Mingxue Yin ,Bo Jia ,Kuiyi You ,Bo Jin ,Yangqiang Huang ,,Xiao Luo ,,Zhiwu Liang ,

1 College of Chemistry and Chemical Engineering,the Engineering Research Center of Advanced Catalysis,Ministry of Education,Hunan University,Changsha 410082,China

2 College of Chemical Engineering,Xiangtan University,Xiangtan 411105,China

Keywords:Methanol Urea alcoholysis Dimethyl carbonate Kinetic model

ABSTRACT In this paper,the highly efficient ZnAlLa layered double oxide (ZnAlLa-LDO) catalyst was evaluated and used in methyl carbamate (MC) alcoholysis synthesis of dimethyl carbonate.Under optimal conditions,the MC conversion was 33.5% and the dimethyl carbonate (DMC) selectivity was up to 92.4% at 443 K and in 9 h.The prepared catalysts were well characterized to investigate the effect on the catalytic performance and reaction catalysis mechanism.The experimental results show that the addition of La adjusted the structure and chemical properties of ZnAl composite oxide and that the synergistic effect among Zn,Al and La play a key role in adjusting the acid-base properties and stability of the catalyst,which definitely improved the DMC selectivity and catalytic stability.Based on the proposed reaction mechanism,two kinetic models of the catalytic reaction were established and modified: Langmuir-Hinshelwood and power-rate law kinetic model.The good agreement between kinetic models and experimental data showed that the power-rate law kinetic model based on the elementary reactions is a suitable model for providing a theoretical basis.The pre-exponential factor and activation energy of the main reaction are 5.77 × 107 and 77.60 kJ·mol-1,respectively.

1.Introduction

Dimethyl carbonate (DMC,Fig.S1 in Supplementary Material),as a raw material for organic carbonates and an important homologue of the di-carbonate family [1],can selectively and efficiently replace some of the harmful reagents or additives in chemical industrial processes [2-4],such as phosgene,dimethyl sulfate,chloromethane,methyl chloroformate and methyl tertiary butyl ether (MTBE) [3,5-7].In addition,DMC can react with a variety of compounds to produce many valuable chemical products or intermediates,such as methylation with phenol to synthesize aromatic-based polycarbonate (PC) products [8,9].Recently,DMC has been widely used in the electrolyte of lithium battery production,the need for which is increasing rapidly [10].

Various process routes have been developed for DMC synthesis:phosgenation of methanol,oxidative carbonylation of methanol,direct synthesis of CO2and methanol,electrochemical synthesis,transesterification,and urea alcoholysis [6,11].The phosgene methanol synthesis of DMC is industrially limited due to the toxic feedstock,use of corrosive gases,risk of explosion,and other hazards.The other methods also face some critical problems such as susceptibility to explosions,and economic limitations [3,12-14].Compared with those methods,the synthesis of DMC by urea alcoholysis has advantages such as cheap feedstock,simple technological process,anhydrous alcohol azeotropes generation and environmental friendliness [15-17].

The overall reactions ((Eq.(1) and Eq.(2)) and the main side reaction (Eq.(3)) for synthesis of DMC by urea alcoholysis is as follows:

By thermodynamic calculation,the Gibbs free energy of equation(1)is ΔG0R=4.3 kJ·mol-1[18].In the first step(Eq.(1)the reaction can obtain high MC yield(97%)and high urea conversion(100%)[19].In contrast,the second step (Eq.(2) is the non-spontaneous reaction,which requires high temperature and efficient catalysts to participate in the reaction,and becomes the rate-determining step of the overall reaction process.Therefore,the catalytic performance of catalysts for the second step becomes vitally important for the synthesis of DMC by MC alcoholysis.

A variety of catalysts such as ion exchange resins,ionic liquids and metal oxides [20,21],have been used for DMC synthesisviathis route.For example,a composite oxide calcined of Zn/Fe was used for synthesizing DMC from methyl carbamate and methanol,with 31.5% DMC yield and 47.2% selectivity [21].Another work showed that DMC was synthesized by a series of Zn-Fe-O mixed oxides,with the yield of DMC at 30.7%,and selectivity of only 66.5% [22].Wanget al.[23] used various lanthanum compounds as catalysts to synthesize DMC from MC alcoholysis,and the results showed that La(NO3)3had the best catalytic performance,with 53.7% DMC yield and 63.3% selectivity.Our previous work also reported that Zn-Al mixed oxide catalyst is an effective solid base catalyst for the reaction of MC alcoholysis [24],producing 16.5% DMC yield,and 58.2% selectivity.

Hydrotalcite like compounds are layered double hydroxides(Fig.S2,LDHs),which can be represented by the general formula·mH2O,where M2+and M3+are divalent and trivalent cations,respectively [25-27].The layered double oxides (LDOs) obtained by thermal decomposition of LDHs exhibit a homogeneous dispersion of M2+and M3+at the atomic level with a high resistance to sintering stability,high specific surface area and tune able properties [27,28].Therefore,compared with the other mixed metal oxides such as physical blends and coprecipitation products,LDOs have the advantages of structural and thermal stability.

