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Việc sử dụng trấu silica như một chất xúc tác. Đây là nghiên cứu mới của ông Đinh Tấn Thành, Công ty TNHH Cao su Kỹ thuật Tiến Bộ, cùng nhóm cộng sự sản xuất silica gel khí từ vỏ trấu, góp phần làm giảm ô nhiễm môi trường, tiết kiệm chi phí. Quy trình chế tạo silica aerogel được thực hiện theo các bước: Vỏ trấu sau khi được rửa sạch và sấy khô để loại bỏ tạp chất, sẽ được tiến hành xử lý làm giảm thành phần kim loại sau đó nung hai cấp ở 500oC và 700oC. Quy trình xử lý tạo ra silica vô định hình, đạt mức độ tinh khiết cao, thành phần silica trong tro trấu trên 90%. Ngoài ra, thành phần oxit kim loại khác với hàm lượng không đáng kể. Sau đó điều chế dung dịch silicat natri. Từ silica thu được ở bước 1, hòa tan với dung dịch sút để điều chế dung dịch silicat natri 6 và 8% Si02. Tiếp đến tạo Silica sol, chất này được tạo thành từ dung dịch silicat natri và axit citric có pH 3,5. Sau đó tạo gel. Silica sol dần dần thành gel nước và được ủ ở nhiệt độ 60oC trong 24 giờ giúp ổn định cấu trúc gel. Gel nước sau khi ủ, được rửa bằng nước nhiều lần loại bỏ muối citrate có trong gel trước khi biến tính bằng hỗn hợp tetramethyl chloro silane (TMCS). Cuối cùng là sấy khô tự nhiên và xử lý nhiệt. Gel sau khi biến tính được để bay hơi tự nhiên ở nhiệt độ thường, sau đó xử lý nhiệt ở 200oC để tạo ra silica aerogel khí. Theo nhóm nghiên cứu, do có diện tích bề mặt cao và cấu trúc xốp nhẹ, vật liệu này có thể dùng làm chất cảm biến, chất xúc tác cho một số phản ứng hóa học, ứng dụng trong việc chế tạo tế bào quang điện, vật liệu cách nhiệt, cách âm và các vật liệu cao cấp khác...
Catalysis Today 190 (2012) 2–14 Contents lists available at SciVerse ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod Review The utilization of rice husk silica as a catalyst: Review and recent progress Farook Adam a,∗ , Jimmy Nelson Appaturi a , Anwar Iqbal b a b School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia Kulliyah of Science, International Islamic University Malaysia, 25200 Kuantan, Pahang, Malaysia a r t i c l e i n f o Article history: Received 15 October 2011 Received in revised form 18 April 2012 Accepted 21 April 2012 Available online 15 June 2012 Keywords: Rice husk Biomass Silica Catalyst Transition metal a b s t r a c t In this review article, we report the recent development and utilization of silica from rice husk (RH) for the immobilization of transition metals and organic moieties Silicon precursor was obtained in the form of sodium silicate and as rice husk ash (RHA) Sodium silicate was obtained by direct silica extraction from rice husk via a solvent extraction method while rice husk ash was obtained by pyrolyzing the RH in the range of 500–800 ◦ C for 5–6 h Transition metals were immobilized into the silica matrix via the sol–gel technique while the organic moieties were incorporated using a grafting method 3(Chloropropyl)triethoxy-silane (CPTES) was used as a bridge to link the organic moieties to the silica matrix All the catalysts exhibited good physical and catalytic potential in various reactions © 2012 Elsevier B.V All rights reserved Contents Introduction Synthesis methodologies 2.1 Silica from rice husk: by calcination and solvent extraction 2.2 Modification of silica: incorporation of metal and immobilization of organic Transition metal-based catalysts from rice husk 3.1 Chromium 3.2 Molybdenum 3.3 Tungsten 3.4 Iron 3.5 Cobalt Metal based catalyst from rice husk ash 4.1 Friedel–Crafts reaction using iron catalysts 4.2 Rice husk ash supported ruthenium catalyst 4.3 RHA supported gallium, indium, iron and aluminum for the benzylation of xylenes and benzene 4.4 RHA supported aluminum, gallium and indium for the tert-butylation of aromatics 4.5 Photocatalysis reaction using silica–tin nanotubes 4.6 Oxidation of benzene over bimetallic Cu–Ce silica catalysts 4.7 Benzoylation of p-xylene on iron silica catalyst 4.8 Synthesis of nanocrystalline zeolite L from RHA Organic–inorganic hybrid catalysts 5.1 One-pot synthesis via sol–gel method 5.2 Grafting method 5.3 Esterification using organic–inorganic hybrid catalysts 5.4 Silica from rice husk ash immobilized with 7-amino-1-naphthalene sulfonic acid 5.5 Silica from rice husk ash immobilized with sulfanilic acid ∗ Corresponding author Tel.: +60 46533567; fax: +60 46574854 E-mail addresses: farook@usm.my, farook dr@yahoo.com (F Adam) 0920-5861/$ – see front matter © 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.cattod.2012.04.056 4 4 5 7 7 8 9 10 10 11 11 11 12 F Adam et al / Catalysis Today 190 (2012) 2–14 Current and future progress Closing remarks Acknowledgments References 12 13 13 13 Introduction Silica is the most abundant oxide in the earth’s crust, yet despite this abundance, silica is predominantly made by synthetic means for its use in technological applications and it is one of the valuable inorganic multipurpose chemical compounds [1] Although silica has a simple chemical formula (SiO2 ), it can exist in a variety of forms, each with its own structural characteristics, as well as chemical and physical properties Silica can exist in the form of gel, crystalline and amorphous material Generally, the structure of SiO2 is based upon a SiO4 tetrahedron, where each silicon atom is bonded to four oxygen atoms and each oxygen atom is bound to two silicon atoms The surface of silica consists of two types of functional group: silanol groups (Si O H) and siloxane groups (Si O Si) The silanol groups are the locus of activity for any process-taking place on the surface, while the siloxane sites are considered non-reactive [1] Porous amorphous silica contains three types of silanol on its surface: isolated, geminal and vicinal [2] The unequal distribution of the silanols in the matrix, resulting from irregular packing of the SiO4 tetrahedral unit as well as the incomplete condensation, results in a heterogeneous surface (i.e., non-uniformity in the dispersion of silanol groups) for synthesized silica The various silanols can have different adsorption activities and current knowledge indicates that the isolated silanols are the more reactive species With increasing temperature of heat treatment, the silica surface becomes hydrophobic due to the condensation of surface hydroxyl groups resulting in the formation of siloxane bridges Commercial silica manufacture is a multi-step process involving high heat and pressure, making it less cost effective and not very environmentally friendly [3] The discovery of mesoporous materials by researchers from the Mobil Oil Company initiated an intense research effort resulting in more than 3000 publications, especially in the area of mesoporous materials made from silica The inertness of silica aided with the ease of structural tailoring has made it a good inorganic material on which to support other organic and inorganic moieties [4] In most published reports, the major silica precursors used were commercially made alkoxysilane compounds such as tetraethylorthosilicate (TEOS), sodium silicate and tetramethylorthosilicate [5] Nakashima et al reported that acute exposure to TEOS can lead to death Thus, there is a need to find a safer, less expensive and more environmentally friendly silica precursor [6] Naturally occurring silicas, especially those found in agro waste, can provide an alternative source to replace commercial silica precursors Rice husk saw dust [7], and rapeseed stalk [8] are among the widely studied agro wastes which have been