Imported: 10 Mar '17 | Published: 27 Nov '08
USPTO - Utility Patents
Provided herein is a new material, periodic mesoporous phosphorus-nitrogen compound, which may be used in a variety of emerging technologies. Its surface properties render it promising as a component in a variety of applications, including gas separation and purification systems in which waste gases such as SO2, SO3, or CO2 are separated from other gases. It may also be used as an interlayer dielectric in microelectronic chips. Its structure and composition are useful due to an advantageous and favorable combination of thermal stability, elastic modulus, and dielectric properties. The surface properties and the regularity of the pores furthermore provides utility as shape selective base catalysts. Protonated forms of the material are expected to be useful as a solid acid, and in applications such as acid catalysis. Additionally, because of the thermal behavior of the material, it is useful as hard template for other porous materials, without the need of an external reagent.
This application claims the priority of application No. 60/940,181, filed May 25, 2007, the entire contents of which are incorporated herein by reference.
Periodic mesoporous phosphorus-nitrogen compounds, related compounds, compositions thereof, uses thereof, and methods of manufacture thereof.
The vast majority of knowledge about inorganic periodic mesoporous materials is concerned with oxidic ceramics. See, e.g. Kresge, et al., Nature 359, 710-12 (1992); Zhao, et al., Science 279, 548-52 (1998). A major reason for this seems to be that surfactant templated assembly in aqueous media is very suitable for the preparation of periodic mesoporous oxides, but far less suitable for the synthesis of non-oxidic compounds. The synthesis of non-oxidic mesoporous ceramics usually requires non-aqueous conditions. Therefore, only a few systems are accessible via surfactant templating routes. See, e.g. Maclachlan, et al., Nature 397, 681-84 (1999); Armatas, et al., Science 313, 817-20 (2006); Kamperman, et al., J. Amer. Chem. Soc. 126, 14708-09 (2004); Attard, et al., Microporous Mesoporous Mater. 44-45, 159-63 (2001); Markham, et al., Appl. Phys. Lett. 86, 011912/1-011912/3 (2005).
Unlike the corresponding mesoporous oxides, very little is known about non-oxidic periodic mesoporous ceramics. The stability of a mesoporous framework strongly depends on the framework connectivity and the bond strengths between the constituting elements. Many non-oxidic ceramics like nitrides, carbides, borides, and phosphides have higher framework connectivities than their corresponding oxides because they can make more bonds to their neighbors in comparison to oxygen. For example, C makes four bonds to neighbor atoms in carbides while oxygen makes only two bonds in oxidic ceramics. Furthermore, the bond strengths in non-oxidic ceramics are often nearly as high as in oxides.
In their dense forms, many non-oxidic ceramics exhibit a much greater thermal stability than their corresponding oxides, and the mesoporous forms may therefore have superior thermal properties in comparison with similar oxides. This has been impressively demonstrated in the case of the SiBCN ceramic prepared by Wiesner. See, Kamperman, op cit. However, porous frameworks have a tendency to convert into a more dense form upon heating. Thus, it is desirable to synthesize new mesoporous frameworks, particularly non-oxidic ceramics, with excellent thermal stabilities.
Many non-oxidic ceramics in their dense forms also exhibit much greater hardness and elastic modulus than their corresponding oxides. In fact, the hardest substances known are non-oxidic ceramics (diamond, c-BN, SiC, Si3N4). Increasing the porosity in a material leads inherently to lower mechanical strength, which can dramatically reduce the practical utility of a porous material. A striking example is low dielectric constant materials for microelectronics. The development of new low-dielectric constant materials is one of the most important problems in microelectronics today, yet it is hampered largely by the low elastic modulus of the porous materials available so far. All materials under investigation so far fail to meet the minimum elastic modulus (6 GPa) at porosities necessary to achieve k values of 1.9. See, e.g. Hatton, et al., Mater. Today 9, 22-31 (2006). Furthermore, the elastic modulus is reduced with increasing porosity in any material. Thus, it is desirable to prepare new mesoporous materials with high porosity and high elastic moduli and hardness.
