A unique opportunity for the utilization of glass wastes as a resource for catalytic applications: toward a cleaner environment

Abstract

Although glass recycling has been conducted since 1970s, and even though recycled glass waste has been used in the construction of various alternative products, the utilization of glass waste for catalytic applications has not been fully considered until now. In the present work, glass waste materials were demonstrated to be accessible, convenient, and inexpensive resources as catalyst supports for the immobilization of sulfonic groups on their surfaces to produce an efficient heterogeneous solid acid nanocatalyst, the so-called nano-glass-waste-supported sulfonic acid (n-glass-waste-SO₃H (n-GW-SA)). Titration and XRD, FE-SEM, EDX, FT-IR, BET, BJH, TEM, and TGA analysis techniques were employed to fully characterize the as-prepared catalyst. The glass-waste-supported sulfonic acid exhibited superior catalytic performance in multicomponent reactions (MCRs) for the one-pot synthesis of biologically useful 3,4-dihydropyrimidin-2(1H)-ones and xanthene derivatives in high to excellent yields. The eco-friendly and economic advantages of the said catalyst include its good recoverability and reusability for several runs, low price, low toxicity, and facile accessibility, manufacture, and handling.

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1. Introduction

Due to the fact that catalysis plays an important role in many industries, such as fine chemicals, commodity chemicals, energy, fuels, and pharmaceuticals, the development of novel, cost-effective, non-toxic, retrievable, and reusable catalysts for organic synthesis has been the center of many researcher's attention due to environmental and economic factors.¹ Homogeneous catalytic technologies have been proven to have numerous drawbacks, including the corrosion of process equipment, a problematic separation of the catalyst, and most importantly, their significant negative environmental impacts through producing a large amount of wastes.² However, today's tight legislation to reduce environmental pollution has shifted the industry onto the implementation of novel clean technologies. Therefore, homogenous catalysts are no longer recommended now because supported-heterogeneous catalysts have been utilized in the chemical industry. In this regard, solid supports (especially nanoparticles (NPs)) have now emerged as viable and appealing options that homogeneous, active protic acid can be immobilized on their surfaces to produce “heterogenized-homogeneous catalysts”, which mimic their homogeneous counterparts in organic transformations.³ To date, different supports, such as silica, carbon, and zirconia as well as organic polymers among others, have been extensively investigated for the immobilization of homogeneous catalysts/reagents and have been applied in chemical syntheses and sustainable transformations.^4–9 These catalysts offer several advantageous in some cases, albeit some have common drawbacks, such as they simply release their active species into solutions (or via leaching) and, therefore, they have poor reusability after completion of the concerned reactions. In light of this situation, there is still ample scope for the development of alternative strategies to design recyclable supported-heterogeneous catalysts from the point of view of improved economics and environmentally benign protocols.

Owing to the fact that energy and products consumption have been globally increasing, the utilization of different waste materials or their recycling has emerged as key issues around the world. Therefore, it is essential for researchers to deal with and circumvent the problems associated with the wastes afflicting future societies through inventing original, imaginative and brilliant solutions as well as by finding alternative and low-cost routes to use waste products/materials instead of disposing of them. To this end, the application of wastes in the context of constricting the negative effects of human industrial activities is still a standard practice and has become a fully-fledged segment of the industrial economy.¹⁰ Among the different types of waste material/product, glass waste is a great source of waste. Since glass plays significant roles in human activity, science, and technology, the utilization of glass wastes not only can contribute to saving energy and resources for manufacturers and governments, but it also can be give environmental benefits in terms of a reduction in energy consumption and pollution.

From a practical standpoint, glass has frequently been used in the formation of different products, e.g., ballotini-small glass spheres, glass wool insulation,¹¹ foamed glass^12,13 ceramics,¹⁴ abrasives,¹⁵ cement,¹⁶ concrete.¹⁷ On account of the fact that there is usually a fairly high content of SiO₂ (70%) in glass materials,¹⁸ it seems that there is still much room to expand the application of glass waste to the chemical industry.

In continuation of our research endeavors on the development of sustainable protocols and applications for nano-catalysts,^2,19–23 recently, we introduced nano-ceramic tile waste for the immobilization of sulfonic acid as stable retrievable and efficient heterogeneous catalysts in multicomponent reactions.²⁴ To the best of our knowledge, there are no reports in the literature on the heterogenization of sulfuric acid by means of immobilizing sulfonic groups on the surfaces of glass waste supports. Therefore, we thought that grafting sulfonic groups on the surfaces of glass particles by means of covalent bonding to the high content SiO₂ in the glass particles could allow the glass waste to serve as an alternative to the current corrosive and toxic sulfuric acid catalysts that result in enormous quantities of hazardous waste. To this end, we were intrigued by the possibility of developing an inexpensive, recoverable, reusable heterogeneous solid acid catalyst, named a nano-glass-waste-supported sulfonic acid, via chlorosulfonic acid as sulfonating agent. Sulfonation with chlorosulfonic acid, which has been the center of great attention since Zolfigol's report on the preparation of silica sulfuric acid,²⁵ was demonstrated to be a convenient, fast, and efficient method for the heterogenization of homogeneous catalysts.^26–32

Multicomponent reactions (MCRs), a multi-purpose synthetic route to construct structurally diverse drug-like chemical entities in addition to other compounds, possess great importance for improving the atom economy compared with conventional linear-type synthetic methods. Hence, the design of a recyclable catalytic system that promotes efficient one-pot synthesis in multicomponent reactions has become a great challenge in organic chemistry. In recent years, the intensive focus of interest has been on the synthesis of 3,4-dihydropyrimidin-2(1H)-ones (in the multicomponent reaction conditions), due to their significant biological activity,^33–35 and on the synthesis of xanthene and its derivatives, owing to their wide range of pharmacological, industrial, and synthetic properties.^36–38 We were, therefore, interested in whether our as-prepared catalyst nano-glass-waste-supported sulfonic acid could react well for the synthesis of these valuable compounds. Therefore, the synthesis of 3,4-dihydropyrimidin-2(1H)-ones (in the condensation reaction of urea, α,β-ketoesters, and aldehydes via Biginelli reactions) was carried out, in addition to the preparation of 1,8-dioxooctahydroxanthene derivatives, and these were tested in the presence of the introduced catalyst.

