4.1. Zeolites

Acid–base properties of different zeolites including HZSM-5, faujasite, SAPO-18, and AlPO-18 were studied in the reaction between methanol and hydrogen sulfide [45, 47, 51, 52]. The results showed that acidic zeolites tend to form DMS and hydrocarbons, where reducing the acidity of zeolites increases the selectivity toward methanethiol. Comparing HZSM-5 with HNaY, NaX, and NaY shows that the activity drastically diminishes with decrease in acidity and increase in alkalinity in the following order:

More alkali metals impregnated in faujasite zeolites were investigated by Ziolek et al. [45]. Both methanethiol and DMS can form on acidic zeolites, where methanol may convert to a hydrocarbon on them. The basic property of zeolites strongly affects the yield of formation of methanethiol (Table 2).

The acidity of zeolites decreases in the following order:

A reaction mechanism on zeolites was proposed according to Scheme 2. Methanol can be adsorbed on both Brønsted basic site and a pair of Lewis acid–base site in two different pathways. The presence of strong Brønsted acidic sites can convert the methanethiol and dimethyl ether (DME) products to DMS and hydrocarbon.

SAPO-18 and AlPO-18 were studied for the production of dimethyl sulfide and methanethiol from methanol and H₂S. The number of acid sites decreases in the following order:

4.2. Metal oxides

The activity and selectivity of some metal oxides including SiO₂, BeO₂, MgO, ZrO₂, ZnO, TiO₂, γ-Al₂O₃, η-Al₂O₃, WO₃, and V₂O₃ in the reaction between methanol and H₂S showed that WO₃ and V₂O₃ have the highest activity and selectivity to methanethiol due to their high specific surface area. They were followed by γ-Al₂O₃, η-Al₂O₃, and TiO₂ with one lower magnitude of activity. The remaining metal oxides with activity thrice less than that of WO₃ and V₂O₃ came third. The activity of metal oxides increases with decrease in their cation’s ionization and electron acceptance potential as shown in Table 3 [18, 53].

Al₂O₃ is the main pure metal oxide utilized for methanol thiolation reaction at all times. It has the highest activity among other pure metal oxides (Table 4). However, its weak basic sites cause reactions to form dimethyl sulfide with high selectivity. MgO shows minimal activity (about 3% as methanol conversion) but 100% selectivity for methanethiol due to very strong basic sites. According to the data from Table 4, ZrO₂ with a yield of 69.1% for methanethiol formation at a H₂S to CH₃OH ratio of 2 is located at the top. The selectivity for dimethyl sulfide was observed to be inversely proportional to the number of basic sites. Metal oxides with very strong Lewis acid (and moderately basic) sites were reported to be suitable for the synthesis of dimethyl sulfide [17].

4.3. Supported catalysts

The principal research regarding the effect of acid–base properties of supported catalysts on the methanol thiolation reaction has been conducted during the past 30 years. The promotion of different supports has been tested including γ-Al₂O₃, SiO₂, ZrO₂, Nb₂O₅, and TiO₂ by WO₃, alkali metals and hydroxide or carbonate of alkali metals [1, 2, 4, 5, 39, 43, 44, 46, 47].

Adding WO₃ to γ-Al₂O₃ reduces the number of Lewis acid sites and basic sites, but this increases the production rate of methanethiol and lowers the production rate of dimethyl sulfide [47]. The addition of K₂WO₄ to γ-Al₂O₃ was found to cause more reduction in the number of Lewis acid sites and basic sites. This resulted in a near-zero production yield for dimethyl sulfide, enhancing the selectivity for methanethiol. Such behavior has also been observed for SiO₂ and AlSi (Table 5). It was concluded that acidic catalysts have higher activities, but their production rate for methanethiol and dimethyl sulfide is 50–50. Catalysts with strong Lewis acid sites have a higher tendency to produce dimethyl sulfide, and for the selective production of methanethiol, it is necessary to use catalysts with strong basic sites on the surface [39].

In a similar study, some γ-Al₂O₃ based catalysts were promoted by K₂WO₄, K₂CO₃, KOH, and NaOH. K₂WO₄ led to the formation of relatively weaker Lewis acid sites and moderate basic sites, but the other three catalysts had strong Lewis acid sites and basic sites. As shown in Table 6, K₂WO₄ created more selectivity for methanethiol, but it exhibited low activity. For example, at 360 ^°C, the selectivity values for methanethiol were 96%, 95%, 92%, and 90%, but the methanol conversion values were 47%, 56%, 55%, and 53% for K₂WO₄, K₂CO₃, KOH, and NaOH, respectively. The other three catalysts, on the other hand, were more active and less selective. It was concluded that strong acidic and basic sites are more active but less selective for producing methanethiol [43].

