The Role of Zeolite Acidity in Coupled Toluene Hydrogenation and

Jan 11, 2008 - Runs have been carried out in a fixed bed reactor under 250−450 °C and 20−80 bar. A positive effect of HZSM-5 zeolite acidity on ...
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Ind. Eng. Chem. Res. 2008, 47, 665-671

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KINETICS, CATALYSIS, AND REACTION ENGINEERING The Role of Zeolite Acidity in Coupled Toluene Hydrogenation and Ring Opening in One and Two Steps Pedro Castan˜ o,*,† Barbara Pawelec,‡ Andre´ s T. Aguayo,† Ana G. Gayubo,† and Jose M. Arandes† UniVersity of the Basque Country, Department of Chemical Engineering, P.O. Box 644, E-48080 Bilbao, Spain, and Institute of Catalysis and Petrochemistry, CSIC, c/ Marie Curie, 2, Cantoblanco, E-28049 Madrid, Spain

In this work, the effect of HZSM-5 zeolite acidity on hydroconversion of methylcyclohexane and toluene has been studied. These are test reactions for the second step and the single step of aromatic valorization process, respectively, with the aim of obtaining C2+ n-alkanes and isoalkanes. Monofunctional HZSM-5 zeolite catalysts (Si:Al ratio between 15 and 140) have been studied in methylcyclohexane ring opening while bifunctional catalysts (hybrid Pt/Al2O3-HZSM-5, same zeolites) have been used in the hydrocracking of toluene. Runs have been carried out in a fixed bed reactor under 250-450 °C and 20-80 bar. A positive effect of HZSM-5 zeolite acidity on methylcyclohexane conversion and C2+ n-alkane selectivity is evident at certain conditions, whereas the maximum selectivity to isoalkanes requires an intermediate value of acidity. On the basis of the relationship between conversion and the Si:Al ratio of the HZSM-5 zeolite, the hydrogenolytic cracking of methylcyclohexane is proposed as a test reaction to determine the Si:Al ratio. Acidity has a highly favorable effect in the hydrocracking of toluene given that it avoids the thermodynamic restrictions for toluene hydrogenation and enhancing all the cracking steps during methylcyclohexane (MCH) transformation, which increases selectivity to C2+ n-alkanes and isoalkanes. 1. Introduction Catalytic hydrotreatment is the preferred route for valorizing the aromatic excess generated in refineries by the imbalance between production and consumption.1-3 This imbalance is a consequence of increasing the production of aromatics (byproducts), particularly during steam cracking, which is unavoidable for meeting increasing demand for light olefins.1,2 On the other hand, the output of processes using aromatics as raw materials is almost stagnant.3 Furthermore, more restricted environmental regulations drastically limit the incorporation of aromatics in fuels.4-6 Hydrotreatment is a highly versatile process that, depending on its severity, allows for obtaining either isoalkanes for adding to the gasoline pool (mild ring opening, MRO),7,8 or C2+ n-alkanes (severe ring opening, SRO), which are a suitable feed for a steam cracking unit. Basically, the required reactions are (1) primary hydrogenation of aromatics and (2) subsequent endand exocyclic scissions of the naphthenes generated in the first stage.2 The process can be undertaken in one or two reaction steps, and in the former case (hydrocracking) the catalyst needs a metallic function to produce hydrogenation-hydrogenolysis and an acidic function to originate cracking.9 In the two-step reaction strategy, these catalytic functions are used separately in two fixed bed reactors in series.10 The selectivity of the hydrotreatment, in either one or two steps, is conditioned by the ring opening, which is slower and * To whom correspondence should be addressed. Tel.: +34 94 6012511. Fax: +34 94 6013500. E-mail: [email protected]. Pedro is currently working as Postdoctoral Researcher in Delft University of Technology, Department of Catalysis Engineering. † University of the Basque Country. ‡ Institute of Catalysis and Petrochemistry

