原位合成法製備含金屬奈米粒子之有機金屬骨架衍生擔體及其應用於多種催化反應

Abstract

擔體型觸媒因其穩定及分離的方便性已被廣泛的運用在工業生產上；然而在生產擔體型觸媒的製程上容易產生：金屬觸媒的團聚、失活以及分布不均等問題。對此，我們提出一套新穎的方法”原位合成（de novo approach）”用以改善製備擔體型觸媒的程序並提升觸媒的效能。金屬有機骨架（MOFs）為一由金屬節點與有機配體所組成的孔洞材料，這類材料可做為自體模板，進一步轉化為多孔碳擔體以及多孔金屬氧化物擔體。第一章將介紹MOF以及擔體型觸媒的發展沿革，接著在二到五章中，我們將探討五種由鑲埋金屬的MOF所衍生而來的擔體型觸媒。我們成功的以原位合成法將不同金屬置入MOF內，並進一步轉化成鑲埋金屬粒子的多孔碳擔體以及多孔金屬氧化物。在不同的催化反應中，本研究所製備之觸媒都有著比後合成法製備之觸媒更優秀的催化能力，原因在於MOF提供一個均勻分布單體前驅物的環境，同時原位合成法可加強金屬與擔體間的作用，使催化效果往上提升。 首先，富含氮之8號類沸石咪唑酯結構（ZIF-8）作為碳擔體之模板，金與鈀的前驅物分別被導入建構中之ZIF-8內並進一步將之裂解成富含氮之金粒子鑲埋多孔碳球（Au@NC）以及均勻錨定鋅鈀合金之多孔碳球（PdZn@NC）；本觸媒因其富含氮原子使得擔體在水中有良好的分散性，在硝基苯酚的還原中有著極佳的表現（specific conversion rate, SC: 1185 g-1s-1）；此外，鋅與鈀的偕同效應使其在硝基苯酚的還原中也有著極佳的表現（SC: 1344 g-1s-1）。另一方面，我們同時將金與鈀導入ZIF-8，並裂解形成鑲埋金鈀合金之多孔碳球（AuPd@NC），此觸媒體現可調控組成比例之合金以及其在苯甲醇氧化反應中偕同效應所提升的反應效果。此研究結果收錄於第二章及第三章。 接著，銅前驅物被導入富含鈦之MOF (MIL-125)內，形成含銅之MIL-125；將銅導入MIL-125後，此複合物即藉由鍛燒的方式轉化成氧化銅嫁接之中孔洞錠型氧化鈦（CuO@MT）。光學分析顯示在二氧化鈦及氧化銅間有著強烈的介面電荷轉移現象，程序控溫分析以及X光吸收光譜則顯示部分銅原子鑲嵌在二氧化鈦的結構內部。而本觸媒憑藉著均勻分散的氧化銅及其與二氧化鈦的交互作用，而在光催化產氫上有著顯著的產氫效果（4760 mol h-1）。此研究結果收錄於第四章。 最後，金與鈀的前驅物同時導入富含鈷的67號類沸石咪唑酯結構（ZIF-67），並進一步鍛燒成金鈀合金鑲嵌之氧化鈷籠（AuPd@Co3O4）。X射線光子能譜的分析結果顯示本觸媒在金與氧化鈷的介面上帶有活性極高的一價金離子，而本觸媒與雙氧水結合轉化5-甲基糠醛（HMF）至2,5-呋喃二甲酸（FDCA），於常壓下一小時即完全反應並產生95%的FDCA，我們將此出類拔萃之催化效果歸因於金鈀合金的偕同效應、金及氧化鈷間的一價金離子以及雙氧水所產生的超氧化氫自由基所造成的結果。此研究結果收錄於第五章。 整體來說，我們利用原位合成法將各種不同的金屬前驅物導入MOF內，並將MOF進行鍛燒或裂解形成鑲埋金屬粒子的多孔碳擔體或多孔金屬氧化物擔體。原位合成法搭配MOF可有效提升擔體及金屬粒子間的作用，使擔體型觸媒的效果大幅提升。在未來，我們希望能將此法推廣到更多的組合上，藉由MOF本身的特性搭配金屬的效用，使材料的應用性更加廣泛。

