A study on the preparation of supported metal oxide catalysts using JRC-reference catalysts. I. Preparation of a molybdena–alumina catalyst. Part 4. Preparation parameters and impact index

Applied Catalysis A: General 170 (1998) 359±379

A study on the preparation of supported metal oxide catalysts using JRC-reference catalysts. I. Preparation of a molybdena±alumina catalyst. Part 4. Preparation parameters and impact index Yasuaki Okamotoa,*, Satoshi Umeno1,b, Yasushi Shirakib, Yusaku Arimac, Kazuyuki Nakaid, Osamu Chiyodae, Hisao Yoshidaf, Kei Uchikawag, Kazuhiro Inamurag, Yoshio Akaig, Sadao Hasegawah, Tetsuya Shishidoi, Hideshi Hattorii, Naonobu Katadaj, Koichi Segawak, Naoto Koizumil, Muneyoshi Yamadal, Isao Mochidam, Atsushi Ishiharan, Toshiaki Kaben, Akio Nishijimao, Hideyuki Matsumotop, Miki Niwaj, Toshio Uchijimaq a

Department of Materials Science, Shimane University, Matsue 690, Japan b Idemitsu Petrochemical Co. Ltd., Tokuyama 745, Japan c Catalysts and Chemicals Ind. Co. Ltd., Kitakyushu 808, Japan d Bel Japan Inc., Ebie, Osaka 553, Japan e Cosmo Research Institute, Energy Research Laboratory, Satte 340-01, Japan f Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-01, Japan g Idemitsu Kosan Co. Ltd., Sodegaura 299-02, Japan h Department of Chemistry, Tokyo Gakugei University, Koganei 184, Japan i Center for Advanced Research of Energy Technology, Hokkaido University, Sapporo 060, Japan j Department of Materials Science, Tottori University, Tottori 680, Japan k Department of Chemistry, Sophia University, Tokyo 102, Japan l Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai 980-77, Japan m Institute of Advanced Material Study, Kyushu University, Kasuga 816, Japan n Department of Applied Chemistry, Tokyo University of Agriculture and Technology, Koganei 184, Japan o National Institute of Materials and Chemical Research, Tsukuba 305, Japan p Technology Marketing Department, Business Development Division, JGC Corp., Yokohama 232, Japan q Department of Materials Engineering, Tsukuba University, Tsukuba 305, Japan Received 16 July 1997; received in revised form 10 February 1998; accepted 11 February 1998

Abstract The effects of the volume and pH of the impregnation solution and of the calcination conditions were examined on the physicochemical and catalytic properties of a 13 wt% MoO3/Al2O3 extrudate catalyst. The Al2O3 support and drying procedures (static conditions without ¯owing air) were ®xed in the preparations. In the present series of catalysts, the amount of crystalline MoO3 was marginally small. It was found that the dispersion of Mo oxide species increased as the volume of the impregnation solution increased, gradually approaching a maximum value. The increase in pH (2±8) of the impregnation solution was found to reduce the dispersion of Mo oxide species. The Mo dispersion increased slightly for the impregnation *Corresponding author. Tel.: +81 852 32 6466; fax: +81 852 32 6429; e-mail: [email protected] 1 Present address: Idemitsu Petrochemical Co. Ltd., Shiba, Tokyo 108, Japan. 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00067-2

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catalysts as the calcination temperature increased (673±873 K), whereas it decreased for the equilibrium adsorption catalysts. The effects of the calcination atmosphere (with or without ¯owing air, or with ¯owing humid air) were very small on the dispersion of Mo oxide species under the present preparation conditions. On the other hand, the methanol oxidation activity of MoO3/Al2O3 was sensitive to the preparation parameters examined here. It was demonstrated by means of EPMA and XPS that a considerable migration of Mo took place during the calcination. In the present study on the preparation of a 13 wt% MoO3/Al2O3 catalyst, an impact index is proposed to measure the magnitude of the effects of the respective parameter(s) on the physicochemical and catalytic properties. With the Mo dispersion, the effects of the preparation parameter decreased in the order, surface area of the support >> drying process > volume of the impregnation solution > pH, calcination temperature and atmosphere. The size of the impact index for the dispersion of Mo sul®de species is 70±75% of that for the Mo oxide species. The HDS activity of the catalyst was less affected by the preparation parameters than the Mo sul®de dispersion. The preparation parameters affected the segregation of Mo on the outer surface of extrudates in a decreasing order: drying process > volume of the impregnation solution > pH, calcination conditions. It was found that the oxidation of methanol was affected most intensely by the drying procedures. The volume of the impregnation solution, calcination conditions and pH of the impregnation solution also strongly affected the oxidation activity. The impact index suggests that the sensitivity to the preparation variables of the physicochemical and catalytic properties of MoO3/Al2O3 decreases in the order, methanol oxidation activity > surface Mo segregation > Mo oxide dispersion > Mo sul®de dispersion > HDS activity. # 1998 Elsevier Science B.V. All rights reserved. Keywords: MoO3/Al2O3; Preparation; Dispersion of Mo; Distribution of Mo; Methanol oxidation; HDS; Impact index

1. Introduction The performance of a catalyst is strongly in¯uenced by preparation procedures [1±9]. The dispersion, distribution and chemical states of catalytically active species must be ®nely controlled to provide optimum catalytic properties. However, there are numerous preparation parameters which would modify these properties of the active species. In addition, it is sometimes dif®cult to predict how and to what extent the parameter affects the performance of the catalyst. Since the effects of the preparation variables depend on particular physicochemical properties of the catalyst systems, for instance, pore structure of the support and interaction modes between the precursor salts and support surface, the details of the preparation procedures have to be studied for respective catalysts. Many systematic and tedious experimental works are usually required for a group to reach optimum preparation procedures. It is sometimes encountered that the report of a catalyst performance in the literature is not reproduced despite ``the same'' preparation procedures, conceivably owing to incomplete description of the catalyst preparations. This fact suggests another approach to study the effects of the preparation method on the catalytic properties: a detailed study of the differences in the catalyst performance among the catalyst sys-

tems prepared by several groups using their customary ways. This kind of a group study will provide a clue to ®nd predominant preparation parameters in practical preparations. In the present series of this study, we have adopted the latter approach to the preparation of a MoO3/Al2O3 catalyst possessing 13 wt% MoO3. An analogous strategy has been adopted in the characterization of EUROCAT and has proven very bene®cial [10]. In the ®rst part of the present study [11], it was demonstrated that the surface area of the catalyst or support was a predominant preparation parameter. In the second part of the study [12], it was shown that the volume of the impregnation solution was an important factor in the practical preparation of an extrudate catalyst. Subsequently, in the preceding report [13], it was established that drying processes were very important for the preparation of a 13 wt% MoO3/ Al2O3 extrudate catalyst. It was found that drying of a wet impregnation sample under static conditions without ¯owing air provided a relatively homogeneous distribution of Mo oxide species. The formation of microcrystalline MoO3 was minimum in these impregnation catalysts. In the present study, we ®xed the surface area of the support and drying processes as well as the MoO3 content. The variables examined here were the amount and pH of an impregnation solution and the calcination temperature and atmo-

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

sphere. A series of catalysts were prepared in one laboratory to remove the effects of individuals and different apparatus used in the preparations. Aliquots of the catalysts were distributed to the group members for physicochemical characterizations and catalytic uses. It was found in this study that the parameters examined here were less important as compared with the surface area of the catalyst and drying processes. However, they still modify the properties of the resultant catalysts. Finally, general conclusions of the present series of the group study are summarized. A parameter, impact index, is proposed to measure the size of the effect(s) of the preparation parameters on the catalyst performance. 2. Experimental 2.1. Catalyst preparation An alumina-support used in the present study was supplied by Catalysis Society of Japan as a Reference Catalyst (JRC-ALO-4) [12±14]. The Al2O3 support was used in an extrudate form as received (0.15 cm in diameter). Before use, Al2O3 was kept over a saturated NH4Cl aqueous solution in a desiccator (H2O content, 10.6 wt%). The Mo content was 13 wt% MoO3.

