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Selective tert-butylation of phenol over molybdate- and tungstate-promoted zirconia catalysts
Applied Catalysis A: General 332 (2007) 183–191 www.elsevier.com/locate/apcata
Selective tert-butylation of phenol over molybdate- and tungstate-promoted zirconia catalysts Benjaram M. Reddy *, Meghshyam K. Patil, Gunugunuri K. Reddy, Baddam T. Reddy, Komateedi N. Rao Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500007, India Received 2 April 2007; received in revised form 18 July 2007; accepted 19 July 2007 Available online 25 July 2007
Abstract The catalytic performance of molybdate- and tungstate-promoted zirconia catalysts for tert-butylation of phenol with tert-butanol as alkylating agent has been investigated. The reaction was carried out at 433–493 K in a fixed bed micro-reactor at normal atmospheric pressure. The promoted zirconia catalysts were synthesized by immersing the finely powdered hydrous zirconium hydroxide in aqueous ammonium heptamolybdate or ammonium metatungstate solution and subsequent oven drying and calcination at 923 K. The hydrous Zr(OH)4 was prepared from aqueous zirconium oxychloride by hydrolysis with a dilute aqueous ammonia solution. To investigate the structural and textural properties, we characterized the synthesized catalysts by X-ray powder diffraction (XRD), BET surface area, temperature programmed reduction (TPR), temperature programmed desorption (TPD) of ammonia, scanning electron microscopy (SEM), Raman spectroscopy (RS), and FT-infrared spectroscopy (FTIR) techniques. All the characterization results reveal that the incorporated promoter cations show a significant influence on the surface and bulk properties of the ZrO2. In particular, the impregnated cations stabilize the metastable tetragonal phase of zirconia at ambient conditions and enhance the total number and the strength of the acid sites. A good substrate (phenol) conversion and excellent product selectivity were obtained over the promoted zirconia catalysts. # 2007 Elsevier B.V. All rights reserved. Keywords: Alkylation; Phenol; Solid acid; Characterization; Tetragonal zirconia
1. Introduction Short chain alkylation of phenols is an industrially significant reaction, because many alkylated phenols are important intermediates in the manufacture of anti-oxidants, ultra-violet absorbers, phenolic resins, drugs, dyes, polymerisation inhibitors, lube additives, and heat stabilisers for polymeric materials [1]. The catalytic reaction of phenol with tert-butanol has been extensively studied by using various homogeneous and heterogeneous catalysts [2–8]. tert-Butyl derivatives of phenol are intermediates in the synthesis of various agrochemicals, fragrances, thermo-resistant polymers, protecting agents for plastics, and precursors for numerous commercially important anti-oxidants [1]. 4-tert-Butyl phenol
* Corresponding author. Tel.: +91 40 27160123; fax: +91 40 27160921. E-mail addresses: [email protected], [email protected] (B.M. Reddy). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.07.026
(4-TBP) imparts improved performance properties to the class of metallic detergents used in the lubricating oils, and in the production of substituted tri-aryl phosphates. 2,4-di-tert-Butyl phenol (2,4-DTBP) is used in the manufacture of its triphosphite, which is employed as a co-stabiliser for polyvinyl chloride [2]. The tert-butylation of phenol is a typical FriedelCrafts alkylation and can be catalysed by different acid sites. In general, tert-butylation of phenol is carried out in homogeneous media using sulfuric acid, flurosulphonic acid, arenesulphonic acid, phosphoric acid, and aluminium chloride with boric acid or boron trifluoride etherate as catalysts [1]. Several problems are associated with these catalysts: toxicity, corrosiveness, the disposal of effluents, and the separation of products from the catalysts. In recent years, catalytic chemists have turned their attention to the use of environmentally friendly catalysts in the place of mineral acids. Thus, several efforts are being reported in the literature to replace the conventional homogeneous Friedel-Crafts catalysts by solid acids to fit in well with the concept of environmentally benign
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Scheme 1. Reaction scheme of tert-butylation of phenol with possible products depending on acidity.
systems for eco-friendly processes [9]. Thus zeolites, acid treated clays, ion exchange resins and heteropolyacids have been investigated by several groups for their applications in pharmaceutical, perfumery, agrochemical, and dyestuff intermediate and specialty chemical industries. A few studies have been reported on the tert-butylation of phenols using various solid acids such as micro-porous molecular sieves, cation exchange resins, clays, zeolites, and supported metal oxides [10–13]. The presence of a phenolic (–OH) group in the reactant kinetically favors the O-alkylation [14]. However, due to steric hindrance, thermodynamically unfavored oisomer, viz., 2-tert-butyl phenol (2-TBP) readily isomerizes into less hindered, partially kinetically favorable p-isomer [14,15]. Further, the selectivity of the product depends on the nature of the acid sites present in the catalysts as well as on the reaction temperature [4–6,8,16,17]. A schematic representation of the reaction is shown in Scheme 1. As can be noted from Scheme 1, the weak acidic catalyst, e.g. Naand K-ion-exchanged zeolite-Y leads to O-alkylated product, viz., tert-butyl phenyl ether (t-BPE) as the major product [5,15]. With the strong acidic catalysts like zeolite-b [18] or at high temperatures, the reaction produces Calkylated product, viz., 3-tert-butyl phenol (3-TBP), which is formed by the secondary isomerization of initially formed o- and p-isomers [4,8,18]. On the other hand, the moderate acidic catalysts such as ZSM-12 [16], SAPO-11 [17], and zeolite-Y [4,5] are suitable for the formation of p-isomers. Promoted zirconia solid acids, in particular, have been the target catalysts among various solid acids described in earlier reports due to their proven advantages. Zirconia is the only metal oxide that possesses all four chemical properties: namely acidic, basic, reducing ability and oxidizing ability [19–23]. Higher thermal stability, extreme hardness and high specific mass make the zirconia as an attractive material for catalytic applications. When dopants like VOx, MoOx and WOx are incorporated into the zirconia lattice, the interactions between the zirconia surface and the dispersed metal oxides lead to
drastic changes in the acidic properties of the catalyst [24,25]. Reports are also available in the literature on the alkylation of phenol over sulfated zirconia with high conversion and moderate selectivity [26,27]. However, the easy loss of sulfate ion in the form of sulfur compounds (SO2 or H2S) during thermal treatments or regeneration of the catalyst and also water poisoning limits their application [28,29]. The best alternative for sulfated zirconia could be molybdate- or tungstate-promoted catalysts, which are considered to be Green catalysts [30–34]. The so-called tungstated zirconia catalysts have been the subject of numerous investigations in view of their commercial significance [31,33]. Therefore, the present study was undertaken against the above background. In this study, molybdate- and tungstate-promoted zirconia catalysts were prepared by a wet impregnation method and investigated for vapour phase tert-butylation of phenol. The synthesized catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), FT-infrared spectroscopy (FTIR), temperature programmed reduction (TPR), temperature programmed desorption (TPD) of ammonia, scanning electron microscopy (SEM), and BET surface area techniques. 