Periodic mesoporous phosphorus-nitrogen compounds and related compounds as described herein are useful in a variety of emerging technologies. Their surface properties render them useful as components in gas separation and purification systems that employ reversible adsorption and desorption of Lewis acidic gases techniques in which waste gases such as SO2, SO3, or CO2 are separated from other gases. Gas separation of such Lewis acidic gases are of importance in addressing energy and environmental problems associated with these gases. They are also useful as an interlayer dielectric in microelectronic chips. Their structures and compositions provide advantageous and favorable combinations of thermal stability, elastic modulus, and dielectric properties. Their surface properties and the regularity of the pores also provide utility as shape selective base catalysts. These materials are archetypal of a new class of solids that have industrial application in heterogeneous base catalysis. Additionally, protonated forms of these materials may be used as solid acids, e.g. in heterogeneous acid catalysis.
Because of the thermal behavior of these materials, they can be used as hard templates for other porous materials without the need of an external reagent. Removal of these materials will not require an external reagent, which makes the process simpler and potentially cheaper, while permitting access to new classes of porous materials not accessible by other routes. Moreover the possibility that an external reagent harms the product is eliminated. Accordingly, the preparation of new mesoporous materials that are difficult to obtain with conventional methods is made possible. Furthermore, improved methods for etching to remove silica templates is also provided. With such etching methods, higher etching selectivity is achieved, and a product with better uniformity and periodicity of mesopores is produced. The methods are generally applicable and may be applied to the synthesis of a variety of mesoporous materials. Also described herein are melt-infiltration techniques for nitridic materials, which is highly efficient for the synthesis of nitridic periodic mesoporous materials. Such methods provide significant advantages over solution infiltrations so far used for the preparation of mesoporous nitridic materials with periodic mesopores because no solvent needs to be present and the precursor does not compete with the solvent for the pore.
Advantages over the only known nitridic high surface area material with periodic mesopores (i.e., boron nitride) include the presence of an electron lone pairs on nitrogen atoms, which provides enhanced Lewis base characteristics. Furthermore, in comparison to high surface area nitrides having disordered mesopores, the new materials described herein have periodic mesopores, shape selectivity, and improved adsorption-desorption properties, as well as better defined mass transport properties. Furthermore, phosphorus-nitrogen compounds offer an advantageous combination of high mechanical strength, high thermal stability, and low dielectric constant.
Additional features may be understood by referring to the accompanying Drawings, which should be read in conjunction with the following detailed description and Examples.
Nanocasting is a technique that has recently been introduced as an alternative to surfactant directed soft assembly. Lu, et al., Adv. Mater. 18, 1793-805 (2006). A periodic mesoporous framework is used as so-called hard template producing a negative replica of the hard template structure. A molecular precursor is infiltrated into the pores of the hard template and subsequently polymerized within the pores of the hard template at elevated temperatures. Then the hard template is removed by a suitable method. This nanocasting route is appealing because no cooperative assembly processes between the template and the ceramic precursors are needed. Moreover, nanocasting can be performed fairly easily in non-aqueous media. The only hard templates that have been used so far are mesoporous silica and mesoporous carbon. Mesoporous silica can be inverted to form mesoporous carbon by infiltration of organic precursors like sugar into the pores, heat treatment of the composite material, and subsequent selective etching, such as by exposure to HF, NaOH or other suitable etching solvents and chemicals.
Periodic mesoporous carbon allowed for the synthesis of a number of periodic mesoporous oxidic materials because the carbon template can be removed by oxygen at modest temperatures (400-500 C.). Under these conditions, the carbon reacts selectively with oxygen under maintenance of the mesostructure of the oxidic ceramic. It has been shown that carbon templates also react with NH3, however, temperatures above 1000 C. are needed. Dibandjo, et al., Adv. Mater. 17, 571-74 (2005); Dibandjo, et al., Microporous Mesoporous Mater. 92, 286-91 (2006); Dibandjo, et al., J. Mater. Chem. 15, 1917-23 (2005). A boron nitride with periodic mesopores has been synthesized this way, which is the only nitridic material with periodic mesopores that has been made so far by nanocasting. The same group also produced mesoporous boron nitride from silica templates, but in this case the pores were not ordered (i.e., periodic).
Phosphorus(V)-nitrogen frameworks have a modestly high thermal stability in their dense forms (ca. 800 C.). Above this point they decompose into gaseous PN and N2 molecules. This makes periodic mesoporous PN compounds useful as hard template for other ceramic materials because its removal would not require an external reagent. They are simply be removed by heating. At the decomposition temperature of phosphorus(V)-nitrogen frameworks many other ceramic materials are still stable. At this temperature a ceramic inside the pores of a phosphorus(V)-nitrogen compound could stay unreactive towards it because of the covalent inert PN bonds. On the other hand, the decomposition point of a phosphorus(V)-nitrogen framework is high enough that the ceramic material that has formed in its pores from a precursor in a thermally directed polymerization process has already a sufficiently high framework connectivity preventing the collapse of the porous structure upon template decomposition.
