A nanocrystalline ZSM–5 aggregate having a hierarchical architecture for the catalytic cracking of waste cooking oil

Abstract

Abstract A series of hierarchical ZSM–5 nanocrystalline aggregates (HNZ–5) was synthesized using a hydrothermal method with tetrapropylammonium hydroxide as a structural guide. The morphologies and structures of these materials were characterized by X–ray diffraction, scanning electron microscopy, N2–Brunauer–Emmett–Teller surface area analysis and assessment of pore size distribution. HNZ–5 specimens having different Si/Al molar ratios all showed suitable hierarchical architectures and the maximum surface area was found to be 329 m2/g. Temperature–programmed desorption with NH3 indicated that the total acidity of this material increased with increases in the Al3+ content to a maximum of 0.47 mmol/g at a Si/Al ratio of 15. In addition, 27Al magic angle spinning nuclear magnetic resonance spectra showed that the acid sites were associated with tetrahedral and octahedral Al sites. This HNZ–5 was applied to the catalytic cracking of a model waste cooking oil compound and gave a light olefins yield as high as 43.9% at 550°C. Extending the reaction time to 6000 min provided a yield of light olefins in excess of 30%. These data confirm that the hierarchical pore structure of this HNZ–5 resulting from the self–assembly of nanocrystals, together with exposed acidic sites, promoted the catalytic conversion of large molecules.