Transesterification of Diols and Glycerol with ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Thermal (Catalyst-Free) Transesterification of Diols and Glycerol with Dimethyl Carbonate: A Flexible Reaction for Batch and ContinuousFlow Applications Sandro Guidi, Roberto Calmanti, Marco Noè, Alvise Perosa, and Maurizio Selva* Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari Venezia, Via Torino 155, 30172 Venezia-Mestre, Italy S Supporting Information *

ABSTRACT: An innovative thermal transesterification protocol for the synthesis of linear and alkylene carbonates was investigated under both batch and continuous-flow (CF) conditions. Accordingly, model 1,n-diols (n = 2−4) and glycerol were set to react with dimethyl carbonate (DMC) at T and p of 150−260 °C and 1−50 bar, respectively, in the absence of any catalyst. 1,2-diols afforded the corresponding five-membered ring carbonates as the main products with a quantitative conversion and a selectivity up to 94%, whereas 1,3-diols gave the six-membered ring products along with linear mono- and dicarbonate derivatives. A complete conversion was attained also for glycerol, but the products distribution depended on reaction conditions: the CF mode allowed the synthesis of glycerol carbonate, whereas batch reactions yielded either glycerol carbonate or its derivative from a further transesterification reaction, i.e., methyl (2-oxo-1,3-dioxolan-4-yl)methyl carbonate. The selectivity toward these two compounds was in the range of 83%−94%. An addition of gaseous CO2 (up to 20 bar) allowed to control further the selectivity of batch reactions. KEYWORDS: Transesterification, Catalyst-free, Glycerol, Diols, Continuous-flow, Dimethyl carbonate

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INTRODUCTION In the past 2 decades, the growing demand for eco-friendly products in the field of solvents, lubricants, fuel additives, and organic intermediates for pharmaceuticals, has fueled a massive interest in organic carbonates (OCs) as both linear and cyclic compounds.1−4 The preparation of such derivatives makes an extensive use of transesterification protocols for both the laboratory practice and large scale applications.5 To cite only one remarkable case, the recent Asahi-Kasei process for the industrial production of polycarbonate involves as many as three transesterification reactions in the overall synthetic sequence: these include the reaction of (i) ethylene carbonate with methanol to produce dimethyl carbonate (MeOCO2Me, DMC), (ii) DMC with phenol to obtain diphenyl carbonate (PhOCO2Ph, DPC), and (iii) DPC with bis-phenol A to get the desired polycarbonate product.6 This clearly exemplifies the crucial role of the transesterification, or to say it more appropriately, of the transcarbonation reaction for the synthesis of OCs. Such processes are usually catalyzed under acid, basic or even neutral conditions.7,8 Also, we noticed that ionic liquids of the class of carbonate phosphonium salts act as very efficient organocatalysts for the transesterification of dialkyl carbonates with diols: depending on the substrate structure, the reaction selectivity may shift from cyclic to linear dicarbonate products that can be isolated in very good yields.9 In a continuation of © XXXX American Chemical Society

this study aimed at a further greening of these transesterification protocols, the implementation of thermal (noncatalytic) reactions has attracted our attention. Modern technologies for the integrated heat and energy recovery allow a rather cheap access to high temperature (150−200 °C) and mid-to-low pressure (40−50 bar) conditions that are necessary, for example, to induce thermal transesterification processes. Not surprisingly, recent studies have estimated that large scale production of biodiesel by supercritical (sc, catalystfree) transesterification of vegetable oils with methanol is even less energy intensive and has a lower PEI (potential environmental impact) per mass of product than conventional base-catalyzed transesterification processes.10,11 This holds true notwithstanding T and p of 270−400 °C and 10−65 MPa may be required for sc-transformations. On the basis of these considerations, we have recently reported the continuous-flow (CF) monotransesterification of dialkyl carbonates (DMC, diethyl- and dibenzyl-carbonate) with biobased derivatives such as glycerol acetals (solketal and glycerol formal).12 This work Special Issue: Building on 25 Years of Green Chemistry and Engineering for a Sustainable Future Received: July 14, 2016 Revised: August 31, 2016

