Development and sorption characterization of some model mesoporous and microporous silica adsorbents

Mesoporous and microporous silica substrates, each of these kinds endowed with unique textural characteristics, have been prepared via sol-gel and micelle-templating techniques and characterized through N₂ sorption. Materials consisting of a lattice of cage-like monodisperse cavities interconnected by narrow throats (SBA-16), a set of individual cylindrical pores of about the same size (SBA-15), a network of long sinuous ganglion-like sequences of bulges and throats (mesoporous glass, MG), or microporous arrangements of globular nanoparticles (microporous SiO₂) have been developed. The N₂ sorption isotherms of microporous silica at 76 K correspond to International Union of Pure and Applied Chemistry (IUPAC) type I, while those of mesoporous SBA-16, SBA-15, and MG silica match a type IV with hysteresis loops of types H1 or H2 depending on the regularity and nature of the void elements constituting the substrate. A characteristic common to all kinds of porous SiO₂ solids herein synthesized is the rigidity of the arrangements, since the total pore volumes attain limiting values. Pore-size distributions of mesoporous SiO₂ materials can be calculated by density functional theory (DFT) and sometimes by Barrett-Joyner-Halenda (BJH) methods. The microporous SiO₂ adsorbent depicts a larger amount of big micropores (supermicropores) rather than small micropores (ultramicropores), this being a very useful characteristic for the entrapment and ulterior release of volatile compounds. MG substrates depict a percolation vapor threshold while the SBA-16 material shows a cavitation phenomenon during capillary evaporation. Additional properties of SBA-16, SBA-15, MG silica substrates are studied via primary desorption scanning curves.