Oxidant concentration effects in the hydroxylation of phenol over titanium-based zeolites Al-free Ti-Beta and TS-1
Date
2006-03
Authors
Burton, Robert M
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : University of Stellenbosch
Abstract
This work focuses on the effects of hydrogen peroxide concentration on the catalytic activity
and product selectivity in the liquid-phase hydroxylation of phenol over titanium-substituted
zeolites Al-free Ti-Beta and TS-1 in water and methanol solvents. Hydroquinone is typically
the desired product, and these solvents employed have previously been shown to be of
importance in controlling the selectivity of this reaction.
Different volumetric quantities of an aqueous 30 wt-% peroxide solution were added to either
water or methanol solutions containing the catalyst and phenol substrate, and the reaction
monitored by withdrawing samples over a period of 6-8 hours.
For Al-free Ti-Beta catalysed reactions, the peroxide concentration affects the selectivity and
activity differently in water and methanol solvents. Using methanol solvent, the selectivity to
hydroquinone formation is dominant for all peroxide concentrations (p/o-ratio > 1), and
favoured by higher initial peroxide concentrations (> 1.27 vol-%), where p/o-ratios of up to
can be reached; in water solvent, increasing the peroxide concentration above this level
results in almost unchanging selectivity (p/o-ratio of ca. 0.35). For lower peroxide
concentrations in water, the p/o-ratio increases slightly, but never exceeds the statistical
distribution of ca. 0.5. Using water as a solvent, higher phenol conversion is obtained as the
initial peroxide concentration increases; in methanol the phenol conversion is largely
independent of peroxide concentration.
As expected for the smaller pore TS-1, higher hydroquinone selectivity is obtained in
methanol than for Al-free Ti-Beta, which is consistent with shape-selectivity effects enhanced
by the use of this protic solvent. Interestingly, with TS-1 the p/o-ratio is higher at lower phenol
conversions, and specifically when the initial peroxide concentration is low (p/o-ratio
exceeding 3 were obtained at low phenol conversion), and decreases to a near constant
value at higher conversions regardless of the starting peroxide concentration. Thus, low
peroxide concentrations favour hydroquinone formation when TS-1 is used as the catalyst.
Comparing the performance of the two catalysts using methanol solvent, the phenol
conversion on TS-1 is more significantly influenced by higher hydrogen peroxide
concentrations than Al-free Ti-Beta. However, with higher initial concentrations the
unselective phenol conversion to tars is more severe since the hydroquinone selectivity is not
higher at these high peroxide concentrations. The increased tar formation, expressed as tar
deposition on the catalyst or as the tar formation rate constant, confirms that the greater amount of free-peroxide present is mainly responsible for the non-selective conversion of
phenol.
Kinetic modelling of the reaction data with an overall second-order kinetic model gave a good
fit in both solvents, and the phenol rate constant is independent of changing hydrogen
peroxide concentration for the hydroxylation over Al-free Ti-Beta using water as the solvent
(kPhenol = 1.93 x 10-9 dm3/mmol.m2.s). This constant value suggests that the model developed
to represent the experimental data is accurate. For TS-1 in methanol solvent the rate
constant is also independent of peroxide concentration (kPhenol = 1.36 x 10-8 dm3/mmol.m2.s).
The effect of the method of peroxide addition was also investigated by adding discrete
amounts over a period of 4.5 hours, and was seen to improve hydroquinone selectivity for
reaction on both catalysts, and most significantly for Al-free Ti-Beta in methanol solvent. With
TS-1, the mode of peroxide addition had little influence on phenol conversion, but the initial
selectivity to hydroquinone was ca. 1.6 times higher than for an equivalent single-portion
addition (at a similar phenol conversion). Discrete peroxide addition for hydroxylation in
methanol over Al-free Ti-Beta gave greatly improved hydroquinone selectivities compared to
the equivalent single-dose addition. Compared to TS-1, the initial selectivity was not as high
(p/o-ratios of 0.86 and 1.40 respectively at 10 mol-% phenol conversion), but this can be
explained on the basis of geometric limitations in the micropores of TS-1 favouring
hydroquinone formation. The final selectivity, however, is marginally higher (using the same
mode of peroxide addition, and at the same phenol conversion).
Discrete peroxide addition has an additional benefit in that it also reduces the quantity of
free-peroxide available for product over-oxidation, and consequently reduces the amount of
tars formed.
Thus, the interaction of the effects of peroxide concentration and the solvent composition and
polarity on the product selectivity and degree of tar formation is important. Particularly with
TS-1, lower peroxide concentrations in bulk methanol solvent are highly beneficial for
hydroquinone formation, because of the implicit geometric constraints in the micropores, the
lower water concentration, and the decreased tar formation associated with high methanol
concentrations. This could have significant reactor design implications, as the results
obtained here suggest that the reaction should be terminated after approximately 30 minutes
to maximise hydroquinone production (under the conditions evaluated in these experiments),
even though the corresponding phenol conversions are low (ca. 10 mol-%). The higher
hydroquinone selectivities reached at low phenol conversions for the discrete peroxide
addition experiments also confirm this. Practically, to enhance the hydroquinone selectivity for reaction over TS-1, the initial phenol-peroxide molar ratio should be ca. 10, methanol
should constitute not less than 90 vol-% of the reaction volume, and the peroxide should be
added in discrete amounts.
For reaction over Al-free Ti-Beta, methanol solvent also enhances the hydroquinone
formation as expected. At low phenol conversions (ca. 10 mol-%) hydroquinone is still the
preferred product, although in contrast to TS-1 the selectivity increases with phenol
conversion, and is higher with higher initial peroxide concentrations. Under the best
conditions evaluated here for optimal hydroquinone formation, the initial phenol-peroxide
molar ratio should be ca. 2.5, with methanol making up at least 90 vol-% of the total volume.
Discrete peroxide addition in methanol solvent for the Al-free Ti-Beta catalysed hydroxylation
gives excellent improvements in hydroquinone selectivity (2.5 times higher than water
solvent), and the addition in more discrete portions might further improve hydroquinone
formation, and should therefore be examined.
Description
Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006.
Keywords
Dissertations -- Process engineering, Theses -- Process engineering, Zeolite catalysts, Hydrocarbons -- Oxidation, Phenol