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Pulp bleaching by hydrogen peroxide activated with copper 2,2′-dipyridylamine and 4-aminopyridine complexes
Abstract
Oxygen-prebleached kraft pulp (OKP) was bleached with H2O2 activated with a copper complex coordinated with 2,2′-dipyridylamine (dpa) or 4-aminopyridine (4-ap) under alkaline conditions. Bleaching of OKP by H2O2 activated with the Cu(II)–dpa complex decreased the kappa number (k) and viscosity (v) of the pulp by 26 and 0.7%, respectively. In contrast pulp bleaching without the coordinator resulted in a decrease in k and v values of 25 and 7.8%, respectively. Thus, selectivity for delignification, expressed by k/v, was increased 12-fold by the coordination with dpa. ESR demonstrated that the coordination with dpa suppressed the production of OH by 57%. These results support the involvement of a hydroperoxo complex of Cu(II) formed by the reaction of −OOH with [Cu(II)(dpa)(H2O)3]2+. When the OKP was bleached with H2O2 activated with a Cu(II)–4-ap complex, selectivity (k/v) in pulp bleaching increased by 2.6-fold and production of OH decreased by 47%. We conclude that Cu(II)–dpa is a catalyst potentially applicable to totally chlorine free (TCF) bleaching sequences due to its high selectivity for delignification.
... The inhibition of •OH production by a diffusible fungal metabolite accounts for the extracellular system of the fungus that attenuates the formation of •OH in the presence of iron, molecular oxygen and free radicals produced during lignin biodegradation (Fig. 1). Recently, we reported that 1-nonadecene-2,3-dicarboxylic acid (ceriporic acid B), an extracellular metabolite of C. subvermispora, strongly inhibits •OH production and the depolymerization of cellulose by the Fenton reaction in the presence of iron ions, cellulose, H 2 O 2 , and a reductant for Fe 3+ , hydroquinone (HQ), at the physiological pH of the fungus (Fig. 2) (Rahmawati et al., 2005). The extracellular fungal system to control free radicals, active oxygen species and the peroxo-complex of transition metals can be applied to biomimetic lignin degradation and pulp bleaching (Rahmawati et al., 2005). ...
... Recently, we reported that 1-nonadecene-2,3-dicarboxylic acid (ceriporic acid B), an extracellular metabolite of C. subvermispora, strongly inhibits •OH production and the depolymerization of cellulose by the Fenton reaction in the presence of iron ions, cellulose, H 2 O 2 , and a reductant for Fe 3+ , hydroquinone (HQ), at the physiological pH of the fungus (Fig. 2) (Rahmawati et al., 2005). The extracellular fungal system to control free radicals, active oxygen species and the peroxo-complex of transition metals can be applied to biomimetic lignin degradation and pulp bleaching (Rahmawati et al., 2005). ...
... The reactions were carried out by mixing 0.5mM FeC1 3 , 0.25mM HQ, 100mM H 2 O 2 and 0.1g cellulose in the presence and absence of 2.5mM ceriporic acid B. Control contained cellulose and H 2 O 2 . (Rahmawati et al., 2005). ...
To avoid serious global warming by the emission of carbon dioxide from fossil fuels, there is a growing demand to produce energy and chemicals from carbon-neutral renewable resources, known as bio- mass. To convert wood biomass by enzymatic sac- charification and fermentation, degradation of the lignin network is necessary because cell wall poly- saccharides are covered with lignin in lignified plant cell walls. One potential approach to degrade lignin prior to saccharification and fermentation is to use the ligninolytic systems of white rot fungi. In the JSPS project "Production of cellulosic materials and bio- mass chemicals from unutilized plant resources", we studied conversion of lignified plant resources to energy, chemicals and feedstuff using the ligninolytic systems of white rot fungi. Among the numerous fungi so far isolated, a white rot fungus, Ceriporiopsis subvermispora, is characterized as one of the best biopulping fungi that degrade lignin without intensive damage of cellulose. Previous studies revealed that the selective ligninolysis by this fungus is catalyzed by low molecular mass compounds at a site far from extracellular enzymes and fungal hyphae. As a possible ligninolytic system by this fungus we fo- cused on a mechanism involving in situ lipid peroxi- dation and suppression of the Fenton reaction by fungal metabolites.
