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Catalytic reactions of methylcyclohexane (MCH) on partially reduced MoO3
Applied Catalysis A: General 275 (2004) 141–147 www.elsevier.com/locate/apcata
Catalytic reactions of methylcyclohexane (MCH) on partially reduced MoO3 H. Belatela, H. Al-Kandarib, F. Al-Khorafib, A. Katriba,*, F. Garina a
LMSPC-UMR 7515 du CNRS-ECPM, University Louis Pasteur-25, rue Becquerel, 67087 Strasbourg, France b Kuwait University, Department of Chemistry, P.O. Box 5969, Safat 13060, Kuwait Received 8 July 2004; accepted 22 July 2004 Available online 9 September 2004
Abstract XPS and UPS characterization of partially reduced MoO3 by hydrogen at different temperatures enabled us to define the metallic character of MoO2. Hydrogen dissociation by this state results in the formation of Bro¨nsted acidic group(s) Mo–OH as characterized by O 1s and catalytic properties. A bifunctional MoO2(Hx)ac phase is formed on the outermost sample surface layer. Two hydrocarbons were studied: methylcyclohexane (MCH) and n-heptane (nC7). The catalytic behavior of MCH on this system shows two different pathways depending on the reaction temperature. A selectivity close to 95% in dimethyl and ethyl-cyclopentanes (DMCP and EtCP) products were obtained at temperatures between 493 and 573 K. At higher temperatures up to 653 K, toluene is the major dehydrogenation product. The general tendency of this catalytic system seems to behave as a bifunctional; to confirm this hypothesis, an acid-supported catalyst, i.e. sulfated zirconia-supported Pt–Ir, was tested under the same experimental conditions. The possible formations of dimethyl or ethylcyclopentanes as intermediate products in the catalytic reaction of n-heptane have been explored using both catalysts. # 2004 Elsevier B.V. All rights reserved. Keywords: Methylcyclohexane, Toluene, Cyclic isomerization, Dehydrogenation, XPS-UPS of MoO3 and MoO2
1. Introduction Combined XPS and UPS measurements of the chemical changes which occur in the oxidation state of molybdenum in MoO3 induced by in situ reduction with hydrogen constitutes the most accurate way in defining the chemical structure of the surface after each treatment. Catalytic experiments carried out in a catalytic reactor under similar conditions as above will enable to get a close correlation between structure and catalytic activity. The use of UPS with its detection sensitivity in the range of 3–4 surface monolayers reveals to be of considerable importance. The presence of a metallic character in the case of MoO2 sub-oxide, measured as a density of states (DOS) at the Fermi level is determinant in defining the metallic character of the surface. Moreover, it is expected that hydrogen is dissociated on MoO2 surface leading to the formation of * Corresponding author. Tel.: +33 3 902 427 56; fax: +33 3 902 727 61. E-mail address: [email protected] (A. Katrib). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.029
Bro¨nsted Mo–OH acidic group(s) on the sample surface. The presence of such OH group(s) on the surface could be observed by XPS in the form of a shoulder at the high binding energy side of the oxidic O 1s energy region. This acidic character has been verified by studying the dehydration of isopropanol. The presence of this bifunctional system on the same site represents a valuable tool to study catalytic reactions which may require monofunctional (metal, acid) or both. Dehydrogenation of methylcyclohexane (MCH) to toluene (Tol) is considered as an important catalytic-reforming process used to enhance the octane number of gasoline [1–3]. Moreover, this catalytic reaction has a considerable interest to extract the hydrogen stored in toluene in the form of MCH as a safe way for hydrogen storage and transportation [4]. In general, dehydrogenation of MCH, takes place on Pt-based catalysts. Other catalytic reactions of great interest are dearomatization required for environmental control [5]. In this respect, MCH and Tol are involved, and the rearrangement of MCH in terms of isomerization reactions leads to
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dimethyl (1,1-, 1,2-cis and -trans and 1,3-cis and -trans) and ethyl-cyclopentanes (DMCP and EtCP). In this work, several catalytic reactions which require different active site(s) were studied. On the one hand, isomerization of MCH to DMCP and EtCP is supposed to take place on acidic site(s), while the dehydrogenation process to toluene is performed by a metal function. On the other hand, the stability of these isomerization compounds and their formation as intermediate species (cyclic mechanism) in the isomerization process of n-heptane to mono or di-branched alkanes is explored. Knowing that isomerization of n-heptane may occur following two reaction mechanisms: (i) one taking place on a mono-functional metal site such as Pt deposited on SiO2, a pathway via 1,2-DMCP and EtCP intermediates leads to the formation of 3-methylhexane (3MH) with almost no 2-methylhexane (2MH) formation [6], and (ii) one involving bifunctional mechanism in which 2- and 3-methylhexanes are almost at thermodynamic equilibrium (2MH/3MH = 1.2). The behavior of the hydrogen reduced MoO3 towards these chemical reactions as monitored by the type and relative concentrations of the products will enable to determine the nature of the active catalytic function(s) present in this system.
