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Article

Supported Ionic Liquid Catalysts for the Oxidation of S- and N-Containing Compounds—The Effect of Bronsted Sites and Heteropolyacid Concentration

by
Vladislav Gorbunov
1,
Aleksey Buryak
2,
Kirill Oskolok
1,
Andrey G. Popov
1 and
Irina Tarkhanova
1,*
1
Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia
2
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31-4 Leninsky Prospect, 119071 Moscow, Russia
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(4), 664; https://doi.org/10.3390/catal13040664
Submission received: 1 March 2023 / Revised: 24 March 2023 / Accepted: 26 March 2023 / Published: 28 March 2023
(This article belongs to the Special Issue Heterogeneous Catalysts for Petrochemical Synthesis and Oil Refining, 2nd Edition)
Browse Figures
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Abstract

:
In this article, a series of effective catalysts based on betaine and sulfuric or phosphomolybdic acids was obtained. These compositions were characterized by various physicochemical methods and tested in the oxidation of sulfur- and nitrogenous-containing compounds by H2O2. An increase in the amount of heteropolyacid (HPA) leads to a non-linear change in acidity, and the degree of removal of sulfur-containing compounds correlates with the concentration of Bronsted acid sites on the surface. On the contrary, the degree of pyridine removal is determined primarily by the content of heteropolyacids in the catalyst.

Graphical Abstract

1. Introduction

The ever-increasing demand for fossil fuels leads to higher amounts of hazardous emissions, which pollute the environment. Therefore, requirements for the quality of oil-derived products increase from year to year. Fuel combustion produces nitrogen- and sulfur-containing compounds and carbon monoxide, which are released into the atmosphere. For this reason, a goal pursued by many researchers is to find a way to minimize harm to the environment by thorough purification of fuel from toxic compounds. Nitrogen and sulfur compounds are adverse to the environment and human health. The amount of residual sulfur has been strictly regulated since long ago [1,2], but this is not the case for nitrogen compounds. A quantitative restriction of the nitrogen oxide emission for diesel engines of light motor vehicles was introduced only recently [3,4,5]. It is commonly known that hydrogenation is the key industrial process used to remove sulfur compounds from oil products [6]. However, as the molecular weight of the sulfur substrate increases, more drastic conditions are required for this process [7] to increase efficiency. Therefore, in some cases, hydrogenation can be replaced by alternative hydrogen-free processes [8], in particular, catalytic oxidation using hydrogen peroxide, which is easy to implement and environmentally friendly [9,10].
The catalysts used for desulfurization include Brønsted acids and ionic liquids (ILs) with acid sites and derivatives of some transition metals, in particular, polyoxometalates [11,12].
It is efficient to combine catalytic oxidation with the subsequent removal of products by extraction or adsorption. The oxidation products are more polar than the initial heteroatomic compounds and more so than fuel hydrocarbons; hence, they can be extracted with organic polar solvents [13] or with ionic liquids [14,15].
The adsorption removal of sulfur compounds and their oxidized derivatives is based on extraction caused by selective interaction between the adsorbate molecules and the adsorbent surface. The commonly used adsorbents are solid metal-organic frameworks [16,17,18], nanocomposites [19], organic polymer supports [20], and active carbon-based fibers [21].
A promising method is to remove heteroatomic compounds from oil distillates using ionic liquids as catalysts or extractants, owing to their unique properties and variable structure [22,23,24].
The most interesting approach is to use ionic liquids simultaneously as catalysts of hydrogen peroxide oxidation and as extractants for sulfur compounds [25,26].
The question arises of what criteria should be used to select the appropriate catalyst for a particular oxidation reaction of sulfur compounds. It was found that the strength and the type of acid sites in the catalysts are one of the most important criteria [27]. Systems are effective if their acidity is provided by a sulfonic acid (−SO3H) linked by a covalent bond to the organic cation of IL and by a Brønsted acid counter-ion, hydrogen sulfate (HSO4−), or dihydrogen phosphate (H2PO4−) [28].
Although the recycling techniques for these compositions are known [29], immobilization of the catalytically active component on the adsorbent surface to obtain supported ionic liquids (SIL) is appropriate to decrease IL consumption and facilitate the catalyst recovery [30].
Oxidative desulfurization supplemented by extraction or adsorption is a promising technique. The ILs that have active sites in both cation and anion moieties are the most efficient catalysts. For example, the cation may contain a Brønsted acid site, and the anion may contain a polyhydric acid residue of phosphoric, sulfuric [31], or phosphomolybdic, phosphotungstic acid, which form peroxo complexes upon reactions with oxidants [12,32]. The fabrication of these complex polyfunctional heterogeneous compositions gives rise to catalysts that are active and stable in oxidative desulfurization.
Nitrogen compounds poison hydrotreating catalysts; therefore, in some cases, their presence requires additional steps in the process [33].
The nitrogen compounds present in oils are subdivided into basic (pyridine deriva- tives) and non-basic (pyrrole derivatives). Oil hydrotreating can convert non-basic nitrogen compounds to strong bases before amine is eliminated as ammonia [34].
Catalytic hydrogenation is most often used in industry to remove nitrogen from oils. Among heteroatomic compounds present in oil feedstock (S, N, and O), nitrogen compounds are the most difficult to eliminate by this method. This is especially true for pyridine nitrogen. The influence of nitrogen compounds on oxidative desulfurization for various catalysts was studied. For example, the presence of quinoline and indole does not affect oxidative desulfurization, while pyrrole and pyridine decrease the rate of thiophene removal in the oxidation with H2O2 [33].
Here we studied the catalytic compositions obtained from 4-(3′- ethylimidazolium) butanesulfonate (betaine) derivatives and containing two types of catalytically active sites, Brønsted sites, due to protonation of betaine with sulfuric or phosphomolybdic acid (PMA) (see Figure 1), and oxometalate sites.
The catalysts were deposited on a solid adsorbent, silica gel. The catalysts were tested in the hydrogen peroxide oxidation of model substrates, dibenzothiophene, thioanisole, and pyridine, taken as examples of heteroatomic components of diesel fractions [35]. In addition, we used thiophene, which is present in light oil fractions and is the heterocyclic compound most difficult to oxidize [30].
A significant factor for these catalytic systems is the number and type of acid sites; therefore, the purpose of this study was to elucidate the effect of the Brønsted acidity and the content of heteropolyacid on the catalytic properties of samples towards the oxidation of test substrates with hydrogen peroxide.

