Mesoporous SAPO-5 (MESO-SAPO-5): a potential catalyst for hydroisomerisation of 1-octene

Abstract

Mesoporous silicoaluminophosphate was assembled from microporous SAPO-5 secondary building unit precursors. The resultant material possessed both mesoporous channel properties and microporous wall properties. The catalyst showed promising results for vapour phase isomerisation of 1-octene. The presence of strong acidic sites favoured the formation of considerable skeletal isomerised products at elevated temperatures (400–450 °C). However, the catalytic conversion remains constant and active for several hours.

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1. Introduction

Successful discovery of surfactant mediated synthesis of inorganic mesoporous materials was pioneered by Mobil scientists,^1,2 and their analogous molecular sieves SBA-1,³ HMS,⁴ and SBA-15,^5,6 with immense attention being paid to various applications in the fields of separation, adsorption, support and heterogeneous catalysis.^7–15 These materials possessed high surface areas, large uniform tunable pore structures and well-ordered morphology.^1,2 In contrast, the feeble thermal, hydrothermal, mechanical stability and reduced acidity shown compared to their zeolitic counterparts means that the practical applications of these materials are considerably retarded. Researchers have adopted several methodologies such as “Bottom-up” and “Top-down”^16,17 to improve the thermal and hydrothermal stability and crystalline properties of mesoporous materials. The bottom-up strategy involves multistep synthesis methods, i.e., either by first preparing the mesoporous phase and then coating the pore-wall with zeolite secondary building units (SBUs)^16–20 or allowing the zeolitic seed precursors to grow in various mesophases in surfactant-driven media.^21–23 Attempts were also made to stabilise the mesoporous materials^24–27 by impregnating them with organic moieties (e.g. tetrapropylammonium cation) followed by hydrothermal treatment to obtain crystalline wall properties. In case of the top-down approach, demetallation of preformed zeolite was carried out to generate mesoporosity without using any surfactant molecules. The resultant modified mesoporous aluminosilicates with intrinsic zeolitic counterparts like BEA or MFI exhibit high activity on various catalytic applications^28–34 such as esterification of benzyl alcohol and hexanoic acid,²⁸cracking of high- and low-density polyethylene, HDPE,²⁹ and the synthesis of α-tocopherol³⁰ etc.

It is worth mentioning here that aluminophosphate (AlPO) molecular sieves are structurally analogous to zeolites, but have more framework flexibility and structural diversity.³⁵ The incorporation of several heteroatoms resulted in dual functionality (acidic and redox properties), which is essential in various organic transformations and chemical processes.^35–40 Many researchers have attempted to synthesize mesoporous aluminophosphate (MESO-AlPO) based molecular sieves and explore their catalytic application in various organic transformations.^41–43 Numerous methodologies such as liquid crystal templating (LCT)^40,44 cooperative self-assembly,⁴⁵ sol–gel,⁴⁶ acid–base pair,⁴⁷ solvent-evaporation and spray drying⁴⁸ methods have been adopted for the synthesis of MESO-AlPO. To date, the soft templating hydrothermal LCT mechanism is the most widely accepted and comprehensively studied path for the synthesis of (MESO-AlPO) mesoporous aluminophosphates^40,44 The synthetic chemistry in mesoporous AlPO phase formation is quite different and more complex than that involved in corresponding silica based mesophases. The major complexity in the synthesis of mesoporous AlPO lies in the irregular organisation of two different inorganic precursors around the surfactant assemblage. The above fact results in the incomplete condensation of P in the AlPO framework. Further framework aluminium presents in different environments such as tetrahedral penta or hexa coordinated.⁴⁹ The synthesized materials often suffer inherent drawbacks of lamellar phase formation and structural collapse during calcinations. Regardless of the challenges regarding synthesis and structural stability, vast structural diversity and catalytic demand, the researchers have been motivated to espouse innovative synthetic routes and explore the mechanistic aspects.

