Ga-promoted sulfated zirconia systems. II. Surface features and catalytic activity

Microporous and Mesoporous Materials 94 (2006) 40–49 www.elsevier.com/locate/micromeso

Ga-promoted sulfated zirconia systems. II. Surface features and catalytic activity q G. Cerrato a

a,*

, C. Morterra a, M. Rodrı´guez Delgado b, C. Otero Area´n b, M. Signoretto c, F. Somma c, F. Pinna c

Department of Chemistry IFM, University of Turin, and Consortium INSTM, UdR Turin University, via P. Giuria, 7-10125 Turin, Italy b Department of Chemistry, Universidad de las Islas Baleares, 07122 Palma de Mallorca, Spain c Department of Chemistry, University of Venice, and Consortium INSTM, UdR Venice, Italy Received 15 December 2005; received in revised form 7 March 2006; accepted 8 March 2006 Available online 27 April 2006

Abstract Sulfated zirconia samples having a variable Ga2O3 content (in the 1–15% molar range) were synthesized. The promoting effect of gallium was studied in the catalytic isomerization of n-butane at 523 K, by feeding n-butane and H2 (with a 1:4 ratio). Catalytic activity was found to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga2O3 doubled the activity of sulfated zirconia, whereas for a sample with 15 mol% Ga2O3 the catalytic activity was completely lost. Surface chemistry of these materials was studied by means of FTIR spectroscopy and adsorption microcalorimetry, using selected probe molecules (CO and 2,6-dimethylpyridine). IR spectroscopy showed that gallium-containing sulfated zirconia samples exhibit both Lewis and Brønsted acidity. Lewis acidity is attributed to coordinatively unsaturated Zr4+ ions located in defective surface sites, whereas Brønsted acidity is associated to surface sulfate groups. Samples with a Ga2O3 content between 1 and 9 mol% show a combination of Lewis and Brønsted acidity that quantitatively decreases with increasing gallium oxide content. Samples having a Ga2O3 content equal to 9 mol% or greater show the following specific features: (i) these samples are much more difficult to dehydrate than those with smaller gallium content, and the hydroxy groups interact by hydrogen bonding, (ii) the sulfate groups progressively lose their covalent character, and (iii) both Lewis and Brønsted acidity of the samples decreases drastically. Couples of Lewis and Brønsted acid sites appear to be needed for catalytic activity in n-butane isomerization, and they present an optimum ratio when the catalyst is brought to a medium-high dehydration degree and when its Ga2O3 content is of about 3–5 mol%.  2006 Elsevier Inc. All rights reserved. Keywords: Sulfated zirconia; Gallium oxide promotion; n-butane isomerization; Physico-chemical characterization; Surface acidity

1. Introduction Catalytic systems based on sulfated zirconia (SZ) are of high interest in the petrochemical industry, since these catalysts play an important role in the isomerization of light linear alkanes, such as n-butane [1], n-pentane and n-hexane [2]. Catalysts currently employed in the above mentioned reactions consist of Pt supported on chlorinated q *

Part I: Ref. [9b]. Corresponding author. Tel.: +39 011 6707534; fax: +39 011 6707855. E-mail address: [email protected] (G. Cerrato).

1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.03.018

Al2O3 or similar systems, such as Pt supported on mordenite [3]. However, some of these systems operate at an elevated temperature and require constant addition of alkyl chlorides to recover acid functionalities. Owing to increasingly strict environmental regulations, researchers are paying increasing attention to solid acids in the search for stable and environmentally friendly catalysts. Among solid acids, SZ systems attract considerable interest because they are almost environmentally benign, highly active, and quite selective for the transformation of hydrocarbons. In order to overcome the fast deactivation affecting plain SZ systems, the addition of small amounts of Pt was tested

