Synthesis of Peptide-based Porous Materials

Integrated Master in Chemical Engineering Synthesis of Peptide-based Porous Materials Master’s Thesis by Seyedali Emami Supervisor: Prof. Adélio Mendes Co-Supervisor: Prof. Luis Gales Department of Chemical Engineering July 2013 "تر شوخ مدیدن قشع نخس زا دنامب راود دبنگ نیا در که یراگدای" ظفاح “I have never seen a more beautiful reminder Than the words of love that linger in this turning dome” -Hafez (1325-1389 C.E.) Synthesis of Peptide-based Porous Materials i Acknowledgments I would like to thank my advisors Prof. Adélio Mendes and Prof. Luis Gales for their invaluable supports and encouragement on this thesis. My special gratitude goes to Prof. Gales for his assistance and guidance during every single moment of the work, without his endless support this research would not be a success. I would also like to express my special gratitude to Dr. Filipe Paz for collecting x-ray diffraction data and solving the crystal structure of my sample at European Synchrotron Radiation Facility (ESRF). Special thanks to Joana Durão for her great assistance, guidance and valuable time spent with me during adsorption measurement of my samples. I am very indebted to Narges and Reza, my lovely sister and her husband. I love you dearly and I am grateful for your wonderful support. Reza’s infinite support and guides during my stay in Porto cannot be described in words, thank you for being there for me. I would also like to acknowledge my best friend, Behdad for his help and guidance to organizing my work, thank you so much. Finally, I would not be who I am without my parents love and support. My biggest thanks and love to them, which I truly believe they deserve most of the credit for this thesis. Synthesis of Peptide-based Porous Materials ii Synthesis of Peptide-based Porous Materials iii Abstract Metal organic frameworks attract significant attention during the last decades, due to their wide range application in gas storage, adsorption, and drug delivery. In this work, we synthesized two new metalpeptide frameworks (MPF) through assembling GlyAsp dipeptide with Zn(II) and Co(II) metal ions. The structures of Zn and Co porous frameworks exhibited two-dimensional and three-dimensional topologies, respectively. In fact, cobalt-based framework is among the few examples of reported MPFs, which has a 3-D structure. Thermogravimetric analysis of these compounds showed thermal stability up to 250 oC, which is a decent stability compared to materials with similar structure. Keywords: Metal-peptide framework (MPF), Metal-organic framework (MOF), Porous material, GlyAsp, CO2 Adsorption. Synthesis of Peptide-based Porous Materials iv Synthesis of Peptide-based Porous Materials v Resumo As estruturas de materiais híbridos metal-orgânico têm atraído uma atenção significativa nas últimas décadas, devido à sua ampla gama de aplicações que vão desde a concentração/separação de gases por adsorção à utlização como veículos para transporte de fármacos. Neste trabalho, sintetizou-se duas novas estruturas de metal-peptídeo (MPF) através da combinação do dipéptido GlyAsp com Zn(II) e Co(II). As estruturas exibem tipologias bidimensionais e tridimensionais, respetivamente. Na verdade, a estrutura de base de cobalto é um dos poucos exemplos de MPFs reportados, que tem uma estrutura 3D. A análise termogravimétrica destes compostos mostrou uma estabilidade térmica superior à da maior parte dos materiais desta classe, e que se situa acima dos 250 oC. Synthesis of Peptide-based Porous Materials xii Synthesis of Peptide-based Porous Materials xiii Nomenclature Symbol Description Default Unit t time s P pressure bar V volume cm3 m mass g ρ density g.cm-1 Mw molecular weight g.mol-1 n molar number mol R gas constant cm3.bar.K-1.mmol-1 nads molar number adsorbed mmol T temperature K q adsorption uptake mol.kg-1 Synthesis of Peptide-based Porous Materials xiv Synthesis of Peptide-based Porous Materials xv Acronyms and Abbreviations AA Amino Acid Ac Acetyl Ala Alanine Arg Arginine Asp Aspartic acid BDC 1,4-benzenedicarboxylic acid BDC-(OH)2 2,5-dihydroxybenzenedicarboxylate BPB 1,2-bis(4’-pyrazolyl)benzene BPE 1,2-bis(4-pyridyl)-ethane BTC 1,3,5-benzentricarbozylate DMF Dimethylformamide DPNI N,N’-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydimiimide DSCP c,c,t-Pt(NH3)2Cl2(OOCCH2CH2CO2H)2 EDS Energy Dispersive Spectrometry Glu Glutamate Gly Glycine HEPES 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid HKUST Hong Kong University of Science and Technology IBU Ibuprofen INA Isonicotinic acid IUPAC International Union of Pure and Applied Chemistry MBioF Metal-biomolecule Framework MEA Membrane Electrode Assembly MIL Materials of Institut Lavoisier MOF Metal-organic Framework MPF Metal-peptide Framework NCP Nanoscale Coordination Polymer NDC 2,6-naphthalendicarboxylate ORR Oxygen Reduction Reaction Synthesis of Peptide-based Porous Materials xvi PEM Proton Exchange Membrane PSA Pressure Swing Adsorption SBU Secondary Building Unit SEM Scanning Electron Microscope TEOS Tetraethyl orthosilicate TG Thermogravimetric TGA Thermogravimetric Analysis Thr Threonine TSA Thermal Swing Adsorption WDS Wavelength Dispersive Spectrometry XRPD X-Ray Powder Diffraction ZIF Zeolitic Imidazolate Framework Synthesis of Peptide-based Porous Materials xvii List of Figures Figure 1.1-The MOF-5 structure shown as metal center cluster (ZnO4 tetrahedra) joined by organic linker (benzene dicarboxylate) to give an extended 3-D cubic framework. Yellow sphere represents the largest sphere that can occupy the pores without coming within the van der Waals size of the framework. ........ 2 Figure 1.2Examples of SBU from carboxylate MOFs. O, red; N, green; C, black. In organic units metal oxygen polyhedral are blue, and polygon defined by carboxylate carbon atoms (SBUs) are red. a) Triangle (three points of extension), b) Square paddle-wheel (four points of extension). .......................... 6 Figure 1.3Hydrogen adsorption isotherms for MOF-5 prepared without exposure to air. Data were measured at 77 K by Jeffrey R. Long et al. and are shown as excess (filled red squares) and total (filled blue triangles) uptake. Open circles indicates the volumetric capacity of hydrogen (right-hand scale).. ... 8 Figure 1.4Schematic illustration of selective gas adsorption in rigid MOFs (top: the molecular sieving effect, bottom: thermodynamic equilibrium effect). ................................................................................. 11 Figure 1.5CO2 and CH4 adsorption isotherm of dehydrated MIL-53 at 304 K. ......................................... 12 Figure 1.6-Schematic representation of a PEM fuel cell consisting of catalyst layers, gas diffusion electrodes and proton exchange membrane. ............................................................................................ 14 Figure 1.7Polarization curves for PEM fuel cell of H2/O2. State-of-the-art Pt-based cathode (0.3 mgPt cm2, green squares), Fe-based cathode catalyst synthesized using ZIF-8 (blue stars) and Fe-based cathode catalyst synthesized using high surface area carbon black instead of ZIF-8 (red circles). . ........................ 15 Figure 1.8Ibuprofen delivery for MIL-100 and MIL-101. .......................................................................... 17 Figure 1.9Pt% released profile for NCP-1, NCP-1’-a, and NCP-1’-b. ......................................................... 