Studies on the Mechanism of Zeolite X Crystallization. II. Effect of Silica Sources of Starting Materials on the Gas-phase Adsorption at Solid Phases

Abstract

Abstract The mechanism of zeolite X crystallization was studied by using two kinds of silica sources, tetraethyl orthosilicate and silica sol. The adsorption of benzene and cyclohexane was subjected to a measurement to clarify the distribution of cation Na+ and the change in the pore structure and surface of solid phases with reaction progress. The relation between specific surface area and reaction time has been revealed by means of adsorption of nitrogen gas, benzene, and cyclohexane as follows: 1) In the early stage of reaction or in the amorphous solid state, N2≈C6H6≈C6H12 for tetraethyl orthosilicate and N2≈C6H6>C6H12 for silica sol; 2) for crystals in the early stage, N2>C6H12>C6H6 for tetraethyl orthosilicate and N2>C6H12≈C6H6 for silica sol; 3) for crystals in a sufficiently developed stage, N2>>C6H12>C6H6 for tetraethyl orthosilicate and N2≈C6H6>C6H12 for silica sol. In the case of silica sol, the crystallization mechanism is based on inhomogeneous nucleation; a number of critical nuclei may be generated but their dimension is small. In the case of tetraethyl orthosilicate critical nuclei are fewer in number and larger in size than in the case of silica sol; the mechanism of crystallization is based on homogeneous nucleation.