- Email: [email protected]
Acid properties of iron oxide catalysts dispersed on silica–zirconia supports with different Zr content
Applied Catalysis A: General 367 (2009) 113–121
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Acid properties of iron oxide catalysts dispersed on silica–zirconia supports with different Zr content A. Gervasini a,*, C. Messi b, D. Flahaut c, C. Guimon c a
Dipartimento di Chimica Fisica ed Elettrochimica & Centro di Eccellenza CIMAINA, Universita` degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy c IPREM-ECP – UMR(CNRS)5654, Universite´ de Pau, 2 Av. P. Angot, 64053 Pau Cedex 09, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2009 Received in revised form 24 July 2009 Accepted 28 July 2009 Available online 5 August 2009
The acid properties of surfaces comprising dispersed iron oxide supported on pure silica and silica– zirconia supports with variable zirconia to silica ratio (ZrO2/SiO2 molar ratio from 0.03 to 0.4) have been studied. The surface acidity of the bare supports and relevant iron oxide covered supports were studied by thermal programmed desorption (TPD) of 2-phenylethylamine (PEA) probe, performed in a thermogravimetric analyzer (TGA). Results from N 1s XPS study of the surfaces saturated with PEA probe complemented the study, permitting the determination of the nature of the acid sites on the different samples. A regularly increasing amount of the number of acid sites and acid strength of the supports was observed increasing the zirconia content on the silica matrix. After the iron oxide support coverage, the number of acid sites of the Fe-samples changed, it increased or decreased compared with that determined on the relevant supports depending on the acidity of the bare support. The average acid strength of the Fe-samples was higher than that of the supports and turned towards a prominent Lewis acidity, in any case. The predominant Lewis nature of all the Fe-catalysts was also confirmed by the results of the reaction of a-pinene oxide (POX) isomerization that gave the corresponding aldehyde with good selectivity. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Silica–zirconia Iron oxide catalysts Surface acidity Thermal programmed desorption XP-spectroscopy
1. Introduction Solid acids are extensively used as catalysts in petroleum chemistry and organic synthesis from many years, they provide several advantages if compared with the liquid or gaseous ones (process safety, easiness of product recover, absence of reactor corrosion) [1–5]. The practical importance of the solid acids in catalysis has caused the dramatic increase of studies on the acid properties of various solids, performed by a large number of different techniques able to determine the nature, the site density, the acid strength and the nature and strength distribution of the acid sites at the surfaces [6–12]. In general terms, a solid acid could be defined as a Lewis acid, when it shows the tendency to create dative bond due to electronic vacancies in its structure (aprotic sites), or as a Brønsted acid, when it is able to release proton (protonic sites). In order to elucidate and control the catalytic actions of solid acids, it is then necessary to distinguish between Brønsted or Lewis acidity [7,13–14], besides all the other properties. Improved knowledge on the structure and properties of the solid acid sites in connection with their catalytic action is of
* Corresponding author. Tel.: +39 02 50314254; fax: +39 02 50314300. E-mail address: [email protected] (A. Gervasini). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.044
primary importance to design and develop highly active and selective acid catalysts for particular reactions [1,5]. Among the solid acids, siliceous mixed oxides (silica–alumina, silica–zirconia, etc.) have been found of real interest because of their easy preparation, possibility to tune the surface properties by controlling the composition, and the good acidic properties for various reactions of applicative interest [1,15–19]. In general, these oxides are considered to possess acid sites of both Brønsted and Lewis type, with the majority of acid silanol groups on their surface compared with the minor amount of Lewis species associated to the electronic deficient metal species. The acid properties of these solids can completely change when a second metal oxide is deposited over them. The acid nature of the surfaces can turn towards a prevalent Lewis acidity with modification of acid strength, too. This is the case of the class of dispersed metal oxides (CuOx, FeOx, MoOx, etc.) over ceramic supports that is well known and extensively used in the catalytic scenario [18,20–26]. Highly dispersed iron(III) oxide displays high acid property and plays an important catalytic role in several heterogeneous reactions, in particular those requiring strong Lewis acid sites, such as isomerizations, Friedel–Crafts reactions, etc. [4,27,28]. The Lewis acidity of nanosized Fe oxide clusters is expected to arise from the high degree of Fe-coordinative unsaturation, besides the electronic effects played by metal–support interaction [22,29,30].
114
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
The support nature plays an important role on the catalyst acidity not only promoting the Lewis acidity of the supported phase but also by its intrinsic acidity. The acidity of the support can add to that of the supported phase thus leading to bifunctional acid catalysts. By a proper choice of the support, it is possible to tune the resultant acidity of catalysts with significant following on the activity-selectivity pattern of the catalyzed reaction. The aim of the present study is to characterize the acidic properties of a series of Fe2O3 based catalytic surfaces supported over silica and modified silicas with variable zirconia content (SixZr1 xO2, 0.715 < x < 1, corresponding to ZrO2 amount from 5 to 45 mass%). On each support, a Fe molecular-complex was grafted giving rise, after calcination, to the final oxide catalysts (4 mass% < Fe2O3 < 7 mass%) [29]. The acid properties of the so obtained catalysts are expected to depend on the support acidity, directly linked to the amount of zirconia at the surface, and on the Fe-supported phase. By modifying the support composition, the number of acid sites, the acid strength, and the acid site nature, in particular the Brønsted to Lewis site ratio, are expected to vary. The study of the acid properties of the supports and Fe-catalysts was carried out by the use of a basic molecular probe (2phenylethylamine, PEA) which permitted the quantitative analysis of the surface acidity by XPS spectroscopy, for the nature, and the determination of amount and strength of the acid sites by thermal desorption (TPD). Moreover, the acid properties of the Fe-catalysts have been compared with the results obtained in the catalytic isomerization of a-pinene oxide (POX) that was chosen as it is a recognized test for Lewis acidity [31] and for its relevance to environmental chemistry [32–34]. Many positive effects are expected to come from a judicious development of such class of supported Fe-catalysts. 2. Experimental 2.1. Materials and characterizations Iron catalysts were prepared by an adsorption equilibrium method from Fe(III)-acetylacetonate precursor that was adsorbed on a pure silica (S) and on a series of silica–zirconia oxides (SZ) (containing from 5 to 45 mass% of ZrO2) prepared by sol–gel route. Details on the support and catalyst preparation and characterization have been presented elsewhere [29]. 2.2. Acidity determination 2.2.1. Thermodesorption of basic probe (PEA-TPD) The thermogravimetric analyses of 2-phenylethylamine (PEA) desorption from the saturated powders (PEA-TPD) were performed in a TGA 7 PerkinElmer thermal analyzer. The calibration of temperature was performed by measuring the Curie transitions (TC) of high-purity reference materials (alumel, nickel, perkalloy, and iron, TC of 163, 354, 596, and 780 8C, respectively) at the same heating rate (b) employed during the TPD analysis. Prior to the TPD analysis, the powder sample was outgassed for 16 h at 350 8C under a residual pressure of 10 3 mbar. The activated powder was then transferred in a glass cell equipped with connections for vacuum/gas line and liquid PEA (purity > 99% from Fluka) was introduced in the cell up to complete covering of the powder. The slurry rested at room temperature under N2 flowing for 3 h. Then, the excess of nonadsorbed PEA was removed by filtration, the operation was carried out under N2. The obtained saturated powder was loaded on the pan of the TGA (6–10 mg) and a two-step analysis was carried out under flowing N2 at 30 ml min 1. The first isothermal step was carried out at 50 8C to remove the excess of adsorbed PEA from the surface and it lasted up to the achievement of a constant
