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• The current chemical and energy industries heavily depend on petroleum, coal and natural gas, which have limited reserve and nonrenewable exploiting. In this case, most countries worldwide have devoted their researchers to studying utilization of renewable biomass to ease resource constrains^[1]. Lignocellulose has been regarded as a promising alternative source to substitute fossil fuels to produce bio-based chemicals and fuels owing to its abundance and renewability^{[2, 3]}. Cellulose, lignin and hemicellulose constitute the main components of lignocellulose and they have exhibited huge research value, as showed in Figure 1. Recently, the lignin has been adopted as the feedstock to synthesize phenolic compounds and derived fuels while the hemicellulose has been extensively explored to produce C₅ chemicals^[4-7]. So far, there has been several studies on the conversion of cellulose to glucose, polyols, 5-hydroxymethyl furfural (5-HMF), organic acids, ethanol and methyl glycolate^[8]. Low carbon (C_{2, 3}) polyols, including ethylene glycol (EG), propylene glycol (PG) and glycerol (Gly), are predominating intermediates in the manufacture of plastics, food additives, pharmaceuticals and cosmetics, etc.

  Figure 1

  

  Figure 1.  Representative bio-based chemicals derived from lignin, hemicellulose and cellulose

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  However, recalcitrance of cellulose exhibits great challenges for oriented depolymerization under mild environmental conditions while some enzymolysis and physical pretreatment strategies were investigated for deforming its robust structure^[9-11]. In order to satisfy more rigid environmental regulations, the researchers face the choices of using the various catalysts for degradation of cellulose because of its recalcitrance. According to previous studies, cellulose could be transformed into C_{2, 3} polyols in the presence of hydrogen at high temperature (220-245 ℃) via chemical catalytic process^{[12, 13]}. In order to crack the bonds of β-1, 4-glycosidic bond and C-C bonds in inner molecular of cellulose, several active metals in nanoscale supported by inorganic materials were further investigated^[14]. The employed catalyst reagents can roughly be divided into non-noble metals such as Ni^[15], Cu^[16] and W^[17], etc. and noble metals such as Pt^[18] and Ru^{[19, 20]}, etc. Successfully, the presence of different active metals on supports promoted the occurrence of various complex reaction pathways in a one-pot reaction of cellulose conversion. Among the active metals, catalysts containing nickel and different forms of tungsten (W/C^[21], WO₃^[22] and NiW alloys^[23]) were revealed extremely high activity and selectivity for formation of C_{2, 3} polyols, especially for EG. The catalysts with tungsten sites were highly active in promoting the selective cleavage of C-C bonds and the retro-aldol condensation which was the most acceptable mechanism for formation of low carbon unsaturated intermediates in liquid medium^[24]. Nickel sites mainly played an important role in hydrogenation of unsaturated intermediates to form polyols^[25]. W and Ni sites occurred the compatible and synergetic catalysis in cellulose conversion. Although the metallic catalysts at the nanoscale can be good candidates for degradation of cellulose, many important factors including particle size, morphology, electrical conductivity and surface modification also had an obvious influence on their activities^[14]. In order to consolidate the active sites resisting to leaching under hydrothermal acid conditions, some researchers dedicated to investigate the activities of metallic catalysts hybridized with nitrogen-doping in hydrogenation reaction^{[26, 27]}. Benefiting from the excellent thermal stability and corrosion resistance against acid and alkali, carbon from biomass materials was widely studied and applied in this catalytic system. Due to its neutral chemical properties, bio-carbon exhibited extreme compatibility for loading of active metals, such as Ni, W, Ru and Pt. Moreover, recently, some reports about the catalysts construction for oxygen reduction revealed that the graphitic carbon nitride (g-C₃N₄) phase showed high chemical stability in hydrothermal circumstance^{[28, 29]}. The usual binding configuration of N dopants in a given carbon matrix were pyridinic, pyrrolic and graphitic N, which are available for complex metal cations including Ni²⁺, Fe²⁺, Cu²⁺ and Pt²⁺. This function could inspire a new strategy for catalysts preparation. Thus, N-containing complex agent was adopted to have a complex reaction with metal cations and eventually producing Metal-N_x-C_y or specific stable N_x-C_y phase.

  In order to prove the scientific hypothesis mentioned above, it was aimed to synthesize nickel-tungsten catalysts supported on N-doping porous carbon through calcining specific catalyst precursors and organic nitrogen sources. The carbon would be from carbonization of raw bamboo powder. These fabricated catalysts would be applied into bamboo pulp conversion into ethylene glycol under hydrothermal reaction condition. The dispersion of particles, leaching resistance and catalytic efficiency of these nickel-tungsten catalysts would be further studied in this catalytic system.

