Adsorption Characteristics of MOF-199 And MOF-5 Prepared Using Renewable Template

Transcription

1 The 3rd National Graduate Conference (NatGrad5), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 5. Adsorption Characteristics of MOF-99 And MOF-5 Prepared Using Renewable Template N.S Jamaludin H. Misran N. Mahadi S.Z Othman Abstract- In this work, the adsorption characteristics of metal-organic frameworks (MOFs) with two different types of nanoporous structure were analyzed. MOF-99 consists of copper-based metal clusters and organic linkers. It has octahedral morphology. MOF-5 consists of zinc-based metal clusters and organic linkers that have cubic morphology. Five samples of MOF-99 materials and three samples of MOF-5 that were successfully synthesized using non-surfactant template (fatty alcohols) from our group [- ] were analyzed using nitrogen adsorption analysis. The adsorption-desorption isotherms of MOF-99 and MOF-5 exhibited Type I isotherm generally observed for microporous materials suggesting that the material produced in this study are of microporous (pore size less than nm) nature. The surface area of MOF-99 estimated using BET method and Langmuir method were at ca. 3-7 m /g and MOF-5 were at ca. 5-5 m /g. The total pore volume obtained at P/P =.99 for MOF-99 were at ca..5 to.49 cm 3 /g and MOF-5 were at.9 to.4 cm 3 /g. The average pore widths of MOF-99 were at ca..53 nm to.56 nm and MOF-5 were at.6 to.53nm. The surface area and pore volume of the materials decreased with longer straight chain fatty alcohol possibility due to mobility of the long carbon chain. Keywords- Microporous, metal-organic framework, MOF- 99, MOF-5, adsorption characteristics I. INTRODUCTION In recent years, metal-organic frameworks (MOFs) which are constituted by metal ions and organic linkers have received much attention as new hybrid materials that show unique features such as versatile structures, tunable porosity and interesting physicochemical properties. Moreover, MOFs are tailorable materials such that the structure and properties can be rationally designed and improved through various ways. As such, they possess an extensive range of potential applications including materials for gas storage, gas separation, catalysis, luminescence, and drug delivery [3-4]. In 999, Chui et al. discovered MOF-99 which also known as HKUST-. MOF-99 is a highly porous MOF with the formula [Cu 3 (BTC) (H O) 3 ] n derived typically from ethanol/water media at 453K. In 6, Mueller et al. developed an electrochemical synthesis route for BASF. MOF-99 has interconnected [Cu (O CR) 4 ] units (R: an aromatic ring), which create a face-centered cubic crystal (Fig. ) built of large octahedral secondary building unit (SBU, yellow) that is, constructed by placing four BTCs at four alternated faces of the eight triangular faces with Cu dimers at the six vertices. These SBUs are assembled by sharing vertices (direct edge sharing of the Cu Cu metal bond) to form an intersecting 3D system of large squareshaped pores (9 9Å) with accessible porosity of about 4% in the solid. The aqua ligands (water) can be removed to leave open copper sites that can be occupied by pyridines [5-6]. Meanwhile, formerly defined by Yaghi et al. in 999, MOF-5 consists of Zn 4 O units connected by linear,4- benzenedicarboxylate (BDC) struts to form a cubic network (Fig. ). The core of the cluster consists of a single oxygen atom bonded to four zinc atoms, forming a regular Zn 4 O tetrahedron whose edges are capped by BDC carboxylic groups to form Zn 4 O(CO ) 6 clusters. [7-8]. 9

