AFI-CHA hybrid crystal zeolite and NH3-SCR catalyst using same as carrier, and preparation methods thereof

Abstract

An AFI-CHA hybrid crystal molecular sieve and an NH.sub.3-SCR catalyst using the AFI-CHA hybrid crystal molecular sieve as a carrier, and preparation methods thereof are disclosed. The AFI-CHA hybrid crystal molecular sieve includes an AFI-type SAPO-5 molecular sieve and a CHA-type SAPO-34 molecular sieve, with hybrid crystal grains of AFI and CHA. The hybrid crystal molecular sieve is synthesized by a hydrothermal synthesis method and can be obtained by changing the structure directing agent, the heating rate and the calcinating temperature in the preparation process. Further, copper is loaded on the basis of the hybrid crystal molecular sieve to prepare copper-based NH.sub.3-SCR catalyst and corresponding monolithic catalyst. The catalytic activity and hydrothermal stability of the catalyst are significantly improved by the hybrid crystal molecular sieve.

Claims

1. A Cu/AFI-CHA supported zeolite catalyst based on an AFI-CHA hybrid crystal zeolite, wherein Cu is supported on the AFI-CHA hybrid crystal zeolite; the AFI-CHA hybrid crystal zeolite comprises an AFI-type SAPO-5 zeolite and a CHA-type SAPO-34 zeolite, grains of the AFI-CHA hybrid crystal zeolite are hybrid crystal grains of AFI and CHA; and the AFI-CHA hybrid crystal zeolite is prepared by the following method: (1) performing a feeding according to a molar ratio of aluminum:phosphorus:silicon:a structure directing agent:water in the AFI-CHA hybrid crystal zeolite of (0.06-0.10):(0.06-0.10):(0.040-0.050):(0.12-0.35):(0.5-6), mixing pseudoboehmite, phosphoric acid, and water uniformly to obtain a first mixture, stirring and aging the first mixture at the room temperature to prepare a P/Al oxide gel, and then adding a Si source, the structure directing agent and water into the P/Al oxide gel to obtain a second mixture, aging the second mixture at room temperature to obtain a first mixed solution; and (2) then keeping the first mixed solution at 120° C.-160° C. for 2-20 h, then heating to 190° C.-210° C. and holding for 5-30 h, cooling to the room temperature and separating a solid from the first mixed solution, then calcinating the solid at 500° C.-600° C. for 5-8 h to remove the structure directing agent to obtain an AFI-CHA hybrid crystal zeolite powder; wherein a heating rate of step (2) is 3-6° C..Math.min.sup.−1; in step (1) and step (2), zeolites with different AFI/CHA hybrid crystal proportions are obtained by selecting different values for at least one of the molar ratio of the structure directing agent, a keeping temperature of the first mixed solution, a calcinating temperature of the solid, and the heating rate.

2. The Cu/AFI-CHA supported zeolite catalyst according to claim 1, wherein the pseudoboehmite is 8-15 parts by weight, the phosphoric acid is 13-23 parts by weight, the Si source is 5-8 parts by weight, and the structure directing agent is 1-41 parts by weight.

3. The Cu/AFI-CHA supported zeolite catalyst according to claim 1, wherein the structure directing agent is triethylamine or tetraethyl ammonium hydroxide.

4. A monolithic catalyst, comprising a substrate and a powder of the Cu/AFI-CHA supported zeolite catalyst according to claim 1, wherein the powder of the Cu/AFI-CHA supported zeolite catalyst is washcoated on a cordierite monolith substrate.

5. A method for preparing the Cu/AFI-CHA supported zeolite catalyst according to claim 1, comprising using a copper salt compound as a precursor to prepare a copper salt solution, mixing and stirring the AFI-CHA hybrid crystal zeolite with the copper salt solution to obtain a third mixture until the third mixture is viscous, to make the Cu evenly disperse into the AFI-CHA hybrid crystal zeolite; after drying the third mixture in a water bath, performing a calcination on the third mixture in a muffle furnace at 300° C.-600° C. for 3-6 h to obtain a powder of the Cu/AFI-CHA supported zeolite catalyst.

