Entry Subject Page No. [618463]

1
Index
Entry Subject Page No.
1 Introduction 2
2 Synthesis free radicals and characterizes 6
2.1 Synthesis and characterization of Stable Organic Polyradicals 8
2.2 Exploring porous nanosilica -TEMPO as heterogeneous 12
2.2.1 Characterization of the catalysts 15
2.2.2 Catalyst preparation 22
2.3 Covalently grafted TEMPO on graphene oxide 23
2.3.1 Expterrntal 27
2.3.1.1 Materials and methods 27
2.3.1.2 Synthesis of GO/iGO 28
2.3.1.3 Functionalization of graphene oxides with TEMPO 28
2.3.2 Synthesis and charectrisis 29
2.3.2.1 Synthesis and characterization of GO and iGO 29
2.3.2.2 GO functionalized with 4 -amino -TEMPO (GO -T and iGO -T) 29
4 Conclosin 41
5 References 42

2
1. Introduction
There are many free radicals in chemistry, and as such the study of free radicals in
organic chemistry has been chosen because of the features and effectiveness of the se chemical s.
There are many free radicals that have been prepared dur ing the previous decades .This was first
started in 1900 by Gomberg , when he discovered triphenylmethyl.[1] Many examples of stable
free radicals were prepared.[2] Various kinds of stable free radicals having one or more unpaired
electrons are known, and re cently , several classes of stable free radicals were developed for
applications in medicine, synthesis and materials science.
We have prepared several kinds of free radicals used as catalysis, and we focused our
attention on the oxidation of alcohols , In general , the oxidation processes in chemistry require
harsh experimental conditions . The selective oxidation of alcohols to aldehyde and ketone
required difficult conditions for the reactions , and includ e toxic waste.
There are a number of oxidants reagent s available for use in the oxidation of alcohols ,
such as Pyridinium Dichromate, the Jones reagent and Collins oxidation, and extensive work -up
is required to isolat e the compound from the se reactions.
We have been chosen to apply and synthesize several ki nds of free radicals , such as the
stable free radicals TEMPO (2,6,6 -tetramethylpiperidine -Noxyl) , due to properties of this
compound, TEMPO is communally used for oxidation and is stable under normal conditions.
Free radicals TEMPO can be used in combinati on with transition metals such iron and copper.
TEMPO has many applications , for example Liu, S. et al.[3] used TEMPO -containing
polymer brushes , which were grafted onto cross -linked polystyrene microspheres for the
selective oxidation of alcohols, Ahn, S.D. et al.[4] studied rocking disc electrode of the TEMPO –
mediated catalytic oxidation of primary alcoh ols and Meng. J. et al.[5] prepared a silica TEMPO
complex for oxidation of cinnamyl alcohol.
Free radical TEMPO have been used for the oxidation of alcohols, and under some
condition s, they were used for the selective of oxidation of primary alcohols . In fact , the primary
oxidant is an oxoammonium salt.
For example , for the selective oxidation of alcohols Anelli‟s protocol for oxidation of
alcohols contain s a biphasic DCM -water mixture with TEMPO catalyst, and NaOCl (sodium
hypochlorite act ing as a secondary oxidant), give s high selectivity for oxidation of primary

3
alcohols in the presence of secondary alcohols,[6-8] and is used in organic chemistry synthesis .[9-
12] See Fig. 1.

Fig. 1. Selective oxidation of primary alcohols in presence of secondary alcohols with 90%
yield

Piancatelli et al. in 1997,[13] used TEMPO with (BAIB) [bis(acetoxy)iodo]benzene act ing
as the secondary oxidant for the selective oxidation of primary alcohols in the presence of
secondary alcohols.

Fig. 2. TEMPO -PhI(OAc) 2 using to selective primary alcohols in the presence of secondary
alcohols[14]

There are more example s of TEMPO -mediated oxidations to select a primary alcohol in
the presence of secondary alcohols , which include NaBrO 2[15], CuCl 2/O2[16], NCS[17-18] and
trichloroisocyanuric acid.[19]

OOH
HO
MeMe
Me MeO
OMe
NS
Me
O OHOO
HO
MeMe
Me MeO
OMe
NS
Me
O OHH
1.5 eq TEMPO, 1.1eq NaOCl
KBr, DCM, 0.5 h, 0 Co
MeHO
Me
OTBSMeMeMeMe
OH OTBSMeO
Me
OTBSMeMeMeMe
OH OTBS0.2 eq. TEMPO, 1.5 eq. PhI(OAc)2
DCM, 1h, 25 CoH

4
Silica supported TEMPO
Oxidation processes often require harsh experimental conditions, involving transition
metals and strong acids.[20] The oxidation of alcohols leads in the first step to the formation of the
corresponding aldehydes or ketones and , in the second step , aldehydes can be converted into
carboxylic acids by common oxidants, including air.
Frequently, alcohol oxidation cannot be stopped at the carbonyl derivative, therefore
selective oxidation methods are highly desirable.[21] Many syntheses of medicines or other fine
chemicals require aldehydes and ketones as intermediates ,[22] and there is a high demand for
specific aldehyde s and ketones: for example, menthone and octanal are used in the fragrance
industry, while cyclohexanone is a precursor in the plastic s industry.
The Jones reagent (CrO 3, aq H 2SO 4) and pyridinium dichromate (Cornforth reagent,
PDC) are some of the most use d oxidants. Because a stoichiometric amount of reagents is needed
to convert primary alcohols into aldehydes,[20-23] isolation of the desired compound from the
reaction mixture requires extensive work -up and generates a large amount of toxic chemical
waste . Nowadays , literature data is rich in novel methods of using transition -metalfree aerobic
oxidation, that is, those involving hypervalent iodine compounds (Dess –Martin oxidation),
oxalyl chloride/DMSO (Swern oxidation), sodium hypochlorite, nitric acid or its salts and
nitroxide free radicals.[20-29]
One of the commonly used compounds for oxidation is the stable free radical 2,2,6,6
tetramethylpiperidinyloxy (TEMPO).[30-33] Although it possesses an unpaired electron, this
chemical is stable under normal conditions, and does not dimerize or react with dioxygen from
the air. This free radical can be involved in acid –base and redox type processes, as shown in
Figure 3 . The oxoa mmonium salt of TEMPO is also well known as a strong yet specific
oxidant.[34] On the other hand, TEMPO is often used in combination with transition metals and
seldomly with metal -free cocatalysts. For example, copper, iron, manganese and silver have
commo nly been used as transition metal cocatalysts,[35-38] while halogens and/or acids have been
employed in the metal -free systems.[39,40 ] Furthermore, iron oxide nanoparticles together with
TEMPO (free or covalently attached) were employed as catalytic system s for the oxidation of
benzylic alcohols.[41-43]