18 Figure 2.1Presentation of potential coordination modes for the amino group of GlyGly dipeptide; a) monodentate mode, b) five-membered chelate ring. ................................................................................ 23 Figure 2.2- (a) Structure of [Cd(AlaThr)2].4H2O. (b) View along the crystallographic b axis (blue networks shows the hydrogen bonds). ....................................................................................................................... 25 Figure 2.3- (a) Structure of [Zn(GlyAla)2] above 298 K. (b) Representation of 1-D pore along the c axis at 298K (after solvent evacuation). ................................................................................................................. 26 Figure 2.4- (a) Thermogravimetric analysis of [Zn(GlyAla)2].(solvent) (b) CO2 sorption isotherms of [Zn(GlyAla)2] at 273 K. ................................................................................................................................. 27 Figure 2.5- (a) Octahedral coordination of the peptide around Zn(II) ion of [Zn(GlyThr)2].MeOH. (b) Space-filling representation of [Zn(GlyThr)2], showing the 1-D pores along the a axis.. ............................ 27 Figure 2.6Thermogravimetric analysis of [Zn(GlyThr)2].MeOH.. ............................................................. 28 Synthesis of Peptide-based Porous Materials xviii Figure 2.7- (a) Selective sorption of CO2 (squares) over CH4 (triangles) of [Zn(GlyThr)2].MeOH at 273 K. (b) CO2 sorption isotherms of [Zn(GlyThr)2].MeOH at 273 K (squares) and 298 K (triangles). Filled and empty symbols showing the adsorption and desorption, respectively. ..................................................... 29 Figure 2.8Representation of the structures of GlyGlyGly (top) and AlaAlaAla (bottom) tripeptides. ..... 29 Figure 2.9- (a) Structure of [Cd(GlyGlyGly)2].H2O, showing a 3-D coordination of cadmium ions. (b) View along the c axis, showing an 8-membered rings formed by linking two Cd ions to two carboxylate groups. . ................................................................................................................................................................... 30 Figure 2.10- (a) Structure of [Cd(AlaAlaAla)2]. (b) View along the b axis, exhibiting 1-D pores. ................ 31 Figure 3.1Schematic representation of base diffusion method. .............................................................. 34 Figure 3.2Structure of GlyAsp crystal, oxygen (red), nitrogen (light blue), carbon (white), and hydrogen (light grey). .................................................................................................................................................. 35 Figure 3.3Energy dispersive spectrometry analysis of sample A. ............................................................ 36 Figure 3.4- (a) Microscopic and (b) SEM images of [Zn(GlyAsp)].H2O. ....................................................... 37 Figure 3.5- (a) Microscopic and (b) SEM images of [Co(GlyAsp)].H2O. ...................................................... 38 Figure 3.6Representation of the structure of GlyAsp............................................................................... 40 Figure 3.7Metal ions coordination modes for [Zn(GlyAsp)] (left) and [Co(GlyAsp)] (right). .................... 41 Figure 3.8Representation of Zn(II) ion coordination in [Zn(GlyAsp)]. ...................................................... 41 Figure 3.9Structure of [Zn(GlyAsp)].H2O; (a) Polyhedral representation, zinc (blue polyhedral), hydrogen (white), carbon (grey), oxygen (red), and nitrogen (light blue). (b) Stick representation, Zn-N bond (blue-light blue), Zn-O bond (blue-red), and water molecules (yellow). ........................................... 42 Figure 3.10Structure of [Co(GlyAsp)].H2O; (a) Polyhedral representation, cobalt (pink polyhedral), hydrogen (white), carbon (grey), oxygen (red), and nitrogen (light blue). (b) Stick representation, Co-N bond (pink-light blue), Co-O bond (pink-red), and water molecules (yellow). (c) Octacoordinated Co(II) ions in [Co(GlyAsp)].H2O complex. .............................................................................................................. 43 Figure 3.11Thermogravimetric analysis of [Zn(GlyAsp)].H2O (blue solid line). Green dashed line indicates the derivative weight loss (right-hand scale), and apparent weight losses are highlighted with red boxes. .................................................................................................................................................... 44 Figure 3.12Thermogravimetric analysis of [Co(GlyAsp)].H2O (blue solid line). Green dashed line indicates the derivative weight loss (right-hand scale), and apparent weight losses are highlighted with red boxes. .................................................................................................................................................... 45 Figure 3.13Schematic representation of volumetric adsorption apparatus. ........................................... 47 Synthesis of Peptide-based Porous Materials xix Figure 3.14Schematic representation of cumulative adsorption data collection for volumetric method. .................................................................................................................................................................... 48 Figure 3.15CO2 adsorption isotherm of [Co(GlyAsp)].H2O at 288 K. ........................................................ 50 Synthesis of Peptide-based Porous Materials xx Synthesis of Peptide-based Porous Materials xxi List of Tables Table 3.1Selected conditions of the experiments done through solvothermal synthesis method. ........ 35 Table 3.2Crystal data and structure refinement for [Zn(GlyAsp)].H2O and [Co(GlyAsp)].H2O. ................ 39 Table 3.3Lattice shape and symmetry operators for [Zn(GlyAsp)].H2O and [Co(GlyAsp)].H2O. .............. 40 Table 3.4CO2 amount adsorbed for [Co(GlyAsp)].H2O at 288 K. .............................................................. 49 Synthesis of Peptide-based Porous Materials Metal-organic framework 6 collapse. This collapse, releases the stored energy in the bubble with heating and cooling rate of >1010 Ks-1, temperatures of approximately 5000 K, and pressures of approximately 1000 bar[9]. 1.2. MOF structure Structure of metal-organic framework can be classified by their secondary building units (SBUs). SBU refers to the geometry of the units defined by the points of extension. However, the organic linker plays an important role in the topology of MOF, but SBUs dictate the final geometry of these materials. Figure 1.2 exhibits two secondary building units according to their inorganic units[5]. A complete review on the secondary building units according to their points of extension could be found in reference [10]. Figure 1.2Examples of SBU from carboxylate MOFs. O, red; N, green; C, black. In organic units metal oxygen polyhedral are blue, and polygon defined by carboxylate carbon atoms (SBUs) are red. a) Triangle (three points of extension), b) Square paddle-wheel (four points of extension)[5]. In 2008, Collins and co-workers proven that the geometry of the SBU is dependent on the characteristics such as structure of the ligand, type of metal, metal to ligand ratio, solvent, and the source of anions to balance the charge of the metal ion[11]. MOFs are mostly porous; which means that they have void spaces within their structure. According to IUPAC pores are classified by their size range, which is micropores (<2 nm), mesopores (2-50 nm), and macropores (>50nm). Mesoporous and macroporous are attractive for catalysis applications because of Synthesis of Peptide-based Porous Materials Metal-organic framework 7 their large pores. Microporous materials can be good candidates for gas storage and separation due to the strong interactions between gas molecules and the pore walls[1]. Commonly the pores of MOFs are filled with the solvent molecules, which must be removed for most applications. One of the possible problems in removal of guest molecule is structural collapse. Generally, in large pores the possibility of collapse is higher. 