Stock Sebastian, Schlander Annika, Kempin Maresa, Geisler Ramsia, Stehl Dmitrij, Spanheimer Kai, Hondow Nicole, Micklethwaite Stuart, Weber Ariane, Schomäcker Reinhard, Drews Anja, Gallei Markus, von Klitzing Regine
Department of Physics, Soft Matter at Interfaces, Technische Universität Darmstadt, Darmstadt, Germany.
Phys Chem Chem Phys. 2021 Jan 28;23(3):2355-2367. doi: 10.1039/d0cp06030e.
Pickering emulsions (PEs), i.e. particle stabilized emulsions, are used as reaction environments in biphasic catalysis for the hydroformylation of 1-dodecene into tridecanal using the catalyst rhodium (Rh)-sulfoxantphos (SX). The present study connects the knowledge about particle-catalyst interactions and PE structure with the reaction results. It quantifies the efficiency of the catalytic performance of the catalyst localized in the voids between the particles (liquid-liquid interface) and the catalyst adsorbed on the particle surface (liquid-solid interface) using a new numerical approach. First, it is ensured that the overall packing density and geometry at the droplet interface and the size of the water droplets of the resulting w/o PEs are predictable. Second, it is shown that approximately all particles assemble at the droplet surface after emulsion preparation and neither the packing parameter nor the droplet size change with the particle surface charge or size when the total particle cross section is kept constant. Third, studies on the influence of the catalyst on the emulsion structure reveal that irrespective of the particle charge the surface active and negatively charged catalyst Rh-SX reduces the PE droplet size significantly and decreases the particle packing parameter from s = 0.91 (hexagonal packing in 2D) to s = 0.69 (shattered structure). In this latter case, large voids of the free w/o interface form and become covered with the catalyst. With a deep knowledge about the PE structure the reaction efficiencies of the liquid-liquid vs. the solid-liquid interface are quantified. By excluding any other influence factors, it is shown that the activity of the catalyst is the same at the fluid and solid interface and the performance of the reaction is explained by the geometry of the system. After the reaction, the catalyst retention via membrane filtration is shown to be successfully achieved without damaging the emulsions. This enables the continuous recovery of the catalyst, i.e. the most expensive compound in PE-based catalytic reactions, being a crucial criterion for industrial applications.
皮克林乳液（PEs），即颗粒稳定乳液，在双相催化中用作反应环境，以使用铑（Rh）-磺基黄原膦（SX）催化剂将1-十二碳烯氢甲酰化生成十三醛。本研究将关于颗粒-催化剂相互作用和PE结构的知识与反应结果联系起来。它使用一种新的数值方法量化了位于颗粒间空隙（液-液界面）中的催化剂以及吸附在颗粒表面（液-固界面）上的催化剂的催化性能效率。首先，要确保所得水包油型PEs的液滴界面处的总体堆积密度和几何形状以及水滴大小是可预测的。其次，结果表明，乳液制备后几乎所有颗粒都聚集在液滴表面，当总颗粒横截面积保持恒定时，堆积参数和液滴大小均不会随颗粒表面电荷或尺寸而变化。第三，关于催化剂对乳液结构影响的研究表明，无论颗粒电荷如何，表面活性且带负电荷的催化剂Rh-SX都会显著减小PE液滴大小，并使颗粒堆积参数从s = 0.91（二维六方堆积）降至s = 0.69（破碎结构）。在后一种情况下，形成了自由水包油界面的大空隙并被催化剂覆盖。基于对PE结构的深入了解，量化了液-液界面与固-液界面的反应效率。通过排除任何其他影响因素，结果表明催化剂在流体和固体界面处的活性相同，并且反应性能由系统的几何形状来解释。反应后，通过膜过滤成功实现了催化剂的保留，而不会破坏乳液。这使得能够连续回收催化剂，即基于PE的催化反应中最昂贵的化合物，这是工业应用的关键标准。
