Journal of Porous Materials 7, 469–473 (2000) c 2000 Kluwer Academic Publishers. Manufactured in The Netherlands. Surface Areas of Nitrated Hydrotalcites G. FETTER∗ 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 lamellar structure whose chemical formula is: [M2+ 1−x M3+ (OH)2 ]x+ [An− ]. mH2 O where x is the molar x x/n M3+ ratio: M2+ +M3+ [1–10]. Usually M3+ is Al3+ , Cr3+ , Fe3+ or Co3+ and M2+ may be Mg2+ , Co2+ , Fe2+ , Mn2+ , Zn2+ or Ni2+ . Although the hydrotalcite-like 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 300◦ C for 5–15 hours to ensure total dehydration [15]. Unfortunately, hydrotalcite-like materials are only sta- ble up to 300–400◦ C and they are, then, expected to be structurally altered if usual pretreatment is applied. The values reported in the literature for comparable ma- terials vary between 14 m2 /g and 250 m2 /g. Authors, 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 M3+ ratio. M2+ +M3+ 470 Fetter et al. Table 1. Surface areas and treatment conditions of various hydrotalcite-like materials. Compensation anion CO2− 3 CO2− 3 CO2− 3 CO2− 3 CO2− 3 CO2− 3 Mo 7 O6− 24 6− V10 O28 V10 O6− 28 terephthalate terephthalate Mox O4 Mox O4 V x O4 V x O4 n.d. n.d. n.d. n.d. n.d. CO2− 3 CO2− 3 V10 O6− 28 Mo 7 O6− 24 V10 O6− 22 Cl− CO2− 3 CO2− 3 CO2− , Cl− 3 CO2− , Cl− 3 CO2− , 3 CO2− , 3 CO2− , 3 CO2− , 3 Cl− Cl− Cl− Cl− Surface Calcining area temperature (m2 /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. adipic2− dodecane-dicarboxylic2− CO2− 3 [SiV3 W9 O40 )7− (H2 W12 O40 ]6− Cl− Surface Areas of Hydrotalcites 471 We have chosen to study a nitrated HT sample where M3+ was 0.25. We report the BET surface area M2+ +M3+ 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 contain- ing 125.58 g of Mg(NO3 )2 ·6H2 O (Baker) and 61.24 g of Al(NO3 )3 ·9H2 O (Baker), with vigorous stirring at 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 all the preparation was not purged with N2 and NaOH was not CO2 free, as all samples were synthesized in N2 atmosphere, the amount of carbonates is expected 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, degasixcation time. For instance HT 100/2 means a hydrotalcite degasixed at 100◦ C for 2 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 a hydrotalcite degasixed at 100◦ C for 2 hours and 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 (m2 /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 – 28.4 472 Fetter et al. Figure 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 − times more NO3 ions than CO2− ions to balance the 3 charge. Therefore, nitrated hydrotalcites have a high density of nitrate ions packed between the layers and N2 cannot diffuse through the space. In carbonated hydrotalcites where the anion density is lower, N2 can 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. Surface 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 concentration of NO− ions in the interlayer region. 3 From the results found in the literature, chloride hy- drotalcites also show a low surface area which can be attributed to high anion density limiting N2 diffusion [23]. Above 100◦ C, higher surface areas (from 3.0 to 99.6 m2 /g) result from higher dehydration temperatures and longer heating times. Surface areas of samples de- hydrated are similar only for low degasixcation tem- peratures (Table 2). However, for 300◦ C or additional 2 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. Degassing at 300◦ C was performed for different 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 degasixcation at 100 or 200◦ C does not eliminate water, 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 temperature is 300◦ C, the water, hydroxyls and nitrates 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 300◦ C for the hydrotalcite structure to be maintained. In order to achieve full dehydration the degasixcation temperature (ca. 200◦ C) has to be maintained for at least 5 hours. The surface area should be in the range of 5 to 15 m2 /g for nitrated hydrotalcites. If reported 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, Col- 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. 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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). . 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. Surface Areas of Hydrotalcites 473 Mg/Al ratios have lower surface. pretreatment temperature determined the surface area of the hydrotalcites. Keywords: hydrotalcites, nitrated hydrotalcites, microporous materials, BET surface area, mixed oxides Introduction Hydrotalcite-like. 3 [SiV3 W9 O40 )7− (H2 W12 O40 ]6− Cl− Surface Areas of Hydrotalcites 471 We have chosen to study a nitrated HT sample where M3+ was 0.25. We report the BET surface area M2+ +M3+ values obtained