Zeolites infrared spectroscopy

Not many methods can be used for the study of the chemical properties of metals supported on zeolites. Infrared spectroscopy of adsorbed CO is one of them. It gives very valuable results which help for a better understanding of the catalytic properties of these systems. 

Karge FI G, Flunger M and Beyer FI K 1999 Characterization of zeolites—infrared and nuclear magnetic resonance spectroscopy and x-ray diffraction Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L... 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

In the present study, we synthesized in zeolite cavities Co-Mo binary sulfide clusters by using Co and Mo carbonyls and characterized the clusters by extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and high resolution electron microscopy (HREM). The mechanism of catalytic synergy generation in HDS is discussed. 

For infrared spectroscopy, 20-50 mg of the cobalt-exchanged zeolite was pressed into a self-supporting wafer and placed into an infrared cell similar to that described by Joly et al. [21], Spectra were recorded on a Digilab FTS-50 Fourier-transform infrared spectrometer at a resolution of 4 cm-i. Typically, 64 or 256 scans were coadded to obtain a good signal-to-noise ratio. A reference spectrum of Co-ZSM-5 in He taken at the same temperature was subtracted from each spectrum. 

Keywords Two-Dimensional correlation, Infrared spectroscopy, Dealuminated H-Y zeolite, Bronsted acidity, MQ-MAS NMR. 

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

Three zeolite crystallite sizes were used, namely 0.02, 1-1.5 and 10-15 pm range The first zeolite sample was obtained from PQ Corporation, the two other were kindly synthesized by the Laboratoire des Materiaux Mineraux , UMR 7016, Mulhouse, France. The three samples had the same framework Si/Al ratio (15) and the same Bronsted acidity (measured by infrared spectroscopy of adsorbed pyridine). They were used in their protonic form (HBEA). 

All the catalysts contained 1 wt.% platinum, introduced into the zeolite through ion exchange by [Pt(NH3)4)]2+ in competition with NH4+ (NH4+/Pt=100), followed by calcination under dry air flow at 450°C for 4h. The dispersion of platinum, measured using CO adsorption followed by infrared spectroscopy, was about 60%. Whatever the crystallite size, the catalysts were first pelletized, then crushed and sieved to obtain 0.2 - 0.4 mm particles. 

Yasenkov, S. and Frei, H. (1998). Time-resolved FT-infrared spectroscopy of visible light-induced alkene oxidation by 02 in a zeolite. J. Phys. Chem. B 102, 8177-8182... 

Acid properties. The acid properties of zeolites, including those of aluminum-deficient zeolites, have been described in several reviews (e.g. 33-35). The methods used to study the acidity of aluminum-deficient Y zeolites include infrared spectroscopy (primarily pyridine and ammonia sorption studies), n-butylamine titrations in the presence of Hammett or arylmethanol indicators, and to a lesser extent potentiometric titrations and calorimetric measurements. 

Infrared spectroscopy has proven to be a very informative and powerful technique for the characterization of zeolitic materials. Most infrared spectrometers measure the absorption of radiation in the mid-infrared region of the electromagnetic spectrum (4000-400 cm or 2.5-25 xm). In this region of the spectrum, absorption is due to various vibrational modes in the sample. Analysis of these vibrational absorption bands provides information about the chemical species present. This includes information about the structure of the zeolite as well as other functional... 

Many excellent reviews on the application of infrared spectroscopy for characterization of silicoaluminates have been written and some are referenced here if a more thorough coverage of this topic is desired [81-89]. The purpose of this section is not to cover infrared spectroscopy of zeolites in great detail, but to give the reader an overview of the utility of the technique and the types of information that can be obtained. References are provided if more in-depth information is desired. 

Infrared spectroscopy can be used to obtain a great deal of information about zeolitic materials. As mentioned earlier, analysis of the resulting absorbance bands can be used to get information about the structure of the zeolite and other functional groups present due to the synthesis and subsequent treatments. In addition, infrared spectroscopy can be combined with adsorption of weak acid and base probe molecules to obtain information about the acidity and basicity of the material. Other probe molecules such as carbon monoxide and nitric oxide can be used to get information about the oxidation state, dispersion and location of metals on metal-loaded zeolites. 

Infrared spectroscopy has been used to help solve or determine the structure of zeolites. The technique is particularly useful for identifying the presence of double four- and six-rings as well as five-membered pentasil rings. In the structural characterization of beta zeolite, Newsam and coworkers used a variety of techniques including IR, electron microscopy (TEM), X-ray diffraction (XRD) and sorption data to solve the stacked, faulted structure [57]. The presence of IR absorption bands at 1232 and 560cm indicated that the structure contained five-member pentasil building units. 

Infrared Spectroscopy Characterization of Zeolitic Systems 129 Bronsted Acid Site Distribution... 

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

Flanigen, E.M. (1975) Structural analysis by infrared spectroscopy. ACS Monogr., 171, 80-117 Rabo, ). (ed.) (1975) Zeolite Chemistry and Catalysis, American Chemical Sodety, Washington, D.C. 

Coke formation during xylene isomerization has been studied using in situ infrared spectroscopy [78]. A study done on EB isomerization with a bifunctional catalyst containing EUO zeolite indicated that poor initial selectivity of the catalyst improves after a period of fast deactivation, during which micropores are blocked [79]. 

Vibrational spectroscopy of adsorbed probe molecules is one of the most powerful tools to assess the acidic properties of catalysts. Acidity studies of dealumi-nated Y zeolites (main active component of FCC catalysts) or other zeolitic catalysts are reported using mostly Fourier Transform Infrared Spectroscopy (FTIR) with CO adsorption at 77 K or FTIR-pyridine/substituted pyridines adsorption at 425 K [22-26]. FTIR acidity studies of commercial FCC catalysts are even more scarce... 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

Recently, we reported that an Fe supported zeolite (FeHY-1) shows high activity for acidic reactions such as toluene disproportionation and resid hydrocracking in the presence of H2S [1,2]. Investigations using electron spin resonance (ESR), Fourier transform infrared spectroscopy (FT-IR), MiJssbauer and transmission electron microscopy (TEM) revealed that superfine ferric oxide cluster interacts with the zeolite framework in the super-cage of Y-type zeolites [3,4]. Furthermore, we reported change in physicochemical properties and catalytic activities for toluene disproportionation during the sample preparation period[5]. It was revealed that the activation of the catalyst was closely related with interaction between the iron cluster and the zeolite framework. In this work, we will report the effect of preparation conditions on the physicochemical properties and activity for toluene disproportionation in the presence of 82. ... 

The object of the present investigation was to study systematically the effect of monovalent cations on the lattice vibrations in the synthetic zeolites Linde A and Linde X. It was reasoned that mid-infrared spectroscopy might yield information on cation siting in these zeolites. 

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

Ward (, 6) determined the acidity of several transition metal X and Y zeolites by infrared spectroscopy but could find no simple relationship between the proton acid concentration and physical parameters of metal ions or catalytic activity for o-xylene isomerization. 

The preparation of ammonium ion exchanged Y zeolites has long been known to be a precursor step in the preparation of H Y zeolites. The former materials have been characterized by thermo-gravimetric experiments, while the latter zeolites that contain the H atoms as hydroxyl groups have been intensively examined by infrared spectroscopy (1 .2 A systematic description of the... 

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