| 摘要: | 本研究透過合成改質含胺基奈米纖維素與二氧化矽混合膠體，並進一步評估吸附CO2的可行性。該材料製備過程首先對棉布廢棄物(cotton cloth waste, CCW)進行鹼處理和脫色處理，再使用鹽酸進行酸水解處理後得到微晶纖維素(microcrystalline cellulose, MCC)，接下來使用硫酸水解和反應曲面法(response surface methodology, RSM)將微晶纖維素轉換為奈米纖維素(cellulose nanocrystal, CNC)。反應曲面法主要透過Box-Behnken design (BBD)設計三個獨立變數，分別為硫酸濃度(58¬64%)、水解溫度(50-70℃)和水解時間(40¬80分鐘)，然後使用溶膠凝膠法(sol-gel method)在常溫條件下進行乾燥，將奈米纖維素與二氧化矽合成為混合膠體(CNC/silica hybrid aerogel, CSA)，接著浸入聚乙烯亞胺(Polyethyleneimine, PEI)提高CO2吸附效果，最後探討溫度(30¬120℃)和PEI濃度等參數對CO2吸附的影響。
根據二次迴歸模型的判定係數顯示，對產物產率、結晶度指數(crystallinity index, CI)和平均粒徑的預測具有非常高的可靠性，顯示獨立變數和產物改變有顯著的相關性，而本研究發現當硫酸濃度為61.27 wt.%、水解溫度及水解時間分別在50℃及56分鐘時，具有提取CNC的最佳酸水解條件，此時CNC的產率、CI及平均粒徑分布則分別為44.57%、86.29%和160 nm，同時符合膠體製備的要求，具有成為CO2吸附劑的潛力。
在CO2吸附的試驗結果顯示，最佳的吸附劑 (CSA¬PE150) 在70℃和50 wt％的PEI濃度條件下具有最佳的CO2吸附能力，達到2.35 mmol g-1，透過多種吸附動力學模型解釋CSA-PEI50的吸附機制發現，Avrami動力學模型對CSA-PEI在不同溫度和PEI濃度下的CO2吸附行為最相符，反應階數介於0.352¬0.613之間，均方根誤差則可忽略不計。透過速率控制動力學(rate-limiting kinetic)分析結果顯示，膜擴散(film diffusion)和粒子間擴散阻力(intraparticle diffusion resistance)會影響吸附速率隨後控制吸附的過程，最後針對CNC進行十次的吸附¬脫附試驗中，發現CSA-PEI50具有非常高的穩定性，本研究證明CSA-PEI50具有成為捕捉燃燒後CO2吸附劑的潛力。
;The aim of the work is to synthesize and evaluate performance of amine-modified cellulose nanocrystal/ silica hybrid aerogel for CO2 adsorption. Firstly, cotton cloth waste (CCW) was subjected to alkali and decoloring treatments, and subsequent hydrochloric acid hydrolysis to obtain microcrystalline cellulose (MCC). The resulted MCC was furthered converted into cellulose nanocrystal (CNC) using sulfuric acid hydrolysis and response surface methodology (RSM). The RSM based on the Box-Behnken design (BBD) was designed with three independent variables: including sulfuric acid concentration (58-64 wt%), hydrolysis temperature (50-70 oC), and hydrolysis time (40-80 min). Then CNC/silica hybrid aerogel (CSA) was synthesized by hybridization of CNC and sodium silicate hybridization based on the one-step sol-gel method under atmospheric drying. Polyethyleneimine (PEI) was impregnated on CSA to improve CO2 adsorption performance. The parameters governing CO2 adsorption performance on CSA-PEI, such as temperatures (30-120 oC) and PEI concentrations (40-60 wt%), were investigated systematically.
According to the analyzed regression models, the predicted quadratic polynomial models for yield, crystallinity index (CI), and average particle size exhibited high reliability, corresponding to the close interaction between independent variables and responses. The study found the optimum acid hydrolysis conditions for CNC extraction were 61.27 wt%, 50 oC, and 56 min. At the optimal points, the results of experimental CNC yield, CI, and particle size distribution were 44.57%, 86.29%, and 160 nm respectively. The obtained CNC properties met the required specifications for aerogel preparation as an adsorbent for CO2 adsorption process.
For the CO2 adsorption performance, the optimum adsorbent (CSA-PEI50) exhibited an excellent CO2 adsorption capacity of 2.35 mmol g-1 at 70 oC and a PEI concentration of 50 wt%. The adsorption mechanism of CSA-PEI50 was elucidated by analyzing many adsorption kinetic models. The CO2 adsorption behaviors of CSA-PEI at various temperatures and PEI concentrations had the goodness of fit with the Avrami kinetic model, which can correspond to the multiple adsorption mechanism. The Avrami model also showed fractional reaction orders in a range of 0.352-0.613, and the root mean square error was negligible. Moreover, the rate-limiting kinetic analysis showed that film diffusion and intraparticle diffusion resistances controlled the CO2 adsorption process. The CSA-PEI50 also exhibited excellent stability after ten adsorption-desorption cycles. This work proved that CSA-PEI was a potential adsorbent in post-combustion CO2 capture. |
