Acetalization of glycerol with acetone to bio fuel additives over supported molybdenum phosphate catalysts

Accepted Manuscript Title: Acetalization of Glycerol with Acetone to Bio Fuel Additives over Supported Molybdenum Phosphate Catalysts Author: Sailaja Gadamsetti N. Pethan Rajan G.Srinivasa Rao Komandur V. R. Chary PII: DOI: Reference:

S1381-1169(15)30082-0 http://dx.doi.org/doi:10.1016/j.molcata.2015.09.006 MOLCAA 9622

To appear in:

Journal of Molecular Catalysis A: Chemical

Received date: Revised date: Accepted date:

30-3-2015 5-9-2015 7-9-2015

Please cite this article as: Sailaja Gadamsetti, N.Pethan Rajan, G.Srinivasa Rao, Komandur V.R.Chary, Acetalization of Glycerol with Acetone to Bio Fuel Additives over Supported Molybdenum Phosphate Catalysts, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.09.006 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.

Acetalization of Glycerol with Acetone to Bio Fuel Additives over Supported Molybdenum Phosphate Catalysts SailajaGadamsetti, N. PethanRajan, G.SrinivasaRaoandKomandur V. R. Chary* Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India ___________________________________________________________________

* Corresponding author: Komandur V.R. Chary, Catalysis Division, CSIR-Indian Institute of Chemical Technology,Hyderabad,India. E-mail: [email protected] Tel: +91-40-27193162; Fax: +91-40-27160921

Graphical abstract fx1 Schematic representation of acetalization of glycerol with acetone over various loadings of MoPO/SBA-15 catalysts Highlights     

SBA-15 supported molybdenum phosphate was prepared. The well dispersed (MoO2)2P2O7 phase was formed over the SBA-15 support. Acetalization of glycerol with acetone was performed at room temperature. Supported catalyst (40wt%) performed better compared to pure catalyst. High activity of 40wt% MoPO/SBA-15 (>97%) was explained by its acidity.

Abstract: The acetalization of glycerol with acetone was carried out over a series of molybdenum phosphate catalysts supported on SBA-15 with varying MoPO loadings ranging from 550wt%. These catalysts were characterized byX-ray diffraction, FT-IR, Laser Raman Spectroscopy, Ultraviolet–visible diffuse reflectance spectroscopy (UV DRS), NH3-TPD analysis,

ex-situ

pyridine

adsorbed

FT-IR

analysis

and

pore

size

distribution

measurements.The XRD results of unsupported MoPOshowthe formation of (MoO2)2P2O7 phase and this phase is present in a well dispersed state on the SBA-15.Ramanspectrareveal

1

the presence ofMoPOspecies in the form of (MoO2)2P2O7 phase in the samples above 40wt% MoPO/SBA-15.

The presence of both isolated tetrahedrally and isolated octahedrally

coordinated Mo centers in the unsupported and supported MoPO are confirmed by the UVDRS findings. Ammonia TPD analysis suggests that the total acidity increased with MoPO loading and acidity of the catalysts was proved to be detrimental to assess the catalytic performance. The conversion and selectivity during the acetalizationdepends strongly on the reaction time, catalyst loading and glycerol to acetone molar ratio. Acetalizationof glycerol suggests that 40wt% MoPO/SBA-15 sample exhibited better catalytic propertiesthanother catalysts investigated.The catalytic properties are well correlatedwith the acidic functionalities of the catalysts. Key words: Molybdenum phosphate, glycerol, solketal, fuel additive, acetalization. 1. Introduction: In recent years due to the gradual declining of petroleum reserves, the world energy crisis has become an important topic to explore other possible alternate sources of energy [1-3]. Hence, the use of biofuels has attracted significant attention as a renewable and biodegradable fuelin recent years from researchers in both academic and industries. Biodiesel is generally produced by transesterification of vegetable oils with methanol, where glycerol is the main by-product[4]. This glycerol cannot be utilized for food and pharmaceutical industries due to its high contamination with methanol. However, it can be converted into value added chemicals by different catalytic processes involving oxidation, hydrogenolysis, etherification, dehydration, esterification and acetalization[5-7]. Among various catalytic processes of glycerol conversion, acetalization is found to be one of themost important chemical transformations of glycerol into high value oxygenated fuel additives[8].This improves the quality of diesel by reducing the emissions of carbon monoxide and unregulated aldehydes. The acetalization of glycerol with acetone produces

2

branched

oxygenated

compounds,

namely

(2,2-dimethyl-[1,3]dioxane-4-yl)-methanol

(solketal) and 2,2-dimethyl-[1,3]dioxane-5-ol. Solketal is an excellent component in the formulation of gasoline, diesel and biodiesel fuels. The acetals of glycerol have numerous applications infragrances, pharmaceuticals, detergents, lacquer industries, cosmetics and also as ignition accelerators and antiknock additives in combustion engines [9-11]. Glycerol acetals can also be used as a basis for surfactants [12]. Acetalization of glycerol with acetone is an acid catalysed reaction (Scheme 1) and conventionally carried out using mineral acids as catalysts. In view of stringent environmental requirements, it is essential to develop an effective and inexpensive solid acid catalyst for the acetalization of glycerol. Different types of solid acids such as amberlyst, zeolites, supported metaloxides,metal phosphates and supported heteropoly acids have been recently reported as the catalysts for glycerol acetalization [13-14]. Umbarkar et al. [15]studied the acetalization of glycerol with various carbonyl compounds using mesoporous MoO3/SiO2 catalyst and extensive investigationwas made to determine the physicochemical and acidic properties. Molybdenum oxide promotedwith ZrO2 and SnO2were alsoemployed earlier for the acetalizationof glycerol [16-17].SBA-15 supported molybdenumoxide has been studied for oxidation reactions such as oxidation of propene and oxidation of ethane [18-19]. The promotion of molybdenumoxide to basic material enhances activity of the catalyst due to an increase of the strong acidic character [20-21].However, the presence of both Brønsted and Lewis acid sites in phosphated metal catalysts makes them to use as selective and active catalysts for acidcatalysed dehydration [22-25], ketalization of ketones with diols[26] and also in the isomerisation reactions. Molybdenum phosphate was employed as a catalyst for the ammoxidation of picoline, oxidation of propane and oxidation of ethane[27-29]because of their ability to stabilize metal ion in various oxidation states, i.e., M6+, M5+, M3+ and even mixed valences such as M5+/M6+.

