Topics in Catalysis Vol. 27, Nos. 1–4, February 2004 (# 2004)

137

Selective oligomerization of glycerol over mesoporous catalysts J. Barraulta,
, J.-M. Clacensb, and Y. Pouillouxa a

LACCO, UMR CNRS 6503; 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex France b IRC, UPR CNRS 5401; 2, Avenue Albert Einstein, 69626 Villeurbanne Cedex France

In the general context of the development of the use of agricultural products for nonfood applications, particularly in the field of glycerol valorization (coproduct of triglyceride hydrolysis and methanolysis processes), the selective etherification of glycerol to di- and triglycerol was studied. Part of this study consisted in the synthesis and the modification, by different techniques, of cesium containing mesoporous solids of the MCM-41 type in order to make them active, selective and stable catalysts for the target reaction. The catalytic results obtained show that the impregnation method gives the highest activity. Concerning the selectivity of the modified mesoporous catalysts, the best values to ðdi- þ tri-Þ glycerol (>90%) are obtained over solids prepared by the impregnating or the grafting methods. The cesium-impregnated catalysts can be reused without loss of selectivity. In the presence of lanthanum- or magnesium-containing catalysts, the glycerol dehydration to acrolein is important, whereas this unwanted product is not formed when cesium is used as promoter. Moreover, when compared to homogeneous catalysts, the mesoporous solids induce a different regioselectivity. Finally, as far as the catalyst leaching and stability is concerned, the best results are obtained with the grafted solids, which retain their structure and their specific area after the promoter addition or incorporation. Such property is not observed over impregnated catalysts. KEY WORDS: glycerol etherification; mesoporous catalysts; base catalysis; polyglycerols.

1. Introduction Glycerol is now mainly obtained from the hydrolysis and methanolysis of vegetable oils and is used in the preparation of various products in food and nonfood industries. For example, polyglycerols and polyglycerol esters are obtained from the oligomerization of glycerol and the esterification or transesterification of the oligomers with fatty acids or methyl esters. Generally, the reactions are performed in the presence of homogeneous catalysts so that a mixture of polyglycerols (scheme 1) as well as a mixture of esters is obtained. In fact, besides linear polyglycerols, branched polyols as well as oxygenated heterocyclic compounds can be obtained from cyclization reactions of glycerol and acrolein (scheme 1(d)) from glycerol dehydration. It is rather difficult to get selectively one type of polyglycerol or to control the mixture and the quality of the product, and, for example, the HLB (hydrophilic lipophilic balance) properties after esterification. If one wants one polyglycerol (ester), new catalytical ways have to be found [1, 2]. We have discovered that wellchosen basic and mesoporous catalysts were quite selective catalysts for the direct synthesis of di- and triglycerol from glycerol without the use of any solvent [3]. In comparison with previous results [4, 5] obtained
To whom correspondence should be addressed. E-mail: [email protected]

with homogeneous systems, resins or zeolites, there is a significant increase in the selectivity and yield without formation of cyclic compounds or acrolein. In this paper, some of the most significant results as well as the catalysts prepared for that purpose are presented.

2. Literature Different industrial processes of preparation of polyglycerols were reported in the literature. As these reactions are not selective, different filtration steps as well as neutralization and purification so as to eliminate solvents and homogeneous catalysts are necessary [6]. Over sodium hydroxide at a temperature of 503 K, only a 12.5% weight fraction of diglycerol is obtained [7]. Two distillation steps (under vacuum) are necessary to get 98% purity of diglycerol. The mixtures obtained over solid catalysts such as zeolites [8,9] and sodium carbonate [10] are presented in tables 1 and 2. In table 1, the [di- þ triglycerol] fraction is below or equal to 65% over sodium zeolites and sodium silicate, showing that there is no shape selectivity. Presumably, the outer surface of the catalyst plays an important role in the case of NaA sample. Some patents claim the use of zeolite or homogeneous catalysts; the (di- þ triglycerol) yields are quite similar while higher selectivities were reported (table 2), which confirm the results obtained at the laboratory scale. Though the precise analysis of the products was 1022-5528/04/0200–0137/0 # 2004 Plenum Publishing Corporation

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J. Barrault et al./Oligomerization of glycerol OH

-H2O

OH

2 HO

OH

OH

OH

O

HO

glycerol

+ x glycerol

polyglycérols

A

- x H2O

diglycerol O

polyglycerols

+

R OH (Me)

OH

B

O

C

OH

OH

O

polyglycerol esters + H2O (MeOH)

