Preparation of Cu/ZnO/Al2O3 catalyst by intercalated hydrotalcite precursor and its catalytic performance in methanol synthesis
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摘要: 分别采用共沉淀法、离子交换法和焙烧复原法制备
$ {\rm{Cu(}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{)}}_2^{2 - } $ 络合阴离子插层ZnAl-LDH水滑石纯物相前驱体，经焙烧得到片层结构的Cu-ZnAl-LDO催化剂。通过XRD、TEM、ICP、H2-TPR、N2O-H2氧化还原滴定及N2物理吸附-脱附等方式对前驱体和催化剂进行表征分析，考察了不同制备方法对插层结构催化剂结构性质及其合成甲醇催化活性及稳定性的影响。结果表明，相比于离子交换法和焙烧复原法，共沉淀法制备的CP-Cu-ZnAl-LDH前驱体具有较完整的水滑石二维层状结构，且插层Cu组分含量最大，焙烧后层间高分散的Cu物种颗粒尺寸较小，与ZnO发生相互作用，表现出很好的催化活性，单位质量Cu的甲醇收率可达3412 mg/(gCu·h)；同时，层板对Cu物种的限域作用有效抑制了Cu颗粒的团聚长大，连续反应60 h过程中催化剂未发现失活。Abstract: The$ {\rm{Cu(}}{{\rm{C}}_{\rm{2}}}{{\rm{O}}_{\rm{4}}}{\rm{)}}_2^{2 - } $ complexed anion intercalated ZnAl-LDH hydrotalcite pure phase precursors were prepared by co-precipitation, ion exchange and calcination-reconstruction methods; after that, the Cu-ZnAl-LDO catalysts with lamellar structure were obtained by calcination of the precursors. The precursors and catalysts were characterized by XRD, TEM, ICP, H2-TPR, N2O-H2 TPD titrations and N2 physisorption; the effect of preparation method on the confinement degree of intercalated active Cu species and the catalytic activity and stability of Cu-ZnAl-LDO in methanol synthesis was investigated. The results indicate that compared with the ion exchange and calcination-reconstruction methods, the CP-Cu-ZnAl-LDO catalyst prepared by co-precipitation method has a higher interlayer Cu content as well as a stronger confinement effect of layer plate on the Cu species. In addition, the unique ordered layered structure of hydrotalcite can also improve the dispersion of active Cu species between the layers and inhibit the sintering of Cu particles. As a result, the CP-Cu-ZnAl-LDO catalyst prepared by co-precipitation method exhibits excellent performance in methanol synthesis; the space-time yield of methanol reaches 3412 mg/(gCu·h) and basically no deactivation is observed after continuous reaction for 60 h.-
Key words:
- hydrotalcite /
- copper-based catalyst /
- confinement effect /
- methanol synthesis
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图 1 不同方法制备插层Cu-ZnAl-LDH前驱体（a）和Cu-ZnAl-LDO催化剂（b）的XRD谱图
Figure 1 XRD patterns of the precursors (a) and catalysts (b) prepared by different methods
图 2 不同前驱体和催化剂的TEM照片及CP-Cu-ZnAl-LDO催化剂的STEM-Mapping照片
Figure 2 TEM images of different precursors and catalysts and STEM-Mapping diagrams of CP-Cu-ZnAl-LDO catalyst
(a): CP-Cu-ZnAl-LDH; (b): IE-Cu-ZnAl-LDH; (c): RE-Cu-ZnAl-LDH; (d): CP-Cu-ZnAl-LDO; (e): IE-Cu-ZnAl-LDO; (f): RE-Cu-ZnAl-LDO; (g)−(j): Mapping diagrams of Cu, Zn and Al elements in CP-Cu-ZnAl-LDO catalyst
图 3 不同方法制备插层Cu-ZnAl-LDO催化剂的H2-TPR谱图
Figure 3 H2-TPR profiles of the Cu-ZnAl-LDO catalysts prepared by different methods
图 4 不同催化剂的甲醇时空收率（a）和TOF值（b）
Figure 4 Space time yield of methanol (a) and TOF values (b) of different catalysts
图 5 不同方法制备插层Cu-ZnAl-LDO催化剂反应后的XRD谱图
Figure 5 XRD patterns of spent Cu-ZnAl-LDO catalysts prepared by different methods
图 6 催化剂Cu颗粒尺寸、Cu有效比表面积、Cu分散度与甲醇时空收率的关系
Figure 6 Relationship between the Cu particle size, specific surface area and dispersion of Cu and the space time yield of methanol
表 1 不同水滑石前驱体结构参数
Table 1 Structural parameters of different hydrotalcite precursors
Sample 2 θ of (003) diffraction peak /(°) Basal spacing d003 /nm Interlayer distance①/nm CP-Cu-ZnAl-LDH 9.02 0.98 0.50 IE-Cu-ZnAl-LDH 9.05 0.98 0.50 RE-Cu-ZnAl-LDH 9.06/11.56 0.98/0.77 0.50/0.29 ${\rm{ZnAl } }{\text{-}} {\rm{NO} }_3^{ - } {\text{-}} {\rm{LDH} }$ 9.98 0.89 0.41 $ {\rm{ZnAl {\text{-}} CO}}_3^{2 - } {\text{-}} {\rm{LDH}} $ 11.56 0.77 0.29 ①interlayer distance = d003−thickness of the brucite layer (0.48 nm)[16] 
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表 2 不同插层结构Cu-ZnAl-LDO催化剂的织构性质
Table 2 Textural properties of Cu-ZnAl-LDO catalysts with different intercalated structures
Catalyst Cu content①w/% SA/(m2·g−1) DCu④/% v⑤/(cm3·g−1) dCu⑥/nm catalyst② Cu③ CP-Cu-ZnAl-LDO 10.7 91.2 45.1 62.5 0.52 2.1 IE-Cu-ZnAl-LDO 6.0 59.7 33.5 41.9 0.20 8.2 RE-Cu-ZnAl-LDO 7.9 48.1 36.3 51.8 0.35 4.5 ①: the mass percentage of Cu in the catalyst was calculated by ICP; ②: the specific surface area of the catalyst was calculated by B.E.T method; ③④: the effective specific surface area and Cu dispersion were calculated by N2O-H2 oxidation-reduction titration; ⑤: the mesoporous volume was calculated by t-plot method; ⑥: based on the half peak width of Cu diffraction peak at 2θ = 43.4°, the Cu particle size after catalyst reaction was calculated by Scherrer formula
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表 3 不同结构Cu/ZnO/Al2O3催化剂的甲醇时空收率对比
Table 3 Comparison of various Cu/ZnO/Al2O3 catalysts with different structures in the methanol yield
Method Cu content w/% p/t(MPa·℃−1) STYMeOH/($ {\rm{m}}{{\rm{g}}_{{\rm{MeOH}}}} \cdot {\rm{g}}_{{\rm{cat}}}^{ - 1} \cdot {{\rm{h}}^{ - 1}} $) STYMeOH/($ {\rm{m}}{{\rm{g}}_{{\rm{MeOH}}}} \cdot {\rm{g}}_{{\rm{Cu}}}^{ - 1} \cdot {{\rm{h}}^{ - 1}} $) Reference CP 56.9 5/250 642 1128 [21] CP 25.6 5/250 280 1093 [22] CP 67.5 5/250 568 841 [23] CP 10.7 3/250 355 3398 this work IE 6.0 3/250 58 971 this work RE 7.9 3/250 88 1118 this work
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