Synthesis of Methyl Mercaptan on Mesoporous Alumina Prepared with Hydroxysafﬂor Yellow A as Template: The Synergistic Effect of Potassium and Molybdenum

: K-promoted Mo-based catalysts showed great promise for the hydrogenation of CS 2 to methyl mercaptan (CH 3 SH). However, the research on the synergistic effect of K and Mo, and the active site of CS 2 hydrogenation to CH 3 SH were unexplored widely. To solve this problem, the synergistic effect of K and Mo in the K-promoted Mo-based catalysts for CS 2 hydrogenation to prepare CH 3 SH was investigated. The mesoporous alumina was the support and loaded the active components potassium and molybdenum to prepare the catalyst. The results suggested that the active components K and Mo can not only cooperatively regulate the acid-base sites on the catalyst surface, but also stabilize the molybdate species at +5 valence during the reduction process and increase the Mo unsaturated coordination sites. Combined with the results of the catalytic activity evaluation, indicating that the main active site of the catalysts is the weak Lewis acid-base site, and the strong acidic site and strong alkaline site are not conducive to the formation of CH 3 SH. Moreover, the possible catalytic mechanism of CS 2 hydrogenation to CH 3 SH on the weak Lewis acid-base sites of the catalysts was proposed. The research results of this paper can provide an experimental basis and theoretical guidance for the design of high-performance CH 3 SH synthesis catalyst and further mechanism research.


Introduction
Methanethiol, also known as methyl mercaptan (CH 3 SH), is one of the raw materials for the industrial production of methionine. At present, methyl mercaptan is mainly synthesized by methanol thiolation (CH 3 OH-H 2 S method) in industry [1][2][3][4][5][6], which has some disadvantages, such as high reaction temperature (340~400 • C), many byproducts, and serious three wastes. More and more researchers are mainly focused on finding a more economical and environmentally friendly synthesis routes in recent years. Among them, the catalytic synthesis of CH 3 SH from sulfur-containing synthesis gas has aroused great interest among researchers [7][8][9][10][11][12][13][14], but the conversion and yield of CH 3 SH are relatively low, which does not meet the requirements of large-scale industrialization. CS 2 is relatively surplus and the price is low at present, and the one-step synthesis of CH 3 SH by CS 2 hydrogenation has the advantages of low reaction temperature (260~280 • C) and fewer The relationship between the selectivity of the products and the reaction temperature over Al, Mo/Al, K/Al, and K-Mo/Al catalysts is shown in Figure 1. Since the conversion of CS2 is 100% in the whole reaction temperature range, it is not shown in the figure. It can be seen from Figure 1 that the Al2O3 (Al) itself is a highly active catalyst for the hydrogenation of CS2. For Al catalyst, the selectivity of CH3SH, CH3SCH3 and CH4 was 31.5%, 67.5% and 1% at 260 °C, respectively. When the active component Mo is introduced, the changing trend of the selectivity of the products is similar to that of Al catalyst with the increase of reaction temperature, except that the selectivity of CH3SCH3 decreases slightly, while the selectivity of CH3SH increases slightly. As far as K/Al catalyst is concerned, it can be concluded that the addition of potassium alone can greatly reduce the selectivity of CH3SCH3, while increasing the selectivity of CH3SH, that is, the addition of potassium promotes the disproportionation of CH3SH to CH3SCH3, which is especially significant at low temperature (T < 300 °C). When potassium and molybdenum were introduced into the support at the same time, the selectivity of CH3SH was greatly improved over K-Mo/Al catalyst, and the selectivity of CH3SH reached 91.2%, the selectivity of CH4 was only 0.1% at 260 °C; even at 300 °C , the selectivity of CH3SH was 85.8%, and the selectivity of CH4 was 1.5%; which shows that the disproportionation and further hydrogenation process of methyl mercaptan are inhibited with the addition of potassium and molybdenum. In addition, for all catalysts, the selectivity of CH4 increases with the increase of reaction temperature, which is consistent with the reports in the literature [15,16]. For example, the selectivity of CH4 on K-Mo/Al catalysts is 0.1%, 1.5%, 9.5%, and 25.2% at 260, 300, 340 and 380 °C, respectively, which increased about 16.8 times from 300 to 380 °C ( Figure S1). To sum up, we can conclude: when the active component potassium or molybdenum is introduced alone, it is not conducive to the formation of CH3SH, and when potassium and molybdenum are added at the same time, potassium and molybdenum can play a synergistic catalytic role and jointly promote the formation of CH3SH.

