Hydrogenation of Toluene on Zirconium-Modified Hexagonal

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Hydrogenation of Toluene on Zr-modified HMS supported Pt, Pd catalysts Shyamal Roy, and SIDHHARTHA DATTA Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 31 Oct 2013 Downloaded from http://pubs.acs.org on November 10, 2013

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Industrial & Engineering Chemistry Research

Hydrogenation of Toluene on Zr-modified HMS supported Pt, Pd catalysts Shyamal Roy a, Siddhartha Datta b a

Department of Chemical Technology, Sant Longowal Institute of Engineering and Technology, Sangrur, Punjab 148106, India b Department of Chemical Engineering, Jadavpur University, Kolkata-700032, West Bengal, India

HMS, Zr–HMS materials were synthesized and employed as supports for preparation of hydrogenation catalyst. Supports and catalysts were characterized by using ICP-AES, XRD, EDX, SEM, TPD, BET surface area and CO chemisorptions techniques, respectively. Results from SEM and EDX confirm the hexagonally ordered mesoporous structure and incorporation of Zr into the HMS support, respectively. The detail kinetic study of hydrogenation of toluene over a (0.15 wt %) Pt-(0.15 wt %) Pd / HMS and (0.15 wt %) Pt-(0.15 wt %) Pd / Zr-HMS were performed in a continuous-upflow stainless steel catalytic fixed bed reactor at varied WHSV, hydrogen partial pressure and temperature, respectively. It was observed that toluene conversion was increased on increasing H2 partial pressure and decreased with increasing WHSV. The conversion is dependent on temperature and shows a well-defined maximum. It was found that incorporation of Zr into the HMS structure increased the toluene conversion and reducibility of the catalysts. It was found that (0.15 wt %) Pt- (0.15 wt %) Pd / Zr-HMS catalysts showed better performance in hydrogenation of toluene than the (0.15 wt %) Pt- (0.15 wt %) Pd/ HMS catalyst. Keywords: Hydrogenation catalysts, Zr-HMS, Pt-Pd/ HMS, Pt-Pd/ Zr-HMS, Activation energy.

Introduction The oil industry is under increasing pressure from legislators to improve the quality of diesel fuel with a view to reducing exhaust emissions. In recent years, considerable attention has been paid to develop new catalysts and processes for aromatics saturation in diesel fuel. Technically, the aromatics impart poor ignition quality and low cetane number to diesel and enhance the smoke point of jet fuel and increased emission of particulate matters. These particulate matters as well

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as aromatics emissions are environmental hazards, and these are known to be responsible for various diseases when inhaled by humans. Therefore, stringent environmental regulations are being directed to lower these hazardous emissions from vehicle exhausts. Consequently, this along with the growing demand for high quality diesel fuels has brought hydrotreating processes center stage in the modern refinery strategies.1 The drawbacks of the conventional hydrotreating catalysts include severe operating conditions such as high temperatures, low space velocities, high pressures, and hydrogen/feed mole ratios to achieve acceptable aromatic reduction. Furthermore, conventional hydrotreating catalysts comprising sulfided mixed oxides (Ni-Mo, NiW, Co-Mo) can only accomplish moderate levels of aromatic saturation under typical hydrotreating conditions in a single-stage operation.2-4 Using these systems, increasing operation severity (temperature and pressure ) do not result in deep levels of aromatics saturation because of thermodynamic limitations. Therefore, noble-metal based catalysts are preferred for aromatic saturation since they can work at lower temperatures, thus avoiding the thermodynamic constraints encountered with the sulfided oxides.5-6 Pt-Pd bimetallic supported on large pore βzeolites have been claimed to display dearomatization/cetane improvement.7,8 The large pore zeolites preferably allow fast diffusion and reaction of bulky aromatic compounds. It is known that noble metals deposited on acidic supports show higher turnover frequencies (TOF) compared to those supported on nonacidic supports.9-11 The positive effect of the acidity on the noble metal sulfur resistance was explained by the changes in the metal electron deficiency induced by metal / support interactions.12–14 However, high acidity of zeolite is also known to favor excessive cracking and coke formation. In order to avoid it without suppressing the sulfur tolerance of catalysts, less acidic supports such as silica–alumina or nonacidic supports has been recently investigated.15-18


