Escaping undesired gas-phase chemistry: Microwave-driven selectivity enhancement in heterogeneous catalytic reactors

Abstract

Research in solid-gas heterogeneous catalytic processes is typically aimed toward optimization of catalyst composition to achieve a higher conversion and, especially, a higher selectivity. However, even with the most selective catalysts, an upper limit is found: Above a certain temperature, gas-phase reactions become important and their effects cannot be neglected. Here, we apply a microwave field to a catalyst-support ensemble capable of direct microwave heating (MWH). We have taken extra precautions to ensure that (i) the solid phase is free from significant hot spots and (ii) an accurate estimation of both solid and gas temperatures is obtained. MWH allows operating with a catalyst that is significantly hotter than the surrounding gas, achieving a high conversion on the catalyst while reducing undesired homogeneous reactions. We demonstrate the concept with the CO2-mediated oxidative dehydrogenation of isobutane, but it can be applied to any system with significant undesired homogeneous contributions.

Beyond electrolysis: old challenges and new concepts of electricity-driven chemical reactors

“…Irrespectively of that, however, the microwave heating of solid catalysts offers interesting opportunities from the reaction engineering viewpoint: (i) one can selectively heat-up the metal nanoclusters within an otherwise microwave-neutral catalyst support, 103 and (ii) one can operate at bulk gas temperature much lower than the catalyst temperature, thereby limiting the occurrence of the unwanted, homogeneous sidereactions. 104 Consequently, the main challenges in applying microwave reactors on the industrial scale are different in (gas-)liquid and gas-solid systems. In liquid systems, the primary challenge consists in bringing and releasing the microwave radiation inside the reactor.…”

Overcoming Stability Problems in Microwave-Assisted Heterogeneous Catalytic Processes Affected by Catalyst Coking

“…This affects the electromagnetic field distribution along the sample and makes the heating control extremely challenging (Figure 2b). On the other hand, the use of β-SiC as structured support [9] for the Mo/ZSM-5 catalyst resulted in fairly homogeneous temperature distribution along the sample both in the presence of air and methane flows, regardless of the presence of unevenly coated monolith channels, i.e., catalyst accumulation at the channel corners (Figure 2c,d). The unavoidable formation of coke did not disturb the temperature profile along the Mo/ZSM-5@SiC sample and the MNOC reaction could be run for more than 5 h. This represents a promising catalyst configuration for long-term MW-assisted MNOC operation.…”

“…Microwave-assisted heating has emerged as an energy-efficient heating solution for a number of processes involving chemical transformations [1][2][3][4][5][6][7][8]. In particular, microwave (MW) irradiation has exciting prospects for gas-solid heterogeneous catalysis, since the selective dielectric heating of suitable catalytic materials is capable to establish a significant gas-solid temperature gradient between the heated catalytic sample and the comparatively colder surrounding gas [9][10][11]. This temperature gap is deemed responsible for the selectivity shift and the more efficient energy use reported in previous works of our laboratory.…”

“…Essentially, MW-heating of structured catalysts promotes a significant temperature gradient between the catalyst wall and the comparatively colder gas phase within the monolith channels. The beneficial effect of the selective MW-heating on the process productivity enhancement was reported for several catalytic processes such methane [10,17,18], ethane [19] or n-hexane [20] dehydroaromatization, methane dry reforming [21], and oxidative n-butane dehydrogenation [9]. However, from an industrial standpoint, most of the reported results on MW-assisted hydrocarbon conversion refer to insufficiently long and stable reaction periods.…”

Drastic Microwave Heating of Percolated Pt Metal Nanoparticles Supported on Al2O3 Substrate

“…These MW effects are a big advantage, which enables short time processes, enhancement of reaction rate, efficient energy usage and simple apparatuses without thermal insulators. The MW effects have been also observed in the supported metal nanoparticles (NPs) catalyst [9][10][11][12][13][14][15][16][17][18]. Jie et al applied the MW heating to the catalytic system of the supported Fe NPs, where dehydrogenation at the Fe NPs was selectively driven while the side reactions were prevented [14,17].…”