SBA-15 Anchored Metal Containing Catalysts in the Oxidative Desulfurization Process

Abstract

Recalcitrant sulfur compounds are common impurities in crude oil. During combustion they produce SO_x derivatives that are able to affect the atmospheric ozone layer, increasing the formation of acid rains, and reducing the life of the engine due to corrosion. In the last twenty years, many efforts have been devoted to develop conventional hydrodesulfurization (HDS) procedures, as well as alternative methods, such as selective adsorption, bio-desulfurization, oxidative desulfurization (ODS) under extractive conditions (ECODS), and others. Among them, the oxidative procedures have been usually accomplished by the use of toxic stoichiometric oxidants, namely potassium permanganate, sodium bromate and carboxylic and sulfonic peracids. As an alternative, increasing interest is devoted to selective and economical procedures based upon catalytic methods. Heterogeneous catalysis is of relevance in industrial ODS processes, since it reduces the leaching of active species and favors the recovery and reuse of the catalyst for successive transformations. The heterogenization of different types of high-valent metal transition-based organometallic complexes, able to promote the activation of stoichiometric benign oxidants like peroxides, can be achieved using various solid supports. Many successful cases have been frequently associated with the use of mesoporous silicas that have the advantage of easy surface modification by reaction with organosilanes, facilitating the immobilization of homogeneous catalysts. In this manuscript the application of SBA-15 as efficient support for different active metal species, able to promote the catalytic ODS of either model or real fuels is reviewed, highlighting its beneficial properties such as high surface area, narrow pore size distribution and tunable pore diameter dimensions. Related to this topic, the most relevant advances recently published, will be discussed and critically described.

1. Introduction

Santa Barbara Amorphous (SBA) materials are mesoporous silicate-aluminosilicates characterized by uniform pore size (4.6–30 nm), well-defined pore structure and size-distribution, high surface area, high thermal stability and the capability to support a large panel of active species [1,2]. A wide variety of SBA materials has been reported in the literature like SBA-1 (Pm3n, cubic), SBA-15 (P6mm, hexagonal) and SBA-16 (Im3m, cubic) [3]. SBA-15 is one of the most promising components of this family, due to its several applications, including selective adsorption processes, heterogeneous catalysis, different chemical transformations and gas storage [4]. SBA-15 is usually prepared by supramolecular non-ionic self-assembly between ethylene oxide/propylene oxide copolymer-template (Pluronic P123) and appropriate silica precursors (tetraethoxysilane TEOS and tetramethoxysilane TMOS) in a range of temperature from 30 °C to 120 °C, followed by calcination at high temperature to degrade the copolymer-template (Figure 1) [5,6].

In this process, the value of the ratio of ethylene oxide to propylene oxide can control the distribution function of the pores inside the basic hexagonally-arrayed channel framework. At low values of this ratio, the p6mm hexagonal morphology is favored instead of the cubic mesoporous arrangement prevailing at higher ratio values [7,8]. Alternative reaction pathways for the preparation of SBA-15 derivatives have been also reported [9,10,11]. SBA-15 materials have been well recognized as a promising support template for the synthesis of catalytic materials due to its uniform, hexagonally-arrayed channels with a narrow pore size distribution. These features, together with high surface area and hydrothermal stability, make it as an ideal support for the incorporation of various active molecules on its surface, provided that its mesoporous surface is previously and appropriately functionalized with various functional groups (i.e., amines, thiol, nitriles, halides etc.), grafting them either by co-condensation or by post synthesis grafting procedures [12].

SBA-15 has been applied in a large variety of industrial processes after functionalization with active species on both the bulky silica framework or on the surface of the material [13,14,15]. These processes include: (i) oxidative transformation of hydrocarbons, such as alkanes [16,17], alkenes [18,19] and aromatic derivatives [20,21]; (ii) oxidative transformation of alcohols [22]; (iii) CO oxidation [23]; reduction processes [24] Knoevenagel reactions [25]; waste water treatments [26,27]; and carrier applications in drug-delivery [28,29].

Environmental concerns associated with novel stringent European regulations for the tolerance limits of pollutants in the atmosphere [30] highlighted the interest for the application of SBA-15 materials in the removal of sulfur-containing compounds from petroleum, a process that is indicated as “deep desulfurization” [31]. Sulfur-containing compounds, such as thiols, sulfides and disulfides, produce sulfur oxides (SO_x) upon combustion, that act as pollution agents at the global level, inducing acid rain and the formation of fine particulate matter of metal sulfates [32,33,34]. To avoid this problem, reductive hydrodesulfurization (HDS) is a well-stablished technology applied at the industrial level to purify crude fuel oils [35]. As a general procedure, this reaction is performed by using hydrogen in the presence of organometallic transition metal species as heterogeneous catalysts at both high temperature (up to 400 °C) and pressure (up to 100 atm) [36,37,38]. The application of SBA-15 in HDS processes has been reported and reviewed, encompassing the incorporation of Al³⁺, Ti⁴⁺ and Zr⁴⁺ cations into the original material to improve the catalyst activity. In the functionalization procedure, the post-synthesis grafting was more effective than the direct synthesis method, leading to a larger pore diameter and the limited diffusion of reactants [39]. However, HDS can yield only limited desulfurization, since refractory sulfur derivatives, such as alkyl-substituted benzothio-phenes (BTs) and dibenzothiophene (DBT), are resistant to reductive treatment [40,41] due to their high alky chain steric hindrance that makes the approach to the catalyst surface difficult [42]. Moreover, the reactivity of thiophene derivatives further decreases in the presence of polyaromatics and nitrogen-containing compounds, which are often components of low quality diesel fuel compounds [43,44].

For this reason, high temperature and pressure, as well as costly hydrogen consumption, are necessary to achieve ultra-low deep desulfurization (that is, to reach a sulfur concentration lower than 10 ppm) [45]. The oxidative desulfurization (ODS) is considered the most promising technology alternative to HDS [46]. In this latter case, the recalcitrant sulfur compounds are oxidized to corresponding sulfoxides and sulfones, followed by distillation, liquid–liquid extraction, or solid–liquid adsorption removal procedures [47]. ODS is accomplished using t-butyl-hydroperoxide (TBHP) [48], superoxides [49], sodium bromate (NaBrO₃) [50] and hydrogen peroxide (H₂O₂), along with mixtures of formic acid with H₂O₂ [51,52,53,54] as primary oxidants. Hydrogen peroxide is the oxidant of choice for ODS, since it is a low cost, mild and environmentally benign reagent [55]. In this latter case, highly active transition metal derivatives have been selected for the activation of H₂O₂, often after immobilization on appropriate supports [56,57,58,59], and in alternative reaction solvents [60] [61,62]. Examples on the application of microporous and mesoporous material in ODS procedures have been previously reported [63], focusing on the application of mesoporous TiO₂ [64], titanium-containing mesoporous molecular sieves [65] and MCM-41 [66], mesoporous phosphotungstic acid/TiO₂ nanocomposites [67], mesoporous TS-1 using a hybrid SiO₂-TiO₂ xerogel procedure [68,69], vanadium polyoxometalate supported on mesoporous MCM-41[70] and phosphotungstic acid containing ionic liquid immobilized on magnetic mesoporous silica rod systems [71].

In the following sections, we will focus on the application of SBA-15 based catalysts in ODS, classifying them in terms of different families depending on the properties of the prevalent catalytic species as follows: (i) titanium oxides; (ii) vanadium oxides; (iii) molybdenum oxides; (iv) iron oxides; (v) tungsten oxides; (vi) silver oxides; (vii) polyoxometalates and (viii) miscellanea. This choice helps the reader to focus upon the role played by the metal (or metals) species in the oxidation system, highlighting the occurrence of specific synergy events with the SBA-15 support.