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

[4], and was found to be of great help in enhancing both activity and stability of SZ. Successively, in order to avoid the use of Pt for economical reasons, promotion with other elements, such as Fe [5], Ni [6] and, more recently, either Al [7,8] or Ga [2,8a,9] was investigated. Aims of the present contribution are: (i) to investigate the physico-chemical aspects of the introduction of Ga2O3 as a promoting agent in SZ systems; (ii) to find out the range of Ga2O3 concentration that leads to the most effective promotion role; and (iii) to see whether a correlation can be found between surface acidity features and promoting effects. 2. Experimental 2.1. Catalyst preparation Ga-promoted SZ systems, termed SZGn (where n stands for the mol% Ga2O3) were prepared by a co-precipitation method, starting from solutions of ZrOCl2 Æ 8H2O (Alfa Aesar, 99.9%) and Ga(NO3)3 Æ xH2O (Alfa Aesar, 99.9%) mixed in the appropriate proportion to obtain nominal n values of 1%, 3%, 5%, 9%, and 15%, after precipitation with an ammonia solution at pH 8 (±0.1), as described in detail elsewhere [9b]. The precipitate was dried at 383 K for 20 h and then sulfated by incipient wetness impregnation with a 1.8 M aqueous solution of (NH4)2SO4 (0.5 ml solution per gram of solid). The amorphous sulfated samples thus obtained were dried at 383 K overnight followed by calcining for 3 h at 923 K, leading to crystallization of amorphous SZ precursors (mostly) in the tetragonal (tZrO2) form, which is known to have enhanced catalytic activity [10]. Table 1 reports relevant data for calcined SZ (reference) and SZGn samples. 2.2. Catalyst characterization The effect of gallium content on catalytic activity of the SZGn samples was studied in the n-butane isomerization reaction, as described elsewhere [9b]. FTIR spectra were obtained at 2 cm 1 resolution with a Bruker spectrophotometer equipped with a MCT criodetector. All powders were inspected in the form of a thin layer deposited on a pure Si platelet, starting from aqueous suspensions. The Si platelet was the same for all samples, so that the geometrical area of samples was constant. Table 1 Specific surface area, Ga2O3 and sulfate content in SZ and SZGn samples Sample

Specific surface area (BET, m2 g 1)

SO24 (wt%)

SO24 (nm 2)

Ga2O3 (mol%)

SZ SZG1 SZG3 SZG5 SZG9 SZG15

98 109 113 112 102 94

3.7 3.9 4.3 4.1 3.9 3.2

2.4 2.3 2.4 2.3 2.4 2.1

– 1.1 3.4 5.6 9.7 15.5

41

The thickness of deposited samples was kept as constant as possible (10 mg cm 2), in order to allow an acceptable normalization of band intensities against a virtually constant sample weight. Home-made quartz infrared cells, equipped with KBr windows, suitable for either beam temperature or low (liquid nitrogen) temperature measurements, were connected to a conventional vacuum line (residual pressure <1.3 · 10 5 mbar) and allowing to perform, in strictly in situ conditions, both thermal treatments on the samples, and probe molecules adsorption/desorption cycles on the activated samples. All IR spectra were recorded at either beam temperature (BT), i.e., the temperature reached by samples in the IR beam (BT is estimated to be some 30 K higher than the actual room temperature [11]), or at approximately liquid nitrogen temperature, i.e., 100 K. XPS measurements were performed with an M-Probe Instrument (SSI) equipped with a monochromatic Al Ka source (1486.6 eV) with a spot size of 200 · 750 lm and a pass energy of 25 eV, providing a resolution of 0.74 eV. The energy scale was calibrated with reference to the 4f7/2 level of a freshly evaporated gold sample, at 84.00 ± 0.1 eV, and with reference to the 2p3/2 and 3s levels of copper at 932.47 ± 0.1 and 122.39 ± 0.15 eV, respectively. With a monochromatic source, an electron flood gun was used to compensate the build up of positive charge on the insulating samples during the analyses: a value of 10 eV was selected to perform measurements on these samples. For all samples, the C1s peak level was taken as internal reference at 284.6 eV. The accuracy of the binding energies is approximately ±0.2 eV. Microcalorimetric measurements were carried out at 303 K using a heat-flow microcalorimeter (standard Tian-Calvet type, Seratam, France) connected to a gas-volumetric apparatus, which enabled the simultaneous determination of evolved heats (integral adsorption heats, qint) and adsorbed amounts (na). The two sets of experimental data, converted into intensive parameters by normalization per unit surface area, led to the calculation of differential molar adsorption heats (qdiff). To check for the possible presence of irreversible CO adsorption processes, the first adsorption run (primary isotherm) of each sample was followed by overnight outgassing at the adsorption temperature and then by a second adsorption run at the same temperature and up to the same final pressure as before (secondary isotherm). Note that the possible occurrence of irreversible adsorption or surface reactions would lead to the non-coincidence of primary and secondary adsorption runs. 3. Results and discussion 3.1. Catalytic activity The effect of gallium content on the catalytic activity for n-butane isomerization at 523 K was investigated for all samples. The profile of activity vs. time on stream is shown in Fig. 1. After an initial decrease, the activity stabilizes

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

12 10

-1

-2

Activity (mmol h m )

Table 2 ˚ layer) of SZ and SZGn Surface atomic composition (external 40–50 A samples as determined from XPS spectroscopy

SZ SZG1 SZG3 SZG5 SZG9 SZG15

14

8 6 4 2 0 0

200

400

600

800

1000

1200

Time (min.) Fig. 1. Catalytic activity vs. time on stream for the reaction of n-butane isomerization at 523 K over SZGn catalysts. Reaction conditions: 0.5 g catalyst, T = 523 K, feed n-C4H10/H2 = 1/4, total flux = 25 ml min 1.

