FT-IR Spectroscopy Study of Surface Acidity and 2-Propanol

Langmuir 1997, 13, 2303-2306

2303

FT-IR Spectroscopy Study of Surface Acidity and 2-Propanol Decomposition on Mixed Oxides Obtained upon Calcination of Layered Double Hydroxides Fathi Kooli,† Cristina Martı´n, and Vicente Rives* Departamento de Quı´mica Inorga´ nica, Universidad de Salamanca, Salamanca, Spain Received June 13, 1996. In Final Form: January 17, 1997X

Mixed Ni-Cr-V, Mg-Cr-V, and Ni-Al-V oxides have been prepared by thermal decomposition of layered precursors with the hydrotalcite structure (brucite-like layers containing divalent and trivalent cations, and interlayer anions balancing the extra positive charge of the layers) at 300 or 500 °C. Surface acidity (Bro¨nsted and Lewis) has been assessed by FT-IR monitoring of pyridine adsorption. Ni-Al-V samples are more acidic (Bro¨nsted and Lewis), while those with Cr contain only surface Lewis acid sites. Adsorption of 2-propanol takes place dissociatively, leading to alcoholate that is oxidized to acetone at 300 °C via a Mars-van Krevelen mechanism; such an oxidation is more extensive in the less acidic (Cr-containing) samples. Both the nature of the cations and the previous calcination temperature of the hydrotalcite control the oxidative properties of the mixed oxides.

Introduction

Experimental Section

Decomposition of alcohols on mixed oxides, well via dehydration or via dehydrogenation, is a very interesting process, because of its application to yield alkenes and carbonyl compounds, such as formaldehyde or acetone. The main factors governing the activity and selectivity of these catalysts in the named reaction are textural effects and the acid/base and electronic properties of these solids.1-4 A rather new family of compounds, the so-called hydrotalcite-like materials, are being studied in the last few years5 because of their use as catalysts or as catalyst precursors. As it is well-known, these are compounds with the general formula [MII1-xMIIIx(OH)2](Am-)x/m‚nH2O: the positive charge in excess existing in partially M(II)/ M(III)-substituted brucite-like layers of M(OH)2 is balanced by anions (Am-) located, together with water molecules, in the interlayer space. Upon decomposition, these materials yield mixed oxides MII1-xMIIIxO(2+x)/2, if the interlayer anion (e.g., carbonate) is removed during thermal decomposition. Location of polyoxometalates (e.g., decavanadate) in the interlayers permits synthesis of mixed oxides containing up to three different metal cations. We here report on the use of mixed oxides obtained upon decomposition of Mg-Cr, Ni-Al, and Ni-Cr hydrotalcites, containing decavanadate in the interlayers, for 2-propanol decomposition. The surface acidity properties of these metal oxides have been checked by FT-IR monitoring of pyridine adsorption.

Synthesis of the precursor hydrotalcites has been described elsewhere.6,7 Briefly, carbonate-containing hydrotalcites were synthetized following the method by Reichle,8 and were then used to exchange decavanadate for carbonate at different pH values.6,7 Total carbonate/decavanadate exchange is achieved when the exchange is performed at a pH between 4.5 and 5.5 for the Ni-Al system, below 6.5 for the Mg-Cr system, and above 5.5 for the Ni-Cr system. We here report on samples calcined at 300 or 500 °C. After calcination at 300 °C, the X-ray diffraction diagrams for the samples Ni-Cr-V (27.0% Ni, 12.2% Cr, 19.8% V, weight) and Ni-Al-V (28.7% Ni, 5.8% Al, 22.2% V) show a decrease in the basal spacing, indicating depolymerization of vanadate,9 and also broad peaks due to NiO were detected. However, an amorphous phase was formed from sample MgCr-V (11.6% Mg, 18.9% Cr, 21.7% V). Calcination at 500 °C led to formation of M3V2O8 (M ) Ni or Mg), the crystallinity of which increases with further increasing of the calcination temperature; lower amounts of Ni2V2O7 were also found in samples prepared from Ni-Al-V and Ni-Cr-V hydrotalcites. However, no crystalline Cr-containing phase was detected by X-ray diffraction in any case. The elemental chemical compositions for the samples calcined at 500 °C were 13.5% Mg, 22.1% Cr, and 25.3% V for the sample MgCrV, 33.0% Ni, 6.6% Al, and 25.5% V for the sample NiAlV, and 30.9% Ni, 13.9% Cr, and 22.7% V for the sample NiCrV. Specific surface areas were in the range 58 ( 5 m2 g-1 for samples calcined at 300 °C, decreasing to 16 ( 5 m2 g-1 after calcination at 500 °C. Samples are named as MM′V/T, where M and M′ stand for the cations originally existing in the brucite-like layers and T stands for the calcination temperature, in °C. Adsorption of pyridine or 2-propanol was monitored by FT-IR spectroscopy, using a Perkin-Elmer PC-16 spectrometer, connected to an Ataio 386-SX computer, using special cells with CaF2 windows. The nominal resolution was 2 cm-1, and 100 runs were averaged to improve the signal-to-noise ratio. Samples were submitted to a conditioning treatment in situ, which consisted of outgassing at 400 °C for 2 h (residual pressure 10-3 N m-2) and then equilibrating with a low pressure of gas (pyridine or 2-propanol), the spectrum being recorded after heating at increasing temperatures. In all cases, the spectrum of the solid was subtracted using the facilities provided by the computer software.