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In the present study noble metals Pt, Pd supported on HMS and Zr-HMS has been explored as a hydrodearomatization catalyst. Toluene was used as a model compound to simulate the aromatics in diesel fuels, because the hydrogenation of toluene is known to be more difficult than that of benzene and naphthalene.19 Hydrogenation activity of toluene over Pt-Pd / HMS and PtPd / Zr-HMS catalysts were studied in a fixed bed reactor at varied temperatures, pressure and toluene feed WHSV. A modification of HMS silica was made by frame modification with Zr+4 ions. Zr-containing HMS sample was prepared using Zr-isopropoxide as Zr precursor, dodecylamine as the surfactant and tetraethoxysilane (TEOS) as the silica source. To our knowledge, no published work has been reported using Pt-Pd / Zr-HMS hydrotreating catalyst and Zr-HMS was synthesized using Zr-isopropoxide. Thus, the aim of this work was to study the catalyst activity using Zr on HMS-supported Pt, Pd catalysts and compare the catalyst activities using Pt-Pd / HMS catalysts.

Experimental Synthesis of HMS and Zr-HMS Hexagonal molecular sieve (HMS) material was prepared via a neutral (S0I0) templating route proposed by Tanev and Pinnavaia.20 The primary amines-dodecylamine was used as neutral structure directors in order to control a pore size of the resulting mesoporus silicas. In a typical preparation, dodecylamine (DDA) 4.15 g was dissolved in 55 ml of ethanol, and then 75 ml of H2O were added to the solution. The synthesis was carried out at 550C. Tetraehoxysilane (TEOS) 20 ml was added to the surfactant solution resulting in a reaction mixture with a ratio TEOS: DDA: EtOH: H2O of 4 : 1 : 41.5 : 170. The mixture was vigorously stirred in sealed flask at 55 °C for 20 h. The resulting white precipitate was filtered, washed by water and dried at room



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temperature. Template removal was performed by calcination at 550 °C for 4 h (heating rate: 2 °C / min). The final product was air dried over night. The Zr-HMS material with Si / Zr molar ratio 40 was prepared using the procedure described by Tuel21 using dodecylamine as the surfactant, tetraehoxysilane (TEOS) as the silica source and zirconium (IV) isopropoxide as Zr source, mesitylene as the swelling agent. In typical preparation of Zr-HMS, an initial solution was made by dissolving 13.39 g dodecylamine in a mixture of 173.57 ml H2O and 99 ml ethanol. Mesitylene (20.19 ml) was added to this solution with vigorous stirring for 15 minutes. The second solution was prepared by adding 60 ml of TEOS in the required amount of zirconium isopropoxide mixed with 10 ml of 2-propanol, and added slowly to the first solution while mechanical stirring was maintained for 20 h at ambient temperature. The obtained white solid was recovered by filtration, washed with distilled water and dried at room temperature, followed by drying at 100 °C for 24 h. The organic template was then removed by calcination at 550 °C for 6 h. Synthesis of Pt-Pd / HMS and Pt-Pd / Zr-HMS The catalyst was prepared by excess solution impregnation (ESI) method. Catalysts supported on HMS and Zr-HMS with different Pt, Pd loadings were prepared using prepared HMS, Zr-HMS supports and tetra amine platinum nitrate (Aldrich) and tetra amine palladium (II) nitrate (Aldrich) as metal precursor. Double distilled water was used in all these experiments. The quantities of platinum and palladium salts required for the specific catalyst with pre-determined % Pt and % Pd content were calculated from the stoichiometry. The amount of Pt and Pd salts of desired concentration was used for 10 g of support and stirred continuously for 4 h at pH 11. The metals dispersion on silica HMS support at varied pH was studied and results are plotted in


