Systematic study of Lewis acid-catalyzed bromination and ......The present work is a broad and systematic study to optimize the bromination and bromoalkylation yield of multi-walled

ObjectivesBromination and bromoalkylation reactions of CNT are very promising pathways to grafting of electrophilic sites. However, only sparse experimental data is available in the literature. The present work is a broad and systematic study to optimize the bromination and bromoalkylation yield of multi-walled CNT (Baytubes® C150P) using Lewis acids as catalysts. Bromination and bromoalkylation of multi-walled CNT were investigated by systematic variation of processing parameters. Eight different reaction times, three reaction temperatures, nine solvents, twelve catalysts and eleven electrophile reagents were studied with respect to bromination yield. Also the substitutability of the introduced bromine was studied. For this, the reaction of brominated CNT with 4-(trifluoromethyl)benzyl mercaptanewas investigated. The substitution efficiency of bromine by this fluorinated compound was quantified by XPS analysis. The optimum bromimation procedure will be used to study thecoupling of specifically-synthesized mercaptanes with brominated or bromoalkylatednanotubes.

HH

HH

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Br

Br

Br

Br

BrH

H

H

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H

HHH

HH

HHH

HH

H

H

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H HH

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BrBrBrBr

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Studied reaction parameters• Solvents: DCM, Et2O, iPr2O, nBu2O, n-C6H14 , n-C9H20 , n-C12H26 , Diglyme, Triglyme,

MeCN/Dioxane, 1,2-Dichlorobenzene, Bromoforme, Ethylendibromide,H2O/Dioxane

• Lewis/Brönstedt-Acids: BF3•Et2O, BBr3, AlBr3, FeBr3, SiBr4, SnBr4, VBr3•Diglyme, ZnBr2• (THF)2, TiBr4, DBPO, MsOH, H3PO4

• Reaction Temperatures: RT, 50 ºC, 95 ºC, 200 ºC• Durations: 1h, 3h, 4h, 5h, 1d, 3d, 1 Wo, 2 Wo• Reagents for bromination/alkylation: Br2, 1,6-Dibromohexane, p-Xylylendibromide, SOCl2,

SOBr2, SO2Cl2, Allylbromide, trans-1,4-Dibromo-2-Butene, 4-Methylbenzylbromide, 6-Bromo-1-Hexanole, 4-Chloromethylbenzylalcohol,5-Bromo-1-Pentene

BF3

BB

r3

AlB

r3

FeB

r3

ZnB

r2

TiB

r4

SiB

r4

SnB

r4

Et2On-C6H14

CH2Cl2iPr2O0

0,51

1,52

2,5

3

XPS at% Br

Lewis acid

Solvent

Baytubes bromination

Et2O

n-C6H14CH2Cl2

iPr2O

BF3

BB

r3

AlB

r3

FeB

r3

ZnB

r2

TiB

r4

SiB

r4

SnB

r4

Et2On-C6H14CH2Cl2iPr2O

0

0,5

1

1,5

2

2,5

3

XPS at% Br

Lewis acid

Solvent

Baytubes bromohexylation with 1,6-Dibromohexane

Et2On-C6H14

CH2Cl2

iPr2O

BF3

BBr

3

AlBr

3

FeB

r3

ZnBr

2

TiB

r4

SiB

r4

SnB

r4

Et2On-C6H14CH2Cl2iPr2O

0

0,5

1

1,5

2

2,5

3

XPS at% Br

Lewis acid

Solvent

Baytubes 4-Bromomethylbenzylation

Et2O

n-C6H14CH2Cl2

iPr2O

High temperature bromination at 200 deg. Celsius

0

5

10

15

20

25

SnBr4 n-Dodecane

AlBr3 Di-n-Hexylether

AlBr3Triglyme

AlBr3 n-Dodecane

FeBr3 Di-n-Hexylether

SiBr4 n-Dodecane

ZnBr2Triglyme

TiBr4Diglyme

VBr3Diglyme

Br2 in n-Dodecane

Reaction parameters for 2,5h

at.%

Br

at%Br

Observed general trends• Bromination is more effective than bromoalkylation• The Lewis acids AlBr3,FeBr3,SnBr4 are most reactive in this order• In contrast to bromination bromoalkylation leads to 100% substitutable bromine at least at

90°C or below• In low-temperature reactions unpolar solvents are superior to ethers• In high-temperature bromination di-n-Hexylether (though expensive) is superior to

dimethylethers of glycols and alkanes• High-temperature bromination leads to about ten times as much bromine

than low-temperature bromination including intercalated bromine• Further and concluding experiments are in progress

Substitution with 4-Trifluoromethylbenzylmercaptane

0

0,5

1

1,5

2

2,5

Bromine AlBr3 DCM 4,5h RT p-Xylylenedibromide SnBr4 1h 95deg n-Nonane

1,6-Dibromohexane 5h 95 degSnBr4 n-Nonane

non-brominated Baytubes

Pretreatment before substitution

at.%

F o

r at.%

Br X

PS

at%Br before deriv. at%F after deriv. at%Br after deriv.

ConclusionsOptimization of bromination or bromoalkylation may be necessary for each type of CNT material due to individual reactivity. For Baytubes, this reaction is efficiently catalyzed byAlBr3 either in DCM at lower temperature or in di-n-Hexylether at 200 °C. Bromoalkylationleads to 100% substitutable bromine but at a lower bromine concentration. The problem of intercalated bromine, which would not be substitutable, remains to be solved if Br2 is used.

Systematic study of Lewis acid-catalyzed bromination and bromoalkylation of multi-walled carbon nanotubesS. Hanelt, Jörg F. Friedrich, A. Meyer-Plath

BAM – Bundesanstalt für Materialforschung und –prüfung, Unter den Eichen 87, 12205 Berlin, Germany