5
A breakthrough in such systems was the use of nitrogen oxides as mediators between
dioxygen and TEMPO.[25,30,32,39 ,44,45] Initially, NOx was obtained in situ from sodium nitrite and
acetic acid, but the drawbacks associated with removing acetic acid from the reaction led to the
use of gaseous NOx. However, working with gaseous NOx represents a problematic issue from a
practical point of vi ew. This inconvenience was recently overcome by our group by absorbing
gaseous NOx onto silica supported TEMPO, thus yielding as an easily handled solid catalyst
which does not require either an additional cocatalyst or an acid.[45]

N
N N
NOH
O
OO-e-
+e–e-+e–H+
+H++H
-H

Fig. 3 . Acid–base and redox processes of TEMPO free radical (H. may stand also for H+ + e_)

Bobbitt‟s salt (4-acetamido -2,2,6,6 -tetramethylpiperidine -1-oxoammonium
tetrafluoroborate),[46] an inexpensive TEMPO derivative, can be easily prepared in a green
manner, using water as the solvent and minimizing the use of environmentally unfriendly
materials. Such oxoammonium salts are metal -free, nontoxic and, after use, the spent oxidant can
be recovered and reused, thereby making the process recyclable.[46]
Heterogeneous catalysts are preferred in large scale or industrial applications. Therefore,
in this work we continue our research into conducting practical oxidations of different types of
alcohols employing new NOx and TEMPO based systems. The newly developed metal -free
system employing commercially available silica supported TEMPO as the catalyst and a

6
nitrosonium tetrafluoroborate cocatalyst could be used under very mild reaction condition s to
obtain a variety of aldehydes or ketones in high yields.

2. Synthesis free radicals and characterizes
They are many methods to prepare polyradicals due to their applications in organic
reaction s. They have paramagnetic properties and have applications in many processes
(chemical, physical, biological).[47- 49] di-t- alkylnitroxids we have prepare d various di – tri –
tetra- radicals[50] and polynitroxide,[51] see Fig. 4.

Fig. 4. Some polyradicals

Oxidation reactions are among the most important in preparative organic chemistry.
Many compounds of interest , like medicines or other fine chemicals contain an aldehyde or
ketone moiety; also, aldehydes and ketones are often key intermediates in chemical industrial
processes.[52,53 ]
Usually, the synthesis of aldehydes or ketones is performed through the oxidation of the
corresponding alcohols. These reactions can require transition metal ions as oxidant and
OC NH2
CCO
OO
N O O
N N N NO ON N O O
PO O
ON
N
NOO
O

7
frequently cannot be stopped at the carbonyl derivative, the final product being the ca rboxylic
acid; therefore selective oxidation methods are highly attractive.[54- 56]
Organic stable free radicals of nitroxide type, like 2,2,6,6 -tetramethylpiperidinyl -1-oxyl
(TEMPO) or 2,2,5,5 -tetramethyl -1-pyrrolidinyloxy (PROXYL) were proved to work as a redox
reagent, meaning that they can be employed in selective or greener oxidation reactions under a
broad range of conditions.[54,57-59] Many derivatives of TEMPO or PROXYL free radicals are
known {60}, and nowadays they are even commercially available as free radicals immobilized
on organic (polymer) or inorganic materials (silica).
These materials can be easily used as recoverable heterogeneous catalysts in such
selective oxidation processes.[60-64] Stable organic polyradicals are also acknowledged as
materials of special interest, with high potential as probes, sensors or markers in many physical,
biological or chemical processes.[47,65,66]
The molecules bearing two or more paramagnetic moieties (polyradicals) can bring a
number of advantages in compa rison with monoradicals; their Electron Spin Resonance (ESR)
spectra usually have particular features due to the appearance of new lines attributed to
intramolecular spin exchange. Polyradicals are also known in literature as high spin compounds,
showing i nteresting magnetic properties; an attractive asset of such compounds is the developing
of some co -operative property (i.e. metallic state).[67- 69]

8
2.1 Synthesis and characteriz ation of Stable Organic
Polyradicals

Merck or Chimopar. NMR spectra were recorded on a Bruker Fourier apparatus at 300
MHz using CDCl 3 as the solvent (isotopic purity 99.9%) and TMS as the internal standard. ESR
spectra were recorded on a Jeol JES FA100 apparatus using the following typical settings:
frequency 8.99 GHz, field 3330 G, sweep width 100 -200 G, sweep time 60 -120 s, time constant
30 ms, gain 50 -500, modulation frequency 100 kHz, modulation width 1 G. HR -MS were
recorded on a ThermoSc ientific (LTQ XL Orbitrap) apparatus.
4-Isocyanato -TEMPO was obtained in a si milar way as literature data {7 0} showed; 2 g
of 4-amino -TEMPO were dissolved in 15 mL of cold DCM and 0.25 mL of diphosgen (also
dissolved in 15 mL of cold DCM) were added under vigorous stirring; the mixture was
maintained below 0o C with an external cooling (ice and salt); after few minutes, 70 mL DCM
were added, and the final solution was extracted twice with 100 mL of aqueous hydrochloric acid
(1 M) and once with 100 mL of sodium hydroxide (1 M); the organic layer was dried over
anhydrous magnesium sulfate, filtered off and the solvent removed under vacuum (below 30o C).
The y ield was around 50%.
2,2‟-succinic acid disulphide was obtained by oxidation of 2 -mercapto -succcinic ac id
{74}; to 300 mg of the acid dissolved in 30 mL of methanol were added under stirring 300 mg of
iodine; after 2 hours, a solution of sodium thiosulfate was added drop -by-drop until the colour of
iodine disappear ed. To the mixture was added 150 mL of DCM and 100 mL of water, and the
organic layer was separated, dried over anhydrous magnesium sulfate, filtered off and the solvent
removed. The y ield was quantitative.
DI-T radical was prepared by adding an equal amount of 4 -isocyanato -TEMPO and of 4 –
amino -TEM PO in THF and leaving the mixture at room temperature overnight; removal of the
solvent affords the pure material in almost quantitative yield. HR -MS: m/z calculated for
C19H36N4O3 [M+] 368.2782 was found to be 368.2807.
TRI-P radical was prepared by addin g 75 mg of tris(2 -aminoethyl)amine, 625 mg of
EEDQ and 372 mg of 3 -carboxy -PROXYL free radical to a mixture of 50 mL DCM and THF

