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Frontiers in Catalysis ·

Volume 5 - 2026 | https://doi.org/10.3389/fctls.2025.1721217

Activated carbon sheets from pomegranate peel with ionic liquid for Knoevenagel condensation: synthesis of aryledene and xanthene derivatives

  • Dhaneshwar Prasad1
  • Archana Asatkar2
  • Swetlana Prerna Ram3
  • Shripal Singh3
  • Santosh Kumar Prajapati4*corresponding author
  • Subhash Banerjee1*corresponding author
  1. 1Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, India
  2. 2Department of Chemistry, Government Nagarjuna Post Graduate College of Science, Raipur, Chhattisgarh, India
  3. 3CSIR-CIMFR Regional Research Centre, Bilaspur, Chhattisgarh, India
  4. 4Department of Botany, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, India
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Abstract

Development of a mild and sustainable protocol for the carbon-carbon bond formation via Knoevenagel condensation is essentially desirable because the products, aryledene derivatives, are useful intermediates and are widely used in the manufacture of fine chemicals, pharmaceutically active molecules, calcium channel blockers, natural products, as well as in the production of flavours and fragrances. Activated carbon sheets were fabricated from pomegranate peels via calcination, activation by KOH and were characterization using Raman spectroscopy, powder XRD, FESEM, FESEM-EDX, and TGA studies. The ionic liquid, [pmIm]Br was synthesized by reacting a 1:1 mixture of N-methyl imidazole and n-pentyl bromide under microwave irradiation. Pomegranate peel–derived two-dimensional graphitic activated carbon (PPAC) nanosheets, with lateral dimensions of 40–200 nm and lengths of 4–10 μm, were confirmed through Raman spectroscopy, powder XRD, FESEM, and other studies. The PPAC, combined with the ionic liquid [pmIm]Br, demonstrated remarkable catalytic performance in the Knoevenagel condensation of aromatic aldehydes with active methylene compounds, producing the corresponding aryledene derivatives in excellent yields (90%–95%) within 5–20 minutes under mild conditions. Furthermore, the PPAC/[pmIm]Br catalytic system efficiently facilitated the synthesis of xanthene derivatives via a tandem condensation–cyclization pathway. The catalyst was easily recovered and reused over five consecutive cycles with minimal loss of activity. This work highlights a renewable, biomass-derived carbon framework as a dual-function, environmentally friendly catalyst for effective C–C bond formation and heterocycle synthesis, offering a scalable approach aligned with green chemistry and waste valorization principles.

Keywords

1 Introduction

Knoevenagel condensation is one of the useful and frequently used condensation reactions in synthetic organic chemistry for the construction of carbon–carbon bonds using active methylene compounds and aldehydes or ketones as substrates (Panja et al., 2015; Sayed et al., 2022; Li et al., 2012; Elhamifar et al., 2016; Dumbre et al., 2015; Wach et al., 2015). The products, aryledene derivatives, are useful intermediates and are widely used in the manufacture of fine chemicals, such as substituted alkenes, carbocyclic compounds, biologically and pharmaceutically active molecules, calcium channel blockers, natural products, functional polymers, and coumarin derivatives, as well as in the production of flavours and fragrances (Appaturi et al., 2021). Structures of a few biologically active functionalized alkenes, which are synthesized by Knoevenagel condensation reactions, are provided in Figure 1 (Tokala et al., 2022; Beutler et al., 2007).

Examples of some biologically active molecules.
Figure 1. Examples of some biologically active molecules.

Traditionally, Knoevenagel condensation reactions have been performed using toxic organic nitrogenous bases such as piperidine (Dalessandro et al., 2017), triethylamine (Pawar et al., 2016), pyridine (Doebner, 1901), and ammonia (Knoevenagel, 1898), 1,8-bisdimethylamino naphthalene (Rodriguez et al., 1999), poly-D-glucosamine (Sakthivel and Dhakshinamoorthy, 2017), etc. Later, various modern methods employing Lewis acids (Muralidhar and Girija, 2014; Ogiwara et al., 2015; Khan and Khan, 2019; Arafa et al., 2024) (e.g., GaCl3 (Muralidhar and Girija, 2014), InCl3/Ac2O (Ogiwara et al., 2015), ZnCl2 (Khan and Khan, 2019), Ru(III) complex/Cs2CO3 (Arafa et al., 2024), oxides-based catalysts (Gawande and Jayaram, 2006; Cabello et al., 1984; Katkar et al., 2010; Appaturi et al., 2018a; Appaturi et al., 2018b) (e.g., MgO/ZrO2 (Gawande and Jayaram, 2006), AlPO4-Al2O3 (Cabello et al., 1984), InCl3/MCM-41 (Katkar et al., 2010), alanine/MCM-41 (Appaturi et al., 2018a), urea/SBA-15 (Appaturi et al., 2018), and tetraethylammonium-BEA zeolite (Chaves et al., 2015), zeolitic imidazolate framework-8/Na12Al12Si12O48.27. H2O (Zhang G. et al., 2015), etc.), nano-based catalysts (Şen et al., 2018; Wang et al., 2019; Sayed et al., 2022; Kumar et al., 2011; Li et al., 2016; Wang et al., 2018) (e.g., Rh-Pt NPs/thiocarbamide/graphene oxide (Şen et al., 2018), Pd NPs/triazine-based g-C3N4 nanotubes (Wang et al., 2019), Ag NPs/TiO2 (Sayed et al., 2022), ZnO NPs/MW (Kumar et al., 2011), NiFe2O4 nanoparticles (Li et al., 2016), [3-(2-aminoethyl) aminopropyl]triethoxysilane/SiO2/Fe3O4 NPs (Wang et al., 2018), etc.), metal-organic frameworks (Cai et al., 2020; Dhakshinamoorthy et al., 2017; Wang et al., 2016; Burgoyne and Meijboom, 2013; Li et al., 2011; Mangala and Sreekumar, 2013) (e.g., Terbium-based (Cai et al., 2020), Al-based (Dhakshinamoorthy et al., 2017), Au-Cu(II) (Wang et al., 2016), Zn-based (Burgoyne and Meijboom, 2013), etc.), a few cage catalysts (Murase et al., 2012; Pei et al., 2021) (e.g., Pd-based cage (Murase et al., 2012), In/Zn calixarene-based cage (Pei et al., 2021)) as well as different basic ionic liquids (ILs) (Xu et al., 2010; del Hierro et al., 2018; Ranu and Jana, 2006) (e.g., [C4-dabco][BF4] (Xu et al., 2010), choline hydroxide/SBA-15 (del Hierro et al., 2018), [bmIm]OH (Ranu and Jana, 2006), etc.) have been developed to promote the Knoevenagel condensation reactions. Each offers unique advantages in terms of catalytic performance, product selectivity, and yields. However, the use of homogeneous and costly Lewis acids or metal complexes, poor stability under moisture, limited substrate accessibility of MOFs, leaching of active components, non-reusability or low reusability of existing catalysts, tedious synthesis procedures, nanoparticles, and cage structures are major drawbacks of these catalytic systems.