Textural Properties
The fundamental physical and chemical properties of Al, Mo/Al, K/Al and K-Mo/Al catalysts, including BET specific surface area, pore volume and average pore size, are shown in Figure 2. It can be seen from the figure that the specific surface area and pore volume of alumina decrease rapidly with the addition of potassium and molybdenum. For instance, the specific surface area and pore volume of alumina (Al) are 327 m 2 ·g −1 and 0.41 cm 3 ·g −1 respectively, while that of the K-Mo/Al catalyst are 196 m 2 ·g −1 and 0.31 cm 3 ·g −1 respectively, which may be largely due to the increase of catalyst density after the addition of potassium and molybdenum, causing the surface area per unit mass is reduced. At the same time, it was found that the average pore size increased with the addition of potassium and molybdenum, which were 2.55, 2.98, 3.64 and 2.99 nm respectively on Al, Mo/Al, K/Al and K-Mo/Al catalysts, which may be due to the formation of some large particle species with the introduction of potassium and molybdenum during the preparation of the catalyst, resulting in some small pores being blocked.
The fundamental physical and chemical properties of Al, Mo/Al, K/Al and K-Mo/Al catalysts, including BET specific surface area, pore volume and average pore size, are shown in Figure 2. It can be seen from the figure that the specific surface area and pore volume of alumina decrease rapidly with the addition of potassium and molybdenum. For instance, the specific surface area and pore volume of alumina (Al) are 327 m 2 ·g −1 and 0.41 cm 3 ·g −1 respectively, while that of the K-Mo/Al catalyst are 196 m 2 ·g −1 and 0.31 cm 3 ·g −1 respectively, which may be largely due to the increase of catalyst density after the addition of potassium and molybdenum, causing the surface area per unit mass is reduced. At the same time, it was found that the average pore size increased with the addition of potassium and molybdenum, which were 2.55, 2.98, 3.64 and 2.99 nm respectively on Al, Mo/Al, K/Al and K-Mo/Al catalysts, which may be due to the formation of some large particle species with the introduction of potassium and molybdenum during the preparation of the catalyst, resulting in some small pores being blocked.

Crystalline Phase and Morphology
The crystalline phases of Al, Mo/Al, K/Al and K-Mo/Al catalysts before and after reaction were characterized by X-ray diffraction, the XRD patterns are shown in Figure 3. Because the crystalline phase of pure alumina (Al) does not change before and after the reaction, the spectra of the spent Al are not listed in the diagram.  (Figure 4b). The results of SEM showed that new species were formed on the surface of the K/Al catalyst. However, only the diffraction peak belonging to K2SO4 [JCPDS File Number: 00-0030608] was detected on the spent catalyst. When potassium and molybdenum were introduced at the same time, no diffraction peaks belonging to potassium and molybdenum species were detected on the fresh K-Mo/Al catalyst, indicating that they were amorphous or highly dispersed on the surface of γ-Al2O3, which was consistent with the results of the corresponding EDS mapping images (Figure 4e-h). While on the spent K-Mo/Al catalyst, diffraction peaks

