Strongly coupled Mn₃O₄–porous organic polymer hybrid: a robust, durable and potential nanocatalyst for alcohol oxidation reactions

Abstract

Herein we describe a novel strategy for noble-metal-free Mn₃O₄@POP (porous organic polymer) hybrid synthesis by encapsulation of Mn₃O₄-NP in the interior cavity of a porous organic polymer which exhibited enhanced catalytic activity and stability for oxidation of diverse activated and nonactivated alcohols relative to the conventional catalysts to demonstrate the benefits of such a nanoarchitecture in heterogeneous nanocatalysis. The use of a non precious catalyst, tremendous recyclability (upto 15 catalytic runs) and exceptional stability make our system innovative in nature, addressing all the profound challenges in the noble-metal-free heterogeneous catalysts development community.

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Metal NPs dispersed on support materials with a high surface area have emerged as growing research interest in petroleum refining, fine chemical production and some promising catalytic applications with the exposure of a high fraction of accessible atoms on the surface.^1,2 However, development of this nanocatalysis research area for practical applications has been limited by some obdurate associated problems owing to the uncontrollable growth and aggregation of metal nanoparticles, detachment of metal nanoparticles from supports resulting in a decrease in catalytic activity and structural deformation in a single use in harsh reaction conditions. During the past few decades, widespread efforts have been adopted for homogenous dispersion and encapsulation of metal or metal oxide nanoparticles on high surface area based nanoporous materials in order to increase catalyst stability thereby avoiding metal leaching and agglomeration with successive improvement of catalytic performance.^3–5 In contrast with the other functionalized porous materials like metal–organic frameworks (MOFs), porous organic polymers (POPs) have attracted an ever-increasing demand as a promising functionalized support for catalytically active particles owing to their easily tuneable structural integrity by changing straightforward effective synthesis strategy and selection of monomers, well-defined three dimensional (3D) rigid symmetric skeletons with exceptional robustness to heat, moisture, acid and base.^6–8 Very recently, POP-supported metal or metal oxide nanoparticles have been also employed as heterogeneous nanocatalyst for coupling,⁹ hydrogenation,¹⁰ oxidation¹¹ and photocatalytic reactions.¹²

Selective oxidation of alcohols to aldehydes or ketones has been considered as the most important chemical transformation for construction of a diverse range of important intermediates and fine chemical products in industry and pharmaceuticals. Luis and co workers reported microwave-assisted selective oxidation of 1-phenyl ethanol in water with Au-NPs and Pd-NPs immobilized onto polymeric backbone which is modified with ionic liquid like moieties.¹³ Solvent free aerobic oxidation of alcohol was performed with Pd-NPs supported on titanate nanobelts by Lu et al.¹⁴ Magnetically recyclable core–shell Pd nanocatalyst was designed by Hyeon et al. for conducting oxidation of various primary and secondary alcohols to the corresponding aldehydes and ketones.¹⁵ Photocatalytic dehydrogenation of alcohol was achieved by MOF based Pd/TiO₂@MIL-101 (ref. 16) and Au–Pd@ZrO₂ catalysts.¹⁷ Oxidation of 1-phenyl ethanol were also achieved employing hydroxyapatite supported palladium nanoclusters and zeolite-supported Pd nanoparticles having mean diameter 2.8 nm.¹⁸ Pd cluster@silica core–shell nanospheres was designed by Dai research group to carry out oxidation of aromatic alcohols. Graphene supported palladium nanoparticles exhibited excellent catalytic activity in solvent-free alcohol oxidation reaction.¹⁹ Although some advancement in catalytic oxidation of alcohols has been achieved but some disadvantages in most of these methods are still associated with the tedious processing steps, use of expensive noble metal catalyst (mainly Pd and Au), high catalyst loading, low stability of catalyst and use of large excess of base additives. In addition, a limited set of literature survey reports demonstrated that Co₃O₄–N@C,²⁰ nano Fe₃O₄@APTES@Ni(OH)₂,²¹ Fe₃O₄@MIL-101(Cr),²² Co@C–N700 (ref. 23) non noble metal catalysts have been employed for alcohol oxidation reactions. A hydrotalcite-supported Mn oxide has been reported as an efficient heterogeneous catalyst for aerobic alcohol oxidation by Kaneda et al.²⁴

