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Aluminosilicate MCM-41 materials prepared by post-synthesis alumination using aqueous aluminium chlorohydrate exhibit remarkably high mechanical and hydrothermal stability and in addition the materials, after steaming at high temperatures, possess stronger Brønsted acid sites compared to the parent material.
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... SBA-15 is a promising support in catalysis. A lot of interest has been focused on the grafting of alumina onto the surface of ordered mesoporous silica . The alumination procedure has a strong effect on the pore structure, the acidity and the surface properties of aluminated materials . ...
... A lot of interest has been focused on the grafting of alumina onto the surface of ordered mesoporous silica . The alumination procedure has a strong effect on the pore structure, the acidity and the surface properties of aluminated materials . This work focuses on the analysis of VO x species distribution at the aluminated SBA-15 materials (Al-SBA-15) and the catalytic activity/selectivity of V-Al-SBA-15 in oxidative dehydrogenation (ODH) of ethane. ...
... The grafting of alumina onto SBA-15 (Si) was performed by the modified method of Mokaya, originally developed for the post-synthesis alumination of siliceous MCM-41 using aqueous solution of aluminum chlorohydrate . 5 g of the dried SBA-15 was mixed with 125 ml aqueous solution containing from 0.2 to 8 g of Al 2 Cl(OH) 5 . ...
This paper focuses attention on the grafting of alumina onto SBA-15 material and the subsequent impregnation of vanadium oxide species (VOx
). The role of alumina loading on the distribution of VOx species was studied in V–Al2O3–SBA-15 catalysts and its activity/selectivity in oxidative dehydrogenation of ethane. The activity of V–Al2O3–SBA-15 materials are correlated with the textural properties (N2-adsorption/desorption isotherms), the distribution of VOx
species (H2-TPR and DR UV–vis) and the acidic properties (NH3-TPD). While V–SBA-15 mainly contains monomeric tetrahedrally coordinated VOx species, grafting of alumina on SBA-15 catalysts result in decreasing population of isolated monomeric VOx units in V–Al2O3–SBA-15 catalysts, increasing population of Td
oligomeric VOx species and the formation of a low amount of octahedrally coordinated VOx species. V–Al2O3–SBA-15 catalysts are more selective to ethane (at the same conversion of ethane) compared to V–SBA-15 and V-alumina catalysts.
... 35 Mokaya et al. reported the synthesis of Al-MCM-41 by reacting calcined MCM-41 with solution of aluminum chlorohydrate at 80°C followed by subsequent treatment. 50 Ryoo et al. incorporated Al 3+ to ordered mesoporous silica (MCM-41, MCM-48, SBA-1, KIT-1, and MSU-1) by mixing them into nonaqueous (e.g. ethanol) solutions of AlCl 3 or Al(NO 3 ) 3 at 20-60°C. ...
...  Since the presence of the strained siloxane bonds, which are easily hydrolyzed in the presence of water or even of moisture,  is mainly responsible for the low hydrothermal stability of silica materials, some attempts to stabilize the structure of mesoporous materials have been reported in the literature. For instance, post-synthesis treatments like coating the pore interiors by a hydrophilic layer,  grafting and ion exchange [17,19] and post-synthesis treatment in water in the presence a surfactant afford more stable silica frameworks.  The key idea is, however, the preparation of materials with more silica condensation in the pore walls in order to enhance the hydrothermal stability and to have the less possible terminal hydroxyl groups that make the structure unstable under hydrothermal and steam conditions. ...
In this study, we have investigated the thermal and hydrothermal stability of a dual mesostructured silica and of a macro‐mesoporous silica. Concerning the thermal stability, results obtained by SAXS, nitrogen adsorption‐desorption analysis and infrared spectroscopy show that the macro‐mesoporous material is stable up to 550°C, the mesostructure being only slightly damaged, but a contraction of both the macropores and the mesopores is observed upon calcination. Considering the dual mesoporous silica, regardless the calcination temperature, the matrix with small pores is completely degraded whereas the one with the larger pores is only weakly damaged. With regard to hydrothermal stability, all the investigated porous silica are damaged when they are plunged into boiling water because of the hydrolysis of the superficial Si–O–Si linkages, involved by water molecules adsorbed on the silanol groups. The extracted and calcined macro‐mesoporous material is stable for one hour in boiling water, whereas the non‐calcined sample is stable for only 30 minutes. The collapse reaches only the mesopores, the macropores remain unaltered. The dual mesostructured silica is also sensitive to boiling water, but two kinetics of degradation are observed. The network having small pores disintegrates after 3 hours, while the matrix having large pores begins to collapse after 8 hours. This behavior has been attributed to the difference in the mesopore wall thickness between both matrixes.
... There have been a number of advances that the Materials Chemistry Group introduced to the chemistry of MCM-41, not least being a method for post-synthetic grafting of aluminium onto this purely silica mesoporous material, leading to an increase in important Brønsted acid groups, 40 along with retention of chemical properties, overall hexagonal architecture and a remarkable improvement in stability -resistant upon hours of steaming at 750 °C or extended exposure to boiling water ( Figure 5b). 33 Another major class of inorganic materials addressed and advanced by the Jones Materials Chemistry Group are LDH solids, composed of cationic mixed-metal hydroxide layers separated by layers of other types of often exchangeable anions. Bill's work on LDHs has included experimental studies on their ion exchange properties, 41 as well as theoretical investigations 42 of their structures -especially the possibility of controlling the separation between and arrangement of catalytically active LDH layers through intercalation of inorganic and organic guests. ...
An introduction to the virtual special issue in honor of Prof. William Jones, wherein the authors reflect on his scientific career and his many outstanding contributions to organic solid-state chemistry.
... The insertion of alumina into the mesoporous silica framework, for instance, has been considered beneficial to water adsorption because it may alter the acidity/basicity of the surface, increasing its hydrophilicity 9 . Moreover, the addition of alumina has been said to strengthen the structure of mesopores against corrosion in the presence of water  . ...
