Comparative study between gas phase and liquid phase for the production of DMC from methanol and CO2

Available online at www.sciencedirect.comJOUPRNLOFScienceDirectNATURAL GAS; CHEMISTRYELSEVIERJoumal of Natural Gas Chemistry 21(2012)189- -193www.elsev1er.com locatejngcComparative study between gas phase and liquid phase forthe production of DMC from methanol and CO2Ahmed Aouissi*, Salem S. Al-DeyabDepartment of Chemistry, King Saud University, P. O. Box 2455, Riyadh 11451, Saudi ArabiaI Manuscript received August 15, 2011; revised October 9, 2011]AbstractDirect synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide over Co1 sPW 12040 in liquid and in gas phase is investigated.The synthesized catalyst has been characterized by means of FTIR and XRD. Liquid phase experiment results showed that high pressures arefavorable for the synthesis of DMC. However, DMC formation is limited by the reaction with co-produced water. DMC selectivity is morestrongly dependent on the temperature than on the pressure of CO2. As for the reactions in gas phase, it has been found that both CH3OHconversion and DMC selectivity decreased with increasing temperature, owing to the decomposition of DMC at high temperatures. Hightemperatures and more amount of Co1.sPW 12040 catalyst favor the fornation of dimethoxymethane (DMM) and methyl formate (MF).Key wordsheteropolyanion; Keggin structure; methanol; dimethyl carbonate; carbon dioxide1. Introductionbility of an explosion. Recently, the conversion of methanolin the presence of carbon dioxide has drawn much attention,due to the abundance, the cheapness, the non-toxicity andTwo routes can be used to perform the natural gas up~the non-flammability of carbon dioxide, compared with cur-grading: direct conversion of methane and indirect conver-rently used phosgene and carbon monoxide [18,20]. There-sion via methanol. In fact, methanol is manufactured fromfore, the development of efficient heterogeneous catalytic sys-syngas, which is typically produced from the steam reform-tems has attracted more attention. Bian et al. [21] studieding of methane. Thus, the conversion of methanol into valu-the reaction over Cu-Nigraphite nanocomposite catalyst inable chemicals is within the framework of upgrading naturalgaseous phase. They obtained 10.13% CH3OH conversiongas. Among the products obtained from methanol, dimethyland 89.04% DMC selectivity at 105 °C. Wu et al. [22] studiedcarbonate is considered as an important product in chemi-the synthesis of DMC from gaseous methanol and CO2 overcal syntheses as well as in industry [1- 3], which is an en-H3PO4 modified V2Os catalyst with various molar ratios ofvironmentally benign chemical product with a wide rangeH3PO4N2Os (P/N). The best conversion (1.95%) and selec-of applications. It has been used as a good solvent [4], antivity of DMC (92.12%) was obtained at 130 °C over the cata-alkylation agent [5] and a substitute for highly toxic phos-lyst H3PO4/V2Os (P/V = 0.20). The relatively low DMC yieldgene and dimethyl sulfate in many chemical processes [6-9].obtained from the direct synthesis from methanol and CO2 isIt is also used as an intermediate in the synthesis of poly-due to the fact that CO2 is thermodynamically stable and ki-carbonates and isocyanates [10,11]. In addition, it is ex-netically inert and also the deactivation of catalysts inducedpected to replace the gasoline oxygenate methyl tert-butylby water formation during reaction process [23,24]. The inert-ether (MTBE), because of its high oxygen content, low toxi-ness of carbon dioxide and the reaction of water produced bycity and rapid biodegradability [3,4,12- 14]. Currently, DMCthe reaction bring serious drawbacks for further applications.is produced mainly by oxidative carbonylation of methanolThe main problem is to find an efficient catalyst, as well as(non-phosgene route) [15]. The synthesis can be carried