2. Results and discussion

In continuation of our investigation into developing new and efficient heterogeneous solid acid catalysts,^2,20,21 in the present paper, we aim to report the synthesis of n-glass-waste-supported sulfonic acid (n-GW-SA) and discuss its performance as a solid acid catalyst. The n-glass-waste-supported sulfonic acid was prepared according to an earlier report with magnetic particle-supported sulfonic acid catalysts.² Glass waste can provide a cost-effective, accessible, low-toxic, recoverable, and reusable support for the heterogenization of sulfuric acid, with the process performed through the reaction of glass waste powder with chlorosulfonic acid at room temperature, which gave the n-glass-waste-supported sulfonic acid catalyst (n-glass-waste-SO₃H). The concise route for preparation of the n-glass-waste-SO₃H is presented in Scheme 1. The attractive feature of the reaction, thanks to the rapid rise of HCl gas from the reaction vessel, is that it's an easy and clean procedure. One of the key merits of this work is the simple catalyst preparation by using inexpensive and easily accessible materials to help produce a heterogeneous, stable, solid acid catalyst. The “inorganic solid acid catalysts” were fully characterized via FT-IR, XRD, TGA, EDX, FE-SEM, TEM, BET, BJH, and the Hammett acidity function method. The loading of SO₃H per g was calculated by pH analysis, and was found to be 2.64 mmol g⁻¹ of n-glass-waste-SO₃H. After the initial characterization of the catalyst, its catalytic performance was investigated in a multicomponent reaction for the one-pot synthesis of some heterocyclic compounds, such as 3,4-dihydropyrimidin-2(1H)-ones and xanthene derivatives (Schemes 2 and 3).

Scheme 1 Preparation of the nano-glass-waste-supported sulfonic acid.

Scheme 2 Synthesis of 3,4-dihydropyrimidin-2(1H)-ones under optimal conditions.

Scheme 3 Synthesis of 1,8-dioxooctahydroxanthene derivatives under optimal conditions.

2.1. Characterization of n-GW-SA

2.1.1. pH analysis of the catalyst. The determination of n-GW-SA acid capacities was carried out by means of acid–base potentiometric titration of an aqueous suspension of a weighed amount of the thoroughly washed catalyst with standard NaOH solution. To this end, 100 mg of n-GW-SA was initially dispersed in 20 ml H₂O through sonication for 60 min. The acid amount on the catalyst was neutralized by the addition of standard NaOH solution (0.08 N) to the equivalence point of titration. The required volume of NaOH to this point was 3.30 ml. The optimum concentration of H⁺ sites was 2.64 mmol g⁻¹ of catalyst (values calculated according to the weight of n-GW-SA) at 25 °C. The concentrations of the residual H⁺ on the recovered catalyst, measured when consecutive experiments were performed, showed a very small or marginal loss of H⁺. This information confirmed that the SO₃H moieties were tightly anchored to the glass base, probably through a covalent linkage.

2.1.2. Surface acidity of the catalyst. The Hammett acidity function (H₀) was utilized as an effective way to determine the acid strength of n-GW-SA.³⁹ It was calculated using the following equation:

H₀ = pK(I)_aq + log([I]_s/[IH⁺]_s)

where, pK(I)_aq is the pK_a value of an aqueous solution of the indicator, and [IH⁺]_s and [I]_s are the molar concentrations of the protonated and unprotonated forms of the indicator in the solvent, respectively. Based on the Beer–Lambert law, the value of [I]_s/[IH⁺]_s can be determined and calculated through the UV-visible spectrum. In this work, we chose 4-nitroaniline (pK(I)_aq = 0.99) as the basic indicator, and CCl₄ as the solvent. As can be observed from Fig. 1, the maximal absorbance of the unprotonated form of the indicator was observed at 331 nm in CCl₄. As Fig. 1 shows, the absorbance of the unprotonated form of the indicator in n-GW-SA was weakened in comparison with the sample of the indicator in CCl₄, indicating that the indicator was partially in the form of [IH⁺]. The full results obtained are listed in Table 1, also showing the acidity strength of n-GW-SA (Table 1).

Fig. 1 Absorption spectra of: (a) 4-nitroaniline (indicator), (b) n-GW-SA (catalyst) in CCl₄.

Table 1 Hammett acidity function (H₀) data for n-GW-SA^a

Entry	Catalyst	Amax	[I]s (%)	[IH^+]s (%)	H0
a Conditions for the UV-visible spectrum measurements: solvent, CCl4; indicator, 4-nitroaniline (pK(I)aq = 0.99), 1.44 × 10^−4 mol l^−1; catalyst, n-GW-SA (20 mg), 25 °C.
1	—	2.74	100	0	—
2	n-GW-SA	0.40	14.37	85.62	1.05

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2.1.3. FT-IR spectral analysis of the catalyst. The infrared spectra of the n-glass-waste (n-GW) and n-GW-SA are shown in Fig. 2. In the case of n-GW, the peaks at 3460 and 1623 cm⁻¹ are, respectively, attributed to the stretching and bending modes of the SiOH groups and the adsorbed water. The strong peaks at 1066, 775, and 468 cm⁻¹ are assigned to the asymmetric stretching, symmetric stretching, and bending modes of SiO₂, respectively.⁴⁰ The n-GW-SA was also characterized. In the IR spectrum of n-GW-SA, the new bands at 1170, 1176, 576, 887, and 1008 cm⁻¹ correspond to the symmetric, asymmetric O=S=O, and S–O stretching vibration of the sulfonic groups (–SO₃H), respectively. The spectrum also shows a relatively broad band around 2800 to 3700 cm⁻¹, due to OH stretching absorption of the SO₃H group.⁴¹ All these observations confirm that the sulfonic groups have functionalized the surface of the n-glass-waste.

Fig. 2 The FT-IR spectra of n-glass-waste and n-GW-SA.

