Copper based macromolecular catalysts for the hydroxylation of phenols

Article abstract of DOI:10.1016/j.jorganchem.2020.121579

Copper containing macromolecules, viz., Copper-meso-tetrakis-(4-carboxyphenyl)porphyrin (CuTCPP) and Copper-meso-tetra-phenylporphyrin (CuTPP) complexes were synthesized and heterogenized on the surface of aminated SBA-15 molecular sieves. The ¹H NMR, mass spectral and elemental (CHN) analyses were performed to confirm the formation of these copper complexes (CuTCPP and CuTPP). The absence of ¹H NMR signal, characteristic of N[sbnd]H bond (-2.79 ppm in TCPP and -2.74 ppm in TPP) in CuTCPP and CuTPP clearly confirms the incorporation of copper ion in to the cavities of the TCPP and TPP ligands. In addition, the decreased intensity of the soret band and the reduction of the number of Q bands also supports the formation of copper complexes. Further the homogeneous CuTCPP and CuTPP were grafted on the amino functionalized SBA-15. The grafting of copper complexes on amino functionalised SBA-15 was evident from the surface area analysis, powder XRD and N₂ sorption isotherm. In the powder XRD, the shift in 2θ value compared to parent SBA-15 materials, and the broadening of the signal supports the grafting of copper complexes in to the amino functionalized SBA-15. Both homogeneous and heterogeneous copper macromolecular complexes were used as catalysts for the hydroxylation of phenolic compounds. The catalysts showed comparable conversion (72%) with the formation of ortho- and para- substituted products which have industrial importance. The hydroxylation reaction using the copper based macromolecular catalyst was found to be first order with an activation energy of 26.03 KJ/mol. Subsequently various substituted phenolic compounds, anisole and benzene also yielded the corresponding hydroxylated product with comparable conversion.

Full text of DOI:10.1016/j.jorganchem.2020.121579

Journal Pre-proof
Copper based macromolecular catalysts for the hydroxylation of
phenols
Kaiprathu Anjali , Venkatachalam Ganesh , Jayaraj Christopher ,
Ayyamperumal Sakthivel
PII:
S0022-328X(20)30482-4
DOI:
Reference:
https://doi.org/10.1016/j.jorganchem.2020.121579
JOM 121579
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Journal of Organometallic Chemistry
Received date:
Revised date:
Accepted date:
14 August 2020
29 September 2020
12 October 2020
Please cite this article as: Kaiprathu Anjali , Venkatachalam Ganesh , Jayaraj Christopher ,
Ayyamperumal Sakthivel , Copper based macromolecular catalysts for the hydroxylation of phenols,
Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2020.121579
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Highlights
 Copper containing macromolecules, viz., (CuTCPP) and (CuTPP) were synthesized.
1
 The H NMR, mass spectral and elemental (CHN) analyses confirmed the structure of
CuTCPP and CuTPP.
 CuTCPP and CuTPP were grafted on the amino functionalized SBA-15.
 The catalysts used for the hydroxylation of phenolic compounds.
 CuTCPP showed comparable conversion of phenol (72 %) with an activation energy of
26.03 KJ/mol.
Copper based macromolecular catalysts for the hydroxylation of
phenols
Kaiprathu Anjali[a], Venkatachalam Ganesh[b], Jayaraj Christopher[c], and Ayyamperumal
Sakthivel [a*]
[a] Inorganic Materials & Heterogeneous Catalysis Laboratory, Department of Chemistry,
School of Physical Sciences, Sabarmati Building, Central University of Kerala Kasaragod,
Tejaswini Hills, Kasaragod 671320, India., Email: sakthivelcuk@cukerala.ac.in
[b] Electrodics & Electrocatalysis Division, CSIR-Central Electrochemical Research Institute
(CSIR-CECRI), Karaikudi 630003, Tamil Nadu, India
[c]Indian Oil Corporation Ltd., R & D Centre, Faridabad, India
Abstract
Copper containing macromolecules, viz., Copper-meso-tetrakis-(4-carboxyphenyl)porphyrin
(CuTCPP) and Copper-meso-tetra-phenylporphyrin (CuTPP) complexes were synthesized and
heterogenized on the surface of aminated SBA-15 molecular sieves. The 1H NMR, mass spectral
and elemental (CHN) analyses were performed to confirm the formation of these copper
complexes (CuTCPP and CuTPP). The absence of 1H NMR signal, characteristic of N-H bond (-
2.79 ppm in TCPP and -2.74 ppm in TPP) in CuTCPP and CuTPP clearly confirms the
1

