Although the local geometry of Mo in Mo/HZSM-5 has been characterized before, we present a systematic way to manipulate the configuration of Mo and link it to its catalytic properties. The location and geometry of cationic Mo-complexes, the precursor of the active metal site for methane dehydroaromatization, are altered by directing the way they anchor to the framework of the zeolite. The feature used to direct the anchoring of Mo is the location of Al in the zeolite framework. According to DFT calculations, the local geometry of Mo should change, while UV-vis and pyridine FTIR spectroscopy indicated differences in the dispersion of Mo. Both aspects, however, did not influence the catalytic behavior of Mo/HZSM-5, indicating that as long as enough isolated Mo species are present inside the pores of the zeolite, the catalytic behavior is unaffected. This paves the way to better understand how the Mo oxo precursor transforms into the active phase under the reaction conditions.

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.

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.