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Synthesis of mesoporous ZrO 2 -SiO 2 and WO 3 /ZrO 2 -SiO 2 solid acids

Synthesis of mesoporous ZrO 2 -SiO 2 and WO 3 /ZrO 2 -SiO 2 solid acids DOI: 10.2478/v10063-008-0008-5 ANNALES UNIVERSITATIS MARIAE CURIE-SKLODOWSKA LUBLIN ­ POLONIA VOL. LXIV, 7 SECTIO AA 2009 Institute for Sorption and Endoecology Problems, National Academy of Sciences of Ukraine, General Naumov Str., 13, 03-164 Kiev, Ukraine e-mail: brei@ukr.net The method for preparation of mesoporous ZrO2-SiO2 and WO3/ZrO2-SiO2 oxides with silicic acid sol is proposed. It was shown that these acidic oxides demonstrate high activity in the reaction of tetrahydrofuran oligomerization. 1. INTRODUCTION Solid acid catalysts are widely used in oil refinery and organic synthesis, especially medium acidic catalysts on the basis of zeolites and mixed oxides [1]. During the last 25 years, significant attention has been devoted to synthesis and study of stable solid superacids, such as sulphated [2] and tangstated [3] zirconia. These oxides effectively catalyze many important reactions at average temperatures, for instance isomerization of linear alkanes and acylation of aromatic compounds [2, 3]. Howevet present the synthesis of solid acids which occupy the intermediate position between zeolites with their acidity function value (0 -8) and the superacid catalysts (0 -12) is of interest. High surface area and suitable texture parameters are also important for oxide material (see, for instance [4]). Relatively low specific surface area, which is lower than 100 m2/g [3], is a certain disadvantage of superacids on the basis of zirconia. This is connected with the high coordination number for Zr4+ ions (7 and 8) and, correspondingly, This article 65th birthday is dedicated to Professor Roman Leboda on the occasion of his with dense packing of 2- ions in the ZrO2 lattice. At the same time, it is known that joining of silicon-oxygen tetrahedrons allows to obtain silica materials as silicagel, with high surface area and considerable pore volume. The synthesis of mixed ZrO2-SiO2 oxide is described in literature [5-10], but there is almost no information on the preparation of ternary WO3/ZrO2-SiO2 system. As starting reagents, water solutions of hexafluorosilicic acid (H2SiF6) and hexafluorozirconic acid (H2ZrF6) [5], tetraethoxysilane, zirconium isopropylate [6, 7] and nitrate [8], sodium silicate and zirconium chloride [9] or a zirconium carbonate complex [10] were used to prepare ZrO2-SiO2 samples. In the present work ZrO2-SiO2 and WO3/ZrO2-SiO2 oxides are prepared on the basis of silicic acid sol. 2. EXPERIMENTAL Zirconyl chloride octahydrate ZrOCl28H2O, ammonium meta-tungstate (NH4)6H2W12O40·xH2O, potassium silicate K2Si2O5, nonionic surface active substance Triton CF-10 (Dow Chemical), and carbamide were used as starting substances. Water sol of oligomers of silicic acid was obtained treating potassium silicate with the cation-exchange resin KU-2 (sulfuretted styrene and divinylbenzene copolymer) [11, 12]. In order to synthesize mixed ZrO2-SiO2 oxide, zirconium oxychloride solution in a small amount of water was added to 1 L of polysilicic acid solution (SiO2 concentration equal 0.4 ) with the mole ratio Zr:Si=1:2 according to [10]. Then 48 g of carbamide and 10 g of Triton CF-10 were added. The resultant solution was heated to boiling and kept at this temperature for 2 h while stirring to transform sol into gel. The obtained gel was dried at 1200C for 48 h. Further treatment of xerogel was performed in two ways: · Dry method. The obtained xerogel was heated (20C/min) up to 7000C and kept at this temperature for 2h. During the treatment, volatile and combustible