Manganese oxides especially Mn₃O₄ have received a considerable interest as energy-storage material for anodes of lithium-ion batteries, promising sensor material for detecting volatile organic compounds and catalysts for oxidation reactions owing to their easy availability, low toxicity, low cost, high durability, polymorphism and coexistence of mix-valence.^25,26 Herein we demonstrate encapsulation of Mn₃O₄ NP in the interior cavity of nitrogen rich three dimension porous organic polymer by facile two-step liquid-phase procedure through strong electronic interaction between N-rich POP and manganese center of Mn(OAc)₂ to obtain a new non-precious hybrid material Mn₃O₄@POP. Our finding reveals potential application of Mn₃O₄@POP material as a robust nanocatalyst for oxidation of diverse range of primary, secondary and α,β-unsaturated nonactivated alkanols. The designed nanocatalyst exhibited enhancement in catalytic performance with tremendous recyclability (upto 15^th catalytic run) for oxidation of diverse alkanols relative to the conventional catalysts including Mn₃O₄@SiO₂, Mn₃O₄@TiO₂, Mn₃O₄@Al₂O₃ and Mn₃O₄@C to demonstrate the benefits of such nanoarchitecture in heterogeneous nanocatalysis.

Triazine based N-rich porous organic polymer (POP) was synthesized by non aqueous polymerization of 2,4,6-triallyoxy-1,3,5-triazine with cross linker divinyl benzene under hydrothermal condition in presence of azobisisobutyronitrile (AIBN) radical initiator. The dark brown color Mn₃O₄@POP hybrid was obtained after irregular growth of Mn₃O₄ nuclei on POP surface followed by solvothermal technique (Fig. 1A) in presence of Mn(OAc)₂ and NH₃ solution. N and O-atoms from each 1,3,5-triazine organic moiety coordinates to the Mn(II) ions strongly and then during this process, irregular growth of Mn₃O₄ nuclei along the c axis in the direction of 001 plane leading to the formation of needle shape was observed on POP surface via heterogeneous nucleation which is supported by Mn₃O₄@N-doped graphene hybrid by Shi Zhang Qiao et al.²⁵ The structure and morphology of the as-synthesized POP and respective Mn₃O₄@POP composite were examined by powder X-ray diffraction (PXRD), TEM, XPS, FE-SEM, ¹³C CP MAS NMR, N₂-adsorption desorption isotherms, FT-IR spectroscopy techniques. Mn loading in the Mn₃O₄@POP hybrid is 0.456 mmol g⁻¹, as determined by ICP-MS analysis. C, H and N analysis data of the as-synthesized POP and Mn₃O₄@POP composite are provided in the Table S1, ESI.†

Fig. 1 (A) Synthesis of Mn₃O₄@POP hybrid material using hydrothermal technique. (B–E) TEM images of Mn₃O₄@POP hybrid material.

Transmission Electron Microscope (TEM) images of Mn₃O₄@POP composites (Fig. 1B–E) at different magnifications show that small needle shape Mn₃O₄-NP are homogeneously dispersed throughout the surface of porous polymer with the needle size and width varying 30–250 nm and 7–12 nm, respectively (Fig. 1E). TEM images (Fig. 1B–D) demonstrated that Mn₃O₄ nanoneedles are tightly surrounded by porous organic polymer (the arrows mark the edges of Mn₃O₄ nanoneedle and POP) and a 3D flame like appearance possessing an interconnected three-dimensional porous network. In every TEM images the monodispersity of the Mn₃O₄ NPs can be well maintained. TEM images (Fig. 1B–E) exhibited hierarchical wrinkled nanostructure which can be assumed to the high temperature hydrothermal technique and strong interactions between the N-rich functional groups and the Mn₃O₄ nuclei. TEM images of as-synthesized porous polymer are provided in Fig. S1B, ESI.† An inter-planar lattice spacing of 0.276 nm corresponding to the (103) crystalline plane of hausmannite Mn₃O₄ was observed from the high resolution TEM images (Fig. S2, ESI†). Selected-area electron diffraction (SAED) pattern of the hybrid material also signifies (Fig. S3, ESI†) crystalline feature of Mn₃O₄ needle in the Mn₃O₄@POP hybrid.^27,28

