Apparatus for Scalable Functionalization of Single-Walled Carbon Nanotubes Via the Billups-Birch Reduction

Journal of C Carbon Research

Article Apparatus for Scalable Functionalization of Single-Walled Carbon Nanotubes via the Billups-Birch Reduction

David Pham 1, Kevin S. Zhang 1, Olawale Lawal 1, Saunab Ghosh 1,2, Varun Shenoy Gangoli 1,2, Thomas J. Ainscough 3, Bernie Kellogg 1, Robert H. Hauge 1,†, W. Wade Adams 1,2,4 and Andrew R. Barron 2,3,4,* 1 Smalley-Curl nanoCarbon Center, Rice University, Houston, TX 77005, USA; [email protected] (D.P.); [email protected] (K.S.Z.); [email protected] (O.L.); [email protected] (S.G.); [email protected] (V.S.G.); [email protected] (B.K.); [email protected] (W.W.A.) 2 Department of Chemistry, Rice University, Houston, TX 77005, USA 3 Energy Safety Research Institute, Swansea University Bay Campus, Swansea SA1 8EN, UK; [email protected] 4 Department of Materials Science and Nanoengineering, Rice University, Houston, TX 77005, USA * Correspondence: [email protected] or [email protected]; Tel.: +1-713-348-5610 † Died 17 March 2016.

Received: 25 May 2017; Accepted: 14 June 2017; Published: 17 June 2017

Abstract: A prototype design of a reactor for scalable functionalization of SWCNTs by the reaction of alkyl halides with Billups-Birch reduced SWCNTs is described. The Hauge apparatus is designed to allow for the safe handling of all the reagents and products under an inert atmosphere at controlled temperatures. The extent of reaction of Li/NH3 solution with the SWCNTs is measured in-situ by solution conduction, while homogenous mixing is ensured by the use of a homogenizer, and thermocouple are placed at different heights within the reactor ﬂask. Addition of an alkyl halide yield alkyl-functionalized SWCNTs, which may be isolated by solvent extraction leaving a solid sample that is readily puriﬁed by hydrocarbon extraction. As an example, reaction of SWCNT/Li/NH3 with 1-iododecane yields dodecane-functionalized SWCNTs (C12-SWCNTs), which have been characterized by TG/DTA, XPS, and Raman spectroscopy. Sample extraction during the reaction allows for probing of the rate of the reaction in order to determine the end point of the ◦ reaction, which for C12-SWCNTs (at −78 C) is 30 min.

Keywords: carbon nanotube; lithium; reduction; functionalization; ammonia

1. Introduction An unfortunate side effect of the all-carbon structure of single walled carbon nanotubes (SWCNTs) is they are highly hydrophobic in nature, which results in insolubility in common solvents and immiscibility in many polymeric materials. Both of which limits the ability to process them for many applications [1]. Thus, an ongoing area of interest is the formation of individual SWCNTs rather than bundles or aggregates. Several methods have been used to functionalize SWCNT bundles with solubilizing substituents; however, only some methods exfoliate the bundles. Solubilzation (miscibility) is made possible through either wrapping (surfacting) the SWNTs [2], end-group functionalization [3,4], or sidewall functionalization [5,6]. Of these, it is the latter that has generated the most interest because of the wide range of functional groups it is possible to employ and the diverse methods for functionalization, including: free radical additions [7], reactions with acids [8], aryldiazonium salts [9], carbenes and nitrenes [10]. Furthermore, ﬂuorination of the sidewalls leads to tubes that may be further functionalized by organolithium or Grignard reagents or primary amines [11,12] as well

C 2017, 3, 19; doi:10.3390/c3020019 www.mdpi.com/journal/carbon C 2017, 3, 19 2 of 9 as Diels–Alder (4 + 2) cycloaddition reaction [13]. The majority of these approaches have only been carried out on a small scale; however, as SWCNTs become more economically viable, developing large scalable process for their functionalization becomes increasingly important. One non-destructive approach that we believe has potential for large scale production of functionalized SWCNTs is the reaction of alkyl halides with reduced SWCNTs formed upon the reaction of SWCNTs with Group 1 metals [14–16]. Using the Wilds and Nelson modification (Li metal in liquid NH3) of the Birch reduction (Na metal in liquid NH3)[17,18], Billups and co-workers reported that SWCNTs were reduced to form solubilized anions that react with alkyl halides via a alkyl radical intermediates [14]. Nanotubes that are highly soluble in solvents such as chloroform, THF and water may be prepared depending on the choice of substituent. The advantage of the Billups-Birch route is that a wide range of substituents may be employed [19,20]. For the effective functionalization of SWCNTS, a typical Billups–Birch reaction process is performed in a three-neck flask/Dewar condenser setup for 24 h with the flask submerged in an acetone-dry ice mixture. However, this generic simple setup has several limitations, as we don’t get actual information on the reaction kinetics and the duration of reaction. The scalable synthesis of highly solubilized undamaged nanotubes requires more robust and controllable approach. In this report, we introduce a state-of-the-art apparatus to functionalize SWCNTS in highly reproducible manner. The use of Li/NH3 at commercial scale is has been investigated previously, and much can be learnt from these efforts. In particular, Joshi et al., have discussed the issues with running a Li/NH3 reduction reaction, at production plant levels, as part of the synthesis of a drug candidate: Sumanirole Maleate [21], in particular challenges relating to the design of the equipment, choice and handling of materials, operations, and waste treatment. We have approached the creation of a flexible, safe and controllable process through the design of a reactor apparatus that can be reproduced in other laboratories. Herein we report an apparatus, designed by our late colleague and collaborator Robert Hauge that allows for not only scalability in running simultaneous samples and re-cycling the NH3 solvent (an important concern for commercialization), but provides in-situ monitoring and sample extraction. We have named the system in his memory.

