Adsorption properties of Fe-containing dealuminated BEA zeolites as revealed by FTIR spectroscopy

Times Cited: 1 Article English Cited References Count: 83 582viAdsorption properties of Fe-containing dealuminated BEA zeolites were investigated by FTIR spectroscopy of adsorbed CO and NO. Two Fe-containing SiBEA zeolite samples were prepared by a two-step post-synthesis method: creation of vacant T-atom sites (T = Si, Al) by dealumination of tetraethylammonium BEA zeolite with nitric acid followed by impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO3)(3). The two samples differed in iron content (0.9 and 4.2 wt.%, for Fe(0.9)SiBEA and Fe-4.2-SiBEA, respectively). The parent SiBEA sample was characterized by IR bands at 3735 cm(-1) (isolated internal silanols), 3705 and 3515 cm-1 (associated with hydroxyl nests at vacant T-atom sites). Upon the impregnation step, the bands at 3705 and 3515 cm(-1) practically disappeared, indicating consumption of the corresponding hydroxyls and incorporation of iron into the framework of SiBEA zeolite (also confirmed by XRD). In agreement with this, the IR spectra of the two samples revealed acidic bridging hydroxyls of a Fe3+-O(H)-Si type characterized by a band at 3632 cm(-1) in higher concentration for Fe(4.2)SiBEA. The 3632 cm(-1) band shifted to 3352 cm(-1) after low-temperature CO adsorption (Delta nu similar to 280 cm(-1)) evidencing a high acidity of the bridging OH groups. Low-temperature CO adsorption experiments revealed the presence of mainly two families of Fen+ sites, evidenced by carbonyl bands at 2215 and 2186 cm(-1), respectively. The latter sites were in higher concentration for Fe(4.2)SiBEA. In addition, a minor fraction of iron sites were found to be able to form tricarbonyls (bands at 2155, 2123 and 2115 cm(-1)). It was also deduced that the majority of iron introduced was in a Fe3+ state and the majority of these ions did not interact with probe molecules. Adsorption of NO leads to appearance of different mononitrosyls (1901, 1869 and 1842 cm(-1)). With time and in the presence of NO, polynitrosyls (1920 and 1815 cm(-1)) were also formed. Experiments on coadsorption of CO and NO reveal that the iron sites producing the 2215 cm(-1) carbonyls form nitrosyl species absorbing at 1901 cm(-1). It is suggested that highly electrophilic Fe3+ ions act as adsorption sites in this case. Treatment of the samples with CO at 673 K generated new Fe2+ sites monitored by CO at 2183, 2174 and 2166 cm(-1). NO adsorption revealed different mono-, di- and polynitrosyl species. A peculiarity in this case was that interconversion between poly- and dinitrosyl species was well observed. The amount of reduced iron was much higher for Fe(4.2)SiBEA than for Fe(0.9)SiBEA. (C) 2009 Elsevier Inc. All rights reserved

Purification of Hydrogen from CO with Cu/ZSM-5 Adsorbents

The transition to a hydrogen economy requires the development of cost-effective methods for purifying hydrogen from CO. In this study, we explore the possibilities of Cu/ZSM-5 as an adsorbent for this purpose. Samples obtained by cation exchange from aqueous solution (AE) and solid-state exchange with CuCl (SE) were characterized by in situ EPR and FTIR, H2-TPR, CO-TPD, etc. The AE samples possess mainly isolated Cu2+ cations not adsorbing CO. Reduction generates Cu+ sites demonstrating different affinity to CO, with the strongest centres desorbing CO at about 350 °C. The SE samples have about twice higher Cu/Al ratios, as one H+ is exchanged with one Cu+ cation. Although some of the introduced Cu+ sites are oxidized to Cu2+ upon contact with air, they easily recover their original oxidation state after thermal treatment in vacuum or under inert gas stream. In addition, these Cu+ centres regenerate at relatively low temperatures. It is important that water does not block the CO adsorption sites because of the formation of Cu+(CO)(H2O)x complexes. Dynamic adsorption studies show that Cu/ZSM-5 selectively adsorbs CO in the presence of hydrogen. The results indicate that the SE samples are very perspective materials for purification of H2 from CO