Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2005. / Although selective oxidation catalysts are widely used and extensively studied for their
industrial and academic value, their complex mechanisms are, to a large extent, still
unclear. The field of so-called allylic (amm)oxidations reactions was chosen for further
investigation, in particular the simplistic selective oxidation of propene to acrolein over an
α-Bi2Mo3O12 catalyst.
One of the most important approaches in selective oxidation is to try to correlate the
physicochemical properties of catalysts with their catalytic performance (activity and
selectivity). The most interesting, and seemingly most widely invoked parameter, is
lattice oxygen mobility. The problem, however, is the difficulty encountered in measuring
oxygen mobility.
It is hypothesised that the depth of oxygen utilisation and lattice oxygen mobility of
bismuth molybdate during the partial oxidation of propene to acrolein may be determined
by measuring the rate of acrolein formation and lattice oxygen usage over a range of
discrete particle sizes that could be synthesised using reverse micelle technology.
Catalyst Preparation
A preliminary investigation into the reverse micelle technique showed that discrete nanosized
particles could be synthesised, but that there was no size control over the outcome
and that, in most cases there were some degree of particle agglomeration. It was also
found that nanorod formation occurred due to adsorbtion of surfactant. More in-depth
investigation had to be done in order to achieve particle size control and the liberation of
the calcined α-Bi2Mo3O12 catalyst particles required for kinetic experiments. Simple
precipitation methods, the catalyst calcination step, and the formation and stability of
reverse micelles were investigated.
A simple precipitation method to prepare α-Bi2Mo3O12, suitable to be integrated into the
reverse micelle technique was found by buffering the mixture of bismuth nitrate and
ammonium molybdate solutions with an excess of molybdate. This prevented the pH
from decreasing below a critical value of 1.3 (at which β-Bi2Mo2O9 forms as an impurity). The excess molybdenum caused the formation of MoO3 in the calcined product, which
was selectively and successfully removed using a warm ammonium wash followed by a
water rinse and a recalcination step.
XRD of a temperature range calcination shows that the calcination starts at temperatures
as low as 200°C and almost complete calcination of the catalyst at 280°C. DSC analyses
show a 47.15 J/g crystal formation peak only at 351°C. The Mo18O56(H2O)8
4- anion or its
double, Mo36O112(H2O)16
8-, is responsible for the formation of α-Bi2Mo3O12 in the
precipitation calcination reaction.
Reverse micelles were investigated using a Malvern Zetasizer and showed a complex
dynamic system in which the reverse micelle sizes and size distributions change over
time as a function of surfactant and aqueous concentrations, the salt used and aqueous
phase salinity. Although much was accomplished in this study, more investigations into
the constituent steps of the reverse micelle technique are needed to develop a method
to synthesise the range of discrete catalyst particle sizes required for kinetic studies.
Kinetic Studies
For the purpose of kinetic experiments a metal reactor was found to be superior to that
of a glass reactor. The reactor rig was adequate for these kinetic studies but do not meet
the requirements for detailed reaction order experiments. The analysing apparatus could
not measure CO2 formation accurately and it had to be calculated using a carbon
balance.
Only the model proposed by Keulks and Krenzke [1980a] was able to describe the kinetic
result, but the model parameter describing the oxidative state of the catalyst surface
could not be calculated due to the lack compatibility between published data. Values
were awarded to this parameter so to give an Arrhenius plot which corresponded to
published data. The parameter describing the oxidative state vs. temperature took on a
function that was consistant with the reasoning of Keulks and Krenzke [1980a].
Comprehensive preliminary kinetic studies are needed, both in catalyst reduction and reoxidation,
in order to determine the reaction conditions, explore more advanced kinetic
models and investigate model parameters that are theoretically and/or empirically
obtainable and quantifiable.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2972 |
| Date | 03 1900 |
| Creators | Van Vuuren, Peter |
| 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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