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Langmuir 1998, 14, 2756-2760
Formation of Surface Basicity through the Decomposition of Alkali Metal Nitrates on γ-Al2O3 Jianyi Shen,*,† Mai Tu,† Chen Hu,‡ and Yi Chen† Department of Chemistry and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received September 4, 1997. In Final Form: March 3, 1998 The γ-Al2O3-supported alkali metal nitrate samples with various loadings were prepared by the incipient wetness impregnation method. Differential thermal analysis (DTA), X-ray diffraction (XRD), and magicangle-spinning (MAS) 23Na and 27Al NMR were conducted regarding the decomposition of supported nitrates and the chemical states of alkali metals on γ-Al2O3. The surface basicity and acidity of these samples evacuated at different temperatures were determined by microcalorimetric adsorption of CO2 and NH3, which were correlated to the states of alkali metals, as revealed by the various physical characterizations.
Introduction Many chemical reactions are known to be catalyzed by bases. The use of liquid bases, however, brings about various problems such as corrosion of reactor and difficulty in separating catalysts from reaction mixtures.1 Consequently, development of solid base catalysts has recently attracted much attention.1-5 Solid bases may be prepared by adding alkali metals onto porous supports such as MgO, SiO2, and Al2O3.6-8 These bases have been tested for some base-catalyzed reactions, e.g., oxidative coupling of methane and oxidative methylation of toluene with methane.7-9 However, solid bases are not so widely studied as solid acids.2 This is partly due to the fact that the surface basic sites, especially the strong ones, are easily poisoned by CO2 when exposed to air. The γ-Al2O3-supported alkali metal nitrates are neutral at the beginning and their basicity can be developed after evacuation at some appropriate temperatures.10 Therefore, the base catalysts produced this way may avoid being poisoned before use and they have been declared to be very active for isomerization of n-butene. However, to our knowledge, decomposition of alkali metal nitrates and formation of base sites on γ-Al2O3 have not been fully understood. In addition, the knowledge of the decomposition of alkali metal nitrates and the dispersion of alkali metal cations on γ-Al2O3 is of importance since alkali metals are widely used as promoters in various catalysts and their nitrates are usually the first choice of precursors because they may be easily mixed with other precursors of catalyst components in aqueous solution and the nitrates may be removed without undesired residues after calcination. * To whom correspondence should be addressed. † Department of Chemistry. ‡ National Laboratory of Solid State Microstructures. (1) Tanabe, K. Appl. Catal. A 1994, 113, 147. (2) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases; Elsevier: Amsterdam, 1989. (3) Tanabe, K.; Hattori, H.; Yamaguchi, T.; Tanaka, T. Acid-Base Catalysis; Kodansha and VCH: Tokyo and Weinheim, 1989. (4) Kijenski, J.; Baiker, A. Catal. Today 1989, 5, 1. (5) Corma, A.; Liopis, F.; Monton, J. B.; Weller, S. J. Catal. 1993, 142, 97. (6) Malinowski, S.; Kijenski, J. Catalysis 1980, 4, 130. (7) Ruckenstein, E.; Khan, A. Z. J. Catal. 1993, 141, 628. (8) Voyatizs, R.; Moffat, J. B. J. Catal. 1993, 142, 45. (9) Khan, A. Z.; Ruckenstein, E. J. Catal. 1993, 143, 1. (10) Zhu, J. H.; Wang, Y.; Tsutomu, Y. Chin. J. Catal. 1996, 4, 286.