more complex than the hydrogenation reaction. In the one-step reaction, n-alkane and isoalkane formation occurs mainly over the acidic surface of the bifunctional catalyst.11-15 The HZSM-5 zeolite is considered a suitable acid function because of its moderate acid strength, shape selectivity, and hydrothermal stability. With the aim of optimizing C2+ n-alkanes selectivity and composition for their use as a feed for the steamcracker, several studies had been carried out on the modification of the HZSM-5 zeolite by incorporating Ga or by treating it with tetraethoxysilane (TEOS).16,17 In the one-step reaction process, the performance of the Pd/HZSM-5 zeolite has been improved by substituting Pd by Pt given that the latter accelerates two key steps of the process: the saturation of CdC bonds and the hydrogenolysis of C-C bonds.18-21 The aim of this paper is to study the effect of catalyst acid structure in the hydrotreatment of aromatics by carrying out the process in either one or two steps and by using toluene and methylcyclohexane (MCH, product of toluene hydrogenation) as models of compounds, respectively. Although the cracking of MCH and other naphthenes has been studied in detail in the literature,22-24 there is a gap concerning the study of this process in a reaction medium with high hydrogen pressure. Raichle et al. proved indirectly the importance of acidity when they were studying the effect of the HZSM-5 zeolite Si:Al ratio on the product distribution of methylcyclohexane (MCH) cracking.15 These authors established an optimum value of Si:Al ) 35 to reach a balance between ethane conversion and selectivity and, consequently, an optimum stream to feed into a steam cracker. 2. Experimental Section 2.1. Catalyst Preparation. Monofunctional acid catalysts have been obtained from four HZSM-5 zeolites (Zeolist

10.1021/ie070921j CCC: $40.75 © 2008 American Chemical Society Published on Web 01/11/2008

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International) with different Si:Al ratios (15, 25, 40, and 140) agglomerating those zeolites (25 wt %) with bentonite (30 wt %) and γ-Al2O3 (45 wt %) as inert charge. The acid catalysts have been named as z15, z25, z40, and z140. The presence of inert Al2O3 (calcined at 1000 °C) and bentonite does not change zeolite acidity and, hence, does not affect its intrinsic activity and selectivity but does improve its mechanical resistance, mass and heat transfer, and stability during coke deposition and combustion (catalyst regeneration). The catalysts have subsequently been ground, sieved (0.15 < dp < 0.30 mm), and calcined at 550 °C for 3 h. Bifunctional (metal-acid) catalysts have been obtained by physically mixing a platinum-supported catalyst (Pt/γ-Al2O3, Johnson Matthey), named Pt/A, with the previous HZSM-5 zeolite acid catalysts in the proper quantities to achieve 0.5 wt % Pt/HZSM-5, which has allowed for obtaining the catalysts Pt/A + z15, Pt/A + z25, Pt/A + z40, and Pt/A + z140. Previously, the metal function was crushed and sieved to obtain particle sizes in the 0.05 < dp < 0.10 mm range for optimum Pt-zeolite contact. 2.2. Characterization Techniques. The Brunauer-EmmettTeller (BET) surface area of the catalysts was measured with a Micromeritics ASAP 2010 apparatus. The samples were previously desorbed at 10-3 mmHg total pressure and 150 °C for 8 h. The analysis was carried out at -196 °C in the range of partial pressures from 0.01 to 1. Data on the structure and acidity of the zeolite have been obtained by means of Fourier transform infrared (FTIR) analysis of samples using a Nicolet 740 SX with a SPECAC transmission cell attached. In the former case, the catalysts were pelletized in wafers with KBr (1 wt % of zeolite), whereas the Bro¨nsted to Lewis acid site ratio (B:L) was determined by adsorbing pyridine over the catalyst surface. The latter experimental procedure consisted of a pretreatment at 400 °C in a He flow for 2 h. Subsequently, the wafer sample was cooled to 150 °C, and a background scan was recorded. The sample was then saturated with pyridine and was analyzed. The B:L ratio was computed from the peaks at 1545 cm-1 and 1453 cm-1 corresponding to the vibration modes of pyridine interacting with Bro¨nsted and Lewis acid sites, using the molar extinction coefficients φBro¨nsted ) 1.67 cm µmol-1 and φLewis ) 2.22 cm µmol-1.25 The coupled thermogravimetric-calorimetric measurement of NH3 adsorption was carried out in a Setaram TG-DSC 111 apparatus attached to a Harvard syringe pump and a Balzers Quadstar 422 mass spectrometer to detect the NH3 flow desorbed during temperature-programmed desorption (TPD). Following this methodology, the acidity quantification and qualification can be carried out.26-28 The methodology involves a preliminary heating of the sample to 550 °C in He (20 cm3 min-1 flow rate). After cooling to 150 °C, a constant flow of 50 µL min-1 of NH3 was introduced and thermogravimetric and calorimetric adsorption data were recorded. Once the sample was saturated, the temperature was increased (5 °C min-1) to 550 °C and the amount of NH3 desorbed was recorded in the quadrupole to obtain the TPD profiles. The chemical analysis of the Pt/A catalyst (Pt content) was undertaken in a Perkin-Elmer Optima 3300DV ICP apparatus. The sample was previously dissolved in HF, HCl, and HNO3, was homogenized in a microwave oven, and was diluted in 50 mL of Milli-Q H2O. The metallic dispersion was measured using a Micromeritics ASAP 2010C apparatus using the following experimental procedure: reduction at 350 °C in a H2 flow, desorption of impurities at this temperature for 7 h, and analysis with hydrogen at 35 °C.