sample is pressed as a pellet form. The XPS of the Mo 3d, O 1s and the VB energy regions (XPS-UPS) were recorded. Mo (3d5/2,3/2) spin-orbit components of the different oxidation states of Mo are defined as follows: MoO3 (232.7, 235.8 eV), Mo2O5 (231.7, 234.9 eV), MoO2 (229.1, 232.3 eV) and Mo (227.7, 230.9 eV). Two well-defined spectral lines at 232.8 and 236.0 eV characteristics of MoO3 are observed in the sample prior to any treatment (Fig. 1a). The O 1s energy region shows the presence of one symmetrical line at 530.4 eV (Fig. 2a) which is attributed to the oxide oxygen. Fig. 3a presents the XPS of the VB of MoO3. The major broad and relatively intense band is attributed to the O 2p. We do not observe any DOS structure at the Fermi level in the UPS spectrum (Fig. 4a). This is expected on the basis of the insulating properties of MoO3. Exposure of the sample to hydrogen for 2 h at 573 K results in slight reduction as could be observed from the presence of low intensity lines at lower BE attributed to Mo2O5 state (Fig. 1b). This slight reduction does not affect the binding energies nor the structures of the O 1s and VB (Figs. 2b and 3b). However, a low-intensity band
2. Experimental Catalytic reactions of MCH were studied in this work by introducing the reactant as pulses of 5 ml over a fixed-bed quartz reactor under atmospheric hydrogen pressure. A continuous H2 flux of 30 cm3/min passes through 0.1 g of the catalyst. Gaseous products were analyzed with an on-line GC using a 60 m (CP-SIL-5 CB) column and a flame ionization detector. Prior to catalytic reactions, this quantity of MoO3 was exposed to atmospheric hydrogen flow at 653 K for 12 h. XPS-UPS measurements were used in order to characterize the chemical state of Mo following its reduction in situ by hydrogen at different temperatures. A VG Scientific ESCALAB-210 spectrometer with a vacuum below 4 10 9 mbar in the analysis chamber is employed. A Mg Ka radiation source operated at 300 W (15 kV, 20 mA) was applied. Binding energies were determined with respect to carbon contamination C 1s at 284.4 eV. Spectral data were ˚ analyzed using ECLIPS VG program. He (I) 584 A (21.217 eV) radiation were employed in order to record the valence band (VB) energy region, taking into consideration the O 2p band as a reference energy in the absence of DOS structure. Binding energies are reported with an experimental error of 0.2 eV.
3. XPS-UPS results The XPS-UPS spectra of MoO3 before and during the sample exposure to hydrogen were studied as follows: a
Fig. 1. The XPS of the Mo 3d energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
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Fig. 3. The XPS of the valence band (VB) energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
Fig. 2. The XPS of the O 1s energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
structure with maxima at 0.7 and 1.8 eV could be observed in the UPS spectrum (Fig. 4b). The first maximum at 0.7 eV is characteristic of the p band of MoO2 [7,8]. The second spectral line at 1.8 eVis assigned to the s band between two adjacent Mo atoms placed along the C-axis of the deformed rutile structure of MoO2. These results reveal that exposure of the MoO3 at 573 K for 2 h results in the formation of a small amount of MoO2 on the sample surface. Continuous increase in the reduction temperature to 623 K for 2 h leads to an increase in the relative concentration of low binding energy sub-oxide(s) (Fig. 1c). Curve-fitting of this spectrum shows that the majority of this low BE structure is composed of Mo2O5 state. The shape and the BE of the O 1s line remain unchanged (Fig. 2c). Also, no DOS structure in the XPS of the VB is observed (Fig. 3c). More surface-sensitive UPS
measurements reveal a slight increase in the relative concentration of the two maxima at 0.7 and 1.8 eV characteristics of MoO2 (Fig. 4c). The higher detection sensitivity of UPS in this energy region, as compared to XPS is an important factor in defining the chemical composition of the outermost surface layer(s) in this reduction process. Substantial reduction takes place when the sample is further exposed to hydrogen for more than 2 h at 653 K (Fig. 1d). Curve-fitting of this complex structure reveals the presence of a considerable amount of MoO2, most probably present on the sample surface, bulk MoO3 and Mo2O5 in the interphase. It is interesting to note the presence of a shoulder at 531.6 eV beside the more intense line at 530.6 eVof the oxide oxygen in the O 1s energy region. The O 1s line at 531.6 eVis assigned to the OH group(s) formed on the sample surface. Molybdenum dioxide seems to be the major sub-oxide species present in the few monolayers measured by XPS following further exposure of the sample to hydrogen at 673 K for more than 2 h (Fig. 1e). Within the detection limit, 10–15 layers, measured by XPS, bulk MoO3 is still observed as well as Mo2O5 in the interphase. The low-intensity shoulder at 531.4 eV beside the more intense O 1s oxide oxygen at 530.6 eV is still observed (Fig. 2e). Also, the XPS of the VB (Fig. 3e) shows a relatively intense unresolved band, at low BE of O 2p, attributed to the p and s bands of MoO2. The structure of these two bands is better resolved in the UPS spectrum (Fig. 4e). It is apparent that the relative MoO2 concentration on the sample surface
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formation of MoO2, with its metallic character, on the sample surface in the reduction process of MoO3 enables to dissociate hydrogen into H atoms. The polar bonding of H to surface O atoms leads to the formation of Bro¨ nsted acidic Mo–OH group(s) on the sample surface. Considering the maximum number of oxygen atoms bonded to one Mo surface atom in MoO2, we expect that the maximum number x of H atoms, per one Mo atom, present on the sample surface in the form of Bro¨ nsted Mo–OH does not exceed 2. As a result, a bifunctional phase having a stoichiometry of MoO2(Hx)ac is formed on the surface of bulk MoO2 produced by hydrogen reduction of MoO3. The term (Hx)ac represents the relative number (x 2) of acidic (ac) groups per Mo atom present on the sample surface.
