Organic–inorganic hybrid tinphosphonate material with mesoscopic void spaces: an excellent catalyst for the radical polymerization of styrene

Abstract

A new porous tinphosphonate HSnP-1 has been synthesized through the reaction of butylene-1,4-diphosphonic acid and SnCl₄·5H₂O under hydrothermal conditions at 453 K for 3 days in the absence of any structure directing agent. Powder X-ray diffraction, transmission and scanning electron microscopies (TEM and SEM), N₂ sorption, solid state ¹³C CP MAS and ³¹P MAS NMR, UV-vis and FT IR spectroscopic tools are used to characterize the material. The crystallinity of HSnP-1 is sufficiently high under mildly acidic pH conditions and the crystal structure of the material has been indexed to the tetragonal phase with unit cell parameters a = b = 11.52 Å, c = 15.84 Å, α = β = γ = 90°. The surface area and the pore volume of the material were 338 m² g⁻¹ and 0.54 cc g⁻¹, respectively. HSnP-1 showed outstanding catalytic activity in the radical polymerization of styrene at room temperature in the presence of aqueous H₂O₂ as an initiator under solvent-free conditions, whereas in the presence of aprotic solvent it facilitates partial oxidation of styrene to phenylacetaldehyde and acetophenone.

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Introduction

Designing functionalized porous molecular architectures has received overwhelming attention in the last two decades.¹ Various inorganic porous materials are prepared by employing different organic molecules as templates.² The shape, size and nature of functional groups present in the template molecules play a crucial role in tuning the pore diameter, surface area and the crystallinity of the porous matrix.³ However, the removal of templates or structure directing agents (SDA) keeping the porous network structure unperturbed is a big challenge. Thus, the synthesis of a novel porous framework without the aid of any SDA molecules is highly desirable.⁴ Unique structural features of the porous materials make them useful in a wide range of research areas, viz. gas storage,⁵ catalysis,⁶ selective separation,⁷ metal ion sensing,⁸ photocatalysis⁹ and drug delivery.¹⁰ On the other hand, varieties of metal phosphonate based porous hybrid materials have been designed for gas adsorption,¹¹ catalysis,¹² photocatalysis,¹³etc. using a suitable metal center and phosphonate based donors.¹⁴ In the synthesis of hybrid porous metal organophosphonates, bi-phosphonic acid is a quite well-known cross-linking agent between the inorganic layers.¹⁵

However, there are very few reports on the synthesis and characterization of porous tin(IV) phosphonates using biphosphonic acid as the spacer group. In most of the reports the focus was given to the variation of solvents.¹⁶ It has already been established that the maximum surface area of these materials could arise when polar solvents (H₂O, EtOH, DMSO) or a mixture of polar solvents are used in the synthetic medium.¹⁷ Our aim is to design a new microporous tin(IV) phosphonate having catalytically active Sn(IV) species, which can behave as redox or acid–base centers depending upon the synthesis conditions. On the other hand, polymerization of styrene is usually carried out in an inert organic solvent via a free radical, anionic or cationic polymerization route.¹⁸ The most common solvent used for this reaction is 1,2-dichloroethane or chlorobenzene. The preferred initiator for this reaction involving cationic polymerization is a mixture of boron trifluoride and water or concentrated sulfuric acid in the absence of water. Both the organic solvents and inorganic initiators used in these reactions are not environment friendly. Further, polymer colloids recovered from the reaction mixture should be purified by ultrafiltration, dialysis, ultracentrifugation, ion exchange or by a combination of these methods and require complicated waste disposal and clean-up procedures. Hence, eco-friendly and high-throughput synthesis conditions for the large scale production of polystyrene via simple catalytic polymerization of styrene are highly desirable.