Described herein is the first example of a periodic mesoporous phosphorus nitrogen compound, which was obtained in a nanocasting process employing mesoporous silica templates. The material shows a high degree of mesostructural periodicity and high surface area. The lone pairs of electrons at N in amine molecules are strong Lewis and Brnsted bases. In phosphorus(V)-nitrides, N can carry formally a negative charge counterbalancing the five positive charges at P in the PN4 building units, and thus it could be basic. See, e.g. Horstmann, et al., Angew. Chem. Int. Ed. 36, 1873-75 (1997).
Two other interesting properties of the phosphorus(V)-nitride compound P3N5 in its dense form are its low dielectric constant and its hardness. See, e.g. Hirota, et al., J. Appl. Phys. 53, 5037-43 (1982); Landskron, et al., Zeitschrift fr Anorganische und Allgemeine Chemie 628, 1465-71 (2002). Furthermore, P3N5 shows a hardness greater than SiO2 glasses. The k value of P3N5 is 3.7, which is lower than the respective value for dense silicon dioxide (4.5). This is even more remarkable considering that P3N5 has a higher framework connectivity than silicon dioxide.
Described herein are the first examples of a mesoporous phosphorus-nitrogen compounds. They were prepared using hexagonal and cubic mesoporous silicas SBA-15 and SBA-16 as a hard template using a direct melt infiltration technique. This is the first successful synthesis of an inorganic nitridic compound with periodic mesopores made directly from a mesoporous silica. Moreover, melt-infiltration techniques have be successfully employed to the synthesis of nitridic materials with periodic mesopores.
The synthesis of periodic mesoporous hexagonal (p6mm) and cubic (Im
In the examples herein, typically, 2 g of PNC was molten at 120 C. in nitrogen atmosphere and infiltrated into 1 gram of the silicas under stirring. Prior to the infiltration the silicas were kept overnight at 150 C. in vacuum. The resulting PNC-SiO2 was kept in a closed bottle in an oven at 120 C. for another 24 hours under N2. Removal of the excess PNC from the outside of the silica particles was carried out by heating the PNC-SiO2 composites at 120 C. in a tube furnace with flowing nitrogen for 1 h. By this technique, the capillary forces keep the infiltrated PNC inside the pores, while PNC that is deposited outside of the pores gets transported outside of the heating zone by the nitrogen stream. Subsequently, the temperature of the furnace was raised to 700 C. with a heating rate of 10 C./min and kept at 700 C. for 20 minutes. This heat treatment was done in an ammonia atmosphere to nitridize the PNC to form the PON-SiO2 composites under the elimination of NH4Cl. Despite the high ion diffusion rates that one might expect under such high-temperature conditions, the mesostructure was retained with only very minor decrease of order, which can be explained by the rather short reaction time of only about 20 minutes at 700 C. To support this interpretation of the findings, the inventors carried out the reaction also for 1 hour at this temperature. Indeed, the inventors found complete loss of mesoscale periodicity at this prolonged reaction time. Shorter reaction times than 20 minutes did not lead to a further increase of order.
The silica template was removed by stirring the PON-SiO2 composites with a solution of 1.4 ml 48-50 wt % aqueous HF in 40 mL acetone for 1 hour. The EDX analysis of the so-prepared PON-1 and PON-2 samples showed that Si was practically completely absent. This shows that the siliceous part can be highly selectively etched, even though P(V) and Si(IV) have both a high affinity to fluoride. This finding may be due to the highly covalent inert character of the PN bonds relative to the comparatively ionic labile character of SiO bonds. The presence of acetone was desirable to maintain mesoscale periodicity inasmuch as removal of the silica template using only aqueous HF solutions of various concentrations may result in loss of mesoscale periodicity.
