Donghui Jo

Fully Copper-Exchanged High-Silica LTA Zeolites as Unrivaled Hydrothermally Stable NH3-SCR Catalysts

Diesel engine technology is still the most effective solution to meet tighter CO2 regulations in the mobility and transport sector. In implementation of fuel-efficient diesel engines, the poor thermal durability of lean nitrogen oxides (NOx ) aftertreatment systems remains as one major technical hurdle. Divalent copper ions when fully exchanged into high-silica LTA zeolites are demonstrated to exhibit excellent activity maintenance for NOx reduction with NH3 under vehicle simulated conditions even after hydrothermal aging at 900 °C, a critical temperature that the current commercial Cu-SSZ-13 catalyst cannot overcome owing to thermal deactivation. Detailed structural characterizations confirm the presence of Cu2+ ions only at the center of single 6-rings that act not only as a catalytically active center, but also as a dealumination suppressor. The overall results render the copper-exchanged LTA zeolite attractive as a viable substitute for Cu-SSZ-13.

Unseeded hydroxide-mediated synthesis and CO2 adsorption properties of an aluminosilicate zeolite with the RTH topology

We have synthesized an aluminosilicate RTH-type zeolite with Si/Al = 10 using 1,2,3-trimethylimidazolium (123TMI+) as an organic structure-directing agent (OSDA) together with Na+ or K+ in hydroxide media and without the use of seed crystals. The zeolite obtained is characterized by a cuboid morphology made of very small ill-defined crystallites, largely different from the plank-like morphology typically observed for RTH-type zeolite crystals thus far. More interestingly, we show experimental evidence demonstrating that two 123TMI+ ions are located within each [46586484] cavity of the RTH framework, forming antiparallel dimers, as found by Rietveld refinement. When hydrothermally aged at 1023 K, Cu-RTH is much less active for NO reduction with NH3 than Cu-SSZ-13, the best catalyst known for this reaction to date. However, while the CO2 uptake (3.2 mmol g−1) on Na-RTH at 298 K and 1.0 bar is lower than that (4.5 mmol g−1) on zeolite Na-Rho, a well-studied small-pore zeolite that selectively adsorbs CO2, it exhibits much faster CO2 sorption kinetics. This renders our RTH zeolite potentially useful as a selective CO2 adsorbent.

Synthesis and Structural Characterization of a CHA-type AlPO4 Molecular Sieve with Penta-Coordinated Framework Aluminum Atoms

The structure-directing effects of a series of polymethylimidazolium cations with different numbers of methyl groups as organic structure-directing agents (OSDAs) in the synthesis of aluminophosphate (AlPO4)-based molecular sieves in both fluoride and hydroxide media are investigated. On the one hand, among the OSDAs studied here, the smallest 1,3-dimethylimidazolium and the largest 1,2,3,4,5-pentamethylimidazolium cations were found to direct the synthesis of a new variant of the triclinic chabazite (CHA)-type AlPO4 material, designated AlPO4-34(t)V, and the one-dimensional small-pore silicoaluminophosphate (SAPO) molecular sieve STA-6 in hydroxide media, respectively. On the other hand, the intermediate-sized 1,2,3,4-tetramethylimidazolium cation gave SSZ-51, a two-dimensional large-pore SAPO material, in fluoride media. Synchrotron powder X-ray diffraction and Rietveld analyses reveal that as-made AlPO4-34(t)V contains penta-coordinated framework Al species connected by hydroxyl groups, as well as tetrahedral framework Al, which contrasts with the distortions arising from the two F- or OH- bridges between octahedral Al atoms in all already known AlPO4-34 materials. The presence of Al-OH-Al linkages in this triclinic AlPO4-34 molecular sieve has been further corroborated by thermal analysis, variable-temperature IR,27Al magic-angle spinning NMR, and dispersion-corrected density functional theory calculations.

Mechanisms of the Reverse Skeletal Isomerization of n-Butenes to Isobutene over Zeolite Catalysts

The use of 13C‐enriched isobutene in the reverse skeletal isomerization of 1‐butene over zeolite catalysts allowed us to corroborate that the mechanisms for the reverse reaction were essentially the same as those for the forward reaction, regardless of the structure type of the catalyst employed.