974
Langmuir 1997, 13, 974-978
Determination of the Surface Heterogeneity of MgO-SiO2 Sol-Gel Mixed Oxides by Means of CO2 and Ammonia Thermodesorption† M. E. Llanos,‡,§ T. Lopez,*,‡ and R. Gomez‡ Department of Chemistry, Universidad Autonoma MetropolitanasIztapalapa, P.O. Box 55-534, Mexico D.F. 09340, Mexico, and Instituto Mexicano del Petroleo, Gerencia de Catalisis, Eje Central Lazaro Cardenas No. 500, Mexico D.F. 07000, Mexico Received September 28, 1995. In Final Form: September 12, 1996X Magnesia-silica mixed oxides were prepared by the sol-gel method. Simultaneous hydrolysis of magnesium diethoxide and tetraethoxysilane was made at pH 3 and 9. The solids obtained in acid medium show large surface areas between 441 and 522 m2/g, and those prepared in basic medium show surface areas between 132 and 174 m2/g. The solids show acidity and basicity, and they are able to retain ammonia and CO2 at 200 °C. They can develop basicity by contact with a sulfuric acid solution. Sulfation of the solids notably increases the basicity. The basicity of sulfated oxides is explained in terms of a magnesia segregation. Sulfation segregates magnesium oxide, and hence well-dispersed magnesia on silica is obtained.
Introduction A large number of papers related to the study of acidity on metal mixed oxides and sulfated mixed oxides have been reported.1-3 However, few examples are known concerning the synthesis and characterization of basic mixed oxides, i.e., TiO2-MgO4 and MgO-SiO2.5 In TiO2-MgO mixed oxides the basicity is obtained by the incorporation of titanium atoms to the magnesium oxide. Deformation of the magnesia network and an unbalanced electron charge distribution are responsible for the high basicity. The ionic ratio of the transition metal incorporated to the magnesium oxide is of great importance. The basicity is maximum when the transition metal and magnesium(+2) atoms have similar ionic ratio. Basicity can also be developed by the synthesis of binary magnesia-silica oxides. The basicity in magnesia-silica mixed oxides prepared by coprecipitation of soluble salts or mechanic mixture strongly depends of the MgO content. A maximum in basicity has been reported when the magnesia concentration in the silica is around 50 wt %.6,7 Magnesia-silica mixed oxides can be prepared by the sol-gel method, i.e., simultaneous hydrolysis of silicium and magnesium alkoxides. The rate of hydrolysis and condensation of the alkoxides strongly depends of the hydrolysis catalyst used. The solids prepared by this method yield solids showing high surface areas. However, * To whom correspondence should be addressed: e-mail,
[email protected]; fax, (+52) 57 24 46 66. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. ‡ Universidad Autonoma MetropolitanasIztapalapa. § Instituto Mexicano del Petroleo. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Tanabe, K. Solid Acid and Bases; Academic Press: New York, 1970. (2) Tanabe, K.; Sumiyoshi, T.; Shibata, K.; Kiyoura, T.; Kitagawa, J. Bull. Chem. Soc. Jpn. 1974, 47, 1064. (3) Tanabe, K. Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 2, p 369. (4) Tanabe, K.; Hattori, H.; Sumiyoshi, T.; Tamaru, K.; Kondo, T. J. Catal. 1978, 53, 1. (5) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acid and Bases; Kodansha: Tokyo, 1989. (6) Lercher, J. A.; Noller, H. J. Catal. 1982, 79, 152. (7) Youssef, A. M.; Khalil, L. B.; Girgis, B. S. Appl. Catal. 1992, 1, 1.