converted into more valuable end products Rice (Oryza sativa L.) is a primary source of food for billions of people and it covers 1% of the earth’s surface Globally, approximately 600 million tonnes of rice are produced each year For every 1000 kg of paddy milled, about 220 kg (22%) of husk is produced [9] Rice husk (RH) is therefore an agricultural residue abundantly available in rice producing countries Much of the husk produced from the processing of rice is either burnt or dumped as waste RH is composed of 20% ash, 38% cellulose, 22% lignin, 18% pentose and 2% other organic components [10,11] Even though some of this husk is converted into end products such as feedstock [12] and adsorbent [13] most is burnt openly, causing environmental and health Scheme Various utilizations of the rice husk silica problems especially in poor and developing countries Therefore, it is very important to find pathways to fully utilize the rice husk Silica can be pyrolyzed at elevated temperature to form rice husk ash (RHA) or it can be extracted from rice husk in the form of sodium silicate by using a solvent extraction method In most applications, rice husk ash is more favorable compared to rice husk Rice husk ash is a general term describing all forms of the ash produced from burning rice husk In practice, the form of ash obtained varies considerably according to the burning temperature The silica in the ash undergoes structural transformations depending on the conditions (time, temperature, etc.) of combustion At 550–800 ◦ C amorphous ash is formed and at temperatures greater than this, crystalline ash is formed [14] These types of silica have different properties and it is important to produce ash of the correct specification for the particular end use (see Scheme 1) Even though the use of sodium silicate extracted from rice husk using solvent is still limited, our studies have shown that it can be utilized for many purposes Transition metals can easily be supported on silica via sodium silicate extracted from rice husk These transformed metal silicates have good potential as heterogeneous catalysts Several researchers have reported different types of synthesis procedures to prepare mesoporous silica from rice husk for incorporation of metals Tsay et al [15] have used aluminum sulfate, nickel nitrate and aqueous ammonia to prepare Ni/RHA–Al2 O3 via simple impregnation and ion exchange methods Chen et al [16] have reported the preparation Cu/RHA using the deposition–precipitation method and calcination at 673 K, and the material was tested for partial oxidation of methanol (POM) to obtain H2 Chang et al [17] described the synthesis of Cu/RHA for the dehydrogenation of ethanol using copper nitrate trihydrate as the copper source via an incipient wetness impregnation route Due to the high interest in using rice husk silica in adsorption and catalysis, several studies have been carried out on the synthesis of mesoporous molecular sieve M41S materials Grisdanurak et al [18] reported the synthesis of MCM-41 mesoporous materials using CTAB as structure-directing agent (SDA), for the adsorption F Adam et al / Catalysis Today 190 (2012) 2–14 of chlorinated volatile organic compounds and photocatalytic degradation of herbicide (alachlor) [19] and tetramethylammonium [20] Some researchers had also used direct hydrothermal synthesis [21] and gasification processes [22] to obtain MCM-41 from rice husk ash In 2009, Jang et al [23] synthesized highly siliceous MCM-48 from RHA using a cationic neutral surfactant mixture as the structure-directing template The materials were used for CO2 adsorption To date, various parameters such as the source of silica, effect of surfactant and concentration, temperature and pH have been considered as major pivotal factors that influence the formation of structural material with the desired pore size distribution for catalytic studies In the present article, we review work performed on the use of silica, obtained from rice husk either via combustion or by solvent extraction, to support various transition metals and organic moieties for heterogeneous catalysis Synthesis methodologies 2.1 Silica from rice husk: by calcination and solvent extraction Initially, adhered dirt and soil on RH can be removed by washing with plenty of tap water and rinsing with distilled water The metallic impurities in RH can be reduced to negligible levels by stirring with nitric acid [24] or refluxing with hydrochloric acid [17] Direct extraction of silica can be performed by stirring the acid treated RH (after drying) with sodium hydroxide solution During this process, silica is extracted in the form of sodium silicate together with other organic moieties, according to the method patented by Adam and Fua [25] The sodium silicate obtained is converted to silica by adding suitable amounts of mineral acid Rice husk ash (RHA) can be obtained by pyrolyzing the RH at temperatures ranging from 500 ◦ C to 800 ◦ C for 5–6 h in a muffle furnace The RHA was then dissolved using sodium hydroxide to obtain sodium silicate Modifications were undertaken to this procedure according to catalyst preparation parameters Thus, Chang et al [17] pyrolyzed RH at 900 ◦ C for h in a furnace and N2 flow to obtain a black crude product, which was then pyrolyzed again in air under the same conditions to obtain a white ash In 2006, Chandrasekhar et al [26] studied critically the effect of acid treatment, calcination temperature and the rate of heating of RH and showed that these parameters influenced the surface area, reactivity toward lime and brightness of the ash 2.2 Modification of silica: incorporation of metal and immobilization of organic Silica precipitation from RH and framework transition metal incorporation was undertaken using the sol–gel technique A greater degree of control on the final properties of a catalyst can be obtained by using the sol–gel technique, which is due to the ability of the metal precursor to be mixed homogeneously with the molecular precursor of the support [27] Metal oxide can be trapped within the polymerizing gel, permitting precipitation from solution where the metal ion can occupy neighboring positions in the gel matrix Further processing and calcination decomposes the resultant amorphous mixture of metal oxide, hydroxides and metal salts leading to the formation of an M O M bond [28] To obtain a structural material, cetyltrimethylammonium bromide (CTAB) as a SDA was added into the sodium silicate solution Several researchers have reported different synthetic routes for preparation of silica incorporated metal catalysts Chang et al [29] incorporated nickel nitrate into the silica matrix via an ion exchange method They also used aqueous copper and chromium nitrate solutions to synthesize Cu/Cr/RHA via incipient wetness impregnation The metal salt solution was added slowly to the support and thoroughly stirred at room temperature Recently, Chen et al [16] used deposition–precipitation to incorporate the copper nitrate in RHA In this technique, the metal salt was dissolved in urea solution and added to RHA to yield a suspension The suspension was heated at 90 ◦ C and the pH was adjusted to 2–3 by adding nitric acid The immobilization of organic moieties was carried out in two steps First the CPTES was reacted with the sodium silicate from RHA in a single step This led to the formation of RHACCl, which contained the Cl functional group at the end of the organic chain This chlorine