Herein, we report a new tetragonal tinphosphonate (HSnP-1) having flake-like crystal morphology with mesoscopic void spaces by using butylene-1,4-diphosphonic acid as the organophosphorus precursor (spacer) under mild hydrothermal conditions. The uniqueness of this new hybrid tinphosphonate material is its catalytic potential in the presence or absence of solvent using dilute aqueous H₂O₂ as an initiator/oxidant. The material polymerizes styrene to polystyrene at room temperature in the absence of any solvent within 1–3 h reaction time, whereas it can also be used as a catalyst in liquid phase partial oxidation of styrene to phenylacetaldehyde and acetophenone in the presence of various aprotic solvents.¹⁹

Experimental section

Materials

Commercially available tin(IV) chloride pentahydrate (SnCl₄·5H₂O; 98%, M_r = 350.58) was purchased from Loba Chemie. 1,4-Dibromobutane (C₄H₈Br₂, 99%) and triethyl phosphate [P(OEt)₃, 98%] were purchased from Aldrich. Hydrogen peroxide (30%) was purchased from E-Merck. All the reagents were used without further purification.

Preparation of butylene-1,4-diphosphonic acid (H₂O₃P–C₄H₈–PO₃H₂). In a typical synthetic procedure (Scheme 1) 6.804 g (0.0315 mol) 1,4-dibromobutane and 15.483 g (0.093 mol) distilled triethyl phosphite were taken in a round bottom flask fitted with a water cooled condenser. The flask was purged with N₂ gas and heated at 433 K for 8 h.²⁰ The crude diethyl phosphate ester (9.23 g) was collected by vacuum distillation and the ester was hydrolyzed by 6 N HCl for 72 h. The hydrolyzed product was dried in a rotary evaporator to collect the crude diphosphonic acid (5.7 g). The crude product was crystallized from water and characterized by ¹H NMR and FT IR spectroscopy.²¹ The yield of the diphosphonic acid was 83%.

Scheme 1 Reaction pathway for the formation of butylene-1,4-diphosphonic acid ligand.

Synthesis of the porous tin(IV) phosphonate HSnP-1. In the typical synthesis of HSnP-1, 2.18 g of 1,4-butylene-diphosphonic acid (10 mmol) was dissolved into 20 ml of distilled water and the pH of the solution was raised up to 5.0 by adding aqueous ammonia (25% aqueous). In another beaker 3.5 g of the Sn(IV) precursor, SnCl₄·5H₂O (10 mmol), was dissolved into 10 ml of distilled water. The synthesis gel was prepared by adding the former solution to the highly acidic metal salt solution and maintaining the pH of the final solution at 5–6 by adding additional aqueous ammonia solution. The reaction mixture was stirred overnight and transferred to a Teflon-lined pressure vessel and kept at 453 K for 3 days. Then the resulting product mixture was centrifuged and the solid was washed with distilled water several times and finally dried at 343 K. The pH of the final solution was acidic in nature. The weight of the dried product (HSnP-1) was 3.2 g. The yield of the reaction is 99% with respect to the metal content in the hybrid framework.

Characterization techniques. Powder X-ray diffraction patterns are recorded on a Bruker D8 Advance SWAX diffractometer operated at 40 kV voltage and 40 mA current. The instrument has been calibrated with a standard silicon sample, using Ni-filtered Cu K_α (λ = 0.15406 nm) radiation. A JEOL JEM 6700 field emission scanning electron microscope (FE-SEM) with an energy dispersive X-ray spectroscopic (EDS) attachment was used for the determination of morphology of the particles and their surface chemical compositions, respectively. For the FE-SEM analysis, a pinch of solid sample (HSnP-1) was spread over the carbon tape and analyzed. A JEM 2010 transmission electron microscope operated at an accelerating voltage ranging from 100 kV to 200 kV has been employed for the determination of nanostructure and pore size. For TEM analysis, 10 mg hybrid material (HSnP-1) was dispersed into anhydrous ethanol by 5 minute sonication. Then one drop of the dispersed solution is dropped onto the carbon coated copper grid and dried before analysis. Nitrogen adsorption/desorption isotherms are obtained by using a Beckmann Coulter SA 3100 surface area analyzer at 77 K. Prior to gas adsorption, samples are degassed for 3 h at 453 K under high vacuum. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) of the sample were carried out in a TGA instrument thermal analyzer TA-SDT Q-600 under air flow. FT IR spectra of the samples are recorded using a Nicolet MAGNA-FT IR 750 spectrometer Series II. ¹H NMR experiments were carried out on a Bruker DPX-300 NMR spectrometer. Solid state MAS NMR studies have been carried out using a Chemagnetics 300 MHz CMX 300 spectrometer. The Sn content in the HSnP-1 sample was measured using a Shimadzu AA-6300 atomic absorption spectrophotometer. Before the analysis suitable stock solutions are prepared by digesting the solid powder in an aqueous solution containing a minimum amount of HF/HCl and its slow evaporation.