... Historically, pulp and paper production has been recognized as a significant source of pollution, since large amounts of solid residues, toxic effluents and gases are generated from the different steps of the pulping and bleaching processes. For example, pulping black liquors contain high amounts of phenolic compounds, which have a powerful toxic effect on microorganisms even at low concentrations (Mishra et al. 1995;Fortuny et al. 1998); and the pulps bleached with chlorinated chemicals (chlorine and chlorine dioxide) produce chlorolignins highly resistant to biodegradation, as well as other polluting compounds that could be acutely toxic, mutagenic and carcinogenic (Rahmawati et al. 2005). Although the chlorine and its derivatives are effective and cheap bleaching agents (Bianchi et al. 1999), environmental concerns have created a need for new technologies using other non-polluting bleaching sequences for pulp, based on the use of elemental chlorine-free or totally chlorine-free procedures (Rahmawati et al. 2005;Tanaka et al. 2004;Tutus 2004). ...
... For example, pulping black liquors contain high amounts of phenolic compounds, which have a powerful toxic effect on microorganisms even at low concentrations (Mishra et al. 1995;Fortuny et al. 1998); and the pulps bleached with chlorinated chemicals (chlorine and chlorine dioxide) produce chlorolignins highly resistant to biodegradation, as well as other polluting compounds that could be acutely toxic, mutagenic and carcinogenic (Rahmawati et al. 2005). Although the chlorine and its derivatives are effective and cheap bleaching agents (Bianchi et al. 1999), environmental concerns have created a need for new technologies using other non-polluting bleaching sequences for pulp, based on the use of elemental chlorine-free or totally chlorine-free procedures (Rahmawati et al. 2005;Tanaka et al. 2004;Tutus 2004). Totally chlorine-free pulp bleaching using oxygencontaining oxidative chemicals (such as molecular oxygen, ozone and hydrogen peroxide) has proved to be among the best potential alternatives to conventional chlorine-based industrial pulping and bleaching technologies both with respect to environment and economy (Shatalov and Pereira 2007;Weinstock et al. 1996). ...
... Among these chemicals, hydrogen peroxide is recognized as an environmentally friendly and strong oxidizing agent, being considered as one of the most important bleaching chemicals. In alkaline media, the equilibrium of H 2 O 2 shifts to the formation of hydroperoxide anion (–OOH) that is the principal active species in peroxide bleaching (Rahmawati et al. 2005;Brooks and Moore 2000). This anion is a strong nucleophile which, during bleaching, converts electron-rich chromophores typified by unsaturated aldehydes and ketones, and phenolic ring-conjugated ethylenic or carbonyl groups to their non-chromophoric counterparts. ...
Brewer’s spent grain (BSG) was evaluated for bleached pulp production. Two cellulose pulps with different chemical compositions
were produced by soda pulping: one from the original raw material and the other from material pretreated by dilute acid. Both
of them were bleached by a totally chlorine-free sequence performed in three stages, using 5% hydrogen peroxide in the two
initial, and a 0.25N NaOH solution in the last one. Chemical composition, kappa number, viscosity, brightness and yield of
bleached and unbleached pulps were evaluated. The high hemicellulose (28.4%w/w) and extractives (5.8%w/w) contents in original
BSG affected the pulping and bleaching processes. However, soda pulping of acid pretreated BSG gave a cellulose-rich pulp
(90.4%w/w) with low hemicellulose and extractives contents (7.9%w/w and <3.4% w/w, respectively), which was easily bleached
achieving a kappa number of 11.21, viscosity of 3.12cp, brightness of 71.3%, cellulose content of 95.7%w/w, and residual
lignin of 3.4%w/w. Alkaline and oxidative delignification of acid pretreated BSG was found as an attractive approach for
producing high-purity, chlorine-free cellulose pulp.
... Melanins were often considered as non-biodegradable polymers and defined on the basis of their supposed inertness and resistance to chemical attack (Prota, 1992). However, contradictory reports suggested slow fungal melanin biodegradation (Luther and Lipke, 1980;Liu et al., 1995;Rättö et al., 2001). ...