6. A method for preparing the monolithic catalyst according to claim 4, comprising preparing the powder of the Cu/AFI-CHA supported zeolite catalyst into a slurry and washcoating the slurry on the cordierite monolith substrate to obtain a washcoated substrate, wherein an amount of the Cu/AFI-CHA supported zeolite catalyst has a loading amount on the cordierite monolith substrate is 120-130 g.Math.L.sup.−1; drying the coated substrate at 70° C.-150° C., and then calcinating the coated substrate at 300° C.-600° C. for 3-6 h.

7. The Cu/AFI-CHA supported zeolite catalyst according to claim 2, wherein the structure directing agent is triethylamine or tetraethyl ammonium hydroxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an x-ray diffraction (XRD) diagram of Cu/SAPO-34, Cu/SAPO-5 and Cu/AFI-CHA catalysts;

(2) FIG. 2A is a scanning electron microscope (SEM) diagram of the Cu/SAPO-34 catalyst prepared by comparative example 1;

(3) FIG. 2B is an SEM diagram of the Cu/AFI-CHA catalyst prepared by embodiment 1;

(4) FIG. 2C is an SEM diagram of the Cu/AFI-CHA catalyst prepared by embodiment 2;

(5) FIG. 2D is an SEM diagram of the Cu/AFI-CHA catalyst prepared by embodiment 3;

(6) FIG. 2E is an SEM diagram of the Cu/SAPO-5 catalyst prepared by comparative example 2;

(7) FIG. 3 is an XRD diagram of Cu/SAPO-34 and Cu/AFI-CHA catalysts after high-temperature hydrothermal aging;

(8) FIG. 4A is an SEM diagram showing the fresh Cu/SAPO-34 catalyst of comparative example 3;

(9) FIG. 4B is an SEM diagram showing the fresh Cu/AFI-CHA catalyst of embodiment 5;

(10) FIG. 4C is an SEM diagram showing the Cu/SAPO-34 catalyst after high-temperature hydrothermal aging of comparative example 4;

(11) FIG. 4D is an SEM diagram showing the Cu/AFI-CHA catalyst after high-temperature hydrothermal aging of embodiment 6;

(12) FIG. 5 is a diagram showing NH.sub.3-SCR activities of Cu/SAPO-34, Cu/SAPO-5 and Cu/AFI-CHA catalysts; and

(13) FIG. 6 is an SEM diagram showing Cu/SAPO-34 and Cu/AFI-CHA catalysts before and after high-temperature hydrothermal aging.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(14) The present invention is further described hereinafter with reference to the specific embodiments. The method of the present invention is further described below through specific embodiments. The following embodiments are merely a part of the embodiments of the present invention rather than all. All other embodiments derived based on the embodiments of the present invention by those of ordinary skill in the art without creative efforts shall be considered as falling within the protective scope of the present invention.

Embodiment 1

(15) 15 g pseudoboehmite, 23 g phosphoric acid and 72 g water are mixed uniformly, stirred for 12 h, and aged at room temperature for 12 h to prepare a P/Al oxide gel. 6 g of 44 nm Si sol provided by Shanghai Nalco Company as a Si source, 20 g of structure directing agent TEA and 36 g of distilled water are added into the P/Al oxide gel, which is mixed evenly and stirred for 24 h, and then aged at room temperature for 12 h to obtain a mixed solution. The mixed solution is transferred to a hydrothermal kettle, followed by the hydrothermal reaction in an oven at a heating rate of 5° C..Math.mind to 160° C. and held for 10 h, and then heated to 200° C. and held for 20 h. After being cooled into the room temperature, centrifugation and washing are carried out for 4 times. Final, the structure directing agent is removed at 550° C. for 6 h to obtain SAPO-34 and SAPO-5 hybrid AFI-CHA zeolite powders. According to the main diffraction peak intensities of SAPO-5 and SAPO-34, the ratio of SAPO-5/SAPO-34 is 3.16.

Embodiment 2

(16) 15 g pseudoboehmite, 23 g phosphoric acid and 72 g water are mixed uniformly, stirred for 12 h, and aged at room temperature for 12 h to prepare a P/Al oxide gel. 6 g of 44 nm Si sol provided by Shanghai Nalco Company as a Si source, 35 g of structure directing agent TEA and 36 g of distilled water are added into the P/Al oxide gel, which is mixed evenly and stirred for 24 h, and then aged at room temperature for 12 h to obtain a mixed solution. The mixed solution is transferred to a hydrothermal kettle, followed by the hydrothermal reaction in an oven at a heating rate of 4° C..Math.min.sup.−1 to 160° C. and held for 10 h, and then heated to 200° C. and held for 20 h. After being cooled into the room temperature, centrifugation and washing are carried out for 4 times. Final, the structure directing agent is removed at 550° C. for 6 h to obtain AFI-CHA zeolite powders.

