- Email: [email protected]
Facile fabrication of aluminum-promoted vanadium phosphate: A highly active heterogeneous catalyst for isopropylation of toluene to cymene
Journal of Catalysis 289 (2012) 190–198
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Facile fabrication of aluminum-promoted vanadium phosphate: A highly active heterogeneous catalyst for isopropylation of toluene to cymene Gobinda Chandra Behera, K.M. Parida ⇑, D.P. Das Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, Odisha, India
a r t i c l e
i n f o
Article history: Received 5 December 2011 Revised 1 February 2012 Accepted 13 February 2012 Available online 27 March 2012 Keywords: Isopropylation p-Cymene Toluene Vanadium phosphate
a b s t r a c t Vanadium phosphate is well known as a heterogeneous catalyst in gas phase oxidation reactions. To date, there has been little interest in carrying out liquid-phase reactions with vanadium phosphate as catalyst. Herein, we report the catalytic activity of vanadium phosphate and aluminum-promoted vanadium phosphate toward liquid-phase isopropylation of toluene to cymene. The catalysts were unambiguously characterized by X-ray diffraction, N2 adsorption–desorption, FT-IR technique, UV–vis DRS, and FE-SEM. The total acid sites were estimated by an NH3 TPD analyzer. XPS was used as a powerful tool to know the electronic environment of the catalysts. The optimization of the reaction was carried out by varying temperature from 75 to 150 °C and molar ratio (toluene: isopropanol) from 1:1–1:3. Under optimum reaction conditions, 5 wt.% aluminum-promoted vanadium phosphate showed 90% conversion with 85% selectivity to p-cymene. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Due to emerging applications, metal phosphates and related solids having low-dimensional structures (i.e., 1D or 2D) are currently the subject of increasing interest [1,2]. Particularly, oxovanadium phosphate chemistry is enjoying interest in view of its industrial uses [3]. This has resulted in a rich crystal chemistry that is associated, in part, with the ability of vanadium to adopt different coordination polyhedra and to accede easily to various oxidation states [4,5]. Many well-characterized crystalline vanadium phosphate phases have been identified, whose structures and catalytic properties have been well documented. Some of the most widely studied are the V5+ vanadyl orthophosphates (a-, b-, c-, d-, e-, and x-VOPO4 and VOPO42H2O) and the V4+ vanadyl hydrogen phosphates (VOHPO44H2O, VOHPO4½H2O, VO(H2PO4)2), vanadyl pyrophosphate ((VO)2P2O7), and vanadyl metaphosphate (VO(PO3)2) [6–13]. Of these compounds, VOHPO4.½H2O (vanadyl hydrogen phosphate hemihydrate) is of particular interest as a catalyst precursor [14–17], which on activation gives a catalyst mainly comprising (VO)2P2O7 (vanadyl pyrophosphate) [18]. A literature survey reveals that the resulting catalyst comprises a complex mixture of (VO)2P2O7 with primarily a-II and d-VOPO4 phases [19]. Some researchers favor V4+ phases, e.g., (VO)2P2O7, as the active phase [20,21]. However, in situ Raman spectroscopy
⇑ Corresponding author. Fax: +91 674 2581637. E-mail address: [email protected] (K.M. Parida). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.02.004
[22] suggests that combinations of V4+ and V5+ phases are required for the catalyst to exhibit high activity and selectivity. In view of the above context, vanadium phosphorus oxide (VPO) catalysts are of great interest due to their potential applications toward various organic transformation reactions. Although the VPO catalyst system has been studied extensively in gas phase oxidation, including catalyst structural stability, morphology, crystalline structure, and surface composition after hundreds of hours of testing, these types of studies are very limited in liquidphase reactions where VPO catalysts have been used [23–27]. It is well known that the VPO catalysts are structurally sensitive to its catalytic activation and reaction conditions [28–30]. A literature survey reveals that the best industrial catalyst from the VPO family is generally not used in the form of pure VPO phases, but incorporates a broad range of promoter compounds [31,32]. Promoter compounds are added to induce enhancement in activity or selectivity or to ensure the structural integrity of the catalyst surface. In an earlier comprehensive review of the literature on this topic, promoters were assumed to act in two ways; namely, structural effects due to enhancement of the surface area of the activated catalysts and electronic effects [33]. Here, we explore the promotion of vanadium phosphate catalysts by aluminum. Some research groups consider the acidity of the catalysts to be an important aspect in controlling the catalyst performance. An IR study of the acid sites using NH3, pyridine, and acetonitrile as probe molecules showed the existence of Lewis and Brønsted acid sites [34–39]. The presence of acid sites in VPO materials encourages us to carry out acid-catalyzed reactions. But the presence of acid sites in VPO is not sufficient for acid-catalyzed
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
reactions. Promoting the catalyst with aluminum enhances the Lewis acid sites on the surface of the catalyst, which play an important role in the alkylation of alkyl aromatics. The alkylation of the aromatic nucleus has been traditionally carried out with well-known Lewis acids or organometallic reagents. Isopropylation of benzene and toluene with isopropyl bromide as alkylating agent and AlCl3 as catalyst has been studied in detail by Olah et al. [40]. Alkylation with various alkylating agents and Friedel–Crafts catalysts has provided insight into the trends, in which activity and selectivity are mostly considered [41–43]. Alcohols and alkenes can also serve as sources of electrophiles in Friedel–Crafts reactions in the presence of strong acids. In many cases, more than a stoichiometric amount of AlCl3 is used for the reaction, giving poor selectivity because of degradation, polymerization, and isomerization. Al-promoted VPO catalysts play an important role in this area. The products of isopropylation of toluene with isopropanol are o-cymene, p-cymene, and mcymene. p-Cymene is an important intermediate used in the pharmaceutical industries and for the production of fungicides and pesticides. It is also used as a flavoring agent and as a heat transfer medium. 2. Materials and methods 2.1. Materials synthesis 2.1.1. Preparation of the bulk VPO precursor (VOHPO40.5H2O) The VPO precursor was prepared according to the following procedure: V2O5 (5.0 g, SBMC, 98.5%) and o-H3PO4 (30 ml, 85% Aldrich) were refluxed in deionized water (120 ml) for 24 h. A yellow solid was recovered by vacuum filtration, washed with cold water (100 ml) and acetone (100 ml), and dried in air (110 °C, 24 h). Powder X-ray diffraction studies confirmed that the solid was the dihydrate VOPO42H2O [44]. The dihydrate (4 g) was refluxed with isobutanol (80 ml, 99%, Spectrochem) for 21 h, and the resulting hemihydrate was recovered by filtration, dried in air (110 °C, 16 h), refluxed in deionized water (9 ml H2O/solid(g)) for 2 h, filtered hot, and dried in air (110 °C, 16 h). 2.1.2. Preparation of promoted VPO precursor Different wt.% Al-promoted VPO catalysts were prepared by the wetness impregnation method using isopropanol as solvent. The requisite amount of promoter source (Al as isopropoxide) was dissolved in isopropanol (30 ml, 99.8%, Spectrochem). The solution was warmed to 70 °C in a water bath for some minutes, and the desired amount of the precursor compound VOHPO40.5 H2O was added in powder form. The resulting slurry was evaporated to dryness on a water bath followed by oven-drying at 120 °C for 16 h. All the materials were calcined at 400 °C for 5 h in air. The final promoted VPO catalysts consisted of 5, 10, 15, and 20 wt.% of Al. Most of the investigations were carried out using bulk VPO and Al-promoted VPO catalysts. 2.2. Characterization The BET surface areas and pore volume distributions of the catalysts were determined by N2 adsorption at 77 K (ASAP2020, Micromeritics). A known amount of catalyst sample was evacuated for 2 h at 110 °C to remove physically adsorbed water prior to surface area measurements. Phase analysis of all materials was performed by XRD (PANalytical, X’pert PRO) using CuKa radiation at 1.543 Å. IR spectra of bulk and promoted VPO catalysts were recorded on a Varian 800 FTIR spectrophotometer. Self-supporting pellets were prepared with KBr and catalysts by applying pressure.
