Glycerol valorization under continuous flow conditions-recent advances

Accepted Manuscript Glycerol valorization under continuous flow conditions-Recent advances Rajender S. Varma, Christophe Len PII:

S2452-2236(18)30056-7

DOI:

https://doi.org/10.1016/j.cogsc.2018.11.003

Reference:

COGSC 223

To appear in:

Current Opinion in Green and Sustainable Chemistry

Received Date: 2 August 2018 Revised Date:

7 November 2018

Accepted Date: 8 November 2018

Please cite this article as: R.S. Varma, C. Len, Glycerol valorization under continuous flow conditionsRecent advances, Current Opinion in Green and Sustainable Chemistry, https://doi.org/10.1016/ j.cogsc.2018.11.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Glycerol valorization under continuous flow conditions-Recent advances.

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Rajender S. Varma1 and Christophe Len2,3

Adresses

Regional Center of Advanced Technologies and Materials, Faculty of Science, Palacký

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1

University, Olomouc, Šlechtitelů 27, 783 71 Olomouc, Czech Republic.

Sorbonne Universités, Universite de Technologie de Compiegne, F-60200 Compiegne,

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France 3

PSL University, Chimie ParisTech, F-75005 Paris, France Corresponding author: Len,

Abstract

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Christophe ([email protected])

This report describes the recent advances in glycerol valorization to valuable products under

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liquid phase continuous flow systems using different types of catalysts and processes. The

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main biobased-chemicals obtained from glycerol, namely acrolein, lactic acid, glyceric acid, propanol, propanediols, glycerol carbonate, solketal, acetin, and oligomers, are highlighted.

Highlights •

Glycerol has received ever-incresing attention due to its abundant nature as a byproduct from the biodiesel industry;

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Glycerol can be valorized under various conditions to produce valuable chemical entities such as acrolein, lactic acid, glyceric acid, propanol, propanediols, glycerol carbonate, solketal, acetin, and related oligomers;

•

An overview of glycerol valorization to useful entities in liquid phase continuous flow

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systems are summarized.

Keywords: Glycero; continuous flow system; acrolein; lactic acid; glyceric acid; propanol;

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propanediols; glycerol carbonate; solketal; oligomer

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Introduction

Biorefinery concept and utilization of vegetable oils is in the spotlight of the chemical industry as a promising alternative to meet future challenges [1,2]. Among the bio-based platform chemicals, glycerol embodies an exceptional example that has significant potential

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in terms of conversion to valuable products (Figure 1) [3-11]. Glycerol, propan-1,2,3-triol, is a symmetrical polyol encompassing three hydroxy (OH) groups : two identical primary OH and one secondary OH having similar pKa. Various chemical transformations such as

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oxidation, hydrogenolysis, etherification, esterification, dehydration and oligomerization can be executed to create a large number of value-added chemicals with specific applications in

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the polymer, agrochemical, and pharmaceutical industries [3-11]. In recent years, interesting alternative technologies, such as continuous flow [12-20], microwave- [21-35], and ultrasound-assisted [36-39] processes, micellar catalysis [40-43], and critical solvents [44] have been explored to develop sustainable processes in green chemistry and engineering. Among them, continuous flow and continuous processing offer significant processing advantages compared to the traditionl batch chemistry; the main improvements being thermal management, mixing control, adaptation to a wider range of reactions conditions, scalability, energy efficiency, waste reduction, safety, use of heterogeneous catalysts, multi-step

ACCEPTED MANUSCRIPT synthesis, among others. This review focuses on recent selected advances of glycerol chemistry under liquid-phase continuous-flow processing conditions during the last 3 years; utility of glycerol for the production of hydrogen and for the generation of nanoparticles has

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been deliberately ommitted.

Figure 1. Roadmap of selected glycerol valorization applications under batch and/or continuous flow conditions.