mass. The second step was carried out at constant heating rate (b) of 10 8C min 1 from 50 to 800 8C, to completely remove PEA from the surface. To identify the product evolved from PEA desorption, a series of isothermal desorption experiments of PEA from selected support and catalyst samples saturated with PEA, as above reported, in the 350–700 8C temperature range were realized, maintained the temperature for 30 min. All the products evolved from the sample were trapped in a vessel maintained at 70 8C by an acetone/dry ice bath, then 3 ml of heptane were introduced in the vessel and the organic mixture was analyzed by GC/MS (Agilent 5975 Series MSD, coupled with Agilent 7890 GC mounting a HP-5 (5%-phenyl)-methylpolysiloxane column). The chromatographic analysis started at 60 8C (for 3 min), then temperature increased at 2.5 8C min 1 to 90 8C (for 1 min), and then it increased at 15 8C min 1 to 280 8C (for 12 min). The solid sample after isothermal desorption at given temperature was flushed out with heptane and the collected organic mixture was analyzed too by GC/MS. On selected samples saturated with PEA, other types of experiments of PEA desorption in the TGA apparatus were realized. A series of isothermal desorption runs were performed at defined temperatures (each one carried out on fresh portions of PEA saturated sample) starting from the lowest, 200 8C, to the higher ones (300, 400 and 500 8C), and maintaining each temperature for 2 h. After each isothermal run, PEA-TPD analysis was carried out as described above. The amount of the sample acid sites was determined by quantifying the total mass loss of PEA during the PEA-TPD run (that took place in the TGA apparatus). By subtracting the final sample mass, determined at 800 8C, from the mass at the end of the isothermal step, 50 8C, the total moles of PEA desorbed could be determined. Assuming a 1:1 stoichiometry for the PEA adsorption on the acid site, the total number of sites could be known and expressed as equivalent of acid site per unit mass (mequiv g 1) [17]. 2.2.2. X-ray photoelectron spectroscopy of adsorbed probe (PEA-XPS) XPS analysis was performed at room temperature with an SSI 301 spectrometer using monochromatic and focused (spot diameter = 600 mm, 100 W) Al Ka radiation (1486.6 eV) under a residual pressure of 10 9 mbar (ultra-high vacuum, UHV, conditions). Charge effects were compensated by the use of a flood gun (5 eV). The hemispherical analyzer worked at constant pass energy of 50 eV for high resolution spectra and of 150 eV for quantitative analyses. The experimental bands were fitted to theoretical bands (80% Gaussian, 20% Lorentzian) with the leastsquares algorithm using a non-linear baseline. Quantitative analyses were performed using the appropriate Scofield factors [35]. Reference binding energy (BE) was Si 2p (103.5 eV, SiO2) in agreement with the literature. XPS investigations was performed on the sample saturated with PEA (same conditions as described in Section 2.2.1); the saturated samples were desorbed during 1 h under helium at 80 8C and then transferred into the introduction chamber of the spectrometer via a glove box. 3. Results and discussion 3.1. Thermodesorption of basic probe (PEA-TPD) The composition and main properties of the SixZr1 xO2 oxides used as supports is presented in Table 1. All the oxides are amorphous, high surface areas materials. XPS analysis confirmed the unique presence of Si4+ and Zr4+ species with regular surface enrichment of Zr with the increasing of zirconium content in the oxide sample [29]. The surface zirconia enrichment guaranteed the
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
115
Table 1 Properties of the silica–zirconias supports including number and nature of acid sites determined by PEA-TPD and PEA-XPS. Supports
S SZ-5 SZ-15 SZ-30 SZ-45 a b c d
Composition
Surfacea
ZrO2 (mass%)
Zr/(Zr + Si)
– 5 15 30 45
0 0.003 0.041 0.155 0.223
S.A. (m2 g
1
)
367 235 330 470 431
Total acid sitesb (mequiv g 1)
Nature and amount of acid sitesc
0.432 0.451 0.921 0.979 1.354
0.410 0.406 0.829 0.734 0.812
Lewis (mequiv g (95)d (90) (90) (75) (60)
1c
)
Brønsted (mequiv g 0.022 0.045 0.092 0.245 0.542
(5)d (10) (10) (25) (40)
N 1s binding energies 1c
)
Lewis (eV)
Brønsted (eV)
399.5 399.8 399.9 400.1 400.4
401.3 401.5 401.6 401.8 402.4
From XPS analysis [29], atomic ratio. Evaluated from PEA thermodesorption profiles at 200 8C. Evaluated from PEA-XPS. Percent of Lewis and Brønsted acid sites population.
desired development of even more acidic oxide surfaces, from silica to the highest content zirconium sample (45 mass% of ZrO2). The acid properties of the silica–zirconia mixed samples are the typical feature of these materials. Several hypotheses on the acidity generation of binary oxides have been proposed in the literature [36–38]. According to Tanabe’s hypothesis [36], the acidity generation in any binary oxide composition is caused by an excess of negative or positive charge that has been formed. In the model structure of silica–zirconia (Scheme 1), in which silica is always the major component, Brønsted acidity is assumed to appear upon the presence of an excess of negative charge following the two Tanabe’s postulates: (i) the coordination number of the positive elements of the two metal oxides are maintained even when they mixed; (ii) the coordination number of oxygen of the major component is retained for all the oxygens in the binary composition. It is then expected that the acidity of the SixZr1 xO2 samples increased with increasing zirconia concentration in the composition. The acidity of all the samples (supports and Fe-catalysts) was quantitatively determined by PEA thermal desorption (TPD) from the saturated surfaces with PEA, working in a thermogravimetric apparatus. PEA was selected as base probe for acidity studies due to its high molecular mass which permitted to accurately evaluate its desorption when working in a TGA apparatus. Questions arise about PEA stability in the temperature range used in the TPD experiments. Because of the low vapor pressure of PEA, it was not possible to make experiments of PEA-TPD on line with a mass detector analyzer to detect the evolution of the different products. Then, isothermal desorptions of PEA in the temperature range from 350 to 700 8C were performed on selected samples (both oxide supports ands Fe-catalysts), collecting and analyzing all the products evolved from the sample (see Section 2.2.1 for the experimental details). All the obtained results, at any desorption temperature, converged to a same conclusion, that is, PEA was the main product desorbed accompanied by 1–2% of styrene, while only traces of other compounds (e.g., benzonitrile, benzylamine, benzylnitrile, benzene ethanamine N-phenyl methylene) could be observed. Fig. 1 depicts the GC-MSD profiles of the products obtained from desorption of PEA, performed at different temperatures, from the Fe/S sample, as a representative example. The profiles indicate that styrene was present in constant amount
Scheme 1. Model structures following the Tanabe’s hypothesis for the acidity generation in mixed silica–zirconia.
among the desorbed products and that its amount was very low in any case in comparison with the desorbed PEA. Only PEA was present on the solid after desorption (for temperatures lower than 550 8C), as confirmed by the GC-MSD analyses of the hetpane mixture after flushing out the sample. Based on our observations, we consider that the thermodesorption curves of PEA obtained by TGA could be interpreted as PEA evolution from the sample surfaces, disregarding the little PEA decomposition. The curves of thermodesorption of PEA (from 200 to 800 8C) for the five oxide supports are depicted in Fig. 2 (left axis), together with the differential distribution of PEA desorbed as a function of temperature. The amount of PEA still present at 200 8C (i.e., residual PEA) on the different samples regularly increased from S to SZ-45 reflecting the increasing amount of total amount of acid sites of the surface. In Table 1, the total amount of the determined acid sites per unit mass has been reported; an increase of acid site number of about three-time was quantified, from 0.432 to 1.354 mequiv g 1 passing from SiO2 (S) to Si0.715Zr0.285O2 (SZ45). Not only the amount of sites but also the acid strength was modified with increasing the zirconia presence in the samples. The PEA thermodesorption curves showed a continuous decrease with temperature (from 200 to 800 8C), attaining the PEA complete removal at 800 8C, in any case. The loss of PEA from the samples as a function of temperature followed different shapes, reflecting a different distribution of the acid sites strength among the different samples. On S and SZ-5, PEA quite completely desorbed at ca. 300 8C, while on SZ-15, SZ-30, and SZ-45, PEA remained still adsorbed on the sample surfaces up to 500 8C or more. All these information indicated that both the amount and strength of the acid sites increased on the oxide surfaces with zirconia concentration. The close relation between the amount of the total acid sites determined on the sample surfaces by PEA-TPD and the surface amount of zirconia, determined by XPS analysis in our previous work [29], is shown in Fig. 3. The acid site distribution of each surface directly obtained from the thermodesorption profiles is indicated in the bar-chart of Fig. 2. On the SixZr1 xO2 oxide samples, iron was deposited on each sample by the so called adsorption equilibrium method [39], from the Fe–acetylacetonate molecular-complex precursor, which guaranteed good dispersion and distribution of the metal phase on the surfaces with final formation of nanosized Fe oxide species, as recently shown by Gervasini et al. [29,40,41]. The Fe oxide concentration, calculated as Fe2O3, was not the same on all the catalysts, but it ranged in 4–7 mass% (Table 2). This because the amount of adsorbed Fe-complex from each oxide support depended on its characteristics that directed not only the amount but also the aggregation state of the metal phase: Fe3+ isolated species, 3d-, and 2d-Fe oxide nanoparticles. Independently of the Fe-concentration and nature, amorphous character of all the samples was observed. Concerning the acid properties of the Fecatalysts, a complex situation was expected because the presence of uncovered support areas (consisting of siloxanes and silanols
116
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
Fig. 1. Results of the GC-MSD analysis of (A) styrene (STY) plus PEA reference mixture in heptane (STY/PEA molar ratio = 1.09 and STY/PEA chromatographic area ratio = 3.20) and of the products formed from PEA thermal desorption from Fe/S; (B) from desorption at 350 8C for 30 min (STY/PEA chromatographic area ratio = 0.050); (C) from desorption at 550 8C for 30 min (STY/PEA chromatographic area ratio = 0.036); (D) from desorption at 700 8C for 30 min (STY/PEA chromatographic area ratio = 0.035). Trace compounds are indicated: a, benzonitrile; b, solvent impurity; c, benzene ethanamine N-phenyl methylene.
