Application of liquid state nuclear magnetic resonance techniques for the study of porous materials

This thesis describes the work undertaken to develop and apply the liquid state NMR techniques of cryoporometry and diffusometry for the study of porous materials. Solid/liquid interfaces are at the heart of modern applied science. Understanding liquid dynamics, when located within a confining environment, such as a pore, is of utmost importance when expanding knowledge in this field. Thereare many novel solid/liquid systems that have yet to be studied; NMR offers the opportunity to probe these systems in their working states.The technique of cryoporometry was initially implemented using a series of highly ordered controlled pore glasses with varying pore diameters. This preliminary work built the foundations for further application. However, one key drawback in the thermoporometry area is the lack of consensus on the two key parameters for converting melting point data into pore size distributions. This work offers values for both. NMR cryoporometry is also limited by the lack of suitable liquids that can not only probe a wide range of pore size, but can also match the chemistry of the solid being studied. Two novel cryoporometric liquids are introduced here; t-butanol and menthol, with the aim of reducing the significance of this limitation. The suitability of these liquids is then investigated further by analysing the change in porosity, with swelling, of polymer systems.The physical and chemical behaviour of a confined liquid is often rate limiting in catalytic science.Diffusion NMR has been utilized for the first time to distinguish whether in-pore diffusion, or diffusion from the bulk media into a pore network, was the rate-limiting step for catalytic esterification over sulfonic acid silica analogues. Rate of reaction and liquid diffusion were found to be uncorrelated, discounting the former hypothesis.NMR spectrometers have near ubiquitous use in materials research. The work detailed here should offer to make NMR cryoporometry a more widely used technique in the analysis of porous materials. The future design of porous materials, such as the optimisation of silica architectures for catalytic activities,will also be aided by the physiochemical insights obtained here from diffusion NM

Hydrogen Bonding Aggregation in Acrylamide: Theory and Experiment

Hydrogen bonding plays a role in the microphase separation behavior of many block copolymers, such as those used in lithography, where the stronger interactions due to hydrogen bonding can lead to a smaller period for the self-assembled structures, allowing the production of higher resolution templates. However, current statistical thermodynamic models used in descriptions of microphase separation, such as the Flory–Huggins approach, do not take into account some important properties of hydrogen bonding, such as site specificity and cooperativity. In this combined theoretical and experimental study, a step is taken toward the development of a more complete theory of hydrogen bonding in polymers, using polyacrylamide as a model system. We begin by developing a set of association models to describe hydrogen bonding in amides. Both models with one association constant and two association constants are considered. This theory is used to fit IR spectroscopy data from acrylamide solutions in chloroform, thereby determining the model parameters. We find that models with two constants give better predictions of bond energy in the acrylamide dimer and more realistic asymptotic behavior of the association constants in the limit of high temperatures. At the end of the paper, we briefly discuss the question of the determination of the Flory–Huggins parameter for a diblock copolymer with one self-associating hydrogen bonding block and one non-hydrogen bonding block by means of fitting the scattering function in a disordered state

Extending the range of liquids available for NMR cryoporometry studies of porous materials

Nuclear magnetic resonance (NMR) cryoporometry, although well established, can be limited by the inability of any one liquid to probe a broad range of pore sizes, a relatively small number of commonly-used probe liquids and the requirement to match the probe liquid to the chemistry of the material being studied. Here we demonstrate, for the first time, the use of menthol and t-butanol as probe liquids in NMR cryoporometry measurements. Using appropriate estimates for the values of the melting point depression constant, kc, and the non-freezing surface layer, 2sl, NMR melting data was converted into pore size distributions. The melting point depression constant for t-butanol is similar to that of cyclohexane; however due to its functionality, t-butanol may be the preferred liquid used to study the porosity of hydrophilic materials. Menthol, having a larger value of kc, can accurately analyze larger pore sizes up to 100 nm. This represents the first use of menthol and t-butanol to accurately probe pore dimensions in NMR cryoporometry

NMR cryoporometric measurements of porous silica:A method for the determination of melting point depression parameters of probe liquids

Nuclear magnetic resonance (NMR) cryoporometry is a non-invasive method for determining the pore size distributions of materials such as porous silica. Cryoporometry has several advantages over other porometric techniques. It is able to measure the melting process in a series of discrete steps, whereas transient heat flow techniques, such as differential scanning calorimetry (DSC), have a minimum rate of measurement, and, secondly, NMR cryoporometry can analyze pore shapes with any geometry, where nitrogen porosimetry is complicated for samples with spherical pores with narrow necks. However, one key drawback of the method is that, for any one liquid observed in any one material, there is a lack of consensus in the two parameters, kckc andView the MathML source2sl , used to convert experimental NMR melting point depression data into a pore size distribution. By considering two decades worth of literature data, values for both were obtained for water in porous silica supports, in particular an estimate of a non-freezing layer between the solid ice and the inner surface of the pore. These values were used to produce pore size distributions for three silica materials, SBA-15 and KIT-6, both with cylindrical pores but possessing different structures, and SBA-16, which has spherical pores. This represents the first time KIT-6 has been characterized by the NMR method. Furthermore, this work demonstrates a general method for obtaining values for kckc and View the MathML source2sl which can be applied to any liquid for which suitable literature data is available

Diffusion NMR characterization of catalytic silica supports:a tortuous path

Mesoporous silicas have found widespread application within the field of heterogeneous catalysis. Acid functionalization of such materials, through one-pot or postsynthetic grafting of sulfonic acid groups, imparts activity for fatty acid esterification, with the studious choice of pore geometry facilitating significant rate enhancements. Diffusion NMR has been utilized for the first time to characterize the structure of mesoporous silicas through the transport behavior of systematically related carboxylic acids confined within their mesopore networks. A reduced diffusion coefficient is obtained for species constrained within the 3-dimensional interconnected pores of KIT-6 relative to the 2-dimensional noninterconnected pore channels of SBA-15. The effective tortuosity of both porous silicas increases with the acid chain length, with the diffusion behavior of long-chain acids dominated by the alkyl chain and silica architecture. Carboxylic acid diffusion within these two pore networks is unlikely to be rate-limiting in catalytic esterification over sulfonic acid silica analogues. Physicochemical insights from diffusion NMR will aid the future design of optimal silica architectures for catalytic applications

NMR Cryoporometry of Polymers: Cross-linking, Porosity and the Importance of Probe Liquid

The morphology of cross-linked polymers plays an important role in their physical and chemical properties. NMR cryoporometry allows for the investigation of these structures over different length scales, through appropriate choice of probe liquid. The different structures of two different polymeric samples, one a cross-linked polymer hydrogel, the other a pore-expanded ion-exchange polymer, are analysed here. The ability for NMR cryoporometry to analyse both polymeric materials in the swollen state is successfully demonstrated, as is the importance of probe-liquid choice for the analysis of different regions of the pore structure. In both cases, water is used to identify populations of pores smaller than ca. 5 nm. The use of t-butanol and menthol reveals the presence of additional mesoporous structures in the ion-exchange resin as well as the responsiveness of the pore structure to the liquid used to swell it