2. Experimental 2.1. Catalyst preparation The zirconium hydroxide was prepared from zirconium oxychloride by hydrolysis with dilute aqueous ammonia solution. For this purpose, the requisite quantity of ZrOCl28H2O (Loba Chime, GR grade) was dissolved in doubly distilled water and to this clear solution, aqueous NH3 was added drop-wise with rigorous stirring until the pH of the solution reached 8. The obtained precipitate was washed with hot distilled water several times until free from chloride ions and then dried at 393 K for 24 h. On the obtained Zr(OH)4, a constant 6-mol% molybdate or tungstate ions were doped by adopting a wet impregnation method. To achieve this, we dissolved the requisite quantities of ammonium heptamolybdate (JT Baker, England, GR grade) and ammonium metatungstate (BDH Chemical Ltd., AR grade) separately in doubly distilled water. To these clear solutions the desired quantity of Zr(OH)4 was added and the excess water was evaporated on a water-bath. The resulting samples were oven dried at 393 K for 24 h and calcined at 923 K for 6 h in a flow of oxygen. A small portion of the hydrous zirconia was also calcined at 923 K for 6 h to obtain the ZrO2. 2.2. Catalyst characterization The BET surface area measurements were made using a Gemini 2360 instrument by N2 physisorption at liquid nitrogen temperature. Prior to analysis, samples were degassed at 393 K under vacuum for 8 h to remove the adsorbed moisture and other volatiles. The surface densities expressed as the number of atoms per nanometer square area
B.M. Reddy et al. / Applied Catalysis A: General 332 (2007) 183–191
were calculated using the equation
2.3. Catalytic activity
Surface density ¼
fPromoter loadingðwt%Þ=100g 6:023 10 Formula weight of promoter BET surface areaðm2 g1 Þ 1018
23
The XRD patterns were recorded on a Siemens D-5000 diffractometer using Cu Ka radiation source and a scintillation counter detector. The XRD phases present in the samples were identified with the help of ASTM powder data files. The amount of monoclinic and tetragonal phases present in the samples was estimated by comparing the areas under the characteristic peaks of monoclinic (2u = 28.58 and 31.68 for the (1 1 1) reflections) and tetragonal phases (2u = 30.48 for the (1 1 1) reflection), respectively. Gaussian curves were first fitted to the XRD pattern at the characteristic peaks and the height (h) and width at half-height (w) were then obtained. The percent composition of each phase was calculated from the Gaussian areas h w: %Monoclinic ¼
%Tetragonal ¼
185
Sðh wÞmonoclinic Sðh wÞmonoclinic and tetragonal Sðh wÞtetragonal Sðh wÞmonoclinic and tetragonal
The Raman spectra were obtained on a DILOR XY spectrometer that was equipped with a liquid-nitrogen cooled charge coupled device (CCD) detector. The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics) was focused on the sample under a microscope, and the width of the analyzed spot was 1 mm. The power of the incident beam on the sample was 3 mW. Time of acquisition was adjusted according to the intensity of Raman scattering. The wave number values reported from the spectra are accurate to within 2 cm1. For each solid, the spectra were recorded at several points of the sample to ascertain the homogeneity and the averages of all these spectra were plotted in the figure presented in this study. The FTIR spectra were recorded on a Nicolet 740 FTIR spectrometer at ambient conditions using KBr discs with a nominal resolution of 4 cm1 and averaging 100 spectra. TPR and TPD studies were carried out on an Auto Chem 2910 (Micromeritics, USA) instrument. Prior to experiments the samples were preheated with appropriate gas mixtures at the desired temperatures. The TPR experiment was carried out from room temperature to 1273 K under a flow of 5% hydrogen in N2 at a constant heating rate of 5 K min1. Prior to each TPD run, the sample was saturated with 10% ultra pure anhydrous ammonia gas at 353 K for 2 h and subsequently flushed with He gas at 373 K for 2 h to remove the physisorbed ammonia. SEM analyses were carried out with a Hitachi model-520 instrument. The sample was mounted on a silver sample holder with the help of an adhesive to make the sample surface conductive and was coated with gold metal at 10 mm Hg pressure.
The tert-butylation of phenol was carried out in a fixed bed down flow micro-reactor made up of Borosil glass (25 cm long and 8 mm i.d.). About 0.2 g of catalyst sample mixed with 0.5 g of inert glass particles was placed in the reactor and supported on either side with a thin layer of quartz wool and ceramic beds. The catalyst was activated at 573 K in a flow of N2 for 3 h, followed by cooling to the desired reaction temperature. The reactant mixture, viz., phenol and tert-butyl alcohol, with a desired ratio and weight hourly space velocity (WHSV) was fed into the reactor using a liquid injection pump and nitrogen as the carrier gas. The gaseous products were cooled and the condensed liquid products were collected at 30 min intervals. The products, viz., p-TBP, o-TBP and 2,4-DTBP were identified by gas chromatography (CIC 2011) with SE-30 column, while m-TBP was identified by employing AT1000 column. Further, the products were conformed using a combined gas chromatograph–mass spectrometry (GC–MS) using HP-5 capillary column. 3. Results and discussion 3.1. Catalytic characterization The X-ray powder diffraction patterns of promoted zirconia samples along with unpromoted ZrO2 for the purpose of comparison are shown in Fig. 1. The corresponding monoclinic and tetragonal phase composition is presented in Fig. 2. As shown in Fig. 1, the unpromoted zirconia obtained by calcination of the hydrous zirconia at 923 K is in poorly crystalline form with monoclinic ZrO2 phase dominating over the tetragonal phase. The sharp diffraction lines at 2u = 28.3, 24.46 and 31.58 degrees correspond to the monoclinic form and
Fig. 1. X-ray diffraction patterns of unpromoted and promoted zirconia catalysts calcined at 923 K; (!) lines due to tetragonal phase; (~) lines due to monoclinic phase.
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Fig. 2. Phase composition of unpromoted and promoted zirconia catalysts.
the lines at 2u = 30.27 and 49.21 degrees are due to tetragonal form of ZrO2. As can be noted from Fig. 1, the tetragonal phase is dominating over the monoclinic phase when dopants (molybdate and tungstate) were incorporated into the hydrous zirconia structure. A through examination of the XRD lines further indicates that the molybdate promoter shows a relatively stronger influence than tungstate on the monoclinic to tetragonal phase modification of ZrO2. As can be noted from Fig. 2, both the promoted samples possess more tetragonal phase (84 and 68 %) than the unpromoted zirconia (24%). This could be due to incorporation of promoter cations into the structure of zirconia, forming solid solutions, which substantially decrease the specific surface free energy of zirconia and favour the tetragonal phase, which has a lower surface free energy than the monoclinic form [35–37]. Further, no reflections corresponding to crystalline MoO3 or WO3 were observed, indicating a strong interaction of the dispersed promoters with the zirconia. Dispersed molybdenum oxide on various supports can exist as MoOx monomers, two-dimensional oligomers, and bulk MoO3 [38]. The poly-molybdate saturation capacity on zirconia support has been reported to be 5 Mo nm2 [39] and the Mo surface density obtained in this study is about 4.6 Mo nm2, close to the saturation value. Thus, the molybdenum oxide probably exists as isolated or twodimensional oligomers instead of the as bulk MoO3. A similar analogy could be extended to WOx/ZrO2 catalyst. Also, there is no evidence for the formation of new phases like Zr(MoO4)2 and Zr(WO4)2, although the existence of Mo–O–Zr and W–O– Zr bonds are highly possible. Zirconia normally exists in the monoclinic, tetragonal and cubic structures. The pure zirconia monoclinic phase in the absence of any impurities is stable up to 1373 K, and transforms into tetragonal phase as the temperature increases to 1473 K. On cooling the high temperature tetragonal phase transforms back to the monoclinic phase with a large hysterisis. However, in the present study, the
Catalyst
BET SA (m2 g1)
NH3 desorbed (a.u.)
