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Definitions

• the invention relates to compositions of carbide-based and oxycarbide-based nanorods, carbon nanotubes including carbide and/or oxycarbide compounds, rigid porous structures including these compositions, and methods of making and using the same. More specifically, the invention relates to rigid three dimensional structures comprising carbon nanotubes bearing carbides and oxycarbides, carbide and/or oxycarbide-based nanorods having high surface areas and porosities, low bulk densities, substantially no micropores and increased crush strengths.
• the invention also relates to using the compositions of carbide-based nanorods, oxycarbide-based nanorods, carbon nanotubes comprising carbide and oxycarbide compounds and the rigid porous structures including these compositions as catalysts and catalyst supports, useful for many types of heterogenous catalytic reactions frequently encountered in petrochemical and refining processes.
• Heterogeneous catalytic reactions are widely used in chemical processes in the petroleum, petrochemical and chemical industries. Such reactions are commonly performed with the reactant(s) and product(s) in the fluid phase and the catalyst in the solid phase. In heterogeneous catalytic reactions, the reaction occurs at the interface between phases, i.e., the interface between the fluid phase of the reactant(s) and product(s) and the solid phase of the supported catalyst.
• the properties of the surface of a heterogeneous supported catalyst are significant factors in the effective use of that catalyst. Specifically, the surface area of the active catalyst, as supported, and the accessibility of that surface area to reactant chemiabsorption and product desorption are important.
• catalytic activity is proportional to catalyst surface area. Therefore, a high specific area is desirable. However, that surface area must be accessible to reactants and products as well as to heat flow.
• the chemiabsorption of a reactant by a catalyst surface is preceded by the diffusion of that reactant through the internal structure of the catalyst.
• the accessibility of the internal structure of a support material to reactant(s), product(s) and heat flow is important.
• Porosity and pore size distribution of the support structure are measures of that accessibility.
• Activated carbons and charcoals used as catalyst supports have surface areas of about 1000 square meters per gram and porosities of less than one milliliter per gram. However, much of this surface area and porosity, as much as 50%, and often more, is associated with micropores, i.e., pores with pore diameters of 2 nanometers or less. These pores can be inaccessible because of diffusion limitations. They are easily plugged and thereby deactivated.
• high porosity material where the pores are mainly in the mesopore (>2 nanometers) or macropore (>50 nanometers) ranges are most desirable.
• catalysts are particularly sensitive to contamination that can either promote unwanted competing reactions, i.e., affect its selectivity, or render the catalyst ineffective, i.e., "poison" it.
• Charcoal and commercial graphites or carbons made from petroleum residues usually contain trace amounts of sulfur or nitrogen as well as metals common to biological systems and may be undesirable for that reason. Since the 1970s carbon nanofibers or nanotubes have been identified as materials of interest for such applications. Carbon nanotubes exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon- containing gases at metal surfaces. Nanofibers such as fibrils, bucky tubes and nanotubes are distinguishable from continuous carbon fibers commercially available as reinforcement materials.
• continuous carbon fibers In contrast to nanofibers, which have, desirably large, but unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios (L/D) of at least 10 4 and often 10 6 or more.
• the diameter of continuous fibers is also far larger than that of nanofibers, being always >1.0 ⁇ and typically 5 to 7 ⁇ .
• U.S. Patent No. 5,576,466 to Ledoux et al. discloses a process for isomerizing straight chain hydrocarbons having at least seven carbon atoms using catalysts which include molybdenum compounds whose active surface consists of molybdenum carbide which is partially oxidized to form one or more oxycarbides. Ledoux et al.
• Nanofiber mats, assemblages and aggregates have been previously produced to take advantage of the increased surface area per gram achieved using extremely thin diameter fibers. These structures are typically composed of a plurality of intertwined or intermeshed nanotubes.
• compositions including a multiplicity of oxycarbide nanorods having predominately diameters between 2.0 nm and 100 nm.
• compositions of matter which comprise three-dimensional rigid porous structures including oxycarbide nanorods, carbide nanorods, carbide nanorods comprising oxycarbides, or carbon nanotubes comprising a carbide portion and optionally an oxycarbide portion. It is a further object of the present invention to provide methods for the preparation of and using the rigid porous structures described above. It is still a further object of the invention to provide improved catalysts, catalyst supports and other compositions of industrial value based on composition including a multiplicity of carbide nanorods, oxycarbide nanorods and/or carbon nanotubes comprising carbides and oxycarbides.
• the present invention which addresses the needs of the prior art provides a composition including nanorods which contain oxycarbides.
• Another composition provided by the present invention includes carbide-based nanorods which also contain oxycarbides.
• Another composition provided by the invention relates to carbon nanotubes which bear both carbides and oxycarbides. In one composition the carbides retain the structure of the original aggregates of carbon nanotubes. However, a composition is also provided which includes carbide-based nanorods where the morphology of the aggregates of carbon nanotubes is not retained.
• the invention also provides a composition of carbides supported on carbon nanotubes where only a portion of the carbon nanotubes have been converted to carbide-based nonorods and/or carbides.
• the present invention also provides rigid porous structures including oxycarbide nanorods and/or carbide-based nonorods and/or carbon nanotubes bearing carbides and oxycarbides. Depending on the morphology of the carbon nanotubes used as a source of carbon, the rigid porous structures can have a uniform or nonuniform pore distribution. Extrudates of oxycarbide nanorods and/or carbide- based nanorods and/or carbon nonotubes bearing oxycarbides and/or carbides are also provided. The extrudates of the present invention are glued together to form a rigid porous structure.
• the invention also provides for the compositions and rigid porous structures of the invention to be used either as catalysts and/or catalyst supports in fluid phase catalytic chemical reactions.
• the present invention also provides methods of making oxycarbide-based nonorods, carbide-based nanorods bearing oxycarbides and carbon-nanotubes bearing carbides and oxycarbides.
• Methods of making rigid porous structures are also provided.
• Rigid porous structures of carbide-nonorods an be formed by treating rigid porous structures of carbon nanotubes with a Q-based compound. Depending upon temperature ranges the conversion of the carbon nanotubes to carbide-based nanorods can be complete or partial.
• the rigid porous structure of carbide nanorods and/or carbon nanotubes can be further treated with an oxidizing agent to form oxycarbide nanorods and/or oxycarbides.
• the rigid porous structures of the invention can also be prepared from loose or aggregates of carbide-based nonorods and/or oxycarbide- based nanorods by initially forming a suspension in a medium, separating the suspension from the medium, and pyrolyzing the suspension to form rigid porous structures.
• the present invention also provides a process for making supported catalysts for selected fluid phase catalytic reactions.
• Figures IB and 1C are SEM micrographs of sample 12 as set forth in Table 1.
• Figure 2 A is an XRD graph of sample 12 as set forth in Table 1.
• a reference XRD pattern of hexagonal Mo C is also shown immediately below.
• Figure 2B is an HRTEM micrograph of sample 12 as set forth in Table 1.
• Figure 3 A is an XRD graph of sample 10 as set forth in Table 1. Reference XRD patterns of hexagonal Mo 2 C, cubic Mo C and graphite are shown immediately below.
• Figure 3B is an HRTEM micrograph of sample 10 as shown in Table C.
• Figure 4 is a thermogravimetric analysis of sample 12 as set forth in Table 1.
• Figure 5 A is an SEM micrograph of SiC extrudates.
• Figure 5B is an SEM micrograph illustrating micropores among the aggregates of the extrudates shown in Figure 5 A.
• Figure 5C is an SEM micrograph illustrating micropores in the networks of the intertwined SiC nanorods present in the extrudates shown in Figure 5A.
• nanotube Each refers to an elongated hollow structure having a diameter less than 1 micron.
• nanotube includes “nanofiber” or “fibril” (which refers to an elongated solid, (e.g. angular fibers having edges) structures having a cross section of less than 1 micron.
• nanotube also includes “bucky tubes” and graphitic nanofibers the graphene planes of which are oriented in herring bone pattern.
• Grapheme carbon is a form of carbon whose carbon atoms are each linked to three other carbon atoms in an essentially planar layer forming hexagonal fused rings.
