N-Butane cracking

In later work by Haag and Dessau product selectivity data were provided for n-butane cracking at 426-523 °C over HZSM-5 with Si/Ah = 70 [90]. The selectivity results at 496 °C and 1-10 kPa for n-butane were extrapolated to zero percent conversion in Table 13.6 to be able to identify the primary products and to assess the decomposition pattern of the n-butyl carbonium ion. Similar selectivities to methane and propylene implied, as expected, that the decomposition of the car-... 

Table 13.7 Biomolecular reactions during n-butane cracking over MFI at 496°C [90],...

Krannila, H., Haag, W.O., and Gates, B.C. (1992) Monomolecular and bimolecular mechanisms of paraffin cracking n-butane cracking catalyzed by HZSM-5./. Catal, 135, 115-124. 

Predictions for n-butane prices are not readily available. The picture for n-butane is more complicated since large amounts of this material are also used as a blending stock in gasoline. Future trends in refinery uses of n-butane will certainly affect price levels. The current price for n-butane on the Gulf Coast is about 1 /lb (ca. 5 /gal). If the price goes to, say, 1.2 /lb (ca. 6 /gal), the ethylene production cost associated with n-butane cracking will rise to about 2.5 /lb vs. 2.05 /lb at the lower level. 

The n-butane cracking values obtained with the titanium substituted zeolites all show an increase in kA value over that obtained with the starting zeolite. This is notable in that the Ti is tetra-valent, and does not require a cation. With reduced cation content, the acidity should be reduced. However, the result obtained was the reverse. With iron-substituted products, the resulting kA values varied with the zeolite. Although not discussed in detail in this paper, all Fe-containing products did show indications of metals activity there was a dramatic increase in the amount of olefins produced. 

The importance of recognizing and dealing with zeolite synthesis as a kinetic process that involves the isolation of metastable phases is pointed out in this book in a variety of ways. An examination of the extensive scientific and patent literature on zeolite synthesis rapidly convinces one that a lack of understanding of this point has been a major bottleneck in the characterization of zeolite chemical and physical properties. The zeolite properties are defined not only by synthesis parameters, but also by treatment following synthesis for example, most synthesis treatment of zeolites with fluorine can be used to modify hydrophobicity drastically and increase catalytic activity for n-butane cracking. 

Dilute fluorine gas (0-20%) can be used to treat zeolites at near-ambient temperature and pressure. Most of the resulting materials retain very high crystallinity even after 600°C postcalcination for two hours. Both framework infrared spectra and X-ray powder diffraction patterns clearly show structural dealumination and stabilization. The hydrophobic nature of the fluorine-treated and 600sC-calcined material is shown by a low water adsorption capacity and selective adsorption of n-butanol from a 1 vol.% n-butanol-water solution. Fluorination also changes the catalytic activity of the zeolite as measured by an n-butane cracking method. 

Finally, the catalytic activity was measured by a 2 mole % n-butane cracking method described in detail on another paper (27). 

The results for the n-butane cracking test expressed in terms of pseudo-first-order rate constants are tabulated in Table VI. Fluorinated water-washed and fluorinated calcined samples were also tested. Water washing promotes removal of entrapped soluble fluoride compounds. The calcination was carried out on LZ-105 and erionite samples treated under relatively severe conditions. 

Figure 12. Transition structures of propane and n-butane cracking in the chabazite framework.

The structural properties of the samples were characterised by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRFS), nitrogen adsorption and 27Al MAS NMR. The acid properties of the zeolite were also investigated using n-butane cracking as a test reaction. 

Figure 5. Product selectivity for n-butane cracking over (a) hydrothermally-treated sample H3, and (b) AHFS-treated sample FS9.363. The reaction products are methane (0), ethane and ethene (A), propane (O), isobutane (b), but-l-ene ( ) and but-2-ene (a).

Table IX. Maximum n-Butane Cracking Values (kA) for Selected MeAPO Molecular Sieves...

Direct fluorination of zeolites has been reported as a method for increasing their acidity for hydrocarbon cracking reactions. Lok et al. treated various zeolites with fluorine gas and reported zeolite dealumination and stabilisation and in certain cases an increase in n-butane cracking activity.20 It appears that the optimum fluorine content for maximum cracking activity is about 1% m/m. 

The catalytic properties of several SAPO materials have been inspected by several other authors, for transformations such as n-butane cracking HI, xylenes isomerization 12, propylene oligomerization and toluene methylation 13l. Most of them, and specifically SAPO-37, presented mild acid character, similar to the one revealed in this study. 

Because the concentration of tetrahedrally coordinated metal cations of a kind is frequently not constant throughout the crystal, it might be difficult to rationalize the catalytic activity or the acid - base properties as function of the overall lattice charge or the overall composition of the material (3). This complicated situation is also reflected in widely varying values for the n-butane cracking rate constants for even a material of one given kind, e.g. 

Table 2 Pseudo-first-order rate small-pore molecular sieves constants for n-butane cracking on, 19,20 ...

Table 4 n-Butane cracking over AlPO-n based molecular sieves ... 

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