Graphene metal organic framework composites and their potential applications 4

Chapter 4: A Catalytically Active Graphene-Porphyrin MOF Composite for Oxidation of Cyclohexane Abstract: Pyridine ligand-functionalized graphene (reduced graphene oxide) can be used as a building block in the assembly of metal organic framework (MOF). By adding the functionalized graphene to iron-porphyrin, a graphene-metalloporphyrin hybrid MOF which exhibits high specific surface area and robust catalytic activity for the oxidation of cyclohexane at 150 OC and 150 psi O2 can be synthesized. The structure and property of the hybrid MOF was investigated as a function of the weight percentage of the functionalized graphene added to the iron-porphyrin framework. 4.1 Introduction Graphene, a two-dimensional sheet of sp2 conjugated carbon atoms,1 can be considered as a giant polyaromatic platform for performing chemistry due to its open ended structure. The combination of high surface area (theoretical value of  2600 m2/g),3,4 high electrical conductivity5,6 and low manufacturing cost makes graphene sheets highly promising as catalysts support.8 When oxidized, graphene is solution-processible in the form of graphene oxide (GO). The presence of epoxy and hydroxyl functional groups on either side of the GO sheet imparts bifunctional properties on the material10 which allow it to act as structural nodes in metal organic framework (MOF).11,12 One attractive approach to MOF-based catalyst design is to heterogenize known homogeneous molecular catalysts by employing them as struts, linking organometallic nodes. Metalloporphyrins are well-known catalyst for hydrocarbon oxidations. 13 Indeed, some of the earliest reports on crystalline MOFs emphasized the potential of porphyrins as building blocks.14 However, there are few reports of catalysis by porphyrin struts in well-defined MOFs because of structural stability problems and the reactivity of the porphyrin ligands towards random metal coordination.15 Supported metalloporphyrins have been identified as effective 77 catalysts for the selective oxidations of hydrocarbons.16,17 It is well known that the choice of organic linker affects the structure and properties of MOF.18 The high specific surface area and thermal stability of GO provides the motivation for employing these materials as catalyst supports.19,20 In this work we employed reduced GO (r-GO) sheets that are functionalized on either side of the basal plane with pyridine ligands. These function as struts can link metalloporphyrin nodes in the MOF framework. The resulting materials were applied as catalyst in solvent-free selective oxidation of cyclohexane. We found that the presence of r-GO in MOF actually increases the catalytic property of the metalloporphyrin. Scheme 4.1 shows how G-dye is synthesized from chemically reduced GO (r-GO) sheets via diazotization with 4-(4-aminostyryl) pyridine. The chemical structures of the various subunits in the assembled MOF are illustrated in Scheme 4.2. The porphyrin used in this structure is 5,10,15,20-Tetrakis (4-carboxyl) - 21H, 23H porphyrin, which is abbreviated as TCPP. The MOF is created by linking TCPP and FeCl2.4H2O, and herewithal abbreviated as (Fe-P)n MOF. G-dye represents chemically reduced GO (r-GO) 78 sheets that were functionalized with donor--acceptor dye which terminate in pyridinium moieties (electron-withdrawing group), Scheme 4.1. The pyridine ligand improves the solubility of the systems by stabilizing the electron-rich phenylethyl group, and prevents aggregation. The composite formed by the combination of G-dye and (Fe-P)n MOF is named as (G-dye-FeP)n MOF, Scheme 4.2. Scheme 4.2 Schematic of the chemical structures of TCPP, G–dye, (Fe-P)nMOF, and (G–dye -FeP)n MOF. In order to study the structure-composition relationship, different weight percentages of Gdye (5, 10, 25, 50 wt %) were mixed with the chemical precursors of (Fe-P)n MOF to synthesize (G-dye 5, 10, 25, 50 wt % -FeP)n MOF composites. Owing to the fact that TCPP has a square planar symmetry decorated by carboxylic groups around the porphyrin site, it is perfectly suited for supramolecular assembly.21 Sumod George et. al. reported the synthesis of 3D frameworks by 79 dissolving of Mn (Cl)–TCPP in nitrobenzene under solvothermal condition. 22 Similarly, 3D MOF based on (Fe-P)n where P=porphyrin is synthesized by dissolving TCPP and FeCl2.4H2O. Graphene sheets decorated by pyridine groups on either side of the sheets are analogous to pillar connectors such as bpy, 4,4-bipyridine used in MOF synthesis.23,24 4.2 Experimental Section Graphite Oxide (GO) and reduced GO sheets were prepared following the procedure reported in chapter 3. 