This paper will introduce the reader to some of the “classical” and “new” families of ordered porous materials which have arisen throughout the past decades and/or years. From what is perhaps the best-known family of zeolites, which even now to this day is under constant research, to the exciting new family of hierarchical porous materials, the number of strategies, structures, porous textures, and potential applications grows with every passing day. We will attempt to put these new families into perspective from a synthetic and applied point of view in order to give the reader as broad a perspective as possible into these exciting materials.
Considering the history of mankind, its development is unavoidably linked to technology, more precisely to the development of materials and methods which have enabled us to go beyond our own frontiers. Focusing more on the matter at hand, which is porous materials, we can find several remarkable examples throughout ancient and modern history about the use of porous carbon materials for a wide variety of applications. For instance, in 3700 BC we find the earliest use of a porous form of carbon. Charcoal was used by Egyptians and Sumerians for the reduction of different metal ores (mainly copper, tin, and zinc) in the manufacture of bronze. This material was also used as domestic smokeless fuel. This example alone clearly shows how even some of the most advanced civilizations of their time have employed porous materials in their technological tree. The earliest recorded example in which porosity of the material comes fully into play is in 1500 BC, in Egyptian papyri describing the use of charcoal to adsorb odorous vapors from infected wounds and from within the intestinal tract. The fact that we find these very early examples linked to a civilization which dominated a vast empire for several thousands of years should come as no coincidence. In 450 BC we can find excellent examples of how the porosity of carbon materials is employed as a means to purify drinking water in Hindu documentation and in Phoenician trading ship records. In 400 BC Hippocrates and Pliny the Elder recorded the use of charcoal to treat a wide range of illnesses and maladies including epilepsy, chlorosis, and anthrax. In short, porous materials have greatly helped modern civilizations from the very infancy of mankind, and even if in ancient times the people could only guess as to how or why they worked, the fact is that their usefulness is very well documented. Presently, carbon materials are undoubtedly crucial in many areas of technological development, with applications ranging from household use in water purification systems or as odour filters [
This starting paragraph should be taken merely as an illustrative standpoint which shows the importance of porous materials. During this paper we will attempt to analyse in detail some of the most prominent classes of ordered porous materials from both a synthetic and applied point of view. Thus, classical carbon materials as those described in the previous paragraph fall outside the scope of this paper. In this respect, it must be mentioned that the term “ordered” does not necessarily mean that the prepared materials should possess high structural order, but in fact the term applies to the porosity of the material itself. Throughout the paper we will come across several examples of materials which, despite possessing walls formed by amorphous solids possess a regular (i.e., ordered) porous array which, in turn, results in clear indications of a crystalline or, at least, of a crystal-like structure. Perhaps one of the earliest and best-known examples is that of the material known as MCM-41 [
Since this paper encompasses a broad array of porous materials, we will not go into excessive detail on any of the topics described henceforth. Should more information on one specific class or type of materials be required, the reader is strongly encouraged to go through some of the references which will be presented here for more detailed information.
The term
Some important milestones in the history of zeolites are shown in Table
1858 | Eichorn points out the ion-exchange capability of zeolites [ |
1862 | de St. Claire-Deville synthesizes the first artificial zeolite, levynite [ |
1876–1905 | Studies on ion-exchange in zeolites [ |
1930 | Taylor reveals the structure of analcime [ |
1932 | McBain introduces the term |
1938 | Barrer presents a remarkable work on molecular sieving [ |
1940–1945 | Reports on zeolite molecular sieves with different pore sizes |
1950–1972 | Synthesis of Zeolites A [ |
In short, the history of man-made zeolites can be traced back to the laboratory preparation of levynite by de St. Claire-Deville in 1862. Nevertheless, zeolite synthesis as it is known and commonly carried nowadays had its origins in the research of Barrer and Milton, commencing in the late 1940s. Barrer began his work by investigating the conversion of mineral phases in the presence of strong salt solutions at moderate-to-high temperatures (between 170 and 270°C) [
In 1960, Flanigen and Breck studied in detail the formation mechanism of zeolites. They reported the use of X-Ray measurements to follow the crystallization of different zeolites over time [
The interest raised in zeolites throughout the scientific community has been growing ever since the aforementioned pioneering works up to the point that research related to both synthesis and application of these materials and some of their related families has been growing for the past decades. Perhaps the best example to illustrate this point is the fact that the number of synthetic zeolites has widely surpassed that of natural ones.
