Carbon Modification of Nickel Catalyst for Depolymerization of

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Carbon modification of nickel catalyst for depolymerization of oxidized lignin to aromatics Min Wang, Xiaochen Zhang, Hongji Li, Jianmin Lu, Meijiang Liu, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03475 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Carbon modification of nickel catalyst for depolymerization of oxidized lignin to aromatics Min Wang, Xiaochen Zhang, Hongji Li, Jianmin Lu, Meijiang Liu and Feng Wang* State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 (China)

Abstract: Catalytic valorization of lignin is a sustainable way to provide aromatics for the human society, which depends on the electronic structure of catalytic sites. We herein report the preparation of carbon-modified nickel catalyst via carbothermal reduction of Ni-doped layered double hydroxides. Lignosulfonate (LS), a lignin resource from pulp industry, was used as renewable carbon precursor. The carbon residues in the nickel surface layer changed the 3d electron distribution of nickel, which was highly selective for the C–O bond hydrogenolysis of lignin into aromatics, and 22 wt % total mass yields of aromatics was achieved from hydrogenolysis of oxidized birch lignin.

Keywords: C–O cleavage • heterogeneous catalysis • nickel • lignin • supported catalyst

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Introduction Lignin is an important aromatic biopolymer that accounts for nearly 30% of the non-fossil organic carbon on Earth.1 The depolymerization of lignin is a sustainable way to provide aromatics for the human society in the future and recently has aroused great interest, but still remains a challenge because of its complex connectivity through relatively stable C–O and C–C linkages.2 C-O bond accounts for two third of the all linkage and therefore the selective hydrogenolysis cleavage of C–O bond and avoid the over-hydrogenation of aromatic ring is pivotal for the lignin fragmentation to aromatics, which heavily lies in the electronic structure of catalytic metal sites.3 Nickel is a suitable catalyst for hydrogenolysis of lignin to aromatics because of its low cost and moderate activity, and recently, considerable attention has been paid to increase the activity of nickel for lignin C-O bond hydrogenolysis via modulation of the electron structure.4 Various supports were used to tune the reactivity via metal-support interaction, but usually resulting in the formation of some saturated products.5 Doping Ni with other atoms is another efficient method. Yan and et al. reported the preparation of bimetallic NiM (M=Ru, Rh, Pd, Au) catalyst for lignin model hydrogenolysis. The introduction of nobel metal significantly increased the C–O bond hydrogenolysis activity, but inevitable lead to the over-hydrogenation of the aromatic ring. It has been long recognized that carbon atoms can penetrate into nickel surface as an initial state for subsequent carbon growth in the pyrolysis process.6 Large carbon formation will deactivate the catalyst, such as the formation of coke, but minor carbon atoms on the metal surface or in the interstitial metal lattice sites have been shown as active phase for Pd7 and Co8 catalyst. We envisioned that utilization of the initial nickel-carbon state will be a promising strategy for the electron structure modulation.


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Scheme 1. The method for preparation of carbon–modified Ni/MgAlO-C. Herein, we report the modulation of the electronic structure of MgAl-oxide supported nickel catalyst via carbon modification, which is achieved from guest-host mediated layered double hydroxides (LDH) (scheme 1). LDH is a class of 2D nanostructured anionic clays consisting of positively charged layers with charge-balancing anions between them.9 In our strategy, the nickel cations were incorporated in the LDH. Lignosulfonate (LS), a lignin resource from pulp industry, was used as counter anions and carbon precursor. After carbonization under Ar, LDH-LS composite was transformed into MgAl-oxide sheet, and nickel cations were reduced by the carbon to form well dispersed nanoparticles. During the high temperature carbothermal reduction process, small amount of carbon atoms resides in the nickel surface, and greatly modified the nickel 3d electronic structure and was highly selective for the hydrogenolysis of oxidized lignin C–O bond to aromatics. Experimental section Catalyst preparation For the preparation of NiMgAl-LDH-LS, Mg(NO3)2•6H2O, Al(NO3)3•9H2O, Ni(NO3)2•6H2O and hexamethylenetetramine (HMT) were dissolved in 100 mL deionized water with [Mg2+] +


