Application of zeolite-encapsulated Cu(II) [H₄]salen derived from [H₂]salen in oxidative delignification of pulp

Abstract

Since the disclosure that salen complexes were potent low-temperature oxidation catalysts with exceptional selectivity, considerable efforts by our group have focussed on their development towards the efficient catalytic delignification of pulp, principally with environmentally benign oxygenic chemicals. These efforts have resulted in additional developments including the heterogenisation of catalytic systems as well as the hydrogenation for application in catalytic delignification of pulp. Herein, heterogeneous complexes of two ligands: [H₂]salen and [H₄]salen with copper as a transition metal were synthesized and encapsulated in zeolite cages by the impregnation (IM) and ship-in-a-bottle (SB) method. The resulting complexes were employed as catalysts for oxidative delignification of pulp, using peracetic acid (CH₃COOOH) as an oxidant. The observed trends in catalytic activity showed that the encapsulation and hydrogenation of complexes played a key role in the selective delignification of pulp. By encapsulating and hydrogenating the complexes, their spatial separation increased which strongly enhanced the activity of complexes. The encapsulation and hydrogenation of complexes resulted in a significant increase in delignification and properties of pulp. The delignification was dependent on the nature of the complexes. The reasons were investigated by spectroscopic techniques.

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1 Introduction

Lignin is a complex and heterogeneous mixture of polymers resulting from the oxidative combinatorial coupling of 4-hydroxyphenylpropanoids. The main building blocks of lignin are the coniferyl alcohol and sinapyl alcohol, with typically minor amounts of p-coumaryl alcohol. Lignin is the main source of color in pulp due to the presence of a variety of chromophores naturally present in the wood or created in the pulp mill. Chemical pulps (kraft and sulphite) contain residual lignin less than 5%. Subsequent bleaching is required to remove essentially all of the residual lignin from the pulp to meet brightness requirements. The bleaching of pulp is the chemical processing carried out on various types of pulp to decrease the color of the pulp, so that it becomes whiter. Bleaching of pulp is extremely important and useful reaction in pulp and paper industry.^1,2

In making pulp products, elemental chlorine bleaching is the chlorine bleaching process that produces and releases into the environment, large amounts of chlorinated organic compounds including dioxin and furans. It is a cheap gas to produce and is extensively used in pulping industries. Elemental chlorine-free bleaching (ECF bleaching) is not free of chlorine at all but only free from chlorine gas. Instead chlorine dioxide is used. ECF pulp production results in the release of high levels of chlorinated organic pollutants into the environment. Many of these chlorinated pollutants have been demonstrated to cause numerous health effects on humans including reproductive, developmental, immune and hormonal problems. Totally chlorine-free or TCF bleaching is one which uses normally oxygenic chemicals. The use of TCF bleaching is the safest process for the environment as it does not release toxins into the environment. For this reason, the pulp and paper industry has directed pulp practices to TCF bleaching processes.^3–6

TCF pulps tend to have lower brightness and reduced strength properties compared with ECF pulps. Selective delignification with TCF bleaching processes has received much attention recently but it is still difficult to activate oxygenic agent with available catalysts. Increasing restrictions on pulp selective delignification demand the use of sustainable and environmentally friendly processes.^7,8 At the same time, the biomimetic catalysts offer an alternative for conventional biological and chemical processes and have many advantages over natural enzyme system, allowing development of more robust catalysts at relative low cost. Such catalysts are pH and thermally more stable, may expand the scope of substrates and increase the scale of production. Moreover, their stability and selectivity may be improved by chemical modifications of the active sites by altering the immobilization procedures. The immobilization of active sites within solid host materials can lead to the creation of highly active and selective catalysts that can facilitate highly selective transformation.^9–11 In this respect, a well studied zeolite-encapsulated system involves the salen ligand (N,N′-bis (salicylaldehyde)ethylenediimine) that possesses two oxygen and two nitrogen atoms and can form stable complexes with transition metal ions. Salen-type complexes are a fundamental class of compounds in coordination chemistry. An extremely wide variety of reactions catalysed by salen complexes has been investigated.¹² Oxidations of lignin and lignin model compound by the use of such salen complexes with oxidants like O₂ and H₂O₂ have become of interest as models for biological oxidations, since salen complexes can mimic the lignin degrading enzymes. Co(salen)/O₂ oxidation of phenolic and non-phenolic β-O-4 aryl ether lignin model compounds showed very high conversion values within 30 min.^13–15 A series of biomimetic oxidation of lignin and lignin model compound using Co(salen) have been successfully studied in our laboratory.^16–19 Ambrose et al. reported the ionic liquid tagged Co(salen) with which veratryl alcohol, a lignin model compound, was selectively oxidized to veratraldehyde using air or pure oxygen as the source of oxygen.⁹ Co(salen) supported in SBA-15 showed higher activity for the oxidation of a lignin model phenolic dimmer [1-(4-hydroxy-3-methoxyphenoxy)-2-(2-methoxyphenoxy)-propane-1,3-diol] than that unsupported.²⁰

