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.

Separation of High-Purity C2H2 from Binary C2H2/CO2 Using Robust Al-Based MOFs Comprising Nitrogen-Containing Heterocyclic Dicarboxylate.

In this study, three nitrogen-containing aluminum-based metal-organic frameworks (Al-MOFs), namely, CAU-10pydc, MOF-303, and KMF-1, were investigated for the efficient separation of a C2H2/CO2 gas mixture. Among these three Al-MOFs, KMF-1 demonstrated the highest selectivity for C2H2/CO2 separation (6.31), primarily owing to its superior C2H2 uptake (7.90 mmol g-1) and lower CO2 uptake (2.82 mmol g-1) compared to that of the other two Al-MOFs. Dynamic breakthrough experiments, using an equimolar binary C2H2/CO2 gas mixture, demonstrated that KMF-1 achieved the highest separation performance. It yielded 3.42 mmol g-1 of high-purity C2H2 (>99.95%) through a straightforward desorption process under He purging at 298 K and 1 bar. To gain insights into the distinctive characteristics of the pore surfaces of structurally similar CAU-10pydc and KMF-1, we conducted computational simulations using canonical Monte Carlo and dispersion-corrected density functional theory methods. These simulations revealed that the secondary amine (C2N-H) groups in KMF-1 played a more significant role in differentiating between C2H2 and CO2 compared to that of the N atoms in CAU-10pydc and MOF-303. Consequently, KMF-1 emerged as a promising adsorbent for the separation of high-purity C2H2 from binary C2H2/CO2 gas mixtures.

Design of Pore Properties of an Al-Based Metal-Organic Framework for the Separation of an Ethane/Ethylene Gas Mixture via Ethane-Selective Adsorption.

A series of Al-based isomorphs (CAU-10H, MIL-160, KMF-1, and CAU-10pydc) were synthesized using isophthalic acid (ipa), 2,5-furandicarboxylic acid (fdc), 2,5-pyrrole dicarboxylic acid (pyrdc), and 3,5-pyridinedicarboxylic acid (pydc), respectively. These isomorphs were systematically investigated to identify the best adsorbent for effectively separating C2H6/C2H4. All CAU-10 isomorphs exhibited preferential adsorption of C2H6 over that of C2H4 in mixture. CAU-10pydc exhibited the best C2H6/C2H4 selectivity (1.68) and the highest C2H6 uptake (3.97 mmol g-1) at 298 K and 1 bar. In the breakthrough experiment using CAU-10pydc, 1/1 (v/v) and 1/15 (v/v) C2H6/C2H4 gas mixtures were successfully separated into high-purity C2H4 (>99.95%), with remarkable productivities of 14.0 LSTP kg-1 and 32.0 LSTP kg-1, respectively, at 298 K. Molecular simulations revealed that the exceptional separation performance of CAU-10pydc originated from the increased porosity and reduced electron density of the pyridine ring of pydc, leading to a relatively larger decrease in π-π interactions with C2H4 than in the C-H···π interactions with C2H6. This study demonstrates that the pore size and geometry of the CAU-10 platform are modulated by the inclusion of heteroatom-containing benzene dicarboxylate or heterocyclic rings of dicarboxylate-based organic linkers, thereby fine-tuning the C2H6/C2H4 separation ability. CAU-10pydc was determined to be an optimum adsorbent for this challenging separation.

Tuning Hydrophilicity of Aluminum MOFs by a Mixed-Linker Strategy for Enhanced Performance in Water Adsorption-Driven Heat Allocation Application.

