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Applied Catalysis A: General 488 (2014) 208–218

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom epage: www.elsev ier .com/ locate /apcata

ffect of Si precursor on structural and catalytic properties of nanosizeagnesium silicates

atabathini Narasimharaoa,∗, Tarek T. Alia,b, Salem Bawakeda, Sulaiman Basahela

Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Kingdom of Saudi ArabiaChemistry Department, Faculty of Science, Sohag University, P.O. Box 82524, Sohag, Egypt

r t i c l e i n f o

rticle history:eceived 7 July 2014eceived in revised form 15 August 2014ccepted 28 September 2014vailable online 6 October 2014

eywords:agnesium silicate

a b s t r a c t

Two Si precursors, inorganic (sodium silicate) and organic (tetraethoxy silane), were used to synthe-size magnesium silicate (MgSil) nanomaterials. The effect exerted by the nature of Si precursors on themorphology and structural properties of the samples was studied by chemical analyses, X-ray diffrac-tion, HRTEM, FTIR spectroscopy, XPS, N2 adsorption, solid-state NMR spectroscopy and TPD of CO2 andNH3 techniques. The characterization results show that MgSil-org sample possessed hollow nanosphereswhich are composed of small platelets and sheets; in contrast, MgSil-inorg sample showed nanotubularstructure due to more alkaline nature of the inorganic Si precursor. Additionally, MgSil-org sample have

anomaterialsetraethoxy silaneodium silicateiodieselransesterification.

different textural characteristics, acidic and basic properties. MgSil-org sample have higher surface areas,more uniform mesoporous pores, and more number of acidic and basic sites as well as higher activitiesin transesterification of tributyrin and esterification of palmitic acid with methanol than MgSil-inorgsample. MgSil-org sample is stable and showed excellent reusability for more than five cycles withoutany loss of activity in transesterification and esterification reactions.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Study of the structure and properties of nanostructural materi-ls (particles, tubes, rods, and sheets) is of great importance fromoth the theoretical and for the use in various practical areas suchs materials science, catalysis, medicine, and microelectronics [1].any efforts have focused on new types of silicate nanotubes

ecause these materials, as mesoporous materials, are also promis-ng candidates for various applications [2]. The physicochemicalroperties of silicate-based materials are very much comparable tohose of carbon, and the rich porous structures and tunable compo-ition of silicates make them rather appealing for real applications,ncluding catalysis, gas adsorption, and separation processes.

Natural or synthetic magnesium silicates (MgSil) belonging tohe clay minerals group have industrial importance [3]. The mag-esium ions in the crystal lattice of magnesium silicate clay-type

aterials are exchangeable with transition metal ions and the

esultant materials are capable of adsorbing both acidic and basicons [4]. These materials were tested as refining and purifying

∗ Corresponding author. Tel.: +966 538638994; fax: +966 26952292.E-mail addresses: nkatabathini@kau.edu.sa, katabathini@yahoo.com

K. Narasimharao).

ttp://dx.doi.org/10.1016/j.apcata.2014.09.050926-860X/© 2014 Elsevier B.V. All rights reserved.

agents in the production of polyether polyols and found that theyare excellent, deodorizing, potassium ion adsorbing agents [5]. TheMgSil material and its derivatives were also used for advancedapplications such as specific catalyst [6], fire-retardant paintingmaterial [7], and a template for the synthesis of carbon nanofibers[8]. The structure of naturally occurring MgSil (sepiolite) is derivedfrom talc-like ribbons that expand with a width of three pyroxenechains. Each ribbon is connected to the next through an invertedSi–O–Si bond, resulting in a staggered talc layer with a continuoustetrahedral sheet and a discontinuous octahedral sheet. The dis-continuous nature of the octahedral sheet allows for the formationof rectangular, tunnel-like micropores, which run parallel to thefiber axis and are filled completely by zeolitic water under ambientconditions [9].

It is known that components of the SiO2–MgO system easilyreact with each other, yielding MgSil material. Temuujin et al. [10]reported the formation of a poorly crystalline layer-lattice typeof MgSil materials. Torro-Palau et al. [11] modified the structureof MgSil by treating the material at high temperature (1000 ◦C).Jesionowski et al. [12] studied the production of highly dispersed

MgSil materials at a pilot scale. Korytkova et al. [13] studiedthe effect of the hydrothermal conditions on MgSil formationand showed that the MgSil nanotubes formed most rapidly at350–400 ◦C, when SiO2 and MgO were used as reactants. The

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uthors also revealed that formation of the tubes occurred via ini-ial formation of thin silicate nanoplatelets. Jancar and Suvarov [14]eported that Mg3Si2O5(OH)4 tubes could form at lower tempera-ures providing highly basic reaction mixtures were used. Wangt al. [15] synthesized porous silicate nanotubes (such as mainroup metal and transition metal silicates) by hydrothermal syn-hesis method under strong basic conditions at 180 ◦C. The authorsbtained thermally stable silicate nanotubes with large surfacereas and narrow size distributions.

Corma and Martin-Aranda [16] reported that strong base cata-ysts can be prepared by substituting a part of the Mg ions locatedt the borders of the channels of MgSil with alkaline ions and thoseaterials exhibited higher basicity than the alkaline X-zeolites. The

uthors also reported that these catalysts were able to catalyze theondensation of benzaldehydes with active methylene compoundst moderate temperatures. Recently, we reported synthesis, char-cterization, and catalytic application of nitridated crystalline andmorphous MgSil materials [17].

