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MoO3–SiO2 for heptane isomerization
Applied Catalysis A: General 362 (2009) 40–46
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Effects of the amount of MoO3 on the catalytic properties of H2-reduced Pt/MoO3–SiO2 for heptane isomerization Takeshi Matsuda *, Tomoya Ohno, Yuuki Hiramatsu, Zhiou Li, Hirotoshi Sakagami, Nobuo Takahashi Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090 8507, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 January 2009 Received in revised form 8 April 2009 Accepted 8 April 2009 Available online 18 April 2009
The heptane isomerization activity of Pt/MoO3–SiO2 reduced by H2 at 773 K for 6 h was investigated, in particular its relation to the physicochemical properties. H2-reduced Pt/MoO3–SiO2 with a larger amount of MoO3 had a higher isomerization activity. The same tendency was observed in the activity for 2propanol dehydration. The isomerization activity was related to the number of acid sites, which was determined from NH3-TPD. XRD studies of H2-reduced Pt/MoO3–SiO2 showed that the formation of MoOxHy was promoted as the MoO3 loading was increased. The conversion of MoO3 to H1.68MoO3 in the initial stage of reduction varied with the amount of MoO3; only a part of MoO3 was converted to H1.68MoO3 in Pt/MoO3–SiO2 with a small amount of MoO3, while the complete conversion to H1.68MoO3 was observed in Pt/MoO3. Based on these findings, we conclude that the isomerization activity of H2reduced Pt/MoO3–SiO2 can be governed by the formation of MoOxHy, which is yielded from the reduction of MoO3 through H1.68MoO3. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Molybdenum oxide Silica Hydrogen reduction Isomerization Heptane
1. Introduction Hydrocarbon upgrading processes such as reforming, hydrocracking, and isomerization play key roles in petroleum refining. Concern over the negative effects of fuel and oil usage on the environment has caused changes in regulations with severe impacts on gasoline, other jet fuels and lubricating oils. Catalytic reforming is widely carried out to produce high-octane rating gasoline from heavy naphtha, but this process produces fuel with high contents of aromatics that have a high environmental impact. To boost the octane quality of a gasoline fraction, the refinery industry uses some high-octane rating components that are paraffinic in nature. The octane index is improved by increasing the degree of isoalkane branching. For instance, 2,2,3-trimethylbutane has a research octane number (RON) 112, whereas the RON of n-heptane is zero. Since these highly branched isomers have a relatively low environmental impact, the skeletal isomerization of n-alkane can be a key technology for gasoline supply that can cope with future gasoline regulations. The isomerization of C4–C6 alkanes is carried out very successfully using bifunctional catalysts, such as Pt/chlorinated Al2O3, Pt/zeolite, and Pt/SO42–ZrO2. However, no isomerization process exists for hydrocarbons larger than heptane because of their higher tendency to be cracked.
* Corresponding author. Tel.: +81 157 26 9448; fax: +81 157 26 4973. E-mail address: [email protected] (T. Matsuda). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.04.018
The use of transition metal carbides as catalysts has received great interest because their catalytic properties in hydrocarbon reactions are similar to those of noble metals. Since the early works of Boudart and co-workers [1–4], WC and Mo2C-based isomerization catalysts have been studied with the aim of finding more efficient and cheaper substitutes to the Pt catalysts used in the industrial isomerization process. Ledoux and co-workers [5–10] reported that high surface area oxygen-modified Mo2C and carbon-modified MoO3 catalyzed the isomerization of linear hydrocarbons (C6–C8) very selectively. On these catalysts, the active phase was identified as molybdenum oxycarbide, MoOxCy, after detailed characterization by XRD, XPS and HRTEM. A bond shift mechanism via metallocyclic intermediates has been proposed to explain the high selectivity of MoOxCy for heptane isomerization. This mechanism does not involve carbenium ion intermediates, which are easily cracked if the carbon number is 7 and more. In the case of oxygen-modified WC, on the other hand, a bifunctional mechanism with the C–C bond rearrangement steps on acid sites (WOx) and the dehydrogenation–hydrogenation steps on sites with a metallic character (WCx) has been suggested [1–4]. We reported [11–14] that H2 reduction of MoO3 yielded an active and selective catalyst for heptane isomerization, and that H2-reduced MoO3 catalyzed the dehydration and dehydrogenation of 2-propanol simultaneously. The isomerization activity depended on the extent of reduction in a similar way to the dehydration activity, whereas the dehydrogenation activity had a different relationship with the extent of reduction. These findings indicate that the isomerization activity of H2-reduced MoO3 can be
T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46
controlled by the activity as an acid catalyst. Based on some experimental facts, Meunier et al. have suggested that the bifunctional mechanism operated in butane isomerization over H2-reduced MoO3, and that the rate-determining step can be the isomerization of n-butene intermediate to iso-butene [15,16]. Loading of a noble metal onto MoO3 was effective to enhance the heptane isomerization activity [17–20]. This phenomenon can usually be explained by promotion of the dehydrogenation– hydrogenation steps by the noble metal. However, the dehydration activity of H2-reduced MoO3 was also enlarged in the presence of the noble metal. The catalytic activity of H2-reduced MoO3 with a noble metal varied with the ability of the noble metal to promote the formation of hydrogen molybdenum bronze, HxMoO3 [21]. H2reduced Pt/MoO3 had a high isomerization activity only when reduction of Pt/MoO3 yielded the molybdenum oxyhydride phase, MoOxHy, through the formation of HxMoO3 [22]. We have suggested from these results that generation of the isomerization activity can be related to reduction of HxMoO3 to MoOxHy. The Mo metal and MoO2 [23], MoO2 alone [24–26], and MoO [27,28] have also been proposed to act as the active phase for alkane isomerization, since MoO3 became an active and selective catalyst for the isomerization after incomplete reduction with pure H2. The isomerization reaction over these phases has been interpreted in terms of the conventional bifunctional mechanism. The physico-chemical properties of supported metal oxides are considered to differ from those of bulk metal oxides because of different interactions with supports. Generally, the most widely used support is alumina, with high surface area and high mechanical strength. Ledoux and co-workers [29] reported that no molybdenum oxycarbide was formed from MoO3 supported on Al2O3, due to the strong interaction between them, resulting in an inactive catalyst for alkane isomerization. In contrast, MoO3 on a SiC support was transformed to the oxycarbide phase without losing the peculiar alkane isomerization property. Katrib et al. [25,30] reported that TiO2 was a convenient carrier to form the active MoO2 phase from MoO3 by H2 reduction. Studies on the formation of MoOxHy from supported MoO3 are very rare. SiO2 is a chemically inert material and has no strong interaction with an active component. The active component is usually more highly dispersed on SiO2 than on TiO2 and SiC, due to its larger surface area. Hence, this work addresses the catalytic properties of H2reduced Pt/MoO3–SiO2 with different amounts of MoO3 for the conversions of heptane and 2-propanol in association with their physicochemical properties. 2. Experimental 2.1. Materials H2, N2, He and Ar were purified by passage through a molecular sieve and an Mn/SiO2 oxygen trap. Commercially available [Pt(NH3)4]Cl2 was used without further purification. Heptane and 2-propanol were dried using a molecular sieve prior to use. SiO2 used in this study was a CARIACT Q-15 purchased from Fuji Silysia Chemical Ltd., with a surface area of 191 m2/g, a pore volume of 0.99 mL/g, and an average pore diameter of 16 nm. MoO3 supported on SiO2 was prepared by a conventional impregnation method using an aqueous solution of (NH4)6Mo7O264H2O. MoO3 supported on SiO2 is denoted to MoO3(20)–SiO2, MoO3(60)–SiO2, etc. The values in parentheses represent the weight percentage of MoO3. After calcination at 773 K for 3 h, Pt was loaded onto MoO3–SiO2 by impregnation using an aqueous solution of [Pt(NH3)4]Cl2. Pt/MoO3 was prepared by the impregnation method using MoO3 obtained from calcination of H2MoO4 at 673 K for 3 h. The loading of Pt was adjusted to be 0.1 wt% for all samples. The Pt-loaded samples were dried overnight at 393 K, and calcined in air at 773 K for 3 h.
