M.L. Mohammed et al. / Applied Catalysis A: General 466 (2013) 142–152
143
optimum reaction conditions obtained from batch experiments
would form the basis for continuous epoxidation in a continu-
offer substantial benefits including increased selectivity, scalability
and reproducibility and therefore have enormous potential as pro-
cess alternatives for carrying out liquid phase chemical reactions
[31–33].
Nomenclature
AAS
atomic absorption spectrophotometric
AMP
ANN
ASAP
BET
2-(aminomethyl) pyridine
artificial neural network
accelerated surface area and porosimetry
Brunauer–Emmett–Teller
divinylbenzene
DVB
ECTM-5 Econo-CapTM-5
2. Experimental methods
FID
FMR
GC
MSE
Mo
flame ionisation detector
feed mole ratio
gas chromatography
mean square error
molybdenum
2.1. Materials
1,2-Dichloroethane (anhydrous, 99.8%), 1-hexene (≥99%), 2-
(aminomethyl) pyridine (AMP) (99%), 2-ethylhexanol (≥99.6%),
4-vinyl-1-cyclohexene (97%), acetone (>95%), divinylbenzene
(DVB) (80%), ethanol (≥99.5%), iso-octane (anhydrous, 99.8%),
molybdenyl acetylacetonate (MoO2(acac)2) (99%), styrene (≥99%),
TBHP solution in water (70%, w/w), toluene (anhydrous, 99.8%)
and vinylbenzyl chloride (VBC) (97%) were purchased from
Sigma–Aldrich Co. Ltd., and microporous polybenzimidazole (PBI)
resin beads were supplied by Celanese Corporation. The purity of all
chemicals was verified by gas chromatography (GC). TBHP was ren-
using the procedure reported by Sharpless and Verhoeven [34]. All
other chemicals were used without further purification. The con-
centration of TBHP in toluene was found to be 3.57 mol L−1 and was
determined by iodometric titration [34].
MoO2(acac)2 molybdenyl acetylacetonate
NN
PBI
neural network
polybenzimidazole
PBI.Mo polybenzimidazole supported Mo(VI) complex
Ps.AMP polystyrene 2-(aminomethyl) pyridine
Ps.AMP.Mo polystyrene 2-(aminomethyl) pyridine sup-
ported Mo(VI) complex
RDC
TBHP
t
reactive distillation column
tert-butyl hydroperoxide
VBC
vinylbenzyl chloride
[21,22]. Polymer supported molybdenum complexes have been
shown to be effective for alkene epoxidation with alkyl hydroper-
oxide as oxygen source [23–27].
2.2. Preparation and characterisation of polymer supported
Mo(VI) catalysts
Due to the complex nature of catalytic reactions, it is challeng-
ing to adopt the conventional method of modelling the kinetic data
that involves assuming an empirical mathematical equation and
finding the unknown parameters by non-linear regression analy-
sis. Neural network (NN) models are very useful in such cases as
multi-dimensional problems that are found in catalysis. NN models
such as artificial neural network (ANN) have the capacity to estab-
lish quantitative relationships between the input and output data
without prior knowledge of the correlation between the variables
involved in the system [28,29].
In this work, a novel process in which an efficient and selective
polybenzimidazole supported molybdenum complex (PBI.Mo) and
a polystyrene 2-(aminomethyl) pyridine supported molybdenum
complex (Ps.AMP.Mo) have been used as catalysts for epoxidation
and uses environmentally benign tert-butyl hydroperoxide (TBHP)
as an oxidant. On the other hand, tert-butanol is also formed as
a co-product during epoxidation. Hence, this process is atom effi-
cient. Oku et al. [30] reported that tert-butanol can be efficiently
recycled in the Sumitomo process through hydrogenolysis and oxi-
dation.
An extensive assessment of PBI.Mo and Ps.AMP.Mo catalysts
has been carried out in a batch reactor to evaluate their stabil-
ity, catalytic activity and reusability. The suitability and efficiency
of both catalysts have been compared by studying the effect of
catalyst loading, feed molar ratio (FMR) of alkene to TBHP and
reaction temperature on the yield of epoxides for optimisation of
the reaction conditions in a batch reactor. A detailed evaluation
of molybdenum (Mo) leaching from the polymer supported cata-
lyst has been conducted by assessing the catalytic activity in the
residue found in the supernatant solutions of the reaction mixture
after the removal of heterogeneous catalyst. All the batch experi-
mental results have been modelled using ANN to predict the trend
in the rate of epoxidation under different reaction conditions. The
PBI has a high degree of thermal stability and chemical resis-
tivity. As a result of these properties, PBI resin is employed in
a wide variety of applications such as ion-exchange, separations,
purifications and support for polymer supported catalysts [35]. On
the other hand, crosslinked polystyrene 2-(aminomethyl) pyridine
(Ps.AMP) was prepared by suspension polymerisation of known
mixtures of divinylbenzene (DVB), vinylbenzyl chloride (VBC) and
followed by amination of the formed resin with an excess of 2-
(aminomethyl) pyridine in ethanol. The crosslinked resin beads
are commonly used as catalyst support due to their high poros-
ity, large surface area and robust spherical particles with uniform
size distribution [36]. PBI and Ps.AMP resins were each loaded
with Mo by reaction with an excess of MoO2(acac)2, relative to
polymer bound ligand under reflux in toluene for four days. The
excess MoO2(acac)2 was removed by exhaustive extraction with
acetone in a Soxhlet. Fresh acetone was replaced at regular time
intervals until the refluxing solvent remained colourless. Molyb-
denum in the prepared catalysts was found to be homogeneously
distributed in the polymer. The particle size distribution of PBI.Mo
and Ps.AMP.Mo catalysts was carried out using a Malvern Mas-
surface area and porosimetry) 2010. Molybdenum content of the
supported catalysts was assayed using atomic absorption spectro-
2.3. Batch epoxidation studies
Fig. 1 shows reaction scheme for alkene epoxidation. Epoxi-
dation of 1-hexene and 4-vinyl-1-cyclohexene with TBHP as an
oxidant in the presence of polymer supported catalyst was car-
ried out in a jacketed four necked glass reactor of 0.25 L capacity.