and no further deactivation is observed within 20 h. Steady-state activity was found to be dependent on gallium loading: a promoting effect is clearly evident for Ga2O3 loadings between 1 and 9 mol%. SZGn samples with n = 3 or 5 showed the best conversion output, whereas for the SZG15 sample the catalytic activity is lost. This shows that an appreciable promoting effect is obtained only with relatively low Ga2O3 concentrations. The selectivity to iso-butane was found to be higher than 95%, with only minor amounts of propane and methane being formed. A thermal treatment in air at 723 K restores completely the catalytic activity of all active samples, but the steep initial decrease is still present in a second catalytic run. This suggests that the initial deactivation may be due to the formation of a carbonaceous deposit on a fraction of very active sites, which can be eliminated by thermal treatment in an oxidizing atmosphere. The positive effect of hydrogen on the catalytic activity is probably ascribable to the formation, in the presence of hydrogen, of a more limited amount of carbonaceous deposits. However, it can also be considered that this gas (at the catalytic test temperature) could selectively interact with Ga3+ ions, increasing their catalytic activity.

Atomic ratio

Zr/S

Zr/Ga

Ga/S

SZ SZG1 SZG5 SZG9 SZG15

8.3 8.4 8.2 6.0 3.0

– 50.2 8.6 4.6 2.8

– 0.2 1.0 1.3 1.0

most layers of ZrO2 crystallites tend to loose part of their Zr component and become enriched in the Ga component with increasing gallium loading. When Ga2O3 content is P9 mol%, the formation of an amorphous overlayer coating the ZrO2 particles has been observed [9b]. It is also worth noting that the best catalytic activity towards n-butane isomerization (in mild conditions), which has been observed for SZGn samples containing a mediumlow Ga2O3 amount, seems to be related to: (i) a high and nearly unchanged Zr/S ratio; (ii) a low Ga/S ratio; and (iii) absence of a Ga-rich phase over-coating the highly crystalline materials [9b]. 3.3. FTIR spectroscopy—surface functionalities In situ FTIR spectroscopy indicates that both, intrinsic functionalities (mainly OH groups, absorbing in the 3850– 3000 cm 1 range; Fig. 2) and added functionalities (sulfate

3650 0.5 3760

1

Absorbance

42

2

3 4

3.2. XPS spectroscopy XPS data reported in Table 2 show that, with increasing Ga loading, Zr/Ga ratio decreases much more than expected on the basis of pure stoichiometric figures. Moreover, up to 5% of Ga2O3 content, the Zr/S ratio remains virtually unchanged with respect to the unpromoted SZ reference sample, whereas it decreases fast for Ga2O3 contents P9 mol%, even when the overall SO24 concentration remains almost constant (Table 1), and it is known that in crystalline ZrO2-based systems sulfates are located at the surface [12]. These observations indicate that the outer-

5 6 3800

3600

3400

Wavenumber

3200

(cm-1)

Fig. 2. Absorbance FTIR spectra in the 3850–3000 cm 1 range (mOH stretching region) of SZGn samples activated in vacuo at 673 K. 1, Nonpromoted SZ reference material; 2, SZG1; 3, SZG3; 4, SZG5; 5, SZG9; and 6, SZG15.

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

ν S=O

ν S-O

1390

*

1077

933 910 1015

Absorbance

1

2

3 4

43

sharpness of the free OH bands decline. For the highest Ga2O3 loading (spectrum 6 of Fig. 2), the whole OH stretching range becomes deeply modified: the two IR bands due to free OH species are either absent or hardly resolved at all, whereas a broad and unresolved envelope centred at m < 3600 cm 1 becomes dominant. This spectral behaviour is indicative of the existence of an extended system of non well-defined OH-containing species mutually interacting by H-bonding, in spite of the relatively high vacuum activation temperature to which samples were subjected. It should be noted that, in these conditions (i.e., when n > 9), the SZGn system presents an overall amorphous habit, as shown by structural and morphological data reported elsewhere [9b], and the sulfated ZrO2 particles exhibit a thin amorphous over-layer that, on the basis of the broad mOH band discussed above, should be ascribed to a sort of amorphous hydroxide-like species. A higher structural disorder could thus justify the observed gradual broadening of the IR absorption bands ascribed to m(O–H) stretching modes.

