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Microwave-induced degradation of nitrosamines trapped in zeolites.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
Published online 23 July 2008 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.176
Research Article
Microwave-induced degradation of nitrosamines
trapped in zeolites
Jia Hui Xu, Yu Zhou, Jing Jia Wen, Zheng Ying Wu, Chun Fang Zhou and Jian Hua Zhu*
Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210 093, China
Received 30 May 2007; Accepted 13 December 2007
ABSTRACT: A new method is reported for the degradation of nitrosamines trapped by zeolites. Significant amount
of N -nitrosodiphenylamine (NDPA) in dichloromethane solution, up to one-third, could be degraded by microwave
within 1 min once they were adsorbed by zeolite. The nitrosamine molecule, resulting from the electrostatic interaction
between the N–NO group of NDPA and the cation of zeolite, could be partially anchored inside the channel of zeolite
depending on the degree of structure matching between the adsorbent and the adsorbate. Anchoring of nitrosamines
limited their movement so that they were degraded by microwave irradiation. Mesoporous SBA-15 and amorphous
silica in solution could also promote the microwave-induced degradation of NDPA, owing to the interaction between
the surface Si–OH and the N–NO group of NDPA, and introducing copper oxide increased the conversion of NDPA.
In addition, the influence of pore structure of zeolite on the microwave-induced degradation of volatile nitrosamines
was examined and discussed, and NaA exhibits a stronger adsorption than NaY for N -nitrosopyrrolidine (NPYR).
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: microwave-induced degradation; N -nitrosodiphenylamine (NDPA); zeolite; adsorption; mesoporous
materials
INTRODUCTION
Nitrosamines are the carcinogens existing in workplace,
salted fish and other salted and preserved foods, tobacco
and tobacco smoke and beer,[1] and they can react
with DNA to induce tumors in a variety of organs,
including liver, lung, kidney, and bladder depending
on the species.[2,3] These compounds are amines with
two organic groups and one NO group bonded to central nitrogen; however, their carcinogenic ability could
be weakened and even removed if their N–NO function group was broken. When the nitrosamines were
irradiated by ultraviolet (UV) light for 2 h at room temperature, about 4–16% of them could be degraded.[1]
Cobalt-60 irradiation with large doses of 5–20 kGy
was also able to reduce the nitrosamines content in
sauce.[4] To separate nitrosamines from environment,
zeolites and mesoporous materials were employed to
adsorb the carcinogens in gas stream and solution,[5 – 13]
and the captured nitrosamines could be decomposed in
the temperature-programmed surface reaction (TPSR),
in which the nitrosamines were degraded to nitrogen
*Correspondence to: Jian Hua Zhu, Key Laboratory of Mesoscopic
Chemistry, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210 093, China. E-mail: jhzhu@netra.nju.edu.cn
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
oxides and amines as well as other fragments.[7,8] However, this thermal process took longer time than that of
irradiation. Thus a new method to degrade nitrosamines
is sought.
Microwaves are electromagnetic waves consisting of
electric and magnetic field components. The electric
field applies a force on charged particles to start the
movement of these charged particles, migrating or rotating; and the concerted forces applied by the electric
and magnetic components of microwaves are rapidly
changing in direction, which causes warming because
the assembly of molecules cannot respond instantaneously to the changing direction of the field and this
creates friction that manifests itself as heat. With many
advantages such as faster internal heating rate and high
efficiency in energy, microwave irradiation has been
applied in chemical reaction and synthesis,[14 – 17] though
the impacts of microwave irradiation on catalytic systems are not fully understood.[18] Owing to the thermal
effect caused by the occurrence of local spots with
higher temperature, microwave irradiation was done
to reduce the content of tobacco specific nitrosamines
(TSNA) in the tobacco by heating the uncured tobacco
leaves with high power (2–10 kW) and long time
(2–5 min) at 333–363 K.[19,20] On the other hand,
microwave irradiation was utilized to detect the content of volatile nitrosamines in tobaccos,[9] through
