Effect of Si precursor on structural and catalytic properties of nanosize magnesium silicates

Accepted Manuscript Title: Effect of Si precursor on structural and catalytic properties of nanosize magnesium silicates Author: Katabathini Narasimharao Tarek Ali Salem Bawaked Sulaiman Basahel PII: DOI: Reference:

S0926-860X(14)00612-7 http://dx.doi.org/doi:10.1016/j.apcata.2014.09.050 APCATA 15031

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

7-7-2014 15-8-2014 28-9-2014

Please cite this article as: K. Narasimharao, T. Ali, S. Bawaked, S. Basahel, Effect of Si precursor on structural and catalytic properties of nanosize magnesium silicates, Applied Catalysis A, General (2014), http://dx.doi.org/10.1016/j.apcata.2014.09.050 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of Si precursor on structural and catalytic properties of nanosize magnesium silicates Katabathini Narasimharaoa* Tarek Alia,b, Salem Bawakeda, Sulaiman Basahela a

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

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Two Si precursors, inorganic (sodium silicate) and organic (tetraethoxy silane)

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were used to synthesize magnesium silicate (MgSil) nano materials. The effect exerted by nature of Si precursor on the morphology and structural properties of samples was

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studied by chemical analyses, X-ray diffraction, HRTEM, FTIR spectroscopy, XPS, N2 adsorption, solid state NMR spectroscopy and TPD of CO2 & NH3 techniques. The

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characterization results show that MgSil-org sample possessed hollow nanospheres which are composed of small platelets and sheets, in contrast MgSil-inorg sample showed

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nanotubular structure due to more alkaline nature of the inorganic Si precursor.

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Additionally, MgSil-org sample have different textural characteristics, acidic and basic

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properties. MgSil-org sample have higher surface areas, more uniform mesoporous pores, and more number of acidic & basic sites as well as higher activities in transesterification of tributyrin and esterification of palmitic acid with methanol than MgSil-inorg sample. MgSil-org sample is stable and showed excellent reusability for more than five cycles without any loss of activity in transesterification and esterification reactions. Keywords: Magnesium silicate; Nano materials; Tetraethoxy silane; Sodium silicate; Biodiesel; Transesterification _______________________________________________________________________ * To whom correspondence should be addressed, Corresponding author: Tel: +966-538638994, Fax: +966-26952292 E-mail: [email protected]

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1. Introduction Study of the structure and properties of nano structural materials (particles, tubes, rods and sheets) is of great importance from both the theoretical and for the use in various

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practical areas such as materials science, catalysis, medicine, and microelectronics [1]. Many efforts have focused on new types of silicate nano tubes because these materials, as

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mesoporous materials are also promising candidates for various applications [2]. The

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physico-chemical properties of silicate-based materials are very much comparable to those of carbon, and the rich porous structures and tunable composition of silicates make

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them rather appealing for real applications, including catalysis, gas adsorption and separation processes.

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Natural or synthetic magnesium silicates (MgSil) belonging to the clay minerals group have industrial importance [3]. The magnesium ions in the crystal lattice of

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magnesium silicate clay-type materials are exchangeable with transition metal ions and

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the resultant materials are capable of adsorbing both acidic and basic ions [4]. These

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materials were tested as refining and purifying agents in the production of polyether polyols and found that they are excellent, deodorizing, potassium ion adsorbing agents [5]. MgSil materials and its derivatives were also used for advanced applications such as specific catalyst [6], fire retardant painting material [7], and a template for the synthesis of carbon nanofibers [8]. The structure of naturally occurring MgSil (sepiolite) is derived from talc-like ribbons that expand with a width of three pyroxene chains. Each ribbon is connected to the next through an inverted Si-O-Si bond, resulting in a staggered talc layer with a continuous tetrahedral sheet and a discontinuous octahedral sheet. The discontinuous nature of the octahedral sheet allows for the formation of rectangular,

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tunnel-like micropores, which run parallel to the fiber axis and are filled completely by zeolitic water under ambient conditions [9]. It is known that components of the SiO2-MgO system easily react with each other,

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yielding MgSil material. Temuujin et al [10] reported the formation of a poorly crystalline layer-lattice type of MgSil materials. Torro-Palau et al [11] modified the

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structure of MgSil by treating the material at high temperature (1000 oC). Jesionowski et

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al [12] studied the production of highly dispersed MgSil materials at a pilot scale. Korytkova et al [13] studied the effect of the hydrothermal conditions on MgSil

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formation and showed that the MgSil nanotubes formed most rapidly at 350-400 oC, when SiO2 and MgO were used as reactants. The authors also revealed that formation of

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the tubes occurred via initial formation of thin silicate nano platelets. Jancar and Suvarov

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[14] reported that Mg3Si2O5(OH)4 tubes could form at lower temperatures providing

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highly basic reaction mixtures were used. Wang et al [15] synthesized porous silicate nanotubes (such as main group metal and transition metal silicates) by hydrothermal

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synthesis method under strong basic conditions at 180 oC. The authors obtained thermally stable silicate nanotubes with large surface areas and narrow size distributions. Corma and Martin-Aranda [16] reported that strong base catalysts can be prepared

by substituting a part of the Mg ions located at the borders of the channels of MgSil with alkaline ions and those materials exhibited higher basicity than the alkaline X-zeolites. The authors also reported that these catalysts were able to catalyze the condensation of benzaldehydes with active methylene compounds at moderate temperatures. Recently, we reported synthesis, characterization and catalytic application of nitridated crystalline and amorphous MgSil materials [17].

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The depletion of world petroleum reserves and increased environmental concerns has stimulated the search for alternative renewable fuels that are capable of fulfilling an increasing energy demand [18]. Biodiesel fuel (fatty acid methyl esters), synthesized

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from vegetable oils, has similar physical properties to petrochemical diesel and is considered the best alternative fuel candidate for use in diesel engines [19]. Biodiesel

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production involves the catalytic transesterification of long and branched-chain

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triglycerides with alcohols to produce monoesters and glycerol [20]. Current syntheses use homogeneous alkaline agents, such as K or Na alkoxides or hydroxides [21];

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however, removal of the soluble base after reaction is a major problem, because aqueous quenching results in the formation of stable emulsions and saponification, rendering

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separation and purification of the methyl ester difficult. As a result, biodiesel production

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by these routes is still not cost-competitive with petrochemical diesel fuel [22]. Kiss et al

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[23] summarized the pros and cons of manufacturing biodiesel via fatty acid esterification using metal oxide solid catalysts. Recently Grecea et al [24] developed superior robust

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superacid catalyst for multiproduct fatty acid esterification. Use of a solid base catalyst offers several process advantages, including the elimination of a quenching step (and associated contaminated water waste) to isolate the products and the opportunity to operate a continuous process [25]. Solid bases, including zeolites [26], alkali earth oxides [27] and hydrotalcites [28], have been investigated in transesterification reactions. In the present study, hydrothermal synthesis method under autogenous pressure was used to synthesize nanosize MgSil materials. The effect of Si precursor and thermal treatment on the structure of nanosize MgSil and its application as catalyst for transesterification of glyceryl tributyrate with methanol for biodiesel production was also

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studied. The structural and textural properties, namely, morphology, crystallite size, surface area and pore structure of nanosize MgSil samples are investigated by using elemental analysis, powder XRD, HRTEM, FTIR, XPS, solid state NMR, TPD and N2

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properties of the MgSil materials with their transesterification activity.

