Selective CO₂ Fixation to Styrene Oxide by Ta-Substitution of Lindqvist-Type [(Ta,Nb)₆O₁₉]⁸⁻ Clusters

Abstract

Metal oxide clusters composed of group 5 metal ions, such as Nb and Ta, exhibit catalytic activities for CO₂ fixation to styrene oxide (SO) due to the highly negative natural bonding charge of the terminal O atoms that could work as CO₂ activation sites. In this study, tetrabutylammonium (TBA) salts of [Ta_xNb_6−xO₁₉]⁸⁻ (TBA-Ta_xNb_6−x, x = 0–6) were prepared and Ta-substitution effect on the catalytic properties of TBA-Ta_xNb_6−x for CO₂ fixation to SO was investigated. We found that TBA-Ta₁Nb₅ shows the highest styrene carbonate (SC) selectivity (95%) among TBA-Ta_xNb_6−x, although the SO conversion monotonously increases with the incremental Ta substitution amount. The CO₂ fixation to SO under various conditions and in situ X-ray absorption fine structure measurements reveal that CO₂ is activated on both terminal O sites coordinated to the Ta (terminal O_Ta) and Nb (terminal O_Nb) sites, whereas the activation of SO proceeds on the terminal O_Ta and/or bridge O sites that are connected to Ta. Density functional theory (DFT) calculations reveal that the terminal O_Ta of TBA-Ta₁Nb₅ preferentially adsorbs CO₂ compared with other O_Nb base sites. We conclude that the selective CO₂ activation at terminal O_Ta of TBA-Ta₁Nb₅ without SO activation is a crucial factor for high SC selectivity in the CO₂ fixation to SO.

1. Introduction

Several CO₂ usage approaches have been implemented to reduce the impact of emitted greenhouse gases and to achieve a carbon neutral level. CO₂ utilization in chemical production gained significant attention in replacing traditionally used C1 sources such as phosgene or CO, which post more toxicity. However, the activation of CO₂ is challenging due to its thermodynamic stability, which requires appropriate catalysts, such as solid base catalysts, for instance, alkaline earth metal oxides. One of the useful reactions is CO₂ cycloaddition to epoxides, which form cyclic carbonates. The resultant cyclic carbonates could be applied as solvents [1,2,3,4,5], monomers [6,7,8,9], electrolytes [10,11,12,13], and pharmaceuticals [14,15].

Recent applications of metal oxide clusters, namely, polyoxometalates, as base catalysts have been reported [16,17,18,19,20,21,22]. One of the advantages of utilizing metal oxide clusters over bulk solid base catalysts is that it does not require the surface activation of catalysts [23]. Up to now, the basicity of metal oxide clusters depends on the structures and the type of metal ions. The Lindqvist-type polyoxotungstate [W₆O₁₉]²⁻ shows the basicity with pK_a value of 11.1 and defective Keggin-type Ge-incorporated polyoxotungstate [γ-H₂GeW₁₀O₃₆]⁶⁻ exhibits high basicity with a pK_a value of 21.9 [24]. The series of group 5 metal polyoxometalates exhibit superior basicity to those of group 6 metal polyoxometalates, and the basicities of [Nb₁₀O₂₈]⁶⁻, [Nb₆O₁₉]⁸⁻, and [Ta₆O₁₉]⁸⁻ increase to a pK_a value of 23.8 [17,18]. Recently, Uchida’s group reported that porous ionic crystals containing Nb/Ta were applied to Knoevenagel condensation reactions as base catalyst [19]. Density functional theory (DFT) calculations reveal that the base strength of the clusters is related to the natural bond orbital (NBO) charges of the surface O atoms and the higher negativity of the NBO charges leads to stronger basicity [16,18]. We reported that Lindqvist-type polyoxometalates with group 5 metal ions (Nb, Ta) had higher negative NBO charges compared to group 6 metal ions (Mo, W) and [Nb₆O₁₉]⁸⁻ and [Ta₆O₁₉]⁸⁻ could activate CO₂ and worked as catalysts for CO₂ fixation and conversion reactions [18,25]. The CO₂ was activated on the terminal O sites of metal oxide clusters, which are Lewis base sites, and activated CO₂ reacts with epoxides to form carbonates [17,18,21]. The cycloaddition of CO₂ to epichlorohydrin proceeded on Keggin-type Na₁₆[SiNb₁₂O₄₀] [21]. In the case of Lindqvist-type [M₆O₁₉]⁸⁻ (M = Nb, Ta), [Ta₆O₁₉]⁸⁻ showed higher activity for CO₂ fixation to styrene oxide (SO) at 403 K than [Nb₆O₁₉]⁸⁻ [18]. However, the styrene carbonate (SC) selectivity of [Ta₆O₁₉]⁸⁻ was lower than 90% and byproducts were formed. In our previous study, Brønsted basicity was investigated using sodium salts of [Ta_xNb_6−xO₁₉]⁸⁻ as solid base catalyst in Knoevenagel condensation reactions and local symmetry of NbO₆ and TaO₆ units in the clusters affected base catalytic properties [23]. In this study, tetrabutylammonium (TBA) salts of mixed metal oxide clusters [Ta_xNb_6−xO₁₉]⁸⁻ (TBA-Ta_xNb_6−x, x = 0–6) were prepared and applied to CO₂ fixation to SO to elucidate the Ta-substitution effect on the catalytic activities and selectivity. It was found that single-Ta-substituted TBA-Ta₁Nb₅ exhibited the highest SC selectivity among TBA-Ta_xNb_6−x. We demonstrated that the high SC selectivity was achieved by the selective adsorption of CO₂ on the terminal O_Ta without SO activation under reaction conditions.

