Comparative Study of Acidic Properties of UZM-5 Nanosheets by Stepwise Adsorption Assisted by a Selective External Poisoning Approach

UZM-5 zeolite was hydrothermally synthesized using a TMA(+)/TEA+ mixed-template as a structure-directing agent (SDA). The resulting sample displays a sheet-like morphology and a uniform particle size of 100-300 nm. The thickness of the particles is ca. 15 nm, corresponding to 5 unit cells. UZM-5 has an 8-membered ring (8-MR) micropore system with a pore diameter of 3.8 angstrom, a secondary mesopore system with a narrow pore size distribution centered at 3.5 nm, specific surface area of 576 m(2)/g and significantly large external surface area of 122 m(2)/g. The nature and the location of acid sites were determined from the temperature-programmed desorption (TPD) of basic probes with various sizes and an FT-IR method, combined with topological analysis. It was found that the density of the acid sites on the external surface of particles is higher than that in the micropores. Regarding the acid sites on the external surface of particles, the highly exposed double 4-ring (D4R) units represent dominantly strong Bronsted-type acidity, while the strong Lewis type acid sites distribute mainly inside and/or very close to the wbc cages at the bottom of the 16-membered ring (16-MR) cavities. Besides the topological constraints, the separated acid sites on the large external surface of UZM-5 particles play a negative role on its inferior catalytic performance in the methanol-to-olefin (MTO) process.