ABSTRACT
Mn oxides are promising materials for thermochemical heat store, but slow reoxidation of Mn3 O4 to Mn2 O3 limits efficiency. In contrast, (Mn1- x Fex )3 O4 oxides show an enhanced transformation rate, but fundamental understanding of the role played by Fe cations is lacking. Here, nanoscale characterization of Fe-doped Mn oxides is performed to elucidate how Fe incorporation influences solid-state transformations. X-ray diffraction reveals the presence of two distinct spinel phases, cubic jacobsite and tetragonal hausmannite for samples with more than 10% of Fe. Chemical mapping exposes wide variation of Fe content between grains, but an even distribution within crystallites. Due to the similarities of spinels structures, high-resolution scanning transmission electron microscopy cannot discriminate unambiguously between them, but Fe-enriched crystallites likely correspond to jacobsite. In situ X-ray absorption spectroscopy confirms that increasing Fe content up to 20% boosts the reoxidation rate, leading to the transformation of Mn2+ in the spinel phase to Mn3+ in bixbyite. Extended X-ray absorption fine structure shows that FeO length is larger than MnO, but both electron energy loss spectroscopy and X-ray absorption near edge structure indicate that iron is always present as Fe3+ in octahedral sites. These structural modifications may facilitate ionic diffusion during bixbyite formation.
ABSTRACT
Catalytic hydrodeoxygenation (HDO) is an effective technology for upgrading pyrolysis bio-oils. Although, in the past years, this process has been extensively studied, the relevance of the cross-reactivity between the numerous chemical components of bio-oil has been scarcely explored. However, molecular coupling can be beneficial for improving the bio-oil characteristics. With the aim of gaining a better understanding of these interactions, this work investigates the catalytic hydrodeoxygenation of mixtures of two typical components of pyrolysis bio-oils: guaiacol and acetic acid. The catalytic tests were carried out employing a bifunctional catalyst based on nickel phosphide (Ni2P) deposited over a commercial nanocrystalline ZSM-5 zeolite. The influence of both hydrogen availability and temperature on the activity and product distribution, was evaluated by carrying out reactions under different H2 pressures (40-10 bar) and temperatures (between 260 and 300 °C). Using blends of both substrates, a partial inhibition of guaiacol HDO occurred because of the competence of acetic acid for the catalytic active sites. Nevertheless, positive interactions were also observed, mainly esterification and acylation reactions, which could enhance the bio-oil stability by reducing acidity, lowering the oxygen content, and increasing the chain length of the components. In this respect, formation of acetophenones, which can be further hydrogenated to yield ethyl phenols, is of particular interest for biorefinery applications. Increasing the temperature results in an increment of conversion but a decrease in the yield of fully deoxygenated molecules due to the production of higher proportion of catechol and related products. Additional experiments performed in the absence of hydrogen revealed that esterification reactions are homogeneously self-catalyzed by acetic acid, while acylation processes are mainly catalyzed by the acidic sites of the zeolitic support.
ABSTRACT
Exâ situ catalytic biomass pyrolysis was investigated at both laboratory and bench scale by using a zeolite ZSM-5-based catalyst for selectively upgrading the bio-oil vapors. The catalyst consisted of nanocrystalline ZSM-5, modified by incorporation of ZrO2 and agglomerated with attapulgite (ZrO2 /n-ZSM-5-ATP). Characterization of this material by means of different techniques, including CO2 and NH3 temperature-programmed desorption (TPD), NMR spectroscopy, UV/Vis microspectroscopy, and fluorescence microscopy, showed that it possessed the right combination of accessibility and acid-base properties for promoting the conversion of the bulky molecules formed by lignocellulose pyrolysis and their subsequent deoxygenation to upgraded liquid organic fractions (bio-oil). The results obtained at the laboratory scale by varying the catalyst-to-biomass ratio (C/B) indicated that the ZrO2 /n-ZSM-5-ATP catalyst was more efficient for bio-oil deoxygenation than the parent zeolite n-ZSM-5, producing upgraded bio-oils with better combinations of mass and energy yields with respect to the oxygen content. The excellent performance of the ZrO2 /n-ZSM-5-ATP system was confirmed by working with a continuous bench-scale plant. The scale-up of the process, even with different raw biomasses as the feedstock, reaction conditions, and operation modes, was in line with the laboratory-scale results, leading to deoxygenation degrees of approximately 60 % with energy yields of approximately 70 % with respect to those of the thermal bio-oil.