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[Al3+] =0.3 mol/L, n(Mg) : n(Al) = 2:1, HMT= 3.0 mol/L. The molar ratio of Ni to Al is 0.4. Then, 3 g of sodium ligninsulfonate was dissolved in the above solution. Sodium ligninsulfonate (AR grade) was purchased from Tokyo Chemical Industry (TCI). The as-obtained solution was kept in an autoclave reactor with an internal Teflon insert at 140 °C for 24 h. After cooling to the room temperature, the brown precipitate were filtered, washed with water three times and dried under 80 °C for 12 h. Ni/MgAl-LDH was prepared through the same process without the addition of sodium ligninsulfonate. MgAl-LDH-LS was prepared the same with that of NiMgAlLDH-LS without the addition of Ni(NO3)2•6H2O. Ni/MgAlO-C was prepared by the carbonization of Ni/MgAl-LDH-LS at 800 °C (5 °C/min) under Ar (10 ml/min) for 2 h. Nickel was reduced to metallic nanoparticles by the carbon. Ni/MgAlO was prepared by heating NiMgAl-LDH at 800 °C (5 °C/min) for 2 h under N2 (15 ml/min) and then reduced by H2 (5 ml/min H2 and 10 ml/min N2) at 800 °C for 0.5 h. NiMgAlO was prepared by calcination of NiMgAl-LDH-LS at 800 °C (5 °C/min) under air (10 ml/min) for 2 h. Nickel supported on activated carbon catalysts (Ni/C, 10 wt%) were prepared by an incipient-wetness impregnation method and further treated by a carbothermal reduction at 800 °C (5 °C/min) under Ar (10 ml/min) for 2 h. 10 wt% Ni/TiO2, Ni/MgO and Ni/Al2O3 was prepared by an incipient-wetness impregnation method and further reduced at 800 °C (5 °C/min) by H2 (5 ml/min H2 and 10 ml/min N2) for 2 h. Ni(NO3)2•6H2O was used as nickel source. Extraction of birch lignin To birch sawdust (40g) was added 1,4-dioxane (200mL) followed by 2N HCl (16 mL), then the mixture was heated at 120 oC under N2 atmosphere for 2.5 h. The mixture was cooled and


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filtrated. The collected liquid was partially concentrated in vacuo and precipitated by water (200 mL). The lignin was collected by filtration and dried in vacuo to give a birch lignin (3.0 g). Oxidation of the birch lignin To a solution of birch lignin in 2-methoxyethanol/1,2-dimethoxyethane (2:3, 17 mL) was added DDQ (120 mg, 10 wt%) and tBuONO (105 µL). The mixture was stirred at 80 °C under O2 atmosphere (balloon) for 14 hours. The solution was precipitated by Et2O (160 mL). The oxidized lignin was collected by filtration and dried in vacuo. Depolymerization of lignin models Typically, 0.2 mmol of lignin model, 2 mL of methanol as solvent and 20 mg catalyst were added into the stainless steel autoclave with an internal Teflon insert. 1 MPa H2 was charged and then heated to 200 °C under magnetic stirring. For the depolymerization of real lignin, 50 mg of oxidized birch lignin was used as reactant. After the reaction was completed, the reactants and products were analyzed and quantified by gas chromatography−mass spectrometry (GC−MS) using an Agilent 7890A/5975C instrument equipped with an HP-5 MS column (30 m in length, 0.25 mm in diameter). The conversion and yields was determined using internal standard method. The selectivity was determined using area normalization method. Characterization Fourier transform infrared (FT-IR) spectra were collected on a Bruker Tensor 27 FT -IR spectrometer in KBr media. Samples were thoroughly dried before measured. The temperatureprogrammed oxidation (TPO) was carried out in a tubular furnace connected with thermal star mass spectroscopy. Samples were preheated at 100 °C for 30 min to remove the adsorbed water,


