Aerosol processing of mesoporous silica particles for air pollution control application
|關鍵字:||中孔洞材料;氣膠;揮發性有機物;沸石分子篩;吸附;金屬化中孔洞材料;Mesoporous materials;Aerosol;Volatile organic compounds (VOCs);Zeolite Molecular sieves;Adsorbent and adsorption;Metallic mesoporous material; Ammonia|
中孔洞吸附劑合成技術上，有別於傳統的水相合成，如水熱模板，膠凝法等，本研究以氣膠技術(aerosol route)之揮發誘導自我聚集組裝(EISA)製備中孔洞矽質顆粒(mesoporous silica particles, MSP)吸附劑，以其快速合成的特性，縮短製備時間。研究內容主要為中孔洞材料之合成技術與其應用做為吸附劑去除揮發性有機物之吸附特性探討，並進一步延伸其應用，將中孔洞顆粒進行鋁金屬化與胺基官能化，並分別進行氨氣與二氧化碳去除可行性研究。研究中亦進行最佳吸附效能參數與中孔洞吸附劑表面物性比較，評估取代商業吸附劑之可行性。
研究結果指出以氣膠法所合成之中孔洞顆粒(MSP)，可藉由調整反應前驅液之介面活性劑與矽基質莫耳比例，成功製備出擁有高均勻度規則排列的中孔洞孔道架構與高比表面積；反應藥劑中界面活性劑與矽基質之莫耳比例對於吸附劑比表面積與丙酮吸附容量改變有顯著的影響，且於特定莫耳比例內，比表面積與吸附量呈現一高度線性關連。而比表面積的增加有助於提昇其吸附效能，但亦會受限於孔洞樣式不同，而產生不同之吸附行為。中孔洞吸附劑之等溫吸附模式回歸結果，均可以Langmuir Isotherm及Freundlich Isotherm此兩種等溫吸附模式來描述其吸附行為。在與現行商用ZSM-5型沸石及Si-MCM-41分子篩相互比較其效能之研究結果顯示，合成中孔洞矽質顆粒具備大比表面積(>1000m2/g)與較大孔徑(2.3-2.5 nm)之中孔洞結構，可提供更多活性吸附位置供丙酮去除，其對於丙酮之吸附量與Si-MCM-41分子篩相當接近，均可達140 mg/g，遠大於H-ZSM-5沸石之吸附量85 mg/g；除此之外，由於中孔洞矽質顆粒具備高體密度與低壓降差之特性，使得其吸附效能及實廠應用上優於Si-MCM-41分子篩，以單位體積吸附劑所能吸附之VOCs吸附量進行比較，中孔洞微粒之吸附量為49 mg/cm3，幾乎是Si-MCM-41分子篩吸附量10 mg/cm3的5倍之多。對於實際的工業應用上，空氣污染去除設備像是吸附塔等的規模設計會嚴重受限於廠房空間大小，故若能使用中孔洞矽質顆粒作為吸附劑取代H-ZSM-5沸石或Si-MCM-41分子篩，設計規劃吸附塔，則可以使得吸附塔的體積大幅度縮小，減小初設成本開銷以及廠房空間的浪費。
本研究亦延伸MSP之製備與應用範疇，以一步氣膠合成方式，製備出鋁金屬化中孔洞顆粒；鋁金屬化中孔洞顆粒合成可藉由改變前驅液鋁金屬濃度，形成不同配位比例之鋁金屬，且應用至空氣汙染物NH3的吸附去除上，相較於商用氧化鋁混以中孔洞微粒之樣品，鋁金屬化中孔洞微粒可得到較佳之吸附效果。而在胺基官能化之中孔洞材料合成與應用至溫室氣體CO2捕捉部分，由於中孔洞微粒本身具有相當均勻的孔洞孔徑與孔道架構，使其擁有高比表面積，對於承載更多胺官能基數量提供一最佳擔體環境，使得其在吸附能力測試上能吸附更多的CO2，其在70oC之吸附測試溫度下之吸附量可超過100 mg CO2/g；再生方面，本研究比較不同再生方式對其吸附量影響。結果指出，經過變壓式再生(pressure swing adsorption, PSA)方式處理之吸附劑，其再生吸附效能維持在90%，優於變溫式再生(thermal swing adsorption, TSA)處理。考量至能源消耗上，PSA再生不需增設任何之加熱脫附爐體，對於應用推廣至工業廢CO2處理，有著較大的利基。|
Mesoporous silica materials have received wide attentions due to their surface properties such as high specific surface area, uniform pore size and heat stability. Although several synthetic pathways are well known to prepare the mesoporous silica materials, most of the them are tedious and time-consumed with several days of operation time. A new aerosol-processing route of evaporation-induced self-assembly (EISA) method for synthesizing ordered mesoporous silica particles (MSPs) is used in this study. The advantage of the aerosol process lies in that it can continuously produce MSP in a very short processing time of a few seconds plus a few hours of calcination. The objectives of this study were to synthesize mesoporous silica particles (MSPs) via aerosol process and to investigate its adsorption performance on air pollutants. A systematic analysis on the effects of specific surface area and pore structure of MSP adsorbent on the acetone adsorption behaviors is performed. In addition, further application extensions of MSP are investigated as adsorbent for NH3 removal and CO2 capture. The results show that optimal Surfactant/Si molar ratio of precursor was found to