... It is also very encouraging that a thorough visual inspection of treated melanized paper samples did not reveal any significant damage to the integrity of the paper as a consequence of the treatment, regardless of the procedure employed. It was reported previously that the oxidative reaction intermediates are highly selective for aromatic structures, oxidizing aromatic lignin structures selectively and not affecting cellulose (Rahmawati et al., 2005). The near-neutral conditions (pH ca. ...
Fungal melanin staining is a problem on many cultural objects, ranging from the French Palaeolithic cave at Lascaux to books and papers in museum collections. Melanin, because it is insoluble and resistant to bleaching, may leave behind undesirable stains long after the fungal infestation has been controlled. Research into removal of melanin stains from paper and other sensitive substrates using industrial biomimetic oxidizing systems has shown considerable success. We studied relative concentration of the bleaching reagents and the reaction kinetics both in liquid suspensions of melanin and on melanized paper samples. Liquid suspension samples were tested for changes in their chemical composition (appearance and relative representation of functional groups and chemical bonds) with FTIR spectrometry. Changes in color of melanized paper samples were investigated with a CIE L*a*b system, where the effectiveness of the treatment (bleaching) was determined as a change in lightness (Delta L). Melanin was oxidized in the liquid suspensions, and the intensity of modification depended on the procedure employed. Bleaching of melanin with the biomimetic copper-pyridine complex proved to be far superior to the effect of white-rot fungal oxidizing enzymes, previously reported on by this group. (c) 2012 Elsevier Ltd. All rights reserved.
... Hydrogen peroxide (H 2 O 2 ) is a green oxidant [1,2] that is used in the pulp bleaching [3], wastewater treatment [4], chemical and pharmaceutical industries [5,6], causing no secondary pollution to the environment. Currently, the main industrial method for producing H 2 O 2 is anthraquinone oxidation, which suffers from high energy consumption and pollution creation [7]. ...
Photocatalytic H2O2 synthesis (PHS) via solar-driven process has emerged as a coveted orientation in the future low-carbon industry. However, the performance of conventional graphitic carbon nitride (g-C3N4) is still constrained by the poor charge transfer and inefficient oxygen reduction reaction (ORR). Cellulose, as its low cost and eco-friendly properties, can serve as a green electron mediate to promote PHS through hydrogen bonds. Nevertheless, the effect of hydrogen bond strength on photocatalytic activity has not been thoroughly explored. Herein, we report a facile method to boost photocatalytic H2O2 generation by modifying g-C3N4 with carboxymethyl cellulose (CMC) of different substitution degrees (DS=1.2, CMCH; DS=0.7, CMCL) via multiple hydrogen bonds. The photocatalytic H2O2 production rate of the CN/CMCL composite catalyst was up to 35.7 μmol·L-1·h-1, which was approximately 3.5 times than that of the pristine g-C3N4. Experimental characterization and density functional theory (DFT) calculations demonstrated that both CMCH and CMCL can act as electron mediates, and CMCL possesses a stronger interaction of hydrogen bonds with g-C3N4. As a result, CN/CMCL exhibits superior charge transfer efficiency, thus promoting the two-step single-electron ORR for PHS. This research significantly contributes to the understanding of the hydrogen bond theory over PHS and sheds light on the potential applications of biomass materials containing rich hydroxyl groups significantly enhancing photocatalytic performance.
... 1 Introduction H 2 O 2 is an environment-friendly chemical product that has been enormously used in various fields, such as chemical synthesis, 1,2 environmental remediation, 3,4 pulp/paper bleaching, 5 and fuel cells. 6,7 At present, the anthraquinone process is still the main industrially scalable process to realize H 2 O 2 production, which requires a large amount of energy input as well as massive amounts of toxic organic solvents. ...