• the layers are platelets only a few rings in diameter or they may be ribbons, many rings long but only a few rings wide.
• Grapheme analogue refers to a structure which is incorporated in a graphenic surface.
• Graphitic carbon consists of layers which are essentially parallel to one another and no more than 3.6 angstroms apart.
• nanorod refers to a rod-like structure having a surface and a substantially solid core with a diameter less than or equal to 100 nm and at least 1.0 nm.
• the structure has an aspect ratio between 10 and 500 and a length up to 50 ⁇ .
• the diameter of a nanorod is substantially uniform along the entire length of the nanorod.
• a nanorod is solid without being neither hollow with one or two open ends, nor hollow with two sealed ends.
• carbides refers to well known compounds of composition QC or Q
• Q is selected from the group consisting of transition metals (groups 3b, 4b, 5b,6b,7b,8 of periods 4, 5,6 of the Periodic Table) rare earths (lanthanides) and actinides. More preferably, Q is selected from the group consisting of B, Ti, Nb, Zr, Hf, Si, Al, Mo, V and W.
• the term also includes crystalline structures characterized by x-ray diffraction (XRD) as QC or Q C by themselves and/or in combinations with Q or C, for instance remaining after the sythesis step is substantially complete. Carbides can be detected and characterized by x-ray diffraction (XRD).
• the carbides are prepared by carburization of metal oxides or by oxidation of elemental carbon, a certain amount of "non- stoichiometric" carbide may appear, but the diffraction pattern of the true carbides would still be present.
• Metal rich non- stoichiometric carbides such as might be formed from a synthesis wherein the metal is carburized, are simply missing a few of the carbons that the metal matrix can accommodate.
• Carbon rich non- stoichiometric carbides comprise domains of stoichiometric carbides embedded in the original carbon structure. Once the carbide crystallites are large enough they can be detected by XRD.
• Carbides also refers to interstitial carbides as more specifically defined in "Structural Inorganic Chemistry” by A.F. Wells, 4th Edition, Clarendon Press, Oxford 1975 and in “The Chemistry of Transition Metal Carbides and Nitrides", edited by S.T. Oyama, a Blackie Academic & Professional publication, both of which are inco ⁇ orated herein by reference as if set forth in full.
• carbides-based nanorod refers to a Q-based nanorod predominantly having a diameter greater than 2.0 nm but less than 50 nm, wherein Q is an element capable of forming a carbide, Q being selected from the group consisting of B, Ti, Ta, Nb, Zr, Hf, Si, Al, Mo, V, W, and having an aspect ratio from 5 to 500.
• oxycarbides-based nanorod refers to an M- based nanorod having a substantially uniform diameter greater than 1.0 nm but less than or equal to 100 nm, wherein M is any metal capable of forming a oxycarbide such as Ti, Ta, Nb, Zr, Hf, Mo, V, W, B, Si and Al. It has an aspect ratio of 5 to 500. Oxycarbides, unlike carbides, are inherently non-stoichiometric.
• the oxycarbides of the present invention have the formula:
• M selected from the group consisting of transition metals (groups 3b, 4b, 5b, 6b, 7b, 8 of periods 4, 5, 6 of the Periodic Table) rare earths (lanthanides) and actinides, and more preferably Ti, Ta, Hf, Nb, Zr, Mo, V, W, Si, Al, B; n and x are selected to satisfy a known stoichiometry of a carbide of Q, where Q is the same as M; y is less than x and the ratio [y/(x-y)j is at least 0.02 and less than 0.9 and more preferably between 0.05 and 0.50.
• oxygen also includes but is not limited to products formed by oxidative treatments of carbides present in connection with carbon nanotubes as a source of carbon or in connection with carbide nanorods as a source of carbides.
• Oxycarbides can also include products formed by carburization of metal oxides.
• Oxycarbides also comprise mixtures of unreacted carbides and oxides, chemisorbed and physisorbed oxygen.
• M is selected from the group consisting of Mo, W, V, Nb, Ta, Ti, Zr, Hf, B, Si and Al.
• oxycarbides have a total amount of oxygen sufficient to provide at least 25% of at least one monolayer of absorbed oxygen as determined by temperature programmed deso ⁇ tion (TPD) based on the carbide content of the carbide source.
• TPD temperature programmed deso ⁇ tion
• Oxycarbides also refer to compounds of the same name as defined in "The Chemistry of Transition Metal Carbides and Nitrides", edited by S.T. Oyama, a Blackie Academic & Professional publication inco ⁇ orated herein by referenced as if set forth in full.
• Examples of oxycarbides include polycrystalline compounds, wherein M is a metal preferably in two valent states. M can be bonded to another metal atom or only to an oxygen or only to a carbon atom. However, M is not bonded to both an oxygen and carbon atoms.
• the term “aggregate” refers to a dense, microscopic particulate structure. More specifically, the term “assemblage” refers to structures having relatively or substantially uniform physical properties along at least one dimensional axis and desirably having relatively or substantially uniform physical properties in one or more planes within the assemblage, i.e. they have isotropic physical properties in that plane.
• the assemblage may comprise uniformly dispersed individual interconnected nanotubes or a mass of connected aggregates of nanotubes. In other embodiments, the entire assemblage is relatively or substantially isotropic with respect to one or more of its physical properties.
• the physical properties which can be easily measured and by which uniformity or isotropy are determined include resistivity and optical density.
• pore traditionally refers to an opening or depression in the surface of a catalyst or catalyst support. Catalysts and catalyst supports comprising carbon nanotubes lack such traditional pores. Rather, in these materials, the spaces between individual nanotubes behave as pores and the equivalent pore size of nanotube aggregates can be measured by conventional methods (porosimetry) of measuring pore size and pore size distribution. By varying the density and structure of aggregates, one can vary the equivalent pore size and pore size distribution.
• micropore refers to a pore which has a diameter of less than 2 micrometers.
• meopore refers to pores having a cross section greater than 2 nanometers.
• nonuniform pore structure refers to a pore structure occurring when individual discrete nanotubes are distributed in a substantially nonuniform manner with substantially nonuniform spacings between nanotubes.
• uniform pore structure refers to a pore structure occurring when individual discrete nanotubes or nanofibers form the structure. In these cases, the distribution of individual nanotubes in the particles are substantially uniform with substantially regular spacings between the nanotubes. These spacings (analogous to pores in conventional supports) vary according to the densities of the structures.
• bimodal pore structure refers to a pore structure occurring when aggregate particles of nanotubes and/or nanorods are bonded together.
• the resulting structure has a two-tiered architecture comprising a macrostructure of nanotube aggregates having macropores among the bundles of nanotube aggregates and a microstructure of intertwined nanotubes having a pore structure within each individual bundle of aggregate particles.
• surface area refers to the total surface area of a substance measurable by the BET technique.
• accessible surface area refers to that surface area not attributed to micropores (i.e., pores having diameters or cross-sections less than 2 nm).
• isotropic means that all measurements of a physical property within a plane or volume of the structure, independent of the direction of the measurement, are of a constant value. It is understood that measurements of such non-solid compositions must be taken on a representative sample of the structure so that the average value of the void spaces is taken into account.
• internal structure refers to the internal structure of an assemblage including the relative orientation of the fibers, the diversity of and overall average of nanotube orientations, the proximity of the nanotubes to one another, the void space or pores created by the interstices and spaces between the fibers and size, shape, number and orientation of the flow channels or paths formed by the connection of the void spaces and/or pores.
• the structure may also include characteristics relating to the size, spacing and orientation of aggregate particles that form the assemblage.
• relative orientation refers to the orientation of an individual nanotube or aggregate with respect to the others (i.e., aligned versus non-aligned).
• the “diversity of and “overall average” of nanotube or aggregate orientations refers to the range of nanotube orientations within the structure (alignment and orientation with respect to the external surface of the structure).
• the term “physical property” means an inherent, measurable property of the porous structure, e.g., surface area, resistivity, fluid flow characteristics, density, porosity, etc.
