Porous materials: microporosity induced by the shape and length of the pores

Abstract

There are insufficient parameters to explain the appearance of microporosity in porous materials. One of the parameters associated with micropores is the generalized pore shape factor F, which includes, as special cases, the known slit, circular and spherical model pore shapes. F covers the shapes between the slit (F = 2000) and spherical (F = 6000), as well as beyond. For the intermediate shape, one can estimate proportions of model motifs. The transition of the shape from one model to another is accompanied by the appearance of micropores (or vice versa); corresponding dependencies are given. Nanotechnologies, such as self-assembly of ordered mesoporous materials (OMMs), the production of carbon OMMs as replicas of silica matrixes, supercapacitors made of carbon nanofibres (CNF), a hybrid CNF/MWCNT for use in lithium-ion batteries, carbon xerogels with Ni additive for storage of H2, catalysis and others are discussed; they cover materials with F in the range 1,100 ÷ 32,600. A pore surface length index Lsi is proposed for any pore shape; it supplements the generalized parameters causing stresses, deformations and micropores. Using F and Lsi, it was discovered, that activated carbon, obtained as a replica of non-circular matrix of silica, behaves like a compressed spring, which, after removal of silica, expands and its pores become circular. A concept of molecular sieving based on the shape of molecules is proposed and demonstrated by the example of lipase immobilization. Application of the proposed parameters improves understanding of many published and new results.