The Role of Hydrogen Partial Pressure in the Gas-Phase Hydrogenation of Aromatics over Supported Nickel

Abstract

The effect of varying the hydrogen partial pressure (from 0.19 to 0.96 atm) on turnover frequencies (TOFs) in the gas-phase hydrogenation of benzene, toluene, and o-, m-, and p-xylene over a Ni/SiO 2 catalyst has been studied. Each system is characterized by well-defined reversible temperature related activity maxima (T max ) where T max was shifted in the conversion of benzene and toluene to lower values as the hydrogen partial pressure was reduced but remained unaffected in reactions involving the isomers of xylene. The range of TOFs reported for each aromatic system and the occurrence of T max are explained on the basis of interplay between the supply and reactivity of active surface species. The observed relationships between reaction temperature and TOF are represented by means of a common extended power rate model. The nature of true and apparent reaction kinetics is discussed, and a compensation effect is established wherein the only reaction variable is the hydrogen partial pressure. The experimentally determined activation energies do not represent the true catalytically relevant energetics because of the existence of preequilibria to the rate-determining step. The measured Arrhenius parameters are shown to be composite terms and are pressure dependent. Catalytically significant heats of adsorption have been extracted from the reaction data, and the TOFs have been found to be inversely proportional to the magnitude of the heat of adsorption of hydrogen.

Hydrogenation of alkylaromatics over Rh/silica

“…As expected as the alkyl chain length is increased the rate of hydrogenation decreases. It has been proposed that this decrease in rate is due the inductive effect of the alkyl chain increasing the electron density of the ring and hence resulting in a stronger -bond to the surface [8,9,16,17], therefore suggesting that it is the strength of the ortho meta para aromatic adsorption that is inhibiting the reaction. Our results confirm this hypothesis: the reaction order changes from zero-order with toluene to negative half-order with ethylbenzene to negative first-order with propylbenzene (table 1) indicating a stronger adsorption as the alkyl chain increases in length.…”

“…The hydrogenation of alkylbenzenes has only been subject to limited study and even that is focused on toluene and the xylenes [5][6][7][8][9][10]. To find studies of ethylbenzene or propylbenzene hydrogenation we must go back to 1945 [11].…”

“…Although there is a more recent study on ethylbenzene hydrogenation [12] examining the kinetics in the gas phase, no other literature on propylbenzene could be found. The xylenes, which have been examined slightly more extensively, are still poorly considered [5][6][7][8][9][10] compared to the hydrogenation of benzene, which has been the subject of numerous studies [13].…”

“…In general the rate of hydrogenation decreases as the length of the substituent increases, while for the xylenes the para-isomer has been found to be the most active [5,6,9]. Toppineen et al [14,15] investigated in detail the hydrogenation of five aromatics (di-and tri-substituted alkyl benzenes, xylenes, mesitylene, and p-cymene) over Ni/Al2O3.…”

“…Many of the kinetic studies have been gas phase studies at moderate temperatures [8,9], while the early work in the liquid phase did not undertake any kinetic analysis. Also there is a dearth of information on rhodium as the active phase, with no study examining the xylenes or ethyl-or propylbenzene [5,16].…”