11 4-(4-Nitrostyryl) pyridine: A mixture of 4-nitro benzaldehyde (3 g, 20 mmol) and 4picoline (2.3 g, 25 mmol) in 15 mL acetic anhydride was refluxed over night for 12 h. 47 The cooled mixture was poured onto ice and neutralized with 40 % NaOH aqueous solution. Extraction was carried out with ethyl acetate and the organic layer was concentrated by rotary evaporator. The residue was recrystallized with EA/HEX=1/1 to give 2.0 g (9 mmol, 45 %) 4-(4Nitrostyryl) pyridine. 4-(4-aminostyryl) pyridine : To a mixture of 4-(4-Nitro styryl) pyridine (2 g, mmol) and Pd on activated carbon (10 %, 50 mg) in ethanol (100 mL) was added hydrazine monohydrate (2 mL).48 The mixture was heated and refluxed for hours and checked with thin layer chromatography. When the reaction is finished, the hot solution was filtered and the solvent was concentrated. The residue was recrystallized in ethanol to give 1.45 g (7.4 mmol, 82 %) 4-(4aminostyryl) pyridine. 4-styrylpyridine - Functionalized Graphene (G-dye): The 4-styrylpyridine diazonium salt was prepared by the following procedures: 343 mg of 4-(4-aminostyryl) pyridine (1.75 mmol) and 131.5 mg of sodium nitrite (1.89 mmol) were added to 40 mL water in ice bath. 49 This solution was added quickly to mL HCl solution (10 %, 3.2 M, 9.6 mmol) and stirred for 45 min. The temperature was maintained at 0-5 oC during the reaction and the solution turned orange at the end. 80 The preparation of G-dye was performed by sonicating 150 mg of r-GO dispersed in wt % aqueous sodium dodecylbenzensulfonate (SDBS) surfactant. 10 The diazonium salt solution was added to the r-GO solution in an ice bath under stirring and the mixture was maintained in ice bath at - oC for around hours. Next, the reaction was stirred at room temperature for another hours. Finally, the solution was filtered using 0.2 µm polyamid membrane and washed several times with water, ethanol, DMF, and acetone. (Fe-P)n MOF: This synthesis follows previous report with some modifications.24 5,10,15,20–Tetrakis (4-carboxyl) - 21H, 23H-porphine (TCPP) (40 mg, 0.05 mmol) and FeCl2.4H2O (29.82 mg, 0.15 mmol) and 0.1 M HCl acid (1.2 mL, 0.12 mmol) in ethanol were dissolved in a mixture of mL DMF and mL ethanol. The final mixture was sealed in a small capped vial and sonicated to ensure homogeneity. The vial was heated at 150 °C in an oven for 48 h, followed by slow cooling to room temperature. The crystals were collected via filtration and washed with DMF and ethanol (G-dye -FeP)n MOF: TCPP (40 mg, 0.05 mmol), FeCl2. 4H2O (29.82 mg, 0.15 mmol) and 0.1 M HCl acid (1.2 mL, 0.12 mmol) in ethanol were dissolved in a mixture of mL DMF and mL ethanol. Varying amounts of G-dye (5, 10, 25, and 50 wt %) were added to the mixtures. The final mixture was sealed in a small capped vial and sonicated to ensure homogeneity. The vial was heated at 150 °C in an oven for 48 hours, followed by slow cooling to room temperature. The crystals were collected via filtration and washed with DMF and ethanol. (GO-FeP)n MOF composites were synthesized using the same methods as (G-dye FeP)n MOF composites, except with the use of GO instead of G-dye. Sample preparation for catalytic reaction: The oxidation of cyclohexane by molecular oxygen was carried out in a 200 mL Parr batch reactor. Typically, 20 ml of cyclohexane and 10 mg solid catalyst were added into the reactor. After purging with O 2, the reactor was heated to 150 °C and the O2 pressure was adjusted to 150 psi. During the oxidation process, the O pressure was 81 kept at 150 psi with a stirring rate of 300 rpm. After hour of reaction, the reactor was cooled down to 30 ºC and the mixture was dissolved in ethanol. An excessive amount of triphenylphosphine (Ph3P) was added to the reaction mixture to completely reduce the cyclohexyl hydroperoxide (CHHP), an intermediate in the cyclohexane oxidation, to cyclohexanol. The products were analyzed using a gas chromatogram (HP 7890 series GC) with a mass spectrometer detector (HP 5973 mass selective detector) and a capillary column (HP 5MS). The performance of the reused catalyst was studied by repeated cyclohexane oxidations. The reaction was first carried out as described above. Then, at the end of the oxidation, (G-dye 10 wt % -FeP)n MOF was recovered by filtration, washed with ethanol, and reused in four consecutive runs. 4.3 Results and Discussion The first question we like to address is whether there is any difference in structure between graphene-metalloporphyrin MOF and metalloporphyrin MOF without graphene. This is important for understanding how graphene influences the crystallization of the MOF, and its subsequent properties and functionality. The functional groups present in the starting material and the different hybrids are characterized by FTIR and UV optical absorption (Figure 4.1). The formation of new chemical bonds in the MOF-hybrids can be judged from the optical absorption. Figure 4.1 (a) shows UV-vis absorption spectra of GO and G-dye in DMF. The absorption peak of GO25 at 268 nm is due to the characteristic -plasmon absorption.26 The red-shifted -* absorption band at 319 nm of G-dye is consistent with the partial recovery of conjugated network 27 and also a coupling effect of functional groups on the surface of graphene. 