S0743-7463(95)00805-5 CCC: $14.00
by this method large amounts of small magnesium oxide particles could be encapsulated by the silica, and hence they will not be accessible to reactants. Additional treatments of the mixed oxides like sulfation with sulfuric acid can produce magnesia phase segregation and hence highly dispersed magnesium oxide particles. A good dispersion of mixed oxides can be then obtained by the sol-gel method. With this in mind (the synthesis of highly dispersed magnesia on a silica surface), in the present work we prepare magnesia-silica mixed oxides using as precursors magnesium diethoxide and tetraethoxysilane and as hydrolysis catalysts hydrochloride acid or ammonium hydroxide. Additional treatment was done by treating the mixed oxide with a sulfuric acid solution. The MgO content in SiO2 was calculated to be in the solid solution range, 1.3, 4.0, and 6.7 mol % MgO.8 The determination of the basic and acid properties was done by temperature programmed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy of CO2 and NH3 chemisorbed molecules, respectively. Experimental Section Mixed Oxides Preparation. The gels were prepared from tetraethoxysilane (TEOS, Alfa Products, 99%) and magnesium diethoxide (MDE, Alfa Products, 97%) by the sol-gel method.9-12 MgSi, pH 3, x%. For the acid preparations HCl (Baker, 36% in water) was used to reach a pH 3 in the initial sol. TEOS and MDE were dissolved in ethanol and refluxed at constant stirring. To the refluxing solution, water is added dropwise (H2O/TEOS ) 8). The reflux was maintained until the gel was formed and then dried at 70 °C for 4 h. x ) 1.3, 4.0, and 6.7 mol % magnesia in silica. MgSi, pH 9, x%. For the basic preparations NH4OH (Baker, 33% of NH3 in water) was used to reach a pH 9 in the initial sol. TEOS and MDE were dissolved in ethanol and refluxed at constant stirring. To the refluxing solution, water is added dropwise (H2O/TEOS ) 8). The reflux was maintained until the gel was formed and then dried at 70 °C for 4 h. x ) 1.3, 4.0, and 6.7 mol % magnesia in silica. (8) Greig, J. W. Am. J. Sci. 1927, 13, 133. (9) Klein, L. C. Annu. Rev. Mater. Sci. 1985, 15, 227. (10) Lopez, T.; Navarrete, J.; Gomez, R.; Armendariz, H.; Figueras, F. Appl. Catal. 1995, 125, 217. (11) Lopez, T.; Asomoza, M.; Gomez, R. Thermochim. Acta 1993, 223, 233. (12) Lopez, T.; Gomez, R.; Ferrat, G.; Dominguez, J. M.; Schifter, I. Chem. Lett. 1992, 1941.
© 1997 American Chemical Society
Determination of Surface Heterogeneity by TPD
Langmuir, Vol. 13, No. 5, 1997 975
Figure 2. FTIR spectra of adsorbed CO2 in magnesia-silica sulfated oxides: (a) MgSi-(pH 9, 1.3%)-S; (b) MgSi-(pH 9, 4.0%)-S; (c) MgSi-(pH 3, 4.0%)-S; (f) MgSi-(pH 3, 6.7%)-S.
Figure 1. FTIR spectra of adsorbed CO2 after outgassing at 200 °C in magnesia-silica unsulfated oxides: (a) MgSi (pH 9, 1.3%); (b) MgSi (pH 9, 4.0%; (c) MgSi (pH 9, 6.7%); (d) MgSi (pH 3, 1.3%); (e) MgSi (pH 3, 4.0%); (f) MgSi (pH 3, 6.7%). MgSi, pH 3 and pH 9, x% S. The sulfated samples were obtained from the previous two series of magnesia-silica mixed oxides, using a 1 M solution of H2SO4 (5 mL of sulfuric acid per gram of solid). In all cases the H2SO4 solution and the solids were mixed and stirred for 1 h, later the solvent is evaporated and the sulfated samples were dried at 70 °C for 24 h. x ) 1.3, 4.0 and 6.7 mol % magnesia in silica. Mixed Oxides Characterization. The BET surface area was calculated from the adsorption isotherm using nitrogen as adsorbate (Micromerictics Sorptometer). CO2 FTIR Adsorption. Infrared spectroscopy in the 20001300 cm-1 range was performed using a Nicolet 710 spectrophotometer. The sample was pressed into a thin self-supported wafer and placed in the infrared cell. After evacuation at 450 °C for 2 h, the cell was cooled to room temperature. The evacuated samples were then put in contact with CO2 for 30 min. The CO2 excess was eliminated by vacuum at room temperature, and then the FTIR spectra were recorded “in situ” at 200 °C. For the sulfated samples, a reference was recorded before CO2 adsorption. CO2 Thermodesorption (TPD). The number and strength of basic sites were determined by carbon dioxide thermodesorption in an automatic Altamira AMI-3 TPD apparatus. Before CO2 adsorption the sample was heated in He flow at 500 °C and then cooled to room temperature. CO2 was admitted and the temperature was increased at a program rate of 10 °C/min. The amount of desorbed CO2 was quantified from the automatized apparatus. Ammonia Thermodesorption (TPD). The number of the acid centers of the samples was determined by the stepwise thermal desorption of ammonia. The solids were first activated “in situ” in He flow saturated with ammonia at 100 °C and then swept by a flow of dry He while the temperature was raised. The amount of ammonia desorbed by the solid was monitored by conductometry.