2. Results

The fabricated TBA-Ta_xNb_6−x were characterized by X-ray absorption spectroscopy (XAS), electrospray ionization mass spectrometry (ESI–MS), Fourier-transformed infrared (FT-IR) in attenuated total reflectance (ATR) mode, and elemental analysis (Figure 1, Figure S1 and Figure S2, and Table S1, respectively). ESI–MS suggests that the various components of Ta–Nb mixed metal oxide clusters are contained in the TBA-Ta_xNb_6−x. Ta L₃-edge Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) of TBA-Ta_xNb_6−x indicates that peaks of Ta–M (M = Nb or Ta) shift to a longer length with increasing Ta content (Figure 1a). A similar peak shift of Nb–M (M = Nb or Ta) is observed in the Nb K-edge FT-EXAFS spectra (Figure 1b). Those indicate the Ta-substitution to Nb sites in [Ta_xNb_6−xO₁₉]⁸⁻. Elemental analysis reveals that TBA/[Ta_xNb_6−xO₁₉]⁸⁻ ratio is 5~6.

The prepared clusters were employed in the catalytic CO₂ fixation to SO. Figure 2 shows the SO conversion and SC selectivity for TBA-Ta_xNb_6−x catalysts. The SO conversion gradually increases with incremental addition of Ta content and TBA-Ta₆ exhibits the highest SO conversion among them. On the other hand, the trend of SC selectivity for the composition of clusters differs from that of SO conversation (Figure 2). Interestingly, single-Ta-substituted TBA-Ta₁Nb₅ provides the highest SC selectivity (95%) among the mixed metal oxide clusters. Further Ta-substitution decreases SC selectivity and the SC selectivity becomes constant in Ta-rich TBA-Ta_xNb_6−x catalysts (x = 4–6). The number of TBA counteractions has a negligible impact on this reaction, although TBA/[Ta_xNb_6−xO₁₉]⁸⁻ ratio varies with the composition [17]. The byproducts in this reaction over TBA-Ta₆ are polymers derived from the polymerization of SO, because the SO conversion is found for TBA-Ta₆ at 100 °C under N₂ atmosphere without CO₂ despite negligible SO conversion for TBA-Nb_6, as shown in Figure S3. The high SC selectivity (95%) of TBA-Ta₁Nb₅ maintains a high SO conversion (88%) for a 24 h reaction (Figure 2). In addition, the >95% SC selectivity is only achieved by TBA-Ta₁Nb₅ among the TBA-Ta_xNb_6−x catalysts at >80% conversion (Figure S4). Thus, the selective CO₂ fixation to SO is achieved by the single Ta substitution to TBA-Nb₆.

