Selective Liquid Phase Oxidation Of Benzyl Alcohol And Ethylbenzene Catalyzed By Carbon Nanotubes And Nitrogen-doped Carbon Nanotubes

Carbon nanotubes (CNTs) as catalyst supports and metal-free catalysts have attractedmuch attentions, owing to their unique physicochemical properties such as high specifcsurface areas, environmental acceptability, corrosion resistance, easy recovery of preciousmetals by support burning, thermal stability under nonoxidative condition and readily surfacemodifcation. At present, most of the studies on CNTs and nitrogen-doped carbon nanotubes(NCNTs) as metal-free catalysts have focused on the oxygen reduction reaction and gas phaseoxidative dehydrogenation reaction. However, it is rarely reported on selective liquid phaseoxidation reaction. In order to enlarge their application, the research on reaction mechanismof different liquid phase oxidation reactions is of great scientific and practical significance. Inthis thesis, the free radical or not free radical oxidation reactions catalyzed by CNTs andNCNTs were investigated, that is, selective liquid phase oxidation of benzyl alcohol orethylbenzene (EB) catalyzed by CNTs and NCNTs to prepare highly valuable chemicalproducts benzaldehyde or acetophenone (AcPO), respectively, under the mild reactionconditions. This study can reduce traditional catalysis dependency on precious and transitionmetals and reduce environment impact from great quantities of waste. Moreover, the effectsof surface oxygen functional groups and surface structure of carbon materials on the catalyticactivity of selective liquid phase oxidation of benzyl alcohol or EB were investigated andplausible catalytic mechanisms of the benzyl alcohol or EB aerobic oxidation were proposed.These results can provide important theoretical basis for promoting carbon materials’application in selective liquid phase oxidation reactions. The main contents are described asfollows:(1) CNTs and NCNTs were successfully prepared by the chemical vapor deposition(CVD) method. The results are showed as follows:(a) The CNTs were prepared by CVD withliquefied petroleum gas as carbon source over a Fe-Mo/Al2O3catalyst in a horizontal tubularquartz furnace with4cm inner diameter (i.d.) and their average outer and inner diameter are20.8and6.6nm, respectively. The CNTs were prepared by the same CVD with xylene ascarbon source and their average outer and inner diameter are33.5and7.7nm, respectively.(b)NCNTs were synthesized by the same CVD with xylene as carbon source and aniline asnitrogen source and their outer and inner diameter are29-40and10-14nm, respectively. Thecontents of total nitrogen increase from0.3to3.11at.%with increasing aniline amount inprecursors. The contents of pyridinc nitrogen and quaternary nitrogen increase with increasinganiline amount in precursors, while the pyrrolic nitrogen content reaches its maximum at the aniline amount of50%in precursors and decreases with further increasing aniline amount.The graphitized structure of CNTs is unchanged obviously, while ID/IGratio of CNTsincreases with increasing aniline amount in precursors after the introduction of nitrogen atomsinto the carbon lattice. Moreover, the thermal stability of NCNTs are slightly lower than thatof the CNTs, but they no remarkable weight loss occurs before500oC.(2) Selective oxidation of EB to AcPO in liquid-phase using oxygen as oxidant wasstudied on the carbon nanotube catalysts, and the effects of reaction conditions on the catalyticactivity of CNTs, the correlation between the performance and structures and probablereaction mechanism were investigated. The results are showed as follows:(a) At155oC,nCNTs/nEB=0.2and1.5MPa O2, EB conversion is38.2%with60.9%selectivity for AcPOafter4h of reaction.(b) In our catalytic system, the oxidation of EB proceeds by a free-radical mechanism, and the1-phenylethyl hydroperoxide (PEHP) is the most importantintermediate and the main chain propagator in the oxidation of EB, and the CNTs play animportant role in the decomposition of PEHP, owing to π-π interactions between the peroxideand the graphene sheets of the CNTs.(c) The contain residual Fe species of CNTs may notplay an active role in the oxidation of EB under our reaction conditions.(d) Surfacecarboxylic groups and defects of the CNTs are unfavorable to EB oxidation, it is probablycaused by the localization of electrons as a result of the introduction of carboxylic groups anddefects, which may be adverse to the π-π interaction between the PEHP or radical andgraphene sheets.(e) The CNTs catalyst shows outstanding recyclability, EB conversion is36.3%with60.6%selectivity for AcPO after six consecutive usages.(f) Doping nitrogenatoms into the carbon lattice are unfavorable to EB oxidation, it is probably attributed to theincreased defects in CNTs, resulting in a decrease of conductivity and electron mobility, thecharge transfer is suppressed, thus decrease π-π interaction between the radical or PEHP andthe graphene sheets of the CNTs.(3) The catalytic performance and probable mechanism of CNTs and NCNTs in theselective liquid phase oxidation of benzyl alcohol were investigated. The results are showedas follows:(a) Neither CNTs nor NCNTs as a alone catalyst has lower catalytic activity forbenzyl alcohol oxidation with conversion of less than10%. HNO3is the most effectivepromoter achieving a benzyl alcohol conversion of96.2%and a benzaldehyde selectivity of88.3%after5h of reaction. However, HNO3alone cannot effectively improve the oxidationof benzyl alcohol in the absence of CNTs, and only obtained a benzyl alcohol conversion of31.4%and a benzaldehyde selectivity of75.3%after5h of reaction.(b) It is not free-radicaloxidation reaction. In our catalytic system, it is clarified that benzyl nitrite is an important intermediate, O2is the really terminal oxidant, while HNO3only initiates the oxidation cycle.(c) The contain residual Fe species of CNTs do not play an active role in the oxidation ofbenzyl alcohol under our reaction conditions, while electron transfer in graphene sheets playsan important role in the decomposition of benzyl nitrite.(d) The surface carboxylic groupsand defects of the CNTs have hardly influence on the benzyl alcohol conversion.(e) Propernitrogen doping indeed enhances the catalytic activity of CNTs in benzyl alcohol oxidation,probably arising from the enhanced electron transfer due to nitrogen dopant, thus enhances π-π interaction between the the intermediate and the graphene sheets of the CNTs. When thecontents of total nitrogen is1.02at.%, the NCNTs can obtain a benzaldehyde yield of78.5%after3h of reaction, while that is68.2%on CNTs catalyst under the same conditions.(f) TheCNTs and NCNTs catalysts have outstanding recyclability showing an excellent potential forindustrial application of benzyl alcohol oxidation to benzaldehyde.