Nanoporous aluminosilicate mediated transacetalization reactions: application in glycerol valorization

Abstract

Nanoporous silicate materials, produced using an evaporation-induced self-assembly (EISA) approach, are highly effective catalysts for transacetalization reactions. Both acyclic acetals and ketals undergo rapid exchange in the presence of low loadings of nanoporous aluminosilicate materials and diols and triols to generate the corresponding cyclic acetals and ketals in excellent yield. The protocol is rapid, employs mild conditions, and the catalyst can be recycled a number of times without loss of activity.

---

1. Introduction

Since the initial disclosure of synthetic routes to ordered mesostructured silicate materials, surfactant directed self-assembly has developed into one of the most versatile and widely applied approaches for the synthesis of inorganic materials with controlled composition, structure and function.¹ The materials produced from this approach have found wide application in a range of areas, including adsorption technologies, separation science and molecular encapsulation.² The application of these silicate materials as heterogeneous catalysts in synthetic chemistry in particular has attracted considerable interest, as they are ideally suited to this task due to their high surface areas and their chemical stability, which allows for recycling and reuse.³ The recent advent of increased environmental and economic awareness has impacted strongly on the use of single-use, air and moisture-sensitive metal catalysts commonly employed in industrial processes. It is therefore not surprising that there has been significant interest in the development of reusable, hydrolytically stable catalytic materials that generate minimal chemical waste by-products.⁴

We recently reported a simple and flexible route for the synthesis of nanoporous silicate and aluminosilicate materials using an evaporation-induced self-assembly (EISA) approach, and the use of these silicate materials as catalysts in a range of synthetic transformations.⁵ The EISA process is a highly flexible, inexpensive, operationally simple and extremely versatile route to structured silicate materials. Furthermore, it is highly reproducible and does not require specialist equipment or extended reaction times. As part of an ongoing programme to develop highly efficient and environmentally benign reaction processes,⁶ and as an extension of our studies on the use of silicate materials in synthesis, we disclose that silicate materials produced using our EISA approach function as highly efficient catalysts for transacetalization reactions.

2. Experimental

2.1 Materials

All materials employed were purchased from commercial sources and were used as received, or were synthesised using literature procedures.⁷

2.2 Catalyst preparation

The aluminosilicate materials AS-(56) and AS-(14), in addition to the unmodified silica material (S-1), were synthesized and characterized using a range of standard techniques as described previously.^5a–c A typical preparation for the synthesis of the aluminosilicate AS-(14) catalyst is as follows: cetyltrimethylammonium bromide (4.0 g, 11 mmol) was dissolved in a solution of hydrochloric acid (2.5 ml, 0.1 M) and ethanol (17.5 ml). TEOS (25 ml, 112 mmol) was then added and the mixture stirred for 10 minutes at 40 °C. The solution was cooled to room temperature and aluminium nitrate nonahydrate (3.35 g, 8.95 mmol) was added in one portion. The mixture was stirred for 20 minutes and then left to age at room temperature for 1 week. The resultant orange solid was crushed into a fine powder and calcined in air at 550 °C for 12 hours to remove the organic template to give a fine white powder. All materials were stored at 120 °C for 12 hours prior to use.

2.3 Catalyst characterisation

Specific surface areas were obtained by the BET method at liquid nitrogen temperatures using a Micromeritics Gemini or a Quantachrome Autosorb-1 automated gas sorption instrument. Samples were degassed at 120 °C under a flow of helium for 2 hours prior to analysis. Pore-sizes were obtained using a Quantachrome Autosorb-1 automated gas sorption instrument. Samples were degassed at room temperature under a stream of helium for 3 hours prior to analysis. Pore-sizes were calculated by applying the non-local density functional theory (NLDFT) method to the N₂ sorption at 77 K employing Quantachrome AS-1 software data reduction parameters. Elemental compositions were obtained with a JOEL scanning electron microscope fitted with an EDX detector using a 20 keV accelerating voltage. MAS-NMR spectra were obtained courtesy of the EPSRC National Solid State NMR Service, Durham University. Aluminium spectra were obtained using a Varian VNMRS system, with a DP pulse sequence and results are reported in ppm using a 1 M aqueous AlCl₃ solution as internal reference. Silicon spectra were obtained using a Varian Unity Inova spectrometer with a DP or CP pulse sequence and results are reported in ppm with respect to tetramethylsilane.

