novel supported rh, pt, ir and ru mesoporous aluminosilicates as catalysts for the hydrogenation of...

Applied Catalysis A: General 251 (2003) 131–141

Novel supported Rh, Pt, Ir and Ru mesoporous aluminosilicates ascatalysts for the hydrogenation of naphthalene

Mélanie Jacquina, Deborah J. Jonesa,∗, Jacques Rozièrea, Simone Albertazzib,Angelo Vaccarib, Maurizio Lenardac, Loretta Storaroc, Renzo Ganzerlac

a Laboratoire des Agrégats Moléculaires et Matériaux Inorganiques, UMR CNRS 5072, Université Montpellier II,2 Place Eugène Bataillon, 34095 Montpellier Cedex 5, France

b Dipartimento di Chimica Industriale e dei Materiali, Università degli Studi di Bologna, INSTM UdR di Bologna,Viale del Risorgimento 4, 40136 Bologna, Italy

c Dipartimento di Chimica, Università Università di Venezia, INSTM UdR di Venezia, Via Torino 155/B, 30173 Venezia-Mestre, Italy

Received 17 January 2003; accepted 9 April 2003

Abstract

Rh, Pt, Ir and Ru have been directly incorporated in mesoporous aluminosilicates prepared using direct liquid crystaltemplating with a non-ionic surfactant via addition of the corresponding metal complexes to the synthesis gel. In this approach,the non-ionic surfactant plays the role both of porogen and dispersant for the metal ions throughout the aluminosilicate,leading to controlled metal particle formation. Chemical–physical characterisation, simulated coking and regeneration tests,and catalytic behaviour towards naphthalene hydrogenation and ring opening are described. The catalyst activity in thehydrogenation of naphthalene has been studied at atmospheric pressure at 200 and 300◦C, and at 6 MPa between 220 and340◦C. At atmospheric pressure, all four catalysts give a naphthalene conversion in the range of 95–96% at 200◦C, whileat 300◦C, the Rh and Ru catalysts have the highest selectivity to high molecular weight (HMW) hydrogenolysis and/orring-opening products, as shown by the relatively high formation of alkylbenzenes and decadiene. The effect of parameterssuch as temperature, contact time and H2/naphthalene ratio on conversion and selectivity to partially or fully hydrogenatedproducts, HMW products and low molecular weight gases, was investigated under 6 MPa for the supported Rh catalyst. Itprovides a naphthalene conversion up to 98–99% and interesting selectivity, with a high proportion of HMW products between260 and 300◦C, and a good thermal stability.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Mesoporous aluminosilicates; Noble metal supported catalysts; Naphthalene hydrogenation/ring opening/hydrogenolysis

1. Introduction

There is an increasing demand for friendly hy-drocarbons and clean-burning high performance fu-els and, particularly, for a decrease of particulate

∗ Corresponding author. Tel.:+33-4-67-14-33-30;fax: +33-4-67-14-33-04.E-mail address: [email protected] (D.J. Jones).

emission in diesel exhaust gases[1]. To satisfy in-creasingly stringent regulations, the improvement ofpetroleum fractions such as middle distillates has be-come one of the main objectives of the refining indus-try. These middle distillate fuels have a high contentof multi-ring aromatic compounds which leads to alow cetane quality, and the conversion of these aro-matics into hydrocarbons of higher cetane number istoday a key process in the modern refining industry.

0926-860X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0926-860X(03)00314-4

132 M. Jacquin et al. / Applied Catalysis A: General 251 (2003) 131–141

Current processes for dearomatisation use catalystscombining the acidity of a support and the hydrogena-tion and hydrogenolysis/ring-opening activity of anincorporated metal. Hydrogenation/hydrocracking ismost often practised on cyclic molecules over primar-ily acidic zeolite, alumina or silico-alumina-supportednoble and other Group VIII metal catalysts. Differentprocesses have used catalysts such as NiMo, CoMo,NiW, Pt and or Pd on various supports[2–12]. Keymechanisms controlling product distributions in hy-drogenolysis and hydrocracking reactions have beenreported previously[13–15]. The dominance of theacid function can lead to excessive cracking and thus,a primary focus is the optimisation of the acid func-tion, and it has been shown recently that significantenhancements in ring opening selectivity are made byfocusing on the metal function. The metal functionis usually provided by Pt and/or Pd but it has beenshown that Ir, Ru and Rh also have exceptional activ-ity and selectivity for the target reaction of selectivering opening[16–18].

