CâC Coupling Catalyzed by Zeolites: Is Enolization the Only Possible

Article pubs.acs.org/JPCC

C−C Coupling Catalyzed by Zeolites: Is Enolization the Only Possible Pathway for Aldol Condensation? Dennis Palagin,*,† Vitaly L. Sushkevich,‡,§ and Irina I. Ivanova*,‡ †

Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom ‡ Department of Chemistry, Lomonosov Moscow State University, Leninskye Gory 1, Building 3, Moscow 119991, Russia § ETBCat LLC, Leninskye Gory, 1, Building 75v, Moscow 119991, Russia S Supporting Information *

ABSTRACT: MBEA zeolites are known to catalyze carbon−carbon coupling reactions such as acetaldehyde condensation, which is an important step in a range of industrially relevant processes, e.g., a sustainable butadiene synthesis. The widely accepted mechanism of the reaction includes a separate enolization step via an α-proton transfer to zeolite. By combining the H−D exchange activity studies, FTIR spectroscopy, and DFT calculations, we show that such a mechanism is indeed feasible for the SnBEA zeolite. For the ZrBEA and TiBEA zeolites, on the other hand, experimental evidence suggests that a separate enolization step is unlikely. We propose the possibility of an alternative concerted single-step mechanism that involves coadsorption of two aldehyde molecules at the open M(IV) Lewis acid site and a subsequent proton transfer between the adsorbates in a collective transition state stabilized by the M−OH group of the open site. The study suggests that the nature of a zeolite dopant can thus be used to control the activity of the catalyst by modifying the reaction mechanism.

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INTRODUCTION Environmental challenges modern chemical industry is facing require development of efficient synthetic routes for a wide variety of organic substances used for the production of highdemand chemicals such as rubbers,1 elastomers,2 and fuels.3 For instance, a carbon−carbon (C−C) coupling reaction, involving precursors ranging from small aldehydes to large sugars, is an important step in various biomass conversion schemes.4−6 A representative example of such an industrially important process is a sustainable synthesis of butadiene from ethanol,7 for which acetaldehyde condensation is the key reaction step,8 while the major production route via isolation of butadiene from naphtha steam cracker fractions of paraffinic hydrocarbons is neither efficient nor green.9 An efficient catalyst for the C−C coupling step is a crucial prerequisite for developing a successful butadiene synthesis methodology. Zeolite-based materials have recently become an important new class of solid catalysts due to their unique porous structure and the possibility of controlling the composition, the amount, and the configuration of the Lewis acid sites by embedding different metals to form active catalytic centers.10−26 For instance, the so-called MBEA zeolite framework allows the introduction of uniformly distributed M(IV) Lewis acid sites with, e.g., Sn27 (known as SnBEA) and Zr28 (ZrBEA), which resulted in an unrivaled catalytic performance in a number of industrially important processes undergoing the C−C coupling step.29−31 © XXXX American Chemical Society

However, the nature of this outstanding catalytic activity is not fully understood. A dominant mechanistic rationalization of the observed zeolite activity relies on the aldol condensation mechanism of the C−C coupling,30 with the zeolite framework believed to abstract an α-proton from an aldehyde to generate a enolate intermediate arguably required for a C−C bond formation upon adsorption of the second aldehyde molecule, as such a enolate would be very reactive. This view is supported by the observed ability of the doped zeolites to activate carbonyl compounds.32 This widely suggested mechanism is presented in Scheme 1 for the example of acetaldehyde condensation. However, such a mechanism cannot fully explain all of the observed phenomena. First, different metal dopants exhibit very different activity in the process of the C−C coupling. A prominent example is the recently reported condensation of acetone with benzaldehyde.30 The experimentally observed reaction conversion amounts to 94% in the case of ZrBEA, while SnBEA only yields 32%. One might speculate that this effect is perhaps caused by the difference in the size and the acidic strength of the zeolite sites doped with different metals. However, such a substantial difference might potentially indicate the possibility of an alternative reaction mechanism. Received: July 20, 2016 Revised: September 12, 2016

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DOI: 10.1021/acs.jpcc.6b07273 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

reagents. The product content and the deuterium distribution were analyzed by the gas chromatography−mass spectrometry method (GC−MS). Reversible keto−enol tautomerization in the presence of zeolite catalysts results in a formation of a enol form of acetaldehyde with a mobile OH group proton, which could easily be exchanged with a D2O deuterium and which can further substitute the protons in a methyl group of CH3CHO (Scheme 2).

Scheme 1. Commonly Suggested Mechanism for the C−C Coupling via the Aldol Condensation over Metal-Substituted MBEA Zeolites30,a

Scheme 2. Substitution of a Proton in a Methyl Group of an Acetaldehyde with Deuterium during the H−D Exchange with D2O via Enolization

The key step is the abstraction of an α-proton by the framework Si− O−M fragment acting as a base, forming an Si−OH group and a metal-coordinated enolate, which readily undergoes C−C coupling with the second aldehyde molecule. a

The data on the deuterium substitution kinetics presented in Figure 1 illustrate that SnBEA is up to 1 order of magnitude

Second, the configuration of the metal sites of the MBEA zeolite is known to influence its catalytic activity. For instance, we have recently shown that the content of the so-called open Zr(IV) Lewis acid sites of ZrBEA, represented by isolated Zr atoms in tetrahedral positions of the zeolite crystalline structure connected to three −O−Si linkages and one OH group, correlates with the catalytic activity in the process of conversion of ethanol into butadiene.33 Similar observations were previously made for SnBEA.34−36 The observed difference in the activity might therefore be interpreted as an indication of the ZrBEA zeolite open fragments being involved in the stabilization of certain reaction intermediates, which is perhaps not possible for the closed sites. Moreover, as the closed sites are thought to be able to participate in an α-proton transfer,30 the possibility of an alternative C−C coupling mechanism for open sites seems plausible. These open questions motivate the present study of the mechanism of acetaldehyde condensation catalyzed by SnBEA, TiBEA, and ZrBEA zeolite frameworks. Using the hydrogen− deuterium exchange (H−D exchange) activity studies, Fourier transform infrared (FTIR) spectroscopy, and density functional theory (DFT) calculations, we look into the possible mechanistic reasons for the observed differences in the catalyst activity. We investigate the possibility of the enolate formation via an α-proton transfer to zeolite and critically assess the feasibility of different C−C coupling mechanisms.