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Figure 3. The excess water from the slurry was removed in a rotary vacuum evaporator at 80 °C. The residue was then dried at 110 °C for 24 h in an oven followed by calcination at 400 °C in controlled flow of oxygen for 4 h for complete decomposition of Pt and Pd oxide salts and deposition of the metals on the support structure. The prepared catalyst was further reduced using H2. Catalyst activity measurements Hydrogenation of toluene was performed in a continuous-up flow stainless steel catalytic fixed bed reactor (Metal: 316 SS, Diameter: 1.5 cm, Length: 49.5 cm) with the ability to operate up to 200 bar pressure and 923 K temperature, equipped with high pressure piston pump (Knauer, Germany). The schematic diagram is shown in Figure 1. The catalyst extrudates were kept at the centre of the reactor throughout the kinetic study. The bulk density of the catalyst was 0.721 g / cc and exact length of the catalytic bed was 12 cm when the amount of catalyst is 5 g. The remaining space of catalyst bed at the top and the bottom of reactor were filled with glass beads (2–3 mm diameter) to avoid the entrance and exit effects. Catalyst was dried at 403 K for 24 h and reduced in situ by hydrogen at 553 K for 5 h. The reactor was brought to the desired temperature before the reactant was introduced. Toluene hydrogenation was carried out at varied temperature (423–523 K), hydrogen pressure (5–30 bar), feed WHSV (1–6 h-1). Liquid sample was withdrawn from the sample-collecting vessel and analyzed by gas chromatograph (Agilent Technologies 6890 N, USA) equipped with a flame ionization detector (FID) and DB-Petro, 12210A6 column and helium gas (3.4 mL / min) was used as a carrier gas. Activity is described in terms hydrogenation of toluene using a specific reaction rate according to Eq. (1)

r

(1)



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where ri is the specific rate (gmol / gcat. min), Xi the conversion of reactant i (i=toluene , F is the molar flow rate of the reactant i (gmol / min), and W is the catalyst weight (g). Toluene conversion at the end of the 40 min of operation was taken as a steady state conversion. It is also essential for kinetic studies that the catalytic reactor operates in the differential mode. The reactor exhibited no temperature or pressure gradients across the catalyst bed and there are no diffusional limitations for the transport of reactant and product molecules, in this condition.22 Hydrogenation of toluene was observed to occur readily within studied temperature range and the selectivity for methylcyclohexane was found to be between 40 and 100 %. No other products like, 1, 2- and 1, 3-dimethyl cyclopentane and ethylcyclopentane were also observed due to isomerization and cracking reactions. Experimental runs were repeated 5 times and the results compared with those of the previous runs in order to ascertain the reproducibility of the data obtained and the error was found to be ± 3%.

Characterization ICP-AES The chemical compositions of the catalysts (Pt, Pd) were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), using an IRIS Intrepid (Thermo Elemental) instrument. The solid samples were first digested (in a mixture of HCl and H2O2) in a microwave oven for 2 h, and then aliquots of solution were diluted to 50 ml using deionized (18.2 mΩ quality) water. The elemental compositions of calcined Pt-Pd / HMS and Pt-Pd / Zr–HMS catalysts were measured by ICP-AES and are given in Table 1. It indicates that the compositions of most of the catalysts match the targeted values. Measurement of N2 adsorption–desorption isotherms The BET surface area, pore volume and pore size distribution of the samples were measured with an Autosorb-1C instrument using low temperature N2 adsorption–desorption isotherms. Before