In this paper, we describe the preparation of newmesoporous catalytic materials containing highly andhomogeneously dispersed particles of Rh0, Pt0, Ir0 orRu0 after reduction, and investigate their activity forhydrogenation and hydrogenolysis and/or ring open-ing of aromatic compounds under various reactionconditions, using naphthalene as a model substrate(Fig. 1). The metal particle size of supported metalcatalysts is an important factor affecting the catalyticbehaviour and the support, in particular, the pore

Fig. 1. Proposed reaction pathway for the vapour-phase hydrogenation of naphthalene, including parallel and consecutive reactions.

structure, can affect the metal dispersion. We haveinvestigated a novel preparation method leading to amesoporous matrix using direct liquid crystal templat-ing with a non-ionic surfactant and have previouslyreported[19] the use of this method to obtain materi-als with surface acidity of strength lower than that ofzeolites[20]. The pore dimension of the channel sys-tem can be tailored by selecting the Si/Al ratio and thenature of the non-ionic template. This preparation ofthe mesoporous silico-aluminate matrix differs fromthat used for MCM-41 type materials[21–23], in par-ticular by the concentration range of surfactant usedand the ready formation, by controlling the kinetics ofthe reaction, of monolithic blocks instead of powders.

Several different methods for metal incorporationin porous supports are described in the literature. Oneapproach is the direct addition of metal salts, alkoxidesor complexes to the sol–gel mixture, which enablesintroduction of the metal into the silico-aluminatebody and/or the pores[24,25]. Such an approach hasbeen used to directly prepare rhodium functionalisedMCM-41 [26,27]. Other methods include impreg-nation or ion-exchange, the former generally of acalcined support, but also of surfactant containingmesophases[28]. Metal nanoparticles have also beenprepared in the micelle pores of block copolymers,then used as templates for porous silica[29]. Weconcentrate here on the direct incorporation of metals(Rh, Pt, Ir or Ru) into the synthesis gel followed bythermal treatment to remove surfactant porogen andreduction under hydrogen.

M. Jacquin et al. / Applied Catalysis A: General 251 (2003) 131–141 133

2. Experimental

The first step of catalyst preparation is the disper-sion of 4.5 g of a non-ionic C11–C15(EO)9 surfac-tant (Aldrich) in 4.5 g of a dilute HNO3 solution (a65% nitric acid solution was obtained from Aldrich).To this solution are added 13.0 g of tetraethoxysi-lane (TEOS, Fluka) and 1.17 g of aluminium ni-trate (Al(NO3)3·9H2O, Merck). Rh(NO3)3 solution(0.80 g) (Strem chemicals), is then incorporated forthe rhodium catalyst, and Pt(CH3COCHCOCH3)2(0.16 g), Ir(CH3COCHCOCH3)3 (0.25 g) or Ru(CH3COCHCOCH3)3 Alfa Aesar (0.21 g) for the Pt, Ir andRu containing materials, respectively. The solutionis stirred until the total dissolution of the reagents isreached. The system is then maintained under vacuumfor 2 h and left in a closed vessel at room temperaturefor 3 days after which time a transparent solidified gelis recovered. The surfactant is then removed by calci-nation at 560◦C for 4 h using a ramp rate of 1◦C/min.The catalysts are denoted 2RhSiAl20, 2PtSiAl20,2IrSiAl20 and 2RuSiAl20 in the following.