Figure 1. Deuterium exchange probes the keto−enol tautomerization of the acetaldehyde adsorbed on SnBEA (black diamonds), TiBEA (red squares), and ZrBEA (green triangles). SnBEA is significantly more active, while for TiBEA and ZrBEA the stabilization of a enolate is unlikely.

more active in such an H−D exchange process than either TiBEA or ZrBEA (see Table 1 for rate constants). This indicates that the enolate stabilization takes place in the case of SnBEA, while for Ti- and Zr-doped zeolites this process is much less likely.

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RESULTS AND DISCUSSION Possibility of Enolate Stabilization. Reliable identification of the mechanism of acetaldehyde condensation is a key to understanding the possible approaches to optimize catalysts for such a process. To study the feasibility of alternative mechanisms we have chosen MBEA zeolite framework based catalysts doped with Sn, Zr, and Ti. The above metals were chosen on the basis of their extraordinary Lewis acidic properties and also due to the fact that they contain metals of different nature: Sn is a p-element, while Ti and Zr are transition d-metals from the fourth and fifth period, respectively. This allows direct observation of the influence of the chemical nature of the material on its catalytic performance and preferable mechanistic behavior. To investigate the possibility of the enolate stabilization, we rely on the H−D exchange activity studies of acetaldehyde with heavy water (D2O). The reaction was carried out in a dioxane solution at 358 K. A batch reactor equipped with a leak valve allowed sampling the reaction mixture without the loss of

Table 1. Metal Content Measured by Elemental Analysis and Rate Constants of the H−D Exchange and Acetaldehyde Aldol Condensation Processes over Metal-Substituted Zeolites rate constant, h−1 catalyst

metal loading (μmol/g)

H−D exchange

aldol condensation

TiBEA ZrBEA SnBEA

70 69 65

0.028 ± 0.010 0.027 ± 0.010 0.207 ± 0.008

18.1 ± 0.5 46.2 ± 0.4 62.7 ± 0.4

How is such enolate stabilization achieved for SnBEA, and why is it not observed for the case of ZrBEA and TiBEA? Does the SnBEA zeolite framework take part in the proton abstraction? In the generally accepted mechanism, a proton transfer with formation of an enol occurs upon adsorption of the first aldehyde molecule (step 2 in Scheme 1). However, no B


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The Journal of Physical Chemistry C experimental evidence confirming the cleavage of an Si−O−M bond has been reported. In order to rationalize a much higher probability of the aldehyde enolization in the case of SnBEA and to assess the feasibility of the proposed proton-transfer mechanism, we simulate the enolization process for all three zeolite catalysts by means of density functional theory (see the Experimental and Computational Setup section for the calculation details). Figure 2 summarizes possible acetaldehyde enolization reaction pathways calculated for the molecule adsorbed on the open sites of Sn-, Ti-, and Zr-based zeolites. Only open sites are presented in Figure 2 for the following reasons. First, previous studies identified higher activity of the open sites for SnBEA34−36 and ZrBEA.33 Second, open fragments provide two possible adsorption sites for an acetaldehyde molecule: either a direct adsorption on the metal or adsorption on the M−OH hydroxyl group. Finally, according to the previously suggested mechanism,30 the Si−O−M fragment of the zeolite is responsible for the proton abstraction. As such a fragment is present in both the closed and the open sites, we expect the open sites to allow simulation of an entire mechanistic manifold. Indeed, in all three cases our calculations identified similar reaction paths and free energies of activation for the closed centers (see the Supporting Information). The results for SnBEA indicate that a direct adsorption of acetaldehyde on Sn is preferable to the adsorption on a hydroxyl group. In the case of a direct adsorption (red dashed lines in Figure 2a), the possible reaction path involves proton transfer from the CH3 group to the CO group within the acetaldehyde molecule, resulting in formation of a locally stable enol. The corresponding transition-state barrier amounts to 1.85 eV. Subsequent proton shift from the HCO group to the Sn−O−Si fragment of the zeolite forms the enolate, which is more stable than the enol form (0.38 eV lower in energy) and is activated for the concomitant condensation with the second acetaldehyde molecule. Adsorption on the OH group yields the possibility of either reconfiguration of the adsorbate toward the direct contact with metal or desorption (green dashed lines). The desorbed acetaldehyde can undergo self-enolization (blue dashed lines) with subsequent coordination of the formed enol with the Sn−OH fragment, which yields a relatively stable aggregate. The free energy of activation for such a process is somewhat lower (1.63 eV), which, however, assumes energetically unfavorable desorption. Therefore, taking into account that acetaldehyde prefers direct adsorption on Sn, while neither adsorption on OH nor desorption have energetical advantages, and also that the reverse reaction has a barrier comparable to the forward process, thus suggesting a rather high probability of forming the enolate, it is safe to assume that an α-proton transfer to the zeolite is a plausible reaction pathway in the case of SnBEA. In the case of TiBEA (Figure 2b), the adsorption on the OH group is slightly less favorable than the direct adsorption on Ti. Consequently, the molecule is very likely to readsorb on the metal (green dashed lines). The direct adsorption on the metal allows spatial reorientation of the acetaldehyde molecule in such a way as to facilitate the direct proton transfer toward the zeolite (red dashed lines). Unlike in the case of SnBEA, this is a one-stage process with a much lower free energy of activation of 1.27 eV. However, the reverse reaction has a barrier of only 0.68 eV, which renders the de-enolization process more favorable than the target enolization reaction, as the rate of the reverse reaction is expected to be higher than that of

