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Article

Catalytic Properties of Mechanochemically Exfoliated MoS2 in the Hydrogenation of Bromoquinolines

by
Anastasia V. Terebilenko
1,2,
Andrii S. Kondratyuk
1,
Maryna V. Olenchuk
3,
Pavlo S. Yaremov
1,
Andrii M. Zhuchenko
1,
Volodymyr V. Buryanov
2,4 and
Sergey V. Kolotilov
1,2,*
1
L.V. Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, 31, pr. Nauky, 03028 Kyiv, Ukraine
2
Enamine Ltd., 78 Winston Churchill St., 02094 Kyiv, Ukraine
3
Institute of Physics, National Academy of Sciences of Ukraine, 46, pr. Nauky, 03028 Kyiv, Ukraine
4
Institute of High Technologies, Taras Shevchenko National University of Kyiv, Volodymyrska Street 60, 01601 Kyiv, Ukraine
*
Author to whom correspondence should be addressed.
Surfaces 2026, 9(2), 34; https://doi.org/10.3390/surfaces9020034
Submission received: 19 December 2025 / Revised: 12 March 2026 / Accepted: 25 March 2026 / Published: 30 March 2026
(This article belongs to the Special Issue Recent Advances in Catalytic Surfaces and Interfaces, 2nd Edition)
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Abstract

This study aimed to develop new catalysts, based on MoS2, for the hydrogenation of bromoquinolines without C-Br bond cleavage. The mechanochemical exfoliation of the bulk MoS2 in the presence of NaCl resulted in the formation of the material (MoS2-1), consisting of flat plates of size between ca. 40 × 100 and ca. 250 × 400 nm2. Similar grinding of MoS2 in the presence of NH4Cl produced smaller nanoplates of size between ca. 10 × 30 and ca. 50 × 300 nm2 (MoS2-2). These materials were characterized using powder XRD, TEM, SEM, Raman spectroscopy and XPS. The specific surface area of the MoS2-1 and MoS2-2 samples was estimated using the analysis of N2 adsorption isotherms. Both materials were catalytically active in the hydrogenation of quinoline; 1,2,3,4-tetrahydroquinoline (THQ) was the sole product and its yield grew proportionally to the accessible surface area of the catalyst. The hydrogenation of 5- and 8-bromoquinolines in the presence of MoS2-1 and MoS2-2 led to the respective bromo-THQs with almost quantitative yields, while the hydrogenation of 6-bromoquinoline resulted in the formation of the respective 6-bromo-THQ with the yield up to 30%. In the case of 7-bromoquinoline, N-methylated 7-bromo-THQ was formed almost quantitatively.

Graphical Abstract

1. Introduction

Catalytic hydrogenation is one of the most important processes in modern fine organic chemistry and the chemical industry. Hydrogenation has been widely used for the preparation of saturated compounds for medical chemistry and agriculture [1]. In particular, the catalytic hydrogenation of substituted aromatic compounds is the simplest way to produce substituted saturated carbo- and heterocycles. One of the reasons is that the introduction of various functional groups in aromatic systems (in particular, electrophilic aromatic substitution) can be easily performed in contrast to the functionalization of sp3 carbon in the saturated analogs [2]. In industry, hydrogenation is used for the hydrocracking of oil, the production of fuel or solvents from biomass, and the hydrogenation of palm oil for its conversion to a solid state [3,4,5,6].
The majority of hydrogenation catalysts contain platinum group metals, first of all Pd or Pt, or nickel, mainly in the form of Raney nickel [7,8,9,10,11,12]. Co-containing catalysts have also been attracting attention in recent decades [13,14,15,16]. As a rule, the hydrogenation of halogen-containing organic compounds (especially bromo- and iodo-containing ones) in the presence of these catalysts results in their dehalogenation [17,18,19,20]. However, halogen-containing compounds bearing saturated rings are important building blocks for fine organic synthesis, for the production of compounds for medicinal chemistry. The search for new selective catalysts for the hydrogenation of halogen-containing compounds, which do not induce cleavage of C-Hal bonds, is an important task in modern physical organic chemistry.
Sulfides of 3d and 4d metals are considered promising systems for the selective hydrogenation of organic compounds bearing “sensitive” functional groups [21,22]. Previously, we reported the formation of a series of nanostructured flower-like MoS2 samples, which had high catalytic performances in the hydrogenation of quinoline and bromoquinolines. A specific feature of these materials was their capacity to perform the hydrogenation of bromoquinolines without C-Br bond cleavage [23]. The storage of such flower-like samples for several months resulted in their oxidation and a drastic decrease in their catalytic performance.
It was not clear whether the high catalytic performance of the nanostructured MoS2 samples was associated with the formation of the specific active sites. Such sites could be related to the X-ray amorphous nature of the materials. On the other hand, such a high performance might be attributed simply to a high specific accessible surface area.
In this study two samples of MoS2 were prepared through the mechanochemical grinding of the bulk MoS2 in the presence of different salts—NaCl and NH4Cl. The samples obtained are hereinafter referred to as MoS2-1 (treatment with NaCl) and MoS2-2 (treatment with NH4Cl). Such treatment led to the exfoliation of MoS2 and the formation of nanostructured crystalline samples with increased specific surface areas. The MoS2 materials were characterized using X-ray diffraction, TEM, SEM, XPS, Raman spectroscopy, and the hydrogenation of quinoline and a series of bromoquinolines in the presence of such samples was studied.

