Solid-acid and ionic-liquid catalyzed one-pot transformation of biorenewable substrates into a platform chemical and a promising biofuel

Abstract

A wide variety of polymeric carbohydrate-rich weed species were directly converted to a platform chemical, 5-hydroxymethylfurfural (HMF), and a promising next-generation biofuel, 5-ethoxymethyl-2-furfural (EMF), with homogeneous and heterogeneous catalysts under mild reaction conditions. Brønsted acidic IL catalysts, [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻, were found to be effective enabling maximum 58 and 52 wt% HMF yields, respectively, from foxtail weed. Strong Lewis acidic silica supported heteropolyacid (HPA-SiO₂) catalyst was also effective producing a maximum 32 wt% HMF from the same weed substrate. Both IL catalysts were effective for high-purity EMF production from HMF and weeds. HMF was quantitatively converted to EMF in 2 h. In the case of weed substrates, EMF was formed as the major product. The ratio of EMF and ethyl levulinate (EL) in the isolated product was 7 : 1. To address the sustainability issue and potential industrial application opportunity of the current method, a larger scale experiment under conventional heating demonstrated to produce 55 wt% HMF in 4 h. Most importantly, the spent catalyst and the solvent system were efficiently recycled for four consecutive catalytic cycles without a significant loss in yield.

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1. Introduction

The fossil resources, e.g., oil, coal and natural gases, are projected to shrink significantly in the next few decades, which necessitates the utilization of biorenewable resources such as cellulosic biomass for chemicals and biofuels production.^1,2 At the verge of the energy crisis and to deal efficiently with the environmental threat by CO₂ emissions, development of sustainable catalytic routes for the production of biofuels and chemicals from inexpensive biomass and biomass derived substrates has been a major target. Biomass conversion has been considered as one of the most important processes because of its renewable and carbon-neutral properties. In this context, production of 5-hydroxymethylfurfural (HMF), levulinic acid and other value added chemicals from carbohydrates, cellulose and lignocellulosic biomass has become increasingly important.^3–5 HMF serves as a platform chemical for the production of a wide range of chemicals and biofuels.⁶ Numerous studies have been performed for the transformation of carbohydrates^7,8 and cellulosic biomass⁹ into HMF. Inexpensive lignocellulosic biomass^9a,10 and macro-algae-derived polymeric carbohydrates¹¹ have also been used for HMF production. Because of the abundance of lignocellulosic biomass, this non-edible crop has been an attractive source for sustainable HMF production.¹² The production of HMF with moderate yields have been reported using several catalytic systems including CrCl₂/HCl/1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]Cl) in DMA–LiCl,^10a Brønsted–Lewis–surfactant-combined heteropolyacid (HPA) Cr[(DS)H₂PW₁₂O₄₀]₃ micellar catalyst,^10c CrCl₃/LiCl in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl),¹³ and combined CrCl₂-RuCl₃ metal chlorides in [EMIM]Cl.^10a Biomass derived D-mannose and D-galactose substrates have also been used for HMF production.¹⁴ Most recently, HMF production from raw biomass, e.g., wheat straw, reeds from phragmites communis, corn stover and husk of Xanthoceras sorbifolia have been reported.^10a,10c Besides HMF, 5-ethoxymethyl-2-furfural (EMF),¹⁵ one of the promising next-generation biofuels, has been synthesized from biomass substrates. EMF has a comparable energy density (31.3 MJ L⁻¹) with those of standard gasoline (31.1 MJ L⁻¹) and diesel fuel (33/6 MJ L⁻¹).¹⁶ A previous literature report has shown the production of EMF from corn stover by treating with LiCl in the presence of HCl in chlorinated solvent, followed by further treatment with ethanol.¹⁷ In this process 72% 5-chloromethylfurfural (CMF) was produced as intermediate from corn stover using concentrated HCl catalyst under high pressure. CMF product was then converted to EMF in nearly quantitative yield by treatment with ethanol.¹⁵ Kim et al. reported the conversion of macro-algae derived agar to a mixture of EMF and ethyl levulinate (EL) (5 : 2) with a total of 30% yield using Dowex 50WX8 solid acid catalyst in the presence of CrCl₂ in EMIM]Cl solvent.¹⁰

Notably, inexpensive and readily abundant weed plants, a rich source of polymeric carbohydrates, have not been explored as substrates for the production of value added chemicals and biofuels. Cyperus rotundus and Cynodon dactylon weed species are known to contain holocellulose (59.7%), glucan and xylan including lignin components.¹⁸ Foxtail weed species contains a high percentage of starch and other carbohydrates as components.¹⁹ Apart from these, several other weed species have high carbohydrate contents.²⁰ Therefore, the weed plants can be transformed into value added chemicals and biofuels with suitable acid catalysts. With this aim, the present report demonstrates the one-pot production of HMF and EMF from different weed substrates with ionic liquid (IL) and silica supported immobilized heteropolyacid (HPA) catalysts under microwave-assisted heating. To the best of our knowledge, this is the first report of HMF and EMF production from weeds biomass.