9
(9/1 v/v) as solvent. After five days, the solution was extracted with 50 mL aqueous hydrochloric
acid (1 M) and with 50 mL of sodium hydrogen carbonate (1 M); the organic layer was separated
and dried over anhydrous magnesium sulfate, filtered off and the solvent removed under
vacuum. The crude mixture was purified by column chromatography using silica as the
stationary phase and ethyl acetate as the eluent. The y ield was around 30%. HR -MS: m/z
calculated for C 33H61N7O6 [M+H+] 651.4678; it was found to be 651.5704.
TE-T radical was obtained by reacting 150 mg of the disulphide with 400 mg of 4 -amino –
TEMPO in the presence of 500 mg EEDQ dissolved in 100 mL DCM. After 3 days, the solution
was extracted with 100 mL aqueous hydrochloric acid (1 M) and with 100 mL of sodium
hydrogen carbonate (1 M); the organic layer was separated and dried over anhydrous magnesium
sulfate, filtered off and the solvent removed under vacuum. The crude mixture was purified by
column chromatography using silica as the stationary phase and ethyl acetate as the eluent. The
yield was 30%. HR -MS: m/z calculated for C 44H80N8O8S2 [M+2H+] 912.5535 was found to be
912.5584.
TE-P radical was obtained by dissolving 30 mg DAB -Am-4 dendrimer
(polypropylenimine tetramine dendrimer, generation 1) with 1 g EEDQ and 750 mg of 3 –
carboxy -PROXYL free radical into a mixture made up of 80 mL DCM and 20 mL THF as the
solvent. After five days, 50 mL of DCM were added and the solution was extracted with 100 mL
of sodium hydrogen carbonate (1 M); the organic layer was separated and dried over anhydrous
magnesium sulfate, filtered off and the solvent removed under vacuum. Purification was
performed by preparative TLC, using silica as the stationary phase and a mixture of DCM and
methanol 9/1 (v/v) as the eluent. The y ield was around 20%. HR -MS: m/z calculated for
C52H97N10O8 [M+H+] 989.7485 was found to be 989.7488.
The polyradicals synthesized in this work and very similar ones are known { 70-75} and
they were obtained by similar experimental procedures, as shown in literature (see Experimental
part). Summarizing, the synthesis consists in a si mple coupling reaction (such as an amide bond
formation, Fig . 5), which was achieved by linking together an acid and an amine derivative
(commercially available) in the presence of the coupling agent N-ethoxycarbonyl -2-ethoxy -1,2-
dihydroquinoline (EEDQ).

10

N
N
O NH N
HOO
N
O NH2
N
O NCO
DI-Tdiphosgene4-NH2-TEMPO

N
N NNNH
N
HN
HO O
O O OO
NO
O OH
NNH2
NH2NH2+
TRI-PEEDQ

NNN
NONH
SONH
SO
O
O
N
HON
HO
O OSH
O
OH
OHO
SOOH
S
O
O OHOH
OHI2 TE-T EEDQ

NNNN
NNNH
OO
N
HNHO
O
NHOO
OO NNNH2
NH2NH2
NH2NO
OOH.
..
+
TE-PEEDQ

Fig. 5. Synthesis of the polyradicals

11
The ESR spectra of the polyradicals thus synthesized showed the expected triplet with
additional features due to the spin -spin coupling. The presence of two or more nitroxide moieties
in a molecular structure is usually accompanied by the appearance in the ESR spectra of
additional lines at tributed to spin -spin coupling.
The intensity and number of lines depend on the ratio between the coupling constant ( J)
and the hyperfine constant ( aN), and the probability of collision with solvent mo lecules (the
solvent nature) { 71}. As a consequence, the polyradicals used in this study show the expected
triplet attributed to the interaction of the unpaired electron with the N nucleus and additional
lines attributed to spin -spin interactions.
The values of line -width of the central line increase in the case of polyradicals compared
with those observed for TEMPO and PROXYL monoradicals as an effect of spin -spin
interactions, as well. HR -MS spectra also confirmed the structure of the compounds.

DI-T TRI -P

TE-T TE -P

Fig. 6. ESR spectra of the polyradicals (solvent DCM)

12
2.2 Exploring porous nanosilica -TEMPO as heterogeneous

Heterogeneous catalysis is one of the most used industrial processes, being used for the
completion of a large variety of oxidation reactions, among others . The choice for such processes
is due first to the eas y recycling of the catalyst and the use of simpler procedures that allow a
greener and more environmental ly friendly approach.[59,76,77]
Selective oxidation of primary and secondary alcohols to the corresponding aldehydes and
ketones is one of the most required processes in the production of fine chemicals.[80,22]
Traditionally, such oxidations are made using excess of transition metal compounds (like
manganese or chromium derivatives), with their usual high drawb acks – large amount of toxic
waste, time consuming processes, expensive procedures.[82,34,45]
A promising alternative for these methods is the use of stable free radicals, namely
2,2,6,6 -tetrametylpiperidine -Noxyl (TEMPO) derivatives, as the catalyst in oxidation
processes.[59,76, 21,22,78 -81,34,28,82 ] Such nitroxide free radicals have quite interesting properties,
showing redox behavior (Fig. 7); moreover, they are indefinitely stable under ambient
conditions, do not dimerize nor do they react with atmosp heric oxygen.[34,45 ,83,84] However,
TEMPO itself doesn‟t have the ability to oxidize alcohols, a cocatalyst being required.[39,37,44]
The role of the cocatalyst is to extract one electron from TEMPO, yielding the
oxoammonium ion (Fig. 7), which is the real oxidant in such processes. As cocatalyst were
employed a wide range of chemicals, including nitrogen oxides, nitric acid, transition metal ions,
halogens, etc.[39,25, 32]

N
Oe-
e-N
Oe-
e-N
O

Fig. 7. Redox behavior of TEMPO

13
In the last decades gold catalyzed processes became a follow -up trend, starting mainly
after the discovery that the noble metal gold may have extremely powerful catalytic activity (i.e.
gold clusters can oxidize carbon monoxide).[85] Literature data regarding novel approaches for
selective oxidation showed that gold nanoparticles (Au NPs) can also convert benzylic alcohols
into aldehydes or even esters (through an oxidative coupling). Such processes do not require an
additional co catalyst, as is usual in the case of TEMPO.[86-88]
Interestingly, recently it was reported that TEMPO can greatly extend the lifetime of the
gold (III) chloride catalyst (the turnovers of it being increased by 3300% and the catalytic
activity maintained fo r 33 cycles); this procedure can lower the cost of the gold catalyst in
applied synthesis.[89]
Gold nanoparticles contain a diversity of conjugated and non -conjugated spherical gold
nanoparticles varying in size between 2 nm to 250 nm. Gold nanoclusters have applications in
catalysis .[90] In 2008, Su, FangZheng et al.[91] used gold nanoparticles on polymorphs of gallia (α –
β- ϫ- Ga2O3) for the oxidation of benzyl alcohol. A gold nanoclusters ( PI-Au) polymer[92] was
used in the aerobic oxidation of alcohols to methyl esters.
Ester groups are found in numerous pharmaceuticals, fragrances, agrochemicals and so
on, and these compounds are obtained usually by the well -known esterification or
transesterification processes, always requiring the presence of a strong acid or base as a catalyst.
Direct transformation of the alcohols into esters via oxidative coupling between aldehydes and
excess alcohols is a very environmental ly friendly process, performed in a single step; moreover,
it can be adapted to use as air as the final oxidant, yielding as water as the byproduct . The sol -gel
is procedure for making solid material from small molecul es. TEMPO free radical can be
covalently bonded to (nano ) silica in various ways, employing different methods, like sol –gel or
surface modification reactions preformed on the solid material. Literature data showed that there
are two types of linkers between the silica surface and the TEMPO moiety, either contai ning an
oxygen atom (like in ethers or esters) or a nitrogen one (like in amines).[93-95] In our work we
used both types, for comparison reason s.
Cat. A has been obtained by a sol –gel method, mixing together tetramethoxysilane
(tetramethyl orthosilicate, T MOS) with 4 -hydroxy -TEMPO and gelling the combination by the
addition of ammonia.

