Brendan Coughlan

Reduction of Ni2+ cations in Y zeolites: I. Effect of sample pretreatment

Abstract A range of NiNa-Y and NiK-Y zeolites was prepared by ion exchange. The location of Ni 2+ cations within the zeolite framework was monitored after various stages of thermal treatment and the reduction process of the transition metal ion in a flowing hydrogen atmosphere was investigated and correlated with such factors as the hydrogen flow rate, reduction time, reduction temperature, sample precalcination, and NH 3 pretreatment. Cation location was probed by means of Cs + back exchange and confirmed by the observation of the IR spectra of CO adsorbed on the activated zeolites. Iodometric titrations and Na + back-exchange techniques were used to measure the levels of Ni 2+ reduction. The data generated reveal how the interrelated Ni 2+ ion location and the nature of the charge-balancing alkali metal co-cation serve to influence the level of Ni 2+ reduction.

The effects of the environment on the growth of nickel metal particles supported on a range of Y zeolites prepared by ion exchange and impregnation

The nature of the nickel metal phase supported on a range of alkali metal cation-exchanged Y zeolite catalysts was characterized by X-ray diffraction line broadening. The sintering process is shown to occur via a crystallite migration mechanism. Nickel crystallite size was measured and correlated with such diverse factors as reduction time, reduction temperature, heating rate, sample precalcination, and ammonia adsorption. The nature of the alkali metal co-cation (Li+, Na+, K+, Rb+, or Cs+) is shown to strongly influence the dimensions of the supported metal, yielding the following order of decreasing crystallite size: NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. In the case of the CeNiNaY and CeNiKY samples, the introduction of an additional level of acidity, due to the presence of the highly polarizing Ce3+ ions, during thermal activation, serves to suppress crystallite growth. For comparative purposes, data on the sizes of nickel metal crystallites formed on the reduction of nickel-impregnated Y zeolites and silica and alumina supports are also presented; the nature of the alkali metal co-cation has a much greater influence on the dispersion of nickel supported on the impregnated zeolites. Under identical reduction conditions, the amorphous alumina and silica carriers exhibit the smallest supported crystallite sizes.

The hydrogenation of benzene over nickelsupported Y zeolites. Part 1. A kinetic approach

The kinetics of benzene hydrogenation were studied on a NiKY zeolite in the temperature range 403–523 K. The reaction orders with respect to benzene and hydrogen partial pressures are reported. Apparent activation energies of 59.5 kJ mol−1 in the temperature range 403–473 K and −11.5 kJ mol−1 in the range of 473–523 K were determined. The effects of hydrogen partial pressure and temperature on the reaction rate are considered. The kinetic data generated are compared to those previously reported for catalysis over unsupported nickel and nickel supported on amorphous carriers as well as the existing limited data for zeolite systems. The appearance of well-defined maxima in the rate vs. temperature curves is also discussed.

The hydrogenation of benzene over nickel-supported Y zeolites. Part 2. A mechanistic approach

From a combination of kinetics, adsorption data, and the isolation and identification of reactive and unreactive intermediates, a detailed mechanism for the hydrogenation of benzene over zeolitesupported nickel is presented. Cyclohexane was identified as the major product with methylcyclopentane, a secondary product resulting from catalysis over the more acidic samples. Cyclohexene was isolated as a reactive intermediate that underwent a secondary catalysis involving both metal and acid sites to yield cyclohexylbenzene as the principal coke product. The partial hydrogenation of benzene to cyclohexene was found to be the rate-determining step. A model is proposed that best fits the experimental data.

Ruthenium zeolite catalysts: A characterization by gas adsorption, thermogravimetry and catalytic activity for the hydrogenation of benzene

Abstract Several ruthenium A, X, Y, L, and mordenite zeolites have been prepared and characterized. The ruthenium zeolites have been studied by thermogravimetry and adsorption techniques and their catalytic activity for the hydrogenation of benzene in the range 353–433 K has been investigated. All the zeolites retain their crystalline structure after several outgassings at 623 K and sorption of CO 2 ; they are also stable to reduction at 723 K in flowing hydrogen. Turnover numbers for benzene hydrogenation increase smoothly with increasing metal surface area in all the zeolites.