• relatively means that ninety-five percent of the values of the physical property when measured along an axis of, or within a plane of or within a volume of the structure, as the case may be, will be within plus or minus 20 percent of a mean value.
• substantially means that ninety-five percent of the values of the physical property when measured along an axis of, or within a plane of or within a volume of the structure, as the case may be, will be within plus or minus ten percent of a mean value.
• substantially isotropic or “relatively isotropic” correspond to the ranges of variability in the values of physical properties set forth above.
• nanotubes refers to various carbon tubes or fibers having very small diameters including fibrils, whiskers, buckytubes, etc. Such structures provide significant surface area when inco ⁇ orated into a structure because of their size and shape. Moreover, such nanotubes can be made with high purity and uniformity.
• the nanotube used in the present invention have a diameter less than 1 micron, preferably less than about 0.5 micron, and even more preferably less than 0J micron and most preferably less than 0.05 micron.
• Carbon nanotubes can be made having diameters in the range of 3.5 to 70 nanometers.
• nanotubes, buckytubes, fibrils and whiskers that are referred to in this application are distinguishable from continuous carbon fibers commercially available as reinforcement materials.
• continuous carbon fibers In contrast to nanofibers, which have desirably large, but unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios (L/D) of at least 10 4 and often 10 6 or more.
• L/D aspect ratios
• the diameter of continuous fibers is also far larger than that of fibrils, being always >1.0 ⁇ m and typically 5 to 7 ⁇ m.
• Continuous carbon fibers are made by the pyrolysis of organic precursor fibers, usually rayon, polyacrylonitrile (PAN) and pitch. Thus, they may include heteroatoms within their structure.
• organic precursor fibers usually rayon, polyacrylonitrile (PAN) and pitch.
• PAN polyacrylonitrile
• They may include heteroatoms within their structure.
• the graphitic nature of "as made" continuous carbon fibers varies, but they may be subjected to a subsequent graphitization step. Differences in degree of graphitization, orientation and crystallinity of graphite planes, if they are present, the potential presence of heteroatoms and even the absolute difference in substrate diameter make experience with continuous fibers poor predictors of nanofiber chemistry.
• Carbon nanotubes are vermicular carbon deposits having diameters less than
• United States Patent No. 4,663,230 to Tennent hereby inco ⁇ orated by reference, describes carbon nanotubes or fibrils that are free of a continuous thermal carbon overcoat and have multiple ordered graphitic outer layers that are substantially parallel to the fibril axis. As such they may be characterized as having their c-axes, the axes which are pe ⁇ endicular to the tangents of the curved layers of graphite, substantially pe ⁇ endicular to their cylindrical axes. They generally have diameters no greater than 0J ⁇ and length to diameter ratios of at least 5.
• the Tennent invention provided access to smaller diameter fibrils, typically 35 to 700 A(0.0035 to 0.070 ⁇ ) and to an ordered, "as grown" graphitic surface. Fibrillar carbons of less perfect structure, but also without a pyrolytic carbon outer layer have also been grown.
• the carbon planes of the graphitic nanotube take on a herring bone appearance.
• These are termed fishbone fibrils.
• These carbon nanotubes are also useful in the practice of the invention.
• oxidized nanofibers are used to form rigid porous assemblages.
• McCarthy et al. U.S. Patent Application Serial No. 351,967 filed May 15, 1989, hereby inco ⁇ orated by reference, describes processes for oxidizing the surface of carbon nanotubes or fibrils that include contacting the nanotubes with an oxidizing agent that includes sulfuric acid (H 2 SO 4 ) and potassium chlorate (KClO 3 ) under reaction conditions (e.g., time, temperature, and pressure) sufficient to oxidize the surface of the fibril.
• the nanotubes oxidized according to the processes of McCarthy, et al. are non-uniformly oxidized, that is, the carbon atoms are substituted with a mixture of carboxyl, aldehyde, ketone, phenolic and other carbonyl groups.
• Nanotubes have also been oxidized nonuniformly by treatment with nitric acid.
• International Application PCT/US94/10168 discloses the formation of oxidized fibrils containing a mixture of functional groups.
• Hoogenvaad, M.S., et al. Metal Catalysts supported on a Novel Carbon Support", Presented at Sixth International Conference on Scientific Basis for the Preparation of Heterogeneous Catalysts. Brussels, Belgium, September 1994
• Such pretreatment with acid is a standard step in the preparation of carbon-supported noble metal catalysts, where, given the usual sources of such carbon, it serves as much to clean the surface of undesirable materials as to functionalize it.
• McCarthy and Bening Polymer Preprints ACS Div. of Polymer Chem. 30 (1)420(1990) prepared derivatives of oxidized nanotubes or fibrils in order to demonstrate that the surface comprised a variety of oxidized groups.
• the compounds they prepared, phenylhydrazones, haloaromaticesters, thallous salts, etc., were selected because of their analytical utility, being, for example, brightly colored, or exhibiting some other strong and easily identified and differentiated signal. These compounds were not isolated and are of no practical significance.
• the nanotubes may be oxidized using hydrogen peroxide, chlorate, nitric acid and other suitable reagents.
• the nanotubes within the structure may be further functionalized as set forth in U.S. Patent Application No. 08/352,400, filed December 8, 1995, by Hoch and Moy et al., entitled “Functionalized Fibrils", hereby inco ⁇ orated by reference.
• Carbon nanotubes of a mo ⁇ hology similar to the catalytically grown fibrils or nanotubes described above have been grown in a high temperature carbon arc (Iijima, Nature 354 56 1991, hereby inco ⁇ orated by reference). It is now generally accepted (Weaver, Science 265 1994, hereby inco ⁇ orated by reference) that these arc-grown nanofibers have the same mo ⁇ hology as the earlier catalytically grown fibrils of Tennent. Arc grown carbon nanofibers are also useful in the invention.
• Nanotube Aggregates and Assemblages The "unbonded" precursor nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes or both.
• the aggregates when present, are generally of the bird's nest, combed yarn or open net mo ⁇ hologies.
• nanotubes aggregate in several stages or degrees.
• Catalytically grown nanotubes produced according to U.S.S.N. 08/856,657 filed on May 15, 1997 are formed in aggregates substantially all of which will pass through a 700 micron sieve.
• the size of as-made aggregates can, of course, be reduced by various means, but such disaggregation becomes increasingly difficult as the aggregates get smaller.
• Nanotubes may also be prepared as aggregates having various mo ⁇ hologies (as determined by scanning electron microscopy) in which they are randomly entangled with each other to form entangled balls of nanotubes resembling bird nests ("BN"); or as aggregates consisting of bundles of straight to slightly bent or kinked carbon nanotubes having substantially the same relative orientation, and having the appearance of combed yarn ("CY") e.g., the longitudinal axis of each nanotube (despite individual bends or kinks) extends in the same direction as that of the surrounding nanotubes in the bundles; or, as, aggregates consisting of straight to slightly bent or kinked nanotubes which are loosely entangled with each other to form an "open net” (“ON”) structure.
• CY combed yarn
• the mo ⁇ hology of the aggregate is controlled by the choice of catalyst support.
• Spherical supports grow nanotubes in all directions leading to the formation of bird nest aggregates.
• Combed yarn and open nest aggregates are prepared using supports having one or more readily cleavable planar surfaces, e.g., an iron or iron- containing metal catalyst particle deposited on a support material having one or more readily cleavable surfaces and a surface area of at least 1 square meters per gram.
• Nanotube mats or assemblages have been prepared by dispersing nanofibers in aqueous or organic media and then filtering the nanofibers to form a mat or assemblage.
• the mats have also been prepared by forming a gel or paste of nanotubes in a fluid, e.g. an organic solvent such as propane and then heating the gel or paste to a temperature above the critical temperature of the medium, removing the supercritical fluid and finally removing the resultant porous mat or plug from the vessel in which the process has been carried out.
• a fluid e.g. an organic solvent such as propane
• the carbon rigid porous structures comprise extrudates of carbon nanotubes. Aggregates of carbon nanotubes treated with a gluing agent or binder are extruded by conventional extrusion methods into extrudates which are pyrolyzed or carbonized to form rigid carbon structures having bimodal pore structure.