In this regard, we have recently stabilised the synthesis of mesoporous SAPO by utilising microporous SAPO-34 and SAPO-37 precursors,^50,51 In continuation of our earlier work, here we are reporting the synthesis of hierarchical mesoporous SAPO-5 (MESO-SAPO-5) assembled from microporous SAPO-5 secondary building unit precursors. It is worth mention here that isomerization of linear alkane/alkenes to branched alkane/alkene is an important reaction in petrochemical and fine chemical industries as it improves octane value of gasoline.⁵² SAPO-n based materials are shown as promising catalyst for such transformation.⁵³ In this context the catalyst developed in the present study viz., MESO-SAPO-5 possessing both micro and mesoporous properties might be potential material for isomerisation of hydrocarbon. Thus, the catalytic activity of MESO-SAPO-5 was studied for hydroisomerisation of 1-octene.

2. Experimental

The materials used in the synthesis were aluminium isopropoxide (98% Spectrochem), tetraethylorthosilicate (TEOS 99%; Aldrich), orthophosphoric acid (85%, Merck) and triethylamine (TEA 99%, Hi Media Laboratories), cetyltrimethylammonium bromide (CTAB 99%; Spectrochem), and 25 wt.%; aqueous solution of tetramethylammonium hydroxide (TMAOH 25 wt.%; Tritech chemical).

2.1. Synthesis of microporous SAPO-5

Microporous SAPO-5 was synthesized as per the procedure described elsewhere⁵⁴ with the molar gel composition of 1.0Al₂O₃ : 1.0P₂O₅ : 0.2SiO₂ : 1.0TEA : 40H₂O. In a typical synthesis, a known quantity of aluminium isopropoxide (18.6 g) was soaked in distilled water (25 ml) and was allowed to age overnight. The gel was stirred for an hour and dilute ortho-phosphoric acid(10.5 g of 85 wt% ortho-phosphoric acid in 5 ml distilled water) was added to it drop wise, with constant stirring for 1 h, until it became homogenised. Then, the required quantity (4.6 g) of structure directing agent TEA was added to the mixture and it was again stirred for 1 h. Finally, TEOS(1.9 g) was added and stirring was continued for another 1 h. The resultant final gel was subjected to crystallisation at 175 °C for different durations in a Teflon-lined stainless steel autoclave under autogenous pressure.

2.2. Synthesis of mesoporous SAPO-5 (MESO-SAPO-5)

The synthesis of mesoporous SAPO-5 (MESO-SAPO-5) using preformed microporous SAPO-5 precursors was carried out in two step hydrothermal process: (i) Preformed microporous SAPO-5 was obtained as per the above-mentioned procedure with a crystallisation time of 2 h; and (ii) the surfactant (CTAB) solution was prepared by mixing 9.5 g of CTAB in 35 ml of distilled water under constant stirring. The preformed microporous SAPO-5 obtained from above procedure and calculated amount of 25 wt% TMAOH solution (35 g) was introduced to the surfactant solution. The final gel was allowed to aged for overnight followed by crystallisation at 70 °C, in polypropylene bottle, for different durations. As-synthesized samples were washed thoroughly with distilled water and ethanol before it was dried and calcined at 550 °C for 6 h in an air oven. The final composition of MESO-SAPO-5 was 1Al₂O₃ : 1.0P₂O₅ : 0.2SiO₂ : 1.0TEA : 0.6CTAB : 2.0TMAOH : 300H₂O.

2.3. Characterization

Powder XRD was performed to determine the nature of the materials using a Bruker D8 diffractometer with Cu-Kα radiation (λ = 1.54184 Å). The diffraction patterns were recorded in the 2θ range of (1.5°–40°) with a scan speed and step size of 0.5° min⁻¹ and 0.02°, respectively. FT-IR spectra were obtained using a Perkin–Elmer 2000-FTIR in the range 400–4000 cm⁻¹ using KBr pellets. Scanning electron microscopy (SEM) images were obtained on an EVOMA15 Zeiss operated at 18–20 kV. The morphology and size of the materials were further analysed using a Phillips Technai G²30 TEM operated at 300 kV. Nitrogen adsorption/desorption isotherm was measured using an automatic micropore physisorption analyser (Micromeritics ASAP 2020, USA). The analysis was carried out at −196 °C, over samples already degassed at 300 °C for 12 h under 0.1333 Pascal pressure. The BET surface area was calculated in the relative pressure range 0.05–0.3, over the adsorption branch of isotherm. Various other textural properties like DFT surface area, pore volume (BJH, DFT, HK) and pore size distribution (BJH with Fass correction) were elucidated from the isotherm data. ³¹P and ²⁷Al MAS NMR patterns were recorded using a Bruker Avance III 400 MHz NMR machine with basic frequencies of 161.923 and 104.229 MHz respectively. Single pulse experiment with pulse duration of 4.5 μs and a relaxation delay time of 6 seconds were used for recording both ³¹P and ²⁷Al MAS NMR patterns. The samples were packed in 4 mm zirconia rotors and subjected to a spinning speed of 10 kHz for all the NMR experiments. All chemical shift values are expressed with respect to 85% H₃PO₄ for ³¹P nucleus and 0.1 M aqueous solution of Al(NO₃)₃ for ²⁷Al nucleus.

