Synthesis, characterization and investigation of catalytic activity of Cu1-xCoxFe2O4 nanocatalysts in t-butylation of p-cresol

Article abstract of DOI:10.1007/s12039-012-0269-6

In this work, tertiary butylation of p-cresol was carried out in the presence of Cu_1-xCoxFe₂O₄ (x = 0 to 1) nanocatalysts by employing methyl-tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) as alkylation agents. Effects of temperature, mole ratio, type and catalyst composition, time and solvent in reaction conditions were investigated. These nanocatalysts were synthesized using hydrothermal method. The characterization of these catalysts was investigated by means of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Fourier Transform Infrared (FT-IR). These nanocatalysts can be recovered and recycled. A good correlation was found between the activity, in terms of p-cresol conversion and various product selectivities for this reaction, and also the acid-base properties of the catalysts. Nano-sized Cu_0.5Co_0.5Fe ₂O₄, in comparison to the other nanocatalysts discussed in this report is the most active nanocatalyst. The only product of this reaction is 2-t-butyl p-cresol with selectivity of 100% and p-cresol conversion is 70%. The possible mechanism for this reaction system was discussed based on the reaction results. The reaction mechanism proposed involves the interaction of phenoxide from phenol and the tert-butyl cation from isobutene on Cu _1-xCo_xFe₂O₄. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-012-0269-6

c
J. Chem. Sci. Vol. 124, No. 4, July 2012, pp. 827–834. ꢀ Indian Academy of Sciences.
Synthesis, characterization and investigation of catalytic activity
of Cu1−xCoxFe2O4 nanocatalysts in t-butylation of p-cresol
REZA FAREGHI ALAMDARIa,∗, ZAHRA HOSSEINABADIa and
MASOUD FARHADI KHOUZANIb
aFaculty of Material and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran
15875-1774, Iran
bInstitute of Nanoscience and Nanotechnology, University of Kashan, Kashan 87317-51167, Iran
e-mail: reza_fareghi@yahoo.com
MS received 23 April 2011; revised 18 December 2011; accepted 26 December 2011
Abstract. In this work, tertiary butylation of p-cresol was carried out in the presence of Cu1−xCoxFe2O4 (x =
0 to 1) nanocatalysts by employing methyl-tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) as alkylation
agents. Effects of temperature, mole ratio, type and catalyst composition, time and solvent in reaction condi-
tions were investigated. These nanocatalysts were synthesized using hydrothermal method. The characteriza-
tion of these catalysts was investigated by means of X-ray diffraction (XRD), Scanning Electron Microscopy
(SEM) and Fourier Transform Infrared (FT-IR). These nanocatalysts can be recovered and recycled. A good
correlation was found between the activity, in terms of p-cresol conversion and various product selectivities for
this reaction, and also the acid–base properties of the catalysts. Nano-sized Cu0.5Co0.5Fe2O4, in comparison to
the other nanocatalysts discussed in this report is the most active nanocatalyst. The only product of this reac-
tion is 2-t-butyl p-cresol with selectivity of 100% and p-cresol conversion is 70%. The possible mechanism for
this reaction system was discussed based on the reaction results. The reaction mechanism proposed involves
the interaction of phenoxide from phenol and the tert-butyl cation from isobutene on Cu1−xCoxFe2O4.
Keywords. Tert-butylation; Cu1−xCoxFe2O4; spinel nanostructures; nanocatalysis; acidity.
