Direct Dual‐Template Synthesis of MWW Zeolite Monolayers

Abstract

A two dimensional zeolite with the topology of MWW sheets has been obtained by direct synthesis with a combination of two organic structure directing agents. The resultant material is formed in about 70% by single and double layers with a well structured external surface area of about 300 m2·g-1. The delaminated zeolite prepared by means of this single step synthesis route presents not only a high delamination degree, but a well preserved structural integrity of the MWW layers. It presents an excellent activity, selectivity and catalyst life for alkylation of benzene with propylene, being better than catalysts used today for producing cumene.The importance of zeolites as adsorbents and heterogeneous catalysts is well recognized [1]. When used as catalysts, the microporous structure of these crystalline solids confers the reactants a confined environment that, besides pre-activating the molecules, may direct the reaction towards the formation of the desired products by stabilizing the reaction transition states. However, the narrow pore dimensions of most of the zeolites, in the range of 4 to 7 Å, limit their use to processes involving relatively small molecules. Many efforts have been directed to increase the accessibility to the active sites, and to reduce the diffusional problems of bulkier reactants while preserving a degree of confinement. Traditionally, the accessibility to the active sites has been increased by means of post-synthesis methods that combine acid and hydrothermal treatments, such as in ultra-stabilized Y based catalysts for FCC [2] or mordenite based catalysts for hydroisomerization [1,3], or by combining acid and basic treatments [4]. These top-down procedures are effective but enclose several steps and, depending on the conditions employed, may result in crystallinity and micropore volume loss. Addition of soft or hard templates to the synthesis gel has also been described as a possible route to obtain hierarchical zeolites presenting micro and mesoporosity [4b, 5]. Direct synthesis of single layers of ZSM-5 was achieved by using diquaternary ammonium surfactants and tetraalkylphosphonium as directing agents (OSDA) [6]. The use of these type of OSDAs has also allowed the synthesis of other micro-mesoporous zeolites, with structural mesoporosity [7] or intercrystalline mesoporosity due to small crystallite size [8].A different approach for increasing zeolite's accessibility was described in the middle 1990's, and consisted in the delamination of layered zeolite precursors. The concept was proved to be applicable to different structures, and new delaminated zeolites ITQ-2 [9], [10], were prepared starting from MCM-22(P), PRE-FER, NU-6(1), and ITQ-19, respectively [13]. The resultant zeolitic materials showed very high accessible surface areas and interesting catalytic properties for a large number of reactions [14]. The first of those delaminated zeolites described, ITQ-2, is formed by disordered MWW layers in a "house of cards" disposition. The layers, with a 2.5 nm thickness, pr...

Controlled direct synthesis of single- to multiple-layer MWW zeolite

“…Moreover, the uptake of nitrogen for SL-MWW 0.1/0.35 in the relative pressure ranges of 10 −6 < P/P 0 < 10 −3 is lower than that for MCM-22 (the inset of Fig. 1 (b)), indicating the absence of 12-MR supercages over the delaminated structure of SL-MWW 0.1/0.35 [ 15 , 18 , 19 ]. The external surface area and pore volume of SL-MWW 0.1/0.35 (446 m 2 /g and 1.21 cm 3 /g) are significantly increased compared to that of MCM-22 (Table 1 ).…”

“…With a similar strategy, the delaminated MWW zeolite nanosheets named MIT-1 are recently reported [ 18 ]. In addition, the delaminated MWW zeolite (DS-ITQ-2) containing a large proportion of MWW monolayers is directly synthesized by hexamethyleneimine (HMI)-assisted bifunctional SDAs [ 19 ]. Nevertheless, these specially designed SDAs requiring complicated synthetic procedure can only be used for a specific zeolite, which limits their potential industrial application [ 22–27 , 32 ].…”

New Trends in Layered Zeolites

“…This one-pot synthetic method using rational designed bifunctional SDA to prepare delaminated material avoids the conventional multistep process and also the amorphization of layer structure in the swelling treatment. Another directly synthesized delaminated MWW zeolite, DS-ITQ-2, was reported by Corma et al, under the co-directing effect of hexamethyleneimine (HMI) and a bifunctional SDA of N-hexadecyl-N′-methyl-DABCO (C 16 DC 1 ) [52]. HMI, the traditional template for the crystallization of layered MWW zeolite, was applied to direct the crystallization of MWW layers.…”

Rational Manipulation of Stacking Arrangements in Three‐Dimensional Zeolites Built from Two‐Dimensional Zeolitic Nanosheets

“…[1] As a sub-family of zeolitic materials, two-dimensional (2D) lamellar zeolites have gained much attention as promising materials in catalysis and separation fields due to the unit-cell-thin nanosheet thickness and flexible stacking feature. [2] In fundamental zeolite chemistry, zeolitic nanosheets provide a unique platform to construct zeolites by linking 2D building blocks in different patterns. [3] Various stacking modes and linkages between zeolitic nanosheets have expanded the types of zeolite structures with different pore entrances, [4] which intimately determine its size/shape selectivity in applications.…”