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Effect of catalyst acidity and porosity on the catalytic isomerization of linoleic acid to obtain conjugated linoleic acids (CLAs)
Chemical Engineering Journal 183 (2012) 459–465
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Effect of catalyst acidity and porosity on the catalytic isomerization of linoleic acid to obtain conjugated linoleic acids (CLAs) Xavier Cardó, Olga Bergadà, Yolanda Cesteros ∗ , Pilar Salagre Facultat de Química, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 19 December 2011 Accepted 20 December 2011 Keywords: Linoleic acid Isomerization Zeolite Acidity Porosity Microwaves CLA
a b s t r a c t The role of catalyst acidity and porosity was studied for the isomerization of linoleic acid by testing two commercial Na-zeolites (mordenite and ZSM-5) and their acid modified forms (H+ , Ni2+ ) obtained by cation exchange. Conversion of linoleic acid increased in the presence of higher amounts of weakmoderate acid centres (Brønsted and Lewis) together with a higher accessibility of the acid sites. Additionally, the synergic effect between Brønsted and Lewis centres led to higher selectivity to the isomer cis-9, trans-11-CLA whereas higher selectivity to trans-10, cis-12-CLA was achieved with higher amounts of Brønsted acidity. Interestingly, the use of microwaves for the Ni-exchange of Na-mordenite resulted in the formation of new weak Lewis acid centres, not observed for the rest of catalysts, yielding higher conversion, and higher selectivity to cis-9, trans-11-CLA than the sample conventionally exchanged. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The term conjugated linoleic acid (CLA) is used to designate the whole of dienoic conjugated isomers in position and geometry of linoleic acid (LA, cis-9, cis-12-octadecadienoic acid). Linoleic acid has double bonds located on carbons 9 and 12, both in the cis configuration whereas CLA has either the cis or trans configuration or both located along the carbon chain [1]. Over the past 15 years, some studies showed that a small number of unsaturated fatty acids present interesting biomedical and nutritional properties, thus demonstrating their potential application as functional foods. Specifically, the cis-9, trans-11-CLA isomer (c9, t11-CLA) has been proposed as potential anti-tumour agent whereas the trans10, cis-12-CLA isomer (t10, c12-CLA) affects the regulation of lipids (cholesterol) and body mass [1–4]. CLAs are naturally present in meat and dairy products from ruminant animals where they are synthesized from linoleic acid by rumen bacteria [2]. Purification of natural fatty acid mixtures, chemical synthesis or biotechnology methods have been used for the production of CLAs but no good separation
∗ Corresponding author at: Dpt. Quimica Fisica i Inorgànica, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain. Tel.: +34 977558785; fax: +34 977559563. E-mail address: [email protected] (Y. Cesteros). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.088
of the isomers was achieved [5]. Homogenous catalysts, such as tris(triphenylphospine)chlororhodium or arene chromiumcarbonyl complexes, have been tested for the isomerization of linoleic acid but the process is not environmentally friendly and the catalyst is difficult to separate [6,7]. There are few publications related to the use of heterogeneous catalysis for the selective obtention of CLA isomers from the isomerization of linoleic acid [8]. The isomerization of linoleic acid involves a change in the position of one of the C C double bonds in the initial LA molecule. The activation of this transformation can take place either through the -bonds along the carbon chain or through the C H bonds adjacent to the double bonds. On the whole, in the presence of hydrogen, double bond migration proceeds via half-hydrogenated intermediates [9]. Bernas et al. tested a great variety of supported-metal catalysts (Ru, Ni, Pd, Pt, Rh, Ir, Os, PtRh supported on C, Al2 O3 , SiO2 Al2 O3 , H-MCM-41, MCM-22) for the isomerization of linoleic acid in the liquid phase [10–14]. Catalysts were activated with hydrogen before the catalytic test, which was made under nitrogen. These studies confirmed that the isomerization of linoleic acid to CLA and the hydrogenation of linoleic acid and CLA are two competing parallel reactions. Low concentration of chemisorbed hydrogen on the catalyst surface favours isomerization while high concentration favours hydrogenation; however, chemisorbed hydrogen increases the isomerization reaction rate. Kreich and Claus reported the synthesis of CLAs over Ag/SiO2 catalyst in the constant presence of hydrogen taking into account that
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silver is the metal with the lowest hydrogen-binding energy [15]. The best catalytic results with Ag/SiO2 at 438 K yielded selectivity values of 35% to c9, t11-CLA and 26% to t10, 12c-CLA for a 69% of conversion with a 12% of selectivity to hydrogenation products. More recently, Au supported on different materials such as Al2 O3 , SiO2 , Fe2 O3 , CeO2 , MnO2 , TiO2 , ZrO2 , activated carbon, mesoporous carbon, titanium silicalite (TS-1) have been also tested for this reaction [16,17]. Strong reduction of Au is beneficial for the overall activity but stimulates undesired hydrogenation. This reveals the high difficulty to convert linoleic acid selectively to the desired products. On the other hand, it is well known that double bond migration in olefins can occur through a Brønsted acid catalysed pathway. The protonated species, namely carbenium ions, are regarded as important intermediates for many acid-catalysed reactions [18]. The importance of protonated intermediates and the mechanism is also emphasized for catalysis of solid acids [19,20]. Zeolites are well known materials widely used as catalysts in petrochemical industry due to their large surface areas, shape selectivity and controllable acidity [21]. Kondo et al. studied by FTIR the catalytic behaviour of mordenite, ZSM-5, and ferrierite for the double bond migration of butene at low temperatures (<230 K) [22–24]. They concluded that the different acidity strength and porosity of these zeolites had an important effect on the conversion and cis/trans isomerization of butane. However, there are not specific studies about the use of acid materials as catalysts for the isomerization of linoleic acid. The aim of this work was to study the role of catalyst acidity and porosity for the isomerization of linoleic acid by testing two commercial Na-zeolites (mordenite and ZSM-5), with different Si/Al ratio (6.5 and 100, respectively), and with different channels dimensions: mordenite has two kinds of channels with pore diameters of 6.7 × 7.0 A˚ (main channels) and 2.6 × 5.7 A˚ (compressed channels) whereas ZSM-5 has channels with pore diameter of 5.1 × 5.5 A˚ [25]. Additionally, the acidic properties of the two zeolites were modified by cation exchange (H+ , Ni2+ ). Fresh and used catalysts were characterized by a wide number of techniques.
microscope slides. The patterns were recorded over a range of 2Â angles from 5◦ to 70◦ and crystalline phases were identified using the Joint Committee on Powder Diffraction Standards (JCPDS) files (43-0171 corresponds to mordenite, and 37-359 to ZSM-5). 2.4. Nitrogen physisorption BET areas were calculated from the nitrogen adsorption isotherms at 77 K using a Micromeritics ASAP 2000 surface analyser (serial number 200-00000-20/261, USA). Samples were pretreated in vacuum at 573 K for 6 h prior to the analysis. Pore volumes and surface areas of micropores and mesopores were determined from their isotherms using the Horvath–Kawazoe method and the BJH method, respectively. BET area errors were in the range from ±1.0 to ±2.5 m2 /g for fresh catalysts and from ±0.19 to ±0.45 m2 /g for used catalysts. 2.5. Temperature-programmed desorption-mass spectrometry experiments (NH3 -TPD) NH3 -TPD studies were made using a TPD/R/O 1100 Thermo Finnigan, equipped with a programmable temperature furnace and TCD detector. The gas outlet was couplet to a quadrupole mass spectrometer Pfeiffer GSD300 to identify the peaks. Experiments were performed with 3% NH3 /He flowing through the sample which was previously activated at 673 K for 1 h. The desorption of NH3 was made by flowing He 20 cm3 /min from room temperature to 1073 K at 5 K/min. 2.6. Adsorbed pyridine FTIR Samples were pressed into self-supported wafers, and activated at 573 K. Pyridine was adsorbed at 313 K, and infrared spectra were recorded on a Bruker-Equinox-55 FTIR spectrometer at room temperature. The spectra were acquired by accumulating 64 scans at 4 cm−1 resolution in the range of 400–4000 cm−1 .
