WO2024062017A1

WO2024062017A1 - Water removal in a process for preparing methylal from carbon dioxide

Info

Publication number: WO2024062017A1
Application number: PCT/EP2023/076037
Authority: WO
Inventors: Emiel Jan KAPPERT; Christian Mueller; Thomas Schaub; Katharina Stefanie Ludwina RUECK; Edward Richmond
Original assignee: Basf Se
Priority date: 2022-09-22
Filing date: 2023-09-21
Publication date: 2024-03-28

Abstract

The present invention relates to a process for preparing methylal from carbon dioxide, wherein the catalyst system and methanol used in said process are recycled. The present invention further relates to a production unit for preparing methylal from carbon dioxide.

Description

Water removal in a process for preparing methylal from carbon dioxide The present invention relates to a process for preparing methylal from carbon dioxide and to a production unit for preparing methylal from carbon dioxide. Dialkoxymethanes, in particular dimethoxymethane (methylal), are of particular commercial in- terest. Since they are able to increase the octane number, lower soot and NOx formation, they are attractive candidates for the use as gasoline or diesel additives. Methylal currently finds use in a variety of applications including perfumes, resins, adhesives, coatings, sealants and putties. In addition, methylal is a valuable compound in pharmaceutical, cosmetic and polymer applica- tions. Therefore, new and simple processes for the production of methylal are of great interest. In order to minimize production costs, a highly active catalyst system is necessary in addition to being highly selective for the formation of methylal. Methylal can be produced by oxidation of an alcohol or the reaction of formaldehyde with the corresponding methanol. Formaldehyde itself is produced by the oxidation of methanol. An alternative method to produce methylal is the direct reduction of carbon dioxide with hydro- gen in the presence of an alcohol using transition metal catalysts and Lewis acidic co-catalyst. An example of production of acetals, and in particular methylal, is disclosed in WO 2020/161175 A1, wherein a Ru-catalyst is used in combination with Al(OTf)3 as a co-catalyst. There is how- ever still a need to produce methylal while being able to recycle the catalysts as well as metha- nol in order to improve the processability and reduce the potential environmental impact of such production. Surprisingly, it was found that the process of the present invention according to which methylal is produced from carbon dioxide and hydrogen shows great performance while allowing the re- cycling of methanol and the catalyst. This allows to reduce costs compared to other processes. Hence, using a process for preparing methylal from carbon dioxide according to the present in- vention is a solvent-efficient process which permits to reduce the CO₂ footprint. Therefore, the present invention relates to a process for preparing methylal from carbon dioxide, the process comprising (i) preparing a stream SMW containing methylal, methanol, water and a catalyst system, com- prising (i.1) providing a gas stream G containing CO₂ and H₂; (i.2) providing a liquid stream S_L containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream SL provided according to (i.2) into a reactor unit RU; (i.4) contacting G with S_L to carbon dioxide reduction conditions, obtaining the stream SMW containing methylal, methanol, water and the catalyst system; (i.5) removing SMW from RU; (ii) separating methylal from the stream S_MW removed from RU in a purification unit PU, ob- taining a stream SM comprising methylal and methanol and a liquid stream SW comprising methanol, water and the catalyst system; (iii) removing water from the liquid stream S_W via a separation unit SU, obtaining a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein at least a part of R is recycled into step (i.2) as a component of S_L. Preferably, according to (i.1) providing a gas stream G, comprises admixing a gas stream G1 comprising CO2 with a gas stream G2 comprising H2, obtaining G. Preferably, the molar ratio of H₂ to CO₂ in G is in the range of from 1:100 to 100:1, more prefer- ably in the range of from 1:30 to 30:1, more preferably in the range of from 1:10 to 10:1, more preferably in the range of from 1:1 to 10:1, more preferably in the range of from 1:1 to 5:1. In the context of the present invention, it is preferred that G consists essentially of, more prefer- ably consists of, H2 and CO2. It is however possible that G comprises other components such as inert gases (N₂), methanol, methylal and/or water. The amount of such additional components would be of at most 5 vol-%, preferably of at most 2 vol-%, more preferably of at most 1 vol-%, more preferably of at most 0.5 vol-%, based on the total weight of G. Preferably, the catalyst system comprises a catalyst complex and an acidic co-catalyst, wherein the catalyst complex comprises a transition metal complex and at least one polydentate ligand comprising at least one P atom. Preferably, the catalyst complex is a homogeneous catalyst complex, which means that the cat- alyst complex is dissolved in the liquid reaction medium, namely methanol, under the reaction conditions. In other words, the catalyst complex is in the same phase as the reactants. Preferably, the transition metal of the transition metal complex is selected from the group con- sisting of ruthenium, manganese, cobalt, iron, osmium, rhodium, rhenium, iridium, nickel, plati- num and palladium, more preferably selected from the group consisting of ruthenium, manga- nese and cobalt, more preferably selected from the group consisting of ruthenium and cobalt, more preferably is ruthenium. Preferably, the transition metal complex is one or more of [Ru(acetylacetonate)₃], [Ru(COD)(methylallyl)2], [Co(acetylacetonate)3], RuCl3*H2O, [Ru(p-cymene)Cl2]2, [Ru(ben- zene)Cl₂]_n, [Ru(CO)₂Cl₂]_n, [Ru(CO)₃Cl₂]₂, [RuCl₃H₂O], [Ru(DMSO)₄Cl₂], [Ru(PPh₃)₃(CO)(H)CI], [Ru(PPh₃)₃(CO)Cl₂], [Ru(PPh₃)₃(CO)(H)₂], [Ru(PPh₃)₃Cl₂], [Ru(Cp)(PPh₃)₂CI], [Ru(Cp)(CO)₂CI], [Ru(Cp)(CO)2H], [Ru(Cp)(CO)2]2, [Ru(Cp*)(CO)2CI], [Ru(Cp*)(CO)2H], [Ru(Cp*)(CO)2]2, [Ru(in- denyl)(CO)2CI], [Ru(indenyl)(CO)2H], [Ru(indenyl)(CO)2]2, ruthenocen, [Ru(binap)(Cl)2], [Ru(2,2'- bipyridin)₂(Cl)₂·H₂O], [Ru(COD)(Cl)₂H]₂, [Ru(Cp*)(COD)CI], [Ru₃(CO)₁₂], [Ru(tetraphenylhy- droxycyclopentadienyl)(CO)₂H], [Ru(PMe₃)₄(H)₂], [Ru(PEt₃)₄(H)₂], [Ru(Pn-Pr₃)₄(H)₂], [Ru(Pn- Bu₃)₄(H)₂], and [Ru(Pn-octyl₃)₄(H)₂], more preferably one or more of [Ru(acetylacetonate)₃] and [Ru(COD)(methylallyl)2], more preferably [Ru(acetylacetonate)3]. Preferably, the at least one polydentate ligand comprising at least one P atom is selected from the group consisting of tris(diphenylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinome- thyl]ethane, tris[di(3,5-dimethylphenyl)phosphinomethyl]ethane, tris(diphenylphosphinome- thyl)methane, tris(diphenylphosphinomethyl)amine, bis(2-diphenylphosphinoethyl)phe- nylphosphine and tris[2-(diphenylphosphino)ethyl]phosphine, preferably selected from the group consisting of tris(diphenylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinomethyl]ethane, tris[di(3,5-dimethylphenyl)phosphinomethyl]ethane and tris(diphenylphosphinomethyl)amine, more preferably is tris(diphenylphosphinomethyl)ethane, more preferably 1,1,1-tris(diphe- nylphosphinomethyl)ethane (Triphos). Preferably, the molar ratio of the transition metal complex relative to the at least one polyden- tate ligand is in the range of from 1:3.0 to 1:1, more preferably in the range of from 1:2.0 to 1:1, more preferably in the range of from 1:1.5 to 1:1, more preferably in the range of from 1:1.2 to 1:1. Preferably, the molar ratio of the acidic co-catalyst relative to the catalyst complex is in the range of from 1:50 to 1:1, more preferably in the range of from 1:20 to 1:1, more preferably in the range of from 1:10 to 1:1. Preferably, the acidic co-catalyst is one or more of Bronsted acid and a Lewis acid. Preferably, the acidic co-catalyst is selected from the group consisting of methane sulfonic acid (MeSO₃H), Al(OTf)₃, Bi(OTf)₃, AlCl₃, ZnCl₂, SnCl₄, TiCl₄, Fe(OTf)₃, HCl, H₂SO₄, tricholoroacetic acid, p-TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, more prefera- bly selected from the group consisting of methane sulfonic acid (MeSO₃H), Al(OTf)₃, Bi(OTf)₃, H₂SO₄, p-TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, more pref- erably selected from the group consisting of MeSO3H, Al(OTf)3, H2SO4, p-TsOH, trifluoro- methanesulfonic acid and mixture of two or more thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃, p-TsOH and a mixture thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃ and a mixture thereof, more preferably selected from the group consisting of MeSO3H and Al(OTf)3. Preferably, the acidic co-catalyst is a Bronsted acid, being an acid selected from the group con- sisting of methane sulfonic acid, HCl, H2SO4, tricholoroacetic acid, p-TsOH, trifluoromethanesul- fonic acid and a mixture of two or more thereof, more preferably selected from the group con- sisting of methane sulfonic acid, HCl, H₂SO₄, tricholoroacetic acid, p-TsOH and trifluoro- methanesulfonic acid, more preferably being methane sulfonic acid. Alternatively, preferably, the acidic co-catalyst is a Lewis acid, being selected from the group consisting of Al(OTf)3, Bi(OTf)₃, AlCl₃, ZnCl₂, SnCl₄, TiCl₄, Fe(OTf)₃, and a mixture of two or more thereof, more prefer- ably selected from the group consisting of Al(OTf)₃, Bi(OTf)₃, AlCl₃, ZnCl₂, SnCl₄, TiCl₄, Fe(OTf)₃, more preferably selected from the group consisting of Al(OTf)₃ and Bi(OTf)₃, more preferably being Al(OTf)3. In the context of the present invention, preferably, the catalyst system comprises [Ru(acety- lacetonate)₃], 1,1,1-tris(diphenylphosphinomethyl)ethane and MeSO₃H. Preferably, the reduction of carbon dioxide to methylal is performed according to (i.4) at a pres- sure p_R in the range of from 40 to 200 bar, more preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 130 bar. Preferably, the reduction of carbon dioxide to methylal is performed according to (i.4) at a tem- perature TR in the range of from 20 to 200 °C, more preferably in the range of from 50 to 180 °C, more preferably in the range of from 60 to 170 °C, more preferably in the range of from 80 to 140 °C. Preferably, the residence time in