25 The absorption spectra of G-dye, TCPP , (Fe-P)n MOF and (G-dye 10 wt % -FeP)n MOF in phosphate buffer are shown in Figure 4.1(b). The UV–vis spectrum of TCPP (black line) displays six characteristics bands, including an intense Soret band (409 nm) [deeper  levels LUMO] and four characteristic visible absorption bands (Q-bands) at 525, 564, 596 and 651 nm [  * 82 electronic transition from the HOMO to the LUMO]. 28 Coordination with iron atoms in the iron porphyrinate (green line) results in a reduction of the Q bands from four to two in the UV-vis spectra and a red shift in the Soret band of (Fe-P)n MOF. 28 The presence of graphene in (G-dye 10 wt % -FeP)n MOF (red line) creates a new band at 303 nm and a blue shift in Soret band compared to (Fe-P)n MOF (green line). Figure 4.1 (a) UV-vis absorption spectra of (a) G-dye (3.7 mg L-1) and (b) GO (4.3 mg L-1) in DMF. Insert image: comparison between solubility in DMF of r-GO (I) and G-dye (II). (b) black plot: UV-vis absorption spectra of TCPP (3.2 mg L -1), red plot: (G-dye 10 wt % -FeP)n MOF (5.2 mg L-1) , green plot: (Fe-P)n MOF (4.9 mg L-1), and blue plot: G-dye (4.7 mg L-1) in phosphate buffer. (c) FTIR spectra of (i) GO, (ii) G-dye, (iii) (G-dye 10 wt % -FeP)n MOF and (iv) TCPP. (d) Fluorescence spectroscopic changes observed for (i) (Fe-P)n MOF , (ii) (G-dye wt % -FeP)n MOF, (iii) (G-dye 10 wt % -FeP)n MOF , (iv) (G-dye 25 wt % -FeP)n MOF, (v) (G-dye 50 wt % -FeP)n MOF (all concentrations:2 mgL-1 in phosphate buffer solution), Excitation wavelength (426 nm). The functional groups present in the starting material and the different hybrids are characterized by FTIR. As shown in Figure 4.1(c)-(i), the vibrational peaks of GO are consistent 83 with the presence of fingerprint groups such as carboxylic species, hydroxyl species and epoxy species (C=O, 1734 cm-1; OH deformation, 1400 cm-1 ; the C-OH stretching, 1230 cm-1 ; C-O-C (epoxy group) stretching, 1061 cm-1 ; skeletal ring stretch, 1624 cm-1).29 In the spectrum of G-dye (Figure 4.1(c)-(ii)), the vibration of the C-O-C (epoxy group) is missing due to the fact that the skeletal framework in G-dye is made of reduced GO. Distinctive absorption bands which emerge at 798 cm-1, 1150 cm-1, 1331 cm-1, 1605 cm-1, 1740 cm-1 are assigned to C-H pyridine,30 C-C bending,31 C-N pyridine,32 phenyl C=C ring stretch,33 and C=O vibration of COOH, 34 respectively, in the G-dye. The spectrum of TCPP (Figure 4.1(c)-(iv)) shows a triplet band at 1020, 985, and 966 cm-1 due to the well-resolved C-H rocking vibrations of the pyrrole ring.35 The C=O stretching vibration in the COOH group in TCPP was seen at 1701 cm-1, the bands in the range 1500 – 1600 cm-1 are due to stretching vibration of C=C in the pyridyl aromatic ring.36 The other absorption bands of TCPP are seen at 806 cm-1 (vibration of C-H bond from pyrrole),36 1396 cm-1 (stretching vibration of C-N from pyrrole),37 and 1268 cm-1 (C-OH stretching vibrations). The FTIR spectrum of (G-dye 10 wt % -FeP)n MOF (Figure 4.1(c)-(iii)) largely resembles that of TCPP. A fingerprint band present at 1675 cm-1 is assigned to the C=O stretch of carboxylate group. The downshift of the C = O stretch from 1701 cm-1 to 1675 cm-1 as well as an intense fingerprint Fe-N stretching at 1008 cm-1 compared to that of TCPP reflects the metallation of porphyrin ring. 32,35 Fluorescence spectra of the (Fe-P)n MOF and (G-dy -FeP)n MOF composites were recorded to examine the electronic interactions of G-dye sheets and Porphyrin-MOF units (Figure 4.1(d)). The observed luminescence quenching of the (Fe-P)n MOF, which has a strong fluorescence peak at 575.8 nm, reveals that there is a strong interaction between the excited state of TCPP and graphene in the hybrid. The fluorescence quenching of the excited TCPP may be due to photoinduced electron transfer or energy transfer to the r-GO scaffold, which acts as a charge sink due to its conjugated network. 