KR102541017B1 - Nitrided nin-zeolite catalysts for the dry reforming of methane - Google Patents

Abstract

In the present invention, N-ZSM-5 and N-beta zeolite with enhanced basic properties were prepared by replacing framework oxygen with nitrogen through high-temperature nitration with ammonia. Basic zeolites and their acidic counterparts were impregnated with Ni by incipient impregnation and their catalytic properties for dry reforming (DRM) of methane were investigated. The physicochemical properties of the catalysts, such as nickel chemical state and framework nitrogen and acid-base properties, were analyzed by elemental analysis, ²⁹ Si MAS NMR, NH ₃ and CO ₂ TPD, IR, XPS, and H ₂ chemisorption. In the case of all catalysts, the DRM activity was compared according to the reaction time by changing the temperature under the same conditions. According to the overall results, nitride Ni/N-ZSM-5 exhibited superior DRM activity compared to Ni/H-ZSM-5, especially at high temperatures, due to the enhanced basic properties of Ni active species and improved dispersion.

Description

Nitrided NiN-zeolite catalyst for methane dry reforming reaction

The present invention relates to a nitrided NiN-zeolite catalyst for methane dry reforming.

Syngas is composed of CO and H ₂ , and is a raw material for producing H ₂ , synthetic fuel, methanol and dimethyl ether through WGS (water-gas shift) reaction, Fischer-Tropsch (FT) synthesis, and methanol synthesis, respectively. It is considered a valuable chemical component because it can be used as a The three representative technologies used for syngas production are steam methane reforming (SMR), partial oxidation (POX), and dry reforming of methane (DRM; Equation 1). Among them, DRM has been extensively studied, and this reaction is regarded as a promising prospect for producing syngas because it simultaneously converts CH ₄ and CO ₂ , the two most harmful greenhouse gases. FT synthesis is suitable for syngas produced by DRM and can produce long-chain hydrocarbons due to its low H ₂ /CO ratio (less than 1.0).

Equation 1 (DRM): CH ₄ + CO ₂ → CO + H ₂ (ΔH° ₂₉₈ = 247 kJ mol ^-1 )

Equation 2 (RWGS): CO ₂ + H ₂ ↔ CO + H ₂ O (ΔH° ₂₉₈ = 41 kJ mol ^-1 )

Equation 3 (MC): CH ₄ ↔ C + 2H ₂ (ΔH° ₂₉₈ = 75 kJ mol ^-1 )

Equation 4 (BR): 2CO ↔ C + CO ₂ (ΔH° ₂₉₈ = -172 kJ mol ^-1 )

Equation 5 (SCG): C + H ₂ O → CO + H ₂ (ΔH° ₂₉₈ = 131 kJ mol ^-1 )

In addition to the main DRM reaction (Equation 1), various side reactions described in Equations 2-5 may occur simultaneously under the DRM reaction conditions. Reverse water gas shift (RWGS, Equation 2) reaction in which H ₂ generated by DRM reacts with CO ₂ again to produce CO and water is the most representative side reaction. This reduces the H ₂ /CO ratio, while increasing the CO ₂ conversion.

Methane cracking (MC, Equation 3) and the Budard reaction (BR, Equation 4) are related to coke formation, but the characteristics of the two reactions are different. Since MC is an endothermic reaction, it is more dominant at relatively high temperatures. In addition, coke produced in MC can be removed by reacting again with CO ₂ through reverse BR (RBR) or by reacting again with H ₂ O produced through steam coke gasification in RWGS (SCG, Equation 5).

Conversely, since BR is an exothermic reaction, it is more active at low temperatures, and the coke produced by this reaction is not removed and accumulates on the catalyst surface. Since DRM is an endothermic reaction that is stronger than side reactions, it dominates various side reactions when the temperature is high. Conversely, at low temperatures, side reactions such as RWGS (spontaneous < 820 °C), MC (spontaneous > 557 °C), and BR (spontaneous > 700 °C) predominate.

Among transition metals, Ni is known as a representative active species of DRM catalysts. Its performance is comparable to noble metals such as Ru, Rh, and Pt. However, it can be easily sintered at high temperatures (> 590 °C) because the Tamman temperature at which Ni atoms acquire sufficient energy required to have bulk mobility is low. In addition, the biggest obstacle to be overcome for commercialization of the DRM reaction using Ni is the formation of coke leading to catalyst deactivation.

It is known that overall catalyst properties can be controlled by adjusting the physicochemical properties of the catalyst support. The use of an appropriate porous, highly stable support can improve the dispersion of Ni active species and improve the thermal stability of the catalyst. It has also been reported that the use of Co together with Ni increases the hydrogenation of atomic carbon and inhibits the formation of carbide species in metal crystals. In addition, various promoters such as Cr ₂ O ₃ , La ₂ O ₃ , CeZrO _x , Mg, and Gd reduce the sintering and carbon deposition rates of Ni active metal species to improve the catalytic properties and enhance the catalytic performance to improve the metal-support interaction. known to strengthen. In addition, through the support with improved physical properties, the chemical adsorption of CO ₂ , which is a weak acid gas, is increased to further activate carbon gasification, thereby increasing coke resistance.

Zeolites are promising supports for DRM catalysts due to their excellent thermal stability and porosity. Although zeolites have inherently weak alkali properties due to lattice oxygen, strong alkali properties can be produced even in zeolites by replacing framework oxygen with nitrogen at high temperatures (usually above 800 °C) and under ammonia flow. As a result, nitride zeolite (N-zeolite) has been reported to maintain crystallinity and porosity even after high-temperature treatment, and has been applied to general base catalysts such as Knoevenagel condensation.

Basic nitrogen sites on the outer surface of N-ITQ-2 acted as active sites for Knoevenagel condensation of bulky aromatic aldehydes with ethylcyanoacetate and showed great advantages.

