Hybrid catalytic systems employing carbon nanotubes as a support for the solvent-free aerobic oxidation of ethylbenzene

doi:10.21203/rs.3.rs-4913672/v1

Download PDF

Article

Hybrid catalytic systems employing carbon nanotubes as a support for the solvent-free aerobic oxidation of ethylbenzene

https://doi.org/10.21203/rs.3.rs-4913672/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

Carbon nanotubes have gained significant interest as catalysts and catalytic supports in hydrocarbon oxidation processes. In this study, pristine multiwalled carbon nanotubes and copper(II) functionalized multiwalled carbon nanotubes were coated with [bmim] cationic ionic liquids (ILs) containing dissolved N-hydroxyphthalimide (NHPI) to produce novel SILP and SCILL-SILP hybrid catalytic systems, respectively (SILP: supported ionic liquid phase and SCILL: solid catalyst with an ionic liquid layer). The catalytic activities of the produced systems were investigated for the solvent-free oxidation of ethylbenzene (80 ^oC, 0.1 MPa, 6 h) using molecular oxygen as a green oxidant. Among the SILP systems, the [bmim][Cl]-based SILP system exhibited the highest conversion of ethylbenzene (14.4%) with the highest selectivity (92.1%) towards acetophenone (AcPO). The catalytic activity of the SILP system increased with increasing lipophilicity of the alkyl group in the IL cation. Conversely, among the SCILL-SILP systems, the highest conversion of ethylbenzene (23.4%) was achieved using [bmim][NTf₂] as the IL phase. Recyclability and reusability studies showed that the catalytic activities of the SILP and SCILL-SILP hybrid systems generally decreased in subsequent cycles, except for [bmim][OcOSO₃]-based catalytic systems.

SILP system

SCILL-SILP system

Hydrocarbon oxidation

Carbon nanotubes

Ionic liquids

N-Hydroxyphthalimide

Carbon materials, particularly carbon nanotubes (CNTs), have gained significant interest in various fields, including catalysis and catalytic support, due to their unprecedented physical and chemical properties, such as low cost, environmental friendliness, elevated conductivity, high specific surface area, and tunable chemical functionality [1, 2]. Among various catalytic processes, the oxidation of hydrocarbons to more valuable oxygenated compounds, such as hydroperoxides, alcohols, ketones, aldehydes, acids, and epoxides, is of crucial importance [3, 4]. Transition-metal salts and complexes have been employed as catalysts in these processes [5, 6]; however, the required conditions are harsh, and the processes have limited selectivity. In addition, the separation of homogeneous catalysts from the reaction mixture is challenging, leading to product contamination. Furthermore, toxic metal oxides [7], peroxides [8–10], and ozone [11] have been utilized as oxidants in these processes; however, molecular oxygen is known as a greener alternative [12]. Organic solvents have also been employed frequently to enhance the efficiency of these processes, but their use raises environmental and economic concerns because of issues related to solvent toxicity, flammability, and recovery [13]. Consequently, the implementation of a more sustainable and environmentally friendly alternative catalytic process becomes imperative. Carbon nanotubes could serve as heterogeneous catalysts and catalytic supports in these oxidation processes as a greener alternative, with molecular oxygen as an oxidant.

CNTs have been demonstrated as active metal-free catalysts in hydrocarbon oxidation processes [14]. Research has revealed that the superior catalytic activities of sp² carbons, such as graphene and CNTs, are attributed to their ability to accelerate the decomposition of intermediate hydroperoxides in the liquid-phase aerobic oxidation of hydrocarbons [15]. The transfer of electrons from the graphene sheets of CNTs is a crucial factor in determining their catalytic activity in free-radical chain reactions, as observed in the case of hydrocarbon oxidation in the liquid phase. The delocalized electrons on the graphene sheets of pristine CNTs facilitate π-π interactions between radicals and intermediate hydroperoxides, leading to the decomposition of intermediate hydroperoxides into free radicals, thus improving the overall catalytic activity [16]. For example, Luo et al. [17] demonstrated the catalytic activity of pristine CNTs as metal-free catalysts in the aerobic oxidation of ethylbenzene to acetophenone. The enhanced catalytic activity was attributed to the capability of CNTs to expedite the decomposition of intermediate hydroperoxides, a result of the π-π interactions between the delocalized electrons on the CNTs surface and the intermediate hydroperoxide, as well as the free radicals. The same phenomenon was observed in the case of cumene oxidation to cumene hydroperoxide using CNTs as a catalyst [18].

Functionalization of CNTs is another advantageous way to tailor their catalytic properties [19, 20]. Functionalized CNTs have been extensively investigated for the liquid-phase aerobic oxidation of hydrocarbons. Yu et al.[16] studied the catalytic activities of multiwalled CNTs (MWCNTs) and nitrogen-doped CNTs (N-CNTs) in the aerobic oxidation of hydrocarbons in the liquid phase. The N-CNTs displayed remarkable performance compared to pristine MWCNTs due to nitrogen doping, which facilitated electron transfer in the graphene sheet. Chao et al.[21] also investigated the catalytic activities of heteroatom-doped CNTs using nitrogen (N), phosphorus (P), and boron (B) as heteroatoms. N-CNTs and P-CNTs performed quite well, whereas B-CNTs were not active in the liquid-phase aerobic oxidation of cyclohexane. Researchers have found that doping heteroatoms, for example, nitrogen doping in CNTs, elevates the interactions between radicals, intermediate hydroperoxides, and the graphene sheets of CNTs, which in turn augments the catalytic activity of the modified CNTs [22–25]. In addition, heteroatom doping accelerates the decomposition of intermediate hydroperoxides, which again improves their catalytic activity [26–28].

The catalytic activity of CNTs in the liquid-phase aerobic oxidation of hydrocarbons can be further enhanced by improving the surface electron transfer of CNTs through metal functionalization. This involved the incorporation of metals such as Co, Fe, Pd, Au, etc. [29–32] with CNTs. Various metallic compounds, including metallic salts of Cu(II), Co(II), and Mn(II) [33], metal oxides such as MnO₂ [34] and Mn₃O₄ [35], metal complexes such as Pt and Pd pincer complexes [36], and Mn(III) porphyrins [37], among others, have also been used in the functionalization of CNTs to augment their catalytic activity in the liquid-phase aerobic oxidation of hydrocarbons. Moreover, CNTs and functionalized CNTs were stable and recyclable in all of the above-reported cases. In addition, researchers have reported the elevated catalytic activity of N-hydroxyphthalimide (NHPI) in hydrocarbon oxidation reactions [38–40]. In fact, the elevated catalytic activity of NHPI is associated with the formation of the phthalimide N-oxyl radical (PINO) (Scheme 1), which can abstract hydrogen atoms from hydrocarbons more effectively than peroxyl radicals in autocatalytic processes [41]. A variety of additives, such as azo compounds, peroxides, transition metal salts, aldehydes, quinones, and their derivatives, are used to generate the PINO radical [42].

Peckh et al. studied the impact of NHPI as a cocatalyst in the presence of azobisisobutyronitrile (AIBN) (0.2 mol%) as a radical initiator with CNTs functionalized by transition metal salts of Cu(II), Co(II), and Mn(II) for the liquid phase aerobic oxidation of ethylbenzene (80 ^oC, 0.1 MPa, 6 h). Following the introduction of 0.5 mol% NHPI, the catalytic activity increased by 2.3, 3.4, and 1.6 times for MWCNT-COO-Co, MWCNTs-COO-Cu, and MWCNT-COO-Mn, respectively. The catalytic activity of carboxylated MWCNTs remained unchanged before and after the addition of NHPI [33]. It is evident from the literature that the use of ionic liquids (ILs) can improve the catalytic performance of NHPI [44–46]. For example, when [bmim][OcOSO₃] was added to the NHPI/Co(II) system during the aerobic oxidation of ethylbenzene (80°C, 6 h, 0.1 MPa), the conversion increased from 14.9–29.2% [46]. The idea of a "supported ionic liquid phase (SILP)," in which a thin layer of IL is placed over solid support, was proposed by Mehnert et al. [47, 48] and Riisager et al. [49, 50] to heterogenize ILs. As a "solid catalyst with ionic liquid layer (SCILL)”, ILs have also been reported to be coated over solid catalysts [51, 52]. Both the SILP and SCILL systems combine the benefits of homogeneous catalysts, such as elevated catalytic activity, and heterogeneous catalysts, such as ease of catalyst separation and recycling.

