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Brawny Silver-Hydrogel Based Nanocatalyst for the Reduction of Nitrophenols. Studies on Kinetics and Mechanism. Rohini Kuttiplavil Narayanan, and Sudha Janardhanan Devaki Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5038352 • Publication Date (Web): 14 Jan 2015 Downloaded from http://pubs.acs.org on January 15, 2015

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Industrial & Engineering Chemistry Research

Brawny Silver-Hydrogel Based Nanocatalyst for the Reduction of Nitrophenols. Studies on Kinetics and Mechanism. Rohini Kuttiplavil Narayanan and Sudha Janardhanan Devaki* Center for Scientific and Industrial Research - National Institute of Science and Technology, Trivandrum, India.

1 Environment ACS Paragon Plus


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ABSTRACT: The present work demonstrates the preparation and characterization of silver nanoparticles ensnared hydrogel and demonstrated its catalytic efficiency for the reduction of a series of nitrophenols. SPAG was synthesized via the simultaneous polymerization of acrylic acid and in-situ reduction of silver nitrate in the presence of amidodiol. Reduction process in presence of SPAG exhibited a first order reaction with less activation energy path (28.0, 30.7 and 33.8 KJ/mol for para nitrophenol, ortho nitrophenol and meta nitrophenol, respectively) and the reduction mechanism found to be obeying Langmuir Hinshelwood model.


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1. INTRODUCTION A great deal of research has been carried out in the area of metal nanoparticles based heterogeneous catalyst as they possess high surface to volume ratio and reusability, ease of separation and enhanced catalytic efficiency.1-5 Silver nanoparticles are particularly attractive due to their strong surface plasmon resonance, high electrical, optical and catalytic properties. 6-9 Due to the presence of high surface charge and surface to volume ratio, these nanoparticles may aggregate easily. The use of capping agents will prevent the aggregation of metal nanoparticles. However the stability, catalytic performance, facile access to the catalyst, ease of separation, reuse and absence of shape/size alteration during reuse makes a catalyst genuinely superior. The supramolecular hydrogel network formed by the various noncovalent interactions in supramolecular receptors and cyclodextrins have great potential for the stabilisation of metal nanoparticles and found to be promising in the field of bio-chemical sensing, controlled drug release, catalysis etc.10-11 Polymers, block copolymers, micelles, and reverse micelles have been widely studied for the stabilisation of metal nanoparticles.12-15 Polymer matrixes can tune the guest-host interaction for providing well defined spatial distribution, conferring kinetic stability for metal nanoparticles and also can provide good mechanical properties for the composites. Polymeric hydrogels have generated significant interest because of their high degree of shrinking and swelling that are controlled by stimuli such as solvent polarity, solutes, pH, temperature, electric field and light.16-18 Polymeric hydrogel metal nanocomposites are viable catalysts because they prevent the aggregation of nanoparticles and the loosely bound dynamic fibrous structure expected to enhance the easy access to the metal nanoparticles.19-21 Chemical reduction of metal salts with reducing agents such as sodium borohydride, ascorbic acid, lithium aluminium hydride, sodium citrate etc. is the commonly employed method for the synthesis of



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metal nanoparticles.22-25 The catalytic performance and stability of metal nanoparticles can be enhanced by incorporating functional groups such as –NH2, -CONH, -SiH.

