JP7302933B2 - Method for producing methanation catalyst - Google Patents

Description

The present invention relates to the production of molybdenum sulfide catalysts.

Demand for natural gas is increasing in line with global economic and population growth, but natural gas production may not be able to keep up with this increased demand. Therefore, there is a need to produce alternative natural gas to meet the increased energy demand. A possible method for producing alternative natural gas is the gasification of coal to produce syngas as a feedstock for methanation (ie, methanogenesis). At the current state of the art, methanation catalysts are transition metals such as nickel (Ni) that have high methanation activity. Nevertheless, these catalysts have operational limitations due to their vulnerability to poisoning by sulfur compounds present in the synthesis gas obtained from coal gasification.

Molybdenum sulfide, _MoS2 , is one of the few methanation catalysts for carbon monoxide that is unaffected by sulfur impurities. It is widely used for mixed alcohol synthesis from hydrodesulfurization, hydrodenitrogenation, and CO hydrogenation. Sulfur-tolerant _MoS2 catalyst can overcome the disadvantage of sulfur poisoning, thus allowing the raw material from coal gasification to be used for direct methanation without sulfur removal method.

The basal and edge surfaces of MoS ₂ have very different physical and chemical properties. The surface energy for the edge is approximately 100 times that of the basal plane (001). The edges have therefore been viewed as active surfaces for chemical reactions. Therefore, to enable alternative natural gas production as the energy market grows, produce more active _MoS2 catalysts with more exposed edges that can achieve high activity for syngas methanation. It is desirable to develop a method to

An effective supported _MoS2 catalyst should be well dispersed on the support while maintaining the desired morphology. To improve the dispersion of the catalyst on the substrate, it is desirable to depolymerize the precursor, while chemical interaction is required to maintain the desired morphology by preserving the self-assembly process during depolymerization. Autoreduction on the support due to the strength of action should be suppressed.

The present invention provides a method of achieving this.

A first aspect of the present invention is a method of making a molybdenum sulfide catalyst that catalyzes a methanation reaction , comprising heating a precursor in the presence of a reducing agent, the precursor being molybdate or thiomolybdenum acid salt and the precursor is a molybdate, the reaction is carried out at a temperature of 400-500° C. in a reducing atmosphere in the presence of elemental sulfur ,
When the precursor is a thiomolybdate, the reaction can be carried out in (a) in the absence of solvent and in the presence of elemental sulfur or (b) in the absence of elemental sulfur and in the presence of solvent. Either provides a process that is carried out at a temperature of 180-220°C .

The following options may be used with the first aspect individually or in any suitable combination.

The salt may be an ammonium salt. Thus, the molybdate may be ammonium molybdate and the thiomolybdate may be ammonium thiomolybdate. The ammonium molybdate may be ammonium heptamolybdate tetrahydrate (AHM). The ammonium thiomolybdate may be ammonium tetrathiomolybdate (ATM).

Heating may be performed in the presence of a carrier. In this case, the method may produce a supported molybdenum sulfide catalyst. The support may be selected from the group consisting of alumina, magnesium oxide, silica, aluminum oxide/magnesium oxide, hydrotalcite, activated carbon, carbon nanotubes, mixtures of any two or more of these, or other Any carrier may be used.

The reaction may be carried out in the absence of solvent and in the presence of elemental sulfur. In this case, the reaction may comprise heating the precursor and elemental sulfur in a reducing atmosphere. Sulfur may be able to decompose the polymolybdate or polythiomolybdate that may be used as a precursor or formed in situ from the precursor. The method may then include the step of pulverizing the precursor and elemental sulfur together prior to the step of heating. The reducing atmosphere may comprise hydrogen gas. Hydrogen gas may be mixed with an inert gas such as, for example, nitrogen. Heating may be performed to a temperature of about 400 to about 500°C. The reaction may be conducted at a suitable ambient pressure, ie, about 1 atmosphere. The reaction may be carried out at a pressure of about 1 to about 2 bar, or a pressure of 1-1.5 bar, 1.5-2 bar, or 1.2-1.7 bar. well, for example about 1 bar, 1.1 bar, 1.2 bar, 1.3 bar, 1.4 bar, 1.5 bar, 1.6 bar, 1.7 bar, 1.8 bar, 1 bar It may be done at a pressure of 9 bar, or 2 bar.

When the precursor is a thiomolybdate (optionally a polythiomolybdate), the reaction can be in the absence of a liquid solvent and the precursor can be or be formed in situ from the precursor. It may be carried out in the presence of a compound capable of decomposing polythiomolybdate. Suitable examples of such compounds include, for example, water or some other common solvent, generally having a dielectric constant greater than about 30. These are described elsewhere in this specification. In such cases, the reaction may comprise heating the precursor and a compound capable of decomposing the intermediate in a reducing atmosphere. The reducing atmosphere may comprise hydrogen gas and may also comprise vapors of compounds capable of decomposing intermediates. Hydrogen gas may be mixed with an inert gas such as nitrogen. Heating may be performed to a temperature of about 400 to about 500°C. This option may be performed in the absence of elemental sulfur.

The catalyst thus produced may then be exposed to a mixture of oxygen and an inert gas after heating as described above. In this case, the oxygen concentration in the inert gas may be less than about 2% v/v.