In the present report, Al-SBA mesoporous materials were investigated for use as water adsorbents. Different precursors for aluminum were investigated: aluminum sulfate and nitrate, boehmite, and aluminum tri-sec-butoxide. Furthermore, three different procedures for addition of the silica and alumina sources were tested. Samples were characterized by nitrogen adsorption to observe their textural characteristics and by thermogravimetric desorption of water to address their performance. The sample synthesized with aluminum tri-sec-butoxide as aluminum precursor and a pre-hydrolysis step of the silica source presented the highest adsorption capacity: 1057 g.kg-1. It was also observed that water attaches more strongly to Al-containing samples than to pure silica, and that water adsorption capacity is more related to the pore volume of samples than to their surface area. This work has shown that functionalizing SBA-15 materials with aluminum is a promising strategy for producing water adsorbents with improved performance and potential for many applications.
... Incorporation of heteroatoms such as aluminium into the highly structured SBA-15 materials [7,8] enhance and optimize its catalytic activity. Up to now, there are two typical approaches to synthesize conventional mesoporous aluminosilicates: direct synthesis [9,10] and post-synthesis [11,12]. Ma et al.  discovered that, in comparison with direct synthesis method, the acidity and catalytic activity of Al-SBA-15 synthesized by post-grafting method has shown to be higher due to formation of Lewis and Brønsted acid sites. ...
Al–SBA-15 is an interesting mesoporous material
having highly ordered nanopores and a large surface
area, which is widely employed as catalysts and adsorbents,
but relatively few studies on the surfaces properties of this
type of materials have been carried out. The purpose of
the present work was to advance knowledge on the textural
properties of Al–SBA-15 by applying the accurate NLDFT
method as well as to gain insight into the surface characterisation
and acidic trend of this material. Mesoporous
Al–SBA-15 molecular sieves, in three SiO2/Al2O3 ratios:
50, 75 and 100 were accomplished by post-synthesis alumination
in aqueous solution of a purely siliceous SBA-15material. The obtained solids were characterized using
powder X-ray diffraction (XRD), X-ray fluorescence
(XRF), N2 adsorption–desorption (BET/DFT), transmission
electron microscopy (TEM), water adsorption and zeta
potential measurements. The results indicate that Al atoms
have been successfully incorporated into the framework of
the hexagonal mesoporous SBA-15. The aluminum introduced
amount has remarkably affected the surface properties
of the SBA-15 solid, indeed microporosity decreased.
Furthermore, the esterification test showed that the Al–
SBA-15 material exhibit Brønsted acid properties with an
interesting activity leading to yields of ~90% of biodiesel.
... MCM-41 were synthesized with different CnTMABr (n-Alkyltrimethylammonium bromides, n means alky chain lengths from 10 to 20) named as MCM-Cn. Al-MCM-41 were synthesized by post modification according to Mokaya's studies (Mokaya and Jones 1998;Mokaya 2000), which was designated as Al-Cn (corresponds to dipping with 0.48 mol/L ACH). Cations doped Al-MCM-41 were obtained by blending Al-Cn with different cation solutions(1 mol/L LiNO 3 , NaNO 3 and 0.5 mol/L MgCl 2 ) and designated as Al-Cn-M (M stands for Na, Li and Mg). ...
Post modification methods were used to synthesis MCM-41 with different pore sizes, modified by Al grafting and cation doping. Water adsorption characteristics were measured by dynamic water vapor adsorption and analyzed regarding equilibrium and kinetics. Results showed that Al grafting and cations doping modifications on MCM-41 changed water adsorption characteristics. Doped cations strengthened the water adsorption capacity at low RH following the order of Al–C12–Mg > Al–C12–Li > Al–C12–Na. Kinetics analysis indicated that capillary condensation process developed slowly due to low diffusion constant. But the adsorption transient uptake rates in capillary condensation sections were larger compared to other sections, Larger RH step not only resulted in larger uptake rate, but also promoted capillary condensation. Instead of calculating or measuring isotherms and diffusion coefficients respectively, parameters like transient uptake rate corresponding to the effects of both transport ability and adsorption capacity should be considered.
... The catalysts with the hexagonal structure, such as Al-MCM-41, offered an opportunity for processing bulky molecules 6 and attracted much interest. 7 Unfortunately, the poor hydrothermal stability of the MCM-41-type materials restricts their wider applications in industrial catalysis. Nevertheless, improved hydrothermal stability of cubic Al-MCM-48 materials was achieved by supercritical uid (SCF) impregnation. ...
A novel non-hydrolytic sol-gel (NHSG) synthesis of mesoporous aluminosilicate xerogels is presented. The condensation between silicon acetate, Si(OAc)4, and tris(dimethylamido)alane, Al(NMe2)3, leads to homogeneous aluminosilicate xerogels containing Si-O-Al linkages through acetamide elimination. The addition of Pluronic P123 and F127 templates provides stiff gels that are, after calcination at 500 [degree]C, converted to stable mesoporous xerogels with high surface area (? 600 m2 g-1) and wormhole-type pores (d = 5.1 nm). The xerogels exhibit high catalytic activity in aminolysis of styrene oxide (82 % conversion) with the turnover frequency up to 100.
Hexagonally mesoporous 5–30%Ni-AlSBA-15 (30) sorbents were prepared by one-step strategy synthesis and the adsorptive desulfurization performances were investigated in a fixed bed adsorber via model and commercial liquid fuels at ambient pressure. The highest sulfur capacity (17.46 mg-S/g-sorbent) over 20%Ni-AlSBA-15 (30) correlated closely with high Lewis acid sites and larger SBET of 20%Ni-AlSBA-15 (30). The characterization results of N2 adsorption, FT-IR, Py-FT-IR, ICP, HRTEM, EDXA, small- and wide-angle XRD revealed that aluminum or nickel atoms could enter effectively the Si-O-Al (or Ni) framework of SBA-15 and were randomly dispersed highly on the hexagonal pore walls of SBA-15. The hexagonal p6mm symmetry mesoporous structure of 20%Ni-AlSBA-15 (30) still remained intact after four successive desulfurization/regeneration cycles, indicating that 20%Ni-AlSBA-15 (30) presents ultrahigh structural stability and regenerable ability. As comparison, SBA-15 and AlSBA-15 (SiO2/Al2O3 = ∞, 100, 50, 30, 20, 10) were also prepared and evaluated on physical properties.