Dobras et al. [53] investigated the catalytic activity of various SCILL systems for the solvent-free oxidation of ethylbenzene using molecular oxygen as an oxidant (80 ^oC, 0.1 MPa, 6 h). SCILL systems were prepared by coating different ionic liquids (ILs), such as [bmim][OcOSO₃], [bmim][Cl], and [bmim][CF₃SO₃], containing dissolved CoCl₂ onto NHPI-immobilized silica gel. The results revealed that all SCILL systems converted ethylbenzene more efficiently than the system comprising only a mixture of IL and CoCl₂. The highest conversion of ethylbenzene (12.1%) was obtained for the [bmim][OcOSO₃]-based SCILL system. The recyclability data showed that the [bmim] cation-based SCILL system containing [OcOSO₃] and [CF₃SO₃] anions showed a drop in conversion after the fourth cycle, whereas the conversion of the [bmim][Cl]-based SCILL system dropped after the third cycle, which was due to the leaching of the mixture of IL and CoCl₂ from the solid support surface. Talik et al. [54] coated a mixture of IL, [emim][OcOSO₃], and CoCl₂ onto an NHPI-immobilized polystyrene support. The resultant SCILL/SILP catalytic system was then employed for the oxidation of ethylbenzene using dioxygen as an oxidant in a solvent-free environment (80 ^oC, 0.1 MPa, 6 h). The SCILL/SILP (PS-NHPI-4@CoCl₂@[emim][OcOSO₃]) system achieved a 7.2% conversion of ethylbenzene with significantly improved selectivity (76.5%) toward acetophenone. The recyclability data indicated that the SCILL/SILP system could be reused without a significant loss in catalytic performance.

In our previous research [55], we developed a SCILL and SCILL-SILP hybrid catalytic system utilizing [emim][OcOSO₃] IL, which was chosen for its remarkable catalytic performance and stability, as demonstrated by repeated recyclability in the SCILL/SILP system developed by Talik et al. [54]. The SCILL system was prepared by coating IL, [emim][OcOSO₃], onto Cu(II) immobilized industrial-grade multiwalled carbon nanotubes (MWCNT-COO-Cu), while the SCILL-SILP hybrid catalytic system was prepared by coating a thin layer of dissolved NHPI in IL ([emim][OcOSO₃]) onto MWCNT-COO-Cu. The synthesized catalytic systems were then applied for the solvent-free oxidation of ethylbenzene using molecular oxygen as an oxidant (80 ^oC, 0.1 MPa, 6 h). It was observed that coating only IL over the MWCNT-COO-Cu, as in the case of the SCILL system, reduced the catalytic activity due to the high viscosity of IL, which hindered the radical formation required to initiate the oxidation reaction. Compared with previous studies, the SCILL-SILP hybrid catalytic system exhibited impressive catalytic activity with 27% ethylbenzene conversion and 77% selectivity toward acetophenone. However, the recyclability test of the SCILL-SILP system revealed that part of the mixture of NHPI and IL was leached out, which reduced its catalytic performance in the corresponding cycles.

In this study, an investigation has been conducted to produce more resilient and recyclable SILP and SCILL-SILP hybrid catalytic systems with the synergetic effects of CNTs and/or functionalized CNTs, ILs, and NHPI for the solvent-free aerobic oxidation of ethylbenzene using molecular oxygen as a green oxidant. The SILP systems were prepared by coating NHPI dissolved in ILs over pristine MWCNTs, whereas the SCILL-SILP hybrid catalytic systems were prepared by coating NHPI dissolved in ILs over MWCNT-COO-Cu. To the best of our knowledge, the reported SILP and SCILL-SILP hybrid catalytic systems are novel and have not been reported previously except for the [emim][OcOSO₃]-based SCILL-SILP system, which we reproduced just for the sake of comparison from our previous work. The recyclability and reusability of the SILP and SCILL-SILP catalytic systems have also been thoroughly examined.

2.1 Materials

Ethylbenzene (99.8%), purchased from Acros Organics, was washed with sulfuric acid, and distilled under vacuum. Multiwalled carbon nanotubes (MWCNTs) with purity = 95 + wt%, outer diameter (OD) = 20–40 nm, and length (L) = 1–2 µm and carboxylated multiwalled carbon nanotubes (MWCNT-COOH) with purity = 95 + wt%, OD = 8–15 nm, and L = 10–50 µm were purchased from IOLITEC Ionic Liquids Technologies GmbH, Heilbronn, Germany. Copper chloride dihydrate (CuCl₂⋅2H₂O) was purchased from POCH, Poland. Ammonia solution (25%), acetone (99%), and ethanol (99.8%) were purchased from Chempur, Poland. Acetone was dried over molecular sieves. N-hydroxyphthalimide (NHPI) and nylon membrane filters (pore size 0.22 µm and diameter 47 mm) were purchased from Sigma-Aldrich, Steinheim, DE, USA. Ionic Liquids: 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), 1-butyl-3-methylimidazolium bromide ([bmim][Br]), 1-butyl-3-methylimidazolium octylsulfate ([bmim][OcOSO₃]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][NTf₂]), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][NTf₂]), 1-ethyl-3-methylimidazolium octylsulfate [emim][OcOSO₃], 1-octyl-3-methylimidazolium chloride ([omim][Cl]), and 1-decyl-3-methylimidazolium chloride ([C₁₀mim][Cl]), were commercial materials and were dried under vacuum before use (50 ^oC, 0.01 MPa).

2.2 Preparation of MWCNT-COO-Cu

MWCNT-COO-Cu was synthesized according to a previously defined technique [56]. Initially, 1 g of MWCNT-COOH was added to 40 mL of a 10% aqueous ammonia solution. The mixture was then sonicated for 3 h, filtered, thoroughly washed with deionized water, and dried at 90°C for 3 h to produce ~ 1 g of MWCNT-COO-NH₄. 0.2 g of the obtained MWCNT-COO-NH₄ was combined with 25 mL of ethanol, to which 5 mL of a 0.3 M CuCl₂·2H₂O solution was added. The resulting mixture was subjected to sonication for 1.5 h, filtered, thoroughly washed with ethanol, and then dried for 3 h at 90°C, yielding ~ 0.2 g of the MWCNT-COO-Cu product. The recyclability and reusability of the catalyst were determined by separating it from the reaction mixture by employing a filtration process, washing it by using ethanol, and then employing it in the oxidation reaction.

2.3 Preparation of the SILP and SCILL-SILP catalytic systems

The SILP catalytic systems were prepared by coating MWCNTs with a mixture of selected IL and NHPI. The coating procedure involved mixing 0.025 g of IL and 0.027 g of NHPI with 0.025 g of MWCNTs in 5 mL of acetone. The mixture underwent 30 min of sonication, followed by stirring at 1000 rpm for 3 h at room temperature. Acetone was then evaporated using a vacuum evaporator, keeping the bath temperature at 60°C and gradually decreasing the pressure from 600 mbar to 300 mbar. The SCILL-SILP hybrid catalytic systems were prepared using the same technique, with the exception that MWCNTs were replaced with MWCNT-COO-Cu. The recyclability and reusability of the SILP and SCILL-SILP systems were performed by isolating the catalytic systems from the reaction mixtures using a filtration technique. The separated systems were thoroughly washed with hexane and then dried using a vacuum desiccator for 12 h. The dried catalysts were then used in the oxidation reactions.

2.4 Ethylbenzene oxidation

The oxidation reactions of ethylbenzene were performed using a gasometric apparatus [56]. The desired reaction mixture was placed in a 10 mL flask linked to a burette. Oxygen was introduced at atmospheric pressure, and the reaction mixture was maintained at 80 ^oC with continuous stirring at 1200 rpm using a magnetic stirrer. The volume of the consumed oxygen (O₂) in mL was recorded from the burette, and the moles of O₂ were determined using the following equation:

$$\:{n}_{{O}_{2}}=\frac{{V}_{{O}_{2}}*273.15*P}{101325*22415*T}\:\left(mol\right)\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\left(1\right)$$

In Eq. (1), $\:{V}_{{O}_{2}}$ is the volume (mL) of consumed oxygen, and P and T represent the atmospheric pressure (Pa) and room temperature (K), respectively. Subsequently, the conversion of ethylbenzene was determined using the following equation:

$$\:\alpha\:=\frac{{n}_{{O}_{2}}}{n}*100\:\left(\%\right)\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:..\left(2\right)$$

In Eq. (2), $\:{n}_{{O}_{2}}$ denotes the moles (mol) of used O₂ and $\:n$ represents the moles (mol) of ethylbenzene used.

