J. Phys. Chem. C 2009, 113, 2903–2910
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Probing Cu-USY Zeolite Reactivity: Design of a Green Catalyst for the Synthesis of Diynes P. Kuhn,†,‡ P. Pale,§ J. Sommer,‡ and B. Louis*,† Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse (LMSPC, UMR 7515 du CNRS), ECPM-ULP, 25, rue Becquerel, F-67087 Strasbourg Cedex 2, France, Part of the ELCASS (European Laboratory of Catalysis and Surface Science), Laboratoire de Physico-Chimie des Hydrocarbures (LPCH, UMR 7177), UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France, and Laboratoire de Synthe`se et Re´actiVite´ Organique (LSRO, UMR 7177), UniVersite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France ReceiVed: NoVember 5, 2008; ReVised Manuscript ReceiVed: December 22, 2008
Cu-exchanged USY zeolite was synthesized by vapor-phase exchange of parent HUSY with CuCl. Reaction parameters were varied in order to investigate the relationship between the structure of Cu+-promoted FAU zeolites and their catalytic properties in C-C bond homocoupling reaction between two acetylenic molecules (Glaser reaction). A proper choice in the CuCl chemical vaporization duration and temperature led to an increase in extra-framework aluminum species formation and thus to higher Lewis acidity. Moreover, such treatment under nitrogen flow during two days at 623 K kept the material highly crystalline. A drastic influence of the zeolite topology and properties (channels or cages, pore diameter, acidity) was noticed in the conversion of phenylacetylene. Observation of the catalyst before, during, and after the reaction, by multinuclear magicangle spinning nuclear magnetic resonance, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and H/D isotope labeling, revealed the necessity for the zeolite to still exhibit both Bro¨nsted and Lewis acidity, after exchange with Cu+ cations. Indeed, the reversible change in copper cations oxidation state from + I to + II was demonstrated by XPS, thus confirming a heterogeneous catalytic process over zeolites to perform terminal alkyne homocoupling reaction. In contrast to its homogeneous Glaser version, this study describes therefore a green route for the quantitative synthesis of diynes via bifunctional heterogeneous catalysis. 1. Introduction
SCHEME 1: Cu-USY Zeolite Catalyzed Diyne Synthesis via Aryl Acetylene Homocoupling
Zeolites containing transition metal cations are of primary importance because of their high catalytic activity and selectivity in various chemical reactions.1-7 Cu(I)-loaded zeolites have recently received an increased attention in the reduction of NO to N2,8,9 in the conversion of syngas to methanol,10 and in the carbonylation of methanol to dimethyl carbonate.11-13 Whereas Cu(II) zeolites can be easily prepared by ion-exchange with Cu(II) aqueous solutions, Cu(I)-promoted zeolites have to be prepared by other techniques due to the insolubility of Cu(I) salts in water and their notorious unstability. Up to now, a solid-state reaction between ground mixtures of zeolite and a copper salt or oxide remains the more suitable technique.1,11,14 Indeed, at relatively high temperature, Cu+ ions migrate within the crystalline framework and thus replace zeolite cations. This anhydrous high-temperature reaction is usually performed between CuCl and the H-zeolite form. Cu+ ions can highly disperse within the zeolite framework (mainly MFI and FAU-types) while heating the solids for several hours at temperatures ranging between 623-923 K.11-17 This ionexchange process which leaves Cu+ cations embedded in the zeolite frame was first described separately by Rabo18 and Clearfield.19 * To whom correspondence should be addressed. E-mail: blouis@ chimie.u-strasbg.fr. Phone: +33.3.90242760. Fax: +33 0.3.90242761. † Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse. ‡ Laboratoire de Physico-Chimie des Hydrocarbures, Universite´ Louis Pasteur. § Laboratoire de Synthe`se et Re´activite´ Organique, Universite´ Louis Pasteur.
