開發觸媒用於催化轉移氫化與醇類脫氫反應

Abstract

本論文目標為開發觸媒用於催化轉移氫化反應(catalytic transfer hydrogenation, CTH)與醇類脫氫反應，並針對觸媒之物化與表面性質對催化表現的影響進行研究。首先，我們成功以釕金屬離子與雙邊的鄰二氮菲配體製備金屬超分子聚合物(Ru-MSP)，並透過調整金屬/配體的比例與合成時間分別來調控Ru-MSP的配位結構與聚合程度。活性測試結果指出，在Ru-MSP觸媒中，Ru配位未飽和點(coordinatively unsaturated site)是催化CTH反應非常重要的活性位點，並且，Ru-MSP觸媒活性遠大於商售的Ru/C、RuCl3、Ru(DMSO)4Cl2和Ru(phen)2Cl2。根據DFT理論計算，Ru-MSP的Ru電子密度與電子親和力會隨著聚合度增加而提升，因此，我們認為Ru-MSP活性優異的原因是：(1)高電子密度的Ru原子，能有助於甲酸根上C-H鍵解離，以利於甲酸根的脫氫；(2)較低的LUMO能量(較高的電子親和力)，有助於吸附與活化反應物上羰基的氧，以利於後續的氫化步驟。最適化的Ru-MSP觸媒有被證明有良好的穩定性、再利用性與泛用性。本研究結果證實了Ru-MSP觸媒是相當有潛力的異相觸媒，能用於含羰基化合物的CTH反應生成對應的醇類化合物。 第二部分，我們通過鹼性水熱法以平板狀的Bi4Ti3O12 (BIT)作為前趨物進行拓撲化學轉化，合成了具有不同鈦酸鍶(SrTiO3, STO)含量的異質結構BIT-STO平板顆粒。利用異丙醇(isopropanol, IPA)的程序升溫表面反應對BIT-STO的表面性質進行討論，發現所有BIT-STO觸媒在IPA脫氫(產生丙酮)中的選擇性皆高於IPA脫水(產生丙烯)反應。與水熱合成的STO和商售STO相比，BIT衍生的STO觸媒在IPA脫氫反應中表現出更高的活性和選擇性，這歸因於在拓撲化學轉化過程中形成的具有大量表面缺陷的(100)晶面。原位紅外光譜結果顯示，在IPA吸附後，BIT衍生的STO觸媒有更高比例的橋接異丙醇(IPO)存在於表面上。我們認為，BIT衍生的STO表面存在的大量氧空缺可以促進IPO中間體的橋接吸附，有利於進一步分解成丙酮，進而促進IPA選擇性脫氫。 第三部分，探討擔載銅觸媒之擔體對於CTH反應的影響，我們使用商售的ZrO2、Al2O3、TiO2和SiO2作為比較的擔體，以含浸法擔載銅金屬於擔體表面，以IPA作為氫源、羥甲基糠醛(5-hydroxymethylfurfural, HMF)為反應物生成二甲基呋喃(2,5-dimethylfuran, DMF)，並透過一系列鑑定方法瞭解觸媒之物化性質、觸媒和反應物(HMF)之間的作用力、氫源(IPA)在觸媒表面的反應選擇性。結果指出，反應中間體2,5-呋喃二甲醇(2,5-bis(hydroxymethyl)furan, BHMF)的氫解為HMF生成DMF的瓶頸步驟，並且此步驟會同時與醚化以及脫水聚合反應進行競爭，在這四個Cu/MOx觸媒上的反應選擇性有顯著的不同，以Cu/ZrO2有最佳的目標氫解產物(DMF)選擇率。經由HMF-IR的分析，我們推測Cu/SiO2表面與HMF過強的作用力會抑制IPA的吸附，進而導致其難以催化HMF的氫化步驟。透過IPA-TPSR與CO-IR的結果，發現Cu/ZrO2與Cu/Al2O3相較於Cu/SiO2與Cu/TiO2有較低的表面金屬銅價態(電子較聚集)，有助於IPA的脫氫反應，進而有較高的DMF選擇率。