2. Titanium Oxides

Titanium dioxide TiO₂/SBA-15 materials have been applied as catalytic adsorptive desulfurization systems (CADS) [72], by coupling between the oxidation of recalcitrant sulfur compounds and the selective adsorption of polar products on the SBA phase [73]. As an example, a large panel of TiO₂/SBA-15 catalysts, differing in the loading of the active species, was prepared by an application of the facile wetness impregnation technology [74]. In this procedure, SBA-15 was treated with tetrabutyl titanate in ethanol under assisted ultrasound conditions [75]. Irrespective of the loading value, the titanium anatase phase largely prevailed in the samples, the active species being well dispersed on SBA-15 as a consequence of the strong electrostatic adsorption of the TiO₂ precursor on SBA-15 with a low point of zero charge [76]. The efficacy of novel TiO₂/SBA-15 was evaluated in the removal of DBT, MDBT and DMDBT as selected substrates, by comparing the simple adsorptive desulfurization capacity (ADS) [77] with the properties of the CADS approach (Figure 2). In this latter case, cumene hydroperoxide was used as the primary oxidant in acetonitrile at 35 °C. As a general trend, these CADS were two orders of magnitude more efficient than simple ADS, showing a desulfurization capacity higher than the previously reported UV-photocatalytic CDAS-TiO₂/SiO₂ procedure [78]. Under optimal experimental conditions, high desulfurization uptakes of 19.3, 12.7 and 7.2 mg-S/g-sorb were obtained. Different interaction sites were operative during these ADS and CADS processes. For CADS, TiO₂ was considered as the main adsorption site for recalcitrant sulfur compounds [79], while for ADS, the recognition site was localized in the correspondence of the silanol groups of SBA-15 [80], probably involved in the formation of hydrogen bonds [81] with sulfur oxidized products. Additionally, TiO₂/SBA-15 catalysts were used for five consecutive adsorption-regeneration cycle tests by acetone washing of the system followed with oxidative air treatment.

Mesoporous Ti-silica (pore size > 2 nm) showed higher catalytic activity and longer lifetimes than its microporous counterpart (pore size < 2 nm), such as titanium silicalite TS-1, thanks to larger access to the active site and the presence of coordinatively unsaturated tetrahedral titanium able to interact with the primary oxidant [82,83]. In accordance with these data, large cylindrical mesopores Ti-SBA-15 catalysts (pore size > 7 nm) have been prepared by post-grafting insertion of titanium alkoxide/acetylacetone (acac) chelates favoring the dispersion of tetrahedral Ti⁴⁺ sites on the mesopore surface of the material, followed by the generation of hydrophobic surface sites by treatment with tetramethyldisilazane (TMDS) to avoid poisoning effects of the polar sulfoxide and sulfone products [84,85]. TEOS was used as the silicon source and Pluronic P123 as the structure-directing agent, working at different temperatures (35 °C, 100 °C and 140 °C) in order to control the size of the mesopores [86,87]. This procedure allows the deposition of highly dispersed Ti⁴⁺ sites on the surface of the parent material without any appreciable modification of the original morphology, making easy the accessibility to active species [88]. The titanium contents [Ti/Si molar (atomic) ratio] in the novel Ti-SBA-15 catalysts were included in the range between 0.7% to 4.7%, and the pore size and pore volume increased by increasing the aging temperatures [89,90]. The novel catalysts have been applied in the oxidation of benzothiophene (BT), dibenzothiophene (DBT), 4-methyldibenzo--thiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) as representative sulfur-containing compounds present in middle distillates [91]. The reaction was performed in an n-heptane/toluene mixture (8:2 wt %) at 80 °C under atmospheric pressure using an excess of cumene hydroperoxide (CHP) (CHP: Sulfur compounds 2:1) as its primary oxidant, to yield the corresponding sulfone derivatives (Figure 3).

All of the novel Ti-SBA-15 catalysts showed largely superior ODS activity compared to Ti-SBA-15 prepared by the conventional impregnation method, as a consequence of the presence of the desired active tetrahedral Ti⁴⁺ sites. Irrespective of the nature of the sulfur-containing substrates, quantitative conversion was obtained within half an hour at 80 °C. Similar rate constants were observed in the oxidation of DBT, 4-MDBT and 4,6-DMDBT for each catalyst at a given Ti loading, suggesting that the efficacy of Ti-SBA-15 materials was independent from the actual electron density and steric hindrance of the sulfur substrates. The loading of titanium, as well as the porosity degree of the catalyst, were found to be critical parameters for the activity of the system. The novel Ti-SBA-15 catalysts were also effective in the ODS of a residue hydrodesulfurization (RHDS) diesel sample containing 200 ppmw of refractory sulfur compounds, under fixed bed reactor conditions at 80 °C. The highest reactivity was again obtained for catalysts with the largest titanium loading and pore size.

With the aim to evaluate the effect of nitrogen compounds in the efficacy of the Ti-SBA-15 based ODS procedure, a catalyst produced by tetrabutyl orthotitanate (TBOT) grafting procedure [92] was applied for the oxidation of recalcitrant sulfur compounds in the presence of indole, carbazole and quinoline, as selected compounds commonly found in feed oils [93]. Reactions were performed by treating model sulfur and nitrogen compounds with an excess of tert-butyl hydroperoxide (TBHP) (TBHP/S-compound ratio = 2.5) for 1 h at 80 °C [94]. With respect to the simple ODS treatment, the Ti-SBA-15 activity was found to be drastically decreased in the presence of nitrogen compounds, in the following order: indole > quinoline > carbazole. These data were in accordance with results previously reported for the inhibitory effect of nitrogen compounds on MoO₃/Al₂O₃ [95] and V₂O₅ catalysts [96] in ODS processes. The mechanism of inhibition may depend on the basic or non-basic properties of the nitrogen compound, the ODS of thiophene being inhibited only by basic conditions, while that of BT being affected by both basic and non-basic nitrogen compounds [97]. Remarkably, the overall ODS activity of Ti-SBA-15 increased in the presence of aromatic and aprotic solvents, such as tetralin, 1-methylnaphthalene and acetonitrile, respectively, due to the detected high solubility of both oxidized sulfur and nitrogen compounds in these reaction media.

As an alternative, trimethylammonium-functionalized SBA-15 polyoxometalate ((PW₁₁Ti)₂OH@TMS-SBA-15) has been applied in ODS procedures [98]. Ti(IV) easily substitutes the W(VI) metal center in the Keggin-type POMs, creating multicenter catalytic active sites with corner- or edge-sharing TiO₆ octahedra, associated with oligomeric species by the formation of Ti–O–Ti bonds [99,100]. The preparation of the catalyst was performed through an impregnation technology specifically designed for the immobilization of polyoxometalates in SBA-15 materials [101,102], starting from freshly functionalized SBA-15 with N-trimethoxysilypropyl-N,N,N-trimethyl-ammonium chloride (TMS-SBA-15) and (PW₁₁Ti)₂OH. Under these experimental conditions a loading of 0.045 mmol/g of (PW₁₁Ti)₂OH was obtained, the absence of peaks characteristic for (PW₁₁Ti)₂OH in the XRD analysis strongly suggesting that the inorganic anions have been successfully incorporated inside the TMS-SBA-15 channels. The catalyst (PW₁₁Ti)₂OH@TMS-SBA-15 was then applied in the oxidation of a mixture of BT, DBT, 4-MDBT and 4,6-DMDBT in n-octane/acetonitrile (total sulfur concentration of 2350 ppm), using an excess of H₂O₂ as the primary oxidant (H₂O₂/S-compounds = 1:9) at 70 °C (Figure 4). The reaction proceeded by the transformation of the µ-hydroxo dimeric (PW₁₁Ti)₂OH into the active peroxo compound [HPW₁₁O₃₉TiO₂]⁴⁻ [103]. After 2 h, complete desulfurization was achieved. The lower activity observed for (PW₁₁Ti)₂OH@TMS-SBA-15 with respect to the homogeneous counterpart (PW₁₁Ti)₂OH was probably due to the presence of kinetic barriers for the access of substrate at the heterogeneous active site. The oxidative reactivity order DBT > 4-MDBT ≥ 4,6-DMDBT > BT was in accordance with electronic density effect and with some steric hindrance [104,105]. Moreover, (PW₁₁Ti)₂OH@TM-SBA-15 retained its structure and morphology after five consecutive ODS cycles.

