LITERATURE REVIEW
2.1 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.
2.2 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].
NANO-STRUCTURED SILICA FROM RICE HUSK
Synthesis of biogenic silica nanoparticles from rice husks (RHs) as the raw material via controlled pyrolysis and catalysis by potassium ions to convert amorphous silica to crystalline phase were investigated by Wang et. al. [31]. He was successful in separating out silica nanoparticles of 20-30 nm dia. which had potential to replace fumed silica for various applications. Sol gel method was used by Le et. al. [32] to synthesize silica nanoparticles from Vietnamese rice husk. They studied the effect of surfactant surface coverage, aging temperature and aging time to produce 3 nm sized amorphous nano-silica with highest surface area of 340 m2/g. Similar sol-gel techniques were also used by Adam et. al. [33] to produce mesoporous silica nanoparticles with an average diameter of 50.9 nm and high specific BET surface area of around 245 m2g-1. Cetyltrimethylammonium bromide (CTAB) has been used as the surfactant at room temperature by Adam and Iqbal [24] to develop catalysts. They had developed silica–tin catalytic materials using simple sol–gel method.
HETEROGENEOUS CATALYST
Heterogeneous catalysis refers to the group of reactions where the catalyst and the substrate are in different phases. Heterogeneous catalysts are mostly solids and the reactants are basically gases or liquids. A system where the catalyst is solid and the reactant is gaseous is the normal consideration while mentioning heterogeneous catalyst. Heterogeneous catalysis were mostly developed by the petrochemicals and bulk- chemicals industries and were usually built to withstand high temperatures, a requirement of chemical industries at large. Thus solid catalysts and gaseous reactants was the order of the use of catalyst in reactions like cracking. The facility of easy separation of a heterogeneous catalyst from the reaction medium is the most important advantage. The catalyst can be easily separated and cleaned in gas-solid systems, and easily filtered in in liquid/solid systems.
Heterogeneous catalysts are extensively used nowadays in various organic reactions like production of bio-diesel, oxidation of phenol, acetone, benzene or higher molecular weight compounds like styrene. Hydrogenation of acetone with heterogeneous catalysts has been achived at temperatures in the range of 100–300oC. Hydrogenation of ace-tone to isopropanol with raney nickel and various nickel catalysts has also been reported [34]. Superactive catalyst consisting of nickel nano catalysts supported on silica obtained from tetraethyl orthosilicate (TEOS) have been studied and proved to be useful particularly for oxidation of acetone to isopropanol. There is an expanding research in the area of mono-dispersed silica nanospheres with lead on two-dimensional (2D) and three- dimensional (3D) ordered superstructures [35]. The temperatures of burning or pyrolysis have been found to alter the structural characteristics of the silica obtained. Higher temperatures have been found to form more crystalinity affecting the surface structure and catalytic effect in turn [36]. Acetone hydrogenation reaction with efficient selectivity has been performed by Gandhia et. al. [37]. The catalyst used by them had showed good conversion at temperatures ranging from 180 to 220oC. Ni/Ru bimetallic catalysts have been reported to be excellent catalysts for various reactions. These were found not to form solid solution or complexes between Ni and Pd or Pt [38, 39]. E-stilbenes have been synthesized with the aid of silica-supported palladium complex [40]. Silica supported molybdenum catalyst have also been synthesized, characterized and applied in the oxidation of various sulfides and olefins to their corresponding sulfoxides/sulfones and epoxides [41].
2.5 AMORPHOUS SILICA FROM RICE HUSK AS CATALYST SUPPORT AND SYNTHESIS METHODOLOGIES
Rice is one of the major crop in India producing about 22 million tonnes of rice husk being produced annually as a by-product most of which goes to waste or burnt in open fields. About 600 million tonnes of rice are produced each year. Every 1000 kg of paddy milled yields about 220 kg (22%) of husk[42]. The various components of RH are 20% ash, 38% cellulose, 22% lignin, 18% pentose and 2% other organic components [43]. Rice husk contains high percentage of amorphous silica mostly on its outer epidermis. Silica extracted from RH had been shown to be a good catalyst support for the synthesis of fine chemicals where iron and 4-(methylamino) benzoic acid have been incorporated into the sodium silicate, obtained from rice husk by a simple cost effective solvent extraction method. The resulting complex had been washed as a catalyst in the benzylation of toluene [44]. Researchers have found out that a conversion of around 27% was possible using chromium modified silica on oxidation of cyclohexane to ketone and alcohol products [45]
Rice husk is pyrolysed at high temperatures to generate RHA after which silica is extracted via an alkaline extraction route. All forms of the ash produced from burning rice husk in general is termed as Rice Husk Ash (RHA).The temperature of burning or pyrolysis varies the various forms of ash evolved. The time. temperature etc. of combustion effects the structural transformations of the silica sh. Amorphous silica is formed at around 550–800oC and crystalline ash at greater temperatures. Amorphous silica is formed at around 550–800oC and crystalline ash at greater temperatures. Sodium silicate when extracted from RH is hardly used directly. It can be utilized after incorporation of transition metals on the silica to produce high potential heterogeneous catalyst via alkaline extraction routes. Various industrial reactions like oxidation of cyclohexane, cyclohexene and cyclohexanol, oxidation of phenylmethanol and decomposition of cyclohexanol has been performed with catalysts having silica as catalyst support [46]. The high surface area of the catalysts obtained gives it the high catalytic activity and conversion percentage.
