Dimethyl carbonate (DMC) has attracted much attention in the chemical field as an environmentally benign and biodegradable chemical in recent years ^{[1, 2, 3, 4]}. The applications of DMC have covered a variety of fields ^{[5, 6]}. As reported previously,several reaction routes have been presented for DMC formation ^{[7, 8]}. The direct synthesis of DMC from methanol and CO₂ is highly favorable since such a method is environmentally benign by nature ^[9]. A great many catalysts have been employed in the direct synthesis of carbonic ester over recent decades,such as Cu based catalysts ^{[10, 11]},cerium-based catalysts ^{[12, 13, 14]},and zirconia-based catalysts ^{[1, 15]}. Among them,nanoceria has shown the favorable catalytic performance for the reaction of methanol and CO₂ ^[16].

However,the DMC yield is far from satisfactory because of the thermodynamic limitation of the direct synthesis of DMC reaction. In situ removal of water coproduced in the reaction system is an important strategy to effectively improve the yield of DMC. Several dehydration agents have been applied to remove H₂O from the reaction system ^[17]. It is convenient to add physical drying agents to decrease H₂O because there is no other product coproduced in the reaction system,but it is extremely difficult for physical drying agents to absorb water from the mixture of methanol and CO₂ at high temperature ^[18]. Therefore,a variety of chemical dehydrated reagents,such as butylene oxide ^[19],CH₃I ^[20] and acetonitrile ^[21],have been exploited for in situ removal of water,which has the advantage of working even far above 100 ℃ Compared with other dehydration agents,2-cyanopyridine is easy to hydrolyze and regenerate by dehydration very conveniently,especially,the hydrolysis of 2-cyanopyridine which also can be catalyzed by CeO₂ ^[22]. This is more favorable for a co-existed reaction system using the same catalyst for DMC formation. Therefore,the DMC yield improved effectively using 2-cyanopyridine as the dehydrated agent over CeO₂ catalyst ^{[23, 24]}. CeO₂ possesses three crystal faces (1 1 0),(1 1 1),and (1 0 0). The theoretical simulations have indicated that the different crystal faces exhibit different properties, further influencing the catalytic activity ^[25]. Our preliminary studies have shown that rod-CeO₂ with the most (1 1 0) crystal face showed more favorable catalytic performance than cube-CeO₂ (1 0 0) and octahedron-CeO₂ (1 1 1) for the direct synthesis of DMC ^[26]. It is therefore necessary to study the effect of the crystal face of CeO₂ on the hydrolysis of 2-cyanopyridine,and its further influence on DMC formation.

On the other hand,the hydrolysis performance of the derivatives of 2-cyanopyridine is likely to be different because the derivatives of 2-cyanopyridine with a similar structure possess a different electron charge of the carbon in the cyanogroup. Thus, it is essential to evaluate the influence of the derivatives of 2-cyanopyridine with the different substituents on DMC formation. This paper reports the effective synthesis of DMC from methanol and CO₂ with the co-existed hydrolysis reaction of 2-cyanopyridine and its derivatives over CeO₂ catalysts. The electronic charge number of the carbon in the cyano group of the 2-cyanopyridine and its derivatives with different substitute atoms and heterocyclic ring atoms were evaluated through DFT calculations. The influence of morphologies of CeO₂ with different crystal faces (1 1 0),(1 0 0) and (1 1 1) on the hydrolysis of the 2-cyanopyridine was carried out to determine the active crystal face for the hydrolysis reaction and co-existed formation of DMC.

Amorphous CeO₂ was prepared by calcinating the Ce(NO₃) ₃▪6H₂O at 600 ℃ for 3 h. This kind of catalyst was used to optimize the dehydration agents.

CeO₂ catalysts with the different morphologies were prepared by the hydrothermal method. For the octahedron-CeO₂,4 mmol Ce(NO₃)₃▪6H₂O and 0.04 mmol Na₃PO₄▪12H₂O were dissolved in distilled water,respectively with 80 mL of distilled water. After being stirred at room temperature for 1 h,the solution was put into the 100 mL Teflon-lined stainless autoclave. Subsequently,the autoclave was transferred to the drying oven and heated for 12 h at 160 ℃ The precipitates were obtained by filtration,washed with ethanol and deionized water several times,and then dried at 80 ℃ for 12 h and calcined at 600 ℃ for 5 h to get octahedron CeO₂.

