Study Of Clean Synthesis Of Dimethyl Carbonate By Catalytic Oxidative Carbonylation Of Methanol

Dimethyl carbonate (DMC) is an environmentally benign chemical which expect to be used widely in industry near future. The research and development of clean technology to synthesize DMC by oxidative carbonylation of methanol have attracted more and more attention in recent decades, but the quick deactivation and heavy corrosion during the synthesis process due to the formation of HC1 from the existing Cl" in catalysts are the major problems to restrict its commercial application. Therefore, to develop a new technology for preparing catalysts with less or free Cl" and to study the catalytic mechanism in order to avoid the deactivation and corrosion problems will be more significant both theoretically and practically. Catalysts of CuVzeolite(H) prepared by heating the physical mixture of cuprous chloride with H zeolite, i.e., by solid state ion-exchange method, have less or no Cl^- and consequently will be less deactivating. In this dissertation, the process of preparing supported Cu1 catalyst by solid state ion-exchange method and its mechanism were researched and discussed in details. The two catalytic reaction processes: liquid-phase slurry process and gas-phase fixed bed process were used to evaluate the catalytic performance, and XRD, XPS, SEM, TEM, TG, etc. were used to study the structures, surface properties, active species of catalysts in order to understand the catalytic mechanism. The major research work and conclusions are listed as following:(1) Thermodynamical calculation and discussion of all nonphosgene reactions to synthesize DMC. The reaction heats and equilibrium constants of all nonphosgene reactions were calculated from the thermodynamic data ΔH_f^°, ΔG_f^° and C_p, of which some substances estimated by the Benson Group Contribution method. The calculation results show that the reactions using CO and O₂ as reactants are much more favorable thcrmodynamically than using CO₂ as reactant. The Δ G_r^° of oxidative carbonyiation of methanol to DMC is far below zero and the reaction is thermodynamical favorable, but the reaction of methanol or dimethyl ether with CO₂ to DMC is not. Because the ΔG_r^° of some suggested side reactions are also far below zero and their equilibrium constants K⁰ are very high, the competition between the main reaction and the possible side reactions in theprocess of oxidative carbonylation of methanol will be much intensive. Therefore, it is more and more important to develop a catalyst with high activity and selectivity.(2) Studied of the conditions and mechanism of solid state ion-exchange between CuCl with HY zeolite. There are many complicated physical and chemical changes occurred during the process of heating the physical mixture of CuCl and HY zeolote under the flow of inert nitrogen gas. In the process, CuCl was easy to sublimate and disperse on the surface of the zeolite. The ion-exchange of Cu1 in CuCl solid with H+ in HY zeolite with the release of HC1 gas occurred at least at 300 °C, and the maximum of ion-exchange rate were reached at 340 °C. CuCl sublimated when the heating temperature was below 430 °C and evaporated when the temperature was above 430°C, accompanying with the dispersion of CuCl on the zeolite surface; The -Si-OCu+ formed by the reaction of -Si-OH on the surface of zeolite with CuCl at 650 °C above; CuCl which adsorbed on the inner cage surface of zeolite was very difficult to desorb below 800 °C. In addition, CuCl could promoted the dealumination of the framework aluminum of zeolite and reacted with the extra framework aluminum to form (AlO)Cl.(3) The activities of catalysts with the kind of zeolite were different in batch slurry reaction and its sequence is Hp>HZSM-5>DASY. The maximum of STY 0.56 gg"'-h"' and selectivity of 93.3 % were obtained when reaction was run at 140 °C for 2h under 3MPa pressure with the ratio of CO/O2 of 2 in the present of CuCl-ZSM-5 catalyst which was prepared at 450 °C for 4h. XRD spectra showed that the fresh catalyst has less phase of CuCl but used catalyst has no more CuCl phase and a new phase of CuCl2-3Cu(OH)2 was observed. The spectrum of FTIR has -OH peak in the used catalyst and the SEM indicated that clustering occurred during the catalytic reaction.(4) In continuous slurry reaction process, the STY of CuCl-Hp catalyst was higher than that of CuCl-HZSM-5. The active sequence was: Hp>HZSM-5>DASY and the activity of the same kind of zeolite support increased with the ratio of Si/Al decreasing. New -OH group peak were found in the used catalysts by XRD, XPS and FTIR spectrum and there existed Cu1 and Cu11 in both fresh and used catalysts, but the intensity of Cu1 in fresh catalyst was higher than that of used catalyst and the species of CuO, CuCl2 and Cu(OH)2 increased in used catalyst.(5) In gas-phase continuous fixed bed reaction, the active sequence of the catalysts is DASY(8.1) > Hp(20-30) >HZSM5(55.6). The catalytic activity increased with the ratio of Si/Al decreasing for the Hp zeolite support. It was found that the catalyst was treated by H2O after prepared by solid state ion-exchange, new Cu2(OH)3Cl phase formed on catalyst surface. Those kinds of catalyst had higher activity, selectivity and longer life than those untreated. The catalytic activity was directly relate to the concentration of Cu2(OH)3Cl. The characteristics of XPS showed that the ratio of Cun/Cu" of CuCl-DASY (650 °C, 4 h) was apparently lower than the catalyst treated with H2O, which means Cu" increased after the catalyst was treated with H2O.(6) A serial mesoporous MCM-41 material with different ratios of Si/Al was synthesized and MCM-41 supported Cu1 catalyst was prepared by solid ion-exchange method. It was calculated that 25 % Si atoms on the surface of MCM-41 had silanol groups by TG and TGA analysis. The Si-O-Cu+ which is active centre for oxidative carbonylation of methanol to DMC was formed by the reaction of the terminal silanol of MCM-41 with CuCl at 350 °C under flowing inert nitrogen gas.(7) When MCM-41 (oo)-Cl material which was prepared by anchoring the organic group 3-chloropropyl on the surface of MCM-41 (00) was treated at 550 °C for 4 h, the organic group decomposed while the mesoporous structure was still kept. The MCM-41(oo)-Cl material decomposed its organic groups at 320°C. Prepared by solid state ion-exchange under flowing nitrogen, CuCl-MCM-41 catalyst had a 100 % selectivity to DMC based on conversion of methanol and the conversion of methanol was around 5-1 Owt. % . With the mole ratio of Si/Al decreased, e.g. the aluminum content in the material increased, the catalytic activity increased. It is because much more Cu1 was loaded on the material by solid state ion-exchange. But the selectivity decreased with the increasing of reaction temperature due to the by-products formed.