Al-FSM-16 mesoporous silicas were synthesized either by direct method, from Al-kanemite (Al-FSM-16/D), or by post-synthesis impregnation of purely siliceous FSM-16 with Al(NO3)3 (Al-FSM-16/P) and characterized with XRD, XRF, SEM, TEM, nitrogen sorption isotherms, ²⁷Al and ²⁹Si MAS NMR, FTIR, XPS, NH3-TPD, FTIR of pyridine adsorption and catalytic decomposition of ethanol. Only substitutional Al sites exist in Al-FSM-16/D, while in Al-FSM-16/P some Al remains in extra-lattice positions. Upon transformation of Al-FSM-16/D into hydrogen form a certain amount of extra-framework Al is formed. Direct alumination introduces a higher degree of structural disorder. In Al-FSM-16/D, Al is preferentially accumulated at inner pore walls, while in Al-FSM-16/P external surface is Al-rich. Post-synthesis alumination is more efficient in introducing acid sites into FSM-16. The generated acidity is of Brønsted and Lewis nature, the latter being stronger than the former.
The catalytic properties of zeolites can be improved in principle by anchoring them to mesoporous aluminosilicate networks that should allow diffusion of the species of interest towards the active sites. In this work the formation of composites between commercial zeolites and a sol precursory of aluminosilicates was studied in order to obtain this type of material.
The silica-alumina/zeolite composites were prepared at room temperature and basic pH by using the sol-gel method in combination with a coprecipitation process. Aluminum nitrate and tetraethylortosilicate were used as aluminum and silicon precursors and the biopolymer chitosan added to the solution was used as a templating agent. Finally between 5 and 10% in weight of the zeolite (ZSM-5) was added to this solution. When the resulting white and dense gel was dried, calcined at 550°C/12 h, composites with high surface area (400 m2/g) and pore diameter of around 30 A0 were obtained. The materials are mesoporous but they maintain the microporosity of the constituent zeolite. On the other hand, when dry material undergoes hydrothermal treatment before calcination, the surface area diminishes ca.100 m2/g, but the mean pore diameter increases to 150 A0 with a pore volume of 1,3 cm3/g. The total micropore volume remains constant indicating that the structure of zeolite was not affected with the treatment, which was also corroborated by XRD analysis. It was also demonstrated that the presence of Chitosan lead to a narrower distribution of the pore size. These results allowed us to conclude that the composites are conformed by zeolite supported on a mesoporous silica-alumina matrix where its porosity is modeled by the HT-treatment and by the presence of chitosan.
Mesoporous molecular sieves such as hexagonally ordered MCM-41 have found potential applications in catalysis, adsorption and separation, preparation of advanced materials, and molecular devices since they were firstly discovered in 1992. However, compared with those of zeolites, the catalytic activity and hydrothermal stability of these mesostructured materials are relatively low, which strongly influence their practical applications in industrial catalytic reactions of petroleum. Here it is systemically summarized the syntheses of a series of catalytically active and hydrothermally stable mesoporous materials, which include (1) ordered mesoporous aluminosilicates assembled from preformed nanosized aluminosilicate zeolites in basic media; (2) ordered mesoporous aluminosilicates assembled from preformed nanosized aluminosilicate zeolites in strongly acidic media; (3) ordered mesoporous titanosilicates assembled from preformed nanosized TS-1 precursors; and (4) ordered mesoporous materials containing heteroatoms assembled from preformed nanosized zeolite precursors containing heteroatoms such as Fe, V, Cr, and Ga.
Mesoporous molecular sieves such as hexagonally ordered MCM-41 have potential applications in catalysis, adsorption and separation, preparation of advanced materials, and molecular devices since they are firstly discovered in 1992. However, as compared with zeolites, the acidity and hydrothermal stability of these mesostructured materials are relatively low, which strongly affect their practical applications in industrial catalytic reactions of petroleum. Here it is systematically summarized that the research work on improvement of acidity and hydrothermal stability for ordered mesoporous materials, which include (1) the assembly of super acidic components into the mesopores; (2) the addition of inorganic salts, organic ammonium compounds during the synthesis of mesoporous materials or post-synthesis of mesoporous materials; (3) the synthesis of mesoporous materials from novel templates such as triblock copolymers; (4) the assembly of mesoporous materials from pre-formed zeolite structure-directing-agent(zeolite seeds solution) with zeolite primary and secondary building structure units.
The direct synthesis of templated super-microporous silica with spherical and cylindrical shapes was achieved in a water/acetonitrile/n-dodecylamine system, using 3-mercaptopropyltrimethoxysilane and 3-trifluoromethyltrimethylsilane as co-template. The benefits of this system to prepare such silicas were high surface area, much enhanced wall thickness, and consequently to greater structural stability. The particles obtained from this route were spherical and cylindrical. The sample was characterized by using N2 adsorption /desorption, XRD, FTIR, SEM, TG-DSC and 13C CP MAS NMR methods, and the process of the direct preparation was given.
Obtaining materials with a structuring that is perfectly controlled and objects of nanometer size may have interesting applications in the fields of electronics, environment, biology or medicine. Therefore, properties of three mesoporous materials have been studied using adsorption isotherms of phenol at 25 °C Two of these materials have a hexagonal structure of the type MCM-41 and FSM-16, whereas the third one has a cubic structure of the type MCM-48. This helped to determine the type of adsorption; it obeys both the Freundlich isotherm and the Langmuir one. The experiments of phenol's adsorption show that the MCM-48 grafted (G) with TMCS is a good adsorbent with maximum adsorption capacities respectively equal to 22,4 × 10-3 g/L and a percentage of adsorption equal to 72% at 25 °C
The recent progress in the supramolecular-templating synthesis of ordered mesoporous silica has been reviewed. The discussion is focused on the control of the internal structure (the mesostructure and the related structural parameters) and exterior morphology of the mesoporous silica templated by both cationic surfactants and poly(ethylene oxide) based block copolymers. The potential development of the research on mesoporous silica is also proposed.
MCM-41's limited hydrothermal stability has been often related to the hydrolysis of Si-O-Si bonds due to the low degree of condensation, its thin walls or a combination of them. In this work, evidence for an additional factor is provided; a physical effect that occurs during the drying of the hydrothermally treated calcined material due to the intense capillary stress exerted in water. Depending on both physical (i.e. mechanical) and chemical (i.e. hydrolysis) resistances, the structure undergoes differently. Three MCM-41 samples with different degree of condensation were investigated. The most remarkable results are found with un-aged TEOS based material, which gets fully disordered and shrunk for all applied hydrothermal temperatures in water. Comparison between water and a low-surface-tension-solvent drying revealed that capillarity is responsible for the loss of ordering (and shrinkage) at moderate hydrothermal temperatures. The material's structure is hexagonal and shrinkage-free under the low-surface-tension-solvent route. At a high hydrothermal temperature, hydrolysis is extensive and responsible for the loss of ordering. The other remarkable finding regards the aged MCM-41 mesostructure that maintains the hexagonal features at all applied temperatures in water, and it is more stable against capillarity at high temperature. The Na-metasilicate based material is mechanically very stable and gets disordered at high temperature due to hydrolysis.