To overcome the limitations of the procedures mentioned above, a few functionalized carbonaceous materials, such as Rh-Pt NPs@thiocarbamide@graphene oxide (Şen et al., 2018), alkaline-doped multiwall carbon nanotubes (Delgado-Gómez et al., 2017), graphene oxide (Islam et al., 2014), ethylene diamine functionalized graphene oxide (Xue B. et al., 2015), have been introduced. These catalysts often suffer from limited stability under harsh reaction conditions, which can hinder their long-term applicability. Additionally, their synthesis and functionalization may involve time-consuming procedures and costly reagents, reducing their scalability for industrial applications.

In recent times, the use of biomass feedstock-derived chemically active carbonaceous materials has attracted much attention compared to commercially available carbonaceous materials. These are abundant, easily available, and biodegradable (Meng et al., 2019). The conversion of biomass into tailored carbon frameworks is pivotal, as it significantly reduces the carbon footprint while enhancing cost-effectiveness and sustainability. Several biomass feedstocks such as rice husk (Patel et al., 2019a; Patel et al., 2020; Asatkar et al., 2020a; Asatkar et al., 2020b; Patel et al., 2019a; Patel et al., 2019b; Patel et al., 2024), walnut peel (Yang et al., 2015), orange peel (Asatkar et al., 2020a; Subramani et al., 2017), winter melon (Feng et al., 2015) silkworm chrysalis (Feng et al., 2016), Phyllanthus acidus (Atchudan et al., 2018), bee pollen (Zhang J. et al., 2015), garlic (Zhao et al., 2015), goose feather (Liu et al., 2015) lychee seeds (Xue M. et al., 2015) and pomegranate peels (PP) have been used to produce activated carbon with high surface areas and tuneable pore size. Among others, pomegranate peels (PP) are one of the major food wastes, which creates 1.6 billion tonnes of garbage every year (Sulieman et al., 2016; Singh et al., 2023; Sheikh, 2006). Several works have been published on the valorization of pomegranate crops using their peels and seeds (Alcaraz-Mármol et al., 2017; Hasnaoui et al., 2014; Uçar and Karagöz, 2009; Pehlivan and Özbay, 2018; Saadi et al., 2019). Due to their high carbon content and low mineral content, pomegranate peels have been utilised as a suitable precursor material for producing activated carbon, thereby creating efficient adsorbents (Saadi et al., 2019; Senthilkumar et al., 2017; Nemr, 2009; Turkmen et al., 2015; Amin, 2009). However, the applications of pomegranate peel-derived activated carbon (PPAC) in organic transformations have not been investigated. As a part of our continuous research in biomass feedstock-derived activated carbon (Patel et al., 2019a; Patel et al., 2020; Asatkar et al., 2020a; Asatkar et al., 2020b; Patel et al., 2019a; Patel et al., 2019b; Patel et al., 2024) and ionic liquids (Ranu and Banerjee, 2005a; Ranu, and Banerjee, 2006; Ranu et al., 2008a; Ambati et al., 2020; Suryawanshi et al., 2020; Saha et al., 2017; Payra et al., 2017; Saha et al., 2016; Banerjee et al., 2012; Ranu et al., 2008b; Ranu et al. 2008c; Ranu et al., 2007a; Ranu et al. 2007b; Ranu et al. 2007c; Ranu et al. 2007d; Ranu et al. 2006; Ranu, and Banerjee, 2005b; Ranu et al., 2007e; Prasad and Banerjee, 2025), herein, we report, pomegranate peel derived activated carbon (PPAC) catalyzed, [pmIm]Br, IL mediated Knoevenagel Condensation leading to the synthesis of aryledene and xanthene derivatives at room temperature under mild, neutral and metal, acid and base-free reaction conditions (Scheme 1).

PPAC-catalyzed Knoevenagel condensation in [pmIm]Br.
Figure 2. PPAC-catalyzed Knoevenagel condensation in [pmIm]Br.
Preparation of [pmIm]Br.
Figure 3. Preparation of [pmIm]Br.

2 Materials and methods

2.1 Preparation of the catalyst and ionic liquid

2.1.1 Preparation of PPAC

Initially, pomegranate peels were collected from a local juice vendor in Bilaspur, Chhattisgarh. The peels were first cleaned with water and then treated with a few drops of concentrated HCl with boiling for an hour. After that, these pomegranate peels were dried for 24 h at 80 °C in an oven. After cooling, these dried pomegranate peels were then crushed into a small size. Then, these small-sized pomegranate peels were carbonized for 3 hours at a temperature of 500 °C in a muffle furnace to create a black carbon material, pomegranate peel carbon (PPC), which was subsequently activated with KOH. For 3 hours, the KOH–carbon mixture was calcined at 700 °C. The calcinated samples were rinsed with distilled water to remove alkali until the pH reached 7. The calcinated pomegranate peels were then dried for 15 h at 80 °C (Patel et al., 2019b). Powder X-ray diffraction (XRD) and Raman spectroscopy measurements were used to analyse the physicochemical properties of the PPC both before and after activation. The preparation of PPAC is illustrated in Figure 2.