To determine the specific surface area and theporosity of the materials, adsorption–desorption ofnitrogen at 77 K was performed using an automatedvolumetric Analsorb 9011 instrument. Samples wereoutgassed overnight at 200◦C under vacuum prior tothese experiments. The surface area was determinedusing the BET formalism[30] and the pore diameterwas estimated using theαs method. Various methodswere used to estimate the size of the metal particlesand the metal dispersion, including H2 chemisorption,X-ray powder diffraction (XRD) and transmissionelectron microscopy (TEM) on the reduced materials.XRD data were recorded on an automated PhilipsX’Pert diffractometer with Cu K� radiation, in aninterval of 18–27◦ 2θ which corresponds to the re-gion of the metal (1 1 1) reflection. The breadth ofthis line (full width at half height) allowed a first es-timation of the average metal particle size by use ofthe Scherrer relation[31], considering contributionsfrom particle stress and instrumental broadening to benegligible. The validity of this hypothesis is justifiedby the closely similar values for the average metalparticle size obtained from XRD and TEM. TEMobservations were made on samples prepared as ex-tractive replicas using a JEOL 1200 EX operating at100 kV. The accessible metal surface was measured

by H2 chemisorption using a Micromeritics instru-ment (ASAP 2010C). Samples were first flushed in aflow of helium, and then evacuated before chemisorp-tion with H2 at 300◦C. The metal dispersion value soderived is directly related to the metal particle size bythe relation:D = 1000/d, whereD (%) is the metaldispersion andd the metal particle diameter in Å.

Simulation of coking and oxidative regeneration ofthe 2RhSiAl20 catalyst enables an assessment of thestability under these conditions. A saturated solutionwas prepared with 0.1 g of anthraquinone in 20 mlof THF, and the reduced catalyst impregnated with aquantity of this solution. The catalyst was left to dry for12 h in a closed vessel at 30◦C, then dried at 65◦C for1 h. Three impregnation/drying cycles were employedto assure a high level of adsorption of carbonaceousmaterial. To simulate coking, the carbon-containingcatalyst was then heated under nitrogen at 100◦C for2 h and at 400◦C for 2 h (ramp rate 2◦C/min) andthen regenerated in air at 120◦C for 2 h and at 560◦Cfor 2 h (ramp rate 2◦C/min). The coked/regeneratedcatalyst was re-analysed by H2 chemisorption, so al-lowing any changes in the metal particle size and thetextural properties of the material to be followed. Inaddition, the surface area and pore characteristics werere-determined by N2 adsorption.

The activity of the four catalysts in the hydro-genation of naphthalene was first investigated in afixed bed lab-scale microreactor, operating at atmo-spheric pressure, at 200◦C and 300◦C. The bed,containing 200 mg of the catalyst first reduced underH2/N2, is in contact with a 10 ml naphthalene/He(0.1 g/10 ml) continuous flow. The reaction productswere analysed on-line by a Hewlett-Packard 5890 gaschromatograph.

Following this evaluation at atmospheric pressure,the catalytic activity was fully investigated at 6.0 MPausing 2RhSiAl20. Here, the set-up made use of a stain-less steel tubular reactor (i.d. 8 mm; length 54 cm),heated by an electric. The catalyst was crushed andsieved to 14–20 mesh and 6 cm3 were placed in theisothermal zone of the reactor. Before the tests, thecatalyst was activated in a 200 ml/min H2 flow, us-ing a programmed increase of the temperature fromroom temperature up to 350◦C. The catalyst temper-ature was determined during the tests using a 0.5 mmJ-thermocouple sliding in a stainless steel capillarytube inside the catalytic bed. A 10 wt.% naphthalene


solution inn-heptane was fed by a Jasco pump, in agas flow of H2/N2. After an initial 1.5 h period un-der reaction conditions to allow steady state activityto be reached, each catalytic test was performed for5 h, and the products collected in a trap cooled at−10◦C. Reaction conditions including temperature,contact time and H2/organic feed ratio were varied.The reaction products were first identified using anHP GCD 1800A system, equipped with a column of5 wt.% methylphenylsilicone (30 m× 0.25 mm, filmwidth 0.25�m), comparing the experimental GC–MSpatterns with those present in the instrument library.Quantitative analysis was performed by a Carlo Erba6000 GC, equipped with FID and a PS 264 column(5 wt.% methylphenylsilicone, 25 m× 0.53 mm, filmwidth 1.5�m), comparing the GC patterns with thoseobtained for pure reference compounds. For the mainreagent/products (decalins, tetralin and naphthalene),GC calibration factors were obtained using pure ref-erence compounds, while for the other molecules(mainly high molecular weight (HMW) products)calculated average GC factors were adopted.

