Throughput Optimization of Molybdenum Carbide Nanoparticle Catalysts in a Continuous Flow Reactor Using Design of Experiments

Abstract

Transition metal carbides (TMCs) have attracted significant attention because of their applications toward a wide range of catalytic transformations. However, the practicality of their synthesis is still limited because of the harsh conditions in which most TMCs are prepared. Recently, a solutionphase synthesis of phase-pure α-MoC 1−x nanoparticles was presented. While this synthetic route yielded nanoparticles with exceptional catalytic performance, the reaction parameter space was not explored, and catalyst throughput was not optimized for scale-up. Continuous flow platforms coupled with statistical design of experiments (DoE) can provide a powerful method for understanding the reaction parameter space for optimizations. Here, we demonstrate the use of statistical DoE in tandem with response surface methodology for a parametric screening analysis to optimize the throughput of a MoC 1−x nanoparticle synthesis utilizing a millifluidic flow reactor. A full factorial design was implemented to evaluate four input variables (reaction temperature, flow rate, solvent fraction of oleylamine, and precursor concentration) that carry statistically significant effects on three responses (throughput, residence time, and isolated yield). A Doehlert matrix was implemented to investigate each significant variable at a higher number of levels to optimize throughput. Our results give a nonintuitive set of experimental conditions that resulted in an optimized throughput of 2.2 g h −1 . This translates to a 50-fold increase in throughput compared to the previously reported batch method. The catalytic performance of the MoC 1−x nanoparticles produced under optimized throughput was demonstrated in the CO 2 hydrogenation reaction. This DoE screening analysis and throughput optimization of MoC 1−x synthesis open the door to an increased feasibility for scale-up.

Activating Molybdenum Carbide Nanoparticle Catalysts under Mild Conditions Using Thermally Labile Ligands

“…The carbon-supported α-MoC 1– x nanoparticle catalysts were evaluated for their performance in the CO 2 hydrogenation reaction following similar conditions to those previously described. , Catalyst (ca. 0.4–0.7 g) was loaded in a 1/4 in.…”

“…The α-MoC 1– x nanoparticles were evaluated for thermocatalytic CO 2 hydrogenation and shown to exhibit an increased activity on a per-site basis compared to bulk α-MoC 1– x . We further demonstrated that the α-MoC 1– x nanoparticle catalyst synthesis could be scaled up using continuous flow chemistry, while retaining the same catalytic performance as the nanoparticles prepared in the small-scale batch method …”

“…One common approach for removing surface ligands on nanoparticles is through high-temperature reductive annealing. For example, in our previous thermocatalytic CO 2 hydrogenation reactions with α-MoC 1– x nanoparticles, the catalysts were activated at 450 °C under a reducing atmosphere (95% H 2 /5% Ar flow) for 2 h to remove the oleylamine ligands; this activation temperature is significantly higher than the synthesis temperature of the α-MoC 1– x nanoparticle catalysts (290–320 °C) and the CO 2 hydrogenation temperature (300 °C). , Aside from being energy-intensive, this method may lead to particle sintering or coking of the surface ligands, which blocks surface active sites and decreases catalytic activity. , Treating the nanoparticles with acid is another common approach for stripping away the surface ligands, although this method is insufficient if the ligands are tightly bound to the surface and, in some cases, can poison or etch the nanoparticle catalysts …”

Effect of nanostructuring on the interaction of CO2with molybdenum carbide nanoparticles

“…The set of studied nanoparticles includes stoichiometric and non-stochiometric cases, in order to provide information that is expected to be useful for catalysis experiments using these systems, 16,17,21 for instance, in the understanding of their interaction with CO 2 , paving the way towards their improvement through a rational design. 44 For the smallest members of the series, global optimization techniques were used to explore the low-energy isomers. For Mo 4 C 6 and Mo 5 C 6 several isomers exist close in energy to the lowest energy one, where only two low-energy isomers have been found for Mo 6 C 5 , while the same isomer for stoichiometric Mo 6 C 6 was the same for both techniques.…”

“…The set of studied nanoparticles includes stoichiometric and non-stochiometric cases, in order to provide information that is expected to be useful for catalysis experiments using these systems, 16,17,21 for instance, in the understanding of their interaction with CO 2 , paving the way towards their improvement through a rational design. 44…”