compounds (carbamide and its transformation product, ammonium chloride) were eliminated. The resultant sample of mixed ZrO2-SiO2 oxide is named ZrSi(dry). · Wet method. The obtained xerogel was washed with water several times to eliminate chloride ions, dried, and then thermally treated in air for 2 h at 7000C. This sample is further coded as ZrSi(aq). The synthesis of WO3/ZrO2-SiO2 samples was performed in the same way, but ammonium metatungstate was added to sol at the atom ratio W:Zr=0.2:1. Further treatment of corresponding gels was performed according to two routes described above for the ZrO2-SiO2 samples. The samples of mixed oxides WO3/ZrO2-SiO2 would be referred to as WZrSi(dry) and WZrSi(aq). X-Ray (CuK) patterns for all obtained samples were registered using a DRON-4-07 diffractometer. Nitrogen adsorption isotherms were measured using Nova 2200e Surface Area and Pore Size Analyzer. Reflectance spectra of the powdered samples were registered with a Specord M40 device. The reflectance coefficient R was calculated according to the MgO standard (R=Rsample/RMgO). The band gap width E0 was estimated from the reflectance spectra using the Kubelka-Munk equation = (h(1-R)2/2R)1/2. The E0 values were determined from the nearly linear long-wave segment of absorption band plot extrapolated to interception with the abscissa [13]. Total acid sites concentration was determined by the method of reverse titration using n-butylamine adsorbed on the surface of a sample from a solution in cyclohexane using bromthymol blue as an indicator. 20 ml of 0.1 M solution of n-butylamine in purified and dried cyclohexane were added to 300­400 mg of a sample. After stirring for 30 minutes, an aliquot of the solution was titrated with 0.05 M solution of hydrochloric acid. The strength of acid sites was estimated by the method of direct titration of the surface of synthesized samples with n-butylamine using the Hammett indicators: (benzalacetophenone (pKa = -5.6), antraquinone (pKa = -8.2), 4-nitrotoluene (pKa = -11.35), 1-chloro-3nitrobenzene (pKa = -13.16), 2,4-dinitrotoluene (pKa = -13.75) and 2,4-dinitro-1fluorobenzene (pKa = -14.52). All samples were dried for 1 hour at 5000C before testing. The activity of the samples was determined in the oligomerization reaction of tetrahydrofuran [14] and in the test reaction of 2-methyl-3-butyn-2-ol transformation [15]. 3. RESULTS AND DISCUSSION Figure 1 presents the X-ray patterns for ZrSi and WZrSi calcinated at 7000C and 8000C. As can be seen from the figure, the ZrSi samples are amorphous. Broad maxima around 300 and 510 approximately correspond to the most intensive peaks of zirconia. The formation of tetragonal crystalline ZrO2 phase was observed for the WZrSi(dry) sample after treatment at 7000C (curve 5). Zirconium dioxide crystals are observed in all WZrSi samples after calcinations at 8000C. Thus, insertion of W6+ ions promotes the crystallization of zirconia in this system. The ZrO2 crystallite size calculated from the peak half-width using the Sherrer equation is 4­5 nm for the WZrSi(aq) sample calcinated at 8000C and 9­10 nm for the WZrSi(dry) sample. Thus burning of the volatile templates without preliminary washing leads to crystallization of ZrO2 at lower temperature and causes the formation of larger ZrO2 crystals. The crystalline ZrO2 phase does not appear under any synthetic conditions in the absence of tungstated ions. It is interesting that WO3 slows down crystallization of zirconia in the WO3-ZrO2 systems not containing silicon [3, 15]. 