Rational design of mesoporous gallium oxide and gallium-based mixed oxide catalysts.

摘要

In the present study, we report the synthesis of thermally stable mesoporous gallium oxide and novel gallium-niobium mixed oxides employing Evaporation-Induced Self-Assembly (EISA), Self-Assembly Hydrothermal-Assisted (SAHA) and Self-Assembly Microwave-Assisted approaches. These methods offer the possibility to synthesize thermally stable mesoporous oxides with controlled morphological, textural and structural properties.;For the epoxidation of cyclooctene to epoxycyclooctane carried out at 60°C the mesoporous gallium oxide displayed 100% selectivity towards epoxide with the conversion of cyclooctene in the 4 to 16% range. As the reaction temperature was increased to 80°C, an increase in the cyclooctene conversion was observed. The highest cyclooctene conversion observed was ∼52% with a selectivity of 83% toward the epoxide. A clear correlation was observed between the cyclooctene conversion and gallium oxide particle size at both reaction conditions. Agglomerate size between 2--3 mum led to higher cyclooctene conversion, whereas the agglomerate sizes between 4.5--7.5 mum led to lower cyclooctene conversions. For the isomerisation of methyl oleate, highest conversion of 57% with the selectivity of 86% and yield of ∼50% was observed over a sample with gallium-niobium composition of 0.3:0.7 wt%. The superior catalytic performance of the gallium-niobium mixed oxide was attributed to its high acidity, crystallinity and mesoporosity.;EISA led to partially crystalline mesoporous gallium oxide phases displaying unimodal pore size distribution in the ∼2--5 nm range and surface areas as high as 300 m2/g. SAHA led to nanocrystalline mesoporous uniform micron-sized gallium oxide spheres (∼0.3--6.5 mum) with narrow size distribution displaying cubic spinel type structure. These mesophases displayed surface areas as high as 220 m2/g and unimodal pore-size distribution in the 5--15 nm range. Microwave-assisted approach led to the formation of nanocrystalline mesoporous gallium oxide phases at low reaction temperature (130°C) and short reaction times (∼15--120 min). Novel semi-crystalline mesoporous Gallium-Niobium mixed oxide phases were prepared via Self-Assembly Hydrothermal-Assisted (SAHA) method. This method led to the formation of uniform ∼ 0.3--2 mum micron-sized mesoporous mixed gallium-niobium oxide spheres with narrow size distribution displaying surface areas as high as 360 m2/g and unimodal pore size distribution in the 3--6 nm range. Due to their high surface areas, tunability of pore sizes and their acidic nature these single phase and mixed mesoporous gallium-niobium oxides were employed as catalysts in the epoxidation of cyclooctene and isomerization of methyl oleate.