Two-step Synthesis of Dialkyl Carbonates through Transcarbonation

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Two-step Synthesis of Dialkyl Carbonates through Transcarbonation and Disproportionation Reactions Catalyzed by Calcined Hydrotalcites Lisa Cattelan, Giulia Fiorani, Alvise Perosa, Thomas Maschmeyer, and Maurizio Selva ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02106 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Sustainable Chemistry & Engineering

Two-step Synthesis of Dialkyl Carbonates through Transcarbonation and Disproportionation Reactions Catalyzed by Calcined Hydrotalcites Lisa Cattelan,a,b Giulia Fiorani,a Alvise Perosa,a Thomas Maschmeyer,b and Maurizio Selva*,a a

Department of Molecular Sciences and Nanosystems, Università Ca’ Foscari Venezia

Via Torino 155, 30172 Venezia Mestre (Italy) b

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, F11

The University of Sydney, Eastern ave, Sydney, NSW 2006 (Australia) E-mail: [email protected]

Abstract A two-step methodology was implemented to prepare dialkyl carbonates from both primary and secondary alcohols, including glycerol acetals, tetrahydrofurfuryl alcohol and cyclohexanol. Accordingly, alcohols were initially subjected to a batch transcarbonation (carbonate interchange) reaction with the non-toxic dimethyl carbonate (DMC), providing the corresponding asymmetrical methyl alkyl carbonates (ROCO2Me). These compounds were then used as reactants for a continuous-flow (CF) disproportionation reaction, producing the corresponding symmetrical dialkyl carbonates (ROCO2R). Although transcarbonation and

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dismutation took place at different operating temperatures (90 and 180-275 °C, respectively), both reactions were catalyzed by the same heterogeneous Mg/Al mixed oxides catalyst, obtained upon calcination of commercially available hydrotalcites (HTs). Yields and selectivities for methyl alkyl carbonates were excellent, in the range of 93-96% and 95-98%, respectively. CFdisproportionation reactions were strongly affected by the nature/structure of reactants, nevertheless, they provided the corresponding dialkyl carbonates with selectivities and productivities up to 92% and 164 mgprod·(gcat·min)-1, respectively. Overall, the reported methodology displays attractive sustainability features, including a straight-forward upgrade of biomass derivatives, development of a continuous-flow intensified process, improved catalyst recycle and products’ purification, while disclosing stimulating perspectives for further investigations on the reaction mechanism and on the role of HTs as catalyst precursors for the synthesis of organic carbonates. KEYWORDS dialkyl carbonates, hydrotalcites, carbonate interchange reactions, continuous flow, green chemistry

Introduction Dialkyl carbonates (DAlCs) are characterized by an ever increasing number of applications in chemical synthesis, energy, electronic consumption industry and transports.1 This widespread interest for DAlCs is mostly due to the appealing features associated with their preparation/use, including (but not limited to): accessible sustainable synthetic methodologies, their environmentally benign nature, their non-toxicity and biodegradabilty, their established role as sustainable solvents for large-scale manufacturing processes (e.g. ethylene carbonate, EC, as solvent for Li-based batteries)2 and, last but not least, their tunable reactivity, witnessed by the extensive body of literature available on applications of DAlCs as selective carboxylalkylating


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and/or alkylating agents,3,4 and “thermodynamically accessible” CO2 equivalents in polycarbonate manufacturing industry.5 Within this scenario, a key role is played by DAlCs in synthetic methodologies, which directly affect their toxicological profiles, their market viability and consumers acceptance. Although phosgenation and phosgenation-like reactions are traditionally used for the preparation of DAlCs, such procedures are characterized by major drawbacks, particularly for large-scale applications, due to the toxicity of phosgene and its derivatives, the formation of overstoichiometric quantities of hazardous salts as by-products, and the need for large volumes of harmful chlorinated solvents like methylene chloride.6 A far more eco-compatible protocol is represented by Carbonate Interchange Reactions (CIRs) which can be triggered by the lighter alkyl groups in the dialkyl carbonate series, i.e. dimethyl carbonate (DMC) (Scheme 1).