Continuous dehydration Dehydration of glycerol at elevated temperature, particularly in presence of acid catalysts, is well known to produce wide-ranging products, from acrolein to syngas. The production of

ACCEPTED MANUSCRIPT acrolein has been reported recently using zirconium phosphate as catalyst [45] wherein an aqueous solution of glycerol (10 wt%) and H2 (2 MPa) were injected simultaneously with a flow rate of 0.04 mL·mn-1 and 30 mL·mn-1, respectively over zirconium phosphate catalyst at 315 °C. A complete conversion of glycerol with good selectivity to acrolein (82%) was

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discerned and no trace of side products such as propan-1-ol, propan-1,2-diol, ethanol and methanol were observed with only minimal amounts of acetol (11%) being detected. Interestingly, this catalyst has proven to be highly hydrothermally stable and water-tolerant

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solid acid and no deactivation was seen over 50 h under N2 (30 mL·mn-1). The surface of the calcinated ZrP catalyst possess a large amount of medium strength Bronsted acid sites

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favoring the dehydration of glycerol to acrolein whereas small amont of Lewis acid site was apparently responsible for the generation of acetol. In contrast, with the double dehydration of glycerol, deoxydehydration of glycerol towards allyl alcohol under continuous flow process

Continuous oxidation

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has been reported using triethyl orthoformate [46].

Lactic acid is a highly prized chemical platform molecule, as well as a precursor for valuable

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biopolymers, whose biotransformation from glycerol is well known [47]. The chemical conversion of glycerol to lactic acid can be realized using three different routes:

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hydrogenolysis to propan-1,2-diol in the presence of metal supported catalysts and then selective oxidation (route 1); selective dehydrogenation to dihydroxyacetone, dehydration to pyruvaldehyde on solid acid catalyst and then Cannizaro rearrangement under acidic conditions (route 2); dehydration to hydroxyacetone on acid catalysts, oxidation to pyruvaldehyde and then Cannizaro rearrangement via acidic treatment (route 3) (Figure 2).

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Figure 2. Plausible reaction pathways to convert glycerol into lactic acid.

Manfro’s group reported the production of lactic acid from glycerol under alkaline conditions

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using Cu and Pt supported on Al2O3, ZnO and MgO [48,49]; oxidation was carried out at 240 °C under 35 atm to keep the reagent in liquid phase. With space velocity (WHSV) of 2 h-1, the aqueous glycerol (10 wt%, 1.37 M) in presence of NaOH (1 eq) and Cu/MgO furnished lactic acid in 86 % yield (selectivity 90%, conversion 95%) [48]. Two by products were identified in liquid phase, namely propan-1,2-diol and acetol. The same group followed similar approach to furnish lactic acid in the presence of Pt/ZnO as catalyst with lower selectivity [49]. In 2018, Prati and co-workers reported the use of AuPt/AC and Bi-AuPt/AC for the oxidation

ACCEPTED MANUSCRIPT of glycerol under continuous flow [50]. Interestingly, AuPt/AC furnished glyceric acid (selectivity of 68% and conversion of 29%) via oxidation of the primary hydroxyl group under the following conditions: aqueous glycerol 5 wt%, O2 flow 15 mL·mn-1, 60 °C, LHSV 0.046 h-1 (Figure 3) whereas Bi-AuPt/AC produced dihydroxyacetone (selectivity of 48% and

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glycerol 5wt%, O2 flow 5 mL mn-1, 60 °C, LHSV 0.183 h-1.

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conversion of 24%) by oxidation of the secondary hydroxyl group under similar conditions:

Figure 3. Continuous flow device for the oxidation of glycerol to glyceric acid.

Selective oxidation of glycerol has been reported to furnish glyceric acid with a selectivity of 80% for a glycerol conversion of 10%; the use of continuous flow electrocatalytic reactor secured higher selectivity to glyceric acid than that in batch reactor [51].

ACCEPTED MANUSCRIPT Continuous hydrogenolysis Hou and co-workers described the catalytic production of propan-1-ol starting from glycerol by combining zirconium phosphate and Ru/SiO2 successively [45]. Zirconium phosphate permitted to produce acrolein from glycerol as mentioned above and Ru (2%)/SiO2 then

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furnished propan-1-ol from acrolein. This process highlights full glycerol conversion and propa-1-ol with a selectivity of 77% at 315 °C with a flow rate of aqueous glycerol solution

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(10 wt%) of 0.04 mL·mn-1 and a flow rate of hydrogen of 30 mL·mn-1 at 20 bar (Figure 4).

