Localization and Coordination of Mg²⁺Cations in Ferrierite: Combined FTIR Spectroscopic and Computation Investigation of CO Adsorption Complexes

Abstract

Localization and coordination of Mg2+ cations in Mg-FER were investigated by a combination of IR spectroscopy of CO probe molecule and periodic DFT calculations employing the ω/r correlation for calculations of CO stretching frequencies. Experimental data were interpreted on the basis of a very good agreement between theoretical and experimental results. The most intense band in the spectra centered at 2205 cm−1 was assigned to monocarbonyl complexes formed on Mg2+ cations located in the six-member ring separating two adjacent FER cages (P6 site). Dicarbonyl complexes can be formed on these Mg2+ sites (characterized by a band at 2194 cm−1) where each of the CO molecules approaches Mg2+ from a different FER cage. The band at 2211 cm−1 observed for the intermediate CO coverage was assigned also to the monocarbonyl complexes on Mg2+ sites inside the FER cage (P6 and P8 sites); the difference between 2205 and 2211 cm−1 bands is due to the different geometry of Al pair within the ring. At very low CO coverages, IR spectra show a band at 2222 cm−1 assigned to carbonyl complexes on Mg2+ cations at the channel intersection and in the main channel. Because of a significant difference in the intensity of the bands at 2222 and 2211 as compared to that of the band at 2205 cm−1, it is concluded that Mg2+ cations exchange preferably into the six-member ring between two FER cages. An unusually high site-specificity of νCO observed for CO complexes formed in Mg-FER can be exploited in the investigation of preferable exchange sites of Mg2+ in zeolites and in the investigation of the localization of Al pairs in zeolites.

The nature of cationic adsorption sites in alkaline zeolites—single, dual and multiple cation sites

“…Carbon monoxide is (arguably) the probe molecule most frequently used for IR characterization of basic zeolites, and main results were summarized in several review articles; [43][44][45][46][47] other probe molecules widely used are carbon dioxide, [48][49][50][51] dinitrogen, [52][53][54] dihydrogen [55][56][57][58] and acetonitrile. 48,59 Based on a wealth of experimental results (mainly from IR spectroscopy) and ever-increasing insight from theoretical quantum-chemical studies, [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77] the 1 : 1 adsorption model emerged; meaning that each single cation site acts as an adsorbing site for incoming (probe) molecules. This is not tantamount to say that every single cation-site type would form only one kind of adsorption complex with a particular adsorbed gas; since, for instance, for adsorbed CO both alkali and alkaline-earth metal cations in zeolites are known to form mono-, di-, and sometimes tricarbonyls (usually depending on equilibrium pressure and cation size); [78][79][80][81] as well as iso-carbonyls in which the metal cation coordinates the CO molecule through the oxygen atom (instead of through the carbon atom).…”

Basis set effects on Cu(I) coordination in Cu-ZSM-5: a computational study

“…For the sake of comparison with other computational studies [45][46][47][48][49][50][86][87][88] where the PBE [73,74] functional was adopted, additional calculations were performed employing the PBE1PBE [75] functional. Such a modified version of the PBE functional includes a 25% of HartreeFock exchange and was chosen in order to get results comparable at the same time to those obtained by the pure PBE and by the hybrid B3LYP functionals.…”

“…The main challenge related to the application of quantum mechanical calculations is to find a compromise between accuracy and computational costs, taking into account that zeolites are complex solids with large unit cells. The most simple and the most complex computational approaches are, respectively, quantum mechanical calculations on small and nonstructure-specific cluster models [36][37][38][39][40][41], or the application of quantum-mechanical methods (mostly based on planewaves DFT) to the full periodicity of the zeolite lattice [42][43][44][45][46][47][48][49][50]. Apart from the above two extreme cases, the most widely employed approaches are nowadays molecular orbital calculations which treat only a portion of the S. Morpurgo (&) Á G. Moretti Á M. Bossa Dipartimento di Chimica, Università degli Studi di Roma ''La Sapienza'', P.le Aldo Moro 5, 00185 Rome, Italy e-mail: simone.morpurgo@uniroma1.it structure (i.e., the active site for a given reaction) taking into account in a variable way the geometrical constraints imposed by the remaining part of the crystal [51,52].…”

A DFT study on Cu(I) coordination in Cu‐ZSM‐5: Effects of the functional choice and tuning of the ONIOM approach

“…In this field, the most popular approaches are quantum mechanics/molecular mechanics calculations such as the QM‐Pot method, which treats the full periodicity of the crystal, or the ONIOM method, available in several computational codes, which allows to build large clusters to be partitioned in different shells treated at a variable computational level. The most sophisticated approach is the application of quantum‐mechanical methods, generally based on plane‐waves DFT, to the full periodicity of the zeolite lattice . The latter method is the one with less approximations, but it is also very demanding from a computational point of view.…”

“…As far as the functional choice is involved, for more than a decade B3LYP was by far the most widely used functional in the majority of the studies on metal‐exchanged zeolites . In more recent times, however, the use of the pure Perdew–Burke–Ernzerhof (PBE) functional became popular, possibly for reasons of computational economy (no HF exchange is to be calculated) within the computationally demanding plane‐waves DFT approach. However, many authors shifted from the use of the B3LYP to that of the PBE functional without showing how the new results may be related to those of older but still significant works.…”