Catalytic Preparation of Methyl Formate from Methanol over Silver

CHEM. RES. CHINESE U.2005 ,21(5),573- -576Catalytic Preparation of Methyl Formate from Methanol over SilverYANG Zhi ,LI Jing , YANG Xiang-guang** and WU Yue1. Changchun Institute of Applied Chemistry , Chinese Academy of Sciences , Changchun 130022 ,P. R. China ;2. Graduate School , Chinese Academy of Sciences , Beijing 100039 ,P. R. ChinaReceived Oct. 27 , 2004A catalytic reaction over a silver catalyst performed in an unregarded temperature region( 473- -873 K ) with a .long catalytic lifetime for the production of methyl formate from methanol was provided as a potential preparing route.The optimal yield of methyl formate( ca. 14. 8% ) with a selectivity > 90% was obtained at about 573 K. Because ax-oxygen species and bulk oxygen species coexist in the unregarded temperature region ,a synergistic process concerninga-oxygen species and bulk oxygen species was proved over 0。-rich and 0, -rich samples.Keywords Silver , Methanol , Methyl formate , Oxygen species , Synergistic effectArticle ID 1005 _9040( 2005 )-05-573-04Introductionmost copper-based catalysts are not stable enough be-MethyI formate is considered as an important prod-cause of the congregation of copper atoms , the cokinguct of Cl chemistry. It can be used as an intermediateon active sites 10 , and the polymerization of formalde-in the production of formic acid and formamide in in-hyde on active sites "1 ].dustrial scale1l. It can also be used as a starting mate-It is well known that silver-based catalysts are therial in the production of carbon monooxide with a high-most important ones used in industrial scale produc-purity and it can undergo a great variety of reactionstion , such as the manufacturing processes of formalde-leading to the formation of acetic acid , acetaldehyde，hyde and ethylene oxide. Recently , it has been foundmethyl acetate , and acetic anhydride 2- .Carbonyla-that a ZnO enhanced Ag/SiO2 catalyst exhibits a verytion of alcohols [671 is an effective route to produce es-high activity to the direct catalytic synthesis of indoleters , while the dehydrogenation of methanol is still thefrom aniline and ethylene glycok 12 . The production ofmain route to produce methyl formate. To the best ofethylene oxide is usually performed at a temperature be-our knowledge , the studies on catalytic dehydrogenationtween 473 and 573 K over a silver catalyst , and theof methanol to methyl formate are mainly focused on theproduction of formaldehyde is commonly at a tempera-systems such as V2O3-TiO2 , SnO2-MoO3 and Cu-SiO2.ture higher than 873 K' 1. Methyl formate is consideredand three types of mechanisms have been summarizedls a byproduct during the manufacturing of formalde-by Louis et al. for the catalytic preparation of methylhyde over a silver catalyst 13.14. So , it seems that theformate[8].region between 473 and 873 K for methanol oxidation isCopper-based catalysts have been applied to the .unregarded.industrial scale for dehydrogenation of methanol intoThe work of Wachs et al.I15] has shown that meth-methyl formate by Mitsubishi Gas Chemical Co. ( withyl formate is a main product of the methanol oxy-dehy-approximately 50% conversion and a methyl formatedrogenation process over the single silver( 110 ) surfaceselectivity of ca. 90% ) and Air Products Co.[1l.under an ultrahigh vacuum( UHV ) condition. BecauseHowever , the copper-based catalysts still have someof the" pressure and material gaps"Hol , we can onlyshortcomings. The preparation methods and activationfind a few records of the methyl formate has been a pre-manners of these catalysts strongly affect the catalyticdominant product over the catalysts based on silver tillactivity and selectivity to methyl formate , and the cata-now中国煤化Ilrogenation of methanollytic performance for the production of methyl formate isintomed over an Ag-Cd-ZnCHCNMHGesensitive to the structures of their active sites" J andalloywlunoul uxygen s parucpation'* Supported by the 973 Program of the Ministry of Science and Technology of China( No. 2003CB615800 ) and the NationalNatural Science Foundation of Chin:( No. 29973041 ).