Water adsorption-driven heat transfer (AHT) technology has emerged as a promising solution to address crisis of the global energy consumption and environmental pollution of current heating and cooling processes. Hydrophilicity of water adsorbents plays a decisive role in these applications. This work reports an easy, green, and inexpensive approach to tuning the hydrophilicity of metal-organic frameworks (MOFs) by incorporating mixed linkers, isophthalic acid (IPA), and 3,5-pyridinedicarboxylic acid (PYDC), with various ratios in a series of Al-xIPA-(100-x)PYDC (x: feeding ratio of IPA) MOFs. The designed mixed-linkers MOFs show a variation of hydrophilicity along the fraction of the linkers. Representative compounds with a proportional mixed linker ratio denoted as KMF-2, exhibit an S-shaped isotherm, an excellent coefficient of performance of 0.75 (cooling) and 1.66 (heating) achieved with low driving temperature below 70 °C which offers capability to employ solar or industrial waste heat, remarkable volumetric specific energy capacity (235 kWh m-3 ) and heat-storage capacity (330 kWh m-3 ). The superiority of KMF-2 to IPA or PYDC-containing single-linker MOFs (CAU-10-H and CAU-10pydc, respectively) and most of benchmark adsorbents illustrate the effectiveness of the mixed-linker strategy to design AHT adsorbents with promising performance.

An Amine-Functionalized Ultramicroporous Metal-Organic Framework for Postcombustion CO2 Capture.

Among the most promising methods by which to capture CO2 from flue gas, the emission of which has accelerated global warming, is energy-efficient physisorption using metal-organic framework (MOF) adsorbents. Here, we present a novel cuprous-based ultramicroporous MOF, Cu(adci)-2 (adci- = 2-amino-4,5-dicyanoimidazolate), which was rationally synthesized by combining two strategies to design MOF physisorbents for enhanced CO2 capturing, i.e., aromatic amine functionalization and the introduction of ultramicroporosity (pore size <7 Å). Synchrotron powder X-ray diffraction and a Rietveld analysis reveal that the Cu(adci)-2 structure has one-dimensional square-shaped channels, in each of which all affiliated ligands, specifically NH2 groups at the 2-position of the imidazolate ring, have the same orientation, with a pair of NH2 groups therefore facing each other on opposite sides of the channel walls. While Cu(adci)-2 exhibits a high CO2 adsorption capacity (2.01 mmol g-1 at 298 K and 15 kPa) but a low zero-coverage isosteric heat of adsorption (27.5 kJ mol-1), breakthrough experiments under dry and 60% relative humidity conditions show that its CO2 capture ability is retained even in the presence of high amounts of moisture. In a Monte Carlo simulation and a radial distribution analysis, the preferential CO2 binding site of Cu(adci)-2 was predicted to be between two ligands, forming a sandwich-like structure and implying that its CO2 adsorption properties originate from the enhancement of Lewis base-acid and London dispersion interactions due to the amino groups and ultramicroporosity, respectively.

Synthesis and structure of the medium-pore zeolite PST-35 with two interconnected cages of unusual orthorhombic shape.

The search for new zeolite structures and compositions remains important in synthetic materials science due to the high impact on developing new chemical technologies, as well as on improving existing ones. Herein we present the synthesis and structure of PST-35, a novel medium-pore germanosilicate (Si/Ge = 2.1-6.6) zeolite, achieved by combining the excess fluoride approach and the unique structure directing ability of Ge in the presence of 1,2,3-triethylimidazolium ions as an organic structure-directing agent. PST-35 contains a zig-zag 10-ring (4.6 × 6.7 Å) channel system constructed of strictly alternating large 28-hedral ([48·58·68·82·102]) and smaller 18-hedral ([46·54·64·82·102]) cages of anomalous orthorhombic shape. The PST-35 structure is built from the connection of pst-35 layers consisting of small 8-hedral ([43·54·6]) cages, previously unobserved zeolite building layers, through single 4-rings.

High-Performance Adsorbent for Ethane/Ethylene Separation Selected through the Computational Screening of Aluminum-Based Metal-Organic Frameworks.

