Abstract:
Porous solids can be defined as solids having adsorbate accessible pores, channels,
cavities and empty spaces available for physisorption. Due to the presence of porous
architecture in the bulk of solid, porous solids exhibit high adsorbate accessible surface area
in comparison to non-porous solids which only show surface adsorption and hence low
adsorbate accessible surface area. Various gas molecules and adsorbates depending on the
dimension of the pores and channels available can easily permeate and pass through the bulk
of these solids. The porous architecture of solids can be uniform and well-ordered as in the
case of zeolites, metal-organic frameworks (MOFs), and ordered mesoporous silica such as
MCM-41 or SBA-15 or random with a non-uniform distribution of cavities pores and
channels as in activated charcoal.[1] Porous solids, although having well-ordered porous
architecture, may or may not be crystalline. For example, MOFs, zeolites, MCM-41 and SBA-
15 have uniform well-ordered porous architecture but MOFs and zeolites are crystalline with
well-defined atomic positions, but the walls of MCM-41 and SBA-15 are amorphous. Voids in
these porous solids are generally occupied by guest or template molecules and can be easily
removed by heating or evacuation. Based on the International Union of Pure and Applied
Chemistry (IUPAC) classification of pores, porous solids can be classified into three
categories. Porous solids with a width not exceeding about 2 nm are called microporous,
solids having pore width between 2 nm and 50 nm are mesoporous solids, whereas solids
with pore width more than 50 nm can be defined as macroporous solids.
