Surface Areas of Nitrated Hydrotalcites Instituto Polit´ cnico Nacional, ESIQIE, SEPI UPALM Edixcio 8, Zacatenco C.P.. Thermal pretreatment temperature determined the surface area of the
Trang 1Journal of Porous Materials 7, 469–473 (2000)
c 2000 Kluwer Academic Publishers Manufactured in The Netherlands.
Surface Areas of Nitrated Hydrotalcites
Instituto Polit´ cnico Nacional, ESIQIE, SEPI UPALM Edixcio 8, Zacatenco C.P 07738, M´ xico, D.F., M´ xico e
e
e
M.T OLGU´
IN
Instituto Nacional de Investigaciones Nucleares, A.P 18-1027, Col Escand´ n, Delegaci´ n Miguel Hidalgo C.P o
o
11801, M´ xico, D.F
e
P BOSCH
Universidad Aut´ noma Metropolitana-Iztapalapa, Michoac´ n y Pur´sima, A.P 55-532, Iztapalapa C.P 09340,
o
a
ı
M´ xico, D.F
e
S BULBULIAN
Instituto Nacional de Investigaciones Nucleares, A.P 18-1027, Col Escand´ n, Delegaci´ n Miguel Hidalgo C.P o
o
11801, M´ xico, D.F
e
Received February 11, 1999; Revised May 7, 1999
Abstract Hydrotalcites in the nitrate form were prepared using microwave irradiation in the hydrotreatment step
The surface area (BET) of nitrated hydrotalcites was evaluated Solids were characterized by atomic absorption, X-ray diffraction and BET analysis Thermal pretreatment temperature determined the surface area of the hydrotalcites Keywords:
hydrotalcites, nitrated hydrotalcites, microporous materials, BET surface area, mixed oxides
Introduction
Hydrotalcite-like compounds are anionic clays with a
1−x
x
x/n
M 3+
natural clays exist, they are generally synthesized
[11]
These materials have found interesting applications
as catalysts, anionic exchangers or as pharmaceutical
components [1, 12, 13] Their performance is most
often determined by surface chemistry mechanisms
∗ Author
to whom correspondence should be addressed.
Surface area measurement is, therefore, a crucial char-acterization parameter [14] Conventionally, this value
is obtained by nitrogen adsorption and the results are interpreted following the BET equation Before nitro-gen sorption, samples have to be dehydrated It is well known that microporous materials have to be treated at
Unfortunately, hydrotalcite-like materials are only
be structurally altered if usual pretreatment is applied The values reported in the literature for comparable
generally, do not specify the pretreatment conditions of the samples as shown in Table 1.The area values seem
to depend on the particle size, the anion, as well as the
M 3+
ratio
M 2+ +M 3+
Trang 2470 Fetter et al.
Table 1.
CO 2−
3
CO 2− 3
CO 2− 3
CO 2− 3
CO 2− 3
CO 2− 3
Mo 7 O 6− 24
6−
V 10 O 28 V 10 O 6− 28
terephthalate terephthalate Mo x O 4 Mo x O 4 V x O 4 V x O 4 n.d n.d n.d n.d n.d CO 2− 3
CO 2− 3
V 10 O 6− 28
Mo 7 O 6− 24
V 10 O 6− 22
Cl − CO 2− 3
CO 2− 3
CO 2− , Cl − 3
CO 2− , Cl − 3
CO 2− , 3
CO 2− , 3
CO 2− , 3
CO 2− , 3
Cl − Cl − Cl − Cl − Surface Calcining area temperature (m 2 /g) ( ◦ C) 121
110
211
72
36
62
26
40
123
35
298
27
32
30
32
40
24
128
192
159
42
180
59
57
35–41 10
128
240
313
167
122
235
120
122
233
239
210
155
63
26
300
300
Type of hydrotalcite Mg/Al = 13 Mg/Al = 6 Mg/Al = 6.6 Mg/Al = 2.7 Mg/Al = 2.4 Mg/A = 2.19 Mg/Al = 1.66 Mg/Al = 1.92 Mg/Al = 1.92 Mg/Al Mg/Al Mg/Al Mg/Al Mg/Al Mg/Al Zn/Cr Ca/Al Ni/Al Mg/Al Mg/Al Mg/Al = 1.8 Mg/Al = 1.8 Mg/Al = 1.8 Mg/Al = 2.2 Zn/Al = 2 Zn/Al = 2 Mg/Al = 2.6 Mg/Al = 2.6 Mg/Al = 2.1 Mg/Al = 2.4 Mg/Al = 2.5 Mg/Al = 3 Mg/Al = 3 Mg/Al = 3 Mg/Al = 2.2 Mg/Al = 2.2 Mg/Al = 2.0 Zn/Al = 2 Zn/Al = 2 Zn/Al = 2 n.c = Not calcined n.d = No data adipic 2− dodecane-dicarboxylic 2− CO 2− 3
[SiV 3 W 9 O 40 ) 7−
(H 2 W 12 O 40 ] 6−
Cl −
Trang 3Surface Areas of Hydrotalcites 471
We have chosen to study a nitrated HT sample where
M 3+