* To whom all correspondence concerning this paper should be addressed. Fax.: +34 23 29 45 15. E-mail: [email protected]. † Present address: University of Cambridge, Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, England. X Abstract published in Advance ACS Abstracts, April 1, 1997. (1) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans 1. 1989, 85, 1723. (2) Ai, M.; Suzuki, S. Bull. Chem. Soc. Jpn. 1973, 46, 321. (3) Ramis, G.; Busca, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1591. (4) Martı´n, C.; Martı´n, I.; Rives, V.; Grzybowska, B.; Grassel, I. Spectrochim. Acta Part A 1996, 52, 733. (5) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173.

S0743-7463(96)00583-5 CCC: $14.00

(6) Reichle, W. T. J. Catal. 1985, 94, 547. (7) Kooli, F.; Rives, V.; Ulibarri, M. A. Inorg. Chem. 1995, 34, 5114. (8) Kooli, F.; Rives, V.; Ulibarri, M. A. Inorg. Chem. 1995, 34, 5122. (9) Twu, J.; Dutta, P. K. J. Catal. 1990, 124, 503.

© 1997 American Chemical Society

2304 Langmuir, Vol. 13, No. 8, 1997

Kooli et al.

Figure 1. FT-IR spectra recorded after adsorption of pyridine on the samples (A) NiCrV/300, (B) MgCrV/300, and (C) NiAlV/ 300, outgassed at (a) room temperature, (b) 100 °C, (c) 200 °C, and (d) 300 °C.

Figure 2. FT-IR spectra recorded after adsorption of pyridine on the samples (A) NiCrV/500, (B) MgCrV/500, and (C) NiAlV/ 500, outgassed at (a) room temperature, (b) 100 °C, (c) 200 °C, (d) 300 °C.

Results

1444, and 1440 cm-1. The band at 1592 cm-1 should be ascribed to physisorbed pyridine, as it is removed by mere outgassing at 100 °C, while the bands at 1606 and 1440 cm-1 are due to pyridine adsorbed on surface Lewis acid sites. These last bands are recorded even after outgassing the sample at 300 °C, a treatment also leading to a shift of the first band toward higher wavenumbers. Bands similar to those recorded here have been previously reported for vanadia/magnesia and chromia/magnesia systems, and so they should be ascribed to pyridine coordinated to surface Lewis acid sites, V5+ or Cr3+. The spectrum recorded after adsorption of pyridine at room temperature on the sample calcined at 500 °C, (Figure 2B) is rather similar to that recorded for the same sample but outgassed at 300 °C; however, the bands are better defined, probably due to the improved crystallinity of the phases, as shown by X-ray diffraction.8 In addition, weak bands (absent in the spectrum of the sample calcined at 300 °C) are recorded at 1640 and 1536 cm-1, characteristic of pyridinium ions, thus suggesting development of surface Bro¨nsted acid sites in this sample; however, these sites should be rather weak, as they are removed after outgassing at 100 °C. NiAlV Sample. The bands recorded after adsorption of pyridine on the sample calcined at 300 or 500 °C (Figure 1C) are very well defined and intense; they are recorded at 1614 (shoulder), 1607 ( 1, 1595 ( 2 (weak), 1489, and 1446 cm-1, all of them due to modes 8a and 19b of pyridine coordinated to surface Lewis acid sites, V5+ (band at 1606 cm-1) and Ni2+ (band at 1593 cm-1). Adsorption of pyridine on alumina gives rise to bands (mode 8a) at 1625, 1615, and 1595 cm-1,13 ascribed to pyridine coordinated to tetrahedrally coordinated Al3+ species, tetrahedral species close to cation vacancies, and octahedrally coordinated Al3+ species, respectively. So, the shoulder recorded in our case at 1614 cm-1 should be ascribed to pyridine coordinated to Al3+ sites, together with the weak band at 1593 cm-1, although this band could also be due to pyridine coordinated to Ni2+ species. In this sample, bands due to pyridinium ion, at 1638 and 1540 cm-1, are also recorded, indicating the presence of surface Bro¨nsted acid sites. When the sample is outgassed at increasing temperatures the band at 1593 cm-1 is removed after outgassing at 200 °C, while all other bands are still recorded. 2-Propanol Adsorption. NiCrV Samples. The FTIR spectrum recorded after adsorption of 2-propanol at room temperature on the sample NiCrV/300 (Figure 3A) shows

