Extraction Of Silica From Rice Husk Ash

Project and Seminar Material for Chemical Engineering

Extraction Of Silica From Rice Husk Ash


A simple method of alkaline extraction followed by acid precipitation of rice husk (RH) led to production of amorphous silica which was found to have high specific surface area and minimal mineral contaminants. In the present work pyrolysis and leaching with alkali helped to obtain amorphous silica with high surface area. Rice hulls were pyrolysed at different temperatures in a furnace and then treated with NaOH of various concentrations. For the samples extracted at higher pH levels also type III isotherm curves were obtained. Thermogravimetric studies were also performed on the silica extracted from rice husk which was pyrolysed at 650, 750, 850 and 1000oC. A four stage decrease in mass was observed with about 20% decrease in mass up to 800oC. FTIR analysis of the catalyst thus obtained showed the presence of the silanol and siloxane bonds on the surface of silica.

Presence of Si-O-Cr and Si-O-Cu bonds were also evident from the FTIR analysis. Strene was oxidised in a laboratory scale with the aid of the prepared catalysts. The oxidant used was H2O2 and acetonitrile was also used as a solvent. The products thus formed were analyzed by GC-MS. The catalysts were found to give a good conversion of styrene to benzaldehyde with maximum conversion for catalyst samples extracted at lower pH. A comparison with the catalyst extracted at same pH but with higher metal (copper) content showed that the increase in copper content help to obtain better conversion and product selectivity. These studies finally help us to get a biogenic catalyst with high conversion and selectivity which can act as a heterogeneous catalyst on oxidation of styrene.

Chapter One


1.1 Background Of The Study

The population growth and its requirement for improvement of human living standards has led to huge increase in the amount of agricultural waste both in volume and in variety. Usually such waste is burned in open and when left in the open decomposes down elevating the disposal problem to higher extents. Decomposition and putrefaction emits methane and other obnoxious gases while burning produced gases which add to air pollution and also form ash as solid waste further increasing the pollution. Thus improper waste management adds to climate change, soil and water contamination and also aids air pollution.

If properly managed, waste agricultural biomass is of high worth because the energy that it contained in it is recovered. In parity with global organization surroundings findings (UNEP) findings, five billion metric a lot of biomass is generated each year from agriculture. Which is thermal equivalent to 2 billion a lot of oil, concerning a pair of 5% of current international production [1]

With growth of population there has been a massive increase in global energy demand and non-renewable sources are withering out at a fast rate. Use of carbon based fuel in the present world along with its increasing demand is not a sustainable source of energy as it results in CO2 emission and effects the global climate. Development of alternative energy sources is the most worthy strategy to protect the global climate, reduction in use of nonrenewable fuel being the other. Processes to convert biomass into energy have been developed like gasification and use of biomass as fuel for boilers have been developed. Various techniques of conversion of biomass to energy and production of synthesis gas and generation of heat for various industrial processes have been reported in literature like biomass may be burnt in air, gasified, pyrolysed, fermented, digested or undergone mechanical extraction, with gasification being one of the most important and common method to obtain energy from burning and pyrolysing biomass and generating heat and synthesis gas. Pyrolysis or gasification of biomass results in the formation of char, which at times is formed in significant quantities and poses a serious disposal problem and causes environmental hazards. Various types of biomass give rise to different types of chars having varied properties. Characterizations of these types of chars have not been reported extensively.

Amorphous silica is known to be bioactive, biocompatible, biodegradable and non-toxic to the living human tissue, unlike silica in crystalline which induces adverse biological effects. In the present times amorphous silica along with fumed silica is increasingly used in biomedical applications, drug delivery and diagnostics. Potential silica based materials like, silica spheres, nano-spheres, mesoporous silica gel, silica aerogel, and silica star gel are used widely for and drug delivery and preparing artificial bone tissue. Since the discovery of silica-based bioactive glass by Hench in 1971 and then ordered mesoporous silica materials in the early 1990s‘ by scientists of the Mobil Corporation [2] and Waseda University [3], these materials lead to the development of a wide area of research on new silica-based bioactive materials for biomedical applications. In general, amorphous silica is prepared using silicon alkoxide raw materials. Tetra methyl ortho silicate (TMOS), tetra ethyl ortho silicate (TEOS) and poly ethoxy disiloxane (PEDS), etc are the commonly used alkoxides to prepare mesoporous amorphous silica. However, such precursors are fairly expensive. Thus alternative low cost amorphous silica precursors are required to substitute alkoxides. In that respect, a cheap source of amorphous silica of bio-origin would be the rice husk ash (RAH). Although RHA contains minor amount of other metallic impurities; it may be safely used in physiological environment as it is generated from biomass.