OH

O

O

HO

O

OH

O OH

HO

OH

HO

OH

HO

O HO OH HO HO

O 3

OH

OH OH

HO

2

O

OH

O

OH

O

1

OH

OH

OH

O

O OH

-H2O

OH

OHC

HO

-H2O OH

glycerol

CHO

D

acrolein

Scheme 1. (a) Glycerol oligomerization, (b) polyglycerol esterification, (c) some examples of di- and triglycerol isomers (1, 2, and 3 being the linear diglycerol isomers), (d) acrolein formation from glycerol.

not reported in these patents, they contained all the isomers and by-products presented above as confirmed by experiments and analyses performed in our group. The identification of most of the main isomers or groups of isomers was possible owing to samples prepared by Cassel et al. [1,11] and Debaig et al. [2,12] via new unambiguous chemical routes.

3. Experimental 3.1. Preparation and characterization of the catalysts Impregnated Mesoporous Materials are prepared as follows: a certain amount (see notation) of alkali, alkaline earth or/and rare earth salt is added to 5 g of pure silica or aluminosilica MCM-41 material, prepared (with cetyltrimethylammonium bromide) as previously described [6] and 50 g of methanol. The mixture is stirred at ambient temperature for 2 h, the solvent is then rapidly evaporated under vacuum and the solid is calcined under air at 723 K overnight at a heating rate of 1 K min1 . Notation: Elementðamount of element impregnated;104 mol g1 Þ AlðSi=Al ratioÞ ,

example: Cs25 Al20 or Cs25 if the mesoporous support does not contain aluminum. Exchanged Mesoporous Materials are prepared as follows: to a solution containing 0.32 g of cesium nitrate in 150 mL of ethanol, a suspension containing 2 g of pure silica MCM-41 (Naþ ) in 50 mL of ethanol is added. After 10 min of stirring under inert atmosphere (Ar) at ambient temperature, the mixture is kept for 24 h without stirring under argon. The mixture is then filtered and the resulting solid is washed with ethanol and calcined under air at 823 K (heating rate: 5 K min1 ); another preparation has been done by replacing ethanol with hexane; these two methods are respectively called Type I and Type II. Notation: Cse Si-MCM-41ðIÞ (see cesium content in table 4). Aluminum Grafted Mesoporous Materials are prepared following a method inspired by Mokaya et al. [13]: to a 150-mL hexane solution containing 0.34 g of aluminum isopropoxide a suspension of 2 g of pure silica MCM-41 in 50 mL of hexane is added. After 10 min of stirring under inert atmosphere (Ar) at ambient temperature, the mixture is allowed to stand for 24 h under argon. The mixture is then filtered and the resulting solid is washed with hexane and calcined under air at 823 K (heating rate: 5 K min1 ). Notation: Alg Si-MCM-41. The other catalysts used have been described previously [6]. Some of the solids have been recovered after reaction by filtration using hot water and/or ethanol to dissolve the polyglycerols formed, and reused without reactivation (Notation: R after the catalyst code; example: Cs25 R). Characterizations of the solids were done using X-ray diffraction, N2 adsorption (BET method), transmission electronic microscopy and chemical analysis.

3.2. Reaction Glycerol etherification is carried out at 533 K in a batch reactor at atmospheric pressure under N2 in the presence of 2 wt% of catalyst, water being eliminated and collected using a Dean–Stark system. Reagents and products are analyzed by GPC after silylation [14]. Analysis conditions: GPC equipped with an on-column injector, an FID, and a polar column (HT5) supplied by

Table 1 Weight composition (%) of the mixtures obtained by reacting glycerol over zeolites [8] Catalysts Na Na Na Na

A Z X silicate

T ¼ 513 K.

Glycerol

Diglycerol

Triglycerol

Other polyglycerols

15.4 9.5 9.6 8.5

32.3 27.6 30.9 29.7

20.5 20.0 22.0 23.0

31.8 42.9 37.5 38.8

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Table 2 Comparison of the conversions and selectivities (and yields) of the different processes (weight %) leading specifically to di- and triglycerol Patents assigned to

Unichema [9] Henkel [8] Homogeneous a

Catalyst

Zeolite  (20 wt%) Zeolite A (2.4 wt%) Na2CO3 (2 wt%)

T (8K)

Glycerol conversion

Diglycerol selectivity (yield)

Triglycerol selectivity (yield)

Di- þ triglycerol selectivity (yield)

473 513 533

100a 84.6 87.5

60 (36)a 38.2 (32.3) 50.4 (44.1)

30 (18)a 24.2 (20.5) 26.0 (22.8)

90 (54)a 62.4 (52.8) 76.4 (66.9)

Balance calculated over 60 wt% of the recovered products, so that there is still 40 wt% of the other products that are not polyglycerols.