Scheme 1. Carbonate Interchange Reaction (CIR): synthesis of dialkyl carbonates via DMC Among the features accounting for the success of CIRs, a major contribution has been made by technologies and processes available for the large-scale synthesis of DMC. Since the mid-80s DMC could be prepared by oxidative carbonylation reactions,7,8 but even more interestingly, in the late 2000s, Asahi-Kasei Corp. developed one of the most successful examples of sustainable industrial chemical process available today, i.e. the syntheses of DMC from carbon dioxide integrated within polycarbonate manufacturing.9 Not only were these phosgene-free catalytic processes characterized by the reduced formation of by-products and use of solvents, but they


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also contributed to making DMC available in the range of several tens of tonnes/year as a nontoxic chemical, an ideal candidate to implement CIRs in a greener way. DAlCs synthesis via CIRs is, however, still a challenging reaction, even when using an environmentally benign reagent like DMC. The thermodynamic limitations of CIRs result in small equilibrium constants, while the reaction kinetics are often not favorable due to the moderate leaving group ability of the methoxide group: overall, approaches such a large excess of DMC, the (azeotropic) distillation of stoichiometric quantities of the co-product MeOH, and if necessary, high reaction temperatures and (autogenic) pressures up to 300 °C and 50 bar, respectively, are required. Representative CIRs examples are summarized in Scheme 2.

Scheme 2. Representative examples of conditions to promote CIRs by DMC. Ishihara and co-workers reported on a CIR catalyst prepared in situ starting from La(OiPr)3 and OH(CH2CH2O)2Me (Eq. 1). When used with mixtures of an alcohol (either primary, secondary or tertiary) and DMC (24 equiv.), the corresponding alkylmethyl or dialkyl carbonates were selectively obtained in good to excellent isolated yields (55–95 %) by monitoring the reaction in


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time and distilling the MeOH/DMC azeotrope (b.p. = 64 °C) over 5Å molecular sieves.10 Similarly, when reacting DMC and catechol in the presence of a basic catalyst (NaOMe or MgO), a substantially quantitative yield of catechol carbonate (CC) was obtained upon continuous MeOH/DMC azeotrope removal (Eq. 2, left).11,12 CC in turn, was active towards CIRs with a variety of alcohols, including glycerol, to prepare the corresponding symmetrical carbonate products (Eq. 2, right): the stability of the leaving group, i.e. the cathecolate anion derived from CC allowed for mild operating conditions (T = 40 - 60 °C and ambient pressure) and provided fast kinetics. When the MeOH/DMC azeotropic mixture, formed during CIR, was not removed by distillation, higher reaction temperatures were required to push the carbonate interchange equilibrium to the right. As an example, in presence of an organocatalyst like methyl trioctylphosphonium acetate [P8881][AcO], DMC-promoted CIRs with several different primary and secondary alcohols occurred only at T = 150-220 °C and 5-15 bar of autogenous pressure (autoclave; Eq. 3).13 These transformations selectively provided alkyl methyl carbonates (ROCO2Me) with the formation of the corresponding symmetrical products in trace quantities. A similar result was also obtained under thermal (catalyst-free) conditions, when performing the CIR between glycerol acetals (e.g. solketal and glycerol formal) and DMC in continuous-flow mode (Eq. 4):14 although the operating T and p (250-300 °C and 20-50 bar, respectively) were higher than those of catalytic processes, thermally induced CIRs were characterized by conversions and selectivities >98 % towards the corresponding alkyl methyl carbonates, simplified downstream operations for product separation, and easy recycle of excess reactants. This catalyst-free protocol was also successfully extended to several different 1,n-diols and


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glycerol for the formation of the corresponding 5-membered and 6-membered cyclic carbonates.15 To ensure the selective formation of either asymmetrical or symmetrical carbonates, the product distributions can also be controlled using multi-step protocols, rather than more straightforward procedures. A remarkable example of this type of strategy is represented by the above-mentioned Asahi-Kasei process, in which a two-step sequence, combining carbonate interchange and disproportionation (self-CIR) reactions, is employed to obtain diphenyl carbonate (Scheme 3).9

