Influence of Monomer Connectivity, Network Flexibility, and Hydrophobicity on the Hydrothermal Stability of Organosilicas

Abstract

It is generally assumed that the hydrothermal stability of organically modified silica networks is promoted by high monomer connectivity, network flexibility, and the presence of hydrophobic groups in the network. In this study a range of organosilica compositions is synthesized to explore the extent to which these factors play a role in the hydrothermal dissolution of these materials. Compositions were synthesized from hexafunctional organically bridged silsesquioxanes (OR1)3Si–R–Si(OR1)3 (R = −CH2–, –C2H4–, –C6H12–, –C8H16–, –p-C6H4–; R1 = −CH3, –C2H5), tetrafunctional (OEt)2Si(CH3)–C2H4–Si(CH3)(OEt)2 and Si(OEt)4, trifunctional silsesquioxanes R′-Si(OMe)3 (R′=CH3, n-C3H7, cyclo-C6H11, phenyl), and bifunctional Si(i-C3H7)2(OMe)2. The bond strain, connectivity and hydroxyl concentration of all networks were estimated using 29Si cross-polarized magic angle spinning nuclear magnetic resonance and Fourier-transform infrared spectroscopy. The hydrophilicity was characterized by monitoring the water uptake of the materials in moisture treatments with thermogravimetric analysis, differential scanning calorimetry, and Fourier-transform infrared spectroscopy. 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 inductively coupled plasma optical emission spectroscopy and X-ray fluorescence. 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.

Organic groups influencing microporosity in organosilicas

“…This indicates that for such long organic segments their overall exposure does not depend on their configuration. The lower water uptake in MTMS-derived material with smaller organic groups as compared to PTMS-derived material can be explained by the reported lower hydroxyl concentration of the MTMS-derived material [5].…”

“…(Organo)silica materials were synthesized as reported elsewhere [5]. The HNO3 : H2O : alkoxy ratio was 0.064 : 1.1 : 1.0, as derived from Castricum et al [12].…”

“…replacement of Si-O-Si linkages with Si-R or Si-R-Si linkages, influences various functional properties such as mechanical flexibility and fracture resistance [1,2], hydrophobicity [3][4][5] and hydrothermal stability [5][6][7][8]. For applications that make use of the microporosity of organosilica materials, such as separation membranes, varying the nature and location of the organic groups allows tuning of micropore sizes and surface chemistries.…”

“…The bulkiness of these bridges is still sufficiently low to allow high network condensation degrees [5], but they transform the network towards a flexible polymer-like structure through which molecules can pass based on affinity rather than through pre-existing pore channels [12,20].…”

Water sensitivity and microporosity in organosilica glasses

“…An overview of all deconvolutions is shown in supplementary material published online. 59 The condensation degrees indicate the as-prepared state without thermal consolidation and reflect intrinsic as well as processing-dependent material properties. From these condensation degrees the hydroxyl concentration and actual connectivity (number of siloxane bonds per monomer) were calculated.…”