Patent Application
20250065319
This application claims priority to and the benefit of Korean Patent Application No. 10-2023-0108975 filed in the Korean Intellectual Property Office on Aug. 21, 2023, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a dehydroaromatization catalyst and a preparation method thereof, and more particularly, to a dehydroaromatization catalyst capable of preparing an aromatic compound at a high yield using methane and a method for preparing the same.
Natural gas, the main source of methane (CH4), is a new alternative energy source to compensate for the depletion of traditional sources such as coal and petroleum. In addition to energy production, methane, which is 70-90% of natural gas, becomes a valuable feedstock for the production of general-purpose chemicals such as methanol (CH3OH) and acetic acid (CH3COOH).
In general, in major industrial methods using methane as a supply raw material, high energy is required due to a strong endothermic reaction, and it is practically possible only on a large scale, and CO2 emission is inevitable. Therefore, the direct conversion pathways of eco-friendly methane, which can be used at small and medium-sized natural gas fields, have been studied for many years.
An object of the present invention is to provide a dehydroaromatization catalyst prepared by a novel preparation method and performing methane dehydroaromatization (MDA) with high efficiency, and a preparation method thereof.
In addition, another object of the present invention is to provide a dehydroaromatization catalyst which can be effectively used in a small-scale reaction device as well as a large-scale reaction device and is environmentally friendly due to a reduction in carbon dioxide emission, and a preparation method thereof.
According to an aspect of the present disclosure, embodiments of the present disclosure include a preparation method of a dehydroaromatization catalyst and a dehydroaromatization catalyst. In one embodiment, provided is a preparation method of a dehydroaromatization catalyst, the method comprising: dealuminating a zeolite by hydrothermal treatment at a first temperature; and supporting molybdenum (Mo) in the dealuminated zeolite, wherein a Fourier-transform infrared (FT-IR) spectrum is repeatedly measured a total of 50 to 300 times in the range of 1600-1400 cm−1, and the BrØnsted acid sites obtained by integrating the wavelength of a peak of 1545 cm−1 are 300 μmol·g−1 to 850 μmol·g−1.
In an embodiment, the method may further include primarily calcining the zeolite at a temperature range of about 300° C. to about 700° C. for about 10 hours to about 24 hours before performing the hydrothermal treatment.
In an exemplary embodiment, the first temperature may be 300° C. to 500° C., and the hydrothermal treatment may be performed in a tubular furnace at a flow rate of 800 mL·min−1 to 1500 mL·min−1 of air containing 5 mol % to 20 mol % water vapor for 6 hours to 48 hours.
In an embodiment, the zeolite may have a Si/Al of 10 to 20.
In an embodiment, the content of molybdenum (Mo) may be about 4 wt % to about 8 wt %.
In an exemplary embodiment, the supporting molybdemun (Mo) may include impregnating the dealuminated zeolite in an aqueous solution containing Mo for 10 minutes to 30 minutes at a temperature range of 20° C. to 30° C., drying the dealuminated zeolite at a temperature range of 50° C. to 90° C. for 8 hours to 24 hours, heating the dealuminated zeolite at 0.5° C.·min−1 to 2° C.·min−1. in an air of 200 mL·min−1 to 500 mL·min−1 in order to perform a second calcination for 10 hours to 24 hours at a temperature range of 300° C. to 700° C.
In an embodiment, the Mo-containing aqueous solution may include one or more of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), ammonium phosphate hydrate ((NH4)3PMo12O40·xH2O), phosphorus molybdate hydrate (12MoO3·H3PO4·xH2O), sodium phosphate hydrate (Na3[P(Mo3O10)4]·xH2O), sodium molybdate 2hydrate (Na2MoO4·2H2O), molybdenum contaminant (MoCl5), molybdenum(VI) tetrachloride oxide (MoOCl4), sodium molybdate (Na2MoO4), and molybdenum(IV) sulfide (MoS2).
In an embodiment, the dehydroaromatization catalyst may include soft coke and hard coke, and the hard coke may be 25 parts by weight to 42 parts by weight, the soft coke may be graphite, and the hard coke may be polyaromatic hydrocarbons or polycyclic aromatic compounds based on 100 parts by weight of the soft coke and the hard coke.
In an exemplary embodiment, the dehydroaromatization catalyst may include an internal coke provided inside the pores of the zeolite and an external coke present on the surface of the zeolite, and the external coke may be 60 parts by weight to 75 parts by weight based on total 100 parts by weight of the internal coke and the external coke.
In an embodiment, when the first temperature increases, the amount of BrØnsted acid sites decreases, and the molybdenum may be present in pores of the zeolite.
In an embodiment, the dehydroaromatization catalyst may have an average particle size of 150 μm to 250 μm, and a methane conversion rate according to the following formula may be 2.5% or more.
Methane conversion rate (%)=(number of moles of CH4 reacted/number of moles of CH4 supplied)×100 (Formula)
In an embodiment, in the X-ray diffraction (XRD) analysis spectrum using Cu—Kα, the peak corresponding to the (021) plane may not be present, and the peak of (101) may be stronger than the peak of (200/020) and the peak of (111).
In an embodiment, a dehydroaromatization catalyst comprising zeolite and molybdenum (Mo) and prepared according to the above-described method is provided.
In an embodiment, the zeolite may be a ZSM-5-based zeolite, the content of molybdenum (Mo) may be about 4 wt % to about 8 wt %, and the molybdenum may be present in pores of the zeolite.
In an embodiment, the BrØnsted acid sites obtained by repeatedly measuring a Fourier-transform infrared (FT-IR) spectrum a total of 50 to 300 times in a range of 1600-1400 cm−1, and integrating the wavelength of a peak of 1545 cm−1 may be 300 μmol·g−1 to 850 μmol·g−1.
In an embodiment, the dehydroaromatization catalyst may include soft coke and hard coke, and the hard coke may be 25 parts by weight to 42 parts by weight, the soft coke may be graphite, and the hard coke may be polyaromatic hydrocarbons or polycyclic aromatic compounds based on 100 parts by weight of the soft coke and the hard coke.
