| With the increase of people's environmental protection and health consciousness, the government have established strict standards to limit vehicle emission pollutants, which mainly include CO, hydrocarbon, NOx, and so on. The future ultra-low emission standard particularly focus on reducing hydrocarbons during cold-start . Recently, one of cold-start solutions is trapping hydrocarbons using adsorbent during cold-start and releasing them after the TWC has reached operating temperature. Zeolites are found to be the preferred adsorbents due to their stability under elevated temperature and high hydrocarbon adsorption capacity. Therefore, to investigat hydrocarbon adsorption performance of zoelite is very helpful for designing the hydrocarbon traps.The propane adsorption breakthrough curve and the propane temperature programmed desorption on a variety of zeolites were investigated to research the propane adsorption performance of these zoelites. The microstructure and adsorption performance of 13X zoelites were modified by metal ion-exchanging. The change of crystal structure and the propane adsorption performance of 13X zoelites were investigated afterion-exchanging by K+,Ag+,Ba2+,Ce3+,Li+,Mn2+. And the effect of the concentration of ion-exchanging solution, ion-exchanging time, activating temperature, bi-metal ion-exchanging on ion-exchanging degree and the propane adsorption performance of 13X zoelites were also studied in the research.1. Comparative research on the propane adsorption performance of zoelitesThe propane adsorption breakthrough curves of SAPO-5, SAPO-34, MCM-41, 13X, 5A,ZSM-5 were measured. Generally, the lower Si/Al ratio resulted in the higher adsorption intensity and capacity for the SAPO-5. But the higher Si/Al ratio lead to higher adsorption intensity and capacity for the SAPO-34. The breakthrough time of 5A (especially from nankai) was longer than that of 4A and 3A, and it has higher adsorption capacity than 4A and 3A. The propane and propylene breakthrough time of ZSM-5-4 was longer than other ZSM-5 zelites. The MCM-41 has adsorption capacity as low as SAPO-34. The adsorption capacity of 13X was comparable to that of 5A from nankai. In a word, SAPO-5,SAPO-34,MCM-41 have a low adsorption capacity. ZSM-5 with a certain Si/Al ratio,13X and 5A were the preferred propane adsorbents.From the propane TPD of zeolites, we know that the adsorption capacity of ZSM-5-3,13X and 5A were the highest in all investigated zelites. The propane desorption temperature of 13X and 5A were high (430℃).From above, 13X and 5A were the preferred propane adsorbents. So, the detailed ion-exchanging experiment was carried out to study the propane adsorption performance of 13X and 5A zoelites.After 5A zoelites were ion-exchanged with Ag+ and Ba2+, the weak desorption peak at low temperature decreased and the strong desorption peak at high temperature increased. So, the 5A zoelites after ion-exchanging with Ag+ and Ba2+ have higher desorption temperature than fresh 5A zoelites. After 13X and 5A zoelites were ion-exchanged with Ag+, the propane desorption temperature or adsorption capacity increased2. The effect of ion-exchanging modification on adsorption performance for 13X zoelitesFrom the XRD spectra of the modified 13X zeolites, we know that the number and intensity of diffraction peaks have not changed much. Metal ion exchange does not affect the skeleton, just changes the local pore in zeolites.The C3H8-TPD of 13X zoelites after K and Ag ion-exchanging were similar to that of the fresh 13X zoelites, which had weak desorption peak at low temperature and strong desorption peak at high temperature. But the adsorption capacity of K-13X decreased. The Mn ions modified 13X zeolite has a smaller adsorption capacity than 13X. The weak desorption peaks at low temperature of Ce, Li, Ba ions modified 13X become smaller, high-temperature desorption peaks intensity become larger. Their desorption temperature can reach to 300℃, higher than the three-way catalyst light-off temperature 225℃.As the concentration of Ag+ solution increase, the strong desorption peak of 13X zoelites keep unchanged, but the weak desorption peak increased. It made the total adsorption capacity increased. Therefore, Ag ion-exchanging probably only happened on the 13X zoelite's surface, and Ag can become propane adsorption active site when Ag on 13X zoelite's surface increase to a extent.With the increase of the concentration of cerium ion exchange, ion exchange degree of Ce ion modified 13X zoelite (76.60%) increased and high temperature desorption capacity also increased. So Ce ion exchange can increase 13X zeolite desorption temperature of propane.Barium ion-exchange of 13X zeolite have greater peaks and area than unmodified 13X, also greater than the peak (1.1) and area of Ag-13X. The 13X zoelite with high Ba ion-exchanging degree not only has high totle adsorption capacity, but also a strong desorption peak at about 570℃. So Ba ion-exchanging has good effect on 13X zoelite's adsorption performance.With the increase of ion exchange time, ion-exchanging degree of sample increase and desorption capacity also increased. With the increase of calcination temperature, ion exchange degree of cerium ion modified 13X zeolite samples was reduced, but the desorption volume increased and then decreased. The propane adsorption performance of 13X zoelite with bi-metal ion-exchanging was between the adsorption performance of the two 13X zoelite with single metal ion-exchanging respectively.The optimal conditions for ion-exchanging of Ce-13X and Ba-13X: the concentration of ion-exchanging solution is 0.2 mol/L, ion-exchanging time is 3 h, calcination temperature is 450℃. At this point, the adsorption capacity and desorption temperature of Ce-13X and Ba-13X is higher, so is suitable for low temperature cold start of the HC adsorption material.
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