The quantity of 1-phenylethyl hydroperoxide (PEHP) produced in each reaction mixture was determined using iodometric analysis [57]. A sample weighing approximately 0.2 g (± 0.0001 g) was taken from the reaction mixture shortly after the completion of the reaction and placed in a flask. To this, 20 mL of glacial acetic acid was added. The solution underwent carbon dioxide flushing to eliminate air, followed by the addition of approximately 2 g of sodium iodide. The flask was then sealed and placed in a dark place for 30 min to facilitate iodide formation. Subsequently, 20 mL of distilled water was added to the mixture, which was titrated using a standard 0.1 M sodium thiosulfate solution. The titration was performed employing a Brand Digital II apparatus with a measuring range of 25 mL and a 0.01 mL subdivision. The selectivity of the hydroperoxide was then determined using the following equation:

$$\:{S}_{PEHP}=\frac{{n}_{PEHP}}{{n}_{{O}_{2}}}*100\:\left(\%\right)\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\dots\:\left(4\right)$$

In Eq. (4), $\:{S}_{PEHP}$ represents the selectivity of the produced PEHP, $\:{n}_{PEHP}$ is the quantity (mol) of PEHP in the reaction mixture, and $\:{n}_{{O}_{2}}$ is the quantity (mol) of O₂ consumed after the completion of the reaction.

PEHP is thermally unstable and readily decomposes into 1-phenylethanol (PEA) and/or acetophenone (AcPO). Therefore, it is imperative to reduce it to PEA before gas chromatography (GC) analysis. This reduction process involves the application of triethyl phosphite ((EtO)₃P), and the utilized (EtO)₃P undergoes oxidation, resulting in the formation of triethyl phosphate (EtO)₃PO [58]. Consequently, the amount of PEA measured through GC analysis is the combined result of the PEA produced in the reaction and the decomposed PEHP. The determination of the selectivity of PEA involves subtracting the selectivity of PEHP, as determined by iodometric analysis, from the selectivity of PEA acquired from GC analysis.

2.5 Analytical methods

Gas chromatography (GC) analyses were performed using a Shimadzu GC-2010 Plus gas chromatograph from Kyoto, Japan. The instrument was equipped with an FID (Flame Ionization Detector) and ZB-5HT column (29.5 m in length and 0.25 mm in internal diameter). The morphologies of the synthesized catalytic systems were analyzed by employing the transmission electron microscope (TEM) of the Thermo Fisher Scientific TecnaiG2 F20 X-TWIN/FEI. The stability of the produced catalysts was investigated using thermogravimetric analyses (TGA) with the TGA/DSC STAR system analyzer from Mettler Toledo. The temperature range was from 25–800 ^oC with a heating rate of 10 ^oC min^− 1 with a constant nitrogen flow of 60 mL min^− 1.

In this study, novel SILP and SCILL-SILP hybrid catalytic systems were developed. SILP systems were synthesized by coating NHPI dissolved in IL onto pristine MWCNTs, whereas SCILL-SILP hybrid catalytic systems were synthesized by coating NHPI dissolved in IL onto Cu(II) functionalized carboxylated MWCNTs (Fig. 1). To develop more stable and active SILP and SCILL-SILP hybrid catalytic systems, ILs with various anions and cations have been utilized, i.e., [emim], [bmim], [omim] or [C₁₀mim] cations and [Cl], [Br], [BF₄], [PF₆], [OcOSO₃] or [NTf₂] anions. The recyclability and reusability of the SILP and SCILL-SILP catalytic systems have also been thoroughly examined. In addition, leaching tests were conducted for both catalytic systems.

As an example, the obtained [bmim][OcOSO₃]-based SILP and SCILL-SILP hybrid catalytic systems were characterized by employing TEM and TGA analyses. For comparison, a TEM image of used commercial MWCNTs was also taken. The smooth and defect-free nanotubes having a 20–40 nm outer diameter with no impurities can be observed from the TEM image of pristine MWCNTs (Fig. 2(a)). The non-uniform thin layer of dissolved NHPI in IL over the surface of pristine MWCNTs (SILP system) and Cu(II) functionalized CNTs (SCILL-SILP hybrid catalytic system) can be observed from the TEM images in Fig. 2(b) and (c), respectively (S. 1). It was already demonstrated in our previous paper [55] that coating the mixture of dissolved NHPI in IL onto the surface of Cu(II) functionalized MWCNTs significantly reduced the BET surface area, total pore volume, and total pore area (S. 2).

The catalytic activities of the produced SILP and SCILL-SILP hybrid catalytic systems were examined in the solvent-free aerobic oxidation of ethylbenzene, which served as a model hydrocarbon. All reactions were performed at 80°C, 0.1 MPa, and 1200 rpm for 6 h. The conversion of ethylbenzene was determined by measuring the quantity of oxygen consumed, and the composition of the reaction mixture was analyzed using both iodometric and GC methods. The identified products are 1-phenylethyl hydroperoxide (PEHP), 1-phenylethanol (PEA), acetophenone (AcPO), benzaldehyde (BA), and benzoic acid (BzA), as illustrated in Scheme 2.

3.1 Catalytic activities of the SILP systems

SILP systems were produced to combine the known catalytic activities of CNTs, NHPI, and ILs in the free-radical liquid-phase oxidation of hydrocarbons. Their catalytic activities in the solvent-free oxidation of ethylbenzene have been presented in Table 1. It was observed that both NHPI and MWCNTs slightly increased the ethylbenzene conversion (entries 2, and 3). The lower selectivity of PEHP in the presence of MWCNTs confirmed its activity in the decomposition of hydroperoxides. When NHPI was added with CNTs, the selectivity of PEHP increased significantly. This enhancement could be attributed to the known higher selectivity of hydroperoxides in the presence of NHPI [59, 60].

All the [bmim] cationic IL-based SILP systems exhibited higher catalytic activities in the oxidation reactions compared to pristine MWCNTs and MWCNTs/NHPI (entries 5–10). The observed effects, among others, could be a result of IL activity in catalyzing hydroperoxide decomposition, NHPI-IL interaction enhancing PINO formation, reagents and NHPI solubility in the IL layer, ILs polarity and viscosity, as well as ILs and NHPI leaching from MWCNTs surface.

The highest conversion of ethylbenzene (14.4%) was achieved for the SILP system based on [bmim][Cl], with the highest selectivity to acetophenone (92.1%). The elevated activity of the SILP system, MWCNTs@[bmim][Cl] compared to MWCNTs@[bmim][Br] aligns with previous results on the influence of halide ILs in NHPI-catalyzed hydrocarbon oxidation reactions reported by Talik et al.[61]. They discovered that the activity of ILs containing 1-alkyl-3-methylimidazolium cation and halide anion, [Rmim][X] in NHPI-catalyzed oxidation reactions was decreased in the order of [Cl]>[Br]>[I]. Their research revealed that there is an interaction between the halide anion in the IL and the hydrogen atom in the -NOH group of NHPI (Fig. 3), which can significantly impact the reaction rate. This interaction may weaken the NO-H bond, facilitating PINO generation. The strength of the interaction can be correlated with the acidity of the HX acids, as determined by the specific anions [X], with their strength increasing in the series [Cl] < [Br] < [I]. At the same time, the weaker the acid, the stronger its conjugate base, indicating that the -NOH-[Cl] interactions are stronger than the -NOH-[Br] and -NOH-[I] interactions, resulting in faster PINO generation and ultimately improving the reaction rate.

