Iron(III)-Modified Tungstophosphoric Acid Supported on Titania Catalyst

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Iron(III) modified tungstophosphoric acid supported on titania catalyst: Synthesis, characterization and Friedel-Craft acylation of m-xylene Manman Mu, Wangwang Fang, Yunlong Liu, and Li-Gong Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01861 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on August 29, 2015

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Industrial & Engineering Chemistry Research

Iron(III) modified tungstophosphoric acid supported on titania catalyst: Synthesis, characterization and Friedel-Craft acylation of m-xylene Manman Mu,†,‡ Wangwang Fang, † Yunlong Liu, † Ligong Chen*,†,‡

†

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, PR China

AUTHOR INFORMATION Corresponding author * Tel.: +86 22 27406314, fax: +86 22 27406314. E-mail address: [email protected]

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ABSTRACT The Friedel-Craft acylation of m-xylene with benzoyl chloride over iron modified tungstophosphoric acid supported on titania was investigated. It was found that FeTPA/TiO2 catalyst displayed excellent catalytic performance for this reaction. Furthermore, a series of catalysts were prepared and characterized by FT-IR, XRD, BET, NH3-TPD and Py-IR. The results indicated that both the Lewis acidity and the textural properties presented significant influences on their catalytic performance. Moreover, the influence of catalyst calcination temperature to the above reaction was also studied. Besides, the reaction parameters, including reaction temperature, catalyst dose and molar ratio of m-xylene to benzoyl chloride, were optimized and a 95.1% yield of 2, 4-dimethylbenzophenone was obtained under optimal conditions. Finally, the kinetics of the benzoylation of m-xylene over 30% FeTPA/TiO2 was established.

KEYWORDS: Acylation, M-xylene, Benzoyl chloride, FeTPA/TiO2, Calcination temperature

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1. Introduction Aromatic ketones are the key intermediates or final products in fine chemical and pharmaceutical industries. 1, 2 The synthesis of aromatic ketones is usually accomplished through the Friedel-Crafts acylation of arenes with acyl chloride and traditional catalysts are Lewis acid (such as AlCl3 and FeCl3) or strong Brönsted acids (such as concentrated H2SO4 and H3PO4). 3 However, these catalysts normally cause a large amount of highly toxic and corrosive waste water, resulting in serious environmental problems. Therefore, numerous heterogeneous catalysts, such as zeolites 4 or modified zeolites 5, clay, Nafion-H and superacids 6, have been developed and widely used for the benzoylation of arenes in recent years. Due to their strong Brönsted acidity and high redox properties, heteropolyacids (HPAs) have been proven to be efficient catalysts applied in acid-catalyzed reactions. 7 Nevertheless, the major disadvantages of HPAs as catalysts lie in solubility in polar solvents, low thermal stability and low surface area (1-10 m2/g). Recently, some researchers have discovered that HPAs supported on acidic or neutral solids like SiO2, TiO2, active carbon, ZrO2, Al2O3 and Hβ zeolite can not only enhance the acidity, but also the thermal stability. 8-11 Therefore, HPAs supported on some solids have been mainly used in the alkylation of arenes 12-14 but rarely applied in the acylation of arenes. Kunmei Su et al reported Friedel-Craft acylation of toluene with acetic anhydride over HPW/TiO2 in 46.2% yield of p-methylacetephenone. 15 Another method for improving the catalytic efficiency of HPAs has been achieved by exchanging their protons with metal ions. 16 The formation of HPA salts like FeTPA or AlTPA enhance the Lewis acidity efficiently, meanwhile the increase of the Lewis acidity might greatly improve the catalytic performance on Friedel-Craft acylation. However, these salts usually have low surface area and high solubility in water, so their applications are also limited. Therefore, metal modified HPA supported on the suitable solid is an efficient method to improve the catalytic performance. Recently, benzylation and oxidation reactions over metal modified HPAs supported on some solids have been studied, 17-19 but the benzoylation reaction is still rarely reported. In this work, iron modified tungstophosphoric acid (FeTPA) supported on some solids were prepared and evaluated for the benzoylation of m-xylene (scheme 1). Moreover, the catalytic activities of a series of FeTPA/TiO2 for this reaction were presented and these catalysts were characterized by Fourier Transform Infrared Spectoscopy (FT-IR), X-ray diffraction (XRD), Py- Infrared Spectoscopy (Py-IR), NH3-temperature programmed desorption (NH3-TPD) and nitrogen adsorption/desorption measurement. Furthermore, the influence of catalyst calcination temperature (473K-973K) was also studied. Moreover, the reaction parameters (temperature, catalyst dose, molar ratio) were optimized. Finally, the kinetics of the benzoylation of m-xylene over FeTPA/TiO2 was established.

