Hybrid organosilica membranes and processes: Status and outlook

Abstract

In the past, the research in molecular separation membranes prepared through sol-gel technologies has been dominated by ceramic membranes. Especially, silica membranes have been studied in great depth. Steps towards hybrid organosilica membranes were taken by using pendant organic groups. However, only with the appearance of organically bridged silica, stable and reliable membranes that are suitable for large scale industrial utilization have become available. In this paper, we provide an overview of recent development of hybrid silica membranes that contain organic bridges. The freedom of choice in precursor allows for a flexible approach towards tailoring of the membrane properties. New support materials can be used by applying alternative deposition methods, such as expanding thermal plasma chemical vapor deposition. The robustness of the membrane concept allows for the design of novel separation process concepts in which the demonstrated stability is required.

Structure–property tuning in hydrothermally stable sol–gel-processed hybrid organosilica molecular sieving membranes

“…Such effective hydrothermal stability cannot be classified as an intrinsic material property if it does not hold for other possible network organizations as well, but these molecular arrangements are at least in part dictated by the molecular design of the organosilica monomer. Monomers that are most likely to form hydrophobic surfaces are the ones with terminating organic groups [37,40,[52][53][54][55][56][57][58][59][60][61] or with long and flexible organic bridges [53,62]. Short or rigid bridges such as ethylene and p-phenylene do not have the length and spatial freedom to ''step out'' to the surface and completely shield the underlying siloxane bonds; thus, these materials keep on having a significant affinity for water [47,48,[62][63][64].…”

“…The size differences between gas molecules are usually smaller than between molecules in the liquid phase. Moreover, the interaction between pore wall and gas molecule is usually less intensive, partly because hydrogen bonding is absent, and partly because gases like H 2 , CH 4 and even CO are spherical and nonpolar so that they do not interact strongly with the pore wall [61]. Hence, the difference between the molecular sizes and the size of the pore is often the main factor determining gas separation selectivity.…”

“…Just as with normal silica the diffusion of CO 2 was largely suppressed, leading to enhanced H 2 /CO 2 permselectivity. Qi et al explained these effects by the formation of acidic Nb sites in the matrix structure [20,87], but also by possible densification effects due to the very high thermal annealing temperature for BTESE (400-550°C in N 2 ) [86], temperatures at which thermal degradation of the organic bridges may have occurred [61,86]. Other dopants that have been introduced into BTESE and BTESM matrices by co-condensation are aluminum (Al) [88], boron (B) and tantalum (Ta) [89].…”

Hybrid Materials for Molecular Sieves

“…The size differences between molecules in the gas phase are usually smaller than in the liquid phase, while polar (or even electrostatic) interactions between pore wall and gas molecule are absent in many cases. This is the reason that many gas separations work on the principle of molecular sieving by size selection, in particular for industrially relevant nonpolar gases like H 2 , N 2 , CH 4 , and CO (Agirre et al 2014). The kinetic diameters of small gases differ less than 0.1 nm from each other (H 2 0.29 nm, CO 2 0.33, N 2 0.36, CO 0.37, CH 4 0.38 nm), so a very sharp pore size cutoff is required to obtain highly selective membranes.…”

“…Low CO 2 permeances were observed when the system was annealed at high temperatures (400-550 C in N 2 ) (Qi et al 2012) but not when lower annealing temperatures (300 C in N 2 ) were employed . BTESE is known to be stable at 300 C, but (partial) thermal degradation of the organic bridges may occur above 400 C (Agirre et al 2014;Qi et al 2012), and this may have influenced the permselectivity of the membrane toward CO 2 in those respective cases. Other dopants in BTESE and BTESM matrices that have been tested are Al (Kanezashi et al 2013), B, and Ta (all 300 C in N 2 ) , but also in these cases, no specific influence on CO 2 permeability was observed.…”