Influence of formation temperature on the morphology of MoS₂ and its catalytic properties in the hydrogenation of isomeric bromoquinolines to bromo-1,2,3,4-tetrahydroquinolines

Abstract

Eight samples of MoS₂ were prepared by the hydrothermal reaction of paramolybdates with thiourea where the synthesis temperature was varied from 120 to 180 °C. It was shown by XPS and Raman spectroscopy that the samples mainly consisted of MoS₂ but contained significant quantities of oxidized species. All samples had a flower-like morphology, as evidenced by TEM and SEM. The flower-like structure was built of nanosheets aggregated in conglomerates with sizes ranging from 50 nm to ca. 1 μm. The maxima of the quantity vs. size distribution curves of such conglomerates gradually shifted to higher values upon an increase in formation temperatures. All samples were catalytically active in the hydrogenation of quinoline; however, the highest yields of 1,2,3,4-tetrahydroquinoline were achieved for the MoS130–MoS145 samples. The hydrogenation of the isomeric bromo-substituted quinolines in the presence of MoS130–MoS140 was examined. In these cases, the respective bromo-1,2,3,4-tetrahydroquinolines were formed with high selectivity, except for 6-bromoquinoline. The results of the study may be applied to the development of selective catalysts for the hydrogenation of halogen-containing aromatic compounds.

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Introduction

Catalytic hydrogenation of functionalized aromatic organic compounds is one of the most common methods for obtaining substituted saturated carbo- and heterocycles. Such hydrogenation processes have been widely used to prepare new substances for pharmaceuticals, agrochemicals, and other valuable fine chemicals.^1–5 At present, those systems based on noble metals (primarily, Pd, Pt, and Ru)^6–10 and nickel^11–14 as hydrogenation catalysts are considered in most cases for these processes. Cobalt-based composites have also been attracting attention as promising candidates for hydrogenation catalysts due to their high performance and low prices, and such systems may have good potential for application.^15–18 The advantages of platinum group metals include high activity, but their high cost is the main obstacle to their wider application.¹⁹ In addition, there are some concerns regarding the toxicity of platinum group metals, though toxicity may be a general problem of many metal-containing catalysts used in organic synthesis.²⁰

A huge number of different classes of compounds can serve as hydrogenation catalysts. Besides the abovementioned metallic catalysts, many multimetallic systems,^21,22 metal oxides,^23–27 metal-free carbon materials,^28,29 nitrides, carbides, borides, and phosphides^30–34 and others materials¹⁵ have been proposed for such applications. The catalysts containing non-precious metals are significantly cheaper compared to those with Pd, Pt and other metals of the platinum group, but their activity and overall performance are also noticeably inferior. The search for new hydrogenation catalysts is an urgent task of modern physical and organic chemistry and catalysis.

Regarding selectivity, the hydrogenation of halogen-containing organic compounds on metallic catalysts almost always results in reductive hydrodehalogenation.^35–37 Such halogen-containing molecules bearing saturated carbo- or heterocyclic rings are important building blocks for the design of new biologically active compounds for MedChem applications, in particular the development of active pharmaceutical ingredients. Hydrogenation of respective halogen-containing aromatic precursors seems to be the simplest route to accessing such species, but it does present a challenging task. Nevertheless, several examples of hydrogenation catalysts, the use of which does not lead to hydrodehalogenation of organic compounds, are known^18,38,39 indicating that the hydrogenation of organic compounds can be performed in principle without cleavage of C–Br and even C–I bonds.

The systems not containing metallic particles could be promising catalysts for the hydrogenation of organic compounds while preserving the C–Hal bond, owing to their different energy profiles for substrate surface adsorption and activation compared to metallic catalysts, where such specific adsorption can promote activation of bonds other than C–Hal.⁴⁰ In particular, metal sulphides have been proposed as chemo-selective hydrogenation catalysts.^41–46 One of the simplest examples, Pd₄S, exhibits significant selectivity for the partial hydrogenation of alkynes to alkenes in hydrogenation processes compared to metallic palladium, which favours over-hydrogenation to form alkanes.^47,48 Selective hydrogenation of halonitrobenzenes to haloanilines without hydrodehalogenation has been reported, and platinum sulphide PtS has been established as the most versatile and efficient catalyst for such processes (among the considered sulphides).⁴⁹

Much attention has been devoted to layered sulphides, such as MoS₂ and WS₂,⁵⁰ and Ni- and Co-substituted WS₂ ^41,51 and Co-containing MoS₂.⁴² The catalysts based on MoS₂ have been widely used in industrial processes involving hydrogen, such as hydrodesulfurization.^52,53 A highly efficient low-temperature hydrogenation of CO₂ to methanol in the presence of MoS₂ nanosheets has been reported; the efficiency of this material was attributed to the dissociation of CO₂ at sulfur vacancies.⁵⁴ A method for the hydrogenation of anthracene using a MoS₂ catalyst was proposed.⁵⁵ Quantitative yields of the hydrogenation products were achieved in this process. Mixed molybdenum and cobalt sulphides, Co–Mo–S-X, with different molar ratios of Mo/Co (X = Co/(Mo + Co)), were used for the hydrogenation of quinoline and its derivatives.⁴² Hydrogenation of a large number of functionalized quinolines was performed using the most efficient catalyst from this series, Co–Mo–S-0.83: under a hydrogen pressure of 12 bar and a temperature of 150 °C, the yields of the hydrogenation products ranged between 50 and 99%. The hydrogenation of a series of halogen-containing quinolines, such as 6-fluoro-quinoline, 6-fluoro-2-methylquinoline, 7-fluoro-2-methylquinoline, 8-chloro-2-methylquinoline, and 7-bromo-quinoline, was considered in this work. In addition, the catalytic hydrogenation of 3-nitrostyrene over a series of previously mentioned mixed Co–Mo–S-X sulphides was described.⁵⁶ Hydrogenation of a wide range of derivatives of nitroarenes was studied in the presence of the most efficient catalyst in this series, Co–Mo–S-0.39. The yields of the hydrogenation products were from 16% to quantitative. The performance of such catalysts was very sensitive to the molar ratios of the components.

