English

• 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 co-precipitation 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 N₂ atmosphere 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 N₂ for 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-n and FCMA-n (n is 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 Cu Kα (0.15406 nm) radiation. The tube voltage and tube current were 40 kV and 50 mA. The scan rate was 3(°)/min and the 2θ was in the range of 5°–80°.

  Thermal gravity-differential thermal gravity (TG-DTG) analyses were measured by a Rigaku TG 8120 instrument. The samples were heated at 10 K/min from room temperature to 1173 K under N₂ atmosphere.

  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 Tristar Ⅱ 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 Al Kα (1486.8 eV) radiation under ultrahigh vacuum. The binding energy (BE) values were calibrated internally by adventitious carbon deposit C 1s peak with BE = 284.8 eV (experimental error within ± 0.1 eV).

  The element chemical analysis of Ca, Mg and Al was measured with the inductively coupled plasma-optical (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.

  CO₂ temperature 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 CO₂ at 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 CO₂ was detected by a thermal conductivity detector.

  1.3   Catalytic tests

  All catalysts were treated under N₂ at 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:

  $ \begin{split} &{\text{C}}_{\text{4}}{\text{H}}_{\text{6}}{\text{O}}_{\text{3}}\text{(PC) + 2C}{\text{H}}_{\text{3}}\text{OH}\stackrel{{\rm{catalyst}}}{\iff }\\ &{\text{CH}}_{\text{3}}\text{OCOOC}{\text{H}}_{\text{3}}\text{(DMC) + OHCH(C}{\text{H}}_{\text{3}}\text{)C}{\text{H}}_{\text{2}}\text{OH(PG)} \end{split} $

  (1)

  The PC conversion, DMC selectivity and DMC yield were defined as

  $ {\rm{PC}}\;{\rm{conversion}} = \frac{{{m_{{\rm{PC1}}}}{\rm{/}}{M_{{\rm{PC}}}}{\rm{ - }}{m_{{\rm{PC2}}}}{\rm{/}}{M_{{\rm{PC}}}}}}{{{m_{{\rm{PC1}}}}{\rm{/}}{M_{{\rm{PC}}}}}} \times {\rm{100}}\% $

  (2)

  $ {\rm{DMC}}\;{\rm{selectivity}} = \frac{{{m_{{\rm{DMC}}}}{\rm{/}}{M_{{\rm{DMC}}}}}}{{{m_{{\rm{PC1}}}}{\rm{/}}{M_{{\rm{PC}}}}{\rm{ - }}{m_{{\rm{PC2}}}}{\rm{/}}{M_{{\rm{PC}}}}}} \times {\rm{100}}\% $

  (3)

  $ {\rm{DMC}}\;{\rm{yield}} = \frac{{{m_{{\rm{DMC}}}}{\rm{/}}{M_{{\rm{DMC}}}}}}{{{m_{{\rm{PC1}}}}{\rm{/}}{M_{{\rm{PC}}}}}} \times {\rm{100}}\% $

  (4)

  where m_PC1 and m_PC2 are the mass of PC in the feed and product; m_DMC is the DMC mass of the product; M_PC and M_DMC are 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 FCMAP-n are 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

  

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

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  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 Ca₁₂Al₁₄O₃₃ phase (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 MgAl₂O₄ phase (2θ = 21.2°) are observed for all catalysts, and CaF₂ phase (2θ = 34.9°, 42.4° and 45.3°) is detected for the fluorine modified catalyst.

  Thermal decomposition of the catalyst precursors FMCAP-n was 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 subject-object structure.

  Figure 2

  

  Figure 2.  TG-DTG curves of FMCAP-n

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  The results of TG-DTG indicate that the catalyst precursors FCMAP-n should 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-n also manifests that there are the different properties of FCMAP-n and FCMA-n, respectively.

  Figure 3 shows the SEM images of catalyst precursors FCMAP-n and 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.

  The structure parameters of FCMA-n were 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.

