catalysts Review Conversion of Green Methanol to Methyl Formate Doreen Kaiser, Luise Beckmann, Jan Walter and Martin Bertau * Institute of Chemical Technology, TU Bergakademie Freiberg, Leipziger Straße 29, 09599 Freiberg, Germany; [email protected] (D.K.); [email protected] (L.B.); [email protected] (J.W.) * Correspondence: [email protected]; Tel.: +49-3731-392384 Abstract: Methyl formate is a key component for both defossilized industry and mobility. The current industrial production via carbonylation of methanol has various disadvantages such as high requirements on reactant purity and low methanol conversion rates. In addition, there is a great interest in replacing the conventional homogeneous catalyst with a heterogeneous one, among other things to improve the downstream processing. This is why new approaches for methyl formate are sought. This review summarizes promising approaches for methyl formate production using methanol as a reactant. Keywords: carbonylation; green chemistry; methanol; methyl formate; synfuel 1. Introduction Methyl formate (MF) is one of the most important building blocks in C1 chemistry and it is of great interest as an emission reducing fuel additive. MF can be synthesized from CO and methanol and can be understood as chemical storage for CO. If methanol is produced Citation: Kaiser, D.; Beckmann, L.; from CO2 and hydrogen, which is generated through water electrolysis by renewable Walter, J.; Bertau, M. Conversion of energy, an access to green MF is created. This way a (partial) substitution for classical i.e., Green Methanol to Methyl Formate. fossil feedstock borne petrochemicals can be realized while reducing CO2 emissions. Since Catalysts 2021, 11, 869. https:// 1925, MF has been produced by carbonylation of methanol with sodium methanolate as doi.org/10.3390/catal11070869 a catalyst on an industrial scale. With a global production capacity of >6 million tons in 2016 [1]. This is an industrial chemical of major interest. However, development has gone Academic Editor: Jaime Soler on since, and there are alternatives to the sodium methanolate process that may be more efficient or economically more attractive, or they are more sustainable. Received: 7 July 2021 Accepted: 17 July 2021 2. Why Convert Methanol into Methyl Formate? Published: 20 July 2021 MF is one of the most important industrial products and has been widely used for the production of more than 50 chemicals, including formic acid, N,N-dimethylformamide Publisher’s Note: MDPI stays neutral (DMF), formamide and dimethyl carbonate (DMC) (Figure1). Particularly, the synthesis of with regard to jurisdictional claims in formic acid is of great importance. In 2016, worldwide production of the acid was estimated published maps and institutional affil- to be 621,000 t/a, whereby about 80% was produced by hydrolysis of MF (Kemira-Leonard iations. process). Formic acid is largely used in pharmaceutical, food and textile industry [2]. Formamide and DMF are synthesized by the reaction of MF with ammonia or N,N- dimethylamine, respectively. Formamides perform as extraction solvents and are applied as solvents for inorganic salts, especially in polymer chemistry. Being dipolar aprotic Copyright: © 2021 by the authors. solvents, they are ideal for nucleophilic substitutions. Therefore, DMF and formamide are Licensee MDPI, Basel, Switzerland. mainly directly used by the manufactures [3]. MF can also react with olefins or halogenated This article is an open access article compounds to form esters via hydroesterification or alkoxyacarbonylation reactions [4]. distributed under the terms and Trichloromethyl carbonochloridate (diphosgene) is produced by radical chlorination of MF conditions of the Creative Commons under UV light. It was originally developed as a pulmonary agent for chemical warfare. Attribution (CC BY) license (https:// Today it is used as an alternative to phosgene, which is easier and safer to handle. Thus, creativecommons.org/licenses/by/ diphosgene has replaced phosgene in carbonates, polyurethane and isocyanate production 4.0/). Catalysts 2021, 11, 869. https://doi.org/10.3390/catal11070869 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, x FOR PEER REVIEW 2 of 22 Catalysts 2021, 11, 869 2 of 21 ation of MF under UV light. It was originally developed as a pulmonary agent for chemi- cal warfare. Today it is used as an alternative to phosgene, which is easier and safer to handle. Thus, diphosgene has replaced phosgene in carbonates, polyurethane and isocy- processes,anate production which act processes, as starting which materials act as for starting high valuematerials plastics, for high resins, value plant plastics, protection resins, productsplant protection and