was 0.25 We report the BET surface area
M 2+ +M 3+
values obtained for different pretreatment conditions
(time, temperature) in an effort to establish reference
points in such a controversial topic
Materials and Methods
Synthesis of Nitrated Hydrotalcite-Like Compounds
Nitrated hydrotalcite-like compounds with an
Al/(Mg + Al) molar ratio of 0.25 were synthesized as
follows: 830 mL of a 1.86 M NaOH (Baker) aqueous
solution was added dropwise during 10 minutes to a
118.4 mL of a total 2.5 M aqueous solution
298 K for 3 minutes The pH varied up to a xnal value
of 13 The obtained gel was treated in a commercial
microwave oven (Philco) operating at 2.45 GHz and
power level of 80 W for xve minutes The use of
mi-crowave irradiation is recomended as it mainly
short-ens the hydrotreatment time [10] The obtained solids
were washed with water and the precipitate was
recov-ered by decantation and dried under reduced pressure
at 373 K Although, deionized water used throughout
to be very low [10]
Characterization
Atomic Absorption Elemental composition (Al and
Mg) was determined by atomic absorption in a Perking
Elmer 2300 instrument The molar ratio Al/(Mg+Al)
was then determined
X-ray Diffraction A Siemens D-500
diffractome-ter with a copper anode tube and a diffracted beam
monochromator was used to identify the obtained
com-pounds
BET Surface Area Analysis Samples (400 mg) were
xrst dehydrated in vacuum at different temperatures for
2 to 18 hours The BET surface areas were determined
in duplicate by standard multipoint technique
adsorb-ing nitrogen A Micromeritics Gemini 2360 instrument
was used
Samples Samples were labeled as follows: HT, degasixca-tion temperature, degasixcadegasixca-tion time For instance HT
hours For samples analyzed for the second time, an
R was added meaning that the analyses was repeated When samples were not degasixed prior to the sec-ond analysis, an N was added For samples degasi-xed a second time at the same temperature, a Y was added instead of N For example, HT 100/2 RN means
then analyzed again without any further degasixcation The mass was mesured before and after degasixcation process
Results and Discussion The experimental molar ratio of the original sam-ple, determined by atomic absorption method, was Al/(Mg + Al) = 0.248 This value is in agreement, within the experimental error range, with the nomi-nal value Figure 1 shows the successful formation of hydrotalcites without any signixcant amount of other crystalline materials The pattern can be fully inter-preted in terms of JCPDS card 22-0700 We have found that nitrated hydrotalcites have, in general, very low surface areas, Table 2 The values are much lower than those measured in carbonated hydrotalcites, similarly pretreated, Table 1 These differences, more than 10 times, can be explained as follows: carbonated and ni-trated hydrotalcites have c parameter values of 7.65
Table 2 Surface area and loss of mass of the samples.
Sample
HT 100/2
HT 100/2 RY
HT 100/2 RN
HT 200/2
HT 200/2 RY
HT 200/2 RN
HT 300/2
HT 300/2 RY
HT 300/2 RN
HT 300/18 Surface area (m 2 /g) 3.0 ± 0.1 3.0 2.2 4.0 ± 0.6 2.7 4.2 7.3 ± 1.0 13.2 7.2 100.0 ± 5.0 Loss of mass (w%) 5.0 1.7 – 14.2 1.0 – 16.0 2.0 – 28.4
Trang 4Figure 1 X-ray diffraction patterns of HT 300/18 sample compared to the original sample.
˚
and 8.34 to 8.79 A, respectively [1, 7] and there is two
−
3
charge Therefore, nitrated hydrotalcites have a high
density of nitrate ions packed between the layers and
diffuse more easily This interpretation can also ex-plain the reason why nitrated hydrotalcites with lower
Figure 2 X-ray diffraction patterns of HT 100/2 RN, HT 200/2 RN and HT 300/2 RN samples.
