Green Synthesis of ZSM-5 Zeolite Prepared by Hydrothermal Treatment of Perlite. Effect of Chemical Composition and Characterization of the Product

摘要

We studied the influence of the chemical composition of the starting synthesis gel (SiO_2/Al_2O_3, H_2O/SiO_2, amount of seed and pH) on the preparation of ZSM-5 zeolite by hydrothermal treatment of expanded perlite. Gels with molar compositions of SiO_2/Al_2O_3 = 20-55 and H_2O/SiO_2 = 25-45, formed at pH values varying from 10.1 to 10.5, produced ZSM-5 zeolite, crystallizing at 453 K for 24 h in a seeded system. Compared with the conventional autoclave treatment, this method avoids the use of environmental-toxic organic structure directing agents. The highly crystalline ZSM-5 zeolite obtained was characterized employing a variety of analytical techniques, including X-ray powder diffraction, scanning electron microscopy, nitrogen adsorption-desorption isotherms, Fourier transform infrared spectroscopy (FTIR), infrared studies of pyridine sorption, ~(27)Al MAS NMR, ~(29)Si MAS NMR and thermogravimetric analysis. At pH values of 13.0 and 13.3, the MFI phase changed to Phillipsite and Analcime, with intermediate formation of amorphous phase at pH 12.0, rearranging the 5-5-1 elements into S4R and S6R units. It was found that the use of an amorphous and low-cost silicoaluminate as starting material can lead, through an inexpensive methodology, to a highly valuable microporous material with many potential applications.