Salen complex catalysed oxidation of lignin offers a way to solve the difficult to activate oxygenic agent on pulp selective delignification with TCF bleaching processes. Catalytic delignification also could make pulp bleaching more environmentally friendly and more cost effective. Lignin and model compound studies enable us to develop the neat and encapsulated salen complex ([H₂]salen complex) used as an effective catalyst for the selective delignification of kraft pulp.²¹ From this and related studies, in the present work, in order to improve the cost-effectiveness and performance of catalysts the reduction and encapsulation have been used for the Cu([H₂]salen) complex. It would be more desirable to have more flexible complexes where [H₂]salen ligands are converted to tetrahydro-salen ligand ([H₄]salen) by the hydrogenation of C=N to C–N in the present of NaBH₄. The reduced derivatives [H₄]salen complexes have showed higher activity than the parent [H₂]salen complexes and are an alternative ligand system for oxidation catalysis.^22,23 Cu([H₄]salen) was encapsulated inside the pores of NaY by the impregnation (IM) and ship-in-a-bottle (SB) method. Furthermore, pulp was delignified with two NaY-encapsulated Cu([H₄]salen) in the presence of CH₃COOOH. Very little is known on the catalytic delignification of pulp with encapsulated [H₄]salen complex, which was suggested by us as a promising catalyst for the biomimetic bleaching of pulp.

2 Experimental

2.1 Materials

All chemicals are commercially available and were used as received without further purification, unless otherwise noted.

Laboratory oxygen delignified eucalyptus (E. urophylla × E. grandis) kraft pulp (46.4% ISO brightness, 13.0 kappa number, 1130 mL g⁻¹ viscosity and 83.34 N m g⁻¹ tensile index) was used for the catalytic TCF bleaching. Before bleaching, the pulp was thoroughly washed with tapwater and distilled water to remove all residual black liquor and stored at 0 °C.

2.2 Instrumentation

X-ray power diffraction (XRD) patterns of the samples NaY, Cu([H₂]salen)/IM, Cu([H₂]salen)/SB, Cu([H₄]salen)/IM, Cu([H₄]salen)/SB were recorded on a Rigaku Dmax X-ray diffractometer (Ni-filtered, CuKa radiation). The Fourier transform infrared (FTIR) spectra of the samples NaY, Cu([H₂]salen), Cu([H₂]salen)/IM, Cu([H₂]salen)/SB, Cu([H₄]salen), Cu([H₄]salen)/IM, Cu([H₄]salen)/SB were recorded on Equinox 55 FT-IR spectrometer in KBr. Atomic absorption spectrometer, Shimadzu AA-6800 was used for the estimation of copper of the samples Cu([H₂]salen), Cu([H₂]salen)/IM, Cu([H₂]salen)/SB, Cu([H₄]salen), Cu([H₄]salen)/IM, Cu([H₄]salen)/SB. Diffuse reflectance (DR) UV-vis measurements of the samples Cu([H₂]salen), Cu([H₂]salen)/SB, Cu([H₂]salen)/IM, Cu([H₄]salen), Cu([H₄]salen)/SB, Cu([H₄]salen)/IM in the range of 200–800 nm were performed on a Perkin-Elmer Lambda Bio40 spectrophotometer equipped with an integration sphere. The surface area (by BET method) of the samples Cu([H₂]salen)/IM, Cu([H₂]salen)/SB, Cu([H₄]salen)/IM, Cu([H₄]salen)/SB was determined by adsorption and desorption of nitrogen at the temperature of liquid nitrogen (77 K) using volumetric adsorption set-up (Micromeritics ASAP-2020, USA).

2.3 Preparation of zeolite-encapsulated Cu(II) [H₄]salen and [H₂]salen complexes

Cu(II) [H₄]salen and [H₂]salen complexes were encapsulated in NaY by the impregnation method (IM) and ship-in-a-bottle method (SB). The encapsulated catalysts were designated by the following notation: complex/encapsulation method.