Because of the large number of organic reactions known to be promoted by copper(I) salts or complexes,20,21 a heterogeneous version which can stabilize Cu(I) species would thus be an important breakthrough for the corresponding applications. Despite an impressive number of homogeneous catalysts based on transition metals already used in organic synthesis,22 and beside aforementioned applications, Cu(I)-modified zeolites have never been applied in organic chemistry. However, such heterogeneous catalysts have very recently shown their power by conveniently promoting various organic transformations.23 The aim of the present study is therefore to extend this palette of organic reactions and, more precisely, to prepare green and sustainable Cu(I)-containing FAU-type zeolites for the synthesis of diynes (Scheme 1), which are important building blocks in fine chemistry.24-26 Since traditional methods for the synthesis of diynes usually involve cupric salts along with a base, the development of a heterogeneous version remains useful, especially in terms of ecofriendliness.26 Cu-USY zeolite should offer both acidic and redox properties and thus induce tailored shape selectivity, with an ease of recycling. 2. Experimental Section 2.1. Cu-USY Zeolite Preparation. HUSY zeolite (CBV500 Zeolyst international, Si/Al ) 2.8) was obtained after calcination of its NH4 form during 5 h at 823 K. Cu+-exchanged USY
10.1021/jp809772n CCC: $40.75 2009 American Chemical Society Published on Web 01/27/2009
2904 J. Phys. Chem. C, Vol. 113, No. 7, 2009 zeolite was prepared by mixing 1 g of dry HUSY with an appropriate mass of CuCl (Aldrich, >99% purity, melting point 703 K) to make a physical mixture containing Cu/Al ) 1.1. The mixture was ground with a mortar and pestle rapidly (to avoid moisture) to achieve an intimate mixture of the two solids. The ground zeolite/CuCl mixture was then loaded into a tubular reactor. The reactor was sealed and connected to a flow manifold. The zeolite/CuCl mixture was either heated to 623 or 923 at 1 K/min under N2 flow 40 mL/min). The exchange temperature was held isothermal (623 or 923 K), and the duration was varied between 15 and 69 h. The as-prepared materials had a brown/tan color and were stored, protected from light, in an Ar-purged drybox until further use. 2.2. Characterization of Cu-USY Zeolites. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer, with a Ni detector side filtered Cu KR radiation (1.5406 Å) over a 2θ range of 5-50° and a position sensitive detector using a step size of 0.02° and a step time of 2 s. XRD was also used to estimate the crystallinity of copper-modified zeolites with respect to the starting HUSY zeolite. The degree of crystallinity (Q) was calculated on the basis of the ratio between the sum of (331), (533), (642), (555) reflection intensities, referred to this sum of the most crystalline sample, set arbitrary to unity. Specific surface areas (SSA) of the different zeolites were determined by N2 adsorption-desorption measurements at 77 K employing the Brunauer-Emmett-Teller (BET) method (Micromeritics sorptometer Tri Star 3000). Prior to nitrogen adsorption, the samples were outgassed at 573 K for 4 h to remove moisture adsorbed on the surface and inside the porous network. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo VG Scientific photoelectron spectrometer equipped with a twin anode providing both nonchromatized Al KR and Mg KR radiations (1486.6 and 1453.6 eV, respectively). The spectrometer equipped with a multichannel detector operated in the constant resolution mode with pass energy of 20 eV. The total resolution of the system was estimated at 0.55 eV. Spectra were referenced to the aliphatic hydrocarbon C1s signal at 285 eV. The Bro¨nsted acidity of the materials was evaluated by means of our H/D isotope-exchange technique reported elsewhere.27-29 27 Al (I ) 5/2) magic angle spinning nuclear magnetic resonance (MAS NMR) was carried out with a Bruker DSX 400 spectrometer operating at B0 ) 9.4 T (Larmor frequency ν0 ) 104.2 MHz). A single pulse of 0.7 µs with a recycle delay of 1 s was used for all experiments. The spinning frequency was 8 kHz. Measurements were carried out at room temperature with [Al(H2O)6]3+ as external standard reference. 19 F MAS NMR spectra were recorded on a Bruker MSL 300 spectrometer operating at 282.4 MHz using a 4-mm Doty probe and single-pulse excitation. A pulse length of 4 µs (π/2) with a recycle delay of 6 or 10 s and a spinning rate of 7-8 kHz were used. Spectra are referenced to external CFCl3.30 1 H-13C cross polarization (CP) MAS NMR spectra were recorded on a Bruker Avance II 300 spectrometer (B0 ) 7.1T) operating at 75.47 MHz. Samples were packed in a 4-mm diameter cylindrical zirconia rotor and spun at a spinning frequency of 12 kHz. These experiments were recorded with a proton π /2 pulse duration of 5 µs, a contact time of 1.5 ms and a recycle delay of 8 s. 2.3. Alkyne Homocoupling Reactions. The procedure for the Cu(I) zeolite catalyzed homocoupling of terminal alkynes was as follows: 70 mg of Cu-USY (0.3 mmol, 0.3 equiv based
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Figure 1. XRD of as-synthesized Cu-USY zeolites.