2. Experimental

2.1 Experimental procedure

Weed samples were collected from various fields. These samples were oven dried at 80 °C and crushed into fine powder prior to use. Authentic HMF, cellulosic materials and polysaccharides were purchased from Sigma–Aldrich and used without further purification. N,N-Dimethylacetamide (DMA), N-methyl-2-pyrrolidinone (NMP), diethyl ether, LiCl and methane sulfonate were purchased from Spectrochem, India and used as received. The microwave assisted conversions of all substrates were performed on a CEM Matthews WC Discover Microwave reactor (model: Discover System, no. 908010 DV9068) at the standard operating frequency (2.45 GHz, power 300 W). Powder X-ray diffraction analysis of HPA-SiO₂ material for wide-angle was carried out in a Rigaku MiniFlex II XRD machine. ¹H NMR spectra were recorded on a JEOL JNM ECX-400 P 400 MHz instrument and NMR data were processed with JEOL DELTA program version 4.3.6. HMF yields were measured by HPLC and UV-visible spectrophotometric techniques. A Shimadzu HPLC instrument (model: 20 AD) equipped with UV detector and pressure gradient pumps was used for determining HMF yields from the reaction mixture. Unless otherwise mentioned, the given substrate concentrations were calculated with respect to the total mass of the reaction mixture and the catalyst concentrations were calculated with respect to the substrate concentrations. For the temperature programmed desorption (TPD) of ammonia studies, sample was activated and then ammonia was injected at room temperature in the absence of carrier gas flow. Then the temperature was raised in a stepwise fashion at a heating rate of 10 °C min⁻¹. The desorbed ammonia in the temperature range of 100 to 750 °C was analyzed by using a MicromeriticsChemiSorb 2720 containing a thermal conductivity detector.

2.2 Preparation of ILs

[DMA]⁺[CH₃SO₃]⁻. DMA (8.7 g, 0.1 mol) was added into a 50 mL round-bottom flask equipped with a magnetic stirrer bar and the flask was placed in a ice-bath. To this flask, methyl sulfonic acid (9.6 g, 0.1 mol.) was slowly added over approximately 30 min. The reaction mixture was stirred for another 4 h at room temperature. The resulting solid was washed with ethyl acetate three times and dried at 90 °C under vacuum. ¹H NMR (400 MHz, DMSO-d₆): δ 2.92 (s, 3H, CH₃–S), 2.76 (s, 6H, CH₃–N), 1.95 (s, 3H, CH₃–C=O), 1.94 (s, 3H, CH₃). ¹³C NMR (dmso-d₆): δ 170.01 (s, CO), 37.69 (s, CH₃–S), 34.78 (s, 2CH₃–N), 21.10 (s, CH₃–CO).

[NMP]⁺[CH₃SO₃]⁻. NMP (9.9 g, 0.1 mol) was added into a 50 mL round-bottom flask containing a magnetic stirrer bar and the flask was placed in a ice-bath. To this flask, methyl sulfonic acid (9.6 g, 0.1 mol.) was slowly added over approximately 30 min. The reaction mixture was kept stirring for another 3 h at room temperature. The resulting solid was washed with ethyl acetate three times and dried at 90 °C under vacuum. ¹H NMR (400 MHz, DMSO-d₆): δ 4.05 (s, 3H, CH₃–S), 3.28 (t, 2H), 2.66 (s, 3H, CH₃–N), 2.14–2.18 (t, 2H), 1.83–1.85 (m, 2H). ¹³C NMR (dmso-d₆): δ 174.60, 49.12, 30.62, 29.55, 17.69.

2.3 Preparation of SiO₂ supported phosphotungstic acid catalyst

H₃PW₁₂O₄₀ (0.627 g) was dissolved in 4 mL water–ethanol mixed solvent (1 : 1 v/v) in a glass vessel. To the same glass container, SiO₂ (1 g) was slowly added and continuously stirred for 72 h so that HPA could get adsorbed on the silica surface. The catalyst was separated out by centrifugation and dried in an oven. Finally it was calcined at 200 °C for 2 h. The material was characterized by solid-state X-ray diffraction (XRD) study and the corresponding spectrum is deposited in ESI†.

