Preparation of highly superacidic sulfated zirconia via combustion synthesis and its application in Pechmann condensation of resorcinol with ethyl acetoacetate

Journal of Catalysis 292 (2012) 99–110

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Preparation of highly superacidic sulfated zirconia via combustion synthesis and its application in Pechmann condensation of resorcinol with ethyl acetoacetate Ganapati D. Yadav a,⇑, Naishadh P. Ajgaonkar a, Arvind Varma b a b

Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400 019, India School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA

a r t i c l e

i n f o

Article history: Received 27 January 2012 Revised 1 May 2012 Accepted 2 May 2012 Available online 21 June 2012 Keywords: Sulfated zirconia Superacidity Combustion synthesis 7-Hydroxy 4-methyl coumarin Pechmann condensation Kinetics

a b s t r a c t A novel zirconia-based catalyst, with high sulfur content (15% w/w) and preservation of the tetragonal phase of zirconia, was synthesized for the first time via a solution combustion synthesis approach. Such high sulfur content with preservation of the tetragonal phase has not been reported so far. The catalyst synthesis parameters were optimized using a probe reaction of Friedel–Crafts alkylation. The optimized catalysts, fuel-lean sulfated zirconia (FLSZ) and fuel-rich sulfated zirconia (FRSZ), were characterized by XRD, FTIR, TPD, EDAX, SEM, and BET surface area and pore size analysis. Further, the activity and stability of FLSZ was tested using a Pechmann condensation reaction between resorcinol and ethyl acetoacetate to produce 7-hydroxy 4-methyl coumarin selectively. A complete theoretical and experimental analysis is presented, and a kinetic model is developed for this reaction. The model explains the experimental data well. The characterization and reaction studies show that combustion-synthesized sulfated zirconia exhibits higher superacidity than the conventional sulfated zirconia catalyst. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Environmentally friendly processes and technologies have gathered considerable attention worldwide, and continuous efforts are made to achieve these goals. Heterogeneous catalysts and especially solid acid catalysts play an important role in this pursuit. Among all the solid acid catalysts, special attention has been given to the preparation, characterization, and catalytic investigation of sulfated zirconia [1–4]. Holm and Bailey were the first to disclose in the early 1960s that Pt containing sulfate-modified zirconia was an acid catalyst suitable for alkylation of hydrocarbons [2]. Ever since Hino et al. [3] reported in the late 1970s that sulfated zirconia (SZ) was a superacid, it has been the subject of many investigations due to its ability to activate light alkanes at lower temperatures. These authors also claimed that this catalyst is able to catalyze the isomerization of n-butane at temperatures as low as room temperature. The amount of sulfur retained and the pore structure play important roles in determining the activity and selectivity of sulfated zirconia catalyst, respectively [1,5–7]. Some control on sulfate loading can be achieved by changing the calcination temperature, but the tetragonal phase and crystallinity are affected [4,8]. Wender [8] has reported that sulfated zirconia derived from impregnation with 0.25 M sulfuric acid and calcined at 600 °C for 3 h possessed 2.5% w/w sulfur content (in the form of sulfate ions). ⇑ Corresponding author. Fax: +91 22 410 2121. E-mail address: [email protected] (G.D. Yadav). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.05.004

Parera [9] studied the optimum amount of sulfuric acid to be used for sulfation and its effect on percentage sulfate loading. He found that percolation with sulfuric acid at concentrations varying from 0.05 to 2 M led to about the same amount of sulfur in sulfated zirconia after calcination. Nascimento et al. [4] and Morterra et al. [10] found an increase in Brønsted acidity with an increase in sulfate concentration up to a certain maximum, after which the amount of Brønsted acidity remained constant. Thus, sulfur in the form of sulfate present above 4% w/w is lost during thermal activation and represents a thermally more labile fraction when prepared with conventional methods. Fârcasiu and co-workers [11] reported that the sulfur content could be controlled and zirconia with high sulfur loading could be achieved. Their report suggests that zirconia exhibits a pure tetragonal phase at low values of sulfur content but at higher values (S > 4 wt%), its crystallinity is strongly affected and a monoclinic phase of zirconia is formed in addition to the tetragonal phase. In another study, sulfated zirconia was prepared by different procedures using a colloidal sol– gel technique and impregnation [12]. The colloidal sol–gel technique leads to the formation of sulfated zirconia with a high sulfur content along with mono- and polynucleate sulfate species, as well as supported sulfuric acid. Yadav and Murkute [6,7] obtained the highest sulfate loading when zirconium hydroxide was treated with chlorosulfonic acid. This catalyst designated as UDCaT-5 showed 9 wt% sulfur retention without the presence of a polynuclear sulfate group. The other important parameter that decides the selectivity of the catalyst is the pore structure of S-ZrO2, which is largely

