Stabilization of Super Electrophilic Pd+2 Cations in Small-Pore SSZ-13 Zeolite

Abstract

<p>We provide the first observation and characterization of super-electrophilic metal cations on a solid support. For Pd/SSZ-13 the results of our combined experimental (FTIR, XPS, HAADF-STEM) and density functional theory study reveal that Pd ions in zeolites, previously identified as Pd<sup>+3</sup> and Pd<sup>+4</sup>, are in fact present as super electrophilic Pd<sup>+2</sup> species (charge-transfer complex/ion pair with the negatively charged framework oxygens). In this contribution we re-assign the spectroscopic signatures of these species, discuss the unusual coordination environment of “naked” (ligand-free) super-electrophilic Pd<sup>+2</sup> in SSZ-13, and their complexes with CO and NO. With CO, non-classical, highly positive [Pd(CO)<sub>2</sub>]<sup>2+</sup> ions are formed with the zeolite framework acting as a weakly coordinating anion (ion pairs). Non-classical carbonyl complexes also form with Pt<sup>+2</sup> and Ag<sup>+</sup> in SSZ-13. The Pd<sup>+2</sup>(CO)<sub>2</sub> complex is remarkably stable in zeolite cages even in the presence of water. Dicarbonyl and nitrosyl Pd<sup>+2</sup> complexes, in turn, serve as precursors to the synthesis of previously inaccessible Pd<sup>+2</sup>-carbonyl-olefin [Pd(CO)(C<sub>2</sub>H<sub>4</sub>)] and -nitrosyl-olefin [Pd(NO)(C<sub>2</sub>H<sub>4</sub>)] complexes. Overall, we show that zeolite framework can stabilize super electrophilic metal (Pd) cations, and show the new chemistry of Pd/SSZ-13 system with implications for adsorption and catalysis.<br></p>

Biomimetic CO oxidation below −100 °C by a nitrate-containing metal-free microporous system

“…SSZ-13 zeolite was selected due to its prevalence in the natural world 13 . SSZ-13 is a robust, hydrothermally stable framework used extensively to decrease pollution from vehicle operation [14][15][16][17][18] and it also contains only 1 equivalent framework T-site 15 , reducing complexity in experimental interpretation and modeling relative to frameworks with a broad distribution of T-sites. Figure 1a depicts the in-situ FTIR data collected during sequential adsorption of NO 2 on H-SSZ-13 with Si/Al ~12 at 25 °C.…”

Catalytic activation of ethylene C-H bonds on uniform d8 Ir(I) and Ni(II) cations in zeolites: toward molecular level understanding of ethylene polymerization on heterogeneous catalysts

“…The bond is not split heterolytically on the M-O bond but instead it is activated homolytically via oxidative addition to an electrophilic d 8 metal center in the zeolite micropore. High coordinative unsaturation and superelectrophilicity of M cations in zeolite have been recently quantified for isoelectronic d 8 Pd(II) ions, 22 explaining why this reaction is favored over heterolytic activation of C-H bonds on covalent M-O bond. It is important to note that such a homolytic pathway of C-H bond activation has been previously overlooked in the metal/zeolite and M/oxide literature.…”

“…These reactions proceed via activation of C-H bonds of ethylene on a super electrophilic cationic metal center recently observed for a metal/zeolite system. 22 A modified IWI method was previously used to produce atomically dispersed Pt and Pd in SSZ-13. 9 We slightly altered this procedure to synthesize 0.4 wt% Ni on BEA by reacting aqueous nickel nitrate with excess ammonia to produce a mononuclear Ni hexamine complex.…”

Increasing Al-Pair Abundance in SSZ-13 Zeolite via Zeolite Synthesis in the Presence of Alkaline Earth Metal Hydroxide Produces Hydro-Thermally Stable Cobalt and Pd-SSZ-13 Materials for Pollutant Abatement Applications

“…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.…”

“…The main challenge for application of Metal/zeolite materials for industrial vehicle use has always been their hydrothermal stability. They may be exposed in vehicles to harsh steam treatment (during diesel particular filter regeneration, for example) [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. In general, it is desirable to develop materials that can survive hydrothermal aging at temperatures as harsh as 800 ⁰C and higher.…”