3

These

materials

are

built

up

from

the

linkage

of

PO4

tetrahedral

with

(MoO6)6−octahedral[30]. Moreover, these materials (MoPO) have received increasing attention in the last decade for the use as new cathode materials for lithium and sodium batteries [31].Hence, in the present work, we have employed this material in the acid catalysed reaction such as glycerol acetalization reaction. The use of unsupported molybdenum phosphate catalyst has disadvantages such as low surface area and low pore size. Therefore, molybdenum phosphate issupported on mesoporoussupport such as SBA-15 in order to impart stability of the catalyst and dispersion of the active phase.SBA-15 is a purely siliceous mesoporous molecular sieve with high thermal stability and having potential to use as a catalyst support for the active phase in various applications. This material possesses a high surface area (700–1000 m2 g-1)which should promote a high dispersion of the active phasewith pore diameter in the mesoporousrange (2–7 nm)[32]. In the present work, we have investigated the efficiency of molybdenum phosphate catalystsupported on SBA-15 for the acetalization of glycerol with acetone to produce bio fuel additives. The reaction was systematically investigated by varying several reaction parameters to optimise the conditions to find a catalyst exhibitinghigh activity/selectivity with fairly good stability.These catalysts were characterized by X-ray diffraction, pore size distribution, FT-IR,Raman, UV-DRS and NH3-TPD methods. Our results provide a basis mainly for correlating the catalyst acidity by varying MoPO content on SBA-15 support and the effect of various reaction parameters such as the molar ratio of acetone to glycerol, catalyst amount and reaction time.

2. Experimental section: 2.1. Catalyst preparation: The mesoporous SBA-15 support was prepared by the procedure described elsewhere[33-34].

4

Briefly, it involves using a tri-block copolymer poly-ethylene glycol–block-poly-propylene glycol–block-poly-ethylene glycol (P123, average molecular mass  5800 g, Aldrich) as a template. About 2g of P123 copolymer was dissolved in a mixture of 15g of water and 45g of 2M followed by addition of 0.2g of cetyltrimethyl ammonium bromide (CTMABr) and 5.9g of tetraethylorthosilicate (TEOS) with continuous stirring of the contents. The final molar ratio of the synthesis mixture was 1 TEOS: 0.02 CTMABr: 3.1 HCl: 115 H2O: 0.012 Polymer. The synthesis mixture was introduced into a Teflon lined autoclave, sealed and kept at 100°C for 24 h. Subsequently,it was cooled, filtered and washed with deionised water and ethanol to remove the excess template from the mixture prior to calcination in air at 500°C (5°C /min) for 5h. The molybdenum phosphate was prepared by the procedure described elsewhere [3536].Typically, the white Mo (VI) containing precursor material was prepared by dissolving 7.5 g of MoO3 (99.5%, Aldrich) in 22.5 cm3 85% H3PO4 (331 mmol, Aldrich) at approximately 180°C. The solution was then cooled to room temperature and 200 cm3of 15.8M HNO3 (Fisher) was added to it andrefluxed further for 12 h. Upon cooling, small crystallites of the target compound was precipitated from the solution. These crystals were vacuum-filtered, washed with acetone, dried in air and subsequently calcined at 550°C (5°C /min) for 6 h. Molybdenum phosphate supported on SBA-15 was prepared by impregnation method by addingrequired amount of aqueous solution of molybdenum phosphate to the calcined SBA-15 support. The samples were subsequently dried at 110°C for 12 h and calcined in air using muffle furnace at 550°C for 6 h. 2.2.Catalyst characterization: X-ray diffraction (XRD) patterns were obtained on Rigakuminiflexdiffractometer using graphite filtered Cu Kα (K = 0.15406 nm) radiation.FT-IR spectra of the catalysts were taken

5

on an IR (Model: GC-FT-IR Nicolet 670) spectrometer by the KBr disc method under ambient conditions. The UV-Vis diffused reflectance spectra were recorded on a GBC UVVisible Cintra 10e spectrometer with an integrating sphere reflectance accessory. The spectra were recorded in air at room temperature and the data were transformed according the Kubelka-Munk equation f(R) = (1-R) 2/2r. The specific surface areaof the prepared catalysts were estimated using N2 adsorption isotherms at -196°C by the multipoint BET method, taking 0.162 nm2 as its cross-sectional area using Autosorb1 (Quantachrome instruments). The pore size distribution measurements were also performed using the same instrument by N2 adsorption-desorption and by applying the BJH analysis.

