Thesis (M. Tech. Chemistry, Dept. of Chemistry, Faculty of Applied and Computer Sciences)--Vaal University of Technology, 2010. / Membrane separation processes offer a promising alternative to energy-intensive
separation processes such as distillation and solvent extraction. NF and RO are
among the most investigated membrane processes with a potential use in the
chemical industry. Carbon-carbon coupling reactions feature in the top ten most
used reactions in the chemical industry. These reactions often use homogeneous
palladium, nickel and other precious catalysts which are often difficult to separate
from reaction products. This leads to potential product contamination and loss of
active catalysts. This not only poses a threat to the environment but is also costly
to the chemical industry.
The purpose of this study was to investigate the efficiency of the recovery of the
metal catalysts by selected membrane processes. Four commercial polymeric
NF and RO membranes (NF90, NF270, BW30 and XLE) were selected for the
study. Palladium catalysts commonly used in Heck and Suzuki coupling reactions
were selected. These are Pd(OAc)2, Pd(OAc)2(PPh3)2, PdCl2 and Pd(PPh3)2Cl2.
A range of organic solvents were also selected for the study. All the membranes
were characterized for pure water permeability, pure solvent permeability,
swelling, surface morphology and chemical structure.
The chemical and catalytic properties of the catalysts were determined. Catalytic
activity was investigated by performing coupling reactions. These catalysts
generally performed well in the Heck coupling reaction with sufficient yields
realized. The catalysts showed poor activities in the Suzuki and Sonogashira
coupling reactions. These coupling reaction systems were affected by rapid
palladium black formation.
vi
Catalyst retention studies showed the influence of membrane-solute interactions
such as steric hindrance and size exclusion. The larger catalyst,
Pd(OAc)2(PPh3)2 was rejected better by all the membranes irrespective of the
solvent used. The smaller catalyst, Pd(OAc)2 was the most poorly rejected
catalyst. This catalyst showed signs of instability in the selected solvents. An
interesting finding from this study is that of higher rejections in water compared to
other solvents for a particular catalyst. In this regard, the influence of solventsolute
effects was evident. Generally, higher rejections were observed in
solvents with higher polarity. This has been explained by the concept of
solvation. It has been shown that solvents with different polarity solvate solutes
differently, therefore leading to a different effective solute diameter in each
solvent.
Catalyst separation using NF90 membrane was attempted for the Heck coupling
reaction system. The reaction-separation procedure was repeated for two
filtration cycles with rapid activity decline evident. This was regarded as very poor
showing of the catalyst separation efficiency of the membrane. Other authors in
similar studies using SRNF membranes have reported reaction-separation
processes of up to seven cycles. This observation shows the inferiority of
polymeric membranes in organic solvent applications such as catalyst
separation.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:vut/oai:digiresearch.vut.ac.za:10352/104 |
| Date | 07 1900 |
| Creators | Xaba, Bongani Michael |
| Contributors | Modise, S. J., Naidoo, E. B. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | xiv, 148 leaves: ill., graphs.; 30 cm. |
| Relation | PDF. Adobe Acrobat Reader |
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