Semiconductor photocatalysis is deemed as a novel and promising process that can produce H2O2 from earth-abundant water and gaseous dioxygen using sunlight as the energy supply. The searching of novel catalysts for photocatalytic H2O2 production has received increasing attention in the last few years. Herein, size-controlled growth of ZnSe nanocrystals was realized via a solvothermal method by varying the amount of Se and KBH4. The performance of the as-obtained ZnSe nanocrystals towards photocatalytic H2O2 production depends on the mean size of the synthesized nanocrystals. Under O2-bubbling, the optimal ZnSe sample presented an excellent H2O2 production efficiency (8.596 mmol g-1 h-1), and the apparent quantum efficiency for H2O2 production reaches as high as 2.84% at λ = 420 nm. Under air-bubbling, the accumulation of H2O2 was as high as 1.758 mmol L-1 after 3 h irradiation at the ZnSe dosage of 0.4 g L-1. The photocatalytic H2O2 production performance is far superior to the most investigated semiconductors such as TiO2, g-C3N4, and ZnS.
... 5,6 For household cleaning applications, it is used as an oxygen-based bleaching agent. 7 However, under ambient conditions, the reaction rate is too slow, and a long reaction time is required. Because of that, catalysts are needed to accelerate the reaction rate. ...
In most chemical reactions, reaction rates increase with increasing reactant concentrations. In this study, we report an unusual catalytic oxidation reaction with a negative reaction order, in which the reactant concentration inversely affected the oxidation rate. In the reaction, trypan blue was oxidized by hydrogen peroxide with copper-triglycine as a catalyst. Under a strong alkaline condition, the reaction rate was inversely proportional to the hydrogen peroxide concentration (i.e., the reaction rate was faster when the hydrogen peroxide concentration was lower). Without the copper-triglycine catalyst, the phenomenon did not happen. A possible explanation was that hydrogen peroxide competed with the reactive species of the catalyst and slowed down the reaction. This phenomenon had an important implication in advanced oxidation processes for wastewater treatments. To achieve a faster oxidation rate of trypan blue, one should add hydrogen peroxide slowly to keep its concentration low at all times. On the basis of this principle, we developed a continuous microdroplet injection process to deliver the hydrogen peroxide solution as droplets. This process was faster and more efficient than a batch process for the degradation of trypan blue.
... Unfortunately, a common bleaching process still involves chlorine-based chemical i.e. natrium chlorite (NaClO 2 ) as bleaching agent. In spite of its cheapness and effectiveness as bleaching agent [11], chlorine bleaching has famous with its toxic waste [12]. Thus, alternative environmentally bleaching agent is inevitably needed. ...
Cellulose is a versatile polymer which can be extracted from various agricultural waste. Sugar palm ( Arenga pinnata ) fibre (SPF) is one of potential cellulose source. The cellulose extraction from SPF was conducted by alkaline treatment with sodium hydroxide (NaOH) solution (10%) and peroxide treatments with hydrogen peroxide (H 2 O 2 ) solution (5, 10, 15%). The alkaline treatment was supposed to remove most of hemicellulose and some of lignin. The following peroxide treatment aimed to remove remaining lignin. The results of each step were analyzed its composition, visual appearances, colour and morphological aspect. The results showed that increasing of H 2 O 2 solution concentration not only tends to increase cellulose content and whiteness but also reduce the diameter size of fibre which indicates the removal of impurities (hemicellulose and lignin) from the fibre. In conclusion, cellulose was successfully extracted from SPF by alkaline treatment followed by peroxide treatments which the best result was in condition of 15% H 2 O 2 solution concentration.
... This anion is a strong nucleophile, during bleaching, which converts electron-rich chromophores typified by unsaturatedaldehydes and ketones, and phenolic ringconjugated ethylenic or carbonyl groups to their nonchromophoriccounterparts. The reactions of lignin with peroxide are not reversible and lead to the permanent removal of most of the chromophoric groups present in the lignin molecule (Rahmawati et al. 2005). In contrast with hydrogen peroxide, sodium dithionite is classified as reductive bleaching agent. ...