2. Experimental Section

2.1. General Procedures

Hydrogen (99.99%) was purchased from Galogas Ltd. (Kyiv, Ukraine). MoS2 (98%, particle size <2µ, Sigma-Aldrich, St. Louis, MO, USA), NaCl (99.7%, Klebrig, Rivne, Ukraine), NH4Cl (99.52%, Klebrig, Rivne, Ukraine) were used as received. All other starting materials and reagents were available from Enamine Ltd. (Kyiv, Ukraine) and UkrOrgSintez Ltd. (Kyiv, Ukraine).
A planetary ball mill PM 100 (Retsch GmbH, Haan, Germany) equipped with a 250 mL alumina grinding bowl and 50 alumina balls (d = 10 mm, total mass ≈ 95 g) was used for the mechanochemical treatment of the samples. The process was performed in continuous mode at a rotation speed of 500 rpm in an air atmosphere. The powder batch mass was approximately 6.8 g, maintaining a ball-to-powder weight ratio (BPR) of 14:1.
Transmission electron microscopy (TEM) studies were performed using a PEM-125K SELMI microscope with an accelerating voltage of 100 kV. The samples were applied to copper grids coated with an amorphous carbon film from alcohol suspensions. Scanning electron micrographs (SEM) were obtained using an FEI Inspect S50 instrument. The samples were placed on a carbon film without any special treatment. The energy-dispersive X-ray (EDX) analysis was performed with an Apollo XL SDD EDAX. Powder X-ray diffraction patterns were measured using a Bruker D8 Advance diffractometer with CuKα radiation (λ = 1.5406 Å) in the range 2θ = 10–60° with an increment of 0.05°. Measurements of nitrogen adsorption at 77 K were performed on a Micro300C-02-Analysis Station3 (Altamira Instruments, Pittsburgh, PA, USA); the MoS2-2 sample was briquetted under very low pressure (less than 1 bar) prior to testing due to its low bulk density. Prior to measurements, samples were kept at 130 C in 10−4 Torr vacuum for 2 h. Raman spectra of the samples were recorded on a Renishaw spectrometer (λ = 514 nm). Before application to a silicon substrate, the sample powders were mixed with NaCl to prevent their burning under the laser beam. X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5600LS spectrometer equipped with a monochromatic Alα X-ray source. The instrument work function was calibrated to give a binding energy (BE) of 84 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.6 eV for the Cu 2p3/2 line of metallic copper. Survey scan analyses were carried out with a pass energy of 93.9 eV and 0.4 eV step. High-resolution spectra were collected with a pass energy of 11.75 eV and 0.1 eV step. Before analysis the samples were not pre-etched with an ion gun. Binding energies were calibrated by referencing the adventitious C 1s peak to 284.8 eV; no further charge correction was applied to the HR spectra. The spectra were analyzed using CasaXPS 2.3.15 software. A Shirley background was employed for baseline modeling [24], and peak profiles were described using Gaussian–Lorentzian line shapes (30% Lorentzian). The value of the statistical indicator Residual STD did not exceed 1.55. During fitting, the following constraints were applied: spin–orbit splitting was fixed at 1.18 eV (S 2p) and 3.15 eV (Mo 3d); doublet area ratios were set to 2:1 (S 2p) and 3:2 (Mo 3d); and full width at half maximum (FWHM) values for doublet components were linked as equal.
Proton nuclear magnetic resonance (1H NMR) spectra was measured on a Varian Unity Plus 400 spectrometer operating at 400 MHz. Mass spectrometry data for organic products in hydrogenation experiments were acquired on an Agilent 5890 Series II 5972 GCMS instrument (Santa Clara, CA, USA) utilizing electron impact ionization, EI.

2.2. Synthesis of MoS2-1 and MoS2-2

The MoS2-1 sample was obtained using the method described in [25]. For this purpose, a dry mixture of 0.85 g of bulk MoS2 and 6 g of NaCl was continuously ground in a ball planetary mill in air for 1 h with a ball-to-powder ratio ≈ 14:1. No external heating was applied. The NaCl was subsequently removed by washing with deionized water, and the resulting nanostructured material was dried under vacuum at 80 °C.
MoS2-2 was obtained in a similar way, but NH4Cl was used as the exfoliating agent instead of NaCl. After grinding NH4Cl was removed by sublimation at 350 °C in an Ar flow for 6 h (heating rate = 5 °C/min; Ar flow rate ≈ 10 mL/min), followed by natural cooling to room temperature.

2.3. Catalytic Hydrogenation of Quinoline and Substituted Quinolines

All hydrogenation experiments were carried out in a stainless-steel autoclave equipped with a manometer, a magnetic stirrer, and a temperature controller, following a previously reported methodology [23]. Briefly, 0.016 g (1 × 10−4 mol) of the catalyst was added to a solution containing 0.129 g of quinoline (1 × 10−3 mol) or 0.209 g of bromoquinoline (1 × 10−3 mol) in 10 mL of methanol. The autoclave was purged by hydrogen several times, pressurized, and heated to a set temperature. During the experiment the pressure was controlled to compensate for a pressure increase (due to temperature expansion) and hydrogen consumption. The hydrogenation was carried out during 24 h for quinoline or 45 h for bromoquinolines. The products were analyzed as described before [23]. The yields were determined using 1H NMR, while GCMS patterns were used for control identification of the products and estimation of minor impurities content (less than 5%).

3. Results and Discussion

3.1. Synthesis of the Materials and SEM and TEM Characterization

The mechanochemical grinding of the bulk crystalline MoS2 led to its exfoliation, which predominantly occurred in directions parallel to the 2D layers. Such exfoliation was assisted by the addition of solid salts, NaCl and NH4Cl. The salts acted as mechanical auxiliary exfoliating agents and also could facilitate exfoliation via the stabilization of the resulting MoS2 sheets due to adsorption on the surface.
The structure and morphology of the prepared samples were studied using electron microscopy, powder XRD, XPS and Raman spectroscopy.
Thin flat plates of MoS2 can be distinguished using SEM (Figure 1). The size of plates in the MoS2-1 sample lies in the range from ca. 3 to 10 μm, while their thickness is less than 0.1 μm. In the case of the MoS2-2 sample, the plates are smaller; their linear size is about 1–3 μm, while the thickness is apparently less than 0.1 μm.
The MoS2 plates are oriented edge-on in multiple directions and assembled into micron-sized aggregates. Such an organization is typical for the products of mechanochemical exfoliation [26]. The morphology of MoS2-1 (Figure 1a–c) is characterized by agglomerates of 1–5 μm in size with a rather compact and dense structure, formed by thicker opaque flake-shaped particles. In contrast, in the case of the MoS2-2 sample (Figure 1d–f), “fluffy” agglomerates with dimensions of approximately 3–10 μm are observed. These agglomerates exhibit a highly developed surface and contain numerous interparticle voids, while the flakes of dispersed molybdenum disulfide appear thin and translucent.
On TEM images of the MoS2-1 sample, the flat plates can be found; the size of the smallest is about 40 × 100 nm, while the size of the largest is about 250 × 400 nm (Figure 2a). In contrast, the MoS2-2 sample consists of smaller nanoplates; their size is between ca. 10 × 30 nm and ca. 50 × 300 nm (Figure 2b). In the case of both samples, the plates are assembled into aggregates. The particles of MoS2-1 have higher contrast on TEM images compared to MoS2-2; this difference may be an indicator of the greater thickness of the former. The overall conclusions on the morphology of the samples, revealed by TEM, are consistent with the results of the SEM studies.