* *万对数掘rrespondence should be addressed. E-mail : xgyang@ ciac. jl. cn74CHEM. RES. CHINESE U.Vol.21Experimentalyield of methyl formate was obtained under the “hard"The silver catalyst was prepared through the reduc-condition. In Fig.2, a maximum yield of methyl for-tion of silver nitrate by a sodium borohydride water so-mate appeared at the initial period of time, then it de-luton. For comparison, electrolytie silver was also pre- creased slightly and leveled off during the fllowing lestpared. The activity experiments were carried out underperiod. When the yield of methyl formate was ca. 16%atmospheric pressure in a vertical fixed-bed quartz tu-( molar fraction), the yield of carbon dioxide was ca.bular reactor. The catalysts were pretreated in a flow of 6% ( molar fraction). Electrolytic silver was also testedair(10 mL/min) at 573 K for 180 min and then cooled and the similar results were obtained. .to 373 K. The tubular oven(i. d.: 20 mm) wasequipped with a Eurotherm temperature controller30 t( model 808). The catalyst particle sizes tested were60- -80 mesh. An on -line gas chromatograph equipped20 twith a TCD detector was used for the analysis of the10 tproducts.Results and DiscussionIt is well known that temperature can affect the5001000 15002000distribution of oxygen species on silverl18). Therefore,t/minit is necessary to investigate the effect of temperature onFig.2 The plots of methanol conversion and thethe reaction. In order to associate the reaction with theyields of products s. reaction time.industrial production process of formaldehyde, the rati-◆Conversion;■yield of CO2;★yield of CH20;os of methanol to oxygen or CHSV were similar to those▲yield of HCOOCHy.under the“hard”condition9， which has been usedIt is well known that a-oxygen species, which isin industry scale production. The plots of methanoldirectly formed from the gas phase oxygen, can causeconversion and yields of the products ts. temperaturedeep oxidation of methanol to carbon dioxidel41.21.221.are shown in Fig. 1. In a range from 400 to 500 K,In order. to reduce the deep oxidation of methanolcarbon dioxide was a predominant product caused bycaused by a-oxygen species, an 8% 02-in-N2 mixturethe active ac-oxygen species on the surface of silver(20was used instead of air. A flow rate of the mixture wasWith the temperature rising, the yield of methyl formate 600 mL/ min and the feed rate of methanol( liquid) wasrose and reached a maximum yield at about 573 K. The1 mL/h. The results are shown in Fig. 3. A maximumyield of formaldehyde rose with the temperature rising yield of methyl formate(ca. 14. 8% ) was also obtainedand reached a maximum at the maximal experimentalat about 573 K, and the yield of formaldehyde intemperature. The yield of carbon dioxide kept relatively creased with temperature increasing. It is noteworthyconstant over 500 K.that the yield of carbon dioxide was about 1. 8% at 57300「K, and it almost kept constantly on temperature rising.80102(00 400 500 600 700 800 900T/K350 400 450 500 550 600 650 700 750Fig. 1 The plots of methanol conversion and theyields of products s. temperature.Fig.3 The plots of CH,OH conversion and yields of◆Conversion; ■yield of CO2;★yield of CH2O;中国煤化工perature.▲yield of HCOCHy.CNMH G1 ofCO2;★yield ofThe“soft”condition [ air: 600 mL/h; methanolYH(liquid): 1 mL/h] test was also performed to test theIn order to investigate the roles of 0。and 0,20]inlifetime of the catalyst at 573 K, at which a maximum the catalytic processes, O_-rich and O, -rich silver cata-.No. 5YANG Zhi et al.575lysts were prepared by the methods of Veen et al. [22].methyl formate appears is about 573 K , which is theThe results are shown in Fig. 4. Compared with theboundary of desorption temperature regions of x-oxygenproducts over an O, - rich silver catalyst , carbon dioxidespecies and bulk oxygen species 25. Because the het-was the main product over the O。