A two in one approach: renewable support and enhanced catalysis for sweetening using chicken feather bound cobalt(II) phthalocyanine under alkali free environment

Abstract

Poultry waste chicken feathers, an inexpensive and abundantly available material has been used as a renewable support for immobilizing a cobalt phthalocyanine catalyst. The synthesized heterogeneous cobalt(II) phthalocyanine catalyst was used for the aerobic oxidation of mercaptans to the corresponding disulfides using ultrasonic irradiation under alkali free conditions. The significantly higher catalytic activity of the heterogeneous catalyst as compared to a homogeneous one can be attributed to the synergistic effect of the support matrix. In addition, the catalyst could easily be recovered and recycled for several runs without loss of activity, which makes the process greener and more cost-effective.

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Poultry waste chicken feathers (CF) is an inexpensive and abundantly available material which holds great potential to be used as a raw material for various applications. According to a recent estimate about 5 million tonnes of CF is generated annually in the US alone,¹ which is disposed as landfill due to the lack of an effective solid waste management strategy. This kind of disposal not only leads to the discarding of a re-cyclable biopolymeric renewable material but also causes serious environmental concerns as the biodegradation rate of CF is very slow. Recently, extensive efforts have been focused towards the development of valuable materials for example, biodegradable thermoplastics,² carbon nanotubes,³ and N-doped carbon materials.⁴ On similar lines, our group has recently reported a new application of CF as a support material for catalytic applications.⁵ The use of CF as support not only provides stability but also exerts a synergistic effect due to the presence of functional groups such as –NH₂, –COOH, –OH and –SH groups which results into significant enhancement in the reaction rates. Furthermore, it can be easily modified chemically or physically and is insoluble in common organic solvents which make it a versatile supporting material.

Oxidation of mercaptans (RSH) in the petroleum products which are undesirable due to their foul odor and corrosive nature to less deleterious disulphides is an essential and important process.⁶ Apart from stoichiometric oxidants such as permanganates,⁷ metal oxides,⁸ a number of catalysts such as VOCl₃,⁹ halogens,¹⁰ Al₂O₃/KF,¹¹ Ni₂FeO₄/H₂O₂,¹² and activated carbon/air¹³ are reported for the oxidation of mercaptans. In addition, some photocatalytic methods using eosin Y,¹⁴ Bi₂WO₆,¹⁵ and iron phthalocyanine immobilized on graphene oxide¹⁶ have been reported for this transformation. However, most of the catalytic and photocatalytic methods reported so far either uses homogeneous reaction conditions or requires costly reagents, long reaction times, formation of toxic byproducts and tedious workup steps. The commercial known process for oxidation of mercaptans also known as “sweetening” is mainly carried out using air as oxidant and cobalt phthalocyanine as catalyst under alkaline conditions. However, the use of caustic for alkalinity is undesirable as it is hazardous in nature and its disposal leads to the environmental pollution.^17–20 Furthermore, homogeneous nature of the catalyst poses the limitations of difficult recovery and therefore non-recyclability of the catalyst. To overcome these limitations, efforts are being made to develop alkali free sweetening process by using heterogenized homogeneous cobalt phthalocyanine catalysts. In this context, several supports such as Mg–Al mixed oxides,²¹ magnetic nanoparticles and Mg–Al layered double hydroxides (MgAl-LDH) have been used for immobilizing cobalt phthalocyanine to develop alkali free sweetening process.²² However these methods are associated with certain drawbacks such as the use of expensive precursors and difficult multi-step synthesis of support materials.

In continuation to our on-going research programme on sustainable chemical processes, herein we report the first successful synthesis of CF grafted cobalt(II) phthalocyanine and its catalytic application for aerobic alkali free oxidation of mercaptans under sonochemical conditions. The use of ultrasonication provides a significant enhancement in the reaction rates, energy input and the reactions could be completed within shorter reaction time with higher product yields.^23,24

The present work describes the use of amino functionalized poultry waste CF as a support for immobilization of tetrasulfonated cobalt phthalocyanine (CoPcS). The synthesized heterogeneous, inexpensive and easily recoverable material was used for the oxidation of thiols into disulfides under mild conditions (Scheme 1).

Scheme 1 Sonicated driven oxidation of thiols to disulphides.

Results and discussion

Prior to the immobilization, the powdered chicken feather was treated with 3-aminopropyltriethoxysilane (APTES) to get more –NH₂ functionalities on the surface for better immobilization. Afterwards, the amino functionalized chicken feather (NH₂@CF) was treated with tetrasulfonated cobalt phthalocyanine (CoPcS) to get desired catalyst. The chicken feather immobilized cobalt phthalocyanine was termed as CoPcS-NH@CF which was washed thoroughly with ethanol, distilled water (1 : 1) and then dried in oven at 60 °C (Scheme 2). Prior literature reports supported the formation of –NH–SO₂ bonding between amino functionalized CF and CoPcS for covalent immobilization.^25,26

Scheme 2 Schematic illustration for synthesis of CoPcS-NH@CF catalyst.