Table 1. Properties of the Acid Catalysts Prepared Using Several HZSM-5 Zeolites % Al in zeolite (mol) Sg. BET (m2 gcat-1) micropore area (cm2 gcat-1) micropore volume (cm3 gcat-1) TS (cm-1) TA (cm-1) TA (cm-1) DB (cm-1) B:L1453cm-1 (mol mol-1) total acidity (µmolNH3 gcat-1) acid distribution (µmolNH3 gcat-1) weak; 150-280 °C medium; 280-420 °C strong; 420-550 °C

z15

z25

z40

z140

6.25 220 91.9 0.040

3.85 184 95.4 0.040

2.44 240 51.8 0.028

0.71 223 85.5 0.039

808 1101 1229 547 2.62

808 1103 1228 551 2.22

808 1106 1230 551 1.30

809 1108 1232 551 1.27

175

164

115

73

83 84 8

98 61 5

85 31 0

59 13 0

2.3. Equipment and Reaction Conditions. The reactions of hydroconversion of MCH (on acid catalysts) and of toluene (on bifunctional catalysts) have been carried out in a continuous down flow fixed-bed catalytic reactor under a total pressure of 20-80 bar. The catalyst was diluted in 1 g of CSi (particle size, 0.5 mm; previously calcined at 700 °C for 2 h). The results of each reaction condition have been obtained in a single run. Weight hourly space velocity (WHSV) was kept at 2.8-4 h-1, referring to the mass of pure zeolite, to obtain comparable kinetic results between indirect and direct hydroconversion since hydrogenation over the metallic surface is much faster than cracking over the zeolite surface.12 The temperature range used was in the 250-450 °C range, and H2 pressure was varied from 19 to 79 bar, keeping a constant hydrocarbon pressure of 1 bar at the reactor inlet and using N2 to complete the total pressure if required. Product analysis was carried out online in an HP5890 Series II gas chromatograph provided with an FID detector and a Tracer TRB-1 capillary column (60 m and i.d. 0.2 mm). To assess the complete reduction of the Pt species of bifunctional catalysts, they were activated by heating to a reduction temperature of 400 °C using a H2:N2 mixture of vol ratio ) 1:2 and flow rate ) 90 cm3 min-1 at atmospheric pressure followed by isothermal reduction at this temperature for 2 h. Stabilization of mono- and bifunctional catalysts has been conducted as a standard reaction carried out at 40 bar and 400 °C for 2 h to eliminate the strongest acid sites normally yielding coke and polyaromatics in the HZSM-5 zeolite. These sites suffer fast deactivation and, hence, they have no interest for the catalyst application.29,30 3. Results 3.1. Acidity of the Catalysts. Table 1 summarizes the main properties of monofunctional catalysts. The values of BET surface area and micropore area-volume are usual in this type of agglomerated catalysts and do not follow any trend with the zeolite Al content.31,32 Besides, Table 1 shows the wave numbers of three vibration modes in the 800-1200 cm-1 range corresponding to symmetric tension, asymmetric tension, and double bonds in the structure, which are proven to be a function of Al content in the zeolite.33-35 The Bro¨nsted:Lewis acid site ratio decreases from 2.7 to 1.3 when the Si:Al ratio of the HZSM-5 zeolite increases. The results of total acidity in Table 1, determined by measuring the total amount of chemisorbed NH3 (the adsorption results determined by TG analysis coincide with those determined by TPD), show that this acidity and, consequently, the concentration of acid sites on the surface, increases as the zeolite Al content is increased. Figure 1 shows the results of acid strength distribution of the catalyst measured by monitoring the

