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The utilization of rice husk silica as a catalyst Reviewand recent progress (1)

Catalysis Today 190 (2012) 2–14

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Catalysis

Today

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c a t t o

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 School of Chemical Sciences,Universiti Sains Malaysia,11800Penang,Malaysia

Kulliyah of Science,International Islamic University Malaysia,25200Kuantan,Pahang,Malaysia

a r t i c l e

i n f o

Article history:

Received 15October 2011

Received in revised form 18April 2012Accepted 21April 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 extrac-tion 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 5–6h.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.

Contents 1.Introduction (3)

Synthesis methodologies ............................................................................................................................42.1.Silica from rice husk:by calcination and solvent extraction ................................................................................42.2.Modi?cation of silica:incorporation of metal and immobilization of organic ..............................................................43.

Transition metal-based catalysts from rice husk ....................................................................................................43.1.Chromium ....................................................................................................................................43.2.Molybdenum .................................................................................................................................53.3.Tungsten ......................................................................................................................................53.4.Iron ...........................................................................................................................................63.5.Cobalt .........................................................................................................................................74.

Metal based catalyst from rice husk ash .............................................................................................................74.1.Friedel–Crafts reaction using iron catalysts ..................................................................................................74.2.Rice husk ash supported ruthenium catalyst ................................................................................................74.3.RHA supported gallium,indium,iron and aluminum for the benzylation of xylenes and benzene ........................................84.4.RHA supported aluminum,gallium and indium for the tert-butylation of aromatics ......................................................84.5.Photocatalysis reaction using silica–tin nanotubes ..........................................................................................84.6.Oxidation of benzene over bimetallic Cu–Ce silica catalysts ................................................................................94.7.Benzoylation of p -xylene on iron silica catalyst .............................................................................................94.8.Synthesis of nanocrystalline zeolite L from RHA.............................................................................................95.

Organic–inorganic hybrid catalysts ..................................................................................................................105.1.One-pot synthesis via sol–gel method .......................................................................................................105.2.Grafting method ..............................................................................................................................115.3.Esteri?cation using organic–inorganic hybrid catalysts .....................................................................................115.4.Silica from rice husk ash immobilized with 7-amino-1-naphthalene sulfonic acid.........................................................115.5.Silica from rice husk ash immobilized with sulfanilic acid ..................................................................................

?Corresponding author.Tel.:+6046533567;fax:+6046574854.

E-mail addresses:farook@usm.my ,farook dr@https://www.wendangku.net/doc/9c10640683.html, (F.Adam).

F.Adam et al./Catalysis Today190 (2012) 2–143

6.Current and future progress (12)

7.Closing remarks (13)

Acknowledgments (13)

References (13)

1.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 SiO2is based upon a SiO4tetrahedron,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,result-ing from irregular packing of the SiO4tetrahedral unit as well as the incomplete condensation,results in a heterogeneous sur-face(i.e.,non-uniformity in the dispersion of silanol groups)for synthesized silica.The various silanols can have different adsorp-tion activities and current knowledge indicates that the isolated silanols are the more reactive species.With increasing temper-ature of heat treatment,the silica surface becomes hydrophobic due to the condensation of surface hydroxyl groups resulting in the formation of siloxane https://www.wendangku.net/doc/9c10640683.html,mercial silica manufacture is a multi-step process involving high heat and pressure,mak-ing 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 than3000publications,especially in the area of meso-porous materials made from silica.The inertness of silica aided with the ease of structural tailoring has made it a good inorganic mate-rial 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 tetraethy-lorthosilicate(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?nd 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 stud-ied 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 covers1%of the earth’s surface.Globally,approxi-mately600million tonnes of rice are produced each year.For every 1000kg of paddy milled,about220kg(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 of20%ash,38%cellulose,22%lignin,18%pentose and2% other organic components[10,11].Even though some of this husk is converted into end products such as feedstock[12]and adsor-bent[13]most is burnt openly,causing environmental and

health

Scheme1.Various utilizations of the rice husk silica.

