High-Temperature Batch and Continuous-Flow Transesterification of

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 3964−3973

pubs.acs.org/journal/ascecg

High-Temperature Batch and Continuous-Flow Transesterification of Alkyl and Enol Esters with Glycerol and Its Acetal Derivatives Roberto Calmanti, Manuele Galvan, Emanuele Amadio, Alvise Perosa, and Maurizio Selva* Dipartimento di Scienze Molecolari e Nanosistemi dell’Università Ca’ Foscari Venezia, Via Torino 155, 30172 − Venezia Mestre, Italy

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S Supporting Information *

ABSTRACT: A new procedure for the transesterification of alkyl acetates and formates with model glycerol acetals (GAs: Solketal and glycerol formal) was explored in the absence of any catalysts at 180−275 °C. Highly selective transformations occurred in both batch and continuous-flow (CF) modes; particularly, the enol derivative isopropenyl acetate (iPAc) was the best performing reactant by which quantitative acetylation reactions were achieved with yields on GAs acetates >95%. An excess acylating agent was necessary (2−20 molar equivs), but the unconverted ester was fully recovered and could be reused. The reaction plausibly involved multiple mechanisms where either the electrophilic and the nucleophilic activation of reagents took place through both traces of acetic acid (formed in situ by the hydrolysis of esters) and the autoprotolysis of GAs. iPAc confirmed a superior performance than other esters also for the high-temperature conversion of glycerol; in this case, although acylation and acetalization processes were simultaneously possible, conditions were optimized to achieve the exhaustive transesterification of glycerol to triacetin, in both batch and CF modes. Triacetin was isolated in 99% yield. KEYWORDS: Transesterification, Alkyl esters, Isopropenyl acetate, Glycerol, Glycerol acetals, High-temperature (HT) reactions

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INTRODUCTION Biobased glycerol and its acetal derivatives (GAs: glycerol acetals) fit in the current list of top biomass-derived platform chemicals which are used as building blocks for higher valueadded chemical products and materials.1−6 Among the procedures for the chemical conversion of glycerol and GAs, an emerging field is related to high-temperature (HT) transformations carried out in the absence of any catalyst. The study of HT-driven routes was originally aimed at improving biodiesel production not only through the transesterification of natural oils, but also by the simultaneous conversion of the coproduct glycerol into glycerol dicarbonate (GDC) or triacetin (TA), respectively.7−11 To this end, supercritical fluid-based (SCF) reactions with nontoxic acyl acceptors such as dimethyl carbonate (DMC) or methyl acetate were used (Scheme 1). At 250−380 °C and 150−200 bar, such SCF-technologies proved highly tolerant toward common impurities (FFA and water) present in the reactant oils, and they allowed extremely fast kinetics and yields >90% with simplified downstream processing in which high-standard glycerol-free biodiesel as blends of FAMEs and GDC or TA were obtained without further purification.12−17 Moreover, energy costs and capital investments of plants running on SCF-technologies (SCFplants) could be efficiently mitigated by integrating the SCFbased reactions within biorefinery units able to recover or exchange waste/excess heat.18 Comparative studies and © 2018 American Chemical Society

Scheme 1. Transesterification of Natural Triglycerides with DMC or Methyl Acetate

simulations demonstrated that the total energy consumption and the output potential environmental impact per mass of product obtained from SCF-plants for the biodiesel production could be even lower than that of conventional base-catalyzed transesterification processes.19−21 HT-reactions were also successfully investigated by us for the upgrading of model glycerol acetals, specifically, glycerol formal and Solketal and glycerol by using dialkyl carbonates.22−24 For example, at 275−300 °C and 20−40 bar, the continuous-flow (CF) reaction of DMC with GAs yielded monotranscarbonated products with a 98% selectivity at complete conversion, while Received: November 19, 2017 Revised: January 10, 2018 Published: January 15, 2018 3964