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Figure 1. Acid strength distribution of monofunctional catalysts on the basis of HZSM-5 zeolite using different characterization techniques: (a) TG-DSC of NH3 adsorption at 150 °C and (b) NH3-TPD.

Figure 2. Evolution of MCH conversion as a function of the time on stream for the most acidic catalysts z15 and z25. Experimental conditions: (a) 450 °C, WHSV ) 2.8 h-1, P ) 5 bar; (b) 350 °C, WHSV ) 2.8 h-1, P ) 5 bar; (c) 350 °C, WHSV ) 4.0 h-1, P ) 5 bar; (d) 325 °C, WHSV ) 2.8 h-1, P ) 10 bar; (e) 350 °C, WHSV ) 2.8 h-1, P ) 40 bar; (f) 300 °C, WHSV ) 2.8 h-1, P ) 40 bar.

differential adsorption of NH3 by the TG-DSC (differential scanning calorimetry) technique. The acid strength of the catalysts is uniform and decreases as the Si:Al ratio of the corresponding zeolite is increased as has already been observed.36 In Figure 1a, this decrease takes place from 152 kJ (mol of NH3)-1 (for Si:Al ) 15, catalyst z15) to 117 kJ (mol of NH3)-1 (Si:Al ) 140, catalyst z140). The NH3-TPD data lead to the identification of the levels of acid strength corresponding to the desorbed amounts of base along different temperature ranges.37,38 The bimodal distribution in Figure 1b for the catalysts with HZSM-5 zeolites having Si: Al ratios of 15 and 25 had been observed previously for these type of zeolites.39 Table 1 also summarizes the amounts of desorbed NH3 interacting with weak (150-280 °C), medium (280-420 °C), and strong acid sites (420-550 °C). These acidity levels are consistent with the Al content of the HZSM-5 zeolite as is observed in the results presented in Figure 1a. Furthermore, there is a clear increase in strong acid sites for low Si:Al ratio zeolites. The main properties of the metallic function (Pt/Al2O3) used for bifunctional catalyst preparation are Pt content, 0.51 ( 0.02 wt %; BET surface area, 118 m2 g-1; pore volume, 0.26 cm3 g-1; metallic dispersion, 81%; and total acidity, 160 µmol of

NH3 g-1. Metallic dispersion is very high when acidity is reasonably high. In principle, both metallic and acid sites produce endocyclic scission by means of hydrogenolysis and cracking respectively, hence it is required to investigate the extent of ring opening caused by the metallic function of the hybrid catalyst. 3.2. Effect of Acidity on the Ring-Opening Step. The results of catalyst stability are displayed in Figure 2, where the MCH hydroconversion profiles are plotted versus time-on-stream (TOS). The data displayed in Figure 2 correspond to catalysts with stronger acid sites (z15 and z25), which are more prone to deactivation by coke fouling. However, only small deactivation is seen in the first 2 h of reaction for the strongest acid sites (z15). This small deactivation is more significant when cokeprecursor formation is faster, that is, by increasing zeolite acidity, by increasing temperature, or by decreasing hydrogen partial pressure in the reaction medium. Thus, to avoid the masking effect of initial deactivation under certain conditions, the results have been obtained subsequent to 2 h TOS. The presence of hydrogen is responsible for the attenuation of deactivation by coke under certain conditions, since some of its precursors are directly hydrogenated. Furthermore, the formation of coke in methylcyclohexane cracking is lower compared to that in methylnaphthalene or n-heptane cracking.40 The results of MCH conversion for the acid catalysts prepared using different HZSM-5 zeolites are compared in Figure 3 in the 250-450 °C range. It is observed that as temperature and Al content of the zeolite (acidity) are increased, conversion increases significantly as has already been observed.2,15 From the experiments in quasi-differential reactor, at 300 °C, using the different catalysts, reaction rate data have been obtained as

