ZSM-12分子筛的化学修饰及其催化性能研究

【作者】 孙可一；

【导师】 吴伟；

【作者基本信息】 黑龙江大学 ， 化学工艺， 2008， 硕士

【摘要】 本文用水热法合成了HZSM-12沸石分子筛,分别用化学液相沉积法(CLD)和脱铝方法对HZSM-12分子筛进行了改性,并采用XRD、FT-IR、SEM、N2吸附、27Al NMR、29Si NMR、NH3-TPD、Py-IR等分析手段对其改性前后的结构和酸性进行了表征,以萘和甲醇的烷基化合成2,6-二甲基萘(2,6-DMN)为探针反应,在固定床微型反应器中评价了改性HZSM-12沸石的催化反应性能,考察了不同改性方法对HZSM-12分子筛的结构、酸性和催化性能的影响规律。对HZSM-12分子筛进行MgO和SiO2的表面化学液相沉积改性。当MgO的沉积量小于0.4 %时,MgO主要沉积在分子筛孔道的内表面,MgO的沉积量超过0.4 %以后,MgO主要在分子筛的外表面沉积,而且高度分散。MgO与HZSM-12分子筛的表面羟基发生作用,分子筛的Br?nsted酸中心减少,产生了新的Lewis酸中心。MgO沉积改性后HZSM-12分子筛的弱酸中心数增加,而强酸中心数减少,积炭程度明显减轻,抗失活能力提高。反应6 h后萘的转化率下降的幅度由使用未改性分子筛催化剂的26.1%降低至9.1%。TEOS的沉积改性主要发生在HZSM-12分子筛的外表面,由于SiO2沉积改性后HZSM-12分子筛外表面非选择性的酸性位减少,有效地抑制了异构化、歧化等副反应,提高了目标产物2,6-DMN的选择性。分别用无机酸、有机酸和高温水蒸气对HZSM-12分子筛进行了脱铝改性。研究结果发现,脱铝改性对该分子筛的孔结构、酸性及其催化反应性能有不同的影响。采用盐酸脱铝改性对分子筛的骨架结构没有显著影响,同时减少了分子筛的强酸和弱酸中心,有效地提高了催化反应的稳定性。当采用10 mol/L的HCl处理后,分子筛脱铝的同时还伴随有脱硅现象的发生。柠檬酸则优先脱除分子筛中的强酸位,并导致二次介孔结构的产生。水蒸气脱铝改性后,HZSM-12分子筛的酸性位减少,萘的转化率有所降低,但明显提高了2,6-/2,7-DMN比。更多还原

【Abstract】 ZSM-12 molecular sieve was prepared by hydrothermal method, and the chemical liquid deposition (CLD) and dealumination have been imposed in the HZSM-12. The structure and acidity for modified HZSM-12 were analyzed and characterized by means of XRD, FT-IR, SEM, N2 adsorption, 27Al NMR, NH3-TPD, Py-IR . The methylation of methylnaphthalene with methanol to synthesis 2,6-DMN as a probe reaction been investigated in a fixed bed reactor. The effects of modification method for HZSM-12 zeolite on their structure, acidity and catalytic properties were investigated .The surface chemical liquid deposition technique for HZSM-12 by MgO and SiO2 was studied. The results show that MgO deposition mainly occurred on the internal surface of HZSM-12, but when the amount deposition more than 0.4% MgO high dispersion on the external surface of HZSM-12 .The interaction of MgO and bridging hydroxyl groups decrease Br?nsted acid sites of HZSM-12 and formed new Lewis acid sites. MgO deposition increase the weak acidic sites ,but significantly decrease the strong acidic sites , leading to the improvement of catalytic stability .After reaction 6h , the conversion decrease 9.1% over modified HZSM-12 compared to 26.1% over untreated HZSM-12. In the contrast, SiO2 deposition mainly on the external surface of HZSM-12. Own to the decrease of the non-selective acidic sites, the probability of side reaction including disproportionation , isomerization of 2,6-DMN decreased and the 2,6-/2,7-DMN increase.Dealumination of HZSM-12 with inorganic acid ,organic acid, steam has studied. The results show that the different dealumination methods have different effects on the pore structure, acidic properties and catalyst preference of modified HZSM-12. Hydrochloric acid eliminates the strong and weak acid sites simultaneously, without destroying the skeleton structure of the zeolite and improving the catalytic activity and stability. 10 mol/L hydrochloric acid treatment induces severe dealumination associated with desilication. In contrast, the citric acid preferred to remove the strongly acid sites and also formed secondary mesopores in the sample, resulting in improved reaction performance. After dealumination the amount of acid sites are markedly reduced, resulting in a lower conversion of naphthalene, but a significantly improved 2,6-/2,7-DMN ratio.更多还原