Dehydrochlorination of polyvinylchloride using Al-modified graphitic-C₃N₄

Abstract

One of the efficient chemical recycling processes for waste polyvinylchloride (PVC) is the dehydrochlorination reaction using an environmentally benign solvent such as polyethylene glycol. Hydrogen chloride (HCl) is simultaneously formed through the unzipping degradation mechanism of PVC due to its lower thermal stability, this causes severe corrosion problems for the operating equipment. For the dehydrochlorination of PVC, a novel graphitic carbon nitride (g-C₃N₄) possessing a large amount of basic sites was prepared via melem-based thermal synthesis with an aluminum species structural modifier during the g-C₃N₄ preparation to verify the dehydrochlorination activity for PVC according to the aluminum content in the g-C₃N₄ matrix. The optimal 10 wt% aluminum modified g-C₃N₄ catalyst showed an excellent dehydrochlorination activity for PVC in an environmentally benign polyethylene glycol solvent due to the superior characteristics of the large amount of basic sites in g-C₃N₄. The positive effects of Al₂O₃ were mainly attributed to the incorporation of the structural modifier Al₂O₃ in the matrix of g-C₃N₄, where Al₂O₃ was a chemically inert material that enhanced the basicity of g-C₃N₄.

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1. Introduction

Polyvinylchloride (PVC) is one of the general-purpose polymers, which can be produced on a large scale with a lower cost, and PVC also has superior physical and chemical properties with easy processing properties during the calendering or extrusion processes.¹ Due to the excellent processing characteristics of PVC, it has been widely used for building timbers, household goods and so on. However, the lower thermal stability of PVC can lead to a well-known unzipping mechanism and yellowing phenomena during the manufacturing process that is responsible for the simultaneous production of hydrogen chloride (HCl), which also causes a severe corrosion problem for the equipment.^2–4 Since the PVC resin has excellent mechanical properties and a strong chemical resistance with nonflammable characteristics, the production of PVC has been steadily increased and it has become one of the most widely consumed general-purpose polymers. Commercially, PVC has been synthesized through a polymerization reaction of vinylchloride monomer (C₂H₃Cl, VCM), which is produced from a thermal cracking reaction of dichloroethane (C₂H₄Cl₂, DCE), producing an equal amount of HCl byproduct. Recently, the amount of global waste plastic has been dramatically increased due to the capacity saturation of the traditional waste disposal facilities such as landfill or incineration and so on. Therefore, some new disposal concepts for waste plastics through appropriate recycling technologies have been widely investigated to regenerate the waste plastics through physical or chemical recycling methods.³

Generally, the recycling technologies for the waste polymers can be categorized into three main methods. The material recycling method (MR) is to grind waste plastics physically followed by blending them for the preparation of recycled products.⁴ The chemical recycling method (CR) is to utilize appropriate reactions such as pyrolysis, chemical treatment, oxidation, or selective dehydrochlorination and so on to produce useful byproducts such as HCl and chemical intermediates with chlorinated hydrocarbons. The HCl product especially can be used after HCl purification for further transformations to Cl₂ or for the oxidative chlorination of ethylene to DCE. Finally, the energy recycling method (EG) is to produce energy sources such as high pressure steam or electricity through the incineration of the waste plastics.^1,3 Among these methods, the recycling method for waste PVC using CR has been highly attractive from the scientific point of view since the CR can efficiently recycle the waste PVC with the simultaneous production of HCl and useful chlorinated hydrocarbons under various reaction conditions.