The goal of this thesis is to develop catalysts for transfer hydrogenation and dehydrogenation of alcohols, and to study the effects of catalyst physicochemical and surface properties on catalytic performance. In the first part of this thesis, Ru-based metallo-supramolecular polymer (Ru-MSP) were successfully prepared from ruthenium metal ions and ditopic phenanthroline ligands, and the coordination structure and polymerization degree of Ru-MSP were tailored by tuning the ratio of metal/ligand and the synthesis period, respectively. The catalytic results showed that the coordinatively unsaturated Ru sites are found to be the critical active sites in Ru-MSP. The Ru-MSP catalysts are much more active than commercial Ru/C (heterogeneous catalyst), RuCl3, Ru(DMSO)4Cl2, and Ru(phen)2Cl2 (homogeneous catalysts). By DFT calculation, the Ru electron density and electron affinity of Ru-MSP will increase with the increase of polymerization degree. Therefore, we attribute the high CTH activity of Ru-MSP catalyst to (1) electron-enriched Ru ions that help the dissociation of C-H bonds on formate to facilitate the dehydrogenation of formate (hydrogen source), and (2) the lower lowest unoccupied molecular orbital (LUMO) that assists the adsorption of carbonyl oxygen (in the substrate). The optimized Ru-MSP catalyst displays stability, reusability and capability of catalyzing a wide scope of carbonyl compounds. The results of this report confirmed that Ru-MSP catalysts are promising candidates of heterogeneous catalysts, which can be used for the CTH reaction of carbonyl compounds selectively into their corresponding alcohols under ambient conditions. In the second part of this thesis, we synthesized heterostructural Bi4Ti3O12-SrTiO3 (or BIT-STO) platelets with various STO contents by the topochemical conversion of BIT platelets via the alkaline hydrothermal treatment. The surface properties of synthesized BIT-STO platelets were probed by the temperature-programmed surface reaction of isopropanol (IPA). It is found that all BIT-STO catalysts show high selectivity in IPA dehydrogenation (to yield acetone) rather than IPA dehydration (to yield propene). Compared with the hydrothermally-synthesized STO and the commercial STO, The BIT-derived STO exhibits much higher activity and selectivity in IPA dehydrogenation, which is attributed to the desirable (100) facet with abundant surface defects formed during the topochemical conversion. In-situ infrared spectroscopy indicates that upon IPA adsorption, the BIT-derived STO shows a higher proportion of bridged isopropoxide (IPO) presents on the surface than the reference STO samples. It is proposed that the abundant oxygen vacancies present on the surface of BIT-derived STO could facilitate the bridged adsorption of IPO intermediate that favors further decomposition into acetone, thus promoting selective IPA dehydrogenation. In the third part of this thesis, we discussed the support effect of copper-supported catalysts on the CTH reaction. Commercially available ZrO2, Al2O3, TiO2, and SiO2 were used as supports, and Cu was loaded by the impregnation method. The IPA-assisted CTH of 5-hydroxymethylfurfural (HMF) to produce dimethylfuran (2,5-dimethylfuran, DMF) was used as the model reaction. The physicochemical properties of catalysts, the interaction between reactant (HMF) and catalysts, and the reaction selectivity of hydrogen source (IPA) on catalysts’ surface were characterized and discussed. The results indicated that the hydrogenolysis of the reaction intermediate, 2,5-bis(hydroxymethyl)furan (BHMF), was the bottleneck step for the production of DMF from HMF. Also, this step would be completed by etherification and dehydration polymerization reaction. The reaction selectivity on these four Cu/MOx catalysts is significantly different, with Cu/ZrO2 having the best target hydrogenolysis product (DMF) selectivity. Based on the analysis of HMF-IR, we suspect that the strong interaction between the Cu/SiO2 surface and HMF will inhibit the adsorption of IPA, which leads to the low activity of HMF hydrogenation. Through the results of IPA-TPSR and CO-IR, it is found that Cu/ZrO2 and Cu/Al2O3 have a lower valence of surface Cu (electron-enriched) than Cu/SiO2 and Cu/TiO2, which is helpful for the dehydrogenation of IPA reaction, and cause a higher DMF selectivity.