In Situ Solid-state ～(13)C MAS NMR Study Of N-Butane Isomerization Over Solid Acid Catalysts

The n-butane isomerization is of significant importance in producing cleaner fuel for the petroleum refining industry. The iso-butane produced is the main feedstock for the synthesis of branched paraffins with high octane numbers, whereas its dehydrogenation product, iso-butene, is the main intermediate to manufacture oxygenate additives, such as methyl tert-butyl ether (MTBE) and ethyl tert-butyl ether (ETBE). With the increasing of environmental constrains, the environmental unfriendly catalysts in petrochemical industry are to be replaced urgently by new "green" catalysts.Investigation on reaction mechanism is of key importance in deep understanding the behavior of catalysis and designing better catalysts. Different from other techniques, such as GC and GC-MS, in situ ¹³C MAS NMR has the advantages of tracing the fate of ¹³C labels both in gaseous and adsorbed state quantitatively during reaction. In this work in situ ¹³C MAS NMR was employed to the study of the mechanism and kinetics of n-butane isomerization. The main work is focused on n-butane isomerization under different conditions over a series of solid acid catalysts in the acidic strength order of SO₄²-/ZrO₂, Cs_xH_3-xPW₁₂O₄₀ and H-MOR. The information obtained can be summarized in the following.The mechanism of n-butane isomerization is dependent on the type of solid acid catalyst. Under low temperature, n-butane isomerization on SO₄²-/ZrO₂ and Cs_xH_3-xPW₁₂O₄₀ catalyst proceeds through a monomolecular pathway in the initial stage of the reaction. At the later stage, very small amount of C3 and C5 is detected, giving the evidence of the contribution of the bimolecular process. On H-MOR catalyst, n-butane isomerization occurs primarily via a bimolecular pathway.For all the catalysts, the ¹³C scrambling reaction in 1-¹³C-n-butane proceeds faster than the isomerization in the initial stage of the reaction. The concentration of 2-¹³C-n-butane goes through a maximum then decreases, suggesting that scrambling and isomerization of 1-¹³C-n-butane proceed via concurrent-consecutive reaction pathways as follows:1-¹³C-n-butane¹³C-iso-butane1-¹³C-n-butane2-¹³C-n-butane¹³C-iso-butaneIn these two reactions ¹³C scrambling in n-butane seems to be a more facile process, whereas n-butane isomerization is the slow rate-determining step. So, in the initial stage of the reaction the concentration of 2-¹³C-n-butane exceeds that of the labeled iso-butanes.The kinetics of monomolecular isomerization of n-butane under low temperature is well represented by a Langmuir-Hinshelwood rate equation for a reversible first-order surface reaction with the reaction of the adsorbed species being rate determining. The apparent activation energy on SO₄²-/ZrO₂ and Cs_2.5H_0.5PW₁₂O₄₀ catalyst is 10.9 and 19.2 kcal mol-1 respectively, suggesting that the stronger acidity of the catalyst leads to the lower energy barrier of the monomolecular isomerization. Also, the catalyst with strong acidity is able to work at low temperature, which results in high selectivity to iso-butane.The influence of promoters on n-butane isomerization was investigated. On a series of solid acid catalysts with different promoter, the product distribution and the change trend are similar except that the rate of n-butane isomerization is somewhat different. This indicates that the promoter will not alter the reaction pathway. However, the promotion mechanism of Pt on different solid acid catalysts is quite varied in the presence of hydrogen.The results of investigating the influence of gas additives on n-butane isomerization show that nitrogen has practically no effect on n-butane isomerization. In contrast, hydrogen strongly inhibits n-butane isomerization, especially via the bimolecular process. This is due to the bimolecular reaction involves C8 intermediate which is formed by the alkylation of butyl carbenium and butene molecules, and the formation of the latters is suppressed under hydrogen. Therefore,...