The time courses of CO₂ fixation to SO over TBA-Ta₆, TBA-Ta₁Nb₅, and TBA-Nb₆ are shown in Figure 3 and Figure S5. The trends of conversion and selectivity depend on the composition of the clusters. TBA-Ta₆ exhibits the highest reaction rate among them, and SC selectivity increases with reaction time. TBA-Ta₁Nb₅ and TBA-Nb₆ show high SC selectivity at the initial stage of the reaction and the SC selectivity is maintained at a high SO conversion. Thus, TBA-Nb₆ and single-Ta-substituted TBA-Ta₁Nb₅ have the specific active sites for selective SC formation. The increment in the SC selectivity over TBA-Ta₆ is due to the consumption of SO and suppression of undesired reactions during the reaction.

Effects of SO concentration and CO₂ concentration on the CO₂ fixation to SO were studied for TBA-Ta₁Nb₅ (Figure 4). When increasing the SO concentration in the dimethyl sulfoxide (DMSO) solution, the SC selectivity slightly increases while maintaining the SO conversion. On the other hand, the SC selectivity dramatically decreases with decreasing CO₂ concentration (Figure 4b), suggesting the CO₂ activation is a key step in the CO₂ fixation to SO. We reported that CO₂ fixation to SO proceeds due to the fact that the CO₂ is activated on the terminal O sites and activated CO₂ reacts with SO to form SC [17,18]. The rate determining step of CO₂ fixation to SO is the nucleophilic attack of activated CO₂ to SO. The above results could be explained by the reaction mechanism. The decrease in the SC yield by reducing SO concentration, as shown in Figure 4a, is due to the inhibition of the reaction of activated CO₂ with SO by a low SO concentration. There are two reasons for the drastic decrease in SC selectivity by reducing CO₂ concentration. One is the decrease in the amount of activated CO₂. The other is that SO can be activated on TBA-Ta₁Nb₅ in low CO₂ concentration conditions. In fact, the SO conversion of TBA-Ta₁Nb₅ under N₂ atmosphere without CO₂ is higher than that of TBA-Nb_6, as shown in Figure S3, suggesting that the SO activation occurs by single Ta-substitution at a low CO₂ concentration.

3. Discussion

We reported that the CO₂ fixation to SO proceeded on the terminal O sites of TBA-Ta₆ because the terminal O sites, which have the negatively charged O, work as Lewis base sites [18]. The CO₂ is activated on the terminal O sites and the activated CO₂ reacts with SO to form SC. The CO₂ adsorption on TBA-Nb₆, TBA-Ta₁Nb₅, and TBA-Ta₆ in SO was examined by in situ XAFS measurements (Figure 5). Nb K-edge XANES spectrum of TBA-Nb₆ in SO exhibits a pre-edge peak at 18,980.5 eV assigned to electron excitation from 1s to hybridized 4d−5p [26,27]. This pre-edge peak intensity gives us the information on the distortion from NbO₆ octahedral (Oh) symmetry. The pre-edge peak intensity decreases with the CO₂ introduction to TBA-Nb₆ in SO, which indicates that the Oh symmetry of NbO₆ in TBA-Nb₆ is improved by the CO₂ addition. Similar results were obtained for TBA-Ta₁Nb_5, as shown in Figure 5b. Ta L₁-edge XANES spectra indicate that the pre-edge peak at 11,686 eV, which is assigned to electron transition from Ta 2s orbitals to hybridized 5d−6p orbitals [26,27], decreases with the introduction of CO₂. This change of pre-edge peak intensity reveals that TaO₆ Oh symmetry is also improved by the CO₂ addition to TBA-Ta₆ and TBA-Ta₁Nb₅. These results suggest that the Oh symmetry of TaO₆ unit increases, while NbO₆ symmetry is slightly improved in TBA-Ta₁Nb₅ by CO₂ adsorption. Actually, the optimized structure of [Ta₁Nb₅O₁₉]⁸⁻ with CO₂ adsorbed on the terminal O_Ta site has highly Oh symmetric TaO₆ units compared to the bare [Ta₁Nb₅O₁₉]⁸⁻ (Figure S6) CO₂ is also adsorbed on TBA-Ta_xNb_6−x in SO solution. This structural change induced by CO₂ adsorption is also observed in FT-IR (Figure S7). FT-IR spectra of TBA-Ta₁Nb₅ in DMSO solvent show the characteristic absorption band assignable to the stretching vibration between the metal and terminal O atoms in MO₆ units (M=O bond). The absorption band shifts to high energy, owing to the slight shrink in the O=Nb bond in NbO₆ units. Those results indicate that CO₂ is preferentially adsorbed on terminal O sites of TaO₆ unit and induces the structure change in TBA-Ta_xNb_6−x.