problems especially in poor and developing countries.Therefore,it is very important to?nd 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 con-siderably according to the burning temperature.The silica in the ash undergoes structural transformations depending on the conditions (time,temperature,etc.)of combustion.At550–800?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 speci?cation for the particular end use(see Scheme1).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.Tran-sition 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 incor-poration of metals.Tsay et al.[15]have used aluminum sulfate, nickel nitrate and aqueous ammonia to prepare Ni/RHA–Al2O3 via simple impregnation and ion exchange methods.Chen et al.[16]have reported the preparation Cu/RHA using the deposition–precipitation method and calcination at673K,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-41mesoporous materials using CTAB as structure-directing agent(SDA),for the adsorption

4 F.Adam et al./Catalysis Today190 (2012) 2–14

of chlorinated volatile organic compounds and photocatalytic degradation of herbicide(alachlor)[19]and tetramethylammo-nium[20].Some researchers had also used direct hydrothermal synthesis[21]and gasi?cation processes[22]to obtain MCM-41 from rice husk ash.In2009,Jang et al.[23]synthesized highly siliceous MCM-48from RHA using a cationic neutral surfactant mixture as the structure-directing template.The materials were used for CO2adsorption.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 in?u-ence 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.

2.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 metal-lic impurities in RH can be reduced to negligible levels by stirring with nitric acid[24]or re?uxing 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 pro-cess,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 from500?C to800?C for5–6h in a muf?e furnace.The RHA was then dissolved using sodium hydroxide to obtain sodium silicate.Modi?cations were undertaken to this pro-cedure according to catalyst preparation parameters.Thus,Chang et al.[17]pyrolyzed RH at900?C for1h in a furnace and N2?ow to obtain a black crude product,which was then pyrolyzed again in air under the same conditions to obtain a white ash.In2006,Chan-drasekhar et al.[26]studied critically the effect of acid treatment, calcination temperature and the rate of heating of RH and showed that these parameters in?uenced the surface area,reactivity toward lime and brightness of the ash.

2.2.Modi?cation 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?nal 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 resul-tant amorphous mixture of metal oxide,hydroxides and metal salts leading to the formation of an M O M bond[28].To obtain a struc-tural 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 syn-thesize 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 to2–3by 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 immobi-lized RHAC-R catalysts,where R is the ligand.

3.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 cat-alytic 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

0.542m2g?1.Subsequent preparation resulted in a surface area of

1.20m2g?1when it was calcined at500?C for5h.Surface direct-ing 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 cata-lyst had improved the selectivity of cyclohexanone but lowered the selectivity of cyclohexanol in the oxidation of cyclohexane.The conversion of cyclohexane was27.13%when the as-synthesized chromium–silica catalyst was used while the conversion was 1

2.69%when calcined chromium–silica catalyst was used instead. Only a slight change was observed in terms of cyclohexene conver-sion and product selectivity when these catalysts were used.Both catalysts yielded100%cyclohexanone selectivity.

By prolonging the aging period and by incorporating surface directing agent,the surface area could be increased.The surface area was increased to3.95m2g?1,and the conversion of cyclo-hexane was100%in6h.Cyclohexanol and cyclohexanone were formed in approximately80:20ratio.The selectivity of the products were improved when4-(methylamino)benzoic acid was added to the catalyst preparation medium to increase the surface hydropho-bicity and the selectivity of cyclohexanol and cyclohexanone was found to be a ratio of50:50.The greater hydrophobic character of chromium–silica catalyst modi?ed with4-(methylamino)benzoic acid enhances the interaction of the cyclohexane molecule with the polar catalyst surface for adsorption and subsequent transforma-tion.The nitrogen atom lone pair in4-(methylamino)benzoic acid may form hydrogen bonds with the hydroxyl groups thus retarding the conversion of cyclohexanol to cyclohexanone.Again only Cr3+ species were identi?ed to be the active site[31].