2. Results and Discussion

2.1. Physico-Chemical Characteristics of the Synthesized Ionic Liquids and Heterogeneous Catalysts

Figure 2 shows the IR spectra of H3PMo12O40, betaine, and their derivative (PMoIL1) in the frequency ranges of 500–1800 and 500–900 cm−1, and oxometalate sites.
Spectra 1 and 3 show absorption bands for the PMo12O40 heteropolyanions. The P-O bond gives rise to a characteristic band at 1064 cm-1 (vas POa). The bands at 960, 872, and 789 cm−1 correspond to the vas Mo-Od, vas Mo-Ob-Mo, and vas Mo-Oc-Mo modes. The 1560 cm-1 band present in the spectrum belongs to the stretching vibrations of the imidazolium moiety (1600–1500 cm−1).
According to published data [36], the decomposition of the acid produces two additional weak bands at 775 cm−1, which can be attributed to monoclinic molybdenum oxide (beta- MoO3). In spectrum 3, there are no such bands; this means that the heteropolyanion structure is retained after interaction with betaine. Apart from the band at 872 cm−1 (vas Mo-Ob-Mo), spectrum 3 (Figure 2b) shows absorption at 880 cm−1, which can be assigned to S-OH vibrations of the SO3H group in sulfonic acids [37]. Thus, from analysis of the spectra, it can be seen that the sulfo group is protonated by the heteropolyacid.
Figure 3 shows micrographs of PMo1 catalyst samples and element distribution maps for S, P, and Mo.
Analysis of the micrographs indicates that the coatings are dense and continuous and that the location areas of S and Mo coincide. Table 1 presents the element distributions on the PMo1 catalyst surface according to EDA data.
Figure 4 shows the micrographs of the SA catalyst samples and S and N distribution maps. The coating is distributed uniformly, and it can be seen that the location areas of S and N coincide. These data are also given in Table 2.
Figure 5a,b show the nitrogen sorption–desorption curves for the PMo1 and SA catalysts. The hysteresis loop attests to the presence of mesopores.
The BJH (Barrett–Joyner–Halenda) average pore diameter and specific volume and the BET (Brunauer–Emmett–Teller) specific surface area for all of the catalysts are summarized in Table 3. As can be seen, the deposition of the active phase leads to a considerable decrease in the specific surface area and in the pore volume of the PMo1 catalyst, which is caused by the filling and partial blocking of pores. In the case of SA, the average pore diameter markedly decreases, while the specific surface area and the volume of pores barely differ from those of the initial support within the determination error. This difference in the textural characteristics of the two catalysts containing equimolar amounts of the active phase on the surface can be attributed to the large difference between the sizes of the HSO4- and HPMo12O402−anions. Table 3 also presents data from X-ray fluorescence analysis and results of measurements of catalyst acidity by pyridine adsorption.
As can be seen in Table 3, an increase in the HPA concentration on the surface leads to a monotonic decrease in the pore-specific surface area, volume, and diameter. Meanwhile, as follows from the analysis of the acidity data, the BAS concentration is maximum when the Mo content is medium (PMo2), which corresponds to HPA to betaine molar ratio of 1:3. In this case, the HPA concentration is 45 µmol/g, while the acidity is 88 µmol/g. On going to the PMo1 sample containing a higher concentration of HPA, the acid sites accessible for pyridine adsorption are apparently partially blocked. This is confirmed by the decrease in the pore-specific surface area, volume, and diameter. As a result, for the 80 µmol/g concentration of HPA on the surface, the acidity is only 57 µmol/g. Conversely, a decrease in the molybdenum concentration from 5 to 2.4% (PMo3) leads to a proportional change in the BAS concentration from 88 to 39 µmol/g.
A different picture is observed for LAS: in this case, the concentration of acid sites monotonically decreases with a decrease in the amount of HPA on the surface.
As will be shown below, the highest activity towards the oxidation of sulfur-containing substrates was inherent in the PMo2 catalyst with betaine to HPA ratio of 3:1; therefore, it was studied by X-ray photoelectron spectroscopy.
The survey XPS spectrum of PMo2 (Figure A1a) and high-resolution spectra of the lines of elements (Figure A1b, Figure A2, Figure A3 and Figure A4) were resolved into components corresponding to different states of atoms in the sample (Table 4). The Mo3 XPS spectrum (Figure A3b shows a state with the Mo3d5/2 binding energy of 233.2 eV, which can be assigned to molybdenum atoms in the +6 oxidation state. The second state present in the spectrum with the Mo3d5/2 binding energy corresponding to 232.0 eV corresponds to Mo+5 [38], which apparently appears as a result of the partial reduction of molybdenum during the recording of the spectra. The partial reduction of the sample induced by X-ray radiation is additionally evidenced by a color change (darkening) of the sample at the site exposed to X-rays.
It follows from the analysis of the atomic concentrations of elements in PMo2 (Table 4) that the S:Mo ratio is 3.4, which is somewhat lower than the theoretical value, amounting to 4, while the P:Mo ratio is 6.4, which is also markedly lower than the expected value. The difference between the actual ratios and the theoretical ones may be due to the partial decomposition of HPA during the synthesis to give molybdenum oxides. Because of the smaller size compared to heteropolyanions, molybdenum oxides can diffuse into the inner layer inaccessible for XPS analysis, for example, into the space inside the pores.
The data of the SALDI mass spectrum of the catalyst PMo2 shown in Figure 6 are in line with this conclusion (mass spectra of PMo1 and PMo3 are in Figure A5 and Figure A6). As can be seen, the system with the maximum content of heteropolyacid (PMo1) shows molecular ions, while in the region of low mass, mainly polymeric molybdenum oxides are detected. In the PMo2 and PMo3 catalysts, anions containing less molybdenum than the Keggin structures are present.
According to literature data, these systems often have a catalytic activity higher than the initial heteropolyanions [39].