Next, NBO charges of the surface O atoms of TBA-Ta_xNb_6−x were calculated to elucidate the catalytic activity and selectivity of TBA-Ta_xNb_6−x for CO₂ fixation to SO (Figure 6). The average NBO charge was also evaluated in Figure 6. The terminal O atoms coordinated to Ta (terminal O_Ta) have the most negative NBO charges (highest basicity) followed by the bridge O atom between Ta ions (bridge O_Ta–Ta) in TBA-Ta_xNb_6−x. The values of NBO charge of terminal O_Ta hardly change with the Ta content. On the other hand, the NBO charge of the terminal O connecting to Nb (terminal O_Nb) has lower negativity than that of terminal O_Ta. The order of NBO charges in TBA-Ta_xNb_6−x is terminal O_Ta, bridge O_Ta–Ta, bridge O_Ta–Nb (Ta–O–Nb), terminal O_Nb, and bridge O_Nb–Nb (Nb–O–Nb). As a result, the average NBO charges of TBA-Ta_xNb_6−x gradually increase with increasing the Ta content. The catalytic activities of TBA-Ta_xNb_6−x, which increase with incremental addition of Ta content in Figure 2, could be explained by the average NBO charges, indicating the increase in the active sites of terminal O_Ta by Ta substitution.

Finally, the CO₂ adsorption sites of [Ta₁Nb₅O₁₉]⁸⁻ were also predicted by DFT calculations. We reported that CO₂ was preferentially adsorbed on terminal O sites rather than bridge O sites [18]. [Ta₁Nb₅O₁₉]⁸⁻ has three terminal O sites (see Figure 7). To determine the CO₂ activation sites of [Ta₁Nb₅O₁₉]⁸⁻, the CO₂ adsorption energy was calculated using three possible configurations (Figure 7). Among the three structures, the lowest energy is found in a structure with CO₂ adsorbed on the terminal O_Ta site, which has the highest negative NBO charge among the surface oxygen atoms in [Ta₁Nb₅O₁₉]⁸⁻. This result indicates that the CO₂ is preferentially adsorbed and activated on the terminal O_Ta. The SO adsorption energy was also calculated to gain the insight into SO activation sites (Figure S8). The adsorption energy of SO on O_Ta is lower than that of CO₂ on O_Ta in [Ta₁Nb₅O₁₉]⁸⁻. In addition, adsorption energies reveal that SO is more likely to be activated on O_Ta than O_Nb. These results suggest that CO₂ preferentially adsorbs on O_Ta site and it is unlikely that SO activation occurs on O_Nb in TBA-Ta₁Nb₅. Therefore, high SC selectivity is achieved in TBA-Ta₁Nb₅. The low SC selectivity in Ta-rich TBA-Ta_xNb_6−x is explained that not only by the fact that CO₂ but also SO is activated on O_Ta sites by competitive adsorption due to the large number of O_Ta adsorption sites.