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2. Experimental 2.1. Materials Tungstophsphoric acid (H3PW12O40·8H2O, TPA), phosphomolybdic acid (H3P4Mo12O40·8H2O, PMoA), tungstosilicic acid (H4Si4W12O40·12H2O, TSiA), iron chloride hexahydrate, titania, m-xylene and benzoyl chloride were obtained from Tianjin Guangfu Fine-chemical institute, Tianjin, china. Commercially available reagents were used without further purification. 2.2. Catalyst preparation A series of X% FeTPA/ TiO2 catalysts were prepared by an impregnation method (X means the weight percentage of FeTPA in the catalyst). Take 30% FeTPA/ TiO2 as an example. 1.50 g TPA was dissolved in water (10 mL) and added to FeCl3·6H2O (0.14 g) under stirring. An hour later, titania (3.52 g) was added to the reaction mixture. It was kept stirring for 3h and the excess water was evaporated to dryness. The obtained catalyst was dried in an air oven at 110 ℃ for 8h and then calcined at 300 ℃ for 2h. The other catalysts were similarly prepared. 2.3. Characterization of catalysts X-ray powder diffraction (XRD) patterns were recorded on a Rigaka D/max 2500 X-ray diffractometer using Cu-Kα radiation (40kV, 100mA) in the range of 5~90℃. FT-IR spectra were obtained using the KBr method on a Nicolet system. Specific surface areas were determined by the Brunauer−Emmett−Teller (BET) method with N2 adsorption-desorption measurements at liquid nitrogen temperature using a NOVA 2000e analyzer. The acidity of catalysts was measured by temperature programmed desorption of ammonia (NH3-TPD). NH3-TPD was conducted with a TP-5000 instrument in a thermal conductivity detector (TCD) device. Lewis and Brönsted acid sites of catalyst were determined with FT-IR spectra of adsorbed pyridine (Py-IR). Py-IR spectra were recorded on a Thermo Nicolet Nexus 470 spectrometer equipped with a heatable and execuatable IR cell containing CaF2 windows. 2.4. General acylation procedure A mixture of m-xylene (40 mmol), benzoyl chloride (10 mmol) and catalyst (70 mg, 5 wt%) were magnetically stirred and heated to reflux (403K) for 6h. The catalyst was recovered by simple filtration and the filtrate was analyzed by gas chromatography (SE-30 capillary column: 60m×0.25mm, 0.2 um film thickness), and the composition of the reaction mixture was confirmed by GC-MS (HP-1 capillary column: 30m×0.25mm, 0.2 um film thickness).

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O

O

Cl

O

Cat.

