Sulfonated asphaltene as a highly efficient catalyst for the Mannich reaction with excellent diastereoselectivity and recyclability

Abstract

This study investigates the repurposing of asphaltene, a petroleum waste product, as a catalyst for organic reactions. Sulfonated asphaltene was synthesized and evaluated for its efficacy in catalyzing the Mannich reaction, displaying notable diastereoselectivity and operating effectively under mild conditions. Characterization of the catalyst’s chemical composition, structure, and thermal stability was conducted using FT-IR, TGA, XRD, CHN, BET-BJH, SEM, and EDS analyses. Employing the sulfonated asphaltene catalyst in a one-pot multi-component reaction to generate β-amino carbonyl derivatives in ethanol resulted in high product yields after 4–7 h. Notably, the catalyst exhibited recyclability, demonstrating reusability for at least 10 reaction cycles.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-80297-2.

Introduction

Asphaltene (As), a supramolecular compound derived from crude oil, has historically been considered a waste product, leading to pipeline blockages¹. However, recent investigations have revealed its similarities to graphene in terms of structural, physical properties, and chemical composition². Asphaltene contains heteroatoms such as oxygen, nitrogen, and sulfur, as well as saturated and unsaturated segments, aliphatic and aromatic rings, and even transition metals like vanadium³. Just as graphene can be oxidized to graphene oxide, asphaltene can also undergo oxidation and be utilized as a catalyst⁴. The use of asphaltene as a catalyst offers several advantages, including lower cost compared to graphene preparation, conversion of industrial waste into a value-added product, and the potential to address environmental concerns associated with asphaltene disposal⁵. It can be used as a fuel additive to improve combustion properties, in asphalt production for road construction, and as a precursor for carbon materials like activated carbon and carbon nanomaterials⁶. Asphaltene can also serve as a feedstock for chemical production, be utilized as a catalyst or adsorbent, and play a role in environmental remediation processes⁷. Its versatile nature and unique properties make asphaltene a valuable resource with significant contributions to energy, infrastructure, and environmental sustainability⁸. It is important to note that the application of asphaltene as a catalyst in organic synthesis is still an emerging field, and currently, there are two reported examples in the literature^9,10.

A one-pot multicomponent reaction (MCR) is a powerful synthetic strategy that allows for the simultaneous incorporation of multiple reactants into a single reaction vessel, resulting in the formation of structurally complex molecules in a single step^11–16. In these reactions, three or more starting materials react together under suitable reaction conditions, leading to the efficient formation of a single product with high efficiency and atom economy¹⁷. One-pot MCRs offer several advantages in organic synthesis, including time and resource efficiency, fewer purification steps, and the potential for the rapid generation of diverse chemical libraries¹⁸. They have found significant applications in medicinal chemistry, drug discovery, and the synthesis of complex natural products. ¹⁹ The key to a successful one-pot MCR lies in the careful selection of reactants and reaction conditions, such as temperature, solvent, and catalysts²⁰. By judiciously choosing appropriate starting materials and reaction parameters, chemists can orchestrate multiple bond-forming events simultaneously, allowing for the rapid construction of complex molecular frameworks^21–23.

The Mannich reaction is a widely recognized multicomponent reaction that typically requires the presence of an acidic catalyst^24–27. It involves the condensation of a non-enolizable aldehyde, a primary or secondary amine, and an enolizable carbonyl compound, resulting in the formation of an aminomethylated product. ²⁸ The enolizable carbonyl compound’s symmetry can lead to the generation of different diastereomeric isomers as products²⁹. Notably, certain catalysts not only promote high reaction yields but also induce enantioselectivity, allowing for the production of chiral amino methylated compounds³⁰. This capability opens up possibilities for the synthesis of complex molecules with specific stereochemical properties through the Mannich reaction^31–37.

