An insight into possibility of chemical reaction between dense carbon dioxide and methanol

    An insight into possibility of chemical reaction between dense carbon dioxide and methanol R.D. Oparin, M.A. Krestyaninov, E.A. Vorobyev, O.I. Pokrovskiy, O.O. Parenago, M.G. Kiselev PII: DOI: Reference:

S0167-7322(16)30798-X doi: 10.1016/j.molliq.2016.12.027 MOLLIQ 6710

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

1 April 2016 21 July 2016 10 December 2016

Please cite this article as: R.D. Oparin, M.A. Krestyaninov, E.A. Vorobyev, O.I. Pokrovskiy, O.O. Parenago, M.G. Kiselev, An insight into possibility of chemical reaction between dense carbon dioxide and methanol, Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2016.12.027

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ACCEPTED MANUSCRIPT AN INSIGHT INTO POSSIBILITY OF CHEMICAL REACTION BETWEEN DENSE CARBON DIOXIDE AND METHANOL

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R.D. Oparina [email protected]; M.A. Krestyaninova; E.A. Vorobyeva; O.I. Pokrovskiyb; O.O.

G.A. Krestov Institute of Solution Chemistry of RAS, Ivanovo, Russia

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Kurnakov Institute of General and Inorganic Chemistry of RAS, Moscow, Russia

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b

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Parenagob and M.G. Kiselev a

ACCEPTED MANUSCRIPT Abstract Possibility of chemical reaction between components of binary mixture of CO2-MeOH was demonstrated by analyzing in situ IR spectroscopy data. The kinetics of this reaction was studied in

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a wide range of temperatures under isochoric conditions showing that the process is considerably

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slow. Dependences of product and intermediates yields on temperature were obtained by means of qualitative analysis of in situ IR spectroscopy data as well. Temperature growth leads to an increase in the overall reaction speed, although its influence on various process stages is not straightforward.

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It has been found that the reaction between MeOH and CO2 is a two-stage process. The intermediate formed at the first stage is monomethyl carbonate which then reacts with methanol

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producing dimethyl carbonate and water. We believe that this synthesized water further partly reacts with CO2 producing carbonic acid which is responsible for the increased acidity of CO2-

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MeOH mixtures. Quantum chemical calculations reveal that the first stage is only possible when some sort of catalyst is involved. We assume that since all experiments with supercritical carbon

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dioxide are performed in a stainless steel vessel, the inner surface of the latter might serve as a

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catalyst. Fe(OH)2 was chosen as a model compound representing the type of acidic catalytic centre on the surface of stainless steel that might promote interaction between CO2 and methanol. With

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Fe(OH)2 involved, the potential barrier of the reaction first stage dramatically decreases, from 192 kJ/mol to 8.5 kJ/mol. And the energy of the complex of reaction products with Fe(OH)2 is lower than the energy of the complex of Fe(OH)2 with initial components, which confirms the hypothesis of vessel walls being able to catalyze a reaction between CO2 and methanol.

ACCEPTED MANUSCRIPT Introduction Carbon dioxide is the most widely used solvent in supercritical fluid (SCF) technologies as it is non-toxic, cheap, readily available, chemically inert, non-flammable and has low critical

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parameters. The biggest CO2 disadvantage is its limited solvating strength, especially for polar

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substances. This fact limits considerably the sphere of applicability of the CO2 as a solvent. The most frequently used approach to overcome this limitation is addition of so-called «co-solvents» or «modifiers» to CO2. A co-solvent is a polar organic solvent miscible with CO2 at required process

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parameters that is added to CO2 in a quantity that leads to a substantial increase in solvating strength, but does not cause the loss of benefits associated with using a SCF. Lower alyphatic

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alcohols such as methanol, ethanol and isopropanol are the most frequently used co-solvents. CO2ROH mixtures are used as extractants in supercritical fluid extraction of polar biologically active

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substances [1], as mobile phases in supercritical fluid chromatography [2], as media for organic synthesis [3], as solvents for forming polymer microparticles by rapid expansion of supercritical

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solutions (RESS) [4] and so on.

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It is generally assumed that alcohols do not react with CO2 [5-7]. All the available physicochemical data for such mixtures, e.g. phase diagrams, were obtained based on this assumption.