The fiber morphology, the content of hydrogen bonds (HBs) of different models, cellulose crystalline structure, water retention value (WRV), and strength properties of eucalyptus pulp bleached by different bleaching methods (hydrogen peroxide bleaching and sodium dithionite bleaching) were investigated. The results of fourier transform infrared spectrometer (FTIR) showed that the content of intramolecular hydrogen bonds (HB intra ) increased by 11.6 % and 4.8 % after hydrogen peroxide bleaching and sodium dithionite bleaching, respectively. The energy of the hydrogen bonds was changed after bleaching treatment. The hydrogen bonding distances showed a small change after different bleaching treatment. The results of X-ray diffraction (XRD) demonstrated a decrease in the average width of crystallite size in the (002) lattice plane after different bleaching treatment, which was the same trend with the variability of cellulose crystallinity. Compared with the unbleached pulp, the WRV and strength properties of the bleached pulp increased after each bleaching process. Tear index of handsheets made from the hydrogen peroxide and sodium dithionite bleaching pulps were 46.0 % and 54.8 %, respectively. The sodium dithionite bleaching treatment had more significant effects on fiber swelling capability.
... One of the most promising methods of enhancing the H 2 O 2 reactivity toward pollutant degradation involves the use of activators based on transition metal complexes (Aravindhan et al., 2003;Chen et al., 2012;Liott et al., 2009;Sorokin and Kudrik, 2011;Yan et al., 2012). The activation of H 2 O 2 with a copper complex of 2, 2-dipyridylamine or 4-aminopyridine under alkaline conditions was used to decolorize oxygen pre-bleached Kraft pulp due to its high selectivity for delignification (Rahmawati et al., 2005). Heterogeneous catalysis for the degradation of resistant pollutants is preferred over homogeneous catalysis as it reduces secondary contamination due to the release of metals from the catalyst. ...
This paper reports an improved chlorolignin (CL) oxidative decolorization in alkaline media by the use of a heterogeneous system consisting of hydrogen peroxide and a silica [bis(dibenzoylmethido) copper H] (SI) catalyst. The catalytic functions of SI in the presence of H2O2 were evaluated by comparing the chlorolignin decolorization in media with and without addition of catalyst. Since decolorization is associated with CL degradation, the pH variation and chloride release assays were carried out to monitor both processes. The influence of several factors such as H2O2 and chlorolignin concentration initial pH, reaction time, temperature, and catalyst dose were assessed to improve the catalytic reactions. Also, the stability and potential reusability of the catalyst were studied. It was found that the oxidative reaction follows pseudo-first order kinetics with respect to chlorolignin concentration. Activation energy, as well as activation enthalpy and entropy, were calculated using the Arrhenius and Eyring equations, respectively. © 2017, Gh. Asachi Technical University of Iasi. All rights reserved.
... Many efforts have been devoted to develop the enzyme-free sensors with simplicity and wide responding range [9]. Recently, H 2 O 2 plays an increasing important role in the alkaline processes of pulp bleaching [10], advanced oxidation processes (AOPs) [11][12][13] and environmental remediation [14]. Limited information for the H 2 O 2 determination in alkaline media is available. ...
... These processes have been utilized commercially for wood pulp delignification and bleaching since the early 1980s [21], and for the commercial production of vanillin derived from the lignin removed during sulfite pulping of softwoods since the 1940s [22]. In addition, homogeneous catalysts and activators of O 2 and H 2 O 2 have been extensively investigated for their abilities to enhance delignification, for pulp brightening, and for their selectivities toward the desired lignin-directed oxidation reactions versus cellulose scission reactions [23][24][25][26][27][28][29]. In one study, Korpi et al. screened 189 different metalligand combinations for the oxidation of veratryl alcohol to veratraldehyde and found that copper complexes of 2,2′-bipyridine and 1,10-phenanthroline were especially active [30,31]. ...
Alkaline hydrogen peroxide pretreatment catalyzed by Cu(II) 2,2'-bipyridine complexes has previously been determined to substantially improve the enzymatic hydrolysis of woody plants including hybrid poplar as a consequence of moderate delignification. In the present work, cell wall morphological and lignin structural changes were characterized for this pretreatment approach to gain insights into pretreatment outcomes and, specifically, to identify the extent and nature of lignin modification.