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measuring, the sample was degassed in vacuum at 200 °C. The surface area was computed from these isotherms using the multi-point Brunauer–Emmett–Teller (BET) method based on the adsorption data in the partial pressure P / P0 range from 0.01 to 0.2. The value of 0.1620 nm2 was taken for the cross-section of the physically adsorbed N2 molecule. The mesopore volume was determined from the N2 adsorbed at a P / P0 = 0.4. The total pore volume was calculated from the amount of nitrogen adsorbed at P / P0 = 0.95, assuming that adsorption on the external surface was negligible compared with adsorption in pores. The pore diameter and pore volume were determined using the Barret-Joyner-Halenda (BJH) method. In all cases, correlation coefficients above 0.999 were obtained. The specific surface area (SBET), average pore volume and average pore diameter of supports, calcined and spent (after reaction) catalysts are listed in Table 1. The HMS support shows the highest specific area value in comparison with the other samples (682 m2 /g). The specific area follows the order: HMS > Zr-HMS > Pt-Pt-Pd / Zr-HMScal > Pt-Pd / Zr-HMSspent > Pt-Pd / HMScal >Pt-Pd / HMSspent. The Zr-HMS shows the higher average pore diameter value (2-4.6 nm). The average pore diameter follows the order: Zr-HMS > Pt-Pd / Zr-HMScal > HMS > Pt-Pd / Zr- HMSspent > Pt-Pd / HMScal > Pt-Pd / HMSspent. Finally, the average pore volume follows the order: HMS > Zr-HMS > Pt-Pd / Zr-HMScal > Pt-Pd / HMScal > Pt-Pd / Zr-HMSspent > Pt-Pd / HMSspent. The specific SBET values of the supports decrease upon incorporation of the Pt and Pd components (Table 1). The highest specific area value is observed in the calcined Pt-Pd / ZrHMS sample in comparison with calcined Pt-Pd / HMS catalyst. The average pore diameter in the calcined Pt-Pd / Zr-HMS, Pt-Pd / HMS samples decrease slightly with the deposition of the active components, ca. 0.1 and 0.3 nm, respectively. By contrast, the Pt-Pd / Zr-HMS sample



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almost preserves the pore diameter in comparison with the homologous Zr-HMS support. Once again, the lower pore volume is observed in the calcined Pt-Pd / HMScal and spent Pt-Pd / HMSspent samples. Besides, it seems that the type of mesostructured material has an important role in the structural parameters of the supported catalysts. The Pt-Pd / HMSspent catalyst shows the higher decrease in specific area of the sample might indicate that, under the conditions of hydrogenation employed in this study, larger carbon, Pt and Pd particles are formed, which block the pores. This is an agreement with the pore diameter value. Pore diameter decreases ca. 0.7 nm in Pt-Pd / HMSspent sample, whereas in the Pt-Pd / ZrHMSspent sample it decreases ca. 0.12 nm, almost it remains the same. Comparing the structural parameters of the calcined catalysts with the spent catalyst in the hydrogenation of toluene, we can note that the Pt-Pd / Zr-HMSspent catalysts largely preserve the structural parameters of the freshly calcined state. Contrary to this sample, one can observe an important decrease in the structural parameters in the Pt-Pd / HMSspent catalyst. The origin of this behavior is not very clear, but it proves, that the Pt-Pd / HMS catalyst undergo major changes in it’s structural parameters during the reaction and this should be reflected in their catalytic performance. CO Chemisorptions Volumetric CO chemisorptions isotherms at 298 K were recorded separately in order to estimate metal dispersion using an Autosorb-1C instrument. For CO chemisorptions catalyst was pretreated as for the catalytic experiments and then outgassed at 9 ×10−6 Torr. After cooling the sample to 303 K, CO was admitted at pressures below 20 Torr. Since, in the region of low pressures, the CO isotherm can be described by the Langmuir equation, only one isotherm was recorded. The extrapolation of its linear part to zero pressure gives the amount of CO adsorbed in


