Current Finding

From Rice Husk to Heterogeneous Catalyst

Rice husk is a waste product of the agriculture activity in most countries in Asia and in particular Malaysia. Rice husk has posed a major problem of disposal to the rice milling industry in Malaysia and elsewhere in the world. Efforts have been made in the past 20 years to use rice husk and its ash in various ways [1]. Moreover, efforts have been made in recent times to extract useful products from rice husk. It has been shown by many researchers that rice husk contains 15 – 20% silica (SiO2), which contributes to its hard and abrasive protective casing covering the rice grain. The silica in rice husk can be obtained by removing the organic components through control burning in muffle furnace in the laboratory. Our recent studies have, however, yielded a more environment friendly technique, whereby the silica from rice husk is obtained by solvent extraction (i.e. without the need for burning) [2]. This chemical procedure has been developed in our laboratory and we have filed a patent for the process. The silica obtained was found to have a high specific surface area of 300 – 400 m2g-1. The purity was determined by x-ray fluorescence and it was shown to be 99.99% pure silica. In contrast, the pyrolysis process produces 93-95% silica. This can be further purified to > 99% by treating the silica with mineral acids like HCl, H2SO4 and HNO3. The silica, from pyrolysed rice husk-ash (RHA) has been shown to adsorb fatty acids from palm oil [3]. A model study showed the adsorption of fatty acids could be described by the Langmuir isotherm [4].

Currently, we are studying the rice husk ash as a possible heterogeneous catalyst. Several interesting results have emerged form our studies. The RHA has been modified with selected transition metals by co-precipitating the sodium silicate and the metal salt in nitric acid so that the metal can be incorporated into the silica matrix. Tests by SEM and EDX on the solids obtained show that the transition metals are indeed incorporated chemically into the silica matrix. The resulting solid was sometimes amorphous and at other times crystalline depending on “the speed of titration (neutralization)”. Fig. 1 shows the SEM micrographs of RHA and the iron-incorporated catalyst RHA-Fe. Initial experiments conducted show promise for these new materials as heterogeneous catalyst. The Fe loaded RHA-Fe was used in the Friedel-Craft benzylation of toluene with benzyl chloride [5].

Fig. 2 shows the GC spectrum of the reaction products. The peak at 1.09 is toluene, which was present in excess. Benzyl chloride appeared at 2.33, which has been completely converted to products. The ortho and para- substituted products were successfully separated in the GR chromatogram. The mono substituted product accounted for 93 % while the di-substituted product yield was 4 %. Recently 4-(methylamino)benzoic acid was incorporated together with Fe [6] to form RH-Fe(amine). This catalyst was shown to reduce the di-substituted products to ca. than 1%.

In a related study, the incorporation of silver into RH resulted in RH-Ag. This resulted in the formation of a narrow pore catalyst as shown in Fig. 3.

This catalyst was successfully used in the oxidation of benzyl alcohol to benzaldehyde and dibenzyl ether.

It was shown that under different condition of the experiment, product (1 or (2) may predominate [7].

The chromium incorporated RH-Cr and RH-Cr(amine) was found to give ca. 100 % conversion in the oxidation of cyclohexane into cyclohexanol and cyclohexanone as shown bellow:

This is an important discovery as cyclohexane conversions were reported to be never more than 50 % in the literature. That too in many cases, the reaction were in the gas phase at elevated temperatures and pressure. However, in our case the reaction was conducted simply in a round bottom flask and refluxed at the boiling point of the solvent, acetonitrile. The product distribution achieved in the above reaction was ca. 50 % each. In our ongoing research, we are looking into the selectivity and the mechanism of some of these catalyst.

References:

1.        Farook Adam, Ismail Ab. Rahman and M. I. Saleh, (1989). “Production and characterization of rice husk ash as a source of pure silica”. Seramik Nusantra: Proceedings of the first National Seminar on Ceramic Technology, pp. 261-273.

2.        Farook Adam and Fua Hock Kiong, Unpublished results, (2003).

3.        Farook Adam, MSc Thesis, Universiti Sains Malaysia , (1992).

4.        F. Adam and S. Ravendran, J. Am. Oil Chem. Soc., 77 (2000) 437-440.

5.       Farook Adam, Kalaivani Kandasamy and Saraswathy Balakrishnan, “Iron incorporated heterogeneous catalyst from rice husk ash”. J. colloid and Interface Sci., 304 (2006) 137 – 143.

6. Farook Adam and Jeyeshelly Andas, “Amino benzoic acid modified silica - an improved catalyst for the mono substituted product in the benzylation of toluene with benzyl chloride”. J. Colloid and Interface Sci. Accepted for publication (2007).

7. Farook Adam, Adil Elhag Ahmed and Sia Lih Min, “Silver modified porous silica from rice husk and its catalytic potential”. Journal of Porous Material. Accepted for publication (2007).

Consultancy/Public Service

The School of Chemical Sciences encourage cooperation between its researchers and the industrial sector and other private agencies. Consultancy work can be undertaken in all areas of chemical analysis (e.g. AA, FT-IR, FT-NMR, elemental analysis, GC/MS, TGA etc.), product development, synthesis and characterization of new and novel compounds which are of benefit especially to the chemical industry and the general public can be undertaken. Interested parties and prospective postgraduate students are welcome to contact me directly for a friendly discussion.

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