Surface Acidity. NiCrV Samples. The spectrum recorded after adsorption of py at room temperature on the sample NiCrV/300 is shown in Figure 1A. Bands at 1609, 1592, 1488, 1449, and 1439 cm-1 are ascribed to different vibration modes of py adsorbed on surface Lewis acid sites (coordinatively unsaturated cations, Mcus). The splitting observed for bands due to modes 8a and 19b clearly indicates the presence of different Lewis surface acid sites, probably corresponding to the different cations existing in the surface of the solids, i.e., coordinatively unsaturated Ni2+, Cr3+, and V5+. Adsorption of py on NiO yields an IR spectrum10 with a band similar to that here recorded at 1592 cm-1 that has been ascribed to the Nipy system. The position of the band at 1609 cm-1 is closer to that reported11,12 for systems containing Cr or V. It should be noted that no band due to pyridinium species is recorded; i.e., it seems that surface Brønsted acid sites do not exist in these samples. The strength of the surface Lewis acid sites can be inferred from the outgassing temperature required to remove these bands. So, the band ascribed to py adsorbed on Ni2+ sites is still recorded as a shoulder after outgassing at 200 °C, but it is completely removed after outgassing the sample at 300 °C. These should then be rather medium-strength Lewis acid sites. However, the band ascribed to py coordinated to V5+ or Cr3+ sites (1609 cm-1) is still recorded after outgassing the samples at higher temperatures, indicating that these are strong Lewis acid sites. A stronger interaction of py with surface Lewis acid sites also gives rise to a shift of the position of the band due to mode 8a toward larger wavenumbers. The spectrum recorded after adsorption of py on the sample NiCrV/500 (Figure 2A) is very similar to that for the sample NiCrV/300; however, the bands are more intense and are better defined, mainly the band at 1592 cm-1, which is still detected even after outgassing at 200 °C. These results suggest that calcination at 500 °C somewhat enhances the acceptor ability of Ni2+ species, probably due to the simultaneous presence of other Lewis sites (V5+ and Cr3+). MgCrV Samples. The spectrum recorded for the sample calcined at 300 °C, MgCrV/300, in Figure 1B is rather similar to that previously shown for the sample NiCrV/ 300, with absorption bands at 1606, 1592, 1575, 1487, (10) Busca, G.; Lorenzelli, V.; Sa´nchez-Escribano, V. Chem. Mater. 1992, 4, 595. (11) Miyata, H.; Nakagawa, Y.; Ono, T.; Kubokawa, Y. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2340. (12) Martı´n, C.; Martı´n, I.; Rives, V.; Palmisano, L.; Schiavello, M. J. Catal. 1992, 134, 434.

(13) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. Soc., Farday Trans. 1 1979, 75, 271.

