Characterization of Tungsten-Based Catalyst Used for Selective Oxidation of Cyclopentene to Glutaraldehyde

Chinese Journal of Chemical Engineering, 16(6) 895ü900 (2008)

Characterization of Tungsten-Based Catalyst Used for Selective Oxidation of Cyclopentene to Glutaraldehyde ZHU Zhiqing (ᅋᄝ௉)* and BIAN Wei (ή᤭)

Institute of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China Abstract Tungsten-containing hexagonal mesoporous silica (W-HMS) supported tungsten oxide catalysts (WOx/W-HMS) was prepared for the selective oxidation of cyclopentene with aqueous hydrogen peroxide to glutaraldehyde. X-ray diffraction (XRD) results indicated that the crystal form of the active phase (tungsten oxide) of the WOx/W-HMS catalysts was dependent on the W loading and calcination temperature. X-ray photoelectron spectroscopy (XPS) analysis revealed that the dispersed tungsten oxides on the surface of W-HMS support consisted of a mixture of W(V) and W(VI). It was found that a high content of amorphous W species in (5+) oxidation state resulted in the high catalytic activity. When the W loading was up to 12% (by mass) or the catalyst precursor was treated at temperature of 623 K, the catalytic activity decreased due to the presence of WO3 crystallites and the oxidation of W(V) to W(VI) on the catalyst surface. Furthermore, NH3-temperature-programmed-desorption (NH3-TPD) analysis showed that the effects of W loading and calcination temperature on the acidity of the catalysts were related to the catalytic activity. A high selectivity of 80.2% for glutaraldehyde with a complete conversion of cyclopentene was obtained over 8%WOx/W-HMS catalyst calcined at 573 K after 14 h of reaction. Keywords cyclopentene, glutaraldehyde, tungsten, hexagonal mesoporous silica, oxidation

1

INTRODUCTION

Glutaraldehyde (GA) has been extensively used for disinfections and sterilization in many fields. An advantageous way to produce GA is the one-step route through the selective oxidation of cyclopentene (CPE) with aqueous hydrogen peroxide as the oxidant, since a great quantity of CPE could be easily obtained as a main by-product from the C-5 fraction in the petrochemical or coking industry [1, 2]. Research has been focused on the selective oxidation of CPE to GA on various tungsten-containing homogeneous catalysts. The challenge is that it is difficult to separate and recover catalysts from the product mixture [24]. It is well known that catalysts containing tungsten oxides have been used as solid acid catalysts. However, bulk WO3 was reported to be inactive at low reaction temperature for several reactions because of its weak acid sites [5]. Furthermore, the charge state of tungsten atoms in supported tungsten oxide catalysts was considered to be mainly W(VI) ions [69]. Recently, a series of tungsten-containing heterogeneous catalysts, such as WO3/TiO2-SiO2, WO3/SiO2, W-MCM41, W-SBA15, W-MCM48, W-HMS and WO3/HMS, were synthesized and used for the selective oxidation of CPE to GA [4, 1016]. Although it is easy to remove catalysts from the product mixture, the performance of these catalysts has been shown to be unsatisfactory because of the low reaction rate. Yang and coworkers [12] reported that the active phase in these types of catalysts was amorphous WO3 and the activity increased as the atomic ratio of Si/W decreased. According to their result, it seems that increasing the amorphous WO3 loading was an effective method for improving the activity. However, the features of interacting species, the various tungsten dispersion states and their quantification were not well understood yet, although some research has been carried out to char-

acterize W-containing heterogeneous catalysts. In our study, the novel heterogeneous WOx/W-HMS catalyst, which has been shown to be significantly active for the selective oxidation of CPE to GA, was synthesized through a combination of sol-gel and impregnation methods. In this paper, we investigated samples containing W species up to 12% (by mass) on the tungsten promoted HMS support. The effect induced by thermal treatments on catalytic activity was determined. Tungsten species present as a dispersed form on the catalyst surface was characterized and their distribution was quantitatively determined with various techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and NH3-temperatureprogrammed-desorption (NH3-TPD). We also obtained a possible correlation between the W valence state and the catalytic activity regarding the oxidation of CPE. 2 2.1

