Index constraint

Cl measurements were later revisited by remeasuring the familiar zeolites, but this time including some of the new structures that were invented. These results are in Table 13.4. While most of these results were within expected ranges, there 

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Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

ZSM-5 and ZSM-11 samples were prepared as previously described (11) using tetrapropy 1 ammonium hydroxide and tetrabutyl ammonium bromide, respectively. The nature and crystallinity of the materials were verified by X ray diffraction, ir spectroscopy of lattice vibrational bands ( 1 2 ), n-hexane adsorption capacity at room temperature and constraint index (13) measurements. All samples correspond to highly crystalline ZSM-5 or ZSM-11 materials. The chemical compositions of the samples as determined from chemical analysis of A1 and Na contents, are given in table 1. 

Table 13.3 Constraint index for numbers in parentheses that are various zeolites (Cl based on tt from the open literature)) 3].

Table 13.4 Constraint index revisited with new framework types[59].

Figure 13.26 Modified constraint index for various zeolites [13].

The constraint index test revisited anomalies based upon new zeolite structure types. Micropor. Mesopor. Mater., 35-36, 31 5. 

R.M. (1981) Introduction of constraint index as a diagnostic test for shape selectivity using cracking rate constants for n-hexane and 3-methylpentane. 

The results on olefin isomers (Table Vll) can also be explained by the observation that the constraint index of ZSM-5 is approximately unity under the conditions of this study. Shape selectivity or preferential conversion of straight chain olefins by ZSM-5 cannot be expected at 500 C. Thus, under the conditions of this study, olefin isomer distribution was not significantly affected by deactivated ZSM-5. At temperatures lower than that employed in the present study, it is conceivable that distribution of olefin isomers could be altered by steam deactivated ZSM-5. 

The catalytic activities of the modified Na,H-ZSM-5 and the fully protonated H-ZSM-5 were measured for several reactions. The constraint index experiment was carried according to the literature A 1 1 molar mixture of n-hexane and 3-methylpentane was fed over the catalyst bed at 300°C at a rate of 1 ml/hr with a helium diluent at 12.5 ml/min. The effluent stream was sampled after 20 minutes by syringe and analyzed by gas chromatography. The constraint index is given by Equation 5. 

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. 

SSZ-24 has been produced along with various impurities when aluminum at a level of Al/Si = 0.01 was introduced into the syntheses. A variety of aluminum sources were used including aluminum sulfate, metakaolin, colloidal alumina dispersed on silica (Nalco ISJ61 2), and even other zeolites (K-L, K-A, and K-offretite). However, accompanying zeolites and other impurities could not account for the catalytic activity and selectivity found in a constraint index test, described in Example 7 of the patent reference (12). The assumption then is made that the catalysis is caused by a low alumina form of the large pore SSZ-24. 

Steric inhibition of the formation of bulky reaction intermediates can also lead to unexpected selectivity. This is known as spatioselectivity or transition state selectivity. Csicsery first proposed this in 1978 in his study with mordenite (1 1). With medium pore zeolites, the relatively slow rate of cracking of 3-methylpentane compared to that of n-hexane, known as the Constraint Index of these zeolites (12) is a typical example of this type of shape selectivity. The reaction intermediate for 3-methylpentane is sterically constrained in the pores of ZSM-5. 

The catalytic characterization of zeolites is generally carried out with the aid of test reactions [8]. For example, the constraint index Cl (Table 7-3) compares the relative rate of cracking of a 1 1 mixture of n-hexane (molecular diameter 0.49 nm) and 3-methylpentane (molecular diameter 0.56 nm). 

Table 7-3 Constraint index (Cl) for some typical catalysts at 316°C [T28]...

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. 

In order to differentiate the available reaction space, Frillette et al. [122] introduced the constraint index (Cl), which is determined from the ratio between the relative rates of w-hexane and 3-methylpentane cracking ... 

It is speculated that the mechanism concerning pentacoordinated carbonium ion intermediates (eg. Ri-CH3 -CH2-R2, Ri-CH2 =CH-R2, CcHy" ) occurs at temperatures above 500°C with the intermediates undergoing ft-scission to smaller paraffins and carbenium ions. As well, the carbonium ions are converted to carbenium ions through the loss of hydrogen, present as molecular hydrogen in the cracking products. This mechanism is also favoured by low conversion, low hydrocarbon partial pressure and high constraint indexed zeolites (Scherzer, 1989). 

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. 

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