Figure 4. Continuous flow device for the oxidation of glycerol to propan-1-ol using

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sequential packing of ZrP and Ru/SiO2.

Friedrich and co-workers developed the hydrogenolysis of glycerol over supported Mo and W [52] wherein an aqueous solution of glycerol (60 wt%) was passed through a cartridge containing Mo/Al2O3 at 325 °C under hydrogen pressure (60 bar); propan-1,2-diol (selectivity of 70%) was the major product obtained whereas in presence of Mo/SiO2 propan-1-ol (selectivity 90%) was obtained. The authors mentioned that the activity of the catalyst was related to their Bronsted acidity.

ACCEPTED MANUSCRIPT In 2017, continuous conversion of glycerol to propan-1,3-diol and derivatives was developed using biocatalysts [53] possessing varying microbial structures [54] as new alternatives for the glycerol valorization.

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Continuous carbonatation

Starting from glycerol, trans-carbonatation furnished glycerol carbonate as value-added chemical under environmentally acceptable conditions. The main hurdle for the

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implementation of continuous flow reactions is the absence of mutual miscibility between glycerol and dimethylcarbonate. In this context, the use of solvent such as methanol or tert-

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butanol is required. Recent work has documented the use of lipase B from Candida antartica immobilized on Accurel MP1000 (CalBAcc) as biocatalyst for the production of glycerol carbonate [55]. Glycerol (5 mmol) and dimethylcarbonate (3 eq) in the presence of Brij 76 (15 wt%) as surfactant in tert-butanol at 60 °C was passed over CalBAcc (1 g) to produce

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glycerol carbonate. Varying the flow rate showed that good selectivity was possible (57%) with a short residence time (3.4 min) since there is not enough time to produce side products. The same group published a cascade process starting from vegetable oils [55]; work entailed

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triacylglycerol hydrolysis, biodiesel synthesis, esterification of the remaining glycerol with dimethylcarbonate via trans-carbonatation. The ideal result was obtained starting from a

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mixture of Soybean oil-dimethylcarbonate (1:10, v/v) in tert-butylmethyl ether with trace of water in the presence of CalBAcc (20 wt%) with a residence time of 176 minutes (flow rate of 0.05 mL·mn-1); minimal amount of tert-butylmethyl ether was used until the formation of a homogeneous system which deliverednearly quantitative conversion and selectivity. In this work, both strategies emanating from either glycerol or from vegetable oils were efficient but continuous flow approach was more appealing when vegetable oils was used as starting material in term of sustainability.

ACCEPTED MANUSCRIPT Interestingly, complete conversion of glycerol was described by Selva and co-workers under continuous flow in the absence of any catalyst [56] wherein a mixture of glyceroldimethylcarbonate-methanol (1:6:10, v/v/v) and flow rate at 0.1 mL·mn-1 was used. At 230 °C, the conversion and selectivity were 78% and 92% whereas at 250 °C, the conversion and

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selectivity were switched 94% and 83%. The process could operate indefinitely due to the absence of any catalyst and the use of subcritical methanol (230 °C, 50 bar or 250 °C, 50 bar)

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provides specific properties of methanol.

Continuous ketalization

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Selective ketalization of glycerol into solketal and derivatives have been developed (i) to obtain protected glycerol in a strategy of protection/deprotection steps and (ii) to produce oxygenated fuel additives. The sustainable production of solketal in batch and in continuous flow reactors has been recently reviewed by Xu and co-workers [57]. Lately, the main

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heterogeneous catalysts used for the ketalization reaction were organic resins such as Amberlyst and Purolite [58-60]; a simple and efficient continuous production of solketal deployed heterogeneous organic resin Purolite® PD206 [60]. A mixture of glycerol and

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acetone (1:5, v/v) was introduced to a cartridge filled with the acid catalyst (0.77 g) at 0.1 mL·min-1 at 20 °C and 120 bar to produce solketal in 95% yield with a selectivity of 100%;

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enhancement could be made possible by increasing the pressure and amount of acetone used.

