Efficient decomposition of a melamine–formaldehyde foam into melamine via selective disconnection of bonds

Abstract

The precise disconnection of one type of chemical bond in polymers realizes high-yield recovery of valuable chemicals. In this study, we demonstrate a sustainable and efficient strategy to selectively cleave the specific sp³C–sp³N bond of a thermoset melamine–formaldehyde foam, by which valuable melamine of 99.5% purity was obtained with a yield of 95.3%.

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The recovery of high value-added chemicals from waste plastics is of great significance for the plastic recycling industry and environmental management.¹ Melamine formaldehyde (MF) resin is a thermoset resin made by the polymerization of melamine and formaldehyde, in which melamine are interlinked via dimethyl ether bonds (pH > 9), forming a reticular network (Fig. 1).² MF resin has been widely applied in vehicle manufacturing, home decoration, daily cleaning products and other fields.³ The widespread production and utilization of MF resin inevitably causes the fast accumulation of the end-of-life MF resin waste, causing environmental protection problems.⁴ Developing an effective MF resin recovery method is crucial to address the issue. However, thermosetting MF resin has good thermal and chemical stability, making its degradation a challenge.⁵

Fig. 1 Structure of MF resin and products formed by the cleavage of different types of chemical bonds.

Presently, the primary methods for recovering thermosetting resins include mechanical, thermal, and chemical processes.⁶ The process of mechanical recycling involves pulverizing scrap resin into powders of specified particle size and utilizing it directly as one of the feedstocks for composite materials or as a reinforcing agent for other resins.⁷ These simplistic treatments (crushing, cutting and grinding, etc.) of MF resin in the mechanical method difficult the fully utilization of the high-value units within the structure of MF resin. In the thermal recovery method, waste resins are commonly decomposed into low-molecular-weight oil or gas products at elevated temperatures.⁸ This process facilitates the irregular breaking of the chemical bonds within the resin, complexing product distributions and the subsequent product separation process. Additionally, the release of toxic gases is unavoidable in the thermal recycling process.⁹ Alternatively, the chemical recovery process involves the cleavage of the chemical bonds of the resin in a degradation system under relatively mild reaction conditions.¹⁰ It is ideal but challenging to develop an efficient and green degradation system, in which the specific chemical bonds in the resins can be selectively cleaved, yielding monomers that can be easily separated and recycled. Noticeably, the majority of chemical bonds existing in MF resin are the C–N bonds of different types, including the sp²C–sp²N bond in the 1,3,5-triazine (Tr) ring, the sp²C–sp³N bond on the Tr ring, and the neighbouring sp³C–sp³N bond (see Fig. 1). These C–N bonds have similar chemical activities. Hence, it remains a great challenge to selectively break one type of C–N bond without destroying the other types of C–N bonds.

H⁺ or OH⁻ ions in acid or base catalysts can disconnect the C–N bonds in polymers. This bond disconnection is commonly non-selective due to the high catalytic activity of H⁺ or OH⁻ ions. Wu et al.¹¹ applied the acid or base recovery system, i.e., the methane sulfonic acid-tetrahydrofuran-10wt% H₂O or 10wt% NaOH–H₂O, to cleave the C–N bonds of MF resin. They found that either the acid or base system disconnected the sp²C–sp³N and sp³C–sp³N bonds of MF resin, producing a blend of ammelide and ammeline. If a catalytic system enables the precise cleavage of the sp³C–sp³N bond without influencing the other types of C–N bonds, the main products will be melamine instead of a complex mixture of ammelide and ammeline. In such a case, the product separation will be largely simplified. Surprisingly, we report herein NH₃·H₂O as a selective and efficient system to precisely cleave the sp³C–sp³N bond of melamine formaldehyde resin foam (MFF) with 100% selectivity to recover melamine of high purity (99.5%) with a high yield of 95.3% (entry 9, Table 1). Besides sound-absorbing MFF, other two types of commercial MFFs, namely the flame-retardant melamine compression foam and the cleaning-type melamine foam, can be efficiently degraded to melamine in the NH₃–H₂O system, showing high melamine yields of 94.8% and 93.1%, respectively (entries 27, 28, Table 1 and Fig. S1, ESI†). Compared with the previous MF resin degradation technology,¹¹ in this work, NH₃ was used as a catalyst, and it is a more ecologically environment-friendly substance (raw material for chemical fertilizer production). A high-purity melamine monomer was obtained instead of a hard-to-separate mixture of ammelide and ammeline. NH₃ can be separated directly through volatilization, product melamine is insoluble in H₂O and can be directly obtained, the post-treatment process is simple, compared with the post-treatment of products in previous work, and this work greatly reduces the consumption of energy and resources. Therefore, the NH₃·H₂O system degradation of MF resin is greener and more efficient (Table S1, ESI†).