The pyridine FT-IR spectra were collected with a Thermo Scientific Nicolet 6700 FTIR single beam spectrometer using a liquid nitrogen-cooled MCT detector. Pyridine vapour adsorption was carried out in a Harrick Scientific HVC-DR2 reaction chamber with a detachable ZnSe window dome mounted inside a Harrick DRA-2 Praying Mantis diffuse-reflectance accessory designed to minimise parasite specular reflectance. About 100 mg (10% of sample was mixed with KBr) of sample was placed in the sample cup and was pre-activated at 350 °C for 6 h. For pyridine adsorption, helium gas was passed through a pyridine saturator. A partial pressure of 27 mm Hg of pyridine was maintained in the saturator. During the pyridine adsorption, sample cup temperature was maintained at 150 °C. This was continued for one hour. After pyridine adsorption, the sample was heated to 150 °C with ultra-high pure helium flush for one hour to ensure that the physically adsorbed pyridine was recovered completely. Sample spectrum was collected with KBr background once the sample temperature reached25 °C. Subsequently, the sample was degassed at a desired temperature and spectra were collected at different temperatures.

2.4. Catalytic studies on MESO-SAPO-5

Catalytic activity of MESO-SAPO-5 was studied for vapour phase hydroisomerisation of 1-octene using a fixed-bed reactor (Lab India Scientific Instrument, India). The carrier gas flow was controlled by using MFC (mass flow controller). Liquid feed was introduce into the reactor by using a liquid injection pump (Inkarp Instruments Pvt Ltd, model no. SP-22 series) with flow in the range of 0.01–0.2 mL min⁻¹. The catalytic testing was performed by loading 0.4 g of catalyst (MESO-SAPO-5) at different reaction temperatures (200 °C–450 °C) and WHSV (20, 12 and 8 h⁻¹) by maintaining a H₂ flow of 25 mL min⁻¹, as the carrier gas. The products were analysed using gas chromatography equipped with FID (Agilent 7890A Series) connected to a HP-5 capillary column (30 m; HP-5). For comparison purposes, the reaction was also studied with conventional microporous SAPO-5, under identical conditions.

3. Result and discussion

The nature of the microporous SAPO-5 precursor was studied using FTIR at various time intervals of crystallisation, the results of which are shown in Fig. 1. It is evident from the spectra that the vibrational band at 630 cm⁻¹ was observed from 2 h crystallisation, which is attributed to the T–O–T stretching in the D6R (double six membered ring) of the SAPO-5 secondary building unit (SBU). Further, an additional vibrational band at 560 cm⁻¹, corresponding to D4R (double four membered ring), was observed after 4 h of crystallisation. The observation is attributed to the fact that, the first of all D6R were formed and joined in a customary fashion to form a periodic building unit (PerBU) with 12 membered ring opening.⁵⁵ Later on, with an increase in crystallisation hours, these SBUs were joined together via D4R, giving rise to a complete framework of SAPO-5.

Fig. 1 FT-IR spectra of microporous SAPO-5 precursor synthesized at different duration.

Complete crystallisation was achieved after 24 h. The powder XRD pattern of SAPO-5 seeds shown in Fig. 2 depicts that the SAPO-5 precursor unit started to develop from 2 h of crystallisation. The XRD reflections as a result of the SAPO-5 structure were resolved with an increase in crystallisation time, and crystalline SAPO-5 was formed at 24 h. It is worth mentioning here that with our objective of assembling microporous SAPO-5 precursors into mesoporous material (MESO-SAPO-5), the premature SAPO-5 precursor obtained at 2 h of crystallisation was utilised for the mesoporous phase synthesis.

