Catalysis Communications 16 (2011) 234–239
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Short Communication
Catalytic reforming of n-heptane over Pt/Al-HMS catalysts
T. Hamoule a, M.H. Peyrovi a,⁎, M. Rashidzadeh b, M.R. Toosi c
a Department of Chemistry, Faculty of Science, Shahid Beheshti University, Tehran, 1983963113, Iranb Research Institute of Petroleum Industry (RIPI), Tehran, 1485733111, Iranc Department of chemistry, Islamic Azad University, Qaemshahr Branch, Qaemshahr, Iran
⁎ Corresponding author. Tel.: +98 21 29902892; fax:E-mail address: [email protected] (M.H. Peyrov
1566-7367/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.catcom.2011.09.020
a b s t r a c t
a r t i c l e i n f oArticle history:Received 22 July 2011Received in revised form 14 September 2011Accepted 15 September 2011Available online 23 September 2011
Keywords:Al-HMSPt catalystCatalytic reformingn-Heptane
Pt catalysts supported on Al-HMS materials were prepared and characterized by XRD, NH3-TPD, TGA andpyridine-adsorbed FTIR techniques. Catalytic performance of samples was investigated for reforming of n-heptane and compared as a function of Al content. TPDmeasurements showed that the acidity of Al-HMSma-terials enhanced with the increasing Al content. Results showed that the ratio of BrÖnsted/ Lewis acidic sitesdecreased with the increasing Al content. The catalytic evaluation revealed that Pt/Al-HMS catalysts had ahigh activity for conversion of n-heptane. The distribution of products was described based on the natureof the acidic sites.
+98 21 22431663.i).
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© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Catalytic reforming of alkaneswhich is the conversion of convention-al petroleum light naphta feeds (C6 and C7 hydrocarbons) into branchedaliphatic hydrocarbons and aromatics has been studied extensively inthe last two decades because at the same time the requirements to thegasoline quality are toughened [1,2]. It has been widely accepted thatthis reactions are achieved over bifunctional catalysts consisting ofnoble metal particles supported on a matrix which contains acid sites[3]. Themajor reactions promoted by bifunctional catalysts are hydroge-nation, dehydrogenation, isomerization, cyclization and hydrocracking[4]. Pt/Al2O3-Cl is the commercial monometallic reforming catalysts.Owing to environmental concerns, the Pt/Al2O3-Cl catalyst is not thebest choice for this reaction because it requires continuous regenerationwith chlorine in order to regain the catalyst acidity, whichmay cause se-vere corrosion problems. In addition, this kind of catalyst is very sensitiveto water and sulfur in the inlet stream. Their use is, however, character-ized by high hydrogenolysis activity at high temperatures. In the last de-cade, researchers paid more attention to hydroconversion of alkanesover solid acid catalysts with large pore diameter including mesoporousmolecular sieves in order to prepare new catalysts for the conversion ofheavier hydrocarbons [5–11]. Among them, Al-containing mesoporousmolecular sieves which possess the acidic sites and good hydrothermalstability are favored. HMS is a hexagonal mesoporous silicate with a par-ticular wormlike pore structure. It has a simple preparation method andcheap primary alkylamines and can be used as a modified support in
catalytic reactions. However, no studies have so far been reported inthe open literature on the use of Al-HMS for reforming of n-alkanes.In this work, we have investigated the catalytic activities of Pt catalystssupported on Al-HMS for the reaction of n-heptane reforming.
2. Experimental
2.1. Materials and methods
The HMS and Al-HMS materials were synthesized by sol–gelmethod similar to Pinnavaia and coworkers [12], Mokaya and Jones[13]used TEOS (Merck) as the silica source, aluminum isopropoxide(Merck) as the aluminum source and dodecylamine (Merck) as thesurfactant. The samples by the various Si/Al ratios were obtained bytaking appropriate amounts of aluminum isopropoxide and TEOS forSi/Al ratio of 5, 10, 20, and 35 following the procedure. Each solidproduct was separated by filtration and dried at 110 °C overnightand calcined at 540 °C for 6 h in the flowing air.