-rMCH )

XFMCH X‚WHSV ) W MMCH

(1)

Figure 4 shows the results of reaction rate (-rMCH) versus alumina content in the HZSM-5 zeolite of the catalysts. The results fit satisfactorily to straight lines whose slope follows a second-order equation with pressure and, consequently, the results in Figure 4 fufill the following expression:

nAl -rMCH ) nAl + nSi (0.13P - 0.014P2)

(2)

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Figure 3. Effect of temperature on methylcyclohexane conversion for catalysts with HZSM-5 zeolites of different Si:Al ratio. WHSV ) 2.8 h-1. PMCH ) 1 bar. P ) 40 bar.

Figure 4. Effect of Al content in the HZSM-5 zeolite of the catalyst on the transformation rate of methylcyclohexane under different pressures. 300 °C; WHSV ) 2.8 h-1. Table 2. Apparent Activation Energies for the Different Monofunctional Acid Catalysts E (kJ mol-1)

z15

z25

z40

z140

153.3 ( 2.2

156.9 ( 1.8

163.5 ( 0.3

157.7 ( 0.1

Equation 2 is useful for determining the Al content of an HZSM-5 zeolite from an experiment of MCH hydrogenolytic cracking carried out in a quasi-differential reactor. The endocyclic cracking of methylcycloalkanes has already been used as test reaction for studying metallic catalysts supported on nonacidic solids41-42 and HY zeolites in the cracking of methylcyclopentane under fluid catalytic cracking (FCC) conditions.43 Nevertheless, this background corresponds to processes without hydrogen in the reaction medium. Table 1 shows that an increase of Al content in the zeolite leads to a nonlinear rise in the total acidity, the acid strength, and the Bro¨nsted:Lewis acid site ratio of the catalyst. These properties contribute to the final activity of the catalyst, but a specific effect of each one on the kinetic behavior of the catalyst cannot be established, which however can be done for the Al content of the zeolite as shown in Figure 4.

Figure 5. Arrhenius plot of methylcyclohexane cracking results obtained using the different monofunctional acid catalysts.

Figure 6. Evolution of the selectivity to C2+ n-alkanes and methane with methylcyclohexane conversion. WHSV ) 2.8 h-1; 40 bar.

By fitting the reaction rate results for the different catalysts to the Arrhenius equation (Figure 5), the apparent activation energies set out in Table 2 have been obtained. The results show that MCH ring opening on zeolite catalysts has an apparent activation energy in the 153-163 kJ mol-1 range, and this value is almost independent of total acidity and of the acid strength level of the zeolite sites idicating that there is no significant change in the reaction network for the studied zeolite catalysts. Figures 6 and 7 show the selectivity of the most interesting products, that is, C2+ n-alkanes, methane, isoalkanes, and aromatics, as a function of conversion for the whole family of HZSM-5 catalysts studied. These graphs allow us to compare the reaction networks for the same conversion regime, whereas the selectivity to each component or lump i has been determined as the quotient between its yield (Yi) and methylcyclohexane conversion (X):

Si )

Yi X

(3)

Both Figures 6 and 7 indicate that the reaction network does not change significantly when the number of acid sites is increased from z15 to z140. The conclusion from these figures is that the selectivity to the main reaction products is only a

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Figure 7. Evolution of the selectivity to isoalkanes and aromatics with methylcyclohexane conversion. WHSV ) 2.8 h-1; 40 bar. Table 3. Results of Methylcyclohexane Conversion, Selectivities to Different Products, and CMR Index for Monofunctional Acid Catalysts with Different HZSM-5 Zeolitesa

