G.A. Sill

Studies of the selective reduction of nitric oxide by hydrocarbons

Abstract The selective reduction of nitric oxide with isobutane in the presence and absence of oxygen has been studied over Cu-ZSM-5-14-114 and compared to the simple decomposition reaction. In the presence of sufficient oxygen, complete combustion of the hydrocarbon was observed, whereas in its absence, acid catalysis accompanied by dehydrogenation associated with coking occurred. The effect of adding nitric oxide to the HC/He stream in the absence of oxygen was small; the conversion to nitrogen was little affected by the hydrocarbon. When oxygen was added, however, the decomposition was complete at 573 K even at high space velocities (e.g., SVH ∼ 10 4 ) even though the combustion was not. Isobutane (an alkane) was more effective than C 3 H 6 (an alkene) for the selective reduction; this is not generally the case. The presence of excess oxygen inhibited aging of the catalyst.

The assay of acid sites on zeolites as measured by ammonia poisoning

Abstract The decomposition of neopentane was studied with and without selective site poisoning with NH3. The lethal dose, which was just sufficient to kill the catalytic activity, was estimated for each of a series of H-mordenites, HY and HZSM-5 zeolites, all with Si Al ratios higher than 7.0, i.e., over silica-rich compositions where ideally all the sites are supposedly identical. It was found that poisoning no more than 10% of the sites (counted as lattice aluminum ions) was sufficient to eliminate the activity. Some ramifications of these observations are discussed. Unpoisoned and partially poisoned zeolites were investigated using 13C-MASNMR following adsorption of isopropanol-2-13C. These findings clearly show the efficacy of adsorbed NH3 in repressing formation of polymeric surface residues.

Study of the mechanism of the cracking of small alkane molecules on HY Zeolites

Cracking of i-butane and n-pentane was studied on HY zeolites. These reactions were initiated by the protonation of C-H and C-C bonds by the Bronsted acid sites. The pentacoordinated carbonium ions thus formed decomposed into carbenium ions and the products of the initiation reactions, viz., H2, CH4, and C2H6. These carbenium ions propagated chain reactions, mainly by isomerization followed by hydride transfer. Disproportionation occurred concomitantly. For these small alkanes the contribution of /gb-scission to product formation was negligible. “Chain length” (the number of chain cycles per initiation) was defined as the ratio of reactant molecules consumed by hydride transfer to those reacted by protonation, i.e., the ratio of rates of bimolecular propagation to unimolecular initiation. Thus, chain length reflected the average lifetime of the carbenium ions. The extent of protonation was found to increase with the strength of acid sites while the chain length remained relatively constant for preparations of a similar nature. The product distribution obtained was therefore critically dependent on the steady-state population of carbonium ions. Finally the chain reactions were terminated by decomposition of carbenium ions into corresponding alkenes. Mass balances derived from these initiation, propagation, and termination steps were in agreement for both the substrates. The product distributions obtained for i-butane and n-pentane cracking were satisfactorily explained on this basis.

The selective reduction of NO and combustion of paraffins over MFI zeolites

Abstract Paraffins of increasing complexity have been studied as reducing agents in the SCR of NO over a Na-ZSM-5 zeolite containing Co 2+ , Ni 2+ or Cu 2+ base-exchange cations. A correlation was established between the rates of N 2 and CO 2 formation which was independent of the hydrocarbons employed. The data suggest that NO participates selectively in the combustion process and that the two reactions are coupled.

On the role of free radicals NO2 and O2 in the selective catalytic reduction (SCR) of NOx with CH4 over CoZSM-5 and HZSM-5 zeolites

The reactions of CH 4 over CoZSM-5 and HZSM-5 zeolites with the mixtures of NO+O 2 and NO 2 +O 2 and with three oxidizing components separately were studied. Differential reaction rates were determined. Comparison of the “light-off” temperatures as well as the activation energies of these reactions led to the conclusion that the SCR of NO x into N 2 and, consequently, CH 4 oxidation into CO x are initiated by the reaction of CH 4 with NO 2 . At low temperatures (300–400°C) O 2 does not compete with NO x for CH 4 , and its role is limited to the oxidation of NO into NO 2 . However, at higher temperatures a strong competition between NO x and O 2 for CH 4 results in a decrease in the selectivity of the SCR process. It is shown that this competition is stronger with CoZSM-5 catalyst than with HZSM-5, and this explains the higher selectivity observed with the latter catalyst. Based on these observations the formation of the CH 3 • free radical is postulated and possible pathways of the SCR of NO x into N 2 are discussed.

The Concentration of Catalytically Active Bronsted Sites on Zeolites

The effects of NH 3 -poisoning and of variation of the temperature and catalysts on the rates and selectivities in isobutane decomposition have been investigated. The addition of NH 3 equivalent to less than 10% of the Al T-sites was sufficient to eliminate over 90% of the catalytic activity. Also, the rates of the primary reactions yielding H 2 and isobutene, together with CH 4 and propene, were less affected than those of secondary carbenium ion processes. In contrast, raising the temperature increased both the overall conversion rates and the per cent primary reaction. Secondary reactions increased with the “acidity” of the zeolite. The underlying chemistry leading to these observations is discussed.