Scheme 3. Two-step synthesis of diphenyl carbonate combining carbonate interchange (top) and disproportionation (bottom) reactions. The CIR between DMC and PhOH initially produces methyl phenyl carbonate (MPC, top) which, in turn, undergoes disproportionation to diphenyl carbonate (DPC) and DMC (bottom). To overcome both thermodynamic and kinetic barriers, the two reactions are run separately in two continuous multi-stage distillation columns at T = 195-200 °C and variable pressures (0.2 to 8 bar). DPC is obtained in 99 % yield and selectivity, in the presence of Pb(OPh)2 as catalyst. It should be noted that in a typical one-pot CIR procedure between DMC and phenol, catalyzed by samarium trifluoromethanesulfonate, DPC was synthesized in a 31% yield.16 As part of our long-standing interest regarding the integration of green protocols that are mediated by dialkyl carbonates with the chemical valorization of biobased platform molecules,1112,14-15,17,18,19,20 ,21

we were prompted to expand the scope of the reaction sequence depicted in


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Scheme 3 to produce carbonates from renewable alcohols. Two aspects were considered to define the scope: i) setup of a continuous-flow (CF) procedure, by which key reaction parameters (T, p and reactant molar ratio) could be flexibly tuned to optimize process kinetics, productivity and products separation, while improving the overall process safety; ii) selection of catalysts within the class of Mg/Al mixed oxides obtained by calcination of hydrotalcite (HT) solid precursors. Although applications of calcined HTs (c-HTs) to catalyze reactions of dialkyl carbonates were already reported in the literature,22,23,24 only recently we discovered that such solids were suitable for CF-reactions mediated by DAlCs at relatively high temperatures (210275 °C).25 c-HTs were not only stable and active, but contrary to other solid catalysts (e.g. alkaline carbonates, aluminas or zeolites), they were far less effective at decomposing DAlCs through decarboxylation, hydrolysis or other side-reactions.26,27 In this context, the present work demonstrates the implementation of an original and reliable two-step synthetic procedure in which methyl alkyl carbonates, prepared via an initial batch CIR between DMC and primary or secondary aliphatic alcohols, undergo a subsequent CF-disproportionation reaction producing the corresponding symmetrical dialkyl carbonates in good-to-excellent selectivity, yields and productivity, up to 98%, 96%, and 164 mgprod·(gcat·min)-1, respectively. The protocol was successfully extended to bio-based primary alcohols obtained from glycerol and sugar-derived furans and cyclohexanol as a model secondary alcohol. The two reactions occurred under different conditions, i.e. T = 90 °C and T = 180-275 °C for the CIR and the disproportionation processes, respectively, but they were conveniently catalyzed by the same solid systems comprised of Mg/Al mixed oxides (Mg/Al ratio ranging from 30:70 to 70:30), obtained upon calcination of commercially available HTs. Moreover, since DMC acted as a reactant for step one and was formed as a co-product in step two, the procedure highlighted multiple advantages


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beyond the preparation and recyclability of the catalysts, including recovery and reuse of unconverted reagents and solvents.

Results and Discussion Substrates. Primary and secondary alcohols used in this study included bio-based glycerol derivatives such as solketal (1a: 2,2-dimethyl-1,3-dioxolane-4-methanol) and glycerol formal (a 2:3 commercial mixture of five- and six-membered ring isomers, respectively: 2a: 5-hydroxy1,3-dioxane, and 2a’: 4-hydroxymethyl-1,3-dioxolane), tetrahydrofurfuryl alcohol (3a), a sugarbased furan derivative, and cyclohexanol (4a), which was chosen as a model for secondary alcohols. Although cyclohexanol is still primarily sourced from benzene,28 several promising routes for its production from lignin-derived phenolics and renewable cyclohexanone are under investigation.29,30 The structures of reagents 1a-4a are summarized in Figure 1.