The effect of pH on the oxidation state of chromium and its in?uence in the oxidation of styrene was also studied to identify which chromium species was more active in the oxidation reac-tion[32].The catalysts were prepared at pH10,pH7and pH3. At pH10,only Cr(VI)species were found while at pH7and pH 3,Cr(VI)and Cr(III)species co-existed.Chromium loading at pH 10(7.3w/w%)was highest,and it was lowest at pH3(2.3w/w%). At pH10,the interaction between the negatively charged silicate particles and positively charged chromium ion is high,thus increas-ing the possibility of Si O Cr bond formation and the adsorption of chromium hydroxide,Cr(OH)3on the silica support.As nitric acid was further added to reduce the pH,adsorbed Cr(OH)3can be re-dissolved into the solution as Cr(III)ions thus resulting in

F.Adam et al./Catalysis Today190 (2012) 2–14

Fig.1.The SEM image of tungsten–silica catalysts from rice husk prepared at(a)pH10,(b)pH7and(c)pH3[37].

lower chromium content.It is hypothesized that the Si O Cr bond especially in the catalyst prepared at pH3was strong enough to prevent the oxidation of Cr(III)species to Cr(VI)species during cal-cination.The Cr(VI)species found in the chromium–silica catalyst prepared at pH10and pH7was 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 to143–564m2g?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 catalyz-ing the oxidation of styrene using hydrogen peroxide as oxidant. Benzaldehyde was obtained as the major product.The maximum conversion of styrene was99.9%with63.1%selectivity to benzalde-hyde.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.Re-characterization of the catalyst after reaction indicated a reduction in chromium content and the chromium detected by AAS anal-ysis was1.0w/w%after catalytic testing.However,the leached chromium species did not contribute signi?cantly to catalytic activ-ity.This was con?rmed 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 high-est 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 cat-alyst prepared at pH3while only Mo(VI)species was detected on the surface of the catalyst prepared at pH10and pH7.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 pH10and pH7,the pres-ence 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 sili-cate species and surfactant,resulting in smaller pores being formed by the template.The larger pores were formed by the agglomer-ation of silica nanoparticles during the hydrolysis–condensation process[35,36].Short ordered pore arrangements existed in the molybdenum–silica catalysts prepared at pH10and 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 pH3showed a higher styrene conversion and benzaldehyde selectivity compared to the other two catalysts.Benzaldehyde(Bza)was obtained as the major product.The conversion was82.2%and the Bza selectivity was ca.

82.8%.A signi?cant amount of molybdenum leached out from the support when it was used for the?rst time.Due to the loss of the active sites,styrene conversion dropped about50%when the cat-alyst 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 re-characterization 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 3while the lowest was found in the catalysts prepared at pH10. 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 pH10,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

6 F.Adam et al./Catalysis Today 190 (2012) 2–14

O Si

O O

W O

6+(b)

Si O

Si O W O

Si O

(a)

Fig.2.The structure of (a)isolated tungsten species and (b)isolated (WO 4)2?

species.

of silica (～pH 2).Thus,the catalysts with higher amounts of tung-sten 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.1.EDX analysis detected a slightly higher tungsten concentra-tion on the bright spots compared to the dark areas as shown in the SEM images of the catalyst (Fig.1).Isolated tetrahedral (WO 4)2?species were the only tungsten species found on the surface of the tungsten–silica catalyst prepared at pH 10.

UV–vis diffuse re?ectance spectroscopic analysis suggested the presence of different kinds of tungsten species.Isolated tetrahe-dral (WO 4)2?species,isolated tungsten species or low oligomeric tungsten oxide species was found in the catalyst prepared at pH 7.On the other hand,tungsten oxide was detected together with iso-lated tetrahedral (WO 4)2?species,isolated tungsten species or low oligomeric tungsten oxide species on the surface of tungsten–silica catalyst prepared at pH 3.