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However, in the last 20 years, a number of observations have been made indicating that CO2 and ROH are not inert to each other. It has been repeatedly observed that mixtures of supercritical СО2 with alcohols possess acidity which exceeds that of pure alcohols. Zheng et al. [8] proved solution acidity increase by observing the change of its color after the barbotage of gaseous СО2 through the solutions of various pH indicators in methanol. The intensity of color change depended proportionally on CO2 pressure. The authors of [9] showed that СО2 /alcohol mixtures converted the Reichardt’s

dye

into

protonated

form

which

in

turn

gave

acidic

reaction

with

diazo(diphenyl)methane [10]. They made an assumption that CO2 reacts with alcohols in a similar manner to its interaction with water. This reaction results in a monoalkyl ester of carbon acid, which is responsible for the acidic medium of this system [9-11]. Subsequently, this group of

ACCEPTED MANUSCRIPT authors proposed and successfully tested the usage of СO2-MeOH gas-expanded liquids as in situ acid catalysts [11]. Based on their conclusions, the authors of [12, 13] gave an explanation of the unusual behavior of chromatographic sorbents with amino-groups in supercritical fluid

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chromatography with СО2-MeOH mobile phases. It was hypothesized that in supercritical fluid

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chromatography amino-groups of sorbents react with СО2 and MeOH with the formation of ionic moieties similarly to the formation of so-called reversible ionic liquids [14]. It was further hypothesized but not proved that the first stage of this process is the formation of

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monomethylcarbonic (MMC) acid in solution from CO2 and MeOH with subsequent MMC reaction with amines.

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This set of assumptions has several weak points. First of all, MMC is an extremely unstable, short living substance, which may exist only at low temperatures and in absence of water and

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alcohols [15, 16]. The possibility of MMC formation resulting from direct interaction of MeOH and CO2 is regarded to be extremely low [16]. So it is doubtful that this particular compound is solely

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responsible for the observed increased acidity of CO2-ROH mixtures. Second, the above-mentioned

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considerations did not account for the possible role of residual water. Water can be present in alcohols in large enough quantities to be responsible for acidity growth resulting from contact with

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CO2. Third, the chemical interaction of CO2 and methanol is an extensively-studied process [17-25] as it is a synthetic route towards dimethylcarbonate (DMC), a promising green methylation agent. It is generally stated that carbon dioxide and methanol do not react spontaneously. Presence of heterogeneous catalysts is necessary to implement this chemical reaction. The rate of this reaction depends on the catalytic agent, the catalyst quantity and the experimental conditions. The authors of [26] have studied a large number of substances of various catalytic activity with respect to reaction of direct DMC synthesis from MeOH and CO2. They have shown that the usage of different types of catalytic agents may considerably change the reaction time and product yield. The kinetics of DMC synthesis in the presence of m-ZrO2 catalyst at ambient temperature is studied in [27]. This work investigates the evolution of IR spectra of DMC within 180 minutes until reaching the

ACCEPTED MANUSCRIPT equilibrium concentration of components. It shows that MMC may be formed during this process but it is only covalently bonded to catalyst surface and does not exist in bulk solution as a free molecule. In addition, no acidic reaction products, which might form in solution, triggering an

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acidic reaction of the CO2 and MeOH mixture, were found, even in the presence of catalytic agents.

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Therefore, although the acidity increase of [СО2–ROH] mixtures is observed and confirmed on multiple occasions, the reason for this phenomenon is not clear yet.

Not taking into account the cases when supercritical CO2 does not react with MeOH even in

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absence of a targeted catalyst may have undesirable consequences. The appearance of a new component may change equilibrium parameters, displace the critical point of mixture (see e.g. [17,

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28]). In this case it is imperative to know dynamic characteristics of this hypothetical reaction in order to be able to decide whether or not it should be taken into consideration. Depending on the

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process, on the chemical nature of new compound(s), and on the speed of their formation it may be either negligible or on the contrary may play a significant role.

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Consequently, the aim of the present work was to study the possibility of a reaction between

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CO2 and MeOH without a targeted catalyst in a wide range of state parameters. IR spectroscopy was chosen as the main investigation tool because it allows in situ estimation of components

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concentration in the mixture, as well as studying the influence of state parameters on reaction yield.

Experimental section A binary mixture of scCO2 and MeOH with the component molar ratio of [MeOH]:[CO2]≈1:39 was chosen as a subject of inquiry. It was dictated by the necessity of maximal proximity to a number of technological processes using CO2-MeOH mixtures, e.g. supercritical fluid chromatography. The experimental investigations were carried out under isochoric heating conditions in the temperature range of 60-170ºС with the step ΔT=10ºС. The density of carbon dioxide phase was 11.032 mol·L-1. The corresponding pressure range was 127333 bars.

ACCEPTED MANUSCRIPT Materials The CO2 gas (99.995% purity, with residual water of 0.001%) was purchased from ‘‘Linde Gas

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Rus’’. Methanol «Fischer Scientific UK Limited» (HPLC grade, 99.95% purity) was purchased

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from ‘‘Sigma Aldrich’’. The methanol underwent additional drying. The water concentration in dry methanol was measured by K. Fischer titration and did not exceed 0.001 % of the weight.