Wide angle powder X-ray diffraction pattern (Fig. 2Aa) of as-synthesized POP material shows a single characteristic diffraction peak at 2θ = 20.1 which could be attributed to the amorphous framework structure. New crystalline reflection patterns are observed in the PXRD pattern of Mn₃O₄@POP hybrid (Fig. 2Ab) which could be assigned to the tetragonal Mn₃O₄ structure (hausmannite, I4₁/amd, a₀ = b₀ = 5.76 Å, and c₀ = 9.47 Å; JCPDS card no. 24-0734).^{29 13}C CP MAS NMR spectra (Fig. 2B) of the as-synthesized POP and Mn₃O₄@POP hybrid materials displayed three signals at chemical shift (δ) 112.0, 126.8 and 136.7 ppm attributable to the characteristic aromatic carbon atoms of organic skeleton unit (overlapping of C7–9 signals). In addition characteristic peak featured for triazine-C in the melamine unit appear at C1 (144.1 ppm). The strong signal at 39.8 ppm could be recognized for the aliphatic carbons (overlapping of C3–6) on the bridging units.^30,31 The N₂-adsorption desorption isotherms of the as-synthesized POP and Mn₃O₄@POP materials display a typical type-II isotherms a sharp nitrogen uptake in the high P/P₀ pressure region corresponding to the existence of interparticle void space (Fig. 2C). The calculated BET surface areas of POP and Mn₃O₄@POP hybrid materials are found to be 620 m² g⁻¹ and 178 m² g⁻¹, respectively with the corresponding total pore volumes are 0.51 cm³ g⁻¹ and 0.169 cm³ g⁻¹, respectively. Pore size distributions of the respective materials obtained by employing NLDFT methods are provided in the Fig. S4 (ESI†). A considerable decrease in surface area, pore volume and pore dimension for Mn₃O₄@POP hybrid compared with the as-synthesized material suggested occupation of Mn₃O₄-NP in the interior cavity of the nanoporous polymer.³² All the characteristic peaks at the region 1565 cm⁻¹, 1332 cm⁻¹ and 801 cm⁻¹ signifying to the presence of distinctive triazine framework and 2923 cm⁻¹ (CH₂ stretching vibrations) similar to that of as-synthesized POP material are observed in the FT-IR spectrum of Mn₃O₄@POP hybrid (Fig. 2D). The presence of Mn₃O₄-NP tethered on porous organic polymer skeleton is clearly confirmed by the characteristic Mn–O stretching vibrations (625 and 517 cm⁻¹).³³ The scanning electron microscopy (SEM) images (Fig. S5, ESI†) of as-synthesized POP and Mn₃O₄@POP hybrid predict the flake like irregular blocks with rough surfaces composed of aggregated nanoparticles. Elemental mapping analyses with energy-dispersive X-ray spectrometry (EDS) for Mn, N and O designate the successful incorporation of Mn₃O₄-NP with a homogeneous distribution (Fig. S6 and S7, ESI†).

Fig. 2 (A) Wide angle powder X-ray diffraction patterns, (B) ¹³C CP solid state MAS NMR spectra, (C) N₂-adsorption–desorption isotherms and (D) FT-IR spectra of as-synthesized POP (a) and (b) Mn₃O₄@POP hybrid materials.

X-ray photoelectron spectroscopy (XPS) survey scan of Mn₃O₄@POP hybrid signifies the presence of Mn, O, C and N elements (Fig. S8, ESI†). In the Mn 2p XPS spectrum of Mn₃O₄@POP hybrid the binding energies of Mn 2p_1/2 and Mn 2p_3/2 appeared at 651.7 eV and 640.2 eV, respectively, with a splitting width of 11.5 eV demonstrating the presence of Mn₃O₄-NP (Fig. 3Ab). In the Mn 2p spectrum of bare Mn₃O₄-NP, a 2p_3/2–2p_1/2 doublet with the corresponding splitting width of 11.7 eV is observed (Fig. 3Aa) is in agreement with an earlier report on Mn₃O₄ by Raj et al.³⁴ The negative shift in the XPS binding energy of Mn 2p region for Mn₃O₄@POP hybrid compared with the bare Mn₃O₄-NP can be attributed to the strong electronic interaction of nitrogen rich porous polymer skeleton with Mn center. XPS spectrum in the N 1s region with the binding energy at 397.9 eV of as-synthesized porous polymer can be recognized to the existence of single type pyridinic nitrogen structure (Fig. 3Ba). A positive shift in the binding energy to the higher binding energy (∼1.0 eV) in N 1s spectrum of Mn₃O₄@POP hybrid signifies strong coordination of Mn centers with the pyridinic N atoms of polymeric skeleton via electron donation thereby changing electron density over the N atoms making slightly positively charged (Fig. 3Bb).³⁵ Manganese oxidation state was also confirmed by Mn 3s region XPS spectrum (Fig. 3C) with the binding energy splitting width 5.3 eV, in a good agreement with the previously reported XPS spectrum of Mn₃O₄. The major Raman bands at 317.6, 373.6 and 659.1 cm⁻¹ in the Raman spectrum (Fig. 3D) can be assigned to the presence of Mn₃O₄-NP in the Mn₃O₄@POP hybrid material.³⁶