Fig. 2 Powder XRD pattern of microporous SAPO-5 precursor synthesized at different duration.

TG/DTA analysis (Fig. 3) of as-synthesized MESO-SAPO-5 exhibit three stage weight loss with corresponding exo and endothermic transitions. First stage endothermic weight loss in the temperature range of 25–200 °C is due to the loss of physisorbed water molecules. However, the major weight loss (36 wt%) occurred in the temperature range of 200–400 °C, with exothermic DTA due to decomposition of long chain surfactant. Final weight loss beyond 400 °C with exothermic DTA is due to decomposition of TEA and TMAOH. The above fact supports the presence of different porous environment on MESO-SAPO-5.

Fig. 3 TG/DTA profile of MESO-SAPO-5.

Powder X-ray diffraction pattern of as-synthesized and calcined MESO-SAPO-5 are depicted in Fig. 4. The as-synthesized sample shows a single reflection at 2θ region of 1.8° which corresponds to (1 0 0) plane of an ordered hexagonal mesoporous structure. The calcined sample shows a well resolved peak at 2θ of 1.9°; the observed shift in 2θ value towards higher angle is attributed to the lattice contraction after surfactant removal. Furthermore, a weak reflection evident at 4.4° and 5.2°, corresponding to (2 0 0) and (2 1 0), supports the hypothesis that the material is typical of MCM-41 type structures. Although microporous precursors showed XRD reflection (see Fig. 2), this corresponds to nanosized SAPO-5. The absence of XRD reflection corresponding to microporous SAPO-5 units indicates that our material did not have any segregated phase; rather,a secondary building unit was assembled on the surface of the mesoporous phase.

Fig. 4 Powder XRD pattern of (a) as synthesized MESO-SAPO-5 and (b) calcined MESO-SAPO-5.

The N₂ sorption isotherm (Fig. 5) of the MESO-SAPO-5 shows the feature of type-I and type-IV isotherms⁵⁶ indicating the presence of micro and mesoporous surfaces. The sharp uptake in the isotherm at a relative pressure (p/p_o) of 0.1 is associated with microporous SAPO-5 units present in the structure. The continuous multilayer adsorption between the relative pressure(p/p_o) of 0.35–0.7 is due to capillary condensation in the mesopore region. The isotherm is associated with a H2 hysteresis loop, which is attributed to the presence of interconnected pores with “ink-bottle” type geometry.^56,57

Fig. 5 N₂ adsorption/desorption isotherm of MESO-SAPO-5 and insert pore size distribution.

The textural properties (Table 1) reveal that MESO-SAPO-5 is associated with high BET (487 m² g⁻¹)and DFT (655 m² g⁻¹) surface areas, high pore volume (0.36 cm³ g⁻¹), and a narrow pore size distribution of 3.0 nm (inset Fig. 5), which is a typical characteristic of well-ordered mesoporous materials.⁵⁷ The total pore volume obtained by DFT calculations was found to be 0.34 cm³ g⁻¹, which is in good agreement with the single point adsorption total pore volume (0.35 cm³ g⁻¹). The observed high DFT surface area supports the suggestion of MESO-SAPO-5 having composite (microporous and mesoporous) properties.^50,58 Material morphology is shown in SEM images (Fig. 6). It consists of an array of elongated hexagonal sheet-like plates with 2–3 μm length. The presence of small spherical granules embedded in the surface is ascribed to nanosized microporous SAPO-5 precursors.

Table 1 Textural data of MESO-SAPO-5

Sample code	Surface area (m^2 g^−1)		Pore volume (cm^3 g^−1)			Pore Size (nm)
	BET	DFT	BJH	DFT	HK	BJH
MESO-SAPO-5	467	655	0.4	0.34	0.14	3.0

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Fig. 6 SEM images of MESO-SAPO-5.

Fig. 7 displays the transmission electron microscopic (TEM) image of as-synthesized and calcined MESO-SAPO-5 along (1 0 0) direction. The image specifies that the material has uniform arrays of hexagonal symmetry with long range order and with an average pore diameter of about 3.0 nm, which is consistent with the results from N₂ adsorption/desorption measurement.

Fig. 7 TEM images of (a) as synthesized and (b) calcined MESO-SAPO-5.