3. Results and discussion

3.1. Catalyst characterisation

Chemical analysis for silicon, aluminium and metalcontents (CNRS elemental analysis service at Vernai-son, France) indicated the Si/Al ratios and the metalcontent to be close to those used in the synthesis.

Table 1Textural properties, dispersion and metal particle size of 2RhSiAl20, 2PtSiAl20, 2IrSiAl20 and 2RuSiAl20

2RhSiAl20 2PtSiAl20 2IrSiAl20 2RuSiAl20

Fresh Regenerated Used Fresh Fresh Fresh

Surface area (m2/g) 660 670 610 760 653 641Pore diameter (nm) 2.2 2.2 2.1 2.2 2.2 2.2Metal dispersion (%) 18 15 1 34.4 50 29Metal surface area (m2/g metal) 78 66 4.4 85 121 120

Particle diameter (nm) 5.5a 6.7a 2.9a 2.2a 3.6a

4.0b 5.0b 4.0b 3.0b 2.8b

4.0c 3.0c 2.0c 3.0c

a From metal dispersion.b From XRD.c From TEM.

Fig. 2. Nitrogen adsorption isotherm at 77 K of 2RhSiAl20 beforecatalytic tests (the isotherm is fully reversible, and the desorptionisotherm is not given).

Precipitation is avoided in the direct liquid crystaltemplating approach, and all components of the cat-alysts are recovered in the product. The BET surfacearea and the pore diameter were determined for thefour fresh catalysts, and were re-determined after sim-ulated coking/regeneration and catalytic test at 6 MPafor 2RhSiAl20. The fresh catalysts all showed fullyreversible adsorption isotherms (example providedby 2RhSiAl20 in Fig. 2) with a quasi-linear regionfrom 0.1 to 0.3 indicative of super-microporosity. Thederived surface areas are between 641 and 760 m2/g,and the mean pore diameter of 2.2 nm, is around thelower limit of mesoporosity (Table 1). These values


can be compared to those for silico-aluminates de-scribed in previous work[19] pertaining to mate-rials free from additional metal components. TheSiO2/Al2O3 (Si/Al = 20) system had a specific sur-face area of 999 m2/g and a pore diameter of 2.6 nm.The incorporation of a metal in the structure there-fore leads to a slight decrease in the surface areabut the pore diameter and the mesoporous characterare maintained. These textural properties can also becompared to those for other reported Rh-, Pt-, Ir- andRu-containing catalysts prepared by sol–gel methods.Rh–SiO2 and Rh–SiO2/Al2O3 systems have surfaceareas of 550–700 and 375 m2/g, respectively[32,33],while those for Ru/Al2O3, Ru/SiO2, Ir/Al 2O3 andPt/Al2O3 systems vary from 100 to 304 m2/g [34–37].

Importantly, in the context of catalytic application,the specific surface area and pore diameter of the2RhSiAl20 catalyst are unaffected by the simulatedcoking and regeneration process, and the textural prop-erties are essentially identical to those of the fresh ma-terial. Finally, following catalytic testing at 6 MPa andup to 340◦C, the surface area was re-determined andfound to have decreased by ca. 7%, while the natureof the porosity remains unchanged (Table 1).

The extractive replica TEM of Rh particles from2RhSiAl20 is shown inFig. 3, and the X-ray diffrac-tion pattern over the region of diffraction from (1 1 1)planes of rhodium metal inFig. 4. The 2% Rh-, Pt-,Ir- and Ru-containing materials are characterised bymetal dispersions of 18, 34, 50 and 29% and metalsurface areas (per gram of metal) of 78, 85, 121 and120 m2/g, respectively,Table 1. The metal particle sizecalculated from these dispersion values, is 5.5 nm forRh, 2.9 nm for Pt, 2.2 nm for Ir and 3.6 nm for Ru.Measurement of the diameter of >500 particles in theTEM micrograph of RhSiAl20 provided an indica-tion of the distribution of particle size. The methodemployed is that described by Gallezot and Leclercq[38] and leads to the histograms shown inFig. 5.The first of these provides a distribution in diameterwith a mean diameter of