  1.   Experimental

  1.1   Catalyst synthesis

  The bimetallic 15%Ni-20%W/MBC@Y-x catalysts were prepared by incipient-wetness impregnation method (Y: nitrogen source; x: the mass fraction of nitrogen source based on the mass of MBC). MBC was from carbonization of bamboo powder. All the mass fractions were calculated on the base of supports weight. Briefly: the nickel nitrate (Ni(NO₃)₂·6H₂O, Energy Chemical Co., Ltd., China.) and phosphotungstic acid (H₃PW₁₂O₄₀·xH₂O, Sinopharm Chemical Reagent Co., Ltd., China.) were impregnated on the mesoporous bamboo carbon (MBC) under vigorous stirring while the nitrogen source (melamine, ethanediamine, carbamide or arginine) was mixed with MBC. The analytical grade melamine (M), ethanediamine (EDA), carbamide (CAR) and arginine (ARG) were purchased from Aladdin Co. Ltd (Shanghai, China). Subsequently, the catalyst samples were oven dried at 90 ℃ for 12 h and followed by milling and screening into powders. Finally, the dry samples were calcined in a tubular calciner under a N₂ flow of 20 mL/min at 650 ℃ for 3.0 h with the heating rate of 3 ℃/min. The calcined samples were reduced in a H₂/N₂ (1:4 in volume) flow with a rate of 10 mL/min at 500 ℃ for 3.0 h.

  1.2   Catalytic experiments and products analysis

  The hydrogenolysis of bamboo pulp was performed in a batch reactor (Yanzheng Equipment Co., Ltd., 100 mL stainless steel autoclave) at a stirring speed of 600 r/min. Typically, 0.5 g bamboo pulp and 0.2 g catalyst mixed with 50.0 mL deionized water were introduced into the autoclave. Afterwards, the reactor was purged with 5.0 MPa H₂ after removing air by N₂ for three times. Subsequently, the reactor was heated to 240 ℃ and the reaction process lasted for 1.5 h. The liquid phase after reaction were filtrated and analyzed by high performance liquid chromatography (HPLC, Waters, USA) equipped the refraction index detector (RID 2414, Waters, USA). 20 μL liquid samples were injected to HPLC under 0.5 mL/min of deionized water (mobile phase) and then detected by Welch X'mate-Ca (7.8 mm×300 mm, 5 μm) column at 80 ℃ heated through an external column oven. The conversion of bamboo pulp and yields of polyols are calculated based on the following formulas^{[12, 30]}:

  $ {x_{{\text{ bamboo}}\;{\text{pulp }}}} = \frac{{{M_0} - {M_{\text{R}}}}}{{{M_0}}} \times 100\% $

  (1)

  $ {w_{{\text{ plovols }}}} = \frac{{{M_{\text{p}}}}}{{{M_0}}} \times 100\% $

  (2)

  Where x and w are the conversion of feedstocks and yield of polyols, respectively; M₀ represents the weight of bamboo pulp before reaction, M_R is the residual weight of bamboo pulp after reaction and M_P is the polyols mass in liquid products.

  1.3   Catalysts characterization

  N₂ adsorption-desorption isotherms were measured at -196 ℃ by the Micromeritics ASAP 2420 degassed at 200 ℃ to release the physically adsorbed impurities for 8 h in vacuum. BET area and BJH pore size distribution were calculated according to the desorption branch of the isotherms.

  The thermogravimetric analysis (Thermogravimetry, TG) of the catalyst samples was done by STA 449 of F3 Jupiter synchronization comprehensive thermal analyzer (Germany NETZSCH, Germany). The heating rate was set at 10 ℃/min from room temperature to 900 ℃ under the mixed flow rate of air (20 mL/min) and the nitrogen (5 mL/min).

  The phase compositions of the catalytic species were characterized by X-ray diffraction (XRD) on a SmartLab X-ray diffractometer (Rigaku, Japan) in the 2θ range of 5°-85° using a Cu Kα1 X-ray radiation operating at 40 kV and 100 mA.

  X-ray photoelectron (XPS) was measured on a Kα spectrometer (Thermo Fisher Scientific, USA) with an Al Kα X-ray source (1486.6 eV, 15 kV, 150 W). The obtained binding energies were calibrated using the C 1s peak at 284.6 eV as the reference. The photo-peak area was determined after the subtraction of the linear spectral background. The spectral simulations were based on a Gaussian profile.

  The catalyst particles distribution was observed by transmission electron microscope (TEM) on a JEOL JEM-2100F microscope at accelerating voltage of 80 kV. Catalyst samples were suspended in ethanol with an ultrasonic dispersion for 20 min and deposited on copper grids coated with amorphous carbon films.

