Patent Application: US-201013513334-A

Abstract:
a process for producing cellulose nanocrystals involves providing a cellulosic material , contacting the cellulosic material with an inorganic persulfate at an elevated temperature to produce cncs , and recovering the cncs . the process permits one - step production of cncs from vegetative biomasses such as flax and hemp . cellulose nanocrystals produced by the process with carboxylic groups are more uniform and have higher aspect ratios than cncs produced by prior art processes .

Description:
ammonium persulfate , potassium persulfate , and sodium persulfate were obtained from aldrich ( st . louis , mo ., usa ). avicel ph102 microcrystalline cellulose was obtained from fmc corp ( philadelphia , pa ., usa ). whatman cfi cellulose powder was purchased from whatman inc . ( piscataway , n . j ., usa ). flax ( linum usitatissimum ) and flax shives from saskatchewan , canada and hemp ( cannabis sativa ) from quebec , canada . triticale straw extract was obtained from dr . g . ( joe ) mazza , agriculture and agri - food canada , summerland , bc , canada . this freeze dried extract comprises 54 . 5 % cellulose , 12 % hemicelluloses and 20 % lignin . bacterial cellulose was obtained from dr . w . k . wan , university of western ontario , london , on , canada . this bacterial cellulose is produced by the gram - negative bacteria acetobacter xylinum bpr2001 . the bacterial cellulose is produced in the form of fibers of diameter less than 50 nm and a degree of polymerization of between 2000 and 6000 . detailed information about the production , purification , and characteristics of the bacterial cellulose are known in the art . the process described in this example is an environmentally friendly , one - step procedure for the preparation of cncs from different cellulosics . ammonium persulfate has a very high solubility in cold water ( 85 g / 100 ml ), while the sodium ( 55 . 6 g / 100 ml ) and potassium ( 5 . 3 g / 100 ml ) counterparts ( weast , 1983 ) have lower solubility . cncs were prepared by simply heating cellulosic materials at 60 ° c . in 1 m persulfate for 16 h with vigorous stirring , as described in more detail below using ammonium persulfate as an example . lignocellulosic fibers such as flax and hemp were cut into short fragments ( about 2 - 3 mm ) prior to the persulfate treatment . prolonged reaction time and persulfate concentrations above 1 m led to excessive hydrolysis , thereby reducing the yield of cncs . thus , in one example , starting biomass material ( 0 . 1 g ) was added to 10 ml of 1 m ammonium persulfate solution ( conductivity about 230 ms · cm − 1 ). the suspension was heated to 60 ° c . for 16 h ( only 3 h for bacterial cellulose ) to give a white suspension of cncs . the suspension was centrifuged ( 18 , 000 rpm , rcf = 25 , 400 ) for 10 min . the solution was decanted , and about 50 ml of water was added to the cnc pellet , followed by 5 min of vigorous mixing and repeated centrifugation . the centrifugation / washing cycles were repeated 4 times until the conductivity of the solution was about 5 μs · cm − 1 ( ph about 6 ), close to the conductivity of deionized water . the product was lyophilized to yield a white solid . cncs prepared using ammonium persulfate were sonicated and atomic force microscopy ( afm ) micrographs of such resulting cncs were obtained using a nanoscope ™ iv ( digital instruments , veeco , santa barbara , calif .) with a silicon tip operated in tapping mode . particle analysis of the afm micrographs was performed using scion ™ image ( http :// www . scioncorp . com / pages / scion_image_windows . htm ). tem micrographs were obtained by a hitachi transmission electron microscope ( tem ) at 60 kv ( model h - 7500 , tokyo , japan ). tems were obtained as follows . a small amount of cncs was suspended in methanol and sonicated to disperse the material . a 20 μl drop of well dispersed suspension was then dried on a formvar - carbon coated grid and analyzed . low voltage transmission electron microscopy ( lvtem ) micrographs were obtained by a delong lvem ( soquelec ltd ., montreal , qc , canada ) low - voltage tem at 5 kv , with lower accelerating voltages generating higher contrast between the carbon mesh and cncs . afm ( fig1 a and fig1 b ) and tem micrographs confirmed the rod - shape geometry of the cncs . such rod - shapes were highly uniform compared to those obtained by prior art acid treatment . for flax ( fig1 a ), the diameter of the cncs was 3 . 