Microcalorimetry is an effective technique for investigating the strength and number of acid-base sites on solids by measuring the differential adsorption heat evolved when probe molecules contact the surface sites.11 In recent years, a few studies using this technique have been published regarding the acid-base properties of unsupported and supported metal oxides.12-16 For example, microcalorimetric adsorptions of CO2 and NH3 were used to investigate the surface acid-base properties of γ-Al2O3-supported potassium, magnesium, and lanthanum oxides,14 in which the K+/γ-Al2O3 catalysts were prepared by using KOH solutions. In the present work, γ-Al2O3-supported base catalysts were prepared by using alkali metal nitrates as the precursors. Differential thermal analysis (DTA), X-ray diffraction (XRD), and magic-angle-spinning nuclear magnetic resonance of 23Na and 27Al (MAS 23Na and 27Al NMR) were conducted to study the decomposition processes of the supported nitrates and the states of alkali metal species. The acid-base properties of the samples were measured by microcalorimetric adsorption of NH3 and CO2. Experimental Section Different nitrates (Li, Na, or K) of different amounts (0.5, 1.5, and 6.3 mmol/g) were deposited onto γ-Al2O3 by the incipient wetness impregnation method. The resulting samples were dried at 100 °C for 8 h and were evacuated at different temperatures for 2 h. All the nitrates used in this work were AR chemicals, and the γ-Al2O3 support was a commercial product (Taiyuan, China). DTA was performed in N2 (60 mL/min) on a TA SDT 2960 thermoanalyzer. The temperature was linearly increased from 30 °C at a rate of 20 °C/min up to 1000 °C. XRD was carried out on a Rigaku D/MAX-RA X-ray diffractometer using Cu KR radiation at a scan rate of 4°/min. The surface area was measured by N2 adsorption at -195.8 °C using the BET method. The results for all the samples studied are shown in Table 1. MAS 23Na and 27Al NMR spectra were recorded with a Bruker MSL-300 spectrometer operating at a magnetic field of 7 T with (11) Cardona-Martinez, N.; Dumesic, J. A. Adv. Catal. 1992, 38, 149. (12) Auroux, A.; Gervasini, A. J. Phys. Chem. 1990, 94, 6371. (13) Gervasini, A.; Auroux, A. J. Phys. Chem. 1993, 97, 2628. (14) Shen, J. Y.; Cortright, R. D.; Chen, Y.; Dumesic, J. A. J. Phys. Chem. 1994, 98, 8607. (15) Shen, J. Y.; Lochhead, M. J.; Bray, K. L.; Chen, Y.; Dumesic, J. A. J. Phys. Chem. 1995, 99, 2384. (16) Gervasini, A.; Bellussi, G.; Fenyvesi, J.; Auroux, A. J. Phys. Chem. 1995, 99, 5117.
S0743-7463(97)01001-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/17/1998
Alkali Metal Nitrate Decomposition on γ-Al2O3
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Table 1. BET Surface Areas of the γ-Al2O3 and the γ-Al2O3-Supported Nitrate Samples sample γ-Al2O3 LiNO3/γ-Al2O3 NaNO3/γ-Al2O3
KNO3/γ-Al2O3
alkali metal loading (mmol/g)
evac temp (°C)
BET area (m2/g)
1.5 0.5 1.5 1.5 1.5 6.3 1.5 1.5
500 500 400 500 600 500 500 600
158 170 179 188 140 169 71 140 142
Figure 2. DTA profiles of 6.3 mmol/g LiNO3 (a), NaNO3 (b), and KNO3 (c) supported on γ-Al2O3.
Figure 1. DTA profiles of γ-Al2O3 (a), NaNO3/γ-Al2O3 with loadings of 0.5 (b), 1.5 (c), and 6.3 (d) mmol/g, and NaNO3 (e). sample spinning rates of 3-4.5 kHz. The chemical shifts of 23Na and 27Al were measured relative to 1 M aqueous solutions of NaCl and Al(NO3)3, respectively. Microcalorimetric adsorptions of CO2 and NH3 were performed at 150 °C on a Tian-Calvet microcalorimeter.17 The apparatus was linked to a gas-handling and volumetric adsorption system, equipped with a Baratron pressure sensor (MKS) for precision pressure measurements. Heat-flow signals were detected by a transducer assembly manufactured by ITI Inc. The differential heat versus adsorbate coverage was determined by measuring the heats evolved when doses of gas (1-3 µmol/dose) were introduced sequentially onto the sample until the final equilibrium pressure reached about 6 Torr. Before measurement, the sample was evacuated at a certain temperature for 2 h.