The Raman spectra of the catalyst samples were collected with a HorbiaJobinYvon Lab Ram HR spectrometer equipped with a confocal microscope, 2400/900 grooves/mm gratings and a notch filter. The Visible Laser excitation at 532 nm (visible/green) was supplied by a YAG doubled diode pumped laser (20mW). The scattered photons were dried and focused onto a single stage monochromator and measured with a UV sensitive LN2 cooled CCD detector (HorbiaJobinYvon CCD 3000V). TPD experiments were conducted on AutoChem 2910 instrument. In a typical experiment for TPD studies, the sample was pretreated by passage of high purity helium (50 ml/min) at 400°Cfor 1h. After pretreatment, the sample was saturated with 10% NH3-He (50 ml/min) at 80°Cfor 1h and subsequently flushed with He flow (50 ml/min) at 100°Cfor 1 h to remove physisorbed ammonia. TPD analysis was carried out from ambient temperature to 600°Cat a heating rate of 10°C/min. The amount of NH3 desorbed was calculated using GRAMS/32 software. The ex-situ experiments of the FT-IR spectra of pyridine adsorbed samples were

6

carried out to find the nature of acidic sites.i.e., Brønsted and Lewis acid sites. Pyridine was adsorbed on the activated catalysts at 120°C until saturation. Prior to adsorption experiments, the catalysts were activated in N2 flow at 300°Cfor 1 h to remove the adsorbed moisture from the samples. After such activation, the samples were cooled to room temperature. The IR spectra were recorded using a FT-IR model: GC-FT-IR Nicolet 670 spectrometer by the KBr disc method under ambient conditions. 2.3. Catalytic reaction: The liquid phase acetalization reaction of glycerol with acetone was carried out at room temperature and atmospheric pressure. In a typical experiment, 0.92 g of glycerol and 0.581.74gof acetone were taken in a 25 ml round bottom flask with 25-100 mg of catalyst. Before the reaction, the catalysts were pre-treated at 300°C for 1h. The reaction products were collected periodically for analysis using a gas chromatograph GC-2014 (Shimadzu) equipped with a DB-wax 123-7033 (Agilent) capillary column (0.32 mm i.d., 30 m long) and a flame ionization detector (FID). 3. Results and Discussion: 3.1. Nitrogen adsorption-desorption analysis: To understand the textural properties of the catalyst, the N2 adsorption-desorption analysis was carried out to measure the surface area, pore size of the samples and the respective isotherms are shown in Fig.1.All isotherms exhibit a sharp uptake of nitrogen at relative pressures (P/Po) of >0.6, showing the type IV isotherm with an H1 hysteresis loop confirms that the pore sizes are present in the ranges of mesoporous region. Hence, the pure SBA-15 and supported MoPO samples possess a hexagonal arrangement of mesopores and the broad hysteresis loop observed in the isotherms of thesesamples reflects the mesopores, which limit the emptying and filling of the accessible volume[33]. The pore size distribution analysis of various catalysts determined by the BJH method

7

are shown in Fig.1 (inset) and it reveals that a narrow pore size distribution is observed for pure SBA-15 and also in the supported MoPO catalysts centred around 60-70 Å. All the samples have shown uni-model pore size distribution. As can be seen from the results of Table 1 the MoPO loading in the samples has shown a clear impact on the surface area of the SBA-15 support. The surface area of the pure SBA-15 was found to be 840 m2/g and decreases as a function of the MoPO content (Table 1). This decrease of surface area with increasing of MoPO loading might be due to blocking of the mesopores of the SBA-15 by MoPO species. The surface area of pure MoPO sample was found to be 2 m2/g. The BJH pore size distribution results also reveal that the pore volume decreases gradually with MoPO loading on SBA-15 due to the presence of added MoPO components in the pores of SBA-15 support. However, the average pore diameter increases marginally with MoPO loading in the catalyst.

Pure SBA-15

Volume (cc/g)

Dv(logdv)(cc/g)

5wt% MoPO/SBA-15 10wt% MoPO/SBA-15 20wt% MoPO/SBA-15 30wt% MoPO/SBA-15 40wt% MoPO/SBA-15 50wt% MoPO/SBA-15

50

100

150

O

BJH Pore diameter(A )

200

(a) 250

(b) (c) (d) (e) (f) (g)

0.2

0.4

0.6

0.8

1.0

P/PO

Fig.1: BJH isotherms and pore size distribution profiles of various MoPO/SBA-15 catalysts. a) Pure SBA-15 b) 5wt% MoPO/SBA-15 C) 10wt% MoPO/SBA-15 d) 20wt% MoPO/SBA-15 e) 30wt%MoPO/SBA-15 f) 40wt%MoPO/SBA-15 g) 50wt%MoPO/SBA-15 8

Table1: Structural properties of various MoPO/SBA-15 catalysts S.No

Surface Area (m2/g)

Pore Volume (cc/g)

1. 2.

MoPO loadings (wt%) 0 5

840 688

1.34 1.29

Average pore diameter (Å) 66.4 65.0

3. 4. 5.

10 20 30

573 442 359

1.13 1.03 0.84

65.2 65.3 62.4

6. 7

40 50

164 125

0.59 0.50

69.6 70.0

3.2.Low angle X-ray diffraction analysis: The low angle XRD patterns of pure SBA-15 and various MoPO /SBA-15 catalysts are shown in Fig.2. The pure SBA-15 sample shows the characteristic diffraction peaks at 2θ = 0.92, 1.64, 1.85 can be indexed to the (100), (110) and (200) planes. This confirms the hexagonally ordered mesoporous structure of SBA-15 [33].