Table 1 Decomposition of MFF^a

Entry	System	System mass/g	Temp/°C	Time/h	R d ^b/%	Y ma ^c/%
a Reaction conditions: 10 ml Teflon-lined reactor, MFF: 0.15 g. b Degradation ratio = (m0 − m1)/m0, where m0 and m1 are the mass of original MFF and residue MFF after degradation, respectively. c Melamine yield = (m2 − m3)/(m0 × 0.65), where m2 is the mass of the product after drying to remove NH3·H2O after the reaction and m3 is the quality of the remaining insoluble solid after the dried product was dissolved in DMSO, and 0.65 in the equation represents the theoretical mass of melamine rings in 1 g of MFF. d Reaction pressure: 1.2–1.4 MPa, greater than the reaction pressure (0.4 MPa) in entry 9. e Reactant was melamine standard (0.15 g), no MFF. f EDA is ethylenediamine. g Reaction condition: 0.15 g flame-retardant-type MFF. h Reaction condition: 0.15 g cleaning-type MFF.
1	27wt% NH3–H2O	3.00	170	12	100	93.8
2	27wt% (NH4)2CO3–H2O	3.00	170	12	100	90.3
3	27wt% Urea–H2O	3.00	170	12	100	89.6
4	27wt% (NH4)2SO4–H2O	3.00	170	12	100	67.1
5	27wt% NH4Cl–H2O	3.00	170	12	100	27.8
6	27wt% NH3–H2O	2.00	170	12	100	94.5
7	27wt% NH3–H2O	1.00	170	12	100	95.1
8	27wt% NH3–H2O	0.50	170	12	100	94.6
9	27wt% NH3–H2O	0.15	170	12	100	95.3
10	27wt% NH3–H2O	0.05	170	12	100	89.4
11	27wt% NH3–H2O	0.15	170	4	100	65.4
12	27wt% NH3–H2O	0.15	170	6	100	87.4
13	27wt% NH3–H2O	0.15	170	8	100	89.9
14	27wt% NH3–H2O	0.15	170	10	100	91.6
15	27wt% NH3–H2O	0.15	160	12	100	84.9
16	27wt% NH3–H2O	0.15	150	12	100	81.9
17	27wt% NH3–H2O	0.15	140	12	92.6	63.7
18	NaOH–H2O (pH = 13.5)	3.00	170	12	2.7	0
19	KOH–H2O (pH = 13.5)	3.00	170	12	3.5	0
20	5wt% NaOH–H2O	3.00	170	12	92.1	0
21	30wt% Na2CO3–H2O	3.00	170	12	1.2	0
22	30wt% K2CO3–H2O	3.00	170	12	1.9	0
23 ^d	NH3 (1 MPa)	—	170	12	3.2	0
24	H2O	3.00	170	12	1.9	0
25 ^e	27wt% NH3–H2O	3.00	170	12	0	>99
26 ^f	27wt% EDA–H2O	3.00	170	12	100	92.1
27 ^g	27wt% NH3–H2O	0.15	170	12	100	94.8
28 ^h	27wt% NH3–H2O	0.15	170	12	100	93.1

