Calixarenes are macrocyclic compounds containing phenol rings connected in ortho positions by methylene groups. The structural flexibility of calixarene molecules and their capacity for hydrogen bonding represent their important features employed in the complexation of cations, anions and neutral molecules. However, physical adsorption of gases was not studied on such materials since they typically form tightly packed nonporous structures.
Solvates of simple calixarenes are among a few examples that do not follow this trend. The desolvation of calixarenes under controlled conditions allows us to obtain a crystalline material with an active porous phase that absorbs gases under normal conditions. In this research, the effect of polymorphism in the formation and thermal decomposition of the 4-tert- butylcalixarene complex with acetonitrile (tBC-AN) on the formation of microporous structures characterized by substantial surface area and narrow pore size distribution was investigated.
The following experimental methods were used in the study: X-ray, differential scanning calorimetry and thermogravimetric analysis. Besides, a specific surface area and pore size analyzer (NOVA 1200 Series Quantachrome) was used to determine the porosity and surface area of 4-tert-butylcalixarene samples.
Thus, the microporous structure of the obtained material was revealed to be dependable upon the following conditions. 0. The possibility of keeping a tetragonal structure in a partially desolvated tBC-AN complex. 1. The presence of external molecular cavities binding adjacent tBC molecules in columns and layers. 2. The cooperative rotation of tert-butyl groups around C(ar)-C(sp3) bond. It promotes the diffusion of solvent or gas molecules through a nonporous molecular crystal matrix moving from the internal cavity of calixarene to its external cavity. 3. The presence of micropores that appear at the interface of two phases in a hybrid organic crystal and play the role of transport channels upon adsorption.
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