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physisorption techniques. An attempt was made to correlate the structural and textural

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2. Experimental 2.1 Materials

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Magnesium nitriate (Mg(NO3)2 6H2O), sodium silicate (Na2SiO3), tetraethoxy silane (Si(OC2H5)4) and sodium hydroxide (NaOH) were purchased from Aldrich, U.K.

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All reagents were analytical grade and used as received without further purification.

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2.2 Synthesis of magnesium silicate nanomaterials

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Magnesium silicate nanomaterials were synthesized by hydrothermal synthesis method. Mg(NO3)2 6H2O (weight corresponding to 3 moles of MgO) was dissolved in

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water/ethanol (1:6 ratio) and Na2SiO3 or Si(OC2H5)4 (weight corresponding to 4 moles of SiO2) was used to form a white precipitate. Then NaOH (1g) was added to maintain the basicity of the contents [pH10.4 in case of Si(OC2H5)4) and pH13.1 in case of Na2SiO3]. The contents were transferred into a Teflon lined autoclave and hydrothermally treated at 180 oC for two days. The obtained precipitate was filtered and washed with distilled water to remove ions possibly remaining in the final products and dried at 120oC in air. The MgSil samples prepared using sodium silicate and tetraethoxy silane as Si precursor was labeled as MgSil-inorg and MgSil-org respectively. 2.3 Characterization 5 Page 5 of 39

The elemental composition of the materials was determined by ICP-AES, Optima 7300DV, Perkin Elmmer Corporation, USA. X-ray powder diffraction (XRD) studies were performed for all of the prepared solid samples using a Bruker diffractometer

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(Bruker D8 advance target). The patterns were run with copper Kα and a monochromator

calculated using Scherrer’s equation: (1)

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D = Bλ/ β1/2 cos θ

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(λ= 1.5405 Å) at 40 kV and 40 mA. The crystallite size of the MgSil phase was

where D is the average crystallite size of the phase under investigation, B is the Scherer

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constant (0.89), λ is wavelength of the X-ray beam used (1.54056 Å ), β1/2 is the full width at half maximum (FWHM) of the diffraction peak and θ is the diffraction angle.

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The identification of different crystalline phases in the samples was performed by

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files.

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comparing the data with the Joint Committee for Powder Diffraction Standards (JCPDS)

FTIR spectra were recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer.

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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 information limit was better than 0.18 nm. High-resolution images with a pixel size of 0.044 nm were taken with a CCD camera. The textural properties of the prepared samples were determined from nitrogen

adsorption/desorption isotherm measurements at -196 oC using a model NOVA 3200e automated gas sorption system (Quantachrome, U.S.A.). Prior to measurement, each sample was degassed for 6 h at 150 oC. The specific surface area, SBET, was calculated by applying the Brunauer-Emmett-Teller (BET) equation. The average pore radius was

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estimated from the relation 2Vp/SBET, where Vp is the total pore volume (at P/P0 = 0.975). Pore size distribution over the mesopore range was generated by the Barrett-JoynerHalenda (BJH) analysis of the desorption branches, and the values for the average pore

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size were calculated.

The XPS measurements were carried out by using a SPECS GmbH X-ray

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photoelectron spectrometer. Prior to analysis, the samples were degassed under vacuum

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inside the load lock for 16 h. The binding energy of the adventitious carbon (C 1s) line at 284.6 eV was used for calibration, and the positions of other peaks were corrected

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according to the position of the C 1s signal. For the measurements of high resolution spectra, the analyzer was set to the large area lens mode with energy steps of 25 meV and

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in Fixed Analyzer Transmission (FAT) mode with pass energies of 34 eV and dwell times of 100 ms. The photoelectron spectra of the four samples were recorded with the

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acceptance area and angle of 5 mm in diameter and up to 5o, respectively. The base pressure during all measurements was 5x10-9 mbar. A standard dual anode excitation

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source with Mg Kα (1253.6 eV) radiation was used at 13 kV and 100 W. 29

Si MAS NMR spectra of the samples were recorded using Bruker 400 MHz

spectrometer and referenced to TMS, where a pulse delay of 60 s was used. NH3 and CO2- TPD patterns of the samples were recorded using Chembet-3000 (Quantachrome, USA) instrument.

2.4 Transesterification of glyceryl tributyrate and esterification of palmitic acid with methanol Transesterification of glyceryl tributyrate with methanol was performed in a stirred batch reactor with samples withdrawn periodically for analysis on a Shimadzu

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GC17A gas chromatograph fitted with a DB-1 capillary column (film thickness, 0.25 mm; i.d., 0.32 mm; length, 30 m), and AOC 20i autosampler. The reaction was performed at 60 oC using 3 wt.% of catalyst, 0.01 mol (3 cm3) of glyceryl tributyrate

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(98%, Aldrich), and 0.3036 mol (12.5 cm3) of methanol with 2.5 mmol (0.587 cm3) of hexyl ether as an internal standard. The catalyst samples were separated from the reaction

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mixture for recycling by centrifugation. Reactions were run for 6 h with initial rates

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determined at conversions <30%, with reactions continued for 24 h. Catalyst selectivity and overall mass balances (closure >98%) were determined using reactant and product

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response factors derived from multipoint calibration curves. Catalyst stability was verified by performing leaching tests in hot methanol, with MgSil catalysts refluxed for 6

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h in methanol, after which the solid was removed. The presence of soluble species in the

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recovered methanol was subsequently investigated by assessing the activity of the

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residual solvent in transesterification reaction. Esterification was performed at 80 oC using 3 wt.% of catalyst, 0.01 mol of

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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 oC are shown in

Fig.1(A). The diffraction pattern of samples exhibited reflection patterns of MgSil structure with the chemical formula of 3MgO.4SiO2.2H2O [JCPDS No 03-0174]. The apparent broadening of all the peaks indicated that as obtained silicates were composed of nanosize crystals usually have distinctive clay type structures with silicon to oxygen

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ratios within the layers [29]. The particle size of MgSil samples was determined using Scherrer’s equation. The full widths at half maxima (FWHM) of the major peak at 2θ = 34.5o was used for crystallite size calculation. The crystallite sizes of the samples are

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presented in Table 1. The crystallite size of the as-synthesized MgSil-inorg is approximately 20.4 nm, and it was noted to decrease to 12.5 nm after calcination at 500 o

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C. As-synthesized MgSil-org sample showed crystallite size to 45.5 nm and upon

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calcination at 500 oC decrease of crystallite size to 34.4 nm was observed.

