Graphene metal organic framework composites and their potential applications 4

77


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
O
C and
150 psi O
2
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 sp
2
conjugated carbon atoms,
1
can be considered as
a giant polyaromatic platform for performing chemistry due to its open ended structure.
2
The
combination of high surface area (theoretical value of  2600 m
2

/g),
3,4
high electrical
conductivity
5,6
and low manufacturing cost
7
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
9
imparts
bifunctional properties on the material
10
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

78

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 FeCl
2
.4H
2

O,
and herewithal abbreviated as (Fe-P)
n
MOF. G-dye represents chemically reduced GO (r-GO)

79

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)
n
MOF, and (G–dye -FeP)
n
MOF.

In order to study the structure-composition relationship, different weight percentages of G-
dye (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

80

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 FeCl
2
.4H
2
O.
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 4-
picoline (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-(4-
Nitrostyryl) pyridine.
4-(4-aminostyryl) pyridine : To a mixture of 4-(4-Nitro styryl) pyridine (2 g, 9 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 2 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-(4-
aminostyryl) 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 3 mL HCl solution (10 %, 3.2 M, 9.6 mmol) and stirred for 45 min.
The temperature was maintained at 0-5
o
C during the reaction and the solution turned orange at
the end.

81

The preparation of G-dye was performed by sonicating 150 mg of r-GO dispersed in 1 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 0 - 5
o
C for around 4 hours. Next, the reaction was stirred at room temperature for another
4 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
FeCl
2
.4H
2
O (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 4 mL DMF and 7 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), FeCl
2
. 4H
2
O (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 4 mL DMF and 7
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 O
2
pressure was adjusted to 150 psi. During the oxidation process, the O
2
pressure was

82

kept at 150 psi with a stirring rate of 300 rpm. After 1 hour of reaction, the reactor was cooled
down to 30 ºC and the mixture was dissolved in ethanol. An excessive amount of
triphenylphosphine (Ph
3
P) 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 GO
25
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 [  *

83

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 5 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

84

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.

85

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 coordination-
assisted 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 2p
3/2
in

86

(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 2p
3/2
core level spectra for (Fe-P)
n
MOF. (B) The Fe 2p
3/2
core level spectra for (G-
dye 10 wt % -FeP)
n
MOF. (C) Comparison between The Fe 2p
3/2
core level spectra for (a) (Fe-P)
n
MOF ,
(b) (G-dye 10 wt % -FeP)
n
MOF.

87




Figure 4.4 (A)The N 1s core level spectra for (Fe-P)
n
MOF. (B) The N 1s core level spectra for (G-dye 10
wt % -FeP)

n
MOF. (C) Comparison between The N 1s

core level spectra for (a) (Fe-P)
n
MOF , (b) (G-dye
10 wt % -FeP)
n
MOF.


Powder X-ray diffraction was used to examine the phase and structure of the synthesized
products, as shown in Figure 4.5. Increasing the content of G-dye in the composite from 5 wt %
to 50 wt% results in increased lattice distortion of MOF, therefore gradual transformation into the
amorphous phase is expected. G-dye plays the role of an intercalant in the basic framework of
(Fe-P)
n
MOF.
11
(Fe-P)
n
MOF is assigned to the triclinic crystal type with lattice parameter ; a =
9.811 Å, b = 8.287 Å, c = 4.976 Å, α = 88.823°, β = 91.491
o
, γ = 101.272
o
, space group: P -1
(No. 2). By adding 5 wt % of G - dye to the precursor of (Fe -P)
n
MOF, the main peaks shifted

and some peaks vanished. The lattice structure has transformed into monoclinic phase with lattice

88

parameters as follows; a = 20.357 Å, b = 5.215 Å, c = 11.912 Å, α = 90°, β = 99.711
o
, γ = 90
o
,
space group: C2 (No. 5). When the content of G-dye is increased to 10 wt %, the composite
maintains its monoclinic crystal type with a slightly changed lattice parameters a = 4.989 Å, b =
16.883 Å, c = 12.599 Å, α = 90°, β = 92.624
o
, γ = 90
o
, space group: P2/c (No. 13). The broad
peak in the region from 11
o
to 17
o
is attributed to that of G-dye. Powder X-ray diffraction studies
of dehydrated (G -dye 10 wt % -FeP)
n
MOF (Figure 4.6) confirms that the compound retains its
crystallinity after removing the solvent. However increasing the amount of G-dye to 25 wt % - 50
wt % destroys the crystallinity and the structure becomes amorphous.