Figure 2. Possible pathways for the enolization of an acetaldehyde molecule over the open site of the MBEA zeolite doped with (a) Sn, (b) Ti, and (c) Zr. Identified transition states are denoted as TS. Energy values (eV) correspond to Gibbs free energies and are given with respect to the individual aldehyde and zeolite. For clarity, the insets only depict an open metal center with the hydroxyl group and three M−O−Si linkages.

enolization. As a consequence, the most probable end product is the aggregate of an acetaldehyde adsorbed on the zeolite center, and thus, the commonly proposed direct enolization scheme is unlikely to be observed for TiBEA. For ZrBEA (Figure 2c), the direct adsorption on the metal is preferable. In this case (red dashed lines), the transition state is formed due to the double proton transfer first from a CH3 group to the HCO group of acetaldehyde and then from the C


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consequently is turned into the adsorbed Sn−O−CHCH2 enolate fragment. The spectra were obtained at a relatively low temperature (298 K). Adsorption at low temperature allows observing the formation of new Si−OH groups, while the adsorption at higher temperatures leads to immediate transformation of all acetaldehyde surface species into reaction products via aldol condensation. In this case, the results of FTIR spectroscopy would not reveal the presence of any Si−OH due to the fast interaction of enol species with acetaldehyde to give C4 fragments. Such an approach allows observation of the formation of Si−OH groups in FTIR spectra while not excluding the possibility of their fast transformation under actual reaction conditions (473 K). We therefore observe a much higher probability of an aldehyde enolization followed by a proton transfer to zeolite framework in the case of the SnBEA catalyst. Alternative Mechanisms? Both experimental and theoretical results in the above section indicate that SnBEA is very active in the acetaldehyde enolization reaction, while for TiBEA and ZrBEA such a process is significantly less likely. Does this mean that SnBEA should be expected to be the only catalytic system active in the C−C coupling process? The results of the catalytic activity tests presented in Figure 4 and Table 1 show

HCO group of acetaldehyde to the zeolite. Such a complex process yields a prohibitively high free energy of activation of 2.59 eV. Adsorption on a metal−hydroxyl fragment opens the possibility of self-enolization (green dashed lines), similar to the case of SnBEA discussed above, which, however, still proceeds via a very high energy barrier of 2.22 eV. The proton transfer from such an adsorbed enol to the zeolite with a formation of an enolate will require overcoming another high barrier. Such a high energy, two-step proton transfer to the zeolite is therefore very unlikely. Thus, in the case of SnBEA, a direct adsorption of an aldehyde molecule on a Sn metal site followed by enolization with an α-proton transfer to the zeolite appears to be plausible. A relatively high free energy of activation for the reverse reaction ensures that enolate will be present in equilibrium. For TiBEA, on the other hand, such transfer is unlikely due to a higher probability of the reverse process, stabilizing an adsorbed intact aldehyde molecule. For ZrBEA, the enolization pathway requires overcoming very large transition state barriers, both in the case of a direct proton transfer to the zeolite and in the case of self-enolization, which also renders this reaction mechanism unlikely. Furthermore, SnBEA is the only system for which the final enolate intermediate is more stable than the original reactants (−0.11 eV), while for both TiBEA (+0.13 eV) and ZrBEA (+0.19 eV) this is not the case. The FTIR studies have been employed to confirm the above theoretical predictions. As the process of an α-proton transfer leads to the formation of a new Si−OH group on the zeolite surface, one should expect the emergence of a new band in the OH vibration region of the IR spectrum after the aldehyde molecule is adsorbed. Figure 3 shows that such a new peak is

Figure 4. Catalytic activity of SnBEA, TiBEA, and ZrBEA in the process of acetaldehyde aldol condensation at 473 K. All metalsubstituted zeolites yield similar activity, while rate constants do not correlate with those for the H−D exchange.

that this is not the case. In all experiments, the selectivity toward crotonaldehyde was higher than 99%, with only traces of 3-hydroxybutanal and hexadienals observed. The measurement of the catalytic activity versus contact time (Figure 4) reveals that SnBEA is indeed the most active catalyst for the C−C coupling, followed by ZrBEA and TiBEA. However, the difference in the catalytic activity is rather small (Table 1). Most importantly, the rate constants for the condensation process do not correlate with those for the H−D exchange studies probing the keton to enol transformation step (Figure 1). It is therefore reasonable to assume that, as all three zeolite systems are active in the C−C condensation reaction, different reaction mechanisms might be suggested for SnBEA and ZrBEA/TiBEA catalysts. While for SnBEA the mechanism via the enolization step is likely, an alternative mechanism not involving such a process seems necessary to account for C−C coupling over ZrBEA and TiBEA zeolites.