  ICP-OES test of reaction liquid was performed on Perkin Elmer ICP instrument (Optima 5300DV, 40.68 MHz) with an optical resolution of 0.006 nm at the wave length of 200 nm and the detection limit was up to (0.1-1.0)×10^-6.

  2.   Results and discussion

  2.1   Physical properties of the catalysts

  The physical properties of different nickel-tungsten catalysts, including surface area (A_BET), total pore volume (v_total), average pore size and pore-size distribution, were determined by N₂ adsorption-desorption experiments and the results are shown in Table 1 and Figure 2. The melamine content, as observed, has very small effects on enlargement of surface area (from 280.7 to 262.9 m²/g) and average pore volume of MBC (v_total) is held as a constant at 0.12 cm³/g. Comparatively, when ethanediamine (EDA) is adopted as the nitrogen source, the BET surface decreases to 164.2 m²/g. According to Figure 2(a) and 2(b), the adsorption-desorption isotherms of the catalysts can be roughly defined as the isotherm of micro and mesopores. In Figure 2(c), large platforms of isotherm curves are observed when p/p₀ was at 0.1-0.8 and the adsorption capacity increases rapidly at relative lower pressure (p/p₀ < 0.1), which implies the emblematic adsorption of micro pores. By contrast, the isotherms increase swiftly to form an inflection point at the higher relative pressure (p/p₀ > 0.9) in Figure 2(a) and 2(b). This result probably elucidates that a large amount of adsorption condensation occurs on the outer surface of the sample particles or some macro pores are formed during the preparation of MBC.

  Table 1

  

  Table 1.  Physical properties of different supported 15%Ni-20%W catalysts measured by BET

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  Sample	ABET/(m2·g-1)	vtotal/(cm3·g-1)	Average pore size d/nm
  MBC@M-0.15	280.7	0.12	7.1
  MBC@M-0.20	276.1	0.12	6.1
  MBC@M-0.25	271.6	0.12	6.6
  MBC@M-0.30	262.9	0.12	5.4
  MBC@EDA-0.25	164.2	0.07	5.2
  MBC@CAR-0.25	256.1	0.12	9.3
  MBC@ARG-0.25	264.1	0.11	4.1

  Figure 2

  

  Figure 2.  N₂ adsorption-desorption isotherms and the pore size distribution of MBC with various nitrogen sources and quantities

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  2.2   XRD

  The phase compositions of the Ni-W/MBC@M-x (x = 0.15-0.30) and Ni-W/MBC@Y-0.25 (Y=EDA, CAR, ARG and M) catalyst samples are characterized by XRD in Figure 3 for 2θ values ranging from 5°-90°. The peaks at 2θ values of 44.4°, 51.8° and 76.1° are corresponded to metal Ni crystallites (JCPDS file No. 04-0850)^[13]. All samples show these Ni⁰ characteristic peaks in Figure 3(a) and 3(b) while the peak intensities are different. This is the reason why the catalysts after calcination show the strong magnetic in Figure 4 when the organic N-containing compounds are introduced during preparation. The nickel phosphide phase is observed at 54.6° since the introduction of P is from phosphotungstic acid. By contrast, the tungstic phases appeared at 33.3° and 36.4° are associated with WO_x species including WO₃ (JCPDS No. 32-1395) and WO_2.83 (JCPDS No. 36-0103)^[12]. The very small intensity of the peaks at 32.7° and 47.5° could be explained by the formation of tungsten nitride (WN, JCPDS No. 32-1395) on account of melamine addition. Previous studies elucidated that metal-nitrogen (M-N_x) could be engaged as an active catalyst for electrocatalytic performance^[31]. Interestingly, another low-intensity peak at 27.4° is corresponded to graphitic carbon nitride (g-C₃N₄) phase which is known as a good photocatalytic performance for hydrogen production via water splitting under visible-light irradiation^[32]. This metal-free g-C₃N₄ also possesses very high thermal and chemical stability as well as its electronic properties. Moreover, the XRD patterns also indicate the strong interactions between the nickel and tungsten occurring to form Ni_xW_y (x, y represented molar ration in alloys) alloys during the calcination process. According to XRD characterization, the Ni_xW_y alloys mainly consiste of NiWO₄ (JCPDS No. 15-0755) which are corresponded by 2θ of 19.2°, 23.4°, 24.8°, 30.8°, 36.4°, 41.5°, 52.1°, 62.3° and 65.6°^[33]. Interestingly, these Ni_xW_y alloys are formed only when the content of melamine is less than 20% as shown in Figure 3(a). The presence of NiWO₄ formation is the most significant discrepancy among Ni-W/MBC@M and Ni-W/MBC@Y (Y=EDA, CAR and ARG) catalysts. In other words, these results suggested that the high content of melamine could hinder the formation of Ni_xW_y alloys. What's more, WO₃ species also could be observed scarcely at 33.3° or 36.4°. The WO₃ phase had been proved to demonstrate high catalytic activity in hydrogenolysis of cellulose attributing for C-C bonds cracking^{[34, 35]}. In order to analyze the characteristic peaks of the support MBC, the XRD pattern of MBC pretreated by ethanol, alkali and water respectively is recorded in 2θ of 5°-50° in Figure 3(c). Accordingly, the results reveal that the presence of bulky crystals of carbon could not be found, implying that the phase of MBC is amorphous.

