Delaminated and pillared zeolites are an innovative family of molecular sieves which introduced a different concept inside the synthesis of active catalysts or inorganic supports. These types of materials exhibit an elevated accessibility due to their open structure, characterized by the high external surface area without imposed restrictions controlled by the pore sizes. These open zeolites are conformed by crystalline ordered (pillared zeolites) or disordered (delaminated zeolites) individual layers, exhibiting textural properties which are favorable to carry out catalytic processes in which it is necessary to employ catalysts with completely accessible active sites. The elevated external surface area of these zeolites is profitable to generate more specific organic-inorganic materials, acting in this case as stable inorganic matrixes. The preparation of this open type-zeolites family is based on the modification of, previously synthesized, zeolitic precursors which are preexpanded to obtain the final delaminated or pillared zeolites which exhibit very different physicochemical properties compared with the starting precursors. Along this paper, the most relevant MWW-type high accessible zeolitic materials will be considered. Their nature, characteristics, and reactivity will be shown in the function of the employed synthesis method for their preparation and the postsynthesis treatments carried out, tuning their properties.
Layered inorganic precursors are structurally conformed by a consecutive repetition of individual sheets located in parallel spatial planes, being electrostatically bonded by Van der Waals interactions or hydrogen bonds along the perpendicular plane in which the layers are disposed. This structural conformation is present in a great number of inorganic materials, from the simplest such as clays or silicates to another more complex such as layered double hydroxides, metallic layered materials, or zeolitic precursors [
These layered materials exhibit the capacity to modify their structural characteristics through consecutive steps using the free space present between the contiguous inorganic layers. In this way, it is possible to obtain novel mesoporous materials due to the intercalation in the interlayered space of organic or inorganic compounds which act as pillars, avoiding the covalent connection between the inorganic layers after the removing of the ionic molecules which are compensating the surface charges present in the sheets. This type of layered solids is considered as
However, it would be feasible to advance more in this approach, starting from inorganic layered precursors, to generate another type of materials with a complete accessibility, being possible to prepare new materials where the practical totality of the initial microporous surface obtained from the collapsed layered precursors is transformed to an accessible external area. For this, the inorganic precursors are involved in “
Layered materials derived from inorganic precursors.
It is important to remark that although the delaminated materials are disordered solids, it is not correct to consider these materials as amorphous or without any type of structuration. On the contrary, although their study is not easy, it is possible to determine correctly their structural characteristics and physico-chemical properties, using and analyzing suitably the characterization results obtained such as it will be shown along this paper.
There are different approaches [ Chemical processes such as it is the solvation of interlayered cations which facilitates the separation between the layers due to the decrease of the attraction forces [ Intercalation of molecules which are located in the interlayered space increasing the separation between the inorganic layers favoring their random spatial disorder [ Mechanical processes such as ultrasonic methods, stirring, freeze-drying, or centrifugation systems which are mechanisms to carry out the exfoliation of the starting layered precursors [
Accordingly, the delamination process could be performed through one of the above explained approaches, the more suitable being the effective combination of them to obtain delaminated materials with high accessibility and a defined layered framework. The application of combined delamination methods would allow the control of the final properties exhibited by the exfoliated solids, even being adjustable the achieved delamination level, preserving the crystalline structure of each individual inorganic layer.
In the next sections, it will be considered the main delaminated and pillared MWW materials, detailing characteristic structural and physico-chemical properties, and catalytic applications.
Inside zeolites family, it is possible to find different examples which their final 3D structure is achieved through stable intermediate layered zeolitic precursors obtained during the synthesis process. After the removing of the structural directing agents (SDAs) located in the interlayer space, using normally calcination methods, it is favored the connection between the contiguous layers to generate the final zeolite. The presence of individual layers separated between them by ionic compounds, which are compensating the charges of zeolitic sheets, facilitates the swelling, pillarization, or exfoliation processes, being possible to obtain new materials with higher pore size diameters and, so, with elevated accessibility to active sites, without loss of physico-chemical properties characteristic of conventional zeolites [
Into the state of art, there are only three main groups of layered zeolitic precursors related with the structures MWW [
The layered precursors with MWW topology is conformed by parallel and ordered zeolitic layers perpendicularly disposed to axis
Graphical representation of MWW(P) layered zeolitic precursor.
From X-ray diffraction, it is possible to evidence the collapse of the individual MWW layers after the removing of the organic template to generate a 3D zeolitic material (Figure
X-ray diffractograms of (a) MWW(P) and (b) MWW collapsed 3D structure obtained before and after the calcination process, respectively.
The 3D MWW zeolitic material obtained after the elimination of the organic template was patented as MCM-22, in 1990, by Rubin [
Graphical representation of zeolite MCM-22.
