Superior Carbonate Based Catalyst Supports for Carbon Nanotube Production Claimed by Belgian Researchers

Facultes Universitaires Notre-Dame De La Paix (Namur, BE) researchers detail a number of hydroxide and carbonate-based catalyst supports used for manufacturing multiwall carbon nanotubes (MWNT) and single wall carbon nanotubes (SWNT) in U.S. Patent Application 20090325788. The inventors claim the new catalyst supports do not produce large amounts of soot and amorphous carbon along with the carbon nanotubes as do many common catalyst supports now used.  

Inventors Janos B. Nagy, Narasimaiah Nagaraju, Isabelle Willems and Antonio Fonseca prepared carbon nanotubes by the catalytic decomposition of hydrocarbons using a technique called CCVD (Catalytic Carbon Vapor Deposition), carried out in the presence of catalysts to produce both MWNTs and SWNTs. Soot and encapsulated metal nanoparticles are the other by-products. The hydrocarbon can be acetylene, ethylene, butane, propane, ethane, methane or any other gaseous or volatile carbon containing compound. The catalyst, a transition metal, is generally, either pure or dispersed on a support.

The presence of a support for the catalyst affects the activity of the catalysts tremendously in the formation of carbon nanotubes. The selectivity of the catalyst for the production of nanotubes also depends on the type of catalyst support interaction.

The most common supports used to prepare supported catalyst for carbon nanotubes production are oxides i.e., silica, alumina, silica-alumina mixtures, magnesium oxide, calcium oxide, titanium oxide, cerium oxide, zeolites, spinels and graphite. The use of porous materials (i.e., silica, alumina, zeolites, etc.) as supports for catalysts, contaminates the carbon nanotubes produced thereon with a large amount of soot and amorphous carbon, while dissolving the support during the purification of the carbon nanotubes.

The Notre-Dame De La Paix hydroxide and carbonate-based catalyst supports do not present the contamination drawbacks of the catalyst supports of the state of the art.

The carbon nanotubes production on the supported catalyst by CCVD comprises the following steps:

Spreading manually or mechanically an appropriate amount of supported catalyst on a quartz boat to be used as bed for the supported catalyst in the fixed bed reactor. In the case of a moving bed reactor, the supported catalyst is spread continuously or by intermittence mechanically or manually on the moving bed of the reactor.

The reactor, containing the supported catalyst, is either kept initially at the appropriate constant reaction temperature (400-1200.degree. C.), or it is heated to the reaction temperature for an appropriate time of the reaction. Inert or reactant gas(es) can be passed over the supported catalyst during that step.

The pure or diluted hydrocarbon is passed over the supported catalyst at a predetermined temperature. Carbon nanotubes are grown on the supported catalyst as a result of the CCVD reaction. Diluted hydrocarbons are obtained by mixing at least one hydrocarbon with other gases such as nitrogen, argon, helium, hydrogen, CO, etc.

The crude nanotubes, composed of a mixture of carbon nanotubes and spent supported catalyst, is collected either continuously in the case of a moving bed reactor or stepwise in the case of a fixed bed reactor.   Preferably, the carbon nanotubes purification is carried out by dissolving the spent supported catalyst as follows:

Stirring the crude nanotubes in a concentrated basic solution, preferably a concentrated NaOH solution, at a temperature in between 100-250.degree. C. Recovering the solid product by filtration and preferably washing it until a neutral pH is obtained. This first step is not necessary if the catalyst support contains only Mg and/or Ca derivatives.

Stirring the product in a concentrated acidic solution, preferably a concentrated HCl solution, at a temperature in between 0-120.degree. C.

Recovering the solid product (purified carbon nanotubes) by filtration and preferably washing until a neutral pH is obtained.

Finally purified carbon nanotubes are dried by air flow on a filter or by a rotary evaporator or by the use of a vacuum pump or by the use of an oven or a furnace. Preferably, the oven or furnace is heated at temperatures varying from 30.degree. C. to 400.degree. C. in air or from 30.degree. C. to 1200.degree. C. under vacuum or inert atmosphere. 

FIG. 3a represents a low magnification Transmission Electron Microscopy (TEM) image of as made MWNTs, synthesized by acetylene decomposition at 700.degree. C. in a continuous reaction of 60 min, on the supported catalyst SCA2. The catalyst was activated by preheating 10 min in N₂ flow.

 

FIG. 3b represents a higher magnification TEM image of MWNTs synthesized as in FIG. 3a.

FIG. 3c represents a low magnification TEM image of as made carbon fibers, synthesized by acetylene decomposition at 700.degree. C. in a continuous reaction of 60 min, on the supported catalyst SCA63. The catalyst was activated by preheating 10 min in N₂ flow.

FIG. 3d represents a low magnification TEM image of purified SWNTs, in bundles, synthesized by CH₄/H₂ decomposition at 1000.degree. C. for 6 min, on the supported catalyst SCC81. The catalyst was activated by 4 min of in situ preheating from 25 to 1000.degree. C. in a CH₄/H₂ flow.

