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Article

Self-Supporting g-C3N4 Nanosheets/Ag Nanoparticles Embedded onto Polyester Fabric as “Dip-Catalyst” for Synergic 4-Nitrophenol Hydrogenation

by
Abdallah Amedlous
1,*,
Mohammed Majdoub
1,*,
Zakaria Anfar
2 and
Elhassan Amaterz
2
1
Laboratory of Materials, Catalysis & Valorization of Natural Resources, Hassan II University, Casablanca 20000, Morocco
2
Laboratory of Materials & Environment, Ibn Zohr University, Agadir 80000, Morocco
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(12), 1533; https://doi.org/10.3390/catal11121533
Submission received: 17 November 2021 / Revised: 5 December 2021 / Accepted: 9 December 2021 / Published: 16 December 2021
(This article belongs to the Special Issue Advanced Functional Nanomaterials for Catalysis and Environmental Applications: Trends and Outlooks)
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Abstract

:
Herein, we report the design of a cost-effective catalyst with excellent recyclability, simple recuperation and facile recovery, and the examination between the reaction cycles via the development of self-supporting g-C3N4 nanosheets/Ag NPs polyester fabric (PES) using a simple, facile and efficient approach. PES fabrics were coated via a sono-coating method with carbon nitride nanosheets (GCNN) along with an in situ setting of Ag nanoparticles on PES coated GCNN surface producing PES-GCNN/Ag0. The elaborated textile-based materials were fully characterized using FTIR, 13C NMR, XRD, TGA, SEM, EDX, etc. Catalytic performance of the designed “Dip-Catalyst” demonstrated that the as-prepared PES-GCCN/Ag0 has effectively catalyzed the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH4. The 3 × 3 cm2 PES-GCNN/Ag0 showed the best catalytic activity, displaying an apparent rate constant (Kapp) equal to 0.43 min−1 and more than 10 reusability cycles, suggesting that the prepared catalyst-based PES fabric can be a strong nominee for sustainable chemical catalysis. Moreover, the coated fabrics exhibited appreciable antibacterial capacity against Staphylococcus epidermidis (S. epidermidis) and Escherichia coli (E. coli). The present study opens up new opportunities for the future design of a low cost and large-scale process of functional fabrics.