We describe the preparation of crystalline as-synthesized aluminosilicate-surfactant mesophases and surfactant-free aluminosilicate mesoporous materials derived from the layered silicate Na-RUB-18, by performing hydrothermal restructuring in the presence of cetyltrimethylammonium (CTA) surfactant molecules. The hydrothermal treatment, at 150 oC for 48 h, with Na-RUB-18 as silica source and a known amount of aluminium isopropoxide (gel Si/Al ratio 5, 10 or 20) in the presence of cetyltrimethylammonium (CTA) ions generates molecularly ordered aluminosilicate-surfactant mesophases. The transformation from mesophase to mesoporous materials takes place with retention varying levels of crystallinity depending on the mode of template removal. The highest apparent retention of crystallinity in a surfactant-free mesostructure is achieved at Si/Al ratio = 20 after surfactant removal via solvent extraction in acidified ethanol. The textural properties of the mesoporous materials (surface area of 141 – 388 m2g-1 and pore volume in the range 0.12 – 0.46 cm3g-1) depend on the mode of surfactant removal and Si/Al ratio. The mesoporous aluminosilicates are strongly acidic with most of the acid sites generated (> 80% and typically above 95%) classified as strong sites, and exhibit attractive activity as solid acid catalysts.
This review paper deals with proven and potential applications of mesoporous molecular sieves in catalysis. In addition to introduction and conclusion, the text is divided into two parts, respectively, dedicated to the design of solid catalysts and catalyst supports and to some relevant examples of catalytic processes.
An achievement of increasing the both adsorption and release of heparin in drug delivery system is reported. The reduced graphene oxide (rGO) material can efficiently adsorb heparin in aqueous solution up to 112 mg g-1 owing to its layered and stacked structure. Moreover, this carbon vessel is able to release 90 mg g-1 of the drug within 30 days, exhibiting the highest released/adsorbed ratio of 80% up to date and has become a promising candidate as a novel drug releaser.
The aging characteristics and stability of decade (12 years) old mesoporous silica MCM-41 samples of varying pore size and wall thickness has been tracked and compared with freshly synthesized samples. It was found that calcined forms of hydrothermally synthesized MCM-41 samples are, depending on their initial wall thickness, generally stable to storage under ambient conditions over the 12 year period. The calcined samples retain all (i.e., 96–100%) of their surface area and virtually all (87–96%) their pore volume. Rather surprisingly, calcined MCM-41 with wall thickness of ca. 11 Å exhibit greater apparent aging stability with respect to retention of unit-cell parameter compared to thicker walled (22 Å) analogues. After 12 years of storage, both types of samples exhibit pore wall thickness of 11–12 Å representing much greater unit cell contraction for the 22 Å sample. Despite varying levels of initial silica condensation and silanol concentration, both types of aged calcined samples exhibit similar 29Si NMR spectra with similar silanol concentration (ca. 6.5 mmol/g) and silanol density (4.5/nm2). The wall thickness, level of silica condensation, silanol loading, and silanol density of the calcined aged samples closely match values that have been shown by molecular dynamics studies to have the highest stabilization energy for MCM-41 structures. The aged calcined samples are thermally stable to further calcination at 800 °C, wherein they retain 90% of their surface area. Aged as-synthesized forms of MCM-41 also have high stability and remain largely unchanged, and on calcination, behave in a manner similar to that of freshly prepared MCM-41. For aged as-synthesized samples, there is an apparent increase in silica condensation and phase separation of template molecules from the silica, and this forms the basis of their aging stability and stability to subsequent calcination.
AlSBA-3 mesostructured materials with different Si/Al ratios were directly synthesized for the first time. The influence of the synthesis medium (different pH of synthesis mixture) and aluminium source on the efficiency of aluminium incorporation was studied. The increase in pH of reacting mixture allows to introduce much higher number of Al atoms, but on the other hand it causes worse ordering of the structure. The aluminium isopropoxide used as Al source leads to relatively high Al content in the final materials. The presence of Al in the framework positions is proved by Al-27 MAS NMR. The obtained materials show considerable catalytic activity for the acid-catalyzed reactions (dehydration of 2-propanol, cumene cracking).
This paper presents the effect of different types of additives on the morphology and mechanical performance of polypropylene (PP). Three different types of nanoparticles, containing mesoporous silica (MCM-41), Hydroxyapatite (HA) and the composite of MCM-41 and HA (MH) were used. Nanocomposites containing PP, 3 wt.% of maleic anhydride grafted polypropylene (PP-g-MA) and 3 wt.% of different nanoparticles were prepared using the melt-compounding technique in a twin-screw extruder. The bulk mechanical response of the nanocomposites such as tensile, flexural and Izod impact properties were studied. The results of mechanical tests show that at the same nanomaterial content, all the nanofillers cause better tensile, flexural and impact strength than neat PP. The MH nanoparticle improves the mechanical properties of PP, better than the other nanoparticles because this nanofiller contains good properties of both MCM-41 and HA nanoparticles in itself. In order to investigate the effect of foam agent on the mechanical properties of neat PP and nanocomposites based on PP, inorganic azodicarboxamide was added to the aforementioned mixtures as chemical blowing agent and the foamed specimens were resulted using the melt-compounding technique. The results reveal that addition of foam agent to mixtures, leads to increase the flexural characteristics of samples, but the tensile properties and impact strength decrease. Scanning electron microscopy (SEM) was used to assess the fracture surface morphology and the dispersion of the nanoparticles. X ray diffractometry (XRD) was used to examine the intercalation effect on the nanocomposites. The observations show that the nanomaterials were well dispersed in the polymer matrix and the enhancement of the interface between the matrix and fillers was obtained by the incorporation of MH, MCM-41 and HA nanoparticles into PP matrix.