RU for obtaining methylal is in the range of from 1 minute to 10 hours, more preferably in the range of from 5 minutes to 2 hours, more preferably in the range of from 5 minutes to 60 minutes. Preferably, apart from methanol, no other solvent is used in RU. Preferably, the reactor unit RU consists of a reactor. Alternatively, preferably, the reactor unit RU comprises two or more reactors arranged in parallel. Preferably, the stream S_MW has a liquid phase and a gas phase. Preferably, the weight ratio of methylal to methanol in S_MW is in the range of from 0.05:1 to 0.5:1, more preferably in the range of from 0.1:1 to 0.4:1, more preferably in the range of from 0.15:1 to 0.35:1. Preferably, the purification unit PU comprises a distillation column D from which the streams S_M and S_W are removed. Preferably, the purification unit PU is a distillation column D from which the streams SM and SW are removed. The present invention preferably relates to a process for preparing methylal from carbon diox- ide, the process comprising (i) preparing a stream S_MW containing methylal, methanol, water and a catalyst system, com- prising (i.1) providing a gas stream G containing CO2 and H2; (i.2) providing a liquid stream SL containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream S_L provided according to (i.2) into a reactor unit RU; (i.4) contacting G with S_L to carbon dioxide reduction conditions, obtaining the stream SMW containing methylal, methanol, water and the catalyst system; (i.5) removing SMW from RU; (ii) separating methylal from the stream S_MW removed from RU in a distillation column D com- prised in a purification unit PU, obtaining a stream S_M comprising methylal and methanol and a liquid stream SW comprising methanol, water and the catalyst system; (iii) removing water from the liquid stream S_W via a separation unit SU, obtaining a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein at least a part of R is recycled into step (i.2) as a component of SL. Alternatively, preferably the purification unit PU comprises one or more distillation columns D. In the context of the present invention, preferably the pressure of the stream S_M removed from PU, more preferably from the top of D, is in the range of from 0.25 to 5 bar, more preferably in the range of from 0.5 to 2 bar. Preferably, the temperature of the stream S_M removed from PU, more preferably from the top of D, is in the range of from 20 to 60 °C, more preferably in the range of from 30 to 50 °C. Preferably, the temperature of the stream S_W removed from PU, more preferably from the bot- tom of D, is in the range of from 50 to 150 °C, more preferably in the range of from 60 to 90 °C. Preferably, the pressure of the stream S_W removed from PU, more preferably from the bottom of D, is in the range of from 0.25 to 5 bar, more preferably in the range of from 0.5 to 2 bar. Preferably, (ii) comprises (ii.1) passing S_MW through a gas-liquid separation sub-unit GLSU for removing H₂ from S_MW, GLSU being comprised in PU and located upstream of a distillation column D comprised in PU, obtaining a stream S*MW depleted in H2 compared to SMW and comprising methylal, methanol, water and the catalyst system, S*_MW having a pressure p*_MW, and a stream H comprising H₂; (ii.2) optionally passing S*_MW through a sub-unit PSU, PSU being comprised in PU, obtaining a stream S**MW having a pressure p**MW, with p**MW < p*MW, S**MW comprising methylal, methanol, water and the catalyst system; (ii.3) passing S*_MW obtained according to (ii.1), or S**_MW obtained according to (ii.2), through the distillation column D, obtaining SM comprising methylal and methanol and SW comprising methanol, water and the catalyst system. Preferably, the stream S**MV comprises a liquid stream S**MV,L having a pressure p**MW,L and a temperature T**MW,L and a gas stream S**MV,G, having a pressure p**MW,G and a temperature T**_MW,G, with p**_{MW =} p**_{MW,L =} p**_MW,G and T**_MW,L= T**_{MW,G =} T**_MW. Preferably, the stream S_MW has a pressure p_MW in the range of from 40 to 200 bar, more prefera- bly in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 120 bar. Preferably, the stream S_MW has a temperature T_MW in the range of from 20 °C to 200 °C, more preferably in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C. Preferably, the stream S*MW has a pressure p*MW in the range of from 40 to 200 bar, more pref- erably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 120 bar, more preferably p*_MW = p_MW. Preferably the stream S*MW has a temperature T*MW in the range of from 20 °C to 200 °C, more preferably in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C, more preferably T*_MW = T_MW. Preferably, the weight ratio of methylal to methanol in S*MW is in the range of from 0.05:1 to 0.5:1, more preferably in the range of from 0.1:1 to 0.4:1, more preferably in the range of from 0.15:1 to 0.35:1. More preferably, the weight ratio of methylal to methanol in S*_MW is the same as the weight ratio of methylal to methanol in SMW. Preferably, at least a part of stream H removed from GLSU is recycled in step (i.1) as a compo- nent of G. Preferably, PSU is a flash drum. Preferably, p**MW is in the range of from 0.5 to 5 bar, more preferably in the range of from 0.75 to 3 bar. Preferably, the stream S**MW has a temperature T**MW in the range of from 20 °C to 200 °C, more preferably in the range of from 50 °C to 120 °C, more preferably in the range of from 60 °C to 80 °C. Preferably, the weight ratio of methylal to methanol in S**MW is in the range of from 0.05:1 to 1.5:1, more preferably in the range of from 0.1:1 to 1.1:1. Preferably, the weight ratio of methylal to methanol in S**MW,L is in the range of from 0.05:1 to 0.5:1, more preferably in the range of from 0.1:1 to 0.4:1. Preferably, the weight ratio of methylal to methanol in S**MW,G is in the range of from 0.05:1 to 1.5:1, more preferably in the range of from 0.8:1 to 1.1:1. Preferably the stream S_M, in addition to methylal and methanol, further comprises methyl for- mate. Preferably, S_M comprises in the range of from 1 to 20 weight-%, more preferably in the range of from 2 to 15 weight-%, more preferably in the range of from 5 to 12 weight-%, of methanol based on the weight of SM. Preferably from 98 to 100 weight-%, more preferably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, more preferably from 99.9 to 100 weight-%, of SW consist of metha- nol, water and the catalyst system. In other words, preferably, SW consists substantially of, more preferably consists of, methanol, water and the catalyst system. Preferably, SW is essentially free of, more preferably free of, methylal. Preferably, the weight ratio of water to methanol in S_W is in the range of from 0.05:1 to 0.50:1, more preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. Preferably, the separation unit SU comprises one or more membranes, said one or more mem- branes being pervaporation membranes. This means that said one or more preferred mem- branes are each individually operated under pervaporation conditions. The present invention preferably relates to a process for preparing methylal from carbon diox- ide, the process comprising (i) preparing a stream S_MW containing methylal, methanol, water and a catalyst system, com- prising (i.1) providing a gas stream G containing CO2 and H2; (i.2) providing a liquid stream S_L containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream S_L provided according to (i.2) into a reactor unit RU; (i.4) contacting G with S_L to carbon dioxide reduction conditions, obtaining the stream S_MW containing methylal, methanol, water and the catalyst system; (i.5) removing S_MW from RU; (ii) separating methylal from the stream SMW removed from RU in a purification unit PU, ob- taining a stream S_M comprising methylal and methanol and a liquid stream S_W comprising methanol, water and the catalyst system; (iii) removing water from the liquid stream SW via a separation unit SU, SU comprising one or more pervaporation membranes, obtaining a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein at least a part of R is recycled into step (i.2) as a component of SL. More preferably, the present invention relates to a process for preparing methylal from carbon dioxide, the process comprising (i) preparing a stream S_MW containing methylal, methanol, water and a catalyst system, com- prising (i.1) providing a gas stream G containing CO2 and H2; (i.2) providing a liquid stream S_L containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream S_L provided according to (i.2) into a reactor unit RU; (i.4) contacting G with S_L to carbon dioxide reduction conditions, obtaining the stream S_MW containing methylal, methanol, water and the catalyst system; (i.5) removing SMW from RU; (ii) separating methylal from the stream SMW removed from RU in a distillation column D com- prised in a purification unit PU, obtaining a stream S_M comprising methylal and methanol and a liquid stream S_W comprising methanol, water and the catalyst system; (iii) removing water from the liquid stream SW via a separation unit SU, SU comprising one or more pervaporation membranes, obtaining a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein at least a part of R is recycled into step (i.2) as a component of SL. Preferably, the separation unit SU further comprises one or more pumps and one or more heat exchangers, in addition to the one or more membranes. Preferably, SU comprises a membrane loop comprising at least one membrane M of the one or more membranes, at least one pump U of the one or more pumps and at least one heat ex- changer H of the one or more heat exchangers. Preferably, SU comprises a plurality of membrane loops, more preferably from 2 to 10 mem- brane loops, more preferably from 2 to 5 membrane loops, more preferably 2 or 3 membrane loops, each membrane loop comprising at least one membrane M of the one or more mem- branes, at least one pump U of the one or more pumps and at least one heat exchanger H of the one or more heat exchangers. Preferably, the process comprising (iii) removing water from the liquid stream S_W via pervaporation through SU comprising one or more pervaporation membranes, obtaining a permeate gas stream P comprising water and a retentate liquid stream R being depleted in water, compared to SW, comprising methanol and the