3. Vanadium Oxides

The use of vanadosilicates in ODS/ H₂O₂ processes has been reported to achieve a desulfurization yield higher than 80% [106]. In these reactions, mesoporous vanadosilicates showed higher activity than materials having different pore-size distribution, confirming the beneficial role of large-surface-area, wider pore size and thicker pore walls on the interaction between the active site and the substrate [107,108]. On the basis of these results, vanadium oxide (V₂O₅) was incorporated into the SBA-15 framework by a controlled grafting process, affording highly dispersed vanadium sites with distorted tetrahedral VO₄ units on the silica surface [109]. These catalysts showed significative CODS activity when compared with other V₂O₅ supports (alumina, titania, ceria and niobia); the highest activity being obtained in the presence of tetravalent cations [110]. As an improvement of this study, vanadium oxide species were incorporated into SBA-15, and modified aluminum Al-SBA-15 and gallium Ga-SBA-15 materials, by both direct-synthesis and impregnation methodology [111].

In the first case, VCl₃ was added to the pluronic 123/TEOS mixture (V/Si ratio of 1/30 and 1/54) to yield V-SBA-15, while in the second case, VO_x-SBA-15, VO_x-Al-SBA-15 and VO_x-Ga-SBA-15 were obtained by treating the appropriate SBA-15 starting material with an aqueous solution of VCl₃ followed by calcination at 500 °C. As a general trend, the incorporation of Al and Ga improved the dispersion of vanadium species, probably due to a better anchorage of the active species on more acidic supports. Irrespective from the applied procedure, the small angle X-ray diffraction analysis (XRD) showed that SBA-15 retained its mesoporous structure after the incorporation of aluminum and gallium [112], with small clusters of vanadium oxide species being highly dispersed on the surface of the support. However, N₂ adsorption–desorption isotherm analysis showed a partial loss of the structure of the catalysts from the impregnation method in the correspondence of the highest value of vanadium loading, as a consequence of the incorporation of vanadium species into the mesoporous channels. In contrast, pore diameter had a marked increase when vanadium was introduced by direct synthesis, suggesting that vanadium atoms in part replaced Si atoms [113]. VO_x-SBA-15 efficiently converted DBT to corresponding sulfone in acetonitrile using H₂O₂ as a primary oxidant, the highest conversion (100%) being obtained with the ratio V/Si = 1/30, corresponding to the prevalence of pseudo-tetrahedral VO₄³⁻ species on the surface of the catalyst. Samples characterized by the higher loading value of vanadium showed lower activity as a consequence of the formation of less active VO_x microcrystal chains. The modification of SBA-15 with Al and Ga further increased the desulfuration activity of VO_x-Al-SBA-15 and VO_x-Ga-SBA-15 catalysts, probably due to the formation of a larger number of highly dispersed VO₄³⁻ species, associated with synergic effect of Lewis and Bronsted acid sites, derived from Ga or Al incorporation [114]. On the other hand, lower activity was found when vanadium was incorporated via direct synthesis in V-SBA-15, due to the presence of a kinetic barrier for the interaction between the active sites and DBT. The experimental design optimization of the ODS of DBT using the VO_x-Ga-SBA-15 prepared by the impregnation method from Ga(NO₃)₃ has been more recently reported [115]. The higher levels of the oxidation of DBT were obtained employing the catalyst with 4 wt % of gallium and 6 wt % of vanadium, the optimal ratio in weight between DBT and VO_x-Ga-SBA-15 being 4 at the H₂O₂/DBT molar ratio value of 5.

Examples of the application of VO_x-SBA-15, prepared by the impregnation method from ammonium metavanadate (NH₄VO₃) and SBA-15, using molecular oxygen (O₂) as oxidant and the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [Bmim]BF₄ as a selective extraction phase, are reported [116]. In this catalytic system, the recalcitrant sulfur compounds (DBT, 4-MDTB and 4,6-DMDTB) are firstly extracted from the oil phase to the ionic liquid in which the VO_x-SBA-15 catalyst is suspended. After the extraction step, the oxidation proceeds by vanadium activation of ionic liquid adsorbed molecular oxygen (O₂) with formation of the corresponding radical anion derivative and successive oxygen atom transfer to sulfur [117]. The sulfones produced during the oxidation are successively removed from the surface of the catalyst thanks to the high solubility property of the ionic liquid, thus reducing the inhibitory effect that these products normally exert after their adsorption on SBA-15 (Figure 5) [118]. Under the optimal experimental conditions, the removal of DBT, as one of the most stable residues in the hydro-desulfurization (HDS) process, reached up to 99.3% (<3.5 ppm) after 7 h at 120 °C.

4. Molybdenum Oxides

The use of high oxidation states molybdenum Mo(VI) derivatives in the ODS procedure has been reported [119]. For example, MoO₃ supported on amorphous silica afforded 82% reduction of the residual sulfur compounds in pre-hydrotreated diesel samples after activation with cumene hydroperoxide at 70 °C [120]. The activity of MoO₃/SiO₂ further increased after the introduction of different alkaline earth metals (Ca, Ba, Sr and Mg) in the silica phase [121], highlighting the key role played by the Lewis and Bronsted acid sites in the behavior of the oxidation [122]. In order to prepare molybdenum ODS catalysts with larger surface area and larger pore diameter, CoMo/SBA-15 compounds were obtained by impregnating the SBA-15 phase with alcoholic solutions at different concentrations of Co(NO₃)₂·6H₂O and (NH₄)₆Mo₇O₂₄·4H₂O, respectively [123]. The XRD analysis confirmed the retention of the mesoporous structure after the impregnation procedure with a slight alteration of the loop shape and pore distribution with respect to original SBA-15, due to percolation of Co and Mo oxides within mesopores, associated with a high dispersion of the metal oxides on the surface of the catalyst. Only Lewis acid sites were found on the surface of the novel catalysts, corresponding to a mixture of different molybdenum species. In particular, the structural analyses showed that the amount of MoO₃ microcrystals increased with respect to the β-CoMoMoO₄ phase by increasing the overall CoMo concentration [124,125]. CoMo/SBA-15 catalysts were studied in the oxidation of DBT in n-hexadecane, using H₂O₂ as a primary oxidant under a large panel of reaction temperature (from 60 °C to 80 °C) for 1h at atmospheric pressure. Irrespective of the loading of the catalyst, the activity decreased at higher temperature as a consequence of both H₂O₂ decomposition and lowering of the amount of the Lewis acid sites. In the optimal catalyst condition (that is, 20% in weight of (Co+Mo) metal oxide loading), a 90% conversion value of DBT was obtained at 60 °C. As evidenced by Raman spectra, the cobalt modified molybdate MoO₃ microcrystals were the main active species during the oxidation of DBT, while β-CoMoO₄ substantially inhibited the oxidative process.