DOI: 10.1021/acssuschemeng.7b04297 ACS Sustainable Chem. Eng. 2018, 6, 3964−3973

Research Article

ACS Sustainable Chemistry & Engineering

flat-bottomed glass reactor which was placed inside a 100 mL stainless steel autoclave. This expedient avoided the contact of reagents with the autoclave walls, thereby ruling out metal catalysis. The autoclave was sealed, degassed via three vacuum−nitrogen cycles, and then electrically heated at the desired temperature (120−220 °C). The reaction was allowed to proceed from 1 to 24 h, during which the reacting mixture was kept under magnetic stirring. The observed autogenous pressure was in the range of 2−20 bar. At the end of the experiment, the reactor was rapidly cooled to room temperature and vented. The reaction mixtures were analyzed by GC/FID or GC/MS. The same procedure was used also to run experiments with (i) added AcOH in a 1−20 mol % amount with respect to the reacting acetal and (ii) glycerol in place of acetals. In this case, irrespective of the reactants molar ratio (ester:glycerol, from 1 to 20), starting mixtures were biphasic, but turned to homogeneous solutions at the end of the experiments. Continuous-Flow (CF) Reactions. Homogeneous solutions of acetal 1a or 4a−4a′ and the selected ester (2a−f) were prepared by mixing the reactants in different molar ratios from 5 to 20. Before any reaction, the CF-reactor (a stainless-steel tubular reactor: 0.52 m × 1/ 4 in. filled with ground-glass Raschig rings) was primed and conditioned by delivering the chosen solution of reactants (10 mL) at room temperature and atmospheric pressure and at a flow rate of 0.1 mL/min. The reaction was then started by setting the temperature and the pressure at the desired values (200−275 °C and 1−50 bar, respectively). T and p were controlled by a thermostatic oven and a back-pressure regulator. When an amount of the reacting mixture equaling 5 times the inner volume of the reactor (2.1 mL) was allowed to flow, samples at the outlet of the reactor were taken up at regular intervals of 30 min through a Rheodyne valve and analyzed by GC/ FID and GC/MS. At the end of the experiment, the oven was set to 100 °C, and the reactor was flushed with methanol (100 at 0.5 mL/ min), cooled to room temperature, and vented. A similar procedure was used also for the CF-reactions of glycerol with both methyl and isopropenyl acetates. Diglyme was necessary as a cosolvent due to the poor mutual miscibility of the reactants. Homogeneous solutions were achieved by mixing glycerol, the selected ester (2a or 2f), and diglyme in a 1:1−20:33 molar ratio, respectively. This mixture was used to run experiments described in Table 2. The product esters included (2,2-dimethyl-1,3-dioxolan-4-yl)methyl acetate (3a: Solketal acetate, (2,2-dimethyl-1,3-dioxolan-4-yl)methyl formate (3b: Solketal formate), (2,2-dimethyl-1,3-dioxolan-4-yl)methyl 2-hydroxypropanoate (3c: Solketal lactate), a 3:2 isomer mixture of 1,3-dioxan-5-yl formate and (1,3-dioxolan-4-yl)methyl formate (5a−a′: isomers of glycerol formal formate), a 3:2 isomer mixture of 1,3-dioxan-5-yl acetate and (1,3-dioxolan-4-yl)methyl acetate (5b−b′: isomers of glycerol formal acetate), and triacetin (8) (see Schemes 2, 4, and 5 for detailed structures). These compounds were purified and characterized by MS and NMR. All data including also isolated yields are reported in the SI.

the same CF-process with glycerol was optimized to obtain glycerol carbonate in a 83−92% yield. Other authors recently described also a batch reaction between supercritical DMC and glycerol to isolate glycerol carbonate in a 98% yield after only 15 min.25 This scenario prompted us to further investigate HTreactions with the new objective of implementing the conversion of glycerol and GAs into the corresponding ester derivatives. Notwithstanding the many catalytic procedures described with carboxylic acids and anhydrides (mostly AcOH and Ac2O),26−33 the catalyst-free esterification of glycerol has seldom been reported; poor conversions (35−49%) and moderate product yields (70%, triacetin) were achieved with both AcOH and Ac2O, respectively.34−36 Issues related to the corrosivity of reactants, the legal restrictions to the use of Ac2O, and the formation of coproduct water severely limited largescale applications of such methods either in the presence or in the absence of catalysts.37,38 Water not only altered the esterification equilibrium, but also contributed in the deactivation of catalysts if used.26−33 A literature survey indicated that the transesterification reactions of esters with both glycerol and GAs were more promising and safer processes. The acetylation of glycerol with alkyl (methyl, ethyl) acetates was reported over several homogeneous and heterogeneous acid and base catalysts,39−46 while the transesterification of esters with GAs was almost exclusively finalized at the kinetic resolution of racemic GAs esters by lipase-based biocatalysts.47−50 The analysis also proved that the HT-based reactions of esters with glycerol and its acetals still represented a largely unexplored area. In the present work, a systematic inspection of such processes has been carried out by using model reactants including formate, acetate, and lactate esters, isopropenyl acetate (iPAc: an active acetylating agent51−54), glycerol, and its acetals glycerol formal and Solketal. At 200−240 °C and 10−50 bar, in the absence of any catalysts, batch and continuous-flow protocols were achieved for the selective and high-yield synthesis of esters of GAs and triacetin, respectively. Some mechanistic hypotheses were formulated to account for the product distribution and the different reactivity of the esters.

