Systematic study of
Lewis acid-catalyzed
bromination and bromoalkylation of
multi-walled carbon nanotubes
S. Hanelt, Jörg F. Friedrich, A. Meyer-Plath
BAM – Bundesanstalt für Materialforschung und –prüfung, Unter den Eichen 87, 12205 Berlin, Germany
Objectives
Bromination 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 mercaptane
was investigated. The substitution efficiency of bromine by this fluorinated compound was
quantified by XPS analysis. The optimum bromimation procedure will be used to study the
coupling of specifically-synthesized mercaptanes with brominated or bromoalkylated
nanotubes.
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Baytubes bromination
Studied reaction parameters
3
2,5
• 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
Et2O
2
n-C6H14
XPS at% Br 1,5
1
0,5
0
CH2Cl2
SnBr4
ZnBr2
TiBr4
Lewis acid
SiBr4
AlBr3
FeBr3
BF3
BBr3
iPr2O
iPr2O
CH2Cl2
n-C6H14
Solvent
Et2O
Baytubes bromohexylation with 1,6-Dibromohexane
3
2,5
Observed general trends
Et2O
n-C6H14
2
CH2Cl2
XPS at% Br 1,5
• 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
iPr2O
1
0,5
iPr2O
CH2Cl2
n-C6H14
Solvent
Et2O
SiBr4
Lewis acid
SnBr4
ZnBr2
TiBr4
AlBr3
FeBr3
BF3
BBr3
0
Baytubes 4-Bromomethylbenzylation
3
2,5
Conclusions
n-C6H14
CH2Cl2
XPS at% Br 1,5
Optimization of bromination or bromoalkylation may be necessary for each type of CNT
material due to individual reactivity. For Baytubes, this reaction is efficiently catalyzed by
AlBr3 either in DCM at lower temperature or in di-n-Hexylether at 200 °C. Bromoalkylation
leads 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.
iPr2O
1
0,5
iPr2O
CH2Cl2
n-C6H14
Solvent
Et2O
Substitution with 4-Trifluoromethylbenzylmercaptane
SiBr4
SnBr4
ZnBr2
Lewis acid
TiBr4
AlBr3
FeBr3
BF3
BBr3
0
High temperature bromination at 200 deg. Celsius
2,5
25
at%Br before deriv.
at%F after deriv.
at%Br after deriv.
at%Br
2
20
1,5
15
at.% Br
at.% F or at.% Br XPS
Et2O
2
1
10
5
0,5
0
0
Bromine AlBr3 DCM 4,5h RT
p-Xylylenedibromide SnBr4 1h 95 1,6-Dibromohexane 5h 95 deg
deg n-Nonane
SnBr4 n-Nonane
Pretreatment before substitution
non-brominated Baytubes
SnBr4 nDodecane
AlBr3 Di-nHexylether
AlBr3
Triglyme
AlBr3 nDodecane
FeBr3 Di-nHexylether
SiBr4 nDodecane
ZnBr2
Triglyme
Reaction parameters for 2,5h
TiBr4
Diglyme
VBr3
Diglyme
Br2 in nDodecane