Influence of Support Acidity of NiMo Sulfide Catalysts for

Article pubs.acs.org/IECR

Influence of Support Acidity of NiMo Sulfide Catalysts for Hydrogenation and Hydrocracking of Tetralin and Its Reaction Intermediates Sheila G. A. Ferraz,† Bruno Martins Santos,† Fatima M. Zanon Zotin,‡ Lucia R. Raddi Araujo,‡ and José Luiz Zotin*,† †

PETROBRAS S.A., R&D Center, Cidade Universitária, Av. Horácio Macedo, 950, Cidade Universitária. Rio de Janeiro, RJ CEP: 21.941-915, Brazil ‡ Institute of Chemistry, Post-Graduation Program in Chemical Engineering, Rio de Janeiro State University−UERJ, Rua São Francisco Xavier, 524, Maracanã, Rio de Janeiro, RJ CEP: 20.550-900, Brazil S Supporting Information *

ABSTRACT: Aromatic saturation is an important reaction for improving the cetane number of diesel streams. NiMo sulfide catalysts supported on alumina (Alu), silica−alumina (Si−Al), and alumina-Y zeolite (AluZ) were prepared with similar dispersions and variable acidities. These catalysts were tested in the hydroconversion of tetralin, indan, decalins, and alkylbenzenes to evaluate the effect of the support acidity in the overall activity and the distribution of products. NiMo/Alu generated essentially hydrogenated products while the presence of an acid component on the support increased not only isomerization and cracking reactions but also hydrogenated compounds formation, especially on tetralin, indan, and butylbenzene hydroconversions over NiMo/AluZ catalyst. The better hydrogenation activity of NiMo/AluZ for these reactions was associated with the presence of strong acid sites that contribute to creating protonated species which would migrate to the sulfide phase. Such species would be easier to hydrogenate due to the lower stability of the aromatic ring.

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INTRODUCTION

should be preferentially broken for maximizing chain linearity and, therefore, cetane number increase. Although most of the commercial hydrotreating catalysts are still supported on aluminas, many other solids have been investigated as carriers.7−11 Acidic supports like silica−alumina and zeolites are traditionally used in bifunctional catalysts for (mild) hydrocracking applications, where a combination of hydrogenation and acidic functions are necessary for obtaining adequate cracking activity and selectivity.12 More recently, acidic supports were also proposed for other applications like hydrodesulfurization (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), and aromatic saturation (HDA), such as silica−alumina, Y zeolites, and alumina modified by P or B.12−20 In a previous work,21 we studied the gas phase hydroconversion of tetralin over sulfided NiMo catalysts supported on alumina (Alu), silica−alumina (Si−Al), and alumina-Y zeolite (AluZ) supports, which are representative of most of industrial catalysts used in HDT and HCC reactors. While NiMo/Alu catalyst presented essentially hydrogenated products (decalins and their ring contraction isomers), silica−alumina and alumina-Y zeolite supported catalysts presented also a significant amount of compounds issued from ring opening and cracking of tetralin and decalin. The presence of acid sites on the support promoted not only the ring opening and cracking

Fuel quality is continuously improving as a consequence of more stringent regulations for exhaust gas emissions, engines evolution, and use of exhaust gas treatment catalysts. In the case of diesel, although most of the focus is the reduction of the fuel sulfur content, other properties like density, distillation range, aromatic content, and cetane number are concerned by these regulations.1 Hydrotreating (HDT) and hydrocraking (HCC), which comprise the reaction between the oil feedstock and hydrogen over a sulfided catalysts (NiMo, NiW or CoMo), are the key processes in the oil refining scheme for achieving an adequate fuel quality. Aromatic saturation of diesel streams is necessary to increase cetane number, a property which represents the ignition quality of this fuel and increases with the degree of saturation, chain linearity, and molecular weight of the hydrocarbons.2,3 In the hydrogenation of multiring aromatic compounds over sulfided NiMo catalysts, it is usually accepted that reactivity decreases in the following order: polyaromatics > diaromatics > monoaromatics.4,5 Figure 1 illustrates some possible reactions and typical products of the hydroconversion of diaromatics compounds such as naphthalene. The hydrogenation of this molecule to naphthenic compounds contributes to a significant increase in the cetane number. Additional enhancement of this property may be achieved by selective ring opening of the naphthenic structures, provided that dealkylation and cracking reactions are minimized in order to avoid the formation of light compounds with lower cetane numbers. Moreover, Santana et al.6 showed that internal C−C bonds of the naphthenic rings © 2015 American Chemical Society

Received: Revised: Accepted: Published: 2646

November 18, 2014 February 17, 2015 February 23, 2015 February 23, 2015 DOI: 10.1021/ie504545p Ind. Eng. Chem. Res. 2015, 54, 2646−2656

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Industrial & Engineering Chemistry Research

Figure 1. Cetane numbers (CN) of some possible products of naphthalene hydroconversion (CN values according to Santana et al.6).