Referring to the attached Drawings, FIGS. 1a and b show the small angle X-ray diffraction (XRD) patterns of PON-1 and PON-2, respectively, demonstrating the well-ordered structure of the specimens. An intense diffraction peak with d=10.9 nm and two weak peaks at 6.3 and 5.4 nm were observed for the PON-1. These peaks can be assigned to the (100), (110) and (200) reflections of the p6mm hexagonal phase with a lattice parameter a=12.6 nm, similar to SBA-15. This hexagonal symmetry is further confirmed by the TEM image (FIG. 1c), which shows the structure of the PON-1 is exactly the inverse replica of the SBA-15 original. The PON-1 pore walls are 5.5 nm in diameter having a periodicity of 8 nm. The lattice parameter estimated from the TEM analysis is approximately 9.2 nm, which is consistent with the calculated value from XRD.
The XRD pattern of the PON-2 (FIG. 1b) shows an intense peak with the d spacing of 12.9 nm and the lattice parameter a of 18.2 nm, which can be indexed to the (011) reflection of Im
The N2 sorption isotherm (FIG. 2a) of the PON-1 shows a type-IV isotherm with a sharp capillary condensation at P/P0=0.45 to 0.7, which indicates that the material has uniform mesoporous channels. The BET surface area and the total pore volumes are calculated to be 380 m2g1 and 0.53 cm3g1, respectively. A DFT Monte-Carlo analysis showed narrow pore size distribution (PSD) with the average pore width of 5.0 nm, compared to 8.4 nm for the parent SBA-15. The pore width of PON-1 determined from the N2 sorption measurement is consistent with the value obtained from the TEM analysis. The PON-2 shows a type-IV isotherm with a BET surface area of 244 m2g1, and total pore volume of 0.29 cm3g1 with the average pore width of 4.9 nm calculated by DFT (Density Functional Theory) Monte-Carlo analysis.
The composition of PON-1 was investigated in detail by elemental analysis. To ensure complete water desorption, the sample was heated to a temperature of 450 C. in order to ensure removal of adsorbed water. The inventors tested not only for P and N, but also for O, H, and C because of the possibility of surface hydrolysis during the etching step, O/N interdiffusion during synthesis, and chemisorption of acetone. The composition was found to be PN1.4O0.88H1.28Si0.02C0.15. The sum of elements was found to be 99.93 wt %, showing that practically no other elements were overlooked. The investigations confirmed that Si is nearly absent even though some N/O interdiffusion had apparently occurred at the mesoscopic oxide/nitride interface which incorporates some oxide ions into the PN framework. The presence of C may be explained by incidental reaction of a small amount of acetone with the material's surface.
The results also indicate that POH and PNH units may be present. In order to investigate this in more detail, the PON-1 sample was heated to various temperatures and studied by FTIR spectroscopy. FIG. 3 shows the FTIR spectra of PON-1 samples exposed to various temperatures (RT, 350, 450, 550, and 600 C.). Three broad bands appear in all of these samples in the 400 to 1600 cm1 range. Very similar bands appear in bulk phosphorus nitrides, nitride imides, and oxynitrides corroborating the results from elemental analysis. See, e.g. Schnick, et al., Chem. Mater. 8, 281-86 (1996); Meng, et al., Chem. Lett. 1252 (2000); Guo, et al., Mater. Res. 20, 325-30 (2005); Meng, et al., Chem. Commun. 469-70 (2001); Levchik, et al., Die Angew. Makromole. Chem. 264, 48-55 (1999). The bands at around 500, 950, and 1300 cm1 are attributed to PNP bending vibration, PNHP stretching vibration, and PNP stretching vibration, respectively. The broad band around 3200 cm1 indicates the presence of the NH groups in the sample.
In addition to these bands, a band appears at 1400 cm1 in the as-synthesized PON-1 that is absent in all other heated samples. The band indicates the presence of some residual NH4+, which may be a result of some minor pore surface hydrolysis during the etching step. The appearance of the small peaks at 1636 and 3435 cm1, respectively, correspond well to OH bending and OH stretching vibrations of H2O. These peaks strongly decrease in intensity, however, for the heat-treated samples (b)-(e), and therefore the bands at 3400-3700 cm1 are not primarily due to adsorbed water, but due to POH groups that overlap with bands of NH groups that typically have resonance frequencies around 3200 cm1. They also explain the rather high H content found in the elemental analysis. The relative intensity of the bands is decreasing with increasing temperature indicating a continuous condensation of the PON framework at higher temperatures. Accordingly, the material is a periodic mesoporous phosphorus-nitrogen compound.