Generally, the use of MoS₂ as a hydrogenation catalyst in fine organic chemistry is limited because of its relatively low activity and, consequently, harsh reaction conditions required for the process. For example, in our research, bulk commercial MoS₂ was not active in the hydrogenation of quinoline at all. Despite several studies devoted to hydrogenation catalysts based on MoS₂, information on the influence of MoS₂ morphology and particle size on its catalytic properties is scarce. Thus, it is anticipated that a thorough study of the influence of preparation temperature on the morphology and catalytic properties of MoS₂ in hydrogenation processes might allow compounds demonstrating high catalytic performance to be discovered. In addition, such a study could provide information on the influence of preparation temperature on the morphology of MoS₂, which could be interesting for its application in other fields of chemistry and materials science.

The aim of this study was to reveal the influence of synthetic conditions on the fine chemical structure and morphology of MoS₂ and its catalytic properties in the hydrogenation of quinoline and bromo-substituted quinolines. Quinoline and its derivatives were chosen as the objects of this study for several reasons. First, the hydrogenation of quinoline is a suitable reaction to estimate the performance of the catalyst because this process requires a catalyst whose activity exceeds that of average ones.^15,57–60 Second, the tetrahydroquinoline fragment can be found in various natural biologically active compounds^61–63 and active components of synthetic anticancer, antimalarial, antimicrobial, anti-mycobacterial, anti-inflammatory, anti-asthmatic, and other drugs,^64–68 and the development of new methods for the hydrogenation of quinoline and its derivatives is an important task. Hydrogenation of bromo-substituted quinolines can be a convenient model process for testing the selectivity of the catalyst towards reductive hydrodehalogenation.

In this paper, we report the results of the study of a series of MoS₂ samples, prepared by the hydrothermal method with a systematic variation of the reaction temperature. The samples are referred to as MoSX, where X is the temperature of the hydrothermal synthesis (for example, MoS120 means MoS₂ synthesized at 120 °C). The samples were characterized by powder XRD, transmission and scanning electron microscopy, and Raman spectroscopy. The catalytic performance of the MoS₂ samples in the hydrogenation of quinoline was studied; the most active samples were tested in the hydrogenation of bromo- and chloro-substituted quinolines.

Results and discussion

The hydrothermal synthesis of MoS₂ was carried out in water solution containing ammonium paramolybdate and thiourea, similar to a previously reported procedure.⁶⁹ The process can be formally represented by a set of reactions (1)–(4). In this scheme, thiourea acts as a source of H₂S, which, in turn, is consumed for the reduction of Mo(VI) to Mo(IV) and for the formation of sulphide. In eqn (3), sulphuric acid (S(VI)) is written as a product; however, SO₂ or S can also form. The overall pH of the reaction mixture after synthesis is above 7 (typically, 8–9) due to the formation of excess NH₃. The evolution of H₂S and NH₃ takes place, as evidenced by the specific odour of the reaction mixture.

CS(NH₂)₂ + 2H₂O → 2NH₃ + CO₂ + H₂S (1)

(NH₄)₆Mo₇O₂₄·4H₂O → 6NH₃ + 7MoO₃ + 7H₂O (2)

4MoO₃ + H₂S → 4MoO₂ + H₂SO₄ (3)

MoO₂ + 2H₂S → MoS₂ + 2H₂O (4)

According to the proposed reaction scheme, 15.75 moles of thiourea are needed for 1 mole of paramolybdate (Mo₇O₂₄⁶⁻). If one assumes that H₂S is oxidized to SO₂ or S, the required quantity of thiourea should be higher, i.e., 17.5 or 21 moles per one mole of paramolybdate, respectively. In the experiment, we took 30 moles of thiourea per one mole of paramolybdate, which is an excessive quantity in any case. The same ratio was used in the previously reported method.⁶⁹ Excess sulphur can suppress the formation of Mo oxides, but despite this precaution, the samples of MoS₂ contained a significant quantity of oxygen, as found by XPS (vide infra). Upon increasing the synthesis temperature from 120 to 180 °C, the yields of products grew from 37 to 75% (counting for formula MoS₂), respectively (Table S1, ESI†).