 

FIG. 4a represents the inner and outer diameter distribution histograms of the MWNTs synthesized as in FIG. 3a. The average inner and outer diameter of the MWNTs was found to be 4.7 and 9.7 nm, respectively. No amorphous carbon is noticed either in the sample or on the walls of the tubes. The tubes are generally turbostratic with some defects in the outer surface.

FIG. 4b represents the number of walls as a function of the inner diameter distribution of the MWNTs synthesized as in FIG. 3a. These MWNTs are obtained by acetylene decomposition at 700.degree. C. in a continuous reaction for 60 min on the supported catalyst SCA2. The supported catalyst was activated by preheating it for 10 min in a flow of N₂. The number of walls of the MWNTs is in the range of 2-26 and the average value is 8.

LANL Discloses Industrial Scale Method of Quickly Separating Single Wall Carbon Nanotubes by Electronic Type, Size and Chirality

Los Alamos National Laboratory scientists Stephen K Doorn and Sandip Niyogi disclose a centrifugal method of separating carbon nanotubes into chirally enriched fractions, and a kit useful for the method in U.S  Patent Application 20090324483. The centrifugal technique can be scaled to produce chirally enriched fractions of single-walled carbon nanotubes (SWNTs) on an industrial scale.

Carbon nanotubes are seamless, nanometer scale, single-walled or multiple-walled tubes of graphite sheets with fullerene caps. Single-walled carbon nanotubes (SWNTs) are generally either of the metallic-type or the semiconducting-type. SWNTs have shown promise for nanoscale electronics, chemical sensors, biological imaging, high strength materials, field emission arrays, tips for scanning probe microscopy, gas storage, photonics, and other important applications. The realization of the potential of SWNTs for these and other applications will likely depend on the availability of bulk quantities of SWNTs having uniform properties.

Most synthetic methods for producing SWNTs (arc and laser techniques, carbon vapor deposition, catalytic cracking of hydrocarbons, catalytic disproportionation of carbon monoxide, for example) result in mixtures of metallic and semiconducting SWNTs having a broad range of nanotube chiralities, diameters, and energy bandgaps. Mixtures of SWNTs are generally unsuitable for nanoscale electronics and other applications because the properties of SWNT mixtures are not uniform.

Doorn and Niyogi  separate a mixture of single-walled carbon nanotubes ("SWNTs") into fractions of enriched chirality by preparing an aqueous suspension of a mixture of SWNTs and a surfactant, injecting a portion of the suspension on a column of separation medium having a density gradient, and centrifuging the column. The separation medium used is IODIXANOL and the surfactant is sodium dodecyl sulfate

In some embodiments, salt is added prior to centrifugation. In other embodiments, the centrifugation is performed at a temperature below room temperature. Fractions separate as colored bands in the column. The diameter of the separated SWNTs decreases with increasing density along the gradient of the column. The colored bands can be withdrawn separately from the column.

The present invention is concerned with forming chirally enriched fractions of single-walled carbon nanotubes from a mixture, and with kits useful for forming these fractions. The invention may be used to produce a fraction that is enriched in a narrow range of SWNT chiralities, or in a single chirality. Separation of SWNTs into fractions enriched in a single chirality is important for developing applications in areas such as nanoelectronics, sensors, imaging, tagging, photonics and smart materials applications.

The method of separating a mixture of SWNTs into chirality-enriched fractions is a rapid method that can be used to separate mixtures of SWNTs on a milligram scale, on a gram scale, on a kilogram scale, or higher. The method can be scaled to produce chirally enriched fractions of SWNTs on an industrial scale.

A "suspension of SWNTs" refers to an aqueous mixture of SWNTs with a surfactant that has been subjected to ultrasonication. Surfactants useful with this invention typically have a long alkyl chain terminating in a polar head group. Examples of surfactants include those with C₆-C₂₄ alkyl chains having either anionic (SO₄^-, for example) or cationic (NH₄⁺, for example) head groups. A preferred surfactant with an anionic head group is sodium dodecyl sulfate ("SDS"). An example of a surfactant with a cationic headgroup is cetyltrimethylammonium bromide (CTAB).

The mixing and ultrasonication of the suspension is followed by centrifugation to remove large SWNT aggregates and produce a visibly clear, black liquid phase that is stable upon standing at room temperature for 6 months or longer.

FIG. 1A shows a schematic representation of a column of SWNTs separated according to an a centrifugal method using NaCl, and FIG. 1B shows absorbance spectra of separated fractions of SWNTs depicted in FIG. 1A.

