Water sensitivity and microporosity in organosilica glasses
The aim of the research presented in this dissertation is to gain more insight in the water sensitivity and microporosity of organosilica glasses. The research primarily focuses on fundamental material understanding, but stands in close relation with the industrial application of organically bridged silicas as molecular sieving membranes. All organosilica materials were synthesized from alkoxy-substituted precursors via sol-gel chemistry. All chemical reactions studied in this thesis involve either hydrolysis of siloxane bonds (Si-O-Si) into silanol groups (Si-OH) or its reverse, i.e. condensation of silanol groups into siloxane bonds.
Chapter 1 presents a systematic study on the influence of various molecular factors on hydrothermal dissolution of organosilicas. Materials were synthesized with various organic groups in bridging (methylene, ethylene, hexylene, octylene, p-phenylene) and terminal (methyl, n-propyl, phenyl, cyclohexane) positions. The bond strain, connectivity and hydroxyl concentration of all networks were estimated using 29Si CP-MAS-NMR and FTIR. The hydrophilicity was characterized by monitoring the water uptake of the materials in moisture treatments with TGA-DSC and FTIR. The resistance of each network against hydrothermal dissolution in a water/1,5-pentanediol mixture at 80 °C and pH 1, 7 and 13 was analyzed with ICP-OES and XRF. Bond strain appears to significantly increase the tendency to dissolve under hydrothermal conditions. The stabilizing influences of increased connectivity and hydrophobicity were found to be weak.
Chapter 2 zooms in on subtle effects of condensation reactions in ethylene-bridged silica when kept at temperatures up to 300 °C. An explanation is presented for the previously not understood problem of slow flux decline in industrially employed organosilica membranes over periods of months to years. Chemical and structural (micropore) evolution were studied in powders, films and gas permeation membranes with in-situ FTIR, 29Si CP-MAS-NMR, in-situ SE, in-situ GP and in-situ XRR. The common assumption that ethylene-bridged materials reach a stabilized structural state after treatment at 250-300 °C for a few hours is shown to be incorrect. A continuously ongoing decrease in both silanol concentration and film thickness were observed, accompanied by changes in density, thermal expansion and micropore structure. The material evolution continued for days to weeks without approaching an end state and was related to network flexibility. Changes in the micropore structure depended on the pore size, yielding an increasing gas permeance through small pores ~0.3 nm but a decreasing permeance through relatively large pores >0.4 nm.
Chapter 3 presents a route to solve the subtle material instability reported in Chapter 2. Ethylene-bridged silica films, powders and membranes were exposed to in-situ synthesized HCl gas alternated with heat treatments at 150-300 °C. The film thickness, network condensation, chemical integrity and micropore structure were monitored with XRR, 29Si DE-MAS-NMR, FTIR and GP. Treatment with HCl was found to predominantly catalyze hydrolysis of siloxane bonds, enabling network optimization via iterative bond breakage and reformation. Network shrinkage, widening or opening of the smallest pores and densification of the overall pore structure were accelerated while the ethylene bridges remained intact. The achieved acceleration of material evolution makes iterative hydrolysis and condensation a promising approach for increasing the long-term micropore stability of organically bridged molecular sieving membranes.
Chapter 4 presents a new method for characterization of micropores <1 nm with increased accuracy as compared to conventional adsorption isotherm analysis. Thermogravimetry was employed to assess the uptake of water, methanol, ethanol, 1-propanol and cyclohexane vapors in microporous structures at room temperature and derive quantitative micropore volumes and minimum pore entrance sizes together with qualitative information on surface chemistries. Pycnometry was employed to measure the uptake and adsorption of He, Ar and N2 gas at room temperature and derive semi-quantitative surface-to-volume ratios, surface areas and micropore cavity sizes. The method was validated and calibrated by applying it to a series of zeolites with known micropore structures. The results were compared with data from conventional N2 adsorption at ‑196 °C and CO2 adsorption at 0 °C. Main advantages of the demonstrated method are that diffusion limitations due to cryogenic temperatures are eliminated, adsorption is studied with non-polar gases, micropore cavity sizes are probed separate from micropore entrances and data can be interpreted in a straightforward fashion without requiring theoretical models on molecular behavior.
Chapter 5 presents a systematic study on the micropore properties of a series of organosilica materials based on the method reported in Chapter 4. Materials were prepared with various organic groups in bridging (methylene, ethylene, hexylene, octylene, p-phenylene) and terminal (methyl, n-propyl) positions. Accessible pore volumes, entrance sizes and surface chemistries were measured with vapor TG using water, methanol, 1-propanol and cyclohexane vapors. Skeletal densities, semi-quantitative surface-to-volume ratios and surface areas, pore entrance sizes and semi-quantitative pore cavity sizes were measured with gas PM using He, Ar and N2. The known classification of 1) short or rigid organic bridges that open up the pore structure, 2) longer and more flexible bridges that cause pore filling and 3) terminal organic groups that reduce pore formation is further specified. The incorporation of any organic group in the silica network increased the dispersity in micropore entrance sizes as compared to inorganic silica in the probed size range. A critical discussion is given of the commonly accepted ‘spacing concept’ of organic bridges.