on total number of OH groups for Y, MOR, BEA, MFI zeolites) were added to 3 mL of dimethylformamide as solvent. Then, the alkyne (1 mmol, 1.0 equivalent) was added to the suspension. The reactor used was a round-bottomed flask (stirred tank) working under air atmosphere. After a vigorous stirring of 15 h at 383 K, the mixture was recovered with 5 mL of dichloromethane and then filtered on nylon membranes (0.20 µm). The filtrate was washed 6 times with a 1 M HCl solution (2-3 times 25 mL). The organic layer was dried on MgSO4 and filtered. Solvent evaporation provided the resulting crude product, usually at >95% purity as judged by NMR. Column chromatography was performed to purify the products. Isolated yields of the coupling products were reported in the manuscript. Results and Discussion 3.1. Structure of the Different Cu-FAU Zeolites: XRD and BET Studies. The powder XRD patterns of the different asprepared Cu-USY zeolites are displayed in Figure 1. It is noteworthy that all patterns exhibit the FAU structure, whatever the calcination temperature and duration. In spite of HCl release during the Cu+/H+ exchange process, it appears that the zeolite framework remained preserved (at least partially) at these high temperatures. The crystallinity (Q) decreased under severe exchange conditions (923 K for 15 h) as mentioned in Table 1. Whereas the Cu-USY zeolite prepared at 623 K after 48 h remains highly crystalline (86% of pristine HUSY), both a further prolongation of the solid-state reaction with CuCl and a reaction at 923 K led to a drastic decrease in the Q values to 0.64 and 0.65, respectively. The presence of remaining bulk CuCl crystalline phase (reflexions at 2θ ) 28.5, 47.4, 56.3) can be excluded in the different samples. Li et al. have also shown that the reflexions corresponding to CuCl phase disappeared at temperatures above 623 K for FAU zeolites.14 A major part of crystalline CuCl was sublimated and dispersed in the surface of the HUSY zeolite. It appears therefore that even below its melting temperature, CuCl became mobile enough to bind somehow to the zeolite matrix. The same phenomenon has already been observed for molybdate species which were able to bind within the MFI channels in a peculiar manner.29 The loss in SSA values (Table 1) tends to confirm the presence of bonded Cu species (with or without chlorine) inside the zeolite channels and cages when the solid-state reaction temperature was held at 623 K. In contrast, BET values support the absence of such species after a thermal treatment at 923 K. Hence, cationic exchange between Cu+ and H+ ions occurred exclusively as reported elsewhere.8,11 TGA studies have shown that
Probing Cu-USY Zeolite Reactivity
Figure 2.
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Al MAS NMR spectra of Cu-USY-623 K (a) and Cu-USY-923 K (b).
TABLE 1: Chemical Composition and Surface Properties of FAU-Type Zeolites materials
Cu/Al
crystallinity, Q
SSA [m2/g]
no. OH groupsa [mmol/g]
exchange rate by copper cations [%]
HUSY Cu-USY 623 K, 48 h Cu-USY 923 K, 15 h
0 1.1 1.1
1.00 0.86 0.65
660 367 405
3.90 0.86 0
0 78 100
a
Following our H/D technique.27,28
the ion-exchange process was maximal at 613 K releasing HCl gas simultaneously in flowing nitrogen,14 thus confirming the possible ion-exchange below CuCl melting point. 3.2. Surface Properties: Influence of Bro¨nsted and Lewis Acidity. Table 1 shows the number of hydroxyl groups still present on the zeolite surface after ion-exchange process. The total number of Bro¨nsted acid sites in parent HUSY zeolite was measured via our H/D isotope exchange technique27 and set to 3.90 mmol/g. The number of remaining OH groups decreased after Cu+/H+ ion exchange, as expected. The exchange process became complete when the reaction was performed at 923 K. This indicates that, above its melting point, Cu+ cations (1.1 equivalent) replace all protons, thus releasing gaseous HCl which can affect both the zeolite structure and acidity.14 In contrast, the exchange process remained incomplete at 623 K (78%). These results are in line with those from Karge et al.31 for temperatures between 573 and 673 K. It cannot be completely
excluded that Cu species (agglomerates) stay on the zeolite surface as suspected from the BET values. Figure 2 presents the 27Al MAS NMR of the two Cuexchanged USY zeolites. Some extra-framework aluminum (EFAl) located at a chemical shift close to 0 ppm was observed in both samples. It is noteworthy that the dealumination extent (amount of EFAl species) was higher after a preparation at 623 K (Figure 2a). The formation of true Lewis acidity via ejection of Al species from the framework in the pores is favored at 623 K. However, when the CuCl/HUSY mixture was heated at 923 K for 15 h (Figure 2b), the signal of EFAl species was lower than after calcination at 623 K. In spite of usual increase of dealumination process with temperature,32-34 the release of HCl at such temperature probably led to the leaching of part of EFAl species.35 This phenomenon is further supported by a drastic loss in zeolite crystallinity (Table 1). On the contrary, for the Cu-USY prepared at 623 K, CuCl species may react
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Figure 3. Isolated yields of 1,4-diphenylbutadiyne obtained over different FAU-type zeolites.