Figure 1. Primary and secondary alcohols used in this study. Catalysts. Three different mixed Al/Mg oxides solids were used in this study, labelled as cHT30, c-HT63, and c-HT70. The solids possessed a MgO/Al2O3 ratio of 30:70, 63:37, and 70:30, respectively. The preparation of these materials, their chemical composition and characterization data were fully detailed in a recent paper of our group.25 c-HTs were prepared upon hightemperature calcination (N2, T = 450 °C, t = 16 h) of commercial Mg-Al hydrotalcites (HT) sourced from Sasol, Italy. Notably, the thermal treatment (calcination) broke down the typical lamellar structure of the starting hydrotalcite to produce a MgO-like phase (periclase) and a


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magnesia–alumina solid solution. This structural change conferred not only a high stability to the solids, but also significantly improved the performance of c-HTs as catalysts for DAlCspromoted reactions. Carbonate interchange reactions (CIRs). Solketal (1a) was initially used as a model substrate to explore the CIR with dimethyl carbonate (DMC) in presence of c-HTs catalysts. In a typical setup, a mixture of 1a, DMC and the catalyst of choice was heated under stirring at T from 70 °C up to the reflux temperature (90 °C). Several conditions were examined by varying not only the temperature, but also the DMC:1a molar ratio and the catalyst loading in the range of 5-18, and 1-5 % wt. (based on the limiting reactant, 1a), respectively. Moreover, to shift the CIR towards product formation, the reaction flask was equipped with a condenser thermostated at 70 °C, ensuring a continuous removal of the azeotropic mixture MeOH/DMC (70:30 w/w, b.p. = 64-67 °C). Experiments were monitored by GC-MS, sampling the reaction mixtures at given time intervals. In all cases, the desired production of methyl solketal carbonate (1c: 2,2-dimethyl-1,3dioxolan-4-yl)methyl methyl carbonate) was achieved with > 97% selectivity even at complete conversion, while the corresponding symmetrical dialkyl carbonate was detected only in trace amounts (1d < 3%). Reproducibility was checked and confirmed by duplicating each test: values for conversion and products amount (by GC–MS) differed by less than 5% from one reaction to another. Compounds 1c and 1d were both isolated and their structures confirmed by 1H and 13

C{1H} NMR (see the Experimental and SI sections for further details). The best results observed were obtained at 90 °C by using DMC and c-HT in 18 molar equiv.

and 3 wt. %, respectively, relative to reactant 1a. Under such conditions, the three catalysts tested exhibited substantially different performances, as depicted in Figure 2.


9


O

O

c-HT (3 wt.%)

+

O

O

OH

1a 5.29 g (4.0 mmol)

O

O O

90 °C azeotropic removal

O

O

+ MeOH

O 1c Yield: up to 95 % (7.22 g)

DMC 60 mL (18.0 equiv.)

100 90 80

Conversion by GC (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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70 60 50 40

c-HT30 c-HT63 c-HT70

30 20 10 0 0

1

2

3

4

5

6

7

8

9

22

23

24

25

Time (h)

Figure 2. CIR between DMC and solketal (1a): conversion profiles vs. time observed using different c-HT catalysts. Reaction conditions: 1a (4.0 mmol), DMC (60 mL, 18.0 equiv.), c-HT catalyst (3 % wt.), T = 90 °C. In all cases, selectivity towards the desired 1c product was > 97% (determined by GC-MS). As is evident from Figure 2, c-HT30 was by far the most active system, allowing a quantitative conversion of 1a in only 3 hours (black curve, Figure 2). By contrast, longer times (e.g. 8 and 24 h) were required over c-HT70 and c-HT63, respectively (blue and red profiles, Figure 2). This trend followed the same order of catalytic activity already observed by us for the alkylation reactions of 1a with DMC, i.e. c-HT30 > c-HT70 > c-HT63.25 Such behavior was accounted for by the peculiarities of c-HT30: among the catalysts tested, it displayed a different bulk structure, richer in the spinel Al2MgO4 phase, with a lower Mg/Al ratio. These properties allowed for an


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improved amphoteric character, due the simultaneous presence of two types of medium strength basic sites (Mg-O and Al-O pairs) and Lewis-acid sites in the form of coordinatively unsaturated Al3+ species.31 Accordingly, c-HT30 was effective in activating both the nucleophilic (alcohol) and the electrophilic (DMC) partner of the reaction. It was also worth noting that results achieved with c-HT30 were comparable to those reported with homogeneous La-based complexes (Eq. 1, Scheme 2)10 but, as an additional advantage, the solid catalyst could be easily recovered and re-used up to three times without any loss of activity (see SI for further details). Due to its superior performance, c-HT30 was the catalyst chosen to broaden the scope of CIRs between DMC and alcohols as summarized in Figure 1. Accordingly, mixtures of DMC and compounds 2a-4a, in an 18:1 molar ratio, respectively, were set to react at DMC reflux temperature. The preferred loading of c-HT30 was 3 wt. % based on the limiting reactant (alcohols 1a-4a), except for the reaction with cyclohexanol, which required a 9 % wt. catalyst loading to observe appreciable activity. Optimized reaction conditions, including isolated yields for the different alcohols tested, are summarized in Table 1 which, for comparison, reports also the reaction of Figure 2 with solketal (Entry 1, Table 1). Table 1. Synthesis of alkyl methyl carbonates catalyzed by c-HT30.