Table 1

Catalytic activities of SILP systems.
Entry	Catalyst	Con. (%)	Sel. (%)
Entry	Catalyst	Con. (%)	PEHP	PEA	AcPO	BA	BzA
1*	-	4.1	84.5	4.6	10.9	-	-
2*	NHPI	6.4	73.2	21.1	5.7	-	-
3*	MWCNTs	5.2	43.8	51.3	4.9	-	-
4*	MWCNTs + NHPI	6.3	91.2	4.9	-	-	-
5	MWCNTs@[bmim][Cl] + NHPI	14.4	5.2	1.5	92.1	1.2	-
6	MWCNTs@[bmim][Br] + NHPI	9.0	3.0	5.5	88.6	1.5	1.4
7	MWCNTs@[bmim][BF₄] + NHPI	11.0	38.6	10.6	27.8	19.7	3.3
8	MWCNTs@[bmim][OcOSO₃] + NHPI	10.9	48.5	3.5	32.6	13.1	2.3
9	MWCNTs@[bmim][NTf₂] + NHPI	10.8	49.2	11.0	24.3	14.8	0.7
10	MWCNTs@[bmim][PF₆] + NHPI	8.5	24.4	6.0	32.1	32.4	5.1
11	MWCNTs@[omim][Cl] + NHPI	20.8	12.0	1.8	83.8	1.8	0.6
12	MWCNTs@[C₁₀mim][Cl] + NHPI	27.8	8.6	1.8	87.3	1.8	0.5
13	MWCNTs@[emim][NTf₂] + NHPI	6.0	17.5	0.9	14.2	61.0	6.4
14	MWCNTs@[emim][OcOSO₃] + NHPI	10.4	59.6	11.7	19.4	9.0	0.3

Reaction conditions: Ethylbenzene = 16.3 mmol, MWCNTs = 0.025 g, NHPI = 0.027 g, IL = 0.025 g, AIBN = 0.186 mol%, 80 ^oC, 0.1 MPa, 6 h. *Results taken from our previous work [55].

The SILP systems produced using [bmim][NTf₂] and [bmim][OcOSO₃] demonstrated higher selectivity to PEHP. Despite their limited hydroperoxide decomposition activity, they demonstrated rather significant catalytic activity, potentially due to their better oxygen solubility. The oxygen solubility in the [bmim] cationic ILs decreases in the following order: NTf₂ > PF₆ ~ OcOSO₃ > BF₄ [62]. The oxygen solubility in [bmim][PF₆] is almost similar to that in [bmim][OcOSO₃], however, the NHPI displayed very little solubility in this particular IL.

The impact of the cation structure of IL on the catalytic activity of the SILP system has also been investigated. It was found that when the number of carbon atoms in the alkyl group of the cations in the IL of the SILP system increased, the conversion of ethylbenzene also increased with elevated selectivity towards AcPO. It is known that enhancing the lipophilicity of cations in the ILs leads to a higher solubility of hydrocarbons and oxygen in the ILs, as well as the solubility of ILs in nonpolar systems [53, 63]. However, this could potentially cause the undesired leaching of IL from the surface of MWCNTs.

To make a comparison, when the [bmim] cation of [OcOSO₃] and [NTf₂] was replaced with the [emim] cation to make a less lipophilic SILP system, the conversion of ethylbenzene was drastically decreased for the SILP system composed of [emim][NTf₂] with more selectivity towards BA (61.0%). BA formed as a result of the ꞵ-C-C cleavage in the 1-phenylethyloxy radical [64, 65]. This may represent the robust impact of [emim][NTf₂] on the cleavage of the alkoxy radical. The [emim][NTf₂]-based SILP system had the highest selectivity toward BA, followed by [bmim][PF₆] and [bmim][BF₄]. The SILP system based on [emim][OcOSO₃] still had a higher selectivity toward hydroperoxide, with the conversion of ethylbenzene comparable to that of the [bmim][OcOSO₃]-based SILP system.

3.2 Recyclability of SILP systems

The recyclability and reusability of SILP catalytic systems were investigated, and the results are presented in Table 2. The SILP catalytic systems were separated from the reaction mixture via the filtration process and washed with hexane. The dried catalysts were subsequently used to oxidize a fresh batch of ethylbenzene. The SILP system based on [bmim][OcOSO₃] was demonstrated to be the most stable compared to the other SILP systems that were studied. It was observed that the system's reusability decreased as the lipophilicity of the cation increased. For instance, by substituting the [bmim] cation in the [Cl]-based anionic IL with [omim] and [C₁₀mim], the conversion increased substantially for the fresh samples, as shown in Fig. 4 and Table 2 (entries 19, and 22). Nevertheless, their catalytic activities decreased drastically in the subsequent cycles. It could have been due to the undesirable leaching of IL from the catalyst surface.

Table 2

Recyclability and reusability of the SILP systems.
Entry	Cycle	Catalyst	Con. (%)	Sel. (%)
Entry	Cycle	Catalyst	Con. (%)	PEHP	PEA	AcPO	BA	BzA
1*	0	MWCNTs@[bmim][Cl] + NHPI	14.4	5.2	1.5	92.1	1.2	-
2	1		14.3	6.8	3.6	88.9	0.7	-
3	2		7.0	4.5	2.9	92.6	-
4*	0	MWCNTs@[bmim][Br] + NHPI	9.0	3.0	5.5	88.6	1.5	1.4
5	1		5.4	9.8	5.1	84.3	0.8	0.0
6	2		4.7	20.5	2.0	77.5	0.0	0.0
7*	0	MWCNTs@[bmim][BF₄] + NHPI	11.0	38.6	10.6	27.8	19.7	3.3
8	1		14.9	47.7	1.4	45.9	3.9	1.1
9	2		7.3	58.9	8.1	33.0	-	-
10*	0	MWCNTs@[bmim][OcOSO₃] + NHPI	10.9	48.5	3.5	32.6	13.1	2.3
11	1		9.9	63.0	15.4	21.6	-	-
12	2		11.1	65.1	10.9	24.0	-	-
13*	0	MWCNTs@[bmim][NTf₂] + NHPI	10.8	49.2	11.0	24.3	14.8	0.7
14	1		12.3	52.6	32.7	12.8	1.9	-
15	2		7.8	59.0	11.2	26.7	3.1	-
16*	0	MWCNTs@[bmim][PF₆] + NHPI	8.5	24.4	6.0	32.1	32.4	5.1
17	1		5.7	46.5	13.3	37.9	2.3	-
18	2		4.0	31.6	2.3	66.1	-	-
19*	0	MWCNTs@[omim][Cl] + NHPI	20.8	12.0	1.8	83.8	1.8	0.6
20	1		11.7	13.2	0.4	85.2	1.2	0.0
21	2		5.6	50.5	0.7	48.8	0.0	0.0
22*	0	MWCNTs@[C₁₀mim][Cl] + NHPI	27.8	8.6	1.8	87.3	1.8	0.5
23	1		9.3	17.0	5.5	77.5	0.0	0.0
34	2		7.2	37.2	24.7	38.1	0.0	0.0
25*	0	MWCNTs@[emim][NTf₂] + NHPI	6.0	17.5	0.9	14.2	61.0	6.4
26	1		12.0	20.9	4.6	73.4	1.1	0.0
27	2		8.5	76.0	4.4	18.6	1.0	0.0
28*	0	MWCNTs@[emim][OcOSO₃] + NHPI	10.4	59.6	11.7	19.4	9.0	0.3
29	1		14.3	51.0	20.8	22.1	6.1	-
30	2		8.4	73.0	0.6	24.6	1.8	-

Reaction conditions: Ethylbenzene = 16.3 mmol, *MWCNTs = 0.025 g, *NHPI = 0.027 g, *IL = 0.025 g, AIBN = 0.186 mol%, 80 ^oC, 0.1 MPa, 6 h.

To examine the reusability of the SILP system based on [bmim][OcOSO₃], a leaching test was performed. A fresh [bmim][OcOSO₃]-based SILP system was prepared and applied in the oxidation reaction. After 30 min, the SILP system was isolated from the reaction mixture, and the reaction effluent was allowed to be run to complete the specified reaction time. For comparison, reactions were also performed in the presence of SILP system components ([bmim][OcOSO₃], NHPI, and MWCNTs) added separately and directly to the reaction mixture. The results are shown in Table 3 and Fig. 5.

It was observed that, after the separation of the SILP system from the reaction mixture, the reaction rate decreased slightly. The conversions obtained after 6 h in reactions carried out in the presence of SILP and after SILP separation were approximately identical, indicating the undesirable leaching of the catalysts. The product composition suggests that mainly IL was leached out from the surface of MWCNTs. The selectivity to PEHP and AcPO achieved following the separation of the SILP system (entry 4) was comparable to that of the reaction run employing pristine [bmim][OcOSO₃] (entry 1).

The leaching of the mixture of IL and NHPI from the surface of MWCNTs was also confirmed by TGA analysis performed for both the fresh and recycled SILP systems after the second cycle. It can be observed from the TGA curves (Fig. 6) that the major weight loss for both samples was in the range of 150–400 ^oC, and there was still a portion of the coated mixture of IL and NHPI in the recovered catalyst.