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monolayer. Metal particle size was estimated assuming CO: Pt (Pd) = 1:1 surface stoichiometry. In the bimetallic catalysts, platinum was assumed to behave like palladium. X-ray diffraction (XRD) analysis The broad-angle X-ray diffraction patterns of the samples were measured using a Bruker D8 Advance Powder diffractometer with a Ge-monochromator producing a monochromatic Cu Kα radiation. The scanning was made from 5 ° to 80 ° with a scan rate of 2°/min. In all cases, the generator was operated at 40 kV and 30 mA. To avoid the problem of illuminated areas, all the samples were measured using the same sample holder. The wide angle X-ray diffraction patterns of the HMS, Pt-Pd / HMS and Pt-Pd / Zr-HMS catalyst samples exhibited a broad line between the 2θ values of 20–30°, which was attributed to the siliceous amorphous SiO2 phase.23 No reflections belonging to ZrO2 were observed, indicating the homogeneous dispersion of ZrO2 in the framework of mesoporous silica material.23 Energy dispersive X-ray analysis (EDX) Quantitative compositional analysis was carried out with an energy-dispersive X-ray analysis (EDX) system attached to the electron microscope, which was operated at 25 kV. Determination of the chemical composition was based on the average analytical data of individual particles and the result of EDX analyses of HMS, Zr-HMS support, Pt-Pd / HMS and Pt-Pd / Zr-HMS catalysts in the form of energy peaks at different applied voltages showed the presence of silica (Si), platinum (Pt), palladium (Pd), zirconium (Zr) and oxygen in the catalyst and no nitrogen was found. This indicates that during calcinations all the nitrate decomposed completely to their respective state.



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SEM analysis Scanning electron microscopy pictures are shown in Figure 2(a) - Figure 2(d) and SEM of Zr– HMS material are shown in Figure 2(b), which also shows the morphology of this material. It reveals that Zr–HMS is made up of sub-micrometer sized free standing or aggregated sphere shaped particles, which causes the decrease in surface area of Zr–HMS support compared to pure HMS material. Figure 2(a) and 2(c) show particles of irregular morphology which are fairly uniform in size. Similar type of morphology is observed in the case of Pt-Pd /Zr–HMS (Figure 2(d)). Temperature-Programmed Desorption (TPD) Temperature-programmed desorption of ammonia over the catalysts was carried out in a quartz reactor (id=4.5 mm) packed with about 0.1 g catalyst from 323 K to 883 K, at a linear heating rate of 20 K per minute in a flow of moisture free helium (40 ml/min). The adsorbate desorbed in the TPD was measured quantitatively by a TCD detector and TPD results were given in Table 1. Before carrying out the TPD, the catalyst was pretreated in situ at 883 K for 1 hr in a flow of helium. The ammonia was chemisorbed on the catalyst at 373 K by saturating the catalyst with the adsorbate at lower temperature (323-333 K) and then desorbing the physically adsorbed adsorbate in a flow of pure helium (40 ml/min) at 373 K for 1 hr.

Results and Discussion Effect of pH on metal dispersion on HMS support The metal dispersions at varied pH on HMS support were carried out by CO chemisorptions method and plotted against corresponding pH value are given in Figure 3. A range of metal dispersion from 64 – 94 % was obtained and it was demonstrated that there exists a pH optimum at about 11, which results in the highest metal dispersion (94 %). In order to get different metal


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dispersion on HMS support, the pH of the impregnating solution was systematically varied. The pH was adjusted through the addition of dilute NH4OH to initial values of 5-11, and 11.5. One observation was that the solution which was clear became cloudy, perhaps due to the formation of a micro emulsion upon the addition of NH4OH. However at a higher pH, obtained by the addition of NH4OH, the surface charge on the support becomes more negative,24 and attraction between the negatively charged surface and the cationic precursor increases with increasing pH. Giudo and Jacob25 brought attention to the phenomenon by showing that soluble catalyst precursors are fixed to the support either by reaction, exchange with surface OH groups, and /or by adsorption. In the former case, the concentration (density) of the surface OH groups, which depends on the pretreatment of the support, is crucial. In the latter case, the surface charge plays an important role. At a pH value of the so-called Point Zero Charge (PZC) of the surface is electrically neutral and at a pH values above PZC, the surface is negatively charged, while at pH values below PZC the surface is positively charged (Table 2). For silica, this can be illustrated as follows. At pH = 3, the surface is neutral. In a mildly basic environment, H+ ions are removed, and, as a result, the surface is negatively charged. In an acidic environment, the surface will become protonated. If it is intended to deposit anions onto the carrier surface, the preparation should proceed with pH values below the PZC, whereas if cation are to be deposited, a pH value above that of the PZC is preferred. PZC values for alumina, silica and a mixture of alumina (10 %) and silica are 8-9, ca. 3, and ca. 5 respectively. It should be mentioned that the exact PZC values not only depend on the chemical nature of the carrier, but also on to its history and the method by which it was prepared. Of course, for the solid support, a window of applicable pH exists. At improper pH values (e.g. pH >12 for alumina), the carrier itself may be dissolved.



