Solvent-free selective oxidation of toluene over metal-doped MCM-22

Article abstract of DOI:10.1039/C8NJ06247A

Herein, copper- and iron-modified MCM-22 were prepared using a wet impregnation method and characterized using XRD, FE-SEM, EDX, TG, UV-vis-DR and N₂ adsorption desorption techniques. The solvent-free catalytic oxidation of toluene was investigated using the copper- and iron-doped MCM-22 catalyst at 90 °C in the presence of hydrogen peroxide as the terminal oxidant. The selectivity to benzaldehyde as a product was studied by varying several reaction parameters. The recycling studies indicate that the catalyst can be reused for up to 3 cycles without structural degradation. Moreover, optimum conditions for the reaction have been explored, and a possible reaction mechanism has been discussed.

Full text of DOI:10.1039/C8NJ06247A

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Solvent-free selective oxidation of toluene over metal doped
MCM-22
Received 00th January 20xx,
Accepted 00th January 20xx
Maaz Nawab, Sunita Barot and Rajib Bandyopadhyay*
DOI: 10.1039/x0xx00000x
Copper and iron modified MCM-22 were prepared with wet impregnation method and characterized using XRD, FE-SEM,
2
EDX, TG, UV-vis-DR and N adsorption desorption technique. Solvent free catalytic oxidation of toluene were investigated
www.rsc.org/
using copper and iron doped MCM-22 catalyst in the presence of hydrogen peroxide as the terminal oxidant at 90 °C.
Selectivity of benzaldehyde as a product was studied by varying different reaction parameters. Recycling studies indicates
that catalyst can be reused upto 3 cycles without structural degradation. Optimum condition for the reaction was
explored,
and
possible
reaction
mechanism
was
also
discussed.
Oxidation of toluene can be done in vapour phase as well as in
liquid phase. Apart from good conversion and selectivity, liquid
phase oxidation allows catalytic oxidation reactions in mild
conditions compared to the vapour phase and high-pressure
condition. Many researchers have attempted for liquid state
oxidation using various homogeneous catalyst such as 10-methyl-9-
Introduction
Toluene is one of the by-product produced during catalytic
reforming of gasoline from crude oil. It is also one of the
commercially valuable raw material for organic synthesis in
functional group transformation. Toluene can be oxidized into
benzyl alcohol, benzaldehyde and benzoic acid. Benzaldehyde is
most versatile and important raw material for pharmaceutical,
dyestuff, food, agro chemical, perfumery
Commercially, benzaldehyde is produced by chlorination of toluene
followed by saponification. The use of halogens and acidic solvent
makes this process environmentally unfriendly.5,6 Benzaldehyde can
also be obtained by direct oxidation of toluene without using
catalyst. The free radical mechanism gives reactivity of toluene,
benzyl alcohol and benzaldehyde as 0.05, 0.85 and 290,
respectively.7,8 The reactivity of benzaldehyde is more than benzyl
alcohol so that high yield of benzaldehyde from benzyl alcohol
seems impossible. This reactivity can be altered by using catalytic
oxidation.
Many scholars had paid great efforts to develop appropriate
catalyst for oxidation of toluene. Heterogeneous catalyst, which can
be easily recycled after reaction, are widely studied. Xue et al. have
1
0
11
12
phenylacridinium ion , iridium (III) chloride , CTAB-SDS , metal
complexes13–15 and xanthone . Some other heterogeneous
16
3
+
17
18
industries.1–4
catalysts like Fe -Al
2 3 3 3
O /CH CN , Mn (salen) complex/CH CN ,
CN -valine complexes19, and metal containing
Ru(III)-L/CH
3
molecular sieves20–23 under mild condition are also reported. Song
et al.24 reported copper nano particle supported graphene as a
catalyst and methanol as a solvent for oxidation of toluene at 2
MPa pressure using high pressure autoclave. There are other
reports available using hydrogen peroxide along with metal-
2
+
loaded/metal-free zeolites like [Cu(terpy)] @Y, Co
Sn-Beta, Fe/ZSM-5, TS-1, VS-1, Sn-Silicalite-1(MFI), Sn-Silicalite-
(MEL), Sn-MTW, metal free Silicalites, Sn-ZSM-12, ZSM-12, Sn-
ZSM-48 (Al free).25–33 Baochun Ma et al.34 reported tri-vanadium-
substituted phosphotungstate ([n-Bu N] [PW (I)) as
3 4
O @HZSM-5,
2
4
3
H
3
9 3 40
V O ]
effective homogeneous polyoxometalate catalyst for the solvent
free oxidation of toluene. Xin Li et al.35 screened number of iron
containing heterogeneous catalyst, metal oxides and zeolites, like
9
reported V−Ag−O/TiO
C. The selectivity of benzaldehyde and conversion of toluene was
5 % and 7.3 %, respectively. Lv et al. reported ceria nano-cubes
covered with oleic acid as catalyst for toluene oxidation at 2 MPa
oxygen and 153 °C. However, these harsh reaction conditions at
high temperature or pressure limit their industrial applications.
