**2.1 Synthesis of layered zeolites with bifunctional amphiphilic SDAs**

Compared to the continuous expanding in the 3D directions for typical zeolite frameworks, the layered structures only spread in 2D directions, with the growth in the third direction being interrupted. Considering this unique structural property of layered zeolites, Ryoo et al. proposed a novel strategy to synthesize layered zeolites using specially designed bifunctional amphiphilic SDAs with the hydrophilic diquaternary ammonium head to direct the crystallization of intralayer structures and on the other hand the hydrophobic long alkyl chain to prevent the continuous growth in the direction vertical to the layers [3, 41]. MFI-layered zeolite, hardly obtained in the traditional hydrothermal synthesis, was firstly reported using this method with the surfactant of C22H45▬N<sup>+</sup> (CH3)2▬C6H12▬N<sup>+</sup> (CH3)2▬C6H13. A large amount of Na+ in the synthetic gel favored the alternating stacking of 2-nmthick MFI nanosheets and 2.8-nm-thick surfactant micelles, producing multilamellar MFI with an overall thickness of 30–40 nm (**Figure 2A–D**). However, reducing the Na<sup>+</sup> resulted in the formation of unilamellar MFI nanosheets (**Figure 2E**). Both of the two-layered MFI zeolites exhibited a significantly higher surface area than that of the traditional 3D MFI zeolite, due to the formation of mesopores upon calcination. The layered MFI zeolites showed longer lifetime in the methanol-togasoline reaction and higher catalytic activity in the reactions involving large-size molecules, due to the simultaneous presence of mesopores and micropores. Similar superior catalytic results were reported by comparing the performance of layered TS-1 zeolite to that of bulk TS-1 in the epoxidation reactions [42]. The as-synthesized multilamellar MFI zeolite has also been applied as an acid-base bifunctional catalyst in the Knoevenagel condensation reactions. The acid site was derived from the Al-related Brönsted acidity, while the ammonium group located in the pore mouth served as a base site [43].

The structure of the bifunctional amphiphilic SDAs was then proved to be critical for the synthesis of layered MFI zeolite [44]. Too small space between the two ammonium groups would result in bulk MFI, while too large space leads to the disordered stacking of MFI nanosheets. Tuning the number of ammonium group is effective in controlling the thickness of MFI nanosheets. The hydrophobic alkyl chain should be long enough to form a micellar structure. Once two long alkyl chains were present in the bifunctional SDA, hexagonal mesostructure was formed with the pore wall characterized with crystalline MFI structure having the thickness of 1.7 nm [45]. By introducing aromatic groups into the hydrophobic chain of bifunctional SDA molecules, Che et al. have synthesized several lamellar MFI zeolites with different mesostructures, benefiting from the strong *π-π* stacking and the geometrical match between aromatic groups and MFI zeolitic frameworks [46–48]. This strategy of using bifunctional SDAs has been successfully extended to

**Figure 2.**

*SEM (A) and TEM (B) images of layered MFI zeolite. The scheme for the single layer MFI nanosheet (C) and the structural model for multilamellar MFI (D) and unilamellar MFI (E).*

**79**

**Figure 3.**

*New Trends in Layered Zeolites*

**from germanosilicates**

*DOI: http://dx.doi.org/10.5772/intechopen.86696*

which have never been synthesized as 2D zeolites before.

material of ITQ-2 in the liquid phase alkylation of benzene with propene.

*Scheme description of one-pot synthesis of MIT-1 with delaminated MWW structure.*

**2.2 Synthesis of layered zeolites by selective removal of double four rings** 

Germanosilicates, with novel topologies and large-pore channels, have shown their great potential in the catalytic reactions involving bulky substrates. Ge atoms favored the formation of double four ring (D4R) and double three ring (D3R) when building the zeolite frameworks together with silica atoms, due to their longer

the synthesis of nanosheets with MTW [3], \*MRE [49], and AlPO [50] frameworks,

For the well-known MWW-layered zeolite, a bifunctional SDA with the head group resembling the SDA for conventional MWW-layered zeolite and a long alkyl chain connected by diquaternary ammonium linker directed the crystallization of MIT-1 composed of MWW nanosheets with a house-of-cards arrangement (**Figure 3**), which was similar to the delaminated MWW material [51]. MIT-1 exhibited high mesoporosity with the external surface area higher than that of MCM-56 but lower than ITQ-2. 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 (C16DC1) [52]. HMI, the traditional template for the crystallization of layered MWW zeolite, was applied to direct the crystallization of MWW layers. For the bifunctional SDA, the hydrophilic head is located in the pocket of MWW layers, while the long hydrophobic chain prevents the ordering of stacking along *c* axis. The obtained DS-ITQ-2 showed a comparable catalytic activity as the conventional delaminated