14
Cat. B was obtained by precipitating gold on Cat.A (Cat.B is in fact the previous one
which contains supported gold, obtained by the reduction of a gold(III) salt with sodiu m
borohydride).
Cat. C employs a different way to covalently link the TEMPO moiety to the silica
material; for this reason , a mixture of TMOS and (3 – aminopropyl)trimethoxysilane (APTMOS)
has been used as the silica precursor. The TEMPO moiety has been co valently attached in two
steps.
In the first one, 4 -oxo-TEMPO reacted with the amino groups on the silica, yielding the
corresponding Schiff base, and in the second step the C=N bond was reduced by borohydride.
Cat. D was obtained by supporting gold on Cat. C, by reducing the gold salt with sodium
borohydride, in the same way as Cat. B. A n illustration showing a schematic drawing of the
catalyst structure together with a picture of the actual solids is shown in ( Fig. 8).

SiO2OH
OHN
OO
SiO2OH
N
OO
SiO2OHN NH
O
SiO2OHN NH
O
OHAu
AuCatalyst ACatalyst B
Catalyst C Catalyst DAu
Au

Fig. 8. Structure representing Cat. A -D

15
2.2.1 Characterization of the catalysts
All catalysts were obtained as solid materials having a yellow or violet -black colour (
Fig. 8); the yellow color is due to the TEMPO free radical, while the violet colour is due to the
supported gold.
IR spectra ( Fig. 9) do not provide a lot of information; however, there are large bands
visible between 3100 and 3500 cm-1, due to the OH and NH groups (Si –OH and C –NH
moieties); Si –O bonds are also noticed between 1200 and 1400 cm-1. Small peaks before 3000
cm-1 are detected only for sample C and D, and those are attributed to the C–H groups; however,
C–H groups are also visible between 1350 and 1 450 cm-1.

Fig. 9. IR spectra of catalyst samples A -D

TEM analysis showed for all samples A –D that the silica support is formed by the
agglomeration of regular nanoparticles with a size of about 20 nm; regarding the morphology of
the gold clusters deposited on silica, these are bigger in size (20 –50 nm) and more asymmetrical
in shape. Fig . 10. shows the TEM images of samples A –D.

16
The nitr ogen adsorption –desorption isotherms of the A – D samples showed that all are
type IV according to the IUPAC classification and the hysteresis loops are of H 2 type, typical for
mesoporous materials (Fig. 10).
This type of hysteresis is usually associated w ith the presence of interconnected pores of
not well -defined or irregular shape (often referred to as „ink -bottle‟ pores). As can be seen in the
inset (Fig. 10), the pore size distribution for all samples is monomodal and relatively narrow. The
average por e diameters are in the range of about 4 –6 nm.

0.0 0.2 0.4 0.6 0.8 1.0050100150200250300350400450
0 5 10 150.00.10.20.30.40.50.6
A
B
P/P0V (cm3g-1, STP )
D (nm)Pore volume (cm3 g-1 nm-1)
0.0 0.2 0.4 0.6 0.8 1.001020304050
0 5 10 15 200.0000.0020.0040.0060.0080.0100.0120.014
C
D
P/P0V (cm3g-1, STP )
D (nm)Pore volume (cm3 g-1 nm-1)

Fig. 10. N2 adsorption –desorption isotherms of A – D samples

17
The specific surface area for sample A is 323 m2/g. Upon deposition of gold, surface area
decreases to 257 m2/ g (sample B). As the gold clusters are bigger in size comparatively with the
pore size, therefore it is a common assumption that gold clusters are placed on the outer surface
of silica nanoparticles, a fraction of pores being blocked b y the gold nanoparticles.
Sample C has a quite smaller specific surface area (26 m2/g). This value may be
explained by taking into consideration the fact that these sample has been obtained using a
mixture of TMOS and APTMOS instead of plain TMOS, as in t he case of samples A and B, and
also to the low water content used in synthesis.
The presence of the organic linkers of 3 -aminopropyl type (from APTMOS) makes the
material more compact and therefore it has a smaller surface and volume of pores . As in the case
of sample B, deposition of gold on the surface of sample C has as result a decrease of the surface
area and total pore volume (Table 1).
Thermal analysis performed on samples A –D showed a major difference between the
samples A –B (TEMPO linked to silica via an oxygen atom) and C –D (TEMPO linked to silica
via a nitrogen atom). The content lost when heating up to 300 oC for the first two sampl es is 20
and 30%, while for the last two samples it is only 5 and 6 %, respectively (Table 1).
This means firstly that TEMPO linked through the oxygen atom are more easily removed
by heating from the inorganic support comparatively with TEMPO linked throu gh the nitrogen
atom, and secondly, the presence of gold clusters accelerate s that decomposition.
Overall, these can be explained by the weak bonds Si –O–C against Si –C– N (that bound
TEMPO on silica) and by the ability of metals (in our case gold) to cata lyze the decomposition
of organic materials. Moreover, in the cases of A and B samples an endothermic peak is noticed,
while for samples C and D an exothermic one is present (Fig. 12).

18

Table 1 . Characterization of samples A –D

Catalyst Cat. A Cat. B Cat. C Cat. D
BET surface area (m2/g) 323 257 26 17
Average pores diameter (nm) 3.6 6.1 5.6 5.8
Total pore volume 0.41 0.50 0.05 0.03
Organic content lost (%)a 20 30 5 6
TEMPO content (mmol/g)b 0.11 0.10 0.23 0.22
Gold content (%)c 0 5 0 5
a Measured by TG –DTA, b Evaluated from ESR spectra, c Considering that all the gold has been deposited
on the silica by the thebprecipitation –deposition process

ESR technique can give important information about the environment in which the
TEMPO free radi cal is found.[96] Fig. 13. shows the spectra for samples A –D as dry solids (red
line), as well as suspended solids in DCM (black line). Interesting features were noticed by this
technique: first of all, broad lines are present for all samples, and this characteristic is attributed
to imm obilized TEMPO moieties on the surface of the nanoparticles.

19

Fig. 11. TEM pictures of samples A –D (a silica/HO -TEMPO; b sample A on which Au NPs were
deposited; c silica/oxo -TEMPO; d sample C on which Au NPs were deposited)

A second observation is extracted from the comparison of the spectra recorded for dry
samples and wet ones: addition of the solvent (DCM) doesn‟t dramatically alter the shape of the
spectra, meaning that the mobility of the TEMPO moieties is quite restrict ed.
However, some differences are visible between dry and wet samples; furthermore, large
differences are observed between samples A –D. As ESR spectra are very sensitive tp the
microenvironment in which TEMPO moiety is found, significant information can b e extracted

20
from these. Sample A showed mainly a single broad line with small shoulders; in this case
TEMPO is attached very tight to the silica surface, the linker being one single oxygen atom.