∑nidi/

∑ni, while the sec-

ond is a quadratic distribution with a mean diameterof

∑nid

3i /

∑nid

2i (ni: number of particles anddi:

mean diameter, in each successive interval). The sec-ond distribution is probably the most pertinent sincethe catalyst activity is directly linked to the metal sur-face area. The distribution is narrow, with 46% of therhodium metal particles of size between 3 and 4 nm

Fig. 3. Transmission electron micrograph (extractive replica) ofthe Rh, Pt, Ir and Ru particles in 2RhSiAl20 (a), 2PtSiAl20 (b),2IrSiAl20 (c) and 2RuSiAl20 (d) (reduced form).

(76% between 3 and 5 nm), and a mean diameter of3.6 nm. Eighty-one percent of the metal surface arearesults from particles between 3 and 5 nm in diameter.The mean surface diameter, 4.1 nm, is in good agree-ment with that estimated from XRD (4 nm) while both

Fig. 4. XRD pattern in the region of the Rh(1 1 1) and (4 0 0)reflections of 2RhSiAl20 after reduction with H2.


Fig. 5. Number and area distribution of rhodium particles in2RhSiAl20.

are slightly lower than that estimated from chemisorp-tion of hydrogen (5.5 nm). However, the values con-cord sufficiently well to allow us to conclude that allthe metal is accessible to hydrogen, and that no sidephases are formed. The metal particle sizes of thefour catalysts may be compared to others previouslyreported systems containing Rh, Pt, Ir and Ru suchas Rh/SiO2, Rh/Al2O3 or Rh/SiO2/Al2O3, containingfrom 0.5 to 5 wt.% of Rh: 1.3–5 nm[32,33,39–44],such as zeolites containing Pt incorporated by impreg-nation: 7.3–11.3 nm[45], or Ir/�-Al2O3 and Ir/SiO2:1.6–3 nm[46] or Ru/Al2O3: 2.1 and 3 nm[47,48]. Insummary, in these various rhodium metal-containingsystems, the metal particle size depends on the metalcontent, the support, the surfactant and the method ofincorporation.

After impregnation with anthracene, reductionand oxidative regeneration, the dispersion results on2RhSiAl20 show a decrease, linked to an increase

of around 20% in metal particle size. This proba-bly corresponds to metal sintering caused by particleripening and aggregation, and could contribute to aloss of active surface area of the catalyst, which couldin turn exert an influence on the catalytic reactivity[49]. Experimental observations demonstrate that thestability of metals and hence their resistance to sin-tering, increases with increase in melting point of themetal. It has been shown, for example, that stabilityincreases in the sequence nickel, palladium, platinumand rhodium, which concurs with the sequence ofincreasing melting point[50]. In the present case,the sintering is linked to the different thermal treat-ments used during catalyst regeneration. However, theresults obtained are promising since without any op-timisation, the textural properties remain unchangedand the dispersion value shows only a slight decrease.

After catalytic reaction with naphthalene, the metalparticle size estimated from width of the X-ray diffrac-tion line is unchanged (Table 1). However, hydrogenchemisorption measurements provided an unrealisti-cally low dispersion, which certainly is a result of thedeposition of heavy products on the catalyst surfacethat prevent access to the metal by H2.

3.2. Hydrogenation/ring opening of naphthalene

3.2.1. Catalytic activity at atmospheric pressurePreliminary tests to assess hydrogenation and hy-

drogenolysis and/or ring-opening activity of the ma-terials prepared were carried out at atmosphericpressure. The reaction is thermodynamically lim-ited at this pressure, and the conversion decreasesrapidly as the temperature is increased. At 200◦C,none of the catalysts prepared show appreciable ring-opening activity, while at 300◦C the hydrogenationconversions are low (90–14.1%) because of thermo-dynamic limitations leading to low yields to ring-opening products (0.8–3.0%). The conversion andselectivity to decalin (decahydronaphthalene), tetralin(1,2,3,4-tetrahydronaphthalene), hydrogenolysis/ring-opening products (e.g. cyclohexane, toluene, xylene,alkylbenzenes, decadiene) and light (C1–C4) prod-ucts are shown inTable 2. At 200◦C, the naphtha-lene conversion values of all four materials is in therange 95–100% while at 300◦C, the conversion by2RhSiAl20 and 2PtSiAl20 (13.5–14.0%) is signifi-cantly higher than that of the Ir and Ru functionalised