0.5 5 6

1400

1200 Wavenumber

1000 (cm-1)

Fig. 3. Absorbance FTIR spectra in the 1500–850 cm 1 range (mSO stretching region) of SZGn samples activated in vacuo at 673 K. 1, Nonpromoted SZ reference material; 2, SZG1; 3, SZG3; 4, SZG5; 5, SZG9; and 6, SZG15. Broken-line inset: mS–O spectral region of sulfates on a sulfated m-ZrO2 sample.

species, absorbing in the 1500–850 cm 1 range; Fig. 3) are present at the surface of all SZGn samples. It is clear that the presence of Ga does not suppress surface functionalities nor does it introduce new ones, since in both spectral ranges examined the spectral features of SZGn samples look quite similar to those typical of all active SZ-based catalysts [13–15]. However, some differences are noticeable for what concerns the relative intensity of the IR absorption bands of surface functionalities, as reported below. 3.3.1. Intrinsic functionalities All catalysts, activated in a dynamic vacuum at 673 K, were brought to a medium-high dehydration degree, in order to simulate the gas-flow activation step carried out before catalytic tests. When Ga2O3 content is not higher than 9 mol%, it can be noted that the spectral region of surface OH groups (Fig. 2, spectra 1–5) shows a two-bands pattern, with spectral positions and relative intensities typical of all catalytically active SZ-based systems and characteristic of a mainly tetragonal ZrO2 phase [15]. Two H-bond free OH species are present, as revealed by a prominent lowm band (3650 cm 1) due to tri-coordinated OH groups, and a weaker high-m band (3760 cm 1) due to terminal (or mono-coordinated) OH species [15–18]. With increasing Ga2O3 content, the tailing on the low-m side of the free OH bands progressively increases, while both intensity and

3.3.2. Added functionalities The IR spectra of SZGn samples in the 1500–850 cm 1 spectral range are shown in Fig. 3. When gallium oxide content is lower than 9 mol% (Fig. 3, spectra 2–4) the spectra exhibit two main IR absorption bands centred at 1390 cm 1 (relatively sharp and asymmetric, with a tail on the low-m side) and 1015 cm 1 (broader and complex). A large spectral separation characterizes these two wellresolved absorption bands, and this spectral behaviour indicates that the bands are typical of non-coupled mS=O and mS–O vibrational modes of isolated (i.e., with no mutual perturbations) and highly covalent poly-dentate surface sulfates [19,20]. In the case of the unpromoted reference SZ sample (Fig. 3, spectrum 1), the presence of a sharp IR absorption band at 1077 cm 1 and of two weak bands at m < 950 cm 1 clearly indicates the existence of some amount of a monoclinic ZrO2 phase [21] (see, for comparison, the broken-line spectra at the top of Fig. 3, relative to the IR pattern of sulfated m-ZrO2). The absence of these absorption bands in SZGn samples with n > 2 (Fig. 3, spectra 3–6) confirms that one of the main roles played by Ga-doping on SZ systems is the stabilisation of the tetragonal ZrO2 phase in the surface layer, as also suggested by Raman spectra reported elsewhere [9b]. For Ga2O3 loading P9 mol%, the spectral features of surface sulfates become deeply modified (Fig. 3, spectra 5 and 6), even if the overall sulfates loading remains nearly unchanged (Table 1). The bands due to covalent polydentate species are less intense, broader and somewhat less separated in frequency, while new strong, broad and illresolved spectral components, typical of poly-nuclear sulfate species, grow in the 1300–1100 cm 1 range. This aspect is ascribed to the presence of increasing amounts of an amorphous phase coating the particles, as reported above. It can be concluded that the spectral features of the surface functionalities considered (i.e., intrinsic surface

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

OH species and added sulfate groups) are influenced in a non-uniform way by the amount of added Ga2O3. Up to a certain Ga2O3 content, virtually no differences are noted, except that of a stabilisation of the tetragonal ZrO2 phase, whereas for Ga2O3 contents above 5–6 mol% the surface of SZGn samples becomes modified. In order to ascertain whether any relationship exists between the spectral features of surface species and surface activity, a systematic study of both qualitative and quantitative aspects of surface acidity was carried out by IR spectroscopy and adsorption microcalorimetry, as described below.

FTIR spectroscopy of basic probe molecules (CO and 2,6-dimethylpyridine, 2,6-DMP) adsorbed onto Ga-promoted SZ systems has been resorted to in order to test surface acidity. Useful information can thus be obtained about: (i) strong Lewis acidity, as revealed by CO adsorption at BT; (ii) total Lewis acidity and protonic (Brønsted) acidity, as revealed by CO adsorption at 100 K, and by 2,6-DMP adsorption at BT and desorption in mild conditions. 3.4.1. Carbon monoxide adsorption Fig. 4a reports the IR spectra of CO (130 mbar) adsorbed at BT on SZGn systems, activated in vacuo at 673 K. In both reference and Ga-doped systems only one IR absorption band is present, centred at 2197 cm 1, and tailing on the low-m side. If we perform some band fittings on the mCO absorption band centred at 2197 cm 1, and starting from the plain SZ system (curve 1 of Fig. 4b), we can observe the presence of a high-m CO component (2201 cm 1), and a low-m CO component (2194 cm 1). On the basis of this spectral behaviour and of previously reported data [22,23], the 2201 cm 1 band can be ascribed to the C–O stretching mode of carbon monoxide adsorbed on a family of strong Lewis acid sites, represented by coordinatively unsaturated (cus) Zr4+ cations located in crystallographic defective positions, whereas the 2194 cm 1 component can be ascribed to mCO mode of CO molecules interacting with cus Zr4+ present on extended ordered patches of the ZrO2 crystallites. It is important to recall that on t-ZrO2 the presence of surface sulfate species tends to reduce the amount of strong Lewis acid centres and to modify only to a rather limited extent (if at all) the strength of strong Lewis acidity [21,23]. In fact, the strength of Lewis acidity remains virtually unchanged with respect to sulfate-free t-ZrO2 and rather modest if compared, for instance, with that of transition aluminas, which are known to possess strong Lewis acidic sites (and mCO frequencies as high as 2220–2240 cm 1) [24]. In the experimental conditions adopted, no evidence was observed for specific carbonyl-like bands ascribable to CO interacting with strongly acidic cus Ga3+ species, since these species are expected to give a mCO band centred at 2225 cm 1 [25–27]. This indicates that, in spite of a high

