Co-Based Catalyst with High Stable Activity for Syngas Production By Methane Conversion

Abstract: Natural gas is a prospective source for replacement of the traditional oil to produce synthetic oil fractions via syngas. This work deals with the production of synthesis gas by dry and bireforming of methane over the Co-based catalysts supported on a matrix composed of alumina modified with zirconia and rare-earth metal oxides. The multicomponent 5%Co-M/Al₂O₃-ZrO₂-R catalysts showed the high stable activity during its long-term continuous testing for 400 hours. The catalysts were characterized by a number of physico-chemical methods.

Introduction

Lately, there has been considerable interest in the development of more efficient processes to generate syngas and hydrogen. Syngas, a mixture of H₂ and CO in any ratios has been considered as a potential and promising sustainable energy from an economic and environmental point of view. Syngas is an intermediate in the multi-tonnage production of fuels and chemicals, including ammonia, methanol, dimethyl ether, oxo-synthesis and Fischer–Tropsch (F-T) fuels and can be produced by three methane reforming processes, including steam (SRM, Eq.1) or dry reforming (DRM, Eq.2) and partial oxidation (POM, Eq.3), or also by any combination of these reactions – bireforming (BRM), tri-reforming, autho-thermal reforming of methane (ATR).

CH₄ + H₂O ↔ CO + 3H₂ ΔH^o = 206 kJ/mol (Eq. 1)

CH₄ +CO₂ ↔ 2CO + 2H₂ ΔH^o = 247 kJ/mol (Eq. 2)

CH₄ + 1/2 O₂ ↔ CO + 2H₂ ΔH^o = -38 kJ/mol (Eq. 3)

The ratio of H₂/CO is of great importance for the subsequent application of syngas. The required H₂/CO ratio of syngas depending on the target processes can be achieved by the combined steam-dry reforming of methane so-called bireforming of methane (BRM). This is a feasible process to adjust the H₂/CO ratio of syngas with various feed ratios of H₂O and CO₂.

The carbonization of catalysts for these processes is a main challenge, which is not yet solved and requires the development of effective catalysts resistant to coke formation.

This work deals with the design and synthesis of the high active and stable catalyst based on Co and supported on a matrix composed of alumina modified with zirconia and rare-earth metal oxides for dry and bireforming of methane to produce synthesis gas with different ratio depending on a steam amount added.

Experimental

The multicomponent 5%Co-M/Al₂O₃-ZrO₂-R catalysts, where M is a noble metal and R is Ce or/and La were synthesized by impregnation and tested in dry and steam reforming of methane. The processes were carried out in a quarts flow reactor supplied with programmed heating, controlled feed velocity, a programmable syringe pump, and GCs. The dry (DRM) and bireforming (BRM) reforming of methane have been studied under atmospheric pressure, varying gas hourly space velocity (GHSV) within 1000-3000 h^-1 with using a feed with a ratio of CH₄/CO₂=1:1 and varying temperature within 300-800^oC. The 0.5-1.0 vol. parts of steam has been added to a feed to provide bireforming of methane. To elucidate the stability the 5%Co-M/Al₂O₃-ZrO₂-CeO₂-La₂O₃ catalyst was continuously tested for a long-term period: 400 hours in DRM and 100 hours bireforming of methane. The catalysts were characterised by BET, SEM, TEM, and TPR methods.

Results/Discussion

The catalysts perform a high activity in both dry and steam reforming of methane. Methane is almost completely converted at 750-800^oC depending on the catalyst composition and process type. Adding steam in amount 10-50 vol.% to methane always leads to decrease in temperature of methane complete conversion and enriching syngas with hydrogen over the synthesized catalysts.

The 5%Co-M/Al₂O₃-ZrO₂-CeO₂-La₂O₃ catalyst showed a high stable activity for all period of testing in both processes: 400 hours in DR and 100 hours in steam reforming of methane. During 400 hours of DRM process over the catalyst under conditions t=700^oC, P=1 atm, GHSV=1000 h^-1, CH₄:CO₂=1:1, methane conversion was varied within 96.0-96.6%, while the extent of carbon dioxide conversion was insignificantly less and varied within 90.4-92.3%. The ratio of H₂/CO in the syngas formed was 0.9. In steam reforming of methane (CH₄:CO₂:H₂O=1:1:0.5), H₂/CO ratio was increased to 1.3.

A high dispersed state of metals and the effect of the second metal on reducibility of Co was observed by TEM and TPR.

Conclusion

The 5%Co-M/Al₂O₃-ZrO₂-R catalysts developed performs the high activity and stability in syngas production from methane. The synthesis gas composition can be controlled by adjusting the composition of the feed.

Acknowledgments

The work was financed by the Ministry of Education and Science of the Republic of Kazakhstan (Program number # PCF_BR05236739).