2 The 3rd National Graduate Conference (NatGrad5), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 5. feasible as meta-atom. Such morphologies include nanotubes, nanospheres and nanoplates resulted from manipulating the reaction condition, concentration ratios, as well as the starting raw materials compositions. The use of surfactants at high concentration (liquid crystal and micellar solution) as structure directing agents in carbon- templating method to synthesize porous silica spheres materials were extensively studied and had become general procedure since their first discovery []. However, surfactants are expensive and less environmental-friendly and the extensive use will result in abundance of undesired scheduled liquid waste. Therefore, by substituting surfactants with simple hydro- carbons derived from biomass, greener and low-cost synthesis method can be achieved []. The surfactants were substituted with palm oil derived fatty alcohols bearing several carbon chain lengths from C8 to C6 (octyl, decyl, dodecyl, myristyl, and cetyl alcohols) to generate carbon template in situ for the formation of silica spheres. Fig.. Crystal structure of MOF-99 (cavities, yellow and blue balls) [5]. In this work, the samples of MOF-99 and MOF-5 that had been successfully synthesized using non-surfactant method by Mahadi et.al [9] and Misran et.al [] were further analyzed to get the porosity characteristics of the materials. II. EXPERIMENTAL DETAILS Fig.. Crystal structure of MOF-5 [7]. MOFs materials specifically MOF-99 and MOF-5 are widely studied for the unique features such as large surface area, high porosity, low density, good thermal stability, high chemical stability and stable and have high affinity towards water molecules. Thus these materials become excellent candidates for various application including hydrogen and other gases storage, gas purification, catalyst and drug delivery as it has the ability to store the guest molecules inside porous structure [9-]. Five samples of MOF-99 analyzed in this work are copper-based metal clusters and organic linker. The samples used were MOFM-NO FA (without addition of fatty alcohol), MOFM-C8 (addition of octyl), MOFM- (addition of decyl), MOFM-4 (addition of dodecyl), and MOFM-6 (addition of myristyl). Meanwhile, for MOF-5, the samples tested were zinc-based metal clusters and organic linkers, namely MOFN (addition of dodecyl), MOFN4 (addition of myristyl) and MOFN6 (addition of cetyl). All samples were characterized by nitrogen adsorption-desorption analysis using Quantachrome Autosorb iq, that were done at 77K at liquid nitrogen temperature. Prior to nitrogen adsorption, the samples had undergone pre-treatment process in the oven for overnight. After that, the samples undergone 3-steps degassing process for hours. The textural properties such as surface area, total pore volume and average pore width were calculated according to the well-known calculation methods such as the Brunauer Emmett Teller (BET), the Langmuir, and the Horvath Kawazoe (HK). Since their discoveries, MOF-99 and MOF-5 were synthesized from various routes to optimize the production yielded and enhancement of unique features such as porosity characteristics. The properties of this microporous material were highly depending on the synthesis method, reaction temperature, ratio of organic to inorganic materials and amount of solvents used [9]. By using carbontemplating synthesis methods, various morphologies of the nanostructured inorganic oxides were obtained which are III. A. Adsorption Characteristic RESULTS AND DISCUSSION The adsorption-desorption isotherms of MOF-99 in this study are shown in Fig. 3. The closed symbol resembles adsorption isotherm and open symbol resembles desorption 93

3 Adsorbed N amounts (cm 3 /g) Adsorbed N amounts (cm 3 /g) The 3rd National Graduate Conference (NatGrad5), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 5. isotherm. The properties of the structures were determined by nitrogen N adsorption-desorption analysis at 77K liquid nitrogen temperature. Samples of MOF-99; MOFM-NO FA, MOFM-C8, MOFM-C and MOFM-C4 exhibited Type I isotherms suggesting the nature of the microporous materials. The main feature of a reversible Type isotherm is the long horizontal plateau, which is indicative of a relatively small amount of multilayer adsorption on the open surface. Micropore filling may take place either in pores of molecular dimensions (i.e primary micropore filling) at very low P/P or in wider mic) over a range of higher P/P [9]. However, for MOFM-C6 sample, the isotherm shown did not obey Type The nitrogen adsorption isotherms of MOF-5 samples; MOFN, MOFN4 and MOFN6, are shown in Fig. 4. The isotherms observed for MOFN and MOFN4 although essentially Type, the isotherms reveal some degree of thermodynamic irreversibility with hysteresis extending back to very low P/P. This form of hysteresis cannot be attributed to capillary condensation and instead indicative of a more complex change in the adsorption system. The adsorption isotherm exhibited Type H3 hysteresis loop. Type H3 does not terminate in a plateau at high P/P and the limiting desorption boundary curve is more difficult to establish. Type H3 loops are usually given by the aggregates of platy particles or adsorbents containing slit-shaped pores. Meanwhile for MOFN6, the isotherm is resembles Type II with Type H3 hysteresis loop. This suggests the sample did not exhibit microporous phase yet only occur in mesoporous phase. [,]. 5 MOFM-NO FA MOFM-C8 MOFM-C MOFM-C4 MOFM-C Relative pressure, P/P Fig. 3. Nitrogen N adsorption-desorption isotherms of MOF-99 samples B. Porosity characteristic 8 MOFN Table shows the porosity characteristics of MOF-99 samples obtained by nitrogen adsorption. The surface area estimated using Brunauer- Emmett- Teller (BET) method for mesopore structure at ca m /g, and Langmuir method for micropore structure at ca. 3-7 m /g. The sample prepared without the addition of fatty alcohol exhibited higher surface area due to the existence of nondefined amorphous structure. The different length of straight carbon chains in fatty alcohol added in the synthesis method affected the adsorption-desorption ability by each sample. The average pore width of MOF-99 samples was estimated using Horvath- Kawazoe (HK) method at ca.53 to.56 nm. The pore size distribution determined by the HK method evidently shows that the MOF-99 possesses a distribution ranging from.37 nm to.86 nm with maxima peaks centered at ca.53 nm to.56 nm (Fig. 5). MOFN4 6 MOFN Relative pressure, P/P Fig. 4. Nitrogen N adsorption-desorption isotherms of MOF-5 samples 94