The role that the defective structure of dispersed MoO₃ active sites can play in the ODS procedure was successively evaluated by a deep analysis of the oxidation behavior of 4,6-DMDBT with the MoO₃/SBA-15 catalyst [126]. MoO₃/SBA-15 was prepared via impregnation of SBA-15 with different amounts of ammonium heptamolybdate (from 5 wt/% to 25 wt/%) as molybdenum precursor. The integrity of the mesoporous structure was found to be dependent from the loading factor, the regular mesoporous feature being observed within to 15 wt/% of Mo, after which the collapse of the structure gradually occurred [127]. XRD analysis of catalysts with 5, 10 and 15 wt/% of MoO₃ showed the presence of highly dispersed molybdenum species with an average size smaller than 4 nm. At a higher loading value, orthorombic α-MoO₃ crystals, with a certain oxygen deficiency, were formed [128]; the crystallite size being increased by increasing the amount of molybdenum. The oxygen defects are electron-deficient Lewis acid sites and may serve as surface active centers for surface adsorption and reaction. The presence of α-MoO₃ crystals in 20 and 25 wt % MoO₃/SBA-15 catalysts was further confirmed by Raman and XPS analyses [129]. The novel MoO₃/SBA-15 catalysts were evaluated in the oxidation of 4,6-DMDBT with H₂O₂ in n-hexadecane at different reaction temperatures (from 50 °C to 70 °C) for 1 h, in the presence or in the absence of formic acid as co-oxidant species. Performic acid (HC(O)OOH) is easily produced by reaction between H₂O₂ and formic acid [130]. As a general trend, the conversion of 4,6-DMDBT was found to be proportional to the number of Lewis acid sites present on the surface of the catalyst during the oxidation performed with H₂O₂ alone, the highest conversion value being obtained in the correspondence of the higher loading factor (15–20 wt/%). In the presence of formic acid, 4,6–DMDBT oxidation was significantly affected by the formation of surface peroxometallic complex and Lewis acidity. Under the optimal reaction condition using 15 and 20 wt % MoO₃/SBA-15 catalysts and the H₂O₂/formic acid mixture, more than 99% 4,6–DMDBT could be removed at 70 °C within 30 min. Figure 6 describes the suggested mechanism for the oxidation performed with the H₂O₂/formic acid mixture. In this reaction, scheme performic acid is coordinated by Mo⁶⁺ ions to generate a reactive peroxometallic complex [131] able to transfer the oxygen atom to the substrate. In this process, oxygen defective α-MoO₃ sites can perform as molecular recognition sites for 4,6-DMDBT. As suggested by the authors, when the amount of MoO₃ was greater than 20 wt % (i.e., 25 wt % MoO₃), the presence of the largest MoO₃ particles lead to lower dispersion and a lower number of exposed Mo ions, thus disfavoring the 4,6-DMDBT adsorption and oxidation.

5. Iron Oxides

Iron-based ionic liquids with redox properties have been applied in ODS processes, thanks to their high reactivity and capability to extract polar sulfone and sulfoxide derivatives from the crude oil [132]. Examples of desulfuration processes mediated by iron-based ionic liquids are reported, including the use of 1-n-butyl-3-methylimidazolium metal chloride derivatives [C₄mim] Cl/MCl₂ (M = Zn, Fe, Cu, Mg, Sn, Co) [133], dialkylpyridinium tetrachloroferrates ionic liquids [C₄MPy] FeCl₄ [134] and Fenton-like ionic liquids [135,136]. The activity and recyclability of iron-based ionic liquids was significantly improved by the immobilization of appropriate precursors on SBA-15, as in the case of the preparation of 1-methyl-3-(trimethoxysilylpropyl)-imidazolium [pmim] tetrachloro-ferrate/SBA-15 catalyst, namely [pmim]FeCl₄-SBA-15. The synthetic procedure for the synthesis of [pmim]FeCl₄-SBA-15 required two successive steps (Figure 7): (i) the grafting of [pmim]Cl on SBA-15 to yield the [pmim]Cl-SBA-15 intermediate and (ii) the ion exchange of chorine with tetrachloroferrate by treatment of [pmim]Cl-SBA-15 with FeCl₃ in acetonitrile [137].

The XRD analysis of [pmim] FeCl₄-SBA-15 showed a long-range, ordered structure with a well-defined hexagonal lattice (p6mm) motif, associated with the expected functionalization of the mesopore channels, as highlighted by the expected decrease in the intensity of the peaks [138]. The textural properties of SBA-15 were substantially maintained after immobilization of [pmim] Cl and on subsequent anchoring of FeCl₄⁻. The [pmim] FeCl₄-SBA-15 was applied in the ODS of a model fuel sample obtained by dissolving DBT, BT and DT in n-octane, using H₂O₂ as the primary oxidant. The concentration of sulfur compounds in the model fuel decreased in the order of DBT > BT > DT, with an overall 94.3% removal of DBT in the optimal conditions, consisting in the presence of [Omim] BF₄ as a co-solvent. As suggested by the authors, the reaction proceeded with the initial extraction of sulfur compounds from the oil, followed by a Fenton-like oxidation process.

As an alternative, the Fe-SBA-15 catalyst was prepared by an impregnation procedure, using Fe(NO₃)₃·9H₂O as source of iron [139]. Fe-SBA-15 showed well-resolved XRD diffraction peaks, confirming the p6mm hexagonal symmetry of mesostructured SBA-15. The decrease of the peaks’ intensity suggested a slight distortion in the material, with a partial diminution of the pore’s diameter, due to the lower ionic radio of Fe with respect to Si [140,141,142]. In particular, Fe-SBA-15 contained a well-dispersed hematite phase (α-Fe₂O₃) without any presence of the magnetite phase (Fe₃O₄).

To evaluate the ODS capacity, Fe-SBA-15 was applied in the oxidation of BT, DBT and 4,6-DMDBT, in a three-phase system (n-dodecane, acetonitrile and catalyst), with H₂O₂ as primary oxidant at 60 °C. Under these experimental conditions, more than 90% of sulfur was removed in the first 15 min of reaction time, DBT showing the higher reactivity as a consequence of its recognized high electron density on the sulfur atom [143]. Note that Fe-SBA-15 was less reactive than Fe-MCM-48 under similar experimental conditions, probably due to the difference in surface area, pore volume and dispersion of the active sites. Ultrasound-assisted technology improved the efficacy of iron-based SBA-15 catalysts. For example, Fe-SBA-15 and Fe/Zr-SBA-15 catalysts with high ODS capacity have been prepared from the corresponding mineral supports by an ultrasound-assisted procedure, using Fe(NO₃)₃·9H₂O as the iron source [144]. Irrespective of experimental conditions, spectroscopic data indicated the presence of a well dispersed hematite phase (in both α-Fe₂O₃ and γ-Fe₂O₃ forms) as an active species. Both catalysts showed the presence of Lewis acid sites, which amount increased by increasing the iron loading. The catalytic activity of Fe-SBA-15 and Fe/Zr-SBA-15 was evaluated in the oxidation of DBT in n-hexadecane and H₂O₂ as the primary oxidant. As a general trend, the higher activity was observed in the presence of the highest value of loading of Fe (30 wt/%), Fe/Zr-SBA-15 being more active than Fe-SBA-15, probably as a consequence of the role played by the zirconium-increased surface acidity in the promotion of γ-Fe₂O₃ with respect to the formation of amorphous, and less reactive, iron oxide.