EXPERIMENTAL Chemical reagents

Dodecylamine, ammonia tungstic ((NH4)2WO4) (analytic grade), tetraethyl orthosilcate (analytic grade), and oxalic acid (C.P. grade) were purchased from Sinopharm Chemical Reagent Company. Hydrogen peroxide (industrial grade, 30%, 50%), tert-butyl alcohol (C.P. grade), and cyclopentene (C.P. grade) were obtained from Sinopec Shanghai Petrochemical Company. Ethanol (C.P. grade) and hydrochloric acid (C.P. grade, 37%) were purchased from Shanghai Lingfeng Chemical Reagent Company. All the chemicals were used as received without further purification. 2.2 Preparation of catalysts W-containing mesoporous silica (W-HMS) was

Received 2008-02-27, accepted 2008-08-04. * To whom correspondence should be addressed. E-mail: [email protected]

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synthesized at ambient temperature using dodecylamine as templating surfactant. A typical preparation process for W-HMS (atomic ratio Si/W 30) was as follows: Solution A was prepared by mixing tungstic acid (H2WO4·H2O, 0.1 mol), oxalic acid (0.05 mol), distilled water (0.85 mol) and hydrogen peroxide (30%, 0.35 mol). Solution B was composed of tetraethyl orthosilcate (TEOS, 0.3 mol) and ethanol (0.7 mol). Solution C consisted of dodecylamine (0.03 mol), distilled water (3.1 mol), ethanol (1.2 mol) and ˉ hydrochloric acid (5.0 ml, 1 mol·L 1). Solution A and solution B were simultaneously added to solution C under vigorous stirring. Stirring was maintained for about 30 min. Then, the solution was kept at ambient temperature for 12 h. The precipitate generated from the reaction was recovered by filtration. After washed several times with distilled water and ethanol, the precipitate was dried at 393 K for 2 h. Organic reagents were removed by calcining the as-synthesized solids in air at 723 K for 4 h. The W-HMS particle size and surface area was 7580 ȝm (180200 mesh) and 808 ˉ m2·g 1, respectively. All WOx/W-HMS catalysts were prepared by impregnation of W-HMS (Si/W 30) with ammonia tungstic [(NH4)2WO4] as follows: (NH4)2WO4 (0.03 mol) ˉ was dissolved in a solution of oxalic acid (0.4 mol·L 1) at 353 K. A variable amount of W-HMS support obtained previously, corresponding to different W loadings, was stirred into the solution to form a paste. After being kept at ambient temperature for 20 h, the paste was dried in the oven at 393 K for 2 h, followed by calcination at a given temperature in air for another 2 h. The samples were identified by their mass percentage of W content impregnated on W-HMS support. The catalyst particle size was about 7580 ȝm (180200 mesh) and the surface area was varied with the differˉ ent WOx-loading ranging from 780 to 805 m2·g 1. 2.3

sorptions pattern was established by a gas chromatograph system with thermal conductivity detector (TCD). The TCD response was calibrated by a serial standard ammonia samples. The gas chromatograph was operated at the same conditions for all catalysts. 2.4

Catalytic performance

The catalytic oxidation was carried out in a sealed 100 ml reactor in which 2.0 g catalyst was mixed with 14 ml of 50% aqueous H2O2 solution with the presence of 60 ml of tert-butyl alcohol as the solvent. Afterwards, 10 ml of cyclopentene was added via dropping funnel under vigorous stirring, then it was heated and allowed to react at 308 K for 16 h under stirring. The reaction products were analyzed by gas chromatograph (GC-9800) using a 0.32 mmh30 m capillary column (FFAP) and a flame ionization detector (FID). Details of the experiment are given in our previous work elsewhere [17]. 3 3.1