The depletion of world petroleum reserves and increased envi-onmental concerns have stimulated the search for alternativeenewable fuels that are capable of fulfilling an increasing energyemand [18]. Biodiesel fuel (fatty acid methyl esters), synthesizedrom vegetable oils, has similar physical properties to petrochem-cal diesel and is considered the best alternative fuel candidate forse in diesel engines [19]. Biodiesel production involves the cat-lytic transesterification of long- and branched-chain triglyceridesith alcohols to produce monoesters and glycerol [20]. Current

yntheses use homogeneous alkaline agents, such as K or Na alkox-des or hydroxides [21]; however, removal of the soluble base aftereaction is a major problem, because aqueous quenching resultsn the formation of stable emulsions and saponification, renderingeparation and purification of the methyl ester difficult. As a result,iodiesel production by these routes is still not cost-competitiveith petrochemical diesel fuel [22]. Kiss et al. [23] summarized

he pros and cons of manufacturing biodiesel via fatty acid ester-fication using metal oxide solid catalysts. Recently, Grecea et al.24] developed superior robust superacid catalyst for multiproductatty acid esterification. Use of a solid base catalyst offers severalrocess advantages, including the elimination of a quenching stepand associated contaminated water waste) to isolate the prod-cts and the opportunity to operate a continuous process [25].olid bases, including zeolites [26], alkali earth oxides [27], andydrotalcites [28], have been investigated in transesterificationeactions.

In the present study, hydrothermal synthesis method underutogenous pressure was used to synthesize nanosize MgSil mate-ials. The effect of Si precursor and thermal treatment on thetructure of nanosize MgSil and its application as catalyst for trans-sterification of glyceryl tributyrate with methanol for biodieselroduction was also studied. The structural and textural properties,amely morphology, crystallite size, surface area, and pore struc-ure of nanosize MgSil samples, are investigated by using elementalnalysis, powder XRD, HRTEM, FTIR, XPS, solid-state NMR, TPD and2 physisorption techniques. An attempt was made to correlate the

tructural and textural properties of the MgSil materials with theirransesterification activity.

. Experimental

.1. Materials

Magnesium nitrate (Mg(NO3)2 6H2O), sodium silicateNa2SiO3), tetraethoxy silane (Si(OC2H5)4), and sodium hydroxideNaOH) were purchased from Aldrich, UK. All reagents were ofnalytical grade and used as-received without further purification.

s A: General 488 (2014) 208–218 209

2.2. Synthesis of magnesium silicate nanomaterials

Magnesium silicate nanomaterials were synthesized byhydrothermal synthesis method. Mg(NO3)2 6H2O (weight corre-sponding to 3 mol of MgO) was dissolved in water/ethanol (1:6ratio) and Na2SiO3 or Si(OC2H5)4 (weight corresponding to 4 molof SiO2) was used to form a white precipitate. Then NaOH (1 g)was added to maintain the basicity of the contents (pH ≈ 10.4in the case of Si(OC2H5)4) and pH ≈ 13.1 in the case of Na2SiO3.The contents were transferred into a Teflon-lined autoclave andhydrothermally treated at 180 ◦C for 2 days. The obtained precipi-tate was filtered and washed with distilled water to remove ionspossibly remaining in the final products and dried at 120 ◦C in air.The MgSil samples prepared using sodium silicate and tetraethoxysilane as Si precursor was labeled as MgSil-inorg and MgSil-org,respectively.

2.3. Characterization

The elemental composition of the materials was determinedby ICP-AES, Optima 7300DV, Perkin Elmer Corporation, USA. X-ray powder diffraction (XRD) studies were performed for all of theprepared solid samples using a Bruker diffractometer (Bruker D8advance target). The patterns were run with Cu K� and a monochro-mator (� = 1.5405 A) at 40 kV and 40 mA. The crystallite size of theMgSil phase was calculated using Scherrer’s equation:

D = B�

ˇ1/2 cos �(1)

where D is the average crystallite size of the phase under investiga-tion, B is the Scherer constant (0.89), � is wavelength of the X-raybeam used (1.54056 A), ˇ1/2 is the full-width at half-maximum(FWHM) of the diffraction peak, and � is the diffraction angle. Theidentification of different crystalline phases in the samples was per-formed by comparing the data with the Joint Committee for PowderDiffraction Standards (JCPDS) files.

FTIR spectra were recorded on a Perkin-Elmer Spectrum100 FTIR spectrometer. A Philips CM200FEG microscope, 200 kV,equipped with a field emission gun was used for HRTEM analysis.The coefficient of spherical aberration was Cs = 1.35 mm. The infor-mation limit was better than 0.18 nm. High-resolution images witha pixel size of 0.044 nm were taken with a CCD camera.

The textural properties of the prepared samples weredetermined from nitrogen adsorption/desorption isotherm mea-surements at –196 ◦C using a model NOVA 3200e automated gassorption system (Quantachrome, USA). Prior to measurement, eachsample was degassed for 6 h at 150 ◦C. The specific surface area,SBET, was calculated by applying the Brunauer–Emmett–Teller(BET) equation. The average pore radius was estimated fromthe relation 2Vp/SBET, where Vp is the total pore volume (atP/P0 = 0.975). Pore size distribution over the mesopore range wasgenerated by the Barrett–Joyner–Halenda (BJH) analysis of thedesorption branches, and the values for the average pore size werecalculated.

The XPS measurements were carried out by using a SPECS GmbHX-ray photoelectron spectrometer. Prior to analysis, the sampleswere degassed under vacuum inside the load lock for 16 h. Thebinding energy of the adventitious carbon (C 1s) line at 284.6 eVwas used for calibration, and the positions of other peaks werecorrected according to the position of the C 1s signal. For the mea-surements of high-resolution spectra, the analyzer was set to thelarge-area lens mode with energy steps of 25 meV and in Fixed

Analyzer Transmission (FAT) mode with pass energies of 34 eV anddwell times of 100 ms. The photoelectron spectra of the four sam-ples were recorded with the acceptance area and angle of 5 mm indiameter and up to 5◦. The base pressure during all measurements

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10 K. Narasimharao et al. / Applied C

as 5 × 10−9 mbar. A standard dual anode excitation source withg K� (1253.6 eV) radiation was used at 13 kV and 100 W.

