ZrO2 Nanocrystals As Catalyst for Synthesis of Dimethylcarbonate

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ZrO Nanocrystals as Catalyst for Synthesis of Dimethylcarbonate from Methanol and Carbon Dioxide: Catalytic Activity and Elucidation of Active Sites Takayuki Akune, Yusuke Morita, Shinya Shirakawa, Kiyofumi Katagiri, and Kei Inumaru Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01294 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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Clear Copy of Revised Manuscript: REVISION2 ZrO2 Nanocrystals as Catalyst for Synthesis of Dimethylcarbonate from Methanol and Carbon Dioxide: Catalytic Activity and Elucidation of Active Sites Takayuki Akune, Yusuke Morita, Shinya Shirakawa, Kiyofumi Katagiri*, and Kei Inumaru* Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan. E-mail: [email protected] The catalytic activity of zirconium oxide (ZrO2) nanocrystals for the reaction of carbon dioxide (CO2) with methanol to form dimethylcarbonate (DMC) was investigated. ZrO2 nanocrystals prepared by hydrothermal synthesis at various temperatures were compared. The size of the ZrO2 nanocrystals monotonically increased with the hydrothermal temperature, according to specific surface area, transmission electron microscope measurements, and their X-ray diffraction peak widths. The ZrO2 nanocrystals prepared by hydrothermal synthesis were found to exhibit high catalytic activity owing to their high surface area and catalytically active surfaces arising from their high crystallinity. Next, adsorbed species generated from CO2 on the ZrO2 surfaces were measured using CO2 temperature programmed desorption (TPD) and in-situ FT-IR spectroscopy. The results confirmed the presence of several kinds of adsorbed species including bidentate bicarbonate (b-HCO3−), bidentate carbonate (b-CO32−) and monodentate carbonate (m-CO32−). The relationship between the amounts of these surface species and the catalytic activity of the ZrO2 was investigated for the first time. The amount of the bidentate species (b-HCO3− and b-CO32−) was found to well correlate with the catalytic activity, demonstrating that the surface sites which afford these species contribute the catalytic activity for this reaction.

Keywords: nanoparticle, CO2 TPD, in situ IR, XRD

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Introduction Metal oxide nanocrystals have attracted substantial attention as important materials for the realization of novel functions in various fields1,2). One advantage of nanocrystals is that their properties, such as shape, size, crystal structure, and crystallinity, can be controlled by tuning their preparation conditions. Nanocrystals typically have a large surface area because of their small size, and particular crystal planes exposed, so novel surface functions are an important objective in nanocrystal science and technology1). Heterogeneous catalysis is one of the most promising fields for nanocrystals. The conversion of carbon dioxide (CO2) is a current challenge in chemical technology. Several methodologies for CO2 conversion have been proposed, including a shift reaction to produce hydrogen (H2) and carbon monoxide3), and chemical reduction with reducing reagents such as H2. The reduction of CO2 is the goal of a proposed future energy system using methanol as an energy carrier4). Another way to use CO2 is the production of carbonates. Diphenylcarbonate is a major chemical source for polycarbonate resin and can be formed from dimethylcarbonate (DMC) by the ester exchange reaction shown in Eq. 1 5). Thus, DMC is a useful chemical and a suitable target product for the chemical conversion of CO2 Ref. 6). CO2 + 2 CH3OH

⇄

(CH3O)2CO + H2O.

(1)

Tomishige et al.7-11) reported that zirconium oxide (ZrO2) is an effective catalyst for the formation of DMC from CO2 and methanol. They also reported cerium oxides as active catalysts for this reaction.9-12) Their works stimulated many studies on this reaction.13-21) Bell and co-workers13-15) investigated the mechanism of this catalytic reaction in detail using in situ infrared (IR) spectroscopy, and proposed that coordinately unsaturated Zr sites and neighboring hydroxyl groups are important in this reaction. In the present study, the use of ZrO2 nanocrystals as catalysts for the formation of DMC from CO2 and methanol is investigated. The nanocrystals and reaction system were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), CO2-temperature-programmed desorption (TPD) measurements, and in situ FT-IR spectroscopy. Particularly, it is the first time that the relationship between the amounts of

surface

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semi-quantitatively

species

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investigated.