482
J. H. XU ET AL.
Asia-Pacific Journal of Chemical Engineering
which the mean amount of both the nitrosamines
and nitrogen oxides could be quickly measured. In
the present study, we combine microwave irradiation
with zeolite adsorption to degrade nitrosamines, and
N -nitrosodiphenylamine (NDPA) is chosen as the target. NDPA is a possible cancer-causing substance in
humans according to the consideration of the United
States Environmental Protection Agency. With two
rigid phenyls connected to the N–NO functional group
(Scheme 1), NDPA molecule has a large molecular size
and it usually adsorbs on zeolite through the way the
N–NO group inserts into the zeolite channel.[10] The
N–NO bond in NDPA molecule is relatively a weak
one with the bond energy of 11 kcal mol−1 ,[10] so it
is possibly broken by the microwave irradiation with
the assistance of zeolite adsorption. To avoid the catalytic degradation of NDPA caused by acidic zeolite,[10]
only basic zeolites or siliceous materials are employed
to trap the nitrosamine in dichloromethane solution.
Other two nitrosamines, N -nitrosopyrrolidine (NPYR)
and N -nitrosonornicotine (NNN), are also used to
examine the microwave-induced degradation of volatile
nitrosamines and TSNA in zeolites.
EXPERIMENTAL
NNN was purchased from Toronto Research Chemicals
Company, NPYR from Sigma, and NDPA was synthesized in laboratory through the reaction of NaNO2
and diphenylamine (DPA) in acid solution.[16] All
three nitrosamines were dissolved in dichloromethane,
respectively, to prepare the sample solution. The purity
of carrier gas, N2 , was 99.99%, and all agents used were
of AR grade. Mega1200 microwave laboratory system,
with the maximum out power of 1 kW at 2450 Hz, was
a product of Milestone S.r.l. It had the function to control the temperature of chamber during the irradiation.
Four zeolites were used in the experiments,
NaZSM-5 with Si/Al ratio of 12.5, 26 or 500, NaY
with Si/Al ratio of 2.86, NaA, and CaA zeolites with
Si/Al ratio of 1. All these zeolites and amorphous silica
are commercially available powder products.[10] CAS-1
is an abundant calcium-ion zeolite-like porous material with a pore size close to that of zeolite NaA.[21]
Two series of mesoporous silica samples, SBA-15
and MCM-41, were used as the adsorbents. Mesoporous siliceous SBA-15 was prepared in our laboratory
according to the literature.[22] In a typical synthesis,
4.0 g of P123 copolymer was dissolved in 150 g of
1.6 M HCl, followed by addition of 8.50 g tetraethylorthosilicate (TEOS) at 313 K. The resulting mixture
was stirred for 24 h and then placed in an oven at 373 K
for 24 h under static condition. Finally, the silica products were filtered, washed, and dried. The templates of
all the ordered mesoporous materials were removed by
calcination at 823 K for 6 h in air before the physical
mixing or impregnation.
One kind of the modified SBA-15 samples, denoted
as MCSn where n represents the CuO mass percentage in the composites, was prepared through grinding the template-free SBA-15 and the precursor salt
Cu(NO3 )2 · 3H2 O in the mortar at room temperature
to achieve a homogeneous mixture and then heated to
773 K at a rate of 2 K min−1 , and this was held at
this temperature for 6 h.[11] Another kind of the coppermodified SBA-15 sample, denoted as CSn where n
represents the CuO mass percentage in the composites,
was prepared through the one-step synthesis.[12] Two
grams of triblock copolymer P123 (EO20 PO70 EO20 ,
Aldrich) and a calculated amount of Cu (NO3 )2 · 3H2 O
were dissolved in 75 g 1.6 M HCl, then 4.25 g TEOS
was added under stirring at 313 K. The molar composition of the mixture was 1TEOS : 0.02P123 : X Cu
(NO3 )2 · 3H2 O : 6HCl : 192H2 O, where X is 0.034, 0.10,
0.17, and 0.34 for preparation of sample with the mass
percentages of CuO as 1, 3, 5, and 10 mass percent,
respectively. The solution was stirred for 24 h at 313 K
and heated at 373 K for another 24 h under static condition. Finally, the liquid was evaporated at 353 K, the
solid obtained was dried and then calcined at 823 K for
6 h to remove template and to form CuO. Through the
similar procedure, the copper-modified MCM-41 samples CMn were prepared,[13] where n represents CuO
mass percentages. For comparison, CuO was loaded on
NNN
NDPA
NPYR
0.12nm
O
0.40nm
N
0.80nm
012nm
.