Crystalline Phase and Morphology
The crystalline phases of Al, Mo/Al, K/Al and K-Mo/Al catalysts before and after reaction were characterized by X-ray diffraction, the XRD patterns are shown in Figure 3. Because the crystalline phase of pure alumina (Al) does not change before and after the reaction, the spectra of the spent Al are not listed in the diagram.  (Figure 4b). The results of SEM showed that new species were formed on the surface of the K/Al catalyst. However, only the diffraction peak belonging to K 2 SO 4 [JCPDS File Number: 00-0030608] was detected on the spent catalyst. When potassium and molybdenum were introduced at the same time, no diffraction peaks belonging to potassium and molybdenum species were detected on the fresh K-Mo/Al catalyst, indicating that they were amorphous or highly dispersed on the surface of γ-Al 2 O 3 , which was consistent with the results of the corresponding EDS mapping images (Figure 4e-h). While on the spent K-Mo/Al catalyst, diffraction peaks belonging to MoS 2 and K 2 SO 4 were detected at the same time. According to the report of references [16,18], the active phase MoS 2 phase is formed during the pre-sulfurization process, while the K 2 SO 4 phase is formed through the intermediates K 2 MoS 4 and K 2 S in the activity evaluation process. The sulfided K 2 MoS 4 is first transformed into K 2 S, and then the irreversible reaction between K 2 S and oxygen-containing species leads to the accumulation of the K 2 SO 4 phase. Whereas, we did not detect the diffraction peaks of K 2 MoS 4 and K 2 S species in the XRD diffraction pattern, indicating that K 2 MoS 4 and K 2 S species are highly dispersed on the alumina support. In addition, we did not perform a pre-sulfurization process for the catalysts before evaluating the activity of the catalysts, indicating the catalysts can be sulfided by using sulfur-containing raw materials and product H 2 S in the reaction process. Finally, on the spent Mo/Al catalyst, not only the diffraction peak of MoS 2 species was detected, but also the diffraction peak belonging to MoO 2 species was detected, demonstrating molybdenum oxide had not been completely sulfided at this time.
references [16,18], the active phase MoS2 phase is formed during the pre-sulfurization process, while the K2SO4 phase is formed through the intermediates K2MoS4 and K2S in the activity evaluation process. The sulfided K2MoS4 is first transformed into K2S, and then the irreversible reaction between K2S and oxygen-containing species leads to the accumulation of the K2SO4 phase. Whereas, we did not detect the diffraction peaks of K2MoS4 and K2S species in the XRD diffraction pattern, indicating that K2MoS4 and K2S species are highly dispersed on the alumina support. In addition, we did not perform a pre-sulfurization process for the catalysts before evaluating the activity of the catalysts, indicating the catalysts can be sulfided by using sulfur-containing raw materials and product H2S in the reaction process. Finally, on the spent Mo/Al catalyst, not only the diffraction peak of MoS2 species was detected, but also the diffraction peak belonging to MoO2 species was detected, demonstrating molybdenum oxide had not been completely sulfided at this time.   the irreversible reaction between K2S and oxygen-containing species leads to th accumulation of the K2SO4 phase. Whereas, we did not detect the diffraction peaks o K2MoS4 and K2S species in the XRD diffraction pattern, indicating that K2MoS4 and K2S species are highly dispersed on the alumina support. In addition, we did not perform a pre-sulfurization process for the catalysts before evaluating the activity of the catalysts indicating the catalysts can be sulfided by using sulfur-containing raw materials and product H2S in the reaction process. Finally, on the spent Mo/Al catalyst, not only th diffraction peak of MoS2 species was detected, but also the diffraction peak belonging to MoO2 species was detected, demonstrating molybdenum oxide had not been completely sulfided at this time.