2. Results and Discussion

2.1. Apparatus Design and Operation Figure1 shows a schematic representation of the Hauge apparatus, while Figure2 shows a photograph of the twin reactor system. This new apparatus allows large-scale (300–500 mg) functionalization of SWCNTs in a single step. The major components of this advanced apparatus include two customized multiple-neck reaction flasks (RX-1 and RX-2) (for two simultaneous reactions) each attached to Dewar condenser (CD) and a sample extraction chamber (EC). Reaction flasks are connected to a mechanical pump (PU) and multiple inert gas supply lines. In order to monitor temperature and conductivity of the reactants, pressure and gas flow inside the flasks, this apparatus is equipped with various electronic probes attached to digital data collection devices (see Figures1 and2). Thus, the setup allows for the monitoring and recording of important reaction parameters, such as conductivity and temperature of the Li/NH3 solution during reaction process. Prior to functionalization of SWCNTs the apparatus was checked for leaks to ensure that no adventitious oxidation (of the reduced species) occurs. The apparatus was evacuated for 10 min by a mechanical pump (PU) to a pressure of ~20 Torr. After closing the pump inlet valve (V-21/V-44) the pressure inside a reaction flask was continuously recorded for 20 min by an electronic pressure gauge attached to a LabQuest 2 digital data collection device (Vernier, CP/TC connections). As shown by Figure3, only a small pressure change (40 Torr) was observed after an interval of 20 min due small gaps inside the homogenizer shaft. The test confirmed that this apparatus was almost free of leaks and ideal for running reactions in inert environment. C 2017, 3, 19 3 of 9

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Figure 1. Schematic representation of the dual reactor apparatus: P—pipe, V—valve, PI—pressure Figure 1. Schematic representation of the dual reactor apparatus: P—pipe, V—valve, PI—pressure indicator, CD—condenser, LP—lithium purge, HG—homogenizer, FK—round bottom flask, FL— Figure 1. Schematic representation of the dual reactor apparatus: P—pipe, V—valve, PI—pressure indicator,filter, CD—condenser,RX—reactor, EC—extraction LP—lithium chamber, purge, HG—homogenizer, CP—conductivity probe, FK—round TC—thermocouple, bottom ﬂask, CV— FL—ﬁlter, indicator, CD—condenser, LP—lithium purge, HG—homogenizer, FK—round bottom flask, FL— RX—reactor,collection EC—extractionvessel, PU-1D—diaphragm chamber, pump, CP—conductivity PU-2M—mechanical probe, pump. TC—thermocouple, CV—collection vessel,filter, PU-1D—diaphragm RX—reactor, EC—extraction pump, PU-2M—mechanical chamber, CP—conductivity pump. probe, TC—thermocouple, CV— collection vessel, PU-1D—diaphragm pump, PU-2M—mechanical pump.

Figure 2. Photograph of the Hauge twin reactor system: PI—pressure indicator, CD-1—condenser, LP-1—lithium purge, HG-1—homogenizer, RX-1—reactor, CP-1—conductivity probe, TC-1-4— Figure 2. Photograph of the Hauge twin reactor system: PI—pressure indicator, CD-1—condenser, thermocouple, CV-1—collection vessel, PU-1D—diaphragm pump. FigureLP-1—lithium 2. Photograph purge, of theHG-1—hom Hauge twinogenizer, reactor RX-1—reactor, system: PI—pressure CP-1—conductivity indicator, probe, CD-1—condenser, TC-1-4— LP-1—lithiumthermocouple, purge, CV-1—collection HG-1—homogenizer, vessel, PU-1D—diaphragm RX-1—reactor, pump. CP-1—conductivity probe, TC-1-4—thermocouple, CV-1—collection vessel, PU-1D—diaphragm pump. C 2017, 3, 19 4 of 9 C 2017, 3, 19 4 of 9

FigureFigure 3. 3. PressurePressure inside inside reaction reaction flask ﬂask as as a a function function of of time time after after evacuation. evacuation.

