Violation or Abidance of Löwenstein’s Rule in Zeolites Under Synthesis Conditions?

Conventional Hydrothermal Synthesis of MFI Zeolite in Methanol Solution

The synthesis of zeolites through more efficient, environmentally friendly, and cost-effective methods was deemed significant in both industrial applications and academic fields. Conventional hydrothermal synthesis strategies have encountered difficulties in producing pure silica MFI zeolite (silicalite-1) under amine-free conditions. This was primarily attributed to the competitive growth of quartz, keatite, or magadiite during the crystallization process. In this work, it was found that the lack of nucleation ability was an important reason for the poor crystallization stability of the methanol solution. Well-crystallized silicalite-1 zeolites with uniform particle sizes were achieved through the cooperative guidance of methanol and seed crystals. Large-scale experiments with silicalite-1 zeolite demonstrated good reproducibility. Combined with the TG-IR and N2 adsorption-desorption results, it was observed that, when an extremely small amount of seed (0.97 wt %) was introduced, methanol could play a role as a crystallization promoter in the hydrothermal synthesis system. Furthermore, a lower alkaline-to-silica ratio and water-to-silica ratio were conducive to the progression of the crystallization process. In summary, this work presented a hydrothermal synthesis strategy for the synthesis of silicalite-1 zeolite in a methanol solution without the need for a large amount of seeds and provided an effective pathway for the low-cost, large-scale production of silicalite-1 zeolite.

Assessing the stability of Pd-exchanged sites in zeolites with the aid of a high throughput quantum chemistry workflow

Cation exchanged-zeolites are functional materials with a wide range of applications from catalysis to sorbents. They present a challenge for computational studies using density functional theory due to the numerous possible active sites. From Al configuration, to placement of extra framework cation(s), to potentially different oxidation states of the cation, accounting for all these possibilities is not trivial. To make the number of calculations more tractable, most studies focus on a few active sites. We attempt to go beyond these limitations by implementing a workflow for a high throughput screening, designed to systematize the problem and exhaustively search for feasible active sites. We use Pd-exchanged CHA and BEA to illustrate the approach. After conducting thousands of explicit DFT calculations, we identify the sites most favorable for the Pd cation and discuss the results in detail. The high throughput screening identifies many energetically favorable sites that are non-trivial. Lastly, we employ these results to examine NO adsorption in Pd-exchanged CHA, which is a promising passive NOx adsorbent (PNA) during the cold start of automobiles. The results shed light on critical active sites for NOx capture that were not previously studied.

Increasing the Number of Aluminum Atoms in T3 Sites of a Mordenite Zeolite by Low-Pressure SiCl4 Treatment to Catalyze Dimethyl Ether Carbonylation

Controlling the location of aluminum atoms in a zeolite framework is critical for understanding structure-performance relationships of catalytic reaction systems and tailoring catalyst design. Herein, we report a strategy to preferentially relocate mordenite (MOR) framework Al atoms into the desired T₃ sites by low-pressure SiCl₄ treatment (LPST). High-field ²⁷ Al NMR was used to identify the exact location of framework Al for the MOR samples. The results indicate that 73 % of the framework Al atoms were at the T₃ sites after LPST under optimal conditions, which leads to controllably generating and intensifying active sites in MOR zeolite for the dimethyl ether (DME) carbonylation reaction with higher methyl acetate (MA) selectivity and much longer lifetime (25 times). Further research reveals that the Al relocation mechanism involves simultaneous extraction, migration, and reinsertion of Al atoms from and into the parent MOR framework. This unique method is potentially applicable to other zeolites to control Al location.

Layered double hydroxide membrane with high hydroxide conductivity and ion selectivity for energy storage device

Membranes with fast and selective ions transport are highly demanded for energy storage devices. Layered double hydroxides (LDHs), bearing uniform interlayer galleries and abundant hydroxyl groups covalently bonded within two-dimensional (2D) host layers, make them superb candidates for high-performance membranes. However, related research on LDHs for ions separation is quite rare, especially the deep-going study on ions transport behavior in LDHs. Here, we report a LDHs-based composite membrane with fast and selective ions transport for flow battery application. The hydroxide ions transport through LDHs via vehicular (standard diffusion) & Grotthuss (proton hopping) mechanisms is uncovered. The LDHs-based membrane enables an alkaline zinc-based flow battery to operate at 200 mA cm⁻², along with an energy efficiency of 82.36% for 400 cycles. This study offers an in-depth understanding of ions transport in LDHs and further inspires their applications in other energy-related devices.

Membranes with fast and selective ion transport are highly relevant for energy storage devices. Here, the authors report a layered double hydroxide membrane with high ionic selectivity and hydroxide ion conductivity for flow battery applications, and reveal the ions transport mechanism of the membrane.

Thermal resistance effect on anomalous diffusion of molecules under confinement

Diffusion is generally faster at higher temperatures. Here, a counterintuitive behavior is observed in that the movement of long-chain molecules slows as the temperature increases under confinement. This report confirms that this anomalous diffusion is caused by the "thermal resistance effect," in which the diffusion resistance of linear-chain molecules is equivalent to that with branched-chain configurations at high temperature. It then restrains the molecular transportation in the nanoscale channels, as further confirmed by zero length column experiments. This work enriches our understanding of the anomalous diffusion family and provides fundamental insights into the mechanism inside confined systems.

Cooperative and Competitive Occlusion of Organic and Inorganic Structure-Directing Agents within Chabazite Zeolites Influences Their Aluminum Arrangement

We combine experiment and theory to investigate the cooperation or competition between organic and inorganic structure-directing agents (SDAs) for occupancy within microporous voids of chabazite (CHA) zeolites and to rationalize the effects of SDA siting on biasing the framework Al arrangement (Al-O(-Si-O)_x-Al, x = 1-3) among CHA zeolites of essentially fixed composition (Si/Al = 15). CHA zeolites crystallized using mixtures of TMAda⁺ and Na⁺ contain one TMAda⁺ occluded per cage and Na⁺ co-occluded in an amount linearly proportional to the number of 6-MR paired Al sites, quantified by Co²⁺ titration. In contrast, CHA zeolites crystallized using mixtures of TMAda⁺ and K⁺ provide evidence that three K⁺ cations, on average, displace one TMAda⁺ from occupying a cage and contain predominantly 6-MR isolated Al sites. Moreover, CHA crystallizes from synthesis media containing more than 10-fold higher inorganic-to-organic ratios with K⁺ than with Na⁺ before competing crystalline phases form, providing a route to decrease the amount of organic SDA needed to crystallize high-silica CHA. Density functional theory calculations show that differences in the ionic radii of Na⁺ and K⁺ determine their preferences for siting in different CHA rings, which influences their energy to co-occlude with TMAda⁺ and stabilize different Al configurations. Monte Carlo models confirm that energy differences resulting from Na⁺ or K⁺ co-occlusion promote the formation of 6-MR and 8-MR paired Al arrangements, respectively. These results highlight opportunities to exploit using mixtures of organic and inorganic SDAs during zeolite crystallization in order to more efficiently use organic SDAs and influence framework Al arrangements.