The preparation of Cu(II) [H₄]salen and [H₂]salen complexes and the following encapsulations on NaY via ‘IM’ and ‘SB’ process were depicted in Scheme 1.

Scheme 1 The preparation of the Cu(II) [H₄]salen and [H₂]salen complexes and the immobilizations on NaY.

2.3.1 Preparation of [H₂] salen and [H₄]salen ligands. [H₄] salen and [H₂]salen ligands were prepared by following the procedures reported in the literature.^24,25

[H₂]salen (N,N′-bis(salicylidene)-ethylenediamine) was synthesized by dropwise addition of ethylenediamine (4.5 mmol), to a methanolic solution (18 mL) of salicylaldehyde (9 mmol). The reaction mixture was stirred for 30 min in a ice-water bath with a condenser. After standing for 15 min, the yellow precipitate of [H₂]salen was formed which was filtered out, washed with petroleum ether, and dried in vacuum. [H₄]salen was obtained by the stirring of 0.011 mol NaBH₄ with 0.01 mol [H₂]salen in CH₃OH at ambient temperature for 2 h. The solid product was washed with water and dried in vacuum. The purity of the ligands were confirmed by IR and H-1 NMR before coordination to Cu(II) cation.

2.3.2 Preparation of Cu([H₂]salen) and Cu([H₄]salen). Cu([H₂]salen) and Cu([H₄]salen) were prepared followed by a literature method.²⁶

Cu([H₂]salen) (Cu([H₄]salen)) was prepared by the following method: 1.16 g of [H₂]salen ([H₄]salen) was dissolved in 50 mL of hot methanol, followed by the addition of 1.08 g Cu(CH₃COO)₂ in 6.7 mL of hot deionised water. Then the mixture was refluxed for 60 min. The precipitate obtained was filtered out, washed with methanol, vacuum dried.

2.3.3 Preparation of Cu/Y. Cu/Y was obtained by ion-exchanging NaY with Cu²⁺ ions at room temperature in an aqueous solution of copper acetate (0.08 M) with a liquid/solid ratio of 20 (mL g⁻¹).

2.3.4 Preparation of Cu([H₂]salen)/IM and Cu([H₄]salen)/IM. The Cu([H₂]salen) (Cu([H₄]salen)) was supported on the NaY by impregnation: 150 mg of Cu([H₂]salen) (Cu([H₄]salen)) complex was dissolved in t-butanol and added to 300 mg of the NaY molecular sieve. The amount of solvent was just enough to cover the mixture. Then refluxed for 6 h under nitrogen gas flow. The resulting solid after filtering was extracted with t-butanol and acetonitrile using soxhlet extractor to the colorless. Then the solid was allowed to vacuum dry to obtain the final encapsulated complex.

2.3.5 Preparation of Cu([H₂]salen)/SB and Cu([H₄]salen)/SB. Cu/Y was mixed well with excessive [H₂]salen ligand ([H₄]salen) (ligand/metal = 3, m/m), and sealed into a round flask. Then the mixture was heated for 24 h at 150 °C under high vacuum condition. Uncomplexed ligand and complex were removed by extraction with acetone, and uncoordinated Cu²⁺ ions were removed by ion-exchange with NaCl aqueous solution (0.1 M). The sample was further washed thoroughly with deionized water, and air-dried for 60 min to give the Cu([H₂]salen)/SB (Cu([H₄]salen)/SB).

2.4 Catalytic TCF bleaching trials

In this paper, catalytic TCF bleaching sequences consisting of four stages included a treatment with catalytic agent and several bleaching stages such as alkaline extraction stage with hydrogen peroxide addition and two hydrogen peroxide stages, designated Cat, Ep, P₁ and P₂, respectively. After each stage the pulp was washed thoroughly with deionized water in a Büchner funnel and then pressed to 30% consistency.

A series of catalytic bleaching trials (Cat) were performed using Cu([H₂]salen), Cu([H₄]salen), Cu([H₂]salen)/SB, Cu([H₂]salen)/IM, Cu([H₄]salen)/SB, Cu([H₄]salen)/IM for 120 min at 70 °C: oxygen delignified eucalyptus kraft pulp (15 g, o. d. p), obtained as a 30% consistency, was combined with catalyst (0.03% on o. d. p), CH₃COOOH (0.5% on o. d. p), and some amount of deionized water to obtain a 5% pulp consistency. Control experiments were carried out also under identical conditions to examine the ability of catalyst.