84 A linear relationship between absorbance and concentration in DMF is indicative of good dispersion of G-dye because aggregation at high concentration will cause a deviation from linearity in the Beer’s plot (Figure 4.2).50 Figure 4.2 Concentration dependence of UV-vis absorption spectra of G-dye in DMF (concentration are 4.2, 6.6, 9.5, 11.4, 14.8, 16.5, 19.3, and 22.7 mg L-1, from a-h, respectively). The insert shows the plot of optical density at 319 nm versus concentration. The straight lines are linear least-squares fit to the data, indicating G-dye was dissolved homogeneously in DMF. According to CHN elemental analysis, the amount of nitrogen in G-dye is between 7.1 wt % to 9.8 wt %, which is evident of the sufficient amount of pyridine ligands for coordinationassisted assembly. To study the chemical environment of the atoms in the compounds, X-ray photoelectron spectroscopy (XPS) was performed. Figure 4.3 shows that there are two chemically shifted Fe peaks in the XPS spectra of both compounds. The peaks are assignable to the Fe inside the porphyrin core (connected to four electronegative nitrogens from pyrrole groups) and the Fe connected to the four electronegative oxygen groups of two adjacent porphyrin sites, respectively. Accordingly, the electron deficiency of porphyrinic Fe is higher than the bridging Fe, giving rise to its higher binding energy. This gives rise to a higher binding energy for the porpyrinic iron and lower binding energy for the bridging iron. It can be seen that the binding energy of Fe 2p3/2 in 85 (G-dye 10 wt % -FeP)n MOF is lower than that in (Fe -P)n MOF in Figure 4.3(C). This shift is attributed to the coordination of the pyridine ligand to iron in (G-dye 10 wt % -FeP)n MOF, which increases the electron density on the iron (thus decreasing binding energy). Figure 4.4(A) shows one peak for the N 1s of (Fe -P)n MOF assignable to the pyrrole nitrogen. Considering that there are two different nitrogen environment in (G-dye 10 wt % -FeP)n MOF arising from pyrrole groups located in the porphyrin core and pyridine groups from the dye, the two peaks at 400.7 and 402.8 eV are attributed to N 1s of pyrrole and pyridine atoms, 51 respectively, as shown in Figure 4.4(B). Figure 4.3 (A) The Fe 2p3/2 core level spectra for (Fe-P)n MOF. (B) The Fe 2p3/2 core level spectra for (Gdye 10 wt % -FeP)n MOF. (C) Comparison between The Fe 2p3/2 core level spectra for (a) (Fe-P)n MOF , (b) (G-dye 10 wt % -FeP)n MOF. 86 zeolite 39,40 which has typical surface area in the range of 500 m2/g, and is higher than that of porphyrin-based MOFs.39,41,42 The important role of graphene in enhancing the adsorption surface area can be seen clearly from the scaling between the higher volume of adsorbed nitrogen (cm3g1 ) with increasing amount of G-dye in the composite (Figure 4.11) . Figure 4.11 Nitrogen gas sorption isotherms at 77 K for (a) (G-dye wt % -FeP)n MOF , (b) (G-dye 10 wt % -FeP)n MOF , (c) (G-dye 25 wt % -FeP)n MOF, and (d) (G-dye 50 wt % -FeP)n MOF. P/P0 is the pressure (P) to saturation pressure (P0) with P0 = 746 Torr. In order to prove the presence of graphene sheet inside the framework (Figure 4.12), the (G-dye 10 wt % -FeP)n MOF (rod-shaped) sample was dissolved in phosphate buffer followed by sonication (process (1)). After evaporating the solvent, the sample was washed with water and ethanol to remove iron- porphyrin and impurity from the G-dye. The collected sample was dried under vacuum (process (2)). Finally, the sample was dispersed in DMF and sonicated for 30 (process (3)). The presence of r-GO sheets were revealed by optical microscopy, AFM, SEM and Raman spectroscopy (process (4)). Raman spectroscopy shows the D band (1347.5 cm-1) and G band (1586.5 cm-1) of the r-GO sheet (part d). 96 Figure 4.12 Schematic of the process flow to show how r-GO sheets were recovered from the rod-shaped (G-dye 10 wt % -FeP)n MOF after dissolving the iron-porphyrin MOF. Process (1) : After evaporating the solvent, the sample was washed with water and ethanol to remove iron- porphyrin and impurity from the G-dye. Process (2): The collected sample was dried under vacuum. Process (3): The sample was dispersed in DMF and sonicated for 30 mins. Process (4): It was checked by (a) optical microscopy , (b) AFM, (c) SEM and (d) Raman spectroscopy. The catalytic properties of MOF hybrids were evaluated by selective oxidation of cyclohexane in a solvent-free system. The oxidation of cyclohexane by the composite MOF was performed at the optimized conditions of 150 °C and 150 psi of O . The oxygenation reaction 43, 44 is catalyzed mainly by iron (II) porphyrinates and uses molecular oxygen as the oxygen source. To study the effect of temperature on cyclohexane oxidation, reactions were done at 140, ◦ 150, 160 and 170 C using (G-dye 10 wt % -FeP)n MOF as catalyst. No reaction products were ◦ obtained when the reaction temperature was below 100 C. Cyclohexane oxidation became 97 ◦ uncontrollable and formed over-oxidized products above 160 C. 150◦C is identified as the optimal temperature for catalytic activities ( Table 4. 5). Table 4.5 Effect of temperature on the oxidation reaction used (G-dye 10 wt % -FeP)n MOF as a catalyst. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,140,150, and 160, and 170 oC ; RPM, 300; 1h. Temperature 140 oC 150 oC 160 oC 170 oC Cyclohexane conversion 3.8 21.4 21.7 25.3 Cyclohexanol selectivity 40.1 22.6 9.9 8.7 Cyclohexanone selectivity 59.9 65.3 71.7 63.1 Useful products selectivity 100 87.9 81.6 71.8 Byproducts selectivity 12.1 18.2 28.2 Useful products Yield 3.8 18.8 17.7 18.2 We examined the influence of pressure on the oxidation of cyclohexane catalyzed by (Gdye 10 wt% -FeP)n MOF at 100 , 150 , and 180 psi. Table S9 shows the effect of reaction pressure on the conversion, yields and selectivity in cyclohexane oxidation catalyzed by (G-dye 10 wt % FeP)n MOF .When the air pressure was 150 psi, the catalyst had the highest activity. In general, the higher the air pressure, the higher the di-oxygen solubility in the liquid phase ( Table 4.6). Table 4.6. Effect of pressure on the oxidation reaction used (G-dye 10 wt%-FeP)n MOF as a catalyst. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 100, 150, and 180 psi; 150 o C; RPM, 300; 1h. Pressure 100 psi 150 psi 180 psi Cyclohexane conversion 4.2 21.4 10.2 Cyclohexanol selectivity 33.9 22.6 21.9 Cyclohexanone selectivity 66.1 65.3 68.1 Useful products selectivity 100 87.9 90 Byproducts selectivity 12.1 10 Useful products Yield 4.2 18.8 9.2 98 The effect of temperature and pressure were studied for (GO 10 wt % -FeP)n MOF. When the reaction is conducted at 150 oC, reasonably high activity (15.5%) and selectivity (93.5%) were obtained. Increasing the reaction temperature to 160 oC or higher led to increased byproducts (> 20%) yield. When the pressure is increased from 150 psi to 180 psi, a slight increase of cyclohexane conversion was observed with an obvious decrease of useful product selectivity. Considering the high conversion and Ketone/Alcohol selectivity, the optimal reaction conditions are identified to be 150 oC under the pressure of 150 psi ( Table 4.7 & 4.8). Table 4.7. Effect of temperature on the oxidation reaction used (GO 10 wt % -FeP)n MOF as a catalyst. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,140,150, and 160, and 170 oC ; RPM, 300; 1h. Temperature 140 oC 150 oC 160 oC 170 oC Cyclohexane conversion 2.9 15.5 24 26.3 Cyclohexanol selectivity 70.9 66 32.4 25 Cyclohexanone selectivity 29.1 27.5 46.9 42.2 Useful products selectivity 100 93.5 79.3 67.2 Byproducts selectivity 6.5 20.7 32.8 Useful products Yield 2.9 14.5 19 17.7 Table 4.8. Effect of pressure on the oxidation reaction used (GO 10 wt%-FeP)n MOF as a catalyst. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 100, 150,180 and 210 psi; temperature, 150 oC; RPM, 300; 1h. Temperature 100 psi 120 psi 150 psi 180 psi 210 psi Cyclohexane conversion 9.4 13.9 15.5 18.7 7.4 Cyclohexanol selectivity 55.4 50.6 66 39 45.5 Cyclohexanone selectivity 41.8 43.2 27.5 46.2 43.1 Useful products selectivity 97.2 92.8 93.5 85.2 88.6 Byproducts selectivity 2.8 6.2 6.5 14.8 11.4 Useful products Yield 9.1 13 14.5 15.9 6.6 99 The effect of temperature and pressure were studied for (Fe-P)n MOF. The optimized reaction conditions involve using a reaction temperature of 150 oC under the pressure of 150 psi ( Table 4.9 & 4.10). Table 4.9. Effect of temperature on the oxidation reaction used (Fe-P)n MOF as a catalyst. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,140,150, and 160, and 170 oC ; RPM, 300; 1h. Temperature 140 oC 150 oC 160 oC 170 oC Cyclohexane conversion 2.3 5.2 5.9 6.8 Cyclohexanol selectivity 37.9 34.5 18.1 14.8 Cyclohexanone selectivity 62.1 65.3 67.6 48.7 Useful products selectivity 100 99.5 85.7 63.5 Byproducts selectivity 0.5 14.3 36.5 Useful products Yield 2.3 5.2 5.0 4.4 Table 4.10. Effect of pressure on the oxidation reaction used (Fe-P)n MOF as a catalyst. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 100, 150,180 and 210 psi; temperature, 150 oC; RPM, 300; 1h. Temperature 100 psi 120 psi 150 psi 180 psi Cyclohexane conversion 1.8 3.7 5.2 4.3 Cyclohexanol selectivity 32.7 29.5 34.5 25.1 Cyclohexanone selectivity 67.3 70.2 65.3 72.6 Useful products selectivity 100 99.7 99.5 97.7 Byproducts selectivity 0.3 0.5 2.3 Useful products Yield 1.8 3.7 5.2 4.2 100 To check whether the catalytic activity is affected by changing the metal center, we have prepared similar composite in (G-dye -FeP)n MOF and (GO-FeP)n MOF by replacing iron with zinc. Comparison of the data reveals that the trend of catalytic activity was similar between iron and zinc. However the catalytic efficiences were poorer than that of the Fe series. The catalytic activation of metallo-porphyrins is influenced by the valency of the metal atoms and their redox activities, and Fe3+/Fe2+ is clearly a more redox active (E1/2= + 0.77 V) system compared to Zn2+ ( Table 4.11 & 4.12). Table 4.11. Comparison of catalytic activities of (G-dye -ZnP)n MOF series. Experimental conditions: catalyst,10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,150 oC ;RPM,300; 1h. Catalyst (G-dye wt % ZnP)n MOF (G-dye 10 wt % ZnP)n MOF (G-dye 25 wt % -ZnP)n MOF (G-dye 50 wt % ZnP)n MOF Cyclohexane conversion 9.34 14.8 13.2 8.9 Cyclohexanol selectivity 32.9 26.7 29.3 35. Cyclohexanone selectivity 67.1 69.3 68.9 62.2 Useful products selectivity 100 96 98.2 97.7 Byproducts selectivity 1.8 2.3 Useful products Yield 9.34 14.2 12.9 8.7 Zn wt % 12.89 10.06 10.13 7.87 TOF (h-1) 873 1702 1535 1333 Table 4.12. Comparison of catalytic activities of (GO -ZnP)n MOF series. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,150 oC ; RPM, 300; 1h. Catalyst (GO wt % -ZnP)n MOF (GO 10 wt % -ZnP)n MOF (GO 25 wt % -ZnP)n MOF Cyclohexanol selectivity 29.2 28 29.5 Cyclohexanone selectivity 69.4 69.1 68.6 Useful products selectivity 98.6 97.1 98.1 Byproducts selectivity 1.4 2.9 1.9 Useful products Yield 8.3 11.7 9.3 Zn wt % 11.71 11.31 10.09 TOF (h-1) 854 1247 1111 To investigate if the presence of r-GO is making any difference to the catalytic performance, we compare the catalytic performance of (G-dye-FeP)n MOF, (GO-FeP)n MOF as 101 well as (Fe-P)n MOF composites, respectively. It has been reported previously that addition of amines (pyridine, N-methyl imidazole, piperidine) to PFeO2FeP generates moderately stable ferryl (Fe4+) porphyrinate complex.45,46 It is anticipated that the presence of pyridine ligand connecting porphyrin sites in (G-dye-FeP)n MOF should afford higher catalytic activities compared to that by the epoxy groups in GO. Indeed, the results showed that the catalytic activity, which is defined by performance indicators such as cyclohexane conversion, TOF (h -1) (turnover frequency, moles of useful product produced per mole of Fe per hour.), byproduct selectivity and useful products yield of the (G-dye 10 wt % -FeP)n MOF is higher than that of (Fe-P)n MOF and (GO -FeP)n MOF hybrids (Figure 4.13). Figure 4.13 GC/MS specrtra comparison (a) (G-dye 10 wt % -FeP)n MOF ,(b) (GO 10 wt % -FeP)n MOF, and (c) (Fe-P)n MOF. Table 4.13 Comparison of catalytic activities of (G-dye -FeP)n MOF series. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,150 oC ;RPM,300; 1h. Catalyst (G-dye wt % -FeP)n MOF (G-dye 10 wt % -FeP)n MOF (G-dye 25 wt % -FeP)n MOF (G-dye 50 wt % -FeP)n MOF Cyclohexane conversion 18.4 21.4 18.9 16.8 Useful products selectivity 95.9 87.9 92.9 92.2 cyclohexanol selectivity 29 22.6 26.5 26.7 cyclohexanone selectivity 66.9 65.3 65.4 65.5 Byproducts selectivity 4.1 12.1 8.1 7.8 Useful products Yield 17.6 18.8 17.5 15.5 Fe wt % 11.27 10.11 9.56 8.92 -1 1622 1932 1901 1805 TOF (h ) 102 Table 4.14 Comparison of catalytic activities of (GO-FeP)n MOF series. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,150 oC; RPM,300; 1h. Catalyst (GO wt % -FeP)n MOF (GO 10 wt % -FeP)n MOF (GO 25 wt % -FeP)n MOF Cyclohexane conversion 13 15.5 10.5 Useful products selectivity 96 93.5 96.6 cyclohexanol selectivity 29 27.5 28.7 cyclohexanone selectivity 67 66 67.9 Byproducts selectivity 6.5 3.4 Useful products Yield 12.5 14.5 10.1 Fe wt % TOF (h-1) 10.84 1197 8.99 1675 7.2 1457 Table 4.15 Control experiment to check the yield of the oxidation reaction in the absence of catalysts. Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure,150 psi ; temperature,150 o C ; RPM, 300; 1h. Catalyst Pure porpyrine (TCPP) (Fe –P)n MOF Oxidation without catalyst Cyclohexane conversion 3.1 5.2 0.3 Cyclohexanol selectivity 38.7 34.5 61.5 Cyclohexanone selectivity 61.3 65.3 38.5 Useful products selectivity 100 99.5 100 Byproducts selectivity 0.5 Useful products Yield 3.1 5.2 0.3 Catalytic activity was observed only when our hybrid MOF/G-dye catalysts were present in the reaction mixture. No catalytic activity was observed in the control reactions where the catalyst was absent. In the presence of only TCPP and also (Fe–P)n MOF , the catalytic activity is lower compared to the MF/G-dye hybrid (Table 4.15). The obtained byproducts are mainly ring-opened acids such as n-butric, n-valeric, succinic, glutaric and adipic acid and the useful products are cyclohexanone and cyclohexanol. (G-dye 10 wt % -FeP)n MOF shows 21 % of cyclohexane conversion, 12% byproducts selectivity and 19 % useful products yield compared to 5% of cyclohexane conversion, 0.5 % of byproducts selectivity and % useful products yield of (Fe-P)n MOF (Figure 4.14). 103 Figure 4.14 Comparison of catalytic activities of (Fe-P)n MOF, (G-dye 10 wt %-FeP)n MOF, and (GO 10 wt %-FeP)n MOF. Cyclohexane conversion (pink column), byproduct selectivity (blue column), useful product yield ( green column). Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,150 oC; RPM,300; 1h. The catalytic activities improve with increasing amount of G-dye addition in the MOF, as evidenced by comparing (G-dye 10 wt % -FeP)n MOF with (G-dye wt % -FeP)n MOF (Table 4.13). However, the catalytic activity decreases beyond the threshold weight percentage of 10 %, which may be due to the phase segregation of G-dye and MOF (Figure 4.10 & Table 4.16). As judged from SEM and powder XRD data, it appears that the excess amounts of G-dye causes aggregation of the r-GO sheets . According to EDS data the carbon and oxygen content in the hybrids increase with increasing concentration of G-dye. The presence of O and C elements is due to increasing amount of r-GO in G-dye. The relative Fe content is decreasing due to dilution effect by increasing amount of G-dye for a fixed mass of Fe (Table 4.16). Table 4.16 EDS data compares the atomic percentage of the element in (G-dye-FeP)n MOF series (G-dye wt % -FeP)n (G-dye 10 wt % -FeP)n (G-dye 25 wt % -FeP)n (G-dye 50 wt % -FeP)n MOF MOF MOF MOF Element Atom% Atom% Atom% Atom% C 77.18 78.52 80.06 82.10 O 13.45 14.37 15.13 15.79 Fe 9.37 7.11 4.81 2.11 Total 100.00 100.00 100.00 100.00 104 The robustness of the composite MOF catalyst was tested by using the catalyst (G-dye 10 wt % -FeP)n in four consecutive reactions and the results are shown in Table 4.17. After four consecutive cycles, the catalyst retained its catalytic activity of ~ 18 % useful products yield, 21 % conversion of cyclohexane and 87 % useful products selectivity. Table 4.17 Comparison of catalytic activities of reused (G-dye 10 wt % -FeP)n MOF. Number of Runs [a] Run Run Run Run Cyclohexane Conversion 21.4 21.2 20.9 20.5 Cyclohexanol Selectivity 22.6 22.3 22.1 21.8 Cyclohexanone Selectivity 65.3 65.1 64.7 64.5 Useful products Selectivity 87.9 87.4 86.8 86.3 12.1 12.6 13.2 13.7 18.8 18.5 18.1 17.7 Byproducts Selectivity Useful products Yield [a] [b] [c] Experimental conditions: catalyst, 10 mg; cyclohexane, 20 mL; oxygen pressure, 150 psi ; temperature,150 oC ; RPM,300; 1h. acid. [c] [b] By-products are mainly ring-opened acids such as n-butric, n-valeric, succinic, glutaric and adipic Useful products are mainly cyclohexanone and cyclohexanol In order to check the stability of (G-dye 10 wt % -FeP)n MOF after oxidation, the supernatant was analyzed by UV-vis spectroscopy and FTIR (Figure 4.15 & 4.16), and no traces of metallo-porphyrin or degradation products were observed. After the oxidation reaction was completed, i.e., when the reduction of oxygen content in the tail gases ceased, the catalyst was recovered by simple separation from the reaction mixture, followed by washing with ethanol and drying in air. UV-Vis spectroscopy of the reused catalyst in phosphate buffer shows similar soret band (391 nm) and Q bands (564 nm and 611 nm) attesting the robustness of the catalyst. 105 Figure 4.15 UV-vis absorption spectra of (G-dye 10 wt % -FeP)n MOF (4.8 mg L-1) in phosphate buffer, and supernatant solution collected after finishing cyclohexane oxidation. The spectra of the reused catalysts largely resemble that of as-prepared (G-dye 10 wt % FeP)n MOF. The presence of almost all functional groups in the catalyst after reaction attests to the stability of catalyst. Figure 4.16 FTIR spectra of (a) (G-dye 10 wt % -FeP)n , (b) reused (G-dye 10 wt % -FeP)n MOF which collected after run oxidation of cyclohexane, (c) after runs, (d) after runs ,and (e) after runs. For MOF hybrids with to 25 wt % of graphene derviatives, the catalytic activity of all (GO-FeP)n MOF samples is less than that of (G-dye-FeP)n MOF (Table 4.13 & 4.14). One explanation is that functionalizing graphene with pyridine provides stronger framework connectivity in the metal porphyrinate compared to the monodentate coordination of the native epoxy linkage in GO. The pyridine ligand is longer than the epoxy groups on GO, thus it allows the spacing out of the porphyrin site for access by candidate reactants in catalysis. Cofacial 106 porphyrin struts which are anchored on the graphene sheet in parallel fashion favor the bridging of the reactant between the iron center, this will facilitate the breakage of the O–O bond when O2 is bridged between the iron porphyrinate to produce (P)Fe( IV)=O as the catalytic center, the latter has been proposed to participate in a broad spectrum of oxygenation reaction pathways. 45,46 4.4 Conclusions In summary, we have synthesized a graphene-dye-porphyrin MOF by adding pyridinium dye-functionalized r-GO sheets into the metalloporphyrin network. Our studies reveal that functionalized r-GO sheets can influence the crystallization process of MOF and promote the catalytic properties of the composite when an appropriate amount is added. These composite MOF are thermally stable to 400 °C and exhibit robust catalytic activities. 