2. Experimental 2.7. Catalytic activity determination 2.1. Catalysts preparation Commercial Na-mordenite (Zeolyst, Si/Al = 6.5), and commercial Na-ZSM-5 (Akzo Nobel, Si/Al = 100), designated as NaM and NaZ, respectively, were exchanged with NH4 Cl 2.2 M and calcined at 673 K to obtain samples HM and HZ, respectively. Other two catalysts were obtained by exchanging the commercial zeolites with Ni(NO3 )2 ·6H2 O 1 M at 333 K for 24 h (NiNaM1, NiNaZ). One more sample was prepared by exchanging NaM with Ni(NO3 )2 ·6H2 O 1 M at 333 K for 15 min under microwaves (NiNaM2). The particle sizes of catalysts were 100–300 nm for mordenite catalysts, and 5–10 m for ZSM-5 catalysts. 2.2. Atomic absorption The Ni2+ atomic absorption results for the Ni-exchanged samples were obtained with a Hitachi Z-8200 Polarized Zeeman Atomic Absorption Spectrophotometer (serial number, Japan). The digestion of all exchanged mordenites was carried out with an Anton Paar Pressurized Microwave Decomposition System. All measures were made in triplicate. 2.3. X-ray diffraction (XRD) Powder X-ray diffraction patterns of the different samples were obtained with a Siemens D5000 diffractometer (serial number HXC01-497, Germany) using nickel-filtered Cu K␣ radiation. Samples were dusted on double-sided sticky tape and mounted on glass
Linoleic acid (C18 H32 O2 ) (>99.9% purity) and n-decane (>95% purity) were supplied by Sigma–Aldrich. Decane (70 mL) and linoleic acid (0.2 g) were charged into a 200 mL stirred glass reactor, which was provided with a reflux condenser and a heating jacket. The solution was deoxygenated before catalyst addition bubbling nitrogen (100 mL/min) through it for 10 min. Catalysts were dried prior to the catalytic tests. The system was stirred at 700 rpm in all experiments to keep the catalyst uniformly dispersed in the reaction medium, and also to eliminate external mass-transfer problems. Catalytic tests were performed under N2 at 438 K and the reaction products were taken at 3, 10 and 24 h of reaction. Reaction products were silylated following the method reported elsewhere [12,15], and identified by gas chromatography (GC) on a Shimadzu GC-2010 instrument equipped with a 25 m capillary HP5 column (i.d. 0.20 mm, film thickness 0.11 m) and a FID detector operating at 563 K. In the silylation procedures, 10 L of samples from the reactor were added into glass tubes that had been washed with methyl tert-butyl ether (MTBE, 99.8% of purity from Sigma–Aldrich). An amount of 50 L of 0.5 mg/mL C17:0 fatty acid solution (98% of purity, Sigma–Aldrich) was added to each sample as internal standard. MTBE and the solvent were evaporated in a stream of nitrogen with water as heating medium and the samples were furthermore dried at 313 K for 20 min. Then, samples were dissolved in 20 L pyridine (98% of purity, Sigma–Aldrich) and 80 L N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) of > 99% of purity and 40 L trimethylchlorosilane (TMCS) of 98% purity were added, both supplied by Sigma–Aldrich. Solutions were kept
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Table 1 Characterization data of fresh catalysts from nitrogen physisorption. Catalysts
BET surface area (m2 /g)
Micropore area (m2 /g)
NaM HM NiNaM1 NiNaM2 NaZ HZ NiNaZ
380 369 378 370 433 381 390
338 330 333 323 81 68 59
Total pore volume (cm3 /g) 0.22 0.22 0.23 0.22 0.22 0.21 0.20
in an oven at 343 K for 45 min. An aliquot of 2 L of the samples was injected into the GC at the injector temperature 533 K. Helium served as a carrier gas with a flow rate of 0.9 mL/min and a split ratio of 1:20. The temperature program was 423 K (0.5 min)–7 K/min–503 K–10 K/min–563 K (10 min). Peaks corresponding to conjugated dienoic isomers of linoleic acid were identified and quantified from the calibration lines obtained by injecting the standards trans-10, cis-12, cis-9, trans-11 and trans-9, trans-11 CLA isomers, supplied by Matreya Inc. (98% of purity). 3. Results and discussion 3.1. Characterization of the catalysts After cation exchange, modified zeolites maintained their structure with similar crystallinity, as observed by XRD (not shown here). Nitrogen adsorption isotherms obtained at 77 K were classified of type I for all samples, which correspond to microporous materials according to the BDDT classification [26]. Zeolite modifications practically do not affect the isotherm shape and the surface area of the resulting samples (Table 1). For the samples exchanged with Ni2+ , the elemental analyses indicated the presence of 3.14 Ni wt% for NiNaM1, 3.23 Ni wt% for NiNaM2, and 0.3 Ni wt% for NiNaZ. These nickel contents correspond to values of Ni2+ exchange of 46.1%, 47.4% and 40.4%, respectively. These levels of Ni2+ exchange are similar to those found in the literature for zeolites exchanged with Ni2+ in the liquid phase, and can be explained because of the increase in the effective size of the nickel cations in aqueous solution by hydration and/or hydrolysis that difficult the occupation of the more restricted size positions [27–29]. NH3 -TPD experiments were carried out in order to evaluate changes in acidity for the modified zeolites with respect to the commercial ones. NH3 -TPD profiles of Na-commercial zeolites (sample NaM and NaZ) showed the presence of one low-intense peak (less intense for NaZ) with a maximum around 475 K, in agreement with other studies [30], which has been assigned to ammonia weakly held or physically adsorbed on the zeolite, but also to some Lewis acidity due to Na+ cations [30]. NH3 -TPD of samples HM and HZ also showed one peak at higher desorption temperatures (720 K, and 645 K, respectively), with slight higher intensity than the commercial ones, which is due to Brønsted acid sites, as expected [30,31]. Fig. 1 shows the NH3 -TPD profiles of Ni-mordenite samples whereas Table 2 summarizes the maxima of desorption temperature peaks, with their relative intensities in parenthesis, for the
˚ Average pore diameter (A) 17 17 18 17 20 20 21
Ni-exchanged samples. In the literature, there are few studies about NH3 -TPD of Ni-exchanged zeolite samples [31,32]. These studies showed mainly the presence of two wide peaks: a low-temperature peak around 450 K, which was assigned to Lewis acid sites due to dehydrated Ni2+ and residual Na+ cations, and a high-temperature peak at about 735 K, related to Brønsted acid sites due to the protons introduced during the exchange. Regarding these data, for our Ni-exchanged zeolite samples, peak 2 can be associated to Lewis acid sites due to dehydrated Ni2+ cations, and remaining Na+ whereas peak 3, which appeared at higher desorption temperatures with lower intensity (Fig. 1, Table 2), could be associated to some proton exchange occurred during Ni-exchange due to the acidic medium (pH 4), as suggested by Tsipis an co-workers [31], and/or protons obtained from the hydrolysis of some Ni2+ cations (Ni2+ + H2 O → [Ni(OH)]+ + H+ [30]). Additionally, the acidity of these protons can be enhanced as a result of a synergy between protons and nickel species, as reported elsewhere [32], explaining the appearance of peak 4. Interestingly, the sample exchanged with microwaves (NiNaM2) showed another peak (peak 1) which desorbs at temperatures slightly lower than peak 2. Therefore, this peak should be related to the appearance of new sites in this sample due the microwaves action. Microwaves could interact with nickel cations causing the formation of hydrolized nickel species, which were not present in the conventional exchanged sample. This peak 1 could be associated to [Ni(OH)]+ species, which should have weakly Lewis acidic character, as suggested by MosquedaJiménez et al. [29]. FTIR spectra of adsorbed pyridine (not shown here) revealed similar relative amounts of Brønsted and Lewis sites for samples NiNaM1 and NiNaM2, in agreement with their similar Ni content. The effect of microwaves on the Ni-exchange process when compared with conventional heating can be mainly related to the different temperature regime and the most homogeneous heating achieved with microwaves [33,34]. Several authors reported that the heating of zeolites under microwaves proceeds in two steps; in the first step the hydrated zeolite absorbs microwaves through its adsorbed water, and in the second step the heated
Table 2 NH3 -TPD results for the Ni-exchanged samples. Samples
NiNaM1 NiNaM2 NiNaZ
NH3 TD (K) Peak 1
Peak 2
Peak 3
Peak 4
– 420 (m)
513 (h) 500 (h) 511 (l)
813 (l) 813 (m) 821 (vl)
933 (m) 920 (l)
TD : desorption temperature maximum; (vl): very low intensity; (l): low intensity, (m): medium intensity; (h): high intensity.
Fig. 1. NH3 -TPD thermograms of samples NiNaM1 (——) and NiNaM2 (——).
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100 80
0,06
70 60
0,05
50 40 30 20 10 0 NaM
HM
NiNaM1 NiNaM2
NaZ
HZ
NiNaZ
Catalysts
Pore volume (cc/g)
Conversion (%)
0,07
3h 10h 24h
90
0,04
0,03
0,02
Fig. 2. Conversion values at 438 K at different reaction times for all catalysts.