The reaction mechanism of TBA-Ta_xNb_6−x for CO₂ fixation to SO is discussed. In the case of TBA-Nb₆, CO₂ is adsorbed on the terminal O_Nb sites and the activated CO₂ reacts nucleophilically with SO to form SC. The low catalytic activity of TBA-Nb₆ for CO₂ fixation to SO is due to the weak Lewis base strength (low negativity in NBO charges) of terminal O_Nb compared with terminal O_Ta of other TBA-Ta_xNb_6−x. The SO activation hardly occurs on TBA-Nb_6, as shown in Figure S3, which is one of the reasons why TBA-Nb₆ shows high SC selectivity. The SO conversion gradually increases with Ta substitution amount, as shown in Figure 2. This can be explained by the increase in active terminal O_Ta sites. On the other hand, the SC selectivity decreases for high Ta content of TBA-Ta_xNb_6−x (x ≥ 2). The low SC selectivity is due to the SO activation on the surface of TBA-Ta_xNb_6−x (x ≥ 2), as shown in Scheme 1. In fact, TBA-Ta₆ exhibits the highest SO conversion among TBA-Ta₆, TBA-Ta₁Nb₅, and TBA-Nb₆ in the absence of CO₂ accompanied with a viscosity increase (Figure S3). The sharp contrast in SO conversion in the absence of CO₂ conditions for TBA-Ta₆ and TBA-Nb₆ clearly indicates that terminal O_Ta and/or bridge O_Ta−Ta can activate SO. On the other hand, TBA-Ta₁Nb₅ exhibits the highest SC selectivity among TBA-Ta_xNb_6−x despite having a terminal O_Ta site. The DFT calculation (Figure 7) and CO₂ concentration dependence on CO₂ fixation to SO (Figure 4b) reveal that the single terminal O_Ta in [Ta₁Nb₅O₁₉]⁸⁻ preferentially adsorbs CO₂ at 100% CO₂ conditions and SO is not activated on terminal O_Nb, bridge O_Nb−Nb, and bridge O_Ta−Nb (Scheme 1). We conclude that the selective CO₂ activation at the terminal O_Ta in TBA-Ta₁Nb₅ without SO activation is a crucial factor for high SC selectivity in the CO₂ fixation to SO.

4. Materials and Methods

TBA salts of [Ta_xNb_6−xO₁₉]⁸⁻ (TBA-Ta_xNb_6−x, x = 0–6) were prepared by microwave-assisted hydrothermal synthesis (Biotage Initiator⁺ 400 W) using Ta_2(x/6)Nb_2(1−x/6)O₅·nH₂O as the precursors. First, Na₃Ta_x/6Nb_1−x/6O₄ were prepared by modified solid-state reaction method according to the reported procedures [23,28]. M₂O₅ (M = Ta or Nb), Na₂C₂O₄, and (NH₂)₂CO at a molar ratio between (Ta + Nb):Na:(NH₂)₂CO of 1:1:4 was ground to fine powder prior to calcination at 773 K for 4 h to obtain NaTa_x/6Nb_1−x/6O₃. NaTa_x/6Nb_1−x/6O₃ was mixed with Na₂C₂O₄, and (NH₂)₂CO at a molar ratio between NaTa_x/6Nb_1−x/6O₃:Na:(NH₂)₂CO of 1:1:3 followed by calcination at 1173 K for 4 h. The resulting powder, Na₃Ta_x/6Nb_1−x/6O₄, was characterized by XRD (Rigaku Miniflex) having diffraction patterns corresponding to the references (Figure S9). Na₃Ta_x/6Nb_1−x/6O₄ was dissolved in water and 1 M HCl was added until pH of the supernatant reached 1 or less. The white precipitate was collected by centrifugation and washed with pure water until the pH of the supernatant became neutral. After drying in vacuum and oven, the Ta_2(x/6)Nb_2(1−x/6)O₅·nH₂O was obtained. Then, 10% tetrabutylammonium hydroxide (TBAOH) aqueous solution was added to Ta_2(x/6)Nb_2(1−x/6)O₅·nH₂O. The mixture was reacted using microwave-assisted hydrothermal synthesis at 180 °C for 5−15 min. The resultant product was washed with hexane to obtain TBA₆H₂[Ta_xNb_6−xO₁₉]. The fabricated clusters were characterized by ESI–MS (Figure S1) (Bruker, MicroOTOFII-ST1), Fourier-transformed infrared spectrometry (JASCO, FT/IR-4700) equipped with attenuated total reflectance-infrared spectroscopy (JASCO, ATR-PRO ONE) (Figure S2), elemental analysis (Table S1), and X-ray absorption fine structure (XAFS) analysis (BL01B1, SPring-8) (Figure 1). XAFS spectra were recorded in transmittance mode using ionization chambers as detectors at room temperature. Si(111) double-crystal monochromator was used to obtain the incident X-ray beam for Ta L₁- and L₃-edges XAFS. In the case of Nb K-edge XAFS measurements, Si(311) double-crystal monochromator was employed. The data were analyzed using xTunes software [29]. The XANES spectra were extracted as the extended X-ray absorption fine structure (EXAFS) after normalization at edge height. The EXAFS spectra in the k range 3.0–14.0 Å⁻¹ were Fourier-transformed into r space to obtain FT-EXAFS spectra. The illustrations of Ta_xNb_6−xO₁₉ were computed using VESTA [30].