26-29

In this context,

synthesis of efficient, robust and reusable homogeneously dispersed metal nanoparticles supported in polymeric hydrogel with enhanced functionalities is a major challenge. Amino phenols and their derivatives are receiving commercial importance in wide range of areas such as photographic, pharmaceutical and dye industries. The most facile synthesis of amino phenols is the reduction of the respective nitrophenols. The detailed study of mechanism for catalytic reduction is important to alter the catalyst for the better output. Two mechanisms for the reduction of para nitrophenol were reported in the literature. Saha et al. studied the kinetics of the reduction by varying the concentration of metal nanoparticles, nitrophenols and sodium borohydride suggested that the reduction is taking place at the surface of the metal nanoparticles.30 Zhang et al. provided a mechanistic interpretation for the reduction that both the reactants (nitrophenol and borohydride) will adsorb on the surface of the catalyst and the reaction could be modelled through Langmuir-Hinshel wood mechanism.31 Khalavka et al. reported that only hydrogen species get adsorbed on the surface and the reaction proceeds through Eley Rideal mechanism.32 In this paper, we report an efficient heterogeneous catalyst of hydrogel entrapped silver nanocomposite for the reduction of nitrophenols. The catalyst is prepared by the simultaneous polymerisation of acrylic acid and reduction of silver nitrate under ambient conditions. The polymerization is performed in the presence of a novel crosslinking agent amidodiol which is a rigid rod like organic molecule with two amide and two hydroxyl groups to impart rigidity and functionality to the polymeric matrix. The catalyst is characterised with SEM, TEM and XRD.


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Here we have studied the kinetics and mechanism of the reduction reaction and validated that the reaction is surface controlled and proceeds via Langmuir Hinshelwood mechanism.

2. EXPERIMENTAL 2.1 Materials. Hexamethylenediamine, ortho nitrophenol, meta nitrophenol, para nitrophenol (Sigma Aldrich), -butyrolactone (Fluka, Germany), isopropanol, acrylic acid, silver nitrate, sodium borohydride, ammonium persulphate (APS) (E-Merk, India). 2.2 Synthesis of silver nanoparticles entrapped polyacrylic acid hydrogel nanocomposite (SPAG). Acrylic acid (1 mL) and 10 % amidodiol (1 mL) were mixed thoroughly in water. Silver nitrate (0.2 ml of 0.02M solution) and ammonium persulphate are added to the above mixture. The contents were shaken well and kept at room temperature. The gel is formed after 10-20 minutes. Then it is kept at 70oC for 12 hours to ensure complete reduction of silver nanoparticles. Experimental details of preparation of amidodiol are given in the supporting information (Scheme S1). 2.3 Catalytic activity. Aqueous solutions of ortho, meta and para nitrophenol (1 mM) were prepared and the catalytic reduction studies were carried out using 20 mL of the solution and 2 mL of 10 mM sodium borohydride solution and 20 mg of SPAG catalyst at room temperature. 20 mg of the SPAG catalyst contains 2.59 × 10-5g silver. Effect of temperature on the reaction was studied by varying the temperature from 300 K to 325 K. The progress of the reaction was monitored by measuring the decrease in the UV absorbance periodically with small aliquots of solution from the reaction mixture. 2.4 Characterization techniques. SEM measurements were carried out with JEOL JFC-1200 fine coater and the probing side was inserted into JEOL JSM- 5600 LV scanning electron microscope. The sample for TEM analysis is dropped onto a copper formwar coated grids. FEI



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(TECNAI G2 30 S-TWIN) with an accelerating voltage of 100 kV were used for TEM measurements. X-ray diffraction studies were done with X-ray diffractometer (Philips X’pert Pro) with CuK radiation ( ~0.154 nm) employing X’celarator detector and a monochromator at the diffraction beam side. Averaged 2 was used with the 2 resolution of 0.002 degree from 2 to 80°. The absorption spectra were recorded using UV-visible spectrophotometer [Shimadzu model 2100]. Rheological measurements of the sample were conducted with Anton Paar Physica MCR 150 rheometer with parallel stainless steel plates of 50mm diameter, and the gap between parallel plates was set as 1mm.

