Raj Gijo, Swalus Colas, Delcroix Marie, Devillers Michel, Dupont-Gillain Christine, Gaigneaux Eric M
Institute of Condensed Matter and Nanosciences, Division Molecules, Solids and Reactivity, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium.
J Colloid Interface Sci. 2015 May 1;445:24-30. doi: 10.1016/j.jcis.2014.12.035. Epub 2014 Dec 24.
Hybridization of polyoxometalates (POMs) via an organic-inorganic association constitutes a new route to develop heterogeneous POM catalysts with tunable supramolecular architecture. As the structural stability of POMs is strongly influenced by the pH conditions, a quantitative understanding of the POMs-polymer association is important in practical applications. Herein, we use Quartz Crystal Microbalance (QCM) to systematically investigate the interactions of Keggin phosphotungstic acid POM with a polyampholyte polymer-coated QCM sensor as a function of pH. The mass of adsorbed POMs increases when pH decreases from 5.6 to 2, indicating that electrostatic forces play a major role in the formation of POM-polymer hybrids. This finding is complemented by AFM images that show an increase in the size of the hybrid entities from 5 to 12 nm as the pH decreases from 5.6 to 2. The POM adsorbed amount at a particular pH value reaches an equilibrium level with time. The hybrids further gain in adsorbed mass only when lowering the pH value of the POM solution. The hybrid structure formed above pH 2 shows resistance to leaching as indicated by the steady level of the adsorbed mass during a rinsing step with water. However, at pH 2, the rinsing step causes desorption of some weakly adsorbed POMs. It is shown that leached POMs can be re-adsorbed back into the polymer matrix during a second contact with a POM solution at pH 2. This adsorption-desorption cycles of POMs were successfully repeated. Our experiments shed light into the coexistence of tightly as well as loosely bound POMs in hybrid catalyst formed at pH 2. The loosely bound POMs can potentially act as homogeneous catalysts when desorbed. However, these leached POMs can be re-adsorbed back into the matrix, preserving the heterogeneous state of the catalyst. Our results show that QCM is a powerful technique to study in situ the dynamics of the adsorption of POMs on a polymer matrix under different pH conditions.
通过有机 - 无机缔合对多金属氧酸盐（POMs）进行杂化，为开发具有可调超分子结构的多相POM催化剂提供了一条新途径。由于POMs的结构稳定性受pH条件的强烈影响，因此在实际应用中对POMs - 聚合物缔合进行定量理解非常重要。在此，我们使用石英晶体微天平（QCM）系统地研究了Keggin型磷钨酸POM与聚两性电解质聚合物包覆的QCM传感器之间的相互作用，该相互作用是pH的函数。当pH从5.6降至2时，吸附的POMs质量增加，这表明静电力在POM - 聚合物杂化物的形成中起主要作用。原子力显微镜（AFM）图像补充了这一发现，该图像显示随着pH从5.6降至2，杂化实体的尺寸从5纳米增加到12纳米。在特定pH值下，POM的吸附量随时间达到平衡水平。只有在降低POM溶液的pH值时，杂化物的吸附质量才会进一步增加。在pH高于2时形成的杂化结构表现出抗浸出性，这在用水冲洗步骤中吸附质量的稳定水平中得到体现。然而，在pH为2时，冲洗步骤会导致一些弱吸附的POMs解吸。结果表明，在与pH为2的POM溶液再次接触时，浸出的POMs可以重新吸附回聚合物基质中。POMs的这种吸附 - 解吸循环成功地重复进行。我们的实验揭示了在pH为2时形成的杂化催化剂中紧密结合和松散结合的POMs共存的情况。松散结合的POMs在解吸时可能充当均相催化剂。然而，这些浸出的POMs可以重新吸附回基质中，保持催化剂的多相状态。我们的结果表明，QCM是一种强大的技术，可用于原位研究在不同pH条件下POMs在聚合物基质上的吸附动力学。