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EXPERIMENTAL SECTION

Materials and equipment including autoclaves and the continuous-flow (CF) apparatus used in this study are listed and described in the SI section. All the reactions described in the Experimental Section and the Results and Discussion Section were repeated twice to ensure reproducibility. Under the same set of conditions (T, p, flow rates) duplicated tests afforded values of conversion and amounts of products (determined by GC/MS) which differed by less than 5% from one experiment to another. Commercially available esters used in this study were safe and nontoxic compounds; these included methyl acetate (2a), ethyl acetate (2b), propyl acetate (2c), ethyl formate (2d), S-(−)ethyl lactate (2e), and isopropenyl acetate (2f: iPAc). Reactants 2a−2f were flammable products with hazardous (H) phrases and precautionary statements (P) as H225, P210, and P403 + P235. However, they were low toxic compounds. For example, LD50 (oral, acute) for methyl, ethyl, and propyl acetates are 5001, 5620, and 6640 mg/kg, respectively.55 Ethyl lactate is even edible and present in many foods. Overall, such esters were safer than acids or anhydrides as esterification reagents. Batch Reactions. Mixtures of acetal 1a or 4a−a′ (4.0 mmol) and each ester 2a−f were prepared by varying the reactant molar ratio (Q = ester:acetal) from 1 to 20, the (excess) ester serving both as a reagent and a solvent. Each homogeneous solution was charged in a

Scheme 2. Reactions of Solketal with Different Esters at High Temperatures

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RESULTS AND DISCUSSION Batch HT-Transesterification of Different Esters with Solketal (1a). Conditions to begin this study were chosen based on our recent results reported for the transcarbonation of dialkyl carbonates with glycerol acetals.22−24 As described in the Experimental Section, mixtures of 1a and each of the esters 2a− 2f in a 1:20 molar ratio (Q), respectively, were set to react in an autoclave at different temperatures (150−220 °C) and times (1−10 h). In the absence of catalyst, the transesterification process occurred at T ≥ 180 °C. The structures of the products, i.e., Solketal esters 3a, 3b, and 3c (R1 = Me, H, and CH3CH(OH), respectively Scheme 2), were confirmed through their GC/MS and 1H/13C MNR characterization. Compounds 3a and 3b were also isolated in 96% and 60% yields from the reactions of 1a with 2f and 2d, respectively. Each reactant ester 2a−2f gave remarkably different outcomes. As an example, Figure 1 compares the results achieved at 200 °C for 5 h under an autogenous pressure of 8 bar.

100% (blue bars), respectively. Except for ethyl lactate, the transesterification selectivity was always very high (≥98%, red bars). Only trace amounts of monoacetins, diacetins, and triacetin were observed (96% by GC/MS and is therefore not shown.

Figure 1. Batch HT-reaction between Solketal and esters 2a−f: conversion of Solketal and selectivity toward Solketal esters 3a, 3b, and 3c are shown. Conditions: ester:Solketal molar ratio (Q) = 20, 200 °C, 5 h.

The conversion of Solketal did not exceed 10% with acetates 2a−c, while it was substantially higher for ethyl lactate, ethyl formate (2e−d) and i-propenyl acetate (2f): 57%, 67%, and Scheme 3. Transesterification of iPAc with Solketala

a

Right and left: electrophilic and cooperative activation of reactants, respectively. 3966


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Table 1. Reaction of iPAc with Solketal in the Presence of Added AcOH Entry 1 2 3 4

