Energy Fuels 2009, 23, 5737–5742 Published on Web 10/09/2009
: DOI:10.1021/ef900617d
Molybdenum-Based Catalysts for Upgrading Light Naphtha Linear Hydrocarbon Compounds H. Al-Kandari, S. Al-Kandari, F. Al-Kharafi, and A. Katrib* Kuwait University, Department of Chemistry, P.O. Box 5969 Safat, 13060, Kuwait Received June 21, 2009. Revised Manuscript Received September 23, 2009
Controlled reduction by hydrogen of an equivalent of 5 monolayers of MoO3 deposited on TiO2 enabled the reduction of the upper 2-3 monolayers of MoO3 to MoO2 with a bifunctional MoO2-x(OH)y phase on the outermost surface layer. Hydroisomerization reactions of linear C5-C7 hydrocarbons, present either as a pure phase or as a mixture, on this molybdenum bifunctional phase produce branched species of relatively high octane numbers, as compared to reactant molecules. Bench-scale catalytic experiments that were comparable to industrial conditions were conducted using 15 g of catalyst under a hydrogen pressure of 5 bar, a hydrogen flow rate of 30 SLPH, a liquid hourly space velocity (LHSV) of 0.8 h-1, and a reaction temperature of 623 K. Time-on-stream experiments for several days showed no changes in neither the conversion nor the isomerization selectivity. The stability and resistance of the catalytic system toward poisoning by hydrocarbon species, water, and sulfur- and nitrogen-containing compounds is attributed to the moderate strength of the (metal-acid) functions and to the fact that individual Mo atoms are present in alignment positions placed along the c-axis of the deformed rutile structure of MoO2 phase. This particular surface configuration enabled one to perform hydroisomerization reactions of n-heptane under the aforementioned experimental conditions, in contrast to platinum-based catalysts, in which hydrocracking reactions of n-heptane were observed. Possible replacement of the platinum-based catalysts by the bifunctional MoO2-x(OH)y catalyst at the industrial level is under consideration. dehydrogenation of saturated hydrocarbon molecules and the hydrogenation of the resultant olefin isomers are performed by the moderate metallic function of the dispersed small platinum particles. On the other hand, isomerization of the formed olefin from the dehydrogenation step is performed by the acidic function of chlorinated alumina or zeolite via a carbenium ion mechanism. Several problems are encountered in relation to the use of platinum-based catalysts. Platinum is rare and expensive. It is very sensitive to poisoning by trace concentrations of sulfur and water present in light naphtha. Moreover, toxic benzene is formed as a by-product of n-hexane hydroisomerization. This is due, in part, to catalyst preparation as well as sintering problems in which different platinum particle sizes are formed as a result of the mobility of Pt atoms under the effects of reaction temperature and pressure. Moreover, different catalytic reactions occur, as a function of reaction temperature. For instance, high isomerization selectivity of n-pentane was observed at 623 K, while cracking reactions of n-heptane occur at this reaction temperature. To overcome these deficiencies, the replacement of platinum by other catalytic system(s) becomes an important objective, from scientific as well as industrial points of view. Recently, we were able to introduce a bifunctional MO2-x(OH)y phase (where M = Mo, W)12-16 that has similar
1. Introduction Saturated linear C5-C7 alkanes are present in relatively large concentration in light naphtha. These compounds have low research octane numbers (RONs): 61.7 for n-pentane, 24.8 for n-hexane, and 0 for n-heptane. The conversion of these compounds to monobranched and multibranched molecules of much higher octane numbers1 is essential to include it into the gasoline pool. These chemical transformations are performed using catalysts, in terms of hydroisomerization processes. The most commonly used catalysts, in this respect, consist of finely dispersed platinum particles deposited on chlorinated alumina or zeolites.2-10 The addition of another metal such as iridium or rhenium is also employed. Generally, the mechanism by which platinum-based catalysts transform linear C5-C7 to branched molecules is rationalized in terms of a bifunctional mechanism.11 In this catalytic process, the