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and then heated at a heating rate of 10 °C/min from 100 to 800 °C in an air flow of 20 mL/min. The gas effluent was monitored by mass spectroscopy. The X-ray powder diffraction (XRD) patterns were obtained using a Rigaku D/Max 2500/PC powder diffractometer with Cu Kα radiation (λ = 0.15418 nm). High resolution transmission electron microscopy (HRTEM) was performed using JEOL JEM-2100 electron microscope operated at 220 kV. Scanning transmission electron microscopy (STEM) was performed using a FEI Titan 80-200 (“ChemiSTEM”) electron microscope operated at 200 kV, equipped with a spherical-aberration (Cs) probe corrector (CEOS GmbH), and a high-angle annular dark field (HAADF) detector. A probe semi-angle of 25 mrad and an inner collection semi-angle of the detector of 88 mrad were used. Compositional maps were obtained with energy-dispersive X-ray spectroscopy (EDX) using four large-solid-angle symmetrical Si drift detectors. For EDX analysis, Ni K, Mg K, and O K peaks were used. Electron paramagnetic resonance (EPR) tests were performed on a Bruker spectrometer in the X-band at room temperature with a field modulation of 100 kHz. The microwave frequency was maintained at 9.401 GHz. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250Xi spectrometer equipped with a monochromated AlKα X-ray source (hυ = 1486.6 eV, 15 kV, 10.8 mA). The samples were dried in vacuum at 120 °C for 12 h. The charge neutralizer system was used for all of the analyses. The base pressure was 1 × 10–8 Pa. High resolution spectra were recorded with 20 eV pass energy. The pass energies correspond to the Ag3d5/2 FWHM of 0.65 eV. The data were acquired with 0.05 eV steps. The binding energy (BE) was calibrated to the C1s signal (284.6 eV) as a reference. The curve fitting procedure was performed using an approximation based on a combination of the Gaussian and Lorentzian functions with subtraction of a Shirley-type background. The accurate charge-to-mass ratio was measured using FT-ICR mass spectroscopy


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(Bruker). X-ray absorption fine structure (XAFS) spectra at Mo K-edge (E0 = 8.333 keV) were performed at BL14W1 beam line of Shanghai Synchrotron Radiation Facility (SSRF) operated at 3.5 GeV under “top-up” mode with a constant current of 240 mA. The XAFS datas were recorded under transmission mode with ion chambers. The energy was calibrated according to the absorption edge of pure Ni foil. Athena and Artemis codes were used to extract the data and fit the profiles. For the X-ray absorption near edge structure (XANES) part, the experimental absorption coefficients as function of energies µ(E) were processed by background subtraction and normalization procedures, and reported as “normalized absorption”. For the extended X-ray absorption fine structure (EXAFS) part, the Fourier transformed (FT) data in R space were analyzed for the Ni-Ni shell. DFT calculation We have used the VASP10 code to perform all the density functional theory (DFT) calculations. The exchange correlation functional was described by the generalized gradient approximation within the Perdew-Burke-Ernzerhof formulation (GGA-PBE).11 The interactions between ionic cores and valence electrons were described by the projected augmented wave (PAW) method12, while 10 valence electrons of Ni and 4 of C were explicitly taken into account using a plane wave basis set with an energy cutoff of 400 eV. A Hubbard term of U = 6.0 eV for Ni atoms (DFT+U) was used to mitigate the self-interaction error (SIE) as has been used elsewhere.13 The optimized lattice constant of bulk fcc Ni, 3.393 Å, is in accord with the experimental value of 3.52 Å14. The optimized lattice constants of bulk cubic NiC, a = 4.047 Å are also quite close to the reported theoretic value, a=4.073 Å.15 The Ni(111) surface was modeled using a hexagonal p(2×2) supercell with 4 atomic layers separated by vacuum with a depth of 15 Å. The bottom two layers were fixed to their bulk positions while the top two layers were allowed to fully relax.