Results and Discussion DTA profiles of the γ-Al2O3, NaNO3, and γ-Al2O3supported NaNO3 samples are shown in Figure 1. The peaks before 200 °C resulted from the losses of physically adsorbed water in the samples, and the peaks in the region from 200 to 400 °C were generated by melting of NaNO3 for the unsupported and supported samples.18 γ-Al2O3 exhibited a broad peak in the range between 400 and 900 °C, corresponding to the losses of surface hydroxyl groups. The three NaNO3/γ-Al2O3 samples displayed an endothermic peak around 480 °C, which can be assigned to the decomposition of NaNO3 interacting intimately with the support. Besides this one, there was another decomposition peak at higher temperatures around 520, 570, and 680 °C for the samples with loadings of 0.5, 1.5, and 6.3 mmol/g, respectively. For comparison, the bulk NaNO3 showed the decomposition peaks at temperatures higher than 700 °C. Apparently, the decomposition temperature (17) Tu, M.; Shen, J. Y.; Chen, Y. J. Solid. State Chem. 1997, 73, 128. (18) Dean, J. A. Lange’s Handbook of Chemsitry, 13th ed.; McGrawHill Book Co.: New York, 1985.
Figure 3. XRD patterns of 6.3 mmol/g NaNO3/γ-Al2O3 dried at 100 °C (a) and evacuated at 400 (b), 500 (c), 600 (d), and 700 °C (e).
of NaNO3 was lowered when the γ-Al2O3 support was used and the temperature for complete decomposition of NaNO3 increased with the loading. It is noted that the peak higher than 800 °C for the unsupported sample corresponded to the evaporation of sodium species since the weight loss of the sample (as recorded simultaneously) exceeded 80% when heated to 1000 °C. The DTA profiles in Figure 2 compare the decomposition processes of various alkali metal nitrates (with the same loading of 6.3 mmol/g) supported on γ-Al2O3. The LiNO3/ γ-Al2O3 exhibited three endothermic peaks around 110, 230, and 530 °C. The two peaks at 110 and 230 °C resulted from the loss of adsorbed water and the melting of LiNO3,18 respectively, while the peak at 530 °C was due to the decomposition of supported LiNO3. The KNO3/γ-Al2O3 sample also exhibited peaks corresponding to the loss of adsorbed water and the melting of KNO3 before 400 °C.18 In addition, this sample showed a relatively large peak at about 160 °C, which can be attributed to the crystal transformation of KNO3.18 After 400 °C, the sample exhibited a series of endothermic peaks corresponding to the decomposition of KNO3, in which two main peaks at 680 and 760 °C could be clearly observed. These results indicated that the decomposition temperature increased with the alkali metal nitrates from lithium to potassium when they were supported on γ-Al2O3. Figure 3 presents the XRD spectra of the 6.3 mmol/g NaNO3/γ-Al2O3 samples treated at different conditions. A set of broad diffraction lines with d values of 1.39, 1.97, and 2.41 belonging to the γ-Al2O3 phase could be seen in all the spectra. The sample dried at 100 °C also exhibited a set of very intense diffraction lines characteristic of the NaNO3 phase.19 These lines were significantly weakened after the sample was evacuated at 400 °C, indicating the
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Figure 4. XRD patterns of 6.3 mmol/g KNO3/γ-Al2O3 dried at 100 °C (a) and evacuated at 400 (b), 500 (c), 600 (d), and 700 °C (e).
