Acidity control of ruthenium pillared clay and its application as a catalyst in hydrogenation reactions

Applied Catalysis A: General 371 (2009) 131–141

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Acidity control of ruthenium pillared clay and its application as a catalyst in hydrogenation reactions F.C.A. Figueiredo a, E. Jorda˜o a, R. Landers b, W.A. Carvalho c,* a

Laboratory of Catalytic Processes Development, Chemical Systems Engineering Department, Faculty of Chemical Engineering, Universidade Estadual de Campinas, Cidade Universita´ria Zeferino Vaz, CP. 6066, CEP 13083-970, Campinas, SP, Brazil b Laboratory of X-Ray Diffraction, Physical Applied Department, Physical Institute, Universidade Estadual de Campinas, Cidade Universita´ria Zeferino Vaz, CP. 6165, CEP 13083-970, Campinas, SP, Brazil c Centro de Cieˆncias Naturais e Humanas, Universidade Federal do ABC, Rua Santa Ade´lia, 166, CEP 09210-170, Santo Andre´, SP, Brazil

A R T I C L E I N F O

A B S T R A C T

Article history: Received 7 May 2009 Received in revised form 21 September 2009 Accepted 29 September 2009 Available online 3 October 2009

Aluminum pillared clay was used as a support for the preparation of catalysts containing ruthenium as active metal, in the presence or not of a promoter (tin) and an acid reducer (barium). The catalysts were characterized and tested for hydrogenation of dimethyl adipate reactions. The results showed a high conversion, typically above 95%, in all studied systems. This conversion occurs irrespectively of the presence of active metal; therefore, it may be due to the support’s intrinsic activity. The acidity was identified as the main cause of this activity. A barium treatment of these catalysts significantly reduced this acidity, thus making more selective the available systems. The catalysts treated with barium reduced the formation of undesirable products up to 45%. Similarly, the presence of tin helped to obtain valuable products such as gamma-caprolactone and methyl caproate, which reached high selectivity values (18.6% and 16.9%, respectively). ß 2009 Elsevier B.V. All rights reserved.

Keywords: Pillared clay Hydrogenation Ruthenium Acidity control Adipate Lactones

1. Introduction Clay may be used as a catalyst in industrial processes of petroleum cracking, among others. However, only the external surface of the material is active in the catalysis, since large organic molecules cannot penetrate between its layers. Besides, the hydrophilic character of the layers does not allow access to apolar molecules, even with reduced dimensions [1,2]. In trying to obtain thermally stable materials with a greater interlayer distance, the clay has been submitted to the intercalation of cations acting as pillars, keeping the silicate layers separate, even in the absence of solvents. Several types of cations may be used in the construction of these pillars, including alkyl ammonium ions, bicyclic amines, metal chelates and polynuclear hydroxy metals [3]. Both interlayer distance and lateral distance or pillar density are responsible for the catalyst’s selectivity form, since they can control the speed of diffusion of the reagents and products or the formation of reaction intermediaries [4]. One of the great advantages of using pillared clay stems comes from the possibility of altering its properties by varying the

* Corresponding author. Tel.: +55 11 4996 0174; fax: +55 11 4996 0174. E-mail address: [email protected] (W.A. Carvalho). 0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2009.09.039

synthesis’ condition. For instance, in varying the ratio Al:clay in the Al-PILC synthesis, it is possible to obtain materials with different acidity and porosity. Salerno and Mendioroz [5] promoted synthesis with the ratio Al:clay of 5, 10 and 30. In all cases there was pillaring, but the material obtained from ratio 5 presented a considerable fraction of clay in the non-pillared form. The intercalation of redox metal ions in smectites, such as the montmorillonite, may lead to the formation of ‘‘redox pillared clay’’ containing metal oxide species in the interlayer space, with different catalytic activities and stabilities. However, special attention should be given to the acidity of these materials, as the introduction of pillars contributes to the acidity of the structure, which should affect the composition of the products obtained in organic compounds catalytic reactions. In pillared clay, besides the supporting layers, metal oxide pillars contribute to the material’s acidity, leading to the formation of structures presenting both Bronsted and Lewis acidity [6], and this is superior to the original clay. This increase in the acid feature of clay was attributed to two factors: the growth in the specific area with relation to nonpillared clay and the appearance of new acid sites in the pillars and in the connecting sites between the pillars and the clay layers. The reduction of a carbonyl group is a relevant step for synthesizing fine chemicals from renewable raw materials. The hydrogenation of carboxylic acids and their esters is rather