The method may be performed in a solvent. In such cases, the reducing agent may be, for example, a borohydride such as sodium borohydride. According to this option, the precursor may be ATM. The method may be performed in the absence of elemental sulfur. The solvent may have a dielectric constant greater than 30. Solvents can be, for example, water, dimethylformamide, or mixtures thereof. Heating may be performed at a temperature of about 180-220°C. The method may be performed in a sealed container. The method may be performed in a sealed container. The pressure in this case may be the pressure naturally generated by heating the solvent in a sealed container. The pressure may be, for example, from about 2 to about 20 bar, or from about 2-10 bar, 2-5 bar, 5-20 bar, 10-20 bar, 10-15 bar, or 15-20 bar. for example about 2 bar, 3 bar, 4 bar, 5 bar, 6 bar, 7 bar, 8 bar, 9 bar, 10 bar, 11 bar, 12 bar, 13 bar, 14 bar, 15 bar , 16 bar, 17 bar, 18 bar, 19 bar or 20 bar.

In one embodiment, a method of making a molybdenum sulfide catalyst is provided comprising heating a precursor in the presence of a reducing agent and elemental sulfur, wherein the precursor is molybdate.

In another embodiment, a method of making a molybdenum sulfide catalyst is provided comprising heating a precursor in the presence of a reducing agent, wherein the precursor is thiomolybdate. In this embodiment, heating may be performed in the presence of sulfur or in the absence of sulfur.

In another embodiment, a method of making a molybdenum sulfide catalyst comprising: molybdic acid in the presence of elemental sulfur and in the absence of a solvent at a temperature of about 400 to about 500° C. in a reducing atmosphere comprising hydrogen; A method comprising heating salt is provided.

In another embodiment, a method of making a molybdenum sulfide catalyst comprising: thiomolybdic acid in the presence of elemental sulfur and in the absence of a solvent at a temperature of about 400-500° C. in a reducing atmosphere comprising hydrogen; A method comprising heating salt is provided.

In another embodiment, a method of making a molybdenum sulfide catalyst is provided comprising heating thiomolybdate in a solvent in the presence of a borohydride reducing agent at a temperature of from about 180 to about 220°C. do.

In a further embodiment, a method of making a molybdenum sulfide catalyst comprising decomposing, for example, a polymeric thiomolybdate into monomers in a reducing atmosphere comprising hydrogen at a temperature of from about 400 to about 500°C. heating the thiomolybdate in the presence of a substance such as water vapor, optionally in the absence of elemental sulfur, in the absence of a liquid solvent. The pressure in this case may be about 1 atmosphere and may be from about 1 to about 2 bar.

A second aspect of the invention provides a molybdenum sulfide catalyst producible or made by a method according to the first aspect.

The following options may be used with the second aspect individually or in any suitable combination.

The catalyst may comprise nano-sized molybdenum sulfide. The catalyst may comprise molybdenum sulfide particles, the molybdenum sulfide particles each having at least one dimension and optionally two or three dimensions ranging from about 1 to about 999 nm. The molybdenum sulfide may be in the form of molybdenum sulfide flour.

Molybdenum sulfide may be of high purity. The molybdenum sulfide may be at least 90%, optionally about 100% molybdenum disulfide, ie, at least 90%, optionally about 100% pure.

The catalyst may comprise molybdenum disulfide on a support. The support may be selected from the group consisting of alumina, magnesium oxide, silica, aluminum oxide/magnesium oxide, hydrotalcite, activated carbon, carbon nanotubes, mixtures of any two or more thereof. The carrier may be hydrotalcite. The ratio of molybdenum sulfide to support may be about 35:65 on a w/w basis.

A third aspect of the invention provides a method of producing methanation comprising exposing a catalyst according to the second aspect to a gas mixture comprising carbon monoxide and hydrogen.

The following options may be used with the third aspect individually or in any suitable combination.

The method may be performed at a temperature of about 500 to about 600°C.

The mixed gas may further comprise sulfur containing gases such as, for example, hydrogen sulfide. In such cases, the catalyst may not be poisoned by sulfur containing gases such as, for example, hydrogen sulfide.

The invention will be better understood with reference to the following description in view of the non-limiting examples and accompanying drawings.

FIG. 1 shows (A) a sample prepared by decomposition of ATM in inert gas (N ₂ ), (B) a sample prepared by decomposition of ATM in N ₂ with elemental sulfur, and (C ) shows scanning electron microscopy (SEM) images of a sample prepared by decomposition of ATM in reducing gas ( _H2 ) and (D) a sample prepared by decomposition of ATM in _H2 with elemental sulfur. ing.

FIG. 2 shows CO conversion and methane of MoS ₂ catalyst unsupported or supported on hydrotalcite from SASOL (PURAL® MG30, labeled Al—Mg30), SiO ₂ , and Al ₂ O ₃ . (CH ₄ ) selectivity. The selectivities of all catalysts reach equilibrium at steady state and the difference in performance is reflected only in the conversion under these reaction conditions (SV=5000 h ⁻¹ , CO:H ₂ =1:1, reaction temperature 550° C.). be.

FIG. 3 shows (A) unsupported MoS 2 prepared by mixing ATM with sulfur (labeled ATM+S method) _{at low magnification, (B) unsupported MoS 2 prepared} by ATM+S method at high magnification, (C ) 35% MoS ₂ /Al-Mg30 prepared by the ATM+S method, (D) 35% MoS ₂ /Al ₂ O ₃ prepared by the ATM+S method, and (E) 35% MoS ₂ /SiO ₂ prepared by the ATM+S method. Scanning electron microscope (SEM) images.

Figure 4 shows the CO conversion of unsupported or supported _MoS2 catalysts prepared using different Mo precursors. MoS ₂ catalysts prepared by mixing ATM and sulfur are labeled (ATM+S), MoS ₂ catalysts prepared by mixing ammonium molybdate tetrahydrate (AHM) are labeled (AHM+S) . _CH4 selectivity is omitted for clarity, as all catalysts reached equilibrium at steady state.

