Evaluation of gas sorption techniques and computational methods utilized in the analysis of porous compounds

Van Heerden, Dewald Pepler (2014-12)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: A recently proposed mechanism responsible for anomalous thermal expansion in a metal-organic framework (compound 1) is examined through theoretical calculations. A mechanistic model that reproduces the convergent expansion of the material’s coordination spiral is developed. In order to decide on a suitable theoretical method, specifically at the Density Functional Theory (DFT) level of theory, a tricyclicorthoamide that presents a unique nearly-eclipsed methyl group in the solid state (compound 2) is investigated. Since the anhydrous solid form of this molecule presents with the expected staggered conformation, the nearly-eclipsed conformation is ascribed to the presence of water molecules in the trihydrate form. An evaluation of the potential energy profile for rotation of the methyl group by the ab initio wave function methods HF, MP2 and CCSD revealed the global minimum to indeed be the nearly-eclipsed conformation (𝜔ℎ𝑐𝑐𝑛 ≈ 10°) with a barrier to rotation (Δ𝐸𝑟𝑜𝑡) of 1.24 kcal.mol-1 located at ~85° at the MP2/6-31+G(d) level of theory. The CCSD potential energy profiles also feature a local minimum at ~55° that is ascribed to a dispersive intramolecular interaction. Since the Kohn-Sham implementation of DFT functionals ignores the dynamic electron correlation required to describe dispersion, semi-empirical dispersion-correction schemes to the density-dependent energy are evaluated. The mechanistic model describing the thermal expansion of 1 is evaluated with the B3LYP (hybrid GGA) and M06 (hybrid meta-GGA) density functionals augmented by the GD2 and GD3 dispersion correction schemes, respectively, in addition to the dispersion-corrected ωB97XD (LC-hybrid GGA) functional in conjunction with various basis sets. The resulting energy trends are found to compare favorably to the four energies obtained by periodic DFT evaluations of the 100, 190, 280 and 370 K crystal structures of 1. The thermodynamic data for carbon dioxide adsorption measured on a previously constructed in-house volumetric isosteric sorption technique (SIT) device are compared to those obtained through conventional gravimetric instrumentation. Two metallocyclic compounds (3 and 4) that maintain their crystallinity upon generation of the porous apohost, are evaluated. A comparison of the various methodologies that exist in the literature to determine isosteric heats of adsorption (𝑄𝑠𝑡) from adsorption isotherms are presented. In particular, the temperature-dependent dual-site Langmuir and virial-type isotherm equations are compared for the first time and found to be complementary. The 𝑄𝑠𝑡 values at zero occupancy are determined to be 29.2 and 35.5 kJ.mol-1 for compounds 3 and 4, respectively, and trends in 𝑄𝑠𝑡 as a function of occupancy plots are accounted for.

AFRIKAANSE OPSOMMING: ʼn Meganisme wat onlangs voorgestel is om die buitengewone termiese uitsetting van ʼn metaal-organiese raamwerk (verbinding 1) te verduidelik, word ondersoek deur teoretiese berekeninge. ʼn Meganistiese model wat die konvergente uitrekking van dié materiaal se koördinasie spiraal naboots word ontwikkel. Om op ʼn gepaste teoretiese metode te besluit, spesifiek op die dighteidsfunksionaalteorie (DFT) vlak, is ʼn trisiklieseorto-amied wat ʼn metiel groep in die amper-versteekte konformasie in die vaste fase toon (verbinding 2) ondersoek. Aangesien die anhidriese vaste-vorm van hierdie molekuul die verwagte verspringde konformasie besit, word die amper-versteekte konformasie aan die teenwoordigheid van water molekules in die trihidraat toegeskryf. ʼn Evaluering van die potensiële energie profiel vir rotasie van die metiel groep met behulp van die ab initio golffunksie metodes HF, MP2 en CCSD het tentoongestel dat die globale minimum inderdaad die amper-versteekte konformasie is (𝜔ℎ𝑐𝑐𝑛 ≈ 10°) met ʼn hindernis tot rotasie (Δ𝐸𝑟𝑜𝑡) van 1.24 kcal.mol-1 geposisioneer by ~85° by die MP2/6-31G(d) vlak van teorie. Die CCSD potensiële energie profiele beskik ook oor ʼn lokale minimum by ~55° wat aan ʼn dispersie intramolekulêre interaksie toegeskryf word. Aangesien die Kohn-Sham implementering van (DFT) funksionale die dinamiese elektronkorrelasie wat nodig is om dispersie te beskryf, ignoreer, word semi-empiriese dispersie-korreksie metodes tot die dightheidsafhanklike energie ondersoek. Die meganistiese model wat die termiese uitsetting van 1 beskryf word met die B3LYP (hibried GGA) en die M06 (hibried meta-GGA) digtheidfunksionale, onderskeidelik met die GD2 en GD3 dispersie korreksie metodes aangevul, asook die inherente dispersiegekorigeerde ωB97XD (LC-hybried GGA) funksionaal in kombinasie met verskeie basisstelle ondersoek. Die verkrygde energietendense vergelyk goed met die vier energieë verkry deur evaluering van die 100, 190, 280 en 370 K kristal strukture van 1 met behulp van periodiese DFT. Die termodinamiese groothede vir koostofdioksied adsorpsie, voorheen bepaal op ʼn tuis-ontwerpte volumetriese sorpsie isosteriese tegniek (SIT) toestel, word met dié verkry deur konvensionele gravimetriese instrumentasie verglyk. Twee metaalsikliese verbindings (3 en 4) wat hul kristalliniteit behou tydens ontwikkeling van die poreuse apo-vorm, is geëvalueer. Verskeie metodologië wat gebruik word om die isosteriese hitte van adsorpsie (𝑄𝑠𝑡) vanaf adsorpsie isoterme te bepaal word vergelyk. Meer spesifiek word die temperatuur-afhanklike twee-setel Langmuir- en die viriale-tipe isotermvergelykings vergelyk vir die eerste keer, en hul word as komplementêr beskou. Die 𝑄𝑠𝑡 waardes by nul-besetting word as 29.2 en 35.5 kJ.mol-1 onderskeidelik vasgestel vir verbindings 3 en 4, en tendense in die 𝑄𝑠𝑡 as ʼn funksie van besetting kurwes word omskryf.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/95780
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