Crystalline mesoporous molecular sieves have long been sought as solid acid catalysts for organic reactions involving large molecules. We synthesized a series of mesoporous molecular sieves that possess crystalline microporous walls with zeolitelike frameworks, extending the application of zeolites to the mesoporous range of 2 to 50 nanometers. Hexagonally ordered or disordered mesopores are generated by surfactant aggregates, whereas multiple cationic moieties in the surfactant head groups direct the crystallization of microporous aluminosilicate frameworks. The wall thicknesses, framework topologies, and mesopore sizes can be controlled with different surfactants. The molecular sieves are highly active as catalysts for various acid-catalyzed reactions of bulky molecular substrates, compared with conventional zeolites and ordered mesoporous amorphous materials.

Three-dimensional graphene architectures with periodic nanopores—reminiscent of zeolite frameworks—are of topical interest because of the possibility of combining the characteristics of graphene with a three-dimensional porous structure. Lately, the synthesis of such carbons has been approached by using zeolites as templates and small hydrocarbon molecules that can enter the narrow pore apertures. However, pyrolytic carbonization of the hydrocarbons (a necessary step in generating pure carbon) requires high temperatures and results in non-selective carbon deposition outside the pores. Here, we demonstrate that lanthanum ions embedded in zeolite pores can lower the temperature required for the carbonization of ethylene or acetylene. In this way, a graphene-like carbon structure can be selectively formed inside the zeolite template, without carbon being deposited at the external surfaces. X-ray diffraction data from zeolite single crystals after carbonization indicate that electron densities corresponding to carbon atoms are generated along the walls of the zeolite pores. After the zeolite template is removed, the carbon framework exhibits an electrical conductivity that is two orders of magnitude higher than that of amorphous mesoporous carbon. Lanthanum catalysis allows a carbon framework to form in zeolite pores with diameters of less than 1 nanometre; as such, microporous carbon nanostructures can be reproduced with various topologies corresponding to different zeolite pore sizes and shapes. We demonstrate carbon synthesis for large-pore zeolites (FAU, EMT and beta), a one-dimensional medium-pore zeolite (LTL), and even small-pore zeolites (MFI and LTA). The catalytic effect is a common feature of lanthanum, yttrium and calcium, which are all carbide-forming metal elements. We also show that the synthesis can be readily scaled up, which will be important for practical applications such as the production of lithium-ion batteries and zeolite-like catalyst supports.

MFI zeolite nanosheets with a single-unit-cell crystal thickness were syn-thesized under hydrothermal synthesis conditions using diquaternary ammonium surfactants as the zeolite structure-generating agent. The zeolite structure generation process was analyzed from X-ray powder diffraction, N2 adsorption isotherms, scanning electron micrographs, and transmission electron micrographs of the porous materials generated at various reaction times. This analysis indicated that the nanosheets were initially generated as a disordered assembly that transformed into an ordered multilamellar mesostructure through a dissolution−recrystallization process upon prolonged hydrothermal aging. Synthesis factors affecting the rates of the initial generation of the nanosheets and their restructuring process were investigated while varying the temperature, structure of the surfactant tail, synthesis compositions, and basicity. On the basis of the result, it was possible to synthesize MFI zeolite nanosheets as a fully disordered assembly possessing a large mesopore volume even under synthesis conditions using the bromide form of the surfactant and a sodium-containing silica source such as water glass.

MFI zeolite nanosheets tailored to 2.5-nm thickness were synthesized using a surfactant-type zeolite structure-directing agent, [C22H45–N+(CH3)2–C6H12–N+(CH3)2–C6H13](Br–)2. The zeolite nanosheets possessed Brønsted acid sites on their external surfaces as well as in the internal micropore walls. The acid strength and concentration was characterized by the 31P NMR signals of the adsorbed trimethylphosphine oxide and tributylphosphine oxide. The 31P NMR investigation identified three types of Brønsted acid sites with different strengths on external surfaces; there were four types inside the micropores. A linear correlation has been established between the number of the external strongest acid sites and the catalytic activity in decalin cracking for the MFI zeolite catalysts investigated in this work.