191
These pellets were further used for recording FTIR spectra. UV–vis investigations in diffuse reflectance mode were recorded in a UV–vis spectrophotometer (Varian, CARY 100). The spectra were recorded in the range 200–800 nm using boric acid as the reflectance standard. The acid character of the catalysts was studied with an NH3 TPD CHEMBET-3000 (Quantachrome, USA) analyzer equipped with a thermal conductivity detector (TCD). About 0.1 g of powdered sample was contained in a quartz U tube and degassed at 250 °C for 1 h with ultrapure nitrogen gas. After the sample cooled to room temperature, NH3 gas (20% NH3 balanced with helium) was passed over the sample while it was heated at a rate of 10 °C min 1 and the profile was recorded. The electronic states of V, Al, and P were examined by X-ray photoelectron spectroscopy (XPS, Kratos Axis 165 with a dual-anode (Mg and Al) apparatus) using an Mg Ka source. All the binding energy values were calibrated by using the contaminant carbon (C1s = 284.9 eV) as a reference. Charge neutralization of 2 eV was used to balance the charge of the sample. Binding energy values of the samples were reproducible within ±0.1 eV. Field emission scanning electron microscopy (FE-SEM) was performed with a ZEISS 55 microscope. Magnification in the range 15.83–44.90 K was used to get a better micrograph. ICP-OES (Optima 2100 DV, PerkinElmer) using microwave pressure digestion (MDS 200; CEM) with hydrofluoric acid and aqua regia at 9 bar was used to analyze the chemical composition of fresh and used catalysts. All the samples were analyzed three times, and the results presented here are the average values. 2.3. Catalytic isopropylation reaction Isopropylation of toluene was conducted in the liquid phase in a 250-ml round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. In a typical reaction procedure, 0.05 g catalyst was mixed with 25 mmol toluene and 25 ml nitrobenzene. To this solution, 25 mmol isopropanol was added, and the whole solution was heated to the reaction temperature (150 °C) with constant stirring for 7 h. The products were separated by column chromatography and then analyzed by offline GC (Shimadzu, GC-17 A) equipped with a capillary column (ZB-1, 30-m length, 0.5-nm ID, and 3.0-lm film thickness) using a flame ionization detector (FID). 3. Results and discussion 3.1. Characterization 3.1.1. X-ray diffraction Fig. 1i and ii displays the XRD patterns of VPO catalysts. For the precursor, all major diffraction peaks can be attributed to VOHPO40.5H2O. XRD patterns corresponding to the planes at (0 0 1), (1 0 1), (0 2 1), (1 2 1), (2 0 1), (2 2 0), (0 3 1), and (1 0 2) were observed at 2h values of 15.6°, 19.7°, 24.5°, 27.4°, 28.9°, 30.4°, 32.1°, and 33.7°, respectively, matching well with the VOHPO40.5H2O. This is in accordance with JCPDS File 4–880. The intense peak at 2h = 30.4° of the (2 2 0) plane of the VPO catalysts (having d spacing = 2.93 Å, cell parameter a = 8.31 Å) is in agreement with cubic mesostructured VPO [45]. In Fig. 1ii, a small peak at 2h = 12.8° was found which corresponds to the reflectance pattern of VOPO4. This is due to the transformation of VOHPO40.5H2O to VOPO4 after calcination. This shows that the vanadium in the sole VPO catalyst sample is predominantly in the V4+ state, with a small amount of V5+ species. However, this peak was only found in the case of 5 wt.% Al-VPO and thereafter was reduced with the increase of the aluminum loadings. This may presumably be due to the reduction of V5+ to V4+ with the increase of aluminum content in the promoted catalysts. There are three peaks found at 2h values of 37.8°,
192
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 1. (i) X-ray diffraction patterns of (a) c-Al2O3, (b) 5 wt.% Al-VPO, (c) 10 wt.% Al-VPO, (d) 15 wt.% Al-VPO, and (e) 20 wt.% Al-VPO; (ii) X-ray diffraction patterns of (a) VOHPO40.5H2O and (b) VPO (calcined VOHPO40.5H2O).
46.1°, and 67.1° for pure Al2O3. But there are no such peaks found in the case of Al-VPO catalysts. The lack of reflections corresponding to crystalline alumina phases indicates that the aluminum species in the VPO were amorphous or highly dispersed on the VPO surface. Crystalline alumina is generally formed at temperatures >500 °C (calcined temperatures). Since the present material was calcined at 400 °C, amorphous alumina might be formed in VPO. The XRD patterns reveal that there is a reduction of the peak in the Al-VPO relative to the parent precursor. In all Al-VPO materials, the intensity of the peak at 2h = 15.6° slightly increased while the peak at 2h = 30.4° decreased in comparison with the bulk VPO.
3.1.2. BET surface areas and pore volume distributions To understand the textural properties, the VPO and promoted VPO catalysts were subjected to N2 adsorption–desorption measurements. The results are depicted in Fig. 2a and b and also summarized in Table 1. Among the promoted samples, 15 and 20 wt.% Al-VPO showed typical type-IV isotherms, indicating the presence of mesoporous structures. But as for the classification of porosity, the materials having pore sizes between 2 and 50 nm are mesoporous. In this context, all VPO materials fall into the mesoporous range. Again, VPO has a surface area below 15 m2 g 1 and very small pore volume. The Al-VPO samples exhibit significantly higher
Fig. 2. (a) Adsorption–desorption isotherm of VPO and different wt.% of Al-VPO samples; (b) pore size distribution curve of VPO and different wt.% of Al-VPO samples.
193
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198 Table 1 Textural properties and surface acidity of the catalysts. Catalyst
Surface area (m2/g)
Pore size (nm)
Pore volume (cm2/g)
Total acidity (mmol/g)
Total acidity (mmol/m2)
Lewis acid sites
Brønsted acid sites
VPO 5wt% Al-VPO 10 wt% Al-VPO 15 wt% Al-VPO 20 wt%Al-VPO
14 27 76 111 127
10.0 5.8 3.5 2.9 2
0.05 0.16 0.24 0.31 0.34
2.37 6.274 7.02 7.77 9.17
0.169 0.232 0.092 0.07 0.072
1.28 5.9 6.0 6.21 7.03
0.09 0.374 1.02 1.56 2.14
values of these parameters, relative to VPO. It can be seen that 20 wt.% Al-VPO presents the highest surface area and pore volume, with all pores being in the mesopore range. The surface area of the sample was observed to increase drastically with increasing aluminum loading. For this study, the deposit of aluminum did not stabilize the VPO phase because the impregnation method appeared to permit aluminum to be present on the VPO crystallite surface. The surface area increases with the increase of aluminum loading but is not linearly increased, that is why there are no separate phases in the materials. The decrease in pore size indicates the mesoporosity of the materials. The increase in surface area may presumably be due to increased amorphous alumina in the VPO materials.
3.1.3. FTIR studies The FTIR spectra of the bulk and promoted VPO catalysts are depicted in Fig. 3. All the catalysts showed sharp bands in the region of 400–3500 cm 1. The slightly broad spectra at 3370 cm 1 are due to the symmetric stretching mode of OAH groups. The infrared spectra of the catalysts in the region 900–1200 cm 1 correspond to the stretching modes of PAO and V@O groups. The band at 642–415 cm 1 can be attributed to the deformation vibrations of OAPAO groups of phosphate tetrahedral. Almost no shift is observed in the catalysts (bulk and promoted catalysts), especially in an important band at 970 cm 1 that corresponds to symmetric stretching vibrations of V4+@O groups. The peak at 1045 cm 1 can be assigned to symmetric stretching vibrations of PO3 groups, and the rest of the peaks at 1101 and 1200 cm 1 can be ascribed to asymmetric stretching vibrations of PO3 groups. The peak at 2376 cm 1 may be due to the adsorption of atmospheric CO2. It is strange to see there is no change in spectra obtained in Fig. 3. It only confirms different stretching and bending vibrations of
Fig. 3. FT-IR spectra of (a) c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.%Al-VPO.
VPO. However, it has been found that there is only a slight change of the peak at 1639 cm 1 with the increase in aluminum loading. 3.1.4. UV–vis DRS studies Optical absorption spectra of bulk and Al-VPO are depicted in Fig. 4. The investigations were carried out to obtain information on the vanadium oxidation state. The presence of a broad band at 550–650 nm in the bulk and Al-promoted catalysts indicates the presence of V4+ species in these catalysts [46]. However, the broad band at 450 nm in the DR UV–vis spectra of Al-containing samples can be related to the presence of V5+ species (VOPO4). According to this, different VAPAO phases with V ions in different oxidation states have been observed. However, this broad band at 450 nm was only prominently found in 5 wt.% Al-VPO, which indicates the presence of V5+ in the Al-VPO materials. This is also confirmed from the XRD peak. Hutchings et al. [22] reported that combinations of V4+ and V5+ phases are required for the catalyst to exhibit high activity and selectivity. However, the peak at 450 nm gets reduced with increased aluminum content. This may presumably be due to the reduction of the V5+ ion to V4+ with the increased aluminum content, as it contained isopropanol units during materials preparation. 3.1.5. Temperature-programmed desorption studies To comprehend the acidic properties of the catalysts, VPO and Al-VPO catalysts were subjected to NH3 TPD analysis. The typical NH3-TPD profiles are depicted in Fig. 5i and also summarized in Table 1. The total acidity of 2.37 mmol/g found for bulk VPO was lower than that of promoted catalysts. The total acidity of the pure Al2O3 was found to be 2.5 mmol/g. Again, the increase in the Al content in the parent VPO enhances the total acidity of the catalyst. That means that Al could contribute to the total acidity of the catalyst. Typically, NH3 TPD curves showed two clear peaks, which
Fig. 4. UV–vis DRS of (a) c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.% Al-VPO.