3.2. XRD Analysis

The phase composition of the obtained materials was studied using powder X-ray diffraction. The diffraction patterns of both the MoS2-1 and MoS2-2 samples (Figure 3) reveal sharp, well-defined reflections, indicative of their high crystallinity. In both cases, characteristic reflections typical of the hexagonal phase 2H-MoS2 are observed, in agreement with previously reported data [27]. The most intense reflection at ~14° corresponds to the (002) plane, and the high intensity of this reflection confirms the presence of multiple layers in the material parallel to the ab plane (2D layers of MoS2) [27]. The reflections corresponding to the (100), (101), (102), (103), (105), (110), and (112) planes are also clearly seen in the 30–65° range. Slightly broader peaks on the MoS2-2 in the diffraction pattern indicate a smaller size and reduced thickness of nanoparticles compared to MoS2-1; this observation is consistent with the results of the TEM and SEM studies.
The mechanochemical exfoliation of bulk crystalline MoS2 led to the formation of thin flat plates of MoS2, which preserved high crystallinity. In contrast, the hydrothermal synthesis of MoS2 through the reaction of molybdate with thiourea in water resulted in the formation of flower-like MoS2 [23]. The flowers (aggregates) consisted of separate MoS2 particles with sizes ranging from 0.05 to 0.1 μm [23] and the samples had low crystallinity, which is consistent with their small size. Previously, broad and low-intensity reflections of the 2H phase were observed in the case of nanocrystalline MoS2 particles with an average diameter of ~100 nm and for MoS2 nanosheets with a thickness of several nanometers, obtained by solvothermal synthesis; the low intensity and high peak width of the reflections were explained by the small (nanoscale) size of the crystallites [28]. The mechanochemical treatment of micron-sized MoS2 led to the formation of a nanomaterial with an average particle size of 250 nm. This structural change was confirmed by a decrease in the intensity of the (002) and (100) diffraction peaks compared to the initial bulk sample [29]. When the size of MoS2 microspheres increased to ~1 μm (as shown for a sample obtained by a facile solid-state reaction), the characteristic set of hexagonal 2H-phase reflections was retained, but the peak width decreased and their intensity increased [30].

3.3. Surface Area and Textural Properties

In order to estimate the specific surface area of the samples, low-temperature nitrogen adsorption isotherms were measured. Due to the low density of the powders (particularly the highly voluminous MoS2-2), the samples were gently briquetted prior to measurement. This procedure was applied solely for experimental handling purposes to ensure efficient degassing and stable loading of the sample holder; however, it may influence packing density and interparticle porosity.
Both samples exhibit nitrogen adsorption isotherms characteristic of physical adsorption. The isotherm of MoS2-1 (Figure 4a) can be referred to as Type II, according to the IUPAC classification, corresponding to physisorption on non-porous adsorbents [31]. In contrast, the MoS2-2 sample shows a Type IVa isotherm with a hysteresis loop consistent with H3-type behavior (Figure 4b). According to the IUPAC classification, this indicates the presence of mesopores formed by non-rigid aggregates of plate-like particles [31]. The pore size distribution curve for the MoS2-2 sample (Figure S1, Supporting Information) reveals a predominant mesopore diameter (Dmeso) of 14.3 nm, whereas the MoS2-1 sample primarily contains small pores (<5 nm).
The specific surface areas of the samples were calculated using the BET model. The MoS2-2 sample exhibits a higher specific surface area than MoS2-1 (57.6 m2/g vs. 47.7 m2/g, respectively), as well as a larger total pore volume (0.22 cm3/g vs. 0.12 cm3/g). These findings are in good agreement with the results obtained from the TEM, SEM, and XRD analyses and are consistent with a higher dispersion of MoS2 particles in the MoS2-2 sample.
Generally, according to the results obtained using SEM, TEM, powder XRD and nitrogen adsorption, the exfoliation of MoS2 in the presence of NH4Cl resulted in the formation of smaller particles compared to exfoliation in the presence of NaCl. Among the factors that can influence the particle size after mechanochemical treatment, the hardness of the auxiliary reagent and its capacity to stabilize fine particles may play significant roles. The hardness of NaCl is 2.5 on the Mohs scale [32], while the hardness of NH4Cl is about 1.5–2.0 on Mohs scale [33]. Nevertheless, mechanochemical grinding with NaCl resulted in the formation of larger MoS2 plates compared to grinding with NH4Cl. Therefore, the size of MoS2 plates does not correlate with the hardness of the salt, which was added as an auxiliary exfoliating agent. It may be concluded that the stabilization of the MoS2 sheets due to the adsorption of the auxiliary reagents plays a more important role compared to the delamination capacity of the salt. This finding is consistent with the conclusion of a previous study [34], where graphite was exfoliated in the presence of deep eutectic solvents (DESs). In that work, it was found that the efficiency of the exfoliation was mainly governed by the interaction strength between the DESs and graphene.
The method of delaminating agent removal can also contribute to the control of the size of the formed MoS2 plates. For instance, the thermal decomposition of NH4Cl leads to the formation of gaseous products that expand the MoS2 layers, thereby enhancing exfoliation, surface development, and outward-oriented edge agglomeration to form a material with a developed mesoporous structure. In contrast, when NaCl is washed with water and then dried, the opposite process of the sticking together of exfoliated particles may occur, resulting in denser and thicker agglomerates.