Graphical Abstract

1. Introduction

Nitro-aromatic compounds are considered one of the most frequent and stubborn organic pollutants found in industrial wastewaters due to their wide applications in several industries, such as pigments, dyes, plastics, pharmaceuticals, pesticides and fungicides [1]. Among them, 4-nitrophenol (4-NP) has been classified as a major pollutant by the US Environmental Protection Agency (EPA), owing to its potential toxicity toward human beings, fauna and flora [2]. On the contrary, 4-aminophenol (4-AP) (4-NP reductive derivative) is a valuable intermediate product used in the fabrication of paracetamol, clofibrate drugs, antioxygens and petroleum mixtures [3,4,5]. Therefore, it is highly important to adopt a sustainable clean route for treating such compounds in aqueous medium. In this regard, catalytic hydrogenation of 4-nitrophenol into 4-aminophenol has been the focus of several studies during the recent years [6,7,8].
Noble metal nanoparticles (NMNPs) have gained tremendous interests due to their fascinating physicochemical properties that are generally superior to those of the bulk species [9,10]. Outstandingly, silver nanoparticles (Ag NPs) have drawn a great interest from the scientific community owing to their distinctive qualities, including antibacterial traits, relatively low cost and high catalytic activity [11,12]. By controlling factors such as dispersion and size of the nanoscale metallic silver, various silver nanoparticle-based catalysts having multiple features have been developed [13,14,15]. However, silver nanoparticles suffer from some shortcomings, mostly associated to their toxicity, leading to environmental contamination, depletion of rare resources and health issues. Additionally, silver nanoparticles tend to form colloidal agglomerate throughout the catalytic operation, giving rise to unavoidable reduction in the catalyst active surface area leading to a decay of the catalytic behavior in an undergoing procedure [16]. To overcome these limitations, immobilization and stabilization of metallic nanoparticles onto high surface area inert matrices (support) has been demonstrated to be a potent method to prevent agglomeration [17,18,19].
Encapsulating metal nanoparticles on inert matrices has recently shown promise in preserving intrinsic properties and ensuring longevity of chemical stability, as well as achieving unique collective properties and uses by combining the features of separate components. Textile materials have emerged as an intriguing new class of supports owing to their many benefits, such as environmental sustainability, good flexibility, lightweight, high porosity and relatively low cost. During the last decade, functional polyester fabrics with distinctive properties, such as self-cleaning, UV absorption, antimicrobial and photocatalytic activities, have been prepared through immobilization of nanomaterials [20,21,22,23,24,25]. The key parameters for achieving high performance polyester materials are the compatibility and the stability of metal nanoparticles on polyester’s surface. The interfacial interactions and the dispersion of metal nanoparticles on polyester’s surface are two crucial factors to achieve high performance textile materials. Therefore, homogenous and stable dispersion of nanoparticles is very well desired to prevent the aggregation causing uneven distribution of metal nanoparticles on polyester’s surface due to the poor wettability and lack of functional groups [26]. Various types of surface modification approaches have been used to improve polyester properties such as UV irradiation, plasma treatment, ozone oxidation hydrolysis and aminolysis [27,28]. However, the majority of these methods are time and energy consuming and tend to use highly toxic chemicals. To overcome these issues, methods of dip coating with various materials are proposed to be an alternative approach due to their high adhesion, high functionality, reproducibility and suitability to prepare on a large scale. Among them, atomically 2D thin-layered structures, such as graphene and its derivatives [29], graphitic carbon nitride nanosheets (GCNN), hexagonal boron-nitride (h-BN) and transition-metal dichalcogenides (TMDCs), are emerging as fascinating materials for a wide range of applications due to their unique physical properties. In particular, graphite-like carbon nitride nanosheets (GCNN) have attracted a lot of scientific interest because of their high adsorption capacity, good electron conductivity, large specific surface area, thermal stability and acid–alkali chemical resistance [30,31,32]. Moreover, GCNN possess a well crystallized lamellar structure that can promote charge transfer [33] and exhibit many π–π conjugations on the surface due to its nature as a π–conjugated 2D material. Besides, GCNN can be easily loaded by other materials because of their similarity with soft polymers [34,35]. For the prior reasons, GCNN are an ideal material for coating PES fabric, which can enhance the compatibility and the adhesion of Ag NPs on the PES surface; therefore, a superior catalytic activity can be achieved.
A part of our research strategy is devoted to the elaboration of multifunctional textile materials with enhanced physicochemical, catalytic and antibacterial properties for numerous applications, including catalytic conversion toward water decontamination. The present study has been conducted to extend the implementation scope of g-C3N4 nanosheets as 2D modifiers for the modification of textile fabric surface. More specifically, this work aims to confer new functionalities to the polyester fabric by incorporating a low fraction of g-C3N4 nanosheets followed by the deposition of compatible silver nanoparticles. Until now, very limited reports have dealt with the modification of textile materials for metal nanoparticles’ incorporation. In fact, our group has dealt with the surface modification of polyester fabric by 2D MoS2 nanosheets to enhance its compatibility and affinity with metal nanoparticles. The results showed that the MoS2 nanosheets deposition has increased the adhesion properties of silver nanoparticles onto the textile fabric [36].