0,01
zeolite directly microwaves [35,36]. Therefore, the different heating process, which involves the use of microwaves, allowed us to obtain a NiNa-mordenite material with different acidic properties, with their subsequent potential use in catalysis. Differences when using microwaves with respect to conventional heating have been previously observed for other zeolite modifications, such as dealumination [37,38]. The lower intensity observed for the thermograms of ZSM-5 samples can be related to the higher Si/Al ratio of this zeolite. These characterization results will be later correlated with the catalytic results. 3.2. Catalytic activity results Fig. 2 shows the conversion values of catalysts with time for the isomerization of linoleic acid in the liquid phase at the conditions referred in the experimental section. There is a significant deactivation of catalysts, especially for the conventionally Ni-exchanged catalysts. This can be explained by the presence of higher amounts of stronger acid sites, observed for these Ni2+ catalysts by NH3 -TPD (Fig. 1, Table 2), which could favour deactivation. For all catalysts, deactivation was accompanied by a drastic decrease of surface area (Table 3) and pore volume, especially at micropore level, as we can see, e.g. for catalyst NaM in Fig. 3. However, no changes in the zeolite structure were detected by XRD after reaction, as observed in Fig. 4 for two representative samples. Bernas et al. also observed this catalyst deterioration when using supported metal catalysts for this reaction [12,13]. From these results, we can conclude that there is not collapse of the structure during reaction, but blocking of pores by reaction products. This is in agreement with the variation of colour observed for all catalysts after reaction, since they were white as fresh catalysts, and became beige-yellow after reaction, especially after longer reaction times. In order to evaluate the nature of the products remaining in the catalytic pores after reaction, several used catalysts were submitted to extraction with ether for 10 h under refluxing, and later rota-evaporated. The products obtained were sylilated [12,15], and analysed by gas chromatography. All chromatograms showed the presence of several peaks at higher retention times than those corresponding to CLAs. This must be related to the formation of condensation products, which appeared in higher amounts for the samples that had higher amounts of stronger acid sites. It is well known that olefins can undergo proton-catalysed reactions on acid sites, such as isomerization, polymerization and cyclization [39]. Besides, traces of linoleic acid and CLAs were also detected. Neither coke formation nor cracking were observed in any case. Catalysts exhibited moderate-high conversion values (Fig. 2). Modified mordenite samples were more active than NaM at all reaction times. This can be associated with the higher amount
0 10
100
1000
Pore diameter (A) NaM before reaction
NaM after reaction
Fig. 3. Pore size distribution graphic, in the desorption process, for fresh and used catalyst NaM.
of acid sites observed by NH3 -TPD for these catalysts. Interestingly, catalyst NiNaM2 (prepared by Ni2+ -exchange under microwaves) showed higher conversion than the sample conventionally exchanged (NiNaM1). The use of microwaves during
Fig. 4. (A) XRD patterns for NiNaZ (a) fresh catalyst, (b) used catalyst; (B) XRD patterns for NiNaM1 (a) fresh catalyst, (b) used catalyst.
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Table 3 Characterization data of used catalysts, after 24 h of reaction, from nitrogen physisorption. Catalysts
BET surface area (m2 /g)
Micropore area (m2 /g)
Total pore volume (cm3 /g)
˚ Average pore diameter (A)
NaM HM NiNaM1 NiNaM2 NaZ HZ NiNaZ
14 17 16 28 12 11 10
3.2 6.2 6.3 7.0 4.4 3.5 1.4
0.04 0.04 0.05 0.12 0.04 0.03 0.02
124 119 117 177 118 98 83
and Brønsted (H+ ) acidity favour the formation of this isomer. This let us to think that Brønsted and Lewis acidity could have a synergic effect on the catalytic activity. Interestingly, the catalyst that showed higher amounts of weaker Lewis acid sites (catalyst NiNaM2, see Fig. 1, Table 2), suffered a less decrease of this isomer after 24 h of reaction. On the other hand, catalysts with higher amounts of stronger acid centres (NiNaM1, HM) resulted in lower selectivity to cis-9, trans-11-CLA at higher reaction times. Selectivity to trans-10, cis-12-CLA was lower than selectivity to cis-9, trans-11-CLA. The higher selectivity values to trans-10, cis-12-CLA (4–6%) were also obtained after 10 h of reaction. The differences between catalysts were small, but H-zeolite catalysts showed slight higher values than their corresponding NiNa-, and Na-zeolite catalysts. This could be related to the presence of higher amounts of Brønsted acidity associated to H+ in the zeolite structure. The decrease of the selectivity to the two desired isomers at higher reaction times (24 h) was mainly done at expenses of the formation of higher amounts of other CLA isomers, in some cases the trans-10, trans-12-CLA, and condensation products (Table 4). These variations of the selectivity values with time have been also observed by other authors [8,40]. Several catalytic experiments were made by lowering the reaction temperature to 418 K, and also by decreasing the catalyst amount in order to decrease conversion values, and see how this affects the isomers selectivity. The results (not shown here) confirmed the expected decrease of conversion but the selectivity to the desired isomers was not improved. Fig. 5 shows a proposed mechanism for the double bond migration during isomerization of linoleic acid on acid sites through an electrophilic addition to the double bound. If Brønsted acid sites are present, the protonation can occur, resulting in a carbocation intermediate. Subsequent loss of a proton from a different carbon atom results in the isomerization to the corresponding isomer. The presence of Lewis acid sites could increase the positive charge of the carbocation favouring the isomerization reaction. Therefore, these studies show, for the first time, a clear relation between the amount, strength, and accessibility of the acid sites (related to the nature and pore dimensions of the zeolite), and the isomerization of linoleic acid.
exchange resulted in the formation of higher amounts of new weaker acid centres (peak 1, not observed for the rest of catalysts), which together with the fewer amounts of strong acid centres explain the catalytic behaviour of this catalyst. Interestingly, after reaction, this catalyst showed slight lower decrease of surface area than the other catalysts (Table 3). Regarding ZSM-5 samples, conversion increased in the sequence NaZ < NiNaZ < HZ. Again, a higher amount of acid sites favoured conversion but when the catalyst had stronger acid centres (NiNaZ, Table 2) they deactivated faster, and consequently, the conversion was lower. The higher conversion values observed for mordenite catalysts compared to ZSM-5 ones can be mainly attributed to the higher amount of acid centres in mordenite (lower Si/Al ratio) but also to the higher accessibility of its main channels to the reactants. Table 4 shows the selectivity values to the identified CLA isomers: cis-9, trans-11-CLA, trans-10, cis-12-CLA, and trans-9, trans-11CLA with time for mordenite and ZSM-5 catalysts. The major CLA product was always the trans-9, trans-11-CLA isomer, which is the most thermodynamically favourable. Other conjugation products, without interest, and very low amounts of hydrogenation products (mainly oleic acid) were also detected by gas chromatography. The presence of hydrogenation products may be due to hydride transfer type reactions taking place on the acid sites [17,40]. Taking into account the results obtained after extraction of used catalysts, as commented above, where we observed mainly the formation of condensation products during reaction, the difference between 100%, and the selectivity to the isomers cis-9, trans-11-CLA, trans-10, cis-12-CLA, and trans-9, trans-11-CLA was called in Table 4 as other products, being the sum of other nonidentified CLA isomers, hydrogenation and condensation products. The lower selectivity to the desired products (cis-9, trans-11-CLA, trans-10, cis-12-CLA) observed for all catalysts could be related to the microporosity of zeolites which can involve some steric hindrance to the formation of these isomers due to their conformation. Anyway, interesting differences in the selectivity values were observed. The higher selectivity to cis-9, trans-11-CLA (9.6–13.2%) were achieved after 10 h of reaction with the Ni-exchanged and H-zeolite catalysts in the order NiNaM2 > NiNaZ > HM > HZ > NiNaM1. From these results, we can conclude that both Lewis (Ni2+ , [Ni(OH)]+ ),
Table 4 Selectivity values in the isomerization of linoleic acid. Catalysts
Selectivity to c9, t11-CLA
Selectivity to t10, c12-CLA
Selectivity to t9, t11-CLA (%)
Selectivity to other productsa (%)
3h
10 h
24 h
3h
10 h
24 h
3h
10 h
24 h
3h
10 h
24 h
NaM HM NiNaM1 NiNaM2
5.1 6.0 8.6 5.3
8.2 10.2 9.6 13.2
5.6 2.4 4.3 7.3
2.0 2.5 2.4 1.9
2.4 5.8 4.6 3.4
1.4 4.8 1.9 1.8
76.1 40.0 46.9 68.3
63.9 38.1 28.4 42.9
68.1 36.0 65.6 26.6
16.8 51.5 42.1 24.5
25.5 45.9 57.4 40.5
24.9 56.8 28.2 64.3
NaZ HZ NiNaZ
3.9 4.6 7.7
9.3 10.8 12.3
5.5 2.7 8.4
2.2 4.1 2.6
3.1 5.6 3.9
1.8 3.3 2.7
51.3 56.1 69.9
42.0 38.3 51.1
74.2 35.7 14.4
42.6 35.2 19.8
45.6 45.3 32.7
18.5 58.3 74.5
a
Other products are composed by other CLA isomers, hydrogenation and condensation products.
