Improving thermal stability and textural properties of mesoporous γ-alumina granules by Zr-La dopants

Abstract

Improving the thermal stability and modifying the porous properties of γ-alumina by incorporating various transition and rare earth metals in alumina have been widely studied. However, less attention has been paid to investigating the co-doping process for increasing the alumina thermal stability and surface area in temperatures ranging from 1000° to 1200°C. In this research, pure, Zr-doped, and Zr-La doped granules with an average diameter of 1.5–2 mm were synthesized using the sol-gel granulation method. XRD results indicated that the introduction of either Zr or Zr-La dopants improves the thermal stability of transition aluminas up to 1200 °C. DTA/TG results revealed that the incorporation of 1 wt% zirconium along with 1 wt% lanthanum retarded α-Al2O3 phase transformation temperature to around 1335 °C. Compared to the pure sample, the BET surface area of alumina containing 1 wt% zirconium, 1 wt% zirconium along with 1 wt% lanthanum, and 1 wt% zirconium along with 3 wt% lanthanum increased to 234.4, 232.4, and 215.6 m2/g at 750 °C, respectively. Moreover, the sample containing 1 wt% of Zr along with 1 wt% of La dopants maintained a higher specific surface area after calcination at 1000 °C (133.6 m2/g) and 1200 °C (64.7 m2/g). According to NMR and HRTEM investigations, the capability of the co-doped Zr-La alumina to preserve the mesoporous structure and thermal stability as well as suppressing α-alumina formation up to 1200 °C is the result of zirconium and lanthanum dopants occupying tetrahedral and octahedral vacancies in the structure of transition alumina. The reported Zr-Al co-doped alumina granules in this study with high thermal and porous structural stability can be good candidates as a support for industrial catalysts.