ZSM-5 zeolite crystals with secondary mesopores were synthesized by alkaline desilication and surfactant-induced re-assembly of dissolved species (i.e., silicates, aluminosilicates and zeolite crystal fragments) originating from the parent ZSM-5 crystals. The meso-zeolite products exhibit a dual-mesopore structure in which the smaller mesopores (ca. 3 nm) are attributed to surfactant-induced micelle formation involving dissolved species, and larger mesopores (ca. 10-30 nm) result from desilication processes occurring under the alkaline reaction conditions. The external surface area (i.e., the surface area due to mesopores, macropores and the external particle surface) of the meso-zeolite materials depends on the Si/Al ratio, the hydroxide concentration and the presence of surfactant, and it reaches values as high as 327 m 2 g-1 when a surfactant is used. The crystallinity of the highest surface-area meso-zeolite is well preserved, maintaining values of ca. 83% (on the basis of micropore volume) or ca. 77% (on the basis of X-ray diffraction intensities) of the parent zeolite structures. Further physicochemical characterization by 27Al and 29Si magic-angle-spinning solid-state NMR spectroscopy, scanning and transmission electron microscopy, temperature-programmed ammonia desorption measurements, and inductively coupled plasma elementary analysis support the hypothesis that re-assembly of dissolved species of zeolite crystals occurred by surfactant-induced micellization, resulting in the high external surface area of the meso-zeolite materials.
Bibliographical noteFunding Information:
Funding was provided by the NSF (mainly by CMMI-0707610 and in parts by DMR-0704312 , DMR-0212302 and CBET-0522518 ). Parts of this work were carried out in the Institute of Technology Characterization Facility, University of Minnesota, which receives partial support from the NSF through the NNIN program and has received capital equipment funding from the NSF through the MRSEC, ERC, and MRI programs. We thank Professor G. Veglia and Dr. N. Traaseth for obtaining the 27 Al and 29 Si MAS NMR spectra at the University of Minnesota Nuclear Magnetic Resonance Facility, Shanita Jones and Lauren Reimer at Micromeritics for the TPD measurements, and Dr. Jason Myers for the EDS analysis.
- Mesoporous zeolite