Embodiment 3

(17) 15 g pseudoboehmite, 23 g phosphoric acid and 54 g water are mixed uniformly, stirred for 12 h, and aged at room temperature for 12 h to prepare a P/Al oxide gel. 6 g of 44 nm Si sol provided by Shanghai Nalco Company as a Si source, 20 g of structure directing agent TEA and 54 g of distilled water are added into the P/Al oxide gel, which is mixed evenly and stirred for 24 h, and then aged at room temperature for 12 h to obtain a mixed solution. The mixed solution is transferred to a hydrothermal kettle, followed by the hydrothermal reaction in an oven at a heating rate of 6° C. mind to 160° C. and held for 10 h, and then heated to 200° C. and held for 20 h. After being cooled into the room temperature, centrifugation and washing are carried out for 4 times. Final, the structure directing agent is removed at 550° C. for 6 h to obtain AFI-CHA zeolite powders.

Embodiment 4

(18) 15 g pseudoboehmite, 23 g phosphoric acid and 72 g water are mixed uniformly, stirred for 12 h, and aged at room temperature for 12 h to prepare a P/Al oxide gel. 6 g of 44 nm Si sol provided by Shanghai Nalco Company as a Si source, 12 g of structure directing agent TEA and 36 g of distilled water are added into the P/Al oxide gel, which is mixed evenly and stirred for 24 h, and then aged at the room temperature for 12 h to obtain a mixed solution. The mixed solution is transferred to a hydrothermal kettle, followed by the hydrothermal reaction in an oven at a heating rate of 3° C..Math.min.sup.−1 to 160° C. and held for 10 h, and then heated to 200° C. and held for 20 h. After being cooled into the room temperature, centrifugation and washing are carried out for 4 times. Final, the structure directing agent is removed at 550° C. for 6 h to obtain AFI-CHA zeolite powders.

Comparative Example 1

(19) 15 g pseudoboehmite, 17 g phosphoric acid and 48 g water are mixed uniformly, stirred for 12 h, and aged at room temperature for 24 h to prepare a P/Al oxide gel. 8 g of 44 nm Si sol provided by Shanghai Nalco Company as a Si source, 35 g of structure directing agent TEAOH and 24 g of distilled water are added into the P/Al oxide gel, which is mixed evenly and stirred for 12 h, and then aged at the room temperature for 12 h to obtain a mixed solution. The mixed solution is transferred to a hydrothermal kettle, followed by the hydrothermal reaction in an oven at a heating rate of 3° C..Math.min to 160° C. and held for 20 h, and then heated to 200° C. and held for 30 h. After being cooled into room temperature, centrifugation and washing are carried out for 5 times. Final, the structure directing agent is removed at 600° C. for 5 h to obtain SAPO-34 zeolite powders.

Comparative Example 2

(20) 8 g pseudoboehmite, 13 g phosphoric acid and 12 g water are mixed uniformly, stirred for 12 h, and aged at room temperature for 12 h to prepare a P/Al oxide gel. 6 g of 44 nm Si sol provided by Shanghai Nalco Company as a Si source, 10 g of structure directing agent TEA and 8 g of distilled water are added into the P/Al oxide gel, which is mixed evenly and stirred for 1 h, and then aged at the room temperature for 24 h to obtain a mixed solution. The mixed solution is transferred to a hydrothermal kettle, followed by the hydrothermal reaction in an oven at a heating rate of 1° C..Math.min.sup.−1 to 140° C. and held for 10 h, and then heated to 200° C. and held for 20 h. After being cooled into the room temperature, centrifugation and washing are carried out for 2 times. Final, the structure directing agent is removed at 500° C. for 8 h to obtain SAPO-5 zeolite powders.

(21) These synthesized zeolites (embodiments 1-4 and comparative examples 1-2) are applied to prepare catalysts according to the following method.