The layered MWW(P) zeolitic precursor is obtained with high crystallinity up to Si/Al molar ratios between 15 and 70, and conforming the aluminum content decreases into the synthesis gel (Si/Al
In 1987, Zones patented the aluminosilicate named as SSZ-25 which exhibited similar characteristics to MWW materials [
On the other hand, it was obtained the zeolite ITQ-1 which is also isostructural to MWW materials, being remarkable that this material is obtained from a purely siliceous layered MWW zeolitic precursor [
Organic templates used during the synthesis of layered ITQ-1 zeolitic precursors: Hexamethylenimine (HMI) and Trimethyladamantamonium hydroxide (TMAda+OH−).
Microdiffraction spectra of ITQ-1 and MCM-22 zeolites along axis
In 1988, Bellusi et al. prepared the microporous borosilicate named ERB-1 which is isostructural with the aluminosilicate MCM-22, being also obtained through a layered zeolitic precursor conformed by ordered MWW layers [
The influence of the sodium cations in the crystallization of ERB-1 layered precursor is not significant because in the final products it is not detected as a compensating charge ion. This fact would allow that layered zeolitic precursors with boron MWW topology could be obtained in the absence of alkaline metals, favoring the direct incorporation of tetrahedral titanium into the framework of this type of 2D materials. It is known that the presence of alkaline cations in the synthesis slurry facilitates the amorphous silicotitanates precipitation which avoids the integration of tetrahedral titanium coordinated into the network of zeolitic layers [
Advancing inside this approach, it was prepared, from ERB-1 precursors, B-free titanosilicates with the MWW topology (Ti-MWW). In this case, the synthesis first involved the preparation of ERB-1 using piperidine or hexamethylenimine following the methods above exposed. Nextly, the borosilicates were deboronated by calcinations followed by different acid extraction steps to obtain highly siliceous solids. The silicates obtained were treated with titanium alkoxide sources in an aqueous solution of cyclic amines such as piperidine, hexamethylenimine, pyridine, and piperazine. The materials finally synthesized were a B-free zeolites with a conventional MWW lamellar organization containing tetrahedrally titanium coordinated conforming the framework of the individual zeolitic layers (Figure
Synthesis route to obtain B-free Ti-MWW layered precursors (according to [
X-ray diffraction diffractograms of as-synthesized B-MWW (Si/B = 11) (a), deboronated MWW (Si/B > 1000) (b), Ti-MWW (Si/Ti = 30) precursor (c), and sample c treated with 2 M HNO3 and calcined (d) (according to [
UV-visible spectra of as-synthesized Ti-MWW with Si/Ti ratio of 45 (a), 30 (b), as (b) calcined at 823 K for 10 h (c), as (a) treated with 2 M HNO3 and calcined (d), and as (b) treated with 2 M HNO3 and calcined (e) (according to [
Transformation of Ti-MWW and Ti-YNU-1 materials (according to [
Formation of Ti-YNU-1 and 3D Ti-MWW, as well as their structures (according to [
Following the same structural approach considered by Tatsumi et al. in the Ti-YNU-1 materials above described, a new type of stabilized interlayer-expanded MWW zeolitic materials (IEZ-MWW) has been prepared by the intercalation in the interlayer space of monomeric silica puncheons between the individual MWW layers as small pillars conformed by only one silicon atom, using normally SiMe2Cl2 or Si(EtO)2Me2 as stabilizer agents which react with the external surface silanol groups. This way allowed, after the removing of SDA molecules through calcination processes, the generation of derived MWW materials with a novel 12 MR microporous channel between the inorganic sheets in which the strong acidity of zeolitic nature was preserved (Figure
Synthesis route to obtain IEZ-MWW materials by postsynthesis treatments using dialkoxysilylated precursors (according to [
It is remarkable that different layered precursors were prepared containing tetrahedrally coordinated metals incorporated into the framework of the individual MWW layers, such as gallium [
On the other hand, there are other tridimensional zeolites with MWW topology which are not obtained through layered precursors, but directly during the synthesis process, such as MCM-49 [
HRTEM micrographs of MCM-56 (a) (according to [
Inside the family of delaminated zeolitic materials, ITQ-2 was the first described into the state of art [
Graphical representation of the ITQ-2 delaminated zeolite.
The preparation procedure is illustrated in Figure
Preparation route to obtain ITQ-2 delaminated zeolite.
X-ray patterns of the layered MWW materials obtained during the preparation of ITQ-2 delaminated zeolite.
Figure
The appreciated characteristics observed for the ITQ-2, above commented, were completely confirmed from other spectroscopic techniques such as FT-IR. Comparing the IR spectra of the ITQ-2 and 3D MWW calcined materials shown in Figure
Lewis y Brönsted acid sites measured by pyridine adsorption-desorption method for 3D MWW and ITQ-2 materialsa.