Surface density
ZrO2 MoO3/ZrO2 WO3/ZrO2
46 94 64
0.01 11.0 9.0
0 4.65 Mo nm2 6.08 W nm2
stabilization of tetragonal phase is noted at a lower temperature, which is important for catalytic application of these materials. The BET surface areas, the amounts of ammonia desorbed and the surface densities of various samples are presented in Table 1. As can be noted from Table 1, the molybdate- and tungstate-promoted samples exhibit more specific surface areas (94 and 64 m2 g1) than the unpromoted zirconia (46 m2 g1). The observed increase in the specific surface areas may be due to the formation of Mo–O–Zr and W–O–Zr linkages, resulting in the formation of porous materials [40]. Temperature programmed desorption (TPD) of probe molecules like ammonia and pyridine is a popular method for the determination of acidity as well as acid strength of solid catalysts because it is an easy and reproducible method. Ammonia is used frequently as a probe molecule because of its small molecular size, good stability and strong basic strength [38]. The ammonia TPD profiles of molybdate- and tungstate-promoted catalysts exhibited two temperature maximums, indicating the presence of two different acid sites with different acid strength distributions. The total amount of ammonia desorbed at higher temperatures in the case of promoted samples is much larger than that of unpromoted zirconia (Table 1). The difference clearly indicates that impregnated cations show a strong influence on the acidic properties of the zirconia. It appears from the present study that the strong acidity of Mo- and Wpromoted zirconia catalysts is mainly due to the preparation procedure adopted. In the normal preparation, the amorphous Zr(OH)4 loses water and crystallizes predominantly into the monoclinic form with some small amounts of tetragonal zirconia. The modifiers such as ammonium heptamolybdate and ammonium metatungstate profoundly change this crystallization phenomenon. These modifiers influence the crystallization characteristics of the zirconia and result in the formation of tetragonal zirconia. Additionally, these modifiers can also stabilize the formed tetragonal phase. Raman spectra of unpromoted and promoted zirconia samples are presented in Fig. 3. The assignment of Raman bands of metal oxide overlayers in supported oxide catalysts is typically based on comparisons with the spectra of structurally well-characterized reference compounds [41]. According to the literature, the Raman bands of the supported metal oxide catalysts obtained after calcination in O2 are in the range between 1050 and 950 cm1 and can be unambiguously assigned to the symmetric stretching mode of short terminal M O bonds, whereas the bands between 750 and 950 cm1 are attributed to either anti-symmetric stretch of M–O–M bonds or the symmetric stretch of the (–O–M–O–)n bonds. The spectrum of pure ZrO2 calcined at 923 K exhibits the Raman bands
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187
Fig. 3. Raman spectra of unpromoted and promoted zirconia catalysts calcined at 923 K.
pertaining to a mixture of monoclinic (180, 188, 221, 331, 380, 476, and 637 cm1) and tetragonal (148, 290, 311, 454, and 647 cm1) phases and the lines due to tetragonal phase are less intense than the lines due to monoclinic phase [42]. In the spectra of promoted catalysts, bands representing the tetragonal phase are relatively prominent. Crystalline WO3 shows characteristic Raman bands at 807, 715, and 274 cm1 [42]. Absence of these bands indicates that microcrystalline WO3 are not formed on the surface of W-ZrO2 catalyst. The band at 1020 cm1 assigned to W O stretching mode is normally observed for mono and poly tungstate species whose presence is clearly visible from Fig. 3. The appearance of shoulder peaks at higher frequency region indicates the presence of geometrically different WOx species on the surface; these may be octahedral- or tetrahedral-coordinated WOx species. Similarly there were no peaks pertaining to crystalline molybdenum oxide in the Raman results. Hence, the absence of these bands suggests that the impregnated oxides are strongly interacting with the zirconia [43]. FT-IR spectra of uncalcined, calcined and promoterincorporated zirconia samples are presented in Fig. 4. Irrespective of the promoters, all the samples show two common bands, which are situated at around 3350 and 1620 cm1. The band observed at 3350 cm1 is due to surface hydroxyl groups. The intensity of this band is weak in the case of calcined support and promoted zirconia samples. This could be due to loss of hydroxyl groups in the form of water during calcination. The peak at 1620 cm1 could be attributed to the vibrations of acidic –OH groups [44]. In addition to these two bands, the tungstate-promoted sample shows another band at 1000 cm1, which was assigned to poly tungstate structure with tungsten in octahedral coordination [45–47]. Therefore, symmetrical W O stretching vibration was assigned to octahedrally co-ordinated WOx species on the surface of tetragonal zirconia, supporting Raman observa-
Fig. 4. FT-IR spectra of uncalcined and calcined zirconia and promoted zirconia catalysts.
tions. This unique vibration mode was observed in the case of tungstate-promoted sample, because the WOx species are more homogeneously distributed on the surface of zirconia. Normally crystalline WO3 shows peaks around 791 and 1110 cm1. Absence of these bands suggests that the tungsten oxide is in a highly dispersed state on the surface of the zirconia. Generally, in the case of molybdate-promoted samples the region at 1050–900 cm1 represents terminal Mo O stretching vibration and that at 800–700 cm1 represents the region of anti-symmetric Mo–O–Mo or O– Mo–O stretching vibrations. In the present study molybdatepromoted samples also exhibit a band at 1000 cm1. This band may be due to stretching vibration mode of terminal Mo O, in agreement with Raman results. Frusen et al. [48] observed the formation of ZrMo2O8 compound by heating ZrO2 and MoO3 together at 820 K, which showed the IR bands at 989, 920, and 800 cm1. In the present study, no such compound formation was observed, in line with XRD and Raman results. This could be attributed to a different preparation method adopted and the precursors used in the present study. Thus, the IR results corroborate well with the Raman and XRD results. The reduction properties of pure support and promoted samples were studied by TPR. The hydrogen consumption profiles obtained as a function of temperature are plotted in Fig. 5. The TPR profiles indicate a steady increase in the peak height with increase of temperature due to loss of water. The profile of pure zirconia shows that there is a continuous loss of oxygen from the lattice. The reduction of molybdena essentially can take place in two steps. The first step is
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Fig. 5. Temperature programming reduction profiles of unpromoted and promoted zirconia samples.
reduction of MoO3 ! MoO2 (Mo+6 ! Mo+4) and the second step is MoO2 ! Mo (Mo+4 ! Mo). The TPR profile of MoOx/ ZrO2 catalyst exhibits two temperature maximums, one at 648 K and another at 973 K. The first peak observed at 648 K could be due to reduction of Mo+6 to Mo+4 ions and the second peak could be assigned to the reduction of Mo+4 to Mo. Similarly, tungstate-promoted samples also show two temperature maximums at 698 and 1053 K. The peak observed in the low temperature region is due to reduction of W+6 to W+4 and the high temperature region peak may be due to reduction of W+4 to W. To study the surface topography and to assess the surface dispersion of the active components over the support, we performed SEM investigations on various samples calcined at 923 K, as presented in Fig. 6. In the micrograph of Zr(OH)4 calcined at 923 K, though crystallinity is observed, there are also certain cracks on the surface that may be attributed to the loss of water molecules during the calcination. As can be seen from the micrograph of MoOx/ZrO2 catalyst, the active component is equally spread on the surface of the support. Formation of bigger crystals of active components is noted in the case of tungstate-promoted samples. 3.2. Catalytic studies As mentioned earlier, the phenol alkylation with tert-butanol is a typical Friedel-Crafts reaction and can be catalyzed by acid sites. The main products of the reaction are 2-TBP (2-tert-butyl phenol), 4-TBP (4-tert-butyl phenol), 3-TBP (3-tert-butyl phenol), 2,4-DTBP (2,4-di tert-butyl phenol), 2,4,6-TTBP
Fig. 6. SEM micrographs of (a) ZrO2; (b) MoO3/ZrO2; and (c) WO3/ZrO2 calcined at 923 K.
