Microemulsion and Sol-Gel Synthesized ZrO2-MgO Catalysts for the Liquid-Phase Dehydration of Xylose to Furfural.

Two series of catalysts were prepared by sol-gel and microemulsion synthetic procedure (SG and ME, respectively). Each series includes both pure Mg and Zr solids as well as Mg-Zr mixed solids with 25%, 50% and 75% nominal Zr content. The whole set of catalysts was characterized from thermal, structural and surface chemical points of view and subsequently applied to the liquid-phase xylose dehydration to furfural. Reactions were carried out in either a high-pressure autoclave or in an atmospheric pressure multi-reactor under a biphasic (organic/water) reaction mixture. Butan-2-ol and toluene were essayed as organic solvents. Catalysts prepared by microemulsion retained part of the surfactant used in the synthetic procedure, mainly associated with the Zr part of the solid. The MgZr-SG solid presented the highest surface acidity while the Mg3Zr-SG one exhibited the highest surface basicity among mixed systems. Xylose dehydration in the high-pressure system and with toluene/water solvent mixture led to the highest furfural yield. Moreover, the yield of furfural increases with the Zr content of the catalyst. Therefore, the catalysts constituted of pure ZrO2 (especially Zr-SG) are the most suitable to carry out the process under study although MgZr mixed solids could be also suitable for overall processes with additional reaction steps.

Comparison of furfural and biogas production using pentoses as platform.

Coffee cut-stems (CCS), a biomass with high lignocellulosic content, is a coffee crop waste after bean harvesting. The main application of this material is as fuelwood for farmers, disregarding their carbohydrate content for biotechnological processes. In these terms, this work aims to compare three process scenarios for the experimental valorization of C5 fraction from CCS to produce biogas and furfural with and without the ethanol production from remaining C6 fraction under biorefinery concept. Therefore, an experimental stage was performed to obtain these products, based on a previous diluted acid pretreatment. The hydrolysate fraction was used to produce furfural and biogas, achieving yields of 0.34 g of furfural/g xylose and 81.1 mL of CH4 per gram of volatile solids. Concerning the solid fraction after acid pretreatment, it was used to produce ethanol with a previous enzymatic hydrolysis. After fermentation, 0.47 g of ethanol/g of glucose (92% of the theoretical yield) was obtained. These experimental results were fed to simulation models in order to compare three scenarios in technical, economic and environmental terms. As the main results, from technical point of view, the biogas production presents the lowest energy requirements. From the economic perspective, the furfural production presents a prefeasibility at the base scale of processing (e.g., 12.5 ton h-1). Meanwhile, the biogas scenario needs a processing capacity >22.5 ton h-1 to achieve the economic prefeasibility. In the biorefinery case, the positive economic performance is found at processing scales above 83 ton h-1. This work concludes that the C5 sugars platform is identified as a potential alternative for the generation of furfural and biogas, however, in this case a multiproduct biorefinery system is not always the best option to valorize biomass given the very high scale required and the economic indicators.

Niobium on BEA Dealuminated Zeolite for High Selectivity Dehydration Reactions of Ethanol and Xylose into Diethyl Ether and Furfural.

In this work, we investigated the role of solid-state dealumination by (NH4)2SiF6 (25% Al removal and 13% Si insertion), the impregnation of niobium (10, 18, and 25 wt. %) on dealuminated *BEA (DB) zeolite and their catalytic properties in ethanol and xylose transformations. Among all the studied catalysts, 18%Nb-DB showed increased mesoporosity and external areas. A leveling effect in the number and strength of the proposed two sites (Brønsted and Lewis) present in the catalyst (n1 = 0.24 mmol g-1, -ΔH1 = 49 kJ mol-1, and n2 = 0.20 mmol g-1, -ΔH2 = 42 kJ mol-1) in the catalyst 18%Nb-DB, might be responsible for its good activity. This catalyst presented the highest selectivity for diethyl ether, DEE (97%) with 61% conversion after 50 ethanol pulses at 230 °C (turnover number, TON DEE = 1.15). These features allowed catalytically fruitful bonding of the ethanol molecules to the neighboring sites on the channels, facilitating bimolecular ether formation through a possible SN2 mechanism. The same catalyst was active and selective for transformation of xylose at 180 °C, showing 64% conversion and 51% selectivity for furfural (TON Furfural = 24.7) using water as a green solvent.

Selective production of furfural from the dehydration of xylose using Zn doped CuO catalyst.

Furfural is a versatile biomass-derived platform compound used for the synthesis of several strategic chemicals. The sonochemically synthesized Zn doped CuO nanoparticles (NPs) were used for the production of furfural. The catalytic activity of the Zn doped CuO NPs was examined, as a model, during the dehydration reaction of xylose to furfural. In addition to that, we have also compared the catalytic activity of the Zn doped CuO NP with ZnO NPs, ZnO bulk, CuO NPs, CuO bulk, etc. This nanoscale catalyst (Zn doped CuO NP) has a large surface area, which enhances its catalytic activity and enables it to completely convert the xylose to furfural at 150 °C within 12 h without any trace of by-products, as confirmed by HPLC, 13C NMR and 1H NMR. HPLC analysis demonstrated that the yield of furfural is up to 86 mol %, compared to the 45 mol % obtained with ZnO NPs, ZnO bulk, CuO NPs, CuO bulk, etc. as catalysts.

Furfural Synthesis from d-Xylose in the Presence of Sodium Chloride: Microwave versus Conventional Heating.

We investigate the existence of specific/nonthermal microwave effects for the dehydration reaction of xylose to furfural in the presence of NaCl. Such effects are reported for sugars dehydration reactions in several literature reports. To this end, we adopted three approaches that compare microwave-assisted experiments with a) conventional heating experiments from the literature; b) simulated conventional heating experiments using microwave-irradiated silicon carbide (SiC) vials; and at c) different power levels but the same temperature by using forced cooling. No significant differences in the reaction kinetics are observed using any of these methods. However, microwave heating still proves advantageous as it requires 30 % less forward power compared to conventional heating (SiC vial) to achieve the same furfural yield at a laboratory scale.

Acid-catalysed xylose dehydration into furfural in the presence of kraft lignin.

In this study, the effects of kraft lignin (Indulin AT) on acid-catalysed xylose dehydration into furfural were studied in formic and sulphuric acids. The study was done using D-optimal design. Three variables in both acids were included in the design: time (20-80 min), temperature (160-180°C) and initial lignin concentration (0-20 g/l). The dependent variables were xylose conversion, furfural yield, furfural selectivity and pH change. The results showed that the xylose conversion and furfural yield decreased in sulphuric acid, while in formic acid the changes were minor. Additionally, it was showed that lignin has an acid-neutralising capacity, and the added lignin increased the pH of reactant solutions in both acids. The pH rise was considerably lower in formic acid than in sulphuric acid. However, the higher pH did not explain all the changes in conversion and yield, and thus lignin evidently inhibits the formation of furfural.