A hydrothermal process was adopted to obtain rod-CeO₂ and cube-CeO₂. 480 mmol NaOH and 4 mmol Ce(NO₃)₃▪6H₂O were dissolved in 10 and 70 mL of deionized water,respectively. After being stirred at room temperature for 30 min,the purple slurry was transferred into a 100 mL Teflon-lined stainless autoclave and heated at 90 ℃ (170 ℃) for 24 h to obtain rod-CeO₂ (cube-CeO₂). After the hydrothermal treatment,the precipitates were separated by filtration,washed with deionized water and ethanol several times. After drying at 80 ℃ for 24 h,the products were calcined at 600 ℃ for 5 h.

The morphology micrographs of the CeO₂ samples were taken by both a JEM-2100F transmission electron microscope (TEM) and a high resolution transmission electron microscopy (HRTEM) by using an accelerating voltage of 200 kV. The CeO₂ samples were characterized by a scan electron microscope (SEM) conducted under high vacuum on a Hitachi S-4800.

The reaction was carried out in a stainless steel autoclave reactor with an inner volume of 100 mL. In a typical procedure, 15 mL CH₃OH,50 mmol 2-cyanopyridine or its derivatives and 0.1 g catalyst were transferred into the autoclave before the reactor was purged with CO₂. After that,the autoclave was pressured with CO₂ to 5 MPa and heated to 140 ℃ for 3 h with mechanical stirring.

The stable structure of the 2-cyanopyridine and its derivatives was optimized by the DFT/B3LYP/DNP. All the atoms in the stable structure were further conducted by a population analysis.

In a typical procedure for the hydrolysis of 2-cyanopyridine to 2-picolinamide,0.1 g 2-cyanopyridine,3 g H₂O,and 0.03 g CeO₂ with different morphologies were added into the autoclave. The resulting mixture was vigorously stirred at 140 ℃ After the reaction,the mixture was extracted with CHCl₃ three times.

In the absence of 2-cyanopyridine,the yield of DMC is only 8.6mmol g cat^-1 in the direct synthesis reaction frommethanol and CO₂ over amorphous CeO₂ catalyst. It is noticeable that themaximal production of the DMC reached 350.7mmol g cat^-1 with the addition of 50mmol 2-cyanopyridine. It is a much higher yield than that obtained in the presence of 2,2-dimethoxypropaneby using CeO₂-ZrO₂ as a catalyst,which reported that the DMC yield increased from 1.6mmol g cat^-1 to 14.0mmol g cat^{-1 [27]}. Therefore, it isquite effective for the promotionofDMCformationbyusing the in situ hydrolysis of 2-cyanopyridine over a CeO₂ catalyst.

Table 1 illustrates the dependence of DMC formation in the direct synthesis reaction of DMC over amorphous CeO₂ with different dehydration agent. As Table 1 shows,the highest yield of DMC,350.7 mmol g cat^-1,was obtained with the addition of 2- cyanopyridine.

Table 1

Table 1 The yield of DMC with different dehydration agents.

Table 1 The yield of DMC with different dehydration agents.

According to the reaction path reported previously (shown in Fig. 1) ^[22],the nitrile hydration by CeO₂ starts with the dissociation of H₂O on CeO₂ to give H^δ+ and OH^δ- (Step 1). Nitrile and CeO₂ change to the nitrile-CeO₂ adsorption complex (Step 2). This nitrile-CeO₂ complex will undergo an addition of H^δ- to a nitrile carbon atom to give the amide,accompanied by a regeneration of the adsorption site on CeO₂ (Step 3). Step 3 is the rate-determining step.

Fig. 1

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Fig. 1.The mechanism of 2-cyanopyridine hydrolysis [22].