In modern organic synthesis, the utilization of acidic catalysts stands as a cornerstone in enabling and accelerating a wide array of chemical reactions.³⁸ These catalysts, often tailored to exhibit specific reactivity profiles, play a pivotal role in facilitating transformations that might otherwise proceed sluggishly or not at all³⁹. Alongside acidic catalysts, heterocycle reactions form a fundamental aspect of organic chemistry, offering a diverse toolkit for constructing complex molecular frameworks⁴⁰. These reactions encompass a variety of transformations, including cyclizations, ring openings, and functionalizations, each contributing uniquely to the synthesis of intricate organic compounds⁴¹. Moreover, the advent of a bio-based solvents such as Cyrene^42–44 and deep eutectic solvents (DESs) has revolutionized reaction media, providing environmentally friendly alternatives with tunable properties that enhance reaction efficiency and selectivity^42–44. Recent advancements in the synthesis of Mannich reactions have showcased novel strategies and reagents, pushing the boundaries of this versatile transformation.⁴⁵ Furthermore, the development of synthesized catalysts brings both advantages and challenges, offering tailored reactivity while also requiring careful consideration of their limitations and potential drawbacks in practical applications⁴⁶.

We interest lies in green chemistry-oriented reactions that have been carried out in environmentally friendly reaction media in recent years^47–50. This focus on sustainable practices aligns with the principles of green chemistry, emphasizing the importance of minimizing environmental impact throughout the chemical process. Conducting reactions in eco-friendly solvents not only reduces the environmental footprint but also contributes to the development of more sustainable chemical methodologies. Herein, we report for the first time, the synthesis of asphaltene-based acidic catalysts and investigate their application in the Mannich reaction. By utilizing asphaltene, a byproduct of crude oil, as a precursor for the catalyst, we aim to address the dual objectives of utilizing a renewable resource and converting an industrial waste product into a value-added material. The acidic catalysts derived from asphaltene exhibit catalytic activity in the Mannich reaction, which is a multicomponent condensation reaction involving aldehydes, amines, and carbonyl compounds.

Experimental

The crude oil was purchased from Marun petrochemical industries in Khuzestan province, Iran. Other chemical reagents, including aniline, carbonyl compounds, chlorosulfuric acids, and solvents, were purchased from suppliers. To confirm the products¹, H NMR and FT-IR spectroscopy techniques were employed¹. H-NMR spectra were recorded on a 500 MHz spectrometer using CDCl₃ as the solvent, and chemical shifts were reported in parts per million (ppm) relative to TMS. FT-IR spectra were obtained using a Bruker Vector-22 infrared spectrometer with KBr disks. For elemental analysis, EDX studies were performed using a scanning Electron Microscope (VEGA3 TESCAN) at an operating voltage of 20 kV. Melting points were determined using a Büchi B-545 melting point apparatus. Thermal gravimetric analysis (TGA) data were recorded using a Netzsch-TGA 209 F1 instrument with a heating rate of 25 °C/min, covering the temperature range from room temperature to 800 °C. The asphaltene’s elemental analysis was determined using a CHN Perkin-Elmer 2400 II apparatus. X-ray diffraction (XRD) was revealed by using Bruker AXS-D8 Advance, and diffractometer type (Cu, Kα1:1.5405 Å). The surface area was determined by the BET (Brunauer-Emmett-Teller) and the pore size distribution was calculated by the BJH (Barrett-Joymer-Halenda) from the isotherm of the adsorption branch with the BELSORP-mini II – Microtrac instrument.

Asphaltene extraction from crude oil

100 mL of crude oil, 2 L n-heptane, and 10 mL toluene were mixed in a round-bottom flask. After 24 h the mixture was filtrated in a vacuum and the remained black-brownish residue was washed severally with n-heptane to remove impurities. The collected asphaltene was dried in an oven at 40 °C under vacuum for 5 h. (black solid, 10 mg)

Preparation of asphaltene sulfonic acid (As-SO3H)

2 g of asphaltene were mixed in 50 mL dichloromethane and the mixture was sonicated for 10 min. After the sonication process, the mixture temperature was decreased by adding 2 mL chlorosulfuric acid to an ice bath. After 5 h 10 mL methanol was added to the mixture for quenching excess chlorosulfonic acids and then filtered. The filtrated was washed with methanol and water severally and dried at 40 °C under vacuum for 5 h (Fig. 1).