3.4. Raman Spectroscopic Characterization

The Raman spectra of both molybdenum disulfide samples (Figure 5) contain vibrational modes characteristic of multilayer 2H-MoS2: the E12g mode is observed at ~381 cm−1, while the A1g mode appears at ca. 406 cm−1 [35,36]. The narrow full width at half maximum for these main characteristic peaks (5–6 cm−1) indicates a high degree of crystallinity of the materials [37]; this feature is consistent with the results of the XRD studies. The separation between the E12g and A1g modes is approximately 25 cm−1, which is an established indicator of the predominance of a multilayer MoS2 structure. Conversely, the higher ratio of intensities of the A1g and E12g bands (i.e., I(A1g)/I(E12g)) reflects a stronger relative contribution of the out-of-plane A1g vibration. Such behavior is usually associated with an increase in the density of structural defects associated with the edges of nanoplates. The combination of these features indicates that, despite high crystallinity, the Raman response of the materials can be explained by a significant contribution from the edge regions, consistent with the presence of structurally disturbed centers at the boundaries of the crystallites [36,38]. In addition, both spectra display a defect-activated mode E1g at 284.5 cm−1; the weak broad peak with maxima at 450 cm−1 can be attributed to the second-order mode 2LA(M) [39]. The peak at 461.5 cm−1 can be assigned to the A2u(LO) optical mode. Its appearance is attributed to resonance effects and the breakdown of selection rules in nanostructures.
Notably, there were no signals typical for the 1T phase in the Raman spectra of both samples, in contrast to MoS2 samples prepared using a hydrothermal method [23].

3.5. Surface Chemical State Analysis Using XPS

The chemical composition of the MoS2-1 and MoS2-2 surfaces was investigated using X-ray photoelectron spectroscopy (XPS). The survey XPS spectra (Figure 6) reveal characteristic core-level peaks of Mo and S, along with significant signals for C 1s and O 1s. The presence of oxygen is attributed to partial surface oxidation and the formation of a hydrated oxosulfide layer, while the carbon signal originates from atmospheric organic molecules (adventitious carbon) [40]. The maxima of the Mo 3d and S 2p signals in the MoS2-2 sample (raw data, not deconvoluted peaks) were shifted to lower binding energies compared to MoS2-1. The trends for Mo 3d and S 2p were the same, but the shift in values for S 2p were slightly higher (0.4–0.5 eV) than for Mo 3d (0.3–0.4 eV). A significant difference was observed in the surface purity and stoichiometry: the S/Mo atomic ratio was 1.68 for MoS2-1 and 1.93 for MoS2-2, while the carbon content reached 42.99 at.% and 25.01 at.%, respectively. The substantially higher carbon content in MoS2-1, detected using XPS, suggests significant surface coverage by adventitious organic molecules. Besides the signals of adventitious carbon, the most intense signals of impurities were attributed to Cl (1.07 at.%) and Na (0.65 at.%) and these impurities were detected in the MoS2-2 sample. These impurities partially originated from the bulk MoS2, which was used as a starting material, and probably remained because the MoS2-2 sample was not washed with water after exfoliation (NH4Cl was eliminated by thermal decomposition without washing with any solvent. In contrast, the MoS2-1 sample was thoroughly washed out after grinding). However, it cannot be excluded that the use of NH4Cl made a contribution to the increased content of Cl− in MoS2-2. The impurity of Na+ was observed in other studies where the XPS of MoS2 was considered and was explained by the presence of Na+ in solvents [41]. The surface concentrations of elements (at.%) in the MoS2 samples, derived from the XPS survey spectra, are summarized in Table S1 (Supporting Information).
The Mo 3d spectra (Figure 7) confirm the dominance of Mo(IV) species, consistent with the hexagonal 2H-MoS2 phase identified using Raman spectroscopy (E12g and A1g modes) [42,43]. Interestingly, while both samples exhibit similar Raman profiles with a high I(A1g)/I(E12g) ratio of ~2 (indicating an edge-terminated morphology in both cases), the XPS analysis reveals a profound difference in their electronic structures. For MoS2-1, the Mo 3d spectrum was fitted using a standard model with MoIVbasal (228.7/231.8 eV) and MoVI (231.4/234.8 eV) doublets. The second component is commonly associated with surface MoOx formed via the partial oxidation of MoS2 upon exposure to air and is frequently observed in nanoscale or defect-rich MoS2 materials [44,45,46] or Mo oxysulfides [47]. However, for MoS2-2, a more complex deconvolution model was required, introducing an additional MoIVedge component at 228.2/231.3 eV (67.02% of total Mo; Table S2, Supporting Information). The content of MoVI in the samples was 13% in MoS2-1 and 17% in MoS2-2. Taking into account the ratios of MoIV/MoVI and S/Mo in the samples, the composition of MoS2-1 can be formally presented as MoS2·0.15MoO3, while the composition of MoS2-2 can be formally presented as MoS2·0.12MoS2O·0.09MoO2S. Notably, the MoVI signals in MoS2-2 are slightly shifted to lower binding energies, and the signals are broader compared to the respective signals in MoS2-1, which is consistent with the plausible assignment of such MoVI in MoS2-2 to oxysulfides [47].
The higher content of oxygenated species in MoS2-2 is consistent with its higher specific surface and may result from a higher quantity of Mo sites exposed to oxygen.
These results suggest that while the mechanochemical treatment led to a high degree of nanostructuring in both materials (as confirmed by TEM and SBET measurements), only in MoS2-2 did it result in the manifestation of signals, which were assigned to the edge sites. The fact that MoIVedge ions in MoS2-1 were indistinguishable by XPS may be explained by chemical modification, leading to the formation of an oxo-species (S/Mo = 1.68). In contrast, in MoS2-2 a significant content of chlorine (1.07 at.%) and/or sodium (0.65 at.%) could favor the stabilization of these active MoIVedge centers since these elements could act as dopants.
In the S 2p spectra of both samples, MoS2-1 and MoS2-2, the best deconvolution of S 2p3/2 and 2p1/2 doublets was achieved with a pair of signals, which could be assigned to S2− in the basal plane (lower binding energies, for example, 162.7/163.9 eV for MoS2-1) and defective S (higher binding energies, for example, 163.6/164.7 eV for MoS2-1; the values for MoS2-2 are provided in Table S2, Supporting Information). These values are typical for S2− in hexagonal 2H-MoS2 [48]. The full width at half maximum (FWHM) values of the S 2p components of S2− in the basal plane (0.84 eV for MoS2-1 and 0.70 eV for MoS2-2) fall within the typical range reported for MoS2 (≈0.6–0.9 eV) [48]. However, the FWHM values for S 2p signals of defective S are higher (1.00–1.14 eV). Since the width of the S 2p peaks is highly sensitive to surface defects, edge sites, and amorphous components, the observed broadening of these peaks in MoS2-1 and MoS2-2 is consistent with their assignment to defective and/or edge S, which is in line with previous reports on layered and ultra-dispersed MoS2 systems [49]. Taking into account the fact that MoIVedge ions were not found in MoS2-1, the defective S atoms in this sample are probably located in the basal plane, in contrast to the edges. At the same time, in both samples, MoS2-1 and MoS2-2, there were no peaks typical of SIV (expected in 166.0–167.5 eV range) or SVI (expected un 168.0–170.0 eV range), in contrast to MoS2 samples prepared using a hydrothermal method [23]. It can be concluded that MoVI species found in the samples are located in oxides, rather than sulfites or sulphates.
The C 1s spectra of both MoS2-1 and MoS2-2 exhibited a dominant peak assigned to adventitious carbon, which was used for peak position calibration; its position was set to 284.8 eV.
The O 1s spectra further highlight the structural differences between the samples. In the case of MoS2-1 a broad signal at 531.6 eV dominates. This signal can be assigned to Odef/OH groups, which can be bound to MoVI ions. In MoS2-2, the deconvolution reveals an additional peak at 530.0 eV, which can be assigned to lattice oxygen (Olat), and a unique high-energy peak at 535.1 eV; the latter can be assigned to intercalated water (H2Oint) trapped between layers during high-energy milling or adsorbed water. The corresponding spectra are provided in the Supplementary Materials for completeness.