(22) The Cu-supported AFI/CHA hybrid crystal zeolite catalyst is prepared with a copper salt compound (such as Cu(NO.sub.3).sub.2, Cu(Ac).sub.2 and Cu(COOH).sub.2) as a precursor. The copper salt compound is dissolved in deionized water to form a solution with a certain concentration of metal salt. 10 g of the prepared SAPO-34, SAPO-5 and AFI-CHA hybrid crystal zeolites are poured into the copper salt solution and stirred for 30 min to obtain a mixture. The stirring is continued until the mixture becomes viscous to ensure that the copper species are uniformly dispersed into the pore of zeolite. The mixture is left to stand for 10 h and then is dried to a water bath at 80° C. for 6 h, and then calcined in a muffle furnace at 500° C. for 4 h to obtain Cu/CHA, Cu/AFI and Cu/AFI-CHA supported catalyst powders, respectively.

(23) The Cu/SAPO-34, Cu/SAPO-5 and Cu/AFI-CHA zeolite-based catalysts were respectively washcoated on a monolithic cordierite (Corning Corporation, USA, 400 mesh, 2.5 cm.sup.3) by the following technology. The specific method is as follows.

(24) a) The obtained Cu/SAPO-34, Cu/SAPO-5 and Cu/AFI-CHA zeolite-based catalyst powders are mixed uniformly with zirconium acetate and acetic acid respectively to form a coating slurry; a solid content of the slurry is controlled to 45%, a content of the zirconium acetate in the slurry is 3%, and a content of the acetic acid is 3%.

(25) b) The slurry is washcoated on a cordierite honeycomb ceramic substrate, and the loading amount of the catalyst is 180 g.Math.L.sup.−1; and

(26) c) the coated substrate is dried to 100° C., and then calcined at 500° C. for 4 h to obtain monolithic Cu/SAPO-34, Cu/SAPO-5 and Cu/AFI-CHA catalysts, respectively.

(27) When the copper is supported and the monolithic catalyst is prepared as described above, the loading amount of copper is the same as the loading amount of the catalyst on the monolithic catalyst.

(28) The structures of the zeolites prepared in embodiments 1-4 and comparative examples 1-2 are analyzed by XRD, and the results are shown in FIG. 1. It can be found that SAPO-34 with a CHA structure, SAPO-5 with an AFI structure, and AFI-CHA hybrid crystal zeolites with different proportions have been synthesized.

(29) The morphology of pure crystals and hybrid crystal SAPO-n are characterized by SEM, shown in FIGS. 2A-E. SAPO-34 are cubic crystals, which is consistent with the typical SAPO-34 morphology reported in the literature. AFI-CHA hybrid crystals have not only cubic crystals, but also hexagonal prism-shaped grains, which is the typical morphology of SAPO-5.

(30) Catalytic Performance Test

(31) The NH.sub.3-SCR performance is tested in a self-assembled multi-channel fixed continuous flow micro-reactor in the laboratory. The simulated diesel vehicle exhaust composition and experimental conditions are shown in Table 1. Before the test, the monolithic catalyst is pretreated in the reaction gas (simulated diesel vehicle exhaust) at 550° C. Then, the unconverted NO, NO.sub.2 and N.sub.2O are recorded with a Fourier transform infrared (FTIR) gas analyzer (Thermo Fisher Scientific), and then the activity is calculated at the test temperature.

(32) TABLE-US-00001 TABLE 1 Simulated exhaust conditions Simulate diesel Air- vehicle exhaust speed components NO NH.sub.3 O.sub.2 H.sub.2O N.sub.2 (h.sup.−1) Simulated diesel 200 ppm 200 ppm 10 vol. % 5 vol. % Bal- 40,000 vehicle exhaust ance content gas

(33) The NH.sub.3-SCR activity of Cu/CHA, Cu/AFI and Cu/AFI-CHA zeolite catalysts are shown in FIG. 5. The comparative example 2 with an AFI structure exhibits the poor NH.sub.3-SCR activity, with the highest conversion reaching 65% only at 450° C.; the activities of embodiment 2 and embodiment 3 are not significantly different from that of comparative example 1 with a CHA structure; while the activity of fresh Cu/AFI-CHA (embodiment 1) is significantly higher than that of comparative example 1; and the difference in maximum NO.sub.x conversion between embodiment 1 and comparative example 1 is about 13% at 300° C.-150° C.