Trang 5Surface Areas of Hydrotalcites 473
Mg/Al ratios have lower surface areas, as a higher
pos-itive charge in the brucite-like sheets requires a higher
3
From the results found in the literature, chloride
hy-drotalcites also show a low surface area which can be
[23]
and longer heating times Surface areas of samples
de-hydrated are similar only for low degasixcation
hours of degasixcation the surface area was almost
du-plicated (HT 300/2 RY sample) When samples were
not degasixed for the second time the second surface
area measurement was very similar to the xrst one for
all temperatures
du-rations Higher surface areas result from longer heating
times X-ray diffraction patterns show that
hydrotal-cites were already partially decomposed at this
tem-perature The presence of periclase (JCPDS 4-0829) is
observed (Fig 1) This xgure also shows the diffraction
pattern of the original sample
It was observed, as expected, that at constant heating
periods the loss of mass is higher for increasing
tem-peratures, but not for repeat measurements The second
hydroxyls and nitrates as they have been already
des-orbed in the xrst pretreatment However the strongly
sorbed compounds remain at these temperatures If the
strongly adsorbed are eliminated progressively Hence,
in this sample the surface area almost duplicates The
diffractograms of Fig 2 show that hydrotalcite
struc-ture is maintained if the samples are not degasixed in
the second measurement, suggesting that hydrotalcite
collapse is, indeed, due to a dehydroxylation process
Conclusions
From the previous results, the pretreatment temperature
in the measurement of surface area has to be lower than
In order to achieve full dehydration the degasixcation
least 5 hours The surface area should be in the range
values are higher, they correspond to a mixture of ox-ides resulting from hydrotalcite structure alteration Acknowledgments
We thank C Rodr´guez and V.H Lara for technical ı
help G Fetter and M.T Olgu´n thanks CONACyT for ı
xnancial support (Projects 4323P-A and 26769-E) References
1 F Cavani, F Trixro, and A Vaccari, Catal Today 11, 173 (1991).
2 L Chatelet, J.Y Bottero, J Yvon, and A Bouchelaghen, loids Surfaces A111, 167 (1996).
3 C Misra and J Perrotta, Clays Clay Miner 40, 145 (1992).
4 S Miyata and T Kamura, Chem Letters 843 (1973).
5 S Miyata, Clays Clay Miner 23, 369 (1975).
6 S Miyata, Clays Clay Miner 28, 50 (1980).
7 S Miyata, Clays Clay Miner 31, 305 (1983).
8 T Sato, K Kato, T Endo, and M Shimada, React Solids 2, 253 (1986).
9 T Sato, H Fujita, T Endo, M Shimada, and A Tsunashima, React Solids 5, 219 (1988).
10 G Fetter, F Hern´ ndez, A.M Maubert, V.H Lara, and P Bosch, a
J Porous Mater 4, 27 (1997).
11 W.T Reichle, Solid State Ionics 22, 135 (1986).
12 W.T Reichle, J Catal 94, 547 (1985).
13 W.T Reichle, S.Y Kang, and D.S Everhardt, J Catal 101, 352 (1986).
14 A.L McKenzie, C.T Fishel, and R.J Davis, J Catal 138, 547 (1992).
15 F Rey, V Fornes, and J.M Rojo, J Chem Soc Faraday Trans.
88, 2233 (1992).
16 T L´ pez, P Bosch, E Ramos, R G´ mez, O Novaro, D Acosta, o
o and F Figueras, Langmuir 12, 189 (1996).
17 M.A Drezdon, E.J Moore, and M.P Kaminsky, US Patent 4,
843, 168 (1989).
18 M.A Drezdon, in Novel Materials in Heterogeneous Catalysis, edited by R.T.K Baker and L.L Murrell, ACS Symp Series 437 (1990), p 140.
19 C.P Kelkar, A Schutz, and G Marcelin, in Perspectives in Molecular Sieve Science, ACS Symp Series 20, (1988), p 324.
20 K Chibwe and W Jones, Chem Mater 1, 489 (1989).
21 M Doeuff, T Kwon, and T.J Pinnavaia, Synthesis Mater 34,
609 (1989).
22 A Guida, M.H Lhouty, D Tichit, F Figueras, and P Geneste, Applied Catal A164, 251 (1997).
23 T Kwon and T.J Pinnavala, J Molec Catal 74, 23 (1992).