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actual this

catalytic

study

activity

highlights

contribution of specific adsorption sites to the catalytic reaction. 2


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Experimental Experimental

Preparation of ZrO2 nanocrystals ZrO2 nanocrystals were prepared according to the literature22) as follows. ZrO(NO3)2·2 H2O (1.60 g) and urea (3.46 g) were dissolved in pure water (30 mL, Millipore). The solution was charged into an autoclave and heated at a selected temperature (393, 433, 473, 493, or 513 K) for 20 h. The resulting solid was separated by centrifugation and then washed several times by repeated dispersion in water and centrifugation. The sample was dried overnight at 323 K. The obtained ZrO2 nanocrystal samples were denoted ZrO2-HT(XXX), where XXX represents the reaction temperature. For comparison, ZrO2 was prepared by calcination of zirconium hydroxide (ZrO2·xH2O, Nacalai Tesque, Inc., Kyoto, Japan) according to the procedure reported by Tomishige et al.7) This catalyst was denoted ZrO2-HX-C. Mesoporous ZrO2 was also prepared as follows.23) ZrO(NO3)2·2H2O (0.48 g) and P-123 surfactant template (BASF, 0.21 g) were dissolved in pure water (60 mL). The pH of the stirred solution was then adjusted to 11 with 28% NH4OH. The solution was aged at 361 K for 24 h to give a white precipitate. This solid was filtered, washed with water and acetone, and then dried at 383 K overnight. The sample was calcined at 873 K in air for 4 h to remove the template. The obtained mesoporous ZrO2 after calcination was denoted Meso-ZrO2.

Characterization XRD patterns were recorded on a diffractometer (D-8 Advance, Bruker) equipped with a position-sensitive detector (Vantec) and Ni filter. In situ IR spectra were measured with an IR spectrometer (FT/IR-4200, JASCO) equipped with a mercury cadmium telluride detector and glass in situ cell with NaCl windows. CO2 TPD experiments were carried out with a custom-made flow system. After pretreatment at 423 K (ZrO2-HT(XXX)) or 673 K (ZrO2-HX-C, Meso-ZrO2) for 1 h under an Ar flow, the sample was exposed to 4000 ppm of CO2 at room temperature. After reversibly adsorbed species were removed under an Ar flow, the sample was heated under an Ar flow at a temperature ramp rate of 10 K min−1. The gas flow from the outlet was introduced into the Q-Mass detector of a gas chromatograph-mass spectrometer (GC-MS) apparatus (JMS-Q1050GC, JEOL, Tokyo, Japan). The signal with m/z = 44 was recorded to monitor the desorption rate of CO2. The desorption rate of CO2 rd is obtained by the following equation:

rd = CCO2 · F where CCO2 is CO2 concentration at the outlet calculated from the mass spectrometer 3


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signal intensity, F is a total flow rate of outlet gas (~ flow rate of the carrier gas, in mol min-1). The mass spectrometer intensity was calibrated by using standard dilute CO2 gas (Ar balance) with known concentration.

Catalytic reaction ZrO2 catalyst (50 mg, after pretreatment in vacuo at 423 K (ZrO2-HT(XXX)) or 673 K (ZrO2-HX-C, Meso-ZrO2)) and methanol (6.2 mL) were added to an autoclave (190 cm3) and CO2 gas was introduced. The autoclave was heated to 433 K at a pressure of 4.8 MPa. After a certain reaction time, the autoclave was cooled by soaking in ice-water. The product in methanol was analyzed with a GC (GC-14B, Shimadzu, Kyoto, Japan) equipped with a flame ionization detector. The catalytic activities of all the prepared catalysts were evaluated using the amount of DMC produced within a reaction time of 5 h. At first the time course of DMC formation was measured with a large amount of ZrO2 catalyst (0.5 g) to examine the effect of chemical equilibrium. As shown in Fig. S1, the DMC amount was almost proportional to the reaction time before DMC reached ca 0.2 mmol. Therefore we used much smaller amount of catalyst (50 mg) and compared the activity by using the DMC amount at 5 h, so that the DMC amount did not reach 0.2 mmol. Results and Discussion

Particle size, surface area, and structure of catalysts Figure 1 depicts TEM images of the ZrO2 nanocrystal catalysts. ZrO2-HT(393) consisted of aggregated particles of less than 5 nm in size (Fig. 1a). The particle sizes observed in the TEM images increased to ca. 10–20 nm as the hydrothermal temperature was increased from 393 to 513 K (Fig. 1a–e). A dark-field image of ZrO2-HT(513) is given in Fig. 1f. The crystallite size almost coincides with the particle size observed in the bright-field image (Fig. 1e). The Brunauer-Emmett-Teller (BET) surface areas of the nanoparticles are listed in Table 1. ZrO2-HT(393) had the highest surface area (168 m2 g−1) except for that of Meso-ZrO2 of 217 m2 g−1. The surface area of ZrO2-HT(XXX) monotonically decreased as the hydrothermal temperature was increased, consistent with the change in particle size observed in the TEM images. Figure 2 shows XRD patterns of the samples. ZrO2-HT(XXX) samples exhibited clear diffraction peaks indicative of the monoclinic crystal structure. The integrated intensities of the diffraction patterns in the range of 2θ = 20–45° were almost constant for all of ZrO2-HT(XXX) samples. This fact directly showed that the amount of 4