3.6. The Catalytic Properties of MoS2

The effect of mechanochemical treatment on the catalytic properties of MoS2 was studied in the hydrogenation reaction of quinoline. Reactions catalyzed by MoS2-1 and MoS2-2 were carried out in methanol at 100 °C under 100 atm of hydrogen. Catalyst loading in all cases was 10 mol.% (relative to formula MoS2 with formula weight 160.07 g/mol) per quantity of quinoline or bromoquinoline. The yield of 1,2,3,4-tetrahydroquinoline (THQ) in 24 h reached 53% for MoS2-1 and 62% for MoS2-2. These values exceed the yields previously reported for MoS2 prepared using a hydrothermal method [23] and were significantly higher than the performance of the bulk MoS2 (Table 1).
In order to compare the efficiency of different MoS2 catalysts in the hydrogenation of quinoline, the values of the specific performance (SP) of MoS2 samples were calculated. This parameter is defined as the quantity of 1,2,3,4-tetrahydroquinoline (THQ) formed during 24 h in the presence of the catalyst (μmol) in relation to 1 m2 of the accessible surfaces of the catalyst, loaded in the reaction mixture, as follows:
SP [μmol/m2] = n(THQ)/(SBET · m(MoS2))
where n(THQ) is the total quantity of THQ formed in the experiment (μmol), SBET is the specific surface area of MoS2 (m2/g) and m(MoS2) is the mass of the catalyst in the reaction mixture (g). These values are presented in Table 1. For comparison, the table included the data on the previously reported MoS2 catalysts, synthesized under hydrothermal conditions (hereinafter referred to as “hydrothermal MoS2 samples”, in contrast to “exfoliated MoS2 samples” MoS2-1 and MoS2-2) at Mo/S ratio 1:15.75 [23] and Mo/S 1:31.5 [25].
The specific surface area values calculated from N2 sorption isotherms by the BET model (vide supra) were used for the analysis. In addition, the nitrogen adsorption isotherms for the previously reported samples of MoS2 were measured and SBET values were calculated; these values are also presented in Table 1. Since we do not have kinetic data on the hydrogenation reaction, the SP values should not be considered as a measure of the catalysts’ activity or the reaction rate, but they can be used for comparison purposes, especially for the evaluation of the efficiency of the MoS2 samples in the preparative synthesis of hydrogenation products. A similar approach for the comparison of catalysts was used previously for the characterization of catalysts [50,51].
The values of SP for MoS2-1 and MoS2-2 (694 and 673 μmol/m2, respectively) were quite close, and it can be noted that the yields of THQ in reactions catalyzed by these samples were proportional to the values of the accessible surface of MoS2. Thus, it can be argued that the chemical nature of the surface of both samples (e.g., the average quantity of active sites per 1 m2) was similar, and there was no specific influence of the exfoliating agent (NaCl or NH4Cl) on the formation or disappearance of the sites, which are catalytically active in the hydrogenation of quinoline. At the same time, the values of SP for these samples were lower than those reported for MoS2 prepared under hydrothermal conditions, especially for MoS2 samples synthesized at high sulfur loading (MoSS130–MoSS150, Table 1).
The experiments on the hydrogenation of isomeric 5-, 6-, 7-, or 8-bromoquinolines in the presence of MoS2-1 and MoS2-2 were performed under similar conditions as quinoline, except that the reaction time was extended to 48 h (Table 2). The reaction afforded the formation of bromo-substituted 1,2,3,4-tetrahydroquinolines and the product of their debromination (i.e., 1,2,3,4-tetrahydroquinoline) and of the compounds formed upon their N-methylation (i.e., bromo-N-methyl-1,2,3,4-tetrahydroquinolines and N-methyl-1,2,3,4-tetrahydroquinoline). Among these compounds, only bromo-substituted 1,2,3,4-tetrahydroquinolines could be considered as the target products because the value of N-methylated compounds, as well as 1,2,3,4-tetrahydroquinoline, is low due to their limited suitability for further functionalization. Thus, the yields were provided as the ratio or the quantity of the bromo-substituted 1,2,3,4-tetrahydroquinoline (the respective isomer) to the quantity of the starting compound (×100%). The selectivity values were provided as the ratio of the quantity of bromo-substituted 1,2,3,4-tetrahydroquinoline to the total quantity of the reaction products (×100%).
In the case of 5-bromoquinoline, both catalysts provided high yields of 5-bromo-1,2,3,4-tetrahydroquinoline (85 and 100%, respectively), while in the presence of MoS2-1, 12% of 5-bromo-N-methyl-1,2,3,4-tetrahydroquinoline was formed. Such a compound apparently formed due to N-methylation of 1,2,3,4-tetrahydroquinoline, occurring by a hydrogen borrowing mechanism, as previously reported [11,16]. However, in the case of 6-bromoquinoline, the reaction was markedly less selective to the target product: alongside the target product, substantial amounts of N-methylated and debrominated derivatives were detected, reducing the selectivity to 21–32%. In the reaction of 7-bromo-substituted quinoline, no target product was observed; instead, both catalysts led to the formation of N-methylated derivatives (selectivity was zero with respect to the target compound, 7-bromo-1,2,3,4-tetrahydroquinoloine, but 100% with respect to the unwanted N-methylated derivative). Finally, the hydrogenation of 8-bromoquinoline on MoS2-2 occurred with high selectivity and led to complete conversion to the target product, while in the case of MoS2-1 a small fraction of N-methylated by-product was detected.