2 , d eg ree s Fig. 1. XRD patterns (CuK) for the ZrO2-SiO2 and WO3/ZrO2-SiO2 samples: 1 ­ ZrSi(aq)700, 2 ­ ZrSi(aq)800, 3 ­ WZrSi(aq)700, 4 ­ WZrSi(aq)800, 5 ­ WZrSi(dry)700, 6 ­ WZrSi(dry)800. Figure 2 shows the pore size distribution for the WZrSi samples, and Table 1 presents the texture data for prepared WZrSi and ZrSi. The values of specific surface area of the WZrSi and ZrSi samples in the range of 200­350 m2/g, that is 3­5 times higher than those for the W3/Zr2 samples obtained by the coprecipitation technique (50­70 m2/g) [15]. Correspondingly, the pore volume for WZrSi and ZrSi exceeds three times the appropriate values for W3/Zr2. However, the average mesopore size for ZrSi, WZrSi (Table 1) and W3/Zr2 [15] remains approximately the same (1.8­2.2 nm). It should be noted that WZrSi(dry) obtained using burning out of organic template and ammonium chloride possesses larger size pores than the samples obtained from washed out xerogels. Probably, the products of thermal polymerization of carbamide and ammonium chloride perform the role of templates and pore-forming substances. 1,6 1,2 0,0 10 0,5 0,3 0,3 0,1 0,0 10 0,1 2,0 1,5 1,0 0,5 Fig. 2. BJH nitrogen desorption pore radius distributions for the ZrO2-SiO2 and WO3ZrO2-SiO2 samples: a) WZrSi(dry)700, b) WZrSi(dry)800, c) WZrSi(aq)700, d) WZrSi(aq)800, e) ZrSi(aq)700. Synthesis of mesoporous ZrO2-SiO2 and WO3/ZrO2-SiO2 solid acids. Tab. 1. Texture parameters for ZrSi and WZrSi. Specific surface area, S, m2/g 250 140 270 130 380 Pore volume, V, cm3/g 0.16 0.19 0.17 0.10 0.24 Pore radius, nm 2.2 2.2 1.8 1.7 2.0 Sample WZrSi(dry) WZrSi(aq) ZrSi(aq) , 0C 700 800 700 800 700 The effect of ammonium chloride as an ionogenic pore-forming agent is described in [14]. It is known that catalytically active WO3/ZrO2 is characterized by surface tungstated clusters with the band gap width in the range 0 = 3.0­3.2 eV (individual ZrO2 and WO3 possess E0 = 5.6 eV and 2.6 eV respectively) [13]. The UV diffuse reflectance spectra for all WZrSi samples show that they are characterized by E0 values, which are very close to those for WO3/ZrO2 (Figure 3). [h(1-R) /2R] 1/2 2 WZrSi(aq)700 WZrSi(dry)700 0 2,5 3,18 eV 3,0 3,22 eV 3,5 E, eV 4,0 Fig. 3. UV-Vis diffuse reflectance spectra for the WO3/ZrO2-SiO2 samples. Total concentration of acidic sites for ZrSi(aq) and WZrSi(aq) calcinated at 7000C is 1.3 mmol/g and 1.1 mmol/g, respectively. The results of determination of strength distribution for the acid sites of the ZrSi and WZrSi samples are presented in Figure 4. It was found that both ZrSi and WZrSi samples change their color from white to light yellow in the presence of 4-nitrotoluene (H0 = -11.35). It can be seen (Figure 4) that concentration-strength acid site distributions are considerably different for the ZrSi and WZrSi samples. WZrSi is a stronger solid acid because about 80% of its medium-acid sites are characterized by the acidity function value H0 = -8.2. The main contents of sites (60%) on the surface of the ZrSi sample are described by the value of H0 = -5.6. For the WO3/ZrO2 system, a wide range of acid strength is observed, from the medium (-5.6 H0 -8.2) up to superacidic values of H0 = -14.5 (~5%) [15]. Acidity of ZrSi, WZrSi correlates with their activity in the test reaction of 2-methyl-3-butyn-2-ol (MBOH) transformation (Figure 5). This molecule undergoes acid-catalyzed dehydration to 3-methyl-3-butene-1-yne (Mbyne, m/e = 66) and isomerization into 3-methyl-2-butene-1-al (Prenal, m/e = 84). The first reaction proceeds easily over weak acid sites of a catalyst, and therefore the maximum of Mbyne evolution appears at lower temperature (40­500C). However, strong acid sites are required for the isomerization of MBOH into Prenal. The prenal peaks are observed at 1100C for the WO3/ZrO2 samples [15]. For the less acidic WZrSi and ZrSi samples, these peaks are shifted to 130oC and 2200C, respectively (Figure 5). The synthesized WO3/ZrO2-SiO2 and ZrO2-SiO2 samples demonstrate high activity in tetrahydrofuran oligomerization reaction in a flow reactor (Table 2, Figure 6). It should be noted that an industrial catalyst for this process has been developed on the basis of ZrO2-SiO2 [8] The activity of ZrSi and WZrSi only slightly differs at low loading on the catalyst (L < 4 mmol THF/gcatal·h), but higher yield of polytetramethylene ethter (PTME) is observed for WZrSi at higher L values (Figure 6). 