Highly ordered aluminum-containing mesoporous silica (Al-MCM-41) was prepared using attapulgite clay mineral as a Si and Al source. Mesoporous complexes embedded with CuO nanoparticles were subsequently prepared using various copper sources and different copper loadings in a direct synthetic route. The resulting CuO/Al-MCM-41 composite possessed p6mm hexagonally symmetry, well-developed mesoporosity, and relatively high BET surface area. In comparison to pure silica, these mesoporous materials embedded with CuO nanoparticles exhibited smaller pore diameter, thicker pore wall, and enhanced thermal stability. Long-range order in the aforementioned samples was observed for copper weight percentages as high as 30%. Furthermore, a significant blue shift of the absorption edge for the samples was observed when compared with that of bulk CuO. H2-TPR measurements showed that the direct-synthesized CuO/Al-MCM-41 exhibited remarkable redox properties compared to the post-synthesized samples, and most of the CuO nanoparticles were encapsulated within the mesoporous structures. The possible interaction between CuO and Al-MCM-41 was also investigated.
Evaluation of low and ultra-low Al content (Si/Al between 50 and 412) Al-MCM-41 materials prepared via three alumination routes (direct mixed gel synthesis, post synthesis wet grafting and post synthesis dry grafting) indicates that trace amounts of Al introduced via dry grafting can stabilise mesoporous silica MCM-41 to steaming at 900°C for 4 h. It was found that trace amounts of Al (Si/Al > 400) introduced via so-called dry grafting of Al stabilise the virtually purely siliceous MCM-41 to steaming, while Al incorporated via other methods that involve aqueous media such as direct mixed gel synthesis or wet grafting of Al offer only limited protection at low Al content. It is particularly remarkable that a dry grafted sample with a Si/Al ratio of 412 and surface area and pore volume of 1112 m2/g and 1.20 cm3/g, respectively, retains 90% (998 m2/g) of surface area and 85% (1.03 cm3/g) of pore volume after steaming at 900°C for 4 h. Under similar steaming conditions, the mesostructure of pure silica Si-MCM-41 is destroyed with a 93% reduction in surface area from 958 m2/g to 69 m2/g and 88% decrease in pore volume from 0.97 cm3/g to 0.12 cm3/g. The steam stable ultra-low (i.e. trace) Al containing MCM-41 materials is found to be virtually similar to mesoporous pure silica Si-MCM-41 with hardly any detectable acidity. The improvement in steam stability arises from not only the presence of trace amounts of Al, but also from an apparent increase in the level of silica condensation that is specific to dry grafted alluminosilicate MCM-41 materials. The more highly condensed framework has fewer silanol groups and therefore is more resistant to hydrolysis under steaming conditions.
Al-SBA-15 containing plug structures inside straight channels have been hydrothermally synthesized through a one-step synthesis strategy in an environmentally friendly acid-free medium. The effects of steaming, alkali, and acid post-treatments on the structural properties were investigated by powder X-ray diffraction (XRD), nitrogen adsorption–desorption at 77 K, transmission electron microscopy (TEM), scanning electron microscopy (SEM), FT-IR spectrum, 27Al MAS NMR, and inductively coupled plasma (ICP) analyses. The presence of the plugs can significantly enhance structural stability against steaming or alkali post-treatments. Acid post-treatment can be a very simple and convenient technique for adjusting the textural properties of plug-containing Al-SBA-15, and especially for optimizing the accessibility to active sites. Acid post-treatment at the low temperatures of 313 and 323 K results in the formation of a larger specific surface area (905 m2/g) and pore volume (0.94 cm3/g) through an “open door” effect in the plugs, without eliminating any of the plugs from the channels. In particular, nearly half of the secondary micropores on the pore walls were preserved. In contrast, raising the acid post-treatment temperature further to 333 and 343 K caused a “closed doors” effect through the blockage of pores in the plugs by the extracted fragments. After loading with nickel, the catalyst exhibited a higher yield of C10+ olefins in ethylene oligomerization, and more importantly, after steaming and acid post-treatments at higher temperatures, the samples showed shape selectivity for the C16 olefin.
Platinum catalysts (1.5 wt.%) containing AlSBA-15 obtained with various aluminium precursors were tested for activity in 1-methylnaphthalene hydrogenation. Experiments were carried out in a continuous-flow system at atmospheric pressure (240–350 °C, W/F = 0.8 g s/cm3). It was found that 1-methylnaphthalene conversion over Pt-loaded catalysts was not influenced by the aluminium precursor used in the preparation of the Pt/AlSBA-15 catalysts. However, the Pt catalyst prepared with AlSBA-15 obtained from aluminium sulphate provided a higher cis-methyldecalins/trans-methyldecalins ratio in the reaction products as compared to the catalysts obtained with aluminium nitrate or aluminium isopropoxide. Consideration was also given to the influence of platinum amount (0.5, 2.5 and 4.5 wt.%) on the catalytic performance of bifunctional Pt/AlSBA-15 catalysts (AlSBA-15 obtained with aluminium sulphate) in decalin hydroconversion. It was shown that impregnation of AlSBA-15 with H2PtCl6 increased Brønsted acidity. Investigations into decalin conversion were conducted in a continuous-flow system with a fixed-bed reactor (5 MPa, 300–380 °C, H2:CH = 500 N m3/m3; WHSV = 2 h−1). Incorporation of 0.5 wt.% Pt into AlSBA-15 yielded a catalyst with the highest dispersion of the platinum phase and the highest yield of ring opening products, amounting to 26.4 wt.% at 380 °C.
The influence of high temperature synthesis of MCM-41 on structural ordering, textural properties and stability has been investigated. Increasing the crystallization temperature resulted in the enlargement of pore size and some reduction of long-range ordering but at the same time generated thicker pore walls. The overall effect was a gradual decrease in surface area and pore volume accompanied by improvement in stability for higher crystallisation temperature samples. Successful synthesis of well ordered MCM-41 materials was achieved up to 190 °C, which is the highest temperature so far reported for synthesis of alkylammonium ion (cetyltrimethylammonium bromide, CTAB) templated materials. MCM-41 samples synthesized at 180 and 190 °C exhibit thick (up to ca. 23 Å) pore walls wherein the silica is highly condensed and have much higher stability compared to a sample prepared at 150 °C. The sample prepared at 190 °C shows exceptional hydrothermal and thermal stability, even retaining long-range mesostructural ordering after refluxing in boiling water for 24 h or heating at 1000 °C for 4 h, which is unprecedented for pure silica MCM-41 materials. Rather unusually, we observed that the porosity of calcined high temperature pure silica MCM-41 samples may be further modified by refluxing in water. Refluxing of the calcined samples not only increased their porosity (surface area and pore volume) but, remarkably, also stabilised them to subsequent high temperature calcination at 1000 °C.