N
N
0.54nm
0.50nm
N
O
N
N
0.95nm
0.54nm
O
N
Scheme 1. The ichnography of NDPA, NPYR, and NNN molecules.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
MICROWAVE-INDUCED DEGRADATION OF NITROSAMINES
amorphous silica by impregnation with the support of
copper nitrate solution followed by calcination at 773 K
in the procedure reported previously.[11]
The amount of nitrosamines in solution was determined by colorimetric method,[7] into which 0.5 ml of
HBr in glacial acetic acid was added to chemically denitrosate the nitrosamines and to liberate the denitrosated
gaseous product. Purged in N2 carrier gas for 0.5 h,
the gaseous product formed was purified through three
consecutive traps containing 10 ml of 5 mol l−1 NaOH
solutions and oxidized to NO2 by passing through a
CrO3 tube in which 0.4 g CrO3 was mixed with 7.6 g
sea sand and dried at 383 K prior use. The flow rate of
the N2 carrier gas was controlled at 200 ml min−1 . After
being absorbed in the solution of sulfanilamide and N 1-naphthylethylenediamine dihydrochloride,[7] the NO2
was converted to NO2 − and then a Digital Visible Spectrophotometer at 540 nm detected the amount of NO2 − .
Finally, the mean total amount of nitrosamines was
determined through the NaNO2 concentration via an
absorbency curve.[7]
The microwave-induced degradation of NDPA was
performed at ambient temperature in Mega1200
microwave laboratory system. Fifty microliter of
nitrosamines solution was sealed in an isolated flask
and radiated by the microwave with given power
and time. After irradiation, the flask was thoroughly
washed by 10 ml dichloromethane to collect the residual nitrosamines. After the dichloromethane solution
that contained the residual nitrosamines was dried by
anhydrous sodium sulfate and concentrated, the amount
of nitrosamines was determined by spectrophotometric
method as mentioned above.[7] Through the comparison
of the residual and the initial amount of nitrosamines,
the portion of microwave-induced degradation could be
calculated.
To assess the promotion of porous adsorbent for
the microwave-induced degradation of nitrosamines,
all samples were activated at 773 K for 2 h in air
and then stored in desiccators prior to use. Fifty
microliter of dichloromethane solution containing 120
nmol NDPA was mixed with 15 mg sample in a
hermetic glass vessel, and then irradiated inside the
Mega1200 microwave laboratory system as mentioned
above. After irradiation, the total amount of the residual
NDPA, dissolved in the solution or adsorbed in the
porous sample, was determined by colorimetric method,
respectively.[23]
To explore the effect of moisture on the microwaveinduced degradation of NDPA, the dichloromethane
solution containing 50 nmol NDPA was added on the filter paper that had been pretreated at various conditions
to get different moisture content, and then irradiated by
microwave of 1 kW for 45 s. The residual amount of
NDPA was collected and analyzed as mentioned above.
Liquid adsorption of NDPA was completed at room
temperature. All samples were activated at 773 K for
2 h at first and cooled down to 423 K before they were
placed in desecrators. Fifteen milligrams sample was
added into the tube containing 0.1 ml dichloromethane
solution with 0.24 µmol of NDPA. After 24 h of
adsorption, residual concentration of nitrosamines in the
solution was determined by the improved spectrophotometric method.[7] Comparing the absolute amount
of NDPA in solution before and after adsorption by
zeolite, the decreased amount of NDPA means the
nitrosamines capture by the zeolite. All the experiments
were repeated and the data error was within ±2%.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
RESULTS
Figure 1 delineates the microwave-induced degradation
of NDPA in dichloromethane. Without addition of
zeolite, no NDPA was degraded in solution under the
irradiation for 45 s, even when the power of microwave
was changed from 0.1 to 1 kW. Only about 7% of
conversion was observed as the irradiation time was
prolonged to 90 s while the power was kept 1 kW
(Fig. 1(B)). In contrast, addition of NaY zeolite, CAS1 or mesoporous SBA-15 into the solution started the
decomposition of NDPA at the low microwave power
of 0.3 kW, and the percentage of degraded nitrosamines
rose quickly as the power was increased to 0.8 kW.