Schematic representation of the fabrication of PPAC.
Figure 4. Schematic representation of the fabrication of PPAC.

2.1.2 Preparation of [pmIm]Br

The neutral ionic liquid [pmIm]Br was synthesized by reacting a 1:1 mixture of N-methyl imidazole (15 mmol, 1.23 g) and n-pentyl bromide (18 mmol, 2.26 g) under microwave irradiation at 80 °C for 2 min. A clear, yellow-coloured, viscous liquid was formed. Then it was cooled and washed with a small amount of ether to remove unreacted starting materials. Finally, the viscous liquid was dried using a vacuum pump to get pure ionic liquid, [pmIm]Br. (Ranu and Jana, 2005).

3 Results and discussion

3.1 Characterization of PPAC

A Raman spectroscopic study has been performed to degree of graphitization of the sample. The Reman spectrum revealed two prominent peaks located at 1345 cm-1 and 1585 cm-1 as shown in Figure 3a. These peaks correspond to the D-band and G-band, respectively. These bands provide insight into the PPAC activated carbon structural arrangement and the degree of crystallinity. The D-band at 1345 cm-1 is associated with structural imperfections within the carbon framework. This arises from edge effects, lattice defects, and the presence of disordered or amorphous carbon regions. A disruption in the symmetry of the sp2-bonded carbon lattice is formed due to these imperfections. The G-band at 1585 cm-1 is associated with the in-plane stretching vibrations of sp2-hybridized carbon atoms. This indicates the presence of well-ordered graphitic domains within the sample. To estimate the degree of disorder, the intensity ratio of the D-band to the G-band (ID/IG) was calculated. For the PPAC sample, the ID/IG ratio was 0.976, which indicates the partial graphitization, where both ordered and disordered carbon regions coexist (Dubale et al., 2014).

**(a)** Raman spectra, and **(b)** powder XRD of PPAC.
Figure 5. (a) Raman spectra, and (b) powder XRD of PPAC.

Powder XRD pattern of PPAC is shown in Figure 3b, exhibiting two broad peaks at approximately 2θ ≈ 28° and 44°, which correspond to the {002} and {100} reflection planes, respectively. These peaks are associated with the stacking of carbon layers. The peak broadening observed in the XRD spectra suggests the presence of disordered amorphous carbon (Suganuma et al., 2008). Further, the surface morphology of the PPAC was investigated using field emission scanning electron microscopy (FESEM) measurement. The FESEM image Figure 4a) of PPAC shows the formation of two-dimensional carbon sheets, with an approximate 40–200 nm width and 4–10 μm in length. Moreover, FESEM-EDAX analysis of the PPAC reveals the presence of carbon (80.30%), oxygen (14.90%), and a minor amount of silica (4.81%) in PPAC (Figure 4b). The FESEM-EDAX elemental mapping of PPAC is presented in Figures 4c,d. The high carbon content indicates efficient carbonization, which is typical for well-activated carbon materials, and the presence of oxygen suggests the existence of surface functional groups such as hydroxyl (-OH), which enhance the interaction between polar functional groups of organic molecules.

**(a)** FESEM images of PPAC, **(b)** FESEM−EDAX of PPAC, **(c)** Elemental mapping of carbon in PPAC. **(d)** Elemental mapping of oxygen in PPAC.
Figure 6. (a) FESEM images of PPAC, (b) FESEM−EDAX of PPAC, (c) Elemental mapping of carbon in PPAC. (d) Elemental mapping of oxygen in PPAC.

The nitrogen adsorption–desorption graph of PPAC, as depicted in Figure 5a, shows a Type IV isotherm, which is typical for mesoporous materials. The Barrett–Joyner–Halenda (BJH) study also supports the finding that the sample primarily consists of mesopores with nearly uniform sizes. The average pore size is around 3.72 nm, and the total pore volume is about 0.480 cm3/g. Figure 5b.

**(a)** Nitrogen adsorption and desorption isotherm and **(b)** Pore size distribution curve of PPAC.
Figure 7. (a) Nitrogen adsorption and desorption isotherm and (b) Pore size distribution curve of PPAC.

Thermogravimetric analysis (TGA) was employed using LECO-TGA-701(604–100-700) in a temperature range of 30 °C–900 °C at a heating rate of 10 °C/min under nitrogen flow to investigate the thermal stability of both PPBA and PPAC, as depicted in Figure 6.

**(a)** TGA/DTG plot of PPBA, **(b)** TGA/DTG plot of PPAC.
Figure 8. (a) TGA/DTG plot of PPBA, (b) TGA/DTG plot of PPAC.

The TGA profile of PPBA reveals an initial mass loss of ∼8.08% below 150 °C, primarily attributed to the release of physically adsorbed water molecules and surface-bound volatiles. In contrast, PPAC exhibits a markedly higher weight loss of ∼25.16% within the same temperature region, indicating the surface functionalization of carbon during the activation process using KOH. Oxygenated functional groups, such as hydroxyl moieties, incorporated at the surface of carbon during activation, are unstable and readily decompose at lower temperatures, thereby contributing to early-stage mass losses (Li et al., 2014). A progressive decomposition of ∼29.03% was observed for PPBA between 150 °C and 700 °C, whereas PPAC displayed a comparatively higher weight loss of ∼32.05% in a similar temperature range. Above 700 °C, further degradation was evident, with PPBA losing ∼14.69% up to 900 °C, while PPAC recorded a higher loss of ∼16.86%, reflecting continued breakdown of the carbon matrix. The enhanced weight reduction in PPAC can be attributed to the development of a highly porous texture and increased surface functionality induced during KOH activation (Mohan et al., 2016). At 900 °C, the residual carbon yield of PPBA remains relatively high at ∼48%, while that of PPAC decreases significantly to ∼26%. The generation of abundant active sites along with a significantly enlarged surface area renders the activated carbon highly effective and exceptionally valuable as a catalyst in various organic transformations.