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crystalline moiety in all samples was similar despite their preparation at different temperatures. If the samples prepared at low temperatures contain amorphous moiety, the fraction of amorphous moiety should be smaller and the amount of crystalline moiety should be larger when the samples are prepared at higher temperatures. This contradict the experimental result. Thus, it was suggested that all the samples consisted of only crystalline moiety. It is obvious that the diffraction peaks became sharper as the hydrothermal temperature was increased: the crystallite size increased with reaction temperature. The crystalline sizes derived from XRD peak half width were 6.0, 6.6, 14, 15, 16, and 16 nm for the samples prepared at 393, 433, 473, 493 and 513 K. This trend is consistent with the TEM images and changes in surface area. Meso-ZrO2 exhibited very broad diffraction peaks, even following calcination at 873 K (Fig. 2f). The main peak centered at ~30° suggests that this sample contained poorly crystallized tetragonal ZrO2. ZrO2-HX-C prepared by calcination of zirconium hydroxide showed diffraction peaks mainly attributable to the tetragonal ZrO2 phase as well as weaker diffraction signals from the monoclinic phase. The peaks were sharp, indicating that these phases were well-crystallized in this sample. To further study the structure of Meso-ZrO2, its nitrogen adsorption isotherm was measured as shown in Fig. 3. An obvious step caused by capillary condensation was observed around P / Po = 0.5–0.7, indicating the presence of well-developed mesopores. Barrett-Joyner-Halenda (BJH) analysis of this isotherm gave a single peak at around 5.2 nm (not shown). The inset in Fig. 3 shows the low-angle region of the XRD pattern of Meso-ZrO2. The peak at 1.8° suggests the presence of an ordered nanostructure. These results reveal that Meso-ZrO2 possessed a high surface area with well-developed mesopores and pore walls of poorly crystallized, mostly tetragonal phase ZrO2.

Catalytic activity of catalysts The catalytic activity of the ZrO2 catalysts for producing DMC was investigated; the results are listed in Table 1. The reaction rates were normalized by surface area, as also listed in Table 1. It is obvious that ZrO2-HX-C and the ZrO2-HT(XXX) samples showed much higher activity per unit surface area than Meso-ZrO2. The low catalytic activity of Meso-ZrO2 may be attributed to its poor crystallinity, as indicated by XRD. The catalyst ZrO2-HX-C, which adopted a tetragonal crystal structure, showed catalytic activity comparable to that of the ZrO2-HT(XXX) catalysts, which had a monoclinic structure. This shows that the crystal structure of the ZrO2 phase did not have a large effect on the catalytic activity in this study. Some reports described that catalytic activity for this reaction or dynamics of adsorbed species 5


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were different between the two crystal phases.15) Another influential factor is preparation conditions and preparation method. It is possible that hydrothermal synthesis in this study affected the influential factors such as surface crystallinity. As shown in Table 1, there were moderate differences in the surface area-normalized catalytic activity of the samples prepared by hydrothermal synthesis (ZrO2-HT(XXX)). Samples prepared at lower temperature (ZrO2-HT(393) and ZrO2-HT(433)) showed relatively lower activity, while those prepared at higher temperature (493 and 513 K) showed higher activity. More precisely, the surface area-normalized activity showed a trend to increase as the hydrothermal temperature increased to 493 K, while further temperature increase to 513 K brought about a decrease in the activity. This result reflects the nature of the surfaces of the catalysts because these activities were per unit surface area. It is possible that the surface crystallinity affects the nature of the catalysts’ surfaces. For example, it is possible that the number of active sites per unit surface area of the catalysts were different depending on the surface crystallinity. To prove this point, we next obtained further insight into the adsorbed species and their amounts by measuring CO2 TPD and in-situ IR spectra of these catalysts. In terms of total catalytic activity, ZrO2-HT(393) produced the highest amount of DMC, more than that of ZrO2-HX-C prepared by calcination of zirconium hydroxide. The high surface area of the ZrO2-HT(XXX) samples contributed to their high total activity. According to these results, we conclude that the present nanocrystal catalysts prepared by hydrothermal synthesis were advantageous because their high surface area and high reaction rate per unit surface area yielded a high overall catalytic activity, especially ZrO2-HT(393).