Sample | Acidity ( | |||||
---|---|---|---|---|---|---|
150°C | 250°C | 350°C | ||||
Lewis | Brönsted | Lewis | Brönsted | Lewis | Brönsted | |
MWW | 23 | 39 | 15 | 24 | 14 | 15 |
ITQ-2 | 23 | 21 | 20 | 15 | 15 | 9 |
a
Absorption coefficients: Lewis (1455 cm−1): 2.22 cm/
Infrared spectra of the 3D MWW and ITQ-2 zeolitic materials.
Spectroscopic studies related with the acidity of ITQ-2 were carried out by Onida et al., considering several samples of ITQ-2 with different Al content (Si/Al = 50 and 25) and different exfoliation degree, being characterized by IR spectroscopy and compared to 3D MWW zeolites with the same Al content, to evaluate the difference established between the acid sites in the function of the accessibility associated to delamination processes. Brönsted acidity was measured as the propensity of OH species to either protonate ammonia or captures in H-bonds with suitable molecules (CO, N2,
Acidic sites generated during the delamination process to obtain ITQ-2 materials (according to [
Inside the IR studies, the nature of external SiOH species present in all-silica ITQ-2 and conventional mesoporous MCM-41 materials was compared through their respective equilibrium constants of the interaction with different bases. The results showed that the O–H stretching frequency and acidity were rigorously the same in both materials with NH3 and CO. Acetone with MCM-41 exhibited a higher O–H shift and a larger equilibrium constant. These appreciable differences were attributed to secondary interactions between H-bonded acetone and the surrounding of the SiOH species, favored by the curved topology of the MCM-41 mesoporous walls, being this effect not detectable in the ITQ-2 due to the crystalline zeolitic nature of the external surface [
The nitrogen adsorption isotherms showed in Figure
Textural properties of delaminated ITQ-2 and 3D MWW zeolites.
Samples |
|
|
|
|
|
---|---|---|---|---|---|
MCM-22 | 453 | 342 | 111 | 0.524 | 0.169 |
ITQ-2 | 840 | 5 |
790 | 0.948 | 0.853 |
Nitrogen adsorption-desorption isotherms of the delaminated ITQ-2 and 3D MWW zeolites.
Ar adsorption isotherms of delaminated ITQ-2 and 3D MWW zeolites.
Interesting studies confirmed that the textural properties of delaminated ITQ-2 materials are strongly influenced by the role of framework aluminum. Particularly, the amount of aluminum, finally present into the network of MWW layers, was considered during different ultrasound treatments employed as exfoliation technique. The results obtained showed that the delamination process is clearly favored by a decreasing of the aluminum concentration in the MWW starting precursors (Si/Al ratios >20) [
Additionally, the 29Si NMR spectra (Figure
29Si NMR spectra of the ITQ-2 and 3D MWW zeolites.
The micrographs obtained from transmission electronic microscopy (TEM) allow the identification of MWW individual layers which are forming the delaminated ITQ-2 zeolite (Figure
TEM micrographs of (a) ITQ-2 and (b) 3D MWW zeolites.
Definitively, all the results obtained from the different characterization techniques, above exposed, clearly show that the ITQ-2 delaminated zeolite exhibits the characteristics and properties corresponding to high accessible zeolitic structure conformed by individual MWW layers with a random spatial distribution. The ITQ-2 combines the acidity and the hydrothermal stability characteristic of conventional zeolites together with the high accessibility to voluminous molecules typical of the mesoporous aluminosilicates such as M41S materials [
The strong alkaline conditions employed during the delamination process, overall in the swelling step, together with the ultrasonic exfoliating treatment favor the low-silica yields obtained in the ITQ-2 materials due to the partial dissolution of MWW zeolitic layers. For this, different studies have been carried out trying to obtain well-defined and characteristic ITQ-2, employing soft conditions in the preparation method. In this sense, recently, Ogino et al. have synthesized the material so-called UCB-1 which is synthesized through the delamination of zeolite precursor MCM-22 (P), such as occurs in the conventional ITQ-2 zeolite, at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride. Characterization by powder X-ray diffraction, transmission electron microscopy, nitrogen physisorption, 29Si MAS NMR, and infrared spectroscopies showed similarities between UCB-1 and the previously reported delaminated ITQ-2 zeolitic material, which required a pH higher than 12.5 and strong sonication processes in order to achieve exfoliation. However, the reduced external surface area finally detected in the UCB-1 materials, close to standard 3D MWW zeolites, would indicate that the disorder achieved with this methodology is not complete. Probably, crystals conformed by several packed and ordered MWW layers would be the individual units of this partially exfoliated material. Specifically, this methodology is based on exfoliation processes through chemical deprotection steps, involving the breaking of Si–O and Al–O bonds in the interlayer region. For this fluoride anion is used because it is an established reagent for the deprotection of silyl ethers and is known to form strong interactions to Si(IV) cations. Additionally, chloride is used because it is an aggressive anion for eroding anodized aluminum. So, the authors conclude hypothesizing that delamination can be successfully conducted using a mixture of fluoride and chloride anions (Figure