Fig. 12. TG and DTA analysis of samples A –D

For sample B, containing gold clusters, the ESR spectra, both in solid and suspended in
DCM, are very similar, and clearly showed the three lines characteristics of a nitroxide free
radical, together with small outside humps. The interpretation of these fe atures means the
coexistence of two types of TEMPO moieties, one in a rigid environment and one in a more
flexible one. This should be due to the presence of gold clusters, but at this moment, without
further investigation, it is inappropriate to elaborate an indisputable explanation.
It is possible for some TEMPO units to leak, as Si –O–TEMPO bonds can break . Sample
C shows an ESR spectrum consisting of a rigid and a flexible component super imposed upon

21
another; in this case, t he linker between the TEMPO moiety and the silica surface is bigger than
for sample A, being formed from a chain with four atoms.
As expected, the addition of DCM to the dry sample increases the mobile component of
the spectrum. Sample D shows a more rigid component, and this can be explained from the
involvement of gold clusters, that may restrain the mobility of free radicals.

Fig. 13. ESR spectra of samples A –D (black -as solid; red -solid suspended in DCM)

22
2.2.2 Catalyst preparation
Cat. A was prepared by adding under stirring 0.1 g of 4 -hydroxy -TEMPO to a solution of
5 mL TMOS in 25 mL of methanol. After a few minutes , 0.5 mL of concentrated ammonia (25
%) were added, followed by 10 mL of water. Next day the resulting gel was separat ed and
allowed to dry in open air, yielding a pale yellow solid. The solid thus obtained was washed
several times with methanol and allowed to dry in open air.
Cat. B was prepared in the same way as catalyst A, supplementarly adding under stirring,
to the solid dry material obtained, 0.1 g of HAuCl 4 dissolved in 5 mL water, 20 mL of methanol
and 10 mg of sodium borohydride. After 2 h the solid material was separated, washed with
methanol and dried. The final material is a violet –black solid.
Cat. C was prep ared by stirring a mixture of 2.5 mL of TMOS and 2.5 mL of APTMOS
in 45 mL of methanol for 2 h, then adding 2 mL of concentrated ammonia. Next day , 2.24 g of
4-oxo-TEMPO were added, and the mixture stirred overnight. The following day , 0.82 g of
sodium cy anoborohydride was added and the mixture stirred for another 2 days. At the end , the
excess of borohydride was destroyed by adding diluted hydrochloric acid. The solid material was
collected, washed several times with methanol and allowed to dry.
Cat. D wa s prepared by adding under stirring 1 g of catalyst C to 0.1 g of HAuCl 4 in 20
mL of methanol, followed by 20 mg of sodium borohydride dissolved in 1 mL of water. After 2
h the solid material was separated, washed with methanol and dried.

23
2.3 Covalently grafted TEMPO on graphene oxide
Carbon is known since antiquity . It is a naturally occurring element found in large
quantities on Earth , being of tremendous importance for human society, due to its major role as a
fuel. Carbon , in addition to hy drogen, oxygen and helium is the most abundant element in the
universe and it has the most types of organic compounds. Carbon -based materials are cheap and
available in large quantities, and can be used in electronics and energy conversion and storage[97].
In addition, they can be easily functionalized to be employed in more specific applications.
Chemically modified carbon -based materials have the potential to be used on an industrial
scale.[98,99]
Allotropes of carbon such as graphite , diamond , and amorphous ca rbon also have
significant technical applications; more unusual forms of carbon (fullerenes, nanotubes,
graphene) have recently found astonishing use in state -of-the-art technologies .[100-102]
Graphite is a mineral composed of many layers of graphene ; it is inexpensive and readily
available As a natural or artificial material, graphene is a single atomic layer of sp2 carbon atoms,
does not easily exfoliate to monolayer graphene sheets; in contrast, graphite oxide, simply
obtained through oxidation of graph ite (and therefore containing abundant oxygen -based
groups) . When graphite exfoliate s, it is use d for the formation of the strong oxidizing agent
graphene oxide (GO) , one of the nonconductive hydrophilic carbon material s.[103-106] It can be
exfoliated to graphene oxide (GO) nanosheets by using different means, including ultrasonic
devices.[107] Graphene nanosheets were first obtained by mechanical exfoliation.[108]
Although the interest in studying GO began in 1859 with B. C. Brodie , who added a
portion o f potassium chlorate to a slurry of graphite in fuming nitric acid . [109] This protocol is
improved by using sulfuric acid with fuming nitric acid and by the step by step addition of
chlorate over the reaction. The Hummers method[104] published in 1958 can be considered as a
milestone in obtaining GO and is one of the most used widely methods today; the graphite react s
with KMnO 4 and NaNO 3 in the presence of concentrated H 2SO 4. The improved method reported
recently by J. M. Tour[110] has a simpler protocol, another major advantages being the higher
yield and the absence of any toxic gas evolution during synthesis. Such functionalized GO has
new properties which can be difficult to determine , the structure of GO, is already a contiguous

24
aromatic lattice of grapheme which can link to alcohols , epoxide, carbonyls, ketone, and
carboxylic groups.[111-113]
GO is the practical precursor of graphene (reduced GO, rGO ); both of them are known as
functional materials with many possible applications. GO is in fact highly oxidized graphite
which is exfoliated into sheets containing functionalities such as hydroxy, epoxide, carbonyl and
carboxyl groups; these hydrophilic g roups make GO dispersible in highly polar solvents (water,
DMF). Moreover, these oxygen -containing groups facilitate exfoliation, and more importantly,
ensure the possibility of covalent functionalization with organic molecules. Because the
graphene layer can also be regarded as a polyaromatic composition, a large number of physical
interactions with organic molecules, such as π – π stacking, are possible.
There are many reproducible methods to functionalize GO, and these can be mainly
divided into covalent and non -covalent functionalization. Covalent functionalization takes
advantages of the carbon surface chemistry; for example, carboxyl and hydroxyl group s can be
easily derivatized using standard chemistry (i.e. with porphyrins[114], ferrocene,[115] and
polymers ).[116] In addition, another route is the use of aryl diazonium salts, which can also be
tailored by organic chemistry.[117,118] Non-covalent functionalization is based, as mentioned
before, on π – π stacking, ionic, cation -π or Van der Waals interactions.[106]
Such functionalized GO has new properties (which can be chemically tuned) and has
therefore found new and interesting appl ications, from electrochemical energy conversion and
storage to robust and highly selective carbocatalysts.[119] GO have so many applications , used as
the start material , such as ( GFET) , it is a devi ce (graphene based field effect transistor),[120,121 ]
(FETs) Field effect transistors are used as chemical sensors[122-124] and biosensors. Ϫ-MnO 2 is
used on a surface of grapheme oxide for the selective aerobic oxidation of benzyl alcohols to
corresponding carbonyl compounds .[125] It is also used as catalyzes for the oxidation of various
alkenes and alcohols and the hydration of various alkynes into their aldehydes and ketones.[126]
GO is used as a catalyst for the selective oxidation of alcohols to the corresponding aldehydes
and ketones.
One of the most important organic chemistry reactions is the selective oxidation of
alcohols to aldehydes or ketones. Such a process usually requires a difficult management of the
reaction conditions, as transition metal species are involved in at least equimolecular quantities