4. Catalytic results and discussion In this work, we report the catalytic activity of MoO2(Hx)ac phase for the reaction of MCH at temperatures up to 653 K. A comparative study with Pt–Ir/ZrO2–SO24 is carried out in order to elucidate the nature and strength of the active site(s) of the MoO2(Hx)ac phase. The possibility of dimethyl and ethyl-cyclopentane as intermediate reaction products has been explored by studying n-heptane reactant using both catalysts. Fig. 4. The UPS of the s and p bands of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
reached a stable state as could be observed from the relative intensities of the two p and s bands in comparison to O 2p following sample reductions at 653 and 673 K (Fig. 5b). The association of the few surface monolayers of MoO2 with the presence of OH group(s) on the outermost one, measured at 531.6 eV O 1s BE could be rationalized as follows. The
Fig. 5. The UPS of the valence band energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) 623 K; (b) 653 K; (c) 673 K.
4.1. Methylcyclohexane (MCH) reactant An active MoO2(Hx)ac phase has been prepared by exposing a fresh sample of MoO3 to hydrogen at 653 K for 12 h. On such active phase, the initial catalytic activity of MCH was observed at 493 K with a conversion of 1.2% and a selectivity of 95% in isomerization (DMCP and EtCP products). An increase in the reaction temperature results in relative increase in conversion to reach 11% at 553 K while the selectivity in isomerization products remains high. A considerable increase in the dehydrogenation of MCH to toluene takes place at reaction temperature of 573 K. It is apparent from experimental results that isomerization process (Scheme 1, pathway 1) is favored at low reaction temperature over the dehydrogenation process (Scheme 1, pathway 2). In case of MCH, such isomerization process usually named as ring shortening takes place on acidic function. It is clear that the Bro¨ nsted acidic functions present in the MoO2(Hx)ac phase are sufficient to perform such catalytic process. It is relevant to mention that we have tested this catalytic reaction on unreduced MoO3 and partially reduced MoO3 below 573 K, instead of 653 K, which produced an intermediate Mo2O5 state as observed by XPS. It seems that the Lewis and Bro¨ nsted acidities in these states are not sufficient to isomerize MCH. Moreover, on this active phase MoO2(Hx)ac a conversion of 9% and a selectivity of more than 82% to toluene formation were obtained at reaction temperature of 653 K. Two distinct reaction pathways of MCH on MoO2(Hx)ac take place in
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Scheme 1. Different reaction pathways of MCH and n-heptane on MoO2(Hx)ac and Pt–Ir/ZrO2–SO24 catalysts.
function of the reaction temperature as it can be noticed in Fig. 6. From MCH, the apparent activation energy (Ea) for MCH to toluene is higher than the Ea for the reaction MCH to DMCP. From Fig. 7, we can observe that isomerization product distributions seem to follow a certain trend in which 1,3-DMCP is dominant except at 493 K at which EtCP constitutes 32%. Similar reaction behavior has been observed in the case of MCH on Pt–Ir/ZrO2–SO24 system. A conversion of 16% and a selectivity of 100% in DMCP and EtCP isomerization products were obtained at reaction temperature of 493 K (Table 1). Following reactions up to 573 K, an increase in conversion is observed while the selectivity in isomerization remains relatively high regardless of reaction temperature increase. Beyond this reaction temperature, the dehydrogenation process is favored. It is clear from these results that both MoO2(Hx)ac and Pt–Ir/ZrO2–SO24 have comparable behaviors concerning isomerization and dehydrogenation reactions of MCH. No isomerization reaction takes place on ZrO2–SO24 alone which may indicate that the metallic function plays an active role in the isomerization process of MCH.
Fig. 6. Conversion and selectivity of methylcyclohexane on MoO2(Hx)ac in function of reaction temperature.
Fig. 7. Products distribution of methylcyclohexane on MoO2(Hx)ac in function of reaction temperature.
4.2. n-Heptane reactant Isomerization of n-heptane could take place via a cyclic mechanism having only 1,2-dimethylcyclopentane and ethylcyclopentane as intermediate compounds (Scheme 1, pathway 3). Such catalytic process usually takes place on a monofunctional metallic catalysts such as Pt deposited on alumina or silica [6]. The final products in this case are 3methylhexane (3MH), 1,2-DMCP, EtCP and toluene Table 1 Conversion and selectivity of MCH on Pt–Ir/ZrO2–SO24 reaction temperatures Temperature (K) Conversion (%) S isomers (%) S toluene (%) S cracking (%)
493 16.39 99.54 0.46 0.0
523 19.76 95.47 2.15 2.38
Distribution of isomerization products (%) M2H – 0.13 1,1-DMCP 1.19 2.17 1,3-cDMCP 37.49 34.43 1,3-tDMCP 30.88 27.70 1,2-DMCP 19.24 19.90 nC7 – – EtCP 10.74 11.14 Tol 0.46 2.15 Distribution of cracking products C2 – C3 – iC4 – C4 – iC5 – C5 – 2,3-DMB – M2P – M3P – MCP – Bz – cc6 –
(%) – 0.27 0.87 0.08 0.36 – 0.05 0.17 – – 0.57 –
at different
553 22.87 85.39 7.38 7.23
573 29.01 70.56 20.11 9.33
593 28.70 55.93 25.26 18.82
0.36 3.41 29.13 22.90 18.58 – 11.01 7.38
0.25 4.01 21.59 17.29 16.79 0.33 10.31 20.11
0.36 3.44 17.05 13.29 12.81 0.30 8.68 25.26
– 1.88 2.40 0.28 0.77 0.11 0.13 0.31 0.11 – 1.25 –
– 2.56 3.66 0.47 0.73 0.05 0.19 0.35 0.17 – 1.15 –
1.98 6.77 4.66 0.99 1.07 0.46 0.23 0.39 0.17 0.21 1.76 0.12
