Process for preparing a compound of formula RSH by hydrogen sulfide

Abstract

The present invention relates to a process for preparing a compound of the formula RSH by the gas-phase catalytic reaction of hydrogen sulfide with a compound of the formula ROH in the presence of a solid catalyst, wherein R represents an alkyl group; in this process, the reaction is carried out in the presence of a catalyst comprising or consisting of: one or more pure or mixed rare earth oxides, one or more pure or mixed rare earth sulfides, or one or more pure or mixed rare earth oxysulfides, provided that when the rare earth is lanthanum, the catalyst is a mixed oxide of lanthanum and at least one metal selected from rare earth or non-rare earth metals, and when the rare earth is cerium, the catalyst is supported on alumina.

Description

[0001] The present invention relates to a process for the gas-phase catalytic thiolation of alcohols to the corresponding thiols. More particularly, the production of methanethiol from methanol and hydrogen sulfide is described, but the scope is not limited thereto.

[0002] The thiolation reaction for the conversion of methanol to methanethiol via a gas-phase catalytic route is known. Generally, this is carried out in the presence of a catalyst based on tungsten oxide and alkali metal oxide supported on alumina, as described, for example, in patent WO2013092129A1.

[0003] However, there are some studies in the literature regarding catalysts. Thus, the work of Plaisance and Dooley (Catalyst Letters, 2009, 128, 449-458) describes the use of catalysts based on different metal oxides, such as tungsten oxide (WO 3 ) supported on various solids, lanthanum oxide (La 2 O 3 ) or titanium oxide (TiO 2 ). Among all these metal oxides, tungsten oxide is the most active one. These works highlight the fact that the activity of each tested catalyst depends on different parameters and it is difficult to determine the optimal conditions common to several catalysts. In addition, the use of a catalyst based on lanthanum oxide supported on alumina (La 2 O 3 / Al 2 O 3 ) gives a high methanol conversion, but this is accompanied by a very low selectivity to methanethiol, making this catalyst unsuitable for industrial-scale applications.

[0004] In addition, Ziolek et al. (Journal of Molecular Catalysis A: Chemical, 1997, 97, 49-55) described the influence of the adsorption of hydrogen sulfide during the thiolation reaction of methanol in the presence of different catalysts based on magnesium oxide (MgO), titanium oxide (TiO 2 ), zirconium oxide (ZrO 2 ), cerium oxide (CeO 2 ) and alumina (Al 2 O 3 ). In particular, the authors have observed the highest adsorption of hydrogen sulfide on cerium oxide and correlated this phenomenon with an increase in the selectivity to methanethiol and dimethyl sulfide. However, at an average methanol conversion, this catalyst produces a large amount of methane at the expense of the selectivity to methanethiol, making methane the main product of the reaction under certain conditions.

[0005] The results show that, so far, there is still a need to develop the conditions for the catalytic reaction of methanol with hydrogen sulfide in the gas phase at the levels of catalysts and process parameters, which are characterized by high methanol conversion and high methyl mercaptan selectivity, and the reduction of the production of non-recoverable by-products such as light compounds.

[0006] The present invention provides a method that meets these requirements.

[0007] This method allows the preparation of a compound of the formula RSH by the gas-phase catalytic reaction of hydrogen sulfide with a compound of the formula ROH in the presence of a solid catalyst, where R represents an alkyl group, and the catalyst comprises or consists of: one or more pure or mixed rare earth oxides, one or more pure or mixed rare earth sulfides, or one or more pure or mixed rare earth oxysulfides, provided that when the rare earth is lanthanum, the catalyst is a mixed oxide of lanthanum and at least one metal selected from rare earths, and when the rare earth is cerium, the catalyst is supported.