The specific surface areas and pore size distributions were obtained from nitrogen adsorption−desorption isotherms at 77 K using a Micromeritics Tristar 3000 V6.03A equipment. Supports and catalysts acidities in the oxide state were determined by n-propylamine temperature-programmed desorption (TPD), as described by Kresnawahjuesa et al.26 This method allows the quantification of the Brönsted acid sites by measuring the amount of NH3 and propene formed by the decomposition of n-propylamine on such sites. High resolution electron transmission microscopy (HRTEM) analyses of the sulfided catalysts were carried out in HRTEM-JEM 3010 URP equipment operating at 300 keV. Previously sulfided samples (673 K/2 h in 10 vol % H2S/H2 mixture) were collected in isooctane for preventing oxidation. A small sample was sonicated, and a drop of it was placed in a copper grid and transferred to the microscope. At least 50 structures of each sample were measured to determine the size distribution of molybdenum sulfide crystals (slab length and number of layers). Catalyst Evaluation. Tetralin, decalin, indan, and alkylbenzenes (n-butyl, n-propyl, ethyl, and toluene) hydroconversion were studied in a high-pressure plug flow continuous reactor. Typically, 2.0 g of catalyst (−100 + 150 mesh) diluted with 4.0 g of silicon carbide (−100 + 150 mesh) was loaded in a tubular reactor. After the catalyst was dried at 425 K for 30 min in hydrogen flow, sulfidation feedstock (4 wt % CS2/n-hexane solution) was admitted in the reactor at 13.5 mL/h in the presence of 0.6 NL/min of hydrogen at 30 bar of total pressure. Sulfidation was carried out at 623 K for 2 h. This temperature was chosen in order to avoid n-hexane extensive cracking over the more acidic catalyst and so preventing premature deactivation of the catalyst. Reaction feedstock was composed of the aromatic or naphthenic reactant diluted in n-hexane (Vetec, spectroscopic grade), doped with carbon disulfide (Vetec, p.a.) to obtain 1300 mg/kg of sulfur in the liquid feed. Reaction conditions were chosen in such way to minimize tetralin, decalin, and indan dehydrogenation. For tetralin hydroconversion, a broad range of conversions was covered in order to analyze the behavior of the product yields with the reactant overall conversion. Temperature was varied in the range of 563 to 603 K, pressure from 30 to 50 bar, liquid flow rate in the range of 9 to 21 mL/h, and hydrogen flow rate from 0.4 to 0.9 NL/min. For each catalyst, 20 experiments were carried out with at least 3

reactions, as expected, but also the formation of completely hydrogenated compounds. Although this behavior could be associated with an electronic effect for the NiMo/Si−Al catalyst that presents NiMoS structures in close vicinity of the Brönsted sites of the support,22,23 it seems to be not adequate for the NiMoS/AluZ catalyst since the characterization results indicated that NiMoS structures and Brönsted sites are located in different domains, respectively, in alumina and Y zeolite. The conversion of tetralin over bifunctional catalysts follows a complex reaction scheme, which involves many sequential and parallel steps.24,25 In some of them the hydrogenation occurs before the ring opening step and in others it goes the other way around. In this process, intermediate products with reactivity different from the original reactant could be produced and, according to the catalyst, one path may be favored over the other. In the present work, the hydroconversion of tetralin was studied in a wide range of overall conversions with the same catalysts of the previous work in order to have a better understanding of the effect of the catalyst acidity on the reaction network of this molecule and on the catalyst hydrogenation properties. The hydroconversion of some representative intermediate products, such as decalin, indan, and alkylbenzenes, were also studied over NiMo/Alu and NiMo/AluZ catalysts aiming to understand the influence of acidity and hydrogenation properties on the several steps of tetralin conversion.

