Selective Synthesis of Diglycerol Monoacetals via Catalyst-Transfer in

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Selective Synthesis of Diglycerol Monoacetals via Catalyst-Transfer in Biphasic System and Assessment of their Surfactant Properties Qiong Tang, Xu Li, and Jinxiang Dong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04082 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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ACS Sustainable Chemistry & Engineering

Selective Synthesis of Diglycerol Monoacetals via Catalyst-Transfer in Biphasic System and Assessment of their Surfactant Properties Qiong Tang†, Xu Li†, Jinxiang Dong*†‡ †Research Institute of Special Chemicals, College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China, ‡School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China *Corresponding Authors E-mail: [email protected] (Jinxiang Dong)

Mailing address: Taiyuan University of Technology, No.79 West Street Yingze, Wanbailin District, Taiyuan 030024, Shanxi, China

ACS Paragon Plus Environment

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ABSTRACT. Diglycerol obtained from biomass glycerol has been valorized into selective products by acetalization reaction in the presence of ZnCl2. A synthesis method of monoacetals via catalyst-transfer is carried out encompassing two phase-separated reagents, diglycerol with noctanal. Mechanistic analysis has revealed that the formation of unstable emulsion promotes the reaction, and the selectivity is affected by the catalyst-transfer direction. By measuring the Zn content in the two phases, we demonstrated the transfer direction of ZnCl2 and the monoacetal were opposite, resulting in separation of the catalyst and the substrate. And the formation of diacetal was retarded. The products monoacetals exhibit superior surfactant properties, especially in terms of foam stability and alkali tolerance.

KEYWORDS. Catalyst-transfer, Biphasic system, Selective catalysis, Nonionic surfactants


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INTRODUCTION Glycerol is a by-product of biodiesel manufacture, and its supply as a renewable feedstock is predicted to grow steadily.1 However, due to contamination with methanol from biodiesel production, even when refined, this glycerol is deemed unfit for direct utilization in the production of pharmaceuticals and foodstuffs.2 It is crucial to convert glycerol into value-added chemicals for use in other fields through various processes. One possibility is to convert glycerol into small platform molecules, such as dehydration to acrolein,3,4 oxidation to glyceric acid,5,6 and hydrogenation to propylene glycol,7,8 and ultimately to obtain olefins.9 Alternatively, glycerol, as a hydrophilic compound, may be subjected to etherification, esterification, polymerization, and acetalization. The resulting glycerol-based compounds are promising alternatives in the fields of fuel additives,10 solvents,11 and surfactants.12

Scheme 1. Acetalization of diglycerol with n-octanal. In 1980, Burczyk’s group13 explored the products synthesized from glycerol and C8–C16 aldehydes and described their surfactant properties, such as aqueous surface tension and adsorption behavior at the oil/water interface. However, subsequent research indicated that glycerol acetals exhibited poor aqueous solubility and unsatisfactory surfactant properties.14


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Only when a glycerol acetal was derivatized with a hydrophilic group by sulfation15,16 or ethoxylation17 did it become useful in wetting, washing, and foaming agents.18-20 Diglycerol is obtained by the dehydration of two molecules of glycerol and can serve as a hydrophilic building block with a central ether linkage and four hydroxyl groups for the synthesis of surfactants. It can be seen that the acetalization of diglycerol with aldehydes may generate two products, monoacetals and diacetals (Scheme 1). Taking n-octanal as an example, hydrophilic–lipophilic balance (HLB) values were calculated. The HLB values of the monoacetal and diacetal formed with n-octanal are 8.09 and 3.34, respectively, as compared to 4.12 for glycerol acetal (see Table S-1). Hence, the monoacetal of diglycerol should exhibit better hydrophilicity than glycerol acetal, making it promising for use as a surfactant. However, it is still a challenge to directly synthesize a monoacetal with high selectivity. The free energy changes associated with the acetalization of diglycerol with n-octanal were calculated by means of density functional theory (DFT; see Table S-2).21 The free energy changes for formation of the monoacetal and diacetal were calculated as –6.24 and –8.06 kJ/mol, respectively. These values are similar, but formation of the monoacetal is thermodynamically less favorable. Moreover, the reaction is inefficient due to poor miscibility of the reagents (diglycerol and aldehyde).22 Solvents are used to facilitate interaction between the two substances residing in different phases. When the carbon chain of the fatty aldehyde is hexyl or longer, it is difficult to find an appropriate solvent to bridge the gap in polarities between diglycerol and the aldehyde.23-25 Herein, we report the selective acetalization of diglycerol with n-octanal catalyzed by ZnCl2. This work represents an interphase reaction between components in two immiscible liquid


