金屬-有機架構物之合成方法及結構特性之研究

Synthesis and Structural Characterization of Metal-organic Frameworks

辜吉男(Chi-Nan Ku)

指導教授 ： 林錕松

元智大學/工程學院/化學工程與材料科學學系/碩士(2007年)

https://doi.org/10.6838/YZU.2007.00017

摘要

摘要
氫氣是種理想的潔淨燃料，其燃燒過程並不會產生污染產物，但對於氫氣使用而言，氫氣的儲存技術更是相當重要，其中微孔的金屬-有機架構物，又稱為無機-有機共價化合物(MOFs)，乃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，深具儲存氫氣之潛力及後續在燃料電池供氫系統之應用。因此，本研究之主要目的為開發新穎的MOFs儲氫材料，MOFs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、TEM、BET、FTIR、TGA、ESCA或XANES/ESAFS技術來分析鑑定。
實驗部份主要包括利用不同的金屬(Al、Cu、Cr或Zn)硝酸鹽類作為合成原料，提供之金屬配位中心，並可以連接不同之有機配基；合成反應溫度範圍在110~220oC之間，並於不同的溶劑狀態下反應。所合成之MOFs稱為MIL-53-Al、MIL-53-Cr、MIL-69、Cu-BTC及IRMOF-1。FE-SEM分析結果顯示，顆粒大小分別為1~4、1~7、1~4、10~50及50~300 μm；最初合成之產物會具有許多不純物而不具孔洞性，為了使其產生孔洞性及高比表面積，MIL系列之化合物必須經過高溫煅燒之處理程序，可以清除孔洞中之有機物雜質而使孔洞變大；Cu-BTC可利用酒精及水等比例之溶劑清洗，以利去除多餘有機殘餘物；IRMOF-1必須利用丙酮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丙酮後生成。經過適當處理之MIL-53-Al、MIL-53-Cr、MIL-69、Cu-BTC及IRMOF-1，比表面積分別為1029、659、132、1660及1884 m2/g，從吸/脫附曲線判斷皆為Type I，孔徑分佈圖顯示MOFs具有微孔材料之特性；XRD圖譜亦表示MOFs具有良好之結晶性；EDS分析指出，MOFs成分中含有C、O以及不同金屬的成份；FTIR光譜得知MOFs於波長1400~1700 cm-1之C-O官能基及因為水氣造成3000~3500 cm-1處，而有譜線加寬之現象；TGA分析結果顯示MOFs具有較一般之有機化合物優異之熱穩定性，並可達到300~400oC。更應用X光吸收邊緣結構光譜(XANES)及X光吸收邊緣結構光譜分析光譜(EXAFS)，來進一步分析Cu-BTC及MIL-53-Cr的精細結構；XANES分析指出Cu-BTC及MIL-53-Cr主要分別為Cu(II)及Cr(III)成份；EXAFS數據結果顯示Cu-BTC及MIL-53-Cr第一層之Cu-O及Cr-O鍵結之鍵長分別為1.94及1.96 Å，配位數分別為4及5。
為了研究及改進MOFs之儲氫能力，進一步合成金屬/活性碳(metal/AC)混合之MOFs。BET表面積分析結果得知，AC、acid-treatment AC、Pt/AC及Pd/AC表面積分別為1039、1108、739及882 m2/g，其中acid-treatment AC表面積最高。FE-TEM結果可知，Pt/AC及Pd/AC之顆粒大小分佈分別為2~3及5~10 nm；EDS亦顯示材料中具有Pt及Pd之成份；此外，亦利用ESCA及XANES分析，價數近似Pt(0)及Pd(0)；EXAFS分析證實出Pt/AC及Pd/AC之第一層鍵結Pt-Pt及Pd-Pd鍵長分別為2.76及2.74 Å，因與周圍8個原子鍵結成體心立方結構，故其配位數為8。高壓重量熱損失法量測MIL-53-Al儲氫量為0.65 wt%，而使用體積法則測得Cu-BTC儲氫量可達到0.35 wt%。