a

0.1

1 2 3 4 5 6

6 4 2 0 0

20 40 60 80 100 PCO (Torr)

1 2 Absorbance

3.4. FTIR spectroscopy—surface acidity features

A (Integrated area)

44

2220

2200

2180

3

4 5 6

2160

2194

b

2201

1 4 5 2220

2200

2180

2160 -1

Wavenumber (cm ) Fig. 4. (a) Differential absorbance FTIR spectra (i.e., background spectrum subtracted) in the mCO stretching region of CO adsorbed (pCO = 130 mbar) at BT on SZGn samples activated in vacuo at 673 K. 1, Non-promoted SZ reference material; 2, SZG1; 3, SZG3; 4, SZG5; 5, SZG9; and 6, SZG15. Inset: optical adsorption isotherms (integrated absorbance vs. pCO) corresponding to BT CO uptake on the samples presented in this figure. (b) Differential absorbance FTIR spectra in the mCO stretching region of CO adsorbed (pCO = 130 mbar) at BT on SZGn samples activated in vacuo at 673 K. 1, Non-promoted SZ reference material; 4, SZG5; and 5, SZG9. Individual curves (broken lines) show some of the results of a spectral deconvolution carried out in the 2250– 2150 cm 1 range.

Ga/Zr surface ratio, there is no significant amount of Ga3+ ions having a high coordinative unsaturation. On the contrary, Fig. 4a and b indicate that as Ga loading increases, the (area normalised) intensity of the broad band due to Zr4+–CO adducts declines sharply, loosing preferentially the low-m component. This suggests that a substantial decrease of (at least one type of) coordinatively unsaturated Zr4+ ions having strong Lewis acidity is gradually brought about by Ga loading. Considering the IR optical adsorption isotherms (inset of Fig. 4a) corresponding to the BT interaction of CO with SZ and SZGn samples, it can be further seen that: (i) In the case of the lowest Ga2O3 loading (n = 1, curve 2), the very close similarity of spectroscopic profile with the unpromoted SZ system (curve 1) seems to be confirmed also on a semi-quantitative spectroscopic ground, since the two optical adsorption isotherms are nearly coincident in the whole pCO range

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

explored. However, quantitative messages deriving from purely spectroscopic data must be regarded with caution. (ii) In the case of medium Ga2O3 loadings (n = 3–5, curves 3 and 4), the amount of cus Zr4+ ions still present at the surface is very similar and seems to correspond to some 30–50% of the initial value. (iii) For very high Ga2O3 loadings (n P 9, curves 5 and 6), the amount of Lewis acidic sites substantially decreases (apparently, only some 10–15% of the starting spectral intensity remains). Comparing the above data, relative to the adsorption of CO at BT, with those relative to CO uptake at a low temperature (100 K; some of the spectra are shown in Fig. 5), it can be added that: (i) even at a low temperature there is no evidence for the formation of carbonyl-like species involving strongly acidic Ga3+ centres; (ii) the mCO component at 2190 cm 1, due to weaker Lewis acid sites, is broad and complex (i.e., it comprises several un-resolved components) and is far dominant, meaning that at low temperature many more families of sites can be involved in the adsorption process. However, as the Ga loading increases, this spectral component declines in intensity much faster than the high-m component, confirming the preferential 2166 2199 2191 0.5 1

Absorbance

2

3

2200

2150

Wavenumber

2x

4

2x

5

2100

(cm-1)

Fig. 5. Differential absorbance FTIR spectra (i.e., background spectrum subtracted) in the mCO stretching region of CO adsorbed (pCO in the 1– 60 mbar range) at 100 K on SZGn samples activated in vacuo at 673 K. 1: Non-promoted SZ reference material; 2: SZG1; 3: SZG5; 4: SZG9; and 5: SZG15.