A zeolite is a crystalline aluminosilicate with a structure based in an open three-dimensional network which presents pores or channels [
The structure and composition of zeolites depends on the zeolite type in question [
Thus, the empirical formula for a zeolite would be as follows:
Perhaps one of the most intuitive description of a zeolitic structure is based on its pore size; thus, zeolites may be classified according to the number of tetrahedral, (TO4, being “
Types of zeolites attending to their pore size.
Zeolite type | Number of TO4 forming the pore | Pore diameter (nm) | Examples |
---|---|---|---|
Extra-large pore |
|
|
AlPO4-8, VPI-5 |
Large pore | 12 |
|
Y, Mordenite, |
Medium pore | 10 |
|
ZSM-5, ZSM-11 |
Small pore | 8 |
|
Erionite, A, SAPO-34 |
As it is well known nowadays, zeolites are prepared by means of hydrothermal synthesis [ Amorphous precursors containing silica and alumina are dissolved together with a cation source, usually in a basic medium. In this respect, it is common to use alkali ion hydroxides (also known as mineralizing agents) to reach the necessary high pH values; the aqueous reaction mixture is heated, often in a sealed reactor. In other cases, the reactor is directly inserted in a preheated oven; the reactants remain amorphous for some time, even after reaching the onset temperature; after the aforementioned “induction period,” formation of the first zeolites crystals is observed; all amorphous material present in the solution transforms into an approximately equal mass of zeolite crystals. These may be recovered by filtration, washing, and drying.
In a typical representative synthesis, a zeolite may be synthesized from sodium metasilicate (water glass) and sodium aluminate under hydrothermal conditions. Once the aluminate is added on the silicate a gel is formed in which aluminate and silicate oligomer chains and cycles coexist, with different degrees of ramification. This is known as precrystalline phase. The chemical composition (silicon
Left to right, schematic representation of a PBU (oxygen ions represented by the small spheres, the T ion represented by the larger sphere), examples of SBUs (taken from [
Scheme of some zeolites derived from the TBUs. Left to right: Faujasite (FAU, zeolite X and Y), Zeolite A (LTA), and Sodalite (SOD).
During hydrothermal synthesis and in the presence of the aforementioned mineralising agent, the crystalline zeolite product, which contains Si–O–Al bonds, is created. Since the bond type of the zeolite is very similar to that of the precursor oxides (namely, Si–O and Al–O bonds), no significant enthalpy change is involved in the process (Figure
Schematics of hydrothermal zeolite synthesis. The starting materials are converted into the crystalline products under the effect of the temperature, autogenous pressure, and the presence of an aqueous mineralizing agent (OH- and/or F- ions). Scheme taken from [
Kinetic control is the main driving force during zeolite synthesis, especially taking into account that zeolites are metastable phases in most cases. Much of the current knowledge in these industrially important materials involves precise control over the synthesis conditions aimed at product optimisation, so that the materials can be prepared on-demand with a high degree of reproducibility [
One rather surprising feature involving zeolite crystallization is that the exact synthesis mechanism is still under intense scrutiny. From the first proposed mechanism by Barrer and coworkers [
During the 1960s, Flanigen and Breck conducted a series of pioneering studies which led to the following conclusions concerning the zeolite crystallisation mechanism. Nucleation takes place in a heterogeneous way during the formation of highly supersaturated gels within the synthesis solution; the crystalline nuclei do not necessarily represent a unit cell but may consist of more primitive preliminary building units as those suggested by Barrer et al. [ during the induction period (i.e., the time during which very little zeolite is formed), the nuclei grow in order to reach a critical size which will prevent their dissolution and then grow rapidly to form small and uniform sized crystals; growth of the crystalline species proceeds through a type of construction and deconstruction process (cleaving and reforming Si, Al–O–Si, Al bonds), catalyzed by excess hydroxyl ion and involving both the solid and liquid phases (although the solid phase appears to play the predominant role).