From the reaction route of Step 3,the nitrogen (or oxygen) in the ring is adsorbed on the Ceⁿ⁺. The more negative charge (which indicates the more Lewis basicity) of the nitrogen (or oxygen) atom,the more easy it is to hydrolyze. On the other hand,the OH^δ- attacks the carbon in the cyanogroup. The more positive charge of the carbon is a benefit to hydrolyzation. Thus,it can be seen that the electronic charge of the carbon atom in the CN and the nitrogen (or oxygen) atom in the ring are vital to the hydrolysis of the cyanogroup. Fig. 2 shows the DFT calculation results of a carbon charge in the CN and nitrogen charge in the ring of 2-cyanopyridine and its derivatives. From the data we can see that the 2- cyanopyridine possessed the most negative charge of the nitrogen and most positive charge of the carbon in the cyanogroup. Comparatively,for its derivatives having an electron-withdrawing group of F,Cl,and Br,the electronic charge number of the carbon was 0.260,0.241,and 0.198,respectively,which is lower than that of 2-cyanopyridine. Although the substitution of hydrogen of 2- cyanopyridine by the F,Cl,Br is favorable for the enhancement of the positive charge number of the carbon in the cyanogroup,the electronic charge number of the carbon in cyanogroup is also affected by the nitrogen atoms in the ring of the 2-cyanopyridine and its derivatives,which has the opposite effect on the carbon. As shown in Fig. 2,the electronic charge number of the nitrogen in the ring of 2-cyano-5-fluorpyridine,5-chloro-2-cyanopyridine, and 5-bromo-2-cyanopyridine was less negative than that of 2-cyanopyridine. So,the carbon in the cyanogroup of the 2- cyanopyridine has a more positive charge number than its derivatives because of the electron-withdrawing effect of the nitrogen in the ring. With the addition of the 2-cyanopyridine, the yield of DMC is 350.7 mmol g cat^-1. Comparatively,2-cyano-5- fluorpyridine,5-chloro-2-cyanopyridine,and 5-bromo-2-cyanopyridine yielded only 333.8,163.2 and 59.4mmol g cat^-1 of DMC, respectively,which suggests that a substitute group at the 5- position relative to the nitrogen atom is not favorable for the DMC formation. So,the hydration activity of cyano group is a positive correlation with the electronic charge number of the carbon in the cyanogroup. The 2-cyanopyridine,which had the highest electronic charge number of the carbon in the CNand the most negative charge in the nitrogen,was suitable for dehydration and DMC formation.

Fig. 2

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Fig. 2.Electronic charge of C atom and N atom of the different dehydration agents.

As to 2-cyanopyridine and 2-cyanofuran,as shown in Fig. 2,the oxygenof2-cyanofuranhas amorenegative chargenumber thanthe nitrogen of 2-cyanopyridine,indicating its more strong adsorption ability on Ceⁿ⁺. However,the charge number of the carbon atom in the cyanogroup of 2-cyanofuran is less than that in 2-cyanopyridine, leading to the weak adsorbing ability of OH^δ-. With the addition of 2-cyanopyridine and 2-cyanofuran,the yield of DMC was 350.7mmol g cat^-1 and231.9mmol g cat^-1,respectively. Itdemonstrates that the hydrolysis capacity of the cyanogroup with heteroatom N located adjacent to the a carbon in the CN group is superior to the cyanogroup with heteroatom O.

Morphologies and crystal sizes of the three ceria catalysts were characterized by SEM,TEM,and HRTEM,as shown in Figs. 3-5. Fig. 3a is the SEM image of the octahedron CeO₂. The octahedrons possessed perfect morphology and mostly perfect crystallinity with smooth surfaces and clear-cut edges and corners. It is noticeable that the predominately crystal faces of the octahedron CeO₂ (HRTEM image in Fig. 3c) was (1 1 1) planes with an interplanar spacing of 0.311 nm calculated from the FFT pattern (Fig. 3c). From the SEM image of uniform cube-CeO₂ exhibited in Fig. 4a,the edge length of the cube-CeO₂ was about 50 nm. The interplanar spacing was calculated to be 0.263 nm of the mainly exposed planes which was corresponding to (1 0 0) planes of CeO₂, showing that the cube-CeO₂ was comprised by six (1 0 0) planes (Fig. 4c). This could be clearly seen in the staggered distributed rod-CeO₂ from Fig. 5a. Nano-rods had a diameter of 4 nm and a length ranging from 60 nm to 200 nm (Fig. 5a). The TEM images revealed that the nano-rods exposed a large amount of (1 1 0) with small (1 1 1) planes (Fig. 5c).

Fig. 3

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Fig. 3.Images of octahedron-CeO2 particles: (a) SEM, (b) TEM, (c) HRTEM.

