Patent Application
20240269658
The present invention relates to a method of preparing a nickel silicate molecular sieve catalyst for dry reforming of methane (DRM), and more particularly, to a method of preparing a two-dimensional nickel silicate molecular sieve catalyst for DRM, wherein the catalyst, which has a large two-dimensional external surface area and nickel bonded to the surface area, and has structural stability and activity, is prepared through one-step hydrothermal treatment.
Dry reforming of methane (DRM) is a reaction (Reaction Scheme 1) that converts two greenhouse gases, CH4 and CO2, to syngas (a mixture of H2 and CO). Since the DRM reaction is one of the core technologies of building a circular carbon economy, which recycles CO2 into value-added chemicals such as synthetic fuels, methanol, and dimethyl ether to reduce carbon emissions, DRM is attracting attention from environmental and industrial sectors.
However, despite the great impact on the environment and industry, it is difficult to successfully commercialize DRM due to its complicated mechanism and endothermic nature. Indeed, in DRM, carbon deposits are generated due to the Boudouard reaction (2CO↔C+CO2, ΔH°298=−190 KJmol−1) and methane cracking (CH4→C+2H2, ΔH°298=75 KJmol−1), and a side reaction such as the reverse water-gas shift reaction (RWGS, CO2+H2↔CO+H2O, ΔH°298=41 kJmol−1), which reduces the H2/CO ratio of syngas, is involved.
Therefore, it is necessary to develop a highly active and stable catalyst for the successful commercialization of DRM.
Many studies on the possibility of using various transition metals as a catalyst in DRM have been conducted, and especially, catalysts in Group VIII, including platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), nickel (Ni), and cobalt (Co) are being developed. Among them, Pt, Ru, Rh, and Pd have good catalytic activity, selectivity, and stability as noble metals but have low economic efficiency when used for industrial purposes because of their high prices.
Therefore, studies on transition metals, which are cheaper than noble metals, such as tungsten (W), titanium (Ti), vanadium (V), nickel (Ni), iron (Fe), copper (Cu), molybdenum (Mo), and cobalt (Co), have been conducted to use as a catalyst for DRM.
Among these transition metals, since Ni is relatively cheaper than other transition metals and has relatively high selectivity and a relatively high conversion rate in CO2 reforming of methane, Ni is widely used as a catalyst for DRM.
However, when Ni is used as a catalyst, it is difficult to keep catalytic activity for a long time because of deactivation of the catalyst caused by coke deposition and sintering of the active metal in DRM.
Therefore, a catalyst support has been studied extensively to compensate for the above shortcoming of the active Ni metal. This is because catalytic activity and stability of a catalyst can vary significantly depending on not only the type and size of active metals, but also the type and surface area of supports on which the active metal is supported, and their metal-support interaction. Alumina is commonly used as a catalyst support.
Korean Patent Publication No. 10-1959379 (published Mar. 18, 2019) relates to a nickel-based catalyst for the combined reforming of methane and discloses that active metals, nickel (Ni), antimony (Sb), and molybdenum (Mo), are supported on a gamma alumina support. However, when the gamma alumina support is used, it is difficult to improve catalytic activity because dispersion of these active metals is not easy, and the external surface area in contact with a reactant is not large enough.
In addition, Korean Patent Publication No. 10-1487387 (published Jan. 28, 2015) relates to a method of preparing a metal carbide-based methane reforming catalyst and discloses that catalytic activity can be improved by increasing the specific surface area by including a porous carbon material support and active ingredients such as Ni and Mo precursors. However, the preparation method of this related art document may cause a decrease in catalytic activity due to Ni agglomeration or formation of Ni clusters.
Therefore, in order to solve problems in DRM such as a decrease in catalytic activity caused by coke deposition and a reduction of active surface area of Ni due to sintering while including Ni as an active metal, it is necessary to develop a new type of catalyst for DRM and a method of preparing the catalyst, which can improve structural stability and catalytic activity and increase a catalyst lifetime through changing the support, Ni-support interaction, and catalyst framework.
The present invention is directed to providing a method of preparing a two-dimensional nickel silicate molecular sieve catalyst for dry reforming of methane (DRM), and the method simplifies a process for preparing a catalyst through one-step hydrothermal treatment and improves the structural stability and catalytic activity of a catalyst by enabling the catalyst to have a large two-dimensional external surface area and nickel bonded to the surface area through the one-step hydrothermal treatment.
In addition, the present invention is directed to providing a two-dimensional nickel silicate molecular sieve catalyst for DRM, prepared by the method.