Fig. 3 (A) XPS spectra in Mn 2p region of bare Mn₃O₄-NP (a) and Mn₃O₄–POP (b); (B) XPS spectra in N 1s region of as-synthesized POP (a) and Mn₃O₄–POP (b); (C) XPS spectrum in Mn 3s region of Mn₃O₄–POP and (D) Raman spectrum of Mn₃O₄@POP hybrid material.

Catalytic performance of Mn₃O₄@POP hybrid material has been evaluated for oxidation of alcohols to develop a variety of carbonyl compounds using acetonitrile as solvent in presence of tert-butyl hydroperoxide considering 1-phenylethanol as model substrate at 80 °C. Detail catalytic procedure has been provided in the Experimental section (see ESI†). Our newly designed nanocatalyst is very effective for oxidation of nonactivated alkanols to afford corresponding aldehydes and ketones with no observable over-oxidizing byproducts. Among the oxidising agents employed including TBHP, H₂O₂ and O₂, tert-butyl hydroperoxide solution has been considered as the best oxidising agent in oxidation of 1-phenylethanol (Fig. S9, ESI†). We have further optimized the mole ratio of TBHP and substrate for oxidation of 1-phenylethanol over Mn₃O₄@POP in acetonitrile solvent at 80 °C temperature. 99.2% conversion of 1-phenylethanol to acetophenone with 100% selectivity has been achieved when mole ratio of alcohol and TBHP is 1 : 2 (Fig. S10, ESI†). Various solvents were screened for catalytic oxidation of 1-pehnylethanol (Table S2, ESI†) over Mn₃O₄@POP, with observed high conversion (99.2%) and TOF 13.5 h⁻¹ in acetonitrile at 80 °C. Subsequent investigation on the effect of reaction temperature (Table S3, ESI†) indicates that 80 °C is the optimum reaction temperature to furnish 99.6% conversion of 1-phenylethanol with TOF 13.6 h⁻¹. Experimental outcome of alcohol oxidation pointed to an optimized performance of Mn₃O₄@POP (20 mg), which provided an entire conversion with a TOF 13.5 h⁻¹ (Table S4, ESI†). Bare Mn₃O₄-NP underwent 47% conversion of 1-phenylethanol after 15 h with a TOF 5.1 h⁻¹. To better clarify the role of support in this hybrid nanocatalyst for oxidation reaction we have successfully developed a series of Mn₃O₄-NP supported nanocatalysts following same catalyst synthesis procedure and checked the catalytic performance with these different solid supported catalysts. The powder X-ray diffraction (PXRD) patterns (Fig. S11, ESI†) and transmission electron microscopy images (Fig. S12–S15 ESI†) of the Mn₃O₄@SiO₂/TiO₂/Al₂O₃/C materials confirmed the construction of Mn₃O₄-NP on the surface of solid support. The products were extracted with ethyl acetate from the reaction mixture after separation of solid catalyst and then isolated by preparative TLC method using n-hexane/ethyl acetate as an eluent. A detail investigation of 1-phenylethanol oxidation to acetophenone was followed by time-on-stream profiles for different Mn₃O₄-NP based catalysts under optimized reaction conditions (Fig. 4A) which demonstrated that alcohol oxidation progressively increased with time and all the catalysts showed different catalytic performance. All the catalysts showed initially ∼10% conversion with time but for Mn₃O₄@POP catalyst it was changed dramatically after 6 h. The high catalytic efficiency of Mn₃O₄@POP-hybrid material can be assigned to the unique well-defined three dimensional (3D) organic highly cross linked rigid porous framework with accessibility of the catalytically active sites thereby facilitating a “point-to-point” manner strong electrostatic and π–π interactions with easy diffusion of organic substrates.³⁷

