The influence of zeolite surface-aluminum species on the deactivationof CuZnAl/zeolite hybrid catalysts for the direct DME synthesis

Abstract

[EN] The influence of the nature and amount of zeolite-surface Al species on the deactivation behavior ofbifunctional CZA/zeolite hybrid catalysts during the direct DME synthesis (260◦C, 4.0 MPa) from syn-gas (66% H2, 30% CO, 4% CO2) has been studied. To this aim, a series of delaminated ITQ-2 zeolites hasbeen prepared by acid treatment and steaming of an Al-ITQ-2 (Si/Al = 12) sample as well as by impreg-nation of an all-silica Si-ITQ-2 sample with Al(NO3)3(aq.) and calcination, and used as the methanoldehydration function of the hybrids. When prepared by grinding, all the hybrid catalysts experienced aloss of activity with time during the syngas-to-DME reaction under conditions where the synthesis ofmethanol on the copper catalyst (CZA) controlled the overall DME synthesis rate. The decay was neitherrelated to changes in the properties of the Cu species (dispersion, Cu0surface area) nor to depositionof carbon species on the metallic function (as ascertained by TPO) but to a particular mode of deacti-vation of the CZA catalyst due to adverse interactions between the copper component and the zeolite.Interestingly, the extent of the deactivation was seen to increase linearly with a parameter ˇ, arbitrarilydefined as ˇ = Sext× [Al/(Si + Al)]surf× %EFAL/100, where Sextis the external surface area of the zeolite,[Al/(Si + Al)]surfis the relative Al surface concentration (from XPS), and %EFAL is the percentage of penta-and hexacoordinated extraframework Al species with respect to the total Al detected by27Al MAS NMR.The correlation suggests that EFAL species located at the surface contact between the CZA and zeoliteparticles were primarily responsible for this particular mode of deactivation of the copper catalyst. Wehypothesize that EFAL species could migrate to the CZA catalyst during DME synthesis through a water-assisted mechanism, modifying the interaction between Cu and ZnOxat the Cu ZnOxinterface where theactive copper sites are likely located