29Si MAS NMR spectra of the samples were recorded usingruker 400 MHz spectrometer and referenced to TMS, where aulse delay of 60 s was used. NH3 and CO2-TPD patterns of theamples were recorded using Chembet-3000 (Quantachrome, USA)nstrument.

.4. Transesterification of glyceryl tributyrate and esterificationf palmitic acid with methanol

Transesterification of glyceryl tributyrate with methanol waserformed in a stirred batch reactor with samples withdrawn peri-dically for analysis on a Shimadzu GC17A gas chromatographtted with a DB-1 capillary column (film thickness, 0.25 mm; i.d.,.32 mm; length, 30 m), and AOC 20i autosampler. The reactionas performed at 60 ◦C using 3 wt% of catalyst, 0.01 mol (3 cm3)

f glyceryl tributyrate (98%, Aldrich), and 0.3036 mol (12.5 cm3) ofethanol with 2.5 mmol (0.587 cm3) of hexyl ether as an internal

tandard. The catalyst samples were separated from the reac-ion mixture for recycling by centrifugation. Reactions were run

Fig. 1. (A) Powder XRD patterns and (B

s A: General 488 (2014) 208–218

for 6 h with initial rates determined at conversions <30%, withreactions continued for 24 h. Catalyst selectivity and overall massbalances (closure >98%) were determined using reactant and prod-uct response factors derived from multipoint calibration curves.Catalyst stability was verified by performing leaching tests in hotmethanol, with MgSil catalysts refluxed for 6 h in methanol, afterwhich the solid was removed. The presence of soluble species in therecovered methanol was subsequently investigated by assessingthe activity of the residual solvent in transesterification reaction.

Esterification was performed at 80 ◦C using 3 wt% of catalyst,0.01 mol of palmitic acid (98%, Aldrich), and 0.3036 mol (12.5 cm3)methanol (98%, Fisher) with 2.5 mmol (0.587 cm3) of hexyl ether(97%, Aldrich) as an internal standard.

3. Results and discussion

The powder XRD patterns of MgSil samples calcined at 500 ◦C

are shown in Fig. 1A. The diffraction pattern of samples exhibitedreflection patterns of MgSil structure with the chemical formula of3MgO·4SiO2·2H2O (JCPDS No 03-0174). The apparent broadening ofall the peaks indicated that as obtained silicates were composed of

) FTIR spectra of MgSil samples.

atalysis A: General 488 (2014) 208–218 211

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K. Narasimharao et al. / Applied C

anosize crystals usually have distinctive clay-type structures withilicon-to-oxygen ratios within the layers [29]. The particle size ofgSil samples was determined using Scherrer’s equation. The full-idth at half-maximum (FWHM) of the major peak at 2� = 34.5o

as used for crystallite size calculation. The crystallite size of the as-ynthesized MgSil-inorg is approximately 20.4 nm, and it was notedo decrease to 12.5 nm after calcination at 500 ◦C. As-synthesized

gSil-org sample showed crystallite size to 45.5 nm and upon calci-ation at 500 ◦C decrease in crystallite size to 34.4 nm was observed.

The Fourier transform infrared (FTIR) spectra of two MgSil sam-les are shown in Fig. 1B. The band at 1645 cm−1 due to HO–Hending and interstitial water molecules was appeared in both theamples. Broad and strong peaks at 860–1175 and 540–450 cm−1

ere appeared in the spectra of both MgSil samples that could bessigned to the presence of silicate groups [30]. The peaks due tohe silicate groups were well resolved in MgSil-inorg sample com-ared to MgSil-org that related to the presence of amorphous silicaaterial along with MgSil nanomaterial.In the inset of Fig. 1 (B), FTIR spectra in the region of

000–4000 cm−1 was presented. A sharp peak at 3685 cm−1 due toctahedral Mg–O–H unit can be observed in the MgSil-inorg sample31]. A broad intense band at 3400 cm−1 owing to adsorbed molec-lar water appeared in the MgSil-org sample. It is known that theTIR spectrum of sodium metasilicate [32] exhibit bands at 583,16, and 873 cm−1 and sodium form of MgSil additionally showsand at 1390 cm−1 that are related to the presence of Si–O–Na andg–O–Na bonds [31]. The MgSil samples prepared in this study

id not have any of these bands, indicating that the synthesizedamples did not contain the metasilicate structure.

Fig. 2 shows the SEM micrographs of the two MgSil samples.he MgSil-inorg sample in Fig. 2A shows significant quantities ofong nanotubes with lengths of several hundreds of nanometers.he outer diameters of the tubes ranged from 10 to 15 nm. As

hown in Fig. 2B, the MgSil-org sample was composed of uniformpherical particles and the surface was possessed rough and porousorphology. The average size of the spheres was uniformly about

0 nm.

Fig. 3. TEM images of MgSil-inorg: (A) low magnification (B) high magnificat

Fig. 2. SEM images of (A) MgSil-inorg and (B) MgSil-org samples.

To examine the structure of the MgSil nanomaterials, the sam-ples were further characterized by TEM analysis. Fig. 3 shows theTEM micrographs of the two MgSil samples. The low-magnification

view in Fig. 3A shows the nanotubes of regular cylindrical shapeand by bundles of hollow tubes with two ends open. The diam-eters of tubes were about 10–12 nm and lengths up to several

ion; and MgSil-org: (C) low magnification and (D) high magnification.