Isolated tungsten species refers to tungsten ions incorporated inside the silica framework as shown in Fig.2(a),whereas the iso-lated tetrahedral (WO 4)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 2?=23?started to split into 3relatively narrow bands with sharp peaks.New peaks started to appear as well at 2?=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 WO 3crystals on the catalyst surface [38].The split became more obvious in tungsten–silica catalyst prepared at pH 7and pH 3.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.How-ever this did not happen in this case.This can be related to the bulky size of (WO 4)2?species.The bulky size of (WO 4)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 963cm ?1in 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 WO 3crystals leading to the formation of extra-framework WO 3on 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 In 3+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.3.

The highest styrene conversion of 61.9%and 100%selectivity toward benzaldehyde was achieved when a tungsten–silica cata-lyst prepared in acidic medium was used.Higher concentrations of tungsten and the presence of different kinds of tungsten species have been identi?ed to be the main

factors contributing to the higher activity of the catalyst prepared at pH 3.The reaction was

Fig.3.Proposed surface active sites of tungsten–silica catalyst prepared at pH 3[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 reac-tions.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.4.

The surface area of the catalyst increased from 267to 331m 2g ?1after modi?cation.The increase in surface area was accompanied by a pore size reduction as expected.The pore size of the cata-lyst decreased from 9.2to 6.0nm.A series of cross-linked lines arranged in an orderly manner was observed in the TEM image of 4-(methylamino)benzoic acid modi?ed iron–silica catalyst,which were not present in the TEM image of unmodi?ed 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 unmodi?ed iron–silica catalyst was found to be less selective to the mono-substituted (ortho -and para -)product compared to the 4-(methylamino)benzoic acid modi?ed 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 Q 3and Q 4silicon centers at ?100.4and ?108.7ppm were observed when the catalyst was subjected to 29Si 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.10and ?210ppm.

Oxidation of phenol has been associated with a free radical mechanism by many authors [45–47].In this research,we had pro-posed 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 Fe 3+active sites.The polar nature of the cata-lyst strongly suggests the reactants were adsorbed on the catalyst surface via hydrogen bond.

F.Adam et al./Catalysis Today 190 (2012) 2–14

H 3C

O O

CH 3

O O Si Si

+NaOH

Fig.4.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.How-ever,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 2–15nm.The FT-IR spectra of the nanoparticles indicated some similari-ties with tungsten–silica catalysts mentioned in Section 3.3.The band at 967cm ?1disappeared upon cobalt addition into the sil-ica framework.This is due to the presence of cobalt silicate or hydrosilicate [49].FT-IR and XRD analyses indicate that the cobalt nanoparticles comprised Co 3O 4and CoO.Cobalt nitrate decom-posed into an intermediate cobalt silicate phase ?rst and later into Co 3O 4during the drying process.Cobalt rice husk silica nanopar-ticles prepared via this method exhibit both ferromagnetic and antiferromagnetic properties.The corresponding saturation mag-netization (Ms ),coercivity (Hc )and remanent magnetization (Mr )were noted to be 0.245emu/g,340.09Oe and 0.0115emu/g respec-tively.Ms for cobalt–silica nanoparticles was much lower compared to M bulkCo =166emu/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 (>100Oe)[51].The hysteresis of this material show the presence of a ‘curvature ’shape which indicates ferro-magnetic (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 10to 80nm prepared by sol–gel method [52].The antiferromagnetic property of the catalyst is due to the presence of CoO nanoparti-cles.The Co nanoparticles prepared in this work are proposed to follow the core–shell model,in which the core is attributed to fer-romagnetic metallic and the shell consists of antiferromagnetic CoO species [53].

4.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 1h,without solvent.

The catalysts show promising activity with almost equal dis-tribution 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-Fe700as the catalyst.The higher yield of ortho -substituted product was due basically,to the presence of 2ortho -positions for substitution on the toluene molecule.For RHA-Fe,about 48.1%and 44.8%of ortho -and para -substituted products were observed respectively.However,RHA-Fe700gave a signif-icantly lower yield of the di-substituted products compared to RHA-Fe.