Graphitic carbon nitride (g-C₃N₄) has many potential applications such as in catalyst supporting materials and gas storage materials to substitute the amorphous or graphitic carbon materials due to its superior chemical and physical properties. In addition, g-C₃N₄ prepared using the melem-based synthesis has superior catalytic properties for fuel cell applications since g-C₃N₄ has a larger basic site density due to the incorporation of N atoms in the carbon nano-layers by the formation of C–N bonds in the form of aliphatic or aromatic structures.^2,3,5–11 g-C₃N₄ can be synthesized using the polycondensation mechanisms of cyanamide, dicyandiamide or melamine,^2,3,5–7 and some novel methods for the synthesis of g-C₃N₄ have been widely reported using mixtures of melamine and uric acid in the presence of alumina. In addition, environmentally benign solvents such as polyethylene glycols (PEGs), which have various and wide molecular weight distributions, and ionic liquids such as 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄) and 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) and so on^12–15 have also been widely investigated for the dehydrochlorination of PVC.^{13,14,16–19}

In the present study, a novel metal-free and Al-modified g-C₃N₄ (Al-g-C₃N₄) material was synthesized to verify the dehydrochlorination activity for virgin PVC resin in an environmentally benign PEG solvent. The different dehydrochlorination activity according to the concentration of aluminum species in the matrix of g-C₃N₄ was also investigated to verify the positive role of the basic sites on g-C₃N₄ through the characterization of the basic properties and physicochemical nature of the Al-modified g-C₃N₄.

2. Results and discussion

2.1. Characteristics of the Al-modified g-C₃N₄

The textural properties of the Al-modified g-C₃N₄ (Al-g-C₃N₄) were measured using a N₂ adsorption–desorption method, and the BET surface area, pore volume and average pore diameter are summarized in Table 1. The yellowish Al-modified g-C₃N₄ was denoted as Al(x)-g-C₃N₄, where x represents the Al weight% based on the total weight of g-C₃N₄. With an increase of aluminum content in Al-g-C₃N₄, the surface area was slightly increased from 4.9 m² g⁻¹ for Al(0)-g-C₃N₄ to 7.4 m² g⁻¹ for Al(10)-g-C₃N₄. Interestingly, a significant increase of surface area was observed for Al(20)-g-C₃N₄ with a value of 45.6 m² g⁻¹. The pore volume showed a similar trend to the surface area with values of 0.041–0.085 cm³ g⁻¹ and a large pore volume of 0.085 cm³ g⁻¹ for Al(20)-g-C₃N₄. However, the average pore diameter was not significantly altered, showing values of 26.0–39.3 nm in size. The bimodal pore size distribution at around 3.5 and 20–70 nm was also observed, and it can possibly be induced by the insertion of aluminum heteroatoms between the flat nanosheet layers of g-C₃N₄ (ref. 5–8 and 11) forming the new larger mesopore above 50 nm. We believe that the significant changes of the structural properties of Al(20)-g-C₃N₄ can be attributed to the newly formed interparticular mesopores from the segregated Al₂O₃ particles on the surfaces of the g-C₃N₄ particles.^9–11 The N₂ adsorption–desorption isotherms and pore size distributions as shown in Fig. S1† and 1 strongly suggest the enlargement of the interlayer pores through the addition of the aluminum species in the matrix of g-C₃N₄, showing a typical type IV isotherm, especially for Al(20)-g-C₃N₄. Interestingly, the bimodal pore size distribution was observed at around 3.5 nm on all Al-g-C₃N₄ which can be attributed to the interlayer pores of g-C₃N₄. The observed pore diameter of 50–70 nm seems to be attributed to the segregated Al₂O₃ particles forming the interparticular pores, which was clearly observed on Al(10)-g-C₃N₄ forming a somewhat smaller mesopore at around 20–50 nm in size as shown in Fig. 1.