2.2.2.2. Hazards Hazards Associated Associated with with Lithium Lithium Metal Metal and and Ammonia Ammonia OneOne of of the the main goals in the design describeddescribed aboveabove isis toto provide provide a a safe safe process; process; however, however, due due to tothe the hazards hazards associated associated with with the the handling handling of Liof metalLi metal and and liquid liquid NH NH3 it is3 it worth is worth reviewing reviewing risks risks and andappropriate appropriate protocols. protocols. MetallicMetallic Li Li reacts reacts violently violently with with water water to to give give off off flammable flammable explosive explosive H H22. .The The reaction reaction with with air, air, NN22, , and NHNH33result result in thein the formation formation of the of oxide the (Lioxide2O), nitride(Li2O), (Linitride3N), and (Li amide3N), and (LiNH amide2), respectively. (LiNH2), respectively.Once a container Once of a Licontainer is opened, of Li unused is opened, metal unused should metal be stored should in mineralbe stored oil in or mineral under oil argon. or under Solid argon.Li metal Solid will Li cause metal skin will and cause eye burns,skin and as it eye reacts burns, with as moisture it reacts to with form moisture corrosive to hydroxide form corrosive (LiOH). hydroxideWhen fighting (LiOH). Li fires, When use fighting of dry Li graphite fires, use powder of dry or graphite dry LiCl powder is recommended. or dry LiCl Mostis recommended. importantly, Mostwater, importantly, sand, CO2, drywater, chemical, sand, CO or2 halon, dry chemical, should not or be halon used. should not be used. AmmoniaAmmonia is is highly highly corrosive corrosive and and irritating irritating to to the the skin, skin, eyes, eyes, and and respiratory respiratory system. system. The The boiling boiling pointpoint of of liquid liquid ammonia ammonia is is −−3333 °C,◦C, and and so so the the temperature temperature of of all all reactions reactions should should be be monitored monitored carefully.carefully. The The use use of of dry dry ice-acetone ice-acetone bath bath ( (−−7878 °C)◦C) ensures ensures sufficient sufficient cooling cooling at at all all times times and and the the presencepresence of of multiple multiple thermocouples thermocouples being being placed placed at at different different points points in in the the reactor reactor to to ensure ensure that that localizedlocalized heating heating is is not not occurring. occurring. If If the the temperatur temperaturee does does increases increases during during the the reaction reaction there there is is the the riskrisk of of over over pressurization pressurization in in th thee vessel. vessel. Thus, Thus, the the pressure pressure is is monitored monitored and and a a controllable controllable exhaust exhaust lineline allows allows pressure pressure to to be be relieved relieved from from the the flask flask via via a a safety safety valve. valve. It It is is essential essential to to eliminate eliminate all all leaks, leaks, provideprovide good good ventilation, ventilation, and and install install suitable suitable NH NH33 alarmsalarms near near the the equipment. equipment.

2.3.2.3. Example Example of of SWCNT SWCNT Functionalization Functionalization InIn order order to to demonstrate demonstrate the the suitability suitability of of th thee Hauge Hauge apparatus apparatus we we have have performed performed a a typical typical alkylationalkylation reaction reaction that that can can be be compared compared to to previo previousus results results [14,22]. [14,22]. Reaction Reaction with with 1-iododecane 1-iododecane yield yield dodecane-functionalizeddodecane-functionalized SWCNTsSWCNTs (C 12(C-SWCNTs),12-SWCNTs), whose whose solubility solubility allows for allows ease of characterization.for ease of characterization.A sample of C12-SWCNTs A sample was of C prepared12-SWCNTs (see was Experimental) prepared (see and Experimental) characterized byand thermal characterized gravimetric by thermalanalysis gravimetric (TGA) and Ramananalysis spectroscopy (TGA) andin Raman comparison spectroscopy with the in literature comparison reported with derivative the literature [22]. reportedA TGA derivative on a dodecylated[22]. sample (C12-SWCNTs) was performed to determine the extent of functionalizationA TGA on a and dodecylated to correlate sample weight loss(C12-SWCNTs) data with the was carbon/alkyl performed group to determine ratio. Using the continuous extent of ◦ ◦ functionalizationheating of a C12-SWCNTs and to samplecorrelate under weight an argon loss atmosphere data with fromthe 25carbon/alkylC to 700 C,group a mass ratio. loss ofUsing 40.6% continuouswas observed heating (Figure of4 aa). C After12-SWCNTs accounting sample for under the 21% an residualargon atmosphere iron catalyst from content 25 °C in to our 700 sample °C, a mass(from loss Figure of 440.6%b), we was calculate observed that (Figure the mass 4a). loss After during accounting pyrolysis for implied the 21% an initialresidual composition iron catalyst of contentone C12 inH 25ourgroup sample per (from 16.3 SWCNTFigure 4b), carbon we calculate atoms. This that ratiothe mass is consistent loss during with, pyrolysis but slightly implied higher an initialthan, thecomposition ratio (one of C 12oneH25 C12groupH25 group per 24 per SWCNT 16.3 SWCNT carbon carbon atoms) atoms. reported This by ratio Liang is etconsistent al., for similar with, butdodecylated slightly higher samples than, [22 the]. ratio (one C12H25 group per 24 SWCNT carbon atoms) reported by Liang et al., for similar dodecylated samples [22]. C 2017, 3, 19 5 of 9 C 2017, 3, 19 5 of 9

