Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States.
Langmuir. 2013 Jun 4;29(22):6546-55. doi: 10.1021/la4001494. Epub 2013 May 21.
The relationship between polyol adsorption affinity and silanol defect density was investigated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials which vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes were confirmed as crystalline mordenite framework inverted (MFI) by SEM and XRD and were shown to contain ~8.5-0 silanol defects per unit cell by (29)Si MAS, (1)H MAS, and (1)H-(29)Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10(-3)P/P0 evolved with a decrease in silanol defects, and, through features in the XRD and (29)Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic-monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption at ~10(-3) g/mL with an increase from ~0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption isotherms on defect-free silicalite-1 at 333 K, both of which exhibit the Type-V character, indicates that water enhances adsorption by a factor of ~2 in the Henry's Law regime. Henry's constants for aqueous C2-C4 polyol adsorption (concentrations below 0.004 g/mL) at 298 K are shown to have a linear dependence on the silanol defect density, demonstrating that these molecules preferentially adsorb at silanol defects at dilute concentrations. This systematic study of polyol adsorption on silicalite-1 materials highlights the critical role of defects on adsorption of hydrophilic molecules and clearly details the effects of coadsorption of water, which can guide the selection of zeolites for separation of biomass-derived oxygenates.
通过开发在结构和表面性质不同的 silicalite-1 材料上的蒸汽和水溶液吸附等温线，研究了多元醇吸附亲和力与硅醇缺陷密度之间的关系。通过 SEM 和 XRD 证实，通过碱性合成、碱性合成后蒸汽处理和氟化物合成路线制备的 silicalite-1 晶体为结晶方钠石骨架反转（MFI），并通过（29）Si MAS、（1）H MAS 和（1）H-（29）Si CPMAS NMR 显示每个单元含有8.5-0 个硅醇缺陷。随着硅醇缺陷的减少，在 Ar 87 K 吸附等温线中出现滞后现象，并且通过 XRD 和（29）Si MAS NMR 光谱中的特征，推测滞后现象是由于硅醇缺陷密度降低而导致的正交-单斜对称转变的结果。重量法和水溶液测量表明，在 333 K 下，丙烯乙二醇的吸附受到硅醇缺陷的促进，在水溶液中吸附的最大增加 20 倍，在10（-3）g/mL 时观察到，硅醇缺陷从~0 增加到 8.5 个/单元。在 333 K 下，比较无缺陷 silicalite-1 上的蒸汽和水溶液丙烯乙二醇吸附等温线，两者都表现出类型-V 特征，表明在亨利定律区域水增强吸附的因子约为 2。在 298 K 下，水相中 C2-C4 多元醇吸附（浓度低于 0.004 g/mL）的亨利常数显示出与硅醇缺陷密度的线性关系，表明这些分子在稀浓度下优先吸附在硅醇缺陷上。对 silicalite-1 材料上多元醇吸附的系统研究突出了缺陷对亲水分子吸附的关键作用，并详细说明了水共吸附的影响，这可以指导沸石的选择，以分离生物质衍生的含氧化合物。
