Extraction of Silica From Rice Husk Ash

Extraction of Silica From Rice Husk Ash

Chapter One

Content Summary

AIM AND OBJECTIVE

This work aims to assess the extraction of silica from rice husk ash. This will specifically involve the extraction of amorphous silica from rice husk by acid leaching, pyrolysis, and carbon removal as catalyst supports.

CHAPTER TWO

LITERATURE REVIEW

CONCEPTUAL REVIEW

Silica, though having a simple formula of SiO₂, exists in a variety of forms in nature, leave aside synthetic forms. Each form of silica exhibits different physical and also chemical properties, existing in the form of gels, crystalline and amorphous forms. It is also at times fount to exist with other elements in form of ores or minerals. Silica is found in abundant quantities on earth, still for most technological applications silica is prepared by synthetic methods. Synthetic silica possess huge surface are which allows it to be used as adsorbing material and as catalyst support. In general the SiO₂ structure is based upon a SiO₄ tetrahedron. Each silicon atom is bonded to four oxygen atoms and again each oxygen atom being bound to two silicon atoms. Two types of functional group: silanol groups (Si-O-H) and siloxane groups (Si-O-Si) are present on the silica surface. All the chemical processes and even the physical processes like adsorption takes place on the silanol sites, the siloxane sites on the surface being inert to most of the activities. Generally the SiO₂ is a part of the SiO₂ tetrahedron structure, where each Si atom is bonded to four oxygen atoms and each oxygen atom in turn bonded to two silicon atoms [19]. Silica has also been reported to be found in some dicotyledonous plankton their husks or seeds like rice husk and foxtail millet. Porous amorphous silica has been found to contains isolated, germinal and vicinal as the three types of silanol bonds on its surface [20]. Isolated silanols have been found to be the more reactive species among all the three types found. An increase of temperature makes the silica surface more hydrophobic. The surface hydroxyl groups condense and form siloxane bridges. Commercial silica manufacture involves high temperature and pressure which renders it as a less cost efficient and non-environmental compatible process [21]. Considerable attention has been laid in recent years to modify catalytic surfaces, making it more efficient for reactions. Alternatives to conventional heterogeneous catalyst are possible by such chemical modifications of the silica obtained from RH. Amorphous silica which is usually produced commercially is a high energy consuming process, thus leading researchers to look for alternatives. Biogenic silica from rice husk has been extensively studied and various techniques applied on it to modify it with transition metals, metals and organic substances to catalyze various reactions. Heterogeneous catalysts have been prepared by immobilization of transition metal complexes and their nature and mechanism has been studied and developed. This has also helped to manipulate the metal particle size and the crystalinity helping to achieve various catalytic reactions [22]. Side chain oxidation of styrene to produce benzaldehyde and styrene oxide is of considerable industry importance. Benzaldehyde is widely used as a starting material for various compounds in the pharmaceutical, dyes, resins, additives, flavours and organic solvents. Tungsten modified rice husk silica has been found to give 100% conversion of styrene with very less byproducts [23]. Heterogeneous catalysts along with greener oxidant like H₂O₂ and molecular oxygen have been reported to overcome some limitations of other heterogeneous catalysts and have drawn some serious research implications [24].Mobil Oil company developed mesoporous materials in the early 90‘s. Since then an extensive research was initiated in the silica field. Presently there are more than 3000 publications specifically in the area of mesoporous silica. Silica being chemically inert and the ease with which it can be structurally modified with metals and organic substances helped it to be widely considered as a catalyst support [11]. A vast amount of literature is available on amorphous silica, transition metal modified silica, heterogeneous catalyst on silica and rice husk derived silica which are briefly mentioned below.

AMORPHOUS SILICA DERIVED FROM RICE HUSK ASH

Rice husk ash contains 85-95% silica and the rest of other inorganic materials. Research on extraction of this silica has been extensively documented over time. Alyosef et. al. [25] in have characterized biogenic silica generated by thermo chemical treatment of rice husk obtained from Egypt . they had optimized the process for least environmental impact by two routes vis. one by citric acid leaching the husk prior to pyrolysis and second without using acid leaching. Chandrasekhar et. al. [7] have treated rice husk with acetic and oxalic acid and used controlled burning techniques to prepare reactive white silica of high purity and also compared the results with rice husk treated with conventional mineral acids. Della et. al. [8] have also reported preparation and characterization of active silica with high surface area from rice husk ash. XRF, XRD, and particle size analysis had been conducted to characterize the silica formed. Similar work was also performed by P. Deshmukh et. al.[26] where they determined the silica activity index of the silica derived from rice husk ash under controlled heating conditions.

They had also performed XRD, and XRF studies. Kalapathy et. al. [9] had also derived silica from rice husk by simple alkaline extraction techniques but in form of xerogels which were later turned to aerogels. They characterized the materials using EDX, ICP (Inductively Coupled Plasma), and FTIR studies.

Several silica embedded materials and composites have also been developed and reported in literature. Rattanasak et. al. [27] have studied the development of high volume rice husk ash (RHA) alumino silicate composites (ASC) and later added with boric acid to prepare stable ASCs with compressive strengths up to 20MPa. Nayak and Bera [28] have developed a procedure for obtaining and characterizing active humidity indicating blue silica gel from rice husk ash after following the conventional technique of alkaline treatment and acid precipitation with impregnation with CoCl₂. The effect of calcination temperature and heating rate on the reactivity, surface area and optical properties of silica from rice husk has been studied by Chandrasekhar et.al. [29].

Production of amorphous silica from rice husk in fluidized bed system has been reported by Taib [30]. He designed and developed a pilot scale fluidized bed combustion system for the production of amorphous silica from rice husk. A review on processing, properties and applications of reactive silica from rice husk covering controlled burning techniques, production of reactive silica, pore structure and surface area studies and also various advanced material production like SiC, Si₃N₄ and Mg₂Si were summarized by Chandrasekhar et. al. [10].

