| Carbon-based metal-free catalysts are one of the most promising candidates for reducing the dependence on metal-based catalysts in catalytic reactions.This dissertation systematically studies the preparation and catalytic hydrogenation performance of N-doped carbon materials.Carbon materials were modified using different nitrogen sources and doped by various forms of nitrogen via high-temperature pyrolysis.Hydrogenation of aromatic nitro compounds to aromatic amino compounds were used as the model reaction to evaluate the as-prepared N-doped carbon materials under mild conditions.The reaction mechanism was explored based on experimental results,and the reaction kinetics was also studied.Adsorption experiments revealed that polydopamine-doped carbon nanotubes catalyze the hydrogenation of p-nitrophenol?NIP?via a two-layer adsorption process,in which hydrogen molecules adsorb on the catalyst surface and split into active hydrogen atoms.The active hydrogen atom further adsorbs NIP molecule,followed by the hydrogenation to form AMP and the desorption from the active sites.A kinetic analysis of liquid phase hydrogenation of NIP reveals that surface reaction is the rate-determining step,and activation energies(?6?of surface reaction is 100.71 kJ/mol.The adsorption of NIP is exothermic,and the enthalpy change???is-130.78 kJ/mol.In contrast,the adsorption of H2 is endothermic with the enthalpy change??2?of174.44kJ/mol.Among the four selected different types of nitrogen sources,the catalysts doped using amines were all active for NIP hydrogenation.Nucleoside-doped carbon materials were inactive.However,only?tryptophan?7 and poly?4-vinylpyridine?-doped carbon materials are active for NIP hydrogenation in peptide compounds and pyridine compounds.Poly?4-vinylpyridine?-doped CNTs was used to catalyze the hydrogenation of 4-nitroacetophenone,4-nitrobenzoic acid methyl ester,NIP and their mixtures.Because the electron-withdrawing ability of the four groups contained in the substrate is different,it has different binding ability with the Lewis acid-base pair in the catalyst,thereby affecting its catalytic hydrogenation performance.When the substrates were converted separately,the catalyst has the best activity for NIP.When the mixtures of the substrates were converted,there is a competition between various reactants,the activity of the catalyst for 4-nitroacetophenone and 4-nitrobenzoic acid methyl ester is better than that of NIP.A in situ doping method was employed to modify the carbon material using pyrrole?Py?as nitrogen source.The experimental results indicate that the optimal preparation conditions are:10-20nm diameter CNTs as carrier,Py concentration of 0.2 mL/L,polymerization temperature of>16°C,and calcination rate of 5°C/min,calcination time of 2h,calcination temperature of 800°C,and the pyrolysis atmosphere of N2.Both the conversion and selectivity for NIP over the optimal catalyst reached 100%.In addition,more than ten kinds of aromatic nitro compounds were converted over the optimal catalyst with good activity and selectivity.Characterization results show that the catalytic activity of the catalyst is not positively correlated with the amount of doped nitrogen.The proportion of graphite nitrogen and pyrrole nitrogen to total nitrogen plays a key role on affecting the catalytic activity.Compared with PDA,PPy and poly?4-vinylpyridine?-doped CNTs,for the hydrogenation of more than ten kinds of aromatic nitro compounds,PPy-doped CNTs and poly?4-vinylpyridine?-doped CNTs possess better hydrogenation selectivity than PDA-doped CNTs,while PDA-doped CNTs exhibits higher catalytic activity than the other two.While the catalytic activity of poly?4-vinyl pyridine?-doped CNTs is the worst. |
PDF Full Text Request