SHORT COMMUNICATION Synthesis of melamine ...

Chemical Papers 68 (7) 983–988 (2014) DOI: 10.2478/s11696-013-0527-1

SHORT COMMUNICATION

Synthesis of melamine–formaldehyde resin functionalised with sulphonic groups and its catalytic activities a

a Zhejiang

Lin-Jun Shao, b Gui-Ying Xing, a Chen-Ze Qi*

Key Laboratory of Alternative Technologies for Fine Chemicals Process, b School of Medicine, Shaoxing University, Zhejiang Province 312000, China Received 24 March 2013; Revised 27 September 2013; Accepted 3 November 2013

Spherical melamine–formaldehyde resin (MFR) particles were synthesised using the condensation reaction of melamine and formaldehyde with PEG-2000 as an additive. After thermal treatment at 200 ◦C and then sulphonation by chlorosulphuric acid, an MFR-supported strong acid catalyst (SMFR) was prepared with an acidity of 3.23 mmol g−1 . This new acid catalyst was evaluated in the reactions of esterification and acetalisation, with the results indicating that this novel acid catalyst was highly efficient in the traditional acid-catalysed reaction. Its high activity, stability and reusability give it great potential for “green” chemical processes. c 2013 Institute of Chemistry, Slovak Academy of Sciences Keywords: solid acid, acidity, acetalisation, esterification

Acid catalysts are very important in the production of various chemicals (Anastas et al., 2002; DeSimone, 2002). In recent decades, over 15 million tons of sulphuric acid were consumed annually as an unrecyclable homogeneous catalyst, leading to serious environmental pollution and increased purification costs. The heterogenisation of active catalysts in the homogeneous phase is an effective way of increasing overall productivity and cost effectiveness. Moreover, heterogenisation can also afford important advantages in handling, separation and recycling procedures (Jothiramalingam & Wang, 2009; Clark, 2002; Sabitha et al., 2010; Adam et al., 2012; Shaterian & Rigi, 2012). Recently, a number of heterogeneous acid catalysts have been developed to replace some of the non-recyclable acids, especially liquids. Specifically, sulphonated carbonaceous materials have received much attention due to their high acidity and catalytic activities (Xiao et al., 2010; Suganuma et al., 2011; Shao et al., 2012a). However, to date, no solid acid catalyst as active, stable and inexpensive as sulphuric acid has been identified. Hence, preparation of an efficient and inexpensive solid acid represents a great challenge for chemists.

Fig. 1. Schematic representation for synthesis of SMFR.

Melamine–formaldehyde resin (MFR), which has excellent physical and chemical stability, is widely used in industry (Brooker & Mungin, 1983; Tutin, 1998; Baytekin, 2012). MFR contains many secondary amine nitrogen atoms which can be used as an immobilisation site for acidic functionalities (Ji et al., 1996; Qiu et al., 2002; Chehardoli et al., 2011; Hang et al., 2011; Rezaei & Karami, 2011). In this study, a novel MFR-supported acid catalyst (SMFR) was synthesised with chlorosulphuric acid as the sulphonating reagent (Fig. 1). The catalytic activities of the SMFR

*Corresponding author, e-mail: [email protected]

Unauthenticated Download Date | 9/24/15 11:19 PM

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L. J. Shao et al./Chemical Papers 68 (7) 983–988 (2014)

Fig. 2. SEM images of MFR without addition of PEG-2000 (a), MFR (b) and SMFR (c, d).

catalyst were evaluated in acetalisation and esterification. All chemicals were commercial products of the highest purity available and were used in the reactions without further purification. Melamine–formaldehyde resin (MFR) was synthesised as follows: Melamine (1,3,5-triazine-2,4,6-triamine, 32.0 g, 0.25 mol) in 19.8 g of a 30 mass % poly(ethylene glycol) (PEG 2000) aqueous solution in the presence of hexamethylenetetramine (1,3,5,7-tetraazatricyclo[3.3.1.13,7] decane, 150 mg) as catalyst and 36 % aqueous solution of formaldehyde (60.0 g, 0.74 mol) were stirred at 90 ◦C for 5 hours. The reaction was quenched by the addition of an HCl aqueous solution (10 mass %) with a final reaction solution pH value of 3–4. After filtration, washing with deionised water and crushing, the MFR was dried for 3 hours at 200 ◦C and isolated with a yield of 66.1 %. The MFR (1.0 g) was added to a 10 mass % chlorosulphuric acid–dichloromethane solution (20 mL) and stirred gently at 10 ◦C for 12 hours. After completion, the reaction solution was filtered and washed with dichloromethane, deionised water, ethanol and dried under reduced pressure at ambient temperature for 12 hours. The yield of SMFR was 75.6 %.