1.3. MOF applications Recently, metal-organic frameworks have received much attention owing to their wide range of applications. Being porous makes these materials interesting in areas such as hydrogen storage [12], gas adsorption and separation [13], catalyst [14], drug storage and delivery [1], and electrochemical [15]. 1.3.1. Hydrogen storage During past decades, researches focused to find suitable replacement for fossil fuels. An alternative fuel for automotive transportation with less carbon emissions become a first priority. Battery and fuel cell technologies are strong candidates to replace gasoline and diesel engines. In particular, hydrogen is an attractive energy carrier because it is carbon-free and abundantly available from water. Hydrogen must be compressed to very high pressures or stored cryogenically (at low temperature) [16]. Materials with large surface areas and low densities such as MOFs, porous carbons, zeolites and organic polymers, are attractive for hydrogen storage applications. Hydrogen storage capacity in these materials depends on their surface area and pore volume. The main limitation of MOFs usage in H2 storage is the weak van der Waals interaction energy between H2 and the surface of the material [12]. MOF-5 5 has been widely studied since, and turns out to be the best cryogenic storage material currently known. In 2003, the initial H2 storage data were reported for MOF-5 (4.5 wt% at 77 K and 1 atm), although it was found later that the maximum H2 uptake varies from 1.3 to 5.2 excess wt% at 77 K depending on the preparation and handling conditions. In 2010, Jeffrey R. Long et al. studied 6 references of MOF-5 preparation and handling conditions, and they suggested a new condition with better hydrogen uptake. They have minimized the exposure to water and air in their synthesis method, by which they have reached to one of the highest gravimetric capacity (7.1 excess wt% at 77 K and 40 bar) observed for a hydrogen storage material operating at 77 K [17]. Figure 1.3 shows the total and 5 Zn4O(1,4-benzene dicarboxylate)3 Synthesis of Peptide-based Porous Materials Metal-organic framework 8 excess uptake of hydrogen for this material with referred synthesis conditions. In the same year (2010), Omar K. Farha et al. simulated a new metal-organic framework with a structure similar to MOF-5. The simulation results showed the highest excess H2 storage capacity so far for MOFs with 99.5 mg/g at 56 bar and 77 K (NU-100) in Northwestern University. After reaching that high storage capacity through computational design, they synthesized the material successfully. Figure 1.3Hydrogen adsorption isotherms for MOF-5 prepared without exposure to air. Data were measured at 77 K by Jeffrey R. Long et al. and are shown as excess (filled red squares) and total (filled blue triangles) uptake. Open circles indicates the volumetric capacity of hydrogen (right-hand scale). Reprinted with permission from [17].Copyright 2007 American Chemical Society. Excess uptake is the amount of gas taken up in addition to what would be present in the container with a volume equal to the pore volume within the sample. Total uptake is the amount of gas contained within the volume of the crystals, with includes both surface-absorbed molecules and pressurized gas within the pores [17]. 1.3.2. Catalyst As mentioned before, one of the key features of the MOFs is their porosity; this characteristic makes metal-organic interesting in catalyst area. Zeolites are the most commercially important classes of catalyst. MOFs have some of the catalytically relevant specifications of zeolites such as large internal surface areas and uniform pore and cavity sizes. First, comparing to zeolites, MOFs can be synthesized in much chemical variety because they also contain organic base. Second, stability (depends essentially on the cation coordination) of MOFs is lower than zeolites, which makes them weak catalysts for reactions 0 10 20 30 40 50 60 70 80 90 0 1 2 3 4 5 6 7 8 9 10 11 12 020 40 60 80 100 120 140 160 180 Volumetric Capacity (g H2/L) H2 Uptake (%wt) P (bar) Synthesis of Peptide-based Porous Materials Metal-organic framework 9 requiring forcing conditions. Third, persistence of microporosity after solvent evacuation is essential for gas-phase catalysis, but some MOFs collapse when solvent is removed (the larger the pore, the more likely the collapse). However, for catalysis of condensed-phase reactions this feature is not essential [14]. Pore system of the MOFs ranges from ultramicroporous to mesoporous, which gives them a remarkable opportunity for catalysis. The variety choice of structure, which facilitates pore-size tenability, is a great opportunity for designing MOFs with pore openings appropriate for generating size and shape selectivity. In addition to pore size, different pore topologies can be found for MOFs. For example, the pore structure can be one-dimensional (1-D) with straight channels, 2-D, or 3-D [18]. In 2009, David Farrusseng and co-workers reviewed the state of the art of catalytic MOFs, and they reported application of metal-organic frameworks in areas such as Lewis acid catalysis, Brønsted acid catalysis, base catalysis, C-C bond formation, polymerization, etc. Omar K. Farha et al. studied metal-organic frameworks application as catalysts. They reported MOF usage in reactions catalyzed such as oxidation of olefin, oxidation of alkane, oxidation of sulfide, oxidation of polyphenol, oxidation of alcohol, reduction of nitroaromatic etc.[14] For example Y.Lu and co-workers examined the catalytic activity of [Co(BPB)].3DMF 6 for oxidation of cyclohexene by employing tert-butyl hydroperoxide as oxidant . They reported a fast and multiple turnover oxidation of cyclohexene in the presence of mentioned MOF, whereas no reaction occurs in the absence of catalyst under the same condition [19]. 1.3.3. Gas adsorption and separation Separation is a process that divides a mixture into its components, which is caused by a mass separating agent called adsorbent, or sorbent. Adsorptive gas separation includes passing a gas mixture through a columns packed with sorbent particles, or fixed-bed adsorbers to yield a product enriched in the more weakly adsorbed constituent. This is then followed by desorption of the strongly adsorbed component so that adsorbent can be reused. Separation is the opposite process of mixing and normally is not a spontaneous procedure. Selective adsorption leads to separation[20]. 