TEM image of as-made UCB-1 material (according to [
Following this methodology based on the delamination of MWW layered precursors in soft conditions, Tsapatsis et al. identified the swelling step as decisive to avoid substantially the partial loss of crystallinity in each individual MWW layer following the standard conditions proposed by Corma et al. [
On the other hand, during the study above considered related with the description of MWW ordered precursors, it was remarked the preparation of Ti layered materials from Wu et al. after the combination of acidic and deboronation processes with the assistance of cyclic amine through post-synthesis treatments [
HRTEM micrographs of single sheet (a) and pack of three sheets (b) of Del-Ti-MWW (according to [
Grafting of titanocene on ITQ-2 external surface area (according to [
In this last example, it has been shown that as the external surface area exhibited by the ITQ-2 delaminated materials can be used to covalently incorporate another organic active species, organocatalysts, through the numerous accessible SiOH groups located on the MWW individual layers, being generated novel organic-inorganic solid materials with multiple applications, overcoming the limitations imposed by the organic, soluble compounds related with their stability and recovering. Another illustrative example, related with the preparation of hybrid organic-inorganic exfoliated derivative materials, comes from the incorporation of heterogenized porphyrins and metaloporphyrins on robust solids such as purely siliceous ITQ-2 zeolites, acting as inorganic matrixes. In this case, two different strategies for preparation of these catalysts were studied, based on direct immobilization of a functionalized porphyrin (route A) or heterogenization of an aromatic aldehyde followed by porphyrin formation (route B). The results showed that route A leads stable materials, allowing the introduction of a higher amount of active metaloporphyrin onto the support (from 0.3 to 0.4 mmol g−1). On the contrary, route B was not successful for the porphyrin formation onto the external surface of the MWW layers (Scheme
Alternative routes explored to synthesize heterogenized metalloporphyrins (according to [
Such as it occurs with clays, hydrotalcites, and other inorganic lamellar materials, the layered zeolitic precursors can also be successfully pillared by permanent covalent intercalation of organic or inorganic species, or the combination of them, into the interlayer space [
Preparation route of pillared MCM-36 zeolite and XRD patterns associated to MWW derivative materials obtained from starting layered zeolitic precursor (according to [
In Figure
Graphical representation of pillared MCM-36 zeolitic material.
However, this problem can be partially reduced through the intercalation of metal oxide pillars instead of silica pillars which can supply additional active sites, compensating the loss of activity due to the blockage associated to pillarization process, preserving the mesoporous textural properties. In this way, alumina and magnesia-alumina were used for pillaring the MWW layered zeolitic precursors to prepare new varieties of MCM-36 molecular sieves. Specifically, pillaring with alumina generates mesoporous materials with lower surface areas than those pillared with silica, being necessary elongated aging of the alumina pillaring solutions to obtain mesoporous regular materials. Application of magnesia in combination with alumina yields in a higher spatial disorder of the MWW layers. However, the employ of MgO–Al2O3 as pillaring agents implies the preparation of MCM-36 materials with significantly higher mesoporosity (pore size diameters between 2–4 nm) compared with the use of only alumina as structural pillars (Figure
TEM images of MCM-36 derivatives pillared with intercalates MgO–Al2O3 (according to [
On the other hand, additional studies have been carried out with the objective to obtain the most efficient expansion methods of MWW layered zeolitic precursors, being this decisive to prepare homogeneous pillared MCM-36 derivatives with higher accessibility without crystallinity and activity losses. In this sense, the employ of concentrated surfactant solutions with high pH provided swollen MWW materials that were successfully converted to pillared MCM-36 zeolites. Specifically, optimal high pH of the swelling solutions was obtained by the addition of tetrapropylammonium hydroxide and by the partial conversion of the surfactant chloride into hydroxide by ion exchange. The catalytic potential of the solids was analyzed based on IR spectra and specific surface area, showing that there was an overall decrease in acid site concentration due to the incorporation of inert silica pillars. However, the MCM-36 zeolites with the highest external surface showed a marked adsorption increase of 2,6-di-
