Khivantsev Konstantin, Jaegers Nicholas R, Kovarik Libor, Derewinski Miroslaw A, Kwak Ja-Hun, Szanyi Janos
Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA.
Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krakow, Poland.
Molecules. 2022 Apr 6;27(7):2352. doi: 10.3390/molecules27072352.
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. OAl(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate AlO 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.
蒸汽处理的沸石对烃类活化（烷烃裂解和脱氢）表现出改善的催化性能。在过去的六十年里，这一具有实际重要性的现象的本质一直是个谜，有人认为它与脱铝后沸石布朗斯台德酸位点强度的增加有关。我们现在利用最先进的红外光谱测量方法，证明在蒸汽处理过程中，氧化铝簇在沸石微孔中演化（由于骨架外位置的铝水解并随后聚集），其性质与羟基化过渡氧化铝表面的（纳米）晶面非常相似。沸石的布朗斯台德酸性在蒸汽处理过程中不会增加，且布朗斯台德酸位点的总数会减少。氧化铝簇的OAl(VI)-OH表面位点在高温下脱羟基形成五配位的AlO位点，该位点能够通过比质子解C-H键活化更低的势垒（在更低温度下）非常有效地断裂第一个C-H键来引发烷烃裂解，随后的反应步骤由附近的沸石布朗斯台德酸位点催化。这解释了蒸汽处理的沸石烷烃裂解和烷烃脱氢活性提高背后的潜在机制：异裂C-H键断裂发生在限制在沸石孔中的氧化铝簇的Al-O位点上。我们的发现从分子水平解释了蒸汽处理的沸石活性增强的起源，并提供了对蒸汽处理/脱铝沸石中形成的骨架外铝物种本质的缺失理解。
