In Situ Solid-state NMR Study On The Acidity-basicity Of Catalysts And N-Butane Isomerization Reaction

The skeletal isomerization of n-butane is an important reaction in petrochemical industry. Its product isobutane is a raw material for producing high octane number gasoline and non-lead gasoline additives, like methyl tert-butyl ether. However, the industrial catalysts being used in n-butane isomerization are environmental unfriendly and/or not highly catalytic active. Therefore, developing new catalysts with high catalytic performance is desirable for n-butane isomerization reaction. Acidity is essential for a catalyst used in n-butane isomerization reaction. To understand the influence of catalyst acidity on n-butane isomerization reaction can offer guidance for the development of new and efficient n-butane isomerization catalysts.Recently, there has been increasing interest in the use of in situ techniques to study catalysts and catalytic reactions as such studies can provide fundamental understanding on the nature of catalyst behavior and catalytic reaction at the molecular level. As one of the in situ techniques, in situ nuclear magnetic resonance (NMR) has the advantages of easily distinguishable and assignable peaks with quantitative information. Besides, in situ NMR can track the reaction behavior of labeled atoms, detect both gaseous and adsorptive species and thus give more detailed and comprehensive information for the study of reaction mechanism. In this thesis, the rearrangement reaction of 2-C-isobutane over SO4-2/ZrO2 was first studied with controlled atmosphere 13C magic angle spinning (MAS) NMR technique, to illustrate the mechanism of its reverse process, n-butane isomerization reaction. Also, a series of acidic cesium salts of tungstophosphoric acid doped with main group metal, MxCs2.5-mxH0.5PW12O40 (M= Mg, Ga, Al), were prepared via a solid state synthesis method, and their catalytic activities and reaction kinetics for l-13C-n-butane isomerization were studied by 13C MAS NMR technique. Solid state NMR can discriminate acidity via the difference in electron density around the nuclei and is one of the methods used to characterize solid acidity. Through appropriate probe molecules, the acid type, density, strength and strength distribution of MxCs2.5-mxH0.5PW12O40 were characterized thoroughly by in situ NMR techniques, to study the correlation between acidity and catalytic performance of the catalysts. In addition, a new method of characterizing acidity together with basicity was explored by using acidic and basic probe molecules simultaneously with the NMR technique.Rearrangement of isobutane is the reverse reaction of n-butane isomerization. Study on the rearrangement reaction of 2-13C-isobutane over SO42-/ZrO2 is helpful to further understanding the reaction mechanism of n-butane isomerization. Follow the former work on n-butane isomerization reaction mechanism study in our group, in this thesis the reaction of 2-13C-isobutane on SO42-/ZrO2 at different reaction temperature and atmosphere was studied. It has been found that the products are mainly 2-13C-n-butane and l-13C-n-butane in the initial stage of the reaction and the former is more easily formed. Byproducts 13C-propanes and 13C-isopentanes appear after reaction for a long period of time. Higher reaction temperature accelerates the formation of propane and isopentane, while H2 atmosphere inhibits the formation of propane and isopentane. The rearrangement of isobutane over SO42/ZrO2 proceeds, therefore, mainly via a monomolecular mechanism in the early stage of the reaction and then a bimolecular mechanism becomes dominant, which is consistent with the mechanism found in n-butane isomerization reaction.Compared with SO4-2/ZrO2, CS2.5H0.5PW12O40 is weaker in acidity and less active for n-butane isomerization, but has the advantage of not being easily deactivated. Considering the acidity of heteropoly compounds can be tuned through changing composition, a series of main group metal doped acidic cesium salts of tungstophosphoric acid, MxCs2.5-mxHo.5PW12O40 (M= Mg, Ga, Al, et al.), were prepared via a solid state synthesis method. The addition of an appropriate amount of Mg, Ga or Al improves the catalytic activity of Cs2.5H0.5PW12O40 in n-butane isomerization. At 373 K, the activity of the catalysts for n-butane isomerization follows the order of AlxCs2.5-3xHo.5PW12O40> GaxCs2.5-3xHo.5PW12O40> MgxCs2.5-2xHo.5PW12O40> CS2.5H0.5PW12O40. Al0.1CS2.2H0.5PW12O40 is the most active catalyst among AlxCs2.5-3xH0.5PW12O4o, Gao.1Cs2.2H0.5PW1204o among GaxCs2.5-3xHo.5PW12O40 ones, and Mg0.15CS2.2H0.5PW12O40 among MgxCs2.5-2xH0.5PW12O40 ones. Based on the activity on l-13C-n-butane isomerization at different reaction temperatures, the Arrhenius apparent activation energy of 54.4 kJ/mol for Al0.1CS2.2H0.5PW12O40 and 75.4 kJ/mol for Mg0.15Cs2.2H0.5PW12O4o were obtained. Take trimethylphosphine (TMP) and 2-13C-acetone as probe molecules, acidic properties of AI0.1CS2.2H0.5PW12O40, Ga0.1Cs2.2H0.5PWi204o, Mg0.15Cs2.2H0.5PW12O40and CS2.5H0.5PW12O40 were characterized by 31P and 13C MAS NMR spectra. All four catalysts mainly contain Bronsted acid sites. The addition of a second metal decreases the acid density of CS2.5H0.5PW12O40, but increases its acid strength, especially the strength of the strong acid sites. The higher catalytic activity of MxCs2.5-mxH0.5PW12O40(M= Mg, Ga, Al) than CS2.5H0.5PW12O40 possibly originates from the enhanced acid strength due to the doping of Mg, Ga or Al. For the catalysts used in n-butane isomerization, increasing the acid strength rather than the acid density of the Bronsted acid sites will be more helpful to improving their catalytic activity.In an acidic or basic catalyst, acidic (basic) sites coexist theoretically with their conjugate basic (acidic) sites. In the practical catalytic reaction, both the acidic and basic sites may involve in the catalytic reaction. The characterization of acidity together with basicity may be more close to the real reaction nature than those from individual characterization of acidity or basicity. At present, there is no mthod to characterize acidity together with basicity simultaneously, so it is necessary to explore and develop a new method to characterize acidity together with basicity. By using 15NH3 or TMP as the basic probe molecules and 13CO2 as the acidic probe molecule, the acidity together with basicity of ZrO2 were studied by in situ 1H-15N CP/MAS NMR or 31P MAS NMR with 13C MAS NMR techniques. Compare with 15N nucleus, 31P nucleus has 100% natural abundance and is more sensitive to NMR, so TMP is more suitable as a basic probe molecule than 15NH3 in acidity characterization with NMR. Through the co-adsorption of TMP and 13CO2, acidity and basicity of SZ,γ-Al2O3 and MgO were characterized simultaneously. The results show that this new characterization method is worth for being further studied.