2.4 Direct conversion of weeds to HMF with IL catalysts

A microwave tube was charged with 5 wt% (25 mg) of weed substrate, 10 wt% (2.5 mg) of IL catalyst and 0.5 g DMA–LiCl solvent. The microwave tube was then inserted into the microwave reactor pre-set at the desired temperature and reaction time. Upon completion of reaction for the set reaction time, the reactor was opened. The temperature of the reaction mass was cooled to room temperature. HMF was extracted with diethyl ether and purified by repeated (four times) column chromatography with silica gel as stationary phase and diethyl ether as mobile phase. The yield of extracted crude HMF was measured by HPLC. ¹H NMR (400 MHz, CDCl₃): δ 9.58 (s, 1H), 7.20 (d, J = 2.8 Hz, 1H), 6.51 (d, J = 2.8 Hz, 1H), 4.70 (s, 2H). ¹³C NMR (100 Hz, CDCl₃): δ 177.76, 160.97, 152.04, 123.47, 109.94, 57.25.

2.5 Direct conversion of weeds to HMF with HPA-SiO₂ catalyst

A microwave tube was charged with 5 wt% (25 mg) of weed substrate, 20 wt% (5 mg) of HPA-SiO₂ catalyst and 0.5 g DMA–LiCl solvent. The microwave tube was then inserted into the microwave reactor pre-set at the desired temperature and reaction time. Upon completion of reaction for the set reaction time, the reactor was opened. The temperature of the reaction mass was cooled to room temperature. HMF was extracted with diethyl ether and the yield of extracted HMF was measured by HPLC.

2.6 Conversion of HMF to EMF with IL catalysts

The conversion of HMF to EMF was carried out in a 100 mL round-bottom flask under oil-bath heating. For typical experiments, the flask was charged with 0.3 g of HMF, 10 wt% (30 mg) of IL catalysts in ethanol, and refluxed with continuous stirring at 120 °C for 2 h using a long condenser and chilled water so that ethanol vapour can be condensed back to the liquid phase. Upon completion of reaction for 2 h, the reaction mixture was cooled to room temperature, ethanol was evaporated under vacuum and the oily residue was run through a column comprised of silica gel of 200–400 mesh as stationary phase and mixed dichloromethane–diethyl ether solvent (2 : 1 volume ratio) as mobile phase. After separating the [DMA]⁺[CH₃SO₃]⁻ component by column chromatography, the oily EMF liquid product was characterized by ¹H NMR spectroscopy. ¹H NMR (400 MHz, CDCl₃): δ 9.59 (s, 1H), 7.19 (d, J = 2.8 Hz, 1H), 6.50 (d, J = 2.8 Hz, 1H), 4.53 (s, 2H), 3.69 (q, 2H), 1.20 (t, 3H).

2.7 Direct conversion of weeds to EMF with IL catalysts

One-pot production of EMF from weeds was carried out in ethanol by oil-bath heating. A 100 mL round-bottom flask was charged with 1 g of foxtail weed, 6 mL ethanol and 10 wt% (0.1 g) of IL catalysts. The mixture was refluxed with continuous stirring at 120 °C for 15 h. Aliquots were collected at different time intervals (3, 10 and 15 h) to monitor the progression of the conversion. After 15 h reaction, the oily liquid product was isolated in the same way as described above. The ¹H NMR spectrum of the isolated product revealed the presence of both EMF and EL in 7 : 1 ratio (ESI†).

2.8 Determination of HMF yield by HPLC method

HPLC measurements for determining HMF yields were conducted on a LC 20 AD Shimadzu instrument equipped with a UV detector, low-pressure gradient pump and C18 reverse phase column of dimensions 250 mm × 4.6 mm × 5.0 μm. The product solution containing HMF was run using a mobile phase of acidic water (0.05% H₂SO₄) at 35 °C. A 20 μL injection loop was used with a 1.0 mL min⁻¹ flow rate. LC solution software was used for analysis of the data and calculation of HMF yield. The HMF peak was identified by its retention time in comparison with an authentic sample and integrated. The actual concentration of HMF was determined from the pre-calibrated plot of peak area against concentration. Repeated measurement of the same solution shows the percentage error associated with this measurement was ±5%. The yield of HMF for a few samples was also determined by our earlier reported UV-Vis spectrophotometric method.^8c–e HMF yields obtained from UV-Vis measurement compared well with that of the HPLC measurement. The yield of HMF was calculated in wt% with respect to the total mass of the starting pre-treated (dried and ground) weed substrates.

