HZSM-22的粒径调控及Pt/HZSM-22的正十二烷加氢异构催化性能

Particle size regulation of HZSM-22 and Pt/HZSM-22 for n-dodecane hydroisomerization

通过调控合成液中的水含量,采用过饱和溶液法,在静态水热条件下合成了三种不同c轴长度的ZSM-22分子筛,对其进行了X射线衍射、扫描电子显微镜、N2物理吸附、NH3-程序升温脱附和吡啶吸附傅里叶变换红外光谱(Py-FTIR)表征。并以所制备的ZSM-22分子筛为酸性载体,在其上负载0.5%(质量分数)的金属Pt作为加(脱)氢活性位点,制备成Pt/ZSM-22双功能催化剂。以正十二烷为探针分子,进行正构烷烃加氢异构化反应,考察了分子筛合成液中水含量对Pt/ZSM-22催化剂加氢异构化性能的影响。结果表明,随着合成液中水含量减少,ZSM-22分子筛粒径减小。但是当水含量低到一定值时,会有方英石和ZSM-5杂晶生成。当分子筛合成液摩尔组成为SiO_(2)∶Al_(2)O_(3)∶K_(2)O∶DEA∶H_(2)O=1∶0.01∶0.08∶0.29∶28时,获得的HZSM-22在所合成的三种分子筛中拥有最多的中、强Brønsted酸量以及较短的c轴长度,对应的双功能催化剂Pt/HZSM-22表现出最佳催化性能。在320℃时,正十二烷转化率为73.24%,异构烷烃收率为57.92%,异构烷烃选择性为79.09%,尤其是中心位置甲基异构体5-甲基十一烷的选择性达到了17.66%。

ZSM-22 zeolite crystals with different c-axis lengths were synthesized by using supersaturated solution method under static hydrothermal conditions and adjusting the water content in the synthetic solution.The as-synthesized ZSM-22 zeolite crystals were characterized by XRD,SEM,N2 physical adsorption,NH3-TPD and Fourier transformed infra-red spectroscopy of pyridine adsorption(Py-FTIR).By loading Pt on the ZSM-22 zeolite crystals,the Pt/ZSM-22 bifunctional catalysts were prepared,in which ZSM-22 zeolite crystals played the role of acid supports and the 0.5%Pt acts as(de)hydrogenation active sites.The hydroisomerization of n-alkanes was carried out with n-dodecane as the probe molecules.The effect of water content in the zeolite synthesis solution on the hydroisomerization performance of Pt/ZSM-22 catalyst was investigated.The results indicated that the less the water,the smaller the ZSM-22 crystals.However,a few cristobalite and ZSM-5 crystals were formed when the content of water in the synthetic solution was reduced to a certain value.HZSM-22 synthesized in the solution with the molar composition of SiO_(2)∶Al_(2)O_(3)∶K_(2)O∶DEA∶H_(2)O=1∶0.01∶0.08∶0.29∶28 showed the highest medium and strong Brønsted acid amount in the three HZSM-22 samples,and shorter c-axis length.The bifunctional catalyst Pt/HZSM-22 prepared with this kind of HZSM-22 crystals showed the best catalytic performance.The conversion of n-dodecane was 73.24%,the yield of isoalkanes was 57.92%,and the selectivity of iso-alkanes was 79.09%.Moreover,the selectivity of monobranched iso-dodecane with methyl branching positioned in the center of the chain,5-methylundecane,was up to 17.66%at 320℃.