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DOI: 10.1021/acssuschemeng.6b01633 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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mixture of glycerol, DMC, and methanol in 1:10:6 molar ratio, respectively, was used. Methanol (as a cosolvent) allowed the formation of a clear homogeneous solution. Products were purified and characterized as reported in the SI section

has demonstrated that on condition to optimize T and p in the range of 250−300 °C and 30−50 bar, respectively, not only a quantitative reaction takes place without any catalyst but also an excellent selectivity (up to 98%) is achieved. Moreover, CFconditions allow further advantages including the easy scale-up of the process and the separation of products, and the absence of catalysts rules out deactivation phenomena, thereby implying that the process can virtually operate indefinitely.13−15 These results have inspired the present investigation in which the thermal transesterification of dialkyl carbonates (DMC and DEC) with 1,n-diols [n = 2−4; ethylene glycol (EG), 1,2- and 1,3-propanediols (PG and 1,3-PD), 1,3- and 1,4-butanediols (1,3- and 1,4-BD)], and glycerol has been explored. To the best of our knowledge, no previous studies have ever been reported on this subject. The screening of these reactions not only proved the feasibility of the process but it demonstrated that (i) regardless of batch or CF-conditions, 1,2-diols were converted into the corresponding five-membered ring carbonates (ethylene and propylene carbonates: EC and PC, respectively) with a very high selectivity (up to 95%); (ii) glycerol yielded glycerol carbonate in the CF-mode (250 °C and 50 bar), whereas either glycerol carbonate or its transesterification derivative [methyl (2-oxo-1,3-dioxolan-4-yl)methyl carbonate] could be selectively obtained under batch conditions (88% and 82%, respectively at 180 °C); (iii) due to the moderate stability of six-membered ring carbonates, 1,3- and 1,4-BD always formed mixtures of the corresponding cyclic and linear (mono- and di-) transesterification derivatives.

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RESULTS AND DISCUSSION Batch Reactions of 1,2-Diols with DMC. An initial investigation was carried out on the reaction of DMC with PG. Mixtures of DMC and PG (1) were prepared in different molar ratios (Q = DMC:PG) Q = 2.5, 5, 10, and 20, and they were set to react for 5 h, in an autoclave, at 120, 150, 180, and 220 °C, respectively. All tests were repeated at least twice to check for reproducibility.16 The screening definitely proved the feasibility of the thermal reaction: under the best found conditions (150 °C and Q = 5), a quantitative conversion was achieved with an excellent selectivity (≥95%) toward propylene carbonate (1a). The structure of 1a was assigned by GC/MS analyses and by comparison to a commercial sample (Scheme 1A). Minor byproducts were mono- and ditransesterified derivatives 1b, 1b′, and 1c.9 Scheme 1. Catalyst-Free Reaction of 1,n-Diols with DMC

EXPERIMENTAL SECTION

Materials and the equipment used in this study are listed and described in the SI section. Batch Reactions. General Procedure. A 200 mL stainless steel autoclave was charged with a mixture of a diol (1−3) or glycerol (6) (each substrate: 500 mg) and DMC. The reactants molar ratio (Q = DMC:substrate) was varied from 2 up to 80. The autoclave was degassed via three vacuum−nitrogen cycles, and then electrically heated at the desired temperature (120−220 °C). The reaction was allowed to proceed from 0.5 to 24 h, during which the reacting mixture was kept under magnetic stirring. The autogenous pressure was in the range of 2−20 bar. At the end of each run, the autoclave was rapidly cooled at rt (in a water bath), and vented. The final mixture was analyzed by GC/FID and GC/MS. The same procedure was used to run experiments also under an additional pressure of a gas chosen among CO2, N2, He, and Ar. Three purging cycles were run using the selected gas; after that, the autoclave was set under a pressure of the same gas in the range of 5−50 bar. Continuous-Flow Reactions. General Procedure. The in-house assembled apparatus described in the SI section (Figure S1) was used for CF experiments. A mixture of a diol (1−5) and DMC was prepared by varying the reactants molar ratio (Q) from 1.1 to 19. The reactants solution (10 mL) was initially used to prime the CF-apparatus at rt. Then, it (solution) was delivered to the CF-reactor (an empty capillary steel tube: 2.65 m × 1/16″) at a flow rate of 0.1 mL/min, while the operating temperature and pressure were set at the desired values (from ambient to 50 bar, and 150 to 280 °C, respectively) by a backpressure regulator and a thermostated oven. Once an amount of the reacting mixture (∼8.5 mL) equal to 5 times the inner volume of the reactor was allowed to flow, samples were taken up (by a Rheodyne valve) at regular intervals of 30 min and analyzed by GC/FID and GC/MS. Once the experiment was complete, the reactor was cooled to 60 °C, washed with methanol (100 mL at 0.5 mL/min), and finally, vented at ambient pressure. The same procedure was used also for the reaction of glycerol with one difference: due to the poor mutual miscibility of reactants, a