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challenging due to the less susceptibility to nucleophilic attack at the C=O bond when compared to aldehydes or ketones. Therefore drastic conditions (hydrogen pressure from 200 to 300 bar; temperature from 423 to 523 K) are often used in the manufacturing processes to achieve an acceptable productivity over copper– chromite based catalysts [1]. The adipic acid dimethyl ester (dimethyl adipate – DMA) was chosen as model reaction also considering the industrial relevance of the catalytic hydrogenation of C3–C6 dicarboxylic acids in the production of a wide variety of fine chemicals. Attention was given to activity and product distribution in liquid phase hydrogenation, at mild reaction conditions throughout the whole reaction. Among the valuable products that could be obtained in adipate hydrogenation, 1,6hexanediol, lactones and methyl caproate are cited. Drastic conditions are often used though in the manufacturing processes to achieve an acceptable productivity of these compounds [7]. Diols are industrially important as they are used as raw materials for a wide variety of fine chemicals, pharmaceuticals and biodegradable polymers. e-Caprolactone is widely used as a raw material for the production of polyester and other polymers. eCaprolactone is typically produced by the Baeyer–Villiger oxidation of cyclohexanone; for example, when conducted by a cooxidation process in which cyclohexanone is oxidized with air in the coexistence of acetaldehyde or an oxidation process in which an organic peracid such as peracetic acid is used as an oxidizing agent. Gamma-caprolactone is commercially and industrially attractive because of its use as a flavor additive in foods and tobacco and for its potential as an intermediate for insecticides. The methods for the production of gamma-caprolactone are either expensive or do not produce good yields. Generally, they use epsilon-caprolactone as raw material. Methyl caproate is used as flavor and fragrance chemical compound. Selective catalytic hydrogenation of a carbonyl group is usually carried out over Group VIII metal (Pt, Rh, Ru, etc.) catalysts. Ni, Cu and Co catalysts could be another option. However, these base metals tend to require higher hydrogen pressures [7]. Moreover they are generally less active against carboxylic and ester groups [8]. Our group has previously reported a systematic study concerning the use of different noble metals and some potential promoters concerning dimethyl adipate hydrogenation. The results indicated that some alternative catalysts based on noble metals can be applied to this reaction, at relatively low hydrogen pressures and temperatures. Ruthenium appeared as a promising active phase when modified by a second metal, especially tin. Some metal composition, varying from 0.3 to 5 wt% was already tested [9]. A nominal ruthenium loading of 2 wt% demands the use of the most appropriate catalyst [10]. In previous studies, we have evaluated the influence of the support, the active metal and the presence of promoting agents, in the hydrogenation of dimethyl adipate using the catalytic systems Pt or Pd/Al2O3, Pt or Pd/C and Pt or Pd/TiO2 whether containing or not Sn as an additive [11]. In the alumina-supported catalysts, there was a major conversion of dimethyl adipate in the Pd’s catalyst (68.4%). The catalysts supported in titanium promoted a smaller conversion of dimethyl adipate (25.3% and 14.5%, respectively for Pt and Pd). The addition of Sn to the catalysts increased the conversion to 31.4% and 29.5%, respectively for Pt–Sn and Pd–Sn. Ruthenium was also studied and appears as a promising active phase when modified by a second metal such as cobalt, zinc and, especially, tin [12]. Recently, we have evaluated the behavior of a kind of pillared clay containing platinum in the hydrogenation of dimethyl adipate [13]. Results indicate that platinum species are partially occupying the inner porous network of the pillared clay, a solid with an amphoteric character, and these crystallites are small enough to occupy the solid

inner micropores. Alumina and Al-PILC supports are active as catalyst, allowing adipate conversion of 26.3% and 83.3%, respectively, and the products distribution is much influenced by the solid surface acidity. When platinum containing catalysts were tested, the products distribution indicates that cracking and hydroisomerization reactions attributed to the acidity of the supports and to the presence of platinum occur preferentially. In view of the evidence that ruthenium in the presence of tin constitutes an active catalyst in esters hydrogenation [14], in this work we have investigated the preparation, characterization and catalytic activity of a catalyst containing ruthenium and tin supported in aluminum pillared clay. 2. Experimental 2.1. Catalysts preparation Catalysts were prepared by using the method of impregnation in aqueous suspensions, in order to obtain a solid with nominal concentration of 2 wt% in ruthenium. Support applied here was montmorillonite aluminum pillared clay (Al-PILC, Fluka). Precursor salt used was RuCl3.0.5H2O (Aldrich, 98%). Impregnation was done by adding support and solution containing metallic precursor in a rotative evaporator, with bath at 333 K and vacuum for 5 h. Next, solid was oven dried for 12 h, at a temperature of 393 K. Catalysts were then subjected to calcination treatment for 4 h at 673 K, under 50 mL min 1 of synthetic air flow, and reduction for 2 h at 673 K, under flow of 40 mL min 1 of 2% H2 and 98% N2 mixture. In the preparation of bimetallic catalyst, the preparation stages were similar to those used to obtain a monometallic system as described above. The only difference lays in the addition to the balloon, together with the ruthenium solution, another solution containing tin (II), obtained from SnCl2 (Aldrich, 98%), in the proper amount as to respond to an atomic ratio ruthenium:tin of 1:2. These catalysts were submitted to impregnation with barium (Ru/Al-PILC-Ba and RuSn/Al-PILC-Ba), aiming at reducing the surface acidity. Each 2 g of catalyst was treated with 200 mL of 0.1 mol L 1 Ba(NO3)2. The system was kept under magnetic stirring at room temperature for 2 h. Solid phase was separated by vacuum filtration and washed with deionized water until absence of barium cations. This solid was oven dried at 383 K for 6 h and submitted to calcination and reduction treatments. 2.2. Characterization Micrometrics ASAP 2010 device was used to obtain adsorption– desorption measurements at 77 K, under pressure ranging from 0.01 to 1.23 bar. Before each measurement, samples were outgassed at 423 K and 1.3  10 5 mbar for 12 h. Specific areas were obtained according to Brunauer, Emmett and Teller (BET) method. The t-plot method was used to evaluate the micropore volume. Hydrogen sorption was done with catalysts samples of 0.4 g. Initially, their surfaces were cleaned by evacuation of the system under He flow at 393 K. After that, the catalysts were reduced ‘‘in situ’’ at 673 K for 2 h under H2 flow and the system was then evacuated under H2 flow for 1 h until reaching the sorption temperature (308 K). The sorption measurements were carried out under H2 pressure varying from 10 to 400 mmHg. The X-ray photoelectrons spectra (XPS) were obtained in a hemispheric analyzer HA100 VSW, operating in fixed transmission mode (passing energy of 44 eV), which provides a line width of 1.6 eV of Au 4f7/2. The radiation Ka of the aluminum (1486.6 eV) was used for exciting. Pressure was maintained at values below 2  10 8 mbar during the entire analysis.