Through TEM imaging, this catalytic oxidation process was shown to disrupt cell wall layers in hybrid poplar. Cu-containing nanoparticles, primarily in the Cu(I) oxidation state, co-localized with the disrupted regions, providing indirect evidence of catalytic activity whereby soluble Cu(II) complexes are reduced and precipitated during pretreatment. The concentration of alkali-soluble polymeric and oligomeric lignin was substantially higher for the Cu-catalyzed oxidative pretreatment. This alkali-soluble lignin content increased with time during the catalytic oxidation process, although the molecular weight distributions were unaltered. Yields of aromatic monomers (including phenolic acids and aldehydes) were found to be less than 0.2 % (wt/wt) on lignin. Oxidation of the benzylic alcohol in the lignin side-chain was evident in NMR spectra of the solubilized lignin, whereas minimal changes were observed for the pretreatment-insoluble lignin.
These results provide indirect evidence for catalytic activity within the cell wall. The low yields of lignin-derived aromatic monomers, together with the detailed characterization of the pretreatment-soluble and pretreatment-insoluble lignins, indicate that the majority of both lignin pools remained relatively unmodified. As such, the lignins resulting from this process retain features closely resembling native lignins and may, therefore, be amenable to subsequent valorization.
... The strategies used by these organisms include the release of reactive oxygen species produced by redox-active metals and metalloenzymes [27] as well as the excretion of species-dependent combinations of lignin-modifying oxidoreductases [28][29][30][31], monooxygenases [32], and glycan-acting hydrolases, esterases, and lyases. Several abiotic catalytic oxidative treatments that mimic certain features of these successful biological approaches have recently been investigated as technologies for pulp bleaching or delignification [33,34] and the pretreatment of cellulosic biomass for the production of biofuels [35,36]. Leskelä and co-workers developed a pressurized O 2dependent strategy catalyzed by copper-diimine complexes that is effective in both pretreatment processes and pulp bleaching [36][37][38]. ...
One route for producing cellulosic biofuels is by the fermentation of lignocellulose-derived sugars generated from a pretreatment that can be effectively coupled with an enzymatic hydrolysis of the plant cell wall. While woody biomass exhibits a number of positive agronomic and logistical attributes, these feedstocks are significantly more recalcitrant to chemical pretreatments than herbaceous feedstocks, requiring higher chemical and energy inputs to achieve high sugar yields from enzymatic hydrolysis. We previously discovered that alkaline hydrogen peroxide (AHP) pretreatment catalyzed by copper(II) 2,2 -bipyridine complexes significantly improves subsequent enzymatic glucose and xylose release from hybrid poplar heartwood and sapwood relative to uncatalyzed AHP pretreatment at modest reaction conditions (room temperature and atmospheric pressure). In the present work, the reaction conditions for this catalyzed AHP pretreatment were investigated in more detail with the aim of better characterizing the relationship between pretreatment conditions and subsequent enzymatic sugar release.
We found that for a wide range of pretreatment conditions, the catalyzed pretreatment resulted in significantly higher glucose and xylose enzymatic hydrolysis yields (as high as 80% for both glucose and xylose) relative to uncatalyzed pretreatment (up to 40% for glucose and 50% for xylose). We identified that the extent of improvement in glucan and xylan yield using this catalyzed pretreatment approach was a function of pretreatment conditions that included H2O2 loading on biomass, catalyst concentration, solids concentration, and pretreatment duration. Based on these results, several important improvements in pretreatment and hydrolysis conditions were identified that may have a positive economic impact for a process employing a catalyzed oxidative pretreatment. These improvements include identifying that: (1) substantially lower H2O2 loadings can be used that may result in up to a 50-65% decrease in H2O2 application (from 100 mg H2O2/g biomass to 35--50 mg/g) with only minor losses in glucose and xylose yield, (2) a 60% decrease in the catalyst concentration from 5.0 mM to 2.0 mM (corresponding to a catalyst loading of 25 mumol/g biomass to 10 mumol/g biomass) can be achieved without a subsequent loss in glucose yield, (3) an order of magnitude improvement in the time required for pretreatment (minutes versus hours or days) can be realized using the catalyzed pretreatment approach, and (4) enzyme dosage can be reduced to less than 30 mg protein / g glucan and potentially further with only minor losses in glucose and xylose yields. In addition, we established that the reaction rate is improved in both catalyzed and uncatalyzed AHP pretreatment by increased solids concentrations.