2.2. Catalytic Properties of Heterogeneous Compositions

Figure 7 shows the time dependence of the degree of removal of thiophene upon oxidation in the presence of the test catalysts and the degree of removal of thiophene in the presence of SA without H2O2.
It can be seen that the highest activity towards the oxidation of thiophene is inherent in PMo2. The PMo1 catalyst proved to be less active, despite the higher content of Mo. In the initial part of the curves, the reaction rates are similar; however, subsequently, the reaction catalyzed by the system with a higher metal content is retarded. This trend is attributable to the fact that together with the target reaction, hydrogen peroxide decomposition takes place (Figure 8); this reaction is catalyzed by transition metals, including molybdenum [40].
Thus, an increase in the molybdenum content on the surface does not lead to a higher activity of the catalyst but, conversely, accelerates the side reaction. Meanwhile, in the case of SA or PMo3 catalysts, that is, when the transition metal is absent or has a low concentration, no retardation of the reaction is observed in the first three hours. The lowest activity was found for SA since Brønsted acids are poor catalysts for thiophene oxidation.
We investigated our systems in three successive cycles of thiophene oxidation. It was found that the SA catalyst is unstable: by the third cycle, the degree of thiophene removal is only 5%. At the same time, molybdenum-based catalysts are more stable: a slight decrease in conversion after the first cycle, which may be associated with the leaching of excess active phase from the surface, is replaced by stable operation of catalysts: thiophene conversions are 34 and 30% for PMo1 and 41 and 40% for PMo2, respectively.
Figure 9 and Figure 10 depict the time dependencies of substrate removal for various catalytic systems. For DBT and pyridine, such as for thiophene, tests were also carried out without hydrogen peroxide to determine the contribution of adsorption to the overall desulfurization.
Analysis of the curves indicates that the above-noted catalyst activity series, PMo2 > PMo1 > PMo3 > SA, holds for sulfur-containing substrates. That is, among the HPA-based catalysts, the catalyst with the highest BAS concentration proved to be the most active. In the case of pyridine, the catalyst activity correlates with the content of HPA on the surface, while the acidity does not significantly affect the removal of pyridine (Figure 10 and Figure 11). This behavior may be due to the fact that, owing to its strong basic properties, pyridine is adsorbed on the acid sites of the catalyst; hence, the catalytic activity is determined only by the content of heteropolyanions. A comparison of the reactivities of pyridine and sulfur-containing heterocyclic substrates indicates that pyridine is more reactive, especially in the initial stage of the reaction (Figure 11).
It can be seen in Figure 11 that the acidity affects most appreciably the conversion of dibenzothiophene. In this case, the removal is completely consistent with the acidity series of the catalysts. This might be due to the fact that the presence of an acidic proton disrupts the aromaticity of the dibenzothiophene molecule and thus facilitates the nucleophilic substitution reaction [12,32]. For other sulfur compounds, the influence of both factors (Brønsted acid sites and polyoxymetalate concentration) is quite pronounced.

3. Materials and Methods

3.1. Materials

For the study, we used a BASF Perlkat 97-0 Silica Gel support (as per manufacturer’s specifications: specific surface area S = 500 m2/g; available pore diameter dp = 10 nm). Sulfuric acid (98%), phosphomolybdic acid (99%) (PMA), isooctane (99.5%), thiophene (99%), dibenzothiophene (99%) (DBT), methylphenyl sulfide (99%) (MFS), pyridine (99%), hydrogen peroxide (30 wt %, high-purity grade) were all purchased from Sigma–Aldrich (Darmstadt, Germany). Betaine, or 4-(3′- ethylimidazolium)-butanesulfonate, was synthesized by the method described in our previous studies [41,42].