In 1966 Kerr reported an experiment [
At the turn of the 1970s, Zhdanov presented a very important contribution to the 2nd International Zeolite Conference [
From the 1980s onward, the most relevant studies on zeolite growth focused on the study of zeolites of the MFI type, namely, zeolite ZSM-5 with different Si
Scheme for zeolite crystallization adapted from [
One of the most recent and detailed studies on the crystallization mechanism of zeolites is owed to the group of Martens and Jacobs at the Catholic University of Leuven [
From this brief review, it becomes obvious that the formation of zeolites is a complex process which involves not only many chemical species under the hydrothermal synthesis conditions, but also a significant number of processes that are involved in the crystallization processes. More recent studies have highlighted the key importance of the mineralizing agent, whose primary task is to convert the reactants into mobile and reactive forms, thus enabling the synthesis to move forward. For a more detailed review on the synthesis of zeolites and the study of the crystallization mechanism, an excellent review by Cundy and Cox covers this aspect in great detail [
Let us now turn our attention to a concept which is of the highest importance from a synthetic point of view, and which is ultimately the cornerstone of this review. The Oxford English Dictionary defines the term
The applications of both natural and synthetic zeolites are a direct consequence of their specific features. Benefitting from their structure presenting a regular array of channels with molecular sizes, zeolites have been widely used as adsorbent materials and as molecular sieves. Furthermore, the excess negative framework charge which was mentioned above, as a consequence of the presence of tetracoordinated aluminium ions, is compensated by cations which may be easily exchanged, making zeolites excellent ion exchangers. Finally, the presence of framework acid sites, and the possibility of introducing new and tunable acid sites makes zeolites very active catalysts with very important applications, especially in pharmaceutical and petrochemical industry. We will elaborate all these points further, but Table
Properties and applications of zeolites.
Property | Application |
---|---|
Ordered cannel structure | Adsorbent materials |
Molecular sieves | |
Catalyst supports | |
Selective catalysis | |
| |
Ion exchange | Cation exchangers |
Dispersion of supported catalysts (ion exchange followed by reduction) | |
Possibility of producing acid sites (ion exchange of |
|
| |
Acid sites present in the structure | Catalysis |
After having crossed the threshold of the new millennium, the attention of the detergent industry has focused on delivering clear and concise answers against four challenges [ economics, safety and environment, technology, consumer requirements.
The crucial factor of the four is still the consumer. Although washing habits differ greatly from country to country, there is a trend towards easier handling (compact powders, tablets) of detergents with increased efficiency. First and foremost, however, the consumer continues to demand a clean wash combined with maximum protection of the items laundered. The resulting demand to continuously improve on the performance of detergents is leading to the use of new and/or optimized raw materials.
To a large degree, the aforementioned challenges dominated the development of detergents by the end of the 20th century. Zeolites, originally designed as a phosphate substitute for purely environmental reasons, had to meet the increasing demands imposed by modified detergent composition and production technologies. In particular, the trend towards compact detergents increased the demand for builder systems with a high adsorption capacity for liquid components, especially for surfactants. Zeolite A, introduced approximately 35 years ago, proved to be a good surfactants carrier and in the 90s advanced to become the builder leading to compact and supercompact detergents. Nevertheless, the demands from the market to bring on new improvements kept coming.