2.4 Catalyst testing and product analysis

All reactions were carried out in a stirred batch reactor. The catalyst was removed from the sample by filtration through a plug of basic aluminium oxide, which was washed with dichloromethane (2 × 2 ml) and the combined solvents were removed under reduced pressure. Product mixtures were analysed using ¹H NMR or GC-MS techniques and percentage conversions of reactions were determined by integration of the relevant signals from the crude ¹H NMR spectra. GC-MS analysis was performed using a Varian 450GC and Varian 300MS employing a VF-5ms capillary column (30 m, 0.25 mm i.d. and 0.25 μm) and a gradient temperature profile of 50 °C for 3 minutes rising to 280 °C at 20 °C min⁻¹. A typical experimental procedure is as follows.

The aluminosilicate catalyst AS-(14) (50 mg) was added to a stirred mixture of benzaldehyde dimethylacetal (152 mg, 1 mmol) and ethane-1,2-diol (84 mg, 1.10 mmol) in chloroform (2 ml) which was heated to reflux. After 30 minutes, the reaction mixture was cooled to room temperature and the catalyst and unreacted diol were removed by filtration through a plug of basic aluminium oxide which was washed with dichloromethane (2 × 2 ml). The combined solvents were removed under reduced pressure to give the product 2-phenyl-1,3-dioxane as a colourless oil; ¹H NMR (CDCl₃; 400 MHz) δ = 3.95–4.05 (2H, m), 4.10–4.15 (2H, m), 5.80 (1H, s), 7.35–7.40 (3H, m), 7.45–7.50 (2H, m), ¹³C NMR (CDCl₃; 100 MHz) δ = 137.8, 129.1, 128.2, 126.3, 103.6, 65.2; ν_max (film)/cm⁻¹ (neat) 2887, 1456, 1392, 1204, 1096, 968 and 699; MS (EI) m/z 150.

3. Results and discussion

3.1 Catalyst preparation and characterisation

The physical properties of the silicate materials used in this study were characterized using a range of standard techniques (Table 1). EDX analysis demonstrated that aluminium was efficiently incorporated into the silicate material structure during the EISA process, an observation that was further confirmed by ²⁷Al and ²⁹Si MAS NMR analysis of the samples. The ²⁷Al MAS-NMR for the calcined aluminosilicate samples shows a broad resonance at 52 ppm due to tetrahedrally coordinated (framework) aluminium and a broad resonance at approximately −1 ppm due to octahedrally coordinated (non-framework) aluminium in line with previous reports. The ²⁹Si MAS-NMR for the calcined aluminosilicate samples shows a broad signal around 109 ppm accompanying a shoulder at 100 ppm which can be assigned to the Q⁴ and Q³ environments of silicon, respectively. A third weaker shoulder at 92 ppm is present which is associated with the Q² environment.^5c The nitrogen adsorption–desorption isotherms of all the materials appeared to be a transition from type IV for mesoporous materials to type I for microporous materials, exhibiting well-defined capillary condensation at a relative pressure (P/P_o) of 0.01–0.25. Similar phenomena have been observed in the isotherms of other nanoporous materials reported previously.⁸ This was confirmed from the pore-size data, calculated using the NLFDT method, which revealed average pore-sizes ranging between 2.34 and 2.50 nm for these materials (Table 1). The NLDFT method has been shown to be superior to the commonly employed BJH method which has been shown to underestimate the pore-size, giving a more accurate determination of pore-size distribution.⁹ The PXRD data obtained were consistent with our previous observations, displaying a broad diffraction peak at 2θ = 2.57°, suggesting that a long-range ordered nanostructure is present.^5a–c