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phases. The ZnCl2 catalytic system was found to promote the acetalization with excellent activity and selectivity. Moreover, the obtained monoacetals exhibit good surfactant properties. RESULTS AND DISSCUSION Traditionally, the reaction of glycerol with long-chain aldehydes (>C6) has been performed with organic acids as effective catalysts.26-28 Our preliminary work29 established a solvent-free procedure for the synthesis of glycerol acetal by using p-toluenesulfonic acid (PTSA) and pdodecylbenzenesulfonic acid (DBSA). In this study, the reaction of diglycerol (1.3 equiv.) with n-octanal was initially conducted in the presence of PTSA (2 mol%) at 100 °C for 4 h without a solvent. Under these conditions, the n-octanal conversion reached 90.0 %, but the selectivity in favor of monoacetals was only 11.8 %. The major product was the diacetal. Similarly, DBSA gave a conversion of 91.1 %, but the monoacetal selectivity was only 9.4 %. Thus, initial results using organic acid catalysts were disappointing. Inspired by the work of Pierpont et al.,21 who found that coordination between metal ions and glycerol could change the symmetrical structure of the transition state, we envisaged that selective acetalization might be achieved by metal salt catalysts through such coordination. Pesek and co-workers30 demonstrated that diglycerol could coordinate to barium ions. BaCl2 was thus used for the acetalization reaction, providing the monoacetal with 86.0 % selectivity in 35.4 % conversion (Figure 1). In order to increase the conversion, other Lewis acid catalysts were screened. AlCl3, CuCl2, FeCl3 and BF3·OEt2, preferred catalysts for acetalization reactions31-33, were ineffective for addition to monoacetals (Table S-3, entries 1 to 4). In contrast, MgCl2 and CaCl2 gave 68.5% and 82.4 % monoacetal selectivities with 75.3 % and 50.1 % n-octanal conversions, respectively (Table S-3, entries 5 and 6), but ZnCl2 proved to be the most active and effective, giving 80.3 % conversion and 84.4 % monoacetal selectivity (Figure 1).


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Figure 1. Reaction of diglycerol and n-octanal in the presence of different catalysts. a) The reaction was performed with diglycerol (50.0 mmol, 8.30 g), n-octanal (39.8 mmol, 4.98 g), and catalyst (2.0 mol% with respect to n-octanal, 0.11 g) for 4 h at 100 °C under N2. b) Conversion of n-octanal determined by gas chromatography. c) Selectivity in favor of monoacetal (%) = moles of monoacetal/total moles of mono- and diacetal. ZnCl2 was thus chosen as a catalyst for further optimization studies of the acetalization reaction. Various reaction parameters were evaluated (Figure S-1). An increase of the reaction temperature from 90 °C to 100 °C led to a significant increase in conversion from 27.0 % to 80.4 % (Figure S-1-a). However, extending the reaction time had little effect on the conversion, which only increased from 80.3 % after 4 h to 81.2 % after 6 h (Figure S-1-b). A reduction in the catalyst loading to 1 mol% afforded monoacetals with excellent selectivity, but lower conversion (Figure S-1-c). Variation of the reactant ratio led to no obvious improvements (Figure S-1-d). The optimized synthetic conditions involved treating diglycerol (1.3 equiv.) with n-octanal in the presence of 2 mol% ZnCl2 at 100 °C for four hours. Analytical results have shown the isomer distribution of diglycerol to be around 53.4 % α,αdiglycerol and 32.1 % α,β-diglycerol.34 The possible products of acetalization of diglycerol are


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thus monoacetals (products a, b, and c) and diacetals (products d and e) (Scheme 1). GC analysis of the product revealed six peaks at retention times of 19–20 min and seven peaks at retention times of 34–40 min (Figure S-2). Mass spectral analysis allowed the former six peaks to be assigned to monoacetals and the latter seven peaks to be assigned to double-chain acetals. The three monoacetal isomers gave fragment ions with m/z 131 and 177 for product a, 233 and 117 for product b, and 219 and 177 for product c. The two diacetal isomers exhibited the most prominent peak at m/z 287, corresponding to [M–C7H15]+. The detected fragments and their possible structures are shown in Figures S-3 and S-4. The product obtained with ZnCl2 was purified to obtain monoacetals for detailed structural investigations. The possible product configurations of the monoacetals were confirmed by their NMR spectra (Figure S-10). And the kind of catalyst and its influence on the product distribution was intended. The major product is product a under two kinds of catalysts. About the acetal of α, β-diglycerol, the free energy of product c is lower than product b (see Table S-2), both of the two kinds catalysts more preferred to form product c (Figure 1).