Table 3

Leaching test for the SILP system.
Entry	Catalyst	Con. (%)	Sel. (%)
Entry	Catalyst	Con. (%)	PEHP	PEA	AcPO	BA	BzA
1^a	[bmim][OcOSO₃]	4.5	41.3	11.6	47.1	-	-
2^b	[bmim][OcOSO₃] + NHPI	8.7	64.8	4.2	31	-	-
5^c	MWCNTs + [bmim][OcOSO₃] + NHPI	15.0	34.0	8.1	56.7	1.2	-
3^d	SILP system	10.9	48.5	3.5	32.6	13.1	2.3
4^e	SILP leaching test	11.1	28.5	11.3	55.8	4.4	-

Reaction conditions: Ethylbenzene = 16.3 mmol, AIBN = 0.186 mol%, 80 ^oC, 0.1 MPa, 6 h; ^a 0.025 g IL; ^b 0.025 g IL and 0.027 g of NHPI; ^c 0.025 g MWCNTs, 0.025 g IL, and 0.027 g NHPI; ^d SILP system prepared by coating the mixture of [bmim][OcOSO₃] IL (0.025 g) and NHPI (0.027 g) over MWCNTs (0.025 g); ^e SILP system was separated from the reaction mixture during the reaction, and the effluent was allowed to proceed and complete the reaction time.

3.3 Catalytic activities of SCILL-SILP hybrid systems

The SCILL-SILP hybrid catalytic systems were developed to synergistically use the known catalytic activities of CNTs, NHPI, ILs, and copper(II) salts for the oxidation of hydrocarbons in the liquid phase using oxygen as an oxidant. The results of ethylbenzene oxidation in the presence of SCILL-SILP systems are presented in Table 4. For comparison, the catalytic activity of MWCNT-COO-Cu was also investigated in the presence of NHPI as a cocatalyst.

Table 4

Catalytic activities of SCILL-SILP hybrid systems
Entry	Catalyst	Con. (%)	Sel. (%)
Entry	Catalyst	Con. (%)	PEHP	PEA	AcPO	BA	BzA
1	MWCNT-COO-Cu + NHPI	26.8	3.6	8.4	87.0	1	-
2	MWCNT-COO-Cu@[bmim[Cl] + NHPI	14.4	1.3	5.1	93.0	0.6	-
3	MWCNT-COO-Cu@[bmim][NTf₂] + NHPI	23.4	12.5	11.3	67.7	7.9	0.6
4	MWCNT-COO-Cu@[bmim[BF₄] + NHPI	21.2	11.0	2.8	82.4	2.7	1.1
5	MWCNT-COO-Cu@[bmim[PF₆] + NHPI	20.5	7.8	6.5	80.6	4.2	0.9
6	MWCNT-COO-Cu@[bmim][OcOSO₃] + NHPI	20.2	14.5	6.2	77.3	2.0	-
7	MWCNT-COO-Cu@[emim][NTf₂] + NHPI	10.0	3.7	0.7	57.9	34.1	3.6
8	MWCNT-COO-Cu@[emim][OcOSO₃] + NHPI	23.6	16.0	0.2	80.3	2.4	1.1

Reaction conditions: Ethylbenzene = 16.3 mmol, MWCNT-COO-Cu = 0.025 g, NHPI = 0.027 g, IL = 0.025 g, AIBN = 0.186 mol%, 80 ^oC, 0.1 MPa, 6 h.

It was observed that the catalytic activity of Cu(II)-functionalized CNTs increased significantly compared to MWCNTs (Table 1, entry 4) when subjected to similar reaction conditions; the ethylbenzene conversion increased from 6 to ~ 27%. The catalytic activity of the MWCNT-COO-Cu/NHPI system is due to the synergistic effect of Cu(II)-functionalized CNTs and NHPI. Transition metal compounds, including Cu(II) compounds, are known to enhance the decomposition of hydroperoxide into corresponding alcohol and ketone, consequently enhancing the rate of the oxidation reaction [66–69]. MWCNT-COO-Cu also accelerated the formation of PINO radicals from NHPI. Furthermore, the literature has reported that the distribution of products in hydrocarbon oxidation reactions is dependent upon the specific type of transition metal salt employed. The presence of Cu(II) salts produced a higher ratio of ketone to alcohol compared to Co(II) and Mn(II) because of differences in their redox potentials [70]. Therefore, the ratio of ketone to alcohol for the MWCNT-COO-Cu/NHPI system was also high (10.4).

The catalytic activities of the SCILL-SILP hybrid catalytic systems decreased compared to MWCNT-COO-Cu/NHPI and depended on the type of IL used. Among the [bmim] cationic ILs-based SCILL-SILP systems, the obtained ethylbenzene conversion decreases in the following order of anions: [NTf₂]>[BF₄]≈[PF₆]≈[OcOSO₃]>[Cl]. All SCILL-SILP-based systems demonstrated higher selectivity towards AcPO in comparison to the respective SILP systems. The obtained results demonstrated that the availability of Cu(II) played a significant role and could be influenced, among others, by the IL viscosity. Among the studied [bmim] cationic ILs, [bmim][NTf₂] is characterized by high oxygen solubility and the lowest viscosity [62]. The obtained ethylbenzene conversion with remarkable AcPO selectivity for the [bmim][Cl]-based SCILL-SILP and SILP systems was approximately identical (Table 1, entry 5, and Table 4, entry 2), which suggested that the highly viscous [bmim][Cl] limited the availability of Cu(II). Additionally, the high selectivity to AcPO could also be attributed to the solvent cage effect of highly viscous [bmim][Cl]. The coated IL acted as a cage, confining the produced alkoxy radicals and restricting their ability to pass through the cage and react with ethylbenzene to produce PEA. Therefore, the β-scission reaction inside the IL-solvent cage is favored [71].

3.4 Recyclability and reusability of SCILL-SILP hybrid catalytic systems

An investigation was conducted to determine whether the SCILL-SILP catalytic systems were recyclable and reusable, and the results have been presented in Table 5. Unfortunately, the catalytic activities of all SCILL-SILP systems decreased significantly after recovery and reuse. The results demonstrated that the decrease in the catalytic activities is not only associated with the leaching of the mixture of ILs and NHPI but also with the leaching of Cu(II) from the MWCNT-COO-Cu. This is evident from the observation that the catalytic activity of Cu(II) also dropped substantially in the second cycle while exhibiting higher selectivity towards hydroperoxides (entry 3).

Table 5

Recyclability and reusability of the SCILL-SILP system.
Entry	Cycle	Catalyst	Con. (%)	Sel. (%)
Entry	Cycle	Catalyst	Con. (%)	PEHP	PEA	AcPO	BA	BzA
1*	0	MWCNT-COO-Cu + NHPI	26.8	3.6	8.4	87	1	-
2	1		22.7	37.4	6.8	55.2	0.6	-
3	2		18.6	34.6	4.8	60.7	-	-
4*	0	MWCNT-COO-Cu+[bmim][Cl] + NHPI	14.4	1.3	5.1	93.0	0.6	-
5	1		6.3	6.7	1.1	92.2	-	-
6	2		2.9	9.2	8.5	82.3	-	-
7*	0	MWCNT-COO-Cu+[bmim][NTf₂] + NHPI	23.4	12.5	11.3	67.7	7.9	0.6
8	1		14.0	41.5	23.0	31.7	3.8	-
9	2		13.8	44.7	44.0	10.1	1.2	-
10*	0	MWCNT-COO-Cu+[bmim][BF₄] + NHPI	21.2	11.0	2.8	82.4	2.7	1.1
11	1		13.7	40.6	10.3	47.1	2.0	-
12	2		7.2	56.0	6.5	35.1	2.4	-
13*	0	MWCNT-COO-Cu+[bmim][PF₆] + NHPI	20.5	7.8	6.5	80.6	4.2	0.9
14	1		7.3	32.0	6.2	60.6	1.2	-
15	2		5.5	61.5	7.2	31.3	-	-
16*	0	MWCNT-COO-Cu+[bmim][OcOSO₃] + NHPI	20.2	14.5	6.2	77.3	2.0	-
17	1		13.0	24.3	9.3	66.4	-	-
18	2		13.2	6.6	8.1	84.5	0.8	-
19*	0	MWCNT-COO-Cu+[emim][NTf₂] + NHPI	10.0	3.7	0.7	57.9	34.1	3.6
20	1		9.7	21.0	6.1	38.1	32.8	2.0
21	2		4.8	50.2	7.8	31.7	10.3	-
22*	0	MWCNT-COO-Cu+[emim][OcOSO₃] + NHPI	24.3	7.7	3.0	87.3	1.4	0.6
23	1		9.3	54.7	21.8	17.5	6.0	-
24	2		8.7	75.7	4.8	17.0	2.5	-
Reaction conditions: Ethylbenzene = 16.3 mmol, MWCNTs = 0.025 g, NHPI = 0.027 g, *IL = 0.025 g, 80 ^oC, 0.1 MPa, 6 h. AIBN = 0.186 mol%

To examine the leaching of the mixture of IL and NHPI from the surface of the catalyst, a test was performed employing a more stable SCILL-SILP catalytic system, which was the [bmim][OcOSO₃]-based system, and the results have been tabulated in Table 6. The SCILL-SILP system was isolated shortly after running the reaction (30 min), and the reaction mixture was allowed to complete the reaction time, as shown in Fig. 7. It can be observed that the rate of reaction significantly decreased after the SCILL-SILP system separation, and the final conversion was almost half of that attained in the presence of the SCILL-SILP system, demonstrating that only a portion of the catalysts have been leached out from the surface of CNTs.