3. RESULTS AND DISCUSSIONS 3.1 Preparation and characterisation of SPAG. The SPAG is synthesized by insitu reduction of silver nitrate with simultaneous polymerisation of acrylic acid using a radical initiator ammonium per sulphate (APS) and amidodiol as the crosslinking agent. Amidodiol is a synthesized by the aminolysis of

– butyrolactone and is reported earlier from our group for the

synthesis of liquid crystalline polyester amides.33 The two terminal hydroxy groups and two amide groups present in the amidodiol can form extensive hydrogen bonding with acrylic acid and these metal affinity groups can coordinate with silver. Hence the amidodiol provides mechanical stability and functionality to the hydrogel composite and also acting as reducing agent. The mechanism for the formation of SPAG is suggested as the dispersed Ag+ accelerates the generation of radical anion from persulphate ion and initiates the polymerization of acrylic acid at room temperature.34 Simultaneously, the homodispersed Ag+ ions were reduced by reductive particles, amidodiol or solvated electrons present in the system. Thus, acrylic acid is polymerized to polyacrylic acid (PAA) in presence of radical initiator APS and simultaneously


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silver ions present in the system reduced to silver atoms.35,36 The synthesis and detailed characterisation of the SPAG catalyst is reported earlier from our group.37 The morphological characterisation of the catalyst was studied with SEM and TEM. The SEM picture of SPAG showed nanosphere morphology and is shown in Figure S1a. TEM analysis confirmed the formation of silver nanospheres in the gel matrix (Figure S1b). Typical EDS spectrum confirming the presence of silver nanoparticles in the SPAG system is shown in Figure S2. Optical property of silver nanoparticles was studied by recording UV–visible spectrum and is shown in Figure S3. The correlation between the frequency and intensity of absorption band with the size and shape of the nanoparticles were reported.38 The spectrum of SPAG showed a characteristic absorption band at 315 nm indicating the formation of distorted spherical and cuboidal silver nanoparticles. The mechanical stability of SPAG is studied by rheology and is shown in Figure S4. The frequency independent storage and loss modulus obtained in the lower frequency region indicates the true gel formation. The high value of storage modulus (6000 Pa) and the large distance between storage and loss modulus (2338 Pa) is also suggesting the high mechanical strength of the composite. 3.2 Catalytic reduction of ortho, meta and para nitrophenol using SPAG. Amino phenols can be synthesised by the reduction of nitro group using sodium borohydride as the reducing agent. The UV absorption peak of para nitrophenol (PNP) appeared at 400 nm in alkaline conditions is due to the formation of nitrophenolate ion on the addition of sodium borohydride. In the presence of catalyst, the reaction starts immediately with the decrease in the intensity of peak at 400 nm. The new peak at 316 nm gradually appearing is indicating the formation of para aminophenol (PAP).39 The progress of reaction can also be monitored visually by the disappearance of yellow colour which indicates the formation of aminophenol. In the absence of



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catalyst, the peak at 400 nm remained unaltered with time indicating the absence of reduction to PAP. After the catalytic reduction is completed (600 s), the peak due to nitro compound is no longer observed, indicating the complete reduction of PNP. The time dependent reduction profile of PNP in the presence of SPAG with respect to time is given in Figure 1a.

Figure 1 (a) Time dependent UV spectra of para nitrophenol reduction (b) Kinetic plot for the reduction of PNP. ( [PNP]: 1mM, [NaBH4]: 10mM, Amount of catalyst: 20 mg, Temperature: 30°c, pH: 7.5)


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Excess amount of sodium borohydride was used for the reduction of PNP; hence the reaction rates are independent of borohydride concentration. So, first order rate kinetics with respect to the concentration of PNP is used to evaluate the rate of reaction. The rate constant is calculated from the decrease in the peak intensity at 400 nm. The linear correlation obtained for ln (A/A0) versus time confirmed the first order reaction kinetics of the catalytic reduction and is shown in Figure 1b. The rate constant was calculated from the slope of the line and is obtained as 8.16 ± 0.04 × 10-3 s-1. As the initial concentration of BH4- was very high, it remained constant during the reaction. Excess concentration of sodium borohydride used in the reaction medium increases the pH and prevents the degradation of borohydride ions. The hydrogen liberated during the reduction will expel out the air and inhibit the aeral oxidation of PAP. A catalyst with an intermediate potential between the donor and acceptor may help the electron relay process and metal nanoparticles are considered as effective redox catalysts. Metal nanoparticles catalyse the reduction by facilitating the electron relay from the donor sodium borohydride to the acceptor para nitrophenol. Both borohydride and the para nitrophenolate are adsorbed on the surface of the SPAG and facilitate the reduction by lowering the activation energy of the reaction.