Surface Acidity and 2-Propanol Decomposition

Figure 3. FT-IR spectra recorded after adsorption of 2-propanol on the samples (A) NiCrV/300 and (B) MgCrV/300, heated at (a) room temperature, (b) 100 °C, (c) 200 °C, and (d) 300 °C. Spectra labeled e correspond to the sample outgassed at 300 °C.

absorption bands at 1464 (δasCH3), 1383, 1367 (doublet, δsCH3), and 1330 (δout-of-planeCH) cm-1. All these bands can be assigned to different deformation modes of the 2-propyl group. Other bands are recorded at 1164, 1130, and 1108 cm-1, characteristic of the modes ν(C-C), ν(C-O), and F(CH3), respectively, indicating the formation of isopropoxide species,14 through dissociative adsorption of 2-propanol. Another band at 1286 cm-1 could be due to the δ(O-H) mode of molecular 2-propanol coordinated to surface Lewis acid sites,14 as this mode gives rise to a band at 1252 cm-1 for monomeric 2-propanol. The band at 1626 cm-1, due to the δ(H2O) mode of water molecules, is removed after outgassing at room temperature, and indicates formation of water molecules through condensation of surface hydroxyl groups with 2-propanol. When the sample is heated at increasing temperatures (Figure 3A) new bands develop after heating at 200 °C, at 1714, 1423, 1367, and 1232 cm-1, characteristic of acetone coordinated to surface Lewis acid sites through the carbonyl group. The band at 1714 cm-1 is due to the stretching mode of the coordinated carbonyl group, and the band corresponding to this mode is recorded at 1734 cm-1 for gaseous acetone.15 The shift is due to a decrease in the electron density on the C-O bond when the acetone molecule is adsorbed on surface Lewis acid sites. Such an interaction gives rise also to a shift of the band due to the ν(C-C-C) mode, recorded at 1232 cm-1 in this sample, while for gaseous acetone it is recorded at 1225 cm-1. The other bands mentioned above, at 1423 and 1367 cm-1, are due to deformation modes of the methyl group. When outgassed at 300 °C the bands due to acetone are strengthened, while those due to isopropoxide species become weaker; simultaneously, new bands, probably due to acetate species,16,17 develop at 1540 and 1444 cm-1. Although the spectrum recorded after adsorption of 2-propanol on the sample NiCrV/500 is rather similar to that recorded for the sample NiCrV/300, it is worth noting that bands due to carboxylate species are not recorded. The bands due to acetone, detected after heating at 300 °C, are much weaker and are similar to those described below for the system NiAlV/500 (Figure 4); the band close to 1720 cm-1, due to ν(CO), splits into two bands at 1737 and 1711 cm-1; the first one, however, is removed after heating at 400 °C and is due to physisorbed acetone. (14) Rossi, P. F.; Busca, G.; Lorenzelli, V.; Saur, O.; Lavalley, J. C. Langmuir 1987, 52, 3. (15) Delle Piane, G.; Overend, G. Spectrochim. Acta 1966, 22, 593. (16) Martı´n, C.; Martı´n, I.; Rives, V. J. Mol. Catal. 1992, 73, 51. (17) Griffiths, D. M.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1978, 74, 403.

Langmuir, Vol. 13, No. 8, 1997 2305

Figure 4. FT-IR spectra recorded after adsorption of 2-propanol on the sample NiAlV/300, heated at (a) room temperature and (b) 300 °C.

MgCrV Samples. The bands recorded after adsorption of 2-propanol at room temperature on the sample MgCrV/ 300 (Figure 3B) are due to isopropoxide species, 1465 cm-1 (δa(CH3)), 1383, 1367 (δs(CH3)), and 1165, 1132, and 1109 cm-1 (ν(CsC), ν(CsO), and F(CH3)); an additional band at 1286 cm-1 is due to molecularly adsorbed 2-propanol. Formation of acetone has been observed after raising the temperature to 300 °C, with bands at 1706 cm-1 (ν(CdO)) and 1232 cm-1 (ν(CsCsC)), together with very broad bands at 1626 and 1422 cm-1 due to the δ(H2O) of water and the δa(CH3) of acetone, respectively. Outgassing at this temperature removes all the bands, and two new ones develop (although they could be obscured by the former, very strong and broad bands) at 1539 and 1444 cm-1, due to acetate species.16,17 A spectrum similar to that of the sample NiCrV/500 is displayed by the sample MgCrV/500 after adsorption of 2-propanol; at 200 °C a weak band due to coordinated acetone develops at 1713 cm-1 (ν(CdO)). However, carboxylate species are not detected, and all bands are removed after outgassing the sample at 300 °C. Summarizing, the samples NiCrV/300 and MgCrV/300 show very similar behavior with regard to 2-propanol adsorption, such similarity being also observed with respect to adsorption of pyridine. NiAlV Samples. Adsorption of 2-propanol on these samples gives rise to spectra (Figure 4) rather similar to those previously described, with absorption bands at 1465, 1381, 1367, 1165, 1130, and 1108 cm-1, characteristic of isopropoxide species; the band at 1620 cm-1 is due to δ(H2O), water being formed by condensation of 2-propanol with surface hydroxyl groups. Coordinated acetone (bands at 1719, 1701, 1367, and 1232 cm-1) is detected after heating at 300 °C, with the simultaneous disappearance of bands due to isopropoxide, thus suggesting that acetone is formed through oxidation of isopropoxide species. However, carboxylate species are not detected at any temperature; all species are desorbed after outgassing at 300 °C. Discussion All FT-IR spectra here discussed regarding adsorption of pyridine on the samples studied reveal the presence of surface acid sites. The samples NiAlV show the largest acid site density, as the bands are very intense and well defined, both in the sample calcined at 300 °C and in that calcined at 500 °C. In both cases, surface Lewis sites and Bro¨nsted sites are detected. On the contrary, samples containing Cr are less acidic, especially the samples NiCrV/ 300 and MgCrV/300, where no surface Bro¨nsted acid sites are detected, but only surface Lewis acid sites have been found. However, this lack of Bro¨nsted acidity cannot be