Rice husk ash/ char have been reported in many literatures to be used to derive silica in ultrapure amorphous form or simply in amorphous form. RHA is found to contain around 60% or more of silica thus making it useful as an economic raw material for production of silica gels and powders as reported by Kamath and Proctor [4] and also Chakraverty and Kaleemullah [5]. Rice husk has been used as a starting material to develop various silica substances like silica carbide, silicon nitride, silica itself in amorphous form and also elemental silicon. Leaching of rice husk with hot alkali results in formation of sodium silicate, which when treated with mineral acid gives us silica as precipitate. Primary treating of the rice husk prior to thermal treatment with mineral acids like HCl, H2SO4, HNO3, have been reported in literature to remove impurities such as metals (found in traces) and producing white ash with high percentage of silica in it giving silica with high surface area. Biomass i.e. Rice husk is utilised for heat generation and drying in rice mills and has high calorific value. Rice husk is also used in gasification plants to directly generate power or syn-gas. These burning, pyrolysis and gasification operations produce ash in relatively high amounts which also pose a disposal problem.

At present the most intense research work is going on in the area of ordered mesoporous silica for drug delivery and other biomedical applications. The important feature of mesoporous material is to host molecular size of particles within it. Silica aerogel is one such ordered mesoporous material and is also used in biomedical applications for controlled release of drugs, peptides, hormones, etc. [6]

In this work such ash i.e. pyrolysed rice husk ash has been characterized and silica has also been derived from the pyrolysed rice husk ash (RHA). This silica is found to be in amorphous format and was proven to have high surface area. This high surface area amorphous grade silica is impregnated with Copper and Chromium bi-metals and thus an oxidizing catalyst is developed. The prepared catalyst are used in oxidation of styrene and is converted to benzaldehyde which is compared with catalysts extracted at different pH conditions of the solution. Metal modified silica catalysts are prepared with two different metal containments, i.e. 5% and 10% (w/w) for Copper while the Chromium content was fixed at 10% (w/w). These metal modified catalyst are prepared at different solution conditions, i.e. in acidic, neutral and basic media, at pH of 3, 7 and 10 respectively. The oxidation of styrene to benzaldehyde is noted for all these catalysts and the best catalyst is then identified. Cetyl trimethyl ammonium bromide (CTAB) surfactant is used as a Structural Directing Agent (SDA) while incorporating the transition metals into the silica matrix. The SDA forms micelles and helps to maintain a particular size to the silica precursor while incorporating the metal into the matrix.

Rice Husk Ash and Extraction of Amorphous Silica

India produces about 120 million tons of paddy every year and along with it comes 24 million tons of rice husk which on burning would generate 4.4 tons of as (RHA). Rice husk ash is mostly used in steel, cement and refractory industries. Numerous examples of production of amorphous silica, silica nano particles, complex organo-silica matrixes and metal-silica compounds have been widely studied and reported in literature [7 – 10]. The process of obtaining rice from the paddy helps to obtain the husk which corresponds to 23% of the initial weight of the paddy. The ash from rice husk is highly

siliceous, containing 95-97% silica [5, 4]. Potassium, sodium, calcium, magnesium, iron, phosphorus and aluminium are the major impurities which occur in the form of oxides or silicates in RH. Samples obtained from various parts of the earth have been found to have different percentages of the above mentioned impurities, depending on the soil characteristics and the minerals available in it. The organic component of rice husk (RH) is around 72% of the husk by weight [11], remaining being silica. Silica after solubilizing produces soluble silicates. These silicates are widely used for the manufacture of glass, ceramics, and cement. A major portion is used in pharmaceuticals, cosmetics and detergents industries as bonding and abrasive agents. The dry basis analysis of rice husk has led to the finding that it contains 20% ash, 22% lignin, 38% cellulose, 18% pentosans, and 2% other organic matter. The husk surrounds the paddy grain as obtained from the crop. Milling of paddy helps to generates about 78 % of weight as rice and broken rice and bran, with the husk being the remaining 22% left. Rice mills use these husk to to generate steam for the parboiling process by burning them to generate heat for making steam in boilers. During such burning process as boiler fuel leads to formation of 25% ash (RHA) while the remaining 75% of the volatile matter is lost due to high temperatures to generate heat which is utilized. RHA has been found to contain around 85- 90 % amorphous silica. The moisture content has been reported to be around 8•68 – 10•44% with the bulk density ranging from 86 to 114 kg/ m3. The percentage of silica in the rice husk usually varies from region to region. EDX studies have showed that the silica is mainly concentrated on the outer proturbances of the outer epidermis. In certain regions of India, RH is mainly used as cheap source of energy for parboiling paddy in rice mills. The product of combustion is partially burnt RH, which is completely a waste material and contributes to pollution. Utilization of this rice husk for production of silica solves these problems. All the different types of ash obtained by burning or pyrolysis of the rice husk produces ash, in general being called 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. 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.