beginning of decomposition of NaNO3 on γ-Al2O3 at this temperature. After evacuation at 500 °C, only a trace of the NaNO3 phase could be observed. Finally, the NaNO3 phase disappeared completely in the sample after evacuation at 600 °C. At the same time, there were no new phases detected in the samples evacuated at 600 and 700 °C. Figure 4 shows the XRD spectra of the 6.3 mmol/g KNO3/ γ-Al2O3 samples treated at different conditions. The analysis of these diffraction patterns revealed that KNO3 on γ-Al2O3 was converted to KNO2 at 400 °C, which was in turn further decomposed at elevated temperatures.19 This behavior was different from that of the supported NaNO3, which decomposed without the formation of NaNO2. After evacuation at 600 °C, there was still a trace of KNO2 in the sample, indicating that KNO3 was more difficult to decompose completely than NaNO3 when they were supported on γ-Al2O3. MAS 23Na and 27Al NMR were also performed in order to reveal more distinctly the state changes of Na+ and Al3+ species during different decomposition stages. The 23 Na NMR spectra of the 6.3 mmol/g NaNO3/Al2O3 samples treated at various conditions are shown in Figure 5. The sample dried at 100 °C exhibited a very sharp line with the chemical shift of -8.5 ppm, representing a crystalline NaNO3 phase. The peak remained when the sample was evacuated at 400 and 500 °C, respectively. XRD results have shown that the evacuation at 400 and 500 °C led to the partial decomposition of NaNO3. Thus, the NMR peak at -8.5 ppm could be taken as the contribution of both NaNO3 and its decomposition products. The shape of this resonance was very sharp, indicating that all the Na+ species are in a relatively homogeneous surrounding. We suggest that they form a layer on the surface of γ-Al2O3 without penetrating into the bulk of γ-Al2O3 when evacuated at 400 and 500 °C. When the sample was evacuated at 600 °C, the spectrum changed substantially. This sample exhibited a greatly broadened peak around -3.0 ppm besides the sharp peak at -8.5 ppm, indicating the formation of new species between sodium and alumina. This species was proved to be NaAlO2 by the result of 27Al NMR, which will be discussed shortly. After evacuation at 700 °C, there was only seen the broadened peak around -3.0 ppm. This indicates that all the surface Na+ species were transformed into the NaAlO2 phase upon evacuation at 700 °C. Figure 6 shows the 27Al NMR spectra of the γ-Al2O3 and the 6.3 mmol/g NaNO3/γ-Al2O3 samples after various treatments. The 27Al resonances for γ-Al2O3 and NaNO3/ γ-Al2O3 dried at 100 °C were the same and fell into (19) JCPDS Card 7-271; 5-337; 22-840.
Figure 5. MAS 23Na NMR spectra of 6.3 mmol/g NaNO3/γAl2O3 dried at 100 °C (a) and evacuated at 400 (b), 500 (c), 600 (d), and 700 °C (e).
Figure 6. MAS 27Al NMR spectra of γ-Al2O3 (a) and 6.3 mmol/g NaNO3/γ-Al2O3 dried at 100 °C (b) and evacuated at 600 °C (c).
two regions around 71 and 5 ppm, corresponding to Al3+ in tetrahedral (AlT) and octahedral (AlO) sites, respectively.20-24 In addition, the AlO/AlT ratio estimated (20) Akitt, J. W. Prog. NMR Spectrosc. 1989, 21, 1. (21) Muller, D.; Gessner, W. Chem. Phys. Lett. 1981, 79 (1), 59. (22) Ohtani, E.; Taulelle, F.; Angell, C. A. Nature 1985, 314, 78. (23) Lippma, E.; Samoson, A.; Kagi, M. J. Am. Chem. Soc. 1986, 108, 1730. (24) Kostarenko, N. S.; Mastikhin, V. M.; Mudrakovskii, I. L.; Shmachkova, V. P. React. Kinet. Catal. Lett. 1986, 30 (2), 375.