194
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 5. (i) NH3 TPD plot of (a) VPO, (b) 5 wt.% Al-VPO, (c) 10 wt.% Al-VPO, (d) 15 wt.% Al-VPO, and (e) 20 wt.% Al-VPO. (ii) Pyridine TPD plot of (a) c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.% Al-VPO.
determine the existence of at least two types of acid sites. These peaks are called the l-peak (lower temperature) and h-peak (high temperature), respectively. The l-peak is attributed to desorption of weakly bound ammonia. It is relevant to gas sensing applications, due to the influence of the weakly bound ammonia on the proton mobility in VPO. The h-peak reflects desorption of ammonia from the strong acid sites (here its Lewis acid sites), which determines the acidic properties of VPO. Brønsted and Lewis acid sites were also quantitatively characterized by pyridine TPD (Fig. 5ii). Adsorption of pyridine on Al-VPO produced two types of adsorbed species: a pyridinium ion on Brønsted acid sites and a covalently bound species on Lewis acid sites. It has been found that the Lewis acid site increases with increased aluminum content. The peak maximum for 15 wt.% at lower temperature was due to the
increase of Lewis acid sites along with Brønsted acid sites. Since the materials contain two different phases, multiple peaks are observed in the profile. 3.1.6. X-ray photoelectron spectroscopy studies The XPS investigations reported here were aimed at the determination of the vanadium oxidation state present in the catalyst samples. The representative photoelectron peaks of Al2p, V2p3/2, O1s, and P2p are depicted in Fig. 6a–c, respectively. Since controversial interpretations of the broad right V2p3/2 photoelectron peak as well as somewhat contradictory values for the binding energies for the Vn+ contributions can be found in the literature, VOPO4 with phase structure found by UV–vis spectra was used to calibrate the binding energy of the V2p3/2 (V5+) peak. The
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
195
Fig. 6. XPS spectra for the Al-VPO catalysts, with Al loadings (wt.%) as indicated by each spectrum; (a) Al2p, (b) V2p, and O1s, and (c) P2p.
measured value of 517.5 eV for V5+ and the independently obtained value of 516.3 eV for V4+ [47] are in good agreement with the literature [48–50] and were used for the V2p3/2 peak deconvolution for all samples studied. The binding energies, 74.6 and 132.9 eV, correspond to Al2p and P2p in +3 and +5 oxidation states, respectively (Fig. 6a and c). The shift to higher binding energy of the Al2p peak with increasing Al loading (Fig. 6a) may reflect a change from Al2O3 clusters to Al2O3 monolayer deposition or a change in overlayer–support interaction. The surface compositions of the catalysts are shown in Table 2. The binding energy value 531.8 eV of the O1s peak indicated that the oxygen species was O2 in oxides, and therefore, the increase of O/V ratio supports
Table 2 Surface composition of the Al-VPO catalysts determined by analysis of the X-ray photoelectron spectra. Al2O3 (wt.%)
5 10 15 20
at.% O
V
P
Al
N
P/V
Al/V
74.0 74.0 73.7 73.2
9.2 9.5 8.7 8.7
14.8 13.6 13 12.7
1.1 1.6 3.2 3.7
0.8 1.4 1.5 1.7
1.61 1.42 1.50 1.46
0.119 0.168 0.367 0.425
the conclusion that a measurable fraction of V4+ at the equilibrated surface is oxidized to V5+. 3.1.7. Scanning electron microscopy studies The FE-SEM micrograph of the catalysts revealed that the samples possess a slatelike morphology (Fig. 7). Further, aggregates without regular shapes are observed in VPO. This is the reason for the low surface area of the VPO catalyst than of Al-VPO. However, the morphology of VPO was not affected significantly by Al doping. 3.1.8. Estimation of catalyst components by ICP-OES measurement To estimate the actual contents of all catalyst components, all the prepared catalysts undergo characterization using the ICP-OES technique. ICP analysis results of various Al-VPO catalysts are shown in Table 3. The contents of aluminum, vanadium, and phosphorus estimated from ICP are in good agreement with the nominal values. In addition, the contents of all these components were also checked again in used catalysts and were quite comparable to those of fresh samples. This fact suggests that there is no loss of catalyst components during the course of reaction. The P/V ratio detected here indicates the presence of V4+ species in the materials. From the literature, it has been found that a P/V ratio >1 is
196
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 7. Scanning electron micrographs of VPO and Al-VPO.
Table 3 Estimation of Al, V, P contents in wt.% from ICP-OES in Al-VPO catalysts with different Al loadings. Catalyst
VPO 5 wt.% Al-VPO 10 wt.% Al-VPO 15 wt.% Al-VPO 20 wt.%Al-VPO
Al
V
P
P/V
Fresh
Used
Fresh
Used
Fresh
Used
00 5.04 10.09 15.13 20.18
00 5.04 10.09 15.13 20.18
18.7 13.01 12.84 12.40 12.01
18.7 13.01 12.83 12.40 12.01
28.91 19.83 19.58 18.85 18.25
28.90 19.83 19.58 18.84 18.25
1.54 1.52 1.52 1.52 1.52
necessary for high catalytic performance [51]. It has also been found that at P/V ratios of less than 1.0, a number of vanadium sites remain inactive, but at the higher P/V ratios, all surface sites are active [52]. This is in good agreement with our results.
3.2. Catalytic reaction The isopropylation of toluene with isopropanol is an electrophilic substitution reaction. Isopropylation reactions catalyzed by acids are commonly considered as proceeding via a carbonium-ion-type mechanism [53]. The reaction of toluene with isopropanol is shown in Scheme 1. The effects of various parameters on the isopropylation reaction are discussed later. The VPO and Al-promoted VPO catalysts are tested in order to have a com-
+ HO
toluene
Al-VPO
parative understanding of the catalytic activity for the reaction. Electrophilic substitution reactions such as isopropylation are catalyzed by strong Lewis acid sites. It was also reported that surface hydroxyl groups are responsible for the acidic nature of the catalyst [54]. Thus, the surface oxygen and surface hydroxyl group [55] play a crucial role in the catalytic activity of vanadium phosphate. The results of isopropylation of toluene with isopropanol using VPO and different wt.% of Al-promoted VPO in nitrobenzene as solvent are summarized in Table 4. It is clearly evident from Table 4 that VPO gives 80% p-cymene selectivity with 86% conversion in a period of 7 h. Also, 5 wt.% Al-VPO showed higher catalytic activity, giving 85% p-cymene selectivity with 90% conversion. However, the conversion decreases with increased aluminum content of VPO. Presumably the high wt.% of aluminum blocks the active site of the catalyst due to protonation of the hydroxyl groups present in the catalyst. The high selectivity of p-cymene may be due to the smaller steric hindrance of the methyl group at the para position, which is favored from the shape selectivity point of view [56].
3.2.1. Effect of reaction time The effect of the reaction period on the isopropylation of toluene with isopropanol using Al-VPO catalyst was studied at 150 °C with mole ratio 1:2 (toluene: isopropanol). The results are illustrated in Fig. 8. From the figure, it is found that the conversion of
+
+
150 o C ,7h
isopropanol p-cymene
o-cymene
Scheme 1. Isopropylation of toluene to cymene.
m-cymene
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
197
Table 4 Comparison of the activity of the bulk and promoted catalysts. Catalyst
Conversion (%)
VPO Al2O3 5 wt.% Al-VPO 10 wt.% Al-VPO 15 wt.% Al-VPO 20 wt.% Al-VPO
86 81 90 88 88 88
Cymene selectivity (%) Para
Ortho
Meta
Others
80 37 85 83 82 82
12 29 14 14 13 13
5 6 – 2 3 3
2 11 – – 1 1
Note: Conditions: temperature 150 °C, time 7 h, molar ratio 1:1 (toluene: isopropanol.
Fig. 9. Effect of reaction temperature on toluene conversion and p-cymene selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, time 6 h, molar ratio 1:2 (toluene: isopropanol).
Fig. 8. Effect of time period on toluene conversion and p-cymene selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, temperature 150 °C, molar ratio 1:2 (toluene: isopropanol).
toluene increases from 78% at 1 h to a maximum of 90% at 7 h. After 7 h, the conversion of toluene remains constant. This may presumably be due to the blocking of active sites of the catalyst with coke [57]. The selectivity to p-cymene exhibits a small but a significant increase with time. The presence of more Brønsted sites causes faster deactivation, which may be due to higher cracking activity and coke formation at strong acid sites of the catalysts. The coke deposits at the pore mouth and/or within the catalyst may reduce the effective channel dimensions, resulting in a tighter fit of the molecules and finer discrimination between the isomers [58].
150 °C over 5 wt.% Al-VPO catalyst with varying toluene-toisopropanol molar feed ratios from 2:1 to 1:3. The results are summarized in Fig. 10. In all cases, p-cymene was obtained as the major product, along with small amounts of o-cymene and m-cymene. It is clearly evident from the figure that toluene conversion increases with decreasing toluene content in the feed. Moreover, there exists competitive adsorption between toluene and isopropanol at the feed ratio 1:1. Hence with the increase in the molar ratio, the conversion of toluene increases. The p-cymene selectivity increases with the increase in the feed ratio from 2:1 to 1:2. The decrease in the selectivity to p-cymene at a feed ratio of 2:1 compared to 1:2 is due to the presence of more toluene content in the former, which leads to more chemisorption of toluene on the catalyst surface, thereby suppressing the chemisorption of isopropanol, which in turn facilitates the dealkylation rather than isopropylation of toluene. It is observed that isopropylation is favored only if toluene reacts from the vapor phase and isopropanol is chemisorbed on the catalyst surface, which could also be due to the trans alkylation of diisopropyltoluene formed due to the easy availability of isopropyl cations. Further there is a marginal decrease in p-cymene selectivity with the increase in the isopropyl content at a feed ratio of 1:3. This may presumably be due to the dilution of toluene with isopropanol. As said above, at a feed ratio of 2:1, toluene is more prone to
3.2.2. Effect of reaction temperature To accomplish better toluene conversion and maximum p-cymene selectivity, liquid-phase isopropylation of toluene was carried out over 5 wt.% Al-VPO catalyst at various reaction temperature, and the results are depicted in Fig. 9. It has been found that the toluene conversion increases up to 150 °C and remains constant thereafter. This could be attributed to the coke deposition caused by dealkylation, which in turn leads to the oligomerization of olefins. On the other hand, the increase in toluene conversion at lower temperatures may be due to the clustering of alcohols around the Brønsted acid sites of the catalyst through H-bonding. As a result, its dissociation to isopropyl carbocations will be suppressed at higher temperatures. But at higher temperatures, the alcohols will have a decreased tendency to form molecular clusters, and so they can freely form carbonium ions via protonation. 3.2.3. Effect of mole ratio of the reactants To study the effect of feed ratios on toluene conversion and product selectivity, an isopropylation reaction was carried out at
Fig. 10. Effect of molar ratio on toluene conversion and p-cymene selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, temperature 150 °C, time 6 h.