1. Introduction
by using solid oxide acid–base catalysts.6 Among vari-
ous oxides, transition metal oxides based on iron oxide
in combination with other metal oxides are found to
be the most active and selective catalyst for ortho-
alkylation. The reaction is sensitive to the acidic and
basic properties of the catalysts, the mole ratio of reac-
tants and the type of alkylating agent. These studies
represented that there is always a competition between
O- and C-alkylation and that acid–base properties of
the catalyst played an important role in the product
selectivity.7
Ferrites which have reversed spinel-type structure
are found to be highly active toward many aromatic
alkylation reactions such as phenol alkylation, aniline
methylation, and pyridine methylation.8–10 The cata-
lytic effectiveness of these systems is due to the ability
of the metallic ions to migrate between the sublattices
without altering the structure, which makes the catalyst
efficient for many organic transformation reactions. The
Cu1−xCoxFe2O4 system was found to be highly active
for phenol alkylation with different alkylating agents.11
The prime attraction of this catalyst system is its high
ortho products selectivity along with high phenol con-
version. A good correlation was found between activity,
The catalytic reaction of phenol and cresols with tert-
butyl alcohol or isobutene as well as methyl-tert-butyl
ether have important commercial applications because
C-alkylated products such as mono-tert-butyl deriva-
tives of m-cresol are the precursors for a number of
commercially important antioxidants and light protec-
tion agents.1,2 Alkylation of p-cresol with tert-butanol
gives 2-tert-butyl-p-cresol (TBC) and 2,6-di-tert-butyl-
p-cresol (2,6-DTBC), commercially known as buty-
lated hydroxy toluene (BHT).3 The alkylated phenol
products are used as raw materials for the synthe-
sis of a variety of resins, durable surface coatings,
varnishes, wire enamels, printing inks, surface-active
agents, rubber chemicals, antioxidants, fungicides and
petroleum additives.4,5 Disadvantages of these reactions
such as poor selectivity, high capital cost, reactor cor-
rosion, and formation of byproducts that were experi-
enced in the conventional method could be overcome
∗For correspondence
827

828
Reza Fareghi Alamdari et al.
in terms of phenol conversion and selectivity of various 2.3 Catalytic activity tests
products for this reaction, and the acid–base properties
of the catalysts.
Tertiary butylation of p-cresol was carried out in a
reflux system. 10 cc nitromethane as a solvent and var-
ious amounts of p-cresol, MTBE and Cu1−xCoxFe2O4
nanoparticles were used in several catalysis tests for
investigation of effects of different molar ratio of start-
ing materials and catalysis in product selectivity and p-
cresol conversion. The reflux system was heated up to
100◦C under magnetic stirring in an oil bath equipped
with an automatic temperature control system. The
reaction was monitored by TLC. When the reaction
was completed, the mixture was filtered to separate the
Cu1−xCoxFe2O4 catalyst. Afterwards, the solvent was
removed to obtain the combined organic phases and
finally products obtained by washing of organic phases
with mixture of n-hexane ethyl acetate and ethyl acetate
(5:1). All products were characterized on the basis of
FT-IR and 1H NMR spectral data and by comparing the
spectra to those of authentic samples or reported data.
2. Experimental
2.1 Materials measurements
All chemical reagents used in our experiments were of
analytical grade and were used as received without fur-
ther purification. XRD patterns were recorded by the
X-ray diffractometer (Philips-PW1800). Fourier trans-
mission infrared (FT-IR) spectra of the powders (as
pellets in KBr) were recorded using a Fourier trans-
mission infrared spectrometer (Spectrum One FT-IR
Spectrometer, Perkin Elmer Instruments) in the range of
4000 cm1−. Conversion of the reactants was determined
by HPLC (Waters600). The particle size and external
morphology of the fine calcined powders were charac-
terized by scanning electron microscopy, SEM (Philips-
XL30), Samples were coated with gold at 10 mA for
2 min prior to SEM analysis.
3. Results
3.1 Characterization of Cu1−xCoxFe2O4
nanoparticles
2.2 Synthesis of nanocatalysts
Cu1−xCoxFe2O4 nanoparticles were prepared via a hy- Figure 1 shows the indexed x-ray diffraction (XRD)
drothermal process. CTAB (1 g) was dissolved in 35 ml patterns of Cu1−xCoxFe2O4 (x = 0.0, 0.5 and 1.0).
of distilled water. Then 1 g FeCl3·6H2O was added to Cu-rich catalyst (x = 0) exhibits a diffraction pat-
it and was stirred for 10 min (S1). Afterwards, appro- tern attributed to cubic spinel phase and a considerable
priate amounts of CuCl2.2H2O and CoCl2.6H2O salts, amount of CuO and Fe2O3. Substitution of Cu with Co
given in table 1, were dissolved in distilled water (S2) and increases the overall crystallinity of the spinel phase.
