Patent Application: US-60038406-A

Abstract:
a novel microwave - assisted process is described for the rapid removal of catalytic metal and non - desirable carbon impurities in fullerene , single wall , and multiple wall carbon nanotube preparations . the purification process is carried out at various programmed pressures , power levels and reaction times in a suspension of the nanocarbon moieties in the presence of strong acids , in weak acids and in the presence of chelating agents . in one embodiment , high metal removal efficiency of 70 to 90 % is observed .

Description:
as required , detailed embodiments of the present invention are disclosed herein ; however , it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms . the figures are not necessarily to scale , some features may be exaggerated to show details of particular components . therefore , specific structural and functional details disclosed herein are not to be interpreted as limiting , but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention . in some embodiments of the present invention , the process involves the microwave - assisted reaction between contaminants of nanocarbon preparations and other appropriate reagents . these reagents and combinations thereof may include but are not limited to strong and weak acids , oxidizing agents , chelating agents and their mixtures . in other embodiments , other chemicals that can bind to metals and those that can oxidize the contaminant carbons can also be used to remove them . the acids used in this process include , but are not limited to nitric acid , hydrochloric acid , sulphuric acids , organic acids such as acetic acids and mixtures and combinations thereof . an example of an oxidizing agent is hydrogen peroxide . some embodiments of the present invention employ a method to remove a portion of at least one contaminant from a nanocarbon preparation comprising subjecting the nanocarbon preparation to microwave radiation in the presence of at least one removing agent . a portion indicates some or all of the contaminant can be removed . thus , in some embodiments , the preparation emerges substantially purified , while in other embodiments , the preparation may be only partially cleansed of a given contaminant . a nanocarbon preparation is any composition containing nano - sized carbon , no matter how prepared , regardless of any processing that composition may have endured . thus , embodiments of the present invention can be used on freshly - prepared fullerene and carbon nanotube compositions , and on such compositions that have otherwise become contaminated . microwave radiation can be applied by any appropriate source . for example , in some embodiments , the inventive method can be carried out in a microwave oven such as a cem model 205 system . the at least one removing agent indicates a solvent or reagent that can remove at least one contaminant from the carbon nanotube preparation . suitable removing agents include more - polar and less - polar solvents such as water , ethanol , other alcohols , and common organic solvents such as benzene , ether , alkanes , dimethyl formamide , dimethylsulfoxide , acetonitrile , and acetone . suitable removing agents also include acids , both strong and weak acids , oxidizing agents such as hydrogen peroxide , and chelating agents . removing agents can be used singly , sequentially , or in combination . some embodiments of the present invention selectively remove one contaminant versus another . for example , one embodiment may remove a greater portion of the metal present , while leaving behind a greater portion of the non - tubular carbon present in a carbon nanotube preparation . in another embodiment , the method could remove a nanotubular carbon contaminant while leaving behind the spherical nanocarbon . metal removal efficiency is defined by the ratio of metal removed to that originally present . in some embodiments , the metal removal efficiency may range from 20 to 100 %, and in other embodiments from 60 - 100 %. the conditions may be any suitable conditions . for example , in some embodiments the bulk reaction temperature may vary from 25 ° c . to 400 ° c . and pressure from half an atmosphere to 20 , 000 psi ., while reaction time may vary from 5 sec to 10 hours . in some embodiments , the temperature ranges between 50 ° c . and 150 ° c ., pressure ranges between 1 to 10 atmosphere and reaction time ranges between 10 sec to 2 hours . the concentration of removing agents may vary widely . in some embodiments , that concentration ranges from highly concentrated removing agents such as , for example , pure acids , to dilute removing agents as low as 0 . 001 molar . the power of the microwave radiation suitable for a given embodiment varies widely . for some embodiments , the reaction can be carried out with power ranging from 50 to 10000 watts , while in other embodiments , the power range is between 100 to 1000 watts . the reactions may be carried out in a closed , or open vessel , in a batch or continuous mode . in another embodiment , the microwave purification process is preceded by pretreating involving but not limited to chemical reaction , heating , washing , chemical washing , drying , and oxidation . a washing is a solvent rinse , while a chemical wash in some embodiments includes at least one surfactant . such pretreating may include more than one of such treatments , for example , a heating and a chemical reaction . in yet another embodiment , post - treating involving but not limited to chemical reactions , oxidation , washing , chemical washing , drying , and washing can be employed . as with pretreating , post - treating steps may be used singly , sequentially , or in