Table 1 Dehydrochlorination activities of the Al-modified g-C₃N₄

Catalysts	N2 sorption			Basicity (CO2-TPD, mmol CO2 per g)	Activity^a (removal%)
	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)	<450 °C/>450 °C
a The dehydrochlorination activity was measured at T = 170 °C and ambient pressure for 6 h in a liquid-phase of polyethylene glycol (PEG) solvent with 0.05 g catalyst.b The removal% of HCl from the virgin PVC resin was measured at a reaction temperature of 200 °C.
None	—	—	—	—	51
Al(0)-g-C3N4	4.9	0.049	39.3	15.1/15.7	60
Al(5)-g-C3N4	6.3	0.041	26.0	93.3/112.0	63
Al(10)-g-C3N4	7.4	0.051	27.4	124.6/127.1	75 (88)^b
Al(20)-g-C3N4	45.6	0.085	27.8	85.9/724.8	50

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Fig. 1 Pore size distribution of the fresh Al-modified g-C₃N₄.

The crystalline structures and phases of the Al-modified g-C₃N₄ were characterized using powder X-ray diffraction (XRD) analysis, and the XRD patterns are displayed in Fig. 2. Two characteristic diffraction peaks of the crystalline g-C₃N₄ were clearly observed on the Al-modified g-C₃N₄, showing the well-known tri-s-triazine building blocks formed from the melem units.^9–11 The observed larger peak intensity at around 2θ = 27.4° can be attributed to the characteristic interlayer stacking structures of the aromatic units which are assigned to the graphitic carbons of the (002) plane by localizing electrons with a stronger binding force between the g-C₃N₄ layers. In addition, the smaller peak intensity at around 2θ = 13.0° can be assigned to the (100) plane of g-C₃N₄, which is associated with the stacking of carbon interlayers.^7,9,11 The intensities of the diffraction peaks were decreased with an increase of the aluminium content monotonically, showing the smallest intensity for Al(20)-g-C₃N₄ due to the possible collapse of the characteristic interlayer structure of g-C₃N₄ with its lower crystallinity. To further verify the results of the N₂ adsorption–desorption and XRD analysis of the fresh Al-modified g-C₃N₄, transmission electron microscopy (TEM) analysis was carried out and the images are displayed in Fig. 3. From the TEM images, bundle-like structures on g-C₃N₄ (ref. 5 and 10) were clearly observed and sphere-like particles mixed with the g-C₃N₄ bundles were also observed after the aluminum modification during the synthesis of the Al-modified g-C₃N₄. This observation reveals that aluminum species with a size of 10–20 nm can be selectively formed in the interlayers of g-C₃N₄. These species seem to be responsible for the enhanced surface area of the Al-modified g-C₃N₄. The particle sizes of the sphere-type aluminum species were also found to be increased with an increase of the aluminum content in the Al-modified g-C₃N₄ by disrupting the characteristic interlayer structures of g-C₃N₄.

Fig. 2 XRD patterns of the fresh Al-modified g-C₃N₄.

Fig. 3 TEM images of the fresh Al-modified g-C₃N₄.

To elucidate the characteristic structures of the Al-modified g-C₃N₄, FT-IR analysis was carried out on the fresh Al-modified g-C₃N₄ and the FT-IR spectra are displayed in Fig. S2.† The characteristic absorption bands of g-C₃N₄ were clearly observed and an absorption band at 1636 cm⁻¹ was assigned to the C–N stretching vibration modes and those at 1247, 1329, and 1423 cm⁻¹ were assigned to the C–N heterocycle stretching modes of g-C₃N₄. A broad absorption band at 3150 cm⁻¹ can be assigned to the stretching modes of the terminal NH₂ groups on the aromatic rings of g-C₃N₄.^20–22 The characteristic peaks for metal oxides such as Al₂O₃ were also observed with absorption bands at 830 and 1050 cm⁻¹.²³ These results strongly suggest the easy formation of the g-C₃N₄ structures even after the aluminum modification of g-C₃N₄ and the chemical state of the aluminum species seems to be the metal oxide such as Al₂O₃ possibly with a high dispersion in the interlayer matrices of g-C₃N₄.