• the bundles of carbon nanotubes are substantially intact except that they have been splayed (e.g. by sonication) or partially unravelled to provide a bimodal pore structure.
• the space between bundles ranges from points of contact to about 1 micron. Within bundles, spaces between carbon nanotubes range from 10 nm to 30 nm.
• the resulting rigid bimodal porous structure is substantially free of micropores, has surface areas ranging from about 250 m /g to about 400 m /g and a crush strength of about 20 psi for extrudates of 1/8 inch in diameter.
• Carbon nanotube extrudates have densities ranging from about 0.5 g/cm to about 0.7 g/cm , which can be controlled by the density of the extrusion paste.
• the extrudates have liquid abso ⁇ tion volumes from about 0.7 cm /g.
• Gluing or binding agents are used to form the paste of carbon nanotubes required for extrusion processes. Useful gluing or binding agents include without limitations cellulose, carbohydrates, polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides, poly(dimethylsiloxane), phenolic resins and the like.
• the extrudates obtained as described above can be further treated with mild oxidizing agents such as hydrogen peroxide without affecting the integrity of the rigid porous carbon structures.
• the rigid porous structures can be impregnated with catalytic particles by ion exchange, generally a preferred method for deposition of small size particles.
• the rigid porous carbon structure can also be impregnated with catalysts by incipient wetness, or physical or chemical adso ⁇ tion.
• Nanorods refers to rod-like structures having a substantially solid core, a surface and a diameter greater than 1.0 nm but less than 100 nm.
• the structure has an aspect ratio between 5 and 500 and a length between 2nm and 50 ⁇ and preferably between lOOnm and 20 ⁇ .
• the disclosed nanorods are substantially solid, being neither hollow with one or two open ends, nor hollow with two sealed ends.
• Carbide Nanorods can be prepared by using carbon nanotubes as a source of carbon.
• D. Moy and CM. Niu have prepared carbide nanorods or nanofibrils as disclosed in U.S. Application No. 08/414,369 inco ⁇ orated herein by reference as if set forth in full. They reacted Q-based gas with carbon nanofibrils or nanotubes to form, in situ, solid Q-based carbide nanofibrils or nanorods at temperatures substantially less than 1700°C and preferably in the range of about 1000°C to about 1400°C, and more preferably at approximately 1200°C.
• Q- based gases were volatile compounds capable of forming carbides.
• Q is selected from the group consisting of transition metals (groups 3b, 4b, 5b, 6b, 7b, 8 of periods 4, 5, 6) rare earths (lanthanides) and actinides.
• Q was selected from the group consisting of B, Ti, Ta, Nb, Zr, Hf, Si, Al, Mo, V and W.
• Carbide nanorods have also been prepared by reacting carbon nanotubes with volatile metal or non-metal oxide species at temperatures between 500°C and 2500°C wherein the carbon nanotube is believed to act as a template, spatially confining the reaction to the nanotube in accordance with methods described in PCT/US 96/09675 by CM. Lieber, inco ⁇ orated herein by reference. Carbide nanorods formed by methods wherein the carbon nanotube serves as a template are also useful in the present invention.
• Volatile Q precursors are compounds having a vapor pressure of at least 20 torr at reaction temperature. Reaction with the volatile Q compound may or may not take place through a nonvolatile intermediate.
• carbide nanorods include reductive carburization in which the carbon nanotubes are reacted with Q-based volatile metal oxides followed by passing a flow of gaseous CHVH 2 mixture at temperatures between 250°C and 700°C
• volatile Q-based compounds useful in preparation of Q-based carbide nanorods include carbonyls and chlorides such as, for example, Mo(CO) 6 , Mo(V) chloride or W(VI)O chloride.
• vapors of a volatile Q-based compound are passed over a bed of extrudates of carbon nanotubes in a quartz tube at temperatures from about 700°C to about 1000°C
• concentration of the Q-based compound By controlling the concentration of the Q-based compound, the crystallization of the carbides is limited to the space of the nanotube.
• the extent of conversion of the carbon in carbon nanotubes to carbide nanorods can be controlled by adjusting the concentration of the Q-based compound, the temperature at which the reaction occurs and the duration of the exposure of carbon nanotubes to the volatile Q-based compound.
• the extent of conversion of the carbon from the carbon nanotubes is between 40% and 100% and preferably around 95%.
• the resulting carbide nanorods have an excellent purity level in the carbide content, vastly increased surface area and improved mechanical strength.
• the surface area of the carbide nanorods is from 1 to 400 and preferably 10 to 300m 2 /g.
• compositions based on carbide nanorods include catalysts and catalyst support.
• compositions including carbide nanorods based on molybdenum carbide, tungsten carbide, vanadium carbide, tantalum carbide and niobium carbide are useful as catalysts in fluid phase catalytic chemical reactions selected from the group consisting of hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkyation, dealkyation and transalkylation.
• silicon carbide and aluminum carbide-based nanorods are especially useful as catalyst supports for conventional catalysts such as platinum and palladium, as well as for other Q-based carbides such as molybdenum carbide, tungsten carbide, vanadium carbide and the like.
• Oxycarbide-based nanorods can be prepared from carbide nanorods.
• the carbide nanorods are subjected to oxidative treatments known in the art.
• oxidative treatments are disclosed in U.S. Patent No. 5,576,466 to Ledoux, et al.; M. Ledoux, et al. European Pat. Appln. No. 0396 475 Al, 1989; C Pham-Huu, et al., Ind. Eng. Chem. Res. 34, 1107-11 13, 1995; E. Iglesia, et al., Journal of Catalysis, 131, 523-544, 1991, inco ⁇ orated herein by reference as if set forth in full.
• Oxycarbide compounds present in an oxycarbide nanorod, and also present when the conversion of the carbide source is incomplete include oxycarbides having a total amount of oxygen sufficient to provide at least 25% of at least one monolayer of absorbed oxygen as determined by temperature programmed deso ⁇ tion (TPD) based on the carbide content of the carbide source.
• TPD temperature programmed deso ⁇ tion
• Useful oxidizing gases include but are not limited to air, oxygen, carbon dioxide, N 2 O, water vapor and mixtures thereof. These gases may be pure or diluted with nitrogen and/or argon.
• Compositions comprising oxycarbide nanorods are useful as catalysts in many petrochemical and refining processes including hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylation and transalkylation. Supported Carbides and Oxycarbides
• the process parameters for example, the temperature, the concentration of, and the length of exposure to the Q-based volatile compound
• the carbide portion of the carbon nanotube can be engineered as desired.
• the carbide portion of the carbon nanotube can be located entirely on the surface of the carbon nanotube such that only parts of the surface comprise nanocarbide compounds. It is possible to have the entire surface of the carbon nanotube coated with carbides while the core of the carbon nanotube remains substantially carbon. Moreover, it is possible to control the surface coverage of carbon nanotubes with carbide compounds from 1% to 99% of the entire surface area.
• the carbon nanotube comprises carbide covering less than 50% of the surface of the carbon nanotube.
• carbide covering less than 50% of the surface of the carbon nanotube is preferred.
• the carbide portion of the carbon nanotube is retained at the surface, the mo ⁇ hology of the carbon nanotube remains substantially the same.
• the carbide portion of the nanotube into a carbide nanorod thereby obtaining a nanotube-nanorod hybrid structure.
• the carbide portion can be located anywhere on the carbon nanotube. Partial conversion of carbon to carbide compounds preferably varies from about 20% to about 85% by weight. When the content of carbide compounds in the carbon nanotube exceeds 85% by weight, then the carbon nanotubes have been substantially converted to carbide nanorods.
• the embodiment of the invention where the carbon nanotubes contain a carbide portion also encompasses providing the carbide portion of the carbon nanotube in any manner now known or later developed.
• the Q-based metal or metal compound preferably Mo, W or V is placed on the carbon nanotubes or aggregates directly and then pyrolyzed, leaving behind carbon nanotubes coated with carbide compounds.