2.1.4. Thermogravimetric analysis. The thermal stabilities of n-glass-waste and n-GW-SA were determined by thermogravimetric analysis (TGA). As shown in Fig. 3, n-glass-waste (the catalyst support) displayed a mass loss below 100 °C, corresponding to the loss of the physically adsorbed water (4 wt%), and then had a steady weight loss (about 2 wt%) below 600 °C, which is possibly attributed to dehydroxylation of the bare n-glass-waste.⁴² The n-GW-SA exhibited a completely different TGA analysis result from n-glass-waste, comprising a four-stage decomposition. The first weight loss (6 wt%) below 110 °C can be attributed to water desorption, while the second mass loss of 11 wt%, which occurred in the range of 110–220 °C, can be ascribed to the slow mass loss of SO₃H groups. The third loss of weight (about 10 wt%) started between 220 °C and 270 °C, and is related to the sudden mass loss of covalently bound SO₃H groups.²⁰ Further mass losses occurred at higher temperature, resulting from dehydroxylation of the n-glass-waste.⁴² Therefore, as the catalyst was stable up to 220 °C, it could be safely used in organic reactions at temperatures up to 210 °C.

Fig. 3 The TGA curve of n-glass-waste and n-GW-SA.

2.1.5. X-ray diffraction analysis. Fig. 4a and b show the powder X-ray diffraction (XRD) patterns for the n-glass-waste and n-GW-SA, respectively. As can be seen in the n-glass-waste pattern, the broad peak around 2θ equal to 27° clearly indicates that n-glass-waste was mainly in the amorphous form.⁴³ It was found that with addition of the SO₃H function to the material, there was no considerable change in the patterns' form. However, when adding the SO₃H function to the material, there was a reduction in the crystalline phase (in Fig. 4b) in comparison with the counts amounts in the vertical axes in Fig. 4a. From the XRD patterns, it can be understood that after modifying n-GW, the structure of the obtained materials remained almost intact.

Fig. 4 The XRD patterns of n-glass-waste (a) and n-GW-SA (b).

2.1.6. Energy-dispersive X-ray spectroscopy (EDX). The energy-dispersive X-ray (EDX) spectra of the n-glass-waste and n-GW-SA present the elemental composition (Fig. 5a and b). Fig. 5a shows the presence of O, Si, Fe, Na, Ca, and Ti in the bare n-glass-waste structure. However, the EDX spectrum of n-GW-SA indicated the appearance of a sulfur peak in the chemical structure of the synthesized materials (Fig. 5b). However, the acid treatment caused changes in the chemical and physical properties of the n-glass-waste similar to those reported for clay.⁴⁴ It can be observed from the EDX spectrum of n-GW-SA (Fig. 5b) that Fe, Ca, and Ti are eliminated from the n-glass-waste structure with modifying the surface of the n-glass-waste material.

Fig. 5 EDX spectra of n-glass-waste (a) and n-GW-SA (b).

2.1.7. BET and BJH texture analysis of the synthesized catalyst. The n-GW-SA catalyst was characterized for its surface area, pore volume, and pore size with the aid of the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) equations. The BET surface area was evaluated on a Beckman Coulter SA3100 Surface Area Analyzer. Prior to the N₂-physical adsorption measurements, the samples were degassed at 150 °C for 120 min in a nitrogen atmosphere. The specific surface area (SBET) of the as-obtained catalysts was determined with adsorption–desorption isotherms of N₂ at 77 K. The obtained results related to the textural properties of the samples (in terms of surface area, pore width, desorption pore diameter, and desorption surface area) are tabulated in Tables 2 and 3. The surface area and average pore diameter of the bare n-GW and n-GW-SA are shown in Table 2. It can be seen from Table 2 that the average surface areas are about 0.8 m² g⁻¹ and 2.9 m² g⁻¹ for n-GW and n-GW-SA, respectively. Also, Table 3 displays the textural properties of the as-prepared materials. The data summarized in Table 3 shows that the specific surface area of pores in n-GW-SA are larger than those for pure n-GW, but that the pore width of n-GW-SA is smaller than that of bare n-GW. As a result, the investigated results of the BET and BJH measurements suggest that with modifying the material, the surface area in n-GW-SA is greater than that of the pure n-GW, and the modified material had a smaller pore size than that of the bare one.

Table 2 BET data showing the textural properties of n-GW and n-GW-SA

Sample	BET surface area (m^2 g^−1)	Pore width (nm)
n-GW	0.8	4.2
n-GW-SA	2.9	3.6

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Table 3 BJH data showing the textural properties of n-GW and n-GW-SA

Sample	n-GW	n-GW-SA
Desorption pore diameter	9.52	6 (nm)
Desorption surface area	0.08	0.7 (m^2 g^−1)

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2.2. Morphology analyses

2.2.1. Field emission scanning electron microscopy. Fig. 6a–d show FE-SEM images for the n-glass-waste and n-GW-SA. It can be seen in Fig. 6a–c that there are some particles on the surface of the glass materials. It is clear that the particle sizes are not homogenous, and indeed their sizes are about 50 nm to 2 μm. However, it can be observed from Fig. 6d that with modifying the glass material with the –SO₃H function, the morphology of the obtained material was changed considerably. It is clear that the morphology of the obtained material is a sponge-like structure with a nearly homogeneous material size and morphology.

Fig. 6 FE-SEM images of: n-glass-waste (a–c) and n-GW-SA (d).

2.2.2. Transmission electron microscopy (TEM). Fig. 7a–c show the transmission electron microscopy (TEM) images as well as the particle size distribution histogram of n-GW-SA. As shown in Fig. 7a and b, the materials consist of nearly homogeneous, narrow-size-distributed, spherical particles. The particle size distribution from the TEM image shows that 55% of the NPs are in the range of 20–30 nm and that the mean diameter of the observable NPs is 25.3 nm.

Fig. 7 TEM images of n-GW-SA (a and b) and the particle size distribution histogram (c).

2.3. Catalytic performance of n-GW-SA in the synthesis of 3,4-dihydropyrimidin-2(1H)-ones and 1,8-dioxooctahydroxanthene derivatives

In order to investigate the catalytic activity of the as-prepared catalyst in multicomponent reaction conditions, we decided to employ n-GW-SA in the reaction media for the one-pot synthesis of various 3,4-dihydropyrimidin-2(1H)-ones and 1,8-dioxooctahydroxanthene derivatives. Therefore, n-GW-SA was applied in a mixture of aromatic aldehydes, methyl/ethyl acetoacetate, and urea/thiourea as well as in the reaction mixture of 5,5-dimethyl-1,3-cyclohexanedione and aromatic aldehydes to provide 3,4-dihydropyrimidin-2(1H)-ones and 1,8-dioxooctahydroxanthene derivatives, respectively.