Scheme 1 Acylation of m-xylene with benzoyl chloride 3. Results and discussion 3.1 Catalytic activity As mentioned above, TPA is an efficient catalyst for acid-catalyzed reactions due to its strong Brönsted acidity, but normally suffers from low specific surface area and poor thermal stability. TPA immobilized on TiO2 was reported to be a good way to overcome these problems. Moreover, their Lewis acidity was enhanced by the exchange of TPA protons and metal (Fe, Al, Sn). Thus, TPA/TiO2 and FeTPA/TiO2 catalysts which the loading of active component was 30 wt% were prepared and evaluated for the acylation of m-xylene with benzoyl chloride. As shown in Table 1(entry 1, 2), it was obvious that FeTPA/TiO2 exhibited much better catalytic activity than TPA/TiO2. Encouraged by the above result, three heteropoly acid (HPA) salts (FeTPA, FeTSiA, FePMoA) were immobilized on TiO2 and the loading of HPA salts was also 30wt%. Their catalytic properties were evaluated respectively (Table 1, entry 3-5). It was found that the conversion of benzoyl chloride decrease in the follow order FeTPA/TiO2> FeTSiA/TiO2> FePMoA/TiO2. It have been reported that the acid strength of HPAs decreases in this order: TPA> TSiA> PMoA. 20 So, the experimental result was in good agreement with their acid strength of HPAs. Therefore, this result indicated that the acid strength of HPAs might have a significant effect on the catalytic activity for the benzoylation. In order to further enhance the catalytic activity, several metal oxides (TiO2, neutral Al2O3, acidic Al2O3, γ-Al2O3, nanoTiO2 and SnO2) were employed as the supports of FeTPA and the results are summerized in Table 1 (entry 2, 5-9). It was found that FeTPA supported on neutral solids exhibited excellent catalytic performance, and FeTPA/TiO2 displayed more conversion of benzoyl chloride than others. In conclusion, FeTPA/TiO2 was chosen as the catalyst for the following study. Table 1 Acylation of m-xylene with benzoyl chloride over various catalysts Selectivity c Entry Catalyst a Conversion b (%) Yield c (%) (%) 1 TPA/TiO2 44.61 86.62 38.64 2 FeTPA/TiO2 96.85 96.86 93.81 3 FeTSiA/TiO2 95.83 94.62 90.67 4 FePMoA/TiO2 87.14 88.89 77.46 5 FeTPA/neutral Al2O3 47.98 40.51 19.44 6 FeTPA/acidic Al2O3 27.51 73.31 20.17 7 FeTPA/γ-Al2O3 76.28 75.17 57.34 8 FeTPA/nanoTiO2 88.11 96.73 85.23 5



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9 FeTPA/SnO2 88.73 94.12 83.51 Reaction conditions: temperature=403K, catalyst dose=5 wt%, n(m-xylene): n(benzoyl chloride)=4:1, reaction time=6 h. a Loading of active component= 20wt% b Conversion of benzoyl chloride. c Selectivity of 2,4-dimethylbenzophenone. The effect of FeTPA loading on the benzoylation of m-xylene was investigated and the results were presented in Table 2. The conversion of benzoyl chloride increased with the increase of FeTPA loading from 20wt% to 30wt%. 30wt% FeTPA/TiO2 displayed the excellent catalytic performance and 93.81 % yield of 2, 4-dimethylbenzophenone was obtained. However, when FeTPA loading continued to increase from 30wt% to 40wt%, the conversion of benzoyl chloride slightly decreased. To further understand these results, three catalysts (20% FeTPA/TiO2, 30% FeTPA/TiO2 and 40% FeTPA/TiO2) were characterized by BET and NH3-TPD. Table 2 The effect of FeTPA loading on the benzoylation of m-xylene. Selectivity b a Entry Catalyst Conversion (%) Yield b (%) (%) 1 20% FeTPA/TiO2 54.73 81.25 44.47 2 25% FeTPA/TiO2 88.03 93.37 82.19 3 30% FeTPA/TiO2 96.85 96.86 93.81 4 35% FeTPA/TiO2 91.42 93.85 85.80 5 40% FeTPA/TiO2 83.26 86.56 72.07 Reaction conditions: temperature=403K, catalyst dose=5 wt%, n(m-xylene): n(benzoyl chloride)=4:1, reaction time=6 h. a Conversion of benzoyl chloride. b Selectivity of 2,4-dimethylbenzophenone. 3.2 Characterization 3.2.1 FT-IR FT-IR patterns of FeTPA/TiO2 catalysts are shown in Fig. 1. There occur four characteristic bands at 1081, 983, 889, and 800cm−1, which can be assigned to the asymmetric stretching vibrations of P–O, W=Ot, W–Oc–W, and W–Oe–W respectively, related to characteristic Keggin ion. 21 These results indicate that the Keggin structure of FeTPA remain unaltered after immobilization on TiO2.