  Figure 3

  

  Figure 3.  XRD patterns of different catalysts and MBC

  (a): amount of melamine as the variable;
  (b): the catalysts prepared by addition of multifarious nitrogen sources;
  (c): MBC was pretreated by ethanol, alkali and water, respectively

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  Figure 4

  

  Figure 4.  Magnetic experiment of unreduced and reduced Ni-W/MBC@M-0.25 catalysts

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  2.3   TEM

  The morphologies and structure of Ni-W/MBC catalysts were investigated by transmission electron microscopy (TEM) analysis. Three different morphologies about Ni-W/MBC@M-0.25 (a), Ni-W/MBC@ARG-0.25 (b) and Ni-W/MBC@CAR-0.25 (c) are exhibited in Figure 5(a)-5(c).

  Figure 5

  

  Figure 5.  TEM images of (a) Ni-W/MBC@M-0.25, (b) Ni-W/MBC@ARG-0.25, (c) Ni-W/MBC@CAR-0.25 catalysts and (d) the structure of metallic particles surrounded by C₃N₄ layer in high resolution

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  This TEM characterization is mainly performed to study the effect of organic nitrogen sources on particle distribution. Definitely, the results demonstrate that the dispersion of metallic particles show great discrepancy among these catalysts. A small broad and well-distributed metallic particles can be clearly observed on MBC according to Figure 5(a) in different scales. The crystallites size is approximately in range of 3-8 nm. Previous studies implied that smaller particles possessed more accordingly catalytic sites since dominating the same concentration of precursors, which could be responsible for the catalytic activity toward the hydrogenation of C=O bonds (unsaturated aldehydes to saturated polyols)^[36]. However, the Ni-W/MBC@ARG and Ni-W/MBC@CAR catalysts exhibite relatively large particles on the surface of MBC, indicating that the crystal shapes are more pronounced. Comparatively, on the surface of the two catalysts, there existed different types of particles probably including metallic alloys and monocrystals. The clear and continuous lattice spacing is measured with a d-spacing of 0.5808, 0.6603 and 0.6535 nm, which are respectively attributed to the (211) [Ni₄W], (111) [Ni₁₇W₃] and (042) [NiW] planes on surface of Ni-W/MBC@ARG catalyst despite the less amount of Ni₁₇W₃ and NiW alloys are formed according to previous characterization^{[33, 37, 38]}. Accordingly, well resolved lattice fringes with interplanar distance of 0.3868 and 0.3669 nm are observed in Figure 5(c), which is closed to the (022) plane of Ni monocrystal at 5-10 nm scale. This characterization results indicate the successful formation of Ni_xW_y alloys and metallic Ni species. In order to observe the relative position of metallic particles and C₃N₄ phase (characterized by XRD), the catalyst sample was characterized by HRTEM. According to Figure 5(d), it is clearly observed that the metallic particles is surrounded by some disordered layer. Therefore, it is deduced that C₃N₄ phase could generate a protecting layer to inhibit the aggregation under this hydrothermal condition. This structure provided a material basis for remaining the chemical stability of catalysts. However, no further evidence could prove that all the particles on the surface of support were surrounded by C₃N₄ layer. These observations were basically consistent with results of XRD characterization. Successful formation of Ni and Ni-W alloys species might result in a cooperation effects in catalysis for degradation of bamboo pulp.

  2.4   XPS

  XPS analysis was conducted to evaluated the chemical states of the surface tungsten and nickel species. The W 4f and Ni 2p spectra, presented in Figure 6, are obtained for Ni-W/MBC, Ni-W/MBC@ARG, Ni-W/MBC@CAR and Ni-W/MBC@M catalysts.