2.2. Dehydrochlorination activity for PVC in terms of the basicity of the Al-modified g-C₃N₄

The dehydrochlorination activities for virgin PVC on the Al-modified g-C₃N₄ under the reaction conditions of T = 170 °C and ambient pressure for 6 h in the PEG400 solvent are summarized in Table 1. Without the Al-modified g-C₃N₄ catalyst, the removal% of HCl from the PVC by the dehydrochlorination reaction was found to be the somewhat higher value of around 51% due to the facile thermal degradation activity of PVC by the well-known unzipping mechanism.^1,4,13–19 Due to the intrinsic basic properties of g-C₃N₄, attributed to the terminal aliphatic or aromatic C–N structures,^2,3,5–11 the dehydrochlorination activity of PVC was dramatically increased when in the presence of the Al-modified g-C₃N₄ catalyst up to 75% for Al(10)-g-C₃N₄ from 60% for Al(0)-g-C₃N₄ as summarized in Table 1. The preferential interactions of the Cl atoms on the surfaces of the PVC resin with the basic sites of Al-g-C₃N₄ seem to be responsible for the superior activity. In addition, the removal% of HCl from the PVC was further increased up to 88% for Al(10)-g-C₃N₄ at a reaction temperature of 200 °C. However, at the higher aluminum content of Al(20)-g-C₃N₄, the dehydrochlorination activity was decreased to 50% due to the less ordered crystalline-phase formation of g-C₃N₄ which possibly originated from the excess presence of Al₂O₃ particles and the simultaneous structural collapse of the characteristic interlayer structures of g-C₃N₄ as confirmed using the XRD and TEM analyses. In addition, the Al₂O₃ itself showed no significant enhancement of dehydrochlorination activity compared with the results of the no-catalyst experiment, which suggests that Al₂O₃ acts as a structural modifier in the matrix of g-C₃N₄, enhancing the basicity of g-C₃N₄. Therefore, the observed higher dehydrochlorination activity for Al(10)-g-C₃N₄ seems to be mainly attributed to the increased surface basic density with the enhanced crystalline structures of the interlayer structures of g-C₃N₄.

To further verify the oxidation states of the aluminum species and the basic properties of the Al-modified g-C₃N₄, XPS and CO₂-TPD analyses were further carried out on the as-prepared Al-modified g-C₃N₄ and the spectra are displayed in Fig. 4 and S3,† respectively. Based on the previous work,²⁴ the binding energies (BE) of Al 2p for the metallic aluminum and γ-Al₂O₃ can be assigned to 72.5 and 74.0–74.5 eV, respectively. The BEs of the aluminum species on the Al-modified g-C₃N₄ were observed in the range of 73.8–74.8 eV with a small shoulder peak in the lower BE region. The BE of the aluminum species in the higher region was increased with the increase of aluminum content on the Al-modified g-C₃N₄. This observation suggests that the aluminum species on the Al-modified g-C₃N₄ exists mainly in the γ-Al₂O₃ phase irrespective of the amount of aluminum content in g-C₃N₄. The interaction between Al₂O₃ and g-C₃N₄ seems to be getting weaker with the increase of aluminum content in the matrix of the g-C₃N₄ as forming larger γ-Al₂O₃ particles resulted in a decrease of the basic sites which was supported through the BE shifts of Al 2p to higher values with the increase of aluminum content in the g-C₃N₄.^9–11,25 To confirm the basic properties of the Al-modified g-C₃N₄, CO₂-TPD analysis was carried out on the fresh Al-modified g-C₃N₄ and the desorption patterns are displayed in Fig. S3† with the summarized results in Table 1. In general, CO₂ molecules can be adsorbed onto basic sites by forming monodentate carbonates in the temperature region below 200 °C and bidentate or bridged carbonates at the much higher temperature region of 250–650 °C on the basic sites or alumina surfaces.^25–28 A small desorption peak was observed for all the Al-modified g-C₃N₄ at around 150 °C which was attributed to the basic sites of the g-C₃N₄ assigned to the monodentate carbonates. The strongly adsorbed CO₂ molecules were observed at around 350 °C, strongly interacting with the basic sites of the g-C₃N₄ surfaces or with the segregated Al₂O₃ particles.²⁷ These desorption peaks, which are denoted as the basic sites below 450 °C, seem to be the most active sites for the dehydrochlorination of PVC. The desorbed amount of CO₂ was maximized on the most active Al(10)-g-C₃N₄ for the dehydrochlorination of PVC, and these values were found to be 15.1, 93.3, 124.6, and 85.9 mmol CO₂ per g for Al(0)-g-C₃N₄, Al(5)-g-C₃N₄, Al(10)-g-C₃N₄, and Al(20)-g-C₃N₄, respectively. In addition, the broad CO₂ desorption peak at the much higher temperature above 450 °C can be possibly attributed to the decomposition of g-C₃N₄. The desorption peak intensity above 450 °C was increased with an increase of the aluminum content in Al-g-C₃N₄ from 15.7 on Al(0)-g-C₃N₄ to 724.8 mmol CO₂ per g on Al(20)-g-C₃N₄, which is possibly induced from the increased thermal degradation of the g-C₃N₄ structures with the help of the solid-acid Al₂O₃ particles especially at a high concentration of Al₂O₃ in the matrix of the g-C₃N₄.