3.2. Preparation of Ionic Liquids

For the synthesis of an ionic liquid based on PMA (PMoIL1) 4-(3′-ethylimidazolium)- butanesulfonate (betaine) (0.8 mmol, 0.2 g) was mixed with PMA (≈0.8 mmol, 1.53 g) in a molar ratio of 1:1. The PMA was stirred in 8 mL of H2O until the heteropolyacid was dissolved. After that, betaine was added, and the mixture was stirred for another 6 h at room temperature. The resulting yellow powder was dried in air.
For the synthesis of an ionic liquid with a reduced content of PMA (PMoIL2), 4-(3′- ethylimidazolium) butanesulfonate (betaine) (0.8 mmol, 0.2 g) was mixed with PMA (≈0.3 mmol, 0.49 g) in a molar ratio of 3:1 in 5 ml of H2O and the procedure was carried out in the same way.
For the synthesis of an ionic liquid based on sulfuric acid (SAIL), betaine (0.5 mmol, 1.2 g) was mixed with sulfuric acid (0.5 mmol, 0.5 mL) in a 1:1 molar ratio. The mixture was stirred for 24 h at 70 °C, after which it was washed three times with diethyl ether (3 × 15 mL).

3.3. Preparation of Catalysts

To obtain a heterogeneous PMo1 catalyst, 10 mL of H2O was added to 1.8 g of PMoIL1 and stirred until dissolved. Then, 4.79 g of SiO2 was added and the mixture was stirred at room temperature for 13 h. The excess solvent was removed in a stream of air at room temperature and then by heating to 40 °C for 6 h. During the reaction, the IL partially decomposed with MoO3 formation. Therefore, the molybdenum concentration on the surface turned out to be less than the calculated one.
To obtain a heterogeneous catalyst with a possible assignment of PMA (PMo2), 10 mL of H2O was added to 0.69 g of PMoIL2 and changed until dissolved. Then, 4.8 g of silica gel was added, and the mixture was stirred at room temperature for 13 h. In this case, IL was completely adsorbed on the support.
The PMo3 catalyst was obtained without preliminary isolation of the corresponding IL. For this, PMA (0.1 g, 0.06 mmol) was dissolved in 5 mL of water, and then 0.1 g (0.4 mmol) of betaine was added. After the resulting mixture was dissolved, 2.3 g of silica gel was added, and the solution was stirred for 6 h at 40 °C. Next, the solvent was removed similarly to the previous experiment.
To prepare the SA catalyst, 0.2 g of SAIL was dissolved in 10 mL of water, then 3.9 g of silica gel was added, and the mixture was stirred for 5 h at room temperature. The solvent was removed in a stream of air and dried at 50 °C. Thus, the content of the active phase is about 5 wt.%.

3.4. Methods for the Analysis of Ionic Liquids and Catalysts

IR spectra in KBr pellets were recorded on an Infralum FT-801 FT-IR spectrophotometer (LLC Lumex-Siberia, Novosibirsk, Russia) in the range of 4000–400 cm−1.
The concentration of acid sites in the samples was determined using IR spectroscopy of adsorbed pyridine from the intensity of the adsorbed pyridine band at 1547 and 1450 cm−1. The extinction coefficient from work was used in the calculations [43].
The surface of the catalysts was examined by scanning electron microscopy (JEOL Ltd, Tokyo, Japan). To achieve this, we used a JEOL JSM-6000 NeoScope electron microscope with a built-in EX-230 X-ray analyzer for energy-dispersive analysis of particle distribution.
Adsorption measurements were carried out on an ASAP 20000N Micromeritics automatic sorptometer (Micromeritics, Norcross, GA, USA).
For the determination of molybdenum in synthesized samples, the high-aperture sequential X-ray fluorescence wavelength-dispersive spectrometer SPECTROSCAN MAX- G (SPECTRON, St. Petersburg, Russia) was used [30]. The determination was carried out according to the line MoKa (70.9 pm) by the method of an external standard (usual calibration). The composition of heteropolyanions on the surface of the catalysts was determined by mass spectrometry using the SALDI technique (surface-activated laser desorption/ionization).
The mass spectra of the samples were recorded in the RN Pep Mix mode on a Bruker Ultra-flex instrument equipped with a nitrogen laser, and a time-of-flight mass analyzer. The spectra were recorded using a reflectron in the negative ion registration mode. Identification of cluster ions by isotopic distribution was carried out using the IsoPro simulator program.
X-ray photoelectron spectra of the PMo2 catalyst were recorded on an Axis Ultra DLD spectrometer (Kratos) using monochromatic AlKa radiation with a neutralizer.