• solutions of Q-based salts such as, for example, salts of Mo, W or V are dispersed over the carbon nanotubes or aggregates thereof and then pyrolyzed, again forming carbide compounds primarily on the surface of the carbon nanotubes.
• An embodiment wherein the core of the carbon nanotube remains carbon and the location of the metallic carbides is limited is quite desirable as a catalytic system.
• the core of the carbon nanotube acts as a catalyst support or carrier for the metallic carbide catalyst.
• the SiC nanorod can have a MoC portion that could be an outer layer or a MoC-based nanorod.
• the resulting nanorod is a mixed carbide-based nanorod wherein part of the nanorod is SiC-based and another portion is MoC-based.
• the mixed carbide nanotube or nanorods as discussed above are particularly suitable as catalyst carriers or directly as catalysts in high temperature chemical reactions, particularly in the petrochemical field.
• the oxycarbide portion comprises oxycarbide compounds located any place on. in and within the carbon nanotube or carbide nanorod.
• the oxycarbide compounds can be placed on the nanotube in any way now known or later developed.
• the nanotube having a carbide portion can be exposed to air or subjected to carburization or any other means of converting the carbide portion of the nanotube partially or completely into an oxycarbide nanorod portion.
• a carbon nanotube which is partly still a carbon nanotube, partly a carbide nanorod and partly a oxycarbide nanorod also referred to as a carbon-carbide-oxycarbide nanotube-nanorod hybrid.
• the invention also relates to rigid porous structures made from carbide nanorods, oxycarbide nanorods, and supported carbide and oxycarbide carbon nanotubes and methods for producing the same.
• the resulting structures may be used in catalysis, chromatography, filtration systems, electrodes, batteries and the like.
• the rigid porous structures according to the invention have high accessible surface area. That is, the structures have a high surface area which are substantially free of micropores (i.e., pores having a diameter or cross-section less than 2 nm).
• the invention relates to increasing the mechanical integrity and/or rigidity of porous structures comprising intertwined carbon nanotubes and/or carbide and/or oxycarbide nanorods.
• the structures made according to the invention have higher crush strengths than the conventional carbon nanotube or nanorod structures.
• the present invention provides a method of improving the rigidity of the carbon structures by causing the nanotubes and/or nanorods to form bonds or become glued with other nanotubes and/or nanorods at the nanotube and/or nanorod intersections.
• the bonding can be induced by chemical modification of the surface of the nanotubes to promote bonding, by adding "gluing" agents and/or by pyrolyzing the nanotubes to cause fusion or bonding at the interconnect points.
• the nanotubes or nanorods can be in the form of discrete nanotubes and/or nanorods or aggregate particles of nanotubes and nanorods.
• the former results in a structure having fairly uniform properties.
• the latter results in a structure having two- tiered architecture comprising an overall macrostructure comprising aggregate particles of nanotubes and/or nanorods bonded together and a microstructure of intertwined nanotubes and/or nanorods within the individual aggregate particles.
• individual discrete nanotubes and/or nanorods form the structure.
• the distribution of individual nanotube and/or nanorod strands in the particles are substantially uniform with substantially regular spacing between strands.
• These spacings (analogous to pores in conventional supports) vary according to the densities of the structures and range roughly from 15 nm in the densest to an average 50-60 nm in the lightest particles (e.g., solid mass formed from open net aggregates). Absent are cavities or spaces that would correspond to micropores ( ⁇ 2 nm) in conventional carbon supports.
• the distribution of individual nanotubes and/or nanorods is substantially nonuniform with a substantially nonuniform pore structure. Nevertheless, there are no cavities or spaces corresponding to micropores which are frequently present in other catalysts and catalyst supports.
• One embodiment of the invention relates to a rigid porous structure comprising carbide nanorods having an accessible surface area greater than about 10m /gm and preferably greater than 50 m /gm, being substantially free of micropores and having a crush strength greater than about 1 lb.
• the structure preferably has a density greater than 0.5 g/cm and a porosity greater than 0.8 cm /g.
• the structure comprises intertwined, interconnected carbide nanorods and is substantially free of micropores.
• the rigid porous structure includes carbide nanorods comprising oxycarbide compounds, has an accessible surface area greater than about 10 m /gm, and preferrably greater than 50 m /gm, is substantially free of micropores, has a crush strength greater than about 1 lb and a density greater than 0.5 g/cm 3 and a porosity greater than 0.8 cm 3 /g.
• the rigid porous structure includes oxycarbide nanorods having an accessible surface area greater than about 10 m /gm, and preferably greater than 50 m /gm, being substantially free of micropores, having a crush strength greater than about 1 lb, a density greater than 0.5g/cm and a porosity greater than 0.8 cm 3 /g.
• the rigid porous structure includes carbon nanotubes comprising a carbide portion.
• the location of the carbide portion can be on the surface of the carbon nanotube or any place on, in or within the carbon nanotube or the carbide portion can be converted into a carbide nanorod forming a carbon nanotube-carbide nanorod hybrid. Nevertheless, the catalytic effectiveness of these rigid porous structures is not affected by the carbide portion on the resulting composites.
• This rigid porous structures has an accessible surface area greater than about 10 m /gm and preferably than 50 m /gm, is substantially free of micropores, has a crush strength greater than about 1 lb, a density greater than 0.5 g/cm and a porosity greater than 0.8 cm /g.
• the rigid porous structure includes carbon nanotubes having a carbide portion and also an oxycarbide portion.
• the location of the oxycarbide portion can be on the surface of the carbide portion or any place on, in or within the carbide portion. Under certain conditions of oxidative treatment it is possible to convert a portion of the carbide nanorod part of the carbon-carbide nanotube-nanorod hybrid into an oxycarbide.
• the rigid porous structure inco ⁇ orating carbon-carbide- oxycarbide nanotube-nanorod hybrids has an accessible surface area greater than about 10m 2 /gm, is substantially free of micropores, has a crush strength greater than about 1 lb, a density greater than 0.5g/cm and a porosity greater than 0.8 cm g.
• the rigid porous structures described above comprise nanotubes and/or nanorods which are uniformly and evenly distributed throughout said rigid structures. That is, each structure is a rigid and uniform assemblage of nanotubes and/or nanorods.
• the structures comprise substantially uniform pathways and spacings between said nanotubes and/or nanorods.
• the pathways or spacings are uniform in that each has substantially the same cross-section and are substantially evenly spaced.
• the average distance between nanotubes and/or nanorods is less than about 0.03 microns and greater than about 0.005 microns. The average distance may vary depending on the density of the structure.
• the rigid porous structures described above comprise nanotubes and/or nanorods which are nonuniformly and unevenly distributed throughout said rigid structures.
• the rigid structures comprise substantially nonuniform pathways and spacings between said nanorods.
• the pathways and spacings have nonuniform cross-section and are substantially unevenly spaced.
• the average distance between nanotubes and/or nanorods varies between 0.0005 microns to 0.03 microns.
• the average distances between nanotubes and/or nanorods may vary depending on the density of the structure.
• the rigid porous structure comprises nanotubes and/or nanorods in the form of nanotube and/or nanorod aggregate particles interconnected to form said rigid structures.
• These rigid structures comprise larger aggregate spacings between the interconnected aggregate particles and smaller nanotube and/or nanorod spacings between the individual nanotubes and/or nanorods within the aggregate particles.
• the average largest distance between said individual aggregates is less than about 0J microns and greater than about 0.001 microns.
• the aggregate particles may include, for example, particles of randomly entangled balls of nanotubes and/or nanorods resembling bird nests and/or bundles of nanotubes and/or nanorods whose central axes are generally aligned parallel to each other.
• Another aspect of the invention relates to the ability to provide rigid porous particulates or pellets of a specified size dimension.
• porous particulates or pellets of a size suitable for use in a fluidized packed bed The method involves preparing a plurality of nanotubes and/or nanorods aggregates, fusing or gluing the aggregates or nanotubes and/or nanorods at their intersections to form a large rigid bulk solid mass and sizing the solid mass down into pieces of rigid porous high surface area particulates having a size suitable for the desired use, for example, to a particle size suitable for forming a packed bed.