SGE (L ¼ 25 m, ID ¼ 0:22 mm, thickness of the film ¼ 0:10
m). Some of the numerous isomers of diglycerol and triglycerol (scheme 1C and figure 2) are separated (cyclic and linear); the three linear isomers can be distinguished (figure 2). Batch processes are generally used in lipochemistry, especially for the esterification and transesterification reactions (except for the preparation of methyl esters). Acrolein formation was evaluated from the excess of water produced during the reaction.

4. Results and discussion 4.1. Mg- or Cs-impregnated mesoporous materials In a previous work [6], it was shown that from the impregnation of magnesium or cesium precursors over MCM-41 type materials, an active and selective solid was obtained for the transformation of glycerol to a mixture of di- and triglycerols. These results, presented in table 3, show that the impregnation of magnesium or cesium on MCM-41 solids increases the selectivity of diglycerol and triglycerol compared to the results obtained with homogeneous catalysts. For example, with sodium carbonate used in similar experimental conditions [6], the selectivities to di- or triglycerol are respectively of 50 and 26% for a glycerol conversion of 87% (table 2), which is rather lower than the results presented in table 3, especially for cesium catalysts. Such results indicate that mesoporous solids could act as shape-selective materials for the selective conversion of glycerol. However, the comparison of the pore size of the catalysts with the dimensions of short oligomers of glycerol (di- or

triglycerol) cannot justify a direct shape selectivity; i.e., a selectivity resulting only of steric hindrance inside the pores of the catalyst. In that particular case, it seems that, due to a change of the hydrophilic lipophilic character of the MCM-41, the coverage and the adsorption of glycerol are strongly modified (see the isomers distribution below) so that the chain-growth probability (polymerization rate in a Schulz Flory process [15]) is decreased. It could also be that the location of active sites inside the pores or/and especially at the pores aperture favored such a reaction. Presently we are not able to discriminate between all those hypotheses. Another very important point concerns the selectivity of the catalyst during the reaction. Polar compounds, particularly polyols are well known to form alkaline (earth) and soluble alcoholates so that one can expect some decrease of the content of the basic element of the catalyst in the presence of glycerol and its oligomers, a decrease of activity and a rapid change of the selectivity upon reuse. This is indeed observed with magnesium catalysts (see figure 1) and with cesium catalysts after recycling (figure 1) even though the di þ triglycerol fractions are quite similar with the second catalyst at a conversion of 80%. Analysis of the solids after the reaction shows that the magnesium or cesium content decreases significantly and so part of the reaction is catalyzed by the soluble base.

4.2. Addition of cesium to Si- or Alg Si-MCM-41 supports As reported in the experimental part and in table 4, two types of supports were used; a silica mesoporous

Table 3 Etherification of glycerol over mesoporous materials impregnated with magnesium or cesium Element impregnated

Mg6 Mg25 Cs6 Cs25 Note:  ¼ trace.

Conversion (%) 8 h (24 h)

10 80 15 25

(25) (–) (40) (80)

Selectivity (%) 8 h (24 h) Diglycerol

Triglycerol

Tetraglycerol

Others

100 (95) 65 95 (90) 100 (75)

– (5) 20 5 (10) 0 (22)

– 15 – 

–  – –

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J. Barrault et al./Oligomerization of glycerol

Figure 1. Selectivity of di- and triglycerol depending on the conversion for Mg- and Cs-impregnated mesoporous (M) solids; comparison with recycled (R) catalysts.

support and the same support modified by grafting of an aluminum precursor. Over the first one, a cesium precursor was added according to two different processes (I) and (II) while only one method was used for the addition of cesium to the Al-modified mesoporous support. The results of table 4 show that the specific area (600– 800 m2 g1 ) and the cesium content ð7–10  104 mol g1 Þ of all the samples are relatively similar so that it will be easy to directly compare their catalytic properties. Generally, the introduction of cesium, whatever the process used, induces a decrease both in the surface area and the porous diameter (Cs6 Al20 ), but it clearly appears that the surface-area changes due to the introduction of

cesium are less important with these new materials (compared to the first series). It means that the ordered structure (XRD analysis) of silica- or AlSi mesoporous solids still exists after addition of cesium. The catalytic properties reported in table 5 show that the glycerol conversion values are rather similar for all the catalysts as well as the selectivity to di- and triglycerol. It could be that the second generation of solids is less active and more selective but the differences are not very significant. An important factor observed with these catalysts and presented in table 4 is that there is a low or no cesium leaching (respectively entries 7 and 5 of table 4) during the glycerol transformation. The result is quite

