Ethanol is a renewable bio-resource with a quite high hydrogen content. According to the overall stoichiometry of steam reforming of ethanol (SRE), six moles of hydrogen can in principle be produced per mole of ethanol.

                                    C₂H₅OH + 3H₂O ↔ 2CO₂ + 6H₂

The main reactions taking place during SRE are the reforming and water gas shift reactions (WGSR) and thermodynamic limitations of WGSR limit the approach to the maximum hydrogen yield value of six [1,2].

C₂H₅OH + H₂O ↔ 2CO + 4H₂

CO + H₂O ↔ CO₂ + H₂

Occurrence of ethanol dehydrogenation, decomposition, dehydration reactions as side reactions and coke formation due to Boudouard and decomposition reactions also cause limitations in H₂ yield and instabilities in the reactor performance. Two of the main challenges to obtain high hydrogen yield and to achieve stable catalytic reactor performance  are minimization of coke formation and elimination of equilibrium limitations during SRE. Sorption enhanced reforming of ethanol (SESRE) is an attractive possibility to eliminate thermodynamic limitations and to achieve high hydrogen yields. One possibility is to use calcium oxide for in-situ removal of produced CO₂ from the reaction zone of the reactor.

                                    C₂H₅OH + 3H₂O + 2CaO ↔ 2CaCO₃ + 6H₂

In order to make SRE process economically feasible, heating efficiency of the reaction zone of the reactor should be quite high. Heating of the reaction zone of the reactor by a focused microwave source has recently been considered as an efficient way of heating chemical reactors [3]. Direct conversion of electromagnetic energy into heat within the reaction zone has another advantage of achieving uniform temperature distribution within the catalyst bed, which helps to control the occurrence of undesired side reactions.

            In the present study, Ni impregnated CeO₂-ZrO₂-SBA-15 type mesoporous catalysts were synthesized and performance of these catalysts were compared with the performance of Ni impregnated mesoporous alumina, in SRE and SESRE performed in conventionally heated and microwave reactors. CeO₂-ZrO₂-SBA-15 support material with a Zr/Si ratio of 0.3 was synthesized following a one-pot route, using P123 as the surfactant. Surface area of this mesoporous catalyst support and 6 % Ni impregnated catalytic material were 644 and 509 m²/g. Results obtained in a conventionally heated tubular reactor showed very high SRE activity of this catalyst, with quite high inititial hydrogen yield values. However, significant reactor instabilities and a sharp decrease of H₂ yield were observed at 600^oC after about 30 min of reaction time. Sorption enhances steam reforming process (SESRE) was shown to give significant improvement in hydrogen yield values during reforming of ethanol in the temperature range of 500-600^oC. It was also shown that highly stable reactor performance, with coke minimization could be achieved as a result of performing SRE and SESRE in a reactor which was heated by a focused microwave source. Elimination of cold spots within the catalyst bed caused minimization of coke formation due to Boudouard reaction in the microwave reactor. Beter energy utilization and much more stable performance were achieved in the focused microwave reactor.

Acknowledgement: Financial support of TUBITAK and TUBA.

References:

[1] Gundus S., Dogu T., Ind. Eng. Chem. Res., 51 (2012) 8796.

[2] Arslan A., Gunduz S., Dogu T., Int. J. Hydrogen Energy, 39 (2014) 18264.

[3] Gunduz S., Dogu T., Appl. Catal. B: Env., 168 (2015) 497.