3. Results and discussion

3.1 Hammett acidity of Brønsted-acidic ILs

As the dehydration of carbohydrates is directly related with the acidic functionalities of IL catalysts, Hammett acidity functions of the ILs were investigated in methanol solution. Based on the procedure of Gilbert et al.²¹ the acidity of the Brønsted-acidic ILs were measured by UV-Vis spectrophotometric technique using p-nitroaniline as an indicator and molecular probe. [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻ solutions of concentrations 3.0 × 10⁻² mol L⁻¹ and 7.5 × 10⁻² mol L⁻¹ were prepared by adding calculated amounts of respective ILs into p-nitroaniline solution in methanol and stirring the solution for 2 h. The UV-Vis spectra of the blank p-nitroaniline solution and that of IL-containing solutions were recorded. The absorbance value of p-nitroaniline solution decreased, in the wavelength region of 350–400 nm, upon addition of ILs (Fig. S1, ESI†). The protonic acidity of both DMA⁺ and NMP⁺ based ILs were calculated from the measured absorbance values. Hammett acidity function (H_o) of ILs were calculated from the following expression, H_o = pK_a(ln) + log([ln]/[lnH⁺]), where pK_a(ln) is the pK_a value of p-nitroaniline (about 0.99) and [ln] and [lnH⁺] are the molar concentrations of the protonated and unprotonated forms of p-nitroaniline indicator, respectively. Calculation shows the acid strength (H_o) of [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻ are comparable (Table 1).

Table 1 Hammett function values of ILs

IL	Absorbance	[In]^a (%)	[InH^+]^b (%)	H o ^c
a Represents the molar concentration of p-nitroaniline indicator. b [InH^+] represents the molar concentration of the protonated p-nitroaniline. c H o = pKa(In) + log([In]/[InH^+]), pKa = 0.99; solvent: methanol; c(In) = 7.5 × 10^−5 mol L^−1; c(sample) = 3.0 × 10^−2 mol L^−1; T = 25 °C.
None	1.84	100	0	—
[DMA]^+[CH3SO3]^−	1.28	69.6	30.4	1.35
[NMP]^+[CH3SO3]^−	1.27	69.0	31.0	1.34

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3.2 Weeds to HMF with IL catalysts

The syntheses of HMF from weed substrates using IL catalysts, [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻, were carried out under microwave assisted heating at 120 °C for 2 min. A variety of weeds were used as substrates. Different parts of the weed plant were separately employed as substrates to investigate their applicability as substrates. The results as tabulated in Table 2 show that the formation of HMF varied from 11 to 58 wt% based on the variety of the weeds and the parts used. Straw of the foxtail variety produced the maximum amount of HMF (58 wt%) in 2 min under microwave assisted heating. Tuber and shoot parts of red nut sedge grass produced 33 and 35 wt% HMF, respectively. In the case of Gajar ghas variety, its ‘root’ and ‘shoot’ parts produced 37 and 23 wt% HMF, respectively. Among several easily available weed varieties tested, Indian doab (Bermuda grass), yellow dock, dodder, pigweed, Gajar ghas, foxtail, cycus and wild elephant foot yam species produced >20 wt% HMF under mild conditions.

Table 2 Results of direct conversion of weeds (grasses) into HMF with Brønsted acidic IL [DMA]⁺[CH₃SO₃]⁻ catalyst

Entry	Weed	Scientific name	Part used	HMF yield^b (wt%)
a Reaction conditions: 5 wt% substrate with respect to total reaction mass, 10 wt% [DMA]^+[CH3SO3]^− (with respect to substrate concentration), solvent: DMA–LiCl (0.5 g, 10 wt% LiCl), T = 120 °C, t = 2 min, b HMF yield was measured by HPLC.
1	Red nut sedge	Cyperus rotundus	Tuber	33
2	Red nut sedge	Cyperus rotundus	Shoot	35
3	Indian doab	Cynodon dactylon	Whole part	28
4	Marijuana	Cannabis spp.	Whole part	13
5	Water spinach	Ipomoea aquatica	Whole part	11
6	Water hyacinth	Eichhornia	Whole part	19
7	Datura	Datura spp.	Fruit	12
8	Yellow dock	Rumex spp.	Whole part	30
9	Dodder	Cuscuta spp.	Whole part	32
10	Pigweed	Chenopodium spp.	Whole part	27
11	Gajar ghas	Parthenium	Root	37
12	Gajar ghas	Parthenium spp.	Shoot	23
13	Spiny pigweed	Amaranthus spp.	Whole part	20
14	Foxtail		Straw	58
15	Wild elephant foot yam		Root	29
16	Cycus		Leaf	33