No reaction took place at 120 °C, whereas the products distribution did not appreciably change at temperatures above 150 °C, or by using a higher DMC excess (Q > 5: Figure 1). By contrast, at 150 °C and Q = 2.5, the conversion and the

Figure 1. Reaction of PG with DMC. Conversion and selectivity toward 1a. Q = DMC:1 molar ratio. Other conditions: 150 °C, 5 h. B


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reaction conversion as a function of both the pressure and the temperature.

selectivity did not exceed 78% and 67%, respectively: the formation of sizable amounts (33%) of unidentified byproducts was noticed (Figure 1). To rule out any metal catalysis by the reactor, and therefore to prove the authentically catalyst-free nature of the process, a further test was run at 150 °C and Q = 5, by loading the reactant mixture in a glass reactor placed inside the autoclave.17 The result was identical to that described in Scheme 1A (Conversion: 100%; 1a > 95%). The reaction of DMC with EG (2) was then explored in the same range of temperature (120, 150, 180, and 220 °C) and Q molar ratios (DMC:2 = 2.5, 5, 10, and 20) used for PG. As in the case of PG, the reaction took place only at or above 150 °C and it required a moderate DMC excess (Q ≥ 5): under the best conditions, the conversion was quantitative and the selectivity toward ethylene carbonate (2a), though not as good as that for 1a, was still satisfactory (87%; Scheme 1A and Figure 2). Byproducts were mono- and ditransesterification derivatives

Figure 3. CF-reaction of PG with DMC. Conversion of PG in function of temperature and Pressure. Other conditions: DMC:PG molar ratio, Q = 5; flow rate: 0.1 mL/min. In repeated tests, values of conversion differed by less than 5% from one reaction to another.

At 50 bar, the reaction was triggered at 220 °C and went to completion at 240 °C (Figure 3: left to right), i.e., at a quite higher temperature than that required for batch experiments (150 °C). However, given the reactor volume (1.7 mL) and the operating flow rate were (0.1 mL/min), the corresponding residence time (τ) was of only ∼15 min (compared to 5 h of batch reactions). An energy input was therefore necessary to impart sufficiently fast kinetics to the CF-reaction. Different Authors have reported that OH groups of both alcohols and phenols could deprotonate trough autoprotolysis at high temperatures (250−380 °C) in the absence of any catalyst.19−22 If so, in the reaction of Figure 3, the alcohol might play a double role as a reactant and an acid catalyst for the process. Even more interesting was the trend of conversion in function of the pressure. At 240 °C, no reaction occurred below 15 bar, whereas a sharp increase of the substrate conversion (from 1−2 to ∼85%) was noted for small increments of the pressure in the range of 15−20 bar. At higher pressures, only an additional 10% improvement of the conversion (up to 95%) was observed. This peculiar behavior matched our previous results on thermal reactions of glycerol acetals,12 thereby confirming the key role played on these transformations by phase transitions. If the pressure was high enough to maintain the (majority of) reacting mixture as a condensed phase, the contact of DMC and PG was effective for a productive reaction. However, if the pressure dropped below a threshold value, reactants rapidly vaporized as soon as they reached the reactor: τ was therefore dramatically reduced and so was the conversion. To corroborate this hypothesis, a modified Wagner equation (Ambrose 1986) was used to predict the liquid−vapor pressure profile of pure DMC up to its supercritical state (see SI, Figure S2).23,24 Notwithstanding that the reactant mixture composition was obviously different from pure DMC, the abrupt change of conversion observed at 15−20 bar well suited the theoretical profile of Figure S2. Overall, if compared to the limited capacity (1.7 mL) of the used CF-reactor, CF-experiments allowed a satisfactory productivity up to 22 mg/min of propylene carbonate. Once the thermal regime was reached, the CF-system could operate

Figure 2. Reaction of EG with DMC. Conversion and selectivity toward 2a. Q = DMC:2 molar ratio. Other conditions: 150 °C, 5 h.