(0.6 wt %) Pt catalysts were prepared by impregnating the supportwith appropriate concentration of H2PtCl6 (Merck) using HMS or Al-HMS (Si/Al=5, 10, 20, 35) as the support. After evaporation of the sol-vent and drying, the Pt catalysts were calcined in flowing air at 300 °Cfor 4 h. The prepared catalysts are named Pt/Al-HMS(x) that x is Si/Alratio. Also, in the order to investigatemetal loading on the catalytic per-formance 0.3 and 0.9 wt %Pt/Al-HMS-35 catalysts were prepared.
2.2. Characterization of catalysts
The mesoporous materials were characterized by XRD using STOEdiffractometer. The aciditymeasurement of HMS and Al-HMSmaterials
Table 1Acidic properties of mesoporous supports.
Sample PeakTemperatureof TPD (°C)
Acidity(mmolNH3/g)
B/L BrÖnsted acidity(mmol NH3/g)
Lewis acidity(mmolNH3/g)
Al-HMS-35 258.9 0.426 1.8 0. 277 0.153Al-HMS-20 270.8 0.845 1.37 0.489 0. 355Al-HMS-10 278.4 1.398 1.06 0.720 0.677Al-HMS-5 287.1 1.556 0.86 0.719 0.837
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was evaluated by TPD of ammonia in a TPD/TPR analyzer (2900 Micro-meritics). To evaluate and analyze the type of acidic sites, pyridine ad-sorption on the samples was performed on a Fourier-transforminfrared spectrometer (170-SX). The level of coke laid down on the sur-facemeasured by thermogravimetric analysis (TGA) equipment using aSTA503 M instrument.
2.3. Catalytic evaluation
The catalytic conversion of n-heptane in the presence of hydrogenwas carried out at the temperature range of 450–550 °C,LHSV=2 h−1, and P=1 atm in a continuous fixed-bed microreactorpacked with 1.0 g of catalyst. After the treatment of catalyst in a H2
flow (60 ml min−1), n-heptane was fed into the reactor. Hydrogenwas also introduced in the optimized amount of H2/C7 molar ratiofor our samples in order to obtain the best selectivity of desired prod-ucts (H2/HC=5). The performance of the catalysts was tested after0.5 h time on stream (TOS) at noted temperatures for each experi-ment. Also, the catalytic performance of all samples was investigatedat 500 °C for 5.5 h time one stream. The reaction products were ana-lyzed by online gas chromatography (Shimadzu 8A) equipped with aTCD detector.
3. Result and discussion
3.1. Characterization
The XRD diagrams of the HMS and AlHMS(x) materials are similarto those reported in literatures [12,13], as shown in Fig. 1. There is asingle broad reflection that can be assigned to a lattice with theshort-range hexagonal symmetry. The increase of Al content in thesamples results in a broadening of this peak, indicating that incorpo-ration of Al is associated with an increasing lattice disorder. The NH3-TPD results for acidic properties are shown in Table 1. TPD profile ofpure HMS shows no evident peak, indicating that HMS material hasno acid sites (not shown here). There is one asymmetric broad peak
Fig. 1. XRD patterns of HMS and Al-HMS with different Si/Al ratio.
