【作者】 邹艳；

【导师】 贺鹤勇；

【作者基本信息】 复旦大学 ， 物理化学， 2006， 博士

【摘要】 正丁烷异构化是石化工业中的重要反应之一，其产物异丁烷是烷基化反应的主要原料和合成MTBE等汽油添加剂的重要前驱体。目前工业上使用的烷烃异构化催化剂，存在活性低和环境污染等问题，而最有潜力替代这些催化剂的各种固体酸催化剂，由于稳定性差等缺点影响了工业化应用。因此迫切需要研发高活性、高稳定性、使用寿命长的催化剂。运用原位技术在分子水平上探索反应物的活化历程和催化反应机理，是设计、开发和优化新型催化剂的关键。与传统的GC等技术相比，原位固体核磁共振技术有独特的优势，它可以¹³C标记化合物作为反应物分子，跟踪标记原子在反应过程中的行为，并能定量研究所有气态和吸附态的反应物、中间体和产物。本工作采用原位固体核磁共振技术和1-¹³C-正丁烷，系统研究在钨酸氧化锆系列（WO_x／ZrO₂）催化剂上的异构化反应机理和反应动力学，详细研究了影响反应的多种因素，包括助剂Pt、反应气氛和氢气预处理的作用。并探索了新型固体酸催化剂的制备方法，包括室温固相法合成固体超强酸催化剂磷钨杂多酸铯盐(Cs_xH_3-xPW₁₂O₄₀)及低温陈化法制备硫酸氧化锆和钨酸氧化锆。具体结论如下：通过研究393-523K温度范围内1-¹³C-正丁烷在WO_x／ZrO₂上的反应，及在不同温度下1-¹³C-正丁烷重排为2-¹³C-正丁烷、正丁烷异构化为异丁烷以及C₈中间体裂解为C₃、C₅反应的动力学关系，发现：在WO_x／ZrO₂上，1-¹³C-正丁烷的反应存在两个诱导期。在较低温度下，存在较短的异构化诱导期和较长的裂解诱导期；在较高温度下，诱导期消失。基于Langmuir-Hinshelwood动力学理论，在所研究的温度范围内，1-¹³C-正丁烷和2-¹³C-正丁烷之间的分子内重排可按可逆的单分子途径处理，而裂解为不可逆的双分子途径。异构化可以同时处理为可逆的单分子机理和双分子机理。但是，双分子裂解产物¹³C标记C₃、C₅产生前，¹³C标记i-C₄产物生成的同时就伴随着经由双分子机理形成的碳沉积。反应压力较高时，在室温即有4-¹³C-异戊烷生成，加热反应过程中陆续检测到由双分子机理形成的多种标记的C₃、C₅化合物；较低压力下，反应初期未检测到C₃、C₅，但高碳损失表明较低压力下正丁烷的异构化亦为双分子机理。因此，综合实验结果，表明1-¹³C-正丁烷异构化反应以双分子机理为主。在WO_x／ZrO₂上1-¹³C-正丁烷按两个平行的途径反应：途径（1）：经由甲基环丙烷碳正离子中间体的单分子正丁烷碳重排反应。途径（2）：经由C₈中间体的双分子反应。该中间体主要发生以下三个反应：以双分子机理进行的正丁烷异构化反应生成异丁烷，以双分子机理进行的正丁烷裂解反应生成丙烷和戊烷（包括异戊烷），通过形成的丁烯中间体进行的寡聚反应形成碳沉积。详细研究WO_x／ZrO₂上的正丁烷异构化反应的影响因素，结果如下：无氢气存在下，Pt的存在对正丁烷异构化的速率影响不大，同时产物种类及分布与WZ相似，表明在酸性位上进行正丁烷异构化反应。反应中20-30％的大量碳沉积说明在金属位Pt上进行了正丁烷脱氢反应。即Pt有经典双功能催化剂中金属位的脱氢作用。氢气气氛下，较低温度时Pt催化正丁烷氢解为主；只有在较高反应温度下，Pt才显示出一定的金属-酸双功能催化作用。可能的原因是：在Pt表面烷烃和氢气的吸附与活化有竞争性，如果质子迁移速度比C₄中间体（C₄⁼）从活化中心迁移离开速度快。C₄中间体就发生氢解。较高反应温度下，C₄中间体迁移速度加快，部分C₄中间体可成功迁移至酸中心发生异构化。还发现对WO_x／ZrO₂及Pt促进的WO_x／ZrO₂进行氢气预处理可以提高催化剂的活性。本工作还探索了固体酸催化剂的新的制备方法。首先，利用室温固相法合成了固体超强酸催化剂Cs_xH_3-xPW₁₂O₄₀。该方法制备的样品在比表面、XRD、IR及表面酸量等方面的性质与传统液相法制备的样品基本一致。利用正丁烷异构化反应测定固相制备的样品与液相法制备的样品催化活性基本一致。另外，采用Cs₂CO₃或CsNO₃制备的样品其正丁烷异构化活性均无明显差异，表明Cs₂CO₃或CsNO₃均是可用作室温固相法制备磷钨杂多酸铯盐的铯源。固相法制备磷钨杂多酸铯盐方法简单，利于工业化生产。其次，探索了利用低温陈化法制备SZ和WZ。陈化可改善SZ和WZ催化剂的洗涤性能，低温陈化效果更佳。陈化对SZ及WZ的催化活性的影响不同。对SZ系列催化剂，陈化改变了SZ的酸性导致催化剂活性降低。对WZ系列催化剂，陈化对样品催化活性无明显影响。因此，在不影响反应性能的条件下，陈化处理对改善WZ催化剂的总体制备条件有积极意义。更多还原