belonging to silica or zirconia) and areas covered by Fe oxide, typically possessing Lewis acidity due to the high degree of Fecoordinative unsaturation. The change of the acid nature from the prevalent Brønsted acidity of the siliceous supports to the prevalent Lewis acidity of the Fe-catalysts has been already observed for other metal oxide supported systems [20,40,41]. The results of the determination of the acid sites of the Fecatalysts by PEA thermodesorption are shown in Fig. 4. The results show both the residual PEA amount as a function of temperature and the differential distribution of the amount of PEA desorbed for a given interval of temperature. As a general trend, the total amount of acid sites of the Fe-samples determined by PEA had a sharp increase with the amount of zirconia in the supports, indicating a significant contribution of the support acidity that added up to that of the Fe-phase (Table 2 and Scheme 2). Furthermore, the PEA thermodesorption profiles showed that PEA was present on the Fe-surfaces up to 700–800 8C, indicating high acid strength of the surfaces. Because each one of the Fe-catalysts was prepared on a support of different composition, any simple comparison among the Fecatalysts could not be sound. At first, it is interesting to compare the amount of acid sites on each support–catalyst couple (Tables 1 and 2). For the sake of clarity, Fig. 5 displays the comparison among the amounts of total acid sites for each couple of samples. It clearly emerges that the Fe-deposition on the least acidic support (S) led to an acidity gain of the catalyst surface while a clearly decrease of acidity was observed following Fe-deposition on the most acidic supports (SZ-5, SZ-15, SZ-30, and SZ-45). The smaller acidity of the Fe-catalysts compared with that of their supports is due to the coverage of support areas containing a large number of acid sites by the Fe-phase possessing lower surface area (i.e., lower number of acid sites per unit surface). The observed acidity difference between the supports and relevant Fe-catalysts does not only concern the amount of acid sites but also the acid strength. To get an insight into the mean acid
strength of the surfaces, supplementary experiments on selected support–catalyst couples were performed (S and Fe/S and SZ-30 and Fe/SZ-30). The more significant results are reported in Fig. 6 for the S, Fe/S couple and in Fig. 7 for the SZ-30, Fe/SZ-30 couple. Isothermal treatments of the PEA saturated surfaces were performed at various temperatures (from 200 to 500 8C), followed by complete PEA removal by thermal programmed desorption. The amount of PEA isothermally removed and the shape of the desorption profiles obtained indicated the presence of stronger acid sites on the Fe-surfaces than on the relevant supports. The isothermal treatment of the PEA saturated surfaces at increasing temperatures removed even more amount of basic probe from the surfaces. For silica (S, Fig. 6), the residual amount of PEA on the surface after treatment at 200, 300, 400, and 500 8C was of 50%, 40%, 30%, and 22%, respectively, of the total PEA adsorbed (0.432 mequiv g 1, Table 1). On Fe/S (Fig. 6) following the same thermal treatment, the observed residual amount of PEA was of 75%, 66%, 55%, and 35% of the total PEA adsorbed (0.480 mequiv g 1, Table 2). The same evidence was even more clearly visible comparing the acid strength of SZ-30 and Fe/SZ-30 (Fig. 7). For thermal treatment at 200, 300, 400, and 500 8C, the residual amount of PEA on SZ-30 was of 51%, 35%, 13%, and 10%, respectively, of the total PEA adsorbed (0.979 mequiv g 1, Table 1) and on Fe/SZ-30 was of 78%, 70%, 53%, and 40%, respectively, of the total PEA adsorbed (0.642 mequiv g 1, Table 2). 3.2. X-ray photoelectron spectroscopy of adsorbed probe (PEA-XPS) Because XPS is a specific spectrometry for surface studies, beyond the classical analysis of the superficial chemical composition, it can be applied to the study of surface reactivity of solids by using a variety of adsorbed probe molecules [7,20,42–45]. In the present study, XPS investigation of the saturated PEA surfaces was used in complementary way to the TPD approach to determine the Brønsted or Lewis nature of
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
117
Fig. 2. PEA thermodesorption profiles (left axis) and relevant differential distribution of PEA desorbed at each 50 8C interval (right axis) as a function of temperature for all the oxide supports.
Fig. 3. Relation between the amount of acid sites of the supports, determined by PEA desorption, and the amount of surface Zr atomic concentration on each support, determined by XPS [29].
the acid sites. In the case of a nitrogen containing basic probe, the N 1s binding energy of the nitrogen atom is a function of its neighborhood. When the base reacts with a Brønsted site to form a cation (ammonium ion in the present case), the value of BE (N 1s) is generally between 401.5 and 402.8 eV. When it forms a complex with a Lewis site, BE is in the 399–401.5 eV range depending on the chosen base and the resulting chargetransfer which is related to the acid strength of the site. PEA is a strong base that can be retained by weak, medium, and strong acid sites. XPS measurements of adsorbed probe molecules provide information only on the strong and medium strength acid sites and do not titrate the weakest sites, which cannot retain the probe under the ultra-high vacuum conditions (10 9 mbar) of the analyses. The XP spectra of all PEA saturated samples showed broad N 1s bands of BE in the range of 399.5–402.4 eV which could be decomposed into two main peak components (Table 2; Figs. 8 and 9), which indicate the presence of Brønsted (B) and Lewis (L) sites. In all the cases, the sites of Lewis able to retain the basic probe in UHV are in a large majority.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
118
Table 2 Properties of the Fe-catalysts including number and nature of acid sites determined by PEA-TPD and PEA-XPS. Catalysts
Fe/S Fe/SZ-5 Fe/SZ-15 Fe/SZ-30 Fe/SZ-45 a b c d
Surfacea
Composition ZrO2 (mass%)
Fe2O3 (mass%)
Fe/(Zr + Si)
– 4.7 14.2 28.1 43.1
7.3 5.9 5.6 6.4 4.3
0.121 0.142 0.048 0.078 0.087
S.A. (m2 g
239 237 232 298 237
1
)
Total acid sitesb (mequiv g 1)
0.480 0.444 0.591 0.642 0.857
Nature and amount of acid sitesc
N 1s binding energies
Lewis (mequiv g
Lewis (eV)
Brønsted (eV)
400.4 400.4 400.2 400.6 400.5
402.3 402.1 402.1 402.5 402.1
0.480 0.377 0.591 0.578 0.771
1
)
(100)d (85) (100) (90) (90)
Brønsted (mequiv g
1
– 0.067 (15)d – 0.064 (10) 0.086 (10)
)
From XPS analysis [29], atomic ratio. Evaluated from PEA thermodesorption profiles at 200 8C. Evaluated from PEA-XPS. Percent of Lewis and Brønsted acid sites population.
As concerning supports, silica surface showed little or none Brønsted sites covered by the basic probe, which well indicates the weakness of the acidity of the silanol groups. The amount and proportion of the Brønsted sites increased with increasing Zr concentration in the silica, as it could be clearly observed in particular on the SZ-30 and SZ-45 samples (Table 2). In parallel, the binding energy (BE) N 1s values increased with increasing Zr concentration due to a higher charge-transfer. Those results
confirm a strengthening of the Lewis and Brønsted acidity with Zr content, as observed for TPD measurements. The nature of the acid sites turned towards the predominant presence of Lewis acid sites when Fe was deposited on the supports. The N 1s binding energies in Fe/SZ catalyst series were generally higher than those in SZ supports. In the same time, the surface concentration (not reported here) of the basic probes was always larger on Fe/SZ than on SZ series, in which the weakest acid
Fig. 4. PEA thermodesorption profiles (left axis) and relevant differential distribution of PEA desorbed at each 50 8C interval (right axis) as a function of temperature for all the Fe-catalysts.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
119
sites do not retain the basic probes. This clearly indicates an increase of the acidity strength for the Fe-catalysts. Furthermore, the highest DBE of the N 1s contribution was obtained between Fe/ S and S samples, for both Lewis and Brønsted sites, which could confirm that the highest acidity gain was obtained for Fe/S catalyst. These observations confirmed the higher mean acid strength of the Fe-surfaces than the relevant supports, as already observed by PEA-TPD measurements. 3.3. Acidic properties and catalytic activity
Fig. 5. Total acid sites (evaluated from the PEA thermodesorption profiles at 200 8C) of the supports in comparison with those of Fe-catalysts.
All the catalysts proved to be very active in a-pinene oxide isomerization at room temperature and the product distribution found was in agreement with the Lewis acidic character of these materials [29]. It is known that the acid-isomerization of POX gives rise to a large variety of products because of the high substrate reactivity [32]. Among the observed products, campholenic
Scheme 2. Model of the 2-phenylethylamine (PEA) adsorption on surface acid sites of catalysts.
Fig. 6. PEA thermodesorption profiles for S support (left) and Fe/S catalyst (right) after isothermal PEA removal (120 min) in the 200–500 8C range.