Recently,it is reported that the doping of rare earth elements can promote the alkalinity and activity of catalytic synthesis of dimethyl carbonate [29].Joeet al.[30] catalyzed the urea alcoholysis with Ce-Zn mixed oxide as catalyst to synthesize diethyl carbonate (DEC) from ethyl carbamate (EC).The DEC and EC yields of 28.8% and 52.9% were obtained at 463 K after 5 h.Kumaret al.[31] prepared Ce-La catalysts by the coprecipitation method,used in the transesterification of propylene carbonate (PC) to DMC,which showed the highest PC conversion and DMC yield of 72%and 74%,respectively.Wanget al.[32] used La-modified Mg-Al composite oxide catalysts in the synthesis of DMC from MC alcoholysis giving 54.3% DMC yield and 80.9% selectivity.Joeet al.[33] also found that ZnO-CeO2-La2O3can significantly improve the synthesis of DMC and achieved a DMC yield of 50.4%.It can be seen that the MC conversion and DMC selectivity are still not high enough for the industrial applications,and more efficient catalysts are still needed.

In this work,in order to find a highly efficient catalyst for synthesis of DMC by MC alcoholysis,ZnAlLa layered double oxide catalyst (denoted as ZnAlLa-LDO) with three metal active centers of ZnAlLa were prepared and evaluated.The catalytic reaction conditions of ZnAlLa-LDO were optimized by single factor experiment.A series of characterization methods were used to determine the physical and chemical properties of the catalyst.The cyclic stability and catalytic reaction mechanism of the catalyst were explained.Finally,the reaction kinetics were studied and simulated.And the innovation of this paper lies in:

(1) In order to solve the problem that there are many side reactions of MC and methanol,the author proposed and applied ZnAlLa-LDO,a calcined product of rare earth hydrotalcite,as a catalyst through reasonable design optimization,and took the selectivity of DMC product rather than the conversion rate of reactants as the main index to evaluate the catalyst activity.

(2) Compared with ZnAl-LDO,the stability and catalytic performance of the catalyst were obviously improved by screening rare earth element La,increasing active sites and reducing the dissolution of active metal Zn through element coordination.

(3) Summarize and put forward two possible acid-base catalytic mechanisms in different directions in the reaction system of ZnAlLa-LDO,establish the kinetics model and parameters based on the reaction mechanism.By comparing the fitting degree of Langmuir-Hinshelwood kinetic model and the power-rate law kinetic model with the components measurements,and considering the number of parameters,the main reaction kinetic parameters of ZnAlLa-LDO reaction system were obtained by fitting and calculating the optima kinetic model.

2.Experimental

2.1.Preparation of catalysts

The ZnAlLa hydrotalcite analogue catalysts were prepared by the urea precipitation method,using urea and metal nitrates(Zn(NO3)2·6H2O,Al(NO3)3·9H2O and La(NO3)3·6H2O),where the atomic ratio of Mg2+:Al3+:La3+was kept at 3:1:0.9,proportionally weighed and dissolved in 100 ml deionized water.A certain amount of urea was weighed and dissolved in another 100 ml of deionized water as well.Ultrasonication was carried out for 30 min to dissolve it fully.The two solutions were then transferred to a 500 ml three-necked flask and heated at 368 K for 8 h with stirring at 600 r·min-1.The reaction product was cooled down,separated by filtration and washed to pH=7.The solid sample was obtained after drying at 358 K for 12 h and recorded as ZnAlLa-LDHs.

After calcination in a muffle furnace for 6 h at 973 K in air,the complex metal hydroxides were oxidized to complex metal oxides,which were recorded as ZnAlLa-LDO.It should be noted that,the preparation processes of the other ternary composite metal oxide catalysts with different metal element M (M=La,Ce,Fe,Co,Zr)were similar to that of ZnAlLa-LDO.

2.2.Characterization

The powder X-ray diffraction (XRD) measurements were recorded on the XRD-6100 instrument with λ=0.1541 nm,using Cu Kα radiation in the 2θ range of 10°-80° at room temperature.The surface areas of the prepared catalysts were obtained using the multipoint Brunauer-Emmett-Teller (BET) method by N2adsorption-desorption on a Micromeritics 2020-HD88 instrument.Carbon dioxide and ammonia were used as a probe molecule to determine the basicity and acidity of the catalyst by CO2/NH3-TPD.Thermogravimetric (TG) analysis of catalyst precursors was characterized using a NETZSCH STA-409 thermal analyzer.In situFourier-transformed infrared absorption of pyridine(Py-IR)spectra were measured using a Tensor 27 spectrophotometer in order to determine the strength of B/L acid sites.The actual mass ratio of the metal in the catalyst was determined by inductively coupled plasma (ICP) emission spectrometer (Agilent 720ES).