  Figure 3

  

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

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  Table 1

  

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

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  Catalyst	SBET/ (m2·g−1)	vpore/ (cm3·g−1)	CO2 uptake/ (mmol·g−1)				Total basic amount/ (mmol·g−1)
  			α	β	γ	δ
  FCMA-0	42.3	0.30	0.09	0.23	0.54	0.19	1.05
  FCMA-0.4	48.2	0.39	0.24	0.24	0.68	0.47	1.63
  FCMA-0.8	55.6	0.41	0.23	0.25	0.72	0.53	1.73
  FCMA-1.2	45.3	0.38	0.27	0.17	0.65	0.35	1.44
  FCMA-1.6	44.1	0.32	0.20	0.21	0.58	0.39	1.38

  2.2   XPS and ICP analyses

  The compositions of the FCMA-n were 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

  

  Table 2.  Composition of FCMA-n catalysts

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  Catalyst	Composition/ (mol %)			F/Ca (atomic ratio)
  	Ca	Mg	Al
  FCMA-0	50.9 (50.5)	16.9 (16.7)	32.2 (32.8)	0
  FCMA-0.4	51.5 (50.9)	17.2 (17.1)	31.3 (32.0)	0.38 (0.38)
  FCMA-0.8	52.8 (51.4)	17.6 (17.5)	29.6 (31.1)	0.75 (0.76)
  FCMA-1.2	51.4 (50.8)	17.1 (17.0)	31.5 (32.2)	1.14 (1.14)
  FCMA-1.6	51.3 (50.7)	17.0 (16.9)	31.7 (32.4)	1.55 (1.54)
  the values outside and inside the parentheses were obtained by XPS and ICP measurements, respectively

  2.3   Surface basicity of FCMA-n

  The surface basicity of FCMA-n was 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, M^x+−F⁻/ M^x+−O²⁻ pairs, unsaturated F⁻/O^2‒ anions and rearrangement of isolated F⁻/O^2‒ 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.

  Figure 4

  

  Figure 4.  CO₂-TPD profiles of FCMA-n catalysts

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  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-n catalysts. 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.

  Figure 5

  

  Figure 5.  Catalytic performance of the catalysts

  reaction conditions: (a): catalyst weight = 2% of total reactants, 333 K, 2 h; (b): FCMA-0.8 catalyst, n(methanol)/n(PC) = 12333 K; (c), (d): n(methanol)/n(PC) = 12, catalyst weight = 2% of total reactants, 333 K, 2 h

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  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-n catalysts, 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.

  Figure 6

  

  Figure 6.  Correlation between (a): the DMC yield and the amount of strong basic sites; (b): the DMC yield and the amount of total basic sites; (c): the PC conversion and the amount of strong basic sites; (d): the PC conversion and the amount of total basic sites

  reaction conditions: n(methanol) / n(PC) = 12, catalyst weight = 2% of total reactants, 333 K, 2 h

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  Table 3

  

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

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  Sample	PC conversion/%	DMC selectivity/%	DMC yield/%	DMC yield decrease/%*	References
  CaO	68.8 (35.5)	95.6	65.8 (32.6)	33.2 (10)	[17]
  CA-3	−	−	59.9	10.9 (4)	[18]
  CA-2	53.7 (41.9)	92.8	49.8	4.8 (4), 12.6 (10)	[16,17]
  Mg-CA	55.3 (52.6)	96.3 (92.5)	53.3 (48.6)	4.7 (10)	[16]
  CA-F−	65.9(60.1)	95.3	62.8	5.6 (10)	[17]
  FCMA-0.8	66.8	97.4	65.1	3.9 (10)	this work
  reaction conditions: n(methanol) / n(PC) = 12, catalyst weight = 2% of total reactants, 333 K, 2 h−: data were not reported in the literature,*: data in the parenthesis stand for the recycle times, other data in the parenthesis stand for the corresponding values after 10 times recycles

  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 CO₂ desorption 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.

  Figure 7

  

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

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  Table 4

  

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

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  Catalyst	Bulk compositions/mol %
  	Ca a	Mg a	Al a	F b	Ca∶Mg∶Al	F∶Ca
  Fresh FCMA-0.8 catalyst	36.9	12.6	22.3	28.2	2.93∶1∶1.77	0.76
  Used FCMA-0.8 catalyst	35.1	12.0	25.0	27.9	2.92∶1∶2.08	0.79
  a: determined by the ICP, b: determined by the ionic chromatography

  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，MgAl₂O₄ and 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.

