Citrate-mediated sol–gel synthesis of Al-substituted sulfated zirconia catalysts for α-pinene isomerization

• Published in: Molecular catalysis Vol. 458; pp. 206 - 212
• Main Authors: Rabee, Abdallah I.M., Durndell, Lee J., Fouad, Nasr E., Frattini, Lucia, Isaacs, Mark A., Lee, Adam F., Mekhemer, Gamal A.H., Santos, Vannia C. dos, Wilson, Karen, Zaki, Mohamed I.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2018
• Subjects: Citrate mediated sol-gel; Solid acidity; Sulfated zirconia; Terpenes; α-pinene; α-pinene; Terpenes; Solid acidity; Citrate mediated sol-gel; Sulfated zirconia
• ISSN: 2468-8231; 2468-8231
• DOI: 10.1016/j.mcat.2017.10.029

[Display omitted] •A new route to sulfated ZrO2, and Al-substituted ZrO2 (SO4/AlxZrO2) reported.•Citric acid-mediated sol-gel synthesis generates Al3+ substituted t-ZrO2.•Low Al concentrations enhance α-pinene isomerization activity by up to 50%.•Acid strength & selectivity to mono and poly cyclic isomers tuned with Al content. Solid acids are desirable heterogeneous catalysts for green chemistry, with zirconia and its sulfated analogues offering tunable Lewis/Brønsted character. A new route to sulfated ZrO2, and Al-substituted ZrO2 (SO4/AlxZrO2), via carbonization and calcination of metal citrate gels and their subsequent sulfation by (NH4)2SO4 is reported. Structural and acidic properties of these materials were characterized by XRD, Raman, XPS, TGA-MS, N2 porosimetry, and propylamine and pyridine titration. Parent and sulfated materials all adopted the tetragonal zirconia phase, with Al-substitution (evidenced by contraction of the zirconia lattice parameter) at between 0.38–3.50wt% increasing the surface area but decreasing acid strength, accompanied by an overall increase in total acid site loading and Lewis character. Low Al concentrations enhance α-pinene isomerization activity by up to 50% due to the increased acid site loading, however the Turnover Frequency (TOF) remained constant indicating a common active site. The TOF decreased for Al concentrations >3.5wt% due to a further fall in acid strength, manifest as higher selectivity to polycyclic versus monocyclic products.