added by a burette into S1 (S3). An aqueous solution of XRD pattern of CoFe2O4 (figure 1a) shows the for-
NaOH (S4) was introduced dropwise into S3 solution. mation of single phase cubic structure with dominant
The addition of S4 was stopped at pH 10-11 where a peak corresponding to (311) reflection indicating that
black precipitate formed. The reaction mixture was stirred the crystallites are preferentially oriented along (311)
for half hour. After stirring, the same was transferred to plane. The breadth of the Bragg peak is a combina-
an autoclave. This autoclave was kept in an oven and tion of both instrument and sample dependent effects.
heated at 160◦C for 72 h. The autoclave was allowed To decouple these contributions, it is necessary to
to cool down to room temperature by natural convec- collect a diffraction pattern from the line broadening
tion. The product was then washed several times with of a standard material such as silicon to determine
distilled water followed by drying at room temperature. the instrumental broadening. The instrument corrected
Table 1. The amount of starting materials for preparing of Cu1−xCoxFe2O4(x = 0, 0.25,
0.50, 0.75 and 1) nanoparticles.
Entry
x
Cu1−xCoxFe2O4
CoCl2(g)
CuCl2(g)
FeCl3(g)
CTAB(g)
1
2
3
4
5
0
CuFe2O4
0
0.32
0.24
0.16
0.079
0
1
1
1
1
1
1
1
1
1
1
0.25
0.5
0.75
1
Cu0.75Co0.25Fe2O4
Cu0.5Co0.5Fe2O4
Cu0.25Co0.75Fe2O4
CoFe2O4
0.11
0.22
0.33
0.44

Nano Cu1−x Cox Fe2O4 as catalysts in t-butylation of p-cresol
829
Figure 2. FT-IR spectra of (a) CuFe2O4, (b) Co0.5Cu0.5Fe2O4,
(c) Co0.75Cu0.25Fe2O4 and (d) CuFe2O4 nanoparticles
obtained by hydrothermal method.
Figure 1. XRD patterns of (a) CoFe2O4, (b) Cu0.5Co0.5Fe2O4
and (c) CuFe2O4 nanoparticles.
X-ray powder diffraction data of Cu1−xCoxFe2O4 for
most intense reflection are shown in table 2. As we can
see from these data, crystallite sizes of Cu1−xCoxFe2O4
(x = 0.0, 0.5 and 1.0) calculated by Debye–Scherrer
equation, are in the nanometer range.
broadening β corresponding to the diffraction peak of
Cu1−xCoxFe2O4 (x = 0.0, 0.5 and 1.0) was estimated by
using the below relation:
Figure 2 shows FT-IR spectra of Cu1−xCoxFe2O4
nanoparticles. Two main broad metal–oxygen bands
are observed in the FT-IR spectra of all spinels,
and ferrites in particular. The highest one, ν1, gen-
erally observed in the range of 600–550 cm−1, corre-
sponds to intrinsic stretching vibrations of the metal
at the tetrahedral site, Mtetra ↔ O, whereas the low-
β = β1 − β2
The crystallite sizes (Dc) of Cu1−xCoxFe2O4 nanoparti-
cles were calculated using the Debye–Scherrer equation
(see table 2):
Kλ
Dc =
,
β cos θ
where β is the breadth of the observed diffraction line est band, ν2, usually observed in the range of 450–
at its half-intensity maximum, K is the so-called shape 385 cm−1, is assigned to octahedral metal stretching,
factor, which usually takes a value of about 0.9, and Mocta ↔ O. However, no clear peak due to octahe-
λ is the wavelength of X-ray source used in XRD. drally coordinated metal ions has been observed due to
Table 2. X-ray powder diffraction data of Cu1−xCoxFe2O4 for most intense
reflection.
Entry
X
Cu1−xCoxFe2O4
2θ(deg)
FWHM(deg)
D(nm)
1
2
3
0
0.5
1
CuFe2O4
Cu0.5Co0.5Fe2O4
CoFe2O4
35.4
35.4
35.2
0.24372
0.32946
0.32755
18
22
20

830
Reza Fareghi Alamdari et al.