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Nomenclature A AS B BS CA C A0 CB C B0 CBS CE CES CS Ct DAB DBA DI

reactant species A, resorcinol chemisorbed cyclohexene reactant species B, ethyl acetoacetate chemisorbed ethyl acetoacetate concentration of A in mol/cm3 initial concentration of A at catalyst (solid) surface (mol/ cm3) concentration of B in mol/cm3 initial concentration of B in bulk liquid phase, mol/cm3 concentration of B at solid (catalyst) surface, mol/cm3 concentration of E in mol/cm3 concentration of E at solid (catalyst) surface, mol/cm3 concentration of vacant sites, mol/cm3 total concentration of the sites, mol/cm3 diffusion coefficient of A in B, cm2/s diffusion coefficient of B in A, cm2/s deionized

controlled by the method of preparation and calcination temperature. Use of higher calcination temperatures result in collapse of pores or sintering of material, which creates a wider pore size distribution. The conventional preparation method for S-ZrO2 results in microporous material, which is more suitable for reactions of small molecules in the vapor phase and in liquid phase, particularly for reactions not involving water [1]. To form a catalyst that has shape selectivity for larger molecules, mesoporous material with a narrow pore size distribution needs to be created. Many attempts have been made to synthesize mesoporous sulfated zirconia, most commonly using a charged template, but removal of the template by calcination or extraction often causes the material to collapse [7,13–15]. Hudson and Knowles [16] were able to synthesize mesoporous zirconia using cationic surfactants. However, the materials formed had broad pore size distributions [width at half height (WHH) > 2.0 nm] that would not be suitable for shape-selective catalytic reactions. A neutral templating method was successfully used and mesoporous sulfated zirconia that did not collapse upon removal of the template was formed [17]. Another approach to overcoming the lacunae of conventional S-ZrO2 is to make use of highly ordered mesoporous material such as HMS, MCM-41, or SBA-15 as a support for S-ZrO2. In this context, the novel synthesis of S-ZrO2 supported on hexagonal mesoporous silica (HMS) was first reported by Yadav and Murkute [18] and designated as UDCaT-6. A series of new superacid catalysts, MCM-41supported S-ZrO2 were prepared by an impregnation–calcination method by Lei et al. [19]. Shape selectivity has also been achieved by using a combination of S-ZrO2 and carbon molecular sieves in UDCaT-2 catalysts [6]. All previous literature suggests that S-ZrO2 has been prepared so far with a maximum 9 wt% of sulfur with preservation of the tetragonal phase of zirconia, and above this value, the tetragonal phase is strongly affected. Retention of mesoporosity in S-ZrO2 without using any support has also been a challenge. Thus, it will be most advantageous to synthesize sulfated zirconia with high sulfur content, particularly above 9%, with a pure tetragonal phase to exhibit high superacidity and generation of mesopores with narrow pore size distribution that would not collapse on calcination. To achieve these goals, a novel route of combustion synthesis was developed to prepare sulfated zirconia (S-ZrO2). In recent years, combustion synthesis has emerged as a powerful alternative to material synthesis [21–26]. This method is reproducible and less time-consuming and does not involve multistep synthesis. The

dP diameter of catalyst particle, cm E 7-Hydroxy-4-methyl coumarin G/N glycine-to-nitrate ratio 0 K1 surface reaction equilibrium constant, k1 =k1 k2 surface reaction rate constant KA, KB, Ki, . . . adsorption equilibrium constant for A, B, i cm3/mol surface reaction rate constant, cm6 mol1 g-cat1 s1 kR2 M molar ratio of initial concentrations ðC B0 =ðC A0 Þ rA rate of reaction of A based on liquid phase volume, mol cm3 s1 S vacant site t time, s w catalyst loading, g/cm3 of the liquid phase W ethanol XA fractional conversion of A

method produces fine nanoscale metal oxides with high surface area and mesoporosity, which makes it a vital tool for synthesizing a tailor-made catalyst [25,26]. A combustion synthesis approach combined with catalysis techniques can be used to impose better activity and probable shape selectivity. The combustion synthesis method explores an exothermic, generally very fast, and self-sustained chemical reaction between the desired metal salts and a suitable organic fuel, which is ignited at a temperature much lower than the actual phase formation temperature. Its key feature is that the heat required to drive the chemical reaction and accomplish the compound synthesis is supplied by the reaction itself and not by an external source. We report, for the first time, a preparation of sulfated zirconia with high sulfur content (15 wt%) possessing better activity, including preservation of its tetragonal phase via combustion synthesis route and using chlorosulfonic acid as a new source for sulfate ions. This catalyst was also used in an industrially important reaction. Coumarins are benzo-2-pyrone derivatives mainly found in plants of the families Rutaceae and Umbelliferae. Coumarin and its derivatives have been attracting great interest because of their importance in synthetic organic chemistry. Among the various coumarin derivatives, 7-substituted coumarins are an important group showing various bioactivities. 7-Hydroxy-4-methylcoumarin is used as fluorescent brightener, an efficient laser dye, a standard for fluorometric determination of enzymatic activity, and a starting material for the preparation of insecticides and furano coumarins [27]. Chemically, coumarins can be synthesized by various methods, such as the Pechmann reaction [28–33], Knoevenagel condensation [34–37], Claisen rearrangement [38], and Perkin [39–41], Wittig [42–45], Reformatsky [46], and catalytic cyclization reactions [47]. However, the acid-catalyzed Pechmann reaction is a simple and commonly used method for synthesizing coumarins from activated phenols, mostly m-substituted phenols containing electrondonating substituents at the m-position and b-keto-esters or an unsaturated carboxylic acid [28,29,48]. Conventionally, the Pechmann reaction is carried out in the presence of concentrated sulfuric acid catalyst [30,49], phosphorus pentoxide [50], trifluoroacetic acid [51], and aluminum chloride [52]. These acids are corrosive and required in excess. Taking environmental and economic factors into consideration, there has been renewed interest in the preparation of coumarins using heterogeneous catalysts and under benign reaction conditions.