2.3.Catalysts evaluation

The experiments of methanol and MC synthesized DMC were carried out in a 100 ml stainless steel autoclave with mechanical stirrer,which can be seen in Fig.S3.A certain amount of raw materials (molar ratio MC: methanol=1:20) and catalyst were firstly added to the reactor (Fig.S3).After checking the gas tightness of the device,the reactor was heated to a certain temperature and stirred at a speed of 600 r·min-1for a fixed time.Then the reactor was cooled to room temperature when the reaction finished and the remaining gas was collected.The remaining solid product was washed sequentially with methanol 3 times,with deionized water 3 times and then dried for 12 h.The qualitative and quantitative analyses of liquid phase products were carried out with a gas chromatography instrument equipped with hydrogen flame ionization detector (FID) and SE-54 capillary column (50 m × 0.32 m m × 1.0 μm).The catalyst activity was evaluated by conversion rate and selectivity of the main reactants and products which including MC,DMC andN-methyl methylcarbamate (NMMC).The calculation of conversion and selectivity is as follows:

The conversion rate of MC:

The selectivity of DMC:

The selectivity of NMMC:

whereniis the amount of each substance in the reaction solution,fiis the relative correction factor of each substance,Aiis the peak area of each substance,Anis the peak area of the internal standard,Miis the relative molecular weight of each substance in the reaction solution,n0is the amount of MC added before the reaction,nMCis the amount of MC in the reaction solution,nDMCis the amount of DMC in the reaction solution,nNMMCis the amount of NMMC in the reaction solution.

2.4.Kinetics experiments

The kinetics of ZnAlLa-LDO for the catalytic synthesis of DMC was investigated in this work.The kinetic studies of DMC synthesis were carried out in the same stainless steel autoclave within the temperature range 433-473 K.The experimental procedure was similar to our previous work[24].A new kinetic model was established and the kinetic parameters were obtained by analyzing the Langmuir-Hinshelwood kinetic model and power-rate law kinetic model,in order to provide a theoretical basis for future industrialization.

3.Results and Discussion

3.1.Screening and optimization of preparation conditions

The doping with different metal elements(Zn:Al:X=3:1:0.5)resulted in varying influences on the MC conversion and DMC selectivity(Fig.1(a)),the doping of La showed an obvious improvement.La,as one of the rare earth elements with the most proved reserves,was found to be able to enhance the amount and strength of medium alkaline sites.It also increases the amount of metal oxygen pairs (La-O) on the catalyst surface,which can partially break up and lead to the generation of coordinated unsaturated O2-ions,causing the lower stability of the layered structure (Fig.S4 and Fig.S5) and being beneficial to the formation of unsaturated O2-ions[32].Therefore,ZnAlLa-LDO was selected as the optimal ternary composite metal oxide catalyst.

Fig.1.The effects of preparation elements of tri-metal oxides catalysts on catalytic performance:(a)doping elements(Zn:Al:X=3:1:0.5),(b)ratio of doping elements Al:La,(c) calcination temperature on the (3:1:0.9) ZnAlLa-LDO,(d) calcination time on the (3:1:0.9) ZnAlLa-LDO.

A series of catalysts(3:1:A)ZnAlLa-LDO(A=0.3,0.5,0.7,0.9,1.1,1.3)were prepared to investigate catalytic performance in the synthesis of DMC by MC alcoholysis.As shown in Fig.1(b),the DMC selectivity showed a maximum value when the molar ratio of Zn,Al and La was 3:1:0.9.The reason probably was that the doping amounts of La affected the moderate and strong basic sites which played a crucial role in MC conversion.However,a too strong basic strength was not beneficial to obtaining a high DMC selectivity,which might be because the side reactions would be aggravated in the presence of strong basic catalysts[32].Therefore,it was reasonable to draw the conclusion that the enhancement of the catalytic performance by La doping was due to the adjustment of the basic properties by incorporation of a suitable amount of La additive.

The influence of calcination temperature on the (3:1:0.9)ZnAlLa-LDO performance was also explored,as can be seen in Fig.1(c).Considering both MC conversion and DMC selectivity,the optimal calcination temperature is at 973 K.The calcination time also has effect on the product selectivity,as shown in Fig.1(d).It was found that 6 h,as the optimal calcination time,led to the highest MC conversion rate with 64.5% of DMC selectivity.

3.2.Characterization of catalyst

3.2.1.Thermogravimetry and differential thermogravimetry(TG-DTG)

TG-DTG analysis shows in Fig.2 respectively,the mass loss and mass loss rate of ZnAl LDHs and ZnAlLa LDHs during calcination.The thermal stability and thermal decomposition of the samples during conversion to mixed oxides were investigated.Obviously,the TG curve of ZnAl LDHs precursor consists of 2 main steps(Fig.2(a)),while there are 4 obvious thermal mass loss stages in the temperature range of 323-1173 K for ZnAlLa LDHs (Fig.2(c)).First,the removal of water molecules adsorbed on the surface of hydrotalcite like compounds,which belonged to physical mass loss,then the removal of OH-and CO32-of the layered structure,and finally,the layered structure of hydrotalcite like compounds collapsed to form composite metal oxide LDO,the thermal mass loss rate was about 17.61%.In the conversion of La hydroxide and hydroxyl carbonate phase to La2O2CO3[34] and the thermal decomposition of La2O2CO3[28,29],where the mass loss rate was about 9.08%.It should be noted that Fig.2(d) shows only 4.42%of thermal mass loss of ZnAlLa LDO,a good thermal stability of ZnAlLa-LDO.It can be concluded that the doping of La made the catalyst form a variety of complexes and coordination bonds while gradually increasing the decomposition temperature.The coordination bonds were further removed to expose more oxygen defects and vacancies,thus improving the catalytic activity,which is consistent with the phenomena in Fig.2(a) and (c).