  Figure 6

  

  Figure 6.  X-ray photoelectron spectra of (a) W 4f region and (b) Ni 2p of various 15%Ni-20%W/MBC@M-0.25 catalysts

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  The W 4f spectrum contains two overlapping doublets including W 4f_5/2 and W 4f_7/2 at binding energies of 37.5 and 35.6 eV. These peaks at 37.5 and 35.6 eV could be attributed to W⁶⁺ species according to deconvolution^[12], which is mainly ascribed to tungsten oxides such as WO₃ and WO_2.83 characterized by XRD. Comparatively, these binding energies of the two peaks exhibite a small shift to higher energy probably because the strong interactions among metals cause oxygen vacancy (WO_3-x species) during calcination. Previously, it was studied that WO₃ was well known for its non-stoichiometric properties since the lattice could withdraw a considerable number of oxygen vacancies^[39]. The phenomenon of oxygen vacancies was beneficial to promote the surface conductivity of catalysts^{[38, 40]}, which would enhance the prosperity of transferring of electron and charged particles. Accordingly, the binding energy of Ni 2p_3/2 is observed at 857.2 eV while the binding energy of Ni 2p_3/2 has shifted to lower energy at 856.1 eV illustrated by Figure 6(b). These characteristic binding energies were assigned to metallic Ni⁰. The formation of Ni⁰ implied that the nickel nitrate was decomposed to form NiO phase and it was consecutively reduced to Ni⁰ during calcination because of existence of organic carbon source which could be converted to reducing agent under N₂ atmosphere. The relative high content of Ni⁰ contributed to hydrogenating process of unsaturated intermediates such as alcohol aldehydes and glyceraldehyde. More importantly, the binding energy fitting peak of Ni 2p_3/2 at 852.3 eV about NiWO₄ phase can observed according to Figure 6(b) marked by the red dotted circle.

  Moreover, asymmetrical but obvious C 1s peak (in Figure 7) of the XPS spectra at 284.6 eV proves the significant modification of the sp² carbon atoms on the surface of the support, which is mainly from the carbonization of organic nitrogen^[41], and carbon sources (melamine, arginine, ethanediamine and carbamide) accompanied interaction between N and C atoms to form C₃N₄ composite at 398.2 and 400.1 eV demonstrated in Figure 7(b). The C 1s peak at 285.7 eV is usually assigned to the oxygen-containing carbonaceous bands (C-O). The existence of foreign atoms including N and O is due to addition of nitrogen source and its inadequate carbonization. According to N 1s XPS spectra of the different catalysts, the asymmetrical and broad features of the observed N 1s XPS peaks at 398.2 and 400.1 eV indicate the existence of chemically different N species in the C₃N₄ composite deduced from deconvolution after Gaussian curve fitting in Figure 7(b). The N 1s peak at 398.2 eV could be assigned to sp²-hybridized nitrogen (C=N-C)^[42]. The other peak at about 400.1 eV could be attributed to tertiary nitrogen [N-(C)₃] pointed out by red arrow in the figure. The other peak at about 400.1 eV could be attributed to tertiary nitrogen [N-(C)₃]^{[43, 44]}, which confirms the polymerization of nitrogen sources especially for melamine. This characterization supported the well-modified character of the catalyst surface by C₃N₄ composite.

  Figure 7

  

  Figure 7.  X-ray photoelectron spectra of (a) C 1s and (b) N 1s on different nickel-tungsten catalysts

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  2.5   Catalytic conversion of bamboo pulp via N-doping nickel-tungsten catalysts

  Actually, the one-pot conversion of cellulose or lignocellulose was a complex transformation process, where the nickel and tungsten based catalytic species contributed to retro-aldol condensation and hydrogenation of unsaturated intermediates^[45]. Here, the bamboo pulp, removing most of lignin and hemicellulose was adopted as the feedstock to investigate the catalytic performance of prepared nickel-tungsten catalysts under condition of 240 ℃ and 5.0 MPa H₂. The 15%Ni-20%W/MBC@M catalysts exhibite relative high performance obtaining 76%-100% conversion of bamboo pulp in Table 2. Comparatively, the catalytic results demonstrate the perceptible activity discrepancy among these catalysts in Entry 1-3 and 5. A high EG yield of 55.8% is achieved using 15%Ni-20%W/MBC@M-0.25 catalyst prepared with addition of 25% melamine while the 15%Ni-20%W/MBC@M-0.15 only obtains 29.4% EG yield. The control experiment in Entry 6 shows slightly low yield of polyols without any nitrogen source. These findings obviously indicate that the content of melamine has an influence on the catalytic efficiency. According to Figure 8, it can be clearly observed that the introduction of melamine has function on yield of EG but a slight influence on the other products. Yang et al^{[17, 46]}reported that the tungsten species could facilitate the C-O and C-C bonds cracking and improve the selectivity of EG significantly. From this view, it was speculated that the interactions mainly among tungsten participations occurred during the decomposed of nitrogen sources in calcination. However, according to XRD and XPS analysis, the introduction of melamine scarcely affected the formation of N-containing composites such as nitride tungsten and nitride nickel but the interactions among metallic particles and MBC were promoted which would enhanced catalytic activity eventually. Interestingly, for another hand, the glycerol (Gly), 1, 2-propanediol (1, 2-PG) and glucose show similar distributions as below 10% yield, indicating that this series of catalysts present the moderate activity for C₃ products except that the glucose could be transformed to glucuronic acid at 240 ℃^[47]. Afterwards, the traditional carbon materials were employed to investigate the attribution of supports to cellulose conversion and the corresponding catalytic results in Table 2 Entry 6-8 reveal the apparent discrepancy of polyol yields. Even though 15%Ni-20%W/SWCNTs (single-walled nano carbon tubers) showed the common activation in this condition, the exorbitant price of SWCNTs resulted in lower process economy.