Inspired by those findings, two main subjects are of interest in this study. The first one is related to g-C3N4 as a novel 2D carbon-based modifier for the improvement of the adhesion properties of textile fabric. The second concern is related to the investigation of the influence of the deposition of silver nanoparticles on g-C3N4 coated PES fabric on the physicochemical, catalytic and antibacterial properties. To feature the two matters, we first described the operatory protocols to produce g-C3N4 nanosheets from urea alongside its full structural, morphological and thermal characterizations. Then, we incorporated g-C3N4 nanosheets into the PES fabric and, afterwards, the Ag nanoparticles were in situ generation via sono-chemical method producing PES-GCNN/Ag0 fabrics with different Ag0 loadings. The produced coated fabrics were fully characterized by means of several physicochemical characterizations, including XRD, FTIR, SEM/EDX and TGA. Furthermore, the catalytic performance and antibacterial activity of the produced “Dip-Catalyst” toward 4-NP hydrogenation were investigated in detail. The obtained findings open up new opportunities for the future design of the low cost and large-scale process of functional fabrics with promising catalytic and antibacterial properties.

2. Results and Discussion

2.1. Characterization of Graphitic Carbon Nitride (GCN)

The transformation of urea into graphitic carbon nitride nanosheets (GCNN) was conducted via thermal treatment and was confirmed by FTIR, XRD, TGA and 13C NMR, as shown in Figure 1.
Figure 1a displays the FTIR spectra of bulk and exfoliated GCN to further demonstrate their graphitic structures. Both samples exhibited quite similar absorption bands in terms of shape and position but with different intensities. The peaks intensities of exfoliated GCN slightly increased with respect to that of bulk GCN, indicating that the sono-exfoliation process has successfully occurred. The absorption bands located at 810 and 887 cm−1 were attributed to the deformation mode of the N–H bond resulting from the incomplete condensation of amino groups and the typical breathing mode of tri-s-triazine units, while the absorption bands at the region of 1200 to 1700 cm−1 were ascribed to the typical skeletal stretching modes of s-triazine derivatives, such as bridging C–NH–C or trigonal (N–(C)3) units (heterocyclic C–N) [37,38]. Both exfoliated and bulk GCN FTIR spectra displayed a broad absorption band at 3000–3500 cm−1 corresponding to the adsorbed H2O molecules and N–H vibrations of the uncondensed amine groups [39].
X-ray diffraction was used to determine the crystal structure of both bulk and exfoliated GCN (XRD). The layer-to-layer distance of GCN materials was also determined using this method. As shown in Figure 1b, two peaks at 13.2° and 27.7° in both samples’ XRD patterns could be properly indexed. The diffraction peak at 13.2° (0.67 nm) corresponded to the in-planar structural packing motif, whereas the 27.7° peak (0.32 nm) is attributed to the usual layer-by-layer stacking of the conjugated aromatic groups [40]. It is worth mentioning that the exfoliated GCN exhibited a significant decrease in both peaks at 27.7° (002) and 13.2° (100), which suggest that the exfoliation process via ultrasonication has successfully occurred since GCNN were generated via the exfoliation of bulk GCN. Nevertheless, the peak positions remained constant which indicates that the layered GCN structure remains in the exfoliated GCNN, keeping the interlayer distance constant [41].
Thermogravimetric analysis (TGA) was utilized to assess the synthesized material’s thermal stability. As can be seen from Figure 1c, at temperatures between 500 and 700 °C, the GCNN sample showed a rapid and uniform weight loss, corresponding to the thermal breakdown of the carbon nitride backbone into nitrogen and carbon-containing gaseous species.
The 13C cross-polarization magical angle spinning (13C CP-MAS) solid state NMR analysis was also carried out to assist in the structural elucidation of the GCNN sample, with the result being shown in Figure 1d. According to this figure, the 13C MAS NMR of the GCNN sample prepared via thermal treatment exhibited two apparent and typical resonance groups: the first resonance groups at 163.0 and 165.1 ppm were attributed to the carbon atoms (C1) connected with –NH2 groups, also called CN2(NH2) groups, whereas the other resonance at 156 ppm was associated with the carbon species in heptazine units and linked with bridging N atoms(C2), also referred as CN3 groups of the cyameluric nucleus. These assignments are in good accordance with those reported of graphitic carbon nitride [42,43,44], further indicating the prepared GCNN were mainly consisting of heptazine tectonic units.
The morphologies of the as-prepared GCNN sample were examined by scanning electron microscopy (SEM). From Figure 2a,b, it is clearly seen that GCNN were composed of irregular and lamellar folded-like wrinkled sheets, where the layers stacking can be observed. TEM observations were performed to have more insights about the layer–layer morphology of the elaborated GCCNN material. According to Figure 2c,d, the elaborated GCNN consisted principally of multilayer edgeways nanosheets, forging a well-defined 2D morphology. In addition, GCNN were distinguished by the presence of small size characteristic folds and creases, indicating the attainment of a high exfoliation degree using ultrasonication treatment. Furthermore, the examinations demonstrated that the GCNN specimen consists majorly of piled small size nanosheets with a relatively translucent, fluffy and rippled aspect.

