopy. We also wish to kindly acknowledge Dr. Mohamad Hassan Amin and Dr. Ylias Sabri of RMIT University Australia for their helpful discussion in kinetic study. The authors thank Dr. Chaitali Dekiwadia staff of RMIT Microscopy & Micro Analysis Research Facility (RMMF) at RMIT University for scientific assistance. They acknowledge DKIM of IICT (Division of Knowledge and Information Management) for plagiarism checking and providing them the manuscript communication number: IICT/Pubs./2019/202.

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REFERENCES

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