Photographic images of the dry reaction products after hydrothermal synthesis, purified by rinsing with water and ethanol, are shown in Fig. 1. Sample MoS120 was orange, while MoS130 had a deep dark-brown colour, while all samples obtained at higher temperatures were black, which is typical of MoS₂.⁷⁰ The samples prepared at 135–180 °C exhibited Mo : S signal ratios in EDX ranging randomly from 1 : 1.6 to 1 : 2.4 (average over 10 measurements), reflecting the elemental composition and consistent with the MoS₂ stoichiometry within the accuracy of measurement. Sample MoS120 had an Mo : S ratio of 1 : 1.6, which was slightly higher compared to the ratios of the remaining materials in this series. The ICP analysis of MoS130 and MoS140 samples revealed that the total content of Mo + S was 68.1 and 77.2%, respectively, at an Mo : S ratio of 1.84 in both cases, indicating significant amounts of other components. It can be concluded that both samples contained Mo oxides in addition to MoS₂. The presence of a significant quantity of these additional components is consistent with the capacity of layered MoS₂ to intercalate small ions^71,72 and even organic compounds.⁷³ Assuming that the samples contained MoO₃ as Mo oxide and water as the third component, the compositions of the samples can be represented by the formulas MoS₂·0.087MoO₃·4.29H₂O for MoS130 and MoS₂·0.087MoO₃·2.70H₂O for MoS140.

Fig. 1 Optical photographs of the dry MoS120–MoS180 samples.

In the case of MoS120, the solid precipitate was orange, the reaction mixture was brown, and the precipitate yield was much lower than that of the treatments at all other temperatures (Fig. 1). This color is not typical of MoS₂, although the sample does contain a significant quantity of S according to EDX analysis. A significant quantity of Mo-containing species seems to remain in the solution as finely dispersed suspended particles; their morphology was examined by SEM, vide infra (ESI, Fig. S1†). The yields of the products prepared at higher temperatures were close to the theoretical values expected for MoS₂.

The phase composition of the obtained sulphides was analysed by powder XRD. There were no distinct narrow reflections on the powder XRD patterns of all freshly prepared samples (Fig. 2); thus the crystallinity of these samples was poor, or the sizes of crystallites were small. For example, MoS₂ samples containing particles of size more than 100 nm had distinct narrow reflections on their XRD patterns.^74–76 It can be concluded that the sizes of the crystallites in samples reported herein fall into the nanoparticle range and/or MoS₂ layers are stacked with a high level of disorder.

Fig. 2 XRD patterns of the freshly prepared MoS120–MoS180 samples.

Among the fresh samples prepared, MoS180 had the most distinct reflections, although their half-widths exceeded 1° (in the θ scale). These reflections could be assigned to diffraction from the 002 plane (centred at 2θ = 13.5°, theor. 2θ = 14.4°, calculated for MoS₂ in the P6₃/mmc space group⁷⁷), 100 and 101 planes (wide reflection centred at 2θ = 32.4°, theor. 2θ = 32.8 for 100 and 33.6 for 101), 103 plane (centred at 2θ = 35.5°, theor. 2θ = 39.7°), and the 2–10 plane (centred at 2θ = 57.1°, theor. 2θ = 58.6°). The reflection at 2θ = 13.5° disappeared in the case of samples synthesized at lower temperatures, but a similar wide reflection in this area was found again in samples MoS130 and MoS120 (Fig. 2). The absence of a 002 peak may be a sign of the formation of structures containing small quantities of MoS₂ layers.⁷⁸ This can indicate that samples MoS130–MoS150 have a relatively small quantity of layers and may have larger lattice spacing (notably, this feature may favor the formation of more catalytically active sites). In contrast, MoS120, MoS130, and MoS180 exhibited a significant (002) peak, which can indicate the presence of a relatively thick layered structure. Such significant differences in the structure may be reflected in catalytic properties, which will be discussed later. In addition, in the case of MoS120, there was a wide reflection centered at 2θ = 25.5°, which could be assigned to the α-MoO₂ phase.^79,80

It was found that the powder XRD patterns significantly changed with time: while freshly prepared samples did not contain distinct reflections on their XRD patterns (Fig. 2), after 8–10 months of storage, the X-ray patterns contained narrow signals of phases that did not correspond to MoS₂ (ESI, Fig. S2†). These results were reproducible, i.e., freshly prepared samples obtained in several syntheses contained only broad reflections, which became narrow upon sample storage. These structural changes might be related to MoS₂ oxidation. Degradation of catalytic properties was also observed; for example, the yield of the hydrogenation product was ten times lower when quinoline was hydrogenated under the same conditions with “old” MoS130–MoS150 samples.

Some information on the phase composition of the chalcogenide structures, such as MoS₂, can be obtained from the data of Raman spectroscopy. Multiple peaks at 148, 194, 212, 236, 285, 337, 376, and 404 cm⁻¹ were observed in the Raman spectra of MoS140 and MoS160 (Fig. 3).

Fig. 3 Raman scattering spectra of MoS160 (1) and MoS140 (2) in the range of 150–450 cm⁻¹ (a) and 100–1100 cm⁻¹ (b). Spectra were measured at room temperature, with laser power of 1 mW, and laser excitation at 532 nm.