XMCH (%) Smethane SC2+ n-alkanes Sethane Spropane Sbutane SC5+ n-alkanes Sisoalkanes Scycloalkanes Saromatics CMR a

z15

z25

z40

z140

99.81 0.67 65.88 1.89 45.54 16.41 2.03 21.69 1.22 10.54 0.21

98.27 0.45 65.10 1.58 42.09 18.23 3.20 23.48 3.45 7.49 0.16

74.46 0.34 57.50 1.25 31.74 18.54 5.97 19.89 14.68 7.31 0.14

16.71 0.23 36.48 0.94 19.97 11.18 4.39 19.69 39.53 2.96 0.13

Reaction conditions: 350 °C; WHSV ) 2.8 h-1; 40 bar.

function of conversion or, in other words, a function of the severity of the reaction, which is consistent with the results presented in Figure 5 and Table 2. Furthermore, according to Figure 6, the maximum selectivity to C2+ n-alkanes is reached at higher conversion regimes where the selectivity to byproducts (methane in Figure 6 and aromatics in Figure 7) in the steam cracker increases to unacceptable levels and, consequently, the optimum conversion is at ca. 95-98%. Furthermore, Figure 7 shows a peak in the selectivity to isoalkanes for a moderate conversion of approximately 90%, since at more severe conditions this lump yields C2+ n-alkanes. The results in Table 3, corresponding to values of conversion, selectivity, and cracking mechanism ratio (CMR) index, for given reaction conditions, are a basis for analyzing the effect of HZSM-5 zeolite acidity on the performance of the catalysts at the following conditions: 350 °C, WHSV ) 2.8 h-1, and 40 bar. This study is true since the reaction network is not affected by the acidity of the zeolite at a given conversion. The CMR index quantifies the relative importance of monomolecular cracking compared to the bimolecular one being calculated as the quotient of the yields of products characteristic to both mechanisms:44

CMR )

Ymethane + Yethane Yisobutane

(4)

The increase in the acidity of the catalyst leads to an increase in MCH conversion (Figure 3) and selectivity to C2+ n-alkanes, particularly of ethane and propane (Table 3). Nevertheless, it

also provokes an increase in the formation of methane and aromatics, which are undesired byproducts. Also, under lower hydrogen concentrations than that used in this study, coke deposition could be significant and is enhanced by an increase in acidity of the catalyst.45 However, the high hydrogen partial pressure used in the present work avoids any coke deposition and, hence, deactivation. Selectivity to cycloalkanes decreases as the capacity for endocyclic cracking is increased, which is associated with an increase in catalyst acidity. It is also evident that an increase in acidity increases the CMR index and, consequently, monomolecular cracking is favored over the bimolecular one. Selectivity to isoalkanes is sensitive to acidity, and Table 3 shows that this selectivity is maximum for z25 catalyst of intermediate acidity, which means that an HZSM-5 zeolite of acid strength slightly lower than 150 kJ (mol of NH3)-1 is advisable when the aim of the process is the production of isoalkanes for their incorporation to the pool of gasoline in the refinery. 3.3. Effect of Acidity on the Hydrocracking of Toluene. The results of toluene and methylcyclohexane (intermediate product) conversion, selectivity to the different product lumps, and the CMR index are compared in Table 4 for the different bifunctional catalysts. It is observed that toluene conversion is almost complete, which is attributable to (1) the high activity of Pt and (2) to the fact that the transformation of MCH obtained by hydrogenation avoids the thermodynamic restrictions of this reversible step. Consequently, the yield and selectivity of the desired product (isoalkanes or C2+ n-alkanes) are a function of the severity of reaction (acidity of the catalyst, temperature, and hydrogen partial pressure) at a given reaction condition. Comparing the results in Table 4 (toluene hydrocracking) with those in Table 3 (MCH cracking under similar reaction conditions except for a small difference in space velocity), a significant involvement of the metallic function of the bifunctional catalyst in the reactions of MCH transformation is noteworthy. Thus, for the same MCH conversion level, the selectivity to isoalkanes is higher because of the isomerizing capacity of the metallic function.46-48 The lower selectivity to aromatics in the single-step process is also noteworthy, which is due to the hydrogenating capacity of the metallic function. Table 4 shows that the selectivity to aromatics decreases as the acidity of the catalyst acid function is increased and becomes insignificant for the zeolite of higher acidity (z15). This result is explained by the synergism of the two functions in the hybrid catalyst, in which the acid function transforms methylcyclohexane, and this avoids thermodynamic restrictions in toluene hydrogenation. This effect is favored by an increase in acidity. In addition, as happened when MCH was in the feed, when the catalyst acidity is increased the CMR index increases. As a consequence of the combined effect of metallic and acid functions, the effect of catalyst acidity on the selectivity to isoalkanes and C2+ n-alkanes is higher in the single-step process (Table 4) than in the two successive reactions (Table 3). Conclusions The acidity of the HZSM-5 zeolite has a relevant effect on the capacity of the bifunctional catalyst for MCH transformation under hydrogenating atmosphere. The catalyst with the highest Al content in the zeolite is the one that leads to higher C2+ n-alkane conversion and selectivity given that there is a relationship between conversion and selectivity. Nevertheless, the selectivity to isoalkanes requires a moderate acidity level with the aim of limiting the transformation into aromatics and methane.