(2,4,6-tri tert-butyl phenol) and TBPE (tert-butyl phenyl ether). Along with these alkylated products, there is a chance to form C8 and C12 olefins by the oligomerization of isobutene, where isobutene is formed by the acid-catalyzed dehydration of tertbutanol. In the present study, 4-TBP was obtained as the major product along with small amounts of ortho- and di-alkylated products, while the formation of 3-TBP was not observed. The results of tert-butylation of phenol over the promoted zirconia catalysts prepared in the present study with various tert-butanol to phenol ratios (TB/P) at a reaction temperature of
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189
Table 2 Effect of tert-butanol to phenol molar ratio on phenol tert-butylation over promoted zirconia catalysts at 473 K and WHSV = 5 h1 Molar ratio 1:2
1:1
2:1
3:1
MoO3
WO3
MoO3
WO3
MoO3
WO3
MoO3
WO3
Conversion (%)
45
42
55
52
62
58
63
59
Selectivity (%) 4-TBP 2-TBP 2,4-DTBP Others
88.4 6.6 4.3 0.7
88.3 7.0 4.1 0.6
88.1 6.5 4.7 0.7
88.2 6.2 4.8 0.8
87 6.3 6.0 0.7
87.3 6.0 6.1 0.6
85.8 6.0 7.5 0.7
86.4 5.7 7.1 0.8
473 K and at WHSV of 5 h1 are summarized in Table 2. In comparison to promoted catalysts, the unpromoted zirconia showed insignificant conversion (less than 2%) and the product selectivity hence was not investigated thoroughly. As can be seen from Table 2, the two promoted catalysts show good conversion with excellent para selectivity of the products. Further, it is clear from Table 2 that an increase in TB/P ratio increases the phenol conversion. It was reported earlier [17] that polar molecules such as methanol and higher alcohols compete with substrate for absorption sites and an increase in the molar excess of alkylating agent results in an increase in the conversion. However, the selectivity to 2,4-DTBP increases with increasing amounts of tert-butanol in the feed at the expense of the mono alkylated products 4-TBP and 2-TBP. The increase in the selectivity for higher alkylated product 2,4DTBP is probably due to the increased availability of tertbutanol molecules in the reactant feed. Similar results have been reported for AlMCM-41 [49,50] and micro-porous alumino phosphate catalysts [51,52]. The increase in the selectivity towards 2,4-DTBP is more in the case of the molybdate-promoted sample. It may be due to strong acidity of the sample [6]. Hence, for further studies the optimum tertbutanol to phenol ratio of 2 was chosen.
In order to study the deactivation behavior of the catalysts we followed the reaction for several hours at 473 K with a WHSVof 5 h1 using a TB/P molar ratio of 2 (Fig. 7). It is clear from this figure that, unlike the molecular sieve-based solid acid catalysts such as SAPO-11 [17], the catalytic activity of the promoted zirconia catalysts remains constant even after several hours. The selectivity of different products is also similar during the time-on-stream runs. The reaction was also studied at various temperatures from 433 to 493 K at WHSV of 5 h1 and TB/P molar ratio of 2. The variation of phenol conversion and selectivity to para product with temperature are shown in Fig. 8. At 433 K the phenol conversion was 52% and it increased to 62% at 473 K. An increase of reaction temperature above 473 K resulted in the decrease of phenol conversion. This behavior is often observed in alkylation reactions and is mainly due to the thermodynamics of alkylation and de-alkylation. Moreover, the olefins produced by dehydration of tert-butanol are probably consumed in more than one parallel reaction, such as alkylation, oligomerization and cracking. It is well known that oligomerization and cracking are dominant at high reaction temperatures, which result in a reduced phenol conversion at higher temperature even in excess of alkylating agent. In addition, at higher
Fig. 7. Time-on-stream (h) vs. conversion (%) and selectivity (%) of promoted zirconia catalysts for selective tert-butylation of phenol at constant temperature (473 K), WHSV (5 cm1) and TB/P ratio (2).
Fig. 8. Effect of temperature (K) on tert-butylation of phenol over MoO3/ZrO2 and WO3/ZrO2 catalysts at constant WHSV (5 cm1) and TB/P ratio (2).
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gel. An unpromoted ZrO2 was also prepared by calcination of the hydrous zirconia. The resultant samples were characterized by XRD, Raman, FT-IR, TPR, ammonia TPD, SEM and BET SA techniques. No peaks pertaining to either promoter oxides or compounds between the promoter oxides and the support were observed in XRD results. The XRD results further reveal that the promoted zirconia catalysts possess more tetragonal phase than the unpromoted sample. The ammonia TPD results suggest that promoters create more strong acidic sites. The promoted catalysts exhibited a high phenol conversion at 493 K reaction temperature with 5 h1 WHSV and a TB/P ratio of 2. Increasing the tert-butanol to phenol molar ratio increased the phenol conversion. Further, the catalytic activity was quite stable for several hours and did not change with repeated use of the catalysts. Fig. 9. Conversion and selectivity profiles of promoted zirconia catalysts with different WHSV (cm1) and at constant temperature 473 K and TB/P ratio 2.