2306 Langmuir, Vol. 13, No. 8, 1997 Scheme 1. Mechanism for 2-Propanol Adsorption and Oxidation to Acetone on the Surface of the Catalysts Studied

Kooli et al. Scheme 2. 2-Propanol Adsorption and Oxidation on the Surface of the Catalysts Studied

straightforwardly related to the presence of Cr, as this element, when supported on other oxides such as titania or silica, gives rise to an increase in acidity, even with simultaneous development of surface Bro¨nsted acid sites.12 All spectra corresponding to adsorption of 2-propanol at room temperature indicate that this molecule adsorbs dissociatively, leading to alcoholate species, coordinated to surface cationic sites. This reaction takes place through condensation with surface hydroxyl groups (reaction 1); as in all cases, formation of water (as indicated by the band close to 1620 cm-1) has been observed:

(CH3)2-CHOH + OH- f (CH3)2-CH-OsL + H2O (1) At 300 °C the alcoholate undergoes an oxidative dehydrogenation, leading to acetone formation. This process takes place without the presence of oxygen in the gas phase, and so it should proceed through reduction of surface cations, following the Mars-van Krevelen mechanism, already known for vanadium-containing catalysts.18 Among the cations existing in the samples here studied, such a process is most probably taking place on V5+ sites, due to its highest electrode potential, which would become reduced to lower oxidation states, while the alcoholate is oxidized (see Scheme 1). Oxidative dehydrogenation is more selective on the samples MgCrV/300 and NiCrV/300; although a precise quantitative analysis cannot be performed by FT-IR spectroscopy, a qualitative study indicates that the acetone bands are five to six times stronger for these samples than for the other. Such a larger selectivity toward acetone (oxidative dehydrogenation) should itself be related to the lower acidity of these samples. It has been shown19,20 that surface acid sites are responsible for dehydration of (18) Ai, M.; Zuzuki, S. Bull. Chem. Soc. Jpn. 1974, 47, 3074. (19) Bond, G. C.; Flamerz, S. Appl. Catal. 1987, 33, 219. (20) Grzybowska-Swierkosz, B. Mater. Chem. Phys. 1987, 17, 121.

2-propanol to propene, while hydrogen abstraction to form acetone via oxidative dehydrogenation takes place on surface basic sites. In addition, it is noteworthy that only in the Crcontaining samples does acetone become further oxidized to acetate species at 300 °C. These acetate species are very stable and are the only species remaining on the surface of the sample after outgassing at 300 °C; so these samples should contain a large concentration of nucleophilic sites. So both the calcination temperature and the nature of the cations originally existing in the brucitelike layers of the parent hydrotalcite affect markedly the surface acidity of these solids, thereof modifying their activity and selectivity during 2-propanol decomposition. According to these results, the surface reactivity of 2-propanol on the samples studied in this work proceeds via Scheme 2. Acknowledgment. The authors are grateful for financial support from CICYT (Grant MAT93-0787) and Castilla-Leo´n (Consejerı´a de Cultura y Turismo, Grant SA56/94). F.K. acknowledges a grant from Ministerio de Educacio´n y Ciencia (Madrid, Spain, ref. SB92-AE0474743). LA960583J