2.2. Characterization of PES Based Materials

ATR-FTIR spectroscopy was used to examine the chemical structure of non treated and coated PES based catalyst, respectively. By using FTIR, the potential interfacial interactions occurring between PES, GCNN/Ag0 coatings were identified. Figure 3 shows typical ATR-FTIR spectra of neat PES, PES-GCNN and PES-GCNN/Ag0 10 textiles, as well as PES-GCNN/Ag0 fabrics.
As can be seen from this figure, the FTIR spectra of all the fabric based samples exhibited the essential absorption bands of polyester functional moieties, particularly the presence of the following absorption bands: 1712 cm−1 (C=O stretching of aromatic ester), 1250 cm−1 (aliphatic ester C=O stretching vibration), 1018 cm−1 (in-plane vibration of benzene), 1095 cm−1 (ester C=O stretching), 972 cm−1 (C–O stretching of glycol) and 725 cm−1 (out of plane benzene ring group) [45,46]. The FTIR spectra of the PES-GCNN sample exhibited some remarkable differences, counting a well noticed decrease in the carbonyl stretching vibrations’ intensity (1712 and 1250 cm−1). This indicates that GCNN established strong interfacial interactions with the PES fibers, which may occur between the aromatic/aliphatic ester moieties of PES and N–H/C–N groups of the heptazine units in GCNN backbone, or through π–π interactions between the benzene rings of PES fabric and s-triazine derivatives, thus giving rise to a homogeneous GCNN coating on PES surface. Moreover, the emergence of new peaks at 3249 and 3165 cm−1 in the FTIR spectra of PES/GCNN corresponding to the N–H vibrations of the uncondensed amine groups of GCNN gave additional supports to the establishment of strong interactions between the PES and GCNN.
In comparison with the PES-GCNN sample, the PES-GCNN/Ag0 10 fabric showed no noteworthy changes with the absence of silver nanoparticles vibrational bands which were overwhelmed by the PES-GCNN fabric owing to the low loading of Ag nanoparticles on the sample’s surface.
X-ray diffraction (XRD) was used to determine the crystalline phase of uncoated PES, PES-GCCN and PES-GCCN/Ag0 samples (Figure 4). According to this figure, the XRD diffractogram of unmodified PES revealed the emergence of three broad diffraction peaks at 2-theta = 17.51°, 22.71° and 25.61°, indicating the existence of crystal regions allocated to the PES crystalline phase/hard segment (semi-crystalline PES) (Lu et al. 2011). The PES-GCNN fabric did not exhibit any apparent extra peaks associated with GCNN, indicating that the GCNN coating had no effect on the crystallinity of neat PES. After conducting an in-situ reduction of Ag+ to Ag NPs (Ag0) onto PES-GCNN, the XRD patterns of PES-GCCN/Ag0 10 revealed the emergence of four prominent peaks at 2-theta = 37.47°, 43.69°, 63.97° and 77.02°, which were assigned to the cubic Ag0 planes (111), (200), (220) and (311), respectively. The Debye–Scherrer equation was used to determine the average crystallite size, which was 13.3 nm.
The thermal degradation profile of unmodified and modified PES based catalysts was examined using TGA under air flux with 10 °C min−1 rate. The thermograms of all samples are shown in Figure 5, and the parameters derived from them are given in Table 1. As can be seen from Figure 5, it is clearly noticeable that all coated fabrics, regardless of coating nature, principally undertake two recognizable decomposition stages. (1) A first decomposition stage occurs between 300 and 496 °C, assigned to the thermal decomposition of ester bond in the polyester sample, which arises from the chain scission based depolymerization along the polyester giving rise to vinyl- and carboxy-ended chain fragments that produce volatile cyclic oligomers. (2) The subsequent thermal degradation step occurs at up to 610 °C, which is attributed to the cleavage of carbon–carbon linkage, giving rise to the generation of volatile gazes including CO, CO2, CH4, benzene, ethylene, etc. [47]. In comparison with pristine PES, the PES-GCNN and PES-GCNN/Ag0 10 fabrics showed improved thermal stability, which can be assigned to the strong non-covalent interactions setteled between the GCNN and GCNN/Ag0 coatings and the PES fabric. For instance, coating PES fabric with GCNN and GCNN/Ag0 resulted in an increase in the thermal decomposition temperature at maximum rate (Tdmax) for the first degradation stage from 419.1 to 439.3 (Table 1). Generally, the thermal degradation of the coated samples occured at an elevated temperature compared to neat PES. The consequential increase in Tdi (300.1 °C for PES vs. 308.6 °C for PES-GCNN and 315.5 for PES-GCNN/Ag0 10) and Tdm (489.1 °C for PES vs. 463.2 °C for PES-GCNN) for the first stage of thermal decomposition gave further proof that GCNN and GCNN/Ag0 coatings has strongly boosted the general thermal decompostion resistance of PES specimens. However, it is clearly noticeable that the PES-GCNN/Ag0 TGA curve exhibited a rapid second thermal degradation stage, i.e., cleavage of the C–C bond, in comparison with pristine PES and PES-GCNN, which can be ascribed to the catalyzing effect of Ag0 over the C–C scission in the PES-GCNN thermal degradation [48]. Further information worth mentioning is that the residual weight fraction of PES-GCNN/Ag0 was found to be approximetly 10 wt%, which is considered to be the content of Ag nanoparticles in the PES-GCNN/Ag0 10 fabric.
SEM examinations were conducted to investigate the morphology of the textile-based catalyst and GCNN/Ag0 degree of dispersion onto the PES surface. SEM micrographs of textile-based catalysts are given in Figure 6. Pristine PES exhibited a well ordered surface, which is composed of extended, intertwisted and knotted microfibers. SEM micrographs of the elaborated PES-GCNN sample clearly displayed a complete change in the surface morphology, going from a microsmooth morphology for pristine PES (Figure 6a,d) to a microrough morphology for PES-GCNN (Figure 6b,e), owing to the dip-coating process via ultrasonication that grants a full wrapping of PES microfibers with GCNN, thus improving the adhesion characteristics. Following the depostion of silver nanoparticles on the PES-GCNN surface via in situ reduction, SEM micrographs of PES-GCNN/Ag0 10 (Figure 6c,f) revealed similar roughnesss to that of the PES-GCNN sample, accompanied with the appearance of deposited silver nanoparticle (Ag0) onto its surface. The in situ deposited Ag0 mainly consisted of homogeneous and uniform quasi-spherical nanoparticles with no sign of aggregation (inset of Figure 6f). Herein, the preceding GCNN coating on the PES surface had the capability to enhance the compatibility and adhesion properties of PES fibers, thus allowing the homogeneous capture and deposition of Ag nanoparticles.
In order to confirm the uniformity of Ag NPs distribution, EDX mapping was carried out under SEM examinations. The EDX result, as given in Figure 7, in addition to elemental mapping images, demonstrated that the produced PES-GCNN/Ag0 10 fabric was comprised of C, O, N and Ag elements. The mapping images also demonstrated a relatively homogeneous and uniform Ag element distribution in the fabric sample, which implied that Ag NPs were uniformly anchored on the PES-GCNN/Ag0 10 surface.
To test how well the deposited Ag nanoparticles adhered to PES-GCNN fabric, we immersed different substrates, including PES-GCNN, PES-GCNN/Ag010 and PES-Ag0 as a reference fabric, in ethanol solution. Afterwards, the samples were rapidly stirred at 500 rpm for 24 h. A total of 3 mL of the reaction mixture was collected at 1 min intervals throughout the experiment. The excellent adherence of Ag nanoparticles on the PES-coated GCNN differs markedly from the samples that lacked the GCNN where the deposited Ag easily slid off the surface by simple shaking, as displayed by the weight change in Table 2.

2.3. Antibacterial Activity and Antibacterial Adhesion

As can be seen from Figure S1, neat PES and PES-GCNN fabrics did not show any antibacterial activity. In contrast, silver nanoparticles-filled PES-GCNN fabric (PES-GCNN/Ag0) displayed a relatively significant bactericidal efficiency to both Staphylococcus aureus and Escherichia coli. In the case of Staphylococcus aureus, the PES-GCNN/Ag0 sample displayed an inhibition zone up to 3 mm (Figure S1a), while it revealed an inhibition zone of 4 mm for Escherichia coli (Figure S1c). The bactericidal mechanism of silver nanoparticles is not fully understood in the state of the art. The most accepted one consists of the release of Ag+ ions by silver nanoparticles, affecting the metabolic processes and the mechanism of cell division, resulting in serious problems in the membrane permeability, thus the death of the bacteria [49,50].
Figure S1b,d shows the adhesion of both Staphylococcus aureus and Escherichia coli on the surface of PES based samples. There were no bacterial colonies around and under PES-GCNN/Ag0 round pieces after the incubating time. In contrast, some bacterial colonies grew around and under the neat PES and PES-GCNN fabrics. This phenomenon was observed in Staphylococcus aureus and Escherichia coli. Hence, the PES-GCNN/Ag0 fabric exhibited significant improvement in the property of antibacterial adhesion. These findings demonstrated that the produced PES-GCNN/Ag0 fabrics an appropriate candidate in a variety of industrial applications that require materials with long term antibacterial and anti-adhesion properties.