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A Γ-point centered 9×9×1 k-point mesh was used for Brillouin zone sampling. The NiC(001) surface was modeled using a p(2×2) supercell with 4 atomic layers separated by vacuum with a depth of 15 Å. The bottom two layers were fixed to their bulk positions while the top two layers were allowed to fully relax. A 4×4×1 k-point mesh was used for Brillouin zone sampling. The convergence criterion was 10-5 eV for the self-consistent electronic minimization and 10-4 eV for ionic relaxation Results and discussion The carbon modified catalyst Ni/MgAlO-C was prepared from calcination of NiMgAl-LDH-LS under Ar at 800 °C. For comparison, Ni/MgAlO was prepared by the H2 reduction of NiMgAlLDH without LS. As shown by Fourier transform infrared spectroscopy (FTIR) characterization, characteristic benzene ring peaks at 1508 cm-1 and 1458 cm-1 were appeared (Figure S1), indicating the LS were successfully introduced to LDH.16 X-ray diffraction (XRD) results further confirmed this conclusion (Figure S2). The introduction of LS into the interlayer of LDH expands the interlayer spacing and reduces the crystallinity as evidenced by the shift to the lower angle and broadness of the peak. After carbothermal reduction at 800 °C, metallic Ni formed with appearance of Ni(111), Ni(200) and Ni(220) peak. The introduction of LS changed the morphology from plat plate to ultrathin curly sheet (Figure S3a, b). After carbonization, the ultrathin sheet morphology maintained (Figure S3c). Ni nanoparticles in Ni/MgAlO are easily observed in the scanning electron microscope (SEM) image (Figure 1a), but no Ni particles are observed in the SEM image of the Ni/MgAlO-C (Figure S3b). High angle annular dark-field (HAADF) (Figure 1b) elemental mapping (Figure 1c, d) and transmission electron microscope (TEM) (Figure 1e) showed that the Ni nanoparticles


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are buried in the support. The mean Ni particle size is about 21 and 25 nm for Ni/MgAlO and Ni/ MgAlO-C, respectively (Figure S4). Graphitic carbon with spacing of 0.35 nm is observed on the oxide support (Figure 1f). The graphitic carbon was further characterized by Raman spectroscopy. No band is observed in the spectrum of Ni/MgAlO (Figure 2a). The Raman spectrum of Ni/MgAlO-C shows characteristic D and G bands of carbon materials at 1351 cm-1 and 1585 cm-1, respectively.17

Figure 1. SEM image (a) Ni/MgAlO, HAADF image (b), element mapping (c, d) and TEM images (e, f) of Ni/ MgAlO-C. It has been recognized that carbon atoms contacting with nickel surface is easy to be oxidized by oxygen.6g-i Thus it is possible to study the carbon species sample by temperature-programmed oxidation (TPO, Figure 2b). Ni/MgAlO-C gives a broad and unsymmetrical peak in the range of 200–600 °C. It can be well deconvolulated into three peaks with starting temperature at 200 °C, 300 °C and 400 °C, respectively. The peaks starting at 300 °C and 400 °C are ascribed to the oxidation of defected and graphitic carbon on the support, respectively. The CO2 peak starting at


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200 °C is probably ascribed to the carbon residues in the nickel surface, and is easy to be oxidized under low temperature.6g The carbon content in Ni/MgAlO-C, calculated by measuring the amount of CO2, is only 0.9 wt%.

Figure 2. Raman spectra (a) of Ni/MgAlO and Ni/MgAlO-C and TPO curve (b) of Ni/MgAlOC. The surface nickel-carbon structure is difficult to be detected by XRD. We then characterized the nickel samples by extended X-ray absorption fine structure (EXAFS). The Ni K-edge XANES and k3-weighted Ni K-edge EXAFS spectrum of Ni/MgAlO resembles that of the Ni foil, which means that most of Ni in Ni/MgAlO retains metallic state (Figure 3a, b). For Ni/MgAlO-C, the carbon residues in the nickel surface make the Ni a slightly different from the metallic Ni. Compared to the Ni foil and Ni/MgAlO, the emergence of weak peaks at 8332 and 8340 eV and the weakening the peak at 8358 eV is supposed to the formation of nickel carbidelike species.18


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Figure 3. Ni K-edge XANES spectra (a), k3-weighted Ni K-edge EXAFS spectra (b) and CO-IR spectra (c) of Ni/MgAlO and Ni/MgAlO-C. The carbon residues in the nickel surface showed great influence on the electronic state of the nickel, which was further characterized by Ni LMM Auger spectra, CO adsorption infrared radiation (CO-IR) and electron paramagnetic resonance (EPR). The Ni in Ni/MgAlO-C is more positively charged than that in Ni/MgAlO as evidenced by the 3 eV shift to lower kinetic energy in the Ni LMM Auger spectra (Figure S5). CO-IR was further carried out to characterize the structural and electronic properties of Ni (Figure 3c). Besides of the CO gas vibration peaks at 2174 and 2116 cm-1, an adsorption peak at 1890 cm-1 appeared for Ni/MgAlO, which indicated that CO adsorption on hollow sites in metallic Ni are dominant in Ni/MgAlO.19 The introduction of carbon atoms in the surface greatly altered the CO adsorption behavior. Ni/MgAlO-C shows only a strong peak at 2033 cm-1 which is assigned to the CO linear adsorption on the atop site of


