## 1 Introduction
Contrary to processes that use ionic liquids as solvents, the SCILL technique allows their use in minimal amounts, thereby avoiding high additional costs. The aim of SCILL is to modify the activity and selectivity of a heterogeneous catalyst while maintaining the ease of its separation and recycling [32]. To date, the SCILL technique has been used mainly in hydrogenation processes, including those involving cyclooctadiene [31], citral [33; 34], toluene [35], and propyne [36]. Additionally, several studies regarding oxidation of alcohols and oxidative coupling of thiols have been published [37; 38; 39]. In 2011, a SCILL system consisting of the 2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical covalently immobilized on SBA-15 silica (Figure 1a) coated with a [bmim][Br] (bmim = 1-butyl-3-methylimidazolium) ionic liquid effectively promoted oxidation of alcohols to aldehydes using dioxygen [37]. The benzyl alcohol oxidation reactions were carried out in the presence of _tert_-butyl nitrite and acetic acid at 40 \({}^{\circ}\)C for about 3 h. The addition of [bmim][Br] increased the benzaldehyde yield from less than 5% to over 99%. The authors further reported that this SCILL system showed unchanged activity over as many as 11 reaction cycles [37]. There are also several studies that employed SCILL systems containing transition metal salts dissolved in the ionic liquid phase [40; 41]. For example, [PERSON] et al. [40] described the oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde using dioxygen as the oxidant and TEMPO immobilized on polystyrene (PS-TEMPO) (Figure 1b) covered with a [bmim][PF6] ionic liquid containing dissolved CuCl2 as the heterogeneous catalyst. Modification of the PS-TEMPO/CuCl2 system by adding an ionic liquid increased the alcohol conversion from 58% to 85%. Research into the possibility of recycling the SCILL system containing CuCl2 revealed that this system can be used four times with only a slight decrease in activity (i.e., in the fourth cycle, a decrease in aldehyde yield from 93% to 79% was noted). It was determined that this decrease results from partial elution of the ionic liquid from the PS-TEMPO surface. In previous work in our laboratory [41], we used a SCILL system consisting of NHPI immobilized via ester bonds on silica gel (SiOCONHPI; Figure 1c), in combination with various ionic liquids containing CoCl2 (SiOCONHPI@CoCl2@IL) for the oxidation of ethylbenzene. The positive impact of the ionic liquid, [bmim][CoSO3], on the catalytic activity of the studied system was clearly observed. Specifically, the SiOCONHPI@CoCl2@[bmim][CoSO3] system achieved 12.1% conversion of ethylbenzene, while using SiOCONHPI/CoCl2 alone led to only 8.2% conversion. The observed effect, might be a result of interaction between N-OH groups and [bmim][CoSO3], better contact between CoCl2 dissolved in a layer of IL and N-OH groups, as well as between reagents and N-OH groups and CoCl2, and also undesired washing out of ionic liquid containing CoCl2 from the surface of silica. We also demonstrated that it is possible to recycle SiOCONHPI@CoCl2@[bmim][CoCOS3], and that the decrease in activity observed in the fourth reaction cycle (i.e., 8% conversion) was due to partial elution of the IL and CoCl2 from the catalyst surface. Promising results were obtained for the system using a slightly different ionic liquid, SiOCONHPI@CoCl2@[emim][CoCOS3] (emim = 1-ethyl-3-methylimidazolium), which could obtain an ethylbenzene conversion of 9.5% in the fourth reaction cycle. This paper describes investigations regarding the use of a novel SCILL system containing heterogeneous NHPI coated with IL containing CoCl2 (NHPI@CoCl2@IL) to carry out ethylbenzene oxidation reactions.