光電廢棄物資源化製備奈米吸附材料及其應用於二氧化碳捕獲之研究

Abstract

隨著京都議定書正式生效，同時二氧化碳捕獲及封存技術 (Carbon dioxide Capture and Storage, 簡稱CCS) 也於2005年被聯合國之IPCC組織評估為可行方式之一。其中利用固體吸附劑捕獲二氧化碳被視為是現行許多捕獲技術中最具有潛力之一；在眾多吸附劑中，中孔洞二氧化矽因其有高比表面積、可調整之孔徑大小與高抗熱性，目前逐漸被應用至二氧化碳控制。雖然中孔洞二氧化矽藉助於奈米科技的創新與技術上之改良，使其材料製程與發展得以快速進化發展，不過該類型材料價格昂貴且其製造過程亦較費時費能，產物取得較沸石與活性碳困難，目前應用中孔洞氧化矽於二氧化碳控制之研究不若傳統沸石與活性碳廣泛。此外為解決全球溫室效應問題所需削減之二氧化碳氣體排放量相當龐大，因此若採用CCS技術將必須消耗大量之地球資源。另一方面，近年來隨著半導體與光電產業的快速發展，大量含矽之廢棄粉末亦伴隨而生。此類型之廢棄物質輕且體積龐大，需額外花費較多之成本委託廠商進行後續廢棄物處理。相對的，如能有效利用廢棄物，將之加以資源化製成多孔材料，則不僅具有成本效益，且可解決廢棄物處理與處置問題。 本研究旨在利用光電粉末廢棄物做為二氧化矽之前驅物，分別透過液相水熱法以及氣相製程製備中孔洞二氧化矽，並將其應用做為吸附劑進行二氧化碳氣體捕獲之研究。研究中亦探討中孔洞吸附劑孔洞特性對於二氧化碳吸附效能之影響以及利用廢棄粉末製備吸附劑之經濟效益，以評估取代商業吸附劑之可行性。研究結果指出以廢棄粉末做為矽源，透過離子型界面活性劑十六烷基三甲基溴化銨(CTAB)作為模板並加入適量之氫氟酸與氫氧化銨可於常溫下製備出具有高比表面積(788 m2g-1)、大孔徑(4.5 nm)以及大孔洞體積(1.1 cm3g-1)之中孔材料MCM-41(DU)-F。為了進一步縮減吸附劑製備所需之成本，本研究亦嘗試利用非離子型之三崁式界面活性劑F127做為模板；相對於陽離子型界面活性劑(CTAB)不僅在價格上較便宜外，在環境汙染程度上也相對較低。而研究成果顯示，界面活性劑F127濃度於再生製備中孔洞材料MS上有顯著的影響。當F127/Si莫耳比例為0.001時，MS材料為具有籠狀(cage-like)之中孔材料；而當當F127/Si莫耳比例提升於0.0023時，MS材料則是轉變為具有囊泡狀(cellular foam)之材料，而其孔徑與孔體積亦大幅提升。 另一方面，本研究亦開發出利用常溫鹼萃取法可將廢棄粉末分離為矽酸鹽水溶液與沉澱物；沉澱物之成分經鑑定後主要為高純度之氟化鈉(>90%)。由於氟化鈉是工業上常用之化學品，因此所回收之高純度氟化鈉可提供二次再利用的機會；而經分離所得之矽酸鹽水溶液則可作為合成二氧化矽材料之前驅物。透過此萃取法能夠將廢棄粉末轉變為兩種具有高度經濟價值的物質。而利用矽酸鹽經由水熱法所製造之中孔材料MCM-41(AF)其物化特性與利用純化學品所製備出MCM-41(NaSi)之特性相似，顯示由TFT-LCD粉末廢棄物所製得之矽酸鹽的確為一具有潛力的二氧化矽來源。本研究亦延伸以粉末廢棄物所製得之矽酸鹽之製備與應用範疇，以無機鹽類做為模板透過連續式氣相製程製備出MSP(AS)以及MSS(HNO3)中孔材料。在價錢成本估算部分中，使用廢棄物粉末合成之中孔材料MSS(HNO3)可相較於使用化學品合成SBA-15節省約95 %的價錢，更為使用化學品合成MCM-41僅2%的價錢。因此利用廢棄矽酸鹽為前驅物以一步氣膠合成方式預期將可大幅減少化學材料成本以及製造時間，如此本研究所製得之奈米材料即可大量製造，並應用於捕獲CO2溫室氣體上。 在二氧化碳吸附捕獲測試結果顯示，中孔洞材料其孔徑大小以及孔體積對於二氧化碳捕獲效能有顯著的影響。經迴歸分析，可知孔洞體積為最影響吸附效能之關鍵因子，其次為孔洞大小，比表面積之影響則相對較小。其中中孔材料MSS(HNO3)在二氧化碳入流濃度10%、吸附溫度60oC時吸附量可達到122 mg-CO2/g-adsorbent，高於利用純化學品所合成之中孔洞材料MCM-41與SBA-15。因此結果顯示利用廢棄物所合成之樣品MSS(HNO3)具有價格便宜、高二氧化碳吸附量以及快速之製備時間。本研究所製得之奈米材料因此可大量製造，並應用於捕獲CO2溫室氣體上。綜合成本考量和後端二氧化碳應用，使用此材料在未來二氧化碳捕捉的應用上具有前瞻性。

