ABSTRACT
High anthropogenic CO2 emissions are among the main causes of climate change. Herein, we investigate the use of CO2 for the synthesis of organic cyclic carbonates on metal-free nitrogen-doped carbon catalysts obtained from chitosan, chitin, and shrimp shell wastes, both in batch and in continuous flow (CF). The catalysts were characterized by N2 physisorption, CO2-temperature-programmed desorption, X-ray photoelectron spectroscopy, scanning electron microscopy, and CNHS elemental analysis, and all reactivity tests were run in the absence of solvents. Under batch conditions, the catalyst obtained by calcination of chitin exhibited excellent performance in the conversion of epichlorohydrin (selected as a model epoxide), resulting in the corresponding cyclic carbonate with 96% selectivity at complete conversion, at 150 °C and 30 bar CO2, for 4 h. On the other hand, in a CF regime, a quantitative conversion and a carbonate selectivity >99% were achieved at 150 °C, by using the catalyst obtained from shrimp waste. Remarkably, the material displayed an outstanding stability over a reaction run time of 180 min. The robustness of the synthetized catalysts was confirmed by their good operational stability and reusability: ca. (75 ± 3)% of the initial conversion was achieved/retained by all systems, after six recycles. Also, additional batch experiments proved that the catalysts were successful on different terminal and internal epoxides.
ABSTRACT
Vehicular pollution has become a major problem in urban areas due to the exponential increase in the number of automobiles. Typical exhaust emissions, which include nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO), soot, and particulate matter (PM), doubtless have important negative effects on the environment and human health, including cardiovascular effects such as cardiac arrhythmias and heart attacks, and respiratory effects such as asthma attacks and bronchitis. The mitigation measures comprise either the use of clean alternative fuels or the use of innovative technologies. Several existing emission control technologies have proven effective at controlling emissions individually, such as selective catalytic reduction (SCR) and lean NOx trap (LNT) to reduce NOx and diesel particulate filter (DPF) specifically for PM abatement. These after-treatment devices are the most profitable means to reduce exhaust emissions to acceptable limits (EURO VI norms) with very little or no impact on the engine performances. Additionally, the relative lack of physical space in which to install emissions-control equipment is a key challenge for cars, especially those of small size. For this reason, to reduce both volume and cost of the after-treatment devices integrated catalytic systems (e.g., a sort of a "single brick") have been proposed, reducing both NOx and PM simultaneously. This review will summarize the currently reported materials for the simultaneous removal of NOx and soot, with particular attention to their nature, properties, and performances.
ABSTRACT
The effect of water, in the temperature range 25-350 °C, and ammonia at RT on two different surface species formed on Pt-K/Al2O3 and Pt-Ba/Al2O3 NSR catalysts during NO(x) storage-reduction cycles was investigated. The surface species involved are nitrates, formed during the NO(x) storage step, and isocyanates, which are found to be intermediates in N2 production during reduction by CO. FT-IR experiments demonstrate that the dissociative chemisorption of water and ammonia causes the transformation of the bidentate nitrates and linearly bonded NCO(-) species into more symmetric species that we call ionic species. In the case of water, the effect on nitrates is observable at all the temperatures studied; however, the extent of the transformation decreases upon increasing temperature, consistent with the decreased extent of dissociatively adsorbed water. It was possible to hypothesize that the dissociative chemisorption of water and ammonia takes place in a competitive way on surface sites able to give bidentate nitrates and linearly bonded NCO(-) that are dislocated, remaining on the surface as ionic species.