Figure 3. SEM images of Cu1−xCoxFe2O4 nanocatalysts: (a) Cu0.5Co0.5Fe2O4, (b)
Cu0.25Co0.75Fe2O4, (c) Cu0.75Co0.25Fe2O4, (d) CoFe2O4 and (e) CuFe2O4.
the detection limit of our FT-IR instrument, which is distribution is observed, which consists of nearly octa-
expected to be below 400 cm−1. The bands at 2923 and hedral crystals with an average size of about 30 nm.
2845 cm−1 are assigned to stretching of C–H group and The grain sizes measured of Cu1−xCoxFe2O4 (x = 0,
the bands at 1120–1060 cm−1 to C–O stretching. The 0.25, 0.50, 0.75 and 1) are 48, 34, 24, 24 and 29 nm,
broad band centred at 3430 cm−1 can assigned to hydro- respectively.
gen bonded O–H stretching vibration arising from sur-
face hydroxyl groups on nanoparticles. The absorption
3.2 Catalytic activity studies
band at 1630 cm−1 on spectra refers to the vibration of
remainder H2O in the sample.
3.2a The effect of reaction time on tert-butylation of
p-cresol: Tertiary butylation of p-cresol with MTBE
is a typical acid-catalyzed reaction. Alkylation of p-
cresol with MTBE using spinel nanostructures gave
a mixture of the isomers of tertbutylated phenols in
terms of alkyls and the position of alkyl substituent.
Only C-alkylated products can be formed depending
on the reaction conditions such as the reaction tem-
perature and the amount of MTBE, etc. The alkyla-
Surface morphology of the obtained Cu1−xCoxFe2O4
(x = 0, 0.25, 0.50, 0.75 and 1) nanoparticles was
investigated by scanning electron microscopy (SEM)
images. SEM micrographs were used to see the grain
micro-structure of the nanoparticles, which would pro-
vide a better view of the grain development and grain
sizes. SEM micrographs of Cu1−xCoxFe2O4 (x = 0,
0.25, 0.50, 0.75 and 1) are shown in figure 3. From the
micrographs, it is clear that the grains also have dif- tion of p-cresol with MTBE using spinel nanostructures
gives a mixture of TBC and 2,6-DTBC. With increasing
ferent morphologies other than spherical. A broad size
Table 3. Influence of catalyst composition on p-cresol conversion and products selectivity over
Cu1−xCoxFe2O4 (x = 0, 0.25, 0.50, 0.75 and 1).
Entry
Cu1−xCoxFe2O4
p-cresol conversion (%) TBC selectivity (%) DTBC selectivity (%)
1
2
3
4
5
CuFe2O4
55
60
70
40
30
90
80
100
85
10
10
0
15
20
Cu0.75Co0.25Fe2O4
Cu0.5Co0.5Fe2O4
Cu0.25Co0.75Fe2O4
CoFe2O4
80
MTBE: p-cresol: catalyst mole ratio of 2:1:0.8, 100◦C, CH3NO2 solvent and 6.5 h

Nano Cu1−x Cox Fe2O4 as catalysts in t-butylation of p-cresol
831
Table 4. Influence of tert-butylation agent on p-cresol conversion and products selectivity over
Cu0.5Co0.5Fe2O4.
Entry t-butylating agent p-cresol conversion (%) TBC selectivity (%) DTBC selectivity (%)
1
2
TBA
MTBE
30
70
5
0
95
100
t-butylating agent: p-cresol: catalyst mole ratio of 2:1:0.6, 100◦C, CH3NO2 solvent and 6.5 h
reaction time, the conversion of p-cresol and the selec- 3.2c The effect of alkylation agent on tert-butylation
tivity to TBC increase rapidly and reach an equilibrium of p-cresol: For evaluation of the effect of tert-
level after 6.5 h, while the selectivity for 2,6-DTBC butylating agent in the conversion of p-cresol and
decreases rapidly and disappears gradually. The conver- selectivity of various products in the presence of
sion of p-cresol reaches to 70% and the selectivity to Cu1−xCoxFe2O4 (x = 0.5) as a catalyst, two tests were
TBC reaches to about 100% after 6.5 h. The increase in carried out: in the first test, MBTE was used as a tert-
the selectivity to TBC is at the cost of a decrease in the butylating agent and in the other test, tert-butyl alcohol
selectivity to DTBC. The possible reason is that DTBC (TBA) was a tert-butylating agent. Results of these tests
is converted into TBC. Thus, the best reaction time is are summarized in table 4. These results clearly show
6.5 h because the catalyst activity has attained a stable that in tertiary butylation of p-cresol, MTBE as a tert-
higher activity state.
