Inorganic Structure-directing Agent Research Articles

Coupling Different Periodic Building Units for Intergrowth Zeolites.

Multiple-phase disordered zeolites, i.e., intergrowth zeolites, are important industrial catalysts, like single-phase ordered zeolites, but little is known about their rational synthesis and phase competition, mainly due to current poor understanding of the zeolite crystallization mechanism. Here, we theoretically demonstrated that sodalite and cancrinite cage layers, the periodic building units (PerBUs) of FAU/EMT and SBT/SBS structures, respectively, could be nondefectively connected to each other across double rings of 6 tetrahedral atoms when inverted and mirrored. We then synthesized an unprecedented family of FAU/SBT/SBS intergrowths with controllable FAU portions (named as the PST-34 family of intergrowth zeolites) using a multiple inorganic cation approach, providing clear experimental evidence for the layer-by-layer crystal growth mechanism of zeolites. This study shows that control of interactive cooperation extent between different inorganic structure-directing agents in the presence of an unselective organic structure-directing agent may enable repeated stacking of different but structurally related PerBUs in intergrowth zeolite synthesis.

Highly efficient synthesis of zeolite chabazite using cooperative hydration-mismatched inorganic structure-directing agents.

Chabazite (CHA type) zeolite is notoriously difficult to synthesize in the absence of organic structure-directing agents owing to long synthesis times and/or impurity formation. The ability to tailor organic-free syntheses of zeolites is additionally challenging due to the lack of molecular level understanding of zeolite nucleation and growth pathways, particularly the role of inorganic cations. In this study, we reveal that zeolite CHA can be synthesized using six different combinations of inorganic cations, including the first reported seed- and organic-free synthesis without the presence of potassium. We show that lithium, when present in small quantities, is an effective accelerant of CHA crystallization; and that ion pairings can markedly reduce synthesis times and temperatures, while expanding the design space of zeolite CHA formation in comparison to conventional methods utilizing potassium as the sole structure-directing agent. Herein, we posit the effects of cation pairings on zeolite CHA crystallization are related to their hydrated ionic radii. We also emphasize the broader implications for considering the solvated structure and cooperative role of inorganic cations in zeolite synthesis within the context of the reported findings for chabazite.

Synthesis of stable ECR-18 zeolite and its catalytic properties in methanol amination

ECR-18 is the synthetic analog of paulingite (framework type PAU), a naturally-occurring cage-based, small-pore zeolite. Because of its high framework Al content (Si/Al ∼ 3), this third generation of the RHO family of embedded isoreticular zeolites is structurally unstable, thereby hampering any catalytic application. Here we present the synthesis of ECR-18 with Si/Al ratios up to 4.4 using N-methyltropinium as an organic structure-directing agent and Na+ and/or K+ as inorganic structure-directing agents. The proton form of ECR-18 with Si/Al = 4.4 was found to retain the structural integrity. The catalytic properties of this ECR-18 for methylamines synthesis from ammonia and methanol have been compared with those of H-rho and H-PST-29, the two most selective catalysts for this reaction. Despite the similarity in acidity, H-ECR-18 showed a low sum yield of monomethylamine and dimethylamine compared to H-PST-29, mainly due to the lower ratio of scaffold (i.e., lta, pau, d8r, and t-plg) cages to embedded (i.e., t-oto, t-gsm, and t-phi) cages per unit cell of the former zeolite catalyst.

Synthesis of PHI-type zeolite harmotome using barium as an inorganic structure-directing agent inspired from natural mineral compositions