2.3.1. Optimization of the reaction parameters for the one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones. First, we carried out the reaction of methyl acetoacetate (1.0 mmol), aromatic aldehyde (1.0 mmol), and urea (2 mmol) at 100 °C in solvent-free conditions, which resulted in a negligible amount of product (Table 4, entry 1), indicating the fact that the application of the catalyst in the reaction plays a vital role. Therefore, the reaction was performed again but in the presence of 0.01 g of unfunctionalized (bare) glass waste powder (without the sulfonic acid group appended) as a catalyst, which provided low yields of the desired product. In the next step, for the optimization of the reaction conditions, we applied different amounts of n-GW-SA in the reaction media. When 5 mol%, 10 mol%, 15%, and 20 mol% of n-GW-SA were used, the yields were 85%, 96%, 97%, and 90%, respectively (Table 4, entries 2–5). Therefore, 10 mol% of n-GW-SA was adequate for the conversion. With an optimized catalyst loading and by taking into account that the temperature has a prominent role in promoting the reaction efficiency, the reaction was carried out in the temperature range from 60 °C to 120 °C in solvent-free conditions using 10 mol% of n-GW-SA as the catalyst (Table 4, entries 3, 6–8). It was observed that increasing the reaction temperature up to 100 °C resulted in enhanced product yields, but further increases in the temperature had the opposite effect. This phenomenon might be ascribed to the formation of some side products. Consequently, the optimum temperature for performing the reaction was 100 °C (Table 4, entry 3). In the following studies, the above reaction was tested in the presence of 10 mol% of n-GW-SA with various solvents, such as H₂O, CH₃CN, EtOH, MeOH, and CH₂Cl₂. The results indicated that different solvents affected the efficiency of the reaction, in that EtOH and MeOH rendered good efficiency (Table 4, entries 12 and 13) but not CH₃CN, CH₂Cl₂, and H₂O (Table 4, entries 9–11). Additionally, the reaction carried out under solvent-free conditions generated the corresponding product in excellent yields in a short reaction time compared with the reaction carried out in solvent conditions (Table 4, entry 3). Based on the reaction yields and environmental considerations, we chose solvent-free conditions in line with the principles of “green chemistry”. As a result, our preferred reaction conditions were determined to be solvent-free, 100 °C, and 10 mol% of the catalyst (Table 4, entry 3). With the established optimal reaction conditions, to study the generality of the procedure, the scope of the Biginelli reaction was probed with various aldehydes, β-ketoester, and urea or thiourea, and a library of substituted 3,4-dihydropyrimidin-2(1H)-ones adducts was obtained in good to excellent yields in appropriate times under solvent-free conditions (Table 5). Both electron-withdrawing as well as electron-donating substituents on the aldehyde aryl ring were tolerated and reacted with the methyl acetoacetate and urea under the optimized conditions. Here, m-, p- and o-nitrobenzaldehydes as well as o- and p-chloro and p-fluorobenzaldehyde successfully produced the desired products in similarly excellent yields (Table 5, entries 2–7), which indicated that the position of the electron-withdrawing substituent had no significant effect on the yields of the 3,4-dihydropyrimidin-2(1H)-ones adducts. A similar behavior was observed with the electron-releasing groups. p-methyl (Table 5, entries 18, 14, 24), o- and p-methoxy, p-hydroxy, 3-ethoxy-4-hydroxy 3,4-dimethoxy (Table 5, entries 8–13), p-isopropyl, and dimethylamino (Table 5, entries 15 and 16) benzaldehydes produced the expected DHPMs in good quantities. It was also found that the use of ethyl acetoacetate instead of methyl acetoacetate in the three-component reaction gave the corresponding products in excellent yields (Table 5, entries 16–21).

Table 4 Optimization of the reaction condition for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones^a

Entry	Solvent	Condition	Catalyst [mmol]	Time [h [min]]	Yield [%]
a Reaction conditions: methyl acetoacetate (1 mmol), benzaldehyde (1 mmol), and urea (2 mmol).
1	Solvent-free	100 °C	—	3 [30]	70
2	Solvent-free	100 °C	0.05	[35]	85
3	Solvent-free	100 °C	0.1	[20]	96
4	Solvent-free	100 °C	0.15	[15]	97
5	Solvent-free	100 °C	0.2	[25]	90
6	Solvent-free	120 °C	0.1	[30]	93
7	Solvent-free	80 °C	0.1	[60]	87
8	Solvent-free	60 °C	0.1	2	79
9	H2O	Reflux	0.1	[20]	26
10	CH3CN	Reflux	0.1	[20]	70
11	CH2Cl2	Reflux	0.1	[20]	51
12	EtOH	Reflux	0.1	[20]	88
13	MeOH	Reflux	0.1	[20]	79

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Table 5 n-GW-SA-catalyzed one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones^a

