The use of linear azo compounds as probes for the characterization of micropores of molecular sieves

A new method for determining the pore sizes of zeolites is introduced. Data are obtained spectrophotometrically for the uptake, from an isooctane solution, of alkane diazodicarboxylate chromophores. The size of the alkane groups was varied, thereby introducing differences in the sizes of the probe molecules being adsorbed. The method demonstrates no uptake of chromophore for small pore zeolites, somewhat hindered uptake for intermediate pore zeolites, and variable uptake rates for large-pore zeolites because the solvent can compete. Crystallite size, pore system dimensionality, and hydrophobicity (in the case of the large pore zeolites) are shown to affect the uptake data. Determination of products residing in the pores after 1000 min of solution circulating over the zeolite bed was determined by TGA/MS analysis. The method was applied to several high silica zeolites with known structures, as well as to unknown structures like SSZ-25 and 35. The study confirmed that the unusual zeolite SSZ-25 has pore openings larger than silicalite or ZSM-5 but smaller than a number of so-called large pore zeolites. Limitations, and future prospects for the use of this relatively simple system, are discussed.