TiO2âAl2O3 mixed oxides have been prepared by cohydrolysis of Ti4+ and Al3+ isopropoxides. Textural and acidâbase properties were investigated using nitrogen physisorption, adsorption of probe molecules followed by FTIR spectroscopy, and test-reactions. Catalysts were found to exhibit high surface areas and pore volumes. Results obtained from adsorbed CO2 probe molecules and COS hydrolysis activity agreed well and showed that the hydroxy basicity of mixed oxides was lower than that of the two oxide components. Pyridine and 2,6-dimethylpyridine were used to titrate Lewis and Brønsted acidic sites, respectively. A linear increase of the number of strong Lewis acid sites with alumina content was observed, whereas the number of Brønsted acid sites reached a maximum for 30% TiO2â70% Al2O3 compared with that expected from the composition and the activity of pure oxides. Comparison between probe-molecule adsorption and test reaction, points out that both types of site are involved in propan-2-ol dehydration. A transformation of Lewis acid sites into Brønsted sites is proposed when propan-2-ol is chemisorbed on Lewis acid sites.
通过Ti⁴⁺和Al³⁺
异丙醇盐的共
水解制备了TiO₂-Al₂O₃混合
氧化物。利用
氮气物理吸附、探针分子吸附后进行FTIR光谱分析以及测试反应,研究了其结构和酸碱性质。发现
催化剂具有高比表面积和孔体积。通过吸附的CO₂探针分子和C
OS水解活性得到的结果一致,表明混合
氧化物的羟基碱性低于两种单一
氧化物成分。分别使用
吡啶和
2,6-二甲基吡啶对Lewis和Brønsted酸性位点进行滴定。观察到随着
氧化
铝含量的增加,强Lewis酸性位点的数量线性增加,而Brønsted酸性位点的数量在30% TiO₂-70% Al₂O₃时达到最大值,超过了单一
氧化物成分预期的数量和活性。探针分子吸附和测试反应的比较指出,两种类型的位点都参与了
丙醇-2-醇的
脱水反应。提出当
丙醇-2-醇
化学吸附在Lewis酸性位点上时,Lewis酸性位点转化为Brønsted酸性位点。