[0008] It has been observed that, using such a catalyst, the selectivity has been significantly improved compared to the catalysts described in the prior art, and in particular, the reduction of non-recoverable products such as carbon monoxide, carbon dioxide, methane, hydrogen, and dimethyl ether can reach at least 40%. Therefore, the main product is methyl mercaptan, and dimethyl sulfide constitutes the main by-product. Dimethyl sulfide can be cleverly recycled and converted into methyl mercaptan, thereby increasing the overall yield of the methyl mercaptan process. In addition, due to this method, the reaction can be carried out in a wider temperature range that is generally lower than the known methods. More generally, this method has the advantage of flexibility, and the selectivity can be directed to any product depending on operating conditions such as temperature.

[0009] The method of the present invention is described in more detail below, and its features can be considered individually or in combination, regardless of the combination, and preferred implementation variants are provided.

[0010] Before this more detailed description, some terms used herein are defined.

[0011] For rare earths, it should be understood as the 15 lanthanide elements (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium) as well as scandium and yttrium.

[0012] According to the present invention, the catalyst can exist in the form of an oxide (or oxide-hydroxide), a sulfide, or any intermediate containing S and O called an oxysulfide.

[0013] In the formula defining the compounds obtained or involved, the term "alkyl" refers to a straight-chain or branched monovalent hydrocarbon radical having from 1 to 20 carbon atoms, advantageously from 1 to 6 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, n-hexyl or a cyclic monovalent hydrocarbon radical having from 3 to 20 carbon atoms, advantageously from 3 to 6 carbon atoms, such as cyclopropyl, cyclohexyl, but is not limited to these groups.

[0014] According to the method of the invention, the catalyst is selected from rare earth oxides, hydroxides or oxide-hydroxides of rare earths and at least one non-rare earth metal. The catalyst may also be selected from mixed oxides (hydroxides or oxide-hydroxides) of rare earths and at least one non-rare earth metal. For the mixed oxides, it is to be understood oxides based on one or more rare earths. The catalyst may also be selected from mixed sulfides of various rare earths, mixed sulfides of one or more rare earths and at least one non-rare earth metal, oxysulfides of various rare earths, mixed oxysulfides of one or more rare earths and at least one non-rare earth metal, and mixtures of said mixed oxides, mixed sulfides and mixed oxysulfides. Preferably, the non-rare earth metal is zirconium.

[0015] In addition to the above-mentioned oxides, sulfides and / or oxysulfides, the catalyst may also comprise one or more oxides of metals other than rare earths.

[0016] In a variant of the invention, the catalyst is supported, advantageously on pretreated or untreated alumina, which allows to overcome the clogging problems that may be observed in the presence of materials based on powdered rare earths.

[0017] As mentioned above, the method of the invention is particularly interesting for the catalytic thiolation of methanol to prepare methanethiol, but it can be applied to obtain any compound RSH, where R is an alkyl as defined above.

[0018] Among the suitable catalysts according to the invention, some combinations are selected for their effectiveness. Thus, advantageously, the catalyst comprises or consists of a mixed oxide of lanthanum, cerium, neodymium and zirconium; in such a combination, the proportion of zirconium oxide and cerium oxide is preponderant compared to the proportion of lanthanum oxide and neodymium oxide.

[0019] According to the present invention, a support allowing to improve the performance of the catalyst used has also been developed. The support may be alumina modified with potassium, having a potassium content of from 0.1% to 20% (m / m), preferably from 0.5% to 10%, more preferably from 0.5% to 5%. In a variant of the present invention, the use of cerium oxide supported on alumina modified in this way has been reported, the supported cerium oxide content ranging from 0.1% to 50% (m / m), preferably from 0.5% to 30% (m / m). This allows a significant increase in the methanethiol production of some catalysts used according to the present invention (in particular cerium oxide which has a high methane selectivity in the pure state), said high methane selectivity being a drawback in the production of methanethiol on an industrial scale.

[0020] In the process according to the present invention, the ratio of hydrogen sulfide to the compound ROH ranges from 0.5 to 20, preferably from 1 to 15, more preferably from 1 to 10.

[0021] It has been previously pointed out that one of the advantages of the present invention is the ability to widen the range of reaction temperatures. Thus, it can be carried out at a temperature of from 200 °C to 450 °C, preferably from 250 °C to 420 °C, more preferably from 275 °C to 400 °C, advantageously at a pressure of from 2 to 20 bar, preferably from 5 to 15 bar, more preferably from 7 to 14 bar, and the contact time of the compound ROH with the catalyst is from 0.1 second to 60 seconds.