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Table 4. Results of Toluene and Methylcyclohexane Conversion, Selectivities to Different Products, and CMR Index for Bifunctional Catalysts with Different HZSM-5 Zeolitesa Pt/A + z15

Pt/A + z25

Pt/A +z 40

Pt/A + z140

100 99.70 0.72 57.12 2.43 43.30 10.65 0.74 41.41 0.75 0.00 0.20

100 93.63 0.38 42.11 1.12 34.31 5.29 1.40 43.40 13.53 0.58 0.12

100 73.04 0.21 23.86 0.33 20.63 2.47 0.43 26.32 47.45 2.16 0.10

99.77 44.47 0.03 2.46 0.00 2.07 0.30 0.09 5.16 90.20 2.15 0.09

XTOL (%) XMCH (%) Smethane SC2+ n-alkanes Sethane Spropane Sbutane SC5+ n-alkanes Sisoalkanes Scycloalkanes Saromatics CMR a

Reaction conditions: 350 °C; WHSV ) 4 h-1; 40 bar.

The relationship between conversion and Al content has led to a simple equation for determining Al content in the HZSM-5 zeolite on the basis of the transformation of MCH as reaction test. The sole experimental result required is the conversion in one run carried out in a differential reactor under a given hydrogen pressure in the feed. Regarding the hydrocracking of toluene in a single reaction step on bifunctional catalysts (metallic function + acid function), complete conversion is reached when MCH is transformed on the acid function, which avoids the thermodynamic restrictions for hydrogenation. The metallic function significantly affects the reactions for MCH transformation given that it increases selectivity to isoalkanes and decreases that to aromatics. An increase in the acidity of the HZSM-5 zeolite efficiently contributes to decreasing selectivity to aromatics given that it avoids the thermodynamic restrictions for hydrogenation. Acknowledgment This work has been carried out through the financial support of the University of the Basque Country (Project GIU06/21) and of the Ministry of Education and Science of the Spanish Government (Project PPQ2003-07822). P.C. wishes to thank the Basque Government (Dept. of Education, Universities & Research: BFI02.96) for the Fellowship and Laura Santamaria for the valuable comments. Nomenclature B:L ) Bro¨nsted-to-Lewis molar acid site ratio, molBro¨nsted molLewis-1 CMR ) cracking mechanism ratio, eq 4 DB ) double bounds in the zeolite structure dp ) particle diameter, mm E ) apparent activation energy, kJ mol-1 F ) molar flow rate, mol h-1 MCH, TOL ) methylcyclohexane and toluene MMCH ) molecular weight of methylcyclohexane, gMCH mol-1 nAl, nSi ) number of Al and Si atoms P, PH2 ) total pressure and hydrogen partial pressure, bar -rMCH ) methylcyclohexane hydroconversion rate, molMCH (g of zeolite)-1 h-1 Si, Yi ) lump or component i selectivity and yield, wt % T ) Temperature, K TA, TS ) asymmetric and symmetric tension, cm-1 X ) conversion W ) weight of pure zeolite in the used catalyst, g WHSV ) weight hourly space velocity, h-1

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ReceiVed for reView July 6, 2007 ReVised manuscript receiVed October 10, 2007 Accepted October 25, 2007 IE070921J