Date on Master's Thesis/Doctoral Dissertation

5-2011

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Carreon, Moises A.

Author's Keywords

Mesoporous materials; Catalysis; Gallium oxide; Epoxidation; Gallium-niobium oxide; Isomerization

Subject

Molecular sieves; Catalysts

Abstract

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. EISA led to partially crystalline meso porous 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 11m) 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 (l30°C) and short reaction times (~15-120 min). Novel semicrystalline 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 11m 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. 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 11m led to higher cyclooctene conversion, whereas the agglomerate sizes between 4.5-7.5 11m 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.

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