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2.2. Catalytic tests Reaction of heptane was carried out at 523 K under atmospheric pressure in a conventional fixed bed flow reactor equipped with a sampling valve for gas chromatographic analysis. A 0.25 g portion of catalyst was charged into a 10 mm i.d. tubular reactor, and then subjected to H2 reduction at 773 K for 6 h (H2 30 mL/min). After cooling to reaction temperature in a stream of H2, heptane was introduced onto the catalyst bed at a partial pressure of 9.2 kPa with H2 as a complement to atmospheric pressure. Reaction of 2-propanol was performed at 383 K and at a molar He/2-P ratio of 20. The composition of effluent gases was analyzed by FID gas chromatography using a TC-1 glass capillary separation column and using a Porapak Q separation column. 2.3. Characterization methods The surface area was determined from the adsorption isotherm of N2, which was obtained on the sample without exposure to air. The sample was reduced in a H2 flow at 773 K for 6 h, and then was cooled to room temperature, followed by evacuation for 0.5 h at room temperature. The adsorption isotherm of N2 was measured at 77 K with a conventional high-vacuum static system. After the adsorption measurements, each sample was oxidized to MoO3 at 773 K by introducing prescribed amounts of O2. The average valence of Mo was calculated from the amount of O2 consumed in the reoxidation. Crystalline phases of the reduced catalyst were determined by X-ray diffraction using Ni-filtered Cu-Ka radiation (PANalytical, X’Pert PRO). The sample for XRD measurement was obtained as follows: the catalyst was reduced with H2 at 773 K for 6 h, followed by flowing N2 for 0.5 h. After cooling to room temperature under a N2 flow, the reduced catalyst was transferred to a glove box without exposure to air, and was dispersed in a solution of heptane to avoid any bulk oxidation. Temperature-programmed reduction was carried out to investigate the reducibility of Pt/MoO3–SiO2. A 0.4 g portion of catalyst was pretreated at 673 K for 1 h in a stream of O2, and then was cooled to room temperature. O2 was replaced by Ar, followed by heating in a 20% H2–Ar gas mixture (20 mL/min) to 1173 K at a rate of 5 K/min. The effluent gas was injected to TCD gas chromatography with a Porapak N separation column at regular intervals to monitor the concentrations of H2 and H2O. The acidity of Pt/MoO3–SiO2 was determined by a temperatureprogrammed desorption of NH3 technique. A 0.1 g portion of catalyst was reduced at 773 K for 6 h in flowing H2. After cooling to 373 K and evacuation for 0.5 h, the catalyst was saturated with NH3 gas, followed by evacuation for 0.5 h. The temperature of the catalyst bed was raised at a rate of 10 K/min to 883 K in a stream of He (50 mL/min, 20 kPa), and the changes of NH3 concentration in the gas phase were monitored using a integrated mass spectrometer at m/e = 16. 3. Results and discussion Reaction of heptane was conducted at 523 K and at a W (weight of Pt/MoO3–SiO2)/F (flow rate of heptane) ratio of 10 g h/mol under atmospheric pressure. Here, Pt/MoO3–SiO2 catalysts were reduced at 773 K for 6 h. Fig. 1 shows the time course of heptane conversion over the reduced catalysts. The catalytic activity declined gradually with time on stream, irrespective of the amount of MoO3. Pt/ MoO3(20)–SiO2 had a low activity for heptane conversion. The catalytic activity increased in proportion to the amount of MoO3, and bulk Pt/MoO3 had the highest activity for heptane conversion among the catalysts tested.
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T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46 Table 1 Product distributions on H2-reduced Pt/MoO3–SiO2 catalysts in heptane conversion.
Fig. 1. Catalytic activity of H2-reduced Pt/MoO3–SiO2 for the conversion of heptane. MoO3 wt%: (*) 20; (&) 40; (~) 60; (^) 80; (5) 100. Reaction conditions: temperature, 523 K; catalyst weight, 0.25 g; C7 flow rate, 2.5 102 mol/h; H2/C7 molar ratio, 10.
In SiO2-supported catalysts, the role of SiO2 is usually to enhance the dispersion of an active component because of the chemical inertness of SiO2 and the absence of a strong chemical interaction. To evaluate the activity of reduced MoO3 on SiO2 support, we conducted the reaction at a reactant contact time based on the weight of MoO3 being constant. Typical results are shown in Fig. 2. Here, the activity was compared using data after a 1 h run. The conversion level was enlarged by an increase in the amount of MoO3, indicating that the activity of reduced MoO3 on SiO2 depended on the amount of MoO3. In the case of physical mixtures of 0.1 wt%Pt/MoO3 and SiO2, no appreciable differences in the activity were seen among the catalysts tested. It is apparent from these results that H2 reduction did not convert Pt/MoO3–SiO2 with a small amount of MoO3, in other words well-dispersed MoO3
Fig. 2. Effect of the MoO3 loading on the catalytic activity of H2-reduced Pt/MoO3– SiO2 for the conversion of heptane. (*) Pt/MoO3–SiO2; (~) physical mixture of Pt/ MoO3 and SiO2. Reaction conditions: temperature, 523 K; catalyst weight, 0.25 g; MoO3 weight/C7 flow rate, 10 g h/mol; H2/C7 molar ratio, 10.
MoO3 (wt%) Conversion (%)
20 57.3
40 50.0
80 52.0
100 51.5
Selectivity (C%) Isomerization Cracking
88.7 10.7
96.7 3.0
97.5 2.1
97.8 1.9
Distribution (C%) Cracking products C1 C2 C3 C4 C5 C6
23.1 15.6 20.1 18.8 11.6 10.8
18.9 14.1 25 24.4 10.1 7.4
0 3.2 46.1 46.4 2.9 1.4
0 0 48.1 48.8 2.1 1.0
Isomerization products 2-MH 3-MH DMP others
37.2 39.6 18.3 4.9
39.3 41.9 15.6 3.2
38.8 40.4 17.3 3.6
38.7 40.1 17.3 3.8
Reaction conditions: temperature, 523 K; H2/C7 molar ratio, 10. Data after a 1 h run. MH, methylhexane; DMP, dimethylpentane.
on SiO2 support, to an active catalyst for heptane conversion. H2reduced Pt/MoO3(60)–SiO2 and Pt/MoO3(80)–SiO2 had comparable catalytic activities to that of H2-reduced Pt/MoO3. Product distributions in the reaction of heptane are summarized in Table 1, where data after a 1 h run were employed. The conversion levels were adjusted to be about 50% by varying the flow rate of reactant. Under the reaction conditions employed, the cracking and isomerization reactions proceeded simultaneously on all of the catalysts tested. The distribution of cracking products varied with the amount of MoO3. Pt/MoO3(20)–SiO2 yielded C1–C6 hydrocarbons as the cracking products, indicating the hydrocracking of the heptane and/or its isomers being catalyzed. The selectivities for C1, C2, C5, and C6 decreased as the amount of MoO3 was raised, and C3 and C4 hydrocarbons were formed as the major cracking products on Pt/MoO3(80)–SiO2 and Pt/MoO3. These results indicate that the surface property of H2-reduced Pt/MoO3– SiO2 depended on the amount of MoO3. Acid sites seem to be responsible for the cracking reaction on Pt/MoO3(80)–SiO2 and Pt/ MoO3, while sites with a metallic character are likely to catalyze the hydrocracking reaction on the catalyst with a small amount of MoO3. On the other hand, no appreciable differences in the isomerization products were seen among the catalysts tested; heptane was isomerized mainly to 2- and 3-methylhexanes in almost equal amounts. Fig. 3 shows the dependency of the heptane isomerization selectivity on the conversion level. Here, the reactions were carried out at the different W (weight of catalyst)/F (flow rate of reactant) ratios to change the conversion level. Heptane was selectively isomerized on all of the catalysts at the low conversion levels. When the isomerization selectivity was compared at a definite level of conversion, Pt/MoO3–SiO2 with a larger amount of MoO3 exhibited a higher selectivity for heptane isomerization. On Pt/ MoO3(80)–SiO2 and Pt/MoO3 catalysts, the isomerization selectivity decreased with increasing conversion level, but the high selectivity was preserved even at the high conversion levels. A similar dependency was observed on Pt/MoO3(40)–SiO2, although heptane was isomerized with a slightly lower selectivity. In contrast, the isomerization selectivity declined rapidly on Pt/ MoO3(20)–SiO2 as the conversion level was raised. The distribution of the isomerization products varied with the conversion level, irrespective of the amount of MoO3. The selectivity for methylhexanes was lowered and that for dimethylpentanes was enlarged by an increase in the conversion level.