3.2 Catalytic performance test

Well-characterized PPAC was employed as a catalyst in the Knoevenagel condensation of benzaldehyde with malononitrile, using [pmIm]Br as the reaction medium. The optimized reaction conditions are summarised in Table 2. A mixture of benzaldehyde (1 mmol), malononitrile (1 mmol), PPAC (100 mg), and [pmIm]Br (40 mol%) was stirred at room temperature, and the reaction reached completion within 3 min, as monitored by TLC. The desired product, 2-benzylidenemalononitrile, was isolated as a white solid in excellent yield (95%) (Entry 1, Table 1). To evaluate the role of the catalyst, the control experiments were carried out. In the absence of PPAC and IL, notably, no product formation was observed even after 30 min (Entry 2, Table 1). Subsequently, when the model reaction was performed in ethanol in place of IL, the reaction proceeded sluggishly, furnishing the desired product (25%) after 30 min (Entry 3, Table 1). Using only [pmIm]Br (40 mol%) without PPAC produced a 15% yield after 30 min (Entry 4, Table 1). To investigate the synergistic effect of PPAC and [pmIm]Br, reactions were performed varying the catalyst loading. Reducing PPAC loading to 50 mg while maintaining [pmIm]Br at 40 mol% still produced 95% product within 3 min (Entry 5, Table 1). A comparable outcome was observed with a combination of PPAC (50 mg) and [pmIm]Br (20 mol%) (Entry 6, Table 2). However, when the PPAC loading was further reduced to 25 mg with [pmIm]Br (20 mol%), the yield of the product decreased to 72% (Entry 7, Table 1). The effect of solvents was also examined. When water was employed as a solvent and PPAC (25 mg), no product formation was observed (Entry 8, Table 1) even after 30 min. Using methanol (2 mL) with PPAC (25 mg) afforded a 20% yield in 30 min (25 mg) (Entry 9, Table 1). In acetonitrile (2 mL), the reaction gave a 15% (Entry 10, Table 2), while only a trace amount of product formation was detected in THF under similar reaction conditions (Entry 11, Table 2). Thus, the combination of PPAC (50 mg) and [pmIm]Br (20 mol%) with 1 mmol each of benzaldehyde and malononitrile was identified as the optimized reaction condition (Entry 6, Table 1). When coupled with the ionic liquid [pmIm]Br, the resulting PPAC/[pmIm]Br system forms a synergistic catalytic interface in which Br− activates electrophilic carbonyl centers while the basic oxygenated sites on PPAC promote methylene deprotonation, thereby accelerating C–C bond formation in the Knoevenagel condensation of aromatic aldehydes with active methylene compounds.

TABLE 1. Optimization of reaction conditions for the Knoevenagel condensation reaction of benzaldehyde and malononitrile a .

Chemical reaction diagram showing reactants and product. The reactants are Ph-CHO and malononitrile (CN-CH2-CN). Under unspecified conditions, these react to form a compound with a phenyl group (Ph) and two cyano groups (CN) on opposite sides of a double bond.
EntryCatalystReaction mediumTemperature (oC)Time (min)Yield (%)b
1PPAC (100 mg)[pmIm]Br (40 mol%)RT395
2No catalyst-RT30NR
3PPAC (100 mg)Ethanol (2 mL)RT3025
4No catalyst[pmIm]Br (40 mol%)RT3010
5PPAC (50 mg)[pmIm]Br (40 mol%)RT395
6PPAC (50 mg)[pmIm]Br (20 mol%)RT395 d
7PPAC (25 mg)[pmIm]Br (20 mol%)RT1072
8PPAC (25 mg)Water (2 mL)RT30NR c
9PPAC (50 mg)Methanol (2 mL)RT3020
10PPAC (25 mg)Acetonitrile (2 mL)RT3015
11PPAC (25 mg)THF (2 mL)RT30trace

Reaction conditions: Benzaldehyde (1 mmol), malononitrile (1 mmol), PPAC, and the solvent were stirred at room temperature.

Yield of pure isolated products.

No reaction.

Optimum reaction conditions.

TABLE 2. Substrate scope of Knoevenagel condensation reactions using PPAC in [pmIm]Br a .