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Figure 4. TGA of (a) C12-SWCNTs measured under Ar atmosphere and (b) chlorine purified HiPco Figure 4. TGA of (a)C12-SWCNTs measured under Ar atmosphere and (b) chlorine puriﬁed HiPco SWNTs measured under air. SWNTs measured under air. Figure 4. TGA of (a) C12-SWCNTs measured under Ar atmosphere and (b) chlorine purified HiPco Figure 5 compares the Raman spectra of the chlorine purified HiPco SWCNTs and C12H25 SWNTs measured under air. Figurefunctionalized5 compares HiPco theSWCNTs. Raman The spectra tangential of themode chlorine (G band) puriﬁed of the SWCNTs HiPco SWCNTsappears as anda strong C 12 H25 peak at ca. 1590 cm−1 with a shoulder at ca. 1560 cm−1, and the disorder mode (D band) centered at ca. functionalizedFigure HiPco5 compares SWCNTs. the Raman The tangential spectra of modethe chlorine (G band) purified of the HiPco SWCNTs SWCNTs appears and asC12 aH strong25 −1 −1 −1 peak1350functionalized at ca. cm 1590. The cm ratioHiPco withof theSWCNTs.a intensities shoulder The of tangential at Dca. and 1560 G bandmode cm peaks (G, andband) (D:G the ofratio) disorderthe providesSWCNTs mode a appearsqualitative (D band) as aanalysis centeredstrong at of the extent−1 of functionalization. Generally, the higher the D:G ratio, the higher the extent to which ca. 1350peak cm at ca.. 1590 The cm ratio−1 with of thea shoulder intensities at ca. of 1560 D andcm−1 G, and band the peaksdisorder (D:G mode ratio) (D band) provides centered a qualitative at ca. analysisthe1350 SWCNTs of cm the−1. The extent have ratio of beenof functionalization. the functionaliz intensities ofed; D however,and Generally, G band we peaks the note higher (D:Gthat variationsratio) the D:G provides ratio,also aoccur the qualitative higher for changes analysis the extent in to the special distribution of functional groups [23]. However, the comparison is valid for different whichof the the SWCNTsextent of functionalization. have been functionalized; Generally, the however, higher wethe D:G note ratio, that variationsthe higher the also extent occur to forwhich changes samples of the same functional group prepared by similar procedures. The precursor nanotubes have in thethe special SWCNTs distribution have been of functionaliz functionaled; groups however, [23 we]. However, note that variations the comparison also occur is validfor changes for different in athe near special zero distributionD:G ratio as ofa resultfuncti onalof no groups functionalization, [23]. Howe whereasver, the comparisonthe C12H25 functionalized is valid for different sample samples of the same functional group prepared by similar procedures. The precursor nanotubes have shownsamples in of Figure the same 5b functionalhas a D:G groupratio of prepared 0.955. This by similarparticular procedures. sample was The functionalized precursor nanotubes for 30 havemin, a nearand zero it exceeds D:G ratio reported as a resultvalues ofin literature no functionalization, for the same reaction whereas conditions the C12 Hin25 previousfunctionalized iterations sampleof a near zero D:G ratio as a result of no functionalization, whereas the C12H25 functionalized sample shownashown Birch in Figure reductionin Figure5b has 5bapparatus has a D:G a D:G ratio[22]. ratio Based of of 0.955. 0.955.on the This This implementation particularparticular sample sample of identical was was functionalized reaction functionalized conditions for 30 for using min, 30 min, and ittheand exceeds sameit exceeds functionalization reported reported values values chemistry in in literatureliterature as reported for for the the same samepreviously reaction reaction [14,22], conditions conditions we inpropose previous in previousthe iterations functional iterations of of a Birchgroupsa Birch reduction reductionare strongly apparatus attached [22]. to [22 the Based]. BasedSWCNT on the on walls implementation the with implementation a minimal of identical increase of identical reaction in the averageconditions reaction SWCNT conditionsusing usingdiameterthe the same same post-functionalization.functionalization functionalization chemistry chemistry as asreported reported previously previously [14,22], [14 ,22we], propose we propose the functional the functional groupsgroups are stronglyare strongly attached attached to to the the SWCNT SWCNT wallswalls with a a minimal minimal increase increase in the in theaverage average SWCNT SWCNT diameterdiameter post-functionalization. post-functionalization.

Figure 5. Raman spectrum (514 nm) of (a) chlorine-purified HiPco SWNTs and (b) C12-SWCNTs.

The TGA and Raman data are therefore consistent with a slight improvement in the efficiency FigureFigure 5. Raman 5. Raman spectrum spectrum (514 (514 nm) nm) of of ( a(a)) chlorine-purifiedchlorine-purified HiPco HiPco SWNTs SWNTs and and (b) (Cb12)C-SWCNTs.-SWCNTs. of functionalization using the present system. Based upon the forgoing it is clear that12 this system allowsThe for TGA the successful and Raman synthesis data are of therefore alkyl-functionalized consistent with SWCNTs. a slight improvement in the efficiency Theof functionalization TGA and Raman using data the are present therefore system. consistent Based upon with the a slight forgoing improvement it is clear that in thethis efficiencysystem of 2.4. Monitoring the Alkylation Reaction functionalizationallows for the usingsuccessful the presentsynthesis system. of alkyl-functionalized Based upon the SWCNTs. forgoing it is clear that this system allows for the successfulAs reported synthesis in the original of alkyl-functionalized literature [14,22] the SWCNTs. liquid NH3 was evaporated after the addition of the2.4. functionalMonitoring group the Alkylation (alkyl halide). Reaction It was assumed that this provided sufficient time for the reaction 2.4. Monitoringto reach completion. the Alkylation The reactor Reaction described herein offers the advantage that aliquots of the reaction As reported in the original literature [14,22] the liquid NH3 was evaporated after the addition of mixture may be taken at any time. This capability further helps understanding of the reaction Asthe reportedfunctional in group the original(alkyl halide). literature It was [14 assumed,22] the th liquidat this NHprovided3 was sufficient evaporated time after for the the reaction addition of the functionalto reach completion. group (alkyl The halide). reactor described It was assumed herein of thatfers thisthe advantage provided that sufficient aliquots time of the for reaction the reaction to reachmixture completion. may be taken The reactorat any describedtime. This hereincapability offers further the advantagehelps understanding that aliquots of the of reaction the reaction mixture may be taken at any time. This capability further helps understanding of the reaction progress, C 2017, 3, 19 6 of 9

