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

Abstract

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.

Increasing Al-pair abundance in SSZ-13 zeolite via zeolite synthesis in the presence of alkaline earth metal hydroxide produces hydro-thermally stable Co-, Cu- and Pd-SSZ-13 materials

“…This remarkable property of Co/SSZ-13(Sr) material, thus, prompted us to load 1.7 wt% Pd in its micropores and study its PNA properties and hydrothermal stability of the resulting Pd-containing SSZ-13 composite [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. Indeed, the sample shows excellent PNA performance after loading Pd in it (Fig.…”

“…Emissions during vehicle cold start, however, remain a challenge. To circumvent this challenge, Pd/zeolite materials (passive NOx adsorbers) have been introduced industrially that adsorb NO strongly as Pd(II)-NO, Pd(I)-NO, Pd(II)(OH)(NO) and Pd(II)(NO)(CO) complexes (depending on Pd speciation in zeolites) at low temperatures (80-130 ⁰C) during cold start and then release them at higher temperatures (>170-180 ⁰C) when downstream SCR catalysts become very active [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]. The main challenge for application of Metal/zeolite materials for industrial vehicle use has always been their hydrothermal stability.…”

Unusual water-assisted NO adsorption over Pd/FER calcined at high temperatures: The effect of cation migration

“…Although zeolites are now widely utilized, for example, for emission control in vehicles and fluid cracking or hydrocracking in petrochemical industry, there is a lot of chemistry left to discover and to clarify. [10][11][12][13][14] Their unique structures confine molecules into small spaces and alter the reaction pathway by modulating transition states as seen in several examples. [15][16][17] Zeolites, in general, strongly interact with water, due to the hydrophilic nature of these materials.…”

Direct observation of a new aluminum Lewis acid site in a zeolite

“…1B). We recently demonstrated that the extra-framework aluminum (EFAL) species that form in zeolite after thermal/steaming treatment are alumina clusters formed during steaming with a high proportion of coordinatively unsaturated penta Al sites (due to their small size) [25]: CO adsorption on these sites produces the presently observed band with a maximum at 2240-2232 cm -1 . These sites form upon elution of mobile hydrated Al species from the framework and subsequent coalescence of the mobile species into alumina clusters/nanoparticles [25].…”

“…This led to evolution of hydrides with characteristic Al-D stretches in the 1430-1370 cm -1 region (Fig. 4B) [25]. Correspondingly, OD bands appeared in the D stretching region due to heterolytic In order to understand the origin of the C-O vibrational band at 2252 cm -1 , we considered various models.…”