**1. Introduction**

In the early 1990s, microreactors and flow chemistry emerged and established themselves as powerful tools for exploratory research, piloting, and industrial production. Due to the characteristically small diameter of the reaction channel in the micron-to-millimeter range, a very high surface-to-volume ratio is realized and mixing and heating are significantly accelerated, which enable demanding chemical processes to operate safely and under closely controlled conditions [1,2]. The very low holdup of the reaction volume ensures an entirely new level of inherent safety in the process, as evidenced in the application of this technology to toxic or explosive reactions [3–6]. The narrow residence time distribution approaching that of an ideal plug flow reactor [7–9] makes it beneficial for maximizing the selectivity of parallel and sequential reactions and in the synthesis of monodispersed nanoparticles [10–12]. Running reactions under flow microconditions enables much better quality control compared to batch reactions, as processes can be run under continuous monitoring using online sensors. As such, flow microreactors are ideal for fully automated systems to synthesize, screen, and optimize chemical research [13–15].

Metal–organic frameworks (MOFs) are an emerging class of porous materials constructed from metal-containing secondary building units (SBUs) and organic linkers [16]. The highly porous cavities decorated with functional ligands and active metal SBUs produce MOFs with exceptional potential for the fabrication of a variety of heterogeneous catalysts. For example, the hydrolysis of chemical warfare agents (CWAs) and simulants with highly stable Zr(IV)-based MOFs such as **UiO-66**, **NU-901, NU-1000,** and **MOF-808** has been explored because of their ultrahigh stabilities in aqueous solvents. Using Lewis acidic zirconium clusters as active sites has also been studied [17–19]. Although MOF or MOF-based composite catalysts have been proven to be effective for the degradation of CWAs, most examples were tested in batch conditions in basic solutions. In general, there are few examples of catalytic testing of MOF-based catalysts in flow reactors as shown in Scheme 1 [20,21].

**Scheme 1.** Schematic illustration of the catalytic hydrolysis of CWAs under microflow conditions.

Catalyst testing in flow microreactors has many advantages over traditional solid catalyst testing in batch reactors [20]. First, many operating parameters such as temperature, pressure, and feed concentrations can be easily and quickly varied in flow microreactors, which provide an insight into the reaction mechanism and kinetics; second, consumption of chemicals and waste production is significantly reduced; and third, it enables easy testing of catalyst reusability, and catalyst stability under reaction conditions via long-term testing on a stream and changing the feed purity. Additionally, flow microreactors enable the study of the direct leaching of the active metal or organic components by chemical analysis of the filtrate and effluent. Considering the potential of MOFs as catalytic materials and flow microreactors as a powerful tool for catalyst testing, studying how to use MOF catalysts in a flow microreactor is valuable for future research, especially during the exploration and screening phase.

A major limitation to loading the as-synthesized MOF catalyst powder in the flow microreactor is the small particle size, ranging from nanometers up to a few micrometers, which results in a very high pressure drop. To eliminate this drop, the catalyst particles must be enlarged to a sufficient size. Several compounding methods have been proposed in the literature to enlarge MOF catalyst particle size. These include coating the catalyst on the wall of the reaction channel, using a monolith, producing catalyst microfibers, or using 3D printing to construct smart designs [22,23]. Each of these concepts has its own advantages and specific challenges. Although these are promising solutions for final commercial applications, testing catalysts in powder form is sometimes inevitable in the research stage. This is especially true at the initial exploratory phase of research, in which determining intrinsic reaction kinetics is essential, the quantities of available catalysts are small, and supply is limited. The intrinsic kinetics are better tested without the presence of binders, as they could affect activity and catalyst particle size needs to remain small to avoid mass and heat transfer limitations.

The most common method used to increase catalyst particle size is tableting the as-synthesized powder via mechanical compression, followed by crushing and sieving the tablet to obtain particles of the desired size. This is advantageous and easy as no binding material or additional complex treatment steps that could affect the catalyst properties are needed. The only concern is that applying mechanical compression to MOFs could cause a decrease in surface area due to destruction of the crystalline structure [24–26]. Some MOFs collapse when submitted to mechanical compression beyond a certain pressure [27]. The zirconium metal–organic framework **UiO-66** chosen for this study possesses exceptional mechanical stability in a highly porous system and can be processed through the pelleting, crushing, and sieving procedure without significant mechanical damage [28–30].

The aim of this paper is to demonstrate the capability of MOFs as a catalytic material in a flow microreactor. The emphasis is mainly on demonstrating if increasing particle size by tableting and crushing is a viable option for efficient catalyst screening and testing. To this end, the detoxification of ethyl-paraoxon (pesticide) through a hydrolysis reaction was studied in the loaded capillary flow reactor. Several parameters were tested including di fferent MOF crystal sizes and functionality. The operating temperature, concentration, and residence time were also tested. Finally, the catalyst was tested for a long period of time under continuous flow to check its stability.