Fig. 7. PEA thermodesorption profiles for SZ-30 support (left) and Fe/SZ-30 catalyst (right) after isothermal PEA removal (120 min) in the 200–500 8C range.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
120
Fig. 8. N 1s XP spectrum after PEA adsorption and desorption at 80 8C on the SZ-45 support.
distribution during catalytic POX isomerization can give information on the presence of Lewis or Brønsted acid sites. For this reason, this reaction is becoming a recognized test for Lewis acidity [31– 34]. The obtained results in terms of product distribution from POX isomerization over the Fe-catalysts showed that as more pure was the Lewis acidity of the surface (i.e., Fe/S) as higher the selectivity to CPA observed (Table 3). By increasing the Brønsted acidity of the Fe-samples (Fe/SZ-30 and Fe/SZ-45, in particular), a marked increase of substrate conversion and a lowering of selectivity to CPA were observed. The obtained results are in agreement with those obtained from the integrated PEA-XPS and PEA-TPD approach used for the sample acidity property determination. The Fe-dispersed phase on poor acidic supports (i.e., S and SZ-5) created quite pure Lewis acid catalysts due to formation of unsaturated Fe oxide nanosized clusters while on more acidic supports (i.e., SZ-30 and SZ-45), the Lewis acidity of the Fe-dispersed phase summed up to the Brønsted acidity of support creating catalytic systems with a given ratio of Brønsted to Lewis acid sites. 4. Concluding remarks
Fig. 9. N 1s XP spectrum after PEA adsorption and desorption at 80 8C on the Fe/SZ45 catalyst.
aldehyde (CPA) represents an important intermediate for the manufacturing of sandalwood fragrances, currently being investigated together with macrocyclic musks, as possible substitute for nitro and polycyclic musk. The polycyclic musks are thought to be less environmentally damaging than nitro musks but they are also persistent, bio-accumulative and toxic to reproduction. For this reasons, the importance of sandalwood fragrances has been growing in the last decade [33,34]. CPA can be prepared in high yield by using a suitable Lewis acid, while Brønsted acids tend to produce different products (trans-carveol TCV, trans-sobrerol TSB, and dimerization products). Therefore, monitoring the product Table 3 Product distribution obtained in the a-pinene oxide (POX) isomerizationa over the Fe-catalysts [31]. Catalysta
tb(min)
Selectivity (mol%) c
Fe/S Fe/SZ-5 Fe/SZ-15 Fe/SZ-30 Fe/SZ-45
35 5 5 5 5
d
e
f
CPA
PCP
TCV
TSB
72 65 61 56 57
3 1 2 1 4
8 8 9 8 7
4 7 7 6 5
CPA productivityc (mmolprod/gcat h) 8 53 50 44 45
a Catalyst (100 mg), POX (100 mg), toluene (8 mL), RT, reaction mixture analysed by GC-FID analysis (Agilent 6890 (5%Phenyl)95% methylpolysiloxane column, injection T = 120 8C). b Time required to obtain POX complete conversion. c CPA, campholenic aldehyde. d PCP, pinocamphone. e TCV, trans-carveol. f TSB, trans-sobrerol.
The aim of this work was to elucidate the acid properties of complex series of samples comprising siliceous oxides with varying concentrations of zirconia in comparison with the analogous sample series prepared dispersing Fe oxide on each oxide, acting as support. The predominant Lewis acidity of the Fesamples was evidenced by XPS spectroscopy which confirmed the TPD evidences on the higher mean acid strength of the Fe-surfaces than those of the support oxides. Furthermore, this work indicates that catalytic materials composed of well dispersed Fe oxide on acidic supports can be developed with a judicious ratio between Brønsted and Lewis acid sites, leading to catalysts with acid sites that could work in cooperation in many reactions of applicative importance. Acknowledgment This work has been partially supported by the Italian Research Ministry (FIRST project 2007). A.G and C.M. would like to thank Bruno Cortese, student at the Universita` degli Studi di Milano, for his assistance in several experiments. Authors are also indebted with Dr. Vladimiro Dal Santo, ISTM-CNR, Milano, for his assistance during GC-MSD measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
A. Corma, Chem. Rev. 95 (1995) 559–614. K. Tanabe, W. Ho¨lderich, Appl. Catal. A: Gen. 181 (1999) 399–434. H.-U. Blaser, M. Studer, Appl. Catal. A: Gen. 189 (1999) 191–204. G. Sartori, R. Maggi, Chem. Rev. 106 (2006) 1077–1104. G. Busca, Chem. Rev. 107 (2007) 5366–5410. W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615–635. M. Johansson, K. Klier, Top. Catal. 4 (1997) 99–108. A. Auroux, Top. Catal. 4 (1997) 71–89. A. Auroux, Top. Catal. 19 (2002) 205–213. C. Sun, J.C. Berg, Adv. Colloid Interface Sci. 105 (2003) 151–175. M.C. Kung, H.H. Kung, Catal. Rev. Sci. Eng. 27 (1985) 425–460. H.G. Karge, V. Dondur, J. Phys. Chem. 94 (1990) 765–772. G. Busca, Phys. Chem. Chem. Phys. 1 (1999) 723–736. A. Zecchina, C. Lamberti, S. Bordiga, Catal. Today 41 (1998) 169–177. K. Hashimoto, T. Masuda, H. Ueda, N. Kitano, Appl. Catal. 22 (1986) 147–156. J.C. Yori, J.C. Luy, J.M. Parera, Appl. Catal. 41 (1988) 1–11. P. Carniti, A. Gervasini, S. Bennici, J. Phys. Chem. B 109 (2005) 1528–1536. S. Bennici, A. Gervasini, N. Ravasio, F. Zaccheria, J. Phys. Chem. B 197 (2003) 5168– 5176. [19] K. Tanabe, M. Misono, Y. Ono, H. Hattori, in: B. Delmon, J.T. Yates (Eds.), New Solid Acids and Bases, vol. 51, Elsevier, Amsterdam, 1989. [20] A. Auroux, A. Gervasini, C. Guimon, J. Phys. Chem. B 103 (1999) 7195–7205. [21] A. Gervasini, M. Manzoli, G. Martra, A. Ponti, N. Ravasio, L. Bordelli, F. Zaccheria, J. Phys. Chem. B 110 (2006) 7851–7861.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121 [22] A. Gervasini, C. Messi, P. Carniti, A. Ponti, N. Ravasio, F. Zaccheria, J. Catal. 262 (2009) 224–234. [23] T. Herranz, S. Rojas, F.J. Pe´rez-Alonso, M. Ojeda, P. Terreros, J.L.G. Fierro, Appl. Catal A: Gen. 308 (2006) 19–30. [24] J.D. Henao, B. Wen, W.M.H. Sachtler, J. Phys. Chem. B 109 (2005) 2055. [25] S. Rajagopal, J.A. Marzari, R. Miranda, J. Catal. 151 (1995) 192–203. [26] Y. Liu, X. Ma, S. Wang, J. Gong, Appl. Catal. B Environ. 77 (2007) 125–134. [27] Y. Sun, S. Walspurger, J.-P. Tessonnier, B. Louis, J. Sommer, Appl. Catal. A: Gen. 300 (2006) 1–7. [28] H. He, S. Bao, Q. Xu, Appl. Catal. A: Gen. 169 (1998) 29–36. [29] A. Gervasini, C. Messi, A. Ponti, S. Cenedese, N. Ravasio, J. Phys. Chem. C 112 (2008) 4635–4642. [30] K. Chen, Y. Fan, Q. Yan, J. Catal. 167 (1997) 573–575. [31] L. Alaerts, E. Se´guin, H. Poelman, F. Thibault-Starzyk, P.A. Jacobs, D.E. De Vos, Chem. Eur. J. 12 (2006) 7353. [32] W.F. Holderich, J. Roseler, G. Heitmann, A.T. Liebens, Catal. Today 37 (1997) 353– 366. [33] N. Ravasio, F. Zaccheria, A. Gervasini, C. Messi, Catal. Commun. 9 (2008) 1125–1127.