insecticides. products Furthermore, and insecticides. it is used Furthermore, in medical it field is used for synthesize in medical sulfonic field for acidsynthesize methyl pyrimidine,sulfonic acid sulfamonomethoxine, methyl pyrimidine, sulfamonomethoxine, antitussive dextromethorphan antitussive and dextrome- other drugsthorphan [1]. and other drugs [1]. Figure 1. Selection of important products derivable from MF as a C1 building block and their ap- Figure 1. Selection of important products derivable from MF as a C1 building block and their applicationplication [5] [5].. PurePure MF MF is is known known as as a refrigeranta refrigerant (R611), (R611), used used as as a blowinga blowing agent agent for for polymers, polymers, as as a a binderbinder in in foundry foundry process, process, as aas solvent a solvent for nitrocellulosefor nitrocellulose as well as well as cellulose as cellulose acetate acetate and can and becan used be asused a smoke as a smoke fumigant fumigant and bactericide and bactericide for treating for treating tobacco, tobacco, dried fruit dried and fruit grains. and Furthermore,grains. Further it ismore, used it in is the used pharmaceutical in the pharmaceutical industry forindustry producing for producing sulfonic acid sulfonic methyl acid pyrimidine,methyl pyrimidine, sulfamonomethoxine, sulfamonomethoxine, antitussive antitussive dextromethorphan dextromethorphan and other and drugs other [1]. drugs [1].A further application is fuel additive. Its research octane number (RON) and the motor octaneA number further (MON)application are similarlyis fuel additive. high (RON Its research = 115, o MONctane n =umber 114.8, (RON) Table1 )[and6]. the The mo- additiontor octa ofne MF number to fuel (MON) increases are knock similarly resistance high (RON and indirectly = 115, MON the efficiency = 114.8, Table of combustion 1) [6]. The process.addition When of MF blended to fuel withincreases diesel, knock MF lowersresistance the cloudand indirectly point and the avoids efficiency sedimentation of combus- oftion diverse process. fuel componentsWhen blended at low with temperatures. diesel, MF lowers This improves the cloud the point filtration and avoids and cold sedimen- start behavior,tation of as diverse well as fuel delays components aging [6, 7at]. low Moreover, temperatures. with MF This serves improves to reduce the emissions filtration ofand nitrogencold start oxides behavior, (NOx )as and well soot. as Mostdelays importantly, aging [6,7]. MFMoreover, shows lesswith toxicity MF serves compared to reduce to otheremissions hydrocarbons of nitrogen in gasoline oxides (NO or consideredx) and soot. fuel Most additives importantly, [6]. According MF shows to theless Global toxicity Harmonizedcompared to System other (GHS),hydrocarbons methanol in gasoline and higher or considered hydrocarbons fuel are additives classified [6] as. According acute or organ-specificto the Global toxins. Harmonized High protective System (GHS), measures methanol are prescribed and higher by law hydrocarbons for handling, are which classi- reducesfied as the acute acceptance or organ as-specific a fuel additive. toxins. High In contrary, protective MF measures is only classified are prescribed as harmful by (GHSlaw for 07).handling, Furthermore, which MFreduces does the not acceptance participate as in aanti-atmospheric fuel additive. In contrary, photochemical MF is only reactions classi- andfied thus as harmful does not (GHS contribute 07). Furthermore, to the formation MF does of ground-level not participate ozone in andanti- smogatmospheric [8]. pho- tochemical reactions and thus does not contribute to the formation of ground-level ozone Table 1. Properties of different fuels [7]. and smog [8]. Gasoline Methanol Methyl Formate Table 1. Properties of different fuels [7]. RON 97.7 108.7–115 115 MON Gasoline 89 Methanol 88.6 Methyl 114.8 Formate ρ [kg/m3] 720–780 800 957 RON◦ 97.7 108.7–115 115 TBoiling [ C] 25–210 64.6 31.5 Flash Point [◦C] −40 12 −19 Lower heating value (LHV) [MJ/kg] 44 19.7 15.8 Catalysts 2021,, 11,, xx FORFOR PEERPEER REVIEWREVIEW 3 of 22 MON 89 88.6 114.8 ρ [kg/m3] 720–780 800 957 TBoiling [°C] 25–210 64.6 31.5 Catalysts 2021, 11, 869 3 of 21 Flash Point [°C] −40 12 −19 Lower heating value (LHV) [MJ/kg] 44 19.7 15.8 3.3. Synthesis Synthesis Routes Routes ThereThere are are a avariety variety of of synthesis synthesis routes routes to to produce produce methyl methyl formate. formate. On On an an industrial industrial scale,scale, the the carbonylation carbonylation of of methanol methanol plays plays the the most most important important role. role. On On a asmaller smaller scale, scale, dehydrogenationdehydrogenation and and oxidation oxidation of of methanol methanol are