In addition, Guan et al. reported improved para-selectivity of ethylbenzene ethylation with ethanol in N-ZSM-5 compared to H-ZSM-5 due to the reduction of strong Bronsted acid sites in the surface area.

Shen et al. used N-NaY catalyst for aldol condensation of furaldehydes with acetone and propane reaction, where they showed better selectivity towards monomers as well as molecular sieving effect. However, to the best of our knowledge, N-zeolite scaffolds have not yet been investigated for DRM response.

Korean Patent Registration No. 10-2075011

In the present invention, N-ZSM-5 and N-beta zeolite prepared by nitriding ZSM-5 (MFI) and beta ( ^* BEA) zeolite, which are the most representative commercial zeolite catalysts, are Ni-impregnated catalysts (Ni/N-ZSM-5 and Ni/N-beta), and the enhancement of DRM catalytic activity using the nitrogen base site framework was studied. The presence of framework nitrogen atoms and their basic properties, enhanced dispersion of Ni active species were characterized using ²⁹ Si MAS NMR, CO ₂ and NH ₃ TPD, OH IR, XPS and H ₂ chemisorption. For comparison, acidic comparators impregnated with Ni (Ni/H-ZSM-5 and Ni/H-beta) were also investigated.

According to one embodiment of the present invention, a methane dry reforming (DRM) reaction including at least one of nitrogen-substituted Ni / N-ZSM-5 (MFI) zeolite and Ni / N-beta ( ^* BEA) zeolite A solute NiN-zeolite catalyst is provided.

The methane dry reforming (DRM) reaction temperature is characterized in that 700 ℃ or more.

The nitrided NiN-zeolite catalyst for the methane dry reforming (DRM) reaction is characterized in that it is a nitrogen-substituted nitrated Ni/N-ZSM-5 zeolite catalyst.

When the methane dry reforming (DRM) reaction temperature is 700 ° C, side reactions of methane cracking (MC) and Boudouard reaction (BR) are suppressed compared to when the reaction temperature is 650 ° C. It is characterized in that it is less than 20 wt%.

When the methane dry reforming (DRM) reaction temperature is 750 ° C., side reactions of methane cracking (MC) and Boudouard reaction (BR) are suppressed, and the amount of coke produced is less than 3.0 wt%. .

When the methane dry reforming (DRM) reaction temperature is 750 ° C, the CH ₄ and CO ₂ deactivation rates in the reaction using the nitrogen-substituted nitrided Ni / N-ZSM-5 zeolite catalyst are 2.3% and 1.6%, respectively. to be characterized

When the methane dry reforming (DRM) reaction temperature is 750 ° C, the conversion rate of CH ₄ and CO ₂ is 20% in the reaction by the nitrogen-substituted nitrided Ni / N-ZSM-5 zeolite catalyst, H ₂ /CO The ratio of is characterized in that 0.9 or less.

Nitrogen-substituted Ni / N-ZSM-5 zeolite and nitrated Ni / N-beta zeolite have Ni / H-ZSM-5 zeolite and Ni / H-beta zeolite in which nitrogen is not substituted, Ni / N-ZSM-5 zeolite in the 855 eV region It is characterized in that the 2p _3/2 spectrum intensity is 1.8 times or more.

According to the present invention, Ni/N-ZSM-5 and Ni/N-beta catalysts containing nitrogen in a zeolite framework were prepared and used for DRM reactions for the first time.

In the present invention, beta zeolite, which is structurally unstable and has many defects, contained about three times more nitrogen than ZSM-5 under the same nitriding conditions. The framework nitrogen provided new CO ₂ adsorption sites capable of adsorption in the high-temperature region as well as improved dispersion of Ni active sites. In the low-temperature DRM reaction, these nitrogen sites had a negative effect on DRM mainly by accelerating coke formation by BR. On the other hand, nitrogen in the framework of the zeolite promoted DRM at high temperatures and side reactions were suppressed. In the high-temperature DRM reaction, Ni/N-beta exhibited poor catalytic stability due to thermal instability of the N-beta support, whereas Ni/N-ZSM-5 exhibited high catalytic stability. After reacting at 750 °C for 12 hours, Ni/N-ZSM-5 showed about 20% higher conversion of both CH ₄ and CO ₂ than Ni/H-ZSM-5 due to the nitrogen substitution effect.

1 shows (a) powder XRD patterns of Ni/H-ZSM-5, Ni/N-ZSM-5, Ni/H-beta and Ni/N-beta before use and (b) 650, (c) for 12 hours ) shows powder XRD patterns of the used catalysts obtained at 700 and (d) 750 °C (patterns of Ni/N-ZSM-5 used after DRM at 650 °C are for samples after 77 min reaction).
Figure 2 is ^{a 29} MAS NMR spectrum of H-ZSM-5, N-ZSM-5, H-beta and N-beta zeolites used in the present invention: experimental data (top), simulation (middle), decomposed components (bottom) ).
3 shows CO ₂ -TPD (left) and NH ₃ -TPD (right) profiles of H-ZSM-5, N-ZSM-5, H-beta and N-beta zeolites used in the present invention.
4 is an IR spectrum in the region of 3000 to 4000 cm ^-1 of unreduced unused Ni/H-ZSM-5, Ni/N-ZSM-5, Ni/H-beta and Ni/N-beta (for comparison, N - Spectra for ZSM-5 and N-beta zeolites are also shown).
5 is N 1s and Ni 2p XPS spectra of Ni/H-ZSM-5, Ni/N-ZSM-5, Ni/H-beta and Ni/N-beta.
6 is a function of time of DRM for Ni/H-ZSM-5 (■), Ni/N-ZSM-5 (●), Ni/H-beta (▲) and Ni/N-beta (▼) at 650 (left), graphs showing CH ₄ conversion (top), CO ₂ conversion (middle), and H ₂ /CO ratio (bottom) at 700 (middle) and 750 °C (right).
7 shows CH ₄ conversion (left), CO ₂ conversion (middle), H ₂ / A graph showing the CO ratio (right) (x and y represent the flow rate and temperature (°C) of NH ₃ (mL min ^-1 ) in nitration, respectively).