121
[34] G. Neri, G. Rizzo, S. Galvagno, G. Loiacono, A. Donato, M.G. Musolino, R. Pietropaolo, E. Rombi, Appl. Catal. A: Gen. 274 (2004) 243–251. [35] J.H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 129–137. [36] K. Tanabe, in: J.R. Anderson, M. Boudart (Eds.), Catalysis, Science and Technology, vol. 8, Springer, Berlin, 1987, pp. 232–271. [37] T. Seiyama, Metal Oxides and Their Catalytic Actions, Kodansha, Tokyo, 1978. [38] A. Gervasini, G. Bellussi, J. Fenyvesi, A. Auroux, J. Phys. Chem. 99 (1995) 5117– 5125. [39] J.A. Schwarz, Chem. Rev. 95 (1995) 477–510. [40] A. Gervasini, P. Carniti, S. Bennici, C. Messi, Chem. Mater. 19 (2007) 1319–1328. [41] A. Gervasini, S. Bennici, A. Auroux, C. Guimon, Appl. Catal. A: Gen. 331 (2007) 129– 137. [42] R. Borade, A. Sayari, A. Adnot, S. Kaliaguine, J. Phys. Chem. 94 (1990) 5989–5994. [43] C. Guimon, A. Gervasini, A. Auroux, J. Phys. Chem. B 105 (2001) 10316–10325. [44] C. Guimon, H. Martinez, in: S.G. Pandalai (Ed.), Recent Research Developments in Catalysis, vol. 2, Research Signpost, Kerala, India, 2003, pp. 99–120. [45] L. Dussault, J.C. Dupin, E. Dumitriu, A. Auroux, C. Guimon, Thermochim. Acta 434 (2005) 93–99.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Acid properties of iron oxide catalysts dispersed on silica–zirconia supports with different Zr content A. Gervasini a,*, C. Messi b, D. Flahaut c, C. Guimon c a
Dipartimento di Chimica Fisica ed Elettrochimica & Centro di Eccellenza CIMAINA, Universita` degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy Dipartimento di Chimica Fisica ed Elettrochimica, Universita` degli Studi di Milano, via C. Golgi 19, 20133 Milano, Italy c IPREM-ECP – UMR(CNRS)5654, Universite´ de Pau, 2 Av. P. Angot, 64053 Pau Cedex 09, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 June 2009 Received in revised form 24 July 2009 Accepted 28 July 2009 Available online 5 August 2009
The acid properties of surfaces comprising dispersed iron oxide supported on pure silica and silica– zirconia supports with variable zirconia to silica ratio (ZrO2/SiO2 molar ratio from 0.03 to 0.4) have been studied. The surface acidity of the bare supports and relevant iron oxide covered supports were studied by thermal programmed desorption (TPD) of 2-phenylethylamine (PEA) probe, performed in a thermogravimetric analyzer (TGA). Results from N 1s XPS study of the surfaces saturated with PEA probe complemented the study, permitting the determination of the nature of the acid sites on the different samples. A regularly increasing amount of the number of acid sites and acid strength of the supports was observed increasing the zirconia content on the silica matrix. After the iron oxide support coverage, the number of acid sites of the Fe-samples changed, it increased or decreased compared with that determined on the relevant supports depending on the acidity of the bare support. The average acid strength of the Fe-samples was higher than that of the supports and turned towards a prominent Lewis acidity, in any case. The predominant Lewis nature of all the Fe-catalysts was also confirmed by the results of the reaction of a-pinene oxide (POX) isomerization that gave the corresponding aldehyde with good selectivity. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Silica–zirconia Iron oxide catalysts Surface acidity Thermal programmed desorption XP-spectroscopy
1. Introduction Solid acids are extensively used as catalysts in petroleum chemistry and organic synthesis from many years, they provide several advantages if compared with the liquid or gaseous ones (process safety, easiness of product recover, absence of reactor corrosion) [1–5]. The practical importance of the solid acids in catalysis has caused the dramatic increase of studies on the acid properties of various solids, performed by a large number of different techniques able to determine the nature, the site density, the acid strength and the nature and strength distribution of the acid sites at the surfaces [6–12]. In general terms, a solid acid could be defined as a Lewis acid, when it shows the tendency to create dative bond due to electronic vacancies in its structure (aprotic sites), or as a Brønsted acid, when it is able to release proton (protonic sites). In order to elucidate and control the catalytic actions of solid acids, it is then necessary to distinguish between Brønsted or Lewis acidity [7,13–14], besides all the other properties. Improved knowledge on the structure and properties of the solid acid sites in connection with their catalytic action is of
* Corresponding author. Tel.: +39 02 50314254; fax: +39 02 50314300. E-mail address: [email protected] (A. Gervasini). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.07.044
primary importance to design and develop highly active and selective acid catalysts for particular reactions [1,5]. Among the solid acids, siliceous mixed oxides (silica–alumina, silica–zirconia, etc.) have been found of real interest because of their easy preparation, possibility to tune the surface properties by controlling the composition, and the good acidic properties for various reactions of applicative interest [1,15–19]. In general, these oxides are considered to possess acid sites of both Brønsted and Lewis type, with the majority of acid silanol groups on their surface compared with the minor amount of Lewis species associated to the electronic deficient metal species. The acid properties of these solids can completely change when a second metal oxide is deposited over them. The acid nature of the surfaces can turn towards a prevalent Lewis acidity with modification of acid strength, too. This is the case of the class of dispersed metal oxides (CuOx, FeOx, MoOx, etc.) over ceramic supports that is well known and extensively used in the catalytic scenario [18,20–26]. Highly dispersed iron(III) oxide displays high acid property and plays an important catalytic role in several heterogeneous reactions, in particular those requiring strong Lewis acid sites, such as isomerizations, Friedel–Crafts reactions, etc. [4,27,28]. The Lewis acidity of nanosized Fe oxide clusters is expected to arise from the high degree of Fe-coordinative unsaturation, besides the electronic effects played by metal–support interaction [22,29,30].
114
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
The support nature plays an important role on the catalyst acidity not only promoting the Lewis acidity of the supported phase but also by its intrinsic acidity. The acidity of the support can add to that of the supported phase thus leading to bifunctional acid catalysts. By a proper choice of the support, it is possible to tune the resultant acidity of catalysts with significant following on the activity-selectivity pattern of the catalyzed reaction. The aim of the present study is to characterize the acidic properties of a series of Fe2O3 based catalytic surfaces supported over silica and modified silicas with variable zirconia content (SixZr1 xO2, 0.715 < x < 1, corresponding to ZrO2 amount from 5 to 45 mass%). On each support, a Fe molecular-complex was grafted giving rise, after calcination, to the final oxide catalysts (4 mass% < Fe2O3 < 7 mass%) [29]. The acid properties of the so obtained catalysts are expected to depend on the support acidity, directly linked to the amount of zirconia at the surface, and on the Fe-supported phase. By modifying the support composition, the number of acid sites, the acid strength, and the acid site nature, in particular the Brønsted to Lewis site ratio, are expected to vary. The study of the acid properties of the supports and Fe-catalysts was carried out by the use of a basic molecular probe (2phenylethylamine, PEA) which permitted the quantitative analysis of the surface acidity by XPS spectroscopy, for the nature, and the determination of amount and strength of the acid sites by thermal desorption (TPD). Moreover, the acid properties of the Fe-catalysts have been compared with the results obtained in the catalytic isomerization of a-pinene oxide (POX) that was chosen as it is a recognized test for Lewis acidity [31] and for its relevance to environmental chemistry [32–34]. Many positive effects are expected to come from a judicious development of such class of supported Fe-catalysts. 2. Experimental 2.1. Materials and characterizations Iron catalysts were prepared by an adsorption equilibrium method from Fe(III)-acetylacetonate precursor that was adsorbed on a pure silica (S) and on a series of silica–zirconia oxides (SZ) (containing from 5 to 45 mass% of ZrO2) prepared by sol–gel route. Details on the support and catalyst preparation and characterization have been presented elsewhere [29]. 2.2. Acidity determination 2.2.1. Thermodesorption of basic probe (PEA-TPD) The thermogravimetric analyses of 2-phenylethylamine (PEA) desorption from the saturated powders (PEA-TPD) were performed in a TGA 7 PerkinElmer thermal analyzer. The calibration of temperature was performed by measuring the Curie transitions (TC) of high-purity reference materials (alumel, nickel, perkalloy, and iron, TC of 163, 354, 596, and 780 8C, respectively) at the same heating rate (b) employed during the TPD analysis. Prior to the TPD analysis, the powder sample was outgassed for 16 h at 350 8C under a residual pressure of 10 3 mbar. The activated powder was then transferred in a glass cell equipped with connections for vacuum/gas line and liquid PEA (purity > 99% from Fluka) was introduced in the cell up to complete covering of the powder. The slurry rested at room temperature under N2 flowing for 3 h. Then, the excess of nonadsorbed PEA was removed by filtration, the operation was carried out under N2. The obtained saturated powder was loaded on the pan of the TGA (6–10 mg) and a two-step analysis was carried out under flowing N2 at 30 ml min 1. The first isothermal step was carried out at 50 8C to remove the excess of adsorbed PEA from the surface and it lasted up to the achievement of a constant