  Table 2

  

  Table 2.  Distribution of polyol products from bamboo pulp via nickel-tungsten catalysts ^a

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  Entry	Catalyst	Yield of product/%					Conv. x/%
  		EG b	Gly	1, 2-PG	glucose	sor
  1	15%Ni-20%W/MBC@M-0.15	29.4	4.1	6.1	5.4	5.1	83
  2	15%Ni-20%W/MBC@M-0.20	46.2	3.5	7.6	3.5	4.3	100
  3	15%Ni-20%W/MBC@M-0.25	55.8	5.6	10.5	4.8	6.1	100
  4	15%Ni-20%W/MBC@M-0.25 b	25.4	3.3	2.1	1.3	trace	76
  5	15%Ni-20%W/MBC@M-0.30	51.2	4.7	8.1	3.5	5.3	100
  6	15%Ni-20%W/MBC	36.9	6.1	7.2	2.9	3.9	95
  7	15%Ni-20%W/AC	21.6	2.3	7.8	3.1	trace	84
  8	15%Ni-20%W/SWCNTs c	33.1	8.1	5.6	3.6	4.6	100
  9	15%Ni/MBC@M-0.25	10.3	trace	1.2	6.1	1.4	78
  10	20%W/MBC@M-0.25	6.5	trace	trace	1.2	UD.g	95
  11	15%Ni-20%W/MBC@EDA-0.25 d	44.6	4.5	5.3	5.7	4.1	100
  12	15%Ni-20%W/MBC@CAR-0.25 e	31.0	5.6	3.2	4.3	0.9	91
  13	15%Ni-20%W/MBC@ARG-0.25 f	54.1	6.1	3.0	4.1	3.2	100
  a : reaction conditions: bamboo pulp 0.5 g, 5.0 MPa H2, 240 ℃ for 1.5 h; b: feedstock: raw bamboo powder; c: SWNTs: single-walled carbon nanotubes; d: EDA: ethanediamine; e: CAR: carbamide; f: ARG: arginine; g: undetected

  Figure 8

  

  Figure 8.  Effect of melamine addition on distribution of C_{2, 3} polyols: M-x represented the mass ratio of melamine based on the weight of MBC

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  What's more, in the domain of nitrogen sources, the other nickel-tungsten catalysts were also prepared using ethanediamine, carbamide and arginine besides melamine, and the addition amount was brought up to 25% correspondently based on the mass of the support MBC. This process was used to investigate the possible synergistic effect among metals and support composites by adjusting the nitrogen sources, which might be regarded as a surface modification to mesoporous bamboo carbon. According to the catalytic results in Table 2 Entry 11-13, the yields of EG demonstrate relatively great discrepancies because of various adding amount of different nitrogen sources. Compared to Ni-W/MBC@M-0.25 catalyst, the Ni-W/MBC@ARG-0.25 catalyst could obtain 63.2% of C_{2, 3} polyols (54.1% yield of EG) while the others could only achieve 54.4% (EDA) and 39.8% (ARG) yield of C_{2, 3} polyols. Furthermore, the addition of ARG inhibites the bamboo pulp conversion slightly (95% in Entry 6 decreased to 91% in Entry 12). Interestingly, this influence seemingly brings the difference of EG yield while the distribution of C_{2, 3} polyols still remained in range of 3.0%-6.1% as exhibited in Figure 9. From the mechanism point of view, the bio-nitrogen could promote the interactions between metals and even formed new phases including NiWO₄ and C₃N₄ species. According to our previous studies, the Ni-W alloys including NiW, Ni₄W and NiWO₄ phases could supplement and enhance the incorporation of intrinsic nickel and tungsten catalytic activity^[23]. Moreover, the NiWO₄ alloy might inhibit the catalysts deactivation at reaction condition because of its hydrothermal resistibility^{[48, 49]}. Furthermore, Ni-W alloy has been nominated as a promising environmental friendly alloy and its anti-corrosion and-leaching properties probably preserved the stability of nickel under this reaction condition^{[50, 51]}. The XPS analysis behaves as proof to prove the formation of NiWO₄ alloy when the melamine is added during the catalyst is prepared (Figure 7).