temperatures, de-alkylation of the formed butyl phenols becomes more dominant. The selectivity of 4-TBP increased with increasing reaction temperature, whereas for increases in the temperature the selectivities of 2-TBP and 2,4-TBP were decreased. However, at higher temperatures, the observed higher p-TBP selectivity could be due to the absence of a secondary alkylation reaction [6]. The effect of WHSV on phenol conversion and product selectivity over promoted zirconia catalysts was studied at 473 K with a TB/P ratio of 2. The results obtained for different WHSV are illustrated in Fig. 9. It was observed that increasing space velocity increased the phenol conversion, but after 5 h1 WHSV the phenol conversion decreased. The lower conversion at lower space velocity could be due to de-alkylation of the alkylated products as well as coke formation [17,49], while at higher space velocity, it could be due to a high diffusion rate of the reaction mixture through the catalyst. The selectivity towards 4-TBP increased and that towards 2,4-DTBP decreased with increasing space velocity. The increase of 4-TBP selectivity with increasing space velocity has been ascribed to the elimination of inner particle diffusion resistance at higher space velocities [49]. However, at the optimum conditions (TB/ P = 2, temperature 473 K and WHSV 5 h1) the promoted zirconia catalysts provided good conversion and excellent para selectivity. To check the effect of promoters we also carried out the reaction with sulfated zirconia at a reaction temperature of 473 K, WHSV of 5 h1 and TB/P molar ratio of 2. Although slightly higher conversion was observed over sulfated zirconia, the selectivity towards para product is still poor and also more of di-alkylated product is formed. This may be due to strong solid acidity of the sample. 4. Conclusions Molybdate- and tungstate-promoted zirconia catalysts with reasonably high specific surface areas were prepared by a wet impregnation method using uncalcined zirconium hydroxide
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Selective tert-butylation of phenol over molybdate- and tungstate-promoted zirconia catalysts Benjaram M. Reddy *, Meghshyam K. Patil, Gunugunuri K. Reddy, Baddam T. Reddy, Komateedi N. Rao Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500007, India Received 2 April 2007; received in revised form 18 July 2007; accepted 19 July 2007 Available online 25 July 2007
Abstract The catalytic performance of molybdate- and tungstate-promoted zirconia catalysts for tert-butylation of phenol with tert-butanol as alkylating agent has been investigated. The reaction was carried out at 433–493 K in a fixed bed micro-reactor at normal atmospheric pressure. The promoted zirconia catalysts were synthesized by immersing the finely powdered hydrous zirconium hydroxide in aqueous ammonium heptamolybdate or ammonium metatungstate solution and subsequent oven drying and calcination at 923 K. The hydrous Zr(OH)4 was prepared from aqueous zirconium oxychloride by hydrolysis with a dilute aqueous ammonia solution. To investigate the structural and textural properties, we characterized the synthesized catalysts by X-ray powder diffraction (XRD), BET surface area, temperature programmed reduction (TPR), temperature programmed desorption (TPD) of ammonia, scanning electron microscopy (SEM), Raman spectroscopy (RS), and FT-infrared spectroscopy (FTIR) techniques. All the characterization results reveal that the incorporated promoter cations show a significant influence on the surface and bulk properties of the ZrO2. In particular, the impregnated cations stabilize the metastable tetragonal phase of zirconia at ambient conditions and enhance the total number and the strength of the acid sites. A good substrate (phenol) conversion and excellent product selectivity were obtained over the promoted zirconia catalysts. # 2007 Elsevier B.V. All rights reserved. Keywords: Alkylation; Phenol; Solid acid; Characterization; Tetragonal zirconia
1. Introduction Short chain alkylation of phenols is an industrially significant reaction, because many alkylated phenols are important intermediates in the manufacture of anti-oxidants, ultra-violet absorbers, phenolic resins, drugs, dyes, polymerisation inhibitors, lube additives, and heat stabilisers for polymeric materials [1]. The catalytic reaction of phenol with tert-butanol has been extensively studied by using various homogeneous and heterogeneous catalysts [2–8]. tert-Butyl derivatives of phenol are intermediates in the synthesis of various agrochemicals, fragrances, thermo-resistant polymers, protecting agents for plastics, and precursors for numerous commercially important anti-oxidants [1]. 4-tert-Butyl phenol
* Corresponding author. Tel.: +91 40 27160123; fax: +91 40 27160921. E-mail addresses: [email protected], [email protected] (B.M. Reddy). 0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2007.07.026
(4-TBP) imparts improved performance properties to the class of metallic detergents used in the lubricating oils, and in the production of substituted tri-aryl phosphates. 2,4-di-tert-Butyl phenol (2,4-DTBP) is used in the manufacture of its triphosphite, which is employed as a co-stabiliser for polyvinyl chloride [2]. The tert-butylation of phenol is a typical FriedelCrafts alkylation and can be catalysed by different acid sites. In general, tert-butylation of phenol is carried out in homogeneous media using sulfuric acid, flurosulphonic acid, arenesulphonic acid, phosphoric acid, and aluminium chloride with boric acid or boron trifluoride etherate as catalysts [1]. Several problems are associated with these catalysts: toxicity, corrosiveness, the disposal of effluents, and the separation of products from the catalysts. In recent years, catalytic chemists have turned their attention to the use of environmentally friendly catalysts in the place of mineral acids. Thus, several efforts are being reported in the literature to replace the conventional homogeneous Friedel-Crafts catalysts by solid acids to fit in well with the concept of environmentally benign
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Scheme 1. Reaction scheme of tert-butylation of phenol with possible products depending on acidity.
systems for eco-friendly processes [9]. Thus zeolites, acid treated clays, ion exchange resins and heteropolyacids have been investigated by several groups for their applications in pharmaceutical, perfumery, agrochemical, and dyestuff intermediate and specialty chemical industries. A few studies have been reported on the tert-butylation of phenols using various solid acids such as micro-porous molecular sieves, cation exchange resins, clays, zeolites, and supported metal oxides [10–13]. The presence of a phenolic (–OH) group in the reactant kinetically favors the O-alkylation [14]. However, due to steric hindrance, thermodynamically unfavored oisomer, viz., 2-tert-butyl phenol (2-TBP) readily isomerizes into less hindered, partially kinetically favorable p-isomer [14,15]. Further, the selectivity of the product depends on the nature of the acid sites present in the catalysts as well as on the reaction temperature [4–6,8,16,17]. A schematic representation of the reaction is shown in Scheme 1. As can be noted from Scheme 1, the weak acidic catalyst, e.g. Naand K-ion-exchanged zeolite-Y leads to O-alkylated product, viz., tert-butyl phenyl ether (t-BPE) as the major product [5,15]. With the strong acidic catalysts like zeolite-b [18] or at high temperatures, the reaction produces Calkylated product, viz., 3-tert-butyl phenol (3-TBP), which is formed by the secondary isomerization of initially formed o- and p-isomers [4,8,18]. On the other hand, the moderate acidic catalysts such as ZSM-12 [16], SAPO-11 [17], and zeolite-Y [4,5] are suitable for the formation of p-isomers. Promoted zirconia solid acids, in particular, have been the target catalysts among various solid acids described in earlier reports due to their proven advantages. Zirconia is the only metal oxide that possesses all four chemical properties: namely acidic, basic, reducing ability and oxidizing ability [19–23]. Higher thermal stability, extreme hardness and high specific mass make the zirconia as an attractive material for catalytic applications. When dopants like VOx, MoOx and WOx are incorporated into the zirconia lattice, the interactions between the zirconia surface and the dispersed metal oxides lead to