The development of a high-performance ethane (C2H6)-selective adsorbent for the separation of ethane/ethylene (C2H6/C2H4) gas mixtures has been investigated for high-efficiency adsorption-based gas separation. Herein, we investigated Al-based metal-organic frameworks (MOFs) to identify an efficient C2H6-selective adsorbent (CAU-11), supported by a computational simulation study. CAU-11 exhibited numerous advantageous properties (such as low material cost, structural robustness, high reaction yield, and high C2H6/C2H4 selectivity) compared to other Al-based MOFs, indicating immense potential as a C2H6-selective adsorbent. CAU-11 exhibited preferential C2H6 adsorption in single-component gas adsorption experiments, and its predicted ideal adsorption solution theory selectivity of C2H6/C2H4 was over 2.1, consistent with the simulation analysis. Dynamic breakthrough experiments using representative compositions of the C2H6/C2H4 gas mixture confirmed the excellent separation ability of CAU-11; it produced high-purity C2H4 (>99.95%) with productivity values of 0.79 and 2.02 mol L-1 while repeating the cyclic experiment with 1:1 and 1:15 v/v C2H6/C2H4 gas mixtures, respectively, at 298 K and 1 bar. The high C2H6/C2H4 separation ability of CAU-11 could be attributed to its non-polar pore environment and optimum pore dimensions which strengthen the interaction of its pores (via C-H···π interactions) with C2H6 to a greater extent than with C2H4.

An Aluminosilicate Zeolite Containing Rings of Tetrahedral Atoms with All Odd Numbers from Five to Eleven.

Herein we report the synthesis, structure solution, and catalytic properties of PST-31, which has an unprecedented framework topology. This high-silica (Si/Al=16) zeolite was synthesized using a pyrazolium-based dication with a tetramethylene linker as an organic structure-directing agent (OSDA) in hydroxide media. The PST-31 structure is built from new building layers containing four-, five-, six-, and seven-membered rings, which are connected by single four-membered rings in the interlayer region to form a two-dimensional pore system. Its channels consist of [4.56 .6.9.11] and [5.6.7.9.10.11] cavities and are thus delimited by nine-, ten-, and eleven-membered rings. The OSDA cations in as-synthesized PST-31 were determined to reside without disorder in the large [42 .514 .64 .72 .94 ] cavities composed of smaller [4.56 .6.9.11] and [5.6.7.9.10.11] ones, leading to a symmetry coincidence between the OSDA and the surrounding zeolite cavity. The proton form of PST-31 was found to be selective for the cracking of n-hexane to light olefins.

The Origin of Selective Adsorption of CO2 on Merlinoite Zeolites.

The CO2 adsorption behavior at 25-75 °C and 0-1.0 bar of various alkali cation-exchanged forms of merlinoite (framework type MER) zeolites with Si/Al=2.3 and 3.8 is described. The adsorption isotherms at 25 °C on the Na+ , K+ , Rb+ , and Cs+ forms of MER zeolite with Si/Al=2.3 are characterized by a clear step, the CO2 pressure of which differs notably according to the type of their extraframework cations. Structural analysis shows that CO2 adsorption on the former three zeolites includes the relocation of gating cations with high site occupancy and the remarkable concomitant structural breathing. We define this unusual adsorption phenomenon as a cooperative cation gating-breathing mechanism. The overall results suggest that the actual mechanism of selective CO2 adsorption on intermediate-silica small-pore zeolites can change from cation gating to cooperative cation gating-breathing to breathing, depending on a combination of their topological and compositional flexibilities.

PST-24: A Zeolite with Varying Intracrystalline Channel Dimensionality.

Herein we report the synthesis, structure solution, and catalytic properties of PST-24, a novel channel-based medium-pore zeolite. This zeolite was synthesized via the excess fluoride approach. Electron diffraction shows that its structure is built by composite cas-zigzag (cas-zz) building chains, which are connected by double 5-ring (d5r) columns. While the cas-zz building chains are ordered in the PST-24 framework, the d5r columns adopt one of two possible arrangements; the two adjacent d5r columns are either at the same height or at different heights, denoted arrangements S and D, which can be regarded as open and closed valves that connect the channels, respectively. A framework with arrangement D only has a 2D 10-ring channel system, whereas that with arrangement S only contains 3D channels. In actual PST-24 crystals, the open and closed valves are almost randomly dispersed to yield a zeolite framework where the channel dimensionality varies locally from 2D to 3D.

Targeted Synthesis of a Zeolite with Pre-established Framework Topology.