4. CONCLUSIONS Thus, the mesoporous ZrO2-SiO2 and WO3/ZrO2-SiO2 oxides with high content of acidic sites have been obtained on the basis of silicic acid sol. It was shown that WO3/ZrO2-SiO2 is a suitable catalyst for the tetrahydrofuran oligomerization process. C, m m ol/g 1,0 C total = 1.09 m m ol/g -8.2 4 m m ol/g -5.6 -11.35 0,19 m m ol/g 0,06 m m ol/g -8,2 0 > -11,35 -11,35 0 > -13,16 0,0 -5.6 0 > -8.2 H0 C, mmol/g -5,6 Ctotal = 1.31 mmol/g -8,2 mmol/g 8 mmol/g 0,0 0,03 mmol/g -5.6 0> -8.2 -8,2 0> -11,35 -11,35 0> -13,16 -11,35 H0 Fig. 4. Concentration-strength acid site distributions for the WZrSi(dry)700 (a) and ZrSi(aq) (b) samples. 10 I, V 8 130 oC 84 I, V -1 66(*10 ) 220 C T, C T, C Fig. 5. TPR spectra of Mbyne and Prenal formation from MBOH adsorbed on WZrSi (a), ZrSi (b). Tab. 2. Average number (Mn), average weight (Mw), and yield of PTME (THF:acetic anhydride = 8:1). Load, mmol THF/gcat·h 7.5 12.7 7.5 12.7 Catalyst WzrSi WzrSi ZrSi ZrSi Yield, %wt 37 33 30 21 Mn 529 625 487 489 Mw 956 1205 723 962 Mw/Mn 1.8 1.92 1.48 1.97 50 Conversion, % Feed rate, mmol THF/gcat*h Fig. 6. Conversion of THF at different feed rates for WZrSi (1) and ZrSi (2). 5. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Annales UMCS, Chemia de Gruyter

Synthesis of mesoporous ZrO 2 -SiO 2 and WO 3 /ZrO 2 -SiO 2 solid acids

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Publisher
de Gruyter
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Copyright © 2009 by the
ISSN
0137-6853
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2083-358X
DOI
10.2478/v10063-008-0008-5
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Abstract

DOI: 10.2478/v10063-008-0008-5 ANNALES UNIVERSITATIS MARIAE CURIE-SKLODOWSKA LUBLIN ­ POLONIA VOL. LXIV, 7 SECTIO AA 2009 Institute for Sorption and Endoecology Problems, National Academy of Sciences of Ukraine, General Naumov Str., 13, 03-164 Kiev, Ukraine e-mail: brei@ukr.net The method for preparation of mesoporous ZrO2-SiO2 and WO3/ZrO2-SiO2 oxides with silicic acid sol is proposed. It was shown that these acidic oxides demonstrate high activity in the reaction of tetrahydrofuran oligomerization. 1. INTRODUCTION Solid acid catalysts are widely used in oil refinery and organic synthesis, especially medium acidic catalysts on the basis of zeolites and mixed oxides [1]. During the last 25 years, significant attention has been devoted to synthesis and study of stable solid superacids, such as sulphated [2] and tangstated [3] zirconia. These oxides effectively catalyze many important reactions at average temperatures, for instance isomerization of linear alkanes and acylation of aromatic compounds [2, 3]. Howevet present the synthesis of solid acids which occupy the intermediate position between zeolites with their acidity function value (0 -8) and the superacid catalysts (0 -12) is of interest. High surface area and suitable texture parameters are also important for oxide material (see, for instance [4]). Relatively low specific surface area, which is lower than 100 m2/g [3], is a certain disadvantage of superacids on the basis of zirconia. This is connected with the high coordination number for Zr4+ ions (7 and 8) and, correspondingly, This article 65th birthday