An important associated problem to the MCM-36 synthesis method is the similarity between the conditions employed (high pH, high temperatures, and the presence of surfactants in the synthesis media) to prepare the pillared and the conventional mesoporous M41S materials. Although, the suitable combination of different characterization techniques (X-ray diffraction, sorption isotherms, and microscopic images, preferably) allows to clearly evidence the differences between the mesoporosity due to conventional M41S solids or attributed to pillaring process, significant studies have been carried out to find the optimal conditions to obtain pillared zeolites, avoiding the possible formation of standard mesoporous materials, as impurities, during the preparation of MCM-36 derivatives [
TEM image of MCM-36 obtained from MWW layered precursors swollen at room temperature (according to [
Recently, advancing in the pillarization concept, multifunctional hybrid organic-inorganic catalytic materials with a hierarchical system of well-defined micro- and mesopores have been prepared from MWW zeolitic precursors by intercalation and stabilization in the interlayer space of aminoaryl-bridged silsesquioxanes between inorganic MWW zeolitic layers. The organic linkers were conformed by two condensed silyl-acrylic groups from disilane molecules, such as 1,4-bis(triethoxysilyl)benzene (BTEB), which reacted with the external silanol groups of the zeolitic layers. The hybrids contained micropores within the inorganic layers and a well-defined mesoporous system between the organic linkers. An amination posttreatment introduced basic groups in the organic linkers close to the acid sites present in the structural inorganic counterpart due to the presence of framework aluminum. Through this methodology it has been possible to prepare bifunctional acid-base catalysts where the acid sites are of zeolitic nature located in the inorganic building blocks and the basic sites are part of the organic structure (Figure
Graphical representation of a layered hybrid material obtained by pillaring with BTEB silsesquioxane molecules (according to [
Heterogeneous catalysis is an important topic in fine and bulk chemical processes, such as pharmaceutical manufacture and petroleum refining, being hugely used as catalysts based on metal(alumino)silicates to perform these types of reactions, combining high stability with excellent activity. Inside this class of materials, zeolitic microporous crystalline solids with three-dimensional framework structures have occupied a relevant place because they exhibit a similar activity compared with soluble standard catalysts and improve the hydrothermal stability of mesoporous structures which contain amorphous walls. Further, they impart shape selectivity on the reaction products. However, the selectivity achieved through uniformly sized pores and channels, present in the structure of microporous zeolites, implies important size constraints on the accessibility to reactants, intermediates, and products. The use of open zeolites, obtained from pillaring or delaminating mechanisms, would allow expanding the number of reactions that zeolites could carry out. For this, the employ of the ITQ-2 and MCM-36 derivative zeolites as catalysts has been hugely used in the last years with excellent results [
The open MWW structures, generated from ordered layered precursors, have allowed the employ of pillared and delaminated zeolites as efficient catalysts to carry out reactive processes in which take part compounds with elevated molecular sizes. The MCM-36 and ITQ-2 materials are aluminosilicates whose zeolite-type catalytic sites are contained within thin and readily accessible sheets. Several and suitable performance reactive tests showed that the pillarization and delamination processes improved the accessibility to the catalytic sites preserving their intrinsic activity.
Specifically, the catalytic potential of ITQ-2 was evidenced by means of a fixed-bed, small-scale catalytic-cracking test, using n-decane as a model feed. The conversions of the reactant observed at different contact times were used to calculate first-order rate constants, showed in Figure
Comparison of the first-order kinetic rate constants for the cracking of n-decane, 1,3-diisopropylbenzene (DIPB), and vacuum gasoil over the MWW-type zeolite and ITQ-2 delaminated materials (according to [
Following this tendency, the ITQ-2 showed excellent results for the cracking or hydrotreating light cycle oil (LCO) fractions, which are provided directly from fluid catalytic cracking units, being the performances comparable to those obtained for pore and large pore zeolites, such as ZSM-5, MCM-22, USY, or Beta [
Naphthalene hydrogenation process.