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High-temperature-high-pressure IR spectroscopy

For infrared absorption experiment, we used a specially designed high-temperature-high-

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pressure (HPHT) optical cell made of special alkali-acid resistant stainless steel (60% Fe, 12% Cr, 18% Ni, 10% Ti). The inner surface of the cell was finished in order to reach the surface roughness

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of 3.2µm. The cell volume was about 5 cm3. This cell was equipped with two windows made of optical silicon, each 8mm thick. No coating was used for the windows. The optical pathlength was

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constant and its value was 3.5 mm. The usage of an HPHT cell with such windows allows

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measuring the high resolution spectra in the wavenumber range of 900 to 7500 cm-1. A detailed description of this cell as well as the experimental setup was given in one of our recent works [29].

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Using a temperature control system made it possible to reach a low value of temperature gradient. Thus, its value within the reaction medium did not exceed ±1ºС. To eliminate apparatus error and to exclude spectral contribution of the optical windows material, the final sample spectra were calculated by subtracting the spectra of the cell with CO2 from the corresponding spectra of the cell with CO2–MeOH mixture measured at the same thermodynamic conditions. To reduce statistical error, each spectrum was obtained by averaging 96 spectrograms.

Scheme of IR experiment

ACCEPTED MANUSCRIPT The experimental cell was preheated up to the temperature of 60ºС. After pumping the cell up to vacuum (10-3 bar) and venting it by dry CO2, the necessary volume of methanol was injected through the bottom port into the cell. The necessary pressure in the cell was created by means of

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CO2 which was pumped through the upper port of the cell. It is also necessary to note that in our

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experiment we did not use stirring. The infrared absorption measurements were made on a BRUKER Vertex-80V Fourier-spectrometer by the following scheme: 

For T=60ºС, the spectra were measured automatically with certain time intervals until

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reaching total methanol dissolution in CO2. At first, the interval was 15 min. and further this time was increased up to 120 min. This allowed estimating the kinetic parameters of a

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possible reaction between CO2 and methanol. The total time of that process at T=60ºС was 3840 minutes.

For T=70-170ºС, the spectra were measured in the same way, once every 10ºС. The total time

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

of the process at each temperature was 720 minutes.

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Finally, the total IR experiment time including the time required for reaching each working

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temperature was 11775 minutes. The experiment was performed in such a manner that the whole set of experiments was carried out without depressurizing and refilling the cell. Therefore, each final

system.

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spectrum for each temperature contained information about cumulative thermal changes in the

Quantum chemical calculation details Quantum chemical mechanics calculations were carried out using GAUSSIAN 09 software [30]. The reagent and product complexes and transition state geometries were optimized at MP2 level of theory using 6-31++G(d,p) basis set. Optimization and frequency calculation were also performed at B3LYP level using an aug-CC-pVTZ basis set for some reagents (CO2, MeOH, H2O) and products (MMC, DMC, H2CO3) to experimental vibrational spectra decomposition. The vibration spectra of the initial components as well as those of the products did not have any

ACCEPTED MANUSCRIPT imaginary frequencies, whereas the transition states (TS) had only one imaginary frequency, corresponding to reaction coordinates. For the non-catalytic reaction there was only one reagent complex and product. For the reaction involving catalyst we chose a more stable reagent and

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product complex. The transition states were optimized by QST3 method [31, 32] that requires

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reactant and product structures and initial stricture for transition state. The energy values are listed with ZPE corrections.

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Results and Discussions

Figure 1 shows the experimental spectra in a wavenumber range of 1200-7500см-1 for each

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temperature. Each spectrum was measured at τn time which corresponds to the last spectrum of Ti temperature. The spectral region of 1550-1850 cm-1 which corresponds to the stretching vibrations

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of ν(C=O) and bending mode of water of δ(H2O) was chosen as the analytical one. This spectral domain is suitable for our task as a hypothetical reaction between carbon dioxide and water might

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result in appearance of products containing a functional group of C=O as well as water.

Figure. 1. Experimental spectra measured at time τn which corresponds to the last spectrum for each temperature Ti and calculated by subtracting spectra of the cell with CO2 from the corresponding spectra of the cell with CO2–MeOH mixture measured at the same thermodynamic conditions in the wavenumber range of 1200-7500 cm-1.