Chemical reaction schematic showing a reaction between Ar-CHO and an EWG-substituted compound, facilitated by PPAC and [pmIm]Br at room temperature, resulting in an Ar-substituted alkene with EWG groups.
EntryAr-CHOIllustration showing a blue angle bracket pointing left, with the letters "EWG" positioned above and below the bracket.Product [ref]Time (min)Yield b (%)Melting point (oC)c
ObservedReported
1Chemical structure of benzaldehyde, showing a benzene ring with a single aldehyde group (CHO) attached to the top. The structure is depicted in red.Chemical structure diagram showing a bent configuration with two "CN" groups, each connected by a line, indicating a bond. The lines form a less than symbol. The "CN" groups are in blue.Chemical structure of a compound showing a benzene ring connected to a carbon-carbon double bond. Two cyanide groups, labeled "CN", are attached to the end of the double bond. 1a (Panja et al., 2015)39581–8383.5
2Chemical structure of para-tolualdehyde, featuring a benzene ring with a CHO group attached to the top and a methyl group labeled as Me attached to the bottom.Chemical structure illustration showing two cyanide (CN) groups connected to a central carbon atom. The CN groups are positioned at an angle, creating a V-shape.Chemical structure of 1,4-dimethyl-2,3-dicyano-1,3-butadiene. A benzene ring with a red methyl group (Me) is attached to a butadiene chain. The chain has two cyanide groups (CN) in blue. 1b (Panja et al., 2015)493132–133134.5
3Chemical reaction scheme showing the reaction of Ar-CHO with a molecule having two electron-withdrawing groups (EWG) to produce a compound with a double bond flanked by Ar and EWG groups. The reaction uses PPAC and [pmIm]Br as catalysts at room temperature (RT).A blue geometric shape resembling an angle bracket, with the letters "EWG" positioned above and below it, also in blue.Chemical structure of benzaldehyde, showing a benzene ring with alternating double bonds and an aldehyde group (CHO) attached. The diagram is in red. 1c (Panja et al., 2015)493112–114115
4Chemical structure diagram with two cyanide groups (CN) connected by a bent line, indicating a molecular bonding angle. The letters "CN" are in blue.Chemical structure of benzylidene malononitrile. It features a benzene ring on the left connected via a double bond to a group containing two cyano groups (CN) on the right.Chemical structure of 4-methylbenzaldehyde. A benzene ring is shown with a CHO group attached to the top and a methyl group (Me) attached to the bottom. 1d (Muralidhar and Girija, 2014)392162–163161
5Chemical structure with two cyanide (CN) groups, displayed diagonally, connected by a bend. The structure is illustrated in blue.Chemical structure of a compound with a benzene ring marked in red, a methyl group (Me) attached, and a vinylidene dinitrile group (CN) labeled in blue.Chemical structure of anisaldehyde, showing a benzene ring with a methoxy group (OCH₃) attached to the para position and an aldehyde group (CHO) at the opposite para position. 1e (Muralidhar and Girija, 2014)395157–158159
6Chemical structure diagram showing two cyanide groups, labeled "CN", connected by a bent line, indicating a possible angle or bond between them.Chemical structure of a compound featuring a benzene ring with a methoxy group (MeO) in red attached to one carbon. A vinyl group connects to two cyanide groups (CN) in blue.Chemical structure of 4-chlorobenzaldehyde, showing a benzene ring with a chlorine atom attached to the para position and an aldehyde group (CHO) attached to the opposite para position. 1f ( Turpaev et al., 2011)490135–136-
7A chemical structural diagram depicts a central atom bonded to two cyanide groups, represented by "CN". The bonds are angled downward from the center.Chemical structure of p-Chlorobenzylidenemalononitrile, showing a benzene ring with a chlorine atom attached, linked to a malononitrile moiety. The benzene and chlorine are highlighted in red, and the malononitrile is in blue.Chemical structure of 4-nitrobenzaldehyde, showing a benzene ring with a nitro group (NO₂) and an aldehyde group (CHO) attached at opposite positions. 1g (Turpaev et al., 2011)49280–82-
8Chemical structure diagram of a cyanide group connected to another cyanide group at an angle, depicted in blue lines with "CN" labels.Chemical structure of 4-(dicyanomethylene)-2-methyl-6-(4-nitrophenyl)-4H-pyran. The diagram shows a six-membered pyran ring with a dicyanomethylene group and a nitrophenyl group attached.Chemical structure diagram of a methoxy-substituted benzaldehyde. A benzene ring is at the center with three methoxy groups (OMe) attached to the 2, 4, and 6 positions, and an aldehyde group (CHO) at the 1 position. The structure is depicted in red. 1h (Datta et al., 1988)49293–95-
9Chemical structure showing two cyanide (CN) groups connected at an angle, forming a V-shape. The CN groups are in blue, indicating their composition of carbon and nitrogen atoms.Chemical structure of a compound featuring a benzene ring with three methoxy groups attached to the ring in red labeled "MeO." Two cyanide groups are attached to a carbon-carbon double bond in blue labeled "CN."Chemical structure of p-tert-butylbenzaldehyde, featuring a benzene ring with a tert-butyl group bonded to the para position and an aldehyde group bonded to the opposite para position. 1i (Datta et al., 1988)492103–105-
10Chemical structural formula showing two cyanide groups (CN) connected by a single line at an angle, indicating a molecular bond.Chemical structure showing a benzene ring with a tert-butyl group attached at one position and a vinylidene dicyanide group at another position. The benzene ring is in red, and the dicyanide group is in blue.Chemical structure diagram of 4-bromobenzaldehyde, featuring a benzene ring with a CHO group attached to the top and a Br (bromine) atom at the bottom. 1j (Pourshahi and Rabiei, 2023)393137–139-
11Chemical structure diagram showing a V-shape with two identical "CN" groups at each end. The lines appear to represent bonds between atoms.Chemical structure of 4-bromobenzylidenemalononitrile, showing a benzene ring bonded to a bromine atom and a side group with two cyanide groups at one end.Chemical structure of 4-chlorobenzaldehyde, featuring a benzene ring with an aldehyde group (CHO) attached at the first position and a chlorine atom (Cl) at the fourth position. 1k (Sayed et al., 2022)189047–4848–49
12Chemical structure diagram showing cyanogen, CN, linked in a zigzag pattern with two CN groups. The structure is depicted in blue.Chemical structure of 2-(4-chlorophenyl)malononitrile. It features a benzene ring with a chlorine atom attached, shown in red, and a malononitrile group in blue.Chemical structure of 2,4-dichlorobenzaldehyde, showing a benzene ring with two chlorine atoms at the 2 and 4 positions, and an aldehyde group at the 1 position. 1L (Sayed et al., 2022)209086–8787–89
13Chemical structure diagram showing a v-shaped arrangement with two "CN" groups, one above the other, connected by a line.Chemical structure of a compound featuring a benzene ring with two chlorine atoms attached and a side chain with two cyano groups. The chlorines are labeled in red as "Cl" and the cyano groups in blue as "CN".Structural chemical formula of benzaldehyde, showing a benzene ring with a formyl group, CHO, attached. The benzene ring consists of six carbon atoms with alternating double bonds. 1m (Kaumanns et al., 2008)209130–3131–31.2
14Chemical structure showing a cyano group (CN) and an ethyl ester group (CO₂Et) connected at a central carbon atom, forming a branched molecule.Chemical structure of ethyl cyanocinnamate showing a benzene ring on the left connected to an acrylonitrile group, with a CN group above and an ethyl ester group (CO2Et) below.Chemical structure of 4-Chlorobenzaldehyde, showing a benzene ring with a chlorine atom substituted at the para position and an aldehyde group at the opposite para position. Red lines and letters indicate bonds and atoms. 1n (Dalessandro et al., 2017)2091167–168-

Reaction condition: Aryl aldehyde (1 mmol), active methylene compound (1 mmol), PPAC (50 mg), and [pmIm]Br (20 mol%) with continuous stirring at room temperature.