RESULTS AND DISCUSSION Characterization of catalyst

From the wide-angle XRD patterns of W-HMS shown in Fig. 1 (a), no peaks corresponding to crystalline WO3 phase were observed at the atomic ratio Si/W 30, revealing the absence of agglomerated crystalline WO3 species in the solids and the successful incorporation of the added tungsten heteroatoms into the lattice of the HMS framework. The strong characteristic peaks of crystalline WO3 phase appeared when the content of tungsten increased to Si/W 20, suggesting that WO3 begins to congregate as reported [12]. The amount of tungsten added into

Characterization of catalysts

The wide-angle X-ray powder diffraction patterns were recorded on a Rigaku D/MAX-2500 spectrometer with Cu KĮ radiation, which was operated at 60 mA and 40 kV. The X-ray photoelectron spectra were recorded on a Perkin Elmer PHI 5300 ESCA system equipped with a dual X-ray source, of which a C anode and hemispherical energy analyzer were used. The background pressure during data acquisition was ˉ maintained below 10 6 Pa. Measurements were performed at a pass energy of 93.90 eV. All binding energies were calibrated using contaminant carbon (C1s 284.6 eV) as a reference. Temperature-programmed desorption of ammonia (NH3-TPD) were conducted in a quartz U-tube reactor with a sample of 100 mg. Prior to adsorption, sample was heated for 1 h at 423 K in a nitrogen flow ˉ (30 ml·min 1). After the sample was cooled to 298 K, ˉ the adsorption of anhydrous ammonia (5 ml·min 1) was carried out for 15 min, followed by a flushing sequence (N2) at the same temperature for 2 h in order to remove the physisorbed ammonia from the surface. The sample was then heated to 873 K at a speed of 20 ˉ ˉ K·min 1 in a nitrogen flow (30 ml·min 1). The ad-

Figure 1 Wide-angle XRD patterns (a) W-HMS samples with different atomic ratio Si/W; (b) WOx/W-HMS catalyst with varying W loading; (c) 8%WOx/ W-HMS catalyst prepared by varying calcination temperatures

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HMS was regulated when W-HMS was prepared in order to control the crystallization of WO3. Figures 1 (b) and 1 (c) show the XRD patterns of WOx supported on W-HMS (Si/W 30). It was obvious that there were peaks ascribed to crystalline WO3 phase. These peaks indicated the presence of crystalline WO3 phase on the WOx/W-HMS catalyst with the W loading of 12% or prepared at calcination temperature of 623 K. Consequently, XRD results indicated the limitations of W loading and calcination temperature for preparing the amorphous WOx/W-HMS catalysts. Nevertheless, it is advantageous that the total amount of amorphous WOx dispersed on the catalyst can be increased by the impregnation of the W-HMS supports. The chemical states and relative quantities of the compounds presented within the outermost surface layer (5 nm) were measured by XPS. Figs. 2 and 3 show the W 4f XPS profiles for the WOx/W-HMS catalyst prepared with different W loadings and treated at different calcination temperatures. From the XPS pattern of W-HMS shown in Fig. 2, no peaks were observed, which indicates the tungsten is mainly located on the deeper layer of the sample. This distribution pattern is similar to that of the Ti species which located in the mesopores of Ti-HMS instead of being randomly distributed in the silica framework as reported by Tuel [18].