For the catalytic reactions under solvent-free conditions, 10.0 mmol of styrene was mixed with 5.0 mmol of dilute aqueous hydrogen peroxide (30% in water) in a 10 ml round bottom flask. To this mixture, 5 wt% HSnP-1 (with respect to styrene) was added under magnetic stirring at room temperature. The reaction was started immediately after the addition of the catalyst and the mixture becomes turbid. The progress of the reaction was monitored by thin layer chromatographic (TLC) analysis of the reaction mixture in chloroform and methanol [CHCl₃ : CH₃OH = 10 : 1 (v/v)]. The reaction mixture is treated with ethyl acetate and the insoluble catalyst is separated through filtration. The solid polystyrene bead is isolated by evaporation of the solvent. The compound is characterized by ¹H and ¹³C NMR spectroscopy (Bruker DPX-300 MHz). CDCl₃ is used as the NMR solvent for the characterization of the polystyrene product. The yield of polystyrene was finally determined by weighing the dried white solid crystalline product. But when the reaction was carried out in the absence of the catalyst under otherwise identical reaction conditions (in the presence of hydrogen peroxide) no turbidity in the reaction mixture appeared and no polystyrene product was obtained even after 3 days. For the catalytic reactions in the presence of other solvents similar experimental conditions are maintained except that the styrene was first dissolved in 8.0 ml of solvent (THF, toluene or acetonitrile) and the reaction was carried out at 333 K.

Results and discussion

Crystal structure of HSnP-1

The powder X-ray diffraction patterns of the HSnP-1 samples are shown in Fig. 1. The sample synthesized under highly acidic conditions (pH ≈ 1) shows very few broad peaks (Fig. 1, inset), indicating the semi-crystalline nature of the material, but under optimized synthesis conditions (pH = 5.0) the crystallinity of the material increases considerably. In most of the previous reports on porous tin(IV) phosphonates, aromatic rings are utilized as the spacer group with the two connecting phosphonic acid groups at the para positions.^20,22 Due to the presence of an aromatic ring in the spacer group the acidic hydrogen of the phosphite is completely dissociated in the polar solvent, hence the effect of pH on the synthesis of porous tin(IV) phosphonates becomes insignificant in those cases. However, for the synthesis of HSnP-1 where the aliphatic butylene moiety was used as an organophosphorus spacer, the pH of the synthesis gel plays a crucial role. Thus, the condensation and crystallization of the material were greatly enhanced at pH = 5.0 vis-à-vis that synthesized at pH = 1.0 (Fig. 1 and the inset). The dissociation constants (K_a) of the acidic hydrogen of diphosphonic acid depend on the pH of the medium.²³ Under highly acidic conditions (pH ≈ 1) a maximum number of phosphonic acid groups remain protonated and hence the co-ordination with the metal ion does not take place properly. The number of available coordination sites increases with the enhancement of the pH of the medium. This facilitates the formation of three dimensional open framework structure. Hence, the crystallinity of the porous framework is quite high at pH ≈ 5 compared to that at pH ≈ 1. Simultaneously the possibility of oxide formation also increases under higher pH conditions, especially at alkaline pH. Our experimental results revealed that the optimal pH for the synthesis of the porous crystalline framework of hybrid tinphosphonate is ca. 5.0.

