The deactivation of Sn-Beta zeolite catalyst during retro-aldolization and isomerization of glucose is investigated. Confocal fluorescence microscopy reveals that retro-aldolization of glucose in CH₃OH at 160 °C is accompanied with the build-up of insoluble oligomeric deposits in the micropores, resulting in a rapid catalyst deactivation. These deposits accumulate predominantly in the outer regions of the zeolite crystals, which points to mass transport limitations. Glucose isomerization in water is not only accompanied by the formation of insoluble deposits in the micropores, but also by the structural degradation of the zeolite due to desilication and destannation. Enhanced and sustained catalytic performance can be achieved by using ethanol/water mixtures as the reaction solvent instead of water.

The nature and evolution of the hydrocarbon pool (HP) species during the Methanol-to-Olefins (MTO) process for three small-pore zeolite catalysts, with a different framework consisting of large cages interconnected by small eight-ring windows (CHA, DDR, and LEV) was studied at reaction temperatures between 350 and 450 °C using a combination of operando UV-vis spectroscopy and online gas chromatography. It was found that small differences in cage size, shape, and pore structure of the zeolite frameworks result in the generation of different hydrocarbon pool species. More specifically, it was found that the large cage of CHA results in the formation of a wide variety of hydrocarbon pool species, mostly alkylated benzenes and naphthalenes. In the DDR cage, 1-methylnaphthalene is preferentially formed, while the small LEV cage generally contains fewer hydrocarbon pool species. The nature and evolution of these hydrocarbon pool species was linked with the stage of the reaction using a multivariate analysis of the operando UV-vis spectra. In the 3-D pore network of CHA, the reaction temperature has only a minor effect on the performance of the MTO catalyst. However, for the 2-D pore networks of DDR and LEV, an increase in the applied reaction temperature resulted in a dramatic increase in catalytic activity. For all zeolites in this study, the role of the hydrocarbon species changes with reaction temperature. This effect is most clear in DDR, in which diamantane and 1-methylnaphthalene are deactivating species at a reaction temperature of 350 °C, whereas at higher temperatures diamantane formation is not observed and 1-methylnaphthalene is an active species. This results in a different amount and nature of coke species in the deactivated catalyst, depending on zeolite framework and reaction temperature.

A systematic study of the effect of physicochemical properties affecting catalyst deactivation, overall olefin selectivity and ethylene/propylene ratio during the methanol-to-olefins (MTO) reaction is presented for two zeolites with the DDR topology, namely Sigma-1 and ZSM-58. Both catalysts show high selectivity towards light olefins and completely suppress the formation of hydrocarbons bigger than C4, with selectivity to ethane not exceeding 1% and some traces of propane. By applying seeded growth approach, a series of Sigma-1 zeolites with tunable crystal size and acidity was synthesized. For this series the highest methanol throughput at 450 °C before deactivation was found for crystals 0.5 μm in size with an acidity corresponding to 0.5 Al atoms per zeolite cage, and a selectivity to ethylene and propylene reaching 90%. Comparison between ZSM-58 and Sigma-1 catalysts with similar morphologies and acidity under the same reaction conditions revealed a three times higher throughput of methanol in case of ZSM-58. The analysis of functional surface groups, assessed through FT-IR, revealed the presence of silanol defects in Sigma-1 responsible for faster catalyst deactivation. These silanol defects can be selectively removed (confirmed by FT-IR) from the zeolite framework by applying a mild treatment in presence of NaOH/CTAB, leading to an improved catalyst lifetime. Co-feeding experiments with short olefins and water show low reactivity of primary MTO products, which only react at the surface of the catalyst particles. These results demonstrate that migration of the reaction zone in case of DDR catalysts hardly affects catalyst stability, product composition and nature of deactivating species. The nature of these species depends mostly on reaction temperature: at low temperatures deactivation occurs mainly due to the formation of inert adamantane species, while at high temperatures poly-condensed aromatic hydrocarbons play the major role in deactivation.

The front cover artwork for Issue 19/2016 is provided by researchers from the Catalysis Engineering group at TU Delft (The Netherlands). The image shows that after impregnation with Ca, ZSM-5 becomes a “super-catalyst”, which is able to suppress formation of aromatics thus increasing selectivity to the desired propylene in MTO reaction. See the Full Paper itself at http://dx.doi.org/10.1002/cctc.201600650.

Incorporation of Ca in ZSM-5 results in a twofold increase of propylene selectivity (53 %), a total light-olefin selectivity of 90 %, and a nine times longer catalyst lifetime (throughput 792 g_MeOH g_catalyst ⁻¹) in the methanol-to-olefins (MTO) reaction. Analysis of the product distribution and theoretical calculations reveal that post-synthetic modification with Ca²⁺ leads to the formation of CaOCaOH⁺ that strongly weaken the acid strength of the zeolite. As a result, the rate of hydride transfer and oligomerization reactions on these sites is greatly reduced, resulting in the suppression of the aromatic cycle. Our results further highlight the importance of acid strength on product selectivity and zeolite lifetime in MTO chemistry.

In this Perspective, we highlight the main challenges to be addressed in the development of heterogeneous catalysts for the direct functionalization of methane. Along with our personal view on current developments in this field, we outline the main mechanistic, engineering, and catalyst design issues that have hampered implementation of new technologies and highlight possible paths to overcome these problems.