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H 3C
(CH 2)4
(CH 2) 7
COOH
Linoleic acid (c9,c12-octadecanoic acid) -H+
+H+
-H+
+H+
+ H H
(CH 2) 7
COOH
(CH 2) 7
COOH
H 3C (CH 2)4
H H H 3C (CH 2)4
+ +H+
(CH 2) 7
H H
COOH
H H
H 3C (CH 2)4
+
-H+
(CH 2)7
H3C (CH 2) 4
+ +H+
COOH
H3C (CH 2)5
c9, t11-CLA
H3C (CH2)4
(CH 2) 7
COOH
-H+
(CH2)8
COOH
t10, c12-CLA
Fig. 5. Proposed mechanism for the double bond migration during isomerization of linoleic acid on acid sites.
4. Conclusions Conversion of linoleic acid was strongly affected by the amount and strength of the acidity, and also by the porosity of the catalysts. The differences observed in the catalytic activity between mordenite and ZSM-5 samples can be mainly associated to the amount of acid sites (lower Si/Al ratio for mordenite), but also to their different accessibility (channels dimensions). Higher amounts of weak-moderate acid sites (Brønsted and Lewis) favours the conversion of linoleic acid to conjugated CLAs, and, more specifically, the selectivity to the isomer of interest c9, t11-CLA. These catalytic results could indicate a synergic effect between Brønsted and Lewis centres. However, if the acid centres are too much strong, they cause a faster activity loss, and a higher decrease in the selectivity to the two desired isomers with time. Interestingly, the use of microwaves for the Ni-exchange of Na-mordenite involved the appearance of new weak Lewis acid centres, which we did not observe for the rest of catalysts. These new acid centres contribute to the conversion of linoleic acid, and to obtain higher selectivity to c9, t11-CLA. The higher selectivity to the other desired isomer, t10, c12-CLA, has been related to the presence of higher amounts of Brønsted acidity associated to H+ . References [1] P.R. O’Quinn, J.L. Nelssen, R.D. Goodband, M.D. Tokach, Anim. Health Res. Rev. 1 (2000) 35–46. [2] L.D. Whigham, M.E. Cook, R.L. Atkinson, Pharmacol. Res. 42 (2000) 503–510. [3] V. Mougios, A. Matsakas, A. Petridou, S. Ring, A. Sagredos, A. Melissopoulou, N. Tsigilis, M. Nikolaidis, J. Nutr. Biochem. 12 (2001) 585–594. [4] J.D. Palombo, A. Ganguly, B.R. Bistrian, M.P. Menard, Cancer Lett. 177 (2002) 163–172. [5] M.W. Pariza, X.Y. Yang, US 5 856 149 (1999). [6] E. Frankel, J. Am. Oil Chem. Soc. 47 (1970) 33–36. [7] W. DeJarlais, L. Gast, J. Am. Oil Chem. Soc. 48 (1971) 21–24.
[8] A. Philippaerts, S. Goossens, P.A. Jacobs, B.F. Sels, ChemSusChem 4 (2011) 684–702. [9] K.S. Sim, L. Hilaire, F. Le Normand, R. Touroude, V. Paul-Boncour, A. PercheronGuegan, J. Chem. Soc. Faraday Trans. 87 (1991) 1453–1460. [10] A. Bernas, N. Kumar, P. Mäki-Arvela, E. Laine, B. Holmbom, T. Salmi, D. Murzin, Chem. Commun. 10 (2002) 1142–1143. [11] A. Bernas, P. Laukkanen, N. Kumar, P. Mäki-Arvela, J. Väyrynen, E. Laine, B. Holmbom, T. Salmi, D. Murzin, J. Catal. 210 (2002) 354–366. [12] A. Bernas, N. Kumar, P. Mäki-Arvela, N.V. Kul’kova, B. Holmbom, T. Salmi, D. Murzin, Appl. Catal. A 245 (2003) 257–275. [13] A. Bernas, N. Kumar, P. Laukkanen, J. Väyrynen, T. Salmi, D. Murzin, Appl. Catal. A 267 (2004) 121–133. [14] A. Bernas, N. Kumar, P. Mäki-Arvela, B. Holmbom, T. Salmi, D. Murzin, Org. Process Res. Dev. 8 (2004) 341–352. [15] M. Kreich, P. Claus, Angew. Chem. Int. Ed. 44 (2005) 7800–7804. [16] P. Bauer, P. Horlacher, P. Claus, Chem. Eng. Technol. 32 (2009) 2005–2010. [17] O.A. Simakova, A.R. Lleino, B. Campo, P. Mäki-Arvela, K. Kordas, J.P. Mikkola, D.Y. Murzin, Catal. Today 150 (2010) 32–36. [18] G.A. Olah, G.K.S. Prakash, R.E. Williams, J.D. Field, D. Wade, Hydrocarbon Chemistry, John Wiley & Sons, New York, 1987. [19] A. Corma, Chem. Rev. 95 (1995) 559–614. [20] R.A. Van Santen, G.J. Kramer, Chem. Rev. 95 (1995) 637–660. [21] I.E. Marxwell, W.H.J. Stork, Hydrocarbon processing with zeolites, in: H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, NL, 2001. [22] J.N. Kondo, L. Shao, F. Wakabayashi, K. Domen, J. Phys. Chem. B 101 (1997) 9314–9320. [23] J.N. Kondo, K. Domen, F. Wakabayashi, J. Phys. Chem. B 101 (1997) 5477–5479. [24] J.N. Kondo, E. Yoda, M. Hara, F. Wakabayashi, K. Domen, Stud. Surf. Sci. Catal. 130 (2000) 2933–2938. [25] A. Jentys, J.A. Lercher, Techniques of zeolite characterization, in: H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, NL, 2001. [26] S.J. Gregg, K. Sing, Adsorption Surface Area and Porosity, Academic Press, London, UK, 1982. [27] R.M. Barrer, Zeolites and Clay Materials, Academic Press, London, 1979. [28] A. de Lucas, J.L. Valverde, F. Dorado, A. Romero, I. Asencio, J. Mol. Catal. A: Chem. 225 (2005) 47–58. [29] B.I. Mosqueda-Jiménez, A. Jentys, K. Seshan, J.A. Lercher, Appl. Catal. B: Environ. 43 (2003) 105–115. [30] M. Niwa, N. Katada, Catal. Surv. Jpn. 1 (1997) 215–226. [31] A. Godelitsas, D. Charistos, A. Tsipis, C. Tsipis, A. Filippidis, C. Triantafyllidis, G. Manos, D. Siapkas, Chem. Eur. J. 17 (2001) 3705–3721.
X. Cardó et al. / Chemical Engineering Journal 183 (2012) 459–465 [32] Y. Medina, M. Hernández, J. Alcaraz, Catal. Lett. 110 (2006) 107–113. [33] H.M. Kingston, S.J. Haswell, Microwave-enhanced Chemistry: Fundamentals, Sample Preparation and Applications, American Chemical Society, Washington, DC, 1997. [34] C. Gabriel, S. Gabriel, E.H. Grant, B.S.J. Halstead, D.M.P. Mingos, Chem. Soc. Rev. 27 (1998) 213–223. [35] T. Ohgushi, S. Komarneni, A.S. Bhalla, J. Porous Mater. 8 (2001) 23–35. [36] T. Ohgushi, Mg. Nagae, J. Porous Mater. 10 (2003) 139–143.
465
[37] M.D. González, Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, Microporous Mesoporous Mater. 188 (2009) 341–347. [38] M.D. González, Y. Cesteros, P. Salagre, Microporous Mesoporous Mater. 144 (2011) 162–170. [39] R. Oukaci, J.C.S. Wu, J. Catal. 107 (1987) 471–481. [40] D. Mukesh, C.S. Narasimhan, V.M. Deshpande, K. Ramnarayan, Ind. Eng. Chem. Res. 27 (1988) 409–414.