2
as a catalyst for oxidation of toluene at 340
Fe
2 3 2 3 2 3 2 3 2 3
O , Fe O /MCM-41, Fe O /HY, Fe O /MCM-22, Fe O / ZSM-5
°
9
4
and bimetal supported ZSM-5 under solvent free condition.
Unique pore structure of zeolite and its acidic nature makes it an
attractive catalyst for various chemical reactions. MCM-22 contains
pore structure with two-dimensional sinusoidal channels with an
elliptical ring cross section of 4.1 × 5.1 Å. It has cylindrical 12-
member-ring supercage (7.1 Å in diameter and 18.2 Å in height)
that are accessible through 10-membered-ring (4.0 × 5.5 Å)
windows and pockets on the external surface (7.1 Å in diameter and
Department of Science, School of Technology, Pandit Deendayal Petroleum
University, Raisan, Gandhinagar, Gujarat-380055, India
Email: rajib.bandyopadhyay@sot.pdpu.ac.in
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
7
.0 Å in height), as shown in Figure 1. MCM-22 has structural
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similarity to aluminosilicates SSZ-25 and PSH-32, silicalite ITQ-1 and Materials
borosilicate ERB- 1.36
View Article Online
The chemicals used in current studies were:DCOoIl:lo1i0d.1a0l3S9il/iCca8N(SJi0lp62o4n7dA,
0 %), hexamethyleneimine (Aldrich, 99 %), Sodium Aluminate
Merck, 53.3 % Al , 43.9 % Na O), Sodium hydroxide (Merck, 98
), Distilled water (MiliQ), Toluene (Merck, 98 %), Hydrogen
peroxide (Rankem, 30 %), Toluene (Rankem, 99.5 %), Acetonitrile
(Merck, 99.5 %), Cupric nitrate (98 %), Ferric nitrate (98 %). All
Chemicals were used as received.
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Synthesis and modification of MCM-22
MCM-22 was synthesized according to the procedure reported by
the Pengfei Wang et al.43 The molar gel composition taken was SiO
:
2
0.033Al
2
O
3
: 0.18NaOH: 0.35HMI: 45H
2
O. In a typical procedure 2.5
were dissolved in 435.5 g of water
and stirred for 10 min. 22.79 g of HMI was added to the mixture
followed by dropwise addition of 130 g of colloidal silica. The
resulting gel with Si/Al ratio 30.3 was stirred for 2 h. Then it was
transferred into a rotating (100 rpm) high pressure SS autoclave and
g of NaOH and 4.11 g of NaAlO
2
°
set at 150 C for 7 days. The crystallized product was separated
using high-speed centrifuge, washed with de-ionized water and
°
°
dried at 100 C overnight followed by calcined at 560 C for 10 h in
air. The obtained Na-MCM-22 was ion exchanged twice with 1 M
°
NH
4
NO
3
solution at 80 C for 4 h. The resultant mass was filtered,
Figure 1. H-MCM-22 zeolite framework with three types of pores:
pockets, and sinusoidal channel and supercages36
°
dried, calcined at 540 C for 6 h to get H-MCM-22.
The copper and iron modified MCM-22 were prepared by mixing H-
MCM-22 with an appropriate amount of cupric nitrate and ferric
MCM-22 attracts attention as catalyst for various catalytic reactions
due to its unique pore system and acidic nature. The 12-MR
supercages allow bimolecular reactions involving bulky
intermediates and the 10-MR channels within the sheets show
unique shape selectivity for reactants and products.37,38 It has been
°
nitrate in distilled water. The mixture was digested at 80 C for 5 h
°
in stirring condition, dried at 100 C for overnight and calcined at
°
5
50 C in the presence of air for 12 h.
Sample characterization
3
6
used as a catalyst for a variety of reactions such as MTO process ,
The X-ray diffraction (XRD) pattern of the samples were recorded
using X’Pert Pro instrument with CuKα radiation in the 2θ range of
5-50 . The surface morphology was collected on FE-SEM (ZEISS).