3.5. Catalytic Test

For the oxidation of sulfur-containing compounds with hydrogen peroxide, a model mixture in an amount of 10 mL (1 wt % of thiophene, dibenzothiophene, methylphenylsulfide or pyridine in isooctane), as well as a catalyst (0.1 g) and an oxidizer (H2O2, 0.4 mL) were placed in a jacketed reactor with a magnetic stirrer.
The contents of the reactor were stirred while heating (60 °C), and samples of the organic phase were periodically taken for analysis by gas–liquid chromatography on a Kristall-2000 instrument (Chromatec, Yoshkar-Ola, Russia) with a Zebron ZB-1 (Phenomenex Inc, Torrance, CA, USA) capillary column (30 m) and a flame ionization detector.
The determination of the conversion of substrates was established by the decrease in their content in the organic phase of their data GLC by the method of internal standard, which was used as nonane or dodecane.
To carry out experiments to determine the stability of the catalysts in successive cycles, after the completion of the reaction, the liquid phase was poured out, a new portion of the reagents was placed in the reactor, and the tests were carried out in a similar way.
To evaluate the contribution of adsorption to the total degree of removal of heteroatomic substrates for thiophene DBT and pyridine, we also performed a series of experiments on the SA catalyst, similar to those described above, but without the addition of an oxidizing agent.

4. Conclusions

A series of effective catalysts for the oxidation of sulfur and nitrogen compounds were obtained using betaine and sulfuric or phosphomolybdic acids with different contents of the active phase on the support surface. Apart from the catalytic properties, the catalysts combine extractant and adsorbent functions. It was shown that an increase in the amount of heteropolyacid induces a non-linear variation of the acidity: the highest concentration of Brønsted acid sites is observed for a catalyst with a medium content of the heteropolyacid. The conversion of dibenzothiophene is correlated with the concentration of Brønsted acid sites on the surface. In the case of pyridine, the degree of removal is determined, first of all, by the content of the heteropolyacid in the catalyst because the substrate, which is basic, can block the acid-active sites. For other sulfur-containing substrates, both characteristics of the catalyst are significant.

Author Contributions

Conceptualization, I.T.; methodology, I.T.; software, A.G.P.; validation, I.T.; formal analysis, A.B., K.O. and A.G.P.; investigation, I.T. and V.G.; resources, V.G.; data curation, V.G.; writing—original draft preparation, V.G. and I.T.; writing—review and editing, V.G., I.T., A.B. and K.O.; visualization, V.G.; supervision, I.T.; project administration, I.T.; funding acquisition, I.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data is contained within the article.

Acknowledgments

This work was carried out within the framework of the Lomonosov Moscow State University State order (project no. AAAA-A21-121011590090) using equipment purchased with funds from the Lomonosov Moscow State University development program.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study.