45

elimination of weaker Lewis acid sites. In fact, a weak mCO component at 2200–2205 cm 1 remains visible also on high-loaded systems; (iii) there is a spectral component at 2165 cm 1, ascribable to the H-bonding interaction of CO with surface OH species (energetically possible at low temperature). For Ga2O3 loadings up to 5 mol% this component becomes progressively weaker, indicating that on crystalline ZrO2 surface OH species already involved in H-bonding (see in Fig. 2 the OH band tailing at low frequencies) cannot interact by H-bonding with CO molecules. However, for high Ga loadings (n P 9 mol%), the 2165 cm 1 band becomes much stronger and much broader (extending down to 2140 cm 1). This suggests that in the hydroxide-like phase forming on the ZrO2 particles at high Ga contents, formation of H-bonded complexes is the main form of interaction with adsorbed CO. 3.4.2. 2,6-Dimethylpyridine adsorption The unpromoted SZ system and all SZGn samples were also characterized using 2,6-DMP as a probe molecule for IR spectroscopy; procedures were similar to those described in detail elsewhere [28]. Fig. 6a reports, as an example, some spectra corresponding to the adsorption of 2,6-DMP on the SZG3 sample. Protonic (i.e., Brønsted) acidity, witnessed by a characteristic bands envelope at m > 1620 cm 1 is always present, as expected in the presence of surface sulfates. In fact, it is known that surface sulfation of oxides induces protonic acidity of variable strength [29]. At m < 1620 cm 1, other bands are also evident; in particular, when the spectrum is obtained in the presence of an excess of 2,6-DMP (Fig. 6a, spectrum a), two bands can be observed at 1580 and 1594 cm 1, they are ascribed to physisorbed and H-bonded 2,6-DMP species, respectively [28]. In addition, a medium-strength and broad shoulder is observed at 1610 cm 1 that is ascribable (on the basis of its spectroscopic behaviour) to the 8a mode of Lewis-coordinated 2,6-DMP species interacting with cus Zr4+ cations [30]. Upon BT outgassing of the 2,6-DMP excess (Fig. 6a, spectra b and c) some important features can be seen: (i) the overall intensity of the two-bands complex located at m < 1600 cm 1 is drastically decreased, as expected for weakly bound H-bonded and physisorbed species. As for the band at 1608–1610 cm 1, due to the 8a mode of Lewis-coordinated 2,6-DMP, it becomes now quite evident as a well resolved and complex component; (ii) the broad envelope located at m > 1620 cm 1 (due to Brønsted-bound 2,6-DMP species) turns out to be little affected by outgassing. It remains strong and is better resolved than it was at the beginning, due to selective removal of adjoining components ascribable to vibrational combination modes of the more labile 2,6-DMP species. The latter effect is even more evident when evacuation of the base is carried out at 423 K: spectrum d in Fig. 6a, the bands due to protonated 2,6-DMP remain almost unaltered, whereas also the band due to 2,6-DMP interacting with Lewis acid sites (1610 cm 1) reaches a negligible residual concentration.

46

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

a

b

Lu-L

0.2

1643 1628

1580

0.2

Absorbance

1594

1611

1608

1580

Lu-B

1 2 3 4 5 6

a b

1650

1645

c

1630

d

1600

1550

1650

1600

1550

-1

Wavenumber (cm ) Fig. 6. (a) Differential absorbance FTIR spectra (i.e., background spectrum subtracted) in the range of the 8a–8b ring stretching modes of 2,6-DMP adsorbed/desorbed on SZG3 activated in vacuo at 673 K. Curve a: 2,6-DMP adsorption (2.5 mbar) at BT; curves b and c: after evacuation at BT (1 and 15 min, respectively); curve d: after evacuation at 423 K. (b) Differential absorbance FTIR spectra of 2,6-DMP desorbed at BT (15 min) on SZGn samples activated in vacuo at 673 K. 1: Non-promoted SZ reference material; 2: SZG1; 3: SZG3; 4: SZG5; 5: SZG9; 6: SZG15.

In fact, for several acidic oxide systems, it has been reported that, if 2,6-DMP adsorption/desorption is carried out at a temperature slightly above BT, steric hindrance from the two methyl groups of the 2,6-DMP molecules renders the Lewis-coordinated species far more labile than the Brønsted-bound species [31,23b]. Fig. 6b reports some spectra relative to desorption at BT (15 min) of 2,6-DMP for the unpromoted and all the SZ promoted systems of interest. In these experimental conditions the fractions of physisorbed and H-bonded 2,6-DMP are not present anymore, and some differences among the various samples can be seen. In particular, as Ga2O3 content increases: (i) the overall amount of 2,6-DMP that remains adsorbed at the surface is gradually decreased. (ii) The complex 8a mode of Lewis-coordinated 2,6-DMP species (1610 cm 1) decreases and becomes virtually absent for n P 9 mol%. This is in agreement with the data obtained using CO adsorption at both BT and low temperature (see above). (iii) Protonic (Brønsted) acidity is always present, but when the amount of Ga2O3 is maximum (spectra 6) its residual surface concentration is only marginal, even if the overall OH content in the particles over-layer is very high (as also witnessed by the strong H-bonding interaction with CO at low temperature). Since the catalytic activity of SZGn systems was found to reach a maximum for n = 3–5, the simultaneous presence of Lewis and Brønsted acid sites is likely to be a necessary condition for such an activity, as also proposed for non-promoted SZ catalysts [32]. This suggests that catalytically active Lewis acid sites belong to the family of cus Zr4+ ions characterized by the highest coordinative unsat-