C 2017, 3, 19 6 of 9 whichC 2017 in turn, 3, 19 provides important information, such as the effect of catalyst residue and6 extent of 9 of functionalizationprogress, which with in turn time, provides in order important to determine information, the most such efﬁcient, as the effect scalable of catalyst method/protocol residue and of functionalizingprogress,extent of which nanotubes.functionalization in turn provides with important time, in information, order to suchdetermine as the theeffect most of catalyst efficient, residue scalable and Inextentmethod/protocol the presentof functionalization case of functionalizing aliquots with were nanotubes. takentime, in-situin orde tor trackto determine the progress the ofmost the reaction.efficient, Thesescalable results weremethod/protocol comparedIn the present with of those casefunctionalizing aliquots after 24 were h nanotubes. reaction. taken in-situ The toRaman track the spectra progress were of the obtained reaction. These for each results sample wereIn compared the present with case those aliquots after were 24 h taken reaction. in-situ Th toe Ramantrack the spectra progress were of theobtained reaction. for These each sampleresults (Figure6 shows the spectra for the samples with reaction time 1 ≤ t ≤ 20 min). Although the D:G ratio were(Figure compared 6 shows withthe spectra those afterfor the 24 samples h reaction. with Th reactione Raman time spectra 1 ≤ t ≤ were20 min). obtained Although for eachthe D:G sample ratio is often assumed to be a direct measure of functionalization [24], it is also dependent on the distribution (Figureis often 6 assumedshows the to spectra be a fordirect the samplesmeasure with of functionalization reaction time 1 ≤ t[24], ≤ 20 min).it is alsoAlthough dependent the D:G on ratio the of functionalisdistribution often assumed groups of functional [ 23to], be making a groupsdirect it hardmeasure[23], making to directly of functionalization it hard compare to directly different [24], compare it sample is alsodifferent types.dependent sample However, on types. the in the presentdistributionHowever, case where in of the functional the present functionalization case groups where [23], the ismaking functionali occurring it hardzation in ato single isdirectly occurring system, compare in changesa single different system, in special sample changes distribution types. in shouldHowever,special notbe distribution signiﬁcant,in the present should and case not thus where be the significant, the D:G functionali ratio and is th azationus good the is indicatorD:G occurring ratio is ofin a thegooda single extent indicator system, of reaction. of changes the extent in Byspecialof reaction. comparing distribution the should D:G ratios not be forsignificant, each sample and thus the the progress D:G ratio of is thea good reaction indicator can of be the tracked extent and as mayof reaction. beBy seen comparing from Figure the D:G7, ratios 30 min for appearseach sample to bethe an progress adequate of the time reaction for thecan functionalizationbe tracked and as of SWCNTsmayBy tobe comparing completeseen from since theFigure D:G the 7,ratios results 30 minfor areeach appears essentially sample to bethe indistinguishableanprogress adequate of the time reaction for from the can afunctionalization sample be tracked prepared and asof with 24 h reactionmaySWCNTs be seen time. to complete from It should Figure since be 7, the noted30 results min that appears are we essentially use to abe condenser anindistinguishable adequate with time a dry fromfor ice/acetonethe a sample functionalization prepared bath to with preventof SWCNTs24 h reaction to complete time. It should since the be resultsnoted thatare essentiallywe use a condenser indistinguishable with a dry from ice/acetone a sample bath prepared to prevent with loss of NH3 from evaporation before the 30 min. reaction time is done. 24loss h reactionof NH3 from time. evaporation It should be before noted thethat 30 we min. use reaction a condenser time withis done. a dry ice/acetone bath to prevent loss of NH3 from evaporation before the 30 min. reaction time is done.

FigureFigure 6. Raman 6. Raman spectra spectra (633 (633 nm) nm) of of the the products products fromfrom the reaction reaction of of Billups-Birch Billups-Birch reduced reduced HiPco HiPco FigureSWCNTs 6. Raman and 1-iododecane spectra (633 as nm) a fu ofnction the productsof reaction from time the (1 reaction≤ t ≤ 20 min). of Billups-Birch The spectra reducedare normalized HiPco SWCNTs and 1-iododecane as a function of reaction time (1 ≤ t ≤ 20 min). The spectra are normalized SWCNTsto the G peak and 1-iododecanefor ease of comparison. as a function of reaction time (1 ≤ t ≤ 20 min). The spectra are normalized to the G peak for ease of comparison. to the G peak for ease of comparison.