6. Tungsten Oxides

Nitrogen functionalized active materials have been successfully applied in heterogeneous catalysis [145,146], including the oxidative transformations of sulfur derivatives [147,148,149], thanks to the high coordinative capability of the nitrogen-containing framework towards metal species [150,151], associated with tunable acid-base properties. With the aim of obtaining a solid catalyst, to be employed in the ODS of gasoline, with large specific surface and high catalytic activity, WO₃ was introduced, for the first time, into the SBA-15 mesoporous molecular sieve, under conventional hydrothermal conditions in strong acidic solution, using H₂WO₄ as a tungsten source in the presence of P123 triblock copolymer and cetyltrimethylammonium bromide as template. This system showed a promising oxidation activity, being able to reduce the sulfur content of gasoline (540 ppm) up to 91% in 80 min [152]. Examples of the preparation and application of tungstate nitrogen-containing catalysts are reported [153], nitride derivative being used in the oxidation of model sulfur compounds under visible light [154]. With the aim of further improving green applications of tungstate nitrogen-containing catalysts, WOx/N-SBA-15 materials have been prepared and used in the ODS procedure [155]. WOx-SBA-15 was prepared as starting material by direct synthesis in the presence of TEOS as the silica source and P123 as our structure-directing agent, using NaWO₄·2H₂O as tungstate source. Nitrogen rich WOx/N-SBA-15 was successively obtained by treating WOx-SBA-15 with a mixture of ethylene diamine NH₂C₂H₄NH₂/CCl₄ at reflux, followed by calcination at high temperature (600 and 800 °C). The novel catalyst showed the presence of well-dispersed WOx species in a nanocluster form, and was characterized by a channel-like pore arrangement of the 2D hexagonal (honey comb) type. The presence of nitrogen-containing CNx-like frameworks was confirmed by both Raman and X-ray photoelectron (XP) spectra, in association with graphite-like carbon originated by the ethylene diamine degradation. The CNx framework was characterized by the presence of pyridine-nitrogen-like atoms (N-py), nitrile moieties and a C–N–C graphitic-like ring motif, suggesting a complex rearrangement of nitrogen and carbon atoms during the preparation of the catalyst. WOx/N-SBA-15 was a catalyst more efficient than WOx-SBA-15 in the ODS procedure, suggesting a beneficial role of the CNx framework in the oxidation. Under optimal experimental conditions, the quantitative conversion of DBT was obtained with WOx/N-SBA-15 after 2 h at 100 °C, using H₂O₂ as primary the oxidant in acetonitrile. Moreover, the activity of the catalyst increased with the increase of the loading of the WOx species. Most probably, the reaction proceeded by the formation of highly reactive tungstate-peroxo complexes [156], even if the authors have not investigated the possible role of the CNx framework in this process.

7. Silver Oxides

Different supported silver adsorbents have been reported in the selective removal of sulfur compounds from fuels via a strong complexation mechanism, including Ag₂O/titania materials in jet fuel desulfurization [157,158], Ag⁺/SBA-15 and Ag⁺/SiO₂ for the selective adsorption of dibenzothiophene (DBT) [159], and Ag⁺ supported zeolite and activated carbon for the adsorption of both BT and DBT [160,161]. Moreover, supported silver oxides or silver can also act as oxidative catalysts in the epoxidation of alkenes [162,163,164], aldehydes [165], and various organic compounds [166] and carbon oxide [167]. The capability of silver oxides to contemporarily perform as active catalytic species and adsorption phases has been applied in the ODS procedure by preparation of novel Ag_xO-SBA-15 materials and their use in the adsorption/oxidation of BT, DBT, 4-MDBT and 4,6-DMDBT, as model fuel compounds [168]. Ag_xO-SBA-15 catalysts were prepared by an ultrasound-assisted impregnation process of SBA-15 and AgNO₃ water solutions at different concentration. Irrespective from the loading factor, the catalysts retained the original mesoporosity of SBA-15 showing Ag_xO species uniformly dispersed in the hexagonally-ordered network (average diameter value of the particle around 5–6 nm), the presence of larger Ag_xO particles being detected only at the highest loading factor (25 wt/%). In the XRD analysis, AgO particles largely prevailed in the fresh sample, while Ag₂O and Ag become the main silver species in the aged sample, as a consequence of degradation processes occurring during the time for unstable high valence AgO oxide at the nano-size scale [169,170]. Ag_xO-SBA-15-aged samples showed a high desulfurization activity in the presence of air under ambient conditions. The addition of air dramatically enhanced the activity with respect to the simple adsorption processes. On the other hand, freshly prepared Ag_xO-SBA-15 catalysts showed a remarkable desulfurization capacity even in the absence of air. In this latter case the desulfurization capacity followed the order of 4,6-DMDBT > 4-MDBT > DBT > BT, consistent with the order of sulfur electron density on the substrate. Irrespective of the origin of the catalysts, sulfones were detected as reaction products in the presence of air. Noteworthy, sulfones were also detected with freshly prepared Ag_xO-SBA-15 catalyst in the absence of air, suggesting that this catalyst may also perform as an endogenous source of oxygen for the oxidation. The general reaction mechanism for fresh and aged Ag_xO-SBA-15 catalysts is described in Figure 8.

Surface-distributed, nano-size [O]-retaining AgO active sites are responsible for the oxygen atom transfer from freshly prepared Ag_xO-SBA-15 catalyst to substrate in the absence of air (Panel A). A different mechanism occurs with aged Ag_xO-SBA-15 in the presence of air (Panel B), in which case Ag₂O and metallic Ag become the main active species for the direct activation of dioxygen [171]. During the aging AgO is progressively transformed into Ag₂O.

8. Polyoxometalates

Polyoxometalates (POMs) are formed by cations and polyanion clusters having structural diversity (Figure 9), where the basic construction units are based upon the MO_x (x = 5, 6) oxometal polyhedral [172,173,174]. The M species are early transition metals (e.g., V, Nb, Ta, Mo, W, and so on) at their highest oxidation states. Some polyanions are centered by heteroatoms (X) that significantly affect their properties. These heteroatoms are usually main-group elements, including Si, P, S, Ge, As, Se, B, Al and Ga, but are not limited to these. The bulky polyanions have a highly negative charge, and their surfaces are rich of oxygen atoms able to donate electrons; consequently, they can be considered as soft bases. At the same time, the metal ions on the skeleton of polyanions possess unoccupied orbitals, able to accept electrons. By this way, polyanions can also act as Lewis acids. Hence, POMs may play the roles of Lewis acid and Lewis base, under different conditions. Generally speaking, the protonation of an oxometallate ion under particular conditions (e.g., pH, concentration, temperature, solvent) gives rise to the polycondensation of the tetrahedral MO₄²⁻ units and the formation of more complex structures called polyanions:

nMO₄²⁻ + 2mH⁺ ⇌ M_nO_(4n−m)^(2n−m)− + mH₂O

If the condensation occurs between similar species, the reaction yields an octahedral isopolyanion (IPA) having the general formula M_nO_(4n−m)^(2n−m)− (usually n 6). Conversely, if the condensation of the oxoanions occurs around either, a central heteroatom (e.g., X = Si, P, As, Ge, etc.) or another metal atom, the reaction leads to the formation of a heteropolyanion (HPA) with the general formula of X_sM_nO_m^y−. Depending on experimental assembling conditions, POMs are formed by IPAs and HPAs, having two main structures, namely (i) Keggin (X/M = 1/12) and (ii) Dawson (X/M = 2/18) type structures. The many different elements which can act as heteroatoms in HPA complexes with various coordination numbers leads to the formation of several other complex polyhedra structures known as Anderson, Lindqvist, Waugh and Silverton (Figure 9) [175].

The Keggin type HPAs containing tungsten or molybdenum addenda atoms have received great attention for the very variable chemical properties, which may be easily tuned through changes in their composition and structure. The replacement of their protons by large radium cations (i.e., Cs⁺) makes insoluble the Keggin HPAs and increases their surface area and thermal stability [176]. On the other hand, the removal of tungsten or molybdenum atoms from the parent structure generates vacancies (Lacunar Keggin HPA), which can give them alternative catalytic activity [177].

In spite of an overgrowing interest toward the preparation of a huge number of inorganic–organic hybrid architectures based on POMs, due to their extreme versatility of compositions, high structural stability and special properties like strong acidity and rich redox chemistry, at the same time these compounds frequently show several drawbacks. They have, indeed, relatively small specific surface areas that hinder accessibility to their active sites, thus reducing their catalytic activity; in addition, their high solubility in polar solvents prevents their simple recovery from complex mixtures. To avoid such disadvantages and to meet the requirement of sustainable chemistry, great effort has been focused on immobilization of the active POMs on different solid supports, such as silica with high-surface areas, activated carbon, molecular sieves and graphene or graphene oxide, to improve their dispersion and heterogeneity [178,179,180]. For all the above reasons, it is reasonable that the POMs compounds have received considerable interest for what concerns their use for demanding catalytic ODS processes [181], and in particular, as immobilized catalysts on mesoporous SBA-15 materials.

