ALBERT

Description: The hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) was studied in the liquid phase over a Rh/silica catalyst at 303–343 K and 3 barg hydrogen pressure. The following order of reactivity, resorcinol 〉 catechol 〉 hydroquinone (meta 〉 ortho 〉 para) was obtained. Kinetic analysis revealed that catechol had a negative order of reaction whereas both hydroquinone and resorcinol gave positive half-order suggesting that catechol is more strongly adsorbed. Activation energies of ~30 kJ·mol−1 were determined for catechol and hydroquinone, while resorcinol gave a value of 41 kJ·mol−1. Resorcinol, and similarly hydroquinone, gave higher yields of the hydrogenolysis products (cyclohexanol, cyclohexanone and cyclohexane) with a cumulative yield of ~40%. In contrast catechol favoured hydrogenation, specifically to cis-1,2-dihydroxycyclohexane. It is proposed that cis-isomers are formed from hydrogenation of dihydroxycyclohexenes and high selectivity to cis-1,2-dihydroxycyclohexane can be explained by the enhanced stability of 1,2-dihydroxycyclohex-1-ene relative to other cyclohexene intermediates of catechol, resorcinol or hydroquinone. Trans-isomers are not formed by isomerisation of the equivalent cis-dihydroxycyclohexane but by direct hydrogenation of 2/3/4-hydroxycyclohexanone. The higher selectivity to HDO for resorcinol and hydroquinone may relate to the reactive surface cyclohexenes that have a C=C double bond β-γ to a hydroxyl group aiding hydrogenolysis. Using deuterium instead of hydrogen revealed that each isomer had a unique kinetic isotope effect and that HDO to cyclohexane was dramatically affected. The delay in the production of cyclohexane suggest that deuterium acted as an inhibitor and may have blocked the specific HDO site that results in cyclohexane formation. Carbon deposition was detected by temperature programmed oxidation (TPO) and revealed three surface species.

Description: Transhydrogenation of pentane (P) and 1,5-hexadiene (1,5HD) and pentane and 2,4-hexadiene (2,4HD) was studied over a CrOx/alumina catalyst at 523–773 K. Thermodynamic stability differences between the conjugated (2,4-hexadiene) and non-conjugated (1,5-hexadiene) isomers indicated that transhydrogenation was favoured between pentane and 1,5-hexadiene but not pentane and 2,4-hexadiene (+ ve ∆G). At 773 K a significantly enhanced alkene yield was observed for the P/1,5HD system, clearly showing the effect of transhydrogenation. The yield of alkenes was ~ 50% and included alkylated and isomerized alkenes. Alkylation and isomerization were significant reactions under reaction conditions. Pentane was shown to affect the chemistry of 1,5HD and vice versa with the conversion of pentane significantly enhanced at all reaction temperatures, indicating a molecular interaction between the reactants even when transhydrogenation was not obvious. In contrast, no effect on the conversion of pentane was observed when the co-feed was 2,4HD. An unexpected effect of pentane on 2,4HD conversion was observed, with all reactions of cis-2,4-hexadiene (including alkylation and isomerization) being completely inhibited at low reaction temperatures (573 K and 523 K) by the presence of pentane, suggesting that pentane competes for the same sites as cis-2,4-hexadiene. Transhydrogenation activity between pentane and 1,5-hexadiene was less obvious at the lower reaction temperature, which appeared to be a kinetic effect. Direct hydrogenation of 1,5-hexadiene revealed that 1,5HD sampled the same hydrogen population for hydrogenation and transhydrogenation. Comparisons of transhydrogenation of 1-hexyne, 1,5-hexadiene, and 2,4-hexadiene with pentane have revealed significant differences in the adsorption and reaction chemistry of the three isomers.

Description: A MoOx/Al2O3 catalyst was synthesised and tested for oxidative (ODP) and non-oxidative (DP) dehydrogenation of propane in a reaction cycle of ODP followed by DP and a second ODP run. Characterisation results show that the fresh catalyst contains highly dispersed Mo oxide species in the +6 oxidation state with tetrahedral coordination as [MoVIO4]2− moieties. In situ X-ray Absorption Spectroscopy (XAS) shows that [MoVIO4]2− is present during the first ODP run of the reaction cycle and is reduced to MoIVO2 in the following DP run. The reduced species are partly re-oxidised in the subsequent second ODP run of the reaction cycle. The partly re-oxidised species exhibit oxidation and coordination states that are lower than 6 but higher than 4 and are referred to as MoxOy. These species significantly improved propene formation (relatively 27% higher) in the second ODP run at similar propane conversion activity. Accordingly, the initial tetrahedral [MoVIO4]2− present during the first ODP run of the reaction cycle is active for propane conversion; however, it is unselective for propene. The reduced MoIVO2 species are relatively less active and selective for DP. It is suggested that the MoxOy species generated by the reaction cycle are active and selective for ODP. The vibrational spectroscopic data indicate that the retained surface species are amorphous carbon deposits with a higher proportion of aromatic/olefinic like species.