Embodiment 5

(34) In order to investigate the influence of the AFI-CHA hybrid crystal structure on the hydrothermal stability of the monolithic Cu/AFI-CHA catalyst, Cu/AFI-CHA catalyst is prepared using 10 g of AFI-CHA zeolite according to the same method mentioned above (incipient-wetness impregnation method) as the fresh catalyst.

Embodiment 6

(35) In order to investigate the influence of the AFI-CHA hybrid crystal structure on the hydrothermal stability of the monolithic Cu/AFI-CHA catalyst, the monolithic Cu/AFI-CHA catalyst prepared in embodiment 5 is hydrothermally aged at 800° C. for 12 h in the flowing air containing 10 vol. % H.sub.2O, and then its performance is evaluated. The composition of the reaction gas and the test method are the same as those in the performance test of catalyst.

Comparative Example 3

(36) In order to investigate the influence of the SAPO-34 structure with the CHA structure on the hydrothermal stability of the monolithic Cu/CHA catalyst, Cu/CHA catalyst is prepared using 10 g of SAPO-34 zeolite with the CHA structure as a comparative example 3 according to the same incipient-wetness impregnation method above. Then, the obtained Cu/CHA powder was washcoated on the cordierite honeycomb ceramic substrate to obtain the monolithic Cu/CHA NH.sub.3-SCR catalyst for diesel vehicles.

Comparative Example 4

(37) In order to investigate the hydrothermal stability of the monolithic Cu/CHA catalyst, the monolithic Cu/CHA catalyst is hydrothermally aged at 800° C. for different times (24-72 h) in the flowing air containing 10 vol. % H.sub.2O. The composition of the reaction gas and the test method are the same as those in Table 1.

(38) The XRD diagram of FIG. 3 shows that the embodiment 3 is consistent with the diffraction peak of typical SAPO-34 structure reported in the literature. However, in addition to the XRD diffraction peak of SAPO-34, the characteristic diffraction peak of SAPO-5 also appears in the AFI-CHA hybrid crystal zeolite of embodiment 5. After high-temperature hydrothermal aging, the diffraction peak intensities of SAPO-34 of both samples are decreased, but the diffraction peak intensity of SAPO-34 of pure crystals of comparative example 4 decreases more obviously than that of the hybrid crystals of embodiment 6. In addition, the characteristic diffraction peaks of SAPO-5 in the hybrid crystal samples increased in intensity after high-temperature hydrothermal aging. Thus, AFI-CHA hybrid crystals are maintained better after the high-temperature hydrothermal aging.

(39) FIGS. 4A-D shows that the sizes of SAPO-34 crystals over both fresh samples are 2-5 μm. After high-temperature hydrothermal aging, most of the cubic structure of Cu/SAPO-34 are obviously destroyed, and some amorphous appear. In contrast, Cu/AFI-CHA remains better structural integrity than Cu/SAPO-34 after hydrothermal aging. These results show that Cu/AFI-CHA hybrid crystals are more stable after the high-temperature hydrothermal aging, which is conducive to the better high-temperature hydrothermal stability.

(40) FIG. 6 shows the NH.sub.3-SCR activity of comparative example 3 and embodiment 5 before and high-temperature hydrothermal aging. It can be found that the NO.sub.x conversion of both fresh catalysts are similar in the temperature range of 200° C.-450° C., with the highest conversion about 90%. At the low temperature (150° C.-200° C.), the activity of embodiment 5 is better than that of comparative example 3, which may be due to that the larger proportion of mesopores in AFI-CHA is beneficial to the diffusion of reaction gas at the low temperature. At the high temperature (450° C.-550° C.), the comparative example 3 exhibits the better activity. After the high-temperature hydrothermal aging, the activity of embodiment 6 is maintained better than that of comparative example 4, the maximum NO.sub.x conversion of the aged catalyst in embodiment 6 is almost unchanged, while the highest NO.sub.x conversion of comparative example 4 decreases by 8%, indicating that the catalyst prepared by the method as the present invention has the better low-temperature activity and hydrothermal stability.

(41) In addition, the amount of by-product N.sub.2O of catalysts before and after hydrothermal aging are less than 3 ppm, indicating the high N.sub.2 selectivity before and after hydrothermal aging.