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

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

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

We report the formation of a new Lewis acid Al site in H-FAU zeolite upon 650 ⁰C thermal treatment, unknown for any zeolite. Spectroscopy and DFT calculations reveal this site is a naked Al+3 ion which is charge balanced by a triplet of adjacent framework oxygens with net charge of -1 for each Si-O-Al moiety. This is the first reported observation of a +3 cation stabilized in a zeolite and the first confirmation of existence of aluminum triplets (as opposed to Al pairs) in that can stabilize such cations in siliceous zeolites. This site forms a thermally stable carbonyl O3Al-CO complex with the highest known frequency at 2252 cm-1 for a carbonyl complex on any solid material. These findings open new horizons in zeolite chemistry and expand our understanding of polyvalent metals’ interactions with 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

Replacing alkaline for alkaline-earth metal hydroxide in the synthesis of siliceous SSZ-13 zeolite (Si/Al~10) yields SSZ-13 with novel, advantageous properties. Its NH4-form ion-exchanges higher amount of isolated divalent M(II) ions than the conventional one: this is the consequence of increased number of Al pairs in the structure induced by the +2 charge of Sr(II) cations in the synthesis gel that force two charge-compensating AlO4- motives to reside closer together. We characterize the +2 state of Co(II) ions in these materials with infra-red spectroscopy and X-ray absorption spectroscopy measurements, and show their utility for NOx pollutant adsorption from ambient air: the ones derived from SSZ-13 with higher Al pair content contain more isolated cobalt(II) and thus, perform better as ambient-air NOx adsorbers. Notably, Co(II)/SSZ-13 with increased number of Al pairs is significantly more hydrothermally stable than its NaOH-derived analogue. Loading Pd(II) into Co-SSZ-13(Sr) produces an active NOx adsorber (PNA) material that can be used for NOx adsorption from simulated diesel engine exhaust. The critical issue for these applications is hydrothermal stability of Pd-zeolites. Pd/SSZ-13 synthesized in the presence of Sr(OH)2 does not lose its PNA capacity after extremely harsh aging at 850 and 900 ⁰C (10 hours in 10% H2O/air flow) and loses only ~55% capacity after hydrothermal aging at 930 ⁰C. This can be extended to other divalent metals for catalytic applications, such as copper: we show that Cu/SSZ-13 catalyst can survive hydrothermal aging at 920 ⁰C without losing its catalytic properties, metal dispersion and crystalline structure. Thus, we provide a new, simple, and scalable strategy for making remarkably (hydro)thermally stable metal-zeolite materials/catalysts with a number of useful applications

Biomimetic CO Oxidation Below – 100 ⁰C by a Nitrate-Containing Metal-Free Microporous System

CO oxidation is of importance both for organic and inorganic systems. Transition and precious metals on various supports can oxidize CO to CO2. Among them, few systems, like Au/TiO2, can perform CO oxidation at the low temperature of -70 ⁰C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes under ambient temperatures which is an important pathway of their living cycle that enables them to “breathe”/produce energy in the absence of oxygen and leads to the carbonate mineral formation. Herein, we report that CO can be oxidized to CO2 by nitrate at –140 ⁰C in completely inorganic system (zeolite) without metals. The transformation of NOx and CO species in zeolite as well as the origin of this unique activity (catalyzed by Bronsted acid sites) are clarified using spectroscopic and computational approach.</p

The Superior Hydrothermal Stability of Pd/SSZ-39 in Low Temperature Passive NOx Adsorption (PNA) and Methane Combustion

We successfully synthesized uniform SSZ-39 with an average crystal size of about a micron. Pd (0.7 - 3 wt%) was supported on SSZ-39 with Si/Al ratio ~12. The as-synthesized materials were characterized by FTIR, XRD, Helium Ion Microscopy, HAADF-STEM imaging, 27Al, 29Si and H solid state NMR spectroscopic techniques. FTIR studies with CO and NO probe molecules reveal that the 0.7 wt% Pd/SSZ-39 material with Si/Al ~12 has the majority of Pd dispersed atomically as isolated Pd(II) and Pd(II)-OH centers, and thus can be used as a low-temperature passive NOx adsorber. Pd(II)-NO, Pd(II)(OH)(NO) and Pd(II)(CO)(NO) complexes form during PNA in this material. We compare this PNA material directly with the Pd/SSZ-13 system (with Si/Al ratio ~12) and show its superior hydrothermal stability. Remarkably, Pd/SSZ-39 with Si/Al ratio ~12 survives hydrothermal aging up to 815 ºC in 10% H2O/Air vapor for 16 hours without significant loss in activity. The SSZ-39 crystal structure remains intact during hydrothermal aging up to 1,000 ºC as we elucidate it with XRD and HAADF-STEM imaging/EDS mapping. However, changes to the framework during such harsh hydrothermal treatment significantly change the NOx release profiles during PNA as evidenced by high-field 27Al NMR on fresh and aged Pd/SSZ-39 samples as well as PNA performance measurements. Besides PNA application, these hydrothermally very stable materials (3 wt% Pd on SSZ-39 with Si/Al ratio ~12) can be used as a robust methane combustion catalyst under industrially relevant conditions (GHSV~600,000hr-1). This catalyst shows minimal deactivation after both harsh hydrothermal aging at 750 and 800 ºC, and prolonged time on stream (105 hrs) at 425 ⁰C. In contrast, both 3wt% Pd/alumina and 3wt% SSZ-13 supported samples lose a significant portion of their activity.<br /