  Figure 9

  

  Figure 9.  Performance of nickel-tungsten catalysts influenced by adopting various nitrogen sources

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  Otherwise, the XPS analysis also revealed that C₃N₄ phase was formed on the prepared catalysts. During the calcination, the nitrogen sources such as ethanediamine, carbamide, arginine and melamine could be carbonized on the surface of MBC to form limited amount of C₃N₄ phase. Previous studies indicated that graphitic carbon nitride (g-C₃N₄) possessed very high thermal and chemical stability as well as interesting electronic properties^[52]. But this study just focus on the formation of C₃N₄, and its possible functions on how to promote the catalytic efficiency and how the specific structure of C₃N₄ was achieved would be further analyzed next step. Though C₃N₄ phase loaded on the MBC has no drastically promotion for catalytic efficiency seemingly, its hydrothermal stability could afford a preponderance of maintaining surficial physical circumstance of the support, which might encourage the permanence of recycle use of the catalysts. Afterwards, the recyclability of the 15%Ni-20%W/MBC@M-0.25 catalyst was tested and the results are shown in Figure 10.

  Figure 10

  

  Figure 10.  Recycling experiments on the conversion of bamboo pulp into polyols over 15%Ni-20%W/MBC@M-0.25 catalyst: 5.0 MPa H₂, 240 ℃ for 1.5 h

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  Similar conversions were conducted for each run, irrespective of the using time of the catalyst. The trend yield of C_{2, 3} polyols could remain relative stable from the first time to the fourth time (from 71.9% to 58.4%). However, a slight deactivation of tested 15%Ni-20%W/MBC@M-0.25 catalyst could be defined as yield of C_{2, 3} polyols decreased. It was speculated that the metallic particles under this hydrothermal condition were probably gathered or active nickel was leached into acid water. The solution of the reactions conducted with 15%Ni-20%W/MBC (without N source)and 15%Ni-20%W/MBC@M-0.25 catalyst (with 25% melamine) were analyzed by ICP-OES. It is determined that the nickel concentrations of (0.15±0.02) to(0.23±0.03) mg/mL are in recycle use reaction liquids (in Figure 11).

  Figure 11

  

  Figure 11.  Concentrations of Ni cations in reaction liquids catalyzed by 15%Ni-20%W/MBC (with no N source, used for one time) and 15%Ni-20%W/MBC catalysts @M-0.25 (recycle use for five times)

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  In contrast, (0.26 ± 0.02) mg/mL of nickel is detected in the liquid conducted by 15%Ni-20%W/MBC used for the first time. Given that the lower Ni leaching was detected with 15%Ni-20%W/MBC@M-0.25 catalyst, it suggested that adding organic nitrogen source was helpful to immobilize Ni particles and suppress the leaching of Ni species, compared to the catalyst without N source. It was also speculated that the active center (Ni sites) had been anchored on the surface of support surrounded by graphitic nitrogen to form C₃N₄ layer. This nitride carbon layer could promote resistance ability for acid-etching of Ni and W sites through anchoring the metallic sites as shown in Figure 12.

  Figure 12

  

  Figure 12.  Ni-W alloys and the C₃N₄ layer were identified as the main contributors to maintain the catalyst stability in the proposal reaction pathway

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  3.   Conclusions

  In summary, a series of MBC supported nickel-tungsten catalysts were successfully prepared by addition of specific nitrogen sources and applied in bamboo pulp conversion to C_{2, 3} polyols. During calcination of the catalysts, some Ni-W alloys were formed besides metallic Ni⁰ and tungsten species characterized by XRD, XPS and TEM. Besides, the organic N sources were decomposed, and the C₃N₄ phase with high hydrothermal property was formed simultaneously. For catalytic efficiency, Ni-W/MBC@M-0.25 catalyst acquired the highest EG yield (55.8%). Catalyst recycle using investigation implied that Ni-W/MBC@M-0.25 could remain relative stable under this reaction condition. The Ni-W alloys and the C₃N₄ phase was identified as the main contributors to maintain the catalyst stability. Thus, knowledge acquired in this study may inspire further research on analysis of catalyst species when hybridization with heteroatoms and studies of their possible functions on catalytic performance in the field of biomass conversion in liquid.