*Author to whom correspondence should be addressed. Tel.: þ965 4985582. Fax: þ965 4816482. E-mail address: katrib@chimie. u-strasbg.fr. (1) Ghosh, P; Hickey, K. J.; Jaffe, S. B. Ind. Eng. Chem. Res. 2006, 45, 337. (2) Martino, G. Conversion Processes; Editions Technip: Paris, 2001; p 101. (3) Bond, G. C.; Maire, G.; F. Garin, F. Appl. Catal. 1988, 41, 313. (4) Pope, T. D.; Kriz, J. F.; Stanciulescu, M.; Monnier Appl. Catal., A 2002, 233, 45. (5) Chica, A.; Corma, A. J. Catal. 1999, 187, 167. (6) Loften, T.; Blekkan, E. A. Appl. Catal., A 2006, 299, 250 (and references therein). (7) Dmirci, U. B.; Garin, F. Catal. Lett. 2006, 76, 45. (8) Hino, M.; Arata, K. Catal. Lett. 1995, 30, 25. (9) Kumra, T. Catal. Today. 2003, 81, 57. (10) Travers, C. Conversion Processes; Editions Technip: Paris, 2001; pp 229-256. (11) Gault, F. Adv. Catal. 1980, 30, 1. r 2009 American Chemical Society
(12) Katrib, A.; Leflaive, P.; Hilaire, L.; Maire, G. Catal. Lett. 1996, 38, 95. (13) Katrib, A.; May, D.; Maire, G. Catal. Today. 2001, 65, 179. (14) Al-Kandari, S.; Al-Kandari, H.; Al-Kharafi, F.; Katrib, A. Appl. Catal., A 2008, 341, 160. (15) Al-Kandari, H.; Al-Kharafi, F.; Katrib, A. Catal. Commun. 2008, 9, 847. (16) Al-Kandari, H.; Al-Kharafi, F.; Katrib, A. J. Mol. Catal. A 2008, 287, 128.
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catalytic properties to those of platinum-based catalysts. In the case of molybdenum, this phase is obtained following in situ reduction of MoO3 by hydrogen at temperatures of 623-673 K. The reduction of MoO3 to MoO2 results in the formation of π and σ bonds between each two adjacent Mo atoms placed along the c-axis of the deformed rutile structure of MoO2.16 Delocalization of these π electrons produces a metallic character in the MoO2 state. This metallic character could be observed in the form of the density-of-states (DOS) at the Fermi level, measured by ultraviolet photoelectron spectroscopy (UPS) at 0.4 eV.14,17 The presence of hydrogen as a reducing agent, and as a reactant in the reaction mixture, leads to the formation of H atoms, produced from the dissociation of H2 molecules by MoO2. Consequently, the bonding of H atoms to the surface O atoms results in the formation of Br€ onsted Mo-OH acidic group(s). As a result, a bifunctional (metal-acidic) MO2-x(OH)y phase is formed on the sample surface. The reduction process of MoO3 to MoO2 and the presence of the catalytic active MO2-x(OH)y phase are characterized by X-ray photoelectron spectroscopyultraviolet photoelectron spectroscopy (XPS-UPS), as well as by high-resoultion transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. The advantages of the Mo and W catalytic systems reside in the fact that these metals are much less expensive than platinum and easy to prepare and regenerate in case of deactivation. Also, no corrosive acidic material such as chlorine is used. Moreover, toxic benzene is not formed as a by-product of n-hexane isomerization. An important advantage of this catalyst system consists of its capacity to isomerize n-heptane under conditions similar to those used in the case of nC5-nC6 alkanes, in terms of reaction temperature, liquid hourly space velocity (LHSV), and hydrogen pressure. It is well-known that hydrocracking processes are dominant in the case of n-heptane catalytic reactions over platinum-based catalysts. The particular properties of the bifunctional MO2-x(OH)y (M=Mo, W) catalysts are attributed to the fact that Mo or W atoms are placed along the c-axis in the form of a stable atomic wire. The absence of sintering problems in this case seems to favor n-heptane isomerization, and it avoids the formation of benzene as a by-product of the n-hexane reaction. On the other hand, it was observed that cyclizations of linear C5-C7 molecules are structure-sensitive catalytic processes. It seems that large metallic particles sizes and relatively high reaction temperatures are required to perform these cyclic transformations. We were able to perform such reactions by reducing the molybdenum or tungsten oxides to the metallic state. Moreover, it was observed that the aromatization of cyclic compounds such as methyl cyclohexane to toluene occurs over these large aggregates of metallic molybdenum and tungsten particles.14 In this study, we report the efficiency of the molybdenumbased catalyst in increasing the research octane number (RON) of linear C5-C7 alkanes by converting them to more-valuable branched products, which could be added to the gasoline pool as a clean fuel. The stability of the catalytic system as a function of the time-on-stream exposure to hydrocarbon reactants has been studied. Resistance of the catalyst to poisoning by sulfur, nitrogen, and water has been tested. To use the Mo catalytic system at the industrial level, scale-up experiments from the microreactor to benchscale and
