Title : Catalytic performance of galloaluminosilicates in aromatization of lower alkanes: A comparative study with Ga/HZSM-5
A series of gallium-containing HZSM-5 zeolites with different Ga contents (Ga/(Al+Ga) = 0.1–0.6) were prepared by hydrothermal in situ synthesis and post synthesis. Their catalytic performance were compared in the aromatization of propane, butane and propane/butane mixture (1:1 molar). Galloaluminosilicate obtained via hydrothermal in situ synthesis exhibited high fraction of acidic framework Ga3+ with few dispersed extracrystalline Ga2O3. Ga/HZSM-5 obtained by post synthesis showed the presence of extracrystalline Ga2O3 and/or extra framework gallyl ions. The aromatization performance of Ga-containing HZSM-5 followed the following sequence; galloaluminosilicate > Ga/HZSM-5 (ion-exchange) > Ga/HZSM-5 (impregnation) >> HZSM-5. Optimum aromatization performance over galloaluminosilicate was achieved with Ga/(Al+Ga) ratio of 0.3. Propane conversion reached 50.9 wt% over galloaluminosilicate with Ga/(Al+Ga) of 0.3, as compared to 31.8 and 40.7 wt% for the corresponding Ga/HZSM-5 obtained by impregnation and ion exchange, respectively, at gas hourly space velocity of 1,600 h-1, and 540 ?C. Comparison of aromatic selectivity at the same conversion level (~10.0 wt%) revealed that galloaluminosilicate is more selective than Ga/HZSM-5. The superior performance of galloaluminosilicate was attributed to the presence of highly dispersed-reducible extra-framework Ga2O3 (Lewis-dehydrogenating sites) formed by degalliation in close proximity to zeolitic Bronsted sites. Thus, hydrothermal in situ approach can thus be considered as an effective method for improving the aromatization performance of HZSM-5.