FOR DRY REFORMING METHANE TO SYNGAS

BACKGROUND OF THE INVENTION

Conversion of methane, which is naturally abundant, to energy chemicals, e.g., syngas, has gained great attention. Dry reforming methane in the presence of C0 ₂ , a main component of greenhouse gas, to produce syngas is an approach that both protects and improves the environment.

Due to the ultra-stability of methane and C0 ₂ , their activation and transformation requires a high temperature, e.g., 800 °C, during the dry reforming process. Transitional metal catalyst has been used to promote conversion of methane and C0 ₂ into syngas. At high temperature, the transitional metal catalyst is easily sintered, thus lowering its activity.

Further, severe carbon deposition often occurs at elevated temperature. Both catalyst sintering and carbon deposition lead to catalyst deactivation, which results in a short lifespan of the catalyst.

There is a need to develop a robust catalyst that has a long lifespan for dry reforming methane to syngas.

SUMMARY OF THE INVENTION

The present invention relates to a ceria-based transitional metal catalyst for dry reforming methane to syngas. The catalyst exhibits a high activity with an unexpectedly long lifespan.

Disclosed herein are three methods for preparing a transitional metal catalyst of this invention, namely, a deposition-precipitation method, a co-precipitation method, and a sequential precipitation method.

The deposition-precipitation method includes the following steps: (a) mixing a cerium compound and/or a lanthanum compound, a praseodymium compound, a zirconium compound, or a titanium compound with excess urea in de-ionized water to form a homogeneous solution, a molar ratio of Ce/M (M being La, Pr, Zr, or Ti) being 8.5-9; (b) obtaining a ceria-based solid solution support from the homogeneous solution; (c) mixing the ceria-based solid solution support and an iridium compound in de-ionized water; (d) obtaining a dry powder from the resulting solution; (e) calcining the dry powder at

700-800 °C; and (f) collecting the transitional metal catalyst thus obtained.

The co-precipitation method includes the following steps: (a) mixing an iridium compound, a cerium compound, and/or a lanthanum compound, a praseodymium compound, a zirconium compound, or a titanium compound with excess urea in de-ionized water to form a homogeneous solution, a molar ratio of Ce/M (M being La, Pr, Zr, or Ti) being 8.5-9; (b) obtaining a dry powder from the homogeneous solution; (c) calcining the dry powder at 700- 800 °C; and (d) collecting the transitional metal catalyst thus obtained.

The sequential precipitation method includes the following steps: (a) mixing a cerium compound, and/or a lanthanum compound, a praseodymium compound, a zirconium compound, or a titanium compound with excess urea in de-ionized water to form a homogeneous solution, a molar ratio of Ce/M (M being La, Pr, Zr, or Ti) being 8.5-9; (b) obtaining a suspension from the homogeneous solution; (c) adding an iridium compound into the suspension; (d) obtaining a dry powder from the mixture thus formed; (e) calcining the dry powder at 700-800 °C; and (f) collecting the transitional metal catalyst thus obtained.

Typically, the homogeneous solution has a concentration of cerium being

0.01-1.0 mol/L and a concentration of lanthanum, praseodymium, zirconium, or titanium being 0.0012-0.12 mol/L.

Examples of the iridium compound include hydrogen hexachloroiridate, potassium hexachloroiridate, sodium hexachloroiridate, iridium oxide, and iridium chloride. Examples of the cerium compound include cerium nitrate, ammonium cerium nitrate, and cerium chloride. Examples of the praseodymium compound include praseodymium nitrate and praseodymium chloride. Examples of the lanthanum compound include lanthanum nitrate, lanthanum chloride, and lanthanum acetate. Examples of the zirconium compound include zirconium oxynitrate, zirconium nitrate, zirconium chloride, and zirconium acetate.

Examples of the titanium compound include titanyl oxalate and titanium nitrate.

Also within the scope of this invention is a transitional metal catalyst containing an active iridium metal in a content of less than 5 wt% and a ceria-based solid solution support in a content of greater than 95 wt%. The ceria-based solid solution includes pure Ce0 ₂ or includes both Ce0 ₂ and M _x O _y , in which M is La, Pr, Zr, or Ti, 1 < x < 6, and 2 < y < 11.

Preferably, the catalyst contains an active iridium metal in a content of 2-3 wt% and the ceria-based solid solution support in a content of 97-98 wt%. The ceria-based solid solution support can be pure Ce0 ₂ . It can also include both Ce0 ₂ and M _x O _y , e.g., La ₂ 0 ₃ , Pr ₆ On, Zr0 ₂ , or Ti0 ₂ , the molar ratio of Ce/M being 8.5-9.

The details of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the following detailed description of several embodiments, and also from the appending claims.