mass. The second step was carried out at constant heating rate (b) of 10 8C min 1 from 50 to 800 8C, to completely remove PEA from the surface. To identify the product evolved from PEA desorption, a series of isothermal desorption experiments of PEA from selected support and catalyst samples saturated with PEA, as above reported, in the 350–700 8C temperature range were realized, maintained the temperature for 30 min. All the products evolved from the sample were trapped in a vessel maintained at 70 8C by an acetone/dry ice bath, then 3 ml of heptane were introduced in the vessel and the organic mixture was analyzed by GC/MS (Agilent 5975 Series MSD, coupled with Agilent 7890 GC mounting a HP-5 (5%-phenyl)-methylpolysiloxane column). The chromatographic analysis started at 60 8C (for 3 min), then temperature increased at 2.5 8C min 1 to 90 8C (for 1 min), and then it increased at 15 8C min 1 to 280 8C (for 12 min). The solid sample after isothermal desorption at given temperature was flushed out with heptane and the collected organic mixture was analyzed too by GC/MS. On selected samples saturated with PEA, other types of experiments of PEA desorption in the TGA apparatus were realized. A series of isothermal desorption runs were performed at defined temperatures (each one carried out on fresh portions of PEA saturated sample) starting from the lowest, 200 8C, to the higher ones (300, 400 and 500 8C), and maintaining each temperature for 2 h. After each isothermal run, PEA-TPD analysis was carried out as described above. The amount of the sample acid sites was determined by quantifying the total mass loss of PEA during the PEA-TPD run (that took place in the TGA apparatus). By subtracting the final sample mass, determined at 800 8C, from the mass at the end of the isothermal step, 50 8C, the total moles of PEA desorbed could be determined. Assuming a 1:1 stoichiometry for the PEA adsorption on the acid site, the total number of sites could be known and expressed as equivalent of acid site per unit mass (mequiv g 1) [17]. 2.2.2. X-ray photoelectron spectroscopy of adsorbed probe (PEA-XPS) XPS analysis was performed at room temperature with an SSI 301 spectrometer using monochromatic and focused (spot diameter = 600 mm, 100 W) Al Ka radiation (1486.6 eV) under a residual pressure of 10 9 mbar (ultra-high vacuum, UHV, conditions). Charge effects were compensated by the use of a flood gun (5 eV). The hemispherical analyzer worked at constant pass energy of 50 eV for high resolution spectra and of 150 eV for quantitative analyses. The experimental bands were fitted to theoretical bands (80% Gaussian, 20% Lorentzian) with the leastsquares algorithm using a non-linear baseline. Quantitative analyses were performed using the appropriate Scofield factors [35]. Reference binding energy (BE) was Si 2p (103.5 eV, SiO2) in agreement with the literature. XPS investigations was performed on the sample saturated with PEA (same conditions as described in Section 2.2.1); the saturated samples were desorbed during 1 h under helium at 80 8C and then transferred into the introduction chamber of the spectrometer via a glove box. 3. Results and discussion 3.1. Thermodesorption of basic probe (PEA-TPD) The composition and main properties of the SixZr1 xO2 oxides used as supports is presented in Table 1. All the oxides are amorphous, high surface areas materials. XPS analysis confirmed the unique presence of Si4+ and Zr4+ species with regular surface enrichment of Zr with the increasing of zirconium content in the oxide sample [29]. The surface zirconia enrichment guaranteed the
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
115
Table 1 Properties of the silica–zirconias supports including number and nature of acid sites determined by PEA-TPD and PEA-XPS. Supports
S SZ-5 SZ-15 SZ-30 SZ-45 a b c d
Composition
Surfacea
ZrO2 (mass%)
Zr/(Zr + Si)
– 5 15 30 45
0 0.003 0.041 0.155 0.223
S.A. (m2 g
1
)
367 235 330 470 431
Total acid sitesb (mequiv g 1)
Nature and amount of acid sitesc
0.432 0.451 0.921 0.979 1.354
0.410 0.406 0.829 0.734 0.812
Lewis (mequiv g (95)d (90) (90) (75) (60)
1c
)
Brønsted (mequiv g 0.022 0.045 0.092 0.245 0.542
(5)d (10) (10) (25) (40)
N 1s binding energies 1c
)
Lewis (eV)
Brønsted (eV)
399.5 399.8 399.9 400.1 400.4
401.3 401.5 401.6 401.8 402.4
From XPS analysis [29], atomic ratio. Evaluated from PEA thermodesorption profiles at 200 8C. Evaluated from PEA-XPS. Percent of Lewis and Brønsted acid sites population.
desired development of even more acidic oxide surfaces, from silica to the highest content zirconium sample (45 mass% of ZrO2). The acid properties of the silica–zirconia mixed samples are the typical feature of these materials. Several hypotheses on the acidity generation of binary oxides have been proposed in the literature [36–38]. According to Tanabe’s hypothesis [36], the acidity generation in any binary oxide composition is caused by an excess of negative or positive charge that has been formed. In the model structure of silica–zirconia (Scheme 1), in which silica is always the major component, Brønsted acidity is assumed to appear upon the presence of an excess of negative charge following the two Tanabe’s postulates: (i) the coordination number of the positive elements of the two metal oxides are maintained even when they mixed; (ii) the coordination number of oxygen of the major component is retained for all the oxygens in the binary composition. It is then expected that the acidity of the SixZr1 xO2 samples increased with increasing zirconia concentration in the composition. The acidity of all the samples (supports and Fe-catalysts) was quantitatively determined by PEA thermal desorption (TPD) from the saturated surfaces with PEA, working in a thermogravimetric apparatus. PEA was selected as base probe for acidity studies due to its high molecular mass which permitted to accurately evaluate its desorption when working in a TGA apparatus. Questions arise about PEA stability in the temperature range used in the TPD experiments. Because of the low vapor pressure of PEA, it was not possible to make experiments of PEA-TPD on line with a mass detector analyzer to detect the evolution of the different products. Then, isothermal desorptions of PEA in the temperature range from 350 to 700 8C were performed on selected samples (both oxide supports ands Fe-catalysts), collecting and analyzing all the products evolved from the sample (see Section 2.2.1 for the experimental details). All the obtained results, at any desorption temperature, converged to a same conclusion, that is, PEA was the main product desorbed accompanied by 1–2% of styrene, while only traces of other compounds (e.g., benzonitrile, benzylamine, benzylnitrile, benzene ethanamine N-phenyl methylene) could be observed. Fig. 1 depicts the GC-MSD profiles of the products obtained from desorption of PEA, performed at different temperatures, from the Fe/S sample, as a representative example. The profiles indicate that styrene was present in constant amount
Scheme 1. Model structures following the Tanabe’s hypothesis for the acidity generation in mixed silica–zirconia.
among the desorbed products and that its amount was very low in any case in comparison with the desorbed PEA. Only PEA was present on the solid after desorption (for temperatures lower than 550 8C), as confirmed by the GC-MSD analyses of the hetpane mixture after flushing out the sample. Based on our observations, we consider that the thermodesorption curves of PEA obtained by TGA could be interpreted as PEA evolution from the sample surfaces, disregarding the little PEA decomposition. The curves of thermodesorption of PEA (from 200 to 800 8C) for the five oxide supports are depicted in Fig. 2 (left axis), together with the differential distribution of PEA desorbed as a function of temperature. The amount of PEA still present at 200 8C (i.e., residual PEA) on the different samples regularly increased from S to SZ-45 reflecting the increasing amount of total amount of acid sites of the surface. In Table 1, the total amount of the determined acid sites per unit mass has been reported; an increase of acid site number of about three-time was quantified, from 0.432 to 1.354 mequiv g 1 passing from SiO2 (S) to Si0.715Zr0.285O2 (SZ45). Not only the amount of sites but also the acid strength was modified with increasing the zirconia presence in the samples. The PEA thermodesorption curves showed a continuous decrease with temperature (from 200 to 800 8C), attaining the PEA complete removal at 800 8C, in any case. The loss of PEA from the samples as a function of temperature followed different shapes, reflecting a different distribution of the acid sites strength among the different samples. On S and SZ-5, PEA quite completely desorbed at ca. 300 8C, while on SZ-15, SZ-30, and SZ-45, PEA remained still adsorbed on the sample surfaces up to 500 8C or more. All these information indicated that both the amount and strength of the acid sites increased on the oxide surfaces with zirconia concentration. The close relation between the amount of the total acid sites determined on the sample surfaces by PEA-TPD and the surface amount of zirconia, determined by XPS analysis in our previous work [29], is shown in Fig. 3. The acid site distribution of each surface directly obtained from the thermodesorption profiles is indicated in the bar-chart of Fig. 2. On the SixZr1 xO2 oxide samples, iron was deposited on each sample by the so called adsorption equilibrium method [39], from the Fe–acetylacetonate molecular-complex precursor, which guaranteed good dispersion and distribution of the metal phase on the surfaces with final formation of nanosized Fe oxide species, as recently shown by Gervasini et al. [29,40,41]. The Fe oxide concentration, calculated as Fe2O3, was not the same on all the catalysts, but it ranged in 4–7 mass% (Table 2). This because the amount of adsorbed Fe-complex from each oxide support depended on its characteristics that directed not only the amount but also the aggregation state of the metal phase: Fe3+ isolated species, 3d-, and 2d-Fe oxide nanoparticles. Independently of the Fe-concentration and nature, amorphous character of all the samples was observed. Concerning the acid properties of the Fecatalysts, a complex situation was expected because the presence of uncovered support areas (consisting of siloxanes and silanols
116
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
Fig. 1. Results of the GC-MSD analysis of (A) styrene (STY) plus PEA reference mixture in heptane (STY/PEA molar ratio = 1.09 and STY/PEA chromatographic area ratio = 3.20) and of the products formed from PEA thermal desorption from Fe/S; (B) from desorption at 350 8C for 30 min (STY/PEA chromatographic area ratio = 0.050); (C) from desorption at 550 8C for 30 min (STY/PEA chromatographic area ratio = 0.036); (D) from desorption at 700 8C for 30 min (STY/PEA chromatographic area ratio = 0.035). Trace compounds are indicated: a, benzonitrile; b, solvent impurity; c, benzene ethanamine N-phenyl methylene.