T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46
Fig. 3. Variation in the heptane isomerization selectivity on H2-reduced Pt/MoO3– SiO2 with the conversion level. MoO3 wt%: (*) 20; (&) 40; (^) 80; (5) 100. Reaction conditions; temperature, 523 K; H2/C7 molar ratio, 10.
As described above, H2-reduced Pt/MoO3–SiO2 with a small amount of MoO3, in other words well-dispersed MoO3 on SiO2, was an inferior catalyst for heptane isomerization, and the catalytic performances of H2-reduced Pt/MoO3–SiO2 for heptane isomerization were improved by loading a larger amount of MoO3. Alkane isomerization proceeds on catalysts with acid and hydrogenation– dehydrogenation functions, and the balance between them is crucial because it determines the activity, product selectivity, and stability. Hence, the conversion of 2-propanol was conducted to study the bifunctional property of H2-reduced Pt/MoO3–SiO2. Since catalyst deactivation in the conversion of 2-propanol was similar to that in heptane isomerization, the catalytic activity was compared using data after a 1 h run. Under the reaction conditions employed, 2-propanol was converted to propene and diisopropylether (DIPE) by dehydration, and to acetone by dehydrogenation. Fig. 4 shows the dependencies of the dehydration and dehydrogenation activities of H2-reduced Pt/MoO3–SiO2 on the amount of MoO3. The dehydration and dehydrogenation reactions proceeded simultaneously on all of the catalysts tested, indicating the presence of acid sites and metallic sites. The dehydration activity of Pt/MoO3(20)–SiO2 was comparable to the dehydrogenation activity. The dehydration activity was enlarged by an increase in the amount of MoO3, while the MoO3 loading affected the dehydrogenation activity only a little. As a consequence, Pt/MoO3 was much more active for the dehydration than for the dehydrogenation. The differences in the distribution of cracking products and in the isomerization selectivity shown in Table 1 can be explained by these results. The catalyst with a small amount of MoO3 catalyzed the hydrocracking of the heptane and/or its isomers, resulting in the low isomerization selectivity, due to the metallic character. The surface became more acidic, and then the balance between the acid and metal functions was improved to be suitable for the isomerization as a larger amount of MoO3 was loaded. The dehydration activity was affected by the MoO3 loading in a similar way to the isomerization activity, whereas the dehydrogenation activity had a different relationship with the amount of MoO3. The heptane isomerization activity shown in Fig. 1 is plotted as a function of the 2-propanol dehydration activity. Here, the isomerization and the dehydration activities were estimated from
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the formation rate of heptane isomers and that of propene, respectively. As shown in Fig. 5, a good relationship was observed between them. This result allows us to suggest that the isomerization activity of H2-reduced Pt/MoO3–SiO2 was dependent on its activity as an acid catalyst. Temperature-programmed desorption of NH3 (NH3-TPD) was carried out to study the acidity of the catalysts. Fig. 6 illustrates the NH3-TPD spectra of Pt/MoO3–SiO2 reduced at 773 K for 6 h. No desorption of NH3 was seen from the catalysts without H2 reduction. NH3 was desorbed in the temperature range of 373– 673 K from all of the reduced catalysts, and the desorbed amount of NH3 increased in proportion to the amount of MoO3. These results indicate that the generation of acid sites was associated with H2 reduction of MoO3 supported on SiO2. The isomerization activity shown in Fig. 1 is plotted as a function of the concentration of acid sites determined from the NH3-TPD spectra. As shown in Fig. 7, a good relationship was observed between them. The catalyst with acid sites of <0.07 mmol/g was almost inactive for heptane isomerization. Above the concentration of 0.07 mmol/g, the isomerization activity was linearly raised by an increase in the concentration of acid sites. We suggest from these results that the isomerization activity of H2-reduced Pt/MoO3–SiO2 is controlled by the acidity. We showed in previous papers [18,19] that H2 reduction of Pt/ MoO3 was accompanied by an increase in surface area, and that the isomerization and dehydration activities depended on the extent of reduction. Hence, the surface area and the average Mo valence of Pt/MoO3–SiO2 reduced at 773 K for 6 h were measured. The average Mo valence was determined from the amount of O2 consumed in reoxidation to MoO3. Results are summarized in Table 2. Before H2 reduction, loading of MoO3 onto SiO2 lowered surface area, due to the small surface area of MoO3 (<10 m2/g). The surface area of Pt/MoO3–SiO2 was enlarged by H2 reduction, depending on the amount of MoO3. H2 reduction enlarged the surface area of Pt/MoO3(20)–SiO2 slightly, and the surface area of Pt/MoO3(80)–SiO2 increased from 44 to 216 m2/g after H2 reduction. The most significant increase was seen in bulk Pt/ MoO3. As a result, all of the reduced catalysts had comparable surface areas. These results indicate that the surface area of MoO3 was enlarged by H2 reduction even in the case of supported MoO3, although this effect became more pronounced on the catalyst with a larger amount of MoO3. No appreciable difference in the Mo valence was seen among the catalysts tested. Thus, the surface area and the Mo valence cannot explain the dependency of the isomerization activity on the amount of MoO3. Fig. 8 illustrates the XRD patterns of Pt/MoO3–SiO2 reduced at 773 K for 6 h. The amount of MoO3 gave no effect on the XRD pattern of Pt/MoO3–SiO2 before H2 reduction; only diffraction lines corresponding to the MoO3 phase were seen. The XRD pattern of H2-reduced Pt/MoO3–SiO2 varied with the amount of MoO3, although all of the catalysts had almost the same average Mo valence, as shown in Table 2. H2-reduced Pt/MoO3(20)–SiO2 provided a diffraction line at 2u = 40.58, which is assigned to the d(0 0 1) diffraction of Mo metal. In the case of H2-reduced Pt/ MoO3(40)–SiO2, diffraction lines at 2u = 38.18 and 44.38 as well as a line at 2u = 40.58 were observed. The intensities of the lines at these angles were strengthened and that due to Mo metal was weakened by an increase in the amount of MoO3. Ledoux and co-workers [7] assigned the lines at 2u = 38.18 and 44.38 to molybdenum oxyhydride, MoOxHy, which is analogous to molybdenum oxycarbide, MoOxCy. In contrast, Wehrer et al. [27,28] pointed out that the presence of hydrogen in this compound was hard to accept because thermal treatment did not change the XRD pattern, and they proposed the existence of a reduced molybdenum oxide with the composition near to MoO. Wang et al. [31] assigned these lines to MoO with a face-centered
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T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46
Fig. 4. Effect of the MoO3 loading on the catalytic activity of H2-reduced Pt/MoO3– SiO2 for the conversion of 2-propanol. propene (*); DIPE (&); acetone (~). Reaction conditions; temperature, 383 K; catalyst weight, 0.25 g; 2-P flow rate, 2.5 102 mol/h; He/2-P molar ratio, 20.
