Insight into the effects of confined hydrocarbon species on the lifetime of methanol conversion catalysts.

The methanol-to-hydrocarbons reaction refers collectively to a series of important industrial catalytic processes to produce either olefins or gasoline. Mechanistically, methanol conversion proceeds through a 'pool' of hydrocarbon species. For the methanol-to-olefins process, these species can be delineated broadly into 'desired' lighter olefins and 'undesired' heavier fractions that cause deactivation in a matter of hours. The crux in further catalyst optimization is the ability to follow the formation of carbonaceous species during operation. Here, we report the combined results of an operando Kerr-gated Raman spectroscopic study with state-of-the-art operando molecular simulations, which allowed us to follow the formation of hydrocarbon species at various stages of methanol conversion. Polyenes are identified as crucial intermediates towards formation of polycyclic aromatic hydrocarbons, with their fate determined largely by the zeolite topology. Notably, we provide the missing link between active and deactivating species, which allows us to propose potential design rules for future-generation catalysts.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials.

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Determining the storage, availability and reactivity of NH3 within Cu-Chabazite-based Ammonia Selective Catalytic Reduction systems.

Three different types of NH3 species can be simultaneously present on Cu(2+)-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Reduction (NH3-SCR) systems. These include ammonium ions (NH4(+)), formed on the Brønsted acid sites, [Cu(NH3)4](2+) complexes, resulting from NH3 coordination with the Cu(2+) Lewis sites, and NH3 adsorbed on extra-framework Al (EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4(+) ions react very slowly in comparison to NH3 coordinated to Cu(2+) ions and are likely to contribute little to the standard NH3-SCR process, with the Brønsted groups acting primarily as NH3 storage sites. The availability/reactivity of NH4(+) ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu(2+), accompanied by a remarkable enhancement in the low temperature activity. Moreover, the presence of EFAl species could also have a positive influence on the reaction rate of the available NH4(+) ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.

Correction: Correlation between Cu ion migration behaviour and deNOx activity in Cu-SSZ-13 for the standard NH3-SCR reaction.

Correction for 'Correlation between Cu ion migration behaviour and deNOx activity in Cu-SSZ-13 for the standard NH3-SCR reaction' by A. M. Beale et al., Chem. Commun., 2016, 52, 6170-6173.

Correlation between Cu ion migration behaviour and deNOx activity in Cu-SSZ-13 for the standard NH3-SCR reaction.

Here we present the results of a synchrotron-based in situ, time-resolved PXRD study during activation of two Cu-SSZ-13 catalysts under O2/He and one during standard NH3-SCR reaction conditions to obtain insight into the behaviour of Cu ions. The results obtained indicate that deNOx activity is inexorably linked with occupancy of the zeolite 6r.