Fig.2.TG-DTG analysis of (a) ZnAl-LDHs,(b) ZnAl-LDO,(c) ZnAlLa-LDHs,and (d) ZnAlLa-LDO.

3.2.2.X-ray diffraction analysis(XRD)

The XRD patterns of the ZnAlLa-LDHs and ZnAlLa-LDO are shown in Fig.3.The catalyst precursor ZnAlLa-LDHs presented a typical layered structure of hydrotalcite,as shown in Fig.3(a).It also can be seen that,the ZnAlLa-LDHs showed characteristic diffraction peaks of hydrotalcite-like compounds,corresponding to crystalline planes with sharp peaks and good symmetry indicating that the structure of hydrotalcite-like compounds can be formed with Zn,Al,and La as interlayer cations,and precursors had good dispersion of metal elements and high crystallinity.With the doping of La,the LDH phase in the precursor decreased,resulting in other crystalline phases appearing instead.La2O3,La(OH)3and La carbonate[29]were detected in the samples,mainly formed after calcination,which was due to La having the lowest electronegativity compared with other rare earth elements,resulting in the formation of La carbonate species in the coprecipitation[32].At the same time,the relatively large ion radius of La causes it to be in the interlayer channel of LDH as a separate hydroxide and hydroxycarbonate,which produces large distortion of the LDH layer.When the calcination temperature was higher than 773 K,the (003) characteristic diffraction peak of hydrotalcite like compounds disappears completely,indicating that the anions CO32-and OH-were completely removed to form LDO.With increasing temperature,the intensity of the diffraction peak of the sample decreased gradually,and the characteristic diffraction peak of La2O3crystal phase also gradually disappeared.The result was that the crystallinity of the sample decreased,and the sample was gradually sintered at the very high calcination temperature,while the ZnO crystal phase gradually covered the surface of the sample.At the same time,it can be observed that there was no characteristic diffraction peak of Al2O3crystal phase in all the samples,indicating that Al atoms may be evenly dispersed in the lattice composed of Zn and La.

Fig.3.The XRD spectra of (a) ZnAlLa-LDHs and ZnAlLa-LDO prepared at different calcination temperatures: (b) 773 K,(c) 873 K,(d) 973 K,(e) 1073 K.

3.2.3.Surface properties of catalyst

As can be seen from Fig.4(a),the N2adsorption/desorption isotherms of the samples obtained at 4 calcination temperatures showed the internal mesoporous structure of the catalysts.When the calcination temperature was lower than 973 K,typical Ⅳadsorption isotherms with a hysteresis loop can be seen,indicating that the catalysts were mainly composed of small slit pores.However,an H3 type (IUPAC classification) hysteresis loop was found when the calcination temperature was increased to 1073 K,meaning that the pore channels in the catalyst were mainly wedgeshaped pores formed by the accumulation of flaky particles,which was consistent with the gradual removal of anions between layers and the gradual collapse of layered structure when the calcination temperature of hydrotalcite-like compounds increased [24].It can be seen from Fig.4(b) that the pore diameter of the ZnAlLa-LDO catalyst prepared at calcination temperature of 973 K was mostly 7-11 nm,and the average pore diameter was 8 nm.At the same time,Table 1 shows the specific surface area of ZnAlLa-LDO catalyst decreased with the increasing of calcination temperature,which was due to the degree of collapse of the lamellar structure becoming higher and higher until it was completely sintered,resulting in the gradual close fitting of the lamellae and the reduction of the pores of the catalysts.

Table 1 Textural properties of ZnAlLa-LDO at different calcination temperatures

Fig.4.(a) N2 adsorption-desorption curves of catalysts prepared at different calcination temperature.(b) Pore-size distributions of ZnAlLa-LDO at 973 K.

According to the results of Fig.1(c) and Fig.3,it can be concluded that the BET surface area of ZnAlLa-LDO has no significant influence and regularity on the catalytic performance of alcoholysis synthesis of DMC.Therefore,it is speculated that other factors can more obviously affect the performance of the catalyst,for example,the number of moderately strong alkaline or acidic active centers of the catalyst.

3.3.Optimization of reaction conditions

To obtain optimized catalytic process conditions,the effects of various factors including catalyst mass percentage,molar ratio of methanol to MC,reaction temperature,and reaction time on the catalytic performance of the catalyst were studied,and the results are shown in Fig.5.

Fig.5.The effects of reaction conditions on the catalytic performances.(a) Reaction temperature.Reaction conditions: reaction time,9 h;catalyst mass percentage,0.8%;molar ratio of methanol to MC,20:1.(b)Reaction time.Reaction conditions:reaction temperature,443 K;catalyst mass percentage,0.8%;molar ratio of methanol to MC,20:1.(c)Catalyst mass percentage.Reaction conditions:reaction temperature,443 K;reaction time,9 h;molar ratio of methanol to MC,20:1.(d)Molar ratio.Reaction conditions:reaction temperature,443 K;reaction time,9 h;catalyst mass percentage,0.8%.

Fig.5(a) shows the results of the impacts of reaction temperature on the catalytic performance.Note that the MC conversion was very low when the reaction temperature was lower than 433 K,and increased rapidly with the increase of reaction temperature while the DMC selectivity increased firstly and then decreased,and reached the optimal value of 92.4% at 443 K.On the other hand,when the temperature was raised to 493 K,the MC conversion was close to 100%,while the selectivity of DMC and NMMC became very low,which indicated that high reaction temperature caused significant MC and DMC decomposition reactions ((Eq.(8) and Eq.(9)).All of these led to the lower selectivity for DMC.Hence,the optimal reaction temperature in this study was set at 443 K.

Fig.5(b) shows that the MC conversion rate increased steadily with increasing reaction time and that the selectivity of DMC increased at first and then decreased,reaching its maximum value at 9 h.Thus,the optimal reaction time was 9 h.

As shown in Fig.5(c),the MC conversion gradually increased with increasing catalyst dosage,and the DMC selectivity reached the maximum value when the catalyst mass percentage was 0.8%.It also can be seen from Fig.5(d) the conversion of MC decreases with the increase of the molar ratio of methanol to MC.When the molar ratio was smaller than 10,the concentration of MC in the reaction system was large,and the generated DMC reacted easily with MC to produce the by-product NMMC,resulting in low selectivity of DMC,and the degree of MC self-decomposition also increased.In contrast,with the molar ratio of methanol/MC increasing,the concentration of MC became lower and lower,which resulted in the MC conversion and DMC selectivity getting lower.Therefore,the optimum molar ratio of methanol/MC was 20.

In summary,the optimal reaction conditions for synthesis of DMC from MC and methanol over ZnAlLa-LDO catalyst were as follows: the catalyst calcination temperature was 973 K,the molar ratio of methanol to MC was 20,the catalyst mass percentage was 0.8%,and the reaction temperature and reaction time were 443 K and 9 h,respectively.Under these optimal reaction conditions,the MC conversion was 33.5%,the selectivity for DMC reached 92.4%,while the selectivity of the by-product NMMC was only 6.18%.This gave the best catalysis performance compared with the literature data so far,as shown in Table 2.

Table 2 Catalytic performance of the present catalyst compared with other reported catalysts

3.4.Reaction mechanism analysis

3.4.1.Stability of catalysts

Under the optimal reaction conditions,the catalytic stability of the catalyst was investigated.The results of cycling ZnAl-LDO and ZnAlLa-LDO catalysts are shown in Fig.6.It can be seen from the figure that with the increasing of recycling times of the catalyst,MC conversion,DMC and NMMC selectivity gradually decrease,and the downward trend of ZnAl LDO is more obvious,which is caused by the dissolution of the main active element Zn of this reaction,and the exposure of more active sites.With the introduction of La,the prepared ZnAlLa LDO showed obviously higher DMC selectivity,which indicated the introduction of La and a synergistic catalytic effect among Zn,Al and La elements.This might be because the migration of La on the surface enhanced the number and strength of Lewis acid sites,which showed that La played a key role in regulating the acid-base properties of the catalyst.

Fig.6.Stability tests of (a) ZnAl-LDO and (b) ZnAlLa-LDO catalysts for using 3 times;XRD patterns of (c) ZnAl-LDO and (d) ZnAlLa-LDO catalysts for using 3 times,respectively.

In order to investigate the stability and reaction mechanism,a series of characterization analysis were then carried out for the newly prepared catalyst and reused catalyst.

The XRD patterns of ZnAl-LDO and ZnAlLa-LDO samples are shown in Fig.6(b),(d).It can be seen that the positions of all diffraction peaks are basically the same,indicating that the structure of the catalyst has no obvious change before and after participating in the catalytic reaction;compared with the fresh catalyst,the reused catalyst has no new diffraction peak,indicating that there is no new substance or new crystal phase after the catalyst participates in the reaction.

ICP analysis (Table 3) was performed to determine the Zn,Al and La contents in these two types of catalysts (fresh and reused).According to the mass percentage of each element,the molar ratio of Zn,Al and La in the sample can be calculated to be roughly the same as the theoretical molar ratio during preparation.For the reused catalyst,the relative content of Zn decreased,while the relative content of Al and La increased.Al and La were not detected in the reaction solution,but a small amount of Zn was found there,indicating that Zn in both catalysts was partially dissolved in the reaction solution during the reaction process.In comparison,the ZnAlLa-LDO catalyst,the dissolution degree of Zn in the reaction process is greatly reduced in the same trials,indicating that ZnAlLa-LDO has better stability,which is consistent with the phenomenon shown in Fig.6.

Table 3 ICP Analysis results of fresh and reused catalysts

For metal oxide catalysts,two factors including alkaline strength and acidic strength were selected to evaluate their catalytic performance.Here,CO2-TPD and NH3-TPD were used for detection.It can be seen from Fig.7 and Table S1 in Supplementary Material that there were four basic sites and three acidic sites with different strengths on the surface of the ZnAlLa-LDO catalyst,and most of them were moderate and strong basic or acidic sites.According to the description of reported studies,the weak basic sites were related to OH-groups,the moderate basic sites were associated with metal-oxygen pairs (such La-O),and the strong basic sites were assigned to low-coordination unsaturated oxygen atoms [39-41].