The coordination environment of different framework cations was studied by solid state MAS NMR. The²⁷ Al MAS NMR spectra of as-synthesized and calcined samples are shown in Fig. 8; the peak at 46 ppm is assigned to tetrahedral Al(OP)₄ groups, and the signals at 8.5 ppm can be attributed to extralinkages of tetrahedral aluminium to hydroxyl groups, which present as hydrated Aluminium species: Al(OP)_x(OH)_6−x.^50,58–60 The calcined samples showed a slight shift observed with a new hump arising at 9.6–10 ppm, which was assigned to penta-coordinated Al species (i.e., Al(OP)_x(OH)_5−x.^{61 31}P MAS NMR (Fig. 9) of as-synthesized and calcined samples shows a broad spectrum centred around −10 and −13 ppm, which indicated that phosphorous is present in different environments. A peak with a chemical shift of −18.2 was assigned to the P(–OAl–)₃(–OH)₁ group.⁶²

Fig. 8 ²⁷Al MAS-NMR spectra of (a) as-synthesized and (b) calcined, MESO-SAPO-5.

Fig. 9 ³¹P MAS-NMR spectra of (a) as-synthesized and (b) calcined, MESO-SAPO-5.

The surface acidities of MESO-SAPO-5 and SAPO-5 were followed by pyridine desorption FTIR spectra and the results are depicted in Fig. 10(a) and (b), respectively. It is evident that both SAPO-5 and MESO-SAPO-5 are associated with broad vibrational bands at 1440–1450 cm⁻¹ and 1540 cm⁻¹, corresponding to pyridine bound to proton-bonded Lewis acid sites and Brönsted acidic sites respectively.^12,50 As the desorption temperature increases to 400 °C, the Lewis acids sites-bound pyridine intensities decrease. It is interesting to note that MESO-SAPO-5 has relatively less and weaker Lewis acidic centres. The band around 1540–1525 cm⁻¹, related to the pyridine bound to Brönsted acid sites,⁶³ is more intense in MESO-SAPO-5. An additional band around 1490 cm⁻¹ is attributed to both pyridine-bounded Lewis and Brönsted acid sites.^12,50,51,63 The figures elucidate that as the desorption increases from 200 to 500 °C, the intensity of pyridine bound to Brönsted sites remains accountable. This confirms that MESO-SAPO-5 possesses stronger Brönsted acidity as compared to conventional SAPO-5. The presence of secondary building units embedded on mesoporous walls help to generate isolated strong acidic sites on MESO-SAPO-5. The systematically characterised MESO-SAPO-5 was used as a catalyst for hydroisomerisation of 1-octene in vapour phase conditions using a fixed bed reactor at various WHSV and temperatures. Schematic representation of isomerisation of 1-octene and their various isomerisation products are shown in Scheme 1.

Fig. 10 FT-IR pyridine desorption spectra of (a) MESO-SAPO-5 and (b) SAPO-5.

Scheme 1 Hydroisomerisation of 1-octene to various linear and branched products.

The effect of various WHSV on 1-octene conversion at 400 °C was systematically studied; the results are depicted in Fig. 11. It can be seen from the figure that 1-octene conversion was increased with a decrease in WHSV from 20, 12 to 8. The observed lower conversion at higher WHSV might be due to the faster diffusion of reactant molecules within the channel of MESO-SAPO-5. The fast diffusion results in less contact time with active sites. The maximum conversion of 85% 1-octene was observed at WHSV of 8.8. The catalytic activity remains constant over the period of more than 8 h. The linear alkene was observed as the major product (70–80%) in all cases, along with mono branched (20–25%) and bibranched octene. No cracked products were obtained in all cases; similar observations were made in the literature⁶⁴ for mesoporous MCM-41.The selectivity of bibranched octene increases with a decrease in WHSV, which indicates that more contact time, helps with skeletal isomerisation and favoured branched isomerisation products.

Fig. 11 Effect of WHSV on 1-octene isomerisation at 400 °C over MESO-SAPO-5.

Further isomerisation of 1-octene was studied in the temperature range of 200–450 °C by keeping the WHSV of 8.8 h⁻¹ under a constant carrier gas (H₂) flow; the results are shown in Fig. 12. As the temperature increases to 200, 300 and 400 °C, the 1-octene conversion increases to 20, 42 and 85%, respectively.