Fig. 1 XRD pattern of HSnP-1 (synthesized at pH = 5.0). Planes corresponding to the peaks are indexed. Inset: HSnP-1 (synthesized at pH = 1.0).

The powder diffraction peaks of HSnP-1 can be assigned very well with a new tetragonal phase with unit cell parameters a = b = 11.52 Å, c = 15.84 Å, α = β = γ = 90°. The unit cell volume of the porous tin(IV) phosphonate is 2102.507 ų with very low standard deviation. The crystal plane parameters (hkl) and corresponding d spacing values are given in Table 1. The standard deviations of the unit cell parameters and volumes of the unit cell are determined by CELSIZ program. The very small deviations of the calculated data from experimental values are in very good agreement with the estimated tetragonal crystal phase.

Table 1 Indexing of tetragonal structure of HSnP-1

h	k	l	2θ/°	d/Å
1	0	0	7.673	11.521
2	0	0	15.353	5.771
2	1	2	20.647	4.301
0	0	4	22.375	3.973
3	0	0	23.132	3.845
3	1	2	26.914	3.312
3	0	4	32.425	2.761
4	1	2	34.073	2.631
3	2	4	36.063	2.490
3	0	5	36.870	2.437
4	2	4	41.841	2.158
5	2	1	42.504	2.126
4	2	5	45.362	1.999
5	2	3	45.782	1.981
6	1	5	56.698	1.623
5	2	7	59.408	1.555
6	3	6	64.801	1.438

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Unit cell parameters
Parameters	Deviations
a = 11.52179 Å	0.00896
c = 15.83789 Å	0.02423
V = 2102.507 Å^3	ESD = 2.427

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Electron microscopic analyses. A representative TEM image of the HSnP-1 sample is shown in Fig. 2. In the image, low electron density spots (pores) are seen throughout the specimen, and micropores of dimensions ca. 1.1 nm are observed. The FE-SEM image of the HSnP-1 material is shown in Fig. 3. As seen from the image thin flake-like particles having dimensions ca. 200–400 nm length/width and 20–25 nm thickness are distributed throughout the specimen. In between these thin flakes large voids of nanoscale dimensions (5–10 nm) are also observed.

Fig. 2 TEM image of HSnP-1.

Fig. 3 FE-SEM image of HSnP-1.

Mesoscopic void spaces. The N₂ adsorption/desorption isotherms of HSnP-1 are shown in Fig. 4. The Brunauer–Emmett–Teller (BET) surface area, average pore diameter and pore volume of the HSnP-1 sample were estimated from the adsorption/desorption isotherms. The isotherms can be classified as typical type IV isotherms characteristic of mesoporous materials²⁴ together with a H3 type hysteresis loop, indicating substantial textural mesoporosity with narrow slit-like pores.²⁵ A strong capillary uptake for the adsorbed volume of nitrogen is observed at a relative pressure of 0.65–0.85, suggesting the presence of an appreciable amount of mesopores in the tinphosphonate framework. These mesopores could be originated from the aggregation (loose assembly) of flake-like nanoparticles and inter-particle voids. The BET surface area of HSnP-1 is 338 m² g⁻¹ with a total pore volume of 0.54 cm³ g⁻¹. The corresponding pore size distribution employing the non-local density functional theory (NLDFT) model is shown in the inset of Fig. 4. The pore size distribution showed peaks centered at 1.2, 2.6 and 6.8 nm. Thus the N₂ sorption study suggested the highly porous structure and mesoscopic void spaces in HSnP-1 varying from micropores to large mesopores.²⁶

Fig. 4 N₂ adsorption and desorption isotherms for HSnP-1. Adsorption points are marked by filled circles and those for desorption by open circles. NLDFT pore size distribution is shown in the inset.