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EXPERIMENTAL SECTION Catalyst preparation and characterization were already described21 and are briefly recalled below. Catalyst Preparation. Cylindrical extrudates (1/16 in. diameter) of alumina (ex-Pural SB, Condea Chemie), silica− alumina (40 wt % of SiO2, ex-Siral 40, Condea Chemie) and alumina + 30 wt % USY zeolite (unit cell parameter Ao = 2.445 nm, bulk silica/alumina mol ratio of 13.7) were used as carriers. All NiMo catalysts containing 20 wt % of MoO3 and 4 wt % of NiO were prepared by incipient wetness impregnation with ammonium heptamolybdate tetrahydrate and nickel nitrate hexahydrate at pH 2.0, followed by drying at 393 K overnight and calcination at 723 K for 1 h. Catalyst Characterization. Supports and catalysts composition were determined by X-ray fluorescence (XRF) in a Philips X-ray spectrometer model PW 1710. 2647

DOI: 10.1021/ie504545p Ind. Eng. Chem. Res. 2015, 54, 2646−2656

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Table 1. Chemical Composition, Surface Area, Acidity and Dispersion of NiMo Catalysts Supported on Alumina, Silica-Alumina and Alumina-Y Zeolite Carriers metal content

a

MoO3

NiO

surface areaa

n-propylamine TPDb

sample

wt %

wt %

m2/g

μmol/gcat

average stacking

average length (nm)

NiMo/Alu NiMo/Si−Al NiMo/AluZ

19.7 21.3 18.5

4.3 4.6 4.3

150 274 239

471 554 695

2.2 2.0 1.8

3.9 4.3 4.4

TEM of sulfided catalysts

Surface area of nonsulfided samples by BET method. bNH3 formed during n-propylamine TPD of nonsulfided samples.

ring opening, dealkylation, and cracking reactions take place.29 Particularly in more acidic supports, like USY zeolite, the number of isomers can increase considerably with the overall conversion of tetralin. For example, in the present work, more than 100 compounds were identified for the alumina−zeolite supported catalyst (tetralin conversion = 84 mol %). In general, the same compounds (or group of compounds) were present in the reaction products of the three catalysts. To facilitate the analysis of the results of the catalytic tests, the identified compounds were assembled in different groups as indicated in Table 2.

repetition tests for evaluating the catalyst deactivation. For a given reaction condition, catalytic activity was considered stable when the variation of tetralin conversion was less than 1%, which was typically observed for stabilization times of at least 8 h. The same criterion was used for the other reactants Indan, decalin, and n-alkylaromatics hydroconversions were studied at 30 bar of overall pressure and 583 K at the same contact time of tetralin hydroconversion. Reaction products were analyzed by online gas sampling on a HP 6890 gas chromatograph using a HP-5 column (60 m long, 0.32 mm diameter, and 1 μm film thickness) and a FID detector. The concentration of the reaction products were calculated by the ratio between the corresponding peak area and total chromatogram area. The reaction products were identified by collecting a liquid sample of the reactor effluent which was analyzed by GC−MS (HP5973N) using the same chromatographic column and conditions of the reaction system analysis. Since there was some difficulty in classifying compounds with molecular weight (MW) 138 (dinaphthenes or mononaphthene olefins), the reaction product was treated with concentrated sulfuric acid to remove olefinic compounds, and then another GC−MS analysis was performed.

Table 2. Groups of the Products Observed during Tetralin Hydroconversion group

subgroup

A-Ta

A A-RO olefin A-RO

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A-C

RESULTS Catalyst Characterization. The catalysts used in the present study were the same already prepared and characterized in a previous study.21 The main properties of these catalysts are summarized in Table 1. Catalyst acidity of the calcined catalysts varied as expected, that is, NiMo/Alu < NiMo/Si−Al < NiMo/AluZ. As discussed before,21 although the acidities were measured in the oxide form, this ranking is not expected to change after sulfidation since the metal content is similar for all the catalysts and also because the NiMo sulfided phase presents weak acid sites.27,28 Sulfided phase dispersions are quite similar for all the catalysts since slab length and stacking are comparable and the differences are within the method precision. Moreover, the similarity of the temperature-programmed reduction (TPR) patterns and TEM investigation of selected areas indicated that the NiMoS phase on the NiMo/AluZ catalyst is probably associated with alumina domains.21 Therefore, the effect of the support acidity on the hydrogenation and hydrocracking activities of tetralin and other aromatic compounds could be evaluated with little interference from the structure of the sulfided phase responsible for the hydrogenation/dehydrogenation activities of these bifunctional catalysts. Tetralin Hydroconversion. Reaction Products Identification. Tetralin hydroconversion (HCC) over bifunctional catalysts involves a complex reaction network where aromatic ring hydrogenation, naphthenic ring isomerization, naphthenic

H-Ta

H H-RO H-C

C-Ta L

some representative compounds

description C10 aromatics without ring opening (RO)b (MW 132) C10 monoaromatics, olefin, with ROa (MW 132) C10 monoaromatics with ROb (MW 134) monoaromatics < C10c C10 dinaphthenes (MW 138) C10 mononaphthenesb (MW 140) mononaphthenes < C10c A-C + H-C light compounds < C6

naphthalene, methylindans butenylbenzenes, methylpropenylbenzenes butylbenzenes, diethylbenzenes indan, propylbenzene, xylenes, ethylbenzene, toluene, benzene decalins, methylperhydroindans butylcyclohexanes, methylbutylcyclopentanes propylcyclohexanes, ethylcyclohexane, methylcyclopentane pentanes, butanes, propane, ethane, methane

a

A-T: total aromatic (A) compounds; H-T: total hydrogenated (H) compounds; C-T: total cracking (C) compounds. bRing opening (RO) groups: products with 10 carbon atoms presenting at least one naphthenic C−C bond breaking. cCracking (C) groups: products containing six to nine carbon atoms.

