Y型與ZSM-5沸石應用於揮發性有機物之吸脫附特性研究

Abstract

揮發性有機物(Volatile organic compounds)係指在標準狀態下蒸氣壓大於0.1 mmHg之有機物質，因此容易揮發且具有較低的沸點，其不只對於環境有害，也會讓人體產生嘔吐、噁心等不適症狀，某些揮發性有機物更被證實對人體有致癌性，因此如何有效減少揮發性有機物的汙染是當今急需迫切解決的問題。在各種去除方法中，以吸附法擁有高操作彈性、價錢便宜最為廣泛使用，其中又以活性碳最常被當作吸附劑使用，因其吸附能力佳、造價便宜，但其在吸附時容易造成孔洞阻塞，容易吸水，更有自燃等安全上得虞慮。沸石（Zeolite），因其具有獨特的晶體結構、高熱穩定性與表面積等特性，被認為是非常有潛力作為吸附揮發性有機物的替代材料。 本研究利用矽鋁比不同的Y型與ZSM-5型沸石作為吸附劑捕捉代表性揮發性有機物（甲苯、甲基環戊烷、巴豆醛），並將材料做後修飾：包括與鈉、銀、銅、氫離子做離子交換，或者經過酸鹼溶液處理進行介孔製備，同時也合成含鈦之沸石TS-1進行比較，材料以X光粉末繞射鑑定其晶體結構，使用感應耦合電漿質譜分析儀計算其中各元素含量，並利用氮氣吸附-脫附測量表面積以及孔體積，使用掃描式電子顯微鏡鑑定沸石形貌，並以程溫脫附儀進行吸脫附特性的研究。實驗結果顯示，在相同吸附條件下，影響甲苯吸附量的關鍵因素為陽離子含量（矽鋁比），陽離子含量越多，甲苯吸附量越高，但同時也較吸水；次要因素為離子的型式，鈉離子最對於甲苯與水氣的吸附量最高，氫離子則相反，對於甲苯與水氣的吸附量最低；當矽鋁比相近、離子型式相同時，比較面積的貢獻才會較為明顯，比表面積越大的材料，吸附效果越好。在甲基環戊烷的吸附中，發現到當探針分子與材料孔洞大小相近時，會使分子類似卡在孔洞中的狀況，吸附量提升且不易脫附，如Na-Z120對於甲基環戊烷的吸附量為5.18 wt%，高於其對於甲苯的吸附量3.75 wt%。在巴豆醛的吸附中，我們發現到含有銀、銅、氫離子的Y型與ZSM-5型沸石的吸附的同時會伴隨產生氧化裂解等反應，而TS-1對於巴豆醛僅有單純吸脫附之作用，吸附量約為6 ~ 7 wt%。在經過諸多試驗後，我們也找出在不同條件下適用之吸附劑：若吸附環境為無水的狀態，適合使用經過鹼處理之Na-Y3B，其對於甲苯與甲基環戊烷的吸附量最高，達到了21.0 wt%與13.8 wt%。而若要在一般含水氣的環境下吸附，則氫離子型式的H-Z120最為適合，因其對於水氣的吸附量非常低，具有30.3的甲苯/水選擇率與44.8的甲基環戊烷/水選擇率。

Volatile organic compounds (VOCs) are organic chemicals that have high vapor pressure at ambient temperature, and many of them are harmful to the environment and are toxic or even carcinogenic to the human being. Many technologies are available for VOCs control, such as adsorption, condensation, membrane separation, oxidation and biological treatment, among which adsorption is the most applicable technology because of the flexibility of the system, low energy, and inexpensive operation costs. Activated carbon has long been recognized as the most versatile adsorbent due to its low cost and excellent adsorption capacity. However, several drawbacks, such as hygroscopicity, pore clogging,and low thermal stability are associated with its use in adsorption processes. Hence, extensive efforts have been focused on finding alternative adsorbents. Zeolites are microporous molecular sieves with crystalline structures, high specific surfaces and thermal stabilities, have been proposed as potential adsorbents for VOCs adsorption/separation. In this study, zeolite Y and ZSM-5 were exchanged with different cations, including sodium, silver, copper and hydrogen or been treated with acid and base solution to create mesopores in zeolite structure. Futhermore, we synthesized zeolites with different amounts of Ti/Si ratio and used as the adsorbents of VOCs, including toluene, methylcyclopentane and crotonaldehyde. The materials were characterized by X-ray diffraction, nitrogen sorption, scanning electron microscope and elemental analysis. The uptakes of toluene, methylcyclopentane and steam over these zeolites were investigated by temperature programmed desorption. We found that under same experiment conditions, the amounts of cations was the key point to determine the amount of toluene adsorption; then was cation form; only when the adsorbent had similar cation amount and same cation form, the effect of specific surface area would show up. In the experiment of methylcyclopentane adsorption, we found that when the size of adsorbate molecules were similar to the pore size of adsorbent, adsorption capacity would increase and become hard to drsorb. In the experiment of crotonaldehyde adsorption, Y-type and ZSM-5 with silver, copper and hydrogen form would promote oxidation/cracking of crotonaldehyde, TS-1 only shows the adsorption/desorption peak without other reactions happened, the adsorption capacity was around 6 ~ 7 wt%. If we want to adsorption VOCs in dry condition, Na-Y3B is the best choice, because it has the largest adsorption for both toluene (21.0 wt%) and methylcyclopentane (13.8 wt%). H-Z120 is suitable for adsorption VOCs in normal condition, due to its hydrophobicity, has the highest toluene/steam selectivity (30.3) and methylcyclopentane/steam selectivity (44.8).