incorporation of copper ion in to the cavities of the TCPP and TPP ligands. In addition, the
decreased intensity of the soret band and the reduction of the number of Q bands also supports
the formation of copper complexes. Further the homogeneous CuTCPP and CuTPP were grafted
on the amino functionalized SBA-15. The grafting of copper complexes on amino functionalised
SBA-15 was evident from the surface area analysis, powder XRD and N2 sorption isotherm. In
the powder XRD, the shift in 2θ value compared to parent SBA-15 materials, and the broadening
of the signal supports the grafting of copper complexes in to the amino functionalized SBA-15.
Both homogeneous and heterogeneous copper macromolecular complexes were used as catalysts
for the hydroxylation of phenolic compounds. The catalysts showed comparable conversion
(72 %) with the formation of ortho- and para- substituted products which have industrial
importance. The hydroxylation reaction using the copper based macromolecular catalyst was
found to be first order with an activation energy of 26.03 KJ/mol. Subsequently various
substituted phenolic compounds, anisole and benzene also yielded the corresponding
hydroxylated product with comparable conversion.
Keywords: Copper-meso-tetrakis-(4-carboxyphenyl)porphyrin (CuTCPP), Copper-meso-tetra-
phenylporphyrin (CuTPP), amino functionalized SBA-15, Phenol hydroxylation.
2

1.
Introduction
Phenolic derivatives, such as catechol and hydroquinone are highly value-added chemicals
having huge industrial applications for the manufacturing of pesticides, flavors, and fragrances.
In addition, these compounds are widely used in the synthesis of pharmaceuticals, medicinal
drugs, dyes, polymers, agrochemicals etc.1-5 Typical phenol hydroxylation processes in
industries are catalysed by various metal ions, their complexes, sulphuric acid and various
mineral acids.6-8 Traditional industrial processes for the synthesis of phenolic chemicals,
catechol and hydroquinone, involve the oxidation of alkyl-benzene derivatives and aniline
hydroxylation.9, 10 However, these methods have lost their market value because of the low
yields, long synthetic routes, formation of numerous bi-products and its adverse effects to the
environment.10 Recent years, numerous homogeneous and heterogeneous catalysts have been
developed for phenol hydroxylation. Among homogeneous catalytic processes, the well-known
Brichima process is based on Fe2+ and Co2+ complexes such as ferrocene and cobalt containing
salt as catalysts in the presence of 60 % aqueous H2O2 as oxidizing agent.11 Another method,
viz., Rhône-Poulenc process, uses strong mineral acids such as phosphoric acid and per-chloric
acid with 70 % aqueous H2O2.12, 13 On the other hand, Ube-Rhone-Poulenc process involve the
use of acid catalysts such as sulphuric or sulphonic acid with ketone peroxides (α-hydroxy-
hydroperoxides), and 60% aqueous H2O2.8 All these methods are based on homogeneous
catalysts, the removal and reuse of the catalysts after the reaction is a great challenge. Therefore,
the development of heterogeneous catalysts for the phenol hydroxylation is an attractive research
area due to its catalytic efficiency, reusability, environmental friendly nature and more
manufacturing control for the industrial chemist. The widely used heterogeneous catalysts are
based on titanosilicates. Microporous titanosilicates TS-114 and TS-215 used for the production of
3

phenolic compounds such as catechol and hydroquinone through an environmentally benign
process. Later Yoshihiro, et al developed a microporous titanium-based catalyst Ti-MCM-68,
which showed excellent catalytic activity.16 However, the application of the microporous
titanosilicate (TS) based catalyst is limited due to its small pore size and complex synthetic
methods on preparation of TS molecular sieves. In order to address the above limitation, recent
years researchers focuses on the preparation of phenolic derivative by direct hydroxylation of
phenol by using peroxide.17 Commonly used oxidizing agents for the hydroxylation of phenol are
19
alkyl hydroperoxides and hydrogen peroxide.18,
However, a disadvantage of hydrogen
peroxide is its low product selectivity. In addition, hydrogen peroxide leads to further oxidation
of phenolic rings by its vacant hydroxyl group which leads to bi-products such as acids and
polymeric compounds.19 Another class of important oxidizing agents are organic peroxides, but
their applications are limited because of their toxicity, explosive, corrosive, and environmentally
destructive nature..20 Therefore, selection of a suitable oxidizing agent and the catalyst for the
hydroxylation process with good conversion and product selectivity is a crucial step. Transition
metal-based complexes were widely used as catalysts for phenol hydroxylation reactions.
Transition metal-based catalysts facilitate some side reactions and gives bi-products such as
quinones and macromolecular tars.21-24 Among various transition metals, copper-based catalysts
have been extensively studied. Variety of copper based molecular sieves show outstanding
catalytic activity for phenol hydroxylation. Wang et al. reported the copper substituted
mesoporous SBA-15 as an excellent catalyst for hydroxylation of phenol with high selectivity for
catechol.25 The Cu2+-exchanged with HY, Hβ, USHY, NaY and HZSM-5 zeolites were also
displayed good catalytic activity for phenol hydroxylation using hydrogen peroxide as the
oxidant in an atmospheric batch reactor. These catalysts were observed to be more active than TS
4

