M. Ojeda

Furfural: a renewable and versatile platform molecule for the synthesis of chemicals and fuels

The production of future transportation fuels and chemicals requires the deployment of new catalytic processes that transform biomass into valuable products under competitive conditions. Furfural has been identified as one of the most promising chemical platforms directly derived from biomass. With an annual production close to 300 kTon, furfural is currently a commodity chemical, and the technology for its production is largely established. The aim of this review is to discuss the most relevant chemical routes for converting furfural to chemicals, biofuels, and additives. This review focuses not only on industrially produced chemicals derived from furfural, but also on other not yet commercialised products that have a high potential for commercialisation as commodities. Other chemicals that are currently produced from oil but can also be derived from furfural are also reviewed. The chemical and engineering aspects such as the reaction conditions and mechanisms, as well as the main achievements and the challenges still to come in the pursuit of advancing the furfural-based industry, are highlighted.

Selective Conversion of Furfural to Maleic Anhydride and Furan with VOx/Al2O3 Catalysts

Furfural can be converted into maleic anhydride (73 % yield) through selective gas phase oxidation at 593 K with O(2) by using VO(x)/Al(2)O(3) (10 at(V) nm(-2)) as solid catalysts. The use of lower temperatures and/or O(2) pressures result in the additional formation of furan (maximum 9 % yield). Mechanistically, furfural (C(5)H(4)O(2)) is oxidized stepwise to furan (C(4)H(4)O), 2-furanone (C(4)H(4)O(2)), and finally, maleic anhydride (C(4)H(2)O(3)). The specific structure of the supported vanadium oxides and reaction conditions (temperature and reactants pressures) all influence furfural oxidation catalysis. We have found that Al(2)O(3)-supported polyvanadates are intrinsically more active (2.70 mmol h(-1) g-at V(-1)) than monovanadates (VO(4)) and V(2)O(5) crystals (0.89 and 0.70 mmol h(-1) g-at V(-1), respectively) in maleic anhydride and furan formation rates (553 K, 1.6 kPa furfural, 2.5 kPa O(2)). Our alternative approach enables the use of biomass instead of petroleum to synthesize maleic anhydride and furan from furfural. The potential variety of industrial applications is of enormous interest for the development of future biorefineries.

Cyclopentyl methyl ether: a green co-solvent for the selective dehydration of lignocellulosic pentoses to furfural.

The effects of cyclopentyl methyl ether (CPME) addition during the aqueous xylose dehydration reaction to furfural are reported here. These investigations were conducted by using pure xylose and Cynara cardunculus (cardoon) lignocellulose as sugar source and H(2)SO(4) as catalyst. The research was also applied to aqueous solutions containing NaCl, since it has been previously demonstrated that NaCl incorporation to these reaction mixtures remarkably increases the furfural formation rate. It has been found that CPME incorporation inhibits the formation of undesired products (resins, condensation products and humins). Thus, cardoon lignocellulosic pentoses were selectively transformed into furfural (near 100%) at the following reaction conditions: 1 wt.% H(2)SO(4), 4 wt.% biomass referred to aqueous solution, 30 min reaction, 443 K, CPME/aqueous phase mass ratio equals to 2.33, and NaCl/aqueous solution mass ratio of 0.4. In contrast, no effect was observed for cellulosic glucose transformation into hydroxymethylfurfural and levulinic acid at identical reaction conditions.

Support effects on the structure and performance of ruthenium catalysts for the Fischer–Tropsch synthesis

The influence of support and metal precursor on Ru-based catalysts has been studied in the Fischer–Tropsch synthesis (FTS) combining flow reactor and quasi in situinfrared spectroscopy experiments. A series of supported ruthenium catalysts (3 wt.%) have been prepared using two different TiO2 (P25, 20% rutile and 80% anatase; Hombifine, 100% anatase) and SiO2·Al2O3 (28% Al2O3) as supports and RuCl3·nH2O as metal precursor. The catalysts were labeled as RuTi0.8, RuTi1 and RuSA respectively. Another catalyst (RuTi0.8N) has been synthesized with TiO2·P25 and Ru(NO)(NO3)3. After thermal treatments in air at 723 K and hydrogen at 443 K, ruthenium metal particles are agglomerated when pure anatase TiO2 and SiO2·Al2O3 are used as supports, leading to low active catalysts. In contrast, and despite the lower specific surface area of TiO2·P25 as compared to that of the other supports, well dispersed Ru particles are stabilized on titania P25. Remarkably, electronic microscopy studies demonstrate that Ru is deposited exclusively on the rutile phase of TiO2·P25. The catalytic performance shown by all these catalysts in FTS reactions follows the order: RuTi0.8 > RuTi0.8N > RuSA ≫ RuTi1. The same trend is observed during quasi in situ FTS experiments conducted in an infrared (IR) spectroscopy cell. The FTIR spectra of TiO2·P25 supported samples show that both samples behave similarly under the FTS reaction. This work shows that the structure of the support, rather than its specific surface area or the Ru precursor, is the parameter that determines the dispersion of Ru particles, hence their catalytic performance.