Interestingly, cobalt particles were incorporated onto the high external surface of ITQ-2 zeolites, previously silylated, through reverse-micelle synthesis, generating excellent catalysts for Fischer-Tropsch synthesis (FTS) which showed a uniform Co(0) particle size distribution in the 5–11 nm range [
Schematic representation of the preparation of sample Co-ITQ-2 by deposition of cobalt nanoparticles synthesized ex-carrier by reverse microemulsion on the silylated ITQ-2 zeolite (according to [
Bifunctional ITQ-2 derivative catalysts were prepared combining the Brönsted acidity due to the framework aluminum with the presence of molybdenum onto the external surface. These delaminated zeolites were used for the methane dehydroaromatization (MDA) reaction. In this process, the Si/Al ratio, and thus the Brönsted acidity, of the zeolite affected both the conversion and the selectivity to the different products (CO, C2, benzene, toluene, and naphthalene). The highest activity and aromatics yields were obtained for the delaminated zeolite with the lowest Si/Al ratio (Si/Al = 15), exhibiting the maximum Brönsted acidity, while the highest benzene selectivity (
The benefits of the high surface area and the acid sites accessibility, without the associated loss of hydrothermal stability and acidic strength, exhibited by delaminated zeolites were taken as an advantage to carry out skeletal isomerization and dehydroisomerization of 1-butene and n-butene to isobutene or alkylation of toluene with methanol, avoiding in both cases the extensive coke formation, allowing easily the pore diffusion of reactants and products even in steady state (Scheme
Alkylation of toluene with methanol on MWW-type individual layers which are forming ITQ-2 delaminated zeolites (according to [
This last reaction process was also used to compare the activity of different catalysts derived from MWW precursors, that is
On the other hand, interesting catalysts based on Ni and Co supported on purely siliceous delaminated ITQ-2 zeolites were synthesized and catalytically used for bioethanol steam reforming. The study showed that Ni-ITQ-2 zeolite was the most active material while ITQ-2 zeolite containing cobalt exhibited the highest H selectivity and the lowest CO selectivity. Moreover, it was appreciated that coke deposition occurs in both materials, although deactivation was not detected during the reaction. In this case, the structure and physicochemical characteristics of the ITQ-2 zeolites, as accessible supporting matrixes, clearly favor the high activity, selectivity, and stability during the steam reforming processes [
The advantages associated to the employ of more accessible MWW zeolites as catalysts, combined with their intrinsic acidic strength, could be positive when take part in reactive processes at industrial scale. In this sense, delaminated zeolites derivatives materials were efficient catalysts for the synthesis of diaminodiphenylmethane (DADPM), that is
General reaction scheme for the isomerization of the neutral amine to DADPM over zeolitic materials. Processes (1) and (2) involve several steps (according to [
Recently, the open structure of the delaminated zeolites has been investigated for methanol to olefin conversion (MTO) [
On the other hand, pillared MCM-36 zeolites have also been used as excellent catalysts in bulk chemical processes related with the
Recently, interesting studies have been performed by Tsapatsis et al. elucidating the catalytic behavior of Brönsted acid sites in MWW zeolites with dual meso- and microporosity, such as MCM-36 derivatives. In this case, ethanol dehydration and monomolecular conversion of propane and isobutane were considered as test reactions. The results clearly corroborated that the rate and activation energy in zeolites possessing dual meso- and microporosity were comparable to conventional microporous 3D-MWW materials, implying this fact that, in pillared zeolites, the catalytic behavior of Brönsted acid sites is preferentially dominated by the microporous environment because it provides a better fit for adsorption of small alkane or alcohol substrates [
Taking into account the precedent section, it has been corroborated the benefits of open structure exhibited by the pillared and exfoliated materials derived from MWW precursors in which is successfully combined the high accessibility to active sites without diffusion problems and characteristic of amorphous mesoporous materials, together with the crystallinity, hydrothermal stability, and acid strength exhibited by conventional zeolitic materials. These cooperative properties are evident when bulky molecules take part during the catalytic process, being possible to use both MCM-36 and ITQ-2 derivative materials as efficient catalysts to carry out also Fine Chemistry processes to obtain high value products with application in pharmaceutical, perfumes and fragrances, cosmetic, or food [
Inside this research field, delaminated materials obtained by grafting of titanocene was used as an excellent catalyst for epoxidation of olefins, yielding high conversions and selectivity to the desired epoxides [
Transition structures