CO2-TPD and in situ IR spectra Figure 4 shows CO2-TPD profiles measured for the catalysts. For clarity, Fig. 4a shows data for three selected samples, ZrO2-HT(393), ZrO2-HT(493) and ZrO2-HX-C. The TPD data of all samples are shown in Fig. 4b. The TPD profile of ZrO2-HX-C exhibited an intense peak at 375 K accompanied by long tale spreading to higher temperature (Fig. 4a). In contrast, the profile of ZrO2-HT(393) showed distinct two peaks at 375 and 525 K. As for ZrO2-HT(493), The TPD profile gave an intense peak at 375 K and smaller desorption signals beginning at around 490 K. The same location of the lower temperature peak of ZrO2-HT(393) and ZrO2-HT(493) as the single peak of ZrO2-HX-C suggested that they were associated with the same adsorbed species. The in situ IR spectra of the three samples are given in Figure 5. The top, 6


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middle and bottom panels correspond to ZrO2-HX-C, ZrO2-HT(493) and ZrO2-HT(393), respectively. After the pretreatment by evacuation at appropriate temperature (423 K for ZrO2-HT(XXX) and 673 K for ZrO2-HX-C), each sample was dosed to CO2 (1 atm) at room temperature and an IR spectrum was measured in the presence of CO2. This spectrum contains the signals from reversibly-adsorbed species. Then the IR cell was evacuated at room temperature (rt) for 30 min and then an IR spectrum was measured at rt. The evacuation removes reversibly-adsorbed species from the sample and the spectrum is associated with only irreversibly adsorbed species. Next, the sample in the IR cell was evacuated at each elevated temperature for 20 min followed by an IR spectrum recording. In the TPD measurements the samples were placed in an Ar flow to remove

reversibly-adsorbed

species

before

the

heating

started.

Thus

TPD

measurements detect only irreversibly-adsorbed species. Table 2 summarizes the literature data of IR bands of surface species formed by adsorption of CO2 on ZrO2.13-15,24-27) The structure of some surface species was illustrated in Scheme 1. In the IR results of ZrO2-HX-C (Fig. 5 top panel), the spectrum taken in the presence of gaseous CO2 gave signals from bidentate CO32- (b-CO32-, 1580 (shoulder), 1276, and 1042 cm-1) and bidentate HCO3- (b-HCO3-, 1620, 1430 and 1227 cm-1). The signal at 1430 cm-1 is also assignable to ionic HCO3- (i-HCO3-, 1430 cm-1 ref 14). After evacuation at rt, all peaks decreased but still existed. The decrease of the signal at 1430 cm-1 was more significant, probably because the superimposed signal from ionic HCO3- decreased to large extent. At the same time, small new peaks appeared at 1454 and 1415 cm-1. According to Table 2 these two peaks correspond to polydentate CO32- (p-CO32-). After evacuation at 375 K for 20 min, signals from b-HCO3and b-CO32- drastically decreased but signals from p-CO32- almost remained. These results demonstrate that the TPD peak observed at 375 K corresponds to the two bidentate species (b-HCO3- and b-CO32-). After evacuation at 450 K the signals from p-CO32- unchanged but disappeared by evacuation at 525 K. IR spectra of ZrO2-HT(493) (Fig. 5, middle panel) also give useful information about the surface species. The spectrum taken in the presence of 1 atm CO2 gave signals from b-HCO3- (1632, 1420 and 1227 cm-1) and b-CO32- (1590, 1290, and 1038 cm-1). These species were observed after evacuation at rt. In the spectrum after 375 K evacuation, b-HCO3- and b-CO32- decreased drastically. It should be noted that a new signal appeared at 1330 cm-1. This signal is close to that of m-CO32- (Table 2). In the case of ZrO2-HT(393) (Fig. 5c), the discussion is slightly more difficult. In the presence of gaseous CO2, b-CO32- and b-HCO3- were detected. These species remained after evacuation at rt. In the spectrum after evacuation at 375 K, the signals 7


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from these bidentate species drastically decreased and very broad bands was observed at ca 1600 and 1292 cm-1. These positions are also close to that of b-CO32- but at present we cannot give a clear assignment to these broad IR signals. A possibility is a b-CO32--like species interacting strongly with the surfaces. By combining the TPD and IR results, we can conclude that the TPD peaks at 375 K represented the bidentate species (b-HCO3− and b-CO32−). The TPD signals observed at higher temperature around 525 K represented m-CO32− (ZrO2-HT(493)) and/or unidentified species for ZrO2-HT(393) (possibly strongly bound b-CO32−-like species ). Fig. 4b shows TPD profiles for all samples. All the data except for that of ZrO2-HX-C and Meso-ZrO2 showed both TPD peaks. Our interpretation of the TPD peaks is consistent with the previous report in the literature.14)

Scheme 1. b-HCO3−, b-CO32−, and m-CO32− species on the catalysts.