Fig.7.CO2-TPD and NH3-TPD profiles of (a) ZnAl LDH,(b) LDO,(c) ZnAlLa-LDH and (d) LDO.

It was observed that the basic and acidic centers on the catalyst surface increased after the reaction.However,the number of acidbase sites in the reused catalyst increased,but the catalytic effect decreased.The reason may be that too many or different acid sites will cause or promote other side reactions,and thus reduce the selectivity of the target product.In addition,we found that there were many more acid-base centers in ZnAlLa-LDO than in ZnAl-LDO.

The specific surface area of the catalyst was investigated by N2adsorption desorption curve.The results are shown in Fig.8 and Table S2.After the reaction,the specific surface area,the pore volume and pore diameter of the catalyst were increased.

Fig.8.N2 adsorption desorption isotherms of catalyst: (a) fresh,(b) reuse.

Based on the analysis of the above results,the reason for the increase of the specific surface area,pore volume and pore diameter of the reused catalyst may be that during the catalytic reaction(Table S3),the Zn in the catalyst partially dissolved into the reaction solution,the alkaline and acidic sites on the surface of the reused catalyst increased,and the specific surface area increased,so the catalytic effect was better.Zn,as an active metal,played an important catalytic role.However,excessive dissolution of Zn will lead to the decrease of catalyst stability,so the conversion rate of MC,the selectivity of DMC and NMMC all gradually decreased.It was found that the catalytic ability of lanthanum compounds mainly comes from La3+,followed by anionic groups and solubility in methanol.La(NO3)3as a catalyst had good catalytic activity for DMC,but using Al2O3or La2O3alone has little effect on the reaction.This may be due to the good solubility of La(NO3)3in methanol,therefore,the introduction of La improved the catalytic activity and the stability of the catalyst.

In view of this reaction phenomenon,in the subsequent catalyst optimization,it may be possible to further improve the catalytic stability of the ZnAlLa-LDO catalyst by loading the catalyst on the support or adopting different preparation methods to reduce the degree of dissolution of Zn in the reaction process.

3.4.2.FT-IR

The adsorption behavior of the solid catalyst surface played an important role in the reaction pathways.In order to investigate the possible mechanism of the reaction between MC and methanol,FTIR experiments under different conditions were conducted.The infrared spectrum shows that the characteristic bands at 2212 cm-1and 1419 cm-1were the stretching vibration of N-C and the deformation vibration of NCO—,respectively.At the same time,N—H (amide band) had obvious tensile vibration near 3441 cm-1,asymmetric deformation vibration of NH3at 1498 cm-1and 1638 cm-1,and tensile vibration of Zn-NH3at 432 cm-1.At 2212 cm-1,the NCO stretching vibration corresponding to the combination with the metal oxide surface (M-NCO)indicated the adsorption on the catalyst surface,indicating that MC may be decomposed into metal isocyanate groups.These were basically consistent with the infrared characterization results of crystal Zn (NH3)2(NCO)2in the reference [24].

It is known that MC decomposes into HNCO and CH3OH at a certain temperature,and the molecular formula of precipitate after reaction was Zn(NH3)2(NCO)2: firstly,ZnO reacted with HNCO to produce Zn(NCO)2and water.In the reaction system,Zn(NCO)2coordinated with NH3and formed a complex in the formula of Zn(NH3)2(NCO)2,as a real active substance to improve the catalytic effect.In addition,C—H asymmetric and symmetric stretching vibrations of deprotonated methanol or methoxy species appeared at 2360 cm-1,indicating the adsorption of methoxy groups(methanol or MC) on the catalyst surface [32].At the same time,the characteristic peak absorption at 689 cm-1showed the possible vibration of metal oxygen on Zn-O,Al-O,La-O.

3.4.3.Mechanism analysis

On the basis of FTIR,XRD and ICP characterization,a possible reaction mechanism was further proposed (see Fig.10).In ZnAl-LDO cases,MC was easy to decompose into HNCO firstly,which can be reacted by ZnO and produce Zn(NCO)2.Then,Zn(NCO)2could coordinate with NH3to form a complex Zn(NH3)2(NCO)2,which can be dissolved in methanol.Here NH3can be replaced by MC molecules to form an intermediate Zn(NH2COOCH3)2(NCO)2[42].After coordination and activation with Zn2+,DMC and NH3were finally generated through a molecular rearrangement reaction from the intermediate,as shown in gray pathway of Fig.9.It indicated that the catalytic effects of Zn would continuously consume HNCO and enhanced MC decomposition reaction,which was consistent with the results of high MC conversion and low DMC selectivity in ZnAl-LDO catalytic experiments.

Fig.9.FT-IR absorbance spectra of ZnAlLa-LDO catalysts for using 3 times,respectively.

Fig.10.Possible reaction mechanism for the DMC synthesis from MC and methanol over ZnAlLa catalyst.