The Fourier transform infrared (FTIR) spectra of two MgSil samples are shown in

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Fig. 1 (B). The band at 1645 cm-1 due to HO-H bending and interstitial water molecules was appeared in both the samples. Broad and strong peaks at 860-1175 cm-1 and 540-450

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cm-1 were appeared in the spectra of both MgSil samples that could be assigned to

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presence of silicate groups [30]. The peaks due to silicate groups were well resolved in

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MgSil-inorg sample compared to MgSil-org that related to the presence of amorphous silica material along with MgSil nanomaterial.

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In the inset of the Fig.4, FTIR spectra in the region of 3000-4000 cm-1 was

presented. A sharp peak at 3685 cm-1 due to octahedral Mg-O-H unit can be observed in MgSil-inorg sample [31]. A broad intense band at 3400 cm-1 owing to adsorbed molecular water appeared in MgSil-org sample. It is known that the FTIR spectrum of sodium metasilicate [32] exhibit bands at 583, 716, and 873cm-1 and sodium form of MgSil additionally shows band at 1390 cm-1 that are related to the presence of Si-O-Na and Mg-O-Na bonds [31]. The MgSil samples prepared in this study did not any of these bands indicating that the synthesized samples did not contained metasilicate structure.

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

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As shown in Fig. 2 (B) the MgSil-org sample was composed of uniform spherical particles and the surface was possessed rough and porous morphology. The average size

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of the spheres was uniformly about 50 nm.

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To examine the structure of the MgSil nanomaterials, the samples were further characterized by TEM analysis. Fig. 3 shows TEM micrographs of the two MgSil

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samples. The low magnification view in Fig. 3 (A) shows nanotubes of regular cylindrical shape and by bundles of hollow tubes with two ends open. The diameters of

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tubes were about 10-12 nm and lengths up to several hundreds of nano meters. At higher

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magnification in Fig. 3 (B), the hollow, open-ended multiwall tubular structure of the

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material was clearly observed. They were typically wider in the centers tapering off toward the ends, as might be expected for structures formed by the rolling of layers. The

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inner diameters were approximately 5 nm. Fig. 3 (C) shows low magnification view of MgSil-org sample. It can be observed

that the sample is constituted of fully and partially formed hollow spheres. The particle size distribution is around 50 nm. From the HRTEM image, Fig. 3 (D), one can obviously determine that the hollow spheres are composed of small platelets and sheets. It can also seen that the spheres are aggregated together to form a porous layers with a large amount of thin lamellae. The bulk chemical composition of the samples was investigated using ICP-AES analysis and corresponded to the composition of theoretical formula 3MgO.4SiO2.2H2O. 10 Page 10 of 39

The coexistence of Mg and Si indicating the formation of magnesium silicates and the samples also contained small amount of sodium at a concentration of ~0.4 wt % (Table 1). The presence of magnesium and silicon of MgSil-inorg and MgSil-org samples

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indicates that the total atomic ratio of magnesium to silicon is about 3:4. It is consistent with the molecular formula described above.

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Solid state 29Si MAS NMR spectroscopy is a powerful tool to study the chemical

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environment and bonding patterns of the SiO4 units. 29Si MAS NMR experiments for two MgSil samples were carried out at room temperature and NMR spectra are shown in 29

Si MAS NMR of all the samples is mainly

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Fig.4. As can be seen in Fig. 4, the

composed of different tetrahedral sites. The MgSil samples showed a sharp intense peak

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at -95.0 ppm corresponding to Q3 groups (Qn corresponds to the Si atoms that connected

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with n other Si atoms through oxygen bridges, ‘n’ can be varied between 0 and 4). Q3

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type structure is consistent with a talc-like crystal structure with SiO4 structural units forming layers (composition of 3MgO.4SiO2. 2H2O) through networking [33].

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The MAS NMR results are agrees with XRD and Raman analyses results.

Besides, a weak chemical shift which was assigned to Q2 (-85.0 ppm) was observed, which was due to the surface silicon atoms. In addition, two sharp peaks, the MgSil samples showed a broad peak at -110 ppm and which could be due to presence of (SiOH) functional groups. The relative proportion of Q3/Q2 is more in case MgSil-inorg sample than MgSil-org sample, indicating enhancement in the number of Q3 silicon sites and progressive structural polymerization in MgSil-inorg sample [34]. These results are in quite accordance with the elemental analysis and FTIR results.

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The XPS spectra of the MgSil-org and MgSil-inorg samples are shown in Fig. 5. High resolution Si 2p, Mg 2p and O1s peaks are broad and suggesting that a distribution of chemical states is present. The salient features of the XPS results are that there is no

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change in the average Si, Mg and O chemical state in the samples, since they both showed the peaks at same binding energy and the mass percentage of Si, Mg and O

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elements remain the almost same (Table 1). The binding energy of the Si 2p peak for

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these two samples is approximately 102.1 eV (Fig. 7A), 1.7 eV lower than the binding energy of SiO2 (103.8 eV) [35].

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The appearance of Mg2p peak at 49.2 eV implies that the surface Mg components in MgSil samples are not belong to MgO or metallic Mg. Corneille et al [36] observed

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Mg2p peak at approximately 50.5 eV for the MgO sample synthesized using Mg thin film

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in presence of oxygen. They also reported that the binding energy of Mg2p peak for

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oxidized magnesium sample appears at 49.6 eV which is very close to the binding energy observed for Mg2p peaks in MgSil samples.