2b and 2c (Scheme 1A). EG, however, proved remarkably less reactive than PG. This was manifest when a Q ratio as low as 2.5 was used: at 150 °C, the conversion of 1 and 2 were 78% and 2%, respectively (cf. Figures 1 and 2). Overall, the comparison of batch thermal tests with the results of our previous study on the organocatalytic transesterification of DMC with PG and EG indicated that although the catalytic method was less energy intensive (reactions took place at T ≥ 90 °C),9 the two procedures offered comparable selectivities at complete conversion, and proved the same reactivity trend for the investigated 1,2-diols. In particular, the mutual repulsion of substituents (valency deviation) and the effect of reactive rotamers could offer an explanation for the easier formation of propylene carbonate with respect to ethylene carbonate.18 CF-Reactions of 1,2- and 1,3-Diols with DMC. The good results of batch thermal transesterifications of DMC with PG and EG prompted us to explore the same reactions in the CFmode. For initial tests, a solution of DMC and PG in a 5:1 molar ratio was used: four isobaric experiments were run at 50 bar, while the temperature was progressively increased from 150 to 180, 220, and 240 °C; then, five isothermal reactions were carried out at 240 °C, by decreasing the pressure from 40 to 20, 15, 10, 5 bar, and finally, to ambient. Figure 3 reports the C


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respectively. Reactions were performed in an autoclave and were run for 5 h over a range of temperatures (150−220 °C). Tests were repeated twice to check for reproducibility.16 The best conditions were found at 180 °C: homogeneous solutions were always recovered at the end of the experiments. The first unprecedented finding was the occurrence of a thermal (catalyst-free) reaction between glycerol and DMC. The structures of major products (6a, 6b, 6c, 6d, and 6e) were assigned by both comparison to authentic commercial samples and characterization by NMR and GC/MS of isolated compounds (Scheme 2 and SI section). Some unidentified

virtually indefinitely under the desired autogenous pressure; the latter had to be monitored and controlled, but no highly expensive compression/decompression cycles were involved. The high reaction temperature apparently implied an energetic issue which, however, could be efficiently mitigated by integrating the thermal reaction in a waste heat recovery system. Particularly, modern biorefinery units are developing elegant sustainable designs for heat recovery or exchange within heat sinks and the usage of excess heat as part of the cogeneration plants.25 The CF-reactions of DMC with EG (2), 1,3-propandiol (3), 1,3-butanediol (4), and 1,4-butanediol (5) were then explored (Scheme 1A,B). Case by case, the reactants mixture was prepared by using the minimum amount of DMC able to ensure the formation of a homogeneous solution. All experiments were carried out at 50 bar. Although reactions were far from being optimized, Table 1 summarize the best preliminary results achieved in terms of conversion and selectivity toward each of the observed products.

Scheme 2. Reaction of Glycerol with DMC: Major Products and Pathways for Their Formation

Table 1. Reaction of DMC with Diols 2, 3, 4, and 5 selectivityc (%)

entry diol

DMC:diol molar ratio (Q)a

T (°C)

conv.b (%)

1 2 3

2 3 4

14 19 6

240 260 240

99.5 75.0 94.0

4

5

19

280

99.5

reaction products 2a:82 3a:14 4a:17

2b:7 3b:53 4b +4b′:39 5b:18

2c:11 3c:22 4c:32 5c:82

a

The DMC:diol molar ratio was adjusted case-by-case to obtain a homogeneous mixture. bReaction conversion was determined by GC. c Selectivity toward each product was determined by GC. The structure of products 2a, 3c, 4c, and 5c was assigned by comparison with previously prepared authentic compounds;9 products 4a, 3b, and (4b +4b′) were isolated and their structure was assigned by NMR and MS characterization (see SI section); the structure of products 3a, 2b, 2c, and 5b was assigned from their GC/MS spectra. The reaction of diols 3 and 4 also produced unidentified by products (8% and 12% of total products, respectively).

products were also observed and were referred to as “others”. The overall products distribution is reported in Figure 4. The conversion of glycerol was always quantitative. Except for the reaction at Q = 2, which gave a conspicuous formation of glycidol (6d: 23%) and others (31%), tests indicated that the increase of the relative amount of DMC greatly favored glycerol carbonate (6a) and most of all, its derivative of further transesterification with DMC [6b: methyl (2-oxo-1,3-dioxolan-4-yl)methyl carbonate] (green and yellow