Correlation between catalytic activity and catalyst properties Here, we discuss the catalytic activity per unit area of the catalysts in further detail. To obtain deep insight into a catalytic reaction, it is important to obtain information on the adsorption sites which contribute to the rate determining step of the reaction. For this purpose, it is effective to investigate the relationship between the amounts of certain surface species and the catalytic reaction rate. If any of these amounts are strongly correlated to the reaction rate, it is strong evidence that the corresponding adsorption species or related surface sites are involved in the rate determining step. In this study, we have obtained catalytic reaction rates for ZrO2 catalysts prepared using various methods and conditions. Thus, we next investigated the relationships between the reaction rate and the amounts of surface species detected in the CO2 TPD and in situ IR experiments. The CO2 TPD data were used to obtain information about the amounts of these surface species. One method is to separate the TPD profiles into two peaks and use the 8


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integrated areas to calculate the amount of each surface species. However, it is difficult to exclude unexpected error and artefact due to poor fitting between theoretical curve and observed data. Thus, we adopted a simpler method; the intensity of each peak (375 and 525 K) in the TPD profile (i.e., desorption rate) was taken as a measure of the relative amount of each surface species among the different samples. This method works if the TPD peak shape does not change. In fact, the shape of TPD peaks observed in Fig. 4 is similar for many samples, especially for the peak at lower temperature (375 K). Of course, this method cannot be used to compare amounts of different species, but usable to extract essential trend of the sample dependence of each adsorbed species’ amount. This treatment is also valid from the viewpoint of theoretical consideration: the TPD peak height for each surface species is proportional to the relative amount of each surface species when the distribution of activation energy for desorption of the species and other experimental conditions are the same among different samples.28) The unchanged shape of the TPD peaks implies that the above condition was met in our samples and experiments. Figure 6 shows the relationship between the catalytic reaction rate and the relative amounts of each surface species derived from the TPD profiles. The vertical axes of the panels show the catalytic reaction rate per unit surface area. The horizontal axes show the CO2 desorption rate per unit surface area at 375 K (Fig. 6a) and 525 K (Fig. 6b). Therefore, the horizontal axis of Fig. 6a can be regarded as the relative amount of the bidentate species (b-HCO3− and b-CO32−) per unit surface area of the different samples. The horizontal axis of Fig. 6b represents the relative amount of m-CO32− (ZrO2-HT(493)) and/or the unidentified species (ZrO2-HT(393)). Because the amount of CO2 adsorbed on Meso-ZrO2 (per unit surface area) was very small compared with that on the other samples (Fig. 4b), the plots for Meso-ZrO2 are isolated at the lower left in the panels in Fig. 6a and 6b. The plots for the other samples are located from center to higher right in the panels. Fig. 6a demonstrates that the relative amount of the bidentate species was reasonably well correlated with the catalytic reaction rate. That is, the catalytic reaction rate increased with the amount of the bidentate species. The correlation factor between the plots in Fig. 6a and the linear line was as high as 0.95. In contrast, as shown in Fig. 6b, the plots for all samples except Meso-ZrO2 and ZrO2-HX-C, located at the upper right in the panel, showed no positive correlation. The catalytic activity rather decreased as the amount of m-CO32− (ZrO2-HT(493)) and/or the unidentified species (ZrO2-HT(393)) increased. These findings demonstrate that the surface sites on which the bidentate species form play a very important role in the rate determining step of the catalytic 9


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reaction. A b-CO32− species forms on a coordinately unsaturated Zr4+ site and neighboring oxide ion,26, 27) and a b-HCO3− may form from an unsaturated Zr4+ site and a OH group. This is the first time that the real catalytic activity of ZrO2 has been linked to the amount of surface sites or surface adsorption species for this reaction. Bell et al. previously proposed a reaction scheme for this reaction over ZrO2 based on in situ FT-IR and TPD experiments.13-15) According to them, the key step is the adsorption of CO2 molecules on the coordinately unsaturated Zr4+ sites, followed by reaction of the adsorbed species with neighboring methoxy groups. Bell et al. claimed that Lewis acidity of coordinately unsaturated Zr4+ sites and Lewis basicity of neighboring oxide ions play important roles in the catalytic formation of DMC from CO2 and methanol. A b-CO32− species forms on an unsaturated Zr4+ site and its neighboring oxide ion. b-HCO3− may form from an unsaturated Zr4+ site and a OH group. The b-CO32− and b-HCO3− species detected by TPD were adsorbed irreversibly on the ZrO2 surface, which reflects the fact that the adsorption sites have strong interaction. The amount of b-HCO3− and b-CO32− species detected by TPD must represent the amount of catalytic active sites contributing to the key reaction step. Our findings have evidently revealed these active sites by comparing the real catalytic activity of various catalysts and the amounts of species adsorbed on them for the first time. Among the catalysts prepared in this study, Meso-ZrO2 had very low crystallinity, and ZrO2-HT(XXX) had increasing crystallinity with the increasing hydrothermal temperature. As a general trend, catalytic activity per surface area increased as the crystallinity increased (Table 1, except ZrO2-HT(513), the catalyst prepared at the highest hydrothermal temperature). For all catalysts, a good correlation was found in Fig. 6a. This demonstrates that the surface sites which afford the bidentate species (b-HCO3− and b-CO32−) contribute the catalytic activity for this reaction. Conclusions ZrO2 nanocrystals formed by hydrothermal synthesis showed high catalytic activity for the formation of DMC. Nanocrystal catalysts prepared by hydrothermal synthesis were advantageous because their high surface area and high reaction rate per unit surface area gave a high catalytic activity overall. CO2 TPD measurements and in-situ FT-IR spectroscopy confirmed the presence of several kinds of adsorbed species on the ZrO2 surfaces. It was demonstrated for the first time that the amount of surface bidentate species shows a good correlation with the 10