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The benefits of the microwave assisted heating over the conventional heating for HMF production are (i) reduction of reaction time from hours to minutes and (ii) maximization of HMF yields. This benefit of the microwave assisted heating was clearly demonstrated in our recent publication^8c for fructose dehydration reaction, where conventional oil-bath heating experiment produced about 30% less HMF than that of microwave-assisted heating experiment under comparable reaction conditions. To further compare the yield of HMF from conventional and microwave heating methods in the present work, we have studied the conversion of foxtail weed under oil-bath heating. The oil-bath heating experiment under the conditions of 500 mg foxtail weed, 50 mg [DMA]⁺[CH₃SO₃]⁻ catalyst, 10 g DMA–LiCl solvent and at 140 °C produced 55 wt% HMF in 4 h, as compared to 58 wt% HMF in just 2 min from a microwave assisted heating experiment at 120 °C (Table 2). Under similar reaction conditions, the methane sulfonic acid catalyzed reaction produced 27 wt% HMF in 4 h from foxtail weed, suggesting the superior Brønsted acidity of the investigated IL catalysts than the methanesulfonic acid. Of note, the high HMF yield from foxtail weed in the latter reaction carried out in 20 times larger scale under conventional heating revealed the sustainability and potential industrial application opportunity of the current method. Thus, the utilization of these inexpensive and abundant sources of weed plants for HMF production can overcome the current challenge of its high production cost and hence can promote its further application for polyester building block chemical²² and biofuels productions.

To study the effect of reaction time on HMF yields, the reaction time of [DMA]⁺[CH₃SO₃]⁻ catalyzed conversion of red nut sedge weed was varied from 1 to 10 min. The yield of HMF was found to increase from 27 to 37 wt% upon increasing the reaction time from 1 to 5 min, followed by a plateau upon further increasing the reaction time to 10 min (Table 3).

Table 3 The effect of reaction time on HMF yields for the conversion of 5 wt% red nut sedge weed with 10 wt% [DMA]⁺[CH₃SO₃]⁻ catalyst in DMA–LiCl at 120 °C under microwave assisted heating

Entry	t/min	HMF yield (wt%)
1	1	27
2	2	33
3	3	34
4	5	38
5	10	37

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The catalytic effectiveness of [NMP]⁺[CH₃SO₃]⁻ catalyst for the direct conversion of weeds to HMF was tested for the selected weed varieties which produced >40% HMF with [DMA]⁺[CH₃SO₃]⁻ catalyst. As shown in Table 4, [NMP]⁺[CH₃SO₃]⁻ catalyzed reaction produced 52 wt% HMF from foxtail substrate, which is 6 wt% less than that obtained with [DMA]⁺[CH₃SO₃]⁻ catalyst under comparable reaction conditions. A comparative analysis of Tables 2 and 4 results reveal that [DMA]⁺[CH₃SO₃]⁻ catalyzed conversion of dodder and Gajar ghas weed species produced about 2–6 wt% more HMF than that obtained with [NMP]⁺[CH₃SO₃]⁻ catalyst. Similar observation of higher effectiveness of [DMA]⁺[CH₃SO₃]⁻ catalyst than that of [NMP]⁺[CH₃SO₃]⁻ catalyst was noted in the conversions of starch, cellulose varieties and sugarcane bagasse substrates (Table S1, ESI†) even though their acid strengths are comparable as determined from their Hammett acidity measurement (H_o = 1.35 and 1.34). The difference in effectiveness may be due to a better proton donating ability of [DMA]⁺[CH₃SO₃]⁻ (ESI†).