198
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
chemisorption on the catalyst surface, but the intermittent chemisorption of isopropanol would also be accomplished. This would abet the ortho position of the aromatic ring, making it easily accessible for electrophilic substitution. Hence, there is enhanced selectivity to o-cymene at a feed ratio of 2:1. Based on the above observations, the molar feed ratio 1:2 was found to be optimum for further studies. 4. Conclusions In summary, we have successfully synthesized Al-VPO. The XRD patterns of VPO and Al-VPO confirm their structure and good crystallinity. The N2 adsorption studies confirm the mesoporous nature of the materials. The pore diameter and pore volume are found to increase with increased promoting material. The acidity measurements of NH3 by TPD show that promotion by aluminum in a vanadium phosphate framework increases the number of total acid sites. The XPS spectra showed the different oxidation states of vanadium and other elements present in the materials. The isopropylation of toluene with isopropanol over these catalysts shows that toluene conversion increases with increasing temperature up to 150 °C, and further increase in temperature leads to a fall in toluene conversion. Based on toluene conversion, a reactant feed ratio of 1:2 and a reaction temperature of 150 °C are recommended. The present study reveals that 5 wt.% Al-VPO has a potential application in the production of p-cymene with high selectivity. Acknowledgment The authors are thankful to Prof. B.K. Mishra, Director, IMMT, Bhubaneswar, for his interest, encouragement, and kind permission to publish this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Kanazawa (Ed.), Inorganic Phosphate Materials, Elsevier, Tokyo, 1989, p. 1. A. Clearfield, Prog. Inorg. Chem. 47 (1998) 371. G. Centi, Catal. Today 16 (1993) 5. M.I. Khan, J. Zubieta, J. Prog. Inorg. Chem. 43 (1995) 1. P. Amoros, M.D. Marcos, A. Beltran, D. Beltran, Curr. Opin. Solid State Mater. Sci. 4 (1999) 123. V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catal. Today 28 (1996) 275. J.W. Johnson, D.C. Johnston, A.J. Jacobson, J.F. Brody, J. Am. Chem. Soc. 106 (1984) 8123. J.A. Lopez-Sanchez, J.K. Bartley, R.P.K. Wells, C. Rhodes, G.J. Hutchings, New J. Chem. 26 (2002) 1613. M.T. Sananes, I.J. Ellison, S. Sajip, A. Burrows, C.J. Kiely, J.C. Volta, G.J. Hutchings, J. Chem. Soc. Faraday Trans. 92 (1996) 137. F.J.C. Sanchez, R.P.K. Wells, C. Rhodes, J.K. Bartley, C.J. Kiely, G.J. Hutchings, Phys. Chem. Chem. Phys. 3 (2001) 4122. J. Ellison, G.J. Hutchings, M.T. Sananes, J.C. Volta, J. Chem. Soc. Chem. Commun. (1994) 1093.
[12] J.T. Vaughey, W.T.A. Harrison, A.J. Jacobson, D.P. Goshorn, J.W. Johnson, Inorg. Chem. 33 (1994) 248. [13] M. Schindler, W. Joswig, W.H. Baur, Eur. J. Solid State Inorg. Chem. 32 (1995) 109. [14] E. Bordes, Catal. Today 1 (1987) 499. [15] G. Centi (Ed.), Catal. Today. 16 (1993) 1. [16] G.J. Hutchings, Appl. Catal. 72 (1991) 1. [17] G.J. Hutchings, J. Mater. Chem. 14 (2004) 3385. [18] A.E. Badraoui, J.Y. Pivan, M. Maunaye, O. Pena, M. Louer, D. Louer, Ann. Chem. (Paris) 23 (1998) 97. [19] C.J. Kiely, A. Burrows, G.J. Hutchings, K.E. Bere, J.C. Volta, A. Tuel, M. Abon, Faraday Discuss. 105 (1996) 103. [20] J.R. Ebner, M.R. Thomson, Catal. Today 16 (1993) 51. [21] V.V. Guliants, J.B. Benziger, S. Sundaresan, N. Yao, I.E. Wachs, Catal. Lett. 32 (1995) 379. [22] G.J. Hutchings, A. Desmartin Chomel, R. Olier, J.C. Volta, Nature 368 (1994) 41. [23] U.R. Pillai, E. Sahle-Demessie, Appl. Catal. A Gen. 276 (2004) 139. [24] U.R. Pillai, E. Sahle-Demessie, Chem. Commun. (2002) 2142. [25] U.R. Pillai, E. Sahle-Demessie, New J. Chem. 27 (2003) 525. [26] C. Carline, P. Patrono, A.M.R. Galleti, G. Sbrana, V. Zima, Appl. Catal. A Gen. 289 (2005) 197. [27] K. You, L. Mao, L. Chem, D. Yin, P. Liu, H. Luo, Catal. Commun. 9 (2008) 2136. [28] M. Di Serio, M. Cozzolino, R. Tesser, P. Patrono, F. Pinzari, B. Bonellic, E. Santacesaria, Appl. Catal. A Gen. 320 (2007) 1. [29] C. Carlini, P. Patrono, A.M.R. Galletti, G. Sbrana, V. Zima, Appl. Catal. A Gen. 289 (2005) 197. [30] Z.Y. Xue, G.L. Schrader, J. Phys. Chem. 103 (1999) 9459. [31] G.J. Hutchings, Appl. Catal. A 72 (1991) 1. [32] G.J. Hutchings, R. Higgins, J. Catal. 162 (1996) 153. [33] G.J. Hutchings, Catal. Today 16 (1993) 139. [34] F. Cavani, F. Trfiro, Appl. Catal. A Gen. 157 (1997) 195. [35] G. Busca, G. Ramis, V. Lorenzelli, J. Mol. Catal. 50 (1989) 231. [36] L.M. Cornaglia, E.A. Lombardo, J.A. Anderson, J.L. Garcia Fierro, Appl. Catal. A Gen. 100 (1993) 37. [37] L.M. Cornaglia, E.A. Lombardo, Stud. Surf. Sci. Catal. 90 (1994) 429. [38] S. Ben Abdelmalek, H. Batis, A. Ghorbel, J. Soc. Alger. Chem. 3S (1993) 265. [39] M. Abon, J.C. Volta, Appl. Catal. A Gen. 157 (1997) 173. [40] G.A. Olah, S.H. Flood, S.J. Kuhn, M.E. Moffatt, N.A. Overchuck, J. Am. Chem. Soc. 86 (1964) 1046. [41] M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, V. Krishnaswamy, K.B. Lal, J. Mol. Catal. 186 (2002) 173. [42] C. Venkatesan, M. Chidambaram, K.R. Kamble, A.P. Singh, Catal. Lett. 85 (2003) 171. [43] R. Savidha, A. Pandurangan, Appl. Catal. A Gen. 276 (2004) 39. [44] G. Busca, F. Cavani, G. Centi, F. Trifiro, J. Catal. 99 (1986) 400. [45] N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179. [46] B. Solsona, V.A. Zazhigalov, J.M. López Nieto, I.V. Bacherikova, E.A. Diyuk, Appl. Catal. A 249 (2003) 81. [47] Y. Suchorski, L. Rihko-Struckmann, F. Klose, Y. Ye, M. Alandjiyska, K. Sundmacher, H. Weiss, Appl. Surf. Sci. 249 (2005) 231. [48] S. Albonetti, F. Cavani, F. Trifiro, P. Venturoli, G. Calestani, M. Lopez Granados, J.L. Fierro, J. Catal. 160 (1996) 52. [49] M. Abon, K.E. Bere, A. Tuel, P. Delishere, J. Catal. 156 (1995) 28. [50] P. Delishere, K.E. Bere, M. Abon, Appl. Catal. A Gen. 172 (1998) 295. [51] F. Garbasi, J.C.J. Bart, F. Montino, G. Petrini, Appl. Catal. 16 (1985) 271. [52] A. Satsuma, A. Hattori, K. Mizutani, A. Furuta, A. Furuta, A. Miyamoto, T. Hattori, Y. Murakami, J. Phys. Chem. 93 (1989) 1484. [53] P.B. Venuto, L.A. Hamilton, P.S. Landis, J. Catal. 5 (1996) 484. [54] Y. Wang, W. Li, React. Kinet. Catal. Lett. 69 (2000) 169. [55] H. Knozinger, H. Buhl, K. Kochloefl, J. Catal. 24 (1972) 57. [56] P.A. Parikh, N. Subrahmanyam, Y.S. Bhat, A.B. Halegeri, Chem. Eng. J. 54 (1994) 79. [57] P.A. Parikh, N. Subrahmanyam, Appl. Catal. A Gen. 90 (1992) 1. [58] W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein, S.A. Butter, J. Catal. 67 (1981) 159.