functional group was then reacted with the required organic ligand in a substitution reaction giving rise to the immobilized RHAC-R catalysts, where R is the ligand Transition metal-based catalysts from rice husk 3.1 Chromium Our interest in chromium-incorporated silica from rice husk began with the aim of incorporating chromium into the silica matrix from rice husk using the sol–gel technique [30] The catalytic potential of the chromium-loaded catalysts was tested in the oxidation of cyclohexane, cyclohexene and cyclohexanol The as-synthesized chromium–silica catalyst’s surface area was only 0.542 m2 g−1 Subsequent preparation resulted in a surface area of 1.20 m2 g−1 when it was calcined at 500 ◦ C for h Surface directing agent was not added during the preparation These catalysts contained only Cr(III) species Calcined chromium–silica catalyst was observed to be highly hygroscopic Calcination of the catalyst had improved the selectivity of cyclohexanone but lowered the selectivity of cyclohexanol in the oxidation of cyclohexane The conversion of cyclohexane was 27.13% when the as-synthesized chromium–silica catalyst was used while the conversion was 12.69% when calcined chromium–silica catalyst was used instead Only a slight change was observed in terms of cyclohexene conversion and product selectivity when these catalysts were used Both catalysts yielded 100% cyclohexanone selectivity By prolonging the aging period and by incorporating surface directing agent, the surface area could be increased The surface area was increased to 3.95 m2 g−1 , and the conversion of cyclohexane was 100% in h Cyclohexanol and cyclohexanone were formed in approximately 80:20 ratio The selectivity of the products were improved when 4-(methylamino)benzoic acid was added to the catalyst preparation medium to increase the surface hydrophobicity and the selectivity of cyclohexanol and cyclohexanone was found to be a ratio of 50:50 The greater hydrophobic character of chromium–silica catalyst modified with 4-(methylamino)benzoic acid enhances the interaction of the cyclohexane molecule with the polar catalyst surface for adsorption and subsequent transformation The nitrogen atom lone pair in 4-(methylamino)benzoic acid may form hydrogen bonds with the hydroxyl groups thus retarding the conversion of cyclohexanol to cyclohexanone Again only Cr3+ species were identified to be the active site [31] The effect of pH on the oxidation state of chromium and its influence in the oxidation of styrene was also studied to identify which chromium species was more active in the oxidation reaction [32] The catalysts were prepared at pH 10, pH and pH At pH 10, only Cr(VI) species were found while at pH and pH 3, Cr(VI) and Cr(III) species co-existed Chromium loading at pH 10 (7.3 w/w%) was highest, and it was lowest at pH (2.3 w/w%) At pH 10, the interaction between the negatively charged silicate particles and positively charged chromium ion is high, thus increasing the possibility of Si O Cr bond formation and the adsorption of chromium hydroxide, Cr(OH)3 on the silica support As nitric acid was further added to reduce the pH, adsorbed Cr(OH)3 can be re-dissolved into the solution as Cr(III) ions thus resulting in F Adam et al / Catalysis Today 190 (2012) 2–14 Fig The SEM image of tungsten–silica catalysts from rice husk prepared at (a) pH 10, (b) pH and (c) pH [37] lower chromium content It is hypothesized that the Si O Cr bond especially in the catalyst prepared at pH was strong enough to prevent the oxidation of Cr(III) species to Cr(VI) species during calcination The Cr(VI) species found in the chromium–silica catalyst prepared at pH 10 and pH was due to the oxidation of Cr(III) species in Cr(OH)3 [33] With the aid of cetyltrimethylammonium bromide (CTAB) as a surface-directing agent, the surface area of the catalysts was improved to 143–564 m2 g−1 Higher chromium containing catalysts yielded lower surface area and vice versa The surface of the catalysts was composed of rocky particles Catalysts prepared in acidic media were found to be more active in catalyzing the oxidation of styrene using hydrogen peroxide as oxidant Benzaldehyde was obtained as the major product The maximum conversion of styrene was 99.9% with 63.1% selectivity to benzaldehyde The higher catalytic activity of the chromium–silica catalysts prepared in acidic media is related to the higher surface area and the co-existence of Cr(III) and Cr(VI) species The rate of hydrogen peroxide decomposition is increased in acidic reaction media Recharacterization of the catalyst after reaction indicated a reduction in chromium content and the chromium detected by AAS analysis was 1.0 w/w% after catalytic testing However, the leached chromium species did not contribute significantly to catalytic activity This was confirmed by leaching tests The catalyst was found to be reusable several times without loss of catalytic activity 3.2 Molybdenum The same reaction conditions were used to study the effect of pH on the incorporation of molybdenum into the framework of silica from rice husk [34] AAS analysis demonstrated that highest concentration of molybdenum was in the catalysts prepared in acidic media Spectroscopic analyses showed the presence of Mo(V) and Mo(VI) species on the surface of the molybdenum–silica catalyst prepared at pH while only Mo(VI) species was detected on the surface of the catalyst prepared at pH 10 and pH The pore system in the catalysts narrowed as the pH was reduced This is due to the deposition of molybdenum species into the larger pores thus resulting in a unimodal pore system Another reason could be due to the presence of nitrate ions At pH 10 and pH 7, the presence of NO3 − ions can shift the equilibrium of the surfactant and silicate assembly The NO3 − ion blocks the adsorption of silicate ions on micelles and delays the formation of the silica/surfactant mesophases This can cause incomplete interaction between silicate species and surfactant, resulting in smaller pores being formed by the template The larger pores were formed by the agglomeration of silica nanoparticles during the hydrolysis–condensation process [35,36] Short ordered pore arrangements existed in the molybdenum–silica catalysts prepared at pH 10 and started to deteriorate as the pH was reduced The SEM images indicate that the catalysts had rocky particles with spherical surfaces [33] Molybdenum–silica catalyst prepared at pH showed a higher styrene conversion and benzaldehyde selectivity compared to the other two catalysts Benzaldehyde (Bza) was obtained as the major product The conversion was 82.2% and the Bza selectivity was ca 82.8% A significant amount of molybdenum leached out from the support when it was used for the first time Due to the loss of the active sites, styrene conversion dropped about 50% when the catalyst was reused However, the catalysts remained heterogeneous during consecutive reuse Re-characterization of the used catalyst indicated that only Mo(VI) species were found on the surface of the catalyst The pore system of the catalyst changed from being unimodal to bimodal after catalytic reaction due to leaching The recharacterization of used molybdenum–silica catalyst indicates that the majority of the molybdenum species was physically adsorbed on the surface of the catalyst and most probably this was the Mo(V) species 3.3 Tungsten Tungsten species were inserted into the silica matrix using the same method and conditions as mentioned above [37] The