The high external surface area, with a big amount of reactive silanol groups, exhibited by the ITQ-2 materials is useful to anchor different organocatalysts, such as chiral salen or Schiff base complexes, obtaining interesting asymmetric catalysts. It is the case of chiral chromium salen compounds which were grafted on previously aminopropyl-functionalized ITQ-2 materials for enantioselective epoxide ring opening reactions. In this study, was confirmed that in the catalysts in which the anchoring was carried out through coordination bonds with the metal, then high enantiomeric excesses were obtained (up to 70% e.e.), but leaching phenomenon was observed. On the contrary, for the solids in which the complex was covalently attached to the surface then it was not observed leaching, being the
(1) Preparation of the asymmetric complex in homogeneous phase complexes, (2) anchoring of a mixture of complexes (according to [
Into this family of chiral delaminated zeolites, it was observed a cooperative effect between the support (ITQ-2) and stabilized salen (Pd and Ni) complexes. This matter was studied through the immobilization of chiral salen palladium and nickel complexes [salen = (R,R)-N′,N′-bis-(3,5-di-
Stabilization of the transition state by the H+ in the reaction media during the hydrogenation of imines (according to [
Heck vinylation reactions performed in a biphasic mode using ethylene glycol and toluene (according to [
Inside this line, Sabater et al. reported that the catalytic behavior and enantioselectivity of three different chiral Mn(III) salen complexes anchored to ITQ-2-type delaminated zeolitic materials were strongly dependent on the attachment nature established between the complexes and the zeolitic surfaces through chiral equatorial tetradentate salen ligand or via the apical ligand. This conformational factor together with the tuning of the hydrophobicity of the external surface allowed to optimize the selectivity to obtain chiral epoxides. In principle, the attachment of the complex through the equatorial position of the metal complex led to a strong reduction in enantioselectivity. On the contrary, when the complexes were fixed to the respective delaminated supports through the axial coordinating ligand were active and more enantioselective toward epoxidation of several prochiral alkenes (Scheme
Immobilization of chiral manganese salen complexes through equatorial tetradentate salen ligand (a) or apical ligand (b) (according to [
Similar approaches were followed by the stable incorporation of chiral vanadyl Schiff base complexes anchored on ITQ-2 delaminated materials as solid enantioselective catalysts for the formation of cyanohydrins, being possible to optimize the asymmetric induction by external surface modification (Scheme
Reaction scheme of the formation of (a) 2-benzyl-4-hydroxymethyl-1,3-dioxolane (
Heterogeneous hybrid materials based on open MWW-type zeolites have not only been formed by the stable and covalent immobilization of organocatalysts such as chiral salen complexes. In this sense, prefixed chiral triaza ligands onto the ITQ-2 external surface allowed the final incorporation of palladium, rhodium, or iridium complexes which were active for olefin hydrogenation reactions. The results obtained for these catalysts showed that the activity and selectivity were higher to that observed under homogeneous conditions owing to support interaction. Additional absorption analyses of the reaction solutions revealed that there is no metal leaching during the catalytic process which corroborated the high stability of the materials (Scheme
Synthesis of paracetamol by Beckmann rearrangement of 4-hydroxyacetophenone oxime.
Cyanohydrins formation from benzaldehyde and trimethylsilyl cyanide (TMSCN).
More sophisticated mononuclear N-heterocyclic carbene-gold complexes were also immobilized on delaminated zeolite ITQ-2, exhibiting high performances in the hydrogenation of alkenes and the Suzuki cross-coupling reaction, favoring selectively nonsymmetrical biaryls. The elevated accessibility introduced by the structure of the exfoliated supports facilitated the preparation of highly efficient immobilized catalysts with TOFs up to 400 h−1 (Scheme
Synthesis of metal complexes based on chiral triaza backbone ligands (according to [
Different examples have above been shown related with the generation of effective hybrid organic-inorganic catalysts by the stable interaction established between suitable soluble organocatalysts and open MWW materials, such as delaminated ITQ-2 zeolites. However, the acid sites homogeneously distributed along the network of each individual zeolitic layer allow the direct employ of these accessible zeolites in the production of fine chemicals without any post-synthesis treatment to immobilize additional organic functionalities. For instance, it is the case of the preparation of dimethylacetals and tetrahydropyranylation of alcohol and phenols using ITQ-2 delaminated zeolite as acid catalyst. The results obtained confirmed that when the reactions involved bulky reactants, ITQ-2 exhibited the highest activity owing to the combination of its disordered structure and the presence of strong and accessible acid sites into their framework (Scheme
Catalytic Michael addition of ethyl 2-oxocycloalkanecarboxylate to acrolein.