combination . the chemical processing experiments involved in the present invention were carried out in a microwave oven ( a cem model 205 system ) using a 100 ml reaction chamber , which was lined with teflon pfa ® and fitted with a 0 - 200 psi pressure controller . typical single and multiwall carbon nanotube samples containing about 10 weight percent and 1 weight percent of catalytic metals obtained from carbon nanotechnologies inc and cheap tubes inc , respectively , were used for the experiments conducted to demonstrate the invention . all other chemicals used were purchased from sigma aldrich inc . the purified nanotubes were analyzed for metal content with an analyst 4000 atomic absorption spectrometer . the microstructure of the purified nanotubes were determined by field emission scanning electron microscopy using a leo 1530 instrument equipped with an energy dispersive x - ray analyzer . the chemical structure of the functional groups on the sidewalls was ascertained by fourier transform infrared ( ftir ) spectroscopic measurements of the reacted nanotubes in highly purified kbr pellets using a perkin elmer instrument . in a typical reaction , 5 mg to 15 mg of single or multiwall carbon nanotubes were introduced into a reaction chamber together with 25 ml of different extraction solvents and then the reaction vessel was subjected to microwave radiation . metal removal reactions were carried out with the microwave power set at 50 % of a total of 900 watts and the pressure set at 30 psi for the different times respectively . after reaction , the reacted mixture was filtered and both the solid and solution phases were subjected to chemical analyses by atomic absorption and ftir spectroscopy . in order to determine the metal content of the starting cnts 5 mg of single wall carbon nanotubes and 15 mg of multiwall carbon nanotubes were added to a reaction vessel containing 25 ml of a 1 : 1 mixture of 70 % nitric acid and 97 % sulfuric acid . then the reaction vessel was subjected to microwave reaction . with the microwave power set to 50 % of a total of 900 watts and the pressure set at 30 psi , reaction under microwaves was carried out for about 1 hour to totally dissolve the carbon nanotubes in the strong acid . after reaction , the solution was analyzed by atomic absorption spectroscopy . a variety of acids could be used to remove the metals from the cnts . the focus here was to use dilute and weak acids along with complexing agents , so that the cnts were not functionalized . the percentage of metal removed depended upon the acid used , the duration of microwave treatment , and the specific metal ( fe , co or ni ). tables 1 and 2 shows selected data on the efficiencies of metal removal by microwave reaction of iron and cobalt for single and multiwall carbon nanotubes ( mwnts and swnts ) respectively , using the various acids . the solvents used were hno 3 , h 2 so 4 , acetic acid ( hoac ) and a variety of complexing agents such as , ethylenediaminetetraacetic acid ( edta ) and nitrilotriacetic acid ( ntaa ). as used herein , “ complexing agent ” means the same as “ chelating agent .” the acids were mixed with a saturated solution of edta , 1m nitric acid , 1m sulfuric acid , ethanol , 1m hydrochloric acid , 100 % acetic acid , 1m acetic acid and deionized water . typical microwave parameters were 20 - 50 % of the power , 5 to 60 min of reaction time , and 30 psi pressure setting . these conditions needed to be optimized based on the type of nanotube and the source . the metal concentrations were obtained by atomic absorption spectroscopy and compared with energy dispersive x - ray analysis measurements . using atomic absorption spectroscopy the cobalt content in the starting multiwall nanotubes is 0 . 466 % by weight and the iron content in the starting single wall carbon nanotubes is 10 . 77 % by weight . by comparing the metal contents of the starting nanotubes with the metal contents obtained after each removal procedure , the metal removal efficiency for each process is obtained . ni was also removed by microwave treatment : 89 % of ni could be removed by reaction with acetic acid in the presence of ada ( h 2 ncoch 2 n ( ch 2 co 2 h ) 2 ) and hcl . as - prepared multiwall carbon nanotubes with extensive amorphous carbon growth on the surfaces is shown in the scanning electron microscope image in fig4 ( a ). images taken after a 10 minute treatment at a 50 % total microwave power level in a mixture of nitric and sulfuric acid is shown in fig4 ( b ). by comparing the images it is clear that the amorphous carbon growths are removed from the nanotube walls . the average tube diameters remained unchanged in the 20 to 40 nm range . as previously stated , detailed embodiments of the present invention are disclosed herein ; however , it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms . it will be appreciated that many modifications and other variations that will be appreciated by those skilled in the art are within the intended scope of this invention as claimed below without departing from the teachings , spirit and intended scope of the invention . furthermore , the foregoing description of various embodiments does not necessarily imply exclusion . for example , “ some ” embodiments may include all or part of “ other ” and “ further ” embodiments within the scope of this invention . t . guo , p . nikolaev , a . thess , d . t . colber , r . e . smalley , chem . phys . lett . 243 ( 1995 ) 49 . p . c . eklund , p . c . ; pradhan , b . k . ; kim , u . j . ; xiong , q . ; fischer , j . e . ; friedman , a . d . ; holloway , b . c . ; jordan , k . ; smith , m . w ., nano letters ( 2002 ), 2 , 561 - 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