Thermal stability. To investigate the thermal stability of the hybrid porous tinphosphonate network, TG-DTA analysis of the HSnP-1 sample has been carried out under N₂ flow with a heating rate of 10 °C min⁻¹. TGA and DTA curves for this material are presented in Fig. 5, which showed two distinct weight losses: a first exothermic weight loss of ca. 1.8%, which corresponds to the loss of water molecules present in the framework up to ca. 573 K, and a second exothermic weight loss of ca. 11.2% from 573–898 K. The presence of a sharp exothermic peak above 573 K suggested that our new hybrid tinphosphonate framework of HSnP-1 has considerably high thermal stability. The latter weight loss could be attributed to cleavage of C–C of C–P and the burning of organics present in the framework.

Fig. 5 TG (a, dotted line) and DTA (b, line) profiles of HSnP-1.

Spectroscopic studies. In Fig. 6, the FT IR spectrum of HSnP-1 is shown. The broad peak centered at 3448 cm⁻¹ could be attributed to the defect P–OH groups and free water molecules bound at its surface. The peak at 1633 cm⁻¹ corresponds to the bending vibration of water molecules. Peaks at 2991 and 2933 cm⁻¹ can be attributed to the asymmetric and symmetric C–H stretching vibrations of the –CH₂ groups whereas the peak at 1412 cm⁻¹ arises from the bending mode of –CH₂. The band at 1081 cm⁻¹ can be assigned to the stretching vibrations of the tetrahedral –CPO₃ groups. The bending vibrations of the tetrahedrally coordinated –CPO₃ groups and stretching vibrations of Sn–O correspond to the peaks at 554 and 502 cm⁻¹. Thus this spectroscopic result suggested the presence of 1,4-butylene-diphosphonic acid building block in HSnP-1 and its covalent linkage with Sn-species.

Fig. 6 FT IR spectrum of HSnP-1.

Solid state MAS NMR studies. ¹³C CP MAS and ³¹P MAS NMR experiments often provide useful information regarding the chemical environment around the C and P nuclei and the presence of organic functional groups in metal phosphonate frameworks. In Fig. 7, the ¹³C CP MAS NMR spectrum of HSnP-1 is shown. The spectrum exhibits two signals at 26.8 and 29.1 ppm, which resemble two different carbon atoms of butylene-1,4-diphosphonic acid. This result indicates that the butylene-1,4-diphosphonic acid moiety is covalently grafted inside the framework of HSnP-1. On the other hand, the ³¹P MAS NMR spectrum of the HSnP-1 sample (Fig. 8) shows one strong signal at 16.1 ppm and a very weak one at 32.4 ppm. This could be attributed to the (OSn)₃P–C₄H₈–P(OSn)₃ and (OH)(OSn)₂P–C₄H₈–P(OSn)₂(OH) species, respectively.³ This result further suggested the presence of the butylene-1,4-diphosphonic acid moiety in the tin phosphonate framework.

Fig. 7 ¹³C CP MAS NMR spectra of HSnP-1.

Fig. 8 ³¹P MAS NMR spectra of HSnP-1.

Molecular formula of HSnP-1

TG-DTA analysis of the HSnP-1 sample under air flow suggested the presence of 11.2 wt% of the organic precursor ligand butylene-1,4-diphosphonic acid (L). On the other hand CHN chemical analysis data revealed the presence of 8.4 wt% of carbon (C) and 3.01 wt% hydrogen and 0.0% nitrogen (N). The atomic absorption spectroscopic (AAS) analysis of HSnP-1 indicates the presence of 31.06 wt% of Sn. Further, the solid state ¹³C and ³¹P MAS NMR spectroscopic results suggested the presence of butylene-1,4-diphosphonic acid moiety, whereas the AAS FT IR spectroscopic data clearly prove the presence of Sn–O linkage in the porous framework of HSnP-1 material. Hence the molar ratio of carbon (C) : tin (Sn) = 0.7 : 0.26, which suggests the molecular formula of the porous architecture of HSnP-1 as L₂Sn₃ (where L = C₄H₁₂O₆P₂). So the molecular formula of HSnP-1 should be C₈H₁₆O₁₂P₄Sn₃·xH₂O (where x is the water of crystallization). The weight percentages of the elements are also in quite good agreement with the Energy-dispersive X-ray spectroscopy (EDX) measurement (Fig. S1 and Table S1, ESI†).

