1
22
G.D. Yadav, R.V. Sharma / Journal of Catalysis 311 (2014) 121–128
Titanium oxides are extensively used for oxidation reaction. Both
anatase and rutile crystallographic forms of titania are acidic in
nature. However, anatase form of titania is more active than rutile
2.2. Catalyst preparation
3 7 4
Titanium isopropoxide (Ti(OC H ) ) (0.221 mol) was dissolved
[
13,14]. However, no report is available on use of sulfated titania
in anhydrous ethanol (4.67 mol) to make dilute titanium isoprop-
oxide solution. It was taken in additional funnel and added drop-
wise to the flask containing 136 ml of water/ethanol (5:1 mol
ratio) solution with rigorous stirring at room temperature under
nitrogen atmosphere. A white precipitate of hydrous oxide was
produced instantly, and the mixture was further stirred for 2 h at
room temperature. The mixture-containing white precipitate was
subjected to hydrothermal treatment at 80 °C in Parr autoclave
for 24 h. The white precipitate was filtered through G4 funnel
and dried at 120 °C for 24 h to obtain white powder of titanium
hydroxide. A calculated amount of iron nitrate (0.15 g per g of tita-
nium hydroxide) was dissolved in 10 ml of distilled water and
added to the dried titanium hydroxide powder by the wet impreg-
nation technique. The dried material was hydrolyzed by ammonia
gas to obtain iron hydroxide containing material and further
washed with distilled water until a neutral filtrate was obtained.
The material was dried in oven for 24 h at 110 °C. Sulfation was
done as described by Yadav and Murkute [25] by immersing in
15 ml of chlorosulfonic acid (0.5 M) per g of material in ethylene
dichloride. The material was soaked for 5 min in the chlorosulfonic
acid solution under nitrogen flow and then was oven-dried to re-
move the solvent at 120 °C for 30 min. The sample was then kept
in oven at 120 °C for 24 h under nitrogen flow to avoid moisture
absorption on the dried material and calcined thereafter at
for oxidation reaction. It is reported that sulfated titania oxide
can be used as a solid acid catalyst due to its high acid strength;
however, its applications are only limited to Friedel Crafts alkyl-
ation and acylation reactions [6,15].
When titania is synthesized by the sol–gel method, it can be ob-
tained with nanocrystallite size [6]. Sol–gel processing provides
excellent chemical homogeneity and an opportunity of achieving
unique metastable structure at a low temperature. It involves the
formation of metal-oxo-polymer network from molecular precur-
sors such as metal alkoxides or metal salts [16,17]. It is also re-
ported that chemical and catalytic properties of titania can be
modified by loading of metallic ions [6,15,18,19]. Iron plays an
important role in oxidation reaction [3,20–23] and hence incorpo-
ration of iron into titania will provide interesting catalytic
properties.
Thus, it was planned in the current work to prepare a multi-
functional catalyst based on iron and titania with superior activ-
ity, selectivity and robustness. Sulfated mixed metal oxides
based on titania with different iron loadings were prepared for
oxidation and alkylation reactions. Our laboratory has reported
that chlorosulfonic acid acts as a better sulfating agent as com-
pared with sulfuric acid, (NH
etc. It provides different pore sizes and higher superacidity
24–26]. Hence, chlorosulfonic acid was used for sulfation of
4 2 4 4 2 2 3 4 2 3
) SO , (NH ) S O , (NH ) S, SO ,
[
500 °C for 3 h to obtain a solid catalyst, namely, sulfated Fe–TiO
catalyst with 6 wt% of iron loading. Similarly, other two catalysts,
sulfated Fe–TiO (3 wt%) and sulfated Fe–TiO (9 wt%), were pre-
pared by varying the amount of iron nitrate. Detailed catalyst char-
acterization of sulfated Fe–TiO (6 wt%) was done and is referred as
2
iron-loaded titania. This new multifunctional catalyst, having
both acidic centers and redox properties, was evaluated in li-
quid-phase selective oxidation of benzyl alcohol to benzalde-
hyde, and alkylation of toluene with benzyl alcohol as model
reactions. Benzyl alcohol is a model compound which has been
tested for oxidation catalysts and different agents and was thus
chosen. Further, the product benzaldehyde has tremendous
applications in cosmetic, food, dyestuff, and agrochemical indus-
tries and is considered as the second most important aromatic
molecule after vanillin [27,28]. The basic challenge in the oxida-
tion of benzyl alcohol is the consecutive reaction which can lead
to overoxidation to benzoic acid. Different innovative strategies
have been used to oxidize benzyl alcohol including mechanistic
details; however; selective oxidation is still a challenging task
2
2
2
ICaT-3. ICaT is acronym for Institute of Chemical Technology and 3
stands for third catalyst in a series of catalysts developed by us.