Entry	Aldehyde	R	X	T [min]	Yield^b [%]	Mp [°C]	Lit. Mp [°C] ref.	Repeatability (%)		Reproducibility (%)
								Intra-day	Inter-day
a Reaction conditions: β-ketoester (1 mmol), aromatic aldehyde (1 mmol), urea/thiourea (2 mmol), and n-GW-SA (0.1 mmol), solvent-free, 100 °C.b Yields refer to isolated products.
1	Benzaldehyde	–OEt	O	12	98	207–208	206–208 (ref. 45)	2.8	3.0	3.3
2	4-Chlorobenzaldehyde	–OEt	O	30	95	215–216	214–215 (ref. 46)	3.1	3.1	3.2
3	2-Chlorobenzaldehyde	–OEt	O	25	97	214–216	216–218 (ref. 47)	3.0	3.2	3.4
4	4-Fluorobenzaldehyde	–OEt	O	15	99	181–183	182–184 (ref. 48)	3.1	3.2	3.3
5	3-Nitrobenzaldehyde	–OEt	O	15	93	230–232	229–231 (ref. 47)	2.9	3.1	3.5
6	4-Nitrobenzaldehyde	–OEt	O	20	95	209–211	208–211 (ref. 49)	3.3	3.4	3.3
7	2-Nitrobenzaldehyde	–OEt	O	25	94	220–222	221 (ref. 50)	3.1	3.3	3.6
8	2-Methoxybenzaldehyde	–OEt	O	20	99	262–263	262 (ref. 51)	2.9	3.2	3.5
9	4-Hydroxybenzaldehyde	–OEt	O	30	96	229–231	228 (ref. 52)	3.0	3.3	3.3
10	4-Methoxybenzaldehyde	–OEt	O	25	94	202–204	201–203 (ref. 53)	3.2	3.4	3.4
11	4-Methylbenzaldehyde	–OEt	O	35	91	203–204	205–206 (ref. 45)	3.1	3.2	3.2
12	3-Ethoxy-4-hydroxybenzaldehyde	–OEt	O	40	95	232–234	232–233 (ref. 54)	3.2	3.5	3.4
13	3,4-Dimethoxybenzaldehyde	–OEt	O	30	88	175–177	178 (ref. 55)	3.3	3.5	3.3
14	4-Isopropylbenzaldehyde	–OEt	O	35	92	193–195	194 (ref. 56)	3.1	3.3	3.5
15	4-N,N-Dimethylbenzaldehyde	–OEt	O	45	82	251–253	251–252 (ref. 57)	3.4	3.5	3.6
16	Benzaldehyde	–OMe	O	20	96	207–208	207–210 (ref. 46)	2.9	3.2	3.3
17	4-Chlorobenzaldehyde	–OMe	O	40	93	203–204	204–207 (ref. 53)	3.0	3.3	3.2
18	4-Methylbenzaldehyde	–OMe	O	35	91	214–215	214–215 (ref. 58)	3.2	3.5	3.4
19	3-Ethoxy-4-hydroxybenzaldehyde	–OMe	O	40	93	254–256	253–254 (ref. 59)	3.0	3.4	3.4
20	Benzaldehyde	–OEt	S	25	93	209	208–210 (ref. 47)	3.2	3.3	3.5
21	4-Chlorobenzaldehyde	–OEt	S	25	90	179–180	180–182 (ref. 47)	3.3	3.5	3.6
22	4-Methylbenzaldehyde	–OEt	S	25	91	191–193	192–194 (ref. 47)	3.1	3.4	3.6
23	Benzaldehyde	–OMe	S	20	96	222–224	222–224 (ref. 60)	3.0	3.3	3.2
24	4-Fluorobenzaldehyde	–OMe	S	30	93	191–193	190–192 (ref. 61)	3.2	3.5	3.4
25	4-Methylbenzaldehyde	–OMe	S	20	92	155–156	156–158 (ref. 60)	3.3	3.6	3.5

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2.3.2. Optimization of the reaction parameters for the one-pot synthesis of 1,8-dioxooctahydroxanthene derivatives. In order to optimize the reaction conditions, initial experiments were performed between 5,5-dimethyl-1,3-cyclohexanedione (2 mmol) and aromatic aldehyde (1 mmol) without the addition of a catalyst at 110 °C under solvent-free conditions; however, the obtained results were not satisfactory (Table 6, entry 1). Further results were obtained using bare glass powder (0.01 g) as a catalyst in the same reaction conditions, in which a very poor yield of product was generated even in a prolonged time, indicating the weak effect of the lone glass base in the reaction conditions. Therefore, we decided to set the above reaction in the presence of various amounts of n-GW-SA catalyst in solvent-free reaction conditions at 110 °C (Table 6, entries 2–5). According to the results, 10 mol% of the catalyst was sufficient for the excellent changeover to the desired products (Table 6, entry 3). Following these observations, the reactions were designed in the conditions in which different temperatures are applied (ranging from 100 °C to 130 °C and r.t.), because the temperature can change the outcome of the reactions in terms of time and efficiency (Table 6, entries 3, 6–9). Based on the obtained data, it was found that with the rise in the temperature to 130 °C, a reduction in the reaction time was observed to 5 minutes, but the product yields were similar to those obtained at 120 °C; therefore, the best temperature for the reaction chosen was 120 °C (Table 6, entry 6). Additionally, as depicted in Table 3, the reactions were investigated in the presence of some solvents, such as EtOH, MeOH, H₂O, CH₃CN, and CH₂Cl₂ (Table 6, entries 10–14) in addition to the solvent-free reaction conditions. The results showed that different solvents affected the efficiency of the reaction; however, the best results (in terms of time and yields) were related to the reaction under solvent-free conditions. So, we chose solvent-free conditions as the key feature in designing a sustainable protocol. As a consequence, the optimal reaction condition we proposed was: solvent-free, at 120 °C, with 10 mol% of the catalyst (Table 6, entry 6). In the next stage, for the evaluation of the scope and limitations of this methodology, we extended our studies to various aldehydes, including aldehydes with electron-releasing substituents, electron-withdrawing substituents, and halogens on the aromatic ring. The results are summarized in Table 7. As indicated in Table 4, the reaction proceeded smoothly within 5–25 min. The main point that is worth mentioning here is the fact that the aldehydes bearing electron-withdrawing substituents on the ring compared to those with electron-releasing groups possessed the higher reaction rate (Table 7, entries 2–8), which can be ascribed to the more activated carbonyl group as an electrophile center present in the ring-bearing electron-withdrawing groups. In the opposite manner, for aldehydes with electron-donating groups on the ring, the reaction rate was slower; therefore, the reaction times were longer (Table 7, entries 9–17).