Figure 4.5 XRD pattern of (a) (Fe-P)

n
MOF, (b) (G-dye 5 wt % -FeP)
n
MOF, (c) (G-dye 10 wt % -FeP)
n

MOF, (d) (G-dye 25 wt % -FeP)
n
MOF [Note: similar to the XRD pattern of (G - dye 50 wt % -FeP)
n

MOF] , and (e) G-dye.










89


Table 4.1 Lattice information of (Fe-P)
n
MOF, and (G-dye -FeP)
n
MOF composites

(Fe-P)
n
MOF
(G-dye 5 wt % -FeP)
n

MOF
(G-dye 10 wt % -FeP)
n

MOF
Evacuated
(G-dye 10 wt % -FeP)
n
MOF

Rel. FOM

0.345

0.139

0.115

0.184
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
20.3574
4.989
16.4095
b
8.2878
5.2154
16.8836
24.5854
c
4.976
11.9129
12.599
15.9861
α
88.823
90
90

90
β
91.491
99.711
92.624
90
γ
101.272
90
90
90
Volume
396.65
1246.71
1060.13
6449.31
Zero
-0.09518
0.0255
0.03519
0.10703
Extinction Class
P-1
C2
P2/c
Cmca
2theta Div
0.005455
0.021084
0.026762

0.036336
Stability
2
1
3
6
Impurities
0
1
1
1
Out
29
33
33
52
Calc
40
49
51
87

Rel. FOM: is the relative figure of merit.
FOM: is the de Wolff figure of merit.
Peaks Found: indicates how many peaks defined for indexing were successfully indexed
using this solution.

System: is the crystal system of the indexing solution.
Lattice parameters: a, b, c, α, β, γ
Volume: is the unit cell volume for the solution.

Zero: is the zero point correction.
Extinction class: is the representative space group of the indexing solution.
2theta div: is the average absolute difference between the experimental and calculated peak
positions in degrees.

90

Stability: is the stability parameter for the solution. This parameter indicates, roughly, the
minimum number of experimental peaks that need to be removed so that the indexing solution is
no longer uniquely defined. The higher the stability parameter, the more reliable the solution. This
parameter is only valid for 3D searches and is omitted if 1D or 2D searches are performed.
Impurities: indicates the number of impurity peaks.
Out: indicates the number of calculated peaks which have no overlap with observed reflections.
Calc: displays the total number of calculated peaks in the powder pattern between 0 and the
maximum 2-theta of the input experimental data, given the unit cell and extinction class.
Table 4.2 X-ray powder diffraction peaks and indices of (G-dye 5 wt % -FeP)
n
MOF
d(A
o
)
Angle (degree) 2
Intensity (counts)

h
k
l
Angle (degree) 2

16.61

5.31
17.2

0
0
2
5.29
1.71
7.54
28.7

0
1
0
7.04
10.01
8.83
60.8

0
1
1
7.53
8.33
10.61
25.5

0
1
2

8.81
5.91
15.01
16.1

0
0
4
10.59
5.02
17.63
181

0
1
3
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













91

Table 4.3 X-ray powder diffraction peaks and indices of (G-dye 10 wt % -FeP)
n
MOF

d(A
o
)
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
17.7

1
1
1
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
8.1

1
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
44.403

92

Table 4.4 X-ray powder diffraction peaks and indices of (G-dye 10 wt % -FeP)
n
MOF

d(A
o
)
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
19.2

2
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
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 = 16.409 Å, b =
24.585 Å, c = 24.585 Å, α = 90°, β = 90
o
, γ = 90
o
, space group: Cmca (No. 64)) in the
framework structure after removal of the guest molecules as evidenced by presence of discrete
peaks. The asymmetry probably arises from van der Waals forces between the two interpenetrated
frameworks in the absence of solvent. The peak shift in the XRD pattern is attributed to the

93

removal of the coordinated ligands, which result in modification of the overall structure. This
manifests as broadening and shifting in the XRD, which has also been observed in other kind of
MOF previously.
52-58

Figure 4.6 Comparison between XRD pattern of (a) (G-dye 10 wt % -FeP)
n
MOF and (b) evacuated (G-
dye 10 wt % -FeP)
n
MOF.

According to size counting by AFM (Figure 4.7), the average diameter of the graphene
sheets we used is in the range of 300 nm.
Accompanying the change in crystal structure, the morphology of (Fe-P)
n
MOF crystal

changes from plate shape to rod shape
24
as increasing amount of G-dye (5 wt % and 10 wt %) is
added, as shown in SEM images Figure 4.8. Most importantly, graphene flakes can be recovered
from the dissolved (G-dye 10 wt%-FeP)
n
MOF, which proves the presence of graphene
incorporation in the MOF structure (Figure 4.9).