In an exemplary embodiment, the dehydroaromatization catalyst may include an internal coke provided inside the pores of the zeolite and an external coke present on the surface of the zeolite, and the external coke may be 60 parts by weight to 75 parts by weight based on total 100 parts by weight of the internal coke and the external coke.
In an embodiment, the dehydroaromatization catalyst may have an average particle size of 150 μm to 250 μm, and a methane conversion rate according to the following formula may be 2.5% or more.
Methane conversion rate (%)=(number of moles of CH4 reacted/number of moles of CH4 supplied)×100 (Formula)
In an embodiment, in the X-ray diffraction (XRD) analysis spectrum using Cu—Kα, the peak corresponding to the (021) plane may not be present, and the peak of (101) may be stronger than the peak of (200/020) and the peak of (111).
In an embodiment, the dehydroaromatization catalyst may perform a dehydroaromatization reaction with a reactant including methane to prepare an aromatic compound, and the aromatic compound may include any one or more of benzene, toluene, xylene, naphthalene, and coke.
According to the present invention as described above, it is possible to provide a dehydroaromatization catalyst controlled to have a high reaction rate and efficiency by analyzing the path of a dehydroaromatization reaction of methane, and a preparation method thereof.
In addition, according to the present invention, it is possible to provide a dehydroaromatization catalyst which stably maintains dehydroaromatization performance and has an improved lifespan and an improved cumulative yield of an aromatic compound, and a preparation method thereof.
Details of other embodiments are included in the detailed description and the drawings.
Advantages and features of the present disclosure, and methods of achieving the advantages and features will become apparent with reference to embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and unless otherwise specified in the following description, all numbers, values, and/or expressions representing components, reaction conditions, and contents of components in the present invention should be understood to be modified by the term “about” in all cases, as these numbers are essentially approximations reflecting various uncertainties in the measurement occurring in obtaining such values among others. In addition, when a numerical range is disclosed in the present disclosure, the range is continuous, and unless otherwise indicated, the range includes all values from the minimum value of the range to the maximum value including the maximum value. The invention further relates to the inclusion of all integers, including from the minimum value to the maximum value including the maximum value, unless otherwise indicated, when such a range refers to an integer
In addition, in the present disclosure, when a range is described for a variable, it will be understood that the variable includes all values within the described range including the described end points of the range. For example, it will be understood that a range of “5 to 10” includes not only the values of 5, 6, 7, 8, 9, and 10, but also any sub-ranges such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, etc., and also includes any values between integers valid within the scope of the described ranges such as 5.5, 6.5, 7.5, 5.5 to 8.5, 6.5 to 9, etc. For example, it will be understood that a range of “10% to 30%” includes not only values of 10%, 11%, 12%, 13%, etc. and all integers including up to 30%, but also any sub-range of 10% to 15%, 12% to 18%, 20% to 30%, etc., and also includes any value between valid integers within the scope of the range described, such as 10.5%, 15.5%, 25.5%, etc.
A preparation method of a dehydroaromatization catalyst according to an exemplary embodiment of the present invention includes: dealuminating zeolite by hydrothermal treatment at a first temperature; and supporting molybdenum (Mo) on the dealuminated zeolite. In the dehydroaromatization catalyst prepared by the preparation method of the dehydroaromatization catalyst, after adsorbing Py, the Fourier-transform infrared (FT-IR) spectrum may be repeatedly measured a total of 50 to 300 times in the range of 1600-1400 cm−1, and the BrØnsted acid sites obtained by integrating the wavelength of a peak of 1545 cm−1 may be 300 μmol·g−1 to 850 μmol·g−1.
The dehydroaromatization catalyst may produce lower hydrocarbons including methane, ethane, and the like into aromatic compounds including benzene, toluene, and xylene (BTX), and the like by a methane dehydroaromatization (MDA) reaction. Specifically, the dehydroaromatization catalyst may be used as a catalyst for initial methane conversion in the methane dehydroaromatization reaction by supporting a Mo metal in zeolite, and the desired aromatic compound may be prepared using the produced intermediate material with the BrØnsted acid sites of the zeolite support.
Specifically, the zeolite may include ZSM-5 having pores of 10-membered-ring (10-MR.) among zeolites having various structures. In the Mo-supported ZSM-5 zeolite, the Mo species act as a dehydrogenation catalyst for methane to produce C2 intermediates, while the Bronsted acid sites of ZSM-5 can form aromatics in the zeolite pores. The zeolite may have a Si/Al of about 10 to about 20. When the Si/Al is less than 10, an excessive amount of the Bronsted acid sites of the zeolite may be present to further promote coke formation after reaction, and thus the reaction activity may be deteriorated, and when the Si/Al is greater than 20, sufficient Mo as necessary for reaction may not be supported when Mo is supported.
The dehydroaromatization catalyst may dealuminate the zeolite by hydrothermally treating the zeolite at a first temperature. Subsequently, by supporting Mo in the dealuminated zeolite, the Mo may be fixed to the zeolite.
The dehydroaromatization catalyst may have a BrØnsted acid sites of 300 μmol·g−1 to 850 μmol·g−1, and when the BrØnsted acid sites are less than 300 μmol·g−1, the production efficiency of the aromatic compound is low, which is problematic, and when the BrØnsted acid sites exceed 850 μmol·g−1, the reaction proceeds excessively, so that undesired coke, etc. may be produced, thereby reducing the selectivity of the aromatic compound. The BrØnsted acid sites may be adjusted by removing Al atoms in the framework of the zeolite by hydrothermal treatment of the zeolite.
The preparation method of the dehydroaromatization catalyst may further include primarily calcining the zeolite at a temperature range of 300° C. to 700° C. for 10 hours to 24 hours before performing the hydrothermal treatment. By performing the primary calcination, impurities in the zeolite may be removed to prevent the zeolite structure from collapsing during hydrothermal treatment. In the first calcination, if the temperature is less than 300° C., calcination is not sufficiently performed, and if it exceeds 700° C., the physical properties of the zeolite are changed, which is problematic. In addition, by performing the primary calcination within the above-described time, it is possible to prevent unnecessary process cost increase while sufficiently calcining the zeolite.