25
and the resulting toxic waste is hard to process. Novel systems involve more gentle (air, oxygen,
hydrogen peroxide, etc.) or non -conventional (carbon -based materials, stable free radicals, etc.)
oxidants, with certain and large advantages: cle an reactions, mild working conditions, recovery
and re -use of the catalyst, less or no ntoxic by -products, and so on.[127- 133]
Many catalytic processes involve metals or metal ions with high toxicity, therefore the
finding of a new benign catalyst represents an important goal in itself. Organic stable free
radicals of nitroxide type (or their functionalized materials) are nowadays a good practical choice
for the reliable, green and clean synthesis of c arbonylic compounds obtained via oxidation of
alcohols.[129,45,134]
The stable free organic radical 2,2,6,6 -tetramethylpiperidine -Noxyl (TEMPO) can be
involved in redox processes as it can be readily oxidized to the oxoammonium salt or reduced to
the corre sponding hydroxylamine. TEMPO can be grafted onto inert materials in order to exploit
its redox properties in catalytic processes. In literature there are very few papers describing GO
functionalized with TEMPO moieties: i) coupling the HO group from 4 -hydroxy-TEMPO with
the activated COOH group of GO,[135] ii) using a malonyl derivative of 4 -hydroxy -TEMPO
(following the BingeleHirsch reaction)[136] or iii) using the oxoammonium salt of TEMPO[137]
(which in fact is not a free radical). None of those have been used in the catalytic selective
oxidation of alcohols.
In this work, we covalently bound 4 -amino -TEMPO to GO, using standard amide bond
formation in two steps. The first step refers to the activation of COOH groups from GO by
transforming them into th e corresponding acid chloride COCl. The second step is represented by
the reaction of COCl with 4 -amino -TEMPO (Fig. 1 4). The choice of this method is justified by
the highest yield of coupling, as is also shown in literature[135] and by the higher stabilit y of the
amide group. Amides are much more stable than esters (which can easily hydrolyze under basic
conditions); moreover, epoxide groups from GO can also react with 4 -amino -TEMPO, resulting
in a highly functionalized GO.
All of the materials thus obtain ed were first characterized by elemental analysis, infrared
(IR), electron spin resonance (ESR) and Raman spectroscopy, thermo -gravimetric analysis
(TGA) and scanning electron microscopy (SEM), and further tested as heterogeneous catalysts in
the selective oxidation of alcohols.

26

O
O
O
O
OH
O
OH
O HOOOHOOOHOHO
O
HO
O
OH
OHO
1] SOCl2
2]NO
NH2O
N
O
N
OH
O
HN
O N
HOOHOOOH
NHO
O
HO
O
OH
OHONONO
NO
NONOFig. 14. Representation of
TEMPO functionalization of GO.

27
2.3.1 Experimental
2.3.1.1 Materials and methods
Synthetic graphite powder (size less than 20 mm) was used as the starting material. IR
spectra were recorded on a Jasco FTIR 4100 apparatus (KBr discs). ESR spectra were recorded
on a Jeol JES FA100 apparatus using the following typical settings: frequency 8.99 GHz, field
3330 G, sweep width 100-200 G, sweep time 60 -120 s, time constant 30 ms, gain 50-500,
modulation frequency 100 kHz, modulation width 1 G, using 1 mm inner diameter plain glass
tubes.
Raman spectra were measured in a Horiba Jobin -Yvon LabRam spectrometer.
Measurements were carried out in the backscattering geometry , at room temperature, with a 50 x
microscope objective, in the rang e from 50 to 2000 cm-1, with acquisition times of 60 s; using as
excitation source the green line (l ¼ 514.5 nm) of an Ar+ laser, with a power of ~20 mW, the
laser spot size was ~1 -2 mm. Thermal measurements were performed on a Netzsch STA 449 F1
Jupiter Simultaneous Thermal Analyzer apparatus in a dynamic argon atmos phere, with a
heating rate of 5oC min-1.
NMR spectra were recorded on a Bruker Fourier apparatus at 300 MHz using CDCl 3 as
the solvent (isotopic purity 99.9%) and TMS as the internal standard. Specific surface areas were
measured by N 2 adsorption -desorption at 96oC using an automatic adsorption system
(Micromeritics ASAP 2020).
The surface area was calculated using the Brunauere Emmette Teller (BET) method
based on adsorption data in the partial pressure (P/Po) range of 0.05 -0.3. Before analysis, the
samples were degassed for 12 h at 100oC under vacuum. Microstructural studies were carried out
by Field Emission (FE) Scanning Electron Microscopy (SEM) in a Dual Beam 3 D FEG FEI.
Secondary electron images were recorded at accelerating voltages of between 1.2 and 2 kV.
Elemental analysis measurements by Energy Dispersive X -ray (EDX) spectroscopy were carried
out in the same apparatus, operating at an accelerating voltage of 20 kV. Elemental analysis was
performed on a CHNPerkin Elmer 2400 apparatus.

28
2.3.1.2 Synthesis of GO/iGO
Two methods for obtaining graphene oxides were followed {104,110}, with slight
modifications, as follows: i) Hummers methods (GO): to a mixture made up of 1 g of graphite
and 0.5 g of sodium nitrate , 25 mL of cold concentrated sulfuric acid was slowly added under
stirring , also using an external cooling of the reaction mixture with ice, then 3 g of potassium
permanganate was added in portions , and the resulting mixture stirred for about half an hour,
then the cooling system was removed and the mixture stirred for another half an hour. After the
addition of approx.. 50 mL of water , the temperature rose to almost 100oC, and the mixture was
left f or 15 min. Another portion of 100 mL of water was added, and the mixture stirred again for
half an hour, then hydrogen peroxide (30%) was added until the violet solution decolorized
(about 5 mL). The mixture was left overnight and then the supernatant was removed.
The resulting slurry was centrifuged at 4000 rpm and the collected solid washed
extensively with water, aqueous hydrochloric acid and methanol, and after that was left to dry
under an open atmosphere till the n ext day, and then heated at 60oC under vacuum for one hour;
ii) improved Hummers method (iGO): 150 mL of a mixture 9/1 (v/v) of concentrated sulfuric
acid and concentrated phosphoric acid were added to a mixture of 1 g of graphite and 6 g of
potassium permanganate and the resulting mixture w as stirred for about 8 h at 50oC, and then the
mixture was slowly poured into about 150 g of ice, adding 5 mL of hydrogen peroxide (30%).
The resulting solution was left overnight to separate the solid, and the next day the slurry was
centrifuged at 4000 r pm and the collected solid washed extensively with water, aqueous
hydrochloric acid and methanol, left to dry under open atmosphere till the next day and then
heated at 60oC under vacuum for one hour.