One aspect of the present invention provides a method of preparing a two-dimensional nickel silicate molecular sieve catalyst for dry reforming of methane (DRM), which includes: (a) synthesizing a layered borosilicate precursor having an MWW zeolite framework type [B-MWW(P)]; and (b) adding a nickel precursor to the B-MWW(P) and performing hydrothermal treatment.
In one example, step (a) may include synthesizing a B-MWW(P) having an Si/B molar ratio of 5.0 to 20.0.
In one example, a concentration of the nickel precursor in step (b) may be 0.1 to 5.0 M.
In one example, a hydrothermal treatment temperature in step (b) may be 100 to 200° C.
Another aspect of the present invention provides a two-dimensional nickel silicate molecular sieve catalyst for DRM, prepared by the method described above.
In one example, a Si/Ni molar ratio of the catalyst may be 1.5 to 30.
In one example, the catalyst may have a catalyst deactivation rate of less than 5% for methane and carbon dioxide in DRM at 700° C.
The present invention can prepare a two-dimensional nickel silicate molecular sieve catalyst by substituting boron with nickel through one-step hydrothermal treatment of a B-MWW(P) and a nickel precursor and delaminating a three-dimensional layered MWW zeolite, and therefore, process efficiency can be improved.
In the present invention, boron of a B-MWW(P) is substituted with nickel during the hydrothermal treatment, so nickel, as an active metal, can be bonded to a specific location of the MWW zeolite, and as the three-dimensional layered MWW zeolite is delaminated to form a two-dimensional framework with nickel being exposed to the outside, an external surface area, where the active nickel is bonded, is increased so that catalytic activity can be significantly increased.
The present invention relates to a nickel silicate molecular sieve for dry reforming of methane (DRM) and a method of preparing the same. The present invention can improve the stability and activity of a catalyst by using nickel as an active metal to lower catalyst expenses and transforming a 3D zeolite into a 2D molecular sieve to increase an external surface area and bonding a large amount of active metal to a support.
Unless otherwise specified, all technical and scientific terms used herein have the same meaning as they are commonly understood by those skilled in the art. Generally, the nomenclature used herein is well known and commonly used in the art.
In this specification, when a part “includes” a certain component, it means that a part may further include other components rather than excluding other components, unless specifically stated to the contrary.
Hereinafter, the present invention will be described in detail.
In one aspect, the present invention provides a method of preparing a two-dimensional nickel silicate molecular sieve catalyst for dry reforming of methane (DRM), which includes: (a) synthesizing a layered borosilicate precursor having an MWW zeolite framework type [B-MWW(P)]; and (b) adding a nickel precursor to the B-MWW(P) and performing hydrothermal treatment.
In the present invention, step (a) is a step of (a) synthesizing a layered borosilicate precursor having an MWW zeolite framework type [B-MWW(P)].
The MWW is one of the zeolite framework topologies, which has a lamellar structure, and is a three-dimensional zeolite including two independent pore systems composed of a pore system of two-dimensional sinusoidal 10-membered ring (10-MR) channels with an elliptical ring cross-section of 4.1 Å×5.1 Å and a pore system including a 12-MR supercage connected to 10-MR windows.
In step (a), a layered borosilicate precursor having an MWW zeolite framework type B-MWW(P), which having a structural specificity, is synthesized by including boron into a three-dimensional MWW zeolite framework type, wherein boron is bonded in the MWW framework in a form of Si—O—B, and an Si/B molar ratio may be 5.0 to 20.0, preferably 10 to 15. When the Si/B molar ratio is less than 5, Si content is relatively insufficient as an excess amount of B is included in the B-MWW(P), which may make it difficult to form a three-dimensional MWW zeolite framework type. When the Si/B molar ratio is greater than 20, a relative amount of B compared to Si is reduced in the B-MWW(P), and sites that can be substituted with nickel are reduced during hydrothermal treatment, which may cause a decrease in catalytic activity.
For example, in step (a), an organic structure-directing agent (SDA) may be used to synthesize a layered B-MWW(P), and a three-dimensional MWW zeolite may be formed through removal of the organic SDA by calcination and condensation during the synthesis process, but step (a) is not limited thereto and any commonly known method may be used for the synthesis, so a detailed description thereof is omitted herein.
In the present invention, step (b) is a step of performing hydrothermal treatment by adding an aqueous acidic nickel precursor solution of pH 4.0 or less to the B-MWW(P). Specifically, in step (b), an aqueous acidic nickel precursor solution is added to the synthesized B-MWW(P), and the mixture solution is hydrothermally treated at 100 to 200° ° C. Therefore, the B-MWW(P) is delaminated under one-step while boron is substituted with nickel to prepare a two-dimensional multilayered nickel silicate molecular sieve.