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2.1.1. IRS series The Al2O3 extrudates were dried at 423 K for 24 h using an electric oven before impregnation. Ammonium heptamolybdate (AHM, (NH4)6Mo7O24
4H2O, Wako, analytical grade, 7.33 g) was dissolved in distilled water at an elevated temperature (ca. 320 K). The pH value of the solution, originally 5.1, was adjusted using NH3 or HNO3 aqueous solutions. The AHM solution was added dropwise to 40 g of Al2O3 over 10 s and the mixture was shaked well. A slight exothermic temperature rise was observed. After cooling to room temperature, the sample was dried using a rotary evaporator under static conditions without ¯owing air. The apparatus used has been illustrated in a previous paper [13]. It took about 10 min before the slurry was subjected to drying. The drying temperature was raised to 393 K at a rate of 18 K minÿ1. Since the present drying procedures correspond essentially to IRS in the previous series of 13 wt% MoO3/Al2O3 [13], the present series of catalysts are denoted IRS-n, depending on the preparation variables. Table 1 summarizes the preparation conditions of IRS-n catalysts. 2.1.2. ERS series Another series of 13 wt% MoO3/Al2O3 was prepared by an equilibrium adsorption method [15,16] as described in the preceding studies (Cat-B [11] or

Table 1 Preparation parameters of 13 wt% MoO3/Al2O3 Catalyst

Vimpa (cm3 g-Al2O3ÿ1)

pH

Calcination temperatureb (K)

Calcination atmospherec

IRS-1 IRS-2 IRS-3 IRS-4 IRS-5 IRS-6 IRS-7 IRS-8 IRS-9 ERS-10 ERS-11 ERS-12

1.3 0.65 3.25 1.3 1.3 1.3 1.3 1.3 1.3 50.0 50.0 50.0

5.1 5.1 5.1 4.0 8.1 5.1 5.1 5.1 5.1 2.0 2.0 2.0

773 773 773 773 773 673 873 773 773 773 673 873

A A A A A A A B C A A A

a

Volume of the impregnation solution. Calcination time: 5 h. c A ± in flowing dry air (50 cm3 minÿ1); B ± in flowing humid air (ca. 25 Torr H2O, 50 cm3 minÿ1); C ± without flowing air. b

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Cat-B2 [12]). The Al2O3 support (10 g) was dispersed in 500 cm3 of an aqueous solution of AHM (4.32 g). The temperature of the slurry was held constant at 323 K for 24 h while adjusting the pH to 2.0 using HNO3 and NH3 solutions. After readjusting the pH to 2.0, the slurry was held at this temperature for another 24 h, followed by ®ltration. The ®ltered catalyst precursor was dried as described above. The equilibrium adsorption catalysts are denoted ERS-n, according to the previous nomenclature of the drying procedures (R: rapid temperature rise; S: static conditions). Preparation parameters of ERS-n are also shown in Table 1. Calcination was carried out using an electric furnace. The temperature was raised stepwise in one of the procedures as shown below: calcination at 673 K: 1h

1h

1h

rt ! 523 K …1 h† ! 623 K ! 673 K …5 h† calcination at 773 K: 1h

2h

1h

rt ! 523 K …1 h† ! 723 K …2 h† ! 773 K …5 h† calcination at 873 K: 1h

3h

1h

rt ! 523 K …1 h† ! 823 K ! 873 K…5 h† 2.2. Characterization 2.2.1. Chemical analysis The chemical composition of the catalyst was obtained using X-ray ¯uorescence (XRF) techniques (RIX 3000, Rigaku). The accuracy was usually better than 0.1 wt%. Detailed procedures have been presented previously [11]. 2.2.2. X-ray powder diffraction (XRD) X-ray diffraction patterns of powdered catalysts were measured on a RINT 1200 (Rigaku) using Cu Ka radiation with a Ni ®lter. 2.2.3. Nitrogen adsorption The BET surface area, pore volume and pore size of the catalysts were obtained using N2 adsorption techniques at a liquid-nitrogen temperature on a Belsorp 36 (Bel Japan). The accuracy limits of the surface area and pore volume were better than 3% and 0.01 cm3 gÿ1, respectively. Detailed procedures have been described in a previous paper [11].

2.2.4. X-ray photoelectron spectroscopy (XPS) The XPS data for the catalyst were accumulated on a JPS-9000MC (JEOL) using an Al Ka radiation (1486.6 eV) at a pass energy of 50 eV. The catalyst sample, extrudates as received or a powder sample prepared from the extrudates using a mortar and pestle, was mounted on a holder using a double adhesive tape. The XP spectra were also measured for uncalcined samples. The binding energy was referenced to the Al 2p level at 74.3 eV owing to the support Al2O3. 2.2.5. Laser Raman Spectroscopy (LRS) The LR spectra of the catalyst samples were measured using a 514.5 nm line of Ar‡ laser under ambient conditions on a NR-1800 LR spectrophotometer (JASCO) equipped with a CCD detector. The sample was ground using a mortar and pestle before the measurements. 2.2.6. Electron probe micro-analysis (EPMA) The catalyst sample was ®xed in MMA plastics. The cross section of the calcined catalyst was polished using abrasive paper (no. 400, 1000 and ®nally 1500) and subsequently using felt-paper while dropping ethanol containing a small portion of 0.3 mm Al2O3 powder. With uncalcined samples, ethanol was not used to avoid a possible Mo redistribution during the sample preparations. EPMA pro®les (line and two dimensional analyses) were obtained on a JXA8600 (JEOL) at an electron acceleration of 20 kV and electron current of 110ÿ7 A. The space resolution was set at 1 mm in the present measurements. 2.2.7. Electron spin resonance spectroscopy (ESR) The ESR spectra of the catalyst were measured at room temperature on a JES-ME3X (JEOL) at 9.16 GHz (X-band) after evacuation for 30 min at elevated temperatures. The evacuation temperature was successively raised up to 473, 573, 673 and 773 K. The signal intensity and g value were referenced to a Mn2‡ standard (Mn2‡ in MgO). 2.2.8. Benzaldehyde±ammonia titration (BAT) A BAT technique was applied to estimate the dispersion of Mo oxide species supported on alumina. The amount of benzaldehyde preferentially adsorbed on an Al2O3 surface uncovered by Mo oxide species

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

was titrated by NH3 as benzonitrile. The detailed procedures has been described elsewhere [11,17]. 2.2.9. NO adsorption MoO3/Al2O3 was sul®ded at 673 K for 2 h in a stream of 5% H2S/H2 (30 cm3 minÿ1). After replacing the H2S/H2 stream by a He stream at room temperature, the NO adsorption capacity was determined by a pulse technique. The accuracy was usually 0.01 mol molÿ1 for NO/Mo ratios. Detailed procedures have been presented in a previous paper [11,18].

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at 633 K for 2 h in a 5% H2S/H2 stream. The reaction products were analyzed after 1 h using FID-GC (Yanaco G3800). The apparatus used in the in situ sul®dation has been shown in the previous report [12]. HYD of 1-methylnaphthalene (1 g 1-MN/9 g tetraline) was conducted at 573 K over sul®ded MoO3/ Al2O3 extrudate catalysts. The reaction procedures were the same as those for the HDS except for the reactants.