F.C.A. Figueiredo et al. / Applied Catalysis A: General 371 (2009) 131–141

Assays of temperature programmed reduction (TPR) were held on a quartz reactor, using 100 mg of each catalyst under reductive gas mixture flow containing 1% hydrogen and 99% helium, at outflow of 25 mL min 1. Analysis temperature was programmed to vary from 298 to 1073 K, with 5 K min 1 heating rate. Throughout the analysis, hydrogen consumption profiles in function of the temperature were recorded by using a Balzers Omnistar Quadrupole Mass Spectrometer, model QMS200. FTIR spectra of chemisorbed pyridine were obtained using a Nicolet Prote´ge´ 460 FTIR spectrometer. Each 10 mg sample was pressed (for 15 min at 10 ton cm 2) into a self-supporting wafer of diameter 12 mm. The wafers were calcined in air at 773 K for 2 h followed by exposure to pyridine vapor for 2 h at room temperature. To remove physisorbed pyridine, the wafers were held under vacuum for 1 h. The wafers were submitted to thermal treatments at 298, 373, 473, 573, 673, 773 and 873 K, and the corresponding spectra were recorded. Pyridine’s temperature programmed desorption (TPD) was performed by monitoring the weight loss of the same samples tested by FTIR, in a Thermogravimetric Analyser Netzsch TG209. Temperature range varied from 298 to 1173 K, at a heating rate of 20 K min 1 and flow of synthetic air (100 mL min 1). The samples (approx. 8 mg) were placed in an alumina crucible. The results of weight loss of each sample containing pyridine were obtained after subtracting the weight loss of an original sample (without pyridine) under the same test conditions, so that the result reflects only the pyridine content incorporated in each sample according to temperature. 2.3. Catalytic tests Dimethyl adipate hydrogenation was held on a high-pressure 300 mL stirred batch Parr reactor (Model 4566), made of AISI 316 stainless steel equipped with mechanical stirring axis and speed controller. Reactions were carried out in liquid medium with 1,4dioxane as suitable solvent to keep the reaction phase homogeneity. The system was kept at 523 K and 50.7 bar of H2 [14], under constant mechanical agitation speed of 1500 rpm. Typically, 1.40 g of catalyst, 80 mL of 1,4-dioxane, 7.09 g of dimethyl adipate and 3.6 g of tetradecane (internal standard) were used. Samples were collected for each reaction for 15 h. Reaction products quantification was done by using gas chromatography through a Thermo Quest Trace GC 2000 chromatograph with a HP1 capillary column and for internal standard, tetradecane. Identification of the main products was previously held in a gas chromatograph attached to a HP5988A mass spectrometer (GC–MS). 3. Results and discussion 3.1. Catalysts characterization All nitrogen adsorption isotherms for the solids presented Type IV features, according to Brunauer, Deming, Deming and Teller classification [15], indicating the presence of mesopores. As an example, the isotherm of Ru/Al-PILC in Fig. 1 is presented. Desorption curves follow a different route until a critical value of P/P0 is reached. According to the IUPAC classification, the pillared clay isotherms present Type H4 hysteresis cycle, without any indication of plateau at high P/P0 values. As shown by Sing et al. [16], this behavior may be obtained with aggregate laminated particles, and corresponds to the adsorption in slitshaped pores formed between blade-shaped particles. In this type of pore, we have a multi-layer formation during the adsorption process, while desorption occurs by capillary evaporation.

133

Fig. 1. N2 adsorption-desorption isotherms of Ru/Al-PILC.

Table 1 Area and volume of pillared material pores. Catalyst

Al-PILC Ru/Al-PILC Ru/Al-PILC C RuSn/Al-PILC

Area (m2 g

1

)

Volume (mL g

1

)