The Regents of the University of California are the assignee for U.S. Patent Application 20090324483

This technology is available for licensing from the LANL Technology Transfer Office.  It provides a rapid, high-throughput, highly selective method for separating single-walled carbon nanotubes by electronic type and chirality.  While other techniques are limited to batch processes of microgram to gram quantities, this invention could potentially be scaled to kilogram or larger quantities.  It could also be performed in either a batch process or continuous flow process.  Existing separation technologies take a half-hour or longer to use, while this invention can separate within minutes. 

Other separations can only separate metallic from semiconducting nanotubes, but this invention can also distinguish between the many chiralities, separating chiralities with such high resolution that single chiralities can be isolated.  This method is also tunable to selected the range of chiralities that are needed.  This invention is cheaper in that it uses off-the-shelf reagents that are much cheaper than the reagents needed for existing DNA separations.

Advantages

• Higher Throughput


• Scalable to kilogram quantities or continuous flow processes


• Faster


• Minutes instead of hours


• Better Chirality Resolution


• Only effective method to isolate single chiralities - Not just Metallic vs. Semiconducting


• Better Selectivity and Tunability


• Any range of chiralities can be separated


• Cheaper


• Easily accessible reagents are much cheaper than reagents needed for DNA separation

John Mott, Technology Transfer Division
Los Alamos National Laboratory
P.O. Box 1663, MailStop C334
(505) 665-0883
jmott@lanl.gov

UOP Patents New Class of Nanoporous Membranes for Dramatically Enhanced Gas Separation

UOP LLC (Des Plaines, IL) earned U. S. Patent 7,637,983 for new nano metal-organic framework (MOF)-polymer mixed matrix membranes (MOF-MMMs)  that show “dramatically enhanced gas separation permeability performance for CO₂ removal from natural gas.”   

Gas separation processes with membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The design of new materials and efficient methods will further advance the membrane gas separation processes within the next decade.

The gas transport properties of many glassy and rubbery polymers have been measured, driven by the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, an important limitation in the development of new membranes for gas separation applications is a well-known trade-off between permeability and selectivity. By comparing the data of hundreds of different polymers, Robeson demonstrated that selectivity and permeability seem to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.

Despite concentrated efforts to tailor polymer structure to improve separation properties, current polymeric membrane materials have seemingly reached a limit in the tradeoff between productivity and selectivity.  UOP inventors have overcome some of those limitation with a new class of nanoporous gas separation membranes.

According to inventors Chunqing Liu, Beth McCulloch,  Stephen T. Wilson, Annabelle I. Benin  and Mark E. Schott, metal-organic framework (MOF)-polymer mixed matrix membranes (MOF-MMMs) were prepared by dispersing high surface area MOFs (e.g. IRMOF-1) into a polymer matrix (e.g. Matrimid 5218). The MOFs allow the polymer to infiltrate the pores of the MOFs, which improves the interfacial and mechanical properties of the polymer and in turn affects permeability.

Pure gas permeation tests show the incorporation of 20 wt-% of IRMOF-1 in Matrimid 5218 polyimide matrix results in 280% improvement in CO₂  permeability without a loss of CO₂/CH₄ selectivity compared to those of the pure Matrimid 5218 membrane. This type of MOF-MMMs has significantly improved gas separation performance with dramatically CO₂  permeability (>35 barrer) and higher than 29 CO₂/CH₄ selectivity at 50.degree. C. under 100 psig pressure, which are attractive candidates for practical gas separation applications such as CO₂ removal from natural gas.

The inventors found the new type of metal-organic framework (MOF)-polymer or metal-organic polyhedra (MOP)-polymer MMM achieves significantly enhanced gas separation performance compared to that of cellulose acetate membranes.

U. S. Patent 7,637,983 describes the design and preparation of  UOP’s new class of metal-organic framework (MOF)-polymer MMMs containing high surface area MOF (or IRMOF or MOP, all referred to as "MOF" herein) as fillers. These MMMs incorporate the MOF fillers possessing micro- or meso-pores into a continuous polymer matrix. The MOF fillers have highly porous crystalline zeolite-like structures and exhibit behavior analogous to that of conventional microporous materials such as large and accessible surface areas and interconnected intrinsic micropores.

Moreover, these MOF fillers may reduce the hydrocarbon fouling problem of the polyimide membranes due to their relatively larger pore sizes compared to those of zeolite materials. The polymer matrix can be selected from all kinds of glassy polymers such as polyimides (e.g., Matrimid 5218 sold by Ciba Geigy), polyetherimides (e.g., Ultem 1000 sold by General Electric), cellulose acetates, polysulfone, and polyethersulfone.

These MOF-polymer MMMs combine the properties of both the continuous polymer matrix and the dispersed MOF fillers. Pure gas separation experiments on these MMMs show dramatically enhanced gas separation permeability performance for CO₂ removal from natural gas (i.e., 2-3 orders of magnitude higher permeability than that of the continuous Matrimid 5218 polymer matrix without a loss of CO₂ over CH₄ selectivity). These separation results suggest that these new membranes are attractive candidates for practical gas separation applications such as CO₂ removal from natural gas.