Research On The Synthesis And Modification Of New Catalytic Materials For Hydrocarbon Trapping In Automobile Exhaust Gas

With the rapid increase of the automobile, the increasingly serious air pollution problem has been caused by its emission. Hydrocarbon trap is required in automotive catalytic system to improve the cold start hydrocarbon emissions. Generally, the catalysts used in hydrocarbon trap possess high adsorption capacity, high hydrocarbon desorption temperature and good hydrothermal stability. Here, we focused on the design and development of new hydrocarbon trapping catalysts based on two kinds of molecular sieves. SSZ-33 molecular sieve with intersecting 10- and 12- membered ring (MR) pores is regarded as a promising candidate for hydrocarbon trapping. The synthesis process, and route have been thoroughly studied and optimized. In light of the catalytic performance of SSZ-33 catalyst, we develop a new-structure hydrocarbon trap catalyst using layered FER zeolites. Moreover, the correlation of their structure and properties are also investigated involving the study on the synthesis, modification, and crystallization mechanism of molecular sieve formation. The specific contents are as follows:1. Although SSZ-33 is the most promising hydrocarbon trap material, the study on the principle of synthesis and the structure and properties is limit. Here, we succeeded in dynamically studying the crystallization process of B-SSZ-33 formation using N,N,N-trimethyl-8-ammonium tricyclo [5.2.1.0] decane iodide (RI) as the structure-directing agent. The effects of temperature, seed crystal and anions of the structure-directing agent on the crystallization rate have been thoroughly investigated, a low-cost and high-efficiency synthesis method was proposed. The optimal conditions of B-SSZ-33 molecular sieve synthesis were proposed using the mixture (ROH:RI= 1:4) as the structure-directing agent. Also, the effect of Si/B and Na+/Si ratios in the synthetic gel on the catalytic performance of B-SSZ-33 was investigated. With the decrease of Si/B molar ratio, the temperature of toluene desorption on B-SSZ-33 increases because of the increase of the amount of acid and the strength of acid of B-SSZ-33 enhancing the interactions between toluene and molecular sieves. With the increase of Na+/Si molar ratio, the amount of the acid and surface areas of B-SSZ-33 may also increase, which result in a growth of toluene desorption temperature and amount.2. To improve the acidic properties of SSZ-33 molecular sieves, Al incorporated SSZ-33 (Al-SSZ-33) was synthesized by post-modification procedure and in-situ procedure. For the post-modification, A1-SSZ-33 was obtained by three different procedures, in which the resulting molecular sieves are addressed as Al-SSZ-33-DEX (direct exchange), Al-SSZ-33-INS (insertion), and Al-SSZ-33-IMP (impregnation). For the formation Al-SSZ-33-DEX with suitable acid properties, the B removal and the Al incorporation simultaneously occurred. Moreover, Si/Al ratio and pore properties can be changed along with the exchange time, the concentration of solution and the exchange temperature. Unlike the post-modification, Al-SSZ-33 was also first In-situ synthesized by temperature process control. And the difference of physical and chemical properties of Al-SSZ-33 molecular sieves between in-situ method and direct exchange method were investigated. The difference in synthesis mechanisms causes different aluminum distribution, acidity and pore properties. The temperature programmed desorption of toluene on H-Al-SSZ-33 was carried to study the performance in hydrocarbon trapping. Compared with H-B-SSZ-33, H-Al-SSZ-33-DE3 has a higher acid strength and shows a comparatively higher toluene desorption temperature (Tmax at 220 ℃). The presence of extra framework Al and Si species in Al-SSZ-33-DE3 modifies its pores and results in a higher desorption end temperature (Tend at 270℃). Therefore, the synthesized Al-SSZ-33-DE3 is a good candidate for hydrocarbon trapping. On the other hand, the uneven distribution of aluminum in H-Al-SSZ-33 (in-situ) molecular sieves might produce adsorption sites with different acid strength, which leads to a higher desorption end temperature (Tend> 300℃). Therefore, Al-SSZ-33 (in-situ) exhibits certain advantages not only in the synthesis method, but also in hydrocarbon trapping performance.3. Based on the plasticity of layered zeolites, the synthesis and in-situ modification of FER zeolite were also studied. The crystallization process and mechanism of FER zeolite formation was studied using in-situ solid-transformation method. By changing the OH-/Al2O3 molar ratios, the FER samples with different compositions, morphologies, pore structures and acid properties were prepared. With the increase of OH-/Al2O3 molar ratio in the synthesis gel, FER zeolite was in-situ delaminated in mesoscale instead of in microcosmic structure. In the temperature programmed desorption of toluene, toluene desorbs giving rise to a much larger peak during desorption at lower temperature and a second small peak during desorption at higher temperature. The maximum occurring at lower temperature was attributed to the toluene which was either adsorbed on the external surface of the zeolite crystals or at weak adsorption sites. The second peak was assigned to the toluene which was strongly adsorbed on acid sites. With the increase of OH-/Al2O3, the desorption temperature in the second peak increases because of more acid amount of H-FER and more surface area for adsorption from the secondary pores formatted by the gaps between the layers.4. In order to improve the hydrocarbon trapping performance of FER zeolite, FER composites have been developed by the post-treatment modification of FER zeolite. Composite micro/mesoporous molecular sieve with FER structure and ordered hexagonal mesoporous molecular sieve (ReFM) with FER secondary units were obtained in basic synthesis system using the dissolution of FER zeolite method. Composite micro/mesoporous materials possess the structure with microporous zeolite FER and mesoporous sieve MCM-41, and the composite material with proper micro-mesoporous proportion and acid strength and acid amount can be controlled by changing dissolution time of FER zeolite into alkaline solution. Moreover, compared to conventional MCM-41, mesoporous ReFM shows much stronger acidity and thicker pore walls and much higher hydrothermal stability, which owe to FER zeolite secondary units in its pore walls. The hydrocarbon trapping experiments showed that the curve of toluene temperature desorption on micro/mesoporous composite molecular sieve is similar to that on FER zeolite, in which both desorption peaks that representing weak adsorption sites and strong adsorption sites were observed respectively. With dissolution time of FER zeolite into alkaline solution extending, two peaks of toluene desorption on composite molecular sieves gradually moved closer to each other and the area of the two peaks gradually increased. For mesoporous ReFM its toluene desorption curve is similar to that of MCM-41 that there is only one large toluene desorption peak at lower temperature. While in the toluene desorption curve of ReFM, the peak span is larger, and the end temperature of toluene desorption comes close to 200 ℃. These show that two types of materials prepared by the post-treatment modification of FER zeolites showed somewhat improved hydrocarbon trapping performance.The research of present work will play a positive promoting role in the further application of molecular sieves for hydrocarbon trapping and the development of new catalytic materials.