The catalytic activity of SPAG was also tested for the reduction of ortho nitrophenol (ONP) and meta nitrophenol (MNP). In this case also, in the presence of SPAG, peak of meta nitrophenolate (398 nm) and ortho nitrophenolate (416 nm) is gradually decreases and a new peak of meta aminophenol (MAP) and ortho aminophenol (OAP) is appearing with time. The time dependent UV spectra of ONP and MNP and the corresponding kinetic plots are given in the Figure 2a and 2b.



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Figure 2 (a) Time dependent UV spectra of MNP and the kinetic plot of ln (A/A0) versus time for the reduction of MNP is shown in the inset. (b) Time dependent UV spectra of ONP and the kinetic plot of ln (A/A0) versus time for the reduction of ONP is shown in the inset. ( [MNP]: 1mM, [ONP]: 1 mM, [NaBH4]: 10mM, Amount of catalyst: 20 mg, Temperature: 30 °c, pH: 7.5). The ortho and para nitrophenolate is more stable than meta nitrophenolate. The –I effect of nitro group and the resonance effect stabilise ortho and para isomer. Ortho isomer is less stable than para due to the steric hindrance which makes –I effect is less effective comparing with the para


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isomer. Due to the absence of conjugation, meta isomer is the least stable one. Hence the rate of reduction of nitrophenols followed the order PNP (8.2 ± 0.04 × 10-3 s-1) > ONP (6.3 ± 0.03× 10-3 s-1) > MNP (5.2 ± 0.05 × 10-3 s-1). 40,41 The catalytic conversion efficiency of the catalyst is 99%, 96% and 95% for PNP, MNP and ONP respectively. The activity factor is the ratio of rate constant over the total weight of the catalyst. Higher value of activity suggests the better performance of the catalyst. The activity of the catalyst is 0.41, 0.31 and 0.21 PNP, ONP and MNP respectively.The rate constant for the reduction of PNP is compared with the polymeric hydrogel supported silver nanoparticles and found to be higher than reported in literature 45

19,20, 41-

and is given in Table 1.The present catalytic studies were carried out with 1mM PNP and 10

mM NaBH4 at room temperature. In the present study, 20 mg of the hydrogel stabilised catalyst (2.59 × 10-5g of silver) is employed. The catalytic dose is also comparable with the references. In the present system, silver nanoparticles were stabilised with carboxyl and hydroxyl groups of the fibrous network and are mostly exposed to the solution system and enhances the rate of diffusion of reactants. The absence of induction period during the reduction process of nitrophenol revealed the fast diffusion of reactants and products in and out of the hydrogel. Thus diffusion rate is extremely quick and hence the rate of reduction is very fast. In this situation, limitation due to the diffusion of reactants inside the hydrogel is not taken into consideration. Similar observations were made by other researchers. 46-48

Catalyst

Rate constant

Ref

PVA/PS-PEGMA/Ag

7.80 × 10-5s-1

19

PVA/Ag hydrogel

7.31 × 10-5s-1

19

Ag–Alg biohydrogel

0.36 min−1

20

PVA/Ag

3.78 × 10-7s-1

41

Ag–PVA/PVA/Ag–PVA

5.3 × 10-3 s-1

42

0.0923 min-1

43

poly-(acrylamide glycolic acid)/Ag composites



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pyridine–pyrazole based 7.8 ± 1.5 × 10-3 s-1

44

trisamide ligand- Ag hydrogel alginate–magnetite hybrid 0.27 min-1

45

(Ag@AMH) biohydrogels Acrylic acid - Amidodiol /Ag

Present -3 -1

-1

8.16 × 10 s (0.489 min ) hydrogel (SPAG)

work

Table 1: Comparative study of the catalytic activity of polymeric hydrogel supported silver nanoparticle for the reduction of nitrophenol.

