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Furfural production through traditional processes is accompanied by acidic waste stream production and high energy consumption. Modern furfural production process concepts will have to consider environmental concerns and energy requirements besides economics, moreover will have to be integrated within widened biorefinery concepts. In this paper, some particular aspects of the chemistry of D-xylose reaction to furfural are addressed, with the aim to clarify the reaction mechanism leading to furfural and to define new green catalytic pathways for its production. Specifically, reducing the use of mineral acids is addressed by the introduction of alternative catalysts. In this sense, chloride salts were tested in dilute acidic solutions at temperatures between 170 and 200 • C. Results indicate that the Clions promote the formation of the 1,2-enediol from the acyclic form of xylose, and thus the subsequent acid catalyzed dehydration to furfural. For this reason the presence of Clions led to significant improvements with respect to the H 2 SO 4 case. The addition of NaCl to a 50 mM HCl aqueous solution gave 90% selectivity to furfural. Among the salts tested FeCl 3 showed very interesting preliminary results, producing exceptionally high xylose reaction rates.

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A high pressure semicontinuous batch electrolyzer is used to convert CO2 to formic acid/formate on a tin-based cathode using bipolar membranes (BPMs) and cation exchange membranes (CEMs). The effects of CO2 pressure up to 50 bar, electrolyte concentration, flow rate, cell potential, and the two types of membranes on the current density (CD) and Faraday efficiency (FE) for formic acid/formate are investigated. Increasing the CO2 pressure yields a high FE up to 90% at a cell potential of 3.5 V and a CD of ∼30 mA/cm2. The FE decreases significantly at higher cell potentials and current densities, and lower pressures. Up to 2 wt % formate was produced at a cell potential of 4 V, a CD of ∼100 mA/cm2, and a FE of 65%. The advantages and disadvantages of using BPMs and CEMs in electrochemical cells for CO2 conversion to formic acid/formate are discussed.

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We use a high-pressure semicontinuous batch electrochemical reactor with a tin-based cathode to demonstrate that it is possible to efficiently convert CO 2 to formic acid (FA) in low-pH (i.e., pH < pK a ) electrolyte solutions. The effects of CO 2 pressure (up to 50 bar), bipolar membranes, and electrolyte (K 2 SO 4 ) concentration on the current density (CD) and the Faraday efficiency (FE) of formic acid were investigated. The highest FE (∼80%) of FA was achieved at a pressure of around 50 bar at a cell potential of 3.5 V and a CD of ∼30 mA/cm 2 . To suppress the hydrogen evolution reaction (HER), the electrochemical reduction of CO 2 in aqueous media is typically performed at alkaline conditions. The consequence of this is that products like formic acid, which has a pK a of 3.75, will almost completely dissociate into the formate form. The pH of the electrolyte solution has a strong influence not only on the electrochemical reduction process of CO 2 but also on the downstream separation of (dilute) acid products like formic acid. The selection of separation processes depends on the dissociation state of the acids. A review of separation technologies for formic acid/formate removal from aqueous dilute streams is provided. By applying common separation heuristics, we have selected liquid−liquid extraction and electrodialysis for formic acid and formate separation, respectively. An economic evaluation of both separation processes shows that the formic acid route is more attractive than the formate one. These results urge for a better design of (1) CO 2 electrocatalysts that can operate at low pH without affecting the selectivity of the desired products and (2) technologies for efficient separation of dilute products from (photo)electrochemical reactors.

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