2.4. Catalytic Hydrogenation of 4-Nitrophenol over PES-GCNN/Ag0

To evaluate the catalytic performance of various as-prepared PES-GCNN/Ag0 catalysts, reduction of 4-NP to 4-AP in the presence of excess NaBH4 was chosen as a model reaction. As it is well known, the light-yellow 4-NP aqueous solution (0.03 mM) exhibits an absorption peak at around 318 nm. After addition of NaBH4 (0.1 M), the color of the solution changes into greenish yellow and the maximum of this peak shifted to 400 nm due to the formation of 4-nitorphenolate ions (Figure 8a,e).
The changes related to the concentration (Ct/C0) of 4-nitorphenolate as a function of reaction time using a series of Ag0-containing catalysts is shown in Figure 8c,d. It should be noted that no conversion of 4-NP to 4-AP was obtained after 3 days in the absence of a catalyst. In addition, it can be seen from the obtained result that PES-GCNN is inactive for reduction of 4-nitorphenolate. In contrast, the addition of different amounts of Ag0, the catalytic activity of PES-GCNN/Ag0 samples gradually elevated with the increased amount of Ag NPs. This may suggest that Ag0 NPs can facilitate electron transfer from BH4− to 4-nitrophenolate, and thus lowers down the barrier of activation energy for reducing 4-NP to 4-AP.
The reaction using the PES-GCNN/Ag0 10 catalyst showed the best catalytic activity. Figure 8b shows ultraviolet–visible spectra for successive reduction of 4–NP with NaBH4 using the PES/GCNN-Ag0 10 catalyst. When the PES/GCNN-Ag0 10 catalyst is introduced into the solution, the absorption peak at 400 nm decreases with the time of reaction; in contrast, a new peak appears at λmax = 300 nm attributed to 4-AP and its intensity simultaneously elevates. It is worth mentioning that increasing the loaded amount of Ag NPs increased the reaction rate. For instance, the conversion of 4-NP displayed 90% after 5 min of reaction time using the PES/GCNN-Ag0 10 sample, with apparent rate constant (Kapp) up to 0.462 min−1 (Figure 8d).
It was assumed that the surface porosity of fabric provides high sorption of both acceptor (4-NP) and donor (BH4−) species, which enhanced the rate of reduction by increasing electron transfer between the reacting substances [51]. This assumption was further reinforced by studying the effect of the surface of PES fabric coated with GCNN/Ag0 NPs on the 4-NP reduction process. Accordingly, we performed experiments varying the surface of catalyst (0.25, 1, 4 and 9 cm2), while reaction parameters such as concentration of 4-NP and NaBH4 were kept constant. Conversion profiles of the different surface of the catalyst are shown in Figure 9a. The rate constants for 0.25, 1, 4 and 9 cm2 are 0.032, 0.098, 0.154 and 0.462 min−1, respectively (Figure 9b). It was observed that the rate constant increased gradually with the increase in the surface of catalyst. The result means that: (1) the increase in the surface of the catalyst, the more active sites are generated, (2) a higher availability of surface for the adsorption of reacting species and (3) homogenous coating was obtained.

2.5. Recyclability and Stability of Catalyst

The recyclability and stability of the catalyst are the most important parameters of its application as a heterogeneous catalyst. As shown in Figure 10, the PES-GCNN/Ag010 catalyst can be reused for ten cycles in succession without significant reduction in its catalytic activity. In order to verify whether soluble catalytic species play a role in 4-NP reduction reaction, the PES-GCNN/Ag0 fabric is removed after 3 min of reaction and the filtrate obtained was stirred for an additional 30 min. Conversion percentage of 4-NP in the presence of the catalyst after 2 min of reaction showed 51%, while no increase in conversion of 4-NP was obtained in the catalyst-free. This result demonstrates that our catalyst was responsible for the reaction, and clearly rules out the possibility of leached Ag catalyzing the reaction. The catalytic and structural stability of PES-GCNN/Ag0 could be expected as a promising catalyst for the industrial production.
The 4-NP conversion over PES-GCNN/Ag0 NPs was compared with some other catalysts previously reported at different reaction conditions (concentrations of 4-NP, catalyst loading and reaction time), as summarized in Table 3. From this comparison data, we can clearly show that our catalyst exhibited an enhanced catalytic activity for 4-NP conversion, which is comparable and noticeably higher than previous reported silver-based catalysts.