2.3. Catalyst characterization
Infrared spectra of the samples were obtained at a resolution of
À1
À1
2 cm from 4000 to 400 cm . Spectra were collected with a Per-
kin–Elmer instrument, and in each case, the sample was referenced
against a blank KBr Pellet.
Thermogravimetry analysis (TGA) of the catalyst was carried
out by DTG-60 H (Shimadzu, Japan) in an aluminum pan. The cat-
alyst samples were heated from room temperature to 500 °C with a
ramp rate of 5 °C per minute under 15 ml/min flow of nitrogen. The
change in weight of the sample with increasing temperature was
recorded.
Acidic sites of the catalyst were determined by temperature-
programed desorption (TPD) analysis by using Autochem II 2910
(Micromeritics, USA) with ammonia as probe molecule. A quantity
of 30 mg of the catalyst was taken in a quartz tube and degassed up
to 500 °C under the flow of nitrogen. After cooling to 30 °C, the
[
29]. Friedel Crafts alkylation reaction is also widely practiced
in the industry using acidic catalysts and hence, the efficacy of
the catalyst was further tested in the alkylation of toluene with
benzyl alcohol. Toluene alkylation with benzyl alcohol or chlo-
ride is a reference reaction used in evaluation of activity of solid
acids in liquid-phase reactions. The objective of the present
study to enhance redox activity of iron-loaded titania-based cat-
alyst by induced acidity via sulfation, and also to confirm its
superior acidity in an alkylation reaction.
3 2
sample was exposed to a flow of 10% NH –N gas mixture at a rate
2
. Experimental section
of 20 ml/min for 30 min. Physisorbed gas was removed by passing
nitrogen gas. Chemisorbed ammonia was desorbed by using tem-
perature-programed desorption and detected by TCD.
2.1. Chemicals
2 2
The surface properties of Fe–TiO and sulfated Fe–TiO were
All the chemicals were procured from the following firms and
measured by the Brunauer–Emmett–Teller (BET) method using
ASAP 2010 (Micromeritics, USA) instrument. The catalyst samples
were degassed under vacuum at 300 °C for 4 h. The measurements
used without further purification: 1,2-dichloroethane, benzyl alco-
hol, ethanol (AR), Fe(NO O, aqueous ammonia solution (all
3
)
3
Á9H
2
from s.d. Fine Chemicals, Mumbai, India), toluene, titanium iso-
propoxide (all from Spectrochem Ltd. Mumbai), 30% W/V aqueous
hydrogen peroxide (Merck Ltd. Mumbai), cetyl trimethyl ammo-
nium bromide (CTAB), as gift from Dishman Pharmaceuticals and
Chemicals Ltd., Ahmadabad, India); other analytical reagents (all
from s.d. Fine Chemicals).
were made using N
method. Isotherms were measured at liquid nitrogen temperature.
Surface area and pore volume were calculated from N -adsorp-
2
gas as the adsorbent and with a multipoint
2
tion–desorption isotherms using the conventional BET method.
The chemical composition of the catalysts (average of three data
points at different locations of the solid) was determined by