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(Scheme 1, pathway 4). In the case of bifunctional mechanism, isomerization of n-heptane follows a sequence of catalytic processes involving dehydrogenation and hydrogenation steps as well as isomerization. In this case, 2MH and 3-MH constitute the major products. The introduction of n-heptane on MoO2(Hx)ac at 523 K results in a conversion of 24% and a selectivity of 88% in isomerization products, i.e. 2MH and 3MH (Scheme 1, pathway 5). The ratio of these two products is at the equilibrium ratio of 2MH/3MH = 1.2 (Table 2). An increase in the reaction temperature to 573 K results in considerable increase in conversion to 52% while the selectivity in isomerization decreases to 49% in favor of hydrocracking products, mainly C3, iC4 and C4. In the case of Pt–Ir/ZrO2– SO24 catalyst, a different reaction mechanism seems to take place. At 493 K, a conversion of 26% and a selectivity of only 28% in 2MH and 3MH isomerization products were obtained. Central C–C bond scission (Scheme 1, pathway 6) seems to be the dominant reaction which takes place on this Pt–Ir catalysts leading to the formation of C3 and C4 compounds (Table 2). Further increase in the reaction temperature to 523 K increases the conversion to 38% with a selectivity of 78% in C3–C4 hydrocracking products. These results show that in the case of n-heptane, we have two parallel pathways: one giving isomers with a rate constant k1, the second yield cracked products with a rate constant k2. In the case of partially reduced MoO3, it seems that k1 is higher than k2. In other words, a selective dehydrogenation process to an olefin as a first step in the isomerization of nheptane via the bifunctional mechanism seems to be favored. In the case of the Pt–Ir catalysts, k2 > k1 leading to central
Table 2 Conversion and selectivity of n-heptane on MoO2(Hx)ac and Pt–Ir/ZrO2– SO24 at different reaction temperatures Catalyst
MoO2(Hx)ac
Temperature (K) Conversion (%) S isomers (%) S cracking (%) 2MH/3MH C3/C4
523 24.3 88.2 11.8 1.2 1.1
Pt–Ir/ZrO2–SO24 573 51.5 49.3 50.7 1.1 1.1
493 25.7 27.9 72.1 0.8 1.0
523 37.6 21.6 78.4 0.9 1.1
Distribution of isomerization products (%) 2,2-DMP 2.0 1.3 2,4-DMP 5.7 3.5 223TMB 0 0.3 3,3-DMP 0.9 0.8 2MH 37.8 19.7 2,3-DMP 6.2 4.4 3MH 32.2 17.4 Et-Pent 3.4 2.0 Toluene 0 0
1.4 2.0 0 0 9.5 2.8 11.2 1.1 0
0 1 0 0 8.1 2.1 9 1 0
Distribution of cracking products (%) C3 6.2 iC4 5.0 C4 0.6 iC5 0
35.8 33.5 0.9 0
40.5 37 1 0
25.9 19.7 4.4 0.76
C3–C4 bond scission. Such process could be attributed to a more pronounced metallic character of the Pt–Ir system. These preliminary catalytic results clearly indicate that ring opening of MCH to alkanes does not take place on a bifunctional or mono-metallic functional catalysts at atmospheric pressure regardless of the reaction temperature. Alternative experimental conditions such as an increase in pressure are under consideration. The experimental catalytic results discussed above clearly show that isomerization and dehydrogenation of MCH on MoO2(Hx)ac phase confirms the presence of both acidic and metallic functions in this system. Both functions (bifunctional) operate sequentially in the case of hydroisomerization of n-heptane. The presence of an oxicarbide MoOxCy species in this case is excluded on the basis that MoO3 is activated under atmospheric hydrogen at 653 K. The first contact of this partially reduced phase with MCH was at 493 K leading to the formation of 95% of MCH isomerization products (DMCP and EtCP). There are no cracking products (C1, C2) in this reaction, and it is not clear how carbon could be inserted in the molybdenum sub-oxide species in order to activate it via the formation of an oxicarbide species. Similar arguments apply to the exclusion of a carbide species formation, such as Mo2C for molybdenum and WC for tungsten, at these experimental conditions, as postulated by Ribeiro et al. [9,10] for the interpretation of the isomerization of alkanes via the bifunctional mechanism. Just to mention that partially reduced MoO3 and WO3 show comparable catalytic behavior for the isomerization of an alkane at a given reaction temperature [11,12]. Therefore, the formation of a Mo carbide, having metallic character, under the experimental conditions used in this work is excluded. The metallic function in our catalytic system is completely due the conjugated p electrons in MoO2 or in WO2 as characterized by XPS-UPS techniques. On the other hand, the acidic function is mainly of a Bro¨ nsted Mo–OH type characterized by the O 1s spectral line at 531.6 eV and verified by the dehydration of isopropanol [11,12].
5. Conclusion In situ characterization by XPS-UPS surface techniques of the reduction of bulk MoO3 at temperatures between 623 and 673 K clearly demonstrate the formation of a bifunctional MoO2(Hx)ac phase on the sample surface. Catalytic reactions of MCH on this system reveal the presence of two reaction mechanisms depending on the reaction temperature. Cyclic isomerization of MCH to DMP and Et-CP takes place at temperatures between 493 and 573 K. On the other hand, dehydrogenation of MCH to toluene takes place at temperatures between 573 and 653 K. A conversion of 97% and a selectivity of 82% to toluene were obtained. Similar catalytic behavior was obtained using Pt–Ir/ZrO2–SO24 catalyst. Hydroisomerization of n-heptane on partially
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reduced MoO3 follows the pathway of a bifunctional (dehydrogenation/carbenium ion) mechanism with no intermediate cyclic compounds formation.
Acknowledgements H. Al-Kandari and F. Al-Korafi would like to acknowledge the support of Kuwait University through research grant No. SC04/004. ANALAB and SAF Grant No. GS01/01.