catalyst system, wherein at least a part of R is recycled into step (i.2) as a component of S_L. In the context of the present invention, in case SU comprises 2 or more loops, the permeate stream P corresponds to the permeate stream coming from the most downstream loop, and the retentate stream R is the retentate stream coming from the most downstream loop. Preferably (iii) comprises (iii.1) passing S_W through SU comprising one or more pervaporation membranes, wherein at least one membrane, of the one or more pervaporation membranes, has a water/methanol pervaporation selectivity ßpervap of at least 1, obtaining from SU - a permeate gas stream P comprising water, and optionally methanol; and - a retentate liquid stream R comprising water and methanol at a weight ratio x(R)=w(H2O):w(CH3OH) with x(R)<x(SW), and the catalyst system, x(SW) being the weight ratio of water to methanol in S_W with 0<x(S_W)≤0.5. In the context of the present invention, the term “pervaporation membrane” means that said membrane is operated under pervaporation conditions. Preferably, the at least one membrane, of the one or more pervaporation membranes, has a water/methanol pervaporation selectivity ßpervap of at least 2, preferably in the range of from 2 to 100, more preferably in the range of from 2.1 to 50, more preferably in the range of from 2.25 to 30, more preferably in the range of from 2.5 to 10, more preferably in the range of from 3 to 8, more preferably in the range of from 4 to 6. Preferably, each pervaporation membrane of SU has a pervaporation selectivity ß_pervap of at least 1, more preferably at least 2, more preferably in the range of from 2 to 100, more prefera- bly in the range of from 2.1 to 50, more preferably in the range of from 2.25 to 30, more prefera- bly in the range of from 2.5 to 10, more preferably in the range of from 3 to 8, more preferably in the range of from 4 to 6. Preferably, said at least one membrane, more preferably each membrane, comprises a porous substrate and a porous material disposed on the substrate, wherein from 70 to 100 weight-% of the porous material consists of carbon. Preferably, the porous substrate is made of one or more of alumina, titania, zirconia, and car- bon, more preferably alumina. Preferably, the porous substrate comprises one or more channels, wherein the porous material is disposed on the surface of the walls of the one or more channels of the substrate. Preferably, the channels are made of one or more layers, the one or more layers comprising alumina and pores having a pore diameter of from 2 to 20 nm or of more than 50 nm. Prefera- bly, the outermost layer in contact with the porous material is mesoporous (2-20 nm). Preferably, the porous material comprises carbon and micropores having a pore diameter of less than 2 nm, more preferably less than 0.6 nm (also called ultramicroporous). Preferably, the one or more pervaporation membranes, preferably each pervaporation mem- brane, are/is obtained or obtainable by a process according to US2012/079943 A1 (Example 1). The pervaporation membranes used in the present invention are commercially available. Preferably, x(R) is of at most 0.1:1, more preferably of at most 0.05:1, more preferably in the range of from 0:1 to 0.05:1. More preferably x(R) is of at most 0.025:1. Preferably, the permeate stream P has a pressure p_P of less than 1 bar, more preferably in the range of from 5 to 100 mbar, more preferably in the range of from 10 to 75 mbar. Preferably, the permeate stream P has a temperature in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C. Preferably, the permeate stream P comprises methanol in addition to water. More preferably the weight ratio of methanol to water in P is in the range of from 19:1 to 0.05:1, more preferably in the range of from 4:1 to 0.05:1, more preferably in the range of from 1:1 to 0.05:1. Preferably, the process further comprises passing P through a purification unit PUP, more pref- erably being one or more of a distillation column and a membrane, more preferably a distillation column, obtaining a stream P1, depleted in methanol compared to P, comprising water and a stream P2 comprising methanol, wherein at least a part of P2 is recycled into step (i.2) as a component of S_L. Preferably, the process is a continuous process, a semi-continuous process or a batch process, more preferably a continuous process. Preferably, the process is computer-implemented. Further, the present invention further relates to a production unit for preparing methylal from carbon dioxide, comprising a reactor unit RU configured for preparing a stream SMW containing methylal, methanol, water and a catalyst system, a purification unit PU configured for separating methylal from the stream S_MW so as to obtain a stream SM comprising methylal and methanol and a liquid stream SW comprising methanol, wa- ter and the catalyst system, and a separation unit SU configured for removing water from the liquid stream S_W so as to obtain a stream P comprising water, and a stream R being depleted in water, compared to S_W, compris- ing methanol and the catalyst system, wherein the production unit is configured for recycling at least a part of R into the rector unit RU. Preferably, the reactor unit RU is configured for preparing a stream S_MW containing methylal, methanol, water and a catalyst system by (i.1) providing a gas stream G containing CO₂ and H₂; (i.2) providing a liquid stream S_L containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream SL provided according to (i.2) into a reactor unit RU; (i.4) contacting G with S_L to carbon dioxide reduction conditions, obtaining the stream S_MW containing methylal, methanol, water and the catalyst system; (i.5) removing S_MW from RU. Preferably, the production unit is configured for preparing methylal from carbon dioxide by a process according to the present invention. Preferably, the purification unit PU comprises a distillation column D from which the streams SM and S_W are removable. Preferably, the purification unit PU comprises, in addition to column D, a gas-liquid separation sub-unit GLSU located upstream of D. More preferably, PU further comprises a sub-unit PSU for reducing the stream pressure, PSU being located upstream of D and downstream of GLSU. Preferably, the separation unit SU comprises one or more membranes, said one or membranes being pervaporation membranes. The present invention further relates to a computer program comprising instructions which, when the program is executed by the production unit according to the present invention, cause the production unit to perform the process according to the present invention. The present invention further relates to a computer-readable storage medium comprising in- structions which, when the instructions are executed by the production unit according to the pre- sent invention, cause the production unit to perform the process according to the present inven- tion. The present invention further relates to a non-transient computer-readable medium including in- structions that, when executed by one or more processors, cause the one or more processors to perform the process according to the present invention. The present invention is further illustrated by the following set of embodiments and combina- tions of embodiments resulting from the dependencies and back-references as indicated. In par- ticular, it is noted that in each instance where a range of embodiments is mentioned, for exam- ple in the context of a term such as "The process of any one of embodiments 3 to 5", every em- bodiment in this range is meant to be explicitly disclosed for the skilled person, i.e. the wording of this term is to be understood by the skilled person as being synonymous to "The process of any one of embodiments 3, 4 and 5". Further, it is explicitly noted that the following set of em- bodiments represents a suitably structured part of the general description directed to preferred aspects of the present invention, and, thus, suitably supports, but does not represent the claims of the present invention. 1. A process for preparing methylal from carbon dioxide, the process comprising (i) preparing a stream SMW containing methylal, methanol, water and a catalyst system, comprising (i.1) providing a gas stream G containing CO₂ and H₂; (i.2) providing a liquid stream S_L containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream SL provided according to (i.2) into a reactor unit RU; (i.4) contacting G with S_L to carbon dioxide reduction conditions, obtaining the stream S_MW containing methylal, methanol, water and the catalyst system; (i.5) removing SMW from RU; (ii) separating methylal from the stream S_MW removed from RU in a purification unit PU, obtaining a stream S_M comprising methylal and methanol and a liquid stream S_W comprising methanol, water and the catalyst system; (iii) removing water from the liquid stream SW via a separation unit SU, obtaining a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein at least a part of R is re- cycled into step (i.2) as a component of SL. 2. The process of embodiment 1, wherein according to (i.1) providing a gas stream G, com- prises admixing a gas stream G1 comprising CO2 with a gas stream G2 comprising H2, obtaining G. 3. The process of embodiment 1 or 2, wherein the catalyst system comprises a catalyst com- plex and an acidic co-catalyst, wherein the catalyst complex comprises a transition metal complex and at least one polydentate ligand comprising at least one P atom. 4. The process of embodiment 3, wherein the transition metal of the transition metal complex is selected from the group consisting of ruthenium, manganese, cobalt, iron, osmium, rho- dium, rhenium, iridium, nickel, platinum and palladium, preferably selected from the group consisting of ruthenium, manganese and cobalt, more preferably selected from the group consisting of ruthenium and cobalt, more preferably is ruthenium. 5. The process of embodiment 3 or 4, wherein the transition metal complex is one or more of [Ru(acetylacetonate)3], [Ru(COD)(methylallyl)2], [Co(acetylacetonate) [Ru(p-cymene)Cl₂]₂, [Ru(benzene)Cl₂]_n, [Ru(CO)₂Cl₂]_n, [Ru(CO)₃Cl₂]₂, [Ru(DMSO)₄Cl₂], [Ru(PPh₃)₃(CO)(H)CI], [Ru(PPh₃)₃(CO)Cl₂], [Ru [Ru(PPh₃)₃Cl₂], [Ru(Cp)(PPh₃)₂CI], [Ru(Cp)(CO)₂CI], [Ru(Cp)(CO)₂H], [Ru(Cp*)(CO)2CI], [Ru(Cp*)(CO)2H], [Ru(Cp*)(CO)2]2, [Ru(indenyl)