be 0.12-0.18 to obtain the well-ordered porous structure and high surface area ( > 1000 m2/g) MSPs adsorbents. The relationship between the physical characteristics of MSP adsorbents and the acetone adsorption behaviors were examined for the first time. It indicates that an increase in the specific surface area results in an increase in the acetone adsorption capacity. But a further increase in the surface area could cause a less porous-structured adsorbent and the acetone adsorption capacity could become less even though the specific surface area is the highest value of 1337 m2/g. The superior performance of MSPs as an acetone adsorbent was demonstrated by comparing to the performance of ZSM-5 zeolite and the common mesoporous silica-based materials of Si-MCM-41. The results showed that the surface area and pore diameter of MSPs are similar to those of Si-MCM-41. But the synthesis of Si-MCM-41 frequently requires longer time and tedious procedure as compared to that of MSPs. The bulk density of MSPs is 3.0 - 5.0 times higher than that of Si-MCM-41. The mass-based acetone adsorption capacities of these two materials are almost similar. This implies that MSPs have a higher volume-based acetone adsorption capacity than Si-MCM-41 so that less space is required for VOCs adsorption using MSPs as the adsorbent. The pressure drops of both powder and pellet forms of MSPs are also smaller than those of Si-MCM-41 for adsorbing the same amount of acetone. In addition, as compared to commercial H-ZSM-5 zeolite, both MSPs and Si-MCM-41 reveal better performances on the regeneration ability. As a result, the MSPs are better as novel regenerative adsorbents for AMCs control in the clean room environment. This study extends the preparation and application of MSP by functional MSP with aluminum and amine chemicals. Aluminum metallic MSP is synthesized via one-step aerosol route. The results indicated that Al coordination composition can be adjusted via controlling Al metal concentration of precursor. On the application of NH3 removal, Al-MSP adsorbent shows a superior performance as compared to the commercial Al2O3 mixing with pure MSP adsorbent. The amine-functionalized MSP adsorbents are synthesized via post-treatment (impregnation) with amine functional reagent in this study. The results indicated that the CO2 adsorption capacities of amine-MSP adsorbents are greatly improved by properly loaded with amine-functionalized reagents into its nano-sized pore channels, the CO2 adsorption capacities could be well above 100 mg-CO2/g adsorbent under the operation temperature of 70℃. Pressure-swing cyclic analysis is undertaken, which shows that both chemical and physical sorption occur on the surface of MSPs. But the regeneration ability of around 86~90% is still achievable after several cyclic tests of pressure swing adsorption.
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