ACCEPTED MANUSCRIPT Presented in Figure 2, the evolution of spectral curves with temperature increase allows us to suppose the behavior of chemical reaction. Two new signals occur: one at 1605 cm-1, which should at 1745 cm-1 attributed to the stretching

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be attributed to the water bending mode and the other

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vibrations of ν(C=O). The intensities of these spectral contributions increase with temperature. The procedure of spectral band deconvolution using «Fityk» software (ver. 1.3.0) [33] was used for quantitative analysis of spectral changes. Taking into account the fact that the molar ratio of the

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initial components was [MeOH]:[CO2]≈1:39, one may suppose that the concentration of possible reaction products even after full methanol conversion is quite small. Therefore, it is safe to assume

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that the product molecules exist in CO2 solution as monomers and there is no direct correlation between them. The possibility of hydrogen bond formation between them is also negligible.

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Consequently, for deconvolution of spectral bands we used an approximation model which contains the Lorentz profiles only. Indeed, the usage of Gaussian profiles gave the unsatisfactory

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reproducibility of experimental spectral bands.

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In order to get a good agreement between the experimental spectral curve and the approximated one we used different sets of Lorentz profiles for different temperature ranges. In the

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temperature range of T=60-100ºС and T=170ºС, four Lorentz profiles as well as an empirically obtained base line were used. In the temperature range of T=110-160ºС, one more additional Lorentz profile was used (see reasoning below). Figure 3 shows examples of spectral deconvolution for several temperatures. To assign the obtained spectral contributions to the vibrations of certain functional groups, the data obtained from quantum chemical calculations (See Tab. 1) were used. The calculations were carried out by DFT/B3LYP method in aug-CC-pVTZ basis set for several substances which might potentially be possible products of chemical reaction between the initial components and between the products themselves.

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Figure. 2. Evolution of spectral contributions in wavenumber range corresponding to the stretching

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vibrations of ν(C=O) and bending mode of water of δ(H2O)

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Taking into account the appearance of water and substances containing carbonyl group, we

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propose a two-stage mechanism of MeOH - CO2 reaction. The first stage is an equimolar reaction leading to formation of «MMC»:

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CH3OH + CO2 → [(CH3O)COOH]*

As stated previously, direct synthesis of MMC from MeOH and CO2 in solution is an

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improbable scenario. Hereafter “MMC” is used to denote this compound in whatever form it might exist in the reaction media, not referring specifically to a “free” (non bonded) molecule in solution. Some insight into a possible form of MMC existence in high pressure CO2/MeOH mixture is given below. The second stage is a reaction of MMC with the second mole of MeOH, giving DMC and water. [(CH3O)COOH]* + CH3OH → (CH3O)CO(CH3O) + H2O Additionally, some part of this water synthesized from CO2 and MeOH may react with CO2 with the formation of carbonic acid. H2O + CO2 ↔ H2CO3

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Figure. 3. Examples of approximation of experimental spectral curve by sets of Lorentz profiles and empirically obtained base line for several temperatures.

Therefore, assuming full methanol conversion, this reaction results in the formation of four

ACCEPTED MANUSCRIPT products (water, MMC, DMC and H2CO3) in the mixture. Moreover, according to the results of quantum chemical calculations (See Tab. 1), the new weak spectral contribution, which occurs in the temperature range of T=110-160ºС may be assigned to the associates of MMC with CO2

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(denoted here and further in the text as MMC_CO2). However, in the temperature range of T=60-

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100ºС and T=170ºС, it is not possible to extract the spectral component assigned to this associate because its signal is lower than the detection threshold. As a result, these mixture components are

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taken into consideration only for the temperature range of T=110-160ºС.

Table 1. Assignment of spectral contributions obtained from deconvolution of experimental

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spectral band to vibrations of functional groups obtained by quantum chemical calculations for potentially possible chemical reaction products

Experiment, ν, cm-1

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Vibration

(in the temperature

Quantum chemical calculations ν, cm-1

α

1607

1627

75.76

1740

1781

438.70

ν(C=O) of DMC

1743

1784

325.38

ν(C=O) of MMC

1755-1760

1800

431.47

ν(C=O) of H2CO3

1796-1817

1818

550.66

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δ(H2O)

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range of 60-170 ºС)

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ν(C=O) of MMC_CO2

In order to estimate the dynamic parameters of the chemical reaction between CO2 and MeOH at T=60ºС, we recorded the spectra in an automatic mode by the scheme described in the experimental section. Using the procedure of spectral band deconvolution, we obtained the time dependences of integral intensities of the spectral profiles assigned to stretching vibrations of C=O

ACCEPTED MANUSCRIPT functional group of molecules MMC, DMC and H2CO3 as well as bending mode of water molecules. These functions are presented in Figure 4. There are two intervals on each A(τ) graph presented in Figure 4. The first interval may be

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approximated by the exponential equation:

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A=A0 – С·exp(–τ/t),

where A is the integral intensity, C is a constant, τ is the time, t is the logarithmic decrement, A0 is the integral intensity at time τ=∞.