Hereafter, the main results obtained in the last years in the oxidative desulfurization of authentic or model fuels promoted by heterogeneous catalysts based on POMs derivatives, will be reviewed grouping the results into two main sub-categories formed, respectively, by phosphomolybdic or phosphotungstic containing POMs.

8.1. Phosphomolybdic Containing POMs

Starting about fifteen years ago, a novel phosphomolybdic (HPMo)/SiO₂ mesoporous composite was prepared by the sol–gel method, working with 12-phosphomolybdic acid, TEOS and the triblock copolymer EO₂₀PO₇₀EO₂₀ (Pluronic P123) as a template [182]. Authors declared that in this compound, the HPMo was highly dispersed inside the silica framework, while maintaining its original Keggin structure. The heterogeneous catalytic system was active and stable toward the ODS of a model fuel only formed by DBT, with H₂O₂ as oxidant in a large excess (O/S molar ratio = 12). Some of the same authors published another paper where the phosphomolybdic (HPMo) and phosphotungstic (HPW) heteropolyacids, were chemically anchored onto the amino functionalized SBA-15 channels, with the aim to study their catalytic activity for the ODS of model fuels [183]. The SBA-15 functionalization with aminosilane groups, through aminopropyltriethoxysilane (APTES) [184], is aimed to increase the electrostatic interactions between the amino moiety of the support and the heteropolyacid groups, thus increasing the chemical stability of the heterogeneous catalyst. Interestingly, by comparing the two catalytic systems, it was evident that the catalyst HPW/H₂N-SBA-15 was slightly more active in comparison with HPMo/H₂N-SBA-15, justifying these experimental data with the conversion of the original Keggin structure of starting HPA (with its original redox potential) into a new polyoxoperoxo species, after the oxidation with H₂O₂. On the other hand, it is known that the ability of tungsten atoms to combine with TBHP is higher than molybdenum atoms. Thus with increasing the tungsten loading, the ability of HPAs to produce more active peroxometalate intermediates becomes stronger, also in the presence of TBHP as main oxidant.

An interesting method for the deep desulfurization of model fuels and exploitable as an alternative to severe HDS, indicated that the condensation reaction with formaldehyde catalyzed by phosphomolybdic acid inside selected supports such as SBA-15, acting both as support and adsorbent of the reaction products, was effective for the specific removal of thiophenic and benzothiophenic compounds, while preserving the fuel quality [185]. Indeed, as assessed by authors, the phosphomolybdic acid promoting the condensation reaction with formaldehyde, affording polymerization products, can occur only with thiophenic or benzothiophenic compounds, thus preserving olefins or the other aromatic components of fuels. The coupling with an oxidant as peracetic acid allowed a strong reduction in the total sulfur content lower than 15 ppm.

Frequently, the phosphomolybdic-based HPA has been modified with the addition of other metals in order to tune the catalytic activity of the corresponding POMs, according to specific requirements. This is the case of an heterogeneous catalyst based on a molybdovanadophosphoric acid framework supported on both pure SBA-15 or H₂N-SBA-15, after the silica functionalization with APTES [186].

It was observed that, in spite of a comparable high activity in the oxidation of sulfur compounds of a model oil (DBT in n-hexane), the catalyst PMoV₂/SBA-15-NH₂ formed through grafting of molybdovanadophosphoric acid onto H₂N-SBA-15, showed good recyclability in comparison of the catalyst PMoV₂/SBA-15 impregnated on unmodified SBA-15 silica. The higher stability against leaching of the grafted catalyst, has been attributed to the strong electrostatic binding between cation and anion due to the in situ formation of the salt ≡ Si(CH₂)NH₃·PMoV₂, not possible in the unmodified silica, where only weak interactions between silanol groups and high soluble PMoV₂ moiety, can occur. In another case, the stability of immobilized molybdovanadophosphoric acid was increased by using zirconium modified mesoporous SBA-15 able to form insoluble salts of HPAs on the silica surface. The 11-Molybdo-vanadophosphoric acid supported on Zr-modified silica (MoV/Zr/SBA-15) worked as an active and stable catalyst for the TBHP promoted ODS of model oil [187]. It is known that, in particular cases, the length and pore size of mesoporous silica channels imply both crucial factors and effectiveness in fluid phase reactions [188]. As a consequence, channels with the larger pore size and shorter length can overcome limitations like mass transfer, accessibility and diffusion in the liquid phase reactions. For this reason platelet SBA-15, characterized by shorter channels parallel to its thickness, has been considered more promising to serve as a support for catalytic systems. Accordingly, cesium salts of tungsten-substituted molybdophosphoric acid, Cs_xH_3−x [PMo_12−yW_yO₄₀], (x = 1–3, y = 2–10), supported on platelet SBA-15 have been synthesized, via a two-step impregnation method, and used as oxidative desulfurization catalysts, using TBHP as oxidant [189]. It is interesting to note that for the selected H₃PMo₈W₄O₄₀ HPA, loaded with different amounts of cesium ion, the observed initial catalytic performances followed the order Cs₂Mo₈W₄/SBA > Cs₁Mo₈W₄/SBA > Cs₃Mo₈W₄/SBA. As known, HPA supported on mesoporous silica shows strong Bronsted acidity (higher Bronsted acid sites vs Lewis acid sites), being the Lewis acidity increased by replacing of H⁺ with Cs⁺ cations. It has been clarified [190] that both kinds of acid sites may be present in Cs₁Mo₈W₄/SBA and Cs₂Mo₈W₄/SBA species, even if the number of Lewis acid sites of the former catalyst is lower than that of the latter, and being the Bronsted acidity irrelevant for Cs₃Mo₈W₄/SBA, due to the absence of H⁺ ions. As well accepted, the oxidation of sulfur substrates activated by molybdenum- or tungsten-polyoxometalate catalyst implies the formation of active peroxo species at the metal site, followed by transferring the oxygen atom to the sulfur acceptor (Figure 10).

From literature, it is known that the formation of peroxo-metallate species, is energetically favored by the combined presence of Lewis and Bronsted acid sites [191]. As a consequence, it has been supposed that the presence of both Lewis and Bronsted sites on the Cs₂Mo₈W₄/SBA catalyst, facilitates the formation of peroxo-HPA intermediates, thus increasing the oxidation reaction efficiency.

Among several advantages on the use of mesoporous SBA-15 as versatile support for heterogeneous catalysis, one of the main drawbacks may be found on its hydrophilic nature which could be a detrimental aspect if we are dealing with the catalysis of reactions containing both aqueous phase and hydrophobic starting materials, as in the case of ODS of authentic fuels. In some cases, it has been reported that the hydrophobization of mesoporous supports could markedly enhance the stability and performance of the catalyst system. To achieve this goal, the most explored approach was based on the functionalization of SBA-15 with imidazole-based ionic liquids (ILs). Noteworthy, due to the introduction of IL, the heterogeneous catalyst exhibited good wettability for the model oil, which could provide easier access to catalytically active sites, for reagents. As a well explored general method, the doping of SBA-15 by IL was based firstly on the preparation of the appropriate precursor 1-Methyl-3-(triethoxysilylpropyl)-imidazolium chloride ([pmim]Cl) ionic liquid, by using N-methyl imidazolium and 3-chloropropyltriethoxysilan, and then by grafting the latter IL on SBA-15, under usual conditions. At this point, several different HPAs may be immobilized affording several different types of hybrid functional materials (Figure 11) depending on the nature of metals (M, M’) and heteroatoms (X).