the pilot plant were used. Experiments were performed by changing the mass of the catalyst, the LHSV, the hydrogen pressure, and the hydrogen-to-hydrocarbon ratio. Improvement in the octane number was evaluated on a synthetic mixture of C5-C7 linear compounds, which resembles its natural presence in light naphtha. 2. Experimental Section The equivalent of five monolayers of molybdenum trioxide (MoO3) were deposited on titanium dioxide (TiO2) using ammonium heptamolybdate ((NH4)6Mo7O24 3 4H2O, 99.9%) that was supplied by STREM Chemicals. The TiO2 material is Degussa P-25 (25% rutile) with a pore volume of 0.5 cm3/g and a BET surface area of 50 ( 5 m2/g. Supported catalysts are prepared by impregnating the appropriate amount of molybdenum in ammonium heptamolybdate salt, following the method described by Pines et al.18 The exact amount of the molybdenum (0.3 g of salt per gram of the support) is dissolved in distilled water followed by the impregnation procedure. The excess water has been eliminated by evaporation; then, the catalyst is dried at 383 K for 8 h and calcined at 773 K for 12 h. The catalytic experiments were performed using different masses of catalyst (500 mg and 15 g) in fixed-bed reactors. The prepared MoO3/TiO2 sample is exposed to hydrogen at 623 K for 12 h for complete conversion of the sample surface to MoO2. Additional exposure time of the sample to hydrogen did not produce any modification in the Mo oxidation state or surface structure. Scale-up measurements were performed using a larger amount of catalyst. Catalytic tests for nC5-nC7 isomerization were conducted in continuous-flow time-on-stream mode under industrial conditions, using a commercial bench-scale catalytic unit (Xytel of India) that was equipped with a stainless steel reactor (with a volume of 235 cm3). The catalyst sample is conditioned by exposing the calcined MoO3/TiO2 to a hydrogen flow of 30 SLPH at a temperature of 623 K for 12 h. In both reactors, the catalytic products were analyzed by online gas chromatography using a 100-m column (Petrocol-DH) and a flame ionization detector. Catalytic experiments were repeated several times to ensure the reproducibility of the results. Optimization of the catalytic isomerization experiments for the nC5-nC7 compounds were obtained under the following conditions: hydrogen pressure, 5 bar; space velocity (LHSV), 0.8 h-1; hydrogen flow rate, 30 SLPH; and reaction temperature, 623 K. Characterization of the samples was performed using a combination of XPS-UPS techniques. Both facilities are present in a VG Scientific Model ESCALAB-200 spectrometer. The XPS radiation source was Mg KR operating at a power of 300 W (15 kV, 20 mA). UPS He(I) resonance 584 A˚ radiation of 21.217 eV was applied for the VB energy region measurements. Vacuum in the analysis chamber was 6 h at 653 K results in the presence of a complex structure in the Mo 3d energy region. Curvefitting enabled determination of the presence of the three MoO3, Mo2O5, and MoO2 states (see Figure 1d). The relative intensities of these spectral lines did not change after extended exposure of the sample to hydrogen at 653 K. Apparently, this combination of multistructure states is stable at this reduction temperature. UPS analysis of this sample reveals the presence of two low-intensity spectral lines at 0.4 and 1.4 eV (see Figure 1f), characteristics of the π and σ bands of the MoO2-phase structure.16 The XPS of the O 1s energy region shows the presence of a shoulder at ∼531.6 eV, in addition to the more-intense line at 530.6 eV of the oxide oxygen (see Figure 1e). Taking into consideration that the analysis in the UPS technique is limited to less than four monolayers from the surface, it is concluded that the sample surface is composed of the MoO2 phase. As a result, Mo2O5 is present in the interphase. The O 1s line