Numerous techniques have been reported in the literature while all are following the basic technique of treatment with an alkali to solubilize the silica in form of alkali- silicate followed by precipitation with a concentrated mineral acid to obtain silica. Numerous cases of prior acid leaching/treatment of the rice husk before combustion or pyrolysis have been reported, where both mineral acids and organic acids have been reported to be used. Inorganic parts of the rice husk are removed by acid leaching and the breakdown of lignin and organic matter is accomplished, thus helping to obtain pure form of amorphous silica with lesser contaminants. Use of various acids for acid leaching like H2SO4, HCl, HNO3, H3PO4, and also use of organic acids like oxalic acid, citric acid and acetic acid have been reported in literature. Precipitation of the sodium silicate has also been tried with various acids like HCL, H2SO4, H2CO3 etc. In the present work silica prepared from RH with and without acid leaching has been identified to be able to be used for impregnation with transition metal, presently with bi-metals, to produce evenly dispersed metals on the silica framework. Use of silica as catalyst support was popularized due to its high surface area available and its quality of having a narrow pore size distribution. The morphologies and properties of silica obtained from RH pyrolysed at various temperatures are also compared with each other. The structural changes and agglomeration of the silica obtained from RH pyrolysed at different temperatures are also observed.

1.2 Statement Of Problem

Silica has been found to exist in form of gel, crystal and also in amorphous forms. SiO2 structure is found to be based on the SiO4 tetrahedral. Each Silicon atom is bonded to four oxygen atoms with each oxygen atom in turn getting attached to two silicon atoms. The silica surface consists of two types of functional groups, namely silanol (Si- O-H) and siloxane (Si-O-Si), with silanol being the more reactive one and with more adsorptive properties. Commercial silica is manufactured in a process involving multiple steps which involves high temperatures and pressure. This makes such a process less cost effective and not very environmentally friendly. Mobil Oil Company developed the mesoporous material and it led to a huge research on such material and various metals incorporated on them. The inertness of silica and its ability to be custom tailored led to publication of around 3000 papers on mesoporous materials. The most common method for extraction of silica is solvent extraction. It is achieved either directly from the husk or the ash produced after pyrolysis of the husk. Better results are obtained on extraction of silica from the RHA instead of RH. Combustion at temperatures ranging from 550-800oC leads to formation of silica on which metal can be impregnated, with higher temperature resulting formation of crystalline silica and reducing the available surface area.

Various types of synthesis procedures for preparation of mesoporous silica from rice husk and metals incorporation, have ben reported by several researchers. Aluminum sulfate, aqueous ammonia and nickel nitrate have been used by Tsay and Chang [12] to prepare Ni/RHA–Al2O3 via simple impregnation and ion exchange methods. Chen et. al.
[13] have reported the preparation of copper impregnated RHA using the deposition– precipitation method followed by calcination at 673 K. Copper nitrate trihydrate has been used by Chang et. al. [14] as the copper source to produce copper impregnated silica. He achieved this via an incipient wetness impregnation method. 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.

A lot of research is being carried out presently to produce environment friendly catalysts. Development of heterogeneous catalyst is advancing at a great pace which is evident from the huge number of research papers being available online in various journals. As we know, over the years, silica powders and gels have been widely used in industry as fillers, adsorbents, chromatographic agents, catalyst and as catalytic supports. Selection of a specific mechanism and a raw material for production of a catalyst is judged by its reusability and the ease with which it can be prepared. The effects of leaching on the catalyst is also an important criterion for catalyst selection. Use of industrial waste, specifically rice husk, to prepare catalyst by sol-gel technique has been studied by Adam et.al. [16].

They reported the benzylation reaction of toluene with benzyl chloride. Insipient wetness method has also been used to develop metal supported catalysts from rice husk which has been reported by Chang et. al. [17]. High surface are asolids in particulate form can be developed by metal and metal oxide impregnation techniques on silica matrixes. Such composite oxides formed, differ from the binary oxides which are currently used as catalyst supports. The composite oxide are well dispersed on the support matrix and thus helps to obtain better catalytic results. 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. Amorphous silica, extracted from rice husk by acid leaching, pyrolysis and carbon removal has been used as catalyst supports, owing to their high porosity and high surface area and has also been found to be impregnated with nickel metal, using ion exchange mechanism and used as an oxidation catalyst.

1.3 Aim and Objective

The aim of this work is 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 Five

5.0 Conclusion and Recommendations

5.1 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 1500o C, while this process used in this work has been made possible in a temperature range of 550-750o 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.

5.2. 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.

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