After introducing of La,the competitive route of DMC formation became significant.Here Al and La have different catalytic effects and ways [43].The amino N atom of MC was coordinated with the cationic surface metal (Al or La) of mixed metal oxide and adsorbed on the catalyst surface.As a result,the electrons of C-N group of MC were redistributed,while the protons were transferred in the form of carbon cations and nitrogen anions.At the same time,the catalyst extracts Hδ+to activate methanol and form a strong nucleophilic methoxy group.Finally,a pair of lone electrons of the O combined with the electrophilic carbonyl carbon can form DMC,while the hydrogen proton was separated from the methanol molecule and combined with the free amino group to form NH3.

According to the results of ICP and stability experiments,the doping of La element is beneficial to stabilize ZnO,and the catalytic reaction increases the La path from the original Zn path,which increases the selectivity of DMC.At the same time,the doping of La also weakens the reactivity of Zn,thus reducing the dissolution of Zn and increasing the stability of catalyst.

3.5.Kinetics study

The kinetic model of MC alcoholysis to DMC in the presence of ZnAlLa-LDO catalyst was established,and its kinetic parameters were fitted and estimated in order to determine the reaction law of the reaction at different temperatures.At the same time,the kinetic model that can be used for calculation was established and improved for the reaction system,hoping to provide a theoretical basis for industrial application and optimization of catalytic reaction process.

It is known that it is necessary to eliminate the influence of internal diffusion and external diffusion when determining the kinetic data of catalytic reactions.The catalyst used in this study was fully ground.Therefore,it could be considered that the particle size of the catalyst was small and the influence of internal diffusion control could be eliminated.The external diffusion was related to the turbulence of fluid.The effect of stirring speed on catalytic performance was studied,and the results are shown in Fig.11.Note that when the stirring speed reaches 500 r·min-1,further increasing the stirring speed had little effect on the reaction.In this case,the external diffusion effect could also be eliminated.

Fig.11.Effects of stirring rate on DMC selectivity (a),MC conversion (b),and NMMC selectivity (c).

The kinetic model proposed in this paper was based on the following assumptions: ignoring the influence of internal and external diffusion,and at the same time,the concentration of all points in the reaction system were the same under vigorous stirring;methanol was both a reactant and a solvent in the reaction system,so its concentration can be regarded as a constant.Finally,the change of solution volume during the reaction can be neglected.

3.5.1.Kinetics experiments

Kinetic experiments were carried out at different reaction temperatures (433-473 K) and different reaction times (0-14 h).The experimental conditions of each group are shown in Table S4.The concentration change of each material under different reaction temperatures is shown in Fig.12.When the reaction temperature was higher than 473 K,the concentration change curve of NMMC first increased and then decreased,which indicates that in the ZnAlLa-LDO catalytic reaction system,NMMC also reacted with MC in the reaction solution to produce (CH3)2NCOOCH3[44].

Fig.12.Concentration curves of DMC (a),MC (b) and NMMC (c) under different reaction temperatures.

In the establishment of a kinetic model and estimation of kinetic parameters of ZnAlLa-LDO catalytic reaction system,by comparing the fitting degree of two different kinetic models to the concentration values of various substances measured in kinetic experiments,and considering the number of parameters in the model and its simplicity in industrial application,the most suitable kinetic model for the reaction system was explored.

3.5.1.1.Langmuir-Hinshelwoodkineticmodel.Langmuir-Hinshelwood mechanism is a heterogeneous catalytic mechanism,in which the surface reaction is controlled by two adsorbed molecules.Taking Eq.(2) as an example,according to this mechanism,MC and methanol were adsorbed on acidic sites and weak basic sites of the catalyst,respectively [44].The reaction processes can be expressed as follows:

where*1and*2denote acid sites and weak basic sites,MC*1and CH3OH*2denote the MC and CH3OH occupying the active sites,respectively.

According to the law of surface mass action,the reaction rate can be expressed as:

There are Eqs.(16)-(19)reaction processes in the catalytic reaction system of ZnAlLa-LDO,namely:

Similarly,assuming that the active sites of ester compounds adsorbed on the catalyst surface in the reaction system are the same,and the self-decomposition of MC and DMC is not controlled by the adsorption process,the reaction rate of Eqs.(16)-(19)can be expressed as:

whereriis the reaction rate,θiis the coverage of each substance at the active site,kiis the surface reaction rate constant,niis the reaction order of each reactant,andCiis the concentration of each substance.

3.5.1.2.Power-rate law kinetic model.In the Power-rate law kinetic model,the reaction rate was directly proportional to then-th power of reactant concentration.Based on this model,the rate equations of Eqs.(24)-(28) are:

whereriis the reaction rate,kiis the reaction rate constant,niis the reaction order of each reactant,andCiis the concentration of each substance.

Therefore,based on the above two catalytic reaction kinetic models,the concentration change rate of DMC,MC and NMMC in the reaction system can be expressed as:

3.5.2.Estimation of dynamic parameters

In essence,the process of estimating kinetic parameters is to solve the numerical solution problem of ordinary differential equations based on a set of initial values,that is,the concentration of each substance when the reaction timet=0 h.The concentrations of DMC,MC and NMMC at the beginning of the reaction were 0,1.6496 and 0 mol·L-1,respectively.DefinitionF:

whereais the number of components (a=3),bis the number of time points(b=8),Ci,t,expandCi,t,calare the experimental and calculated concentration values of componentiat each time point.