Added AcOH (mol %)a none 1 5 20

d

Qb 20 20 20 20

T/tb 180 180 180 180

°C/3 °C/3 °C/3 °C/3

Conv. (%)c h h h h

57 68 69 83

a

mol % with respect to Solketal. bQ was the iPAc:Solketal molar ratio; Q, T, and t (temperature and time, respectively) were kept constant throughout runs 1−4. cConversion of Solketal by GC/MS. dTraces of AcOH (97%, and the structure of products (5a−a′and 5b−b′) was confirmed by GC/MS and 1 H/13C MNR analyses (Scheme 4). Under the same reaction conditions (220 °C, 5h), the reversibility of the reaction of conventional esters (Scheme 4, top) was proved by heating a mixture of glycerol formal acetate (5a−5a′, 4.0 mmol) and MeOH in a 1:20 molar ratio, respectively: GlyF (4a−a′) was achieved with almost quantitative conversion (98%) and full selectivity. From the above-mentioned HT-reactions of GlyF with iPAc and 2d, products as mixtures of isomers 5a−a′ and 5b−b′ were isolated in 92% and 45% yields, respectively, in the same 3:2 isomeric ratio of the reagent (4a−4a′).

Figure 2. Conversion of Solketal as a function of the temperature and the reactants molar ratio (Q) during the batch reaction of Solketal and iPAc. All tests were carried out for 5 h. The selectivity toward product 3a was always >96%.

The conversion was favored both by higher temperature (bars of the same color) as well as by higher Q ratio (left to right). Yet, a quantitative reaction took place also with a moderate excess of iPAc (Q = 5, 220 °C: green bar). This behavior was further substantiated by a separate experiment, not reported in Figure 2, demonstrating that at the same T (220 °C) the transesterification could be completed even at Q = 2, albeit over 10 h. Whichever the conditions used, vacuum distillation of the final mixtures allowed an almost complete recovery of the unconverted excess reagent; more than 95 wt % of iPAc could be reused as such for further transesterification reactions. GC/MS analyses of the final mixtures also provided some insights to speculate on the reaction mechanism, particularly the detection of traces of acetic acid among the reaction products. The presence of AcOH was plausibly due to a hydrolysis side reaction of iPAc occurring at high temperature.60 In fact, both commercial Solketal and iPAc had water contents of ∼1 and 0.1−0.15 mol %, respectively (see Karl Fischer tests in the SI for details), and attempts to dehydrate Solketal failed because of its highly hydrophilic nature. This suggested that the investigated transesterification could be an acid-catalyzed process. To explore this hypothesis, further reactions of Solketal and iPAc in the presence of added AcOH (1, 5, and 20 mol % respect to Solketal) were run. Most representative results are summarized in Table 1 (Experiments were followed for 5 h: details on the trend of conversion vs time are reported in the SI, Figure S25). With respect to the blank test (entry 1), the conversion of Solketal only slightly increased by using 1 and 5 mol % of AcOH (entries 2−3), while the selectivity toward ester 3a was not altered (>98%). The effect still appeared limited even by quadrupling the acid amount (20 mol %, entry 4). A similar poor catalytic performance of AcOH was reported also for the closely related esterification of fatty acids with supercritical methyl acetate.61 This led to conclude that an acid-catalyzed pathway was hardly the only active mechanism for reactions of Figures 1 and 2. 3967


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ACS Sustainable Chemistry & Engineering Scheme 4. Batch HT-Transesterification of Esters 2a, 2d, and 2f with Glycerol Formal