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The current work deals with synthesis and characterization of a novel material, combustion-synthesized sulfated zirconia [53], and its application in the synthesis of 7-hydroxy-4-methylcoumarin by Pechmann condensation. Both the method of preparing the strongest superacid and its application in Pechmann condensation are novel.

2. Experimental

101

2.3.2. Synthesis of fuel-lean sulfated zirconia In synthesis of FLSZ, the glycine and zirconium oxynitrate hexahydrate were taken in a 1:2 mol ratio. The above-mentioned procedure of combustion synthesis was followed to obtain fuel-lean zirconium dioxide powder. This material was immersed in 15 cm3/g of 1 M chlorosulfonic acid in ethylene dichloride. Without moisture absorption being allowed, the material was transferred to an oven, and the heating was started to evaporate the solvent. It was kept in an oven at 120 °C for 24 h and calcined at 650 °C for 3 h to obtain the active catalyst FLSZ.

2.1. Chemicals A.R. grade glycine, toluene, ethyl acetoacetate (EAA), resorcinol, diphenylmethane, benzyl chloride, n-undecane, 98 wt% sulfuric acid, and ammonium sulfate were obtained from S.D. Fine Chem. Pvt. Ltd., Mumbai. Chlorosulfonic acid was obtained from Spectrochem Pvt. Ltd., Mumbai. Zirconium oxynitrate hexahydrate was purchased from Sigma Aldrich, Germany.

2.2. Optimization of synthesis parameters Synthesis of any sulfated metal oxide via combustion synthesis has not been reported so far, and thus, the current work is the first attempt of its kind to synthesize sulfated zirconia. To obtain a catalyst with high activity and required textural properties, a number of parameters need to be optimized. The important parameters are glycine-to-nitrate ratio, acid to be used for sulfation and its normality, and calcination temperature prior to acid treatment. All these parameters were varied systematically, and the activity of the catalyst was optimized using a probe reaction of Friedel–Crafts alkylation of toluene with benzyl chloride [5]. Three different G/N ratios were selected (G/N = 0.5, 1.1, 2). Using these ratios, zirconium dioxide was synthesized using the method of solution combustion synthesis, and the three oxides were used for further experiments. Sulfuric acid, ammonium sulfate, and chlorosulfonic acid were used as sources of sulfate ion. The amount of acid used was 15 cm3/g with molarity 0.5 and 1 M. Calcination prior to acid treatment is necessary when the oxide contains considerable amounts of residual carbon. This phenomenon is observed at a G/N ratio of 2. Trials were conducted to determine the calcination temperature on the basis of the color of the calcined oxide. It was observed that most of the residual carbon is burnt off at 550 °C. The effect of calcination time was studied for 2 and 4 h.

2.3. Catalyst preparation On the basis of optimization results, two catalysts were screened: fuel-lean sulfated zirconia (FLSZ) and fuel-rich sulfated zirconia (FRSZ).

2.3.1. Synthesis of zirconium dioxide by combustion synthesis In a typical application of combustion synthesis to produce zirconium dioxide, predetermined amounts of glycine and zirconium oxynitrate hexahydrate were taken in minimum amounts of DI water (2 cm3/g of precursors). The solution was stirred on a magnetic stirrer at 80 °C for 2 h to transform the aqueous solution into a highly viscous gel. The temperature and time of stirring depended on the amount of precursors used. The viscous gel was then transferred to a silica crucible and kept in a muffle furnace maintained at 350 °C. The gel underwent self-ignition and transformed into a fluffy mass, which upon mild crushing resulted in a fine crystalline zirconium dioxide powder.

2.3.3. Synthesis of fuel-rich sulfated zirconia In synthesis of FRSZ, glycine and zirconium oxynitrate hexahydrate were taken in a 2:1 mol ratio. The same procedure of combustion synthesis was followed to obtain fuel-rich zirconium dioxide powder. The fuel-rich oxide contained a considerable amount of residual carbon. To remove this unwanted carbon, the zirconium dioxide powder was calcined at 550 °C for 4 h prior to acid treatment. The material was then immersed in 15 cm3/g of 1 M chlorosulfonic acid in ethylene dichloride. Without moisture absorption being allowed, the material was transferred to an oven and the heating started to evaporate the solvent. This material was kept in the oven at 120 °C for 24 h and calcined at 650 °C for 3 h to get the active catalyst FRSZ. UDCaT-5 was prepared by immersing dried zirconium hydroxide in 15 cm3/g of a 0.5 M solution of chlorosulfonic acid in ethylene dichloride, followed by drying at 120 °C for 24 h and calcination at 650 °C for 3 h [10]. Sulfated zirconia (S-ZrO2) was prepared by immersing zirconium hydroxide in 15 cm3/g of 0.5 M of sulfuric acid, followed by drying at 120 °C for 24 h and calcination at 650 °C for 3 h [49]. 2.4. Catalyst characterization Crystallinity and textural patterns of the catalysts were predicted from XRD data, which were recorded using a Philips PW 1729 powder diffractometer with Cu Ka (1.54-Å) radiation. FTIR was used to confirm the presence of bidentate sulfated ligands in S-ZrO2 solid acid catalyst. Infrared spectra of samples pressed in KBr pellets were obtained at a resolution of 2 cm1 between 4000 and 350 cm1. Spectra were collected with a Perkin–Elmer (Spectrum BX instrument), and in each case, the sample was referenced against a blank KBr pellet. The wafers were subjected to 32 scans, after which the spectra were recorded. Ammonia TPD experiments were performed in an Autochem II (Micromeritics Model 2920) instrument equipped with a TCD detector. Approximately 0.1 g of catalyst was loaded in the sample holder, heated to 573 K in argon, and subsequently cooled to 298 K. After being dosed with 5 vol.% of ammonia in argon mixture at a flow rate of 30 ml min1 for 30 min, the system was purged with argon for 1 h at the same temperature. The physisorbed ammonia was removed by passing argon gas at RT for 30 min. After cooling to 298 K, the temperature was raised to 923 K at 10 K/min, and the outlet gases were analyzed by a thermal conductivity detector. Nitrogen adsorption–desorption for BET surface area, pore volume, and pore size distribution were carried out by BJH and multipoint BET methods using a Micromeritics ASAP-2010 instrument. The analysis was carried out at 77 K, maintained by liquid nitrogen, after the sample was preheated to 300 °C for 4 h. SEM micrographs and electron-dispersive scanning (EDS) data for the catalysts were obtained on a JEOL JSM 7400 microscope operated at 10 kV with a working distance of 2.6–8 mm and resolution up to 1 nm. The dried samples were mounted on specimen studs and sputter-coated with a thin film of gold to prevent charging. The gold-coated surface was then scanned at various magnifications using a scanning electron microscope.