Product W Entry

a

b

t

Conv.

(h)

(%, GC)

Sel.

c

Substrate (% wt)

(%, GC)

Isolated Structure yield (%)

1

1a

3

3

100

97

O

O O

O

95

1c O


11


2d

2a/2a’

3

24

100

98

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96 2c/2c’ ratio = 2:3

3

3a

3

120

98

95

93

4

4a

3

48

12

100

-

5

4a

9

24

99

98

95

Conditions: Reflux temperature; alcohol (40.0 mmol), DMC (720.0 mmol, 18.0 equiv.). aW = c-HT30:alcohol weight ratio (% wt); bAlcohol conversion, determined by GC-MS. cSelectivity towards alkyl methyl carbonate product, determined by GC-MS; dGlycerol formal was used as a 3:2 mixture of isomers 2a and 2a’, respectively. All the reactions investigated afforded the corresponding alkyl methyl carbonates (2c-4c) in excellent yields and selectivity of 93-96 % and > 97 %, respectively, though, in all cases prolonged reaction times were necessary compared to solketal (Entry 1, Table 1). In a representative case, the CIR of glycerol formal proceeded eight times slower than that of the homologue acetal 1a (Entries 1 and 2, Table 1: 3 and 24 hours, respectively). This result reflected the same trend recently noticed by us and others when reacting glycerol formal and solketal with light dialkyl carbonates or esters, either in the presence or in the absence of catalysts.14,15,18,25,32 Although a stronger structuration of liquid glycerol formal due a larger extent of hydrogen bonding, as compared to solketal, was hypothesized as one of the reasons for the different reactivity of the two acetals,33 the current interpretation of experimental and modeling results is still far from explaining the behavior observed at a molecular level.34,35,36 In this respect, it was also noted that the original 2:3 ratio of isomeric reactants 2a and 2a’ was retained in the carbonate products 2c and 2c’ both during the reaction and in the final mixture


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(Entry 2, Table 1), thereby suggesting, quite surprisingly, that the primary alcohol 2a reacted at substantially the same rate of the secondary one 2a’. The CIR of tetrahydrofurfuryl alcohol (3a) was complete in 120 h (Entry 3, Table 1), meaning a 40-fold time increase compared to solketal. This behavior mirrored that observed for Omethylation of alcohols with DMC catalyzed by c-HT30:25 compounds 1a and 3a were both primary substrates, but the difference in polarity determined by the oxygen content of the corresponding cyclic structures, was likely responsible for a poorer adsorption of 3a at the catalytic surface, particularly when compared to solketal. This aspect was even more pronounced for cyclohexanol (4a) where, in addition, the larger steric hindrance at the OH function further contributed to reduce its reactivity. In this case, a quantitative conversion was achieved in 24 h only by tripling the catalyst loading to 9 % wt. (Entry 5, Table 1). Notably, also under homogeneous conditions and with ionic liquids as catalysts, the CIR between DMC and cyclohexanol was remarkably less effective than with primary alcohols (Eq. 3, Scheme 3).13 The resulting alkyl methyl carbonates 1c-4c were fully characterized by 1H and 13C{1H} NMR, spectroscopy and GC-MS spectrometry (see Experimental and SI for details). The procedure for CIRs could also be scaled up by a factor of 10, thereby obtaining some tens of grams (40-50 g) of each of the desired alkylmethylcarbonates. Under such conditions, the continuous distillation of the MeOH/DMC azeotrope during the transcarbonation process, allowed an efficient recovery of the excess DMC: up to 80 % yield (based on the initial DMC loading), and purity > 99 % by 1H NMR, were achieved in all cases. CF-disproportionation reactions for the synthesis of symmetrical dialkyl carbonates Alkyl methyl carbonates 1c-4c were used as reactants for the synthesis of symmetrical dialkyl carbonates according to the two-step disproportionation strategy depicted in Scheme 3. Aiming