The manufacturers of detergent zeolites responded to the demand for higher standards of performance and processing by developing new zeolite grades. These include the zeolites of types P, X, and AX, which have all been introduced into the market during the past years. The main drawback of Zeolite A as zeolite builder was that it had a poor magnesium exchange capacity due to the hydrate shell surrounding the
Following the development of a more economically viable production process, Zeolite X was recently introduced into the market for detergents [
A further new development on the market is a cocrystallite comprised of 80% Zeolite X and 20% Zeolite A [
Historic use of inorganic materials as detergent builders.
Year | Builder |
---|---|
1907 | Silicate (water glass) + carbonate |
1933 | Diphosphate |
1946 | Triphosphate |
1976 | Zeolite A + triphosphate |
1983 | Zeolite A + carbonate + cobuilders* |
1994 | Zeolite A + special silicates** + cobuilders |
1994 | Zeolite P + carbonate (+cobuilders) |
1997 | Zeolite X + carbonate (+cobuilders) |
1999–present | Zeolite AX (+carbonate + cobuilders) |
We have covered the first one of the two major applications of zeolites in modern industry. The second field in which zeolites have by far the most outstanding impact from an economical perspective is of course the petrochemical industry [
During the second half of the 20th century, the industrial application of zeolites and more general purpose molecular sieves emerged in full. The majority of the chemicals that constitute our consumers goods basis and energy carriers/vectors like transportation fuels have passed through the micro- and mesopores of these materials or materials derived from them. Some zeolite structures, mainly MFI and FAU and to a minor extend also MOR, are very versatile materials; that is, their properties can be tuned to the specific requirements of different applications, thanks mainly to the fine tuning of the Si
It must be considered that oil refining is in itself a very complex industry, involving many different processes. This is covered in some excellent reviews [ adjust the crude oils supply in order to increase the H remove pollutants present in feedstocks or during the refining processes (metals, N, S, be efficient, both in terms of thermal efficiency and carbon/hydrocarbon efficiency. Typical carbon efficiency for a refinery is above 90%, which results in the best
Some of the processes in which we may find zeolites inside a refinery include the following. Light hydrocarbon ( Cracking and hydrocracking of heavy feedstocks: this includes Fluid Catalytic Cracking (FCC) and HydroCracking (HDC). The aim of these processes is to adjust the H Aromatics production: despite the fact that the majority of aromatics is still obtained in the catalytic reformer of a refinery or recovered as byproduct in cracking of liquid hydrocarbons [ Aromatics processing: this includes disproportionation, intramolecular isomerization, aromatic alkylation, and hydrodealkylation. These processes have as main goal the production of benzene and xylene, and the different zeolites involved include ZSM-5 (MFI) loaded with platinum, mordenite (MOR), zeolite BETA (BEA*), and Ba or K-exchanged zeolite X (FAU) [ Emerging technologies: a few examples include the hydrocracking of heavy aromatics into light paraffins [
In addition to these two very prominent applications, zeolites have found many other uses in our daily life. At the beginning of the 21st century, zeolites are applied in the development of new construction materials since they bring forth both an ion-exchange and an adsorption function, to solve problems in construction engineering according to different practical requirements [
By the end of the 20th century, zeolites had more than a firm foothold in different fields thanks to their exceptional properties, but unfortunately they were not devoid problems. Since most zeolites are synthesized by means of hydrothermal synthesis [
