Prof.  John  Hu
West Virginia University,  USA

Title: Activation of Stable Chemical Bonds by Microwave Catalysis

Abstract:

Introduction

Microwave catalytic processing is an emerging technology in line with the principles of process intensification. The interaction of microwave with solid catalyst through dielectric heating has been widely explored for numerous applications.¹ Experimental evidence showed that microwave-induced activation process has significantly lowered the activation energy of endothermic reactions, therefore enabling the reaction to proceed under lower bulk temperature. Microwave-driven catalytic process can quickly switch between startup and shutdown mode, exhibiting flexibility in handling intermittent renewable power supply. Microwave can also induce plasma to activate stable molecules. In the presence of a catalyst, the pre-activated plasma species can react on the catalyst surface under reaction pathways that are different from thermally heated reaction. A synergy of the plasma species with catalyst can be established if the post-plasma species influence the distribution and selectivity of the final products. An optimum catalyst design can maximize the inter-molecular interaction of the plasma species. This paper presents the applications of microwave in CH₄ conversion, NH3 synthesis and CO₂ capture and utilization.

Materials and Methods

Microwave testing was performed in a 2.45 GHz, 3 kW, magnetron generator with a mono-mode cavity microwave from Sairem (model GMP20K). The temperature was measured using a laser aligned infrared pyrometer (IR) from Micro-Epsilon (model CTLM-3SF75H2) with a pre-calibrated temperature range between 200 and 1500 °C and a 6.5 mm radial spot size. In the scenario of microwave plasma-driven catalysis, the CH₄ and N₂ are activated into plasma species in the microwave reactor cavity. Plasma generation is performed using a 2.54 GHz, 3 kW, fixed frequency microwave (Sairem, GMP20K). The quartz tube was placed in the waveguide at 300 W power in continuous wave mode, plasma was ignited using an external spark in pure 50 sccm Ar (UHP, Matheson).

Results and Discussion

Brief Experiments were carried out to form ethylene from ethane without using ZSM-5 catalyst. Figure 1 illustrates oxidative dehydrogenation of ethane using CO₂ for ethylene production over CsRu based catalysts. The microwave reactor reached 90% conversion at 650 °C, which is higher than that of a thermally heated fixed-bed reactor operated at 800 °C. Ethylene yield is also higher under the MW condition. Under microwave irradiation, the ratio of Ce^3+/Ce⁴⁺ in CeO₂ changes affecting the electron shift between Cs- Ru. This could alter reaction pathways, leading to lower activation energy. The outcomes of this work include decarbonizing the olefin industry and introducing electrification into the process rather than simply changing the combustion fired heater. The introduction of microwave heating to the process results in “co-benefits” of improved energy efficiency and product yield.

The effect of variation in pressure on the performance of ammonia synthesis under microwave irradiation was investigated. As shown in Figure 2, the stability test was performed

Figure 1. Oxidative dehydrogenation of ethane by CO₂ over Cs/Ru/CeO₂ catalyst: comparison of microwave and conventional heating(a) conversion (b) selectivity.

for 7 days, during which 6 cycles of startup- shutdown were performed.² The total cumulative on-line time was 80 hours. There was no loss in productivity either at 60 or 80 psig when microwave irradiation was resumed after shutdown. Five more cycles of startup–shutdown was repeatedly performed at 80 psig, during the period of time each cycle was held at 12 h on-line time. Ammonia concentration remained constant during 6 cycles of repeated startup-shutdown operation. This highlights that the microwave catalytic ammonia synthesis over the CsRu/CeO₂ catalysts is a robust process with the flexibility of coping with intermittent nature of renewable power supplies for distributed production.

Significance

Microwave-driven catalysis exhibits a strong influence on the activation of stable molecules such as CH₄, ethane N₂ and CO₂. Microwaves improve energy intensity and reduce carbon intensity, leading to decarbonization of chemical process via electrification.

References

1. J. Hu, Advances in Microwave-assisted Heterogeneous Catalysis, Book, RSC. 2023

2. Y. Wang, Catalysis Communication, 2021, 159, 2021, 106344-.

Biography: