Design of Scandium-Doped USY Zeolite: An Efficient and Green

Article pubs.acs.org/JPCC

Design of Scandium-Doped USY Zeolite: An Efficient and Green Catalyst for Aza-Diels−Alder Reaction A. Olmos,† S. Rigolet,‡ B. Louis,*,† and P. Pale*,† †

Laboratoire de Synthèse Réactivité Organique et Catalyse (LASYROC), Institut de Chimie, UMR 7177, Université de Strasbourg, 1 rue Blaise Pascal, F-67000 Strasbourg Cedex, France ‡ Institut de Science des Matériaux de Mulhouse (IS2M), Matériaux à Porosité Contrôlée, CNRS LRC 7228, Ecole Nationale Supérieure de Chimie de Mulhouse, 3, rue Alfred Werner, 68093 MULHOUSE Cedex, France ABSTRACT: Scandium-exchanged USY zeolites were synthesized by vapor-phase exchange of parent HUSY with scandium trifluoromethanesulfonate (triflate) Sc(OSO2CF3)3. Several FAU zeolites having scandium loadings ranging from 2 to 30 mol % were prepared and applied as catalysts to aza-Diels− Alder reaction in order to establish structure and catalytic properties relationships. Calcination at 623 K for 72 h led to preservation of the zeolite crystallinity along with the presence of different extra-framework aluminum species (EFAl) and scandium species, thus providing high Lewis acidity. Observation of the catalyst before, during, and after reaction, by multinuclear MAS NMR techniques and H/D isotope labeling, revealed the modification of aluminum and scandium environments within the zeolite framework. Recycling experiments stated for a truly heterogeneous catalytic path over these Sc−USY zeolites. In contrast to the homogeneous version for the aza-Diels−Alder reaction, the present heterogeneous new version provided a green route for quantitative synthesis of tetrahydropyridines via bifunctional heterogeneous catalysis.

1. INTRODUCTION Zeolites containing transition metal cations (TM) are of primary importance due to their promising activity and selectivity as heterogeneous catalysts in several chemical reactions.1−9 Faujasite-type zeolites doped with TM show remarkable catalytic activity and selectivity in several petrochemical processes such as alkylation, desulfurization, fluid catalytic cracking, and isomerization.10−13 These materials have also been used in olefin paraffin separation, purification of light olefins, or CO removal in fuel cells.14−17 To design an active and a stable catalyst, the zeolite itself as well as the way to introduce TM are crucial parameters. Indeed, the presence of extra-framework aluminum species (EFAl) stabilizes the framework.18 In addition, the presence of TM inside the zeolite pores may further affect the structure and thus the catalyst behavior. It is therefore of high importance to properly introduce and localize these metal cations in a proper amount to tailor the remaining Brønsted acidity.19,20 Since aqueous solutions of TM can be acidic, the classical cationic exchange reaction can favor side reactions and partial lattice destruction. Furthermore, some TM are nonsoluble in aqueous medium. In order to avoid or at least reduce these side effects, solid-state exchange techniques have been developed.21−24 However, upon high-temperature treatment, TM can agglomerate, thus forming dimeric or oligomeric species or even metallic nanoparticles.20,25 © 2012 American Chemical Society

Incorporation of rare-earth elements in zeolites is known to stabilize the framework. In addition, Lercher et al. demonstrated an increase in the Brønsted acid strength when La3+ cations are present in extra-framework positions.26 The Sc3+ cation is the first member of trivalent ions of Group 3, and its ionic radius (0.75 Å) makes it the smallest rare-earth element, in comparison with La3+ (0.86−1.03 Å).27 Scandium has been shown to be rather labile toward ligand substitution in liquid medium, rendering it an interesting Lewis acid for homogeneous and heterogeneous catalyses.28−33 Nevertheless, reports dealing with preparation of scandium-containing zeolites remain surprisingly scarce and restricted to cationic exchange34 and isomorphous substitution.35,36 We aim herein to design active and stable Sc-doped USY zeolites in order to develop a new family of green catalyst for synthesis of fine chemicals. On the basis of our earlier studies devoted to metalated zeolites,20,37−40 slow vapor diffusion of scandium triflate inside the porous network was used to attempt achieving highly dispersed and homogenized scandium cationic species. In the case of success, one can therefore extend the palette of organic reactions promoted by scandium cations to green and sustainable Sc(III)-containing USY zeolites. The present study remains restricted to synthesis of tetrahydroquiReceived: April 3, 2012 Revised: May 28, 2012 Published: June 4, 2012 13661

dx.doi.org/10.1021/jp303169h | J. Phys. Chem. C 2012, 116, 13661−13670

The Journal of Physical Chemistry C

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noline derivatives, which are important building blocks in fine chemistry and key components of pharmaceuticals41 as well as natural products, for which they represent a subclass of alkaloids.42 Due to the various roles of such compounds, several synthetic routes have been developed,43 among which the [4 + 2] cycloaddition reaction between N-arylimines and alkenes is one of the most convenient and rapid reaction (Scheme 1).

27

Al (I = 5/2) magic angle spinning nuclear magnetic resonance (MAS NMR) was carried out with a Bruker Avance II 400 spectrometer operating at B0 = 9.4 T (Larmor frequency ν0 = 104.2 MHz) equipped with a Bruker 2.5 mm doublechannel probe. Samples were spun at 25 kHz, and free induction decays (FID) were collected with a π/12 rf pulse (0.5 μs) and a recycle delay of 1 s. Measurements were carried out with [Al(H2O)6]3+ as external standard reference. To separate all Al sites, 27Al multiple quantum MAS (MQMAS) NMR was employed using a z-filter pulse sequence with three rf pulses. First and second pulse durations were adjusted to 4 and 1.5 μs, respectively, so that the triple-quantum coherence was created by a radiofrequency field strength of 100 kHz. The zeroquantum coherence was converted into single-quantum coherence with a third ‘‘soft’’ pulse of 13 μs duration. The spinning frequency was set to 30 kHz, and rotationsynchronized acquisition was used. Shearing of the 27Al3QMAS NMR spectra was performed with the program XfShear, which aligns the anisotropic axis of the 2D spectra parallel to the F2 dimension. The 19F MAS NMR spectrum was recorded on a Bruker Avance II 400 spectrometer operating at a Larmor frequency of 376.57 MHz using a 2.5 mm Bruker double-channel probe. A pulse length of 4 s (π/2) with a recycle delay of 20 s and a spinning rate of 30 kHz were used. Spectra are referenced to external CFCl3.51 45 Sc MAS NMR spectra were collected with a Bruker Avance II 400 MHz spectrometer operating at a Larmor frequency of 97.22 MHz, equipped with a Bruker 2.5 mm double-channel probe. Samples were spun at 25 kHz, and free induction decays (FID) were collected with a π/16 rf pulse (0.38 μs) and a recycle delay of 1s. 45Sc NMR chemical shifts were referenced to that of ScCl3. The 1H−13C cross-polarization magic angle spinning (CPMAS) NMR spectrum was recorded on a Bruker Avance II 400 spectrometer operating at 100.6 MHz. Samples were packed in a 4 mm diameter cylindrical zirconia rotor and spun at a spinning frequency of 12 kHz. Experiments were acquired using a ramp for Hartmann−Hahn matching with a proton π/2 pulse duration of 5 μs, a contact time of 1 ms, and a recycle delay of 3 s depending of 1H spin−lattice relaxation times (t1) estimated with the inversion−recovery pulse sequence. 29 Si MAS NMR spectra were recorded with a Bruker Avance II 300 MHz spectrometer operating at B0 = 7.2 T (Larmor frequency ν0 = 59.62 MHz) equipped with a Bruker 7 mm double-channel probe, and samples were spun at 4 kHz. In a view to obtain quantitatively reliable 29Si data, single-pulse magic angle spinning (SPE-MAS) experiments have been performed using a pulse angle of π/6 and a recycling delay of 80 s. 1H−29Si CPMAS NMR spectrum was acquired using a ramp for Hartmann−Hahn matching with a proton π/2 pulse duration of 5.9 μs, a contact time of 5 ms, and a recycle delay of 3 s. 13C and 29Si NMR spectra were recorded under high-power proton decoupling conditions, and 13C and 29 Si chemical shifts are relative to tetramethylsilane. All NMR measurements were carried out at room temperature. 2.3. Aza-Diels−Alder Reactions. Scandium-doped zeolites catalytic activities have been evaluated with a model azaDiels−Alder reaction using the simplest reagents (Scheme 2). Catalytic tests were thus performed by reacting at room temperature 0.5 mmol of N-benzylidenaniline with 1.5 mmol of cyclopentadiene in the presence of enough Sc−USY to provide 0.025 mmol of Sc3+ (5 mol %) in 2 mL of acetonitrile. The

Scheme 1. Sc−Zeolite-Catalyzed [4 + 2] Cycloaddition Route to Tetrahydroquinolines

Usually performed under acidic conditions, such cycloadditions have been reported with various Lewis acid catalysts.44 However, few heterogeneous and recyclable catalysts have been used so far.45

2. EXPERIMENTAL SECTION 2.1. Scandium−USY Zeolite Preparation. HUSY zeolite (CBV500 Zeolyst international, Si/Al = 2.8) was obtained after calcination of its NH4 form for 15 h at 823 K. Sc3+-exchanged USY zeolite was prepared by mixing 1 g of dry HUSY with an appropriate mass of scandium triflate Sc(CF3SO3)3 (Aldrich, >99% purity, melting point 573 K) to obtain a physical mixture containing loadings of Sc between 2 and 30 mol %. The mixture was ground with a mortar and pestle rapidly (to avoid moisture) in order to achieve an intimate mixture of the two solids. The ground HUSY/Sc(CF3SO3)3 mixture was then loaded into a tubular reactor. The reactor was sealed and connected to a flow manifold. The intimate mixture of the solids was heated to 723 at 10 K/min under N2 flow (40 mL/ min). The exchange temperature was held isothermal, and the duration was set to 72 h. The as-prepared materials had a gray color and were stored in a drybox until further use. In coming sections, these samples were named as xScUSY in which x represents the scandium loading, varying from 2 to 30 mol %. 2.2. Characterization of Sc−USY Zeolites. X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advance diffractometer with a Ni detector side filtered Cu Kα radiation (1.5406 Å) over a 2θ range of 5−60° and a positionsensitive detector using a step size of 0.02° and a step time of 2 s. XRD was also used to estimate the crystallinity of scandiumdoped zeolites with respect to the starting HUSY zeolite. The crystallinity was estimated on the basis of the ratio between the intensities of zeolite main peaks and their background as already reported.46,47 The degree of crystallinity (Q) could therefore be deduced from this former ratio referred to the one of pristine HUSY zeolite, thus arbitrarily set to Q = 1. The Brønsted acidity of the materials was evaluated by means of our H/D isotope-exchange technique.48−50 SEM images were acquired with a Hitachi S4800 FEG microscope equipped with an EDS system (EDX) for elemental analysis. Samples were loosely dispersed on a conductive carbon tape to preserve the as-prepared morphology as much as possible. SE micrographs were acquired at low accelerating voltage, i.e., 1.5 kV, for better sensitivity to surface features. EDX spectra were acquired at an accelerating voltage of 15 kV. 13662



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Scheme 2. Model Aza-Diels−Alder Reaction Used in the Present Study

Table 1. Specific Surface Area, Pore Volume, and Relative Crystallinity of Sc-Doped Faujasite Zeolites

reaction was monitored by TLC and GC, and after completion, the reaction mixture was diluted with 10 mL of dichloromethane and filtrated through Nylon membrane (0.20 μm) to separately recover solid catalyst and crude solution mixture. The latter was then evaporated under vacuum and purified by flash chromatography when necessary.

catalyst

SSA [m2/g]

pore volume [cm3/g]

crystallinity

HUSY 2ScUSY 5ScUSY 10ScUSY 30ScUSY

623 610 597 645 359

0.51 0.55 0.58 0.58 0.49

1 0.98 0.95 0.91 0.72

observed for molybdate species20 or mobile CuCl species37 which were able to bind within zeolite channels in a particular environment. Table 1 also presents the SSA and pore volumes values for the various materials. It is important to note that both SSA values and pore volumes remained only barely affected by the scandium loading up to 10 mol %. In contrast, 30ScUSY exhibited a drastic loss in its SSA down to 359 m2/g. Such loss in SSA values, as well as in crystallinity, tends to confirm partial destruction of the zeolite framework by triflic acid released after heating the solids at 723 K (Scheme 3). Figure 2 shows the SEM image of 10ScUSY zeolite. The morphology of the FAU crystals is octahedral and their size

3. RESULTS AND DISCUSSION 3.1. Structure of the Different Sc−FAU Zeolites: XRD, SEM, and BET Studies. The XRD patterns of pristine HUSY and various as-prepared Sc−USY zeolites are presented in Figure 1. These patterns were not accompanied by any

Figure 1. XRD patterns of HUSY and Sc−USY zeolites.

characteristic reflection of neither the metallic salt nor oxide such as Sc2O3,52 thus indicating that the final zeolite contains only dispersed scandium-docked species. Nevertheless, no shift in the diffraction lines toward lower 2θ values could be observed in Sc-doped zeolites, excluding therefore any unit cell expansion due to Sc3+ cation introduction in a T-atom position within the zeolite frame.35 It is noteworthy that all patterns exhibit the FAU structure whatever the scandium loading is. In spite of trifluorosulfonic acid53 release during the Sc3+/H+ exchange process (Scheme 3), it appears that the zeolite

Figure 2. SEM image of 10ScUSY zeolite.

comprised between 500 and 800 nm. Like XRD and BET data, the presence of scandium oxide particles can be excluded at the outer surface of the crystals. Furthermore, EDX analysis also excluded the existence of scandium-enriched zones in the various materials. The proper metal content was evidenced for all samples containing more than 2% of scandium. The scandium content in the latter zeolite is in the range of the apparatus detection limit. Interestingly, an unexpected F/Sc ratio of 3 ± 0.3 was observed for these catalysts, thus supporting the presence of one remaining triflate as a ligand surrounding the Sc3+ cation after solid-state preparation. 3.2. Surface Properties: Influence of Brønsted and Lewis Acidities. Figure 3 shows the number of hydroxyl groups still present on the zeolite surface after the Sc3+/H+ cationic exchange process. The total number of Brønsted acid sites in parent HUSY zeolite and metalated zeolites was measured via our H/D isotope exchange technique.48,49 The number of acid sites was measured as 3.90 mmol/g for the parent HUSY zeolite. The number of OH groups after doping with scandium triflate linearly decreased upon raising the metal

Scheme 3. Sc3+/H+ Exchange Process

framework remained mostly preserved up to a 10% scandium loading at such high temperature. Indeed, the degree of crystallinity was kept at Q > 0.90 for these metal-promoted zeolites when compared to pristine HUSY zeolite. In contrast, the crystallinity (Q) drastically decreased for the 30ScUSY sample, with Q = 0.72 (Table 1). These results showed that scandium triflate was sublimated and dispersed in the zeolite matrix to bind somehow onto the zeolite surface. The same phenomenon has already been 13663



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Figure 3. Dependence between the amount of Brönsted acid sites determined by H/D isotope exchange technique and the scandium loading of the different Sc-containing faujasite zeolites.

centered at 52 ppm corresponding to framework tetrahedral aluminum (AlF) and an intense signal centered at −5 ppm which is attributed to octahedral extra-framework Al (EFAl).47,54−56 In addition, a third resonance in the 27Al spectrum can be observed at nearly 24 ppm, which has already been assigned to five-coordinated extra-framework aluminum.57−59 However, one cannot exclude the presence of one more signal due to the contribution from hydrated aluminate having a strong interaction with neighboring scandium cation. Indeed, such signal may appear between 10 and 20 ppm as reported for alumino−molybdate species in earlier studies.20,60,61 Surprisingly, the peak corresponding to AlF was shifted toward higher fields: −5 ppm after Sc introduction with respect to the usual signal at nearly 0 ppm. This can be due to modification of the chemical environment around AlF atoms, presumably via anchoring of scandium in the neighboring of oxygen atom. Kucera and Nachtigall have shown that such shift observed in the Al spectrum of zeolites is due to the extraframework cation coordination as well as to T−O−T angle distortion.62 The presence of both octahedral and pentacoordinated EFAl species, especially in the FAU framework, raises the Lewis acidity of the material but can also induce an increase in the acid strength of the remaining Brønsted acid sites.63−65 The activity of Sc-doped zeolites in the aza-Diels−Alder reaction can therefore be influenced by the presence of both EFAl species and remaining Brønsted acid sites. Interestingly, the signal attributed to pentacoordinated Al seems to be drastically decreased after having performed the aza-Diels−Alder reaction (Figure 4b). In order to assess this observation, 27Al multiple quantum MAS (MQMAS) NMR was performed over 10ScUSY before and after reaction (Figure 5). Two-dimensional MQ MAS NMR shows the disappearance of the former resonance after reaction (Figure 5b). Hence, this unambiguously confirms the increase in the structuring around Al nuclei after chemical reaction, with complete disappearance of the signal at 24 ppm, usually present when the framework exhibits numerous defects.58,66 The signal corresponding to AlF at 55 ppm was amazingly enhanced with respect to octahedral EFAl (−2 ppm), thus indicating again important modifications on the zeolite surface after reaction. A healing of the framework seems therefore to occur during the catalytic run at room temperature and can barely be explained at the present stage. Nevertheless,

content, varying from 3.49, 3.06, and down to 1.32 mmol/g for, respectively, 5ScUSY, 10ScUSY, and 30ScUSY. It is noteworthy that the loss of OH groups per Sc3+ cation introduced remained nearly equal to 2 whatever the metal loading was. These results suggest that one scandium ion displaced two protons but not three, thus keeping one of its original triflate ligand. Indeed, these results are in perfect agreement with the EDX analysis (see above in section 3.1), showing that one triflate ligand remained around the Sc3+ cation. The other two triflates were obviously released as gaseous triflic acid, leaving the scandium cation chelated by two oxygen atoms from the zeolite framework and one triflate. Scandium-doped USY zeolite has been further characterized by the 27Al MAS NMR technique to get insights about the surface acidity of the zeolite via investigation of the nature and environment of Al species.54−56 Figure 4 shows the 27Al MAS NMR of as-synthesized 10ScUSY zeolite (Figure 4a) and after having performed the aza-Diels−Alder reaction (Figure 4b). 10ScUSY material exhibits at least three signals: a resonance

Figure 4. 27Al MAS NMR spectra of 10ScUSY (a) as synthesized and (b) after performing reaction. 13664



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Figure 5. 27Al multiple quantum MAS (MQMAS) NMR of 10ScUSY (a) as prepared and (b) after reaction.

neighbors. The resonances at nearly −103 ppm may be assigned to Q3 signals, referred to as either Si(OSi)3(OH) or Si(OSi)2(OAl)(OH) species.50 These relatively broad signals can be due to the presence of an Al atom that induces distortion of the lattice, thus to a distribution of chemical shifts depending on the different distances between Al atoms and Q3 and even a few Q2 sites (−96 ppm). In addition, the former effect may be further emphasized by the presence of defects in the lattice mainly due to Si(OSi)3(OH) species. However, the Q3 signal (−102 ppm) drastically diminished after the catalytic reaction (Figure 6b). In contrast, Q4 contributions, Q4(0Al) and Q4(1Al), were increased and shifted toward higher fields down to −117.1 and −112.1 ppm, respectively. In general, such shielding of nearly 5 ppm is observed when Si−O−Si substitutes Si−O−Al linkages,47,69,70 thus inducing profound internal changes in the framework structure. This phenomenon may be explained by the partial silicon healing of the structure as proposed by Lutz et al.47 This interpretation can be further

this phenomenon, usually encountered at higher temperatures,67,68 will be further discussed in the coming sections. 3.3. Multinuclear MAS NMR Study: Sc−USY Structure and Its Evolution upon Catalysis. In order to further investigate the structure and properties of scandium-doped faujasites, a multinuclear solid-state NMR study has been adopted to establish structure−activity relationships, as already reported in our earlier contribution devoted to Cu(I)− zeolites.37 Figure 6 presents the 29Si MAS NMR spectra of pristine 10ScUSY (Figure 6a) and after recovery of the former catalyst after aza-Diels−Alder reaction (Figure 6b). The local environment of Si atoms, or SiO4 units, is mainly constituted by Q4 and Q3 signals in the zeolite framework.50,69 The main large peak located at −107.8 ppm, in Figure 6a, is attributed to Si(OSi)4 and Si(OSi)3(OAl) signals, named Q4(0Al) and Q4(1Al), respectively. Si atoms are therefore located at the T site of the framework, surrounded by either 4 Si atoms (zero Al) in the nearest T crystallographic positions or 3 Si and 1 Al 13665



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the partial healing of the zeolite framework by recombination of defect sites into Si−O−Si and Si−O−Al linkages,71 thus raising the signal corresponding to tetrahedral coordinated aluminum atoms (Figure 4). To go one step further, 45Sc NMR experiments were performed. Figure 8a shows the 45Sc NMR spectra of as-

Figure 6. 29Si MAS NMR spectra of (a) 10ScUSY (as prepared) and (b) after the catalytic test.

verified by 1H−29Si CPMAS spectra of 10ScUSY before and after performing the catalytic test (Figure 7a and 7b,

Figure 7. 29Si{1H} cross-polarization magic angle spinning (CP MAS) NMR spectra of (a) parent 10ScUSY and (b) after reaction. Figure 8. 45Sc MAS NMR of (a) parent 10ScUSY, (b) after reaction, and (c) superposition of spectra.

respectively). Indeed, the latter technique allows one to evidence Si atoms possessing protons in their close vicinity, due to a strong magnetic dipolar coupling between the two nuclei.67 Looking at such proton−silicon interactions clearly confirmed the healing hypothesis. Indeed, the ratio Q4/Q3 is greatly increased after performing the reaction. The Q3 signal due to Si(OSi)3(OH) and Si(OSi)2(OAl)(OH) species is noticeably reduced. Furthermore, no signal at −112 ppm could be observed, thus confirming the absence of protons in the vicinity of Si atoms (Q4 signal). The CP MAS technique therefore sheds light on the structural reorganization of the zeolite framework after reaction. In line with 27Al NMR experiments, 29Si NMR also supports

prepared 10ScUSY zeolite. A sharp resonance centered at −50.8 ppm can be observed and unambiguously attributed to scandium present in an octahedral environment.35,36 This result revealed that such hexacoordinated scandium can exclusively be present as an extra-framework cation. Likewise, the same chemical environment can be deduced after the catalytic reaction since a unique and sharp signal located at −51.6 ppm can be observed (Figure 8b). The full width at half-maximum (fwhm) of the 45Sc peak was slightly reduced after the catalytic reaction, from 1144 to 920 Hz, thus supporting an increase in 13666



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the homogeneity around the scandium nucleus (Figure 8c). Again, this result confirms a number of structural defects. Interestingly enough, such reorganization and Sc−zeolite healing was further nicely supported by 19F MAS NMR monitoring. Indeed, 19F MAS NMR spectra for the 10ScUSY zeolite before and after the first catalytic run showed the disappearance of a narrow signal at −122.6 ppm (Figure 9a and

Table 2. Yield in Tetrahydroquinoline Obtained via AzaDiels−Alder Reaction over the Various Sc−USY Zeolites X−USY

time (h)

yield (%)

HUSY 2ScUSY 5ScUSY 10ScUSY 30ScUSY

24 2 5 22 24

traces 99 97 78 64

Monitoring of the so-formed tetrahydroquinoline revealed the dramatic changes in reaction rates upon Sc loading in this USY zeolite series (Figure 10). 2ScUSY and 5ScUSY zeolites

Figure 9. 19F MAS NMR spectra of 10ScUSY (a) before and (b) after aza-Diels−Alder reaction.

Figure 10. Correlation between different Sc loadings in the catalyst and their efficiency.

9b). This signal could be attributed to the CF3 group of the triflate ligand around the scandium cation remaining after catalyst preparation (cf. sections 3.1 and 3.2). Its disappearance after reaction (Figure 9b) perfectly agreed with the better homogeneity around scandium observed in 45Sc NMR as well as with the structure reorganization observed by 29Si and 27Al NMR upon reaction. The 19F chemical shift is in line with an interaction between the CF3 group and the zeolite surface, placing its vicinity to a Brønsted acid site within the FAU pores.51,72 Since the influence of the zeolite magnetic susceptibility can be neglected,51 the 19F spectrum tends to confirm the presence of interactions between the fluorine of the triflate group and the acid sites. Moreover, the relatively broad signal at −148 ppm and the presence of spinning side bands indicate that the FAU topology plays a role of confinement medium, where steric constraints might be involved and thus prevent the isotropic orientation of the molecules. Earlier studies have shown that the triflate group can be anchored to the zeolite surface via involvement of hydrogen bonds with the OH groups.73,74 These results are in agreement with an adsorption, or coadsorption, of the reactants on acid sites present in the vicinity of scandium cations. 3.4. Activity Relationships in Aza-Diels−Alder Reaction. Sc−zeolites have been used as catalysts in cycloaddition of N-benzylidenaniline with cyclopentadiene. Table 2 presents the yield in tetrahydroquinoline product (Scheme 2) obtained after reaction completion over the different scandium-doped zeolite catalysts. Pristine HUSY zeolite led only to traces of product after 24 h of reaction. Once scandium cation is introduced in its porous host, excellent yields were achieved over the catalysts containing up to 10 mol % of metal. At higher loading, conversion was slower and only 60−80% of the final product was achieved after 1 day.

proved to be very active as catalysts; they indeed converted almost quantitatively the starting materials to the expected product within only a few hours. In sharp contrast, 10ScUSY zeolite allowed reaching a high yield of product after 22 h of reaction, whereas 30ScUSY proved to convert even slower. 30ScUSY acted as the worse catalyst, as a priori expected from the physical characterization data gained (see section 3.1), but the marked difference between the catalysts with a low metal loading and the ones loaded at 10 or 30 mol % of scandium ion is quite surprising. These results suggest that a higher dispersion of scandium cations improves the catalytic performance of Sc−zeolites. It is also worth noting that at low metal loading more Brønsted acid sites are present within the zeolites. This higher Brønsted acid site density might also contribute to the higher efficiency of these Sc−zeolites. Similar cycloadditions have indeed been reported with protic acids such as trifluoroacetic acid,75 ptoluenesulfonic acid,76 oxalic acid,77 and 4-nitrophthalic acid78 as promoters. 1 H−13C CP MAS NMR has been used to study the interactions between the reactants and the products and the metal-doped zeolite surface (Figure 11). The reaction catalyzed by 10ScUSY zeolite was stopped after 5 h, and catalysts were recovered and dried overnight. The spectrum of the corresponding zeolite exhibits a large peak between 110 and 140 ppm centered around 130 ppm, revealing the presence of different carbons from aromatic and vinylic groups as expected from the starting imine and product structures. The small peak near 170 ppm could be assigned to the imine carbon. The other peaks around 60 ppm and some within the large peak between 10 and 40 ppm could be ascribed to the product. The latter also corresponds to the cyclopentadiene and to its dimer, which could be formed from the remaining cyclopentadiene. 13667



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Figure 12. Activity of 5ScUSY for the aza-Diels−Alder reaction over several runs. Reactivation of the catalyst between runs performed by calcination at 823 K for 1 night.