denyl)(CO)₂H], [Ru(indenyl)(CO)₂]₂, ruthenocen, [Ru(binap)(Cl)₂], [Ru(2,2'-bipyri- din)₂(Cl)₂·H₂O], [Ru(COD)(Cl)₂H]₂, [Ru(Cp*)(COD)CI], [Ru₃(CO)₁₂], [Ru(tetraphenylhy- droxycyclopentadienyl)(CO)2H], [Ru(PMe3)4(H)2], [Ru(PEt3)4(H)2], [Ru(Pn-Pr3)4(H)2], [Ru(Pn-Bu₃)₄(H)₂], and [Ru(Pn-octyl₃)₄(H)₂], preferably one or more of [Ru(acety- lacetonate)₃] and [Ru(COD)(methylallyl)₂], more preferably [Ru(acetylacetonate)₃]. 6. The process of any one of embodiments 3 to 5, wherein the at least one polydentate lig- and comprising at least one P atom is selected from the group consisting of tris(diphe- nylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinomethyl]ethane, tris[di(3,5-dime- thylphenyl)phosphinomethyl]ethane, tris(diphenylphosphinomethyl)methane, tris(diphe- nylphosphinomethyl)amine, bis(2-diphenylphosphinoethyl)phenylphosphine and tris[2-(di- phenylphosphino)ethyl]phosphine, preferably selected from the group consisting of tris(di- phenylphosphinomethyl)ethane, tris[di(p-tolyl)phosphinomethyl]ethane, tris[di(3,5-dime- thylphenyl)phosphinomethyl]ethane and tris(diphenylphosphinomethyl)amine, more pref- erably is tris(diphenylphosphinomethyl)ethane, more preferably 1,1,1-tris(diphe- nylphosphinomethyl)ethane (Triphos). 7. The process of any one of embodiments 3 to 6, wherein the molar ratio of the transition metal complex relative to the at least one polydentate ligand is in the range of from 1:3.0 to 1:1, preferably in the range of from 1:2.0 to 1:1, more preferably in the range of from 1:1.5 to 1:1, more preferably in the range of from 1:1.2 to 1:1. 8. The process of any one of embodiments 3 to 7, wherein the molar ratio of the acidic co- catalyst relative to the catalyst complex is in the range of from 1:50 to 1:1, preferably in the range of from 1:20 to 1:1, more preferably in the range of from 1:10 to 1:1. 9. The process of any one of embodiments 3 to 8, wherein the acidic co-catalyst is one or more of Bronsted acid and a Lewis acid, wherein preferably the acidic co-catalyst is se- lected from the group consisting of methane sulfonic acid (MeSO₃H), Al(OTf)₃, Bi(OTf)₃, AlCl₃, ZnCl₂, SnCl₄, TiCl₄, Fe(OTf)₃, HCl, H₂SO₄, tricholoroacetic acid, p-TsOH, trifluoro- methanesulfonic acid and a mixture of two or more thereof, more preferably selected from the group consisting of methane sulfonic acid (MeSO₃H), Al(OTf)₃, Bi(OTf)₃, H₂SO₄, p- TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃, H₂SO₄, p-TsOH, trifluoro- methanesulfonic acid and mixture of two or more thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃, p-TsOH and a mixture thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃ and a mixture thereof, more pref- erably selected from the group consisting of MeSO3H and Al(OTf)3. 10. The process of any one of embodiments 3 to 9, wherein the catalyst system comprises [Ru(acetylacetonate)₃], 1,1,1-tris(diphenylphosphinomethyl)ethane and MeSO₃H. 11. The process of any one of embodiments 1 to 10, wherein the reduction of carbon dioxide to methylal is performed according to (i.4) at a pressure p_R in the range of from 40 to 200 bar, preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 130 bar. 12. The process of any one of embodiments 1 to 11, wherein the reduction of carbon dioxide to methylal is performed according to (i.4) at a temperature TR in the range of from 20 to 200 °C, preferably in the range of from 50 to 180 °C, more preferably in the range of from 60 to 170 °C, more preferably in the range of from 80 to 140 °C. 13. The process of any one of embodiments 1 to 12, wherein the purification unit PU com- prises a distillation column D from which the streams SM and SW are removed. 14. The process of any one of embodiments 1 to 13, wherein (ii) comprises (ii.1) passing SMW through a gas-liquid separation sub-unit GLSU for removing H2 from S_MW, GLSU being comprised in PU and located upstream of a distillation column D comprised in PU, obtaining a stream S*_MW depleted in H₂ compared to S_MW and comprising methylal, methanol, water and the catalyst system, S*MW having a pres- sure p*MW, and a stream H comprising H2; (ii.2) optionally passing S*_MW through a sub-unit PSU, PSU being comprised in PU, ob- taining a stream S**_MW having a pressure p**_MW, with p**_MW < p*_MW, S**_MW comprising methylal, methanol, water and the catalyst system; (ii.3) passing S*_MW obtained according to (ii.1), or S**_MW obtained according to (ii.2), through the distillation column D, obtaining S_M comprising methylal and methanol and SW comprising methanol, water and the catalyst system. 15. The process of embodiment 14, wherein the stream S_MW has a pressure p_MW in the range of from 40 to 200 bar, preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 120 bar; wherein preferably the stream S_MW has a temperature T_MW in the range of from 20 °C to 200 °C, more preferably in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C. 16. The process of embodiment 14 or 15, wherein the stream S*_MW has a pressure p*_MW in the range of from 40 to 200 bar, preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 120 bar, more preferably p*_MW = p_MW; wherein preferably the stream S*_MW has a temperature T*_MW in the range of from 20 °C to 200 °C, more preferably in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C, more preferably T*_MW = T_MW. 17. The process of any one of embodiments 14 to 16, wherein at least a part of stream H re- moved from GLSU is recycled in step (i.1) as a component of G. 18. The process of any one of embodiments 1 to 17, wherein the stream S_M, in addition to methylal and methanol, further comprises methyl formate. 19. The process of any one of embodiments 1 to 18, wherein from 98 to 100 weight-%, prefer- ably from 99 to 100 weight-%, more preferably from 99.5 to 100 weight-%, more prefera- bly from 99.9 to 100 weight-%, of S_W consist of methanol, water and the catalyst system. 20. The process of any one of embodiments 1 to 19, wherein the weight ratio of water to methanol in SW is in the range of from 0.05:1 to 0.50:1, preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. 21. The process of any one of embodiments 1 to 20, wherein the separation unit SU com- prises one or more membranes, said one or membranes being pervaporation membranes. 22. The process of embodiment 21, wherein the separation unit SU further comprises one or more pumps and one or more heat exchangers, in addition to the one or more mem- branes. 23. The process of embodiment 21 or 22, comprising (iii) removing water from the liquid stream SW via pervaporation through SU comprising one or more pervaporation membranes, obtaining a permeate gas stream P comprising water and a retentate liquid stream R being depleted in water, compared to S_W, compris- ing methanol and the catalyst system, wherein at least a part of R is recycled into step (i.2) as a component of SL. 24. The process of any one of embodiments 1 to 23, wherein (iii) comprises (iii.1) passing SW through SU comprising one or more pervaporation membranes, wherein at least one membrane, of the one or more pervaporation mem- branes, has a water/methanol pervaporation selectivity ß_pervap of at least 1, ob- taining from SU - a permeate gas stream P comprising water, and optionally methanol; and - a retentate liquid stream R comprising water and methanol at a weight ratio x(R)=w(H2O):w(CH3OH) with x(R)<x(SW), and the catalyst system, x(S_W) being the weight ratio of water to methanol in S_W with 0<x(S_W)≤ 0.5. 