2.3. Catalytic reaction

3. Results

2.3.1. Oxidation of methanol The oxidation of methanol was carried out at 498 K using a ®xed bed ¯ow reactor ( reaction gas: 3.4% methanol/15.2% O2/81.4% N2) at an atmospheric pressure. Detailed procedures have been presented elsewhere [13,19].

The MoO3 content, BET surface area, pore size and pore volume are summarized in Table 2. The surface areas and pore volumes of ERS catalysts were slightly larger than those of IRS catalysts, even when the amount of supported MoO3 content is taken into consideration. Similar tendencies were observed previously [13]. Fig. 1 depicts the XRD powder pattern of the catalyst sample. In addition to broad diffraction peaks due to a g-Al2O3 phase, very weak peaks ascribable to micro-crystalline MoO3 (2ˆ23.38) were detected only for a few catalysts, IRS-5, IRS-7, IRS-8 and IRS-9. Comparing the MoO3 XRD peak intensity (2ˆ23.38) for a physical mixture of MoO3 and Al2O3 [12] with those for the present catalysts, it is

2.3.2. Hydrodesulfurization (HDS) and hydrogenation (HYD) The catalytic activity of sul®ded MoO3/Al2O3 was studied at 573 K at 2.5 MPa of H2 for the HDS of 4,6dimethyldibenzothiophene (4,6-DMDBT) using an autoclave reactor (50 cm3) ( 0.01 g 4,6-DMDBT/ 10 cm3 of n-decane). The catalyst was used in an extrudate form as received, after in situ presul®dation

Table 2 MoO3 content, surface area, pore volume and pore size of 13 wt% MoO3/Al2O3 Catalyst

MoO3 content (wt%)

Surface area (m2 g-catÿ1)

Pore volumea (cm3 g-catÿ1)

Pore sizea (nm)

Al2O3 IRS-1 IRS-2 IRS-3 IRS-4 IRS-5 IRS-6 IRS-7

Ð 13.3 13.0 12.4 12.6 12.8 12.9 12.7 12.4b 12.8 12.9 11.2 11.1 11.2

174 159 154 161 161 156 157 156

0.755 0.625 0.629 0.662 0.638 0.666 0.669 0.626

12.0 9.4 10.7 9.4 9.4 9.4 9.4 9.4

155 158 166 161 166

0.590 0.639 0.669 0.685 0.689

9.4 9.4 9.4 9.4 9.4

IRS-8 IRS-9 ERS-10 ERS-11 ERS-12 a b

Calculated from an adsorption branch. Before calcination.

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Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

Fig. 1. XRD patterns of 13 wt% MoO3/Al2O3. IRS-7*: uncalcined sample.

estimated that the fraction of crystalline MoO3 in the catalyst is below 1±2% of the Mo oxide species. The LRS techniques were also used to obtain information on the Mo oxide species in MoO3/ Al2O3. Representative spectra of the catalysts are shown in Fig. 2. Broad peaks at ca. 970 and 850 cmÿ1 are ascribed to polymeric Mo oxide species in octahedral con®gurations interacting with the Al2O3 surface [20±22]. In addition to these bands, three weak bands characteristic of crystalline MoO3 are observed at 1008, 829 and 678 cmÿ1 for the catalysts IRS-2, IRS-4, IRS-5, IRS-6 and IRS-9. The band intensities due to MoO3 are signi®cantly small compared with those at 970 cmÿ1 due to the interaction species. IRS-9 shows the most intense MoO3 bands. The XRD and LRS results on the series of 13 wt% MoO3/Al2O3 show that the amount of MoO3 formation is markedly smaller than those in the preceding studies [11±13]. The XPS results for the calcined and uncalcined (after drying) catalysts in extrudate and powder forms are compared in Table 3. The binding energies of the Mo 3d5/2 level was 232.90.3 eV, indicating the formation of Mo6‡ oxide species [20,22,23]. The dispersion of Mo oxide species over the Al2O3 support was evaluated on the basis of the model proposed by

Fig. 2. LR spectra of IRS-8, IRS-9 and IRS-10. The spectrum for MoO3 is shown for comparison.

Kerkhof and Mouljin [24]. The degree of dispersion thus calculated is also summarized in Table 3. With the powder samples, the dispersion of Mo is ca. 85%

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Table 3 Surface analysis of 13 wt% MoO3/Al2O3 by means of XPS Catalyst

(Mo/Al)s

IRS-1 IRS-2 IRS-3 IRS-4 IRS-5 IRS-6 IRS-7 IRS-8 IRS-9 ERS-10 ERS-11 ERS-12

(Mo/Al)s/(Mo/Al)b

Mo 3d/Al 2p XPS intensity ratio a

(Mo/Al)bb

Uncalc.

Calc.

Uncalc.

Calc.

Uncalc.

Calc.

0.84 (94%)c 1.07 (123%) 0.87 (105%) 0.79 (94%) 0.83 (97%) 0.83 (96%) 0.94 (111%) 0.91 (107%) 0.91 (109%) 0.80 (109%) 0.81 (111%) 0.83 (113%)

1.05 (117%) 1.35 (155%) 1.10 (133%) 1.09 (129%) 1.03 (120%) 1.13 (131%) 1.08 (127%) 1.08 (127%) 1.07 (128%) 0.90 (123%) 0.92 (126%) 0.89 (121%)

0.73 (81%) 0.72 (83%) 0.66 (80%) 0.63 (75%) 0.73 (86%) 0.70 (81%) 0.68 (80%) 0.70 (82%) 0.70 (84%) 0.64 (87%) 0.61 (84%) 0.63 (85%)

0.79 (89%) 0.83 (96%) 0.79 (95%) 0.80 (95%) 0.77 (89%) 0.80 (93%) 0.82 (97%) 0.76 (89%) 0.79 (94%) 0.69 (94%) 0.69 (95%) 0.69 (94%)

1.15

1.33

1.49

1.63

1.32

1.39

1.25

1.36

1.14

1.34

1.19

1.41

1.38

1.32

1.30

1.42

1.30

1.35

1.25

1.30

1.33

1.33

1.32

1.29

a

Mo 3d/Al 2p XPS intensity ratios for extrudate samples. Mo 3d/Al 2p XPS intensity ratios for powder samples. c The value in parentheses is the dispersion of Mo in % of monolayer as calculated using an equation proposed by Kerkhof and Mouljin [24]. b

before calcination, while it increases to 90±95% after calcination. Accordingly, the XPS results in Table 3 indicate that the dispersion of Mo in the calcined catalysts is very high irrespective of the preparation parameters examined here. In addition, it is considered that the dispersion of Mo increases on calcination, in agreement with the results by others [23,25] and in the preceding study [12]. With the surface analysis of the extrudates, the dispersion of Mo in Table 3 exceeds 100%, in particular after calcination. This fact apparently indicates a Mo-segregation on the extrudate surface. The ratio of the Mo 3d/Al 2p XPS intensity ratios for the extrudate and powder samples, (Mo/Al)s/(Mo/ Al)b, re¯ects the magnitude of Mo segregation on the outer surface of the extrudate catalyst. The (Mo/Al)s/

(Mo/Al)b ratios are also listed in Table 3. The calcined catalysts show similar values (1.3±1.4) except for IRS2 (1.6). There is a tendency that ESR catalysts show a slightly lower ratios than IRS catalysts. The ratios in Table 3 suggest that a small surface segregation of Mo is observed on the extrudate surface of all the catalysts examined here. However, these values are considerably smaller than those observed previously [12,13], indicating that Mo distributions in the present catalysts are relatively homogeneous and that the preparation parameters varied here modify the Mo distribution to a much lesser extent compared with the variables examined in the previous studies. There are clear tendencies that the (Mo/Al)s/(Mo/ Al)b ratio increases on calcination. However, the ratio decreases (IRS-7 and ERS-12) when the catalyst is