Table 2Conversion and product distribution at 200 and 300◦C and atmospheric pressure given by mesoporous silico-aluminates containing dispersedRh, Pt, Ir and Ru

Catalyst Conversion(%)

Selectivity (%) Yield (%)a

Decalin Tetralin Cyclohexane C1–C4 Toluene,o-xylene

Alkyl-benzenes

Decadiene HMW hydrogenolysisand/or ring-openingproducts

At 200◦C2RhSiAl20 96.3 2.3 97.7 – – – – –2PtSiAl20 100 97.4 0.7 – – – – –2IrSiAl20 95.2 16.5 83.3 – – – 0.2 – 0.22RuSiAl20 95.0 2.8 97.0 – – – 0.2 – 0.2

At 300◦C2RhSiAl20 14.1 – 75.3 1.4 3.5 5.6 7.1 7.1 3.02PtSiAl20 13.5 – 94.0 – – – 3.0 3.0 0.82IrSiAl20 9.6 2.1 83.4 1.0 1.0 2.1 7.3 3.1 1.32RuSiAl20 9.0 – 77.9 1.1 1.1 5.5 8.8 5.6 1.9

a Sum of cyclohexane, toluene,o-xylene, alkylbenzenes and decadiene.

catalysts (9–10%). Further, at 200◦C the presence ofPt, Rh and Ru gives pronounced selectivity to decalin(Pt: 97.4%) or tetralin (Rh and Ru: 97–98%) whilethe iridium-containing sample provides a mixture ofdecalin (17%) and tetralin (83%). Tetralin is alsothe main product of the naphthalene hydrogenationover Pd-containing catalysts at 100◦C [51], and thisobservation has been interpreted in terms of interac-tion of Pd with naphthalene similar to the�2-olefinco-ordination in a mononuclear transition metal com-plex. Indeed, it has been suggested that naphthalenemay adsorb on different precious metals (Ru, Rh,Pt, Pd) as an olefin or an aromatic[52] and this willinfluence the rates of product formation. By analogywith the above, it may be that in the present catalysts,the adsorption interaction of Rh and Ru with thesubstrate is more “olefinic” (giving tetralin), and thatof the Pt catalyst more an aromatic type interaction(leading to decalin).

Table 3Catalytic activity as a function of temperature of 2RhSiAl20 towards hydrogenation and ring opening of naphthalene at 6 MPa

T (◦C) Conversion(%)

Tetralin(yield %)

Decalintrans(yield %)

Decalin cis(yield %)

HMW products(yield %)

C balance(� %)

220 95.6 84.2 2.1 5.9 1.6 −1.8260 98.0 79.2 6.3 8.5 3.6 −0.4300 98.7 29.3 28.1 6.5 30.8 −4.0340 95.1 23.2 9.1 1.5 33.7 −27.6260 98.3 78.6 6.0 8.5 3.6 −1.6

Contact time: 6.8 s; H2/naphthalene: 21 mol/mol.

Most interesting is the change in product distribu-tion observed at 300◦C, with a significant degree offormation of molecules resulting from ring-openingreaction, representing ca. 20% of the reaction prod-ucts given by supported Rh and Ru catalysts, 13% ofthose given by the Ir functionalised sample and 6%given by material containing Pt. The high selectivityin high molecular weight products (mainly decadi-ene) of the 2RhSiAl20 to decadiene is of particularinterest in the context of increase of cetane numberof diesel by hydrodearomatisation, and the reactiv-ity towards hydrogenation and hydrogenolysis/ringopening of naphthalene by this catalyst was thus in-vestigated more completely at high pressure, usingvarious reaction conditions.