Appendix A. XPS and SALDI Spectrums

Catalysts 13 00664 g0a1 550
Figure A1. Panoramic (a) and Si2p (b) XPS spectrum of the test sample.
Figure A1. Panoramic (a) and Si2p (b) XPS spectrum of the test sample.
Catalysts 13 00664 g0a1
Catalysts 13 00664 g0a2 550
Figure A2. C1s (a) and O1s (b) XPS spectrum of the test sample.
Figure A2. C1s (a) and O1s (b) XPS spectrum of the test sample.
Catalysts 13 00664 g0a2
Catalysts 13 00664 g0a3 550
Figure A3. P2p (a) and Mo3d (b) XPS spectrum of the test sample.
Figure A3. P2p (a) and Mo3d (b) XPS spectrum of the test sample.
Catalysts 13 00664 g0a3
Catalysts 13 00664 g0a4 550
Figure A4. Mo3p/N1s (a) and S2p (b) XPS spectrum of the test sample.
Figure A4. Mo3p/N1s (a) and S2p (b) XPS spectrum of the test sample.
Catalysts 13 00664 g0a4
Catalysts 13 00664 g0a5 550
Figure A5. SALDI mass spectra of the PMo1 catalyst—in the region of low (a) and high (b) masses in the negative ion detection protocol.
Figure A5. SALDI mass spectra of the PMo1 catalyst—in the region of low (a) and high (b) masses in the negative ion detection protocol.
Catalysts 13 00664 g0a5
Catalysts 13 00664 g0a6 550
Figure A6. SALDI mass spectra of the PMo3 catalyst in the negative ion detection mode.
Figure A6. SALDI mass spectra of the PMo3 catalyst in the negative ion detection mode.
Catalysts 13 00664 g0a6