Micropores

Total

Micropores

Total

131.5 94.1 116.8 94.9

225.2 158.2 181.5 164.9

0.0697 0.0502 0.0542 0.0587

0.2536 0.1751 0.1829 0.1858

As indicated in Table 1, the incorporation of the metal causes a small reduction in the volume of nitrogen adsorbed in the entire P/ P0 range values, similar to that observed by Campanati et al. [17]. This effect should be related to the blocking of some pores by metal species deposited on the support’s surface. Even after calcination, there was no significant change in the surface area, showing that Ru has a reduced mobility in the pillared clay surface. This is in accordance with the low dispersion obtained for this metal. If we consider that the mesopores’ volume can be estimated by the difference between the total pores volume and the micropore volume [18], it is possible to evaluate that ruthenium particles affect the micropores (reduction of 28.0%) as well as the macro and mesopores (reduction of 32.1%). After calcination, the micropores are reduced (in relation to Al-PILC) by 22.2%, while the reduction of the macro and mesopores is of 30.0%. This suggests that there are solid ruthenium particles whose average diameter is small enough to occupy the micropores originally present in the solid. Analyses of hydrogen chemisorption have indicated that the ruthenium dispersion on the surface of pillared clay is low (3.4%). This reduced dispersion influenced the calcination process on the metal, but did not prevent an adequate reduction. Results of XPS from the non-calcined catalyst (Fig. 2), indicated the presence of signals relative to Ru(III), originated from the metallic precursor. After calcination, we have the RuO2 signal and with a fraction of 82.7%, the signal from Ru(0). As the ruthenium was identified with a low dispersion on the catalyst surface, we may assume that the calcination thermal treatment promoted the decomposition of the metallic precursor, but oxidized only the metal present on the ruthenium particles surface (Fig. 3). In the bimetallic catalyst (spectra not showed in this work), we have the signals relative to a small parcel of Ru(0) and to RuO3, with a fraction of 78.3%. Tin was found mainly in oxidized form (94.2%) and a small quantity in the form of Sn(0) (5.8%). As the

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Fig. 2. High-resolution XPS spectra of Ru 3p in Ru/Al-PILC (left) and Ru/Al-PILC calcined (right) catalysts.

binding energy values of species Sn(II) and Sn(IV) are very close, it was not possible to establish to which species the signal at 487.8 eV is related to. Different from the monometallic catalyst, the ruthenium could be easily oxidized in the presence of tin, with the formation of Ru(VI), not identified in the monometallic catalyst. Concerning metallic species reduction, it shows only one peak of hydrogen consumption, relative to reduction of Ru(IV) into Ru(0) [19]. The uncalcined material also shows, besides the main peak at 452 K, a shoulder at 420 K, probably related to the Ru species interacting less vigorously with the support. In the calcined material, the only peak observed, at 479 K, may be related to the reduction of the oxide RuO2, formed during calcination [20]. Bimetallic catalyst presents two reduction peaks, at 493 and 670 K, corresponding respectively to Ru and Sn reduction. Ru reduction occurs at the same temperature observed in monometallic samples, indicating that alloys with Sn were not formed. The theoretical consumption of H2 for the non-calcined catalyst, considering the conversion from Ru(III) to Ru(0), is of 14.8 mmol H2/mg metal. As for the calcined material, for a conversion from Ru(IV) to Ru(0), the consumption would be of 19.8 mmol H2/mg metal. We obtained the values of 16.7 and 21.6 mmol H2/mg metal, respectively, for the materials before and after calcination. These values are close to the theoretical ones, which indicate that the reduction process, essential for the activation of the catalyst before

its addition to the reaction medium, was adequate even considering the low metallic dispersion. The infrared analyses performed on the solids impregnated with pyridine offered spectra such as those presented in Fig. 4. Peak at around 1620 cm 1 is due to water bending vibrations, while a narrow band below 1300 cm 1 (maximum around 1050 cm 1, not shown in the figure) was ascribed to the Si–O–Si antisymmetric vibration [21]. We may observe the bands related to the interaction of pyridine with the Bronsted (1580 cm 1) and Lewis (1445 cm 1) acid sites, besides the bands in 1490 cm 1, where the contribution of the two types of acid sites are present in the material. The presence of a considerable quantity of these sites at the working temperature of catalytic reaction (523 K) contributes both to the acidity of the solids and to the activity of the catalytic system. The acid characteristics of the catalyst were modified through barium treatment. We promoted the deposition of Ba(II) on the pillared clay surface, which already contains the impregnated active metal, providing the material identified as Ru/Al-PILC-Ba). In this process, we got the partial recovery of the surface of the solid with Ba (II), which has caused at least two important modifications: the parameter of dispersion obtained through analysis of chemisorption diminishes and Lewis acidity is practically eliminated. In the dispersion case, we have that part of the metallic particles present on the surface of the solid recovered by Ba (II), which causes a small reduction in ruthenium dispersion from 3.4% to 3.1%. As a consequence of the deposition of Ba (II), a reduction also occurs on the surface area, mainly related to shrinkage of 13.2% in the area of micropores. Corbos et al. [22] demonstrated

Fig. 3. TPR profiles of Ru/Al-PILC catalysts. In detail, RuSn/Al-PILC.

Fig. 4. FTIR spectra of Ru/Al-PILC with pyridine, after thermal treatments.