parameters (T, p, reactants ratio) and an optimization of recycling and recovery operations, and productivity.69,70 This prompted us to explore the HT-transesterification of esters with both Solketal and glycerol formal also in the CF-mode. The CF-apparatus used in this work was similar to that described elsewhere,22−24 with a difference; instead of an empty tubular reactor, a steel tube (1/4 in. × 52 cm = diameter × length, inner void volume = 2.1 mL) filled with ground-glass Raschig rings was used. This inert filler (Raschig rings) could improve both mass/heat transfer and the contact between reagents and avoid preferential pathways of the reactants stream throughout the reactor (further details are in the Experimental Section).71,72 Initial tests were carried out to study the CF-reaction of Solketal with the same model esters used in batch (Figures 1 and 2), specifically, methyl, ethyl, and propyl acetates (2a−c), ethyl formate (2d), and iPAc (2f). Solutions of the selected compound 2 and Solketal in a 20:1 molar ratio, respectively, were delivered at 0.1 mL/min to the CF-reactor; at first, isobaric experiments were run at 50 bar by progressively increasing the temperature from 225 to 250 and 275 °C. Then, in the same range of T, the effect of the pressure from ambient to 50 bar was investigated only for the case of iPAc. Results are reported in Figure 4a and b which show the conversion of Solketal as a function of (i) T for different esters (4A) and (ii) p and T for the reaction of iPAc (4B). The selectivity toward the expected products, esters 3a and 3b, was always above 97% and is not indicated. Irrespective of the structure of the ester 2, CF-runs required a higher temperature than batch experiments (Figures 1−3: 120−220 °C; Figure 4A: 225−275 °C). This was consistent with the features of the CF-arrangement; given the volume of the reactor (2.1 mL) and the operating flow rate (0.1 mL/min), the residence time (τ) in the continuous mode was of only 20 min compared to 5 h of batch reactions. Extra energy input was therefore necessary to impart sufficiently fast kinetics to the CF-processes. Notwithstanding this, Figure 4A shows that esters 2 followed the same trend of reactivity of Figure 1. For example, when methyl acetate, ethyl formate, and iPAc were used, the corresponding conversions of Solketal were 15%, 51%, and 93% and 27%, 67%, and 100% at 250 and 275 °C, respectively, thereby confirming iPAc as the best reagent also for the HT-transesterification reaction in the CF-mode. iPAc could afford a substantial conversion (71%) even a 225 °C. The transesterification of iPAc with Solketal was clearly influenced by the pressure (Figure 4B). At each of the selected temperatures, the conversion was more than doubled when the pressure was increased from 10 to 30 bar; particularly, a quantitative reaction was achieved at 275 °C. Only minor

Figure 3 shows some illustrative results of the effect of the temperature on conversion and selectivity of the HT-transesterification of iPAc with GlyF.

Figure 3. Conversion and selectivity toward isomer esters 5a−a′ for the batch reaction between glycerol formal and iPAc in the range of 120−220 °C. Reaction conditions: molar ratio iPAc:GlyF, Q = 20, t = 5 h.

The conversion gradually improved from 24% up to a quantitative value as the temperature was increased from 120 to 220 °C. In this interval, however, the comparison of Figures 2 and 3 showed that GlyF was systematically less reactive than Solketal. The same trend was reported by us in the reaction of GlyF and Solketal with light dialkyl carbonates,22 and similar findings were recently described also by others.68 More in general, the application/implementation of the Hansen approach and the COSMO-RS model indicated that glycerol formal had not only a stronger structuration in the liquid state than Solketal, but formaldehyde-based acetals were less reactive than ketal acetone-based homologues, under acidic (hydrolytic) conditions. Although this offers an interesting basis for discussion, the interpretation of experimental and modeling results is still far from explaining the different behavior of GAs at a molecular level. Further investigations will be necessary to clarify such aspects. For the sake of completeness, a minor competitive process of transacetalization of GlyF with acetone (coproduced from iPAc) was also observed under the conditions of Figure 3. Traces of Solketal acetate (3a, < 2%) were detected. Continuous-Flow Reactions. Continuous-flow (CF) conditions generally allow a better control of reaction 3968


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condensed liquid phase in which the contact between iPAc and Solketal was effective for a productive reaction. On the contrary, an abrupt decrease of the conversion occurred below a threshold value of about 30 bar when reactants, especially the more volatile iPAc (bp = 97 °C), preferentially partitioned in the vapor phase inside the reactor. To get further insights into this aspect, the phase diagram of pure iPAc was also predicted by using an extended Antoine equation (for details, see Figure S2, SI).73 Under the assumption that the theoretic liquid−vapor pressure profile of pure iPAc could be an acceptable approximation for the binary mixtures of Figure 4 (that contained a large excess of the ester 2f: Q = 20), the change of conversion shown in the range of 10−30 bar of Figure 4B well matched the predicted liquid−vapor transition of iPAc (Figure S2). A similar dependence of conversion vs pressure was noticed also in the HT-transcarbonation of glycerol acetals;22 in that case, however, the effect of the pressure was even more pronounced because reactions took place in the absence of any (inert) filler inside the CF-reactor, thereby making the contact of reactants more difficult. Under the optimized conditions for T and p (275 °C and 30 bar), the iPAc:Solketal molar ratio (Q) was scaled down from 20 to 2 to examine the effect of the relative amounts of reactants. Experiments proved that the Q ratio could be substantially decreased; a quantitative process was still observed at Q = 5, i.e., 4-fold below the initial value. However, a further reduction impacted on the conversion of 1a. This (conversion) did not exceed 82% at Q = 2. In all cases, the selectivity toward ester 3a was >98%. Considering the (limited) capacity of the CF-reactor, the results confirmed not only the reliable performance of the process, but also the perspective for largescale applications and further process intensification, in line with the principles of green engineering.74 The CF-arrangement could operate virtually indefinitely once the hightemperature regime was reached. No catalyst had to be prepared, activated, recovered, or disposed of, thereby simplifying both upstream and downstream operations for the delivery of reactants, the recovery of products, and the recycle of the unconverted iPAc. Not to mention that iPAc was a cheap reactant available on a large scale and that the integration of the process in a waste heat recovery system provided by a biorefinery could efficiently relieve the energy demand of the reaction. In short, if the framework of energy existing within an industrial park could be used, high-temperature reactions could be driven at very competitive, if any, costs with a further valorization of “waste” energy.18,74 The CF-protocol proved efficient also for the reaction of iPAc with glycerol formal; although, in analogy to batch reactions, GlyF was less reactive than Solketal. Under the