Results CO2 FTIR Adsorption. Figure 1 shows the FTIR spectra of CO2 adsorption of the samples at 200 °C. Intense absorption bands can be seen in the 1700-1300 cm-1
range. Four bands can be chosen as the most representative ones. The bands observed at 1632 and 1626 cm-1 are assigned to a bidentade carbonate vibration.13 This carbonate is formed by the interaction of CO2 with the hydroxyls of the sol-gel magnesia-silica materials and is indicative of the presence of strong basic sites. In nonsulfated MgSi (pH 3 and pH 9) samples, the intensity of the bands is a function of the magnesia content. In Figure 1 is observed a peak at 1529 cm-1 with splitting band at 1430 cm-1 due to the unidentate carbonate vibration, which is indicative of the existence of medium strength basic sites.14 Figure 2 shows the substracted spectra at 200 °C of CO2 chemisorption on sulfated samples and outgassed sulfated reference sample. The most representative bands are those at 1626 and 1408 cm-1. The band at 1408 cm-1 is assigned to -SdO stretching vibration.15-17 This band appears as a negative peak since the intensity of the reference sulfated mixed oxide is higher than that observed in the CO2 adsorbed samples. The CO2 desorption regenerates the intensity of the reference sulfated band. This behavior is clear evidence of the interaction between the -SdO species and CO2. CO2 Thermodesorption (TPD). The number and strength of basic sites can be determined by calorimetric methods like CO2 TPD thermodesorption.18 The CO2 is adsorbed on basic sites at least in four different chemisorbed states on MgO-SiO2 mixed oxides.19 Thermal program desorption characterizes some of them. The main feature is a low-temperature peak (weak basic sites) with a maximum around 80 °C and a high-temperature broad peak above 500 °C (strong basic sites), Figures 3 and 4. The basicity of pH 3 preparations (µmol of CO2/m2) calculated from the TPD thermograms is reported in Table 1. The basicity of the acid preparations is in general of the same order for unsulfated and sulfated samples. A (13) Evans, J. V.; Whathely, T. I. Trans. Faraday Soc. 1967, 63, 2789. (14) Zhang, G.; Hattori, H.; Tanabe, K. Appl. Catal. 1988, 36, 189. (15) Saur, O.; Benistel, M.; Saad, A. B. M.; Lavelley, J. C.; Morrow, B. A. J. Catal. 1986, 99, 104. (16) Matsuhashi, H.; Hino, M.; Arata, K. Appl. Catal. 1990, 59, 203. (17) Yamaguchi, T.; Jin, T.; Tanabe, K. J. Phys. Chem. 1986, 90, 3148. (18) Tejuca, L. G.; Bell, A. T.; Cortes Corberan, V. Appl. Surf. Sci. 1989, 37, 353. (19) Schubart, W.; Kno¨zinger, H. Z. Phys. Chem. (Munich) 1985, 144, 130.
976
Langmuir, Vol. 13, No. 5, 1997
Llanos et al. Table 2. Basic Site Distribution of Magnesia-Silica Oxides Prepared at pH 3 basicity [µmol of CO2/g] sample
total
weak (77 °C)
MgSi (pH 3, 1.3%) MgSi (pH 3, 4.0%) MgSi (pH 3, 6.7%) MgSi-(pH 3, 1.3%)-S MgSi-(pH 3, 4.0%)-S MgSi-(pH 3, 6.7%)-S
488 819 1292 481 661 816
316 534 765 302 401 620
medium (227 °C)
strong (450 °C) 172 253 252 179 260 196
32 275
Table 3. Specific Surface Area and Acid and Basic Site Density of Magnesia-Silica Prepared at pH 9
sample
Figure 3. CO2 thermodesorption (TPD) of unsulfated magnesia-silica oxides: (s) silica; (a) MgSi (pH 9, 1.3%); (b) MgSi (pH 9, 4.0%); (c) MgSi (pH 9, 6.7%); (d) MgSi (pH 3, 1.3%); (e) MgSi (pH 3, 4.0%); (f) MgSi (pH 3, 6.7%).