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Figure 1. FT-IR spectra of (a) TiO2, (b) TPA, (c) FeTPA, (d) 20% FeTPA/TiO2, (e) 30% FeTPA/TiO2 and (f) 40% FeTPA/TiO2. 3.2.2 XRD The X-ray patterns of the FeTPA/TiO2 catalysts are described in Fig. 2. The catalysts mainly exhibited peaks at 2θ value of 25.36°, 37.84°, 48.06°, 53.88°, 53.99° and 62.76°, corresponding to the anatase phase of titania, respectively. 22 It has been reported that the characteristic peaks related to Keggin ion in the structure of FeTPA were shown at 2θ value of 10.5°, 14.7°, 18.1° and 21°. 23 The weak peaks related to Keggin ion are visible for these catalysts with FeTPA loading above 30 wt%, indicating poor dispersion or agglomeration of FeTPA crystals. It was demonstrated that FeTPA is highly dispersed on the surface of titania when the loading is below 30 wt%.

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Figure 2. XRD patterns of Fe exchanged TPA supported on titania catalysts. (a) 30 %TPA/TiO2, (b) 20 %FeTPA/TiO2, (c) 30 %FeTPA/TiO2, (d) 40 %FeTPA/TiO2. 3.2.3 The textural properties of the catalysts The textural properties of FeTPA/TiO2 are listed in Table 3. As can been seen, with the increase of FeTPA loading from 20wt% to 30wt%, the surface area increases from 11.56 to 12.31 m2/g. However, with further increase of FeTPA loading beyond 30wt%, the surface area decreases. This is due to the multilayer generated on the surface of support. It was further demonstrated by the XRD patterns. Meanwhile, pore blocking also takes place in the presence of the excess FeTPA. Therefore, the micropore area and volume shrinks. These were the reasons that 30% FeTPA/TiO2 exhibited better catalytic activity than other catalysts. Thus, it could be speculated that the catalyst, which had larger surface area and micropore volume, exhibited better catalytic performance in the benzoylation of m-xylene. Table. 3 The textural properties of these three catalysts. Catalyst SBETa(m2/g) SMico (m2/g) Vpb (cm3/g) VMico (cm3/g) Dpc(nm) 20% FeTPA/TiO2 11.56 6.49 1.82 1.06×10-2 3.05×10-3 -2 -3 30% FeTPA/TiO2 12.31 6.51 1.83 1.12×10 3.18×10 -2 -3 40% FeTPA/TiO2 11.26 4.42 2.47 1.39×10 2.18×10 a Calculated by the nitrogen adsorption-desorption at 77K. b Calculated by BJH method. c Mean pore diameter = 4VP N2/SBET.

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3.2.4 NH3-TPD and Py-IR In order to study the acidic strength of these catalysts, NH3-TPD analysis was preformed. As shown in Fig. 3(A), 30% TPA/TiO2 only exhibited one desorption peaks in the range of 393 to 523 K corresponding to the weak acid sites. However, 20% FeTPA/TiO2 、 30% FeTPA/TiO2 and 40% FeTPA/TiO2 all displayed two desorption peaks in the low(393-664 K) and high(above 673 K) temperature range. These two peaks were assigned to the weak and strong acid sites, respectively. Then, the acidities of these catalysts are summarized in Table S1. The total acidity clearly increased when the FeTPA loading increased from 20wt% to 30wt%. This also explained why the conversion of benzoyl chloride increased with the increase of loading (Table 2, entry 1-3). However, there was no appreciable increase in the total acidity with the FeTPA loading further increased. Moreover, it was obvious that the total acidic strength of 30% TPA/TiO2 was larger than other catalysts but the conversion of benzoyl chloride over this catalyst was lower than others. Thus, Py-IR spectra were performed and the ratios of Lewis acid sites to total acid sites are summarized in Fig. 3(B) and Table S1. The bands at 1540 cm-1 and 1450 cm-1 were attributed to the pyridine adsorbed on the Brönsted and Lewis acid sites, respectively. 24 The 30% TPA/TiO2 catalyst showed Lewis acidity due to the coordinately unsaturated Ti4+ species on the surface. 25 It was observed that 30% FeTPA/TiO2 exhibited more Lewis acidity than 30% TPA/TiO2. Meanwhile, the enhancement of the Lewis acidity was due to the exchange of iron with the protons of TPA. Therefore, although the total acidity of 30% FeTPA/TiO2 was weaker than that of 30% TPA/TiO2, the Lewis acidity was stronger. This was the main reason that 30% FeTPA/TiO2 exhibited better catalytic performance compared with the TPA/TiO2 (Table 1, entry 1, 2). However, the obtained results indicated that there was no appreciable increase in Lewis acidity when the FeTPA loading further increased.