based catalysts.26 Additionally, the copper based catalysts such as Cu/MCM-41,1, 27 Cu/ZSM-5,28
Cu/HMS29 also exhibited good catalytic activity for phenol hydroxylation. Further, Karakhanov
et al. reported, iron and copper based complexes with N,O and N,N ligands for the hydroxylation
of phenol which was a comparatively good method with excellent selectivity for catechol under
mild reaction conditions.17 However, these catalysts were observed to be less stable, emphasising
the need for improvements in the phenol hydroxylation reaction. Metal phthalocyanine
complexes of copper, vanadium and cobalt encapsulated in zeolite-Y are known to have good
catalytic activity for phenol hydroxylation.30 Studies showed that the transition metal complexes
of biologically relevant ligands act as efficient catalysts for a variety of industrially important
organic transformations. Porphyrin is one of the potential biological ligands and porphyrin based
metallo-complexes are widely applied in several organic transformation reactions.31-38 Based on
this metallo-complexes of naturally existing macrocyclic ligand, porphyrin, and the catalytic
activity of porphyrin in several biologically important processes have drawn our attention. In
particular, we have investigated copper based porphyrin systems as catalysts for the phenol
hydroxylation reaction.
2.
Experimental Procedures
The solvents and the starting materials required for the synthesis and purification of metal
complex and heterogenised samples were dried as per the standard procedure previously
reported.33-37 Further, the ligands (meso-tetrakis-(4carboxyphenyl)porphyrin
tetraphenylporphyrin) and its copper complex were prepared using Schlenk line set up. The
complete synthetic procedures for the synthesis of copper-meso-tetrakis(4-
&
meso-
carboxyphenyl)porphyrin (Cu-TCPP), copper-meso-tetraphenylporphyrin (Cu-TPP), aminated
5

SBA 15 (SBA-AM), grafting of metal complex, i.e. Cu-TCPP and Cu-TPP on amino
functionalized SBA-15 are summarized below.
2.1. Synthesis of Copper-meso-tetrakis-(4-carboxyphenyl)porphyrin (Cu-TCPP):
The synthesis of ligand meso-tetrakis-(4-carboxyphenyl)porphyrin (TCPP) was achieved by
taking terephthaladehydic acid (0.338 g, 2.25 mmol) and dried pyrrole (0.2 mL, 0.193 g, 2.88
mmol) in two neck round bottom flask containing propionic acid (100 mL). The reaction mixture
was refluxed in inert atmosphere (N2 atmosphere) for two hours. After the completion of the
reaction, the mixture was cooled to room temperature and the precipitated product was filtered,
washed with cold methanol and dried. Subsequently, the product was recrystallized in 1:1
chloroform-acetone mixture and purple coloured crystals were obtained (12 % yield). For the
preparation of copper-meso-tetrakis-(4-carboxyphenyl)porphyrin (Cu-TCPP) complex, metal salt
cupric chloride was dissolved in 5 mL of dry methanol. Dropwise addition of the dissolved salt
was then carried out in to the round bottom flask containing solution of 1 mmol of TCPP in dry
acetone (45 mL) under nitrogen atmosphere. The final reaction mixture was refluxed for 24 h
under N2 atmosphere. After reaction completion, the solvent was evaporated and impurities were
removed by washing the crude product with methanol and acetone. Finally, the dark green
coloured solid product was isolated, (Yield: 86 %).33-37(1H NMR, 400 MHz, DMSO-d6: δ (ppm)
8.77 (8H, d, β-pyrrole proton), 8.30 (8H, d, o-phenyl), 8.25 (8H, d, m, p-phenyl). (13C NMR, 400
MHz, DMSO-d6: δ (ppm) 167.90 (COOH carbon), 145.91, 134.93, 130.98 (phenyl carbons),
119.82 (meso carbon), 128.38 (pyrrolic carbon). HR-MS (ESI-MS) Calcd:
851.1203(C48H28N4O8Cu).
6

2.2. Synthesis of Copper-meso-tetra-phenylporphyrin (Cu-TPP):
First the ligand meso-Tetra-phenylporphyrin (TPP) was prepared by refluxing a mixture of
benzaldehyde (0.028 mol) and dried pyrrole (0.027 mol) in the propionic acid (120mL) under N2
atmosphere as per the reported procedure33-37 (Yield : 12 %). Afterwards, preparation of Cu-
TPP was carried out by dissolving cupric chloride (1mmol) in 5mL dried methanol and added
dropwise to the round bottom flask containing brown coloured solution of 1 mmol of ligand TPP
in 45 mL of dried acetone. The obtained dark green colour reaction mixture was refluxed for 24
h. The reaction mixture was cooled to room temperature after the completion of the reaction. The
dark green coloured precipitate formed was collected, washed several times with acetone and
methanol to eliminate the unreacted reactants and the final product obtained was dried, (Yield:
89 %).33-37 (1H NMR (400 MHz, DMSO-d3): δ (ppm) 8.84 (8H, d, β-pyrrole proton), 8.24 (8H, s,
o-phenyl), 7.85 (12H, m, m, p-phenyl), (13C NMR, 400 MHz, DMSO-d6: δ (ppm) 134.59,
126.70, 127.73 (phenyl carbons), 120.17 (meso carbon), 142.22 (pyrrolic carbon), HR-MS (ESI-
MS) Calcd: 675.1610 (C44H28N4Cu).
2.3
Synthesis of amino functionalized SBA 15 (SBA-AM)
Mesoporous SBA-15 molecular sieve was synthesized by the previously reported procedure. 3941
The triblock co-polymer pluronic P123 and tetraethyl orthosilicate were used as surfactant and
silica source, respectively. Treatment of calcined SBA-15 (3 g) with 3-
aminopropyltrimethoxysilane (3mmol per gram) yielded the amino functionalized SBA-15
(SBA-AM).39-44
2.4
Grafting of Cu-TCPP and Cu-TPP complex on amino functionalized SBA-15
For the grafting of Cu-TPP and Cu-TCPP complexes on the surface of aminated SBA-15
material, 0.05 mmol of Cu-TPP and Cu-TCPP were dissolved in dry methanol (5 mL). The metal
complex was introduced dropwise in to the round bottom flask containing 1 g of amino
7