6 Cobalt (II) 1,2-bis(4’-pyrazolyl)benzene in dimethylformamide. Synthesis of Peptide-based Porous Materials Metal-organic framework 10 Adsorptive gas separation processes can be divided into two types: bulk separation (adsorption of a significant fraction, 10% by weight or more from a gas stream) and purification (less than 10% 7 by weight of a gas stream is adsorbed)[20]. High separation power can be reached by continuous contact and equilibration between the gas and adsorbent, for this purpose cyclic process such as thermal swing adsorption (TSA) cycles, pressure swing adsorption (PSA) cycles are available[21]. Every component has its own adsorption capacity as an adsorbent. The difference between adsorption capacities is the foundation of gas separation in adsorptive separation. The performance of any adsorptive separation or purification process is directly determined by the characteristics of the adsorbents in both adsorption equilibrium and kinetics. In addition to suitable mechanical properties, a promising adsorbent should have favorable adsorption kinetics and regenerability as well as good adsorption capacity and selectivity. To satisfy these requirements, the adsorbent should possess not only reasonably high surface area, but also relatively large pore sizes for porous materials to allow adsorbate molecules to approach the interior surface[21]. There are four major mechanisms for gas adsorptive separation by a porous material which can be reached by one or several of these mechanisms[22][20]: (1) Molecular sieving effect; gas mixture components are either allowed or prevented from entering to the pores of an adsorbent, because of size and/or shape exclusion. The allowed components are subsequently adsorbed while the prevented components don’t adsorb. (2) Thermodynamic equilibrium effect; because of different adsorbate surface and/or adsorbate packing interaction, preferential adsorption of certain components over others occurs on the surface of and adsorbent. (3) Kinetic effect; because of different diffusing rates, certain components enter the pores and become adsorbed faster than other components. (4) Quantum sieving effect; because of quantum effect, some light molecules have different diffusing rates in narrow micropores, which allows such molecules to be separated. 7 Usually <2% Synthesis of Peptide-based Porous Materials Metal-organic framework 11 Figure 1.4Schematic illustration of selective gas adsorption in rigid MOFs (top: the molecular sieving effect, bottom: thermodynamic equilibrium effect)[21]. MOFs can be categorized into flexible and rigid. Flexible MOFs have dynamic, “soft” frameworks which are sensitive to external stimuli, such as temperature, guest molecules, and pressure, while rigid MOFs have relatively stable and robust porous frameworks with permanent porosity[23]. Metal-organic frameworks features such as large surface area, adjustable pore size and thermal stability, gives them a decent opportunity to be used as adsorbents for gas separation. Research on MOFs is in the early stage, and a few numbers of them have been tested for their adsorption properties. The adsorption selectivity in rigid MOFs may be related to the molecular sieving effect and/ or preferential adsorption based on the different strengths of adsorbent-adsorbate and adsorbate-adsorbate interaction. Figure 1.4, presents a schematic illustration of selective gas adsorption in rigid MOFs[21]. Synthesis of Peptide-based Porous Materials Metal-organic framework 12 Adsorption behavior of MIL-53 with chemical formula of Cr(OH)(BDC), which is a flexible MOF, has been studied by Bourrelly and co-workers. As synthesized MIL-53 has a molecule of water inside its structure, dehydration of this material can occur by heating it at temperature of 100 oC. Dehydrated MIL-53 shows a typical CH4 adsorption behavior for a microporous material, while the adsorption of CO2 reveals two steps (Figure 1.5). The adsorption of CO2 is higher than CH4, which is expected because CO2 has a significant quadrupole moment, whereas CH4 is nonpolar[24]. Figure 1.5CO2 and CH4 adsorption isotherm of dehydrated MIL-53 at 304 K. Reprinted with permission from [24].Copyright 2005 American Chemical Society. In 2008, Youn-Sang Bae et al. synthesized a MOF with formula of Zn2 (NDC)2(DPNI) 8 by two different routes: first at 80oC for two days with conventional heating, and second at 120oC for 1 hour using microwave heating. They reported a selectivity of 30 for CO2 over CH4 for the microwave sample, which is among the highest selectivities reported for this separation[25]. 8 NDC=2,6-naphthalendicarboxylate DPNI=N,N’-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydimiimide Synthesis of Peptide-based Porous Materials Metal-organic framework 13 1.3.4. Electrochemical The electrochemical application of MOFs is quite a new investigation on this type of materials. Metalorganic frameworks may be used for energy storage and conversion. For instance, MOFs can be used for supercapacitors, rechargeable batteries and fuel cell. Electrochemistry, is the study of chemical reactions, include electron transfer, at the interface of electrode and electrolyte. A chemical reaction that involves electron transfer between molecules is called oxidation/reduction or “Redox” reaction. Metal cations are redox source inside the structure of metal-organic frameworks; these cations can provide a pathway for electrons. In addition, decent selecting of organic linker may improve the charge transfer inside the framework[15]. Two applications of MOFs in electrochemical area are briefly reported in this work; Li-ion battery and Proton Exchange Membrane (PEM) fuel cell. Li-ion battery: One of the most common rechargeable batteries is lithium-ion (Li-ion) battery and it is widely used for portable electronics. Lithium intercalated/alloyed in the anode and during discharge; Li is oxidized to lithium cation (Li+) and transfers to the cathode. Three attempts have made to apply MOFs for positive electrode in Li-ion batteries and only one of them was successful. Zinc and Nickel based MOFs was used to achieve this goal, using of Ni-based microporous phosphate led to transformation of the solid into a nanocomposite electrode made of Ni nanoparticles and Li2O matrix. The other attempt with Zn-based MOF is also came up with the similar result to nickel based, in the presence of Li ions Zn-based framework decomposed into a zinc based nanocomposite matrix containing Li2O. In contrast, Ferey et al. realized that stronger metal-oxygen bonds my lead to suitable stability; therefore, they used metals such as Cr3+ and Fe3+ with higher oxidation state than Zn or Ni. The suitable metal-organic framework was founded to be MIL-53 (Fe). H2O, 15 % wt carbon was added to this material to be used in the positive electrode, while the negative electrode was Li-metal. Li0.6Fe(OH)0.8F0.2(BDC).H2O showed a maximum uptake of lithium upon discharge while it was fully reversible for at least 50 cycles [26][15]. Synthesis of Peptide-based Porous Materials Metal-organic framework 14 PEM fuel cell: The need to switch from internal combustion engines to a low emission engines was gained much attention during past decades, fuel cells are one of the main leader for this replacement. Fuel cells can be characterized by the electrolyte of them. Every fuel cell is consisting of three parts; Anode where fuel is oxidized into electrons and protons, Cathode where oxygen is reduced to oxide species, and Electrolyte where oxide ion or protons (depending on the type of electrolyte) are combined with oxide or protons to produce electricity power and water. Figure 1.6-Schematic representation of a PEM fuel cell consisting of catalyst layers, gas diffusion