3.2.2. Catalytic activity at 6 MPaAt 6 MPa and at temperatures between 260 and

340◦C (Table 3), a naphthalene conversion of 98–99%


was observed. Formation of tetralin is predominantat low temperatures. This preference was alreadyidentified at atmospheric pressure and is clearly con-served at high pressure. Using Pt- and Pd-supportedzeolites, the selectivity tocis or trans decalin fromnaphthalene at 200◦C, 10 MPa has been reported todepend on the nature of the metal, the zeolite and itssurface acidity[53]. An increase intrans isomer wasobserved over Pd and at higher Si/Al ratios, i.e. withan increase in weak acid sites. In the present case, de-calin is never the dominant product over either tetralinor HMW products, but a change in selectivity of theminor amounts formed fromcis to trans decalin isobserved as the reaction temperature increases above260◦C, in agreement with the higher thermal stabilityof the latter[53]. Corma has related the selectivityto the naphthalene conversion, with hydrogenationof the second ring occurring to a significant extentonly at very high conversions using Al-MCM-41 sup-ported Pt. Under 5 MPa and at 225–275◦C, <0.1%of ring opening and isomerisation products wereobserved[54]. In contrast, in the present work, ahigh proportion of hydrogenolysis/ring-opening prod-ucts is also observed. At 300◦C, the yield in suchhigh molecular weight products reaches 31%, whilethe carbon balance indicates a loss by formationof light gases of only 4%. Moreover, very similaractivity is observed on repeating the test at 260◦Cafter that at 360◦C, indicating good stability withtemperature.

Complete hydrogenation of naphthalene to decalinrequires, in principle, a ratio of 5 moles of hydro-gen per mole of naphthalene. Investigation was madehere of the influence at two temperatures (260 and300◦C) of the H2/naphthalene ratio on the conver-sion and selectivity (flow was maintained constantby completing with N2). The results are representedgraphically in Fig. 6. The detailed effects of thechange in ratio are compounded with the influence ofthe different temperatures, but common trends maybe identified. Firstly, the conversion value falls as theratio H2/naphthalene is lowered from 21 to 5 mol/mol.This effect becomes particularly significant belowca. 10 mol/mol, and it is slightly compensated by in-creasing the temperature to 300◦C. Such lowering ofthe H2/naphthalene ratio also modifies the selectivity.Fig. 6 shows that below a value of 21 mol H2/molnaphthalene, the yield of HMW hydrogenolysis and/or

Fig. 6. Influence of the H2/naphthalene molar ratio on conver-sion and product selectivity at 260◦C (a) and 300◦C (b), using2RhSiAl20. (×) Conversion; (+) carbon balance; (�) tetralin; (�)decalin trans; () decalincis; (�) HMW.

ring-opening products drops suddenly, while that oftetralin increases. This effect is all the more notableat 300◦C, where the yield of high molecular prod-ucts decreases from 30.8 to 5.4% on reducing theH2/naphthalene ratio from 21 to 15 mol/mol. Sincethe carbon balance also generally worsens under theseconditions, it may be concluded that it is essentialto operate with a large excess of hydrogen over thestoichiometric quantity in order to favour formationof high molecular weight products, and avoid partial


hydrogenation, and loss of yield by formation of lowmolecular weight products (light gases). In the reac-tion pathway ofFig. 1, high molecular weight com-pounds are formed mainly by consecutive reactionsof hydrogenolysis and/or ring opening on decalin (cisand trans). Such reaction is more strongly favouredby a higher excess of hydrogen, than is hydrogena-tion, compatible with the experimentally observedhigh H2 excess requirement, and the decrease in theyield in HMW products observed at the first decreaseof the H2/naphthalene molar ratio.

In a further attempt to identify the most appropriateoperating conditions for catalytic hydrogenation/ringopening of naphthalene with this catalyst, the con-tact time was varied between 3.4 and 22.5 s, and theconversion and selectivity at 260 and 300◦C weremonitored (Fig. 7). TA maximum in the yield of highmolecular weight products andcis and trans decalin,and a minimum in the carbon balance are observedwith a contact time of 6.8 s (operating temperature300◦C). At longer contact times, the yield of LMWlight gases is excessive (>50%), while at lower con-tact times the reaction does not proceed beyond theformation of the partially hydrogenated tetralin. Thesedata clearly indicate that a contact time of 6.8 s is themost favourable in terms of overall product yield, andselectivity to high molecular weight compounds, andare further compatible with the hypothesis that thelatter are formed via isomerisation/hydrogenolysis ofdecalin.