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The second interval may be approximated by a linear equation. Such functional behavior at the first stage may be a consequence of two parallel processes simultaneously taking place in the

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system. One of them is related to methanol dissolution in carbon dioxide. The other is related to the reaction between MeOH and CO2. In case of the second interval, its linear behavior is only

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associated with the chemical reaction. In case of DMC, the function A(τ) becomes linear at τ ~1200 minutes from the process start. For the other products this time was ~1800 minutes. The relative

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increase in the integral intensity, defined as A(τn)/A(τ1), was ~2.45 for DMC, ~2.73 for MMC,

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~2.26 for water.

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Figure. 4. Time dependences of integral intensities of the spectral profiles assigned to stretching

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vibrations of C=O functional group of molecules MMC, DMC, H2CO3 and bending mode of water

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molecules at T=60ºС

Following the assumption that the chemical reaction between MeOH and CO2 has two stages, and taking into consideration the obtained kinetics results, one may suppose that at T=60ºС the possibility of the first stage leading to MMC appearance is higher than the possibility of the second stage leading to DMC and H2O. Moreover, only a certain part of MMC may react with MeOH, producing DMC, while the residual MMC remains in the system. The growth in δ(H2O) integral intensity is noticeably less than that in ν(C=O) for DMC. However, according to stoichiometry of the second stage of reaction, the change of water concentration should be equal to the change of DMC concentration. In our opinion, this fact proves the hypothesis that a certain portion of water reacts with CO2 giving H2CO3. Since at the temperature of T=60ºС, starting from the time of

ACCEPTED MANUSCRIPT τ~1800 minutes as well as for the temperature range of T=70-170ºС for the whole time interval, the time dependences of A(τ) are linear, we analyzed only the spectra recorded at time τn for each of the temperatures. Deconvolution of these spectra provided parameters of spectral profiles, their

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temperature dependences are presented in Figure 5.

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The presented results show that the temperature increase has virtually no effect on the positions of spectral profiles of ν(C=O) (for DMC molecules and associates of MMC_CO2) and δ(H2O). However, the temperature dependence of ν(C=O) for free molecule MMC has its maximum

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at T=120ºС. Dispersions of spectral profiles depend on the temperature. For spectral contribution assigned to δ(H2O), the value of Δν1/2 linearly grows with the temperature. In case of ν(C=O) (for

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MMC and DMC molecules), the value of Δν1/2 also increases under heating but the dependences of Δν1/2(T) have an inflection point at T=120ºС. For the spectral contribution assigned to ν(C=O) for

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MMC_CO2 associate, the temperature dependence of dispersion has a maximum at T=140ºС. The functional dependences of A(T) for these spectral profiles are also presented in Figure 5. The value

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of A(νC=O(MMC)*) is a sum of integral intensities of spectral bands assigned to C=O stretching

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vibrations of free MMC molecules and associates of MMC_CO2. The temperature dependence of this value is proportional to the total yield of MMC at each temperature. Each A(T) curve may be

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approximated by the following sigma-like function: A=A2 –(A2 –A1)/(1+exp((T – T0)/t)),

where A is the integral intensity, (A1 and A2 are integral intensities at the first and last temperatures respectively), T is the temperature, T0 is the temperature at inflection point, t is the logarithmic decrement. Since the value of integral intensity of spectral contribution is proportional to the corresponding mixture component concentration, one may calculate the change of concentration of these components with the temperature. For this purpose we used the temperature dependences of reduced integral intensity which was calculated as c'=A/α. Here α is the band activity obtained from quantum chemical calculations (See Tab. 1). It is also important to note that in order to calculate the

ACCEPTED MANUSCRIPT total water concentration at each of the temperatures we took into consideration water which was consumed for the formation of carbon acid. One mole of water is required for the formation of one mole of H2CO3. Therefore, the total concentration of water synthesized as a result of the reaction

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between MeOH and CO2 may be calculated as A(H2CO3)/α(H2CO3)+A(H2O)/α(H2O). The values

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of integral intensities of ν(C=O) spectral band for H2CO3 molecules as a temperature function are also presented in Figure 5. The nature of C=O group stretching vibrations in the molecules of substances containing carbonyl and water molecule bending mode is different. Thus, in order to

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compare the reduced integral intensities of spectral bands assigned to these vibrations, the values of c' should be presented on the same scale. The temperature dependencies of c' for spectral bands

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assigned to vibrations of ν(C=O) and δ(H2O) are presented in Figure 6.