Table 1 Physical characteristics of the nanoporous catalysts

Catalyst	Si/metal ratio (gel composition)	Si/metal ratio (EDX)^a	BET surface area^b/m^2 g^−1	Pore volume^c/cm^3 g^−1	Average pore width^d/nm
a Determined by EDX analysis. b Surface areas were obtained by the BET method. c Pore volumes determined by the NLDFT method. d Pore width determined by the NLDFT method.
S-1	—	—	704	0.3450	2.34
AS-(56)	54	56	639	0.3632	2.34
AS-(14)	13	14	580	0.2494	2.50

---

3.2 Catalytic activity of nanoporous silicate materials

The development of new protocols for the protection of carbonyl compounds in multistage synthetic sequences continues to attract significant attention, and a diverse array of transformations has been reported.¹⁰ The generation of acyclic and cyclic acetals in particular is one of the most widely used and popular protection strategies, and recent interest in this area has led to the development of more efficient methodologies for both the formation and deprotection of acetals and ketals.^5h Additional interest in this transformation has been generated by recent reports which have highlighted the significant benefits of employing acetals in place of aldehydes and ketones in a diverse range of synthetic transformations.^5a,11 While our original work in this area demonstrated that nanoporous aluminosilicate materials were efficient catalysts for the generation of dimethyl- and diethyl acetals, they were less effective for the formation of cyclic acetals. These transformations generally required extended reaction times and furnished only moderate isolated yields.^5h As part of an ongoing programme to develop highly efficient and environmentally benign reaction processes, we were attracted to the potential of developing an acetal exchange strategy as a facile, flexible and operationally simple route to access cyclic acetals and ketals. Such exchange processes are well established in carbohydrate chemistry,¹² but have attracted significantly less attention in carbonyl protection strategies.¹³

We initially investigated the ability of the nanoporous aluminosilicate materials to catalyze the transacetalization reaction of benzaldehyde dimethylacetal (BDMA) 1 with 2,2-dimethyl-1,3-propanediol (Table 2). Our previous work demonstrated that the incorporation of aluminium into the silica structure results in the production of highly effective Lewis acid catalytic material, and our initial studies employed low loadings of the AS-(14) material (50 mg mmol⁻¹) in dichloromethane at room temperature. These reactions, however, gave only moderate conversions to the cyclic acetal product even after extended reaction times. On changing the solvent to chloroform, and carrying out the reactions at reflux temperatures, however, complete conversion of the starting material to the cyclic product was achieved within 30 minutes in an extremely clean and highly efficient reaction. Interestingly, reactions employing the AS-(56) catalyst, which has a significantly lower aluminium content, gave similar conversions to the AS-(14) catalyst (entry 2). High aluminium content has previously been reported as a requirement for effective catalytic activity in aluminosilicate materials, and is related to high levels of 4-coordinate aluminium incorporated into the silicate framework, which generates both Brønsted and Lewis acid sites in amounts increasing with the degree of aluminium content.¹⁴ A similar observation with regards to high catalytic activity of low aluminium containing silicates has been reported for the zeolite-mediated formation of glycerol formal from formaldehyde and glycerol, where the presence of the increased number of acid sites in a high aluminium content material was off-set by the adsorption of reactants onto these catalytic sites.¹⁵ Expectedly, and in line with our own and previous literature observations, the unmodified silica S-1 demonstrated no catalytic activity (entry 3), as did reactions with no catalyst present, and in these cases the starting materials were recovered unchanged. Given our previous experience with high aluminium containing silicate materials, we employed the AS-(14) catalyst for all subsequent exchange reactions.