The zeolite may be hydrothermally treated at a first temperature after the primary calcination. The first temperature may be 300° C. to 500° C., and the hydrothermal treatment may be performed for 6 hours to 48 hours in a tubular furnace at a flow rate of 800 mL·min−1 to 1500 mL·min−1 of air containing 5 mol % to 20 mol % water vapor. When the first temperature is less than 300° C., dealumination is not sufficiently performed, which is problematic, and when the first temperature is more than 500° C., too few BrØnsted acid sites are formed, which makes it impossible to efficiently produce aromatic compounds.
In the present embodiment, when the first temperature increases, the amount of BrØnsted acid sites may decrease.
In the supporting of molybdenum (Mo), the dealuminated zeolite may be impregnated in an aqueous solution containing Mo at a temperature range of 20° C. to 30° C. for 10 minutes to 30 minutes. Subsequently, the method may include drying the mixture at a temperature range of 50° C. to 90° C. for 8 hours to 24 hours, and then heating the mixture at 0.5° C.·min−1 to 2° C.·min−1 under air of 200 mL·min−1 to 500 mL·min−1 to perform secondary calcining at a temperature range of 300° C. to 700° C. for 10 hours to 24 hours. Specifically, the dealuminated zeolite may be impregnated in the aqueous solution containing Mo at room temperature for 15 minutes.
If the temperature is less than 300° C., it is difficult for the molybdenum to be delivered and fixed in the pores of the zeolite support, and if the temperature is greater than 700° C., the physical properties of the zeolite support may change, which is problematic.
In the secondary calcination, it is possible to remove impurities remaining in the reaction while stably fixing the molybdenum in the support, and when it is less than 300° C., it is difficult to remove impurities after the reaction, and when it is more than 700° C., the molybdenum is formed of an oxide and is provided on the surface of the zeolite support, thereby deteriorating catalyst efficiency.
The content of the molybdenum (Mo) may be 4 wt % to 8 wt %, and the molybdenum may be present in pores of the zeolite. The molybdenum may act as a dehydrogenation catalyst for converting methane, which is a raw material, into the C2 intermediate, and when the content is less than 4 wt %, it is not sufficient to act as a dehydrogenation catalyst for methane, and when the content is more than 8 wt %, it blocks fine pores of zeolite to reduce reaction efficiency. In addition, when the molybdenum is present on the outer surface of the zeolite, the activity thereof is lower than that present therein, which is problematic. Molybdenum present on the outer surface can be identified using TEM or XPS.
The aqueous solution including Mo may include at least one of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), ammonium phosphate hydrate ((NH4)3PMo12O40·xH2O), phosphomolybdate hydrate (12MoO3·H3PO4·xH2O), sodium phosphate hydrate (Na3[P(Mo3O10)4]·xH2O), sodium molybdate two hydrate (Na2MoO4·2H2O), molybdenum contaminant (MoCl5), molybdenum(VI) tetrachloride oxide (MoOCl4), sodium molybdate (Na2MoO4), molybdenum(IV) sulfide (MoS2).
The dehydroaromatization catalyst may comprise soft coke and hard coke. Based on 100 parts by weight of the total weight of the soft coke and the hard coke, the hard coke may be 25 parts by weight to 42 parts by weight. The soft coke may be graphite, and the hard coke may be polyaromatic hydrocarbons or polycyclic aromatic compounds. Specifically, the soft coke may be artificial graphite, and the hard coke may include any one or more of biphenylene, acenaphthylene, phenanthrene, anthracene, fluoranthene, pyrene, xylene, naphthalene, benzo(A)pyrene (BaP), benzo[E]pyrene (BeP), benzo[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbFA), benzo[j]fluoranthene (BjFA), benzo[k]fluoranthene (BkFA), and di-benzo[a,h]anthracene (DBAhA).
When the hard coke is less than 25 parts by weight, the catalytic reaction efficiency is deteriorated, and when the hard coke is more than 42 parts by weight, the fine pores of the zeolite are blocked by the hard coke, thereby deteriorating the reaction efficiency.
In addition, the dehydroaromatization catalyst may comprise an internal coke provided inside the pores of the zeolite, and an external coke present on the surface of the zeolite. Based on 100 parts by weight of the total of the internal coke and the external coke, the external coke may be 60 parts by weight to 75 parts by weight. Specifically, the external coke may be 63 parts by weight to 73 parts by weight. The external coke may be provided within the above-described range, thereby improving the yield of the aromatic compound. Here, the internal coke or the external coke may be determined by a location, for example, the inside or the surface of the pores, present with respect to the zeolite, regardless of the type of the hard coke and the soft coke.
The dehydroaromatization catalyst may have an average particle size of 150 μm to 250 μm, and a methane conversion rate according to the following formula may be 2.5% or more.
Methane conversion rate (%)=(number of moles of CH4 reacted/number of moles of CH4 supplied)×100 (Formula)
The methane conversion rate according to the above (Formula) may be measured by supplying a reaction gas containing methane of CH4 and Ar (CH4/Ar=9/1, v/v) at a rate of 5 mL·min−1 and fixing the reaction gas with 0.2 g of the dehydroaromatization catalyst to maintain a gas capacity per hour of 1500 mL·g−1·h−1. Specifically, the catalyst is thermally activated by heating the catalyst at 500° C. for 1 hour under a flow of He having a heating rate of 2.5° C.·min−1, and the catalyst is measured by maintaining a temperature of 500° C., and then heating the catalyst to 700° C. with a heating rate of 2.0° C.·min−1. At this time, the initial state of the catalyst starts to be measured 10 minutes after the reaction gas is supplied so that the experiment is performed in a state where the methane completely covers the catalyst.