ACCEPTED MANUSCRIPT Figure 5. The temperature dependences of position (ν, cm-1), dispersion (∆ν1/2, cm-1) and integral intensity (A, cm-1) of spectral contributions for spectra recorded at time τn for each of temperatures.

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Here A(νC=O(MMC)*)=A(νC=O(MMC))+A(νC=O(MMC_CO2)).

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As Figure 6 shows, c' values which are proportional to the concentrations of the corresponding components increase with the temperature growth. Starting from T=110ºС, the signal from MMC_CO2 associates becomes higher than the detection threshold. Moreover, it continues to

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grow reaching its maximum at T=130ºС. Then it decreases with the temperature increase up to T=170ºС. Comparing c'(T) curves for ν(C=O) of DMC molecules with δ(H2O), we may assume that

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in the temperature range of T=60-140ºС all the water in the system is produced at the second stage of the reaction between CO2 and MeOH. However, heating above 140ºС leads to a considerable

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increase in water concentration in the mixture as compared to the DMC concentration, the latter remaining constant. This feature leads us to an assumption about another chemical reaction taking

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place in this system under these conditions. The authors of [34] show that a side reaction of simple

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etherification between two methanol molecules may take place in CO2-MeOH mixture: CH3OH + CH3OH → CH3OCH3 + H2O,

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It is shown [34] that this reaction occurs only at high temperatures. In the temperature range of T=80-180ºС the possibility of dimethyl ether (DME) formation is less than that of DMC, but it increases with temperature. Water forming in this reaction leads to an increase in the total water concentration in the mixture. This is in good agreement with the c'(T) curves behavior for DMC and H2O in the temperature range of T=140-170ºС.

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Figure. 6. Temperature dependences of reduced integral intensities c' for spectral bands assigned to

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vibrations of ν(C=O) and δ(H2O)

The temperature dependences presented in Figure 6 of c' are sigma-like functions except the

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one for MMC_CO2 associates. In case of ν(C=O) of DMC molecules and δ(H2O), the value of T0 parameter is ~113ºС. Thus, in the temperature range of 60-113ºС, the increase in DMC and water

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yields is related to the growing possibility of MeOH and CO2 reaction. However, further heating

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leads to a decrease in methanol concentration due to its self-etherification. It results in deceleration of DMC concentration growth. The reaction between methanol and carbon dioxide has two stages

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(see the text above). The concentration of the unstable MMC which is the product of the first stage is less than that of stable DMC in the temperature range of 60-160ºС, but at temperature above 160ºС the MMC concentration exceeds the DMC concentration. It is also related to a decrease in methanol concentration in the mixture and, hence, to a decrease in the possibility of the second reaction stage. In Figure 5 we show that the position of maximum of ν(C=O) of MMC is not constant, but its shift does not exceed 5 cm-1. Therefore, one may suppose that most of these molecules are not involved in specific interactions. However, as one can see from the temperature dependence of A(T) (see. Figure 5), only a very small part of MMC are able to participate in associates formation with CO2 (MMC_CO2). In the temperature range of 110-130ºС, the concentration of these associates, being proportional to integral intensity of the spectral band, grows. At the temperatures

ACCEPTED MANUSCRIPT above 130ºС the concentration of MMC_CO2 associates decreases. In turn, the whole concentration of MMC, being proportional to the sum of integral intensities of the spectral bands assigned to C=O stretching vibrations of free MMC molecules and associates of MMC_CO2, increases. Then the

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temperature dependence of A(T) associated with this sum is a sigma-like function with the T0

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parameter value equal to ~130ºС.

An estimation of DMC concentration at different temperatures may be made based of data for water concentration. In order to determine the latter, we conducted an additional experiment at

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60ºС. A certain amount of water was added to the initial mixture of methanol and carbon dioxide. We used a priori known concentration of water which was equal to ~5.2 mol % with respect to CO2.

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Along with this, the concentration of methanol in the mixture was the same as in the main experiment. The recorded spectra for this additional experiment are shown in Figure 7.

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It is obvious that adding water leads to an increase in the intensity of the spectral band assigned to vibrations of ν(C=O) of H2CO3 molecules and δ(H2O). However, this water does not

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affect the reaction between methanol and carbon dioxide and, in particular, the yields of MMC and

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DMC. This fact is confirmed by the absence of contributions of ν(C=O) of MMC and DMC molecules into the difference spectrum which was obtained by subtracting the spectrum of

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anhydrous initial mixture from the spectrum of the initial mixture containing water (see Figure 7).

a

b

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Figure 7. a - Spectra recorded at 60ºС for anhydrous initial mixture of CO2 and MeOH and for

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initial mixture containing water in the spectral range of 1480-2000 cm-1. b - Difference spectrum

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obtained by subtracting the anhydrous initial mixture spectrum from the spectrum of initial mixture containing water.