with remaining EFAl to form (AlO)Cl species, which exhibit Lewis acidity.14 The presence of EFAl species, especially in the FAU framework, raises the Lewis acidity of the material but can also induce an increase of the acid strength of remaining Bro¨nsted acid sites.36-41 The activity for the coupling reaction of copper-modified zeolites can therefore be influenced by the presence of both EFAl species and remaining O-H groups. 3.3. Cu-USY Structure and Activity Relationship in Terminal Alkyne Coupling Reaction. Preliminary experiments performed with phenyl acetylene in various solvents revealed that the reaction was highly dependent on solvent polarity. Whereas reactions performed in nonpolar toluene mostly returned the starting material, the expected diyne was obtained in excellent yield in highly polar DMF solvent in a very clean manner. Solvents of intermediate polarity, such as CH2Cl2, THF, or CH3CN, did not allow any transformation of the starting alkyne.26b On the basis of homogeneous catalysis considerations, DMF solvent might help in copper complexation, thus raising somehow its activity. Homogeneous Glaser coupling reactions usually involve Cu(I) salts together with a base (pyridine, ammoniacal solution), which can act as complexing agent.26b,44 Figure 3 shows the yields in isolated homocoupling product, i.e., 1,4-diphenylbutadiyne, obtained over the different FAUtype zeolites. Since blank experiments showed that no reaction occurred with parent HUSY zeolite, Cu(I)-USY acted as a true catalyst for phenyl acetylene homocoupling. In Glaser-type acidic conditions, it has been shown that the rate of coupling increased while raising cuprous ion concentration.42,43 The same trend was observed with Cu(I)-USY (prepared at 623 K) as catalyst since yields increased from 78% with 10 mol % cuprous loading to 97% with 30 mol % loading. As suspected from our physicochemical studies described above, the catalytic performance of Cu-USY materials differed depending on the exchange temperature applied for their formation. The zeolite that contains many EFAl species (Figure 2a) and still possesses 22% of its Bro¨nsted acidic sites (Table 1) allowed a complete transformation of phenyl acetylene into coupling product. On the contrary, Cu(I)-USY zeolites prepared at 713 and 923 K did not contain residual Bro¨nsted acidic sites and less EFAl species were observed by 27Al MAS NMR (Figure 2b). However, these catalysts were active in the homocoupling reaction in the presence of air (Hay conditions44), but with these Bro¨nsted acid free zeolites, only 50 and 52% were converted into corresponding diyne, respectively. Despite Cu cations are without any doubts responsible for the activity in Glaser coupling, it seems also that acid sites might be required to reach an optimum dimerization of alkynes.
Figure 4. 1
1
H-13C CP MAS NMR spectra of Cu-USY zeolites.
H-13C CP MAS NMR has been used to study the interactions between the reactants and products and the zeolite surface (Figure 4). The reaction was stopped after 2 h, and the catalysts were recovered and dried overnight. The spectra for the two zeolites prepared at 623 and 973 K are similar, with signals present with in the same chemical shift area. The presence of different carbons from a phenyl ring between 120-140 ppm can be ascertained. The acetylenic carbons expected around 70 ppm can barely be observed. Surprisingly, a broad signal was observed between 25-40 ppm for both samples that cannot be attributed to traces of DMF solvent. While trying to prepare diamond-like carbon from copper acetylides, Cataldo and Capitani have noticed the presence of a similar broad signal between 25-50 ppm.45 In line with Raman studies, they were able to attribute this signal to a carbyne chain. Carbyne corresponds to an acetylenic carbon chain with a sequence of alternating single and triple bonds. This result accounts for a simultaneous adsorption of two phenyl acetylene molecules on a copper cation, acting as the active site (see section 3.4). Since this reaction was performed under clean conditions, quantitatively and selectively toward the homocoupling product, only the latter remains present at the zeolite surface. However, the difference in the intensities of the signals from the aromatic carbons indicates a different adsorption mode and hence a different chemical environment around the active site. These results are in agreement with an adsorption, or coadsorption, of the alkyne compound on acid sites present in the vicinity of copper cations. To verify this statement, we have performed the Glaser coupling with trifluorophenyl acetylene reactant and investigate
Probing Cu-USY Zeolite Reactivity
Figure 5.