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat
Facile fabrication of aluminum-promoted vanadium phosphate: A highly active heterogeneous catalyst for isopropylation of toluene to cymene Gobinda Chandra Behera, K.M. Parida ⇑, D.P. Das Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, Odisha, India
a r t i c l e
i n f o
Article history: Received 5 December 2011 Revised 1 February 2012 Accepted 13 February 2012 Available online 27 March 2012 Keywords: Isopropylation p-Cymene Toluene Vanadium phosphate
a b s t r a c t Vanadium phosphate is well known as a heterogeneous catalyst in gas phase oxidation reactions. To date, there has been little interest in carrying out liquid-phase reactions with vanadium phosphate as catalyst. Herein, we report the catalytic activity of vanadium phosphate and aluminum-promoted vanadium phosphate toward liquid-phase isopropylation of toluene to cymene. The catalysts were unambiguously characterized by X-ray diffraction, N2 adsorption–desorption, FT-IR technique, UV–vis DRS, and FE-SEM. The total acid sites were estimated by an NH3 TPD analyzer. XPS was used as a powerful tool to know the electronic environment of the catalysts. The optimization of the reaction was carried out by varying temperature from 75 to 150 °C and molar ratio (toluene: isopropanol) from 1:1–1:3. Under optimum reaction conditions, 5 wt.% aluminum-promoted vanadium phosphate showed 90% conversion with 85% selectivity to p-cymene. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Due to emerging applications, metal phosphates and related solids having low-dimensional structures (i.e., 1D or 2D) are currently the subject of increasing interest [1,2]. Particularly, oxovanadium phosphate chemistry is enjoying interest in view of its industrial uses [3]. This has resulted in a rich crystal chemistry that is associated, in part, with the ability of vanadium to adopt different coordination polyhedra and to accede easily to various oxidation states [4,5]. Many well-characterized crystalline vanadium phosphate phases have been identified, whose structures and catalytic properties have been well documented. Some of the most widely studied are the V5+ vanadyl orthophosphates (a-, b-, c-, d-, e-, and x-VOPO4 and VOPO42H2O) and the V4+ vanadyl hydrogen phosphates (VOHPO44H2O, VOHPO4½H2O, VO(H2PO4)2), vanadyl pyrophosphate ((VO)2P2O7), and vanadyl metaphosphate (VO(PO3)2) [6–13]. Of these compounds, VOHPO4.½H2O (vanadyl hydrogen phosphate hemihydrate) is of particular interest as a catalyst precursor [14–17], which on activation gives a catalyst mainly comprising (VO)2P2O7 (vanadyl pyrophosphate) [18]. A literature survey reveals that the resulting catalyst comprises a complex mixture of (VO)2P2O7 with primarily a-II and d-VOPO4 phases [19]. Some researchers favor V4+ phases, e.g., (VO)2P2O7, as the active phase [20,21]. However, in situ Raman spectroscopy
⇑ Corresponding author. Fax: +91 674 2581637. E-mail address: [email protected] (K.M. Parida). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.02.004
[22] suggests that combinations of V4+ and V5+ phases are required for the catalyst to exhibit high activity and selectivity. In view of the above context, vanadium phosphorus oxide (VPO) catalysts are of great interest due to their potential applications toward various organic transformation reactions. Although the VPO catalyst system has been studied extensively in gas phase oxidation, including catalyst structural stability, morphology, crystalline structure, and surface composition after hundreds of hours of testing, these types of studies are very limited in liquidphase reactions where VPO catalysts have been used [23–27]. It is well known that the VPO catalysts are structurally sensitive to its catalytic activation and reaction conditions [28–30]. A literature survey reveals that the best industrial catalyst from the VPO family is generally not used in the form of pure VPO phases, but incorporates a broad range of promoter compounds [31,32]. Promoter compounds are added to induce enhancement in activity or selectivity or to ensure the structural integrity of the catalyst surface. In an earlier comprehensive review of the literature on this topic, promoters were assumed to act in two ways; namely, structural effects due to enhancement of the surface area of the activated catalysts and electronic effects [33]. Here, we explore the promotion of vanadium phosphate catalysts by aluminum. Some research groups consider the acidity of the catalysts to be an important aspect in controlling the catalyst performance. An IR study of the acid sites using NH3, pyridine, and acetonitrile as probe molecules showed the existence of Lewis and Brønsted acid sites [34–39]. The presence of acid sites in VPO materials encourages us to carry out acid-catalyzed reactions. But the presence of acid sites in VPO is not sufficient for acid-catalyzed
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
reactions. Promoting the catalyst with aluminum enhances the Lewis acid sites on the surface of the catalyst, which play an important role in the alkylation of alkyl aromatics. The alkylation of the aromatic nucleus has been traditionally carried out with well-known Lewis acids or organometallic reagents. Isopropylation of benzene and toluene with isopropyl bromide as alkylating agent and AlCl3 as catalyst has been studied in detail by Olah et al. [40]. Alkylation with various alkylating agents and Friedel–Crafts catalysts has provided insight into the trends, in which activity and selectivity are mostly considered [41–43]. Alcohols and alkenes can also serve as sources of electrophiles in Friedel–Crafts reactions in the presence of strong acids. In many cases, more than a stoichiometric amount of AlCl3 is used for the reaction, giving poor selectivity because of degradation, polymerization, and isomerization. Al-promoted VPO catalysts play an important role in this area. The products of isopropylation of toluene with isopropanol are o-cymene, p-cymene, and mcymene. p-Cymene is an important intermediate used in the pharmaceutical industries and for the production of fungicides and pesticides. It is also used as a flavoring agent and as a heat transfer medium. 2. Materials and methods 2.1. Materials synthesis 2.1.1. Preparation of the bulk VPO precursor (VOHPO40.5H2O) The VPO precursor was prepared according to the following procedure: V2O5 (5.0 g, SBMC, 98.5%) and o-H3PO4 (30 ml, 85% Aldrich) were refluxed in deionized water (120 ml) for 24 h. A yellow solid was recovered by vacuum filtration, washed with cold water (100 ml) and acetone (100 ml), and dried in air (110 °C, 24 h). Powder X-ray diffraction studies confirmed that the solid was the dihydrate VOPO42H2O [44]. The dihydrate (4 g) was refluxed with isobutanol (80 ml, 99%, Spectrochem) for 21 h, and the resulting hemihydrate was recovered by filtration, dried in air (110 °C, 16 h), refluxed in deionized water (9 ml H2O/solid(g)) for 2 h, filtered hot, and dried in air (110 °C, 16 h). 2.1.2. Preparation of promoted VPO precursor Different wt.% Al-promoted VPO catalysts were prepared by the wetness impregnation method using isopropanol as solvent. The requisite amount of promoter source (Al as isopropoxide) was dissolved in isopropanol (30 ml, 99.8%, Spectrochem). The solution was warmed to 70 °C in a water bath for some minutes, and the desired amount of the precursor compound VOHPO40.5 H2O was added in powder form. The resulting slurry was evaporated to dryness on a water bath followed by oven-drying at 120 °C for 16 h. All the materials were calcined at 400 °C for 5 h in air. The final promoted VPO catalysts consisted of 5, 10, 15, and 20 wt.% of Al. Most of the investigations were carried out using bulk VPO and Al-promoted VPO catalysts. 2.2. Characterization The BET surface areas and pore volume distributions of the catalysts were determined by N2 adsorption at 77 K (ASAP2020, Micromeritics). A known amount of catalyst sample was evacuated for 2 h at 110 °C to remove physically adsorbed water prior to surface area measurements. Phase analysis of all materials was performed by XRD (PANalytical, X’pert PRO) using CuKa radiation at 1.543 Å. IR spectra of bulk and promoted VPO catalysts were recorded on a Varian 800 FTIR spectrophotometer. Self-supporting pellets were prepared with KBr and catalysts by applying pressure.
