Microwave Encounters Ionic Liquid: Synergistic Mechanism, Synthesis and Emerging Applications

Progress in microwave (MW) energy application technology has stimulated remarkable advances in manufacturing and high-quality applications of ionic liquids (ILs) that are generally used as novel media in chemical engineering. This Review focuses on an emerging technology via the combination of MW energy and the usage of ILs, termed microwave-assisted ionic liquid (MAIL) technology. In comparison to conventional routes that rely on heat transfer through media, the contactless and unique MW heating exploits the electromagnetic wave-ions interactions to deliver energy to IL molecules, accelerating the process of material synthesis, catalytic reactions, and so on. In addition to the inherent advantages of ILs, including outstanding solubility, and well-tuned thermophysical properties, MAIL technology has exhibited great potential in process intensification to meet the requirement of efficient, economic chemical production. Here we start with an introduction to principles of MW heating, highlighting fundamental mechanisms of MW induced process intensification based on ILs. Next, the synergies of MW energy and ILs employed in materials synthesis, as well as their merits, are documented. The emerging applications of MAIL technologies are summarized in the next sections, involving tumor therapy, organic catalysis, separations, and bioconversions. Finally, the current challenges and future opportunities of this emerging technology are discussed.

Intergrowth Zeolites, Synthesis, Characterization, and Catalysis

Microporous zeolites that can act as heterogeneous catalysts have continued to attract a great deal of academic and industrial interest, but current progress in their synthesis and application is restricted to single-phase zeolites, severely underestimating the potential of intergrowth frameworks. Compared with single-phase zeolites, intergrowth zeolites possess unique properties, such as different diffusion pathways and molecular confinement, or special crystalline pore environments for binding metal active sites. This review first focuses on the structural features and synthetic details of all the intergrowth zeolites, especially providing some insightful discussion of several potential frameworks. Subsequently, characterization methods for intergrowth zeolites are introduced, and highlighting fundamental features of these crystals. Then, the applications of intergrowth zeolites in several of the most active areas of catalysis are presented, including selective catalytic reduction of NOx by ammonia (NH3-SCR), methanol to olefins (MTO), petrochemicals and refining, fine chemicals production, and biomass conversion on Beta, and the relationship between structure and catalytic activity was profiled from the perspective of intergrowth grain boundary structure. Finally, the synthesis, characterization, and catalysis of intergrowth zeolites are summarized in a comprehensive discussion, and a brief outlook on the current challenges and future directions of intergrowth zeolites is indicated.

Effect of linearly polarized microwaves on nanomorphology of calcium carbonate mineralization using peptides

Microwaves are used for diverse applications such as mobile phones, ovens, and therapy devices. However, there are few reports on the effects of microwaves on diseases other than cancer, and on physiological processes. Here, we focused on CaCO₃ mineralization as a model of biomineralization and attempted to elucidate the effect of microwaves on CaCO₃ mineralization using peptides. We conducted AFM, ζ potential, HPLC, ICP-AES, and relative permittivity measurements. Our findings show that microwaves alter the nanomorphology of the CaCO₃ precipitate, from sphere-like particles to string-like structures. Furthermore, microwaves have little effect on the mineralization when the mineralization ability of a peptide is high, but a large effect when the precipitation ability is low. Our findings may be applicable to not only the treatment of teeth and bones but also the development of organic-inorganic nanobiomaterials. This methodology can be expanded to other molecular/atomic reactions under various microwave conditions to alter reaction activity parameters.