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Satterfield26 reported that most reagents are adsorbed to varying degrees on most supports, but the characteristics of the process are complicated since various types of adsorptions are possible. The ultimate degree of dispersion of metal through the catalyst pellet is also determined by the interplay of a large number of factors whose relative importance varies with circumstances. These include the method of impregnation, the strength of adsorption, the extent to which the metal compound is present as occluded solute (that in the bulk liquid in the pores) in contrast to adsorbed species on pore walls, and the chemical reactions that occur upon heating and drying. It is also possible to control the deposition by competitive adsorption, e.g., by adding a citrate to the impregnating solution. This procedure has been used to embed a catalytically active layer slightly inside a catalyst particle.27 The interaction between the positively adsorbed cations, Pt(NH3)4+ and the negatively charged surface is strong. Under these conditions, dispersions are much higher. The use of Pt(NH3)4(NO3)2 results in a high Pt metal dispersion (94 %). This is interpreted in terms of strong metal-support interaction because of the positive nature of the cation in solution and the negatively charged surface. Effect of feed Weight Hourly Space Velocity (WHSV) on toluene conversion Figure 4 depicts the effect of feed WHSV on the conversion of toluene at 498 K temperature, 20 bar pressure. Toluene conversion was found to decrease with an increase in feed WHSV of toluene that can be explained by two different ways. With increase in liquid feed rate, wetted fraction of the catalyst as well as the gas–liquid and liquid–solid mass transfer coefficients also increase.28 At lower liquid feed rate, catalyst particles are partially wetted, and under these conditions, conversion increases due to direct transfer of the gas-phase reactant to the catalyst surface, already wetted internally due to the capillary forces. Therefore, with an increase in


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liquid feed rate an increase in the wetted fraction is expected to retard the conversion, while an increase in the external mass transfer coefficients will enhance the conversion. Another possible reason for observed decrease in conversion is due to the shorter residence time of reactants under high feed WHSV. However, the former explanation is applicable to the large catalyst bed but in this study low amount of catalyst around 5 g (smaller catalyst bed) was used, hence the latter explanation is more appropriate for this study.18,29 Effect of hydrogen pressure on toluene conversion Figure 5 shows the plot of toluene conversion at 498 K as a function of pressure using hydrogen as a carrier gas. From the plot, it can be observed that toluene conversion increased with pressure for up to 20 bar beyond which it remains fairly constant up to 30 bar. The initial increase in toluene conversion between 5 and 20 bar can be attributed to the fact that within this pressure range increasing the pressure enhanced the adsorption of toluene on the active sites, leading to increased toluene conversion. Further increase in pressure beyond 20 bar only resulted in a slight increase in conversion probably because the adsorption of toluene had already reached saturation with the occupation of most of the active sites. Hydrogen is adsorbed along with toluene and takes part in the reaction. It should be noted that, because of its smaller size, the diffusion of hydrogen is faster than that of toluene. Thus, there are competing reactions where hydrogen occupies some of the active sites and dissociates into atomic hydrogen which participates in the reactions. This competing adsorption, diffusion, and activation of hydrogen molecule on the active sites may lower toluene conversion. This phenomenon can also be explained on the basis of Bronsted and Lewis acidity. The presence of hydrogen may lead to a decrease in Bronsted acidity and thus decrease in the concentration of intermediates benzylic carbocations, leading to lower toluene conversion.29,30