In order to study how the order of the mesostructure would behave at elevated temperatures, the small angle XRD patterns of the PON-1 heated at 350, 450, 550, and 600 C. for 1 h in N2 atmosphere were studied. From the XRD patters, it can be clearly seen that the periodicity of mesostructure remains up to a temperature of 550 C. but vanishes on heating to a higher temperature as high as 600 C.
Accordingly, the inventors have successfully synthesized hexagonal and cubic periodic mesoporous phosphorus-nitrogen compound by direct melt infiltration of phosphonitrilic chloride into the pores of mesoporous silica. A novel etching process was utilized in this work to produce high order in the mesostructure. The synthesized PON-1 and PON-2 are expected to be useful in a variety of applications such as catalysis, microelectronics, gas adsorption, purification and separation media, and as new template. For example, the lone electron pairs of high surface area nitrides can be strong bases, and therefore they can be useful as heterogeneous base catalysts. See, e.g. Kaskel, et al., Chem. Mater. 17, 181-85 (2005). High surface area nitrides have also shown to active otherwise inert organic molecules such as benzene. See e.g. Goettmann, et al., Angew. Chem. Int. Ed. 46, 2717-20 (2007). The basic sites at the surface may also be excellent adsorption sites for Lewis acidic gases, such as waste gases like CO2 or SO3. Thus, they may be of interest in gas purification and separation systems. This includes CO2 capture from waste gases from combustion of fossil fuels in electricity generation plants.
Hexagonal and cubic periodic mesoporous phosphorus-nitrogen compounds made by direct melt infiltration of phosphonitrilic chloride into the pores of mesoporous silica represents the first nitridic material of an element of the 3rd or higher period of the periodic table of the elements having periodic mesopores, as well as the first periodic mesoporous nitridic material with electron lone pairs at N. It was also heretofore unknown that an inorganic material could be used as a hard mesogenic template without the need of an external reagent to remove it. Likewise, the novel porous nitridic materials as described herein have a high thermal stability (550 C.). They suggest a low dielectric constant, and a high elastic modulus. Furthermore, melt infiltration applied to periodic mesoporous nitridic materials is novel.
As used herein, the term mesopores or mesoporous refers to a material having pores (as opposed to dense, non-porous materials) with an average size of from about 2 nm to about 50 nm. Also as used herein, the term periodic refers to a material having translational symmetry in at least two directions, and preferably in all possible directions. As used herein, the term nitridic materials comprises materials with N3, NH2, and NH2 units. As used herein, min herein refers to minutes, and h refers to hours.
Accordingly, in an embodiment, a novel composition as provided herein includes at least one mesoporous phosphorus-nitrogen compound. The at least one mesoporous phosphorus-nitrogen may include mesoporous nitrides, imides, and amides of phosphorus(V). Likewise, the mesoporous phosphorus-nitrogen compound may include at least one additional element, such as oxygen, sulfur, or carbon.
In another embodiment, a mesoporous phosphorus-nitrogen compound is characterized by mesopores having an average size of from about 2 nm to about 50 nm. Similarly, a mesoporous phosphorus-nitrogen compound may be a periodic mesoporous compound. For example, a mesoporous phosphorus-nitrogen compound may include periodic mesoporous phosphorus(V)-nitride or phosphorus(V)-oxynitride. Additionally, a mesoporous phosphorus-nitrogen compound may include periodic mesoporous phosphorus(V)-oxyimide. Similarly, a mesoporous phosphorus-nitrogen compound may include hexagonal periodic mesoporous phosphorus(V)-oxyimide or cubic periodic mesoporous phosphorus(V)-oxyimide.
In still another embodiment, as described herein, a method of making a mesoporous phosphorus-nitrogen compound includes steps of (1) providing a mesoporous silica template and a polymerizable phosphorus source; (2) combining the mesoporous silica template and the polymerizable phosphorus source; (3) polymerizing the polymerizable phosphorus source under nitridizing conditions; and thereafter (4) isolating at least one mesoporous phosphorus-nitrogen compound. The polymerizable phosphorus source may include phosphorus(V) having, or being capable of forming, at least one PN covalent bond. For example, the polymerizable phosphorus source may be PNC, PCl5, tetraminophosphazenium, hexaminodiphosphazenium, or a phosphorus-sulfide. Also, the polymerizable phosphorus source may include P(III) or elemental phosphorus, in addition to an oxidizing agent.
In the foregoing method, the polymerizing step may be induced by application of heat. Furthermore, the isolating step may include leaching the silica template to thereby produce at least one isolatable mesoporous phosphorus-nitrogen compound. Also, the leaching step included contacting the mesoporous silica template with an aqueous solution of acetone and hydrofluoric acid.