Multidimensional Building Blocks for Molecular Sieve Membranes

Chemical separations aiming for high-purity commodities are critical to modern society. Compared to distillation, chemical absorption, and adsorption, membrane separation is attractive for its energy efficiency, ease of operation, and compact footprint. Molecular sieve membranes (MSMs) are broadly defined as membranes that are constructed from intrinsically and artificially porous materials. On the basis of our recent studies, this Account will first summarize the evolution of MSMs from the viewpoint of dimensionality of building blocks, which fundamentally determines the stacking architectures, intercrystalline gaps, and mass transfer channels of MSMs. Intergrowth of three-dimensional (3D) crystals as primary building blocks gives rise to classical MSMs. However, the poor connection between crystals inherent to those membranes results in intercrystalline gaps that are catastrophic for separation selectivity. We adopted a variety of strategies to close the crystal boundary gaps, including microwave synthesis, electrochemical-ionothermal synthesis, and modular integration. These efforts make us better understand the structure-performance relationship in membranes and create solutions for industrial processes. Excitingly, we first scaled-up the microwave synthesis of a Linde type A (LTA) zeolite membrane and built the world's largest ethanol dehydration membrane unit with an annual capacity of 100,000 tons. MSMs can also be made of two-dimensional (2D) nanosheets as primary building blocks. Those strike a balance between permeation rate and selectivity because the nanometer thickness ensures the minimization of the mass-transfer resistance of the membrane and the layer-by-layer stacking mode can significantly reduce the intercrystalline gaps. By publishing our first report on metal-organic framework (MOF) nanosheet membranes in Science, we committed to establishing top-down and bottom-up methods for assembly of laminae. Once the stacking, orientation, and connection between the layers are meticulously controlled, nanosheet building blocks with diversity open the door for ultrapermeable and selective MSMs. We recently proposed a supramolecule array membrane (SAM) with zero-dimensional (0D) molecules as primary building blocks, which has great potential to absolutely eliminate intercrystalline gaps in membranes. In contrast to the classical transport through nanopores of membranes, selective transport through the intermolecular spacing of supramolecules is creatively realized within the SAM, which marks a new breakthrough in ultraprecise sieving of molecules with tiny differences in size and revolutionizes MSMs in regard to stacking modes, intercrystalline gaps, and transport channels. MSMs have proven to be successful in diverse applications and have triggered wide interest. A unique perspective on the dimensionality evolution of building blocks will accelerate the progress of MSMs. The synergy of multidimensional MSMs will be a positive response to fundamental bottlenecks and industrial questions of membranes and will unlock the potential of membranes to displace the existing separation technologies in the future.

Hierarchy Control of MFI Zeolite Membrane towards Superior Butane Isomer Separation Performance

Microstructural optimization (such as thickness and preferred orientation) is a major concern for performance enhancement of zeolite membranes. In this study, we demonstrated that the introduction of hierarchy easily enabled concurrent thickness reduction and orientation control of zeolite membranes. Specifically, hierarchical MFI zeolite membranes comprising higher degree of (h0h) preferentially oriented ultrathin (ca. 390 nm) selective top layers and porous intermediate layers on porous α-Al₂ O₃ substrates were fabricated. The use of hollow-structured MFI nanoseeds and the employment of single-mode microwave heating during membrane processing were found indispensable for the preparation of MFI zeolite membranes with superior butane isomer separation performance, thereby surpassing the current n-/i-butane selectivity versus n-butane permeance trade-off limits of MFI zeolite membranes prepared via solution-based synthetic protocols.

Single-Mode Microwave Heating-Induced Concurrent Out-of-Plane Twin Growth Suppression and In-Plane Epitaxial Growth Promotion of b-Oriented MFI Film under Mild Reaction Conditions

In this study, single-mode microwave heating was applied in epitaxial growth of b-oriented MFI seed monolayer prepared by facile manual assembly, resulting in the formation of well-intergrown and highly b-oriented MFI film with few twins. It exhibited a precise molecular sieving property at a reaction temperature no higher than 100 °C within 2 hours, therefore making it possible for easy operation in an open environment. The capability for concurrent suppression of undesired out-of-plane twin growth and promotion of in-plane epitaxial growth rate under mild reaction conditions was attributed to the obvious superiority of single-mode microwave heating in comparison with conventional multi-mode microwave heating in aspects of microwave field uniformity and intensity. Our research indicated that the single-mode microwave heating technique could potentially be a useful tool for improving the microstructure and therefore the performance of diverse zeolite films.