functionalized SBA-15 (SBA-AM) in dry methanol (45 mL). The reaction mixture obtained was
refluxed for 24 h under inert N2 atmosphere. After the completion of the reaction, the
precipitated product was collected by filtration, washed thoroughly with dried methanol followed
by dichloromethane.33-36 The Cu-TCPP and Cu-TPP grafted materials were designated as Cu-
TCPP−SBA-AM and Cu-TPP-SBA-AM respectively.
2.5. Catalytic reaction
In a typical reaction (Scheme 1), 9 mmol of phenol was added to the round bottom flask
containing solution of required amount of catalyst (0.01 g in the case of Cu-TCPP and Cu-TPP
and 0.05 g in the case of Cu-TCPP-SBA-AM and Cu-TPP-SBA-AM) in 3 mL of methanol.
Subsequently, 18 mmol of oxidant cumene hydroperoxide was added to the reaction mixture and
the reaction was stirred at room temperature for 1h. After completion of reaction, the products
were separated,45 and identified by gas chromatography (GC) analysis using commercially
available authentic samples (hydroquinone and Catechol; SRL chemicals). The results were
analysed quantitatively by GC (Mayura Analytical model 2100) in temperature program mode
(heating from 70 to 180 °C, in 10° per minute) using an FID detector equipped with an HP 88
column.
2.6
Characterization Methods
In order to identify functional groups in ligands (TCPP, TPP) and copper complexes (Cu-TCPP,
Cu-TPP), FT-IR was performed in the range of 400−4000 cm−1 with 4 cm−1 resolutions and 120
scans by the KBr method using a PerkinElmer spectrum-2 FT-IR spectrometer. UV−Vis spectra
were measured using Shimadzu UV2600 spectrometer to find out the coordination nature of the
13
copper complexes. The 1H and C nuclear magnetic resonance (NMR) spectra of the molecules
TCPP, TPP, Cu-TCPP and Cu-TPP were recorded on a Brꢀcker AVANCE III 400 instrument
8

using tetramethylsilane as internal standard. The CDCl3-d6 and DMF- d7 were used as the NMR
solvents for ligand and copper complex respectively. Bruckerma Xis mass spectrometer was
used to analyse mass spectra by electrospray ionization method and methanol as solvent.
Elemental analyses were carried out on the Elementar-vario MICRO cube by using fine
powdered sample packed in an aluminium foil. The N2 sorption measurements were performed
to obtain the textural properties of the heterogenised samples using an automatic micropore
physisorption analyser (Quanta Chrome Nova 2200e) at −196 °C. Prior to each run, the samples
were degassed under a pressure of 10−3 Torr at 110 °C for 10 h. Shimadzu MAXima_X XRD-
700 X-ray diffractometer with Cu Kα radiation (α = 1.5418) were used for recording the powder
X-ray diffraction patterns of all the heterogenised samples in the 2θ range of 0.5−10° with a scan
speed of 0.5°/min and step size of 0.02°. The EVO MA15 Zeiss scanning electron microscope
(SEM) operated at 10−20 kV was used to obtain the external morphology and particle sizes of in
heterogenised samples. The LaB6 FEI-TECNAIG2-20 TWIN filament was used for
Transmission electron microscopy (TEM) images of the heterogenised samples.
3.
Results and Discussion
FT-IR spectra (Figure 1 and 2) of TCPP, CuTCPP, TPP, and CuTPP showed characteristic
vibration peaks in the range of 4000 to 450 cm−1. Most of the bands appeared in the region of
450−970 cm−1 corresponding to the in-plane bending, out-of-plane bending, ring rotation, and
ring torsion modes of the TCPP and TPP skeleton.35 In TCPP, the vibrational band at 964 cm−1 is
characteristic of the bending mode of N−H plane of the pyrrole ring which disappeared upon the
formation of the Cu-TCPP complex. A new band appeared at 828 cm−1 corresponding to Cu-N
bond (Figure 1). Similarly, the peak corresponding to N−H bending mode of pyrrole ring present
in TPP skeleton at 980 cm-1 disappeared upon Cu incorporation and a new shoulder peak at 963
9