Fig. 6. NH3-TPD spectra of Pt/MoO3–SiO2 reduced at 773 K for 6 h. MoO3 wt%: ( ) 20; (- - -) 60; (—) 100.
Fig. 5. Relationship between the dehydration activity and the isomerization activity. Data taken after a 1 h run.
Fig. 7. Effect of the concentration of acid sites on the rate of heptane isomerization over Pt/MoO3–SiO2 reduced at 773 K for 6 h.
cubic lattice. We reported previously [32] that H2O was evolved during thermal treatment of H2-reduced MoO3, of which the XRD pattern had diffraction lines at 2u = 38.18 and 44.38, indicating the presence of hydrogen in the reduction product. We have considered from this result that diffraction lines at 2u = 38.18 and 44.38 correspond to the MoOxHy phase, although the exact nature of this compound is still under discussion. The XRD patterns shown in Fig. 8 indicate that the formation of Mo metal was suppressed and that of MoOxHy was promoted as the amount of MoO3 increased. These XRD results can be helpful to account for the differences in product distributions shown in Table 1; the distribution of cracking products was shifted from the mixture of
C1–C6 hydrocarbons to C3 and C4 hydrocarbons as the content of Mo metal in the catalysts was decreased. The reduction process of Pt/MoO3–SiO2 was studied by means of a temperature-programmed reduction (TPR) technique to elucidate why the reduction product varied with the amount of MoO3. Typical results are demonstrated in Fig. 9. Pt/MoO3–SiO2 reacted with H2 without generating an equivalent amount of H2O in the temperature region below 473 K, indicating the formation of HxMoO3. This phenomenon can be understood by hydrogen spillover. A large amount of H2O was evolved compared with the amount of H2 consumed in the range of 400–550 K. This implies that the decomposition of HxMoO3 proceeded in this temperature
T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46 Table 2 Physical properties of H2-reduced Pt/MoO3–SiO2. MoO3 (wt%)
0 20 40 60 80 100
Before reduction
After reduction
Surface area (m2 g1)
Average valence
Surface area (m2 g1)
172 152 112 79 44 9
– 1.9 2.0 2.1 2.0 2.1
– 186 175 191 216 243
H2 reduction: 773 K, 6 h.
range. The amount of H2 consumed was consistent with that of H2O evolved at higher temperatures. These results show that reduction of MoO3 supported on SiO2 proceeded through the formation of HxMoO3, and its decomposition, irrespective of the amount of MoO3. However, the amount of H2 consumed in the low temperature region (< 473 K) varied with the amount of MoO3; the amounts of H2 reacted with Pt/MoO3(20)–SiO2, Pt/MoO3(40)– SiO2, and Pt/MoO3(80)–SiO2 were calculated from the TPR spectra as 7.8 104, 2.8 103, and 4.8 103 mol/g-MoO3, respectively. Fig. 10 shows the XRD patterns of Pt/MoO3–SiO2 heated to 473 K under the TPR conditions. Pt/MoO3(80)–SiO2 gave no diffraction peaks due to the MoO3 phase, and the lines corresponding to the H1.68MoO3 phase were seen. H1.68MoO3 was also formed, but the formation of HxMoO3 with the lower hydrogen content such as H0.34MoO3 and H0.93MoO3 was not observed in Pt/MoO3(20)–SiO2 and Pt/MoO3(40)–SiO2. These results imply that MoO3 was converted to H1.68MoO3 in the initial stage of reduction, irrespective of the amount of MoO3. However, Pt/MoO3(20)–SiO2 and Pt/ MoO3(40)–SiO2 contained the MoO3 phase after heating to 473 K, indicating the incomplete conversion to H1.68MoO3. To study the formation of H1.68MoO3 quantitatively, we calculated the degree of MoO3 conversion to H1.68MoO3 from the amount of H2 consumed below 473 K in the TPR measurement. As shown in Fig. 11, the
Fig. 8. XRD patterns of Pt/MoO3–SiO2 reduced at 773 K for 6 h. MoO3 wt%: (A) 20; (B) 40; (C) 60; (D) 80. MoOxHy (*) Mo metal (&).
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conversion of MoO3 to H1.68MoO3 in Pt/MoO3(20)–SiO2 was about 10%. The degree of MoO3 conversion was enlarged by an increase in the amount of MoO3, and MoO3 was completely converted to H1.68MoO3 in Pt/MoO3. Insertion of hydrogen atoms into the lattice of MoO3 leads to the formation of HxMoO3. This reaction seems to be inhibited in well-dispersed MoO3, probably due to its low crystallinity. The TPR profile at temperatures above 600 K varied with the amount of MoO3. In TPR of Pt/MoO3(20)–SiO2, two peaks were seen, at 790 and 1050 K, and Mo valence after TPR was calculated as zero from the TPR spectrum. Pt/MoO3(40)–SiO2 also gave two peaks and the reduction was completed in the TPR, but the peak at 790 K was weakened and the other peak appeared at higher temperature. In contrast, Pt/MoO3(80)–SiO2 gave very broad peaks, and the Mo valence after TPR was calculated as 1.5 from the TPR spectrum. We deduce from these results that the reduction process was changed by the amount of MoO3. We reported that reduction of Pt/MoO3 yielded no MoO2 when the reduction proceeded through the formation of H1.68MoO3, and vice versa [22]. Thus, it is possible to consider that two types of reduction processes appear in Pt/MoO3–SiO2. Only a part of MoO3 was converted to H1.68MoO3
Fig. 9. TPR spectra of Pt/MoO3–SiO2. MoO3 wt%: (A) 20; (B) 40; (C) 80. H2 consumed (*) H2O evolved (&). Conditions: sample weight, 0.4 g; gas, 20%H2–Ar; flow rate, 20 mL/min; heating rate, 5 K/min.
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[21]. Pt/MoO3(60)–SiO2 and Pt/MoO3(80)–SiO2 had almost comparable activity value to that of Pt/MoO3 when the catalytic activity was compared based on the weight of MoO3, although these catalysts had lower conversions of MoO3 to H1.68MoO3. This result can result from the support effect that allows the active phase to contribute to the catalytic reaction efficiently.
4. Conclusion The catalytic activity of H2-reduced Pt/MoO3–SiO2 for heptane isomerization was investigated. H2 reduction did not convert Pt/ MoO3–SiO2 with a small amount of MoO3, in other words welldispersed MoO3 on SiO2, to an active and selective catalyst for heptane isomerization. The isomerization activity was enlarged by an increase in the amount of MoO3, and was related to the concentration of acid sites. The formation of MoOxHy was promoted and that of Mo metal was suppressed as the amount of MoO3 increased. A good relationship was observed between the conversion of MoO3 to H1.68MoO3 in the initial stage of reduction and the isomerization activity. We conclude from these results that the isomerization activity of H2-reduced Pt/MoO3–SiO2 is governed by the formation of H1.68MoO3, the reduction of which yields the MoOxHy phase with acidity. Fig. 10. XRD patterns of Pt/MoO3–SiO2 heated to 473 K under the TPR conditions. MoO3 wt%: (A) 20; (B) 40; (C) 80. H1.68MoO3 (*) MoO3 (!).