electrodes and proton exchange membrane. PEM fuel cell is among the low temperature operating (50-100 oC) fuel cell, its electrolyte is a solid polymer (such as Nafion). The heart of a PEM fuel cell is its membrane electrode assembly (MEA) with a thickness of less than a few hundred microns. Liquid water should be present in the membrane for effective proton conducting, and this is why PEM fuel cells should operate in low temperatures. MEA performance depends on its electrocatalyst technology. The catalysts form thin gas-porous electrode layers on either side of the membrane[27]. Synthesis of Peptide-based Porous Materials Metal-organic framework 15 Today, the most famous and applicable catalyst used in MEA is platinum-based materials. One of the key sources of voltage loss in H2/air fuel cells is the slow kinetics of the Oxygen Reduction Reaction (ORR); even in the best Pt-based catalyst, this problem faced. Finding an alternative for platinum-based catalyst is one of the major research areas since platinum is expensive and its availability is low. Suitable and low cost alternative catalysts for ORR may lead to decrease the overall cost of PEM fuel cells. MOFs have some features such as available metal cations in their structure, high micropore surface area and high volumetric density of metal-ion sites, which make them suitable to be pyrolyzed, in order to synthesize non-precious metal catalysts for ORR[15]. Figure 1.7Polarization curves for PEM fuel cell of H2/O2. State-of-the-art Pt-based cathode (0.3 mgPt cm-2, green squares), Fe-based cathode catalyst synthesized using ZIF-8 (blue stars) and Fe-based cathode catalyst synthesized using high surface area carbon black instead of ZIF-8 (red circles). Preprinted by permission from Macmillan Publishers Ltd: Nature Communications [28], copyright 2011. In 2011, Proietti and co-workers produced ORR-catalyst precursor by mixing zinc based ZIF-8 (ZnII zeolitic-imidazolate-framework) with ferrous acetate and 1,10-phenanthroline. ZIF-8 used for that study had chemical formula of Zn(MeIm)2 9 and its structure exhibits a nanopore topology resulting from bridging the Zn(II) centers to nitrogen atoms of imidazolate ligands. A cathode with best electrocatalyst 9 MeIm=2-methylimidazolate Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 22 Table 2.1Selected potential coordination modes for metal ion in amino acids. Structure Coordination mode Main AAs chain/side chain bidentate (µ2-N1O1:O2) main chain bidentate (µ2-O1:O2) main chain tridentate (µ3-N1O1:O1:O2) main chain monodentate (µ1-O3O4) side chain bidentate (µ2-O3O4:O4) side chain monodentate (µ1-O3) side chain bidentate (µ2-O3:O4) side chain tridentate (µ3-O3:O3O4:O4) side chain Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 23 2.2. Dipeptide-based MBioFs Dipeptides are composed of two amino acids linked by peptide bond. Peptide bond in a chemical bond formed between two molecules when the carboxyl group of one molecule links to the amino group of the other molecule, which follows by release of water (reaction 2.1). Reaction 2.1 Both carboxyl group and amino group of a peptide can coordinate metal ions via different coordination modes. The amino group usually coordinates to metal ions in a monodentate or chelating fashion(figure 2.1) [2]. The first dipeptide-based MBioF was reported in 1996 by Takayama et al. They used GlyGly dipeptide with Zn(II) and Cd(II) metal salts. These authors synthesized three MBioFs by adjusting the pH to 6 and 9. At pH 6, they obtained 2-D frameworks with formula of [M(GlyGly)2].2H2O (where M is Zn(II) or Cd(II)). Each octahedral metal ions of zinc or cadmium was linked to four other metal ions through GlyGly ligands. Each GlyGly ligand bridges two metal ions; a five-membered chelate ring was formed between the terminal amino group and the adjacent O, and a monodentate mode through the terminal carboxylate group. At pH 9, only Cd(II) framework formed, a novel 2-D MBioF formulated as [Cd(GlyGly)2].H2O, which each octahedral cadmium ions was bridged to six other Cd(II) ions by four GlyGly ligands. The terminal carboxylate group links to two metal ions, and amino group bridges to another Cd(II) ion in a monodentate mode[32]. Figure 2.1Presentation of potential coordination modes for the amino group of GlyGly dipeptide; a) monodentate mode, b) five-membered chelate ring. Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 24 To date, small number of dipeptide-based MBioFs has been reported. Some of these frameworks are summarized in table 2.2. Here, in this work three of these frameworks ([Cd(AlaThr)2].4H2O, [Zn(GlyAla)2].(solvent), and [Zn(GlyThr)2].CH3OH) were selected to discus in more details. Table 2.2A summary of dipeptide-based metal-biomolecule frameworks. Dipeptide Metal ion(s) MBioF(s) formula Ref. Cd(II), Zn(II) [Cd(GlyGly)2].2H2O, [Zn(GlyGly)2].2H2O, [Cd(GlyGly)2].H2O [32] Zn(II) [Zn(GlyThr)2].CH3OH [33] Cd(II) [Cd(AlaAla)] [34] Zn(II) [Zn(GlyAla)2].(solvent) [35] Pb(II), Cd(II) [Pb(GlyGlu)(H2O)1/2].ClO4, [Cd(GlyGlu)2].3H2O [36] Cd(II) [Cd(AlaThr)2].4H2O [34] Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 25 [Cd(AlaThr)2].4H2O[34]: This framework was synthesized at room temperature using base diffusion method with ultrasound assistant. A solution of 0.06 mmol of CdAc2 and 0.12 mmol of AlaThr in 2 mL of water was sonicated for 5 min with gentle shaking at 150 w using ultrasonic bath. This solution was placed in a 5 mL glass vial, which was in turn was placed in a 20 mL screw-capped vial. Two milliliters of 2% triethylamine solution was added to the 20 mL vial and sealed with cap. Needle crystals with 60% yield (based on cadmium) were grown after about 3 weeks. The crystal of this framework was monoclinic with space group of C2. Its structure exhibited a 2-D topology in which four peptides were linked together by four cadmium ions (figure 2.2). As shown in figure 2.2a each octahedral Cd(II) ions are linked to another four metal ions through four peptide ligands; two of which through the C-terminus carboxylate group (monodentate), and the remaining four via two different chelate rings through the N-terminus (NH2and amide carbonyl). Figure 2.2- (a) Structure of [Cd(AlaThr)2].4H2O. (b) View along the crystallographic b axis (blue networks shows the hydrogen bonds).Reprinted with permission from [34], Copyright 2008 American Chemical Society. [Zn(GlyAla)2].