Among three porous adsorbents, SBA-15 showed an
inferior function (Fig. 1(A)). In the case when the power
of microwave was fixed at 1 kW, addition of CAS1 or SBA-15 in the solution degraded NDPA in 30 s
whereas the effect of NaY zeolite emerged at 45 s. As
the irradiation time was prolonged to 90 s, about onethird of NDPA was degraded in the presence of SBA-15
or CAS-1 (Fig. 1(B)).
Table 1 shows how the structure of zeolite affects the
microwave-induced decomposition of NDPA in solution. Zeolite NaA, NaZSM-5(26) and NaY had the small
(0.4 nm), middle (0.56 nm), and large (0.76 nm) micropore, and the percentage of NDPA that was adsorbed
and decomposed in them was 4.1, 16.6, and 22.3%,
respectively. Clearly, the large pore size of zeolite
is beneficial for the adsorption and decomposition of
NDPA in zeolite. This inference is supported by the
24-h-adsorption results in which the adsorbed NDPA by
zeolite was in the sequence: NaY > NaZSM-5(Si/Al =
26) > NaA (Table 1). The ratio of Si/Al governed the
adsorption and catalysis of NaZSM-5 zeolite. As the
Si/Al ratio increased from 12.5 to 500, the percentage of
NDPA decomposed on NaZSM-5 decreased from 18 to
9.6%. The kind and distribution of cation of zeolite also
affected the actual function of zeolite, for instance the
ion exchange of zeolite NaA with Ca2+ could improve
the adsorption and decomposition of NDPA, enhancing
the conversion of NDPA from 4.1 to 15.4%.
Figure 2 and Table 1 illustrate the promotion of copper on the microwave-induced degradation of NDPA
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
DOI: 10.1002/apj
483
J. H. XU ET AL.
Asia-Pacific Journal of Chemical Engineering
Table 1. Microwave-induced degradation of NDPA
with the assistance of zeolite and other porous
materials (Microwave power: 1 kW; irradiation time:
45 s).
(A) 40
Amount of NDPA: 120 nmol,
irradiation time: 45 sec
Degraded NDPA (%)
NaY
30
CAS-1
SBA-15
Degraded Reduction Adsorbed
NDPA percentage NDPA
(µmol/g)
(%)
(µmol/g)
none
Sample
20
10
0
0
(B) 50
200
400
600
800
Power of microwave (W)
1000
Amount of NDPA: 120 nmol,
microwave power: 1 kW
40
Degraded NDPA (%)
CAS-1
SBA-15
NaY
none
30
20
10
0
0
20
40
60
80
100
Time (second)
SiO2
Imp CuO/SiO2 (1)
Imp CuO/SiO2 (3)
Imp CuO/SiO2 (5)
Imp CuO/SiO2 (10)
NaA
CaA
NaZSM-5 (Si/Al = 12.5)
NaZSM-5 (Si/Al = 26)
NaZSM-5 (Si/Al = 500)
NaY
SBA-15
MCS1
MCS3
MCS5
MCS10
CS1
CS3
CS5
CS10
MCM-41
CM1
CM3
CM5
CM10
Figure 1. Promotion of zeolite on the microwave-induced
degradation of NDPA at ambient temperature.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
10
12
10
10
12
4.1
15.4
18
16.6
9.6
22.3
8.4
9.8
14
16.8
33.3
15.3
18.1
29
35
13
13
14.9
18.5
7.4
6.42
6.49
5.45
6.39
3.58
3.67
5.50
6.42
4.59
4.59
5.50
8.91
10.74
10.85
10.95
10.85
10.95
9.99
9.04
9.17
4.59
4.59
3.67
3.67
3.67
CS-n
2.5
in SBA-15. The conversion of NDPA on the sample of MCS1 was similar to that of SBA-15. As the
amount of copper oxides loaded on SBA-15 rose from
1 to 10% (w/w), the conversion of NDPA gradually
increased from 0.75 to 2.66 µmol g−1 . However, further addition of copper oxides in SBA-15 had no
effect on the degradation of NDPA, and the conversion on the sample of MCS20 (1.88 µmol g−1 , not
shown) was lower than that on MCS10 instead. With
the same loading amount of copper oxide, CSn sample
that was prepared through one-step synthesis exhibited a higher catalytic activity than the corresponding MCSn analogue, hence more NDPA was thus
degraded. For example, 2.32 µmol g−1 of NDPA was
degraded on CS5 sample, 73% more than that on MCS5
(1.34 µmol g−1 ). Different situation was observed on
CMn composite where the highest catalytic activity emerged on the sample of CM5 (1.48 µmol g−1 )
whereas the value on CM10 dramatically declined to
0.59 µmol g−1 (Table 1).