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When degraded in the 27 wt% ammonia solution (NH₃·H₂O), the white foam-like MFF gradually disappeared and a yellow crystal product precipitated from the aqueous system after the degradation reaction (Fig. 2a). The crystal product was characterized by NMR and IR analysis. The ¹H- and ¹³C-NMR spectra of product showed a single unipolar peak at chemical shifts of 6.13 ppm and 167.5 ppm, respectively, which was identical to that of the melamine standard (Fig. 2b and c). Besides, the IR spectrum of solid product was similar to that of the melamine standard, showing the peaks for the N–H bond on the NH₂ group exterior to Tr (at 3467 cm⁻¹ and 3413 cm⁻¹) and for the triazine (Tr) ring (at 1528 cm⁻¹, 1423 cm⁻¹ and 809 cm⁻¹). The IR analysis is in good agreement with the NMR result, identifying the solid crystal product as melamine (Fig. 2d). In addition, the side products in the degradation solution were analyzed by ¹H- and ¹³C-NMR characterizations. As displayed in Fig. S3 and S4,† the side products were either volatile or unstable molecules such as methanol, methylamine, dimethyl ether, and diamine methyl ether, their content was very low in the degradation solution.

Fig. 2 (a) MFF depolymerization in NH₃·H₂O to produce a yellow transparent melamine crystal. (b) ¹H NMR spectrum of the MFF degradation product in NH₃·H₂O and melamine standard. (c) ¹³C NMR spectrum of the MFF degradation product in NH₃·H₂O and melamine standard. (d) FT-IR spectrum of the MFF degradation product in NH₃·H₂O and melamine standard. (e) Nuclear magnetic quantification of MFF degradation products in NH₃·H₂O, the mass of the melamine product is 10.6 mg and the quality of piperazine is 4.1 mg. Deuterated reagent of all NMR analytic is DMSO-d₆.

On the basis of ¹H-NMR analysis, a high selectivity to melamine (99.5%) and a yield (93.8%) were obtained in 27 wt% NH₃·H₂O (entry 1, Table 1). Besides the product yield, the product purity is a key factor affecting the efficiency of whole degradation process. The purity of melamine product was determined by ¹H-NMR quantitative analysis (Fig. 2e).¹² The result showed that the melamine product had a purity as high as 99.5%. The elemental analysis showed that the content of C, N and H elements in the melamine product (28.57%C, 65.94%N and 4.83%H) was very close to those in the melamine standard (28.58%C, 66.55%N and 4.80%H) (Table S2, ESI†), further proving the melamine product of high purity. The high yield and purity of melamine will benefit the application of NH₃·H₂O in the industrial degradation of MFF.

Several types of aqueous ammonium salt solutions with the same mass concentration to NH₃·H₂O, were also employed for the degradation reaction (entries 2–5, Table 1). They all exhibited activity for the degradation reaction, resulting in the melamine yield ranging from 27.8% to 90.3%. These ammonium salts decomposed at the reaction temperature to produce NH₃, as indicated by the pungent NH₃ smell after reaction. These released NH₃ enabled the decomposition of MFF. Noticeably, NH₃·H₂O showed a higher melamine yield than the ammonium salt solutions. NH₃·H₂O can release NH₃ directly without the salt decomposition step, accelerating the reaction. Moreover, it did not have the anions existed in the ammonium salts, including CO₃²⁻, SO₄²⁻, and Cl⁻. In such case, the possible negative effect of these anions on the degradation reaction can be avoided.

Furthermore, the impact of reaction conditions on the degradation reaction was explored, including the NH₃·H₂O amount, the reaction time and reaction temperature. A reduction of NH₃·H₂O amount from 3.00 g to 0.15 g led to the same melamine selectivity and comparable melamine yield, whereas a further decrease in NH₃·H₂O amount to 0.05 g resulted in a declined degradation efficacy (entries 1, 6–10, Table 1). The highest melamine yield of 95.3% was reached with the NH₃·H₂O amount of 0.15 g. Hence, the effects of reaction time and temperature on the degradation reaction were studied with 0.15 g of NH₃·H₂O (entries 9, 11–14 and 15–17, Table 1). An increased reaction time and an elevated reaction temperature enhanced the MFF degradation reaction.