Fig. 4 (A) Comparison study in catalytic performance in terms of 1-phenylethanol conversion as a function of time employing all Mn based nanocatalysts. (B) Time-on-stream profile for catalytic oxidation of cinnamyl alcohol with Mn₃O₄@POP catalyst. Reaction conditions: 1-phenylethanol (1.5 mmol, 180 mg), CH₃CN (10 mL), TBHP (70 wt% in H₂O, 3 mmol, 0.38 mL), 80 °C, catalyst (20 mg).

Cinnamaldehyde become the major product with 70% conversion of cinnamyl alcohol after 8 h reaction evaluation with Mn₃O₄@POP nanocatalyst (Fig. 4B). But with the further reaction progress a drop in cinnamaldehyde selectivity is observed with concomitant formation of epoxide minor product suppressing other competitive side reactions including C=C hydrogenation, via surface hydrogen liberated during the initial alcohol activation step and decarbonylation of –CHO group which are likely to be observed for allylic alcohol oxidation with Pd-based catalysts.^38,39 Inspired by these intriguing results, we have next examined substrate scopes for representative oxidation reaction with Mn₃O₄@POP nanocatalyst employing a variety of activated and nonactivated alkanols under optimized reaction conditions (Table 1). Cyclohexyl methyl alcohol was oxidised to the corresponding aldehyde (Table 1, entry 1) with 97.9% conversion exhibiting 13.4 h⁻¹ TOF. Oxidation of benzyl alcohol proceeded (Table 1, entry 2) with high selectivity of benzaldehyde, although with somewhat moderate TOF value 4.6 h⁻¹. 9-Fluorenemethanol (Table 1, entry 3) oxidation afforded 76.2% conversion, with remarkably high TOF value 13.9 h⁻¹. Successive catalytic oxidations of chemically inert cyclic alkanols to the corresponding ketones were achieved (Table 1, entries 4–8) with notably high TOF values irrespective on the ring size. Cyclohexanol and 4-methylcyclohexanol also underwent oxidation reaction very smoothly to the corresponding desired ketones, furnishing TOF values 7.7 h⁻¹ and 8.3 h⁻¹, respectively (Table 1, entries 4 & 5). Cyclooctanol having a large ring size could be transformed into corresponding ketone with high selectivity providing 8.6 h⁻¹ TOF (Table 1, entry 8). Our catalyst is also very effective for large scale oxidation reaction (Table 1, entry 9). The catalyst also worked well in the oxidation of some aliphatic alcohols (Table 1, entries 10 & 11). α,β-Unsaturated alcohols including 2-cyclohexen-1-ol and cinnamyl alcohol (Table 1, entries 12, 13) could be transformed into corresponding ketone and aldehyde with high selectivity. Several substituted benzyl alcohols afforded corresponding aldehydes (Table 1, entries 14 & 15) in excellent yields. The present catalytic system was also applicable to the oxidation of heterocyclic alcohol with desired product yield (Table 1, entry 16).

Table 1 Catalytic oxidation of different activated and nonactivated alkanols catalyzed by Mn₃O₄@POP nanocatalyst^a

Entry	Substrate	Product	Con (%)	TOF (h^−1)
a Reaction conditions: alcohols (1.5 mmol), CH3CN (10 mL), TBHP (70 wt% in H2O, 3 mmol, 0.38 mL), time (12–15 h), Mn3O4@POP catalyst (20 mg), temperature 80 °C. TOF (turn over frequency) = moles of substrate converted per mole of active site per hour.
1			97.9	13.4
2			45.2	4.6
3			76.2	13.9
4			56.8	7.7
5			61.2	8.3
6			71.6	7.8
7			67.9	7.4
8			63.4	8.6
9			98.6	13.6
10			68.6	7.5
11			59.2	6.9
12			58.9	6.9
13			70.3	9.6
14			78.9	10.8
15			72.5	6.6
16			88.7	8.1
17			46.3	3.8