Evolution of the bulk structure and surface species on Fe–Ce catalysts during the Fischer–Tropsch synthesis

Two Fe–Ce catalysts were prepared by wet impregnation of Ce onto iron oxyhydroxide (FeOOH) and hematite iron oxide (α-Fe2O3), respectively. Their performance in the Fischer–Tropsch (FT) synthesis was investigated and compared with that obtained with a Ce-free α-Fe2O3 catalyst. It was observed that the behavior of the different catalysts changed along the course of the FT reaction. The catalysts were tested for different periods of time, carefully passivated, recovered from the reactor and characterized by different techniques. The FT activity of the Ce-loaded and Ce-free catalysts decreased initially, but at a certain point the catalytic activity started to increase. The time needed to reach this inflection point depended on the catalyst composition, being shorter for the Ce-promoted catalysts. The catalytic activity of the Ce-free catalyst increased when the Fe3C species were transformed into χ-Fe2.5C, which are suggested to be the carbide phase present when polymerized carbon species (Cβ) are formed. The addition of Ce to the iron oxyhydroxide developed solids with a higher BET surface area. Besides, these samples displayed a higher FT activity at long time-on-stream (TOS). Moreover, Ce addition also facilitated the formation of the Cβ species previous to the evolution of Fe3C into χ-Fe2.5C, and therefore, promoted the FT synthesis reaction.

TiO2-supported heteropoly acid catalysts for dehydration of methanol to dimethyl ether: relevance of dispersion and support interaction

Two heteropoly acids (HPAs) with Keggin structures, namely, H3PW12O40 (HPW) and H4SiW12O40 (HSiW), have been deposited on TiO2 and used as catalysts for the production of dimethyl ether (DME) from methanol. The catalysts have been prepared by incipient wetness impregnation of HPW and HSiW on TiO2 with HPA loadings ranging between 0.9 and 9.0 Keggin units (KU) per square nanometer. The structure and acid properties of the final catalysts have been thoroughly characterized by N2 adsorption–desorption isotherms, TGA, XRD, Raman spectroscopy, XPS, NH3 adsorption isotherms, DRIFT and 1H NMR. All catalysts exhibit very high DME productivities and high methanol conversion rates at temperatures as low as 413 K. The effect of the HPA loading on TiO2 for the production of DME has been correlated with the structure and acid properties of the final catalyst. We find an optimum loading for both TiO2-supported HPW and HSiW of 2.3 KU nm−2. At this level, both HPW and HSiW are well dispersed onto the support, thus permitting the access of methanol to the active acid sites. On the contrary, higher HPA loadings on TiO2 result in the formation of larger HPA units that prevent the access of methanol to the inner acid sites within the HPA. On the other hand, HPA loadings below 2.3 KU nm−2 result in a strong interaction between the acid protons of the HPAs and the support, hence preventing the participation of such protons in the methanol dehydration reaction, which results in lower methanol conversion rates.

Influence of residual chloride ions in the CO hydrogenation over Rh/SiO2 catalysts

Silica-supported rhodium catalysts prepared from nitrate and chloride precursors were tested in the CO hydrogenation reaction. Similar reaction rate and selectivities to the different product families were found for both catalysts, however the ex-chloride catalyst showed a lower 1-olefin/n-paraffin ratio. Characterisation by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), H2 chemisorption and Fourier transform infrared spectroscopy (FTIR) of CO chemisorbed did not evidence disparate features between ex-nitrate and ex-chloride samples. However, a much higher H2 desorption was found in the ex-chloride catalyst according to the hydrogen temperature-programmed desorption (TPD) studies. Residual chloride species seems to be involved in the H2 adsorption on silica enhancing the spillover of hydrogen from metal particles to the silica support. It is suggested that spilt-H atoms can create active sites on the silica surface where olefins can be hydrogenated to the corresponding paraffins.