Through MATLAB software,the fourth-order Runge-Kutta method is used to solve the ordinary differential equations.Finally,the fmincon function in MATLAB is used to optimize the calculation results,and the optimal solution to minimize the value of functionFis found.The solutions of kinetic equations are to determine the values of the parameters.The total average relative deviation was calculated as follows:

whereCexpandCcalare the experimental and calculated concentration values of each component respectively,andnis the total number of experiments.

3.5.2.1.Parameter estimation of Langmuir-Hinshelwood dynamic model.Firstly,the Langmuir-Hinshelwood kinetic model was used to fit the reaction rate equation.In order to calculate the kinetic parameters of each reaction,Arrhenius equation was used to describe the dependence of reaction rate constant on reaction temperature.After the value ofKof each reaction rate equation calculated by MATLAB software at each reaction temperature,lnkis linearly fitted withT-1.The values of the kinetic constants (pre exponential factor and activation energy) of each reaction can be calculated by the slope of the linear regression equation and the intercept with theYaxis respectively.The unit of the preexponential factor was the same as that of the reaction rate constant.Since the reaction order should be a constant that does not change with temperature,the average value of the reaction order calculated at each temperature was taken as the final result.The calculation results of each reaction rate constant,reaction order and reaction kinetic parameters are shown in Table 4 and Table S5 respectively.

Table 4 Langmuir-Hinshelwood pre-exponential factor and activation energy of each reaction in the kinetic model

According to the calculation results,the curve of the concentration of each substance over time predicted by the reaction kinetic model can be obtained.In addition,the comparison of experimental data and corresponding predicted values were performed in Fig.13 which showed 0.3340 of the total average relative deviation(Fig.S6).At the same time,we also calculated the ARD of DMC,MC and NMMC under the Langmuir-Hinshelwood dynamic model by using Eq.(33),which were 0.2700,0.3573,0.3360.

Fig.13.Langmuir-Hinshelwood the experimental values and predicted concentrations of components of DMC(a),MC(b),NMMC(c)at different temperatures in the kinetic model and comparison between experimental and predicted value for all points (d).

3.5.2.2.Theparameterinpower-ratelawkineticmodel.For the power-rate law kinetic model,the kinetic model based on elementary reaction was discussed respectively.Fig.14(a)-(c) are the curves of the experimental concentration values of DMC,MC and NMMC and the predicted concentration of the model in the powerrate law kinetic model based on the elementary reaction,respectively,Fig.14(d) is the comparison between the experimental concentration values of each component and the predicted concentration values of the model.According to the above calculation results (see Fig.S7 and Table S6),the total average relative deviation of the fitting results of the power series kinetic model based on the elementary reaction is 0.2772.The ARD of DMC,MC and NMMC under the power-rate law kinetic model by using Eq.(33),which were 0.2172,03040,0.2791 respectively.

Fig.14.Experimental values and predicted concentration curve of DMC (a),MC (b),NMMC (c) at different temperatures in the power series kinetic model based on elementary reaction and comparison between experimental and predicted value for all points (d).

Similarly,comparing the fitting results of the above models,the total average relative errors of the two different forms of kinetic models in ZnAlLa-LDO catalytic system were not much different,while the number of parameters required to be fitted by the Power-rate law kinetic model based on elementary reaction is the least,so the application of the model in industry is relatively simple.According to Table 5,under this model,the preexponential factor and activation energy of the main reaction in the reaction system of MC alcoholysis to DMC catalyzed by ZnAlLa-LDO catalyst are 5.77 × 107and 77.60 kJ·mol-1respectively.

4.Conclusions

In summary,the high-efficiency ZnAlLa-LDO catalyst was evaluated and used to catalyze the synthesis of DMC by MC alcoholysis.The optimum reaction conditions were investigated and determined by single factor experiment: reaction temperature 443 K,reaction time 9 h,catalyst dosage 0.8% (mass),molar ratio of MC to methanol 1:20.The results revealed that under optimal reaction conditions,the selectivity of DMC reached 92.4%.The experimental results of catalytic stability of ZnAlLa-LDO show that the partial dissolution of active metal Zn is the main reason for the decrease of activity of the catalyst after three cycles.After that,we can also start from the theoretical calculation of mechanism and other aspects to find more efficient active metals and optimize their preparation process,so as to improve the catalytic stability of the catalyst by reducing the content of active metals dissolved in the reaction system.The results also show that the addition of La has a significant effect on the catalytic performance and stability of DMC synthesis from MC and methanol,especially the highest DMC selectivity in the literature so far.The kinetic model is in good agreement with the experimental data,indicating that the powerlaw kinetic model based on the elementary reaction is a suitable model to provide theoretical basis.At 77.60 kJ·mol-1,the activation energy of the main reaction in ZnAlLa-LDO catalytic reaction system is low,which indicates that ZnAlLa-LDO can significantly reduce the activation energy of DMC synthesis from MC and methanol.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The financial support from the National Natural Science Foundation of China (22178089) is gratefully acknowledged.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.005.