(solvent)[35]: Solvothermal method is used to synthesize this framework. A solution of 0.5 mmol of GlyAla and 0.25 mmol of zinc nitrate hexahydrate in a mixture of 90% methanol and 10% 1M aqueous NaOH, was heated to 85 oC for 1 hour with heating rate of 2 oC.min-1 and cooling rate of 0.2 oC.min-1, resulting crystalline product with 75% yield. The crystal of this framework was orthorhombic belonging to the space group P21212. A 2-D structure was formed which, had a 1-D square-shaped pore along the c axis (figure 2.3b). In which, each Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 26 tetrahedral zinc ions were linked to four other metal ions through four peptide ligands; two dipeptide ligands were coordinated by the C-terminal Ala carboxylate groups (in monodentate mode) and the other two via N-terminal Gly amine groups (in monodentate mode) (figure 2.3a). Figure 2.3- (a) Structure of [Zn(GlyAla)2] above 298 K. (b) Representation of 1-D pore along the c axis at 298K (after solvent evacuation). Reprinted with permission from supporting online material of [35], Copyright 2010, American Association for Advancement of Science. The solvent within the structure of this framework can be removed reversibly to obtain desolvated framework. According to thermogravimetric analysis (TGA) of the [Zn(GlyAla)2]. (solvent), the first weight loss (15.4%) occurs in the temperature range of 35-250 oC which, corresponds to solvent loss. The next weight loss (64.5%) occurs in temperature range of 250-700 oC which, agrees with decomposition of [Zn(GlyAla)2] to ZnO (figure 2.4a). This framework shows a phenomenon called “breathing”. Breathing phenomenon consists of two successive crystal-to-crystal transformations during adsorption process. In which, the pore volume of flexible framework is changing from large pore state to narrow pore state, and back again to the large pore structure[37]. Breathing phenomenon reduced the pore volume of the desolvated framework during CO2 adsorption by changing the ϕ torsion angle of the methyl group of the Ala. The CO2 adsorption isotherm of desolvated framework at 273 K indicates two steps, first at pressure range of 0-2 bar showing an small adsorption (non-porous structure), and second at pressure range of 2-15 exhibiting higher mass uptake (porous structure) (figure 2.4b). Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 27 Figure 2.4- (a) Thermogravimetric analysis of [Zn(GlyAla)2].(solvent) (b) CO2 sorption isotherms of [Zn(GlyAla)2] at 273 K (closed and open symbols represent adsorption and desorption, respectively). Reprinted with permission from [35], Copyright 2010, American Association for Advancement of Science. [Zn(GlyThr)2].CH3OH[33]: This framework was synthesized by the same group who produced the [Zn(GlyAla)2].(solvent). The synthesis (solvothermal method) procedure is similar to that MBioF with small change in the solvent. A solution of 0.05 mmol of zinc nitrate hexahydrate and 0.1 mmol of GlyThr in methanol and 0.08 mmol of NaOH (aq) 1M, was heated to 85oC with heating rate of 2 oC.min-1 and cooling rate of 0.5 oC.min-1, resulting in colorless crystals with yield of 70%. Figure 2.5- (a) Octahedral coordination of the peptide around Zn(II) ion of [Zn(GlyThr)2].MeOH. (b) Space-filling representation of [Zn(GlyThr)2], showing the 1-D pores along the a axis. Reprinted with permission from[33], Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 28 The crystalline material of this framework had monoclinic structure belonging to I2 space group. The framework exhibits 2-D structure with 1-D pores along a axis (figure 2.5b). Each octacoordinated Zn(II) ions are linked to four dipeptides; two peptides ligands are coordinated through the C-terminus Thr carboxylate group (in bidentate mode) , and the other two are forming a five-membered chelate with amine and oxo group via N-terminus Gly residue (figure 2.5a). This MBioF is soluble in water and nonsoluble is organic solvents such as, ethanol, methanol, and acetone. Methanol guest molecules can be evacuated by heating the framework up to 100oc under vacuum overnight. According to TGA of [Zn(GlyThr)2].MeOH, the first weight loss occurs at temperatures below 50 oC which, corresponds to methanol removal. The structure remains stable up to 250oC. The first decomposition of the framework takes place in temperature range of 250-380oC, and the second decomposition occur around 500oC, which corresponds, to the formation of ZnO (figure 2.6). Figure 2.6Thermogravimetric analysis of [Zn(GlyThr)2].MeOH. Reprinted with permission from[33], Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 29 The adsorption isotherm of this framework was fitted to BET theory to get the BET surface area of 192 m2.g-1. The most interesting point of this framework is its adsorption selectivity of CO2 over CH4. Figure 2.7a shows the preferential CO2 over CH4 adsorption with single-component separation ratio of 14:1 (wt%:wt%) at 1 bar. The highest reported CO2:CH4 separation ratio for MOFs is 24:1 for [Zn2(BPDC)2BPE] at 298 K and 1 atm[38]. Figure 2.7- (a) Selective sorption of CO2 (squares) over CH4 (triangles) of [Zn(GlyThr)2].MeOH at 273 K. (b) CO2 sorption isotherms of [Zn(GlyThr)2].MeOH at 273 K (squares) and 298 K (triangles). Filled and empty symbols showing the adsorption and desorption, respectively. Reprinted with permission from[33], Copyright 2012 WileyVCH Verlag GmbH & Co. KGaA, Weinheim. 2.3. Tripeptide-based MBioFs Up to date, there are only two reported tripeptide-based metal-organic frameworks by Lee and coworkers on 2007. They assembled two MBioFs with Cd(II) metal ions and AlaAlaAla and GlyGlyGly tripeptides, structures of these two tripeptides are presented in figure 2.8. Figure 2.8Representation of the structures of GlyGlyGly (top) and AlaAlaAla (bottom) tripeptides. Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 30 [Cd(GlyGlyGly)2].H2O: The synthesis route of this framework is similar to earlier reported synthesis for [Cd(AlaThr)2].4H2O. A solution of 0.05 mmol of CdAc2 and 0.10 mmol of GlyGlyGly in 2:2:3 mL of DMF, water, and ethanol, respectively, was sonicated for 5 min with gentle shaking at 150 w using ultrasonic bath. This solution was placed in a 5 mL glass vial, which was in turn was placed in a 20 mL screw-capped vial. One milliliter of 2% triethylamine in ethanol solution was added to the 20 mL vial and sealed with a cap. Needle crystals with 59% yield (based on cadmium) were grown after about 3 months. The crystalline framework of this MBioF was monoclinic belonging to the space group C2/c. Each octahedral Cd(II) ion was linked to six peptides to form a 3-D complex (figure 2.9a); two of which through the N-terminus of two peptides (monodentate), and four via the C-terminus carboxylates of four peptides (monodentate). Figure 2.9- (a) Structure of [Cd(GlyGlyGly)2].H2O, showing a 3-D coordination of cadmium ions. (b) View along the c axis, showing an 8-membered rings formed by linking two Cd ions to two carboxylate groups. Reprinted with permission from [34], Copyright 2008 American Chemical Society. [Cd(AlaAlaAla)2]: This framework was also assembled through the base diffusion method with ultrasound assistant described for [Cd(AlaThr)2].4H2O and [Cd(GlyGlyGly)2].H2O. A solution of 0.15 mmol of CdAc2 and 0.15 mmol of GlyGlyGly in 10:10:5 mL of DMF, water, and ethanol, respectively, was sonicated for 5 min with gentle shaking at 150 w using ultrasonic bath. This solution was placed in a 20 mL glass vial, which was in turn was placed in a 100 mL screw-capped jar. Ten milliliter of 2% triethylamine solution was added to the 100 mL jar and sealed with a cap. Colorless transparent needle crystals with 43% yield were grown after about 3 weeks. Synthesis of Peptide-based Porous Materials Peptide-based MBioFs 31 This framework crystallized as monoclinic crystals belonging to the chiral group space C2. The resulting framework had a 3-D topology with 1-D pores along its b axis. Each Cd(II) ion was octahedrally coordinated by four trialanine peptides; two through the C-terminus carboxylate group of two peptides (monodentate), two other through the N-terminus amine group of two peptides, and finally the remaining two via N-terminus amide carbonyl group of the two other peptides (figure 2.10a). Figure 2.10- (a) Structure of [Cd(AlaAlaAla)2]. (b) View along the b axis, exhibiting 1-D pores. Reprinted with permission from [34], Copyright 2008 American Chemical Society. Synthesis of Peptide-based Porous Materials Experimental procedure and results 38 [Zn(GlyAsp)].H2O demonstrate 1:1 stoichiometry of zinc and dipeptide in the framework. Therefore, another way to synthesis of this material is to use 1:1 stoichiometry of dipeptide and zinc salt. This new synthesis route can reduce the amount of dipeptide used and thus reducing the final cost of framework assembly of 1. 