As did PG, also EG yielded the corresponding cyclic carbonate with a high selectivity (2a:82%, entry 1); whereas both 1,3- and 1,4-diols favored the formation of linear products derived from mono- or ditransesterification reactions (3b, 4b +4b′, and 5b, and 3c-5c, respectively; entries 2−4). The situation was particularly evident for 1,4-butanediol (5c: 82%). This reflected the general trend that favors 5-exo-dig over 6exo-dig ring closure (C5- and C6-, respectively):26 specifically, for organic carbonates, the greater thermodynamic stability of C5- with respect to C6-cyclic derivatives was reported in many cases.27,28An analogue behavior was observed for the catalytic transesterification of DMC with 1,n-diols, where for n ≥ 3, dicarbonate derivatives were by far the preferred products.9 However, thermal CF-reactions of Table 1 allowed us to isolate and characterize monotransesterification compounds [3b and isomers (4b+4b′)] as well as the less common C6-carbonate 4a.29 Batch Reaction of Glycerol with DMC. At rt, glycerol (6) and DMC were not mutually soluble. Initial screening tests were carried out by using biphasic mixtures of DMC and glycerol (0.5 g, 5.4 mmol) in different molar ratios (Q = DMC:6) varying over the range 2, 5, 10, 20, 40, 60, and 80,

Figure 4. Product distribution for the batch thermal reaction of glycerol with DMC as a function of the DMC:glycerol molar ratio. Other conditions: 180 °C, 5 h. D


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of the thermal reaction.34 Moreover, in the same work, the selectivity toward glycerol carbonate did not exceed 85%. In Figure 5, as the reaction time was increased, the trend of the products distribution corroborated the mechanism outlined in Scheme 2. The amounts of 6a and 6b progressively reduced and increased to ∼30% and ∼65%, respectively, due to multiple transesterification processes between DMC, glycerol, and glycerol carbonate. The onset of decarboxylation reactions explained the not negligible formation of glycidyl methyl carbonate (6e, 17%) observed after 24 h. Besides the synthesis of 6b (Figure 4), tests of Figure 5 disclose the synthetic potential of the thermal reaction also for the preparation of glycerol carbonate (6a). However, because this required a high temperature (180 °C) for a short time (1 h), an issue was the regulation of the heating rate and the thermal inertia of the reactor (autoclave): the reaction could run out of control toward products of multiple transesterification and “others”. To overcome such a problem, a simple expedient was devised by performing experiments under an additional pressure of CO2, whose use was originally thought to minimize the observed decarboxylation side-processes. Once reactants were charged in the autoclave, gaseous CO2 was introduced by a syringe pump to reach at rt, a preselected pressure of 5, 10, 20, 30, 40, and 50 bar, respectively. Temperature and time screening tests demonstrated that the best reaction conditions were at 180 °C for 5 h (Figure 6). Tests were repeated twice to check for reproducibility.16,35

bars): in particular, from Q = 5 to 60, the overall process became more and more effective toward 6b whose selectivity was enhanced from 19 to 82%, respectively. At the same time, side-reactions were substantially suppressed (≤5%). Minor variations of the products distribution were appreciated when Q was further increased to 80. Once the experiment at Q = 40 was complete, the final reaction mixture was purified: the DMC excess was completely recovered (by distillation) and recycled, and product 6b was isolated in a 78% yield (see also SI). Scheme 2 summarizes the plausible reaction pathways for the formation of the observed products. The process likely involved the double transesterification of glycerol to produce 2,3-dihydroxy propyl methyl carbonate (6*) followed by the (more stable) five-membered ring product glycerol carbonate 6a (eq 1 in Scheme 2). Results of Figure 4 clearly suggested that these equilibrium reactions were affected by the Q ratio. The higher the relative amount of DMC (Q), the larger the extent of a further (third) transesterification yielding carbonate 6b (eq 2, path (i) in Scheme 2). A direct methylation of 6a also occurred to produce compound 6b (eq 2, path (ii) in Scheme 2): this compound, however, was observed only in trace amounts consistently with the higher activation barrier of methylations with respect to transesterification reactions promoted by DMC.1,12,30 Finally, glycidol (6d) and its methyl carbonate derivative [6e: methyl (oxiran-2-ylmethyl) carbonate] plausibly derived from decarboxylation processes of carbonates: these reactions have been previously described by us and by others (eqs 3 and 4 in Scheme 2).31−33 At 180 °C, the effect of the reaction time was investigated in detail. A mixture of DMC and glycerol in a 20:1 molar ratio (Q = 20) was set to react in autoclave for 0.5, 1, 2, 3, 4, 5, 10, and 24 h (Figure 5).