Yield of pure isolated products.

The melting point was determined for the pure recrystallised products.

Following the successful optimization of reaction parameters, a series of Knoevenagel condensation products were synthesized at ambient temperature under environmentally benign conditions—specifically, in the absence of metals and volatile organic solvents. A broad range of aromatic aldehydes bearing diverse substituents on the aromatic ring efficiently underwent condensation with active methylene compounds in the presence of PPAC and [pmIm]Br as catalysts, affording α, β-unsaturated products in excellent yields. The detailed substrate scope is provided in Table 2. When malononitrile served as the active methylene donor, the corresponding products (1a-n) were obtained within a remarkably short reaction time of 3–4 min. In contrast, the use of alternative methylene compounds such as diethyl malonate, ethyl cyanoacetate, or acetylacetone extended the reaction time to approximately 20 min. Nonetheless, all transformations proceeded rapidly and delivered high yields in the range of 90%–95%. The resulting products were purified by straightforward recrystallization in ethyl acetate and characterized through comparison with literature-reported melting points, as well as detailed analysis of their 1H NMR and 13C NMR spectra.

All the aldehydes gave good yields under mild conditions. Aldehydes having electron-withdrawing groups reacted a little faster because these groups make the carbonyl carbon more reactive. The reaction usually finished in about 3–4 min and gave nearly 95% yield. On the other hand, aldehydes with electron-donating groups like methyl or methoxy reacted a bit slower, but the yield still remained high, around 90%. When active methylene compounds other than malononitrile were used, the reaction took a longer time, about 18–20 min, to complete. Even then, the products were obtained in good yield. These results show that both the substituent on the aromatic ring and the type of active methylene compound influence the reaction rate, while the catalyst continues to perform efficiently in all cases.

3.3 Plausible mechanism

A plausible mechanistic pathway for the Knoevenagel condensation between benzaldehyde and malononitrile, catalyzed synergistically by PPAC and the ionic liquid [pmIm]Br, is shown in Figure 7. The reaction starts with PPAC abstracting a proton from the active methylene group of malononitrile, forming a resonance-stabilized carbanion (Int-I). This nucleophilic intermediate then attacks the electrophilic carbonyl carbon of the aromatic aldehyde. The ionic environment created by [pmIm]Br significantly facilitates this step by engaging in electrostatic interactions with the carbonyl group, increasing its susceptibility to nucleophilic attack and promoting the formation of an alkoxide intermediate. Next, the alkoxide captures the proton originally abstracted by PPAC, resulting in a β-hydroxy intermediate (Int-II). The final step involves the elimination of water from the β-hydroxy compound, a process stabilized by the ionic liquid through its influence on the transition state. This dehydration yields the desired α,β-unsaturated product, completing the catalytic cycle.

Plausible mechanism of knoevenagel condensation of benzaldehyde and malononitrile using PPAC in [pmIm]Br.
Figure 9. Plausible mechanism of knoevenagel condensation of benzaldehyde and malononitrile using PPAC in [pmIm]Br.

In this process, nearly all atoms from the aldehyde and active methylene compound become part of the arylidene product, while only a small amount of water is released. This shows that the reaction uses the materials efficiently and produces very little waste. The recyclability of the PPAC and [pmIm]Br catalytic system was evaluated over multiple reaction cycles under optimized conditions. The results are shown in Figure 8. After each reaction, the product was separated from the reaction mixture by the addition of 10 mL of ethyl acetate. PPAC-[pmIm]Br combination is insoluble in ethyl acetate, and was washed 2 times, dried, and recycled for subsequent runs. The catalytic performance showed only a slight decrease after five cycles, with product yields remaining consistently high, indicating that both PPAC and [pmIm]Br preserved their catalytic efficiency and structural integrity, as indicated from FESEM measurement of reused PPAC (Figure 9). This demonstrates the durability and reusability of the system, making it a promising approach for greener and more cost-effective chemical transformations.

Percentage yield retention after each recycling cycle.
Figure 10. Percentage yield retention after each recycling cycle.
FESEM images of reused PPAC.
Figure 11. FESEM images of reused PPAC.

In Table 3, a comparison is presented with previously reported catalytic systems, including various biomass-based catalysts, showing that superior performance is achieved when PPAC is used in [pmIm]Br under mild, metal-free, and reusable conditions.

TABLE 3. Comparative study of present and previously reported methods for Knoevenagel condensation.

