Development of Mesoporous Nanocatalysts for Production of Hydrogen and Fisher Tropsch Studies

• Main Author: Abrokwah, Richard Yeboah
• Language: EN
• Published: North Carolina Agricultural and Technical State University 2016
• Subjects: Alternative Energy|Chemistry|Materials science
• Online Access: http://pqdtopen.proquest.com/#viewpdf?dispub=10117803

<p> The primary aim of this study was to develop mesoporous nanocatalysts for (i) hydrogen production via steam reforming of methanol (SRM) in a tubular reactor, and (ii) syngas conversion to hydrocarbons via Fisher-Tropsch synthesis using silicon microchannel microreactors. The mesoporous catalysts for SRM were prepared by an optimized one-pot hydrothermal synthesis procedure. The catalysts were investigated for SRM activity in a packed bed tubular reactor using metals, namely, Cu, Co, Ni, Pd, Zn, and Sn. The metals were incorporated in different supports -MCM-41, SBA-15, CeO₂, TiO₂, and ZrO₂ to investigate the influence of support on catalyst properties. A sharp contrast in catalyst performance was noticed depending on the type of support employed. For example, in SRM at 250 &deg;C, Cu supported on amorphous silica SBA-15 and MCM-41 produced significantly less CO (&lt; 7%) compared to other crystalline supports Cu-TiO₂ and Cu/ZrO₂ that showed high CO selectivity of &sim;56% and &sim;37%, respectively. Amongst all the metals studied for SRM activity using 1:3 methanol:water mole ratio at 250 &deg;C, 10%Cu-MCM-41 showed the best performance with 68% methanol conversion, 100% H₂ , &sim;6 % CO, 94% CO₂ selectivities, and no methane formation. Furthermore, 10%Cu-CeO₂ yielded the lowest CO selectivity of 1.84% and the highest CO2 selectivity of &sim;98% at 250 &deg;C. Stability studies of the catalysts conducted for time-on-stream of 40 h at 300 &deg;C revealed that Cu-MCM41 was the most stable and displayed consistent steady state conversion of &sim;74%. Our results indicate that, although coking played an influential role in deactivation of most catalysts, thermal sintering and changes in MCM-41 structure can be responsible for the catalyst deactivation. For monomtetallic systems, the MCM-41 supported catalysts especially Pd and Sn showed appreciable hydrothermal stability under the synthesis and reaction conditions. While bimetallic Pd-Co-MCM-41 and Cu-Ni-MCM-41 catalysts produced more CO, Cu-Zn-MCM-41 and Cu-Sn-MCM-41exhibited better SRM activity, and produced much less CO and CH4. In spite of the improved the stability and dispersion of the monometallic active sites in the support, no noticeable synergistic activity was observed in terms of H₂ and CO selectivities in the multimetallic catalysts. For the Fisher-Tropsch (F-T) studies, Co-TiO₂, Fe-TiO₂ and Ru-TiO₂ catalysts were prepared by the sol-gel method and coated on 116 microchannels (50&mu;m wide x 100&mu;m deep) of a Si-microreactor. The F-T process parameters such as temperature, pressure and flow rates were controlled by an in-house setup programmed by LabVIEW^&reg;. The effect of temperature on F-T activity in the range of 150 to 300&deg;C was investigated at 1 atm, a flow rate of 6 ml/min and a constant H₂:CO molar ratio of 2:1. In our initial studies at 220 &deg;C, 12%Ru-TiO₂ showed higher CO conversion of 74% and produced the highest C₂-C₄ hydrocarbon selectivity-of &sim;11% ethane, 22% propane and &sim;17% butane. The overall catalyst stability and performance was in the order of 12%Ru-TiO₂>> 12%Fe-TiO₂ > 12%Co-TiO₂.</p>