Fig. 11. Effect of the MoO3 loading on the formation of H1.68MoO3 over Pt/MoO3– SiO2.
on Pt/MoO3(20)–SiO2 under the TPR condition. This implies that reduction of MoO3 to MoO2 can proceed on this catalyst, although no MoO2 was detected by XRD measurements. In contrast, reduction of Pt/Mo(80)–SiO2 is likely to proceed without the formation of MoO2, due to the high conversion to H1.68MoO3. As shown in Figs. 3 and 11, the dependency of the heptane isomerization activity on the amount of MoO3 is very similar to that of the MoO3 conversion to H1.68MoO3, suggesting that the formation of H1.68MoO3 is the key step to generate the isomerization activity. This result is compatible with the reported one; the isomerization activity of H2-reduced MoO3 with a noble metal depended on the ability of the noble metal to promote the formation of H1.68MoO3
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Effects of the amount of MoO3 on the catalytic properties of H2-reduced Pt/MoO3–SiO2 for heptane isomerization Takeshi Matsuda *, Tomoya Ohno, Yuuki Hiramatsu, Zhiou Li, Hirotoshi Sakagami, Nobuo Takahashi Department of Materials Science, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090 8507, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 January 2009 Received in revised form 8 April 2009 Accepted 8 April 2009 Available online 18 April 2009
The heptane isomerization activity of Pt/MoO3–SiO2 reduced by H2 at 773 K for 6 h was investigated, in particular its relation to the physicochemical properties. H2-reduced Pt/MoO3–SiO2 with a larger amount of MoO3 had a higher isomerization activity. The same tendency was observed in the activity for 2propanol dehydration. The isomerization activity was related to the number of acid sites, which was determined from NH3-TPD. XRD studies of H2-reduced Pt/MoO3–SiO2 showed that the formation of MoOxHy was promoted as the MoO3 loading was increased. The conversion of MoO3 to H1.68MoO3 in the initial stage of reduction varied with the amount of MoO3; only a part of MoO3 was converted to H1.68MoO3 in Pt/MoO3–SiO2 with a small amount of MoO3, while the complete conversion to H1.68MoO3 was observed in Pt/MoO3. Based on these findings, we conclude that the isomerization activity of H2reduced Pt/MoO3–SiO2 can be governed by the formation of MoOxHy, which is yielded from the reduction of MoO3 through H1.68MoO3. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Molybdenum oxide Silica Hydrogen reduction Isomerization Heptane
1. Introduction Hydrocarbon upgrading processes such as reforming, hydrocracking, and isomerization play key roles in petroleum refining. Concern over the negative effects of fuel and oil usage on the environment has caused changes in regulations with severe impacts on gasoline, other jet fuels and lubricating oils. Catalytic reforming is widely carried out to produce high-octane rating gasoline from heavy naphtha, but this process produces fuel with high contents of aromatics that have a high environmental impact. To boost the octane quality of a gasoline fraction, the refinery industry uses some high-octane rating components that are paraffinic in nature. The octane index is improved by increasing the degree of isoalkane branching. For instance, 2,2,3-trimethylbutane has a research octane number (RON) 112, whereas the RON of n-heptane is zero. Since these highly branched isomers have a relatively low environmental impact, the skeletal isomerization of n-alkane can be a key technology for gasoline supply that can cope with future gasoline regulations. The isomerization of C4–C6 alkanes is carried out very successfully using bifunctional catalysts, such as Pt/chlorinated Al2O3, Pt/zeolite, and Pt/SO42–ZrO2. However, no isomerization process exists for hydrocarbons larger than heptane because of their higher tendency to be cracked.
* Corresponding author. Tel.: +81 157 26 9448; fax: +81 157 26 4973. E-mail address: [email protected] (T. Matsuda). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.04.018
The use of transition metal carbides as catalysts has received great interest because their catalytic properties in hydrocarbon reactions are similar to those of noble metals. Since the early works of Boudart and co-workers [1–4], WC and Mo2C-based isomerization catalysts have been studied with the aim of finding more efficient and cheaper substitutes to the Pt catalysts used in the industrial isomerization process. Ledoux and co-workers [5–10] reported that high surface area oxygen-modified Mo2C and carbon-modified MoO3 catalyzed the isomerization of linear hydrocarbons (C6–C8) very selectively. On these catalysts, the active phase was identified as molybdenum oxycarbide, MoOxCy, after detailed characterization by XRD, XPS and HRTEM. A bond shift mechanism via metallocyclic intermediates has been proposed to explain the high selectivity of MoOxCy for heptane isomerization. This mechanism does not involve carbenium ion intermediates, which are easily cracked if the carbon number is 7 and more. In the case of oxygen-modified WC, on the other hand, a bifunctional mechanism with the C–C bond rearrangement steps on acid sites (WOx) and the dehydrogenation–hydrogenation steps on sites with a metallic character (WCx) has been suggested [1–4]. We reported [11–14] that H2 reduction of MoO3 yielded an active and selective catalyst for heptane isomerization, and that H2-reduced MoO3 catalyzed the dehydration and dehydrogenation of 2-propanol simultaneously. The isomerization activity depended on the extent of reduction in a similar way to the dehydration activity, whereas the dehydrogenation activity had a different relationship with the extent of reduction. These findings indicate that the isomerization activity of H2-reduced MoO3 can be
T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46
controlled by the activity as an acid catalyst. Based on some experimental facts, Meunier et al. have suggested that the bifunctional mechanism operated in butane isomerization over H2-reduced MoO3, and that the rate-determining step can be the isomerization of n-butene intermediate to iso-butene [15,16]. Loading of a noble metal onto MoO3 was effective to enhance the heptane isomerization activity [17–20]. This phenomenon can usually be explained by promotion of the dehydrogenation– hydrogenation steps by the noble metal. However, the dehydration activity of H2-reduced MoO3 was also enlarged in the presence of the noble metal. The catalytic activity of H2-reduced MoO3 with a noble metal varied with the ability of the noble metal to promote the formation of hydrogen molybdenum bronze, HxMoO3 [21]. H2reduced Pt/MoO3 had a high isomerization activity only when reduction of Pt/MoO3 yielded the molybdenum oxyhydride phase, MoOxHy, through the formation of HxMoO3 [22]. We have suggested from these results that generation of the isomerization activity can be related to reduction of HxMoO3 to MoOxHy. The Mo metal and MoO2 [23], MoO2 alone [24–26], and MoO [27,28] have also been proposed to act as the active phase for alkane isomerization, since MoO3 became an active and selective catalyst for the isomerization after incomplete reduction with pure H2. The isomerization reaction over these phases has been interpreted in terms of the conventional bifunctional mechanism. The physico-chemical properties of supported metal oxides are considered to differ from those of bulk metal oxides because of different interactions with supports. Generally, the most widely used support is alumina, with high surface area and high mechanical strength. Ledoux and co-workers [29] reported that no molybdenum oxycarbide was formed from MoO3 supported on Al2O3, due to the strong interaction between them, resulting in an inactive catalyst for alkane isomerization. In contrast, MoO3 on a SiC support was transformed to the oxycarbide phase without losing the peculiar alkane isomerization property. Katrib et al. [25,30] reported that TiO2 was a convenient carrier to form the active MoO2 phase from MoO3 by H2 reduction. Studies on the formation of MoOxHy from supported MoO3 are very rare. SiO2 is a chemically inert material and has no strong interaction with an active component. The active component is usually more highly dispersed on SiO2 than on TiO2 and SiC, due to its larger surface area. Hence, this work addresses the catalytic properties of H2reduced Pt/MoO3–SiO2 with different amounts of MoO3 for the conversions of heptane and 2-propanol in association with their physicochemical properties. 2. Experimental 2.1. Materials H2, N2, He and Ar were purified by passage through a molecular sieve and an Mn/SiO2 oxygen trap. Commercially available [Pt(NH3)4]Cl2 was used without further purification. Heptane and 2-propanol were dried using a molecular sieve prior to use. SiO2 used in this study was a CARIACT Q-15 purchased from Fuji Silysia Chemical Ltd., with a surface area of 191 m2/g, a pore volume of 0.99 mL/g, and an average pore diameter of 16 nm. MoO3 supported on SiO2 was prepared by a conventional impregnation method using an aqueous solution of (NH4)6Mo7O264H2O. MoO3 supported on SiO2 is denoted to MoO3(20)–SiO2, MoO3(60)–SiO2, etc. The values in parentheses represent the weight percentage of MoO3. After calcination at 773 K for 3 h, Pt was loaded onto MoO3–SiO2 by impregnation using an aqueous solution of [Pt(NH3)4]Cl2. Pt/MoO3 was prepared by the impregnation method using MoO3 obtained from calcination of H2MoO4 at 673 K for 3 h. The loading of Pt was adjusted to be 0.1 wt% for all samples. The Pt-loaded samples were dried overnight at 393 K, and calcined in air at 773 K for 3 h.