outoptimal operating conditions. Therefore, the direct synthesisin both liquid- and gas-phases [16- 19]. However, both routesof DMC from methanol and carbon dioxide in liquid and gasuse poisonous gas of carbon monoxide and there is the possi-phase is still中国煤化工igations in order to●Coresponding author. Tel: 00 966 1 4675958; Fax: 00 966 1 4675992; E-mail: aouissed@yahMYHCNMHGCopyrightO2012, Dalian Institute of Chemnical Physics, Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003- 959311)60353-8.190Ahmed Aouissi et aL/ Journal of Natural Gas Chemistry Vol. 21 No. 22012optimize all the parameters (conversion, selectivity, reactionsaturator. Prior to the reaction, the catalyst was pretreated atconditions, etc.) required for specific reactions of economic300 °C with CO2 for 2 h. The reaction products were analyzedimportance. Here, we report the direct synthesis of DMCwith a gas phase chromatograph (Agilent 6890N) equippedfrom methanol and COr in gas and in liquid phase overwith a flame ionization detector and a capillary column (HP-Co1.sPW:2O40 regarded as a Keggin-type heteropolyanionPLOT Q length 30 m D 0.53 mm).catalyst. The effect of the reaction conditions on DMC syn-thesis is investigated. It is worth noting that heteropoly com-3. Results and discussionpounds do not undergo deactivation by water, and they areknown as efficient catalysts in both liquid and gas phases3.1. Characterization of the catalyst[25- -27]. .The FT-IR spectrum of Co1 sPW12O40 is shown in Fig-2. Experimentalure 1. The IR spectrum has been assigned according to refer-ences [28,29]. The main characteristic features of the Keggin2.1. Catalyst preparationstructure are observed at 1080- 1060cm-1, 990- 960 cm~900-870cm 1 , and 810- - 760 cm~ I , assigned to the stretch-The heteropolyacid H3PW 12O40 was prepared accordinging vibration vs (P- _Oa), vs (W -0d), vs (W- _Od- W), andto the method of Rocchiccioli-Deltcheff et al. [28]. The cobaltvs (W- Oc -W). The result of X-ray powder diffraction analy-salt taken form of Co1 sPW 12O40, was obtained from the het-sis of the catalyst is shown in Figure 2. In each of the foureropolyacid H3PW12O40 as precipitate by slowly adding theranges of 20=7°-10, 16°-230, 250- -300 and 31°- -380,required amount of Ba(OH)2 8H2O into the aqueous solutionthe compound shows a characteristic peak of heteropolyan-of H3PW 12O40 to neutralize the three proton, then the requiredions (HPA) having Keggin structure [30]. So the presenceamount of CoSO4.7H2O was added. After eliminating theof the primary Keggin structure in the synthesized catalyst isformed BaSO4 precipitate, the obtained solution was aged forconfirmed by IR and XRD.a few days at 4°C. The Co1.sPW12O40 salt was recoveredfrom the solution by filration.18162.2. Physicochemical techniques14The purity and the Keggin structure of the samples were10 Echaracterized by means of IR and XRD. IR spectra wererecorded in the range of 4000- -400cm-1 as KBr pellets, us-ing a GENESIS II-FTIR spectrometer. XRD powder patternswere recorded on a Rigaku diffractometer Ultima IV using CuEKa radiation.2.3. Reaction procedure4000 3500 3000 2500 2000 1500 1000 500Wavenumber (cm~)For liquid phase reactions, experiments were conductedFigure 1. IR spectrum of Co1,sPW 12O40 Keggin-type beteropolyanionin a stainless steel autoclave equipped with magnetic stirring.The temperature of the autoclave was adjusted by a heatingjacket. In a typical procedure, 20 mL methanol and 0.1 gcatalyst were loaded into the autoclave. A low pressure ofCO2 was injected into the reactor, and then released severaltimes in order to remove the air in the reactor. Next, the re-actor was pressurized with CO2 to 2.5 bar. The system was恩|stirred and heated at 80°C for 5 h. After the reaction, the re-actor was cooled down to below 5 °C with a circulator andthen depressurized. As for gas phase reactions, experimentswere carried out at the temperature