Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2009. / Aromatic precursor compounds are derivatives that play an important role in
biosystems and are useful in the production of fine chemicals. This work
focuses on the catalytic synthesis of 2-methyl-1, 4-naphthoquinone and cresols
(para- and ortho) using aqueous hydrogen peroxide as an oxidant in liquidphase
oxidation of 2-methylnaphthalene and toluene over titanium-substituted
zeolite TS-1 or Ti-MCM-41.
Catalysts synthesised in this work were calcined at 550°C, extensively
characterised using techniques such as X-ray Fluorescence for determining the
catalyst chemical composition; BET for surface area, pore size and micropore
volume; Powder X-ray diffraction for determining their crystallinity and phase
purity and SEM was used to investigate the catalyst morphologies. The BET
surface areas for Ti-MCM-41 showed a surface area of 1025 m2/g, and a 0.575
cm3/g micropore volume. However, zeolite TS-1 showed a BET surface area of
439 m2/g and a 0.174 cm3/g micropore volume.
The initial experiments on 2-methylnaphthalene hydroxylation were performed
using the normal batch method. After a series of batch runs, without any
success as no products were generated as confirmed by GC, a second
experimental tool was proposed. This technique made use of the reflux system
at reaction conditions similar to that of the batch system. After performing
several experimental runs and optimising the system to various reactor
operating conditions and without any products formed, the thought of continuing
using the reflux was put on hold. Due to this, a third procedure was brought into perspective. This process made use of PTFE lined Parr autoclave. The reactor
operating conditions were changed in order to suit the specifications and
requirements of the autoclave. This process yielded promising results and the
formation of 2-MNQ was realised. There was a drawback when using an
autoclave as only one data point was obtained, at the end of each run.
Therefore, it was not possible to investigate reaction kinetics in terms of time.
Addition of aqueous hydrogen peroxide (30 wt-%) solution in the feed was done
in one lot at the beginning of each reaction in all oxidation reactions, to a
reactor containing 2-methylnaphthalene and the catalyst in an appropriate
solvent of choice (methanol, acetonitrile, 2-propanol, 1-propanol, 1-pentanol,
and butanol), with sample withdrawal done over a period of 6 hours (excluding
catalytic experiments done with a Parr autoclave as sampling was impossible).
As expected, 2-methylnaphthalene oxidation reactions with medium pore
zeolite TS-1 yielded no formation of 2-methyl-1, 4-naphthoquinone using
various types of solvents, with a batch reactor, reflux system, or a Parr PTFE
autoclave. This was attributed to the fact that 2-methylnaphthalene is a large
compound and hinders diffusion into zeolite channels.
With the use of an autoclave, Ti-MCM-41 catalysed reactions showed that the
choice of a solvent and reaction temperature strongly affect 2-
methylnaphthalene conversion and product selectivity. This was proven after
comparing a series of different solvents (such as methanol, isopropanol, npropanol,
isobutanol, n-pentanol and acetonitrile) at different temperatures.
Only reactions using acetonitrile as a solvent showed 2-MNQ. Formation of 2-
MNQ, indicating that acetonitrile is an appropriate choice of solvent for this system. The highest 2-methylnaphthalene conversion (92%) was achieved at
120 ˚C, with a relative product selectivity of 51.4 %. Temperature showed a
major effect on 2-MN conversion as at lower reaction temperature 100˚C, the
relative product selectivity (72%) seems to enhance; however, the drawback is
the fact that lower 2-methylnaphthalene conversions (18%) are attained.
Another important point to note is the fact that using an autoclave (with
acetonitrile as a solvent), 2-methyl-1-naphthol was generated as a co-product. In conclusion, it has been shown that the hydroxylation of different aromatic
compounds over zeolites conducted in this study generated interesting findings.
In 2-MN hydroxylation over Ti-MCM-41 as a catalyst, only acetonitrile is an
appropriate choice of solvent using an autoclave. In addition, zeolite TS-1 is not
a suitable catalyst for 2-MN hydroxylation reactions. It is ideal to optimise an
autoclave in order to investigate reaction kinetics and optimum selectivity.
Toluene hydroxylation reactions yielded para and ortho-cresol as expected with
either water or acetonitrile as a solvent. No meta-cresol was formed. The kinetic
model fitted generated a good fit with water as a solvent or excess toluene, with
acetonitrile as a solvent generating a reasonable fit.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2564 |
| Date | 03 1900 |
| Creators | Gqogqa, Pumeza |
| Contributors | Callanan, L. H., University of Stellenbosch. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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