In general, CO₂ fixation to SO over TBA-Ta_xNb_6−x were carried out using 5 µmol of catalyst, SO (0.6 mL, ca. 5 mmol), 100% CO₂ (0.1 MPa) at 100 °C for 6 h using biphenyl as an internal standard. The product solutions were analyzed using gas chromatography equipped with flame ionization detector (GC-FID, Shimadzu, GC-2014 with column Restex, Rtx-1) and gas chromatography–mass spectrometry (GC–MS, Shimadzu, GCMS-QP2010 SE with column Agilent, DB-1MS). Time course of CO₂ fixation to SO reactions were carried out using 10 µmol of catalyst, SO (1.2 mL, ca. 10 mmol), 100% CO₂ (0.1 MPa) at 100 °C for 24 h. Small amount of solution (ca. 20 µL) was drawn to measure at specified reaction times. The peak areas from GC-FID chromatograms were used to calculate with this formula:

$$\begin{gathered} Conversion \left(\%\right)=\left(1-\frac{Are{a}_{Sub.}}{Are{a}_{IS}}\times \frac{Are{a}_{IS0}}{Are{a}_{Sub.0}}\right)\times 100 \\ Selectivity\left(\%\right)=\frac{Are{a}_{Pro.}}{Are{a}_{IS}}\times \frac{EC{N}_{Sub.}}{EC{N}_{Pro.}}\times \frac{Are{a}_{IS0}}{Are{a}_{Sub.0}}\times \frac{100}{Conversion \left(\%\right)}\times 100 \end{gathered}$$

where Sub. = substrate (SO), IS = internal standard (biphenyl), Pro. = product (SC), ECN = equivalent carbon number, subscripted 0 = initial value before reaction.

The DFT calculations were conducted using Gaussian 16 program as previously reported [18]. The structural optimization for [Ta_xNb_6−xO₁₉]⁸⁻ was performed by B3LYP with the solvation effect of DMSO using PCM (dielectric constant = 46.826). LanL2DZ basis sets were employed for Ta and Nb atoms and 6−31 + G(d) basis sets for O and C atoms to investigate the effect of the composition of the clusters on the NBO charge of O atoms and the adsorption energies of CO₂ on [Ta₁Nb₅O₁₉]⁸⁻.

5. Conclusions

In conclusion, TBA-Ta_xNb_6−x (x = 0–6) were prepared by microwave-assisted hydrothermal reaction and were used as catalysts for CO₂ fixation to SO to produce SC. Among TBA-Ta_xNb_6−x, TBA-Ta₁Nb₅ shows the highest selectivity toward SC, whereas the SO conversion increases with the Ta content in the clusters. The effects of SO concentration and CO₂ concentration for CO₂ fixation to SO indicate that high SO concentration and 100% CO₂ atmosphere are required to obtain high SC selectivity for TBA-Ta₁Nb₅ because the rate-determining step of CO₂ fixation to SO is the reaction of the activated CO₂ with SO. The SO conversions in the absence of CO₂ suggest that the SO activation hardly occurs in TBA-Nb₆. DFT calculations reveal that the increase in the SO conversion with Ta content is the increment of the active terminal O_Ta sites that have the highest negative NBO charges. In addition, [Ta₁Nb₅O₁₉]⁸⁻ preferentially adsorbs CO₂ at terminal O_Ta sites compared to other O_Nb sites. In conclusion, the selective CO₂ activation at terminal O_Ta in TBA-Ta₁Nb₅ without SO activation is a crucial factor for high SC selectivity in the CO₂ fixation to SO.