Table 4 Results of direct conversion of weeds into HMF with Brønsted-acidic IL [NMP]⁺[CH₃SO₃]⁻ catalysts

Entry	Weed	Scientific name	Part used	HMF yield^b (wt%)
a Reaction conditions: 5 wt% substrate with respect to total reaction mass, 10 wt% [NMP]^+[CH3SO3]^− (with respect to substrate concentration), Solvent: DMA–LiCl (0.5 g, 10 wt% LiCl), T = 120 °C, t = 2 min. b HMF yield was measured by HPLC.
1	Red nut sedge	Cyperus rotundus	Shoot	32
2	Dodder	Cuscuta spp.	Whole part	26
3	Gajar ghas	Parthenium	Root	35
4	Foxtail		Straw	52
5	Cycus	Cycus	Leaf	33

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3.3 Recyclability of [DMA]⁺[CH₃SO₃]⁻ catalyst and solvent

The recyclability of the catalyst and solvent was tested by performing an experiment under the following reaction conditions: 500 mg foxtail weed, 50 mg [DMA]⁺[CH₃SO₃]⁻ catalyst, 10 g DMA–LiCl (10 wt% LiCl), 140 °C and 4 h reaction time under oil-bath heating. After separating HMF by diethyl ether extraction, the bottom layer of the reaction mixture containing spent catalyst and solvent (DMA–LiCl) was reused for the next run. The catalytic activity of the spent reaction mixture was tested by adding fresh foxtail weed into the spent reaction mixture. Fresh catalyst was not added to compensate any loss of the catalyst in the prior run. In a similar fashion, the spent reaction mixture was reused for four catalytic cycles. The yields of HMF obtained from each cycle are shown in Fig. 1. The result shows a minimal loss of activity of the spent catalyst; in terms of HMF yield, the loss is only 3 wt% after four cycles. The result shows the potential application opportunity of the present ionic liquid catalyst for industrial scale HMF production from weed biomass.

Fig. 1 Recyclability study of [DMA]⁺[CH₃SO₃]⁻ catalyst and the reaction medium for the conversion of foxtail weed to HMF.

3.4 Weeds to HMF with SiO₂-HPA catalyst

Previous reports have shown that heteropolyacid catalysts, FePW₁₂O₄₀ and Cs_2.5H_0.5PW₁₂O₄₀, were highly effective for the conversion of fructose and glucose, giving 91–97% HMF yields under evacuation (0.97 × 10⁵ Pa) at 120 °C for 2 h.^12a A recent article reported excellent activity of Ag₃PW₁₂O₄₀ catalyst, producing 78% HMF from fructose in 60 min at 120 °C.²³ Similar activity of Brønsted–Lewis–surfactant-combined heteropolyacid (HPA), Cr[(DS)H₂PW₁₂O₄₀]₃, has also been reported, giving 53 mol% HMF from cellulose substrate in aqueous medium at 150 °C in 2 h.^10c In this context, the beneficial effect of immobilized heteropolytungstic acid (H₃WP₁₂O₄₀) on silica support has been reported for efficient production of furfural from xylose because of easy separation and recyclability of the catalyst.²⁴ This warranted to test the effectiveness of the silica supported HPA catalyst for one-pot conversion of weeds into HMF. The reactions between 5 wt% weed substrate and 20% SiO₂-HPA catalyst (with respect to substrate concentration) were carried out in DMA–LiCl solvent under microwave assisted heating at 120 °C for 2 min. The results as shown in Table 5 demonstrate the moderate activity of HPA-SiO₂ catalyst, giving a maximum of 32 wt% HMF from foxtail weed.

Table 5 Results of direct conversion of weeds (grasses) into HMF with HPA-SiO₂ catalyst

Entry	Weed	Scientific name	Part used	HMF yield^b (wt%)
a Reaction conditions: 5 wt% substrate with respect to total reaction mass, 20 wt% HPA-SiO2 (with respect to substrate concentration), Solvent: DMA–LiCl (0.5 g, 10 wt% LiCl), T = 120 °C, t = 2 min. b HMF yield was measured by HPLC.
1	Indian doab	Cynodon dactylon	Whole part	11
2	Water hyacinth	Eichhornia	Whole part	8
3	Foxtail		Straw	32
4	Wild elephant foot yam		Root	19

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Although the Hammett acidity of HPA-SiO₂ catalyst cannot be calculated as for IL catalysts, strong Lewis acidity of the HPA supported catalyst was confirmed by the NH₃-TPD method. NH₃-TPD results (Fig. S11, ESI†) showed very high desorption temperature (400–650 °C) with peaks at ca. 500 and 620 °C. Total acidity of the HPA-SiO₂ material amounts to 2.08 mmol g⁻¹.