Zeolites are nanoporous aluminosilicates widely used as commercial adsorbents, heterogeneous catalysts, and ion-exchange materials due to their unique porosity, acidity, and thermal stability. The vast majority of synthetically-realized zeolite structures are prepared under hydrothermal conditions in alkaline media, and often in the presence of an organic structure-directing agent that facilitates the crystallization of diverse porous networks. Due to economic and environmental disadvantages of organic-based syntheses, it is often desirable to produce zeolites in organic-free media using alkali metal ions as the most commonly employed inorganic structure-directing agents (ISDAs). In this study, we were inspired by the elemental compositions of natural zeolite minerals to examine the use of an alkaline earth metal ion as the ISDA to prepare zeolites with the PHI framework type. Two natural minerals exhibiting this crystal structure are phillipsite and harmotome. Both structures contain sodium and potassium as extra-framework cations, but only harmotome contains barium cations. Here we examined how the combination of alkali and alkaline earth ISDAs promote the formation of PHI-type zeolites. Our findings reveal that each isostructure is prepared under different synthesis conditions where simply adding barium to a growth mixture does not always lead to harmotome formation. Harmotome is formed in more dilute and alkaline growth media; however, time-resolved analyses of zeolite crystallization reveal that harmotome can be synthesized at low temperatures (ca. 65 °C) with rates that are 5-times faster than syntheses of phillipsite. This seemingly indicates that barium reduces the energetic barrier for PHI-type zeolite nucleation in ways that are not fully understood. Moreover, harmotome crystals exhibit an average particle size of ca. 60 nm, which is nearly 40-fold smaller than phillipsite crystals prepared under similar conditions. Collectively, these findings identify routes to control crystallization of PHI-type zeolites and may provide a broader strategy for using the composition of natural zeolite minerals to optimize the preparation of synthetic analogues.

Metal cations as inorganic structure-directing agents during the synthesis of phillipsite and tobermorite

Metal cation identity determines the zeolite topology. Framework topology determines the total zeolite cationic content. Potassium predominantly counterbalances Al anions; sodium and calcium are predominantly structure-directing agents.

Synthesis and Crystal Growth Mechanism of PST-2: An Aluminosilicate SBS/SBT Zeolite Intergrowth.

The synthesis of PST-2, an aluminosilicate zeolite intergrowth of cage-based, large-pore SBS and SBT topologies, and its intergrowth characteristics are presented. With the Si/Al ratio and crystallization inorganic structure-directing agent in zeolite synthesis mixtures fixed to 8.0 and Cs+ ions, respectively, pure PST-2 is obtained at 120 °C using tetraalkylammonium ions with C/N+ ratios of 5-9 as a charge density mismatch (CDM) organic structure-directing agent (OSDA). More interestingly, the intergrowth ratio between SBS and SBT in PST-2 was found to vary notably not only with the type of CDM OSDA employed but also with the crystallization time, unlike the case of other well-known zeolite intergrowths such as β and MFI/MEL. When tetraethylammonium ions are used as a CDM OSDA at 100 °C in the presence of Cs+, the SBS portion in PST-2 decreases from over 60% to less than 45% with increasing crystallization time from 2.5 to 14 days, suggesting that SBS formation is kinetically more favorable than SBT formation. A thorough characterization of changes in the crystallite dimension of PST-2 with crystallization time, together with those in the chemical composition, allowed us to propose a plausible crystal growth mechanism of this large-pore zeolite intergrowth.

Evolution of Framework Al Arrangements in CHA Zeolites during Crystallization in the Presence of Organic and Inorganic Structure-Directing Agents

Evolution of Framework Al Arrangements in CHA Zeolites during Crystallization in the Presence of Organic and Inorganic Structure-Directing Agents

Crystallization of potassium-zeolites in organic-free media

A ubiquitous phenomenon in zeolite synthesis is the generation of polymorphs leading to either single or multi-phase products. The use of organic structure-directing agents (OSDAs) often ensures a high degree of phase purity for a desired zeolite; however, OSDAs can be economically and environmentally prohibitive. The use of alkali metals as inorganic structure-directing agents (ISDAs) is a common alternative, although reductions in polymorph specificity and/or purity in organic-free media are significant limitations. Developing more controlled, facile synthesis strategies without the aid of OSDAs relies on understanding how zeolite growth conditions impact polymorph selection and alter the physicochemical properties of crystalline products. Relatively few studies of organic-free syntheses have systematically explored potassium growth media. In this study, we examine synthesis conditions that promote the formation of five K-zeolites: CHA, LTL, MER, EDI, and BPH frameworks. Time-resolved analyses reveal distinct trends in crystallization kinetics and interzeolite transformations for potassium compared to prior studies that have largely focused on Na-zeolites. We also highlight methods to achieve nanosized crystals and markedly reduce crystallization times for zeolites, such as chabazite, which are notoriously difficult to synthesize in organic-free media at reasonable timescales. Collectively, these findings identify routes to control K-zeolite polymorphism and provide general design strategies to tailor their crystallization.