Hereinafter, the present invention will be described in more detail through Examples and Experimental Examples. However, these examples are only for helping the understanding of the present invention, and the scope of the present invention is not limited to these examples in any sense.

Experiment

1.1 Catalyst preparation

H-zeolite was prepared by mixing commercial NH ₄ -ZSM-5 (Zeolyst CBV2314, SiO ₂ /Al ₂ O ₃ = 23) and NH ₄ -beta (Zolyst CP 814E, SiO ₂ /Al ₂ O ₃ = 25 at 550 °C for 2 h). ) was obtained by firing. N-zeolite was prepared by nitration of NH ₄ -zeolite according to a procedure reported in a previous study. About 1.0 g of NH ₄ -zeolite powder was put into the pipe and heated from room temperature (RT) to 500 ° C, and then maintained in the same state in N ₂ (100 mL min ^-1 ) for 2 hours. At this time, the flowing gas was pure NH ₃ stream (100 mL min ^-1 ) and then increased to 900 °C at a rate of 2 °C min ^-1 . In order to prepare N-zeolites having different nitrogen contents, the flow rate of NH ₃ and/or the nitriding temperature were adjusted to 200 mL min ^-1 and 500 °C, respectively. The furnace was maintained at the target temperature (500 °C or 900 °C) for 20 hours and then cooled in a flowing N ₂ (100 ml min ^-1 ) atmosphere. A Ni-supported catalyst with a metal content of 5 wt% was prepared through an initial impregnation method using an aqueous solution of Ni(NO ₃ ) ₂ 6H ₂ O (98% Samchun) and zeolite as a support. In order to minimize zeolite pore blocking due to excessive Ni content, all catalysts were prepared with the same Ni content of 5 wt %. The Ni/zeolite was dried overnight at 100 °C and then calcined at 500 °C for 3 h under air exposure. For comparison, a 5 wt% Ni-impregnated Ni/γ-Al ₂ O ₃ was also prepared in the same way.

1.2 Characterization

Powder X-ray diffraction (XRD) patterns were measured on a Rigaku SmartLab X-ray diffractometer with Cu-Kα radiation in the 2θ range from 3 to 50° (scan rate = 4° min ⁻¹ ). Elemental analysis of Al, Si, and Ni was performed on a PerkinElmer OPTIMA 7300DV inductively coupled plasma (ICP) spectrometer. The nitrogen content of the catalyst was measured using a Vario Micro CHNS elemental analyzer. Crystal morphology and average size were measured using a Jeol JSM-7800F scanning electron microscope (SEM) operating at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images were acquired with a FEITALOS F200X operating at 200 kV. N ₂ uptake experiments were performed on a Micromeritics Tristar II analyzer. H ₂ chemisorption measurements were performed using the dynamic pulse method (Micromeritics Autochem II 2920). After reduction at 700 °C with a flow of 10% H ₂ /Ar for 3 hours, the catalyst (0.1 g) was cooled to 40 °C in an Ar atmosphere, and H ₂ chemisorption was performed at this temperature. ^{The 29} Si MAS NMR spectrum was measured using a 500 MHz Avance III HD Bruker solid-state NMR spectrum at a rotation speed of 5 kHz, a ²⁹ Si frequency of 99.36 MHz, a π/2 rad pulse length of 4 μs, a recycle delay of 60 seconds, and a pulse delay of 300 pulses. . ²⁹ Si chemical shifts have been reported relative to TMS. IR spectra of the OH stretching region were measured on a Nicolet 6700 FT-IR spectrometer using about 12 mg self-supporting zeolite wafers with a diameter of 1.3 cm. Before the IR measurement, the samples were pretreated in vacuum at 550 °C for 2 h in an IR cell equipped with a ZnSe window. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 Versa Probe II with Al-Kα radiation (hυ = 1,486.6 eV). XPS samples were prepared as about 20 mg self-supporting zeolite wafers of 1.3 cm diameter in the dry state. Thermogravimetric analysis (TGA) was performed in air on a Hitachi STA7000 thermal analyzer, and weight loss associated with coke combustion was further confirmed by differential thermal analysis (DTA) using the same analyzer. All spectral decompositions and simulations were performed using the Origin 9.0 curve-fitting program.

The basic and acidic properties of H-zeolite and N-zeolite were determined by temperature programmed desorption (TPD) of CO ₂ and NH ₃ , respectively. CO ₂ TPD measurements were performed in a home TPD device equipped with a thermal conductivity detector (TCD). 0.1 g of the sample was activated in flowing Ar (30 ml min ^-1 ) at 550 °C for 2 h before CO ₂ adsorption. Then, pure CO ₂ was pulsed to the sample at room temperature (RT) for 0.5 hour, and then physically adsorbed CO ₂ was removed by blowing in He (30ml min ^-1 ) for 1 hour at the same temperature. CO ₂ TPD was obtained through flowing He (30ml min ^-1 ) at a rate of 10 °C min ^-1 from room temperature (RT) to 700 °C. NH ₃ TPD was also measured using the same analyzer. About 0.1 g of the sample was activated in flowing Ar (30 ml min ^-1 ) at 550 °C for 2 hours. Then, 10% NH ₃ (10ml min ⁻¹ ) was passed over the sample at 150° C. for 0.5 hour. The treated sample was then stripped with He at the same temperature for 1 hour to remove physically adsorbed NH ₃ . NH ₃ TPD was performed in flowing He (30 ml min ^-1 ) at a heating rate of 10 °C min ^-1 at 150 to 700 °C.