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observed catalytic activity of these catalysts. Our findings show that that surface sites which afford the species play an important role in the rate determining step of this catalytic reaction. Acknowledgments This work was supported by the Advanced Catalytic Transformation Program for Carbon Utilization (JST ACT–C Grant Number JPMJCR12Y2) of the Japan Science and Technology Agency (JST), and a JSPS KAKENHI Grant Number JP25288108. This work was also partially supported by a KAKENHI Grant Number JP22107011 in the Innovative Area: “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (Area No. 2206) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work is also partly supported by the Centre for Smart Materials, the Centre for Functional Nano Oxide at Hiroshima University. The authors thank Dr. M. Maeda for TEM measurements.

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References (1) Shipway, A. N.; Katz, E.; Willner, I. Nanoparticle Arrays on Surfaces for Electronic, Optical, and Sensor Applications. ChemPhysChem 2000, 2000 1, 18-52. (2) Vanmaekelbergh, D.; Liljeroth, P. Electron-Conducting Quantum Dot Solids: Novel Materials Based on Colloidal Semiconductor Nanocrystals. Chem. Soc. Rev. 2005, 2005 34, 299-312. (3) Bradford, M. C. J.; Vannice, M. A. CO2 reforming of CH4. Catal. Rev. –Sci. Eng. 1999, 1999

41, 1-42. (4) Olah, G. A.; Goeppert, A.; Prakash, G. K. S. Beyond Oil and Gas: The Methanol Economy, 2009, 2009 Wiley-VCH. (5) Ono, Y. Catalysis in the Production and Reactions of Dimethyl Carbonate, an Environmentally Benign Building Block. Appl. Catal. A-General, 1997, 1997 155 (1997) 133-166. (6) Aresta, M.; Dibenedetto, A. Utilisation of CO2 as a Chemical Feedstock: Opportunities and Challenges. Dalton Trans. 2007, 2007 2975-2992. (7) Tomishige, K.; Sakaihori, T.; Ikeda, Y.; Fujimoto, K. A Novel Method of Direct Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide Catalyzed by Zirconia. Catal. Lett. 1999, 1999 58, 225-229. (8) Tomishige, K.; Ikeda, Y.; Sakaihori, T.; Fujimoto, K. Catalytic Properties and Structure of Zirconia Catalysts for Direct Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide. J. Catal. 2000, 2000 192, 355-362. (9) Tomishige, K.; Furusawa, Y.; Ikeda, I.; Asadullah, M.; Fujimoto, K. CeO2-ZrO2 Solid Solution Catalyst for Selective Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide. Catal. Lett. 2001, 2001 76, 71-74. (10) Tomishige, K.; Kunimori, K., Catalytic and Direct Synthesis of Dimethyl Carbonate Starting from Carbon Dioxide using CeO2-ZrO2 Solid Solution Heterogeneous Catalyst: Effect of H2O Removal from the Reaction System. Appl. Catal. A. General, 2001, 2001 237, 103-109. (11) Honda, M.; Suzuki, A.; Noorjahan, B.; Fujimoto, K.; Suzuki, K.; Tomishige, K. Low Pressure CO2 to Dimethyl Carbonate by the Reaction with Methanol Promoted by Acetonitrile Hydration. Chem. Commun. 2009, 2009 30, 4596-4598. (12) Honda, M.; Tamura, M.; Nakagawa, Y.; Nakao, K.; Suzuki, K.; Tomishige, K. Organic Carbonate Synthesis from CO2 and Alcohol over CeO2 with 2-Cyanopyridine: Scope and Mechanistic Studies. J. Catal., 2014, 2014 318, 95-107. (13) Jung, K. T.; Bell, A. T. An in situ Infrared Study of Dimethyl Carbonate Synthesis 12


