Preparation Of Carbon Nanotube Catalysts With Metal-encapsulated Structure And Their Catalytic Performance For Aromatic Alkanes Oxidation

Carbon nanotubes(CNTs)have many advantages,such as high specific surface area,acid and alkali resistance,stable chemical properties,easy modification and surface functionalization,etc.They are excellent new catalytic materials,not only in the fields of photocatalysis and energy storage,but also widely used as catalysts supports and metal-free catalysts in heterogeneous catalytic oxidation.At present,various kinds of carbon nanotubes catalysts reported in the literature mainly depend on the selection of different synthesis methods and the replacement of different precursors.Therefore,it is very important to choose the suitable precursors and develop a simple and efficient preparation strategy for the synthesis of carbon nanotubes with unique structure and high catalytic performance.Melamine is a common and cheap raw material in the chemical industry.As an organic compound rich in nitrogen(N content is 66.7%),melamine is widely used as a nitrogen precursor in the synthesis of N-containing carbon materials.Therefore,the use of high-temperature pyrolysis melamine precursor has become a common choice for the preparation of heteroatom nitrogen doped carbon nanotubes.In addition,the use of transition metal salts as the source of metal and citric acid as metal complexing agent and additional carbon source,which can help coordinate the construction and formation of metal encapsulation.Generally speaking,the doped heteroatom N forms a coordination structure with the transition metal atom to provide a large number of M-N active centers,the stability of the active center is effectively improved.However,due to the incomplete structure of the encapsulated structure and the lack of effective measures to regulate the encapsulated metal and nitrogen content,the relationship between the encapsulated metal content and the nitrogen content and the catalytic activity has not been clarified.Therefore,it is necessary to develop a catalyst with a better encapsulation structure and to determine the relationship between the encapsulation metal content,nitrogen content and catalytic performance.Based on the above discussion,this article comprehensively and systematically explored the preparation of carbon nanotubes catalyst with high catalytic performance by one-pot method with melamine,citric acid and transition metal salt as precursors,and analyze it by various characterizations.The main research contents are:1)As the main nitrogen and carbon source,melamine is easily soluble in hot water and can self-assemble with citric acid.Using deionized water as a solvent,dissolve melamine under the condition of heating in a water bath,and then use citric acid to complex the metal ions and self-assembled protonated melamine to form a homogeneous complex precipitate simultaneously.Calcined at 900 ℃ to form N-doped bamboo-like carbon nanotubes composite catalysts(Co@CNTs-X)with different metal content and encapsulation structure,as well as the high specific surface area.After the investigation of the metal content,the results of TEM analysis show that the increase of the metal content helps to promote the growt h of carbon nanotubes.Compared with other carbon nanotubes with different metal contents,when the metal content is 0.8 g,which forms a large number of stable and efficient N-doped carbon nanotubes with a bamboo-like metal encapsulation structure.In addition,the experimental results show that the Co@CNTs-0.8 catalyst has excellent catalytic performance in the selective oxidation of ethylbenzene under molecular oxygen and solvent-free conditions.The conversion of ethylbenzene is 63.7%,and the selectivity of acetophenone is 91.5%.Through the further characterization analysis of the catalysts and different experiments,it is shown that this special metal encapsulation structure can not only effectively utilize the advantages of bamboo-like carbon nanotubes,but also fully improve the activity and stability of the metal encapsulated within carbon nanotubes,and promote the coordination between the metal and N to form more Co-N active sites.XPS results and control experiment data show that the synergistic effect between the bamboo-like carbon nanotubes and Co-N active sites play an important role in promoting the catalytic performance.2)A series of NCNTs-X bamboo-like carbon nanotube catalysts were prepared by one-pot pyrolysis of melamine,citric acid an d cobalt chloride hexahydrate complexes with different melamine contents.The experimental resuluts indicate that the activity of catalyst is closely related to the content of melamine,that is,the content of nitrogen.The NCNTs-1.6 shows the best catalytic activity.The conversion of ethylbenzene oxide is 65.9%,and the selectivity of acetophenone is 92.2%.In addition,TEM and HRTEM analysis results confirm that the increase in nitrogen content can contribute to the formation of obvious bamboo-like structures,thereby affecting the encapsulation of metal nanoparticles in carbon nanotubes.Furthermore,the XPS analysis shows that when the melamine content is 1.6 g,the doped nitrogen can form more active sites with the coordination of the encapsulated meta l,and the Co-N species content on the catalyst surface is the highest at this time.Combined with the ethylbenzene and arylalkanes oxidation test data,at this time,the catalyst has the best catalytic performance,which further proved that the active cen ter of the catalyst was Co-N species and it was beneficial to the improvement of catalyst activity.3)The complexes of melamine,citric acid and cobalt chloride hexahydrate were pyrolyzed at 900 ℃.The obtained black powders were etching at 80 ℃ and 2 mol/L hydrochloric acid for different times to prepare a series of AT-Co@CNTs-X catalysts.The results show that the AT-Co@CNTs-6 catalyst after an etching time of 6 hours has a good catalytic activity for the oxidation of ethylbenzene to acetophenone with a conversion of 98.1%and a selectivity of 99.8%under the condition of TBHP as oxidant.In addition,the TEM and BET characterization of the catalyst show that acid etching is helpful to optimize the structure of bamboo-like carbon nanotubes.Compared with Co@CNTs catalysts,the AT-Co@CNTs-X catalysts have a more complete encapsulation structure and higher specific surface area.The results indicate that acid etching can not only be helpful to improve the specific surface area,but also help to remove unstable metal particles and obtain carbon nanotubes with stable bamboo-like and metal-encapsulated structures.Furthermore,XPS characterization analysis,the investigation of ethylbenzene oxidation and cycle stability prove that acid etching at a suitable time is conducive to the formation of well-stabled Co-N active species,and the synergistic effect between Co-N species and acid treated bamboo-like nitrogen doped carbon nanotubes is an important factor for the higher efficiency of AT-Co@CNTs-X catalysts.