In the dehydroaromatization catalyst, in the X-ray diffraction (XRD) analysis spectrum using Cu—Kα, the peak corresponding to the (021) plane may not be present, and the peak of (101) may be stronger than the peak of (200/020) and the peak of (111). The peak corresponding to the (021) plane is a representative XRD peak of MoO3 shown at about 27.3°, and in the dehydroaromatization catalyst according to the present embodiment, an oxide of Mo, for example, MoO3, or the like, is hardly present on the outer surface of the zeolite, and Mo is uniformly dispersed in the zeolite channel. In addition, the peak of (101) was stronger than the peak of (200/020) and the peak of (111) in the dehydroaromatization catalyst, which means that the crystallinity of the zeolite was increased by hydrothermal treatment.
According to another aspect of the present invention, the present invention includes a dehydroaromatization catalyst prepared according to the above-described method.
The dehydroaromatization catalyst uses ZSM-5-based zeolite as a support, and 4 wt % to 8 wt % of molybdenum (Mo) may be uniformly sprayed inside the pores of the zeolite.
In addition, in the dehydroaromatization catalyst according to the present embodiment, the size of the micropores hardly changes even by hydrothermal treatment, and the BrØnsted acid sites obtained by repeatedly measuring the Fourier-transform infrared (FT-IR) spectrum 50 to 300 times in a range of 1600-1400 cm−1, and integrating the wavelength of the peak of 1545 cm−1 may be 300 μmol·g−1 to 850 μmol·g−1. In the dehydroaromatization catalyst, the Mo is involved in dehydrogenating methane to produce a C2 intermediate product, and the BrØnsted acid site can produce an aromatic compound of the C2 intermediate product.
The dehydroaromatization catalyst may perform a dehydroaromatization reaction with a reactant including methane to prepare an aromatic compound, and the aromatic compound may include any one or more of benzene, toluene, xylene, naphthalene, and coke.
Hereinafter, Examples and Comparative Examples of the present invention will be described. However, the following Examples are only preferred embodiments of the present invention, and the scope of the present invention is not limited by the following Examples.
A commercial NH4+-ZSM-5 zeolite having a Si/Al ratio of 11.5 (Zeolyst International) was calcined to produce a H+-ZSM-5 support. At this time, calcining was performed at 550° C. in a furnace at 1° C.·min−1 for 12 hours under 300 mL·min−1 of air. After the calcining was completed, the H+-ZSM-5 support was dealuminated by performing a hydrothermal treatment. The hydrothermal treatment was performed at 400, 500, 600, and 700° C. with different temperatures, respectively, but the other conditions were the same.
The hydrothermal treatment was carried out for 24 hours in a tubular furnace consisting of two heating zones under an air stream (ca. 1000 mL·min−1) containing 10 mol % water vapor. At this time, the H+-ZSM-5 support (about 1.0 g) was distributed to two quartz boats to be provided before injecting air into the tubular furnace, and distilled water was injected at a fixed ratio of 4 mL·h−1 into the tubular furnace when the temperature reached 400, 500, 600, and 700° C.
The H-ZSM-5 support (Mo/Z) not subjected to hydrothermal treatment through an initial wetting method, and the H-ZSM-5 supports (Mo/Z_400, Mo/Z_500, Mo/Z_600, Mo/Z_700) subjected to hydrothermal treatment at each temperature of 400, 500, 600 and 700° C. were impregnated in an aqueous solution of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, 99%, Sigma-Aldrich) to load the same amount with 6 wt % Mo. Then, after drying at 70° C. overnight, all the dried particles were calcined at 550° C. for 12 hours in a heating lamp at 1° C.·min−1. under 300 mL·min−1 of air to prepare a Mo-supported H-ZSM-5 zeolite catalyst (hereinafter, referred to as a Mo-supported catalyst) in the form of particles.
In addition, for comparison, a catalyst having a Mo loading of 3 wt % and 9 wt % and a commercial NH4+-ZSM-5 zeolite (Zeolyst International) having a Si/Al ratio of 25 were prepared using the same method except that hydrothermal treatment was not performed after calcination, a catalyst having a Mo loading of 6 wt % was not hydrothermal treatment, and the rest were prepared using the same method except that hydrothermal treatment was performed on the catalyst having a different Mo content and the rest had a Mo loading of 3 wt % and 9 wt % when the same Si/Al ratio was 11.5.
Here, the H-ZSM-5 support catalyst in which Mo is supported without performing hydrothermal treatment is referred to as “Mo/Z”, and the H-ZSM-5 support catalyst in which hydrothermal treatment is performed and Mo is supported is referred to as “Mo/Z_x”. Here, x represents a hydrothermal treatment temperature (° C.). Table 1 below shows the catalysts prepared in this Example.
For every 0.02 deg step using a D/MAX-2500V/PC diffractometer (Rigaku, Japan), a powder X-ray diffraction (XRD) pattern was obtained using Cu Kα radiation (40 kV, 100 mA, λ=0.154 nm) at a rate of 0.5 deg min−1.
Argon (Ar) adsorption-desorption isotherms were measured using a ASAP 2020 analyser (Micromeritics, Inc., USA) at 87 K to determine the texture properties of the catalyst. In addition, all samples were pretreated at 350° C. for 12 hours under vacuum before the Ar adsorption-desorption measurement.
The size distributions of the micropores and mesopores of the catalyst were calculated from the adsorption branches using the Horvath-Kawazoe (H-K) and Barrett-Joyner-Halenda (BJH) methods, respectively.
29Si and 27Al MAS NMR (Magic angle spinning nuclear magnetic resonance) spectra were obtained using a AVANCE III HD 500 MHz spectrometer (Bruker, USA) having a magnetic field of 11.7 T. A 29Si MAS NMR spectrum was detected 1024 times at a rotation speed of 5 kHz at 99.3 MHz, and octakis(trimethylsiloxy)silsesquioxzne (Q8M8) was used as a reference at 11.5 ppm. In addition, the 27Al MAS NMR spectrum was detected 4096 times at a rotation speed of 10 kHz at 130.3 MHz, and 1 M AlCl3 (aq) was used as a reference of 0 ppm.
