One-pot synthesis of MWW zeolite nanosheets using a rationally designed organic structure-directing agent (original) (raw)
Related papers
ACS Catalysis
Porous solids containing internal pores with sizes ranging from angstroms to nanometers are highly useful and valuable in the catalysis, separation, and storage of molecules because these materials provide large surface areas and void spaces for the interaction and adsorption of molecules. In particular, two-dimensional zeolites (2D, sometimes called layered zeolites) with layer thickness of 2−3 nm (1−2 unit cells) have enabled the synthesis of advanced materials and their application in catalysis for the transformation of bulky substrates unable to enter zeolite pores, thereby substantially increasing the number of zeolite applications and modifications. Accordingly, this Review aims to highlight recent developments in the synthesis, characterization, and application of 2D zeolites, focusing on the two most important representatives, MWW and MFI.
Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts
Nature, 2009
Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis 1-3 because strong acid sites within their uniform micropores enable size-and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations 3-5 that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion 4,5 . This has been realized by synthesizing zeolite nanocrystals 6 , by exfoliating layered zeolites 7-9 , and by introducing mesopores in the microporous material through templating strategies 10-17 or demetallation processes . But except for the exfoliation, none of these strategies has produced 'ultrathin' zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-togasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.
Lamellar MWW-Type Zeolites: Toward Elegant Nanoporous Materials
Applied Sciences
This article provides an overview of nanoporous materials with MWW (Mobil twenty two) topology. It covers aspects of the synthesis of the MWW precursor and the tridimensional zeolite MCM-22 (Mobil Composition of Matter number 22) as well as their physicochemical properties, such as the Si/Al molar ratio, acidity, and morphology. In addition, it discusses the use of directing agents (SDAs) to obtain the different MWW-type materials reported so far. The traditional post-synthesis modifications to obtain MWW-type materials with hierarchical architectures, such as expanded, swelling, pillaring, and delaminating structures, are shown together with recent routes to obtain materials with more open structures. New routes for the direct synthesis of MWW-type materials with hierarchical pore architecture are also covered.
Catalysis Today
Zeolite MCM-56 is a high alumina, monolayered form of the commercially useful framework MWW but it is a transient product during crystallization, so many factors influence its state and quality. This work examines properties of a series of MCM-56 and MCM-49 samples synthesized for different times using Aerosil, Ultrasil and Ludox as silica sources, hexamethyleneimine as the structure directing agent and additionally aniline as structure promoting agent. It was found that the most important parameter, governing the catalytic activity in the test reaction of Friedel-Crafts alkylation of mesitylene with benzyl alcohol, was availability of the Brønsted acid centers located at the external surfaces of the crystals. The key role in correlating physical characteristic with activity was played by infrared spectroscopy as it enabled the study of many properties of the tested materials, starting from the total concentration of acid centers, and their type (Brønsted or Lewis acids) through the concentration of centers available for the reagent molecules, to investigating the correlation of acidity with the degree of zeolite crystallinity.
Journal of Porous Materials
Layered MCM-22(P) was synthesized in the presence of hexamethonium (HM) cations. Compared to zeolite EU-1 (EUO-type structure), which crystallizes in similar conditions, its formation is favored at high HM concentrations (HM/SiO2 ≥ 0.3) and in the absence of Na2O. HM-containing MCM-22(P) was used as starting material for zeolite formation. Upon hydrothermal treatment, HM-MCM-22(P) transforms into zeolite EU-1 and upon calcination into a MWW-type zeolite. Transformation mechanisms were studied by standard characterization techniques such as XRD, SEM and TEM. Catalytic properties of the MWW-type zeolite obtained from this precursor were evaluated in a m-xylene isomerization reaction. Compared to zeolite MCM-22 prepared with hexamethyleneimine, a higher catalytic activity and an increased isomerization selectivity were observed and discussed.