191
These pellets were further used for recording FTIR spectra. UV–vis investigations in diffuse reflectance mode were recorded in a UV–vis spectrophotometer (Varian, CARY 100). The spectra were recorded in the range 200–800 nm using boric acid as the reflectance standard. The acid character of the catalysts was studied with an NH3 TPD CHEMBET-3000 (Quantachrome, USA) analyzer equipped with a thermal conductivity detector (TCD). About 0.1 g of powdered sample was contained in a quartz U tube and degassed at 250 °C for 1 h with ultrapure nitrogen gas. After the sample cooled to room temperature, NH3 gas (20% NH3 balanced with helium) was passed over the sample while it was heated at a rate of 10 °C min 1 and the profile was recorded. The electronic states of V, Al, and P were examined by X-ray photoelectron spectroscopy (XPS, Kratos Axis 165 with a dual-anode (Mg and Al) apparatus) using an Mg Ka source. All the binding energy values were calibrated by using the contaminant carbon (C1s = 284.9 eV) as a reference. Charge neutralization of 2 eV was used to balance the charge of the sample. Binding energy values of the samples were reproducible within ±0.1 eV. Field emission scanning electron microscopy (FE-SEM) was performed with a ZEISS 55 microscope. Magnification in the range 15.83–44.90 K was used to get a better micrograph. ICP-OES (Optima 2100 DV, PerkinElmer) using microwave pressure digestion (MDS 200; CEM) with hydrofluoric acid and aqua regia at 9 bar was used to analyze the chemical composition of fresh and used catalysts. All the samples were analyzed three times, and the results presented here are the average values. 2.3. Catalytic isopropylation reaction Isopropylation of toluene was conducted in the liquid phase in a 250-ml round-bottomed flask equipped with a reflux condenser and a magnetic stirrer. In a typical reaction procedure, 0.05 g catalyst was mixed with 25 mmol toluene and 25 ml nitrobenzene. To this solution, 25 mmol isopropanol was added, and the whole solution was heated to the reaction temperature (150 °C) with constant stirring for 7 h. The products were separated by column chromatography and then analyzed by offline GC (Shimadzu, GC-17 A) equipped with a capillary column (ZB-1, 30-m length, 0.5-nm ID, and 3.0-lm film thickness) using a flame ionization detector (FID). 3. Results and discussion 3.1. Characterization 3.1.1. X-ray diffraction Fig. 1i and ii displays the XRD patterns of VPO catalysts. For the precursor, all major diffraction peaks can be attributed to VOHPO40.5H2O. XRD patterns corresponding to the planes at (0 0 1), (1 0 1), (0 2 1), (1 2 1), (2 0 1), (2 2 0), (0 3 1), and (1 0 2) were observed at 2h values of 15.6°, 19.7°, 24.5°, 27.4°, 28.9°, 30.4°, 32.1°, and 33.7°, respectively, matching well with the VOHPO40.5H2O. This is in accordance with JCPDS File 4–880. The intense peak at 2h = 30.4° of the (2 2 0) plane of the VPO catalysts (having d spacing = 2.93 Å, cell parameter a = 8.31 Å) is in agreement with cubic mesostructured VPO [45]. In Fig. 1ii, a small peak at 2h = 12.8° was found which corresponds to the reflectance pattern of VOPO4. This is due to the transformation of VOHPO40.5H2O to VOPO4 after calcination. This shows that the vanadium in the sole VPO catalyst sample is predominantly in the V4+ state, with a small amount of V5+ species. However, this peak was only found in the case of 5 wt.% Al-VPO and thereafter was reduced with the increase of the aluminum loadings. This may presumably be due to the reduction of V5+ to V4+ with the increase of aluminum content in the promoted catalysts. There are three peaks found at 2h values of 37.8°,
192
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 1. (i) X-ray diffraction patterns of (a) c-Al2O3, (b) 5 wt.% Al-VPO, (c) 10 wt.% Al-VPO, (d) 15 wt.% Al-VPO, and (e) 20 wt.% Al-VPO; (ii) X-ray diffraction patterns of (a) VOHPO40.5H2O and (b) VPO (calcined VOHPO40.5H2O).
46.1°, and 67.1° for pure Al2O3. But there are no such peaks found in the case of Al-VPO catalysts. The lack of reflections corresponding to crystalline alumina phases indicates that the aluminum species in the VPO were amorphous or highly dispersed on the VPO surface. Crystalline alumina is generally formed at temperatures >500 °C (calcined temperatures). Since the present material was calcined at 400 °C, amorphous alumina might be formed in VPO. The XRD patterns reveal that there is a reduction of the peak in the Al-VPO relative to the parent precursor. In all Al-VPO materials, the intensity of the peak at 2h = 15.6° slightly increased while the peak at 2h = 30.4° decreased in comparison with the bulk VPO.
3.1.2. BET surface areas and pore volume distributions To understand the textural properties, the VPO and promoted VPO catalysts were subjected to N2 adsorption–desorption measurements. The results are depicted in Fig. 2a and b and also summarized in Table 1. Among the promoted samples, 15 and 20 wt.% Al-VPO showed typical type-IV isotherms, indicating the presence of mesoporous structures. But as for the classification of porosity, the materials having pore sizes between 2 and 50 nm are mesoporous. In this context, all VPO materials fall into the mesoporous range. Again, VPO has a surface area below 15 m2 g 1 and very small pore volume. The Al-VPO samples exhibit significantly higher
Fig. 2. (a) Adsorption–desorption isotherm of VPO and different wt.% of Al-VPO samples; (b) pore size distribution curve of VPO and different wt.% of Al-VPO samples.
193
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198 Table 1 Textural properties and surface acidity of the catalysts. Catalyst
Surface area (m2/g)
Pore size (nm)
Pore volume (cm2/g)
Total acidity (mmol/g)
Total acidity (mmol/m2)
Lewis acid sites
Brønsted acid sites
VPO 5wt% Al-VPO 10 wt% Al-VPO 15 wt% Al-VPO 20 wt%Al-VPO
14 27 76 111 127
10.0 5.8 3.5 2.9 2
0.05 0.16 0.24 0.31 0.34
2.37 6.274 7.02 7.77 9.17
0.169 0.232 0.092 0.07 0.072
1.28 5.9 6.0 6.21 7.03
0.09 0.374 1.02 1.56 2.14
values of these parameters, relative to VPO. It can be seen that 20 wt.% Al-VPO presents the highest surface area and pore volume, with all pores being in the mesopore range. The surface area of the sample was observed to increase drastically with increasing aluminum loading. For this study, the deposit of aluminum did not stabilize the VPO phase because the impregnation method appeared to permit aluminum to be present on the VPO crystallite surface. The surface area increases with the increase of aluminum loading but is not linearly increased, that is why there are no separate phases in the materials. The decrease in pore size indicates the mesoporosity of the materials. The increase in surface area may presumably be due to increased amorphous alumina in the VPO materials.
3.1.3. FTIR studies The FTIR spectra of the bulk and promoted VPO catalysts are depicted in Fig. 3. All the catalysts showed sharp bands in the region of 400–3500 cm 1. The slightly broad spectra at 3370 cm 1 are due to the symmetric stretching mode of OAH groups. The infrared spectra of the catalysts in the region 900–1200 cm 1 correspond to the stretching modes of PAO and V@O groups. The band at 642–415 cm 1 can be attributed to the deformation vibrations of OAPAO groups of phosphate tetrahedral. Almost no shift is observed in the catalysts (bulk and promoted catalysts), especially in an important band at 970 cm 1 that corresponds to symmetric stretching vibrations of V4+@O groups. The peak at 1045 cm 1 can be assigned to symmetric stretching vibrations of PO3 groups, and the rest of the peaks at 1101 and 1200 cm 1 can be ascribed to asymmetric stretching vibrations of PO3 groups. The peak at 2376 cm 1 may be due to the adsorption of atmospheric CO2. It is strange to see there is no change in spectra obtained in Fig. 3. It only confirms different stretching and bending vibrations of
Fig. 3. FT-IR spectra of (a) c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.%Al-VPO.
VPO. However, it has been found that there is only a slight change of the peak at 1639 cm 1 with the increase in aluminum loading. 3.1.4. UV–vis DRS studies Optical absorption spectra of bulk and Al-VPO are depicted in Fig. 4. The investigations were carried out to obtain information on the vanadium oxidation state. The presence of a broad band at 550–650 nm in the bulk and Al-promoted catalysts indicates the presence of V4+ species in these catalysts [46]. However, the broad band at 450 nm in the DR UV–vis spectra of Al-containing samples can be related to the presence of V5+ species (VOPO4). According to this, different VAPAO phases with V ions in different oxidation states have been observed. However, this broad band at 450 nm was only prominently found in 5 wt.% Al-VPO, which indicates the presence of V5+ in the Al-VPO materials. This is also confirmed from the XRD peak. Hutchings et al. [22] reported that combinations of V4+ and V5+ phases are required for the catalyst to exhibit high activity and selectivity. However, the peak at 450 nm gets reduced with increased aluminum content. This may presumably be due to the reduction of the V5+ ion to V4+ with the increased aluminum content, as it contained isopropanol units during materials preparation. 3.1.5. Temperature-programmed desorption studies To comprehend the acidic properties of the catalysts, VPO and Al-VPO catalysts were subjected to NH3 TPD analysis. The typical NH3-TPD profiles are depicted in Fig. 5i and also summarized in Table 1. The total acidity of 2.37 mmol/g found for bulk VPO was lower than that of promoted catalysts. The total acidity of the pure Al2O3 was found to be 2.5 mmol/g. Again, the increase in the Al content in the parent VPO enhances the total acidity of the catalyst. That means that Al could contribute to the total acidity of the catalyst. Typically, NH3 TPD curves showed two clear peaks, which
Fig. 4. UV–vis DRS of (a) c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.% Al-VPO.
