Synthesis And Characterization Of Pt-Supported Mesoporous LTA Zeolites

Mesopore-micropore hierarchical zeolites, in which the mesopore walls are comprised of microporous crystalline zeolite frameworks, are able to overcome diffusion limitations, and maintain stability and strong acidity. It combines advantages of mesoporous materials and zeolite crystals. In addition, Metal-supported materials are very important hydrogenation catalysts, with high hydrodearomatization activity. The issue integrates the advantages of two kinds of materials, synthesizing of Pt-supported mesoporous LTA zeolites.In this paper, mesoporous LTA zeolites and Pt-supported mesoporous LTA zeolites were successfully synthesized by performing a direct hydrothermal methods. The as-synthesized materials were characterized using complementary combination of X-ray diffraction(XRD), nitrogen adsorption-desorption measurements, scanning electron microscopy(SEM), transmission electron microscopy(TEM) and thermogravimetric analysis (TGA). The conclusions are as follow.1. Mesoporous LTA zeolites were successfully synthesized using sodium metasilicate nonahydrate (Na2SiO3·9H2O) and TPTAC(or TPOAC) as the silicon source and the mesopore-generating agent, respectively. The study has found that NaA zeolites were synthesized from n(surfactant)/n(SiO2) molar ratios of 0 to 0.08. The BET surface area, the mesoporous volume and the pore size increased with the n(surfactant)/n(SiO2) ratio increasing. The largest BET area of samples was 147 cm2/g using TPTAC as the mesopore-generating agent. and The largest BET area of samples was 164 cm2/g using TPOAC as the mesopore-generating agent. SEM images indicated that samples had wormhole-like mesopores.2. Mesoporous LTA zeolites were successfully synthesized using tetraethylorthosilicate (TEOS) and TPTAC as the silicon source and the mesopore-generating agent, respectively. The study has found that NaA zeolites were synthesized from n(TPTAC)/n(SiO2) molar ratios of 0 to 0.10. The mesoporous volume and the BET surface area increased with the increasing of n(TPTAC)/n(SiO2) ratio. However, BET specific surface area no longer increased after up to 244 m2/g. The mesopore-size distribution curves were centered at 4.6 nm from n(TPTAC)/n(SiO2) molar ratios of 0.04 to 0.08 and 5.7 nm at n(TPTAC)/n(SiO2) molar ratios of 0.10. In addition, addition sequence had a great influence on the mesoporous pore structure. That TPTAC was added before TEOS was more conducive to creating mesopores. SEM and TEM images further proved that the M-LTA samples had transgranular mesopores.3. We studied the different pore structure properties of mesoporous LTA zeolites synthesized with different silicon sources. The BET area synthesized with TEOS was 244 m2/g, was larger than that synthesized with Na2SiO3·9H2O (147m2/g). And mesoporous LTA zeolites synthesized with TEOS showed smaller apertures than those synthesized with sodium metasilicate nonahydrate. The TG curves suggest that there are more TPTAC organosilanes bonded to the LTA zeolite framework surface synthesized with TEOS than that synthesized with Na2SiO3·9H2O. The amount of TPTAC bonded to the LTA zeolite framework surface has a significant effect on the BET area of the samples.4. Mesoporous LTA zeolites were successfully synthesized using tetraethylorthosilicate (TEOS) and TPOAC as the silicon source and the mesopore-generating agent, respectively. The study has found that NaA zeolites were synthesized from n(TPOAC)/n(SiO2) molar ratios of 0 to 0.12. In addition, elevating temperature and increasing alkalinity can reduce the crystallization time, but it is easy to generate sodalite crystal at too high temperature and alkalinity conditions. N2 physisorption illustrated that the samples had a mesoporous structure. Mesoporous volume increased and then decreased with the increasing of n(TPOAC)/n(SiO2) ratio. The largest mesoporous volume could be up to 0.59 cm3/g. The BET surface area increased and then decreased as the n(TPOAC)/n(SiO2) ratio increases. The largest BET surface area could be up to 212 m2/g. The mesopore-size increased and then decreased with the increase of n(TPOAC)/n(SiO2) ratio. SEM and TEM images furtherproved that the M-LTA samples had transgranular mesopores.5. Pt-supported mesoporous LTA zeolites were successfully synthesized using sodium metasilicate nonahydrate (Na2Si03-9H20) and TPTAC as the silicon source and the mesopore-generating agent, respectively. XRD patterns of the samples exhibit diffraction peaks of platinum after adding Pt(NH3)4Cl2 into the synthetic gels. The best n(SiO2)/n(Al2O3) ratio is 1.1724. The crystallization time is 17-21h. Diffraction peaks of A-type zeolite dropped and finally disappear with the amount of Pt(NH3)4Cl2 increasing. As TPTAC increased, A-type zeolite diffraction peaks decreased and diffraction peaks of Pt(111) crystal face increased. In addition, the BET surface area, the mesopore size and the mesoporous volume decreased as the n(Pt(NH3)4Cl2)/n(Si02) ratio increased. The isotherms for the PtA zeolites contained no hysteresis loop. This reveals that PtA zeolites does not exist mesoporous structure. As the n(TPTAC)/n(SiO2) ratio increased, the BET surface area and the mesopore size increased, but the mesoporous volume decreased. SEM images indicated that samples had wormhole-like mesopores. H2-TPD spectra indicates that the peak intensity decreased and the desorption temperature moved forward with the increase of n(Pt(NH3)4Cl2)/n(Al203). Two desorption peaksγandδappeared, corresponding to Pt-H and Pt-H-Pt two adsorption states, respectively.6. Pt-supported mesoporous LTA zeolites were successfully synthesized using sodium metasilicate nonahydrate (Na2SiO3-9H2O) and TPOAC as the silicon source and the mesopore-generating agent, respectively. XRD patterns of the samples exhibit diffraction peaks of platinum after adding Pt(NH3)4Cl2 into the synthetic gels. The crystallization time is 24-32h. Diffraction peaks of A-type zeolite dropped and finally disappear with the amount of Pt(NH3)4Cl2 increasing. As TPOAC increased, A-type zeolite diffraction peaks decreased and diffraction peaks of Pt (111) crystal face increased. The higher the calcination temperature was, the higher the Pt peaks was. In addition, as the n(Pt(NH3)4Cl2)/n(SiO2) ratio increased, the BET surface area increased, but the mesopore size a remained almost at around 3.5nm and the mesoporous volume remained almost invariable. As the n(TPOAC)/n(SiO2) ratio increased, the BET surface area and the mesopore size increased, but the mesoporous volume decreased. SEM images indicated that samples had wormhole-like mesopores. H2-TPD spectra indicates that the peak intensity decreased and the desorption temperature moved forward with the increase of n(Pt(NH3)4Cl2)/n(Al2O3). Two desorption peaksγandδappeared, corresponding to Pt-H and Pt-H-Pt two adsorption states, respectively.