Abstract: The common zeolite catalYst HZSM-5 is modified by treating with phosphoric acid to improve its reaction activity towards o-chlorotoluene isomerization. The reaction is carried out at atmospheric pressure and in a microreaetor with o-continuous flow of nitrogen gas. The modified catalyst P-HZSM-5 shows a higher activity in chlorotoluenxc isomerization and gives a higher yield of P-isomer than HZSM-5 under the same reaction conditions.

Key words: Isomerization, Chlorotolucne, Zeolite catalyst, Phosphoric acid