M. López Granados

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.

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.

Partial oxidation of methane to formaldehyde on silica-supported transition metal oxide catalysts

Abstract This work describes the partial oxidation of methane on high surface area silica-supported redox oxide catalysts (MO x /SiO 2 ; M = V, Mo, W and Re). Formaldehyde, C 2 H n , and CO 2 are primary products obtained in this reaction, while CO originates from further oxidation of formaldehyde and hydrocarbons. Supported vanadium oxide was found to be the most reactive due to its higher reducibility, thus providing additional sites for oxygen activation. Rhenium oxide exhibited high specific activity and selectivity, however it deactivates from sublimation of metal oxide under on-stream operation at high temperatures, typically above 773 K. Results suggest a reaction scheme where oxygenates and oxygen-free intermediates are present yielding HCHO, CO x and C 2 H n hydrocarbons.

Effect of Pd on Cu-Zn catalysts for the hydrogenation of CO2 to methanol: Sstabilization of Cu metal against CO2 oxidation

A palladium–copper–zinc catalyst (PdO:CuO:ZnO=2:28:70), prepared by sequential precipitation of the respective cations, was tested in the hydrogenation of CO2 at high pressure (conditions: 60 bar, CO2:H2=1:3 (molar), W/F=0.0675 kg h/m3, 453-513 K). The methanol yield was improved on using this Pd-containing catalyst at all temperatures with respect to the reference copper–zinc catalyst (CuO:ZnO=30:70). This improvement was not due to an additional effect in which palladium was acting as an independent catalytic site but was caused by a synergetic effect of Pd on the active Cu sites. This effect was explained in terms of hydrogen spillover and an increased stability against CO2 oxidation of the surface copper. Therefore, the present contribution not only supports previous literature findings concerning the hydrogen spillover mechanism but also resulted in a complementary view regarding the role of palladium in Pd-modified CuO-ZnO-based catalysts.

o-xylene hydrogenation on supported ruthenium catalysts

The influence of the support on the surface properties and catalytic activity of finely divided ruthenium catalysts is reported. The catalysts were prepared using an organometallic precursor, Ru(acac)2, and three different supports, Al2O3, TiO2 and SiO2. In order to study the influence of the particle size on the catalytic performance, the effect of the calcination temperature was also evaluated. XPS suggests that the state of ruthenium is essentially Ru0, and chemisorption measurements indicate a decrease in metal dispersion from catalysts supported on Al2O3 > TiO2 > SiO2. The turnover number in the o-xylene hydrogenation showed significant differences depending on the support and on the particle size. Additionally, an increase in the selectivity to cis-dimethylcyclohexane with particle size was observed.

Aqueous-phase catalytic oxidation of furfural with H2O2: high yield of maleic acid by using titanium silicalite-1

This investigation explores the selective liquid-phase oxidation of furfural to maleic acid (MA) using hydrogen peroxide as an oxidant and titanium silicalite (TS-1) as a catalyst. The effect of temperature and of the concentration of H2O2, furfural and catalyst on the MA yield was studied. The highest yield, 78 mol%, was obtained under the following reaction conditions: 4.6 wt% of furfural, 4.6 wt% of catalyst, a H2O2/furfural mol ratio of 7.5, corresponding to 12.3 wt% of H2O2, 323 K and 24 hours of reaction. To reduce the amount of H2O2 employed, a two-step sequence of reactions was conducted using TS-1 and Amberlyst 70 consecutively as catalysts in the first and second steps, respectively. In this case, a H2O2/furfural mol ratio = 4.4 was required, which is quite close to the stoichiometric ratio (3.0), and a maleic acid yield close to 80% was obtained under 4.6 wt% of furfural, 4.6 wt% of catalyst and 28 h of reaction at 323 K; after 52 h of reaction, the MA yield reached 92%. Fresh and used catalysts were characterised by X-ray diffraction (XRD), Raman spectroscopy, total reflection X-ray fluorescence (TXRF), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption isotherms and thermogravimetric analysis. Ti was largely incorporated within the silicalite framework, but the presence of some TiO2 anatase was also confirmed. Ti leaching was observed, especially during the first run but became much less important in successive cycles. Leaching affects both anatase and Ti species within the silicalite framework. Notwithstanding the leaching, when using pure furfural, TS-1 could be reused for six runs without noticeable deactivation, whereas when using furfural directly derived from biomass, weak but visible deactivation occurred upon reutilisation; this deterioration must be related to the presence of some organic products other than furfural.

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.

Selective oxidation of o-xylene to phthalic anhydride on V2O5 supported on TiO2-coated SiO2

A ternary V-Ti-Si catalyst was prepared by depositing the vanadium oxide precursor on a SiO2 previously coated by TiO2. The ternary V-Ti-Si catalyst and the corresponding binary V-Ti and V-Si counterparts were tested for the selective oxidation of o-xylene to phthalic anhydride. Silica-supported vanadium oxide shows lowest activity and selectivity, meanwhile selectivity to phthalic anhydride is higher on titania-containing catalysts. The best yield to phthalic anhydride (82.5%) is obtained at 573 K on the ternary V-Ti-Si catalyst which is comparable to that claimed for the best conventional V-Ti catalysts described in literature.

Thermal decomposition of a hydrotalcite-containing Cu–Zn–Al precursor: thermal methods combined with an in situ DRIFT study

A Cu–Zn–Al precursor (CZA) was synthesized efficiently by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. The starting precursor contained a mixture of two hydroxycarbonate phases: rosasite and a Cu–Zn hydrotalcite-like phase. The thermal decomposition was studied by conventional thermal methods (TGA, DTA and EGA-MS) as well as by in situ FTIR spectroscopy (DRIFT). Analysis of the CZA precursor showed similar results by both procedures. Dehydration, dehydroxylation and decarbonation of the precursor were analysed in situ by monitoring the hydroxyl and carbonate infrared bands. A Cu–Zn hydrotalcite phase, one of the components of the CZA precursor, was also prepared independently. A detailed FTIR study revealed an interesting effect upon heating this hydrotalcite. At 373–423 K, a carbonate rearrangement in the interlayer space takes place during the loss of interlayer water. Carbonate groups change from their symmetrical coordination with interlayer water molecules to an arrangement involving the OH groups of the octahedral M(OH)m layers. This phenomenon certainly takes place in the CZA material as well but, in this case, it cannot be observed, probably due to the complexity of the material formed by two hydroxycarbonate phases.

Deactivation on vehicle-aged diesel oxidation catalysts

This paper studies oxidation catalytic converters that were vehicle-aged in a diesel automobile. The deposition of several atoms (S, P, C, Si, Ca, Zn and Fe) was detected by several chemical analysis techniques. Moreover, AlPO4 and Al2(SO4)3 were identified by XRD. The catalytic performance of the aged catalysts with different mileages showed a similar degree of deactivation for CO and C3H6 oxidation. By contrast, laboratory sintering carried out on fresh catalysts revealed that it may have certain impact in the deactivation of diesel catalysts. Nevertheless, the possibility that the impact of chemical deposition on deactivation has a threshold value cannot be discarded.