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2.2. Catalytic tests Reaction of heptane was carried out at 523 K under atmospheric pressure in a conventional fixed bed flow reactor equipped with a sampling valve for gas chromatographic analysis. A 0.25 g portion of catalyst was charged into a 10 mm i.d. tubular reactor, and then subjected to H2 reduction at 773 K for 6 h (H2 30 mL/min). After cooling to reaction temperature in a stream of H2, heptane was introduced onto the catalyst bed at a partial pressure of 9.2 kPa with H2 as a complement to atmospheric pressure. Reaction of 2-propanol was performed at 383 K and at a molar He/2-P ratio of 20. The composition of effluent gases was analyzed by FID gas chromatography using a TC-1 glass capillary separation column and using a Porapak Q separation column. 2.3. Characterization methods The surface area was determined from the adsorption isotherm of N2, which was obtained on the sample without exposure to air. The sample was reduced in a H2 flow at 773 K for 6 h, and then was cooled to room temperature, followed by evacuation for 0.5 h at room temperature. The adsorption isotherm of N2 was measured at 77 K with a conventional high-vacuum static system. After the adsorption measurements, each sample was oxidized to MoO3 at 773 K by introducing prescribed amounts of O2. The average valence of Mo was calculated from the amount of O2 consumed in the reoxidation. Crystalline phases of the reduced catalyst were determined by X-ray diffraction using Ni-filtered Cu-Ka radiation (PANalytical, X’Pert PRO). The sample for XRD measurement was obtained as follows: the catalyst was reduced with H2 at 773 K for 6 h, followed by flowing N2 for 0.5 h. After cooling to room temperature under a N2 flow, the reduced catalyst was transferred to a glove box without exposure to air, and was dispersed in a solution of heptane to avoid any bulk oxidation. Temperature-programmed reduction was carried out to investigate the reducibility of Pt/MoO3–SiO2. A 0.4 g portion of catalyst was pretreated at 673 K for 1 h in a stream of O2, and then was cooled to room temperature. O2 was replaced by Ar, followed by heating in a 20% H2–Ar gas mixture (20 mL/min) to 1173 K at a rate of 5 K/min. The effluent gas was injected to TCD gas chromatography with a Porapak N separation column at regular intervals to monitor the concentrations of H2 and H2O. The acidity of Pt/MoO3–SiO2 was determined by a temperatureprogrammed desorption of NH3 technique. A 0.1 g portion of catalyst was reduced at 773 K for 6 h in flowing H2. After cooling to 373 K and evacuation for 0.5 h, the catalyst was saturated with NH3 gas, followed by evacuation for 0.5 h. The temperature of the catalyst bed was raised at a rate of 10 K/min to 883 K in a stream of He (50 mL/min, 20 kPa), and the changes of NH3 concentration in the gas phase were monitored using a integrated mass spectrometer at m/e = 16. 3. Results and discussion Reaction of heptane was conducted at 523 K and at a W (weight of Pt/MoO3–SiO2)/F (flow rate of heptane) ratio of 10 g h/mol under atmospheric pressure. Here, Pt/MoO3–SiO2 catalysts were reduced at 773 K for 6 h. Fig. 1 shows the time course of heptane conversion over the reduced catalysts. The catalytic activity declined gradually with time on stream, irrespective of the amount of MoO3. Pt/ MoO3(20)–SiO2 had a low activity for heptane conversion. The catalytic activity increased in proportion to the amount of MoO3, and bulk Pt/MoO3 had the highest activity for heptane conversion among the catalysts tested.
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T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46 Table 1 Product distributions on H2-reduced Pt/MoO3–SiO2 catalysts in heptane conversion.
Fig. 1. Catalytic activity of H2-reduced Pt/MoO3–SiO2 for the conversion of heptane. MoO3 wt%: (*) 20; (&) 40; (~) 60; (^) 80; (5) 100. Reaction conditions: temperature, 523 K; catalyst weight, 0.25 g; C7 flow rate, 2.5 102 mol/h; H2/C7 molar ratio, 10.
In SiO2-supported catalysts, the role of SiO2 is usually to enhance the dispersion of an active component because of the chemical inertness of SiO2 and the absence of a strong chemical interaction. To evaluate the activity of reduced MoO3 on SiO2 support, we conducted the reaction at a reactant contact time based on the weight of MoO3 being constant. Typical results are shown in Fig. 2. Here, the activity was compared using data after a 1 h run. The conversion level was enlarged by an increase in the amount of MoO3, indicating that the activity of reduced MoO3 on SiO2 depended on the amount of MoO3. In the case of physical mixtures of 0.1 wt%Pt/MoO3 and SiO2, no appreciable differences in the activity were seen among the catalysts tested. It is apparent from these results that H2 reduction did not convert Pt/MoO3–SiO2 with a small amount of MoO3, in other words well-dispersed MoO3
Fig. 2. Effect of the MoO3 loading on the catalytic activity of H2-reduced Pt/MoO3– SiO2 for the conversion of heptane. (*) Pt/MoO3–SiO2; (~) physical mixture of Pt/ MoO3 and SiO2. Reaction conditions: temperature, 523 K; catalyst weight, 0.25 g; MoO3 weight/C7 flow rate, 10 g h/mol; H2/C7 molar ratio, 10.
MoO3 (wt%) Conversion (%)
20 57.3
40 50.0
80 52.0
100 51.5
Selectivity (C%) Isomerization Cracking
88.7 10.7
96.7 3.0
97.5 2.1
97.8 1.9
Distribution (C%) Cracking products C1 C2 C3 C4 C5 C6
23.1 15.6 20.1 18.8 11.6 10.8
18.9 14.1 25 24.4 10.1 7.4
0 3.2 46.1 46.4 2.9 1.4
0 0 48.1 48.8 2.1 1.0
Isomerization products 2-MH 3-MH DMP others
37.2 39.6 18.3 4.9
39.3 41.9 15.6 3.2
38.8 40.4 17.3 3.6
38.7 40.1 17.3 3.8
Reaction conditions: temperature, 523 K; H2/C7 molar ratio, 10. Data after a 1 h run. MH, methylhexane; DMP, dimethylpentane.
on SiO2 support, to an active catalyst for heptane conversion. H2reduced Pt/MoO3(60)–SiO2 and Pt/MoO3(80)–SiO2 had comparable catalytic activities to that of H2-reduced Pt/MoO3. Product distributions in the reaction of heptane are summarized in Table 1, where data after a 1 h run were employed. The conversion levels were adjusted to be about 50% by varying the flow rate of reactant. Under the reaction conditions employed, the cracking and isomerization reactions proceeded simultaneously on all of the catalysts tested. The distribution of cracking products varied with the amount of MoO3. Pt/MoO3(20)–SiO2 yielded C1–C6 hydrocarbons as the cracking products, indicating the hydrocracking of the heptane and/or its isomers being catalyzed. The selectivities for C1, C2, C5, and C6 decreased as the amount of MoO3 was raised, and C3 and C4 hydrocarbons were formed as the major cracking products on Pt/MoO3(80)–SiO2 and Pt/MoO3. These results indicate that the surface property of H2-reduced Pt/MoO3– SiO2 depended on the amount of MoO3. Acid sites seem to be responsible for the cracking reaction on Pt/MoO3(80)–SiO2 and Pt/ MoO3, while sites with a metallic character are likely to catalyze the hydrocracking reaction on the catalyst with a small amount of MoO3. On the other hand, no appreciable differences in the isomerization products were seen among the catalysts tested; heptane was isomerized mainly to 2- and 3-methylhexanes in almost equal amounts. Fig. 3 shows the dependency of the heptane isomerization selectivity on the conversion level. Here, the reactions were carried out at the different W (weight of catalyst)/F (flow rate of reactant) ratios to change the conversion level. Heptane was selectively isomerized on all of the catalysts at the low conversion levels. When the isomerization selectivity was compared at a definite level of conversion, Pt/MoO3–SiO2 with a larger amount of MoO3 exhibited a higher selectivity for heptane isomerization. On Pt/ MoO3(80)–SiO2 and Pt/MoO3 catalysts, the isomerization selectivity decreased with increasing conversion level, but the high selectivity was preserved even at the high conversion levels. A similar dependency was observed on Pt/MoO3(40)–SiO2, although heptane was isomerized with a slightly lower selectivity. In contrast, the isomerization selectivity declined rapidly on Pt/ MoO3(20)–SiO2 as the conversion level was raised. The distribution of the isomerization products varied with the conversion level, irrespective of the amount of MoO3. The selectivity for methylhexanes was lowered and that for dimethylpentanes was enlarged by an increase in the conversion level.
