LI Feng ，LIAO Yun-hui ，ZHAO Ning ，XIAO Fu-kui

（1. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China；2. College of Chemistry and Bioengineering, Hunan University of Science and Engineering, Yongzhou 425199, China）

Abstract: Versatile and environmentally benign dimethyl carbonate (DMC) synthesized by propylene carbonate (PC) and methanol via transesterification is green and energy efficient. A series of solid base catalysts derived from F-Ca-Mg-Al layered double hydroxides (LDHs) with different NaF amount were prepared, characterized and tested for the transesterification reaction. The properties of the catalysts modified by fluorine have improved obviously. The catalytic activity increases in the order of: FCMA-0.8 > FCMA-0.4 > FCMA-1.2 > FCMA-1.6 > FCMA-0, which is consistent with the total basic sites amount and the strong basic sites amount. FCMA-0.8 has the best catalytic activity as pure CaO catalyst, and the PC conversion, DMC selectivity and DMC yield are 66.8%, 97.4% and 65.1%, respectively. Furthermore, the DMC yield for FCMA-0.8 just decreased 3.9% (33.2% for CaO catalyst) after 10 recycles. FCMA-0.8 has good prospects in the transesterification of PC with methanol to DMC for industrial application.

Key words: dimethyl carbonate；methanol；solid base；NaF；transesterification

Dimethyl carbonate (DMC) is a versatile and environmentally benign feedstock for organic synthesis, and can be widely employed in methylation,carbonylation and methoxylation reactions as reagent in place of the more toxic, corrosive and hazardous substances phosgene, methyl chloroformate and dimethyl sulfate[1?4]. It can also be used as a monomer for the polycarbonate synthesis[5]. In addition, DMC can be used as an effective gasoline additive due to its high octane value and oxygen content[6,7]. Moreover, DMC can be applied in the lithium ion cell field as electrolyte because of its strong solvation to lithium ion[8].

DMC is now industrially produced via transesterification from propylene carbonate (PC) and methanol due to its simplicity, no corrosion, safety,mild reaction conditions, high yield and co-production of valuable propanediol (PG) as by-product[9,10]. The traditional homogeneous catalyst such as sodium methoxide is used for this reaction because of the high reaction rate and high yield, but it is hard to separate the products and catalysts and recover the catalyst,which leads to the DMC production cost increase.Therefore, the demand of the heterogeneous catalysts for transesterification is high on account of the advantage of simple separation of products and catalysts.

It has been concluded that basic catalysts are effective for this transesterification reaction[11]. Calcium oxide solid base catalyst shows better catalytic activity but the stability is poor caused by the active phase leaching, so a lot of works have been done to improve the catalytic activity and recyclability of dimethyl carbonate synthesis by transesterification of methanol with propylene carbonate over Ca-based solid basic catalyst[5,12?18]. It was found that Ca-based solid base catalysts derived from layered double hydroxides(LDHs) have good performance for the reaction. The works screened out that magnesium and fluorine were the best modified metal cation[16]and halogen anion[17]respectively for Ca-Al based solid base catalyst.

Then, this paper did the following work:Magnesium and fluorine were adding simultaneously to the Ca-Al based solid base catalyst derived from layered double hydroxides, i.e. form the F-Ca-Mg-Al hydrotalcite precursor. A series of catalysts with different NaF amount adding were prepared, and their physico-chemical properties and DMC synthesis activities were investigated.

1 Experimental

1.1 Catalyst preparation

F-Ca-Mg-Al LDHs were synthesized by a coprecipitation method. 30 mmol CaCl2, 10 mmol MgCl2·6H2O and 20 mmol AlCl3·6H2O were dissolved in 100 mL distilled water. 0.1 mmol NaOH and appropriate amount of NaF were dissolved in 100 mL distilled water. Subsequently, the above two solutions were added dropwise to 250 mL water-ethanol (2∶3,v/v) solution, which was stirred in N2atmosphere at 333 K. The mixture was aged under this condition for 24 h. Then the precipitate was filtered and washed with deionized water. After that, the sample was dried at 333 K under vacuum for 24 h, which was called the catalyst precursor. The sample was calcined at 1073 K under N2for 6 h to attain the solid base catalyst. A series of samples which the molar ratios of F/Ca were 0, 0.4, 0.8, 1.2 and 1.6 were prepared, and then the catalyst precursors and calcined catalysts were labeled as FCMAP-nand FCMA-n(nis the molar ratio of F/Ca), respectively.

1.2 Characterization of catalysts

Powder X-ray diffraction (XRD) was performed on a D8 Advance (Bruker, Germany) diffractometer with CuKα (0.15406 nm) radiation. The tube voltage and tube current were 40 kV and 50 mA. The scan rate was 3(°)/min and the 2θwas in the range of 5°–80°.