Thermal properties were analyzed by TGA (Mettler Toledo Star ͤ) .
Nitrogen adsorption/desorption isotherms of samples were
isomerization , disproportionation and alkylation37 reactions.
With the objective of applying MCM-22 for fine chemical and
petrochemical reactions involving bulky organic molecules, it is
desirable to make full utilization of surface pockets and the acid
sites of interlayer supercages. Chu et al.40 reported a new hybrid
micro-mesoporous MCM-36 by swelling and pillaring MCM-22
lamellar precursor. Corma et al.41 prepared delaminated thin sheets
of ITQ-2 by treating swollen MCM-22 with ultrasound. These
materials show enhanced activity in acid-catalyzed reactions
involving bulky molecules. It is still desirable to find convenient and
3
9
38
°
°
measured at -196 C on Micromeritics ASAP 2020 instrument. The
specific surface area and the micropore volume were calculated by
BET and t-plot methods, respectively. Optical properties of metal
impregnated samples were investigated by diffuse reflectance UV-
visible spectrometry (Shimadzu UV 2600).
cost effective way that will enhance acidity by introducing transition Catalytic reaction
metal ion into MCM-22. M. Milanesio et al.42 found with the 2.5 mL of toluene and 0.2 g of catalyst were taken into two neck
support of DFT combined with synchrotron radiation X-ray powder round bottom flask fitted with reflux condenser and mechanical
diffraction analysis that Cu sits inside the supercage, whereas no Cu stirrer. The mixture was heated at 90 C in the oil bath, and then 7.5
2 2
ions were found within the sinusoidal channels. Based on these mL 30 % H O was added. The mixture was stirred at set
results, we have planned to impregnate Cu and Fe ions on MCM-22 temperature for 4 h. At the end of the reaction, the solid catalyst
and use it for oxidation of bulky molecule like toluene. In this study, was separated by centrifugation from the reaction mixture. Product
we explore copper and iron impregnated MCM-22 zeolites for their identification done by GC-MS (Agilent 7890) and product analysis
catalytic activity in selective solvent-free oxidation of toluene by during reaction was monitor by GC (Shimadzu 2025).
following possible reaction pathways (Scheme 1). Various reaction
°
parameters are studied to get optimized reaction condition.
Results and discussion
Experimental
Catalyst Characterization
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% Cu-MCM-22
52.4
2.8
39.6
5.2View Article Online
DOI: 10.1039/C8NJ06247A
The surface morphology of MCM-22 was investigated by FE-SEM.
The crystals were mostly agglomeration of platelets with uneven
size. Calcination of the material did not affect the morphology of
the crystals.
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Figure 2: XRD patterns of H-MCM-22 and copper doped MCM-22
The powder XRD patterns of the calcined H-MCM-22 and Cu-MCM-
2
2 samples are shown in Figure 2. It confirms MWW type crystal
structure. There is no change in crystal structure after impregnation
of copper on it.
Figure 4: TG curve of MCM-22
Thermal stability of as-synthesized material was determined by TG.
Figure 4 show TG curve of H-MCM-22 sample. The first weight loss
between 100-150 C is due to removal of moisture and second
weight loss up to 500 C is due to degradation of organic moieties.
Beyond 500 °C, no significant weight loss was observed, which
indicates good thermal stability of H-MCM-22 sample.
°
°
Figure 3: FE-SEM image of as made and calcined MCM-22
Table 1: EDX analysis report of copper impregnated MCM-22
Sample
"O"
wt %
61.6
"Al"
wt %
2.5
"Si"
wt %
33.9
"Cu"
wt %
2.1
Figure 5: UV-vis-DR spectra of Copper and iron impregnated MCM-
2
2
2
3
4
% Cu-MCM-22
% Cu-MCM-22
% Cu-MCM-22
Figure 5 shows UV-vis-DR spectra of Fe-MCM-22 and Cu-MCM-22
with different copper concentration. The spectra of Cu-MCM-22
shows a strong absorption below 300 nm. The absorption band
between the regions of 200–250 nm represents monomeric Cu2+
ions to O2− charge transfer absorption band in the zeolite crystal
50.9
47.2
3.1
3.3
43.2
45.6
2.8
4.0
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2
−
2+
Cu2+
H2O2
Cu3+
lattice (LMCT, O →Cu ). The broad band at about 600–800 nm is
usually assign to the d–d transition in Cu2+ located in a distorted
+
+
OH
+
OH
View Article Online
DOI: 10.1039/C8NJ06247A
octahedral coordination.44 The intensity of the band corresponds to
the total amount of introduced Cu into the samples, determined
using EDX technique and given in Table 1.