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to develop a robust and scalable CF protocol and, given the results observed for transcarbonations with DMC (Figure 1 and Table 2), c-HT30 was the chosen heterogeneous catalyst also for the study of the dismutation process. Accordingly, powdered c-HT30 was charged in a tubular steel reactor (l = 12 cm, Ø = 1/4“, inner volume = 1.16 cm3) as uniformly and evenly as possible, to minimize preferential pathways of the reactant mixture through the solid bed.37 The catalyst was used in a 0.50 g amount according to its apparent density. Methyl solketal carbonate 1c was selected to begin the investigation. Due to the availability of this compound on a 50 g-scale only (see above) and the relatively large volumes required by CFtests over prolonged times, a 1.42 M solution of 1c in cyclohexane was used as the reactant mixture. Cyclohexane was selected as a solvent for its mid-range boiling point (b.p. = 81 °C), chemical inertness at high temperatures and easy separation from the reaction products upon distillation. Operating conditions were chosen based on preliminary results on the reactivity of 1c:25 accordingly, CF-experiments were carried out in a temperature range between 210-275 °C at atmospheric pressure, by feeding the reactor at a volumetric flow rate (F) of 0.1 mL min-1 (contact time ~10 min). The periodic GC-MS analysis of the mixture collected at the reactor outlet proved the formation of the expected symmetrical carbonate bis((2,2-dimethyl-1,3-dioxolan-4-yl)methyl) carbonate (disolketal carbonate, 1d) as a major product, along with solketal (1a) and solketal methyl ether (1b) as side-products. The structures of 1a, 1b and 1d were further confirmed by 1H and 13C{1H} NMR, and by comparison to authentic samples. Figure 3 shows the results achieved after 5 hours of on-stream time. It was noted, however, that both conversion and products distributions remained substantially unchanged after the first


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120 min. Each run was duplicated to ensure reproducibility, using a fresh sample of the same

90

90

80

80

70

70

60

60

Conversion 1b Sel. 1a Sel. 1d Sel.

50 40

50 40

30

30

20

20

10

10

0 200

Products' Selectivity (% by GC)

batch of calcinated catalyst.

Conversion (% by GC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 210

220

230

240

250

260

270

280

Temperature (°C)

Figure 3. Conversion and products distribution observed in the CF-reactions of methyl solketal carbonate (1c) catalyzed by c-HT30 at different temperatures. Other conditions: 1c/cyclohexane = 0.2 v/v; F=0.1 mL·min-1, p = 1 bar. Selectivity towards the three major products (1b, 1a, and 1d) is shown by the colored curves (blue, green, and orange, respectively). Raising the operating temperature from 210 to 275 °C, improved the conversion of 1c from 71% to 86%, respectively (black profile). However, the selectivity towards the desired symmetrical carbonate 1d dropped from 82% to 62% (orange curve) in the same temperature range, because of the increased formation of by-products 1a and 1b (green and blue profile, respectively). This behavior was consistent with the occurrence of reaction pathways described


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in Scheme 4. All products 1a, 1b, and 1d could be obtained directly from reactant 1c through three competitive processes: 1) a disproportionation reaction, which was the major transformation, yielding the desired carbonate 1d and DMC [i), solid box, mid right]. The formation of DMC was proven by GC-MS analysis of the reaction mixtures; 2) a hydrolysis reaction, producing compound 1a with a selectivity of up to 13 % at 275 °C [ii), dashed box, bottom, and Figure 3]. This process was likely induced by the presence of trace amounts of water (up to 0.15 % w/w) in the starting reagent 1c, which was confirmed by Karl Fischer titration (see SI for details).38 This hydrolysis did not only generate solketal 1a, but also methyl hydrogen carbonate (MeOCO2H) as an unstable co-product, which then decomposed into CO2 and MeOH.39 3) a decarboxylation reaction, generating solketal methyl ether 1b [iii), dashed box, mid left]. Dialkyl carbonates are known to release CO2 upon heating over several heterogeneous catalysts including zeolites, alkaline carbonates,27and hydrotalcites.40 Although we already observed that c-HT30 was poorly active at catalysing this process,25 decarboxylation of 1c could not be completely ruled out under the conditions of Figure 3.


