Fully recyclable gold-based nanocomposite catalysts with enhanced reusability for catalytic hydrogenation of p-nitrophenol

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Abstract

Nanocomposite (NC) materials, which demonstrate catalytic activity under mild conditions, have garnered considerable attention due to the environmental hazards associated with aromatic nitro compounds. Herein, a novel in-situ synthesis of NCs with zero-valent Au (Au°) is proposed. These materials are based on suspension copolymers with molecular reactors that enable the fabrication of Au° and the enhancement of both the catalytic activity and reusability. NCs were obtained using amino-based nanoreactors (3–14 mmol per gram of a polymer) derived from polyethyleneimine (PEI), 1-(2-hydroxyethyl)piperazine (HEP), 1,4-bis(3-aminopropyl)piperazine (APP). The resultant Au@PEI, Au@HEP, and Au@APP NCs were investigated using optical microscopy, SEM, and TEM. Additionally, the physiochemical structures of both the polymeric matrix and Au° were evaluated using, i.a., FT-IR and XRD. The so-obtained NCs were used as nanocatalysts (NCats) for the catalytic hydrogenation of 4-nitrophenol (4-NP) under mild conditions. The method developed for the synthesis of NCs produced gold-based NCats with 25–37 % Au° content. The synergism between the polymeric matrix and Au° enabled the hydrogenation of 4-NP within approx. 30―38 min with a rate constants of 0.071 min−1 (Au@HEP) and 0.028 min−1 (Au@APP) and, respectively. The spherical millimetric morphology of the polymeric support facilitated the facile recyclability of NCats that maintained their catalytic activity for more than 10 cycles of 4-NP reduction.

Introduction

Several types of chemical industries rely on aromatic nitro compounds. Among these, 4-nitrophenol (4-NP) is particularly important because it is indispensable in agricultural and pharmaceutical industries for the production of plant protection products and pharmaceutical formulations, including drugs containing paracetamol or acetaminophen (alternative to aspirin) [1]. However, 4-NP exhibits toxic, mutagenic, and cariogenic characteristics, which, combined with its increased release into water, render it one of the most hazardous chemicals. Consequently, 4-NP is recognised as a high priority risk by the American Environmental Protection Agency (EPA). Thus, 4-NP must be removed from production sources [2].

The most popular method for neutralising 4-NP involves its reduction to 4-aminophenol (4-AP). The reaction is readily controlled and yields only one product (4-AP), which is an important intermediate in the synthesis of paracetamol [3]. Moreover, if a suitable catalyst is utilised, the reduction of 4-NP to 4-AP can be conducted in water under mild conditions. Compared to other different catalysts, those based on noble metal nanoparticles (NPs) have been found to be particularly effective. Nanomaterials (NMs) such as AuNPs, PtNPs, PdNPs, and AgNPs demonstrate increased activity resulting from the high active surfaces [[4], [5], [6]] and are active in a wide range of conditions [5]. However, there are also limitations in that NPs reveal limited stability because they tend to sediment. Furthermore, it is extremely challenging to remove NPs from the reaction environment, even if a stabiliser is applied [7].

To resolve the issues limiting the utility of NPs as catalysts, it is necessary to identify a suitable support that does not restrict the functionality to stabilising NPs but also enhances the catalytic activity [5,8,9]. To date, considerable effort has been devoted to the research of novel nanocomposites (NCs) that address the issues regarding the utility of noble metal NPs. Recently, 4-NP was successfully reduced to 4-AP in the presence of nanocatalysts (NCats) containing AgNPs immobilised on carbon nanotube-modified Stachys lavandulifolia extract [10], PdNP NCats loaded onto reduced graphene oxide [11], AuNPs supported on aluminium oxide and titanate nanotubes [12], and Au/AgNPs coated on alkali-activated sand [13]. Some reports indicate the synergistic effect between NPs and an inorganic support leading to enhanced NCat activity towards the reduction of 4-NP [14]. However, the most desirable solutions include organic-inorganic NCs because they can lead to significant increased NCat activity by the formation of highly porous metal-organic frameworks (MOFs) [[15], [16], [17]] or styrene (St)-based copolymers, thereby revealing affinity towards 4-NP [18,19]. According to the literature, the increased activity of organic-inorganic NCats can be enhanced even further by stabilising the noble metal NPs into amino functionalities loaded on both the inorganic or polymeric supports [[17], [18], [19], [20], [21], [22], [23]]. Further, based on previous studies [18,19,24], different amino derivatives can also play a capping- stabilising role, thus leading to an efficient reduction of Au(III) and Pd(II) to AuNPs and PdNPs without any external reducing agents. As compared to other studies, this approach enables the control of size and the amount of reduced Au that is essential for NCat activity. Moreover, the method enables the free adjustments of a polymeric matrix, thus making NC catalysts extremely easy to be used [5,18].