The procedures employed in the SMFR-catalysed acetalisation and esterification were as previously reported (Shao et al., 2012a, 2012b). Quantitative analysis was performed on a Shimadzu (GC-14B) gas chromatograph. FT-IR/ATR spectra were recorded on a FT-IR spectrometer (Nicolet, Nexus-470, USA) equipped with accessories to attenuate total reflection. The morphologies of the samples were characterised using a scanning electron microscope (SEM) (Jeol, Jsm-6360lv, Japan). Samples for SEM were sputtered with a 20–30 ˚ A layer of Au to render them conductive. The diameters of the particles were determined from the SEM images. The elemental analysis was performed on a EuroEA 3000 from Leeman, USA. Poly(ethylene glycol) (PEG) has been widely used as a dispersant to synthesise regular inorganic nanoparticles (Yan et al., 2008; G¨ oz¨ uak et al., 2009; Choi et al., 2010). In the present study, PEG was initially used as an additive to synthesise MFR particles. Fig. 2, shows that the addition of PEG-2000 could promote the formation of regular spherical MFR particles with diameters of (3.97 ± 0.64) m. The specific surface area of MFR also increased from 1.56 m2 g−1 to 8.09 m2 g−1 after the addition of PEG-2000. Fig. 3 shows the FT-IR spectra of the MFR


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Table 1. Acetalisation of various carbonyl compounds and diolsa Conversion/%b

Yield/%b

1

98

98

2

99

99

3

93

93

4

86

86

5

99

99

6

89

88

7

99

99

8

90

88

Entry

Substrate

Product

a) Catalytic conditions: 20 mg of SMFR, 20 mmol of cyclohexanone, 24 mmol of ethylene glycol, 5 mL of cyclohexane, reflux, 2 hours, with Dean–Stark apparatus; b) conversion and yield were determined by GC based on carbonyl compounds.

Fig. 3. FT-IR spectra of MFR and SMFR.

and SMFR. The absorption bands at 1626 cm−1 , — 1334 cm−1 and 786 cm−1 assigned to C—N, C— —N and the triazine ring clearly indicated the synthesis of MFR. The appearance of the absorption band at 1043 cm−1 shows that the sulphonic acid groups were successfully immobilised on the MFR (Liang et al., 2010). The elemental analysis gave the results: N: 42.8 %; C: 29.2%; H: 4.5%; S: 3.1%, indicating that the amount of sulphonic acid groups was 0.99 mmol g−1 . Hence, the acidity of SMFR was expected to be 0.99 mmol g−1 . In effect, the acidity of the SMFR was 3.23 mmol g−1 , as determined by neutralisation

titration (Margelefsky et al., 2008). The titration was carried out as follows: MFR or SMFR (40 mg) and 2 M aqueous NaCl (4 mL) were stirred at ambient temperature for 24 hours. The solids were removed by filtration and washed with water (4 × 2 mL). The combined filtrate was titrated with 0.01 M NaOH using phenol red as the indicator. Accordingly, other acidic groups besides the sulphonic groups would appear to be present in the SMFR. As the MFR contains many secondary amine nitrogen atoms, which not only can be used as the sites to immobilise the sulphonic groups, but also can adsorb the H+ in reaction solution, the chlorosulphuric acid can decompose into HCl during the sulphonation step. Therefore, the physical adsorption of H+ on SMFR might be a significant factor in the high acidity of SMFR. Acetalisation is an important strategy in protecting carbonyl groups. The effects of reaction time on the conversion and yield were investigated (Fig. 4), indicating that the catalyst was very efficient in the acetalisation. The yield was 90 % after reacting for half an hour, and achieved the maximum at 2 h. Various carbonyl compounds and diols were employed to evaluate the catalytic activities of the SMFR. Table 1 shows that all aldehydes and ketones could be converted to the corresponding 1,3dioxolanes with moderate to good yields (Entries 1, 3, 5 and 7). As a seven-membered ring is not as stable as a five-membered ring, the yields are lower for certain substrates (Entries 4, 6 and 8) using butane-1,4-diol.