The efficiency of Rh-supported catalysts towardshydrogenation and hydrogenolysis/ring opening ofnaphthalene has also been demonstrated by others.For example, in the selective ring opening ofn-pentyl-cyclopentane (PCP) with 0.5 wt.% Rh-Al2O3, there is68.3% PCP conversion with a full ring disappearanceand a selectivity of 87% in ring-opened paraffins[20].The catalytic activity of Rh-supported on�-Al2O3at 300◦C and 63 MPa towards benzene, naphthaleneand biphenyl hydrogenation has also been exam-ined. The specific reaction rate constant shows thatbiphenyl is the most reactive over Rh metal, followedby benzene and naphthalene, with pseudo first-orderrate constants of, respectively, 11.6, 2.2 and 0.42×10−6 (ml/(min g mol) Ms) [43]. The activity ofrhodium-supported catalysts in toluene hydrogenationin the presence also of a sulphur-containing poison at100◦C was studied for two different supports (SiO2

Fig. 7. Influence of the contact time on conversion and productselectivity at 260◦C (a) and 300◦C (b), using 2RhSiAl20. (×)Conversion; (+) carbon balance; (�) tetralin; (�) decalin trans;() decalincis; (�) HMW.

and Al2O3) and two different precursors (RhCl3 andRh(acac)3) with greater sulphur tolerance on the twosupports impregnated with the latter[44].

4. Conclusion

Highly dispersed metal particles embedded withina supermicroporous/mesoporous silicoaluminate sup-port may be prepared by an approach exploiting


the pore forming and metal dispersant propertiesof non-ionic surfactants. In the direct liquid crystaltemplating route to porous materials, precipitation isavoided through control of hydrolysis conditions andkinetics, and since all inorganic components added tothe synthesis gel are recovered, none of the preciousmetal is lost to a solution. Metal particles increasein size in the order platinum and iridium (2–3 nm),ruthenium (3–4 nm) and rhodium (4–5 nm) and pro-vide a metal surface area of up to ca. 120 m2/g metal.Of the four catalysts, it has been shown by reactionwith naphthalene at 300◦C and 1 atm that rhodium,ruthenium and iridium show higher selectivity to-wards hydrogenolysis and/or ring-opening productsthan the platinum-containing catalyst. This obser-vation agrees with recent reports on the activity ofalumina and zeolites impregnated with metals such asrhodium, iridium, platinum, palladium and ruthenium[16–18]. Further, it should be related to the nature ofthe metal and not to the SiAl ratio, identical in eachcase, nor to the metal particle size, which is generallylarger for ruthenium and rhodium than for platinum.Such hydrogenolysis/ring-opening activity must becompounded with an effective amount of controlledacidity to give catalysts of optimally high activityand with limited loss of yield through non-selectivecracking to light products. Under the less thermody-namically limited conditions of operation at 6 MPa,the rhodium-containing catalyst at 300◦C leads tosignificant amounts of high-molecular weight andfully hydrogenated (cis, trans decalin) products fromnaphthalene, with a favourable carbon balance, inparticular, under conditions of a high excess of hy-drogen and adequate contact time. Moving towardsthe stoichiometric value of the hydrogen/naphthaleneratio, the conversion undergoes a sudden drop, whileconversion is relatively insensitive to the contacttime. Conversion is also unchanged in the presenceof dibenzothiophene (remaining at 98–99% at upto 3000 ppm of DBT) although the hydrogenationand hydrogenolysis activities decrease resulting in apoorer carbon balance.

Acknowledgements

Financial support under Brite-EuRam ProjectBRPR-CT97-0560 is gratefully acknowledged.

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novel supported rh, pt, ir and ru mesoporous aluminosilicates as catalysts for the hydrogenation of...

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