is dedicated to Professor Roman Leboda on the occasion of his with dense packing of 2- ions in the ZrO2 lattice. At the same time, it is known that joining of silicon-oxygen tetrahedrons allows to obtain silica materials as silicagel, with high surface area and considerable pore volume. The synthesis of mixed ZrO2-SiO2 oxide is described in literature [5-10], but there is almost no information on the preparation of ternary WO3/ZrO2-SiO2 system. As starting reagents, water solutions of hexafluorosilicic acid (H2SiF6) and hexafluorozirconic acid (H2ZrF6) [5], tetraethoxysilane, zirconium isopropylate [6, 7] and nitrate [8], sodium silicate and zirconium chloride [9] or a zirconium carbonate complex [10] were used to prepare ZrO2-SiO2 samples. In the present work ZrO2-SiO2 and WO3/ZrO2-SiO2 oxides are prepared on the basis of silicic acid sol. 2. EXPERIMENTAL Zirconyl chloride octahydrate ZrOCl28H2O, ammonium meta-tungstate (NH4)6H2W12O40·xH2O, potassium silicate K2Si2O5, nonionic surface active substance Triton CF-10 (Dow Chemical), and carbamide were used as starting substances. Water sol of oligomers of silicic acid was obtained treating potassium silicate with the cation-exchange resin KU-2 (sulfuretted styrene and divinylbenzene copolymer) [11, 12]. In order to synthesize mixed ZrO2-SiO2 oxide, zirconium oxychloride solution in a small amount of water was added to 1 L of polysilicic acid solution (SiO2 concentration equal 0.4 ) with the mole ratio Zr:Si=1:2 according to [10]. Then 48 g of carbamide and 10 g of Triton CF-10 were added. The resultant solution was heated to boiling and kept at this temperature for 2 h while stirring to transform sol into gel. The obtained gel was dried at 1200C for 48 h. Further treatment of xerogel was performed in two ways: · Dry method. The obtained xerogel was heated (20C/min) up to 7000C and kept at this temperature for 2h. During the treatment, volatile and combustible compounds (carbamide and its transformation product, ammonium chloride) were eliminated. The resultant sample of mixed ZrO2-SiO2 oxide is named ZrSi(dry). · Wet method. The obtained xerogel was washed with water several times to eliminate chloride ions, dried, and then thermally treated in air for 2 h at 7000C. This sample is further coded as ZrSi(aq). The synthesis of WO3/ZrO2-SiO2 samples was performed in the same way, but ammonium metatungstate was added to sol at the atom ratio W:Zr=0.2:1. Further treatment of corresponding gels was performed according to two routes described above for the ZrO2-SiO2 samples. The samples of mixed oxides WO3/ZrO2-SiO2 would be referred to as WZrSi(dry) and WZrSi(aq). X-Ray (CuK) patterns for all obtained samples were registered using a DRON-4-07 diffractometer. Nitrogen adsorption isotherms were measured using Nova 2200e Surface Area and Pore Size Analyzer. Reflectance spectra of the powdered samples were registered with a Specord M40 device. The reflectance coefficient R was calculated according to the MgO standard (R=Rsample/RMgO). The band gap width E0 was estimated from the reflectance spectra using the Kubelka-Munk equation = (h(1-R)2/2R)1/2. The E0 values were determined from the nearly linear long-wave segment of absorption band plot extrapolated to interception with the abscissa [13]. Total acid sites concentration was determined by the method of reverse titration using n-butylamine adsorbed on the surface of a sample from a solution in cyclohexane using bromthymol blue as an indicator. 