Supported catalyst has been involved in most of industrial production owing to the benefits of long-term stability and easy recycle. The aggregation, deactivation and random distribution of metal nanoparticles during procedures are three issues in the preparation of supported metal catalysts. In this dissertation, a novel synthetic methodology called de novo approach is proposed to improve the quality of supported catalyst. Metal-organic frameworks (MOFs) are a porous structure composed of abundant metal joints and organic ligands which serve as self-template for porous carbon structure and metal oxide structure. We successfully use MOFs as the host and different metal precursors as metal sources to synthesize metal nanoparticles embedded MOF by de novo approach, and further converted the MOF composite into MOF-derived substrates with even metal nanoparticles distribution. The development of supported catalyst, MOFs and its derivatives are introduced in Chapter 1. Five kinds of metal nanoparticles embedded, MOF-derived supported catalyst would be discussed from Chapter 2 to Chapter 5. The synthesized supported catalysts showed apparently better performance on different catalytic reactions than those prepared by general post-treatment methods owing to the homogeneous substrate precursor on framework of MOF and strong interaction between metal composites and substrates contributed by de novo approach. Nitrogen-rich zeolitic imidazolate framework-8 (ZIF-8) is selected as the host material, and Au and Pd precursors are introduced into ZIF-8 structure using de novo method to prepare Au@ZIF-8 and Pd@ZIF-8, respectively. After pyrolysis, ZIF-8 framework would become nitrogen-contained carbon framework and the resulting nitrogen doped, Au nanoparticles embedded nanoporous carbon nanoparticles (namely Au@NC) showed excellent performance (specific conversion rate [SC]: 1185 g-1s-1) on reduction of 4-nitrophenol due to N-rich carbon framework originated from 2-methylimidazole. Similarly, the Pd@ZIF-8 can be carbonized into even Pd-Zn anchored nanoporous carbon nanoparticles (namely PdZn@NC), which also shows good performance (SC:1344 g-1s-1) on reduction of 4-nitrophenol owing to N-rich carbon substrate and the synergistic effect of Pd-Zn alloy. Finally, Au and Pd co-loaded ZIF-8 is carbonized into Au-Pd alloy embedded nanoporous carbon nanoparticles (namely AuPd@NC), demonstrating tunable composition of alloy and synergistic effect of alloy on the oxidation of benzyl alcohol. The detail is included in chapter 2 and chapter 3. Another MOF material (i.e. MIL-125) is selected as the host material because it contains titanium in the framework. Cu precursor is then introduced into MIL-125 by de novo method, and the synthesized copper ion encapsulated MIL-125 is calcined into CuO-loaded mesoporous titania tablets (namely CuO@MT). The results of H2-TPR and XAS show that part of CuO are inserted in TiO2 layer. The resulting CuO@MT catalysts demonstrate enhanced hydrogen evolution in methanol-containing solution (4760 mol h-1) owing to even distribution of CuO embedded in titania and effectively interfacial charge transfer between CuO and TiO2. The detail of experiment and discussion is included in chapter 3. A Co-containing MOF (ZIF-67) is selected as the host material. Both Au and Pd precursors are introduced into ZIF-67 during assembly, and the resulting composites are calcined into Au-Pd alloy nanoparticles embedded cobalt oxide cages (namely AuPd@Co3O4). XPS spectrum shows the appearance of highly active Au+ on the interface of catalyst and substrate. The AuPd@Co3O4 cages are then applied as an efficient solid catalyst for H2O2-assisted conversion of hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA). The results show a high yield of FDCA (95%) is achieved under atmosphere in one hour, which is attributed to the hydroperoxyl radicals, synergistic effect of Au-Pd alloy, strong interaction between Co3O4 and Au nanoparticles. The detail of experiment and discussion is included in chapter 5. In summary, metal nanoparticles encapsulated MOFs are synthesis with de novo approach, and the composites are further converted into supported catalyst. We demonstrate a reliable methodology (i.e. de novo approach) for synthesis of supported catalyst with enhanced activity on various catalytic reactions. The future works and perspective of this new method is described in Chapter 6.