Consequently, the aim of this study is to completely exploit the synergistic effect between the polymeric matrix and amino functionalities on three different levels. First, the amino derivatives incorporated into St-based copolymers containing N and O chelating atoms should dramatically increase the amount of reduced Au, thus producing NCs covered by Au nanodust. Second, the so-obtained NCs should reveal satisfactory catalytic activity towards the reduction of 4-NP, which will be attributed to the synergism between the polymeric matrix, capping/stabilising amine, and reduced Au. Third, the type of polymeric matrix should enable easy and facile reuse of a catalyst without losing its efficiency. To achieve these goals, a series of NCs based on vinylbenzyl chloride-co-divinylbenzene (VBC-co-DVB) copolymers was synthesised during the reduction-coupled adsorption of AuCl4 on the functionalities derived from polyethyleneimine (PEI), 1-(2-hydroxyethyl)piperazine, and 1,4-bis(3-aminopropyl)piperazine. The synthesis of polymeric bases (amino-functionalised VBC-co-DVB copolymers) was monitored using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and by determining the concentration of N introduced with the functionalities. The NCs were analysed using digital-optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Additionally, the crystalline structure and the amount of Au0 loaded into the NCs were analysed using X-ray powder diffraction (XRD) and atomic absorption spectroscopy (AAS) techniques. The NCats based on the obtained NCs were then used as heterogenous catalysts for the reduction of 4-NP to 4-AP within multiple runs. The reaction was monitored using ultraviolet-visible light spectrophotometry (UV–vis).

Section snippets

Materials

All the reagents used for the synthesis of the polymeric matrix and tests on the catalytic reaction, including VBC (99 %, mixture of o and p isomers), DVB (80 %), branched poly(ethylene imine) (PEI, 100 %, ∼25,000 g mol−1), 1-(2-hydroxyethyl)piperazine (98 %, HEP), 1,4-bis(3-aminopropyl)piperazine (>99 %, APP), and 4-nitrophenol (4-NP, 99 %) were acquired from Sigma-Aldrich Chemical Co. (Polish department, Poznan, Poland). Other chemicals used were purchased from Avantor Performance Materials

Synthesis of nanoreactors

The synthesised NMs are based on the amino-modified suspension VBC-co-DVB copolymer. Au0 obtained on the polymer surface resulted in the formation of NCs. This was dependent on the PEI, HEP, and APP because the amines act as molecular rectors for the reduction of AuCl4. Hence, it was necessary to determine whether the amines were actually introduced onto the surface of the VBC-co-DVB copolymer by analysing the ATR-FTIR spectra displayed in Fig. 2.

All the positions of the discussed bands are

Conclusion

A series of new nanocomposite catalysts were synthesised in the present study. The synthesis involved the preparation of nanoreactors introduced into a polymeric matrix, in which the N and O chelating atoms spontaneously reduced the AuCl4. The in-situ reduction of AuCl4- was dependent on the number of nanoreactors introduced in association with the amino functionalities. As a result, this self-propagating process proved to be extremely effective, as the NCs exhibited increased content of Au0

CRediT authorship contribution statement

Piotr Cyganowski: Conceptualization, Methodology, Validation, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing, Visualization, Project administration, Project administration, Formal analysis, Funding acquisition, Methodology, Visualization, Project administration.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

This work was supported by the subsidy granted to the Wroclaw University of Science and Technology from the funds of the Polish Ministry of Science and Higher Education.

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