Solid-liquid separation of xylene isomers using metallocycles

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