H 2 -TPR Studies
The H 2 -TPR characterization of the fresh catalysts is shown in Figure 5. It can be seen that, in terms of Al 2 O 3 , there is a weak low-temperature peak at 202 • C and a wide overlapping peak in the range of 250~600 • C which is mainly attributed to the reduction process of Al species. When the active component is loaded, the low-temperature peak shifts to low temperature, while the wide overlapping peak shifts to a higher temperature, indicating that the interaction between the active component and the Al 2 O 3 support becomes stronger after loading the active component and a new H 2 consumption peak appears at 633 • C on the Mo-containing catalysts, which is attributed to the reduction of Mo species. For the attribution of the reduction peak, it is generally believed that Mo species exist in two states: one is a highly dispersed state (monolayer dispersed structure), and the other is aggregated state (double layer or multilayer structure). Monolayer dispersed Mo-O species are easier to be reduced than double-multilayer dispersed Mo-O species. Brito et al. [45] observed two kinds of reduction peaks on MoO 3 /Al 2 O 3 catalysts named Mo I (455-500 • C) and Mo II (780-860 • C). The former belongs to the partial reduction of Mo species with high dispersion, while the latter belongs to the deep reduction of this species and the reduction of Mo species which are more difficult to reduce (such as tetrahedral coordination Mo species). Feng et al. [46] found that the low-temperature reduction peak can be attributed to the reduction process of +6 to +4 valence of octahedral coordination Mo(Oh) species, and the high-temperature reduction peak is the reduction process of +6 to +4 valence of tetrahedral coordination Mo(Td) species. It should be pointed out that the distribution of Mo species on the support is often affected by preparation methods, such as the properties of the support, molybdenum precursors, calcination conditions, and other factors. When the temperature is less than 800 • C, the supported molybdenum-oxygen catalyst can't be reduced to the metal state [46,47]. It is generally believed that the molybdenum oxygen species on the support mainly exist in the form of tetrahedral coordination and octahedral coordination, and the molybdenum oxygen species with tetrahedral coordination structures are difficult to be reduced [48]. To sum up, for the Mo/Al and K-Mo/Al catalysts in this paper, the H 2 consumption peak in the range of 250~500 • C can be classified as the Mo (VI)→Mo (IV) reduction process of octahedral molybdenum oxygen species, and at 550~800 • C, it can be attributed to the Mo(VI)→Mo(IV) reduction process of tetrahedral molybdenum species. With the addition of alkali auxiliaries, the surface Mo-O species structure has changed, so that the surface Mo-O structure is more transformed into a tetrahedral structure, and the octahedral coordinated Mo-O species decreases. For the H 2 consumption peak of K-Mo/Al catalyst at low temperature (250~500 • C), the two splitting peaks may be attributed to the Mo (VI) →Mo (V)→Mo (IV) reduction process of octahedral molybdenum species, indicating the addition of alkali can stabilize Mo-O species at +5 valence during the reduction process, which is consistent with the results reported in reference [49,50]. The effect of alkali is considered to delay the reduction of Mo. Chen et al. [51] believe that the addition of alkali strengthens the Mo-O bond and leads to the increase of reduction activation energy.