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Catalysts 13 00664 g001 550
Figure 1. The composition of the obtained catalysts.
Figure 1. The composition of the obtained catalysts.
Catalysts 13 00664 g001
Catalysts 13 00664 g002 550
Figure 2. IR spectra (1)—H3PMo12O40; (2)—4-(3′-ethylimidazolium)-butanesulfonate; (3) PMoIL1 in the range (a) 500–1800 (b) 500–900 cm−1.
Figure 2. IR spectra (1)—H3PMo12O40; (2)—4-(3′-ethylimidazolium)-butanesulfonate; (3) PMoIL1 in the range (a) 500–1800 (b) 500–900 cm−1.
Catalysts 13 00664 g002
Catalysts 13 00664 g003 550
Figure 3. SEM micrograph and distribution of sulfur, phosphorus, and molybdenum on the surface of the PMo1 molybdenum catalyst.
Figure 3. SEM micrograph and distribution of sulfur, phosphorus, and molybdenum on the surface of the PMo1 molybdenum catalyst.
Catalysts 13 00664 g003
Catalysts 13 00664 g004 550
Figure 4. SEM micrograph and distribution of sulfur and nitrogen on the surface of the SA catalyst.
Figure 4. SEM micrograph and distribution of sulfur and nitrogen on the surface of the SA catalyst.
Catalysts 13 00664 g004
Catalysts 13 00664 g005 550
Figure 5. Curve sorption—desorption of nitrogen on the catalyst: (a) PMo1, (b) SA.
Figure 5. Curve sorption—desorption of nitrogen on the catalyst: (a) PMo1, (b) SA.
Catalysts 13 00664 g005
Catalysts 13 00664 g006 550
Figure 6. SALDI mass spectra of the PMo2 catalyst in the negative ion detection mode.
Figure 6. SALDI mass spectra of the PMo2 catalyst in the negative ion detection mode.
Catalysts 13 00664 g006
Catalysts 13 00664 g007 550
Figure 7. Thiophene removal versus time.
Figure 7. Thiophene removal versus time.
Catalysts 13 00664 g007
Catalysts 13 00664 g008 550
Figure 8. Processes occurring in the system sulfur-containing compound—H2O2—catalyst.
Figure 8. Processes occurring in the system sulfur-containing compound—H2O2—catalyst.
Catalysts 13 00664 g008
Catalysts 13 00664 g009 550
Figure 9. Methylphenyl sulfide (MPS) removal versus time.
Figure 9. Methylphenyl sulfide (MPS) removal versus time.
Catalysts 13 00664 g009
Catalysts 13 00664 g010 550
Figure 10. Dibenzothiophene (a) and pyridine (b) removal versus time.
Figure 10. Dibenzothiophene (a) and pyridine (b) removal versus time.
Catalysts 13 00664 g010
Catalysts 13 00664 g011 550
Figure 11. Influence of acidity and metal content in catalysts on substrates removal in 1 h (MFS 30 min).
Figure 11. Influence of acidity and metal content in catalysts on substrates removal in 1 h (MFS 30 min).
Catalysts 13 00664 g011
Table
Table 1. Distribution of elements on the PMo1 surface according to EDA data.
Table 1. Distribution of elements on the PMo1 surface according to EDA data.
Element(keV)Weight, %SigmaAtom, %K
C K0.27716.130.1823.233.06
N K0.3925.940.247.364.67