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Table 2. Esterification of acetic acid with various alcoholsa Conversion/%b

Yield/%b

1

91

91

2

93

93

3

90

88

4

89

89

Entry

Substrate

Product

a) Catalytic conditions: 50 mg of catalyst, 20 mmol of alcohol, 24 mmol of acetic acid, reflux, 3 h; b) conversion and yield were determined by GC based on carbonyl compounds.

Fig. 4. Time-dependence of SMFR-catalysed acetalisation of benzaldehyde with ethylene glycol, conversion ( ), yield ( ).

◦

Fig. 5. Reuse of SMFR catalyst, conversion (light grey bar), yield (dark grey bar).

Examination of entries 1–4 in Table 1 shows that the aliphatic aldehydes were more active in acetalisation than the aromatic aldehydes.

In addition to acetalisation, esterification was investigated using SMFR as the catalyst. It is recognised that water as a by-product could significantly decrease the conversion and yield of esterification (Peters et al., 2006; Bhorodwaj & Dutta et al., 2011). Interestingly, using SMFR as catalyst, there was no need to remove the water continuously from the reaction mixture using a Dean–Stark apparatus. Table 2 shows that the alcohols were converted to the corresponding esters with conversions of 89 % or greater, and with yields of at least 88 %. These results demonstrated the effectiveness of the catalyst in the esterification reaction. The recovery and reuse of heterogeneous catalysts could greatly facilitate the purification of products and increase overall yield of the product and reduce production costs, resulting in a “greener” and more sustainable chemical transformation process. The catalytic activity of the recovered SMFR catalyst was carefully investigated through the acetalisation of cyclohexanone with ethylene glycol. Fig. 5 shows that the catalytic activity of SMFR did not changed even after four runs, indicating the excellent stability of the SMFR catalyst. The heterogeneous catalyst is usually less active than the corresponding homogeneous one, due to the less favourable kinetics of the biphasic catalytic system. A comparison of the catalytic activity of the SMFR with traditional homogeneous catalysts was performed using the acetalisation of benzaldehyde with ethylene glycol. Table 3 shows that the catalytic activity of SMFR is slightly lower than that of the homogeneous H2 SO4 and p-toluene sulphonic acid catalyst. However, the SMFR could conveniently be recovered and reused, which would avoid the purification step and reduce overall costs. In summary, a highly active and recyclable solid acid catalyst was synthesised by sulphonation of MFR. This new solid acid is as effective as the homogeneous acid catalysts commonly used for acetalisation and es-


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Table 3. Comparison of different catalysts for the acetalisation of benzaldehyde with ethylene glycola Entry

Catalyst

Amount/mgb

Conversion/%c

Yield/%c

1 2 3 4

H2 SO4 Benzene sulphonic acid Amberlyst SMFR

3.2 10.2 13.7 20.0

99 77 99 91

99 77 99 91

a) Catalytic conditions: 20 mmol of cyclohexanone, 24 mmol of ethylene glycol, reflux, 30 min, with Dean–Stark apparatus; b) amounts of H+ for all catalysts were 6.46 × 10−5 mol; c) conversion and yield were determined by GC based on benzaldehyde.

terification. The remarkable stability and recyclability of this unique environmentally-benign heterogeneous acid catalyst render it attractive for large-scale industrial applications. Acknowledgements. The authors wish to acknowledge the financial support received from the Zhejiang Science and Technology Innovation Team of Zhejiang Province Science and Technology Hall (2012R10014-16) and Shaoxing University.

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