Given their great potential as new industrial catalysts and adsorbents, the search for new zeolite structures is of major importance in nanoporous materials chemistry. However, although innumerable theoretical frameworks have been proposed, none of them have been synthesized by a priori design yet. We generated a library of diazolium-based cations inspired from the organic structure-directing agents (OSDAs) recently reported to give two structurally related zeolites (PST-21 and PST-22) under highly concentrated, excess-fluoride conditions and compared the stabilization energies of each OSDA cation in ten pre-established hypothetical structures. A combination of the ability of the OSDA selected in this way with the excess-fluoride approach has allowed us to crystallize PST-30, the targeted aluminosilicate zeolite structure. We anticipate that our approach, which aims to rationally couple computational predictions of OSDAs with an experimental setup, will advance further development in the synthesis of zeolites with desired properties.

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.

An open-framework silicogermanate regularly constructed from natrolite zeolite chains and Ge9O18(OH)4 clusters.

Here we describe the synthesis and structure of PST-18, a novel open-framework silicogermanate with Si/Ge ∼ 0.6, which contains a three-dimensional pore system consisting of large cuboid-shaped cavities with 8-ring windows, as well as with 7-rings interrupted by one OH group. Synchrotron single-crystal X-ray diffraction reveals that the structure of PST-18, synthesized using only tetramethylammonium fluoride as a structure-directing agent, is built up of natrolite zeolite chains and Ge9O18(OH)4 clusters in a fully ordered manner. The discovery of such a hybrid structure of zeolitic building units and non-zeolitic oxide clusters provides a new direction for expanding the structural regime of inorganic microporous crystalline materials.

Two Aluminophosphate Molecular Sieves Built from Pairs of Enantiomeric Structural Building Units.

Herein we report the synthesis and structures of two new small-pore aluminophosphate molecular sieves PST-13 and PST-14 with mutually connected 8-ring channels. The structure of PST-13, synthesized using diethylamine as an organic structure-directing agent, contains penta-coordinated framework Al atoms bridged by hydroxy groups and thus edge-sharing 3- and 5-rings. Upon calcination, PST-13 undergoes a transformation to PST-14 with loss of bridging hydroxy groups and occluded organic species. The structures of both materials consist "nonjointly" of pairs of previously undiscovered 1,5- and 1,6-open double 4-rings (d4rs) which are mirror images of each other. We also present a series of novel chemically feasible hypothetical structures built from 1-open d4r (sti) or 1,3-open d4r (nsc) units, as well as from these two enantiomeric structural building units.

A Zeolite Family Nonjointly Built from the 1,3-Stellated Cubic Building Unit.

From a technological point of view, the synthesis of new high-silica zeolites is of prime importance owing to their high potential as industrial catalysts and catalyst supports. Two such materials have been synthesized which are made up of the 1,3-stellated cubic unit (hexahedral ([42 54 ]) bre unit) as a secondary building unit, with the aid of existing imidazolium-based structure-directing agents under "excess fluoride" conditions. One of them, denoted PST-21, is the first aluminosilicate zeolite consisting of 9-ring apertures solely; it displays exceptional activity towards steering the skeletal isomerization of 1-butene to isobutene and bridges the gap between small- and medium-pore structures. A series of hypothetical structures are also described that are nonjointly built from the bre unit; all of these structures are chemically feasible and will thus be helpful in designing the synthesis of novel zeolites containing 9-ring and/or 10-ring channels.

Charge distribution and conformational stability effects of organic structure-directing agents on zeolite synthesis.

While the use of the so-called organic structure-directing agents (OSDAs) is no stranger to zeolite synthesis at all, the effects of their charge distribution and conformational stability on the phase selectivity of the crystallization remain elusive. To systematically investigate these effects, we have prepared a series of imidazolium and pyrazolium derivatives with the same geometric shape but different positions of nitrogen atoms in the five-membered heterocyclic aromatic ring and used them as OSDAs for the synthesis of zeolites with pure-silica composition. Meaningful differences in the zeolite product were observed.

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.

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.