194
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 5. (i) NH3 TPD plot of (a) VPO, (b) 5 wt.% Al-VPO, (c) 10 wt.% Al-VPO, (d) 15 wt.% Al-VPO, and (e) 20 wt.% Al-VPO. (ii) Pyridine TPD plot of (a) c-Al2O3, (b) VPO, (c) 5 wt.% Al-VPO, (d) 10 wt.% Al-VPO, (e) 15 wt.% Al-VPO, and (f) 20 wt.% Al-VPO.
determine the existence of at least two types of acid sites. These peaks are called the l-peak (lower temperature) and h-peak (high temperature), respectively. The l-peak is attributed to desorption of weakly bound ammonia. It is relevant to gas sensing applications, due to the influence of the weakly bound ammonia on the proton mobility in VPO. The h-peak reflects desorption of ammonia from the strong acid sites (here its Lewis acid sites), which determines the acidic properties of VPO. Brønsted and Lewis acid sites were also quantitatively characterized by pyridine TPD (Fig. 5ii). Adsorption of pyridine on Al-VPO produced two types of adsorbed species: a pyridinium ion on Brønsted acid sites and a covalently bound species on Lewis acid sites. It has been found that the Lewis acid site increases with increased aluminum content. The peak maximum for 15 wt.% at lower temperature was due to the
increase of Lewis acid sites along with Brønsted acid sites. Since the materials contain two different phases, multiple peaks are observed in the profile. 3.1.6. X-ray photoelectron spectroscopy studies The XPS investigations reported here were aimed at the determination of the vanadium oxidation state present in the catalyst samples. The representative photoelectron peaks of Al2p, V2p3/2, O1s, and P2p are depicted in Fig. 6a–c, respectively. Since controversial interpretations of the broad right V2p3/2 photoelectron peak as well as somewhat contradictory values for the binding energies for the Vn+ contributions can be found in the literature, VOPO4 with phase structure found by UV–vis spectra was used to calibrate the binding energy of the V2p3/2 (V5+) peak. The
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
195
Fig. 6. XPS spectra for the Al-VPO catalysts, with Al loadings (wt.%) as indicated by each spectrum; (a) Al2p, (b) V2p, and O1s, and (c) P2p.
measured value of 517.5 eV for V5+ and the independently obtained value of 516.3 eV for V4+ [47] are in good agreement with the literature [48–50] and were used for the V2p3/2 peak deconvolution for all samples studied. The binding energies, 74.6 and 132.9 eV, correspond to Al2p and P2p in +3 and +5 oxidation states, respectively (Fig. 6a and c). The shift to higher binding energy of the Al2p peak with increasing Al loading (Fig. 6a) may reflect a change from Al2O3 clusters to Al2O3 monolayer deposition or a change in overlayer–support interaction. The surface compositions of the catalysts are shown in Table 2. The binding energy value 531.8 eV of the O1s peak indicated that the oxygen species was O2 in oxides, and therefore, the increase of O/V ratio supports
Table 2 Surface composition of the Al-VPO catalysts determined by analysis of the X-ray photoelectron spectra. Al2O3 (wt.%)
5 10 15 20
at.% O
V
P
Al
N
P/V
Al/V
74.0 74.0 73.7 73.2
9.2 9.5 8.7 8.7
14.8 13.6 13 12.7
1.1 1.6 3.2 3.7
0.8 1.4 1.5 1.7
1.61 1.42 1.50 1.46
0.119 0.168 0.367 0.425
the conclusion that a measurable fraction of V4+ at the equilibrated surface is oxidized to V5+. 3.1.7. Scanning electron microscopy studies The FE-SEM micrograph of the catalysts revealed that the samples possess a slatelike morphology (Fig. 7). Further, aggregates without regular shapes are observed in VPO. This is the reason for the low surface area of the VPO catalyst than of Al-VPO. However, the morphology of VPO was not affected significantly by Al doping. 3.1.8. Estimation of catalyst components by ICP-OES measurement To estimate the actual contents of all catalyst components, all the prepared catalysts undergo characterization using the ICP-OES technique. ICP analysis results of various Al-VPO catalysts are shown in Table 3. The contents of aluminum, vanadium, and phosphorus estimated from ICP are in good agreement with the nominal values. In addition, the contents of all these components were also checked again in used catalysts and were quite comparable to those of fresh samples. This fact suggests that there is no loss of catalyst components during the course of reaction. The P/V ratio detected here indicates the presence of V4+ species in the materials. From the literature, it has been found that a P/V ratio >1 is
196
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
Fig. 7. Scanning electron micrographs of VPO and Al-VPO.
Table 3 Estimation of Al, V, P contents in wt.% from ICP-OES in Al-VPO catalysts with different Al loadings. Catalyst
VPO 5 wt.% Al-VPO 10 wt.% Al-VPO 15 wt.% Al-VPO 20 wt.%Al-VPO
Al
V
P
P/V
Fresh
Used
Fresh
Used
Fresh
Used
00 5.04 10.09 15.13 20.18
00 5.04 10.09 15.13 20.18
18.7 13.01 12.84 12.40 12.01
18.7 13.01 12.83 12.40 12.01
28.91 19.83 19.58 18.85 18.25
28.90 19.83 19.58 18.84 18.25
1.54 1.52 1.52 1.52 1.52
necessary for high catalytic performance [51]. It has also been found that at P/V ratios of less than 1.0, a number of vanadium sites remain inactive, but at the higher P/V ratios, all surface sites are active [52]. This is in good agreement with our results.
3.2. Catalytic reaction The isopropylation of toluene with isopropanol is an electrophilic substitution reaction. Isopropylation reactions catalyzed by acids are commonly considered as proceeding via a carbonium-ion-type mechanism [53]. The reaction of toluene with isopropanol is shown in Scheme 1. The effects of various parameters on the isopropylation reaction are discussed later. The VPO and Al-promoted VPO catalysts are tested in order to have a com-
+ HO
toluene
Al-VPO
parative understanding of the catalytic activity for the reaction. Electrophilic substitution reactions such as isopropylation are catalyzed by strong Lewis acid sites. It was also reported that surface hydroxyl groups are responsible for the acidic nature of the catalyst [54]. Thus, the surface oxygen and surface hydroxyl group [55] play a crucial role in the catalytic activity of vanadium phosphate. The results of isopropylation of toluene with isopropanol using VPO and different wt.% of Al-promoted VPO in nitrobenzene as solvent are summarized in Table 4. It is clearly evident from Table 4 that VPO gives 80% p-cymene selectivity with 86% conversion in a period of 7 h. Also, 5 wt.% Al-VPO showed higher catalytic activity, giving 85% p-cymene selectivity with 90% conversion. However, the conversion decreases with increased aluminum content of VPO. Presumably the high wt.% of aluminum blocks the active site of the catalyst due to protonation of the hydroxyl groups present in the catalyst. The high selectivity of p-cymene may be due to the smaller steric hindrance of the methyl group at the para position, which is favored from the shape selectivity point of view [56].
3.2.1. Effect of reaction time The effect of the reaction period on the isopropylation of toluene with isopropanol using Al-VPO catalyst was studied at 150 °C with mole ratio 1:2 (toluene: isopropanol). The results are illustrated in Fig. 8. From the figure, it is found that the conversion of
+
+
150 o C ,7h
isopropanol p-cymene
o-cymene
Scheme 1. Isopropylation of toluene to cymene.
m-cymene
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
197
Table 4 Comparison of the activity of the bulk and promoted catalysts. Catalyst
Conversion (%)
VPO Al2O3 5 wt.% Al-VPO 10 wt.% Al-VPO 15 wt.% Al-VPO 20 wt.% Al-VPO
86 81 90 88 88 88
Cymene selectivity (%) Para
Ortho
Meta
Others
80 37 85 83 82 82
12 29 14 14 13 13
5 6 – 2 3 3
2 11 – – 1 1
Note: Conditions: temperature 150 °C, time 7 h, molar ratio 1:1 (toluene: isopropanol.
Fig. 9. Effect of reaction temperature on toluene conversion and p-cymene selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, time 6 h, molar ratio 1:2 (toluene: isopropanol).
Fig. 8. Effect of time period on toluene conversion and p-cymene selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, temperature 150 °C, molar ratio 1:2 (toluene: isopropanol).
toluene increases from 78% at 1 h to a maximum of 90% at 7 h. After 7 h, the conversion of toluene remains constant. This may presumably be due to the blocking of active sites of the catalyst with coke [57]. The selectivity to p-cymene exhibits a small but a significant increase with time. The presence of more Brønsted sites causes faster deactivation, which may be due to higher cracking activity and coke formation at strong acid sites of the catalysts. The coke deposits at the pore mouth and/or within the catalyst may reduce the effective channel dimensions, resulting in a tighter fit of the molecules and finer discrimination between the isomers [58].
150 °C over 5 wt.% Al-VPO catalyst with varying toluene-toisopropanol molar feed ratios from 2:1 to 1:3. The results are summarized in Fig. 10. In all cases, p-cymene was obtained as the major product, along with small amounts of o-cymene and m-cymene. It is clearly evident from the figure that toluene conversion increases with decreasing toluene content in the feed. Moreover, there exists competitive adsorption between toluene and isopropanol at the feed ratio 1:1. Hence with the increase in the molar ratio, the conversion of toluene increases. The p-cymene selectivity increases with the increase in the feed ratio from 2:1 to 1:2. The decrease in the selectivity to p-cymene at a feed ratio of 2:1 compared to 1:2 is due to the presence of more toluene content in the former, which leads to more chemisorption of toluene on the catalyst surface, thereby suppressing the chemisorption of isopropanol, which in turn facilitates the dealkylation rather than isopropylation of toluene. It is observed that isopropylation is favored only if toluene reacts from the vapor phase and isopropanol is chemisorbed on the catalyst surface, which could also be due to the trans alkylation of diisopropyltoluene formed due to the easy availability of isopropyl cations. Further there is a marginal decrease in p-cymene selectivity with the increase in the isopropyl content at a feed ratio of 1:3. This may presumably be due to the dilution of toluene with isopropanol. As said above, at a feed ratio of 2:1, toluene is more prone to
3.2.2. Effect of reaction temperature To accomplish better toluene conversion and maximum p-cymene selectivity, liquid-phase isopropylation of toluene was carried out over 5 wt.% Al-VPO catalyst at various reaction temperature, and the results are depicted in Fig. 9. It has been found that the toluene conversion increases up to 150 °C and remains constant thereafter. This could be attributed to the coke deposition caused by dealkylation, which in turn leads to the oligomerization of olefins. On the other hand, the increase in toluene conversion at lower temperatures may be due to the clustering of alcohols around the Brønsted acid sites of the catalyst through H-bonding. As a result, its dissociation to isopropyl carbocations will be suppressed at higher temperatures. But at higher temperatures, the alcohols will have a decreased tendency to form molecular clusters, and so they can freely form carbonium ions via protonation. 3.2.3. Effect of mole ratio of the reactants To study the effect of feed ratios on toluene conversion and product selectivity, an isopropylation reaction was carried out at
Fig. 10. Effect of molar ratio on toluene conversion and p-cymene selectivity over 5 wt.% Al-VPO. Conditions: catalyst 0.05 g, temperature 150 °C, time 6 h.