Fig. 4 XPS spectra of Al 2p on the fresh Al-modified g-C₃N₄.

In summary, the observed higher dehydrochlorination activity over Al(10)-g-C₃N₄ can be attributed to the larger number of active basic sites with the help of the structural modifier Al₂O₃ particles which are possibly inserted in the interlayers of the g-C₃N₄. The dehydrochlorination activity for PVC in an environmentally-benign PEG400 solvent is well correlated with the total amount of basic sites on the g-C₃N₄. The reaction mechanism was schematically proposed as summarized in Scheme 1. The intrinsic basic properties of g-C₃N₄ from the terminal aliphatic or aromatic C–N groups can initially attack the Cl atoms on the PVC surfaces. And the activated Cl atoms can selectively produce HCl by reacting with the adjacent H atoms on the PVC surfaces which results in the simultaneous formation of C=C bonds. The structural modification of g-C₃N₄ with an optimal amount of Al₂O₃ can increase the basic properties without a significant loss of the crystallinity of g-C₃N₄ due to the highly dispersed Al₂O₃ particles in the interlayers of g-C₃N₄. Furthermore, the HCl produced by the dehydrochlorination of the PVC can be further utilized for an oxidative chlorination reaction of ethylene to dichloroethane in the PVC industry using an environmentally benign chemical recycling process of the waste PVC.

Scheme 1 Proposed mechanism of the dehydrochlorination of PVC on the basic sites of the Al-modified g-C₃N₄.

3. Conclusions

A novel graphitic carbon nitride (g-C₃N₄), having sufficient basic sites after modification with an optimal amount of aluminum, showed excellent dehydrochlorination activity in polyethylene glycol solvent at a lower temperature of 170 °C. The optimum concentration of Al₂O₃ in the g-C₃N₄ matrices was found to be around 10 wt%, which can enhance the surface area of the Al-modified g-C₃N₄ slightly by maximizing the amount of active basic sites and by increasing the terminal aliphatic or aromatic C–N groups of the g-C₃N₄. The dehydrochlorination of PVC seems to be selectively initiated by attack on the Cl atoms of PVC by the basic sites of g-C₃N₄, followed by the simultaneous production of HCl product by reaction of the Cl atoms with the adjacent H atoms to produce dehydrochlorinated C=C bonds in the PVC resin. The structural modification of g-C₃N₄ with an appropriate amount of chemically inert Al₂O₃ can increase the number of basic sites with the help of the highly dispersed Al₂O₃ particles in the interlayers of the matrix of g-C₃N₄.