3.1.2. Synthesis of [Co(GlyAsp)].H2O We used a similar solvothermal synthesis method to assemble cobalt-based peptide framework. Thus instead of zinc salt, cobalt nitrate salt is used for synthesis of MPF [Co(GlyAsp)].H2O (2). A 20 mL scintillation vial was charged with GlyAsp (0.1 mmol, ≥99.0% from Aldrich) and cobalt (II) nitrate hexahydrate (0.05mmol, 98% from Sigma-Aldrich) in a water/methanol mixture (1.9/1.9 mL respectively). The following steps carried out to prepare the solution: 1. 0.0145 g (equal to 0.05 mmol) of cobalt nitrate salt was carefully weighted and charged to flask. 2. 0.0190 g (equal to 0.01 mmol) of GlyAsp was added to scintillation vial. 3. 1.9 mL of methanol and 1.9 mL of deionized water was added. 4. The solution was stirred for approximately 45 min. A transparent (with pink color) and clear solution had reached. 5. 1 M aqueous sodium hydroxide was added to basify the solution. The pH was adjusted to 5.99 while stirring. 6. The reagents were heated at 85 ⁰C for 15 hours, and cooled down to room temperature for 30 min (the sample left in the oven to cool down to room temperature for about 5 hours). As shown in figure 3.5a, needle shape crystals were grown in the edge of powders of 2. Figure 3.5- (a) Microscopic and (b) SEM images of [Co(GlyAsp)].H2O. Synthesis of Peptide-based Porous Materials Experimental procedure and results 39 Comparing to 1, this framework has advantages such as, no undesired precipitation of cobalt hydroxide during assembly, and higher crystal yield (22.23% based on cobalt). In contrast, although the framework of 2 has 1:1 stoichiometry of dipeptide and Co but the yield of crystallization was highly decreased when this stoichiometry was applied thus, 1:1 stoichiometry is not suitable for assembly of this framework. 3.2. X-ray diffraction X-ray diffraction (XRD) is a method used for determining the structure of a crystal. When X-ray beams interact with crystalline substance, they diffract and as a result, a diffraction pattern is obtained. Every crystalline substance gives a unique pattern therefore, XRD pattern of a pure substance is like a fingerprint of the substance. By measuring the angle and intensity of diffracted X-ray beams, the structure of crystal can be solved. Table 3.2 shows the crystal data and structure refinement for 1 and 2. Table 3.2Crystal data and structure refinement for [Zn(GlyAsp)].H2O and [Co(GlyAsp)].H2O. Metal-peptide framework [Zn(GlyAsp)].H2O [Co(GlyAsp)].H2O Empirical formula C6H10N2O6Zn C6H9N2O6Co Formula weight (g/mol) 271.54 264.09 Temperature (K) 100 120 Crystal system Monoclinic Orthorhombic Space group P21 P212121 a (Å) 4.807 6.086 b (Å) 9.472 8.954 c (Å) 9.701 16.757 α (o) 90 90 β (o) 95.76 90 γ (o) 90 90 Volume (Å3) 439.5 913.17 Z 2 4 ρcal (g/cm3) 2.0519 1.9135 R indexes14 (all data) R1=0.0676 , wR2=0.1694 R1=0.0693 , wR2=0.1687 14 R1=∑ΙIFoI-IFcII/∑IFoI and wR2= [w(Fo 2-Fc 2)2]/[w(Fo 2)2]1/2. Synthesis of Peptide-based Porous Materials Experimental procedure and results 40 As reported in table 3.2, crystals system of 1 and 2 are monoclinic and orthorhombic, respectively. Both of these frameworks have primitive (P) lattice centering, in which lattice points are only coordinated on the cell corners. Space group describes the symmetry of a crystal, and Z value indicates the number of the symmetry operators (table 3.3). Table 3.3Lattice shape and symmetry operators for [Zn(GlyAsp)].H2O and [Co(GlyAsp)].H2O. Metal-peptide framework [Zn(GlyAsp)].H2O [Co(GlyAsp)].H2O Crystal system Monoclinic Orthorhombic Lattice shape Space group P21 P212121 Symmetry operators (x,y,z) (-x,y+1/2,-z) (x,y,z) (1/2-x,-y,1/2+z) (-x,1/2+y,1/2-z) (1/2+x,1/2-y,-z) 3.3. Structure Here we describe the structures and metal coordination modes for 1 and 2. Both of these frameworks were assembled with GlyAsp dipeptide ligands. The structure of GlyAsp illustrated in figure 3.6. Figure 3.6Representation of the structure of GlyAsp. Synthesis of Peptide-based Porous Materials Experimental procedure and results 41 As mentioned before, two different metal salts of zinc and cobalt nitrates were used for synthesis of these frameworks. Coordination modes for Zn (II) and Cobalt (II) metal ions in frameworks 1 and 2 are presented in figure 3.7. Figure 3.7Metal ions coordination modes for [Zn(GlyAsp)] (left) and [Co(GlyAsp)] (right). 3.3.1. Structure of [Zn(GlyAsp)].H2O As shown in figure 3.7, four Zn(II) metal ions are linked to each GlyAsp dipeptide. Each pentahedral Zn(II) ions are linked to four other metal ions by four dipeptide ligands; one forming a five-membered chelate with amine and oxo group through N-terminus Gly residue, two through C-terminus carboxylate group of Asp α-carbon (in bidentate mode), and finally one via C-terminus carboxylate group of β-carbon of Asp residue (monodentate). Therefore, adapting tetradentate coordination mode of µ4-N1O1:O2:O3:O4 (figure 3.8). Figure 3.8Representation of Zn(II) ion coordination in [Zn(GlyAsp)]. Synthesis of Peptide-based Porous Materials Experimental procedure and results 42 The framework of 1 exhibits 2-D topology with 1-D pores along the a axis. Figure 3.9 illustrates the extended structure of 1, in which the solvent molecules (water) are placed in channels of the framework. These solvent molecules can be removed by heating up to 100oC. Figure 3.9Structure of [Zn(GlyAsp)].H2O; (a) Polyhedral representation, zinc (blue polyhedral), hydrogen (white), carbon (grey), oxygen (red), and nitrogen (light blue). (b) Stick representation, Zn-N bond (blue-light blue), Zn-O bond (blue-red), and water molecules (yellow). 3.3.2. Structure of [Co(GlyAsp)].H2O Five cobalt (II) metal ions are coordinated with each GlyAsp dipeptide (figure 3.7). Each octahedral Co(II) ions are bridged to five other metal ions by five dipeptide ligands; one forming a five-membered chelate with amine and oxo group through N-terminus Gly residue, four via C-terminus carboxylate group of Asp α and β-carbons (each two in bidentate mode). As a result, adapting pentadentate coordination mode of µ5-N1O1:O2:O3:O4:O5 (figure 3.10c). Compound 2, shows three-dimensional porous structure, which has one-dimensional pores along a axis. These pores are filled with water guest molecule. As mentioned earlier in chapter 2, metal-peptide frameworks commonly have 1-D topology and a few of them showing 2-D structure, 3-D topologies are scarce. Figure 3.10 represents the structure of this framework in two models of polyhedral and sticks. The fivemembered chelate ring formed through the N-terminus of Gly is clearly shown in the stick model. Synthesis of Peptide-based Porous Materials Experimental procedure and results 43 Figure 3.10Structure of [Co(GlyAsp)].H2O; (a) Polyhedral representation, cobalt (pink polyhedral), hydrogen (white), carbon (grey), oxygen (red), and nitrogen (light blue). (b) Stick representation, Co-N bond (pink-light blue), Co-O bond (pink-red), and water molecules (yellow). (c) Octacoordinated Co(II) ions in [Co(GlyAsp)].H2O complex. 