Green and cost-effective preparation of small-sized ZSM-5

Small-sized zeolite ZSM-5 for a wide SiO₂/Al₂O₃ ratio range was prepared using a small amount of colloidal silicalite-1 as the active seeds. The thus-prepared small-sized ZSM-5 samples have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption analysis, temperature-programmed ammonium desorption (NH₃-TPD) analysis, and adsorbed pyridine infrared spectroscopy (Py-IR). The use of the active silicalite-1 seeds was effective in directing the reaction towards the formation of the MFI phase, avoiding the impure phases and reducing the crystal sizes. The prepared sample exhibited aggregated morphologies when a lower ratio of starting batch SiO₂/Al₂O₃ (SiO₂/Al₂O₃ ratio = 30) was used. The aggregates, with the size of ~500 nm, were formed with nano-sized primary crystals 50 nm in size, possessing large external surface area (84.9 m^2·g^s−1) and secondary pore volume (0.22 cm^3·g−1) and relatively regular mesopores. Different morphologies could be observed when the SiO₂/Al₂O₃ ratio was increased (SiO₂/Al₂O₃ ratio = 60–120). ZSM-5 with the size of 200 nm could be prepared, with the external surface area and the secondary pore volume being ~60 m^{2·g −1} and 0.10 cm^3·g−1, respectively. It should be highlighted that all the prepared samples could be directly ion-exchanged to obtain the acidic H-form samples without complete blocking of the micropores due to the low dose of the organic structure-directing agent. The obtained acidic H-form samples exhibited acidic properties similar to the samples ion-exchanged after calcination and the conventional ZSM-5 possessing similar SiO₂/Al₂O₃ ratio, showing catalytic performance comparative to the catalytic conversion of methanol to olefins. Compared with conventional methods, this method largely reduced the use of organic templates and avoided the subsequent combustion before ion-exchange. The method is green and cost-effective, possessing wide potentials in the industrial processes.