in the range of 150–500 °C on TPD profiles of Al-HMS samples, attrib-uted to the distribution of acid site fromweak to strong acid sites (notshown here). Maximum of desorption peak is in the range of 250–300 °C corresponded to the medium acid sites. Details of Table 1 (sec-ond column) indicate that the number of acidic sites of Al-HMS in-creases with the decrease of Si/Al ratio. Results also indicate thatmaximum of the TPD diagram shifts to the high temperatures withthe decrease of Si/Al ratio. It means that the strength of acid sites in-creased with the increasing Al content. The type of acid sites (Lewisacid and Bronsted acid) was also studied by FT-IR spectra of pyridineadsorption. It is well known that the vibration bands in the 1400–1650 cm−1 regions of the IR spectrum of the chemisorbed pyridinecould be distinguished between the BrÖnsted and Lewis acid sites[14,15]. The infrared spectra of the pyridine-adsorbed Al-HMS (notshown here) revealed three peaks at ca 1449 and 1540 cm−1 due topyridine adsorbed on Lewis and BrÖnsted acid sites, respectivelyand near 1480 cm−1 corresponded to Lewis and BrÖnsted (L+B)sites. It is observed that the intensities of bands for adsorbed pyridineon BrÖnsted and Lewis acid sites increased with the increase of Alcontent. The infrared spectra of all the samples were recorded atroom temperature. BrÖnsted to Lewis pyridine ratio (B/L) wasobtained by measuring the integrated absorbance of bands forBrÖnsted and Lewis acid-site chemisorbed pyridine and by usingthe correlation developed by Emeis for porous aluminosilicates(1.67 cm/μmol for the 1545 cm−1 band characteristic of pyridine ona BrÖnsted acid site and 2.22 cm/μmol for the 1455 cm−1 band ofpyridine on a Lewis acid site) [16]. The B/L acid site ratio decreaseswith increasing Al content. It is in agreement with this fact that thetetrahedrally coordinated framework aluminum (potential BrÖnstedacid site) decrease with increasing Al content while octahedrallyextra framework aluminum (potential Lewis acid site) increases[17,18]. The number of BrÖnsted and Lewis acid sites was calculatedfrom the NH3-TPD results by using the B/L ratios (Table 1).
3.2. Hydroconversion of n-C7
In order tomake an evaluation of the activity of catalysts, n-heptanehydroconversion was performed over mesoporous catalysts between450 and 550 °C. The major products obtained were classified into(1) hydrogenolysis, (2) hydrocracking, (3) C7 isomers and (4) aromaticproducts.
The activity and selectivity of various products over Pt/HMSand Pt/Al-HMS catalysts are given in Fig. 2(a–e). It can be seen thatPt/Al-HMS(x) catalysts are highly active for conversion of n-heptane(Fig. 2a). Fig. 2 shows that n-heptane conversion linearly increasedwith increasing reaction temperatures. The highest conversion forn-heptane appeared for the Pt/Al-HMS-10 catalyst. It was observedthat the catalytic activity does not follow that the surface acidity var-iation may be due to the fact that catalytic activity are influenced bymore activity-determining factors including textural properties, me-tallic function and others. The selectivity of isoheptanes as a functionof temperature is presented in Fig. 2b. It can be seen that the isomer-ization selectivity depends on the reaction temperature. For all sam-ples, the isomerization selectivity decreases with the increasingtemperature in agreement with the fact that isomerization and
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Pt-HMSPt-Al-HMS-35Pt-Al-HMS-20Pt-Al-HMS-10Pt-Al-HMS-5
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Fig. 2. n-C7 conversion and selectivity of various products vs. temperature over Pt / HMS and Pt /Al-HMS(x) catalysts at H2/HC=5.