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Catalytic reactions of methylcyclohexane (MCH) on partially reduced MoO3 H. Belatela, H. Al-Kandarib, F. Al-Khorafib, A. Katriba,*, F. Garina a
LMSPC-UMR 7515 du CNRS-ECPM, University Louis Pasteur-25, rue Becquerel, 67087 Strasbourg, France b Kuwait University, Department of Chemistry, P.O. Box 5969, Safat 13060, Kuwait Received 8 July 2004; accepted 22 July 2004 Available online 9 September 2004
Abstract XPS and UPS characterization of partially reduced MoO3 by hydrogen at different temperatures enabled us to define the metallic character of MoO2. Hydrogen dissociation by this state results in the formation of Bro¨nsted acidic group(s) Mo–OH as characterized by O 1s and catalytic properties. A bifunctional MoO2(Hx)ac phase is formed on the outermost sample surface layer. Two hydrocarbons were studied: methylcyclohexane (MCH) and n-heptane (nC7). The catalytic behavior of MCH on this system shows two different pathways depending on the reaction temperature. A selectivity close to 95% in dimethyl and ethyl-cyclopentanes (DMCP and EtCP) products were obtained at temperatures between 493 and 573 K. At higher temperatures up to 653 K, toluene is the major dehydrogenation product. The general tendency of this catalytic system seems to behave as a bifunctional; to confirm this hypothesis, an acid-supported catalyst, i.e. sulfated zirconia-supported Pt–Ir, was tested under the same experimental conditions. The possible formations of dimethyl or ethylcyclopentanes as intermediate products in the catalytic reaction of n-heptane have been explored using both catalysts. # 2004 Elsevier B.V. All rights reserved. Keywords: Methylcyclohexane, Toluene, Cyclic isomerization, Dehydrogenation, XPS-UPS of MoO3 and MoO2
1. Introduction Combined XPS and UPS measurements of the chemical changes which occur in the oxidation state of molybdenum in MoO3 induced by in situ reduction with hydrogen constitutes the most accurate way in defining the chemical structure of the surface after each treatment. Catalytic experiments carried out in a catalytic reactor under similar conditions as above will enable to get a close correlation between structure and catalytic activity. The use of UPS with its detection sensitivity in the range of 3–4 surface monolayers reveals to be of considerable importance. The presence of a metallic character in the case of MoO2 sub-oxide, measured as a density of states (DOS) at the Fermi level is determinant in defining the metallic character of the surface. Moreover, it is expected that hydrogen is dissociated on MoO2 surface leading to the formation of * Corresponding author. Tel.: +33 3 902 427 56; fax: +33 3 902 727 61. E-mail address: [email protected] (A. Katrib). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.029
Bro¨nsted Mo–OH acidic group(s) on the sample surface. The presence of such OH group(s) on the surface could be observed by XPS in the form of a shoulder at the high binding energy side of the oxidic O 1s energy region. This acidic character has been verified by studying the dehydration of isopropanol. The presence of this bifunctional system on the same site represents a valuable tool to study catalytic reactions which may require monofunctional (metal, acid) or both. Dehydrogenation of methylcyclohexane (MCH) to toluene (Tol) is considered as an important catalytic-reforming process used to enhance the octane number of gasoline [1–3]. Moreover, this catalytic reaction has a considerable interest to extract the hydrogen stored in toluene in the form of MCH as a safe way for hydrogen storage and transportation [4]. In general, dehydrogenation of MCH, takes place on Pt-based catalysts. Other catalytic reactions of great interest are dearomatization required for environmental control [5]. In this respect, MCH and Tol are involved, and the rearrangement of MCH in terms of isomerization reactions leads to
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dimethyl (1,1-, 1,2-cis and -trans and 1,3-cis and -trans) and ethyl-cyclopentanes (DMCP and EtCP). In this work, several catalytic reactions which require different active site(s) were studied. On the one hand, isomerization of MCH to DMCP and EtCP is supposed to take place on acidic site(s), while the dehydrogenation process to toluene is performed by a metal function. On the other hand, the stability of these isomerization compounds and their formation as intermediate species (cyclic mechanism) in the isomerization process of n-heptane to mono or di-branched alkanes is explored. Knowing that isomerization of n-heptane may occur following two reaction mechanisms: (i) one taking place on a mono-functional metal site such as Pt deposited on SiO2, a pathway via 1,2-DMCP and EtCP intermediates leads to the formation of 3-methylhexane (3MH) with almost no 2-methylhexane (2MH) formation [6], and (ii) one involving bifunctional mechanism in which 2- and 3-methylhexanes are almost at thermodynamic equilibrium (2MH/3MH = 1.2). The behavior of the hydrogen reduced MoO3 towards these chemical reactions as monitored by the type and relative concentrations of the products will enable to determine the nature of the active catalytic function(s) present in this system.