The conversion of weed biomass to HMF involves the following steps: hydrolysis of weed polysaccharides to monosaccharides, isomerization of glucopyranose to fructofuranose, and finally dehydration of fructofuranose to HMF.^9a Thus, a multifunctional catalytic system is required to perform these steps. In an attempt to investigate the direct transformation of weed biomass into HMF, we have used acidic ionic liquids and silica supported heteropolyacid (HPA-SiO₂) catalysts. The hydrolysis of weed polysaccharide is believed to occur under acidic conditions in DMA–LiCl. Since commercial 99% DMA was directly used, it is likely that a considerable amount of water was present in the solvent, which assisted the hydrolysis of ether linkages of polysaccharide units catalyzed by H⁺ in the presence of water. Additionally, DMA–LiCl solvent facilitates the dissolution of weed polysaccharides by forming DMA·Li⁺ macrocations, resulting in a high concentration of weakly ion-paired Cl⁻,^9a and hence disrupting its extensive network of intra- and interchain hydrogen bonds.

Similar to the previous report by Raines et al.,^9c the present acidic ILs catalyzed the direct transformation of weed substrates into HMF, involving polysaccharide hydrolysis and subsequent dehydration of resulting sugars into HMF, assisted by the H⁺ ions of the ILs. In a separate study, we have investigated the effectiveness of [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻ IL catalysts for the conversion of easily hydrolysable polysaccharide (starch) and difficult to hydrolyse polysaccharides (cellulose, sugarcane bagasse) in DMA–LiCl (LiCl 10 wt%) solvent. Under comparable reaction conditions, the reaction produced more HMF from starch than the cellulose substrates (Table S1, ESI†).

3.5 Weeds to EMF with IL catalysts

The production of EMF, a promising biofuel, from biomass and biomass derived HMF is the most recent research focus in this area. Mascal et al.^15,17 reported a process for EMF production via CMF route, similar to the one reported by Cho et al. for the conversion of red macro-algae derived agar into a mixture of EMF and EL in 5 : 2 ratio with CrCl₂ catalyst in [EMIM]Cl.¹¹ In a recent study, Riisager et al. demonstrated that EMF formation depends on the acid strength of IL catalysts when using a series of task-specific hydrogen sulfate anion, –SO₃H, containing ILs. The authors reported that EMF was formed as a major product within 2 h from fructose, glucose and sucrose substrates, but continuing the reaction for 25 h resulted in the formation of EL as the major product.^6a

To test the effectiveness of the recently prepared Brønsted acidic IL catalysts, [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻, for EMF production, a preliminary experiment was carried out using HMF as a substrate under the conditions of 0.3 g HMF, 10 wt% [DMA]⁺[CH₃SO₃]⁻ catalyst in 6 mL ethanol at 120 °C under oil-bath heating. The ¹H NMR spectrum of the resultant product confirmed almost quantitative conversion of HMF within 2 h. The total isolated yield of EMF from 0.3 g starting HMF substrate was 0.31 g (Table 6, entry 2). Under similar reaction conditions, [NMP]⁺[CH₃SO₃]⁻ catalyzed HMF conversion also produced EMF with high selectivity; 0.28 g isolated yield in 2 h from 0.3 g HMF (Table 6, entry 1). The detailed preparative method and isolation of EMF are shown in the Experimental section.

Table 6 Results of EMF production from weeds and HMF using IL catalysts

Entry	Substrate/g	t/h	Ethanol/mL	IL (10 wt%)^b	EMF yield^c/g
a Reaction conditions: T = 120 °C. b 10 wt% with respect to substrate. c Isolated yield after separating the IL by column chromatography.
1	HMF, 0.3	2	6 mL	[NMP]^+[CH3SO3]^−	0.28
2	HMF, 0.3	2	6 mL	[DMA]^+[CH3SO3]^−	0.31
3	Foxtail, 1.0	15	20 mL	[NMP]^+[CH3SO3]^−	0.40
4	Foxtail, 1.0	15	20 mL	[DMA]^+[CH3SO3]^−	0.46
5	Red nut sedge, 0.5	15	10 mL	[NMP]^+[CH3SO3]^−	0.27
6	Red nut sedge, 0.5	15	10 mL	[DMA]^+[CH3SO3]^−	0.28

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In the case of weed substrates, a reaction between 1.0 g of foxtail weed and 10 wt% [DMA]⁺[CH₃SO₃]⁻ catalyst in 20 mL ethanol at 120 °C produced 0.46 g isolated oily product (Table 6, entry 4). The ¹H NMR spectrum of the isolated product (ESI†) confirmed the presence of EMF and EL in the product.