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Ni.19-20 The absence of hollow site adsorption is probably due to the isolation of nickel atoms by carbon atoms. Density functional theory (DFT) calculation shows that the vibration wavenumber of CO adsorption on the NiC is 2045 cm-1 which is very close to that on Ni/MgAlO-C. This result further indicates the presence of Ni-C structure.

Figure 4. DOS of Ni and Ni-C. The blue and yellow indicate the decrease and increase of the electron density. Blue ball: Ni; Brown ball: C. We then calculated the electronic state of surface Ni-C by DFT (Figure 4). The interaction of the carbon with Ni significantly changed the 3d electron density of state (DOS). From the differential charge density, the 3d electrons in Ni are partially transferred to the carbon and make Ni positive charged in accordance with the Ni LMM Auger spectra results. Barden charge analysis shows that 0.67 electrons are transferred from Ni to carbon. Compared with Ni metal, the valence band was narrowed and shifted to Fermi level. The increased DOS near the Fermi level will enhance the formation of chemical bonds between the catalyst and reactant because of


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the increased availability of symmetry-allowed and energetically favored electronic states from the valence band.21 274 kJ mol-1

OH O

OH

1 β -O-4

C=O hydrogenation

O 1

228 kJ mol-1

O O 2

Eq. (1)

Catalyst H2, methanol

O OH

β -O-4 ketone

+ C-O cleavage

3

4

Figure 5. Catalytic results of the hydrogenation of 2-phenoxy-1-phenylethanone. Reaction conditions: 20 mg catalyst, 0.2 mmol 2-phenoxy-1-phenylethanone, 2 mL methanol, 1 MPa H2, 200 °C, 2 h. Recently, a two-step strategy has been explored in the transformation of the dominant β-O-4 lignin models (comprising around 50% of all linkages). The pre-oxidation of Cα-OH of β-O-4 alcohol to β-O-4 ketone lowers the Cβ–O bond dissociative energy by 40-50 kJ·mol–1, which is promising for the subsequent C–O bond cleavage.3a-f The key issue for the hydrogenolysis of


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oxidized lignin in H2 is to avoid the hydrogenation of β-O-4 ketone back to β-O-4 and the overhydrogenation of aromatic ring. We then investigate the hydrogenation of 2-phenoxy-1phenylethanone (2) (Figure 5). The reaction did not occur without catalyst. NiMgAlO mixed oxide was inactive. Ni/TiO2 showed low activity and moderate Cβ–O bond cleavage selectivity. Ni supported on MgO and Al2O3 showed high activity but poor Cβ–O bond cleavage selectivity with monomer [acetophenone (3) and phenol (4)] selectivity below 20%. In the case of Ni/MgAlO, 63% conversion was obtained and the monomer selectivity was only 34%. Ni/C showed high conversion, but moderate monomer selectivity (62%). Ni/MgAlO-C not only exhibited high activity but also showed >99% Cβ–O bond cleavage selectivity. If the ketone is hydrogenation back to alcoholic group will strength the C–O bond and make it hard break [eq. (1)]. With 2-phenoxy-1-phenylethanol (1) as substrate, only 22% conversion was obtained. When Ni/MgAlO-C was first oxidized by O2 and then reduction by H2 to remove the carbon, the conversion and monomer selectivity were both decreased to 40% and 60%, respectively. The monomer 3 and 4 are stable under the reaction conditions and no benzene ring hydrogenation occurs even extending the reaction time to 6 h (Figure S6). Even decreasing the reaction temperature to 160 °C, C=O hydrogenation is still could not avoided over Ni/MgAlO. Ni/MgAlO-C shows high C-O bond cleavage and monomer selectivity in the reaction temperature range of 160 oC-200 °C (Figure S7). The substrate scope was further investigated. The Ni/MgAlO-C catalyst is effective and selective for a wide range of β-O-4 ketones. The β-O-4 ketones with methoxyl substituents were converted into ketones and phenols in 80-99% yields (Table 1). The relative lower yield of methoxylpenols than that of phenol is because some of the electron rich methoxyphenols were


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polymerized induced by the base support.22 Even the synthetic β-O-4 ketone polymers were depolymerized into p-hydroxyacetophenone in 90% yields.

