(617b) Hydrogen Production By Steam Reforming of Bio-Oil Derived Acetic Acid over Ni Based Zr-SBA-15 Type Mesoporous Catalysts

Acetic acid (AcOH) is a renewable, non-toxic and a safe bio-based hydrogen carrier. It is one of the main components of bio-oil, which can be produced by pyrolysis of biomass. Acetic acid can also be produced from biomass through fermentation. Hence, it is considered as an attractive renewable resource for the production of synthesis gas and/or hydrogen, through a catalytic steam reforming process [1]. As a result of steam reforming process of AcOH, a gas mixture which is mainly composed of H_2, CO and CO₂, is produced. Product distribution and the ratio of CO/CO₂ in the produced syngas is expected to be strongly related to the catalyst composition and reaction conditions. This gas mixture can be used as a fuel in solid oxide fuel cells or to obtain high purity hydrogen for chemical applications. Syngas thus produced can also be used as a resource for the production of valuable chemicals and fuels through Fischer-Tropsch or dimethyl ether synthesis routes. In such cases, a gas composition with equimolar quantities of H₂ and CO is generally preferred. Production of syngas from acetic acid mainly involves steam reforming (R.1) and water gas shift reactions (R.2). Occurrence of thermal decomposition, methanation, decarboxylation and coke formation reactions would also contribute to the product distributions and catalyst deactivation.

CH₃COOH + 2H₂O â?? 4H₂ + CO₂ (R.1)

CO + H₂O â?? H₂ + CO₂ (R.2)

Synthesis of active, stable and selective catalysts for steam reforming of acetic acid is a hot research area for the development of new renewable energy processes. Nickel is one of the most active catalytic metals for reforming reactions. Type of support material is expected to have strong influence on the performance of Ni based catalysts in steam reforming of AcOH. Invention of silicate structured mesoporous materials, like MCM-41 and SBA-15, opened new avenues in catalysis research. These materials, with ordered pore structures, were reported to be less susceptible to deactivation due to coke formation than the conventional microporous catalyst supports. They also cause less resistance to diffusion of reactants to the active sites. However, some of the silicate structured mesoporous materials were known to have low hydrothermal stability in the presence of water vapor. Zirconia, with its high thermal and chemical stability attracted the attention of catalysis researchers in recent decades. Strong interaction of Ni clusters with zirconia and quite high water dissociation capacity of zirconia were reported as attractive properties of this material in steam reforming reactions. More recently, zirconia incorporated SBA-15 was reported as a highly stable catalyst support for steam reforming of ethanol [2]. In the present study, zirconia incorporated SBA-15 type mesoporous materials with different Zr/Si ratios were synthesized following a one-pot hydrothermal route, characterized and used as the catalyst support materials for the synthesis of new Ni based catalysts to be used in steam reforming of AcOH. Some of these Ni based catalysts were modified by addition of tungsten, during the impregnation step. Catalytic performances of these materials were tested in steam reforming of AcOH at 750 ^oC. Synthesized Zr-SBA-15 type support materials were shown to have ordered mesopore structures with high surface area values. For instance, the material containing 25% zirconia in SBA-15 (25Zr-SBA-15) had a surface area of 608 m²/g, while some decrease of surface area was observed as a result of Ni and W impregnation. For instance, the catalysts containing 5% Ni (5Ni@25Zr-SBA-15) and 5% NÄ°+10% W (5Ni-10W@25Zr-SBA-15) had surface area values of 561 m²/g and 151 m²/g, respectively. Catalytic activity tests proved that the performance of the Zr-SBA-15 supported Ni based catalysts were highly stable and they also showed very high activity in steam reforming of acetic acid, giving complete conversion at temperatures over 700 ^oC, at space time of 0.072 s.g.ml^-1. Product distributions were shown to be strongly influenced by the composition of the catalyst. In the case of 5Ni@25Zr-SBA-15, syngas produced at 750 ^oC contained about 54 % H₂, 22% CO, 20% CO₂ and 4% CH₄. These results indicated that thermal decomposition of AcOH to methane and CO₂ was minimized over this catalyst. Results were considered to be highly promising from the point of view of production of hydrogen rich syngas. It was most interesting to observe that modification of this catalyst by the addition of tungsten caused significant changes in the product distribution. For instance, syngas produced over 5Ni-10W@25-SBA-15 at the same reaction conditions, contained equimolar quantities of H₂ and CO (about 47.5 % each) with very small amounts of CO₂ and CH₄ (about 3% and 2%, respectively). Production of a syngas with such a composition was considered to be highly attractive from the point of view of a resource gas for DME and Fischer-Tropsch synthesis. Activity test results obtained with different catalyst compositions and at different reaction conditions, as well as the characterization results of the synthesized catalytic materials, will be reported in this presentation.

Acknowledgements: TUBITAK Project (214M578) and TUBA.

References:

[1] Goicoechea S., Kraleva E., Sokolov S., Schneider M., Pohl M.M., Kockmann N., Ehrich H., Appl. Catal. A: Gen. 514 (2016) 182.

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