There was some coelution of dinaphthenic (MW 130) and alkylnaphthenic (MW 140) compounds. Although this represents less than 10% of the amount of reaction products, these two classes of compounds will be considered in the same group (H + H-RO). Additional information on reaction products identification can be found in the Supporting Information. Product Yields versus Tetralin Conversion. The distributions of the main product groups are shown in Figure 2 as a function of the overall conversion of tetralin. In such figures, trend lines were drawn to facilitate the visualization of the 2648


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Figure 2. Aromatic, hydrogenated, and cracking products distributions as a function of the overall tetralin conversion (reaction temperature = 310 °C). See text for the group acronyms.

Figure 3. Product distribution inside the groups of aromatic and hydrogenated products as a function of the overall tetralin conversion for the NiMo/Alu catalyst. See Table 2 for the group acronyms.

experimental data, without any compromise, for instance, with their kinetic modeling. According to Figure 2, it can be inferred that the yield of both total aromatic and total hydrogenated products present a behavior of primary reaction products (slope > 0 at low tetralin conversions), instead of cracking products which clearly show a behavior of secondary reaction products (slope → 0 at low tetralin conversions). A comparison of similar overall conversion of tetralin shows that NiMo/Alu is the most selective for the formation of hydrogenated products although less active than the other catalysts (see Catalytic Activity section below). The formation

of aromatics, RO, and cracking products are minimized in the NiMo/Alu sample. In the opposite side, NiMo/AluZ is more active but less selective for hydrogenated products, presenting higher yields of aromatic, RO, and cracking products. Si−Al supported catalyst presents an intermediate behavior between Alu and AluZ supported ones. It is important to point out that the formation of aromatic, RO, and cracking products on these three catalysts follows the same trend of their Brönsted acidity as measured by n-propylamine TPD (Table 1). The product distributions inside the aromatic and hydrogenated product groups for each catalyst are shown in Figures 3 to 5. NiMo/Alu presents essentially products issued from the 2649


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Figure 4. Product distribution inside the groups of aromatic and hydrogenated products as a function of the overall tetralin conversion for the NiMo/Si−Al catalyst. See Table 2 for the group acronyms.

Figure 5. Product distribution inside the groups of aromatic and hydrogenated products as a function of the overall tetralin conversion for the NiMo/AluZ catalyst. See Table 2 for the group acronyms.

hydrogenation of the aromatic ring (decalins and methylindans), with low formation of aromatic compounds, mostly naphthalene (Figure 3). RO and cracking products are formed in very small amounts for both groups, up to an overall tetralin conversion of 29%. For NiMo/Si−Al, the hydrogenated products are still predominant but RO and cracking products appear in both groups (Figure 4). In the aromatic group, A-RO compounds are predominant (mostly butyl-benzene) and small amounts of A-RO olefin (butenyl−benzene) and cracking compounds (propylbenzene and indan) are observed. In the hydrogenated group, decalins, alkyl-dinaphthenic, and ring opening compounds are the most abundant products (H + H-RO); cracking compounds are formed in lower amounts. So, it can be concluded that, for NiMo/Si−Al catalyst, most of the RO and cracking products still contain an aromatic ring. NiMo/AluZ catalyst presents a more equilibrated proportion of aromatic and hydrogenated products as compared with the previous catalysts (Figure 5). In the aromatic group, cracking products (A-C) are predominant over the other aromatic compounds, in the entire range of tetralin conversions analyzed. The presence of A-RO olefin products are more important than the preceding catalysts and tends to decrease at higher tetralin conversions, either by hydrogenation to the corresponding ARO compounds or by cracking/dealkylation to form A-C compounds, as suggested by the marked increase in the yields of aromatic cracking products at high conversions. The formation of dinaphthenic and ring opening compounds (H