highest tungsten concentration was found in the catalysts prepared at pH while the lowest was found in the catalysts prepared at pH 10 The increasing trend in the immobilization of tungsten content as the pH was decreased can be related to the interaction between tungstate species (WO4 )2− and the silicate species At pH 10, lack of interaction between these two species due to negative charge repulsion, yielded catalysts with lower incorporation of tungsten The interaction became stronger as the negative character of the silica oligomers reduced as the pH approached the isoelectric point F Adam et al / Catalysis Today 190 (2012) 2–14 (a) Si Si Si O O Si W O (b) O O O O O O W 6+ O O O Si Si O O Fig The structure of (a) isolated tungsten species and (b) isolated (WO4 )2− species of silica (∼pH 2) Thus, the catalysts with higher amounts of tungsten formed under conditions of acidic pH The SEM image of the catalyst showed that bright spots started to appear as the acidity of the catalysts preparation was increased The images are shown in Fig EDX analysis detected a slightly higher tungsten concentration on the bright spots compared to the dark areas as shown in the SEM images of the catalyst (Fig 1) Isolated tetrahedral (WO4 )2− species were the only tungsten species found on the surface of the tungsten–silica catalyst prepared at pH 10 UV–vis diffuse reflectance spectroscopic analysis suggested the presence of different kinds of tungsten species Isolated tetrahedral (WO4 )2− species, isolated tungsten species or low oligomeric tungsten oxide species was found in the catalyst prepared at pH On the other hand, tungsten oxide was detected together with isolated tetrahedral (WO4 )2− species, isolated tungsten species or low oligomeric tungsten oxide species on the surface of tungsten–silica catalyst prepared at pH Isolated tungsten species refers to tungsten ions incorporated inside the silica framework as shown in Fig 2(a), whereas the isolated tetrahedral (WO4 )2− species is the species that was formed on the surface of the catalyst as shown in Fig 2(b) XRD analysis indicates that the peak related to the amorphous silica at 2Â = 23◦ started to split into relatively narrow bands with sharp peaks New peaks started to appear as well at 2Â = 27◦ , 29◦ , 33◦ , 34◦ , 42◦ , 47◦ and 48◦ when the pH of the synthesis medium was decreased, indicating phase segregation leading toward the formation of larger WO3 crystals on the catalyst surface [38] The split became more obvious in tungsten–silica catalyst prepared at pH and pH All the catalysts have a bimodal pore system The formation of the bimodal pore system could be due to the presence of nitrate ions [39] and due to the tungsten precursor Normally metal species are able to speed up the condensation and hydrolysis process However this did not happen in this case This can be related to the bulky size of (WO4 )2− species The bulky size of (WO4 )2− species may have prevented the hydrolysis and condensation process from taking place leading to the formation of different pore sizes Some researchers concluded that the FT-IR band around 963 cm−1 in the tungsten–silica material indicated the incorporation of tungsten species inside the silica matrix This band started to diminish when the acidity was decreased This indicates the agglomeration of WO3 crystals leading to the formation of extra-framework WO3 on the support surface, especially in tungsten–silica catalyst prepared in acidic medium [39,40] A similar phenomenon was also observed by us when we incorporated indium into the matrix of silica from rice husk ash [41] As the In3+ ion concentration was increased, this band started to disappear indicating the formation of extra-framework metal oxide on the surface The structure of surface active sites of tungsten–silica catalysts prepared in an acidic medium is shown in Fig The highest styrene conversion of 61.9% and 100% selectivity toward benzaldehyde was achieved when a tungsten–silica catalyst prepared in acidic medium was used Higher concentrations of tungsten and the presence of different kinds of tungsten species have been identified to be the main factors contributing to the higher activity of the catalyst prepared at pH The reaction was Fig Proposed surface active sites of tungsten–silica catalyst prepared at pH [37] proposed to be catalyzed by pertungstic acid like intermediates, with styrene oxide as the intermediate active reagent A small amount of tungsten species was found to be leached from the support and catalyze the reaction homogeneously The physical properties of the catalyst were not affected by the loss of tungsten active sites [38] due to leaching 3.4 Iron Iron is a cheap transition metal which is non-toxic to human health and which has been known to catalyze many organic reactions When 4-(methylamino)benzoic acid was used as a surface directing agent it increased the catalyst surface area [42] The 4(methylamino)benzoic acid was proposed to be attached to the silica matrix via the nitrogen atom The formation of the Si N bond is shown in Fig The surface area of the catalyst increased from 267 to 331 m2 g−1 after modification The increase in surface area was accompanied by a pore size reduction as expected The pore size of the catalyst decreased from 9.2 to 6.0 nm A series of cross-linked lines arranged in an orderly manner was observed in the TEM image of 4-(methylamino)benzoic acid modified iron–silica catalyst, which were not present in the TEM image of unmodified catalyst This could be due to the amine acting as a template during the syntheses Both catalysts were tested in the Friedel–Crafts benzylation of toluene giving 100% toluene conversion The mono-substituted (ortho- and para-) products were found to be the major components in the product The unmodified iron–silica catalyst was found to be less selective to the mono-substituted (ortho- and para-) product compared to the 4-(methylamino)benzoic acid modified iron–silica catalyst In another study, a purely iron–silica catalyst was found to be very active in the oxidation of phenol using hydrogen peroxide as oxidant under mild conditions [43] Oxidation of phenol using this catalyst yielded catechol and hydroquinone as the only products Two signals related to the Q3 and Q4 silicon centers at −100.4 and −108.7 ppm were observed when the catalyst was subjected to 29 Si MAS NMR analysis Signals related to the spinning side bands were observed, suggesting the paramagnetic nature of Fe(III) species [44] which was detected at ca 10 and −210 ppm Oxidation of phenol has been associated with a free radical mechanism by many authors [45–47] In this research, we had proposed a non-free radical mechanism Free radical mechanisms are known to produce benzoquinone which can later be transformed to polymeric materials and tar However these products were not detected in our study The intermediate was formed on the surface of the catalyst assisted by the formation of coordinate bonds by the reactants to the Fe3+ active sites The polar nature of the catalyst strongly suggests the reactants were adsorbed on the catalyst surface via hydrogen bond F Adam et al / Catalysis Today 190 (2012) 2–14 O O Si O- Na+ Si O H3C O N H Strongly basic CH3 Si Si N O + NaOH O Si O HO HO O Fig The formation of Si N bond [42] 3.5 Cobalt Cobalt catalysts, including nanoparticles, have been prepared using rice husk silica as the support Most of the procedures in the literature were expensive, tedious and time consuming