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Effect of temperature on toluene conversion Low mechanical strength and collapsing of the structure at elevated temperature in the presence of water are the main drawbacks of mesoporous materials limiting their use as catalysts.31,32 In order to investigate the stability of the synthesized materials under real reaction conditions, the Pt (0.15 wt %)-Pd (0.15 wt %) / HMS and Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS were tested in toluene hydrogenation (Figure 6). Only methylcyclohexane was found as the reaction product. The toluene conversion of 100 % was achieved over the Pt (0.15 wt %)-Pd (0.15 wt %) / ZrHMS catalyst at 498 K, 20 bar pressure and WHSV = 2 h-1. The catalytic activity was stable during the long-term operation. The measurements performed after the reaction revealed that the specific surface area of the spent Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalyst dropped from 665 m2 /g to 460 m2/g (Table 1), and the average metal particle size only slightly increased from 3.2 to 3.4 nm (Table 1) but in case of spent Pt (0.15 wt %)-Pd (0.15 wt %) / HMS catalyst the specific surface area dropped significantly from 682 m2 /g to 342 m2 /g and average metal particle size increased from 3.1 to 3.7 nm. The Zr-HMS support and Pt, Pd nanoparticles confined within the mesopores under the mentioned reaction conditions were seen to be quite resistant against structure collapsing and metal sintering. The reaction rate constants were calculated from the experimental data and the results are plotted as ln (k) vs 1/T as shown in Figure 7 for Pt (0.15 wt %)-Pd (0.15 wt %) / HMS and Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalysts separately. The activation energy (E2) of the reaction using Pt (0.15 wt %)-Pd (0.15 wt %) / HMS catalyst was found to be 71.64 kJ/mol while the activation energy (E1) using the Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalyst was found to be 53.98 kJ/mol. The value of activation energy for Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalyst is lower than the activation energy of Pt (0.15 wt %)-Pd (0.15 wt %) / HMS catalyst


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indicating higher activity of Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalyst. The activation energy obtained in the present study is compared with the activation energy reported in the literature for hydrogenation of toluene. Navarro et al.33 reported that activation energy was found to be 129.2 kJ/mol for hydrogenation of toluene over Pt-Pd / Silica catalyst at 523 K. Coughlar and Keane

34

reported that the activation energy was found to be 78.2 kJ/mol for hydrogenation

of toluene over nickel loaded Y-zeolite. It can be noticed that value of activation energy (E1) using Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalyst for hydrogenation of toluene in the present study is roughly one third to half of that reported in literature. This is due to the fact that in the present study hydrogenation was carried out using a catalyst which facilitates a chemical reaction proceeds by a different pathway with a lower energy barrier. Activity is described in terms hydrogenation of toluene using a specific reaction rate at 498 K, 20 bar pressure and 4 h on-stream according to Eq.(1) are given in Table 1. It is observed that reaction rate for toluene hydrogenation is higher on Pt (0.15 wt %)-Pd (0.15 wt %) / Zr-HMS catalyst than that of Pt (0.15 wt %)-Pd (0.15 wt %) / HMS catalyst. The hexagonal nature of the mesoporous materials and their thermal and hydrothermal stability are increased when transition metals are anchored into the framework of mesoporous silicate. Substitution of foreign ion (Al+3, Ti+4, Zr+4) into the silicate framework is an efficient route in order to enhance acidity and stability of the mesoporous silica. It can be further noticed from Table 1 that addition of Zr on HMS support increases the metal dispersion and acidity. Some authors have reported that Al, Ti, or Zr-containing SBA-15 provided a better dispersion of metal species as compared to the pure SBA-15 and alumina-supported catalysts.35-38 Thiele modulus and the Weisz-Prater criterion were applied to determine the rate limiting step. When the value of Thiele modulus is small, the surface reaction is usually rate limiting. In the



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present study, the value of Thiele modulus was found to be 0.87, indicating that the surface reaction is the rate-limiting step. The Weisz-Prater criterion was applied to determine if internal diffusion is limiting the reaction. The value of Weisz-Prater criterion was found to be 2.71 x 10-5 ( PZC

Surface charge

positive

Neutral

negative

Species

MOH2+

MOH

MO-

Adsorption

anions

-

cations

pH increase---------------------------------------------------->


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Hydrogenation of Toluene on Zirconium-Modified Hexagonal

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