cm-1 was observed corresponding to the Cu-N bond in Cu-TPP (Figure 2).46 Furthermore, in
TCPP and Cu-TCPP the vibrational modes of C=O aromatic C=C and C-O were observed at
1687 cm–1, 1600 cm–1 and 1170 cm-1, respectively. The other vibrational bands for TPP and Cu-
TPP complex followed a similar trend. 33-37
The CuTPP and CuTCPP formation was analyzed by 1H NMR studies (Table 1). In the spectra, a
typical peak corresponding to the N−H protons of the pyrrole unit in TPP appeared at δ −2.74
ppm. This peak disappeared upon copper incorporation into CuTPP, (Figure S1 in ESI*)
supporting the copper linkage to the nitrogen center of the TPP ligand.34-37 In the case of
CuTCPP complex, no peak was observed at δ -2.79 ppm, due to the presence of N–H protons in
TCPP, again supporting the linkage of copper ion with “N” center of TCPP (Figure S2 in
ESI*).34-37 The remaining peaks in the NMR spectra of CuTPP and CuTCPP, remained identical
to that of the pure ligands (TPP and TCPP), characteristic of the TPP and TCPP frameworks
which is summarized in table 1.33-37 The 13C NMR spectra of CuTCPP and CuTPP shows
characteristic peaks at 120 and 119 ppm respectively, corresponding to meso-carbon.34-37 The
additional signals appeared at 145, 134, 130, 128 and 167 ppm correspond to phenyl, pyrrolic,
and carboxylic moiety of the CuTCPP frame-work (Figure S3 in ESI*).34-37 Similarly, in CuTPP
the peaks at 126, 127, 134, 130, 145 ppm corresponds to phenyl and pyrrolic carbons (Figure S4
in ESI*).34-37 CHN analysis also supports the formation of copper complex and the results are
summarized in table 2. The theoretical value (%) of C, H, and N are well matches with the
experimental value.
The ligand TCPP showed soret band at 420 nm and Q bands at 514, 551, 592 and 646 nm in the
UV-Vis spectra (Figure 2) which are characteristic peaks of porphyrin skeleton.34-37 After the
formation of copper complex (Cu-TCPP), it was observed that the intensity of soret band
10

decreased with a small blue shift towards 414 nm. 34-37 In addition, the number of Q bands is
reduced to one at 537 nm. This may be due to the reduction in energy gap and increase in
structural symmetry (D4h) of the copper porphyrin complex as compared to the symmetry of the
pure ligand (D2h).33-37, 47, 48 A similar trend was observed in the TPP and Cu-TPP complex as
shown in figure 2.
After the confirmation and thorough characterization of Cu-TCPP and Cu-TPP structures using
various characterization techniques, the copper complexes were grafted on the amino
functionalized SBA-15 (SBA-AM) to synthesize Cu-TCPP-SBA-AM and Cu-TPP-SBA-AM.
The synthesized heterogenized samples were also characterized using various analytical and
spectral techniques. The grafting of the Cu-TCPP and Cu-TPP complex on the surface of the
aminated SBA-15 material (SBA-AM) was observed and confirmed through FT-IR, N2 sorption
studies and powder XRD. Amine (NH2) functional groups on the surface of SBA-AM was
evident from the wagging modes of vibration at 1441 cm-1 in the FT-IR spectra (Figure 1 & 2).
In the FT-IR spectra of Cu-TCPP-SBA-AM, the vibrational band at 1656 cm-1 is characteristic of
amide linkage between the Cu-TCPP complexes and amino functionalized SBA-15 material. A
typical band around 1080–1200 cm-1 was observed in both Cu-TCPP-SBA-AM and Cu-TPP-
SBA-AM, complexes, characteristic of the as-symmetric Si-O-Si stretching vibrational mode of
the SBA-15 framework. Even after the immobilization of bulk Cu-TCPP and Cu-TPP complexes
on the amino functionalized SBA-15, the molecular structural framework SBA-15 remains
intact.
The grafting of bulk copper complexes i.e. CuTPP and CuTCPP on the functional molecular
sieve was evident from the low-angle diffraction patterns of CuTPP and CuTCPP grafted
materials (Figure 4). The X-ray pattern of CuTPP and CuTCPP heterogenized samples showed
11