The leaching of IL and NHPI could be associated with the fact that as the reaction time proceeded, more polar products formed and the solubility of IL and NHPI in the reaction mixture increased. To confirm the statement, the [bmim][OcOSO₃]-based SCILL-SILP system was washed thoroughly with a nonpolar solvent (hexane) before being applied to the oxidation reaction (Table 6, entry 5). It is worth noting that washing the SCILL-SILP system with the nonpolar solvent does not affect its activity, and the conversion of ethylbenzene (24.5%) and the selectivity towards AcPO (84.7%) were even higher. TGA analysis was also performed for the fresh and recycled (after the second cycle) [bmim][OcOSO₃]-based SCILL-SILP system (Fig. 8). It can be observed that the maximum weight loss was between 150–400 ^oC for both samples and despite leaching, there was still a portion of the IL and/or NHPI over the surface of the hybrid catalysts.

Table 6

Leaching test for the SCILL-SILP system.
Entry	Catalyst	Con. (%)	Sel. (%)
Entry	Catalyst	Con. (%)	PEHP	PEA	AcPO	BA	BzA
1^a	[bmim][OcOSO₃]	4.5	41.3	11.6	47.1	-	-
2^b	[bmim][OcOSO₃] + NHPI	8.7	64.8	4.2	31	-	-
6^c	MWCNT-COO-Cu + [bmim][OcOSO₃] + NHPI	25.5	18.8	1.5	77.3	2.4	-
3^d	SCILL-SILP system	20.2	14.5	6.2	77.3	2	-
4^e	SCILL-SILP Leaching test	12.54	8.4	5.5	81.6	4.5	-
5^f	Washed SCILL-SILP system	24.5	7.3	6.8	84.7	1.2	-

Reaction conditions: Ethylbenzene = 16.3 mmol, AIBN = 0.186 mol%, 80 ^oC, 0.1 MPa, 6 h; ^a 0.025 g IL; ^b 0.025 g IL and 0.027 g of NHPI added separately to ethylbenzene; ^c mixture of 0.025 g MWCNTs, 0.025 g IL and 0.027 g NHPI added separately to ethylbenzene; ^d SCILL-SILP system prepared by coating the mixture of IL [bmim][OcOSO₃] (0.025 g) and NHPI (0.027 g) over MWCNT-COOH-Cu (0.025 g); ^e SCILL-SILP system was separated from the reaction mixture during the reaction and the effluent was allowed to proceed and complete the reaction time; ^f Fresh SCILL-SILP system was washed with hexane before applying in the oxidation reaction.

3.5 Plausible reaction mechanism

A plausible reaction mechanism was proposed for the SILP and SCILL-SILP hybrid catalytic systems, which catalyzed the oxidation reactions of ethylbenzene in a solvent-free environment using molecular oxygen as an oxidant (Fig. 9). AIBN was utilized to produce the PINO radical from NHPI, which initiated the reaction. The generated PINO radical abstracts a hydrogen atom from the ethylbenzene, yielding the 1-phenylethyl radical (PE) while regenerating the NHPI. In the following step, the produced PE radical quickly reacted with the absorbed oxygen to produce the 1-phenylethyl peroxyl (PEPO) radical, and the produced PEPO radical abstracted the hydrogen atom from the NHPI, regenerating the PINO radical while producing PEHP, and the cycle continued [72].

The produced PEHP is a crucial intermediate in the ethylbenzene oxidation reaction and serves as the major chain propagator. PEPO can abstract the α-H atom of PEHP, and the PEHP promptly converts to AcPO and ^•OH radical [17] while reproducing PEHP. The PEHP can also decompose to PEO, which can react with ethylbenzene, producing the PEA and the PE radical. The PE radical can react with oxygen, producing PEPO, and the produced PEPO can either react with ethylbenzene or the PINO radical to produce PEHP again, and the cycle continues. The PEO can also undergo a β-scission reaction to produce BA and the produced BA can further be oxidized, producing BzA. According to the results and discussion section, the amounts of BA and BzA produced are quite low, except for the [emim][OcOSO₃] IL-based SILP and SCILL-SILP systems.

In PEHP decomposition, both the rate of reaction and product composition are influenced by the type of catalyst employed, i.e., the type of IL and MWCNTs, or MWCNT-COO-Cu. The high ketone to alcohol ratio obtained when MWCNT-COO-Cu was used indicates that the MWCNT-COO-Cu played a vital role in accelerating the decomposition of the PEHP into AcPO. It is consistent with the literature that among transition metal catalysts, Cu(I/II) compounds accelerated the hydroperoxide decomposition and favored ketone formation [5, 66–69]. The comparison of various SILP and SCILL-SILP systems indicated that the presence of IL also favored the hydroperoxide decomposition to AcPO. It could be an effect of the higher polarity of the studied systems, which promotes the β-scission of alkoxy radicals more than hydrogen atom abstraction reaction. Additionally, the solvent cage effect also affects the product composition.

In this study, pristine multiwalled carbon nanotubes (MWCNTs) and Cu(II) functionalized multiwalled carbon nanotubes (MWCNT-COO-Cu) were employed as catalytic support for NHPI dissolved in various [bmim] cationic ILs to produce SILP and SCILL-SILP hybrid catalytic systems. The catalytic systems were employed in the solvent-free oxidation of ethylbenzene using molecular oxygen.

It was demonstrated that the produced [bmim] cationic IL-based SILP systems were more active than pristine MWCNTs with and without NHPI. This could be due to the synergistic effect of the coating of the mixture of NHPI and ILs over the MWCNTs. The [bmim][Cl] IL-based SILP system had the highest catalytic activity, with an ethylbenzene conversion of 14.4% and elevated AcPO selectivity (92.1%) compared with the other SILP systems. This enhanced catalytic activity was attributed to the halide anion in the [bmim][Cl] IL-based systems, which had a specific interaction with NHPI that accelerated the production of PINO radicals and decomposed the hydroperoxides at higher rates. An increase in the lipophilicity and/or chain length of the alkyl group in the [Cl] anionic-based ILs in the SILP systems increased the catalytic activity. The recyclability data showed that an increase in the lipophilicity of the IL in the SILP system drastically decreased the recyclability. However, the [bmim] [OcOSO₃]-based SILP system was recyclable without significant loss of catalytic activity. The leaching test and TGA curves of the fresh and recyclable [bmim][OcOSO₃]-based SILP system showed that although the catalytic activity remained approximately the same, a portion of the mixture of IL and NHPI was leached in each of the corresponding cycles.

SCILL-SILP systems have lower catalytic activities compared to MWCNT-COO-Cu, which could be due to the higher viscosities of the coated ILs with dissolved NHPI, which covered the Cu(II) active sites of the catalysts and prolonged the time of absorption and desorption of the reagents and products. Nonetheless, [bmim][NTf₂] had the highest conversion of ethylbenzene (23.4%) compared with other [bmim] cationic IL-based SCILL-SILP systems. The recyclability data showed that the catalytic activities of all the SCILL-SILP systems decreased dramatically, but the percentage reduction in the catalytic activity for the subsequent cycle of the [bmim][OcOSO₃]-based SCILL-SILP system was lower than that of the other systems. The leaching test and TGA curves for the fresh and recycled [bmim][OcOSO₃]-based SCILL-SILP system showed that only a part of the mixture of IL and NHPI was leached in the subsequent cycles. Furthermore, the SCILL-SILP catalytic systems have elevated selectivities toward AcPO, which could be attributed to the solvent cage effect.