8 ± 0 . 1 nm while the length was 144 ± 5 nm . for comparison , prior art acid hydrolysis of flax gave cncs with a diameter of 21 ± 7 nm and a length of 327 ± 108 nm ( cao et al ., 2007 ). for hemp ( fig1 b ), the diameter of cncs was 5 . 8 ± 0 . 1 nm , while the length was 148 ± 3 nm . for comparison , prior art acid hydrolysis of hemp gave cncs with a diameter of 30 ± 10 nm and a length that was micrometers in size ( cao et al ., 2008 ). nanocrystal size , shape , and size distribution were dependent to a certain extent upon the starting cellulosic material . as an example , the cross - section dimension of cncs prepared from hemp and flax centered on about 2 - 6 nm , reflecting elementary fibrils ( about 3 - 7 nm in diameter ) initially present in the starting materials . such cncs are much more uniform and significantly smaller than cncs obtained by prior art acid hydrolysis ( diameters ranging from 10 - 20 nm ; mahmoud et al ., 2009 ). tables 1 and 2 provide yield , crystallinity index ( cri ) and dimensions of cncs prepared from various sources using ammonium persulfate . cris are estimated using the integral method and from peak heights ( in parentheses ). length and diameter are reported at 95 % confidence interval . other cnc samples prepared from different cellulosics also showed a similar mean particle length and length polydispersity . in comparison , cncs produced from flax and hemp using prior art acid hydrolysis procedures have higher average diameters of 16 - 28 nm and 20 - 40 nm , respectively . further , the average length of cncs prepared in accordance with the present invention was about 90 - 150 nm such that the average aspect ratio of the cncs was determined to be 30 compared to 10 for cncs obtained by acid hydrolysis . these are very important findings since the uniformity , small size , and high aspect ratio of cellulose nanocrystals are critical for their intended applications as nanofillers . considering the dimension of one unit cell ( 7 . 8 å , 8 . 2 å , and 10 . 4 å ), or one glucose unit of the cellulose chain is equal to about 0 . 5 nm , the degree of polymerization of cncs was estimated to be 20 to 500 using a known method in the art ( nishiyama et al ., 2002 ). sem analysis was performed on a hitachi s 2600n scanning electron microscope at 2 . 8 kv . sem revealed morphological changes on the surface of the fibers upon ammonium persulfate treatment , indicating the destruction of the amorphous regions . in contrast , the fiber remained intact when subjected to heating without ammonium persulfate . ammonium persulfate was able to in situ produce clean cncs by dissolving lignin , hemicellulose , pectin , and other plant contents . free radicals are formed when the solution containing persulfate is heated ( s 2 o 8 2 − + heat → 2so 4 − ) ( hsu et al ., 2002 ). therefore , persulfates are often used as initiators for emulsion polymerization reactions in the preparation of polymers and synthetic rubber . in addition , under the acidic condition used in this study ( ph 1 . 0 ), hydrogen peroxide was formed ( s 2 o 8 2 − + 2h 2 o → hso 4 − + h 2 o 2 ) ( edgar and gray , 2003 ; stiernstedt et al ., 2006 ). collectively , such free radicals and h 2 o 2 should be capable of penetrating the amorphous regions to break down the β - 1 , 4 linkage of the cellulose chain to form cncs . both free radicals 2so 4 − and h 2 o 2 also opened the aromatic rings of lignin to decolorize this material . prior art acid hydrolysis procedures require alkaline or bleaching agents to remove the other fiber contents and this treatment often affects the crystallinity and structure of cellulose ( conversion of cellulose i to cellulose 11 ; krassig , 1996 ). wide angle x - ray scattering analyses were obtained at room temperature on a panalytical x &# 39 ; pert pro diffractometer equipped with a copper ( cukα , λ = 1 . 