It was found that the RHA-Fe700gave slightly higher yield (～97.1%)for the mono-substituted product and signi?cantly 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 sur-face 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 (2?=22?)with some crystalline peaks present in RHA-Ru700.Ruthenium was shown to be present in the form of its dioxide (RuO 2)in RHA-Ru700.These materials were further investigated using N 2sorption studies.The isotherm and hysteresis loop were shown to be of type IV with type H3hysteresis respectively for both catalysts according to IUPAC classi?cation.The BET surface area of RHA-Ru was 65.1m 2g ?1compared to RHA-Ru700(10.4m 2g ?1).The signi?cant 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 ?at elongated pieces of ?ber with sharp edges and of nano dimension.The width of the needles was estimated to be about 200nm.However,this was not

8 F.Adam et al./Catalysis Today190 (2012) 2–14

Table1

The effect of different xylene isomers on the percentage conversion and product distribution at80?C and Xyl/BC molar ratios of15:1[55].

Catalyst Xylene50%BC conversion90%BC conversion TOR a

Time(min)Selectivity(%)Time(min)Selectivity(%)

RHA-Ga o-Xylene1597.73594.671.7 m-Xylene2398.7b4296.5b46.8 p-Xylene32.397.36194.833.3

RHA-In o-Xylene10.497.123.894.2103.4 m-Xylene13.598.324.596.4b79.7 p-Xylene15.996.423.594.667.6

RHA-Fe o-Xylene 2.393.5 4.092.7467.4 m-Xylene 3.497.8b 5.596.3b316.2 p-Xylene 6.095.710.293.3179.2

a Turnover rate for50%conversion in?mol g?1s?1.

b Two mono-substituents2,4-DMDPM and2,6-DMDPM in a percentage ratio of about79:21.

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 at80?C[55].The differences in activity and selectivity between the xylene isomers and catalysts are shown in Table1.

From Table1,the RHA-Fe showed the highest catalytic activity whereas RHA-In and RHA-Ga gave higher selectivity to2,5-dimethyldiphenylmethane(2,5-DMDPM)within a shorter time. The rate of reaction decreased in the following order:RHA-Fe>RHA-In>RHA-Ga.Iron has a redox potential of+0.77V while gallium and indium have a redox potential of?0.44V.The higher redox property of iron was expected to play a crucial role for initiat-ing 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 signi?cant change in their activity and selectivity[55].

In2009,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,RHA-Al 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 at80?C[57].At the initial stage of the reaction,the tert-butyl cation was formed subsequently via the rad-ical mechanism process,which in turn attacks the benzene ring for the formation of tert-butyl benzene(TBB)and di-tert-butyl ben-zene(DTBB)via the S N1mechanism(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 signi?cantly with time,indicating the reversibility of the oligomer-ization reactions.The catalysts were stable against leaching and were reusable several times but with an observable drop in cat-alytic activity.RHA-Ga lost almost20%of its activity after each run, whereas,RHA-In was stable until the3rd run and then lost～13% of its activity at the5th 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 conver-sion of TBC to tert-butyl carbocations.However,the carbocations remained adsorbed on the catalyst,possibly at the framework posi-tion replacing the extra-framework Na+ions forming tert-butoxide. These tert-butoxide species can either attack the aromatic to form the tert-butyl products(S N1)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 Br?nsted).It is noteworthy that the polymerization reaction can be initiated by unconverted tert-butyl 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 S N1reaction as well as the high nucle-ophilicity 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 2–4nm and internal diameter of1–2nm 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 speci?c surface area of RHA-10Sn was found to be 607m2g?1compared to RHA-silica(315m2g?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 be111.5and322.6(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 pho-tocatalytic studies.About96%of MB remained unchanged after 60min in the dark reaction.The degradation of MB was con?rmed with the reduction in concentration after960min.The catalyst RHA-10Sn gave maximum degradation compared to RHA-silica. This behavior is due to the wide band gap(E g=3.6eV)of Sn and high