Figure 6. Conversion and products distribution for the batch thermal reaction of glycerol with DMC in the presence of additional CO2. Other conditions: Q = 20, 180 °C, 5 h.

At 5 bar, no substantial variations were appreciated with respect to previous results (Figures 4 and 5). However, the increase of the CO2 pressure up to 20 bar allowed a remarkably better control of the product distribution: the process was slowed down and the selectivity toward glycerol carbonate was improved to 88% at a conversion of 94%. Byproducts were substantially suppressed. Once the experiment was complete, the excess DMC was simply removed by distillation, and 6a was isolated in a 84% yield (see SI). A further increase of the CO2 pressure at 30, 40, and 50 bar, brought about a significant drop of the reaction conversion (44, 15, and 11%, respectively), though glycerol carbonate was the almost exclusive product. Experiments proved that the addition of CO2 allowed to control the thermal reaction of DMC and glycerol for a reproducible and selective synthesis of glycerol carbonate. To investigate further on this aspect, gases different from CO2 were

Figure 5. Conversion (of glycerol) and products distribution for the batch thermal reaction of glycerol with DMC in function of the reaction time. Other conditions: 180 °C, Q (DMC:glycerol molar ratio) = 20.

An excellent result was obtained after 1 h: glycerol was almost quantitatively converted (94%), and glycerol carbonate (6a) was achieved with a selectivity as high as 86%. This corresponded to a reaction rate of ∼0.53 mol L−1 h−1 (with respect to glycerol). Among the most effective catalytic procedures recently reported for the transesterification of DMC with glycerol, it was claimed that in the presence of Ca− La mixed-oxide catalysts, the reaction proceeded at 90 °C, but the overall rate (0.14 mol L−1 h−1) was 3 times lower than that E


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ACS Sustainable Chemistry & Engineering also tested. Experiments were carried under the same conditions of Figure 6 by substituting CO2 with an equal pressure of He, Ar, or N2, respectively. In no case was the reaction selective: at best, the amount of glycerol carbonate was ∼35% at complete conversion, thereby suggesting that not only the mere action of the operative pressure but also the nature of the added gas, played a crucial role. For sure, the direct carboxylation of glycerol with CO2 did not occur under the explored conditions. This transformation not only requires high T and p (180 °C and 50 bar) but also highly active catalysts (CeO2) and powerful dehydrating coreagents.36 Other factors should therefore be considered. For example: (i) at 180 °C, in the range 10−50 bar, the density of CO2 varies between 0.015 and 0.06 g/mL, similar to Ar, but, on average, 2 times as high as N2 and 10-fold higher than He;37 (ii) at 25 °C, the Henry constants for CO2, Ar, N2, and He in a model polar liquid such as methanol are 601, 10 330, 17 310, and 85 500 Pa m3 mol−1, meaning that CO2 is from 17- to 143-fold less soluble than other considered gases;38 (iii) at 25 °C and 70 bar, the viscosities for CO2, Ar, N2, and He are of 54.2, 24.1, 19.1, and 19.6 μPa·s−1.39,40 Not to mention the clustering effects that may operate in the proximity of the supercritical state (sc) of CO2 (Figure 6: at PCO2 = 50 bar).41 Whichever the interaction, the reactivity of liquid/vapor phases and their relative partitioning during the investigated thermal process were obviously affected by the properties of CO2. If DMC allowed the formation of carbonates 6a and 6b, the coproduct methanol favored the reverse processes (Scheme 2): the balance of such reversible reactions was controlled by CO2 that reduced the overall reaction rate, thereby limiting the further transesterification of 6a to the double carbonate 6b thus improving the selectivity. In this respect, CO2 might also decrease, if not prevent, the occurrence of the above-described decarboxylation reactions. A similar result has been described also in the catalytic liquid phase oxidation of p-xylene carried out in CO2-expanded solvents.42 This behavior was substantiated by two additional experiments in which a mixture of glycerol carbonate and DMC (molar ratio DMC:6a = 20) was set to react at 180 °C for 5 h, both without and with CO2 (20 bar). Although the first experiment gave 6b with conversion and selectivity of 83% and 96%, respectively, the presence of CO2 inhibited any reaction (conv. ≤ 5%). Although far from being exhaustive, this analysis could offer a speculative basis to account for the results of Figure 6. CF-reaction of Glycerol with DMC. The absence of mutual miscibility between glycerol and DMC was a tricky problem for the implementation of CF-reactions. Irrespective of their relative amounts and flow rates, when reactants were delivered separately to the CF reactor, undesired side-reactions could not be avoided: in particular, extensive decarboxylation reactions producing glycidol derivatives. These products rapidly formed polymeric-like materials43 which eventually clogged the reactor. A cosolvent was therefore required. Among the several solvents tested, methanol offered the best option. Screening experiments of T, p, and flow rates proved that good results could be achieved at 50 bar and 230−250 °C, by feeding the above-described CF-reactor at 0.1 mL/min with a mixture of DMC, methanol, and glycerol in a 10:6:1 molar ratio (Q), respectively (Figure 7). As observed for the transesterification of diols (Figures 1−3), the moderate residence time (τ ∼ 15 min) implied the need of a higher T for the CF-reaction of glycerol with DMC with