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stationary phase of 10% OV-17 supported on Chromosorb-WHP. Typically ethyl acetoacetate was taken in excess and conversions were based on the limiting reactant, resorcinol. The product was confirmed by GC–MS (Perkin–Elmer Clarus 500 with software Turbomass version 5.0.0) and melting point.

2.5. Experimental setup The Friedel–Crafts benzylation reaction was conducted in a glass reactor of i.d. 5 cm and height 10 cm with four glass baffles and a four-bladed disk turbine impeller located 0.5 cm from the bottom of the vessel and mechanically agitated with a motor. In a typical reaction, 0.5 mol of toluene was reacted with 0.05 mol of benzyl chloride at 90 °C with 0.018 g/cm3 catalyst loading, and 1000 rpm speed of agitation. All experiments for Pechmann condensation of resorcinol with ethyl acetoacetate were carried out in a high-pressure autoclave (Amar Equipments, Mumbai) of capacity 100 ml. The autoclave was equipped with a 45°-inclined four-bladed pitched turbine impeller, temperature controller (±1 °C), pressure indicator (kg/ cm2), and speed regulator (±5 rpm). Predetermined quantities of reactants and the catalysts were charged into the autoclave. The reaction mixture was heated to the required temperature, and the samples were withdrawn at specific intervals of time starting from zero time. A standard (control) experiment consisted of 0.03 mol resorcinol and 0.09 mol ethyl acetoacetate. The total reaction volume was maintained at 50 ml with makeup of toluene. The temperature was maintained at 150 °C and the speed of agitation at 1200 rpm with a catalyst loading of 0.03 g/cm3. After the completion of the reaction, the reaction mixture was cooled to room temperature, and the contents were poured into ice-cold water. The products were collected by filtration, washed with ice-cold water, and then recrystallized from hot ethanol to provide the coumarin derivative. All the coumarin derivatives could be identified by comparison with reported physical and spectral data.

Table 1 shows various experiments carried out in order to optimize the synthesis parameters to attain maximum activity for the benzylation of toluene to give benzyltoluene. First, sulfuric acid was used for sulfation of zirconia with three G/N ratios. The conversion obtained was not significant as compared to reported values under otherwise similar conditions [10]. Then ammonium sulfate was used for sulfation. The catalyst showed even less activity compared to sulfuric acid-treated zirconia. Finally, chlorosulfonic acid was used for sulfation. The strength of chlorosulfonic acid and calcination temperature were varied systematically to get maximum conversion in the probe reaction. Two catalysts were screened on the basis of catalytic performance (Sr. No. 10 and Sr. No. 13). The catalyst with fuel-lean composition (G/N ratio of 0.5) was named FLSZ, and the one with fuel-rich composition (G/ N ratio of 2) was named FRSZ. Detailed characterization of these catalysts was conducted in order to study their properties.

2.6. Analysis

3.2. Catalyst characterization

Analysis of the reaction mixture was performed by GC (Chemito, Model 8610) using a flame ionization detector and a stainless steel column (diameter 3.25 mm and length 4 m) packed with a

3.2.1. X-ray diffraction studies The crystal structure of sulfated zirconia is strongly affected by the sulfur content. When the sulfur content is low or medium

HO

OH

O

2.7. Reaction scheme Scheme 1 shows the described reaction. 3. Results and discussion 3.1. Optimization of synthesis parameters

HO

O

O

+ C 2H 5 OH + H2O

+ H3C

Resorcinol

O

OC2H5

Ethyl acetoacetate CH3

7-Hydroxy-4-methyl-coumarin Scheme 1. Pechmann condensation of resorcinol with ethyl acetoacetate.

Table 1 Experiments for optimization of parameters. No.

G/N ratio

Calcination temperature prior to acid treatment

Acid

Strength and amount

Conversion after 1 h in probe reaction (%)

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

1.1 0.5 2 1.1 0.5 2 1.1 1.1 0.5 0.5 2 2 2

550 °C – 550 °C 550 °C – 550 °C 550 °C 550 °C – – 550 °C 550 °C 550 °C

SA SA SA AS AS AS CSA CSA CSA CSA CSA CSA CSA

15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g, 15 cm3/g,

10.5 13.5 12.0 0.58 1.48 1.10 0.0 15.0 45.0 85.0 25.0 55.0 84.0

for 2 h for 2 h for 2 h for 2 h for 2 h for 4 h

for 2 h for 4 h for 4 h

Note. G: glycine, N: nitrate, SA: sulfuric acid, AS: ammonium sulfate, CSA: chlorosulfonic acid.