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Scheme 4. Major reaction pathways occurring during the CF reaction of solketal methyl carbonate 1c. Moreover, the formation of 1a and DMC from hydrolysis and disproportionation pathways, plausibly triggered additional competitive reactions (dashed box, top). For example, acetal 1a could undergo a CIR with DMC and/or 1c. Although these processes were limited by the moderate availability of 1a, the first one restored the starting reagent 1c [v), Scheme 4], while the second transformation contributed further to the production of 1d [vi), Scheme 4]. Finally, carbonates 1c and DMC could act as methylating agents for 1a, thereby increasing the presence of methyl ether 1b as a by-product [vii), Scheme 4; for simplicity, only the reaction of DMC is shown]. It should be noted that such methylations mediated by DMC or alkyl methyl carbonates are not only irreversible reactions, but they are usually more energy-demanding than transcarbonation and disproportionation equilibrium processes.3,18,21,25 This observation might explain the trend of Figure 3, where the selectivity towards methyl derivative 1b increased


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progressively with temperature, at the expenses of carbonate 1d (cfr. orange and blue profiles). On the other hand, decarboxylation of the starting reagent 1c to produce ether 1b had plausibly a minor relevance (path iii), as substantiated by the total absence of disolketal ether [4,4'(oxybis(methylene))bis(2,2-dimethyl-1,3-dioxolane] in the reaction mixture. A fact suggesting that even the decarboxylation of the homologue carbonate 1d was a rather unlikely reaction. Overall, Figure 3 indicated that the desired disproportionation of solketal methyl carbonate was feasible over c-HT30 requiring, however, improvements in terms of selectivity. The operational flexibility of the CF-procedure was therefore exploited tuning the reaction parameters, aiming at optimising the formation of disolketal carbonate 1d. Experiments were carried out at 210 °C, varying both pressure and flow rate in the range of 1 to 50 bar, and 0.1 to 0.4 mL min-1, respectively. Other experimental conditions, including catalyst loading (0.5 g) and reactant concentration (1.42 M solution of 1c in cyclohexane) were kept unchanged. In general, the outcome of the reaction did not improve upon increasing the pressure from 1 to 50 bar: 1c conversion and selectivity towards carbonate 1d decreased from 71 to 66 %, and 82 to 68 %, respectively, while the formation of both by-products, solketal 1a (9 %) and solketal methyl ether 1b (23 %), was favored at higher pressures (see Figure S2 in SI for further details). Pressure variations plausibly altered the liquid/vapor partition of reactants and products, but only moderate effects on the relative rates of competing reactions of Scheme 4 were noticed. Accordingly, optimization studies were continued at ambient pressure. Under such conditions (T = 210 °C and p = 1 bar), a remarkable improvement in conversion and selectivity was noticed by varying the flow rate. Results are reported in Figure 4, which additionally includes the reaction productivity (P) for a more detailed comparison. P is calculated as the mass of desired product 1d obtained per unit of time and per gram of catalyst (P = [mgprod·(gcat·min)-1]). Similarly to the


18

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experiments summarized in Figure 3, each run was repeated twice to ensure reproducibility, using a fresh sample from the same batch of calcinated catalyst for each run. 160

100

140

-1

80 70

120

60 100

50 40

Conversion 1b Sel. 1a Sel. 1d Sel. Productivity

30 20

80

60

10 0 0.05

Productivity [mgprod (gcatmin) ]

90

% by GC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

-1

Flow rate (mL min )