F.Adam et al./Catalysis Today190 (2012) 2–14

Fig.5.(a)The TEM micrographs at110K,and(b)the3-D AFM topography image of RHA-10Sn[24].

surface area.The degradation products were identi?ed as inorganic anions such as nitrate,chloride and sulfate using ion chromatogra-phy 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,RHA-10Cu20Ce,and RHA-10Cu50Ce.TG/DTG analysis of the catalysts con?rmed the complete removal of the template at773K.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 of20–30?2?.However,an observed shift of the diffrac-tion band for RHA–10Cu50Ce,to the25–35?2?region can be due to the poor crystallization of CeO2with increase in Ce loading[58].

These catalysts were used for a single step oxidation of benzene with H2O2as oxidant and acetonitrile as solvent at343K under atmospheric pressure.The incorporation of two different metals with silica plays a crucial role in the catalytic activity due to a syn-ergy effect between the metal ions.The equation for the catalytic oxidation is presented in Scheme2.

In a typical run,84.3%benzene conversion and96.4%phe-nol selectivity was achieved using70mg of RHA-10Cu20Ce at 343K with other parameters kept constant(H2O2=22mmol;ben-zene=11mmol;acetonitrile=116mmol and reaction time of5h). The high activity and phenol selectivity observed under mild reaction conditions could be correlated to the enhanced textural properties such as the speci?c surface area(329m2g?1),large pore volume(0.95m3g?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%or47.7%)and phenol selectivity(34.6%or79.4%) 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 cata-lysts resulted in the same products.However,the selectivity for phenol was signi?cantly lower and as a consequence,a higher per-centage of hydroquinone and1,4-benzoquinone were obtained. The catalytic oxidation followed the order RHA-10Cu5Ce

RHA was used to synthesize RHA-5Fe,RHA-10Fe,RHA-15Fe and RHA-20Fe via the sol–gel technique(pH5.0)at room temperature [59].The acidity of the catalysts was con?rmed by pyridine adsorp-tion,and FT-IR spectra show typical bands around1551cm?1 and1565cm?1(attributed to Br?nsted acid sites)and1450cm?1 (attributed to Lewis acid sites).The surface of the catalysts exhib-ited 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 as-synthesized catalyst.The RHA-10Fe catalyst exhibited the highest activity for benzoylation of p-xyl.The conversion of BzCl and the selectivity toward2,5-dimethylbenzophenone(2,5-DMBP)were found to be98.4and88.9%respectively at413K[59].

As the molar ratio increased from1:5to1:20,(BzCl:p-xyl)the BzCl conversion also increased.At a molar ratio of1:20,high con-version 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 forma-tion of2,5-DMBP was not affected as the molar ratio was changed from1:5to1:20.The benzoylation over different metal loaded cat-alysts resulted in the same products.However,the selectivity of 2,5-DMBP was reduced slightly after the Fe loading increased more than10wt.%.When the amount of iron increased from5to10wt.%, the BzCl conversion increased from77.7to98.4%.However,further increase of metal loading to15and20wt.%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.How-ever the amount of Fe on the catalyst was found to be reduced from7.22to4.96w/w%.A decrease in conversion(42.4%)was also observed for the second cycle with insigni?cant decrease in selec-tivity of2,5-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 form2,5-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–

Scheme 2.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 one-dimensional 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 solu-tion.Fully crystallized zeolite L was achieved after 24h resulting in a product with a mean crystallite size of 210nm.TEM images (Fig.6)con?rmed the arrangement of hexagonal pattern,which is the distinctive feature of zeolite L.

https://www.wendangku.net/doc/9c10640683.html,anic–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 cova-lent bonds.These are post-synthetic functionalization (grafting),co-condensation (direct synthesis),production of periodic meso-porous organosilanes (PMO)and “ship-in-bottle”techniques.More recently,Adam et al.had successfully immobilized chloropropy-ltriethoxysilane (CPTES)onto the silica network via a one-pot synthesis using the sol–gel method [61].

