1. Introduction The production of renewable fuels can be achieved by photocatalysis in the frame of an integrated biorefinery. In this context, we deal with two main processes. At first, the production of hydrogen through photoreforming of aqueous solutions of organic compounds is considered as a way to exploit solar energy storage in the form of hydrogen. It can be used either as energy vector or as a reactant in the biorefinery itself, such as for hydrotreating processes, for the Fischer Tropsch synthesis of higher hydrocarbons, or for much less mature, still under development processes, such as those aiming at lignin valorization. The photocatalytic reforming occurs through the following general reaction1: which is promoted by a photocatalyst, i.e. a semiconductor, which can drive the H+ ion reduction thanks to an electron which is photopromoted from the valence to the conduction band of the material upon light absorption. To close the circuit, the hole which is formed in the valence band should be filled with electrons coming from different reactive species, that is, holes oxidise other reactants. In principle holes may oxidise water, but the reaction is very slow and not competitive with the very fast relaxation phenomena occurring in the semiconductor. Therefore, is more readily oxidisable compounds are available in solution, the may consume holes, making hydrogen production faster. Many efforts have been devoted to the research of suitable materials for the photocatalytic H2 production and to the screening of various organic sacrificial agents, predominantly focusing on simple molecules. In this work, we dealt with the use of different sugars, namely glucose, xylose and arabinose, as well as levulinic acid. They were used as examples of compounds that may be rather easily obtained from the hydrolysis of biomass. In the second example, we investigated the photoreduction of CO2 as a mean to regenerate fuels in form of methane and methanol, depending on the photocatalyst and operating conditions adopted2. Interestingly, the liquid phase products of this reaction include different amounts of HCOOH, HCHO and CH3OH, which may undergo photoreforming themselves, thus allowing the formation in gas phase of a substantial amount of H2. This makes the two reactions very tightly correlated. For both reactions our attention was predominantly focused on the development of innovative reactors, possibly operating under unconventional conditions, with fine tuning of the operation parameters, rather than of materials properties. 2. Experimental The selected photocatalysts were based on TiO2, since the focus was reactor optimization. The materials were prepared by flame spray pyrolysis and compared with commercial samples of nanostructured TiO2 P25 by Evonik. Different metals, such as Pt, Pd, Ag and Au, with loading ranging from 0.1 to 0.5 wt% were either added during the FP synthesis or added by post synthesis impregnation. The role of the metals was that of electron sinks, to inhibit the electron-hole recombination. Some of them were also selected due to the formation of a plasmon resonance band which improves visible light absorption (Ag, Au), other due to their specific affinity for H2. The samples were characterized by N2 adsorption-desorption, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and temperature programmed reduction/oxidation (TPR/TPO). The photoreforming reaction was carried out in different prototypes of photoreactors specifically developed in our lab, as described in the next section. 3. Results and discussion As for photoreforming, in every case, an external 200 W lamp was used, with emission wavelengths centred around 365 nm. A first photoreactor was developed with internal capacity ca. 0.3 L, with big head space for gas collection and very efficient mixing of the suspension thanks to an optimized length/diameter ratio (L/D) ca. 2. A drawback was the poor irradiation efficiency of the suspension, which limited the overall productivity, irrespectively or the substrate. A different reactor layout was tested, with D =200 mm and L ca. 100 mm, which ensured a better uniformity in light distribution. Mass transfer issues were partially solved by improving mixing, but heat transfer was not optimal, leading to the increase of reaction temperature during operation. The present configuration includes an external cooling jacket, which should be better replaced by internal cooling coils to better control the fluid temperature. The highest H2 productivity achieved was 0.28 mmol/h gcat using the first reactor configuration. As for the photoreduction of CO2 a completely different concept of innovative photoreactor was designed, to cope with the very low solubility of CO2 in water, which usually limits the reaction yield when operating in liquid phase. The photoreactor was set up to operate up to 20 bar, which is an extreme pressure for photocatalytic applications due to the need of realizing windows with suitably transparent materials. In this case an annular configuration with internally irradiating lamp (same specifications given above) was chosen and the operating conditions were finely tuned as for pressure, temperature (up to 90°C), power of the 2 emitters of the lamp, addition of inorganic hole scavengers, photocatalyst. The photoreactor performance widely changed depending on the material. When pure titania was used, higher productivity of HCOOH and HCHO was achieved, besides some hydrogen, whereas much higher H2 yield, CH4 and CH3OH as primary liquid product was obtained with 0.2 wt% Au/TiO2. The optimal pressure was an intermediate 7 bar, allowing high productivities both in gas and liquid phase, and intermediate temperature (ca. 65°C). By considering the time dependence of products distribution we also evidenced that the formation of H2 takes place when the inorganic hole scavenger is fully consumed at the expenses of liquid organic products. This confirms the photoreforming of organics as the formation route for hydrogen, taking place as a consecutive step, not a parallel one with respect to CO2 photoreduction. The highest productivities achieved were (under different conditions): 100 mmol/kgcat h for H2, 2954 mmol/kgcat h for HCOOH, 16537 mmol/kgcat h for HCHO and 350 mmol/kgcat h for methanol. 4. Conclusions In the present work we developed different prototypes of photoreactors to accomplish hydrogen production from biomass derived organic compounds and for the photoreduction of CO2 for the regeneration of fuels and chemicals. Reactor modelling is in progress for both applications, including the optimization of radiation distribution in the photoreactor. Fondazione Cariplo (UP-Unconventional Photoreactors) and MIUR (HERCULES - Heterogeneous robust catalysts to upgrade low value biomass streams are gratefully acknowledged for financial support. References 1. I. Rossetti, ISRN Chemical Engineering, vol. 2012, Article ID 964936, 21 pages, 2012. doi:10.5402/2012/964936 2. F. Galli, M. Compagnoni, D. Vitali, C. Pirola, C. Bianchi, A. Villa, L. Prati, I. Rossetti, Appl. Catal. B: Environmental, 200 (2017) 386.
Photoreactors design in the exploitation of biorefinery processes / I.G. Rossetti, E. Bahadori, G. Ramis. ((Intervento presentato al 4. convegno International Conference on Catalysis for Biorefineries tenutosi a Lione nel 2017.
|Titolo:||Photoreactors design in the exploitation of biorefinery processes|
|Data di pubblicazione:||2018|
|Settore Scientifico Disciplinare:||Settore ING-IND/25 - Impianti Chimici|
|Citazione:||Photoreactors design in the exploitation of biorefinery processes / I.G. Rossetti, E. Bahadori, G. Ramis. ((Intervento presentato al 4. convegno International Conference on Catalysis for Biorefineries tenutosi a Lione nel 2017.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|