Figure 3. Subtraction results of the FTIR spectra in the region of 4000−3500 cm−1 region of OH vibrations for (a) TiBEA, (b) ZrBEA, and (c) SnBEA after the adsorption of acetaldehyde. A band at 3736 cm−1 for SnBEA indicates the formation of a new Si−OH group achieved via an α-proton transfer from acetaldehyde to zeolite. The adsorption of acetaldehyde was carried out at 298 K to avoid fast transformation into reaction products. The three lines in each spectrum correspond to different acetaldehyde pressures, namely, 0.01, 0.02, and 0.05 Torr.

only observed in the case of SnBEA, with no additional peaks observed for either ZrBEA or TiBEA, which is in line with our H−D exchange results (Figure 1). A pronounced new high frequency band at 3736 cm−1 indicates that a Si−O−Sn bond is broken, and a new silanol group (Si−OH) is formed with the proton taken from the adsorbed acetaldehyde molecule, which D


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The Journal of Physical Chemistry C SnBEA: Mechanism with Enolization. In order to be able to suggest an alternative mechanism, one has to fully understand the originally proposed enolization process via an α-proton transfer. For this we simulate the further process of the second acetaldehyde molecule adsorption and a subsequent C−C coupling for the case of SnBEA. Figure 5 depicts the steps

aldehyde molecules should occur simultaneously with the coupling without a separate proton-transfer step. At the same time, it should not necessarily indicate that the zeolite center does not take part in this process. In our previous study,33 we have shown that in the case of ZrBEA open sites are much more active than the closed ones. Following the same approach, we employed FTIR spectroscopy to study the nature of the active catalytic sites of ZrBEA in the C−C coupling process. FTIR spectra of the CO adsorbed over fresh ZrBEA and ZrBEA with preadsorbed acetaldehyde are shown in Figure 6 (see the

Figure 5. Possible C−C coupling pathway for SnBEA following the αproton transfer. Identified transition states are denoted as TS. Energy values (eV) correspond to Gibbs free energies and are given with respect to the individual aldehyde and zeolite.

of this reaction. After the zeolite framework abstracted the αproton (Figure 2a), the second acetaldehyde molecule is adsorbed. Metal enolate formed after the proton transfer attacked the second aldehyde molecule, readily undergoing the C−C coupling. The transition state is only 0.58 eV higher in energy than the two adsorbed precursors, and overcoming this barrier is rewarded by the formation of a very stable complex. The subsequent back-transfer of a proton from zeolite to the adsorbate forms the desired 3-hydroxybutanal product. Having just 0.38 eV of binding energy to the zeolite center, the product easily desorbs at higher temperatures. The following dehydration of 3-hydroxybutanal leads to the formation of crotonaldehyde, which completes the acetaldehyde conversion reaction cycle. Although the free energy of activation for C−C coupling is therefore lower than that of the enolization process, we do not expect this result to contradict the experimental FTIR data indicating the presence of Si−OH groups (Figure 3), since the latter was obtained at low temperature (298 K) and therefore does not exclude the possibility of the fast transformation under actual reaction conditions. Such a two-step mechanism of the condensation reaction is therefore equivalent to the previously suggested process30 shown in Scheme 1, assuming first the formation of a enol and a subsequent C−C coupling, each involving overcoming a transition state barrier. The highest free energy barrier is computed as 1.85 eV for the proton abstraction step, which should be possible to overcome at the experimental reaction temperature. TiBEA and ZrBEA: Mechanism without Enolization. As opposed to SnBEA, for TiBEA and ZrBEA the enolization is not observed. However, as we have seen in Figure 2, adsorption of the acetaldehyde molecule either directly on the metal center or on the M−OH hydroxyl group is exothermic. The C−C coupling activity studies presented in Figure 4 suggest that condensation does happen for TiBEA and ZrBEA, and the latter two are only marginally less active than SnBEA. We therefore assume that the activation of one of the two adsorbed

Figure 6. FTIR spectrum of the 2200−2100 cm−1 region of adsorbed CO vibrations for (a) fresh ZrBEA and (b) ZrBEA with preadsorbed acetaldehyde. Decreased intensity of the peaks corresponding to the open centers indicates preferential adsorption on open sites.

Supporting Information for the details). As can be seen from the spectra, a decreased intensity of the vibrational bands corresponding to the open centers of ZrBEA after acetaldehyde adsorption (at 2185 and 2163 cm−1) indicate that mainly open sites adsorb acetaldehyde strongly, since the intensity of the bands assigned to the closed sites (2176 cm−1) does not show any change. This suggests additional configurational possibilities for the reaction pathway. First, adsorption of two aldehyde molecules can occur both directly on the metal center and on the hydroxyl group. Second, the M−OH group can take part in the proton transfer and/or the transition-state stabilization. In order to establish the possible mechanism, we simulate the condensation reaction pathways for the open centers of TiBEA and ZrBEA. For ZrBEA, the optimal reaction pathway corresponds to the adsorption of two acetaldehyde molecules on different sites: while the first molecule is absorbed directly on the metal dopant, the second aldehyde forms a hydrogen bond stabilized complex with the α-proton of the first aldehyde molecule and the open OH group of the metal center (Figure 7a). Such an adsorbate configuration is slightly less stable than the case when both aldehydes adsorb directly on Zr (−0.80 eV vs −0.98 eV). However, it allows the second acetaldehyde molecule to assume an optimal configuration to receive an α-proton from the CH3 group of the first aldehyde activated by the Zr center. The proton transfer is immediately followed by the C−C coupling, and the newly formed product of condensation is coordinated with the metal center, as also was the case for the originally proposed mechanism observed for SnBEA. E


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the observed transition barrier is at 2.57 eV, notably higher than in the case of an open center, hence confirming the higher activity of open sites compared to the closed ones in ZrBEA.33 For TiBEA, we found that the overall mechanism is similar to the case of ZrBEA (Figure 7b). The notable difference, however, lies in the way the open OH group helps to stabilize the transition state. At first, the second aldehyde molecule is only coordinated to the first acetaldehyde adsorbed directly on the metal center. Again, such a configuration is energetically higher than both aldehydes adsorbed directly on Ti (−0.53 eV vs −0.66 eV). However, it facilitates an easy reorientation of the second acetaldehyde molecule with respect to the first molecule and the open OH group. The transition state corresponds to the concerted α-proton transfer from the CH3 group of the first aldehyde molecule via the open OH group to the CO group of the second molecule. Interestingly, the concomitant condensation does not lead to the coordination of the product’s newly formed OH group back to the metal center, as was the case for ZrBEA. Instead, the adsorbate assumes an almost vertical position in respect of the adsorption center, with the OH group of the product forming a hydrogen bond with the O atom of the open center’s hydroxyl group. The resulting transition state barrier in the case of TiBEA is at 1.95 eV very similar to that of the ZrBEA zeolite (2.02 eV), and is also comparable to the corresponding barrier for SnBEA (1.85 eV). The relative heights of these barriers correlate with the experimentally observed similar activities (Figure 4). As is the case with ZrBEA, the closed center of TiBEA is unable to provide additional transition-state stabilization, which leads to a significantly higher barrier of 2.38 eV (see the Supporting Information), and renders open sites potentially more active. Do Different Dopants Require Different Mechanisms? We therefore observe the preference for different mechanisms for Sn-, Ti-, and Zr-doped zeolites. For SnBEA, the optimal condensation process requires a separate step of enolization of the first adsorbed acetaldehyde molecule via the transfer of an α-proton from the CH3 group of the aldehyde to the zeolite framework. The second molecule then undergoes a C−C coupling attack by the first activated enolate immediately upon adsorption. As we have seen in Figure 2, in the case of TiBEA such a two-step mechanism implies a very high probability of the reverse de-enolization process, while for ZrBEA the enolization mechanism requires overcoming a very high transition-state barrier. ZrBEA and TiBEA are, therefore, expected to undergo the C−C coupling without a separate enolization step. Instead, both zeolites are likely to first adsorb one aldehyde molecule directly on the metal center with the second molecule coordinated either to the first one (TiBEA) or to both the first molecule and the OH group of the open center (ZrBEA). A subsequent concerted single-step C−C coupling process involves an α-proton transfer from the CH3 group of the first aldehyde molecule to the CO group of the second molecule. Although the complexity of the transition states differs from Ti to Zr, the overall reaction mechanisms are similar. For both catalytic systems, the M−OH hydroxyl group of the open center is involved in either the proton transfer or the stabilization of the transition state. Open sites are therefore expected to play a crucial role in the reaction mechanism. This view is supported by our calculations of the possible reaction mechanisms for the closed centers, suggesting higher transitionstate barriers due to lower stability of the intermediate

Figure 7. Possible C−C coupling pathways for (a) ZrBEA and (b) TiBEA without the initial enolization step. Identified transition states are denoted as TS. Energy values (eV) correspond to Gibbs free energies and are given with respect to the individual aldehyde and zeolite.

Remarkably, during the proton transition step the second aldehyde molecule stays coordinated with the OH group of the open center up to the stage of condensation, after which the open OH group assumes its original orientation (see the transition trajectory available as Supporting Information). One of the transition snapshots even reveals the temporary formation of the diol-like configuration of the second aldehyde molecule after receiving the α-proton. As such, by stabilizing the transition-state structure, the hydroxyl group of the zeolite open center is indirectly taking part in the proton transfer during the condensation step. Therefore, for ZrBEA we observe an important role of the open site in the process of C−C coupling. Such involvement of the OH group in the reaction mechanism is not unique to ZrBEA and was also observed for SnBEA, e.g., in the process of glucose to fructose isomerization.23,37,38 The free energy of activation is calculated as 2.02 eV and corresponds to a simultaneous process of the proton transfer and desorption of the second aldehyde molecule from the hydroxyl group of the open center. Interestingly, an analogous transition-state stabilization is less probable in the case of the closed center. Due to the full saturation and limited sterical accessibility of the Zr−O−Si fragments, the second aldehyde molecule could not coordinate directly to the oxygen center (see the Supporting Information). Although the α-proton transfer is in principle possible due to the activation of the first aldehyde molecule, the resulting transition state corresponds to a detached proton being transferred to an unstabilized aldehyde molecule. As a result, F


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agrees with our findings. As the electronic structure of Ti is much closer to that of Zr than Sn, it is reasonable to expect a similar reaction mechanism also for TiBEA. This effect can be illustrated by the Mulliken charge distribution analysis. While both Ti and Zr donate about 1.0 e to the neighboring oxygen atoms, resulting in about 0.4 e excessive charge on oxygen atoms corresponding to three M−O−Si linkages, Sn is donating about 1.6 e, resulting in up to 0.7 e excessive charge on adjacent oxygen atoms. Interestingly, such a concerted mechanism does not seem to be possible for SnBEA. Our test local optimizations inevitably led to the configuration with two acetaldehyde molecules adsorbed directly on the Sn metal center, as opposed to the OH group mediated adsorption of the second molecule observed for Zr and Ti. This is in line with the larger size and higher propensity of Sn to transfer excess charge toward oxygen atom, which results in a higher probability of the enolization mechanism.

configurations. The transition-state barriers are in the case of both TiBEA and ZrBEA comparable to that of SnBEA, which correlates with the experimentally observed catalytic activity. The proposed mechanism is summarized in Scheme 3 for the case of ZrBEA. The crucial difference from the original mechanism presented in Scheme 1 is the absence of the extra enolization step. Scheme 3. Newly Proposed Mechanism for C−C Coupling without a Separate Aldol Formation Step, as Proposed for Open Sites of ZrBEAa

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CONCLUSIONS In summary, we have systematically studied the possibility of an alternative mechanism of the aldehyde condensation catalyzed by the MBEA zeolite framework doped with different metals. The most commonly suggested mechanism relies on the basicity of the metal-attached oxygen atoms and includes the separate enolization step via an α-proton transfer to zeolite. By combining the H−D exchange activity studies, FTIR spectroscopy, and DFT calculations, we show that such a two-step mechanism is indeed very likely for the SnBEA zeolite, as the electronic structure of Sn allows activation of the basic Si−O− Sn center. The electronic manifold of Ti and Zr, on the other hand, is not expected to be consistent with the α-proton abstraction mechanism. Indeed, experimental evidence suggests that such proton transfer via a separate enolization step is very unlikely. The DFT-based simulation of the possible reaction pathways revealed that an alternative concerted single-step mechanism is possible, which involves coadsorption of two aldehyde molecules at the open M(IV) Lewis acid site and a subsequent proton transfer between the adsorbates in a collective transition state stabilized by the M−OH group of the open site. Moreover, the results of the calculations for the closed centers indicate that such open hydroxyl stabilization leads to notably lower free energies of activation, thus making the catalyst more effective. Therefore, open sites play an important role in the proposed mechanism, which is in line with our previous study suggesting the superior activity of the ZrBEA open sites in the process of catalytic conversion of ethanol into butadiene.33 Most importantly, the current results emphasize the crucial role of the chemical nature of the metal dopant, which is able to change the chemical environment of the zeolite adsorption sites and thus influence the catalytic activity. This opens an exciting possibility of using the metals with different electronic structures to tune the properties of the obtained catalytic materials. By using dopants of different chemical nature, it is possible not only to adjust the catalytic activity by, e.g., tuning the adsorption strength or acidity/basicity of the given site but even to change the entire reaction mechanism. The recently suggested possibility of forming mesoporous molecular sieves with bimetallic dopants41 opens an additional dimension to such tunability of the zeolite properties. This specifically renders metal-doped MBEA zeolite lattice a natural choice for

a

TiBEA catalyzes the aldehyde condensation via a similar mechanism (see text).

It should be emphasized that the direct coupling mechanism proposed here is not unique and bears similarities with other known catalytic processes, such as, for instance, the recently proposed direct hydrogen transfer between crotonaldehyde and ethanol over Ca−O sites39 or a direct coupling in the Mukaiyama aldol condensation process.40 This potentially opens the possibility of applying the ideas proposed here to the description of aldol condensation over other types of Lewisbased catalysts. The necessity of an alternative mechanism can be explained by the difference in the chemical nature of the considered dopants. Ti and Zr, being d-elements, should be expected to have similar properties that differ from those of Sn, a p-metal. It is known that the catalytic activities of MBEA zeolite lattices depend, among other things, on the Brønsted basicity of the associated oxygen atom.30,35,38 It has been shown35 that the different nature of the lowest unoccupied molecular orbitals (LUMO) in SnBEA and ZrBEA is responsible for the different levels of basicity of the oxygen atoms attached to the metals. As was thoroughly investigated in ref 34, antibonding σ*(Sn−O) orbitals constituting the LUMO of the SnBEA framework are capable of transferring electronic density toward the orbital lobes located on the oxygen atoms, thus increasing the basic character of such an adsorption site. Having an active base is, of course, beneficial for the abstraction of an α-proton from an acetaldehyde molecule. This is in line with the originally proposed mechanism of the C−C coupling via aldol condensation, which we found likely for the SnBEA catalyst. The LUMO of ZrBEA, on the other hand, is a dz2 orbital of Zr, which is able to accommodate the excess electronic density without increasing the basic character of the neighboring oxygen atoms. In lieu of an active base, the proton transfer to zeolite is naturally less likely, and ZrBEA should be expected to catalyze the C−C coupling via an alternative mechanism, which G


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The Journal of Physical Chemistry C rational design of novel catalytic materials with engineered properties.

effective core potential basis set for Sn, Ti, and Zr atoms and the standard 6-31G(d,p)49 basis set for C, O, Si, and H atoms. Grimme’s DFT-D3 semiempirical dispersion correction50 has been applied to the B3LYP-optimized geometries to account for the dispersion component of the van der Waals energies arising from the attraction between induced dipoles formed due to charge fluctuations in the interacting species. All geometries reported herein correspond to the locally optimized intermediate configurations. All energy values are computed as free Gibbs energies corresponding to an experimental temperature of 473 K and a pressure of 1 atm. In the spirit of ab initio thermodynamics,51 we calculated the translational, rotational, and vibrational contributions to the Gibbs free energy according to the following formula

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EXPERIMENTAL AND COMPUTATIONAL SETUP Experimental Details. The preparation of the MBEA catalysts was performed using the hydrothermal synthesis in the fluoride medium carried out as proposed elsewhere.42 The Si/ M ratio in the final gel was kept at 200. As made materials were calcined in a flow of dry air at 823 K for 5 h. A detailed synthesis procedure and material characteristics are given in the Supporting Information. The H−D exchange experiments were carried out in a batch reactor equipped with a leak valve to collect the samples during the reaction. The reaction mixture containing 100 mg of acetaldehyde, 250 mg of D2O, and 20.0 mg of catalyst in 1.0 g of 1,4-dioxane was placed in the reactor and heated at 358 K in an oil bath with a magnetic mixer. 1,4-Dioxane has been used as a solvent for the H/D exchange reaction, and the pH of the solution was controlled to remain constant. Deuterated ethanol and tert-butyl alcohol have also been used as D sources and proved to be reactive toward acetaldehyde, leading to the formation of stable byproducts, namely, acetals and hemiacetals. The products were analyzed with a Thermo Trace chromatograph equipped with the DSQ II mass detector using a matrix deconvolution method that accounts for the mass fragmentation patterns of acetaldehyde. Activity of the catalyst in the aldol condensation reaction was studied in a fixed-bed microreactor at 473 K. In a typical experiment, a 40 mg catalyst sample (fraction of 0.25−0.5 mm) was placed into the tubular reactor, purged at 773 K, and cooled to the reaction temperature in a flow of He. Acetaldehyde was fed through a saturator kept in an ice bath; WHSV was varied within the range of 0.4−1.3 h−1. Gaseous products were analyzed online using a Crystal 2000 M gas chromatograph with the 50 m SE-30 column. The reaction rates were calculated from the slopes of the kinetic curves (Figures 1 and 4) at low conversion levels. The adsorption sites of ZrBEA were studied by FTIR spectroscopy of adsorbed CO43 and acetaldehyde. IR spectra were recorded with a Nicolet Protégé 460 FT-IR spectrometer at the optical resolution of 4 cm−1. Prior to the measurements, the catalyst was pressed in self-supporting disks and activated in an IR cell attached to a vacuum line at 723 K for 4 h. Adsorption of CO and CH3CHO was carried out in the cell cooled with liquid nitrogen. Difference spectra were obtained by the subtraction of the spectra of the activated catalyst samples from the spectra of the samples with the adsorbate. The spectra processing was carried out using the OMNIC 7.3 package. Computational Details. DFT calculations have been carried out using two cluster models, corresponding to the so-called closed (M(OSi)4) and open (M(OSi)3OH) sites of the MBEA zeolite framework. The initial geometries of the adsorption centers were obtained by cutting a cluster of atoms containing an Si atom in the so-called T9 position, with four coordination spheres around it, from the periodic structure of pure silica BEA zeolite, as proposed in ref 35. While a zeolite structure is periodic, the behavior of adsorbates is largely local, and due to the rigidity of inorganic systems, choosing a reasonably large cluster model is often considered sufficiently reliable.33,36,44−46 The geometries of every intermediate discussed below were optimized within the DFT framework with the hybrid functional B3LYP47 using a LANL2DZ48

G(T , p) = G(electr) +

⎧ ⎪

∑ ⎨NA ⎪

i

⎩

ℏνi 2

⎫ ⎡ ⎛ −hνi ⎞⎤⎪ ⎬ + RT ln⎢1 − exp⎜ ⎟⎥⎪ ⎢⎣ ⎝ kBT ⎠⎥⎦⎭ ⎛ (2πk Tm)3/2 k T ⎞ B B ⎟⎟ − RT ln⎜⎜ p ⎠ h3 ⎝ ⎤ ⎡ 8π 2 ⎛ 2πk T ⎞3/2 ⎜ 2B ⎟ (IAIBIC)1/2 ⎥ − RT ln⎢ ⎥⎦ ⎢⎣ σ ⎝ h ⎠

with the first term of the vibrational free energy accounting for zero-point vibrations. Here, T is temperature, p is pressure, NA is Avogadro’s constant, h is Planck’s constant, kB is the Boltzmann constant, R is the universal gas constant, m is the mass of a system, ν is the vibrational frequency, σ is the symmetry index, and Ix is moment of inertia. The translational, rotational, and vibrational contributions are easily calculated from the structural data and the calculated vibrational spectrum. All atoms present in the system, i.e., both zeolite and adsorbate, were included in the frequency calculations. The G(electr) term is equal to the calculated total energy of the system. As it has been shown that electronic energy profiles can be misleading52,53 and interplay between enthalpy and entropy might play an important role in zeolites,54 we base all mechanistic discussions on the Gibbs free energy reaction profiles. Following the approach by Kozuch and Shaik,55 our simplified analysis of the relative feasibility of considered mechanisms is based on the heights of the corresponding transition-state barriers, determined as the free energy difference between the most stable intermediate and the least stable transition state. The nudged elastic band (NEB)56 method as implemented in the ASE57 package was used to find the transition paths and corresponding energy barriers between given initial and final states, which were defined as the locally optimized structures of adjacent reaction intermediates. A chain of 12 replicas of the system has been constructed for every transition-state calculation, and the forces were allowed to relax with a threshold of 0.05 eV/Å. The two states are assumed to be connected by a single barrier path. All calculations were performed using the NWChem package.58 H


