Shape Selective Cracking

The structure of zeolites often modifies the selectivity of catalytic cracking with respect to both reactants and products, depending on the effective pore size of the zeolites. 

The relative cracking rates of heptanes and hexanes over HZSM-5 zeolites are shown in Table 4.27.The rates of cracking are in the following decreasing order. 

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. 

The constraint index therefore approximates the ratio of the cracking rate constants for the two hydrocarbons. The values of the constraint index of various zeolites are summarized in Table 4.28. 

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. 

The use of a small pore ZSM-5 co-catalyst with REUSY catalysts increases the octane number of gasoline by a process known as shape-selective cracking. Straight-chain Ce-Cio olefins produced by normal cracking reactions and which are the precursors of low-octane paraffins are selectively cracked by ZSM-5 catalysts. 

So-called centre cracking produces a C4-C5 olefin fraction which rapidly isomerises to isobutene and isoamylene. These products were converted to methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) by reaction with methanol to produce octane-enhancing additives for use in reformulated gasoline. Propane and n-butane are also produced. Fresh ZSM-5 also cracks paraffins imtil the acid site density decreases. Eventually, olefin cracking activity dechnes but isomerization activity is retained. Regular addition of fresh ZSM-5 is therefore required to maintain the shape-selective activity. 

TABLE 5.11. Time Taken to Change Catalyst Inventory.  

Days after addition begins % New catalyst at 2% addition 

Catalyst producers provide information on the time taken to change an existing catalyst. With a relatively low daily replacement rate of 2% it can take 20 days to change 40% of the old catalyst inventory. Because the zeolite in an FCC catalyst is rapidly deactivated, more than 50% of the cracking activity is supplied by catalyst less than 20 days old. Older catalyst still has some activity in the matrix, which converts heavier fractions. 

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Isodewaxing A catalytic dewaxing process developed by Chevron Research Technology. It incorporates catalysts that achieve both wax isomerization and shape-selective cracking. 

The applications of the ZSM-5 family of zeolites for shape-selective cracking of paraffins in the gasoline (2, 10), distillate (11) and lube oil range (12) have all been reported. In this paper, we have established evidence of the converse reaction, shape-selective polymerization, to produce hydrocarbons in the same product range. 

In 1960, Weisz, Frilette, and co-workers first reported molecular-shape selective cracking, alcohol dehydration, and hydration with small pore zeolites (6,7), and a comparison of sodium and calcium X zeolites in cracking of paraffins, olefins, and alkylaromatics (8). In 1961, Rabo and associates (9) presented data on the hydroisomerization of paraffins over various zeolites loaded with small amounts of noble metals. Since then, the field of zeolite catalysis has rapidly expanded,... 

ZSM-5 zeolites were shown to perform shape-selective cracking (called Selectoforming with a later improved M-forming process by Mobil) [8]. 

N. Y. Chen, J. Garwood, and F. G. Dyer, Shape Selective Cracking in Industrial Applications, Dekker, New York, 1989. 

A different kind of shape selectivity is restricted transition state shape selectivity. It is related not to transport restrictions but instead to size restrictions of the catalyst pores, which hinder the fonnation of transition states that are too large to fit thus reactions proceeding tiirough smaller transition states are favoured. The catalytic activities for the cracking of hexanes to give smaller hydrocarbons, measured as first-order rate constants at 811 K and atmospheric pressure, were found to be the following for the reactions catalysed by crystallites of HZSM-5 14 n-... 

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

Castaneda, R., Corma, A., Eornes, V., Martinez-Triguero, J., and Valencia, S. (2006) Direct synthesis of a 9 x 10 member ring zeolite (Al-lTQ-13) a highly shape-selective catalyst for catalytic cracking. J. Catal., 238, 79-87. 

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

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

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