at 531.6 eV is assigned to the OH group(s) that formed on the sample surface. The formation of Br€ onsted Mo-OH acid groups is attributed to a sequence of processes in which hydrogen molecules, which are present as reducing agents, were dissociated to active H atoms by the metallic character of the delocalized π electrons present on the sample surface. Chemical bonding of H atoms with surface O atoms of MoO2 results in the formation of the Mo-OH groups. As a result, the simultaneous presence of both metallic and acidic functions on the sample surface constitutes a bifunctional MO2-x(OH)y phase. It is important to emphasize at this point that the reduction of MoO3 by hydrogen at 653 K for more than 6 h produces a combination of three distinct phases with the following structures from bulk to surface:
of this phase structure, present in a stable state, on the sample surface is based on both spectroscopic and catalytic measurements. As mentioned in the Experimental Section, the initial molybdenum compound, ammonium heptamolybdate, has been calcined at 773 K for 12 h to convert all the Mo atoms to MoO3. The presence of this state is verified by in situ surface XPS-UPS measurements. The reduction of molybdenum trioxide by hydrogen to different lower-valency oxidation states, as a function of temperature, is studied using in situ XPS-UPS techniques. In parallel, catalytic activity evolution, as a function of the surface Mo structure, under similar experimental conditions, is monitored. Consequently, a correlation between catalytic activity and surface electronic structure will be established. Based on the aforementioned results, optimization of the catalyst performances will be established. The stability of the surface structure will be verified following extended reduction time at a given temperature. In the catalytic measurements, the presence of a stable surface structure will be monitored by the stability of the catalysts performances, in terms of conversion, isomerization selectivity, and products distribution. In the following, we present the XPS-UPS characterization results after the calcination process of the catalyst and after the formation of the stable bifunctional MO2-x(OH)y phase. 3.1. Characterization by XPS-UPS and HRTEM. In the following, we present the XPS-UPS spectra of the calcined Mo phase deposited on TiO2, prepared following the previously mentioned procedure. Also, the spectra of the stable structure obtained either by a gradual reduction process by hydrogen, as a function of temperature up to 653 K, or by continuous exposure of a fresh sample to hydrogen at 653 K for 12 h. Both methods of reduction produce the same stable surface structure, in which maximum conversion and isomerization selectivity of nC5-nC7 alkanes were obtained. Identification of the different Mo oxidation states by XPS was based on the binding energies of the Mo 3d3/2,5/2 spin-orbit components. These energies are 235.85 and 232.65 eV for MoO3, 234.9 and 231.7 eV for Mo2O5, 232.3 and 229.1 eV for MoO2, and 230.85 and 227.7 eV for molybendum metal (Mo(0)). In the case of the calcined sample, two well-defined spectral lines in the Mo 3d energy region at 232.8 and 236.0 eV, which are characteristic of MoO3, are observed (see Figure 1a). The O 1s energy region shows the presence of one symmetrical line at 530.4 eV
MoO3 f Mo2 O5 f MoO2 ½MO2 -x ðOHÞy This well-defined phase structure is in opposition to the undefined MoOx structure deduced from the calculation of O2 consumed in the reoxidation of the reduced Mo states to 5739
Energy Fuels 2009, 23, 5737–5742
: DOI:10.1021/ef900617d
Al-Kandari et al. Scheme 1. Bifunctional Mechanism for the Isomerization of n-Pentane