This article covers the recent development in the remarkably growing field of synthesis, characterization and particularly catalytic investigation of zeolite‐based materials combining micro‐ and mesoporous features. New synthetic approaches for preparation of micro/mesoporous composites including recrystallization of originally amorphous matter, utilization of nanocrystalline zeolite seeds and formation of mesoporous zeolite single crystals are the first focus of this article. The advantages and disadvantages of composite materials in comparison with pure micro‐ and mesoporous molecular sieves will be discussed, as well. The relevance of individual experimental techniques for analysis of the composites, i.e., their structure, porosity, chemical composition, morphological features, and so on, are described in the second section of the article. The last Section is focused on the application of micro/mesoporous composites and mesoporous zeolites as catalysts in acid‐catalyzed reactions, oxidation reactions and synthesis of fine chemicals. The potential of the composites in challenging areas of catalysis for future applications is the final objective of the review.
The growth of hydrothermally stable ordered meso-porous silica with M41S type structure interconnected around zeolite crystals was achieved from seeds normally nucleating the crystallization of micro-porous zeolite. The solid state NMR and FT-IR studies showed that species like Si(OSi)3OH and Si(OSi)2(OH)2 (or Q3 and Q2 sites, respectively) are formed during first hydrothermal treatment with surfactant and NaOH. These species and surface silanol group of zeolite crystal condense to give meso-porous silica structure interconnected around zeolite crystal during re-crystallization at lower pH. Resulting meso-micro-porous hybrid material is found to be highly active, selective and stable in the synthesis of 2-(phenylamino) ethanol from aniline and ethylene carbonate compare to parent ZSM-5 and pure MCM-41.
Ordered hexagonal mesoporous aluminosilicates molecular sieves, designated as MSAMS-2A, MSAMS-2G and MSAMS-2, have been synthesized via re-crystallization of mesoporous SBA-15 within the diluted solution of aluminosilicate sol–gel, glycerol, and both of them, respectively. The three materials have been characterized by XRD, N2 adsorption–desorption, FT-IR, FE-SEM, 27Al MAS NMR and 29Si MAS NMR, and the corresponding hydrothermal stability of these three sieves is in the order of MSAMS-2>MSAMS-2G≫MSAMS-2A. The hydrothermal stability difference between MSAMS-2 and MSAMS-2G might be attributed to the synergistic effect of the higher condensation of silanol groups and insertion of all non-framework Al atoms into the framework of MSAMS-2. The hydrothermal stability of MSAMS-2G is higher than that of MSAMS-2A, which is likely because the high viscosity of glycerol will be in favor of the silanol groups interacting with zeolite-like subunits and moreover glycerol can act as a stabilizing guest molecule.
Micro- and mesoporous MFI / MCM-41 like phases, synthesized using mixtures of C6H13(CH3)3NBr and C14H29(CH3)3NBr templates / surfactants, were tested in 100% steam for 5 hours at temperatures of 650, 760 and 815°C, respectively. A generally improved hydrothermal stability was observed for the composite samples obtained using this mixture of surfactants in the synthesis gels. The most stable sample synthesized with a C6H13(CH3)3NBr / C14H29(CH3)3NBr ratio of 75%/25% at a temperature of 175°C, consisting of about 10% MFI and 90% MCM-41 structure, maintained a N2-BET surface area (BET-SA) of 243 m2/g after steaming at 815°C, corresponding with a retention of 62% for this sample, which is very high for regular mesoporous phases. The XRDs and N2 adsorption isotherms reveal that the typical features of the mesoporous materials (low angle peak in the XRD; inflection point in the N2-isotherm) are partly maintained. Consequently, this material is an interesting candidate for catalytic cracking and other applications if properly modified.
The thermal stability and acidity of a series of mesoporous aluminosilicates obtained by the cationic surfactant route were investigated by means of 27Al and 29Si MAS NMR and IR spectroscopy as a function of Si/Al ratio (Si/Al = 0.2-5). High temperature treatment applied to remove the occluded molecules of the surfactant results in a partial dealumination of the mesostructured solids. The extent of the dealumination depends on the chemical composition of the calcined material and is considerably higher for Al rich samples. Sorption of pyridine shows that both Brönsted- and Lewis acid sites are present in all mesoporous aluminosilicates under study. The number of the Brönsted sites depends on the Si content and increases together with the increase of the silicon concentration. The nature of the Lewis sites depends on the composition of the starting mesostructured material. For samples with high Al content the Lewis sites are mainly the extraframework octahedral aluminium species. For the more thermally stable silicon rich aluminosilicates, the trigonally coordinated Al atoms, formed during the dehydroxylation of the neighbouring Brönsted sites, mainly account for the high aprotonic acidity.
Different possibilities for application of molecular sieves in catalytic processes involving large size molecules are described. Preparation and reactivity of zeolites with mesopores, nanocrystalline zeolites, delaminated zeolites and amorphous and partially ordered mesoporous molecular sieves are presented.
Structurally well ordered and crystalline (molecularly ordered) mesoporous silica materials have been prepared from layered silicate precursors. The materials were derived from crystalline layered silicate, Na-RUB-18, by performing secondary hydrothermal synthesis (i.e., restructuring) in the presence of cetyltrimethylammonium (CTA) surfactant molecules. The mesoporous silica materials show retention of molecular ordering (crystallinity) in their framework depending on the method used to remove surfactant molecules. Crystalline RUB-18–surfactant mesophases were first obtained by hydrothermal treatment of Na-RUB-18 in CTA–NaOH–H2O systems at 150 °C for 48 or 120 h. Benign template removal via solvent extraction in acidified ethanol retained a significant level of crystallinity or molecular ordering in the resulting mesoporous material. H2O2-mediated oxidation of the surfactant at room temperature was also found to be an effective method for generating porous surfactant-free materials with significant crystallinity. Template removal via calcination generated porous materials that exhibited no crystallinity. The textural properties of the mesoporous materials (surface area: 150–260 m2 g−1, and pore volume: 0.2–0.3 cm3 g−1) depend on the mode of surfactant removal, and follow the trend: calcined > extraction > oxidation. The level of mesostructural ordering was not significantly affected by the surfactant removal method. The morphology of the layered silicate, i.e., plate-like particles, was unchanged during the transformation to mesostructured materials.