Figure 3. (A)NH3-TPD curves and (B)Py-IR spectra for (a) 30% TPA/TiO2, (b) 20% FeTPA/TiO2, (c) 30% FeTPA/TiO2, (d) 40% FeTPA/TiO2.

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3.3 The influence of catalyst calcination temperature Tungstophosphoric acid itself is poor thermal stability and easily decomposed at high temperature. Therefore, the influence of catalyst calcination temperature was studied. The catalysts calcined at 573K, 673K, 773K and 973K were used in the benzoylation of m-xylene and the results are presented in Table S2. The conversion of benzoyl chloride decreased from 96.85% to 37.01% when calcination temperature was increased from 573K to 973K. This was probably due to the decomposition of FeTPA. In order to confirm the above speculation, the catalysts calcined at 573K, 773K and 973K were respectively characterized by FT-IR and XRD. FT-IR patterns of the FeTPA/TiO2 catalyst calcined at 573K, 773K and 973K are shown in Fig. 4(A). It was observed that the main peaks of curve (a) were related to characteristic Keggin ions. However, these characteristic bands were difficultly observed on curve (b) and curve (c). This suggested that the decomposition of FeTPA occurred above 573K. Furthermore, XRD patterns of these catalysts are also presented in Fig. 4(B). It could be clearly found from curve (b) and curve (c) that the peaks observed at 2θ value of 23.16°, 23.58° and 24.36° were related to WO3 phase, which was formed by the decomposition of TPA salts. 26 This result is consistent with FT-IR.

Figure 4. (A) FT-IR spectra and (B) XRD patterns of 30%FeTPA/TiO2 catalyst calcination temperatures. (a) 573K, (b) 773K, (c) 973K. 3.4 Optimization of reaction parameters The benzoylation of m-xylene was firstly carried out at the temperatures ranging from 343 to 403K and the results are summarized in Fig. 5(A). It was clearly observed that the conversion of benzoyl chloride and selectivity of 2,4-dimethylbenzophenone increased with the increase of reaction temperature. Thus, 403K was chosen as the optimal reaction temperature. Next, the influence of catalyst dose on the benzoylation of m-xylene was investigated, the obtained results were presented in Fig. 5(B). It was found that as catalyst dose increased from 0.035g (2.5wt%) to 0.14g (10wt%), the conversion of benzoyl chloride increased from 90.83% to 100%, but the selectivity of 2,4-dimethylbenzophenone increased slightly. So, 0.14 g (10wt%) was selected as the 10


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suitable catalyst dose. Finally, the influence of molar ratio of m-xylene to benzoyl chloride was also studied. As shown in Fig. 5(C), it was no doubt that with the increase of molar ratio from 1 to 4, the conversion of benzoyl chloride significantly increased from 61.58% to 94.6% and the selectivity also increased. When molar ratio continued to increase, the conversion didn’t increase obviously. Thus, the optimal molar ratio was 4.

Figure 5(A) Effect of the temperature on this reaction (reaction condition: catalyst amount=0.07 g, m-xylene: benzoyl chloride=4:1, reaction time=5h); (B) Effect of catalyst amount (reaction condition: temperature=403K, m-xylene: benzoyl chloride=4:1, reaction time=5h); (C) Effect of molar ratio of m-xylene to benzoyl chloride (reaction condition: catalyst amount=0.07 g, temperature=403K, reaction time=5h). 3.5 Reusability of 30% FeTPA/TiO2 The reusability of 30% FeTPA/TiO2 was investigated under the optimal conditions. It was found that the activity of this catalyst basically kept unchanged in the first two cycles and the conversions of benzoyl chloride were respectively 87.74% and 85.14%. However, this conversion sharply decreased to 63.79% in the third cycle and the catalyst was obviously deactivated. The main reason for the catalyst deactivation might be attributed to the leaching of the active species. The similar phenomenon could be observed in the acylation of anisole with benzoic acid over 11