54184 å ) rotating anode source , along with instrumental settings of 45 kv and 40 ma . samples were carefully deposited on glass slides and inserted in the chamber . the collected data were analyzed using winplotr ( hllp : llwww . llb . cea . fr / fullweb / winplotrlwinplotr . htm ), a graphic tool for powder diffraction to provide peak position ( 2θ ), fwhm ( full width half maximum ), peak deconvolution , and integration intensity for calculation of the crystallinity index ( cri ). the d hkl - spacing is calculated as λ / 2 sin θ with λ = 1 . 54184 å . the crystal size is estimated as kλ / fwhm . cos θ with the form factor or scherrer constant ( k ) taken as 1 ( scherrer , 1918 ). cncs prepared from various biomass sources were characterized by x - ray diffraction ( xrd ). results are provided in fig2 and in tables 3 and 4 . fig2 shows pxrd spectra of cncs produced from hemp before and after treatment with 1 m ammonium persulfate . the inset shows the deconvoluted cellulose peaks . table 3 provides a comparison of crystallite size , d hkl - spacing and crystallinity index ( cri ) before and after treatment of biomass samples . table 4 provides a comparison of the peak position ( 2θ ) values for the most intense peaks before and after treatment of biomass . the cnc diffractograms exhibited the most intense peak ( 002 ) with a shoulder ( 021 ) and two lower peaks ( 101 and 10 - 1 ). in some samples , a very small peak ( 040 ) at 35 ° was observed . in all cases , the ( 002 ) peak position remained virtually unchanged during the course of treatment . such features , including the d - spacing and average crystallite size as determined by the debye - scherrer formula ( debye , 1915 ) resembled the diffraction pattern of cellulose i and confirmed the integrity of the material during the course of treatment with ammonium persulfate . the crystallinity index ( cri ) of cncs was then estimated using an integral method based on the ratio of the areas of crystallines to total scattered intensity ( jayme and knolle , 1964 ) with the results summarized in table 1 . cri estimated using the maximum intensity ( peak height ) from ( 002 ) plane and the intensity of the background scatter measured at 20 of about 18 ° often results in overestimated crystallinity ( segal et al ., 1959 ; thygesen et al ., 2005 ). cncs prepared in this work can be used to cast smooth thin films , which are suitable for surface force and friction measurements ( stienstedt et al ., 2006 ). in general , cris of cncs were noticeably higher than that of their parental counterparts except for cncs prepared from mcc and whatman cfi . fwhm ( full width half maximum ) of the 002 peak was also smaller than that of the starting material , indicative of less dissolution of cncs . for flax shives and hemp , the initial cri was about 50 - 70 %, in agreement with the literature data ( bhatnagar and sain , 2005 ). with very high initial crystallinity , the treatment of mcc and whatman cfi with ammonium persulfate just resulted in shorter fibers whereas the cri remained unchanged . sodium and potassium persulfate were also capable of producing cncs . similarly to ammonium persulfate treatment , cris of the cncs produced with sodium and potassium persulfate increased by about 8 - 16 % from their starting materials . the dimensions of the cncs obtained from sodium and potassium persulfate are summarized in table 5 . cris are estimated using the integral method and from peak heights ( in parentheses ). length and diameter are reported at 95 % confidence interval . xps analysis was performed using an axis ™ ultra spectrometer ( kratos analytical ltd ., manchester , uk ) equipped with a monochromatic al kα source at a power of 225 w . the elemental composition of the analyzed surface areas was obtained from survey spectra collected at pass energy of 160 ev . high - resolution c ( 1s ) and o ( 1s ) spectra were collected at 20 ev . the pressure in the analytical chamber was lower than 10 − 6 pa and the apparatus was calibrated against the following lines : au ( 4f ), ag ( 3d ) and cu ( 2p ). since the sample was charging , an electron flood gun was used during the xps experiments . atomic concentrations of each element were calculated using casaxps ( casa software ltd .) by determining the relevant integral peak areas , and applying the sensitivity factors supplied by the instrument manufacturer ; a shirley background was used . to compare the high - resolution c ( 1s ) and o ( 1s ) peak positions , the spectra were shifted to ensure that the leading edges of the