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Soc. 1998, 120, 2186-2187. 110 111 [...]... 10.62 4. 56 19 .42 20.2 0 1 4 12.73 4. 17 21.26 61.7 0 2 0 14. 12 3.88 22.86 36.3 0 2 1 14. 37 3. 34 26.67 21.1 0 2 2 15.09 3.22 27.63 16 .4 0 2 3 16.22 2. 94 30.31 75.5 0 2 4 17.69 2.85 31.26 15 .4 0 2 5 19 .42 2 .48 36.01 12 .4 0 3 0 21.25 2. 34 38.37 23.3 0 3 3 22.72 2.12 42 .47 20.1 1 0 -4 26. 64 1.85 48 .95 19.1 0 4 1 28.59 1 1 -1 33.28 1 2 0 37.60 1 3 -1 39.15 1 2 2 41 . 94 1 3 1 43 .01 1 1 4 44. 80 90 Table 4. 3 X-ray... 0 4 34. 36 2.01 45 .01 7.5 0 4 4 35.55 2 0 0 36.01 2 1 1 37 .46 2 0 -2 38.19 2 2 2 40 . 94 2 3 -2 41 . 54 2 4 0 42 .07 2 1 3 43 .37 2 2 3 44 .40 3 91 Table 4. 4 X-ray powder diffraction peaks and indices of (G-dye 10 wt % -FeP)n MOF d(Ao) Angle (degree) 2 Intensity (counts) h k l Angle (degree) 2 16.01 5.51 12 .4 0 0 2 5.5 12. 14 7.27 45 .8 2 0 0 7.22 10.26 8.61 58.1 2 0 2 9.08 9.82 9.18 37.6 0 0 4 11.02 7. 94 11.12... 0 4 13.19 6.71 13.16 14. 5 1 1 1 13 .4 5.69 15. 54 12.1 1 1 2 14. 23 4. 85 18.25 80.9 1 1 3 15.52 4. 41 20.01 33.9 3 1 1 16.88 4. 25 22.29 39 .4 3 1 2 17.55 3. 84 23 .47 43 .6 3 1 3 18.62 3.51 25.33 17.7 3 1 4 20.02 3.28 27. 14 13.9 5 1 1 22.31 3.09 28.81 16.5 0 2 0 25.36 2. 94 30.28 10.9 2 2 1 26.55 2. 84 31 .42 26.9 2 2 3 27.71 2.51 35.79 14. 5 2 2 4 28.69 2.36 38.07 11.6 4 2 2 29.86 2.25 39.88 16.8 4 2 4 31 .42 ... 20. 14 4.21 21.05 18.3 1 2 0 20.67 3.88 22.88 26.8 1 2 -1 21.59 3.69 24. 04 12.5 1 2 1 22.12 3.51 25. 34 12.3 1 0 -2 22.22 3.31 26. 84 12.3 1 0 2 23. 24 3.21 27. 74 12.5 1 1 2 23. 84 3. 04 29.27 7.31 1 2 -2 24. 62 2. 94 30.33 33.5 1 3 1 25.1 2.82 31.61 18.7 1 3 2 28.19 2.61 34. 26 17.5 1 2 -3 29.15 2 .45 36.61 6.9 1 2 3 30.35 2.32 38.62 9.8 1 3 -3 31.51 2.25 39.93 5.9 1 3 3 32.63 2.18 41 .27 8.3 1 1 -4 33.37 2.11 42 .81... 2.25 39.88 16.8 4 2 4 31 .42 2.11 42 .83 15.9 4 2 5 32. 54 2.01 45 .06 17.1 6 2 0 33.68 1. 94 46.63 12.7 1 3 1 38. 74 1 3 2 39.05 1 3 3 39.58 3 3 1 40 .17 3 3 4 41.68 3 3 5 42 .57 5 3 4 44. 35 Powder X-ray diffraction studies of the evacuated (G–dye 10 wt % -FeP)n MOF confirms that the compound changes from monoclinic to orthorhombic crystal type (a = 16 .40 9 Å, b = 24. 585 Å, c = 24. 585 Å, α = 90°, β = 90o , γ... (G-dye 10 wt % -FeP)n MOF 0.139 0.115 0.1 84 0. 345 Rel FOM FOM 658 238 182 143 Peaks Found 11 of 12 13 of 14 15 of 16 21 of 22 System Triclinic Monoclinic Monoclinic Orthorhombic a 9.8117 20.35 74 4.989 16 .40 95 b 8.2878 5.21 54 16.8836 24. 58 54 c 4. 976 11.9129 12.599 15.9861 α 88.823 90 90 90 β 91 .49 1 99.711 92.6 24 90 γ 101.272 90 90 90 Volume 396.65 1 246 .71 1060.13 644 9.31 Zero -0.09518 0.0255 0.03519 0.10703... 20 04, 8 , 41 8 109 (44 ) Zampronio, E C.; Gotardo, M.; Assis, M D.; Oliveira, H P Catal Lett 2005, 1 04 , 53 (45 ) Balch, A.; Chan, Y.; Cheng, R.; La Mar, G.; Latos-Grazynski, L.; Renner, M J Am Chem Soc 19 84, 106 , 7779 (46 ) Chin, D H.; Lamar, G N.; Balch, A L J Am Chem Soc 1980, 102, 43 44- 4350 (47 ) Williams, J.; Adel, R.; Carlson, J.; Reynolds, G.; Borden, D.; Ford Jr, J J Org Chem 1963, 28 , 387 (48 )... Chem 2007, 46 , 5 544 (39) Smithenry, D W.; Wilson, S R.; Suslick, K S Inorg Chem 2003, 42 , 7719 (40 ) Suslick, K S.; Bhyrappa, P.; Chou, J H.; Kosal, M E.; Nakagaki, S.; Smithenry, D W.; Wilson, S R Acc Chem Res 2005, 38, 283 (41 ) Ma, B Q.; Mulfort, K L.; Hupp, J T Inorg Chem 2005, 44 , 49 12 (42 ) Ohmura, T.; Usuki, A.; Fukumori, K.; Ohta, T.; Ito, M.; Tatsumi, K Inorg Chem 2006, 45 , 7988 (43 ) Yuan, Y.;... 120 psi 150 psi 180 psi 210 psi Cyclohexane conversion 9 .4 13.9 15.5 18.7 7 .4 Cyclohexanol selectivity 55 .4 50.6 66 39 45 .5 Cyclohexanone selectivity 41 .8 43 .2 27.5 46 .2 43 .1 Useful products selectivity 97.2 92.8 93.5 85.2 88.6 Byproducts selectivity 2.8 6.2 6.5 14. 8 11 .4 Useful products Yield 9.1 13 14. 5 15.9 6.6 99 The effect of temperature and pressure were studied for (Fe-P)n MOF The optimized reaction... peaks and indices of (G-dye 10 wt % -FeP)n MOF d(Ao) Angle (degree) 2 Intensity (counts) h k l Angle (degree) 2 16.77 5.26 20.9 0 1 0 5.22 11.58 7.62 15.6 0 1 1 8.75 9.96 8.86 120.1 0 2 0 10 .47 8.38 10. 54 21.1 0 2 1 12.61 6.93 12.66 23.6 0 0 2 14. 061 5.88 15.03 27.5 0 1 2 15.012 5 .49 16. 14 7.7 0 2 2 17.56 5.15 17.18 42 .5 1 0 0 17.78 4. 98 17.78 103.2 1 1 0 18. 54 4.76 18.81 15.5 1 1 -1 19.55 4. 52 19.61 . 0 4 34. 36 2.01 45 .01 7.5 0 4 4 35.55 2 0 0 36.01 2 1 1 37 .46 2 0 -2 38.19 2 2 2 40 . 94 2 3 -2 41 . 54 2 4 0 42 .07. 2.85 31.26 15 .4 0 2 5 19 .42 2 .48 36.01 12 .4 0 3 0 21.25 2. 34 38.37 23.3 0 3 3 22.72 2.12 42 .47 20.1 1 0 -4 26. 64 1.85 48 .95 19.1 0 4 1 28.59 . 1 1 1 13 .4 5.69 15. 54 12.1 1 1 2 14. 23 4. 85 18.25 80.9 1 1 3 15.52 4. 41 20.01 33.9 3 1 1 16.88 4. 25 22.29 39 .4 3 1 2 17.55 3. 84 23 .47 43 .6 3 1 3