+ H-RO) over NiMo/AluZ is predominant and presents a maximum yield for tetralin conversions of approximately 60%. At higher conversions, H and H-RO compounds are transformed into cracking (H-C) compounds in a larger extension, presenting a decrease in the yield of this group. These results suggest that, over a catalyst presenting strong acid sites as NiMo/AluZ, once the C−C bond of the naphthenic ring is broken, the formed compound, alkyl-benzene or alkylmononaphthene, is easily cracked or dealkylated to lighter compounds. Reaction Scheme and Mechanisms. Considering the distribution of reaction products observed in this work as well as previous studies dealing with the conversion of tetralin over acidic and bifunctional catalysts, the simplified reaction scheme shown in Figure 6 can be proposed for the hydroconversion of tetralin over NiMo sulfide catalysts. Aromatic hydrogenation and isomerization steps were considered as reversible reactions.4,30,31 For alumina-supported NiMoS catalysts, presenting essentially hydrogenated products, only the initial steps of the proposed reaction scheme are observed, at least at moderated overall tetralin conversions (95%) for NiMo/Alu catalyst but this proportion decreases for NiMo/Si−Al and NiMo/AluZ catalysts (respectively, 60% and 39%). The effect of acidity on the formation of hydrogenated compounds during the hydroconversion of tetralin was somehow surprising considering that all catalysts showed similar dispersions of the sulfided metals, which are responsible for the catalytic sites for aromatic ring hydrogenation. The analysis of product yields with overall conversion of tetralin helped with understanding the effect of acidity on the formation of ring opening and cracking products but not with the higher hydrogenation activity of the more acidic catalysts. The acidity of the supports may be contributing to modify the activity of these catalysts, either by changing the intrinsic activity of the sulfide phase or by modifying the reaction scheme or mechanism. These aspects will be further developed in the next section by analyzing the reactivity of intermediate products of tetralin hydroconversion. Hydroconversion of Representative Intermediates of Tetralin Conversion. To have more insight on the tetralin reaction scheme over the acidic catalysts, the hydroconversion of some representative intermediate compounds were also studied with NiMo/Alu and NiMo/AluZ catalysts. Indan was selected because it has a similar structure of tetralin and can contribute to understand the effect of ring contraction on the overall activity. Decalin was studied because it is the primary product of tetralin hydrogenation and it is important to evaluate the reactivity of this compound in respect to ring opening and cracking reactions. Finally, the n-alkylbenzenes were tested aiming to evaluate the reactivity of the tetralin ring opening aromatic products vis-à-vis the hydrogenation and cracking reactions. As previously observed with tetralin hydroconversion, with acidic catalyst these molecules may present a lot of reactions products. To facilitate the analysis, reaction products were grouped according to the same approach used for tetralin products, except that no aromatic ring opening products group was considered in the case of n-alkylbenzenes hydroconversion. Comparative results of activity and product distribution for hydroconversion of tetralin, indan, and decalin are shown in Figure 8 and those for hydroconversion of n-alkylbenzenes are presented in Figure 9. Indan. The reaction pathway of indan over these catalysts can be considered similar to the one of tetralin. Primary hydrogenation products are cis- and trans-octahydroindan which can be isomerized to different dinaphthenic structures.

Figure 8. Activities for tetralin, indan and decalin hydroconversion on NiMo/Alu and NiMo/AluZ catalysts. Experimental conditions: pressure, 3 MPa; temperature, 583 K; feed flow rate, 13.5 mL/h; hydrogen flow rate, 0.6 NL/h; reactant concentration, 4 mol % in nhexane.

Figure 9. Activities for alkylbenzenes hydroconversion on NiMo/Alu and NiMo/AluZ catalysts. Experimental conditions: pressure, 3 MPa; temperature, 583 K; feed flow rate, 13.5 mL/h; hydrogen flow rate, 0.6 NL/h; reactant concentration, 4 mol % in n-hexane.

Dinaphthenic compounds and indan can also undergo naphthenic ring opening to form respectively alkylcycloalkanes and alkylbenzenes (C9 hydrocarbons). Subsequently these structures can be further dealkylated or cracked to form lighter compounds (C6 to C9 aromatic or hydrogenated compounds). Heavy compounds (C10 and C11 hydrocarbons) are mainly constituted by methylindans, tetralin, and C10 dinaphthenic compounds and were formed in higher yields as compared to tetralin hydroconversion. The formation of such compounds can be associated with (trans)alkylation reactions of indan and cracking products (e.g., toluene). Moreover, the formation of alkylated products was also previously reported on hydrotreating reactions in the presence of CS2 as H2S precursor.39 However, the higher yields as compared to the ones on tetralin reaction are not clear based on the present results. The results of indan hydroconversion are presented in Figure 8 (see also Supporting Information, Table S2, for detailed data on product distributions). Indan is more difficult to hydrogenate than tetralin on both catalysts. On the acidic NiMo/AluZ catalyst, there is an increase of hydrogenated, isomerization and cracking products as compared to the nonacidic catalyst, however the overall indan conversion and the hydrogenated product yields are clearly smaller than the ones obtained with tetralin. 2652