the characteristic X-ray reflections at 2θ values of around 1.4°, with the typical line broadening
as well as reduction in relative intensities as that of mesoporous SBA-15 structure.41-44 The
presence of organo-linker and bulk CuTPP and CuTCPP resulted, shift in 2θ value towards
higher angle in both the samples compared to parent SBA-15 materials and also the broadening
the signal. 34-37, 41-44 In both cases, the structure remains intact even after incorporation of bulk
macromolecules.
The N2 sorption studies showed a decrease in pore volume and surface area. The SBA-15 shows
surface area of 824.2 m2g1 with pore size of 7.2 nm. After the grafting of copper complex, it
showed surface area of 203 and 200 m2g1 with pore size 4.9 nm and 4.8 in the case of Cu-TCPP-
SBA-AM and Cu-TPP-SBA-AM, respectively (Table 3). The decrease in pore size, surface area,
and pore volume of the immobilized samples compared to the amine functionalized sample
(SBA-AM) supported the grafting of Cu-TCPPP and Cu-TPP into the mesoporous channels of
the aminated SBA-15 through axial position.34-37 In addition, the pore size distribution appeared
in the range of 4.8  4.9 nm supporting the mesoporous intactness, which is beneficial for the
molecular diffusion and catalytic transformations.33-37
As per the IUPAC classification, the nitrogen sorption isotherm (Figure 5) showed typical type
IV isotherm A well-defined sharp inflection appeared between relative pressure range of 0.4 and
0.8 due to capillary condensation of nitrogen inside the primary mesoporous channels. After
thorough characterization of the CuTCPP and CuTPP, homogeneous complexes and their
heterogenised analogues were explored for their catalytic activity towards phenol hydroxylation
under liquid phase condition.
4.
Catalytic studies
12

The catalytic activity of synthesized copper complexes catalytic activity for the hydroxylation of
phenol in liquid phase medium using organic-peroxide were explored (Scheme 1). The reaction
conditions were optimized using different oxidants, solvents, various temperatures, etc., and the
results are summarized in the following section.
4.1
Effect of different oxidant
The reaction was initially carried out using various oxidants such as 30 % aq. H2O2, TBHP in
water (70 %) and cumene hydro peroxide and the results are summarized in table 4. The use of
H2O2 as oxidant with CuTCPP catalyst showed only 17% conversion of phenol with 34%
selectivity of catechol and 66% selectivity of hydroquinone. Similarly, TBHP in water showed
about 18% conversion with 46% catechol and 54% of hydroquinone selectivity. The use of
cumene hydroperoxide as an oxidant and the CuTCPP as a catalyst yielded comparably good
conversion of 72 % with the formation of catechol as major product (98% selectivity). The
presence of water in the aqueous peroxide may interact with macromolecules, and hence block
the active sites suggesting much lower conversion.37 The use of organic peroxide with organic
solvent thus facilitates better conversion. Hence, for the further studies, cumene hydroperoxide
was chosen as oxidant for phenol oxidation. Relatively, higher stability of cumene hydroperoxide
may facilitate good conversion and selectivity for the phenol hydroxylation.
4.1
Effects of Solvent
In general, solvents play a key role on the catalytic activity in any chemical reaction and
especially in the case of homogeneous catalysts. Attempts were made to understand the exact
role of a solvent in the phenol hydroxylation reaction catalyzed by Cu-TCPP. The results are
summarized in table 5. The reactions were monitored in various solvents such as methanol,
tertiary-butyl alcohol, isopropyl alcohol and acetonitrile. Among these solvents, maximum
13

conversion of 72% was observed with methanol yielded catechol as the major product (98%).
The above finding is in agreement with the literature.49,50 The conversion of phenol using
different solvents follows the decreasing order as methanol > tertiary-butanol > isopropyl alcohol
> acetonitrile. The polarity of the chosen solvents are as follows acetonitrile > methanol >
tertiary-butanol ~ isopropyl alcohol. Although, acetonitrile is strongly polar in nature, the non-
protic acetonitrile gives comparable conversion. Among the various protic solvents, phenol
50
conversion steadily increases with increase in the polarity of the solvent.49,
methanol was used as the solvent for further studies.
Therefore,
4.3
Effects of temperature
The reaction was carried out at various temperatures ranging from 0 to 60 °C and the results are
displayed in Figure 6. The increase in phenol conversion was observed with increase in
temperature from 0- 25 °C with exclusive formation of catechol as product. Further increase in
temperature from 25 to 60 °C, yields no appreciable change in conversion of phenol and hence
the 25 °C (i.e. at room temperature) was considered as optimal temperature condition for the
chosen reaction. In all of the cases, catechol was obtained as the major product.
4.4
Effects of time
The reaction was carried out at different intervals such as 10, 20, 30, 40, 50, 60, 120 and 180
minutes at different reaction temperatures (Figure 7). It was observed that as the reaction time
increases from 10 to 60 minutes, phenol hydroxylation increases linearly and the conversion was
steady state afterwards. At 60 minutes the catalyst CuTCPP, showed almost 70% conversion
with the formation of catechol as the major product. Hence 60 minutes was chosen as reaction
time for phenol hydroxylation. The reaction was carried out at various temperatures and
14