Catalysis: polymerization vs. partial oxidation. Usually styrene polymerizes to polystyrene at high temperatures (333–353 K) in the presence of a suitable initiator and solvent.²⁷ Controlled radical polymerization of styrene in the presence of molecular iodine could proceed in the presence of an organic initiator like bis(4-tert-butylcyclohexyl) peroxydicarbonate.²⁸ However, over HSnP-1 the reaction is very simple and it proceeds very fast at room temperature under eco-friendly conditions (water being the only byproduct of H₂O₂ decomposition). Within a minute of the initiation of the reaction, the mixture becomes turbid. Within 2–3 h reaction time almost quantitative conversion (85% yield of polystyrene) of styrene takes place and solid polystyrene nanoparticles precipitate out. The reaction mixture is treated with ethyl acetate, the insoluble catalyst is separated by filtration and the solid polystyrene bead is isolated by evaporation of the solvent. A close investigation has been made by varying the amount of oxidant for the production of polystyrene at room temperature over HSnP-1 material (Table 2). The probable reaction mechanism is shown in Scheme 2. Possibly hydrogen peroxide produces the hydroxyl radical (˙OH) and it is stabilized in the presence of the HSnP-1 surface, which initiates the polymerization of styrene under solvent-free conditions. After removal of the catalyst from the reaction mixture we have characterized the powder solid polystyrene product. The powder XRD pattern of this polymer showed the semicrystalline nature (Fig. 9), which nicely resembles polystyrene.²⁹ Moreover, the FE SEM image (Fig. 10) suggested that the polystyrene product is composed of nanospheres of dimensions ca. 19.4 nm and they are almost monodispersed. The average crystallite size of polystyrene particles was calculated employing the Debye–Sherrer equation³⁰
where D is the average crystallite size (Å), λ is the X-ray wavelength (Cu K_α = 1.5406 Å), β is the full width at half maximum (FWHM, in radians) and θ is the Bragg diffraction angle. The average particle size using this equation was 20.1 nm, which is in good agreement with the FE-SEM image analysis (Fig. 10). Such ultrasmall polymeric nanoparticles are very useful in designing functional hybrid materials and soft nano-objects.³¹ The observed turnover number (TON) of the catalyst HSnP-1 in the polymerization reaction is 65.8. Indium halides in combination with alkyl halides could catalyze the polymerization of styrenes at a little higher temperature of 50–60 ^°C,³² but being homogeneous in nature catalyst recovery is a major problem there. Cu(I)Br and Me₆TREN in the presence of DMSO solvent could polymerize styrene with bromine chain-ends in very short time.³³ But the catalyst could not be recycled for further use and the catalytic system is not eco-friendly. Our catalytic system for the polymerization of styrene is eco-friendly (solvent-free: devoid of hazardous organic solvents), proceeds at room temperature at a very fast rate (high TON) and can be recycled (see below).

Table 2 Conversion of styrene with varying amounts of hydrogen peroxide^a

Entry	Styrene/mmol	Hydrogen peroxide (30%) (mmol)	Time/h	Yield (%)
a Reaction conditions: 5.0 wt% catalyst, 298 K. b Product isolated without a catalyst.
1	10	5.0	3	85.0
2	10	2.5	3	72.0
3	10	1.0	3	63.0
4^b	10	5.0	24	—

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Scheme 2 Probable reaction mechanism for the polymerization of styrene over HSnP-1.

Fig. 9 Powder XRD profile of the polystyrene product.

Fig. 10 FE-SEM image of the polystyrene product.