Figure 7. Plot of D:G ratio from the Raman spectra of the products from the reaction of Billups-Birch Figurereduced 7. HiPcoPlot of SWCNTsD:G ratio andfrom 1-iododecane the Raman spectra as a func of thtione products of reaction from time. the Thereac 30-mintion of Billups-Birchsample has a Figure 7. Plot of D:G ratio from the Raman spectra of the products from the reaction of Billups-Birch reducedD:G ratio HiPco that matches SWCNTs reported and 1-iododecane literature values as a func fromtion reactions of reaction for 24 time. h. The 30-min sample has a reducedD:G HiPcoratio that SWCNTs matches and reported 1-iododecane literature asvalues a function from reactions of reaction for 24 time. h. The 30-min sample has a D:G ratio3. Experimental that matches Section reported literature values from reactions for 24 h. 3. Experimental Section 3. Experimental3.1. Materials Sectionand Characterization 3.1. Materials and Characterization Pristine HiPco single-walled carbon nanotubes (SWCNTs), 09-HiPco-0093 batch No. 190.1, were 3.1. MaterialsobtainedPristine andfrom HiPco Characterization the single-walledCarbon Nanotube carbon Laboratory nanotubes (C (SWCNTs),NL) at Rice 09-HiPco-0093 University and batch used No. as190.1, received. were Pristineobtained HiPcofrom the single-walled Carbon Nanotube carbon Laboratory nanotubes (C (SWCNTs),NL) at Rice 09-HiPco-0093University and batchused as No. received. 190.1, were obtained from the Carbon Nanotube Laboratory (CNL) at Rice University and used as received. C 2017, 3, 19 7 of 9

Lithium (granule, high sodium, 99%), tetrahydrofuran (≥99.9%), hexane (≥98.5% concentration), 1-iododecane (98% concentration), acetone, ethanol (200 proof), and Millipore PTFE (0.2 µm) ﬁlter paper were obtained from Sigma-Aldrich (St. Louis, MO, USA) and used as received unless otherwise noted. All tubing used for the reactor was Teﬂon™ obtained from Cole-Palmer or United States Plastic Corp (Shanghai, China). Chlorine treatment of SWCNTs was adapted from the literature [25]. Thermogravimetric analysis (TGA) was performed on a Seiko TG/DTA 200. The iron percentage and extent of functionalization was measured using ca. 10 mg of sample placed in a platinum pan and heated under ambient conditions up to 800 ◦C in dry air with a ramp rate was 5 ◦C min−1, and sampling interval of 3 s. Raman was carried out on a Renishaw inVia Raman Microscope using three laser lines for characterization; these were 514 nm (2.41 eV), 633 nm (1.96 eV), and 785 nm (1.58 eV). To maximize the Raman signal a continuous scan was carried out of the G peak (~1600 cm−1), while the focus of the beam was altered to maximize the G peak intensity. When the maximal intensity was found, data was acquired using an integration of 10 accumulations at a power of 10%, with cosmic ray background removal applied. Each sample was probed a number of times at a variety of locations, to acquire data that represented the entirety of the sample. Raman data was acquired for a range of wavenumbers ranging from 100–3300 cm−1.

3.2. Apparatus A schematic representation of the apparatus is shown in Figure1, while a photograph of the actual apparatus is provided in Figure2. Argon flow originates from a tank and enters reaction flask, RX, through homogenizer, HG, general chamber, CD, or an external tube, LP, for degassing lithium before insertion into the reactor flask, RX. Flowmeters on each side display Ar flow, allowing controlled flow of inert gas during reaction. An adjustable inlet with pressure control manages NH3 gas flow into the system (V-19, V-42). Prior to reaction SWCNT samples are dried in vacuum using a mechanical pump, PU-2M. The diaphragm pump, PU-1D, allows transfer of large amounts of liquids and gases to a collection flask, CV-1, or small aliquots of solids to an extraction line, EC. Each reactor has a line to the middle flask with a detachable Swagelok filter, FL. Filters pores range from 15–40 µm. Liquid solvents can be extracted to the sample collector (CV), leaving solid sample behind in the reactor. A CAT ×120 homogenizer (HG-1, HG-2) with shear mixing serves as the dissolution mechanism for improved homogenous mixing relative to a standard stir bar. A LabQuest Pro conductivity probe (Vernier, LP-1 and LP-2) measures conductivity in-situ. Probe tips were changed to titanium electrodes. The probe determines conductivity by measuring current when a potential difference is applied. A 2 point calibration is performed with Nanopure 18 MΩ water and LabQuest standard 500 mg/L (1000 µS/cm) NaCl solution in Nanopure water. Three K-type (Ni-Cr/Ni-Al) thermocouple wires (TC-1–TC-8) are placed at different heights within a reactor flask to read temperatures in-situ at varying levels in the reactor. Reaction temperatures are displayed using a Vernier Labquest Pro digital display. Mechanical and electronic pressure sensors maintain control of the flask environment. A controllable exhaust line allows pressure to be relieved from the flask. A safety valve automatically releases gases if the pressure 1 is too high. Lithium is held in a 4 ” glass tube outside of the reactor and purged with argon for 1 min prior to insertion. A needle valve allows passage of lithium into reactor from insertion chamber after purging. Aliquots can be taken to monitor reaction kinetics. A combination of positive pressure and vacuum extract the sample from a narrow hole to the collection filter membrane.