EntryCatalystSolventTime (min)Temperature (°C)Yield (%)References
1UIO-66-NH-RNH2THF300RT84Luan et al. (2015)
2Au@Cu-MOFToluene420RT99Wang et al. (2016)
3Pd-Based MOF-1DMSO5RT42.5Ezugwu et al. (2016)
4Fe3O4@ZIF-8Toluene180RT94Schejn et al. (2015)
5ZIF-8Toluene360RT97Tran et al. (2011)
6Ni induced ZIF-8 nanoframesToluene120RT68.8Zhang N. et al. (2018)
7FeBTCXylene36013093Opanasenko et al. (2013)
8TMU-5Methanol30RT68Masoomi et al. (2014)
9CAU-1-NH2Ethanol4204091Dhakshinamoorthy et al. (2017)
10UPC-30DCM300RT94Fan et al. (2017)
11Fe-MIL-101-NH2Toluene1808090Hartmann and Fischer (2012)
12Zn-MOFDCM180RT80Neogi et al. (2009)
13Ni-MOFp-Xylene36013078Almaši et al. (2014)
14Mn-MOF-PiDMF180RT65Rambabu et al. (2017)
15Calcinated eggshellWater15RT97Patil et al. (2013)
16Amine-Functionalized Sugarcane Bagasse95% ethanol∼15RT∼95%Qiao et al. (2017)
17N-doped bamboo carbonAcetonitrile1207016.1%Mi et al. (2018)
18Zinnia grandiflora derived graphitic carbon nitride (BC@GCN) in IL (1-vinyl-3-butylimidazolium chloride)Water120RT100Sadjadi et al. (2020)
19Water Extract of Orange Fruit Peel AshEthanol45RT90Badiger and Kamanna (2021)
20Electrochemical CDs derived from orange peelsSolvent-free30RT98Michenzi et al. (2023)
21PPAC in [pmIm]BrSolvent-free3RT95Present work a

Represents present work.

After successful synthesis of α,β-unsaturated compounds using the combination of PPAC and [pmIm]Br via Knoevenagel condensation reaction, we tried to explore the catalytic activity of this catalytic system in the synthesis of xanthene derivatives via carbon-carbon bond formation and intramolecular cyclization (Scheme 3). Xanthene derivatives have a wide range of applications in various areas of research and industry. These compounds play an important role in pharmaceutical applications, for example, laser applications (Shankarling and Jarag, 2010; Ahmad et al., 2002; De et al., 2005) and fluorescent materials (Guo et al., 2018; Liu et al., 2018; Qi et al., 2011; Ebaston et al., 2019; Guo et al., 2019; Wan et al., 2019; Zhang P. et al., 2018). In addition, dyes such as rose bengal and phloxine B are often used as coloring agents in food products (Qi et al., 2011; Weisz et al., 2018a; Weisz et al., 2018b; Rasooly, 2005). A mixture of aromatic aldehyde (1 mmol), dimedone (2 mmol), PPAC (50 mg), and [pmIm]Br (20 mol% with respect to aromatic aldehyde) was taken in a round-bottom flask and stirred for 20 min at room temperature. A white solid product was formed and separated by using ethyl acetate. It was then purified by recrystallization using hot ethanol. A detailed substrate scope of xanthene derivatives is provided in Table 4.

Synthesis of xanthene derivatives using PPAC in [pmIm]Br.
Figure 12. Synthesis of xanthene derivatives using PPAC in [pmIm]Br.

TABLE 4. Substrate scope of xanthene derivatives using PPAC in [pmIm]Br a .

EntryArCHOProductYield b (%)Melting point (°C)
ObservedReported
1Chemical structure diagram showing a compound with a cyano group (CN) at the top and an ethyl ester group (CO2Et) branching below. The bonds are highlighted in blue.Chemical structure of a compound showing a benzene ring with a chlorine atom, labeled Cl, at one position. Attached is a cyano group, CN, and an ethyl ester group, CO₂Et, via a double bond. Elements are color-coded for clarity. 2a (Zabihzadeh et al., 2021)92204–206205–207
2Chemical structure of benzaldehyde, showing a benzene ring with alternating double bonds and a formyl group (CHO) attached to the ring.Chemical structure showing two ester groups labeled "CO₂Et" connected to a central point, arranged in a V shape. The text is in blue. 2b90222–225-
3Chemical structure of diethyl cinnamate, featuring a benzene ring connected to an alkene chain with two ester groups, each labeled "CO₂Et" in blue. The benzene is in red.Chemical structure of benzaldehyde, showing a benzene ring with a CHO group attached to the top carbon atom. The structure is drawn in red lines. 2c (Zabihzadeh et al., 2021)90230–232232–234
4The image shows the chemical notations "COMe" and "OMe" in blue text.Chemical structure of a compound featuring a benzene ring connected to an ethenone group. The ethenone is bonded to two methoxy groups (COMe) in blue. 2d (Zabihzadeh et al., 2021)89245–246247–249
5Chemical structure of 2,4,5-trimethoxybenzaldehyde. A benzene ring with methoxy groups (OMe) attached at positions 2, 4, and 5, and an aldehyde group (CHO) at position 1.Chemical structure diagram of a molecule with a central benzene ring substituted with three methoxy groups labeled "OMe" and "MeO". Below it, two cyclic ketone structures are connected by an oxygen atom. The benzene and methoxy groups are in blue, while the cyclic ketones are in red. 2e (Venkatesan et al., 2008)90186–188187–189
6Chemical structure of 4-Bromobenzaldehyde, featuring a benzene ring with a bromine (Br) atom and a formyl group (CHO) attached on opposite sides.Chemical structure diagram showing a bromobenzene group connected to a diketone with two cyclohexane rings. The bromobenzene is in blue, while the diketone and cyclohexane components are in red. 2f (Venkatesan et al., 2008)89189–190190–192
7Chemical structure of benzaldehyde, featuring a benzene ring connected to a formyl group (CHO). The benzene ring is represented by a hexagon with alternating double bonds.Chemical structure diagram showing a molecule with a central carbon atom connected to a benzene ring in blue, and two symmetrical six-membered rings with ketone groups in red. 2g (Mosaddegh et al., 2012)90247–249248–250
8Chemical structure of 4-ethylbenzaldehyde, featuring a benzene ring with an ethyl group (Et) and an aldehyde group (CHO) attached at opposite positions.Chemical structure of a compound featuring a central aromatic ring with an ethyl group labeled "Et" attached. The ring is bonded to two hexagonal structures with oxygen double bonds, highlighted in red. 2h (Zabihzadeh et al., 2021)92220–222222–224
9Chemical structure of 4-chlorobenzaldehyde, showing a benzene ring with a chlorine atom (Cl) attached to the para position, and an aldehyde group (CHO) at the opposite para position.Chemical structure of a compound featuring a chloro-substituted benzene ring attached to a central carbon, which is bonded to two six-membered carbon rings each containing a keto group. The benzene is in blue, and the rest in red and black. 2i (Alsalhi et al., 2025)91130–131130–132
10Chemical structure of anisaldehyde, showing a benzene ring with a methoxy group (OCH₃) attached to the top and an aldehyde group (CHO) attached to the bottom.Chemical structure of a compound featuring a methoxybenzene group in blue, connected to a central ketone group, which is linked to two red cyclohexanone rings. 2j (Zukić et al., 2018)90170–171171–174

