EPR investigation, before and after adsorption of naphtalene, of mordenite containing Fe3+ and Cr5+ ions as impurities

Abstract

During the study of mordenite with the EPR technique, three EPR signals centered at g = 4.3; g = 2.4 and g = 1.98 were evidenced. The first two were attributed to Fe 3+ ions (signal at g = 4.3 typical to isolated framework iron species, signal at g = 2.4 characteristic to agglomerated non-framework ones). The signal at g = 1.98 was assigned to Cr 5+ ions. All these cations are present in the solid as impurities. Furthermore, under certain conditions the Cr 5+ ions can be in interaction with two equivalent 27 Al nuclei according to a formation of a superhyperfine structure in the EPR signals. After adsorption of naphtalene on the mordenite samples, organic p-radicals are formed via secondary reactions of primary cation-radicals nearby the aluminum atoms of the mordenite framework. The formation of radicals seems to be correlated to the presence of impurities in the solids. Thus not only acidity but also content of transition metals may be an important factor in the formation of radicals.

Direct Synthesis of Acetone by Aerobic Propane Oxidation Promoted by Photoactive Iron(III) Chloride under Mild Conditions

“…The signal at g = 1.91 was clearly visible, along with two more at g = 4.31 and g = 2.01 (Figure S13). Usually transition lines around g ∼ 4.3 are associated with Fe 3+ in tetrahedral coordination, while those close to g ∼ 2.0 are assigned to octahedral chemical environments. − The existence of more than one signal suggests a mixture of complexes in our system, which is consistent with the co-presence of FeCl 4 – and other tetrahedral Fe species like those identified by ESI-MS and DFT calculations.…”

Oxidation State and Local Structure of Chromium Ions in LaOCl

“…Cr 5+ is a S = 1/2 system; therefore, there are no effects related to ZFS, and the EPR signal is not as complicated as for Cr 3+ . A notable feature occasionally observed in the spectra of Cr 5+ is HF structure caused by an interaction with the nearest cations [69][70][71][72]. We can observe a similar effect for the 2 mol% sample; however, due to the high Cr concentration, it is partially obscured.…”

Особенности Применения Ароматических Молекул-Зондов Для ЭПР -Спектроскопии В Исследовании Кислотных Свойств Поверхности Модифицированных Алюмооксидных Каталитических Систем

“…Целая серия работ была посвящена изучению цеолитов (морденит, L-цеолит, ZSM-5 и др.) методом ЭПР с нафталином [21,22], периленом [23] и бензолом [24,25] в качестве молекул-зондов. Например, используя нанесение бензола из газовой фазы в in situ ЭПР-исследованиях, коллектив авторов [25] получил данные по количеству катионрадикалов бензола и димера бензола, предполагая образование катион-радикалов только при адсорбции зонда на ЛКЦ цеолита за счёт одноэлектронного переноса.…”

“…Другой коллектив [21] применил нафталин для оценки «окислительной способности» цеолитов, прошедших различные варианты термической и химической обработки. В работе [22] была обнаружена корреляция концентрации образующихся катион-радикалов нафталина с содержанием примесных ионов Fe 3+ и Cr 5+ , в связи с чем авторы предположили влияние не только кислотных центров морденита, но и примесных катионов переходных металлов на образование ароматических катион-радикалов.…”