Table 6 Optimization of the reaction conditions for the synthesis of 1,8-dioxooctahydroxanthene derivatives^a

Entry	Solvent	Condition	Catalyst [mmol]	Time [h [min]]	Yield [%]
a Reaction conditions: 5,5-dimethyl-1,3-cyclohexanedione (2 mmol), and benzaldehyde (1 mmol).
1	Solvent-free	110 °C	—	4	70
2	Solvent-free	110 °C	0.05	[25]	92
3	Solvent-free	110 °C	0.1	[15]	98
4	Solvent-free	110 °C	0.15	[30]	90
5	Solvent-free	110 °C	0.2	[35]	88
6	Solvent-free	120 °C	0.1	[7]	97
7	Solvent-free	130 °C	0.1	[5]	98
8	Solvent-free	100 °C	0.1	[35]	85
9	Solvent-free	60 °C	0.1	2 [45]	72
10	H2O	Reflux	0.1	[35]	20
11	CH3CN	Reflux	0.1	[35]	48
12	CH2Cl2	Reflux	0.1	[35]	30
13	EtOH	Reflux	0.1	[35]	60
14	MeOH	Reflux	0.1	[35]	55

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Table 7 n-GW-SA-catalyzed one-pot synthesis of 1,8-dioxooctahydroxanthene derivatives^a

Entry	Aldehyde	T [min]	Yield^b [%]	Mp [°C]	Lit. Mp [°C] ref.	Repeatability (%)		Reproducibility (%)
						Intra-day	Inter-day
a Reaction conditions: 5,5-dimethyl-1,3-cyclohexanedione (2 mmol), aromatic aldehyde (1 mmol), and n-GW-SA (0.1 mmol), solvent-free, 120 °C.b Yields refer to isolated products.
1	Benzaldehyde	7	97	204–205	203–204 (ref. 62)	3.0	3.3	3.0
2	4-Chlorobenzaldehyde	5	94	231–233	230–232 (ref. 62)	3.3	3.4	3.3
3	2-Chlorobenzaldehyde	12	92	225–226	225–227 (ref. 62)	3.3	3.5	3.3
4	4-Fluorobenzaldehyde	5	95	224–226	226–227 (ref. 63)	3.2	3.3	3.2
5	3-Nitrobenzaldehyde	7	95	170–171	169–170 (ref. 62)	3.5	3.5	3.5
6	4-Nitrobenzaldehyde	4	96	226–228	222–224 (ref. 64)	3.1	3.4	3.1
7	2-Nitrobenzaldehyde	10	93	252–254	251–253 (ref. 65)	3.3	3.5	3.3
8	4-Bromobenzaldehyde	20	86	239–241	240–242 (ref. 62)	3.4	3.5	3.4
9	2-Methoxybenzaldehyde	14	90	209–210	209–210 (ref. 66)	3.2	3.3	3.2
10	4-Methoxybenzaldehyde	15	91	242–243	243–245 (ref. 66)	3.1	3.4	3.1
11	2-Hydroxybenzaldehyde	15	88	203–205	202–205 (ref. 67)	3.5	3.6	3.5
12	3-Hydroxybenzaldehyde	25	90	256–258	255–257 (ref. 68)	3.4	3.3	3.4
13	4-Hydroxybenzaldehyde	10	93	247–248	248–250 (ref. 67)	3.1	3.3	3.1
14	4-Methylbenzaldehyde	10	91	215–217	216–218 (ref. 69)	3.3	3.6	3.3
15	3,4-Dimethoxybenzaldehyde	25	88	176–178	175–176 (ref. 70)	3.4	3.5	3.4
16	4-Isopropylbenzaldehyde	25	92	170–173	170–172 (ref. 71)	3.2	3.1	3.2
17	4-N,N-Dimethylbenzaldehyde	13	93	222–224	221–223 (ref. 66)	3.4	3.3	3.4

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2.3.3. Precision of the synthesis methods. The precision of the proposed method was determined by studying the repeatability (intra-day and inter-day precisions) and reproducibility. Intra-day precision was evaluated by synthesizing five replicates of the product in the same day under the same conditions (including the same laboratory, apparatus, reagents, temperature, and interval of time), at their optimal amounts; whereas, inter-day precision was calculated during five consequent days using three replicates of the synthesis process at the same optimal conditions. Reproducibility was calculated during five consequent days using three replicates of the synthesis process in two different laboratories, at the same optimal conditions. The results of the precision study (in terms of R.S.D.) are depicted in Tables 5–7, which show that the methods were reliable and reproducible.

Next we set out to draw a comparison between n-GW-SA and the other catalysts, as reported in the literature, for the synthesis of 1,8-dioxooctahydroxanthene (Table 9) and 3,4-dihydropyrimidin-2(1H)-ones (Table 8). The n-GW-SA catalyst has merits over most of the other catalysts in terms of higher yields, shorter reaction time, lower reaction temperature, and not using a volatile solvent. According to these findings, it was found that n-GW-SA is a very efficient catalyst and is useful in the synthesis of 3,4-dihydropyrimidin-2(1H)-ones as well as for 1,8-dioxooctahydroxanthene derivatives.

Table 8 Comparison of the efficiency of n-GW-SA with different catalysts in the synthesis of 3,4-dihydropyrimidin-2(1H)-ones

Entry	Catalyst	Condition	Time [h [min]]	Yield [%]	Ref.
a Nano-preyssler HPA on NiFe2O4@SiO2.b Nano-silica phosphoric acid.
1	PC8-PEG-SO3H	Dioxane + isopropanol/80 °C	10	80	72
2	Silica sulfuric acid	Reflux in EtOH	6	91	54
3	CuI	Reflux in H2O	4	89	73
4	Zeolite	Reflux in toluene	12	80	74
5	SiO2·KAl(SO4)2·12H2O	Solvent-free/80 °C	4	92	75
6	H3PMo12O40	Reflux in acetic acid	5	80	76
7	Bf3·OEt2/Cu(OAc)2	Acetic acid/65 °C	18	71	77
8	Yb(III)-resin	Solvent-free/120 °C	48	80	78
9	n-Fe3O4@SBA-15	Reflux in EtOH/90 °C	6	85	79
10	n-YALO3:EU^3+	Reflux in EtOH/80 °C	1 [30]	86	80
11	n-NFS-PRS^a	Reflux in EtOH/78 °C	[30]	93	81
12	n-SPA^b	Solvent-free/80 °C	2	91	82
13	Et-PMO-Me-PrSO3H	Solvent-free/90 °C	2 [25]	90	61
14	n-GW-SA	Solvent-free/100 °C	[20]	96	Present work

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Table 9 Comparison of the efficiency of n-GW-SA to different catalysts in the synthesis 1,8-dioxooctahydroxanthene derivatives