Table 2 Comparison of catalyst conversion for the formation of cyclic acetals from 1

Entry	Catalyst	Conversion^a^,^b (%)
a 1 mmol of BDMA and 1.1 mmol of 2,2-dimethyl-1,3-propanediol in 2 ml CHCl3. b Determined by ^1H NMR analysis of the crude reaction mixture.
1	AS-(14)	>95
2	AS-(56)	>95
3	S-1	0

---

We next investigated the reactions of a range of acyclic acetals with propane-1,3-diol and 2,2-dimethyl-1,3-propanediol to assess the generality of the transacetalization protocol, and were gratified to observe that in all cases the cyclic acetal products were produced in excellent isolated yields (Table 3). The reaction proceeds rapidly with dimethyl- or diethyl acetals derived from either aldehydes of ketones (entries 2, 3 and 8, 9), and products required minimal purification, typically involving only removal of the catalyst and unreacted diol.

Table 3 Acetal exchange reactions of acyclic acetals and ketals

Entry	Acetal	Time/h	Product^a	Yield^b (%)
a All compounds gave satisfactory spectroscopic data. b Isolated purified yield.
1		0.5		89
2		0.5		91
3		0.5		87
4		0.5		88
5		0.5		93
6		3		86
7		4		79
8		1		83
9		1		92
10		1		94

---

With our preliminary studies completed, and with a highly efficient acetal exchange protocol established, we turned our attention to the use of glycerol in transacetalization reactions. Glycerol is produced as a by-product in the transesterification of vegetable oils and animal fats in biodiesel production, or by fermentation of biomass,¹⁶ and while a number of established uses for this glycol exist, the large quantities of glycerol produced during biodiesel manufacture in particular cannot be absorbed by these existing markets. Glycerol is an ideal candidate as a renewable and highly flexible chemical feedstock, given its wide availability and biosustainable production, its rich chemical functionality, and its low toxicity and biodegradability. It is therefore unsurprising that processes for the valorization of glycerol have attracted considerable recent interest.¹⁷

The generation of acetals and ketals, derived from glycerol, is an emerging area of importance that has attracted recent attention given the potential of these materials as fuel additives, surfactants and flavouring agents.¹⁸ Our initial studies employing BDMA and the AS-(14) catalyst with glycerol under our established reaction conditions produced disappointing conversions to the cyclic acetal products. This poor reactivity is due to the catalyst coagulating in the presence of glycerol, and is easily overcome by changing to a more polar solvent, such as acetonitrile. The use of polar solvents has the added advantage of aiding product desorption from the active sites and so minimising catalyst deactivation. Under these modified reaction conditions we were gratified to observe a rapid conversion of the acyclic acetal which was complete within one hour (Table 4, entry 1).

Table 4 Glycerol reactivity and selectivity in AS-(14) mediated acetal exchange reactions

Entry	Acetal	Product	Yield^a (%)	Product selectivity^a (trans)-2 : (cis)-2 : (trans)-3 : (cis)-3	Ratio 2 : 3
a All compounds gave satisfactory spectroscopic data.
1		2a and 3a	87	1.2 : 1.5 : 1.3 : 1	55 : 45
		R^1 = Ph R^2 = H
2		2b and 3b	83	1.0 : 1.0 : 1.8 : 3.0	30 : 70
		R^1 = Pent R^2 = H
3		3c	89	—	—
		R^1 = R^2 = CH3
4		3d	86	—	—
		R^1 = R^2 = (CH2)5

---

Analysis of the crude reaction mixture indicated that the reaction proceeded to give a mixture of the dioxane 2 and dioxolane 3, which consisted of approximately equal quantities of the four possible conformational isomers (trans)-2a, (cis)-2a, (trans)-3a and (cis)-3a. These product ratios are similar to those reported previously for zeolite mediated exchange reactions, and reflect the existence of a temperature-dependent acid catalyzed equilibrium.^18e Reactions of hexanal dimethylacetal proceeded to give a product distribution which favours the formation of the dioxolane 3b (entry 2), and reflects the slower isomerization of the dioxolane 2b to the dioxane 3b due to the reduced stability of the intermediate cation formed during the isomerization process. Reactions of dimethyl- or diethyl ketals also proceeded rapidly to give excellent yields of the corresponding dioxolane products 3c and 3d (entries 3 and 4) which were produced as the only isolatable products.^18c,e Finally, to demonstrate the stability and recyclability of these nanoporous aluminosilicate materials, a series of consecutive experiments employing 1 and glycerol were carried out using the AS-(14) catalyst. After each cycle of the reaction, the catalyst was separated, washed with acetonitrile and reused without showing a significant decrease in its ability to catalyze the transacetalization reaction (Fig. 1).