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Fig. 3. EPMA line analysis of uncalcined 13 wt% MoO3/Al2O3 extrudate catalysts.

calcined at 873 K. It is deduced that Mo oxide species move toward the extrudate surface upon calcination at <773 K. On the other hand, calcination at 873 K induces a Mo migration toward the inside of the extrudates or a loss of the Mo oxide species on the outer surface by sublimation during the high temperature calcination. At a calcination temperature higher than 1073 K, extensive migration of Mo oxide species toward the center of the pellets was observed by Duncombe and Weller using EPMA [26]. The distribution of Mo in the extrudate catalyst was examined by means of EPMA. Figs. 3 and 4 illustrate line analyses of the Mo distribution across the extrudate cross sections for uncalcined (after drying) and

calcined catalysts, respectively. The macroscopic Mo distribution in Fig. 3 for the uncalcined sample seems rather ¯at irrespective of the preparation procedures. However, it appears in Fig. 3 that the catalyst precursors, in particular IRS-5, show considerable local ¯uctuations in the Mo content, whereas ERS catalysts have a more homogeneous distribution of Mo in extrudates. It should be noted that the uncalcined samples, IRS-1, IRS-6, IRS-7, IRS-8 and IRS-9, prepared in the same impregnation procedures, show essentially identical EPMA results, indicating a reproducible catalyst preparation and EPMA observations. Comparing the EPMA results in Figs. 3 and 4, it is evident that the Mo distribution is varied by calcina-

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367

Fig. 4. EPMA line analysis of calcined 13 wt% MoO3/Al2O3 extrudate catalysts.

tion. A concave type or egg shell type distribution is developed in most of the catalysts, IRS-1, IRS-2, IRS3, IRS-4, IRS-7 and IRS-9, indicating the migration of Mo species toward the extrudate surface during calcination. On the other hand, IRS-6 shows rather ¯at Mo distribution, possibly owing to a lower calcination temperature (673 K). The local heterogeneity in Mo distribution in Fig. 3 for IRS-5 is reduced on calcination. The ERS series of the catalyst exhibits a rather homogeneous distribution of Mo even after calcination in conformity with the previous EPMA observations [13]. The dispersion of Mo oxide species in MoO3/Al2O3 was estimated on the basis of the average area occu-

pied by one Mo atom (SMo: nm2 Mo-atomÿ1) [17]. The SMo values are summarized in Table 4 for the 13 wt% MoO3/Al2O3 catalysts. These values are very close to 2 the cross section of MoO2ÿ 4 (0.252 nm ) [27], suggesting that Mo oxide species are well dispersed to form almost a monolayer. This is consistent with the XPS results in Table 3. It should be noted that the coincidence of SMo with the cross section of MoO2ÿ 4 [27] does not mean an exclusive formation of Mo oxide species in tetrahedral con®gurations. Scrutinizing Table 4, the SMo values slightly but evidently depend on the preparation variables. Fig. 5 shows a correlation between SMo and the volume of the impregnation solution per g-Al2O3 for the catalysts,

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Table 4 Surface area occupied by Mo of 13 wt% MoO3/Al2O3 as measured by a BAT method and the turnover frequency of formaldehyde formation Catalyst

Surface area of bare Al2O3 (m2 g-catÿ1)

Surface area occupied by Mo (SMo: nm2 Mo-atomÿ1)

TOFa (10ÿ3 sÿ1 )

IRS-1 IRS-2 IRS-3 IRS-4 IRS-5 IRS-6 IRS-7 IRS-8 IRS-9 ERS-10 ERS-11 ERS-12

20.3 22.7 20.5 23.0 22.1 20.5 17.8 29.1 30.4 36.4 31.9 41.0

0.249 0.242 0.271 0.262 0.250 0.253 0.261 0.238 0.245 0.276 0.279 0.267

1.78 1.22 1.82 1.63 1.03 1.02 1.70 2.12 1.45 1.72 0.84 1.50

a

TOF: turnover frequency of formaldehyde formation (mol/mol-Moÿ1 sÿ1) at 498 K.

IRS-1, IRS-2 and IRS-3, with other parameters being ®xed. ERS-10 is included for comparison. Apparently, the SMo value increases as the volume of the impregnation solution increases and, possibly, approaches a maximum value of 0.27±0.28 nm2 Mo-atomÿ1 for 13 wt% MoO3/Al2O3. Figs. 6 and 7 show the dependencies of SMo on the pH of the molybdate solution and the calcination temperature, respectively. The dispersion of Mo decreases slightly as the pH of the solution increases.

With the effects of the calcination temperature, opposite behaviors are observed for the IRS and ERS catalysts, i.e., the SMo value of IRS increases slightly at 873 K, while that of ERS decreases slightly. ERS shows higher SMo than IRS but they approach the same value as the calcination temperature increases. The ESR spectra of Mo5‡ species in axial symmetries were found to increase in intensity almost linearly with increasing evacuation temperature of 13 wt%

Fig. 5. Correlation between SMo and the volume of an impregnation solution for 13 wt% MoO3/Al2O3.

Fig. 6. SMo as a function of pH of the impregnation solution for 13 wt% MoO3/Al2O3.

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

Fig. 7. SMo as a function of calcination temperature for 13 wt% MoO3/Al2O3.

MoO3/Al2O3, accompanying a gradual shift in the g value from 1.946 between room temperature and 473 K to 1.950 at 773 K, as observed previously [13]. The signal intensity depended on the preparations; IRS-8 showed the most intense signal, whereas ERS-11 exhibited the least one. The shift in the g value is considered to indicate the change in the coordination number of Mo5‡ species from six to ®ve [28,29].

369

The oxidation of methanol was conducted at 498 K over the MoO3/Al2O3 catalysts. The reaction products were exclusively formaldehyde (FA) and dimethylether (DME). Fig. 8 illustrates the catalytic results for IRS and ERS catalysts. FA is the oxidation product but DME is formed by acidic sites of Al2O3 [19]. IRS8 shows the highest oxidation activity. No clear relation was observed between SMo and turnover frequency (TOF, Table 4) calculated on the basis of the total amount of Mo. Fig. 9 shows a relation between the rate of FA formation and ESR signal intensity due to Mo5‡ species observed after evacuation at 673 K. Although somewhat scattered, it is likely that the methanol oxidation activity of MoO3/ Al2O3 is correlated to the Mo oxide species easily reducible to the Mo5‡ species. Niwa et al. [30] reported a similar correlation between TOF and Mo5‡ ESR signal intensity. The NO adsorption capacities of sul®ded MoO3/ Al2O3 catalysts are presented in Table 5. Taking into consideration empirical errors, it is likely that the NO/ Mo ratio is approximately in proportion to SMo. Table 6 summarizes the catalytic activities and selectivities of sul®ded MoO3/Al2O3 (extrudates) for the HDS of 4,6-DMDBT and the HYD of 1-MN. The products in the HDS are shown in Fig. 10. The main products in the HDS reactions are B4,6 and H, as shown in Table 6. With the HYD of 1-methylnaphtha-

Fig. 8. Activity of the MoO3/Al2O3 catalyst for the oxidation of methanol. Closed: formaldehyde formation; open: dimethylether formation.

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Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379 Table 5 Amount of NO adsorbed on sulfided 13 wt% MoO3/Al2O3

Fig. 9. Correlation between the rate of formaldehyde formation and Mo5‡ ESR signal intensity obtained after evacuation at 673 K.

lene over sul®ded MoO3/Al2O3 catalysts, approximately the same amounts of 1-methyltetralin (1MT) and 5-methyltetralin (5-MT) are produced. No evident relation can be established between the NO/ Mo ratio and HDS or HYD activity corrected for the MoO3 content to 13 wt% MoO3.