Thermal gravity-differential thermal gravity (TGDTG) analyses were measured by a Rigaku TG 8120 instrument. The samples were heated at 10 K/min from room temperature to 1173 K under N2atmosphere.

The morphologies of the samples were investigated using a JSM-7001F scanning electron microscope (SEM) with an accelerating voltage of 10 kV. The samples were coated with a gold film.

The BET specific surface areas and pore volumes of the samples were tested on a Micromeritics TristarⅡ 3020 instrument. The samples were pre-degassed at 473 K for 2 h.

X-ray photoelectron spectroscopy (XPS)measurements were performed on an AXIS ULTRA DLD spectrometer equipped with monochromatic AlKα (1486.8 eV) radiation under ultrahigh vacuum. The binding energy (BE) values were calibrated internally by adventitious carbon deposit C 1speak withBE=284.8 eV (experimental error within ± 0.1 eV).

The element chemical analysis of Ca, Mg and Al was measured with the inductively coupled plasmaoptical (ICP) emission spectroscopy on a Thermo iCAP 6300 instrument. Fluorine anion was measured with an ionic chromatography (881 Compact IC Pro, Metrohm)with conductivity detection.

CO2temperature programmed desorption (CO2-TPD) was performed on a Builder PCA-1200 chemical adsorption instrument. Each sample was first pretreated in a He flow of 40 mL/min at 1073 K for 1 h. After cooling to room temperature, the catalyst was saturated with pure CO2at 40 mL/min for 30 min, and then He flow was switched for 30 min to remove residual CO2.Afterward, the desorption was started under He flow(40 mL/min) with a heating rate of 10 K/min from 323 to 1173 K, and the desorbed CO2was detected by a thermal conductivity detector.

1.3 Catalytic tests

All catalysts were treated under N2at 1073 K for 1 h and transferred immediately to a three neck flask with a spiral condenser. The reaction temperature was 333 K. DMC synthesis from methanol and PC can be described as the following:

wheremPC1andmPC2are the mass of PC in the feed and product;mDMCis the DMC mass of the product;MPCandMDMCare the molar mass of PC and DMC.

For recyclability tests, the catalyst was treated as follows: when the reaction finished and the temperature decreased to the room temperature, the solid catalyst and the liquid were separated by centrifugation, and then the solid catalyst was dried at 333 K under vacuum for 24 h; afterwards, the catalyst was put into the reactor to the next reaction.

2 Results and discussion

2.1 Structural characterization

The XRD patterns of catalyst precursors FCMAPnare shown in Figure 1(a). The crystallinity of these samples is high, and symmetric characteristic peaks assigned to the reflections of the (002), (004) and (020)planes of the Ca-Al LDHs are observed[19]. Compared with catalyst precursor FCMAP-0, the crystallinity of the fluorine modified catalyst precursors is higher.With the increasing of fluorine amount, the characteristic peaks strength of the fluorine modified catalyst precursors first increases and then decreases,and the maximum is attained when the fluorine amount is 0.8.

Figure 1 XRD patterns of (a) FCMAP-n precursors and (b)FCMA-n catalysts

It can be seen that the layer structure of LDHs is collapsed after calcination for all samples (Figure 1 (b)).Cubic CaO phase (2θ= 32.2°, 37.4° and 53.8°),mayenite Ca12Al14O33phase (2θ= 18.1°, 27.7°, 29.7°,33.3°, 36.6°, 41.1°, 46.5°, 52.7°, 55.0°, 57.2°, 66.8°and 71.8°) and MgAl2O4phase (2θ= 21.2°) are observed for all catalysts, and CaF2phase (2θ= 34.9°,42.4° and 45.3°) is detected for the fluorine modified catalyst.

Thermal decomposition of the catalyst precursors FMCAP-nwas tested by TG analysis and the TG-DTG curve was depicted in Figure 2. It shows the typical thermal decomposition stages of LDHs structures which contains three major weight losses[20]. The first weight loss at 300?440 K can be assigned to the removal of physically absorbed and interlayer water.With the increasing of fluorine amount, the weight loss peak increases, which might be generated by the enhancement of interaction between water molecule and interlayer F-[21]. The second weight loss over the range of 440?660 K can be ascribed to the removal of water through de-hydroxylation from the LDHs network, which results in the collapse of the layered structure. Due to the ion interaction between layers and layers increase, the decomposition temperature also moves to the higher when fluorine added. The former two weight loss peaks variation indicates that the fluorine amount affects the interaction of the sample ions. The third weight loss at 660?850 K can be attributed to the decomposition of the LDHs subjectobject structure.

