吸附在活性炭上的二氧六环类固相和类液相性质的NMR研究

物理吸附在活性炭上的有机分子的性质,一直是人们感兴趣的问题。近年来,人们开始用NMR梯度场和核弛豫的方法,研究活性炭上吸附分子的扩散运动和分子交换运动.由于吸附分子在活性炭上处于不同的状态,其核弛豫表现出快、慢两个过程。通过拟合磁化矢量随时间变化的曲线,可以得到不同状态分子的百分含量。本文采用核弛豫的方法,研究活性炭的表面酸性基团及孔径对被吸附的二氧六环(DO)类固相和类液相形成的影响.并对不同吸附量下,DO在活性炭上的吸附行为进行讨论。

THE NMR STUDY OF THE SOLID AND LIQUID LIKE PHASES OF DIOXANE ADSORBED ON CHARCOALS

In this paper a study of the spin lattice relaxation times of dioxane(DO) adsorbedon various charcoals is presented. Results show that the magnetziation of DO decays biexponentially (Table 1) as observed in the case of adsorbed hydrocarbons. Thefast relaxing molecules are considered to be those adsorbed in micropores with radiiless than 10A whose motion is restricted and slowed down, thus they form a solidlike phase. However, the slow relaxing ones are those condensed in the micropores,they are free to move and form the liquid like phase. Molecules condensed in the in-terior of the micropores exchange rapidly with those situated in the vicinity of thepore wall according to Eq. (2). Comparison of data in Table 1 may lead to a conclusion that charcoal with greaterdensity of surface acidic groups and greater percentage of pore capacity with radius<10A (V_(10)/V_(30))has a greater percentage of solid like phase(P_f). Although V_(10)/V_(30) ofcharcoal 3~# is less than charcoal 2~#, P_f of 3~# is greater than 2~# because of its higherdensity of surface acidic groups. It is apparent that surface acidic groups in micro-pores play an important role in the formation of solid like phase of DO on charcoals.It seems that DO form hydrogen bond with the surface acidic groups.It is this hy-drogen bond and the hinderance effect of pores restrict the DO from moving freely,which results in the formation of the so called solid like phase.It can be found fromFig.2 that liquid like DO still cosists 50% even when the adsorption (Q) is verysmall (0. 10 ml/gC). This implies that DO vapour may first coudense in the micro-pores having more acidic groups and higher activation energy, and then fill in otherpores step by step as the adsorption process proceeds.