Uniform hierarchical MFI nanosheets prepared via anisotropic etching for solution-based sub-100-nm-thick oriented MFI layer fabrication

Zeolite nanosheets have shown unprecedented opportunities for a wide range of applications, yet developing facile methods for fabrication of uniform zeolite nanosheets remains a great challenge. Here, a facile approach involving anisotropic etching with an aqueous solution of tetrapropylammonium hydroxide (TPAOH) was developed for preparing uniform high-aspect ratio hierarchical MFI nanosheets. In addition, the mechanism associated with the formation of MFI nanosheets was proposed. In the next step, a dynamic air-liquid interface-assisted self-assembly method and single-mode microwave heating were used for b-oriented MFI nanosheets monolayer deposition and controlled in-plane solution-based epitaxial growth, respectively, ensuring the formation of well-intergrown b-oriented MFI layers with sub-100-nm thickness. Moreover, our study indicated that b-oriented ultrathin MFI layers could be fabricated on diverse substrates demonstrating excellent anticorrosion capacity, ionic sieving properties, and n-/i-butane isomer separation performance.

In-Plane Epitaxial Growth of Highly c-Oriented NH2 -MIL-125(Ti) Membranes with Superior H2 /CO2 Selectivity

Preferred-orientation control has significant impact on the separation performance of MOF membranes. Under most conditions the preferred orientation of MOF membranes is dominated by the Van der Drift mechanism of evolutionary growth selection so that the obtained orientation may not be optimized for practical application. In this study, highly c-oriented NH₂ -MIL-125 membranes were prepared on porous α-alumina substrates by combining oriented seeding and controlled in-plane epitaxial growth. Dynamic air-liquid interface-assisted self-assembly of c-oriented NH₂ -MIL-125(Ti) seed monolayers, the use of layered TiS₂ as the metal precursor, and single-mode microwave heating were crucial in ensuring the preferred c-orientation while simultaneously suppressing undesired twin growth. Owing to reduced grain boundary defects, the prepared c-oriented membranes showed an ideal H₂ /CO₂ selectivity of 24.8, which was 6.1 times higher than that of their randomly oriented counterparts under similar operating conditions.

One-Pot Synthesis of High-Flux b-Oriented MFI Zeolite Membranes for Xe Recovery

We demonstrate that b-oriented MFI (Mobil Five) zeolite membranes can be manufactured by in situ crystallization using an intermediate amorphous SiO₂ layer. The improved in-plane growth by using a zeolite growth modifier leads to fusion of independent crystals and eliminates boundary gaps, giving good selectivity in the separation of CO₂/Xe mixtures. The fast diffusion of CO₂ dominates the overall membrane selectivity toward the CO₂/Xe mixture. Because of the straight and short [010] channels, the obtained CO₂ permeation fluxes are several orders of magnitude higher than those of carbon molecular sieving membranes and polymeric membranes, opening opportunities for Xe recovery from waste anesthetic gas.

Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite

Nanoscale crystal growth control is crucial for tailoring two-dimensional (2D) zeolites (crystallites with thickness less than two unit cells) and thicker zeolite nanosheets for applications in separation membranes and as hierarchical catalysts. However, methods to control zeolite crystal growth with nanometer precision are still in their infancy. Herein, we report solution-based growth conditions leading to anisotropic epitaxial growth of 2D zeolites with rates as low as few nanometers per day. Contributions from misoriented surface nucleation and rotational intergrowths are eliminated. Growth monitoring at the single-unit-cell level reveals novel nanoscale crystal-growth phenomena associated with the lateral size and surface curvature of 2D zeolites.

Submicrometer-thick b-oriented Fe–silicalite-1 membranes: microwave-assisted fabrication and pervaporation performances

The well-intergrown, purely b-oriented and ca. 1 μm-thick Fe–silicalite-1 membranes on porous α-Al₂O₃ supports possess superior pervaporation performances.

Oriented MFI Membranes by Gel-Less Secondary Growth of Sub-100 nm MFI-Nanosheet Seed Layers

A zeolite membrane fabrication process combining 2D-zeolite nanosheet seeding and gel-free secondary growth is described. This process produces selective molecular sieve films that are as thin as 100 nm and exhibit record high permeances for xylene- and butane-isomers.

Zeolite membranes – a review and comparison with MOFs

The latest developments in zeolite and MOF membranes are reviewed, with an emphasis on synthesis techniques. Industrial applications, hydrothermal stability, polymer-supported and mixed matrix membranes are some of the aspects discussed.