25. The process of any one of embodiments 1 to 24, being a continuous process, a semi-con- tinuous process or a batch process, preferably a continuous process. 26. The process of any one of embodiments 1 to 25, wherein the process is computer-imple- mented. 27. A production unit for preparing methylal from carbon dioxide, comprising a reactor unit RU configured for preparing a stream S_MW containing methylal, methanol, water and a catalyst system, a purification unit PU configured for separating methylal from the stream SMW so as to ob- tain a stream SM comprising methylal and methanol and a liquid stream SW comprising methanol, water and the catalyst system, and a separation unit SU configured for removing water from the liquid stream S_W so as to ob- tain a stream P comprising water, and a stream R being depleted in water, compared to SW, comprising methanol and the catalyst system, wherein the production unit is configured for recycling at least a part of R into the rector unit RU. 28. The production unit of embodiment 26, wherein the reactor unit RU is configured for pre- paring a stream S_MW containing methylal, methanol, water and a catalyst system by (i.1) providing a gas stream G containing CO2 and H2; (i.2) providing a liquid stream SL containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream S_L provided according to (i.2) into a reactor unit RU; (i.4) contacting G with SL to carbon dioxide reduction conditions, obtaining the stream S_MW containing methylal, methanol, water and the catalyst system; (i.5) removing S_MW from RU. 29. The production unit of embodiment 27 or 28, wherein the production unit is configured for preparing methylal from carbon dioxide by a process according to any one of embodi- ments 1 to 26. 30. The production unit of any one of embodiments 27 to 29, wherein the purification unit PU comprises a distillation column D from which the streams S_M and S_W are removable. 31. The production unit of any one of embodiments 27 to 30, wherein the separation unit SU comprises one or more membranes, said one or membranes being pervaporation mem- branes. 32. A computer program comprising instructions which, when the program is executed by the production unit according to any one of embodiments 27 to 31, cause the production unit to perform the process according to any one of embodiments 1 to 26. 33. A computer-readable storage medium comprising instructions which, when the instruc- tions are executed by the production unit according to any one of embodiments 27 to 31, cause the production unit to perform the process according to any one of embodiments 1 to 26. 34. A non-transient computer-readable medium including instructions that, when executed by one or more processors, cause the one or more processors to perform the process ac- cording to any one of embodiments 1 to 26. In the context of the present invention, the terms “methylal” and “dimethoxymethane” are used interchangeably. The term „bar“ as used in the context of the present invention refers to „bar(abs)“. In the context of the present invention, the term “gas stream” means that the stream has a gas phase. In the context of the present invention, the abbreviation “Cp” stands for cyclopentadienyl, the abbreviation “Cp*” stands for pentamethylcycopentadienyl and the abbreviation “binap” stands for 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl. In the context of the present invention, a term “X is one or more of A, B and C”, wherein X is a given feature and each of A, B and C stands for specific realization of said feature, is to be un- derstood as disclosing that X is either A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. In this regard, it is noted that the skilled person is capable of transfer to above ab- stract term to a concrete example, e.g. where X is a chemical element and A, B and C are con- crete elements such as Li, Na, and K, or X is a temperature and A, B and C are concrete tem- peratures such as 10 °C, 20 °C, and 30 °C. In this regard, it is further noted that the skilled per- son is capable of extending the above term to less specific realizations of said feature, e.g. “X is one or more of A and B” disclosing that X is either A, or B, or A and B, or to more specific reali- zations of said feature, e.g. “X is one or more of A, B, C and D”, disclosing that X is either A, or B, or C, or D, or A and B, or A and C, or A and D, or B and C, or B and D, or C and D, or A and B and C, or A and B and D, or B and C and D, or A and B and C and D. The present invention is further illustrated by the following examples. Examples The following abbreviations are used in the examples: MeFo: methyl formate DMM: dimethoxymethane Triphos: 1,1,1-tris(diphenylphosphinomethyl)ethane COD: 1,5-cyclooctadiene Ph: phenyl Tol: tolyl All components used in the examples are commercially available components. Pervaporation membrane The pervaporation membrane used in the inventive examples is an single channel carbon mo- lecular sieve membrane obtained from Fraunhofer IKTS prepared according to a process de- scribed in Example 1 of US 2012/079943A1. Said membrane has a length of 25 cm, an inner diameter of 7 mm and an outer diameter of 10 mm. The binary pervaporation selectivity ß_pervap for water/methanol measured according to the procedure outlined in 1.3 was of 3.6. Analytics 1.1 GC Analysis of reaction mixtures Retention times of methylal and methyl formate were determined by comparison to samples of the authentic materials. Relative response factors of methylal and methyl formate were deter- mined by calibration against 1,4-dioxane as internal standard. The relative quantities of the two species in the crude reaction mixture were thus determined according to the peak areas and de- termined response factors. Methylal and methyl formate contents were analyzed by gas chromatography according to the following procedure: To a 0.75 gram aliquot of the crude reaction mixture was added 0.25 g 1,4- dioxane as an internal standard. The sample was then analyzed according to the following GC method: GC column: CP-Sil 5 CB for formaldehyde 60 m x ID 320 micrometers x 8 micrometers Carrier gas: He Inlet temperature: 250 °C – Pressure: 3.138 bar – Split ratio: 50:1 Constant flow rate: 8 mL/min – Injection volume: 1 microliter Oven temperature: 45 °C for 3 min, then increase of 10°C/ min ramp rate up to 225 °C, then at 25 °C for 8.25 min (total run time = 29.25 min) Flame Ionization Detector (FID) 290°C 1.2 Water content analysis - Karl Fischer Analysis The water content of the samples was determined by a calibrated volumetric one-component Karl-Fischer titration using a Metrohm Dosimat 805 with 803 Ti Stand, with which the endpoint of the titration is detected by electrometric indication. Methanol was used as the solvent and Hy- dranalTM Composite 5 (Honeywell) was used as the Karl-Fischer titrant. The presence of traces of water in the methanol was compensated for by dosing titrant until the water was reacted away. Then, about 100-400 mg sample was accurately weighted and added to the solvent and the titration was performed until the endpoint was reached. The mass fraction of water is calcu- lated based on the amount of titrant dosed using the Metrohm software. 1.3 Pervaporation selectivity ßpervap of a given membrane The pervaporation selectivity for water/methanol was calculated according to the following equation: H 2 O,