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of hydrogenated products were quite similar to the ones observed with alumina supported catalyst, indicating that there was not a clear effect of the support acidity on the overall hydrogenation activity of C1 to C3 benzenes. The acidity effect was observed in the product distribution of n-butylbenzene since there was an increase in the yields of isomerization and hydrogenated cracking products on NiMo/AluZ catalyst. The formation of hydrogenated heavy compounds was small for the four n-alkylbenzenes. The behavior of n-butylbenzene is quite different from the other alkylbenzenes since the total yield of hydrogenated products on NiMo/AluZ is much higher than on NiMo/Alu, resembling tetralin and indan hydroconversion on these catalysts. Mostly of the hydrogenated products are issued from cracking or dealkylation reactions, and C4 cyclohexanes or C5 cyclopentanes (ring opening products) are present in very low amounts. Jik et al.45 also observed a much higher reaction rate of dealkylation of t-butylbenzene as compared to other C1 to C3 benzenes, although the catalyst and reaction conditions were very different from the ones of the present work (reduced NiMo/alumina catalysts at 550 °C). The authors proposed that an acid-catalyzed mechanism was responsible for the higher conversion of the C4 benzene compound but a hydrogenolytic mechanism was predominant for the other alkylbenzenes. For all reactants, hydrogenated ring opening products were not observed indicating that either they are not formed or, otherwise, they are rapidly cracked to lighter compounds.

Isomerization of tetralin naphthenic ring forming methylindan favors ring opening reactions since the five-member ring is more constrained and less stable than the six-member naphthenic ring and because the tertiary carbon forms carbenium ions easier than secondary carbons.40 This behavior was not observed for indan with the present catalysts. Probably the presence of a tertiary aliphatic carbon on methylindan, as compared to the indan molecule, would favor ring opening reactions. Decalin. NiMo/Alu catalyst was almost inactive for decalin HCC (overall conversion of ca. 1%) as shown in Figure 8. In contrast, the activity of NiMo/AluZ catalyst was very high (almost complete at the same reaction conditions) and most of the reaction products were mono- and dinaphthenic compounds with important formation of cracking products (at this conversion level). Some alkylbenzenes were also observed, probably by hydroconversion of some tetralin or methylindan formed by dehydrogenation of decalin. A higher conversion of decalin as compared to tetralin over NiMo/USY catalyst was previously reported by Li et al.,41 in the frame of a study comparing different zeolites for HDS and HCC reactions. In that study, only the USY-supported catalyst presented significant reaction rates for these two reactions as compared to mordenite and ZSM-5 zeolites, which was attributed to a synergetic effect between the strong Brönsted acid sites present on the USY zeolite and the NiMo sulfided phase. Monteiro,42 using noble metal catalysts supported on USY zeolites, showed that decalins can be converted into ring opening and cracking products by direct protonation of the molecule over the strong acid sites of the support, without intervention of a classical bifunctional mechanism usually considered for the hydroconversion of (cyclo)alkanes over noble metal/acid support catalysts. Therefore, on the basis of these results, one can expect that the decalins (and their dinaphthenic isomers) formed by hydrogenation of tetralin on NiMo/AluZ catalyst are easily converted in their ring opening and cracking products. n-Alkylbenzenes. By increasing the number of carbon atoms in the alkyl chain of the homologue series, the formation of hydrogenated compounds increases slightly from toluene to nbutylbenzene over NiMo/Alu catalyst, with very little formation of cracking and isomerized naphthenic compounds, as shown in Figure 9 (see also Supporting Information, Table S3, for detailed data on product distribution). The formation of aromatic and heavy compounds was very low with this catalyst. For example, toluene conversion produced essentially hydrogenated products (methylcyclohexane and some C2 cyclopentane). The other n-alkylbenzenes presented the same behavior. Increasing hydrogenation activity with alkyl length or number of alkyl substituents in the benzene ring was already observed with NiMo or CoMo sulfided catalysts,43 and was attributed to the weakening of the aromatic character of the molecule by the presence of electron-donating substituents. On NiMo/AluZ catalyst, the overall conversions increased markedly from toluene to n-butylbenzene, mostly by the formation of aromatic cracking products. On toluene and ethylbenzene conversions, benzene was formed together with heavier compounds (HV) such as xylenes and diethylbenzenes, respectively, probably by transalkylation reactions on the USY zeolite.44 n-Propyl and n-butylbenzene presented small formation of heavier compounds and most of the aromatic products were issued from cracking of the alkyl chain and dealkylation reactions. Except for n-butylbenzene, the total yield