3.3. Alkylation of SWCNTs Functionalization proceeds according to methods described by Billups’ alkylation of nanotubes using Birch reduction [14,22]. In a typical reaction, SWCNTs (50 mg) were added to the 700 mL reaction flask and heated under vacuum at 180 ◦C for 10 min. in order to remove adsorbed gases. Argon gas continuously flows through the system to create an inert atmosphere. The flask is submerged in the dry ice-acetone bath to condense NH3 (300 mL). Li metal was added in molar quantities (C:Li = 1:1–1:3), via the lithium purge (LP-1/LP-2), until a conductive solution is created as measured by the conductivity C 2017, 3, 19 8 of 9 probes (CP1/CP2). The shear-mixing homogenizer (HG1/HG2) ensures homogeneity of the solution. 1 h after the Li addition, 1-iododecane (4 mL) was added using a syringe via lithium purge inlet along with THF solvent (6 mL) to prevent reactant from solidifying under reaction conditions. Aliquots were taken to monitor reaction kinetics via extraction chamber (EC-1/EC-2). The sample collects for approximately 1 min per aliquot sample. The solution is stirred for 2 h, after which the liquid NH3 is extracted to the sample collector (CV-1) to prevent contamination of the functionalized product from any side products formed during reaction. C12-SWCNTs are extracted into a separatory funnel, separate from the reactor. To this was added ethanol (150 mL), DI water (150 mL), and hexane (200 mL). A solution of diluted HCl (10 mL) was added to the funnel and the solution is vigorously shaken. The hexane layer is filtered through a cellulose filter paper after separation from the water/ethanol layer. Products are vacuum-dried overnight.

4. Conclusions We have reported a flexible apparatus for the scalable alkylation of SWCNTs using the Billups-Birch reduction process in liquid NH3. The advantages of our new apparatus include: the solid reactants are treated in an inert atmosphere to prohibit unwanted oxygen inclusion, the liquid reactants are automatically added to the reaction flask by a controlled injection system, and the intermediate partially functionalized products can be systematically extracted to monitor the reaction. The conductivity versus time profile indicates that a typical alkylation reaction of SWCNTs ends much earlier (within 30 min) and requires less metallic lithium than earlier reported [14,22]. The cost-efficient features of this apparatus: at the end of a reaction cycle the excess NH3 can be separated and transferred to the second reaction flask reutilized for another functionalization reaction cycle. The reproducibility of reaction process further confirms that this apparatus is ideal for bulk functionalization of SWCNTs.

Acknowledgments: This work was supported by the Office of Naval Research (N00014-15-2717), the Welsh Government Sêr Cymru National Research Network in Advanced Engineering and Materials (NRN-150), the Robert A. Welch Foundation (C-0002), and Lockheed Martin Corporation. We also acknowledge Steve Ripley’s help in the construction of the apparatus. Author Contributions: David Pham and Kevin S. Zhang were involved in the design, construction and testing of the apparatus; Olawale Lawal helped build the apparatus, performed experiments and characterization; Saunab Ghosh built and leak checked the apparatus, and performed functionalization experiments and characterization; Varun Shenoy Gangoli helped leak check the apparatus and contributed to discussions of the experiments and results; Thomas J. Ainscough created the P&ID for the apparatus; Bernie Kellogg was involved in discussions on the reactor design and directly contributed to the apparatus construction; Robert H. Hauge designed the apparatus and contributed to experimental discussions; W. Wade Adams contributed towards the construction of the apparatus and in discussions about the experiments and results; Andrew R. Barron was involved in the simplification of the apparatus, providing guidance to experiments. All the authors contributed to writing the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Barron, A.R.; Khan, M.R. Carbon nanomaterials: Opportunities and challenges. Adv. Mater. Process. 2008, 166, 41–43. 2. O’Connell, M.J.; Bachilo, S.M.; Huffman, C.B.; Moore, V.C.; Strano, M.S.; Haroz, E.H.; Rialon, K.L.; Boul, P.J.; Noon, W.H.; Kittrell, C.; et al. Band gap ﬂuorescence from individual single-walled carbon nanotubes. Science 2002, 297, 593–596. [CrossRef] 3. Chen, J.; Hamon, M.A.; Hu, H.; Chen, Y.; Rao, A.M.; Eklund, P.C.; Haddon, R.C. Solution properties of single-walled carbon nanotubes. Science 1998, 282, 95–98. [CrossRef] 4. Hamilton, C.E.; Barron, A.R. Phosphene functionalized single walled carbon nanotubes. Main Group Chem. 2009, 8, 275–281. [CrossRef] 5. Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Functionalized carbon nanotubes: Properties and applications. Acc. Chem. Res. 2002, 35, 1096–1104. [CrossRef] C 2017, 3, 19 9 of 9