observed that irrespective of the reaction temperature the conversion of phenol showed almost
constant, i.e. after 60 minutes, there was no appreciable change observed in phenol conversion.
4.5
Determination of rate and activation energy of phenol hydroxylation
The reaction was carried out at various temperatures and time intervals in order to find out the
rate of reaction and activation energy (Figure 8). The plot with log of the concentration of the
reactant versus time yielded a straight-line indicting that the reaction follows first-order kinetics
with respect to substrate phenol (Figure 8a). In addition, the lnK versus 1/T yielded a negative
slope of 2774.4. The activation energy of the reaction was calculated and found to be 23.06
KJ/mol., which is comparable to the activation energy reported in the literature.51-56
4.6
Recyclability Study
The optimal reaction conditions, the catalytic efficiency of both homogeneous (Cu-TCPP, Cu-
TPP) and heterogeneous catalysts (Cu-TCPP-SBA-AM, Cu-TPP-SBA-AM) were evaluated
(figure 9). Among CuTPP and CuTCPP homogeneous catalysts, Cu-TCPP showed better phenol
conversion (72%) and Cu-TPP shows a slight lower conversion i.e.; 65%. Both of the
homogeneous catalysts gave exclusively catechol as the product (98 %). Both the heterogeneous
catalysts, Cu-TCPP-SBA-AM and Cu-TPP-SBA-AM, showed almost similar conversion of
phenol which is about 65 %. In all the cases catechol was obtained as the major product (>
95 %). The recyclability of both homogeneous (Cu-TCPP and Cu-TPP) and heterogeneous (Cu-
TCPP-SBA-AM and Cu-TPP-SBA-AM) catalysts was further explored (Figure 10). In the case
of homogeneous catalysis, 9 mmol of reactant (phenol) was added into the reaction vessel after
each run, and the reaction was continued further under identical experimental conditions. Both
Cu-TCPP and Cu-TPP showed a gradual decrease in phenol hydroxylation. The Cu-TCPP
displayed 72% conversion approximately in the first run and was decreased to 43% in the fifth
15

run. In addition, the selectivity of catechol was decreased from 98% to 79%. Similar tends were
observed in the case of Cu-TPP catalysts and heterogeneous catalysts, (Cu-TCPP-SBA-AM and
Cu-TPP-SBA-AM). In the first run both of the catalysts showed conversion of almost 65% with
a 95% selectivity of catechol and in the fifth run it showed 43 % conversion with 78% of
catechol selectivity, which is represented in figure 10. The water formed as by product during the
reaction may interact through axial position of the complexes and may reduce the activity during
the recycle studies.
4.7
Substrate scope
The scope of homogeneous (CuTCPP) and heterogeneous (Cu-TCPP-SBA-AM) catalyst were
explored for hydroxylation of different phenolic substrates and the results are summarized in
table 6. All of the reactions were performed in methanol for 1 h at room temperature. Both
homogeneous and heterogeneous catalysts showed comparable conversion. Substituted phenols
such as 2,5-dimethylphenol, 3,5-dimethyl phenol and 2,3,5-trimethylphenol gave comparable
conversion with the formation of corresponding hydroquinone as a major product. Benzene
showed very less conversion, with the formation of phenol as the major product with comparably
good selectivity. Anisole yielded both para- and ortho- substituted hydroxylated products in
which the para- substituted product, 4-methoxyphenol was formed as major product (73%).
Whereas the isoeugenol, yielded vanillin and vanillic acid with 36% conversion with the
oxidation of propenyl chain.
4.8
Mechanism of phenol oxidation by cumene hydroperoxide
Based on the results and previous studies57-61, it is proposed that the reaction may proceed via the
pathway as shown in Scheme 2. Initially in the presence of solvent, the proximal oxygen of
peroxo species will coordinate with the metal center i.e. copper center which acts as Lewis acid.
16

Subsequently, the hydroxyl species will remove from the peroxide and attack the ortho- or para-
position of phenol and form corresponding products, viz., catechol and hydroquinone
respectively. Finally, the catalysts were regenerated and forming hydroxy cumene as bi-product.
5.
Conclusion
Copper-containing porphyrin complexes (CuTCPP, CuTPP) were synthesized for the first time
and heterogenized on the amino functionalized SBA-15 molecular sieve. The synthesized copper
complexes were characterized and confirmed through FT-IR, CHN, 1HNMR, 13CNMR and mass
spectral analyses. The heterogenization of copper complex on aminated SBA-15 was confirmed
by powder XRD, FT-IR, and adsorption-desorption analyses. The catalytic application of the
synthesized copper complexes, CuTPP, CuTCPP heterogeneous complex, CuTPP-SBA-AM and
CuTCPP-SBA-AM for the hydroxylation of phenolic compounds, anisole, benzene and
substituted phenols were studied. The catalysts showed comparable conversion (65-72 %) with
the formation of ortho- and para- substituted products. The reaction was found to be first order
with activation energy of 23.06 KJ/mol., which is comparable to the previously reported values.
Further the homogeneous and heterogeneous catalysts showed comparable recyclability and
reusability.
Conflicts of interest
There are no conflicts to declare.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
17