uration and charge-withdrawing power, and that active Brønsted acid sites are associated to surface sulfate groups. 3.5. Adsorption microcalorimetry In order to analyse CO adsorption at ambient temperature on more quantitative grounds, adsorption microcalorimetry was used. This approach demonstrated to be particularly successful in providing information on population and energetics of the sites present at the surface of zirconia-based and other oxide systems [20,33–35]. Carbon monoxide adsorption microcalorimetry was performed at 303 K after activation of the samples at 673 K, i.e. after the same activation treatment used in the case of in situ FTIR measurements. Fig. 7a reports the volumetric adsorption isotherms (na vs. pCO) for the reference SZ and for some SZGn systems. The following can be observed: (i) CO adsorption turned out to be fully reversible, since the curves relative to primary isotherms (solid symbols) and secondary isotherms (open symbols) are closely overlapping. (ii) In the presence of gallium oxide, the overall population of Lewis acid sites active at 303 K becomes rapidly lower the higher the amount of Ga2O3 species: compare curves b–d, for SZGn with n = 1, 3, and 9 mol%, respectively, with curve a for the unpromoted SZ sample. Note, in particular, that these quantitative data indicate that 1% Ga2O3 reduces CO uptake by some 25–30%, whereas IR data

-2

qint (J m )*10

2

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

1.0

1.2 1.0 0.8 0.6 0.4 0.2 0.0

-2

a

0

0.8

na (μ mol m )

c

20 40 60 PCO (Torr)

a

80

b 0.4 c d

0.0 0

20

40

60

80

PCO (Torr) 60

b

50 -1

(iv) Also, on the energetic ground, the process of CO adsorption/desorption is fully reversible (see the inset to Fig. 7a). Moreover, the spectrum of energies involved in CO uptake must be quite broad, since the qint vs. pCO plot increases in the first adsorption step much faster than does the corresponding na plot. This aspect becomes clearer by using qdiff plots, as reported below.

0.6

0.2

qdiff (kJ mol )

47

40 30

a

b

c

d

20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

na (μmol m-2) Fig. 7. (a) Volumetric adsorption isotherms for CO uptake at 303 K on SZGn samples vacuum activated at 673 K. Inset: calorimetric adsorption isotherm for CO uptake at 303 K on SZG3 vacuum activated at 673 K. (b) Differential heats of adsorption as a function of surface coverage for CO uptake at 303 K on SZGn samples vacuum activated at 673 K. Curves a: the reference non-promoted SZ system. Curves b–d: SZGn systems, with n = 1, 3 and 9, respectively. Solid symbols: primary adsorption isotherm; open symbols: secondary adsorption isotherm.

(Fig. 4a) gave for the CO band of SZ and SZG1 virtually unchanged spectral profiles and intensity. By contrast, a reduction of CO uptake by some 50% upon loading 3 mol% Ga2O3, as suggested by IR data in Fig. 4a, is roughly confirmed by quantitative volumetric data. (iii) In the pCO range examined, the completion of the monolayer (h = 1) is not easily attained (especially in the case of SZ and of low-loaded SZGn samples), as indicated by curves still growing at the highest equilibrium pressure reached (100 mbar). This shows that, especially in the case of reference SZ and of low-loaded SZGn samples, a broad range of Lewis acid strength is likely to exist, which is increasingly probed by CO at BT upon increasing the equilibrium pressure.