The vibrational spectra of unmodified CF and modified CF (viz.; NH₂@CF and CoPcS-NH@CF) are shown in Fig. 1. In the FTIR spectrum of unmodified CF (Fig. 1a), the characteristic two absorption bands at 1649 and 1535 cm⁻¹ were attributed to the N–H bending vibration and C–N stretching in amide group, respectively. The broad band appeared at 3430 cm⁻¹ corresponds to N–H and –OH stretching, revealing the presence of plenty hydroxyl and amino groups on the surface of CF, which are major grafting sites for APTES, whereas the two bands at 2958 and 2917 cm⁻¹ were assigned to the aliphatic C–H symmetric and asymmetric stretching vibrations respectively. Further, the successful functionalization of CF with APTES (Fig. 1b) was confirmed by the appearance of characteristic two additional peaks at 1132 and 1028 cm⁻¹ corresponding to the stretching vibration of C–NH₂ and Si–O stretching respectively. The other significant evidence observed was the reduced intensity of the –OH stretching band at 3430 cm⁻¹. Furthermore, the presence of characteristic ring vibration peak of CoPcS at 611 cm⁻¹ and slight shifting in peaks suggested the successful grafting of tetrasulfonated cobalt phthalocyanine (CoPcS) to amino functionalized CF (Fig. 1c). The lower intensity of the CoPc bands may be attributed due to the low loading of the catalyst.

Fig. 1 FT-IR spectra of (a) CF (b) NH₂@CF and (c) CoPcS-NH@CF.

The X-ray diffraction pattern of CF, NH₂@CF and CoPcS-NH@CF are represented in Fig. 2. XRD pattern of neat CF (Fig. 2a) shows a broad hump at 2θ value around 21.36 which is assumed due to the diffraction of β-sheet structure of the feather keratin.²⁷The XRD pattern of amino functionalized CF (Fig. 2b) and CoPcS-NH@CF (Fig. 2c) remained almost similar to the unmodified CF, which suggested the lower loading of complex units on the CF support.

Fig. 2 The XRD of (a) CF, (b) NH₂@CF and (c) CoPcS-NH@CF.

Thermal stability of the synthesized materials was determined by thermogravimetric analysis (Fig. 3). In the TG curve of unmodified CF, two weight losses were observed (Fig. 3a). The first mass loss (6%) of CF (Fig. 3a) from 100–150 °C is due to the loss of adsorbed water molecules. The second weight loss occurred from 222 to 380 °C with a decrease in fiber mass ranging from 10 to 79% was associated to a general rupture of disulfide and peptide bonds. Due to strong inter- and intramolecular hydrogen bonds leading to close packing of polypeptide chains in the form of β-sheets, CF shows decomposition in the temperature range of 220–380 °C. It is obvious that the introduction of a functional silane group increases the thermal stability. The thermal stability of NH₂@CF (Fig. 3b) is higher than that of CF, was due to the APTES moieties. Furthermore, the CoPcS immobilized CF showed initial weight loss near to 100 °C due to loss of absorbed water molecules. Next, weight loss was observed between 220 and 400 °C which is mainly due to the degradation of polypeptide chains in CF as well as phthalocyanine moieties of grafted complex units. Afterwards the material was found to be quite stable mainly due to the formation of oxides of cobalt.

Fig. 3 TG/DTA curves of (a) CF, (b) NH₂@CF, and (c) CoPcS-NH@CF.

The phase behaviour of the prepared CF and CoPcS-NH@CF was determined by DSC. The heat flow curves of synthesized materials are shown in Fig. 4. The heat flow curves of both neat CF and modified CF were found to be nearly similar due to the lower loading of CoPc to CF support. A low temperature broad peak in the range of 100–150 °C is mainly attributed to the evaporation of residual moisture/denaturation of the protein. However an exothermic peak nearly at 230 °C is slightly shifted towards lower temperature in CoPcS-NH@CF is attributed to the decomposition/denaturation of the CF²⁸ (Fig. 4a and b).

Fig. 4 DSC curve of (a) CF and (b) CoPcS-NH@CF.

UV-Vis absorption spectra are shown in (Fig. 5). Weak absorption band of NH₂@CF was observed in the UV region. After immobilization of CoPcS, characteristic broad peaks at 350 nm (Soret band) and 650 nm due to π–π* macrocyclic ring transition in the spectra of CoPcS-NH@CF (Fig. 5b) which indicated the successful attachment of phthalocyanine moiety onto the solid support.