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from Carbon Dioxide and Methanol over Zirconia. J. Catal. 2001, 2001 204, 339-347. (14) Pokrovski, K.; Jung, K. T.; Bell, A. T. Investigation of CO and CO2 Adsorption on Tetragonal and Monoclinic Zirconia. Langmuir 2001, 2001 17, 4297-4303. (15) Jung, K. T.; Bell, A. T. Effects of Catalyst Phase Structure on the Elementary Processes Involved in the Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide over Zirconia, Topics Catal. 2002, 2002 20, 97-105. (16) Kindermann, N.; Jose, T.; Kleij, A. W. Synthesis of Carbonates from Alcohols and CO2. Top. Current Chem., 2017, 2017 375, 15. (17) Prymak, I.; Kalevaru, V. N.; Wohlrab, S.; Martin, A. Continuous Synthesis of Diethyl Carbonate from Ethanol and CO2 over Ce-Zr-O Catalysts. Catal. Sci. Technol. 2015, 015 5, 2322-2331. (18) Li, H. G.; Jiao, X.; Li, L.; Zhao, N. Xiao, F. K.; Wei, W.; Sun, Y. H. Zhang, B. S. Synthesis of Glycerol Carbonate by Direct Carbonylation of Glycerol with CO2 over Solid Catalysts Derived from Zn/Al/La and Zn/Al/La/M (M = Li, Mg and Zr) Hydrotalcites. Catal. Sci. Technol. 2015, 2015 5, 989-1005. (19) Tamura, M.; Honda, M.; Nakagawa, Y.; Tomishige, K. Direct Conversion of CO2 with Diols, Aminoalcohols and Diamines to Cyclic Carbonates, Cyclic Carbamates and Cyclic Ureas using Heterogeneous Catalysts. J. Chem. Technol. Biotechnol. 2014, 2014 89, 19-33. (20) Santos, B. A. V.; Pereira, C. S. M.; Silva, V. M. T. M.; Loureiro, J. M.; Rodrigues, A. E. Kinetic Study for the Direct Synthesis of Dimethyl Carbonate from Methanol and CO2 over CeO2 at High Pressure Conditions. Appl. Catal. A-General, 2013, 2013 455, 219-226. (21) Wang, S. P.; Zhao, L. F.; Wang, W.; Zhao, Y. J.; Zhang, G. L.; Ma, X. B.; Gong, J. L. Morphology Control of Ceria Nanocrystals for Catalytic Conversion of CO2 with Methanol, Nanoscale, 2013, 2013 5, 5582-5588. (22) Li, W.; Huang, H.; Zhang, W.; Liu, H. Facile Synthesis of Pure Monoclinic and Tetragonal Zirconia Nanoparticles and Their Phase Effects on the Behavior of Supported Molybdena Catalysts for Methanol-Selective Oxidation. Langmuir 2008, 2008 24, 8358-8366. (23) Rezaei, M.; Alavi, S. M.; Sahebdelfar, S.; Liu, X. M.; Qian, L.; Yan, Z. F. CO2-CH4 Reforming over Nickel Catalysts supported on Mesoporous Nanocrystalline Zirconia with High Surface Area. Energy Fuels, 2007, 2007 21, 581-589. (24) Kondo, J.; Abe, H.; Sakata, Y.; Maruya, K.; Domen, K.; Onishi, T. Infrared Studies of Adsorbed Species of H2, CO and CO2 over ZrO2, J. Chem. Soc., Faraday Trans. I, 1988, 1988

84, 511-519. (25) Fisher, I. A.; Bell, A. T. In-Situ Infrared Study of Methanol Synthesis from H2/CO2 13


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over Cu/SiO2 and Cu/ZrO2/SiO2, J. Catal. 1997, 1997 172, 222-237. (26) Bachiller-Baeza, B.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Interaction of Carbon Dioxide with the Surface of Zirconia Polymorphs. Langmuir 1988, 1988 14, 3556-3564. (27) Morterra, C.; Orio, L. Surface Characterization of Zirconium-Oxide. 2. The Interaction with Carbon-Dioxide at Ambient-Temperature. Mater. Chem. Phys. 1990, 1990 24, 247-268. (28) Masuda, T; Fujikata, Y; Ikeda, H; Matsushita, S; Hashimoto K. A Method for Calculating the Activation Energy Distribution for Desorption of Ammonia Using a TPD Spectrum Obtained under Desorption Control Conditions, Appl. Catal. A: General 1997, 1997 162, 29-40.