Alkaline extraction with 0.5% (on o. d. p) addition of H₂O₂ (Ep) was performed by heating the pulp at a consistency of 10% at 70 °C for 60 min in 0.0375 M NaOH and using MgSO₄ (0.1% on o. d. p) and DTPA (0.05% on o. d. p) as stabilizer at this stage.

The consistency of pulp in P₁ and P₂ stage was maintained at a value of 10%. NaOH, MgSO₄, Na₂SiO₃, DTPA, H₂O₂ were used as bleaching chemicals. P₁-stage: NaOH 1.5% (on o. d. p), MgSO₄ 0.1% (on o. d. p), Na₂SiO₃ 1.0% (on o. d. p), DTPA 0.1% (on o. d. p), H₂O₂ 2.0% (on o. d. p), and 80 °C for 150 min; P₂-stage: NaOH 1.0% (on o. d. p), MgSO₄ 0.05% (on o. d. p), DTPA 0.1% (on o. d. p), H₂O₂ 1.5% (on o. d. p), and 80 °C for 90 min.

All experiments were run at least three. The relative error in the parallel experiments was lower than the standard level.

2.5 Analytical methods of pulp

Kappa numbers of pulp were determined using TAPPI Test Method T 236 om-06. The kappa number of a pulp is a standard pulp and paper industry index of its lignin content. Pulp viscosity (T 230 om-08), a measure of its strength properties depending on molecular mass, were obtained using capillary viscometer method. Brightness of pulp (T452) was measured for samples obtained in each bleaching sequence. Sample sheets (T 205 sp-06) for physical tests of pulp were formed by means of a handsheet former. The handsheet weight varied from 60 to 65 g m⁻². Physical properties (T 220 sp-10) of pulp handsheets were measured for tensile index.

3 Results and discussion

3.1 Characterization of the catalyst

3.1.1 Chemical analysis and XRD. The analytical data of neat and encapsulated Cu(II) [H₄]salen and [H₂]salen complexes were given in Table 1. The encapsulation methods employed led to the decrease in copper amount of the complex. However, it maybe noted that in Table 1 the amount of copper was higher in sample obtained by ‘IM’ rather than the ‘SB’ method. This suggests that the probability of retention of the complex inside the supercage during the synthesis was higher for complex obtained by ‘IM’.¹²

Table 1 Characteristics of neat and encapsulated Cu(II) [H₄]salen and [H₂]salen complexes^a

Sample	Cu content (wt%)	SBET (m^2 g^−1)	VBJH (cm^3 g^−1)	DBET (nm)
a Specific surface area, SBET (m^2 g^−1); pore volume, VBJH (cm^3 g^−1); pore diameter, DBET (nm).
NaY	—	584.38	0.32	3.22
Cu([H2]salen)	19.00	—	—	—
Cu([H2]salen)/IM	1.88	270.10	0.15	2.17
Cu([H2]salen)/SB	1.21	581.77	0.26	2.22
Cu([H4]salen)	19.22	—	—	—
Cu([H4]salen)/IM	3.99	308.38	0.17	2.21
Cu([H4]salen)/SB	0.86	528.03	0.28	2.28

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Cu(II) [H₄]salen and [H₂]salen complexes encapsulated in NaY showed lower surface area (S_BET) and pore volume (V_BJH) as well as pore diameter (D_BET) compared with pure NaY. The lowering of S_BET, V_BJH and D_BET indicated the presence of the Cu(II) [H₄]salen and [H₂]salen complex within the supercages of the NaY structure (Table 1).^27,28

XRD patterns of Cu(II) [H₄]salen and [H₂]salen complexes encapsulated within NaY confirmed that the complexes were structurally unaltered after the encapsulation procedure (Fig. 1).²⁹

Fig. 1 XRD spectra of (a) NaY, (b) Cu([H₂]salen)/IM, (c) Cu([H₂]salen)/SB, (d) Cu([H₄]salen)/IM, (e) Cu([H₄]salen)/SB.

3.1.2 FTIR. The characterization of neat and encapsulated complexes by FTIR spectroscopy was analysed in comparison (Fig. 2).

Fig. 2 FTIR spectra of (a) NaY, (b) Cu([H₂]salen), (c) Cu([H₂]salen)/IM, (d) Cu([H₂]salen)/SB, (e) Cu([H₄]salen), (f) Cu([H₄]salen)/IM, (g) Cu([H₄]salen)/SB.