sample is pressed as a pellet form. The XPS of the Mo 3d, O 1s and the VB energy regions (XPS-UPS) were recorded. Mo (3d5/2,3/2) spin-orbit components of the different oxidation states of Mo are defined as follows: MoO3 (232.7, 235.8 eV), Mo2O5 (231.7, 234.9 eV), MoO2 (229.1, 232.3 eV) and Mo (227.7, 230.9 eV). Two well-defined spectral lines at 232.8 and 236.0 eV characteristics of MoO3 are observed in the sample prior to any treatment (Fig. 1a). The O 1s energy region shows the presence of one symmetrical line at 530.4 eV (Fig. 2a) which is attributed to the oxide oxygen. Fig. 3a presents the XPS of the VB of MoO3. The major broad and relatively intense band is attributed to the O 2p. We do not observe any DOS structure at the Fermi level in the UPS spectrum (Fig. 4a). This is expected on the basis of the insulating properties of MoO3. Exposure of the sample to hydrogen for 2 h at 573 K results in slight reduction as could be observed from the presence of low intensity lines at lower BE attributed to Mo2O5 state (Fig. 1b). This slight reduction does not affect the binding energies nor the structures of the O 1s and VB (Figs. 2b and 3b). However, a low-intensity band
2. Experimental Catalytic reactions of MCH were studied in this work by introducing the reactant as pulses of 5 ml over a fixed-bed quartz reactor under atmospheric hydrogen pressure. A continuous H2 flux of 30 cm3/min passes through 0.1 g of the catalyst. Gaseous products were analyzed with an on-line GC using a 60 m (CP-SIL-5 CB) column and a flame ionization detector. Prior to catalytic reactions, this quantity of MoO3 was exposed to atmospheric hydrogen flow at 653 K for 12 h. XPS-UPS measurements were used in order to characterize the chemical state of Mo following its reduction in situ by hydrogen at different temperatures. A VG Scientific ESCALAB-210 spectrometer with a vacuum below 4 10 9 mbar in the analysis chamber is employed. A Mg Ka radiation source operated at 300 W (15 kV, 20 mA) was applied. Binding energies were determined with respect to carbon contamination C 1s at 284.4 eV. Spectral data were ˚ analyzed using ECLIPS VG program. He (I) 584 A (21.217 eV) radiation were employed in order to record the valence band (VB) energy region, taking into consideration the O 2p band as a reference energy in the absence of DOS structure. Binding energies are reported with an experimental error of 0.2 eV.
3. XPS-UPS results The XPS-UPS spectra of MoO3 before and during the sample exposure to hydrogen were studied as follows: a
Fig. 1. The XPS of the Mo 3d energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
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Fig. 3. The XPS of the valence band (VB) energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
Fig. 2. The XPS of the O 1s energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
structure with maxima at 0.7 and 1.8 eV could be observed in the UPS spectrum (Fig. 4b). The first maximum at 0.7 eV is characteristic of the p band of MoO2 [7,8]. The second spectral line at 1.8 eVis assigned to the s band between two adjacent Mo atoms placed along the C-axis of the deformed rutile structure of MoO2. These results reveal that exposure of the MoO3 at 573 K for 2 h results in the formation of a small amount of MoO2 on the sample surface. Continuous increase in the reduction temperature to 623 K for 2 h leads to an increase in the relative concentration of low binding energy sub-oxide(s) (Fig. 1c). Curve-fitting of this spectrum shows that the majority of this low BE structure is composed of Mo2O5 state. The shape and the BE of the O 1s line remain unchanged (Fig. 2c). Also, no DOS structure in the XPS of the VB is observed (Fig. 3c). More surface-sensitive UPS
measurements reveal a slight increase in the relative concentration of the two maxima at 0.7 and 1.8 eV characteristics of MoO2 (Fig. 4c). The higher detection sensitivity of UPS in this energy region, as compared to XPS is an important factor in defining the chemical composition of the outermost surface layer(s) in this reduction process. Substantial reduction takes place when the sample is further exposed to hydrogen for more than 2 h at 653 K (Fig. 1d). Curve-fitting of this complex structure reveals the presence of a considerable amount of MoO2, most probably present on the sample surface, bulk MoO3 and Mo2O5 in the interphase. It is interesting to note the presence of a shoulder at 531.6 eV beside the more intense line at 530.6 eVof the oxide oxygen in the O 1s energy region. The O 1s line at 531.6 eVis assigned to the OH group(s) formed on the sample surface. Molybdenum dioxide seems to be the major sub-oxide species present in the few monolayers measured by XPS following further exposure of the sample to hydrogen at 673 K for more than 2 h (Fig. 1e). Within the detection limit, 10–15 layers, measured by XPS, bulk MoO3 is still observed as well as Mo2O5 in the interphase. The low-intensity shoulder at 531.4 eV beside the more intense O 1s oxide oxygen at 530.6 eV is still observed (Fig. 2e). Also, the XPS of the VB (Fig. 3e) shows a relatively intense unresolved band, at low BE of O 2p, attributed to the p and s bands of MoO2. The structure of these two bands is better resolved in the UPS spectrum (Fig. 4e). It is apparent that the relative MoO2 concentration on the sample surface
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formation of MoO2, with its metallic character, on the sample surface in the reduction process of MoO3 enables to dissociate hydrogen into H atoms. The polar bonding of H to surface O atoms leads to the formation of Bro¨ nsted acidic Mo–OH group(s) on the sample surface. Considering the maximum number of oxygen atoms bonded to one Mo surface atom in MoO2, we expect that the maximum number x of H atoms, per one Mo atom, present on the sample surface in the form of Bro¨ nsted Mo–OH does not exceed 2. As a result, a bifunctional phase having a stoichiometry of MoO2(Hx)ac is formed on the surface of bulk MoO2 produced by hydrogen reduction of MoO3. The term (Hx)ac represents the relative number (x 2) of acidic (ac) groups per Mo atom present on the sample surface.
4. Catalytic results and discussion In this work, we report the catalytic activity of MoO2(Hx)ac phase for the reaction of MCH at temperatures up to 653 K. A comparative study with Pt–Ir/ZrO2–SO24 is carried out in order to elucidate the nature and strength of the active site(s) of the MoO2(Hx)ac phase. The possibility of dimethyl and ethyl-cyclopentane as intermediate reaction products has been explored by studying n-heptane reactant using both catalysts. Fig. 4. The UPS of the s and p bands of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) as received; (b) 573 K; (c) 623 K; (d) 653 K; (e) 673 K.
reached a stable state as could be observed from the relative intensities of the two p and s bands in comparison to O 2p following sample reductions at 653 and 673 K (Fig. 5b). The association of the few surface monolayers of MoO2 with the presence of OH group(s) on the outermost one, measured at 531.6 eV O 1s BE could be rationalized as follows. The
Fig. 5. The UPS of the valence band energy region of bulk MoO3 reduced in situ by hydrogen at consecutive temperatures each for 2 h: (a) 623 K; (b) 653 K; (c) 673 K.