關鍵字

金屬-有機架構物 ； 無機-有機共價化合物 ； 儲氫能力 ； 燃料電池 ； 孔洞材料

並列摘要

ABSTRACT
Hydrogen is a clean power source, it provides energy without producing any pollutions. For the utilization of hydrogen, hydrogen storage technology is quite important. Recently, microporous metal-organic frameworks (MOFs) are crystalline compounds, so called inorganic-organic colvent compound. Highly porous frameworks held together by strong metal–oxygen–carbon bonds and with exceptionally large surface area, thermal stability, and capacity for hydrogen storage. Furthermore, the hydrogen uptake is correlated with the specific surface area for crystalline microporous material of MOFs. Therefore, the main objective of the present study was to develop new synthetic routes for MOFs hydrogen storage materials. The fine structural characterization of MOFs and the capacity of hydrogen storage were also performed by XRD, FE-SEM, TEM, BET, TGA, ESCA or XANES/EXAFS technique.
Experimentally, MOFs were synthesized with different metal nitrates and combined with different organic linkers, reaction temperatures range of 110 to 220oC, and different solvents. These MOFs named MIL-53-Al, MIL-53-Cr, Cu-BTC, MIL-69, and IRMOF-1 have the particle sizes were 1~4, 1~7, 1~4, 10~50, and 50~300 μm, respectively by FE-SEM microphotos. Since as-synthesized MOFs having many impurities that may cause low porosity, therefore the cleaning methods to improve the specific surface area and porosity included MIL series calcined at high temperature, Cu-BTC washed with ethanol/water several times or IRMOF-1 immersed in the acetone and dried under Ar atmosphere. The specific surface area of MIL-53-Al, MIL-53-Cr, Cu-BTC, MIL-69 and IRMOF-1 were 1029, 659, 132, 1660, and 1884 m2/g, respectively. Isothermal adsorption/desorption curves of MOFs were all type I, the distribution of pore diameter curves revealed that MOFs were microporous materials. The XRD patterns of MOF showed that well crystallinity of MOFs after treatment was found. EDS data indicated that MOFs consist of C, O elements and different kinds of metals. FTIR spectra exhibited vibrational bands in the usual region of 1400~1700 cm-1 for the carboxylic function and 3000~3500 cm-1 for H2O of these MOFs. TGA curves showed that these MOFs were more stable around 300~400oC than other organic compounds. XANES/EXAFS spectroscopy was performed to identify the fine structures of Cu-BTC and MIL-53-Cr. The XANES spectra indicated that the valencies of Cu-BTC and MIL-53-Cr were Cu(II) and Cr(III), respectively. The EXAFS data also revealed that Cu-BTC and MIL-53-Cr have a first shell of Cu-O and Cr-O bonding with bond distances of 1.94 and 1.96 Å, respectively. The coordination numbers of Cu-BTC and MIL-53-Cr were 4.and 5, respectively.
In order to investigate and improve the hydrogen storage capacity of MOFs, metal/activated carbons mixed with MOFs were thus prepared. The BET surface area of AC, acid-treatment AC, Pt/AC and Pd/AC were 1039, 1108, 739 and 882 m2/g, respectively. FE-TEM microphotos of Pt/AC and Pd/AC indicated that the particle sizes were 2~3 and 5~10 nm, respectively. The EDS data showed that Pt/AC and Pd/AC are composed of Pt and Pd nanoparticles, respectively. By using ESCA and XANES spectra, the valancies of Pt and Pd species were both zero. The EXAFS data revealed that Pt/AC and Pd/AC have a first shell of Pt-Pt and Pd-Pd bonding with bond distances of 2.76 and 2.74 Å, respectively. Coordination numbers of both nanoparticles with BCC structures are close to 8. The hydrogen storage capacity of MIL-53-Al was 0.65 wt% by using high-pressure thermogravimetric analysis. Similarly, the volumetric analysis of hydrogen storage capacity for Cu-BTC was 0.35 wt%.

並列關鍵字

Metal-organic Framework ； Inorganic-organic Colvent Compound ； Hydrogen Storage ability ； Fuel Cell ； Porous Material

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