0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 0.5 M 1M 0.5 M 0.5 M 1M

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103

ble for increased crystallinity and tetragonal phase. The fuel-rich oxide shows the highest crystallinity and minimum monoclinic phase. The crystallinity decreases as the oxides are sulfated.

Fig. 1. XRD of (1) ZrO2, (2) FLSZ, and (3) FRSZ. T—tetragonal phase; M—monoclinic phase.

(S < 4 wt%), the sulfated zirconia crystallizes in the tetragonal form. When the sulfur content is high (S > 4 wt%), it exists in monoclinic form in addition to the major tetragonal form [7,11]. In this case, the sulfur content in both the sulfated catalysts is very high (15 wt%), which is the reason behind the presence of monoclinic phases in the XRD of FLSZ and FRSZ (Fig. 1). It is generally observed that pure zirconia transforms into the monoclinic phase from the tetragonal phase above the calcination temperature of 600 °C [1]. The XRD patterns of combustion-synthesized zirconia prior to acid treatment with different G/N ratios are shown in Fig. 2. The splitting of 013 and 121 peaks is the characteristic feature of tetragonal zirconia. Also, the XRD result shows the presence of a monoclinic zirconia phase along with major tetragonal zirconia, in the oxides produced with G/N = 0.5 (fuellean). The hump at 2h = 28° in the fuel-lean sample is characteristic of the monoclinic phase. In all three compositions, the percentage of the monoclinic phase is less than 5%. Thus, FLSZ has more tetragonal phase. Table 2 shows percentage crystallinity of pure zirconia and corresponding sulfated samples. The crystallinity increases with increased G/N ratio in all combustion-synthesized zirconia powders. As the G/N increases, the amount of fuel taking part in combustion increases. This results in higher temperatures in the combustion flame. Formation at higher temperatures is responsi-

Fig. 2. XRD results of combustion-synthesized ZrO2. T—tetragonal phase; M— monoclinic phase.

Table 2 Crystallinity of zirconia before and after acid treatment. Sample

Glycine-to-nitrate ratio

Crystallinity (%)

Fuel-lean ZrO2 Stoichiometric ZrO2 Fuel-rich ZrO2 Fuel-lean sulfated ZrO2 Fuel-rich sulfated ZrO2

0.5 1.1 2 0.5 2

35 37.5 41 24.4 31.4

3.2.2. Fourier transform infrared spectroscopy FTIR spectroscopy was used to study the nature of sulfur retained on the surface and the nature of the acidic sites generated. The spectra of acid-treated zirconia exhibit peaks in the range of 1000–1250 cm1, whereas pure zirconia does not show any bands in this range. The IR spectra of FLSZ (Fig. 3) show a broad peak having shoulder peaks at 1321.2, 1289, 1119.45, 1009.87, and 972 cm1, which are typical of a chelating bidentate sulfate ion (SO2 4 ) coordinated to a metal cation. The absence of a band at 1400 cm1 indicates that there is no formation of polynuclear sul1 fates S2 O2 is attributed to 7 on the surface. The band at 1637 cm the dO–H bending frequency of water molecules associated with sulfate groups [50]. The feature at 752 cm1 is due to Zr–O2–Zr asymmetric and Zr–O stretching modes, which confirms formation of ZrO2 phases. The IR bands between 450 and 800 cm1 are characteristic of crystalline zirconia. An additional broad band at 3402 cm1 corresponds to the stretching vibration of vOH of the hydroxyl group. The IR spectra of FRSZ (Fig. 4) show a similar pattern. A broad peak with shoulder peaks in the range from 1000 to 1300 cm1 confirms the presence of bidentate SO2 4 ions coordinated to metal cations. The absence of bands at 1400 cm1 indicates that there is no formation of polynuclear sulfates S2 O2 on the surface. The 7 band at 1633 cm1 is attributed to the dO–H bending frequency of water molecules associated with sulfate groups [50]. The IR bands between 450 and 800 cm1 are characteristic of crystalline zirconia. A sharp band at 754.62 cm1 is due to Zr–O2–Zr asymmetric and Zr–O stretching modes, which confirms the formation of ZrO2 phases. An additional broad band at 3398 cm1 corresponds to the stretching vibration of vOH of the hydroxyl group. 3.2.3. Ammonia temperature-programmed desorption Temperature-programmed desorption using NH3 was used to measure the total acidity and the nature of acidic sites generated on the catalyst surface. Fig. 5 shows a typical desorption curve for FLSZ. Well-defined peaks were obtained at 150 and 400 °C, indicating the presence of intermediate and strong acid sites. The desorption curve of FRSZ is shown in Fig. 6. The peak at 400 °C is wider than that for FLSZ, indicating that strong acidic sites are more dominant in the case of FRSZ. FRSZ has considerable amounts of residual carbon present before acid treatment as compared to FLSZ. When the oxide is calcined at 650 °C after acid treatment, this carbon escapes from the inner crust of the oxide in the form of CO2. This evolution of gases results in violent changes in the morphology of the oxide, generating large numbers of pores and surface defects. This phenomenon is practically absent in fuel-lean oxide with relatively small amounts of residual carbon. This is likely the reason behind generation of stronger acidic sites in FRSZ than in FLSZ. Table 3 shows a comparison between the acidity of combustion-synthesized and conventional sulfated zirconia catalysts. FLSZ shows remarkably higher acidity than other catalysts. It must be noted, however, that higher concentrations of acid were used in preparation of FLSZ (1 M, 15 cm3/g) and FRSZ (1 M, 15 cm3/g) than of UDCaT-5 (0.5 M, 15 cm3/g) and S-ZrO2 (0.5 M, 15 cm3/g). More acid is required in the case of combustion-synthesized catalysts because of the presence of excess residual carbon in oxides prior to acid treatment. These results can now be compared with UDCaT-5 and UDCaT5b vis-à-vis S-ZrO2, as reported in our previous work [7]. The most striking feature of the spectra of the UDCaT-5 and UDCaT-5b is the generation of an additional peak at 550 °C in both. Corma et al. [54]