(see Scheme 1). This isomerization process is traditionally performed by a platinum-based bifunctional catalyst. Introduction of n-pentane at 623 K on the bifunctional MoO2-x(OH)y phase deposited on TiO2 at 623 K results in 58.5% conversion and 89.4% isomerization selectivity to isopentane. These catalytic results are comparable to those reported in the literature using platinum-based catalysts.20,21 The stability of the catalyst active phase toward n-pentane isomerization and conversion has been studied as a function of the time-on-stream for more than 94 h. We did not observe any changes in either the conversion or the isomerization selectivity. 3.2.2. n-Hexane. Hydroisomerization reactions of n-hexane produce monobranched and dibranched isomers with higher RON values, compared to the parent molecule (24.8): 2-methylpentane (abbreviated hereafter as 2MP) (73.4), 3-methylpentane (abbreviated hereafter as 3MP) (74.5), 2,3-dimethylbutane (abbreviated hereafter as 2,3 DMB) (100.5), and 2,2-dimethylbutane (abbreviated hereafter as 2,2 DMB) (91.8). These catalytic reactions were performed by a bifunctional catalyst in a manner similar to the n-pentane isomerization reaction. The introduction of n-hexane on the MoO2-x(OH)y/TiO2 catalyst at LHSV = 0.8 h-1 and a reaction temperature of 623 K results in a conversion of 71.6% and an isomerization selectivity of 91.3%. The isomers distribution consists of 2 MP (45.8%), 3 MP (31.9%), 2,2 DMB (5.1%), and 2.3 DMB (8.5%). Isomerization products are present in thermodynamic equilibrium, as can be concluded from the 2MP:3MP ratio at 1.4. Low concentrations of isopentane (1.8%) and isobutane (0.4%) were also obtained. Chemical transformation of n-hexane to its aforementioned isomers results in a considerable increase in RON value, from 24.8 for n-hexane to 65.7 for the monobranched and dibranched products. The relative concentrations of the different isomers are obtained using the Mo catalyst are comparable to those obtained using the most efficient platinum-based catalysts.6 Interestingly, no benzene, as a by-product, was formed in this catalytic process, compared to platinum catalysts. This could be attributed to the fact that the catalytic active site(s) consists of the Mo atoms of mild metallic character, which are placed in arrays along the c-axis of the deformed rutile structure of the MoO2 phase. This atomic array structure is not suitable for the cyclization of n-hexane to cyclohexane, followed by its aromatization to benzene. In fact, cyclization and aromatization of n-hexane were observed on relatively large aggregates of molybdenum metal present on the sample surface. Such structure is obtained following extended reduction of the molybdenum catalyst by hydrogen at 873 K for several hours.22 Based on this observation, it is postulated that benzene formation, as a by-product of the
Figure 2. HRTEM micrograph of the equivalent of five monolayers of MoO3/TiO2 after its reduction by hydrogen at 653 K for 12 h.
MoO3 as proposed by Matsuda et al.19 The XPS of the Ti 2p binding energy (BE) did not change following sample reduction by hydrogen at 653 K. The use of TiO2 in this case is limited to provide a convenient support of relatively large surface area with no strong electronic interaction with the Mo bifunctional phase. HRTEM analysis of a partially reduced MoO3/TiO2 sample (see Figure 2) clearly shows the alignment of Mo atoms along the c-axis of the deformed rutile structure, with alternate Mo-Mo bond lengths of 2.5 and 3.0 A˚, which are characteristic of the MoO2 phase.15 3.2. Catalytic Results. In the following investigation, we will present the catalytic performances of MoO2-x(OH)y phase for each of the nC5-nC7 compounds, performed under similar experimental conditions. Also, a synthetic mixture of nC5-nC7 compounds is prepared by respecting its natural relative concentrations in light naphtha. The catalytic reactions and the overall modification of the RON will be reported. 3.2.1. n-Pentane. The transformation of n-pentane to isopentane is an important catalytic process. The relatively high octane number of isopentane (92.3), as compared to that for n-pentane (61.7), enables one to enhance the octane number of clean gasoline. To convert n-pentane to isopentane, concerted sequences of catalytic actions occur. The first process consists of the dehydrogenation of n-pentane to 2-pentene (see Scheme 1), which is performed by a metallic function, followed by the isomerization of the olefin via a carbenium ion mechanism, which is assured by an acidic function. The last step consists of hydrogenation of the olefin isomer by the metallic function to produce isopentane
(20) Essayem, N; Ben Taarit, Y; Feche, C; Gayraud, P. Y.; Sapaly, G; Naccache, C. J. Catal. 2003, 219, 97. (21) Brito, A; Garcia, F. J.; Alvarez-Galvan, M. C.; Bourges, M. E; Diaz, C; de la Pena O’Shea, V. A. Catal. Commun. 2007, 8, 2081. (22) Al-Kharafi, F; Al-Kandari, H.; Katrib, A. Catal. Lett. 2008, 123, 269.