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Microporous Crystals in Catalysis by Advances in Materials Design. Chem. Soc. Rev. 2008, 37, 2530−2542. (12) Derouane, E.; Védrine, J.; Pinto, R. R.; Borges, P.; Costa, L.; Lemos, M.; Lemos, F.; Ribeiro, F. R. The Acidity of Zeolites: Concepts, Measurements and Relation to Catalysis: A Review on Experimental and Theoretical Methods for the Study of Zeolite Acidity. Catal. Rev.: Sci. Eng. 2013, 55, 454−515. (13) Moliner, M. State of the Art of Lewis Acid-Containing Zeolites: Lessons from Fine Chemistry to New Biomass Transformation Processes. Dalton Trans. 2014, 43, 4197−4208. (14) Moliner, M.; Román-Leshkov, Y.; Davis, M. E. Tin-Containing Zeolites are Highly Active Catalysts for the Isomerization of Glucose in Water. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 6164−6168. (15) Román-Leshkov, Y.; Moliner, M.; Labinger, J. A.; Davis, M. E. Mechanism of Glucose Isomerization Using a Solid Lewis Acid Catalyst in Water. Angew. Chem., Int. Ed. 2010, 49, 8954−8957. (16) Bermejo-Deval, R.; Gounder, R.; Davis, M. E. Framework and Extraframework Tin Sites in Zeolite Beta React Glucose Differently. ACS Catal. 2012, 2, 2705−2713. (17) Dijkmans, J.; Gabriels, D.; Dusselier, M.; de Clippel, F.; Vanelderen, P.; Houthoofd, K.; Malfliet, A.; Pontikes, Y.; Sels, B. F. Productive Sugar Isomerization with Highly Active Sn in Dealuminated β Zeolites. Green Chem. 2013, 15, 2777−2785. (18) Dusselier, M.; Van Wouwe, P.; Dewaele, A.; Makshina, E.; Sels, B. F. Lactic Acid as a Platform Chemical in the Biobased Economy: the Role of Chemocatalysis. Energy Environ. Sci. 2013, 6, 1415−1442. (19) van der Graaff, W. N. P.; Li, G.; Mezari, B.; Pidko, E. A.; Hensen, E. J. M. Synthesis of Sn-Beta with Exclusive and High Framework Sn Content. ChemCatChem 2015, 7, 1152−1160. (20) Dijkmans, J.; Demol, J.; Houthoofd, K.; Huang, S.; Pontikes, Y.; Sels, B. Post-Synthesis Snβ: an Exploration of Synthesis Parameters and Catalysis. J. Catal. 2015, 330, 545−557. (21) Dijkmans, J.; Gabriëls, D.; Dusselier, M.; Houthoofd, K.; Magusin, P. C. M. M.; Huang, S.; Pontikes, Y.; Trekels, M.; Vantomme, A.; Giebeler, L.; et al. Cooperative Catalysis for Multistep Biomass Conversion with Sn/Al Beta Zeolite. ACS Catal. 2015, 5, 928−940. (22) Roy, S.; Bakhmutsky, K.; Mahmoud, E.; Lobo, R. F.; Gorte, R. J. Probing Lewis Acid Sites in Sn-Beta Zeolite. ACS Catal. 2013, 3, 573− 580. (23) Rai, N.; Caratzoulas, S.; Vlachos, D. G. Role of Silanol Group in Sn-Beta Zeolite for Glucose Isomerization and Epimerization Reactions. ACS Catal. 2013, 3, 2294−2298. (24) Yang, G.; Pidko, E. A.; Hensen, E. J. M. A Mechanistic Study of Ni-catalyzed Carbon Dioxide Coupling with Ethylene towards the Manufacture of Acrylic Acid. ChemCatChem 2014, 6, 800−807. (25) Yang, G.; Pidko, E. A.; Hensen, E. J. M. Structure, Stability, and Lewis Acidity of Mono and Double Ti, Zr, and Sn Framework Substitutions in BEA Zeolites: A Periodic Density Functional Theory Study. J. Phys. Chem. C 2013, 117, 3976−3986. (26) Román-Leshkov, Y.; Moliner, M.; Labinger, J. A.; Davis, M. E. Mechanism of Glucose Isomerization Using a Solid Lewis Acid Catalyst in Water. Angew. Chem., Int. Ed. 2010, 49, 8954−8957. (27) Corma, A.; Nemeth, L. T.; Renz, M.; Valencia, S. Sn-Zeolite Beta as a Heterogeneous Chemoselective Catalyst for Baeyer−Villiger Oxidations. Nature 2001, 412, 423−425. (28) Zhu, Y.; Chuah, G.; Jaenicke, S. Al-Free Zr-Zeolite Beta as a Regioselective Catalyst in the Meerwein−Ponndorf−Verley Reaction. Chem. Commun. 2003, 21, 2734−2735. (29) Van de Vyver, S.; Odermatt, C.; Romero, K.; Prasomsri, T.; Román-Leshkov, Y. Solid Lewis Acids Catalyze the Carbon−Carbon Coupling between Carbohydrates and Formaldehyde. ACS Catal. 2015, 5, 972−977. (30) Lewis, J. D.; Van de Vyver, S.; Román-Leshkov, Y. Acid−Base Pairs in Lewis Acidic Zeolites Promote Direct Aldol Reactions by Soft Enolization. Angew. Chem., Int. Ed. 2015, 54, 9835−9838. (31) Van de Vyver, S.; Román-Leshkov, Y. Metalloenzyme-Like Zeolites as Lewis Acid Catalysts for C−C Bond Formation. Angew. Chem., Int. Ed. 2015, 54, 12554−12561.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07273. Details of catalyst synthesis and characterization, reaction, and FTIR spectroscopy measurements; calculated reaction pathways for the closed sites of the SnBEA, ZrBEA, and TiBEA zeolites; atomic coordinates for all intermediates(PDF) transition trajectory file for Sn enolization step TS (XYZ) transition trajectory file for Ti open TS path (XYZ) transition trajectory file for Zr open TS path (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +44 1865 275159. *E-mail: [email protected]. Tel: +7 095 939 3570. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.L.S. and I.I.I. gratefully acknowledge the Russian Science Foundation for financial support (Grant No. 14-23-00094). We thank Mr. Pavel Kots for the synthesis of Ti-, Zr-, and SnBEA.

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