ARTICLE pubs.acs.org/JPCC
H-Bonding of Zeolite Hydroxyls with Weak Bases: FTIR Study of CO and N2 Adsorption on H-D-ZSM-5 Kristina Chakarova and Konstantin Hadjiivanov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
bS Supporting Information ABSTRACT: Adsorption of CO (13CO) and 15N2 at 100 K on H-ZSM-5 samples has been followed by FTIR spectroscopy. For better interpretation of the spectra, experiments with H-D-ZSM-5 and D-ZSM-5 were also carried out. It was established that zeolite bridging hydroxyls displaying at 100 K a band at 3616 cm-1 formed 1:1 adducts with CO, and as a result, the band disappeared, and two new bands at 3306 and 3415 cm-1 emerged at its expense. These two bands changed in concert together with a carbonyl band at 2175 cm-1. Experiments with H-D-ZSM-5 and D-ZSM-5 indicated that the OD band due to bridging deuteroxyls (2667 cm-1) was shifted, upon CO adsorption, again to two IR bands (at 2460 and 2568 cm-1). However, the shift is smaller than expected from the OH f OD isotopic shift factor, which indicates a smaller acidity of OD groups as compared with OH. In addition, the difference in the positions of the two bands (calculated again on the basis of the isotopic shift factor), as well as their relative intensities, deviate from those observed for the OH bands. The results exclude the phenomenon to be due to heterogeneity of the bridging OH/OD groups and indicate a spectral origin. It was concluded that the appearance of two shifted bands was due to Fermi resonance with the second excitation mode of δ(OH). At high CO equilibrium pressure solvatation occurs, and the band at 3306 cm-1 is shifted to lower frequencies. Simultaneously, a shift of the carbonyl band to 2172 cm-1 was observed. At intermediate CO coverages, AlOH 3 3 3 CO complexes are also formed. It is clearly demonstrated that the CO-induced shift of the AlO-H band at 3667 cm-1 is ∼-190 cm-1, and the often reported higher values are due to confusion with the 3415 cm-1 band described above. The AlOD groups have been found to manifest a lower acidity than that of AlOH. Here again, solvatation occurred at high CO coverages. Another effect observed at high CO coverages is the interaction of CO with silanol groups. Two kinds of interaction are unambiguously distinguished: (i) the well documented CO-induced shift of isolated silanols (3745 cm-1) to 3650 cm-1 and (ii) a shift of silanols absorbing in the 3730-3710 cm-1 region to 3580 cm-1. No solvatation was evidenced in this case. Experiment on low temperature adsorption of 15N2 confirmed that the bridging OH(OD) groups of H-ZSM-5 were homogeneous. Upon 15N2 adsorption, the 3616 cm-1 OH band shifted by 118 cm-1, and the respective OD band at 2667 cm-1, by 81 cm-1. Solvatation was detected at high coverages. The 15N2 induced shift of AlO-H modes was found to be 77 cm-1, and for AlO-D, 51 cm-1. Here again, two kinds of interaction with two different silanols were established. A good correlation between the shifts produced by the two probe molecules of different OH and OD groups was found.
1. INTRODUCTION Thousands of investigations have dealt with determination of the acidity of hydroxyl groups on various oxide surfaces or in zeolites. This interest is mainly due to the fact that many catalytic reactions require the presence of Brønsted acid sites, and the catalyst performance depends, to a great extent, on the density and acidity of the accessible hydroxyl groups. Among others, the spectral methods are usually preferred because they selectively monitor separate hydroxyl families. Along with NMR, FTIR spectroscopy is the most used technique for that purpose. There are two IR methods for determination of Brønsted acidity: the socalled ion-pair method and the hydrogen bond method.1 The ion-pair method is based on protonation of strong bases, typically pyridine and ammonia. Its advantage is that proton transfer occurs in these cases, which is unambiguous proof for the acidity of the corresponding hydroxyls. However, the ion-pair method does not give direct quantitative results. One should combine it with thermodesorption experiments or to use a series of bases r 2011 American Chemical Society
with various strengths to scale the acidity of different hydroxyls. In contrast, the hydrogen bond method, in the ideal case, gives direct values on the acidity of different hydroxyls.1,2 Its principle is based on the interaction of OH groups with weak bases. As a result, a H-bond is formed, the O-H bond order decreases, and the respective O-H stretching bands are shifted to lower frequencies. The stronger the H-bond, the larger the shift value and the higher the acidity of the hydroxyls. Different probes have been applied to measure hydroxyl acidity according to the H-bond method: simple diatomic molecules2-45 (e.g., H2, O2, CO, N2), hydrocarbons2,5,21,39,46,47 (alkanes, alkenes, aromatic compounds), nitriles,5,21,46,47 etc. The use of relatively strong bases (however, weak enough not to be protonated) is accompanied by some complications when Received: December 16, 2010 Revised: January 31, 2011 Published: March 02, 2011 4806