3.3 Effect of temperature. Effect of temperature on the catalytic reduction of nitrophenol was studied using SPAG and is shown in Figure 3a. The rate of reduction increases with increase in temperature. As the temperature increased, the surface area of the catalyst and the diffusion of reactants onto the surface of catalyst is increased. These factors contribute for the enhancement in the rate constant at higher temperature. Activation energy of the reduction process was calculated using Arrhenius equation.

ln

ln

Where k is the rate constant, A is the pre exponential factor, Ea is the activation energy, R is the gas constant (8.314 Jmol-1K-1) and T is the temperature. A straight line is obtained on plotting ln k versus 1/T for the SPAG catalyzed reaction and is given in Figure 3b. The activation energy of the SPAG catalysed reduction reaction is 28, 30.7 and 33.8 KJ/mol for PNP, ONP and MNP respectively.


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Figure 3.(a) Effect of temperature on the reduction of nitrophenols (b) Arrhenius plot for the reduction of nitrophenols. ([PNP]: 1mM, [ONP]: 1mM, [MNP]: 1mM, [NaBH4]: 10mM, Amount of catalyst: 20 mg, Temperature: 30 °c, pH: 7.5)

Finally, the reusability of the SPAG for the catalytic reduction was studied and as shown in Figure 4. The variation of rate constant for nitrophenols upto 20 cycles is given the Table S1. There was only a slight decrease in the catalytic efficiency as evidenced from rate of reaction



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even after 20 cycles, which suggests that SPAG as an efficient catalyst for the reduction of nitrophenols. The storage stability of SPAG was also checked. SPAG was stored at 4 °c in sealed covers. The graph showing the variation of rate constant of SPAG in the reduction of para nitrophenol for six months is given in Figure S5. The SPAG catalyst is stable upto 6 months without any appreciable loss in catalytic efficiency. The High rate constant, high reusability, good storage stability, easy separation and the absence of induction period suggest SPAG as an efficient catalyst for the reduction of organic molecules and the process is rapid, efficient and economical.

Figure 4. Reusability of the SPAG catalyst for 10 successive cycles ([PNP]: 1mM, [ONP]: 1mM, [MNP]: 1mM, [NaBH4]: 10mM, Amount of catalyst: 20 mg, Temperature: 30 °c, pH: 7.5)

3.4 Investigation of the reaction mechanism. The most facile method for the synthesis of aminophenols is the reduction of their respective nitrophenols. The study of the reaction mechanism helps to design a catalyst with better output. The catalytic reduction of nitrophenols with hydrogel supported metal nanoparticles is a heterogeneous catalytic reaction. The heterogeneous catalytic reaction can be proceeding through either Eley- Rideal model or


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Langmuir – Hinshelwood model. In the Eley Rideal model, only one of the educts will adsorb on the surface of the catalyst and reacts with the other educt in the solution. In the case of Langmuir- Hinshelwood model, both the educts get adsorb on the surface of the catalyst and react on its surface. The valid reaction mechanism for the reduction of nitrophenols can be suggested by performing two sets of experiments i.e., by studying the rate constant dependence on the concentration of PNP and sodium borohydride. In Eley- Rideal model, the rate is increasing with an increasing concentration of PNP which is not adsorbed on the surface. According to Langmuir – Hinshelwood model, the rate constant is decreasing with an increasing concentration of PNP whereas for an increasing concentration of sodium borohydride, rate is reaching maximum.49,50 The reduction of nitrophenol is taking place only in the presence of metal nanoparticles. If an excess sodium borohydride is used for the reaction, the reaction is first order with respect to the concentration of para nitrophenol. The apparent rate constant is proportional to the surface area of the nanoparticles. The kinetic constant can be calculated as

For the quantitative comparison of the data, the catalytic reduction can be modelled in terms of Langmuir-Freundlich isotherm.