236 T. Hamoule et al. / Catalysis Communications 16 (2011) 234–239
hydrocracking/aromatization are consecutive reactions. Fig. 2c and dalso shows the selectivity of hydrocracking and aromatization of hep-tane plotted against the reaction temperature. It can be seen thatthere is practically little formation of aromatization product (toluene)on the Pt/HMS. However, the selectivity of aromatization product isnot negligible on the Pt/Al-HMS(x) catalysts, indicating that the aro-matization mainly occurs via bifunctional mechanism. According tothis mechanism, the aromatization involves two processes: dehydro-genation on the metallic sites and aromatization into the channels ofmesopore [19,20]. It is clear that aromatization and/or cracking aremajor reactions for all mesoporous samples. The product distribu-tions can be attributed to acidity and porosity of these materials be-cause according to the literature reports [21] the presence ofmesoporosity leads to facilitated hydrogen transfer reactions be-tween olefinic intermediates and cyclic intermediates to obtain en-hanced yields of aromatics and LPG wherein this ability increasedwith Al content for Al-HMS materials [18]. On the other hand, itseems that Si/Al ratio influenced this competition via a change inthe acidity (number, strength, and nature of acid sites). For lowestAl content (Si/Al=35), having lowest acidity (number and strength),aromatization increased with increasing temperature. It is wellknown that both cracking and aromatization are favored at high tem-perature and are therefore competing reactions and this competitionis affected by the balance between the acid and metal functions.Therefore at high temperatures, the higher the Si/Al ratio (and thuslower acid content) the lower the selectivity to cracking and thus aro-matization predominate. At higher Al contents the nature of acid siteis a determinative factor that influences the product distribution. Forcatalysts with Si/Al=10 and 20, having high BrÖnsted acidity, crack-ing is one of the major reactions. It is due to this fact that the incres-cent of BrÖnsted acidity favors the cracking. For Pt/Al-HMS-5 catalyst,having high Lewis acidity, aromatization (mainly toluene production)
occurs mostly in agreement with the literature reports indicating thatincrease of Lewis acidity enhances the toluene selectivity in the n-heptane hydroconversion [22].
Results show that our mesoporous catalysts have a good selectiv-ity for desirable products such as isomerization and aromatizationproducts. Selectivity to various aromatic and C7 isomers are presentedin Table 2. The isomerization products are categorized into twogroups as monobranched (MoB) and multibranched (MuB) isomers.Results show that these catalysts exhibited good selectivity to MuBisomers that is very promising because it is well known that onekey objective of reforming catalysts modifications has been the mod-ulation of the catalytic selectivity in order to increase the yield of mul-tibranched isoparaffins in the reformate to prove a good octanenumber [23]. From Table 2, it is observed that toluene is the only aro-matization product for Si/Al=35, 20 and the main product in associ-ation with xylene for Si/Al=10, 5. The production of xylene increasedwith the Al content, due to an increase of alkylation ability with an in-crease in the Al content of mesoporous catalysts [21]. It is noteworthythat one of the most advantages of these mesoporous catalysts is theabsence of benzene in the aromatic products because it is well knownthat due to environmental restrictions, at present, the production ofreformulated gasoline with low content of benzene is one of themain problems of petrochemical industry [24].
The selectivity of C1–C2 species increased with increasing the tem-perature. Also, these products increased with decreasing the Si/Alratio from 35 to 10 and then decreased for Si/Al ratio of 5 (Fig. 2e).It seems that at high temperatures there is a competition betweenaromatization, cracking and hydrogenolysis that is affected by prop-erties of catalyst such as acidity, type of acid site, geometry, and bal-ance between acid and metallic sites.
Resistance of catalytic active phase towards hydrocarbon poisoningas well as possible changes in the catalyst activity and selectivity of
Table 2Selectivity of various isomerization and aromatic products over Pt-Al-HMS(x) catalysts.
Catalyst T,°C
Pt-Al-HMS-35 Pt-Al-HMS-20 Pt-Al-HMS-10 Pt-Al-HMS-5
450 500 550 450 500 550 450 500 550 450 500 550
Selectivity%MoB 14.7 10.3 5.5 15.5 15.1 6.3 11.2 7.6 4.6 8.7 3.1 2.0MuB 11.3 9.0 5.8 15.6 10.1 7.3 9.0 6.4 3.4 7.3 2.9 1.2Toluene 28.5 33.5 53.7 33.1 34.8 32.4 35.4 40.1 40.0 42.5 53.6 59.5xylene – – – – – – 5.0 6.2 9.1 9.5 20.4 18.1
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various products has been explored. These experimentswere continuedfor 5.5 h. A graph of conversion and selectivity of various products ver-sus time on streamat 500 °C for all catalysts is given in Fig. 3 (a–e).Mostof the changes in reaction characteristics happened in the first 1–2 h onstream. The important features illustrated are (1) a decline in the tolu-ene production and increase in the C3–C4 production in the first 1–2 h
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Pt-HMSPt-Al-HMS-35Pt-Al-HMS-20Pt-Al-HMS-10Pt-Al-HMS-5
Fig. 3. n-C7 conversion and selectivity of various products vs. time
on stream that this trend can be reversed after 2 hours (expect Pt/Al-HMS); (2) a slight increase in the C7 isomers; and (3) a decline in theC1–C2 production with time on stream. We observed a decrease in theconversion of all the catalysts (probably due to operating at atmospher-ic pressure), but the decrease in the case of Pt/Al-HMS-5 is higher thanthat of other Al containing samples. To interpret these results it is
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on stream over Pt/HMS and Pt/Al-HMS(x) catalysts at 500 °C.
