M-Xylene Isomerization Over Silicon-Modified HZSM-5Zeolite Catalyst

HZSM-5zeolite was modified by chemical liquid deposition (CLD) with tetraethyl orthosilicate (TEOS). A series of zeolites modified by different content of TEOS were prepared for m-xylene isomerization. The reaction performances of these zeolites were evaluated using an isothermal fixed bed integral reactor. The suitable deposition conditions for modified HZSM-5were investigated including the CLD solvent type,the amount of CLD agent, CLD time, CLD cycle numbers. The results showed that with the increase of SiO2deposition, the conversion of m-xylene and xylene loss decreased, para-xylene selectivity and PX/OX radio increased. The appropriate amount of TEOS was between0.04g/g and0.05g/g for the single deposition. The appropriate CLD time was more than6h. The appropriate CLD solvent type was cyclohexane. Multi-cycle deposition could improve the effectiveness of SiO2deposition. The properties of HZSM-5and modified HZSM-5were characterized by means of XRD, NH3-TPD, Py-IR, FT-IR, BET and SEM. The results showed that TEOS mainly deposited on the external surface of HZSM-5which formed SiO2after calcination. The acid sites on the external surface of HZSM-5were deactivated and the pore openings of HZSM-5were narrowed by the formed SiO2without affecting the internal structure of HZSM-5.The m-xylene isomerization over acids and alkali modified HZSM-5was preliminary discussed in this paper. The conversion of m-xylene decreased but para-xylene selectivity increased over the inorganic acids-treated HZSM-5. No effect was observed by using the organic acid. The conversion of m-xylene increased but para-xylene selectivity decreased on the alkali-treated HZSM-5, the amount of xylene loss was significantly improved.A series of shaped HZSM-5catalyst with different amount of HNO3, different kinds of hole-expanding agents, different amount of PEG20000were modified by TEOS. The mechanical strength and catalyst properties of these shaped HZSM-5catalyst were investigated. The results showed:(1) The conversion of m-xylene decreased and para-xylene selectivity increased with the amount of HNO3increasing; The mechanical strength increased first and then decreased with the amount of HNO3increase.(2) The conversion of m-xylene and para-xylene selectivity were both high with using PEG20000as hole-expanding agent.(3) The conversion of m-xylene increased while the mechanical strength decreased with the amount of PEG20000increase. The appropriate amount of PEG20000was between5%and10%.