The nature and distribution of iron species in zeolite sample have
three characteristic regions in the UV–vis-DR spectra. Absorption
bands below 300 nm are attributed to the presence of tetrahedral
CH3
CH2
CH OH
CHO
COOH
2
OH
H2O
H2O
2
OH
H2O
2
2H2O
H2O2
H2O
[O]
(A)
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Fe3+
Fe3+
Fe2+
H2O2
HO2
HO2
H
+
+
+
+
+
+
(200–250 nm) and octahedral (250–300 nm) coordinated
Fe2+
O2
H
3+
mononuclear Fe . Absorption bands in the range of 300–400 nm
are due to the presence of small oligonuclear Fe clusters in the
3+O
zeolite channels, whereas the bands above 400 nm could be Scheme 1: Mechanism of product formation
attributed to bulky Fe O aggregates located possibly on the
2 3
external surface of the zeolite crystals. The sample of Fe-MCM-22
(B)
x
y
The reaction mechanism can be proposed as shown in Scheme 1.48
The reaction starts with formation of radical as well as ionic
hydroxyl molecule in presence of solid acid catalyst. The radical
hydroxyl accepts acidic proton of methyl group of toluene resulting
into benzyl alcohol that further reacts with transition metal-based
active sites and undergoes dehydrogenation process to form
benzaldehyde. Cu-MCM-22, which contains Cu2+ (confirmed by DR-
UV-Vis), promotes reaction shown in Scheme A, leading to the
formation of benzaldehyde and benzoic acid. On the other hand,
Fe-MCM-22, which contains Fe3+ (confirmed by UV-Vis-DR),
shows combination/mixture of three intensive absorption bands
characteristic of all above mentioned species.45–47
Table 2: BET analysis report of copper impregnated MCM-22
Material
Surface area
Micropore
volume
(
0.158
0.152
0.147
0.145
2
(
m /g)
3
cm /g)
H-MCM-22
483
466
448
435
2
3
4
% Cu-MCM-22
% Cu-MCM-22
% Cu-MCM-22
2 2
converts H O rapidly into oxygen without participating in the
oxidation reaction, which makes it inactive for the oxidation of
toluene (Scheme B).
The BET surface area and micropore volume of the H-MCM-22 and
Cu-MCM-22 were given in the Table 2. As expected, all the copper
impregnated samples have lower surface area as well as lower
micropore volume compared to the parent H-MCM-22 sample.
Surface area reduces from 483 m /g to 435 m /g with the
increment in copper loading on the parent H-MCM-22. This is due
to the binding of Cu2+ with the surface adhered O2− present in the
Catalytic activity of Cu-MCM-22 towards selective oxidation of
toluene
At the initial screening, different amount of copper impregnated
MCM-22 and 4 % iron impregnated MCM-22 were examined to
evaluate the effect of copper and iron concentration on the
reaction at specific reaction parameters (Figure 6). Fe-MCM-22
showed negligible catalytic activity as compared to Cu-MCM-22.
With the increase in amount of copper from 2 to 5 % on MCM-22,
conversion of toluene increases from 3.0 to 9.3 % respectively.
Selectivity of benzaldehyde increases from 53.3 to 71.1 % as the
amount of copper concentration increases from 2 to 4 %. Further
2
2
H-MCM-22. Similarly, micropore volume decreases from 0.158
3
3
cm /g to 0.145 cm /g with the increase in copper concentration
because of blockage of pores by copper ions.
Reaction mechanism
The result of toluene oxidation catalyzed by metal impregnated increasing copper concentration from 4 to 5 %, shows decrease in
MCM-22 is given in Table S1 (supplementary information). Copper- the selectivity of benzaldehyde from 71.1 to 46.2 %. This is because
based catalyst was found to more selective than iron based one, increased amount of copper ions results into the side reactions to
which yields more undesired products like benzyl alcohol, form more by-products (o-cresol, p-cresol and benzoic acid). 4 %
benzaldehyde and benzoic acid. In the first step, addition of oxygen copper impregnation was found to be the optimum concentration
to toluene forms benzyl alcohol that undergoes oxy- for the reaction.
dehydrogenation to form benzaldehyde further in step 2. In the 3rd
step, benzoic acid is produced from benzaldehyde by
dehydrogenation reaction.
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Figure 6. The effect of copper loading on the oxidation of toluene.