Reaction condition: Aryl aldehyde (1 mmol), dimedone (2 mmol), PPAC (50 mg), and [pmIm]Br (20 mol%) with continuous stirring at room temperature.

Yield of pure isolated products.

The melting point was determined for the pure recrystallized products.

In Table 5, the results are compared for synthesis of xanthene derivatives with previously reported catalytic systems. The data shows that PPAC works better in [pmIm]Br, as the reaction runs under mild conditions.

TABLE 5. Comparative study of present and previously reported methods for Knoevenagel condensation.

EntryCatalystTemperatureSolventTimeYield (%)References
1[TEASA][TFA]100 °CSolvent-free10 m80Saadat et al. (2018)
2H5PW10V2O40/MCM-48100 °CSolvent-free6 h95Tayebee, and Maleki (2013)
3HClO4–SiO2 and PPA–SiO2RefluxAcetonitrile6 h54Kantevari et al. (2007)
4Fe3O4@Agar-Ag NPsRefluxEthanol6–10 h96Hoseinzade et al. (2022)
5CAN-supported HY-zeolite80 °CSolvent-free45 min94Sivaguru and Lalitha (2014)
6nano-TiO2100 °CSolvent-free15 min96Khazaei et al. (2013)
7Fe3O4@SiO2-imid-PMArefluxEtOH75 min93Esmaeilpour et al. (2014)
8CaCl290 °CDMSO5 h87Ilangovan et al. (2013)
9PPAC in [pmIm]BrRTSolvent-free20 min92Present work

In this study, the Knoevenagel condensation of benzaldehyde with malononitrile exhibits excellent atom economy, nearly 97.66%, since most of the reactant atoms contribute to the formation of the desired product, with water being the only minor by-product.

We have also examined the large-scale feasibility of the proposed catalytic system by carrying out the Knoevenagel condensation using gram-level quantities of the reactants. A mixture of benzaldehyde (10 mmol, 1.06 g) and malononitrile (10 mmol, 0.66 g) was treated with the PPAC (50 mg) and [pmIm]Br (20 mol%) at room temperature. The reaction was completed in approximately 20 min, affording 2-benzylidenemalononitrile (1.45 g, 95%) after filtration and recrystallisation. This observation confirms the applicability of the proposed catalytic system even when applied to a gram-scale reaction.

4 Conclusion

In summary, we demonstrated the fabrication of pomegranate peel-derived activated carbon (PPAC). The formation of two-dimensional carbon sheets, with an approximate width of 40–200 nm and 4–10 μm in length, was evident from the FESEM measurement. The absence of impurity of any other metal ions was indicated from EDX studies. The catalytic activity of PPAC has been demonstrated in the Knoevenagel condensation reactions of active methylene compounds and aromatic aldehyde in a green reaction medium, ionic liquid, [pmIm]Br, for the synthesis of α,β-unsaturated compounds. The methodology has also been extended for the synthesis of xanthene derivatives. The notable increase in reaction rate is due to the combined functionality of PPAC’s catalytic activity and the ionic interactions provided by [pmIm]Br. This catalytic approach delivered impressive product yields (90%–95%), demonstrating high efficiency. Key advantages of this method include utilizing renewable biomass-based materials, simple catalyst preparation, a recyclable and eco-friendly solvent system, rapid reaction times (3–20 min), excellent yields of products, easier purification, and a clean, atom-efficient process. Overall, these features position this method as a strong and sustainable alternative to traditional techniques, and the strong synergistic effect of PPAC and [pmIm]Br imparts the system with enhanced catalytic characteristics, indicating its promising applicability in diverse organic transformations beyond those demonstrated in this study.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fctls.2025.1721217/full#supplementary-material

Supplementary material Supplementary material at the source

Footnotes

Edited by: Raju Dey, National Institute of Technology Calicut, India

Reviewed by: Biplob Borah, Royal Global University, India

Sudip Mondal, Seth Kesarimal Porwal College, India

Statements

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The authors declare that financial support was received for the research and/or publication of this article. We are pleased to acknowledge ANRF PAIR project (FNANRF/PAIR/2025/000003/PAIR-B) funded by the Department of Science and Technology (DST), Government of India for financial support for the characterization of materials.

Data availability

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

DP: Software, Methodology, Data curation, Investigation, Writing – original draft, Validation. AA: Data curation, Software, Formal Analysis, Writing – original draft. SR: Formal Analysis, Software, Writing – original draft, Data curation. SS: Supervision, Writing – original draft, Investigation. SP: Resources, Supervision, Writing – original draft, Conceptualization. SB: Writing – review and editing, Writing – original draft, Supervision, Conceptualization, Formal Analysis.

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Citation

Prasad, D., et al. (2026). Activated carbon sheets from pomegranate peel with ionic liquid for Knoevenagel condensation: synthesis of aryledene and xanthene derivatives. Frontiers in Catalysis, Volume 5. https://doi.org/
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5 - 2026

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Copyright © 2026 Prasad, Asatkar, Ram, Singh, Prajapati and Banerjee.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Santosh Kumar Prajapati; Subhash Banerjee

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