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DISCUSSION From the results presented in Figures 7 and 8, it is clear that the presence of the acidic Y zeolite in the support increases significantly the overall activity for the conversion of tetralin, affecting not only C−C bond cracking, as expected, but also the total yield of hydrogenated products, as compared with a nonacidic supported catalyst (NiMo/Alu). This effect was also observed, in a lesser extension, for the silica−alumina supported catalyst. The acid sites associated with the sulfide phase are considered weak for promoting C−C rearrangement or cracking27,28 as well as those present in the alumina surface. As a matter of fact, the activity of NiMo/alumina catalyst for ring opening and cracking reactions is very low as compared to the other catalysts. Therefore, for NiMo/Si−Al and NiMo/ AluZ catalysts, the acid sites associated with the support play a key role in the activity and product distribution observed in the hydroconversion of the different reactants. The interaction between the support acid sites and the hydrogenation sites (reduced metal or sulfided metal) on bifunctional catalysts has been studied by several authors and can be divided in two groups: (a) spillover of activated hydrogen from hydrogenation sites to the acidic support, increasing the hydrocracking activity and (b) formation of electron deficient metal or sulfide particles in close vicinity of the support acid sites, increasing the activity or sulfur tolerance. As examples of the first group, Stumbo et al.46,47 observed the formation of additional Brönsted sites on silica−alumina support by activated hydrogen spillover from a sulfided CoMo/ alumina catalyst and Camorim et al.48 suggested such a mechanism for explaining the enhancement of cumene dealkylation activity with the increase of metal content of sulfided NiMo/silica−alumina catalysts. The influence of the support acidity over metallic particles is well-known for noble metal-based catalysts for which the 2653


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As depicted in Figure 6, the hydroconversion of tetralin follows a complex network in which sequential and parallel steps of aromatic ring hydrogenation and C−C bond cracking take place. In such a way, some intermediate compounds of tetralin hydroconversion, such as methylindan or alkylbenzenes, could be easier to hydrogenate than the original reactant, favoring the formation of hydrogenated products. The results obtained with the hydroconversion of indan and alkylbenzenes showed that these intermediates have similar or lower activities than tetralin for hydrogenation reactions. Considering that indan and alkylbenzenes isomers would be present in lower concentrations than tetralin along the reactor, the net hydrogenation rate of these intermediates must be lower than the direct hydrogenation of tetralin to decalins. The slightly higher hydrogenation activity observed for n-butylbenzene cannot be invoked as an alternative path for the formation of hydrogenated products on tetralin conversion since the formation of hydrogenated ring opening products on this reaction, at low conversion levels, is much smaller than the formation of dinaphthenic compounds indicating that the hydrogenation of tetralin to decalins is the preferred route. Once formed, decalins can be easily converted to ring isomerization and ring opening products as shown by the isolated study of hydroconversion of this molecule on NiMo/ AluZ catalyst. A possible explanation for the influence of the acidic sites of the zeolite in the hydrogenation activity of the NiMoS/AluZ catalyst may be related to the migration of protonated species from the zeolite cages (or from the zeolite−alumina interface) to the NiMoS sites. Such protonated species would be easier to hydrogenate than tetralin or other monoaromatic compound due to the lack of stability of the aromatic ring. Apparently, the formation of these species would be favored on the conversion of tetralin and indan but not on n-alkylaromatic compounds (except n-butylbenzene).