FIG. 5 shows SEM images of (A) 35% MoS ₂ (AHM+S)/Al-Mg30 and (B) 35% MoS ₂ (ATM+S)/Al-Mg30.

FIG. 6 depicts the CO conversion of MoS ₂ catalysts with different loadings from 25 to 100% (ie unsupported) prepared by the ATM+S method.

FIG. 7 shows (A) 25% MoS ₂ (ATM+S)/Al-Mg30, (B) 35% MoS ₂ (ATM+S)/Al-Mg30, and (C) 50% MoS ₂ (ATM+S)/Al-Mg30. SEM image of.

Figure 8 depicts the CO conversion of the _MoS2 catalyst prepared by the hydrothermal method. MoS ₂ catalysts were supported with Al ₂ O ₃ , SiO ₂ , hydrotalcite (Al-Mg30), activated carbon (Act-C), and carbon nanotubes (CNT). All catalysts were prepared using dimethylformamide as solvent except 35% MoS ₂ (H ₂ O)/Al-Mg30-hydro which was prepared using deionized water as solvent.

FIG. 9 shows (A) SEM images of 35% MoS ₂ (H ₂ O)/Al-Mg30-hydro and (B) SEM images of the same catalyst at higher magnification.

The methods of preparation of the catalysts described herein involve the formation of precursor (or polymerized from precursor) monomers, optionally on a support, so as to preserve the self-assembly of nanostructured _MOS2 . and its autoreduction. This method yielded effective MoS ₂ catalysts demonstrated in the examples provided herein. For example, nano-sized molybdenum disulfide (MoS ₂ ) catalysts supported on hydrotalcite were prepared through a method that favored the self-assembly process during synthesis. Specific examples have been demonstrated through chemical vapor deposition and hydrothermal methods.

The precursors for the reactions described herein are molybdates or thiomolybdates. These terms should be considered to include the polymeric forms of these salts, ie, polymolybdates or polythiomolybdates, unless otherwise indicated.

Thus, we provide a method for synthesizing nano-sized hydrotalcite-bearing MoS ₂ catalysts by allowing self-assembly methods during synthesis. One embodiment of this method begins by mixing ammonium tetrathiomolybdate, a carrier, and a large amount of elemental sulfur in a mortar. The reaction of the mixture is carried out in a reduced environment at elevated temperature in a tubular furnace. Another method is carried out by mixing ammonium tetrathiomolybdate, carrier, and reducing agent in a solvent. The mixture is then transferred to a Teflon-lined autoclave and heated to elevated temperature. Self-assembly methods are facilitated by control of precursors, degradation environment, and support materials.

Without being bound by theory, it is believed that the molybdate or thiomolybdate precursor is first thermally or hydrothermally converted to the polymeric form (i.e., polymolybdate or polythiomolybdate, respectively). ). This is then decomposed into monomers, ultimately leading to the nano-sized molybdenum sulfide catalysts described herein. It is understood that if the precursor is initially in polymeric form (i.e. molybdenum is polybdate and thiomolybdic acid is polythiomolybdic acid), no conversion step to polymeric form occurs. will be done.

One embodiment of the present invention includes a method of producing nano-sized _MoS2 catalysts. The process begins by mechanically mixing ammonium tetrathiomolybdate (ATM) and elemental sulfur in a mortar or ball mill. This mixture is transferred to a tube furnace. Reaction of the mixture is carried out in a tubular furnace at elevated temperature. In this embodiment, the reaction occurs in a sulfur-rich, reducing environment to allow precursor decomposition and self-assembly into layered _MoS2 nanostructures.

In one embodiment, a support material suitable for nano-sized _MoS2 catalysts is used. The support serves to offload the _MoS2 used for methanation while maintaining the nano-sized layered morphology and activity of the catalyst. For example, a suitable carrier material can be a basic carrier. This could improve the depolymerization properties of intermediate polymolybdates or polythiomolybdates and alleviate the autoreduction on the support that inhibits the decomposition and self-assembly of layered dispersed nano-sized _MoS2 catalysts. It is considered to be a thing.

In other embodiments, the supported _MoS2 catalyst can be prepared under hydrothermal conditions. The process begins by mixing ammonium tetrathiomolybdate, carrier, and reducing agent in a suitable solvent, after which the mixture is transferred to a Teflon-lined autoclave and heated at 200° C. for 10 hours. . This provides a low cost way of preparing _MoS2 catalysts at lower temperatures. A suitable solvent helps reduce polymerization of the precursor, improves the dispersibility of MoS ₂ on the support, and prevents strong chemical interactions between the precursor and support leading to autoreduction. Decomposition of the precursor from decomposition to intermediate polythiomolybdate or polymolybdate monomers may be achieved by selecting a suitable solvent in which the precursor can be effectively dispersed. It is believed that sulfur can act as an agent to effect the decomposition of polymolybdate or thiomolybdate intermediates into monomers. However, the decomposing agent into monomers may be replaced by a solvent such as, for example, water. In solvent-free systems, suitable vapors, such as, for example, water vapor, may play a similar role. Thus, for example, low concentrations (eg, about 0.5 to about 5% w/w or v/v, or about 0.5-2%, 1-2%, 1-3 %, 3-4%, or 2-4%, for example about 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%). The gas may be static within a sealed chamber, or may flow within a vessel or reaction chamber where the reaction takes place.