Fig. 1.

The step-by-step preparation As-SO₃H from As.

General procedure

In a test tube with a magnetic stirring bar, aldehyde (0.5 mmol), aniline (0.5 mmol), enolizable carbonyl (0.5 mmol), ethanol (0.2 mL), and 20 mg As-SO₃H were mixed in a flask at room temperature for 5–7 h. The reaction completion was monitored by thin layer chromatography (TLC) (mobile phase 1:5 ethyl acetate: n-hexane) After this time had elapsed the reaction was quenched by adding water (5 mL) and ethyl acetate (5 mL) and As-SO₃H was isolated by centrifugation and washed Twice times with ethyl acetate (5 mL) to reuse again. The organic phase was separated and the ethyl acetate was evaporated by a rotary evaporator and the resulting residue was purified by recrystallizing with a suitable solvent column chromatography using silica gel to give a pure product. All synthesized compounds are known and compared with literature melting point.

Selected data

2-(phenyl(phenylamino)methyl)cyclohexanone (B14): 0.5mmol (0.051 mL) benzaldehyde, 0.5 mmol (0.046mL) aniline, 0.5 mmol (0.052 mL) cyclohexanone 20 mg As-SO₃H and 0.2 mL ethanol were mixed in test tube for 6 h at room temperature. The reaction was monitored by TLC and after 6 h the reaction was quenched by adding 5 mL water and 5 mL ethyl acetate. The catalyst was separated by centrifugation and the organic layer was evaporated by rotary evaporator. Remained residue recrystallized in ethanol. Melting point 114–116 °C.

δ_H: (500 MHz; CDCl₃; Me₄Si): 1.69–1.98 (6 H, m, aliphatic H), 2.34–2.39 (1 H, m, aliphatic H), 2.44–2.50 (1 H, m, aliphatic H), 2.77–2.81 (1 H, m, aliphatic H), 4.66–4.67 (1 H, d, J = 6.82 Hz, aliphatic H, anti-isomer), 4.77 (0.73, broad, N-H, D₂O exchangeable), 6.56–6.58 (2 H, d, J = 8.31 Hz, aromatic H), 6.66–6.68 (1 H, t, J = 7.32 Hz, aromatic CH), 7.08–7.11 (2 H, dd, J = 8.30 Hz - J = 7.32 Hz, aromatic H), 7.23–7.26 (1 H, t, J = 7.33 Hz, aromatic H), 7.32–7.35 (2 H, dd, J = 7.8 Hz – J = 7.34 Hz, aromatic H), 7.40–7.42 (2 H, d, J = 7.8 Hz, aromatic H).

2-(phenyl(phenylamino)methyl)cycloheptanone (B15): 0.5mmol (0.051mL) benzaldehyde, 0.5 mmol (0.046) aniline, 0.5 mmol (0.059mL) cycloheptanone 20 mg As-SO₃H and 0.2 mL ethanol were mixed in test tube for 6 h at room temperature. The reaction was monitored by TLC and after 6 h the reaction was quenched by adding 5 mL water and 5 mL ethyl acetate. The catalyst was separated by centrifugation and the organic layer was evaporated by rotary evaporator. Remained residue recrystallized in ethanol. Melting point 139–141 °C.

δ_H: (500 MHz; CDCl₃; Me₄Si): 1.27–1.73 (5 H, m, aliphatic H), 1.85–1.92 (4 H, m, aliphatic H), 2.01–2.10(2 H, m, aliphatic H), 2.27–2.56 (2 H, m, aliphatic H), 2.74–2.79 (1 H, m, aliphatic H), 2.90–2.97 (1 H, m, aliphatic H), 4.55–4.57 (0.54 H, d, J = 7.67 Hz, aliphatic H, anti-isomer), 4.68–4.69 (0.46 H, d, J = 4.84 Hz, aliphatic H, syn isomer), 5.48( 0.6 H, broad, D₂O exchangeable, N-H), 6.57–6.60 (2 H, t, J = 7.1 Hz, aromatic H), 6.65–6.68 (1 H, t, J = 7.46 Hz, aromatic H), 7.09–7.13 (2, dd, J = 7.46 Hz – J = 7.1 Hz, aromatic H), 7.25–7.43 (7 H, m, aromatic H).