a)


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b)

Figure 2. a) GC traces of the compositions of the two phases after 4 h. b) Phase separation times and conversions over reaction times. The reaction was occurred between two immiscible phases. Before the reaction, the upper phase contained n-octanal and the lower phase contained diglycerol. After the reaction, the compositions of the two phases were analyzed by GC. The upper phase contained n-octanal, monoacetal, and diacetal, whilst the lower phase contained only diglycerol (Figure 2-a). Thus, the products appeared in the upper phase along with n-octanal. During the reaction, it was observed that the two immiscible phases were gradually emulsified. The emulsion formed after 2 h was analyzed by GC and was found to contain n-octanal, diglycerol, and monoacetal (Figure S5), which implied that the initially formed small amounts of monoacetals served as an emulsifier. Thus, the conversion was low, and the process involved accumulation of the emulsifier in the first 2 h. The emulsification was an endothermic process, with the conversion not exceeding 27.0 % when the reaction temperature was lower than 100 °C (Figure S-1-a). However, the emulsified state was very unstable, and the droplet sizes were inhomogeneous (Figure S-6). Consequently, phase separation occurred easily without stirring (Figure S-6). The phase separation time (details in the Supporting Information) increased sharply from 2 to 4 h,


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consistent with the conversion (Figure 2-b). This suggested that the reaction occurred at the twophase interface by emulsification. As the phase separation time increased, the emulsion became more stable and more n-octanal could take part in the reaction. a)

b)

c)

Figure 3. a) Reaction of diglycerol and n-octanal. The reaction was performed with diglycerol (12.0 mmol, 2.00 g), n-octanal (15.0 mmol, 1.92 g), and catalyst (2.0 mol% with respect to noctanal, 0.04 g) for 4 h at 100 °C under N2. b) Reaction of monoacetal and n-octanal. Monoacetal (10.0 mmol, 2.76 g) and n-octanal (15.0 mmol, 1.92 g), the other condition was same. c) Reaction of monoacetal and n-octanal with diglycerol. Diglycerol (12.0 mmol, 2.00 g), monoacetal (10.0 mmol, 2.76 g), n-octanal (15.0 mmol, 1.92 g), the other condition was same. Conversion of monoacetal and n-octanal determined by gas chromatography.


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Figure 4. Zn content and distribution in the n-octanal phase and the selectivities after different reaction times.

Figure 5. Mechanistic hypothesis supported by control experiments. Given the unique chemoselectivity of the acetalization catalyzed by ZnCl2, we became interested in the reaction mechanism. The catalysts, AlCl3, ZnCl2 and CuCl2, showed inconsistent consequence with acid strength in biphasic system (Table S-1). However, when the catalysts were used in the reaction of monoacetal and n-octanal in one phase, the results were consistent with the acid strength (Figure 3-b). Strong Lewis acid, AlCl3, was showed high monoacetal conversion, 95.8 %. The mild Lewis acids, ZnCl2 and CuCl2, offered monoacetal conversion of 43.3 % and 32.3 %. And the color change was observed for CuCl2. At the beginning, the catalyst was dissolved in diglycerol, and the n-octanal was gradually added dropwise to the system. Then the CuCl2 was able to present in the monoacetal and n-octanal phases (Figure S-8). We further envision that the selectivity may be caused by the catalysttransfer between the two phases. Furthermore, the diglycerol added in the monoacetal and n-


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octanal was studied (Figure 3-c). In the presence of CuCl2, the conversion of n-octanal was 76.2 %, which was lower than the reaction of diglycerol and n-octanal under the same condition (Figure 3-a). As a part of the catalyst moved to the upper layer (Figure S-8), the conversion of noctanal was reduced. About ZnCl2, the conversion of n-octanal was 24.6 % and the amount of monoacetal was increased by 23.7 %. The conversion was lower than the reaction of monoacetal to the diacetal and the reaction of diglycerol and n-octanal, which may be caused by the incompetence of ZnCl2 to transfer to the upper layer. Then the Zn contents of the two phases were determined by ICP-OES to establish the distribution of the catalyst. During the reaction, the Zn content in the diglycerol phase gradually increased from 5,971 ppm to 10,510 ppm, and the proportion in the diglycerol phase decreased from 97.9 % to 79.5 %.The Zn content in the noctanal phase increased from 148.3 ppm to 1078 ppm and the proportion in the n-octanal phase increased from 2.1 % to 20.5 % (Figure 4). Therefore, monoacetal, which was formed at the interface, was ultimately transfered in the n-octanal phase. ZnCl2 had the opposite transfer direction with monoacetal. Thus the content of ZnCl2 in the n-octanal phase was low, resulting in less efficient further reaction with the monoacetal, and the formation of diacetal was retarded (Figure 5).