1.3 Catalytic reactions

DRM was performed under atmospheric pressure in a continuous flow instrument equipped with a fixed bed microreactor. Prior to the experiment, the catalyst was reduced with pure H ₂ (50 mL min ^-1 ) flowing at 700 °C for 3 hours, removed with N ₂ (60 mL min ^-1 ) at the same temperature for 0.5 hour, standard reaction conditions set up. The reaction temperature (650, 700 or 750 °C) was maintained to fix the allowable time for reactant/carrier gas partitioning. The flow rate of the feed gas composed of CH ₄ , CO ₂ , and N ₂ was 20, ²⁰ , ^and 10 mL ^min rate, which was fixed to compare the catalytic activity between the catalysts prepared in this invention. The reaction products were analyzed online on a TCD and a Scion 456 gas chromatograph equipped with a Carbosphere 60-80 mesh packed column (1830 Х 3.2 Х 2.0 mm), with the first analysis performed after 10 minutes. Molar balance conversion and H ₂ /CO ratio were calculated as follows.

CH ₄ conversion (%) = {(CH ₄ ) _in - (CH ₄ ) _out }/(CH ₄ ) _in Х 100

CO ₂ conversion (%) = {(CO ₂ ) _in - (CO ₂ ) _out }/(CO ₂ ) _in Х 100

H ₂ /CO ratio = [(H ₂ ) _out /2(CH ₄ ) _in ]/[(CO) _out /{(CH ₄ ) _in + (CO ₂ ) _in }]

2. Results and discussion

2.1 Physical and chemical properties of zeolite support and catalyst

Figure 1 is the powder XRD pattern of Ni/Zeolite (Fig. 1a) which was not used and the catalyst used after DRM at 650-750 °C (Fig. 1b-1d). Ni/H-ZSM-5 before use and Ni/N-ZSM-5 maintained their MFI structures after nitriding and/or Ni impregnation. However, the Ni/H-beta before use slightly lost crystallinity after Ni impregnation compared to the XRD pattern of the H-beta support. The crystallinity of Ni/N-beta was greatly reduced compared to that of Ni/H-beta, and an amorphous phase was formed. This indicates that the ^* BEA structure partially collapsed during nitridation at a high temperature of 900 °C because of its lower thermal stability than the MFI structure. ^* The low framework density of the BEA structure was more detrimental in high-temperature nitridation. On the other hand, X-ray peaks on NiO at 2θ = 37.3° (111) and 43.2° (200) were not observed in the four catalysts before use, indicating that NiO is in a highly dispersed state with small particles. In addition, as shown in FIGS. 1b-1d, among the catalysts used after DRM, neither the NiO phase nor the metallic Ni(111) phase was observed at 2θ = 44.5°, which means that the Ni particles were highly dispersed. Comparing the TEM images of the catalyst before and after DRM, it can be seen that the Ni particle size of the sample after reaction at 750 °C was larger than that of the catalyst before use, but the dispersion was still high.

^a Prepared with 100 cc/min of NH ₃ in a nitration reaction at 900 °C. ^b Determined by ICP analysis.

^c Determined by CHNS analysis. ^d Determined by SEM analysis. ^e Calculated from N ₂ adsorption data. ^f Values given in parentheses are BET areas for samples before initial impregnation. ^g 750 °C, catalyst used obtained after DRM for 12 hours. ^h Determined using catalyst reduced at 700 °C in a flow of 100 cc/min of 10% H ₂ /Ar.

Table 1 shows the physicochemical properties of the catalyst prepared in the present invention. The Si/Al ratios of Ni-impregnated catalysts can be compared (16.8-18.4). The same amount of Ni (5 wt.%) was impregnated, and the Ni/Al ratios (0.80–0.96) of all catalysts were almost similar. On the other hand, the N contents of Ni/N-ZSM-5 and Ni/N-beta were determined to be 0.7 and 2.2 wt.%, respectively. Under the same conditions, the nitrogen displacement framework appears to be easier in the beta zeolite structure than in ZSM-5. In fact, it is known that the degree of nitrogen substitution depends on the concentration of silanol (Si-OH) defect sites in the zeolite. In the OH IR spectrum of beta zeolite, a strong OH stretching vibrational peak for the external Si-OH group was observed. ^* The structure of BEA is well known to be highly disordered and consists of the random intergrowth of three polymorphs, A, B and C, which produce an unusually large number of defective silanols. Thus, framework nitrogen substitution appears to be easier in the beta zeolite structure than ZSM-5, and as discussed above, beta zeolites that are less thermally stable than ZSM-5 may break framework TOT bonds (T = Si and/or Al). Nitrogen replacement can also be made easier, as is the case. Nitrogen replacement of the zeolite framework by nitration can be confirmed through ²⁹ Si MAS NMR chemical shifts known to be observable in a relatively low magnetic field region. As shown in FIG. 2, a weak but clear band for N-ZSM-5 was additionally observed around -90 ppm compared to H-ZSM-5. N-beta is more pronounced in the Si-NT ²⁹ Si band around -91 ppm, which may be due to the higher nitrogen content of N-beta.

Table 1 also shows the particle size and BET surface area of the catalyst after DRM at 750 °C, before and after the reaction. H-beta with smaller crystal size (0.1 vs 1.0 μm) had a larger BET surface area than H-ZSM-5 (610 vs 400 m ² g ^-1 ). After nitration, the surface area of N-ZSM-5 and N-beta decreased by 15% and 36%, respectively, compared to the top zeolite. The significant decrease in surface area of N-beta is due to the partial collapse of the zeolite structure upon high-temperature nitration at 900 °C, which is in good agreement with the powder XRD results. In addition, after Ni impregnation, the BET area of Ni/zeolite decreased by 10~15% compared to the scaffold. This may be due to partial pore blocking by NiO and/or additional structural collapse due to additional heat treatment during the impregnation process. However, all pre-reaction Ni-impregnated catalysts still maintained a high surface area of 300 m ² g ^-1 or more.