• the above-described rigid porous structures are formed by causing the nanotubes and/or nanorods to form bonds or become glued with other nanofibers at the fiber intersections.
• the bonding can be induced by chemical modification of the surface of the nanofibers to promote bonding, by adding "gluing” agents and/or by pyrolyzing the nanofibers to cause fusion or bonding at the interconnect points.
• U.S. Patent Application No. 08/857,383 filed May 15, 1997, inco ⁇ orated herein by reference describes processes for forming rigid porous structures from carbon nanofibers or nanotubes.
• rigid porous structures comprising carbide nanorods are prepared by contacting a rigid porous carbon structure made of carbon nanotubes with volatile Q-based compounds under conditions sufficient to convert all of the carbon or only part of the carbon of the carbon nanotubes to carbide-based compounds.
• the rigid, high porosity structures can be formed from regular nanotubes or nanotube aggregates, either with or without surface modified nanofibers (i.e.. surface oxidized nanotubes).
• Surface oxidized nanotubes can be crosslinked according to methods described in U.S. Patent Application No. 08/856,657 filed on May 15, 1997 and U.S. Patent Application No. 08/857,383 also filed on May 15, 1997, both inco ⁇ orated herein by reference, and then carbonized to from a rigid porous carbon structure having a uniform pore structure, substantially free of micropores.
• rigid porous structures comprising carbide nanorods.
• the rigid porous carbon structures prepared as described above are contacted with Q-based compounds under conditions of temperature and pressure sufficient to convert the carbon nanotubes of the rigid porous carbon structure to carbide nanorods.
• the location of the carbide portion of the carbon nanotubes of the rigid porous carbide structure can be on the surface of the carbon nanotube or any place on, in or within the carbon nanotube, or when the conversion is complete then the entire carbon nanotube is transformed into a substantially solid carbon nanorod.
• the carbide-based rigid porous structures of the present invention have high accessible surface areas between 10 m /gm and 100 m /gm and are substantially free of micropores. These structures have increased mechanical integrity and resistance to attrition by comparison to individual carbide-based nanorods.
• Carbide-based rigid porous structures have a density greater than 0.5 g/cm and a porosity greater than 0.8 cm /g. The structure has at least two dimensions of at least 10 microns and not greater than 2 cm.
• the porous structure of the carbide-based rigid porous structure can be uniform, nonuniform or bimodal.
• the rigid porous structure When the rigid porous structure is uniform the average distance between said carbide-based nanorods is less than 0.03 microns and greater than 0.005 microns.
• the rigid porous structure comprises carbide-based nanorods in the form of interconnected aggregate particles wherein the distance between individual aggregates ranges from point of contact to l ⁇ .
• the carbide-based nanorod rigid porous structures was formed from rigid porous carbon structures comprising nanotube aggregates, the structure has aggregate spacings between interconnected aggregate particles and carbide nanorod spacings between nanorods within the aggregate particles. As a result the rigid porous structure has a bimodal pore distribution.
• One embodiment of the invention relates to rigid porous structures comprising extrudates of aggregate particles of carbide nanorods, wherein the carbide nanorods are glued together with binding agents such as cellulose, carbohydrates, polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides, poly(dimethylsiloxane) and phenolic resins.
• binding agents such as cellulose, carbohydrates, polyethylene, polystyrene, nylon, polyurethane, polyester, polyamides, poly(dimethylsiloxane) and phenolic resins.
• rigid porous structures comprising oxycarbide nanorods and/or nanotubes comprising a carbide portion and further an oxycarbide portion.
• carbide based rigid porous structures are subjected to oxidative treatments as disclosed in the art and in U.S. Patent No. 5,576,466 to Ledoux et al. issued November 13, 1996.
• rigid porous structure comprising carbon nanotubes having an oxycarbide portion and/or a carbide portion are prepared by subjecting to oxidative treatments disclosed in the art rigid porus carbon structures which have been partially converted to carbide nanorods.
• discrete carbide nanorods are subjected to oxidative treatments and then assembled into rigid porous structures according to methods similar to those disclosed in U.S. Patent Application No. 08/857,383 filed May 15, 1997 inco ⁇ orated herein by referenced.
• discrete carbon nanotubes or aggregate of carbon nanotubes which have been partially converted to carbide nanorods are further subjected to oxidative treatments and then assembled into rigid porous structures according to methods disclosed in U.S. Patent Application No. 08/857,383 filed May 15, 1997.
• the carbide and/or oxycarbide nanorods and nanotubes having carbide and/or oxycarbide portions of the invention have superior specific surface areas as compared to carbide and oxycarbide catalysts previously taught in the art. As a result, they are especially useful in the preparation of self-supported catalysts and as catalyst supports in the preparation of supported catalysts.
• the self-supported catalysts of the invention include catalytic compositions comprising nanotubes and/or nanorods and rigid porous structures comprising the same.
• Self-supported catalysts of the invention constitute the active catalyst compound and can be used without any additional physical support to catalyze numerous heterogenous reactions as more specifically described herein.
• the supported catalyst of the invention comprises a support including a nanofiber and/or nanorod rigid porous structure and a catalytically effective amount of a catalyst supported thereon.
• the uniquely high macroporosity of carbon nanotube structures greatly facilitates the diffusion of reactants and products and the flow of heat into and out of the self-supported catalysts.
• This unique porosity results from a random entanglement or intertwining of nanotubes and/or nanorods that generates an unusually high internal void volume comprising mainly macropores in a dynamic, rather than static state. Sustained separability from fluid phase and lower losses of catalyst as fines also improves process performance and economics.
• Other advantages of the nanotube and/or nanorod structures as self-supported catalysts include high purity, improved catalyst loading capacity and chemical resistance to acids and bases.
• carbon nanotube and/or nanorod aggregates provide superior chemical and physical properties in porosity, surface are, separability and purity.
• Self-supported catalysts made of nanotubes and/or nanorods have a high internal void volume that ameliorates the plugging problem encountered in various processes. Moreover, the preponderance of large pores obviates the problems often encountered in diffusion or mass transfer limited reactions. The high porosities ensure significantly increased catalyst life.
• One embodiment of the invention relates to self-supported catalyst which is a catalytic-composition comprising carbide-based nanorods having a diameter between at least 1.0 nm and less than 100 nm, and preferably between 3.5 nm and 20 nm.
• the carbide-based nanorods have been prepared from carbon nanotubes which have been substantially converted to carbide nanorods.
• the carbide nanorods retain substantially the structure of the original carbon nanotubes.
• the carbide nanotubes can have uniform, nonuniform or bimodal porous structure.
• catalytic compositions can be used as catalysts to catalyze reactions such as hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylation and transalkylation.
• the rigid porous structures of the invention can be used as both self-supported catalysts and as catalyst supports.
• catalysts and catalyst supports comprising the rigid porous structures of the invention have unique properties. They are exceptionally mesoporous and macroporous. They are also pure and resistant to attrition, compression and shear and consequently can be separated from a fluid phase reaction medium over a long service life.
• the increased rigidity of the rigid porous structures of the present invention enables catalysts and catalyst supports comprising the structures to be used in fixed bed catalytic reactions.
• a packing containing the sized rigid structures can be formed and a fluid or gas passed through the packing without significantly altering the shape and porosity of the packing since the rigid structures are hard and resist compression.
• Rigid structures formed from nanorod aggregates, preferably silicon carbide and aluminum carbide-based nanorods are particularly preferred structures for use as catalyst supports.
• nanorod structures The combination of properties offered by nanorod structures is unique. No known catalyst supports combine such high porosity, high accessible surface area and attrition resistance.
• the combination of properties offered by the nanorod structures is advantageous in any catalyst system amenable to the use of a carbide catalyst support.
• the multiple nanorods that make up a nanorod structure provide a large number of junction points at which catalyst particles can bond to multiple nanorods in the nanorod structures. This provides a catalyst support that more tenaciously holds the supported catalyst.