MgSi (pH 9, 1.3%) MgSi (pH 9, 4.0%) MgSi (pH 9, 6.7%) MgSi-(pH 9, 1.3%)-S MgSi-(pH 9, 4.0%)-S MgSi-(pH 9, 6.7%)-S
acid sites basic sites BET area density (µmol density (µmol (m2/g) of NH3/m2) of CO2/m2) 132 136 174 117 86 127
0.99 3.91 2.96 6.33 7.40 4.67
4.40 4.76 4.44 13.27 23.87 26.12
Table 4. Basic Site Distribution of Magnesia-Silica Oxides Prepared at pH 9 basicity [µmol of CO2/g]
Figure 4. CO2 thermodesorption (TPD) of sulfated magnesiasilica system: (s) silica; (a) MgSi (pH 9, 1.3%); (b) MgSi (pH 9, 4.0%); (c) MgSi (pH 9, 6.7%); (d) MgSi (pH 3, 1.3%); (e) MgSi (pH 3, 4.0%); (f) MgSi (pH 3, 6.7%). Table 1. Specific Surface Area and Acid and Basic Sites Density of Magnesia-Silica Prepared at pH 3
sample MgSi (pH 3, 1.3%) MgSi (pH 3, 4.0%) MgSi (pH 3, 6.7%) MgSi-(pH 3, 1.3%)-S MgSi-(pH 3, 4.0%)-S MgSi-(pH 3, 6.7%)-S
acid sites basic sites BET area density (µmol density (µmol (m2/g) of NH3/m2) of CO2/m2) 522 415 441 318 321 317
0.53 1.55 0.91 2.76 4.06 2.47
0.93 1.97 2.93 1.51 2.06 2.57
small effect of the magnesia content in basicity can be observed. The total amount of desorbed CO2 reported in Table 2 is 488-1292 µmol of CO2/g and 481-816 µmol of CO2/g for unsulfated and sulfated samples, respectively. A basic distribution can be seen in which the weak basic sites dominate. On the other hand, for basic preparations it can be seen in Table 3 that the basicity is smaller on unsulfated
sample
total
weak (77 °C)
medium (227 °C)
strong (450 °C)
MgSi (pH 9, 1.3%) MgSi (pH 9, 4.0%) MgSi (pH 9, 6.7%) MgSi-(pH 9, 1.3%)-S MgSi-(pH 9, 4.0%)-S MgSi-(pH 9, 6.7%)-S
580 647 773 1553 2053 3318
432 484 428 1134 1543 1078
64 93 114 254 920
147 99 252 305 256 1320
samples than on sulfated ones. The magnesia content effect can be seen on sulfated samples; the basicity increases as a function of magnesia content. The total of CO2 desorbed reported in Table 4, is in the range of 580773 µmol of CO2/g and 1553-3318 µmol of CO2/g for unsulfated and sulfated samples, respectively. It can be noted that for pH 9 preparations the basic site distribution of weak and strong sites is modified by sulfation. Nitrogen adsorption BET specific surface areas of the various magnesia-silica oxides are listed in Table 1 for pH 3 preparations and in Table 3 for pH 9 preparations. In general the specific surface area is higher for acid preparations. Magnesia is a basic catalyst, nevertheless its applications are limited because of its low specific surface area (usually between 25 and 60 m2/g). Solids showing comparable basicity to magnesium oxide but with high specific surface areas like that reported in Table 1 are promising catalytic materials. NH3 Thermodesorption (TPD). The TPD results are shown in Figures 5 and 6. All TPD profiles have asymmetric shape with broadened peaks, well-known for energetically inhomogeneous surfaces. The low-temperature peak (277-300 °C) corresponds to weak acid centers and the high-temperature peak (400 °C) corresponds to strong acid centers. The acid site density of pH 3 and pH 9 preparations is reported in Table 1 and Table 3, respectively. In general it can be seen in the tables that the acid site density is increased by sulfation. Discussion The specific BET area of the various magnesia-silica mixed oxides obtained by the sol-gel method show that when the hydrolysis was done in acid medium the largest areas were obtained (Table 1). Such behavior can be
Determination of Surface Heterogeneity by TPD
Figure 5. NH3 thermodesorption (TPD) of non-sulfated magnesia-silica system: (s) silica; (a) MgSi (pH 9, 1.3%); (b) MgSi (pH 9, 4.0%); (c) MgSi (pH 9, 6.7%); (d) MgSi (pH 3, 1.3%); (e) MgSi (pH 3, 4.0%); (f) MgSi (pH 3, 6.7%).