However, we introduced a simple way to prepare cobalt–silica catalyst and nanoparticles The sol–gel method was used to prepare the cobalt rice husk silica nanoparticles under mild conditions [48] The cobalt nanoparticles prepared were in the range of 2–15 nm The FT-IR spectra of the nanoparticles indicated some similarities with tungsten–silica catalysts mentioned in Section 3.3 The band at 967 cm−1 disappeared upon cobalt addition into the silica framework This is due to the presence of cobalt silicate or hydrosilicate [49] FT-IR and XRD analyses indicate that the cobalt nanoparticles comprised Co3 O4 and CoO Cobalt nitrate decomposed into an intermediate cobalt silicate phase first and later into Co3 O4 during the drying process Cobalt rice husk silica nanoparticles prepared via this method exhibit both ferromagnetic and antiferromagnetic properties The corresponding saturation magnetization (Ms), coercivity (Hc) and remanent magnetization (Mr) were noted to be 0.245 emu/g, 340.09 Oe and 0.0115 emu/g respectively Ms for cobalt–silica nanoparticles was much lower compared to MbulkCo = 166 emu/g [50] Decrease in Ms is mostly due to the smaller cobalt rice husk silica nanoparticles synthesized in this study Hard magnet behavior is shown from the hysteresis loop by showing large Hc (>100 Oe) [51] The hysteresis of this material show the presence of a ‘curvature’ shape which indicates ferromagnetic (FM) nature and a ‘straight’ shape which correspond to antiferromagnetic (AFM) properties Similar magnetic hysteresis plot was reported for CoO nanoparticles with size ranging from 10 to 80 nm prepared by sol–gel method [52] The antiferromagnetic property of the catalyst is due to the presence of CoO nanoparticles The Co nanoparticles prepared in this work are proposed to follow the core–shell model, in which the core is attributed to ferromagnetic metallic and the shell consists of antiferromagnetic CoO species [53] Metal based catalyst from rice husk ash 4.1 Friedel–Crafts reaction using iron catalysts The Friedel–Crafts (benzylation) reaction between toluene and benzyl chloride has been carried out using solid, environmentally friendly and reusable catalysts (RHA-Fe and RHA-Fe700) [11] The mono-substituted benzyltoluene was the major product and both catalysts yielded more than 92% of the product at 100 ◦ C, in h, without solvent The catalysts show promising activity with almost equal distribution of ortho- and para-isomers Sixteen minor products consisting of various di-substituted isomers were also detected The ortho-substituted product was present in larger proportion (49.53%) compared to the para-substituted (46.01%) product when using RHA-Fe700 as the catalyst The higher yield of orthosubstituted product was due basically, to the presence of ortho-positions for substitution on the toluene molecule For RHAFe, about 48.1% and 44.8% of ortho- and para-substituted products were observed respectively However, RHA-Fe700 gave a significantly lower yield of the di-substituted products compared to RHA-Fe It was found that the RHA-Fe700 gave slightly higher yield (∼97.1%) for the mono-substituted product and significantly lower yield (∼2.8%) for the di-substituted products during the second reusability studies However, there was not much difference in the distribution of the ortho- and para-derivatives 4.2 Rice husk ash supported ruthenium catalyst RHA-Ru (as-synthesized) and RHA-Ru700 (calcined at 700 ◦ C) heterogeneous catalysts were prepared similarly using rice husk ash silica as the support The effect of calcination on the surface and bulk structure of the catalyst was investigated and compared with as-synthesized RHA-Ru catalyst using several physico-chemical techniques [54] XRD studies showed RHA-Ru was largely amorphous (2Â = 22◦ ) with some crystalline peaks present in RHA-Ru700 Ruthenium was shown to be present in the form of its dioxide (RuO2 ) in RHA-Ru700 These materials were further investigated using N2 sorption studies The isotherm and hysteresis loop were shown to be of type IV with type H3 hysteresis respectively for both catalysts according to IUPAC classification The BET surface area of RHA-Ru was 65.1 m2 g−1 compared to RHA-Ru700 (10.4 m2 g−1 ) The significant reduction in the surface area was attributed to a collapse in the pore structure at 700 ◦ C due to the condensation of adjacent silanol groups Fine needle like structure was seen in the SEM micrographs for RHA-Ru700 The needles looked like thin flat elongated pieces of fiber with sharp edges and of nano dimension The width of the needles was estimated to be about 200 nm However, this was not F Adam et al / Catalysis Today 190 (2012) 2–14 Table The effect of different xylene isomers on the percentage conversion and product distribution at 80 ◦ C and Xyl/BC molar ratios of 15:1 [55] Xylene Time (min) Selectivity (%) Time (min) Selectivity (%) RHA-Ga o-Xylene m-Xylene p-Xylene 15 23 32.3 97.7 98.7b 97.3 35 42 61 94.6 96.5b 94.8 71.7 46.8 33.3 RHA-In o-Xylene m-Xylene p-Xylene 10.4 13.5 15.9 97.1 98.3 96.4 23.8 24.5 23.5 94.2 96.4b 94.6 103.4 79.7 67.6 RHA-Fe o-Xylene m-Xylene p-Xylene 2.3 3.4 6.0 93.5 97.8b 95.7 4.0 5.5 10.2 92.7 96.3b 93.3 467.4 316.2 179.2 a b 50% BC conversion TORa Catalyst 90% BC conversion Turnover rate for 50% conversion in mol g−1 s−1 Two mono-substituents 2,4-DMDPM and 2,6-DMDPM in a percentage ratio of about 79:21 observed in RHA-Ru RHA-Ru in general had a porous matrix due to the amorphous structure of the catalyst 4.3 RHA supported gallium, indium, iron and aluminum for the benzylation of xylenes and benzene Liquid phase Friedel–Crafts reaction of xylenes (o-Xyl, m-Xyl and p-Xyl) with benzyl chloride (BC) over the prepared catalyst (RHA-Fe, RHA-Ga and RHA-In) was carried out at 80 ◦ C [55] The differences in activity and selectivity between the xylene isomers and catalysts are shown in Table From Table 1, the RHA-Fe showed the highest catalytic activity whereas RHA-In and RHA-Ga gave higher selectivity to 2,5dimethyldiphenylmethane (2,5-DMDPM) within a shorter time The rate of reaction decreased in the following order: RHAFe > RHA-In > RHA-Ga Iron has a redox potential of +0.77 V while gallium and indium have a redox potential of −0.44 V The higher redox property of iron was expected to play a crucial role for initiating the BC carbocation and showed superior catalytic activity over the rest However, the higher activity of RHA-In over RHA-Ga could be due to the lower amount of non-framework Ga species present on the surface of RHA-Ga The catalyst could be reused several times without significant change in their activity and selectivity [55] In 2009, Ahmed and Adam [56] used aluminum, gallium and iron incorporated RHA for the benzylation of benzene (Bz) with BC Iron based catalyst, showed excellent activity, whereas RHA-Ga gave good selectivity toward diphenylmethane (DPM) However, RHAAl was almost inactive in this reaction due to the low redox property of the Al3+ ion Among the main advantages of these catalyst was that there was, no need for calcination after catalyst preparation and more important was the fact that RHA-Ga and RHA-Fe were not moisture sensitive and can be handled and stored under normal conditions 4.4 RHA supported aluminum, gallium and indium for the tert-butylation of aromatics The tert-butylation of some substituted benzenes (toluene and chlorobenzene) with tert-butyl chloride (TBC) was carried out using RHA-Al, RHA-Ga and RHA-In at 80 ◦ C [57] At the initial stage