drastic changes in the acidic properties of the catalyst [24,25]. Reports are also available in the literature on the alkylation of phenol over sulfated zirconia with high conversion and moderate selectivity [26,27]. However, the easy loss of sulfate ion in the form of sulfur compounds (SO2 or H2S) during thermal treatments or regeneration of the catalyst and also water poisoning limits their application [28,29]. The best alternative for sulfated zirconia could be molybdate- or tungstate-promoted catalysts, which are considered to be Green catalysts [30–34]. The so-called tungstated zirconia catalysts have been the subject of numerous investigations in view of their commercial significance [31,33]. Therefore, the present study was undertaken against the above background. In this study, molybdate- and tungstate-promoted zirconia catalysts were prepared by a wet impregnation method and investigated for vapour phase tert-butylation of phenol. The synthesized catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), FT-infrared spectroscopy (FTIR), temperature programmed reduction (TPR), temperature programmed desorption (TPD) of ammonia, scanning electron microscopy (SEM), and BET surface area techniques. 2. Experimental 2.1. Catalyst preparation The zirconium hydroxide was prepared from zirconium oxychloride by hydrolysis with dilute aqueous ammonia solution. For this purpose, the requisite quantity of ZrOCl28H2O (Loba Chime, GR grade) was dissolved in doubly distilled water and to this clear solution, aqueous NH3 was added drop-wise with rigorous stirring until the pH of the solution reached 8. The obtained precipitate was washed with hot distilled water several times until free from chloride ions and then dried at 393 K for 24 h. On the obtained Zr(OH)4, a constant 6-mol% molybdate or tungstate ions were doped by adopting a wet impregnation method. To achieve this, we dissolved the requisite quantities of ammonium heptamolybdate (JT Baker, England, GR grade) and ammonium metatungstate (BDH Chemical Ltd., AR grade) separately in doubly distilled water. To these clear solutions the desired quantity of Zr(OH)4 was added and the excess water was evaporated on a water-bath. The resulting samples were oven dried at 393 K for 24 h and calcined at 923 K for 6 h in a flow of oxygen. A small portion of the hydrous zirconia was also calcined at 923 K for 6 h to obtain the ZrO2. 2.2. Catalyst characterization The BET surface area measurements were made using a Gemini 2360 instrument by N2 physisorption at liquid nitrogen temperature. Prior to analysis, samples were degassed at 393 K under vacuum for 8 h to remove the adsorbed moisture and other volatiles. The surface densities expressed as the number of atoms per nanometer square area
B.M. Reddy et al. / Applied Catalysis A: General 332 (2007) 183–191
were calculated using the equation
2.3. Catalytic activity
Surface density ¼
fPromoter loadingðwt%Þ=100g 6:023 10 Formula weight of promoter BET surface areaðm2 g1 Þ 1018
23
The XRD patterns were recorded on a Siemens D-5000 diffractometer using Cu Ka radiation source and a scintillation counter detector. The XRD phases present in the samples were identified with the help of ASTM powder data files. The amount of monoclinic and tetragonal phases present in the samples was estimated by comparing the areas under the characteristic peaks of monoclinic (2u = 28.58 and 31.68 for the (1 1 1) reflections) and tetragonal phases (2u = 30.48 for the (1 1 1) reflection), respectively. Gaussian curves were first fitted to the XRD pattern at the characteristic peaks and the height (h) and width at half-height (w) were then obtained. The percent composition of each phase was calculated from the Gaussian areas h w: %Monoclinic ¼
%Tetragonal ¼
185
Sðh wÞmonoclinic Sðh wÞmonoclinic and tetragonal Sðh wÞtetragonal Sðh wÞmonoclinic and tetragonal
The Raman spectra were obtained on a DILOR XY spectrometer that was equipped with a liquid-nitrogen cooled charge coupled device (CCD) detector. The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics) was focused on the sample under a microscope, and the width of the analyzed spot was 1 mm. The power of the incident beam on the sample was 3 mW. Time of acquisition was adjusted according to the intensity of Raman scattering. The wave number values reported from the spectra are accurate to within 2 cm1. For each solid, the spectra were recorded at several points of the sample to ascertain the homogeneity and the averages of all these spectra were plotted in the figure presented in this study. The FTIR spectra were recorded on a Nicolet 740 FTIR spectrometer at ambient conditions using KBr discs with a nominal resolution of 4 cm1 and averaging 100 spectra. TPR and TPD studies were carried out on an Auto Chem 2910 (Micromeritics, USA) instrument. Prior to experiments the samples were preheated with appropriate gas mixtures at the desired temperatures. The TPR experiment was carried out from room temperature to 1273 K under a flow of 5% hydrogen in N2 at a constant heating rate of 5 K min1. Prior to each TPD run, the sample was saturated with 10% ultra pure anhydrous ammonia gas at 353 K for 2 h and subsequently flushed with He gas at 373 K for 2 h to remove the physisorbed ammonia. SEM analyses were carried out with a Hitachi model-520 instrument. The sample was mounted on a silver sample holder with the help of an adhesive to make the sample surface conductive and was coated with gold metal at 10 mm Hg pressure.
The tert-butylation of phenol was carried out in a fixed bed down flow micro-reactor made up of Borosil glass (25 cm long and 8 mm i.d.). About 0.2 g of catalyst sample mixed with 0.5 g of inert glass particles was placed in the reactor and supported on either side with a thin layer of quartz wool and ceramic beds. The catalyst was activated at 573 K in a flow of N2 for 3 h, followed by cooling to the desired reaction temperature. The reactant mixture, viz., phenol and tert-butyl alcohol, with a desired ratio and weight hourly space velocity (WHSV) was fed into the reactor using a liquid injection pump and nitrogen as the carrier gas. The gaseous products were cooled and the condensed liquid products were collected at 30 min intervals. The products, viz., p-TBP, o-TBP and 2,4-DTBP were identified by gas chromatography (CIC 2011) with SE-30 column, while m-TBP was identified by employing AT1000 column. Further, the products were conformed using a combined gas chromatograph–mass spectrometry (GC–MS) using HP-5 capillary column. 3. Results and discussion 3.1. Catalytic characterization The X-ray powder diffraction patterns of promoted zirconia samples along with unpromoted ZrO2 for the purpose of comparison are shown in Fig. 1. The corresponding monoclinic and tetragonal phase composition is presented in Fig. 2. As shown in Fig. 1, the unpromoted zirconia obtained by calcination of the hydrous zirconia at 923 K is in poorly crystalline form with monoclinic ZrO2 phase dominating over the tetragonal phase. The sharp diffraction lines at 2u = 28.3, 24.46 and 31.58 degrees correspond to the monoclinic form and
Fig. 1. X-ray diffraction patterns of unpromoted and promoted zirconia catalysts calcined at 923 K; (!) lines due to tetragonal phase; (~) lines due to monoclinic phase.
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B.M. Reddy et al. / Applied Catalysis A: General 332 (2007) 183–191 Table 1 The BET surface area, amount of ammonia desorbed and surface density values of pure zirconia and promoted zirconia catalysts
Fig. 2. Phase composition of unpromoted and promoted zirconia catalysts.
the lines at 2u = 30.27 and 49.21 degrees are due to tetragonal form of ZrO2. As can be noted from Fig. 1, the tetragonal phase is dominating over the monoclinic phase when dopants (molybdate and tungstate) were incorporated into the hydrous zirconia structure. A through examination of the XRD lines further indicates that the molybdate promoter shows a relatively stronger influence than tungstate on the monoclinic to tetragonal phase modification of ZrO2. As can be noted from Fig. 2, both the promoted samples possess more tetragonal phase (84 and 68 %) than the unpromoted zirconia (24%). This could be due to incorporation of promoter cations into the structure of zirconia, forming solid solutions, which substantially decrease the specific surface free energy of zirconia and favour the tetragonal phase, which has a lower surface free energy than the monoclinic form [35–37]. Further, no reflections corresponding to crystalline MoO3 or WO3 were observed, indicating a strong interaction of the dispersed promoters with the zirconia. Dispersed molybdenum oxide on various supports can exist as MoOx monomers, two-dimensional oligomers, and bulk MoO3 [38]. The poly-molybdate saturation capacity on zirconia support has been reported to be 5 Mo nm2 [39] and the Mo surface density obtained in this study is about 4.6 Mo nm2, close to the saturation value. Thus, the molybdenum oxide probably exists as isolated or twodimensional oligomers instead of the as bulk MoO3. A similar analogy could be extended to WOx/ZrO2 catalyst. Also, there is no evidence for the formation of new phases like Zr(MoO4)2 and Zr(WO4)2, although the existence of Mo–O–Zr and W–O– Zr bonds are highly possible. Zirconia normally exists in the monoclinic, tetragonal and cubic structures. The pure zirconia monoclinic phase in the absence of any impurities is stable up to 1373 K, and transforms into tetragonal phase as the temperature increases to 1473 K. On cooling the high temperature tetragonal phase transforms back to the monoclinic phase with a large hysterisis. However, in the present study, the
Catalyst
BET SA (m2 g1)
NH3 desorbed (a.u.)