dx.doi.org/10.1021/jp111961g | J. Phys. Chem. C 2011, 115, 4806–4817
The Journal of Physical Chemistry C testing strong Brønsted acid sites: due to Fano-type Fermi resonance with 2δ(OH) or 2γ(OH) modes, the so-called ABC (AB, BC) structure of the shifted OH bands is formed in the IR spectra.5,21,48 This makes it difficult to determine the exact maximum of the shifted band. This Fermi resonance occurs when the adsorbate-induced OH shift, Δν(OH), is higher than ∼400 cm-1. This fact forced researchers, to avoid spectral complications, to prefer as molecular probes bases that induce a shift of the OH modes smaller than 400 cm-1. However, bases that are too weak practically do not interact with OH groups of weak acidity. In addition, the observed shifts are too small, which often leads to superimposition of different bands and poor resolution. That is why CO seems to be the ideal probe for measuring acidity of hydroxyls. The CO-induced shift of the OH modes reaches the value of 390 cm-1,3,4 just below the limit for occurrence of Fermi resonance. In addition, the CO molecule is small, which eliminates the problems with the accessibility of some OH groups (mainly in zeolites) that are encountered with some bulky molecules. Careful analysis of the literature data on zeolites and related materials indicates that the acidic bridging hydroxyls very often produce a composite shifted band upon CO adsorption.2-41 This seems to be always the case of OH groups absorbing at 3620-3600 cm-1 and shifted by about 300 cm-1. Very often, the complex contour of the shifted OH band has been disregarded;2,5,7-17,19-22,25,26,30-33,36,38,40,41 however, many authors have taken it as evidence of heterogeneity of the OH groups.6,24,29,35,39 A research group from Torino22,23 pointed out that the components of the composite shifted OH band changed in concert, and only one carbonyl band at 2173 cm-1 followed these changes. These authors suggested interaction of OH modes with intermolecular OH 3 3 3 CO modes around 150 cm-1. In the pilot communication of this study,18 we proposed that the phenomenon under consideration arises from Fermi resonance with the 3δ(OH) modes. Evidently, additional experiments are needed to support one or another hypothesis. It is also to be noted that many other details on the OH 3 3 3 CO interaction in zeolites are still under debate. For instance, the published data on the acidity of the AlOH groups remarkably differ.5,8-11,18,19,27,28,30,31 There are also various opinions on the acidity of silanol groups. The studies of interaction of zeolite hydroxyls with nitrogen are most restricted5,7,20-22,34,43-45 and focused mainly on the acidity of bridged OHs. Nitrogen is a weaker base than CO, and consequently, the N2-induced shift of the band around 36003610 cm-1 is reported, with few exceptions,45 to be around 110120 cm-1.7,20,22,34,43 In contrast to the case of CO adsorption, testing ZSM-5 and other zeolites with N2 indicates homogeneity of the bridging hydroxyls because a single shifted band is observed. The aim of the present investigation is to revisit the OH 3 3 3 CO and OH 3 3 3 N2 interactions in zeolites. We have chosen H-ZSM-5 as a model system because of the availability of many literature data with this material.2,5-19,43,44,46,47,49-53 To make adequate conclusions, we have used different isotopically labeled molecules.
2. EXPERIMENTAL SECTION Two H-ZSM-5 samples were investigated in this study: one was from Zeolyst (Si-to-Al ratio of 25) and another from Degussa
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(Si-to-Al ratio of 27). Most of the results presented here concern the sample from Zeolyst, and it will be referred to hereafter simply as H-ZSM-5. When the experiments are performed with the Degussa sample, it will be specially noted. The IR investigations were carried out using a Nicolet Avatar 360 spectrometer at a spectral resolution of 2 cm-1 and accumulating 200 scans. Self-supporting pellets (≈10 mg cm-2) were prepared by pressing sample powders at 104 kPa and were directly treated in the IR cell. Prior to the adsorption measurements, the samples were activated by heating for 1 h at 673 K under oxygen and evacuation for 1 h at the same temperature. Carbon monoxide (>99.5% purity) was supplied by Merck. 13CO (99 atom % 13C,
