南昌大学杨维冉教授团队GEE | 催化剂的神奇相变——hcp晶相Ni纳米颗粒促进HMF氢化合成BHMTHF

热重（TG）曲线和FT-IR光谱结果表明催化剂前驱体在400 °C可以实现完全碳化，进而实现将Ni-CA前驱体原位合成为Ni@C-X催化剂。XRD图谱及TEM结果表明 Ni@C-X（X=1, 2, 3）催化剂中同时存在非常规的hcp相和fcc相Ni金属纳米颗粒。

Fig. 1. (a) Schematic illustration of the catalysts preparation, (b) Thermogravimetric curve of precursors, (c) FT-IR spectra of the catalysts, (d) XRD patterns of the catalysts. (e) Refined crystal structure obtained by XRD refinement (red in pie graph represents hcp Ni, blue represents fcc Ni). 

Fig. 2. (a-d) TEM images and particle size distribution of Ni@C-X (X=0,1,2 and 3) catalysts; TEM images of (e) Ni@C-0 and (f) Ni@C-3; (g and i) Enlarged images of Ni@C-3; (h) inverse FFT of (g); (j) inverse FFT of (i); (k)HAADF-STEM and (l) the corresponding elemental mappings of Ni@C-3.

^a Reaction conditions: 0.5 mmol HMF, 20 mg catalyst, 2 mL methanol, 2 MPa initial H₂ Pressure, 70 ℃, 4 h; ^b 50 mmol (6.30 g) HMF, 200 mg Ni@C-3, 20 mL methanol, the H₂ pressure of reactor was maintained at 2 MPa by connecting the H₂ input device, 90 ℃, 24 h; ^c Ni@C-3 was exposed to air for 30 days (0.5 mmol HMF, 20 mg catalyst, 2 mL methanol, 2 MPa initial H₂ Pressure, 70 ℃, 4 h); ^d Isolated yield. N.D. represents not detected.

基于上述结果，以Ni@C-3为催化剂，考察了溶剂、反应温度、H₂压力、催化剂用量以及反应时间等因素对反应的影响。结果表明最佳反应条件为：甲醇为溶剂，反应温度为70 ℃，H₂压力为2 MPa，催化剂用量为20 mg，反应时间为4 h。此外，该催化剂在10次循环使用中依旧保持良好稳定性。

Fig. 3. (a) Effect of reaction solvent (2 MPa initial H₂ pressure, 4 h, 0.5 mmol HMF, 70 ℃, 20 mg Ni@C-3); (b) Effect of reaction temperature (2 MPa initial H₂ pressure, 4 h, 0.5 mmol HMF, MeOH, 20 mg Ni@C-3); (c) Effect of H₂ pressure (4 h, 0.5 mmol HMF, MeOH, 70 ℃, 20 mg Ni@C-3); (d) Effect of catalyst dosage (2 MPa initial H₂ pressure,4 h, 0.5 mmol HMF, MeOH, 70 ℃); (e) Effect of reaction time (2 MPa initial H₂ pressure, 0.5 mmol HMF, MeOH, 70 ℃, 20 mg Ni@C-3); (f) The reusability of the Ni@C-3 catalysts (2 MPa initial H₂ pressure, 1 h, 0.5 mmol HMF, 70 ℃, 20 mg Ni@C-3).

DFT结果表明，fcc-Ni （111）晶面有利于HMF醛基的加氢，而hcp-Ni （101）晶面有利于H₂的活化以及呋喃环的氢化。由此推测，在HMF加氢合成BHMTHF中，Ni@C-3优异的催化性能归因于催化剂中fcc-Ni和 hcp-Ni的协同催化作用。

Fig. 4. Energy barrier diagram and corresponding optimized structures of (a) H₂ dissociation, (b) C=O hydrogenation in HMF and (c) furan ring hydrogenation in BHMF on the surface of fcc-Ni (111) facets (black line) and hcp-Ni (101) facets (red line), respectively. (d) Scheme of the proposed mechanism for the selective total hydrogenation of HMF to BHMTHF over Ni@C-3. Insert: “TS” denotes a transition state and “*” stands for the adsorbed state of reactants, Ni blue, C gray, O red, H white.

如表2所示，进一步考察了Ni@C-3催化剂的普适性。结果表明Ni@C-3催化剂对其他生物基平台化合物均表现出优异的催化活性。其中，高浓度（2500 mmol L^-1）的HMF可获得89.7%产率的BHMTHF。以糠醛为原料时，在无溶剂条件下成功实现了糠醛加氢生成四氢糠醇，此外，Ni@C-3还对糠酸、环己酮、硝基苯及乙酰丙酸等也表现出了优异的催化活性。

^a Reaction conditions: 0.5 mmol substrate, 20 mg Ni@C-3, 2 mL methanol, 2 MPa initial H₂ Pressure, 70 ℃, 4 h; ^b 50 mmol (6.30 g) HMF, 300 mg Ni@C-3, 20 mL methanol, the H₂ pressure was maintained at 2 MPa by connecting the H₂ input device, 90 ℃, 24 h; ^c 15 mL (16.8 g) furfural, 500 mg Ni@C-3, solvent-free, the H₂ pressure was maintained at 3 MPa by connecting the H₂ input device, 90 ℃, 48 h; ^d 0.5 mmol substrate, 20 mg Ni@C-3, 2 mL IPA, 2 MPa initial H₂ Pressure, 100 ℃; ^e 0.5 mmol substrate, 20 mg Ni@C-3, 2 mL THF, 2 MPa initial H₂ Pressure, 100 ℃; ^f Isolated yield.

本文开发了一种具有hcp相镍纳米颗粒的碳包覆镍基催化剂（Ni@C-3）。对于HMF氢化合成BHMTHF的决速步骤（呋喃环加氢），hcp Ni(101) 晶面较fcc Ni (111) 晶面具有更低的反应能垒。此外，Ni@C-3还表现出优异的稳定性和广泛的底物适用性。该研究不仅为亚稳相Ni基催化剂的开发提供了新方法，还为发展镍基催化剂在生物质加氢的规模化应用提供了新策略。

相关研究以“Hexagonal close-packed Ni (101) facets boost furan ring activation for selective total hydrogenation of biomass-derived 5-hydroxymethylfurfural over carbon-encapsulated Ni catalysts”为题在Green Energy & Environment期刊发表，该论文第一作者为南昌大学博士研究生库从灏，通讯作者为南昌大学化学化工学院杨维冉教授。

撰稿：原文作者

编辑：GEE编辑部