The 29Si MAS NMR spectrum of the resulting organo-silica product,RHACCl (Fig.7(a))shows chemical shifts attributed to Q 4and Q 3[Q n =Si(Osi)n (OH)4?n ],i.e.at ?=?109.92and ?100.65ppm.A chemical shift at ?65.2ppm indicates the formation of Si O Si linkage of CPTES to the silicon atom of the silica via three

siloxane

Fig.6.HR TEM images of solid after heating at (a)0h,(b)4h,(c)8h,and (d)12h [60].

F.Adam et al./Catalysis Today190 (2012) 2–14

Fig.7.The MAS NMR spectra of RHACCl:(a)the29Si MAS NMR spectrum for RHACCl and(b)the13C MAS NMR spectrum for RHACCl[61].

bonds,SiO2(O)3Si CH2CH2CH2Cl(T3).The chemical shift at ?57.4ppm was due to two siloxane bonds to the silica matrix, i.e.SiO2(O)2Si(OH)CH2CH2CH2Cl.The13C MAS NMR of RHACCl (Fig.7(b))showed three peaks with chemical shift at10.37,26.70 and47.69ppm which corresponds to the C1,C2and C3carbons 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 arti?cial sweetening agent)[62]and melamine(Mela)[63].The synthesis of silica-saccharine(RHAC-Sac)and silica-melamine(RHAPrMela)catalysts were carried out using dry toluene and triethylamine(deprotonat-ing agent)under re?ux conditions at110?C.

EDX con?rmed the presence of chlorine(RHACCl;3.07%),nitro-gen(RHAPrMela;3.65%)and sulfur(RHAC-Sac;2.29%)respectively. RHAPrMela exhibited a hollow nanotube like structure and RHAC-Sac showed agglomerated particles.

The results of29Si MAS NMR studies for both RHA-Sac and RHAPrMela indicated the successful immobilization of these organic molecules on the solid support.Chemi-cal shifts were observed which were attributed to Q4and Q3silicon atoms.A chemical shift at?64.78and?57.41 (ppm)indicates the formation of Si O Si linkages via three siloxane bonds,(SiO2)(O)3Si CH2CH2CH2Sac and (SiO2)(O)3Si CH2CH2CH2Mela(T3)respectively.A chemi-cal shift at?57.4and?49.16(ppm)indicates the formation of two siloxane linkages,i.e.(SiO2)(O)2Si(OH)CH2CH2CH2Sac and(SiO2)(O)2Si(OH)CH2CH2CH2Mela(T2),to the silica respectively.

The13C MAS NMR of RHA-Sac is shown in Fig.8(a).Several broad chemical shifts at124and130ppm which were easily assignable to the aromatic carbon at C8,C4,C6and C9are apparent.The chemical shift of the carbon of the lactam ring(C10)can be seen at160ppm.

The13C MAS NMR for RHAPrMela shows two strong chemical shifts at161.52and169.67ppm 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, the13C MAS NMR was recorded at different spin frequencies of 7MHz(Fig.8(b)),and5MHz(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 161.52ppm was assigned to the two carbon atoms with free amine groups C5(Scheme3)which are chemically equivalent.The second chemical shift at169.67ppm was assigned to the carbon atom of the melamine ring which is bonded to the propyl group C4(Scheme3) through the C3carbon atom.

5.3.Esteri?cation using organic–inorganic hybrid catalysts

A simple,environmentally friendly,cheap,time-saving and non-toxic catalyst(RHA-Sac[62]and RHAPrMela[63])was used for the esteri?cation reaction using ethanol and acetic acid.A conversion of 66%was achieved at85?C,and6h reaction time with(acid:alcohol) 1:1molar ratio.The catalyst contains weak basic sites(strong con-jugate 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 ef?ciency was observed when the solid catalyst was reused after regeneration at150?C.