Various types of synthesis procedures for preparing mesoporous silica incorporating metals and transition metals and also organic compounds from rice husk silica has been reported. Aluminum sulfate, nickel nitrate and aqueous ammonia had been used by Tsay and Chang [12] to prepare nickel modified RHA-Al2O3 catalyst using impregnation and ion exchange methods. Calcination at 673K has been used by Chen et. al. [39] to prepare Cu/RHA using the deposition–precipitation method. Methanol was oxidized partially to generate hydrogen using the above mentioned catalysts. Also Chang et. al. [14] described the synthesis of copper in RHA, using it for dehydrogenation of ethanol using copper nitrate trihydrate via an incipient wetness route. Mesoporous molecular sieve (M41S) materials have also been extensively studied in details. Grisdanurak et. al. [15] have used CTAB as structure-directing agent (SDA) and synthesized MCM-41 mesoporous materials. Chlorinated volatile organic compounds were adsorbed and photocatalytic degradation of herbicide undergone using such materials developed. Manufacturing silica structural materials with desired pore size is influenced by varying parameters like silica source, type and concentration of surfactant, pH and the temperature of obtaining the silica precursor.
The general procedure for extraction of silica followed is an alkaline extraction route followed by gel formation by addition of mineral acid. Later this gel is aged, centrifuged or filtered and finally dried. Addition of structural directing agent (SDA) to the gel during the extraction process helps to regulate the size of the ultimate particles. This SDA can later be removed from the dried powdered metal modified silica by calcination up to temperatures in which the SDA decomposes. Rice Husk Ash (RHA) is obtained by pyrolysis of the rice husk at the temperatures range of 500oC to 800oC for 5-6 hours in muffle furnace. These procedures have been modified according to the needs of the catalyst required. A black crude product had been developed by Chang et. al. [10] pyrolyzed RH at 900oC for 1 h in a furnace with N2 flow. They also developed white ash by pyrolysis in atmospheric conditions. The effect of acid treatment, calcination temperature and the rate of heating of RH were studied by Chandrasekhar et. al. [47] reporting the effect on surface area.
Styrene was oxidized in its liquid phase with the help of tungsten modified silica as catalyst, as reported by Adam and Iqbal [23], using H2O2 as green oxidant for production of benzaldehyde. It was also reported that the pH of the preparation media had a strong influence on catalytic activity of the structure of the resulting silica-tungsten species. Zhang et. al. [48] reported that Mn–MCM-41 decomposes H2O2 very rapidly leading to very low styrene conversion. Other materials such as Cr–MCM-41, Fe–MCM- 41, Mo–MCM-41 and V–MCM-41 had also been studied by them. Chromium incorporated rice husk silica synthesized at different pH was reported by Adam and Iqbal [24], where oxidation of styrene using hydrogen peroxide as the oxidant was studied. Chromium supported on mesoporous silica have been reported to also greatly steer the transformation of cyclohexane to cyclohexanone and cyclohexanol, which is required for the production of nylon-6 and nylon-66. Yao et al. reported a conversion of cyclohexane ca. 95% over Ce-MCM-41 at 100oC over 12 hr giving 82% selectivity to cyclohexanol [49]. Silica-chromium catalyst incorporating 4-(methylamino)benzoic acid (MBA) into the RH silica framework has been reported to give 100% conversion of cyclohexane in a much shorter time, by Adam et. al. [46]. Adam and Fook [50] have also reported incorporation of Chromium into Rice husk without prior conversion of the husk to ash. They have used this to produce cyclohexanone and cyclohexanol from cyclohexane.
There is also number of publications based on silica from rice husk with metal incorporations by incipient wetness methods, but almost everywhere it has been found out that the catalyst is prone to metal leaching due to reaction. Among the preparation routes, incipient wetness impregnation is the one most frequently adopted in industry due to its simplicity and convenience but its utility is still limited by poor metal dispersion. To enhance metal dispersion, an inert material termed as ‗‗textural promoter‘‘ when introduced during preparation to separate the metal particles from contact with one another minimize the coalescence of metal particles. Such a textural promoter gives a relatively high melting point and helps to have somewhat smaller particles than those of
the active metal species. Owing to its high melting point (2708 K) and fine particle size, chromia (Cr2O3) was found to be a good textural promoter [51]. Chromia-promoted copper catalysts supported on RHA and on commercial silica gel have been prepared for comparison, to investigate the effects of chromia content on both the surface properties and the catalytic activity in ethanol dehydrogenation [52]. Template assisted sol-gel precipitation method has been used by Adam and Thankappan [53] to generate high surface area Cu-Ce incorporated rice husk silica catalysts. These were used for one-step oxidation of benzene using H2O2 as the oxidant in acetonitrile. They investigated the presence of Ce and Cu together in the silica matrix and the mechanism in which they enhanced catalytic activity.
Aluminium ions have been chemical incorporated into rice husk ash silica matrix by the sol-gel technique and the fatty acid adsorption capability have also been studied [54]. Actephenone has been oxidized using H2O2 in aqueous phase under mild conditions using vanadium incorporated RH silica leading to a cost-effective and green preparation route and has been reported by Adam et. al. [55]. Characterization of rice husk ash- supported nickel catalysts prepared by an ion exchange method has been reported by Tesh et. al. [18] to understand the interrelationship between the physical and chemical properties. It has also been reported that catalysts prepared by deposition-precipitation is stronger than that in the catalysts prepared by impregnation. Cluster compounds of metal oxides prepared by impregnation of supporting oxides with a precursor compound has been found to modify dramatically the interfacial properties of the bulk oxide and is referred to as composite oxides.