Entry

Sample name

HLB

CMC (g/L)

Surface tension at CMC -1 (mN m )

Foam volume (mL) 40 °C 0s

5min

Difference contact a angle (°)

Cloud point (°C)

Solubility (1 g/L with 7.5 wt% aqueous b NaOH) Clear

1

Monoacetal

8.09

0.029

27.4

300

250

33.97

>95

2

AEO-7

12.6

0.023

28.2

300

80

32.66

58

3

AEO-3

8.49

0.008

25.8

80

50

17.33

29

4

Diacetal

3.34

NA

NA

0

0

-

-


Severe turbid Severe turbid -

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[a] Calculated as the difference between pure water and 1 g/L aqueous surfactant in the first cycle. The greater the value, the better the wetting ability. [b] After 2 weeks at room temperature. [c] NA: not available.

Table 1. Surfactant properties of diglycerol acetals compared with those of AEO-3 and AEO-7 The surfactant properties of the monoacetals and diacetals obtained after purification have been compared with those of the traditional fatty alcohol polyoxyethylene ethers AEO-7 and AEO-3. Although HLB value of the single-chain acetal (8.09) is slightly less than AEO-3 (8.48), we observed a critical micelle concentration (CMC) of 0.029 g/L and a surface tension at the CMC of 27.4 mN/m and these values are very close to those observed for AEO-7. The results are listed in Table 1. Due to poor solubility in water, the diacetal could not be dispersed in aqueous solution. Hence, its surfactant properties could not be assessed, and this product is not discussed further in the following text. The dynamic contact angle was then measured on canvas to characterize the wetting ability (see Table S-4).35-37 The value was obtained as the difference between pure water and 1 g/L aqueous surfactant. The difference contact angles of AEO-3 and AEO-7 were measured as 17.33° and 32.66°, respectively, and that of the monoacetal was 33.97°. Thus, the monoacetal showed better hydrophilicity than that of AEO-3, similar to that of AEO-7. Acetal derivatives, which have a number of free hydroxy groups, are stable at high temperatures and in basic media.14 The cloud point of the monoacetal (>95 °C) was found to be much higher than those of AEO-7 (58 °C) and AEO-3 (29 °C), opening a broad usable temperature range. The foam performance of this product was then evaluated at 40 °C and compared with those of AEO7 and AEO-3. Its foaming ability proved to be far better than that of AEO-3, and the foam stability was better than that with AEO-7. The foam volume with AEO-7 rapidly decreased from 300 mL to 80 mL after 5 min, whereas that with the monoacetal remained at 250 mL over this period. Moreover, alkali tolerance was examined by treating 1 g/L aqueous surfactant solution with 7.5 wt% NaOH over a period of two weeks. AEO-3 was limited in its effectiveness by its


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solubility, and the addition of NaOH led to turbidity. Aqueous AEO-7 showed severe turbidity and low tolerance to alkali, whereas aqueous monoacetal was clear at the same NaOH concentration and showed better performance. Characterization of the surfactant properties of the monoacetals has indicated that they are comparable surfactants to traditional non-ionic surfactants, with superior properties in terms of alkali tolerance and foam stability. CONCLUSION In summary, we have demonstrated selectivity for the monoacetal over the diacetal in the ZnCl2-catalyzed acetalization of diglycerol with n-octanal. This provides an example of the direct catalytic synthesis of a monoacetal in a two-phase system through an unstable emulsification process. As part of a preliminary mechanistic study, we have noted the monoacetals and ZnCl2 had different transfer direction, which affects the selectivity of the reaction. Finally, the surfactant properties of the monoacetals have been determined and showed that they might be applicable as new non-ionic surfactants. EXPERIMENTAL SECTION General information: All materials were purchased from commercial sources and utilized without further purification. The reactions were monitored by GC and GC-MS. Samples of the product mixtures were cooled to room temperature. An aliquot (0.1 g) of the upper organic layer was diluted with ethyl acetate (10 mL) and the solution was washed with saturated aqueous Na2CO3 solution (10 mL). It was then centrifuged for 3 min at 3000 rpm, dried over anhydrous Na2SO4, and filtered prior to GC and GC-MS analyses. Analysis was carried out on a Shimadzu