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135

Fig. 5. FTIR spectra of Ru/Al-PILC (left) and Ru/Al-PILC Ba (right) samples impregnated with pyridine, after treatment at room temperature.

that the addition of barium leads to a reduction in the surface area of platinum catalysts supported in alumina and silica, this effect being more pronounced with the increase in the quantity of incorporated barium. Consistent with the results obtained in this work, the authors stated that the reduction in the BET area is mainly related to a reduction in the pores volume caused by its filling with barium. In the case of acidity reduction, various authors have suggested that the aluminol groups present on the surface of solids are neutralized by the addition of Ba(II) [23,24]. Kwak et al. [25] demonstrated that the barium oxide deposited on the g-alumina is present in pentacoordinated Al(III) sites. These sites are associated to the acid character identified in a number of materials such as zeolites [26] and pillared clay [27]. The FTIR analysis of the samples with pyridine (Fig. 5) showed a significant reduction in bands intensity related to acid sites after treatment with Ba (II). This result, associated to a reduced modification in the metallic dispersion on the surface area and in the volume of pores, proves that the barium treatment was adequate, significantly altering only the acidity of the solids. After this qualitative analysis, we performed the quantification of acid sites by weight loss in the samples impregnated with pyridine. The results are shown in Fig. 6, and confirm that treatment with barium deeply modifies the acidity of the solid surface. In the original sample we observed a reduction in the number of acid sites with increasing temperature. At 298 K, we found about 3 acid sites per nm2. This value is reduced to half after treatment at 873 K. This behavior is in agreement with literature data for pillared clays [28]. On the other hand, the sample treated with barium presented less than 1 acid site per nm2 at 298 K, which demonstrates the efficiency of this treatment in reducing acidity.

generated are typical from hydroisomerization and cracking reactions, and related to the support acidity [13]. We promoted adipate hydrogenation reaction in the presence of catalysts containing ruthenium. The results are summarized in Table 2. There was practically total consumption of dimethyl adipate in the systems containing ruthenium, with conversions higher than 90%, and reaction times lower than that observed in the Al-PILC catalyzed system. After 20 min reaction time, the conversion in systems without Ba reached 65.3% (Ru) and 24.0% (RuSn). In Ba containing systems, the conversion within the same time was 54.4% (Ru) and 21.0% (RuSn). Monometallic catalyst was more active, but less selective than bimetallic RuSn catalysts. Barium slightly reduces the conversion, but it deeply changes the selectivity of the system. The distribution of the formed products is presented in Figs. 7–10, respectively for Ru/Al-PILC, Ru/Al-PILCBa, RuSn/Al-PILC and RuSn/Al-PILC-Ba. Although the total conversion of dimethyl adipate in the monometallic system is high, the selectivity values are rather low. The system presents a high amount of products not quantified

3.2. Catalytic activity A previous evaluation of the reaction medium activity, in the absence of the catalyst and in presence of the support (Al-PILC) as catalyst, was performed and presented in a previous work [13]. It was demonstrated that the consumption of dimethyl adipate in the absence of a catalyst occurs in the first hour and corresponds to only 3.6% of the substrate originally added. On the other hand, when using Al-PILC as catalyst, 83.3% of the adipate initially present in the reaction medium is consumed. The products

Fig. 6. Number of acid sites per surface area of samples impregnated with pyridine, after treatment at various temperatures: Ru/Al-PILC (*) and Ru/Al-PILC Ba (&).

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136

Table 2 Concentration of hydrogenation products from dimethyl adipate reaction catalyzed by monometallic catalysts, modified or not with Ba. Catalyst

Conversion (%)

Al-PILC Ru/Al-PILC Ru/Al-PILC Ba RuSn/Al-PILC RuSn/Al-PILC Ba

83.3 97.0 93.8 98.4 98.9

Concentration (mmol L

1

)

Ane

Ol

e-one

Oic

Diol

g-one

MME

CM

Others

2.5 24.1 34.9 42.0 145.1

0.5 3.5 – – 6.2

0.5 11.0 18.1 15.0 14.9

– – – 32.2 21.4

– 3.0 16.0 7.6 8.8

– – – 58.9 74.5

8.6 40.0 102.7 24.3 17.9

0.8 11.2 10.3 71.3 67.7

318.8 294.2 202.5 140.3 37.5

Ane = hexane, Ol = hexanol, e-one = e-caprolactone, Oic = caproic acid, Diol = 1,6-hexanediol, g-one = g-caprolactone, MME = adipic acid monomethyl ester, CM = methyl caproate.

individually (‘‘others’’), resulting from the undesirable substrate reactions. This behavior has already been observed in systems catalyzed by Pt, Pd and Ru/Al2O3 [12], and it seems related to an intrinsic activity of the support. Upon analysis of the formation of products over time, we can identify the occurrence of a significant increase in concentration of

Fig. 7. Products selectivity obtained with catalyst Ru/Al-PILC at 523 K and 50 atm of H2 ((*) hexanol, (*) e-caprolactone, (~) 1,6-hexanediol, (&) methyl caproate, (*) monomethyl ester, (~) hexane).

Fig. 8. Products selectivity obtained with catalyst Ru/Al-PILC-Ba at 523 K and 50 atm of H2 ((*) e-caprolactone, (~) 1,6-hexanediol, (&) methyl caproate, (*) monomethyl ester, (~) hexane).

adipic acid monomethyl ester (MME) in the first two hours of reaction, followed by its consumption along the reaction. The assumption is that the rapid increase in MME concentration is related to a conversion (Eq. (1)) mediated by active metal. In fact,

Fig. 9. Products selectivity obtained with catalyst RuSn/Al-PILC at 523 K and 50 atm of H2 ((*) hexanol, (*) e-caprolactone, (~) 1,6-hexanediol, (&) methyl caproate, (*) monomethyl ester, (~) hexane), (&) g-caprolactone, (") caproic acid).

