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

Abstract

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.

Theoretical Assessment of the Mechanism and Active Sites in Alkene Dimerization on Ni Monomers Grafted onto Aluminosilicates: (Ni–OH)+ Centers and C–C Coupling Mediated by Lewis Acid–Base Pairs

“…The energies of the slab with the two Al atoms (Figure S2) did not show any discernible preference for Al pairing in the MCM-41 framework or any evident trends in electronic energies with Al−Al distance for a series of Al substitution combinations (1.256−1.287 eV for 0.4−0.5 nm distance and 1.257−1.274 eV for a 1.5−2.1 nm distance). The high Si/Al ratio (40) in the samples used to derive these inferences about mechanisms and site requirements, as well as the absence of synthetic attempts at Al pairing in these samples through the use of structure-directing templates 46 or alkaline earth cations 47 that encourage Al−Al pairs, make it unlikely that such pairs exist to act as grafting points for Ni 2+ cations; such species, in any case, would lead to the titration of all grafting sites at 0.5 Ni 2+ per H + , in contradiction with experimental observations. Alkene dimerization rates (per Ni) that remain constant up to a grafting stoichiometry of unity and decrease thereafter (thus leading to constant mass-normalized rates) indicate that NiO domains are not responsible for catalysis and that they merely act as unreactive spectators after grafting sites are depleted by exchange.…”

Synthesis–Structure–Activity Relationship in Cu-MOR for Partial Methane Oxidation: Al Siting via Inorganic Structure-Directing Agents

“…Acidic solutions of Co 2+ ions preferentially exchange on paired Al sites. 20,65,66 The Langmuir-type cobalt-ion exchange isotherm of the acidic cobalt acetate solution (pH 3−4) showed comparable cobalt uptake for both MOR zeolites and the absence of cobalt oxide nanoparticles. This was irrespective of their Al source discounting varying degrees of Al pairing as a possible explanation for the differing MeOH productivity (Figure S13a).…”

Challenges and Opportunities for Exploiting the Role of Zeolite Confinements for the Selective Hydrogenation of Acetylene

“…When Co 2+ is introduced into the zeolite via ion exchange it exclusively titrates paired Al . Therefore, performing an ion exchange with Co(NO 3 ) 2 and the zeolite sample followed by analysis with inductively coupled plasma (ICP) quantifies the amount of Co 2+ incorporation and thereby the number of Al pairs. , This information offers an upper limit in terms of the potential TMI loading in the form of SACs. Additionally, it can be used to study the impact of synthesis methods and SDAs on the formation of Al pairs in the framework to further tailor the catalyst design to specific loadings of TMIs.…”