Catalyst

NO uptake (cm3 g-catÿ1)

NO/Mo (mol mol

IRS-1 IRS-2 IRS-3 IRS-4 IRS-5 IRS-6 IRS-7 IRS-8 IRS-9 ERS-10 ERS-11 ERS-12

2.99 3.13 3.01 2.84 2.98 2.98 2.98 3.04 3.02 2.99 3.22 2.68

0.131 0.141 0.142 0.132 0.136 0.135 0.137 0.140 0.141 0.156 0.170 0.140

ÿ1

)

4. Discussion 4.1. Dispersion and distribution of Mo species The surface area and pore volume of ERS catalyst are larger than those of the corresponding IRS catalyst. It is deduced that a longer contact time of Mo anions during the equilibrium adsorption leads to stronger interactions of Mo with the Al2O3 surface even in the

Table 6 Activities of sulfided 13 wt% MoO3/Al2O3 for the HDS reaction of 4,6-dimethyldibenzothiphene at 573 K and for the HYD of 1-methylnaphthalene at 573 K Catalyst

HDS reaction

HYD reaction

a

Product yield (%)

IRS-1 IRS-2 IRS-3 IRS-4 IRS-5 IRS-6 IRS-7 IRS-8 IRS-9 ERS-10 ERS-11 ERS-12 a b

conv. (%)

A4,6

B4,6

C4,6

H

8.4 6.0 8.7 6.2 8.7 9.0 12.6 11.1 9.9 7.5 7.3 11.8

13.7 20.9 13.1 17.0 11.2 12.6 15.2 12.0 13.5 8.6 9.8 9.7

1.8 2.9 1.9 3.5 1.5 1.9 2.0 1.6 2.5 1.7 2.8 4.0

15.5 13.4 16.9 12.1 17.7 16.4 17.7 17.6 16.8 18.5 19.2 18.3

Products are shown in Fig. 10. 1-MT: 1-methyltetralin; 5-MT: 5-methyltetralin.

39.4 43.2 40.6 38.8 39.1 39.9 47.5 42.3 42.7 36.2 39.2 43.8

Selectivityb (%)

conv. (%)

1-MT

5-MT

50 50 51 49 51 50 50 51 52 51 51 52

50 50 49 51 49 50 50 49 48 49 49 48

27.9 32.0 28.0 27.9 30.9 31.2 33.0 27.2 27.8 29.3 23.3 27.5

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

371

Fig. 10. Reaction products in the HDS of 4,6-dimethyldibenzothiophene over sulfided 13 wt% MoO3/Al2O3.

narrowest pores, this preventing the Mo species from agglomeration in these pores. The dispersion of Mo oxide species over calcined MoO3/Al2O3 was estimated by means of the XPS and BAT techniques. In the latter technique, the dispersion of Mo oxide species is evaluated by the (average) area occupied by each Mo species [11]. Both techniques lead to a conclusion that with the present catalyst, the dispersion of Mo oxide species is considerably high and close to a monolayer dispersion irrespective of the preparation variables. This is consistent with the XRD and LRS results which show only a very small amount of crystalline MoO3 formation (<1±2% of Mo oxide species). In addition, the 970 cmÿ1 band in LRS indicates that a majority of the Mo oxide species are polymeric molybdates in octahedral symmetries [20±22] irrespective of the present preparations. Taking account of the surface Mo concentration in the present series of catalysts (3.6 Mo nmÿ2), a nonuniform distribution of Mo over the Al2O3 surface results in a formation of local sites possessing a surface Mo concentration higher than a critical value (3.2±3.8 Mo nmÿ2 [11,12,22]) to produce bulk-like Mo oxide species. Accordingly, the high dispersion of Mo oxide species in 13 wt% MoO3/Al2O3 is considered to result from a homogeneous distribution of Mo within the extrudates [11,12]. In conformity with this, the formation of only a moderate egg shell type distribution of Mo is demonstrated by EPMA (Fig. 4) for the present IRS and ERS catalysts.

The increase in the dispersion of Mo species during calcination are suggested by XPS. One of the driving forces to cause a better Mo dispersion is considered to be surface reactions between the Mo oxide species and Al2O3 surface to form Mo±O±Al chemical bonds [21,22,31,32]. These surface reactions are promoted by extensive Mo migrations induced by the heattreatment, resulting in a monolayer type dispersion of Mo oxide species. It is considered that after drying, Mo anions and salts are partly agglomerated in the Al2O3 pores owing to incomplete surface reactions or large nuclearities of Mo anions adsorbed or deposited on the Al2O3 surface. After drying a wet impregnation sample, the EPMA results in Fig. 3 indicate that the radial distribution of Mo looks considerably uniform for both IRS and ERS. These results suggest that during the impregnation (ca. 10 min in the present study), Mo species are rapidly distributed within the extrudate by capillarity. However, since the EPMA resolution is limited (ca. 1 mm), microscopic information about Mo distribution in the pores cannot be obtained. Rather, it should be noted that the diffusion of Mo species into the pores of primary Al2O3 particles is not completed after drying, since theoretical calculations [33] predict a much longer time for the establishment of adsorption±desorption equilibrium in the pores, which was experimentally con®rmed by Fierro et al. [8] and Goula et al. [9]. A considerable local ¯uctuation of Mo content in IRS catalysts, in particular IRS-5, suggests that a

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considerable portion of Mo anions and/or salts is accumulated between Al2O3 granules in the extrudates. In the case of IRS-5, the high pH of the impregnation solution causes slightly positive or negatively charged Al2O3 surface (vide infra) and prevents a large fraction of Mo anion species from adsorption [31,32]. Accordingly, a poor dispersion of Mo anions is expected after drying. The XPS results indicate that these agglomerated Mo species are dispersed better during calcination. The migration of Mo species toward the outer surface of the extrudates and resultant agglomeration are observed upon calcination of IRS by means of XPS and EPMA. The EPMA shows formations of moderate egg shell type pro®les of Mo in the IRS catalyst systems. In conformity with the EPMA observations, the increased Mo concentration ratio, (Mo/Al)s/(Mo/ Al)b, also indicates the increase in the Mo content in the outer surface of the extrudates for IRS. It is concluded that with the present impregnation catalysts, egg shell type distributions of Mo are mainly established during calcination. On the other hand, ERS shows a relatively uniform Mo distribution even after calcination. In the case of the impregnation catalysts, a sizable part of Mo species are weakly interacting with the Al2O3 surface and, accordingly, considerably mobile. With the catalysts prepared by the equilibrium adsorption method, on the other hand, it is considered that a much longer contact time (48 h) of Mo anions with the Al2O3 surface at a higher temperature (353 K) leads to an establishment of adsorption equilibrium and that Mo anions interact strongly with the positively charged Al2O3 surface (pHˆ2) before the calcination. Consequently, the migration of Mo species is considered to be greatly suppressed in ERS by the strong interactions of Mo species with the Al2O3 surface already established before the calcination. The activity of MoO3/Al2O3 for the oxidation of methanol is not correlated simply to the dispersion of Mo oxide species (SMo), since IRS-8 with the lowest dispersion shows the highest activity, and ERS-11 with a moderate dispersion shows the least activity. These ®ndings are not surprising because the oxidation is suggested to require a particular structure of Mo oxide species as active sites [19], the fraction of which may not be determined by the Mo dispersion alone. The higher activity of IRS-8 suggests that the presence