of the reaction, the tert-butyl cation was formed subsequently via the radical mechanism process, which in turn attacks the benzene ring for the formation of tert-butyl benzene (TBB) and di-tert-butyl benzene (DTBB) via the SN mechanism (main reaction) However, a proton elimination reaction (side reaction) also occurred, resulting in the formation of isobutene dimmers (IBD) and isobutene trimers (IBT) The extent of these side products was found to decrease significantly with time, indicating the reversibility of the oligomerization reactions The catalysts were stable against leaching and were reusable several times but with an observable drop in catalytic activity RHA-Ga lost almost 20% of its activity after each run, whereas, RHA-In was stable until the 3rd run and then lost ∼13% of its activity at the 5th run The deactivation was suggested to be induced by the poisoning effect of the bulky side products that were strongly adsorbed on the catalyst surface Based on the product analysis, a mechanism was proposed for the tert-butylation of aromatics It was suggested the reaction proceeds initially through the radical mechanism for the conversion of TBC to tert-butyl carbocations However, the carbocations remained adsorbed on the catalyst, possibly at the framework position replacing the extra-framework Na+ ions forming tert-butoxide These tert-butoxide species can either attack the aromatic to form the tert-butyl products (SN 1) or can undergo elimination reaction (E1) for the formation of IB monomers The latter species (i.e., IB) has extraordinary reactivity toward polymerization under all types of acidic conditions (i.e., Lewis or Brønsted) It is noteworthy that the polymerization reaction can be initiated by unconverted tertbutyl carbocation or librated HCl The capability of the catalyst for converting the TBC to TB carbocation depends merely on its redox potential and the number of active sites on its surface However, the production of tert-butylated products depends on its ability to activate the aromatic for the SN reaction as well as the high nucleophilicity of the aromatic, i.e., the presence of electron donating and not electron withdrawing substituents in the benzene ring [57] 4.5 Photocatalysis reaction using silica–tin nanotubes Silica–tin nanotubes (RHA-10Sn) with external diameter of 2–4 nm and internal diameter of 1–2 nm were made by a simple sol–gel method at room temperature [24] These nanotubes possess a hollow inner core with open tube ends (Fig 5(a)) The specific surface area of RHA-10Sn was found to be 607 m2 g−1 compared to RHA-silica (315 m2 g−1 ) The increase in surface area suggests that tin particle were well dispersed within the silica matrix No crystalline phase was detected in the high angle powder XRD analysis The root-mean-square roughness and height distribution of RHA-10Sn were found to be 111.5 and 322.6 (nm) from AFM analysis (Fig 5(b)) These high values correlate well to the highly porous tubular material with a high BET surface area The photocatalytic activity of RHA-10Sn was studied toward degradation of methylene blue (MB) under UV-irradiation As a control experiment, dark reaction (without UV and catalyst) and photolysis was conducted to compare with the adsorption and photocatalytic studies About 96% of MB remained unchanged after 60 in the dark reaction The degradation of MB was confirmed with the reduction in concentration after 960 The catalyst RHA-10Sn gave maximum degradation compared to RHA-silica This behavior is due to the wide band gap (Eg = 3.6 eV) of Sn and high F Adam et al / Catalysis Today 190 (2012) 2–14 Fig (a) The TEM micrographs at 110 K, and (b) the 3-D AFM topography image of RHA-10Sn [24] surface area The degradation products were identified as inorganic anions such as nitrate, chloride and sulfate using ion chromatography analysis [24] 4.6 Oxidation of benzene over bimetallic Cu–Ce silica catalysts A series of mesoporous RHA silica supported Cu–Ce bimetal catalyst was prepared with cetyltrimethylammonium bromide (as a template) These catalysts were labeled as RHA-10Cu5Ce, RHA10Cu20Ce, and RHA-10Cu50Ce TG/DTG analysis of the catalysts confirmed the complete removal of the template at 773 K The XRD pattern showed that RHA and metal incorporated silica catalysts have amorphous characteristics due to the presence of a broad peak in the region of 20–30◦ 2Â However, an observed shift of the diffraction band for RHA–10Cu50Ce, to the 25–35◦ 2Â region can be due to the poor crystallization of CeO2 with increase in Ce loading [58] These catalysts were used for a single step oxidation of benzene with H2 O2 as oxidant and acetonitrile as solvent at 343 K under atmospheric pressure The incorporation of two different metals with silica plays a crucial role in the catalytic activity due to a synergy effect between the metal ions The equation for the catalytic oxidation is presented in Scheme In a typical run, 84.3% benzene conversion and 96.4% phenol selectivity was achieved using 70 mg of RHA-10Cu20Ce at 343 K with other parameters kept constant (H2 O2 = 22 mmol; benzene = 11 mmol; acetonitrile = 116 mmol and reaction time of h) The high activity and phenol selectivity observed under mild reaction conditions could be correlated to the enhanced textural properties such as the specific surface area (329 m2 g−1 ), large pore volume (0.95 m3 g−1 ) and good dispersion of loaded Cu and Ce ions which gave more active centers on the amorphous silica However, the mono metal ceria (RHA-20Ce) or copper (RHA-10Cu) showed low activity (23.5% or 47.7%) and phenol selectivity (34.6% or 79.4%) in comparison to the bimetallic catalysts This is an indication that the existence of copper and ceria together in the catalytic system was necessary for improving the oxidation of benzene The oxidation of benzene over different metal loaded catalysts resulted in the same products However, the selectivity for phenol was significantly lower and as a consequence, a higher percentage of hydroquinone and 1,4-benzoquinone were obtained The catalytic oxidation followed the order RHA-10Cu5Ce < RHA10Cu20Ce < RHA-10Cu50Ce while the order of phenol selectivity was RHA-10Cu50Ce < RHA-10Cu5Ce < RHA-10Cu20Ce The catalyst, RHA-10Cu20Ce was found to be the most suitable for this reaction based on its reusability (up to three recycles with some loss in catalytic activity) [58] 4.7 Benzoylation of p-xylene on iron silica catalyst RHA was used to synthesize RHA-5Fe, RHA-10Fe, RHA-15Fe and RHA-20Fe via the sol–gel technique (pH 5.0) at room temperature [59] The acidity of the catalysts was confirmed by pyridine adsorption, and FT-IR spectra show typical bands around 1551 cm−1 and 1565 cm−1 (attributed to Brønsted acid sites) and 1450 cm−1 (attributed to Lewis acid sites) The surface of the catalysts exhibited irregular shaped particles, compared to RHA-silica which showed agglomerates of spherical particles The liquid phase Friedel–Crafts acylation reaction of p-xylene (p-xyl) with benzoyl chloride (BzCl) was carried out over the assynthesized catalyst The RHA-10Fe catalyst exhibited the highest activity for benzoylation of p-xyl The conversion of BzCl and the selectivity toward 2,5-dimethylbenzophenone (2,5-DMBP) were found to be 98.4 and 88.9% respectively at 413 K [59] As the molar ratio increased from 1:5 to 1:20, (BzCl:p-xyl) the BzCl conversion also increased At a molar ratio of 1:20, high conversion of BzCl (86.0%) was observed At the lower concentration of BzCl, more active sites of catalyst are available for adsorption, which results in the formation of active electrophilic benzoylinium cations that can react with p-xyl In addition, the selective formation of 2,5-DMBP was not affected as the molar ratio was changed from 1:5 to 1:20 The benzoylation over different metal loaded catalysts resulted in the same products However, the selectivity of 2,5-DMBP was reduced slightly after the Fe loading increased more than 10 wt.