From that starting point, scientists around the world dedicated their efforts to come up with strategies to synthesize zeolites which incorporated mesopores so as to improve diffusivity within the zeolite crystals. Coming back to the phenomenon known as “templating” which was described in detail in the previous section, the idea around which the first studies were founded was that quaternary ammonium ions had been by far the most versatile templates for zeolite synthesis, and the length of the alkyl chain linked to the nitrogen had a specific impact on the type of zeolite Tertiary (composite) Building Units formed during hydrothermal treatment. If one of this alkyl functions was substituted by a longer hydrocarbon chain, there might be a chance that the shorter alkyl functions formed the desired TBUs which would organize themselves around the longer alkyl chain, giving rise to a crystalline, porous structure with the properties of zeolites while having significantly higher accessibility. The initial results were not entirely those expected, but a significant turning point in materials science was reached. In 1992, following up on Japanese researchers which hinted at the possibility of the existence of ordered mesoporous phases [
All these features have caused a dramatic increase in the research related to this kind of materials [ Despite the fact that trivalent ions can be incorporated into these materials by a wide variety of approaches [ These materials are generally (especially in the case of all-silica materials) rather unstable when undergoing treatment with hot water or steaming [
These two features derive from the fact that the walls forming the M41s are not crystalline. The silica found forming the walls is very similar to amorphous
At this point, it becomes critical to discuss one point on which rests the very title of this review paper, even if it is just for clarification purposes. Despite the clear mention to “nanoporous” in the keywords, no mention of such a term has been hitherto made. In order to refer to this point we should briefly consider how materials are classified according to their porosity, or more precisely, according to their pore size. For many decades the pores were divided into micropores (diameter smaller than 2 nm), mesopores (diameter between 2 and 50 nm), and macropores (diameter larger than 50 nm), according to the IUPAC classification [
New pore size classification as compared with the current IUPAC nomenclature (top). Adapted from [
If we consider the materials that we have described so far (zeolites and ordered mesoporous materials), they both fit in the “nanoporous” materials description according to Mays, even if according to the IUPAC classification they are microporous and mesoporous solids, respectively. As it will be illustrated throughout this review, the vast majority of the materials that will be discussed will fall in the “nanoporous” and “micro/mesoporous” category, inasmuch as a few of them will also possess a porosity which will fit into the “submicroporous” or “macroporous” region. Bearing this in mind, we shall refer to the porosity of the materials using the IUPAC nomenclature, indicating the new nomenclature between parentheses. Thus, zeolites are microporous (sub-nanoporous) materials, whereas ordered mesoporous materials may possess inter- or even supernanopores.
As we discussed with zeolites, there are many hypotheses concerning the mechanism of formation of M41s, or other ordered mesoporous materials synthesized using surfactants. The most relevant mechanisms described to date are formation mechanism from liquid crystals: silicate tubes assembly [ piling of silicate sheets [ “charge density coupling” [ “folded sheets” [ “silicatropic liquid crystals” [ silicate tubes aggregates [ formation mechanism based on electrostatic interactions [ formation mechanism based on hydrogen-bond interactions [
Despite the large variety of mechanisms that have been proposed to explain the formation of these materials, and even from numerous recent experiments carried out by Electronic Paramagnetic Resonance [
As a very illustrative example, we will describe in detail the mechanism proposed by Firouzi and co-workers [
Schematic diagram of the cooperative organization of silicate-surfactant mesophases. Taken from [
The mechanism may be divided into the following stages. Dissolution of organic and inorganic precursors: depending on the surfactant concentration, the starting organic precursor will consist in micelles with different geometries that will be in dynamic equilibrium with individual surfactant molecules. Dissolution of the inorganic precursor consists mainly of silicate anions with different charges. When the two solutions are mixed, the silicate oligomers exchange with different anions to form organic-inorganic hybrids, with a structure that may be different to that of the organic precursor micelles. Multidentate interactions between the silicate oligomers and the surfactant molecules take place, giving rise to the screening of the electrostatic double-layer repulsion forces. This causes the self-assembly of the silicatropic liquid crystal mesophases.