Nano-sized _MoS2 catalysts show significant improvement over conventional catalysts due to their unique morphology. By using a suitable support material, the nanomorphology and its high activity can be maintained despite the lower loading of _MoS2 . CO conversions obtained using such catalysts reached 69% when using hydrothermal synthesis catalysts and with catalysts produced in the chemical vapor process in direct syngas methanation. more than 80% in some cases.

Raw syngas, which normally contains equivalent molar concentrations of carbon monoxide (CO) and hydrogen (H ₂ ), can be directly methanated by nano-sized MoS ₂ catalysts. Therefore, the H ₂ /CO ratio of the feed does not need to be adjusted to higher ratios using the water gas shift reaction as required with Ni catalysts, which will help reduce operating costs. Additionally, the sulfur tolerant properties of the _MoS2 catalyst also help reduce the capital cost of acid gas removal units.

Accordingly, the method of the present invention relates to the production of molybdenum sulfide catalysts. In this method, a molybdenum-containing precursor is heated in the presence of a reducing agent. If the precursor does not contain sulfur, such as the molybdate precursor, it is necessary to include a sulfur containing reagent. A convenient way to achieve this is to carry out the reaction in the presence of elemental sulfur. However, if the precursor contains sulfur, for example in the case of a thiomolybdate precursor, the presence of additional sulfur-containing reagents is not necessary, although this may optionally be used. In one embodiment, this embodiment involves producing a molybdenum sulfide catalyst by heating either molybdate or thiomolybdate in the presence of elemental sulfur and another reducing agent. . The reducing agent may be, for example, hydrogen gas or a borohydride salt, depending on the details of the experiment.

Convenient precursors are therefore molybdates or thiomolybdates. These may conveniently be ammonium salts, but alternatively may be any other salt such as an alkali metal or alkaline earth metal. Particular examples include sodium, potassium, magnesium, calcium, or rubidium salts. These salts may be anhydrous or in the form of hydrates. As mentioned above, the molybdate or thiomolybdate may be in polymeric form. These treatments may be carried out under conditions similar to those used for the monomeric molybdate and thiomolybdate, respectively. In some cases, higher pressures and/or higher temperatures may be used. These conditions are discussed elsewhere in this specification.

The reaction may be carried out in the presence of a support, since it is usually convenient to support the catalyst as the product. This results in a supported catalyst. Suitable carriers include metal oxides, mixed metal oxides, hydrated forms thereof, eg silica, magnesia, dolomite, diatomaceous earth, hydrotalcite. Other suitable supports include activated carbon, carbon nanotubes, graphite, graphene, and other forms of carbon.

The reducing agent may be a solid reducing agent. The reducing agent may be a borohydride, for example sodium borohydride. The reducing agent may be used in solution, eg, an aqueous solution.

The reaction may conveniently be carried out in the absence of solvent. This avoids the need for solvents that are costly, hazardous (eg, flammable and/or toxic), and can pose disposal or recycling issues. To accomplish this, the precursor may simply comprise heating the precursor and elemental sulfur in a reducing atmosphere. The reducing atmosphere may comprise, for example, hydrogen. The hydrogen may be pure hydrogen or it may be in combination with a second gas that does not react with hydrogen under the reaction conditions. Suitable gas mixtures include nitrogen, argon, helium, neon, or mixtures of two of these. Hydrogen (or any other reducing gas or steam) is about 1 to about 100%, or about 10-100%, 50-100%, 80-100%, 1-50%, 1-20% by mass , 1-10%, 10-50%, 20-80%, 50-80%, such as about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 60%, 70%, 80%, 90%, or 100% reducing atmosphere may exist in Prior to heating, the precursor and sulfur may be well mixed. A way to achieve this is simply to grind the two together. The sulfur to molybdate ratio is about 2:1 to 10:1, or about 2:1 to 5:1, 2:1 to 3:1, 2.5:1 to 10:1, 3 on a molar basis. : 1 to 10:1, 5:1 to 10:1, 2.5:1 to 5:1, or 2.1:1 to 2.5:1, for example about 2:1, 2.1 : 1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1 , 6:1, 7:1, 8:1, 9:1, or 10:1. This ratio should be at least 2:1, but may be substantially higher, such as at least 5:1, at least 10:1, at least 20:1 or more. Therefore, sulfur may be in excess of molybdenum by at least a factor of ₂ to increase the yield of MoS2.

Heating may be at a temperature of about 350 to about 550°C, or about 350-500°C, 350-450°C, 400-550°C, 450-450°C, 400-500°C, 400-450°C, or at 450-500°C, for example at about 350°C, 400°C, 450°C, 500°C, or 550°C. The reaction may be run for a time sufficient to achieve at least 90% conversion to _MoS2 . Sufficient time may be about 1-10 hours, or about 1-5 hours, 1-2 hours, 2-10 hours, 5-10 hours, or 2-5 hours, such as , about 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours. It will be appreciated that the lower the temperature, the longer the reaction will take.

After formation of the catalyst, it may be cooled, for example, to about ambient temperature (eg, about 20 to about 30 degrees). It may then be exposed to an atmosphere containing oxygen. Oxygen may be mixed with gases that do not oxidize under exposure conditions. This serves to passivate the catalyst. Oxygen may be at a concentration of about 0.5-5 wt.%, or about 0.5-2 wt.%, 0.5-1 wt.%, 1-5 wt.%, 2-5 wt.%, or Concentrations of 1-2% by weight, for example about 0.5% by weight, 1% by weight, 1.5% by weight, 2% by weight, 2.5% by weight, 3% by weight, 3.5% by weight %, 4%, 4.5%, or 5% by weight. In some cases, this concentration may be less than about 2%, or less than about 1.5% or less than about 1%. This step may be performed at ambient temperature. This step may be performed at a temperature of about 20 to about 40°C, or may be performed at a temperature of about 20-30°C, 30-40°C, or 25-30°C, such as 20°C, It may be performed at a temperature of 25°C, 30°C, 35°C, or 40°C. This step may be carried out for about 0.5-5 hours, or about 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours. , 4.5 hours, or 5 hours.