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Effect of catalyst acidity and porosity on the catalytic isomerization of linoleic acid to obtain conjugated linoleic acids (CLAs) Xavier Cardó, Olga Bergadà, Yolanda Cesteros ∗ , Pilar Salagre Facultat de Química, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 19 December 2011 Accepted 20 December 2011 Keywords: Linoleic acid Isomerization Zeolite Acidity Porosity Microwaves CLA
a b s t r a c t The role of catalyst acidity and porosity was studied for the isomerization of linoleic acid by testing two commercial Na-zeolites (mordenite and ZSM-5) and their acid modified forms (H+ , Ni2+ ) obtained by cation exchange. Conversion of linoleic acid increased in the presence of higher amounts of weakmoderate acid centres (Brønsted and Lewis) together with a higher accessibility of the acid sites. Additionally, the synergic effect between Brønsted and Lewis centres led to higher selectivity to the isomer cis-9, trans-11-CLA whereas higher selectivity to trans-10, cis-12-CLA was achieved with higher amounts of Brønsted acidity. Interestingly, the use of microwaves for the Ni-exchange of Na-mordenite resulted in the formation of new weak Lewis acid centres, not observed for the rest of catalysts, yielding higher conversion, and higher selectivity to cis-9, trans-11-CLA than the sample conventionally exchanged. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The term conjugated linoleic acid (CLA) is used to designate the whole of dienoic conjugated isomers in position and geometry of linoleic acid (LA, cis-9, cis-12-octadecadienoic acid). Linoleic acid has double bonds located on carbons 9 and 12, both in the cis configuration whereas CLA has either the cis or trans configuration or both located along the carbon chain [1]. Over the past 15 years, some studies showed that a small number of unsaturated fatty acids present interesting biomedical and nutritional properties, thus demonstrating their potential application as functional foods. Specifically, the cis-9, trans-11-CLA isomer (c9, t11-CLA) has been proposed as potential anti-tumour agent whereas the trans10, cis-12-CLA isomer (t10, c12-CLA) affects the regulation of lipids (cholesterol) and body mass [1–4]. CLAs are naturally present in meat and dairy products from ruminant animals where they are synthesized from linoleic acid by rumen bacteria [2]. Purification of natural fatty acid mixtures, chemical synthesis or biotechnology methods have been used for the production of CLAs but no good separation
∗ Corresponding author at: Dpt. Quimica Fisica i Inorgànica, Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n, 43007 Tarragona, Spain. Tel.: +34 977558785; fax: +34 977559563. E-mail address: [email protected] (Y. Cesteros). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.088
of the isomers was achieved [5]. Homogenous catalysts, such as tris(triphenylphospine)chlororhodium or arene chromiumcarbonyl complexes, have been tested for the isomerization of linoleic acid but the process is not environmentally friendly and the catalyst is difficult to separate [6,7]. There are few publications related to the use of heterogeneous catalysis for the selective obtention of CLA isomers from the isomerization of linoleic acid [8]. The isomerization of linoleic acid involves a change in the position of one of the C C double bonds in the initial LA molecule. The activation of this transformation can take place either through the -bonds along the carbon chain or through the C H bonds adjacent to the double bonds. On the whole, in the presence of hydrogen, double bond migration proceeds via half-hydrogenated intermediates [9]. Bernas et al. tested a great variety of supported-metal catalysts (Ru, Ni, Pd, Pt, Rh, Ir, Os, PtRh supported on C, Al2 O3 , SiO2 Al2 O3 , H-MCM-41, MCM-22) for the isomerization of linoleic acid in the liquid phase [10–14]. Catalysts were activated with hydrogen before the catalytic test, which was made under nitrogen. These studies confirmed that the isomerization of linoleic acid to CLA and the hydrogenation of linoleic acid and CLA are two competing parallel reactions. Low concentration of chemisorbed hydrogen on the catalyst surface favours isomerization while high concentration favours hydrogenation; however, chemisorbed hydrogen increases the isomerization reaction rate. Kreich and Claus reported the synthesis of CLAs over Ag/SiO2 catalyst in the constant presence of hydrogen taking into account that
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silver is the metal with the lowest hydrogen-binding energy [15]. The best catalytic results with Ag/SiO2 at 438 K yielded selectivity values of 35% to c9, t11-CLA and 26% to t10, 12c-CLA for a 69% of conversion with a 12% of selectivity to hydrogenation products. More recently, Au supported on different materials such as Al2 O3 , SiO2 , Fe2 O3 , CeO2 , MnO2 , TiO2 , ZrO2 , activated carbon, mesoporous carbon, titanium silicalite (TS-1) have been also tested for this reaction [16,17]. Strong reduction of Au is beneficial for the overall activity but stimulates undesired hydrogenation. This reveals the high difficulty to convert linoleic acid selectively to the desired products. On the other hand, it is well known that double bond migration in olefins can occur through a Brønsted acid catalysed pathway. The protonated species, namely carbenium ions, are regarded as important intermediates for many acid-catalysed reactions [18]. The importance of protonated intermediates and the mechanism is also emphasized for catalysis of solid acids [19,20]. Zeolites are well known materials widely used as catalysts in petrochemical industry due to their large surface areas, shape selectivity and controllable acidity [21]. Kondo et al. studied by FTIR the catalytic behaviour of mordenite, ZSM-5, and ferrierite for the double bond migration of butene at low temperatures (<230 K) [22–24]. They concluded that the different acidity strength and porosity of these zeolites had an important effect on the conversion and cis/trans isomerization of butane. However, there are not specific studies about the use of acid materials as catalysts for the isomerization of linoleic acid. The aim of this work was to study the role of catalyst acidity and porosity for the isomerization of linoleic acid by testing two commercial Na-zeolites (mordenite and ZSM-5), with different Si/Al ratio (6.5 and 100, respectively), and with different channels dimensions: mordenite has two kinds of channels with pore diameters of 6.7 × 7.0 A˚ (main channels) and 2.6 × 5.7 A˚ (compressed channels) whereas ZSM-5 has channels with pore diameter of 5.1 × 5.5 A˚ [25]. Additionally, the acidic properties of the two zeolites were modified by cation exchange (H+ , Ni2+ ). Fresh and used catalysts were characterized by a wide number of techniques.
microscope slides. The patterns were recorded over a range of 2Â angles from 5◦ to 70◦ and crystalline phases were identified using the Joint Committee on Powder Diffraction Standards (JCPDS) files (43-0171 corresponds to mordenite, and 37-359 to ZSM-5). 2.4. Nitrogen physisorption BET areas were calculated from the nitrogen adsorption isotherms at 77 K using a Micromeritics ASAP 2000 surface analyser (serial number 200-00000-20/261, USA). Samples were pretreated in vacuum at 573 K for 6 h prior to the analysis. Pore volumes and surface areas of micropores and mesopores were determined from their isotherms using the Horvath–Kawazoe method and the BJH method, respectively. BET area errors were in the range from ±1.0 to ±2.5 m2 /g for fresh catalysts and from ±0.19 to ±0.45 m2 /g for used catalysts. 2.5. Temperature-programmed desorption-mass spectrometry experiments (NH3 -TPD) NH3 -TPD studies were made using a TPD/R/O 1100 Thermo Finnigan, equipped with a programmable temperature furnace and TCD detector. The gas outlet was couplet to a quadrupole mass spectrometer Pfeiffer GSD300 to identify the peaks. Experiments were performed with 3% NH3 /He flowing through the sample which was previously activated at 673 K for 1 h. The desorption of NH3 was made by flowing He 20 cm3 /min from room temperature to 1073 K at 5 K/min. 2.6. Adsorbed pyridine FTIR Samples were pressed into self-supported wafers, and activated at 573 K. Pyridine was adsorbed at 313 K, and infrared spectra were recorded on a Bruker-Equinox-55 FTIR spectrometer at room temperature. The spectra were acquired by accumulating 64 scans at 4 cm−1 resolution in the range of 400–4000 cm−1 .