The decline in catalytic activities of the SILP and SCILL-SILP systems in the subsequent cycles could be associated with the leaching of the mixture of ILs and NHPI, as the leaching test and TGA results demonstrated that the systems were not completely heterogeneous, which could be due to the polarity of the reaction system. As the reaction time proceeded more polar products formed and caused the leaching of the portion of the mixture of IL and NHPI.

CNTs: Carbon nanotubes, MWCNTs: Multiwalled carbon nanotubes, NHPI: N-Hydroxyphthalimide, AIBN: Azobisisobutyronitrile, ILs: Ionic liquids, SILP: Supported ionic liquid phase, SCILL: Solid catalyst with an ionic liquid layer, [bmim][Cl]: 1-butyl-3-methylimidazolium chloride, [bmim][Br]: 1-butyl-3-methylimidazolium bromide, [bmim][OcOSO₃]: 1-butyl-3-methylimidazolium octylsulfate, [bmim][PF₆]: 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][BF₄]: 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][NTf₂]: 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf₂]: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [emim][OcOSO₃]: 1-ethyl-3-methylimidazolium octylsulfate, [omim][Cl]: 1-octyl-3-methylimidazolium chloride, [C₁₀mim][Cl]: 1-decyl-3-methylimidazolium chloride, PEHP: 1-phenylethyl hydroperoxide, PEA: 1-phenylethanol, AcPO: Acetophenone, BA: Benzaldehyde, and BzA: Benzoic acid

Data availability

All data generated or analysed during this study are included in this published article [and its supplementary information files].

Competing interests

The authors declare no competing interests.

Funding

This research was funded by the Silesian University of Technology (grants No. BKM-555/RCH5/2024).