Entry	Catalyst	Condition	Time [h [min]]	Yield [%]	Ref.
a Phosphomolybdic acid supported on silica.b Polyphosphoric acid supported on silica.c Nano-silica-supported preyssler.d Nano-WO3-supported sulfonic acid.
1	PMA^a/SiO2	Reflux in CH3CN	5	95	83
2	InCl3·4H2O	[bmim][BF4]/80 °C	4	87	84
3	Silica sulfuric acid	Neat/80 °C	1 [30]	82	85
4	PPA^b/SiO2	Neat/140 °C	10–12	54–82	86
5	Amberlyst-15	Reflux in CH3CN	5	92	87
6	Fe^3+/montmorillonite	EtOH/100 °C	6	94	88
7	DOWEX-50w ion exchange resin	Solvent-free/100 °C	1.5	78	89
8	H2SO4	H2O/80 °C	3	90	90
9	NaHSO4–SiO2	Reflux in CH3CN	6 [30]	90	91
10	n-SPNP^c	Reflux in water	3	93	92
11	n-ZnS	Solvent-free/90 °C	[18]	90	93
12	n-ZnO	Reflux in EtOH	1 [30]	94	94
13	Fe3O4@SiO2-imid-PMA^n	Reflux in EtOH	1 [30]	94	95
14	n-WSA^d	Solvent-free/100 °C	1	92	96
15	n-GW-SA	Solvent-free/120 °C	[7]	97	Present work

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2.4. Possible mechanisms for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones and xanthene derivatives catalyzed by n-GW-SA

According to the mechanism suggested by Kappe,⁹⁷ we present here a plausible mechanistic pathway for the formation of (3) using n-GW-SA, as presented in Scheme 4. Here, protonation of the carbonyl group (in benzaldehyde) by the Brønsted acid site on the catalyst generates an electrophilic center on the carbonyl carbon atom, which is easily attacked by the urea/thiourea to form an acyl imine intermediate (1). The intermediate (1) is intercepted by β-dicarbonyl to produce an open chain ureide (2), which subsequently cyclizes through a dehydration process, affording compounds (3).

Scheme 4 Proposed mechanism for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones.

Considering the general mechanistic pathway, the plausible mechanism for the synthesis of 1,8-dioxooctahydroxanthenes using n-GW-SA as the catalyst is exhibited in Scheme 5. In the suggested mechanism, at first, the carbonyl group of the aromatic aldehyde is activated by the –SO₃H groups of n-GW-SA to give intermediate (2). Subsequently, 1,3-cyclicdiketone attacks the activated aldehyde to afford intermediate (3). After removing H₂O from intermediate (3), (4) is prepared as a Michael acceptor. Once again, the SO₃H group of n-GW-SA activates intermediate (4). Then, the Michael addition of dimedone with intermediate (4) affords (5). Intermediate (5) changes to form (6) subsequent to the ring-closing reaction and removing the catalyst. Eventually, the 1,8-dioxooctahydroxanthene derivative (7) is produced through removing H₂O from compound (6). The proposed mechanism clearly illustrates the catalytic role of the n-GW-SA catalyst.

Scheme 5 Proposed mechanism for the synthesis of 1,8-dioxooctahydroxanthene derivatives.

2.5. Reusability of the catalyst

When the activity and versatility of the n-GW-SA for different reactions were established, its recyclability and reusability, in the model aforementioned reaction conditions, were then examined. After separation of the catalyst from the reaction mixture by filtration, it was washed with ethanol and water. In the final step, it was dried at 110 °C for 2 h before another cycle of the reaction. For the recycling experiment between fresh reactant under the same reaction conditions, the recycled catalyst could be reused for at least five consecutive runs without a significant decrease in the reaction yield (Fig. 8). After successive experiments, the concentrations of the residual H⁺ on the recovered catalyst were measured, which showed a very small or only marginal loss of H⁺. This clearly demonstrated that the catalyst was stable after each run and that the SO₃H group was tightly anchored to the glass support, probably by a covalent linkage.

Fig. 8 Recyclability of n-GW-SA.

3. Experimental section

3.1. Instrumentation, analysis, and starting materials

All the chemicals were purchased from Merck and Aldrich companies and used without any further purification. Glass waste was from Istac (Iran). Glass powder was from a two-cup planetary ball mill. All the yields refer to the isolated products after purification. The products were characterized by their physical constants and by comparison with authentic samples. The purity of the products was checked by thin layer chromatography (TLC) on glass plates coated with silica gel 60 F254 using an n-hexane/ethyl acetate mixture as the mobile phase. Melting points were determined in open capillaries using an Electrothermal 9100 without further corrections. Fourier transform infrared spectroscopy (FT-IR) was recorded on a Shimadzu 8400s spectrometer using KBr pressed powder disks. The NMR spectra were measured with a Bruker Avance 300 spectrometer (¹H NMR 300 MHz and ¹³C NMR 75 MHz) in pure deuterated chloroform with tetramethylsilane (TMS) as the internal standard. Thermogravimetric analyses (TGAs) were carried out on a Du Pont 2000 thermal analysis apparatus at a heating rate of 5 °C min⁻¹ under an air atmosphere. X-ray diffraction (XRD) was detected by a Philips instrument using Cu-Kα radiation of wavelength 1.54 Å. The BET surface area was acquired on a Beckman Coulter SA3100 surface area analyzer. TEM analyses were performed through a transmission electron microscope (TEM; Philips – CM300, 150 kV). The average particle size distribution was determined using Image software. Field emission scanning electron microscopy (FE-SEM) images were acquired using a Zeiss field emission scanning electron microscope (Sigma, Germany) instrument operating at 15 kV, equipped with an Oxford X-ray detector (EDX; Oxford Instruments, Oxford, UK). The presented UV-vis spectra were obtained as carbon tetrachloride solutions (10⁻⁵ M) on a Shimadzu UV-1650PC spectrophotometer.

3.2. Preparation and purification of the glass waste powder

At first, all the glass wastes were washed out to remove the marks on their surfaces and to clean their insides. Then, all the organic compounds were washed away by 200 ml methanol and ethanol. After crushing them into fine powder, 20 g of the powder was carefully washed with an excess amount of distilled water three times. Subsequently, the precipitated glass powder was filtered and rinsed with an excess amount of water and then dried at 120 °C.