2. Experimental 2.7. Catalytic activity determination 2.1. Catalysts preparation Commercial Na-mordenite (Zeolyst, Si/Al = 6.5), and commercial Na-ZSM-5 (Akzo Nobel, Si/Al = 100), designated as NaM and NaZ, respectively, were exchanged with NH4 Cl 2.2 M and calcined at 673 K to obtain samples HM and HZ, respectively. Other two catalysts were obtained by exchanging the commercial zeolites with Ni(NO3 )2 ·6H2 O 1 M at 333 K for 24 h (NiNaM1, NiNaZ). One more sample was prepared by exchanging NaM with Ni(NO3 )2 ·6H2 O 1 M at 333 K for 15 min under microwaves (NiNaM2). The particle sizes of catalysts were 100–300 nm for mordenite catalysts, and 5–10 m for ZSM-5 catalysts. 2.2. Atomic absorption The Ni2+ atomic absorption results for the Ni-exchanged samples were obtained with a Hitachi Z-8200 Polarized Zeeman Atomic Absorption Spectrophotometer (serial number, Japan). The digestion of all exchanged mordenites was carried out with an Anton Paar Pressurized Microwave Decomposition System. All measures were made in triplicate. 2.3. X-ray diffraction (XRD) Powder X-ray diffraction patterns of the different samples were obtained with a Siemens D5000 diffractometer (serial number HXC01-497, Germany) using nickel-filtered Cu K␣ radiation. Samples were dusted on double-sided sticky tape and mounted on glass
Linoleic acid (C18 H32 O2 ) (>99.9% purity) and n-decane (>95% purity) were supplied by Sigma–Aldrich. Decane (70 mL) and linoleic acid (0.2 g) were charged into a 200 mL stirred glass reactor, which was provided with a reflux condenser and a heating jacket. The solution was deoxygenated before catalyst addition bubbling nitrogen (100 mL/min) through it for 10 min. Catalysts were dried prior to the catalytic tests. The system was stirred at 700 rpm in all experiments to keep the catalyst uniformly dispersed in the reaction medium, and also to eliminate external mass-transfer problems. Catalytic tests were performed under N2 at 438 K and the reaction products were taken at 3, 10 and 24 h of reaction. Reaction products were silylated following the method reported elsewhere [12,15], and identified by gas chromatography (GC) on a Shimadzu GC-2010 instrument equipped with a 25 m capillary HP5 column (i.d. 0.20 mm, film thickness 0.11 m) and a FID detector operating at 563 K. In the silylation procedures, 10 L of samples from the reactor were added into glass tubes that had been washed with methyl tert-butyl ether (MTBE, 99.8% of purity from Sigma–Aldrich). An amount of 50 L of 0.5 mg/mL C17:0 fatty acid solution (98% of purity, Sigma–Aldrich) was added to each sample as internal standard. MTBE and the solvent were evaporated in a stream of nitrogen with water as heating medium and the samples were furthermore dried at 313 K for 20 min. Then, samples were dissolved in 20 L pyridine (98% of purity, Sigma–Aldrich) and 80 L N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) of > 99% of purity and 40 L trimethylchlorosilane (TMCS) of 98% purity were added, both supplied by Sigma–Aldrich. Solutions were kept
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Table 1 Characterization data of fresh catalysts from nitrogen physisorption. Catalysts
BET surface area (m2 /g)
Micropore area (m2 /g)
NaM HM NiNaM1 NiNaM2 NaZ HZ NiNaZ
380 369 378 370 433 381 390
338 330 333 323 81 68 59
Total pore volume (cm3 /g) 0.22 0.22 0.23 0.22 0.22 0.21 0.20
in an oven at 343 K for 45 min. An aliquot of 2 L of the samples was injected into the GC at the injector temperature 533 K. Helium served as a carrier gas with a flow rate of 0.9 mL/min and a split ratio of 1:20. The temperature program was 423 K (0.5 min)–7 K/min–503 K–10 K/min–563 K (10 min). Peaks corresponding to conjugated dienoic isomers of linoleic acid were identified and quantified from the calibration lines obtained by injecting the standards trans-10, cis-12, cis-9, trans-11 and trans-9, trans-11 CLA isomers, supplied by Matreya Inc. (98% of purity). 3. Results and discussion 3.1. Characterization of the catalysts After cation exchange, modified zeolites maintained their structure with similar crystallinity, as observed by XRD (not shown here). Nitrogen adsorption isotherms obtained at 77 K were classified of type I for all samples, which correspond to microporous materials according to the BDDT classification [26]. Zeolite modifications practically do not affect the isotherm shape and the surface area of the resulting samples (Table 1). For the samples exchanged with Ni2+ , the elemental analyses indicated the presence of 3.14 Ni wt% for NiNaM1, 3.23 Ni wt% for NiNaM2, and 0.3 Ni wt% for NiNaZ. These nickel contents correspond to values of Ni2+ exchange of 46.1%, 47.4% and 40.4%, respectively. These levels of Ni2+ exchange are similar to those found in the literature for zeolites exchanged with Ni2+ in the liquid phase, and can be explained because of the increase in the effective size of the nickel cations in aqueous solution by hydration and/or hydrolysis that difficult the occupation of the more restricted size positions [27–29]. NH3 -TPD experiments were carried out in order to evaluate changes in acidity for the modified zeolites with respect to the commercial ones. NH3 -TPD profiles of Na-commercial zeolites (sample NaM and NaZ) showed the presence of one low-intense peak (less intense for NaZ) with a maximum around 475 K, in agreement with other studies [30], which has been assigned to ammonia weakly held or physically adsorbed on the zeolite, but also to some Lewis acidity due to Na+ cations [30]. NH3 -TPD of samples HM and HZ also showed one peak at higher desorption temperatures (720 K, and 645 K, respectively), with slight higher intensity than the commercial ones, which is due to Brønsted acid sites, as expected [30,31]. Fig. 1 shows the NH3 -TPD profiles of Ni-mordenite samples whereas Table 2 summarizes the maxima of desorption temperature peaks, with their relative intensities in parenthesis, for the
˚ Average pore diameter (A) 17 17 18 17 20 20 21
Ni-exchanged samples. In the literature, there are few studies about NH3 -TPD of Ni-exchanged zeolite samples [31,32]. These studies showed mainly the presence of two wide peaks: a low-temperature peak around 450 K, which was assigned to Lewis acid sites due to dehydrated Ni2+ and residual Na+ cations, and a high-temperature peak at about 735 K, related to Brønsted acid sites due to the protons introduced during the exchange. Regarding these data, for our Ni-exchanged zeolite samples, peak 2 can be associated to Lewis acid sites due to dehydrated Ni2+ cations, and remaining Na+ whereas peak 3, which appeared at higher desorption temperatures with lower intensity (Fig. 1, Table 2), could be associated to some proton exchange occurred during Ni-exchange due to the acidic medium (pH 4), as suggested by Tsipis an co-workers [31], and/or protons obtained from the hydrolysis of some Ni2+ cations (Ni2+ + H2 O → [Ni(OH)]+ + H+ [30]). Additionally, the acidity of these protons can be enhanced as a result of a synergy between protons and nickel species, as reported elsewhere [32], explaining the appearance of peak 4. Interestingly, the sample exchanged with microwaves (NiNaM2) showed another peak (peak 1) which desorbs at temperatures slightly lower than peak 2. Therefore, this peak should be related to the appearance of new sites in this sample due the microwaves action. Microwaves could interact with nickel cations causing the formation of hydrolized nickel species, which were not present in the conventional exchanged sample. This peak 1 could be associated to [Ni(OH)]+ species, which should have weakly Lewis acidic character, as suggested by MosquedaJiménez et al. [29]. FTIR spectra of adsorbed pyridine (not shown here) revealed similar relative amounts of Brønsted and Lewis sites for samples NiNaM1 and NiNaM2, in agreement with their similar Ni content. The effect of microwaves on the Ni-exchange process when compared with conventional heating can be mainly related to the different temperature regime and the most homogeneous heating achieved with microwaves [33,34]. Several authors reported that the heating of zeolites under microwaves proceeds in two steps; in the first step the hydrated zeolite absorbs microwaves through its adsorbed water, and in the second step the heated
Table 2 NH3 -TPD results for the Ni-exchanged samples. Samples
NiNaM1 NiNaM2 NiNaZ
NH3 TD (K) Peak 1
Peak 2
Peak 3
Peak 4
– 420 (m)
513 (h) 500 (h) 511 (l)
813 (l) 813 (m) 821 (vl)
933 (m) 920 (l)
TD : desorption temperature maximum; (vl): very low intensity; (l): low intensity, (m): medium intensity; (h): high intensity.
Fig. 1. NH3 -TPD thermograms of samples NiNaM1 (——) and NiNaM2 (——).
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100 80
0,06
70 60
0,05
50 40 30 20 10 0 NaM
HM
NiNaM1 NiNaM2
NaZ
HZ
NiNaZ
Catalysts
Pore volume (cc/g)
Conversion (%)
0,07
3h 10h 24h
90
0,04
0,03
0,02
Fig. 2. Conversion values at 438 K at different reaction times for all catalysts.
