N-ALKANES CRACKING

Number of moles of cracked products with carbon number i [moles per 100 moles n-alkane cracked J... 

The apparent first-order rate constant of n-alkane cracking over ZSM-5 at 380°C. [Reprinted from J. Wei, Adsorption and Cracking of N-Alkanes over ZSM-5 Negative Activation Energy of Reaction, Chem. Eng. Sci., 51 (1996) 2995, with permission from Elsevier Science.]... 

The temperature of zeolite samples containing various adsorbed molecules was switched from room temperature to 500-600 K within 30-40 seconds by means of a laser beam. Catalytic n-alkane cracking and H-D exchange with deuterated cyclohexane were monitored by IH MAS NMR in time steps of down to one second. A two-dimensional representation of the chemical shift and the chemical reaction of the species will be given, allowing a good characterization of reaction steps. At low temperature a weak proton transfer without chemical reaction can be observed, whereas at 430 K and 530 K the proton transfer is accompanied, respectively, by an isomerization or a decomposition to methane and coke. In addition to the effect of high temperature, the laser radiation itself can force the conversion of alkanes to methane and coke. 

The anomalous decrease in the activation energies of n-alkanes cracking after Cie the C- sixteen effect[31]", could be explained in terms of the contribution from the internal vibrations of zeolites whose frequencies are in resonance with the skeleton vibrations of hydrocarbons at isocatal)4 ic temperature and has been described in detail elsewhere[32]. 

Barthomauf 0 Compared formation of light products In n-alkane cracking In SAPO-37 and Si-AI faujasites energy gradient selectivity. AppI Ceial 1995,126 187-194. 

Homologous n-a-olefins by pyrolysis of high molecular weight C20-C30 n-alkanes (wax cracking)... 

Table 5 gives typical results of the wax cracking process to surfactant olefins. Compared with the pure a-olefins produced by the oligomerization reactions of ethylene the crack olefins are decreased in quality, especially due to the conjugated diene part (2-4%). Moreover, there are some problems in guaranteeing the wanted amounts of C20-C30 n-alkanes. Therefore in industrially de-... 

The formation of lipid components in an aqueous phase at temperatures from 370 to 620 K was studied by Rushdie and Simoneit (2001), who heated aqueous solutions of oxalic acid in a steel vessel for 2 days the yield of oxidized compounds reached a maximum (5.5% based on oxalic acid) between 420 and 520 K. A broad spectrum of compounds was obtained, from n-alkanes to the corresponding alcohols, aldehydes and ketones. At higher temperatures, i.e., above 520-570 K, cracking reactions competed with the synthetic reactions. 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

The analysis of the literature data shows that zeolites modified with nobel metals are among perspective catalysts for this process. The main drawbacks related to these catalysts are rather low efficiency and selectivity. The low efficiency is connected with intracrystalline diffusion limitations in zeolitic porous system. Thus, the effectiveness factor for transformation of n-alkanes over mordenite calculated basing on Thiele model pointed that only 30% of zeolitic pore system are involved in the catalytic reaction [1], On the other hand, lower selectivity in the case of longer alkanes is due to their easier cracking in comparison to shorter alkanes. 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

This A cracking was shown to be 20 times faster than the rearrangement of type B involved in n-alkane hydroisomerization. 

Much progress has been made in understanding the catalytic activity of zeolites for several type of reactions. The number of reactions catalyzed by zeolites has been extended, and new multi-component polyfunctional catalysts with specific properties have been developed. In addition to cracking and hydrocracking, reactions such as n-alkane isomerization, low temperature isomerization of aromatic C8 hydrocarbons, and disproportionation of toluene are industrially performed over zeolite-containing catalysts. Moreover, introduction of various compounds (C02, HCl) into reaction mixtures allows one to control the intensity and selectivity of the reactions. There are many reviews on the catalytic behavior of zeolites and even more original papers and patents. This review emphasizes the results, achievements, and trends which we consider to be most important. 

The cracking reactions of normal alkanes such as n-C7°, n-Cio°, n-Ci2° and n-Ci6° were performed in a pulse microreactor at 500 °C with N2 flow rate of 15 ml/min and pulse amount of 0.5 ul. 100 mg of catalysts were put into a quartz tube with diameter of 4 mm. For n-Ci6° cracking, similar conversions were obtained by varying the amount of the catalysts used. 

Comparison of cracking performance of n-alkanes over various samples (reaction temperature 500 °C) ... 

In Figure 5 the generally accepted reaction path (14) for hydroisomerization of n-alkanes has been represented along with different possibilities for the cracking step. The n-alkane molecules are adsorbed at a dehydrogenation/hydrogenation site where n-alkenes are formed. After desorption and diffusion to an acidic site chemisorption yields secondary carbenium ions that rearrange... 

Four principle cracking reactions, all of them being irreversible, have to be taken into account hydrogenolysis of the n-alkane feed /3-scission of straight chain carbenium ions ... 

Hence, primary products may be obtained which is not the case in catalytic cracking over monofunctional catalysts where formation of carbenium ions occurs by hydride abstraction from the n-alkane rather than via n-alkenes. 

High hydrogenation/dehydrogenation activity of the bifunctional catalyst is a prerequisite for pure primary cracking of long chain n-alkanes (4, 19). Thus it is the same type of catalyst that... 

Figure 7. Hydrocracking of n-alkanes with an even carbon number. Distribution of the cracked products.

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