From a purely synthetic point of view, the principle is the same as with zeolites: the key idea is to organize the inorganic oxide species around a template which will give the material its final structure. A representative synthesis would be that of MCM-41 [
Different MCM-41 materials with pore sizes ranging from (a) 20, (b) 40, (c) 65, to (d) 100
The discovery of MCM-41 together with a family of new mesoporous molecular sieves obtained by what the authors called a liquid-crystal templating mechanism was a significant leap in materials science for it effectively filled the existing gap between microporous (subnanoporous-internanoporous) and macroporous (upper half of supernanoporous and beyond). This motivated several research groups worldwide to dedicate outstanding research efforts towards the synthesis of novel mesoporous silicates by modifying the synthesis conditions such as pH, hydrothermal conditions, or the use of surfactants of very different natures (Figure Even though Mobil scientists claimed that they used hydrothermal synthesis for the preparation of MCM-41, the differences between “traditional” hydrothermal synthesis (i.e., the one used for zeolite synthesis) and the “new” one are (1) lower temperatures and narrow temperature range, (2) significantly higher formation rate in the case of ordered mesoporous materials, and (3) while zeolites require water as solvent in their synthesis, several reports have successfully prepared ordered mesoporous materials in nonaqueous media by different strategies [ In order to improve the quality or thermal stability of ordered mesoporous materials, several postsynthesis treatments have been devised. For instance, submitting an MCM-41 material with AlCl3 vapour [ In order to control the final structure of the materials, the control must be carried out on the mesocale, which includes the mesophases formed by the surfactants employed, as well as the resulting pore size and pore wall thickness. In this respect, parameter such as the critical micellar concentration, the phase diagram of the given surfactant in the reaction medium, or the hydrophilic
Some examples of surfactants of different chemical nature. Left: ionic surfactants (CTAB would belong to this category); Center: zwitterionic surfactant; right: nonionic surfactants. For an extensive list of surfactants commonly used in mesoporous materials synthesis please refer to the excellent review by Wan and Zhao [
Although a large number of methods have been reported to prepare all-silica and silica-rich mesostructures (see [
The soft templating method refers to those strategies developed for the synthesis of ordered mesoporous materials using species which may be removed either by calcination [
Unlike the soft templating method, in which the surfactant arranges itself during the synthesis process in order to give rise to the desired structure if the appropriate synthesis conditions are met, the hard templating method relies on the use of preshaped solid forms as templates, which, upon deposition of the desired oxide species and removal of the “mold” gives rise to the final desired material. This method may be found in the literature with other names such as nanocasting [
A schematic diagram of mesoporous material synthesis by hard templating synthesis using SBA-15. Taken from [
In this particular example, the silica mesopores are first infiltrated with a precursor solution of the species that will give rise to the negative replica of the material. The precursor is converted to a solid by reduction or decomposition inside the pores. Then the mesoporous silica template is removed using a suitable solution (aqueous NaOH or HF are amongst the most popular for silica hard templates), and after washing, a material replicating the mesostructure of the hard template is obtained. The presence of disordered micropores between the 1D mesopores of SBA-15 ensures that the replica carbon or metal oxide nanowire arrays are connected by bridges so as to form mesopores (a good example is the material known as CMK-3) [
In the hard templating protocol, it is not straightforward to fill a mesoporous silica template completely, because there are complex interactions between the silica and filtrated ion precursor, which include Coulombic interactions, hydrogen bonding, coordinating interactions, and van der Waals interactions [
Ordered mesoporous solids may also be prepared by the hard templating method without the template having to undergo surface functionalization, following alternative methodologies such as the dual solvent method, solvent evaporation method, solid-liquid method, combustion method, or the impregnation-precipitation-calcination method. In the dual solvent method a suspension of the hard template in a highly hydrophobic solvent (e.g., hexane) is mixed with a concentrated aqueous solution of the oxide precursor. Generally, the solution volume is equal to the pore volume of the template in order to maximize the impregnation quantity and prevent the growth of solid phase on the outer surface of the hard template [