(19) Matsuda, T.; Ohno, T.; Hiramtsu, Y.; Li, Z.; Sakagami, H.; Takahashi, N. Appl. Catal., A 2009, 362, 40.
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n-hexane isomerization reaction(s) on platinum-based catalysts, is due, in part, to sintering problems. The stability of the catalytic active phase, as a function of the time-on-stream exposure to the n-hexane reactant, has been tested for more than 50 days. These results indicate that the catalyst electronic structure is stable as a function of experiment time and resists poisoning by carbonaceous species. 3.2.3. n-Heptane. The transformation of n-heptane, which has a RON value of zero, to different branched isomers of relatively high octane numbers is certainly beneficial, in terms of gasoline pool efficiency, by improving the octane number as well as meeting environmental requirements. However, hydrocracking reactions of n-heptane are dominant on platinum-based catalysts. Most probably, this is due to sintering problems. As a result, different modifications of these platinum catalysts, such as the addition of cesium or zinc, were considered.23,24 Interestingly, the relative stability of the Mo atoms along the c-axis in the MoO2-x(OH)y catalyst phase seems to favor the isomerization reactions of n-heptane. The catalytic reactions of n-heptane on the bifunctional molybdenum phase, as a function of reaction temperature, shows a certain trend, in which the conversion increases from 6.4% at 553 K to 85.1% at 623 K, while the isomerization selectivity decreases from 93.5% to 72.9% at these temperatures. In all the experiments conducted at different temperatures, the 2MH:3MH ratio remains constant at 0.95. This is consistent with the carbenium ion mechanism reported by Mahos et al.,25 in which the 2MH:3MH ratio is equal to 1, compared to 2MH:3MH = 0.5 in the protonated cyclopropane mechanism. The relatively high conversion and isomerization selectivity of n-heptane on the molybdenum catalyst at 623 K constitutes an important advantage of this system, compared to the platinum-based catalysts, especially that good performances of the Mo catalyst for nC5-nC6 were realized at this reaction temperature. Therefore, it is possible to conduct catalytic reactions on a mixture of nC5-nC7 alkane compounds using this molybdenum catalyst at 623 K. This is not possible in the case of platinum catalysts, because the n-heptane reactant in the mixture will undergo extensive hydrogenolysis reactions. The stability of the catalytic active MoO2-x(OH)y phase toward n-heptane at 623 K and LHSV = 0.8 h-1 has been monitored by timeon-stream exposure of the catalyst to the reactant for more than 70 h. We observed no changes in neither the conversion nor the isomerization selectivity. Interstingly, a substantial increase in the RON value from 0 for n-heptane to 64.1 has been obtained. Hydroisomerization reactions, as previously reported, were performed using only molybdenum oxide that had been deposited on TiO2. The catalytic active phase consists of the bifunctional MoO2-x(OH)y phase present on the sample surface. It is very important to note that TiO2 has no catalytic activity toward nC5-nC7 alkanes. It only provides a large surface area for the active molybdenum oxide phase and mechanical strength to the catalyst. We strongly emphasize
