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Carbon-based adsorbents were prepared through the physical activation of sewage sludge with CO2. The effect of the acid treatment was investigated, either before or after the physical activation. The post-washing increases the porosity due to the removal of the inorganic fraction and the improvement in the accessibility of the carbon fraction. The post-washing is essential for high activation temperatures since the high reactivity of CO2leads to a high pore blockage. In fact, the material activated at 900°C has a very low surface area (43 m2/g), while after washing it reaches 654 m2/g. The effect of the acid treatment prior to the physical activation depends on temperature: at 600°C, it results in a huge pore development (mainly macropores), while at 900°C, the microporosity increases. The adsorption capacity is not directly related to the BET surface area. For both adsorbates, the uptake ability depends on the textural properties (it is favoured by mesoporosity for methylene blue (MB) and microporosity for phenol) and surface chemistry (favourable effect of oxygen- and nitrogen-containing functional groups for MB and unfavourable effect for phenol). [ABSTRACT FROM AUTHOR]

Abstract: A kinetic model has been established for the transformation of (H2 +CO2) into dimethyl ether (DME) on a CuO–ZnO–Al2O3/γ-Al2O3 bifunctional catalyst with excess γ-Al2O3 acid function. The model considers the attenuation of methanol synthesis and the formation of paraffin byproducts due to CO2 and water in the reaction medium. Furthermore, the catalyst''s low deactivation (lower than for (H2 +CO) feed) is quantified by means of an equation that considers coke deposition on the metallic function due to the degradation of oxygenates (methanol and DME) in the reaction medium as the cause of deactivation. Deactivation attenuates as water and CO2 concentrations in the reaction medium are increased. The model fits the experimental results for the evolution with time on stream of both CO2 conversion and the yields of DME, methanol and paraffins in the range of operating conditions studied: 225–325°C; 20–40bar; H2/CO2 molar ratio in the feed, between 2/1 and 4/1; time on stream, up to 30h. The following operating conditions are suitable by striking a balance between activity at zero time on stream and deactivation: temperature, 275°C; pressure, 40bar; H2/CO2 molar ratio in the feed, 4/1. [Copyright &y& Elsevier]

Abstract: Co-feeding water with syngas in the direct synthesis of dimethyl ether has been studied in order to reduce the deactivation by coke deposition on the CuO–ZnO–Al2O3 metallic function in the CuO–ZnO–Al2O3/γ-Al2O3 bifunctional catalyst. The runs (synthesis of methanol and synthesis of DME) have been carried out in an isothermal fixed bed reactor under conditions avoiding catalyst deactivation due to the sintering of the metallic function (<325°C) and the deactivation of the acid function (with an excess of γ-Al2O3 in the catalyst). The results of long-term experiments evidence that water in the reaction medium attenuates coke deposition during the methanol synthesis step, which is the limiting one. Water is understood to partially inhibit the formation of coke precursor methoxy ions. Furthermore, the formation of paraffins is attenuated by co-feeding water. [Copyright &y& Elsevier]

Abstract: This paper deals with the regeneration of a CuO-ZnO-Al2O3/γ-Al2O3 catalyst used in the direct synthesis of dimethyl ether. When coke combustion (subsequent to aging treatment) is performed in a thermobalance, two coke fractions with different combustion kinetics are identified. One coke fraction, deposited on the metallic sites and whose combustion is activated by these sites, has a kinetic constant of combustion 7–26 times higher than that corresponding to the coke located at the Al2O3 support. Uninterrupted operation is feasible in reaction–regeneration cycles in a fixed bed reactor as long as the reaction and regeneration steps are carried out at 325°C, which are limiting conditions in order to avoid irreversible deactivation by sintering of the metallic function. The regeneration with O2 diluted with He (5% of O2) allows the complete combustion of coke. Under conditions of partial coke combustion it is observed that the removal of the fraction deposited on the metal makes it possible to recover the initial activity of the catalyst, which subsequently undergoes fast deactivation. [Copyright &y& Elsevier]

A kinetic model has been established for the deactivation of a CuO−ZnO−Al2O3/γ-Al2O3catalyst in the direct synthesis of dimethyl ether. The model allows calculating the effect of the operating conditions and the evolution of component concentration in the reaction medium (dimethyl ether, methanol, paraffins, H2, CO, CO2, and H2O) with time on stream. The results of the model fit the experimental results in a wide range of conditions: 225−350 °C; 10−40 bar; space time, 0.1−68.0 (g of catalyst) h (mol of reactants)−1; molar ratio (H2/CO) in the feed, 2−4; time on stream, 30 h. The model considers the cause of deactivation to be coke deposition on the metallic function and that this coke is formed by degradation of oxygenates (dimethyl ether and methanol). The effect of water in the reaction medium has been quantitatively considered in the kinetic model, whose drawback is the attenuation of the methanol synthesis rate and whose benefit is the attenuation of deactivation by coke deposition. The kinetic model is useful for predicting the influence of water in the feed with syngas as a strategy to attenuate deactivation, which is insignificant for a H2O/(H2+ CO) molar ratio of around 0.20, although it causes a slight decrease in initial catalyst activity. [ABSTRACT FROM AUTHOR]