When the reaction is carried out in a solvent, suitable solvents include solvents for reagents. These typically have a high dielectric constant, for example greater than about 30, optionally greater than 40, 50, 60, 70, or 80. The dielectric constant may be from about 30 to about 90, or from about 30 to 80, from about 30 to 60, from about 40 to 90, from about 60 to 90, or from about 40 to 60, such as about It may be 30, 40, 50, 60, 70, 80, or 90. These include water, DMF, and mixtures thereof. More generally, a number of amphoteric aprotic solvents may suitably be used alone, mixed with each other, or mixed in water. Such solvents include DMSO, HMPT, HMPA, dioxane, and the like. In such cases, a temperature of about 150-250°C may be used, and a may be used, for example about 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, or 250°C. Such reactions may suitably be carried out in a sealed container. This causes the temperature to exceed the normal boiling point of the solvent or any solvent in the mixed solvent used in the reaction.

As previously mentioned, when the precursor contains sulfur, the need for separate addition of sulfur may be eliminated. Thus, in this case the reaction may be carried out in the absence of sulfur (ie elemental sulfur). Preferred precursors for the synthesis of sulfur-free MoS ₂ are thiomolybdates such as ammonium thiomolybdate. However, in one embodiment, thiomolybdate may be used in the presence of elemental sulfur.

The catalyst produced by the method described above may have a nano-size. By this is meant a catalyst having at least one range and optionally two or three dimensions which are in the range 1-999 nm, ie in the nano-dimension range. The nanodimension or each nanodimension independently ranges from about 1 to about 500 nm, about 1-200 nm, about 1-100 nm, about 1-50 nm, about 1-20 nm, about 1-10 nm, about 10-999 nm, about 20- 999 nm, about 50-999 nm, about 100-999 nm, about 200-999 nm, about 500-999 nm, about 10-500 nm, about 10-100 nm, about 10-50 nm, about 50-500 nm, about 50-100 nm, or about 100 ~500 nm, for example about 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 950 nm, or 999 nm.

MoS ₂ produced by the method may be pure. This should not exclude the presence of catalyst supports (discussed above) which are not taken into account when determining the purity of the catalyst. The purity of the catalyst may be at least about 80%, optionally at least 85%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, on a weight or molar basis. Or it may be 99.9%.

When MoS ₂ is supported, the carrier may be as previously described. The ratio of MoS ₂ to carrier may be from about 1:1 to about 1:3, or from about 1:1 to 1:2, from 1:2 to 1:3, or from 2:3 to It may be 2:5, for example about 1:1, 2:3, 1:2, 2:5, 1:3, or 35:65.

The catalyst of the invention, whether supported or unsupported, may be used to produce methane from carbon monoxide and hydrogen. Carbon monoxide and hydrogen may be in a ratio of about 1:1 to about 1:4, or about 1:1 to 1:3, 1:1 to 1:2, 1:1 on a molar or mass basis. 2 to 1:4, or 1:2 to 1:3, for example about 1:1, 2:3, 1:2, 2:5, 1:3, 2:7. Or it may be 1:4. However, the catalysts of the present invention are highly tolerant of variable proportions, and proportions other than those described above may be used. The reaction is, for example, at about 400 to about 700°C, or about 400 to 600°C, 400 to 500°C, 500 to 700°C, 600 to 700°C, or 500 to 600°C, for example about 400°C, 450°C , 500°C, 550°C, 600°C, 650°C, or 700°C. The catalysts of the present invention are significantly more resistant to poisoning by sulfur present. Accordingly, the supplied mixed gas may comprise a sulfur-containing gas such as, for example, hydrogen sulfide.

As noted above, prior to use as a catalyst, the materials of the invention may be treated with a reducing agent such as, for example, hydrogen sulfide. Suitable conditions are, for example, 5% H ₂ S in H ₂ and 1 hour. This pretreatment may serve to remove surface oxides.

Example 1

An unsupported _MoS2 catalyst was prepared by mixing ammonium tetrathiomolybdate (ATM) with pristine sulfur and then transferred into a tube furnace. The reaction was carried out in a mixture of hydrogen and nitrogen environments at a temperature of 450° C. for 5 hours. Prior to catalyst recovery, the catalyst was passivated with 1% _O2 in _N2 at a flow rate of 30 mL/min for 1 hour. The resulting catalyst was in the form of a black powder. This method is identified as the ATM+S method.

A supported _MoS2 catalyst was prepared by a chemical gas-phase reaction method using a tubular furnace. ATM was used as the precursor and pyrolysis was carried out in a quartz tube reactor. Typically, 1.0 g of support, 1.461 g of ATM, and 4.3828 g of sulfur powder were mixed in a mortar prior to transferring the mixture to a ceramic boat placed inside a quartz tube. The mixture was heated to 450° C. for 5 hours at a heating rate of 5° C./min in a H ₂ /N ₂ (1:2) atmosphere with a total gas flow of 30 mL/min. The reactor was then cooled to room temperature. Prior to catalyst recovery, the catalyst was passivated with 1% _O2 in _N2 for 1 hour at a flow rate of 30 mL/min. This catalyst was recovered and tested in a fixed bed microreactor.