Into this thematic, interesting acetalization processes were carried out, catalyzed by MWW delaminated materials, for the synthesis of orange blossom and apple fragrances [
Synthesis of gold-carbene complexes (according to [
High value products, such as nonsteroidal drugs, with anti-inflammatory and analgesic activities, were obtained through the oximation of acetophenone derivatives, followed by the solid acid-catalyzed Beckmann rearrangement to give the corresponding amides. Delaminated ITQ-2 zeolites were employed as catalysts, being obtained excellent activity and selectivity for amides when both accessibility of the reactants to the active sites and the surface polarity of the catalysts were optimized (Scheme
Protection of hydroxyl groups by tetrahydropyranyl ethers (according to [
Liquid-phase Beckmann rearrangement of cyclododecanone oxime into 2-azacyclotridecanone or
Analysis of the peculiar structure of the delaminated materials and its influence on the catalytic reactivity was also evaluated through the performances of a different solid acids including conventional zeolites (HY, mordenite, and ZSM-5), mesoporous Al-MCM-41 materials, and amorphous silica-aluminas compared with the layered zeolite ITQ-2 for the liquid-phase condensation of 2-methoxynaphthalene and naphthalene with paraformaldehyde to form dinaphthylmethane derivatives (Scheme
Products of the condensation of paraformaldehyde with 2-methoxynaphthalene (according to [
Another interesting fine chemical processes, such as Diels-Alder and retro-Diels-Alder reaction between cyclopentadiene and
Distribution of products in the Diels-Alder reaction between cyclopentadiene and
Synthesis of Nopol through
Simplified representation of the conversion of carbohydrate biomass into furfural (according to [
The benefits of the open zeolitic structures, derived from MWW layered precursors, can be exploited in other catalytic fields, more related with electro- or photo-catalysis. It is the case of the delaminated materials in which were supported and heterogenized metalloporphyrins containing Co or Fe, being active as catalysts for electrochemical oxygen reduction without detecting catalysts desorption from the electrode [
Inside the photochemistry field, it is important the generation of electron transfer species with high stability and durability. In this sense, the prevalence of the external surface over the internal porosity in the delaminated zeolites (ITQ-2) favors the spontaneous generation of tetrathiafulvalene radical cations. This effect is strongly marked in MWW open structures compared with conventional microporous Y, mordenite, and ZSM-5 zeolites as well as to Al-MCM-41 and amorphous silica-alumina. Moreover, through selective silylation of the external cups in the surface of individual MWW sheets, it was found that the formation of a significant concentration of stable organic radical cations occurred within these open cups rather than inside the sinusoidal internal 10 MR channels (Figure
Representation of the best docking of tetrathiafulvalene (a), 1-phenyl-naphtalene (b), and hydroxyquinolate ligands inside the open cups of ITQ-2 (c) (according to [
On the other hand, interesting biocatalysts can be obtained using delaminated zeolites as supports of enzymes. This type of zeolitic disordered materials exhibits suitable physicochemical properties such as a very high, hydroxylated, and ordered external surface together with elevated thermal stability. So, the preparation of catalysts based on zeolitic sheets with immobilized enzymes onto their external surface area was viable, being noticeable the improved activity and stability of the final products compared with the action of soluble enzymatic compounds. Specifically, the enzymes were supported either by means of ionic or covalent bonds, improving the stability of the enzymes without reactivity loss. Moreover, after reaction, the zeolitic supports can be recycled and reused to immobilize additional enzymes. Definitively, these results have opened the possibility of using open zeolitic structures for processes which involve enzymes as catalysts (Scheme
Covalent or electrostatic immobilization of enzymes onto delaminated zeolites used as inorganic supports.
Alternatively, open MWW structures have been used as sensors by the covalent anchoring of fluorophore compounds, such as pyrene, being the resulting solid a selective heterogeneous sensor for iodide in the presence of other halides in the medium (Scheme
Procedure to anchor pyrene fluorophore onto ITQ-2 individual layers (according to [
Into the catalysis field, zeolites have promoted a revolution as acid catalysts. Moreover, their microporosity introduces shape selectivity in reactive processes and this matter will influence on reaction products. This apparent advantage can represent, in some cases, a serious inconvenient, since in a large number of processes take part molecules of elevated size, which are not feasible with microporous catalysts of restricted accessibility. This problem can be avoided with innovative zeolitic materials, with their characteristic acidity, but with a higher accessibility in order to facilitate the reactive molecules access to active sites, overcoming the difficulties imposed by their conventional microporosity.
This proposal has successfully carried out with bi-dimensional MWW layered zeolitic precursors which are delaminated or exfoliated with the objective to obtain new delaminated materials, ITQ-2 zeolites, composed by disordered layers, and characterized by an elevated and accessible external surface area. on the other hand, when the layered MWW precursors are pillared, innovative materials with high accessibility are obtained, but formed by interlayered mesoporous galleries, called MCM-36 zeolites. Both MWW materials families are currently being employed in interesting applications related with catalytic reactions of industrial relevance inside petrochemistry and Fine Chemistry. Furthermore, they are excellent inorganic supports of functional molecules, as enzymes or specific organocatalysts, owing to their elevated hydrothermal stability, structural homogeneity, and high external surface area.
These peculiar characteristics would allow the stable incorporation, onto the individual MWW sheets, of several and suitable organic, organometallic, and/or metal functional species. These active and accessible sites could even be combined with additional centers included into the framework of the individual inorganic layers. The consequence would be the generation of organic-inorganic multifunctional hybrid catalysts to carry out multicomponent or cascade reactions in one-pot processes, using only one heterogeneous and recyclable catalytic system (Figure
Schematic representation of cascade reaction catalyzed by multifunctional materials based on individual zeolitic layers which contain several active groups.
The author thanks the Spanish Government (Consolider Ingenio 2010-MULTICAT (CSD2009–00050) and MAT2011–29020-C02-01) for the financial support.