Thesis (MSc)--Stellenbosch University, 2022.ENGLISH ABSTRACT: Host-guest chemistry is an important concept in supramolecular chemistry. Porous hosts may
be able to selectively accommodate guests, making them attractive candidates for potential
applications such as sorption, storage, separation, drug delivery, etc. In this study, two porous
materials were investigated for their solid-liquid xylene separation ability. These materials are
the metallocycles MC1 and MC2. Xylenes are an important chemical feedstock to produce
consumer products. However, they are always produced as a mixture of isomers and thus
their separation is essential.
The first material investigated was MC1·2(MeOH). Its phase purity could not be
established directly by comparison of the simulated and experimental powder patterns. This
is because the MeOH guest readily escapes from the host at ambient temperatures. However,
the phase purity can be confirmed after the material is fully activated (MC1a). Solvent
exchanges of the as-synthesized MC1·2(MeOH) with xylenes, as well as liquid sorption of
MC1a with xylenes were attempted. Both investigations showed that MC1 is not a suitable
material for xylene separation. The host structure remained unchanged after exposure to
xylene. Further investigation revealed that the kinetic diameters of the xylenes are too large
for these molecules to fit in the host cavity without a phase transformation.
The second material explored was MC2·2(DMSO). It can undergo multiple single-
crystal to single-crystal transformations upon guest exchange and removal and exhibits
solvatochromism. The activation of MC2 by different methods results in two possible activated
forms (open form 3 and collapsed form 4). In this study, the main obstacle was to obtain phase
pure sample of either activated form. The problem was resolved by trialing variations of
temperature and time. Vapor and liquid sorption of xylenes were carried out using form 3.
Form 3 is able to separate xylenes in the order of preference of para-xylene >> meta-xylene >
ortho-xylene under mild conditions. The pockets within form 3 merge and become channels
to accommodate the xylenes, as established by single-crystal X-ray diffraction analysis. The
selectivity trend was rationalized as being dependent on the kinetic diameter of the respective
xylene isomers. The non-porous form 4 is not suitable for xylene separations even at high
temperature. Thus, as the phase purity problem has been solved, porous form 3 with its ability
to change pore sizes, is a worthy substrate for xylene purification, and this also has
implications for future studies involving gas sorption and storage.AFRIKAANS OPSOMMING: Gasheer-gas-chemie is 'n belangrike konsep in supramolekulêre chemie. Poreuse gashere
mag dalk gaste selektief kan akkommodeer, wat hulle aantreklike kandidate maak vir
potensiële toepassings soos sorpsie, berging, skeiding, geneesmiddelaflewering ens. In
hierdie studie is twee poreuse materiale ondersoek vir hul vastestof-vloeistof xileen
skeidingsvermoë. Hierdie materiale is die metallosiklusse MC1 en MC2. Xileen is 'n belangrike
chemiese grondstof om verbruikersprodukte te produseer. Hulle word egter altyd geproduseer
as 'n mengsel van isomere en dus is hul skeiding noodsaaklik.
Die eerste materiaal wat ondersoek is, was MC1·2(MeOH). Die fasesuiwerheid
daarvan kon nie direk vasgestel word deur die gesimuleerde en eksperimentele poeierpatrone
te vergelyk nie. Dit is omdat die MeOH gas maklik by omgewingstemperature van die gasheer
ontsnap. Die fasesuiwerheid kan egter bevestig word nadat die materiaal ten volle geaktiveer
is (MC1a). Oplosmiddeluitruilings van die soos-gesintetiseerde MC1·2(MeOH) met xileen,
sowel as vloeistofsorpsie van MC1a met xileen, is gepoog. Beide ondersoeke het getoon dat
MC1 nie 'n geskikte materiaal vir xileenskeiding is nie. Die gasheerstruktuur het onveranderd
gebly na blootstelling aan xileen. Verdere ondersoek het aan die lig gebring dat die kinetiese
diameters van die xileen isomere te groot is vir hierdie molekules om sonder 'n
fasetransformasie in die gasheerholte te pas.
Die tweede materiaal wat ondersoek is, was MC2·2(DMSO). Dié materiaal kan
veelvuldige enkelkristal-tot-enkelkristal-transformasies ondergaan tydens gasuitruiling en -
verwydering, en vertoon solvatochromisme. Die aktivering van MC2 deur verskillende
metodes lei tot twee moontlike geaktiveerde vorms (oop vorm 3 en toe vorm 4). In hierdie
studie was die grootste struikelblok om fase-suiwer monsters van beide geaktiveerde vorms
te verkry. Die probleem is opgelos deur variasies van temperatuur en tyd te toets. Damp- en
vloeistofsorpsie van xileen is uitgevoer op vorm 3. Vorm 3 toon voorkeur in xileen skeiding in
die volgorde van para-xileen >> meta-xileen > orto-xileen onder matige toestande. Die holtes
binne vorm 3 smelt saam en word kanale om die xileen te akkommodeer, soos vasgestel deur
enkelkristal X-straaldiffraksie-analise. Die selektiwiteitstendens is gerasionaliseer as afhanklik
van die kinetiese deursnee van die onderskeie xileen isomere. Die nie-poreuse vorm 4 is nie
geskik vir xileenskeidings selfs by hoë temperatuur nie. Dus, omdat die
fasesuiwerheidsprobleem opgelos is, is poreuse vorm 3 met sy vermoë om holtegroottes te
verander, 'n waardige substraat vir xileensuiwering, en dit het ook implikasies vir toekomstige
studies wat gassorpsie en berging behels.Master
