Abstract
Activated carbon monoliths (ACMs), with 25 cells/cm2, were prepared from the direct extrusion of Alcell, Kraft lignin and olives stones particles that were impregnated with phosphoric acid, followed by activation at 700 °C. These ACMs were used as catalysts for methanol dehydration reaction under air atmosphere. ACM that was prepared from olive stone and at impregnation ratio of 2, OS2, showed the highest catalytic activity, with a methanol conversion of 75%, a selectivity to dimethyl ether (DME) higher than 90%, and a great stability under the operating conditions studied. The results suggest that the monolithic conformation, with a density channel of 25 cells/cm2 avoid the blockage of active sites by coke deposition to a large extent. Methanol conversion for OS2 was reduced to 29% in the presence of 8%v water, at 350 °C, although the selectivity to DME remained higher than 86%. A kinetic model of methanol dehydration in the presence of air was developed, while taking into account the competitive adsorption of water. A Langmuir-Hinshelwood mechanism, whose rate-limiting step was the surface reaction between two adsorbed methanol molecules, represented the experimental data under the conditions studied very well. An activation energy value of 92 kJ/mol for methanol dehydration reaction and adsorption enthalpies for methanol and water of −12 and −35 kJ/mol, respectively, were obtained.
Highlights
Dimethyl ether (DME) is recognized as a clean diesel substitute, being environmental friendly with zero ozone depletion potential and a key intermediate for other important chemicals, such as olefins.DME can be produced by methanol dehydration reactions while using acid catalysts, such as zeolite materials, γ-Al2 O3, and acid modified γ-Al2 O3
Cylindrical Activated carbon monoliths (ACMs) were obtained with preparations yields that ranged from 30 to 40% (ACMs mass/raw material mass)
Small quantities of diisppropyl ether were observed at low temperatures and IPA conversions, showing a selectivity to propylene that was higher than 90% for temperatures higher than 250 ◦ C, for all the ACMs. These results suggested that these ACMs developed a relatively large amount of surface acidity during the preparation procedure and, there would be no need for additional treatments to increase the acidity of these catalysts, such as activated carbon oxidation with nitric acid [37]
Summary
DME can be produced by methanol dehydration reactions while using acid catalysts, such as zeolite materials, γ-Al2 O3 , and acid modified γ-Al2 O3. The look for alternative catalysts with improved levels of performance is considered to be an important challenge. In this sense, the use of carbon materials as catalysts and/or catalyst supports is receiving great attention [3]. Only few works analyzed the use of activated carbons for methanol dehydration due to their lowest surface acidity This acidity can be eventually increased by different methods [4,5,6], but no outstanding results were obtained due to the low stability of the acidic surface sites produced
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