Figure 2 TG-DTG curves of FMCAP-n

The results of TG-DTG indicate that the catalyst precursors FCMAP-nshould be decomposed thoroughly after 1073 K which leads to the collapse of LDHs structure. This is agreed with the results of XRD. Furthermore, the different decomposition process of the catalyst precursors FCMAP-nalso manifests that there are the different properties of FCMAP-nand FCMA-n, respectively.

Figure 3 shows the SEM images of catalyst precursors FCMAP-nand catalyst FCMA-n. It can be seen that the precursors consist mainly of homogeneous and thin plate-shaped crystals, which suggests the formation of the layered structure[22]. With the increase of fluorine amount, the size of the crystals is not different obviously, while there are some small particles on the plates for FCMAP-1.6.

After 1073 K calcination, the LDHs structure collapses and transforms into amorphous particles as shown in Figure 3(b). It is clear that the sizes of FCMA-0.4 and FCMA-0.8 are smaller than other samples.

Figure 3 SEM images of (a) FCMAP-n and (b) FCMA-n

The structure parameters of FCMA-nwere listed in Table 1. After 1073 K calcination, the BET surface area and pore volume of fluorine modified catalysts are higher than that of no fluorine modified catalyst(FCMA-0), which maybe lead by the smaller particles forming[23,24]. The values of the two parameters follow the same trend: FCMA-0.8 > FCMA-0.4 > FCMA-1.2 >FCMA-1.6 > FCMA-0.

Table 1 Structure parameters and basicity of the FCMA-n catalysts

2.2 XPS and ICP analyses

The compositions of the FCMA-nwere tested by XPS and ICP, and the data were listed in Table 2. In catalyst preparation process, the input values of Ca, Mg and Al were 30 mmol (50%), 10 mmol (16.7%), and 20 mmol (33.3%), while the data in Table 2 shows that the eventual ratio of Ca, Mg is higher and Al is lower.The Ca and Mg composition (mol %) has the same order: FCMA-0.8 > FCMA-0.4 > FCMA-1.2 > FCMA-1.6 > FCMA-0. Eventual F/Ca atomic ratio is approximate with the input value. The Ca and Mg composition (mol %) order is not in agree with the fluorine amount adding, which means that NaF has selective effect on metal elements in the catalyst preparation process. Furthermore, comparing the data of XPS (surface composition) and ICP (bulk composition), it is clear that the catalyst surface was enriched in Ca and Mg. The basicity of Ca and Mg is higher than Al, so the surface enrichment of Ca and Mg is good for surface basicity of the sample.

Table 2 Composition of FCMA-n catalysts

2.3 Surface basicity of FCMA-n

The surface basicity of FCMA-nwas determined by CO2-TPD, and the profiles are showed in Figure 4 All profiles were deconvoluted into four Gaussian peaks, which could be assigned to weak (α peak),moderate (β peak), strong (γ peak) and super strong (δ peak) basic sites, respectively. These sites are associated with basic OH?groups, Mx+?F?/ Mx+?O2?pairs, unsaturated F?/O2?anions and rearrangement of isolated F?/O2?ions, respectively[16]. The amounts of the sites were listed in Table 1. Compared with the peaks of FCMA-0 which does not contain fluorine, the four peaks move to higher temperature for all the fluorine adding catalysts, which means that the basicity of all basic site types increases when fluorine adding to the Ca-Mg-Al system. The total basic amount and strong basic amount have the same trend: FCMA-0.8 >FCMA-0.4 > FCMA-1.2 > FCMA-1.6 > FCMA-0. The total basic amount trend is agreed with the BET surface area order and (Ca, Mg) composition (mol %) order but not the fluorine amount order, which indicates that the surface basic amount is contributed by (Ca, Mg) exposed amount but not fluorine amount.

2.4 Catalytic performance

The reaction conditions of DMC synthesis by methanol and PC were optimized and the results were showed in Figure 5(a) and 5(b). The best reaction condition is:n(methanol)/n(PC) = 12, catalyst weight =2% of total reactants, 2 h, 333 K. Figure 5(c) exhibits the catalytic performance in the best reaction condition for FCMA-ncatalysts. It is clearly that the PC conversion is improved obviously when the Ca-Mg-Al catalyst is modified by fluorine. The PC conversion and DMC yield have the same trend: FCMA-0.8 >FCMA-0.4 > FCMA-1.2 > FCMA-1.6 > FCMA-0.FCMA-0.8 catalyst has the highest PC conversion,DMC selectivity and DMC yield, which are 66.8%,97.4% and 65.1%, respectively. The recyclability test of FCMA-0.8 was investigated and the result was showed in Figure 5(d). It indicates that the FCMA-0.8 catalyst is very stable, the PC conversion and DMC selectivity remain higher with almost no decline after 10 recycles.