Fig. 4. Amount of coke deposited after 5.5 h of n-heptane conversion for various catalysts.
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Fig. 5. Effect of Pt loading on the n-C7 conve
238 T. Hamoule et al. / Catalysis Communications 16 (2011) 234–239
important to follow the coke formation on the catalyst during this pro-cess. The results of TGA analysis for all catalysts are plotted in Fig. 4. Thisfigure shows that coke deposition is higher on catalysts containing Althat can be correlated with the presence of acid sites on these catalysts.It is noticeable that the coke builds up to relative similar levels on thePt/Al-HMS-35/20 and 10, but is higher on Pt/Al-HMS-5. This moreamount of coke on this type of catalyst can be correlated with thehigher deactivation observed after 5.5 h of reaction and also with thehigher amount of acid sites on Pt/Al-HMS-5. Also, these results suggestthat most of the coke is laid down within the first 1–2 h on stream andtoluene or a toluene precursor molecule is the source of the coke [25]that, at the beginning of the reaction, is converted to cracking productsand coke but between 2 and 5.5 h on stream, the strong acid sites, re-sponsible for cracking, are covered by coke that is in agreement with
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rsion and selectivity of various products.
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the increase in the aromatic selectivity and isomer selectivity, asreported in the literatures [26–28].
The effect of the amount of metal loading on the overall activityand selectivity of Pt/Al-HMS-35 materials was investigated. The re-sults are shown in Fig. 5. It can be seen that the amount of Pt on thecatalysts (within the range 0.3–0.9 wt.%) does not affect the total con-version and selectivity to isomers. The selectivity to aromatic prod-ucts increases with increasing Pt loading. This trend is vice versa forhydrocracking. Indeed, it seems that the increase in aromatizationhappens at the expense of cracking especially at 0.9% loading. This be-havior may be explained by this fact that both cracking (on acid sites)and aromatization (on metal sites) are favored at high temperatureand are therefore competing reactions. It is clear that this competitionis affected by a balance between metal and acid sites [6]. High selec-tivity to C1–C2 species has been observed in the case of Pt (0.3)/Al-HMS-35 at 550 °C. This trend can be attributed to secondary crackingon the acid sites at higher temperatures that is due to poorer balancebetween metal and acid sites [29].
4. Conclusion
Highly ordered mesoporous molecular sieves have been synthe-sized. The molar Si/Al ratio influences the structural regularity andsurface acidity. The number and strength of acid sites on Al-HMS ma-terials increase with the decrease of Si/Al ratio. It has been proventhat the Pt/Al-HMS catalysts exhibit superior catalytic activity in thereforming n-heptane. The coke deposition induces a decrease of thecatalytic activity especially at the first 1–2 h. High selectivity to aro-matic and multibranched isomers products is the main advantage ofthese catalysts. High activity and selectivity for desirable productscan be attributed to porosity, acidity and nature of acid site. This im-portant result for the first time reveals the possibility to obtain highoctane-number gasoline by using Al-HMS containing catalysts.
Acknowledgments
The authors gratefully acknowledge financial support from the Re-search Council of Shahid Beheshti University and Catalyst Centre ofExcellence (CCE).
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