4. Experimental section

4.1. Preparation of g-C₃N₄ and the dehydrochlorination activity test

For preparing the Al-modified g-C₃N₄, aluminum isopropoxide (AIP) and melamine precursor, at a fixed weight of melamine (1.5 g) with a different weight of AIP, were completely mixed in 50 ml of 2-propanol solvent. The mixture was vigorously stirred at room temperature for 1 h, and the 2-propanol solvent was evaporated at 60 °C under vacuum conditions using a rotary evaporator. The prepared gel-like mixture was dried in an oven kept at 80 °C overnight, and the product was carburized by elevating the temperature to 250 °C and keeping for 0.5 h, 350 °C for 0.5 h, 550 °C for 4 h, and finally cooling down to room temperature under a flow of N₂ with a ramping rate of 1 °C min⁻¹. The as-prepared yellowish Al-modified g-C₃N₄ was denoted as Al(x)-g-C₃N₄, where x represents the Al metal weight% based on the weight of the g-C₃N₄ from 0 to 20 wt%.

For measuring the dehydrochlorination activity for the PVC resin, 35 ml of polyethylene glycol (PEG400, Aldrich) with a low molecular weight of 380–420 g mol⁻¹, 1.0 g of PVC (Duksan Chem., Korea), and 0.05 g of the Al-modified g-C₃N₄ catalyst were added to and thoroughly mixed in a 3-neck round flask. The mixture was reacted at T = 170 °C at ambient pressure for 6 h. The extent of the dehydrochlorination of the PVC was measured using a titration method using NaOH solution, and the removal% of the HCl on the PVC resin was calculated by dividing the measured amounts of HCl by the titration with the theoretical amount of HCl in the virgin PVC resin.

4.2. Characterization of the Al-modified g-C₃N₄

N₂ adsorption–desorption isotherms were measured at −196 °C using a Micromeritics Tristar II instrument to verify the effects of aluminum species on the g-C₃N₄ porosity and surface area. Before analysis, the sample was treated under vacuum at 350 °C for 4 h under a He flow. The specific surface area of the Al-modified g-C₃N₄ was calculated using the Brunauer–Emmett–Teller (BET) method, and the pore volume, and the average pore diameter with a pore size distribution were also determined using the Barrett–Joyner–Halenda (BJH) method using the data from the N₂ desorption isotherm. Powder X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer with Cu Kα (λ = 0.15406 nm) irradiation in the range of 2θ = 10–60° to verify the crystalline structures of the Al-modified g-C₃N₄. Transmission electron microscopy (TEM) images were also obtained using a TECNAI G2 T-20S instrument to confirm the morphology of the Al-modified g-C₃N₄.

X-ray photoelectron spectroscopy (XPS) analysis was carried out using a VG Multilab 2000 instrument using a monochromatic Al Kα X-ray source (1486.6 eV) to verify the electronic and oxidation states of the aluminum species in the Al-modified g-C₃N₄. The binding energy (BE) of aluminum was adjusted using the reference BE of C 1s of 284.6 eV.

Temperature-programmed desorption of CO₂ (CO₂-TPD) analysis on the Al-modified g-C₃N₄ was performed using a BELCAT-M instrument. Before analysis, the sample was pretreated at a He flow rate of 30 ml min⁻¹ and 250 °C for 2 h, and it was then cooled down to 100 °C. The adsorption analysis of the adsorbed CO₂ was subsequently carried out on the pretreated sample, and CO₂-TPD patterns were obtained in the temperature range of 100–600 °C with a heating rate of 10 °C min⁻¹.

Fourier transformed infrared spectroscopy (FT-IR) analysis was carried out to characterize the surface functional groups of the aluminum and g-C₃N₄ using a self-supported thin pellet of the fresh Al-modified g-C₃N₄ mixed with KBr.

Acknowledgements

The present work was financially supported by the R&D Center for Valuable Recycling (Global-Top R&D Program) of the Ministry of Environment of Korea (Project No. GT-14-C-01-038-0). The authors would like to acknowledge the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (NRF-2014R1A1A2A16055557).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra24477c

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