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離子液體(ionic liquids; ILs)是公認具有高穩定性、極低揮發性的綠色溶劑,更因其可藉由離子交換調控的陽離子和陰離子組份,該液體鹽類可為質子、非質子或兩性離子(zwitterionic)的型態,是一理想的均相催化觸媒。本論文部份研究利用本實驗室所開發之31P-TMPO NMR酸性鑑定技術,即利用三甲基磷氧化物(TMPO)做為探針分子,透過31P核磁共振(nuclear magnetic resonance; NMR)光譜進行實驗,針對不同類型離子液體之酸性特性加以鑑定,並討論其催化應用性。 雜多酸(heteropolyacids;HPAs)則是一種穩定性極佳的無機鹽類,由於其獨特的氧化、還原特性、可溶性和超強酸特性,常被應用於石化及生質能催化反應做為異相或均相觸媒。然而,由於HPA容易溶解於極性溶劑(如水、醋酸等)中以及表面積低等缺點,使反應後催化劑與產物不易分離以及觸媒循環使用困難等缺點,極大地限制了其在催化領域的工業應用。因此,許多新進研究著重於將HPA固載在高比表面積擔體上面,藉以提升其分散性及穩定性。但單純以物理吸附方式將雜多酸負載在擔體表面則存在嚴重的催化劑浸出(leaching)問題,不利於產物分離以及觸媒回收循環使用。針對此一困難問題,本研究開發一種新穎負載型無機有機複合催化劑,選擇利用1-甲基-3-(三甲氧基甲矽烷)咪唑氯化物(1-methyl-3-(trimethoxysilylpropyl)- imidazolium chloride)離子液體做為連接HPA與具高比表面積的中孔二氧化矽SBA-15擔體之間的橋樑,首先將IL修飾於SBA-15載體表面,再利用IL上的帶負電咪唑環與雜多酸陽離子所形成的離子鍵結達到固載的目的。吾人所製備的HPA-IL/SBA-15觸媒並透過各種物裡化學分析與光譜技術加以鑑定,此外,亦利用31P-TMPO NMR酸性鑑定技術其酸性特性(酸強度與濃度)。最後,並藉由線性烷基苯(LAB)催化反應進行測試,了解其催化效能。 由異核固態NMR及其他實驗結果顯示,吾人成功地負載典型的Keggin-型態HPA,即磷鎢酸(H3PW12O40; HPW),在IL修飾的SBA-15表面,且所製備之HPW-IL/SBA-15觸媒具有超強酸的特性存在。經由改變HPW和IL的負載比例,吾人亦成功的調控負載在擔體上的酸強度與酸量分佈,並發現在適當的HPW/IL比例(即接近1:1)時,反應不僅具有高轉化率且有較好的短鏈線性烷基苯(2-LAB)產物選擇率,說明此時HPW原存在的三個質子活性酸位(proton active sites)當中,平均只有一個質子與IL上的陰離子形成離子鍵而被佔據,所剩餘的兩個質子表現出最高濃度的超強酸質子分佈,因而展現較佳的催化能力。反之,負載過少(或過多)HPW時,極低(或極高)質子酸位的存在,均導致較低的LAB轉化率和選擇率,反而不利於反應的進行。
Ionic liquids (ILs) are well-known green solvents with high stability and extremely low volatility. Moreover, through ion-exchange processes invoked by properly choosing the anionic and ionic components, the nature of such liquid salts may also be either protic, aprotic, or zwitterionic, making ILs favorable homogeneous catalysts or electrolytes. Portions of this study focus on synthesis and characterization of various ILs and their catalytic performances. In particular, the acid properties of these ILs were characterized by the 31P nuclear magnetic resonance (NMR) of trimethylphosphine oxide (TMPO), that is, the 31P-TMPO NMR approach, developed in this laboratory. Heteropolyacids (HPAs) are inorganic salts with excellent stability, solubility, oxidation and reduction capability, and ultra-strong acidity, and have been exploited for heterogeneous or homogeneous catalytic conversions of petro-chemical and biomass. Nonetheless, owing to their high solubility in polar solvents (such as water, acetic acid etc.) and low surface areas, HPAs suffer from difficulties in product separation, post-treatment pollution, hence largely limit their practical industrial applications. Thus, considerable recent R&D attentions have been directed toward immobilization of the HPA catalyst on solid supports, aiming to improve catalyst dispersion and stability. Nonetheless, the supported HPA catalysts are still drawback by undesirable leaching problem, causing problems in product separation and catalyst recycling. Herein, a novel synthesis route is proposed for fabricating HPA-immobilized solid catalysts. This is accomplished by invoking functionalization of a specific-task IL, namely 1-methyl-3- (trimethoxysilylpropyl)-imidazolium chloride on to high surface-area mesoporous SBA-15 silica, followed by ionic coupling of the positively charged pyrrolic ring of the IL with anionic ligand of the HPW. The physicochemical properties of TPA-IL/SBA-15 catalysts so fabricated were characterized by a variety of different analytical and spectroscopic techniques. Whereas, their acidic properties, such as acidic strength and concentration, were assessed by the 31P-TMPO NMR approach. Moreover, their catalytic properties were tested by linear alkyl benzene (LAB) reactions. We successfully immobilized a typical heteropolyacid with Keggin structure, namely tunstophosphoric acid (H3PW12O40; HPW) onto a IL surface-modified mesoporous SBA-15 silicas while preserving the desirable superacidity and acid concentration, as revealed by heteronucleus solid-state NMR studies. Moreover, the acid properties of such HPW-IL/SBA-15 catalysts may be adjusted by controlling the relative loadings of HPW and IL. It was found that, as the HPW/IL ratio reaching 1:1, a superior LAB conversion and short-chain LAB (e.g., 2-LAB) selectivity were simultaneously achieved, indicating the highest preservation of proton active sites. In this case, two out of the total three protic sites remain active, while the other one inactivated by linkage to an IL ligand. Whereas for the cases of inadequate or excessive HPW loadings, which result in diminishing or upmost abundance of protic sites, inferior LAB conversion and 2-LAB selectivity were observed.



離子液體, 中孔二氧化矽, 負載型雜多酸觸媒, ionic liquid, immobilization, heteropolyacids