以稻殼廢棄物資源化製備多孔洞矽基吸附材料及其應用於空氣汙染控制之研究

Abstract

為減緩能源短缺與空氣汙染之問題，本研究擬將稻殼農業廢棄物再利用，藉由調整合成參數，製備成不同孔洞結構特性之多孔洞材料，探討多孔洞材料物性變化對於化學性吸附二氧化碳與物理性吸附揮發性有機物之效能影響。 本研究首先藉由調整稻殼前處理溫度以及矽酸鹽萃取溫度，探討兩種溫度對於中孔洞材料物理特性及其應用於二氧化碳吸附效能之影響。經由數據歸納分析出影響二氧化碳吸附量的關鍵因素為中孔洞材料之總孔洞體積之大小。相較於文獻中多以添加高分子有機物作為擴孔劑以增大多孔材料之孔徑與孔體積；本研究研發出在不使用任何有機高分子擴孔劑的條件下，僅靠簡單控制稻殼高溫熱處理溫度以及矽酸鹽萃取溫度即可製備出具有大孔徑以及高孔體積特性之多孔二氧化矽材料。此技術能大幅減少使用化學藥劑，不僅能夠減少在吸附材料製備程序中可能衍生的環境汙染污染問題，亦能夠縮減吸附材料製備之費用。 為了節省吸附劑製備所需耗費的成本與時間，本研究亦開發出能夠在常溫下快速直接從稻穀中製備出多孔性二氧化矽奈米顆粒(Porous silica nanoparticles, PSNs)。研究結果指出在合成材料過程中形成的氟矽酸銨(NH4)2SiF6，可作為孔洞形成劑並且可於常溫下透過水洗的方式去除，進而形成孔洞材料。相較於傳統文獻中之多孔性吸附劑製備大多需要有機界面活性劑作為模板，且需要數十小時的製備時程，此研究之合成方法可大幅減少吸附劑製備所需耗費之成本與時間。而相比於文獻中常見之二氧化碳中孔洞吸附劑MCM-41、SBA-15及SBA-16，多孔性二氧化矽奈米顆粒亦呈現最佳的吸附能力。多孔性二氧化矽奈米顆粒在製備程序上，具有快速、低能源消耗並且不需要額外添加界面活性劑的優勢，且應用於溫室氣體CO2捕獲，亦有優異的成效。 另一方面，本研究亦探討多孔二氧化矽材料孔洞結構特性應用於揮發性有機物丙酮吸附之影響；在本研究中，首度利用稻殼灰做為二氧化矽前驅物製備出微/中孔洞RSBA-16作為丙酮之吸附劑，並藉由調整界面活性劑與二氧化矽莫耳比例，進而尋求最適應用於丙酮吸附之孔洞特性。結果顯示，孔洞材料具備大比表面積時，可以提供更多活性吸附位置，增加丙酮吸附能力；此外，本研究發現在具有相同總比表面積下，吸附劑若具有較高微孔比表面積有助於提升丙酮吸附能力。其中，RSBA-16因同時具備高總比表面積以及高微孔比表面積，因此其丙酮飽和吸附量(179 mg/g)遠高於RMCM-41 (108 mg/g) 與RSBA-15 (152 mg/g)。而丙酮等溫吸附測試顯示丙酮在吸附劑RSBA-16表面上屬物理性吸附，此現象則與比表面積為最重要決定因子前後呼應。反覆吸脫附測試亦顯示RSBA-16具有相當好之熱穩定性。此外RSBA-16(0.004)其製備成本與RSBA-15相似，並遠低於RMCM-41。故經過吸附效能、製備成本與再生測試後，RSBA-16(0.004)具有最好的吸附劑條件。

As the energy crisis and resource shortage continue, the rice husk becomes an agriculture valuable waste resource which can be made into various kinds of energy products and resource materials. On the other hand, the capture of CO2 and the control of VOCs emitted from industrial sources are two of the most important air pollution issues. This study intends to reutilize the rice husk as the silica source for the synthesis of porous silica materials and to investigate the effects of structural properties of the waste-derived-materials on CO2 and VOCs adsorption performance. In this study, alkali fusion method was employed to extract the silica from rice husk, and the effects of fusion temperature and extraction temperature on the textural properties of waste-derived materials was investigated. Unlike the conventional methods for preparing large-pore silica materials in which toxic and expensive additives were employed as swelling agents, the obtained waste-derived silica materials with large mesopores could be facilely prepared via a simple temperature-controlled approach without adding pore expanders in this study. The correlation between CO2 adsorption capacity and the textural properties (pore volume, pore size and specific surface area) was demonstrated, and a linear correlation between CO2 adsorption capacity and the total pore volume of the adsorbents was clearly observed. This indicated that the total pore volume of the adsorbent plays a dominant role in determining the CO2 adsorption performance. To reduce the energy consumption and processing time for the preparation of porous silica materials as CO2 adsorbents, a rapid and simple method for the preparation of porous silica nanoparticles (PSNs) directly from agricultural waste of rice husk was developed. Compared with the traditional alkaline fusion and surfactant-templated methods for preparing waste-derived porous silica materials, this method possessed important advantages of a cost-effective, and energy-saving process with faster production rate. Results showed that the (NH4)2SiF6 salt formed during the synthetic process was an effective pore structure medium, which can be easily removed and recovered for further reuse by washing with water. Furthermore, compared to MCM-41, SBA-15 and SBA-16, the PSNs showed the best proformance on CO2 adsorption conditions, probably due to its larger pore volume. On the other hand, the influence of pore structural properties of porous silica materials on adsorptive removal of volatile organic compounds (VOCs) was also investigated. In particular, for the first time, micro-/mesoporous RSBA-16 materials were synthesized by rice husk derived sodium silicate as a silica source. The pore structural properties of waste-derived SBA-16 materials were controlled and optimized by adjusting the surfactant/silica molar ratio for achieving the best adsorption performance of acetone, and the relationship between structural properties and acetone adsorption performance of RSBA-16 adsorbents was investigated. The results indicated that specific surface areas in both micro- and meso-pore ranges were the main factor that determined the superiority of acetone adsorption capacity of RSBA-16(0.004) adsorbents (179 mg/g) over other adsorbents such as mesoporous RMCM-41 (108 mg/g) and micro-/mesoporous RSBA-15 (152 mg/g). The results suggested that RSBA-16(0.004) which had high adsorption rate, high adsorption capacity, high cyclic stability and relatively low chemical cost can be considered as a potential adsorbent for VOCs removals.