Reaction Condition: Catalyst 0.2 g Cu-MCM-22, Toluene 2.5 mL,
(30 %) 7.5 mL, Time 4h, Temperature 90 °C
Figure 8. Effect of Toluene/H
2
O
2
ratio (vol./vol.) on oxidation of
toluene. Reaction Condition: Catalyst 0.2 g 4 % Cu-MCM-22,
Toluene 2.5 mL, Time 4h, Temperature 90 °C
2 2
H O
The effect of catalyst amount Cu-MCM-22(4 % Cu) was examined
for the oxidation of toluene (Figure 7). Toluene conversion
increases with increase in the amount of catalyst. However,
benzaldehyde selectivity does not follow the same trend.
Benzaldehyde selectivity increases from 52.0 to 71.1 % with
increase in the catalyst amount from 0.1 to 0.2 g. However, with
further increase in the catalyst amount from 0.2 to 0.3 g; it
decreases drastically from 71.1 to 23.2 %; this is due to the further
oxidation of benzaldehyde into benzoic acid. Therefore, 0.2 g Cu-
MCM-22 (4 % Cu) was considered as optimum amount of catalyst.
Figure 9 show the effect of temperature on oxidation of toluene.
Both conversion of toluene and selectivity of benzaldehyde
increases with increase in temperature. Conversion of toluene
increases with the increase in temperature throughout. While
selectivity of benzaldehyde was maximum (71.1 %) at 90 °C after
that selectivity drastically decreased to 47.1 % at 110 °C due to over
oxidized product (benzoic acid).
Figure 9. Effect of temperature on oxidation of toluene. Reaction
Condition: Catalyst 0.2 g 4 % Cu-MCM-22, Toluene 2.5 mL, H
2 2
O (30
%
) 7.5 mL, Time 4h
Figure 7. Effect of catalyst amount on oxidation of toluene. Reaction
Condition: Catalyst 4 % Cu-MCM-22, Toluene 2.5 mL, H
.5 mL, Time 4h, Temperature 90 °C
2
O
2
(30 %) Experimental result of reused Cu-MCM-22 catalyst is depicted in
Figure 10. At the end of each cycle, the solid catalyst was recovered
by centrifugation, washed with methanol and dried overnight. The
7
The effect of amount of hydrogen peroxide on oxidation reaction
was also investigated (Figure 8). The results indicate the direct
relationship between the selectivity of benzaldehyde and amount
of hydrogen peroxide used. Lower i.e. 1:1 Toluene/H O (volume
2 2
ratio) is not effective as it generates the insufficient amount of
oxygen for toluene to oxidize. While excess amount of hydrogen
peroxide ( beyond 1:3 Toluene/H O ) generates more number of
2 2
oxygen ions that are responsible for further oxidation of
benzaldehyde to benzoic acid as well ring oxidation products i.e. o-
2 2
cresol and p-cresol. Thus, 1:3 Toluene/H O vol. ratio (molar ratio
1:1.32) was optimum ratio for obtaining selectivity of benzaldehyde
as major product.
catalyst was recycled up to 3 times without any modification at the
end of each oxidation reaction. The XRD patterns of the reused
catalyst (Fig. S1) reveal no structure degradation. However, due to
gradual deactivation of catalyst, conversion of toluene and
selectivity of benzaldehyde decrease each time when recycled
catalyst was used.
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Figure 10. The study of catalyst reusability on the oxidation of
toluene. Reaction Condition: Catalyst 0.2 g 4 %Cu-MCM-22, Toluene 14
.5 mL, H (30 %) 7.5 mL, Time 4h, Temperature 90 °C
2
2 2
O
1
5
Conclusions
MCM-22 zeolite modified with different amount of copper and iron
by wet impregnation method was evaluated for selective oxidation
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participating in the oxidation of toluene makes it inactive catalyst,
while valency of copper in Cu-MCM-22 is 2+ that makes it active 21
catalyst for oxidation of toluene. Most of Cu ions are present in the
supercages, which suggest that reaction takes place at the
supercages and not within the sinusoidal channels.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
Maaz Nawab is grateful to MANF UGC, New Delhi for the grant of
SRF. Authors acknowledge SRDC, PDPU for assistance in XRD and
FE-SEM analyses.
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Synopsis:
Synthesis of MCM-22, metal impregnation and its characterization by physico-chemical
technique are reported. Solvent free catalytic oxidation of toluene using hydrogen peroxide and
optimized reaction parameters are explored. Possible reaction mechanism is also described.
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