9

Intensity(a.u)

(100)

Intensity(a.u)

Pure SBA-15

(110)

0.5

1.0

1.5

2.0

2.5

3.0

2 Theta

3.5

4.0

4.5

5.0

(200)

(a) (b) (c) (d) (e) (f)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

2 Theta

Fig.2: Low angle XRD profiles of various MoPO/SBA-15 catalysts a) 5wt% MoPO/SBA-15 b) 10wt% MoPO/SBA-15 c) 20wt% MoPO/SBA-15 d)30wt% MoPO/SBA-15 e) 40wt% MoPO/SBA-15 f)50wt% MoPO/SBA-15

As the MoPO loading increases on the SBA-15 support, the intensity of all XRD peaks decreases. At lower MoPO loadings (<30wt%) the (110) and (200) diffraction peaks of hexagonally ordered mesoporous material are observed in the XRD pattern. However, at higher MoPO loadings (>40wt%) the aforementioned peaks were disappeared (Fig. 2). This indicates that the added MoPO components are occupied the hexagonal mesopores of the SBA-15 and leads to the loss of hexagonally arranged porosity of the SBA-15. However, with the increase of the MoPO loadings over the SBA-15 support, the (110) and (200) diffraction peaks of hexagonally ordered mesoporous material have shifted towards the lower 2 suggesting the presence of MoPO species in the pores of the support. 3.3. Wide angle X-ray diffraction and FT-IR analysis:

10

The

wide

angle

XRD

patterns

of

calcined

and

uncalcinedMoPO(MoO2·HPO4·H2O)areshown in Fig.S1(supplementary file).The X-ray diffraction pattern of uncalcinedMoPO precursor (MoO2·HPO4·H2O) shows the major X-ray diffractionpeak at 2θ = 15.6°, 22.5°, 28.1° and 30.5° confirms the formation of crystalline monoclinic phase of molybdenum hydrogen phosphate hydrate [37]. The calcined MoPOsample exhibits the main X-ray diffraction peaks at 2θ =22.2o.This XRD diffraction peakis related to the (MoO2)2P2O7 phase [38-39]. The FT-IR spectra of pure calcined MoPOsample are shown in Fig.S2.The FT-IR spectra of pure MoPO sample exhibits the IR band at 3600 cm−1 due to the vibration of the OH bond, while the 1620 cm−1 band is assigned to Mo-O-H bond. The IR band at 738 cm-1 and 928 cm-1 are assigned to symmetric and asymmetric stretching frequency of P-O-P bonds respectively confirming the formation of molybdenum pyrophosphate. However, the calcined pure MoPO sample exhibits the symmetric and asymmetric stretching frequency of PO4 groups at 1251 and 1025 cm-1 and stretching vibrations of the oxygen atoms linked to two molybdenum atoms (Mo-O-Mo) are found at 858 cm−1 in (MoO2)2P2O7 [37]. The low wave number peak in the spectra is attributed to Mo-O bonds of molybdenum pyrophosphate. Hence, the XRD and FT-IR analysis is used to find the molybdenum pyrophosphate phase formed in calcined pure MoPO sample. The wide angle X-ray diffraction patternand FT-IR spectra of pure SBA-15 support and various MoPO/SBA-15 catalysts with MoPO loadings ranging from 5 to 50 wt% catalyst are shown in Fig.S3 and Fig.S2 respectively.The results of XRD patterns and FT-IR spectra ofSBA-15 supported MoPO sample did not showany X-ray diffraction peaks or IR bands related to MoPO species indicating that MoPO species is in well dispersed state over the SBA-15 support. 3.4.Raman spectroscopy:

11

(832) (975)

Intensity(a.u)

Pure MoPO

Intensity(a.u)

(583) (727)

400

600

800

(963)

(1156) (1023)

1000

1200

1400

(864)

1600

-1

wave number(cm )

(f) (e) (d) (c) (b) (a)

400

600

800

1000

1200

-1

wave number(cm )

Fig.3:Raman spectroscopy profiles of various MoPO/SBA-15 catalysts a) 5wt% MoPO/SBA-15 b) 10wt% MoPO/SBA-15 c) 20wt% MoPO/SBA-15 d) 30wt% MoPO/SBA-15 e) 40wt% MoPO/SBA-15 f) 50wt% MoPO/SBA-15

The Raman spectra of pure MoPO and SBA-15 supported molybdenum phosphate samples obtained under ambient conditions in the 400-1200 cm−1 region are presented in Fig. 3. The pure MoPO sample exhibits the Raman bands at 583, 727, 832, 975, 1023 and 1156 cm−1. The presence of both MoO4 tetrahedra and MoO6 octahedra terminal, Mo-O stretching vibrations of MoO6 octahedra was observed in the region of 950–1000 cm−1, whereas stretching vibrations of MoO4 tetrahedra were assigned to vibrational bands observed within the region of 900-950 cm−1. Mo-O-Mo stretching vibrations of coupled MoO6 octahedra were observed within the region of 800-900 cm−1 [40]. In the case of supported MoPO samples, the pure SBA-15 shows no visible bands in the Raman spectra. The introduction of MoPO loadings over the SBA-15 support exhibitsa band at 963 cm−1and the intensity of this band is increasing with MoPO loading. This band can be attributed to symmetric stretching frequency of PO43- group which confirms the formation of molybdenum pyrophosphate 12

(MoO2)2P2O7) phase on SBA-15. The other Raman band at 864 cm−1 is noticed for higher MoPO loading (50wt% MoPO/SBA-15) due to the symmetric stretching frequency of Mo-OMo bond [40]. This result reveals the formation of polymolybdate structure over the surface at higher MoPO loadings due to agglomerisation of MoPO species. However, the XRD pattern of 50wt% MoPO/SBA-15 does not suggest any proof related to agglomerisation of MoPO species. The calcined MoPO species exhibits XRD peak at 2θ = 22.2° which is present in the centre of broad reflection pattern of amorphous silicon dioxide (2θ = 15 – 35°). Hence, the formed XRD peak of MoPO could be hidden by broad reflection peak of SiO2. The Raman spectra of the pure MoPO sample exhibits sharp intense peak due to more amount of polymerised structure of MoPO species instead of isolated MoPO species.