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The binding energy (530.9 eV) shown by O1s spectra of both MgSil samples are

in accordance with the O1s binding energy position of both suboxidized magnesium and magnesium silicate which would generally exist between the SiO2 binding energy; 533.3 eV and MgO binding energy; 531.8 eV. The observed result is clearly indicating the majority of oxygen atoms were existed in the form of non- bridging (Mg-O-Si-) mode. Nitrogen adsorption-desorption isotherms of MgSil samples were shown in Fig. 6. The isotherms of the samples can be categorized as type II with H3 type hysteresis loop, indicating that these samples are mesoporous in nature with particles giving rise to slitshaped pores. The hysteresis loop between the two branches did not close completely

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until the relative pressure P/P0 had returned to 0.4 and 0.5 in desorption branch of MgSilorg and MgSil-inorg samples respectively, indicating a broad distribution of pore size in prior sample than later one. This was confirmed by the Barrett-Joyner-Halenda (BJH)

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pore sized distribution obtained from the adsorption branch (inset). A broad distribution of pore sizes ranging from 20 Å to more than 1400 Å was observed with a maximum near

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200 Å in case of MgSil-org sample in contrast MgSil-inorg sample showed distribution

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of pore sizes ranging between 20 Å and 900 Å. The MgSil samples had a significantly high BET surface area; 124 m2g-1 and 255 m2g-1 for MgSil-inorg and MgSil-org samples

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respectively. The total pore volume of MgSil-org sample (1.253 cm3g-1) is much higher than the pore volume of MgSil-inorg (0.902 cm3g-1) sample. The difference in the

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textural properties of the two samples could be due to the difference in extent of fracture

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of bridging bonds and the resultant reduction in the number of silanol groups (Si-OH)

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along the axis of layers can lead to change in the size and number of pores. The concentration of acidic and basic sites of MgSil is an important

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physicochemical characteristic that determines its impact on catalytic performance. The CO2 and NH3-TPD measurements were carried out to determine the base and acid strength of the MgSil samples.

Fig. 7 (A) & (B) represents the CO2-TPD and NH3-TPD patterns of MgSil

samples respectively. It is well known that desorption of probe molecule from weak sites occurs at lower temperature ranges, whilst desorption from strong sites occurs at higher temperature ranges. It is also well established that the basic character of the solids is associated to carbonation of the surface basic sites. The CO2-TPD profile of MgSil-org sample showed desorption peaks due to three basic sites; weak (around 140 oC), medium

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(at 400 oC) and strong (at 620 oC), in contrast MgSil-inorg sample showed small peak due to medium basic sites at 400 oC and a major strong basic sites at 640 oC. The area of desorption peaks on the TPD profiles of MgSil-org sample was much more than that on

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MgSil-inorg sample. This observation indicating that Mg prepared with tetraethoxy silane as Si precursor had much more basic sites, which was much more beneficial to base

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catalyzed reactions. The total basicity of the MgSil-org sample was 0.98 mmol/g. On the

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other hand, low intense CO2 adsorption peak centered at about 410 oC ascribed to moderate basic sites and a more intense peak at 640 oC attributed to strong basic sites

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were detected on MgSil-inorg sample. The total basicity of the MgSil-inorg sample was 0.46 mmol/g.

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MgSil-org sample showed two broad NH3 desorption peaks, first at low temperature (240 oC) and second one centered at high temperature (555 oC), whereas

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MgSil-inorg sample showed only one desorption peak at high temperature, 650 oC. In FTIR analysis, MgSil-org sample showed the presence of zeolitic water. Thus, the

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desorption peak at 240 oC on MgSil-org can be attributed to NH3 replacing the zeolitic water site. As compared with MgSil-inorg sample, NH3 desorption maximum at 240 oC seems to be due to the unique characteristics of the MgSil-org sample structure. This peak disappears in the MgSil-inorg sample, indicating that the zeolitic water site could be the main place to interact with the MgSil core-shell particles. The total amount of desorbed NH3 for MgSil-inorg was only 95 mmol/g, as shown in Table 1. MgSil-org had much higher amount (202 mmol/g) than MgSil-inorg sample. Jung and Grange [37] reported that composite of Ti(OH)4 and Si(OH)4 showed a high increase of the Lewis and Bronsted acid sites. Previously Gao and Wachs also

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observed that TiO2–SiO2 mixed oxides show a high acidity than their parent oxides [38]. Kataoka and Dumesic [39] suggested that the bridging oxygen of Metal–O–Si in mixed oxide is the main location of protons able to act as Bronsted acid sites. The generation of

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strong acid sites in MgSil samples can thus be explained by the formation of Mg–O–Si bonds, due to the connectivity between Mg and Si atoms. For MgSil-inorg sample, both

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of the desorption peaks of NH3 and CO2 are small and their intensity are weak, which can

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be attributed to very small amount of acidic and basic sites. However, MgSil-org sample showed broad NH3 and CO2-TPD profiles, indicating that the surface acid and basic

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strength were widely distributed. The relative amounts of NH3 and CO2 desorbed from MgSil-org sample were two times higher than that from the MgSil-inorg (Table 1). These

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CO2- and NH3-TPD patterns confirm the co-existence of acid sites and basic sites on the

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surface of MgSil samples and their basicity and acidity was influenced by the Si

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precursor used to synthesize the MgSil nano material. Figure 8 (A) & (B) illustrates the conversion of tributyrin and the product

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distribution obtained at different reaction temperatures of 60, 70 and 80 oC on MgSil samples. It is clear that MgSil-org sample exhibit significant different catalytic performances in transesterification. It can be seen that the conversion levels increased with the increase of temperature. After 45 minutes, MgSil-org sample offered 92.2%, 95.1% and 99.5% at 60, 70 and 80 oC, respectively. Under identical reaction conditions, the conversion of tributyrin over MgSil-inorg sample is only 72.3%, 80.4% and 84.8% at these reaction temperatures. The high conversions of tributyrin over MgSil-org sample are related to its superior physico-chemical properties. Even though the conversion of tributyrin is different for MgSil samples, the product distributions over these two

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catalysts are almost same. The lower catalytic activity for MgSil-inorg, can be mainly attributed to its low surface area, less number of active basic sites per unit surface area. The selectivity data is consistent with the reaction Scheme 1, where the

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diglycerides are the first products formed, and the selectivity towards methylbutyrate is around 98 % at low conversions. A drop in selectivity to methylbutyrate observed while

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glycerin was produced with prolonged reaction times. Monoglceride does not reach

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considerable high selectivity values and has a tendency to disappear with the reaction time, while the glycerine was reached maximum of 8% selectivity for MgSil-org sample.