Evidences of Two‐Regimes in the Measurement of Ru Particle Size Effect for CO Dissociation during Fischer–Tropsch Synthesis

This work assesses the effect of the particle size for the Fischer–Tropsch synthesis with catalysts based on Ru particles between 4 and 71 nm. CO dissociation is a structure‐sensitive reaction, that is, the turnover frequency (TOF) is affected by Ru particle size. Herein it is demonstrated that two regimes exist for the effect of Ru particle size. On the one hand, the expected relationship of TOF with particle size, that is, the TOF increases with Ru particles <10 nm, is only observed if measured at steady‐state conditions. On the contrary, the TOF increases constantly with Ru particle size if measured at the initial state. However, the TOF measured for catalysts with Ru particles >10 nm declines during time on stream. These observations suggest that two regimes for the measurement of CO dissociation exist during Fischer–Tropsch synthesis. The reason for this is that two sites for CO dissociation on the Ru particles >10 nm exist at the initial state, terraces and step‐edges, but the latter ones deactivate during time on stream.

Preparation and characterization of Mg-Zr mixed oxide aerogels and their application as aldol condensation catalysts.

A series of Mg-Zr mixed oxides with different nominal Mg/(Mg+Zr) atomic ratios, namely 0, 0.1, 0.2, 0.4, 0.85, and 1, is prepared by alcogel methodology and fundamental insights into the phases obtained and resulting active sites are studied. Characterization is performed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, N(2) adsorption-desorption isotherms, and thermal and chemical analysis. Cubic Mg(x)Zr(1-x)O(2-x) solid solution, which results from the dissolution of Mg(2+) cations within the cubic ZrO(2) structure, is the main phase detected for the solids with theoretical Mg/(Mg+Zr) atomic ratio ≤0.4. In contrast, the cubic periclase (c-MgO) phase derived from hydroxynitrates or hydroxy precursors predominates in the solid with Mg/(Mg+Zr)=0.85. c-MgO is also incipiently detected in samples with Mg/(Mg+Zr)=0.2 and 0.4, but in these solids the c-MgO phase mostly arises from the segregation of Mg atoms out of the alcogel-derived c-Mg(x)Zr(1-x)O(2-x) phase during the calcination process, and therefore the species c-MgO and c-Mg(x)Zr(1-x)O(2-x) are in close contact. Regarding the intrinsic activity in furfural-acetone aldol condensation in the aqueous phase, these Mg-O-Zr sites located at the interface between c-Mg(x)Zr(1-x)O(2-x) and segregated c-MgO display a much larger intrinsic activity than the other noninterface sites that are present in these catalysts: Mg-O-Mg sites on c-MgO and Mg-O-Zr sites on c-Mg(x)Zr(1-x)O(2-x). The very active Mg-O-Zr sites rapidly deactivate in the furfural-acetone condensation due to the leaching of active phases, deposition of heavy hydrocarbonaceous compounds, and hydration of the c-MgO phase. Nonetheless, these Mg-Zr materials with very high specific surface areas would be suitable solid catalysts for other relevant reactions catalyzed by strong basic sites in nonaqueous environments.

Hydrogenation of aromatics over supported noble metal catalysts ex organometallic complexes

Abstract In the present work, organometallic complexes were used as metal precursors for the synthesis of sulphur-resistant noble metal catalysts. The catalysts were tested in the hydrogenation reaction of diesel model-type molecules. Dibenzothiophene (DBT) was used as the source of sulphur. The organometallic-based catalysts activity was compared with the activity displayed by a commercial Pt/Al 2 O 3 hydrogenation catalyst. Comparable activity results were obtained when the reaction was carried in the presence of sulphur containing molecules. However, the organometallic-based catalysts were only active once the ligands had been removed from the metal coordination sphere, thus allowing the reactant molecules to adsorb on the metallic surface centres. Accordingly, the major advantage of this methodology may lie in improved metallic dispersion, which will be reflected in catalysts with greater sulphur resistance, rather than in the electronic or steric effects ascribed to organometallic precursors.