permeate

H 2 O, retentate

methanol, retentate

wherein X_{H2O, permeate}, X_{methanol, permeate}, X_{H2O,retentate} and X_{methanol, retentate} are the mole fractions (mol/mol) of the respective components. The binary pervaporation selectivity ß_pervap of a given membrane at a molar concentration of 25 mol-% is determined as follows: a feed stream with a binary composition of about 30 mol-% wa- ter and 70 mol-% methanol is filled into a batch pervaporation unit equipped with a pervapora- tion membrane. At the permeate side of the membrane, a vacuum of 70±15 mbar is drawn. Then, the feed stream is circulated across the pervaporation membrane at a crossflow velocity of 2.4 m/s and heated to 120 °C. The permeate was condensed using a permeate condenser operated at a temperature of 1 °C. Any permeate collected during the heat-up phase was dis- carded. After reaching the desired temperature, samples are taken from retentate and permeate simultaneously. Since ßpervap may vary with concentration and since it is impossible to measure ßpervap at an ex- act targeted concentration, the value for ß_pervap at X_{water,retentate} = 25 mol-% water is obtained through a linear least squares fit of ßpervap vs Xwater,retentate (mol/mol). The thus obtained pervapo- ration selectivity for the membrane in this example is 3.6.

Example 1: Acid co-catalysts evaluation According to Example 1, methylal was synthesized from CO2, H2 in presence of methanol, the Ru-catalyst source Ru(COD)(methylallyl)2, a ligand Triphos and an acidic co-catalyst. The reac- tion is illustrated by Equation I below.

In an argon filled glovebox, the ruthenium precursor, Ru(COD)(methylallyl)2 (0.041 g, 1.0 eq., 0.127 mmol), triphos ligand (0.083 g, 1.05 eq, 0.133 mmol) and the requisite acidic co-catalyst (4 eq., 0.510 mmol) were charged into a round bottom flask and dissolved in MeOH (78.9 g). This catalyst/solvent mixture was then transferred to a nitrogen flushed steel autoclave (300 mL inner volume) with an overhead stirrer set at 800 rpm, the autoclave sealed and flushed with ni- trogen gas. The autoclave was then charged with 20 bar CO₂ gas pressure and warmed to 30 °C for 30 min. The mass increase of this CO2 dosing was noted as the basis for determining the reaction yield (typically 20 bar corresponded to a CO2 mass of 7-10 g). H2 gas was then added to the autoclave reactor and the autoclave heated to the required reaction temperature (100 °C). The hydrogen gas dosing was controlled so as to obtain a total pressure of 120 bar at 100 °C. The autoclave was once more sealed, and stirred (800 rpm) at 100 °C for the required reaction time (8 h). No further gas dosing was done during the course of the reaction. Heating was then stopped and the autoclave allowed to cool to ambient temperature before being transferred to an ice cooling bath (ca.0 °C). The reactor pressure was then slowly released to minimize loss of the volatile reaction products. The crude reaction mixture was analyzed by gas chromatog- raphy to determine conversion and selectivity for the desired product, methylal. Results are shown below in Table 1. Table 1

*methylal selectivity [%] = methylal [GC wt.%] / (methylal [GC wt.%] + methyl formate [GC wt.%])*100 As may be taken from Table 1, different acidic co-catalysts can be used in combination with the Ru-catalyst. It has been found according to the present invention that, in addition to the known Al(OTf)3 co-catalyst, Bi(OTf)3, MeSO3H and p-TsOH demonstrate great performance as well. Brønsted acids have also been investigated (see entries 1.3, 1.4 and 1.7) but only methanesul- fonic acid (MSA) results in a success as when using trifluoroacetic acid (TFA) as a co-catalyst, no conversion was obtained. We may also note that H₂SO₄ and triflic acid can also be used as co-catalysts (entries 1.6 and 1.8) while no conversion was obtained using HNO3 and very few with BF3 (50% solution in MeOH). Example 2: Ru source evaluation According to Example 2, methylal was synthesized from CO2, H2 in presence of methanol, a Ru- catalyst source, a ligand triphos and MeSO₃H as co-catalyst according to the same proce- dure/recipe as described in Example 1 and with the same molar ratios. The reaction is illus- trated by Equation II below. Equation II:

Table 2

*methylal selectivity [%] = methylal [GC wt.%] / (methylal [GC wt.%] + methyl formate [GC wt.%])*100 † Experiment performed with double the standard quantitites of both ruthenium precursor and triphos ligand As may be taken from Table 2, different sources of Ru-catalyst under different conditions can be used in order to successfully obtained methylal. It has been found according to the present in- vention that Ru(COD)(methylallyl)2 can easily be replaced by Ru(acac)3, a Ru-source cheaper than Ru(COD)(methylallyl)₂ while exhibiting also improved results with a selectivity to methylal of 85% with a reaction time of only 4 hours. Example 3: Water removal according to the present invention A stream S_W having the following composition (mass fraction) was prepared 89.6 % MeOH; 10.0 % H2O; 0.40 % catalyst system: Ru(acac)3 / triphos / Al(OTf)3 (Ru(acac)3: triphos: Al(OTf)3 molar ratio = 1: 1.05: 4). To do so, all components were mixed and heated to 50 °C for 1 hour. This ensured that the triphos ligand and Al(OTf)₃ dissolved fully in methanol prior to passing through the membrane. For this example, the experiment was performed in a batch mode. The stream S_W was then passed through a separation unit. In particular, about 3 kg of S_W was fed into a batch vessel and the stream was then pumped over a pervaporation membrane at a crossflow velocity of 2.5 m/s and heated to 120 °C. The retentate was flowed back to the batch vessel for progressively re- moving water over the pervaporation membrane. The experiment was run for 48 hours. Sam- ples from the retentate streams (R_1-R_7) and a final retentate liquid stream R_8 being de- pleted in water compared to S_W and comprising methanol and the catalyst system were ob- tained. The water content of the retentate at different time was measured (R_1-R_8). The pre- sent membrane separation method using this stream S_W did not result in an increase in pres- sure and was operated successfully until completion, i.e. less than 2 wt.-% residual water con- tent in the retentate as shown below (Table 3). Table 3 Water analysis results of the retentate at different time