Table 1. Substrate scope over Ni/MgAlO-C.[a]

[a]

Reaction conditions: 20 mg of Ni/MgAlO-C, 0.2 mmol of substrate, 2 mL of methanol, 1 MPa

of H2, 200 °C, 6 h. The results were determined by GC with p-xylene as internal standard.


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According to β-O-4 ketone model adsorption on the Ni surface,23 the carbonyl group adsorbs on the nickel atoms, while the C-O bond is suspended above the surface. Thus, carbonyl group tends more easily to be activated and hydrogenated. It is reported that the carbonyl group is activated on the electron rich nickel atoms. The electron deficient nickel will lose its activity for ketone hydrogenation.24 After carbon modification, the nickel is positively charged and therefore weakens the activation of carbonyl group. According to the literature, the resulting electrondeficient Ni might more effectively activate the C-O bond via coordinate the oxygen of ether linkage.25

Figure 6. Partial 2D HSQC NMR spectra of birch lignin (a) before and (b) after oxidation. Finally, we focused on the real lignin as substrate. Real lignin is more complex and is a challenge to convert lignin to aromatics. Birch lignin with 43% content of β-O-4 linkage was first oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/tBuONO according to previous work.3b The disappearance of β-O-4 peak (A) and the emergence of desired α-ketone β-


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O-4 structure (A’) could be readily identified in the 2D HSQC NMR spectrum (Figure 6). The oxidized birch lignin was further subjected to hydrogenolysis over Ni/MgAlO-C and Ni/MgAlO catalyst. The oxidized birch lignin was partially soluble in methanol. With Ni/MgAlO as catalyst, only 5 wt % yields of aromatics were obtained. Comparatively, enhanced performance was observed in the presence of Ni/MgAlO-C catalyst, and a mixture of aromatics was obtained in 22 wt % yields (Figure 7).

Figure 7. GC spectrum of hydrogenolysis of oxidized birch lignin over Ni/MgAlO-C. Reaction conditions: 20 mg of Ni/MgAlO-C, 50 mg of oxidized birch lignin, 2 mL of methanol, 1 MPa of H2, 200 °C, 6 h. Conclusions In summary, we provided a new strategy for tuning the electronic property of the Ni sites via carbon modification. Lignosulfonate (LS), a lignin resource from pulp industry, was used as renewable carbon precursor. The carbon residues in the nickel surface layer changed the 3d


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electron distribution of nickel, which was highly selective for the C–O bond hydrogenolysis of lignin into aromatics. This work points out a new avenue for the preparation of highly selective catalyst for lignin depolymerization. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * F.W. Email: [email protected]; Supporting Information. Synthesis of lignin models; some reaction date; SEM, FTIR, Auger spectra and XRD characterization. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgement This work was supported by the National Natural Science Foundation of China (21603219, 21403216), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300) and DICP (DICP ZZBS201613). References (1) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. Chem. Rev. 2010, 110, 3552-3559. (2) (a) Li, C.; Zhao, X.; Wang, A.; Huber, G. W.; Zhang, T. Chem. Rev. 2015, 115, 1155911624; (b) Crestini, C.; Crucianelli, M.; Orlandi, M.; Saladino, R. Catal. Today 2010, 156, 8-22; (c) Zaheer, M.; Kempe, R. ACS Catal. 2015, 5, 1675-1684; (d) Ren, Y. L.; Yan, M. J.; Wang, J. J.; Zhang, Z. C.; Yao, K. S. Angew. Chem. Int. Ed. 2013, 52, 12674-12678; (e) Galkin, M. V.;


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Table of Contents Graphic and Synopsis


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Carbon Modification of Nickel Catalyst for Depolymerization of

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