Shape selective cracking of n-octane and 2,2,4-trimethylpentane over an alumina-pillared clay

A mixture of n-octane (nC₈) and 2, 2, 4-trimethylpentane (224-TMP) was cracked over an alumina-pillared montmorillonite (Al-PILC) acid catalyst as a means of characterising its pore structure and shape selectivity. The shape selectivity of the catalyst was quantified by a time dependent parameter analogous to the constraint index, called the selectivity ratio (SR), and defined as log (fraction of nC₈ remaining)/log (fraction of 224-TMP remaining). The SR was measured over Al-PILC, a 0.16 wt% Pt loaded Al-PILC, a Y zeolite, and an amorphous silicaalumina. The SR was greater over Al-PILC compared with Y-zeolite even though the pore openings of the Al-PILC were of similar or larger size than those in the Y-zeolite. Doping AlPILC with Pt caused a large increase in the SR. As found with constraint index measurements, internal pore dimensions and reaction mechanisms appear to be the determinants of SR. However, SR is better capable of discriminating larger pore microporous materials.