Shape-selective catalysis by phosphate modified ZSM-5: Generation of new acid sites with pore narrowing

• Phosphate treatment generated new kinds of acid sites with pore narrowing.
• Optimization of P content to form monolayer islands of phosphates on ZSM-5.
• Decrease in pore volume by phosphates has a direct correlation with shape selectivity.
• P-modification remarkably enhanced shape selectivity while retaining good activity.
• Constraint index for modified catalysts showed the extent of pore modification.

Pore narrowing of ZSM-5 was carried out by phosphate modification technique and shape selectivity was evaluated for alkylation, disproportionation reactions and competitive reaction of ethylbenzene and meta-xylene. Catalysts were characterized by XRD, FTIR, MAS NMR, NH3-TPD, PZC etc. Post synthesis modification of ZSM-5 by phosphate treatment generated new kinds of acid sites with pore narrowing. Even though there was a change in acid site strength, these active sites efficiently catalyzed toluene alkylation with methanol, ethylbenzene alkylation with ethanol and disproportionation of ethylbenzene. These active sites formed monolayer islands over catalyst surface and narrowed the pore opening resulting in high para-selectivity. Decrease in pore volume by phosphate modification has a direct correlation with increase in shape selectivity. Constraint index for modified catalysts obtained from competitive reaction of ethylbenzene and m-xylene was used to determine extent of pore narrowing by phosphate modification.

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