Isotopic exchange of oxygen between proton-exchanged zeolites and water

Journal Article · Thu Nov 25 04:00:00 EST 1999 · Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical

DOI:https://doi.org/10.1021/jp992258l· OSTI ID:20013645

Mizuno, N; Mori, H; Mineo, K; Iwamoto, M

The time courses of the title reaction were analyzed by using a closed circulation system. It was found that there are three kinds of framework oxygen, O{sub I}, O{sub II}, and O{sub III}, that is, easily, moderately, and hardly exchanging oxygens. The respective rate constants of the exchange reactions, k{sub 1}, k{sub 2}, and k{sub 3}, were almost constant, or independent of the zeolite structures and silica/alumina ratios. In contrast, the numbers of O{sub I} and O{sub II} (n{sub 1} and n{sub 2}) decreased with increasing silica/alumina ratios, whereas that of O{sub III} (n{sub 3}) increased. n{sub 1} + n{sub 2} and n{sub 3} are approximately in agreement with the numbers of bridging oxygens of Si-O-Al and Si-O-Si, respectively. Next, IR measurement was applied to confirm the origin of the active framework oxygens. Upon the exchange reaction on H-MFI-23, one acidic OH band at 3612 cm{sup {minus}1} readily shifted to lower frequency while the silanol band at 3,735 cm{sup {minus}1} and {nu}(Si-O-T (T = Si or Al)) bands at 1,219 and 1,092 cm{sup {minus}1} shifted slightly. The {sup 18}O contents in the bands, evaluated by the deconvolution of the spectra, decreased in the following order: acidic OH {much{underscore}lt} Si-O-T = silanol. A similar order was observed on H-FAU-5.6, H-MOR-16, or H-LTL-6.0. Most of oxygens corresponding to the acidic OH bands could exchange with {sup 18}O upon treatments in a large excess of H{sub 2}{sup 18}O. It follows that O{sub I} and O{sub II} are the bridging oxygens bound to Al and exchangeable with water by the contribution of neighboring proton, and O{sub III} is the bridging oxygen between two Si atoms. It was further found that the exchange reaction, Si-{sup 18}O-Al + Si-{sup 16}O-Si {yields} Si-{sup 16}O-Al + Si-{sup 18}O-Si, starts to proceed above 673 K and is dominant at 873 K.

• Research Organization: Hokkaido Univ., Sapporo (JP)
• OSTI ID: 20013645
• Journal Information: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical, Journal Name: Journal of Physical Chemistry B: Materials, Surfaces, Interfaces, amp Biophysical Journal Issue: 47 Vol. 103; ISSN 1089-5647; ISSN JPCBFK
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CATALYTIC EFFECTS
ISOTOPIC EXCHANGE
OXYGEN 18
REACTION KINETICS
WATER
ZEOLITES