Search

Wastewater is of general concern for environmental sustainability. At the industrial level, carbon-laden aqueous streams must be treated and should be valorized to reduce environmental and economic concerns. Lignocellulosic biomass processing (such as hydrolysis, pyrolysis, and hydrothermal liquefaction) results in secondary aqueous streams in which a high fraction of the initial carbon content of the biomass is virtually lost; food industry (like breweries and cheese factories) produces streams with a variety of organic load and salinity, which may complicate conventional valorization treatments; biodiesel production leads to glycerol excess in the market, which needs to be valorized. Among other alternatives, aqueous phase reforming (APR) has been proposed as a process driven at relatively mild conditions, able to convert oxygenated molecules into hydrogen. Despite its potential, APR has commonly been investigated with model compounds, and a systematic study on the possible fields of application of this technology is lacking. In the present review, the study of the available literature was focused on the valorization of complex feedstocks, such as real waste streams or synthetic mixtures, showing the outcome derived from laboratory-scale experiments. The results were critically discussed, pointing out the present limitations for the full development of this process and its application to the industrial scale. Despite the challenges of APR, its potential is noteworthy for the development of a circular low-waste economy.

A set of Cu-ZSM-5 samples (Si/Al = 25) was synthesized by wetness impregnation, aqueous phase ion exchange and solid-state ion exchange, SSIE. Copper(II) acetate was used as precursor during both wetness impregnation and aqueous phase ion exchange to prepare excessively-exchanged samples (i.e. Cu/Al > 1), whereas SSIE was carried out by sublimating copper(I) chloride. Additionally, a Cu-ZSM-5 sample with extremely low Al content (Si/Al = 500) was prepared by impregnation. The samples were tested in the catalytic decomposition of N2O, with the aim of studying possible effects of the preparation technique on the isothermal oscillatory behavior of the reaction rate in a series of tests carried out by varying both the temperature and the residence time. The SSIE sample led to a non-oscillating system, whereas the highest Cu content samples prepared by the two aqueous procedures exhibited clear oscillations of the reaction rate. The Cu-ZSM-5 with Si/Al = 500 was quite inactive, but, surprisingly, gave rise to oscillations of both unconverted N2O and produced O2 concentrations at the reactor outlet. A combined IR spectroscopy and H2-TPR study showed the role of oligomeric CuxOy species with extra-lattice oxygen close to the Al atoms on the observed oscillations. Such species are much more abundant in the samples prepared by either impregnation or ion-exchange, when Cu/Al > 1. In contrast, the sample prepared by SSIE resulted to be rich in Cu+ isolated species, active for the reaction, but not inducing oscillation.

Alginate, a carbohydrate abundant in the outer cell wall of macroalgae, was subjected to catalytic aqueous phase reforming (APR) to produce hydrogen using a 3% Pt/C commercial catalyst. The performance of the process was evaluated according to the conversion of the carbon to gas, the hydrogen yield and the hydrogen selectivity. The catalyst and feed amount, temperature, reaction time, pH and the presence of H2 were modified to understand the dependence of the outcome of the process on these parameters. The presence of the catalyst was fundamental in order to increase the hydrogen yield compared to the uncatalyzed reaction, and it can be reused without activity loss. In addition, it was observed that the increase in alginate loading led to a decreasing conversion of the carbon; the yield of hydrogen increases with the increasing temperature and the basic pH had a strong beneficial effect in terms of selectivity. The plateau appeared after 2 h was attributed to the low kinetic tendency of the intermediate compounds to produce hydrogen. The study validated what is present in literature for simpler molecules, moving at the same time towards a more complex feed, closer to a possible industrial application.

Secondary streams in biorefineries need to be valorized to improve the economic and environmental sustainability of the plants. Representative model compounds of the water fraction from the hydrothermal liquefaction (HTL) of biomass were subjected to aqueous phase reforming (APR) to produce hydrogen. Carboxylic and bicarboxylic acids, hydroxyacids, alcohols, cycloketones and aromatics were identified as model compounds and tested for APR. The tests were performed with a Pt/C catalyst and the influence of the carbon concentration (0.3–1.8 wt. C%) was investigated. Typically, the increase of the concentration negatively affected the conversion of the feed toward gaseous products, without influencing the selectivity toward hydrogen production. A synthetic ternary mixture (glycolic acid, acetic acid, lactic acid) was subjected to APR to evaluate any differences in performance compared to the tests with single compounds. Indeed, glycolic acid reacted faster in the mixture than in the corresponding single compound test, while acetic acid remained almost unconverted. The influence of the reaction time, temperature and carbon concentration was also evaluated. Finally, residual water resulting from the HTL of a lignin-rich stream originating from an industrial-scale lignocellulosic ethanol process was tested for the first time, after a thorough characterization. In this framework, the stability of the catalyst was studied and found to be correlated to the presence of aromatics in the aqueous feedstock. For this reason, the influence of an extraction procedure for the selective removal of these compounds was explored, leading to an improvement in the APR performance.