Surface Acid-Base Properties
The acid-base sites on the surface of the catalysts are often determined by CO2/NH3-TPD. From the CO2-TPD spectrum (Figure 6a), it can be seen that all the catalysts have a CO2 desorption peak attributed to the weak base center at about 100 °C . There are only weak basic sites on Al, Mo/Al and K-Mo/Al catalysts, and the quantities of weak base sites decrease greatly when the active component Mo is introduced alone, but when K and Mo are introduced at the same time, the quantities of weak base sites is slightly more than that of the Al catalyst. For the K/Al catalyst, due to the addition of strong basic components, a

Surface Acid-Base Properties
The acid-base sites on the surface of the catalysts are often determined by CO 2 /NH 3 -TPD. From the CO 2 -TPD spectrum (Figure 6a), it can be seen that all the catalysts have a CO 2 desorption peak attributed to the weak base center at about 100 • C. There are only weak basic sites on Al, Mo/Al and K-Mo/Al catalysts, and the quantities of weak base sites decrease greatly when the active component Mo is introduced alone, but when K and Mo are introduced at the same time, the quantities of weak base sites is slightly more than that of the Al catalyst. For the K/Al catalyst, due to the addition of strong basic components, a wide overlapping CO 2 desorption peak belonging to weak, medium and strong basic sites appeared. In addition, according to the NH 3 -TPD spectrum (Figure 6b), there are weak, medium and strong acid sites on the surface of the four catalysts, and their relative quantities are summarized in Table 1. Combining the activity evaluation result of the catalyst, it can be considered that strong acid sites and strong base sites are not conducive to the selectivity of methyl mercaptan.

Surface Acid-Base Properties
The acid-base sites on the surface of the catalysts are often determined by CO2/NH3-TPD. From the CO2-TPD spectrum (Figure 6a), it can be seen that all the catalysts have a CO2 desorption peak attributed to the weak base center at about 100 °C . There are only weak basic sites on Al, Mo/Al and K-Mo/Al catalysts, and the quantities of weak base sites decrease greatly when the active component Mo is introduced alone, but when K and Mo are introduced at the same time, the quantities of weak base sites is slightly more than that of the Al catalyst. For the K/Al catalyst, due to the addition of strong basic components, a wide overlapping CO2 desorption peak belonging to weak, medium and strong basic sites appeared. In addition, according to the NH3-TPD spectrum (Figure 6b), there are weak, medium and strong acid sites on the surface of the four catalysts, and their relative quantities are summarized in Table 1. Combining the activity evaluation result of the catalyst, it can be considered that strong acid sites and strong base sites are not conducive to the selectivity of methyl mercaptan.   The surface acidic sites of the catalysts can also be characterized by pyridine adsorption infrared spectroscopy. As shown in Figure 7, there are five characteristic peaks with different intensities at 1612, 1593, 1577, 1490 and 1444 cm −1 , respectively, which are attributed to the different adsorption forms of pyridine on alumina [52]. The absorption peaks at 1612 and 1593 cm −1 belong to the 8a ring vibration mode of the coordination bond between pyridine and Lewis acid site, which belongs to the strong Lewis acid site [4]. When the active component Mo was introduced, a new pyridine desorption peak belonging to the Brønsted acid site appeared at 1540 cm −1 , and the intensity of the pyridine desorption peak belonging to the Lewis acid site increased. When K is introduced, all the pyridine adsorption peaks shift to low wavenumber, which may be due to the strong interaction between K and the Al 2 O 3 support [4]. The quantities of Lewis acid sites are in the following order: Mo/Al > Al > K/Al > K-Mo/Al. In addition, the intensity of pyridine desorption peak on the surface of K/Al and K-Mo/Al catalysts decreased with the increase of desorption temperature, and pyridine was desorbed completely on K/Al and K-Mo/Al catalysts at 250 • C ( Figure S2), indicating that the Lewis acid on the surface of K/Al and K-Mo/Al catalysts is mainly a weak Lewis acid site. pyridine adsorption peaks shift to low wavenumber, which may be due to the strong interaction between K and the Al2O3 support [4]. The quantities of Lewis acid sites are in the following order: Mo/Al > Al > K/Al > K-Mo/Al. In addition, the intensity of pyridine desorption peak on the surface of K/Al and K-Mo/Al catalysts decreased with the increase of desorption temperature, and pyridine was desorbed completely on K/Al and K-Mo/Al catalysts at 250 °C ( Figure S2), indicating that the Lewis acid on the surface of K/Al and K-Mo/Al catalysts is mainly a weak Lewis acid site.

