Preparation of potential biofuel 5-ethoxymethylfurfural and other 5-alkoxymethylfurfurals in the presence of oil shale ash

Abstract

5-Ethoxymethylfurfural (EMF) can be prepared from the corresponding halomethylfurfural and absolute ethanol in good yield. The use of significantly more affordable 96% ethanol results in formation of levulinic acid or its ester in considerable amount (up to 16%), which is difficult to separate from the desired EMF. In the present study we report that the addition of oil shale ash prevents the hydrolysis of the furan ring and enables the use of 96% ethanol with great success. The developed procedure is applicable to a wide range of aqueous alcohols, is operationally simple and utilizes an inexpensive basic ash, which is deposited in millions of tons per year. Notably, the basicity of the ash is decreased during the process, making its deposits less hazardous to the environment.

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Introduction

Diminishing fossil resources and growing environmental concerns have initiated an intense search for renewable sources for liquid fuels and chemicals. The use of degradable lignocellulosic biomass, as an abundant, inexpensive and CO₂-neutral source of carbon, has been considered one of the most promising ways to substitute the products from non-renewable fossil sources.¹ 5-Ethoxymethylfurfural (EMF) is a promising second-generation biofuel, which has been tested as a possible additive or even substitute for diesel fuel. EMF is a liquid, with a boiling point of 235 °C and an energy density 8.7 kW h L⁻¹. This is higher than that of ethanol (6.1 kW h L⁻¹) and comparable to gasoline (8.8 kW h L⁻¹) and diesel fuel (9.7 kW h L⁻¹).² EMF can be synthesized from 5-hydroxymethylfurfural (HMF) in ethanol or from fructose, glucose or sugarcane through an HMF intermediate.^3,4 However, the furan ring of HMF is prone to hydrolytic cleavage under acidic conditions and, besides the desired EMF, a fair amount of ethyl levulinate (EL) is formed, as well.^4a–e Unfortunately, due to similar polarity and only 10 Dalton difference in molar mass, the EL is not easily separable from EMF by simple distillation or chromatography and this hampers the use of the current routes for large scale EMF production.

EMF 4 and other alkoxymethylfurfurals (AMF) can also be synthesized by combining an alcohol with 5-chloromethylfurfural (CMF) 1 or 5-bromomethylfurfural (BMF) 2.⁵ The latter substances are both readily available from carbohydrates and lignocellulosic biomass⁵ (Scheme 1).

Scheme 1 Synthesis of alkoxymethylfurfurals.

EMF can be achieved in high yield and purity if absolute ethanol is used;⁵ the use of more conventional 96% aqueous ethanol leads to the formation of considerable amounts of EL and formic acid (Scheme 2).⁶

Scheme 2 Hydrolysis of halomethylfurfural.

We have previously shown that addition of sodium or calcium carbonate decreases the amount of EL formed, down to roughly 10 mole percent.⁶ However, herein we are pleased to report that the use of oil shale ash as a basic additive reduces the formation of levulinate down to negligible amounts.

Oil shale ash is a waste, which is formed as a by-product of burning oil shale in power plants. In Estonia, most of the electricity is produced by burning oil shale and as a consequence, 5–7 million tons of ash are deposited yearly in deposit plateaus next to the power plants.⁷ Currently, less than 5% of it is recycled and used in construction materials and agriculture. The content of kerogen (the organic part of oil shale) as well as the chemical composition of the mineral part can vary significantly depending on the country. Estonian oil shale (kukersite) is highly calcareous. The ash remaining after combustion is rich in free lime (CaO), anhydrite (CaSO₄) and silicates.⁸

The ash collected from the various parts of the boiler system contains typically 16–25% free calcium oxide and 8–15% magnesium oxide, while ash from an electrostatic precipitator contains normally less free calcium oxide (6–14%) and magnesium oxide (8–11%).⁹ Hydration of ash upon storage under open atmosphere in plateaus initiates the formation of compounds like Ca-hydroxide (portlandite) and Ca-sulphoaluminate (ettringite) whose dissolution in surface water causes high alkalinity (pH 12–13)¹⁰ and environmental issues.⁷

Our current discovery of using ash for the preparation of EMF has double benefits: (i) waste material can be used in an important chemical transformation, which is difficult to perform otherwise, and (ii) the pH of the ash decreases, making its storage and recycling less problematic.

Experimental

General

All reagents were obtained from commercial sources and used without further purification. CMF and BMF were synthesized according to literature procedures.⁶ Absolute ethanol was obtained by distillation from CaH₂ and stored under argon. Alcohol–water solutions were prepared by mixing corresponding volumes of absolute alcohol with a calculated volume of water. Reactions were monitored by TLC using UV light and KMnO₄ for visualizing spots. NMR spectra were recorded using Bruker AVANCE II 400 MHz spectrometer.