Figure 4. CF-reaction of solketal methyl carbonate 1c catalyzed by c-HT30: effect of the variation in flow rate (F) on conversion, selectivity, and productivity. Reaction conditions: 1c/cyclohexane = 0.2 V/V, p = 1 bar, T = 210 °C. When applying a four-fold flow rate increase, from 0.1 to 0.4 mL·min-1 and corresponding reduction of both the contact time (τ) from 10 to 2.5 min, the conversion of 1c dropped from 71 to 50%, respectively (black profile). The decrease of conversion was rather moderate, less than expected by the variation of τ, thereby suggesting that a further optimization of the catalyst loading was possible. This aspect, however, would have required a substantial change of the reactor design, which was beyond the scope of the investigation. Notably, the flow rate increase improved the selectivity towards 1d up to 92% (orange profile), and the effect on productivity was even more striking. P raised linearly for 0.1 < F < 0.3 mL·min-1, reaching a maximum value of 152 mgprod·(gcat·min)-1 at 0.4 mL·min-1 (red-dashed trace): overall, it was boosted by a factor of


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3.5 in the range of flow rates explored. As can be noticed from Figure 4, this result was mostly due to a decreased formation of by-product 1b, consistent with the fact that the shorter the contact time, the lower the extent of high energy-demanding processes such as methylation/decarboxylation reactions of Scheme 4. The resulting scenario finally corroborated the key role of the dismutation of 1c for the synthesis of carbonate 1d. Moreover, the solvent could be quantitatively recovered at the end of the reaction, by distillation under reduced pressure (rotary evaporator). Reaction scope. The best CF-conditions for the disproportionation of compound 1c that we determined here, were then chosen to expand the reaction scope to alkyl methyl carbonates analogues namely, methyl glycerol formal carbonate (2c/2c’), methyl tetrahydrofurfuryl carbonate (3c) and methyl cyclohexyl carbonate (4c) (Table 1). Regrettably, a preliminary reactivity screening of these substrates indicated the need to adjust experimental conditions on a case-by-case basis, specifically for carbonates 2c/2c’ and 4c which were poorly soluble in cyclohexane and required higher reaction temperatures. Toluene solutions (1.42 M) of the alkyl methyl carbonates tested were thus prepared for CF-tests; while, for comparison, analogous cyclohexane solutions were still used when possible, i.e. for carbonates 1c and 3c. Such mixtures were delivered at 0.3 mL·min-1 (see Figure 4), to a catalytic bed of c-HT30 (0.5 g) operating at 180-275 °C and ambient pressure. All reactions were followed for 5 h, ensuring that steady conversions and selectivities were reached. Similarly to the above-described case of solketal methyl carbonate, some hydrolyses and methylations/decarboxylations were noticed as competitive pathways to the desired dismutation process. The results of this study are summarized in Table 2.


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Among the reactant carbonates, 1c and 3c (derived from solketal and tetrahydrofurfuryl alcohol, respectively) gave the highest selectivity (87-92%) and productivity [P: 123-164 mgprod·(gcat·min)-1] towards the corresponding symmetrical carbonates 1d and 3d; though, the change of the solvent from cyclohexane to toluene induced opposite effects on conversion (Entries 1-2 and 5-6, Table 2). Intriguingly, when using a bulky substituted alcohol such as cyclohexanol 4c, the reaction occurred under milder conditions than for 1c and 3c: lowering the temperature from 210 to 180 °C, but still allowed a 60 % conversion of cyclohexyl methyl carbonate (Entries 6-7, Table 2). In this range, the selectivity and P towards dicyclohexyl carbonate 4d were still satisfactory, reaching 71% and 90 mgprod·(gcat·min)-1, respectively. Reasons for this behavior are still unclear and are likely not to be limited simply to steric effects, but also to a peculiar solvation and interaction with the catalyst surface.

Table 2. CF disproportionation reaction of alkyl methyl carbonates (1c-4c) catalyzed by c-HT30.

Reaction Substrate Entry

T

Pc

Conv.

Solvent ROCO2Me

products, (°C)

(%)a

mgprod·(gcat·min)-1

b

Sel. (%)

1

CyHd

1a

1b

1d

1d

3

5

92

141

76

3

10

87

164

22

2a

2b

2d

2d/2d’

62 210

2

Toluene

3

Toluene

210


21


2c/2c’

=

2:3

mol/mol

4

275 d

5

210

CyH

6

Toluene

7

30

210

64

180

60

210

/2a’

/2b’

/2d’

53

12

35

15

47

22

31

19

3a

3b

3d

3d

Two-step Synthesis of Dialkyl Carbonates through Transcarbonation

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