3.4. Thermogravimetric analysis Thermogravimetric analysis (TGA) is the analysis of weight changes in relation to changes in temperature. A plot of weight loss versus temperature exhibits the composition changes in sample and thermal stability of referred sample. Another curve of derivative weight loss (DTG) can be used to see the point at which weight loss is most apparent. TGA of compounds 1 and 2 were carried out with NETZSCH TG 209 F1 instrument in 30-650oC temperature range with heating rate of 5oC.min-1 and air flow rate of 50 mL.min-1. Sample size of 11 and 21 mg of framework 1 and 2 used for these measurements. Synthesis of Peptide-based Porous Materials Experimental procedure and results 44 3.4.1. Thermogravimetric analysis of [Zn(GlyAsp)].H2O Figure 3.11 displays the TGA of metal-peptide framework of 1. The first weight loss, corresponded to the removal of water guest molecules, takes place at temperature below 100 oC. This 6.5% weight loss is in good agreement with 6.6% water molecule fraction in the framework of compound 1. The framework remains stable up to 250 oC where the second decomposition occurs. Finally, weight loss of 69% at temperature of 500oC, which leaves a final residue with molar weight of 84.12 g.mol-1 associated with the formation of Zn(II) oxide 15 . Figure 3.11Thermogravimetric analysis of [Zn(GlyAsp)].H2O (blue solid line). Green dashed line indicates the derivative weight loss (right-hand scale), and apparent weight losses are highlighted with red boxes. 3.4.2. Thermogravimetric analysis of [Co(GlyAsp)].H2O This framework exhibits different decomposition behavior comparing to framework 1. As shown in figure 3.12, the first decomposition occurs at temperature range of 115-240 oC with 6.15% weight loss that is associated to water removal. Decomposition continues up to 350oC where fast weight loss takes place through 30oC temperature increase. The final 69.89% weight loss after 380oC is in sufficient agreement with formation of Co(II) oxide 16 . 15 ZnO=81.39 g.mol-1 16 CoO=74.93 g.mol-1 approximately equal to 79.78 g.mol-1 (residue molar weight) -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 0 20 40 60 80 100 120 0 100 200 300 400 500 600 700 Deriv. Weight loss (%/min) Weight loss (%) Temperature oC TG DTG Synthesis of Peptide-based Porous Materials Experimental procedure and results 45 As mentioned in section 3.2.2, pores of this framework are filled with these guest molecules therefore; better adsorption behavior can accomplish after evacuation by heating the framework of 2 up to 240oC. Figure 3.12Thermogravimetric analysis of [Co(GlyAsp)].H2O (blue solid line). Green dashed line indicates the derivative weight loss (right-hand scale), and apparent weight losses are highlighted with red boxes. 3.5. Adsorption Every sorption process is consist of two components, sorbate and sorbent. Fluid phase (sorbate) is transferred to sorbing agent (sorbent) which could be suspended in a vessel or packed in a column[39]. Adsorption, ion exchange, and chromatography are sorption operations. In an adsorption process, sorbate (gas/liquid) diffuses to the surface of a sorbent (solid). Molecules, atoms, or ions of sorbate bond with the surface of the sorbate or are held by weak intermolecular forces of it. In general, the solid material is referred to as adsorbent, whereas the adsorbed solutes are called adsorbate. During adsorption, the adsorbent become saturated or nearly saturated with the molecules, atoms, or ions of the adsorbate. The adsorbent is regenerated by desorption the sorbed material, in order to reuse the -1 1 3 5 7 9 11 13 0 20 40 60 80 100 120 0 100 200 300 400 500 600 700 Deriv. Weight loss (%/min) Weight loss % Temperature (oC) TG DTG Synthesis of Peptide-based Porous Materials Experimental procedure and results 46 adsorbent. As mentioned before, adsorption can be classified as purification or bulk separation, depending on the concentration in the feed fluid of the components to be adsorbed 17 [39]. Different types of forces between adsorbate molecule and the molecules of the adsorbent may categorize adsorption into physical adsorption (van der Waals adsorption) and chemisorption (activated adsorption). Physical adsorption from a gas happens when the intermolecular forces of the gas are smaller than the intermolecular attractive forces between molecules of adsorbent and the gas. The resulting adsorption is exothermic and it is similar to condensation. Physical adsorption begins as a monolayer, becomes multilayered, and then, if the pores are close to the size of the molecules, capillary condensation occurs, the pores fill with adsorbate. Capacity of a porous adsorbent depends on the pore volume of it. However, in case of having gas for adsorbate and at temperature beyond its critical temperature, physical adsorption is limited to a monolayer. In contrast, chemisorption occurs when adsorbent and adsorbate form a chemical bond with each other. Chemisorption from a gas adsorbate usually happens at temperatures higher than 200 oC and may be slow and irreversible [39]. 3.5.1. Volumetric method for gas adsorption During the adsorption of gas in solid, the weight of the solid increases and the pressure of the gas decreases. Therefore, the amount adsorbed can be measured from mass or pressure changes. When the weight change is used for measurement, the technique is referred to as gravimetric adsorption method. Alternatively, measurement based on the gas pressure change is called volumetric adsorption method. In volumetric method, the volume of the sample required for measurement. The equilibrium adsorption isotherm is the plot of pressure versus amount adsorbed at a constant temperature. A schematic representation of volumetric apparatus used for this work is shown in figure 3.13. Commonly, volumetric apparatuses are consisting of two sides, injection side and sample side. The volumes of these two sides are also required for measurements. For measuring adsorption at a certain pressure and temperature two steps should carried out. Frist step is to hold the gas inside the injection side while the valve (R2) between two sides is closed. Second step is followed by opening the R2 valve, and allowing the gas into the sample side, where the adsorption takes place. 17 Bulk separation (adsorption of 10 wt% or more from adsorbate) and Purification (> 2 wt %) Synthesis of Peptide-based Porous Materials Experimental procedure and results 47 Figure 3.13Schematic representation of volumetric adsorption apparatus. The initial pressure inside injection and sample side is set to zero through making vacuum in the whole apparatus. After following those two described steps, the adsorption data for the first pressure point can be collected. For collecting adsorption data for the second pressure point, there are two possible routes. First route is to repeat the procedure through making another vacuum between each pressure point. Alternatively, second route is to continue the procedure without making vacuum, which in this case the final equilibrium pressure of previous point is considered as the initial pressure for the sample side. In this work, cumulative measurement (second route) is used for data collection. Schematic representation of cumulative adsorption measurement used for two pressure points is shown on figure 3.14. The following calculation should be considered for measuring the amount adsorbed for this method. Synthesis of Peptide-based Porous Materials References 54 [14] K. A. S. JeongYong Lee, Omar K. Farha, John Roberts and S. T. N. and J. T. 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