0,01
zeolite directly microwaves [35,36]. Therefore, the different heating process, which involves the use of microwaves, allowed us to obtain a NiNa-mordenite material with different acidic properties, with their subsequent potential use in catalysis. Differences when using microwaves with respect to conventional heating have been previously observed for other zeolite modifications, such as dealumination [37,38]. The lower intensity observed for the thermograms of ZSM-5 samples can be related to the higher Si/Al ratio of this zeolite. These characterization results will be later correlated with the catalytic results. 3.2. Catalytic activity results Fig. 2 shows the conversion values of catalysts with time for the isomerization of linoleic acid in the liquid phase at the conditions referred in the experimental section. There is a significant deactivation of catalysts, especially for the conventionally Ni-exchanged catalysts. This can be explained by the presence of higher amounts of stronger acid sites, observed for these Ni2+ catalysts by NH3 -TPD (Fig. 1, Table 2), which could favour deactivation. For all catalysts, deactivation was accompanied by a drastic decrease of surface area (Table 3) and pore volume, especially at micropore level, as we can see, e.g. for catalyst NaM in Fig. 3. However, no changes in the zeolite structure were detected by XRD after reaction, as observed in Fig. 4 for two representative samples. Bernas et al. also observed this catalyst deterioration when using supported metal catalysts for this reaction [12,13]. From these results, we can conclude that there is not collapse of the structure during reaction, but blocking of pores by reaction products. This is in agreement with the variation of colour observed for all catalysts after reaction, since they were white as fresh catalysts, and became beige-yellow after reaction, especially after longer reaction times. In order to evaluate the nature of the products remaining in the catalytic pores after reaction, several used catalysts were submitted to extraction with ether for 10 h under refluxing, and later rota-evaporated. The products obtained were sylilated [12,15], and analysed by gas chromatography. All chromatograms showed the presence of several peaks at higher retention times than those corresponding to CLAs. This must be related to the formation of condensation products, which appeared in higher amounts for the samples that had higher amounts of stronger acid sites. It is well known that olefins can undergo proton-catalysed reactions on acid sites, such as isomerization, polymerization and cyclization [39]. Besides, traces of linoleic acid and CLAs were also detected. Neither coke formation nor cracking were observed in any case. Catalysts exhibited moderate-high conversion values (Fig. 2). Modified mordenite samples were more active than NaM at all reaction times. This can be associated with the higher amount
0 10
100
1000
Pore diameter (A) NaM before reaction
NaM after reaction
Fig. 3. Pore size distribution graphic, in the desorption process, for fresh and used catalyst NaM.
of acid sites observed by NH3 -TPD for these catalysts. Interestingly, catalyst NiNaM2 (prepared by Ni2+ -exchange under microwaves) showed higher conversion than the sample conventionally exchanged (NiNaM1). The use of microwaves during
Fig. 4. (A) XRD patterns for NiNaZ (a) fresh catalyst, (b) used catalyst; (B) XRD patterns for NiNaM1 (a) fresh catalyst, (b) used catalyst.
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Table 3 Characterization data of used catalysts, after 24 h of reaction, from nitrogen physisorption. Catalysts
BET surface area (m2 /g)
Micropore area (m2 /g)
Total pore volume (cm3 /g)
˚ Average pore diameter (A)
NaM HM NiNaM1 NiNaM2 NaZ HZ NiNaZ
14 17 16 28 12 11 10
3.2 6.2 6.3 7.0 4.4 3.5 1.4
0.04 0.04 0.05 0.12 0.04 0.03 0.02
124 119 117 177 118 98 83
and Brønsted (H+ ) acidity favour the formation of this isomer. This let us to think that Brønsted and Lewis acidity could have a synergic effect on the catalytic activity. Interestingly, the catalyst that showed higher amounts of weaker Lewis acid sites (catalyst NiNaM2, see Fig. 1, Table 2), suffered a less decrease of this isomer after 24 h of reaction. On the other hand, catalysts with higher amounts of stronger acid centres (NiNaM1, HM) resulted in lower selectivity to cis-9, trans-11-CLA at higher reaction times. Selectivity to trans-10, cis-12-CLA was lower than selectivity to cis-9, trans-11-CLA. The higher selectivity values to trans-10, cis-12-CLA (4–6%) were also obtained after 10 h of reaction. The differences between catalysts were small, but H-zeolite catalysts showed slight higher values than their corresponding NiNa-, and Na-zeolite catalysts. This could be related to the presence of higher amounts of Brønsted acidity associated to H+ in the zeolite structure. The decrease of the selectivity to the two desired isomers at higher reaction times (24 h) was mainly done at expenses of the formation of higher amounts of other CLA isomers, in some cases the trans-10, trans-12-CLA, and condensation products (Table 4). These variations of the selectivity values with time have been also observed by other authors [8,40]. Several catalytic experiments were made by lowering the reaction temperature to 418 K, and also by decreasing the catalyst amount in order to decrease conversion values, and see how this affects the isomers selectivity. The results (not shown here) confirmed the expected decrease of conversion but the selectivity to the desired isomers was not improved. Fig. 5 shows a proposed mechanism for the double bond migration during isomerization of linoleic acid on acid sites through an electrophilic addition to the double bound. If Brønsted acid sites are present, the protonation can occur, resulting in a carbocation intermediate. Subsequent loss of a proton from a different carbon atom results in the isomerization to the corresponding isomer. The presence of Lewis acid sites could increase the positive charge of the carbocation favouring the isomerization reaction. Therefore, these studies show, for the first time, a clear relation between the amount, strength, and accessibility of the acid sites (related to the nature and pore dimensions of the zeolite), and the isomerization of linoleic acid.
exchange resulted in the formation of higher amounts of new weaker acid centres (peak 1, not observed for the rest of catalysts), which together with the fewer amounts of strong acid centres explain the catalytic behaviour of this catalyst. Interestingly, after reaction, this catalyst showed slight lower decrease of surface area than the other catalysts (Table 3). Regarding ZSM-5 samples, conversion increased in the sequence NaZ < NiNaZ < HZ. Again, a higher amount of acid sites favoured conversion but when the catalyst had stronger acid centres (NiNaZ, Table 2) they deactivated faster, and consequently, the conversion was lower. The higher conversion values observed for mordenite catalysts compared to ZSM-5 ones can be mainly attributed to the higher amount of acid centres in mordenite (lower Si/Al ratio) but also to the higher accessibility of its main channels to the reactants. Table 4 shows the selectivity values to the identified CLA isomers: cis-9, trans-11-CLA, trans-10, cis-12-CLA, and trans-9, trans-11CLA with time for mordenite and ZSM-5 catalysts. The major CLA product was always the trans-9, trans-11-CLA isomer, which is the most thermodynamically favourable. Other conjugation products, without interest, and very low amounts of hydrogenation products (mainly oleic acid) were also detected by gas chromatography. The presence of hydrogenation products may be due to hydride transfer type reactions taking place on the acid sites [17,40]. Taking into account the results obtained after extraction of used catalysts, as commented above, where we observed mainly the formation of condensation products during reaction, the difference between 100%, and the selectivity to the isomers cis-9, trans-11-CLA, trans-10, cis-12-CLA, and trans-9, trans-11-CLA was called in Table 4 as other products, being the sum of other nonidentified CLA isomers, hydrogenation and condensation products. The lower selectivity to the desired products (cis-9, trans-11-CLA, trans-10, cis-12-CLA) observed for all catalysts could be related to the microporosity of zeolites which can involve some steric hindrance to the formation of these isomers due to their conformation. Anyway, interesting differences in the selectivity values were observed. The higher selectivity to cis-9, trans-11-CLA (9.6–13.2%) were achieved after 10 h of reaction with the Ni-exchanged and H-zeolite catalysts in the order NiNaM2 > NiNaZ > HM > HZ > NiNaM1. From these results, we can conclude that both Lewis (Ni2+ , [Ni(OH)]+ ),
Table 4 Selectivity values in the isomerization of linoleic acid. Catalysts
Selectivity to c9, t11-CLA
Selectivity to t10, c12-CLA
Selectivity to t9, t11-CLA (%)
Selectivity to other productsa (%)
3h
10 h
24 h
3h
10 h
24 h
3h
10 h
24 h
3h
10 h
24 h
NaM HM NiNaM1 NiNaM2
5.1 6.0 8.6 5.3
8.2 10.2 9.6 13.2
5.6 2.4 4.3 7.3
2.0 2.5 2.4 1.9
2.4 5.8 4.6 3.4
1.4 4.8 1.9 1.8
76.1 40.0 46.9 68.3
63.9 38.1 28.4 42.9
68.1 36.0 65.6 26.6
16.8 51.5 42.1 24.5
25.5 45.9 57.4 40.5
24.9 56.8 28.2 64.3
NaZ HZ NiNaZ
3.9 4.6 7.7
9.3 10.8 12.3
5.5 2.7 8.4
2.2 4.1 2.6
3.1 5.6 3.9
1.8 3.3 2.7
51.3 56.1 69.9
42.0 38.3 51.1
74.2 35.7 14.4
42.6 35.2 19.8
45.6 45.3 32.7
18.5 58.3 74.5
a
Other products are composed by other CLA isomers, hydrogenation and condensation products.