Ashes used

Oil shale ashes used in the current work were taken from pulverized-fired boiler (ash A) and from electrostatic precipitator (ash B) from an Estonian thermal power plant. Thermogravimetric analysis of ashes showed that there was 11% of calcium hydroxide and 2% of mineral carbon dioxide in both ashes.

The basicity of the ashes was measured by titration in the following way: 35% hydrochloric and 48% hydrobromic acid were used to titrate the basicity of ashes in the presence of bromophenol blue. 0.5 g of ash was stirred in 10 mL of water. Titration was stopped when the color of the aqueous suspension of ash turned from dark blue to yellow and the amount of acid added was measured. It turned out that 1 gram of ash A binds 8 millimoles of hydrogen ions whereas 1 gram of ash B binds 10 millimoles of hydrogen ions. Oil-shale ash consists of variety of compounds (Ca(OH)₂, CaCO₃, CaO, MgO, CaSiO₃, SiO₂, CaSO₄, etc.) with different reactivity towards acidic species. To ensure the presence of sufficient amount of reactive base, an excess of oil-shale ash was used.

Preparation of EMF (4) and other 5-alkoxymethylfurfurals

Optimization experiments were carried out as follows: 10 mL of alcohol or its solution in water was added to the mixture of 0.3–0.6 g BMF or CMF and (if ash additive was used) 0.5 g oil shale ash A or B. In one reaction the ash was substituted with 90 mg Ca(OH)₂. The mixture was stirred under the conditions specified in Tables 1 and 2. Excess alcohol was then removed by rotary evaporation under reduced pressure. 10 mL of water was added to the viscous slurry followed by extraction with diethyl ether (3 × 30 mL). The organic phases were combined, dried over anhydrous sodium sulfate, filtered and concentrated to afford the final product as yellowish oil. The purity and composition of the product were determined by ¹H NMR spectroscopy. No unreacted BMF or CMF was detected in the product unless specifically stated (Table 2, entries 3, 5 and 9).

Table 1 Synthesis of EMF (4)

Entry	Substrate	Additive^a	Time (h)	Temp. (°C)	Combined yield (%, 4 + 8)	Product composition (4  : 8)
a Ash A is from pulverized-fired boiler; ash B is from electrostatic precipitator.b Reaction carried out on 10 g scale.
1	BMF	—	0.5	70	98	87 : 13
2	BMF	Ash A	0.5	70	84	99 : 1
3	BMF	Ash B	0.5	70	73	99 : 1
4	CMF	—	1	70	86	84 : 16
5	CMF	Ash A	2	70	84	99 : 1
6	CMF	Ash B	2	70	80	98 : 2
7	CMF	Ca(OH)2	1	70	67	100 : 0
8	CMF	Ash A	72	r.t.	85	99 : 1
9	CMF	Ash B	72	r.t.	82	99 : 1
10	BMF	Ash A	16	r.t.	87	100 : 0
11	BMF	Ash B	16	r.t.	75	100 : 0
12^b	BMF	Ash A	17	r.t.	88	100 : 0

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Table 2 Conditions, yields and compositions of final products of AMF syntheses starting from BMF

Entry	Alcohol	% (alcohol)	Additive	Time (h)	Temp. (°C)	Comb. yield	Product comp. (AMF : AL : HMF)
a The product also contained 81% of unreacted BMF.b The product also contained 20% of unreacted BMF.c The product also contained 29% of unreacted BMF.
1	MeOH	95	—	2	r.t.	95	91 : 9 : 0
2		95	Ash B	2	r.t.	68	100 : 0 : 0
3	n-PrOH	99	—	2	65	12^a	18 : 1 : 0
4		99	Ash B	2	65	61	100 : 0 : 0
5	i-PrOH	100	—	2	65	71^b	78 : 2 : 0
6		100	Ash B	2	65	97	98 : 2 : 0
7		99	Ash B	5.5	65	75	91 : 0 : 9
8		90	Ash B	3.5	65	72	71 : 0 : 29
9	n-BuOH	97	—	2	65	58^c	66 : 5 : 0
10		97	Ash B	2	65	86	100 : 0 : 0
11		90	Ash B	2	65	70	80 : 4 : 16

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Large scale EMF synthesis: BMF (2, 10.018 g, 53 mmol) was added to the suspension of oil shale ash A (15.0 g) in ethanol (96%, 300 mL) and stirred for 17 hours at r.t. Then ethanol was removed on a rotary evaporator. The residue was diluted with water (300 mL) and extracted with diethyl ether (4 × 300 mL). After drying with Na₂SO₄, solvents were removed to afford EMF 4 (7.204 g, 88%) as yellowish oil. No further purification was needed.