In still yet another embodiment, provided herein are novel and heretofore unknown compounds including periodic mesoporous phosphorus(V)-oxyimide, periodic mesoporous phosphorus(V)-nitride, and phosphorus(V)-oxynitride. Such compound may have a dielectric constant (k) of less than about 2.0, an elastic modulus of larger the 6 gigapascal, and be thermally stable to at least 400 C. Likewise, such compounds may belong to the hexagonal p6mm or cubic Im
The foregoing Detailed Description is further exemplified for the following Examples, which should not be construed as limiting.
In the following Examples, the following chemical reagents were used: triblock copolymers EO20PO70EO20 Pluronic P123 and EO106PO70EO106 Pluronic F127 (BASF Corp., USA), tetraethyl orthosilicate (TEOS, Sigma-Aldrich, Inc.), phosphonitrilic chloride trimer (PNC) (Alfa Aesar), 48-50% HF in water (Fisher Chemicals), nitrogen (Airgas, Inc) and ammonia (GT S, Inc.) gas. All the chemicals were used as-received, without further purification. (PLURONIC is a trademark of BASF Corp., New Jersey).
Synthesis of Mesoporous p6mm Hexagonal Silica (SBA-15)
Mesoporous hexagonal silica SBA-15 was synthesized by using P123 as a structure directing agent and TEOS as silica source. In a typical synthesis, 3.0 g of P123 was dissolved in 114.0 mL of 1.6 M hydrochloric acid. To it, 6.6 g of TEOS was added at 35 C. under stirring with a magnetic stirrer until TEOS was completely dissolved. The mixture was placed in an oven for 24 h at 35 C. for the mesostructure formation, and subsequently for 6 h at 100 C. for hydrothermal treatment. The product was filtered, dried at 100 C. without washing, and then calcined in air at 550 C. for 5 h.
Synthesis of Mesoporous Im
In a typical synthesis procedure, 5.0 g of F127 was dissolved in 240 g of distilled water and 10.5 g of hydrochloric acid (35 wt %). To it, 15.5 g of butanol was added at 45 C. After stirring for 1 h, 24 g TEOS was added and the stirring of the mixture was continued for 24 h for the mesostructure formation. The hydrothermal treatment was carried out by aging the mixture for another 24 h in static condition. The product was filtered, dried at 100 C. without washing, and then calcined in air at 550 C. for 5 h. The molar composition of the starting reaction mixture was 0.0035 F127/1 TEOS/1.8 butanol/0.91HCl/117H2O.
The formation of the mesostructure, study of their textured properties and the compositions were carried out by transmission electron microscopy (TEM), small and wide angle X-ray diffraction (XRD), nitrogen sorption analysis, Fourier-transform infrared (FTIR) spectroscopy and elemental analysis. The TEM images were taken on a JEOL JEM-2000 electron microscope operated at 200 kV. Samples for the TEM analysis were prepared by dispersing the particles in an organic solvent and dropping a small volume of it onto a carbon-coated copper grid. Energy dispersive X-ray analysis (EDS) of samples was carried out using Oxford ISIS EDS with a light element detector. The ordered mesostructure of the specimen was determined by small angle XRD patterns using a Rigaku Rotaflex diffractometer with a Cu K radiation source (k=0.15405 nm). Nitrogen sorption isotherms were measured at 196 C. with an Autosorb-1 analyzer (Quantachrome). Before measurements, the samples were degassed in vacuum at 150 C. overnight. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface areas. FTIR spectra were collected on Thermo Mattson Satellite FTIR spectrometer, using KBr pellets. Elemental analysis of selected samples was carried out by Fa. Pascher (Remagen, Germany). The characteristics and properties of the periodic mesoporous phosphorus-nitrogen compounds are discussed hereinabove.
While this description is made with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings hereof without departing from the essential scope. Also, in the Drawings and the description, there have been disclosed exemplary embodiments and, although specific terms may have been employed, they are unless otherwise stated used in a generic and descriptive sense only and not for purposes of limitation, the scope of the claims therefore not being so limited. Moreover, one skilled in the art will appreciate that certain steps of the methods discussed herein may be sequenced in alternative order or steps may be combined. Therefore, it is intended that the appended Claims not be limited to the particular embodiment disclosed herein.