2.6. Mechanism of 4-NP Reduction Reaction by PES-GCNN/Ag0

By adopting Langmuir–Hinshelwood model, it is possible to describe how the reduction mechanism of the reaction occurs when both reactants (BH4− and 4-NP) are adsorbed on the surface and inside PES pores in distinct catalytic active sites [58,59]. Figure 11 depicts a possible 4-NP catalytic hydrogenation route utilizing the PES-GCNN/Ag0 catalyst in NaBH4 presence. The BH4− ions react with the active surface of metallic Ag0 nanoparticles in reaction medium to generate active hydrogen species. This reaction occurs first in aqueous solution. Meanwhile, the generated 4-nitrophenolate ions present in the solution migrate into pores between the PES fibers, where they proceed to a reaction with active hydrogen-containing species, thus producing 4-hydroxylaminophenol as a stable intermediate that is eventually transformed to 4-aminophenol.

3. Experimental Section

3.1. Materials and Chemicals

A local Fez manufacturer supplied the polyester fabric (90 g/m2). 4-nitropheonol (C6H5NO3) was provided by Merck, as was silver nitrate (AgNO3, 99.9%), urea (CH4N2O), sodium borohydride (NaBH4) and ammonia solution (28%). All the experiments were conducted with bi-distilled water.

3.2. Preparation of Graphitic Carbon Nitride Nanosheets (GCNN)

Graphitic carbon nitride nanosheets (GCNN) were synthesized on the basis of the procedures previously reported in the literature with some modifications [60]. Typically, urea (15 g) was put in a covered crucible and calcined under air atmosphere at 550 °C for 4 h with a ramping rate of 2.5 °C/min. The powder was collected and placed in an open crucible and further heated at 500 °C for 2 h with a ramp rate of 5 °C/min. Then, the obtained brownish powder was washed several times with distilled water to remove any residual alkaline species (i.e., ammonia) and further drying in a vacuum oven at 60 °C overnight. The obtained powder was re-dispersed in aqueous medium and then exfoliated via ultrasonication using Brandson Digital Sonifer (S450D, 35% amplitude) to generate graphitic carbon nitride nanosheets (GCNN). Finally, the product was centrifuged and washed with distilled water and dried at 60 °C overnight.

3.3. Preparation of Polyester Coated GCNN and GCNN/Ag NPs

The PES-GCNN fabric was sono-coated. First, polyester fabric samples (8 × 3 cm) were pretreated with 1 g/L aqueous ammonia and 1 g/L nonionic detergent for 30 min at 60 °C. The textiles were then washed with distilled water and dried in a vacuum oven at 60 °C for 2 h. Exfoliated GCNN aqueous solution (1 mg/mL) was applied to prepare PES/GCNN specimens by dip-coating under ultrasonication for 45 min, washed with water/ethanol mixture to detach residual GCNN, then dried at 70 °C for 2 h. After the GCNN coating, the color was obviously changed from white to brown.
The resulting PES-GCNN (5 wt% of GCNN) fabric was submerged in an aqueous solution of AgNO3 (0.5 M) for 30 min before being subjected to an ultrasonication treatment at room temperature. Later, the sample was withdrawn from the AgNO3 solution and put in NaBH4 aqueous solution (1 M) for 30 min at ambient temperature, thus reducing the Ag+ metallic ions to Ag0 (Ag nanoparticles). Using distilled water, the PES-GCNN/Ag0 samples with different Ag0 wt% were cleaned to detach any non-reduced Ag+. Finally, the samples were dried at 60 °C for 2 h.

3.4. Catalytic Hydrogenation of 4-Nitrophenol

In order to investigate the catalytic performance of the elaborated textile-based catalyst, a freshly prepared NaBH4 aqueous solution (5 mL, 0.1 M) was added to a 4-NP solution (15 mL, 0.03 mM). Afterwards, PES-GCNN/Ag0 catalyst in a squared form was introduced by simple immersion to the above mixture. To analyze UV–vis absorption spectra (6705 UV/vis JENWAY) throughout the 4-NP reduction process, 3 mL of the reaction mixture was collected at 1 min intervals throughout the experiment. Immediately after completion of the reaction, the PES-GCNN/Ag0 fabric was removed from the reaction medium, washed with bi-distilled water and then dried to further examine its reusability.

3.5. Characterization Techniques

Infrared spectroscopy was used to investigate the chemical structure of the GCNN, and the textiles were generated using a Bruker Vector 22 instrument in transmission mode. The GCN and GCNN were softly crushed and pressed into KBr pellets for FTIR characterization. FTIR spectra were collected at 32 cm−1 resolution between 400 and 4000 cm−1. All spectra were collected after 128 scans.
Cross-polarization magic angle spinning (CP-MAS) 13C solid-state NMR spectra were recorded using the combined techniques of magic angle spinning (MAS) and cross polarization (CP). Experiments were performed at room temperature in a Bruker Avance III-600 spectrometer equipped with a 4-mm MAS probe.
The phase purity and composition of the sample were determined by X-ray diffraction (XRD) using a Model: D8 Discover Bruker AXS with CuKα radiation (λCu = 1.5407) and a scan speed of 0.1°/min.
TGA-Model Q500 (TA Instrument) was used to perform thermogravimetric analysis (TGA). The thermal stability was investigated by heating specimens of about 50 mg from 25 to 1000 degrees Celsius at a heating rate of 5 °C/min under air atmosphere with a flow rate of 90 mL/min−1.
The transmission electron microscopy (TEM) was carried out using JEOL ARM200 imaging apparatus. A carbon grid was used to deposit the samples, which were then allowed to dry before being examined.
Scanning electron microscopy (SEM) (AFEI Quanta 200-ESEM) was used to examine the surface morphology of the produced materials, with an accelerating voltage of 40 kV.
The adhesion experiments of the manufactured textile-based catalysts were performed as described by Lee et al. [61]. Typically, 30 mm squared specimens were first dried at 70 °C for 24 h, then weighted (m0) and stirred in ethanol solution at 500 rpm for 24 h. After stirring, the specimens were dried in air for 1 h, then vacuum dried at 80 °C overnight, and then weighed again (m) for the assessment of coating adherence.