Further, broadening of ¹H NMR signals for both aromatic C–H and aliphatic –CH₂ groups (Fig. S1, ESI†) confirms the complete polymerization of styrene to polystyrene.³⁴ The gel permeation chromatographic profile of the polystyrene product (Fig. S2, ESI†) suggested a high molecular weight of ca. 18 900 and polydispersity index (PDI) of 1.4. The observed glass transition temperature is 368 K, which resembles well that of polystyrene with a similar range of molecular weights, and its peak melting point is 593 K. Thus styrene polymerizes to uniform nanospheres of polystyrene at room temperature in the presence of porous tin(IV) phosphonate material HSnP-1 using the H₂O₂ initiator under solvent-free conditions. Uniform spherical nanoparticles of polystyrene may have some unique properties over the bulk polymers.³⁵ The stability of the hydroxyl radical (˙OH) produced during the polymerization reaction plays an important role in the room temperature polymerization of styrene. Though iron (Fe²⁺/Fe³⁺) is a quite well known hydroxyl radical (˙OH) generator from hydrogen peroxide,³⁶ probably due to the instability of the radical in the reaction medium, no polymerization product was obtained under the same reaction conditions.

On the other hand, when the catalyst HSnP-1 is added to a mixture of styrene solution taken in an aprotic solvent (tetrahydrofuran, acetonitrile or toluene) in the presence of H₂O₂ the polymerization reaction is prohibited almost completely. Here mostly partial oxidation of styrene takes place, where H₂O₂ acts as an oxidant. We have studied the oxidation of styrene in different aprotic and protic solvents, the conversions, selectivities and respective turnover frequencies (TOFs) of the catalyst (HSnP-1) are given in Table 3. In the presence of aprotic solvent two major products are phenylacetaldehyde and acetophenone, whereas in the presence of protic solvent like methanol conversion was negligible. For the conversion of styrene to the corresponding oxidized products the observed TOFs over our hybrid tinphosphonate catalyst HSnP-1 are quite higher (Table 3) than the other catalysts reported for the partial oxidation of styrene.³⁷ The reaction pathway for the partial oxidation of styrene over our hybrid tinphosphonate material is shown in Scheme 3. In the presence of an aprotic solvent and H₂O₂ possibly tinhydroperoxo species formed at the surface of HSnP-1.³⁸ This could facilitate the partial oxidation of styrene via initial epoxidation to styrene oxide, which undergoes rearrangement and further oxidation to yield phenyl acetaldehyde and acetophenone. Phenyl-1,2-ethanediol is also produced as the by-product in the presence of these solvents. The formation of phenyl-1,2-ethanediol could be attributed to the hydrolysis of styrene oxide at the acidic phosphate sites of the catalyst surface. The vinyl group attached to the benzene ring is more prone to radical formation than a saturated aliphatic chain³⁹ present in the porous framework. So the catalyst remains intact after the catalytic reaction. The XRD pattern of the material (HSnP-1) remains unchanged after the catalysis.

Table 3 Catalytic activity of HSnP-1 in partial oxidation of styrene over different solvents^a

Solvent	Conversion (%)	Phenyl acetaldehyde (%)	Acetophenone (%)	Phenyl-1,2-ethanediol (%)	Temp./K	TOF/h^−1
a Reaction conditions: 10 mmol styrene, catalyst = 1.0 wt% with respect to styrene, 12 mmol H2O2, 10 ml solvent, reaction time 4 h.
Tetrahydrofuran	83.1	73.5	6.0	20.5	338	80.4
Toluene	80.0	62.5	12.5	25.0	383	76.0
Acetonitrile	77.2	39.0	27.3	33.7	353	73.4
Methanol	0.2	—	—	—	338	0.08

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Scheme 3 Reaction pathway for the oxidation of styrene over HSnP-1.

Reusability of the catalyst

It is quite well known that when the solid acid is used as catalyst in the liquid phase reaction in the presence of H₂O₂, the metal ion could be leached as soluble hydroperoxide in the medium.⁴⁰ Thus when the catalyst is reused for the subsequent cycles there is a possibility of the decrease in catalytic activity. Here for the liquid phase polymerization of styrene, the catalyst HSnP-1 is separated by a simple filtration technique followed by washing with ethyl acetate for regeneration. We have reused the catalyst for five consecutive cycles. Our experimental results revealed only a little loss in the product polystyrene yield in five reaction cycles (85.0 to 69.5%). This result suggested that our organic–inorganic hybrid tinphosphonate can be utilized as an efficient catalyst for the room temperature polymerization of styrene under solvent-free conditions.