Figure 6. NH3 thermodesorption (TPD) of sulfated magnesiasilica system: (s) silica; (a) MgSi (pH 9, 1.3%); (b) MgSi (pH 9, 4.0%); (c) MgSi (pH 9, 6.7%); (d) MgSi (pH 3, 1.3%); (e) MgSi (pH 3, 4.0%); (f) MgSi (pH 3, 6.7%).
understood in terms of the different rate of hydrolysis occurring in acid or basic medium in the first step of the synthesis. For the former case the hydrolysis of TEOS is faster and highly hydroxylated samples are obained, whereas in basic samples the low hydrolysis rate yields low hydroxylated samples of low surface area.20 The hydrolysis of MDE is faster when is compared with TEOS hydrolysis in acid or basic medium.21 Then the main effect (20) Lopez, T. React. Kinet. Catal. Lett. 1992, 46, 45.
Langmuir, Vol. 13, No. 5, 1997 977
in the specific surface area of sol-gel magnesia-silica mixed oxides is given by the TEOS olation/oxolation rates. FTIR spectroscopy of adsorbed molecules is largely reported as an important technique to characterize acid or basic sites on solids. NH3 adsorption is reported for the characterization of acid sites, whereas CO2 adsorption is used to characterize basic sites. The FTIR characterization of basic sites on the magnesia-silica sol-gel insulated mixed oxides is done in Figure 1. The CO2 adsorption bands at 1626-1632 cm-1 assigned to a bidentate carbonate vibration are clearly observed and identify strong basic sites. The FTIR spectra of adsorbed CO2 of Figure 2, show the 1408 cm-1 band assigned to the interaction of sulfate ion and CO2. Thus, by sulfation additional CO2 adsorption is observed by FTIR. Basic sites density of the samples prepared at pH 3 is given in Table 1. In such samples the basicity of unsulfate and sulfate pH 3 oxides is slightly modified. However, in the oxides prepared at pH 9 an important effect of sulfation on the basicity can be seen in Table 3. Mg-O-Si bonds and dispersed MgO particles will be formed durin the cogelation of MDE and TEOS. In acid preparations the TEOS hydrolysis is faster than in basic ones. The MgO-Si bonds suggested, will be then larger in acid preparations. In basic medium the low TEOS hydrolysis rate could induce the formation of MgO such will be found encapsulated by silica. Such dispersed MgO particles could be substracted from the silica during sulfation and then an important increase of the basicity should be observed. The basicity developed by sulfation suggests a magnesia segregation phenomenum induced by the sulfuric acid solution contact. Moreover, silica sol-gel was prepared at pH 9 as a reference and their TPD spectra are shown in Figures 3-5. It can be seen that the surface modifications essentially occur on the mixed oxides. Silica shows small evolution of its basicity by sulfation. The quantitative analysis of the ammonia thermodesorption for pH 3 and pH 9 preparations reported in Tables 1 and 3 let us see that in general the acid site density is smaller on unsulfated oxides than on sulfated ones. The specific area, acidity and basicity of unsulfated magnesia-silica oxides could be considered as a result of the preparation method. The structural and textural properties strongly depend of the alkoxides hydrolysis. However, an important modification of the acidity and basicity of the oxides can be induced by sulfation of the samples. The main effect, important increase of basicity on the sulfated samples, suggests a magnesium oxide segregation effect. The above results show that in the synthesis by the sol-gel method of magnesia-silica oxides, important modifications of the acid-basic properties can be obtained if the samples are treated with sulfuric acid. The extent of the textural modifications can be controlled by varying, by example, the sulfuric acid concentration or the sulfation time. The fine equilibrium between acidity and basicity required on petrochemical applications22 can then reached. Conclusions It is shown that in the preparation of magnesia-silica oxides by the sol-gel method, the hydrolysis of the alkoxides has a strong effect on the surface properties. Hydrochloride acid or ammonium hydroxide used as hydrolysis catalysts produce mixed oxides showing quiet (21) Portillo, R.; Lopez, T.; Gomez, R.; Bokhimi,; Morales, A.; Novaro, O. Langmuir 1996, 12, 40. (22) Corma, A.; Lopez Nieto, J. M.; Paredes, N. Appl. Catal. 1993, 97, 159.
978
Langmuir, Vol. 13, No. 5, 1997
different basic properties. In acid medium are synthesized mixed oxides showing large specific surface area. The characterization of the MgO-SiO2 oxides by CO2 and NH3 chemisorption shows that the acidity and the basicity of the oxides slightly depend of the pH of the alkoxides gelation. By sulfation with sufuric acid the acidity of the mixed oxides is increased. Such results are in agreement with those reported for sulfated silica. However, by
Llanos et al.
sulfation an unexpected increase of the basicity is observed. Such results are explained by the MgO segregation induced during the acid sulfuric impregnation. Acknowledgment. We are indebted to CONACYT, CNRS, and NSF for fnancial support. LA950805Y