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1637.04

20

752.15

%T 15 10

502.51 3402.13

611.87 1119.45 1321.20

5

1289.35

0.0 4000.0

3000

2000

1500

972.09 1009.87

1000

400.0

cm-1 Fig. 3. FTIR spectra of FLSZ.

35.0 30 462.45

25 754.62

20

1633.48 2343.82

%T 15

649.39

10

501.80

3398.36

612.57

5

1116.04 1332.26

0.0 4000.0

3000

2000

1500

cm-1 Fig. 4. FTIR spectra of FRSZ.

Fig. 5. Ammonia TPD of FLSZ.

1000

587.63

400.0

G.D. Yadav et al. / Journal of Catalysis 292 (2012) 99–110

105

Fig. 6. Ammonia TPD of FRSZ.

Table 3 Acidity measurement of various superacids by ammonia TPD in mmol/g. FLSZ

FRSZ

UDCaT-5

S-ZrO2

0.628

0.502

0.584

0.433

assign this peak to superacidic centers of S-ZrO2 calcined at 550 °C. It is thus inferred here that there is a generation of superacidic centers in UDCaT-5, even though it was calcined at 650 °C for 3 h and the same peak is not observed in S-ZrO2 calcined at 650 °C. The total acid sites of each of UDCaT-5 (0.584 mmol g1) and FLSZ (0.628 mmol g1) are much higher than for S-ZrO2 (0.433 mmol g1) and UDCaT-5. The acidity of both UDCaT-5 and FLSZ is dependent on the molar strength of chlorosulfonic acid, and they are more superacidic than S-ZrO2. 3.2.4. BET surface area and pore size analysis The surface area of S-ZrO2 gradually increases at low sulfate content up to 4% w/w (119 m2/g), but it decreases abruptly at the maximum sulfate content of 5.64% w/w (71 m2/g), due to migration of sulfate ions in the bulk phase of zirconia [7,11]. A similar reason could be suggested in the current case. The XRD results also indicate that migration of sulfate ions into the bulk phase of zirconia has resulted in generation of monoclinic phases. The higher sulfur loading may have resulted in reduction of the surface area (Table 4). The combustion-synthesized catalysts have comparatively larger pore size and ordered mesoporosity. Fig. 7 shows the pore size distribution of combustion-synthesized

oxides. Fuel-lean ZrO2 and fuel-rich ZrO2 show narrow pore size distributions prior to acid treatment. In the case of their sulfated counterparts, a narrow pore size distribution can be seen only in FLSZ (width at half height <2 nm), while FRSZ shows a comparatively wider pore size distribution (Fig. 8). The additional step of calcination for FRSZ may be responsible for this feature. 3.2.5. Scanning electron microscopy The SEM images of FLSZ and FRSZ are shown in Figs. 9 and 10, respectively. Formation of lumps and agglomerates can be seen in SEM images of FLSZ. The SEM image of FRSZ shows no such agglomerate formation. Images of FRSZ indicate formation of flakes of uniform size and shape. Agglomeration in FLSZ may be due to the comparatively small amount of fuel used in combustion. Use of less fuel results in relatively less gas formation. On the other hand, in FRSZ, excessive gas formation avoids agglomerate formation. 3.2.6. Energy-dispersive X-ray spectroscopy EDXS was conducted to confirm complete decomposition of chlorosulfonic acid in synthesized catalyst and to check any residual carbon after combustion. Chlorosulfonic acid may decompose to sulfuryl chloride, pyrosulfuryl dichloride, sulfuric acid, sulfur dioxide, chlorine, and water. Elemental analysis (Table 5) showed complete absence of Cl species, and absence of C confirmed complete removal of carbon after combustion and calcination. Table 6 shows sulfur retention of various catalysts compared with combustion-synthesized sulfated zirconia. FLSZ and FRSZ show high retention of sulfur compared with conventionally synthesized S-ZrO2 [1,5,18]. 3.3. Efficacies of various catalysts

Table 4 Surface area characteristics. Catalyst

Surface area (m2/g)

Pore size (nm)

Pore volume (cm3/g)

FLSZ FRSZ UDCaT-5 S-ZrO2 Fuel-lean ZrO2 Fuel-rich ZrO2

53.3 21.6 84 103 17 10

9.85 9.14 4.0 4.1 8.5 6.1

0.131 0.051 0.21 0.11 0.142 0.076

The efficacy of various solid acid catalysts in this reaction was assessed. The catalysts used were FLSZ, FRSZ, UDCaT-5, 20 wt% Cs-DTP/K-10, and conventional S-ZrO2. The order of activity was as follows: FLSZ (highest) > 20 wt% Cs-DTP-K10 >FRSZ > UDCaT5 > S-ZrO2 (lowest). Cs-DTP/K-10 is cesium dodecatungstophoshoric acid supported on K-10 clay (Cs2.5H0.5PW12O40/K-10). Combustion-synthesized

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Fig. 7. Pore size distribution of (a) fuel-lean ZrO2 (FLSZ) and (b) fuel-rich ZrO2 (FRSZ).

