Font Size: a A A

The Different Iron Species In Iron-containing Zeolite: A UV Resonance Raman Spectroscopy Study

Posted on:2005-11-14Degree:MasterType:Thesis
Country:ChinaCandidate:Y X LianFull Text:PDF
GTID:2121360155452319Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Iron-containing zeolites have drawn more and more attentions because of theirexcellent performance as catalysts in many important reactions. Results have shownthat the iron species in different sites show different catalytic activities. Thecharacterization of these different iron species is absolutely necessary for theunderstanding of the nature of these catalysts.In this paper, the UV resonance Raman spectroscopy as well as many othercharacterization methods was employed to distinguish and identify the different ironspecies in the iron-containing zeolites, including the framework iron sites andextraframework iron species on the surface or channel of zeolites. Results show thatthe framework iron sites and extraframework iron species show different propertiesin many aspects. In addition, the evolutions of framework iron sites during thehydrothermal crystallization process are investigated using UV-Raman and UV-visabsorption spectroscopy.UV resonance Raman spectroscopy was first used to detect and identify theframework iron sites in the Fe-silicalite. Four clear resonance Raman bands at 516,1005, 1110 and 1165 cm"1 can be observed when excitation line is chosen at 244 nm,centered at the charge-transfer band of framework iron sites in Fe-silicalite. The twobands at 520 and 1110 cm-1 should be attributed to the symmetric/asymmetricvibrations Fe-O-Si structure in the framework iron sites, which shall be enhanced bythe 244 nm excitation line and cannot be observed in the spectrum excited by 325 nmline. Changes of the Raman bands directly related to the framework iron species areobserved before and after different post-synthesis treatment. The results suggest thatthe framework iron sites show some stability under ambient treatment as HC1solution washing; while these structures will be destructed when a harsh treatmentsuch as severe calcination at 900 °C is used. UV Raman spectra of Fe-silicalite withdifferent iron content are also recorded and relative intensities of the frameworkiron-concerning Raman bands increase linearly in low iron content while level offwhen the iron content is beyond a threshold of Fe/Si molar ratio at 0.006. From thispoint of view we can conclude that the maximum amount of the framework iron sitesshall not run up over 0.006 of Fe/Si molar ratio. This method gives an opportunity todetect the maximum amount of iron that can substitute the framework silicon sitesaccording to the iron content in the Fe-silicalite. In addition, similar bands have alsobeen observed in other iron-concerning zeolites such as Fe-APO-5 and Fe-SBA-15.Using 244 nm excitation line the Raman spectrum of Fe-APO-5 gives a characteristicband at 1130 cm"1 and that of Fe-SBA-15 shows this band at 1100 cm'1. These bandsshall also be attributed to the framework iron sites in concerning zeolites. The slightshifts of these bands from that of Fe-silicalite suggest that the coordinatesurroundings of iron framework sites in these zeolites are different comparing to theFe-silicalite. UV Raman spectroscopy is also employed to detect the extraframework ironspecies and distinguish their structures. When the excitation line is shift to 325 nm,locating in the absorption tail of charge-transfer band, the Raman spectrum shows aseries of clear bands locating at 520, 1005 and 1165 cm'1. These bands showdifferent properties throughout the HC1 solution treatment comparing to those of thebands due to the framework iron sites. Detailed research of different solutiontreatments shows that such surface iron species that gives the Raman band at 520cm'1 will be destructed by Cl" anion in the solution while do not change before andafter normal acid treatment. There is an absorption shoulder in the UV-vis absorptionspectrum of Fe-silicalite after HC1 treatment. This band suggests a kind of new ironspecies with the Fe-Cl coordinated structure emerge in Fe-silicalite after the HC1treatment. From this fact it can be concluded that the 520 cm"1 band should beattributed to surface iron-oxide species with highly dispersion. From the UV Ramanspectra of Fe-silicalite with different iron content we found that the 520 cm"1 bandshows trace intensity in low iron content; however, when the amount of iron in theFe-silicalite moves up to a certain amount (in our case it is 0.006 of Fe/Si molarratio) the relative intensity of this band increases linearly according to iron content.This gives a suggestion that this kind of surface iron species relative to the 520 era"'Raman band shall not appear until the emergence of the framework iron sites inFe-silicalite. Hence we speculate that such surface iron species bond directly to theBransted acid sites in the Fe-silicalite, which come from the substitution offramework silicon atom by iron atom. In addition, such iron-oxide species can alsobe observed in Fe/ZSM-5 with the iron-oxide species bonding to the Bransted acidsites from the framework aluminum structure. These species give similar bandcentered at 520 cm"1 in the Raman spectrum excited by 325-nm laser. However, suchsurface iron species are not observed in Fe-SBA-15 by Raman spectroscopy.Considering that the wall of meso-material is consisted by amorphous silica, so thesubstituted iron framework sites shall not give enough acidity to form such surfaceiron-oxide species. The evolution of framework iron structure during hydrothermal crystallizationprocess is also investigated using UV-Raman and UV-vis absorption spectroscopy.Different evolution routines have been observed in two different hydrothermalsynthesis methods. In the acid synthesis method containing fluoride anions in theinitial gel, a tetrahedral coordinated iron-silicate structure has been observed beforethe full crystallization. We speculate that such species change little by little to formthe framework iron structure in the Fe-silicalite. However, when basis synthesisroutine is used without the adding of fluoride, no such iron precursor can be observedbefore the full crystallization. This result suggests that in such synthesis method theframework iron species should form after the formation of zeolite frameworkstructure.
Keywords/Search Tags:Fe-silicalite, UV Raman, resonance Raman, framework iron sites, extraframework iron species, crystallization mechanism
PDF Full Text Request
Related items