The hydrogenation of toluene over nickel loaded Y zeolites

The catalytic activities of a range of hydrogen reduced nickel Y zeolites for the hydrogenation of toluene were measured and correlated with the following catalytic parameters: reaction temperature; reaction time; coke deposition. The role of the alkali metal co-cation (Li+, Na+, K+, Rb+ or Cs+) in influencing the overall hydrogenation activity of the supported nickel metal was probed. The effect of poisoning the surface Bronsted acidity by the adsorption of ammonia is discussed. For comparative purposes, data on the hydrogenation of benzene over the same catalysts are included.

Adsorption of benzene on a range of activated Y zeolites

The adsorption of benzene on calcined LiY, NaY, KY, RbNaY, CsNaY, CeKY, HY and on a range of calcined and reduced nickel-exchanged Y zeolites has been investigated by infrared and chromatographic techniques. The nature of the alkali-metal cation is shown to influence strongly the extent of the zeolite–benzene interaction; the strength of benzene adsorption increases in the order: CsNaY < RbNaY < KY < NaY < LiY. Benzene interacted weakly with the CeKY and HY samples. Reduction of the nickel-exchanged Y zeolites lowered the overall strength of benzene adsorption. The strongly adsorbed aromatic phase (on LiY, NaY or the Ni2+-exchanged forms) closely resembles crystalline benzene. Benzene desorption from the zeolite surface as a function of temperature is also considered.

Catalyst deactivation during the hydrogenation of benzene over nickel-loaded Y zeolites

The hydrogenation of benzene to cyclohexane was investigated over a range of nickel-exchanged and nickel-impregnated Y zeolites, varying the nickel content and the nature of the alkali metal co-cation (Li+, Na+, K+, Rb+ or Cs+). With a view to optimizing benzene conversion levels, the following catalytic parameters were studied: reaction temperature, reaction time, benzene flow rate and coke deposition. The observed catalytic activities are correlated with previously reported physical characterizations. Benzene hydrogenation increased in the order: NiLiY < NiNaY < NiKY < NiRbNaY < NiCsNaY. Catalyst deactivation results from the deposition of involatile coke on the catalyst surface, which is promoted by increasing zeolite acidity. The effects of poisoning the surface Bronsted acid sites by adsorption of ammonia onto the activated reduced zeolites are considered. The results of catalyst regeneration by high temperature oxidation of the coke deposits are also reported.

Benzene ethylation and cumene dealkylation over nickel-loaded Y zeolites

Abstract The alkylation and dealkylation activities of a range of nickel-loaded Y zeolites, prepared by ion exchange and impregnation, have been measured and correlated to the surface Bronsted acidity. The effect on the level of protonic activity of varying the alkali metal co-cation (Li + , Na + , K + , Rb + , or Cs + ) and the inclusion of Ce 3+ ions is discussed. With a vew to optimising catalytic efficiency, a number of parameters were considered: these were catalyst precursor reduction temperature, catalyst precalcination, reaction temperature, acid poisoning by pyridine adsorption, coke deposition, and catalyst regeneration. The presence of nickel metal on the support served to enhance the ethylation and cracking activities by converting coke precursors. In the case of benzene ethylation, diethylbenzenes were formed over the most active samples; the relative distributions of the ortho-, para- , and metaisomers are reported. For comparative purposes, data on benzene ethylation over nickel-impregnated silica and alumina catalysts are also presented.

A characterization study of the surface acidity generated on the calcination and reduction of a range of Y zeolites

Abstract Zeolite acidity, in the form of Bronsted acid sites generated on calcining or reducing a range of Y zeolites, was probed by infrared spectroscopy and the use of Hammett indicators. In particular, the acidity of a range of reduced NiY zeolites was characterized and correlated with reduction temperature and metal loading. The nature of the alkali metal co-cation (Li + , Na + , K + , Rb + , or Cs + is shown to influence both the acid strength and the location of the Bronsted sites. Acid strength decreases in the order NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. Exchange of Ce 3+ ions into NiY generates additional protonic acidity. Ni 2+ reduction levels are inferred from the infrared data. The effect of the adsorption of three bases (ammonia, pyridine, and quinoline) onto the zeolite surface on the level of acidity was also considered.