Pillared MWW zeolites MCM-36 prepared by swelling MCM-22P in concentrated surfactant solutions
Catalysis Today, 2012
Lamellar zeolite forms like layered MCM-22 precursor, MCM-22P, offer unprecedented opportunities for creating diversity of more open zeolite structures prepared post-synthesis by expanding and modifying the interlamellar space. This is a relatively unexplored area with regard to procedures ensuring easy, least destructive and most efficient expansion of layered zeolites. Herein we explore concentrated surfactant solutions with high pH for their
Journal of Porous Materials
Recently reported groundbreaking discovery of efficient delamination of zeolite MCM-56, producing colloidal suspensions of MWW monolayers dispersed in the liquid phase, created unprecedented possibilities for the synthesis of a zeolite catalyst. Based on this innovation, the concept of using MWW monolayers to prepare silica-supported zeolite nanosheet catalysts suitable for transformations of large organic molecules was explored in this work. A series of silica-MWW preparations was synthesized from colloidal suspensions of the monolayers, using both solid and colloidal silica sources. The synthesized solids were thoroughly characterized with various physicochemical methods and their catalytic performance was tested in alkylation of mesitylene with benzyl alcohol. The obtained results indicate that solids containing MWW layers dispersed on silica show promising catalytic properties. The mixed MWW:silica catalysts synthesized from dispersions of MWW monolayers and liquid silica were f...
Chemistry - A European Journal, 2013
Dedicated to Professor Takashi Tatsumi on the occasion of his 65th birthday Introduction Zeolites have important applications in catalysis, adsorption, and ion-exchange processes as a result of their molecularsized micropores, which confer unique properties on these materials. [1] Over the past decade, various new zeolitic materials with either large (12-membered-ring, 12-R) or extralarge (> 12-R) micropores have been synthesized with the aid of bulky structure-directing agents (SDAs). [2-4] To date, over 200 different zeolite framework type codes (FTCs) [5] have been approved by the International Zeolite Association (IZA). The MCM-68 zeolite (FTC: MSE), first reported by researchers at Mobil Corporation in 2000, is a new type of three-dimensional zeolite with a 12 10 10-R channel system. [6] This zeolite has a characteristic structure in which a straight 12-R channel intersects two independent tortuous 10-R channels and, in addition, it possesses an 18-R 12-R supercage that is accessible only through 10-R channels. [7] These unique features of the MCM-68 zeolite have attracted attention because there are only a handful of acidic zeolites that contain three-dimensional channel systems with large pores. Zeolites of this type are known to exhibit unique acid-catalytic properties [8, 9] and are potentially useful as shape-selective catalysts for the alkylation of aromatics, [10-12] as well as for the production of propylene by naphtha cracking. [13] Their use as hydrocarbon traps has also been reported. [14] In addition, titanium-substituted MCM-68 has demonstrated performance superior to that of TS-1 ([Ti]-MFI) for the oxidation of phenol and olefins with H 2 O 2 as oxidant. [15] MCM-68 has been synthesized under hydrothermal conditions by using N,N,N',N'-tetraethyl-exo,exo-bicycloA C H T U N G T R E N N U N G [2.2.2]oct-7-ene-2,3:5,6-dipyrrolidinium diiodide (TEBOP 2+ (I À) 2) as the SDA. The gel composition window for the successful crystallization of pure MCM-68 is very narrow and the product is limited to a Si/Al molar ratio in the range of 9-12. We have previously overcome this limitation by employing the steam-assisted crystallization (SAC) method [16] to obtain a precursor of the pure silica version of the MSE topology (YNU-2P) or its stabilized microporous version (YNU-2). [17, 18] Even so, the current requirement for a crystallization period of 14 days or more during the synthesis of MCM-68 remains an important unresolved issue. Recently, however, researchers at the Universal Oil Products Company (UOP) [19] reported the synthesis of the UZM-35 zeolite, which has an identical topology to MSE but can be synthesized in only 5-9 days by using a processing temperature of 175 8C in conjunction with dimethyldipropylammonium hydroxide as the SDA. The synthesis of zeolites typically involves the conversion of an amorphous phase into a specific type of zeolite. In reality, though, the formation of a given zeolite proceeds through a process of gradual transformation. This sequential process begins with the amorphous phase, which undergoes a transition through a semi-stable form of the zeolite to the final stable zeolite product. [20, 21] This phenomenon suggests the possibility of an alternative synthetic strategy involving