Synthesis and spectroscopic characterization of Cr III in crystalline zeolitic silicates

Heloise O. Pastore, Edison Steins, Celso U. Davanzo, Eduardo J. S. Vichi, Ossamu Nakamura, Mauro Baesso, Edson Silva, Helion Vargas
1990 Journal of the Chemical Society Chemical Communications  
The incorporation of Crlll into the framework of high content silica zeolite is confirmed by electron spin resonance and photoacoustic spectroscopies. The newest effort in the chemistry of zeolites is the synthesis, characterization, and applications of materials where aluminium or silicon in the conventional lattice are completely or partially replaced isomorphously by other elements which assume tetrahedral co-ordination. Isomorphous substitution of elements such as germanium,' phosphorus,2
more » ... um,' phosphorus,2 boron,3 and gallium4 for silicon or aluminium in zeolite frameworks has been reported. Germanium, phosphorus, and gallium, which have ionic radii close to those of Si4+ and AP+, are expected to replace the latter ions in the zeolite lattice. However, the incorporation of transition metal ions into the zeolite framework is more difficult owing to their differences in ionic radii to silicon and aluminium. Recently pentad-type zeolites with iron substituted at silicon sites have been unambiguously characterized.5-10 Here, we report on the synthesis and characterization of pentad-type zeolites with chromium substituted at silicon sites in the lattice. Our samples have been studied by electron spin resonance (ESR, v = 9.53 GHz) and photoacoustic spectroscopy (PAS), which reveal direct information on the state of the chromium atoms in the lattice. Initially we tried conventional procedures for the synthesis of ZSM-5 zeolites,ll changing the aluminium source for a chromium one. The green powder obtained showed, in the IR and X-ray diffraction spectra, the presence of a highly crystalline zeolitic structure. The CrIII ESR spectrum of this Table 1. Calculated and measured energy levels (nm) for CrI1*. Transition 4r2 4 4r4 4r2 4 4rs Calculated 448 650 Experimental 440 640 v = 9.53 GHz v = 9.53 GHz 0 1 2 3 4 5 6 7 8 9 10 B /kG Figure 1. ESR spectra of Cr"1 in chromium silicate, (a) in cationic positions only, (b) in silicon substitutional sites and in cationic sites. IG = 1 0 -4~. sample [Figure l(a)] consisted of a single symmetric line, centred at g = 2 . This spectrum is attributed to Cr"' ions outside the silicon skeleton, probably linked to oxygen atoms to form oxides similar to Cr203. After changes in the experimental procedure, e.g. , changing the reactants molar ratios [SO2, 0.005; Cr203, 0.04 (TPA)20 (TPA = tetrapropylammonium)], and lowering the pH (6.0-6.8) , we obtained a very light-green powder, highly crystalline as judged by X-ray diffraction analysis. The elemental analysis of the as-synthesized sample showed the presence of aluminium (O.6lYO, Si02/A1203 = 228), iron (0.05y0, Si02/Fe203 = 4650), and chromium (0.41%, Si02/Cr203 = 540). Figure l(b) shows the ESR spectrum of this sample. The spectrum shows a distribution of resonances which includes high g values. These resonances are associated with CrIII in cubic symmetry (octahedral or tetrahedral co-ordination) , probably with low symmetry distortions in the silicon substitutional positions. The line with g = 4.3 is due to iron(m), an impurity that came through the silicon source. Washing this silicate three times with dilute hydrochloric acid caused no change to the ESR spectrum. The optical absorption measurements were carried out at room temperature in the wavelength range 300-700 nm using an EDT model OAS-400 photoacoustic spectrometer.12J3 The spectrum for the Cr"1 in the zeolitic silicate shown in Figure 2 is characterized by two bands centred at 440 and 640 nm. For the level assignrnentl4 the band observed at 440 nm is identified as the 4r2 -+ 4r4 (F), and the band at 640 nm is interpreted as the 4r2 + 4r5 transition. The resulting energy-level values are given in Table 1 . The values obtained for Cr*II in this silicate are comparable with reported values for CrIII in soda-lime silica glass.15 After washing the sample showed the same PAS spectrum. Although previous reports have appeared in the literature,16-17 the ESR and PAS spectra shown in this communication give unambiguous evidence that CrIII is present in substitutional sites, co-incorporated with aluminium in the zeolite, for the first time. Complementary work including OH-IR spectroscopy is now in progress. 300 400 500 600 70 0 h / n m Figure 2. Photoacoustic spectra of Cr"1 in chromium silicate at a modulation frequency of 10 Hz.
doi:10.1039/c39900000772 fatcat:4hz2rzq7wvcjteubxoznfsn4lm