Since CO ₂ and NH ₃ have acidic and basic properties, they can be used as probe molecules to analyze the base and acidic sites of solid materials, respectively. 3 shows the CO ₂ and NH ₃ TPD profiles for the four H-ZSM-5, N-ZSM-5, H-beta and N-beta scaffolds used in the present invention. All zeolite supports show strong CO ₂ desorption peaks in the relatively low temperature region below 350 °C. It has been reported that lattice oxygen in zeolite frameworks acts as a weak Lewis base, resulting in low-temperature desorption of CO ₂ . Here, when comparing H-ZSM-5 and H-beta, N-ZSM-5 showed a weak but distinct additional desorption peak above 350 °C, whereas N-beta showed a much stronger additional high-temperature peak over a wider range. point should be noted. This result may be due to the nitrogen substitution framework leading to the formation of new additional base sites. As a result, the N-zeolite support has a greater CO ₂ adsorption capacity in a high temperature region than the H-zeolite support. On the other hand, H-ZSM-5 and H-beta showed two major NH ₃ desorption peaks at 200 - 300 and 300 - 500 °C, corresponding to typical weak and strong acid sites, respectively. However, unlike the acidic comparator, N-ZSM-5 and N-beta only possessed weakly acidic sites of low intensity, which is an advantage in catalytic activity because acidic sites readily induce coke formation.

Figure 4 shows the IR spectra of the OH stretching region for two N-zeolite supports and four pre-reacted Ni/zeolite. Compared to Ni/H-ZSM-5, N-ZSM-5 and Ni/N-ZSM-5 have both imido (-NH-) and amido (-NH _{2 )} owing to the framework nitrogen species. ) showed three distinct but weakly distinct peaks at 3290 - 3390 cm ^-1 in response to NH stretching vibrations from the group. Therefore, as confirmed in the ²⁹ Si MAS NMR spectrum of the N-ZSM-5 scaffold, nitrogen atoms are still present in the ZSM-5 framework after Ni impregnation. However, it is interesting that an imido (-NH-) vibration band was clearly identified in N-beta, but an NH stretching vibration peak was not found in Ni/N-beta. The disappearance of the NH IR band in Ni/N-beta can be attributed to the shielding of NH functional groups by the Ni–N bond after Ni impregnation.

5 and Table 2 show the N 1s and Ni 2p XPS results for the Ni/zeolite before the four reactions prepared in the present invention. The N 1s spectra of Ni/N-ZSM-5 and Ni/N-beta have two peaks at 397.7 - 398.5 and 399.8 - 400.1 eV that can be assigned to the imido(-NH-) and amido(-NH ₂ ) functional groups, respectively. was fitted with Also, the first strong band (397.7 - 398.5 eV) can also be assigned to the Ni-N functional group. Comparing the total XPS peak areas of Ni/N-ZSM-5 and Ni/N-beta, the relative intensity was about 0.41, which is in good agreement with the results of elemental nitrogen analysis (~0.32, Table 1). In the Ni 2p XPS result, there are four types of spectra, the spectra of Ni 2p _3/2 and Ni 2p _1/2 at 854 - 858 and 872 - 876 eV, respectively, and their shake-up satellite peaks. am. Here, Ni 2p _1/2 is discussed based on Ni 2p _3/2 because it shows almost the same trend in all catalysts. Ni 2p _3/2 XPS spectra can be classified into four types according to binding energy. Namely, 852-853 eV metallic Ni ⁰ , 853-854 Ni ⁺ , 854-856 Ni ²⁺ (reducible free NiO) and NiN, 856-858 Ni ^δ+ (NiO _x interacting with zeolite lattice oxygen). As shown in FIG. 5 and Table 2, metal Ni ⁰ and Ni ⁺ species were not detected because all Ni/zeolites were fresh samples that were not reduced.

Comparison of the Ni 2p _3/2 spectra of Ni/H-ZSM-5 and Ni/H-beta revealed that in both catalysts, two Ni species interacting with the zeolite framework, i.e., reducible free NiO and NiO _x was observed, and the total peak area was similar (1750 vs. 1790). However, the relative intensities of the Ni species of Ni/H-ZSM-5 and Ni/H-beta were 29:71 and 58:42, respectively. That is, the relative ratio of NiO _x was higher in Ni/H-ZSM-5, and the reducible NiO was relatively higher in Ni/H-beta. As a result, it was confirmed that the amount of H ₂ chemisorption to Ni/H-beta was greater than that of Ni/H-ZSM-5 (0.07 vs 0.14 cm ³ g ^-1 , Table 1). On the other hand, when comparing the Ni 2p _3/2 spectra of Ni/H-ZSM-5 and Ni/N-ZSM-5, the intensities of the NiO _x species were found in Ni/H-ZSM-5 and Ni/N-ZSM-5. Although almost identical, the intensity in the 855 eV region representing the Ni-N bond was more than twice as high in Ni/N-ZSM-5. The spectrum of NiO _x in Ni/N-beta was also similar to that of Ni/H-beta, but the spectrum around 855 eV was about 1.8 times higher than that of Ni/H-beta. Therefore, Ni/N-ZSM-5 and Ni/N-beta were significantly different from Ni/H-ZSM-5 and Ni/H-beta (0.07 vs 0.12, 0.14 vs 0.25, table 1), H ₂ chemisorption appears to be greater than that of As a result, the amount of chemisorbed H ₂ increased as the relative intensity of the Ni 2p _3/2 XPS spectrum increased around 855 eV.