Fig. 1 AS-(14) catalyst recycling studies.

Finally, we considered the possibility of employing glycerol–water mixtures in transacetalization protocols. The use of aqueous solutions of glycerol in synthetic transformations is a highly desirable goal, given the abundance of this cheaper source of glycerol generated during biodiesel production. Few reports, however, have addressed the effect of water on the rate of acetalisation or transacetalization,^15,18c due to its detrimental effect on the catalysts typically employed, and the facile hydrolysis of acetals under aqueous conditions. We reasoned that if the rate of transacetalisation of 1 to 2a and 3a was sufficiently rapid, then it may permit the use of glycerol solutions derived from alternative sources. Our initial studies considered the transacetalization of 1 with either a 10%- or 20%/weight solution of water–glycerol under our standard reaction conditions. We were gratified to observe that excellent conversions to cyclic acetal products in reactions employing the 10% solution were achieved, with little apparent deactivation of the catalyst (Table 5, entry 1). These conversions to acetal products were reduced to approximately 50% in reactions employing the 20% solution, with benzaldehyde, produced by hydrolysis of 1, being the major product. Reactions employing a 50% aqueous solution gave even poor conversions, with benzaldehyde again being the major product. We were aware that cyclic acetal products could be generated by a competing acetalization reaction of benzaldehyde generated by hydrolysis of 1, and with this in mind we next investigated the acetalization reactions of benzaldehyde under our standard conditions. Reactions employing benzaldehyde in place of 1 gave approximately 45% conversions to cyclic acetal products when a 10% glycerol–water mixture was employed, and similar conversions were achieved with the 20% solution. While conversions were significantly reduced in reactions employing the higher water content solutions, these experiments do suggest that the use of glycerol–water mixtures in transacetalization procedures is an achievable goal and our studies in this area are currently underway.

Table 5 Transacetalization reactions of 1 and benzaldehyde with aqueous glycerol solutions

Entry	Substrate	Water–glycerol (wt%)	Conversion^a^,^b (%)
a Determined by ^1H NMR analysis and GC-MS analysis of the crude reaction mixture. b Conversion to dioxane and dioxolane products 2a and 3a.
1	1	10%	>95
2	1	20%	45
3	1	50%	35
4	PhCHO	10%	45
5	PhCHO	20%	40

---

4. Conclusions

Synthetic routes to materials with pore-sizes between the upper limit of zeolites and the lower end of mesoporous materials are of considerable interest and remain a significant challenge. The EISA process we have developed provides access to ordered silicate materials with average pore-sizes in the nanoporous range, and is operationally simple, requires no specialist equipment and provides materials with a narrow pore-size distribution. These materials are well characterized heterogeneous solid acid catalysts, which efficiently catalyze the transacetalization reactions of acyclic acetals with diols and glycerol, giving excellent yields of the cyclic acetal products which require minimal subsequent purification. These reactions employ lower catalyst loadings and proceed faster and give higher conversions of cyclic acetal products than previous reports.^18b–e Furthermore, they are highly stable, crystalline materials which are easy to handle, and the operationally simple synthetic protocol requires no special precautions, such as the exclusion of moisture. These materials are also recyclable and can be re-used without a reduction in their catalytic activity. The facile synthesis of these materials, their ease of handling, their high catalytic activity and the simplified reaction and isolation procedures make them a highly attractive alternative to current synthetic methodologies.

Acknowledgements

The authors thank the Royal Society of Chemistry and the Nuffield Foundation for financial support, and the EPSRC National Mass Spectrometry Service, Swansea University.

