Effects of HZSM-5 crystallite size on stability and alkyl-aromatics product distribution from conversion of propanal

The conversion of propanal on large (2–5 µm) and small (0.2–0.5 µm) crystallite HZSM-5 at 400 °C and atmospheric pressure has been studied. Improved catalyst stability was observed on small crystallites due to faster removal of products with the shorter diffusion path length, reducing the formation of coke precursors. As previously shown, C9 aromatics are the initial aromatics produced from propanal via aldol condensation followed by cyclization and these have less opportunity to crack to lighter aromatics on the small crystallites. Thus, a higher ratio of C9/(C8+ C7) aromatics was observed on the small crystallites. The main isomer of the C8 aromatic products observed on small crystallites was the initial cracking product of the C9 aromatics, and thermodynamically preferred, meta-xylene, while the shape-selective preferred para-xylene was the predominant product on large crystallites. The higher internal diffusion rate of the para isomer results in greater shape selectivity with the longer path of the large crystallite zeolite. It is concluded that the use of smaller crystallite HZSM-5 improves results for production of alkyl aromatics from light oxygenates at mild conditions that may prove useful for bio-oil upgrading.