3.5.UV-DRS spectroscopy: UV-Vis spectra of pure and the supported MoPO samples are presented in Fig.4. The UV-DRS spectrum of pure MoPO sample exhibits a broad absorption band between 200 and 400 nm can be assigned to ligand to metal charge transfer (CT) transitions (O2-→Mo6+/ Mo4+) [41]. However, this broad charge transfer band shows a maximum at around 242 nm can be assigned to isolated tetrahedrally coordinated Mo centres. Besides, a shoulder band at around 320 nm also appears in the spectrum can be associated with the octahedrallycoordinated Mo centres (Mo6+). (h)

Absorption(a.u)

(g) (f) (e) (d) (c) (b) (a)

13 200

300

400

500

wavelength(nm)

600

700

Fig. 4: UV DRS profiles of various MoPO/SBA-15 catalysts a) Pure SBA-15 b) 5 wt% MoPO/SBA-15 c) 10 wt% MoPO/SBA-15 d) 20 wt% MoPO/SBA-15 e) 30 wt% MoPO/SBA-15 f) 40 wt% MoPO/SBA-15 g) 50 wt% MoPO/SBA-15 h) pure MoPO

Similarly, the UV-DRS spectrum of SBA-15 supported MoPO sample exhibits a broad absorption band between 200 and 400 nm can be assigned to ligand to metal charge transfer (CT) transitions (O2-→Mo6+/ Mo4+) of isolated tetrahedrally and octahedrally coordinated Mo centres. As the MoPO loadings on SBA-15 increases, the intensity of the charge transfer transition band at 242 and 320 nm also increases, which is the characteristic edgeof the MoPO sample. However, there is no significant change in the shape and position of the charge transfer band is observed with increase of MoPO loadings over the SBA-15 support. These findings are qualitatively in accordance with the results of X-ray diffraction and Ramanspectroscopy.

3.6.Temperature programmed desorption of ammonia: Temperature-programmed desorption (TPD) of ammonia and/or pyridine are the popular methods for determining the acidity of solid catalysts as well as acid strength distribution. Ammonia is frequently used as a probe molecule because of its small molecular

14

size, stability and strong basic strength (pKa = 9.2). In the present investigation, the acidity measurements have been carried out by the NH3-TPD method. The surface acidity is an important factor to assess the acetalization of glycerol with acetone as this reaction proceeds on the surface acidic sites of the solid acid catalysts. The TPD profiles of various samples are shown in Fig. 5 and the respective ammonia uptake values are given in Table 2.The TPD profiles in Fig. 5 clearly demonstrated the effect of the acidic properties of different wt% of MoPO loadings on SBA-15 support. The lower loadings SBA-15 supported MoPO sample exhibits peak in the temperature region of 100-350 °C due to weak and moderate acidic sites. However, it is interesting to note that as MoPO loadings are increasing, the intensity of the weak and moderate acidic region peak is also increasing. This could be due to the formation of well dispersed MoPO phases over the SBA-15 support. However, at high MoPO loadings (50 wt%) the intensity of the weak and moderate acidic region peak is decreasing due to the agglomerisation of MoPO species as evidenced from Raman analysis. It is interesting to note that the total acidity value of supported MoPO is higher than the pure MoPO catalyst.

Volume of NH3 desorbed(ml)

Volume of NH3 desorbed(ml)

Pure MoPO

100

200

300

400 o

Temperature ( c)

500

600

(f)

(e) (d) (c) (b) (a)

200

300

400

500

600

o

Temperature( c)

Fig.5: TPD profiles of various MoPO/SBA-15 catalysts a) 5wt% MoPO/SBA-15 b) 10wt% MoPO/SBA-15 c) 20wt% MoPO/SBA-15

15

d) 30wt% MoPO/SBA-15 e) 40wt%MoPO/SBA-15 f)50wt%MoPO/SBA-15

Table 2:Results of temperature programmed desorption of ammonia of various loadings ofMoPO samples MoPO loadings (wt%)

Desorbed ammonia (mmol/g)

5

Temperature Tmax (°C) 223

10

234

0.30

20

222

0.53

30

221

0.81

40

219

0.96

50

311

0.48

100

298

0.20

0.19

3.7. Ex-situ pyridine FT-IR analysis: The previous discussion does not contain any information on the nature of acidic sites, because the ammonia TPD cannot discriminate Brønsted and Lewis acid sites. For characterizing the nature of surface acidic sites, we have employed ex-situ pyridine adsorbed FT-IR analysis. The FT-IR spectra reveals that the IR bands appeared at 1540–1550 cm−1 and 1445–1460 cm−1 are characteristic bands of Brønsted (B) and Lewis (L) acid sites respectively. Furthermore, the bands correspond to a combination of both Brønsted and Lewis (B+L) acid sites are appearing at 1490-1500 cm-1. It should be noted that the intensity of the IR bands is proportional to the concentration of acidic sites. The pyridine adsorbed FTIR spectra of different MoPO/SBA-15 catalysts are illustrated in Fig.6.