• nanorod structures permit high catalyst loadings per unit weight of nanorod. However, catalyst loadings are generally greater than 0.03 weight percent and preferably greater than 0J, but generally less than 5% weight based on the total weight of the supported catalyst.
• Desirable hydrogenation catalysts are the platinum group (ruthenium, osmium, rhodium, iridium, palladium and platinum or a mixture thereof) and, preferably, palladium and platinum or a mixture thereof.
• Group VII metals including especially iron, nickel and cobalt are also attractive hydrogenation catalysts.
• Oxidation (including partial oxidation) catalysts may also be supported on carbide and oxycarbide nanotubes and nanotube structures.
• Desirable metallic oxidation catalysts include, not only members of the platinum group enumerated above, but also, silver and the group VIII metals.
• Oxidation catalysts also include metal salts known to the art including salts of vanadium, tellurium, manganese, chromium, copper, molybdenum and mixtures thereof as more specifically described in "Heterogeneous Catalytic Reactions Involving Molecular Oxygen," by Golodets, G.I.& Ross, J.R.H, Studies in Surface Science, 15, Elsevier Press, NYC 1983.
• Active catalysts include other carbide compounds such as carbides of Ti, Ta, Hf, Nb, Zr, Mo, V and W. These supported carbides are particularly useful for hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylation and transalkylation.
• carbide nanorod aggregates exhibit high resistance to attack by acids and bases. This characteristic is advantageous since one path to regenerating catalysts is regeneration with an acid or a base. Regeneration processes can be used which employ strong acids or strong bases. This chemical resistance also allows the carbide supports of the invention to be used in very corrosive environments.
• the supported catalysts are made by supporting a catalytically effective amount of catalyst on the rigid nanorod structure.
• the term "on the nanotube and/or nanorod structure” embraces, without limitation, on, in and within the structure and on the nanotubes and/or nanorods thereof. The aforesaid terms may be used interchangeably.
• the catalyst can be inco ⁇ orated onto the nanotube and/or nanorod or aggregates before the rigid structure is formed, while the rigid structure is forming (i.e., add to the dispersing medium) or after the rigid structure is formed.
• Methods of preparing heterogeneous supported catalysts of the invention include adso ⁇ tion, incipient wetness impregnation and precipitation.
• Supported catalysts may be prepared by either inco ⁇ orating the catalyst onto the aggregate support or by forming it in situ and the catalyst may be either active before it is placed in the aggregate or activated in situ.
• the catalyst such as a coordination complex of a catalytic transition metal, such as palladium, rhodium or platinum, and a ligand, such as a phosphine, can be adsorbed by slurrying the nanorods in a solution of the catalyst or catalyst precursor for an appropriate time for the desired loading.
• a catalytic transition metal such as palladium, rhodium or platinum
• a ligand such as a phosphine
• One embodiment of the invention relates to a catalyst comprising a composition including a multiplicity of oxycarbide-based nanorods.
• Each nanorod has substantially uniform diameters between 3.5 nm and 20 nm.
• the oxycarbide-based nanorods have a substantially solid core, form a substantially polycrystalline solid and the individual nanorods are predominantly unfused.
• a catalyst comprising a rigid porous structure including oxycarbide-based nanorods as described above.
• Each catalytic composition can be used as a catalyst in a fluid phase reaction selected from the group consisting of hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylation and transalkylation.
• Another embodiment of the invention relates to a catalyst comprising a composition including a multiplicity of Q-based nanorods, wherein Q is selected from the group consisting of B, Si, Al, Ti, Ta, Nb, Zr, Hf, Mo, V and W.
• Q is selected from the group consisting of B, Si, Al, Ti, Ta, Nb, Zr, Hf, Mo, V and W.
• the resulting carbide nanorods can be distributed nonuniformly, uniformly or can be in the form of interconnected aggregate particles.
• the catalyst comprises a rigid porous structure based on the Q-based nanorods described above which have been formed into extrudates and connected by gluing agents or in any other manner sufficient to form the rigid porous structure.
• Each catalytic composition discussed immediately above can be used as catalysts in a fluid phase reaction selected from the group consisting of hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkylation, dealkylation and transalkylation.
• Another embodiment relates to a catalyst comprising a composition including a multiplicity of carbide-based nanorods which further comprise oxycarbide compounds any place on, in or within the nanorod, preferably on the surface.
• the catalyst comprises a rigid porous structure including the carbide-based nanorods comprising oxycarbides which have been formed into extrudates connected into the rigid porous structure by gluing agents or in any other manner sufficient to form the rigid porous structure.
• a fluid phase reaction selected from the group consisting of hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkyation, dealkyation and transalkyation.
• a catalyst comprising a composition including a multiplicity of carbon nanotubes having substantially uniform diameters.
• the carbon nanotubes comprise carbide compounds anywhere on, in or within the nanotubes, but preferably on the surface of the nanotubes.
• the carbon nanotubes additionally comprise oxycarbide compounds on, in or within the nanotubes, but preferably on the surface as more specifically described in section "Supported Carbides and Oxycarbides" of the of the specification.
• the nanotube mo ⁇ hology is substantially retained.
• the catalyst comprises a rigid porous structure including carbon nanotubes comprising carbide compounds and, in another embodiment, also oxycarbide compounds as described above.
• Each rigid porous structure is useful as a catalyst in a fluid phase reaction to catalyze a reaction selected from the group consisting of hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization. alkyation, dealkyation and transalkyation.
• the catalytic composition includes a multiplicity of carbon nanotubes having a carbide portion which has been converted to a carbide nanorod forming a nanotube-nanorod hybrid structure.
• the catalytic composition includes a multiplicity of carbon nanotubes having a carbide nanorod portion and in addition also an oxycarbide portion which has been converted to an oxycarbide nanorod.
• the foregoing carbon nanotubes can be included in rigid porous structures, wherein the carbon nanotubes are formed into extrudates and/or are otherwise connected to form rigid porous structures.
• the catalytic compositions are useful as catalysts in a fluid phase reaction selected from the group consisting of hydrogenation, hydrodesulfurisation, hydrodenitrogenation, hydrodemetallisation, hydrodeoxygenation, hydrodearomatization, dehydrogenation, hydrogenolysis, isomerization, alkyation, dealkyation and transalkytion.
• Moacac Moacac
• the resulting mixture was dried at 110°C at full vacuum for 18 hours during which the Mo precursor decomposed to a mixture of molybdenum suboxides, generally designated as MoO 3 . x , wherein x is 0 orl.
• the sample was set aside for conversion to carbide catalysts by careful calcination under an inert atmosphere as described in
• Example 2 A similar procedure as used in Example 1 above was followed, except that the impregnating solutions were aqueous solutions containing the correct amount of ammonium heptamolybdate tetrahydrate or (NFL;) 6 Mo 7 O 4 .4H 2 O herein referred to as ammonium molybdate necessary for the desired CMo atom ratio loading.
• the resulting mixtures were dried at 225°C in full vacuum for 18 hours during which the heptamolybdate compound was decomposed to MoO 3 .
• the sample was set aside for conversion to carbide catalysts by careful calcination under an inert atmosphere as more particularly described in Examples 5, 6 and 7 herein.
• CC or CY type aggregates were oxidized with nitric acid as described in U.S. Application Serial No. 08/352, 400 filed December 8, 1994 entitled "Functionalized Nanotubes" to form oxidized CC aggregates having an acid titer of about 0.6 mg/g).
• Five grams of the oxidized CC type aggregates of carbon nanotubes were well-mixed with either an ethanol solution of Moacac or an aqueous solution of ammonium heptamolybdate tetrahydrate, each solution containing the correct amount of Mo compound necessary for the desired CMo loading. The mixing was accomplished by kneading in a Braybender kneader until the paste had a homogeneous consistency.
• oxidized CC aggregates having an acid titer of about 0.6 mg/g.
• Five grams of oxidized CC type aggregates of carbon nanotubes were physically admixed with the correct amount of either ammonium heptamolybdate tetrahydrate or MoO 3 necessary for the desired CMo atom ratio by kneading the sample in a mortar and pestle.