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• 

  Figure 1  Representative bio-based chemicals derived from lignin, hemicellulose and cellulose

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  Figure 2  N₂ adsorption-desorption isotherms and the pore size distribution of MBC with various nitrogen sources and quantities

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  Figure 3  XRD patterns of different catalysts and MBC

  (a): amount of melamine as the variable;
  (b): the catalysts prepared by addition of multifarious nitrogen sources;
  (c): MBC was pretreated by ethanol, alkali and water, respectively

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  Figure 4  Magnetic experiment of unreduced and reduced Ni-W/MBC@M-0.25 catalysts

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  Figure 5  TEM images of (a) Ni-W/MBC@M-0.25, (b) Ni-W/MBC@ARG-0.25, (c) Ni-W/MBC@CAR-0.25 catalysts and (d) the structure of metallic particles surrounded by C₃N₄ layer in high resolution

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  Figure 6  X-ray photoelectron spectra of (a) W 4f region and (b) Ni 2p of various 15%Ni-20%W/MBC@M-0.25 catalysts

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  Figure 7  X-ray photoelectron spectra of (a) C 1s and (b) N 1s on different nickel-tungsten catalysts

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  Figure 8  Effect of melamine addition on distribution of C_{2, 3} polyols: M-x represented the mass ratio of melamine based on the weight of MBC

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  Figure 9  Performance of nickel-tungsten catalysts influenced by adopting various nitrogen sources

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  Figure 10  Recycling experiments on the conversion of bamboo pulp into polyols over 15%Ni-20%W/MBC@M-0.25 catalyst: 5.0 MPa H₂, 240 ℃ for 1.5 h

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  Figure 11  Concentrations of Ni cations in reaction liquids catalyzed by 15%Ni-20%W/MBC (with no N source, used for one time) and 15%Ni-20%W/MBC catalysts @M-0.25 (recycle use for five times)

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  Figure 12  Ni-W alloys and the C₃N₄ layer were identified as the main contributors to maintain the catalyst stability in the proposal reaction pathway

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  Table 1.  Physical properties of different supported 15%Ni-20%W catalysts measured by BET

  Sample	ABET/(m2·g-1)	vtotal/(cm3·g-1)	Average pore size d/nm
  MBC@M-0.15	280.7	0.12	7.1
  MBC@M-0.20	276.1	0.12	6.1
  MBC@M-0.25	271.6	0.12	6.6
  MBC@M-0.30	262.9	0.12	5.4
  MBC@EDA-0.25	164.2	0.07	5.2
  MBC@CAR-0.25	256.1	0.12	9.3
  MBC@ARG-0.25	264.1	0.11	4.1

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  Table 2.  Distribution of polyol products from bamboo pulp via nickel-tungsten catalysts ^a

  Entry	Catalyst	Yield of product/%					Conv. x/%
  		EG b	Gly	1, 2-PG	glucose	sor
  1	15%Ni-20%W/MBC@M-0.15	29.4	4.1	6.1	5.4	5.1	83
  2	15%Ni-20%W/MBC@M-0.20	46.2	3.5	7.6	3.5	4.3	100
  3	15%Ni-20%W/MBC@M-0.25	55.8	5.6	10.5	4.8	6.1	100
  4	15%Ni-20%W/MBC@M-0.25 b	25.4	3.3	2.1	1.3	trace	76
  5	15%Ni-20%W/MBC@M-0.30	51.2	4.7	8.1	3.5	5.3	100
  6	15%Ni-20%W/MBC	36.9	6.1	7.2	2.9	3.9	95
  7	15%Ni-20%W/AC	21.6	2.3	7.8	3.1	trace	84
  8	15%Ni-20%W/SWCNTs c	33.1	8.1	5.6	3.6	4.6	100
  9	15%Ni/MBC@M-0.25	10.3	trace	1.2	6.1	1.4	78
  10	20%W/MBC@M-0.25	6.5	trace	trace	1.2	UD.g	95
  11	15%Ni-20%W/MBC@EDA-0.25 d	44.6	4.5	5.3	5.7	4.1	100
  12	15%Ni-20%W/MBC@CAR-0.25 e	31.0	5.6	3.2	4.3	0.9	91
  13	15%Ni-20%W/MBC@ARG-0.25 f	54.1	6.1	3.0	4.1	3.2	100
  a : reaction conditions: bamboo pulp 0.5 g, 5.0 MPa H2, 240 ℃ for 1.5 h; b: feedstock: raw bamboo powder; c: SWNTs: single-walled carbon nanotubes; d: EDA: ethanediamine; e: CAR: carbamide; f: ARG: arginine; g: undetected

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•