T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46
Fig. 3. Variation in the heptane isomerization selectivity on H2-reduced Pt/MoO3– SiO2 with the conversion level. MoO3 wt%: (*) 20; (&) 40; (^) 80; (5) 100. Reaction conditions; temperature, 523 K; H2/C7 molar ratio, 10.
As described above, H2-reduced Pt/MoO3–SiO2 with a small amount of MoO3, in other words well-dispersed MoO3 on SiO2, was an inferior catalyst for heptane isomerization, and the catalytic performances of H2-reduced Pt/MoO3–SiO2 for heptane isomerization were improved by loading a larger amount of MoO3. Alkane isomerization proceeds on catalysts with acid and hydrogenation– dehydrogenation functions, and the balance between them is crucial because it determines the activity, product selectivity, and stability. Hence, the conversion of 2-propanol was conducted to study the bifunctional property of H2-reduced Pt/MoO3–SiO2. Since catalyst deactivation in the conversion of 2-propanol was similar to that in heptane isomerization, the catalytic activity was compared using data after a 1 h run. Under the reaction conditions employed, 2-propanol was converted to propene and diisopropylether (DIPE) by dehydration, and to acetone by dehydrogenation. Fig. 4 shows the dependencies of the dehydration and dehydrogenation activities of H2-reduced Pt/MoO3–SiO2 on the amount of MoO3. The dehydration and dehydrogenation reactions proceeded simultaneously on all of the catalysts tested, indicating the presence of acid sites and metallic sites. The dehydration activity of Pt/MoO3(20)–SiO2 was comparable to the dehydrogenation activity. The dehydration activity was enlarged by an increase in the amount of MoO3, while the MoO3 loading affected the dehydrogenation activity only a little. As a consequence, Pt/MoO3 was much more active for the dehydration than for the dehydrogenation. The differences in the distribution of cracking products and in the isomerization selectivity shown in Table 1 can be explained by these results. The catalyst with a small amount of MoO3 catalyzed the hydrocracking of the heptane and/or its isomers, resulting in the low isomerization selectivity, due to the metallic character. The surface became more acidic, and then the balance between the acid and metal functions was improved to be suitable for the isomerization as a larger amount of MoO3 was loaded. The dehydration activity was affected by the MoO3 loading in a similar way to the isomerization activity, whereas the dehydrogenation activity had a different relationship with the amount of MoO3. The heptane isomerization activity shown in Fig. 1 is plotted as a function of the 2-propanol dehydration activity. Here, the isomerization and the dehydration activities were estimated from
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the formation rate of heptane isomers and that of propene, respectively. As shown in Fig. 5, a good relationship was observed between them. This result allows us to suggest that the isomerization activity of H2-reduced Pt/MoO3–SiO2 was dependent on its activity as an acid catalyst. Temperature-programmed desorption of NH3 (NH3-TPD) was carried out to study the acidity of the catalysts. Fig. 6 illustrates the NH3-TPD spectra of Pt/MoO3–SiO2 reduced at 773 K for 6 h. No desorption of NH3 was seen from the catalysts without H2 reduction. NH3 was desorbed in the temperature range of 373– 673 K from all of the reduced catalysts, and the desorbed amount of NH3 increased in proportion to the amount of MoO3. These results indicate that the generation of acid sites was associated with H2 reduction of MoO3 supported on SiO2. The isomerization activity shown in Fig. 1 is plotted as a function of the concentration of acid sites determined from the NH3-TPD spectra. As shown in Fig. 7, a good relationship was observed between them. The catalyst with acid sites of <0.07 mmol/g was almost inactive for heptane isomerization. Above the concentration of 0.07 mmol/g, the isomerization activity was linearly raised by an increase in the concentration of acid sites. We suggest from these results that the isomerization activity of H2-reduced Pt/MoO3–SiO2 is controlled by the acidity. We showed in previous papers [18,19] that H2 reduction of Pt/ MoO3 was accompanied by an increase in surface area, and that the isomerization and dehydration activities depended on the extent of reduction. Hence, the surface area and the average Mo valence of Pt/MoO3–SiO2 reduced at 773 K for 6 h were measured. The average Mo valence was determined from the amount of O2 consumed in reoxidation to MoO3. Results are summarized in Table 2. Before H2 reduction, loading of MoO3 onto SiO2 lowered surface area, due to the small surface area of MoO3 (<10 m2/g). The surface area of Pt/MoO3–SiO2 was enlarged by H2 reduction, depending on the amount of MoO3. H2 reduction enlarged the surface area of Pt/MoO3(20)–SiO2 slightly, and the surface area of Pt/MoO3(80)–SiO2 increased from 44 to 216 m2/g after H2 reduction. The most significant increase was seen in bulk Pt/ MoO3. As a result, all of the reduced catalysts had comparable surface areas. These results indicate that the surface area of MoO3 was enlarged by H2 reduction even in the case of supported MoO3, although this effect became more pronounced on the catalyst with a larger amount of MoO3. No appreciable difference in the Mo valence was seen among the catalysts tested. Thus, the surface area and the Mo valence cannot explain the dependency of the isomerization activity on the amount of MoO3. Fig. 8 illustrates the XRD patterns of Pt/MoO3–SiO2 reduced at 773 K for 6 h. The amount of MoO3 gave no effect on the XRD pattern of Pt/MoO3–SiO2 before H2 reduction; only diffraction lines corresponding to the MoO3 phase were seen. The XRD pattern of H2-reduced Pt/MoO3–SiO2 varied with the amount of MoO3, although all of the catalysts had almost the same average Mo valence, as shown in Table 2. H2-reduced Pt/MoO3(20)–SiO2 provided a diffraction line at 2u = 40.58, which is assigned to the d(0 0 1) diffraction of Mo metal. In the case of H2-reduced Pt/ MoO3(40)–SiO2, diffraction lines at 2u = 38.18 and 44.38 as well as a line at 2u = 40.58 were observed. The intensities of the lines at these angles were strengthened and that due to Mo metal was weakened by an increase in the amount of MoO3. Ledoux and co-workers [7] assigned the lines at 2u = 38.18 and 44.38 to molybdenum oxyhydride, MoOxHy, which is analogous to molybdenum oxycarbide, MoOxCy. In contrast, Wehrer et al. [27,28] pointed out that the presence of hydrogen in this compound was hard to accept because thermal treatment did not change the XRD pattern, and they proposed the existence of a reduced molybdenum oxide with the composition near to MoO. Wang et al. [31] assigned these lines to MoO with a face-centered
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Fig. 4. Effect of the MoO3 loading on the catalytic activity of H2-reduced Pt/MoO3– SiO2 for the conversion of 2-propanol. propene (*); DIPE (&); acetone (~). Reaction conditions; temperature, 383 K; catalyst weight, 0.25 g; 2-P flow rate, 2.5 102 mol/h; He/2-P molar ratio, 20.
Fig. 6. NH3-TPD spectra of Pt/MoO3–SiO2 reduced at 773 K for 6 h. MoO3 wt%: ( ) 20; (- - -) 60; (—) 100.