3-(Phenyl(phenylamino)methyl)tetrahydro-4-pyranone (B16): 0.5mmol (0.051mL) benzaldehyde, 0.5 mmol (0.046) aniline, 0.5 mmol (0.046 mL) Tetrahydro-4 H-pyran-4-one 20 mg As-SO₃H and 0.2 mL ethanol were mixed in test tube for 7 h at room temperature. The reaction was monitored by TLC and after 7 h the reaction was quenched by adding 5 mL water and 5 mL ethyl acetate. The catalyst was separated by centrifugation and the organic layer was evaporated by rotary evaporator. Remained residue recrystallized in ethanol. Melting point 186–187 °C.

δ_H: (500 MHz; CDCl₃; Me₄Si): 2.47–2.81 (3 H, m, aliphatic CH), 3.71–3.90 (3 H, m, aliphatic H), 4.17–4.19 (1 H, m, aliphatic H), 4.82–4.84 (1 H, d, J = 9.31 Hz, anti-isomer), 4.97 (0.7 H, s, D₂O exchangeable, N-H), 6.60–6.61 (2 H, d, J = 6.51 Hz aromatic H), 6.60–6.71 (1 H, t, J = 6.87 Hz, aromatic H), 7.08–7.12 (2 H, dd, J = 6.87 Hz- J = 6.51 Hz, aromatic H), 7.34–7.45 (5 H, m).

Result and discussion

The characterization of As and As-SO₃H were performed using various methods, with the first method being FT-IR. The FT-IR spectrum of As and As-SO₃H are shown in Figs. 2 and 3. As FT-IR (Fig. 1) has just revealed two important absorption bands. a strong absorption band at 2926 cm^− 1 is related to C-H saturated stretching and two weak absorption bands at 1457 cm^− 1 and 1606 can be attributed to aromatic C = C ring stretch absorption. As-SO₃H has also shown various absorption bands that were observed in important regions, providing valuable information about the compound’s functional groups. The broadband observed from 2996 cm^− 1 to 3700 cm^− 1, with a centrality at 3435 cm^− 1, can be attributed to the O-H stretching vibration. The absorption band at 2918 cm^− 1, which can be attributed to C-H stretching. This suggests the presence of carbon-hydrogen bonds in the compound. The two strong absorption bands at 1601 cm^− 1 and 1440 cm^− 1 are related to the C = C aromatic ring stretch absorption. These bands indicated the presence of an aromatic ring structure in the compound. The strong absorption band at 1168 cm^− 1 corresponds to the O = S = O asymmetric stretching band, indicating the presence of a sulfur dioxide group. Additionally, another strong absorption band at 1027 cm^− 1 is observed, which is related to the stretching of the C-S bond. These bands indicate the presence of a sulfur-oxygen and carbon-sulfur bond. Furthermore, three important absorptions were identified in the As-SO₃H compound.

Fig. 2.

FT-IR spectrum of As.

Fig. 3.

FT-IR spectrum of As-SO₃H.

The XRD pattern of As-SO₃H, as depicted in Fig. 4, did not exhibit any distinct or notable reflection. This characteristic suggests that As-SO₃H lacks crystalline structures and is instead an amorphous solid. Amorphous materials do not possess long-range order in their atomic arrangement, resulting in the absence of sharp diffraction peaks in XRD patterns. The absence of distinct reflections in the XRD pattern provides further evidence supporting the amorphous nature of As-SO₃H.

Fig. 4.

XRD Diagram of As-SO₃H.