The experimental results demonstrate a pronounced dependence of the reaction direction and its selectivity on the position of the bromine atom in the quinoline core. Specially, the isomers bearing Br in the fifth and eighth positions undergo hydrogenation to the desired bromo-tetrahydroquinolines with high yields. In contrast, the isomer that contains Br in the seventh position undergoes complete conversion into the N-methylated by-product (thus, selectivity to this methylated compound is formally equal to 100%, while selectivity to the target product is zero). 6-bromoquinoline exhibits intermediate behavior with low selectivity to the target product in this series. Such diversity of hydrogenation pathways of different isomeric bromoquinolines is explained by the combination of steric and electronic effects, which govern the adsorption process of the substrate onto the surface, defining the reaction course. The critical role of MoS2 morphology and edge sites in modulating the activation barriers to hydrogenation reactions and the selectivity of transformations was also supported by DFT studies [52,53].
For comparison, Table 2 includes the values of the yields achieved during the hydrogenation of the respective isomers of bromoquinoline in the presence of the most efficient MoS2 catalysts prepared using hydrothermal synthesis. In the case of 5- and 8-bromoquinolines, the performance of the exfoliated MoS2 samples (evaluated using the criteria of the yield of the target product, 6-bromo-1,2,3,4-tetrahydroquinoline, per 1 g or 1 mole of the catalyst) was at the level of the most efficient “hydrothermal” MoS2 samples. In the case of 6-bromoquinoline, the efficiency of exfoliated MoS2 samples was higher compared to hydrothermal ones. However, the SP values for exfoliated MoS2 catalysts in the hydrogenation of all three isomers (5-, 6-, and 8-bromoquinoline) were lower than similar values found for hydrothermal MoS2 catalysts (for example, for 5-bromoquinoline 1.1 mmol/m2 vs. 1.9–3.7 μmol/m2; note, that these SP values were calculated for the yield of the target product, rather than the overall conversion of the starting compound). As in the case of quinoline hydrogenation, the specific performance of the “hydrothermal” MoS2 in the hydrogenation of 6-bromoquinoline was better. Notably, very similar by-products were formed in the cases of exfoliated and hydrothermal catalysts (i.e., N-methylated compound and Br-free quinoline).
It is difficult to distinguish the main factors which control the activity and selectivity of MoS2 catalysts in various reactions, especially taking in mind the diversity of hydrogenation products. We can note that in the case of the hydrogenation of 5- and 8-bromoquinolines (the isomers for which minimal content of the by-products was observed) the higher product yield and selectivity achieved using MoS2-2 (compared to MoS2-1) were consistent with the higher content of the MoIVedge sites found for the former catalyst using XPS. In general, the higher performance of the catalysts obtained using hydrothermal methods over the exfoliated MoS2 catalysts can be explained by the content of the 1T phase, since it was indicated that 1T MoS2 had higher catalytic activity in hydrogenation processes [54]. In addition, the density of the defects per 1 m2 of the surface and the content of edge sites (which were shown to be catalytically active sites [55,56,57,58]) may be higher in the “hydrothermal” samples than in exfoliated ones; such different contents of the active sites may be associated with the X-ray amorphous nature of the former and high crystallinity of the latter. However, the mentioned difference in SP could also be associated with limitations of the reagent diffusion in solution, located in voids of the aggregates of MoS2 plates.
The case of 7-bromoquinoline is very specific because completely different products formed in the presence of exfoliated and hydrothermal MoS2 catalysts: the former led to N-methyl-7-bromo-1,2,3,4-tetrahydroquinoline with a quantitative yield, while the latter led to non-methylated 7-bromo-1,2,3,4-tetrahydroquinoline. Such behavior of the MoS2-1 and MoS2-2 samples apparently was not associated with the specific activation of methanol because in such a case N-methylation would occur in the cases of all isomers of bromoquinoline as well as quinoline itself. It can be supposed that the formation of N-methyl-7-bromo-1,2,3,4-tetrahydroquinoline can be caused by the efficient adsorption of the 7-bromo-1,2,3,4-tetrahydroquinoline onto the surface of the catalysts, facilitating its methylation due to the long “waiting” time at the active site. On the grounds of the comparison of exfoliated and “hydrothermal” samples of MoS2 it can be deduced that the crystallinity of the catalyst may be the factor responsible for N-methylation in the case of this isomer.
In order to check the stability of the catalyst during storage, the hydrogenation of 5-, 7- and 8-bromoquinolines was repeated in the presence of the catalysts, which were kept for 8 months in air in closed vials at +5 °C. The hydrogenation of bromoquinolines in the presence of these “aged” samples led to lower yields (by 15–30%) and lower selectivities, while general trends in the product composition and yield depending on the isomer were the same as in the case of “fresh” catalysts. For example, the hydrogenation of 5-bromoquinoline in the presence of MoS2-1 led to 5-bromo-1,2,3,4-tetrahydroquinoline with 60% yield (compared to 85% in the case of “fresh” catalysts), while in the case of 8-bromoquinoline the respective 8-bromo-1,2,3,4-tetrahydroquinoline formed with 81% yield (compared to 93% in the case of “fresh” catalysts; all results are shown in Table S3, Supporting Information). Notably, the hydrogenation of 7-bromoquinoline in the presence of “aged” MoS2 resulted in the formation of a N-methylated derivative, as it was observed for a “fresh” sample. In the presence of the “aged” MoS2-1, 7-bromo-N-methyl-1,2,3,4-tetrahydroquinoline formed with 86% yield, but 6% of 7-Bromo-1,2,3,4-tetrahydroquinoline was additionally found, while the “fresh” catalyst led to the formation of 7-Bromo-N-methyl-1,2,3,4-tetrahydroquinoline with almost an quantitative yield.