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GC-2014C gas chromatograph equipped with a column injector (280 °C), an FID detector (290 °C), and an Rxi-5 MS column (30 m, 0.25 mm i.d., 0.25 µm film thickness). The column temperature was initially 120 °C for 2 min, then gradually increased to 250 °C at 10 °C min-1, and maintained at this level for 30 min. GC-MS analysis was realized on a Shimadzu GCMSQP2010 Plus instrument equipped with electrospray ionization (EI) (70 eV) for MS; other conditions were the same as for GC. 1H and

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C NMR spectra were obtained on a Bruker

Advance III spectrometer at 400 MHz. Chemical shifts are given with reference to the signal of residual H in CDCl3 at 7.26 ppm (1H) and its

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C signal at 77.0 ppm. The content of zinc ions

was analyzed by inductively-coupled plasma–optical emission spectrometry (Perkin-Elmer ICPOES). Monoacetal : A 250 mL three-necked round-bottomed flask was charged with catalyst (2 mol% with respect to n-octanal, 0.11 g) and diglycerol (50.0 mmol, 1.3 equivalents, 8.30 g) under nitrogen. n-Octanal (38.9 mmol, 4.98 g) was dropped into the flask and the reaction mixture was stirred at 100 °C for 4 h. The resulting residue was dissolved in ethyl acetate, saturated aqueous Na2CO3 solution (40 mL) was added, and the biphasic mixture was centrifuged for 3 min at 3000 rpm. The organic layer was dried over Na2SO4 and concentrated under reduced pressure to afford the product mixture. The crude products were purified by column chromatography on 200–300 mesh silica gel, eluting with petroleum ether/ethanol (19:1, v/v). Yield: 60.8 % ; pale yellow liquid; HRMS (ESI) m/z calcd for C14 H28 O5 [M + Na]+ 299.1829, found 299.1823. Diacetal: A 250 mL three-necked round-bottomed flask was charged with p-toluenesulfonic acid (2 mol% with respect to n-octanal, 0.17 g) and diglycerol (50.0 mmol, 1 equivalent, 8.30 g) under nitrogen. n-Octanal (50.0 mmol, 6.40 g) was dropped into the flask and the reaction


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mixture was stirred at 100 °C for 4 h. The resulting residue was dissolved in ethyl acetate, saturated aqueous NaHCO3 solution (40 mL) was added, and the biphasic mixture was stirred. The organic layer was washed with brine, dried over Na2SO4, and concentrated under reduced pressure to afford the product mixture. The diacetal was purified by recrystallization three times from ethanol at –35 °C for 24 h. Yield: 78.7 % ; pale yellow liquid; HRMS (ESI) m/z calcd for C22 H43 O5 [M + H]+ 387.3105, found 387.3098. ASSOCIATED CONTENT Supporting Information. Computational details, characterization data for compounds, GC traces, MS spectra, 1H and 13C NMR spectra, the mechanism experiments of PTSA and DBSA, and additional supporting data. ACKNOWLEDGMENT We acknowledge generous financial support from the Program for Sanjin Scholars of Shanxi Province of China and the Key Program of National Natural Science Foundation of China (grant No. 21436008). We also thank the computational resource assistance by the Shenzhen Supercomputing Center. REFERENCES 1. Mota, C. J. A.; Peres Pinto, B.; de Lima, A. L. Glycerol: A Versatile Renewable Feedstock for the Chemical Industry; Springer International Publishing: New York, 2017, DOI 10.1007/978-3-319-59375-3. 2. Haider, M. H.; Dummer, N. F.; Knight, D. W.; Jenkins, R. L.; Howard, M.; Moulijn, J.; Taylor, S. H.; Hutchings, G. J. Efficient green methanol synthesis from glycerol. Nat. Chem. 2015, 7, 1028-1032, DOI 10.1038/nchem.2345. 3. Katryniok, B.; Paul, S.; Belliere-Baca, V.; Rey, P.; Dumeignil, F. Glycerol dehydration to acrolein in the context of new uses of glycerol. Green Chem. 2010, 12 (12), 2079-2098, DOI 10.1039/C0GC00307G.


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For Table of Contents Use Only

A direct method of selectivity synthesis to monoacetals in a biphasic system is reported and monoacetals can serve as nonionic surfactants.


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Selective Synthesis of Diglycerol Monoacetals via Catalyst-Transfer in

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