Figure 3 W 4f XPS patterns of 8%WOx/W-HMS catalyst prepared at varying calcination temperatures ƻW(V) 4f5/2;ƶW(V) 4f7/2;ƽW(VI) 4f5/2;ƵW(VI) 4f7/2

Sun and co-workers [19], the binding energies of W 4f7/2 were 35.6 eV and 34.5 eV for W (VI) and W (V), respectively, while the difference between the maximal XPS signals of W 4f7/2 and W 4f5/2 was about 2.0 eV. Taking into account the reference data [19, 20], most tungsten atoms on the supported WO3 presented at the charge state of W(VI) while the left presented as W(V). Thus, in all the XPS profiles, a careful deconvolution was performed [2123]. The W (VI) 4f7/2,5/2 and W (V) 4f7/2,5/2 spectra were observed for all of the WOx/W-HMS catalysts as presented in Figs. 2 and 3. By comparing the areas under the peaks with the background subtraction, the superficial W loading and atomic ratios of W(V)/W(VI) was determined as listed in Table 1. From the XPS data, the W loading was less than the mean values as silica and tungsten directly mixed, showing that the W species incorporated by impregnation dispersion was not only on the external-surface but also in the mesopores of W-HMS support. Furthermore, a large number of W species Table1

XPS data of supported tungsten oxides

Sample

Figure 2 W 4f XPS patterns of catalyst with varying W loading amounts ƻ W(V) 4f5/2;ƶW(V) 4f7/2;ƽ W(VI) 4f5/2;ƵW(VI) 4f7/2

All samples, after being impregnated with tungsten oxide on the support W-HMS, the peak of W 4f7/2,5/2 showed up in the XPS spectra. As reported by

W Calcination Binding energy/eV loading/ W(V)/ temperaW(VI) % (by W(VI) W(V) ture/K 4f7/2 4f7/2 mass)

W-HMS

/

/

/

0

/

4%WOx/W-HMS

573

34.3

35.2

3.7

2.24

8% WOx/W-HMS

573

34.4

35.3

7.0

2.31

12%WOx/W-HMS

573

34.1

35.1

9.2

0.29

8% WOx/W-HMS

523

34.3

35.2

6.8

2.35

8% WOx/W-HMS

623

34.2

35.2

7.1

0.32

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Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008

existed on the surface were in (5+) oxidation state with W loading no more than 8% (by mass). However, when the surface W species increased with the increase of the W loading up to 12% (by mass), the surface ratio of W(V)/W(VI) was decreased to less-than one as given in Table 1. This indicated that a number of superficial W(V) was converted into W(VI). This special phenomenon was also noticed when the catalyst precursor prepared at a calcination temperature of 623 K as shown in Fig. 3. Considering XRD studies mentioned above, it could be concluded that the formation of crystalline WO3 led to a change in the valence state of tungsten on the WOx/W-HMS catalyst. A typical N 1s XPS spectrum for the WOx/W-HMS catalyst prepared at different calcination temperatures is shown in Fig. 4. The XPS binding energies at 401.8 eV corresponding to N (II) [24] were only observed for the WOx/W-HMS catalyst prepared at a calcination temperature of 523 K, suggesting the presence of nitrogen oxide on the surface. It was possible that, as a catalyst starting material, the (NH4)2WO4 did not completely decompose during the calcination procedure under such low temperature.

Figure 4 N 1s XPS patterns of 8%WOx/W-HMS catalyst prepared at varying calcination temperatures

Ammonia adsorption-desorption technique is a popular method for determining the strength of acid sites present on the catalyst surface together with total acidity [25]. The shapes of desorption profiles with different W loading percentages can be seen in Fig. 5. The amount of desorbed NH3 and the maximum temperatures Td for TPD peaks are given in Table 2. All samples show symmetric and broadened peaks on the TPD profiles with Td ranging from 478 K to 496 K, revealing that the surface acid strength is uniformly distributed. It is well known that tungsten oxide species dispersed on supports of oxides by impregnation and subsequent oxidation treatments at high temperature show strong acidity [5, 8]. Therefore, for WOx/W-HMS catalyst, the amount of desorbed ammonia increases with the increase of W loading and reaches maximum at the W loading of 8% (by mass). When the W loading was increased up to 12% (by mass), Td shifted to a lower temperature and the acidic amount decreased, suggesting that the surface acid strength and acid sites also decreased due to WO3 congregated crystal on the catalyst surface as shown in XRD results. The W-HMS support shows lower ammonia desorption because of the lack of tungsten oxide on the surface (see also XPS pattern in Fig. 2).