4 Pore Volume (cm 3 /g.nm) Pore Volume (cm 3 /g.nm) The 3rd National Graduate Conference (NatGrad5), Universiti Tenaga Nasional, Putrajaya Campus, 8-9 April 5. Table : Porosity characteristics of MOF-99 Sample name S BET S LANGMUIR Total pore volume (cm 3 /g) Average pore width 3 (nm) MOFM-NO FA MOFM-C MOFM-C MOFM-C MOFM-C n/a.. MOFN MOFN4 3 By BET plot, By Langmuir plot, 3By HK method MOFM-NOFA...5. Pore Width (nm) Fig.6: Nitrogen N adsorption-desorption isotherms of MOF-99 samples. Fig.5. Nitrogen N adsorption-desorption isotherms of MOF-99 samples Table shows the porosity characteristics of MOF-5 samples obtained by nitrogen adsorption. The surface area estimated using Brunauer- Emmett- Teller (BET) method for mesopore structure at ca. -84 m /g, and Langmuir method for micropore structure at ca. 5-5 m /g. Table : Porosity characteristics of MOF-5 Sample name MOFM-C8 MOFM-C MOFM-C4.5 Pore Width (nm) S BET S LANGMUIR Total pore volume (cm 3 /g) By BET plot, By Langmuir plot, 3By HK method Average pore width 3 (nm) MOFN MOFN MOFN n/a The pore size distribution determined by the HK method evidently shows that the MOF-5 possesses a distribution ranging from.9 nm to.86 nm (Fig. 6). The average pore widths of MOF-5 samples were estimated using Horvath- Kawazoe (HK) method at ca.6 and.53 nm. IV. CONCLUSIONS In this study, we examined the adsorption characteristics of nanoporous MOF-99 and MOF-5. Samples of MOF-99 and MOF-5 exhibited Type I isotherms suggesting the nature of the microporous materials. In addition, the samples also possess high surface area. Thus, fatty alcohols were suggested to be suitable renewable template in the synthesis of MOF-99 and MOF-5. ACKNOWLEDGMENT The authors thank financial support from Ministry of Education under 9ERGS Grant and 36FRGS Grant scheme. REFERENCES. H. Misran, N. Mahadi and S.Z. Othman, Single Crystal Morphology of Metal-Organic Frameworks (MOF-99 and MOF-5) using Renewable Fatty Alcohol, Malaysian J. Microsc. 5, in press.. H.Misran, S.Z. Othman, A. Manap, N.I.M Pauzi and S.Ramesh, Nonsurfactant Synthesis and Characterizations of Metal-Organic Framework MOF-5 Materials Using Fatty Alcohol, Sci. Adv. Mater., 6-7 (4) R. J. Kuppler, D. J. Timmons, Q.-R. Fang, J.-R. Li, T. a. Makal, M. D. Young, D. Yuan, D. Zhao, W. Zhuang, and H.-C. Zhou, Potential applications of metal-organic frameworks, Coord. Chem. Rev., vol. 53, no. 3 4, pp , Dec D.-S. Chen, J.-M. Cheng, L.-B. Sun, Z.-Q. Liang, K.-Z. Shao, C.-G. Wang, H.-Z. Xing, and Z.-M. Su, A new porous D copper(ii) metal organic framework for selective adsorption of CO over N, Inorg. Chem. Commun., vol. 38, pp. 4 7, Dec Chui, S. S. Y., Lo, S. M. F., Charmant, J. P. H., Orpen, A. G., & Williams, I. D. (999). A chemically functionalizable nanoporous material [Cu3(TMA)(HO)3]n. Science, 83(545), B. Xiao and Q. Yuan, Nanoporous metal organic framework materials for hydrogen storage, Particuology, vol. 7, no., pp. 9 4, Apr

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