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

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

Improvement of catalyst stability during methane dehydroaromatization (MDA) on Mo/HZSM-5 comprising intracrystalline mesopores

[EN] Anew Mo/HZSM-5catalyst (3 wt% Mo, Si/Al = 26) comprising a carbon-templated zeolite having intracrystalline mesopores sizing ca. 10-20nm (HZSM-5-BP) has been prepared. For comparison purposes, an equivalent catalyst has also been prepared from a commercial zeolite (HZSM-5-ref). The materials have been extensively characterized by XRD, ICP-OES, N(2) physisorption, SEM-TEM, (27)Al MAS NMR, FTIR-pyridine, NH(3)-TPD, H(2)-TPR, TG-DTG analyses and TPO, and their catalytic activity evaluated for methane dehydroaromatization (MDA) at 700 degrees C, 1 bar, and 1500 ml/g(cat) h. The hierarchical Mo/HZSM-5-BP catalyst displayed a lower deactivation rate during MDA than the reference one, leading to a higher and stable aromatics yield at T.O.S. above 3 h, despite a higher amount of less reactive coke (associated to the zeolite acid sites) was produced in the former. We hypothesize that the enhanced tolerance to carbon deposits of the carbon-templated zeolite could be related to the intracrystalline mesopores acting as a trap for coke molecules and leaving a higher fraction of acid sites within the channels active for aromatization.Financial support by the Comision Interministerial de Ciencia y Tecnologia (CICYT) of Spain through the projects CTQ2007-66614/PPQ and CTQ2006-28341-E/BQU is gratefully acknowledged. Thanks are also due to the UE Network of Excellence IDECAT (FP6 Programme, NMP3-CT-2005-011730) which promoted the collaboration between the two research institutions.Martinez Feliu, A.; Peris, E.; Derewinski, M.; Burkat-Dulak, A. (2011). Improvement of catalyst stability during methane dehydroaromatization (MDA) on Mo/HZSM-5 comprising intracrystalline mesopores. Catalysis Today. 169(1):75-84. doi:10.1016/j.cattod.2010.11.063S7584169

On the nature of extra-framework aluminum species and improved catalytic properties in steamed zeolites

Steamed zeolites have improved catalytic properties for hydrocarbon activation (alkane cracking reaction as well as alkane dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was speculated to be related to increased Bronsted acidity during dealumination. We now 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. Bronsted acidity of 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 the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying reason behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on penta Al(V)1O5 sites of aluminum oxide clusters confined in zeolitic pores. Furthermore, slightly decreased number of adjacent Al framework sites (due to Al dislodgement from the framework) decreases the coking activity, prolonging catalyst lifetime. 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. Furthermore, our findings suggest that similar La2O3 clusters exist for La-containing zeolites and the origin of their cracking activity promotion should be similar

Achieving controllable distribution of metal cations (Pd, Pt, Ni, Cr, Cu) in a zeolite either as [M(II)-OH]/1Al or M(II)/2Al provides novel mechanistic insights for adsorptive and catalytic reactions

Utilizing H-BEA zeolites with similar Si/Al ratios but with different Al site distributions we show that the divalent metal cations (Ni, Pd, Pt, Cr, Cu) can be dispersed predominantly as either M(II)/2Al species (for conventional zeolite prepared in the hydroxide media) or as [M(II)-OH]/1Al species (for H-BEA prepared in HF). M(II) species are active in ethylene dimerization. However, Pd(II)-OH and Ni(II)-OH species, that were not previously prepared or evaluated for this reaction, are even more catalytically active. M(II)-OH species in zeolite can activate ethylene via formation of C2H4--M(II)-OC2H5 species which can eliminate butene restoring M(II)-OH species. We also reveal that Pt(II) and Pt(II)-OH in zeolite, not previously known to catalyze ethylene dimerization on solid materials, are in fact catalytically active. This synthetic realization further exemplifies the different NO adsorption aspects of these materials. Both Pd(II) and Pd(II)-OH are active for NO adsorption, the latter desorbing NO at higher temperature than isolated Pd(II). Notably, Pd(II)-OH is active for Wacker oxidation chemistry of ethylene into acetaldehyde, whereas Pd(II) is less active: this clarifies the missing mechanistic aspects of Wacker oxidation by homogeneous complexes. The presence of OH ligand in the Pd(II) first coordination sphere is important for reactivity. Further, we show that Cr/2Al in H-BEA is inactive for ethylene oligomerization, whereas Cr-OH has ethylene dimerization activity, illuminating a previously unknown possibility that Cr-OH species could be an active species for Cr/silica Phillips ethylene oligomerization catalysts

PdO self-assembly on zeolite SSZ-13 with rows of O3Al(IV)OH selectively incorporated in PdO(101) facets for moisture-resistant methane oxidation

We describe an efficient way to prepare moisture-tolerant methane (hydrocarbon) combustion catalysts based on PdO nanoparticles supported on siliceous SSZ-13 zeolite. Only zeolites with high Si/Al ratios >15 are hydrophobic enough to exclude the Pd from the micropores while forming well-faceted PdO nanoparticles. Simultaneously, during self-assembly mobile Al hydroxo species get incorporated into the as-formed PdO nanoparticles. For the first time, we reveal selective incorporation of rows of O3Al(IV)-OHbridging aluminum hydroxo-species into the (101) facets of PdO nanoparticles that form during thermal self-assembly in Pd/SSZ-13 using state-of-the-art atomically-resolved HAADF-STEM imaging, solid-state NMR, DFT calculations and reactivity measurements. The Al+3-OH moieties form atom-thin rows in place of tri-coordinate Pd ions Pd+2 in Pd1O3 on (101) facets: these tri-coordinate Pd1+2O3 are responsible for C-H bond dissociation of methane and hydrocarbons during catalytic methane oxidation. However, on unmodified or non-zeolite supported PdO nanoparticles in the presence of water vapor from engine exhaust, water competes with methane by forming a deactivated Pdtetra(OH)(H2O)Pdtetra site with two water molecules on contiguous 3-coordinate Pd, which is not active for C-H bond activation. When Al-OH moieties are present in place of some tri-coordinate Pd1O3 sites, water dissociation becomes kinetically unfavorable due to disruption of Pdtetra(OH)(H2O)Pdtetra species formation. Consequently, our catalytic measurements reveal a significantly more stable performance of such catalysts in methane combustion in the presence of water vapor. Our findings provide an unprecedented atomic-level insight into structure-property relationships for supported PdO materials in catalytic methane oxidation and offer a new strategy to prepare moisture-tolerant Pd-containing methane combustion catalysts for green-house gas mitigation by selectively doping atomically thin rows of non-precious metal into specific facets of PdO

Dynamic Adsorption of CO2/N2 on Cation-Exchanged Chabazite SSZ-13: A Breakthrough Analysis

Alkali-exchanged SSZ-13 adsorbents were investigated for their applicability in separating N2 from CO2 in flue gas streams using a dynamic breakthrough method. In contrast to IAST calculations based on equilibrium isotherms, K+ exchanged SSZ-13 was found to yield the best N2 productivity, comparable to Ni-MOF-74, under dynamic conditions where diffusion properties play a significant role. This was attributed to the selective, partial blockage of access to the chabazite cavities, enhancing the separation potential in a 15/85 CO2/N2 binary gas mixture