belonging to silica or zirconia) and areas covered by Fe oxide, typically possessing Lewis acidity due to the high degree of Fecoordinative unsaturation. The change of the acid nature from the prevalent Brønsted acidity of the siliceous supports to the prevalent Lewis acidity of the Fe-catalysts has been already observed for other metal oxide supported systems [20,40,41]. The results of the determination of the acid sites of the Fecatalysts by PEA thermodesorption are shown in Fig. 4. The results show both the residual PEA amount as a function of temperature and the differential distribution of the amount of PEA desorbed for a given interval of temperature. As a general trend, the total amount of acid sites of the Fe-samples determined by PEA had a sharp increase with the amount of zirconia in the supports, indicating a significant contribution of the support acidity that added up to that of the Fe-phase (Table 2 and Scheme 2). Furthermore, the PEA thermodesorption profiles showed that PEA was present on the Fe-surfaces up to 700–800 8C, indicating high acid strength of the surfaces. Because each one of the Fe-catalysts was prepared on a support of different composition, any simple comparison among the Fecatalysts could not be sound. At first, it is interesting to compare the amount of acid sites on each support–catalyst couple (Tables 1 and 2). For the sake of clarity, Fig. 5 displays the comparison among the amounts of total acid sites for each couple of samples. It clearly emerges that the Fe-deposition on the least acidic support (S) led to an acidity gain of the catalyst surface while a clearly decrease of acidity was observed following Fe-deposition on the most acidic supports (SZ-5, SZ-15, SZ-30, and SZ-45). The smaller acidity of the Fe-catalysts compared with that of their supports is due to the coverage of support areas containing a large number of acid sites by the Fe-phase possessing lower surface area (i.e., lower number of acid sites per unit surface). The observed acidity difference between the supports and relevant Fe-catalysts does not only concern the amount of acid sites but also the acid strength. To get an insight into the mean acid
strength of the surfaces, supplementary experiments on selected support–catalyst couples were performed (S and Fe/S and SZ-30 and Fe/SZ-30). The more significant results are reported in Fig. 6 for the S, Fe/S couple and in Fig. 7 for the SZ-30, Fe/SZ-30 couple. Isothermal treatments of the PEA saturated surfaces were performed at various temperatures (from 200 to 500 8C), followed by complete PEA removal by thermal programmed desorption. The amount of PEA isothermally removed and the shape of the desorption profiles obtained indicated the presence of stronger acid sites on the Fe-surfaces than on the relevant supports. The isothermal treatment of the PEA saturated surfaces at increasing temperatures removed even more amount of basic probe from the surfaces. For silica (S, Fig. 6), the residual amount of PEA on the surface after treatment at 200, 300, 400, and 500 8C was of 50%, 40%, 30%, and 22%, respectively, of the total PEA adsorbed (0.432 mequiv g 1, Table 1). On Fe/S (Fig. 6) following the same thermal treatment, the observed residual amount of PEA was of 75%, 66%, 55%, and 35% of the total PEA adsorbed (0.480 mequiv g 1, Table 2). The same evidence was even more clearly visible comparing the acid strength of SZ-30 and Fe/SZ-30 (Fig. 7). For thermal treatment at 200, 300, 400, and 500 8C, the residual amount of PEA on SZ-30 was of 51%, 35%, 13%, and 10%, respectively, of the total PEA adsorbed (0.979 mequiv g 1, Table 1) and on Fe/SZ-30 was of 78%, 70%, 53%, and 40%, respectively, of the total PEA adsorbed (0.642 mequiv g 1, Table 2). 3.2. X-ray photoelectron spectroscopy of adsorbed probe (PEA-XPS) Because XPS is a specific spectrometry for surface studies, beyond the classical analysis of the superficial chemical composition, it can be applied to the study of surface reactivity of solids by using a variety of adsorbed probe molecules [7,20,42–45]. In the present study, XPS investigation of the saturated PEA surfaces was used in complementary way to the TPD approach to determine the Brønsted or Lewis nature of
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
117
Fig. 2. PEA thermodesorption profiles (left axis) and relevant differential distribution of PEA desorbed at each 50 8C interval (right axis) as a function of temperature for all the oxide supports.
Fig. 3. Relation between the amount of acid sites of the supports, determined by PEA desorption, and the amount of surface Zr atomic concentration on each support, determined by XPS [29].
the acid sites. In the case of a nitrogen containing basic probe, the N 1s binding energy of the nitrogen atom is a function of its neighborhood. When the base reacts with a Brønsted site to form a cation (ammonium ion in the present case), the value of BE (N 1s) is generally between 401.5 and 402.8 eV. When it forms a complex with a Lewis site, BE is in the 399–401.5 eV range depending on the chosen base and the resulting chargetransfer which is related to the acid strength of the site. PEA is a strong base that can be retained by weak, medium, and strong acid sites. XPS measurements of adsorbed probe molecules provide information only on the strong and medium strength acid sites and do not titrate the weakest sites, which cannot retain the probe under the ultra-high vacuum conditions (10 9 mbar) of the analyses. The XP spectra of all PEA saturated samples showed broad N 1s bands of BE in the range of 399.5–402.4 eV which could be decomposed into two main peak components (Table 2; Figs. 8 and 9), which indicate the presence of Brønsted (B) and Lewis (L) sites. In all the cases, the sites of Lewis able to retain the basic probe in UHV are in a large majority.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
118
Table 2 Properties of the Fe-catalysts including number and nature of acid sites determined by PEA-TPD and PEA-XPS. Catalysts
Fe/S Fe/SZ-5 Fe/SZ-15 Fe/SZ-30 Fe/SZ-45 a b c d
Surfacea
Composition ZrO2 (mass%)
Fe2O3 (mass%)
Fe/(Zr + Si)
– 4.7 14.2 28.1 43.1
7.3 5.9 5.6 6.4 4.3
0.121 0.142 0.048 0.078 0.087
S.A. (m2 g
239 237 232 298 237
1
)
Total acid sitesb (mequiv g 1)
0.480 0.444 0.591 0.642 0.857
Nature and amount of acid sitesc
N 1s binding energies
Lewis (mequiv g
Lewis (eV)
Brønsted (eV)
400.4 400.4 400.2 400.6 400.5
402.3 402.1 402.1 402.5 402.1
0.480 0.377 0.591 0.578 0.771
1
)
(100)d (85) (100) (90) (90)
Brønsted (mequiv g
1
– 0.067 (15)d – 0.064 (10) 0.086 (10)
)
From XPS analysis [29], atomic ratio. Evaluated from PEA thermodesorption profiles at 200 8C. Evaluated from PEA-XPS. Percent of Lewis and Brønsted acid sites population.
As concerning supports, silica surface showed little or none Brønsted sites covered by the basic probe, which well indicates the weakness of the acidity of the silanol groups. The amount and proportion of the Brønsted sites increased with increasing Zr concentration in the silica, as it could be clearly observed in particular on the SZ-30 and SZ-45 samples (Table 2). In parallel, the binding energy (BE) N 1s values increased with increasing Zr concentration due to a higher charge-transfer. Those results
confirm a strengthening of the Lewis and Brønsted acidity with Zr content, as observed for TPD measurements. The nature of the acid sites turned towards the predominant presence of Lewis acid sites when Fe was deposited on the supports. The N 1s binding energies in Fe/SZ catalyst series were generally higher than those in SZ supports. In the same time, the surface concentration (not reported here) of the basic probes was always larger on Fe/SZ than on SZ series, in which the weakest acid
Fig. 4. PEA thermodesorption profiles (left axis) and relevant differential distribution of PEA desorbed at each 50 8C interval (right axis) as a function of temperature for all the Fe-catalysts.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
119
sites do not retain the basic probes. This clearly indicates an increase of the acidity strength for the Fe-catalysts. Furthermore, the highest DBE of the N 1s contribution was obtained between Fe/ S and S samples, for both Lewis and Brønsted sites, which could confirm that the highest acidity gain was obtained for Fe/S catalyst. These observations confirmed the higher mean acid strength of the Fe-surfaces than the relevant supports, as already observed by PEA-TPD measurements. 3.3. Acidic properties and catalytic activity
Fig. 5. Total acid sites (evaluated from the PEA thermodesorption profiles at 200 8C) of the supports in comparison with those of Fe-catalysts.
All the catalysts proved to be very active in a-pinene oxide isomerization at room temperature and the product distribution found was in agreement with the Lewis acidic character of these materials [29]. It is known that the acid-isomerization of POX gives rise to a large variety of products because of the high substrate reactivity [32]. Among the observed products, campholenic
Scheme 2. Model of the 2-phenylethylamine (PEA) adsorption on surface acid sites of catalysts.
Fig. 6. PEA thermodesorption profiles for S support (left) and Fe/S catalyst (right) after isothermal PEA removal (120 min) in the 200–500 8C range.