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Figure 9 (A) shows tributyrin conversion with methanol at 60 oC for catalyst concentrations of 1 and 3 wt.% of MgSil catalysts in the reaction mixture. It can be seen

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that the conversion of tributyrate over both MgSil catalysts a good linear relationship

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with the catalyst loading. Without any catalyst addition, the transesterification is not

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carried out even after 180 minutes. Kim et al [40] reported that if the transesterification of triglycerides is controlled by the surface reaction on catalysts, then the conversion of

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triglycerides over them should show a linear relationship with the catalyst loading. In this study, the stirring speed is 600 rpm was maintained in all the runs and the external diffusion can be excluded. The conversions over the two MgSil samples may be are controlled by the different rate-determining steps, which result in the differences in catalytic performance with the change of the catalyst loading. This difference could be mainly related to the migration rate of the reactants in the pores of the catalysts. The rate constants were calculated and used to compare the performance of the MgSil catalysts. The rate constants for transesterification of tributyrin with methanol over MgSil catalysts are also compared to those associated with the pure oxides (MgO and

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SiO2) and physical mixture of MgO and SiO2 in Table 2. The simplest way to compare specific activity is to examine the rate constant for the consumption of tributyrin as characterized by k1. The rate constant for dibutyrin conversion to monobutyrin,

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represented by k2, confirmed the activity pattern by tributyrin loss in each catalyst. The rate of monobutyrin consumption to form glycerol during the sequential reaction was not

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included due to the glycerol formation was very low through most of the reaction. The

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rate constants were normalized by the exposed surface areas determined by N2 physisorption experiments.

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The results in Table 2 illustrate the effect of Si precursor on the reactivity of MgSil catalysts. Two important observations can be drawn from the results. First, MgSil-

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org sample showed high catalytic activity than MgSil-inorg sample for transesterification

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of tributyrin. Second, MgSil-org sample was 300% more active than pure MgO, on a

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surface area basis. This finding was significant, and the activity test was repeated two times to confirm the result. SiO2 was inactive for transesterification under the standard

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conditions of our study. And also, a physical mixture of MgO and SiO2 in a ratio of 3:4 converted tributyrin at a rate similar to that of pure MgO (Table 2). To investigate the effects of the methanol to tributyrite molar ratio for the two

catalysts, the transesterification experiments were conducted by changing the molar ratio from 6:1 to 12:1, while keeping the temperature and the catalyst amount constant at 80 °C and 1 wt.%, respectively. Figure 9 (B) demonstrates the effect of molar ratio of methanol to tributyrin, on tributyrin conversion for MgSil-org sample. The reaction developed rapidly within 30 min and the conversion varied from 60% to 80%, depending on the different molar ratios of methanol to tributyrin; in the 45 minutes, the

17 Page 17 of 39

transesterification reached a state of equilibrium, the conversion of tributyrin reached to from 94.2% for 6:1 to 100% for 12:1. A similar pattern was observed in case of MgSilinorg sample; however this catalyst requires longer reaction times to convert tributyrin. It

ip t

is known that the stoichiometry of transesterification requires 3 mol of methanol per mol of triglycerides; an excess of methanol can shift the equilibrium to the right side and

cr

biodiesel yield was improved.

us

The acidic and basic nature of MgSil samples makes these catalysts attractive for use in esterification and transesterification reactions pertinent to biodiesel synthesis.

an

Consequently, we evaluated the activity of MgSil catalysts in the esterification of palmitic acid (a major saturated fatty acid found in palm oil) and transesterification of

M

tributyrin with methanol (Scheme 1). As we can see in the Table 3, MgSil-org sample

d

offered 80% of palmitic acid conversion after 45 minutes of reaction at 80 oC. In both

te

reactions, MgSil-org showed the high catalytic activity than MgSil-inorg sample. The productivity of MgSil-org and MgSil-inorg catalysts was measured by TOF.

Ac ce p

The TOF data for transesterification and esterification reactions was presented in Table S1. MgSil-org catalyst (0.23 h-1 - transesterification, 0.19 h-1 - esterification) showed higher TOF in both reactions than MgSil-inorg (0.18 h-1 - transesterification, 0.12 h-1 esterification) sample. It appears that the catalytic activity is influenced by the particle size of the catalyst. The small particle size of the catalyst increases the growing number of collisions, thus affecting the productivity of a catalyst. Based on Table S1, it is shown that the both MgSil catalysts are more productive than MgO and SiO2 bulk oxides, which could be due to the higher amount of active sites present in both samples. These results are also supported by TON values for all samples.

18 Page 18 of 39

The ability of MgSil catalysts give rise in conversion and yield which could be attributed to presence of MgSil nanostructure. This result is in agreement with the research that was conducted by Taufiq-Yap et al. [41] which studied the behavior of the solid catalyst from

ip t

a mixture of two metal oxides (CaMgO and CaZnO) and compared with CaO, MgO, and ZnO in transesterification reaction.

cr

We also tested the deactivation pattern of MgSil catalysts for transesterification

us

reaction. The two catalysts were stirred with tributyrin for 1 h at 80 oC separately, before adding methanol to perform the reaction. The tributyrin pretreatment of MgSil-inorg

an

sample decreased its activity slightly (tributyin conversion from 84.8% to 80.3%) whereas the catalytic activity of MgSil-inorg sample pretreated with methanol did not

M

change significantly. Apparently, hydrolysis of the ester led to decrease the catalyst

d

activity of MgSil-inorg sample. The decrease of catalytic activity is not observed in case

te

of MgSil-org sample. Corma et al. [42] indicated that butyric acid from the hydrolysis of ester in the presence of adsorbed water would also poison the base sites on the catalysts.

Ac ce p

Shibasaki-Kitakawa et al [43] also reported that the deactivation of anion-exchange resin catalyst for transesterification of triolein with ethanol was due to a direct exchange of hydroxyl with oleate.

The re-usability of the most active catalyst (MgSil-org) was also tested for both

transesterification and esterification reactions. The catalyst was removed by centrifugation, washed with methanol and used for five subsequent reactions. The conversion of tributyrin and the yield of methylbutyrate for each reaction are presented in Table 3. On the basis of tributyrin conversion, the catalyst retained 99% and 98% of its original activity after the first and second recycles, respectively.

19 Page 19 of 39

Usually, the adsorption of methanol on basic sites initiates the transesterification of triglyceride by forming active methoxide ions that react with triglyceride molecules [40]. Therefore, the migration rate of methanol in mesopores to basic sites is important

ip t

for rapid transesterification. The water contained in MgSil with uniform mesopores facilitated the migration of methanol because of the high miscibility between water and

cr

methanol. The high activity of MgSil-org catalyst was responsible for its more basic sites

us

per unit surface area that could produce very active methoxide ions and for the large

an

empty mesopores that could provide rapid migration of hydrophobic triglycerides.