Fig. 7b reports the differential adsorption heat (qdiff, kJ mol 1) as a function of the amount of adsorbed CO (na, lmol m 2), as obtained from volumetric and calorimetric adsorption isotherms by graphic differentiation. These curves seem to present two different parts: the first section of the curves, corresponding to low coverages, shows a gradual and limited heat-versus-coverage decrease for SZ and SZG1 samples (curves a and b), and a much sharper and large decrease for the samples with 3% and 9% Ga2O3 (curves c and d), respectively. The second region of the curves, that is significantly present only in SZ, SZG1 and (to a minor extent) in SZG3 samples (curves a-c), exhibits a differential adsorption heat that is approximately constant or just slowly decreasing with increasing CO coverage. The presence of two regions in the qdiff curves confirms that the CO adsorption process is heterogeneous. In particular, it indicates that there is a relatively small group of highly energetic species (qdiff values between 50 and 35 kJ mol 1) and another group of less energetic species (qdiff values between 40 and 30 kJ mol 1). The indications coming from FTIR spectroscopy data are thus confirmed. SZGn catalysts exhibit the same type of adsorbing sites as those shown by non-promoted SZ systems (i.e., cus Zr4+ cations), even in the presence of very high Ga loadings. However, the surface population of these sites depends on Ga2O3 loading in a complex way: (i) there is a fraction of highly energetic sites (corresponding to the high mCO IR band) that, although decreasing when Ga loading is increased, remains always present and covers the same energy range regardless of the amount of Ga loading; (ii) a second fraction of lower energy sites (corresponding to the mCO component at lower wavenumbers) nearly disappears for samples with high Ga2O3 loading. 4. Conclusions Sulfated zirconia samples having a variable Ga2O3 content (from 1 to 15% molar) were synthesized, with the aim of studying the promoting effect of gallium in the isomerization of n-butane. FTIR spectroscopy was used to study surface functionalities (OH and sulfate groups) of the samples; the adsorption of carbon monoxide and 2,6-dimethylpyridine on gallium promoted sulfated zirconia materials showed that these catalysts combined Brønsted with Lewis acidity. Lewis acidity was attributed to coordinatively unsaturated Zr4+ ions located in defective crystallographic sites, whereas Brønsted acidity was associated to surface sulfate groups. CO adsorption microcalorimetry

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G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

showed that the total acid strength considerably decreases when Ga2O3 content goes over 5 mol%. All these features correlate with catalytic activity in n-butane isomerization. This catalytic activity was shown to reach a maximum precisely at 3–5% Ga2O3, i.e., for those materials which were shown to possess an adequate Lewis/Brønsted acidity ratio. As a first hypothesis, the promoting role of gallia on SZ can be attributed to stabilisation of the tetragonal crystal phase of zirconia. As shown in the first part of this series [9b], addition of gallia delays crystal growth of zirconia particles, thus favouring formation of the tetragonal polymorph. This effect might be sufficient on its own for promotion; note that promotion with Al species causes a similar effect and generates a positive response on catalytic performance of SZ materials [7]. However, the present study on gallia-promoted SZ suggests that an optimum ratio of Lewis/Brønsted acid sites is also a relevant factor for catalytic activity. Finally, it should be noted that Klose et al. [36] have recently proposed that a main role of the promoter (manganese and iron oxides in their study) in SZ is to enhance the oxidation potential of sulfate (SVI) surface species. These species would facilitate alkane isomerization through an initial step involving oxidative dehydrogenation. Such an effect could also be a further source of promotion in gallia-containing SZ, since previously reported TPR-MS data [9b] show that for moderate Ga loadings reduction of SVI species occurs at a temperature significantly lower than in the absence of gallia. Acknowledgments This research was partly financed with funds from MIUR (Project FIRB 2001, code RBAU01X7PT_001) and from INSTM Consortium (Project PRISMA 2002). M.R.D. thanks the Spanish Ministry of Education for a Ph.D. fellowship. Thanks are also due to Dr. C.L. Bianchi (University of Milan, Italy) for XPS measurements. References [1] (a) X. Song, A. Sayari, Catal. Rev.-Sci. Eng. 38 (1996) 329; (b) G.D. Yadav, J.J. Nair, Microporous Mesoporous Mater. 33 (1999) 1; (c) X. Li, K. Nagaoka, R. Olindo, J.A. Lercher, J. Catal. 238 (2006) 39. [2] (a) V. Parvulescu, S. Coman, V.I. Parvulescu, P. Grange, G. Poncelet, J. Catal. 180 (1998) 66; (b) C.-J. Cao, S. Han, C.-L. Chen, N.-P. Xu, C.-Y. Mou, Catal. Commun. 4 (2003) 511. [3] G.L. Frisckorn, P.J. Kuchar, R.K. Olson, Energy Prog. 8 (1988) 154. [4] (a) K. Ebitani, J. Konishi, H. Hattori, J. Catal. 130 (1991) 257; (b) S.Y. Kim, J.G. Goodwin Jr., S. Hammache, A. Auroux, D. Galloway, J. Catal. 201 (2001) 1; (c) K. Arata, H. Matsuhashi, M. Hino, H. Nakamura, Catal. Today 81 (2003) 17; (d) C. Morterra, G. Cerrato, S. Di Ciero, M. Signoretto, F. Pinna, G. Strukul, J. Catal. 165 (1997) 172; (e) M. Signoretto, P. Chies, F. Pinna, G. Strukul, G. Cerrato, C. Morterra, S. Di Ciero, J. Catal. 167 (1997) 522.

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