Fig. 7. PEA thermodesorption profiles for SZ-30 support (left) and Fe/SZ-30 catalyst (right) after isothermal PEA removal (120 min) in the 200–500 8C range.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121
120
Fig. 8. N 1s XP spectrum after PEA adsorption and desorption at 80 8C on the SZ-45 support.
distribution during catalytic POX isomerization can give information on the presence of Lewis or Brønsted acid sites. For this reason, this reaction is becoming a recognized test for Lewis acidity [31– 34]. The obtained results in terms of product distribution from POX isomerization over the Fe-catalysts showed that as more pure was the Lewis acidity of the surface (i.e., Fe/S) as higher the selectivity to CPA observed (Table 3). By increasing the Brønsted acidity of the Fe-samples (Fe/SZ-30 and Fe/SZ-45, in particular), a marked increase of substrate conversion and a lowering of selectivity to CPA were observed. The obtained results are in agreement with those obtained from the integrated PEA-XPS and PEA-TPD approach used for the sample acidity property determination. The Fe-dispersed phase on poor acidic supports (i.e., S and SZ-5) created quite pure Lewis acid catalysts due to formation of unsaturated Fe oxide nanosized clusters while on more acidic supports (i.e., SZ-30 and SZ-45), the Lewis acidity of the Fe-dispersed phase summed up to the Brønsted acidity of support creating catalytic systems with a given ratio of Brønsted to Lewis acid sites. 4. Concluding remarks
Fig. 9. N 1s XP spectrum after PEA adsorption and desorption at 80 8C on the Fe/SZ45 catalyst.
aldehyde (CPA) represents an important intermediate for the manufacturing of sandalwood fragrances, currently being investigated together with macrocyclic musks, as possible substitute for nitro and polycyclic musk. The polycyclic musks are thought to be less environmentally damaging than nitro musks but they are also persistent, bio-accumulative and toxic to reproduction. For this reasons, the importance of sandalwood fragrances has been growing in the last decade [33,34]. CPA can be prepared in high yield by using a suitable Lewis acid, while Brønsted acids tend to produce different products (trans-carveol TCV, trans-sobrerol TSB, and dimerization products). Therefore, monitoring the product Table 3 Product distribution obtained in the a-pinene oxide (POX) isomerizationa over the Fe-catalysts [31]. Catalysta
tb(min)
Selectivity (mol%) c
Fe/S Fe/SZ-5 Fe/SZ-15 Fe/SZ-30 Fe/SZ-45
35 5 5 5 5
d
e
f
CPA
PCP
TCV
TSB
72 65 61 56 57
3 1 2 1 4
8 8 9 8 7
4 7 7 6 5
CPA productivityc (mmolprod/gcat h) 8 53 50 44 45
a Catalyst (100 mg), POX (100 mg), toluene (8 mL), RT, reaction mixture analysed by GC-FID analysis (Agilent 6890 (5%Phenyl)95% methylpolysiloxane column, injection T = 120 8C). b Time required to obtain POX complete conversion. c CPA, campholenic aldehyde. d PCP, pinocamphone. e TCV, trans-carveol. f TSB, trans-sobrerol.
The aim of this work was to elucidate the acid properties of complex series of samples comprising siliceous oxides with varying concentrations of zirconia in comparison with the analogous sample series prepared dispersing Fe oxide on each oxide, acting as support. The predominant Lewis acidity of the Fesamples was evidenced by XPS spectroscopy which confirmed the TPD evidences on the higher mean acid strength of the Fe-surfaces than those of the support oxides. Furthermore, this work indicates that catalytic materials composed of well dispersed Fe oxide on acidic supports can be developed with a judicious ratio between Brønsted and Lewis acid sites, leading to catalysts with acid sites that could work in cooperation in many reactions of applicative importance. Acknowledgment This work has been partially supported by the Italian Research Ministry (FIRST project 2007). A.G and C.M. would like to thank Bruno Cortese, student at the Universita` degli Studi di Milano, for his assistance in several experiments. Authors are also indebted with Dr. Vladimiro Dal Santo, ISTM-CNR, Milano, for his assistance during GC-MSD measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
A. Corma, Chem. Rev. 95 (1995) 559–614. K. Tanabe, W. Ho¨lderich, Appl. Catal. A: Gen. 181 (1999) 399–434. H.-U. Blaser, M. Studer, Appl. Catal. A: Gen. 189 (1999) 191–204. G. Sartori, R. Maggi, Chem. Rev. 106 (2006) 1077–1104. G. Busca, Chem. Rev. 107 (2007) 5366–5410. W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615–635. M. Johansson, K. Klier, Top. Catal. 4 (1997) 99–108. A. Auroux, Top. Catal. 4 (1997) 71–89. A. Auroux, Top. Catal. 19 (2002) 205–213. C. Sun, J.C. Berg, Adv. Colloid Interface Sci. 105 (2003) 151–175. M.C. Kung, H.H. Kung, Catal. Rev. Sci. Eng. 27 (1985) 425–460. H.G. Karge, V. Dondur, J. Phys. Chem. 94 (1990) 765–772. G. Busca, Phys. Chem. Chem. Phys. 1 (1999) 723–736. A. Zecchina, C. Lamberti, S. Bordiga, Catal. Today 41 (1998) 169–177. K. Hashimoto, T. Masuda, H. Ueda, N. Kitano, Appl. Catal. 22 (1986) 147–156. J.C. Yori, J.C. Luy, J.M. Parera, Appl. Catal. 41 (1988) 1–11. P. Carniti, A. Gervasini, S. Bennici, J. Phys. Chem. B 109 (2005) 1528–1536. S. Bennici, A. Gervasini, N. Ravasio, F. Zaccheria, J. Phys. Chem. B 197 (2003) 5168– 5176. [19] K. Tanabe, M. Misono, Y. Ono, H. Hattori, in: B. Delmon, J.T. Yates (Eds.), New Solid Acids and Bases, vol. 51, Elsevier, Amsterdam, 1989. [20] A. Auroux, A. Gervasini, C. Guimon, J. Phys. Chem. B 103 (1999) 7195–7205. [21] A. Gervasini, M. Manzoli, G. Martra, A. Ponti, N. Ravasio, L. Bordelli, F. Zaccheria, J. Phys. Chem. B 110 (2006) 7851–7861.
A. Gervasini et al. / Applied Catalysis A: General 367 (2009) 113–121 [22] A. Gervasini, C. Messi, P. Carniti, A. Ponti, N. Ravasio, F. Zaccheria, J. Catal. 262 (2009) 224–234. [23] T. Herranz, S. Rojas, F.J. Pe´rez-Alonso, M. Ojeda, P. Terreros, J.L.G. Fierro, Appl. Catal A: Gen. 308 (2006) 19–30. [24] J.D. Henao, B. Wen, W.M.H. Sachtler, J. Phys. Chem. B 109 (2005) 2055. [25] S. Rajagopal, J.A. Marzari, R. Miranda, J. Catal. 151 (1995) 192–203. [26] Y. Liu, X. Ma, S. Wang, J. Gong, Appl. Catal. B Environ. 77 (2007) 125–134. [27] Y. Sun, S. Walspurger, J.-P. Tessonnier, B. Louis, J. Sommer, Appl. Catal. A: Gen. 300 (2006) 1–7. [28] H. He, S. Bao, Q. Xu, Appl. Catal. A: Gen. 169 (1998) 29–36. [29] A. Gervasini, C. Messi, A. Ponti, S. Cenedese, N. Ravasio, J. Phys. Chem. C 112 (2008) 4635–4642. [30] K. Chen, Y. Fan, Q. Yan, J. Catal. 167 (1997) 573–575. [31] L. Alaerts, E. Se´guin, H. Poelman, F. Thibault-Starzyk, P.A. Jacobs, D.E. De Vos, Chem. Eur. J. 12 (2006) 7353. [32] W.F. Holderich, J. Roseler, G. Heitmann, A.T. Liebens, Catal. Today 37 (1997) 353– 366. [33] N. Ravasio, F. Zaccheria, A. Gervasini, C. Messi, Catal. Commun. 9 (2008) 1125–1127.
121
[34] G. Neri, G. Rizzo, S. Galvagno, G. Loiacono, A. Donato, M.G. Musolino, R. Pietropaolo, E. Rombi, Appl. Catal. A: Gen. 274 (2004) 243–251. [35] J.H. Scofield, J. Electron Spectrosc. Relat. Phenom. 8 (1976) 129–137. [36] K. Tanabe, in: J.R. Anderson, M. Boudart (Eds.), Catalysis, Science and Technology, vol. 8, Springer, Berlin, 1987, pp. 232–271. [37] T. Seiyama, Metal Oxides and Their Catalytic Actions, Kodansha, Tokyo, 1978. [38] A. Gervasini, G. Bellussi, J. Fenyvesi, A. Auroux, J. Phys. Chem. 99 (1995) 5117– 5125. [39] J.A. Schwarz, Chem. Rev. 95 (1995) 477–510. [40] A. Gervasini, P. Carniti, S. Bennici, C. Messi, Chem. Mater. 19 (2007) 1319–1328. [41] A. Gervasini, S. Bennici, A. Auroux, C. Guimon, Appl. Catal. A: Gen. 331 (2007) 129– 137. [42] R. Borade, A. Sayari, A. Adnot, S. Kaliaguine, J. Phys. Chem. 94 (1990) 5989–5994. [43] C. Guimon, A. Gervasini, A. Auroux, J. Phys. Chem. B 105 (2001) 10316–10325. [44] C. Guimon, H. Martinez, in: S.G. Pandalai (Ed.), Recent Research Developments in Catalysis, vol. 2, Research Signpost, Kerala, India, 2003, pp. 99–120. [45] L. Dussault, J.C. Dupin, E. Dumitriu, A. Auroux, C. Guimon, Thermochim. Acta 434 (2005) 93–99.