% When the amount of iron increased from to 10 wt.%, the BzCl conversion increased from 77.7 to 98.4% However, further increase of metal loading to 15 and 20 wt.% did not have much effect on the catalytic activity The RHA-SiO2 , did not show any activity for the benzoylation reaction under the same reaction conditions Hence, the presence of iron was crucial for boosting the catalytic activity The RHA-10Fe was successfully reused several times However the amount of Fe on the catalyst was found to be reduced from 7.22 to 4.96 w/w% A decrease in conversion (42.4%) was also observed for the second cycle with insignificant decrease in selectivity of 2,5-DMBP (86.8%) The reduction in conversion is due to the reduced number of metal active sites on the catalyst and may also be due to the blockage of the pore system by products [59] The mechanism for the catalysis involves the formation of an adsorbed BzCl transition species (fast step) This reacts with p-xyl to form 2,5-DMBP (a bimolecular slow step) with the simultaneous elimination of HCl [59] 4.8 Synthesis of nanocrystalline zeolite L from RHA Wong et al [60] have reported the microscopic investigation of aluminosilicate zeolite L (structure code LTL) nanocrystals using 10 F Adam et al / Catalysis Today 190 (2012) 2–14 Scheme The oxidation of benzene to phenol with the Cu–Ce silica catalyst [58] RHA as the reactive silica source in a template-free hydrothermal system Unlike the conventional cylindrical-shaped zeolite L, the nanocrystalline zeolite L synthesized from RHA exhibits a onedimensional channel structure with tablet-like features (shorter c-dimension for better diffusion of products and reactants) The framework structure of zeolite L consists of cancrinite (CAN) cages and hexagonal prisms (D6R), alternating to form columns that run parallel to the c-axis The research interest in the synthesis of zeolite L is based on its excellent catalytic properties and wide applications in host–guest chemistry Microscopic and spectroscopic analyses showed that the nucleation of zeolite L took place in the very early part of the reaction This rapid formation of LTL nanocrystals is due to the use of RHA as the reactive silica source in the precursor solution Fully crystallized zeolite L was achieved after 24 h resulting in a product with a mean crystallite size of 210 nm TEM images (Fig 6) confirmed the arrangement of hexagonal pattern, which is the distinctive feature of zeolite L Organic–inorganic hybrid catalysts 5.1 One-pot synthesis via sol–gel method There are various synthesis methods that have been utilized to attach organic groups to silica surface via the formation of covalent bonds These are post-synthetic functionalization (grafting), co-condensation (direct synthesis), production of periodic mesoporous organosilanes (PMO) and “ship-in-bottle” techniques More recently, Adam et al had successfully immobilized chloropropyltriethoxysilane (CPTES) onto the silica network via a one-pot synthesis using the sol–gel method [61] The 29 Si MAS NMR spectrum of the resulting organo-silica product, RHACCl (Fig 7(a)) shows chemical shifts attributed to Q4 and Q3 [Qn = Si(Osi)n (OH)4−n ], i.e at ı = −109.92 and −100.65 ppm A chemical shift at −65.2 ppm indicates the formation of Si O Si linkage of CPTES to the silicon atom of the silica via three siloxane Fig HR TEM images of solid after heating at (a) h, (b) h, (c) h, and (d) 12 h [60] F Adam et al / Catalysis Today 190 (2012) 2–14 11 Fig The MAS NMR spectra of RHACCl: (a) the 29 Si MAS NMR spectrum for RHACCl and (b) the 13 C MAS NMR spectrum for RHACCl [61] bonds, SiO2 ( O )3 Si CH2 CH2 CH2 Cl (T3 ) The chemical shift at −57.4 ppm was due to two siloxane bonds to the silica matrix, i.e SiO2 ( O )2 Si(OH)CH2 CH2 CH2 Cl The 13 C MAS NMR of RHACCl (Fig 7(b)) showed three peaks with chemical shift at 10.37, 26.70 and 47.69 ppm which corresponds to the C1, C2 and C3 carbons from CPTES respectively [61] 5.2 Grafting method Grafting is a method to functionalize or modify the surface of mesostructured silica with organic groups This process was carried out using RHACCl with saccharine (Sac) (an artificial sweetening agent) [62] and melamine (Mela) [63] The synthesis of silicasaccharine (RHAC-Sac) and silica-melamine (RHAPrMela) catalysts were carried out using dry toluene and triethylamine (deprotonating agent) under reflux conditions at 110 ◦ C EDX confirmed the presence of chlorine (RHACCl; 3.07%), nitrogen (RHAPrMela; 3.65%) and sulfur (RHAC-Sac; 2.29%) respectively RHAPrMela exhibited a hollow nanotube like structure and RHACSac showed agglomerated particles The results of 29 Si MAS NMR studies for both RHA-Sac and RHAPrMela indicated the successful immobilization of these organic molecules on the solid support Chemical shifts were observed which were attributed to Q4 and Q3 silicon atoms A chemical shift at −64.78 and −57.41 (ppm) indicates the formation of Si O Si linkages via three siloxane bonds, (SiO2 )( O )3 Si CH2 CH2 CH2 Sac and (SiO2 )( O )3 Si CH2 CH2 CH2 Mela (T3 ) respectively A chemical shift at −57.4 and −49.16 (ppm) indicates the formation of two siloxane linkages, i.e (SiO2 )( O )2 Si(OH)CH2 CH2 CH2 Sac and (SiO2 )( O )2 Si(OH)CH2 CH2 CH2 Mela (T2 ), to the silica respectively The 13 C MAS NMR of RHA-Sac is shown in Fig 8(a) Several broad chemical shifts at 124 and 130 ppm which were easily assignable to the aromatic carbon at C8, C4, C6 and C9 are apparent The chemical shift of the carbon of the lactam ring (C10) can be seen at 160 ppm The 13 C MAS NMR for RHAPrMela shows two strong chemical shifts at 161.52 and 169.67 ppm with their respective spinning side bands (marked *), indicating that the carbon atoms in melamine are not equivalent To prove the existence of the spinning side bands, the 13 C MAS NMR was recorded at different spin frequencies of MHz (Fig 8(b)), and MHz (Fig 8(c)) The result clearly showed the shifting in the spinning side bands while the main chemical shifts of the melamine ring were not affected The chemical shift at 161.52 ppm was assigned to the two carbon atoms with free amine groups C5 (Scheme 3) which are chemically equivalent The second chemical shift at 169.67 ppm was assigned to the carbon atom of the melamine ring which is bonded to the propyl group C4 (Scheme 3) through the C3 carbon atom 5.3 Esterification using organic–inorganic hybrid catalysts A simple, environmentally friendly, cheap, time-saving and nontoxic catalyst (RHA-Sac [62] and RHAPrMela [63]) was used for the esterification reaction using ethanol and acetic acid A conversion of 66% was achieved at 85 ◦ C, and h reaction time with (acid:alcohol) 1:1 molar ratio The catalyst contains weak basic sites (strong conjugate acid) and the amine group which was believed to play an important role in this catalytic activity However, similar catalytic activity was also obtained when the homogeneous catalyst (Sac) was used Minimal loss of catalytic efficiency was observed when the solid catalyst was reused after regeneration at 150 ◦ C The