Two characteristic modes of the hexagonal (2H) MoS₂ were determined at 376 cm⁻¹ (E¹_2g, in-plane vibrations) and 404 cm⁻¹ (A_1g, out-of-plane vibrations^81–84). Simultaneously, the peaks at 148 cm⁻¹ (J₁), 194 cm⁻¹ (J₂), and 337 cm⁻¹ (J₃) in the spectrum correspond to phonon modes characteristic of the octahedral (1T) MoS₂.^82,85–87 Specifically, J1 reflects the presence of Mo–Mo interactions both within and outside the lattice region; J2 is associated with Mo–S atoms displaced in and out of the layers; and J3 corresponds to the out-of-plane vibrations of Mo–Mo atoms within the lattice (Fig. 3a). The emergence of the phonon mode E_1g (280 cm⁻¹) could be associated with the stretching of the Mo–S bond in the crystal lattice. The ratio of the 2H and 1T phases might be evaluated from the ratio of the integral intensities of the A_1g and J₂ modes.^88–96 This assessment suggests that, within the accuracy of evaluation, both MoS140 and MoS160 contain about one half of the 2H phase.

The bands at 148, 194, 236, 285, and 337 cm⁻¹ in the Raman pattern were attributed to different forms of MoO_x, while the peaks at 212 and 236 cm⁻¹, as well as the intense bands at 821 and 994 cm⁻¹, accentuated the significant presence of MoO₃ in sufficient quantities.^97–100 It was suggested that the low intensity and large width of the E¹_2g peak could indicate the presence of significant defect areas in the crystal structure of MoS₂.¹⁰¹ Comparing the spectra, it is clear that the samples have different ratios between the intensities of the E¹_2g and A¹_g peaks. The sample synthesized at a higher temperature has an E¹_2g peak with slightly higher intensity compared to that of the A_1g peak, indicating that the synthesized MoS₂ samples have more intense in-plane vibrations, implying that the edges of the MoS₂ nanoflowers are “highly exposed”.^84,102 Additionally, the relatively low ratios of the E¹_2g to A_1g peaks indicate the small size of the crystallites, which is consistent with the observations of SEM and TEM.

X-ray photoelectron spectroscopy (XPS) was used for assessing the chemical composition of the MoS₂ samples. The anticipated components are revealed in the survey spectra shown in Fig. 4: oxygen, sulfur, molybdenum, and a trace quantity of carbon. The spectra were analysed using the Voigt function (70 : 30 ratio of the Gaussian to Lorentzian profile) with the help of CasaXPS 2.3.15 software. The width at half-height of the components did not surpass physically possible values for these elements, and the number of components was the smallest that could be used to define the spectrum.

Fig. 4 Survey XPS spectra of MoS140 (a) and MoS160 (b).

In addition to the detection of oxygen in the samples, the Mo 3d and S 2p regions in the XPS analysis may be useful for estimating the contributions of the 1T and 2H phases in MoS₂. The spectra of the Mo 3d ground state levels for the MoS140 (Fig. 5a) and MoS160 (Fig. 5c) samples contained peaks with maxima at ∼228.2 eV and ∼231.6 eV corresponding to the Mo(IV) 3d_5/2 and Mo(IV) 3d_3/2 components, respectively. The presence of the 1T phase^{81,86,103,104} was indicated by the lower BE values of the peaks corresponding to the Mo(IV) 3d_3/2 and Mo(IV) 3d_5/2 components for both samples compared to the corresponding peaks for (2H) MoS₂. Therefore, each of the two Mo 3d peaks could be fitted using two components associated with the 1T and 2H phases, indicating that these two polymorphs were present in comparable quantities.^86,88

Fig. 5 Mo 3d (a and c) and S 2p (b and d) XPS spectra and their deconvolutions for MoS140 (a and b) and MoS160 (c and d).

For the sample synthesized at 160 °C, the Mo(IV) 3d_5/2 peak could be represented by two components at 228.4 eV (1T phase) and 230.2 eV (2H phase), while for the Mo(IV) 3d_3/2 peak, two fitted components had values of 231.6 eV (1T phase) and 233.3 eV (2H phase) (Fig. 5c). For the sample synthesized at 140 °C, the Mo(IV) 3d_5/2 component was represented by two peaks at 228.3 eV (1T phase) and 230.2 eV (2H phase), and the Mo(IV) 3d_3/2 component was represented by peaks at 231.4 eV (1T phase) and 233.2 eV (2H phase) (Fig. 5a). The peak localized at 226.2 eV (225.7 eV for the MoS140) corresponded to the S 2s component. In the Mo 3d region, the presence of oxides in the form of MoO₃ (Mo–O bond) was detected, with the position of Mo(VI) 3d_5/2 at an energy of 231.2 eV for MoS160 and 232.3 eV for the other sample.¹⁰⁵ The contribution of the component corresponding to MoO₃ for the MoS160 sample was smaller than for the other sample, indicating a smaller amount of molybdenum trioxide in the sample. These data correlate with the results of Raman spectra analysis (Fig. 3).

