Stefan Smeds

Chemisorption and TPD studies of hydrogen on Ni/Al2O3

Abstract The activities of two supported nickel catalysts, a commercial (17 wt.-% Ni/Al 2 O 3 ) and a non-commercial (10 wt.-% Ni/Al 2 O 3 ) catalyst, were investigated in gas phase toluene hydrogenation. Both catalysts were active in hydrogenation, exhibiting rate maxima at about 443 K. The catalysts were characterized using hydrogen chemisorption and temperature programmed desorption (TPD) techniques. Decreasing hydrogen adsorption capacity was generally found in the temperature interval 298–423 K, the capacity of both the commercial and the non-commercial Ni-catalysts being about 20 cm 3 /g Ni at 423 K. No effect on the total adsorption capacity was found by increasing the pretreatment temperature from 503 K to 773 K on the commercial catalyst. Three adsorption states of hydrogen (I–III) were resolved from the TPD-spectra of both catalysts. Hydrogen desorption was modelled with peak shape analysis as a second order process with free readsorption, giving hydrogen adsorption enthalpies ranging from −108 to −124 kJ mol −1 for adsorption state I. The kinetic data and the TPD studies indicate that the decrease of the toluene hydrogenation activity at temperatures above 443 K is due to the decay of adsorption state I.

Kinetics of ethylbenzene hydrogenation on Ni/Al2O3

Abstract The gas-phase hydrogenation kinetics of ethylbenzene on Ni/Al 2 O 3 was investigated in a differential microreactor at atmospheric pressure and 130–190°C. Rapid deactivation of the catalyst complicated the steady-state measurements. Reaction orders were near zero for the aromatic compound and between 1.2 (130°C) and 2.3 (190°C) for hydrogen. A rate maximum was observed at 160–175°C, depending on the concentration ratio of the reactants. Mechanistic modelling suggested that the reaction rate is governed by three steps of pairwise sequential addition of hydrogen atoms to adsorbed ethylbenzene. The partially hydrogenated surface intermediates were proposed to have aromatic character. The physical reasonability of the model parameters was discussed.

Kinetics of toluene hydrogenation on Ni/Al2O3 catalyst

The gas phase hydrogenation of toluene to methylcyclohexane on a commercial Ni/Al2O3 catalyst was investigated in a differential reactor operating at atmospheric pressure and temperatures between 150 and 210°C. The results revealed that the hydrogenation kinetics is of the order 1–3 with respect to hydrogen at the actual temperature interval and that the reaction order increases with temperature. The reaction order with respect to toluene is negative. The reaction rate exhibited a maximum at approximately 170°C. The rate maximum is explained by the escape of catalytically active hydrogen from the Ni-surface at the highest reaction temperatures, which was confirmed by temperature-programmed desorption studies and chemisorption studies of hydrogen. The kinetics was modelled with an empirical power-law rate expression and with three mechanistic rate models. The latter were based on the assumption of rapid competitive adsorption steps of toluene and hydrogen and rate determining surface reaction steps involving addition of hydrogen atoms to adsorbed toluene and partially hydrogenated intermediate molecules. The best fit to the experimental data were provided by two models; one implying simultaneous addition of hydrogen atoms to adsorbed toluene and the other, being more probable from a mechanistic point of view, implying sequencial addition of hydrogen atoms to adsorbed toluene.

Gas phase hydrogenation of o- and p-xylene on NiAl2O3 — Kinetic modelling

Abstract A mechanistic rate model was developed to describe the gas phase hydrogenation kinetics of o -xylene and p -xylene on a Ni Al 2 O 3 catalyst at temperatures between 403 and 493 K. The mechanistic scheme comprises consecutive and pairwise hydrogen addition steps to the aromatic molecules adsorbed on the catalyst surface. Competitive as well as non-competitive quasi-equilibrated adsorption of the reactants were accounted for. An experimentally observed cyclic olefin is assumed to be the key intermediate, determining the stereoselectivity of the main products ( cis - and trans -dimethylcyclohexanes) by its adsorption properties. All the main features of the observed reaction kinetics, including a rate maximum in the overall rate as a function of temperature as well as the production rates of the main products and the intermediate cyclic olefin, were satisfactorily predicted by the rate model. The calculated model parameters were consistent with thermodynamics and previously reported parameters for elementary steps.