of water during the calcination is favorable for the formation of the active sites. The lowest activities of IRS-6 and ERS-11, which were calcined at 673 K, suggest that the formation of the active sites are promoted at the calcination temperature higher than 773 K. Fig. 9 indicates that the active species are correlated with the Mo oxide phase which easily produces Mo5‡ species by thermal activations. These Mo oxide species have been proposed to be multilayered Mo oxide clusters in the preceding study [13]. Hall [31,32] proposed the formation of a Mo oxide phase about two layers thick. In conformity with this, IRS-8 shows a moderate dispersion of Mo as measured by the BAT technique, but a negligible formation of MoO3. Discussion on the detailed structure of the active sites is beyond the scope of the present study. The HDS and HYD activities are not correlated simply to the dispersion of Mo sul®de species estimated on the basis of the NO adsorption. In the preceding studies [11,12], the activities have been shown to increase as the dispersion of Mo sul®de species increases. The activity±dispersion correlations might be rendered vague by diffusional effects of the reactants since the catalysts were used in the extrudate form. 4.2. Preparation parameters In the previous study [13] on the drying processes in the preparation of 13 wt% MoO3/Al2O3, it was demonstrated that the IRS preparation of a MoO3/ Al2O3 impregnation catalyst provided the least amount of MoO3 formation and a moderate egg shell type distribution of Mo in extrudates. As mentioned above, the amount of MoO3 formation is much smaller in the present IRS and ERS preparations than those in the preceding catalysts [13] prepared under a variety of drying conditions. The radial distribution of Mo in extrudates is signi®cantly uniform in the IRS series of the catalysts and only a moderate egg shell type distribution is observed. These ®ndings strongly suggest that the preparation parameters studied here affect much more weakly the dispersion and distribution of Mo species than the drying processes. However, the volume of the impregnation solution, pH, calcination temperature and atmosphere in¯uence the properties of the catalyst. In the previous study [12], it was suggested that when the volume of the impregnation solution became

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

smaller, the Mo dispersion and distribution were more intensely affected by other preparation prameters. A high dispersion and uniform distribution of Mo were relatively easily obtained by use of a large amount of the impregnation solution, and an extreme case was the equilibrium adsorption method. Fig. 5 con®rms the above suggestions. The increase in the volume of the impregnation solution leads to an increase in the SMo value or dispersion of Mo oxide species under the same drying conditions. The dispersion of Mo is saturated at 3±4 cm3 g-Al2O3ÿ1. Taking account of the pore volume of the alumina (JRC-ALO-4, 0.76 cm3 gÿ1), a use of more than 4±5 times of the pore volume as an impregnation solution is adequate for the preparation of a 13 wt% MoO3/Al2O3 catalyst possessing a high dispersion and considerably uniform distribution of Mo within the extrudates. A small decrease in SMo was observed with increasing pH of the impregnation solution (Fig. 6). This may be interpreted in terms of the interactions between the Mo anions and Al2O3 surface [31,32]. At a relatively high pH (e.g. pH 8), the Al2O3 surface is positively charged or negatively charged owing to the isoelectric point (8.8 [34]) of Al2O3, close to the pH of the solution. Accordingly, most of the Mo anions (namely species) are dissolved in the impregnation MoO2ÿ 4 solution without adsorption, resulting in a formation of agglomerated Mo species during the drying and vice versa. In conformity with this, the EPMA line analysis in Fig. 3 indicates a strong local ¯uctuation in Mo concentration in IRS-5 before calcination. With a series of MoO3/Al2O3 pellet catalysts prepared at various pH values by an equilibrium adsorption technique, it was reported [8] that at a high pH (10.8), a virtually ¯at pro®le inside the pellet was obtained, while at a low pH (2.8), an egg shell type distribution was observed. It should be noted, however, that in these catalyst systems the Mo content depends on the pH and decreases as the pH increases. In the present impregnation catalysts, the Mo content is ®xed and all the Mo salts are forcibly incorporated in the pores even at a high pH. With 12 wt% MoO3/Al2O3 powder catalysts, Houalla et al. [35] reported signi®cant effects of pH of the impregnation solution on the dispersion of Mo measured by XPS and ISS and on the HDS activity. The catalyst prepared at pH 5 showed the highest Mo dispersion and HDS activity, in contrast to the present

373

results. According to their ISS results, the highest dispersion at pH 5 was established only after calcination at 773 K, since the dispersion of Mo decreased with increasing pH for the sample, after drying at 393 K for 12 h as in the present study. The differences from the results in Fig. 6 may be caused during the calcination. On the other hand, with Al2O3 particles Cordero et al. [36] suggested an increasing dispersion of Mo oxides as measured by XRD and TPR with decreasing pH (11 to 2) of the impregnation solution. The amount of the impregnation solution and the form of the support, powder or extrudate, may affect the local pH in the pores, Mo species in the solution 6ÿ (MoO2ÿ 4 or Mo7 O24 ) and the distribution and, accordingly, the dispersion of Mo oxides in the MoO3/Al2O3 catalysts. More systematic studies will be needed to solve the discrepancies among the results obtained by the different workers. The calcination temperature slightly affects the dispersion of Mo oxide species, but in a different manner between IRS and ERS, as shown in Fig. 7. In both catalyst systems, the BET surface area was not varied by the calcination temperature (673±873 K). The slight increase in SMo for IRS results from a redispersion of agglomerated Mo anion species during calcination at the high temperature, forming stable Mo oxide surface species. The slight SMo decrease at 873 K for ERS is considered to be a consequence of a transformation of excessively dispersed Mo species to the stable Mo oxide species, being thermally equilibrated with each other and with Al2O3 surface sites. The higher SMo for ERS causes the slightly higher surface area of the catalyst in Table 2. In both catalyst systems, the optimum calcination temperature for the methanol oxidation was 773 K. It is conceivable that catalytically active Mo oxide species are not fully produced at 673 K but partially destroyed at 873 K. The effects of the calcination atmosphere appear to be also small upon the dispersion of Mo oxide species under the present conditions. Leyer et al. [37] have shown a redispersion of agglomerated Mo oxide species supported on Al2O3 during a heat-treatment in the presence of H2O. In the present catalyst systems, Mo oxide species calcined in ¯owing dry air are already highly dispersed and, accordingly, the effects of water vapor become unclear. However, the oxidation of methanol is in¯uenced more intensively, and IRS-8

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Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

calcined in ¯owing damp air shows the highest activity. These results indicate that the local structure of Mo oxide species is strongly in¯uenced by the calcination conditions. 5. Conclusions The present study on the preparation of a 13 wt% MoO3/Al2O3 extrudate catalyst leads to the following conclusions: 1. The dispersion of Mo increases as the volume of the impregnation solution increases under the present drying conditions. 2. The increase in pH (2±8) of the impregnation solution slightly reduces the dispersion of Mo oxide species. 3. The calcination temperature (673±973 K) slightly affects the dispersion of Mo oxide species; the Mo dispersion increases as the calcination temperature increases for the impregnation catalysts, whereas it decreases for the equilibrium adsorption catalysts. 4. The effects of the calcination atmosphere are very small on the dispersion of Mo oxide species under the present preparation conditions. 5. The activity of MoO3/Al2O3 for the oxidation of methanol is very sensitive to the preparation parameters examined here. 6. With the impregnation catalysts, a considerable migration of Mo takes place during the calcination, developing egg shell type distributions of Mo inside the extrudate. The characterizations and catalytic reactions in Part 4 were conducted by the following groups: chemical analysis and XRD (Catalysts and Chemicals Ind.), N2 adsorption (Bel Japan), XPS (Idemitsu Kosan), LRS (Nagoya University), EPMA (Cosmo Research Institute), BAT and methanol oxidation (Tottori University), ESR (Tokyo Gakugei University), NO adsorption (Tohoku University), and HDS and HYD reactions (Kyushu University). 6. General discussion and conclusions In the present series of the group study, the preparation parameters of a 13 wt% MoO3/Al2O3 catalyst