212 K. Narasimharao et al. / Applied Catalysis A: General 488 (2014) 208–218

Table 1Chemical composition, acidic and basic properties of MgSil samples.

Catalyst Chemical composition Acidic and basic properties

ICP-AES analysis XPS analysis NH3 (mmol g−1) CO2 (mmol g−1)

hlcob5

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surAmsgS0s3

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Mg Si O Na Mg

MgSil-inorg 33.5 11.8 54.2 0.5 31.6

MgSil-org 33.0 12.6 54.0 0.4 30.9

undreds of nanometers. At higher magnification in Fig. 3B, the hol-ow, open-ended multiwall tubular structure of the material waslearly observed. They were typically wider in the centers taperingff toward the ends, as might be expected for structures formedy the rolling of layers. The inner diameters were approximately

nm.Fig. 3C shows the low-magnification view of MgSil-org sample. It

an be observed that the sample is constituted of fully and partiallyormed hollow spheres. The particle size distribution is around0 nm. From the HRTEM image, Fig. 3D, one can obviously deter-ine that the hollow spheres are composed of small platelets and

heets. It can also be seen that the spheres are aggregated togethero form a porous layers with a large amount of thin lamellae.

The bulk chemical composition of the samples was investigatedsing ICP-AES analysis and corresponded to the composition ofheoretical formula 3MgO·4SiO2·2H2O. The coexistence of Mg andi indicating the formation of magnesium silicates and the sam-les also contained small amount of sodium at a concentrationf ∼0.4 wt% (Table 1). The presence of magnesium and silicon ofgSil-inorg and MgSil-org samples indicates that the total atomic

atio of magnesium to silicon is about 3:4. It is consistent with theolecular formula described above.Solid-state 29Si MAS NMR spectroscopy is a powerful tool to

tudy the chemical environment and bonding patterns of the SiO4nits. 29Si MAS NMR experiments for two MgSil samples were car-ied out at room temperature and NMR spectra are shown in Fig. 4.s can be seen in Fig. 4, the 29Si MAS NMR of all the samples isainly composed of different tetrahedral sites. The MgSil samples

howed a sharp intense peak at –95.0 ppm corresponding to Q3roups (Qn corresponds to the Si atoms that connected with n otheri atoms through oxygen bridges, where n can be varied between

and 4). Q3-type structure is consistent with a talc-like crystaltructure with SiO4 structural units forming layers (composition of

MgO·4SiO2·2H2O) through networking [33].

The MAS NMR results agree well with XRD and Raman analy-is results. Besides, a weak chemical shift which was assigned to2 (–85.0 ppm) was observed, which was due to the surface silicon

Fig. 4. 29Si MAS NMR spectra of (A) MgSil-inorg and (B) MgSil-org samples.

Si O Na

11.4 54.8 0.3 95.0 0.4611.7 53.6 0.2 202.0 0.98

atoms. In addition, two sharp peaks, the MgSil samples showed abroad peak at –110 ppm and which could be due to the presenceof (Si–OH) functional groups. The relative proportion of Q3/Q2 ismore in the case of MgSil-inorg sample than the MgSil-org sample,indicating enhancement in the number of Q3 silicon sites and pro-gressive structural polymerization in the MgSil-inorg sample [34].These results are in quite accordance with the elemental analysisand FTIR results.

The XPS spectra of the MgSil-org and MgSil-inorg samples areshown in Fig. 5. High-resolution Si 2p, Mg 2p and O 1s peaksare broad and suggesting that a distribution of chemical states ispresent. The salient features of the XPS results are that there isno change in the average Si, Mg and O chemical state in the sam-ples, since they both showed the peaks at same binding energy andthe mass percentage of Si, Mg and O elements remain the almostsame (Table 1). The binding energy of the Si 2p peak for these twosamples is approximately 102.1 eV (Fig. 5), 1.7 eV lower than thebinding energy of SiO2 (103.8 eV) [35].

The appearance of Mg 2p peak at 49.2 eV implies that the sur-face Mg components in the MgSil samples are not belong to MgOor metallic Mg. Corneille et al. [36] observed Mg 2p peak at approx-imately 50.5 eV for the MgO sample synthesized using Mg thinfilms in the presence of oxygen. They also reported that the bindingenergy of Mg 2p peak for oxidized magnesium sample appears at49.6 eV which is very close to the binding energy observed for Mg2p peaks in the MgSil samples.

The binding energy (530.9 eV) shown by O 1s spectra of bothMgSil samples are in accordance with the O 1s binding energy posi-tion of both suboxidized magnesium and magnesium silicate whichwould generally exist between the SiO2 binding energy (533.3 eV)and MgO binding energy (531.8 eV). The observed result clearlyindicates that the majority of oxygen atoms were existed in theform of non-bridging (Mg–O–Si–) mode.

Nitrogen adsorption–desorption isotherms of the MgSil sampleswere shown in Fig. 6. The isotherms of the samples can be catego-rized as type II with H3-type hysteresis loop, indicating that thesesamples are mesoporous in nature with particles giving rise to slit-shaped pores. The hysteresis loop between the two branches didnot close completely until the relative pressure P/P0 had returnedto 0.4 and 0.5 in desorption branch of MgSil-org and MgSil-inorgsamples, respectively, indicating a broad distribution of pore sizein the prior sample than the later one. This was confirmed bythe Barrett–Joyner–Halenda (BJH) pore-sized distribution obtainedfrom the adsorption branch (inset). A broad distribution of poresizes ranging from 20 A to more than 1400 A was observed witha maximum near 200 A in the case of the MgSil-org sample; incontrast, the MgSil-inorg sample showed distribution of pore sizesranging between 20 and 900 A. The MgSil samples had a signifi-cantly high BET surface area; 124 and 255 m2 g−1 for MgSil-inorgand MgSil-org samples, respectively. The total pore volume of theMgSil-org sample (1.253 cm3 g−1) is much higher than the porevolume of the MgSil-inorg (0.902 cm3 g−1) sample. The differencein the textural properties of the two samples could be due to

the difference in extent of fracture of bridging bonds, and theresultant reduction in the number of silanol groups (Si–OH) alongthe axis of layers can lead to change in the size and number ofpores.