DETAILED DESCRIPTION

Described in detail below are three methods for preparing a ceria-based transitional metal catalyst of this invention, i.e., a deposition-precipitation method, a co-precipitation method, and a sequential precipitation method.

One embodiment of the deposition precipitation method is performed as follows.

A homogeneous aqueous solution, formed of a cerium compound and excess urea or formed of a cerium compound and a lanthanum compound, a praseodymium compound, a zirconium compound, or a titanium compound, and excess urea, is prepared. Subsequently, the homogeneous solution is heated at 65-90 °C for 4-6 hours to hydrolyze the metal compound(s). The mixture thus formed is filtered and washed to afford a solid material, which is then dried at 100-110 °C to obtain a solid slurry. The solid slurry is calcined at 400- 450 °C for 3.5-4.5 hours to provide a ceria-based solid solution support. The solid solution support is mixed with an iridium compound in de-ionized water and dispersed with vigorous stirring. The resulting solution is stirred at 65-75 °C, followed by slow addition of an aqueous sodium carbonate or sodium bicarbonate solution to adjust its pH to 8-9. After standing for 2-4 hours, a suspension is formed. The suspension is filtered and washed with hot water to provide a slurry. The slurry is then dried at 100-110 °C overnight to obtain a dry powder. Finally, the dry powder is calcined at 700-800 °C for 4-5 hours to obtain a transitional metal catalyst.

One embodiment of the co -precipitation method is performed as follows.

A homogeneous aqueous solution, formed of an iridium compound and a cerium compound or formed of an iridium compound, a cerium compound and a lanthanum compound, a praseodymium compound, a zirconium compound, or a titanium compound, is prepared. The homogeneous solution is slowly transferred into a sodium carbonate or sodium bicarbonate solution maintained at 65-75 °C. The mixture thus formed is aged for 2-4 hours to form a suspension, which is then filtered to afford a slurry. The slurry is dried at 100-110 °C to afford a dry powder. Finally, the dry powder is calcined at 700-800 °C for 4-5 hours to obtain a transitional metal catalyst.

One embodiment of the sequential precipitation method is performed as follows.

A homogeneous aqueous solution, formed of a cerium compound and excess urea or formed of a cerium compound, and a lanthanum compound, a praseodymium compound, a zirconium compound, or a titanium compound, and excess urea, is prepared. The homogeneous solution is heated at 65-90 °C for 4-6 hours to hydrolyze the metal compound(s), followed by addition of an iridium compound. The mixture thus formed is aged for 2-4 hours and then filtered to afford a slurry. The slurry is dried at 100-110 °C to afford a dry powder. Finally, the dry powder is calcined at 700-800 °C for 4-5 hours to obtain a transitional metal catalyst.

Also within this invention is a transitional metal catalyst that can be prepared by one of the methods described above.

The catalyst preferably contains less than 5 wt% (e.g., 2-3 wt%) iridium and greater than 95 wt% (e.g., 97-98 wt%) a ceria-based solid solution support.

The ceria-based solid solution support can be Ce0 ₂ , Ce0 ₂ -La ₂ 0 ₃ , Ce0 ₂ -Pr ₆ 0n, Ce0 ₂ -Zr0 ₂ , or Ce0 ₂ -Ti0 ₂ . Preferably, the Ce0 ₂ -La ₂ 0 ₃ , Ce0 ₂ -Pr ₆ 0n, Ce0 ₂ -Zr0 ₂ , or Ce0 ₂ - Ti0 ₂ solid solution support has a molar ratio of Ce/La, Ce/Pr, Ce/Zr, or Ce/Ti being 8.5-9.

The catalyst thus prepared is capable of promoting the conversion of methane and C0 ₂ into syngas during a dry reforming reaction.

More specifically, when using a catalyst of this invention, the dry reforming reaction features a methane conversion rate of 50-80%, a C0 ₂ conversion rate of 75-95%, a product molar ratio (i.e., H ₂ /CO) of 0.95-1.05, and a space velocity of 6000-36000 mlJ(gh).

In one example, a catalyst of this invention unexpectedly maintains high activities at 1073 K or 800 °C for as long as 1000 hours in a methane dry reforming reaction at a space velocity of 36000 mlJ(gh).

Without further elaboration, it is believed that one skilled in the art can, based on the above description, utilize the present invention to its fullest extent. The specific examples below are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

The catalyst prepared in each of the following examples is labeled according to its components, e.g., Ir and Ce0 ₂ -Pr ₆ 0n, and the preparation method, i.e., deposition- precipitation (DP), co-precipitation (CP), or sequential precipitation (SP). For instance, catalyst Ir/CePr-DP refers to a catalyst formed of Ir, Ce, and Pr, and prepared by the deposition-precipitation method.