A comparison of integrated values of –CH₂OEt quartet signals of EMF and EL suggested the formation of EMF and EL occurred in 7 : 1 ratio. [NMP]⁺[CH₃SO₃]⁻ catalyst was also effective for the direct conversion of foxtail biomass to EMF by producing 0.40 g isolated yield.

This method was further extended for the conversion of red nut sedge weed by refluxing 0.5 g substrate with 10 wt% IL catalysts in ethanol for 15 h. The corresponding isolated yields of EMF with [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻ catalysts were 0.28 g and 0.27 g, respectively. The present catalytic method of EMF production demonstrates superior EMF selectively when compared with the previously reported results of the formation of EMF and EL in 5 : 2 ratio.¹¹ The progression of the reaction was monitored by observing the appearance and disappearance of the –CHO proton signal of intermediate HMF product (Fig. 2). The final product, EMF, was formed by etherification of intermediate HMF with ethanol. A stack plot of ¹H NMR spectra showing the progression of foxtail weed biomass conversion to EMF via the formation of HMF as intermediate is presented in Fig. 2.

Fig. 2 Stack plots of ¹H NMR spectra (0–10.2 ppm) showing progression of foxtail weed biomass conversion into EMF for a reaction between foxtail weed and [DMA]⁺[CH₃SO₃]⁻ catalyst in ethanol.

It was also confirmed that the selectivity of EMF remained unchanged upon extending the reaction time from 15 to 30 h. The advantages of the current method of EMF production can be envisaged as: (1) sustainable production of bio-based products using inexpensive and readily abundant weeds biomass, (2) one-pot synthetic route of EMF with high selectivity, and (3) easy EMF purification by simple column chromatography.

A pictorial representation of HMF product obtained from grass substrate using ILs and HPA-SiO₂ catalysts and isolated EMF obtained from the grass samples using IL catalysts is presented in Fig. 3.

Fig. 3 Photos of the weeds samples, HMF and EMF products obtained from red nut sedge weed with ILs and HPA-SiO₂ catalysts. The appearance of darker color in [DMA]⁺[CH₃SO₃]⁻ catalyzed HMF product is due to the formation of humin (oligomeric species between HMF and fructose) as a by-product.^8c

4. Conclusions

In conclusion, Brønsted acidic ILs, [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻, have been utilized as catalysts for the direct conversion of waste plant-materials (weeds) to HMF and EMF under both microwave-assisted and oil-bath heating. Foxtail weed gave maximum HMF yields; 58 and 52 wt% with [DMA]⁺[CH₃SO₃]⁻ and [NMP]⁺[CH₃SO₃]⁻ catalysts, respectively. Strong Lewis acidic (2.08 mmol g⁻¹) silica supported heteropolyacid (HPA-SiO₂) catalyst was also effective producing a maximum of 32 wt% HMF from foxtail weed. Both IL catalysts were effective for the conversion of HMF and one-pot conversion of weeds into EMF, one of the promising biofuels. HMF was almost quantitatively converted to EMF in 2 h. In the case of weed substrates, total isolated yields of a mixture of EMF and EL (7 : 1 ratio) were 0.28 g and 0.46 g from 0.5 g red nut sedge tuber and 1 g foxtail substrates, respectively, with 10 wt% [DMA]⁺[CH₃SO₃]⁻ catalyst. The high selectivity of EMF product remained unchanged for a prolonged reaction. The effectiveness of [NMP]⁺[CH₃SO₃]⁻ catalyst was slightly lower than that of [DMA]⁺[CH₃SO₃]⁻ catalyst for the conversion of all substrates, perhaps due to the higher proton donating ability of [DMA]⁺[CH₃SO₃]⁻. This report disclosed the production of HMF and EMF from inexpensive and abundance source of weed biomass for the first time. These results will direct the future research to develop task-specific solid acid and IL catalysts for the production of EMF and higher HMF-ether derivatives to achieve better liquid fuels from plant biomass and biowaste.

Acknowledgements

The authors gratefully acknowledge the financial support by the University Grant Commission (UGC), India and the University of Delhi. S. Dutta thanks UGC, India for a DS Kothari Postdoctoral Research Fellowship. S. De and I. A. thank UGC, India for a Research Fellowship.

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Footnote

† Electronic supplementary information (ESI) available: Results of recycling, NMR spectral characterization of ILs and reaction products, HPLC plot, results of conversions and HMF yields. See DOI: 10.1039/c2ra20574b

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