NH₃·H₂O (27wt%) was alkaline with a pH value of 13.5. To exclude the catalytic effect of alkaline OH⁻ ions on the depolymerization of MFF, the solutions of NaOH–H₂O and KOH–H₂O that have the same pH values (13.5) as that of NH₃·H₂O (27wt%), were chosen for MFF degradation. MFF did not degrade in the above two solutions (entries 18 and 19, Table 1). In addition, 5wt% NaOH–H₂O was applied to degrade MF resin under the same reaction conditions as that of NH₃·H₂O (entry 20, Table 1). The degradation rate reached 92.1%, and the products included ammelide and ammeline. Therefore, NH₃ has a higher degradation activity and a narrower product distribution than NaOH. The Na₂CO₃–H₂O (30 wt%) and K₂CO₃–H₂O (30 wt%) solutions that have similar mass concentrations to that of NH₃·H₂O were also applied to recycle MFF under the identical condition, both of which were inert toward MFF degradation (entries 21 and 22, Table 1). NH₃ or H₂O alone was employed to react with MFF, and neither of them can degrade MFF. This result suggested that the degradation of MFF by NH₃·H₂O was achieved through the synergy of NH₃ and H₂O (entries 23 and 24, Table 1).

In order to further evaluate the greenness of the NH₃·H₂O system, we analyzed and compared the green metrics¹³ of the NH₃·H₂O system recovery of MF resin with the previous recovery technology. The detailed information is shown in Tables S3 and S4.† Environmental impact factors (EF) and process mass intensity (PMI) were calculated respectively. The results show that simple EF, complete EF, EF, Waste amount and PMI values of the NH₃·H₂O system (1.06, 2.27, 1.06, 0.66, and 3.27) are lower than those of the MSA–THF–H₂O system (2.36, 8.92, 2.93, 1.70, and 9.92) and NaOH–H₂O system (1.65, 11.23, 1.65, 1.03, 12.23). This proves the greenness and economy of NH₃·H₂O to degrade MF resin.

The possible reaction route was explored for the degradation of MF resin to melamine. Two routes were proposed. The first one involved the formation of melamine via a multi-step process, i.e., MF resin degraded into ammelide, ammeline and/or cyanic acid, followed by their transformation to melamine, while in the second one, MF resin degraded directly into melamine without the above intermediate(s). Ammelide, ammeline and cyanic acid were chosen as the model reactant and tested in 27 wt% NH₃·H₂O, respectively. No melamine was formed in all of the tests (Fig. S5, ESI†), which ruled out the first route as the reaction pathway. Additionally, the stability experiment showed that >99% of melamine was stable in NH₃·H₂O (entry 25, Table 1) with the structure unchanged (Fig. S6, ESI†). These results proved the second route as the reaction pathway, i.e., MF resin degraded directly into melamine.

In the direct depolymerization route of MF resin, the nature of selective disconnection of C–N bond was elucidated via model reactions. Two possible paths existed for NH₃·H₂O to directly depolymerize MF resin into melamine (Fig. 3): path1 involved the selective breakage of the sp³C–sp³N bond (without breaking the sp²C–sp³N bond) to produce melamine, while in path 2, the ammonia molecule attacked the Tr ring, by which the sp²C–sp³N bond was cleaved to form melamine. It is noticed that in path 2, the cleavage of the sp²C–sp³N bond led to the leaving of a sp³N atom of MF resin, and this vacancy was replenished by a N atom of NH₃. When NH₃ is replaced by ethylenediamine (EDA), the replenishment of a N atom of EDA will lead to the formation of N²,N⁴,N⁶-tris(2-aminoethyl)-1,3,5-triazine-2,4,6-triamine, if path 2 is the reaction path. EDA was utilized as the model substitute for NH₃ in the NH₃·H₂O solution to cleave the C–N bond of MF resin (entry 26, Table 1). The product was identified to be melamine (Fig. S7, in ESI†), which excluded path 2 as the reaction path. Hence, path 1 was the reaction pathway.