Fig. 5. Relationship between the dehydration activity and the isomerization activity. Data taken after a 1 h run.
Fig. 7. Effect of the concentration of acid sites on the rate of heptane isomerization over Pt/MoO3–SiO2 reduced at 773 K for 6 h.
cubic lattice. We reported previously [32] that H2O was evolved during thermal treatment of H2-reduced MoO3, of which the XRD pattern had diffraction lines at 2u = 38.18 and 44.38, indicating the presence of hydrogen in the reduction product. We have considered from this result that diffraction lines at 2u = 38.18 and 44.38 correspond to the MoOxHy phase, although the exact nature of this compound is still under discussion. The XRD patterns shown in Fig. 8 indicate that the formation of Mo metal was suppressed and that of MoOxHy was promoted as the amount of MoO3 increased. These XRD results can be helpful to account for the differences in product distributions shown in Table 1; the distribution of cracking products was shifted from the mixture of
C1–C6 hydrocarbons to C3 and C4 hydrocarbons as the content of Mo metal in the catalysts was decreased. The reduction process of Pt/MoO3–SiO2 was studied by means of a temperature-programmed reduction (TPR) technique to elucidate why the reduction product varied with the amount of MoO3. Typical results are demonstrated in Fig. 9. Pt/MoO3–SiO2 reacted with H2 without generating an equivalent amount of H2O in the temperature region below 473 K, indicating the formation of HxMoO3. This phenomenon can be understood by hydrogen spillover. A large amount of H2O was evolved compared with the amount of H2 consumed in the range of 400–550 K. This implies that the decomposition of HxMoO3 proceeded in this temperature
T. Matsuda et al. / Applied Catalysis A: General 362 (2009) 40–46 Table 2 Physical properties of H2-reduced Pt/MoO3–SiO2. MoO3 (wt%)
0 20 40 60 80 100
Before reduction
After reduction
Surface area (m2 g1)
Average valence
Surface area (m2 g1)
172 152 112 79 44 9
– 1.9 2.0 2.1 2.0 2.1
– 186 175 191 216 243
H2 reduction: 773 K, 6 h.
range. The amount of H2 consumed was consistent with that of H2O evolved at higher temperatures. These results show that reduction of MoO3 supported on SiO2 proceeded through the formation of HxMoO3, and its decomposition, irrespective of the amount of MoO3. However, the amount of H2 consumed in the low temperature region (< 473 K) varied with the amount of MoO3; the amounts of H2 reacted with Pt/MoO3(20)–SiO2, Pt/MoO3(40)– SiO2, and Pt/MoO3(80)–SiO2 were calculated from the TPR spectra as 7.8 104, 2.8 103, and 4.8 103 mol/g-MoO3, respectively. Fig. 10 shows the XRD patterns of Pt/MoO3–SiO2 heated to 473 K under the TPR conditions. Pt/MoO3(80)–SiO2 gave no diffraction peaks due to the MoO3 phase, and the lines corresponding to the H1.68MoO3 phase were seen. H1.68MoO3 was also formed, but the formation of HxMoO3 with the lower hydrogen content such as H0.34MoO3 and H0.93MoO3 was not observed in Pt/MoO3(20)–SiO2 and Pt/MoO3(40)–SiO2. These results imply that MoO3 was converted to H1.68MoO3 in the initial stage of reduction, irrespective of the amount of MoO3. However, Pt/MoO3(20)–SiO2 and Pt/ MoO3(40)–SiO2 contained the MoO3 phase after heating to 473 K, indicating the incomplete conversion to H1.68MoO3. To study the formation of H1.68MoO3 quantitatively, we calculated the degree of MoO3 conversion to H1.68MoO3 from the amount of H2 consumed below 473 K in the TPR measurement. As shown in Fig. 11, the
Fig. 8. XRD patterns of Pt/MoO3–SiO2 reduced at 773 K for 6 h. MoO3 wt%: (A) 20; (B) 40; (C) 60; (D) 80. MoOxHy (*) Mo metal (&).
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conversion of MoO3 to H1.68MoO3 in Pt/MoO3(20)–SiO2 was about 10%. The degree of MoO3 conversion was enlarged by an increase in the amount of MoO3, and MoO3 was completely converted to H1.68MoO3 in Pt/MoO3. Insertion of hydrogen atoms into the lattice of MoO3 leads to the formation of HxMoO3. This reaction seems to be inhibited in well-dispersed MoO3, probably due to its low crystallinity. The TPR profile at temperatures above 600 K varied with the amount of MoO3. In TPR of Pt/MoO3(20)–SiO2, two peaks were seen, at 790 and 1050 K, and Mo valence after TPR was calculated as zero from the TPR spectrum. Pt/MoO3(40)–SiO2 also gave two peaks and the reduction was completed in the TPR, but the peak at 790 K was weakened and the other peak appeared at higher temperature. In contrast, Pt/MoO3(80)–SiO2 gave very broad peaks, and the Mo valence after TPR was calculated as 1.5 from the TPR spectrum. We deduce from these results that the reduction process was changed by the amount of MoO3. We reported that reduction of Pt/MoO3 yielded no MoO2 when the reduction proceeded through the formation of H1.68MoO3, and vice versa [22]. Thus, it is possible to consider that two types of reduction processes appear in Pt/MoO3–SiO2. Only a part of MoO3 was converted to H1.68MoO3
Fig. 9. TPR spectra of Pt/MoO3–SiO2. MoO3 wt%: (A) 20; (B) 40; (C) 80. H2 consumed (*) H2O evolved (&). Conditions: sample weight, 0.4 g; gas, 20%H2–Ar; flow rate, 20 mL/min; heating rate, 5 K/min.
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[21]. Pt/MoO3(60)–SiO2 and Pt/MoO3(80)–SiO2 had almost comparable activity value to that of Pt/MoO3 when the catalytic activity was compared based on the weight of MoO3, although these catalysts had lower conversions of MoO3 to H1.68MoO3. This result can result from the support effect that allows the active phase to contribute to the catalytic reaction efficiently.
4. Conclusion The catalytic activity of H2-reduced Pt/MoO3–SiO2 for heptane isomerization was investigated. H2 reduction did not convert Pt/ MoO3–SiO2 with a small amount of MoO3, in other words welldispersed MoO3 on SiO2, to an active and selective catalyst for heptane isomerization. The isomerization activity was enlarged by an increase in the amount of MoO3, and was related to the concentration of acid sites. The formation of MoOxHy was promoted and that of Mo metal was suppressed as the amount of MoO3 increased. A good relationship was observed between the conversion of MoO3 to H1.68MoO3 in the initial stage of reduction and the isomerization activity. We conclude from these results that the isomerization activity of H2-reduced Pt/MoO3–SiO2 is governed by the formation of H1.68MoO3, the reduction of which yields the MoOxHy phase with acidity. Fig. 10. XRD patterns of Pt/MoO3–SiO2 heated to 473 K under the TPR conditions. MoO3 wt%: (A) 20; (B) 40; (C) 80. H1.68MoO3 (*) MoO3 (!).
Fig. 11. Effect of the MoO3 loading on the formation of H1.68MoO3 over Pt/MoO3– SiO2.
on Pt/MoO3(20)–SiO2 under the TPR condition. This implies that reduction of MoO3 to MoO2 can proceed on this catalyst, although no MoO2 was detected by XRD measurements. In contrast, reduction of Pt/Mo(80)–SiO2 is likely to proceed without the formation of MoO2, due to the high conversion to H1.68MoO3. As shown in Figs. 3 and 11, the dependency of the heptane isomerization activity on the amount of MoO3 is very similar to that of the MoO3 conversion to H1.68MoO3, suggesting that the formation of H1.68MoO3 is the key step to generate the isomerization activity. This result is compatible with the reported one; the isomerization activity of H2-reduced MoO3 with a noble metal depended on the ability of the noble metal to promote the formation of H1.68MoO3
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