have been examined in a practical manner for better and more reproducible catalyst preparations. The same lots of the catalysts have been subjected to multiprong characterizations and catalytic uses. The main conclusions derived in the present study are summarized below: 1. The surface area of the support is the most predominant parameter for the dispersion of Mo oxide and sul®de species. 2. When the amount of the impregnation solution is large, a homogeneous distribution and high dispersion of Mo oxide species are easily attained irrespective of the other preparation parameters. The dispersion of Mo increases as the amount of the impregnation solution increases up to 3±4 times of the pore volume of Al2O3. 3. A rapid drying, in particular, at a reduced pressure induces a strong segregation of Mo oxides on the outer surface of extrudates, forming a sharp egg shell type distribution of Mo. 4. A slow drying rate (static conditions without flowing air) is favorable for a moderate egg shell type distribution of Mo. 5. When the catalyst precursor was dried under static conditions, the increase in pH (2±8) of the impregnation solution slightly reduces the dispersion of Mo oxide species. 6. The effects of the calcination temperature and atmosphere are relatively small on the dispersion of Mo species. 7. The hydrodesulfurization and hydrogenation over sulfide catalysts are greatly affected by Mo dispersion and distribution in extrudates. 8. The Mo oxide species including MoO3 are strongly influenced by the preparations. The oxidation of methanol is suggested to depend strongly on the Mo oxide species formed on the MoO3/Al2O3. The parameters studied here in¯uence the physicochemical and catalytic properties of the resultant MoO3/Al2O3 catalyst to strongly different extents. In order to estimate the magnitude of the effects of the respective preparation parameters, we examined as a function of the preparation parameters the dispersion of Mo oxide species obtained by the BAT technique (SMo), surface Mo segregation estimated by XPS ((Mo/Al)s/(Mo/Al)b), the dispersion of Mo sul®de species (NO/Mo), HDS reaction and the methanol

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

oxidation activity (TOF of formaldehyde formation). These properties are the most fundamental characteristics of the MoO3/Al2O3 catalysts and quantitative in nature. Tables 7±11summarize the whole preparation parameters examined here and the maximum and

375

minimum values in the preparations for SMo, (Mo/ Al)s/(Mo/Al)b, NO/Mo, HDS conversion corrected for the Mo content and TOF of formaldehyde formation, respectively. We de®ne here an impact index of the preparation variable, e.g., for SMo as

Table 7 Impact indexa of the preparation parameter of 13 wt% MoO3/Al2O3 for the dispersion of Mo oxide species as determined by a BAT technique Partb

(SMo)min

(SMo)max

Impact index

Surface area

Drying process

Vimpc

Calc. temp.

Calc. atmos.

pH

1 2 3

0.182 0.219 0.209 0.209 0.238 0.238 0.238 0.249 0.249 0.249 0.267 0.238 0.249

0.417 0.290 0.271 0.271 0.238 0.279 0.262 0.276 0.271 0.261 0.279 0.249 0.262

0.39 0.14 0.13 0.13f 0.0e 0.08 0.05f 0.05 0.04f 0.02f 0.02e 0.02f 0.03f

*d Fixedd Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

* * * * * Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

* * * Fixed Fixed * Fixed * * Fixed Fixed Fixed Fixed

* Fixed Fixed Fixed Fixed * * Fixed Fixed * * Fixed Fixed

* Fixed Fixed Fixed Fixed * * Fixed Fixed Fixed Fixed * Fixed

* * * Fixed Fixed * * * Fixed Fixed Fixed Fixed *

4

a

Impact index is defined here as [(SMo)maxÿ(SMo)min]/[(SMo)max‡(SMo)min]. Part in the present series of study. c Volume of the impregnation solution. d *: varied parameter; Fixed: fixed parameter. e Only equilibrium adsorption catalysts are concerned. f Only impregnation catalysts are included. b

Table 8 Impact indexa of the preparation parameter of 13 wt% MoO3/Al2O3 for the surface segregation of Mo on extrudates as determined by XPS techniques Partb

Rmin

Rmax

Impact index

Surface area

Drying process

Vimpc

Calc. temp.

Calc. atmos.

pH

2 3

1.14 1.35 1.35 1.75 1.29 1.32 1.30 1.33 1.32 1.29 1.33 1.33

3.03 3.63 3.63 1.83 1.63 1.42 1.63 1.63 1.41 1.33 1.42 1.36

0.45 0.46 0.46f 0.02e 0.12 0.04f 0.11 0.10f 0.03f 0.02e 0.03f 0.01f

Fixedd Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

*d * * * Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

* * Fixed Fixed * Fixed * * Fixed Fixed Fixed Fixed

Fixed Fixed Fixed Fixed * * Fixed Fixed * * Fixed Fixed

Fixed Fixed Fixed Fixed * * Fixed Fixed Fixed Fixed * Fixed

* * Fixed Fixed * * * Fixed Fixed Fixed Fixed *

4

a

Impact index is defined here as (RmaxÿRmin)/(Rmax‡Rmin), where Rˆ(Mo/Al)s/(Mo/Al)b. Part in the present series of study. c Volume of the impregnation solution. d *: varied parameter; Fixed: fixed parameter. e Only equilibrium adsorption catalysts are concerned. f Only impregnation catalysts are included. b

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Table 9 Impact indexa of the preparation parameter of 13 wt% MoO3/Al2O3 for the dispersion of Mo sulfide species as determined by an NO adsorption technique Partb

(NO/Mo)min (NO/Mo)max Impact index

Surface area

Drying process

Vimpc

Calc. temp.

Calc. atmos.

pH

1 2 3

0.089 0.119 0.124 0.124 0.132 0.131 0.131 0.131 0.131 0.131 0.140 0.131 0.131

*d Fixedd Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

* * * * * Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

* * * Fixed Fixed * Fixed * * Fixed Fixed Fixed Fixed

* Fixed Fixed Fixed Fixed * * Fixed Fixed * * Fixed Fixed

* Fixed Fixed Fixed Fixed * * Fixed Fixed Fixed Fixed * Fixed

* * * Fixed Fixed * * * Fixed Fixed Fixed Fixed *

4

0.157 0.146 0.152 0.152 0.141 0.170 0.141 0.156 0.142 0.137 0.170 0.140 0.136

0.28 0.10 0.10 0.10f 0.03e 0.13 0.04f 0.09 0.04f 0.02f 0.10e 0.03f 0.02f

a

Impact index is defined here as [(NO/Mo)maxÿ(NO/Mo)min]/[(NO/Mo)max‡(NO/Mo)min]. Part in the present series of study. c Volume of the impregnation solution. d *: varied parameter; Fixed: fixed parameter. e Only equilibrium adsorption catalysts are concerned. f Only impregnation catalysts are included. b

Table 10 Impact indexa of the preparation parameter of 13 wt% MoO3/Al2O3 for HDS reactions Partb 1 2 3 4

a

(Conv.)min 25.5 14 24.5 24.5 27.0 38.5 38.5 38.5 38.5 38.5 42.0 38.5 38.5

(Conv.)max 32.3 26 29.1 29.1 28.1 50.8 43.0 43.0 43.2 48.6 50.8 43.0 40.0

Impact index 0.12 0.30 0.09 0.09f 0.02e 0.14 0.06f 0.06 0.06f 0.12f 0.09e 0.06f 0.02f

Surface area d

* Fixedd Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

Drying process

Vimpc

Calc. temp.