An amount of the aqueous nickel precursor solution may be adjusted according to a hydrothermal treatment temperature, and the aqueous nickel precursor solution may have a concentration of 0.1 to 5.0 M. When the concentration of the aqueous nickel precursor solution is less than 0.1 M, an increase in pH of the aqueous pretreatment solution causes a decrease in boron release and delamination of the MWW framework, and substitution of nickel becomes difficult. When the concentration of the aqueous nickel precursor solution exceeds 5.0 M, the MWW framework may collapse due to excessively low pH.
The aqueous nickel precursor solution includes an acid component, preferable nitric acid, and has a pH of 4.0 or lower. Under this condition, boron is removed from the B-MWW(P) and is substituted with nickel by the acidic nickel precursor solution during hydrothermal treatment in step (b), and during the process, the MWW zeolite is delaminated as interlayer bonds are broken, and the three-dimensional B-MWW may be transformed into a two-dimensional nickel silicate molecular sieve.
A temperature during the hydrothermal treatment is 100 to 200° C. When the temperature is lower than 100° C., the three-dimensional B-MWW(P) may not be sufficiently delaminated, and the substitution of nickel is reduced at sites where deboronation occurs, which may make it difficult to form a two-dimensional nickel silicate molecular sieve. In addition, while the number of defect sites in the molecular sieve is increased, an amount of nickel bonded to the framework is reduced, and therefore, the number of catalytically active sites may be reduced. On the other hand, when the temperature is higher than 200° ° C., an amount of nickel clusters formed by agglomeration of the nickel precursor increases rather than that of nickel bonded to the delaminated MWW zeolite due to the high temperature, and therefore, it may be difficult to obtain a uniform composition. Therefore, the hydrothermal treatment may be performed at 100 to 200° ° C., preferably at 140 to 160° C.
The hydrothermal treatment time is not limited as the hydrothermal treatment time may be adjusted according to a processing speed and reaction environment, and for example, may be 1 to 4 days.
After step (b), filtration, drying, and sintering may be further included. As these processes may be carried out according to commonly known methods in the art, a detailed description will be omitted herein, and through the calcination step, an occluded organic structure-directing agent in the framework may be removed from the two-dimensional nickel silicate molecular sieve.
According to the preparation method of the present invention, boron is substituted with nickel in the MWW framework, and an Si—O—Ni bond may be formed in the MWW framework, and therefore, controlling nickel bonding sites may become easy and an interaction with a zeolite may be improved. In particular, as the three-dimensional layered MWW zeolite is delaminated to form a two-dimensional framework with nickel being exposed to the outside, an external surface area, where the active nickel is bonded, is increased so that catalytic activity may be significantly increased.
Therefore, a two-dimensional nickel silicate molecular sieve catalyst for dry reforming of methane (DRM) prepared according to the preparation method exhibits improved reactivity in DRM, that is, improved CH4 and CO2 conversion, depending on framework properties and nickel content.
Specifically, a Si/B molar ratio of the two-dimensional nickel silicate molecular sieve catalyst may be 11 to 130, and an Si/Ni molar ratio thereof may be 1.5 to 30. The two-dimensional nickel silicate molecular sieve catalyst may include not only Si and Ni, but also B in the framework, but when a relative amount of B in the Si/B molar ratio is less than 11, active Ni is not sufficient to replace B in the two-dimensional nickel silicate molecular sieve, which may cause a decrease in catalytic activity.
On the other hand, when a relative amount of Ni in the Si/Ni molar ratio of the two-dimensional nickel silicate molecular sieve catalyst framework is less than 1.5, an excess amount of Ni is included in the molecular sieve catalyst, and not only an Si—O—Ni bond, but also a nickel cluster is formed in the molecular sieve or on a surface of the molecular sieve, which may cause a decrease in catalytic activity.
When a relative amount of Ni exceeds 30, active Ni content in the molecular sieve is insufficient, which may cause a decrease in catalytic activity.
Therefore, the two-dimensional nickel silicate molecular sieve catalyst has Si and Ni in the framework, and the Si/Ni molar ratio may be 1.5 to 30, preferably 4 to 25, and the Si/B molar ratio may be 11 or more.
A deactivation rate of the two-dimensional nickel silicate molecular sieve catalyst having such composition and framework characteristics for methane and carbon dioxide in DRM at 700° C. may be less than 5%.
Hereinafter, examples of the present invention will be described in detail.
However, the following examples are merely preferred examples of the present invention, and the present invention is not limited to the following examples.
Hexamethyleneimine (99%) and sodium hydroxide (99%) used in synthesis of a B-MWW precursor were purchased from Sigma-Aldrich (USA), and boric acid (99.5%) was purchased from Junsei Chemical (Japan), and fumed silica was purchased from Evonik (Japan). As a nickel precursor, nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O) (98%) was purchased from Samchun Chemicals (South Korea).