20 ml of 0.1 M solution of n-butylamine in purified and dried cyclohexane were added to 300­400 mg of a sample. After stirring for 30 minutes, an aliquot of the solution was titrated with 0.05 M solution of hydrochloric acid. The strength of acid sites was estimated by the method of direct titration of the surface of synthesized samples with n-butylamine using the Hammett indicators: (benzalacetophenone (pKa = -5.6), antraquinone (pKa = -8.2), 4-nitrotoluene (pKa = -11.35), 1-chloro-3nitrobenzene (pKa = -13.16), 2,4-dinitrotoluene (pKa = -13.75) and 2,4-dinitro-1fluorobenzene (pKa = -14.52). All samples were dried for 1 hour at 5000C before testing. The activity of the samples was determined in the oligomerization reaction of tetrahydrofuran [14] and in the test reaction of 2-methyl-3-butyn-2-ol transformation [15]. 3. RESULTS AND DISCUSSION Figure 1 presents the X-ray patterns for ZrSi and WZrSi calcinated at 7000C and 8000C. As can be seen from the figure, the ZrSi samples are amorphous. Broad maxima around 300 and 510 approximately correspond to the most intensive peaks of zirconia. The formation of tetragonal crystalline ZrO2 phase was observed for the WZrSi(dry) sample after treatment at 7000C (curve 5). Zirconium dioxide crystals are observed in all WZrSi samples after calcinations at 8000C. Thus, insertion of W6+ ions promotes the crystallization of zirconia in this system. The ZrO2 crystallite size calculated from the peak half-width using the Sherrer equation is 4­5 nm for the WZrSi(aq) sample calcinated at 8000C and 9­10 nm for the WZrSi(dry) sample. Thus burning of the volatile templates without preliminary washing leads to crystallization of ZrO2 at lower temperature and causes the formation of larger ZrO2 crystals. The crystalline ZrO2 phase does not appear under any synthetic conditions in the absence of tungstated ions. It is interesting that WO3 slows down crystallization of zirconia in the WO3-ZrO2 systems not containing silicon [3, 15]. 2 , d eg ree s Fig. 1. XRD patterns (CuK) for the ZrO2-SiO2 and WO3/ZrO2-SiO2 samples: 1 ­ ZrSi(aq)700, 2 ­ ZrSi(aq)800, 3 ­ WZrSi(aq)700, 4 ­ WZrSi(aq)800, 5 ­ WZrSi(dry)700, 6 ­ WZrSi(dry)800. Figure 2 shows the pore size distribution for the WZrSi samples, and Table 1 presents the texture data for prepared WZrSi and ZrSi. The values of specific surface area of the WZrSi and ZrSi samples in the range of 200­350 m2/g, that is 3­5 times higher than those for the W3/Zr2 samples obtained by the coprecipitation technique (50­70 m2/g) [15]. Correspondingly, the pore volume for WZrSi and ZrSi exceeds three times the appropriate values for W3/Zr2. However, the average mesopore size for ZrSi, WZrSi (Table 1) and W3/Zr2 [15] remains approximately the same (1.8­2.2 nm). It should be noted that WZrSi(dry) obtained using burning out of organic template and ammonium chloride possesses larger size pores than the samples obtained from washed out xerogels. Probably, the products of thermal polymerization of carbamide and ammonium chloride perform the role of templates and pore-forming substances. 1,6 1,2 0,0 10 0,5 0,3 0,3 0,1 0,0 10 0,1 2,0 1,5 1,0 0,5 Fig. 2. BJH nitrogen desorption pore radius distributions for the ZrO2-SiO2 and WO3ZrO2-SiO2 samples: a) WZrSi(dry)700, b) WZrSi(dry)800, c) WZrSi(aq)700, d) WZrSi(aq)800, e) ZrSi(aq)700. Synthesis of mesoporous ZrO2-SiO2 and WO3/ZrO2-SiO2 solid acids. Tab. 1. Texture parameters for ZrSi and WZrSi. Specific surface area, S, m2/g 250 140 270 130 380 Pore volume, V, cm3/g 0.16 0.19 0.17 0.10 0.24 Pore radius, nm 2.2 2.2 1.8 1.7 2.0 Sample WZrSi(dry) WZrSi(aq) ZrSi(aq) , 0C 700 800 700 800 700 The effect of ammonium chloride as an ionogenic pore-forming agent is described in [14]. It is known that catalytically active WO3/ZrO2 is characterized by surface tungstated clusters with the band gap width in the range 0 = 3.0­3.2 eV (individual ZrO2 and WO3 possess E0 = 5.6 eV and 2.6 eV respectively) [13]. The UV diffuse