Fig. 5 UV-visible spectra of (a) NH₂@CF and (b) CoPcS-NH@CF.

The surface chemical composition of the synthesized NH₂@CF and CoPcS-NH@CF was investigated with XPS (Fig. 6). C 1s and N 1s XPS graphs of NH₂@CF are shown in Fig. 6a and b. Wide scan XPS spectra of NH₂@CF in C 1s region demonstrated three characteristic peak components at 285 eV, 286.56 eV and 288.79 eV assigned respectively to C–C, C–O and C=O. The XPS spectrum of NH₂@CF in N 1s region demonstrated two peaks at 399.23 eV and 400.71 eV due to free –NH₂ and C–N bonds respectively. Further the XPS spectrum of CoPcS-NH@CF is shown in Fig. 6c–f. The XPS spectrum of CoPcS-NH@CF in C 1s region demonstrated three peaks as similarly shown in Fig. 6a but there is little increment in the intensity of the peaks corresponding at 286.56 eV and 288.79 eV is observed. The XPS spectrum in N 1s region demonstrated three peaks at 398.50 eV, 400.11 eV and 401.71 eV due to C=N, –NH₂ and C–N respectively. Peak at 398.50 eV confirmed the presence of cobalt phthalocyanine moiety in the synthesized CoPcS-NH@CF. Presence of two characteristic peak components at 780.5 eV and 795.9 eV assigned respectively to Co 2p_3/2 and Co 2p_1/2 confirmed that cobalt is in +2 state (Fig. 6e).²⁹ Wide scan XPS spectrum of CoPcS-NH@CF in Cl 2p region (Fig. 6f) demonstrated two peaks at 199.5 eV and 200.4 eV corresponding to Cl 2p_3/2 and Cl 2p_1/2, which relates to the unreacted sulfonyl chloride functionalities in the synthesized catalyst.

Fig. 6 XPS spectra of (a and b) NH₂@CF; and (c–f) CoPcS-NH@CF.

The catalytic activity of the synthesized CoPcS-NH@CF was tested for the oxidation of mercaptans using ultrasonic radiation at room temperature with molecular oxygen as terminal oxidant using dimethylformamide (DMF) as solvent under alkali free conditions.

A wide variety of mercaptans consisting of aliphatic, aromatic, and long chain aliphatic were selectively oxidized to their corresponding disulfides in near quantitative yields without any evidence for the formation of corresponding sulphonic acid. The results of these experiments are summarized in Table 1. The conversion of the mercaptan to disulfide was determined by GC-MS and the identity of the products was established by comparing their spectral data (¹H NMR) with those of authentic samples (see in ESI†). In general aromatic mercaptans (Table 1, entries 7–12) were found to be less reactive than the aliphatic ones (Table 1, entries 1–6), however in the case of aliphatic mercaptans the reactivity decreases with the increase in chain length and accordingly the mercaptans with longer chain length require more reaction time (Table 1, entries 4–6).

Table 1 Oxidation of mercaptans using CoPcS-NH@CF^a

Entry	Mercaptan	Disulfide	T/min	Conv.^b (%)	Yield^c (%)
a Reaction conditions: mercaptan (5 mmol), catalyst (0.1 g), DMF (5 ml) using ultra-sonic irradiation using molecular oxygen at room temperature (23 °C).b Determined by GC-MS.c Isolated yield.
1			30	98.5	96
2			30	98	94
3			35	97	92
4			45	96	93
5			45	96	87
6			45	90	85
7			55	96	93
8			50	88	86
9			55	87	83
10			65	91	89
11			70	89	82
12			120	72	68

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Next, in order to evaluate the effect of cobalt catalyst, we performed the blank experiments under described reaction conditions (Fig. 7). There was no oxidation occurred in the absence of catalyst or using bare CF as catalyst even after prolonged time. The oxidation was found to be very slow using amino-functionalized CF. Homogeneous cobalt phthalocyanine (CoPcS) gave 80% product yield whereas nearly quantitative conversion (96%) was obtained when CoPcS-NH@CF was used as catalyst under similar experimental conditions (Fig. 7).

Fig. 7 Experimental results of thiophenol as a model substrate: without catalyst, NH₂@CF, CoPcS and CoPcS-NH@CF.