Table 1. Effect of Time on Selectivity, Activity, Products Distribution, and Total RON on Partially Reduced 15 cm3 (MoO3/TiO2) Five Layers Equivalent of a Synthetic Mixture of nC5-nC8 Linear Alkanes at 623 K, 5 bar, and 30 SLPH Hydrogen Flow for 12 ha products C1-C4 iC4 iC5 C4 þ C5 C5 22DMB 23DMB 2MP 3MP C6 MecycloC5 DMP þ 3EtC5 2MeH 3MeH C7 C8 total RON
as-received
23.7
64.9
6.2 5.2 30.2
5h
26 h
30 h
48 h
72 h
74 h
8.8 3.3 8.7 19.7 17.0 1.2 3.3 17.9 11.9 18.9 0.1 2.1 2.3 2.7 1.8 0 66.8
6.4 2.2 8.8 20.5 18.3 1.30 3.4 19.2 12.9 21.5 0.1 1.3 1.5 1.8 1.4 0.0 65.0
6.9 2.4 9.3 21.8 19.4 1.3 3.4 19 12.7 20.8 0.1 4.0 0.4 0.1 0 0 67.3
5.8 1.8 8.5 19.1 17.1 1.4 3.6 20.1 14.0 22.2 0.2 1.4 1.4 1.6 1.0 0 64.9
7.0 2.1 9.1 23.4 21.0 1.2 3.1 18.4 12.3 21.6 0.1 1.0 1.0 1.2 0.8 0 65.3
6.8 2.0 8.8 22.3 20.0 1.3 3.2 18.7 12.7 21.8 0.1 1.0 1.2 1.4 0.9 0 65.0
a Conditions: reaction temperature, 623 K; pressure, 5 bar; flow rate, 30 SLPH; LHSV = 0.8 h-1.
that the dehydrogenation of the alkane and hydrogenation of produced olefin isomers were performed by the metallic function of the MoO2-x(OH)y phase. In other words, there is no need to use a noble metal such as platinum to perform these dehydrogenation/hydrogenation catalytic reactions as envisaged by Matsuda et al.19 3.2.4. nC5-nC8 Mixture. To evaluate the isomerization catalytic performances of the molybdenum bifunctional catalyst in the case of the simultaneous presence of the above-studied linear hydrocarbon molecules, a synthetic mixture of different linear nC5-nC8 hydrocarbon compounds has been prepared by respecting its natural concentration in Kuwaiti light naphtha: 23.7% n-pentane, 64.9% n-hexane, 6.2% n-heptane, and 5.2% n-octane. Time-onstream experiments were conducted at a reaction temperature of 623 K and LHSV = 0.8 h-1. After 5 h of continuous feedstock passage on the catalyst, the relative concentrations of the reactants decrease considerably, to 1.8% n-heptane, 18.9% n-hexane, and 17% n-pentane, while all n-octane molecules were reacted (see Table 1). The conversions of nC6-nC8 are in agreement with the obtained conversions of individual compounds. However, in the case of n-pentane, the presence of 17% in the products could be from the origin, in part, and from the hydrocracking of heavier nC6-nC8 compounds. Time-on-stream experiments for more than 74 h reveal certain stability of the catalyst performances toward this reaction mixture. Isomerization products 2MP/3MP and 2MH/3MH are present at thermodynamic equilibrium ratios. This implies that catalytic reactions of individual components nC5-nC8 in the mixture were not affected by the presence of the other compounds. Note that the overall catalytic reactions result in a substantial increase in the RON (see Table 1). 3.3. Catalyst Poisoning. Trace concentrations of nitrogen and sulfur compounds, as well as water, are naturally present in light naphtha. Chemical reactions occur between these compounds and platinum, which is commonly used for the isomerization reactions of light alkanes; this is called “catalyst poisoning”. As a result, a decrease in the efficiency and stability of the catalyst is observed. To investigate the resistance of the bifunctional MoO2-x(OH)y phase toward
(23) Snatamaaria, G. E.; Baustita, J. M.; Silva, H.; Munoz, L; Batinam, N. Appl. Catal., A 2002, 231, 117. (24) Sabri, M. A.; Le Van Mao, R.; Martin, M.; Mak, A. W. H. Appl. Catal., A 2001, 241, 229. (25) Mahos, K.; Nakamura, R.; Niiyama. H. In Proceedings of the 7th IZC; Murakami, Y., Iijima, A., Word, J. W., Eds.; Elsevier: Tokyo, 1986; p 973.
