Institute for Catalysis, Hokkaido University , Sapporo, Hokkaido 001-0021, Japan.
Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
Langmuir. 2017 May 2;33(17):4129-4137. doi: 10.1021/acs.langmuir.7b00045. Epub 2017 Apr 19.
The molecular origins of adsorption of lignin-derived phenolics to metal-organic framework NU-1000 are investigated from aqueous solution as well as in competitive mode with glucose present in the same aqueous mixture. A comparison of adsorption equilibrium constants (K) for phenolics functionalized with either carboxylic acid or aldehyde substituents demonstrated only a slight increase (less than a factor of 6) for the former according to both experiments and calculations. This small difference in K between aldehyde and carboxylic-acid substituted adsorbates is consistent with the pyrene unit of NU-1000 as the adsorption site, rather than the zirconia nodes, while at saturation coverage, the adsorption capacity suggests multiple guests per pyrene. Experimental standard free energies of adsorption directly correlated with the molecular size and electronic structure calculations confirmed this direct relationship, with the pyrene units as adsorption site. The underlying origins of this relationship are grounded in noncovalent π-π interactions as being responsible for adsorption, the same interactions present in the condensed phase of the phenolics, which to a large extent govern their heat of vaporization. Thus, NU-1000 acts as a preformed aromatic cavity for driving aromatic guest adsorption from aqueous solution and does so specifically without causing detectable glucose adsorption from aqueous solution, thereby achieving complete glucose-phenolics separations. The reusability of NU-1000 during an adsorption/desorption cycle was good, even with some of the phenolic compounds with greatest affinity not easiliy removed with water and ethanol washes at room temperature. A competitive adsorption experiment gave an upper bound for K for glucose of at most 0.18 M, which can be compared with K for the phenolics investigated here, which fell in the range of 443-42 639 M. The actual value of K for glucose may be much closer to zero given the lack of observed glucose uptake with NU-1000 as adsorbent.
从水溶液以及在相同的水溶液混合物中存在葡萄糖的竞争模式下，研究了木质素衍生酚类物质在金属有机骨架 NU-1000 上吸附的分子起源。对具有羧酸或醛取代基的酚类物质的吸附平衡常数 (K) 进行比较，根据实验和计算，前者仅略有增加（不超过 6 倍）。醛和羧酸取代吸附剂之间 K 的这种微小差异与 NU-1000 的芘单元作为吸附位点一致，而不是氧化锆节点，而在饱和覆盖度下，吸附容量表明每个芘单元有多个客体。吸附的实验标准自由能与分子尺寸和电子结构计算直接相关，证实了这种直接关系，吸附位点为芘单元。这种关系的根本原因在于非共价 π-π 相互作用是吸附的原因，这种相互作用存在于酚类物质的凝聚相中，在很大程度上决定了它们的蒸发热。因此，NU-1000 作为一个预先形成的芳香空腔，从水溶液中驱动芳香客体的吸附，并且特异性地进行，而不会导致从水溶液中检测到葡萄糖的吸附，从而实现完全的葡萄糖-酚类物质分离。NU-1000 在吸附/解吸循环过程中的可重复使用性良好，即使对于一些与水和乙醇在室温下的洗涤不易去除的具有最大亲和力的酚类化合物也是如此。竞争性吸附实验给出了葡萄糖的 K 值的上限，最多为 0.18 M，可以与这里研究的酚类物质的 K 值进行比较，其范围为 443-42639 M。考虑到作为吸附剂的 NU-1000 没有观察到葡萄糖的摄取，葡萄糖的实际 K 值可能更接近零。