The TGA thermogram curve and its first derivative (DTG), as shown in Fig. 5; Table 1, exhibit distinct decomposition events for the As-SO₃H compound. The first mass loss occurs at 98 °C, which can be attributed to the evaporation of solvents and water. This initial weight reduction suggests that the sample may contain residual moisture or volatile organic compounds that are easily vaporized at relatively low temperatures. The second and the third mass loss are observed at 210 °C and 456 °C respectively, and they are associated with the decomposition of the SO₃H group. This indicates that the sulfonic acid groups present in the As-SO₃H compound undergoes thermal decomposition at this temperature range. The fourth main mass loss can be attributed to the decomposition of aliphatic and aromatic components of the compound. This suggests that both aliphatic and aromatic moieties present in the As-SO₃H compound undergo thermal decomposition at higher temperatures.

Fig. 5.

TGA curve of As-SO₃H.

Table 1.

Thermal decomposition data of As-SO₃H in N₂ atmosphere.

Sample	T5 (°C)	T10(°C)	T max1 (°C)	Tmax2 (°C)	Tmax3 (°C)	Char yield (800 °C) (%)
As-SO3H	137	207	210	456	537	2.8

The CHN analysis was employed to determine the carbon, hydrogen, and nitrogen content of the asphaltene sample, as indicated in Table 2. Furthermore, the accuracy and precision of the EDS analysis were validated by confirming the carbon content. In this case, the carbon content was compared between the CHN method and the EDS analysis, and the agreement between the two methods confirmed the accuracy and precision of the EDS analysis.

Table 2.

CHN analysis of As-SO₃H.

Sample	%C	%H	%N
As	78.51	6.61	0.75
As-SO3H	42.55	4.41	0.42

Moreover, the results from the EDS analysis demonstrated an increase in the sulfur content due to the sulfonation process. Additionally, it was observed that the weight% of carbon decreased as a result of the sulfonation process. These findings, presented in Table 3, provide evidence that the sulfonation process was successfully conducted on the asphaltene sample. The EDS analysis revealed an increase in sulfur content and a decrease in carbon weight%, indicating the successful sulfonation process. The data presented in Tables 2 and 3 support these conclusions.

Table 3.

EDS data of as and As-SO₃H.

Sample	C (Weight%)	O (Weight%)	S (Weight%)	N (Weight%)
As	60.97	21.22	12.24	5.56
As-SO3H	40.27	36.97	18.76	3.99

The surface morphology of As and As-SO₃H was studied using SEM, offering crucial insights into their morphology and surface characteristics (Figs. 6 and 7). The SEM images of as suggest a nearly uniform morphology with slight agglomeration and an absence of porosity. In contrast, the SEM images of As-SO₃H exhibit a distinct morphology compared to As, confirming surface modification.

Fig. 6.

SEM images of As.

Fig. 7.

SEM images of As-SO₃H.

The N₂ adsorption–desorption diagram in BET and BJH shows pore size, surface area, and pore volume are shown in Fig. 8. The radius pore was reported 1.86 nm, surface area in BET was reported 3.8477 m²/g.

Fig. 8.

The adsorption/desorption isotherm of As-SO₃H (left) and BJH pore size distribution of As-SO₃H.

To optimize the reaction conditions, various parameters were investigated in the model reaction, and the results are presented in Table 4 to identify the most suitable solvent for the reaction. Different types of solvents, including polar, non-polar, protic, and aprotic ones, were investigated. The highest yield of 98% was achieved with ethanol (Table 4, entry A22). While the lowest yield of 57% was observed with CH₂Cl₂ (Table 4, entry A3). Other solvents such as chloroform (Table 4, entry A6), dimethylformamide (DMF) (Table 4, entry A1), dimethyl sulfoxide (DMSO) (Table 4, entry A2), water (Table 4, entry A8), and others were also examined during the optimization process. Regarding the amount of As-SO₃H catalyst, it was found that adding 20 mg of the catalyst significantly improved the reaction yield, resulting in optimal yield. However, increasing the amount of catalyst beyond this point did not further enhance the yield.

Table 4.

Monitoring solvent and temperature on the model reaction.