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To estimate the concentration of “free” water in the mixture for each of the temperatures, one should know the integral molar absorption coefficient for δ(H2O) spectral band. This coefficient

ε=A/lc,

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may be calculated as

component molar concentration.

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where A is the integral intensity of the spectral band, l is the optical pathlength of sample, с is the

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Since an optical cell has a constant pathlength, we may use reduced integral molar absorption

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coefficient: ε'=A/c. The integral intensity of δ(H2O) spectral band was obtained in the procedure of difference spectrum deconvolution, and its value was 7.56 см-1. Further in order to calculate the

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absorption coefficient on the basis of difference spectrum, we need to know the water concentration in the mixture, but we should not take into consideration water produced by the chemical reaction between CO2 and MeOH. This concentration is calculated assuming that there is a molar relation of the mixture initial components with additional water ([H2O]:[MeOH]:[CO2] = 0.05:0.024:0.926). Taking molar densities of the components at 60ºС equal to 54.877, 23.935 and 11.032 mol·L-1 for water, methanol and CO2, respectively, and knowing the experimental cell volume (4.65 ml), we calculated the sought-for water concentration. This concentration was 0.590 mol·L-1. As a result, we calculated the reduced integral molar absorption coefficient. Its value was 12.81·103 cm2·mol-1. In the initial system which did not contain water, the concentration of water formed as a result of reaction is half the concentration of methanol (in case of its full conversion). Since the

ACCEPTED MANUSCRIPT concentration of initial methanol is small compared to CO2, water molecules formed in the system will exist in monomeric form and will not create H-bonds with each other. In one of our recent works [35] we showed that if molecules of substances in solution exist in monomeric form, or if

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their certain functional groups do not create H-bonds, the integral extinction coefficient of spectral

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band assigned to vibration of these groups is almost constant in a wide range of temperatures. Therefore, in order to calculate water concentration in the mixture at each of the temperatures we used a constant value of reduced integral molar absorption coefficient and the values of integral

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intensity of δ(H2O) spectral band obtained above. The temperature dependence of the yield of water formed as a result of the reaction between CO2 and MeOH is presented in Figure 8. As this figure

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shows, the total water yield is 0.1388 mol∙L-1.

It is possible to calculate the maximum theoretical concentration of water in the system if full

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methanol conversion occurs. The methanol concentration in the initial mixture was calculated based on molar ratios of initial components ([MeOH]:[CO2]=0.025:0.975) as well as the data on molar

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density of MeOH and CO2 and volume of the experimental cell (see text above). The value of

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methanol concentration was 0.2795 mol∙L-1. On the one hand, two methanol molecules reacting with one molecule of carbon dioxide will produce one DMC molecule and one water molecule. On

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the other hand, in case of etherification reaction, two methanol molecules will produce one water molecule and one DME molecule. Therefore, irrespective of the reaction taking place in the system, the maximal theoretical concentration of water will be a half of the initial methanol concentration (0.1398 mol∙L-1).

ACCEPTED MANUSCRIPT Figure 8. Temperature dependence of yield of water formed as a result of the chemical reaction between CO2 and MeOH

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These data show that the maximal concentration of “free” water in the mixture after reaction,

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obtained by spectroscopy (0.1388 mol∙L-1) is less than the theoretical maximum. This may have two reasons. Firstly, in spite of the fact that the temperature shifts the equilibrium of the reaction between H2O and CO2 toward the initial components, a small amount of H2CO3 still remains in the

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system because of high pressure. Thus, a small part of water present in the system is spent on H2CO3 formation. As a result, the presence of H2CO3 in the mixture results in an acid reaction of

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the medium. Secondly, as Figure 8 shows, the c(T) curve does not attain plateau at high

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temperatures, so we may suspect that full conversion of methanol is still not reached.

