Alkane Aromatization Using Microwave Technology

Abstract

The current ever-increasing energy required to sustain human civilisation needs new catalytic and manufacturing processes that can not only minimise energy use but also improve reaction yields. To protect the environment, any new processes must ideally function with reduced toxic emissions and more efficient usage of feedstocks. This thesis focuses on the important aromatisation of alkanes and investigates the use of microwave technology to address the above goals. Aromatics are important starting materials for many applications and industries, including cosmetics, automobiles, medicine, and clothing. Based on their published activities using conventional heating, a range of catalysts have been screened for their activities for the microwave-driven catalysis of the aromatisation of heptane. The greatest yields of aromatics were found using zinc or gallium oxides in combination with an HZSM-5 zeolite. The possibilities of increasing the yields of aromatics by enhancing microwave absorption were then explored. Adding strongly microwave-absorbing compounds such as carbon might create mechanistic complications in the reaction. Hence the effects of doping the metal oxides as a route to enhanced electronic conductivity and associated microwave heating were studied. It was shown for the first time that doping of ZnO with Al increased the efficiency of aromatisation significantly as compared to pure ZnO, while Ga2O3 with zeolite can not be heated significantly until it is doped with Sn. Furthermore, the yields of BTX using microwaves and doping in the case of doped ZnO tended to be better than using conventional heating (39 % vs 34%), while it was lower with Ga2O3- based catalysts with the use of microwaves (32 % vs 37 %). The BTX yield does not follow entirely the abundance of the active phase, and additional factors might be at play. The most obvious factor is the formation of different olefin pools for the aromatisation process. In the case of conventional heating, smaller alkenes were produced while the microwave reactor produced larger alkenes. In addition, doping has been shown to alter the intermediates as suggested by Raman studies and it is more evident with higher doping where increased olefine formation is associated with decreased BTX yield. Carbon deposition was considered to be the major issue for catalyst deactivation, as suggested by TGA, and elevated temperatures increased its rate. Additional factors may be considered like the loss of active species especially in the case of ZnO catalysts as shown by XPS studies. This work introduces a potential novel method to enhance the catalytic capabilities of semiconductor materials by precisely adjusting their electronic properties through appropriate doping, resulting in decreased power consumption. By utilising microwaves, the catalyst bed temperature can be maintained at approximately 200 °C, a significant reduction compared to the conventional heating requirement of 550 °C. While there may be localised hotspots with microwaves, the overall mass of the catalyst at these elevated temperatures remains relatively small compared to the total catalyst mass. This approach aims to improve energy efficiency and lower operational costs in catalytic processes.