A sustainable approach for the synthesis of catalytically active

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A sustainable approach for the synthesis of catalytically active peroxidase-mimic ZnS catalysts Camilla M Cova, Alessio Zuliani, Mario J. Muñoz Batista, and Rafael Luque ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04968 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Sustainable Chemistry & Engineering

A sustainable approach for the synthesis of catalytically active peroxidase-mimic ZnS catalysts

Camilla M. Covaa, Alessio Zuliania, Mario J. Muñoz-Batistaa and Rafael Luquea, b*

aDepartamento

de Quimica Organica, Universidad de Cordoba, Edificio Marie-Curie (C-3), Ctra

Nnal IV-A, Km 396, Cordoba, Spain bPeoples

Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya str., 117198,

Moscow, Russia

Corresponding Author: [email protected] (R. Luque)

KEYWORDS: Zinc sulfide, Pig bristles, Microwave chemistry, selective oxidations

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ABSTRACT Zinc sulfides are emerging as promising catalysts in different fields such as photochemistry or organic synthesis. Nevertheless, the synthesis of ZnS compounds normally requires the utilization of toxic sulfur precursors e.g. thiourea which is a contaminant and carcinogenic agent. As a result, new green and sustainable synthetic methodologies are needed. Herein, an innovative, simple and cheap approach for the synthesis of ZnS carbon composites is reported. Zinc acetate dihydrate was employed as metal precursor while wasted pig bristles were employed as carbon and sulfur source. The phase and the morphology of the compounds were analyzed by XRD, XPS, SEM and EDX and the surface area was determined by nitrogen physisorption. ZnS carbon materials showed remarkable peroxidase-like catalytic activity for two different model reactions: the liquid-phase selective oxidation of benzyl alcohol and toluene to benzaldehyde (conversions up to 63% and 29% and selectivities up to 86% and 87% respectively) using hydrogen peroxide as oxidant under microwave irradiation.


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INTRODUCTION

Zinc sulfide has been studied as an important compound due to its unique physical and photochemical properties. potentiality

for

different

1-3

It has demonstrated an extraordinary versatility and

applications

including

light-emitting

diodes

(LEDs),

electroluminescence, infrared windows, sensors, lasers, and biodevices.4-6 Additionally, ZnS possesses many interesting characteristics such as excellent transport properties, an intrinsically n-type semiconductivity, good thermal stability, high electronic mobility, nontoxicity, water insolubility, and low-production costs.4-7

All reported synthetic protocols for ZnS preparation involve the utilization of highly toxic sulfur sources including extremely hazardous compounds such as H2S, or Na2S as well as thioureas, highly contaminant and carcinogenic agents.8-10 New green and low-toxicity sulfur sources are therefore required for sustainable development. In this context, pig bristles represent a cheap and largely available source of sulfur (and carbon). Pig bristles are considered as a waste toxic-free feedstock, easily collectable and accessible at industrial scale. In fact, ca. ~225k tons of wasted pig bristles are yearly produced only in European slaughterhouses.11 However, only a few works to date are available on the reutilization/use of pig bristles as food fodder supplements, while their valorisation as chemical source is extremely limited.11-13



Herein, a simple and innovative synthesis of ZnS carbon composites derived from wasted pig bristles is reported. The synthetic procedure involved a facile heating step in a diluted aqueous solution of potassium hydroxide. Wasted pig bristles were employed as sulfur and carbon source while zinc acetate dihydrate was used as metal source. The material was characterized by XPS, XRD, N2 physisorption, SEM and EDX, demonstrating the successful formation of zinc sulfides carbon compounds.

In order to validate the practical application of the new material, ZnS carbon composites were tested as catalysts in two different model reactions: the selective oxidation of benzyl alcohol and toluene to benzaldehyde. Among all known oxidative transformations, the selective oxidations of alcohols to the corresponding carbonyl compounds have gained attention due to their broad range of industrial applications.14,

15

Specifically, the conversion of

benzyl alcohol to benzaldehyde has attracted significant interest since benzaldehyde is a widely used chemical in food, pharmaceutical, and perfumery industries as well as building block in other chemical industries.16-22 For example, benzaldehyde is commonly used as a flavor and fragrance agent tasting of almond, peach, marzipan and pistachio. In addition, benzaldehyde can be also synthesized via toluene oxidation.23 Considering that toluene is classified as highly pollutant chemical, its oxidation to beneficial chemicals products i.e. benzyl alcohol, benzaldehyde, benzoic acid and benzoate is extremely attractive.24, 25 For example, benzoic acid is industrially obtained from toluene using (thermal-driven) Co-based catalyzed reactions.26


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ZnS carbon compounds were particularly tested in fast microwave-assisted reactions, using only hydrogen peroxide as oxidant. Less aggressive oxidants such as hydrogen peroxide (H2O2) and molecular oxygen (O2) are nowadays employed aiming to scale up oxidation processes,14 whereas in the past the scale up of the oxidation reactions has been very limited due to the use of heavy metals (e.g. chromium, manganese and permanganate derivatives).27-30 In addition, the utilization of hydrogen peroxide allows the minimization of chemical waste in these catalytic processes, as water is the only reaction side-product. MATERIALS AND METHODS Materials. Zinc acetate dihydrate (Zn(CH3COOH)2·2H2O), potassium hydroxide (KOH), acetone (CH3COCH3), ethanol (CH3CH2OH, 99.8%) acetonitrile (CH3CN, 99.8%), benzyl alcohol (C6H5CH2OH, 99.8%), toluene (C7H8, 99.8%) and hydrogen peroxide (H2O2, 30% v/v) were purchased from Sigma-Aldrich Inc., St. Louis, MO, USA. All reagents were used without any further purification. Synthesis of ZnS carbon catalysts. A sequence of three ZnS carbon structures characterized by different synthesis times were prepared. In a typical procedure, the correct amount of zinc acetate dihydrate (850 mg, 3.8 mmol) was dissolved in 200 mL of 1.0 M KOH solution in a 250 mL round flask equipped with a stirring bar and a reflux condenser. Sequentially, 1.5 g of pig bristles (cut in pieces of ~5 mm) were added. The mixture was kept at room temperature under vigorous magnetic stirring (800 rpm) for 10 minutes before starting the reaction. The mixture was then heated at reflux in an oil bath. After completing the reaction, the solution was naturally cooled down to r.t. and the light-brown precipitate was filtrated and washed several times with acetone and ethanol. Finally, the material was oven-dried at 100 °C for 24 hours. Three different samples were produced by



setting the heating time for 1 hour, 3 hours and 5 hours. The samples were denoted ZnS-1h, ZnS3h and ZnS-5h respectively.

Material characterization. ZnS carbon composites were characterized by Scanning electron microscopy (SEM), Energy Dispersive X-ray Analysis (EDX), Powder X-ray diffraction (XRD), N2 physisorption and X-ray photoelectronic spectroscopy (XPS). Scanning electron microscopy images were recorded with a JEOL JSM-6300 scanning microscope (JEOL Ltd., Peabody, MA, USA) equipped with Energy-dispersive X-ray spectroscopy (EDX) of 15 kV at the Research Support Service Center (SCAI) from University of Cordoba. Powder X-ray diﬀraction (XRD) patterns were recorded using a Bruker D8 DISCOVER A25 diffractometer (PanAnalytic/Philips, Lelyweg, Almelo, The Netherlands) using CuKα (λ=1.5418Å) radiation. Wide angle scanning patterns were collected over a 2θ range from 10° to 80° with a step size of 0.018° and counting time of 5 s per step. Textural properties of the samples were determined by N2 physisorption using a Micromeritics ASAP 2020 automated system (Micromeritics Instrument Corporation, Norcross, GA, USA) with the Brunauer-Emmet-Teller (BET) and the BarretJoyner-Halenda (BJH) methods. Prior to analysis, the samples were outgassed for 24 h at 100 °C under vacuum (P0 = 10−2 Pa) and subsequently analysed. XPS studies were performed at the Central Service of Research Support (SCAI) of the University of Cordoba, using an ultrahigh vacuum (UHV) multipurpose surface analysis system SpecsTM. The experiments were carried out at pressures

A sustainable approach for the synthesis of catalytically active

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