Figure 11. 1H−13C cross-polarization magic angle spinning (CPMAS) NMR spectrum of 10ScUSY zeolite after 5 h of aza-Diels−Alder reaction.

selectivity (only one product recovered). It is however worth mentioning that calcination at 823 K was required between each run to ensure such recyclability. If the catalyst was only separated from the reaction mixture through filtration and reused as such, a regular activity loss was observed, probably due to carbonaceous deposits, coking phenomenon, on the zeolite surface. It appears therefore that the Sc-promoted USY zeolite acts as a true heterogeneous catalyst exhibiting only reversible deactivation. To summarize, a peculiar spatial localization of Sc3+ and Brönsted acid sites being in the vicinity of each other is warranted to design an active, a selective, and an even recyclable scandium-promoted catalyst. In the present case, the zeolite seems to behave analogous to enzymes, thus acting as zeozymes.80 In contrast to the molecular shape-selective properties of zeolites which often hinder reactions, confinement effects arisen from positive interactions guide the reactant toward the active site, recognize it, and hence stabilize this couple in an environment, promoting the desired catalytic reaction. Confinement effects facilitate absorption of reactant molecules in the solid solvent and guide them toward the active sites.65,80−84 Further studies are under progress to investigate the structure of different scandium-loaded zeolite structures.

These results clearly show the concomitant presence of both starting materials and of the resulting product, in agreement with both reaction rates (Figure 10) and a truly heterogeneous reaction involving adsorption of the reactants within the zeolite. In this process, the scandium cation is required, as shown from the lack of activity of pristine H-USY (see Table 2). Our structural studies show that the scandium cation is only coordinated twice to the zeolite framework despite its octahedral geometry within the zeolite, revealed by 45Sc NMR. From the remaining coordination sites, one is occupied by a triflate group, as shown by EDX analysis and 19F MAS NMR, and the others are probably coordinated by either the solvent (acetonitrile) or remaining water. The latter can easily be exchanged, allowing the scandium cation to act as an active site by coordinating the reagent (Scheme 4). With mean Sc−O and Sc−N bond lengths around 2.1 Å,79 an octahedral scandium ion with water and/or acetonitrile as ligands should occupy nearly one-half the size of an USY pore. It is therefore not so surprising that at higher Sc concentration the zeolites become less active. At 30% loading, more than one scandium ion could be incorporated within the zeolite pores, almost obstructing these pores and thus inhibiting catalysis. Furthermore, the known coordination chemistry of scandium shows that μ-oxo dimers might be formed.79 3.5. Recycling. The rational design of metal-promoted zeolite catalyst should also guarantee potential reuse after performing consecutive runs. Figure 12 presents the yield in cycloadduct product as a function of the successive runs peformed over 5ScUSY zeolite. Catalyst was reused over 10 experiments, without a noticeable decrease in efficiency and

4. CONCLUSION Sc(III)−FAU zeolites have been successfully prepared via gas− solid reaction between parent HUSY and scandium triflate vapors. Multinuclear MAS NMR techniques and H/D isotope labeling revealed the modification of aluminum, silicium, and scandium environments within the zeolite framework after

Scheme 4. Proposed Reaction Pathway Involving Ligand Exchange Around Sc3+ Cation (L = water or acetonitrile)

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performing the catalytic reaction. In addition, an interesting correlation between the structure and the activity of different Sc−USY, with loadings up to 30 mol %, could be established. Hence, this further confirmed an exaltation of the catalyst performance due to the confinement effect (framework healing combined to proper surface acidity). Finally, a class of tailored Sc(III)-exchanged FAU zeolites has been developed, which can thus work efficiently as a heterogeneous ligand-free and recyclable catalyst for synthesis of tetrahydroquinolines.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.L.), [email protected] (P.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Jean Daniel Sauer (Institut de Chimie) and Edy Casali (EPFL) for their technical expertise. A.O. thanks the CNRS and the “Ministerio Español de Educación” for postdoctoral fellowships. The authors gratefully acknowledge financial support from the CNRS and the French Ministry of Research.

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REFERENCES

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Design of Scandium-Doped USY Zeolite: An Efficient and Green

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