O K0.52552.000.3756.2243.59
Si K1.73919.490.1112.0038.16
P K *2.0100.040.020.020.07
S K2.3070.030.020.020.83
Mo L2.2936.370.101.1510.44
*. K the phosphorus and molybdenum lines partially overlap.
Table
Table 2. Distribution of elements on the SA surface according to EDA data.
Table 2. Distribution of elements on the SA surface according to EDA data.
Element(keV)Weight, %SigmaAtom, %K
C K0.27711.090.1823.233.06
N K0.3923.910.247.374.67
O K0.52557.560.3756.2243.59
Si K1.73926.090.1112.0038.16
S K2.3070.540.020.290.83
Table
Table 3. Elemental analysis results, textural characteristics, and catalyst acidity.
Table 3. Elemental analysis results, textural characteristics, and catalyst acidity.
CatalystSBET (m2/g)Dp (nm)Vp (m3/g)Mo, wt.%BAS *LAS *
SiO230010.00.75
PMo12468.60.609.05717
PMo22769.10.625.08813
PMo32919.80.662.4393
SA2979.10.72 830
*—the concentration (A mmol/g) of the Brønsted acid sites (BAS) was determined from the intensity of the adsorbed pyridine band at 1547 cm−1, and the Lewis acid site (LAS) concentration was found from the intensity of the 1450 cm−1 band.
Table
Table 4. Element concentrations, binding energies, and fractions of components in XPS spectra, and the corresponding types of bonds in the sample PMo2.
Table 4. Element concentrations, binding energies, and fractions of components in XPS spectra, and the corresponding types of bonds in the sample PMo2.
SpectrumContent Element, at. %Binding Energies, eVFraction, at.%Type of Chemical Bond
O1s69.01530.2
532.8		1.20
67.81		O-Met O-C, SiO2
N1s0.52401.50.52NR4+
P2p3/20.12133.90.12P5+
Si2p25.24103.625.24SiO2
Mo3d5/20.77232.0
233.2		0.42
0.35		Mo5+
Mo6+
C1s4.11285.0
286.2
287.3		1.89
1.53
0.54		C-C
C-O, C-N
N-C=N, C=O
288.80.15O-C=O
S2p3/20.23168.40.23S6+
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Gorbunov, V.; Buryak, A.; Oskolok, K.; Popov, A.G.; Tarkhanova, I. Supported Ionic Liquid Catalysts for the Oxidation of S- and N-Containing Compounds—The Effect of Bronsted Sites and Heteropolyacid Concentration. Catalysts 2023, 13, 664. https://doi.org/10.3390/catal13040664

AMA Style

Gorbunov V, Buryak A, Oskolok K, Popov AG, Tarkhanova I. Supported Ionic Liquid Catalysts for the Oxidation of S- and N-Containing Compounds—The Effect of Bronsted Sites and Heteropolyacid Concentration. Catalysts. 2023; 13(4):664. https://doi.org/10.3390/catal13040664

Chicago/Turabian Style

Gorbunov, Vladislav, Aleksey Buryak, Kirill Oskolok, Andrey G. Popov, and Irina Tarkhanova. 2023. "Supported Ionic Liquid Catalysts for the Oxidation of S- and N-Containing Compounds—The Effect of Bronsted Sites and Heteropolyacid Concentration" Catalysts 13, no. 4: 664. https://doi.org/10.3390/catal13040664

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