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

In the pursuit of controlling the propensity of Cu-mordenite (MOR) for the selective oxidation of CH4, we take a closer look at intrinsic zeolite parameters. Via synthesis design, we vary the relative proportion of Al situated near the 8-rings and 12-rings of MOR zeolite. This is accomplished using different Al sources impacting the local degree of silica dissolution and zeolite formation as evidenced by crystallization times and morphological differences. Interrogating the crystalline system with steric probe molecules in conjunction with spectroscopic techniques such as 1H magic angle spinning (MAS) NMR, infrared spectroscopy, as well as temperature-programmed desorption confirms discrete changes of the Al within the unit cell. The subsequent copper exchange allows for the generation of Cu-MOR materials of different inclinations for the activation of methane in the stepwise formation of MeOH. Here, an increasing degree of acid sites in more easily accessible locations (e.g., 12-ring) correlates with increasing maximum productivity toward MeOH at moderate exchange degrees. X-ray absorption spectroscopy supports this notion, finding a higher concentration of self-reduction-resistant framework-associated Cu2+ species, previously established as the active sites in the selective oxidation of CH4.

Manipulation of amorphous precursors to enhance zeolite nucleation.

Crystallization in media comprised of amorphous precursors is becoming a more common phenomenon for numerous synthetic, biological, and natural materials that grow by a combination of classical and nonclassical pathways. Amorphous phases can exhibit a wide range of physicochemical properties that may evolve during the course of nucleation and crystal growth. This creates challenges for establishing causal relationships between amorphous precursor properties and their effect(s) on the selection of mechanistic pathways of crystallization and ultimately the properties of the crystalline product. In this study, we examine ways to manipulate the composition and colloidal stability of amorphous (alumino)silicate precursors that are prevalent in nanoporous zeolite syntheses. Changes in the amorphous precursor properties are evaluated on the basis of their ability to enhance rates of crystal formation. Here, we use fumed silica as the primary silicon source and examine the effects of infusing the source or growth medium with additional alkali metal, which serves as an inorganic structure-directing agent to facilitate the formation of porous crystal structures. We also assess the impact of adding a polymer additive, which reduces the colloidal stability of precursors, wherein we posit that the confined pockets of solution within the interstitial spaces of the precursor aggregates play an important role in regulating the rate of zeolite crystallization. Three commercially relevant zeolites (mordenite, SSZ-13, and ZSM-5) were selected for this study based on their diverse frameworks and methods of preparation. Our findings reveal that alkali infusion significantly reduces the crystallization times for mordenite and SSZ-13, but has little impact on ZSM-5 synthesis. Conversely, we find that polymer addition markedly enhanced the rates of crystallization among all three zeolites, suggesting that this method may be a general approach to reduce zeolite synthesis times. Given the relatively high costs associated with commercial zeolite production, identifying new methods to improve the efficiency of hydrothermal syntheses can have significant practical implications beyond the fundamental benefits of developing new routes to tailor nonclassical crystallization.

Study of the Relationship between Synthesis Descriptors and the Type of Zeolite Phase Formed in ZSM‐43 Synthesis by Using Machine Learning

AbstractSmall‐pore zeolites possess pores that are constructed of eight tetrahedral atoms. ZSM‐43 is a small‐pore zeolite with two‐dimensional eight‐ring channels. The preparation of ZSM‐43 is influenced by the molar concentration of hydroxide ions, choline based organic structure‐directing agents (OSDA) and inorganic structure‐directing agents (ISDA) such as sodium, potassium and cesium. The synthetic conditions yield a range of products such as ZSM‐43, amorphous, UZM‐15, and other zeolites. There is a significant challenge in relating synthetic descriptors to their implications in zeolite phase formation. As a proof‐of‐concept study, we correlated the type of product formed with the gel molar composition in complex ZSM‐43 synthesis using machine learning algorithms. Seven different supervised machine learning algorithms have demonstrated an accuracy of >95 percent and an F1 score of >95 percent. The decision tree algorithm (DT) demonstrates the relationship between what type of phase is produced by what concentration of ISDA and hydroxide ions, as well as the effects of changing these parameters on phase transformation. DT provides structural and physicochemical insights into zeolite chemistry. From the experimental data of ZSM‐43, it is difficult to obtain detailed information regarding the chemistry of all phase formations. However, machine learning algorithms aid in recognizing hidden patterns in the data, facilitating deeper understanding of zeolite chemistry and phase transformations.