Fig. 3 Ethylenediamine replaces NH₃ in NH₃·H₂O to investigate the decomposition path of MF resin. In the degradation of MF resin to melamine, the cleavage of either NH–CH₂ (sp³N–sp³C) (path 1) or C–NH (sp²C–sp³N) bond (path 2) of MF resin is inevitable, as illustrated in Fig. 3. If the NH–CH₂ bond is cleaved, melamine will be produced (see path 1). If the C–NH bond is cleaved (see path 2), no amino group will be present outside the triazine ring. In such case, the additional amino groups will be needed to produce melamine. These additional amino groups can only come from ethylenediamine (NH₂–CH₂–CH₂–NH₂). As a result, the amino groups outside the triazine ring will be NH₂–CH₂–CH₂–NH₂ groups, producing N²,N⁴,N⁶-tris(2-aminoethyl)-1,3,5-triazine-2,4,6-triamine as the product (other than melamine).

It is essential for NH₃·H₂O to interact with MF resin before it cleaves the C–N bond of MF resin. The interaction between NH₃·H₂O and MF resin was explored by ¹³C-NMR characterization, in order to understand how NH₃·H₂O initiate the cleavage of C–N bond. For a simple and accurate analysis, MF resin prepolymer that has the typical structure unit of MF resin was chosen as to model MF resin (Fig. S8, ESI†). MF resin prepolymer itself and its mixture with NH₃·H₂O were analyzed by ¹³C-NMR (Fig. S9, ESI†). MF resin prepolymer has two distinct types of C atoms, i.e., the sp² C atoms (located at the Tr ring) and sp³ C atoms (located at the chains). Both sp² C and sp³ C were neighbored by N atom(s). Notably, the presence of NH₃·H₂O caused a movement of chemical shift (δ) of the two types of C atoms towards the high δ direction, indicating the decrease in the electron cloud density of the two types of C atoms. This result implied that the C atoms were surrounded by more electron-withdrawing atoms, i.e., N and O atoms of NH₃·H₂O. It is possible that the N and O atoms of NH₃ and H₂O can associate with the H, O and N atoms of MF resin prepolymers via hydrogen bonds (O–H⋯N, N–H⋯N, and N–H⋯O, etc.) to form a stable six-ring structure (Fig. S10, ESI†). In this six-ring structure, the electron transfer from the C atoms of the MF resin prepolymer to the N and O atoms of NH₃ and H₂O was achieved via an inductive effect.

Additionally, ¹³C-NMR characterization was employed to probe the alterations in chemical bonds during the degradation of MF resin at a microscopic level (Fig. S11, ESI†), 0.15 g of MF resin was degraded in 3 g of NH₃·H₂O at 170 °C with different reaction periods (3 h, 6 h, and 9 h), and the resulting degradation solution was directly characterized by ¹³C-NMR. The degradation of MF resin became increasingly intensive as the reaction time progressed. At 3 h, Tr–NH₂ groups and Tr–NH–CH₂–O–CH₂–NH–Tr groups were dominant in the degradation system, whereas Tr–NH–CH₂–OH groups were scarce and few. As the reaction time prolonged, the content of Tr–NH₂ groups increased progressively, accompanied by the significant diminishing of Tr–NH–CH₂–O–CH₂–NH–Tr groups and nearly complete consumption of Tr–NH–CH₂–OH groups. Nevertheless, NMR evidenced that the concentration of Tr–NH₂ groups was substantially higher than that of Tr–NH–CH₂–O–CH₂–NH–Tr groups at 9 h, indicating that the reaction mechanism involved the breakage of ether bonds and sp³C–sp³N bonds.