Table 4 Stability: some characteristics of modified mesoporous compounds Entry

Catalyst

Specific area (m2 g1)

Cs analysis (104 mol g1)

XRD d (nm)

Before After Catalytic test 1 2 3 4 5 6 7

Al20 (MCM-41) Cs6Al20 (MCM-41) Si-MCM-41 CseSi-MCM-41(I) CseSi-MCM-41(II) AlgSi-MCM-41 CseAlgSi-MCM-41

838 620 – 719 744 1438 787

– 6.4 – 8.4 7.6 – 10.0

– 0.1 – – 7.6 – 8.9

3.57 3.38 3.92 3.32 3.29 3.44 3.43

Table 5 Etherification catalytic properties of modified mesoporous compounds Catalyst

Glycerol conversion (%) After 10 h

Cs6Al20 CseSi-MCM-41(I) CseSi-MCM-41(II) CseAlgSi-MCM-41

20 15 15 15

Selectivity (%) at 15% or (50%) Diglycerol 93 100 100 100

(87) (94) (89) (86)

Triglycerol 7 – – –

(13) (6) (11) (14)

Others – – – –

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different from that obtained with Cs6 Al20 (entry 2, table 4) for which the cesium is completely extracted from the solid. An increase in the stability of Cs-MCM-41 catalysts was also mentioned by Kloetstra et al. when cesium was associated with lanthanum in CsLaOx [16,17]. In our case, a similar effect could be obtained with aluminum grafted over the mesoporous supports and the characterizations now performed should bring more information on that particular aspect. One may conclude that the second method for cesium introduction in the mesoporous material is quite convenient for the selective transformation of glycerol to di- and triglycerol.

4.3. Effect of the nature of the catalyst over the diglycerol isomers distribution A careful analysis of the diglycerol fraction (see chromatogram in figure 2) shows the presence of at least three different peaks, which are identified and attributed to the three linear and branched isomers 1, 2, 3 (scheme 1(c)) (without formation of cyclic compounds with mesoporous catalysts). When the isomers distributions obtained with the different catalysts are compared, significant differences are observed (table 6). Surprisingly, the reactivity of the secondary hydroxyl group (position 2) of glycerol is much more important in the presence of mesoporous catalysts (isomers ð1 þ 2Þ  70%Þ than over homogeneous catalysts (isomers ð1 þ 2Þ  35%Þ. This result, not reported in the literature, is a specific property of mesoporous catalysts and is also a strong indication that the reaction takes place inside the pores of the solids. Kloetstra et al. also observed some selectivity changes when Michael reactions were done over Cs(La)-MCM-41 catalysts [16,17]. Nevertheless, it was difficult to discriminate between

Table 6 Effect of the catalyst over the distribution of diglycerol isomers Catalyst

Na2CO3 CsOH Mg25Al20 Cs25Al20 Cs25

Diglycerol isomers (%) 1

2

3

3.6 5.1 17.8 20.9 20.4

30.6 31.1 46.1 49.5 50.4

65.8 63.8 36.1 29.6 29.2

pore size effects and specific activation, especially with lanthanum species well known to coordinate with oxygen esters.

5. Conclusion From these new preparations, stable active and basic mesoporous catalysts were obtained for the chemo- and region-selective conversion of glycerol to linear di- and triglycerols. Such result was not previously reported in the literature and the isomers distribution seems to show that the reaction really takes place over the basic sites dispersed at the surface (inside or/and at the pore mouth) of the solid mesoporous catalyst. The catalysts prepared by the impregnation method give the highest activity, but are subject to metal leaching. Mesoporous solids modified by cesium impregnation or exchange lead to the best selectivity and yield to (di- þ tri-) glycerol. The most stable catalysts are the exchanged ones but though they are less stable, the impregnated catalysts can be reused without major modification of their selectivity to the (di- þ tri-) glycerol fraction.

Acknowledgments The authors gratefully acknowledge support from the FAIR Program no. CT965045 of the European Community; Dr. M. Sto¨cker and Dr. A. Karlson from SINTEF—Oslo (Norway); and Prof. P. Rollin, Lafosse and Dr. S. Cassel from ICOA—Orleans (France).

References

Figure 2. Chromatogram showing the three different peaks of diglycerol.

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