The Role of Organic and Inorganic Structure-Directing Agents in Selective Al Substitution of Zeolite

Organic and inorganic structure-directing agents (SDAs) impact Al distributions in zeolite, but the insights into how SDAs manipulate Al distribution have not been elucidated yet. Herein, the roles of different SDAs such as cyclohexylamine (CHA), hexamethylenimine (HMI), and Na+ in selective Al substitution of MCM-49 zeolite are investigated comprehensively by multinuclear solid-state NMR. The results demonstrate that MCM-49 synthesized with HMI shows relatively more T6 and T7 Al, while more T2 Al is observed using CHA. The formation of T2 Al in both MCM-49(HMI) and MCM-49(CHA) is derived from Na+, while protonated HMIs show bias in incorporation of T6 and T7 Al. Most HMIs are occluded in protonated status, and about half of CHAs are occluded in nonprotonated status. The close spatial proximity between nonprotonated CHAs and Na+ synergistically promotes the formation of zeolite structure, leading to more Na+ ions occluded in the zeolite channel with preferential T2 Al substitution.

Synthesis of thermally stable SBT and SBS/SBT intergrowth zeolites

UCSB-6 (framework type SBS) and UCSB-10 (SBT), two three-dimensional phosphate-based molecular sieves with supercages accessible through 12-ring (circumscribed by 12 tetrahedral atoms) windows, are structurally similar to the hexagonal and cubic polytypes of faujasite or zeolite Y, an industrially relevant catalyst, but the cage structures are substantially different. Nonetheless, their inherent thermal instability has precluded any catalytic application so far. By using multiple inorganic cation and charge density mismatch approaches, we synthesized PST-32 and PST-2, a thermally stable aluminosilicate version of UCSB-10 and the hypothetical SBS/SBT intergrowth family member, respectively. This study suggests that many hypothetical cage-based zeolite structures with multidimensional channel systems can be synthesized as compositionally robust forms by systematically exploring the synergy effect of inorganic and organic structure-directing agents.

Stabilizing large pores

Zeolites Zeolites with very large cages, such as zeolite Y, are rare but extremely useful in processing the large hydrocarbons in petroleum. Lee et al. report a route for preparing thermally stable forms of the large-cage phosphate-based molecular sieves UCSB-6 and UCSB-10 (see the Perspective by Xie). They used mainly sodium and cesium ions as inorganic structure-directing agents, along with an organic molecule, in their charge-density mismatch approach. These zeolites were used as diesel-cracking catalysts at 600°C. Science , abi7208, this issue p. [104][1]; see also abj1834, p. [28][2] [1]: /lookup/doi/10.1126/science.abi7208 [2]: /lookup/doi/10.1126/science.abj1834

Super-ions of sodium cations with hydrated hydroxide anions: inorganic structure-directing agents in zeolite synthesis.

In inorganic zeolite formation, a direct correspondence between liquid state species in the synthesis and the supramolecular decoration of the pores in the as-made final zeolite has never been reported. In this paper, a direct link between the sodium speciation in the synthesis mixture and the pore structure and content of the final zeolite is demonstrated in the example of hydroxysodalite. Super-ions with 4 sodium cations bound by mono- and bihydrated hydroxide are identified as structure-directing agents for the formation of this zeolite. This documentation of inorganic solution species acting as a templating agent in zeolite formation opens new horizons for zeolite synthesis by design.

In situ imaging of two-dimensional surface growth reveals the prevalence and role of defects in zeolite crystallization

Zeolite crystallization predominantly occurs by nonclassical pathways involving the attachment of complex (alumino)silicate precursors to crystal surfaces, yet recurrent images of fully crystalline materials with layered surfaces are evidence of classical growth by molecule attachment. Here we use in situ atomic force microscopy to monitor three distinct mechanisms of two-dimensional (2D) growth of zeolite A where we show that layer nucleation from surface defects is the most common pathway. Direct observation of defects was made possible by the identification of conditions promoting layered growth, which correlates to the use of sodium as an inorganic structure-directing agent, whereas its replacement with an organic results in a nonclassical mode of growth that obscures 2D layers and markedly slows the rate of crystallization. In situ measurements of layered growth reveal that undissolved silica nanoparticles in the synthesis medium can incorporate into advancing steps on crystal surfaces to generate defects (i.e., amorphous silica occlusions) that largely go undetected in literature. Nanoparticle occlusion in natural and synthetic crystals is a topic of wide-ranging interest owing to its relevance in fields spanning from biomineralization to the rational design of functional nanocomposites. In this study, we provide unprecedented insight into zeolite surface growth by molecule addition through time-resolved microscopy that directly captures the occlusion of silica nanoparticles and highlights the prevalent role of defects in zeolite crystallization.