Example 4: Water removal according to the present invention A stream SW having the following composition (mass fraction) was prepared 89.8 wt.-% MeOH; 10.0 wt.-% H₂O; 0.20 wt.-% of the catalyst system: Ru(acac)₃ / triphos / methanesulfonic acid (Ru: triphos: MeSO₃H molar ratio = 1: 1: 3). To do so, all components were mixed and heated to 50 °C for 1 hour. This ensured that the triphos ligand and Al(OTf)3 dissolved fully in methanol prior to passing through the membrane. For this example, the experiment was performed in a batch mode. The stream SW was then passed through a separation unit SU. In particular, 4.0kg of SW was fed into a batch vessel and the stream was then pumped over a pervaporation membrane at a crossflow velocity of 2.5 m/s and heated to 120 °C. The retentate stream was flowed back to the batch vessel for continuously removing water over the pervaporation membrane. The experiments was run for 45 hours. The permeate gas stream P had a pressure of 70 mbar comprising water and methanol. A final retentate liquid stream R_7 being depleted in water compared to S_W and comprising methanol, water and the catalyst system. The water content of the retentate at different time was measured (R_1-R_7). The present membrane separation method using this stream SW did not result in an increase in pressure and was operated successfully until completion, i.e. about 2% residual water content in the retentate R as shown below (Table 4). Table 4 Water analysis results of the retentate at different time

Reference Example 1 Process for preparing methylal not according to the present inven- tion A stream S_MW having the following composition (mass fraction) was prepared 84.6 wt.-% MeOH; 7 wt.-% Methylal; 1 wt.-% MeFo; 6.9 wt.-% H2O; 0.1 % HCO2H; 0.4 wt.-% catalyst system: Ru(COD)(methylallyl)2 / triphos / Al(OTf)3 (Ru(COD)(methylallyl)2: triphos: Al(OTf)3 molar ratio = 1: 1.05: 4). To do so, all components were mixed and heated to 50 °C for 1 hour. This ensured that the triphos ligand and Al(OTf)3 dissolved fully in methanol prior to passing through the membrane. While according to the invention, methylal is removed from S_MW prior to passing through a sepa- ration SU. For Reference Example 1, it has been decided to pass the mixture SMW comprising methylal first through a separation unit, comprising a pervaporation membrane, to remove water and to remove methylal via distillation in a purification unit PU afterwards.3.5 kg of SMW was used to run the experiment using the pervaporation membrane. Upon subjecting the reaction mixture to the conditions of the membrane separation, namely 120 ºC at 8 bar N₂, it was proved that applying such reaction conditions was impossible. Indeed, already upon reaching a temper- ature of 90 °C, a pressure build-up of 12 bar was registered. Because of the continuously rising pressure, the pressure was relieved from the system to avoid an undesired discharge of the unit contents over a safety valve. Over the course of a further 5 hours, the pressure needed to be released frequently to prevent a pressure build-up. The pressure increase continued after reaching a steady temperature, indicating that not only physical effects were taking place, but that a pressure increase continued due to the release of gaseous components. This was con- firmed by analysis results: for 5 samples, all taken within the first 3 hours after heating, the con- tent of methylal decreased rapidly, from 7.0 wt.-% at the start of the experiment, to 2.65 wt.-% after 1.5 h after heating started, to 0.87 wt.-% after 2 hours (see Figure 3). The rapid decrease in concentration is presumably due to a combination of physical release through venting and thermal degradation under presence of the catalyst (and absence of hydrogen and CO2). At the end of the experiments, no methylal could be detected in the retentate anymore. The content of methyl formate decreased correspondingly. Thus, this example clearly demonstrates that the order of the steps of the process of the present invention is of importance. Example 5 Process for preparing methylal according to the present invention (entire process) – simulation The simulation was done with process simulation software (commercial). The components used in the process simulation and their characteristics respectively, were taken from the Dortmund Database. The simulated process is based on the flow diagram of Figure 2. Operating conditions: The following feed streams were used for the simulation. A stream G comprising H2 and CO2 at a H2:CO2 molar ratio of 3:1 and a stream SL comprising 10 weight.-% of the catalyst system and 90 weight-% of methanol. The feed quantity were set such that S*_MW had a catalyst concentration of 550 ppm by weight and the stream S_L was ad- justed such that S*MW contained 10 weight.-% of water. The stream G was introduced into RU with stream S_L having a pressure of 103 bar and a tem- perature of about 88 °C. The reaction took place in RU at 100 bar and 120°C. The conversion of CO2 was set to 100%. A stream SMW was removed from RU said stream had a pressure of 100 bar and a temperature of 120°C and then passed through a purification unit PU comprising a gas liquid separation sub-unit GLSU, a sub-unit PSU and a distillation column D. First, the stream SMW was passed through GLSU, obtaining a stream S*MW depleted in H2 compared to SMW and a stream H comprising H2. S*MW was then passed through the sub-unit PSU (flash drum) for reducing the pressure of the stream S*_MW which was of 100 bar, obtaining a stream S**_MW comprising a liquid stream S**_MV,L and a gas stream S**_MV,G both having a pressure of 1.8 bar. S**MW was then passed through the distillation column D. The column D was set up such that the weight ratio of methylal to methanol in SM was of 9:1. SM had a pressure of 1.1 bar and a temperature of 44 °C. The stream S_W was removed from D and had a temperature of 70 °C and pressure of 1.115 bar. S_W had a water content of 12 weight-%. The stream S_W was then fed to the simulated separation unit SU comprising a pervaporation membrane, a heat exchanger and a pump (one membrane loop). The conditions for the heat exchanger in the loop were set such that the feed stream SW to the pervaporation membrane was at 120 °C, and the ratio of the retentate stream R to the internal loop recycle stream were set such that a temperature drop of 5°C over the pervaporation membrane was achieved. The conditions were also set up such that the permeate gas stream P had a pressure of 70 mbar. The temperature of the stream P exiting SU was of 115°C and P comprises methanol and water with a water to methanol weight ratio of 1:4. The retentate liquid stream R removed from SU was depleted in water compared to both S_W and S_MW. Indeed, the water content was reduced to only 3.6 weight-% in retentate R, permitting to recycle at least a part of R as a component of SL with a minimum content of water, which is beneficial to maximize the yield of methylal in the pro- cess. Therefore, with the process according to the present invention, it is possible to produce methylal from carbon dioxide and hydrogen at high conversion rate while allowing the recycling of methanol and the catalyst. Brief description of the figures Figure 1 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention. The production unit comprises a reactor unit RU, a purification unit PU and a separation unit SU, with SU preferably comprising one or more pervaporation membranes. The gas stream G comprising G1 (carbon dioxide) and G2 (H₂) and a liquid stream S_L, comprising methanol and a catalyst system, are fed into the reactor unit RU and carbon dioxide is subjected to a reduction reaction for obtaining methylal, preferably at a reduction temperature T_RU at a reduction pres- sure p_RU as detailed in the foregoing. A stream S_MW is removed from RU with S_MW comprises methylal, methanol, water and the catalyst system CS. The stream S_MW is then passed through the purification unit PU obtaining a stream SM comprising methylal and methanol, and a liquid stream S_W comprising methanol, water and the catalyst system. Preferably, the pressure of S_M removed from PU is in the range of from 0.25 to 5 bar, more preferably in the range of from 0.5 to 2 bar, and SM has a temperature in the range of from 20 to 60 °C, more preferably in the range of from 30 to 50 °C. Preferably, the pressure of S_W removed from PU is in the range of from 0.25 to 5 bar, more preferably in the range of from 0.5 to 2 bar, and S_W has a temperature in the range of from 50 to 150 °C, more preferably in the range of from 60 to 90 °C. The liquid stream SW is then passed through SU, obtaining a permeate gas stream P comprising water and optionally methanol and a retentate liquid stream R depleted in water compared to S_W and comprising methanol and the catalyst system. Preferably, the permeate gas stream P has a pressure p_P of less than 1 bar, more preferably in the range of from 5 to 100 mbar, more prefer- ably in the range of from 10 to 75 mbar. At least a part of R is recycled into SL to be introduced into RU. The water removed as P from SU can be further treated and used in different pro- cesses or used to create heat for the process of the present invention. Figure 2 is a schematic representation of a production unit used for the process according to preferred embodiments of the invention. The production unit comprises a reactor unit RU, a purification unit PU and a separation unit SU, with SU preferably comprising one or more pervaporation membranes. The purification unit PU comprises a gas-liquid separation unit GLSU and a distillation column D, GLSU being lo- cated upstream of D. The gas stream G comprising G1 (carbon dioxide) and G2 (H₂) and a liq- uid stream SL, comprising methanol and a catalyst system CS, are fed into the reactor unit RU and carbon dioxide is subjected to a reduction reaction for obtaining methylal, preferably at a reaction temperature T_RU at a reaction pressure p_RU as detailed in the foregoing. A stream S_MW is removed from RU, SMW comprising methylal, methanol, water and the catalyst system, and passed through GLU for removing H2, obtaining a stream S*MW depleted in H2 having preferably the same pressure and temperature of S_MW exiting RU. S*_MW is then passed through the distilla- tion column D, obtaining a stream S_M comprising methylal and methanol, and a liquid stream S_W comprising methanol, water and the catalyst system. Preferably, the pressure of SM removed from PU is in the range of from 0.25 to 5 bar, more preferably in the range of from 0.5 to 2 bar, and S_M has a temperature in the range of from 20 to 60 °C, more preferably in the range of from 30 to 50 °C. Preferably, the pressure of SW removed from PU is in the range of from 0.25 to 5 bar, more preferably in the range of from 0.5 to 2 bar, and S_W has a temperature in the range of from 50 to 150 °C, more preferably in the range of from 60 to 90 °C. The liquid stream S_W is passed through SU, preferably comprising one or more pervaporation membranes, obtaining a permeate stream P comprising water and a retentate stream R, depleted in water compared to S_W, comprising methanol and the catalyst system. At least a part of R is recycled into S_L to be introduced into RU. Preferably, the permeate gas stream P has a pressure p_P of less than 1 bar, more preferably in the range of from 5 to 100 mbar, more preferably in the range of from 10 to 75 mbar. The water removed as P from SU can be further treated and used in different pro- cesses or used to create heat for the process of the present invention. Figure 3 represents the feed pressure and temperature as a function of experiment time for Ref. Example 1 not according to the present invention. Cited literature - WO 2020/161175 A1 - US 2012/079943 A1