K. Narasimharao et al. / Applied Catalysis A: General 488 (2014) 208–218 213

99 100 101 102 10 3 10 4 105

600 0

800 0

1000 0

1200 0

1400 0

1600 0

Inte

nsity

(a.u

.)

Binding ener gy (eV)

Si2p

MgSil-orgMgSil -inorg

45 46 47 48 49 50 51 52 532000

4000

6000

8000

10000

12000

14000

16000

Inte

nsity

(a.u

.)

Binding ener gy (e V)

Mg2p

MgSil-or gMgSil-inor g

525 526 527 528 529 530 531 532 53 3 534 535 53 6 537

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

Inte

nsity

(a.u

.)

g ene

O1s

MgSil- orgMgSil -inorg

tra of

tccs

torpoTp

Bindin

Fig. 5. XPS spec

The concentration of acidic and basic sites of MgSil is an impor-ant physicochemical characteristic that determines its impact onatalytic performance. The CO2 and NH3-TPD measurements werearried out to determine the base and acid strength of the MgSilamples.

Fig. 7A and B represents the CO2-TPD and NH3-TPD patterns ofhe MgSil samples, respectively. It is well known that desorptionf probe molecule from weak sites occurs at lower temperatureanges, whilst desorption from strong sites occurs at higher tem-

erature ranges. It is also well established that the basic characterf the solids is associated to carbonation of the surface basic sites.he CO2-TPD profile of the MgSil-org sample showed desorptioneaks due to three basic sites; weak (around 140 ◦C), medium (at

Fig. 6. N2 adsorption–desorption isotherms of MgSil

rgy (eV)

MgSil samples.

400 ◦C) and strong (at 620 ◦C); in contrast, the MgSil-inorg sam-ple showed small peak due to medium basic sites at 400 ◦C anda major strong basic sites at 640 ◦C. The area of desorption peakson the TPD profiles of the MgSil-org sample was much more thanthat on the MgSil-inorg sample. This observation indicates that Mgprepared with tetraethoxy silane as Si precursor had much morebasic sites, which was much more beneficial to base-catalyzed reac-tions. The total basicity of the MgSil-org sample was 0.98 mmol/g.On the other hand, low-intense CO2 adsorption peak centered at

◦

about 410 C ascribed to moderate basic sites and a more intensepeak at 640 ◦C attributed to strong basic sites were detected on theMgSil-inorg sample. The total basicity of the MgSil-inorg samplewas 0.46 mmol/g.

samples, pore size distribution patterns (inset).

214 K. Narasimharao et al. / Applied Catalysis A: General 488 (2014) 208–218

75 150 22 5 300 37 5 450 52 5 60 0 67 5 75 0

Nor

mal

ized

TC

D s

igna

l (m

V)

Temperature (oC)

(A)

MgSil- org

MgSil-in org

100 150 200 250 300 350 400 45 0 500 55 0 600 65 0 700

Nor

mal

ized

TC

D s

igna

l (a.

u.)

Temperature (oC)

(B)

MgSil-ino rg

MgSil -org

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08070670

75

80

85

90

95

100

MgSi-inorg

Con

vers

ion

of tr

ibut

yrin

(%)

Reaction temperature (oC)

MgSi-or g

(A)

groni-liSgMgro-liSgMgroni-liSgMgro-liSgM0

20

40

60

80

10080 oC

Sele

ctiv

ity (%

)

Meth yl butyrate Di glyceri de Monogl ycerid e Glycerine

60 oC (B)

Fig. 8. (A) Conversion of trybutyrin at different reaction temperatures. (B) Selec-◦

produced with prolonged reaction times. Monoglceride does not

Fig. 7. TPD patterns of MgSil samples: (A) CO2 and (B) NH3.

The MgSil-org sample showed two broad NH3 desorption peaks,he first at low temperature (240 ◦C) and the second one centeredt high temperature (555 ◦C), whereas the MgSil-inorg samplehowed only one desorption peak at high temperature (650 ◦C). InTIR analysis, the MgSil-org sample showed the presence of zeoliticater. Thus, the desorption peak at 240 ◦C on MgSil-org can be

ttributed to NH3 replacing the zeolitic water site. As comparedith the MgSil-inorg sample, NH3 desorption maximum at 240 ◦C

eems to be due to the unique characteristics of the MgSil-orgample structure. This peak disappears in the MgSil-inorg sample,ndicating that the zeolitic water site could be the main place tonteract with the MgSil core-shell particles. The total amount of des-rbed NH3 for MgSil-inorg was only 95 mmol/g, as shown in Table 1.he MgSil-org sample had much higher amount (202 mmol/g) thanhe MgSil-inorg sample.