ranging from 200°C to300 °C in a flow-type fixed-bed stainless reactor loaded withMm,|100 mg catalyst under atmospheric pressure. To supply the中国煤化工reactant, a gaseous CO2 at a flow rate of 60 mLminIwas:MYHCNMHGO6passed through the methanol saturator thermostated at 40°C.201(°)The molar ratio of CO2/CH3OH was adjusted by the flux ofFigure 2. XRD pttrm of Co1.sPW12O4o Kegin-type heteropolyoxometa-CO2 controlled by mass flow controller and the temperature oflate catalystJoumal of Natural Gas Chemistry Vol. 21 No.2 20121913.2. Synthesis of DMC from CH3OH and CO2 in liquid phase2.3.2.1. Effect of reaction temperature+ Conversion十Y(DMM)豆1.5-士Y(MF)士Y(DMC)In order to investigate the effect of temperature on the di-rect synthesis of DMC from methanol and CO2, the effect of冒1.0个pressure was minimized by lowering it to 0.5 bar. Figure 3shows the effect of reaction temperature on methanol conver-sion and product selectivities. As expected, the conversion of言0.smethanol increased with increasing temperature. This resultis in agreement with that of Wu et al. [22]. As for the productselectivities, it can be seen that a relatively high temperature0.04(80 °C) was favorable to the production of DMC. However,beyond this point, an evident decrease in DMC selectivity inReaction time (1)favor of DMM and MF production was observed. The se-Figure 4. Effet of reation time on the conversion and product yieldslectivity of the reaction to DMC production decreased from(measalyxt = 0.1 g PCO2 =0.5 bar)73.57% to 41.51% as the temperature increased from 80 to100 °C in favor of MF and DMM.1000+ S(DMC)2.5+ S(DME+DMM+MF)i ∞0f- + - S(DMC)2.00图景40-★Y(DMC)810.5Po, (bar)Figure 5. Effect of CO2 pressure on the conversion and product selectivity0.0(T= 80°C, malyu=0.1g.t=Sh) .50Temperature (C)Figure 3. Effect of reaction temperature on the conversion and product se-3.3. Synthesis of DMC from CH3OH and CO2 in Gas phaselectivities (mcamalym=0.1 g, Pcon =0.5bar,t=5h)3.3.1. Effect of reaction temperature3.2.2. Effect of reaction timeThe effect of reaction temperature on the reaction per-formance was investigated at temperatures ranging fromFigure 4 reflects the effect of reaction time on the con-200- 280 °C. The results are illustrated in Figure 6. It wasversion and product yields. From this figure, we can scefound that that CH3OH conversion decreased dramaticallythat the conversion increased with the reaction time, but thewith increasing temperature. As for the selectivities, it can bechange was not significant afer about 6 h. The selectivity toseen from the figure that DMC selectivity decreased in favorDMC reached its maximum value when the reaction time wasof DMM and MF when the temperature increased from 200about5 h.to 220°C. When the temperature increased beyond 220°C,the selectivity to MF increased slightly to the detriment of3.2.3. Effect of COz pressureDMM, while that of DMC remained almost unchanged. Thedecrease of DMC selectivity as increasing temperature mightThe results of the effect of CO2 pressure on the conver-be due to the decomposition of DMC [31-33]. It is worth not-sion and product selectivity are depicted in Figure5. It caning that the highest conversion of 7.6% and highest selectivitybe seen from the figure that with the increase of CO2 pres-toDMCof8中国煤化工west temperature insure, the degree of conversion increased significantly, whereasthis range ofCNMH(her decrease in tem-DMC selectivity decreased slightly. By raising the total pres-perature will?DMC, because CO2sure from 0.5 to 5.0 bar, DMC selectivity decreased fromrequires higher temperatures to be activated. Thus the conver-73.57% to 65.20%.sion of methanol into DMC is limited.192Ahmed Aoussi et al./ Journal of Narural Gas Chenmistry Vol. 21 No.2 2012methanol and product distribution is shown in Figure 8. It canbe seen that the conversion varied uniformly as Co1 ,sPW 12O40十Conversion x10proportion varied. The