## *New Trends in Layered Zeolites DOI: http://dx.doi.org/10.5772/intechopen.86696*

*Zeolites - New Challenges*

A large amount of Na+

mouth served as a base site [43].

the Na<sup>+</sup>

method with the surfactant of C22H45▬N<sup>+</sup>

the third direction being interrupted. Considering this unique structural property of layered zeolites, Ryoo et al. proposed a novel strategy to synthesize layered zeolites using specially designed bifunctional amphiphilic SDAs with the hydrophilic diquaternary ammonium head to direct the crystallization of intralayer structures and on the other hand the hydrophobic long alkyl chain to prevent the continuous growth in the direction vertical to the layers [3, 41]. MFI-layered zeolite, hardly obtained in the traditional hydrothermal synthesis, was firstly reported using this

thick MFI nanosheets and 2.8-nm-thick surfactant micelles, producing multilamellar MFI with an overall thickness of 30–40 nm (**Figure 2A–D**). However, reducing

of the two-layered MFI zeolites exhibited a significantly higher surface area than that of the traditional 3D MFI zeolite, due to the formation of mesopores upon calcination. The layered MFI zeolites showed longer lifetime in the methanol-togasoline reaction and higher catalytic activity in the reactions involving large-size molecules, due to the simultaneous presence of mesopores and micropores. Similar superior catalytic results were reported by comparing the performance of layered TS-1 zeolite to that of bulk TS-1 in the epoxidation reactions [42]. The as-synthesized multilamellar MFI zeolite has also been applied as an acid-base bifunctional catalyst in the Knoevenagel condensation reactions. The acid site was derived from the Al-related Brönsted acidity, while the ammonium group located in the pore

The structure of the bifunctional amphiphilic SDAs was then proved to be critical for the synthesis of layered MFI zeolite [44]. Too small space between the two ammonium groups would result in bulk MFI, while too large space leads to the disordered stacking of MFI nanosheets. Tuning the number of ammonium group is effective in controlling the thickness of MFI nanosheets. The hydrophobic alkyl chain should be long enough to form a micellar structure. Once two long alkyl chains were present in the bifunctional SDA, hexagonal mesostructure was formed with the pore wall characterized with crystalline MFI structure having the thickness of 1.7 nm [45]. By introducing aromatic groups into the hydrophobic chain of bifunctional SDA molecules, Che et al. have synthesized several lamellar MFI zeolites with different mesostructures, benefiting from the strong *π-π* stacking and the geometrical match between aromatic groups and MFI zeolitic frameworks [46–48]. This strategy of using bifunctional SDAs has been successfully extended to

*SEM (A) and TEM (B) images of layered MFI zeolite. The scheme for the single layer MFI nanosheet (C) and* 

*the structural model for multilamellar MFI (D) and unilamellar MFI (E).*

resulted in the formation of unilamellar MFI nanosheets (**Figure 2E**). Both

(CH3)2▬C6H12▬N<sup>+</sup>

in the synthetic gel favored the alternating stacking of 2-nm-

(CH3)2▬C6H13.

**78**

**Figure 2.**

the synthesis of nanosheets with MTW [3], \*MRE [49], and AlPO [50] frameworks, which have never been synthesized as 2D zeolites before.

For the well-known MWW-layered zeolite, a bifunctional SDA with the head group resembling the SDA for conventional MWW-layered zeolite and a long alkyl chain connected by diquaternary ammonium linker directed the crystallization of MIT-1 composed of MWW nanosheets with a house-of-cards arrangement (**Figure 3**), which was similar to the delaminated MWW material [51]. MIT-1 exhibited high mesoporosity with the external surface area higher than that of MCM-56 but lower than ITQ-2. 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 (C16DC1) [52]. HMI, the traditional template for the crystallization of layered MWW zeolite, was applied to direct the crystallization of MWW layers. For the bifunctional SDA, the hydrophilic head is located in the pocket of MWW layers, while the long hydrophobic chain prevents the ordering of stacking along *c* axis. The obtained DS-ITQ-2 showed a comparable catalytic activity as the conventional delaminated material of ITQ-2 in the liquid phase alkylation of benzene with propene.