2.2. methane dry reforming reaction

2.2.1. Overall DRM Results: Non-Catalytic Thermal Effects

Prior to the catalytic experiments, it was confirmed that the H-ZSM-5 and N-ZSM-5 supports did not show initial activity for CH ₄ or CO ₂ conversion at 750 °C (~0%). Even when CO ₂ is adsorbed on the framework nitrogen base sites, DRM does not proceed unless CH ₄ is adsorbed on the Ni active sites. In addition, side reactions such as RWGS and BR are possible only when H ₂ and CO are generated by DRM, respectively. MC can occur on acidic supports (Fig. 3) but appears to be inhibited at high temperatures (750 °C). Therefore, the Ni active site appears to be essential especially for high-temperature reactions where side reactions are inhibited and DRM is most likely to occur.

^a After 77 min reaction. Determined by the formula ^b (10 min conversion - 12 h conversion)/10 min conversion ×100. ^c Amount of coke determined by TGA/DTA of spent catalyst obtained after DRM for 12 hours unless otherwise specified.

Figure 6 and Table 3 show the DRM results at 650, 700 and 750 °C for the four Ni-impregnated zeolite catalysts prepared in the present invention. CH ₄ conversion < CO ₂ conversion, CH ₄ deactivation rate > CO ₂ deactivation rate _{and H 2} /CO < 1 was always observed. This can be explained by the occurrence of RWGS as a dominant side reaction in all temperature ranges. WGS is possible in the range of temperatures performed, but RWGS is more likely because of the high _CO2 concentration and dry conditions. In addition, in the case of almost all catalysts, the higher the reaction temperature, the higher the initial activity, but the lower the deactivation rate. At higher reaction temperatures, as RWGS becomes less important, the H ₂ /CO ratio increases but is still less than 1. At the lower temperature of 650 °C, more than 56 wt% of coke was formed, which is much higher than the percentage of coke formed at 750 °C (< 3 wt%). At high temperatures, suppression of coke formation by RBR, SCG, etc. is also possible.

On the other hand, CH ₄ and CO ₂ conversions and H ₂ /CO ratios decreased with TOS for all catalysts in all temperature ranges (except for Ni/N-ZSM-5 at 750 °C, see below). The greater the rate of CH ₄ and CO ₂ deactivation, the greater the reduction of the H ₂ /CO ratio with TOS. Considering the Langmuir-Hinselwood mechanism, DRM is possible only when both CH ₄ and CO ₂ are adsorbed on the active site, but RWGS can proceed by adsorbing only CO ₂ . Therefore, when the active site is blocked and inactivated by coke formation according to TOS, RWGS is stochastically higher than DRM, and thus the reduction of the H ₂ /CO ratio is greater.

2.2.2. Catalytic reaction results at 650 °C

As above, DRM at 650 °C is generally characterized by low conversion of CH ₄ (< 56%), CO ₂ (< 63%) and fast deactivation (CH ₄ > 51% and CO ₂ > 45%). . As shown in Table 3, the coke content of the used catalyst after 12 hours in the continuous reaction phase was higher than 56%, indicating that the active site shielding by coke deposition at this temperature is the main cause of catalyst deactivation. As shown in Fig. 1b, X-ray peaks for the graphitic carbon (200) and (100) phases were detected at 2θ = 26° and 43° for the used catalyst obtained after DRM at 650 °C, respectively. In particular, the X-ray peak intensity on graphite (200) was very strong in the order of Ni/N-ZSM-5 > Ni/N-beta > Ni/H-beta > Ni/H-ZSM-5, which was 3) agrees well with the analyzed coke content. All catalysts used after DRM at 650 °C showed an exothermic DTA peak higher than 610 °C, which led to catalyst deactivation due to combustion of graphitic carbon.

As a result of comparing the catalyst results of Ni/H-ZSM-5 and Ni/H-beta at 650 °C, no significant difference was observed in CH ₄ conversion. However, Ni/H-beta showed higher CO ₂ conversion and lower H ₂ /CO ratio at 12 h. This seems to be due to the predominance of RWGS in Ni/H-beta. In addition, this catalyst showed a higher coke content than Ni/H-ZSM-5 (56.3 vs 67.0 wt.%) after reacting at 650 °C for 12 hours. Coke formation is a result of the same side reactions as MC and BR under the DRM reaction conditions, but MC increases both the CH ₄ conversion and the H ₂ /CO ratio, and BR causes a decrease in CO ₂ conversion and an increase in the H ₂ /CO ratio. Therefore, these two responses seem to conflict with current DRM results. Therefore, the unique catalytic performance (ie, higher CO ₂ conversion than CH ₄ and low H ₂ /CO ratio) and high coke formation appear to be mainly caused by the combined effect of RWGS and BR at 650 °C. As described above, Ni/H-beta has a smaller crystal size, larger BET surface area, and higher Ni diffusion characteristics than Ni/H-ZSM-5 (Table 1), leading to more active DRM as well as side reactions.