Calc. atmos.

pH

* * * * * Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

* * * Fixed Fixed * Fixed * * Fixed Fixed Fixed Fixed

* Fixed Fixed Fixed Fixed * * Fixed Fixed * * Fixed Fixed

* Fixed Fixed Fixed Fixed * * Fixed Fixed Fixed Fixed * Fixed

* * * Fixed Fixed * * * Fixed Fixed Fixed Fixed *

Impact index is defined as [(conv.)maxÿ(conv.)min]/[(conv.)max‡(conv.)min] where conv. shows the conversion (%) of the HDS reaction corrected for the Mo content (13 wt% MoO3). b Part in the present series of study. Parts 1 and 3: HDS of dibenzothiophene at 643 K over powder or particle catalysts; Parts 2 and 4: HDS of 4,6-dimethyldibenzothiophene at 573 K over extrudate catalysts. c Volume of the impregnation solution. d *: varied parameter; Fixed: fixed parameter. e Only equilibrium adsorption catalysts are concerned. f Only impregnation catalysts are included.

Y. Okamoto et al. / Applied Catalysis A: General 170 (1998) 359±379

377

Table 11 Impact indexa of the preparation parameter of 13 wt% MoO3/Al2O3 for the oxidation of methanol to formaldehyde at 498 K Partb 3 4

TOFmin 0.32 0.32 0.76 0.84 1.02 1.03 1.22 1.02 0.84 1.45 1.03

TOFmax 1.36 1.36 0.83 2.12 2.12 1.82 1.82 1.78 1.72 2.12 1.78

Impact index 0.62 0.62f 0.04e 0.43 0.35f 0.28 0.20f 0.27f 0.34e 0.19f 0.27f

Surface area d

Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

Drying process d

* * * Fixed Fixed Fixed Fixed Fixed Fixed Fixed Fixed

Vimpc

Calc. temp.

Calc. atmos.

pH

* Fixed Fixed * Fixed * * Fixed Fixed Fixed Fixed

Fixed Fixed Fixed * * Fixed Fixed * * Fixed Fixed

Fixed Fixed Fixed * * Fixed Fixed Fixed Fixed * Fixed

* Fixed Fixed * * * Fixed Fixed Fixed Fixed *

a

Impact index is defined here as (TOFmaxÿTOFmin)/(TOFmax‡TOFmin), where TOF is the turnover frequency of formaldehyde formation (10ÿ3 sÿ1). b Part in the present series of study. c Volume of the impregnation solution. d *: varied parameter; Fixed: fixed parameter. e Only equilibrium adsorption catalysts are concerned. f Only impregnation catalysts are included.

Impact index ˆ ‰…SMo †max ÿ …SMo †min Š=‰…SMo †max ‡ …SMo †min Š to evaluate the size of the influence of the parameter(s). The larger the impact index of a parameter, the more intensely the parameter affects the properties of the catalyst. The impact indices thus calculated are also summarized in Tables 7±11, respectively. With the dispersion of Mo oxide species, it is evident from Table 7 that the impact index including the surface area of Al2O3 is 0.39 and signi®cantly larger than those of the other parameters. The second largest impact index is a drying process (0.13). The effect of the volume of the impregnation solution is of third importance (0.04±0.08). The other parameters affect the Mo dispersion to a lesser extent (ca. 0.02). It is concluded that the effects of the preparation parameters of 13 wt% MoO3/Al2O3 on the dispersion of Mo decrease in the order, surface area of Al2O3 >> drying procedures > volume of an impregnation solution > pH, calcination temperature and atmosphere. A surface segregation of Mo oxides outside the extrudate catalyst is evaluated by the Mo concentration ratio, (Mo/Al)s/(Mo/Al)b, measured by XPS for the extrudate and powder samples. With the surface segregation, the impact index of the drying process is

largest (0.45±0.46) among the parameters examined here. Unfortunately, the effect of the surface area of the support was not studied here. The volume of the impregnation solution is of secondary importance (0.10±0.12). The other parameters affect the Mo segregation to a much lesser extent (<0.03). It is concluded that with the impregnation catalysts, the surface segregation of Mo is predominantly affected by the drying process or by a drying rate. The surface segregation of Mo in the equilibrium adsorption catalysts is minimal and scarcely affected by the preparation variables. The NO adsorption capacity of a sul®ded MoO3/ Al2O3 catalyst is considered to show the dispersion of Mo sul®de species. Table 9 shows the impact indices for the preparation parameters. The surface area affects the Mo sul®de dispersion most intensely (0.28). The drying process (0.10) is the second largest parameter with a decreasing order, volume of the impregnation solution > calcination conditions  pH. The magnitude of the impact index for the Mo sul®de dispersion is 70±75% of that for the Mo oxide dispersion, indicating that the dispersion of Mo sul®de species is less sensitive to the preparation variables. It is deduced that oxygen±sulfur exchange reactions and successive reductions of Mo6‡ to Mo4‡ during the

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sul®dation [38,39] induce redistribution of Mo species and diminish the effects of the preparation parameters. Table 10 indicates that the HDS activity is also considerably affected by the preparation parameter but to a less extent than the Mo sul®de dispersion. In the case of extrudate catalysts, the impact index is enlarged because of larger effects of reactant diffusions and Mo segregations on the external surface. Finally, the effects of the preparation variables of MoO3/Al2O3 are examined upon the oxidation activity of methanol. The impact index for the oxidation is listed in Table 11. It is revealed that the drying processes (0.62) strongly in¯uence the activity per Mo for the impregnation catalysts. The effects of the preparation parameters are in the order, drying process > volume of the impregnation solution  calcination temperature  calcination atmosphere  pH. These results are remarkably different from those for the dispersions of Mo oxide and sul®de species and for the surface Mo segregation; the impact indices in Table 11 are signi®cantly larger than those in Tables 7±10 and even the parameters which in¯uence only slightly the Mo dispersion and distribution also strongly affect the oxidation activity of MoO3/Al2O3. These results are a consequence of the requirements of speci®c local structures for the active sites [19]. It is evident that the dispersion and distribution of Mo species are considerably controlled by the preparation variables examined here, whereas the local structure of the Mo oxide species are not controlled suf®ciently. The impact index for MoO3/Al2O3 suggests that the sensitivity to the practical preparation parameters of the physicochemical and catalytic properties of MoO3/ Al2O3 decreases in the order, methanol oxidation activity > surface Mo segregation > Mo oxide dispersion > Mo sul®de dispersion > HDS activity. The catalytic properties of MoO3/Al2O3 are strongly in¯uenced by the chemical state and local structure of Mo oxide species as well as their dispersion and distribution. A next object of the preparation of the catalyst is to control the chemical and local structure of the surface species and to clarify the effects of the preparation parameters on these species. Multiprong characterizations on a molecular level are required as well as macroscopic characterizations employed here. These studies will lead us to the preparation of designed catalysts.

Acknowledgements This work was supported by a Grant-in-Aid for Scienti®c Research on ``Standardization of Catalyst Preparations by Use of JRC-Reference Catalysts'' (07305035) from the Ministry of Education, Science, Sports and Culture. We express our sincere gratitude to the following coworkers for their cooperation: Prof. Tadashi Hattori (Nagoya University) and Messrs. Hatsutaro Yamasaki (Cosmo Research Institute), Ken-ichi Shimizu (Nagoya University), Mikio Takamatsu (Nagoya University), Tsutomu Oonari (Tokyo Gakugei University), Junya Igarashi (Tottori University), Nobuaki Kotakari (Tottori University), Kazuhiro Hoshino (Sophia University), Satoshi Yoshinaka (Sophia University), Tadashi Shimura (Sophia University), Minoru Iijima (Tohoku University), Masato Yamazaki (Tohoku University), Tomoshige Nagamatsu (Kyushu University), Seiichiro Eguchi (Kyushu University), Kinya Sakanishi (Kyushu University), Yozo Korai (Kyushu University), Kei Cho (Tokyo University of Agriculture and Technology) and Toshio Sato (National Institute of Materials and Chemical Research). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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