Figure 5 NH3-TPD profiles of WOx/W-HMS catalysts prepared at the calcination temperature of 573 K with varying W loadings (a) 0%; (b) 4%; (c) 8%; (d) 12% Table 2 Acidity and maximum desorption temperature obtained from NH3-TPD for catalysts with varying W loadings NH3 desorbed×106/mol·g

Sample

Td/K

W-HMS

491

437

4%WOx/W-HMS

493

671

8% WOx/W-HMS

496

795

12%WOx/W-HMS

478

763

ˉ1

From NH3-TPD profiles as seen in Fig. 6, only for the catalyst obtained at the calcination temperature of 623 K, ammonia was gradually released starting at 373 K, reaching a shoulder at 425 K and then continued to increase slowly to a maximum as TPD temperature reached 493 K. Compared with the catalyst calcined at 573 K, the amount of desorbed NH3 tended to decrease as shown in Table 3. These results indicated that there were two types of adsorption sites available for NH3 on the catalyst when prepared at the calcination temperature of 623 K. The two-adsorption sites include a weak and a strong acidic site on the catalyst.

Figure 6 TPD profiles of 8%WOx/W-HMS catalysts prepared at varying calcination temperatures (a) 523 K; (b) 573 K; (c) 623 K Table 3 Acidity and peak temperature obtained from NH3-TPD for 8%WOx/W-HMS catalyst calcined at varying temperatures Sample

Calcination temperature/K

Td/K

NH3 desorbed×106/ ˉ mol·g 1

a

523

492

699

b

573

496

795

c

623

425, 493

718

899

Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008

3.2

Catalytic oxidation of cyclopentene

The oxidation of CPE with hydrogen peroxide over W-containing catalyst can proceed through a complex network that is composed of several parallel and consecutive reactions [26]. The distribution of the products for the oxidation of CPE over 8%WOx/W-HMS catalyst is shown in Fig. 7. It was observed that the concentration of CPE reduced to zero while the concentration of GA reached to a plateau which remained constant after a reaction time of 14 h. By comparing with the reaction time necessary to reach a similar yield of GA, it is reasonable to conclude that the activity of our catalyst was higher than that of the catalysts reported previously [1012]. The selectivity to GA displayed in Fig. 8 as a function of the reaction time demonstrates the better catalytic behavior of 8%WOx/ W-HMS catalyst. The selectivity increased with an increase of reaction time and remained above 80% at a high CPE conversion. Our results indicated that the higher amorphous W species in the catalyst was related to the oxidation activity of WOx/W-HMS catalyst.

Figure 7 Distribution of products for the oxidation of CPE over 8%WOx/W-HMS catalyst prepared at the calcination temperature of 573 K ƺ cyclopentene;Ƹ1,2-cyclopentanediol; ƻglutaraldehyde; ͪ 2-butoxyl-1-cyclopentanediol

Figure 8 Selectivity to GA as a function of the reaction time for 8%WOx/W-HMS catalyst prepared at the calcination temperature of 573 K

The influence of catalyst preparation, such as W loading and calcination temperatures, on CPE conversion and GA selectivity, was studied by keeping the reaction time constant at 14 h. From the results presented in Table 4, the W-HMS support without W loading gave a CPE conversion of 74.5% and GA selectivity of 72.8% respectively; this was probably due