butylating agent is a far better choice than TBA. The
possible reason is that by using TBA, H2O is formed in
the reaction media which may be absorbed by the acidic
sites of catalyst and thus blocking the acidic sites of
the catalyst. Consequently, the reaction on the surface
of the catalyst is decreased. Besides, the etheric bond is
weaker than alcoholic bond and therefore, carbocation
is formed easily by MBTE than TBA.
3.2b The effect of catalyst composition on tert-
butylation of p-cresol: Table 3 illustrates the cat-
alyst composition dependence of p-cresol conver-
sion and selectivity of tertiary butylated p-cresol on
Cu1−xCoxFe2O4 using MTBE as tert-butylating agent
at 100◦C with tert-butylating agent: p-cresol: cata-
lyst mole ratio of 2:1:0.8, CH3NO2 solvent and 6.5 h.
The important observations are: (i) p-cresol conversion
increases from x = 0 to 0.5 and then decreases with fur- 3.2d The effect of the amounts of Cu0.5Co0.5Fe2O4
ther increase in x. (ii) 2-TBC selectivity decreases as Cu nanoparticles on tert-butylation of p-cresol: The
content decreases. (iii) x = 0.5 is associated with max- influence of the molar ratio of Cu0.5Co0.5Fe2O4 to p-
imum p-cresol conversion and high selectivity towards cresol on the conversion of p-cresol and the selectivi-
2-TBC formation. These results show that for achieving ty to TBC is shown in table 5. As the molar ratio of
the maximum amount of catalytic activity, an optimum the nanocatalyst to p-cresol increases, both the conver-
distribution between Cu atoms and neighboring het- sion of p-cresol and the selectivity to TBC first increase
eroatoms (Co atoms) is needed.7 We can easily find this and then decrease. The selectivity to 2,6-DTBC almost
optimum distribution when x = 0.5. Besides, low con- remains constant. This is attributed to the proportional
version of p-cresol in x ≥ 0.75 may be due to de-tert- increase in the amount of catalysts and the increase of
butylation of tert-butylated products in the presence of acidity in the reaction system, which leads to increase
strong acidic sites. (Based on table 8 x = 1 has a maxi- in p-cresol conversion. However, a strong acid sys-
mum acidity and x = 0 has a maximum basic activity.) tem does not lead to further alkylation of TBC with
Table 5. Influence of catalyst amount on p-cresol conversion and products selectivity over
Cu0.5Co0.5Fe2O4.
Entry
Catalyst (g)
p-cresol conversion (%)
TBC selectivity (%)
DTBC selectivity (%)
1
2
3
0.1
0.15
0.2
40
70
60
100
100
100
0
0
0
MTBE: p-cresol = 2:1, 100◦C, solvent: CH3NO2 and 6.5 h

832
Reza Fareghi Alamdari et al.
Table 6. Influence of reaction temperature on p-cresol conversion and products selectivity over
Cu0.5Co0.5Fe2O4.
Entry Temperature (◦C) p-cresol conversion (%) TBC selectivity (%) DTBC selectivity (%)
1
2
3
4
100
70
45
70
50
20
0
100
95
95
0
0
5
5
0
R.T
MTBE: p-cresol: catalyst mole ratio of 2:1:0.6, CH3NO2 solvent and 6.5 h
MTBE. Considering both the conversion of p-cresol 3.2f The effect of the molar ratio of MTBE to p-cresol
and the selectivity to TBC, the optimum molar ratio of in tert-butylation of p-cresol: The influence of the
Cu0.5Co0.5Fe2O4 to p-cresol is 1:1.
molar ratio of MTBE to p-cresol on the conversion of
p-cresol and the selectivity to TBC in tert-butylation
using Cu0.5Co0.5Fe2O4 are shown in table 7. As the
molar ratio of MTBE to p-cresol increases, the conver-
sion of p-cresol and the selectivity to TBC first increase
and then rapidly reach to a steady state, while the selec-
tivity for 2,6-DTBC is zero. Both high conversion of p-
cresol and the selectivity to TBC are achieved when the
molar ratio of p-cresol to MTBE is 1:2.