By this way, several authors published the preparation of heterogeneous PMOs catalysts, ranging from HPMoV₂ [192], to HPMo [193] based materials, all following the same immobilization procedure over the functionalized IL-SBA-15 silica. These hybrid catalysts displayed high catalytic performances for removing sulfur compounds from model oil (especially in the case of HPMo system, where the H₂O₂ oxidant was used in the lowest amount) and high stability and recyclability properties, acting both as a catalyst and as an adsorbent toward the sulfur oxidized products. Interestingly, XRD analyses confirmed that the ordered structure of SBA-15 is maintained after the IL grafting and the successive HPA loading. Concerning the latter point, an optimal value should be carefully chosen in order to avoid that IL-SBA-15 channels can be blocked by the HPA overloading, thus reducing the surface area and consequently the catalytic activity.

8.2. Phosphotungstic Containing POMs

Since the last fifteen years, several articles reported the preparation of heterogeneous catalysts based on the simple wet impregnation of variable amounts of phosphotungstic acid over mesoporous SBA-15 or mixed HY-SBA-15 composite zeolite [194], with the aim to study their catalytic activity in the ODS process applied either to model fuels [195] or real feeds [196] with H₂O₂, TBHP o air [197] as main oxidants. In general, these catalytic systems showed high activity and stability, being possible to reuse the heterogeneous catalysts after the regeneration step. Supports based on commercial silica showed worst results in comparison with mesoporous SBA-15, with the same tungsten loading, thus evidencing the positive role on the use of a mesoporous support allowing on the one hand the presence of well-dispersed tungsten species and on the other reducing the possible catalyst deactivation caused by sulfone deposition during oxidation. Nevertheless, some problems remain in SBA-15-supported catalysts being possible that phosphotungstic acids easily leached into a solution because of the weak interaction between them and SBA-15. Therefore, the search for catalysts with high activity and good recovery remains essential. From this point of view, the creativity of the researchers to find innovative solutions, ranged from the preparation of micro-mesoporous composite molecular sieves (ZSM-5/SBA-15) doped with different metal ions as Zr, Ag, Ce, thus balancing advantages/disadvantages of both types of supports [198] to the immobilization of phosphotungstic acids (HPW) onto composite supports as pentaethylenehexaamine (PEHA)-preloaded acidic Zr/SBA-15 with short pore channels [199]. In the latter, the synergic role of the zirconium-ion-modified acidic SBA-15 and the auxiliary PEHA molecules toward the high dispersion and stabilization of HPW clusters is believed to endow the resulting nanocomposites with high reusability in comparison to the zirconium free materials. A deep investigation on the potential effects of the preparation methods of mesoporous HPW/SBA-15 catalysts on their catalytic performances on the ODS of a model oil, has been reported [200]. In this work, authors focused their study on two kind of catalysts prepared through silica aminofunctionalization (AF) or evaporation-induced self-assembly (EISA) methods, respectively (Figure 12). The study revealed that the catalyst prepared by EISA method gave better results, in terms of higher catalytic activity and stability in comparison with that obtained by AF procedure. The reasons may be found on the fact that, according to structural and spectroscopic analyses, in the first case the catalyst maintain the Keggin structure of original HPW molecules and has a high surface area; in the second strategy, the mesostructure of SBA-15 resulted partially blocked after the introduction of HPW molecules, and the Keggin structure of HPW molecules was damaged.

Anyway, the easy modification of native SBA-15 surface through a post-grafting procedure, in order to introduce more robust functional group (i.e., APTES to give H₂N-SBA-15 or N-(3-trimethoxysilylpropyl) tributylammonium to give tba-SBA-15) for the anchoring of different types of HPAs, justifies the large number of published articles appeared in literature, in the last decade. Among them, H₂N-SBA-15 and tba-SBA-15 supports were used to heterogenize, through an impregnation method, Keggin-type derivatives like lacunar polyanions [PW₁₁O₃₉]⁷⁻ (PW₁₁), resulting by the removal of one WO⁴⁺ unit, or sandwich-type [Eu(PW₁₁O₃₉)₂]¹¹⁻ anion affording robust and active catalysts for the ODS of model oil and real diesel, either under biphasic (diesel/acetonitrile 1:1) or solvent-free conditions [201,202]. Noteworthy, the PW₁₁/H₂N-SBA-15 composite system showed to maintain its activity for eight consecutive cycles. In another case, the amino functionalization of SBA-15 has been obtained either by one-step synthesis (co-condensation method, that is introducing the APTES into mesoporous silica in the presence of copolymer Pluronic P123) [203] or by the classical post-grafting procedure [204], having in mind the heterogenization, in both cases, of the Keggin-type 12-tungstophosphoric [PW₁₂O₄₀]³⁻ HPA. In this case, the heterogeneous catalysts have been only used for the study of model oils with H₂O₂.

Composite catalysts based on tricopper(II)-substituted sandwich-type polyoxotungstates, having the general formula (Cu₃X₂W₁₈, X = Bi^III, Sb^III) supported over H₂N-SBA-15, have been prepared to be used in the H₂O₂ promoted ODS of thiophene [205]. Authors suggested that the best detected loading amount of Cu₃X₂W₁₈ species over the mesoporous silica was up to 4.7%, while worst activities were observed for higher levels of active species, as a result of the decreasing of surface area and pore diameters, thus hindering the flow of the reaction substrates into the pore channel of H₂N-SBA-15. No relevant differences, on the catalytic activity, have been detected for the two different polyoxotungstates containing Bi^III or Sb^III, respectively. In a recent paper, a contribution based on the use of a composite material formed by a phosphorous-containing tetranuclear peroxotungstate {PO₄[WO₂(O₂)₂]₄}³ Venturello anion symbolized as PW₄, immobilized on trimethylammonium functionalized SBA-15 (PW₄/tma-SBA-15) through an impregnation method, employed as catalyst for the ODS of model and real fuels, has been published [206]. Impressive data concerning activity and stability of heterogenized POM, have been reported by authors; moreover, carefully investigations on the fresh heterogeneous catalyst and on the material recovered after the successive recycling steps, showed that during the heterogenization and oxidation experiments an equilibrium between the two species PW₄ and the α-Keggin heteropolyanion [PW₁₂O₄₀]³⁻ (PW₁₂), cannot be ruled out.

Accordingly to what above reported for the immobilization of phosphomolybdic based POMs, several authors published examples describing the preparation of phosphotungstic acid immobilized over ionic liquid-modified SBA-15 support too, having in mind to enhance the stability and the performances of the catalyst system. An interesting paper described the preparation of a series of hybrid polyoxometalates formed by the same anion α-Keggin phosphotungstate PW₁₂ and three different ionic liquid cations, namely 1-butyl-3-methylimidazolium (BMIM), 1-butylpyridinium (BPy) and hexadecylpyridinium (HDPy), having the general formula [IL]₃PW₁₂ [207]. The activity and stability of these catalysts toward the ODS of model diesel under extractive conditions (ECODS), that is using [BMIM]PF₆ or acetonitrile as extraction solvent, have been studied in comparison with the phosphotungstate PW₁₂ anion immobilized over the functionalized mesoporous tma-SBA-15 (see above). The latter (PW₁₂/tma-SBA-15) showed to be the most stable composite system having similar catalytic performances with the three homogeneous [IL]₃PW₁₂ catalysts and comparable activity both in acetonitrile and [BMIM]PF₆, even if with high recycling capacity. On the other hand, the classical ionic liquid derivatization of SBA-15, based on the preparation of the appropriate precursor 1-Methyl-3-(triethoxysilylpropyl)-imidazolium chloride ([pmim]Cl) ionic liquid and on the next grafting of the latter on SBA-15, allowed the anchoring of different HPA, ranging from the silicotungstic acid H₄[SiO₄(W₃O₉)₄] [208] to the classical phosphotungstic PW₁₂ anion [209]. In both cases, irrespective to the specific type of polyoxometalate composition, the anchoring on the IL functionalized support gave a system with higher catalytic activity for the H₂O₂ promoted ODS of model oils in comparison to the system obtained by unmodified SBA-15. As outlined by the authors, the reasons may be found on the enhanced wettability for the model oil promoted by the IL functionalization of the SBA-15 (changing its nature from hydrophilic to hydrophobic support), thus affording a catalytic system with high surface area, high accessibility of substrate and oxidant toward the active sites and substantially no leaching of the active species in the reaction mixture. Finally, the loading amount of HPA need to be carefully evaluated because beyond a certain value that varies for each species, the sulfur removal efficiency decrease due to the occurrence of mass transfer problems caused by the decrease of surface area and pore size of the support, thus limiting the successful contact between the substrates and the active species.