Y. Yan, J. Miao, Z. Yang, F.-X. Xiao, H. Bin Yang, B. Liu, Y. Yang, Chem Soc Rev 44 (2015) 3295–3346. https://doi.org/10.1039/C4CS00492B
R. Rocha, O. Soares, J. Figueiredo, M. Pereira, C (Basel) 2 (2016) 17. https://doi.org/10.3390/c2030017
R. Sheldon, Metal-Catalyzed Oxidations of Organic Compounds: Mechanistic Principles and Synthetic Methodology Including Biochemical Processes, Elsevier, 2012.
L. Gómez-Hortigüela, F. Cora, C.R.A. Catlow, ACS Catal 1 (2011) 1475–1486.
R.A. Sheldon, R.A. Van Santen, Catalytic Oxidation: Principles And Applications, World Scientific, 1995.
E.T. Denisov, I.B. Afanas’ ev, (2005).
J. Muzart, Chem Rev 92 (1992) 113–140. https://doi.org/10.1021/cr00009a005
M.I. bin Saiman, G.L. Brett, R. Tiruvalam, M.M. Forde, K. Sharples, A. Thetford, R.L. Jenkins, N. Dimitratos, J.A. Lopez‐Sanchez, D.M. Murphy, Angew. Chem. Int. Ed. 24 (2012) 5981–5985. https://doi.org/10.1002/anie.201201059
Y. Wang, J. Zhang, X. Wang, M. Antonietti, H. Li, , Angew. Chem. Int. Ed. 49 (2010) 3356–3359. https://doi.org/10.1002/anie.201000120
H. Zhou, L. Xiao, X. Liu, S. Li, H. Kobayashi, X. Zheng, J. Fan, ChemComm 48 (2012) 6954–6956. https://doi.org/10.1039/C2CC30737E
M. Waser, W.G. Jary, P. Pöchlauer, H. Falk, J Mol Catal A Chem 236 (2005) 187–193. https://doi.org/10.1016/j.molcata.2005.04.017
C.A. Hone, C.O. Kappe, Accounts on Sustainable Flow Chemistry (2020) 67–110. https://doi.org/10.1007/s41061-018-0226-z
R.A. Sheldon, Green Chem. 7 (2005) 267–278. https://doi.org/10.1039/B418069K
S.U. Azam, B. Orlińska, Appl Catal A Gen (2023) 119465. https://doi.org/10.1016/j.apcata.2023.119465
Y. Cao, X. Luo, H. Yu, F. Peng, H. Wang, G. Ning, Catal Sci Technol 3 (2013) 2654–2660. https://doi.org/10.1039/C3CY00256J
H. Yu, F. Peng, J. Tan, X. Hu, H. Wang, J. Yang, W. Zheng, Angew. Chem. 17 (2011) 4064–4068. https://doi.org/10.1002/ange.201007932
J. Luo, F. Peng, H. Yu, H. Wang, W. Zheng, ChemCatChem 5 (2013) 1578–1586. https://doi.org/10.1002/cctc.201200603
S. Liao, F. Peng, H. Yu, H. Wang, Appl Catal A Gen 478 (2014) 1–8. https://doi.org/10.1016/j.apcata.2014.03.024
S. Rathinavel, K. Priyadharshini, D. Panda, Mater. Sci. Eng. B 268 (2021) 115095. https://doi.org/10.1016/j.mseb.2021.115095
E.M. Alosime, Discover Nano 18 (2023) 12. https://doi.org/10.1186/s11671-023-03789-6
Y. Cao, H. Yu, J. Tan, F. Peng, H. Wang, J. Li, W. Zheng, N.-B. Wong, Carbon N Y 57 (2013) 433–442. https://doi.org/10.1016/j.carbon.2013.02.016
X. Duan, H. Sun, Y. Wang, J. Kang, S. Wang, ACS Catal 5 (2015) 553–559. https://doi.org/10.1021/cs5017613.
X. Yang, H. Wang, J. Li, W. Zheng, R. Xiang, Z. Tang, H. Yu, F. Peng, Chemistry – A European Journal 19 (2013) 9818–9824. https://doi.org/10.1002/chem.201300676
W. Li, Y. Gao, W. Chen, P. Tang, W. Li, Z. Shi, D. Su, J. Wang, D. Ma, ACS Catal 4 (2014) 1261–1266. https://doi.org/10.1021/cs500062s
P. Tang, G. Hu, M. Li, D. Ma, ACS Catal 6 (2016) 6948–6958. https://doi.org/10.1021/acscatal.6b01668
M. Martin‐Martinez, R.S. Ribeiro, B.F. Machado, P. Serp, S. Morales‐Torres, A.M.T. Silva, J.L. Figueiredo, J.L. Faria, H.T. Gomes, ChemCatChem 8 (2016) 2068–2078. https://doi.org/10.1002/cctc.201600123
Y. Cao, H. Yu, F. Peng, H. Wang, ACS Catal 4 (2014) 1617–1625. https://doi.org/10.1021/cs500187q
S. Liao, Y. Chi, H. Yu, H. Wang, F. Peng, ChemCatChem 6 (2014) 555–560. https://doi.org/10.1002/cctc.201300909
Y. Su, Z. Chen, J. Huang, H. Wang, H. Yu, Q. Zhang, Y. Cao, F. Peng, ChemCatChem 14 (2022) e202101378. https://doi.org/10.1002/cctc.202101378
J. Luo, H. Yu, H. Wang, F. Peng, Catal Commun 51 (2014) 77–81. https://doi.org/10.1016/j.catcom.2014.03.031
P. Zhang, Y. Gong, H. Li, Z. Chen, Y. Wang, Nat Commun 4 (2013) 1593. https://doi.org/10.1038/ncomms2586
H.M. Alshammari, A.S. Alshammari, J.R. Humaidi, S.A. Alzahrani, M.S. Alhumaimess, O.F. Aldosari, H.M.A. Hassan, Processes 8 (2020) 1380. https://doi.org/10.3390/pr8111380
K. Peckh, B. Orlińska, Materials 14 (2021) 2314. https://doi.org/10.3390/ma14092314
Y. Deng, Z. Chen, J. Huang, G. Yang, Q. Zhang, Z. Liu, Y. Cao, F. Peng, Chem. Eng. J. 444 (2022) 136666. https://doi.org/10.1016/j.cej.2022.136666
Y. Feng, A. Zeng, Catalysts 10 (2020) 623. https://doi.org/10.3390/catal10060623
K. Machado, J. Mishra, S. Suzuki, G.S. Mishra, Dalton Trans. 43 (2014) 17475–17482. https://doi.org/10.1039/C4DT02099E
M. Moghadam, I. Mohammadpoor-Baltork, S. Tangestaninejad, V. Mirkhani, H. Kargar, N. Zeini-Isfahani, Polyhedron 28 (2009) 3816–3822. https://doi.org/10.1016/j.poly.2009.08.016
F. Recupero, C. Punta, Chem Rev 107 (2007) 3800–3842. https://doi.org/10.1021/cr040170k
K. Chen, P. Zhang, Y. Wang, H. Li, Green Chemistry 16 (2014) 2344. https://doi.org/10.1039/c3gc42135j
S. Coseri, Catalysis Reviews 51 (2009) 218–292. https://doi.org/10.1080/01614940902743841
G. Dobras, K. Kasperczyk, S. Jurczyk, B. Orlińska, Catalysts 10 (2020) 252. https://doi.org/10.3390/catal10020252
L. Melone, C. Punta, Beilstein J. Org. Chem. 9 (2013) 1296–1310. https://doi.org/10.3762/bjoc.9.146
R. Amorati, M. Lucarini, V. Mugnaini, G.F. Pedulli, F. Minisci, F. Recupero, F. Fontana, P. Astolfi, L. Greci, J Org Chem 68 (2003) 1747–1754. https://doi.org/10.1021/jo026660z
J.-R. Wang, L. Liu, Y.-F. Wang, Y. Zhang, W. Deng, Q.-X. Guo, Tetrahedron Lett 46 (2005) 4647–4651. https://doi.org/10.1016/j.tetlet.2005.04.136
S. Koguchi, T. Kitazume, Tetrahedron Lett 47 (2006) 2797–2801. https://doi.org/10.1016/j.tetlet.2006.02.077
S. Mahmood, B.-H. Xu, T.-L. Ren, Z.-B. Zhang, X.-M. Liu, S.-J. Zhang, Mol. Catal. 447 (2018) 90–96. https://doi.org/10.1016/j.mcat.2018.01.006
C.P. Mehnert, R.A. Cook, N.C. Dispenziere, M. Afeworki, J Am Chem Soc 124 (2002) 12932–12933. https://doi.org/10.1021/ja0279242
C.P. Mehnert, E.J. Mozeleski, R.A. Cook, Chem. Commun. (2002) 3010–3011. https://doi.org/10.1039/B210214E
A. Riisager, P. Wasserscheid, R. van Hal, R. Fehrmann, J Catal 219 (2003) 452–455. https://doi.org/10.1016/S0021-9517(03)00223-9
A. Riisager, K.M. Eriksen, P. Wasserscheid, R. Fehrmann, Catal Letters 90 (2003) 149–153. https://doi.org/10.1016/S0021-9517(03)00223-9
U. Kernchen, B. Etzold, W. Korth, A. Jess, Chem. Eng. Technol. 30 (2007) 985–994. https://doi.org/10.1002/ceat.200700050
I. Podolean, O.D. Pavel, H.G. Manyar, S.F.R. Taylor, K. Ralphs, P. Goodrich, V.I. Pârvulescu, C. Hardacre, Catal Today 333 (2019) 140–146. https://doi.org/10.1016/j.cattod.2018.07.014
G. Dobras, K. Kasperczyk, S. Jurczyk, B. Orlińska, Catalysts 10 (2020) 252. https://doi.org/10.3390/catal10020252
G. Talik, A. Osial, M. Grymel, B. Orlińska, Catalysts 10 (2020) 1367. https://doi.org/10.3390/catal10121367
S.U. Azam, K. Peckh, B. Orlińska, Chem. Eng. J. 457 (2023) 141207. https://doi.org/10.1016/j.cej.2022.141207
K. Kasperczyk, B. Orlińska, J. Zawadiak, Open Chem 12 (2014) 1176–1182. https://doi.org/10.2478/s11532-014-0565-8
J. Zawadiak, D. Gilner, Z. Kulicki, S. Baj, Analyst 118 (1993) 1081–1083. https://doi.org/10.1039/AN9931801081
D.B. Denney, W.F. Goodyear, B. Goldstein, J Am Chem Soc 82 (1960) 1393–1395. https://doi.org/10.1021/ja01491a027
N.I. Kuznetsova, L.I. Kuznetsova, O.A. Yakovina, I.E. Karmadonova, B.S. Bal’zhinimaev, Catal Letters 150 (2020) 1020–1027. https://doi.org/10.1007/s10562-019-02999-x
M.M. Al-Taher, C. Kalamaras, M.A. Alqahtani, F.S. Alomar, Fuel 324 (2022) 124563. https://doi.org/10.1016/j.fuel.2022.124563
G. Talik, B. Orlińska, ChemCatChem 13 (2021) 4578–4590. https://doi.org/10.1002/cctc.202100990
G. Dobras, B. Orlińska, Appl Catal A Gen 561 (2018) 59–67. https://doi.org/10.1016/j.apcata.2018.05.012
D. Pyszny, B. Orlińska, Reac Kinet Mech Cat 124 (2018) 123–138. https://doi.org/10.1007/s11144-017-1307-7
I. Hermans, J. Peeters, P.A. Jacobs, J Org Chem 72 (2007) 3057–3064. https://doi.org/10.1021/jo070040m
S. Evans, J.R.L. Smith, , J. Chem. Soc. Perkin Trans. 2 (2000) 1541–1552. https://doi.org/10.1039/B000967I
B. Orlińska, I. Romanowska, Open Chem 9 (2011) 670–676. https://doi.org/10.2478/s11532-011-0048-0
N. Тurovskij, E. Raksha, Y. Berestneva, A. Eresko, J Organomet Chem 922 (2020) 121371. https://doi.org/10.1016/j.jorganchem.2020.121371
N. V Ulitin, K.E. Kharlampidi, К.A. Tereshchenko, N.A. Novikov, D.A. Shiyan, T.S. Nurmurodov, N.M. Nurullina, N.N. Ziyatdinov, N.P. Miroshkin, Mol. Catal. 515 (2021) 111886. https://doi.org/10.1016/j.mcat.2021.111886
N. V Ulitin, K.A. Tereshchenko, N.A. Novikov, D.A. Shiyan, Y.L. Lyulinskaya, N.M. Nurullina, M.N. Denisova, V.I. Anisimova, T.S. Nurmurodov, K.E. Kharlampidi, Appl Catal A Gen 653 (2023) 119044. https://doi.org/10.1016/j.apcata.2023.119044
B. Orlińska, Tetrahedron Lett 51 (2010) 4100–4102. https://doi.org/10.1016/j.tetlet.2010.05.128
D.A. Braden, E.E. Parrack, D.R. Tyler, Coord Chem Rev 211 (2001) 279–294. https://doi.org/10.1016/S0010-8545(00)00287-3
G. Dobras, M. Sitko, M. Petroselli, M. Caruso, M. Cametti, C. Punta, B. Orlińska, ChemCatChem 12 (2020) 259–266. https://doi.org/10.1002/cctc.201901737

Schemes are available in the Supplementary Files section.

No competing interests reported.

Scheme1.jpeg
Scheme 1. Reaction mechanism of hydrocarbon oxidation in the presence of NHPI.
Scheme2.jpeg
Scheme 2. Oxidation products of ethylbenzene.

Download PDF

Editorial decision: Revision requested
18 Sep, 2024
Reviews received at journal
10 Sep, 2024
Reviews received at journal
10 Sep, 2024
Reviewers agreed at journal
31 Aug, 2024
Reviewers agreed at journal
30 Aug, 2024
Reviewers invited by journal
28 Aug, 2024
Editor assigned by journal
28 Aug, 2024
Editor invited by journal
27 Aug, 2024
Submission checks completed at journal
25 Aug, 2024
First submitted to journal
14 Aug, 2024

You are reading this latest preprint version

Hybrid catalytic systems employing carbon nanotubes as a support for the solvent-free aerobic oxidation of ethylbenzene

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Section

2.1 Materials

2.2 Preparation of MWCNT-COO-Cu

2.3 Preparation of the SILP and SCILL-SILP catalytic systems

2.4 Ethylbenzene oxidation

2.5 Analytical methods

3. Results and discussions

3.1 Catalytic activities of the SILP systems

3.2 Recyclability of SILP systems

3.3 Catalytic activities of SCILL-SILP hybrid systems

3.4 Recyclability and reusability of SCILL-SILP hybrid catalytic systems

3.5 Plausible reaction mechanism

4. Conclusion

Abbreviations

Declarations

References

Schemes

Additional Declarations

Supplementary Files

Status:

Version 1