以水溶液電漿分解纖維素之製程研究

Abstract

由於在水溶液生成電漿可產生許多不同種類的自由基，如OH、H、O和過氧化氫，可被用來分解有機物。本實驗中，主要是探討水溶液電漿分解纖維素的可能性，並使用高效能液相層析儀(High Performance Liquid Chromatography, HPLC)來分析水溶液中產物的種類及濃度。 在本實驗中，纖維素經過電漿處理後會生成黃色的物質，使得水溶液會變成黃色，且經由高效能液相層析儀鑑定後，發現纖維素可分解成小分子的物質，如葡萄糖(Glucose)、果糖(Fructose)、乙醇酸(Glycolic Acid)、脫水葡萄糖(Levoglucosan)、醋酸(Acetic Acid)、5-羥甲基糠醛(5-Hydroxymethylfurfural, 5-HMF)和一些未知的化合物。為了得到最佳的分解效率，分別改變電解質的化學性(濃度、種類及pH值)及電漿參數。在0.01~0.2 M 範圍中，分解效率會隨著電解質濃度增加而增加；而在不同濃度下不同電解質對於分解效率及產物種類而言各有其特點，其中所使用的電解質有氯化鈉、硝酸鈉、氫氧化納、氯化鋅、硝酸鋅、硫酸鋅、氯化鈣、硝酸鈣、硫酸鈣、氫氧化鈣及鹽酸，在0.01 M下，氫氧化鈣的效率最高，在0.1 M 下則是氯化鋅的整體效率最佳，且只有在氯化鋅及硫酸鋅的環境下能產生脫水葡萄糖；而溶液的pH 值對於產物的顏色及產率也有影響。除了改變溶液性質外，透過施加不同的電壓及電漿模式也會影響效率；對於施加電壓而言會有一個最佳的使用範圍，高於或低於此電壓都會使得效率降低；直流正偏壓、負偏壓及透過交流電所產生的氣泡模式(Bubble Mode)及噴射模式(Jetting Mode)中，透過直流正偏壓所生成的電漿分解纖維素的效率最佳。

Plasmas in salt solution are able to generate active species and can be used to decompose organic compounds. In this study, plasmas driven by AC or DC power sources in various saline-solutions are used to decompose cellulose. The solutions used include NaCl, NaNO3, NaOH, Zn(NO3)2, ZnCl2, CaCl2, Ca(NO3)2, and Ca(OH)2. The electrode at which the plasma is ignited is a platinum wire 0.5 mm in diameter covered by a glass tube while the grounding electrode is a bare platinum wire of the same diameter. The plasma voltage and current waveforms are monitored using electrical probes. Optical emission spectrometer is used to monitor the time-averaged emission spectra emanating from the plasma. The conductivity and the pH of the solution in which the plasma is ignited are monitored using commercially available meters. The decomposed products are identified and quantified using high performance liquid chromatography. It is shown that the plasma is able to effectively decompose cellulose into smaller molecules, such as glucose, fructose, glycolic acid, levoglucosan, 5-HMF, and several unknown products. The decomposition efficiency increases with the salt concentration. When different salt solution is used, it shows significantly different decomposition efficiencies and decomposed products. This observation strongly suggests that the decomposition process using plasmas in salt solution is chemical in nature and obtaining reaction selectivity is highly possible. Preliminary studies show that Ca(OH)2 and ZnCl2 solutions can most effectively decompose cellulose under optimized conditions.