As a result of comparing the results of Ni/H-ZSM-5 and Ni/N-ZSM-5 at 650 °C, Ni/N-ZSM-5 has reduced CH ₄ (55.4 vs. 46.0%) and CO ₂ (54.3 vs. 52.9%). The initial conversion rate was higher in all of them, especially the initial CH ₄ conversion rate was the highest among the four catalysts. However, it should be noted here the excessive formation of coke (77 wt%) which prevented the continuation of the reaction by shutting down the continuous reactor after only 77 minutes. Ni/H-ZSM-5 had a higher concentration of acidic sites in the support than Ni/N-ZSM-5 (Fig. 3), but the former had a smaller amount of coke. This suggests that the coke of Ni/N-ZSM-5 at 650 °C was formed more by BR than by MC. RWGS also occurred with Ni/N-ZSM-5 as with the other three catalysts, but BR predominated and excess coke formed during such short TOS. Stoichiometrically, BR consumes 2 moles of CO and produces 1 mole of CO ₂ , lowering the conversion of CO ₂ and increasing the H ₂ /CO ratio. As a result, among all catalyst results, Ni/N-ZSM-5 showed only CH ₄ > CO ₂ conversion and H ₂ /CO ratio increase, and the TOS was maintained higher than 1. Therefore, it can be speculated that the adsorption of CO ₂ on the additional framework nitrogen base sites could make DRM and/or RWGS dominant and thus BR could dominate. These results demonstrate that framework nitrogen base sites are more likely to negatively affect DRM and form coke at relatively low temperatures. On the other hand, when comparing the catalyst results of Ni/H-beta and Ni/N-beta at 650 °C, there was no significant difference in initial reactivity between the two catalysts. However, the deactivation rate of Ni/N-beta was higher than that of Ni/H-beta, even though the coke content (60.2 vs. 67.0 wt%) of Ni/N-beta was low. As shown in Table 1, partially decayed N-beta is relatively weak to coke deposition and is more easily deactivated than H-beta.

2.2.3. Catalytic reaction results at 700 °C

As shown in Figure 6 and Table 3, CH ₄ (57 - 69%) and CO ₂ (67 - 79%) conversions and H ₂ /CO ratios (0.83 - 0.87) were all catalysts during DRM at 700 °C compared to 650 °C. increased in The initial activities of CH ₄ and CO ₂ for each catalyst at 700 °C were in the order of Ni/N-beta > Ni/H-beta > Ni/N-ZSM-5 > Ni/Ni/N-ZSM-5. Considering almost the same Ni content, this difference in initial activity can be attributed to the difference in the dispersion of Ni. Unlike 650 °C, the initial activation sequence at 700 °C is in good agreement with the amount of H ₂ chemisorbed (Table 1). This reflects that DRM was relatively dominant at 700 °C compared to 650 °C, and that side reactions were more suppressed. After reacting at 700 °C for 12 hours, the amount of coke was significantly reduced to less than 20 wt% compared to 650 °C, which also means that side reactions MC and BR were suppressed. What is interesting here is that Ni/N-ZSM-5 produced the highest amount of coke (17.6 wt.%) among the four catalysts at 700 °C as at 650 °C, but this catalyst produced the same CO ₂ as the other three catalysts. > CH ₄ conversion and H ₂ /CO ratio < 1. This phenomenon may also occur because coke formation by the BR side reaction was suppressed at 700 °C.

When the catalytic reaction results of Ni/H-ZSM-5 and Ni/N-ZSM-5 were compared at 700 °C, Ni/N-ZSM-5 had a higher initial conversion rate of CH ₄ and CO ₂ than Ni/H-ZSM-5. was higher, and apparently maintained higher activity throughout the 12-hour reaction. However, the coke content of Ni/N-ZSM-5 was about 3 times higher than that of Ni/H-ZSM-5 (17.6 vs 5.8 wt.%). When the DTA peak positions of the two catalysts were compared after the reaction, Ni/N-ZSM-5 was 510 to 570 °C, about 20 °C lower than Ni/H-ZSM-5, and also the lowest among all catalysts discussed here. This can be attributed to other properties of coke species, namely amorphous and activated carbon species such as carbon nanotubes (CNTs) and graphite-like coke. In general, the oxidation temperature of multilayer CNTs (about 550 °C) is lower than that of graphite (670 °C). Thus, the coke species formed on Ni/N-ZSM-5 are more CNT-like than graphite and do not block zeolite pores. As shown in FIG. 1c, although the amount of coke was three times higher, it was confirmed that there was no significant difference in the graphite X-ray peak intensities of the two catalysts used because the X-ray diffraction peaks of CNTs were generally lower in intensity than those of graphite. On the other hand, in the case of the reaction at 700 ° C, the initial activity of Ni / N-beta was the highest, but there was no significant difference in activity between Ni / H-beta and Ni / N-beta as at 650 ° C. Ni/H-beta showed a higher amount of coke compared to Ni/N-beta (2.6 vs 1.5 wt.%), but the high deactivation rate of Ni/N-beta also contributed to the structural instability of the N-beta scaffold, as discussed above. attributed to. As a result of comparing the results of Ni/N-ZSM-5 and Ni/N-beta at 700 °C, as at 650 °C, Ni/N-beta showed higher activity during 12 hours of reaction. This is partly due to the higher metal dispersion in Ni/N-beta and additionally to the framework nitrogen base sites (Table 1).

2.2.4. Catalytic reaction results at 750 °C

As shown in Figure 6 and Table 3, DRM at 750 °C showed significantly higher initial CH ₄ and CO ₂ conversions and H ₂ /CO ratio than at 700 °C. Very similar to the results of the catalytic reaction at 700 °C, the conversion rates of CH ₄ and CO ₂ were in the order of Ni/N-beta > Ni/H-beta > Ni/N-ZSM-5 > Ni/H-ZSM-5 . The amount of coke in all catalysts after 12 hours of reaction at 750 °C was less than 3 wt.%, indicating that BR was further suppressed. Ni/N-beta still showed the highest initial activity (CH ₄ conversion = 83.9%, CO ₂ conversion = 88.9%) among the four catalysts due to the CO ₂ adsorption capacity and the enhancement of Ni dispersion by the framework nitrogen base site. . However, as mentioned above, due to the structural instability of the N-beta carrier, the deactivation rates of CH ₄ and CO ₂ were 35.9 and 27.1%, respectively, higher than 700 °C. As shown in Fig. 1d, after DRM at 750 °C, the X-ray peak intensity of the Ni/N-beta used decreased by about 10% compared to that of the fresh sample. In addition, the BET surface area of Ni/N-beta used at 750 °C was 280 m ² g ^-1 , which is the lowest among all values obtained in the present invention (Table 1).