Alkali Metal Nitrate Decomposition on γ-Al2O3
Figure 7. Differential heat versus coverage for CO2 adsorption on γ-Al2O3 (9) and 1.5 mmol/g NaNO3/γ-Al2O3 evacuated at 400 (0), 500 (O), and 600 °C (b).
from the integrated areas under the corresponding peaks for the two samples was about 3/1, similar to that reported by other authors for γ-Al2O3.25 These results indicated that the chemical surrounding of Al3+ in the sample dried at 100 °C was not altered, in agreement with the 23Na NMR results discussed above. The sample evacuated at 600 °C showed two 27Al NMR peaks with chemical shifts at 78.0 and 7.0 ppm, which could be assigned to AlT and AlO, respectively. The peak (AlT) was greatly enhanced and the AlO/AlT ratio was found to be about 1/2 in this sample. It has been reported that Al3+ cations mainly occupy tetrahedral sites in sodium aluminate, which exhibit as 27Al NMR peak at 77 ppm.21 Accordingly, we suggest that NaAlO2 is formed in this sample upon evacuation at 600 °C, consistent with the 23 Na NMR results discussed above. In addition, this sample displayed the NMR peaks with chemical shifts at 78.0 and 7.0 ppm, which were higher than the respective shifts for γ-Al2O3 or the sample dried at 100 °C. It is known that the 27Al chemical shift is sensitive to the second nearest-neighbor coordination sphere and the shift becomes more positive with the smaller electronegativity of elements bonded to Al-O polyhedra.23,24 Thus, the formation of the NaAlO2 phase in the sample evacuated at 600 °C might result in Al3+ cations with higher electron density and therefore in the Al3+ resonances at lower fields. Figure 7 shows the plots of differential heat versus coverage for CO2 adsorption on the γ-Al2O3 and the 1.5 mmol/g NaNO3/γ-Al2O3 samples evacuated at 400, 500, and 600 °C, respectively. The γ-Al2O3 exhibited the lowest basicity with the initial heat of 125 kJ/mol and saturation coverage of 0.3 µmol/m2 for CO2 adsorption. The sample evacuated at 400 °C displayed an increased basicity with the initial heat of 133 kJ/mol and saturation coverage of 0.7 µmol/m2. As discussed above, NaNO3 began to decompose at this temperature, but most of NaNO3 remained, as evidenced by the XRD results. After evacuation at 500 °C, only a trace of NaNO3 was observed in the corresponding XRD pattern and most of NaNO3 was transformed into a kind of surface species (most probably the Na2O), resulting in the strong basicity in both the strength (145 kJ/mol) and the number (2.7 µmol/ m2). However, the evacuation at 600 °C led to an obvious decrease in surface basicity, apparently due to the formation of NaAlO2 in the sample. Figure 8 presents the plots of differential heat versus coverage for CO2 adsorption on the NaNO3/γ-Al2O3 samples with different loadings (0.5, 1.5, and 6.3 mmol/ g). All these samples were evacuated at 500 °C. The (25) McKenzie, A. L.; Fishel, C. T.; Davis, R. J. J. Catal. 1992, 138, 547.
Langmuir, Vol. 14, No. 10, 1998 2759
Figure 8. Differential heat versus coverage for CO2 adsorption on γ-Al2O3 (0) and NaNO3/γ-Al2O3 with loadings of 0.5 (b), 1.5 (O), and 6.3 (9) mmol/g evacuated at 500 °C.
Figure 9. Differential heat versus coverage for CO2 adsorption on γ-Al2O3 (0), 1.5 mmol/g LiNO3/γ-Al2O3 (9), and NaNO3/γAl2O3 (O) evacuated at 500 °C and of 1.5 mmol/g KNO3/γ-Al2O3 evacuated at 500 (b) and 600 °C (4).