MCM-41 had a larger surface area (1122 m2 g−1[24] ),
but a smaller pore volume (0.58 cm3 g−1 ) and pore
0.80
0.95
0.80
0.80
0.96
0.33
1.23
1.44
1.33
0.77
1.78
0.67
0.78
1.12
1.34
2.66
1.22
1.45
2.32
2.80
1.04
1.04
1.19
1.48
0.59
3.0
Degraded NDPA (µmol/g)
484
2.0
MCS-n
1.5
1.0
CM-n
0.5
0.0
0
4
8
10
2
6
The amount of copper oxide loaded (wt.-%)
Figure 2. Promotion of copper oxide modification on the
microwave-induced degradation of NDPA on mesoporous
silica SBA-15 and MCM-41.
size (2.1 nm) than SBA-15, which caused the similar difference between CMn and CSn samples. For
instance, CM5 possessed the pore size of 2.5 nm and the
pore volume of 0.70 cm3 g−1 , obviously smaller than
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
DOI: 10.1002/apj
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
30
NDPA
Degraded nitrosamines (%)
that of CS5 though its surface area (1080 m2 g−1 ) was
larger. Consequently, CMn sample showed a relatively
lower activity than the corresponding CSn composite in
the microwave-induced degradation of NDPA (Fig. 2),
though they were prepared through the one-pot synthesis and the copper content was the same.
Promotion of copper was also found on amorphous
silica whose surface area (350 m2 g−1 ) was smaller than
that of SBA-15 (918 m2 g−1 ,[7] ), but the situation was
complex. Introducing copper oxide of 1% (w/w) on the
silica through impregnation created a surface concentration of copper of 0.36 µmol m−2 and elevated the
conversion of NDPA from 11.3 to 15.6%. This was
similar to that of MCS3 whose surface copper concentration was also 0.36 µmol m−2 and the corresponding conversion of NDPA was 14.1% (Table 1), implying the important role played by copper in anchoring
nitrosamines. Once the nitrosamine molecules contact
the copper sites in the porous adsorbent, they would be
trapped due to the electrostatic interaction from copper cation to the N–NO group, and hence the anchored
nitrosamines could be easily degraded by microwave
irradiation. However, loading 3 or 5% (w/w) of copper
oxide on amorphous silica exhibited negative impact,
and the sample loaded 10% CuO only exhibited the
activity similar to that of Imp CuO/SiO2 [1] .
Figure 3 depicts the microwave-induced degradation
of volatile nitrosamines and TSNA in zeolites. NPYR
consists of five-membered ring,[6 – 8,12,25] and the structure of NNN looks like one of the H atoms in the
five-membered ring of NPYR to be replaced by a pyridine so its molecular size reaches 0.80 nm (Scheme 1).
Neither NPYR nor NNN in solution could be degraded
when the microwave irradiated them with 1 kW for
45 s. The conversion of NNN in the solution containing
NaZSM-5 (Si/Al = 26) sample, 10.2%, was similar to
that of NDPA, 9.9%, while more NPYR (11.6%) was
degraded under the same conditions probably due to
its smaller molecular size related to the fast adsorption in the zeolite. Less NNN was degraded in NaY
zeolite than NDPA, originating from its larger N–NO
bond energy than that of NDPA.[26] The volatility of
NPYR made its microwave-induced degradation in NaY
zeolite complex, and the conversion was 1.4% (Fig. 3).