The esteri?cation of acetic acid with ethanol was also stud-ied at85?C using RHAPrMela.About73%conversion with100% selectivity(ethyl acetate)was achieved in the esteri?cation. The higher conversion was obtained due to the strong basic character of the secondary amine in RHAPrMela compared to RHA-Sac.The esteri?cation of several alcohols were also stud-ied over RHAPrMela.The alcohols studied were1-propanol (conversion=47%),1-butanol(conversion=42%),2-propanol(con-version=25%),tert-butanol(conversion=14%)and benzyl alcohol (conversion=20%).The conversion generally decreased as the rel-ative molecular mass of the alcohol increased.Primary alcohols also showed a higher conversion rate compared to the2?or the 3?derivatives as shown for propanol and butanol.These variations could be due to stearic effects as demonstrated for1-propanol, 2-propanol,tert-butanol and benzyl alcohol.However,it must be noted that these studies were not carried out at the optimal con-ditions 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 with3-(chloropropyl)triethoxysilane and7-amino-1-naphthalene sulfonic acid to prepare a heteroge-neous catalyst for the esteri?cation of n-butyl alcohol with different mono-and di-acids with strong Br?nsted acid sites.Even though the surface area of the catalyst was only111m2g?1,it gave a conversion of88%and100%selectivity toward the ester.The esteri?cation reaction was proposed to take place at the terminal SO3H 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–

Fig.8.The 13C MAS NMR spectrum of (a)RHA-Sac [62],(b)RHAPrMela at 7kHz and (c)RHAPrMela at 5kHz [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 100,110and 120?C showed that the alkylation of phenol followed a pseudo-?rst order rate law.The activation energy deduced from the Arrhenius plot was found to be 10.4kcal mol ?1.The hydroxyl group in tertiary butyl alcohol (TBA)can easily be protonated by the strong Br?nsted 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-?rst 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].

6.Current and future progress

Catalysts with ordered and oriented pore system and nar-row pore size distribution were synthesized recently.Mesoporous molecular sieves such as MCM-41incorporated with metals and organic ligands for catalysis studies have been undertaken.Even tough,over the past 20years,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 3.The prepared catalysts:(a)RHA-Sac [62]and (b)RHAPrMela [63].

F.Adam et al./Catalysis Today190 (2012) 2–14

Fig.9.The characterization of RHA-MCM-41:(a)the N2sorption isotherm,at77K.The inset shows the corresponding BJH pore size distribution,(b)the low angle X-ray diffraction spectra,(c)29Si MAS NMR spectra,and(d)TEM micrographs at450K[66].

due to the promising application of these materials.Various factors, such starting material,structure-directing agent(SDA),reaction parameters(pH,temperature,solvent,etc.)in?uence the formation of these mesoporous structure.

RHA-MCM-41with a high speci?c surface area(1115m2g?1) and narrow pore size distribution(PSD)(2.3nm)with a hexagonal arrangement of the mesopores has been synthesized using CTAB as a SDA at80?C.The characterization of this material is shown in Fig.9.The low angle X-ray diffractogram of RHA-MCM-41con-tained four crucial peaks at2?2.43?,4.20?,4.84?and6.30?which can be attributed to the(100),(110),(200)and(210)diffrac-tion 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-41prepared from rice husk ash was used as a catalyst for the synthesis of?-amino alcohols at70?C with toluene as solvent.

A high selectivity of94.0%of2-phenylamino-2-phenylethanol(iso-mer I)and5.3%of2-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-41can 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.

7.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 com-mercial silica.From the industrial viewpoint,this cheaper silica precursor has made the mass production of expensive heteroge-neous 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 sup-ported 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 schol-arship to A.Iqbal.

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