In order to identify the type of coke produced from the used catalyst and to estimate the amount thereof, thermogravimetric analysis (TGA) was performed using a Q50 thermogravimetric analyser (TA Instruments, USA). Specifically, 10-20 mg of the pretreated sample was heated from room temperature to 900° C. under an air flow of 100 mL·min−1 at a heating rate of 1.0° C.·min−1. TGA measurement and Ar physical adsorption results were used together to estimate the amount of internal coke and external coke present inside and outside the catalyst, respectively. The amount of internal coke was estimated by multiplying the difference in volume of pores of the catalyst before and after the reaction (i.e., the volume of 10 membered-ring (10-MR.) pores, V10-MR), determined by Ar physical adsorption, by the coke density considered (d=1.22 g·cm−3). As a result, the amount of external coke was calculated as the total coke content obtained from the TGA measurement minus the amount of internal coke. The coke density used to estimate the amount of internal coke can represent the density of the polyaromatic coke that is normally assumed to be the composition of the internal coke that is produced during the methane dehydroaromatization (MDA) reaction. Since the coke formed inside the pores of the zeolite is generally polyaromatic, the coke density used to calculate the internal coke may indicate the density of the polyaromatic coke.
Energy dispersive X-ray spectroscopy (EDX) analysis was performed using a field emission scanning electron microscope (FE-SEM, SU-70, Hitachi, Japan), and accordingly, the Mo weight composition and Si/Al ratio of the catalyst were analyzed.
The amount of the total acid sites (i.e., the sum of the Bronsted acid sites and the Lewis acid sites) was estimated by pyridine (Py) adsorption and ammonia (NH3) desorption. Py adsorption experiments were performed using a Hg—Cd—Te (MCT) detector and a Fourier-transform infrared (FT-IR) spectrometer (Nicolet IS50, Thermo Fisher Scientific Inc., USA) equipped with a custom in-situ cell with ZnSe window on both sides. About 1020 mg of the sample was converted to a self-supporting wafer and then placed in a cell and heated to 500° C. under vacuum for at least 6 hours and then cooled to 150° C. to record the background spectrum. Then, helium (He) gas (saturated vapor pressure=2.80 kPa and moving diameter=0.55 nm at 25° C.) passing through the bubbler containing liquid Py was injected into the cell for 20 minutes. Subsequently, the over-adsorbed or weakly adsorbed Py was removed from the sample under vacuum for 1 hour before measuring the FT-IR spectrum. To determine the amount of acid sites, the resulting product, excluding the background spectrum from the obtained FT-IR spectrum, was integrated at wavelengths of 1545 and 1450 cm−1 related to the Bronsted and Lewis acid sites, respectively. The amounts of the BrØnsted and Lewis acid sites were calculated according to the following equation.
Amount of acid sites=FT-IR peak area of corresponding region×[cross-sectional area of self-supporting wafer×(weight of wafer)1]×(extinction coefficient)1 (Formula)
Here, the extinction coefficients used at 1545 and 1450 cm−1 are 1.13 and 1.28 cm·μmol−1, respectively. The extinction coefficient is a coefficient indicating how strongly the corresponding chemical species absorbs light at a given wavelength to reduce the light, and may also be expressed as an absorption coefficient.
In addition to FT-IR analysis, temperature program desorption of NH3 (NH3-TPD) was performed on the Mo-supported catalyst before and after the reaction using a BELCAT (MicrotractBEL Corp., Japan) analyser. The sample (about 50 mg) was preferentially treated to remove previously adsorbed impurities for 1 hour at 500° C. under He flow. After cooling to 50° C., 30 mL·min−1 of a mixed gas containing 5% NH3 (He balance) was supplied to the sample cell for 1 hour. After sufficient adsorption, the sample was heated to 800° C. at a heating rate of 10° C.·min−1., and an NH3 desorption curve was obtained under He 30 mL·min−1. The NH3 desorption curve according to temperature was subdivided into five areas. The two regions with peaks less than 200° C. are known as physically adsorbed NH3, SiOH groups, and weak acid sites. Therefore, the acid concentration was measured in consideration of only the peak of 200° C. or higher. Two peaks at about 200-400° C. are known to be due to moderate strength and strong acidic sites, and a single peak above 400° C. is due to the Lewis acid sites. In NH3-TPD, the term “total BASM+S” was used as the sum of the acid sites of the medium strength (M) and the strong strength (S). The acid site of the medium strength was NH3 desorbed at 230° C. to 280° C., and the acid site of the strong strength was calculated based on NH3 desorbed at 310° C. to 390° C.
In order to evaluate the MDA performance of the Mo-supported catalyst, a fixed bed quartz tube reactor having an inner diameter of 6.5 mm and a length of 420 mm was used. All catalysts were made into pellets and then pulverized, and only particle sizes in the range of 150-250 μm were selected. The reaction gas of CH4 and Ar (CH4/Ar=9/1, v/v) was supplied at a rate of 5 mL·min−1, and the amount of catalyst was fixed at about 0.2 g to maintain a gas hourly capacity of about 1500 mL·g−1·h−1 of the reaction gas of CH4 and Ar. Here, 1500 mL·g−1·h−1 means a space velocity calculated by (gas flow rate of 5 ml·min−1)/(catalyst 0.2 g). In order to precisely analyze, gas chromatograph (GC) analysis was performed using Ar as an internal standard. All reactions were carried out at atmospheric pressure.
In a general setting, a selected particulate catalyst was placed in the middle of a quartz tube reactor into which quartz wool was previously inserted. The quartz wool may be provided before the catalyst is put into the quartz tube, thereby preventing the catalyst from passing as it is. The quartz wool was previously placed inside the quartz tube to fix the cotton in the middle, and then a catalyst was placed on it.