Fig. 10. Products selectivity obtained with catalyst RuSn/Al-PILC Ba at 523 K and 50 atm of H2 ((*) hexanol, (*) e-caprolactone, (~) 1,6-hexanediol, (&) methyl caproate, (*) monomethyl ester, (~) hexane), (&) g-caprolactone, (") caproic acid).

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137

Table 3 Concentration of hydrogenation products originated from dimethyl adipate reaction (conversion of 50%) catalyzed by catalysts supported on different solids. Catalyst

Time (h)

Ru/Al-PILC Ru/Al-PILC Ba RuSn/Al-PILC RuSn/Al-PILC Ba RuSn/TiO2 Ru/Al2O3 RuSn/Al2O3 Pd/Al2O3 Pt/Al2O3 PtSn/TiO2 PdSn/TiO2

0.3 0.3 2.0 1.5 15 4 6 7 15 15 15

Concentration (mmol L

1

Ane

Ol

e-one

Oic

Diol

g-one

MME

CM

Others

10.9 21.9 37.6 28.7 – – – 8.5 4.6 18.6 8.8

3.3 – 5.2 6.2 0.6 – 2.0 2.0 0.2 9.2 7.8

3.7 7.3 2.9 4.8 1.0 – 14.0 7.8 3.6 17.4 30.2

– – 50.3 44.5 – – – – – – –

3.2 26.1 2.2 4.0 26.6 – 6.0 2.5 1.0 83.6 35.0

– – 3.2 10.2 – – – – – – –

83.0 105.8 77.3 60.5 – 150.0 8.0 12.5 7.8 8.0 6.6

7.8 9.5 15.4 12.4 1.5 – 30.0 5.0 5.0 24.8 20.6

88.1 29.5 6.3 30.2 155.7 50.0 134.0 161.7 177.8 38.4 91.0

)

Ref

a a a a

[11] [12] [12] [10] [13] [13] [10]

Ane = hexane, Ol = hexanol, e-one = e-caprolactone, Oic = caproic acid, Diol = 1,6-hexanediol, g-one = g-caprolactone, MME = adipic acid monomethyl ester, CM = methyl caproate a This work.

the O–CH3 bound hydrogenolysis of the ester function, followed by a hydrogenation to generate the acid group, has already been attributed to the presence of noble metals, such as Rh and Ru [29]. The second conversion process leading to the consumption of the MME originally formed generates many other products identified by GC–MS.

later consumption. This is also the behavior of hexane, indicating that these products are undergoing new conversions in the reaction medium. Gamma-caprolactone follows a different pattern. Similar to that observed with methyl caproate, and on a minor scale with e-caprolactone, its concentration increases during the entire reaction period. In this reaction medium, 4-hexenoic acid

(1)

The cyclic product obtained, e-caprolactone, may have been formed by two distinct routes: hydrogenation (Eq. (2)) or intramolecular esterification (Eq. (3)) of dimethyl adipate. Similarly, this conversion may occur from the MME.

and 5-hexenoic acid methyl esters were also identified in small concentrations (lower than 2 mmol L 1). The formation and subsequent conversion of these compounds will be discussed later. After modification with barium, the bimetallic catalyst pre-

(2)

(3)

The presence of acid sites associated to active metal seems to favor the formation of this cyclic product, as suggested by Aurox et al. [30], which may indicate the contribution of the support in the formation of e-caprolactone. The modification of the monometallic barium catalyst kept high the adipate conversion, but the distribution of products, presented in Fig. 8, is quite distinct. With regard to the monometallic catalyst, the modification with barium turned the system more selective to the main products, which reduced the amount of ‘‘other’’ formed products. All products identified in Table 2 had their concentrations substantially increased at the end of the reaction. Furthermore, the formation of 1,6-hexanediol was observed, a product identified only in systems with reduced acidity [13]. The distribution of products for the catalyst Ru–Sn/Al-PILC (Fig. 9) demonstrates that the presence of tin causes a sharp modification in the system‘s behavior. Two new products were identified in the reaction media, caproic acid and g-caprolactone. Caproic acid follows the same pattern already presented by the MME: increased concentration in the initial hours of reaction, with

sented a different behavior compared with the non-modified catalyst (Fig. 10). There was a significant reduction in the caproic acid and a substantial increase of hexane. We may assume that the presence of barium favored the hydrogenation of caproic acid to the corresponding alkane. Furthermore, the concentration of ‘‘other’’ products was substantially reduced, thus rendering the catalyst more selective for the main products. The presence of strong and abundant acid sites in pillared clay (both Bronsted and Lewis) is the main cause of low selectivity of these catalytic systems. Various authors have demonstrated that these acid characteristics catalyze reactions of isomerization and cracking; however, the most of the hydrogenation products originally formed may be under subsequent conversion [31–33]. Thus, cyclic products as well as alcohols and hexane, produced in abundance with the use of less acid supports, are being consumed, therefore increasing the concentration of ‘‘other’’ products indicated in Table 2. Both, the activity and selectivity of the catalysts supported on pillared clays are quite different from those obtained with other supports. In Table 3, comparative data between systems in which