Fig. 8. Pore size distribution of (a) FLSZ and (b) FRSZ.

Fig. 9. SEM image of FLSZ.

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Fig. 10. SEM image of FRSZ.

70

Table 5 Elemental analysis of FLSZ and FRSZ. FLSZ (%)

FRSZ (%)

60

O S Zr

38.8 15.14 46.06

38.6 15.1 46.3

50

% Conversion

Element

Table 6 Sulfur content of different superacids by elemental analysis (w/w %).

40

30

20

a

FLSZ

FRSZ

UDCaT-5

S-ZrO2a

15.14

15.1

9

4

10

Reference material. 0 0

30

60

90

120

150

180

Time (mins) 40

Fig. 12. Effect of speed of agitation. 600 rpm, 800 rpm, 1000 rpm, 1200 rpm, 1400 rpm. Reaction conditions: temperature 150 °C, catalyst loading 0.03 g/cm3, mole ratio (EAA:resorcinol) 3:1, ethyl acetoacetate 90 mmol, resorcinol 30 mmol, reaction volume – 50 ml. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

% Conversion after 1 h

35 30 25 20 15 10

70

5

60 FLSZ

Cs/DTP/K 10

FRSZ

UDCaT -5

S-ZrO 2

Fig. 11. Efficacies of different catalysts for Pechmann condensation reaction. Reaction conditions: temperature 150 °C, catalyst loading 0.01 g/cm3, mMole ratio (EAA:resorcinol) 3:1, ethyl acetoacetate 90 mmol, resorcinol 30 mmol, reaction volume 50 ml.

fuel-lean sulfated zirconia showed the highest activity in comparison with other catalysts (Fig. 11). The selectivity was almost similar. The catalyst showed nearly 95% selectivity with FLSZ. The obvious reason is the high sulfur content and retention of tetragonal phase in this catalyst. Although the surface area is smaller than that of Cs-DTP/K10, the pore sizes are in the meso region, and thus, there is no intraparticle diffusion limitation for formation of a bulky product. Further experiments were carried out using FLSZ due to its better activity toward this reaction. 3.4. Effect of speed of agitation The effect of speed of agitation on the rate of reaction was studied in order to eliminate the external mass transfer resistance. The

50

% conversion

0

40

30 0.005 g/cm3

20

10

0 0

30

60

90

120

150

180

210

Time (min) Fig. 13. Effect of catalyst loading. 0.005 g/cm3, 0.01 g/cm3, 0.02 g/cm3, 0.03 g/cm3, 0.04 g/cm3. Reaction conditions: temperature 150 °C, speed of agitation 1200 rpm, mole ratio (EAA:resorcinol) 3:1, ethyl acetoacetate 90 mmol, resorcinol 30 mmol, reaction volume 50 ml. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

G.D. Yadav et al. / Journal of Catalysis 292 (2012) 99–110

speed of agitation was varied in the range 600–1400 rpm under otherwise similar conditions (Fig. 12), maintaining temperature at 150 °C. The initial rate of reaction increased as the speed of agitation was increased from 600 to 1200 rpm. It remained practically unchanged above 1200 rpm. The reaction is mass transfer controlled below 1200 rpm. All further experiments were carried out at 1200 rpm, which ensured the absence of mass transfer resistance.

3.5. Effect of catalyst loading The initial rate of a reaction is proportional to the number of active sites, when external mass transfer resistance and intraparticle diffusion resistance are absent. Reactions can be carried out at widely varying concentrations of active sites to ensure the elimination of mass transfer limitations. So the catalyst loading based on the total volume of the reaction mixture was varied from 0.005 to 0.04 g/cm3 at 1200 rpm. There was an increase in conversion when the loading was increased from 0.005 to 0.04 g/cm3 (Fig. 13). This trend is due to the increase in active sites with increased catalyst loading. The relative increase in conversion is comparatively less when the reaction is carried out at catalyst loadings of 0.03 and 0.04 g/ cm3, which indicates that there is an onset of intraparticle resistance. This would suggest that the number of sites available is more than that required. This is also confirmed by the Wiesz–Prater criterion (modulus). Hence, further experiments were carried out at 0.03 g/cm3.

3.6. Effect of mole ratio The mole ratio of ethyl acetoacetate to resorcinol was varied from 1:2 to 1:4, keeping the total volume of reactants and volume of solvent constant. It was found that the rate of reaction increased with increased mole ratio from 1:2 to 1:3 (Fig. 14). At lower mole ratios, the number of moles of ethyl acetoacetate in the vicinity of resorcinol is comparatively less, resulting in a decreased rate of reaction. At higher molar ratios, this effect is less dominating. Hence, the molar ratio of 1:3 was used for further reactions.

