Studies On Al- And Ga-Promoted Sulfated Zirconia For Alkane Isomerization

Owing to environmental restrictions on emissions, the transformation of linear light alkanes into their branched isomers plays an important role in improving the octane number in clean gasoline. Since the discovery of its strong acidity, sulfated zirconia has attracted much attention as a potential isomerization catalyst. The main reason is associated with its advantage in lower hazards such as toxicity and corrosivity, with respect to liquid acid catalysts. Although SZ displays good initial activity for catalyzing isomerization, the fast deactivation at high temperature prevents its application in industry. Therefore, it is important to enhance the stability and activity by modifying sulfated zirconia.In this paper, the work was mainly focused on the preparation of the solid acid catalysts SO₄^2-/Ga₂O₃/ZrO₂ and SO₄^2-/Al₂O₃/ZrO₂. The structural information, the acidities and redox properties of Al, Ga-promoted sulfated zirconia was studied by characterizations. The catalytic activity of Al, Ga-promoted sulfated zirconia was examined in light alkane isomerization and the reasons for the enhancement in activity and stability were discussed based on the experimental results.The characterization of Al, Ga-promoted sulfated zirconia was carried out using XRD, TEM, TG, N₂ adsorption-desorption techniques, NH3-TPD, IR, H₂-TPR and chemical analysis. The results showed that the Al, Ga-doped sulfated zirconia can prevent the grain growth and stabilize the tetragonal phase. The gallium and aluminum can retard the decomposition of sulfate groups so that it has a positive effect on retaining surface sulfate. The addition of Al increased bronsted acid sites while it had no effect on redox properties. However, the addition of Ga impacted redox properties and it had no effect on acid properties.To understand the effect of digestion of Zr(OH)₄ on catalytic activity of sulfated zirconia. The precursor was produced by digestion of the hydrous oxide at 100 ℃ forvarious times. The structure, surface area and thermal stability were studied and the catalytic activity of catalyst was examined in n-butane isomerization. The results showed the transformation from the metastable tetragonal phase to the monoclinic phase was retarded and the surface area increased with digestion. However, the hydroxyl of Zr(OH)4 decreased with long time digestion so that it had a passive effect on retaining surface sulfated. Hence, the sulfated content of catalyst decreased with 96 h digestion of Zr(OH)4. The n-butane isomerization reaction on these catalysts was investigated and the excellent catalytic activity was obtained for the sample digested 24 h.The catalytic activity of the Al, Ga-promoted sulfated zirconia was examined in n-butane isomerization and the promotion mechanism of gallium was discussed based on the experimental results. The experiment results showed that the Al, Ga-promoted sulfated zirconia exhibited excellent catalytic activity and stability. In particular, the n-butane conversion on the catalyst containing 3 mol% Ga2C>3, calcined at 700 °C remained at 51% which is closed to its equilibrium conversion after running on stream for 360 min. The n-butane isomerziation reaction on these catalysts was investigated at different reaction temperature and the results showed that the promotion effect of Al and Ga was different. According to the characterization results, it was considered that the improvement of activity by Al was mainly caused by the increase of Bronsted acid sites while the improvement of activity by Ga was related to the higher oxidization ability of Ga(ffi).Isomerization of n-pentane was studied over these catalysts in presence of H2. hi comparison to sulfated zirconia, both promoted catalysts showed improved catalytic activity and selectivity. In particular, the n-pentane conversion on the Al-promoted catalyst remains at 80 %, which is closed to its equilibrium conversion after running on stream for 360 min. The conversion of n-hexane over Al, Ga-promoted sulfated zirconia was greatly improved and the selectivity of 2, 2-DMB was increased as well.Furthermore, the simultaneous promotion of Pt and Ga enhanced catalytic stability and isomer selectivity. The n-pentane and n-hexane isomerization reaction over Al, Ga-promoted sulfated zirconia were studied at higher reaction temperature. It was noted that a large amount of cracking products were observed over Al-promoted catalyst and it resulted in coking on catalytic sites. In contrast, a small amount of cracking products were observed over Ga-promoted catalyst. This result" provided another evidence that the promoting mechanism of gallium was different from that of aluminum.