Study of the sorption hysteresis loop mechanisms in micropore materials Title: Study of the sorption hysteresis loop mechanisms in micropore materials Introduction Hysteresis loop appears when the adsorption and desorption curves do not overlap in sorption isotherms. Research by now shows that all kinds of porous materials can exhibit hysteresis loop regardless of the pore size. For instance, when the pore diameter of a typical mesoporous activated carbon is far larger than the kinetic diameter of the gas molecule, the hysteresis loop appears when the phase transition occurs, resulting in the difficulty of effusion during desorption. Besides, hysteresis loop also occurs in other situations in microporous materials such as metal-organic frameworks （MOFs）and zeolites where the pore apertures are in the same order of the kinetic diameters of gas molecules. This kind of hysteresis has been confirmed to be different from what state above, and is relevant to transition of the framework and/or differences in kinetic energy barriers between adsorption and desorption. Therefore，fundamentally understanding the mechanisms of sorption hysteresis on micropore materials is critical to design and exploit efficient porous materials for adsorbing specific kinds of gases. Usually, the hysteresis in microporous materials can be classified into two categories- with substantial framework structural change and without such change. For flexible materials such as MOFs, hysteresis is usually affected by the type of gas molecules, temperature and pressure, etc. Hysteresis loops usually occur accompanying with transition of the framework, and such change includes the reversible expansion, distortion1 and rotation of the constituents2. But it should also be pointed out that the transition of framework does not necessarily cause hysteresis loop3, and the hysteresis without structural change will be discussed in the paragraph after “3”. As discussed above, sorption conditions (gas type, T and P) have strong influences on hysteresis for flexible materials. But for rigid materials even some flexible materials4, there is no substantial structural change when hysteresis occurs. In such cases, the characteristics of metal cations and pore size in zeolites or metal oxides play important roles in deciding the hysteresis mechanisms once the structure does not change.