Before the catalyst was placed in the quartz tube reactor, zirconia-silicate ceramic beads were placed on the other side of the quartz wool to reduce space. The catalyst was thermally activated by heating at 500° C. for 1 hour under He flow with a heating rate of 2.5° C.·min−1. After maintaining the temperature of 500° C., the catalyst was heated to 700° C. with a heating rate of 2.0° C.·min−1. Upon reaching 700° C., the feed was converted to a reaction gas. The feed means a supplied gas, and He flows until reaching 700° C., and after reaching 700° C., it is converted into CH4/Ar gas, which is a reaction gas. The reaction state of the catalyst was first measured 10 minutes after supplying the reaction gas so that the experiment was performed with methane covering the entire catalyst. All measurements were performed using an online GC (6500 GC Young Lin Instrument Co., Republic of Korea) for about 12 hours at 45 minute intervals. In the GC, a packed column (HAYESEP DB 100-120 mesh, 30 ft×1/8½×2.0 mm, Agilent, USA) connected to the TCD was used to detect CH4 and Ar, and major hydrocarbons such as methane (CH4), ethane (C2H6), ethylene (C2H4), propane (C3H8), benzene (C6H6), toluene (C7H8), and naphthalene (C10H8) involved in the MDA reaction were detected using a capillary column (GS-GASPRO, 30 m×0.32 mm, Agilent, USA) connected to the FID. The temperature of the stainless steel discharge line connected in the GC was maintained at 250° C. to prevent strong adsorption and condensation of aromatic hydrocarbons (e.g., naphthalene). Methane conversion, selectivity of gas hydrocarbons and hard coke and soft coke (solid coke), and hydrocarbon yield were calculated using the following equation.
Methane conversion rate (%)=(number of moles of CH4 reacted/number of moles of CH4 supplied)×100
Hydrocarbon selectivity (%)=(Number of moles of hydrocarbon having X of carbon atoms in the hydrocarbon/number of moles of CH4 reacted)×100
Coke selectivity (%)=100-hydrocarbon selectivity (%)
In (a) of
The peak of (101) in the hydrothermally treated catalyst (Mo/Z_x; x=400, 500, 600, 700) was stronger than the peak of (111) in ca. 8° (200/020) and ca. 9° (111) than in the catalyst (Mo/Z) which was not hydrothermally treated. Table 2 below shows the relative crystallinity of the catalyst, and it was confirmed using the combined area of the normalized signal region of the (101) plane at ca. 8° and the normalized signal region of the (200/020) plane and the (111) plane at ca. 9°.
(Determined using the combined area of the normalized signal region of the ca. 8° plane and the normalized signal region of the (200/020) and (111) planes at ca. 9°)
Referring to Table 2, it was confirmed that the crystallinity of the catalyst subjected to hydrothermal treatment was higher than that of the catalyst not subjected to hydrothermal treatment, and the crystallinity increased as the temperature of hydrothermal treatment increased. The relative crystallinity refers to an increase in crystallinity of zeolite based on Mo/Z, which is a catalyst that is not subjected to hydrothermal treatment, and the increase in relative crystallinity means that Al is removed from the crystal lattice of zeolite and simultaneously crystallinity is improved. That is, it could be confirmed that as the temperature of the hydrothermal treatment increased, the crystallinity was improved similar to that of the zeolite analog containing a high content of silica.
Referring to (b) of
Table 3 shows the textural characteristics of Mo-supported catalysts (Mo/Z and Mo/Z_x; x=400, 500, 600, 700). All the textural characteristics in Table 3 were confirmed from Ar physisorption isotherms at 87 K. Table 4 shows the Mo content and Si/Al ratio (Mo/Z and Mo/Z_x; x=400, 500, 600, 700) of the Mo-supported catalyst.
In addition, the structural strength of the Mo-supported catalyst was confirmed through the tissue characteristics obtained from the results of Ar physical adsorption. The Ar adsorption of all Mo-supported catalysts shows Type I isotherms ((c) of
aBrunauer-Emmett-Teller (BET) equation
bt-plot equation
cHorvath-Kawazoe (H-K) equation
dObtained convoluting the micropore size distribution between the 0.4-0.6 nm
(Obtained through SEM-EDX analysis in various mapping regions of each catalyst for statistical measurement)
Table 5 shows the contents of the BrØnsted and Lewis acid sites of the Mo-supported catalysts (Mo/Z and Mo/Z_x; x=400, 500, 600, 700).
aDetermine after vacuuming crystals at 150° C.
bEstimated by integrating the FT-IR spectrum of the Py-adsorbed sample at 1545 cm−1
c Estimated by integrating the FT-IR spectrum of the Py-adsorbed sample at 1545 cm−1
d Estimated by integrating and adding two NH3 desorption peaks at 200-400° C. (intermediate acid + strong acid) (where intermediate acid is measured at 200-300° C., strong acid at 300° C.)
eEstimation by integrating the NH3 desorption peak at a temperature above 400° C.
The acid concentrations of all Mo-supported catalysts determined by Py-adsorption FT-IR and NH3-TPD analysis are shown in (a) and (b) of
When the H-ZSM-5 support was hydrothermally treated, the total BASM+S (medium BAS and strong BAS) was greatly reduced compared to the Mo/Z sample that was not hydrothermally treated. Compared to Mo/Z, Mo/Z_400 showed a slight decrease in the amount of BASM+S (579.5 μmol·g−1 vs. 538.8 μmol·g−1 of Mo/Z), the NH3 desorption temperature was almost similar, and it was confirmed that the desorption intensity was similar (386° C. and 276° C. vs. 389° C. and 278° C. of Mo/Z). When the temperature of the hydrothermal treatment was further increased to 500, 600, and 700° C., the concentration and strength of the acid were greatly reduced. That is, the amount and strength of BASM+S can be controlled by dealumination in the framework of zeolite by hydrothermal treatment, and the strength decreases as the temperature of the hydrothermal treatment increases.
Since BAS is closely related to Al atoms in the framework of tetrahedral zeolite, the status of Al species contained in the untreated H-ZSM-5 support and the hydrothermally treated H-ZSM-5 support (Z and Z_x; x=400, 500, 600, 700) at each temperature was analyzed by 27Al MAS NMR spectroscopy before loading Mo. Referring to (c) of
Referring to Table 6, it was confirmed that as the temperature of the hydrothermal treatment increased, the peak area around 55 ppm gradually decreased. That is, as shown in the results of the 27Al MAS NMR spectrum, it was confirmed that Al atoms were effectively extracted from the zeolite framework by the hydrothermal treatment according to the present embodiment. The Al peak near 0 ppm representing the Al species present outside of the zeolite framework (outer framework Al species) was found in all kinds of supports. The lost amount of Al species in the framework of all hydrothermally treated H-ZSM-5 supports did not quantitatively correspond to the increase in NMR peaks corresponding to the outer framework Al species.