The preparation of mesoporous oxides by the soft templating method involves different reactions such as the hydrolysis of the inorganic precursors, condensation of the inorganic oxide ions, and self-assembly with the template to give rise to the final mesostructure. All these steps have proven to be highly sensitive to the reaction conditions. If the self-assembly processing is based on the arrangement of already formed monodisperse nanoparticles around the template, the process will be less sensitive to the variable reaction conditions [
Despite the fact that ordered mesoporous materials were firstly envisioned as a means to overcome the limitations of zeolites when operating in large packed beds, the low crystallinity of the walls of the resulting materials resulted in a significantly poorer performance than the one intended. Several groups attempted to improve this aspect by a variety of techniques, such as the development of benzene-silica hybrids to improve wall crystallinity [
Commercial bulk metal oxides are widely available and used extensively in catalysis, sensing, adsorption and separation, purification, energy conversion and storage, and so forth [
In the field of solar cells, Dye Sensitized Solar Cells (DSSCs) use a light absorbing dye which transfers the electrons to a cathode by means of an electrochemical reaction, which in turn gives a current (Figure
Schematic illustration for the operation of a dye-sensitized solar cell (DSSC). Taken from [
Mesoporous metal oxides are also very attractive materials for the construction of electrode materials for lithium ion batteries. The domains of mesoporous metal oxides are micron-sized, thus the fabrication of composite electrodes should be similar to the synthesis of dense micron-sized particles, thus resulting in a similar packing density (i.e. no efficiency loss). The contact between domains is sufficiently good to ensure an efficient electron transport. The thin walls of the oxide framework (in the range of a few nanometers) result in short diffusional pathways for the lithium ions on intercalation. Moreover, thanks to the high surface area that ordered mesoporous materials exhibit and the hydrophilic nature of inorganic oxides, flooding of the pores by the electrolyte will result to enhanced electrode reaction kinetics (this benefit also applies in the case of the previous advantage). Finally, the ordered mesopores have approximately the same dimensions as the walls, thus the lithium ion transportation is practically identical everywhere within the pore and within the wall. Such an ordered structure ensures that a mesopore
Another area in which mesoporous materials have been applied with some promise is that of supercapacitors. These materials are of high interest due to their high energy and power density, which in turn bridges the gap between conventional batteries and classical electric capacitors [
The high internal surface area of mesoporous materials in general and the redox character displayed by some mesoporous metal oxides has been the drive of an enormous research effort to prove the efficiency of these materials in a very wide array of reactions. Even though we will just address a few examples due to the general scope of this review, any readers interested in this topic are recommended to read the excellent paper by Johnson [
A visual one-shot image of the usefulness of ordered mesoporous silica (MCM-41) used as catalyst support in single-step hydrogenation reactions using a
The high accessibility of ordered mesoporous materials also makes them ideal candidates for metallic nanoparticles, as we have shown in a number of papers which clearly present the potential of these materials in different reactions of significant applied interest [
The use of metal oxide-based sensors is of high current interest due to their robustness, cost efficiency, and excellent sensing capabilities. When a chemical reaction takes place on a semiconductor surface, the electron transfer between the gas molecules and semiconducting material surface will result in a quantifiable variation of a measurable parameter such as electric conductivity
Images (top) and schematics (bottom) of an example sensor substrate (UST Umweltsensortechnik GmbH, Germany). Taken from [
Despite all the applications that have been mentioned in this review, in which ordered mesoporous materials have clearly shown their potential, the industrial implementation is still a few years since one of the main obstacles this new family of solids must overcome is the cost effectiveness of the resulting materials