formation of electron deficient particles would contribute for enhancing the hydrogenation activity and sulfur tolerance.49 This concept has been extended to sulfided catalysts more recently. The beneficial influence of the support acidity on some hydrotreating reactions and, specifically, on hydrogenation activity of metal sulfides was already discussed in the literature. Most of these works succeed in obtaining a good dispersion of the sulfided phase inside the zeolite cages so that the close vicinity of the hydrogenation function and the acidic sites was claimed to be at the origin of this effect. In such a way, Breysse and co-workers observed an increase of tetralin hydrogenation activity with support acidity using MoS2/betazeolite50 and for tetralin and toluene hydrogenation using RuS2/USY-zeolite catalysts.22 In both cases, metal precursors were introduced into the zeolite structure by ionic exchange so that metal sulfide clusters inside the zeolite cages were obtained after catalyst sulfidation. Using FTIR spectroscopy of adsorbed CO on such catalysts, the authors observed a displacement of the ν(CO) frequency to higher values with increasing acidity what was claimed as evidence of electron transfer between the sulfide particles and the zeolites. Leyrit et al.51 also observed higher toluene hydrogenation activity of unpromoted MoS2/ USY-zeolite catalysts as compared to promoted NiMo and CoMo/alumina catalysts, especially for lower metal loadings. This behavior was attributed to the formation of molybdenum sulfide clusters inside the zeolite cages, presenting higher hydrogenation activity as compared to MoS2 slabs, and to their proximity to the acid sites of the zeolite structure. Similar explanation was proposed by Vissenberg et al.23 which observed a synergetic effect between the proton acidity of USY zeolite and cobalt sulfide particles in the HDS reaction. Chen et al.,52 using CO adsorption on sulfided CoMo catalysts supported on boria-alumina carriers, observed a good correlation among the Brönsted acidity of the support, the electron deficiency of the CoMoS sites, and the hydrogenation activity in the conversion of 4,6-DMDBT. In the present work, an alumina−zeolite support was used instead of pure zeolite as in the aforementioned works. Characterization results of our catalysts indicate that the sulfide phase of NiMo/AluZ catalyst is mainly deposited in the alumina fraction of the support. Indeed, the similarity of molybdenum sulfide slabs dimensions (length and stacking) and of TPR patterns21 support this hypothesis. Moreover, the hydrogenation activities of toluene, ethylbenzene, and propylbenzene (Figure 9) were very similar for both alumina and alumina−zeolite supported catalysts, indicating that the intrinsic hydrogenation activity of the sulfide phase is apparently not affected by the zeolite acidity as observed by Leyrit et al.51 Therefore, an electronic effect of acid sites over the sulfide phase producing more active electron deficient particles cannot be the main reason for explaining our tetralin hydroconversion results. Although most of the sulfide phase must be present in the alumina support, it is not possible to exclude that part of the impregnated nickel could be exchanged in the zeolite structure since this metal precursor is in the cationic form in the impregnating solution. Even if it would be the case, the effect on the hydrogenation activity is expected to be low as discussed in the work of Francis et al.53 These authors observed the same toluene hydrogenation activity for NiMo/alumina−zeolite catalysts prepared by coextrusion of alumina and USY zeolite exchanged with different amounts of nickel.

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CONCLUSIONS NiMo catalysts supported on alumina and acidic carriers were studied over a large range of operating conditions and overall conversion of tetralin. The NiMoS/AluZ catalyst was more active for tetralin hydroconversion than a NiMoS/Alu catalyst. The presence of the zeolite component in the support promoted not only typical acid sites reactions, such as ring contraction, ring opening, and cracking, but also the formation of hydrogenated compounds. The hydroconversion of some intermediate products in the tetralin hydroconversion network, such as indan, decalin, and alkylbenzenes, were also studied with NiMo/Alu and NiMo/ AluZ catalysts. Indan presented a similar behavior of tetralin, that is cracking and hydrogenation activities were promoted on NiMoS/AluZ catalyst, as compared to NiMoS/Alu, but indan was slightly less active than tetralin in these catalysts. The activity for hydroconversion of a series of nalkylbenzenes (C1 to C4) was also studied. For C1 to C3 benzenes, NiMoS/AluZ catalyst presented the same activity of NiMoS/Alu for the formation of hydrogenated products but the acidic catalyst presented, as expected, higher formation of cracking products. The only exception was butylbenzene for which the acidic catalyst presented higher activity for cracking and hydrogenation, as previously observed for tetralin and indan. The NiMoS/AluZ catalyst also presented a high activity for converting decalin, indicating that this hydrogenated 2654


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intermediate, once formed by hydrogenation of tetralin, is fast isomerized and cracked. Therefore, these results indicate that the higher hydrogenation activity of NiMoS/AluZ catalyst cannot be explained by the formation of intermediates easier to be hydrogenated, such as indans (ring contraction product) or alkylbenzenes (ring opening product) since the hydrogenation activity with these molecules were similar or smaller than that observed with tetralin. Moreover, an electronic effect between the acid and hydrogenation sites is not likely since the characterization results indicate that these sites are located in different domains, respectively, in alumina and Y zeolite. A tentative explanation would be the migration of protonated aromatic species from the zeolite cages (or from the zeolite− alumina interface) to the NiMoS sites. Such protonated species would be easier to hydrogenate than tetralin or other monoaromatic compound due to the lower stability of the aromatic ring.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information is provided on product characterization and product distribution of the hydroconversion reactions described in this study (Figure S1 and Tables S1 to S3). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +55-21-21626630. Fax: +55-21-21626627. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Rosana Cardoso for the contribution of GC−MS characterization of the reaction products, Dayse L. Fonseca and Anilza A.L. Correa for catalyst preparation, and Denise Costa and Denise Filgueiras for acidity and textural characterization.

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Influence of Support Acidity of NiMo Sulfide Catalysts for

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