9. Miscellanea

In close correlation with SBA-15, the SBA-16 material is characterized by a more ordered three-dimensional (3-D) cage-like mesoporous structure, representing a further major achievement in the synthesis of mesoporous materials [210]. Pluronic F127 is used as a template to synthesize SBA-16 instead of P123 [211], providing larger pore diameter and wall thickness, as well as, higher thermal and hydrothermal stability [212]. Unfortunately, the preparation of SBA-16 requires acid conditions which are not compatible with the incorporation of a large panel of metal derivatives by traditional procedures, due to the occurrence of leaching processes. To avoid this drawback, Ti-SBA-16 mesoporous catalysts have been prepared (with various Ti loadings of 5, 10, and 15 wt%) by the evaporation-induced self-assembly method, and successively applied in the ODS procedure [213]. In this procedure, F127 in ethanol was treated with the appropriate amount of tetrabutyl titanate and TEOS under acid conditions (pH 2), followed by removal of the organic solvent and calcination at high temperature. Ti-SBA-16 showed the typical XRD signals of mesostructured material characterized by a slightly disordered structure. The presence of the silica-titanium framework was unambiguously confirmed by FTIR analysis [214] and XPS analysis, this latter highlighting that all Ti ions were incorporated onto the SiO₂ structural motif [215,216,217]. Irrespective from the value of the Ti loading factor, all catalysts demonstrated a high activity in the oxidation of DBT as a model compound in n-octane/MeOH at 60 °C, using H₂O₂ as primary oxidant. A slight decrease in the activity of the catalysts was observed only at the highest 15 wt/% of Ti loading, probably due to the presence of major distortion effects on the mesoporous framework. Under optimal experimental conditions (that is 10 wt/% Ti-SBA-16, 60 °C for 3 h) 99% of sulfur removal was obtained.

10. Conclusions

The best catalysts and experimental conditions for the desulfurization process are summarized in

Table 1. Selection of optimal catalyst modification and operating conditions for SBA-15 anchored metal containing catalysts in the oxidative desulfurization process.

Catalyst Modification	Operating Conditions a	Sulfur Removal (%) b	Catalyst Recycle (n° of Runs) c	Reference
TiO2/SiO2 sol-gel method	BT, DBT, MDBT, and DMDBT, UV-irrad., air, toluene/dodecane, 25 °C, 2 h.	n.c.	5	[78]
TiO2/SBA-15 grafting insertion	BT, DBT, 4-MDBT, and 4,6-DMDBT, CHP, n-heptane/toluene, 80 °C.	>99	n.a.	[91]
V2O5/SBA-15 wetness impregnation	Selective extraction phase DBT, 4-MDTB, 4,6-DMDTB, [Bmim]BF4, O2, 120 °C, 6 h.	99	6	[116]
CoMo/SBA-15 wetness impregnation	DBT, hexadecane, H2O2, 60 °C, 1 h.	90	n.a.	[125]
MoO3/SBA-15 wetness impregnation	Formic acid as co-oxidant species, 4,6-DMDBT, H2O2, hexadecane, 70 °C, 1 h.	99	n.a.	[130]
[pmim]FeCl4/SBA-15 grafting insertion	BT and DBT, H2O2, octane, 30 °C, 1 h.	94.3	n.a.	[137]
Fe/SBA-15 wetness impregnation	BT, DBT and 4,6-DMDBT, H2O2, n-dodecane and CH3CN, 60 °C, 15 min.	90	4	[139]
WOx/SBA-15 grafting insertion	DBT, H2O2, n-dodecane, 100 °C, 2 h.	>99	n.a.	[155]
MoV/Zr/SBA-15 wetness impregnation	DBT, TBHP, n-hexane, 60 °C, 75 min.	98.5	4	[187]
Cs2Mo8W4/SBA-15 wetness impregnation	DBT, TBHP, n-hexane, 60 °C, 80 min.	>99	4	[189]
HPMo/IL-SBA-15 wetness impregnation	DBT, H2O2, n-octane, 60 °C, 90 min.	>90	5	[193]
HPW/PEHA/Zr/SBA-15 layer by layer strategy	DBT, H2O2, n-octane, 60 °C, 60 min.	95	6	[199]
HPW/SiO2-EISA self-assembly	BT, DBT and 4,6-DMDBT, H2O2, petroleum ether/CH3CN, 60 °C, 2 h	>99	7	[200]
PW11/H2N-SBA-15 wetness impregnation	BT, DBT, 4-MDBT and 4,6-DMDBT, H2O2, n-octane and CH3CN, 70 °C, 60 min.	>99	8	[201]
PW4/tma-SBA-15 wetness impregnation	BT, DBT and 4,6-DMDBT, H2O2, n-octane and [Bmim]PF6 or MeCN, 70 °C, 3 h	>99	10	[206]
HSiW/IL-SBA-15 wetness impregnation	BT, DBT and 4,6-DMDBT, H2O2, n-octane, 60 °C, 2 h	96	8	[208]
TiO2/SBA-16 self-assembly	DBT, n-octane, 60 °C, 3 h	>99	5	[213]

^a BT:benzothiophene; DBT: dibenzothiophene; 4-MDBT: 4-methyldibenzothiophene; 4,6-DMDBT: 4,6- dimethyl dibenzothiophene. CHP: cumene hydroperoxide. [Bmim]BF₄: 1-butyl-3-methylimidazolium tetrafluoroborate. [pmim]: 1-methyl-3-(trimethoxysilylpropyl)-imidazolium. ^b n.c.: percentage of S removal, not calculated. ^c n.a.: not available.

, where the selection criteria have been focused upon the nature of the catalyst modification, the operating conditions and the percentage of the sulfur removal. As a general trend, the wetness impregnation and the grafting insertion technologies showed a similar efficacy in the activity of SBA-15 based metal oxide catalysts, with the only exception of TiO₂, in which case the grafting procedure yielded the most reactive system. The low expensive, mild and environmentally friendly H₂O₂ was applied as main oxidant in most of the reported procedure, highlighting the high catalytic performance of the metal oxides. The use of ionic liquids as selective extraction solvent or, in alternative, the employment of carboxylic acid as co-oxidant, does not appear to significantly improve the efficiency of the desulfurization process, with respect to simpler procedures. Among the metal oxides, V₂O₅/SBA-15 was the only catalyst able to activate dioxygen (even if at relatively high temperature, 120 °C), while [pmim] FeCl₄/SBA-15 was operative at the lowest reaction temperature (30 °C). With respect to the recyclability of the systems, Fe/SBA-15, TiO₂/SiO₂ and V₂O₅/SBA-15, were effective systems ranging from 4 to 5 and 6 successive runs, respectively. In the case of SBA-15 based polyoxometalates, wet impregnation or self-assembly strategies were the mainly used strategies for their heterogenization, varying from phosphomolybdic to phosphotungstic containing systems. Excellent results in terms of sulfur removal percentage and recyclability efficiencies have been observed after the POMs heterogenization on previously functionalized SBA-15, through the introduction of amino groups, trialkylammonium moieties or selected ionic liquids, even if excellent results were observed after simple doping of micro-mesoporous composite molecular sieves (ZSM-5/SBA-15) with Zr, Ag and Ce metal ions.