Table 4

Catalytic performance of WOx/W-HMS catalyst in a 14 h experiment

Sample

Calcination temperature/K

Conversion of CPE/%

Selectivity of GA/%

W-HMS

/

74.5

72.8

4%WOx/W-HMS

573

96.3

77.6

8% WOx/W-HMS

573

100.0

80.2

12%WOx/W-HMS

573

87.5

71.3

8% WOx/W-HMS

523

84.3

68.3

8% WOx/W-HMS

623

91.6

73.4

to the W species combined with HMS. Both CPE conversion and GA selectivity increased with an increase in W loading and reached to a maximum selectivity of 80.2% with a complete conversion of CPE at W loading of 8% (by mass). However, as W loading amount increased further up to 12% (by mass), CPE conversion and GA selectivity decreased. Considering the XRD and XPS studies aforementioned, it was obvious that partial WO3 crystallization occurred and the valence state of tungsten changed remarkably at the W loading of 12% (by mass), suggesting that the active species consisted of amorphous W species at (5+) oxidation state. It resembled that the WO2.5 reduced from WO3 was more active than WO3 for dehydrogenation of alcohols as reported [5]. Therefore, the formation of WO3 crystal and the oxidation of W (V) to W (VI) could be responsible for the decrease in catalytic activity and GA selectivity over the WOx/W-HMS catalyst [12]. It can be concluded that the amorphous W species at (5+) oxidation state dispersing not only on the external-surface but also in the mesopores of W-HMS support contributes to increasing the catalytic activity for CPE oxidation. For the same reasons, high calcination temperature can inhibit the activity of catalyst WOx/W-HMS. The data in Table 4 indicates that the conversion for the catalyst calcined at 623 K is lower than the one calcined at 573 K. Moreover, low calcination temperature also leads to the decrease of activity because of the incomplete decomposition of the starting material (NH4)2WO4 in the catalyst precursor as shown in Fig. 4. On the other hand, the catalytic activity essentially depends on the acid-base characteristic of the catalyst surface. According to XPS and NH3-TPD studies above, the density and strength of acid sites on WOx/W-HMS catalyst decreased with the increase of W loading and calcination temperature, which results in the crystallization of WO3 and oxidation of the W (V) centers. The stronger acidic sites of the WOx/W-HMS catalyst clearly represent a higher conversion towards the oxidation of CPE. The stronger acidity of the catalysts indicates a higher rate for the oxidation of CPE. The recyclability of the 8%WOx/W-HMS catalyst was evaluated by the oxidation of CPE with hydrogen peroxide. After each 14 h of reaction, the sedimentary catalyst was separated by filtration and reused with fresh reactants. As shown in Table 5, the catalyst retained 94% of the initial selectivity with a conversion

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Chin. J. Chem. Eng., Vol. 16, No. 6, December 2008

Table 5

Catalyst recycle test

7

Run

Conversion of CPE/%

Selectivity of GA/%

1

100.0

80.2

2

99.3

77.2

3

93.6

77.1

4

90.5

75.5

5

85.5

73.7

8 9

10

11

of 85% after the fifth use. The activity reduction of the used catalyst could result from the blockage of active sites on the catalyst by heavy organics [27]. In summary, the 8%WOx/W-HMS catalyst exhibited a high stability of activity for the selective oxidation of CPE to GA.

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4

CONCLUSIONS

The addition of tungsten into HMS as a catalyst support has a beneficial effect on the catalytic activity of WOx/W-HMS catalyst used for the selective oxidation of CPE to GA. Effects of the W loading amount and calcination temperature on the activity of WOx/W-HMS catalyst were studied. Results of the XRD, W 4f XPS and NH3-TPD indicate that the high amorphous W species in their (5+) oxidation state, indicating a strong acidic property on the surface, are considered to contribute to the superior catalytic behavior. However, the crystal form of W species, the oxidation state of tungsten and the acidic properties of WOx/W-HMS catalyst were sensitive to the calcination temperature as well as the W loading on the W-HMS support. Desired amorphous WOx surface density of catalysts can be obtained by regulating the W contents and of the calcination temperature. The 8%WOx/W-HMS catalyst prepared at the calcination temperature of 573 K exhibits higher CPE conversion and GA selectivity than other catalysts prepared under various preparation conditions that have been investigated. REFERENCES 1

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