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F MAS NMR spectra of Cu-USY-623 K (a) and Cu-USY-923 K (b).
via 19F MAS NMR the interactions between the product and the surface of the 623 and 923 K prepared Cu-USY zeolites. Again, the isolated yield toward the coupling product was very high, i.e., 91% over the optimized catalyst. Figure 5 shows the 19 F MAS NMR spectra for the two different zeolite surfaces. A variation of 2 ppm in the 19F chemical shift of trifluorophenyl acetylene molecules present within the FAU pores was observed between the two zeolites. By use of the same technique, Simon et al. have proven that a chemical shift toward higher values was directly connected to the strong adsorption of organic species on the zeolite surface and hence to the acid strength of the zeolite.30 The Cu-USY zeolite prepared at 623 K, which both presents a higher amount of EFAl species (Figure 2a) and remaining
Bro¨nsted acid sites (Table 1), induced a deshielding of the fluorine nuclei, i.e., - 61.6 ppm vs -63.6 ppm, when compared to the zeolite prepared at 923 K (Figure 5). Since the influence of the zeolite magnetic susceptibility can be neglected,30 these 19 F chemical shift variations can be ascribed to the presence of interactions between the fluorine containing organic molecules, present within the FAU structure and the acid sites (EFAl or Bro¨nsted). Moreover, the relatively broad signal and the presence of spinning side-bands indicate that the FAU topology plays a role of confinement medium, where steric constraints might be enhanced by the presence of EFAl species in the cages. Hence, this would prevent the isotropic orientation of the molecules. On the basis of earlier studies in liquid acids, the
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Figure 6. Influence of zeolite structure on the catalytic conversion of phenyl acetylene into 1,4-diphenylbutadiyne.
TABLE 2: XPS Binding Energy of Cu 2p3/2 electrons in Cu-USY Zeolite before the Reaction, after the Reaction (without Work Up), and after the Removal of Organics material
binding energy (eV) Cu 2p3/2
remarks
CuUSY as-prepared CuUSY intermediate CuUSY after workup Cu Cu2O CuO
932.8 933.3 933.1 932.2 932.8 933.8-934.4
before reaction after reaction with phenylacetylene and DMF solvent removal after complete removal of organics ref 47 ref 47 ref 57
narrow signal present for both zeolites at -122.7 ppm can be attributed to physisorbed molecules on the zeolite surface.46 Table 2 lists XPS binding energy of Cu 2p3/2 electrons in Cu-USY zeolite as prepared at 623 K, after reaction with phenyl acetylene (without workup), and finally after the removal of organics from the zeolite surface. For comparison, Cu 2p3/2 binding energies of Cu (932.2 eV), CuO (934.3 eV), and Cu2O (932.8 eV) are also mentioned.47 It is noteworthy that copper ions present in as-synthesized zeolite are at a degree of oxidation +I. After performing the homocoupling reaction, a shift of 0.5 eV toward higher binding energy values was observed. This shift is clearly caused by the oxidation of Cu+ ions to Cu2+ during the course of the reaction (performed under air).48 Indeed, such intermediate value in binding energy corresponds to the presence of both Cu(I) and Cu(II) species which confirm that the reaction ran over Cu-zeolites probably has a mechanism similar to the classical reaction based on Cu redox cycles.42 Since pore size and shape play a key role in the catalysis by zeolites and often involve shape selectivity, we have screened other zeolite structures, looking for optimal alkyne homocoupling activity. Figure 6 shows the performance of different Cudoped promoted zeolites prepared at 623 K following the same procedure as the one used for Cu-USY (see the experimental section). As expected for reactions in which rod shape molecules are formed, the zeolite nature had a dramatic influence on reaction efficiency. Indeed, medium-pore zeolites possessing either unidimensional or interconnected channels exhibit only weak performance in coupling products formation (6% over CuMOR, 15% over Cu-MFI, and 56% over Cu-BEA zeolite). On the contrary, the FAU cage structure seems to be the framework of choice for performing the reaction. The reaction efficiency is also directly correlated with the zeolite pore sizes: the larger the pore size, the better the yield (Figure 6). These results confirm that the reaction took place into the zeolite frame and not on its external surface, for which such correlation could not be expected.
Since the release of active species in the solution could be the “true” catalyst, we have examined the question of leaching. Cu-USY zeolite was suspended in DMF, and the mixture was stirred overnight and then filtered off (at the reaction temperature) leaving a clear solution.26b Phenylacetylene was added to the later solution and allowed to react during 15 h at 383 K. Only starting alkyne was detected. Indeed, no diyne product was detected or at best as traces level (