EXAMPLE 1: Catalyst Ir/CePr-DP (A) preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -Pr ₆ 0n was prepared following the procedure set forth below.

2.42 g of cerium nitrate, 0.27 g of praseodymium nitrate, and 50 g of urea were dissolved in 500 mL of de-ionized water to form a homogeneous solution. The homogeneous solution was heated at 90 °C for 5 hours to hydrolyze the metal compounds. The resulting slurry, after filtration and washing, was dried at 100 °C overnight and calcined at 400 °C for 4 hours to obtain a Ce0 ₂ -Pr ₆ 0n solid solution support. The solid solution support

Ce0 ₂ -Pr ₆ 0ii (0.98 g) and hydrogen hexachloroiridate (containing Ir 0.02 g) were mixed in 400 mL of de-ionized water. The solution was heated at 65-75 °C. Subsequently, an aqueous sodium carbonate solution (0.1 mol/L) was slowly added until the pH reached 8-9. The resulting suspension was aged for 2-4 hours. After filtration and washing, the resulting solid material was dried at 100 °C overnight to provide a dry powder, which was then calcined at 750 °C for 4 hours. The catalyst Ir/CePr-DP (A) thus obtained was collected.

The thusly prepared catalyst Ir/CePr-DP (A) was evaluated in a methane dry reforming reaction as follows.

The reaction was conducted in a fixed-bed reactor. The Ir/CePr-DP (A) catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. A feedstock of methane and CO ₂ (methane: 15 mL/min; CO ₂ : 15 mL/min) was then passed through the catalyst at a space velocity of 18000 mL/(gh). The products, i.e., ¾ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and CO ₂ were 60% and 90%, respectively. The molar ratio of H ₂ /CO was about 1.05. The catalytic activity persisted for at least 200 h.

EXAMPLE 2: Catalyst Ir/CeLa-DP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -La ₂ 0 ₃ was prepared following the procedure described in Example 1 except that 0.25 g of lanthanum nitrate was used.

The thusly prepared catalyst Ir/CeLa-DP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor. More specifically, the Ir/CeLa-DP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and CO ₂ (methane: 15 mL/min; CO ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mlJ(gh). The products, i.e., ¾ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and CO ₂ were 72% and 92%, respectively. The molar ratio of H ₂ /CO was about 1.05. The catalytic activity persisted for at least 200 h.

EXAMPLE 3: Catalyst Ir/CeZr-DP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce ZrC^ was prepared following the procedure described in Example 1 except that 0.14 g of zirconium oxynitrate was used.

The thusly prepared catalyst Ir/CeZr-DP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CeZr-DP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and CO ₂ (methane: 15 mL/min; CO ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mL/(gh). The products, i.e., ¾ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and CO ₂ were 56% and 83%, respectively. The molar ratio of ¾/CO was about 1.05. The catalytic activity persisted for at least 200 h.

EXAMPLE 4: Catalyst Ir/CePr-DP (B) preparation and evaluation

A catalyst composed of 3 wt% Ir and 97 wt% Ce0 ₂ -Pr ₆ 0n was prepared following the procedure described in Example 1 except that 0.27 g of praseodymium nitrate, 0.97 g of solid solution support Ce0 ₂ -Pr ₆ 0n, and hydrogen hexachloroiridate (containing Ir 0.03 g) were used.

The thusly prepared catalyst Ir/CePr-DP (B) was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CePr-DP (B) catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and CO ₂ (methane: 15 mL/min; CO ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mL/(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that conversions of methane and C0 ₂ were 75% and 95%, respectively. A molar ratio of H ₂ /CO was about 1.02. The catalyst's activity sustained for at least 200 h.

EXAMPLE 5: Catalyst Ir/CeTi-DP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -Ti0 ₂ was prepared following the procedure described in Example 1 except that 0.18 g of titanium nitrate was used.

The thusly prepared catalyst Ir/CeTi-DP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CeTi-DP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and C0 ₂ (methane: 15 mL/min; C0 ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mlJ(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and C0 ₂ were 50% and 80%, respectively. The molar ratio of H ₂ /CO was about 1.02. The catalytic activity persisted for at least 200 h. EXAMPLE 6: Catalyst Ir/CePr-CP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -Pr ₆ 0n was prepared following the procedure set forth below.

2.42 g of cerium nitrate, 0.18 g of praseodymium nitrate, and hydrogen

hexachloroiridate (containing Ir 0.02 g) were dissolved in 500 mL of de-ionized water to form a homogeneous solution. The homogeneous solution was slowly added to an aqueous sodium carbonate solution (0.1 mol/L) at 65-75 °C and its pH was maintained at 8-9. The mixture thus formed was aged for 2 hours. After filtration and washing, the resulting solid material was dried at 100 °C overnight to provide a dry powder. Finally, the dry powder was then calcined at 750 °C for 4 hours. The catalyst Ir/CePr-CP thus obtained was collected.