FTIR spectra showed similarity upon encapsulation of the complexes, providing an evidence that the complex structure remained unchanged in the encapsulated state, in agreement with the XRD analysis.³⁰ The appearance of the bands at 568 cm⁻¹ (Co–N), 470 cm⁻¹ (Co–O), 1500 cm⁻¹ (aromatic ring), 1500–1300 cm⁻¹ (C–N for [H₄]salen) provided a further conclusive evidence of the successful encapsulation of Cu(II) [H₄]salen and [H₂]salen into the NaY through IM and SB methods.³¹

3.1.3 DR UV-vis. DR UV-vis spectroscopy was used to elucidate the nature of Cu species of neat and encapsulated complexes within NaY (Fig. 3). The neat and encapsulated complexes showed strong absorption bands at 200–250 nm ([H₄]salen and [H₂]salen), 300–350 nm (Cu [H₄]salen and [H₂]salen), which indicate that the complexes were dispersed within the zeolite supercages.³¹ However, the weak absorption of encapsulated complexes and hydrogenated samples in the DR UV-vis spectra confirmed the presence of small change in geometry and stability of complexes due to encapsulation and hydrogenation, which is in agreement with the analyses by Yang et al.³²

Fig. 3 DR UV-vis spectra of (a) Cu([H₂]salen), (b) Cu([H₂]salen)/SB, (c) Cu([H₂]salen)/IM, (d) Cu([H₄]salen), (e) Cu([H₄]salen)/SB, (f) Cu([H₄]salen)/IM.

3.2 Catalytic bleaching of pulp

For closer evaluation of the role of encapsulation and hydrogenation and their influence on catalytic activity, catalytic delignification of pulp was studied with neat Cu(II) [H₄]salen and [H₂]salen complexes and their heterogeneous analogues encapsulated in NaY using CH₃COOOH as the oxidant.

From the data presented in Fig. 4 and 5, it is clear that those heterogeneous catalysts which posses well isolated active sites displayed higher activity than their neat homogeneous analogues. In fact, while the encapsulated complexes afforded high value of selectivity (Fig. 4), brightness (Fig. 5) compared to the neat analogues, the [H₄] complexes showed to be more efficient than [H₂] complexes in pulp delignification with complexes. Influence of encapsulation method on catalytic activity was studied. The SB-encapsulation of heterogeneous Cu(II) [H₄]salen and [H₂]salen complexes resulted in increased selectivity, but decreased brightness. Cu([H₂]salen)/SB afforded selectivity of 12 222 that decreased to 9722 when Cu([H₂]salen)/IM complex was employed. By hydrogenating the [H₂]salen complex for Cu([H₄]salen)/SB complex the selectivity increased to 19 789. However, SB-encapsulated Cu(II) [H₄]salen and [H₂]salen complexes obtained a brightness of 87.8% ISO and 86.6% ISO that increased to 88.5% ISO and 87.1% ISO when IM-encapsulated Cu(II) [H₄]salen and [H₂]salen complexes were employed.

Fig. 4 Selectivity for the CatEp bleaching. ; K: kappa number; V: viscosity.

Fig. 5 Brightness for the CatEpP₁P₂ bleaching.

It is clear from the results obtained that the encapsulated and hydrogenated complexes exhibit a superior activity in selectivity and brightness as compared to the neat and un hydrogenated catalysts due to greater dispersion of the metal active sites which play a key role in catalysis in terms of cluster and supported-bound dimer or oxo- and hydroxo-bridged metal species.^33–35

A leaching experiment was conducted to test the effect of the encapsulation and hydrogenation on release of copper from complex. The test showed that only a trace amount of copper was detected (∼0.3 ppm) in solution by spectroscopic method and, the concentration of copper in solution for encapsulated complexes was lower than for neat complexes, while the value was more lower for [H₄](salen) complexes. The encapsulated and hydrogenated complexes showed good stability without significant loss in activity and selectivity within reaction.³⁶ Additionally, our UV-visible studies indicated that oxo species (copper-hydroperoxo, copper-oxo species) may varied with encapsulation and hydrogenation in these reaction. This was the likely reason for the difference in the activity of catalytic system.³⁷

4 Conclusions

The introduction of encapsulation and hydrogenation on Cu([H₂]salen) has a beneficial effect in catalytic delignification of pulp. The enhanced activity in delignification selectivity, pulp brightness compared to the neat and unhydrogenated analogues in pulp delignification with catalysts was studied using spectroscopic methods. The possible reasons could be attributed to the greater dispersion of the metal active sites. The encapsulation and hydrogenation on salen complexes are thus worth to be considered further for oxidative bleaching of pulps or for lignin oxidation.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of P. R. China (no. 21166011).

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