Jung and Grange [37] reported that composite of Ti(OH)4 andi(OH)4 showed a high increase in the Lewis and Bronsted acid sites.reviously, Gao and Wachs also observed that TiO2–SiO2 mixedxides show a high acidity than their parent oxides [38]. Kataokand Dumesic [39] suggested that the bridging oxygen of metal–O–Sin mixed oxide is the main location of protons able to act as Bronstedcid sites. The generation of strong acid sites in the MgSil sam-les can thus be explained by the formation of Mg–O–Si bonds,ue to the connectivity between Mg and Si atoms. For the MgSil-

norg sample, both the desorption peaks of NH3 and CO2 are smallnd their intensity are weak, which can be attributed to very smallmount of acidic and basic sites. However, the MgSil-org samplehowed broad NH3 and CO2-TPD profiles, indicating that the sur-ace acid and basic strength were widely distributed. The relativemounts of NH3 and CO2 desorbed from the MgSil-org sample were

wo times higher than that from the MgSil-inorg sample (Table 1).hese CO2- and NH3-TPD patterns confirm the co-existence of acidites and basic sites on the surface of the MgSil samples and their

tivity of products at 60 and 80 C over MgSil catalysts [3 wt.% of catalyst, 0.01 mol(3 cm3) of glyceryl tributyrate and 0.3036 mol (12.5 cm3) of methanol with 2.5 mmol(0.587 cm3) of hexyl ether as an internal standard].

basicity and acidity were influenced by the Si precursor used tosynthesize the MgSil nanomaterial.

Fig. 8A and B illustrates the conversion of tributyrin and theproduct distribution obtained at different reaction temperatures of60, 70 and 80 ◦C on the MgSil samples. It is clear that the MgSil-orgsample exhibit significant different catalytic performances in trans-esterification. It can be seen that the conversion levels increasedwith the increase in temperature. After 45 min, the MgSil-org sam-ple offered 92.2, 95.1 and 99.5% at 60, 70 and 80 ◦C, respectively.Under identical reaction conditions, the conversion of tributyrinover the MgSil-inorg sample is only 72.3, 80.4 and 84.8% at thesereaction temperatures. The high conversion rates of tributyrin overthe MgSil-org sample are related to its superior physicochemicalproperties. Even though the conversion of tributyrin is differentfor the MgSil samples, the product distributions over these twocatalysts are almost the same. The lower catalytic activity for MgSil-inorg can be mainly attributed to its low surface area and lessnumber of active basic sites per unit surface area.

The selectivity data are consistent with the reaction Scheme 1,where the diglycerides are the first products formed, and the selec-tivity towards methylbutyrate is around 98% at low conversions. Adrop in selectivity to methylbutyrate observed while glycerin was

reach considerable high selectivity values and has a tendency todisappear with the reaction time, while the glycerin was reachedmaximum of 8% selectivity for the MgSil-org sample.

K. Narasimharao et al. / Applied Catalysis A: General 488 (2014) 208–218 215

ributy

cro

Fueo(

Scheme 1. Reaction pathway of (i) transesterification of t

Fig. 9A shows tributyrin conversion with methanol at 60 ◦C for

atalyst concentrations of 1 and 3 wt% of MgSil catalysts in theeaction mixture. It can be seen that the conversion of tributyratever both MgSil catalysts shows a good linear relationship with the

10 20 30 40 50 60 70 80 90 10 0 11 0

20

30

40

50

60

70

80

90

100

Con

vers

ion

of tr

ibut

yrat

e (%

)

Reaction time (min. )

1 wt. % MgSil-org 3 wt. % MgSil-org 1 wt.% MgSil-inorg 3 wt.% MgSil-inorg

(A)

0 15 30 45 600

20

40

60

80

100

Con

vers

ion

of tr

ibut

yrin

(%)

Reaction time (min.)

6:1 9:1 12:1

(B)

ig. 9. (A) Effect of the amount of MgSil catalysts [0.01 mol (3 cm3) of glyceryl trib-tyrate and 0.3036 mol (12.5 cm3) of methanol with 2.5 mmol (0.587 cm3) of hexylther as an internal standard, reaction temperature: 80 ◦C]. (B) Effect of molar ratiof methanol to trybutyrin on tributyrin transesterification over MgSil-org catalystreaction temperature, 60 ◦C; rotation speed, 600 rpm).

rin and (ii) esterification of palmitic acid with methanol.

catalyst loading. Without any catalyst addition, the transesterifica-tion is not carried out even after 180 min. Kim et al. [40] reportedthat if the transesterification of triglycerides is controlled by thesurface reaction on catalysts, then the conversion of triglyceridesover them should show a linear relationship with the catalyst load-ing. In this study, the stirring speed of 600 rpm was maintained inall the runs and the external diffusion can be excluded. The conver-sions over the two MgSil samples may be controlled by the differentrate-determining steps, which result in the differences in catalyticperformance with the change in the catalyst loading. This differ-ence could be mainly related to the migration rate of the reactantsin the pores of the catalysts.

The rate constants were calculated and used to compare theperformance of the MgSil catalysts. The rate constants for trans-esterification of tributyrin with methanol over MgSil catalysts arealso compared to those associated with the pure oxides (MgO andSiO2) and physical mixture of MgO and SiO2 in Table 2. The simplestway to compare specific activity is to examine the rate constant forthe consumption of tributyrin as characterized by k1. The rate con-stant for dibutyrin conversion to monobutyrin, represented by k2,confirmed the activity pattern by tributyrin loss in each catalyst.The rate of monobutyrin consumption to form glycerol during thesequential reaction was not included due to the glycerol forma-tion was very low through most of the reaction. The rate constantswere normalized by the exposed surface areas determined by N2physisorption experiments.

The results in Table 2 illustrate the effect of Si precursor on thereactivity of MgSil catalysts. Two important observations can bedrawn from the results. First, the MgSil-org sample showed highcatalytic activity than the MgSil-inorg sample for transesterifica-tion of tributyrin. Second, the MgSil-org sample was 300% moreactive than pure MgO, on a surface area basis. This finding was sig-nificant, and the activity test was repeated twice to confirm theresult. SiO2 was inactive for transesterification under the standardconditions of our study. And also, a physical mixture of MgO andSiO2 in a ratio of 3:4 converted tributyrin at a rate similar to that ofpure MgO (Table 2).