Authors are thankful to CSIR for the financial support through the Project No: 01(2921/18EMR-
II). Anjali is grateful to CUK and DST-Inspire (IF160353) for the lab facilities and fellowship.
Supporting Information
Figure S1: 1H NMR spectrum of CuTPP (DMSO-d6, 400MHz).
Figure S2: 1H NMR spectrum of CuTPP (DMSO-d6, 400MHz).
Figure S3: 13C NMR spectrum of CuTCPP (DMSO-d6, 400MHz).
Figure S4: 13C NMR spectrum of CuTPP (DMSO-d6, 400MHz).
List of figures
Figure 1: FT-IR spectra of TCPP, Cu-TCPP and CuTCPP-SBA-AM.
Figure 2: FT-IR spectra of TPP, Cu-TPP and CuTPP-SBA-AM.
Figure 3: UV-Vis spectra of TCPP, Cu-TCPP, TPP, Cu-TPP.
Figure 4: Powder XRD pattern of SBA-15, Cu-TCPP-SBA-AM, and Cu-TPP-SBA-AM.
Figure 5: N2 sorption isotherm of Cu-TCPP-SBA-AM, Cu-TPP-SBA-AM.
Figure 6: Effect of temperature on phenol hydroxylation over Cu-TCPP catalyst.
Figure 7: Effect of reaction duration (a) at 25 °C (b) at different temperature using CuTCPP as
catalyst.
Figure 8: Kinetic study on phenol hydroxylation using CuTCPP; (a) Log concentration of the
reactant vs time plot for the first order, and (b) rate constant vs 1/T plot to find out slope and
activation energy.
Figure 9: Effect of various copper macromolecular catalysts on phenol hydroxylation.
Figure 10: Recyclability of (a) Cu-TCPP (b) Cu-TPP, (c) Cu-TCPP-SBA-AM and (d) Cu-TPP-
SBA-AM on phenol hydroxylation.
18

List of schemes
Scheme 1: Schematic representation of hydroxylation of phenol.
Scheme 2: Proposed mechanism for the phenol hydroxylation using cumene hydroperoxide as
oxidant and Cu based catalyst.
List of tables
1
Table 1: H-NMR of macromolecular ligands TPP (tetraphenylporphyrin), TCPP (Tetrakis (4-
carboxyphenyl)porphyrin) and its copper complexes CuTPP (Copper-tetraphenylporphyrin),
CuTCPP (Copper-tetrakis(4-carboxyphenyl)porphyrin).
Table 2: CHNS of macromolecular ligands TPP (tetraphenylporphyrin), TCPP (Tetrakis (4-
carboxyphenyl)porphyrin) and its copper complexes CuTPP (Copper-tetraphenylporphyrin),
CuTCPP (Copper-tetrakis(4-carboxyphenyl)porphyrin).
Table 3: Textural properties of heterogeneous copper porphyrin complexes.
Table 4: Role of oxidant on phenol hydroxylation using using CuTCPP catalyst.
Table 5: Effect of solvent on hydroxylation of phenol using CuTCPP catalyst.
Table 6: Conversion of various phenolic substrates using CuTCPP and CuTCPP-SBA-AM
catalysts.
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Figure 1: FT-IR spectra of TCPP, CuTCPP and CuTCPP-SBA-AM.
25

420
415
414
TPP
CuTPP
TCPP
539
CuTCPP
300
400
500
Wavelength (nm)
600
700
300
400
500 600
wavelength(nm)
700
800
Figure 3: UV-Vis spectra of TCPP, Cu-TCPP, TPP, CuTPP.
Figure 2: Powder XRD pattern of SBA-15, Cu-TCPP-SBA-AM, and Cu-TPP-SBA-AM.
26

Cu-TCPPSBA-AM
CuTPPSBA-AM
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p )
o
Figure 5: N2 sorption isotherm of Cu-TCPP-SBA-AM, Cu-TPP-SBA-AM.
Figure 6: Effect of temperature on phenol hydroxylation over Cu-TCPP catalyst.
(Reaction Conditions: Methanol (Solvent) =3mL, Cu-TCPP (Catalyst) =10mg, Phenol (Substrate)
= 0.8mL (9mmol), Cumene hydroperoxide (oxidant) = 2mL (18mmol), Time = 1 hr.
27

Figure 7: Effect of reaction duration (a) at 25 °C (b) at different temperature using CuTCPP as catalyst.
28

Figure 8: Kinetic study on phenol hydroxylation using CuTCPP; (a) Log concentration of the reactant vs
time plot for the first order, and (b) rate constant vs 1/T plot to find out slope and activation energy.
Figure 9: Effect of various copper macromolecular catalysts on phenol hydroxylation.
Reaction Conditions: Methanol (Solvent) =3mL, Cu-TCPP/Cu-TPP (Catalyst) =10mg, Cu-TCPP-
SBA-AM/Cu-TPP-SBA-AM = 50mg, Phenol (Substrate) = 0.8mL (9mmol), Cumene
hydroperoxide (oxidant) = 2mL (18mmol), Time = 1 hr
29