70

60

70 60 50 40 30 20 10 0 0

30

60

90

120

150

180

210

Time (mins) Fig. 15. Effect of temperature. 130 °C, 140 °C, 150 °C, 160 °C. Reaction conditions: speed of agitation 1200 rpm, catalyst loading 0.03 g/cm3, mole ratio (EAA:resorcinol) 3:1, ethyl acetoacetate 90 mmol, resorcinol 30 mmol, reaction volume – 50 ml. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

3.7. Effect of temperature The reaction was carried out over a wide range of temperatures ranging from 130 to 160 °C. Self-condensation of ethyl acetoacetate takes place above 160 °C, resulting in undesirable side products. The conversion increased with temperature, indicating a kinetically controlled reaction (Fig. 15). This suggests a kinetically controlled mechanism, which is discussed later. 3.8. Catalyst reusability Reusability of FLSZ was tested by conducting two runs. After the completion of the experiment, the catalyst was filtered, washed with 100 ml of methanol, and dried for 3 h at 110 °C. In each run, completely used catalyst was employed without addition of fresh catalyst. It was observed that the there was only a marginal decrease in conversion. Experiments were also done with used catalyst with makeup quantities of fresh catalyst. The conversions were similar, with experimental error of ±2%. Thus, the catalyst was found to be reusable. 3.9. Development of kinetic model

50

% conversion

80

% conversion

108

In the absence of both external mass transfer and intraparticle diffusion resistances, it is possible to develop a kinetic model. Several models were tried, and the following was observed to fit the data well. Chemisorption of A (resorcinol) and B (ethyl acetoacetate) takes place on two adjacent vacant surfaces (S) according to the Langmuir–Hinshelwood–Hougen–Watson (LWHW) mechanism to produce E (7-hydroxy-4-methyl coumarin) and W (ethanol). A standard derivation can be done. If the surface reaction controls the rate of reaction, then the rate of reaction of A is given by

40

30

20

10

r A ¼ 

0 0

30

60

90

120

150

180

210

Time (min) Fig. 14. Effect of mole ratio. 1:2, 1:3, 1:4. Reaction conditions: temperature 150 °C, speed of agitation 1200 rpm, catalyst loading 0.03 g/cm3, reaction volume 50 ml. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

dC A ¼ k2 C AS C BS  k20 C ES C WS ; dt

n o k2 K A K B C A C B  K E K WK C2 E CW C 2t dC A :  ¼ dt ð1 þ þK A C A þ K B C B þ K S C E þ K W C W Þ2 When the reaction is far away from equilibrium,

ð1Þ

ð2Þ

G.D. Yadav et al. / Journal of Catalysis 292 (2012) 99–110

of the rate constants at different temperatures were calculated, and an Arrhenius plot was used to estimate the activation energy of the reaction (Fig. 17). The apparent activation energy was found to be 36.0 kJ/mol. This activation energy also supported the conclusion that the overall rate of reaction is not influenced by either external mass transfer or intraparticle diffusion resistance, and it is an intrinsically kinetically controlled reaction on active sites.

y = 0.006x R² = 0.994 y = 0.004x R² = 0.995

ln {(M -X)/M(1-X)}

109

y = 0.004x R² = 0.994

4. Conclusions y = 0.002x R² = 0.992

Time (min) Fig. 16. Plots of ln{(M  XA)/M(1  XA)} vs time. 130 °C, 140 °C, 150 °C, 160 °C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

We report, for the first time, that sulfated zirconia prepared via a combustion synthesis route, which is treated with chlorosulfonic acid, has superior acidity compared to sulfuric acid-treated S-ZrO2. The catalyst shows mesoporosity with a narrow pore size distribution, making it a shape-selective catalyst. The combustion-synthesized sulfated zirconia catalyst is more stable and also is more active than S-ZrO2. Fuel-lean sulfated zirconia (FLSZ) is more active than fuel-rich sulfated zirconia (FRSZ). Characterization and reaction tests support the above results. The activity and stability of FLSZ was studied by using Pechmann condensation reaction between resorcinol and ethyl acetoacetate to produce selectively 7-hydroxy 4-methyl coumarin. A kinetic model is also developed. Acknowledgments N.P.A. acknowledges support from the UGC as JRF. G.D.Y. acknowledges support from the R.T. Mody Distinguished Professor Endowment and Department of Science and Technology (DST), GoI, as J.C. Bose National Fellow. A.V. and G.D.Y. thank DST for support under the CP-PIO program of the DST. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

Fig. 17. Arrhenius plot for Pechmann reaction.

dC A k2 C 2t K A K B C A C B ; ¼ P dt ð1 þ K i C i Þ2 dC A kR wK B C A C B ; ¼ 2 P  dt ð1 þ K i C i Þ2



ð3Þ ð4Þ

where kR2 w ¼ k2 C 2t K A K B , where w is catalyst loading. If the adsorption constants are small, then the above equation reduces to



dC A ¼ kR2 wK B C A C B : dt

ð5Þ

Let C B0 =C A0 ¼ M be the molar ratio of ethyl acetoacetate to resorcinol at time t = 0. Then Eq. (5) can be written in terms of fractional conversion as

dX A ¼ kR2 wC A0 ð1  X A ÞðM  X A Þ ¼ k1 C A0 ð1  X A ÞðM  X A Þ: dt

ð6Þ

This upon integration leads to

ln fðM  X A Þ=Mð1  X A Þg ¼ k1 C A0 ðM  1Þt:

ð7Þ

Thus, a plot of ln{(M  XA)/M(1  XA)} versus t was made at different temperatures to get an excellent fit, thereby supporting the model (Fig. 16). This is an overall second-order reaction. The values

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