4. Conclusions

It was shown that the mechanochemical exfoliation of bulk MoS2 led to the formation of small plates of a size between some dozen and some hundred nm. The addition of NH4Cl upon grinding MoS2 led to the formation of smaller particles compared to exfoliation in the presence of NaCl, presumably due to the different adsorption of such auxiliary reagents onto the surface area of MoS2 plates. The sample of MoS2-2, consisting of smaller particles, expectedly possessed a higher specific surface area and had a higher content of oxygenated species, but in contrast to MoS2-1 it contained a significant quantity of MoIVedge sites (as concluded from analysis using XPS). Such exfoliated samples preserved high crystallinity (in contrast to the previously reported MoS2 samples prepared using hydrothermal synthesis) and possessed high performances in the hydrogenation of quinoline and bromoquinolines. Thus, the crystallinity or X-ray amorphous nature of the MoS2 catalysts is not a key factor responsible for their catalytic activity in hydrogenation processes. The hydrogenation of 5- and 8-bromoquinolines resulted in the formation of the respective bromo-1,2,3,4-tetrahydroquinolines with high yields, while in the case of 6-bromoquinoline, the yields of the target product, 6-bromo-1,2,3,4-tetrahydroquinoline, were significantly lower. In the case of the hydrogenation of 7-bromoquinoline in the presence of exfoliated samples MoS2-1 and MoS2-2, the complete transformation of the starting compound to 7-bromo-N-Methyl-1,2,3,4-tetrahydroquinoline occurred due to methylation with methanol (solvent). Among the exfoliated samples, MoS2-1 and MoS2-2, the catalytic performance of the second one was superior, but the difference could be explained by increases in the specific surface. At the same time, the performance of the exfoliated MoS2 samples in the hydrogenation of both quinoline and bromoquinolines, estimated as the product yield per 1 m2 of the catalyst, was lower than the performance of the MoS2 obtained using hydrothermal synthesis. This difference can be presumably associated with the presence of the 1T MoS2 phase and higher activity of the sites located at the edges of MoS2 plates in the “hydrothermal” samples, though different limitations of reagent diffusion in solution, located in voids of the catalyst aggregates, could also make a contribution. Nevertheless, the complete hydrogenation of 5- and 8-bromoquinolines to the respective brominated 1,2,3,4-tetrahydroquinolines was achieved in the presence of exfoliated MoS2, indicating that such catalysts can be used for the preparation of the respective products.
Taking into account the simpler preparation method, compared to hydrothermal synthesis, mechanochemically exfoliated MoS2 may have advantages over other nanostructured MoS2 hydrogenation catalysts in the case of easy access to a ball mill. These catalysts can find application for the preparative hydrogenation of two isomeric bromoquinolines, i.e., the ones bearing a Br atom in the fifth and eighth positions. The outcomes of bromoquinoline hydrogenations are apparently isomer-sensitive, and special studies are required for the possible extension of the use of such catalysts for the synthesis of different organic halogen-containing compounds.
We hope that the obtained results may find application in the development of catalysts for the preparative hydrogenation of bromoquinolines in laboratory practice.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/surfaces9020034/s1, Figure S1: Pore size distribution curves for MoS2-1 and MoS2-2, calculated from N2 desorption isotherms by BJH method; Figure S2: The XPS of MoS2-1 (a,c) and MoS2-2 (b,d) samples; Figure S3: The XPS regions of Na 1s (a) and Cl 2p (b) in MoS2-1 and MoS2-2 samples; Table S1: The surface concentrations of elements (at.%) for MoS2-1 and MoS2-2; Table S2: Fitting parameters for deconvolution XPS core levels C1s, O1s, Mo3d, S2p; Table S3: The yields of bromo-1,2,3,4-tetrahydroquinoline (the respective isomer), conversion of the starting compound and selectivity to bromo-1,2,3,4-tetrahydroquinoline in the processes of 5-, 6-, 7-, 8-bromoquinolines hydrogenation catalyzed by MoS2-1, MoS2-2 after 8 months storage.

Author Contributions

A.V.T.—investigation, writing—original draft preparation, writing—review and editing; A.S.K.—investigation, writing—original draft preparation; M.V.O.—investigation; P.S.Y.—investigation; A.M.Z.—investigation; V.V.B.—investigation, methodology; S.V.K.—conceptualization, writing—original draft preparation, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank to the National Academy of sciences of Ukraine and Enamine Ltd. (Ukraine) for support.

Conflicts of Interest

Authors Anastasia V. Terebilenko, Volodymyr V. Buryanov and Sergey V. Kolotilov were employed by Enamine Ltd. The authors declare that this study received funding from Enamine Ltd. The funder had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
a.u.arbitrary units
DmesoDiameter of mesopores, calculated from N2 desorption isotherms using BJH method
GCMSGas chromatography with mass spectral control
NMRNuclear magnetic resonance
SBETSpecific surface, calculated using the model of Brunauer–Emmett–Teller from N2 adsorption isotherm
SEMScanning electron microscopy
SPSpecific performance of the catalyst
TEMTransmission electron microscopy
THQ1,2,3,4-tetrahydroquinoline
XPSX-ray photoelectron spectroscopy
XRDX-ray diffraction

