Water sensitivity and microporosity in organosilica glasses

In this dissertation the water sensitivityand microporosity of organosilica glasses are studied. The research focuses onfundamental material understanding, but stands in close relation with theindustrial application of organically bridged silicas as molecular sievingmembranes.

Chapter1 presents a systematic study on the influence of monomerconnectivity, network flexibility and hydrophobicity on the hydrothermaldissolution of organosilicas. Bond strain appears to significantly increasethe tendency to dissolve under hydrothermal conditions.The stabilizing influences of increased connectivity and hydrophobicity werefound to be weak.

Chapter2 zooms in on subtle effects of condensationreactions in ethylene-bridged silica when kept at temperatures up to 300 °C. Anexplanation is presented for the previously not understood problem of slow fluxdecline in industrially employed organosilica membranes over periods of monthsto years. The common assumption that a stabilized structural state is reachedafter treatment at 250-300 °C for a few hours is shown to be incorrect.

Chapter3 presents a post-treatment to solve the subtlematerial instability reported in Chapter 2, involving exposure to in-situsynthesized HCl gas alternated with heat treatments at 150-300 °C. Treatmentwith HCl was found to predominantly catalyze hydrolysis of siloxane bonds,enabling network optimization via iterative bond breakage and reformation.

Chapter 4 presents anew method based on vapor thermogravimetry and gas pycnometry for characterizationof micropores <1 nm with increased accuracy ascompared to conventional adsorption isotherm analysis. Main advantages of the demonstrated method are that diffusion limitationsdue to cryogenic temperatures are eliminated, adsorption is studied withnon-polar gases, micropore cavity sizes are probed separate from micropore entrancesand data can be interpreted in a straightforwardfashion without requiring theoretical models on molecular behavior.

Chapter5 presents a systematic study on the microporeproperties of a series of organosilica materials based on the method reportedin Chapter 4. The known classification of 1) short or rigid organic bridgesthat open up the pore structure, 2) longer and more flexible bridges that causepore filling and 3) terminal organic groups that reduce pore formation isfurther specified. The incorporation of any organic group in the silica networkincreased the dispersity in micropore entrance sizes as compared to inorganicsilica in the probed size range. A criticaldiscussion is given of the commonly accepted ‘spacingconcept’ of organic bridges.