Fig. 4

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Fig. 4.Images of cube-CeO2 particles: (a) SEM, (b) TEM, (c) HRTEM.

Fig. 5

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Fig. 5.Images of rod-CeO2 particles: (a) SEM, (b) TEM, (c) HRTEM.

Catalytic tests in DMC synthesis from CO₂ and methanol were performed over ceria catalysts with the different morphologies of octahedron-CeO₂,rod-CeO₂and cube-CeO₂,respectively. The results are presented in Table 2. The three samples exhibited different catalytic activities,in the order of rod-CeO₂ > octahedron- CeO₂ > cube-CeO₂. Rod-CeO₂ catalysts exhibited the best catalytic performance,12.8mmol g cat^-1 of DMC,nearly 8.5 times of the DMC yield over cube-CeO₂.

Table 2

Table 2 The DMC yield with the different morphologies of CeO2.

Table 2 The DMC yield with the different morphologies of CeO2.

The results of the addition of 2-cyanopyridine to the DMC direct synthesis system under the same condition with a different morphological catalyst were also displayed in Table 2. It could be clearly seen that the DMC yield increased surprisingly up to 378.5 mmol g cat^-1 from 12.8 mmol g cat^-1,showing that rod- CeO₂ which exposed the most (1 1 0) crystal face,exhibited outstanding catalytic performance in the hydrolysis of 2-cyanopyridine, further speeding up the DMC formation significantly. Comparatively,the least DMC yield of 9.6 mmol g cat^-1 from CO₂ and methanol was obtained over cube-CeO₂ with the (1 0 0) crystal face in the presence of 2-cyanopyridine,meaning that it had the smallest activity for the 2-cyanopyridine hydration among the three ceria catalysts with different morphologies. The (1 1 0) crystal face manifested better activity than (1 1 1) and (1 0 0) for DMC formation co-existing with 2-cyanopyridine hydration. Thus, it can be inferred that (1 1 0) plane is an active factor for hydrolysis of 2-cyanopyridine,further accelerating the formation of DMC.

To further verify the favorable catalytic performance of rod- CeO₂ exposing the most (1 1 0) crystal faces for the 2-cyanopyridine hydration,the hydrolysis of the 2-cyanopyridine catalyzed by CeO₂ with different morphologies was conducted,respectively, and the results were shown in Table 3. It is worth noting that rod- CeO₂with a crystal face of (1 1 0) exhibited remarkable catalytic performance with the conversion of 2-cyanopyridine reaching 93.7%,in comparison to the conversion of 6.7% and 21.4% with CeO₂ (1 0 0) and (1 1 1). Therefore,(1 1 0) is the most active crystal face for 2-cyanopyridine hydrolysis. The much higher DMC yield was obtained because of the effective removal of water by the in situ hydrolysis of 2-cyanopyridine over rod-CeO₂ exposing the most (1 1 0) crystal face in the co-existed reaction system.

Table 3

Table 3 The conversion of 2-cyanopyridine with the different morphologies of CeO2.

Table 3 The conversion of 2-cyanopyridine with the different morphologies of CeO2.

The synthesis of dimethyl carbonate from carbon dioxide and methanol was drastically enhanced by the in situ hydrolysis of 2- cyanopyridine over the CeO₂ catalyst. The hydrolysis properties of cyanogroup of 2-cyanopyridine and its derivatives were related to the electronic charge number of carbon in the cyanogroup. 2- Cyanopyridine with the highest carbon electronic charge in cyanogroup was the most preferable dehydrant,correspondingly, resulting in the highest DMC yield. The DMC yield obtained by the different morphological CeO₂ showed that rod-CeO₂ with a crystal face of (1 1 0) exhibited better activity than cube-CeO₂ (1 0 0) and octahedron-CeO₂ (1 1 1). (1 1 0) planes were active factors for the hydrolysis of 2-cyanopyridine,further enhancing the DMC formation by in situ removing water. The maximum DMC yield reached up to 378.5 mmol g cat^-1 over the rod-CeO₂ catalyst, which was a 30-fold increase in comparison to that obtained without any water removal.

Financial support by Natural Science Foundation of China (NSFC,Nos. 21176179,U1462122),and the Program for New Century Excellent Talents in University (No. NCET-13-0411) is gratefully acknowledged.