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Figure 3. Effect of time on selectivity and activity on partially reduced 15 cm3 (MoO3/TiO2) five layers at 623 K, 5 bar, and 30 SLPH hydrogen flow for 12 h for n-hexane þ 20 ppm thiophene. Conditions: reaction temperature, 623 K; pressure, 5 bar; flow rate, 30 SLPH; and LHSV = 0.8 h-1.
poisoning by these compounds, a systematic study at the microreactor scale has been determined by preparing different n-hexane solutions that contain 20, 60, 120, 200, 300, 500, 700, and 900 ppm of water. Time-on-stream experiments, in terms of conversion, isomerization selectivity, and products distribution, were monitored for 3 h for each concentration. The same experiments were repeated for similar concentrations of thiophene and piperidine compounds. These results, as well as other factors that might affect the catalyst performances, are part of a comprehensive study that is underway. Nevertheless, we report some of the data concerning the catalyst resistance to possible poisoning by trace concentrations of sulfur and nitrogen compounds, as well as benzene, that occurred at the bench-scale catalytic reactor. The catalytic MoO2-x(OH)y active phase shows certain stability and resistance to possible poisoning by these impurities, regardless of its relative concentration in the feedstock. In fact, water is formed in the process of hydrogen reduction of the initial MoO3 to produce the catalytic active phase. The water molecules that are formed seem to desorb without affecting the catalyst structure and activity. In the large-scale feedstock of n-hexane at the bench-scale level, 20 ppm of thiophene was introduced with the n-hexane reactant. As could be observed from Figure 3, the catalyst performances, in terms of conversion and isomerization selectivity, were not affected by continuous catalyst exposure to the n-hexane-thiophene mixture for 25 h. Similar results were obtained in the case of 20 ppm of piperidine and benzene. Possible poisoning effects by thiophene and piperidine have been investigated by adding 10 ppm thiophene and 10 ppm piperidine to a feed mixture of an nC5-nC8 synthetic mixture of alkane molecules, as reported previously. There is no apparent effect of these impurities after 96 h of time-on-stream experiments in a separate study in which molybdenum is reduced to the metallic state (Mo(0)),
following sample reduction at 773 K. Large aggregates and relatively strong metallic function were observed on the sample surface, as revealed from the HRTEM and XPSUPS data.14 In this case, a poisoning effect by carbonaceous species such as benzene, water, and sulfur compounds was observed. As a result, it could be concluded that the surface morphology and electronic structure play in an important role in determining catalyst resistance to poisoning and stability toward continuous time-on-stream performances. 4. Conclusions Controlled reduction of the equivalent of five monolayers of MoO3/TiO2 enabled the production of a bifunctional MoO2-x(OH)y catalyst. Important conversions and isomerization selectivity were obtained at a hydrogen pressure of 5 bar, a liquid hourly space velocity (LHSV) of 0.8 h-1, a hydrogen flow rate of 30 SLPH, and a reaction temperature of 623 K. High conversion (83.2%) and isomerization selectivity (75.8%) of n-heptane were obtained under these industrial experimental conditions. This is opposite to what observed in the case of platinum-based catalysts, in which a hydrocracking of n-heptane occurs. Substantial improvement in the research octane number (RON) from 30.2% to 65.6% occurs following the time-on-stream of a synthetic C5-C8 mixture over the molybdenum catalyst. Moreover, the bifunctional MoO2-x(OH)y catalyst shows strong resistance to poisoning by different concentrations of water, thiophene, and piperidine (in the range of 20-900 ppm). Based on the aforementioned results, it is envisaged to use this molybdenum catalyst as a possible substitute for platinum-based catalysts in different catalytic processes related to the oil industry. Acknowledgment. The support by Kuwait University (through Research Grant No. SC08/06) is gratefully acknowledged.
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