[0022] Although the reaction is highly selective for methanethiol, dimethyl sulfide can also be formed. The catalytic conversion reaction of said dimethyl sulfide to methanethiol can then be further carried out according to techniques well known to those skilled in the art to further increase the yield of the methanethiol process.

[0023] The present invention and its advantages are illustrated in the examples below.

[0024] Example 1: According to the present invention, methanethiol is produced from methanol in the presence of a catalyst based on a mixed oxide

[0025] Preparation of catalyst Cat1 with a composition of LaCeNdZr(2 / 21.3 / 5.1 / 71.6):

[0026] The catalyst has been prepared by a soft chemical synthesis route. For example, it can be obtained according to the methods described in patent FR2907445A1 or patent FR2859470A1. The specific surface area of the catalyst is 75 m 2 .g -1 . The oxide composition in weight percentages is: 2.0% of La 2 O 3 , 21.3% of CeO 2 , 5.1% of Nd 2 O 3 and 71.6% of ZrO 2 .

[0027] Production of methanethiol by thiolation of methanol in the presence of the above catalyst

[0028] The catalytic bed of 2 ml of catalyst diluted in silicon carbide with a particle size distribution of 0.400 to 0.500 nm was placed in a reactor with an inner diameter of 1.26 cm. The inlet gas of the reactor consisted of a mixture of methanol and hydrogen sulfide.

[0029] Different operating conditions were tested and are described below:

[0030] 1) H 2 S / MeOH molar ratio = 0.5 / Temperature = 330 °C / Contact time = 10 s

[0031] 2) H 2 S / MeOH molar ratio = 4 / Temperature = 375 °C / Contact time = 20 s

[0032] 3) H 2 S / MeOH molar ratio = 1.7 / Temperature = 400 °C / Contact time = 4 s

[0033] The pressure in the reactor was 10 bar.

[0034] To compare the performance of the method of the present invention with that of the prior art, the same reaction was carried out under the same conditions in the presence of a catalyst composed of cerium oxide (specific surface area 99 m 2 .g -1 ) similar to that described in the prior art.

[0035] The results are reported in Table 1 below:

[0036] [Table 1]

[0037]

[0038] *CO, CO 2 and CH 4

[0039] It was observed that, regardless of the conditions, the method of the present invention had both a higher methanol conversion rate and a higher methanethiol selectivity, while producing very little light gas to favor a higher dimethyl sulfide selectivity. These results highlight the performance of the catalyst used in the method according to the present invention compared to cerium oxide. They further demonstrate that, in order to achieve the optimal methanethiol yield, it is advantageous for the method to include a supplementary step in which the dimethyl sulfide produced is converted to methanethiol in the presence of a catalyst known to those skilled in the art, such as alumina.

[0040] Example 2: According to the present invention, methanethiol is produced from methanol in the presence of a catalyst based on a mixed oxide

[0041] Preparation of catalyst Cat2 with a composition of LaCeNdZr(1.75 / 30.3 / 5.35 / 62.6):

[0042] Catalyst Cat2 was prepared according to the same technique as catalyst Cat1 described above. The specific surface area of this catalyst is 59 m 2 .g -1 . The oxide composition in weight percentage is: 1.75% La 2 O 3 , 30.3% CeO 2 , 5.35% Nd 2 O 3 and 62.6% ZrO 2 .

[0043] Production of methanethiol by thiolation of methanol in the presence of the above catalyst

[0044] The catalytic performance of this catalyst was determined in the same experimental system as in Example 1, with an H 2 S / MeOH molar ratio = 1.7, a temperature of 375 °C, and a pressure in the reactor of 10 bar.

[0045] By varying the mass of the catalyst introduced and the flow rates of the different reactants, the performance of the catalyst was compared over a wide range of methanol conversions.