XPS Study of Catalysts
The XPS spectra of Mo/Al and K-Mo/Al were shown in Figure 8. The peaks of binding energy at ~229, ~231 and ~232.7 eV represent the +4, +5, and +6 valence states of Mo, respectively, and the acromion at 226.6 eV is the peak of S(2 s) [53,54]. Molybdenum species on fresh catalysts are present mainly in the form of Mo 6+ , and Mo 4+ appears after the reaction, which is consistent with the results of XRD experiments (Figure 3). MoS2

XPS Study of Catalysts
The XPS spectra of Mo/Al and K-Mo/Al were shown in Figure 8. The peaks of binding energy at~229,~231 and~232.7 eV represent the +4, +5, and +6 valence states of Mo, respectively, and the acromion at 226.6 eV is the peak of S(2 s) [53,54]. Molybdenum species on fresh catalysts are present mainly in the form of Mo 6+ , and Mo 4+ appears after the reaction, which is consistent with the results of XRD experiments (Figure 3). MoS 2 (Mo 4+ ) was detected in XRD experiments on both Mo/Al and K-Mo/Al catalysts after the reaction.  The fitting results of the Mo(3d) XPS spectra are shown in Figure 9, and the quantitative results are shown in Table 2. From the results of Table 2, it can be seen that the addition of potassium affects the valence equilibrium among Mo species, increasing the concentration of Mo 5+ species and decreasing the concentration of Mo 4+ species on the sulfided K-Mo/Al catalyst, reducing the reducibility of K-Mo/Al catalyst, which was consistent with the characterization results of H2-TPR. The research results [46,48,55] show that the Mo-O species mainly have octahedral and tetrahedral configurations, and tetrahedral configurations are more difficult to reduce and sulfurization than octahedral configurations. Due to the interaction between K and Mo, some octahedral species are transformed into tetrahedral species, so the addition of potassium reduces the reducibility of molybdenum species, resulting in that the Mo 5+ species being more stable on the K-Mo/Al catalyst, while the Mo 4+ species is more likely to be formed on the Mo/Al catalyst, The fitting results of the Mo(3d) XPS spectra are shown in Figure 9, and the quantitative results are shown in Table 2. From the results of Table 2, it can be seen that the addition of potassium affects the valence equilibrium among Mo species, increasing the concentration of Mo 5+ species and decreasing the concentration of Mo 4+ species on the sulfided K-Mo/Al catalyst, reducing the reducibility of K-Mo/Al catalyst, which was consistent with the characterization results of H 2 -TPR. The research results [46,48,55] show that the Mo-O species mainly have octahedral and tetrahedral configurations, and tetrahedral configurations are more difficult to reduce and sulfurization than octahedral configurations.
Due to the interaction between K and Mo, some octahedral species are transformed into tetrahedral species, so the addition of potassium reduces the reducibility of molybdenum species, resulting in that the Mo 5+ species being more stable on the K-Mo/Al catalyst, while the Mo 4+ species is more likely to be formed on the Mo/Al catalyst, which is consistent with the results reported by Kantschewa et al. [55] and Ozkan et al. [50]. Some scholars [51] believe that the addition of alkaline auxiliaries inhibits the reducibility of MoO 3 , and the existence of alkali enhances the bond energy of the Mo-O bond and increases the activation energy of reduction.   Figure 10 shows the S(2p) XPS spectrum of the sulfided catalyst, in which SH represents the sulfur species of high valence state and SL represents the sulfur species of low valence state. Low-valent sulfur species can be classified into elements S (164.0 eV), S 2− (162.0 eV), S2 2− (162.5 eV), oxy-sulfides (162.3~163.2 eV) and polysulfides (162.9~164.4 eV) [56]. The high valence sulfur species is SO4 2− (169.1 eV), confirmed by the XRD experimental results. The XRD experimental results detected the existence of K2 SO4 (Figure 3), caused by the oxidation of low-valent sulfur species by oxygen-containing species and other oxidants in the reaction system [16,18]. It can be seen from Figure 10 that the addition of K increases the quantities of the high-valence S 6+ species, while the low-valence sulfur species decreases ( Figure 10). For the MoS2-based catalyst, it has been proved that low-valent sulfur ions such as S 2− and S2 2− can activate hydrogen [57,58]. In the process of reduction and sulfurization of oxidized Mo (VI) species to Mo 5+ and Mo 4+ species in CS2/H2 atmosphere, the coordination number of Mo-S and Mo-Mo in the sulfided catalyst is significantly less than that of the oxidized catalyst, and the surface reconstruction occurs, leading to the increase of Mo unsaturated coordination sites on the catalysts surface [49,59], which is beneficial to the formation of methyl mercaptan.   Figure 10 shows the S(2p) XPS spectrum of the sulfided catalyst, in which S H represents the sulfur species of high valence state and S L represents the sulfur species of low valence state. Low-valent sulfur species can be classified into elements S (164.0 eV), S 2− (162.0 eV), S 2 2− (162.5 eV), oxy-sulfides (162.3~163.2 eV) and polysulfides (162.9~164.4 eV) [56]. The high valence sulfur species is SO 4 2− (169.1 eV), confirmed by the XRD experimental results. The XRD experimental results detected the existence of K 2 SO 4 (Figure 3), caused by the oxidation of low-valent sulfur species by oxygen-containing species and other oxidants in the reaction system [16,18]. It can be seen from Figure 10 that the addition of K increases the quantities of the high-valence S 6+ species, while the low-valence sulfur species decreases ( Figure 10). For the MoS 2 -based catalyst, it has been proved that low-valent sulfur ions such as S 2− and S 2 2− can activate hydrogen [57,58]. In the process of reduction and sulfurization of oxidized Mo (VI) species to Mo 5+ and Mo 4+ species in CS 2 /H 2 atmosphere, the coordination number of Mo-S and Mo-Mo in the sulfided catalyst is significantly less than that of the oxidized catalyst, and the surface reconstruction occurs, leading to the increase of Mo unsaturated coordination sites on the catalysts surface [49,59], which is beneficial to the formation of methyl mercaptan.

