Carbon nanotubes : innovative heterogeneous gas phase catalysts
Since the coke deposition on the inorganic oxide in the course of ODEB to styrene has been considered as the true catalytically active substance, various carbonaceous materials have shown to be active and selective catalysts for this reaction. It has been confirmed that oxygenated surface functional groups of catalyst, especially carbonyl/quinoid and hydroxyl groups, are responsible for the catalytic performance. Therefore, the focus in this study is in the creation of oxygen-containing functional groups on the surface of carbon nanotubes by partial oxidation using high potential oxidizing agents, including hydroxyl radicals, and ozone in gas phase followed by investigating the influences of these groups on the ODEB catalytic behavior. The effects of the oxidative treatment methods on the textural, structural and chemistry surface of MWCNTs, as well as comparative investigations on the catalysts before and after the ODEB have been conducted by using a series of analytical techniques including TGA, BET surface area, Raman spectroscopy, TEM, FTIR and XPS. It is found that as-received MWCNTs possessed a number of oxygenated surface groups, as a result of CVD method. Further oxidation treatment with highly active hydroxyl radicals generated via UV/H2O2 process increased substantially the formation of oxygenated surface functional groups compared to as-received MWCNTs, particularly hydroxyl and carbonyl groups. Using hydrogen peroxide solutions with concentrations higher than 35 wt.% for the radical oxidation of MWCNTs did not enhance the surface oxygen content as expected. The effectiveness of the oxidation could be declined by the scavenging of radicals instead. As a result, the catalytic performance of UV/H35 catalyst is better than that of UV/H50 or UV/H60. At 400 oC and molar ratio EB/O2 of 1:1, ethylbenzene conversion and styrene selectivity of UV/H35 catalyst achieved 47% and 91%, respectively, for several hours time on stream. The styrene yield of UV/H35 is two times higher than that of as-received MWCNTs. Purification of as-received MWCNTs followed by ozone corona discharge oxidation of samples generated various oxygenated functional groups on the MWCNTs as identified by FT-IR. The surface oxygen content of the ozonated MWCNT increased significantly with increasing ozone concentration in gas flow, leading to highly stable ethylbenzene conversion and styrene selectivity reaching up to 80 and 92%, respectively, at 450 oC, and EB/O2 molar ratio of 1:2 in long-term experiments under conventional heating. That is the best result for the ODEB community to the best of our knowledge. The catalyst performance does not depend on the porosity of the MWCNTs. At the same reaction conditions (EB/O2 molar ratio of 1:1, 400 °C), the catalytic activity/ selectivity of the MWCNTs treated with ozone is better than that of the MWCNTs oxidized with hydroxyl radicals. The catalytic performance is substantially affected by the supplied heating mode for the reaction. A comparative investigation on catalyst performance under conventional and microwave heating indicated that the conversion difference between both heating methods is negligible, but conventional heating results in higher styrene selectivity than the microwave-assisted process. Generally, coke deposit has been observed for all the tested catalysts after the ODEB. However, the extent of coke formation depends strongly the oxidative treatment of nanotubes. For conventional heating, an increase in coke weight incorporated with oxygenated groups has been occurred, while for microwave heating coke deposit has been observed but the oxygenated groups are reduced after the ODEB. This fact could be the reason for the lower selectivity under microwave heating. In order to investigate the contribution of carboxylic groups on the nanotubes to the catalytic behavior, Boehm’s titration has been employed. By selective neutralization of carboxylic groups on the MWCNTs with NaHCO3 solution, and by comparison of the catalytic activity/selectivity with that of untreated samples, it has been shown that the styrene yield of neutralized MWCNTs is 16% lower than that of non-neutralized catalysts. Examining both catalysts after the reaction indicated that oxygenated coke is not deposited on carboxylic neutralized MWCNTs after the reaction test. This result demonstrates that carboxylic surface groups improve the catalytic performance indirectly. After the ODEB reaction, it is found that an enhancement of oxygen-containing groups on the surface of MWCNTs, particularly hydroxyl and carbonyl groups, have been observed, suggesting that these oxygenated groups could be able to be the main active phases in the reaction.