94


Figure 4.7 (a) AFM topology of G-dye on Si substrate. (b) Histogram of the size distribution of G-dye
flakes.


Figure 4.8 SEM images for (a) (Fe-P)
n
MOF, (b) (G-dye 5 wt % -FeP)
n
MOF,(c) & (d) (G-dye 10
wt %-FeP)
n
MOF, (e) (G-dye 25 wt % -FeP)
n
MOF and (f) G-dye.

The thermal stability of (G-dye-FeP)
n
MOF composites is an important property for

application as catalysts at elevated temperatures, and this was investigated by thermogravimetric
analysis in N
2
flow (Figure 4.9, 4.10). The TGA spectrum show that about 20 – 25 % of the initial
mass loss occurs after heating to 100
◦
C. The presence of functionalized Graphene gives rise to

95

characteristic mass reduction at 250-300 °C caused by pyrolysis of the oxygen-containing
functional groups, generating CO, CO
2
and stream. The MOF structure collapsed at 400 °C.

Figure 4. 9 TGA and DTA plots for evacuated (G - dye 10 wt % -FeP)
n
MOF.



Figure 4.10 (a) TGA plots for freshly synthesized (G-dye - FeP)
n
MOF series (under N
2
, 10
o
C /min) ,(a) (G -
dye 50 wt % -FeP)
n

MOF , (b) (G - dye 25 wt % -FeP)
n
MOF,(c) (G - dye 10 wt % -FeP)
n
MOF, and (d) (G -
dye 5 wt % -FeP)
n
MOF. (b) DTA plots for freshly synthesized (G-dye - FeP)
n
MOF series ,(a) (G - dye 50
wt % -FeP)
n
MOF , (b) (G - dye 25 wt % -FeP)
n
MOF,(c) (G - dye 10 wt % -FeP)
n
MOF, and (d) (G - dye 5
wt % -FeP)
n
MOF

Entrapped water
38
acts as axial ligand in the structure and its easy removal leaves open
catalytic sites. The adsorption surface area of the (G-dye-FeP)
n
MOF hybrids was evaluated using
adsorption isotherm measurements. The adsorption of N
2
follows a type I isotherm with a BET

surface area of 932.75 m
2
/g for (G-dye 50 wt% -FeP)
n
MOF). This compares favorably with

96

zeolite
39,40
which has typical surface area in the range of 500 m
2
/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 (cm
3
g
-
1
) 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 5 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/P
0
is the
pressure (P) to saturation pressure (P
0
) with P
0
= 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
min (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).

97


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
2
. 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

98

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
o
C ; RPM, 300; 1h.


We examined the influence of pressure on the oxidation of cyclohexane catalyzed by (G-
dye 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
0

12.1
10
Useful products Yield
4.2
18.8
9.2


Temperature
140
o
C
150
o
C
160
o
C
170
o
C

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
0
12.1
18.2
28.2
Useful products Yield
3.8
18.8
17.7
18.2

99

The effect of temperature and pressure were studied for (GO 10 wt % -FeP)
n
MOF. When
the reaction is conducted at 150
o

C, reasonably high activity (15.5%) and selectivity (93.5%)
were obtained. Increasing the reaction temperature to 160
o
C 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
o
C 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
o
C ; RPM, 300; 1h.

Temperature
140
o
C
150
o
C
160
o
C

170
o
C
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
0
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
o
C; 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



100

The effect of temperature and pressure were studied for (Fe-P)
n
MOF. The optimized
reaction conditions involve using a reaction temperature of 150
o
C 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
o
C ; RPM, 300; 1h.

Temperature
140
o
C
150
o
C
160
o
C
170
o
C

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
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
o
C; 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
0.3
0.5
2.3
Useful products Yield
1.8
3.7
5.2
4.2



101

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 Fe
3+
/Fe
2+
is clearly a more redox active (E
1/2
= + 0.77 V) system compared to Zn
2+
( 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
o
C ;RPM,300; 1h.

Catalyst
(G-dye 5 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. 5
Cyclohexanone selectivity
67.1
69.3
68.9
62.2
Useful products selectivity
100
96
98.2
97.7
Byproducts selectivity
0
4

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
o
C ; RPM, 300; 1h.

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
Catalyst
(GO 5 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