Dan Farcasiu

Isomerization of hexane by zeolite HZSM-5: The effect of cyclic hydrocarbons

Abstract The main products of hexane (H) isomerization on HZSM-5 at 160°C in liquid phase are the isomers 2-methylpentane (2MP) and 3-methylpentane (3MP), accompanied by cracking and disproportionation products, but no alkenes. 2MP and 3MP are not formed by a simple carbocationic isomerization of H, because the 2MP/3MP ratio, (6–7)/1 is higher than the equilibrium ratio of 2.1 and does not change with conversion between 1 and 10%. The same observation applies to the small amounts of 2,3-dimethylbutane formed. Methylcyclopentane (MCP) has a minor accelerating effect on the conversion of H only at ratios 1:1. No hydride transfer catalysis operates. At the same ratio (1:1), cyclopentane (CP) has a somewhat greater effect than MCP on the conversion of H. The redistribution of the label in the conversion of H-u-d4.3 containing 4–5% MCP and of H containing 5% MCP-1-d showed that the products were formed from alkenes, which are the major intermediates of the reaction, even though none of them is found in the products desorbed. Each alkene exchanges hydrogen with the catalyst several times before being desorbed. The direct H/D exchange between the alkene (or its mechanistic equivalent) and the alkane, if it occurs, is less important. Thus, the usual mechanistic representation for alkane conversion, borrowed from reactions in superacid solutions, does not apply to the reaction with the zeolite catalyst.

Evaluation of hydrogen bonding ability of liquids and solids by C-13 NMR. Silica gel as a strong hydrogen bond donor

The chemical shift difference between signals of C(β) and C(α) of unsaturated ketones, Δδ, which we used before to measure acid strengths, has now been used to evaluate the hydrogen bond donor ability of solvents which are not acidic enough to hydronate the indicator. For such solvents there is no general correlation between H-bond donor ability and acid strength: hexa-fluoroisopropanol is a much weaker acid than acetic acid, but it is a stronger H-bond donor. The method can be applied to evaluate the H-bonding properties of solid surfaces, and it was thus found that silica gel has a much stronger H-bond donor ability than methanol or acetic acid.

One-electron transfer reactions of pyrylium cations

One- and two-step one-electron transfers to pyrylium cations, generating pyranyl free radicals and anions, respectively, as well as one-electron transfers from pyrylium ions, forming dication radicals, and the properties of these species are critically reviewed

Conformational Effects in Heterogeneous Catalysis

Abstract The importance of conformation of a molecule for its interaction with solid catalysts is discussed. The contributions of conformational enthalpy and entropy are examined. For molecules having a large number of conformations the entropy term can dominate the reaction. Cracking of paraffins on small and medium-pore zeolites is entropy-controlled; a simple molecular sieve effect (shape selectivity) cannot explain the higher reactivity of normal isomers.