Surface density
ZrO2 MoO3/ZrO2 WO3/ZrO2
46 94 64
0.01 11.0 9.0
0 4.65 Mo nm2 6.08 W nm2
stabilization of tetragonal phase is noted at a lower temperature, which is important for catalytic application of these materials. The BET surface areas, the amounts of ammonia desorbed and the surface densities of various samples are presented in Table 1. As can be noted from Table 1, the molybdate- and tungstate-promoted samples exhibit more specific surface areas (94 and 64 m2 g1) than the unpromoted zirconia (46 m2 g1). The observed increase in the specific surface areas may be due to the formation of Mo–O–Zr and W–O–Zr linkages, resulting in the formation of porous materials [40]. Temperature programmed desorption (TPD) of probe molecules like ammonia and pyridine is a popular method for the determination of acidity as well as acid strength of solid catalysts because it is an easy and reproducible method. Ammonia is used frequently as a probe molecule because of its small molecular size, good stability and strong basic strength [38]. The ammonia TPD profiles of molybdate- and tungstate-promoted catalysts exhibited two temperature maximums, indicating the presence of two different acid sites with different acid strength distributions. The total amount of ammonia desorbed at higher temperatures in the case of promoted samples is much larger than that of unpromoted zirconia (Table 1). The difference clearly indicates that impregnated cations show a strong influence on the acidic properties of the zirconia. It appears from the present study that the strong acidity of Mo- and Wpromoted zirconia catalysts is mainly due to the preparation procedure adopted. In the normal preparation, the amorphous Zr(OH)4 loses water and crystallizes predominantly into the monoclinic form with some small amounts of tetragonal zirconia. The modifiers such as ammonium heptamolybdate and ammonium metatungstate profoundly change this crystallization phenomenon. These modifiers influence the crystallization characteristics of the zirconia and result in the formation of tetragonal zirconia. Additionally, these modifiers can also stabilize the formed tetragonal phase. Raman spectra of unpromoted and promoted zirconia samples are presented in Fig. 3. The assignment of Raman bands of metal oxide overlayers in supported oxide catalysts is typically based on comparisons with the spectra of structurally well-characterized reference compounds [41]. According to the literature, the Raman bands of the supported metal oxide catalysts obtained after calcination in O2 are in the range between 1050 and 950 cm1 and can be unambiguously assigned to the symmetric stretching mode of short terminal M O bonds, whereas the bands between 750 and 950 cm1 are attributed to either anti-symmetric stretch of M–O–M bonds or the symmetric stretch of the (–O–M–O–)n bonds. The spectrum of pure ZrO2 calcined at 923 K exhibits the Raman bands
B.M. Reddy et al. / Applied Catalysis A: General 332 (2007) 183–191
187
Fig. 3. Raman spectra of unpromoted and promoted zirconia catalysts calcined at 923 K.
pertaining to a mixture of monoclinic (180, 188, 221, 331, 380, 476, and 637 cm1) and tetragonal (148, 290, 311, 454, and 647 cm1) phases and the lines due to tetragonal phase are less intense than the lines due to monoclinic phase [42]. In the spectra of promoted catalysts, bands representing the tetragonal phase are relatively prominent. Crystalline WO3 shows characteristic Raman bands at 807, 715, and 274 cm1 [42]. Absence of these bands indicates that microcrystalline WO3 are not formed on the surface of W-ZrO2 catalyst. The band at 1020 cm1 assigned to W O stretching mode is normally observed for mono and poly tungstate species whose presence is clearly visible from Fig. 3. The appearance of shoulder peaks at higher frequency region indicates the presence of geometrically different WOx species on the surface; these may be octahedral- or tetrahedral-coordinated WOx species. Similarly there were no peaks pertaining to crystalline molybdenum oxide in the Raman results. Hence, the absence of these bands suggests that the impregnated oxides are strongly interacting with the zirconia [43]. FT-IR spectra of uncalcined, calcined and promoterincorporated zirconia samples are presented in Fig. 4. Irrespective of the promoters, all the samples show two common bands, which are situated at around 3350 and 1620 cm1. The band observed at 3350 cm1 is due to surface hydroxyl groups. The intensity of this band is weak in the case of calcined support and promoted zirconia samples. This could be due to loss of hydroxyl groups in the form of water during calcination. The peak at 1620 cm1 could be attributed to the vibrations of acidic –OH groups [44]. In addition to these two bands, the tungstate-promoted sample shows another band at 1000 cm1, which was assigned to poly tungstate structure with tungsten in octahedral coordination [45–47]. Therefore, symmetrical W O stretching vibration was assigned to octahedrally co-ordinated WOx species on the surface of tetragonal zirconia, supporting Raman observa-
Fig. 4. FT-IR spectra of uncalcined and calcined zirconia and promoted zirconia catalysts.
tions. This unique vibration mode was observed in the case of tungstate-promoted sample, because the WOx species are more homogeneously distributed on the surface of zirconia. Normally crystalline WO3 shows peaks around 791 and 1110 cm1. Absence of these bands suggests that the tungsten oxide is in a highly dispersed state on the surface of the zirconia. Generally, in the case of molybdate-promoted samples the region at 1050–900 cm1 represents terminal Mo O stretching vibration and that at 800–700 cm1 represents the region of anti-symmetric Mo–O–Mo or O– Mo–O stretching vibrations. In the present study molybdatepromoted samples also exhibit a band at 1000 cm1. This band may be due to stretching vibration mode of terminal Mo O, in agreement with Raman results. Frusen et al. [48] observed the formation of ZrMo2O8 compound by heating ZrO2 and MoO3 together at 820 K, which showed the IR bands at 989, 920, and 800 cm1. In the present study, no such compound formation was observed, in line with XRD and Raman results. This could be attributed to a different preparation method adopted and the precursors used in the present study. Thus, the IR results corroborate well with the Raman and XRD results. The reduction properties of pure support and promoted samples were studied by TPR. The hydrogen consumption profiles obtained as a function of temperature are plotted in Fig. 5. The TPR profiles indicate a steady increase in the peak height with increase of temperature due to loss of water. The profile of pure zirconia shows that there is a continuous loss of oxygen from the lattice. The reduction of molybdena essentially can take place in two steps. The first step is
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Fig. 5. Temperature programming reduction profiles of unpromoted and promoted zirconia samples.
reduction of MoO3 ! MoO2 (Mo+6 ! Mo+4) and the second step is MoO2 ! Mo (Mo+4 ! Mo). The TPR profile of MoOx/ ZrO2 catalyst exhibits two temperature maximums, one at 648 K and another at 973 K. The first peak observed at 648 K could be due to reduction of Mo+6 to Mo+4 ions and the second peak could be assigned to the reduction of Mo+4 to Mo. Similarly, tungstate-promoted samples also show two temperature maximums at 698 and 1053 K. The peak observed in the low temperature region is due to reduction of W+6 to W+4 and the high temperature region peak may be due to reduction of W+4 to W. To study the surface topography and to assess the surface dispersion of the active components over the support, we performed SEM investigations on various samples calcined at 923 K, as presented in Fig. 6. In the micrograph of Zr(OH)4 calcined at 923 K, though crystallinity is observed, there are also certain cracks on the surface that may be attributed to the loss of water molecules during the calcination. As can be seen from the micrograph of MoOx/ZrO2 catalyst, the active component is equally spread on the surface of the support. Formation of bigger crystals of active components is noted in the case of tungstate-promoted samples. 3.2. Catalytic studies As mentioned earlier, the phenol alkylation with tert-butanol is a typical Friedel-Crafts reaction and can be catalyzed by acid sites. The main products of the reaction are 2-TBP (2-tert-butyl phenol), 4-TBP (4-tert-butyl phenol), 3-TBP (3-tert-butyl phenol), 2,4-DTBP (2,4-di tert-butyl phenol), 2,4,6-TTBP
Fig. 6. SEM micrographs of (a) ZrO2; (b) MoO3/ZrO2; and (c) WO3/ZrO2 calcined at 923 K.