  
  
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  Figure FIG. 1240. 

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

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  Figure 2  TG-DTG curves of FMCAP-n

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

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  Figure 4  CO₂-TPD profiles of FCMA-n catalysts

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  Figure 5  Catalytic performance of the catalysts

  reaction conditions: (a): catalyst weight = 2% of total reactants, 333 K, 2 h; (b): FCMA-0.8 catalyst, n(methanol)/n(PC) = 12333 K; (c), (d): n(methanol)/n(PC) = 12, catalyst weight = 2% of total reactants, 333 K, 2 h

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  Figure 6  Correlation between (a): the DMC yield and the amount of strong basic sites; (b): the DMC yield and the amount of total basic sites; (c): the PC conversion and the amount of strong basic sites; (d): the PC conversion and the amount of total basic sites

  reaction conditions: n(methanol) / n(PC) = 12, catalyst weight = 2% of total reactants, 333 K, 2 h

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

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

  Catalyst	SBET/ (m2·g−1)	vpore/ (cm3·g−1)	CO2 uptake/ (mmol·g−1)				Total basic amount/ (mmol·g−1)
  			α	β	γ	δ
  FCMA-0	42.3	0.30	0.09	0.23	0.54	0.19	1.05
  FCMA-0.4	48.2	0.39	0.24	0.24	0.68	0.47	1.63
  FCMA-0.8	55.6	0.41	0.23	0.25	0.72	0.53	1.73
  FCMA-1.2	45.3	0.38	0.27	0.17	0.65	0.35	1.44
  FCMA-1.6	44.1	0.32	0.20	0.21	0.58	0.39	1.38

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  Table 2.  Composition of FCMA-n catalysts

  Catalyst	Composition/ (mol %)			F/Ca (atomic ratio)
  	Ca	Mg	Al
  FCMA-0	50.9 (50.5)	16.9 (16.7)	32.2 (32.8)	0
  FCMA-0.4	51.5 (50.9)	17.2 (17.1)	31.3 (32.0)	0.38 (0.38)
  FCMA-0.8	52.8 (51.4)	17.6 (17.5)	29.6 (31.1)	0.75 (0.76)
  FCMA-1.2	51.4 (50.8)	17.1 (17.0)	31.5 (32.2)	1.14 (1.14)
  FCMA-1.6	51.3 (50.7)	17.0 (16.9)	31.7 (32.4)	1.55 (1.54)
  the values outside and inside the parentheses were obtained by XPS and ICP measurements, respectively

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

  Sample	PC conversion/%	DMC selectivity/%	DMC yield/%	DMC yield decrease/%*	References
  CaO	68.8 (35.5)	95.6	65.8 (32.6)	33.2 (10)	[17]
  CA-3	−	−	59.9	10.9 (4)	[18]
  CA-2	53.7 (41.9)	92.8	49.8	4.8 (4), 12.6 (10)	[16,17]
  Mg-CA	55.3 (52.6)	96.3 (92.5)	53.3 (48.6)	4.7 (10)	[16]
  CA-F−	65.9(60.1)	95.3	62.8	5.6 (10)	[17]
  FCMA-0.8	66.8	97.4	65.1	3.9 (10)	this work
  reaction conditions: n(methanol) / n(PC) = 12, catalyst weight = 2% of total reactants, 333 K, 2 h−: data were not reported in the literature,*: data in the parenthesis stand for the recycle times, other data in the parenthesis stand for the corresponding values after 10 times recycles

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

  Catalyst	Bulk compositions/mol %
  	Ca a	Mg a	Al a	F b	Ca∶Mg∶Al	F∶Ca
  Fresh FCMA-0.8 catalyst	36.9	12.6	22.3	28.2	2.93∶1∶1.77	0.76
  Used FCMA-0.8 catalyst	35.1	12.0	25.0	27.9	2.92∶1∶2.08	0.79
  a: determined by the ICP, b: determined by the ionic chromatography

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