以硫酸均相觸媒系統催化木糖生成糠醛之動力學研究

Abstract

糠醛是用途廣泛的基礎化工原料，可由農業廢棄物衍生之木糖(五碳糖)在稀酸溶液催化下經脫水反應製備而得，不過由於伴隨副反應發生使糠醛產率受限在50%左右[1]，文獻曾分別提出在稀酸溶液中添加鉻離子(III)[2]及氯化鈉[3]有助於提升反應速率以及糠醛產率。本研究利用實驗級批次反應器，以添加均相觸媒(硫酸、硫酸鉻(III)、氯化鈉)來建立木糖生成糠醛反應之動力學平台。 本研究先以純木糖為原料在批次反應系統下研究糠醛生成之反應動力學，利用實驗設計及統計分析找尋較適生產糠醛之觸媒濃度，研究發現反應中同時存在硫酸、硫酸鉻(III)、氯化鈉具有共觸媒加乘效應，可以使木糖生成糠醛之反應速率大幅增加。此外，在共觸媒效應下，由溫度對反應速率影響顯示，以純木糖液生產糠醛之主反應活化能較各副反應為高，因此在較高反應溫度下能增加糠醛選擇率及產率。在高溫200℃下，以2.5mM硫酸、5ppm硫酸鉻(III)、5wt%氯化鈉之觸媒濃度進行反應，主副反應速率比值k1/k2(糠醛選擇率)可達3.95。 本研究後續以核能研究所木片廢料衍生的實際木糖液為原料，行生產糠醛程序並探討實際反應動力研究。在相同反應條件下與純木糖原料比較，發現木片木糖液中的木糖反應速率較純木糖來的低，糠醛繼續反應的副反應速率則較高，因此糠醛選擇率及產率大幅下降。相關結果應該是受到木片木糖液中雜物的影響。本研究已分析出木片木糖液之部分雜物(糠醛降解物、纖維素及木質素衍生物、金屬離子)，經純木糖液模擬反應比較，僅發現葡萄糖會增加糠醛降解速率，而不會降低木糖反應速率。顯示應還有未測得之其他雜物影響各反應。在雜物存在的實際情況下，以較適之觸媒型式探討溫度對木片木糖液之反應影響，發現溫度增加反而減少糠醛選擇率，與純木糖液之趨勢相反。顯示未來若以不同廢料衍生的木糖液進行糠醛生產，由於雜物的影響，必須個別決定最適反應條件。

Furfural yields from dehydration of xylose in conventional production processes catalyzed by sulfuric acid were limited to fifty percent because of side reactions. In the past, there are some researches about increasing furfural production rate and yield, such as addition of chromium ions (III) or sodium chloride respectively in acidic reaction media. In this study, reaction kinetics of furfural production from pure xylose in a homogeneous catalyst with chromium ions (III) and sodium chloride in sulfuric acid solution has been studied under a batch reaction system. Experimental designs and statistical analysis were used to find optimal concentrations of the catalysts for furfural production. In this research, the synergetic effect of simultaneous addition of sulfuric acid, chromium sulfate (III) and sodium chloride on catalytic property has been found, which can increase the furfural production rate substantially. In addition, the results about effects of reaction temperature showed that the activation energy of the major reaction is higher than side reactions; therefore, furfural selectivity and yield can increase at higher reaction temperature. When the reaction was in the solution with 2.5mM sulfuric acid, 5ppm chromium sulfate(III) and 5wt% sodium chloride at 200℃ high temperature, the value of the ratio between main reaction rate and side reaction rate k1/k2 (furfural selectivity) can reach 3.95. Reaction kinetics of xylose solution, produced from agricultural wastes (wood-chip) in real pretreatment processes developed by Institute of Nuclear Energy Research, have also been studied. Compared with the results of pure xylose at the same reaction conditions, lower reaction rates and higher side reaction rate of furfural degradation were found. Furfural selectivity and yield decreased significantly. It demonstrated that the reaction may be influenced by impurities contained in wood-chip xylose solution. The analysis of the composition of the impurities in wood-chip xylose solution was carried out. It was found that only glucose can increase the degradation rate of furfural. Thence, there were still other unknown impurities in wood-chip xylose solution affecting reaction rates. Moreover, the value of selectivity k1/k2 decreased as the reaction temperature increased. When the reaction was in the solution with 50mM sulfuric acid, 3.27ppm chromium sulfate(III) and 5wt% sodium chloride at 160℃, the value of k1/k2 (furfural selectivity) was only at 2.36.