The influence of Si precursor on mechanism of formation of MgSil nanomaterials

M

The growth mechanism of MgSil core shell particles can be explained as follows.

d

Tetraethoxy silane has the remarkable property of easily converting into silica. The

te

reaction involves a series of condensation reactions that convert the tetraethoxy silane molecule into a colloid like silica via the formation of Si-O-Si linkages. Rates of this

Ac ce p

conversion are sensitive to the presence of acids and bases, both of which serve as catalysts.

During the hydrothermal process, silica colloids were slowly dissolved and then

formed the silicate anions in the alkaline solution. The stable alkaline condition was provided by the dissolution of NaOH pellets in the solution. And then magnesium cations would react with silicate anions and produce magnesium silicate hydroxide hydrate around the surface of SiO2 spherical particles. Afterwards, the MgSil core/shell structure can be formed with gradual release of the silicate anions from the silica spheres. At last the MgSil hollow spheres were produced in the center of the nanostructure after the

20 Page 20 of 39

remaining silica core has been dissolved completely [29]. As shown in HRTEM images, the MgSil-org sample was composed of spherical particles and the surface was rough and porous with a large amount of thin lamellae.

ip t

However, MgSil-inorg sample possessed nanotublar structure. Jancar and Suvorov [14] reported that MgSil nanotubes under hydrothermal conditions grow through

cr

the step mechanism. The authors also studied the influence of the synthesis parameters on

us

the curling of Mg-Si double layers with the formation of MgSil tubes. They demonstrated that the initial stage of the reaction is the formation of lamellar nanocrystals, which curl

an

into helices after attaining certain size. The tube sizes can be controlled by the temperature and time of the hydrothermal reaction.

M

Korytkova et al [30] studied the influence of different physicochemical

d

parameters of synthesis on the MgSil tube growth under hydrothermal conditions. The

te

authors observed that an increase in the alkali concentration in the reaction medium results a significant growth of nanotubes. When we used the sodium silicate as Si

Ac ce p

precursor, the alkali content of reactant mixture was increased considerably and caused the conversion of lamellar nanocrystals into helices and subsequently resulted the formation of MgSil nanotubes.

The catalytic activity of MgSil-inorg sample was lower than that of MgSil-org

sample. From the comparison of activity between these two catalysts, we could interpret that the charge-balancing hydroxyl anions, i.e., the Brønsted base sites, are the most active sites for the transesterification reaction; the hydroxyl groups coordinated to Mg in the brucite-like layers of MgSil structure have less catalytic activity compared to Brønsted base sites.

21 Page 21 of 39

In general, solid base catalysts are more active than solid acid catalysts requiring relatively shorter reaction times and lower reaction temperatures for transesterification reaction [44] In a recent report, Degirmenbasi et al [45] used 10-50 wt.% of K2CO3

ip t

supported MgSil catalysts for transesterification of oil to biodiesel. Methyl ester yields of around 98 % could be obtained using MgSil support loaded with 40 and 50 wt.% K2CO3.

cr

The testing of the recovered K2CO3(50%)/MgSil catalyst particles for their reusability

us

and stability indicated that the initial catalytic activity of the catalyst could be maintained for only few cycles and there is a clear possibility for leaching of active K2O from the

an

MgSil surface. MgSil-org sample exhibits large surface area, large pore volume and hierarchical hollow structure. These characteristics not only make the basic sites more

te

Conclusions

d

M

accessible, but also provide rapid migration of the reactants.

Magnesium silicate nano materials were successfully synthesized using inorganic

Ac ce p

and organic Si precursors. Under the employed hydrothermal synthesis conditions, Si precursor played an important role in controlling the morphology of MgSil nano material. MgSil sample synthesized with organic Si precursor exhibited hollow nanospheres composed of small platelets and sheets. However, alkaline nature of inorganic precursor (sodium silicate) influenced the formation of nanotubular structure. The MgSil sample synthesized using organic Si precursor exhibited higher catalytic activity in transesterification of tributyrin and esterification of palmitic acid with methanol than that synthesized using inorganic Si precursor. The high catalytic activities of this sample in transesterification and esterification reactions are due to the integrated effects of their

22 Page 22 of 39

pore structures, surface areas and number of basic and acidic sites per unit surface area. This work demonstrates that changing the Si precursors is an effective method for obtaining the MgSil nanomaterials with different physico-chemical properties that can

ip t

influence the catalytic performance.

cr

Acknowledgements

us

This project was funded by Saudi Basic Industries Corporation (SABIC) and the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant

an

No. MS/15-357-1434. The authors, therefore acknowledge with thanks SABIC and DSR

M

technical and financial support.

d

References

te

[1] J. Goldberger, R. Fan, P. D. Yang, Acc. Chem. Res., 39 (2006) 239–248. [2] X. Wang, J. Zhuang, J. Chen, K. B. Zhou, Y. D. Li, Angew.Chem. Int. Ed., 43 (2004)

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2017–2020; Y. Zhuang, Y. Yang, G. L. Xiang, X. Wang, J. Phys. Chem. C, 113 (2009) 10441–10445.

[3] Y. Yang, Y. Zhuang, Y. H. He, B. Bai, X. Wang, Nano Res., 3 (2010) 581–593. [4] H. H. Murray, Clay Miner., 34 (1999) 39-49. [5] R. Le Van Mao, E. Rutinduka, C. Detellier, P. Gougay, V. Hascoet, S. Tavakoliyan, S.V. Hoa, T. Matsuura, J. Mater. Chem., 9 (1999) 783-788. [6] S. Damyanova, L. Daza, J.L.G. Fierro, J. Catal., 159 (1996) 150-161. [7] J. Zhu, A.B. Morgan, F.J. Lamelas, C.A. Wilkie, Chem. Mater., 13 (2001) 3774-3780. [8] G. Sandi, R.E. Winans, S. Seifert, K.A. Carrado, Chem. Mater., 14 (2002) 739-742.

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[9] K. Wenxing, G.A. Facey, C. Detellier, B. Casal, J.M. Serratosa, E. Ruiz-Hitzky, Chem. Mater., 15 (2003) 4956-4967. [10] J. Temuujin, K. Okada and D. Mackenzie, J. Am. Ceram. Soc., 1998, 81, 754–756

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[11] A. Torro-Palau, J. Fernandez-Garcia, A. Orgiles-Barcelo, J. Martin-Martinez, Adhes. Adhes. J., 21 (2001) 1–9.