4. Conclusions

In this study, we report a novel catalyst based on graphitic carbon nitride nanosheets (GCNN) coated on PES fabric and decorated with Ag NPs. The catalyst was prepared via dip-coating method followed by in situ reduction of Ag+ to Ag0 in its surface. Various physicochemical characterizations were used to study the possible interfacial interactions between PES, GCNN and GCNN/Ag0. The resulting catalytic activity of PES-GCNN/Ag0 on hydrogenation of 4-NP reaction showed the best catalytic activity and excellent recyclability with unchanged catalytic efficiency for more than 10 consecutive cycles. In addition, the results show that GCNN/Ag0 coatings on PES substrate can be beneficial to overcome difficulties of separation of powder. This study demonstrated that the as-prepared PES-GCNN/Ag0 can be a promising candidate for important organic conversions and industrial applications.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11121533/s1, Figure S1: Inhibition zones and bacterial adhesion of the fabrics based samples toward Saphylococcus aureus and Escherichia coli.

Author Contributions

Conceptualization, A.A. and M.M.; methodology, A.A. and M.M.; investigation, M.M., Z.A. and E.A.; data curation, M.M., Z.A. and E.A.; writing—original draft preparation, A.A. and M.M.; writing—review and editing, A.A., M.M., Z.A. and E.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors express their gratitude to the University Hassan II, Casablanca, and University Ibn Zohr, Agadir, for their commitment and encouragement toward young researchers to develop international works.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Catalysts 11 01533 g001 550
Figure 1. (a) FTIR of bulk and exfoliated GCN, (b) XRD diffractograms of bulk and exfoliated GCN, (c) TGA curve of GCNN, (d) 13C MAS NMR of GCNN and ‘’*” is the spinning sidebands.
Figure 1. (a) FTIR of bulk and exfoliated GCN, (b) XRD diffractograms of bulk and exfoliated GCN, (c) TGA curve of GCNN, (d) 13C MAS NMR of GCNN and ‘’*” is the spinning sidebands.
Catalysts 11 01533 g001
Catalysts 11 01533 g002 550
Figure 2. (a,b) SEM and TEM and (c,d) images of the prepared GCNN.
Figure 2. (a,b) SEM and TEM and (c,d) images of the prepared GCNN.
Catalysts 11 01533 g002
Catalysts 11 01533 g003 550
Figure 3. FTIR spectra of PES, PES-GCNN, PES-GCNN/Ag0 10 and GCNN.
Figure 3. FTIR spectra of PES, PES-GCNN, PES-GCNN/Ag0 10 and GCNN.
Catalysts 11 01533 g003
Catalysts 11 01533 g004 550
Figure 4. XRD patterns of uncoated PES fabric, PES-GCNN and PES-GCNN/Ag0 10.
Figure 4. XRD patterns of uncoated PES fabric, PES-GCNN and PES-GCNN/Ag0 10.
Catalysts 11 01533 g004
Catalysts 11 01533 g005 550
Figure 5. TGA and DTG thermogarms of PES, PES-GCNN, PES-GCNN/Ag0 10.
Figure 5. TGA and DTG thermogarms of PES, PES-GCNN, PES-GCNN/Ag0 10.
Catalysts 11 01533 g005
Catalysts 11 01533 g006 550
Figure 6. SEM micrographs of (a,d) PES, (b,e) PES-GCNN and (c,f) PES-GCNN/Ag0 10.
Figure 6. SEM micrographs of (a,d) PES, (b,e) PES-GCNN and (c,f) PES-GCNN/Ag0 10.
Catalysts 11 01533 g006
Catalysts 11 01533 g007 550
Figure 7. The energy dispersive X-ray (EDX) mapping analysis of the PES-GCNN/Ag0 10 fabric. (a) The explanation for subfigure is EDX Mapping, (b,c) the EDX spectrum of the PES-GCNN/Ag0 in two zones, and the EDX-mapping distribution of carbon (d), oxygen (e), nitrogen (f) and silver nanoparticles (g).
Figure 7. The energy dispersive X-ray (EDX) mapping analysis of the PES-GCNN/Ag0 10 fabric. (a) The explanation for subfigure is EDX Mapping, (b,c) the EDX spectrum of the PES-GCNN/Ag0 in two zones, and the EDX-mapping distribution of carbon (d), oxygen (e), nitrogen (f) and silver nanoparticles (g).
Catalysts 11 01533 g007
Catalysts 11 01533 g008a 550Catalysts 11 01533 g008b 550
Figure 8. (a) UV–vis of control spectra obtained before (4-NP) and immediately after addition of NaBH4. (b) A series of absorbance spectra obtained every 1 min after addition of the PES-GCNN/Ag010 catalyst [3 × 3 cm2] to a solution containing 4-NP and NaBH4. (c) Time-dependent 4-NP conversion over various Ag0 containing catalysts and (d) the corresponding kinetics plots. (e) Reaction scheme and photographs representing reduction of 4-NP to 4-AP by NaBH4, catalyzed by PES-GCNN/Ag0.
