TY - JOUR

T1 - X-ray absorption spectroscopic study on the structure and crystallization of Ga-containing MFI-type zeolites

AU - Behrens, Peter

AU - Kosslick, Hendrik

AU - Vu Anh Tuan, Anh Tuan

AU - Fröba, Michael

AU - Neissendorfer, Frank

N1 - Funding information: We thank Hasylab Desy (Hamburg, Germany) for allocating beamtime and the Hasylab staff for technical assistance. Help during the measurements from Stefanie Assmann (Universit~t Konstanz) and Karin Lochte (Universitfit Hamburg) is gratefully acknowledged. M.F. thanks the Alexander von Humboldt Foundation for a Feodor Lynen Research Fellowship.

PY - 1995/1

Y1 - 1995/1

N2 - The local environment of the Ga atoms in a series of gallosilicates with MFI structure as well as in the dried synthesis gel has been investigated by GaK XANES and EXAFS. In fully crystallized gallosilicate MFI samples, Ga is tetrahedrally coordinated by oxygen atoms with a GaO bond distance of 1.83 Å. Lack of order beyond the first coordination shell is in agreement with Ga located in several different crystallographic sites of the zeolite-type framework. For the gel, the same oxygen coordination is observed for the first coordination shell. As expected for the amorphous gel, no order beyond the first coordination shell is detected. Thus, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) cannot differentiate between tetrahedrally coordinated Ga atoms in a gel phase and Ga atoms incorporated into the framework. Inspection of a series of samples prepared with different crystallization times and with Si Ga ratios>40 show that during crystallization framework and extra-framework Ga remain in tetrahedral coordination. A sample with a low Si Ga ratio of 19 in the gel, a value far below the so far observed maximum Ga framework content of Si Ga≈40, contains also octahedral Ga after prolonged hydrothermal treatment. A quantitative evaluation shows that extra-framework Ga in gallosilicate MFI samples occurs in both tetrahedral and octahedral coordination. This result on the nature of extra-framework Ga is of importance, since these Ga species are believed to play a decisive role in catalytic processes on gallosilicate MFI-type solids.

AB - The local environment of the Ga atoms in a series of gallosilicates with MFI structure as well as in the dried synthesis gel has been investigated by GaK XANES and EXAFS. In fully crystallized gallosilicate MFI samples, Ga is tetrahedrally coordinated by oxygen atoms with a GaO bond distance of 1.83 Å. Lack of order beyond the first coordination shell is in agreement with Ga located in several different crystallographic sites of the zeolite-type framework. For the gel, the same oxygen coordination is observed for the first coordination shell. As expected for the amorphous gel, no order beyond the first coordination shell is detected. Thus, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) cannot differentiate between tetrahedrally coordinated Ga atoms in a gel phase and Ga atoms incorporated into the framework. Inspection of a series of samples prepared with different crystallization times and with Si Ga ratios>40 show that during crystallization framework and extra-framework Ga remain in tetrahedral coordination. A sample with a low Si Ga ratio of 19 in the gel, a value far below the so far observed maximum Ga framework content of Si Ga≈40, contains also octahedral Ga after prolonged hydrothermal treatment. A quantitative evaluation shows that extra-framework Ga in gallosilicate MFI samples occurs in both tetrahedral and octahedral coordination. This result on the nature of extra-framework Ga is of importance, since these Ga species are believed to play a decisive role in catalytic processes on gallosilicate MFI-type solids.

KW - Extended X-ray absorption fine structure

KW - Extra-framework gallium

KW - Gallosilicate

KW - MFI

KW - X-ray absorption near edge structure

UR - http://www.scopus.com/inward/record.url?scp=0000897430&partnerID=8YFLogxK

U2 - 10.1016/0927-6513(94)00054-Y

DO - 10.1016/0927-6513(94)00054-Y

M3 - Article

AN - SCOPUS:0000897430

VL - 3

SP - 433

EP - 441

JO - Microporous Materials

JF - Microporous Materials

SN - 0927-6513

IS - 4-5

ER -