Determination of Zn Speciation, Siting, and Distribution in Ferrierite Using Luminescence and FTIR Spectroscopy

Abstract

A methodology for the analysis of the siting of Zn 2+ ions in extra-framework cationic sites of silicon-rich zeolite was developed and demonstrated on Zn-ferrierite samples (Si/Al 8.5, Zn/Al 0.04−0.33). This methodology is based on the FTIR spectroscopy of antisymmetric tetrahedral−octahedral−tetrahedral (T-O-T) vibrations of the zeolite framework perturbed by Zn 2+ ions, combined with a complementary approach based on Zn 2+ luminescence monitored by laser-induced timeresolved luminescence spectroscopy. Three vibration bands in the FTIR spectra of antisymmetric T-O-T vibrations of dehydrated zeolite reflect Zn 2+ ligation in three extra-framework cationic sites of ferrierite zeolite. The band at 935 cm −1 corresponds to Zn 2+ ions in the α-site of ferrierite, the band at 917 cm −1 correspondsto ions in β-site, and the band at 902 cm −1 correspondsto Zn 2+ ions in the γ-site. The extinction coefficient for quantitative analysis of Zn 2+ ions in cationic sites was estimated and exhibited the same value for Zn 2+ cations in all cationic sites, ε = 49.1 ± 3.8 cm•μmol −1 . In all Zn samples, Zn 2+ siting in the β-site prevails, while Zn 2+ ions in the γ-site are of low population or negligible. Time-resolved luminescence showed that bare Zn 2+ ions in the extra-framework cationic sites can be distinguished from Zn 2+ ions in ZnO by a decay time which is several magnitudes longer and a high sensitivity for quenching. The luminescence spectrum of Zn-ferrierites is composed of three bands at 545, 480, and 425 nm attributable to Zn 2+ ions in the α-, β-, and γ-site with luminescence coefficients (for semiquantitative analysis) ζ α = 10.1, ζ β = 9.4, and ζ γ = 8.8 mmol/g of Zn 2+ ions in the α-, β-, and γ-sites, respectively. The analysis of Zn ions in ferrierites showed that ZnH-ferrierites exclusively contain Zn 2+ ions in cationic sites. In the case of the ZnNa-FER sample with maximum Zn loading (Zn/Al 0.33), a small amount of Zn-oxo species can be formed.

Organization of Cooperating Aluminum Pairs in Ferrierite Evidenced by Luminescence Quenching

“…For the analysis of the siting and distribution of divalent TMI in the extra-framework sites of zeolite, FTIR spectroscopy of antisymmetric skeletal T–O–T vibrations is often used. 14 , 17 , 20 − 22 The accommodation of the cation results in the perturbation of the T–O–T vibrations and the appearance of the complex band at 1000–800 cm –1 , which can be used for qualitative and quantitative analysis of the cations present in the extra-framework positions 21 , 22 (such a band is not observed for the H- and Na-exchanged forms of zeolites). 17 , 23 Positions of the particular bands of Co-FER and Zn-FER are published elsewhere.…”

“… 17 , 23 Positions of the particular bands of Co-FER and Zn-FER are published elsewhere. 14 , 17 FTIR spectra records of the ZnCo-FER samples were compared with those of the Co-FER, Zn-FER and parent Na-FER samples and showed that due to the overlap in the frequency characteristic for Co and Zn 14 , 15 , 17 (see Figure S2 in the Supporting Information), they cannot be distinguished and fitted into particular bands.…”

“…They exhibit similar features to those already reported for Zn-exchanged ferrierite, with Zn(II) located in all cationic sites with maxima at 545, 480, and 425 nm for the α, β, and γ sites respectively, as reported in our previous work. 14 …”

“… 13 Our recent study showed that divalent Zn ions located in extra-framework positions of the ferrierite matrix (accommodated by Al pairs) exhibit similar luminescence properties as Cu(I) ions in zeolites and Zn(II) emission wavelength can be employed for the identification of Zn(II) siting. 14 Therefore, the quenching of Zn(II) luminescence by other divalent cations of transition metal ions (M(II)) can be suggested as a possible way to prove the presence of two M(II) ions in close vicinity forming binuclear TMI sites. In this work, the introduction of Zn(II) (emitter) and Co(II) (quencher) to develop a method for studying the intrinsic property of the chosen FER matrix with Al distribution assuring the formation of binuclear catalytic centers with appropriate geometry, 5 offering O 2 splitting for selective oxidation reactions, is reported.…”

Unusual water-assisted NO adsorption over Pd/FER calcined at high temperatures: The effect of cation migration

“…[62][63][64] The perturbed band with divalent cations in α-site has been observed at 935-950 cm -1 , in β-site is 913-928 cm -1 , and in γ-site at 879-905 cm -1 . 65 Based on this, the newly formed divalent Pd 2+ ions in FER after thermal treatment are highly likely to be located near β-site.…”