6. Zeng, L.; Zhang, L.; Barron, A.R. Tailoring aqueous solubility of functionalized single-wall carbon nanotubes over a wide pH range through substituent chain length. Nano Lett. 2005, 5, 2001–2004. [CrossRef] 7. Peng, H.; Reverdy, P.; Khabashesku, V.N.; Margrave, J.L. Sidewall functionalization of single-walled carbon nanotubes with organic peroxides. Chem. Commun. 2003, 7, 362–363. [CrossRef] 8. Zhang, X.; Sreekumar, T.V.; Liu, T.; Kumar, S. Properties and structure of nitric acid oxidized single wall carbon nanotube films. J. Phys. Chem. B 2004, 108, 16435–16440. [CrossRef] 9. Bhakta, A.K.; Detriche, S.; Martis, P.; Mascarenhas, R.J.; Delhalle, J.; Mekhalif, Z. Decoration of tricarboxylic and monocarboxylic aryl diazonium functionalized multi-wall carbon nanotubes with iron nanoparticles. J. Mater. Sci. 2017.[CrossRef] 10. Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Sidewall functionalization of carbon nanotubes. Angew. Chem. Int. Ed. 2001, 40, 4002–4005. [CrossRef] 11. Boul, P.J.; Liu, J.; Mickelson, E.T.; Huffman, C.B.; Ericson, L.M.; Chiang, I.W.; Smith, K.A.; Colbert, D.T.; Hauge, R.H.; Margrave, J.L.; et al. Reversible sidewall functionalization of buckytubes. Chem. Phys. Lett. 1999, 310, 367–372. [CrossRef] 12. Dillon, E.P.; Andreoli, E.; Cullum, L.; Barron, A.R. Polyethyleneimine functionalized nanocarbons for the efficient absorption of carbon dioxide with a low temperature of regeneration. J. Exp. Nanosci. 2015, 10, 746–768. [CrossRef] 13. Zhang, L.; Yang, J.; Edwards, C.L.; Alemany, L.B.; Khabashesku, V.N.; Barron, A.R. Diels alder addition to fluorinated single walled carbon nanotubes. Chem. Commun. 2005, 14, 3265–3267. [CrossRef] 14. Liang, F.; Sadana, A.K.; Peera, A.; Chattopadhyay, J.; Gu, Z.; Hauge, R.H.; Billups, W.E. A convenient route to functionalized carbon nanotubes. Nano Lett. 2004, 4, 1257–1260. [CrossRef] 15. Chattopadhyay, J.; Sadana, A.K.; Liang, F.; Beach, J.M.; Xiao, Y.; Hauge, R.H.; Billups, W.E. Carbon nanotube salts. Arylation of single-wall carbon nanotubes. Org. Lett. 2005, 7, 4067–4069. [CrossRef] 16. Anderson, R.E.; Barron, A.R. Solubilization of single-wall carbon nanotubes in organic solvents without sidewall functionalization. J. Nanosci. Nanotechnol. 2007, 7, 3436–3440. [CrossRef] 17. Birch, A.J. Reduction by dissolving metals. Part I. J. Chem. Soc. 1944, 430–436. [CrossRef] 18. Wilds, A.L.; Nelson, N.A. A superior method for reducing phenol ethers to dihydro derivatives and unsaturated ketones. J. Am. Chem. Soc. 1953, 75, 5360–5365. [CrossRef] 19. Billups, W.E.; Sadana, A.K.; Liang, F.; Hauge, R.H. Reductive Functionalization of Carbon Nanotubes. U.S. Patent 7,758,841, 20 July 2010. 20. Borondics, F.; Bokor, M.; Matus, P.; Tompa, K.; Pekker, S.; Jakab, E. Reductive functionalization of carbon nanotubes. Fuller. Nanotub. Carbon. Nan. 2007, 13, 375–382. [CrossRef] 21. Joshi, D.K.; Sutton, J.W.; Carver, S.; Blanchard, J.P. Experiences with commercial production scale operation of dissolving metal reduction using lithium metal and liquid ammonia. Org. Process Res. Dev. 2005, 9, 997–1002. [CrossRef] 22. Liang, F.; Alemany, L.B.; Beach, J.M.; Billups, E.W. Structure analyses of dodecylated single-walled carbon nanotubes. J. Am. Chem. Soc. 2005, 127, 13941–13948. [CrossRef] 23. Zhang, L.; Zhang, J.; Schmandt, N.; Cratty, J.; Khabashesku, V.N.; Kelly, K.F.; Barron, A.R. AFM and STM characterization of thiol and thiophene functionalized SWNTs: Pitfalls in the use chemical markers to determine the extent of side-wall functionalization in SWNTs. Chem. Commun. 2005, 5429–5430. [CrossRef] 24. Dresselhaus, M.S.; Pimenta, M.A.; Ecklund, P.C.; Dresselhaus, G. Raman Scattering in Materials Science; Webber, W.H., Merlin, R., Eds.; Springer: Berlin, Germany, 2000. 25. Gomez, V.; Irusta, S.; Lawal, O.B.; Adams, W.; Hauge, R.H.; Dunnill, C.W.; Barron, A.R. Enhanced purification of carbon nanotubes by microwave and chlorine cleaning procedures. RSC Adv. 2016, 6, 11895–11902. [CrossRef]

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