16

(a) (L) (B+L)

(b)

(B)

(c)

%Transmittance

(d) (e)

(f)

(g)

1400

1500

1600

1700

-1

wavenumber(cm )

Fig. 6:Ex-situ pyridineadsorbed FT-IR profiles of various MoPO/SBA-15 catalysts a) 5wt% MoPO/SBA-15 b) 10wt% MoPO/SBA-15 c) 20wt% MoPO/SBA-15 d) 30wt% MoPO/SBA-15 e) 40wt% MoPO/SBA-15 f) 50wt% MoPO/SBA-15 g) pure MoPO

All the catalysts have shown IR bands at 1490 cm−1 corresponding to both Brønsted and Lewis (B+L) acid sites and the other IR band appeared at 1550 cm−1 is attributed to Brønstedacidic sites. However, theseacidicsites (Fig.6)are present in different proportions depending on the wt% of MoPO on SBA-15 catalyst. As can be seen from the pyridine adsorbed FT-IR spectra, the intensity of IR absorption bands at 1498 cm-1 (B+L) and 1540 cm-1 (B) increases with the increase of MoPOwt% on the SBA-15 support. The IR adsorption band at 1450 cm-1 (L) attributed to Lewis acidic sites is present at lower loadings sample (5 wt% MoPO/SBA-15). It is interesting to note that the pure MoPO sample and 50 wt% MoPO/SBA-15 exhibit the low intensity of IR absorption bands than 40 wt% MoPO/SBA-15 sample suggesting that the supported sample possesses higher acidity than pure MoPO sample and otherMoPOsamples. 17

4.Catalytic activity: Acetalization of glycerol A systematic study was undertaken to investigate the catalytic properties of the MoPO/SBA-15 catalysts during the liquid phase acetalization of glycerol with acetone. This reaction mainly yields the five-membered ringketal (solketal) and also forms the sixmembered ring ketal, whose relative formation depends on the acetalization position within the glycerol molecule. Glycerol acetalization with acetone favours the formation of the fivemembered ring transition state, which leads to the production of solketal[11, 13].Acetalization of glycerol with acetone is represented in Scheme 1. O OH OH

OH

Acetone

Glycerol

cat

-H2O

O

O

O

OH 2,2-dimethyl-1,3-dioxan-4-ol

O OH

(2,2-dimethyl-1,3-dioxolan-4-yl)methanol

Scheme 1:Schematic representation ofacetalization of glycerol with acetone over MoPO/SBA-15 catalysts

18

Fig.7:Acetalization of Glycerol with acetone over various MoPO/SBA-15 catalysts. In the present study, the pure MoPO and various wt% loadings of MoPO/SBA-15 catalysts are investigated in the acetalization of glycerol with acetone and the catalytic results are presented in Fig.7. The Fig. 7 clearly illustrates the impact of MoPO loadings on the catalytic activity behaviour towards acetalization of glycerol and the supported MoPO catalyst shows a maximum conversion of glycerol (100%) with 98% solketal selectivity. The catalytic experiments were carried out at room temperature, with 2h of reaction time and 3:1 molar ratio of acetone to glycerol. Pure SBA-15 catalyst has found to be less active for the acetalization reaction probably due to its less acidic nature compared to other supported MoPO catalysts. Among all the catalysts tested, the 40wt% MoPO/SBA-15 has displayed the highest activity, with a maximum conversion of 100%. Activity towards acetalization initially increases upto40wt% MoPO/SBA-15 by increasing MoPO loadings and it decreases at higher MoPO loadings (50 wt%). It is interesting to note that the solketal selectivity was always same (97-98%) irrespective of the MoPO loadings over the SBA-15 support. This is due to 19

the fact thatthe five membered ring compound solketal [(2,2-dimethyl-1,3-dioxolan-4yl)methanol]is thermodynamically more stable thanthe six membered ring compound [2,2dimethyl-1,3-dioxan-5-ol] because the six membered ring compound possesses the methyl group in axial position produces the steric repulsions[42]. Silva et al. [43] investigated the acetalization of glycerol with aqueous formaldehyde solution over various heterogeneous catalysts. Among the catalysts tested, Amberlyst-15 acid resin showed the best catalytic performance of 95% glycerol conversion. Serafim et al.[13] studied the acetalization of glycerol with butanal over a different types of zeolites and the beta zeolite showed the highest catalytic activity of 87% glycerol conversion with79% solketal selectivity. Sudarsanam et al.[16] studied the acetalization of glycerol with benzaldehyde over various solid acid catalysts namely ZrO2, TiO2–ZrO2 and the respective MoO3 promoted catalysts and the MoOx promoted TiO2–ZrO2 catalyst exhibited a better catalytic activity of74% glycerol conversion with 51% 1,3‐dioxane selectivity. Khayoon et al.[44] studied the acetalization of glycerol with acetone over 5%Ni–1%Zr/AC composite catalyst and glycerol was completely converted to a final product containing 76% of solketal. In the present study, we reported the MoPO catalyst supported on SBA-15 showed asuperior catalytic activity of 100% glycerol conversion with 98% solketal selectivity. Mallesham et al.[45] studied the acetalization of glycerol using MoO3 and WO3 promoted SnO2-based solid acid catalysts. They found that the molybdenum promoted SnO2 catalyst showed the 71% glycerol conversion with 96% solketal selectivity due to the presence of large amounts of acidic sites. The same research group[17]also studied the acetalization of glycerol using sulphatedSnO2 solid acid catalyst and it exhibited an outstanding catalytic activity of 95% glycerol conversion and 98% solketal selectivity after 240 min reaction time. This is due to the presence of higher amounts of surface acidic sites associated with super acidic sites. Hence, it is understood that the acidic sites of the catalysts