Experimental and Theoretical Assessments of Aluminum Proximity in MFI Zeolites and Its Alteration by Organic and Inorganic Structure-Directing Agents

The molecular structure and cationic charge density of organic and inorganic structure-directing agents (SDAs) influence the siting and arrangement of Al substituted in zeolite frameworks. Yet, dev...

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.

Rediscovery of the Importance of Inorganic Synthesis Parameters in the Search for New Zeolites.

Zeolites and related crystalline microporous materials with cavities and channels of molecular dimensions are of major importance for applications ranging from ion-exchange to adsorption and to catalysis. Because their unique shape-selective properties are closely related to the size, shape, and dimensionality of the intracrystalline channels and cavities, much interest has been devoted to the discovery of novel zeolitic materials over the last several decades. As a result, a dramatic expansion in the structural domain of crystalline microporous materials, as well as in their compositional range, has been achieved. This is largely due to the development of innovative synthetic strategies, for example, organic structure-directing agent (OSDA) design, introduction of heteroatoms like Ge in OSDA-mediated zeolite synthesis, topotactic transformation of two-dimensional layered zeolite precursors, assembly-disassembly-organization-reassembly method, etc. However, although many of these methodologies are quite successful in finding unprecedented zeolite structures, the resulting materials tend to be (hydro)thermally unstable and are often commercially impractical from a manufacturing perspective because of the high cost of the OSDA and/or heteroatom employed. Therefore, we focused on inorganic synthesis parameters as the key phase selectivity factor that has received relatively little attention in the search for new industrially relevant zeolites. This Account describes our recent efforts to find previously undiscovered aluminosilicate zeolites by boosting the roles of inorganic structure-directing agents (ISDAs) in the presence of OSDAs. They include the multiple inorganic cation and excess fluoride approaches, which aim to promote a synergistic cooperation between ISDAs and/or OSDAs and thus to hold a rational design concept, although the latter is not friendly to the practical zeolite manufacturing process due to the toxicity of fluoride. Using these two approaches, we were able to synthesize not only the second generation (PST-29) and four higher generations (PST-20 (RHO-G5), PST-25 (RHO-G6), PST-26 (RHO-G7), and PST-28 (RHO-G8)) of the RHO family of embedded isoreticular zeolites but also three other novel zeolite structures (EU-12, PST-21, and PST-22). We also explored the synthesis of a number of heteroatom-containing aluminophosphate (AlPO4) molecular sieves with different framework structures and unusually high framework charge density through the cooperative structure direction of alkali metal and small OSDA cations or under wholly inorganic conditions. Although we need to clarify the nature and extent of interactions between the inorganic cations and framework components in synthesis mixtures, we believe that our synthetic concepts, shedding new light on the importance of inorganic synthesis parameters, will open a door for achieving many other novel zeolite structures and compositions.

Organic-Free Synthesis of Silicoaluminophosphate Molecular Sieves.

Silicoaluminophosphate (SAPO) molecular sieves are an important class of microporous materials and are useful for industrial catalysis and separations. They have been synthesized exclusively by the use of expensive and environmentally unfriendly organic structure-directing agents. Now the synthesis of SAPO molecular sieves is reported with MER, EDI, GIS, and ANA topologies under wholly inorganic conditions. Multinuclear MAS NMR analyses demonstrate the presence of Si, Al, and P atoms in their frameworks. These SAPO materials all have unusually high framework charge densities (0.25-0.46), owing to the small size of alkali metal cations used as an inorganic structure-directing agent. A continuous Si increase in the synthesis gel for MER-type SAPO molecular sieves led to the formation of framework Si(0Al) units, decreasing the number of extra-framework cations per unit cell and thus making the resulting solid useful for CO2 adsorption.