(2,4,6-tri tert-butyl phenol) and TBPE (tert-butyl phenyl ether). Along with these alkylated products, there is a chance to form C8 and C12 olefins by the oligomerization of isobutene, where isobutene is formed by the acid-catalyzed dehydration of tertbutanol. In the present study, 4-TBP was obtained as the major product along with small amounts of ortho- and di-alkylated products, while the formation of 3-TBP was not observed. The results of tert-butylation of phenol over the promoted zirconia catalysts prepared in the present study with various tert-butanol to phenol ratios (TB/P) at a reaction temperature of
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Table 2 Effect of tert-butanol to phenol molar ratio on phenol tert-butylation over promoted zirconia catalysts at 473 K and WHSV = 5 h1 Molar ratio 1:2
1:1
2:1
3:1
MoO3
WO3
MoO3
WO3
MoO3
WO3
MoO3
WO3
Conversion (%)
45
42
55
52
62
58
63
59
Selectivity (%) 4-TBP 2-TBP 2,4-DTBP Others
88.4 6.6 4.3 0.7
88.3 7.0 4.1 0.6
88.1 6.5 4.7 0.7
88.2 6.2 4.8 0.8
87 6.3 6.0 0.7
87.3 6.0 6.1 0.6
85.8 6.0 7.5 0.7
86.4 5.7 7.1 0.8
473 K and at WHSV of 5 h1 are summarized in Table 2. In comparison to promoted catalysts, the unpromoted zirconia showed insignificant conversion (less than 2%) and the product selectivity hence was not investigated thoroughly. As can be seen from Table 2, the two promoted catalysts show good conversion with excellent para selectivity of the products. Further, it is clear from Table 2 that an increase in TB/P ratio increases the phenol conversion. It was reported earlier [17] that polar molecules such as methanol and higher alcohols compete with substrate for absorption sites and an increase in the molar excess of alkylating agent results in an increase in the conversion. However, the selectivity to 2,4-DTBP increases with increasing amounts of tert-butanol in the feed at the expense of the mono alkylated products 4-TBP and 2-TBP. The increase in the selectivity for higher alkylated product 2,4DTBP is probably due to the increased availability of tertbutanol molecules in the reactant feed. Similar results have been reported for AlMCM-41 [49,50] and micro-porous alumino phosphate catalysts [51,52]. The increase in the selectivity towards 2,4-DTBP is more in the case of the molybdate-promoted sample. It may be due to strong acidity of the sample [6]. Hence, for further studies the optimum tertbutanol to phenol ratio of 2 was chosen.
In order to study the deactivation behavior of the catalysts we followed the reaction for several hours at 473 K with a WHSVof 5 h1 using a TB/P molar ratio of 2 (Fig. 7). It is clear from this figure that, unlike the molecular sieve-based solid acid catalysts such as SAPO-11 [17], the catalytic activity of the promoted zirconia catalysts remains constant even after several hours. The selectivity of different products is also similar during the time-on-stream runs. The reaction was also studied at various temperatures from 433 to 493 K at WHSV of 5 h1 and TB/P molar ratio of 2. The variation of phenol conversion and selectivity to para product with temperature are shown in Fig. 8. At 433 K the phenol conversion was 52% and it increased to 62% at 473 K. An increase of reaction temperature above 473 K resulted in the decrease of phenol conversion. This behavior is often observed in alkylation reactions and is mainly due to the thermodynamics of alkylation and de-alkylation. Moreover, the olefins produced by dehydration of tert-butanol are probably consumed in more than one parallel reaction, such as alkylation, oligomerization and cracking. It is well known that oligomerization and cracking are dominant at high reaction temperatures, which result in a reduced phenol conversion at higher temperature even in excess of alkylating agent. In addition, at higher
Fig. 7. Time-on-stream (h) vs. conversion (%) and selectivity (%) of promoted zirconia catalysts for selective tert-butylation of phenol at constant temperature (473 K), WHSV (5 cm1) and TB/P ratio (2).
Fig. 8. Effect of temperature (K) on tert-butylation of phenol over MoO3/ZrO2 and WO3/ZrO2 catalysts at constant WHSV (5 cm1) and TB/P ratio (2).
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gel. An unpromoted ZrO2 was also prepared by calcination of the hydrous zirconia. The resultant samples were characterized by XRD, Raman, FT-IR, TPR, ammonia TPD, SEM and BET SA techniques. No peaks pertaining to either promoter oxides or compounds between the promoter oxides and the support were observed in XRD results. The XRD results further reveal that the promoted zirconia catalysts possess more tetragonal phase than the unpromoted sample. The ammonia TPD results suggest that promoters create more strong acidic sites. The promoted catalysts exhibited a high phenol conversion at 493 K reaction temperature with 5 h1 WHSV and a TB/P ratio of 2. Increasing the tert-butanol to phenol molar ratio increased the phenol conversion. Further, the catalytic activity was quite stable for several hours and did not change with repeated use of the catalysts. Fig. 9. Conversion and selectivity profiles of promoted zirconia catalysts with different WHSV (cm1) and at constant temperature 473 K and TB/P ratio 2.
temperatures, de-alkylation of the formed butyl phenols becomes more dominant. The selectivity of 4-TBP increased with increasing reaction temperature, whereas for increases in the temperature the selectivities of 2-TBP and 2,4-TBP were decreased. However, at higher temperatures, the observed higher p-TBP selectivity could be due to the absence of a secondary alkylation reaction [6]. The effect of WHSV on phenol conversion and product selectivity over promoted zirconia catalysts was studied at 473 K with a TB/P ratio of 2. The results obtained for different WHSV are illustrated in Fig. 9. It was observed that increasing space velocity increased the phenol conversion, but after 5 h1 WHSV the phenol conversion decreased. The lower conversion at lower space velocity could be due to de-alkylation of the alkylated products as well as coke formation [17,49], while at higher space velocity, it could be due to a high diffusion rate of the reaction mixture through the catalyst. The selectivity towards 4-TBP increased and that towards 2,4-DTBP decreased with increasing space velocity. The increase of 4-TBP selectivity with increasing space velocity has been ascribed to the elimination of inner particle diffusion resistance at higher space velocities [49]. However, at the optimum conditions (TB/ P = 2, temperature 473 K and WHSV 5 h1) the promoted zirconia catalysts provided good conversion and excellent para selectivity. To check the effect of promoters we also carried out the reaction with sulfated zirconia at a reaction temperature of 473 K, WHSV of 5 h1 and TB/P molar ratio of 2. Although slightly higher conversion was observed over sulfated zirconia, the selectivity towards para product is still poor and also more of di-alkylated product is formed. This may be due to strong solid acidity of the sample. 4. Conclusions Molybdate- and tungstate-promoted zirconia catalysts with reasonably high specific surface areas were prepared by a wet impregnation method using uncalcined zirconium hydroxide
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