Study Of Aggregation Properties And Structure Characterization Of The Heavy Products Of Direct Coal Liquefaction Using Fluorescence Spectroscopy

The association properties and structure characterization of the heavy products of direct coal liquefaction,which had an important guiding significance on improving the process of direct coal liquefaction, liquefaction efficiency and promoting making use of heavy products. In this paper, the fluorescence properties of a series of model compounds were firstly investigated. The correlation between the fluorescence spectrum and the structure, and the influences of the aggregation between aromatic rings on their fluorescence spectra were discussed. Meanwhile, the structures of two types heavy products including asphaltene and preasphaltene, which obtained by a batch hydro-liquefaction of Shenhua coal and a liquefaction residue from Shenhua 6t/d PDU technology, respectively, were characterized. Subsequently, the aggregation properties of two types of AS and PA in different solvents, such as dichloromethane,tetrahydrofuran, toluene(or acetone), were investigated by ultraviolet spectroscopy,fluorescence spectroscopy, gel chromatography and surface tension technique,respectively. The behaviors, nature and structural relativity of aggregation were also discussed. Finally, the excitation, emission and synchronous spectra of heavy products were resolved by combine the spectra of model compounds.The results showed that the fluorescence spectra of aromatic compounds possess the characteristics of its aromatic ring, and show different degrees of aggregates fluorescent. The association between single nuclear aromatic molecules was enhanced by the increased the scale of aromatic systems and the solution concentration. Both of intramolecular and intermolecular aggregations can be observed in polynucleus aromatic compounds linked by bridge bonds, such as dipheylmethane, etc. It was mainly dynamic association of intramolecular exciplex in dilute solutions and static aggregates fluorescence between the ground state of the aromatic rings in the relatively high concentration solutions. The bridging structure in the molecule was the main factor that effected the above associations. Oxygen-containing aromatic compounds had complex fluorescence transfers and low fluorescence quantum efficiency. Especially, aromatic carboxylic acids had strong fluorescence quenching effect. The heavy product was a mixture consisting of different condensed aromatic fragments, and contained some oxygen-containing functional groups. Oxygen functional group contents and condensed aromatic structures in PA were greater thanthose of AS. The structures of AS-I and PA-I were more regular than AS-II and PA-II.Surface tensions, ultraviolet absorption spectra, fluorescence spectra and GPC analyses of AS and PA solutions consistently showed that the AS and PA obtained by two different processes had strong intermolecular association tendency. The critical association concentration of AS and PA were substantially from 12.5 mg/L to 50 mg/L.The association ability of AS-I and PA-I was stronger than corresponding AS-II and PA-II, and the association ability had a certain correlation with their structure regularity. The two kinds of heavy products of coal liquefaction had similar fluorescence characteristics, which showed mainly the fluorescent of naphaltene nucleus and benzene nucleus, and very weak fluorescent of more than 3 rings condensed aromatic nucleus. Meanwhile, there were some poly-nucleuses aromatic structures linked by bridge bonds in these heavy products, which showed strong intramolecular and intermolecular static and kinetic aggregations, so that strong fluorescence energy transfer effects were observed in the heavy product solutions. In addition, the excitation, emission and synchronous fluorescence spectra were successfully resolved, and the fluorescence peaks of monomers and aggregates of heavy products were also identified eventually.