2.3.1.3 Functionalization of graphene oxides with TEMPO
Very little literature data is available on TEMPO functionalized GO, preserving the
nitroxide spins.[135,136] We used a faster and easier protocol, as follows. Graphene oxide 1 g (GO
or iGO) was suspended into 50 mL of dry dichloroethane and then 10 mL of thionyl chloride and
0.5 mL of DMF were added. The mixture was heated to reflux for about 3 h, and then the solvent
and excess thionyl chloride were removed under vacuum, yielding a black solid. To the solid was

29
added 25 mL of dichlorome thane (DCM), 1 g of 4 -amino -TEMPO and 5 mL of triethylamine
and the mixture was kept at room temperature and under stirring for three days.
The solid material recovered by centrifugation was extensively washed with methanol,
left to dry in the open air ov ernight and then heated under vacuum at 40oC to remove any trace of
solvents. The TEMPO -functionalized samples thus obtained from GO and iGO are noted herein
as GO -T and iGO -T, respectively.

2.3.2 Synthesis and charectrisis

2.3.2.1 Synthesis and characterization of GO and iGO
We used synthetic graphite powder (<20 mm) as a reliable starting material[138] because
different sources of natural graphite significantly affect the properties of the GO.[139] Literature
data[140] report s that smaller -size graphite oxidizes more under the same conditions, as shown by
X-ray diffraction and SEM results, thus demonstrating the importance of the precursor size to
obtain easily exfoliated sheets. The synthesis of GO generally followed the Humme rs[104] and
improved Hummers[110] methods, thus yielding two types of GO, named GO and iGO,
respectively.

2.3.2.2 GO functionalized with 4 -amino -TEMPO (GO -T and iGO -T)

As GO contains COOH groups, the next step in our study was to functionalize these with
4-amino -TEMPO, using standard amide bond formation through coupling the NH 2 group with
the COOH group. The COOH groups were activated using thionyl chloride (Fig. 1 4). As
mentioned before, this method afforded the best results. The thus , the obtained materials were
further characterized by elemental analysis, IR, ESR, SEM, Raman and TGA; all these
investigations demonstra te the covalent attachments of the TEMPO moieties to the solid
material.
Raman spectroscopy is one of the most powerful techniques t o characteriz e carbonaceous
materials.[141,142] The Raman spectrum (Fig. 15) of the starting material (graphite) shows the

30
narrow G peak at ~1580 cm-1, corresponding to the vibrations of sp2 carbon in the graphite
lattice, while GO/iGO and GO/iGO functionalized with TEMPO showed two bro ad peaks,
namely the G (1580 cm-1) and D band (at about 1350 cm-1).

Fig. 15. Raman spectra of the materials used in this study

The D band is associated with the disruption of the sp2-bonded lattice of graphite by the
massive formation of C -O bonds in the GO samples, leading to the distortion and opening up of
aromatic rings.
The intensity ratios between both Raman bands I D/IG gives values higher than 1, which
correspond to the reduction of the crystallite domain size in the hexagonal layers from 20 mm to
~40 Å, determined from the T uinstra and Koenig equation.[143]

31
In addition, all of the oxidized samples also show a clear sh ift of the G band towards
higher Raman shift values, as sociated with the incorporation of oxygen -containing
functionalities, disrupting the graphitic bonds.
The TEMPO -functionalized samples show similar Raman features, except for a
noticeable splitting of the G band in the GO-T sample. G band splitting is caused by straining of
the C -C bridges in the sp2-bonded hexagonal layers, caused by pure mechanical strain or by
chemical doping with adatoms ly ing on the grapheme sheets,[144] straining the sp2 bonds without
opening up the aromatic ring.
In our case, doping of the graphene hexagonal layers caused by TEMPO
functionalization seems to be the origin of the Raman G band splitting in the GO -T sample.
IR spectra of both samples of GO and iGO are quit e different from the starting material G
(Fig. 16), demonstrating the presence of the new functional groups, such as carboxyl, hydroxyl
and carbonyl.

Fig. 16. IR spectra of the materials used in this study

Specific ally, the broad band at 3400 cm-1 belong s to the OH vibrations, while C=C
double bonds produce bands at about 1620 cm-1; bands due to carbonyl and carboxyl groups are
also noticed at around 1740 cm-1.

32
The IR spectra of the TEMPO -functionalized samples (Fig. 17) showed that the
intensities of the bands at 1620 cm-1 and 1740 cm-1 are reduced,[114] while new a band emerges at
about 1570 cm-1, corresponding to the C-N stretching vibrations.[135] This band is very intense
and is also present in the IR spectrum of 4 – amino -TEMPO Fig. 18 and 19).

Fig. 17. IR spectrum of 4 -amino -TEMPO

Fig. 18. Superimposed IR spectra of 4 -amino -TEMPO (black) and iGO -T (red)

The possibility of physically adsorbed nitroxide being present on the GO is unlikely, as
the samples were extensively washed with methanol; in addition, the IR spectrum recorded for a
sample used in a c atalytic cycle maintains the same shape (see Fig. 19).

33

Fig. 19. Superimposed IR spectra of iGO -T before (black) and after it was used as a catalyst
(red)

ESR spectroscopy is the most powerful tool to study free radicals, and provides
additional data on the samples used within this study. Unpaired electrons are easily detected by
this technique; moreover, shapes and intensities of the spectra provide information about the
types, number of radicals and their interactions wi th the microenvironment.[146-148]
The ESR spectrum of the pristine graphite s howed the well -known broad band due to the
free electrons that are present in a carbon -based material with extended p -electron systems
(carbon -centred radicals);[143] also, in GO materials a sharp line is noticed (Fig. 20).

34

Fig. 20. ESR spectra of the materials used in this study

However, the origin of the unpaired electron is still unclear and literature data showed
that different types of electrons exhibiting paramagnetic behavior are present at the edge and in
the bulk of such ma terial.[149]
Because phenolic rad icals are stabilized on GO,[145] we do not exclude the presence of
semiquinone -type radicals, as they were identified for example in ci garette tar.[150]
For TEMPO -functionalized samples (GO -T and iGO -T) additional data are noticed: the
ESR spectra showed a very broad line with a triplet feature (Fig. 20), well known for grafted
radicals.[136,151]
These spectra represent a sum of two components: a single broad line determined by the
very short inters pin distances and a second line showing the immobilized s pectrum of TEMPO,
reflecting an increased dis tance between nitroxide spins.
Quantitative ESR analysis of the samples was also performed aiming to evaluate the
degree of func tionalization of the grapheme oxides w ith the 4-amino -TEMPO free radical.
Double integration of the ESR spectra showed an increase i n the number of spins contained by
the composite materials (GO -T and iGO -T) compared with the starting graphene oxides (GO a nd
iGO) of about two orders of magnitude (see Table 2).