Figure 4. HT-transesterification of esters 2a, 2d, and 2f with Solketal in the CF mode. (A) Effect of the temperature on the reaction conversion at a constant p of 50 bar and (B) only for the case of iPAc (2f): effect of the pressure on the reaction conversion at 225, 250, and 275 °C. Other conditions: molar ratio ester, Solketal, Q = 20, flow rate = 0.1 mL/min. The selectivity is always >97%.

improvements (≤8%) were noticed by further rising the pressure up to 50 bar at both 225 and 250 °C. By contrast, the conversion was lower than 15% for reactions run at the same temperatures but at atmospheric pressure. These results highlighted the fundamental role of phase transitions in the investigated reactions. If the pressure was high enough, despite the high T, most of the reacting mixture was present as a Scheme 5. Batch Reaction of Glycerol with Esters 2a and 2f

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ACS Sustainable Chemistry & Engineering Table 2. Batch and Continuous-Flow Reactions of Glycerol with Methyl and Isopropenyl Acetate Products distribution (%)d a

Entry

Cond.

1 2 3 4

Batch

5 6 7 8e 9e

Cont.-flow

a

Q

b

Ester

Cat.

T/p/t(°C/bar/h)

2a

none none 3%Y/SBA-3 CaSn(OH)6

1 20 14 10

180/10/5 180/10/24 110/1/3 70/1/0.25

2f

none none none

1 5 20

2a 2f

none none

20 20

c

Conv. (%)

d

6a−6a′

7a−7a′

8

20 84 97 68

90 63 8 77

10 35 30 23

2 62

180/8/5 180/8/5 180/8/24

73 73 100

43 54

12 16

1 1 100

300/50/5 300/50/5

78 100

65

33

2 100

1a

3a

ref

41 43 32 22

12 7

a

Catalyst (if present) and reactions conditions (batch or continuous-flow) used. bQ = Glycerol:ester molar ratio. cFor batch reactions, p was the autogenous pressure in the autoclave reactor (entries 1−2 and 5−7). dConversion of glycerol and products distribution determined by GC/MS. e Under continuous-flow conditions, a mixture of glycerol, ester, and diglyme as a cosolvent in a 1:20:33 molar ratio, respectively, was used.

moderate Q ratios (1−5), batch reactions of 2f yielded complex mixtures including not only acetins, but also Solketal (1a) and its methyl ester (3a) (entries 6−7). Both 1a and 3a came from a competitive acetalization of glycerol induced by the acetone released during the transesterification with iPAc (Schemes 2 and 5). This was proved by the reaction of a mixture of glycerol and acetone in a 1:10 molar ratio, respectively; at 180 °C, after 5 h, a 70% conversion was achieved with full selectivity toward Solketal 1a. A larger iPAc excess substantially inhibited the acetalization reaction, and at the same time, the irreversible loss of acetone from 2f allowed not only a quantitative process, but also the exhaustive acetylation of glycerol toward triacetin 8 (entry 8, Q = 20). This derivative was isolated in 99% yield. In the continuous-flow mode, mixtures of monoacetin, diacetin, and triacetin products were still obtained with methyl acetate (entry 8), while the reaction of 2f proved successful for the selective synthesis of triacetin (entry 9). Although preliminary, these data further proved the concept, demonstrating that iPAc was not only effective for the conversion of GAs into the corresponding methyl esters, but also for the straightforward transesterification of glycerol into its fully acetylated derivative triacetin. This outcome also raised two challenging perspectives for the implementation of (i) the CF- transesterification of crude glycerol as received from the biodiesel production [The herepresented HT-strategy would avoid costly techniques required for the purification of off-grade glycerol and common drawbacks due to the poisoning of catalysts by the impurities of the crude reagent (salts, soaps, etc.)]76 and (ii) a selective one-pot preparation of Solketal esters (3a and its derivatives) by reacting glycerol with both iPAc and its higher enol ester homologues. These subjects are currently under investigation in our lab.