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the catalyst was supported on pillared clays, alumina and titania are presented. For comparison purposes, all data correspond to a conversion of 50% of the substrate. The use of pillared clay as support makes the system more active, reaching 50% DMA conversion in reaction times below 2 h. In the case of monometallic catalysts, the conversion is faster, but the selectivity decreases (‘‘others’’ concentration). Typically, ruthenium monometallic systems are selective to MME, while methyl caproate concentration increases in the presence of Sn. Pillared clay based catalysts promote the production of MME and its subsequent conversion, allowing the formation of products not identified in other systems, especially gamma-caprolactone. In this case, its concentration reaches 74.5 mmol L 1 after 15 h reaction time. In order to clarify some conversion routes of the main obtained products, we promoted the substitution of DMA by other subtracts. In the reaction with e-caprolactone as substrate, the conversion reached 94.5%. The main obtained product was g-caprolactone, with a maximum concentration of (146.8 mmol L 1) in the first hour of reaction and subsequent consumption. There was also a gradual increase in the concentration of hexane and methyl caproate, which reached 59.3 and 37.4 mmol L 1, respectively. By using 1,6-hexanediol as substrate, we obtained a conversion of 98.0%, having as major product hexane (144.5 mmol L 1), originated from the total hydrogenation of diol. The remaining products were methylcaproate (29.9 mmol L 1), e-caprolactone (3.4 mmol L 1) and hexanol (3.0 mmol L 1). In the reaction with monomethyl ester, the conversion reached 94.3%. The final distribution of the products is much similar to that obtained in the conversion of DMA with this same catalyst. However, formation and consumption of the products during the reaction are much different. When using DMA as substrate, we had a large production of caproic acid and hexane at the beginning of the reaction (and subsequent consumption), but when MME was used, caproic acid remained throughout in low concentrations and hexane had a constant production during the reaction. This indicates that the routes of conversion from these two substrates should be distinct, contrary to what was proposed in previous works [12]. The formation of products may be understood as a process in which there is contribution from both, the metallic and acid sites present in the support’s surface, involving hydrogen migration from the metallic atoms to molecules of the substrate adsorbed on the solids’ surface (Fig. 11). According to this mechanism, metallic sites are responsible for the chemisorption of atomic hydrogen, and as observed in monometallic catalyst systems, ruthenium is also active in the hydrogenolysis of the O–CH3 bound of the ester group (forming MME). Various authors have confirmed that isolated metallic sites present reduced activity in the hydrogenation of carbonyl groups, which is attributed to the low adsorption of carbonyl by the metals [34,35]. On the other hand, the presence of Lewis acid sites in the material would permit carbonyl activation, with subsequent conversion processes into other products through hydrogenation. The Lewis acid sites are of two distinct natures in these catalysts: sites present in the own structure of the support or generated by the introduction of a promoter, such as tin. At least two mechanisms are suggested for the conversion mediated by activation of carbonyl groups. Deshpande et al. [36] proposed a mechanism for the formation of alcohols from esters in bimetallic catalysts, where one of the metals (usually Sn) acts as

Fig. 11. Carbonyl groups hydrogenation mechanism by a metallic catalyst supported in a solid with acid sites. Adapted from Mendes et al. [10].

Lewis acid. According to this mechanism, carbonyl would be activated by SnOx species (Sn(II) or Sn(IV)) that would have high affinity with oxygen from carbonyl groups, once they are considered Lewis acid sites. The polarization of carbonyl by Sn would be the first step in the formation of carbanion, from which an aldehyde would be formed, and in the sequence, alcohol. Another mechanism, proposed by Pouilloux et al. [37] suggests the formation of alcohol via acetal (without the formation of aldehyde). This evaluation is in accordance with the studies made by Toba et al. [14]. In both cases, acid sites are responsible for the activation of carbonyl, which also seems to be the reason for the elevated conversions obtained in this work. It has been observed that the presence of Bronsted acid sites in the proximity of metallic particles results in an increase of hydrogenation activity [38]. In reactions of benzene hydrogenation, Lin and Vannice [39] identified these sites as ‘‘interfacials’’. Their occurrence would be in the interlamellar region, where we have Bronsted sites of pillars surrounding metallic particles formed during the reduction phase of the catalyst. However, in the presence of strong acid sites, the hydrogenation is accompanied by hydrogenolysis, which leads to the formation of hydrocarbons with various chain sizes and isomerization reactions, which may form several ramified products [40]. Isomerization reactions have already been identified in catalysts based on Pt [41] and Pd [42]. To confirm this assumption, the reaction media were analyzed by mass spectrometry, which indicates the formation of various products related to the catalysts’ high acidity. The main sub-products identified by GC–MS are presented in Table 4. We obtained compounds originating from dehydration and hydrogenolysis processes of the main products. The 1,6-hexanediol, in acid environments, undergoes intramolecular condensation to form cyclopentanone [43]. Diacetones, such as actylacetone, provides furans in the presence of acid sites [44]. Dehydration reactions are also common in acid environments. From alcohols, we may have the conversion to alkenes (by dehydration) or cetones (by dehydrogenation), besides isomerization reactions [32]. Also considering the presence of diols, we may have a formation of unsaturated rings through dehydration and cyclization reactions. It is worthy to mention that the formation of hexane was much more prominent with the use of catalysts treated with barium. This product was obtained in various similar systems, with less acid supports. The conversion of the alkane eventually formed occurs due to its dehydrogenation, forming the respective alkane. This would be the same process that generates products such as 5hexenoic acid, methyl ester (dehydrogenation of methyl caproate), cyclopentanone, cyclopentene and 1-methyl-cyclopentene (dehydrocyclization, demethylation or dehydration of the diol). The mechanism for these conversions (Eq. (4)) was proposed by Frennet et al. [45] for metallic catalysts and, subsequently, Issaadi et al. [31] demonstrated that the process is equivalent in acid systems, suggesting the presence of M-H+ (M = Pt, Pd or Ru) sites, active in the mentioned conversions.