Possible explanation for dual catalytic behavior. Hydrogen peroxide generates the hydroxyl radical (˙OH) in the presence of various metal ions.⁴¹ Here in the porous hybrid material (HSnP-1) tin species present at the surface plays the crucial role in the production of the active hydroxyl radical (˙OH). The stability of the free hydroxyl radical (˙OH) directs the mode of the reaction. The stability of the hydroxyl radical is quite high in the acidic medium compared to in neutral or basic medium.⁴² Since neat H₂O₂ is considerably acidic, the absence of solvent could stabilize the hydroxyl radical. For room temperature reaction probably the feasibility of the hydrogen bonding plays a pivotal role. From the solid state MAS NMR it is quite clear that there are considerable amounts of free phosphonic acid groups at the surface of the catalyst HSnP-1. This probably helps stabilize the free hydroxyl radical (˙OH) via hydrogen bonding interactions. Excess hydrogen peroxide present in the medium also plays an important role in stabilizing the free radical via hydrogen bonding.⁴³ Styrene is a hydrophobic molecule (dipole moment = 0.13 D). Due to the presence of the butylene moiety in the framework the surface of the porous tinphosphonate is considerably hydrophobic. In the absence of any solvent, the non-polar styrene molecule adsorbs on the hydrophobic part of the catalyst, where the vinyl group can be exposed at the surface due to the unsymmetrical charge distribution in the styrene molecule. The ˙OH radical generated under solvent-free conditions can react vigorously with the surface vinyl group of the styrene molecule and thus initiation of the radical polymerization proceeds at a very fast rate. The polymeric product is separated out as solid nanoparticles from the reaction mixture. Thus, once initiated the polymerization proceeds smoothly until completion and thus the epoxidation reaction seized completely in the absence of any solvent. On the other hand in the presence of polar aprotic solvents at elevated reaction temperatures and in the presence of H₂O₂, the catalytically active tinhydroperoxo species was generated at the catalyst surface. This reacts with styrene leading to the partial oxidation reaction. The oxidized products have higher polarity than styrene, which are easily soluble in the polar aprotic solvents, and thus the oxidation reaction proceeds smoothly. Methanol can act as an antioxidant, probably the hydroxyl radical (˙OH) is quenched via [MeO˙H˙OH] species in this polar protic solvent.⁴⁴ Thus neither oxidation nor polymerization could proceed when methanol is used as solvent (Table 3).

Conclusions

New organic–inorganic hybrid tinphosphonate material having mesoscopic void spaces can be synthesized by using butylene-1,4-diphosphonic acid as organophosphorus precursor under hydrothermal conditions in the absence of any template molecule. Powder XRD data revealed a tetragonal phase with unit cell parameters a = b = 11.52 Å, c = 15.84 Å, α = β = γ = 90° and N₂ sorption studies suggested mesoporosity with good BET surface area and pore volume for this material. This novel material showed unique catalytic property in the presence of dilute aqueous H₂O₂ as an oxidant. In the absence of any solvent HSnP-1 catalyzes efficiently the polymerization of styrene to polystyrene, which is composed of semicrystalline and highly monodispersed nanospheres having an average particle size of ca. 19.4 nm. On the other hand, in the presence of an aprotic solvent it facilitates the partial oxidation reaction. The novel strategy developed herein for the synthesis of hybrid tinphosphonate material and its unique catalytic activity for the preparation of polystyrene may open up new avenues for the synthesis of value-added polymers under very convenient and energy efficient reaction conditions.

Acknowledgements

MP wishes to thank CSIR, New Delhi, for Junior Research Fellowship. HU wishes to thank Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 21655081) and the Adaptable and Seamless Technology transfer Program through target-driven R&D from Japan Science and Technology Agency (JST). AB wishes to thank DST Unit on Nanoscience for providing instrumental facilities.

References

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy00352j

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