Continuous esterification

Esterification of glycerol under acidic conditions is a challenging proposition since triacetin is a good bio-additive as anti-knocking agent. Mainly, heterogeneous acid catalysts such as Amberlyst and Purolite [61-64] have been used reminiscent of the solketal production. Recently, Ghaziaskar and co-workers published the selective continuous synthesis of

ACCEPTED MANUSCRIPT monoacetin in 62% yield without any catalyst [65]. In the context of green chemistry, this process is more efficient since the system could operate almost indefinitely. Starting from a mixture of glycerol-acetic acid (1:3.7, v/v) at 79 °C and 1 bar with a flow rate of 0.9 mL·min, only low amount of diacetin was obtained (8%) while the conversion and selectivity of

monoacetin were 70% and 89%, respectively.

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Trans-esterification is another strategy for the formation of glycerol esters. Selva and coworkers have developed the selective continuous synthesis of triacetin using isopropenyl

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acetate without any catalyst [66]; a mixture of glycerol, isopropenyl acetate, and diglyme as a cosolvent (1:20:33 molar ratio) was allowed to flow at 300 °C and 50 bar with a residence

Continuous polymerization

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time of 5 hours which furnish selectively 100% of triacetin under the optimized conditions.

Glycerol oligomers have been widely used in the industry for different applications including

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food and cosmetics [67]. Their direct synthesis from glycerol has been accomplished with various catalysts in a batch reactor, but only one report by the Len group described the oligomerization of glycerol under continuous flow mode [68]. In order to optimize the

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conversion of glycerol, the oligomerization was carried out in a cycle by continuously reinjecting the outlet into the reactor. A mixture of glycerol (2.61 mol) and K2CO3 (72.4

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mmol) was magnetically stirred and heated at 250 °C in a 250 mL glass beaker with a flow of 1 mL·min-1 over 300 minutes (Figure 5). It should be noted that a solvent-free process has been realized and under these conditions, the crude mixture contained initial glycerol (39.1 wt%), cyclic dimer (3.0 wt%), dimer (17.7 wt%), cyclic trimer (2.0 wt%), trimer (10.6 wt%), tetramer (7.8 wt%), pentamer (5.1 wt%), and others. After a short path evaporation, two fractions were obtained: distillate and residue, the distillate being almost all the glycerol and cyclic dimer and a few dimers. On the other hand, the residue contained glycerol (0.2 wt%),

ACCEPTED MANUSCRIPT cyclic dimer (0.4 wt%), dimer (20.2 wt%), cyclic trimer (2.3 wt%), trimer (20.8 wt%), tetramer (15.8 wt%), pentamer (10.6 wt%), and oligomers with higher molecular weight. After a complex analysis, the linear/branched dimer regioselectivities after 300 minutes afforded α,α-diglycerol (72%), α,β-diglycerol (26%), and β,β-diglycerol (2%). The

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β,α-diglycerol (45%), and less than 5 wt% for the other trimers.

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regioselectivities for the trimers were : α,α-α,α-diglycerol (46%); α,α-α,β-diglycerol and α,α-

cyclic oligomers.

Conclusions

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Figure 5. Continuous flow device for oligomerization of glycerol into various linear and

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The glycerol transformation in continuous process in academic domain as well as in industry is flourishing. To date, research around glycerol is no longer conducted according to the dogmas of the last century chemistry, namely, protection, functionalization and deprotection. The use of glycerol or triglycerides is favored in order to attain sustainable processes. In this context, the production of various value-added chemicals, such as acrolein, lactic acid, glyceric acid, propanol, propanediols, glycerol carbonate, solketal, acetin, and oligomers, have been reported using homogeneous, heterogeneous, and enzymatic catalysis. As is illustrated here, many examples still present low selectivity and conversion and there is added

ACCEPTED MANUSCRIPT need for co-solvent to counter the viscosity of glycerol and its lack of miscibility with other reagents. For these reasons, it is obvious that the use of hybrid catalyst and multistep reactions using successive continuous flow reactor should be explored on a laboratory scale that can be

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readily adapted by the industry.