Kinetic NMR measurements of EMF synthesis

26 mg of BMF was dissolved in 0.7 mL of 5% D₂O solution in deuterated ethanol C₂D₅OD. The solution was transferred to an NMR tube and proton spectra was measured at specific time intervals. When ash impact was investigated, 40 mg of BMF and 50 mg of ash B were vigorously shaken with 1 mL of 5% D₂O solution in deuterated ethanol C₂D₅OD. Before each NMR measurement, the reaction mixture was centrifuged to remove ash particles and then the supernatant was transferred to the NMR tube. After each NMR measurement, the solution was quickly transferred from the NMR tube back to an ash pellet in micro test tube and was vigorously shaken between NMR measurements. Incubations and measurements of solutions were carried out at room temperature. Mole fractions of mixture components were calculated from corresponding peak integrals in proton spectra.

NMR spectral data

5-Methoxymethylfurfural 3. ¹H NMR (400 MHz, CDCl₃): δ = 9.62 (s, 1H), 7.21 (d, J = 3.5 Hz, 1H), 6.53 (d, J = 3.5 Hz, 1H), 4.49 (s, 2H), 3.42 (s, 3H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 177.7, 158.3, 152.6, 121.7, 111.1, 66.5, 58.7 ppm.

5-Ethoxymethylfurfural 4. ¹H NMR (400 MHz, CDCl₃): δ = 9.59 (s, 1H), 7.19 (d, J = 3.5 Hz, 1H), 6.50 (d, J = 3.5 Hz, 1H), 4.50 (s, 2H), 3.56 (q, J = 7.1 Hz, 2H), 1.21 (t, J = 7.1 Hz, 3H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 177.6, 158.7, 152.5, 121.8, 110.9, 66.5, 64.7, 14.9 ppm.

5-Propoxymethylfurfural 5. ¹H NMR (400 MHz, CDCl₃): δ = 9.58 (s, 1H), 7.19 (d, J = 3.5 Hz, 1H), 6.49 (d, J = 3.5 Hz, 1H), 4.50 (s, 2H), 3.45 (t, J = 6.7 Hz, 2H), 1.60 (m, 2H), 0.90 (t, J = 7.4 Hz, 3H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 177.6, 158.8, 152.5, 121.9, 110.8, 72.8, 64.9, 22.7, 10.3 ppm.

5-Isopropoxymethylfurfural 6. ¹H NMR (400 MHz, CDCl₃): δ = 9.56 (s, 1H), 7.17 (d, J = 3.5 Hz, 1H), 6.48 (d, J = 3.5 Hz, 1H), 4.49 (s, 2H), 3.68 (spt, J = 6.0 Hz, 1H), 1.17 (d, J = 6.0 Hz, 6H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 177.5, 159.3, 152.4, 122.0, 110.6, 72.0, 62.4, 21.8 ppm.

5-Butoxymethylfurfural 7. 1H NMR (400 MHz, CDCl₃): δ = 9.58 (s, 1H), 7.19 (d, J = 3.5 Hz, 1H), 6.49 (d, J = 3.5 Hz, 1H), 4.49 (s, 2H), 3.50 (t, J = 6.6 Hz, 2H), 1.55 (m, 2H), 1.36 (m, 2H), 0.89 (t, J = 7.4 Hz, 3H) ppm. ¹³C NMR (100 MHz, CDCl₃): δ = 177.6, 158.8, 152.5, 121.8, 110.8, 70.9, 64.9, 31.5, 19.1, 13.7 ppm.

Results and discussion

Preparation of EMF (4)

Our results and conditions explored for the synthesis of EMF with 96% ethanol are summarized in Table 1. In general, when ash was used as an additive, a significantly lower ethyl levulinate content was observed compared to conditions where ash was not used (entries 1 and 4). Starting from BMF (2), 99% pure EMF was obtained with both ashes after half-an-hour at 70 °C (entries 2 and 3), whereas from the less reactive CMF (1), two hours at 70 °C was needed (entries 5 and 6). Calcium hydroxide was also a suitable base for this conversion, although the yield of EMF was lower (entry 7). We also tested the conversion at room temperature (entries 8–11). In the case of CMF, the conversion was rather sluggish, requiring 3 days to reach full conversion. However, the yield of EMF was good and it contained 1% of EL (8). More reactive BMF, on the other hand, reacted faster and no EL was observed, affording the EMF with up to 87% yield (entry 10). Both ashes behaved rather similarly and EL contents are basically identical in all cases; some variation in yields can be seen as ash A gives slightly higher yields in all comparable cases. We also scaled up the reaction to 10 g scale (entry 12) and the results were pleasing: EMF was obtained cleanly in 88% yield. It is important to note that the procedure described herein is especially suitable for large scale synthesis; both ethanol and the extraction solvent, diethyl ether, can be reused, and no further purification of any kind is needed to obtain pure EMF.