20

plays an important role in the catalytic performance. The supported MoPO sample showed better performance in the acetalization of glycerol to solketal due to the presence of higher acidic sites. Among the supported MoPO samples, the 40wt% MoPO/SBA-15 showed the 100% glycerol conversion with 98% solketal selectivity. The acidity of the supported MoPO samples also increases until 40wt% MoPO/SBA-15 after that it decreases. It can be evidenced by the resultsof ammonia TPD and ex-situ pyridine adsorbed FT-IR analysis. Besides acidity of the catalyst, texturalproperties and porosity of the catalystalso plays a crucial role in the catalytic performance during the acetalization of glycerol. The effect of crystallite size and pore size of different zeolites in the acetalization of glycerol with acetone was investigated by Manjunathanet al.[46]. They reported that the zeolite with lower crystallite size (H-Y Zeolite) and higher pore size gave a better conversion (74%) and solketal selectivity (98%). Interestingly, the supported MoPO sample possesses the average pore size in the range of 62-70Å (mesoporous range) (Table-1) which eliminates the diffusion limitation of reactants and products and also these samplesis in well dispersed state over SBA-15 support (amorphous in nature) as evidenced from the XRD and FT-IR analysis. The catalytic experiments were also carried out without catalyst at the same reaction condition (blank experiment), which showedvery low glycerol conversion (less than 1%). This confirmed the importance of the catalyst in the glycerol acetalization reaction. As stated earlier, this reaction is promoted by acidic sites (Lewis and Brønsted acidic sites)of the catalyst [47,14]. However, in the present work, the supported MoPO sample exhibited the large amount of Brønsted acidic sites as evidenced from the ex-situ pyridine adsorbed FT-IR analysis. Thus, it can be concluded that solketal formation over the supported MoPO sample is mainly catalyzed by Brønsted acidic sites. Many others also reported in the literature that five membered solketal can be formed selectively by the Brønsted acidic sites (ketal mechanism). Based on the above studies, the possible reaction pathway for the acetalization

21

of glycerol with acetone over MoPO supported on SBA-15 sample is shown in scheme 2.The majority of acetalization reactions involving polyalcohol occur through two reversible steps[48]. In the first step, the lone pair of the oxygen atom of the glycerol attacks the positively charged carbonyl carbon forms the tertiary alcohol intermediate 3-(2hydroxypropan-2-yloxy)propane-1,2-diol called as hemiketal and this intermediate interacts with the Brønsted acidic sites of the MoPO/SBA-15 catalyst through tertiary alcohol group [49].In the next step, the nucleophilic attack of the secondary hydroxyl group of glycerol to the tertiary carbon of the hemiketal intermediate forms the solketal as a major product, whereas the nucleophilic attack of the primary hydroxyl group of glycerol produces the six membered compound as a minor product. The formationof six membered ring compounds is not shown in scheme 2since it formed as a minor product (less than 3%)during the reaction. OH

OH O

H

OH

O

OH

MoPO SBA-15

O

+

OH

O

O

MoPO SBA-15

-H2O O O

H

H

O

OH

Solketal

MoPO SBA-15 Hemikeatal

Scheme 2: Possible reaction mechanism of acetalization of glycerol with acetone over MoPO/SBA-15 catalysts

22

H

4.1.Effect of acetone to glycerol mole ratio: The influence of acetone to glycerol molar ratio on the glycerol acetalization over various MoPO/SBA-15 catalysts is shown in Table 3. The reaction was studied by varying the acetone to glycerol mole ratio from 1:1 to 3:1, at room temperature, 2 h reaction time and using a catalyst with 5 wt% of glycerol. The obtained resultsshow that an increase in glycerol conversion is noticed with an increase in the molar ratio of acetone to glycerol in all the catalysts. The fact that the equilibrium has to be driven towards the production of solketal using large excess of acetone or by removing the water produced by reaction media [42].

Table 3: Effect of acetone to glycerol molar ratio on the glycerol acetalization over various MoPO/SBA-15 catalysts MoPO

Acetone to glycerol molar ratio

loadings (wt%)

1:1

2:1

3:1

Conversion

Selectivity

Conversion

Selectivity

Conversion

Selectivity

(%)

of solketal

(%)

of solketal

(%)

of solketal

(%)

(%)

(%)

5

30

97

57

97

69

98

10

43

98

62

98

75

98

20

47

98

67

98

81

98

30

57

98

77

98

88

98

40

69

98

100

98

100

98

50

30

97

56

97

72

97

100

32

97

64

97

64

97

Reaction conditions: Cat wt: 50mg, Time: 2h, Room temperature, Atmospheric pressure

4.2.Effect of catalyst amount: Table 4: Effect of catalyst amount on the glycerol acetalization over various MoPO/SBA-15

23

Catalysts MoPO

Catalyst amount

loadings (wt%)

25 mg

50 mg

100 mg

Conversion

Selectivity

Conversion

Selectivity

Conversion

Selectivity

(%)

of solketal

(%)

of solketal

(%)

of solketal

(%)

(%)

(%)