On the other hand, Ni/N-ZSM-5 at 750 °C showed the highest activity for 12 hours except for the initial activity. The CH ₄ and CO ₂ deactivation rates of this catalyst were only 2.3 and 1.6%, respectively, the highest among the four catalysts. Compared to Ni/H-ZSM-5, the framework nitrogen effect of Ni/N-ZSM-5 at 750 °C, where DRM is most dominant and side reactions related to coke formation are suppressed, is significantly reduced by 20% of CH ₄ and CO ₂ after 12 h. % led to high conversions and a nearly constant _H2 /CO ratio (~ 0.9) over the entire reaction time studied here. These results are almost the same as those of Ni/γ-Al ₂ O ₃ known as a commercial catalyst for DRM. This suggests that nitrided Ni/N-ZSM-5 is a suitable candidate for DRM catalyst. These results provide a better understanding of why basic supports with high thermal stability are needed for the development of highly active and stable DRM catalysts.

2.2.5. Nitriding effect of zeolite support in DRM

To more clearly confirm the enhancement of DRM activity by nitrogen substitution in the zeolite framework by nitration, a series of experiments were conducted with catalysts prepared with different nitrogen contents (Fig. 7). It is known that the degree of nitrification is influenced by the temperature and/or the flow rate of NH ₃ . In FIG. 7, the Ni/N-ZSM-5 (100-900) and Ni/N-beta (100-900) catalysts were synthesized at 900° C. at an NH ₃ flow rate of 100 mL min ^-1 , and the catalytic reaction results are shown in FIG. It was the same as the 700 ° C reaction result presented. As shown in FIG. 7, the Ni/N-ZSM-5 (200-900) and Ni/N-beta (200-900) catalysts synthesized twice at the same temperature of 900° C. at an NH ₃ flow rate of 200 mL min ⁻¹ did not show a significant difference from the corresponding (100-900) catalysts. However, the Ni/N-ZSM-5 (100-500) and Ni/N-beta (100-500) catalysts synthesized at the same flow rate of 100 mL min ^-1 but at a reduced nitridation temperature of 500 °C correspond to each other. showed obviously lower activity than the (100-900) catalyst. This is a clear evidence that the catalytic performance of the DRM is improved by the nitration of the zeolite support.

conclusion

Ni/N-ZSM-5 and Ni/N-beta catalysts containing nitrogen in a zeolite framework were first prepared and used for DRM reactions. The incorporation of nitrogen into the zeolite framework and its basic properties were confirmed using various analytical methods such as CHNS elemental analysis, ²⁹ Si MAS NMR, CO ₂ TPD, OH IR and XPS. Under the same nitridation conditions, the structurally unstable beta zeolite with many defects contained approximately three times more nitrogen than ZSM-5. The framework nitrogen provided new CO ₂ adsorption sites capable of adsorption in the high-temperature region as well as improved dispersion of Ni active sites. In the low-temperature DRM reaction, these nitrogen sites had a negative effect on DRM mainly by accelerating coke formation by BR. On the other hand, framework nitrogen of zeolite promoted DRM at high temperature and side reactions were suppressed. In the high-temperature DRM reaction, Ni/N-beta showed poor catalytic stability due to the thermal structural instability of the N-beta support, but Ni/N-ZSM-5 showed high catalytic stability. After reacting at 750 °C for 12 hours, Ni/N-ZSM-5 showed about 20% higher conversion of both CH ₄ and CO ₂ than Ni/H-ZSM-5 due to the nitrogen substitution effect. Based on the overall results of the present invention, several useful suggestions are presented for the development of more stable and active DRM catalysts.

The present invention described above is not limited to the above-described embodiments, since various substitutions and changes can be made to those skilled in the art within the scope of the technical idea of the present invention. .

Claims (8)

A nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction comprising at least one of a nitrogen-substituted nitrided Ni/N-ZSM-5 zeolite and a nitrated Ni/N-beta zeolite.
According to claim 1,
Nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction, characterized in that the methane dry reforming (DRM) reaction temperature is 700 ℃ or more.
According to claim 1,
The nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction is a nitrogen-substituted nitrated Ni / N-ZSM-5 zeolite catalyst.
According to claim 1,
When the methane dry reforming (DRM) reaction temperature is 700 ° C, side reactions of methane cracking (MC) and Boudouard reaction (BR) are suppressed compared to when the reaction temperature is 650 ° C. A nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction, characterized in that the content is less than 20 wt%.
According to claim 1,
When the methane dry reforming (DRM) reaction temperature is 750 ° C., side reactions of methane cracking (MC) and Boudouard reaction (BR) are suppressed, so that the amount of coke generated is less than 3.0 wt%. Nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction.
According to claim 1,
When the methane dry reforming (DRM) reaction temperature is 750 ° C, the CH ₄ and CO ₂ deactivation rates in the reaction by the nitrogen-substituted nitrided Ni / N-ZSM-5 zeolite catalyst are 2.3 and 1.6%, respectively. A nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction.
According to claim 1,
When the methane dry reforming (DRM) reaction temperature is 750 ° C, the conversion rate of CH ₄ and CO ₂ is 20% in the reaction by the nitrogen-substituted nitrided Ni / N-ZSM-5 zeolite catalyst, H ₂ /CO A nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction, characterized in that the ratio is 0.9 or less.
According to claim 1,
Nitrogen-substituted Ni / N-ZSM-5 zeolite and nitrated Ni / N-beta zeolite have Ni / H-ZSM-5 zeolite and Ni / H-beta zeolite in which nitrogen is not substituted, Ni / N-ZSM-5 zeolite in the 855 eV region A nitrided NiN-zeolite catalyst for methane dry reforming (DRM) reaction, characterized in that the 2p _3/2 spectral intensity is 1.8 times or more.

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