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X. Cardó et al. / Chemical Engineering Journal 183 (2012) 459–465
H 3C
(CH 2)4
(CH 2) 7
COOH
Linoleic acid (c9,c12-octadecanoic acid) -H+
+H+
-H+
+H+
+ H H
(CH 2) 7
COOH
(CH 2) 7
COOH
H 3C (CH 2)4
H H H 3C (CH 2)4
+ +H+
(CH 2) 7
H H
COOH
H H
H 3C (CH 2)4
+
-H+
(CH 2)7
H3C (CH 2) 4
+ +H+
COOH
H3C (CH 2)5
c9, t11-CLA
H3C (CH2)4
(CH 2) 7
COOH
-H+
(CH2)8
COOH
t10, c12-CLA
Fig. 5. Proposed mechanism for the double bond migration during isomerization of linoleic acid on acid sites.
4. Conclusions Conversion of linoleic acid was strongly affected by the amount and strength of the acidity, and also by the porosity of the catalysts. The differences observed in the catalytic activity between mordenite and ZSM-5 samples can be mainly associated to the amount of acid sites (lower Si/Al ratio for mordenite), but also to their different accessibility (channels dimensions). Higher amounts of weak-moderate acid sites (Brønsted and Lewis) favours the conversion of linoleic acid to conjugated CLAs, and, more specifically, the selectivity to the isomer of interest c9, t11-CLA. These catalytic results could indicate a synergic effect between Brønsted and Lewis centres. However, if the acid centres are too much strong, they cause a faster activity loss, and a higher decrease in the selectivity to the two desired isomers with time. Interestingly, the use of microwaves for the Ni-exchange of Na-mordenite involved the appearance of new weak Lewis acid centres, which we did not observe for the rest of catalysts. These new acid centres contribute to the conversion of linoleic acid, and to obtain higher selectivity to c9, t11-CLA. The higher selectivity to the other desired isomer, t10, c12-CLA, has been related to the presence of higher amounts of Brønsted acidity associated to H+ . References [1] P.R. O’Quinn, J.L. Nelssen, R.D. Goodband, M.D. Tokach, Anim. Health Res. Rev. 1 (2000) 35–46. [2] L.D. Whigham, M.E. Cook, R.L. Atkinson, Pharmacol. Res. 42 (2000) 503–510. [3] V. Mougios, A. Matsakas, A. Petridou, S. Ring, A. Sagredos, A. Melissopoulou, N. Tsigilis, M. Nikolaidis, J. Nutr. Biochem. 12 (2001) 585–594. [4] J.D. Palombo, A. Ganguly, B.R. Bistrian, M.P. Menard, Cancer Lett. 177 (2002) 163–172. [5] M.W. Pariza, X.Y. Yang, US 5 856 149 (1999). [6] E. Frankel, J. Am. Oil Chem. Soc. 47 (1970) 33–36. [7] W. DeJarlais, L. Gast, J. Am. Oil Chem. Soc. 48 (1971) 21–24.
[8] A. Philippaerts, S. Goossens, P.A. Jacobs, B.F. Sels, ChemSusChem 4 (2011) 684–702. [9] K.S. Sim, L. Hilaire, F. Le Normand, R. Touroude, V. Paul-Boncour, A. PercheronGuegan, J. Chem. Soc. Faraday Trans. 87 (1991) 1453–1460. [10] A. Bernas, N. Kumar, P. Mäki-Arvela, E. Laine, B. Holmbom, T. Salmi, D. Murzin, Chem. Commun. 10 (2002) 1142–1143. [11] A. Bernas, P. Laukkanen, N. Kumar, P. Mäki-Arvela, J. Väyrynen, E. Laine, B. Holmbom, T. Salmi, D. Murzin, J. Catal. 210 (2002) 354–366. [12] A. Bernas, N. Kumar, P. Mäki-Arvela, N.V. Kul’kova, B. Holmbom, T. Salmi, D. Murzin, Appl. Catal. A 245 (2003) 257–275. [13] A. Bernas, N. Kumar, P. Laukkanen, J. Väyrynen, T. Salmi, D. Murzin, Appl. Catal. A 267 (2004) 121–133. [14] A. Bernas, N. Kumar, P. Mäki-Arvela, B. Holmbom, T. Salmi, D. Murzin, Org. Process Res. Dev. 8 (2004) 341–352. [15] M. Kreich, P. Claus, Angew. Chem. Int. Ed. 44 (2005) 7800–7804. [16] P. Bauer, P. Horlacher, P. Claus, Chem. Eng. Technol. 32 (2009) 2005–2010. [17] O.A. Simakova, A.R. Lleino, B. Campo, P. Mäki-Arvela, K. Kordas, J.P. Mikkola, D.Y. Murzin, Catal. Today 150 (2010) 32–36. [18] G.A. Olah, G.K.S. Prakash, R.E. Williams, J.D. Field, D. Wade, Hydrocarbon Chemistry, John Wiley & Sons, New York, 1987. [19] A. Corma, Chem. Rev. 95 (1995) 559–614. [20] R.A. Van Santen, G.J. Kramer, Chem. Rev. 95 (1995) 637–660. [21] I.E. Marxwell, W.H.J. Stork, Hydrocarbon processing with zeolites, in: H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, NL, 2001. [22] J.N. Kondo, L. Shao, F. Wakabayashi, K. Domen, J. Phys. Chem. B 101 (1997) 9314–9320. [23] J.N. Kondo, K. Domen, F. Wakabayashi, J. Phys. Chem. B 101 (1997) 5477–5479. [24] J.N. Kondo, E. Yoda, M. Hara, F. Wakabayashi, K. Domen, Stud. Surf. Sci. Catal. 130 (2000) 2933–2938. [25] A. Jentys, J.A. Lercher, Techniques of zeolite characterization, in: H. van Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, NL, 2001. [26] S.J. Gregg, K. Sing, Adsorption Surface Area and Porosity, Academic Press, London, UK, 1982. [27] R.M. Barrer, Zeolites and Clay Materials, Academic Press, London, 1979. [28] A. de Lucas, J.L. Valverde, F. Dorado, A. Romero, I. Asencio, J. Mol. Catal. A: Chem. 225 (2005) 47–58. [29] B.I. Mosqueda-Jiménez, A. Jentys, K. Seshan, J.A. Lercher, Appl. Catal. B: Environ. 43 (2003) 105–115. [30] M. Niwa, N. Katada, Catal. Surv. Jpn. 1 (1997) 215–226. [31] A. Godelitsas, D. Charistos, A. Tsipis, C. Tsipis, A. Filippidis, C. Triantafyllidis, G. Manos, D. Siapkas, Chem. Eur. J. 17 (2001) 3705–3721.
X. Cardó et al. / Chemical Engineering Journal 183 (2012) 459–465 [32] Y. Medina, M. Hernández, J. Alcaraz, Catal. Lett. 110 (2006) 107–113. [33] H.M. Kingston, S.J. Haswell, Microwave-enhanced Chemistry: Fundamentals, Sample Preparation and Applications, American Chemical Society, Washington, DC, 1997. [34] C. Gabriel, S. Gabriel, E.H. Grant, B.S.J. Halstead, D.M.P. Mingos, Chem. Soc. Rev. 27 (1998) 213–223. [35] T. Ohgushi, S. Komarneni, A.S. Bhalla, J. Porous Mater. 8 (2001) 23–35. [36] T. Ohgushi, Mg. Nagae, J. Porous Mater. 10 (2003) 139–143.
465
[37] M.D. González, Y. Cesteros, P. Salagre, F. Medina, J.E. Sueiras, Microporous Mesoporous Mater. 188 (2009) 341–347. [38] M.D. González, Y. Cesteros, P. Salagre, Microporous Mesoporous Mater. 144 (2011) 162–170. [39] R. Oukaci, J.C.S. Wu, J. Catal. 107 (1987) 471–481. [40] D. Mukesh, C.S. Narasimhan, V.M. Deshpande, K. Ramnarayan, Ind. Eng. Chem. Res. 27 (1988) 409–414.