The XPS spectra in the S 2p region are shown in Fig. 5b and d. This doublet (S 2p_1/2 and S 2p_3/2) could be assigned to “edge-sulphur”.¹⁰⁵ This doublet also exhibited a lower binding energy (BE) compared to classical (2H) MoS₂.^81,86,88,104 Therefore, each component of the doublet was fitted with two peaks (Fig. 5b and d). The S 2p_1/2 component was represented by two peaks: 161.7 eV (1T phase) and 164.6 eV (2H phase) for the MoS160 sample (Fig. 5d); and 161.6 eV (1T phase) and 164.2 eV (2H phase) for the MoS140 sample (Fig. 5b). The S 2p_3/2 component has the following BE values: 160.6 eV (1T phase) and 163.2 eV (2H phase) for the sample synthesized at 160 °C; 160.4 eV (1T phase) and 163.0 eV (2H phase) for the MoS140 sample. The decrease in the binding energies of Mo and S in (1T) MoS₂ compared to those of (2H) MoS₂ could be due to an increase in the electron densities of Mo and S atoms, respectively.⁸¹ The peak at 168.8 eV (or 169.2 eV for the lower temperature sample) was identified as the S 2p peak of oxidized sulfur (S(VI)) produced from the S²⁻ in the samples by oxidation.¹⁰⁶ The ratio of intensities of the S(VI) 2p peaks for different samples indicated that the sample synthesized at a higher temperature contained a lower quantity of oxidized species than MoS140, which is consistent with the data described above.

Freshly prepared MoS₂ samples were studied by SEM (Fig. 6 and 7). According to SEM measurements, the samples could be divided into two groups: MoS120 and MoS130 (low-temperature group, LT, Fig. 6) and MoS135–MoS180 (high-temperature group, HT, Fig. 7). Samples of the LT group contained separate spherical particles, which were not assembled into “nanoflowers”, while samples of the HT group contained assemblies resembling “nanoflowers” (though some tendency to form conglomerates was initially noticeable in the Mo130 sample).

Fig. 6 SEM images of the MoS120 (a) and MoS130 (b) samples along with particle quantity vs. particle size distribution curves.

Fig. 7 SEM images and particle size distribution curves of MoS135 (a), MoS140 (b), MoS145 (c), MoS150 (d), MoS160 (e) and MoS180 (f).

Samples of the LT group consisted of particles, for which the particle content vs. particle size curve was centred at 50–100 nm, and no fine structure could be found by SEM. In turn, samples of the HT group could be considered as hierarchically ordered structures, formed by 2D sheets of MoS₂, assembled into spherical conglomerates (hereinafter, the size of the conglomerate is estimated as the diameter of the smallest sphere in which the conglomerate can be placed). Visually, such conglomerates in the HT group resembled “nanoflowers”. This morphology is typical of MoS₂ prepared by the hydrothermal method at a similar temperature.^78,107 Notably, the MoS130 sample contained some admixture of hierarchically ordered “nanoflowers”. It could be concluded that the abovementioned hierarchically ordered structures began to form (in terms of the temperature scale) upon treatment of the reaction mixture at ca. 135 °C, and such formation could be associated with the crystallization of MoS₂, in contrast to the deposition of the amorphous phase.

It is essential to note that the size distribution diagrams for the HT group samples were plotted for the size of the conglomerates because it was difficult to distinguish separate particles (in contrast to LT samples). The sizes of these conglomerates in the MoS135–MoS150 samples ranged from 50 to 650 nm. With an increase in the formation temperature, the maxima on the quantity vs. size distribution graphs gradually shifted to larger values. Two samples, synthesized at the highest temperatures (MoS160 and MoS180), were characterized by a bimodal distribution of conglomerates. In the case of MoS160, two maxima were observed on the distribution diagram at ca. 500 nm and ca. 1600 nm, while in the case of MoS180 the first maximum was located at approximately 500 nm, but the second maximum was not well defined, possibly due to limited statistical data. The width of the first maxima for MoS160 and MoS180 was significantly larger compared to similar peaks on the distribution curves for MoS135–MoS150. In the case of MoS180, in addition to spherical conglomerates, flat sheets were found; these formations had a layered structure.

It can be concluded that the morphology of the obtained samples was affected by the temperature of the hydrothermal synthesis, which could be explained by different rates of the seeding and crystallization processes.

The layered structure of MoS₂ was observed on TEM images (Fig. 8). On the TEM images of MoS130 (Fig. 8a–c), separate 2D layers lying one over another could be distinguished by variations in darkness, indicating different thicknesses of the sample, and the presence of separate layers was additionally confirmed by layer ruptures (edges of broken layers of white color, Fig. 8b). Besides, a relatively large quantity of small particles of 10–20 nm size was detected in the case of MoS130 (Fig. 8c). Similar small particles were observed on the TEM images of the MoS120 sample (Fig. S3†). In contrast, the TEM images of the MoS135–MoS180 samples revealed flat particles assembled into rounded conglomerates (Fig. S3†). The 2D layers were much more distinct on the TEM images of MoS140. These layers resembled crumpled paper sheets and were assembled into flower-like conglomerates (Fig. 8d and e). Separate layers with an interlayer distance of ca. 0.8 nm (6–7 layers per 5 nm) were revealed on the TEM images of MoS140 with a large magnification (Fig. 8f). This interlayer distance is larger than expected for pure MoS₂ but consistent with that previously reported for MoS₂ intercalated with guest molecules.^108,109 Notably, such a 2D layered structure was not observed in the case MoS130. The conglomerates constituted the dominant phase in the case of samples MoS140–MoS180, and their diameter increased with an increase of the reaction temperature (Fig. S3†). These results were consistent with the observations derived from the analysis of the SEM images. The general conclusion is that the LT samples contained amorphous particles, among which 2D species could not be distinguished, while the HT samples consisted of 2D particles of crystalline MoS₂.