esterification of acetic acid with ethanol was also studied at 85 ◦ C using RHAPrMela About 73% conversion with 100% selectivity (ethyl acetate) was achieved in the esterification The higher conversion was obtained due to the strong basic character of the secondary amine in RHAPrMela compared to RHA-Sac The esterification of several alcohols were also studied over RHAPrMela The alcohols studied were 1-propanol (conversion = 47%), 1-butanol (conversion = 42%), 2-propanol (conversion = 25%), tert-butanol (conversion = 14%) and benzyl alcohol (conversion = 20%) The conversion generally decreased as the relative molecular mass of the alcohol increased Primary alcohols also showed a higher conversion rate compared to the 2◦ or the 3◦ derivatives as shown for propanol and butanol These variations could be due to stearic effects as demonstrated for 1-propanol, 2-propanol, tert-butanol and benzyl alcohol However, it must be noted that these studies were not carried out at the optimal conditions for the respective alcohols, but rather the conditions for ethanol was used 5.4 Silica from rice husk ash immobilized with 7-amino-1-naphthalene sulfonic acid RHA was functionalized with 3-(chloropropyl)triethoxysilane and 7-amino-1-naphthalene sulfonic acid to prepare a heterogeneous catalyst for the esterification of n-butyl alcohol with different mono- and di-acids with strong Brønsted acid sites Even though the surface area of the catalyst was only 111 m2 g−1 , it gave a conversion of 88% and 100% selectivity toward the ester The esterification reaction was proposed to take place at the terminal SO3 H group The sulfonic group can adsorb the carboxylic acid and form an eight membered transition state for subsequent attack 12 F Adam et al / Catalysis Today 190 (2012) 2–14 Fig The 13 C MAS NMR spectrum of (a) RHA-Sac [62], (b) RHAPrMela at kHz and (c) RHAPrMela at kHz [63] by n-butanol The prepared catalyst was reusable without loss in catalytic activity [64] 5.5 Silica from rice husk ash immobilized with sulfanilic acid RHA was immobilized with sulfanilic acid via 3(chloropropyl)triethoxysilane to prepare acidic heterogeneous catalyst for the solvent free liquid phase alkylation of phenol Kinetic studies conducted at 100, 110 and 120 ◦ C showed that the alkylation of phenol followed a pseudo-first order rate law The activation energy deduced from the Arrhenius plot was found to be 10.4 kcal mol−1 The hydroxyl group in tertiary butyl alcohol (TBA) can easily be protonated by the strong Brønsted acid sites in the catalyst to form the oxonium ion This oxonium ion can form a carbocation when water was removed as by-product The carbocation formed can attack the ortho- or para-position of phenol via formation of transition state to give the ortho- or para-alkyl phenol in a seemingly pseudo-first order reaction The carbocation can undergo a proton elimination to form an alkene which can further attack the ortho- or para-position of phenol as described by Adam et al [65] Current and future progress Catalysts with ordered and oriented pore system and narrow pore size distribution were synthesized recently Mesoporous molecular sieves such as MCM-41 incorporated with metals and organic ligands for catalysis studies have been undertaken Even tough, over the past 20 years, there has been a dramatic increase in the literature on the synthesis, characterization and application of these molecular sieve materials in catalysis, separation, adsorption and host–guest chemistry, more research needs to be undertaken Scheme The prepared catalysts: (a) RHA-Sac [62] and (b) RHAPrMela [63] F Adam et al / Catalysis Today 190 (2012) 2–14 13 Fig The characterization of RHA-MCM-41: (a) the N2 sorption isotherm, at 77 K The inset shows the corresponding BJH pore size distribution, (b) the low angle X-ray diffraction spectra, (c) 29 Si MAS NMR spectra, and (d) TEM micrographs at 450 K [66] due to the promising application of these materials Various factors, such starting material, structure-directing agent (SDA), reaction parameters (pH, temperature, solvent, etc.) influence the formation of these mesoporous structure RHA-MCM-41 with a high specific surface area (1115 m2 g−1 ) and narrow pore size distribution (PSD) (2.3 nm) with a hexagonal arrangement of the mesopores has been synthesized using CTAB as a SDA at 80 ◦ C The characterization of this material is shown in Fig The low angle X-ray diffractogram of RHA-MCM-41 contained four crucial peaks at 2Â 2.43◦ , 4.20◦ , 4.84◦ and 6.30◦ which can be attributed to the (1 0), (1 0), (2 0) and (2 0) diffraction planes [66] These prominent peaks are clear evidence for the presence of a highly ordered mesoporous hexagonal phase with long-range order which was later proved by TEM images MCM-41 prepared from rice husk ash was used as a catalyst for the synthesis of -amino alcohols at 70 ◦ C with toluene as solvent A high selectivity of 94.0% of 2-phenylamino-2-phenylethanol (isomer I) and 5.3% of 2-phenylamino-1-phenylethanol (isomer II) was produced from aminolysis of styrene oxide (SO) [66] As for the future application of the mesoporous molecular sieves, MCM-41 can be immobilized with other organic moieties via post-synthetic methods The goal is to utilize the organic moieties as the active site and the solid to provide the support to convert homogeneous catalysts into heterogeneous ones Closing remarks Silica from rice husk obtained through the methods described in this review has shown great potential to be developed and utilized in many silica related industries, thus, gradually replacing commercial silica From the industrial viewpoint, this cheaper silica precursor has made the mass production of expensive heterogeneous catalysts possible From the environmental point of view, the extraction of silica from rice husk is safe and does not harm the environment Acknowledgments We would like to thank the Malaysian Government for a Research University Grant (Ac No 1001/PKIMIA/811092) and USM-RU-PRGS grant (1001/PKIMIA/832027) which partly supported this work We would also like to thank the Malaysian Ministry of Higher Education for MyBrain15 (MyPhd) scholarship to J Nelson and International Islamic University Malaysia for a scholarship to A Iqbal References [1] D.J Londeree, Silica–titania composites for water treatment, M Eng Thesis, University of Florida, 2002 [2] T.W Dijkstra, R Duchateau, A Rutger, van Santen, A Meetsma, G.P.A Yap, J Am Chem Soc 124 (2002) 9856–9864 [3] J.A.J Conner, W.A Mallow, R.S Rieber, Patent Genius 6524543 [4] Z Xinhong, W Xiaolai, J Mol Catal A: Chem 261 (2007) 225–231 [5] E.A.M.S Adil, Aluminium, gallium, indium and iron supported onto rice husk ash silica as catalysts for the Friedel–Craft alkylation reactions of aromatic compounds, PhD Thesis, School of 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Khanam, Microporous Mesoporous Mater 156 (2012) 16–21 ... almost inactive in this reaction due to the low redox property of the Al3+ ion Among the main advantages of these catalyst was that there was, no need for calcination after catalyst preparation and... surface of the catalyst assisted by the formation of coordinate bonds by the reactants to the Fe3+ active sites The polar nature of the catalyst strongly suggests the reactants were adsorbed on the. .. molybdenum? ?silica catalyst prepared at pH while only Mo(VI) species was detected on the surface of the catalyst prepared at pH 10 and pH The pore system in the catalysts narrowed as the pH was reduced