Notes and references

1. (a) J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834–10843; (b) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710–712; (c) D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548–552; (d) Y. Wan and D. Zhao, Chem. Rev., 2007, 107, 2821–2860.
2. (a) M. E. Davis, Nature, 2002, 417, 813–821; (b) D. Brühwiler and G. Calzaferri, Microporous Mesoporous Mater., 2004, 72, 1–23; (c) A. Taguchi and F. Schuth, Microporous Mesoporous Mater., 2005, 77, 1–45.
3. (a) D. Trong, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Appl. Catal., A, 2001, 222, 299–357; (b) Q. Yang, J. Liu, L. Zhang and C. Li, J. Mater. Chem., 2009, 19, 1945–1955.
4. (a) A. Corma and H. Garcia, Chem. Rev., 2003, 103, 4307–4365; (b) K. Binnemans, Chem. Rev., 2007, 107, 2592–2614; (c) A. Corma and H. Garcia, Adv. Synth. Catal., 2006, 348, 1391–1412; (d) T. B. Poulson, Chem. Rev., 2008, 108, 2903–2915.
5. (a) T. M. Kubczyk, S. M. Williams, J. R. Kean, T. E. Davies, S. H. Taylor and A. E. Graham, Green Chem., 2011, 13, 2320–2325; (b) M. W. C. Robinson, A. M. Davies, I. Mabbett, T. E. Davies, D. C. Apperley, S. H. Taylor and A. E. Graham, J. Mol. Catal. A: Chem., 2010, 329, 57–63; (c) M. W. C. Robinson, A. M. Davies, I. Mabbett, D. C. Apperley, S. H. Taylor and A. E. Graham, J. Mol. Catal. A: Chem., 2009, 314, 10–14; (d) M. W. C. Robinson, A. M. Davies, I. Mabbett, S. H. Taylor and A. E. Graham, Org. Biomol. Chem., 2009, 7, 2559–2564; (e) M. P. Copley, A. E. Graham, J. D. Holmes, M. A. Morris, R. Seraglia and T. R. Spalding, Appl. Catal., A, 2006, 304, 14–20; (f) M. W. C. Robinson, K. S. Pillinger, I. Mabbett, D. A. Timms and A. E. Graham, Tetrahedron, 2010, 66, 8377–8382; (g) M. W. C. Robinson, R. Buckle, I. Mabbett, G. M. Grant and A. E. Graham, Tetrahedron Lett., 2007, 48, 4723–4725; (h) M. W. C. Robinson and A. E. Graham, Tetrahedron Lett., 2007, 48, 4727–4731; (i) M. W. C. Robinson, D. A. Timms, S. M. Williams and A. E. Graham, Tetrahedron Lett., 2007, 48, 6249–6251.
6. (a) B. M. Smith, E. J. Skellam, S. J. Oxley and A. E. Graham, Org. Biomol. Chem., 2007, 5, 1979–1982; (b) D. Cott, V. Owens, K. J. Zeigler, J. D. Glennon, A. E. Graham and J. D. Holmes, Green Chem., 2005, 7, 105–110; (c) D. J. Phillips and A. E. Graham, Synlett, 2010, 769–773; (d) D. D. Hughes, R. Buckle, M. W. C. Robinson, C. Torborg, M. C. Bagley and A. E. Graham, Synth. Commun., 2008, 38, 205–211; (e) D. J. Phillips, K. S. Pillinger, L. Wei, A. E. Taylor and A. E. Graham, Chem. Commun., 2006, 2280–2282; (f) D. J. Phillips, K. S. Pillinger, L. Wei, A. E. Taylor and A. E. Graham, Tetrahedron, 2007, 63, 10528–10533; (g) B. M. Smith and A. E. Graham, Tetrahedron Lett., 2007, 48, 4891–4894; (h) D. J. Phillips and A. E. Graham, Synlett, 2008, 649–652; (i) M. C. Bagley, V. Lin, D. J. Phillips and A. E. Graham, Tetrahedron Lett., 2009, 50, 6823–6825.