Quantum chemical calculations

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The fact that CO2 and MeOH react with each other even in the absence of deliberately added

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catalyst requires explanation. As it was mentioned in the introduction, the absolute necessity of a catalyst for methanol activation was confirmed by numerous investigations [17-25]. Without a

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catalyst these two compounds should not react under conditions typical for SCF technologies. But, as IR spectroscopy shows, the reaction goes notwithstanding and we assume that there is a substance in the system which can work as a catalyst for this reaction. We suppose that metal oxides and/or hydroxides which are present on the inner surface of high pressure stainless steel reactors can serve as such catalysts. In order to provide a support for this hypothesis we performed quantum chemical simulations of possible reactions between carbon dioxide and methanol in presence of Fe(OH)2 and without it. As an example, Fe(OH)2 was chosen as the most abundant metal hydroxide which might be present on the inner surface of a typical SCF reactor. We calculated the barriers of the first stages of corresponding chemical reactions of one molecule of MeOH and one molecule of CO2 producing the complex of MMC with Fe(OH)2 and MMC without

ACCEPTED MANUSCRIPT complex (see Figures 9 and 10). Through a straightforward calculation of MeOH and CO2 reaction barrier we obtain the value of 192 kJ∙mol-1 (see Figure 9). This proves extremely low possibility of such reaction as the potential barrier is prohibitively high.

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A completely different behavior is observed in the presence of Fe(OH)2 (see Figure 10). The

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MeOH and CO2 reaction barrier dramatically decreases and becomes equal to 8.5 kJ∙mol-1. This value is comparable to the energy of thermal fluctuations at the temperatures used in this study which makes the possibility of this reaction very high. The calculated energy of the complex of

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reaction products with Fe(OH)2 is 38 kJ∙mol-1 lower than that of the complex of Fe(OH)2 with the initial components. This fact confirms our hypothesis too. For comparison, the total reaction

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products energy of the same system without Fe(OH)2 is 40 kJ∙mol-1 higher than the total energy of the initial components (see Figure 9). Therefore, MMC formation as a result of the chemical

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reaction in the medium containing Fe(OH)2 should occur with higher possibility.

Figure 9. Scheme of the first stage of reaction between MeOH and CO2 passing through transition state (TS) without Fe(OH)2 and producing MMC.

It is worth noting that in case of Fe(OH)2 involvement, MMC is not formed as a free molecule with a carboxylic functional group. It exists only as a part that is covalently bonded to the Fe atom

ACCEPTED MANUSCRIPT or to whatever centre on the stainless steel surface, which might catalyze the reaction in question. So it is doubtful that MMC might be responsible for the increased acidity of CO2-MeOH mixtures

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subsequent reaction with CO2 appears to be more plausible.

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at high pressure. Our alternative explanation based on the assumption of water synthesis and

Figure 10. Scheme of the first stage of reaction between MeOH and CO2 passing through transition

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Conclusions

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state (TS) in presence of Fe(OH)2 and producing a complex of MMC with Fe(OH)2.

The possibility of chemical reaction between components of binary mixture of [CO2-MeOH] producing dimethyl carbonate was investigated by analysing in situ IR spectroscopy data. The kinetics of this reaction was studied in a broad range of temperatures under isochoric conditions. The data acquired can be smoothly interpreted within a paradigm of a reaction between MeOH and CO2 happening as a two-stage process. An intermediate forming at the first stage is monomethyl carbonate. The second stage is a reaction between monomethyl carbonate and methanol leading to the formation of dimethyl carbonate and water. We suppose that this synthesized water might be responsible for the increased acidity of CO2-MeOH mixtures as it can react with CO2 producing carbonic acid. Based on the quantitative analysis of IR spectra, we also obtained yields of products

ACCEPTED MANUSCRIPT and intermediates as a temperature function. The first stage of reaction becomes energetically possible only if there is a possibility of complex formation between reagents and some acidic hydroxyl groups carrier, for instance, Fe(OH)2 which might be present on the surface of a high

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pressure stainless steel reactor. As shown by ab initio calculations, the complex formation leads to a

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drastic decrease in the potential barrier of the first stage. The energy of the complex of reaction products with Fe(OH)2 becomes lower than that of the complex of Fe(OH)2 with initial components. The obtained kinetic characteristics show that even at high temperatures the reaction between

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CO2 and MeOH is considerably slow. Practically, the possibility of this reaction should be taken into account when performing accurate thermodynamic measurements requiring a significant

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amount of time, but can be freely ignored in those cases where CO2 and methanol contact for a

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short period of time at moderate temperatures.

Ackowledgements

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This work was supported by Russian Scientific Foundation, grant N. 14-33-00017.

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ACCEPTED MANUSCRIPT Highlights - Reaction between dense CO2 and methanol was studied by in situ IR spectroscopy.

- It is a two stage process giving dimethyl carbonate and waters.

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- Reaction kinetics was monitored in a wide temperature range.

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- Synthesized water might be responsible for increased CO2-MeOH mixture acidity.

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- This reaction might be catalysed by the surface of stainless steel vessels.