The reducing environment during the synthesis of the MoS ₂ catalyst ensures the formation of MoS ₂ instead of MoS ₃ when only inert gas is used. Sulfur is believed to be crucial for the formation of nanostructured _MoS2 . Since ATM contains sulfur, the role of sulfur is not only to act as a sulfur donor, but also to provide saturated sulfur vapor during the reaction, which facilitates the self-assembly of nano-sized _MoS2 flowers in the gas phase. considered to be important in

Overuse of sulfur (S/Mo>>2) and decomposition of ATM in reducing gas ( _H2 ) have a great impact on the morphology of _MoS2 catalysts. Decomposition of ATM in inert gas only produces a smooth-surfaced _MoS2 catalyst (FIG. 1A). The inclusion of sulfur causes surface roughness of MoS ₂ , but does not produce the desired nanoflower-like morphology (Fig. 1B). Decomposition of ATM with H ₂ gas without sulfur will produce the other type of nanostructures shown in FIG. 1C. Past workers have also reported the observation of _MoS2 nanotubes or fibers when ATM is decomposed in _H2 , but these are due to the decomposition of ATM in _H2 and the addition of large amounts of elemental sulfur. is not the desired nanoflower morphology shown in FIG.

For activity evaluation, 0.8 g of _MoS2 catalyst was loaded into a stainless steel reactor. Prior to activity testing, the catalysts were treated with 5% _H2S in _H2 for 1 hour to remove surface oxygen. A synthesis gas ratio (H ₂ /CO) of 1.0 was chosen and a reaction temperature of 550° C. was used during the activity tests. The _H2 and CO flow rates were 23.33 sccm and the residual inert gas was _N2 flowing at 20 sccm. 3000 ppm _H2S was fed into the reaction stream. The main products obtained in the reaction using this catalyst were _CH4 and _CO2 .

Figure 2 shows the CO conversion and _CH4 selectivity of unsupported or supported _MoS2 catalysts on different catalyst supports. Three supports, namely SASOL (PURAL® MG30, labeled Al-Mg30), SiO ₂ and Al ₂ O ₃ were used to compare the activity of MoS ₂ catalysts on supports. Since the selectivity of _CH4 for all these _MoS2 catalyst tests at present conditions can reach the equilibrium selectivity, the main subject of discussion will be the CO conversion. The reaction reached steady state about 3 hours after the reactor had reached the temperature and pressure set points, after which the average CO conversion of the catalyst was calculated. MoS ₂ (ATM+S)-unsupported average CO conversion was 81.9%, 35% MoS ₂ (ATM+S)/Al-Mg30 was 81.6%, 35% MoS ₂ (ATM+S)/SiO ₂ was 80%. .5%, 35% MoS ₂ (ATM+S) Al ₂ O ₃ was 78.1%. The activity of 35% MoS ₂ (ATM+S)/Al-Mg30 is similar to that of MoS ₂ (ATM+S)-unsupported, despite the MoS ₂ loading being 1/3 lower, compared to the other two materials ( SiO ₂ and Al ₂ O ₃ ) were less efficient than the supports for the MoS ₂ catalyst prepared by Method C. The isoelectric points (IEP) of MgO, _Al2O3 , and _SiO2 are known, _with MgO being highly _basic (IEP=11.6) and _Al2O3 slightly basic. (7.2), found that _SiO2 is acidic (2.8). It is believed that the IEP of the support determines the fundamental interaction of the supported molybdenum ions. There is evidence that the higher the IEP of the support, the more depolymerizable the precursor. Therefore, MoS ₂ supported on Al-Mg30, consisting of 30% MgO and 70% Al ₂ O ₃ , has better dispersion than the catalyst supported on Al ₂ O ₃ and SiO ₂ is expected. This is confirmed by observation using a scanning electron microscope (SEM).

FIG. 3 shows an SEM image of the catalyst. MoS ₂ (ATM+S)-unsupported bulk morphology is shown in FIG. 3A. The MoS ₂ particles in FIG. 3A typically have dimensions of about 30-40 microns. FIG. 3B shows MoS ₂ (ATM+S)-unloaded morphology at higher magnification. From the SEM images, the surface of the _MoS2 slab is covered with nanoflowers with petals less than 100 nm. This nanostructure formed through self-assembly during decomposition of the precursor is the reason for the high activity of the _MoS2 catalyst. However, the formation of large _MoS2 slabs from these nanoflowers limited the utilization of active sites in the middle of the _MoS2 blocks. Therefore, a suitable support is required for better dispersion of the _MoS2 catalyst. FIG. 3C depicts 35 wt % MoS ₂ supported by Al—Mg30. Small flakes of _MoS2 nanoflowers can be observed on Al-Mg30. Al ₂ O 3 and SiO ₂ are not effective in dispersing MoS ₂ because slabs of MoS _{2 similar to slabs of unsupported MoS 2 are} still observable, as shown in FIGS. _3D and 3E.

Example 2

Ammonium heptamolybdate tetrahydrate (AHM) can be used as a precursor for mixing with sulfur and then reacting under conditions similar to the resulting unsupported _MoS2 catalyst. The resulting catalyst was labeled MoS ₂ (AHM+S)-unsupported. MoS ₂ (AHM+S)-unsupported CO conversion is shown in FIG. The average CO conversion for this catalyst was 81.8%, which is comparable to MoS ₂ (ATM+S)-unsupported and 35% MoS ₂ (ATM+S)/Al-Mg30. However, the supported catalyst prepared by the AHM+S method, labeled 35% MoS ₂ (AHM+S)/Al-Mg30, shows very low activity.