Proposed Mechanism on the Acid-Base Site
Combined with the activity test data of the catalysts and the aforementioned characterization results, illustrating the surface performance of the catalysts directly affects the distribution of the products. When alumina is treated with alkali, the Lewis acid site on the surface of the catalyst decreases rapidly, and the strong base site increases greatly, so the selectivity of CH3SH decreases significantly, and the selectivity of byproduct CH3SCH3 increases greatly. This is mainly due to the strong adsorption of CH3SH on the surface of the catalyst, and CH3SH cannot be separated from the catalyst surface in time, which leads to the further disproportionation of CH3SH to CH3SCH3. Comparing the activity results of Mo/Al and Al catalysts, the selectivity of CH3SH on Mo/Al catalyst is slightly higher than that of Al, because the introduction of Mo slightly increases the weak acidity site of the catalyst, which is beneficial to the formation of CH3SH. After the simultaneous introduction of K and Mo, alkaline K and acidic Mo have a synergistic effect, which jointly regulates the acid-base sites on the surface of the catalyst, thus affecting the adsorption performance of the products on the catalyst surface, and cooperatively catalyzes the hydrogenation of CS2 to CH3SH. XPS results show that the increase of Mo 5+ species concentration on the K-Mo/Al catalyst leads to the increase of Mo coordination unsaturated sites on the catalyst surface, which is beneficial to the activation of CS2 [15,16,18]. To sum up, as far as the synthesis of CH3SH by CS2 hydrogenation is concerned, we have concluded that the main active site of the catalysts is the weak Lewis acid-base site, and the strong acidic site and strong basic site are not conducive to the formation of CH3SH. In addition, it is generally believed that the active phase for the synthesis of CH3SH over K-promoted MoS2-based catalysts is the K-Mo-S phase [8,10,11,16], but in this paper, we did not pre-sulfurization the catalyst before the reaction, so it can be considered that apart from the K-Mo-S phase, the K-Mo-O phase may also be the main active phase.
The possible reaction mechanism for the hydrogenation of CS2 to CH3SH is presented in Scheme 1. Firstly, the molecule H2 is dissociated and adsorbed on the two adjacent basic centers, while the CS2 molecule is non-dissociated and adsorbed on the acid site (Step I). The S=C=S double bond is broken and interacts with the nearby dissociated adsorbed H to produce the intermediate fragment HSCSH (Step II). Then, another molecule H2 is dissociated and adsorbed on the alkaline center (Step III), after the C-S bond breaks and rearranges, the intermediate fragment HSCH is formed, and a molecule of H2S is released (Step IV). Finally, another molecule H2 is dissociated and adsorbed on the base center (Step V), and interacts with the intermediate fragment HSCH and rearranges to form CH3SH, detached from the surface of the catalyst (step VI). As a result, the catalyst returns to its initial state.

Proposed Mechanism on the Acid-Base Site
Combined with the activity test data of the catalysts and the aforementioned characterization results, illustrating the surface performance of the catalysts directly affects the distribution of the products. When alumina is treated with alkali, the Lewis acid site on the surface of the catalyst decreases rapidly, and the strong base site increases greatly, so the selectivity of CH 3 SH decreases significantly, and the selectivity of by-product CH 3 SCH 3 increases greatly. This is mainly due to the strong adsorption of CH 3 SH on the surface of the catalyst, and CH 3 SH cannot be separated from the catalyst surface in time, which leads to the further disproportionation of CH 3 SH to CH 3 SCH 3 . Comparing the activity results of Mo/Al and Al catalysts, the selectivity of CH 3 SH on Mo/Al catalyst is slightly higher than that of Al, because the introduction of Mo slightly increases the weak acidity site of the catalyst, which is beneficial to the formation of CH 3 SH. After the simultaneous introduction of K and Mo, alkaline K and acidic Mo have a synergistic effect, which jointly regulates the acid-base sites on the surface of the catalyst, thus affecting the adsorption performance of the products on the catalyst surface, and cooperatively catalyzes the hydrogenation of CS 2 to CH 3 SH. XPS results show that the increase of Mo 5+ species concentration on the K-Mo/Al catalyst leads to the increase of Mo coordination unsaturated sites on the catalyst surface, which is beneficial to the activation of CS 2 [15,16,18]. To sum up, as far as the synthesis of CH 3 SH by CS 2 hydrogenation is concerned, we have concluded that the main active site of the catalysts is the weak Lewis acid-base site, and the strong acidic site and strong basic site are not conducive to the formation of CH 3 SH. In addition, it is generally believed that the active phase for the synthesis of CH 3 SH over K-promoted MoS 2 -based catalysts is the K-Mo-S phase [8,10,11,16], but in this paper, we did not pre-sulfurization the catalyst before the reaction, so it can be considered that apart from the K-Mo-S phase, the K-Mo-O phase may also be the main active phase.
The possible reaction mechanism for the hydrogenation of CS 2 to CH 3 SH is presented in Scheme 1. Firstly, the molecule H 2 is dissociated and adsorbed on the two adjacent basic centers, while the CS 2 molecule is non-dissociated and adsorbed on the acid site (Step I). The S=C=S double bond is broken and interacts with the nearby dissociated adsorbed H to produce the intermediate fragment HSCSH (Step II). Then, another molecule H 2 is dissociated and adsorbed on the alkaline center (Step III), after the C-S bond breaks and rearranges, the intermediate fragment HSCH is formed, and a molecule of H 2 S is released (Step IV). Finally, another molecule H 2 is dissociated and adsorbed on the base center (Step V), and interacts with the intermediate fragment HSCH and rearranges to form CH 3 SH, detached from the surface of the catalyst (step VI). As a result, the catalyst returns to its initial state.