Fig. 8 TEM images of MoS130 (a–c) and MoS140 (d–f) samples.

Flower-like conglomerates of MoS₂ have been reported previously, and such a type of aggregation is not a unique feature for the compound reported herein.^108,109 It was shown that pH strongly affected the morphology of MoS₂. The morphology of MoS₂ obtained in this work is close to the one expected for pH = 7, as found in a previous report.¹⁰⁷ The formation of the nanoflowers could be explained by the growth of the MoS₂ sheets on the growth centre (“seed”) belonging to the other sheet.

Hydrogenation of quinoline was used as a reference reaction for studies of the influence of temperature on the performance of the catalysts. The yields of 1,2,3,4-tetrahydroquinoline (THQ) in 24 hours ranged from 42% to 6% with MoS₂ samples (10 mol%) at T = 100 °C and p(H₂) = 100 atm in methanol (the reaction mixtures were analyzed using NMR and gas chromatography). The yields of 1,2,3,4-tetrahydroquinoline are shown in Fig. 9 and the spectra of the reaction mixtures after evaporation of solvent are shown in Fig. S4.† In all cases, no by-products were detected; all reaction mixtures contained the respective quantity of unchanged starting material (thus the conversion of quinoline was equal to the yield of THQ).

Fig. 9 The yields of 1,2,3,4-tetrahydroquinoline in the reactions of quinoline hydrogenation catalyzed by MoS120–MoS180 samples.

Hydrogenation of quinoline under the same conditions using commercial MoS₂ as fine powder led to a 10% yield of 1,2,3,4-tetrahydroquinoline (Fig. S5†), which is lower than the yields in the cases of MoS130–MoS180, and the difference is especially pronounced compared to MoS130–MoS145 samples. In accordance with Chianelli's rim-edge model, the active sites for the catalytic hydrogenation reactions of MoS₂ materials are the rim sites at the edges of external layers with adjacent basal planes, that is, the upper and bottom edges.¹¹⁰ The highest yields of 1,2,3,4-tetrahydroquinoline were observed for MoS130–MoS140, which could be explained by the optimal combination of the arrangement of sides bearing active sites and their accessibility.

The effect of reaction conditions (catalyst content, temperature, or pressure) on the result of quinoline hydrogenation was evaluated for the MoS140 sample. The conditions of 100 °C, 100 atm, and methanol as the solvent were chosen as a reference for correct comparison with Co- and Ni-containing catalysts.^11,16 Under these conditions, the yield of 1,2,3,4-tetrahydroquinoline was approximately proportional to the catalyst loading: it was 20% in the case of 5 mol% MoS₂, and increased to 42 and 58% in the case of 10 and 15 mol% MoS₂, respectively (Fig. S6†). Further experiments were carried out for 10 mol% MoS₂ content. Temperature lowering to 80 °C resulted in a decrease of the product yield to 13% (1,2,3,4-tetrahydroquinoline was the only product here and in all cases), while further temperature lowering to 60 °C led to almost zero conversion (product yield < 1%; Fig. S7†). Curiously, it appeared that the hydrogen pressure in the range from 60 to 100 atm had no effect on the product yield: it was the same within experimental error (42 ± 2%), implying that hydrogen concentration on the catalyst's surface is not the factor limiting the reaction over this range of pressures (Fig. S8†). The use of water or toluene as a solvent instead of methanol led to a decrease in the reaction yield to 20% and 16%, respectively (compared to 42% in methanol; Fig. S9†). Thus, temperature and solvent can be identified as the most important parameters for the reaction.

In attempting to repeat the use of sample MoS140 in the hydrogenation of quinoline (catalyst recycling test), the product yield fell from 42 to 9% after the first run (10% MoS₂ loading; Fig. S10†). It can be concluded that MoS₂ lost its activity after its first use. This effect is similar to the abovementioned significant decrease of catalytic activity of these sulphides after long storage. The loss of activity after the first catalytic run being caused by quick oxidation of the sample after its isolation from the reaction mixture, filtering and drying in air cannot be excluded. Besides, in this experiment, approximately 2–3% of the MoS140 catalyst is dissolved in the reaction mixture during the hydrogenation of quinoline (catalyst leaching). The presence of Mo in solution was confirmed by ICP analysis.