Fig. 12 Effect of reaction temperature on 1-octene conversion at WHSV of 8.8 h⁻¹.

Further increases in reaction temperature to 450 °C do not cause much of a change in 1-octene conversion. Importantly, the catalytic activity remains constant throughout the reaction time. At high temperature, a slightly decreases in catalytic conversion was observed, which might be due to the small amount of coke formed. The products such as 2-octene, 3-octene and 4-octene were obtained as major products (above 70%), through double bond shift.⁶⁴

The skeleton rearrangement gives products such as methylheptene (mono branched, 20–25%), and dimethyl hexenes (5–10%). As the reaction temperature increases from 300 °C to 400 °C and 450 °C, the secondary product distribution such as bibranched and mono branched octene concentration increases (see Fig. 13).

Fig. 13 Product distribution on 1-octene isomerisation at different temperature with WHSV of 8.8 h⁻¹; (a) = 300 °C; (b) = 400 °C and (c) = 450 °C.

At 300 °C, 75% linear 22% mono branched and 2–5% bibranched octene were obtained; as the reaction temperature increases to 400–450 °C, the bibranched octene (Dimethyl hexenes) selectivity increases about 10%with the cost of a decrease in linear octene (70%) selectivity, which indicates that secondary skeletal isomerisation was favoured at high temperatures. Importantly, the catalytic activity remains constant and no cracked products were formed. The presence of strong acidic sites and mesoporous channels with microporous SAPO-5 units favoured the observed high catalytic activity in MESO-SAPO-5. The detailed plausible mechanism of reaction is shown in scheme 2a and b.Reaction proceeds via abstraction of proton from MESO-SAPO-5 surface by olefin, resulted in formation of secondary carbenium ion/carbocation as intermediate. The resultant carbenium ion/carbocation is delocalised over the octene skeleton⁵³ and forms different linear isomers of octene as illustrated in scheme 2a.The secondary carbocation at C3 delocalised over C2 and C4, may undergo cyclic carbenium ion formation.⁵³ The intermediate cyclic carbenium ion experiences heterolytic cleavage to form secondary carbocation. Further, this secondary carbocation intermediate go through 1, 2 hydride shift rearrangement to form more stable tertiary carbocation. The resultant tertiary carbocation undergoes elimination of proton to form mono branched internal or terminal heptenes as major product.

Scheme 2 (a) Proposed mechanism for linear and mono branched olefins formation. (b) Proposed mechanism for di-branched olefins formation.

The formation of branched octene isomers is illustrated in scheme 2b.The protonated cyclic carbenium ion(I) formed from tertiary carbocation can move along the carbon skeleton of intermediate⁵³ and undergoes ring opening/closing and hydride shift to form stable intermediate which finally leads to di-branched olefins.

The catalytic activity of microporous SAPO-5 was further studied under identical reaction conditions the conversion of 1-octene was found to be 65% which is lower than MESO-SAPO-5. Importantly the selectivity of branched isomers was identified less than 5% on microporous SAPO-5. The presence of pore constraint on microporous SAPO-5 resulted in poor branched isomers selectivity compared to MESO-SAPO-5, where it showed 25–30% of branched isomers. Further, in comparison to other microporous SAPO-n studied so far^65,66 to the best of our knowledge MESO-SAPO-5 yielded better 1-octene conversion and comparable branched isomers selectivity.

4. Conclusion

MESO-SAPO-5 of hexagonal structure was assembled from SAPO-5 building units. The presence of microporous SAPO-5 is evident from FT-IR vibrations, sharp N₂ sorption at relative low pressure of p/p_o 0.1 and strong acidity. Mesoporosity was evident from powder XRD, N₂ sorption and TEM analysis. The resultant MESO-SAPO-5 has strong acidity, which has been found to be suitable for long chain hydrocarbon (1-octene) isomerisation. The formation of skeletal isomers was favoured at high temperatures.

Acknowledgements

Authors express their sincere thanks to DST (SR/S1/PC-11/2011) India, for the financial support. Authors are also thankful to USIC, University of Delhi, NCCR, IIT-Madras, for their support on instrumentation facility and Dr R. Nagarajan, University of Delhi for his support on use of powder XRD facility under DST(Nano Mission). AKS and RY are grateful to CSIR and UGC, India, for their JRF and SRF.

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