For the reaction of DMC synthesis from methanol and PC, CH3OH is activated to CH3O?by the basic sites, and then CH3O?reacted with PC to produce DMC. The higher basicity and basic amount are benefit for the catalytic performance[25?27]. For FCMA-ncatalysts, the CO2-TPD results draw a conclusion that there are four types of basic sites and the amount of them are different. Associated catalytic activity with catalyst basicity, it is found that PC conversion and DMC yield are increased with the total basic amount increase and linearly related with the strong basic (γ peak) amount (Figure 6). It can be said that the activity of DMC synthesis from methanol and PC is mainly related to the strong basic sites amount, although the reaction can proceed on the different types of basic sites.

In order to demonstrate the catalytic performance improvement of FCMA-0.8, Table 3 summarized the results of our team about the catalytic activity and recyclability of dimethyl carbonate synthesis by transesterification of methanol with propylene carbonate over Ca-based solid basic catalyst derived from layered double hydroxides. Primary catalytic activity of CaO is higher, but its DMC yield decreases 33.2% to 32.6% after 10 times recycles caused by the CaO leaching[17]. Many studies show that the stability of the Ca-based catalysts is improved but at the expense of activity decrease. Through modifying by metal cation[16]and halogen anion[17], and optimizing Ca/Al molar ratio and fluorine amount, a best catalyst FCMA-0.8 was obtained. The activity of FCMA-0.8 has attained the level of pure CaO catalyst and the DMC yield just decreases 3.9% (33.2% for CaO catalyst)after 10 recycles. That is to say that FCMA-0.8 is a very good solid base catalyst for dimethyl carbonate synthesis by transesterification.

Table 3 Catalytic activity and recyclability over Ca-based solid basic catalyst

In order to understand the change of the FCMA-0.8 before and after recycle reaction, some characterizations of the used FCMA-0.8 catalyst were also investigated, and comparison was done with the fresh one. Figure 7(a) and 7(b) are the XRD patterns and CO2-TPD profiles of the fresh and used FCMA-0.8 catalysts. The XRD patterns of the two catalysts are same, and it means that the phase is not change after ten times reaction. Compared with the fresh FCMA-0.8 catalyst, for the used FCMA-0.8 catalyst, the CO2desorption peaks are same and areas are decrease a little, and these indicate that the basicity is maintain and basic amount is lost a little bit. The elemental compositions of the fresh and used FCMA-0.8 catalysts were listed in Table 4. It can conclude that the amount of Ca, Mg and F decline just slightly, and this may be caused by the surface leaching of Ca and Mg. This conclusion also can explain the reason why the basic amount is lost a little bit. The above properties comparisons of the FCMA-0.8 before and after recycle reaction reveal the reason of the catalyst stability.

Table 4 Elemental compositions of the fresh and used FCMA-0.8 catalysts

Figure 7 (a) XRD patterns and (b) CO2-TPD profiles of the fresh and used FCMA-0.8 catalysts

3 Conclusions

A series of F-Ca-Mg-Al layered double hydroxides (LDHs) precursors with different fluorine amount were synthesized and the correlative solid basic catalysts were obtained after 1073 K calcination. The catalysts were applied for dimethyl carbonate (DMC)synthesis by methanol and propylene carbonate (PC).Besides CaO phase, the phase of the catalyst also contains Ca12Al14O33，MgAl2O4and CaF2, which could contribute to the alleviation of Ca leaching. Compared to the FCMA-0 catalyst which has no fluorine modified, the properties of the catalysts modified by fluorine have improved obviously. The BET area, total basic amount, strong basic amount and catalytic activity have the same trend: FCMA-0.8 > FCMA-0.4 >FCMA-1.2 > FCMA-1.6 > FCMA-0. In addition,fluorine adding to the Ca-Mg-Al catalyst also changes the distribution of Ca, Mg and Al. The properties increase trend is not agree with the fluorine amount increase trend, which means that the modification of fluorine to the Ca-Mg-Al catalyst has selectivity and has the optimized amount. It is found that the activity of DMC synthesis from methanol and PC is mainly related to the total basic site amount and the strong basic site amount. FCMA-0.8 has the best catalytic activity, and the PC conversion, DMC selectivity and DMC yield are 66.8%, 97.4% and 65.1%, respectively.The activity is comparable to that of pure CaO catalyst.The recyclability test of FCMA-0.8 was investigated,and the DMC yield just decreased 3.9% (33.2% for CaO catalyst) after 10 recycles, which shows clearly that the stability of FCMA-0.8 is higher and much better than that of pure CaO catalyst. The XRD pattern and basicity of the used catalyst and the fresh catalyst are basically the same. FCMA-0.8 is a high-efficiency solid base catalyst for the transesterification of PC with methanol to DMC, which has good prospects for industrial application.