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Figure 1. SEM images of MoS2-1 (a–c) and MoS2-2 (d–f).
Figure 1. SEM images of MoS2-1 (a–c) and MoS2-2 (d–f).
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Figure 2. TEM images of MoS2-1 (a) and MoS2-2 (b).
Figure 2. TEM images of MoS2-1 (a) and MoS2-2 (b).
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Figure 3. Powder XRD patterns of MoS2-2 and MoS2-1. Reference XRD pattern was built using the data reported in [27].
Figure 3. Powder XRD patterns of MoS2-2 and MoS2-1. Reference XRD pattern was built using the data reported in [27].
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Figure 4. Nitrogen adsorption/desorption isotherms of MoS2-1 (a) and MoS2-2 (b).
Figure 4. Nitrogen adsorption/desorption isotherms of MoS2-1 (a) and MoS2-2 (b).
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Figure 5. Raman spectra of MoS2-1 and MoS2-2 along with schematic presentation of the vibrations of MoS2 lattice, corresponding to the peaks. Schematic image under the peaks illustrate movement of atoms, associated with occurrence of the peak: grey circles—Mo, yellow circles—S, the arrows show the direction of atoms movement.
Figure 5. Raman spectra of MoS2-1 and MoS2-2 along with schematic presentation of the vibrations of MoS2 lattice, corresponding to the peaks. Schematic image under the peaks illustrate movement of atoms, associated with occurrence of the peak: grey circles—Mo, yellow circles—S, the arrows show the direction of atoms movement.
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Figure 6. The survey XPS of MoS2-1 and MoS2-2 samples.
Figure 6. The survey XPS of MoS2-1 and MoS2-2 samples.
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Figure 7. The XPS of MoS2-1 (a,c) and MoS2-2 (b,d) samples.
Figure 7. The XPS of MoS2-1 (a,c) and MoS2-2 (b,d) samples.
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Table 1. Hydrogenation yields of 1,2,3,4-tetrahydroquinoline, specific performance and specific average reaction rates of hydrogenation in presence of MoS2-1, MoS2-2 and previously reported MoS2 samples, obtained using hydrothermal synthesis.
Table 1. Hydrogenation yields of 1,2,3,4-tetrahydroquinoline, specific performance and specific average reaction rates of hydrogenation in presence of MoS2-1, MoS2-2 and previously reported MoS2 samples, obtained using hydrothermal synthesis.
Sample (1)Specific Surface Area SBET (m2/g)THQ Yield (%)Specific Performance (μmol/m2)Ref.
MoS2-147.753694this work
MoS2-257.662673this work
MoS13032.237718[23]
MoS13524.438973[23]
MoS14016.1421630[23]
MoS15013.514648[23]
MoSS13028.1661468[25]
MoSS14016.2672585[25]
MoSS15015.6753005[25]
Bulk MoS2<1<5 [25]
Notes: (1) Samples MoS130–MoS150, MoSS130–MoSS150 were prepared using hydrothermal synthesis; the digit in the sample code indicates the temperature of the synthesis (for example, MoS130 was prepared at 130 °C), while the second “S” indicates that the reaction mixture contained a two-fold increased quantity of sulfur source (thiourea): Mo/S 1:31.5 vs. Mo/S ratio 1:15.75.
Table 2. The yields of bromo-1,2,3,4-tetrahydroquinoline (the respective isomer), conversion of the starting compound and selectivity to bromo-1,2,3,4-tetrahydroquinoline in the processes of 5-, 6-, 7-, 8-bromoquinolines hydrogenation catalyzed by MoS2-1, MoS2-2 and previously hydrothermally obtained MoS2.
Table 2. The yields of bromo-1,2,3,4-tetrahydroquinoline (the respective isomer), conversion of the starting compound and selectivity to bromo-1,2,3,4-tetrahydroquinoline in the processes of 5-, 6-, 7-, 8-bromoquinolines hydrogenation catalyzed by MoS2-1, MoS2-2 and previously hydrothermally obtained MoS2.
SubstrateProductCatalystYield, %Conversion, %Selectivity, %By-Products (Yield, %)
Surfaces 09 00034 i001Surfaces 09 00034 i002MoS2-18597885-Bromo-N-Methyl-THQ (12)
MoS2-2100100100–
MoS130 (1)99100995-Bromo-N-Methyl-THQ (1)
MoSS140 (2)96100965-Bromo-N-Methyl-THQ (4)
Surfaces 09 00034 i003Surfaces 09 00034 i004MoS2-12133636-Bromo-N-Methyl-THQ (12)
MoS2-23210032 (3)6-Bromo-N-Methyl-THQ (14)
THQ (45)
N-Methyl-THQ (9)
MoS130 (1)239924 (3)6-Bromo-N-MethylTHQ (15)
THQ (44)
N-Methyl-THQ (7)
MoSS140 (2)1520756-Bromo-N-MethylTHQ (5)
Surfaces 09 00034 i005Surfaces 09 00034 i006MoS2-101000 (3)7-Bromo-N-MethylTHQ (98)
THQ (2)
MoS2-201000 (3)7-Bromo-N-MethylTHQ (100)
MoS130 (1)9797100–
MoSS140 (2)4040100-
Surfaces 09 00034 i007Surfaces 09 00034 i008MoS2-193100938-Bromo-N-MethylTHQ (7)
MoS2-2100100100-
MoS130 (1)9497978-Bromo-1-N-MethylTHQ (2)
THQ (1)
MoSS140 (2)4257748-Bromo-N-MethylTHQ (15)
Notes: (1) Data from Ref. [23]; (2) Data from Ref. [25]; (3) According to the definition of selectivity adopted in this study, this value reflects the ratio of the quantity of the target compound—bromo-substituted 1,2,3,4-tetrahydroquinoline—to the total quantity of the reaction products (×100%), even in the case where this product is not a dominating one.
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Terebilenko, A.V.; Kondratyuk, A.S.; Olenchuk, M.V.; Yaremov, P.S.; Zhuchenko, A.M.; Buryanov, V.V.; Kolotilov, S.V. Catalytic Properties of Mechanochemically Exfoliated MoS2 in the Hydrogenation of Bromoquinolines. Surfaces 2026, 9, 34. https://doi.org/10.3390/surfaces9020034

AMA Style

Terebilenko AV, Kondratyuk AS, Olenchuk MV, Yaremov PS, Zhuchenko AM, Buryanov VV, Kolotilov SV. Catalytic Properties of Mechanochemically Exfoliated MoS2 in the Hydrogenation of Bromoquinolines. Surfaces. 2026; 9(2):34. https://doi.org/10.3390/surfaces9020034

Chicago/Turabian Style

Terebilenko, Anastasia V., Andrii S. Kondratyuk, Maryna V. Olenchuk, Pavlo S. Yaremov, Andrii M. Zhuchenko, Volodymyr V. Buryanov, and Sergey V. Kolotilov. 2026. "Catalytic Properties of Mechanochemically Exfoliated MoS2 in the Hydrogenation of Bromoquinolines" Surfaces 9, no. 2: 34. https://doi.org/10.3390/surfaces9020034

APA Style

Terebilenko, A. V., Kondratyuk, A. S., Olenchuk, M. V., Yaremov, P. S., Zhuchenko, A. M., Buryanov, V. V., & Kolotilov, S. V. (2026). Catalytic Properties of Mechanochemically Exfoliated MoS2 in the Hydrogenation of Bromoquinolines. Surfaces, 9(2), 34. https://doi.org/10.3390/surfaces9020034

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