[0046] The results are reported in Table 2 below:

[0047] [Table 2]

[0048]

[0049] *CO, CO 2 and CH 4

[0050] Example 3: According to the present invention, in the presence of a catalyst based on cerium oxide supported on modified alumina, methanethiol is produced from methanol

[0051] Preparation of catalyst CatS3 supported on potassium-modified alumina

[0052] A supported catalyst CatS3 was synthesized by sequentially impregnating and calcining (at 450 °C in air) 100 g of commercial alumina with a specific surface area of 171 m 2 .g -1 with a potassium hydroxide solution (38 g / L) and a cerium(III) nitrate solution (1151 g / L). The potassium content is 1.5 wt%, and the cerium oxide content is 3.5 wt%. The specific surface area of this catalyst is 167 m 2 .g -1 .

[0053] Production of methanethiol by thiolation of methanol in the presence of the above catalyst

[0054] The catalytic performance of these catalysts was determined under the same experimental conditions as in Example 2.

[0055] To compare the performance of the method of the present invention with that of the prior art methods, under the same conditions, the same reaction was carried out in the presence of pure alumina (the same as the alumina used as the support for CatS3 and the alumina used as the support for the catalyst (KS) composed of potassium and supported on alumina), and the catalyst was synthesized according to the method described for CatS3. By varying the mass of the catalyst introduced and the flow rates of the different reactants, the performance of the catalyst was compared over a wide range of methanol conversion rates.

[0056] The results are reported in Table 3 below:

[0057] [Table 3]

[0058]

[0059] *CO, CO 2 and CH 4

[0060] These results demonstrate the effectiveness of cerium oxide supported on modified alumina (especially alumina pre-impregnated with potassium).

Claims

1. A method for preparing a compound of formula RSH by gas-phase catalytic reaction of hydrogen sulfide with a compound of formula ROH in the presence of a solid catalyst, wherein R represents an alkyl group, characterized in that, the reaction is carried out in the presence of a catalyst, the catalyst being pure cerium oxide, the pure cerium oxide being supported on alumina impregnated with potassium, wherein the cerium oxide is supported on the alumina impregnated with potassium in the range of 0.1% to 50% (m / m), and the content of potassium is 0.1% to 20% (m / m).

2. The preparation method according to claim 1, characterized in that the compound of formula RSH is methanethiol, and it is prepared by catalytic thiolation of methanol with hydrogen sulfide in the gas phase.

3. The preparation method according to claim 1, characterized in that the catalyst is supported on alumina impregnated with potassium in the range of 0.5% to 30% (m / m), and the content of potassium is 0.1% to 20% (m / m).

4. The preparation method according to claim 1, characterized in that the catalyst is supported on alumina impregnated with potassium, wherein the content of potassium is 0.5% to 10% (m / m).

5. The preparation method according to claim 1, characterized in that the molar ratio of hydrogen sulfide to the compound ROH is 0.5 to 20.

6. The preparation method according to claim 5, characterized in that the molar ratio of hydrogen sulfide to the compound ROH is 1 to 15.

7. The preparation method according to claim 6, characterized in that the molar ratio of hydrogen sulfide to the compound ROH is 1 to 10.

8. The preparation method according to claim 1, characterized in that the reaction is carried out at a temperature of 200 °C to 450 °C, at a pressure of 2 to 20 bar, and the contact time of the compound ROH with the catalyst is 0.1 second to 60 seconds.

9. The preparation method according to claim 8, characterized in that the reaction is carried out at a temperature of 250 °C to 420 °C, at a pressure of 5 to 15 bar, and the contact time of the compound ROH with the catalyst is 0.1 second to 60 seconds.

10. The preparation method according to claim 8, characterized in that the reaction is carried out at a temperature of 275 °C to 400 °C, at a pressure of 7 to 14 bar, and the contact time of the compound ROH with the catalyst is 0.1 second to 60 seconds.

11. The preparation method according to claim 1, characterized in that the reaction produces dimethyl sulfide, and a catalytic conversion reaction of dimethyl sulfide to methanethiol is further carried out.

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