Z, which is a H-ZSM-5 support not subjected to hydrothermal treatment, and Z_400, which is a H-ZSM-5 support subjected to hydrothermal treatment at 400° C., were compared. For Z_400, a broad peak was observed near 30 ppm. Abroad peak near 30 ppm was mainly shown in Al having a different pentagonal structure or Al of a distorted tetrahedron, and a broad peak near 30 ppm was shown to be stronger as the temperature of hydrothermal treatment increased (Z_500<Z_600<Z_700). In addition, the upward shift of the peak at a high hydrothermal treatment temperature (x≥500° C.) also means a framework Al species organized into a tetrahedron.
Dealumination can be confirmed by quantifying the spectral intensity of the Al species as a whole in the zeolite support, but there were some differences by the framework Al species that were not measured or not organized into tetrahedrons. On the other hand, when hydrothermal treatment was performed at a high temperature, the monotone reduction of the tetrahedral framework Al species ((c) of
In (f) of
Mo/Z and Mo/Z_400 showed the highest BT yield at similar TOS (yield of about 6.4% at Mo/Z., yield of about 7.0% at Mo/Z_400.). Although Mo/Z and Mo/Z_500 had similar catalytic activity (
In order to confirm the MDA catalyst performance of the Mo-supported catalyst, the BT production rate was confirmed ((a) of
Referring to (b) of
Mo/Z_500 showed a similar trend to CH4 conversion of Mo/Z and Mo/Z_400 ((c) of
On the other hand, both Mo/Z_600 and Mo/Z_700 achieved a maximum BT yield at a TOS of about 1 hour. Similar to Mo/Z_500, it is predicted that Mo dispersed in Mo/Z_600 and Mo/Z_700 was affected by the framework Al species rather than the tetrahedron produced by hydrothermal treatment ((c) of
Referring to (c) of
In the Mo-supported catalyst, the outer Mo species located on the outer surface are less active than the Mo species present inside the zeolite pores. That is, it is judged that Mo/Z_600 and Mo/Z_700 are excessively dealuminated and thus most of the Mo species exist on the outer surface, and as a result, both CH4 conversion and BT yield are low.
Referring to
3. Coke Analysis on Mo-Supported Catalysts after Reaction
Table 7 is a linear fit equation for each of the soft coke and the hard coke. In Table 7, numerical values (i.e., the slope of the fit equation) are described to show the estimated accumulation rate (mgcoke·g−1·h−1) for each soft coke and hard coke. Table 8 shows the ratio of soft coke to hard coke.
The Mo-supported catalyst according to this example was subjected to hydrothermal treatment to dealuminate the zeolite support in order to increase the BT intermediate product before the hard coke was produced. In addition, the hard coke reduces the stability of the catalyst, and the content of the hard coke is reduced by the dealumination, thereby making it possible to improve the stability of the catalyst. In order to confirm this, the Mo-supported catalyst was recovered according to each reaction time (about 2, 4, 6, 8, 10 and 12 hours), and soft coke and hard coke were analyzed for this.
Specifically, soft coke and hard coke were analyzed using two approaches: (1) a method using only TGA measurements and (2) a method combining TG and Ar adsorption analysis. The type of coke in the catalyst used was confirmed using the TGA measurement.
In the differential weight percentage profile of the Mo-supported catalyst recovered after 12 hours of reaction ((a) of
The coke whose mass is reduced at a low temperature of about 440° C. is a soft coke generated by the decomposition of intermediate C2 hydrocarbons, and the coke whose mass is reduced at a high temperature of about 480° C. is a hard coke formed by the polycondensation of aromatic substances. It was confirmed that Mo/Z_x subjected to the hydrothermal treatment had a lower ratio of hard coke than Mo/Z. In addition, it could be confirmed that as the temperature of the hydrothermal treatment increased, the ratio of hard coke decreased compared to the ratio of soft coke. That is, it can be seen that the higher the temperature in the process of dealumination, the lower the BrØnsted acid site which promotes the formation of a hard coke.
(b) of
In order to evaluate the formation of coke generated during the MDA reaction, a test was performed with various reaction times, and then the TGA measurement of the catalyst was performed. That is, the amounts of soft coke and hard coke are estimated using the decomposed differential weight percentage profiles of the respective Mo-supported catalysts at different reaction times, as shown in
The accumulation rates of the soft coke formed at Mo/Z., Mo/Z_400, and Mo/Z_500. were almost similar, and the Mo/Z_600 was significantly lower ((c) of
In (a) of
Specifically, the difference in the V10-MR values consumed in the unreacted reaction (reaction time of 0 hours) obtained by decomposing the H-K pore size distribution and the reaction for each time (reaction times of 2, 4, 6, 8, 10 and 12 hours) was confirmed (
(d) of
After the reaction, external coke in the catalyst blocked the pore inlets to prevent access to BAS present inside the micropores. In connection with the deactivation of the catalyst, in order to identify other causes besides blocking of the pore inlets by the external coke, the catalyst was recovered after each reaction time (about 2, 4, 6, 8, 10, and 12 hours), and the amount of BAS in the catalyst was tracked and quantified using the NH3-TPD technique.
(a) of
In particular, the position of the coke was confirmed through TG and Ar adsorption analysis. Referring to (d) of
It was confirmed that Ar was still accessible in the 10-MR pores even though the external coke was laminated when the reaction time (TOS) was about 8 hours or more (
The BT formation rate (see (4a) of
The present experiment confirmed that only the amount of 10-MR pores representing physical properties does not affect the catalyst deactivation in the MDA process. That is, although the roles of the external coke and the BAS could not be completely distinguished from each other, it could be confirmed that both the gradual increase of the external coke and the decrease in the amount of BASM+S (representative of the chemical characteristics) had an effect on the deactivation of the catalyst.