Mesoporous silica (JLU-20-S) is successfully synthesized at high-temperatures (180–220 °C) by the assembly of preformed silicalite-I zeolite nanoclusters with a mixture of triblock copolymer surfactant (P123) and fluorocarbon surfactant (FC-4). The preformed silicalite-I nanoclusters are obtained by heating, at 100–120 °C for 2–3 h, silica gels at SiO2/(TPA)2O/H2O molar ratios of 1/0.14/36. Mesoporous JLU-20-S shows extraordinary stability in steam (800 °C, 4 hours), compared with other mesoporous silica materials (JLU-20 synthesized at 190 °C, MPS-9 prepared from preformed silicalite-I zeolite nanoclusters at 100 °C, and SBA-15). The results of X-ray diffraction and transmission electron microscopy show that JLU-20-S has a hexagonal mesoporous symmetry, and the data obtained from N2 isotherms show that JLU-20-S contains both mesopores and micropores. The high steaming stability of the mesoporous silica of JLU-20-S is possibly related to synergistic advantages of both high-temperature synthesis and preformed zeolite nanoclusters in the synthesis of ordered mesoporous silica materials, and the observed microporous volume in JLU-20-S is attributed to the use of preformed zeolite nanoclusters.
Mesoporous AlMCM-41 materials were prepared by both direct synthesis and the reaction of siliceous MCM-41 with aluminium chloride vapour. The products were characterised using XRD, N2 sorption, EDAX and both MAS NMR and FTIR spectroscopy, and acid catalytic activity was evaluated using the conversion of 1,3,5-triisopropylbenzene (1,3,5-TIPB) as a test reaction. Acid site concentrations (Brønsted and Lewis) were estimated from sorption of pyridine, using FTIR. The concentrations of acid sites were correlated with the fraction of aluminium in the solids and comparisons were made with results published previously for similar mesoporous materials. Both the compositions of the gels used in the synthesis of Al-mesoporous materials and the procedures used to introduce aluminium into mesoporous silica strongly influenced the above correlation. In all cases, the ratio of total acid site concentration to aluminium concentration was less than unity. Consideration of the present catalytic results for conversion of 1,3,5-TIPB along with results published previously for conversion of cumene, demonstrates that acid catalytic activity is better correlated with Brønsted rather than with Lewis site concentration.
The degradation of pure silica MCM-41 materials prepared under various synthesis conditions has been investigated. Materials from preparations titrated with acid during synthesis not only show improved long range ordering of the pore system but also greater stability. This effect is greatest in unheated preparations.
A simple and effective method denoted “pH-adjusting” is used to graft a large amount of heteroatoms such as Al and Ti to mesoporous silica material SBA-15. The products prepared by this method show highly ordered mesostructures with large surface areas and uniform mesopore size distribution. The results of ICP, EDX, 27Al NMR, calcium ion-exchange capacity, and UV−vis spectra show that almost all the heteroatoms added into the initial reaction mixture can be introduced into the products, and moreover, the heteroatoms introduced by this route locate at mainly tetrahedrally coordinated sites.
The hydrothermal stability of Cr-containing MCM-48 in boiling water was found to be higher than that of the pure silica MCM-48, which was first improved with the increasing Cr content at low Si/Cr ratio, then went to the maxima at Si/Cr=50, and finally instead decreases with the increasing Si/Cr ratio. The Cr-MCM-48 sample synthesized at Si/Cr=50 maintained their mesoporous structure after refluxing in boiling water for 36h. The results of diffuse reflectance UV–vis and ESR spectra showed that the calcined Cr-MCM-48 contains Cr(V) and Cr(VI) at low Cr content. Chromium is present as Cr(V), Cr(VI) and Cr(III) species at the rich Cr content. Both of the Cr positioned on the surface of pore walls and in the framework provide the protection against water attack.
The interaction of water vapour with Al-MCM-41, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, aluminium sulfate, hexadecyltrimethylammonium bromide and ammonia, and its effect on the pore structure were studied in order to investigate the stability towards prolonged exposure to water vapour and the influence of the aluminium content. With this purpose two consecutive water adsorption isotherms were determined at 298K on samples with Si/Al ratio between 15 and 100. The samples were characterised by X-ray diffraction and adsorption of nitrogen at 77K and toluene at 298K, prior to and after exposure to water vapour. Pore size distributions were calculated from nitrogen, toluene and water adsorption isotherms using, respectively, the NLDFT method, a recently developed hybrid MC-DBdB method and the DBdB macroscopic approximation. It was found that Al-MCM-41 samples are significantly stable and that the stability improves as the amount of aluminium increases. Upon prolonged exposure to water vapour, there is a small decrease in pore size (3–5%), pore volume (8–16%) and total surface area (3–7%). The structural changes are essentially a consequence of the surface hydroxylation that occurred and not a result of a partial collapse of the pore structure. Although the presence of some extraframework Al can contribute to the improvement of the stability by protecting the surface, it was concluded that tetracoordinated Al plays an important role. The stabilizing effect of the Al incorporated in the walls can result from a higher degree of condensation on the surface of the pore walls and from the mild acidity generated.
Al-SBA-15 supports were prepared by post-synthesis alumination in aqueous solution of a pure silica SBA-15 material. The effects of the aluminium concentration [Al], and of the hydrolysis ratio h=[TMAOH]/[Al] on the speciation of aluminium ([Al(H2O)6]3+, [Al13O4(OH)24(H2O)20]7+, aluminium polymers) in the solution were examined. Depending on h and [Al], samples with various Si/Al ratios and acidities were obtained. The effects of h and [Al] on the characteristics of the final material were assigned to the speciation of the aluminium in the alumination solution: a correlation between the species present in solution and the Si/Al ratio in the final materials leads us to propose that [Al13O4(OH)24(H2O)20] is the most reactive and effective species toward grafting on the silica surface.