conversion increased from 7.60%大SMC)+8士S(DMM)to 15.73% when Co1.sPW 12O40 proportion increased fromR61600.63% to 3.16%. As for DMC selectivity, the result showedthat increasing the catalytic amount from 0.1 g to 0.3 g, DMCselectivity decreased from 86.5% to 68.1% loosing in this way18.4% selectivity. Further increase the catalyst amount did notinfluence the selectivity considerably. This result indicated120that the reaction of preparing DMC from CH3OH and CO2was performed by Co1 sPW 12O40 cataltically.200220402602820h 10Temperabure (C)Figure 6. Dependence of the conversion and product selectivities on reac--815tion temperature over Co1.sPWl2O40 (Reaction conditions: meatalys=0.1g,molar ratio of CH3OH/CO2= 1.9)603.3.2. Effect of time on stream」40Figure 7 shows CH3OH conversion and products distri-+ Conversionr S(DMM+MF)bution as a function of time on stream. It can be seen that+ S(DMC)CH3OH conversion increased rapidly during the first threehours, after then it increased slowly. In fact, the conversion).51..02.53.at 3h (7.05%) became about 2.5 times larger than thatat 1hCatalyst (wr%) (based on CH2OH)(2.86%), whereas when the time was increased from3 hto5 h,Figure 8. Effect of the amount of Co1, sPW12O40 on the conversionthe conversion increased from 7.05% to 7.60% which repre-and product selectivities (Reaction conditions: T = 200°C, molar ratio ofCH3OH/CO2= 1.9)sents only an increase of 7.70%. As for the product distribu-tion, one can see that DMC selectivity was almost stable dur-ing the first two hours of reaction, then it increased slightly4. Conclusionsto the detriment of DMM and MF selectivity. DMC was ob-tained as a major product whereas DMM and MF were ob-In this work, direct synthesis of DMC from CH3OH andtained as minor products.CO2 has been studied in liquid and in gas phase system usingCor1.sPW 2O40 as catalyst. DMC can be synthesized directly10 rn 10from methanol and CO2. However, the yield of DMC is lowbecause of the thermodynamic limitations of the reaction. Forboth liquid and gas phases, DMC selectivity is more stronglydependent on the temperature. Low reaction temperature ismore favorable for DMC formation. High temperatures de-区660 8crease both the conversion of CH3OH and the selectivity to十ConversionDMC. The decrease of DMC selectivity might be due to its士S(DMM+MF)★S(DMC)decomposition at high temperatures. Low temperatures willnot be sufficient to activate CO2 molecules required for thedirect synthesis of DMC. For Co1.sPW12O4o catalyst, the gasphase seems to be better than the liquid phase. There is nowater in the reaction system during reaction that favors theprogress of the formation of DMC. Thus, gas phase processTime on stream (h)can be adopted for the direct synthesis of DMC from CO2Figure 7. Effect of time on steam on theand methanol provided that the catalyst is effective for CO2tities (Reaction conditions:activation at low temperature. However, taking int) accountconditions: T =200°C, malyut=0.1g, molar ratio ofCH;OH/CC2=1.9)that some catalytic systems (such organometallic catalysts)are not therma中国煤化工more efficient in3.3.3. Effect ofcatalyst amountliquid phase whYHC N M H Civity, we cean con-clude that the li.. provided that theco-produced water in the reaction is removed from the mixtureThe effect of Co1 sPW 1204o amount on the conversion ofby pervaporation process, for example..Journal of Natural Gas Chemistry VoL. 21 No.2201293Acknowledgements[18] Jessop P G, Ikariya T, Noyori R. Chem Rev, 999, 99: 475The authors extend their appreciation to the Deanship of Scien-19] Tundo P, Selva M, Bomben A.0rg Synth, 1999, 76: 169tific Research at King Saud University for funding the work through[20] Darensbourg D J, Holtcamp M W, Struck G E, Zimmer M s,the research group project (No. RGP-VP-116).Niezgoda S A, Rainey P, Robertson J B, Draper J D, Reiben-spies J H. 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