The development of a suitable after-treatment system for the abatement of the pollutant emissions contained in the flue gases of wood fired stoves, is the object of this work. All steps from powder catalyst synthesis, to the deposition of the selected catalyst on lab-scale monoliths, and finally the implementation of all developed concepts in a full-scale monolith, to be mounted in the chimney of a commercial stove to experience real working conditions, were tackled. A series of La–Cr perovskite catalysts was developed to undertake this task, from which La 0.9 Na 0.1 CrO 3 demonstrated to be the best compromise in terms of CO, HC and soot oxidation. The addition of 2% of Au enhanced the CO oxidation activity of the catalyst. The selected catalyst was then deposited on a lab-scale structured monolith (200 cpsi) to assess the best catalyst loading. It emerged that, at a 10% oxygen concentration and a GHSV of 3500 h −1 , a 6% loading (referred to the monolith weight) of La 0.9 Na 0.1 CrO 3 + 2%Au completely removed all CO and HC from the gaseous flow around 400 °C. This is a remarkable result since most of the wood combustion cycle produces an exhaust gas stream above this temperature. Finally, a full-scale monolith was mounted on the chimney of a stove, and was exposed to several wood loading cycles and different wood stacking protocols. The catalyzed monolith, with a 10 wt% catalyst loading, but a much lower cell density (50 cpsi), was able to abate all gaseous emissions throughout the entire wood combustion cycle, even at its lowest temperatures (around 200 °C). This was possible because, after some repeated wood loading cycles, the monolith heated up and stayed at suitable thermal levels (thanks to its inertia) viable for CO and HC oxidation, due to the exotermicity of these reactions. The soot emissions were also reduced by more than order of magnitude, thanks to the above described reason, and the high levels of SOF, which confer both reactivity towards combustion and sticking enhancement at the monolith surface. The developed catalyzed filter proved also to be very stable in terms of performances, even after a severe aging protocol of 160 h.

Several Li–Cr delafossite catalysts (LiCrO2, LiCr0.9O2, LiCr0.8O2, LiCr0.7O2, Li0.9CrO2, Li0.8CrO2 and Li0.7CrO2) were prepared via a highly exothermic and self-sustaining reaction, the so-called “solution combustion synthesis (SCS)” method, and characterized by means of XRD, BET, FESEM-EDS, H2-temperature programmed reduction (TPR) and XPS analyses, as catalysts for the combustion of soot, a major pollutant emitted by diesel engines. These catalysts already showed appreciable activity at 350 °C towards the catalytic combustion of soot even under loose contact conditions. The best prepared catalyst (LiCr0.9O2) could ignite soot combustion well below 350 °C, which is inside the range of temperatures reached at the exhaust line of a diesel engine. The correlation between the activity order and the capability to provide surface adsorbed oxygen (O−) by the prepared delafossite catalysts, enabled by a shift of the chromium from a high valence to a low valence state, is pointed out as a peculiar feature of these catalysts. An in situ SCS method was tailored to the preparation of a LiCr0.9O2-catalyzed trap based on a SiC wall-flow monolith. Engine bench tests on these catalytic traps (trap loading and regeneration by induced temperature increase) showed that the presence of the catalyst enabled both a more complete regeneration and a one-third fold reduction of the regeneration time compared to the case of a bare, non-catalytic trap. The catalyzed trap was finally characterized in terms of soot emissions during both the loading and the regeneration phase.

The present paper highlights the features of soot deposition inside the channels of wall-flow diesel particulate filters (DPFs). This investigation is tailored to understand the behaviour of soot loading in DPFs, since the subsequent regeneration step is strongly affected by the cake profile in the inner channels of the filter. In addition, the experimental data here reported are useful to validate detailed mathematical models for the prediction of pressure drop, filtration efficiency and filter loading. The investigation regards the differences in soot deposition profiles of two geometries for the filter housing: the first one was responsible for an uneven distribution of the flow at the inlet of the channel, closer to real cases in the exhaust pipes, while the other one was designed to minimize this effect. The two housings were tested in the same operating conditions, with lab-scale filters and synthetic soot, in order to achieve a better reproducibility of the inlet conditions, rather than with real diesel soot from a diesel engine. The evolution of soot deposition profiles at different loading times was monitored as well. Furthermore, the filtration efficiency was evaluated through measurements of particle size distributions before and after the filter. Some conclusions regarding the most appropriate mathematical model to simulate these phenomena are reported at the end of the paper, with reference to our past modelling activity.