The hydrothermal and mechanical stability of aluminosilicate MCM-48 may be remarkably enhanced by post-synthesis treatment of the as-synthesised materials in water. To obtain materials with enhanced stability, as-synthesised aluminosilicate MCM-48 samples, prepared via direct mixed-gel synthesis, were subjected to further hydrothermal treatment in water at various temperatures prior to calcination. The stability of the (calcined) treated materials, in boiling water and after compaction at high pressure, was investigated using a variety of techniques including powder XRD, N2 sorption studies and elemental analysis. The stability of the treated materials was found, in all cases, to be higher than that of the primary materials. The improvement in stability was dependent on the temperature and duration of the post-synthesis treatment and was observed to go through a maximum within the temperature range 120–165 °C. The optimum post-synthesis temperature for enhancing stability was found to be 150 °C. The effect of the post-synthesis treatment on both the mechanical and hydrothermal stability is related to pore wall thickening and increased crosslinking within the aluminosilicate framework of the treated MCM-48 materials.
The mechanical stability of mesoporous material MCM-41 has been studied by X-ray diffraction (XRD) and N2 adsorption–desorption. It was shown that the diffraction peaks of MCM-41 materials are broad and weak and that adsorption capacity, BET surface area and pore volume considerably decrease upon increasing the pressure from 100 to 480 MPa. These results indicate that high pressure has a remarkable effect on the structural property of MCM-41 materials and that the structural order gradually loses with the increase of pressure.
Post-synthesis alumination of MCM-41 is found to substantially enhance the material's: (1) surface acidity, by increasing its number of Lewis and Bronsted acid sites; (2) physical stability, by improving its hydrothermal stability in boiling water and in high-temperature steam; (3) mechanical stability, by minimizing the collapse of its mesopores under high pressure compression; and (4) chemical stability, by improving its resistance in high pH aqueous solution.
The effect of Al ion implantation on the properties of mesoporous aluminosilicate mineral phases of the SBA-15 type was studied.
The implantation of Al was performed immediately under conditions of the synthesis of SBA-15 in a weakly acidic medium (pH
∼ 2.9). It was found that, under these conditions, the amount of Al that can be implanted into the SBA-15 framework is limited
(a maximum of 7.2 mol %). According to XPS data, aluminum ions were implanted into the matrix of silica rather than occurring
on the surface as an individual phase. The study of nitrogen adsorption at 77 K and the results obtained by X-ray diffractometry
and high-resolution electron microscopy suggest that Al-SBA-15 materials exhibited a hexagonal structure of channel pores
of the same diameter of 8.3 nm, and the unit cell parameter was 12.3 nm. The degree of crystallinity of the material increased
with the concentration of Al.
A new Solid acid catalyst has been prepared by grafted BF3·Et2O on mesoporous molecular sieve MCM-41. The solid catalyst would effectively promote the opening reaction of epichlovohydrin with isobutanol to form 1-isobutoxy-3-chloropropanol, and showed a nice activity and selectivity.
Highly crystalline MCM-41 with a very narrow pore-size
distribution (FWHM=1.5 Å), high surface area (1185
), large grain size and thick
channel walls (ca. 17 Å) was prepared in alkali-free
media. The properties of the product depend on the source and
concentration of the reactants, the gel aging time, the temperature
and the duration of the synthesis. There is no induction period in the
course of the synthesis and Ostwald's rule of successive
transformations applies. The initially produced hexagonal phase is
transformed into the lamellar phase and then into an amorphous phase.
In the 150°C synthesis the most stable product is amorphous
silica. The course of the synthesis is conveniently monitored by pH
measurement. Gel aging, during which a spatial distribution of
silicate polyanions and micellar cations is established, is essential
for preparing high-quality MCM-41. Surfactants with the same cationic
organic group but different counter-anions alter the course of the
synthesis. The degree of polymerization of silica is also important.
Highly basic gels favour the lamellar product; when the gel is weakly
basic the quality of MCM-41 is lower as insufficient TMAOH is
available to dissolve the silica. The result of excess silica is
similar but even more pronounced. Purely lamellar products are made at
concentrations, when the gel is more strongly
basic. The best quality MCM-41 is prepared from a gel of molar
:0.19 TMAOH:0.27 CTABr:40 H
=0.27, similar to the ratio in the solid
product) aged at 20°C for 24 h and synthesis lasting for 48 h.
Al-containing MCM-41 materials are prepared by grafting Al onto purely siliceous MCM-41; the resulting materials retain the hexagonal order and physical properties of the purely siliceous parent MCM-41 and exhibit higher Brønsted acid content compared to Al-MCM-41 prepared by direct hydrothermal synthesis.
Primary amines are used as structure-directing agents in the room temperature assembly of aluminosilicate inorganic species to yield mesoporous materials with physical and textural properties similar to those of MCM-41 but with substantially higher Brønsted acidity. Si and Al are incorporated into the mesoporous framework in proportions dependent on the gel Si/Al ratio. Calcination of the as-synthesised material to remove the occluded amine generates Brønsted acid sites which (depending on Si/Al ratio) are stronger or comparable in strength to those on zeolite-HY (Si/Al=3.65) but weaker than those on ultrastable-Y zeolite (USY). The materials, designated Al-MMS, exhibit higher Brønsted acidity and catalytic activity for the cracking of cumene compared to equivalent aluminosilicate MCM-41 materials or to amorphous silica-alumina and show considerable stability to catalytic deactivation. Al-MMS samples with Si/Al ratio ≤20 have catalytic activity higher than the zeolite-HY but exhibit a lower rate of deactivation compared to the zeolite. Ageing of the materials (for 1 year in the calcined form) has no significant effect on their acidity and catalytic activity. Hexagonal ordering and total Brønsted acidity (but not acid strength) of the Al-MMS materials may be improved by using prepolymerised aluminosilicate inorganic precursors.
We report experimental data on the mechanical stability of the silica mesoporous material MCM-41. Using X-ray diffraction and nitrogen adsorption we show that the ordered mesoporous structure of MCM-41 can be affected considerably by mechanical compression at pressures as low as 86 MPa and essentially destroyed at 224 MPa.
The discovery of mesoporous molecular sieves, MCM-41, which possesses a regular hexagonal array of uniform pore openings, aroused a worldwide resurgence in this field. This is not only because it has brought about a series of novel mesoporous materials with various compositions which may find applications in catalysis, adsorption, and guest−host chemistry, but also it has opened a new avenue for creating zeotype materials. This paper presents a comprehensive overview of recent advances in the field of MCM-41. Beginning with the chemistry of surfactant/silicate solutions, progresses made in design and synthesis, characterization, and physicochemical property evaluation of MCM-41 are enumerated. Proposed formation mechanisms are presented, discussed, and identified. Potential applications are reviewed and projected. More than 100 references are cited.