The carbon dioxide (CO2) capture and storage (CCS) technologies have received out-breaking concerns after the Kyoto Protocol came into force in 2005. Among capturing technologies, adsorption is regarded as one of the feasible approaches which can limit the CO2 emission. Mesoporous silica materials with high surface area, large pore size and large pore volume are considered as good candidates for CO2 capture. However, the requirements of tedious processing time and expensive manufacture costs strongly limited their applications. Furthermore, the global emission quantity of CO2 is so huge that it may consume tremendous amount of resource materials to capture the CO2 greenhouse gas. On the other hand, with the evolution of semiconductor and optoelectronic industries, huge amounts of siliceous waste powder are significantly increased. Such waste powders are light-density with bulky volume and are thus difficult to be transported and disposed. Therefore, additional expenses on waste treatment and landfill disposal are needed. So if the captured sorbent can be obtained from product wastes, the cost-effectiveness of the CO2 capture technology and the waste treatment and disposal problem will be resolved simultaneously. This study intends to reutilize the waste powder as an alternative resource for the production of mesoporous silica materials via either solution precipitation method or aerosol spray approach. The structural properties and cost-effectiveness of the recycled materials on CO2 adsorption performance was investigated as well. The results showed that the waste powder can be directly converted in to mesoporous silica MCM-41(DU)-F with high surface area (788 m2g-1), large pore size (4.5 nm) and large pore volume (1.1 cm3g-1) with the assistance of ionic surfactant of CTAB, hydrofluoric acid as well as ammonium hydroxide. Through similar pathway, silica materials with hierarchically mesocellular structures can be facilely prepared by using single F127 surfactant. The concentrations of hydrofluoric acid and F127 were found to strongly affect the structural properties of the recycled materials. On the other hand, a low-temperature alkali extraction was developed to effectively separate the silicate supernatant and the sediment of sodium fluoride (NaF) from the waste powder. The obtained sediment containing high purity of NaF (>90%), which provides further reuse possibility since NaF is widely applied in chemical industry. The supernatant is a valuable silicate source for synthesizing mesoporous silica material. In other words, the optoelectronic waste powder can be converted into two valuable resources, the supernatant as the silica precursor and the sediment of sodium fluoride. The mesoporous MCM-41 produced from the waste-derived silicate, namely MCM-41(AF), possessed high specific surface areas (1069 m2/g), narrow pore size distributions (3.0 nm) and large pore volumes (0.97 cm3/g), similar with those of the MCM-41(NaSi) fabricated using commercial silica precursors. This clearly suggests that the silicate supernatant from waste powder can be potential silica resource. This study further extends the preparation of mesoporous materials using waste-derived silicate supernatant as precursors. It was demonstrated that mesoporous MSP(AS) and MSS(HNO3) materials can be synthesized by employing inorganic salts as templating media. The cost analysis shows that the synthesized material of MSS(HNO3) is about five percent of the price of SBA-15 and two percent of the MCM-41 made from commercial silica precursors. Furthermore, the correlation between CO2 adsorption capacity and the pore structure properties (pore size, pore volume and specific surface area) is studied. The result of the linear regression indicates that the CO2 adsorption capacity has the strongest correlation with the total pore volume of the mesoporous materials (R2>0.9). The amine-impregnated MSS(HNO3) can achieve 122 mg/g adsorption capacity, which is superior to that of the original MCM-41(115 mg/g) and SBA-15(117) made from commercial precursors under the same conditions. The MSS(HNO3) prepared using optoelectronic industrial waste powder as the silica source via salt-templated aerosol spray approach exhibits several important advantages of simple and rapid synthesis, low manufacturing costs and superior CO2 adsorption performance. Therefore, it could be considered as potential and competitive sorbents for CO2 capture from flue gas.