7. (a) B. M. Smith and A. E. Graham, Tetrahedron Lett., 2006, 47, 9317–9319; (b) B. M. Smith and A. E. Graham, Tetrahedron Lett., 2011, 52, 6281–6283.
8. (a) L. Zhang, Q. Yang, H. Yang, J. Liu, H. Xin, B. Mezari, P. C. M. M. Magusin, H. C. L. Abbenhuis, R. A. van Santen and C. Li, J. Mater. Chem., 2008, 18, 450–457; (b) P. Zhang, L. Wang, L. Ren, L. Zhu, Q. Sun, J. Zhang, X. Meng and F.-S. Xiao, J. Mater. Chem., 2011, 21, 12026–12033; (c) M. Tiemann, M. Schulz, C. Jäger and M. Fröba, Chem. Mater., 2001, 13, 2885–2891; (d) Y. Di, X. J. Meng, L. F. Wang, S. G. Li and F.-S. Xiao, Langmuir, 2006, 22, 3068–3072.
9. (a) P. I. Ravikovitch, G. L. Haller and A. V. Neimark, Adv. Colloid Interface Sci., 1998, 66–67, 203–226; (b) M. L. Ojeda, J. M. Esparza, A. Campero, S. Cordero, I. Kornhauser and F. Rojas, Phys. Chem. Chem. Phys., 2003, 5, 1859–1866; (c) M. Thommes, Chem. Ing. Tech., 2010, 82, 1059–1073.
10. P. J. Kocienski, Protecting Groups, Thieme, New York, 2003.
11. S. Singh and P. J. Guiry, J. Org. Chem., 2009, 74, 5758–5763.
12. K. Toshima and K. Tatsuta, Chem. Rev., 1993, 93, 1503–1531.
13. (a) A. E. Graham, Synth. Commun., 1999, 29, 629–703; (b) B. M. Smith, T. M. Kubczyk and A. E. Graham, RSC Adv., 2012, 2, 2702–2706.
14. R. Mokaya, W. Jones, Z. H. Luan, M. D. Alba and J. Klinowski, Catal. Lett., 1996, 31, 113–120.
15. V. R. Ruiz, A. Velty, L. L. Santos, A. Leyva-Pérez, M. J. Sabater, S. Iborra and A. Corma, J. Catal., 2010, 271, 351–357.
16. (a) L. R. Morrison, Glycerol, Kirk–Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., New York, 2001; (b) Z. Z. J. Wang, J. Zhuge, H. Fang and B. A. Prior, Biotechnol. Adv., 2001, 19, 201–223.
17. (a) C.-H. Zhou, J. N. Beltramini, Y.-X. Fan and G. Q. Lu, Chem. Soc. Rev., 2008, 37, 527–549; (b) S. Papanikolaoua, S. Fakas, M. Fick, I. Chevalot, M. Galiotou-Panayotou, M. Komaitis, I. Marc and G. Aggelis, Biomass Bioenergy, 2008, 32, 60–71; (c) A. Behr, J. Eilting, K. Irawadi, J. Leschinski and F. Lindner, Green Chem., 2008, 10, 13–30.
18. (a) C. J. A. Mota, C. X. A. da Silva, N. Rosenbach, Jr., J. Costa and F. da Silva, Energy Fuels, 2010, 24, 2733–2736; (b) C. Crotti, E. Farnetti and N. Guidolin, Green Chem., 2010, 12, 2225–2231; (c) C. X. A. da Silva, V. L. C. Gonçalves and C. J. A. Mota, Green Chem., 2009, 11, 38–41; (d) P. Ferreira, I. M. Fonseca, A. M. Ramos, J. Vital and J. E. Castanheiro, Appl. Catal., B, 2010, 98, 94–99; (e) J. Deutsch, A. Martin and H. Lieske, J. Catal., 2007, 245, 428–435.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cy20188g

---