The thusly prepared catalyst Ir/CePr-CP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CePr-CP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and C0 ₂ (methane: 15 mL/min; C0 ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mlJ(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and C0 ₂ were 55% and 80%, respectively. The molar ratio of H ₂ /CO was about 1.05. The catalytic activity persisted for at least 200 h.

EXAMPLE 7: Catalyst Ir/CeLa-CP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -La ₂ 0 ₃ was prepared following the procedure described in Example 6 except that 0.25 g of lanthanum nitrate was used.

The thusly prepared catalyst Ir/CeLa-CP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CeLa-CP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and C0 ₂ (methane: 15 mL/min; C0 ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mL/(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and C0 ₂ were 60% and 95%, respectively. The molar ratio of H ₂ /CO was about 1.03. The catalytic activity persisted for at least 200 h.

The same catalyst was evaluated in a similar experiment when the space velocity was 6000 mL/(gh). It was found that the conversion rates of methane and C0 ₂ were 75% and 97%, respectively. The molar ratio of H ₂ /CO was about 1.03. The catalytic activity persisted for at least 200 h.

EXAMPLE 8: Catalyst Ir/CeZr-CP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -Zr0 ₂ was prepared following the procedure described in Example 6 except that 0.14 g of zirconium oxynitrate was used.

The thusly prepared catalyst Ir/CeZr-CP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CeZr-CP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and C0 ₂ (methane: 15 mL/min; C0 ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 6000 mL/(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and C0 ₂ were 73% and 95%, respectively. The molar ratio of H ₂ /CO was about 1.02. The catalytic activity persisted for at least 200 h.

EXAMPLE 9: Catalyst Ir/CePr-SP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ -Pr ₆ 0n was prepared following the procedure set forth below.

2.42 g of cerium nitrate, 0.27 g of praseodymium nitrate, and 50 g of urea were dissolved in 500 mL of de-ionized water to form a homogeneous solution. The homogeneous solution was heated at 90 °C for 5 hours to hydrolyze the metal compounds, followed by addition of hydrogen hexachloroiridate (containing Ir 0.02 g). The resulting mixture was aged for 3 hours. After filtration and washing, the resulting solid material was then dried at 100 °C overnight to provide a dry powder, which was calcined at 750 °C for 4 hours. The catalyst Ir/CePr-SP thus obtained was collected.

The thusly prepared catalyst Ir/CePr-SP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/CePr-SP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and C0 ₂ (methane: 15 mL/min; C0 ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mL/(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and C0 ₂ were 60% and 75%, respectively. The molar ratio of H ₂ /CO was about 0.95. The catalytic activity persisted for at least 200 h.

EXAMPLE 10: Catalyst Ir/Ce-DP preparation and evaluation

A catalyst composed of 2 wt% Ir and 98 wt% Ce0 ₂ was prepared following the procedure set forth below.

5 g of cerium nitrate and 50 g of urea were dissolved in 500 mL of de-ionized water to form a homogeneous solution. The homogeneous solution was heated at 90 °C for 5 hours to hydrolyze the metal compound. After filtration and washing, the slurry thus received was dried at 100 °C overnight and then calcined at 400 °C for 4 hours to obtain a Ce0 ₂ solid solution support. The solid solution support Ce0 ₂ (0.98 g) and hydrogen hexachloroiridate (containing Ir 0.02 g) were mixed in 400 mL of de-ionized water. The solution was heated at 65-75 °C. Then an aqueous sodium carbonate solution (0.1 mol/L) was slowly added until its pH reached 8-9. The suspension thus formed was aged for 2-4 hours. After filtration and washing, the resulting solid material was dried at 100 °C overnight to provide a dry powder, which was calcined at 750 °C for 4 hours. The catalyst Ir/Ce-DP thus obtained was collected.

The thusly prepared catalyst Ir/Ce-DP was evaluated in a methane dry reforming reaction conducted in a fixed-bed reactor.

More specifically, the Ir/Ce-DP catalyst (100 mg) was loaded into the reactor and reduced by 25% H ₂ /N ₂ at 750 °C for 30 minutes. Subsequently, a feedstock of methane and C0 ₂ (methane: 15 mL/min; C0 ₂ : 15 mL/min) was passed through the catalyst at a space velocity of 18000 mL/(gh). The products, i.e., H ₂ and CO, were analyzed by an online GC7890A gas chromatography.

It was found that the conversion rates of methane and C0 ₂ were 40% and 70%, respectively. The molar ratio of H ₂ /CO was about 1.05. The catalytic activity persisted for at least 200 h.

OTHER EMBODIMENTS

All of the features disclosed in this specification may be combined in any

combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

Further, from the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.