Hexamethyleneimine and sodium hydroxide were dissolved in deionized water, and then boric acid was added at 50° C. After the boric acid was completely decomposed, fumed silica was slowly added, and the solution was homogenized.
The homogenized final solution was poured into a Teflon-lined stainless steel autoclave and heated at 175° C. for 7 days while stirring at 100 rpm.
After heating, a white solid was recovered by filtration with distilled water and then dried at room temperature to prepare a B-MWW precursor [B-MWW(P)].
The prepared B-MWW(P) was added to a 0.25 to 1 M aqueous nickel (II) nitrate hexahydrate solution at 0.02 g/mL and mixed. The mixed solution was poured into a hydrothermal synthesis device (Teflon-lined stainless autoclave commercially available from PARR Instrument Company) and heated to 100 to 160° C. for 1 to 4 days while stirring at 0 to 100 rpm.
After heating, the solution was washed with deionized water, filtered, and dried at room temperature and then calcined at 550° C. for 8 hours.
Accordingly, Ni-DML-X-Y-Z was prepared, where X, Y, and Z refer to a hydrothermal synthesis temperature, a hydrothermal treatment time, and a molar concentration of nickel nitrate, respectively, and Examples 1 to 9 were prepared according to the X, Y, and Z.
Using γ-Al2O3 as a support and nickel(II) nitrate hexahydrate as a precursor of an active metal, Ni/γ-Al2O3 impregnated with 5 wt % nickel was prepared using incipient wetness impregnation. The synthesized Ni/γ-Al2O3 was dried at 100° C. overnight and calcined at 550° C. for 3 hours.
The same method used for preparing Ni/γ-Al2O3 was applied, except that a B-MWW precursor was used as a support to prepare Ni/B-MWW.
The B-MWW precursor prepared in the example described above was used as a control group.
Table 1 below shows nickel silicate catalysts for dry reforming of methane, prepared by each of the above-described methods.
In
During preparation of Examples 1 to 4 (Ni-DML-X-4-1), when interlayer delamination and a nickel substitution mechanism were carried out in one-step, at a relatively low temperature of 100° C. or 120° C., some nickel atoms took up the framework space, where boron atoms were removed, but some of the framework space, where boron atoms were removed, remained as defect sites. On the other hand, at a relatively high temperature of 140° C. or 160° C., one or more nickel atoms may be present in an additional framework per substituted nickel atom in the framework.
Accordingly, the catalyst of Example 4 (Ni-DML-160-4-1) included the largest number of nickel atoms in the framework, whose Si/NiFramework ratio was 10.4.
In addition, the hysteresis in the adsorption-desorption isotherm of Example 4 (Ni-DML-160-4-1) shown in
An experiment was performed on a catalyst for dry reforming of methane (DRM) using a continuous flow device with a fixed-bed microreactor under atmospheric pressure, and the product was analyzed using a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). Before the experiment, the catalyst was activated under a flow of pure H2 (50 mL/min) at 700° C. for 3 hours and purged with N2 (60 mL/min) for 0.5 hours at the same temperature. (However, if necessary, the reaction proceeded without H2 pretreatment.)
DRM catalytic activity was measured by supplying a feed gas stream composed of 40 vol % CH4 and 40 vol % CO2 (balanced with N2) at a gas hourly space velocity (GHSV) of 30,000 mL/gcath (supplying a feed gas stream composed of CH4, CO2, and N2 at a rate of 20, 20, and 10 mL/min, respectively) to a reactor containing 0.1 g of the catalyst activated by the above process. An inert gas, N2, was used as an internal standard for GC-TCD analysis, and concentration changes due to an increase in gas volume after DRM were conducted.
In addition, a CH4 conversion, CO2 conversion, and H2/CO ratio were calculated using the following equations.
In general, in DRM, coke is formed much faster at a low temperature due to an increase in Boudouard reaction (2CO++>C+CO2).
The deactivation rate of CO2 conversion of Examples 1 to 4 (Ni-DML-X-4-1) for 12 hours was 0 to 1% while the deactivation rate of CO2 conversion of Comparative Example 1 (Ni/γ-Al2O3) and Comparative Example 2 (Ni/B-MWW) was 10% and 45%, respectively. This is because, in Examples 1 to 4 (Ni-DML-X-4-1), high activity and excellent stability may be maintained due to highly dispersed nickel sites on an external surface area.
In Examples 1 to 4, as a hydrothermal treatment temperature increased, the number of active nickel sites increased, and therefore, CH4 and CO2 conversion was increased.
In
In
The Raman spectra of
Meanwhile,
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2021-0069791 | May 2021 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2021/009424 | 7/21/2021 | WO |