In FIG. 5, SEM images of 35% MoS ₂ (AHM+S)/Al-Mg30 are compared with SEM images of 35% MoS ₂ (ATM+S)/Al-Mg30. In FIG. 5A, MoS ₂ formed edge thicker nanowalls on Al—Mg 30 when AHM was used as the precursor. This morphology is significantly different from MoS ₂ flakes with nanoflowers on the surface, as observed in 35% MoS ₂ (ATM+S)/Al-Mg30 (Fig. 5B). The structure of MoS ₂ in 35% MoS ₂ (AHM+S)/Al-Mg30 is not suitable for methanation and hence lower activity as shown in FIG. The interaction between the precursor (AHM) and the support (Al-Mg30) is apparently too strong, leading to the formation of _MoS2 nanowalls. On the other hand, the interaction between ATM and Al-Mg30 appears to be sufficient to disperse the _MoS2 catalyst through depolymerization of the precursor, but not too strong to chemically react with the support. Therefore, for the synthesis of active _MoS2 catalysts, it seems crucial to use suitable precursors with the desired interaction strength with the support. It is believed that the polymolybdate species are the reason why the AHM precursors show a stronger interaction with the support. The Mo--O--Mo species in the AHM precursor interact strongly with the oxygen carrier compared to the oxygen-free ATM precursor. Furthermore, since oxygen is more electronegative than sulfur, the AHM precursor will have a higher degree of polymerization than ATM. A high degree of polymerization would lead to auto-reduction of AHM precursors, which would be unsuitable for self-assembly of MoS _bilayered structures.

Example 3

FIG. 6 shows the CO conversion of MoS ₂ catalysts with different loadings prepared by the ATM+S method. The activity of the catalyst increased from 68.5% to 81.6% when the _MoS2 loading was increased from 25% to 35% and dropped slightly to 80.5% when the loading was increased to 50%. As a result, 35% was the optimum loading for the supported catalyst. FIG. 7A shows SEM images of 25% MoS ₂ (ATM+S)/Al-Mg30, 35% MoS ₂ (ATM+S)/Al-Mg30, and 50% MoS ₂ (ATM+S)/Al-Mg30. If the loading of 25% MoS ₂ (ATM+S)/Al-Mg30 is lower, the precursor will enhance the interaction with the support, leading to the undesirable formation of MoS ₂ nanowalls on the support (Fig. 7A). ). At higher loading (35%), the _MoS2 flakes with nanoflowers on the support were dispersed (Fig. 7B), but when the loading increased to 50%, the _MoS2 flakes clumped together and some reduces the surface available for reaction in the form of

Example 4

Alternatively, the hydrotalcite-supported _MoS2 catalyst can be prepared in a low-temperature environment, i.e., a low-cost hydrothermal method. The method begins by dissolving 1.461 g of ATM in 80 ml of solvent by sonication for 20 minutes. After the molybdenum sauce is completely dissolved, add 1 g of carrier. The mixture was sonicated for an additional 20 minutes and 0.01 g of _NaBH4 was added to the mixture before transferring the mixture to a Teflon-lined autoclave. The autoclave was heated at 200° C. for 10 hours. After reaction, the mixture was washed with deionized water and filtered. The recovered catalyst was in the form of black powder.

For activity evaluation, 0.8 g of _MoS2 catalyst was loaded into a stainless steel reactor. The reaction conditions were the same as those described in Example 1.

FIG. 8 shows the CO conversion of catalysts prepared using the hydrothermal method. MoS ₂ catalysts were supported on Al ₂ O ₃ , SiO ₂ , hydrotalcite (Al-Mg30), activated carbon (Act-C), and carbon nanotubes (CNT). All catalysts were prepared using dimethylformamide (DMF) as solvent, except 35% MoS ₂ (H ₂ O)/Al-Mg30-hydro, which was prepared using deionized water as solvent. Carbon supports are generally less effective than other supports with observable low CO conversions (~30%). Compared to all hydrothermal catalysts prepared using DMF as solvent, Al-Mg30 was found to be the most effective support with a CO conversion of about 60%. Changing the solvent to water improved the performance of the MoS ₂ catalyst using the same support (Al-Mg30) by almost 10 percent. This indicates that the correct carrier and solvent combination can make the prepared MoS ₂ more effective.

FIG. 9 shows SEM images of 35% MoS ₂ (H ₂ O)/Al-Mg30-hydro. In the images, it can be observed that the _MoS2 formed on the support exhibits different types of morphologies, one similar to nanowalls on the support and the other scattered on the top of the support. Similar to nanoplates. Both structures expose the edge sites of the _MoS2 catalyst, making this catalyst effective for CO methanation, as shown in FIG.

The difference in performance between 35% MoS ₂ /Al-Mg30-hydro prepared with DMF and 35% MoS ₂ /Al-Mg30-hydro prepared with water can be explained by the solvent polarity index. ATM is highly soluble in polar solvents. ATM is more soluble in water because the polarity of water (dielectric constant=80) is significantly higher than DMF (dielectric constant=38). This leads to lower interactions between precursor and support, reducing auto-reduction that may occur with the support. As a result, this helps the self-assembly of _MoS2 catalysts. A more soluble solvent may help reduce precursor polymerization, form smaller agglomerates, or better disperse the _MoS2 catalyst.

In one aspect, the present invention provides a method of producing nano-sized _MoS2 catalysts. The method begins by mixing ammonium tetrathiomolybdate (ATM) and elemental sulfur in a mortar. The reaction of the mixture is carried out in a reduced environment at elevated temperature in a tubular furnace. This demonstrated preservation of the self-assembly method leading to the desired morphology under a sulfur-rich reducing environment.