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[12] T. Jesionowski, F. Ciesielczyk, A. Krysztafkiewicz, Physicochemical Problems of

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Mineral Processing, 38, 2004, pp. 197–205.

[13] E.N. Korytkova, A.V. Maslov, L.N. Pivovarova, I.A. Drozdova, V.V. Gusarov,

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Glass Phys. Chem., 30 (2004) 51-55.

[14] B. Jancar, D. Suvorov, Nanotechnology, 17 (2006) 25-29.

M

[15] X.Wang, J. Zhuang, K. Zhou, Y. Li, Angew.Chem.Int.Ed., 43 (2004) 2017-2020.

d

[16] A. Corma, R.M. Martin-Aranda, J. Catal., 130 (1991) 130-137.

(2013) 4274-4283.

te

[17] K. Narasimharao, M. Mokhtar, S. N. Basahel, S. A. Al-Thabaiti, J. Mater. Sci., , 48

Ac ce p

[18] K. Narasimharao, D.R. Brown, A.F. Lee, A.D. Newman, P.F. Siril, S.J. Tavener and K. Wilson, J. Catal., 248 (2007) 226–234. [19] S. Romano, Vegetable Oils Fuels, in: Proc. Int. Conf. on Plant and Vegetable Oils as Fuels, ASAE, MI, USA, 1982, p. 106. [20] A. C. Dimian, Z. W. Srokol, M. C. Mittelmeijer-Hazeleger, G. Rothenberg, Top. Catal. 53 (2010)1197–1201. [21] F. Ma, M. Hanna, Bioresour. Technol., 70 (1999) 1-15. [22] B. Freedman, E.H. Pryde, T.L. Mounts, J. Am. Oil Chem. Soc., 61 (1984) 16381643.

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[23] A.A. Kiss, A.C. Dimian, G. Rothenberg Energy Fuels, 22 (2008) 598-604. [24] M.L. Grecea, A.C. Dimian, S. Tanase, V. Subbiah, G. Rothenberg, Catal. Sci. Tech.,

[25] Y. Ono, T. Baba, Catal. Today, 38 (1997) 321-337.

ip t

2 (2012) 1500-1506.

[27] S. Gryglewicz, Bioresour. Technol., 70 (1999) 249-253.

cr

[26] E. Leclercq, A. Finiels and C. Moreau, J. Am. Oil Chem. Soc., 2001, 78, 1161-1165.

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[28] D.G. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Appl. Catal. A Gen., 287 (2005)

Ind. Eng. Chem. Res., 45 (2006) 3009-3014.

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183-190; M. Di Serio, M. Ledda, M. Cozzolino, G. Minutillo, R. Tesser, E. Santacesaria,

[29] Y. Wang, G. Wang, H. Wang, C. Liang, W. Cai, L. Zhang, Chem. Eur. J., 16 (2010)

M

3497–3503.

d

[30] I. M. El-Naggar, M. M. Abou-Mesalam, J. Hazar. Mater., 149 (2007) 686-692.

te

[31] R.N. Nyquist and R.O. Kagel, Infrared and Raman spectra of inorganic compounds and organic salts, Academic Press, New York, 1997

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[32] E. N. Korytkova, A. S. Brovkin, T. P. Maslennikova, L. N. Pivovarova and I. A. Drozdova, Glass Phy. Chem., 37( 2011) 161–171. [33] M. C. Davis, W.J. Brouwer, D. J. Wesolowski, L. M. Anovitz, A. S. Liptonc, K. T. Mueller, Phys. Chem. Chem. Phys., 11 (2009) 7013–7021. [34] J. Temuujin, K. Okada, K.J.D. MacKenzie, J. Solid State Chem., 138 (1998) 169– 177. [35] J.F. Moulder, W.F. Stickle, P.E. Sobol and K.D. Bomben. Handbook of X-ray photoelectron spectroscopy, Eden Prairie, Minnesota: Perkin-Elmer; 1992 and references therein

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[36] J. S. Corneille, J-W He, D. W. Goodman, Sur. Sci., 306 (1994) 269-278. [37] S. M. Jung and P. Grange, Appl. Sur. Sci., 221 (2004) 167–177.

[39] T. Kataoka, J.A. Dumesic, J. Catal. 112 (1988) 66-79.

ip t

[38] X. Gao, I.E. Wachs, Catal. Today 51 (1999) 233-254.

[40] M. J. Kim, S. M. Park, D. R. Chang, G. Seo, Fuel Proce. Technol., 91 (2010) 618–

cr

624.

us

[41] Y.H. Taufiq-Yap, H.V. Lee, M.Z. Hussein, R. Yunus, Biomass Bioenergy, 35 (2011) 827-834.

an

[42] A. Corma, S.B. Abd Hamid, S. Iborra, A. Velty, J. Catal. 234 (2005) 340-347. [43] N. Shibasaki-Kitakawa, H. Honda, H. Kuribayashi, T. Toda, T. Fukumura, T.

M

Yonemoto, Biores. Technol. 98 (2007) 416-421

d

[44] M. Hara, ChemSusChem, 2 (2009) 129-135.

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147– 156.

te

[45] N. Degirmenbasi, N. Boz, D. M. Kalyon, Appl. Catal. B: Environ., 150-151 (2014)

26 Page 26 of 39

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

Si

O

MgSil-inorg

33.5

11.8

54.2

MgSil-org

33.0

12.6

54.0

Na

Na

NH3 (m.mol g-1)

CO2 (m.mol g-1)

95.0

0.46

Mg

Si

O

0.5

31.6

11.4

54.8

0.3

0.4

30.9

11.7

53.6

0.2

202.0

0.98

M

an

us

Mg

XPS analysis

cr

ICP-AES analysis

ip t

Catalyst

Acidic and basic properties

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

b

MgSil-inorg MgSil-org MgO SiO2 MgO:SiO2-mixb

124 255 56 292 312

Cumulative pore volume (cm3 g−1) 0.902 1.253 0.024 0.156 0.146

te

d

Surface area (m2 g-1)

Ac ce p

a

Catalyst

k1 (×106) a k2 (×106) a −1 −2 −1 (l mol m s ) (l mol−1 m−2 s−1) 1.35 ± 0.07 2.59 ± 0.03 0.86 ± 0.05 0.00 ± 0.00 0.87 ± 0.04

0.24 ± 0.08 0.33 ± 0.09 0.10 ± 0.03 0.00 ± 0.00 0.12 ± 0.02

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

27 Page 27 of 39

Table 3: Reusability experiments for transesterification on MgSil-org catalyst

99 98 98 97 97

Conversion of palmitic acid (%)b 80 79 79 78 78

a

Yield of methyl paltimate (%)d

ip t

1 2 3 4 5

Yield of methylbutyratec (%)c 93 93 93 92 92

100 100 100 100 100

cr

Conversion of tributyrin(%)b

us

Cyclea

Cycle 1 is for fresh catalyst whereas subsequent runs are after centrifugation and methanol washing. Reaction conditions: 3 wt.%, MgSil-org, T = 80 oC, time = 45 min c Methylbutyrate yield is defined as the moles of methylbytyrate produced divided by the moles of tributyrin reacted divided by 3. d Methylpaltimate yield is defined as the moles of methylpaltimate produced divided by the moles of palmitic acid reacted divided by 1.