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Table 1. 1. Activity of ZrO2 catalysts. BET surface area / m2 g-1

Product (DMC) formed for 5 h

ZrO2-HX-C

71

78

4.4

ZrO2-HT-(393)

168

163

3.9

ZrO2-HT-(433)

148

140

3.8

ZrO2-HT-(473)

115

116

4.1

ZrO2-HT-(493)

96

115

4.7

ZrO2-HT-(513)

82

90

4.4

Meso-ZrO2

218

83

1.5

Sample

/ µmol

Reaction rate per unit surface area / µmol h-1 m-2

. Table 2. 2 IR peak assignments in the literatures. Wavenumber of Species absorption peaks / cm-1 Bidentate carbonate (b-CO32-)

1556, 1304, 1061

Kondo et al24)

1595, 1315 1575, 1555, 1335, 1325 1325

Bidentate bicarbonate (b-HCO3-)

References

(t-ZrO2)

Pokrovski et al14)

(m-ZrO2)

Pokrovski et al14)

(m-ZrO2)

Jung et al13)

1600, 1470, 1225 1622, 1465 (wk) 1620,

Kondo et al24) (ZrO2/SiO2)

1225

(t-ZrO2, m-ZrO2)

1625, 1437, 1225

(m-ZrO2)

Fisher et al25) Pokrovski et al14) Jung et al13)

Monodentate carbonate (m-CO32-)

1385

Polydentate carbonate (p-CO32-)

1450, 1430

(t-ZrO2)

Pokrovski et al14)

Ionic bicarbonate (i-HCO3-)

1695, 1435

(m-ZrO2)

Pokrovski et al14)

(ZrO2/SiO2)

1375, 1355

Fisher et al25) Pokrovski et al14)

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Figure captions Fig. 1. TEM images of ZrO2 nanoparticle catalysts. a) ZrO2-HT(393), b) ZrO2-HT(433), c) ZrO2-HT(473), d) ZrO2-HT(493), e) ZrO2-HT(513), and f) dark field image of ZrO2HT(513). Fig. 2. XRD patterns of ZrO2 nanoparticle catalysts. a) ZrO2-HT(393), b) ZrO2-HT(433), c) ZrO2-HT(473), d) ZrO2-HT(493), e) ZrO2-HT(513), f) Meso-ZrO2, and g) ZrO2-HX-C. h) and i) are calculated patterns of tetragonal and monoclinic ZrO2, respectively. Fig. Fig. 3. Nitrogen adsorption isotherm of Meso-ZrO2. The inset shows the low-angle region of the XRD pattern of Meso-ZrO2. Fig. 4. CO2 TPD profiles of catalysts. (a) ZrO2-HT(393), ZrO2-HT(493) and ZrO2-HX-C, b) all samples. Fig. 5. In situ IR spectra of CO2 adsorbed on ZrO2-HX-C (top), ZrO2-HT(493) (middle), and ZrO2-HT(393) (bottom). Spectra were taken firstly in the presence of 1 atm CO2, and then taken after evacuation for 20 min at each temperature. The spectrum of the ZrO2 catalyst after pretreatment has been subtracted from each spectrum. Fig. 6 Relationship between catalytic activity per unit surface area and CO2 TPD peak height (per unit surface area) at (a) 375 K, (b) 525 K.

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Fig. 1. TEM images of ZrO2 nanoparticle catalysts. a) ZrO2-HT(393), b) ZrO2-HT(433), c) ZrO2-HT(473), d) ZrO2-HT(493), e) ZrO2-HT(513), and f) dark field image of ZrO2HT(513).

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Fig. 2. XRD patterns of ZrO2 nanoparticle catalysts. a) ZrO2-HT(393), b) ZrO2-HT(433), c) ZrO2-HT(473), d) ZrO2-HT(493), e) ZrO2-HT(513), f) Meso-ZrO2, and g) ZrO2-HX-C. h) and i) are calculated patterns of tetragonal and monoclinic ZrO2, respectively. Figure 2

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250

3

200 150

Intensity / a.u.

-1

300

Adsorption / cm (STP) g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

2

4 6 8 2Theta/degree

10

100 50 0

0.0

0.2

0.4

0.6

0.8

1.0

P / P0

Fig. 3. Nitrogen adsorption isotherm of Meso-ZrO2. The inset shows the low-angle region of the XRD pattern of Meso-ZrO2.

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Fig. 4. CO2 TPD profiles of catalysts. (a) ZrO2-HT(393), ZrO2-HT(493) and ZrO2-HX-C, b) all samples.

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Fig. 5. In situ IR spectra of CO2 adsorbed on ZrO2-HX-C (top), ZrO2-HT(493) (middle), and ZrO2-HT(393) (bottom). Spectra were taken firstly in the presence of 1 atm CO2, and then taken after evacuation for 20 min at each temperature. The spectrum of the ZrO2 catalyst after pretreatment has been subtracted from each spectrum.

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Fig. 6 Relationship between catalytic activity per unit surface area and CO2 TPD peak height (per unit surface area) at (a) 375 K, (b) 525 K.

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TOC graphics

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ZrO2 Nanocrystals As Catalyst for Synthesis of Dimethylcarbonate

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