## 1 Introduction  
In 1900, [PERSON] first discovered the coal to liquid fuels (CTL) technique and since then, this technology has evolved rapidly, especially in China [1, 2]. At present, China has built lots of projects to utilize the CTL technology, such as SHENHUA project, YUNNAN project, etc. Actually, CTL is a refinery process to convert coal into liquid hydrocarbons: liquid fuels and petrochemicals, such as petrol, diesel, and liquefied petroleum gas etc. Therefore, CTL is commonly called coal liquefaction. Coal liquefaction technologies generally fall into two categories: direct coal liquefaction (DCL) and indirect coal liquefaction (ICL) processes [3, 4]. DCL processes convert coal into liquids directly, without the intermediate step of gasification, by breaking down its organic structure with the application of solvents or catalysts in a high pressure and temperature environment and is applied widely in China. However, the direct coal liquefaction residue (DCLR) is the main by-product generated in the DCL process, which accounts for 30% by weight of raw coal. Currently, both in China and overseas, DCLR is mainly used as a feedstock for gasification or combustion, which will not only cause serious environmental pollution but also reduce its economic value and lead to a waste of valuable resources. If DCLR were applied for asphalt pavement construction, it would be economical and beneficial, and can potentially reduce the environmental pollution in China [5]. How to utilize DCLR in a friendly and economical way is always a big issue [6,7]. Recently, some asphalt modifiers (e.g., styrene-butadiene-styrene (SBS), rubber, acids) are commonly used in the pavement construction because of their excellent performances. [PERSON], [PERSON] et al. evaluated rheological, elastic and low-temperature properties of SBS, ground tire rubber (GTR) and polyphosphoric acid (PPA) modified asphalt binder respectively. The results showed that all of the modifiers could improve the high-temperature properties and had not significant effects on the low-temperature continuous grade [8, 9, 10]. Being that DCLR contains 30-50% heavy oil and asphaltene materials and is a very valuable resource material, it has the potential to be developed into asphalt modifier [11,12]. [PERSON] et al. tested the properties and microstructure of SK-90, which is a commercial brand asphalt binder, blended with different DCLR contents using various test methods, such as Superpave performance grade specification, Penetration performance grade specification, infrared spectrometer, and gel permeation chromatography. The test results indicated that the high-temperature properties of DCLR modified asphalt binder improved, but the low-temperature properties and fatigue cracking resistance of asphalt binder declined [13]. [PERSON] et al. measured the surface energies of three DCLR modified asphalt binders, the test results showed that the water sensitivities of the three DCLR modified asphalt binders combined with aggregates were better than those of the base asphalt binder [14]. However, although DCLR can improve the high-temperature properties and water sensitivity of asphalt binder, the low-temperature properties and fatigue cracking resistance declined because DCLR contains 45% tetrahydrofuran insoluble fraction. Therefore, some studies tried to pre-treat DCLR and remove the tetrahydrofuran-insoluble fraction. The rest part, such as tetrahydrofuran soluble fraction (THFS), could be developed into asphalt modifier. [PERSON] et al. studied the reaction condition for Shenhua DCLR. The test results presented that optimal conditions were the middle distillate of Shengli coal, 130-150 \({}^{\circ}\)C in temperature, 0.5 MPa initial N\({}_{2}\) pressure, 4:1 or 5:1 of the mass ratio of solvent to DCLR and 15-30 min of residence time as well as stirring [15]. [PERSON] et al. extracted THFS from DCLR and studied the properties of THFS modified asphalt binder.