198
G.C. Behera et al. / Journal of Catalysis 289 (2012) 190–198
chemisorption on the catalyst surface, but the intermittent chemisorption of isopropanol would also be accomplished. This would abet the ortho position of the aromatic ring, making it easily accessible for electrophilic substitution. Hence, there is enhanced selectivity to o-cymene at a feed ratio of 2:1. Based on the above observations, the molar feed ratio 1:2 was found to be optimum for further studies. 4. Conclusions In summary, we have successfully synthesized Al-VPO. The XRD patterns of VPO and Al-VPO confirm their structure and good crystallinity. The N2 adsorption studies confirm the mesoporous nature of the materials. The pore diameter and pore volume are found to increase with increased promoting material. The acidity measurements of NH3 by TPD show that promotion by aluminum in a vanadium phosphate framework increases the number of total acid sites. The XPS spectra showed the different oxidation states of vanadium and other elements present in the materials. The isopropylation of toluene with isopropanol over these catalysts shows that toluene conversion increases with increasing temperature up to 150 °C, and further increase in temperature leads to a fall in toluene conversion. Based on toluene conversion, a reactant feed ratio of 1:2 and a reaction temperature of 150 °C are recommended. The present study reveals that 5 wt.% Al-VPO has a potential application in the production of p-cymene with high selectivity. Acknowledgment The authors are thankful to Prof. B.K. Mishra, Director, IMMT, Bhubaneswar, for his interest, encouragement, and kind permission to publish this work. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Kanazawa (Ed.), Inorganic Phosphate Materials, Elsevier, Tokyo, 1989, p. 1. A. Clearfield, Prog. Inorg. Chem. 47 (1998) 371. G. Centi, Catal. Today 16 (1993) 5. M.I. Khan, J. Zubieta, J. Prog. Inorg. Chem. 43 (1995) 1. P. Amoros, M.D. Marcos, A. Beltran, D. Beltran, Curr. Opin. Solid State Mater. Sci. 4 (1999) 123. V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catal. Today 28 (1996) 275. J.W. Johnson, D.C. Johnston, A.J. Jacobson, J.F. Brody, J. Am. Chem. Soc. 106 (1984) 8123. J.A. Lopez-Sanchez, J.K. Bartley, R.P.K. Wells, C. Rhodes, G.J. Hutchings, New J. Chem. 26 (2002) 1613. M.T. Sananes, I.J. Ellison, S. Sajip, A. Burrows, C.J. Kiely, J.C. Volta, G.J. Hutchings, J. Chem. Soc. Faraday Trans. 92 (1996) 137. F.J.C. Sanchez, R.P.K. Wells, C. Rhodes, J.K. Bartley, C.J. Kiely, G.J. Hutchings, Phys. Chem. Chem. Phys. 3 (2001) 4122. J. Ellison, G.J. Hutchings, M.T. Sananes, J.C. Volta, J. Chem. Soc. Chem. Commun. (1994) 1093.
[12] J.T. Vaughey, W.T.A. Harrison, A.J. Jacobson, D.P. Goshorn, J.W. Johnson, Inorg. Chem. 33 (1994) 248. [13] M. Schindler, W. Joswig, W.H. Baur, Eur. J. Solid State Inorg. Chem. 32 (1995) 109. [14] E. Bordes, Catal. Today 1 (1987) 499. [15] G. Centi (Ed.), Catal. Today. 16 (1993) 1. [16] G.J. Hutchings, Appl. Catal. 72 (1991) 1. [17] G.J. Hutchings, J. Mater. Chem. 14 (2004) 3385. [18] A.E. Badraoui, J.Y. Pivan, M. Maunaye, O. Pena, M. Louer, D. Louer, Ann. Chem. (Paris) 23 (1998) 97. [19] C.J. Kiely, A. Burrows, G.J. Hutchings, K.E. Bere, J.C. Volta, A. Tuel, M. Abon, Faraday Discuss. 105 (1996) 103. [20] J.R. Ebner, M.R. Thomson, Catal. Today 16 (1993) 51. [21] V.V. Guliants, J.B. Benziger, S. Sundaresan, N. Yao, I.E. Wachs, Catal. Lett. 32 (1995) 379. [22] G.J. Hutchings, A. Desmartin Chomel, R. Olier, J.C. Volta, Nature 368 (1994) 41. [23] U.R. Pillai, E. Sahle-Demessie, Appl. Catal. A Gen. 276 (2004) 139. [24] U.R. Pillai, E. Sahle-Demessie, Chem. Commun. (2002) 2142. [25] U.R. Pillai, E. Sahle-Demessie, New J. Chem. 27 (2003) 525. [26] C. Carline, P. Patrono, A.M.R. Galleti, G. Sbrana, V. Zima, Appl. Catal. A Gen. 289 (2005) 197. [27] K. You, L. Mao, L. Chem, D. Yin, P. Liu, H. Luo, Catal. Commun. 9 (2008) 2136. [28] M. Di Serio, M. Cozzolino, R. Tesser, P. Patrono, F. Pinzari, B. Bonellic, E. Santacesaria, Appl. Catal. A Gen. 320 (2007) 1. [29] C. Carlini, P. Patrono, A.M.R. Galletti, G. Sbrana, V. Zima, Appl. Catal. A Gen. 289 (2005) 197. [30] Z.Y. Xue, G.L. Schrader, J. Phys. Chem. 103 (1999) 9459. [31] G.J. Hutchings, Appl. Catal. A 72 (1991) 1. [32] G.J. Hutchings, R. Higgins, J. Catal. 162 (1996) 153. [33] G.J. Hutchings, Catal. Today 16 (1993) 139. [34] F. Cavani, F. Trfiro, Appl. Catal. A Gen. 157 (1997) 195. [35] G. Busca, G. Ramis, V. Lorenzelli, J. Mol. Catal. 50 (1989) 231. [36] L.M. Cornaglia, E.A. Lombardo, J.A. Anderson, J.L. Garcia Fierro, Appl. Catal. A Gen. 100 (1993) 37. [37] L.M. Cornaglia, E.A. Lombardo, Stud. Surf. Sci. Catal. 90 (1994) 429. [38] S. Ben Abdelmalek, H. Batis, A. Ghorbel, J. Soc. Alger. Chem. 3S (1993) 265. [39] M. Abon, J.C. Volta, Appl. Catal. A Gen. 157 (1997) 173. [40] G.A. Olah, S.H. Flood, S.J. Kuhn, M.E. Moffatt, N.A. Overchuck, J. Am. Chem. Soc. 86 (1964) 1046. [41] M. Selvaraj, A. Pandurangan, K.S. Seshadri, P.K. Sinha, V. Krishnaswamy, K.B. Lal, J. Mol. Catal. 186 (2002) 173. [42] C. Venkatesan, M. Chidambaram, K.R. Kamble, A.P. Singh, Catal. Lett. 85 (2003) 171. [43] R. Savidha, A. Pandurangan, Appl. Catal. A Gen. 276 (2004) 39. [44] G. Busca, F. Cavani, G. Centi, F. Trifiro, J. Catal. 99 (1986) 400. [45] N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater. 13 (2001) 179. [46] B. Solsona, V.A. Zazhigalov, J.M. López Nieto, I.V. Bacherikova, E.A. Diyuk, Appl. Catal. A 249 (2003) 81. [47] Y. Suchorski, L. Rihko-Struckmann, F. Klose, Y. Ye, M. Alandjiyska, K. Sundmacher, H. Weiss, Appl. Surf. Sci. 249 (2005) 231. [48] S. Albonetti, F. Cavani, F. Trifiro, P. Venturoli, G. Calestani, M. Lopez Granados, J.L. Fierro, J. Catal. 160 (1996) 52. [49] M. Abon, K.E. Bere, A. Tuel, P. Delishere, J. Catal. 156 (1995) 28. [50] P. Delishere, K.E. Bere, M. Abon, Appl. Catal. A Gen. 172 (1998) 295. [51] F. Garbasi, J.C.J. Bart, F. Montino, G. Petrini, Appl. Catal. 16 (1985) 271. [52] A. Satsuma, A. Hattori, K. Mizutani, A. Furuta, A. Furuta, A. Miyamoto, T. Hattori, Y. Murakami, J. Phys. Chem. 93 (1989) 1484. [53] P.B. Venuto, L.A. Hamilton, P.S. Landis, J. Catal. 5 (1996) 484. [54] Y. Wang, W. Li, React. Kinet. Catal. Lett. 69 (2000) 169. [55] H. Knozinger, H. Buhl, K. Kochloefl, J. Catal. 24 (1972) 57. [56] P.A. Parikh, N. Subrahmanyam, Y.S. Bhat, A.B. Halegeri, Chem. Eng. J. 54 (1994) 79. [57] P.A. Parikh, N. Subrahmanyam, Appl. Catal. A Gen. 90 (1992) 1. [58] W.W. Kaeding, C. Chu, L.B. Young, B. Weinstein, S.A. Butter, J. Catal. 67 (1981) 159.