The use of metals as promoters for methanol thiolation reaction has been taken into consideration for the past 20 years. The addition of Na to γ-Al₂O₃ and ZrO₂ enhances methanethiol selectivity, but it reduces catalyst activity (Tables 7 and 8). Na was loaded on γ-Al₂O₃ from 0.25% up to 4.5%. Therefore, the selectivity increased from 28% to 100%, and at the same time, methanol conversion decreased from 91% to 11%. The addition of Mo to ZrO₂ enhances the activity, yet this reduces the selectivity for methanethiol (Table 9). Moreover, Mo increases the acidity of the catalyst [44].

The addition of alkali metals to niobia leads to the formation of acid–base pairs, thereby increasing the selectivity of methanethiol and the basic sites on alumina and silica. When a support is impregnated with alkali metals, the Brønsted centers are destroyed [19], where the strength of Lewis acid sites decreases together with increase in the basic property [54]. The selectivity for methanethiol strongly depends on the nature of active sites. Table 10 reports higher methanethiol production by modified niobia, which can be attributed to the presence of acid–base pairs on its surface [46].

Although catalysts have been available for the methanol thiolation process since about 1910, the research in this area is still ongoing. Vast and comprehensive studies have been conducted on methanol thiolation since 2017 in Technische Universität München (TUM) and Institute for Integrated Catalysis by impregnating alkali metals (K, Rb, and Cs) on supports of γ-Al₂O₃ and TiO₂ [1, 2, 4, 5]. They claim that tuning the acid–base properties of the catalyst by adjusting alkali metal properties is an approach to preparing novel catalysts.

Alkali metals have two important roles in the final catalyst: lowering the acidic strength and creating suitable sites for the adsorption of reactants (H₂S and CH₃OH). Cs has lower electronegativity than those of Rb and K, which increases the electron density on the neighboring anions and enhances their Lewis basic strength [55, 56, 57]. Adding alkali cations to gamma-alumina blocks Lewis acid sites and prevents the formation of DME. Indeed, very strong Lewis acid sites are substituted with the weaker ones by adding alkali cations to gamma-alumina, which makes the neighboring sulfur oxyanions stronger Lewis basic sites through increasing their electric charge. As such, some acid–base pairs are formed, which help in dissociative adsorption of H₂S. H₂S is dissociated into H⁺ and SH^- as illustrated in Figure 1. Furthermore, SH^- generates nucleophilic attacks on the methoxy species, which leads to the formation of methanethiol [1].

The size and loading of cations on gamma-alumina affect the rate of formation of methanethiol. At lower loadings, the rate of methanethiol formation in Cs⁺ is less than that of Rb⁺ and K⁺ due to better dispersion of smaller cations (Rb and K). However, at higher cation loadings, Cs⁺ provides better results in the formation of methanethiol due to higher electronegativity [1] as shown in Table 11.

Due to the good performance of Cs-promoted catalysts, another attempt was made to find how the acid–base properties of the catalyst can affect the reaction route. The acid–base properties of the final catalysts can be manipulated by altering the Cs content. Catalysts (WS₂/Al₂O₃) without Cs produced DME and methanethiol, but Cs-containing catalysts (Cs–WS₂/Al₂O₃) produced methanethiol at a higher yield (Figure 2).

The addition of Cs reduces both the specific area and the acidity of the catalyst. The decrease in acidity is greater than the area reduction. The addition of Cs⁺ strongly improves the nature of the catalyst and reduces the strength and concentration of Lewis acid sites. In fact, very strong Lewis acid cation sites (Al₃⁺ and W₄⁺) are replaced with the weaker strength Lewis acid sites of Cs⁺ [2].

Cs⁺ induces very strong basic sites, which increases catalytic activity. The very strong basic sites, which associate with neighboring weak Lewis acid site alkali cations, improve the performance of catalysts. The incorporation of Cs⁺ lowers the amount of adsorbed methanol but increases the rate of reaction between SH groups and the adsorbed methanol. Cs⁺ plays a key role in directing methanol conversion to methanethiol selectively. WS₂ does not have any role in the methanol thiolation reaction [4].

Cs/Al₂O₃ and CsW/Al₂O₃ reveal the same reaction rates. The binding of Cs⁺ and WS₂ is strong and makes the catalyst more stable. Two transition metal oxides, TiO₂ and ZrO₂, were examined for comparison. They were impregnated by Cs⁺ but without the addition of WS₂. TiO₂ and ZrO₂ provide Lewis acid–base pairs, while gamma-alumina has a combination of weak and strong Lewis acid sites. The highest methanethiol initial rates were observed with TiO₂. The results indicated that a methanol thiolation catalyst can be developed without WS₂. The main parameter in the design of catalysts is the existence of acid–base pairs, which provide suitable sites for the dissociative adsorption of H₂S and CH₃OH, where SH generates a nucleophilic attack on the methoxy species. Strong basic anions play the main role in the absence of strong Lewis acid sites [5].