Cracking Of Heptane Research Articles

The Effect of Exchange Cations on Acidity, Activity, and Selectivity of Faujasite Cracking Catalysts

Several studies have shown that the removal of exchanged sodium increases activity for paraffin cracking as would be expected of a solid Brønsted acid. However, it has also been observed that at low unit cell sizes the presence of low levels of sodium affects selectivity but not activity for gas oil cracking. This paper describes how cation exchange affects the distribution of acid sites in dealuminated faujasite over a range of unit cell sizes and how this in turn affects the activity and selectivity of USY at a unit cell size of ∼2.440 nm for gas oil and heptane cracking. The acidic properties were determined by NH 3/TPD measurements, by pyridine adsorption followed by IR measurements, and by isopropylamine TPD experiments. Activities and selectivities were measured by gas chromatographic analysis of the products of heptane and gas oil cracking. The results show that alkali exchange of dealuminated faujasite has little or no effect on activities for gas oil cracking, but a very strong effect on activity for heptane cracking. While the results show that exchange by alkali cations reduces the number of cracking sites, the major effect is to reduce the intrinsic activity per site by an order of magnitude or more. The deactivating effect of individual cations is a strong function of size. The activity follows the order H + ∼ Mg 2+ ∼ Li + > Na + > K +

ChemInform Abstract: Catalytic Activity of Large‐Pore High Si/Al Zeolites: Cracking of Heptane on H‐Beta and Dealuminated HY Zeolites

Abstract The catalytic activity, selectivity, catalyst decay, thermal and hydrothermal stability, and acidity of H-Beta and HY zeolites with Si Al ratios of 7.5 and 10, respectively, have been studied during cracking of n-heptane at 450 °C and atmospheric pressure. It has been found that the H-Beta zeolite is more active and decays more slowly than HY. H-Beta presents a higher steric hindrance for dibranched molecules and therefore its open structure should be smaller than that of HY. A lower hydrogen transfer activity and hydrothermal stability is observed for H-Beta in comparison with the corresponding HY.

Catalytic activity of large-pore high [formula omitted] zeolites: Cracking of heptane on H-Beta and dealuminated HY zeolites

The catalytic activity, selectivity, catalyst decay, thermal and hydrothermal stability, and acidity of H-Beta and HY zeolites with Si Al ratios of 7.5 and 10, respectively, have been studied during cracking of n-heptane at 450 °C and atmospheric pressure. It has been found that the H-Beta zeolite is more active and decays more slowly than HY. H-Beta presents a higher steric hindrance for dibranched molecules and therefore its open structure should be smaller than that of HY. A lower hydrogen transfer activity and hydrothermal stability is observed for H-Beta in comparison with the corresponding HY.

Transition-state selectivity in the cracking of n-heptane over modified ZSM-5 catalysts

The rates of cracking of n-heptane on Ni-, Zn-, and H-ZSM-5 incorporated with a silica binder were in the order of Ni-ZSM-5 > Zn-ZSM-5 > H-ZSM-5. Transition state selectivity, dehydrogenation and classical carbonium ion chemistry were used to explain the relative activities of these catalysts. Activation energy for cracking of heptane over Ni-ZSM-5 was found to be 32 ± 3 kcal/ mol.

Controlled pore sizes and active site spacings determining selectivity in amorphous silica-alumina catalysts

Controlled pore sizes in amorphous silica-alumina were achieved by employing tetraalkylammonium cations (TMA (methyl) to TBA (butyl)] as the only counterions to the fourfold coordinated aluminate ions in the gels. The width of the pore size distributions and the median size of the pores in the calcined silica-aluminas (now free of nitrogenous cations) decreased as the size of the cations used in the synthesis increased, except in the butyl case. The median values as determined by N 2 desorption were 15, 5, 4.5, 3.7, and 4.2 nm for the Na NH 4 -, TMA-, TEA-, TPA-, and TBA-silica-aluminas, respectively. The surface area per unit mass of catalyst increased with the size of the cations. The average interexchange site distance increased correspondingly, except in the butyl case. This exception is attributed to constraints set by the specific SiO 2 Al 2O 3 ratio used. In the catalytic cracking of heptane the TPA-silica-alumina produced substantially greater proportions of toluene and cyclic C 6 hydrocarbons than the other silica-aluminas. These materials have potential as sorbents, catalysts, and catalyst supports inter alia in the treatment of heavy residue and coal liquids where zeolites cannot be used because of their small pore sizes (< 1 nm).

Acceleration and retardation of the cracking of hydrocarbons in various media

1. It was found that the pyrogenetic transformations of heptane in the molten metals aluminum, tin, and sodium proceed selectively in a way dependent on the nature of the metal. 2. Aluminum and tin accelerate the cracking of heptane. Sodium and potassium hydroxide exert a powerful retarding action on the cracking of heptane. 3. The mechanism of the action of inhibiting additives appears to amount to chain termination due to the reaction of alkali metals with free radicals or to the saturation of hydrogen liberated by reaction with hydrocarbons. 4. Alkali-metal hydroxides may be reduced by hydrocarbons to metals, which then react by the mechanism indicated.