Experimental Section
For the experimental section, including the preparation, activity tests and characterization of the catalysts, please refer to the supporting information for details.

Conclusions
In the present study, to further understand the effect of active components potassium and molybdenum on the selectivity of CH3SH and how they cooperate in the synthesis of CH3SH, we chose Al2O3 as the support and supported active components potassium and molybdenum to prepare catalysts to study the one-step synthesis of CH3SH by CS2 hydrogenation. The introduction of alkali metal potassium alone will enhance the surface basicity of the catalyst and lead to the stronger adsorption of the product CH3SH, which is beneficial to the formation of the by-product CH3SCH3. After loading potassium and molybdenum at the same time, the basic potassium species will interact with the acidic molybdenum species to coordinate the acid-base sites on the surface of the catalyst to promote the formation of CH3SH. On the K-Mo/Al catalyst, the selectivity of CH3SH reached 91.2%. The characterization results show that due to the interaction between K and Mo, some octahedral species are transformed into tetrahedral species, so the reducibility of molybdenum species decreases with the addition of K, resulting in the Mo 5+ species being more stable on the K-Mo/Al catalyst, while the Mo 4+ species is more likely to be formed on the Mo/Al catalyst. In the process of reduction and sulfurization of oxidized Mo (VI) species to Mo 5+ and Mo 4+ species in the reaction atmosphere of CS2/H2, the coordination number of Mo-S and Mo-Mo in the sulfided catalyst decreased significantly compared with that of the oxidized catalyst, and the surface reconstruction occurred, which led to the increase of Mo unsaturated coordination sites on the catalyst surface, which was beneficial to the formation of CH3SH. In addition, the catalysts were not pre-sulfurization before the reaction, but the presence of MoS2 was detected in the catalysts after the reaction, indicating that the catalysts can use the reaction atmosphere for self-sulfurization during the reaction. It is considered that apart from the K-Mo-S phase, the K-Mo-O phase may also be the main active phase.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: The selectivities toward CH4, CH3SCH3 and CH3SH as a function of reaction temperature over the Al, Mo/Al, K/Al and K-Mo/Al catalysts. Reaction conditions: P = 0.3 MPa, R(CS2) = 1.6 mL/h, R(H2) = 30 mL/min, 2 mL of the Catalyst. Figure

Experimental Section
For the experimental section, including the preparation, activity tests and characterization of the catalysts, please refer to the Supporting Information for details.

Conclusions
In the present study, to further understand the effect of active components potassium and molybdenum on the selectivity of CH 3 SH and how they cooperate in the synthesis of CH 3 SH, we chose Al 2 O 3 as the support and supported active components potassium and molybdenum to prepare catalysts to study the one-step synthesis of CH 3 SH by CS 2 hydrogenation. The introduction of alkali metal potassium alone will enhance the surface basicity of the catalyst and lead to the stronger adsorption of the product CH 3 SH, which is beneficial to the formation of the by-product CH 3 SCH 3 . After loading potassium and molybdenum at the same time, the basic potassium species will interact with the acidic molybdenum species to coordinate the acid-base sites on the surface of the catalyst to promote the formation of CH 3 SH. On the K-Mo/Al catalyst, the selectivity of CH 3 SH reached 91.2%. The characterization results show that due to the interaction between K and Mo, some octahedral species are transformed into tetrahedral species, so the reducibility of molybdenum species decreases with the addition of K, resulting in the Mo 5+ species being more stable on the K-Mo/Al catalyst, while the Mo 4+ species is more likely to be formed on the Mo/Al catalyst. In the process of reduction and sulfurization of oxidized Mo (VI) species to Mo 5+ and Mo 4+ species in the reaction atmosphere of CS 2 /H 2 , the coordination number of Mo-S and Mo-Mo in the sulfided catalyst decreased significantly compared with that of the oxidized catalyst, and the surface reconstruction occurred, which led to the increase of Mo unsaturated coordination sites on the catalyst surface, which was beneficial to the formation of CH 3 SH. In addition, the catalysts were not pre-sulfurization before the reaction, but the presence of MoS 2 was detected in the catalysts after the reaction, indicating that the catalysts can use the reaction atmosphere for self-sulfurization during the reaction. It is considered that apart from the K-Mo-S phase, the K-Mo-O phase may also be the main active phase.