There have been numerous reports on quinoline hydrogenation with high conversion rates in the presence of nanoparticles of palladium,^111,112 ruthenium,^113,114 cobalt,^16,17 nickel,^11,115 and other metals.¹¹⁶ However, information on the hydrogenation of halogenated quinolines is scarce, and preservation of the halogen atom in the heterocycle is quite a difficult task to accomplish. In our study, hydrogenation of chloroquinolines in the presence MoS135 and MoS140 samples and bromoquinolines in the presence of MoS130, MoS135 and MoS140 samples was carried out at T = 100 °C and p(H₂) = 100 atm in methanol (10 mol% of MoS₂ in all cases). Compared to the abovementioned experiments with quinoline, the reaction time was increased to 48 h. While the yields of tetrahydroquinoline formed upon the hydrogenation of quinoline in the presence of MoS130, MoS135, and MoS140 were very similar, the yields of the corresponding chloro- and bromo-1,2,3,4-tetrahydroquinolines formed upon the hydrogenation of the respective chloro- and bromoquinolines varied drastically, even for the same isomer (different catalysts) and the same catalyst (different isomers, Fig. 10 and S11–S18, Tables S2 and S3 in the ESI†). The yields of chloro-1,2,3,4-tetrahydroquinolines were in a range from 51 to 90%; however, selectivity was 100% in all cases (the reaction mixtures contained only the target product and the starting compound). The hydrogenation of three isomeric 5-, 7-, or 8-bromoquinolines in the presence of MoS130 led to the respective bromo-1,2,3,4-tetrahydroquinolines with the highest yields (above 94%). In the case of 6-bromoquinoline, the highest yield of 6-bromo-1,2,3,4-tetrahydroquinoline was only 25%, and it was achieved in the case of MoS135. The most active catalyst for the hydrogenation of quinoline—MoS140—showed the lowest yields in the case of the hydrogenation of its bromo-substituted derivatives but the highest selectivity. Hydrogenation of 6-bromoquinoline in the presence of MoS130 and MoS135 led to hydrodehalogenation and the formation of tetrahydroquinoline (44% and 10%, respectively), resulting in lower selectivity.

Fig. 10 Diagrams showing the selectivity of the hydrogenation reaction and the yields of bromo-1,2,3,4-tetrahydroquinolines in the presence of MoS130, MoS135, and MoS140. The yields are determined using Y = n(bromo-1,2,3,4-tetrahydroquinoline)/n(bromoquinoline) × 100%.

Hydrogenation of bromoquinolines in the presence of MoS₂ without C–Br bond cleavage, in contrast to debromination typically occurring in the case of metallic catalysts, can be explained by the different quinoline molecule adsorption process at the catalyst surface.¹¹⁷ For example, by promoting the coordination of 6-chloroquinoline on the catalyst surface through a nitrogen atom, hydrogenation of 6-chloroquinoline on a series of Au_1−xPd_x bimetallic catalysts occurred with a 92% yield of 6-chloro-1,2,3,4-tetrahydroquinoline without dechlorination.¹¹⁸ The orientation of the halogenated quinoline on the surface of the catalyst can be a factor that controls the reaction route; a large distance from the catalyst surface means the C–Hal bond can prevent the hydrodehalogenation.

Notably, in the case of the hydrogenation of 6-bromo-quinoline in the presence of MoS130–MoS140, the yield of 6-bromo-1,2,3,4-tetrahydroquinoline was the lowest (Fig. 9). It can be speculated that this isomer of bromoquinoline was mainly coordinated on the surface of MoS₂ in a flat plane at the active centers of the S-edges.¹¹⁸ To explain the difference in the yields of hydrogenation products for different isomers of bromoquinoline, it can be noted that the limiting stage of hydrogenation may be related to the energies of substrate/product adsorption and desorption on the catalyst surface. Such energies are definitely different for the bromoquinoline isomers due to the electronic effects of the substituent.¹¹⁹

Experimental

The solvents were purified according to standard procedures.¹²⁰ Hydrogen (99.99%) was purchased from Galogas Ltd (Ukraine) and was used without further purification. Ammonium paramolybdate tetrahydrate (99.0%) was obtained from Klebrig Ltd (Ukraine). All other starting materials and reagents were available from Enamine Ltd (Ukraine) and UkrOrgSintez Ltd (Ukraine).

ICP analysis was carried out using an optical emission spectrometer with inductively coupled plasma (Thermo Fisher Scientific 7600 Duo), equipped with an autosampler (Teledyne Cetac ASX-560). TEM was performed using a Philips CM-20 Super Twin instrument operating at 160 kV and a TEM-125K transmission electron microscope (Selmi LLC, Ukraine) operating at an acceleration voltage of 100 kV. Samples were suspended in methanol upon ultrasonic irradiation for 1 min, and a drop of the suspension was applied onto a Cu grid (300 mesh), covered by a film of amorphous carbon, immediately after the end of the ultrasonic treatment. SEM measurements were carried out using a scanning electron microscope (JEOL JSM 6060LA) working at an acceleration voltage of 30 kV. The samples were not specially prepared (in particular, they were not coated with conducting material) for SEM measurements. Powder X-ray diffraction patterns were recorded using a Bruker D8 Advance diffractometer with CuK_α radiation (λ = 1.54056 Å). Raman spectra of MoS₂ samples were recorded at room temperature in backscattering geometry using a Horiba Jobin Yvon T64000 Raman spectrometer equipped with an Olympus BX41 microscope and a Peltier-cooled Si charge-coupled device (CCD) detector. Light emitted from a Spectra-Physics EXLSR-532-150-CDRH solid-state laser of 532 nm wavelength was used for excitation. The studied powders were placed onto a microscope slide and compacted slightly with a spatula. Laser light was focused on the sample surface using a 50× NA0.75 objective to a spot diameter of ∼1 μm. The laser power at the sample surface was kept below 1 mW to avoid laser heating or oxidation.