To investigate the effects of the methanol-to-tributyrite molarratio for the two catalysts, the transesterification experiments wereconducted by changing the molar ratio from 6:1 to 12:1, while keep-

ing the temperature and the catalyst amount constant at 80 ◦C and1 wt%, respectively. Fig. 9B demonstrates the effect of molar ratioof methanol to tributyrin, on tributyrin conversion for the MgSil-org sample. The reaction developed rapidly within 30 min and the

216 K. Narasimharao et al. / Applied Catalysis A: General 488 (2014) 208–218

Table 2Surface areas and transesterification rate constants for the catalysts.

Catalyst Surface area (m2 g−1) Cumulative pore volume (cm3 g−1) k1 (×106)a (l mol−1 m−2 s−1) k2 (×106)a (l mol−1 m−2 s−1)

MgSil-inorg 124 0.902 1.35 ± 0.07 0.24 ± 0.08MgSil-org 255 1.253 2.59 ± 0.03 0.33 ± 0.09MgO 56 0.024 0.86 ± 0.05 0.10 ± 0.03SiO2 292 0.156 0.00 ± 0.00 0.00 ± 0.00MgO:SiO -mixb 312 0.146 0.87 ± 0.04 0.12 ± 0.02

crrftlcot

cruaescMi

weos(aogc

mdTacrTac

ftf

TR

2

a Errors represent 95% confidence intervals on fitted reaction rate constants.b Physical mixture of MgO and SiO2 in a Mg:Si 3:4 molar ratio.

onversion varied from 60 to 80%, depending on the different molaratios of methanol to tributyrin; in 45 min, the transesterificationeached a state of equilibrium, the conversion of tributyrin reachedrom 94.2 for 6:1 to 100% for 12:1. A similar pattern was observed inhe case of the MgSil-inorg sample; however, this catalyst requiresonger reaction times to convert tributyrin. It is known that the stoi-hiometry of transesterification requires 3 mol of methanol per molf triglycerides; an excess of methanol can shift the equilibrium tohe right side and biodiesel yield was improved.

The acidic and basic nature of the MgSil samples makes theseatalysts attractive for use in esterification and transesterificationeactions pertinent to biodiesel synthesis. Consequently, we eval-ated the activity of MgSil catalysts in the esterification of palmiticcid (a major saturated fatty acid found in palm oil) and trans-sterification of tributyrin with methanol (Scheme 1). As we canee in Table 3, the MgSil-org sample offered 80% of palmitic acidonversion after 45 min of reaction at 80 ◦C. In both reactions, thegSil-org sample showed high catalytic activity than the MgSil-

norg sample.The productivity of the MgSil-org and MgSil-inorg catalysts

as measured by TOF. The TOF data for transesterification andsterification reactions was presented in Table S1. The MgSil-rg catalyst (0.23 h−1–transesterification, 0.19 h−1–esterification)howed higher TOF in both reactions than the MgSil-inorg0.18 h−1–transesterification, 0.12 h−1–esterification) sample. Itppears that the catalytic activity is influenced by the particle sizef the catalyst. The small particle size of the catalyst increases therowing number of collisions, thus affecting the productivity of aatalyst.

Based on Table S1, it is shown that both the MgSil catalysts areore productive than MgO and SiO2 bulk oxides, which could be

ue to the higher amount of active sites present in both samples.hese results are also supported by TON values for all samples. Thebility of the MgSil catalysts give rise in conversion and yield whichould be attributed to the presence of the MgSil nanostructure. Thisesult is in agreement with the research that was conducted byaufiq-Yap et al. [41] which studied the behavior of the solid cat-lyst from a mixture of two metal oxides (CaMgO and CaZnO) andompared with CaO, MgO and ZnO in transesterification reaction.

We also tested the deactivation pattern of the MgSil catalystsor transesterification reaction. The two catalysts were stirred withributyrin for 1 h at 80 ◦C separately, before adding methanol to per-orm the reaction. The tributyrin pretreatment of the MgSil-inorg

able 3eusability experiments for transesterification on MgSil-org catalyst.

Cyclea Conversion of tributyrin (%)b Yield of methylbutyrate (%)c

1 99 93

2 98 93

3 98 93

4 97 92

5 97 92

a Cycle 1 is for fresh catalyst, whereas subsequent runs are after centrifugation and meb Reaction conditions: 3 wt%, MgSil-org, T = 80 ◦C, time = 45 min.c Methylbutyrate yield is defined as the moles of methylbutyrate produced divided by

d Methylpaltimate yield is defined as the moles of methylpalmitate produced divided b

sample decreased its activity slightly (tributyin conversion from84.8 to 80.3%), whereas the catalytic activity of the MgSil-inorgsample pretreated with methanol did not change significantly.Apparently, hydrolysis of ester led to decrease in catalyst activityof the MgSil-inorg sample. The decrease in catalytic activity is notobserved in the case of the MgSil-org sample. Corma et al. [42] indi-cated that butyric acid from the hydrolysis of ester in the presenceof adsorbed water would also poison the base sites on the catalysts.Shibasaki-Kitakawa et al. [43] also reported that the deactivationof anion-exchange resin catalyst for transesterification of trioleinwith ethanol was due to a direct exchange of hydroxyl with oleate.

The re-usability of the most active catalysts (MgSil-org) was alsotested for both transesterification and esterification reactions. Thecatalyst was removed by centrifugation, washed with methanoland used for five subsequent reactions. The conversion of tributyrinand the yield of methylbutyrate for each reaction are presented inTable 3. On the basis of tributyrin conversion, the catalyst retained99 and 98% of its original activity after the first and second recycles,respectively.

