Study On The Reaction Mechanism Of N-Butane Isomerization And Related Catalyst Preparation

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.