Entry	Solvent	Volume (mL)	Catalyst loading (mg)	Temperature (°C)	Yield (%)
A1	DMF	0.5	40	60	80
A2	DMSO	0.5	40	60	79
A3	CH2Cl2	0.5	40	Reflux	57
A4	Methanol	0.5	40	60	90
A5	Acetonitrile	0.5	40	60	86
A6	CHCl3	0.5	40	Reflux	66
A7	Ethyl acetate	0.5	40	50	70
A8	Water	0.5	40	60	79
A9	Solvent-free	-	40	60	67
A10	THF	0.5	40	Reflux	59
A11	PEG-200	0.5	40	60	91
A12	Glycerol	0.5	40	60	88
A13	Ethanol	0.5	40	60	98
A14	Ethanol	0.5	30	60	98
A15	Ethanol	0.5	20	60	98
A16	Ethanol	0.5	15	60	91
A17	Ethanol	0.5	20	40	98
A18	Ethanol	0.5	20	R.T*	98
A19	Ethanol	0.5	20	Reflux	98
A20	Ethanol	0.4	20	R.T	98
A21	Ethanol	0.3	20	R.T	98
A22	Ethanol	0.2	20	R.T	98
A23	Ethanol	0.1	20	R.T	89

Reaction conditions: benzaldehyde, (0.5 mmol, 0.051 mL), aniline (0.5 mmol, 0.046 mL), cyclohexanone (0.5 mmol, 0.052 mL), As-SO₃H (15–40 mg), and solvent (0.1–0.5 mL) for 6 h.

After successful optimization in the Mannich reaction with As-SO₃H as a catalyst, the reaction was further studied using various benzaldehyde derivatives (with electron-donating groups (EDG) and electron-withdrawing groups (EWG)) and different enolizable ketones. The catalyst effectively promoted the reaction, regardless of the presence of either EWGs or EDGs. Although aldehydes with EDGs (methoxy and methyl) and amines with EWGs (NO₂, Cl) do not have an outstanding yield in the Mannich reaction, some amines with EWGs and aldehydes with EDGs exhibit acceptable yields with this catalyst (B11, B13). The highest yields belonged to benzaldehyde and aldehydes with EWGs (B1, B4 and B5). One notable advantage of this catalyst is diastereoselectivity. The catalyst produces a specific diastereomer with 100% (anti) diastereoselectivity (B14-B16) and 54/46% (syn/anti) (B15) (Table 5).

Table 5.

Synthesis of β-aminocarbonyl derivatives in ethanol.

Entrya	Ar1 (aldehyde)	Ar2 (amine)	Ar3 (enolizable carbonyl)	Yield (%)	Time (h)	Syn/anti or Rfb	Melting point (°C)
							Found	Reported(ref)
B1	C6H5	C6H5	Acetophenone	98	6	0.50	165–167	167-16951
B2	4-Cl C6H4	C6H5	Acetophenone	91	6	0.41	118–120	118-12051
B3	4-MeO C6H4	C6H5	Acetophenone	78	7	0.45	150–151	150-15151
B4	3-NO2 C6H4	C6H5	Acetophenone	95	5	0.31	140–142	141-14252
B5	4-NO2 C6H4	C6H5	Acetophenone	93	5	0.28	107–108	107-10951
B6	4-Br C6H4	C6H5	Acetophenone	92	6	0.43	137–139	138-13951
B7	4-Me C6H4	C6H5	Acetophenone	95	7	0.49	132–134	133-13451
B8	C6H5	4-Cl C6H5	Acetophenone	91	6	0.38	165–166	167-16951
B9	4-NO2 C6H4	4-Cl C6H5	Acetophenone	90	6	0.22	128–129	127-13053
B10	4-Me C6H4	4-Cl C6H5	Acetophenone	87	7	0.37	161–163	160-16254
B11	4-MeO C6H5	4-Cl C6H4	Acetophenone	88	6	0.46	120–122	123-12555
B12	4-Me C6H4	4-NO2 C6H4	Acetophenone	81	6	0.46	160–162	165-16557
B13	C6H5	C6H5	4-Cl acetophenone	86	6	0.41	133–134	132-13358
B14	C6H5	C6H5	Cyclohexanone	91	6	0/100
B15	C6H5	C6H5	Cycloheptanone	85	6	54/46
B16	C6H5	C6H5	Tetrahydro-4 H-pyran-4-one	82	7	00/100
B17	C6H5	3-Me C6H4	Cyclohexanone	84	7	0/100
B18	4-ClC6H4	C6H5	Cycloheptanone	78	7	52/48
B19	4-ClC6H4	C6H5	Cyclohexanone	79	7	0/100
B20	C6H5	3-MeC6H4	Cycloheptanone	76	7	68/32
B21	4-ClC6H4	4-BromoC6H4	Cyclohexanone	81	7	69/31
B22	4-BrlC6H4	C6H5	Cyclohexanone	75	7	30/70