35

Table 2. Quantitative ESR analysis

Sample Mass (mg) ESR double integral Spins/mg
DPPH 1.6 1168000 2.446×1018
G 3.1 10626 7.17×1015
GO 6.5 3375 1.08 x1015
iGO 4.1 11666 5.95×1015
GO-T 2.8 902600 6.75×1017
iGO-T 3.5 612300 3.66×1017

For quantitative results all cautions were taken in order to get accurate results (baseline
corrections, integration on all spectrums, acquiring a high signal to noise ratio and so on).

A rou gh evaluation of the content in the organic free radical led to a va lue of 0.6 mmol/g
for iGO -T and 1.1 mmol/g for GO -T. Elemental analysis showed for GO -T and iGO -T samples a
nitrogen content of 3.40% a nd 3.81%, respectively (see Table 3).

Table 3. Quantitative elemental CHN analysis

Sample Carbon (%) Hydrogen (%) Nitrogen (%)
GO 58.14 2.25 0.12
iGO 54.28 2.49 0.09
GO-T 72.16 2.96 3.40
iGO-T 60.32 4.19 3.81

36
As the starting free radical 4-amino -TEMPO has two nitrogen atoms that are incorporated
into the final composite material, these values correspon d to an average of 1.25 mmol/g.
Although there are some divergenc es between the values (provided by different methods), these
can be attributed to different ways of calculation, each of them wit h inherent experimental errors.
However, both methods showed wi thout a doubt the presence of the TEMPO moiety in the
composite ma terial.
The presence of N atoms in the GO -T and iGO -T sampl es (see below). An interesting
feature was no ticed in the t hermo -gravimetric analysis. In general, the thermal degradation of
GO materials occu rs in several steps: up to about 3 00oC water, carbon dioxide and carbon
monoxide derived from t he adsorbed water and oxygenated groups are evolved, while at higher
temperatures (to approximately 900-1000oC) the more stable oxygen -containing functionalities
are decomposed.[123] Several interesting outcomes may be revealed by the thermal analysis (F ig.
21).
The f irst is the efficiency of the improved Hummers procedure in generating materials
with a greater content of oxygenated groups. The statement is supported by the higher mass loss
of the iGO samples compared with GO samples (e.g. 57.05% iGO > 51.15 % GO). The s econd is
the capability of the graphene oxide to be funct ionalized with TEMPO. The lower mass loss
recorded for the samples containing TEMPO proved tha t the new amide groups formed dur ing
TEMPO functionalization are more stable than COOH; in a temperature range characteristic for
oxygenated moi eties decomposi tion (~100 -300oC) a three – and two-fold smaller mass loss of the
GO-T and iGO -T samples is measured compared with the corresponding unfunctionalized
TEMPO samples. Moreover, because o f a higher organic content, the decay process of the
TEMPO sample s occurs at significantly lower temperatures (dotted line marked zone, Fig. 21). A
decomposition step characteristic for the TEMPO samples is the one that occurs between ~300
and 500oC (solid li ne marked zone in Fig. 20), most likely caused by therm ally ind uced
decomposition of the TEMPO moieties, in accordance with literatur e data.[150,151]

37

Fig. 21. TGA analysis of the materials used in this study

The covalent bonding of the TEMPO radical to GO ind uces a significant modification of
its thermo -chemistry, simi lar to those registered for the coordinatively linked ligands: the
linkages determine a sensibl y higher stability of the bonded moieties compared with the free
ones. Moreover, TEMPO‟s charact eristic melting point cannot be identified in the functionalized
samples (see Figs. 22 and 23). FE-SEM micrographs of the sam ples are shown in Fig. 24.

38

100 200 300 400 500 6009080706050403020100
Temperature / oCMass loss / %4 amino TEMPO
-10.0-7.5-5.0-2.50.0DSC /
(mg/mW)DTG /
(%/min)
-10123
exo
Fig.22. TG, DTG and DSC curves for 4 -amino -TEMPO

100 200 300024
GOT
IGOT
Temperature / oCDSC (mW/mg) TEMPO
melting of TEMPO

Fig. 23. DSC curves for 4 -amino -TEMPO comparative with TEMPO functionalized GO

During heating in inert atmosphere, after a melting process (T DSC = 43.65oC), bare 4 –
amino TEMPO undergoes a mass loss of 86.38% in the temperature range 70 -160oC. As
expected, the decomposition is associated with an endothermic effect, determined by the splitting
of the ring and side chain bonds. Two proofs of the TEMPO radical covalent bonding to
graphene oxide can be brought by thermal investigations: i) the higher thermal stabili ty of the
functionalized GO, and ii) the absence of the TEMPO melting process.

39

Fig. 24. SEM micrographs and EDX spectroscopy of the samples G, GO, iGO, GO -T and
iGO-T (from top to bottom)

40
There are clear morphological changes : G morphology is that of flakes with straight
edges and is very easy to delaminate, typical of the grapheme sheets in graphite.
Element al analysis by EDX spectroscopy detects only C. GO is formed of aggregates of
stacked sheets that are significantly corrugated, bent an d with rounded edges. Elemental analysis
by EDX spectroscopy detec ts C and O, indicating that the morphological changes correspo nd to
the oxidation of the grapheme sheets.
The state of aggregation i ndicates that oxidation induces format ion of intersheet bonds in
the GO sample. GO -T shows a similar morphology as GO, with cle aner, flatter surfaces than
GO, and EDX measurements detect C, O in lower content than GO, and N, indicating that in
GO-T the graphit e oxide has been functionalized with TEMPO groups, which c ontain nitrogen.
The iGO sampl e is composed of flat flakes with fairly s traight edges, a microstructure closer to
the initial one of G tha n GO; elemental analysis on the other hand determines an oxyge n content
similar to GO. Sample iGO-T has a similar morphology to iGO; elemental analysis de tects the
presence of N and lower O conten t, confirming the incorporation of TEMPO groups in iGO -T.
BET analysis showed a surface of 4 and 8 m2/g, for iGO -T and GO -T samples, respectively (see
Table 4).

Table 4. BET analysis of the samples

Sample SBET (m2/g)
G 10.4
GO 34.9
iGO 6.5
GO-T 8.2
iGO-T 3.9

41
4. Conclusion
Preparation of free radicals and the interest in application s with different levels of
stability is one of the main reasons for the development and practical application of chemical
theory.
Free radicals have several applications in organic chemistry such as substitutions,
polymerizations and biological pro cesses.
The commercial availab ility o f industrially produced 2,26,6 – Tetramethylpiperidin -1-ly
(TEMPO) has a variety of applications , esspecial ly in the oxidation of alcohols . In this paper we
focused on the synthesis of several types of free radicals , including their characteriz ation using
different type s of techinqe s such as IR, NMR, Raman sepectra and ESR (Electron Spin
Resonance) .
We have synthesized stable di -, tri- and tetra -radicals of TEMPO or PROXYL as a
prelude in their use for the selective oxidation o f primary and secondary alcohols with mild
reaction conditions , and explored porous nanosilica -TEMPO as a heterogeneous catalyst system
and also supported TEMPO on silica nanoparticles containing gold clusters. We also prepared
the TEMPO free radical covalentled grafted to grapheme oxide GO through an amide bond.

42
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