On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites

Khivantsev, Konstantin; Jaegers, Nicholas R.; Kovarik, Libor; Derewinski, Miroslaw A.; Kwak, Ja-Hun; Szanyi, Janos

doi:10.3390/molecules27072352

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites

Journal Article · Wed Apr 06 00:00:00 EDT 2022 · Molecules

DOI:https://doi.org/10.3390/molecules27072352· OSTI ID:1893324

^[1]; ^[1]; Kovarik, Libor ^[1]; Derewinski, Miroslaw A. ^[2]; Kwak, Ja-Hun ^[3]; Szanyi, Janos ^[1]

Pacific Northwest National Laboratory (PNNL), Richland, WA (United States). Institute for Integrated Catalysis
Pacific Northwest National Laboratory (PNNL), Richland, WA (United States). Institute for Integrated Catalysis; Polish Academy of Sciences (PAS), Krakow (Poland)
Ulsan National Institute of Science and Technology (UNIST), Ulsan (Korea, Republic of)

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O₅Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al₁O₅ sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

View Accepted Manuscript (DOE)

Research Organization:: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Sponsoring Organization:: USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division; USDOE Office of Science (SC), Biological and Environmental Research (BER)

Grant/Contract Number:: AC05-76RL01830

OSTI ID:: 1893324

Report Number(s):: PNNL-SA-174113

Journal Information:: Molecules, Journal Name: Molecules Journal Issue: 7 Vol. 27; ISSN 1420-3049

Publisher:: MDPICopyright Statement

Country of Publication:: United States

Language:: English

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On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites

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