4.1. Methylcyclohexane (MCH) reactant An active MoO2(Hx)ac phase has been prepared by exposing a fresh sample of MoO3 to hydrogen at 653 K for 12 h. On such active phase, the initial catalytic activity of MCH was observed at 493 K with a conversion of 1.2% and a selectivity of 95% in isomerization (DMCP and EtCP products). An increase in the reaction temperature results in relative increase in conversion to reach 11% at 553 K while the selectivity in isomerization products remains high. A considerable increase in the dehydrogenation of MCH to toluene takes place at reaction temperature of 573 K. It is apparent from experimental results that isomerization process (Scheme 1, pathway 1) is favored at low reaction temperature over the dehydrogenation process (Scheme 1, pathway 2). In case of MCH, such isomerization process usually named as ring shortening takes place on acidic function. It is clear that the Bro¨ nsted acidic functions present in the MoO2(Hx)ac phase are sufficient to perform such catalytic process. It is relevant to mention that we have tested this catalytic reaction on unreduced MoO3 and partially reduced MoO3 below 573 K, instead of 653 K, which produced an intermediate Mo2O5 state as observed by XPS. It seems that the Lewis and Bro¨ nsted acidities in these states are not sufficient to isomerize MCH. Moreover, on this active phase MoO2(Hx)ac a conversion of 9% and a selectivity of more than 82% to toluene formation were obtained at reaction temperature of 653 K. Two distinct reaction pathways of MCH on MoO2(Hx)ac take place in
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Scheme 1. Different reaction pathways of MCH and n-heptane on MoO2(Hx)ac and Pt–Ir/ZrO2–SO24 catalysts.
function of the reaction temperature as it can be noticed in Fig. 6. From MCH, the apparent activation energy (Ea) for MCH to toluene is higher than the Ea for the reaction MCH to DMCP. From Fig. 7, we can observe that isomerization product distributions seem to follow a certain trend in which 1,3-DMCP is dominant except at 493 K at which EtCP constitutes 32%. Similar reaction behavior has been observed in the case of MCH on Pt–Ir/ZrO2–SO24 system. A conversion of 16% and a selectivity of 100% in DMCP and EtCP isomerization products were obtained at reaction temperature of 493 K (Table 1). Following reactions up to 573 K, an increase in conversion is observed while the selectivity in isomerization remains relatively high regardless of reaction temperature increase. Beyond this reaction temperature, the dehydrogenation process is favored. It is clear from these results that both MoO2(Hx)ac and Pt–Ir/ZrO2–SO24 have comparable behaviors concerning isomerization and dehydrogenation reactions of MCH. No isomerization reaction takes place on ZrO2–SO24 alone which may indicate that the metallic function plays an active role in the isomerization process of MCH.
Fig. 6. Conversion and selectivity of methylcyclohexane on MoO2(Hx)ac in function of reaction temperature.
Fig. 7. Products distribution of methylcyclohexane on MoO2(Hx)ac in function of reaction temperature.
4.2. n-Heptane reactant Isomerization of n-heptane could take place via a cyclic mechanism having only 1,2-dimethylcyclopentane and ethylcyclopentane as intermediate compounds (Scheme 1, pathway 3). Such catalytic process usually takes place on a monofunctional metallic catalysts such as Pt deposited on alumina or silica [6]. The final products in this case are 3methylhexane (3MH), 1,2-DMCP, EtCP and toluene Table 1 Conversion and selectivity of MCH on Pt–Ir/ZrO2–SO24 reaction temperatures Temperature (K) Conversion (%) S isomers (%) S toluene (%) S cracking (%)
493 16.39 99.54 0.46 0.0
523 19.76 95.47 2.15 2.38
Distribution of isomerization products (%) M2H – 0.13 1,1-DMCP 1.19 2.17 1,3-cDMCP 37.49 34.43 1,3-tDMCP 30.88 27.70 1,2-DMCP 19.24 19.90 nC7 – – EtCP 10.74 11.14 Tol 0.46 2.15 Distribution of cracking products C2 – C3 – iC4 – C4 – iC5 – C5 – 2,3-DMB – M2P – M3P – MCP – Bz – cc6 –
(%) – 0.27 0.87 0.08 0.36 – 0.05 0.17 – – 0.57 –
at different
553 22.87 85.39 7.38 7.23
573 29.01 70.56 20.11 9.33
593 28.70 55.93 25.26 18.82
0.36 3.41 29.13 22.90 18.58 – 11.01 7.38
0.25 4.01 21.59 17.29 16.79 0.33 10.31 20.11
0.36 3.44 17.05 13.29 12.81 0.30 8.68 25.26
– 1.88 2.40 0.28 0.77 0.11 0.13 0.31 0.11 – 1.25 –
– 2.56 3.66 0.47 0.73 0.05 0.19 0.35 0.17 – 1.15 –
1.98 6.77 4.66 0.99 1.07 0.46 0.23 0.39 0.17 0.21 1.76 0.12