NaY could adsorb all the NPYR in solution;[6] however, about 45% desorbed while only 1.4% degraded
once the sample is irradiated. With the pore diameter
of 0.4 nm that is close to the minimum molecular size
of NPYR (Scheme 1), zeolite NaA also adsorbed all of
the nitrosamines in solution; however, no desorption of
NPYR occurred while 11.2% of NDPA was degraded
under microwave irradiation. No degradation or desorption of NNN occurred in zeolite NaA after microwave
irradiation, the narrow channel and the plenty cation
of NaA formed a strong force to tightly anchor the
nitrosamine molecules to prevent desorption.[27]
MICROWAVE-INDUCED DEGRADATION OF NITROSAMINES
NNN
NPYR
NaY
20
NaZSM-5
(Si/Al=26)
NaA
10
0
Figure 3. Microwave-induced degradation of NDPA, NNN
and NPYR in different zeolites (microwave power: 1 kW;
time: 45 s).
50
40
Reduction (%)
Asia-Pacific Journal of Chemical Engineering
30
20
10
0
0
4
8
12
16
Concentration of H2O (%)
Figure 4. The influence of moisture content in filter
paper on the microwave-induced degradation of NDPA
(microwave power: 1 kW; time: 45 s).
Figure 4 reveals the impact of moisture on the
microwave-induced decomposition of NDPA. Under the
same irradiation conditions, the NDPA could not be
decomposed on the dried filter paper whose moisture
content was below 5.5%; the reaction became obvious
as the moisture content rose to 8.5% and the conversion
was dramatically enhanced as the moisture content of
filter paper increased from 8 to 14%. It is clear that the
moisture in the support will promote the degradation of
NDPA under the microwave irradiation.
DISCUSSION
One may have two queries on the microwave-induced
degradation of nitrosamines: First, when the microwave
irradiates the nitrosamines that have been adsorbed by
zeolite, whether its energy may induce desorption of the
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
DOI: 10.1002/apj
485
486
J. H. XU ET AL.
adsorbate besides the decomposition. Second, whether
the fragments of the parent nitrosamines with larger
molecular weights convert to some of the nitrosamines
with simpler structures such as N -nitrosodimethylamine
(NDMA) that are more toxic. However, these arguments are not justified by the experiments. The first,
microwave-induced degradation of NDPA was carried
out in solution at room temperature so that the parent nitrosamines, degraded fragments, and the possible
reformed nitrosamines, if any, would be trapped by
zeolite because of the strong adsorptive capability of
zeolite,[10] which also hindered the analysis of reaction products. The only way for these nitrosamines to
leave the zeolite is to be removed and degraded by
HBr solution during the measurement process.[10] All
nitrosamines, trapped in zeolite or dissolved in solution,
would be detected by the spectrophotometric method,
therefore, the difference between the residual and the
initial amount of nitrosamines represents the portion
to be converted in the microwave-induced degradation.
The second, the limited energy of microwave used here
could not break the N–N bond of NDPA (Fig. 1(A)),
let alone the stronger bonds of C–C, C–H, C–N, or
C–Cl. There was no necessary fragment like methyl
group in the solution to form NDMA at all, so the possibility of forming new nitrosamines like NDMA with
simpler structure and larger toxicity was excluded. Once
the NDPA solution consisting of zeolite was irradiated
by microwave, blue color immediately emerged on the
particles of zeolite, indicating the rupture of N–NO
bond of NDPA and formation of diphenylbenzidine and
DPA radical cations.[10] N–NO bond was the weakest
one in the structure of NDPA, hence the microwaveinduced degradation started from the broken N–N bond,
same as that observed in other catalytic degradation
processes.[10,12] Adsorption of NDPA in zeolite limited
the movement of the adsorbate, meanwhile the interaction between the channel wall and the nitrosamine
anchored the molecule. (Scheme 2), which realized the
rupture of N–NO bond by microwave irradiation. Thus
the geometric matching degree between zeolite and
NDPA, and the electrostatic attraction between NDPA
molecule and the cation in the channels of zeolites governed the adsorption and the succedent degradation of
the nitrosamines.