Claims

Claims 1. A process for preparing methylal from carbon dioxide, the process comprising (i) preparing a stream S_MW containing methylal, methanol, water and a catalyst system, comprising (i.1) providing a gas stream G containing CO2 and H2; (i.2) providing a liquid stream S_L containing methanol and the catalyst system; (i.3) introducing the gas stream G provided according to (i.1) and the liquid stream SL provided according to (i.2) into a reactor unit RU; (i.4) contacting G with SL to carbon dioxide reduction conditions, obtaining the stream S_MW containing methylal, methanol, water and the catalyst system; (i.5) removing S_MW from RU; (ii) separating methylal from the stream SMW removed from RU in a purification unit PU, obtaining a stream S_M comprising methylal and methanol and a liquid stream S_W comprising methanol, water and the catalyst system; (iii) removing water from the liquid stream SW via a separation unit SU, obtaining a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein at least a part of R is re- cycled into step (i.2) as a component of S_L. 2. The process of claim 1, wherein the catalyst system comprises a catalyst complex and an acidic co-catalyst, wherein the catalyst complex comprises a transition metal complex and at least one polydentate ligand comprising at least one P atom. 3. The process of claim 2, wherein the transition metal of the transition metal complex is se- lected from the group consisting of ruthenium, manganese, cobalt, iron, osmium, rhodium, rhenium, iridium, nickel, platinum and palladium, preferably selected from the group con- sisting of ruthenium, manganese and cobalt, more preferably selected from the group con- sisting of ruthenium and cobalt, more preferably is ruthenium; wherein more preferably the transition metal complex is one or more of [Ru(acety- lacetonate)₃] and [Ru(COD)(methylallyl)₂], more preferably is [Ru(acetylacetonate)₃]. 4. The process of claim 2 or 3, wherein the molar ratio of the acidic co-catalyst relative to the catalyst complex is in the range of from 1:50 to 1:1, preferably in the range of from 1:20 to 1:1, more preferably in the range of from 1:10 to 1:1. 5. The process of any one of claims 2 to 4, wherein the acidic co-catalyst is one or more of Bronsted acid and a Lewis acid, wherein preferably the acidic co-catalyst is selected from the group consisting of methane sulfonic acid (MeSO₃H), Al(OTf)₃, Bi(OTf)₃, AlCl₃, ZnCl₂, SnCl4, TiCl4, Fe(OTf)3, HCl, H2SO4, tricholoroacetic acid, p-TsOH, trifluoromethanesulfonic acid and a mixture of two or more thereof, preferably selected from the group consisting of methane sulfonic acid (MeSO₃H), Al(OTf)₃, Bi(OTf)₃, H₂SO₄, p-TsOH, trifluoromethanesul- fonic acid and a mixture of two or more thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃, H₂SO₄, p-TsOH, trifluoromethanesulfonic acid and mix- ture of two or more thereof, more preferably selected from the group consisting of MeSO3H, Al(OTf)3, p-TsOH and a mixture thereof, more preferably selected from the group consisting of MeSO₃H, Al(OTf)₃ and a mixture thereof, more preferably selected from the group consisting of MeSO₃H and Al(OTf)₃. 6. The process of any one of claims 2 to 5, wherein the catalyst system comprises [Ru(acet- ylacetonate)₃], 1,1,1-tris(diphenylphosphinomethyl)ethane and MeSO₃H. 7. The process of any one of claims 1 to 6, wherein the reduction of carbon dioxide to meth- ylal is performed according to (i.4) at a pressure p_R in the range of from 40 to 200 bar, preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 130 bar. 8. The process of any one of claims 1 to 7, wherein the reduction of carbon dioxide to meth- ylal is performed according to (i.4) at a temperature TR in the range of from 20 to 200 °C, preferably in the range of from 50 to 180 °C, more preferably in the range of from 60 to 170 °C, more preferably in the range of from 80 to 140 °C. 9. The process of any one of claims 1 to 8, wherein the purification unit PU comprises a dis- tillation column D from which the streams S_M and S_W are removed. 10. The process of claim 9, wherein (ii) comprises (ii.1) passing S_MW through a gas-liquid separation sub-unit GLSU for removing H₂ from S_MW, GLSU being comprised in PU and located upstream of a distillation column D comprised in PU, obtaining a stream S*_MW depleted in H₂ compared to S_MW and comprising methylal, methanol, water and the catalyst system, S*MW having a pres- sure p*_MW, and a stream H comprising H₂; (ii.2) optionally passing S*_MW through a sub-unit PSU, PSU being comprised in PU, ob- taining a stream S**MW having a pressure p**MW, with p**MW < p*MW, S**MW comprising methylal, methanol, water and the catalyst system; (ii.3) passing S*_MW obtained according to (ii.1), or S**_MW obtained according to (ii.2), through the distillation column D, obtaining S_M comprising methylal and methanol and SW comprising methanol, water and the catalyst system. 11. The process of claim 10, wherein the stream S*_MW has a pressure p*_MW in the range of from 40 to 200 bar, preferably in the range of from 80 to 150 bar, more preferably in the range of from 85 to 140 bar, more preferably in the range of from 90 to 120 bar; wherein more preferably the stream S*_MW has a temperature T*_MW in the range of from 20 °C to 200 °C, more preferably in the range of from 50 °C to 180 °C, more preferably in the range of from 60 °C to 170 °C, more preferably in the range of from 80 to 140 °C. 12. The process of claim 10 or 11, wherein at least a part of stream H removed from GLSU is recycled in step (i.1) as a component of G. 13. The process of any one of claims 1 to 12, wherein the weight ratio of water to methanol in S_W is in the range of from 0.05:1 to 0.50:1, preferably in the range of from 0.06:1 to 0.40:1, more preferably in the range of from 0.07:1 to 0.25:1. 14. The process of any one of claims 1 to 13, wherein the separation unit SU comprises one or more membranes, said one or membranes being pervaporation membranes; wherein preferably the separation unit SU further comprises one or more pumps and one or more heat exchangers, in addition to the one or more membranes. 15. A production unit for preparing methylal from carbon dioxide, comprising a reactor unit RU configured for preparing a stream S_MW containing methylal, methanol, water and a catalyst system, a purification unit PU configured for separating methylal from the stream SMW so as to ob- tain a stream SM comprising methylal and methanol and a liquid stream SW comprising methanol, water and the catalyst system, and a separation unit SU configured for removing water from the liquid stream S_W so as to ob- tain a stream P comprising water, and a stream R being depleted in water, compared to S_W, comprising methanol and the catalyst system, wherein the production unit is configured for recycling at least a part of R into the rector unit RU.