Figure 7. Conversion and selectivity of the CF-reaction of glycerol and DMC as a function of the temperature. Other conditions: DMC, methanol, and glycerol in a 10:6:1 molar ratio (Q), respectively; 50 bar; total flow rate: 0.1 mL/min.

respect to the batch-process (cf. Figures 4, 5, and 7). Under the explored conditions, conversion (of glycerol) and selectivity toward glycerol carbonate were of 78% and 92% at 230 °C, and of 94% and 83% at 250 °C, respectively. The CF-system could operate virtually indefinitely because no catalyst had to be recovered/activated or disposed of. This also simplified the workup of reaction mixtures: a complete recycle of the excess DMC was possible with isolated yields of glycerol carbonate in the range of 70−80%.44 It should be noted here that of the many catalytic methods recently reviewed for the transesterification of dialkyl carbonates with glycerol,45,46 most of them were not suitable for CF-applications because conventional bases (even solid compounds) used as catalysts were partially soluble in glycerol and/or DMC. To the best of our knowledge, the only reported CF-method using hydrotalcitebased catalysts was able to operate at 130 °C, but it required a highly noxious solvent such as DMSO and the best found selectivity for 6a was 77% at complete conversion.47 This value was comparable, if not lower, to that of the present CF-thermal reaction. Not to mention the impact of both upstream and downstream operations: the manufacture, activation, and recycle (when possible) of heterogeneous catalysts needed energetically expensive calcination steps.48 On a final note, thermal CF-tests confirmed the role of the partitioning of reactants between liquid and vapor phases. At 250 °C, no reaction took place at ambient pressure, while, at only 5 bar, a quantitative process was observed and a fascinating shift of selectivity was observed: the decarboxylation of glycerol gave glycidol as the major product (6d: ∼83%) and only traces of glycerol carbonate were detected. The synthetic potential of this reaction was, however, limited by the further polymerization of 6d, which caused clogging of the reactor. This result will be the object of future investigations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01633. Description of continuous-flow apparatus and full spectral characterization (1H and 13C NMR, heterocorrelated 1H−13C NMR, and MS) of compounds 3a, 3b, 4a, 4b+4b′, 6a, 6b, 6c, and 6e (PDF) F


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AUTHOR INFORMATION

Corresponding Author

*Maurizio Selva. E-mail: [email protected]. Author Contributions

Maurizio Selva conceived and designed the experiments, and he wrote the paper along with Sandro Guidi; Sandro Guidi and Roberto Calmanti performed the reactivity experiments; Alvise Perosa and Marco Noè contributed to planning of research and interpretation of the data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Ke Jie from the University of Nottingham (UK) is gratefully acknowledged for his contribution to the prediction of the liquid−vapor pressure profile of DMC.

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ABBREVIATIONS: CF, continuous-flow DMC, dimethyl carbonate EC, ethylene carbonate EG, ethylene glycol PC, propylene carbonate PG, propylene glycol τ, residence time

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REFERENCES

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