3.3. Preparation of n-GW-SA

A two-neck round-bottomed flask, equipped with a constant pressure dropping funnel and a gas inlet tube for conducting HCl gas over an adsorbing solution (i.e., water), was charged with 1.0 g of glass powder in dry CH₂Cl₂ (20 ml). Subsequently, chlorosulfonic acid (0.3856 g, 0.0033 mol) was added dropwise over a period of 15 min at room temperature. While the powder was stirred, HCl gas started to evolve from the reaction vessel immediately. After the addition was completed, the mixture was allowed to stir for 1 h at room temperature, while the residual HCl was eliminated by suction.²⁵ Then, the as-obtained solid powder was washed with water (10 ml) and dried at 80 °C. In the final step, solid material, weighing 1.35 g, was collected as a solid acid, and named n-GW-SA for use in the organic reactions.

3.4. Application of the n-GW-SA catalyst for the multicomponent one-pot synthesis of heterocyclic compounds

3.4.1. General procedure for the synthesis of 3,4-dihydropyrimidin-2(1H)-ones. A mixture of aromatic aldehyde (1.0 mmol), methyl acetoacetate (1.0 mmol), and urea (2 mmol) was treated at 100 °C utilizing n-GW-SA (0.03 g, 10 mol% with respect to benzaldehyde) in solvent-free conditions for an appropriate time (Table 2). The reaction was monitored by TLC [7 : 3 hexane : acetone]. After completion of the reaction, the mixture was diluted with hot ethanol and the reaction mixture was separated from the catalyst through centrifugation and filtration (using Whatman 41 filter paper). Finally, the organic solvent was evaporated under reduced pressure and the resulting solid product was then crystallized from hot ethanol.

3.4.2. General procedure for the synthesis of 1,8-dioxooctahydroxanthene derivatives. A mixture of 5,5-dimethyl-1,3-cyclohexanedione (2 mmol), aldehyde (1 mmol), and n-GW-SA (0.03 g, 10 mol%) was reacted at 120 °C in solvent-free conditions for the appropriate time (Table 7). During the procedure, the reaction was monitored by TLC. When the reaction was completed, the reaction mixture was cooled to room temperature and diluted with hot ethanol (5 ml). For the separation of the catalyst, the obtained solution was first centrifuged, and then filtrated under hot conditions. Finally, the solvent was evaporated and the crude product was recrystallized from ethanol to afford the corresponding pure product. The remaining reactions were performed following this general procedure and the physical data (Mp, IR, and NMR) of all the known compounds were identical with those reported in the literature.

4. Nature and composition of the glass used

Glass produced from the vitrification of pure SiO₂ possesses the characteristics of a high melting temperature and viscosity, which make it difficult to work with. Therefore, it is essential to add other substances to simplify the process. For instance, with the addition of small amounts of iron oxide to the glass ingredients, its melting rate will accelerate. Moreover, when added to other components, calcium oxide (CaO) and aluminum oxide (Al₂O₃) can provide a better chemical stability, which is required especially for the storage of beverages and food. Furthermore, titanium dioxide is used to intensify and brighten the glass bodies. According to the manufacturing formula of the Istac factory, the ingredients for production of colorless waste glass bottles are designed and constructed as a mixture of silicon dioxide (SiO₂), calcium oxide (CaO), aluminum oxide (Al₂O₃), sodium oxide (Na₂O), iron(III) oxide (Fe₂O₃), and titanium dioxide (TiO₂), which can facilitate the above-mentioned advantages.

5. Capital cost estimation

During the project, from the early stages of the glass wastes collection through to catalyst synthesis, approximate cost estimates were prepared in order to establish and ensure the commercial viability of the system. The results are shown below:

Annual catalyst production: for the production of 1 kg catalyst, 11 h is needed (that is for washing the glass wastes, crushing and powder making, washing the powder made, drying the powder, producing the catalyst, washing the catalyst, and drying it). In a year with 2496 working hours, 227 kg of catalyst can be produced.

•Cost of glass waste collection ($ year⁻¹): labor costs for one full-time worker annually are $4114 in Iran.

•Cost of glass waste transportation: for 1 vehicle per 10 containers: 10 × 14.28 = $143 (a 7 m³ container will cost $14.28). For a full-time worker: $3429 year⁻¹.

•Cost of equipment ($ year⁻¹): about $2000 for the mixer, oven, flask, dropping funnel, gas inlet tube.

•Cost of chemicals ($ year⁻¹): specific chemical cost per kg × annual catalyst production = 196 × 227 = $44 492 (for 1 kg of the catalyst, about 3000 ml ethanol, 3000 ml methanol, 20 000 ml CH₂Cl₂, 220 ml chlorosulfonic acid, and 1000 ml distilled water are needed).

•Total water cost ($ year⁻¹): specific water cost per kg × annual catalyst production = 1.7 × 227 = $389 (for 1 kg of the catalyst, about 20 000 ml water is required).

•Cost of electricity: specific electricity cost per kg × annual catalyst production = 11.76 × 227 = $2670.

•Cost of labor ($ year⁻¹): specific labor cost per kg × annual catalyst production = 30 × 227 = $6810.

•Cost of powder making by ball mill: using a ball mill with 10 tonne capacity will cost $143 per tonne.

•Total capital cost ($ year⁻¹): $64 190.

6. Conclusion

A GW-SA catalyst was prepared in nano-size, in order to develop the current important areas of the heterogenization of sulfonic acid on the one hand and the utilization of waste materials for catalytic applications on the other hand. The introduced catalyst was proven to be a highly powerful solid acid catalyst in multicomponent reactions for the simple, efficient, and rapid one-pot synthesis of heterocyclic compounds, such as 3,4-dihydropyrimidin-2(1H)-ones and xanthene derivatives. Moreover, the application of n-GW-SA as a highly stable, easy-to-handle, inexpensive, accessible, retrievable, reusable, and low-toxicity catalyst makes the whole process more economical and industrially important. Besides, using a recyclable catalyst in the reactions in high throughput, low-cost, short times, non-toxicity, solvent-free conditions can be considered as a green process because of fulfilling many criteria of green chemistry.

Acknowledgements

We gratefully acknowledge the Faculty of Chemistry of Semnan University for supporting this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22791k

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