Throughout this paper we have dealt with materials which were either microporous (subnanoporous) or mesoporous (inter- or supernanoporous), with either a crystalline structure or formed by amorphous skeletal walls which possessed regular porous ordering. As we previously mentioned, one of the “philosopher’s stone” in materials science and catalysis would be the development of zeolites which retained their outstanding catalytic activity while being able to process larger molecules or to reduce the diffusional limitations in catalyst beds due to their small pore size. In this specific instance is where hierarchical materials fit the requirements perfectly. The Oxford English Dictionary describes the term “hierarchical” as “belonging or according to a regular gradation of orders, classes, or ranks.” This, applied to porous materials usually refers to a type of materials possessing pores (preferentially regular) of different categories, which are effectively combined in a material with superior properties. In principle, the term hierarchical should refer to materials which have different types of ordered porosity, whose sizes lie in the different pore size regions (micro-, meso-, macroporous or sub-, inter- and supernanoporous), but the term is also used for materials in which some of the porosity may not be perfectly ordered, but which nonetheless present a certain degree of structuring. In short, in a hierarchical material each level of porosity in the structure fulfills a distinct complementary task: the micropores hold catalytically active sites and adsorb smaller molecules, whose access is facilitated by the newly introduced mesoporosity which can also adsorb larger molecules, with the enhanced diffusion through the network introduced by the macroporosity. Just to give an account of the importance of hierarchical materials, the journal Science recently appointed the development of such materials as one of the runners-up for the “Scientific Breakthrough of 2012” [
There are essentially three ways to create materials with hierarchical porosity: to take a porous material and create a different family of pores which belong to a different category (top-down techniques), to build a material from scratch designing the appropriate template or a selection of templates (bottom-up techniques), or to use a combination of both. The earlier works dealing with bottom-up techniques described the development of microporous-mesoporous composites [
(a)
In the case of top-down synthetic routes, they involve a series of postsynthetic treatments of previously formed materials by the removal of framework atoms or delamination. Some classical examples include steam [
Schematic representation of desilication by NaOH treatment of MFI-type zeolites. The images show two MFI zeolite crystals before and after alkaline treatment, respectively. Adapted from [
Another recent development in the preparation of hierarchical materials involves both top-down and bottom-up techniques. Following on a previous work [
Schematic illustration of a combined soft- and hard-templating synthesis of mesoporous carbons subjected to postsynthesis physical activation with CO2 or
Concerning the applications of these materials, some recent reviews have already pointed out that hierarchical zeolitic materials can clearly outperform their solely microporous counterparts in different reactions such as isomerization, alkylation, acylation, aromatization, pyrolysis, cracking, and methanol-to-hydrocarbons [
From the first porous carbon that was used by man, it is clear that we have taken many and very significant steps towards the formation of ordered porous materials which combine different pore architectures with tunable pore sizes (and shapes), which can be effectively combined to form what we could call “materials-on-demand.” It is clear that materials science has evolved noticeably since the discovery of zeolites and has found a way to evolve in order to create larger pores when diffusional limitations were the main hurdle to be overcome, giving rise to OMMs. Then, as hydrothermal stability became a serious issue, scientists developed OMMs with highly crystalline walls which showed very high thermal and hydrothermal stability. The last (surely not final) frontier that has recently been breached is that of combining different types of pores into one single material which has given rise to the family known as hierarchical materials. In this respect, great advances have been made in the development of different hierarchical materials such as carbons, silicogermanates, and especially zeolites. In the last years major progress has been achieved in the synthesis and application of mesoporous zeolites prepared by a variety of techniques, both in terms of reported benefits in catalyzed reactions and general understanding of the synthesis mechanism. The industrial application of these exciting materials should be a matter to be solved in the coming years, that is, if they are not being already applied in a medium-to-large scale already. It should be noted that the materials which have the best outlook towards industrial implementation seem to be hierarchical zeolites synthesized with top-down techniques since they do not require the use of large amounts of sophisticated templates which hitherto need to be custom-made. Nevertheless, some fundamental issues concerning the mesopore formation by desilication still need to be fully addressed. Although some general rules and guidelines have been devised, the process of mesopore formation on a molecular level in this type of materials is still unclear. In any case, the next step towards industrial incorporation of these materials was already undertaken by Groen and coworkers [
The author would like to thank the Spanish Ministerio de Ciencia e Innovación (MICINN) (Project RyC-2009-03813), as well as the PROMETEO project from the Generalitat Valenciana and the FEDER program (Prometeo/2009/047) for the financial support for this work.