Spent ashes are easily separated by sedimentation and can be stored in open air storages (ash hills) or put underground in old mines. In addition, aqueous phase from extraction containing calcium chloride/bromide, can after concentration be potentially used as counter-ice reagent on the roads or treated with sulfuric acid to obtain gypsum and HCl/HBr can be recovered.

Preparation of other 5-alkoxymethylfurfurals

We also used other alcohols to synthesize corresponding alkoxymethylfurfurals in addition to EMF. Aqueous methanol, propan-1-ol, propan-2-ol and butan-1-ol were utilized to prepare methoxy-, propoxy-, isopropoxy- and butoxymethylfurfural, respectively, from BMF. The conditions and results from these syntheses are presented in Table 2. All these experiments were carried out using ash B.

In the case of 95% aqueous methanol the ash is crucial to keep levulinate content low (entries 1 vs. 2). Without the ash there was 9% of methyl levulinate 9 present in the final product (entry 1). In case of propane-1-ol the reaction was slow without the ash and a large amount of unreacted BMF was still present after 2 hours (entry 3). However, the addition of ash speeded up the reaction considerably (entry 4) and the desired propoxymethylfurfural 5 was obtained cleanly. When the sterically more hindered propane-2-ol was used, the already low water content (1%) led to a significant amount of HMF (5) (entry 7) which increased to 29% with propane-2-ol containing 10% water (entry 8). In the case of propane-2-ol the ash also had an impact on the reaction speed (entries 5 vs. 6), whereas the content of the corresponding levulinate (10) remained unchanged.

If aqueous BuOH was used as the alcohol, the reaction without the ash did not reach completion in 2 hours (entry 9) and besides the desired product 7, also a fair amount of corresponding levulinate 12 was formed. The addition of ash under otherwise identical reaction conditions afforded butoxymethylfurfural 7 in good yield and high purity (entry 10). The increased water content to 10% resulted in significant amounts of hydrolysis products (12 and 13) and a corresponding decrease in yield (entry 11). In general, it can be said that for the aqueous alcohols investigated, the use of ash increased the conversion to the corresponding alkoxymethylfurfurals significantly and suppresses the formation of hydrolysis products.

Kinetic measurement of EMF synthesis reaction

BMF reaction progress with 95% ethanol-d₆ in the absence or presence of ash B was studied by NMR spectroscopy. The decrease of BMF and increase of the desired product, EMF, and byproducts, EL and acetal (2-(diethoxymethyl)-5-(ethoxymethyl)furan), are depicted in Fig. 1. It can be seen that the acetal formation is fast and its concentration does not change substantially over 6 hours. In addition, the amount of acetal is, at a first approximation, independent of the presence of ash. It is clearly seen that with the ash, levulinate content is lower and the yield of EMF is higher. If the usual extractive work-up is applied then the small amount of acetal formed is hydrolyzed to the corresponding aldehyde and therefore not present in the final product.

Fig. 1 Mole fractions of reaction mixture components in the absence (left) and presence (right) of ash B.

The current work shows that the problem of furan ring acidic hydrolysis of the alkoxymethyfurfurals in the presence of water can be avoided or substantially diminished by the use of basic oil shale ash that neutralizes the acid by-product. Even if levulinate esters are formed, oil shale ash probably catalyzes their basic hydrolysis and thereby purifies the desired product. In addition to preventing the hydrolysis, ash also binds acid species which otherwise must be separated in an additional step. Compared to other bases, oil shale ash is inexpensive or free. Furthermore, it is a waste product available in vast amounts in regions where oil shale is used as a source of energy. Thereby, the present method recycles environmentally hazardous waste and offers a simple solution for the synthesis of alkoxymethylfurfurals with aqueous alcohols at room temperature.

Conclusions

The current work provides a simple method for production of alkoxymethylfurfurals in high yield and purity. This method provides a useful application for oil shale ash, which is an environmentally hazardous industrial waste. Furthermore, the use of ordinary ethanol, low cost additives and no need for chromatography, it has the potential to be applied to large-scale biofuel synthesis.

Acknowledgements

We sincerely thank Andres Trikkel and Rein Kuusik, Tallinn University of Technology, Estonia, for providing thermogravimetry to the ashes and Dr John J. Kane (Immunexcite, Inc., Lexington, MA, USA) for critically reading the manuscript. The authors would like to thank Archimedes Foundation (project no. 3.2.0501.10-0004) and Estonian Ministry of Education and Research (project no. SF0180073s08) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46570e

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