Heterogeneous Catalysts From Rice Husk Modified With Chromium, Molybdenum And Tungsten : Synthesis, Characterization And Application In Styrene Oxidation
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Date
2011
Authors
Mohamed Iqbal, Mohammad Anwar
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Publisher
Universiti Sains Malaysia
Abstract
A series of chromium, molybdenum and tungsten incorporated silica extracted from rice husk were prepared at pH 10, pH 7 and pH 3 media using solvent extraction method and sol-gel technique. Liquid phase oxidation of styrene using hydrogen peroxide as oxidant was used as a probe to test their catalytic potential. The incorporation of chromium into the silica matrix resulted in amorphous catalysts labeled RHCr-10, RHCr-7 and RHCr-3. RHCr-3 has the highest surface area (564 m2 g-1) but lowest chromium loading (2.3 w/w %). RHCr-3 showed better structural and catalytic properties compared to RHCr-7 and RHCr-10. About 99.9 % of styrene was successfully oxidized with 63.1 % selectivity towards benzaldehyde. Acidic pH of the reaction medium, higher surface area and the existence of Cr3+ and Cr6+ were predicted to increase the catalytic activity of RHCr-3. Re-characterization of used RHCr-3 showed slight changes in chromium loading, surface area and pore volume due to leaching. RHCr-3 is proposed to catalyze the reaction via Cr3+ and Cr6+ species. The incorporation of molybdenum into the silica matrix resulted in amorphous catalysts labeled as RHMo-10, RHMo-7 and RHMo-3. Physico-chemical characterization on these catalysts indicates that molybdenum species prefer to be on the surface of the support. Highest molybdenum percentage was found in RHMo-3 (7.6 w/w %) whereas the surface area of RHMo-3 was the lowest (445 m2 g-1). The existence of Mo5+ and Mo6+ species had increased the activity of RHMo-3. About 82.2 % of styrene was oxidized and benzaldehyde selectivity was 82.8 %. RHMo-3 faced massive loss in its catalytic activity due to leaching. Re-characterization of used RHMo-3 indicates rapid changes in its surface area, pore volume, pore size distribution and surface active sites. The oxidation reaction is proposed to proceed via permolybdic acid like intermediate. Tungsten incorporation into the silica matrix resulted in amorphous RHW-10 and RHW-7 and semi-crystalline RHW-3 catalysts.
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Keywords
Heterogeneous catalysts from rice husk modified , with chromium, molybdenum and tungsten