Pore size constraint index

The fully protonated H-ZSM-5 had a constraint index of 10.8 in agreement with the literature values. The modified Na, H-ZSM-5 had a constraint index of 1.1 which is typical of a larger pore zeolite with a pore diameter greater than 6A. The modified zeolite reacts with the substrates primarily at external catalytic sites and in a non-discriminating manner. Size and shape selectivity are not major factors since few catalytic sites are located internally. 

The hydroisomerisation of -decane is one such test reaction where a modified constraint index Cl is defined as the ratio of the formation of 2-methylnonane to that of 5-methylnonane. Cl is found to increase over the range 1-10 as the pore size decreases from large to medium pore (for Y, Cl is ca. 1, whereas for ZSM-5, Cl is ca. 10). However, like Cl, it gives relatively little differentiation between large-pore zeolites, and explanation for the origin of the Cl values remains ambiguous. 

Hydrocracking of bulky cyclic alkanes is found to give a sensitive index in the large-pore region. For example, the hydrocracking of butylcyclohexane in the absence of spatial constraints proceeds almost entirely to methylcyclopentane and isobutane, whereas shape selectivity that occurs as pore size decreases forces the reaction to go via alternative processes, resulting in the formation of -butane (Scheme 8.9). ... 

Frilette et al. proposed a simple test reaction for estimating the effective pore size of zeolites. The determination of the constraint index is done by continuously passing a mixture of hexane and 3-methylpentane over a zeolite at atmospheric pressure. The constraint index is defined as follows. 

An 8-membered ring zeolite, erionite, gives a constraint index of 38, 10-membered ring zeolites, ZSM-5 and ZSM-11, give values of 8.3 and 8.7, respectively, while 12-membered ring zeolites, mordenite and REY, give values of 0.5 and 0.4. In this way, the constraint index is a very useful tool for estimating the pore size of zeolites of unknown structures. 

V.J. Frilette, W.O. Haag, R.M. Lago et al.. Catalysis by ctystaUine aluminosilicates characterization of intermediate pore-size zeolites by the Constraint Index . J. Catal. 67, 218-222 (1981)... 

---