¹H NMR spectra were recorded on a Varian Unity Plus 400 spectrometer at 400 MHz. Mass spectra were recorded on an Agilent 1100 LCMSD SL instrument (chemical ionization (CI)) and an Agilent 5890 Series II 5972 GCMS instrument (electron impact ionization (EI)). High-resolution mass spectra (HRMS) were recorded on an Agilent Infinity 1260 UHPLC system coupled to a 6224 Accurate Mass TOF LC/MS system. Hydrogenation experiments were carried out in a steel autoclave equipped with a manometer, magnetic stirrer, and temperature controller as reported previously.^11,16

Samples were stored in closed vials at room temperature, and no special precautions against oxidation were taken. All measurements and catalytic tests were carried out during one week after the synthesis of the samples, unless explicitly indicated otherwise.

Synthesis of MoS₂

All procedures were carried out in air. Briefly, 1.23 g (0.001 mol) of (NH₄)₆Mo₇O₂₄·4H₂O and 2.285 g (0.03 mol) of thiourea were dissolved in 35 mL of distilled water with stirring for 30 min at 40 °C. The solution was placed in a Teflon reaction beaker in a steel autoclave with a capacity of 100 mL and kept in a thermostat unit for 20 hours at a certain temperature (120 °C for MoS120, 135 °C for MoS135, etc.). After this time, the autoclave was cooled down to room temperature, and the solid phase was separated from the reaction mixture by centrifugation. The resulting black product was washed with distilled water (3 × 20 mL), then with absolute ethanol (20 mL), and dried at 65 °C for 10–12 h. The compounds were stored in air without special precautions. The yields were up to 0.75 g (Table S1†). Anal. MoS130, found Mo 42.2%, S 25.9%. Calcd for MoS₂·0.087MoO₃·4.29H₂O, Mo 42.2%, S 25.0%. Anal. MoS140, found Mo 47.8%, S 29.4%. Calcd for MoS₂·0.087MoO₃·2.70H₂O, Mo 47.8%, S 29.4%.

Catalytic hydrogenation of quinoline

16 mg (0.1 mmol) of the MoS₂ catalyst, 129 mg (1 mmol) of quinoline, and 10 mL of methanol were placed in a 100 mL Teflon beaker, which was placed in a steel autoclave, mounted on a stirrer with a heating thermostat unit. The autoclave was purged with hydrogen three times, then pressurized with H₂ to 100 atm, and the reaction was carried out at 100 °C for 24 hours. During heating and the further course of the reaction, the pressure was adjusted to a constant value of 100 atm. Then the autoclave was cooled down to room temperature, and the reaction mixture was filtered through a glass filter. The solvents were distilled off from the reaction mixture under reduced pressure on a rotary evaporator, and the resulting non-volatile mixture was analyzed by ¹H NMR spectroscopy and gas chromatography.

Catalytic hydrogenation of bromo- and chloro-substituted quinolines was carried out by following the same procedure, as reported above for hydrogenation of quinoline, except that the reaction time was increased to 48 h.

Conclusions

In this study it was shown that reaction of paramolybdates with thiourea under hydrothermal conditions led to the formation of nanostructured MoS₂, which had the morphology of “nanoflowers” – the structures built of nanosheets, aggregated into conglomerates of size from 50 nm to ca. 1 μm. The maxima of quantity vs. size distribution curves of such conglomerates gradually shifted to higher values upon an increase of the formation temperature. Though the composition of samples was consistent with a Mo : S ratio of 1 : 2, the samples contained a significant quantity of oxygen, which could occur due to the surface oxidation of the sulphide or could be trapped during its formation due to an incomplete reduction of Mo(VI).

The catalytic performance of MoS₂ in the hydrogenation of quinoline strongly depends on the temperature of the catalyst's synthesis. The samples prepared at 130–145 °C ensured the formation of 1,2,3,4-tetrahydroquinoline with the highest yields compared to the catalyst materials prepared at higher temperatures. The prepared MoS₂ appeared to be an efficient catalyst for the hydrogenation of bromoquinolines, containing Br in the carbocycle, and the reaction progressed without C–Br bond cleavage.

Samples of MoS₂ reported in this paper can be considered promising hydrogenation catalysts for the replacement of palladium and platinum due to their lower cost. However, the application of MoS₂ for selective hydrogenation of bromoquinolines leading to the respective bromo-substituted 1,2,3,4-tetrahydroqunolines seems to be a more attractive option for fine organic synthesis and medicinal chemistry.

The results of this study may be interesting for the development of various functional materials based on 2D sulphides, as well as for the creation of selective catalysts for the hydrogenation of organic compounds.

Author contributions

Anastasia V. Terebilenko: investigation, writing – original draft, and writing – review and editing; Maryna V. Olenchuk: investigation, formal analysis, and writing – original draft; Denys O. Mazur: investigation; Andrii S. Nikolenko: investigation; Vadym I. Popenko: investigation; Galyna I. Dovbeshko: investigation; Oleksii Bezkrovnyi: investigation; Tomash Sabov: investigation; Boris M. Romanyuk: formal analysis and validation; Volodymir N. Poroshin: validation; Serhiy V. Ryabukhin: methodology; Dmytro M. Volochnyuk: methodology and writing – review and editing; Sergey V. Kolotilov: conceptualization, methodology, writing – original draft, and writing – review and editing. All authors have agreed to their individual contributions ahead of submission.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The manuscript contains the majority of experimental data obtained in the study. Other data supporting this article have been included as part of the ESI.†

Acknowledgements

The authors thank the National Academy of Sciences and Enamine Ltd. for support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5dt00065c

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