results clearly showed that both the number and strength of base sites on the samples increased with the Na+ loadings. Figure 9 shows the plots of differential heat versus coverage for CO2 adsorption on the 1.5 mmol/g γ-Al2O3supported samples with different alkali metal nitrates. LiNO3/γ-Al2O3 and NaNO3/γ-Al2O3 were evacuated at 500 °C. KNO3/γ-Al2O3 was evacuated at 500 and 600 °C, respectively. Li+/γ-Al2O3 displayed the basicity with the initial heat of 135 kJ/mol and saturation coverage of 1.1 µmol/m2, which is higher than that of γ-Al2O3. Na+/γAl2O3 and K+/γ-Al2O3 evacuated at 500 °C showed almost the same basicity in terms of CO2 adsorption: the initial heat was 145 kJ/mol, and the saturation coverage was 2.7 µmol/m2. Unlike Na+/γ-Al2O3, K+/γ-Al2O3 evacuated at 600 °C exhibited a basicity stronger (initial heat 156 kJ/ mol and saturation coverage 3.5 µmol/m2) than that of the sample evacuated at 500 °C. This could be explained by the fact that KNO3 was more difficult to decompose than NaNO3 on γ-Al2O3. In fact, substantial KNO3 remained in the K+/γ-Al2O3 sample after evacuation at 500 °C. Figure 10 represents the plots of differential heat versus coverage for NH3 adsorption on the γ-Al2O3 and the 1.5 mmol/g γ-Al2O3-supported alkali metal nitrate samples evacuated at different temperatures. All the supported nitrate samples were evacuated at 500 °C except that NaNO3/γ-Al2O3 was also evacuated at 600 °C. The acidity of γ-Al2O3 and the samples evacuated at 500 °C decreased with the order γ-Al2O3 > Li+/γ-Al2O3 > Na+/γ-Al2O3 > K+/γ-Al2O3. The Na+/γ-Al2O3 evacuated at 600 °C showed an acidity stronger than that of the sample evacuated at 500 °C, consistent with the results of CO2 adsorption on these samples. The diffusion of Na+ into the bulk of γ-Al2O3 might result in more Al3+ cations on the surface, leading to the increased acidity and the decreased basicity.
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Figure 10. Differential heat versus coverage for NH3 adsorption on γ-Al2O3 (0), 1.5 mmol/g LiNO3/γ-Al2O3 (9), and KNO3/ γ-Al2O3 (b) evacuated at 500 °C and of 1.5 mmol/g NaNO3/γAl2O3 evacuated at 500 (O) and 600 °C (4).
Conclusions The formation of surface basicity from the decomposition of alkali metal nitrates on γ-Al2O3 has been extensively studied in this work by using the technique of microcalorimetric adsorption of CO2 in combination with structural characterizations. The base catalysts obtained this way have the advantage in that the surface basicity can be produced in situ without being poisoned by CO2 in air. The strength of surface basicity was found to depend on the evacuation temperature as well as the type and loading
Shen et al.
of alkali metal nitrates and can be related to the interaction between alkali metal cations and the support. Without the support, the nitrates are hard to decompose completely. The interaction between alkali metals and γ-Al2O3 facilitates the decomposition of these nitrates to produce alkali metal oxides that form the strong base sites on the surface. On the other hand, the interaction also facilitates the diffusion of alkali metal cations into the bulk of γ-Al2O3 to form alkali metal aluminates when the precursors are heated at elevated temperatures so as to decrease the strength of basicity. Hence there must be a temperature for each of the γ-Al2O3-supported alkali metal nitrates at which the evacuation will result in extensive decomposition of nitrate to form a layer of surface alkali metal oxide with the minor formation of aluminate, corresponding to the strongest basicity for the specific alkali. This temperature was found to be 500 and 600 °C when NaNO3 and KNO3 were used, respectively, since the decomposition temperature is higher for KNO3 than for NaNO3 on γ-Al2O3. For the different alkali metal nitrates with the same loading, the basicity produced on γ-Al2O3 follows the order K+ > Na+ > Li+. Acknowledgment. We acknowledge the financial support from the National Natural Science Foundation of China (No. 29673021). LA971001G