^aReaction conditions: aldehyde (0.5 mmol), aniline (0.5 mmol), an enolizable ketone (0.5 mmol), ethanol (0.2 mL), and 20 mg As-SO3H were mixed in a flask at room temperature for 5–7 h.

^b(Rf for n-hexane-ethyl acetate = 5:1);

The successful recycling of the catalyst for multiple reaction cycles highlights its potential for practical applications, as it offers sustainability and cost-effectiveness. The ability to reuse the catalyst without a loss in catalytic activity enhances the overall efficiency and economic feasibility of the reaction process. As a heterogeneous catalyst, As-SO₃H, demonstrates the ability to be recycled. After each reaction cycle in the model reaction (B1), ethyl acetate (5 mL) were added and the catalyst was isolated via centrifugation. The isolated catalyst was then washed with ethanol (0.5 mL) for reuse in subsequent cycles. The recycling performance of the catalyst was assessed over a total of 10 consecutive cycles (Fig. 9). The FTIR spectrum of the used catalyst after 10 cycles (Fig. 10) indicates that its spectrum is similar to sulfonated asphaltene without any major changes. Therefore, based on this FTIR analysis, it can be concluded that after 10 uses, this catalyst possesses the appropriate functional groups required for catalytic activity.

Fig. 9.

Recyclability of As-SO₃H catalyst on model reaction (B1).

Fig. 10.

FT- IR spectrum of Recycled As-SO₃H.

The proposed mechanism of this reaction on this catalyst is illustrated in Fig. 11. In the proposed one-pot synthesis of β-amino carbonyl derivatives utilizing the As-SO₃H catalyst, a conceivable reaction mechanism was envisioned. Initially, the As-SO₃H catalyst initiates the reaction by activating the carbonyl compound through protonation, rendering it more susceptible to nucleophilic attack. Subsequently, an amine nucleophile was added to the activated carbonyl, forming an imine intermediate. This intermediate then undergoes further transformation, possibly attack of the enol derived from the ketone on the protonated imine, ultimately yielding the desired β-amino carbonyl derivative. Proton transfers and potential rearrangements may also play a role in the process, leading to the final product. Throughout these steps, the As-SO₃H catalyst is likely involved in facilitating the reactions and may undergo regeneration to sustain catalytic activity in the synthesis process.

Fig. 11.

The proposed mechanism of synthesis β-amino carbonyl with As-SO₃H as a recyclable catalyst.

Conclusion

In summary, this study showcases the successful repurposing of asphaltene waste as a catalyst for organic transformations, with sulfonated asphaltene proving to be a highly efficient catalyst for the Mannich reaction. The notable diastereoselectivity and recyclability of the catalyst, along with its performance under mild conditions, underscore its potential in green chemistry applications. The comprehensive characterization of the catalyst through various analytical techniques further validates its suitability for catalytic processes. Using the As-SO₃H catalyst resulted in high yields of the desired products and demonstrated excellent diastereoselectivity. The reaction process was straightforward and efficient, making it a practical and efficient method for synthesizing β-amino carbonyl derivatives. Furthermore, the catalyst offered significant advantages in terms of its reusability. It could be recycled and reused for at least 10 consecutive reaction cycles without any noticeable decrease in catalytic activity.

Electronic supplementary material

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Author contributions

Navid Habibnejad performed material preparation, data collection, and analysis. Mohsen Hajibeygi wrote the first draft of the manuscript. Najmedin Azizi supervised and Writing- review & editing. All authors reviewed the manuscript.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data availability

All data generated during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.