CHAPTER THREE

MATERIALS AND METHODS

SAMPLEPREPARATIONAND EQUIPMENTS REQUIRED

Rice husk (RH) was obtained from a local rice mill and initially the husk was washed thoroughly with water to wash off the mud and dirt in the husk. The samples after washing was drained of any water and dried in a drier at 90-100^oC overnight to obtain light weight rice husk. Considerable amount of such water washed RH was kept for the later procedures to be performed on. This water washed rice husk was termed as WWRH.

The equipment required are conical flask (150, 250, 500 ml), beaker (500, 1000 ml), pipette, burette, magnetic bead, magnetic stirrer with digital temperature control, measuring cylinder (50, 250 ml), petridish, Muffle furnace, drier, mortar pestle, ashless filter paper (Watmann 41 grade), normal filter paper (12.5 mm).

CHAPTER FOUR

RESULTS AND DISCUSSION

RESULTS

The X-Ray diffraction intensity of the acid washed pyrolysed ash, AWRH-A650, AWRH-A750, and AWRH-A850 was performed using ―Panalytical X‘Pert³ Powder‖ diffraction system. The X-Ray diffraction patter of the silica sample was recorded at a range of 10 – 80^o 2θ Bragg‘s angle.at a scanning speed 3 deg min^-1. The X-ray being produced from CuK_α radiation and after use of a Ni filter. The powder sample for the XRD was prepared on metal plate and pressed to have a flat surface. The XRD was carried out at a voltage of 40 kV and 30 mA current intensity. The XRD diffraction patterns of the three samples are shown in Fig 4.1, Fig 4.2 and Fig 4.3 below. No definite peaks were identified but a pattern list with a presence of silica, quartz and calcite was identified for samples pyrolysed at 750^oC, a peak at 26.72^o 2θ indicating presence of crystalline quartz which was formed due to increase in pyrolysis temperature.

CHAPTER FIVE

CONCLUSION AND RECOMMENDATIONS

CONCLUSIONS

This study revealed the process for extraction of silica from rice husk and also primarily in amorphous form. The removal of the mineral and organic component from the rice husk helped to obtain high yields of silica from the ash. All existing techniques for obtaining silica are primarily operated at temperatures of around 1500^o C, while this process used in this work has been made possible in a temperature range of 550-750^o C. This proves to be highly energy efficient and also a useful technique for waste disposal and utilization. For cases where even pyrolysis was performed the temperature use was far less than the conventional techniques.

The initial acid washing of the rice husk prior to pyrolysis have shown to improve the quality of the silica and eliminate trace of other organic and inorganic elements. This silica derived can be further used as catalyst framework and can be impregnated with heavy metals to act as catalyst itself. The surface area and adsorption isotherms can be calculated and proper catalyst can be obtained from it. The white silica was proved amorphous by the XRD studies and a proper comparison was made between various samples showing the effect of variation of temperature and pyrolysis conditions. Hence, production of value added materials from rice husk not only facilitates utilization of an abundantly available agro-waste but also reduces environmental pollution.

The surface area was found to be in the microporous range and having a high surface area, thus they can be used as adsorbents or as catalyst framework. The high surface area of the silica will provide as sites for active reactions and charge transfer to facilitate high rates of reactions. Oxidation reaction can be undertaken and effect of this developed catalyst can be studied. Performance of such catalyst over catalyst of copper made on commercial MCM can also be compared. The conversion percentage of styrene to benzaldehyde has been compared with heterogeneous catalysts of chromium and copper alone respectively.

The incorporation of bimetals into the silica matrix has helped to further increase product conversion and selectivity to benzaldehyde with considerable reduction in byproduct formation. The effects of metal loading on the catalytic activity of the catalyst for benzaldehyde conversion and also on the total conversion to product has also been studied. An increase of the amount of copper loading onto the silica matrix during the sol-gel extraction helped to increase the oxidation capacity of the catalyst. The effect of pH of extraction of the silica catalyst were also studied and found out that the maximum conversion is archived for metal modified silica catalyst extracted at lower pH, i.e. at acidic medium. Though the surface area of the catalyst extracted at lower pH had relatively low surface area and low pore volume, yet they were found to be more efficient for styrene conversion and product selectivity, which can be attributed to the copper and chromium present on the surface. The comparison of the catalytic activities of the high surface area catalysts at different pH levels were found to be excellent for oxidation of styrene high benzaldehyde selectivity. Presence of high surface area helps more active suites to be available on the silica surface which accounts for the general better performance of all the metal modified catalysts developed here. Also it can be said the catalytic activity is not only due to the high surface area of the catalyst but also due to other factors.

RECOMMENDATION

The oxidation states and the silica-metal bond on the surface can be identified using UV-Vis diffuse reflectance techniques. Such a study can help to identify the exact reaction mechanism followed and the route to formation of benzaldehyde from styrene and also the cause of formation of intermediates and byproducts. Effect of pyrolysis temperature and effect of concentration of metal ions, effect of aging the precipitate, effect of calcining the dried catalyst, SDA concentration can be checked to obtain best possible method to prepare the catalyst. The reusability of the catalyst can also be checked. Styrene can be a common substance which can be chosen to be oxidized in presence of an oxidizing agent like H₂O₂. The effect of use of different weight of catalyst, varying the recation time and the temperature, styrene to H₂O₂ ratio in the conversion percentage to benzaldehyde and conversion to product can be analyzed. Characterization techniques like ICP and NMR can be used to identify the exact surface bonds and oxidation states.

REFERENCES

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