Accelerating the Crystallization of Zeolite SSZ-13 with Polyamines.

Tailoring processes of nucleation and growth to achieve desired material properties is a pervasive challenge in synthetic crystallization. In systems where crystals form via nonclassical pathways, engineering materials often requires the controlled assembly and structural evolution of colloidal precursors. In this study, we examine zeolite SSZ-13 crystallization and show that several polyquaternary amines function as efficient accelerants of nucleation, and, in selected cases, tune crystal size by orders of magnitude. Among the additives tested, polydiallyldimethylammonium (PDDA) was found to have the most pronounced impact on the kinetics of SSZ-13 formation, leading to a 4-fold reduction in crystallization time. Our findings also reveal that enhanced nucleation occurs at an optimal PDDA concentration where a combination of light-scattering techniques demonstrate these conditions lead to polymer-induced aggregation of amorphous precursors and the promotion of (alumino)silicate precipitation from the growth solution. Here, we show that relatively low concentrations of polymer additives can be used in unique ways to dramatically enhance SSZ-13 crystallization rates, thereby improving the overall efficiency of zeolite synthesis.

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.

Stable Multimetallic Nanoparticles for Oxygen Electrocatalysis.

Nanostructured catalysts often face an important challenge: poor stability. Many factors contribute to catalytic degradation, including parasitic chemical reactions, phase separation, agglomeration, and dissolution, leading to activity loss especially during long-term catalytic reactions. This challenge is shared by a new family of catalysts, multimetallic nanoparticles, which have emerged owing to their broad tunability and high activity. While significant synthesis-based advances have been made, the stability of these nanostructured catalysts, especially during catalytic reactions, has not been well addressed. In this study, we reveal the critical influence of a synthetic method on the stability of nanostructured catalysts through aprotic oxygen catalysis (Li-O2 battery) demonstrations. In comparison to the conventional wet impregnation (WI) method, we show that the carbothermal shock (CTS) method dramatically improves the overall structural and chemical stability of the catalyst with the same elemental compositions. For multimetallic compositions (4- and 8-elements), the overall stability of the electrocatalysts as well as the battery lifetime can be further improved by incorporating additional noncatalytically active elements into the individual nanoparticles via CTS. The results offer a new synthetic path toward the stabilization of nanostructured catalysts, where additional reaction schemes beyond oxygen electrocatalysis are foreseeable.

Adsorptive Nature of Surface Barriers in MFI Nanocrystals.

Zeolite nanocrystals with characteristic diffusion lengths of nanometers are widely used in molecular applications to overcome diffusion limitations. However, with a large fraction of external surface area, mass transport in these materials is often limited by the presence of a surface barrier, which limits their overall potential in catalytic or separation applications. Herein, silicalite-1 crystals of varying sizes were synthesized, and the adsorption and diffusion characteristics of four molecules (ethylcyclohexane, methylcyclohexane, cyclohexane, and cis-1,4-dimethylcyclohexane) were measured to mechanistically evaluate the mass transfer surface barrier. The results observed in this study support the presence of a nonstructural surface resistance associated with the strong enthalpic interaction between the diffusing molecule and zeolite surface. Further analysis indicates that the contributions of structural and nonstructural surface barriers to the mass transport vary greatly with the heat of adsorption. This work suggests that when diffusing molecules have a weak heat of adsorption on the zeolite surface, strategies to mitigate the surface barrier should focus on the structural modification of the zeolite surface using methods such as surface etching to remove pore blockages. When the heat of adsorption is strong, strategies should focus on tuning the adsorbate/adsorbent surface interaction by methods such as depositing a mesoporous silica overlayer to reduce surface adsorption or adding a secondary external surface to minimize re-entering of the micropores.

Broadening the Scope for Fluoride-Free Synthesis of Siliceous Zeolites.

Siliceous zeolites are ideally suited for emerging applications in gas separations, sensors, and the next generation of low-k dielectric materials, but the use of fluoride in the synthesis significantly hinders their commercialization. Herein, we show that the dry gel conversion (DGC) technique can overcome this problem. Fluoride-free synthesis of two siliceous zeolites-AMH-4 (CHA-type) and AMH-5 (STT-type), has been achieved for the first time using the method. Siliceous *BEA-, MFI-, and *MRE-type zeolites have also been synthesized to obtain insights into the crystallization process. Charge-balancing interactions between the inorganic cation, organic structure-directing agent (OSDA), and Si-O- defects are found to be an essential aspect. We quantify this factor in terms of the "OSDA charge/silica ratio" of the as-made zeolites and demonstrate that the DGC technique is broadly applicable and opens up new avenues for fluoride-free siliceous zeolite synthesis.

Exfoliation of two-dimensional zeolites in liquid polybutadienes.

Layered zeolite precursors were successfully exfoliated by brief shearing or sonication with the assistance of commercially available telechelic liquid polybutadienes at room temperature. The exfoliated zeolite nanosheets can form a stable suspension in an organic solvent, providing exciting potential for the fabrication of zeolite membranes, composite materials and hierarchical zeolites.