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(Scheme 1, pathway 4). In the case of bifunctional mechanism, isomerization of n-heptane follows a sequence of catalytic processes involving dehydrogenation and hydrogenation steps as well as isomerization. In this case, 2MH and 3-MH constitute the major products. The introduction of n-heptane on MoO2(Hx)ac at 523 K results in a conversion of 24% and a selectivity of 88% in isomerization products, i.e. 2MH and 3MH (Scheme 1, pathway 5). The ratio of these two products is at the equilibrium ratio of 2MH/3MH = 1.2 (Table 2). An increase in the reaction temperature to 573 K results in considerable increase in conversion to 52% while the selectivity in isomerization decreases to 49% in favor of hydrocracking products, mainly C3, iC4 and C4. In the case of Pt–Ir/ZrO2– SO24 catalyst, a different reaction mechanism seems to take place. At 493 K, a conversion of 26% and a selectivity of only 28% in 2MH and 3MH isomerization products were obtained. Central C–C bond scission (Scheme 1, pathway 6) seems to be the dominant reaction which takes place on this Pt–Ir catalysts leading to the formation of C3 and C4 compounds (Table 2). Further increase in the reaction temperature to 523 K increases the conversion to 38% with a selectivity of 78% in C3–C4 hydrocracking products. These results show that in the case of n-heptane, we have two parallel pathways: one giving isomers with a rate constant k1, the second yield cracked products with a rate constant k2. In the case of partially reduced MoO3, it seems that k1 is higher than k2. In other words, a selective dehydrogenation process to an olefin as a first step in the isomerization of nheptane via the bifunctional mechanism seems to be favored. In the case of the Pt–Ir catalysts, k2 > k1 leading to central
Table 2 Conversion and selectivity of n-heptane on MoO2(Hx)ac and Pt–Ir/ZrO2– SO24 at different reaction temperatures Catalyst
MoO2(Hx)ac
Temperature (K) Conversion (%) S isomers (%) S cracking (%) 2MH/3MH C3/C4
523 24.3 88.2 11.8 1.2 1.1
Pt–Ir/ZrO2–SO24 573 51.5 49.3 50.7 1.1 1.1
493 25.7 27.9 72.1 0.8 1.0
523 37.6 21.6 78.4 0.9 1.1
Distribution of isomerization products (%) 2,2-DMP 2.0 1.3 2,4-DMP 5.7 3.5 223TMB 0 0.3 3,3-DMP 0.9 0.8 2MH 37.8 19.7 2,3-DMP 6.2 4.4 3MH 32.2 17.4 Et-Pent 3.4 2.0 Toluene 0 0
1.4 2.0 0 0 9.5 2.8 11.2 1.1 0
0 1 0 0 8.1 2.1 9 1 0
Distribution of cracking products (%) C3 6.2 iC4 5.0 C4 0.6 iC5 0
35.8 33.5 0.9 0
40.5 37 1 0
25.9 19.7 4.4 0.76
C3–C4 bond scission. Such process could be attributed to a more pronounced metallic character of the Pt–Ir system. These preliminary catalytic results clearly indicate that ring opening of MCH to alkanes does not take place on a bifunctional or mono-metallic functional catalysts at atmospheric pressure regardless of the reaction temperature. Alternative experimental conditions such as an increase in pressure are under consideration. The experimental catalytic results discussed above clearly show that isomerization and dehydrogenation of MCH on MoO2(Hx)ac phase confirms the presence of both acidic and metallic functions in this system. Both functions (bifunctional) operate sequentially in the case of hydroisomerization of n-heptane. The presence of an oxicarbide MoOxCy species in this case is excluded on the basis that MoO3 is activated under atmospheric hydrogen at 653 K. The first contact of this partially reduced phase with MCH was at 493 K leading to the formation of 95% of MCH isomerization products (DMCP and EtCP). There are no cracking products (C1, C2) in this reaction, and it is not clear how carbon could be inserted in the molybdenum sub-oxide species in order to activate it via the formation of an oxicarbide species. Similar arguments apply to the exclusion of a carbide species formation, such as Mo2C for molybdenum and WC for tungsten, at these experimental conditions, as postulated by Ribeiro et al. [9,10] for the interpretation of the isomerization of alkanes via the bifunctional mechanism. Just to mention that partially reduced MoO3 and WO3 show comparable catalytic behavior for the isomerization of an alkane at a given reaction temperature [11,12]. Therefore, the formation of a Mo carbide, having metallic character, under the experimental conditions used in this work is excluded. The metallic function in our catalytic system is completely due the conjugated p electrons in MoO2 or in WO2 as characterized by XPS-UPS techniques. On the other hand, the acidic function is mainly of a Bro¨ nsted Mo–OH type characterized by the O 1s spectral line at 531.6 eV and verified by the dehydration of isopropanol [11,12].
5. Conclusion In situ characterization by XPS-UPS surface techniques of the reduction of bulk MoO3 at temperatures between 623 and 673 K clearly demonstrate the formation of a bifunctional MoO2(Hx)ac phase on the sample surface. Catalytic reactions of MCH on this system reveal the presence of two reaction mechanisms depending on the reaction temperature. Cyclic isomerization of MCH to DMP and Et-CP takes place at temperatures between 493 and 573 K. On the other hand, dehydrogenation of MCH to toluene takes place at temperatures between 573 and 653 K. A conversion of 97% and a selectivity of 82% to toluene were obtained. Similar catalytic behavior was obtained using Pt–Ir/ZrO2–SO24 catalyst. Hydroisomerization of n-heptane on partially
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reduced MoO3 follows the pathway of a bifunctional (dehydrogenation/carbenium ion) mechanism with no intermediate cyclic compounds formation.
Acknowledgements H. Al-Kandari and F. Al-Korafi would like to acknowledge the support of Kuwait University through research grant No. SC04/004. ANALAB and SAF Grant No. GS01/01.
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