Zeolites could adsorb microwave heating energy,
especially on the surface where the heating emerged
Scheme 2. The possible manners of NDPA adsorbed and
degraded in zeolites NaA by microwave.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
much quickly[28] and the microwave heating of NaA
was much easier than that of CaA.[29] When the NDPA
solution containing various zeolites was irradiated by
microwave, however, the conversion of NDPA on CaA
was higher than that on NaA (Table 1), in contrast to the
easiness of microwave heating. Thus microwave heating
of zeolites is not the sole factor governing the degradation of NDPA. In fact, CaA could adsorb more NDPA
than NaA (Table 1) due to its relatively large pore
size (0.5 nm), providing more anchored nitrosamines
molecules for microwave irradiation. On the other hand,
the cation in zeolites had specific electrostatic interaction with the N–NO group of nitrosamine,[10,12] and
both the N and the O atoms in the nitroso group of
NDPA were electronegative, therefore, they were easily attracted by the cation with electrostatic pull. At
the same time, these cations were forced to migrate
or rotate by the electric field of microwave once the
zeolite was irradiated, so that they played an important
role in the microwave-induced degradation of NDPA.
Consequently, the NDPA molecule was agitated in
the microwave field, and the atomic vibration became
more and more strenuous while the atoms N and O
in the N–NO group were bounded near the cation.
With the help of energy transmission caused by the
microwave adsorption of cation,[30,31] the interatomic
distance became larger and larger, meanwhile the chemical bond energy became weaker and weaker until the
microwave broke the connection of the two nitrogen
atoms as a result. Furthermore, for the electrostatic
attraction between cation and phenyl group of NDPA
molecule, the chemical bond energy should be weakened in another way analogously. As seen in Table 1,
the conversion of NDPA increased as the Si/Al ratio of
NaZSM-5 decreased, for instance, NZSM-5 with Si/Al
of 12.5 could catalyze 1.44 µmol/g of NDPA, 8% more
than the NaZSM-5 with Si/Al of 26, 87% exceeded the
analogue with high Si/Al ratio of 500. This phenomenon
implies that high density of sodium cation in NaZSM-5
zeolite is profitable for the degradation of nitrosamines.
For the zeolite with different structure, however, the
content of sodium ion is no longer the predominant
factor to govern the microwave-induced degradation.
Seventeen percent of the NDPA in the solution was
degraded on zeolite NaZSM-5 (Si/Al = 26) while only
4% degraded in the case of NaA, though the former
has much less sodium ions than the latter. The pore
diameter of NaA zeolite is 0.4 nm that was only compatible for the N–NO group of NDPA to be embedded
(Scheme 1). Due to the geometric limitation provided
by the narrow pore, most of the sodium ions inside
the channel of NaA could not contact the bulky NDPA
so that they had no contribution on the degradation.
Adsorption of nitrosamines on the external surface of
zeolite, if any, had a negligible impact on the degradation. With a pore diameter of 0.54 × 0.56 nm, NaZSM5 could adsorb NDPA by pulling the N–NO group
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
MICROWAVE-INDUCED DEGRADATION OF NITROSAMINES
or one phenyl of NDPA toward the channel, allowing
more NDPA molecules to be captured inside the zeolite channels and contact the wall of channel. On the
basis of these results it is clear that suitable pore system of zeolite facilitates the zeolite-assisted microwaveinduced catalytic reaction through the monomolecular mechanism.[18] The larger pore diameter of NaY
(0.7 nm) enables NDPA molecule to go entirely inside
the channel. Consequently, the conversion of NDPA
in the microwave-induced degradation was further elevated to 22%. On the basis of these experiments, geometry factor is proven to be crucial for the application
of zeolite in the microwave-induced degradation of
nitrosamines. Both cation and the pore volume of zeolite
are available only in that case where the nitrosamines
can be trapped into the channel. Consequently, the
pore diameter of zeolite affects the final degradation
of NDPA.