Gas phase hydrogenation of o- and p-xylene on Ni/Al2O3 — Kinetic behaviour

The gas phase hydrogenation kinetics of o-xylene and p-xylene on a Ni/Al2O3 catalyst was investigated at temperatures between 403 K and 493 K in a differential fixed bed reactor. The main products, the respective cis- and trans-dimethylcyclohexanes, were formed in non-equilibrium ratios; the amount of the trans-isomer was clearly below the equilibrium one. The hydrogenation also yielded partially saturated cyclic olefins in non-negligible amounts ( < 13% of the main products). Strong catalyst deactivation, which was reversible in hydrogen flow at elevated temperatures, was observed. Overall hydrogen reaction orders above two were calculated from extrapolated initial reaction rates at the highest experimental temperatures. The overall aromatic reaction order was always close to zero. A rate maximum in the overall reaction rate as function of temperature was observed. The results indicate that the reaction proceeds through consecutive hydrogen addition steps on the catalyst surface, the cyclic olefin being the key intermediate.

Kinetics of m-xylene hydrogenation on NiAl2O3

Abstract Gas phase hydrogenation of m-xylene to cis- and trans-1,3-dimethylcyclohexane, partially saturated 1,3-dimethylcyclohexene-1 being a by-product, was studied in a differential microreactor at atmospheric pressure and 418–493 K over a Ni Al 2 O 3 catalyst. Rapid reversible deactivation of the freshly reduced catalyst prompted for reactivation before every kinetic measurement and extrapolation of the steady-state activity to initial activity. Reaction orders were close to zero (up to 0.2) for m-xylene and ranged from 0.8 (418 K) to 2.6 (493 K) for hydrogen. A rate maximum for the production of the saturated compounds was observed at about 450 K, varying with the pressure ratio of the reactants. Mechanistic modelling of the surface reaction steps, including governed by three steps of pair-wise sequential addition of hydrogen atoms to adsorbed m-xylene. The partially hydrogenated surface intermediates in the first two addition steps were proposed to have aromatic character.

TOLUENE AND METHYLCYCLOHEXANE ADSORPTION ON NICKEL CATALYSTS

The chemisorption of toluene and of methylcyclohexane on nickel catalysts at atmospheric pressure in the temperature range 365–485 K has been investigated by gravimetric method as well as infrared spectroscopy. The latter indicates that toluene attached to the surface with π-bonding is the most realistic surface compound. The coverage of organic compounds at partial pressures close to 1 kPa was found to be equal to 0.16–0.25. The amount of adsorbed compound increased with increasing temperature. These observations led to the application of the isotherm which takes into account lateral interactions, as well as the polyatomic nature of adsorption. A good description of experimental data was obtained.

Dialkylbenzene hydrogenation: Kinetic analysis of rollover mechanism

Kinetic analysis is presented for the so-called rollover mechanism in hydrogenation of dialkylbenzenes. The theory of complex reactions and graph theory are applied for discussing plausible mechanisms. It is demonstrated that mechanisms which involve either desorption-readsorption of some reaction intermediate or some kind of “rollover” of an adsorbed intermediate, result in similar kinetic equations if these steps are reversible.

Gas-phase hydrogenation of ethylbenzene over Ni. Comparison of different laboratory fixed bed reactors

Gas phase ethylbenzene hydrogenation was studied over a supported Ni catalyst in two laboratory fixed bed reactors. Difference in deactivation behavior and reaction kinetics was attributed to existence of interparticle heat transfer in the case of reactor with bigger inner diameter.

Activation of Ni/A12O3 catalysts through modification of the Al2O3 support

The effect of alumina pretreatment on the performance of alumina supported nickel catalysts was demonstrated in gas phase hydrogenation of toluene to methylcyclohexane. The state of the alumina was changed from pure γ to pure θ phase through various heat treatments in air. The catalysts were prepared from vapor phase by saturating the accessible binding sites on the pretreated alumina with the nickel precursor. The highest number of active sites for hydrogenation was observed for catalysts prepared on alumina having an incomplete phase transition and a θ/γ alumina phase ratio between 0.5 and 10. Results from temperature programmed desorption (TPD) studies revealed that a maximum in weakly chemisorbed hydrogen as well as in total amount of desorbed hydrogen was found for the same catalysts. By hydrogen chemisorption studies the total hydrogen uptake was found to correlate with the observed hydrogenation maximum. It is suggested that both the chemical and physical properties of the alumina influence the activity. An optimal metal-support interaction and structural defects on the alumina due to the phase transition can explain the observed maximum in the number of active sites and in hydrogen uptake.