【Abstract】 The isomerization of n-butane to iso-butane is of significant importance in the petroleum refining industry. Iso-butane reacts with olefins to synthesize branched paraffins with high octane numbers, whereas its dehydrogenation product, iso-butene, is the main intermediate to manufanture oxygenate additives, such as methyl tertbutyl （MTBE） and ethyl tert-butyl ether （ETBE）. The most widely applied alkane isomerization catalysts are Pt/Cl-Al₂O₃ and mordenite. The former subjects to stringent environmental regulations, while the latter is significantly less active. Therefore, the development of more efficient and environmental-friendly catalysts operating at lower temperature is highly demanded.Investigation on reaction mechanism is essential for deep understanding the catalysis and designing better catalysts. In situ ¹³C MAS NMR is a powerful technique for the study of reaction mechanism with ¹³C-labeled compounds. This technique has the advantage of quantitative tracing the fate of ¹³C labels both in gaseous and adsorbed state during reaction and monitoring the kinetics of reaction. In this work in situ ¹³C MAS NMR was employed to the study of the mechanism and kinetics of 1-¹³C-n-butane isomerization over tungstated zirconia and other catalysts. The information obtained can be summarized in the following.In the temperature range of 373-523 K over tungstated zirconia the rearrangement of 1-¹³C-butane to 2-¹³C-butane, the isomerization of n-butane to iso-butane and the disproportionation of C₈ intermediates to C₃ and C₅ species were qualitatively studied by ¹³C MAS NMR. Based on kinetic study, the rearrangement reaction between 1-¹³C-n-butane and 2-¹³C-n-butane can be treated by a reversible monomolecular pathway and the disproportionation rraction by a irreversible bimolecular pathway. The isomerization data can be treated either by a reversible monomolecular pathway or a bimolecular pathway. However, the detailed study with the data of ¹³C MAS NMR, in particular, the observation of the ¹³C loss assigning to formation of oligomers strongly indicates that the isomerization of n-butane to isobutane undergoes a bimolecular reaction pathway.There are two parallel reactions of 1-¹³C-n-butane over tungstated zirconia. The first reaction is the ¹³C-labal scrambling in n-butane through the monomolecular pathway, resulting in migration of the ¹³C labal from the methyl group into methylene groups to form 2-¹³C-n-butane. The second reaction involves C₈⁺ cation through abimolecular reaction. The C₈⁺ cation formed by dimerization of C₄⁺ carbenium ions with alkene further reacts to form iso-butane and C3+C5 through bimolecular mechanism. Also, the alkene formed through bimolecular mechanism furhter oligomerized to form ’NMR invisible’ coke species.The influence of Pt supported on tungtated zirconia on n-butane isomerization was investigated. In the absence of hydrogen, Pt does not affect the rate of the isomerization, which means that the isomerization does not take place directly on Pt surface. At the same time Pt catalyzed the dehydrogenation of n-butane leading to 20-30% ¹³C species becoming ’NMR-invisible’. In the presence of hydrogen, Pt has a high hydrogenolytic activity. At higher temperature, A increase in selectivity to iso-butane is caused by the operation of the catalyst in bifunctional mechanism.Treatment of tungtated zirconia and Pt promoted tungtated zironia catalysts in H₂ atmosphere can increase the catalyst acitivity.Cs_xH_3-xPW₁₂O₄₀ （x = 2.25, 2.50 and 2.75） were successfully prepared using a solid synthesis method. The products were characterized by XRD, IR, N₂ sorption, ³¹P MAS NMR. The results indicate that cesium salts prepared via solid method have the same structural features as that via liquid method. The Cs_2.50H_0.50PW₁₂O₄₀ shows a good catalytic activity for the isomerization of n-butane as that of those prepared via the liquid method. The results indicate that the solid method is a feasible substitute for the liquid method, which may be used to prepare metal salts of heteropolyacid. On Cs_2.50H_0.5PW₁₂O₄₀ catalyst, n-butane isomerization occurs primarily via a bimolecular pathway.Tungtated zirconia and sulfated zirconia were prepared by ageing at low temperature. The prepared tungtated zirconia has the same performance as the sample without ageing treatment. However, the former is better to be handled during preparation. The results indicate that the ageing processs has benefit in the preparation of tungtated zirconia catalyst.更多还原