3.2e The effect of the reaction temperature in the
tert-butylation of p-cresol: The influence of the reac-
tion temperature on the conversion of p-cresol and the
selectivity to TBC is shown in table 6. The conver-
sion of p-cresol and the selectivity to TBC reach maxi-
ma at 100◦C. With an increase in the temperature, the
conversion of p-cresol and the selectivity to TBC first
increased and then rapidly reached a steady state. The
conversion of p-cresol increases from 0 to 70 mol% as
the temperature changes from RT to 100◦C. 2,4-DTBC
4. Discussion
4.1 Catalytic performance
selectivity also remains the same from 45 to 70◦C, From the activity data, it is evident that Cu-rich sam-
except at 100◦C. It also indicates that lower tempera- ples are active catalysts for tertiary butylation and Co-
ture favours the formation of 2,6-DTBC, while higher dominant compositions show poor activity, emphasiz-
temperature favours the formation of TBC. Consider- ing the important role of Cu in this reaction. However,
ing both the conversion of p-cresol and the product dis- the equal bulk contents of Cu and Co at x = 0.5 show
tribution, the optimum reaction temperature should be maximum activity. Except at x = 0.5, p-cresol con-
around 100◦C. At this temperature, both the selectivity version is between 30 and 60 mol% for all other com-
to TBC and the conversion of p-cresol reach the high- positions, suggesting that tertiary butylation activity is
est value. These results indicate that no 2,6-DTBC in mainly due to the Cu + Co combination, of which Cu
the 100◦C may be due to de-tert-butylation reactions is the highly active. An important aspect of the buty-
at higher temperature. However, for the precise inves- lation reaction is related to product selectivity too. At
tigation of the effect of temperature in tert-butylation an optimum temperature of 100◦C, MTBE attaches to
reactions, we have carried out the kinetic investigation the phenolic ring to give tertbutyl phenols. Unlike lower
of these reactions in parallel to the work which is pre- alkylations, tertiary butylation of p-cresol gives two
sented in this report, but because of high volume of products, namely, 2-TBC and 2,6-DTBC, and the selec-
kinetic investigation of this research, we are collecting tivity of each product depends on the various reaction
our data to report as a separate paper.
parameters.
Table 7. Influence of MTBE: p-cresol mole ratio on p-cresol and product selectivity over
Cu0.5Co0.5Fe2O4.
Entry MTBE: p-cresol p-cresol conversion (%) TBC selectivity (%) DTBC selectivity (%)
1
2
3
2:1
1.5:1
1:1
70
40
20
100
100
100
0
0
0
p-cresol: catalyst mole ratio of 1:0.6, 100◦C, CH3NO2 solvent and 6.5 h

Nano Cu1−x Cox Fe2O4 as catalysts in t-butylation of p-cresol
833
Table 8. Textural and acid-base characteristic of Cu1−xCoxFe2O4 10.