conditions of Figure 4B (30 bar), conversions of GlyF did not exceed 31% and 76% at 225 and 250 °C, respectively. Only at 275 °C, a substantially quantitative reaction was achieved (conv.: 97%); isomer esters 5a and 5a′ were obtained in a ∼ 3:2 ratio and with an overall transesterification selectivity >96%. Reaction of Glycerol with Methyl and Isopropenyl Acetates. Batch and continuous-flow reactions were performed by using mixtures of glycerol and methyl or isopropenyl acetates (2a and 2f) in a molar ratio (Q = 2:glycerol) variable between 1 and 20. Batch (autoclave) tests were run at 120−220 °C, while CF-experiments required higher temperatures up to 300 °C and dyglime as a cosolvent. In the absence of any catalyst, the reactions proceeded with both esters. The whole spectrum of the observed products is shown in Scheme 5. The structure of products acetals (1a and 3a) and acetins (6, 7, and 8) were assigned by GC/MS and by comparison to authentic samples when available (compounds 1a, 3a, and 8). The reaction conversion and the product distribution achieved under the best conditions found in this work are described in Table 2. For comparison, the table also shows some of the best results already reported for the batch transesterification of glycerol with methyl acetate in the presence of either an acid or a base catalyst [3%Y/SBA-3 and CaSn(OH)6, respectively]. At 180 °C, the batch transesterification of methyl acetate with glycerol yielded monoacetin, diacetin, and triacetin products. As expected, increasing the Q ratio and the reaction time favored the conversion, but it also resulted in the formation of derivatives of multiple transesterification (entries 1−2). The selectivity was elusive also for catalytic processes; although these were faster than HT-reactions, not even triacetin 8 was obtained as a sole product despite the use of a large excess of ester 2a (entries 3−4). The equilibria involved in the consecutive transesterification steps were clearly perturbed by the increasing amount of MeOH originated as a reaction coproduct. In analogy to previously reported studies for catalytic processes,75 the reversible nature of these reactions was confirmed when a solution of triacetin and methanol in a 1:10 molar ratio, respectively, was set to react at 180 °C for 5 h. A mixture of diacetin, monoacetin, and glycerol in 17%, 1%, and 1% amounts, respectively, was achieved. As in the case of acetals, the HT-conversion of glycerol was higher with iPAc (2f) than with methyl acetate. At 180 °C and

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CONCLUSION In conclusion, the here described HT-processes exemplify an archetype of clean reactions for the conversion of glycerol and its acetals to their ester derivatives, by using innocuous reagents and producing minimal, if any, wastes. The overall protocol discloses a new perspective for the upgrading of renewables, although for a genuine sustainable path, the procedure should be conveniently integrated within a biorefinery plant where synthetic operations may take advantage of modern tech3970


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nologies for the recovery of waste/excess heat and the recycle of reactants/solvents.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04297. Description of batch and continuous-flow apparatus and full spectral characterization (1H, 13C NMR, and MS) of compounds. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web: http://www.unive.it/persone/ selva. ORCID

Alvise Perosa: 0000-0003-4544-8709 Maurizio Selva: 0000-0002-9986-2393 Author Contributions

Maurizio Selva conceived the experiments, and he wrote the paper along with Roberto Calmanti. Roberto Calmanti and Manuele Galvan performed the reactivity experiments. Alvise Perosa and Emanuele Amadio contributed to the interpretation and discussion of data and results. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS CF, continuous-flow; iPAc, isopropenyl acetate; GAs, glycerol acetals; τ, residence time

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REFERENCES

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High-Temperature Batch and Continuous-Flow Transesterification of

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