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Catalytic reaction with MOF based catalyst has been considered significantly poorer compared with our POP based catalyst (Table 1, entry 17). Our newly designed Mn₃O₄@POP nanocatalyst could be reused and recycled at least 15 times without any significant loss of catalytic activity (Fig. 5A) and the consistent turn over number (TOF) ∼ 13 h⁻¹ in each catalytic run is observed (Fig. 5B) for 1-phenylethanol oxidation reaction. Hot-filtration test and leaching test were performed considering 1-phenylethanol oxidation to confirm that our nanocatalyst is indeed heterogeneous in nature (see ESI†). Mn content in the reused catalyst after 15^th catalytic run was found to be 0.448 mmol g⁻¹ (still comparable with the fresh catalyst) which indicates strong entrapment of nanoparticles in interior porous cavity of the unique polymeric framework inhibits detectable leaching of metal during catalytic reaction. Furthermore, our Mn₃O₄@POP catalyst system exhibited considerable superior reusability (Fig. S16, ESI†) than the corresponding conventional catalysts such as Mn₃O₄@C, Mn₃O₄@SiO₂, Mn₃O₄Al₂O₃ and Mn₃O₄@TiO₂. Reused catalyst characterization revealed the mechanical and chemical stability of the stability, evading the associated structural degradation of the catalyst (Fig. S17 and S18, ESI†).⁴⁰

Fig. 5 (A) Recycle potential diagram of Mn₃O₄@POP catalyst for 1-phenylethanol oxidation, (B) continuous turn over frequency (TOF) as a function of time obtained during the oxidation of 1-phenylethanol in each catalytic cycle. Reaction conditions: 1-phenylethanol (1.5 mmol, 180 mg), CH₃CN (10 mL), TBHP (70 wt% in H₂O, 3 mmol, 0.38 mL), 80 °C, catalyst (20 mg). Inside TEM image of reused Mn₃O₄@POP catalyst after 16^th catalytic run.

The impressive catalytic performance, high chemical stability, and favorable reusability with no sign of metal leaching during the heterogeneous catalysis providing shorter reaction time employing non-noble metal nanocatalyst compared with the previously reported catalysts can be attributed to the several unique characteristics including (1) well-defined three dimensional (3D) organic highly cross linked rigid porous framework for easy mass transfer/diffusion and (2) introduction of mix-valence manganese ions into interior cavity of porous matrix exposing more reactive active sites and synergic interaction between C–N and Mn to promote the activation of C–H, O–H and O–O bonds by interacting with alkanols/TBHP.⁴¹ A plausible reaction pathway involving free radical for alcohol oxidation is depicted in the Fig. S19, ESI.† TBHP is cleaved into t-butoxide and hydroxyl radicals upon heating which abstracts the proton to generate carbon-centred radical on α-carbon followed by elimination of aldehyde is in good agreement with previous report by Yamamoto et al.⁴²

In conclusion, we have successfully designed noble-metal-free Mn₃O₄@POP hybrid material by encapsulation of mixed valence Mn₃O₄-NP into the interior cavity of nitrogen rich triazine functionalized porous organic polymer. On the basis of the characterization of hybrid nanocatalyst, the catalytic property was explored for oxidation of a diverse range of activated and nonactivated alkanols to afford corresponding carbonyl compounds with high product selectivity, tremendous recyclability and mechanical stability. An enhancement in catalytic performance for our designed Mn₃O₄@POP nanocatalyst compared with the other conventional catalysts revealed potential employment of such three dimensional porous organic highly cross linked framework as a useful platform in heterogeneous catalysis. We strongly believe that our present study and investigation regarding the use of Mn₃O₄-nanocatalyst may open up new opportunities for development and progress of new noble-metal-free heterogeneous catalysts.

Acknowledgements

KD wishes to thankfully acknowledge Department of Science and Technology (DST), New Delhi for providing her DST-INSPIRE-JRF fellowship. AB wishes to thank DST, New Delhi for providing funding though DST-SERB and DST-UKIERI project research grants and DST Unit on Nanoscience for providing instrumental facilities at IACS. RS, BMR and JM thank to Department of Science and Technology, India for DST-INSPIRE Faculty Research project grant (GAP-0522) in CSIR-IICT, Hyderabad.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07200c

‡ These two authors have equally contributed in this work.

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