In a second form, the _MoS2 catalyst can also be produced by mixing ATM, elemental sulfur and a suitable support in a mortar. This mixture is then transferred to a tube furnace and the reaction is carried out in an environment of a mixture of hydrogen and nitrogen at 450° C. for 5 hours. The support plays a role in reducing the dosage of MoS ₂ used for methanation while maintaining the nano-sized morphology as well as the activity of the catalyst. Suitable support materials were selected to improve dispersion and inhibit chemical interaction of the precursor with the support.

In another aspect, the invention provides another method of reacting ATM and a support under hydrothermal conditions. The method begins by mixing ammonium tetrathiomolybdate, carrier, and reducing agent in a solvent. The mixture is then transferred to a Teflon-lined autoclave and heated at 200° C. for 10 hours. Suitable solvents and carriers were chosen to improve decomposition to monomers and reduce autoreduction of precursors.

Thus, herein are methods for promoting self-assembly of nano-sized molybdenum disulfide catalysts with or without supports prepared using chemical vapor deposition in a saturated sulfur source and a reducing environment. explains. Suitable carriers include PURAL® MG30 available from SASOL. The reaction temperature may be 450°C. The sulfur source may be vapor from elemental sulfur. The reducing gas may be _H2 . MoS ₂ catalysts may have over 80% total CO conversion in syngas direct methanation.

A hydrothermal method is also described to promote the self-assembly of nano-sized molybdenum disulfide catalysts. As noted above, according to this method, the carrier may be PURAL® MG30 available from SASOL. The reaction temperature may be 200°C. The solvent may be water.

We also describe the use of hydrotalcite support with _H2 flux in the preparation of _MoS2 catalyst from ammonium tetrathiomolybdate by sulfur vapor synthesis.

Furthermore, the use of hydrotalcite support in the hydrothermal synthesis of _MoS2 catalyst from ammonium tetrathiomolybdate using water as solvent is also described. Effects of the present invention include resistance of the catalyst to _H2S . As a result, syngas can be used without purification. This can result in higher CO conversion compared to conventional impregnation methods. The method of the present invention results in unique morphologies as observed in SEM images. These include clusters of nanoflowers, nanowalls, or nanoplates for hydrothermal catalysis. For example, high CO conversions have been achieved between 70% and 80°C at 550°C.

Claims (21)

A method for preparing a molybdenum sulfide catalyst that catalyzes a methanation reaction, comprising:
heating a precursor in the presence of a reducing agent, said precursor being molybdate or thiomolybdate;
When said precursor is a molybdate, the reaction is carried out at a temperature of 400-500° C. in a reducing atmosphere in the presence of elemental sulfur,
When the precursor is a thiomolybdate, the reaction is carried out at a temperature of 180-220° C. in the absence of elemental sulfur and in the presence of a solvent. 2. The method of claim 1, wherein said salt is an ammonium salt whereby said molybdate is ammonium molybdate and said thiomolybdate is ammonium thiomolybdate. 3. The method of claim 2, wherein the ammonium molybdate is ammonium heptamolybdate tetrahydrate (AHM) and the ammonium thiomolybdate is ammonium tetrathiomolybdate (ATM). A process according to any one of claims 1 to 3, wherein said heating is carried out in the presence of a support, whereby said process produces a supported molybdenum sulfide catalyst. 4. The support is selected from the group consisting of alumina, magnesium oxide, silica, aluminum oxide/magnesium oxide, hydrotalcite, activated carbon, carbon nanotubes, and mixtures of any two or more thereof. The method described in . 6. Any one of claims 1 to 5, wherein the reaction is carried out in the absence of a solvent and in the presence of elemental sulfur, and the reaction comprises heating the precursor and the elemental sulfur in a reducing atmosphere. or the method described in paragraph 1. 7. The method of claim 6, comprising grinding together the precursor and the elemental sulfur prior to the heating step. 8. A method according to claim 6 or 7, wherein the reducing atmosphere comprises hydrogen gas. 9. The method of claim 8, wherein said hydrogen gas is mixed with an inert gas. 10. The method of claim 9, wherein said inert gas is nitrogen. A method according to any one of claims 6 to 10, wherein said heating is carried out to a temperature of 400-500°C. The method according to any one of claims 6 to 11, comprising exposing the catalyst to a mixture of oxygen and an inert gas after the heating, wherein the oxygen concentration in the inert gas is less than 2% v/v. described method. A method according to any one of claims 1 to 5, wherein the method is carried out in a solvent and the reducing agent is borohydride. 14. The method of claim 13, wherein the borohydride is sodium borohydride. 15. A method according to claim 13 or 14, wherein said precursor is ATM. 16. The method of claim 15, wherein said method is performed in the absence of elemental sulfur. The method of any one of claims 13-16, wherein the solvent has a dielectric constant greater than 30. 18. The method of claim 17, wherein the solvent is water, dimethylformamide, or mixtures thereof. The method according to any one of claims 13-18, wherein said heating is performed at a temperature of 180-220°C. A method according to any one of claims 13 to 19, wherein said method is carried out in a sealed container. A method for preparing a molybdenum sulfide catalyst that catalyzes a methanation reaction, comprising:
heating a precursor in the presence of a reducing agent, said precursor being polymolybdate or polythiomolybdate;
When said precursor is a polymolybdate, the reaction is carried out at a temperature of 400-500° C. in a reducing atmosphere in the presence of elemental sulfur,
When the precursor is a polythiomolybdate, the reaction is carried out at a temperature of 180-220° C. in the absence of elemental sulfur and in the presence of a solvent.

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