In order to further confirm the effect of hydrothermal treatment of the Mo-supported catalyst on MDA performance, the amount of hard coke obtained by TGA measurement was corrected by the sum of the BT (
At Mo/Z_400, the amount of corrected hard coke decreased, which means that the hydrothermal treatment at 400° C. reduced the BAS that promoted hard coke formation. In addition, when the temperature of the hydrothermal treatment was increased to 500° C. or 600° C., the amount of hard coke corrected at Mo/Z_500. and Mo/Z_600. was almost similar at all reaction times.
In addition, in the Mo-supported catalyst after the reaction was completed, the ratio of hard coke to soft coke did not change significantly over time ((b) of
In this example, Mo/Z_400 showed the best MDA performance, which is believed to be due to the reduction in BAS that promotes hard coke production. The performance of Mo/Z follows this performance, with Mo/Z having an excess of BAS that further converts the desired product (BT) into coke. In addition, the MDA performance of Mo/Z_500 was different from that of Mo/Z_600 and Mo/Z_700 in terms of CH4 conversion and BT production, but Mo/Z_500, Mo/Z_600, and Mo/Z_700 exhibited lower MDA performance than that of Mo/Z_400 due to excessive dealumination of the zeolite support.
A catalyst (H-ZSM-5 (untreated)) having various Mo contents or Si/Al ratios and not subjected to hydrothermal treatment was prepared and evaluated. Most of the catalyst (indicated by open circles) higher than the Si/Al ratio (11.5) of Mo/Z (CH4 conversion of about 8.6% and selectivity of 65.4%) exhibited low BT or benzene yield. The highest performing catalyst among the activated untreated catalysts known in the literature (about 9.5% CH4 conversion and 81.3% selectivity) had a higher Mo content relative to Mo/Z, but the Si/Al ratio was nearly the same.
As a result of preparing and evaluating the catalyst having the same Si/Al ratio (11.5) as the higher Mo content (9 wt %), the catalyst performance was very low compared to Mo/Z (about 4.3% CH4 conversion and 20.5% selectivity). As a result of evaluating a catalyst having a Si/Al ratio (11.5) equal to the lower Mo content (3 wt %), the catalyst performance was relatively low compared to Mo/Z (about 9.3% CH4 conversion and 48.7% selectivity). That is, when the catalyst was used as the MDA catalyst, it was confirmed that the highest performance was achieved by the Mo content of 6 wt % and the Si/Al ratio of 11.5.
In addition to the untreated catalyst, catalysts which were subjected to further vapor treatment (reduced amount of BAS; triangular, inverted triangular, rhombus, star-shaped symbols) were included in
The steam-treated catalysts may be (1) a catalyst in which the H-ZSM-5 support is pretreated with steam and then subjected to acid treatment (indicated by triangles) or (2) a catalyst in which the NH4-ZSM-5 support is first calcined in humid air and then calcined in dry N2 (indicated by diamonds). Since the steam-treated catalysts had a Si/Al ratio of 25, which is nearly twice the Mo/Z_400, the corresponding MDA performance (about 7.6% CH4 conversion and 47.7% selectivity) was not satisfactory compared to the MDA performance of the hydrothermally treated H-ZSM-5 support as a result of preparing an untreated catalyst having a Mo content of 6 wt % and a Si/Al ratio of 25 to compare them.
It could be confirmed that dealumination by hydrothermal treatment of the H-ZSM-5 support before Mo was supported had an effect on the performance of the MDA catalyst.
Specifically, dealumination of the zeolite support by hydrothermal treatment was performed using steam in the range of 400-700° C. Thereafter, the performance of the catalyst that was not subjected to hydrothermal treatment and the catalyst that was subjected to hydrothermal treatment was evaluated, and the difference in BT production rate was confirmed. There was a difference in the rate of BT production according to the order of increase in the activity of the catalyst, and the rate of BT production was in the order of Mo/Z_400>Mo/Z≈Mo/Z_500>Mo/Z_600>Mo/Z_700.
As a result of TGA measurement of the catalyst recovered after performing the MDA reaction for each reaction time, the production of hard coke was suppressed, and the degree of inhibition was proportional to the temperature of the hydrothermal treatment. In particular, in the case of Mo/Z_400, BAS, which is known to promote hard coke production, was reduced by hydrothermal treatment at 400° C., and a higher BT production rate was shown compared to Mo/Z, which was not subjected to hydrothermal treatment with excess BAS. On the other hand, it could be confirmed that Mo/Z_500, Mo/Z_600, and Mo/Z_700, which are catalysts having increased temperature of hydrothermal treatment, were not advantageous for further increasing the BT production rate. That is, the amount of hard coke was effectively reduced by hydrothermal treatment, but the BAS required for the production of BT, the desired product, was also greatly reduced.
The physicochemical properties of the catalyst recovered after the reaction time elapsed were confirmed. In the Mo-supported catalyst, the amount of BAS and micropore volume were considered as indicators representing chemical and physical properties, respectively. The chemical and physical properties were used to understand the catalyst inert properties in MDA in Mo-supported catalysts.
In the present example, it was confirmed that the catalyst had improved MDA performance due to the optimum amount of BAS of Mo/Z_400, and it was confirmed that the BT yield could be improved through the inhibition of coke formation, which is the final product of the catalyst, by adjusting the amount of BAS to improve the performance of the Mo-containing catalyst.
Those skilled in the art to which the present invention pertains will be able to understand that the present invention may be embodied in other specific forms without changing the technical spirit or essential features thereof. Therefore, it should be understood that the embodiments described above are exemplary and not restrictive in all aspects. The scope of the present invention is indicated by the scope of a patent claim to be described later rather than the detailed description, and it should be interpreted that all changes or modifications derived from the meaning and scope of the scope of the patent claim and the equal concept thereof are included in the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023-0108975 | Aug 2023 | KR | national |