Entry
X
a(A◦)
SBET (m2/g)
Pore volume (cc/g)(10−2
)
Relative acidity(basicity)a
1
2
3
4
5
0
8.3898
8.4051
8.4012
8.3982
8.3998
28.8
34.0
43.8
36.6
36.8
5.1
6.7
10.9
6.4
300 (580)
381 (510)
497 (420)
623 (275)
782 (210)
0.25
0.50
0.75
1.0
5.9
aObtained from FT-IR studies of pyridine and CO2 adsorbed catalysts [23]
4.2 Reaction mechanism
(Cu2+) or nonreducible (Co2+) ions in the neigh-
bourhood. Generally, robust Co2+ in the neighbour-
A detailed investigation of the acid–base properties hood increases the acidity. A simple comparison of p-
of the Cu1−xCoxFe2O4 catalyst system was carried out cresol conversion and acid–base character demonstrates
by Mathew et al.12 The acidic and textural proper- that neither strong acidic nor strong basic character
ties of Cu1−xCoxFe2O4 which well obtained by Mathew helps the tertiary butylation reaction. It is also evident
research group are summarized in table 8. Our catalytic that the intermediate acid–base character observed for
results and their investigation about Cu1−xCoxFe2O4 the x = 0.5 composition with high catalytic activi-
acidic properties are in good agreement. Based on their ty indicates an optimum level of acid–base sites are
investigations and our results, four important points required for better catalytic performance in terms of p-
which help in understanding of tertiary butylation activ- cresol conversion and overall product yield. In accor-
ity are: (i) the Cu1−xCoxFe2O4 surface is dominated by dance to the above discussion, we proposed a mecha-
Lewis acid character (ii) a relatively weak acidic char- nism which is summarized in the figure 4.
acter at x = 0 increases with Co content to highly
Alkylation of p-cresol, just like other aromatic com-
acidic character at x = 1.0 and an increase in num- pounds, follows the Friedel-Craft mechanism. From
ber and strength of acidic sites (iii) the basic charac- electrophilic substitution it is clear that the tert-butyl
ter of the catalyst decreases with increasing x. (iv) the cation attacks the p-cresol ring preferentially in ortho
acid–base character on the catalyst surface is predomi- positions to the –OH group. The ratio of isomers formed
nantly due to octahedral cations, and the Lewis acidity initially is kinetically controlled and is determined by
changes significantly depending on the easily reducible phenolic OH groups and incoming tert-butyl cations in
Figure 4. Schematic diagram of the proposed mechanism of tertiary butylation of p-cresol
by Cu1−xCoxFe2O4 nanocatalysts.

834
Reza Fareghi Alamdari et al.
the present case. Though the mono-tert-butylated p- References
cresol are more active than p-cresol, subsequent ter-
1. Chhabra J S, Athar J, Agranal J P and Singh H 1993
tiary butylation occurs only when there is less steric
Rubber Compos. Process. Appl. 20 305
2. Ajit S, Nilesh S and Shriniwas S 2004 Appl. Catal. A:
Gen. 267 5
hindrance.12
3. Karthik M, Vinu A and Tripathi K N 2005 Micropor.
Mater. 70 207
4. Gonzalez V 1981 Rubber Chem. Technol. 54 134
5. Emil D and Vasile H 2003 J. Catal. 218 249
6. Fiege H 2000 Ullmann’s Encyclopedia of Industrial
Chemistry; Federal Republic of Germany, Bayer A G,
Leverkusen, A19, p. 324
7. Thomas M, Bollapragada S R and Chinnakonda S G
2004 J. Catal. 222 107
8. Rajgopal R, Vetrivel R and Rao B S 2000 Catal. Lett.
65 99
9. Rao B S, Sreekumar K and Jyothi T M 1998 Indian
Patent 2707/98
10. Sreekumar K, Jyothi T M, Mathew T, Talawar M B,
Sugunan S and Rao B S 2000 J. Mol. Catal. A 159 327
11. Mathew T, Shylesh S, Devassy M B and Vijayaraj M
2004 Appl. Catal. A: Gen. 273 35
5. Conclusion
In this research, Cu1−xCoxFe2O4 (x = 0, 0.25, 0.50,
0.75 and 1) spinel nanostructures were synthesized
and characterized, and their catalytic activity in the
tert-butylation of p-cresol were examined. Under opti-
mal reaction conditions, more than 70% conversion
of p-cresol and about 100% selectivity to TBC were
obtained. The catalysts can be recycled after the reac-
tion. In comparison with the solid acid catalysts, the
tert-butylation of p-cresol using spinel nanostrucures
can be conducted at mild reaction conditions. It is
concluded that the Cu0.5Co0.5Fe2O4 nanoparticles have
strong acidic nature and can be used as a substitute for
traditional acids.
12. Mathew T, Vijayaraj M, Pai S, Tope B B, Hegde G S,
Rao S B and Gopinath S C 2004 J. Catal. 227 175