Switchable Stimuli-Responsive Heterogeneous Catalysis

Abstract

Heterogeneous catalytic systems based on the use of stimuli-responsive materials can be switched from an "on" active state to an "off" inactive state, which contributes to endowing the catalysts with unique functional properties, such as adaptability, recyclability and precise spatial and temporal control on different types of chemical reactions. All these properties constitute a step toward the development of nature-inspired catalytic systems. Even if this is a niche area in the field of catalysis, it is possible to find in literature intriguing examples of dynamic catalysts, whose systematic analysis and review are still lacking. The aim of this work is to examine the recent developments of stimuli-responsive heterogeneous catalytic systems from the viewpoint of different approaches that have been proposed to obtain a dynamic control of catalytic efficiency. Because of the variety of reactions and conditions, it is difficult to make a quantitative comparison between the efficiencies of the considered systems, but the analysis of the different strategies can inspire the preparation of new smart catalytic systems.Catalysts 2018, 8, 569 2 of 25 fields [6] and mechanical stress [7], have also been exploited. A large variability can also be observed in the type of system response: variations of chemical properties, solubility, shape, volume, color, aggregation state, electrical properties, magnetic properties, mechanical properties, etc. are all possible. Thanks to this high versatility, artificial smart materials have found application in various sectors [8], ranging from medicine to national security and structural engineering [9]. They are commonly employed for drug delivery [10], self-healing [11], sensing [12], actuations [13], information storage [14], etc. Their use for catalytic purposes has been remarkably less investigated, even if heterogeneous catalysis can fully take advantage of the dynamic behavior of smart materials. For example, it is possible to make a catalyst to pass from a state of activity (an "on" state) to a state of inertness (an "off" state) in a controlled way, with a precise temporal control, obtaining switchable catalysis. In other circumstances, the possibility of modulating the reaction rate in a continuous way, regulating the amount of accessible active sites and accelerating or lowering the reaction rate in a non-discrete way, is more interesting, and in this case we can also exploit stimuli-responsive materials. It is also possible to create microenvironments in which the reaction occurs preferentially, accomplishing a spatial control on the reaction development. In addition, the responsiveness of the employed systems can be exploited to improve the recyclability of the catalytic system, making the catalyst separation from the reaction medium and its reuse easier.In this review, we analyze some of the latest works on dynamic systems in the field of heterogeneous catalysis. Many more examples can be found for homogeneous catalysis, but most of them have been already analyzed in previ...

Dual‐Responsive Photocatalytic Polymer Nanogels

“…Altering the materials solubility results in controlled activation or deactivation of the catalyst by controlling access of reagents to the catalytic sites. [5] In biological systems,f unctions or activities can be enabled or disabled by triggers,w hich lead to structural changes in conformation and, therefore,a ctivity control. For artificial materials,m ultiple stimuli have previously been reported that can be utilized to modify the physical properties of classical polymers most notably change by light, [6] pH value, [7] and temperature.…”

“…Dual-responsive thermo-and light-sensitive polymer nanogels were designed so that photocatalysis could be quenched through changes in temperature.The contraction of the photocatalytic sites within the polymer gel network is demonstrated in Scheme 1, in which the gel network becomes insoluble and precipitates upon an increase in temperature, which shields the photocatalytic units and retards partitioning of the reagents into the network. [5] Furthermore,t he condensation of the nanogel network upon temperature elevation may limit light penetration into the gel network. A decrease in temperature to ambient conditions results in ar ewetting of the gel network increasing the transfer of reagents from the continuous phase to the active regions.This reversible behaviour of the photocatalytic nanogel enables photocatalysis to be turned on or off depending on temperature in amanner analogous to enzymatic functions within the body.…”

Dual‐Responsive Photocatalytic Polymer Nanogels

“…[4] Them ain challenges within these photocatalytic systems is maintaining efficient catalysis whilst producing amaterial that can be easily recovered. [5] In biological systems,f unctions or activities can be enabled or disabled by triggers,w hich lead to structural changes in conformation and, therefore,a ctivity control. Altering the materials solubility results in controlled activation or deactivation of the catalyst by controlling access of reagents to the catalytic sites.…”

“…[5] Furthermore,t he condensation of the nanogel network upon temperature elevation may limit light penetration into the gel network. [5] Furthermore,t he condensation of the nanogel network upon temperature elevation may limit light penetration into the gel network.…”

Rice Husk Derived Porous Silica as Support for Pd and CeO2 for Low Temperature Catalytic Methane Combustion

“…Finally, the CPG is obtained after time consuming acidic and alkaline leaching. In addition, functionalizing Pd nanoparticles in next-generation matrix supports, such as pH-responsive polymers, is another feasible way to obtain high-performance Pd-based catalysts [25].…”