• a small amount of wetting agent such as water or ethylene glycol, was added periodically to keep the oxidized carbon nanotube powder dusting under control and to facilitate the contact between the molybdenum precursor particles and the carbon nanotube aggregates.
• Example 5 The same procedure as described in Example 5 above was followed up to 600°C The samples were then held at 600°C for one hour. Thereafter, heating was resumed at the same rate of l°C/min to 800°C and held at that temperature for another 3 hours. After cooling in argon, the samples were passivated using 3% O /Ar.
• EXAMPLE 7 Calcination of Molybdenum Carbide Carbide Precursors at 1000°C The same procedure as described in Example 6 above was followed up to 800°C at which temperature the samples were held for 1 hours. Thereafter, heating of the samples was resumed at the rate of 1 °C/min to 1000°C, where the temperature was maintained for 0.5 hours. After cooling in argon, the samples were passivated using 3% O 2 /Ar.
• Impregnated powder of aggregates of carbon nanotubes b Impregnated extrudates of aggregates of carbon nanotubes c Powder of aggregates of carbon nanotubes physically mixed with Mo precursor e Calculated Mo 2 C loading in final calcined product assuming full conversion of Mo precursor to Mo 2 C
• the stoichiometric atom ratio to produce pure carbide with no excess C or Mo, i.e., pure Mo C is 3.5.
• the number following in parentheses is the calculated loading of the Mo C contained in the resulting materials.
• Weight loss (theor) refers to the theoretical weight loss according to the equation at the top of Table 1.
• Phases, XRD shows the compounds found in the X-ray diffraction (XRD) analyses. Mo C exists in two distinct crystallographic phases, hexagonal and cubic. 5 Table 2 below summarizes the XRD results for the samples of Table 1. TABLE 2.
• Table 2 summarizes the XRD results for the experiments summarized in Table 1 , identifies the compounds made, the phases present, and the calculated average particle size for the different phases.
• the average particle size is a volume-biased average size, such that the value of one large particle counts more heavily than several medium particles and much more than the volume of many small particles. This is a conventional procedure which is well known to those familiar with XRD methods. Discussion of Results of Examples 1-7
• Samples 1 and 12 provided the clearest evidence of the formation of freestanding Mo 2 C nanorods and nanoparticles. These were obtained by reacting stoichiometric or near stoichiometrix mixtures of MoO 3 and carbon nanotubes, either as powder or as extrudates. Product identification and mo ⁇ hologies were obtained by SEM, HRTEM and XRD. In Example 1, with about 15% excess of C the major product was identified by XRD as the hexagonal phase of Mo C MoO 2 and graphitic carbon were seen as minor components. SEM showed the presence of both nanorods (-10-15 nm diameter) and nanoparticles (-20 nm).
• Samples 11 and 12 resulted by reacting carbon nanotubes either with a stoichiometric mixture of well-dispersed MoO 3 powder, or with impregnated ammonium molybdate.
• Sample 12 which was a stoichiometric mixture, was studied in more detail in order to learn the course of the reaction.
• the reaction was tracked by thermogravimetric analysis (TGA) as shown in Figure 4.
• TGA thermogravimetric analysis
• Figure 4 shows that the stoichiometric reaction has occurred in two distinct steps, namely, reduction of MoO 3 by carbon to MoO 2 at from about 450 to about 550°C, followed by further reduction to Mo 2 C at from about 675°C to about 725°C.
• SEM and XRD analyses taken after calcination at 600°C showed a complete redistribution of oxide precursor from the very large, supra- ⁇ particles of MoO 3 initially present to about 20-50 nm particles of MoO 3-x , well-dispersed amongst individual fibrils.
• Figures 3 and 4 which are copies of HRTEM micrographs taken from Sample 10.
• the particle size can be estimated by direct comaprison with the fibril diameters, which range from 7-10 nm.
• Tungstate The same procedure as used in Example 2 above was followed, except that the impregnating solution was an aqueous solution containing the correct amount of ammonium paratungstate hydrate or (NH 4 ) ⁇ oW 12 O 41 .5H O, 72.% W (herein referred to as ammonium tungstate) necessary for the desired C:W atom ratio loading (CW mole ratios of 3.5:1, 10:1 and 20:1.) The resulting mixture was dried at 225°C in full vacuum for 18 hours during which the paratungstate compound was decomposed to
• Example 8 The same procedure as used in Example 8 above was followed, except that the impregnating solution was an aqueous solution containing the correct amount of phosphotungstic acid, H 3 PO 4 J2 WO 3 .xH 2 O, 76.6% W, herein referred to as PTA, necessary for the desired CW atom ratio loading (CW mole ratios of 3.5:1, 10:1 and 20:1.)
• PTA phosphotungstic acid
• the resulting mixture was dried at 225°C in full vacuum for 18 hours during which the PTA was decomposed to WO 3 .
• the sample was set aside for conversion to carbide catalysts by careful calcination under an inert atmosphere as more particularly described in Example 10 herein.
• Example 7 The same procedure as described in Example 7 above was followed to vonvert precursors of tungsten carbides to tungsten carbides. After cooling in argon, the samples were passivated using 3% O /Ar. Table 3 below summarizes the experimental conditions and XRD results for selected experiments.
• Impregnated powder of CC aggregates of carbon nanotubes by incipient wetness with phosphotungstic acid Impregnated powder of CC aggregates of carbon nanotubes by incipient wetness with ammonium paratungstate hydrate
• SiC nanorods were prepared from Hyperion aggregates of carbon nanotubes in accordance with Example 1 of U.S. Application Serial No. 08/414,369 filed March 31, 1995 (Attorney Docket No. KM 6473390) by reacting the carbon nanotubes with SiO vapor at high temperature.
• the resulting SiC nanorods have a uniform diameter of 15nm on the average and a highly crystallized ⁇ SiC structure.
• the paste was pushed through a syringe to produce extrudates of a green color which were heated under flowing argon atmosphere as follows: at 200°C for 2 hours (Example 11); at 400°C for 4 hours (Example 12); and at 700°C for 4 hours (Example 13).
• a rigid porous structure of SiC nanorods has formed.
• the extrudates obtained in Example 11-13 had a density of 0.97 g/cc and a bimodal pore structure.
• the macropores were 1-5 ⁇ m, as shown in Fig. 5B among aggregates and the mesopores were 10-50 nm, as shown in Fig. 5C in the networks of intertwined SiC nanorods.
• the diameter of the extrudates was around 1.2 mm as shown in Fig. 5 A.
• the specific surface area of the extrudates of SiC nanorods was 97 m 2 /g.
• SiC extrudates are attractive for various applications, including support for catalysts such as platinum, palladium and the like and carbides of Mo, W, V, Nb or Ta.
• catalysts such as platinum, palladium and the like and carbides of Mo, W, V, Nb or Ta.
• the surface properties of SiC nanorods when used as a catalyst support are very close to that of carbon. Therefore conventional carbon supports can be replaced with SiC extrudates and thus extend many properties of carbon supported catalysts to high temperature regions, as required in particular for oxidative conditions.
• a sample of extrudates of carbon nanotubes is placed in a vertical reactor such that a bed is formed.
• the extrudates are heated under flowing H gas at 150°C for 2 hours. Thereafter, the extrudates are cooled to 50°C.
• H gas passed through a saturator containing Mo(CO) at 50°C is flown over the cooled extrudates of carbon nanotubes.
• Mo(CO) 6 becomes adsorbed on the surface of extrudates of carbon nanotubes.
• the temperature of the sample is raised to 150°C in an atmosphere of pure H 2 . The temperature is maintained at 150°C for 1 hour.
• the temperature of the sample is then increased at 650°C and maintained at this temperature for 2 hours under flowing H 2 gas.
• a sample of extrudates of carbon nanotubes bearing molybdenum on their surfaces is obtained.
• This sample is then kept at 650°C for 1 hour and the gas is switched from H 2 to a CH 4 /H 2 mixture (20% Cu t ).
• the molybdenum adsorbed on the surfaces of carbon nanotubes is converted to molybdenum carbides.
• the amount of molybdenum carbide formed on the surface of the extrudate can be controlled.

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