Usually, the adsorption of methanol on basic sites initiates thetransesterification of triglyceride by forming active methoxide ionsthat react with triglyceride molecules [40]. Therefore, the migra-tion rate of methanol in mesopores to basic sites is important forrapid transesterification. The water contained in MgSil with uni-form mesopores facilitated the migration of methanol because ofthe high miscibility between water and methanol. The high activityof the MgSil-org catalyst was responsible for its more basic sites perunit surface area that could produce very active methoxide ions andfor the large empty mesopores that could provide rapid migrationof hydrophobic triglycerides.

4. Influence of Si precursor on mechanism of formation ofMgSil nanomaterials

The growth mechanism of MgSil core-shell particles can beexplained as follows. Tetraethoxy silane has the remarkable prop-erty of easily converting into silica. The reaction involves a seriesof condensation reactions that convert the tetraethoxy silanemolecule into a colloid-like silica via the formation of Si–O–Si link-ages. Rates of this conversion are sensitive to the presence of acids

and bases, both of which serve as catalysts.

During the hydrothermal process, silica colloids were slowlydissolved and then formed the silicate anions in the alkaline

Conversion of palmitic acid (%)b Yield of methyl paltimate (%)d

80 10079 10079 10078 10078 100

thanol washing.

the moles of tributyrin reacted divided by 3.y the moles of palmitic acid reacted divided by 1.

atalysi

slwhAuMt[cp

tuTtMiat

chtcScin

tbbacl

crrsMsesciscbr

5

seamesisct

[[

[

[

[[[[

[

[

[

[[[[

[[[[

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[[

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K. Narasimharao et al. / Applied C

olution. The stable alkaline condition was provided by the disso-ution of NaOH pellets in the solution. And then magnesium cations

ould react with silicate anions and produce magnesium silicateydroxide hydrate around the surface of SiO2 spherical particles.fterwards, the MgSil core/shell structure can be formed with grad-al release of the silicate anions from the silica spheres. At last, thegSil hollow spheres were produced in the center of the nanostruc-

ure after the remaining silica core has been dissolved completely29]. As shown in the HRTEM images, the MgSil-org sample wasomposed of spherical particles and the surface was rough andorous with a large amount of thin lamellae.

However, the MgSil-inorg sample possessed nanotubular struc-ure. Jancar and Suvorov [14] reported that the MgSil nanotubesnder hydrothermal conditions grow through the step mechanism.he authors also studied the influence of the synthesis parame-ers on the curling of Mg–Si double layers with the formation of

gSil tubes. They demonstrated that the initial stage of the reactions the formation of lamellar nanocrystals, which curl into helicesfter attaining certain size. The tube sizes can be controlled by theemperature and time of the hydrothermal reaction.

Korytkova et al. [30] studied the influence of different physico-hemical parameters of synthesis on the MgSil tube growth underydrothermal conditions. The authors observed that an increase inhe alkali concentration in the reaction medium results a signifi-ant growth of nanotubes. When we used the sodium silicate asi precursor, the alkali content of reactant mixture was increasedonsiderably and caused the conversion of lamellar nanocrystalsnto helices and subsequently resulted in the formation of MgSilanotubes.

The catalytic activity of the MgSil-inorg sample was lower thanhat of the MgSil-org sample. From the comparison of activityetween these two catalysts, we could interpret that the charge-alancing hydroxyl anions, i.e., the Brønsted base sites, are the mostctive sites for the transesterification reaction; the hydroxyl groupsoordinated to Mg in the brucite-like layers of MgSil structure haveess catalytic activity compared to Brønsted base sites.

In general, solid base catalysts are more active than solid acidatalysts requiring relatively shorter reaction times and lowereaction temperatures for transesterification reaction [44]. In aecent report, Degirmenbasi et al. [45] used 10–50 wt% of K2CO3-upported MgSil catalysts for transesterification of oil to biodiesel.ethyl ester yields of around 98% could be obtained using MgSil

upport loaded with 40 and 50 wt% K2CO3. The testing of the recov-red K2CO3 (50%)/MgSil catalyst particles for their reusability andtability indicated that the initial catalytic activity of the catalystould be maintained for only few cycles and there is a clear possibil-ty for leaching of active K2O from the MgSil surface. The MgSil-orgample exhibits large surface area, large pore volume and hierar-hical hollow structure. These characteristics not only make theasic sites more accessible, but also provide rapid migration of theeactants.

. Conclusions

Magnesium silicate nanomaterials were successfully synthe-ized using inorganic and organic Si precursors. Under themployed hydrothermal synthesis conditions, Si precursor playedn important role in controlling the morphology of MgSil nano-aterial. The MgSil sample synthesized with organic Si precursor

xhibited hollow nanospheres composed of small platelets andheets. However, alkaline nature of inorganic precursor (sodium sil-

cate) influenced the formation of nanotubular structure. The MgSilample synthesized using organic Si precursor exhibited higheratalytic activity in transesterification of tributyrin and esterifica-ion of palmitic acid with methanol than that synthesized using

[

[

s A: General 488 (2014) 208–218 217

inorganic Si precursor. The high catalytic activities of this sample intransesterification and esterification reactions are due to the inte-grated effects of their pore structures, surface areas and number ofbasic and acidic sites per unit surface area. This work demonstratesthat changing the Si precursors is an effective method for obtainingthe MgSil nanomaterials with different physicochemical propertiesthat can influence the catalytic performance.

Acknowledgements

This project was funded by Saudi Basic Industries Corporation(SABIC) and the Deanship of Scientific Research (DSR), King Abdu-laziz University, Jeddah, under grant No. MS/15-357-1434. Theauthors therefore acknowledge with thanks SABIC and DSR techni-cal and financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.apcata.2014.09.050.

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