fitted aliphatic ch x component were coincident and the detail spectra were fitted with several peaks using a mixed gaussian - lorentzian function . the xps spectrum of cncs from flax prepared by ammonium persulfate treatment is shown in fig3 . the sample shows the main segments for c ( 1s ) and o ( 1s ) due to cncs and only insignificant traces of n ( 1s ), s ( 2p ) and si ( 2p ). fourier transform infrared ( ftir ) spectra were collected from 4000 to 400 cm − 1 for 64 scans at a resolution of 4 cm − 1 using a bruker tensor 27 ftir spectrophotometer . samples were run as kbr pellets . near infrared ( nir ) analysis for cellulose content was measured with an antaris ft - nir analyzer equipped with an integrating sphere . an internal gold flag was used as the instrument background , the spectral resolution was set to 8 cm − 1 and 300 scans were processed mathematically to generate one spectrum . the spectra for each sample were processed with the 2nd derivative transformation to remove the spectral baseline drift due to the color and scattered light from the particles . the pls ( partial least square ) algorithm was used to develop the correlation models for the cellulose (%), lignin (%), and hemicelluloses (%) concentration measurements ( i . e . calibration curves ) ( guhados et al ., 2005 ). cncs prepared from various biomass sources were characterized by ftir spectroscopy . fig4 lists the ftir absorption bands and their assignments for the various cellulosic materials and their corresponding cncs . the increase of cri of various cellulose materials after their treatment with persulfate also correlated well with ftir data . ftir spectra of the cncs showed absorption bands that are typical for cellulosic materials . the presence of signals at 1429 cm − 1 , 1163 cm − 1 , 1111 cm − 1 and 897 cm − 1 indicated that the cncs are primarily in the form of cellulose i β , except for persulfate - treated bacterial cellulose in which characteristic absorption bands at 3241 cm − 1 and 753 cm − 1 confirms the high degree of cellulose type i α in bacterial cellulose . persulfate treatment of bacterial cellulose resulted in the reduction of the mass fraction of cellulose i α to i β . the absorption patterns of the cncs remained unchanged after treatment with persulfate , indicating that there are no significant changes to the conformation of the cellulose structure , i . e . mercerization did not occur . the ratios between crystalline absorption at 1430 cm − 1 and amorphous absorption at 895 cm − 1 of cncs was higher than that of its parental counterpart ( nelson and o &# 39 ; connor , 1964 ). although this ftir ratio has been used to calculate cri , its applicability is somewhat limited , e . g ., not applicable for mercerized cellulose ( cellulose ii ) and provides rather poor correlations with the pxrd data . the ir spectra of cncs prepared from various materials displayed a peak at 1735 cm − 1 , which was absent from cncs obtained by acid hydrolysis . this peak could be attributed to oxidation of the c6 primary hydroxyl groups on the cellulose fibers to form carboxylic acids . the degree of oxidation ( ds ) of the cncs was determined using a conductometric titration method to be about 0 . 08 ( da silva perez et al ., 2003 ). conductometric titration experiments on different samples showed that the degree of oxidation can be in a range of from about 0 . 08 to 0 . 19 . the increase of cri of flax and hemp after their treatment with ammonium persulfate also correlated well with an increase in cellulose content as observed by nir analysis ( kramer and ebel , 2000 ; poke and raymond , 2006 ). cellulose content for flax fibers increased from 70 % to 79 % after persulfate treatment , whereas the cellulose content for hemp increased from 75 % to 83 %. cncs with higher cellulose content may be prepared using fibers treated with pectate lyase , with cellulose content of 84 % and 93 % for flax and hemp , respectively . the observed increase in cellulose content illustrated that the present process is effective in the removal of non - cellulosic content from the natural fibers . the contents of the entirety of each of which are incorporated by this reference . aho o , gadda l , peltonen s , immonen k , liukkonen s , funck h . 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