Where,

i

is the surface coverage of the compound i, Ki is the adsorption constant of the

respective component and ci is the concentration in solution and ‘n’ is related to the heterogeneity of the sorbent. Further rearrangement of the equation 1 leads to equation 3 which can be used to model the catalytic activity.



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Thus, kapp is given by

Here, k is the molar rate constant per square meter of the catalyst and KPNP is the adsorption coefficient of nitrophenol and KBH4 is the adsorption coefficient of BH4-.


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Figure 5.Dependence of apparent rate constant on the concentration of (a) PNP and (b) sodium borohydride.

The Figure 5 showed the nonlinear dependence of rate constant of both PNP and sodium borohydride concentration variation and the solid lines refer to the fit of the experimental data for equation 3 and suggested Langmuir – Hinshelwood model for the reaction. The rate constant is decreasing with an increasing concentration of PNP and increases with an increasing concentration of sodium borohydride. According to this mechanism, borohydride and PNP adsorb on the surface of the catalyst. The diffusion of the reactants to the nanoparticle surface and the adsorption/desorption equilibrium is assumed to be fast. In the rate determining step, the surface hydrogen and PNP react with each other and the PAP desorbs from the surface of the catalyst. Detachment of the product creates a free surface and catalytic cycle can start again. A high concentration of nitrophenol molecules leads to the full coverage of the catalytic surface which slows down the reaction. An increasing concentration of sodium borohydride leads to an increase in the rate constant because it supplants PNP. Further increase in the concentration of borohydride leads to a maximum rate constant due to the saturation on the catalyst surface.



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According to the results, both borohydride ions and PNP simultaneously adsorb on the surface of the catalyst and then the product PAP is desorbed out. A schematic representation for the reduction is given in Figure S6. The reduction rate is observed to be very fast and the diffusion of the reactants and products in and out of the hydrogel is much quicker. So the diffusion rate inside the hydrogel is ignored while calculating the rate constant.51

4. CONCLUSIONS Robust silver nanoparticles entrapped hydrogel composite was prepared by a simple strategy of insitu reduction and polymerization approach and is characterised. Catalytic efficiency of SPAG in the reduction of a series of nitrophenols was checked. The activation energy of the SPAG catalysed reduction reaction is 28, 30.7 and 33.8 KJ/mol for PNP, ONP and MNP respectively. The reaction mechanism is investigated and fitted with Langmuir Hinshelwood model and is suggested that both the reactants are adsorbed on the surface of the catalyst. The proposed hydrogel composites receives novelty in terms of simple low cost strategy for the preparation, high storage modulus, reusability, storage stability, sludge free operation, absence of induction period and superior efficiency in the reduction process.

Author information Corresponding author: J. D. Sudha

Email id: [email protected]

ASSOCIATED CONTENT Supporting Information. Synthesis of amidodiol, Characterisations of SPAG (SEM, TEM, EDS, UV and rheology), reusability and storage stability of SPAG, schematic representation for the reduction of para


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nitrophenol in the presence of SPAG. “This material is available free of charge via the Internet at http://pubs.acs.org.”

Acknowledgements.

We thank CSIR Network project (CSC0101) for the financial support. We

also would thank Dr. Suresh Das, Director, CSIR-NIIST and Dr. A. Ajayaghosh, NIIST, Trivandrum. We are also thankful to Ms. Lucy Paul for SEM analyses and Mr. Kiran Mohan for TEM analyses.



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