Ac ce p

te

d

M

an

b

28 Page 28 of 39

40

(A)

ip t

MgSil-inorg

cr

20

10

MgSil-org

0 5

10

15

20

25

30

35

40

45

50

140

60

M

(B) 120

te

80

80

MgSil-inorg

100

MgSil-org

80

60

3000

3200

3400

3600

3800

4000

-1

Wavenumber (cm )

20

800

75

MgSil-inorg

40

600

70

120

% Transmittance (a.u.)

60

65

MgSil-org

d

100

Ac ce p

% Transmittance (a.u.)

55

an

2degrees)

us

Intensity (a.u.)

30

1000

1200

1400

1600

1800

2000

-1

Wavenumber (cm )

Fig.1: (A) Powder XRD patterns and (B) FTIR spectra of MgSil samples

29 Page 29 of 39

ip t cr us an M d te Ac ce p

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

30 Page 30 of 39

ip t cr us an M

Ac ce p

te

d

Fig. 3: TEM images MgSil-inorg (A) low magnification (B) high magnification and MgSil-org (C) low magnification (D) high magnification

31 Page 31 of 39

ip t cr us an

Ac ce p

te

d

M

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

32 Page 32 of 39

16000

16000

Mg2p

Si2p 14000

14000

Intensity (a.u.)

12000

MgSil-inorg MgSil-org

12000

MgSil-org

10000 8000 6000

8000

4000

6000 2000

99

100

101

102

103

104

105

45

46

47

48

52

53

O1s

us

140000

51

MgSil-inorg MgSil-org

120000

an

Intensity (a.u.)

160000

50

cr

200000 180000

49

Binding energy (eV)

Binding energy (eV)

ip t

10000

100000 80000 60000

M

40000 20000

525 526 527 528 529 530 531 532 533 534 535 536 537

Binding energy (eV)

te

d

Fig. 5: XPS spectra of MgSil samples

Ac ce p

Intensity (a.u.)

MgSil-inorg

33 Page 33 of 39

800

MgSil-org

1.0

400

100

350

80

MgSil-inorg

700

400 0.2

300 0.0 0

200

400

600

800

1000

1200

1400

1600

o

Pore radius (A )

200 100

250

60

40

200 20

150 0 0

100

200

400

600

800

1000

50

0 0.0

0.2

0.4

0.6

0.8

o

1.0

Relative pressure (P/P )

1200 o

Pore radius (A )

0 0.0

0.2

ip t

0.4

1400

1600

1800

cr

500

300 dV(r)

Volume adsorbed (CC)

0.6

dV(r)

Volume adsorbed (CC)

0.8

600

0.4

0.6

o

0.8

1.0

Relative pressure (P/P )

us

Fig. 6: N2 adsorption-desorption isotherms of MgSil samples, pore size distribution

Ac ce p

te

d

M

an

patterns (inset)

34 Page 34 of 39

Normalized TCD signal (mV)

(A)

ip t

MgSil-inorg

75

150

225

300

375

450

525

600

o

(B)

750

an

MgSil-inorg

MgSil-org

M

Normalized TCD signal (a.u.)

675

us

Temperature ( C)

cr

MgSil-org

100 150 200 250 300 350 400 450 500 550 600 650 700 o

d

Temperature ( C)

Ac ce p

te

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

35 Page 35 of 39

(A)

95

MgSi-org

ip t

90

85

MgSi-inorg

80

75

70 70

80

o

Reaction temperature ( C)

o

60 C

(B)

Methyl butyrate Diglyceride Monoglyceride Glycerine

0

d

40

20

80 C

M

60

te

Selectivity (%)

80

o

an

100

us

60

cr

Conversion of tributyrin (%)

100

MgSil-org MgSil-inorg

Ac ce p

MgSil-org MgSil-inorg

Fig. 8: (A) Conversion of trybutyrin at different reaction temperatures (B) Selectivity of products at 60 oC and 80 oC 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]

36 Page 36 of 39

100

(A) 80 70 60 50 40 30 20 20

30

40

50

60

70

80

90

100

110

us

10

Reaction time (min.) 100

an

(B) 80

M

60

40

6:1 9:1 12:1

d

20

0 0

te

Conversion of tributyrin (%)

ip t

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

cr

Conversion of tributyrate (%)

90

15

30 Reaction time (min.)

45

60

Ac ce p

Fig.9: (A) Effect of the amount of MgSil catalysts [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, reaction temperature: 80 oC] (B) Effect of molar ratio of methanol to trybutyrin on tributyrin transesterification over MgSil-org catalyst [Reaction temperature- 60 oC; rotation speed -600 rpm]

37 Page 37 of 39

Graphical abstract

Magnesium Nitrate

ip t

+

us

an

(Tetraethoxy Silane) (Sodium Silicate)

Inorganic

cr

Organic Si Precursor Si Precursor

M

Hydrothermal

Ac ce p

te

d

Synthesis

Nanosize Magnesium Silicate catalysts for biodiesel production

38 Page 38 of 39

(i)

OH O(OC 4H 7 )

O(OC 4H 7 )

MeOH k1

O(OC 4H 7 )

MeOH k2

+ MeO(OC4H7)

OH

OH

OH

O(OC 4H 7 )

MeOH OH

+

k3

+ MeO(OC4H7) Methylbutyrate

MeO(OC4H7) Methylbutyrate

us

Methylbutyrate (ii)

CH3 (CH2) 14COOCH3 + H2O

CH3 (CH2) 14COOH + CH3OH

Methyl palmitate

Methanol

an

Palmitic acid

OH

cr

O(OC 4H 7 )

O(OC 4H 7 )

Glycerol

Monoglyceride

Diglyceride

ip t

Tributyrin

M

Scheme 1: Reaction pathway of (i) transesterification of tributyrin and (ii) esterification of

Ac ce p

te

d

palmitic acid with methanol

39 Page 39 of 39