reflectance spectra for all WZrSi samples show that they are characterized by E0 values, which are very close to those for WO3/ZrO2 (Figure 3). [h(1-R) /2R] 1/2 2 WZrSi(aq)700 WZrSi(dry)700 0 2,5 3,18 eV 3,0 3,22 eV 3,5 E, eV 4,0 Fig. 3. UV-Vis diffuse reflectance spectra for the WO3/ZrO2-SiO2 samples. Total concentration of acidic sites for ZrSi(aq) and WZrSi(aq) calcinated at 7000C is 1.3 mmol/g and 1.1 mmol/g, respectively. The results of determination of strength distribution for the acid sites of the ZrSi and WZrSi samples are presented in Figure 4. It was found that both ZrSi and WZrSi samples change their color from white to light yellow in the presence of 4-nitrotoluene (H0 = -11.35). It can be seen (Figure 4) that concentration-strength acid site distributions are considerably different for the ZrSi and WZrSi samples. WZrSi is a stronger solid acid because about 80% of its medium-acid sites are characterized by the acidity function value H0 = -8.2. The main contents of sites (60%) on the surface of the ZrSi sample are described by the value of H0 = -5.6. For the WO3/ZrO2 system, a wide range of acid strength is observed, from the medium (-5.6 H0 -8.2) up to superacidic values of H0 = -14.5 (~5%) [15]. Acidity of ZrSi, WZrSi correlates with their activity in the test reaction of 2-methyl-3-butyn-2-ol (MBOH) transformation (Figure 5). This molecule undergoes acid-catalyzed dehydration to 3-methyl-3-butene-1-yne (Mbyne, m/e = 66) and isomerization into 3-methyl-2-butene-1-al (Prenal, m/e = 84). The first reaction proceeds easily over weak acid sites of a catalyst, and therefore the maximum of Mbyne evolution appears at lower temperature (40­500C). However, strong acid sites are required for the isomerization of MBOH into Prenal. The prenal peaks are observed at 1100C for the WO3/ZrO2 samples [15]. For the less acidic WZrSi and ZrSi samples, these peaks are shifted to 130oC and 2200C, respectively (Figure 5). The synthesized WO3/ZrO2-SiO2 and ZrO2-SiO2 samples demonstrate high activity in tetrahydrofuran oligomerization reaction in a flow reactor (Table 2, Figure 6). It should be noted that an industrial catalyst for this process has been developed on the basis of ZrO2-SiO2 [8] The activity of ZrSi and WZrSi only slightly differs at low loading on the catalyst (L < 4 mmol THF/gcatal·h), but higher yield of polytetramethylene ethter (PTME) is observed for WZrSi at higher L values (Figure 6). 4. CONCLUSIONS Thus, the mesoporous ZrO2-SiO2 and WO3/ZrO2-SiO2 oxides with high content of acidic sites have been obtained on the basis of silicic acid sol. It was shown that WO3/ZrO2-SiO2 is a suitable catalyst for the tetrahydrofuran oligomerization process. C, m m ol/g 1,0 C total = 1.09 m m ol/g -8.2 4 m m ol/g -5.6 -11.35 0,19 m m ol/g 0,06 m m ol/g -8,2 0 > -11,35 -11,35 0 > -13,16 0,0 -5.6 0 > -8.2 H0 C, mmol/g -5,6 Ctotal = 1.31 mmol/g -8,2 mmol/g 8 mmol/g 0,0 0,03 mmol/g -5.6 0> -8.2 -8,2 0> -11,35 -11,35 0> -13,16 -11,35 H0 Fig. 4. Concentration-strength acid site distributions for the WZrSi(dry)700 (a) and ZrSi(aq) (b) samples. 10 I, V 8 130 oC 84 I, V -1 66(*10 ) 220 C T, C T, C Fig. 5. TPR spectra of Mbyne and Prenal formation from MBOH adsorbed on WZrSi (a), ZrSi (b). Tab. 2. Average number (Mn), average weight (Mw), and yield of PTME (THF:acetic anhydride = 8:1). Load, mmol THF/gcat·h 7.5 12.7 7.5 12.7 Catalyst WzrSi WzrSi ZrSi ZrSi Yield, %wt 37 33 30 21 Mn 529 625 487 489 Mw 956 1205 723 962 Mw/Mn 1.8 1.92 1.48 1.97 50 Conversion, % Feed rate, mmol THF/gcat*h Fig. 6. Conversion of THF at different feed rates for WZrSi (1) and ZrSi (2). 5.

Journal

Annales UMCS, Chemiade Gruyter

Published: Jan 1, 2009

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