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56. Guidi S, Calmanti R, Noe M, Perosa A, Selva M: Thermal (catalyst-free) transesterification of diols and glycerol with dimethylcarbonate: a flexible reaction

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for batch and continuous flow applictions. ACS Sustainable Chem Eng 2016, 4:61446151. •• In this paper an innovative thermal transesterification process (catalyst-free) for the production of carbonate was investigated under continuous-flow conditions. The selectivity of glycerol carbonate was higher than 90%. 57. Nanda MR, Zhang Y, Yuan Z, Qin W, Ghaziaskar HS, Xu C: Catalytic conversion of glycerol for sustainable production of solketal as a fuel additive: a review. Renew Sustain Energy rev 2016, 56:1022-1031.

ACCEPTED MANUSCRIPT 58. Nanda MR, Zhang Y, Yuan Z, Qin W, Ghaziaskar HS, Poirier MA, Xu C: A new continuous-flow process for catalytic conversion of glycerol to oxygenated fuel additive: catalyst screening. Appl Energy 2014, 123:75-81. 59. Oliveira PA, Souza ROMA, Mota CJA: Atmospheric pressure continuous production

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of solketal from the acid-catalyzed reaction of glycerol with acetone. J Braz Chem Soc 2016, 27:1832-1837.

60. Shirani M, Ghaziaskar HS, Xu C: Optimization of glycerol ketalization to produce

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solketal as biodiesel additive in a continuous reactor with subcritical acetone using purolite ® PD206 as catalyst. Fuel Process Technol 2014, 124:206-211.

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61. Rastegari H, Ghaziaskar HS, Yalpani M, Shafiei A: Development of a continuous system based on azeotropic reactive distillation to enhance triacetin selectivity in glycerol esterification with acetic acid. Energy Fuels 2017, 31:8256-8262. 62. Aghbashlo M, Tabatabaei M, Jazini H, Ghaziaskar HS: Exergoeconomic and

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exergoenvironmental co-optimization of continuous fuel additives (acetins) synthesis from glycerol esterification with acetic acid using Amberlyst 36 catalyst. Energy Conv Manag 2018, 165:183-194.

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63. Aghbashlo M, Tabatabaei M, Rastegari H, Ghaziaskar HS: Exergy-based sustainability analysis of acetins synthesis through continuous esterification of glycerol in acetic

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acid using Amberlyst 36 as catalyst. J Clean Prod 2018, 183:1265-1275. 64. Aghbashlo M, Tabatabaei M, Rastegari H, Ghaziaskar HS, Shojaei TR: On the exergetic optimization of solketalacetin synthesis as a green fuel additive through ketalization of glycerol-derived monoacetin with aceton. Renew Energy 2018, 126:242-253. 65. Gorji YM, Ghaziaskar HS: Continuous synthesis of a green fuel additive mixture with highest quantities of solketalacetin and solketal and lowest amount of diacetin from biodiesel-derived glycerol. J Braz Chem Soc 2018, 29:1218-1224.

ACCEPTED MANUSCRIPT 66. Calmanti R, Galvan M, Amadio E, Perosa A, Selva M: High-temperature batch and continuous-flow transesterification of alkyl and enol esters with glycerol and its acetal derivatives. ACS Sustain Chem Eng 2018, 6:3964-3973. • In this paper a new procedure for catalyst-free transesterification was explored in both batch and continuous-

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flow modes using different sources of acetate.

67. Martin A, Richter M: Oligomerization of glycerol – a critical review. Eur J Lipid Sci Technol 2011, 113:100-117.

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68. Galy N, Nguyen R, Blach P, Sambou S, Luart D, Len C: Glycerol oligomerization in continuous flow reactor. J Ind Eng Chem 2017, 51:312-318. •• In this paper direct

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oligomerization of glycerol in presence of K2CO3 in continuous flow process via thermic

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activation is described on a hundred-gram scale of glycerol.

ACCEPTED MANUSCRIPT Highlights •

Glycerol has received ever-incresing attention due to its abundant nature as a byproduct from the biodiesel industry;

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Glycerol can be valorized under various conditions to produce valuable chemical

carbonate, solketal, acetin, and related oligomers;

An overview of glycerol valorization to useful entities in liquid phase continuous flow

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systems are summarized.

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•

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entities such as acrolein, lactic acid, glyceric acid, propanol, propanediols, glycerol