5

64

98

69

98

70

97

10

70

98

75

98

75

98

20

72

98

81

98

82

98

30

80

98

88

98

89

98

40

90

98

100

98

100

98

50

67

98

72

97

74

98

100

60

97

64

97

65

97

Reaction conditions: Acetone and Glycerol ratio: 3:1, Time: 2 h, Room temperature, Atmospheric pressure

The effect of catalyst amount of glycerol conversion over various MoPO/SBA-15 catalysts at room temperature for the acetalization of glycerol is shown in Table 4. The catalytic experiments were studied by varying the catalyst weight from 25 mg to 100 mg. From these studies, it can be observed that the 40 wt% MoPO/SBA-15 catalysts exhibited good conversion of glycerol even at low catalyst amount and also observed that the conversion of glycerol increases with the increase of the weight of the catalyst. This is due to an increase in the availability of total number of acid sites and suggest that the presence of more acidic sites are helpful to the glycerol acetalization with acetone. However, the selectivity towards solketal has been observed over 97% in all the cases during the reaction.

4.3.Effect of reaction time:

24

100

conversion/selectivity(%)

90

80

% conversion % selectivity

70

60

50

40 0

20

40

60

80

100

120

Time(min)

Fig.8:Dependence of glycerol conversion and selectivity of solketal as afunction of time over 40wt% MoPO/SBA-15 catalyst. Reaction conditions: Acetone and Glycerol ratio: 3:1, 50mg catalyst, Room temperature, Atmospheric pressure

Fig. 8 shows the effect of reaction time on the glycerol acetalization with acetone over 40 wt% MoPO/SBA-15catalyst. The reaction conditions are: 50mg catalyst, acetone/glycerol molar ratio 3, room temperature and atmospheric pressure. It can be seen from Fig. 8 that the conversion of glycerol increases with reaction time and it reaches a maximum (100%) at 60 min. However, the solketal selectivity was high from the beginning of the reaction and remains constant with reaction time indicates that the reaction achieves the equilibrium state. 4.4.Catalyst reusability and stability studies: It is important to carry out the reusability of the spent 40 wt% MoPO/SBA-15 catalyst to understand its stability during the acetalization of glycerol with acetone. After the completion of the first catalytic experiment, the catalyst was separated by centrifugation, and washed with small amount of methanol followed by drying it in an oven at 80 °C. Then the sample was activated at 300°C for 1h in the muffle furnace and the reaction was performed under similar reaction conditions mentioned earlier. This procedure was repeated three times 25

toachieve four consecutive recycles and the results are presented in Fig. 9. It can be seen from Fig. 9 that the glycerol conversion decreases significantlyafter the first run, thereafter it did not changed appreciably. The glycerol conversion at the 1st, 2nd, 3rdand 4thcycles was found to be 100, 70, 68, and 62%, respectively. The selectivity of solketal was always found to be 9798%. 120

conversion selectivity

Conversion/Selectivity (%)

100

80

60

40

20

0 1st run

2nd run

3rd run

4th run

Reusability

Fig.9: Reusability studies of40wt% MoPO/SBA-15 catalyst In order to understandabout the deactivation of the supported MoPO catalyst, we have performed the leaching test accordingly [44]. The 40 wt% MoPO/SBA-15 sample was first immersed in the required amount of acetone and the mixture was then stirred at the room temperature for 2h. Then, the mixture was filtered and the required amount of glycerol was added to the filtrate. This mixture was stirred for 2h at the same reaction conditions described above, in the absence of the catalyst and the reaction product was analyzedusing a gas chromatograph. The results showed the significant amount of catalytic activity (glycerol conversion-32%, solketal selectivity-97%) which confirms the leaching of the MoPO samples

26

from the 40 wt% MoPO/SBA-15 catalyst in the liquid phase. However, the same leaching test was also carried out for the spent 40 wt% MoPO/SBA-15 sample (after 2ndrun sample). Interestingly, the spent 40 wt% MoPO/SBA-15 sample didnot show any catalytic activity (glycerol conversion-1%) indicatesthe stability of spent 40 wt% MoPO/SBA-15 sample during the acetalization reaction.This interesting observation is ingood agreement with the reusability studies. The ex-situ pyridine adsorbed FT-IR analysis of the fresh and spent 40 wt% MoPO/SBA-15 catalysts are shown in Fig. S4. FromFig. S4, it is observed that the intensity of the IR band at 1540 and 1490 cm−1due to the Brønsted acidic sites and Brønsted& Lewis acidic sites was decreased appreciably in the spent sample due to the leaching of MoPO species. As a result, thedecrease in the catalytic activity of 40 wt% MoPO/SBA-15 during the glycerol acetalization with acetone is observed.

5. Conclusion: MoPO/SBA-15 catalysts are found to be highly active for the acetalization of glycerol.XRD and Raman studies reveal that MoPO is present in a well dispersed state in the samples below 50wt% MoPO-SBA-15. The SBA-15 supported MoPO catalysts described herein are novel and selective heterogeneous catalysts for the preparation of acetals from glycerol.Among all the catalysts studied,40wt% MoPO/SBA-15 catalysthas shown the best performance, with the highest conversion at 60 min of reaction time. The total acidity mainly due to the Brønsted acidic sites of the catalyst are the most significant factors affecting the catalytic performance of acetalization of glycerol. The catalytic activity of the spent 40wt% MoPO/SBA-15 sample decreased due to the leaching of the MoPO species.

Acknowledgment

27

The author thanks University Grants Commission (UGC), New Delhi for the award of Senior Research Fellowship. This work is supported by the CSIR XII- 5 year plan program under Indus-Magic.

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