Another factor that should be taken into account
for the actual function of zeolite in the microwaveinduced degradation of NDPA is the moisture content of the zeolite, because zeolite can quickly adsorb
the moisture in atmosphere and these water molecules
can efficiently absorb the microwave energy to be
moved or rotated,[9,32] which will affect the adsorbed
nitrosamines, inducing the rupture of N–NO bond and
further decomposition of the carcinogen. As demonstrated in Fig. 4, even the water molecules on filter
paper could improve the degradation of NDPA. Under
the same condition (at the pressure of moisture of
20 Torr and 298 K[33] ), the adsorptive capability of NaA
(0.28 g g−1 ) was smaller than that of CaA (0.30 g
g−1 ) and NaY zeolite (0.35 g g−1 ), and the amount
of NDPA converted on these zeolites was in the same
trend (Table 1). This result also reflects the influence of
water on the microwave-induced reaction in zeolite. In
the case when zeolite was heated to 378 K, the residual
amount of water remaining correlated with the surface
cation density,[34] and these zeolites had different numbers of cation in the sequence of NaA > CaA > NaY.
Although mesoporous SBA-15 and amorphous silica lacked cation owing to the siliceous composition,
they still exhibited a considerable promotion on the
microwave-induced degradation of NDPA as shown
in Fig. 1 and Table 1. This result originated from the
function of their Si–OH groups that interact with the
N–NO group of NDPA through hydrogen bonds.[35]
There are plenty of Si–OH on the surface of silica
and SBA-15, say, about 4.5–4.6 Si–OH groups per
square nanometre on the former and 3.7 Si–OH groups
per square nanometre on the latter.[36 – 38] Once NDPA
molecule enters the channel of SBA-15, it could be
anchored by the surface Si–OH,[39] and then degraded
by microwave. Surface silanol groups played a crucial
role in the adsorption by mesoporous silica because
they were able to form hydrogen bond with the N
atoms of amino groups and/or the O atom in the nitroso
group of nitrosamines.[39] Dispersion of copper oxide
on SBA-15 was at the expense of surface Si–OH for
each copper cation reacted with three Si–OH groups,[40]
so the MCS1 sample showed similar activity to SBA15 and the highest performance emerged on the sample of MCS10 (Table 1). Through the comparison of
MCSn and CSn sample, the influence of preparation on
the copper modification could be seen. Both CS5 and
MCS5 samples had the similar surface area, 706 and
746 m2 g−1 , and pore volume, 1.04 and 1.02 cm3 g−1 ,
as well as the pore size, 9.2 and 9.6 nm, respectively,
but their difference in degradation of nitrosamines was
obvious (Table 1) because copper modifier could be dispersed more equably on SBA-15 by one-pot synthesis
than post-modification,[12,14] due to the promotion of the
micelles in the as-synthesized mesoporous silica.[41 – 43]
Though copper modifier could establish some active
sites on amorphous silica, its activity was only slightly
enhanced because lack of ordered pore structure in the
host made it difficult to anchor the NDPA molecules.
On the basis of these results, it is safe to infer that dispersion of metal oxide such as copper oxide on porous
materials can promote the microwave-induced degradation of NDPA in solution, probably due to the hot-spot
effect[14] ; Likewise, adsorption of NDPA in siliceous
materials such as SBA-15 or amorphous silica seems
involving a kind of synergy between the surface silanol
groups of host and the copper guest[39] so that optimal distribution of copper sites in the porous support is
crucial for adsorption of nitrosamines.
 2008 Curtin University of Technology and John Wiley & Sons, Ltd.
CONCLUSIONS
NDPA could be degraded as much as 33% within 45 s
by microwave irradiation provided it was adsorbed by
zeolite. Assistance of adsorbent dramatically promotes
the degradation of nitrosamines.
Cation of zeolite and the surface Si–OH group of
siliceous adsorbents such as SBA-15 and amorphous silica played the crucial role for the adsorption of NDPA
and succedent microwave-induced degradation. Incorporation of copper strengthened the attraction forwards
NDPA, enhancing the conversion of the nitrosamines in
the microwave-induced degradation.
NaA zeolite had a stronger adsorption than NaY
to volatile nitrosamines such as NPYR, which was
valuable for protection of environment.
Acknowledgments
National Natural Science Foundation of China
(20773601 and 20673053), National Basic Research
Program of China (2007CB613301) and Analysis Center of Nanjing University financially supported this
Asia-Pac. J. Chem. Eng. 2008; 3: 481–488
DOI: 10.1002/apj
487
488
J. H. XU ET AL.
Asia-Pacific Journal of Chemical Engineering
study. Technical assistance from Milestone is also gratefully acknowledged.
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DOI: 10.1002/apj
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