## 1 Introduction  
Hydrocarbon oxidations represent an important family of reactions in organic synthesis, and they play a key role in the chemical industry. For environmental and economic reasons, it is advantageous to use dioxygen or air as the oxidizing agent for such transformations, which is made possible by using catalytic systems containing often transition metal compounds, e.g., Co and Mn [1, 2]. The high activity of \(N\)-hydroxphthalimide (NHPI) in this type of reaction has been also demonstrated repeatedly in the literature [3, 4, 5]. This compound's activity is related to the fact that, during turnover, it generates the phthalimide-\(N\)-oxyl (PINO) radical, which effectively abstracts the hydrogen atom from the hydrocarbon being oxidized (Scheme 1). The PINO radical in this system can be generated by various additives, including azo compounds, transition metal complexes, aldehydes, oximes, and quinones [4, 6]. For example, azobisisobutyronitrile (AIBN) was used in the oxidation of cumene to its hydroperoxide using dioxygen at 60 \({}^{\circ}\)C, aided by 10 mol% NHPI [7]. After allowing the reaction to proceed for three hours in acetonitrile, a 46% conversion of cumene reagent was obtained, with nearly 100% selectivity toward production of cumene hydroperoxide. In contrast, in the presence of NHPI and Co(acac)\({}_{2}\), 2-phenyl-2-propanol and acetophenone were obtained as main products [8]. Transition metal compounds can indeed generate the PINO radical, but they also decompose the hydroperoxide formed during the reaction [3]. NHPI-catalyzed oxidation of hydrocarbons with dioxygen are most often carried out in polar solvents, such as acetonitrile, benzonitrile, or acetic acid [7; 8; 9; 10]. This is because of the low solubility of NHPI in non-polar media and the fact that the catalyst must be fully dissolved to ensure promotion of the catalytic cycle shown in Scheme 1. Recently, researchers have investigated replacing classic organic solvents with supercritical CO\({}_{2}\) (s\(\mathrm{cCO}_{2}\)) [11; 12] or ionic liquids (ILs) [11; 13; 14; 15; 16; 17; 18]. A key issue in the context of applying NHPI in industrial processes involves its potential separation and reuse. Unfortunately, simple isolation methods, such as solvent evaporation [7; 20] or, in the case of \(\mathrm{s\mathrm{cCO}}_{2}\), decompression by changing the pressure and temperature parameters [11; 12], are ineffective if the reaction product is highly polar and effectively dissolves NHPI. However, this issue can be avoided by immobilizing NHPI on solid supports, such as silicas [21; 22; 23], polymers [24; 25; 26; 27; 28; 29], or zeolites [30]. For example, heterogeneous NHPI was used to oxidize toluene with dioxygen in the presence of heterogeneous Co(II) in acetic acid [21]. After 20 h, benzaldehyde was obtained as a product in 12% yield. In the second reaction cycle, the activity of the catalytic system allowed the product to be obtained with a yield of 10%. Heterogenization of NHPI effectively simplifies the separation and therefore reuse of the catalyst; however, it adversely affects the catalytic activity. In the oxidation of cyclohexane with dioxygen at 130 \({}^{\circ}\)C in the presence of heterogeneous NHPI (0.85 mmol/g catalyst; 0.1 mol% NHPI), oxygen consumption after 4 h was about 8.5 mmol [22]. In an analogous process carried out in the presence of 0.1 mol% of homogeneous NHPI in acetonitrile, the oxygen consumption surpassed 11 mmol. Recently, in order to increase the activity of heterogeneous catalysts, scientists have applied the so-called solid catalyst with ionic liquid layer (SCILL) technique, which involves covering the catalyst's surface with a layer of ionic liquid [31]. The addition of an ionic liquid may increase both the activity and the selectivity of the solid catalyst by tuning its chemical properties (i.e., co-catalytic effect) and/or by changing the concentration of intermediates and semi-products at the catalyst surface (i.e., physical solvent effect).