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139

Table 4 Sub-products obtained from hydrogenation of dimethyl adipate, identified by GC–MS. Product

Similarity (%)

Cyclopentene

98

1-Methyl-cyclopentene

86

Cyclopentanone

98

5-Hexenoic acid, methyl ester

96

4-Hexenoic acid, methyl ester

96

2-Methyl,2-cyclopentenone

94

Butanodioic acid, dimethyl ester

95

Pentanodioic acid, dimethyl ester

95

g-Caprolactone

96

3-Methyl cyclopropilpropanoate

84

2-Pentil-cyclopentanone

82

Thus, the formation of 1, 6-hexanediol in the systems studied would be inhibited by at least two different conversions. In regions where the metallic feature of the catalyst predominates, we may have the hydrogenations indicated in Eq. (5), while regions where there is a predominance of acid characteristics, (Ru-H+) would be responsible for the conversions shown in Eq. (6).

Structure

Finally, some considerations concerning the form’s selectivity in these catalysts are presented. Singh et al. [46] demonstrated that reactions of diols catalyzed by Zr-PILC lead the system to the formation of products derived from the activation of only one of the molecule’s alcohol groups. The justification was that the acidity of the catalyst is confined to the interlayer space, which causes

(5)

(6)

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Fig. 12. Reaction scheme for products formed by dimethyl adipate hydrogenation.

steric effects favoring the attack of only one of the diol hydroxyls. This behavior may also be evaluated in the catalysts tested in this work, with the indication that the steric effect favors the production of monomethyl ester, in detriment of adipic acid. In fact, the reaction medium obtained after 15 h reaction time, catalyzed by RuSn/Al-PILC, was submitted to a process of derivatization with H2SO4/methanol and subsequent chromatographic analysis, which did not indicate the presence of adipic acid. In view of the evidence presented in the course of this work, we propose a reaction scheme for the conversion of dimethyl adipate, presented in Fig. 12. In the figure, the routes traced are favored by the acidity of the support. Initially, the dimethyl adipate may be hydrogenated to 1,6-hexanediol, converted into adipic acid monomethyl ester or dehydrated to 4- or 5-hexenoic acid monomethyl esters. The 1,6hexanediol is converted into various products by means of hydrogenation, dehydration, dehydrocyclization and demethylation reactions. Monomethyl ester may generate 1,6-hexanediol, methyl caproate (which may be hydrogenated to hexanol and hexane) and caproic acid. The 5-hexenoic acid monomethyl ester undergoes cyclization reaction to form e-caprolactone, while the 4hexenoic acid monomethyl ester forms g-caprolactone. The ecaprolactone may also be formed from caproic acid, and may be converted into g-caprolactone. The g-caprolactone may also be converted into products not identified in the reaction medium. 4. Conclusions The preparation of catalysts containing the active metal supported on aluminum pillared clay proved to be feasible. For this, the high surface area and porous structure of the support concur, as well as the thermal stability in temperatures as high as 673 K. Different from any other active metals already tested, ruthenium presented low dispersion, which did not prevent its total conversion into the metal state through conventional reduction treatment with hydrogen. The acidity of catalysts was

evaluated, indicating that the materials present Bronsted and Lewis acidity. This acidity may effectively be reduced through solids treatment with barium. The catalytic tests proved that all studied systems presented high conversion, typically higher than 95%. This conversion occurs irrespectively of the presence of active metal, and may therefore be attributed to an intrinsic activity of the support. Catalysts containing ruthenium supported on pillared clays were able to hydrogenate adipic acid dimethyl ester, allowing the formation of valuable products, as 1,6-hexanediol, lactones and methyl caproate. In the monometallic catalysts the values of selectivity remain reduced. The generated cyclic products may be attributed to the acid nature of the support. The reduction of acidity, promoted by treatment with barium, significantly modifies the activity of catalysts. The catalysts treated with barium reduced the formation of undesirable products in up to 45%. The bimetallic catalysts Ru–Sn allowed the attainment of products at high concentrations such as, for instance, caproic acid, reaching a selectivity of 34% after 3 h reaction time. After treatment with barium, this catalyst changes the products’ formation profile, favoring the production of hexane (selectivity of 36%). The evaluation of the products distribution in these reactions, complemented by the use of other substrates (ecaprolactone, adipic acid monomethyl ester and 1,6-hexanediol) in substitution of dimethyl adipate, permitted the proposition of routes for the formation of the varied obtained products. In this proposition, the routes favored by the catalyst acidity are identified, showing that it is possible to direct the catalytic process to certain products through careful tuning of the catalyst properties. Acknowledgements The authors wish to thank FAPESP (2006/04142-0), CAPES and CNPq for their financial support, UNICAMP and PUC-Campinas for their support in the development of the studies and to Dr. P. A.

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