The degradation mechanism of MF resin was further detailed by density functional theory (DFT) on the Gaussian 16 platform (Fig. 4a).¹⁴ The model compound, R1, which possesses a comparable structure to MF resin, was developed using Gaussview 6.0.¹⁵ In the depolymerization process, NH₃ and R1 interacted with each other to form two hydrogen bonds, generating the intermediate species 2 with a six-membered-ring structure, and this step had an energy of 12.25 kcal mol⁻¹. One hydrogen (H) atom of NH₃ attacked and then bound to the (O) oxygen atom of R1, meanwhile one H atom on the N atom of intermediate 2 interacted and then bounded to the N atom of NH₃, leading to the formation of intermediates 3 and 4 through the transition state TS1. The Gibbs activation energy and Gibbs reaction energy for this step were 24.19 kcal mol⁻¹ and 10.26 kcal mol⁻¹, respectively. Subsequently, the additive reaction between the C=N bond of intermediate 3 and H₂O produced intermediate 4, with an activation energy of 25.81 kcal mol⁻¹. Thereafter, NH₃ and intermediate 4 interacted with each other to produce intermediate 5 via hydrogen bonding, showing an activation energy of 4.66 kcal mol⁻¹. Intermediate 5 subsequently cleaved its sp³C–sp³N bond to yield melamine via the transition state TS2 and H transfer. The Gibbs activation energy and Gibbs reaction energy for this step were 27.82 kcal mol⁻¹ and −4.78 kcal mol⁻¹, respectively. Briefly, the principal process of MF resin depolymerization, as shown in Fig. 4b, started from the formation of a six-membered-ring intermediate through the H-bond association between MF resin and NH₃, followed by the breakage of ether bonds and sp³C–sp³N bonds via H transfer.

Fig. 4 (a) The changes in the structure and energy in the process of NH₃·H₂O depolymerizing MF resin were DFT calculated using the Gaussian 16 platform. (b) Degradation mechanism of MF resin.

Conclusions

This study introduces a novel approach to utilize NH₃·H₂O to greenly and efficiently degrade MF resin. The synergy effect of NH₃ and H₂O selectively cleaved the sp³C–sp³N bond within MF resin without damaging the sp²C–sp³N bonds of MF resin at 170 °C for 12 h, by which valuable melamine was recovered with a high yield of 95.3%. The recovered melamine was precipitated from the degradation solution as crystals with a high purity of 99.5%, which simplifies largely the following product separation. Compared with previous MF resin recovery systems, in this work, the catalyst NH₃ is a more ecologically environment-friendly substance, melamine monomer insoluble in NH₃·H₂O was obtained directly, without tedious and resource-intensive post-processing, greatly reducing the consumption of resources and energy, so NH₃·H₂O system degradation MF resin is greener and more efficient. Through the comparison of EF of different systems, it is more intuitive to reflect the NH₃·H₂O system greenness and economy. A viable degradation mechanism was proposed and corroborated by combining NMR and density functional theory calculations. The ability of NH₃·H₂O to precisely cleave the specific C–N bond makes it a great candidate in the controllable recovery of high-value-added chemicals from MF resin and other polymers that contain different types of C–N bonds.

Author contributions

Chaohui Yang: data curation, investigation, and writing – original draft. Xinyu Li: methodology and software. Hongyan Li: methodology and software. Chizhou Wang: methodology. Qianqian Xing: software. Xiaoliang Jia: methodology and software. Xiaojing Cui: writing – review & editing. Xianglin Hou: resources, funding acquisition, investigation, and project administration. Tiansheng Deng: conceptualization, resources, methodology, supervision, validation, project administration, and writing – review & editing.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the autonomous research project of SKLCC (Grant No.: 2024BWZ007), Fundamental Research Program of Shanxi Province (No. 202303021211257 and No. 202303021212375) and the Fund of Shanxi Province patent Transformation Special Plan Project (No. 202306005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4gc04481a

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