A comparative IR characterization of acidic sites on HY zeolite by pyridine and CO probes with silica-alumina and γ-alumina references.
Using IR spectroscopy, three different surface states of HY zeolite were probed by successive adsorption of CO at 143 K followed by evacuation and pyridine adsorption at 523 K: HY zeolite [1] without strong Lewis acid sites (LAS); [2] after high temperature (873 K) evacuation to convert Brønsted acid sites (BAS) to strong LAS; and [3] after water re-adsorption on HY zeolite [2] to recover BAS from LAS. The original surface of HY zeolite [1] seemed to be recovered on HY zeolite [3] after high temperature evacuation and water treatment by CO adsorption, while a part of generated LAS on HY zeolite [2] seemed irreversible on HY zeolite [3] to HY zeolite [1] by pyridine adsorption. To clarify this discrepancy, re-examination of the IR spectra of adsorbed CO and pyridine on γ-alumina and silica-alumina after similar treatments to those on HY zeolite was conducted. Based on the results of CO adsorption on γ-alumina and silica-alumina, the presence of extra-framework aluminium sites on HY zeolite [1] was confirmed. High temperature evacuation of HY zeolite [1] formed very strong LAS, a part of which was irreversible to BAS by water re-adsorption at room temperature. The irreversible sites on HY zeolite [3] were assigned to non-acidic OH groups attributed to silica-alumina. The non-acidic OH groups on HY zeolite [3], which were BAS on HY zeolite [1], hydrogen-bonded to pyridine to show IR spectra similar to those adsorbed on LAS. Thus, LAS on HY zeolite [3] seemed irreversible by pyridine adsorption after water re-adsorption. On the other hand, CO adsorbed on non-acidic OH groups showed a band at only slightly lower frequency (2160 cm(-1)) than that of BAS (2178 cm(-1)), resulting in overlapps and ignoring their presence. Thus, CO adsorption seemed to show that complete recovery of LAS to BAS occurred.
Kondo
Junko N
JN
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
Nishitani
Ryoko
R
Yoda
Eisuke
E
Yokoi
Toshiyuki
T
Tatsumi
Takashi
T
Domen
Kazunari
K
D000143:Acids / Q000737:chemistry; D000537:Aluminum Oxide / Q000737:chemistry; D002248:Carbon Monoxide / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D012822:Silicon Dioxide / Q000737:chemistry; D013055:Spectrophotometry, Infrared / Q000379:methods; D017641:Zeolites / Q000737:chemistry*
D000143:Acids; D011725:Pyridines; D017641:Zeolites; D012822:Silicon Dioxide; D002248:Carbon Monoxide; D000537:Aluminum Oxide; C023666:pyridine