After completion of the reaction, catalyst was recovered by centrifugation, washed with ethanol, dried and reused for recycling experiments by choosing oxidation of thiophenol as a representative example (Fig. 8). As shown the recovered catalyst showed almost similar catalytic activity for five runs without showing any significant decrease in the catalytic activity. Moreover, the resulted filtrate samples were analyzed by ICP-AES analysis to ascertain that no leaching of the active CoPcS catalyst occurred during the reaction. The cobalt content in the recovered catalyst after five runs was found to be 0.18 wt% which was almost same to the fresh one (0.20 wt%), indicating that the developed catalyst is quite stable and the reaction is truly heterogeneous in nature.

Fig. 8 Results of recycling experiments.

Experimental section

Materials

Chicken feather was supplied by local poultry farm, the feathers were cleaned (washed and treated with ethanol), dried in oven. Feather barbs separated from quill and ground in a Retsch ball mill to get feather powder. APTES was purchased from Sigma-Aldrich. Cobalt phthalocyanine, thiols and chlorosulphonic acid were purchased from Alfa Aesar. Thionyl chloride, HPLC grade water and diethyl ether were purchased from MERCK India. All other chemicals were of the analytical grade and were used without further purification. Tetrasulfonated cobalt phthalocyanine (CoPcS) was synthesized by following the literature procedure.³⁰

Characterization techniques

Vibrational spectra (FT-IR) of the solid samples were recorded on Perkin Elmer spectrum RX-1 IR spectrophotometer. Samples for XRD analysis was prepared on glass slide and the diffraction plot of CF, NH₂@CF and CoPcS-NH@CF was analyzed with Bruker D8 Advance diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15418 nm). Thermo-gravimetric analysis of the samples was performed using a thermal analyzer TA-SDT Q-600 in the temperature range 40 °C to 500 °C and temperature ramp was set at 10 °C min⁻¹ under nitrogen atmosphere. UV-visible absorbance of the samples was determined on Perkin Elmer lambda-19 UV-VIS-NIR spectrometer using BaSO₄ as a reference material. The surface chemical composition of the synthesized catalyst was analyzed by using X-ray photoelectron spectroscopy. XPS measurements were obtained on a KRATOS-AXIS 165 instrument equipped with dual aluminum–magnesium anodes using Mg Kα radiation (hν = 1253.6 eV) operated at 5 kV and 15 mA with pass energy 80 eV and an increment of 0.1 eV. The cobalt content in catalyst was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES DRE, PS-3000UV, Leeman Labs Inc, USA). Sample for ICP-AES was prepared by digesting 0.05 g catalyst with conc. HNO₃ at 70 °C for 30 min to oxidize all organic materials and leach out cobalt metal in the oxidized form. The obtained solution was filtered and final volume was made up to 10 ml by adding de-ionized water.

Synthesis of amino functionalized chicken feather (NH₂@CF)

In order to get more NH₂ functionality on the CF surface, the powdered chicken feather (1.5 g) was treated with 3-aminopropyltriethoxysilane (10 ml) in toluene and the resulting suspension was refluxed for 24 h with the continuous stirring under the nitrogen atmosphere. The modified CF was collected by centrifugation and subjected to a Soxhlet extraction with dichloromethane to remove unreacted APTES for 12 h. The obtained material was dried in vacuum oven at 50 °C.

Immobilization of CoPcS onto the amino functionalized CF

Amino-functionalized chicken feather 1.8 g was treated with tetrasulfonated cobalt phthalocyanine (CoPcS, 0.5 g) in methanol (5 ml) at room temperature for 24 h. The immobilized CoPcS-NH@CF was filtered and washed thoroughly with ethanol and distilled water (1 : 1). The obtained material was dried in oven at 60 °C.

Typical procedure for oxidation of thiols

All the experiments were carried out in alkali free medium using DMF as solvent under ultrasonic irradiation at room temperature. In a typical experiment, thiophenol (5 mmol) and CoPcS-NH@CF catalyst (0.1 g, 0.003 mmol) was added in dry DMF (5 ml) and the resulting suspension was put under ultrasonic irradiation at room temperature, in the presence of an oxygen atmosphere. The samples were taken at different intervals and analyzed by GC to monitor the progress of the reaction. After completion of the reaction, the catalyst was recovered by centrifugation, washed with ethanol, dried under vacuum and reused for recycling runs. The resulting organic layer was diluted with diethyl ether, washed with water and dried under reduced pressure to get the crude product which was further purified by column chromatography. The conversion of mercaptans to corresponding disulfides was determined by GC-MS, whereas the identity of the product was confirmed by comparing their spectral data (¹H NMR) with authentic sample.

Acknowledgements

We kindly acknowledge Director, CSIR-IIP for his kind permission to publish these results. DC is thankful to CSIR, New Delhi for providing technical HR under the XII five year projects. Analytical division is kindly acknowledged for providing support in analysis of the samples.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06842a

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