- 學位論文
類沸石咪唑骨架結構材料之合成、特性鑑定及其儲氫能力之研究
Preparation, Characterization, and Hydrogen Storage Capacity of Zeolitic Imidazolate Frameworks
摘要
由於全球暖化及地球石油儲藏量即將告竭，世界各研究單位正在積極開發新型乾淨之氫能源，以代替石化燃料，但因儲存的問題，使得氫氣的應用性受到相當大的限制。為達成此目標，安全的儲放氫氣變成為一個必要的標準。因此本論文的研究課題即是以微孔的金屬-有機架構物，又稱為類沸石咪唑骨架結構材料(Zeolitic Imidazolate Frameworks, ZIFs)當作吸附材，藉由鍵結力強的金屬-氧-碳原子形成之化合物具有高孔洞性、高比表面積、熱穩定性佳及氣體儲存的能力等特性之高性能特殊材料，提供更加適合的氫氣存放點，並開發新穎的ZIFs儲氫材料，ZIFs之合成方法、精細結構或特性及其儲氫能力，更進一步以XRD、FE-SEM、FTIR、TGA技術來分析鑑定。 實驗部份主要包括利用金屬(Zn)硝酸鹽類作為合成原料，提供之金屬配位，並可以連接相同或不同之有機配基；合成反應溫度範圍在110~180℃之間，並於溶劑狀態下反應。所合成之ZIFs稱為ZIF-7及ZIF-8。FE-SEM分析結果顯示，顆粒大小分別為1~3 μm及2~6 μm；最初合成之產物會具有許多不純物質而不具孔洞性，為使其產生孔洞性及高比表面積，必須經過高溫煆燒或溶劑清洗之處理程式，可以清除孔洞中之有機物雜質而使孔洞顯現出來，ZIFs系列必須利用丙酮或其他有機溶劑將孔洞內之難揮發有機溶劑進行交換，再經由Ar帶走及在真空烘乾下去除後生成，才具有孔洞性。經過適當處理之ZIF-7及ZIF-8，比表面積分別為807及1,129 m2/g，從吸/脫附曲線判斷皆為Type I，孔徑分佈圖顯示ZIFs具有微孔材料之特性。當其如果再空氣中暴露過久會與空氣中的水氣吸收而造成孔洞容易被阻塞而造成比表面積下降。EDS分析指出，ZIFs成分中含有C、O以及不同金屬的成份；XRD圖譜亦表示ZIFs具有良好之結晶性；FTIR圖譜則在3,400-2,200cm-1 之間顯示了咪唑化物在鍵結前後的變化，顯示咪唑化合物與金屬原子產生了鍵結，而其他官能基的訊號則完整的被保留下來。TGA分析結果顯示ZIFs具有較一般之有機化合物優異之熱穩定性，並可達到300~500℃， 此外，亦利用X光吸收邊緣結構光譜(XANES)及延伸X光吸收細微結構光譜(EXAFS)，來進一步分析ZIF-7和ZIF-8的精細結構，由XANES分析指出ZIF-7與ZIF-8主要為Zn(II)的成份；EXAFS數據結果顯示ZIF-7第一層之Zn-O鍵結之鍵長為2.01#westeur006#，配位數為3.3，而ZIF-8第一層之Zn-O鍵結之鍵長為1.95#westeur006#，配位數為3.52，另以高壓熱重分析儀測量ZIF-7和ZIF-8在室溫及450 psig (30 atm)下儲氫量分別為0.18和0.25wt%。氫氣溢流法探討ZIF-7和ZIF-8與5%之Pt/AC、Pd/AC混合，ZIF-7和ZIF-8表面經由碳鍵結形成的碳橋在室溫及450 psig (30 atm)下的儲氫量可提升至0.34 wt%與0.41wt%。此外，本研究亦以熱力學推導印證熱力學一致性，吸附熱其主要會影響低壓時的吸附行為，吸附熱的大小會從低吸附量慢慢往高吸附量遞減，可以發現ZIF-7和ZIF-8表面經由碳鍵結形成碳橋之吸附熱在低壓時吸附量可達到4~7kJ/mol。
並列摘要
Both from the point of view of global warming and from that of the inevitable exhaustion of Earth’s oil reserve, worldwide interest is focused on using a clean burning substitute such as hydrogen in place of fossil fuels. However the storage of hydrogen is one of the most important challenges impeding its practical application. (ZIFs) are a new emerging class of crystalline porous materials, displaying very low density, significant thermal stability, and very high surface area. They offer significant opportunities for hydrogen storage. Therefore, the main objectives were to develop and investigate the synthesis methods, fine structural characterization, and capacity of hydrogen storage of ZIFs using XRD, FE-SEM/EDS, TEM, BET, TGA, ESCA, and XANES/EXAFS techniques. Experimentally, ZIFs were synthesized with zinc nitrates in the presence of different solvents combined with organic linkers. Followed by refluxing the solution method was used to synthesize the ZIFs with the reaction temperatures range from 110 to 180oC. These ZIFs were named as ZIF-7 and ZIF-8 having the particle size about 1~3 and 2~6 μm, respectively identified by FE-SEM microphotos. Since as-synthesized ZIFs contain many impurities, it may cause low porosity. Therefore the cleaning methods, such as optimum calcination temperatures or washing several times with different solvents at different warm temperatures were effective and approved to improve higher specific surface area and porosity. The specific surface area of ZIF-7 and ZIF-8 were 807 and 1,129 m2/g, respectively. N2 adsorption isotherms of ZIFs were type I. The distribution of pore diameter curves revealed that ZIFs were microporous materials. The XRD patterns represented that ZIFs had well crystallinity after chemical treatment. EDS data indicated that ZIFs consist of C, O elements and different kinds of metals. FTIR spectra exhibited the azolate signal difference before and after bonding with metal ions locating in wavenumber 3,400-2,200 cm-1 region .It proved the bonding between azolates and metal ions established and another functional group signals were remaining. TGA curves showed that these ZIFs were stable around 300~500oC. XANES/EXAFS spectroscopy was performed to identify the fine structures of ZIF-7and ZIF-8. The XANES spectra indicated that the valence of ZIF-7 and ZIF-8 was Zn(II). The EXAFS data also revealed that ZIF-7 had a first shell of Zn-O bonding with bond distance of 2.01 #westeur006# and the coordination number was 3.3, and ZIF-8 had a first shell of Zn-O bonding with bond distance of 1.95 #westeur006# and the coordination number was 3.2. The hydrogen storage capacity of ZIF-7 and ZIF-8 were 0.18 and 0.25 wt%, respectively at 450 psig (30 atm) and room temperature measured using high-pressure thermogravimetric analyzer. The hydrogen storage capacity of ZIF-7 and ZIF-8 were 0.18 and 0.25 wt%, respectively at 450 psig (30 atm) and room temperature measured using high-pressure thermogravimetric analyzer. The hydrogen adsorption capacity of modified ZIF-7 and ZIF-8 was significantly enhanced up to 0.34 and 0.41 wt% by using the secondary spillover by carbon bridges measured at 450 psig and room temperature. In addition, the adsorption thermodynamic of the data was also confirmed using thermodynamic equations for thermodynamic consistency. Under lower pressures, the adsorption heat is affected by adsorption behaviors. The adsorption heats decrease of increasing adsorption capacities. The adsorption heat of hydrogen onto modified ZIF-7 and ZIF-8 was 4~8 kJ/mol using the secondary spillover of carbon bridges under lower pressures.