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TPA/ MCM-41. 27 Thus, improving the reusability of 30% FeTPA/TiO2 will be a big challenge for the future application. 3.6 Reaction kinetics of the acylation of m-xylene with benzoyl chloride The experimental results are used to build a kinetic model. In this paper, the experimental data is analyzed on the assumption that Langmuir-Hinshelwood mechanism is the preferable one for the acylation of m-xylene with benzoyl chloride catalyzed by FeTPA/TiO2. As a bimolecular reaction, the reaction rate of the acylation of m-xylene with benzoyl chloride is theoretically decided by the concentrations of m-xylene and benzoyl chloride, so the reaction rate equation was shown in Eq. (1), where r is the reaction rate, k is the reaction rate constant, C and Cm-xylene are the concentration of benzoyl chloride and m-xylene respectively, t is the reaction time, n and m is the reaction order, and X is the conversion of benzoyl chloride. However, to the desorption of the product easily from the catalyst at shorter reaction time, the more mole ratio of m-xylene to benzoyl chloride should be selected. As m-xylene was in far molar excess (mole ratio of m-xylene to benzoyl chloride=10:1) in the acylation, these were considered as a pseudo first-order reaction with respect to benzoyl chloride. Therefore, the reaction rate equations of the acylation of m-xylene with benzoyl chloride can be simplified as Eq. (2). dC r=− = kC n C mm− xylene dt (1) dC r=− = kC n dt (2) Hence, the standard first order reaction rate expression which shown in Eq. (3) was used for the kinetic analysis of this reaction. Furthermore, the activation energy of the reaction was estimated using the Arrhenius Equation (Eq. (4)), where Ea is the activation energy, R is the universal gas constant and T is the reaction temperature.

− ln(1 − X ) = kt (3) lnk = −

Ea + lnk 0 RT

(4) The plots of –ln(1-X) against time over FeTPA/TiO2 at the temperatures (363K, 373K, 383K, 393K and 403K) gave straight lines as illustrated in Fig. 6. The plots show that the rate of this benzoylation is first-order dependence over FeTPA/TiO2 with respect to benzoyl chloride which is the limiting reactant.

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Figure 6. Pseudo first-order kinetic plots for this reaction over FeTPA/TiO2 at different temperatures From the plots in Fig. 6, the reaction rate constant k at the temperatures of 363K, 373K, 383K, 393K and 403K could be calculated as 6.22×10-3 min-1, 7.97×10-3 min-1, 10.56×10-3 min-1, 12.42×10-3 min-1 and 14.18×10-3 min-1, respectively. The representative Arrhenius plots for the reaction rate constant obtained by Arrhenius Equation are shown in Fig. 7. It can be seen that the fit of the experimental values to the Arrhenius Equation is good and the Ea and k0 derived from this plot are 27.10 kJ/mol and 12.75 min-1.

Figure 7. Arrhenius plots for the acylation of m-xylene with benzoyl chloride over FeTPA/TiO2

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4. Conclusions A series of the FeTPA/TiO2 catalysts were prepared and exhibited excellent catalytic performance on the acylation of m-xylene with benzoyl chloride. Meanwhile, these catalysts were characterized by FT-IR, XRD, BET, NH3-TPD and Py-IR. It was obviously found that 30 % FeTPA/TiO2 exhibited better catalytic performance than other catalysts, attributed to its larger surface area and micro pore volume. Moreover, the increase of Lewis acidic strength could also efficiently enhance the catalytic activity. Furthermore, the influence of catalyst calcination temperature was studied, and it was observed that the active component started to decompose when catalyst calcination temperature was beyond 573K. Then, the reaction parameters, including reaction temperature, catalyst dose and molar ratio of m-xylene to benzoyl chloride, were optimized and a 95.1% yield of 2, 4-dimethylbenzophenone was obtained under optimal conditions. Finally, the kinetics of the benzoylation of m-xylene over 30% FeTPA/TiO2 was established. Supporting Information Available: Two tables: Table S1, S2.

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Iron(III)-Modified Tungstophosphoric Acid Supported on Titania Catalyst

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