Study On The Association Performance Of Coal Liquefaction Heavy Products And Its Application In The Preparation Of Carbon Quantum Dots

The study on the association behaviors of heavy products from direct coal liquefaction has a great significance to the improvement of coal direct liquefaction technology.At the same time,the preparation of functional carbon materials based on heavy products is helpful to the high valued utilization of coal liquefaction residue.In this paper,coal liquefaction heavy products（APA）,asphaltenes（AS）and preasphaltenes（PA）were separated from the liquefied residue of PDU built by Shenhua group in Shanghai.Then,the association effects and behaviors of APA in different solvents and H₂O/THF media were studied by combination of the structural characterization.Also,the effects of medium on the association structure of APA were investigated.Meanwhile,the preparation method of fluorescent carbon quantum dot（CQDs）with heavy product as carbon source was studied by hydrothermal method,DMF solvent thermal method and H₂O₂ oxidation method.And its structure,fluorescence characteristics and application properties were characterized and evaluated.The results of research on association performance show that the heavy products of coal liquefaction mainly consist of aromatic structures with different condensation degree of aromatic nucleuses.AS mainly consists of single-nucleus aromatic structures with some substituent groups,such as aliphatic and oxygen containing functional groups.The condensation degree of aromatic nucleus in AS is low.PA mainly consists of multi-nucleus in form of"island type"structures,and its condensation degree of aromatic ring is high.There are substantial non-covalent bonding interactions including hydrogen bond,π-πand charge transfer in APA.APA exists in the form of nano-aggregates in the solution,so that their fluorescence is mainly caused by energy transfer excitation.The association behaviors of APA in H₂O/THF media mainly involve the processes of association,de-association and aggregation precipitation.It depends mainly on the role of non-covalent bonds between APA and solvents.Adding water can change the non-covalent bonds and association mode of APA,in which a small amount of water can adjusted the aggregation structure of APA,and more water mainly results in the precipitation of APA aggregates.The results of researches on the modification of heavy products and the preparation of CQDs show that the hydrothermal treatment cannot directly prepare the CQDs,because the resulting solution shows poor fluorescence and narrow the range of excitation wavelength.DMF solvent thermal treatment can form a strong OH…N hydrogen bonding,and improve the fluorescence characteristics of soluble products.It is found that CQDs with better fluorescence performance can be prepared by mechanical grinding pretreatment and subsequent H₂O₂ oxidation.The obtained CQDs have good dispersion,and the average particle size is 3.92 nm.Its fluorescence excitation wavelength range is wide,and the best excitation wavelength is 360 nm.It can be successfully applied for the detection of Fe³⁺and Zn²⁺.At the same time,the CQDs also have good p H dependence and stability,so it is expected to be used in the study of p H sensing.In addition,some nano-tubular structures was accidently observed in the insoluble products of the DMF solvent thermal treatment,and it is worthy of further study.