Figure 8. (a) UV–vis of control spectra obtained before (4-NP) and immediately after addition of NaBH4. (b) A series of absorbance spectra obtained every 1 min after addition of the PES-GCNN/Ag010 catalyst [3 × 3 cm2] to a solution containing 4-NP and NaBH4. (c) Time-dependent 4-NP conversion over various Ag0 containing catalysts and (d) the corresponding kinetics plots. (e) Reaction scheme and photographs representing reduction of 4-NP to 4-AP by NaBH4, catalyzed by PES-GCNN/Ag0.
Catalysts 11 01533 g008aCatalysts 11 01533 g008b
Catalysts 11 01533 g009 550
Figure 9. Comparison of catalytic activity of different area of PES-GCNN/Ag0 catalyst fabric. (a) Conversion profiles for the 4-NP. (b) Plot of ln(C/C0) against the reaction time for the reduction of 4-NP by NaBH4 using different area of PES-GCNN/Ag0 catalyst fabric.
Figure 9. Comparison of catalytic activity of different area of PES-GCNN/Ag0 catalyst fabric. (a) Conversion profiles for the 4-NP. (b) Plot of ln(C/C0) against the reaction time for the reduction of 4-NP by NaBH4 using different area of PES-GCNN/Ag0 catalyst fabric.
Catalysts 11 01533 g009
Catalysts 11 01533 g010 550
Figure 10. Cycling performance of PES-GCNN/Ag0 for 4-NP reduction reaction conditions.
Figure 10. Cycling performance of PES-GCNN/Ag0 for 4-NP reduction reaction conditions.
Catalysts 11 01533 g010
Catalysts 11 01533 g011 550
Figure 11. Possible 4-NP catalytic reduction route by NaBH4 over PES-GCNN/Ag0.
Figure 11. Possible 4-NP catalytic reduction route by NaBH4 over PES-GCNN/Ag0.
Catalysts 11 01533 g011
Table
Table 1. Thermal parameters of decomposition stages of the fabric-based samples.
Table 1. Thermal parameters of decomposition stages of the fabric-based samples.
Sample CodeStage 1: Ester Linkage DecompositionStage 2: C–C Cleavage (Oxidation)
Tdi–Tdf (°C)Tdmax (°C)Tdi–Tdf (°C)Tdmax (°C)
PES300.1–489.1419.2498.5–579.4540.2
PES-GCNN308.6–496.4431.1501.7–606.7590.9
PES-GCNN/Ag010315.5–448.3439.3449.9–495.2458.4
Tdi: The initial thermal degradation temperature. Tdf: The final thermal degradation temperature. Tdmax: The thermal degradation temperature at maximum rate.
Table
Table 2. Comparison of mass change of each sample after the adhesion test.
Table 2. Comparison of mass change of each sample after the adhesion test.
Samplem/m0(t = 0)m/m0(t = 24h)
PES-GCNN11
PES-GCNN/Ag0 1010.99
PES-Ag0 (reference)10.48
Table
Table 3. Comparison of the activity of our catalysts with other reported catalysts for 4-NP to 4-AP.
Table 3. Comparison of the activity of our catalysts with other reported catalysts for 4-NP to 4-AP.
CatalystSupport4-NP (mM)kapp (min−1)Quantity of Catalyst Used (mg)Reaction Time (min)Conve-rsion %Ref.
Ag NPsNone50.192.720100[52]
RGO/AgGraphene oxide0.20.216115100[53]
RN–GO/AgN-doped graphene oxide0.20.44416.6100[53]
Ag/MoS2MoS20.10.291.51294.5[54]
SiO2/AgSiO20.120.213151078[55]
Fe3O4@SiO2–AgFe3O4@SiO250.833496[56]
Ag/CCarbon0.120.3221298[8]
Ag/PANIPolyaniline51.282.73100[52]
g-C3N4/Ag2SGraphitic carbon nitride2.50.03110.0008
(10 mg/L)		4297[57]
PES-GCNN/Ag0PES-GCNN0.030.46220 a590This study
a Not including PES weight.
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Amedlous, A.; Majdoub, M.; Anfar, Z.; Amaterz, E. Self-Supporting g-C3N4 Nanosheets/Ag Nanoparticles Embedded onto Polyester Fabric as “Dip-Catalyst” for Synergic 4-Nitrophenol Hydrogenation. Catalysts 2021, 11, 1533. https://doi.org/10.3390/catal11121533

AMA Style

Amedlous A, Majdoub M, Anfar Z, Amaterz E. Self-Supporting g-C3N4 Nanosheets/Ag Nanoparticles Embedded onto Polyester Fabric as “Dip-Catalyst” for Synergic 4-Nitrophenol Hydrogenation. Catalysts. 2021; 11(12):1533. https://doi.org/10.3390/catal11121533

Chicago/Turabian Style

Amedlous, Abdallah, Mohammed Majdoub, Zakaria Anfar, and Elhassan Amaterz. 2021. "Self-Supporting g-C3N4 Nanosheets/Ag Nanoparticles Embedded onto Polyester Fabric as “Dip-Catalyst” for Synergic 4-Nitrophenol Hydrogenation" Catalysts 11, no. 12: 1533. https://doi.org/10.3390/catal11121533

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