-rich silver catalysterogeneous catalytic process occurs over the catalystbelow 450 K. On contrary , the yield of methyl formatesurface , the a-oxygen and γ-oxygen species are the es-over the O, -rich silver catalysts was higher than thatsential ones for the oxydehydrogenation of methanol. Atover O。-rich silver catalyst in the temptrature rangea low temperature , almost 90% methanol will be oxi-from413 to 473 K. With temperature rising , the cata-dized to carbon dioxide , and the selectivity of carbonlytic performances of the two catalysts became similardioxide will decrease because of the decrease of theand the yields of formaldehyde and methyl formate overcoverage degree of a-oxygen species with temperatureO,-rich catalyst were better than the yields of those overrising. Above 500 K , the yield of methyl formate risesthe O。-rich one below 600 K.linearly with temperature rising until about 573 Kwhich accords with the TDS results 251 in some degree.12 S12FEEoWith temperature rising , the stick time of a-oxygenspecies on catalysis decreased , so the ratio of O。/Odecreased 26400 500 60The work of Waterhouse et al. [14] also showed thatT/Kthe n( Ag- -0 ) mode of y-oxygen reached the maximumvalue at about 550 K. Veen et al.suggested that γ-oxygen lowered the coverage of a-oxygen. And Lam-400500600bert 27 J suggested that subsurface oxygen species com-peted with the adsorbed surface oxygen for metal elec-Fig.4 Effects of O。- and O,- rich siliver cata-tron density ,and thus caused the surface species to be-lysts on the yields of products and cov-come more electrophilic. It is well known that nu-ersion of methanol.cleophilic oxygen species is easy to cause the complete■a-CO2;口γr-CO2 ;★a-HCHO;★r-oxidation over silver. In the catalytic process , the cov-HCHO ;erage of γ-oxygen as a function of temperature controls▲a-HC0OCHs ;o y-HC0OCH3.the coverage and electron density of a-oxygen , both ofWachs et al.[15]proposed that the catalytic con-which affect the degree of oxidation of the absorbedversion of methanol to methyl formate obeyed the Tisch-species.enko mechanism via a surface hemiacetal intermediate.Fig. 4 clearly shows that the 0,-rich silver catalystNew viewpoints on the influences of different atomic ox-favors the formation of methyl formate at a low tempera-ygen species on reaction pathways and selectivity in theture , and the difference in the yield of methyl formateconversion of methanol to formaldehyde have beenover the two catalysts got smaller with temperature risavailable from recent spectroscopic investigations'ing. It is reasonable to associate the formation of methylThe a-oxygen species is considered to contribute to theformate with the ratio of 0。/0, that depends on temper-oxidation of methanol to formaldehyde , formic acid,ature. It should be noted that the catalytic contact timecarbon dioxide in sequence , and the γ-oxygen speciesin Fig.4 is about 5 times those in Figs. 1 ,2 and 3to the selective oxidation to formaldehyde 18]. The con-thus , the maximums of methyl formate were obtained attent of the a-oxygen species that is directly formed froma lower temperature. It also indicates that contact timethe gas phase oxygen decreases with temperature risingcan raise the selectivity to methyl formate. Based onand is almost zero at about 580- -590 K{2- -25]. Abovethe above discussion ,a synergistic process is suggested500 K , bulk oxygen species begin to migrate to thefor the formation of methyl formate with the participat-sub-surface and surface to form y-oxygen and a-oxygening中国煤化工species. 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Sci. ,1978 ,76 ,531Correction of Published PaperCorrections of the errors appeared in the Vol.21 No. 3 , May 2005 of" Chemical Research In Chinese Universi-ties" are made as follows :1. Authof" ZU Long-min” on page 334 should be written as" ZU Long-fei".2. Received date" June 21 ,2005" on page 372 should be written as" June 21 , 2004"3. Received date" May 21 ,2005”on page 376 should be written as”May 21 ,2004" .中国煤化工MHCNMHG.