This work explored the relationship between reaction conditions impacting a catalyzed oxidative pretreatment of woody biomass and identified that significant decreases in the H2O2, catalyst, and enzyme loading on the biomass as well as decreases in the pretreatment time could be realized with only minor losses in the subsequent sugar released enzymatically. Together these changes would have positive implications for the economics of a process based on this pretreatment approach.
... Copper-diimine complexes have been shown to improve oxygen delignification (Korpi et al., 2004) and H 2 O 2 pulp bleaching (Rahmawati et al., 2005). In one especially relevant study, Korpi et al. (2004) screened nearly 200 metaldiimine ligand combinations and found that combinations of iron or copper with o-phenanthroline or 2,2 0 -bipyridine catalyzed the oxidation of phenolic hydroxyl groups to aldehydes on lignin model compounds (veratryl and benzyl alcohol) during alkaline oxygen delignification at near theoretical conversions. ...
Copper(II) 2,2'-bipyridine (Cu(II) (bpy))-catalyzed alkaline hydrogen peroxide (AHP) pretreatment was performed on three biomass feedstocks including alkali pre-extracted switchgrass, silver birch, and a hybrid poplar cultivar. This catalytic approach was found to improve the subsequent enzymatic hydrolysis of plant cell wall polysaccharides to monosaccharides for all biomass types at alkaline pH relative to uncatalyzed pretreatment. The hybrid poplar exhibited the most significant improvement in enzymatic hydrolysis with monomeric sugar release and conversions more than doubling from 30% to 61% glucan conversion, while lignin solubilization was increased from 36.6% to 50.2% and hemicellulose solubilization was increased from 14.9% to 32.7%. It was found that Cu(II) (bpy)-catalyzed AHP pretreatment of cellulose resulted in significantly more depolymerization than uncatalyzed AHP pretreatment (78.4% versus 49.4% decrease in estimated degree of polymerization) and that carboxyl content the cellulose was significantly increased as well (5-fold increase versus 2-fold increase). Together, these results indicate that Cu(II) (bpy)-catalyzed AHP pretreatment represents a promising route to biomass deconstruction for bioenergy applications. Biotechnol. Bioeng. © 2012 Wiley Periodicals, Inc.
... Although the latter method is attractive, mainly because of environmental considerations, the use of hydrogen peroxide resulted in pulps whose ash content was higher than those bleached by the conventional chlorine-based method (Christov et al., 1998). Hydrogen peroxide is generally recognized as an environmentally friendly oxidant (Rahmawati et al., 2005). The length of the fiber is not changed in bleaching, but due to yield loss the linear density of the fiber is reduced. ...
Non-wood fibres are increasingly being used in the pulp and paper industry to help meet the increasing world demand for paper and also to reduce demand on declining forest reserves. In this study, the dissolving pulp (alpha-cellulose) was produced from corn stalk of the agricultural farms of Kermanshah/Iran. The sequences of experimental work were: water and acid prehydrolysis for 30min at 160˚C, then a Kraft pulping for 90min at 170˚C, followed by a HEH and HEHP bleaching process. The influence of two parameters: active alkali (14-20%), and sulphidity (10, 25%) on the pulp properties (yield, kappa number, and degree of polymerization), was investigated. In optimum conditions (water prehydrolysis, pulping with 25% sulphidity, and HEHP bleaching sequences), the alpha-cellulose content, degree of polymerization, and ash content of the produced dissolving pulp were: 94.1%, 269 and 2% respectively. With respect to delignification, active alkali was the most important influent parameter whereas the sulphidity had a minor effect.
... [23][24] Such systems were mostly reported for the decolorization of wastewaters containing synthetic dyes, and are highly enhanced by complexation of Cu(II) with various chelating agents. Only few researches were focused on the decolorization of lignin derivatives and model compounds with Cu(II) systems, [25][26][27][28] all being involved in homogeneous catalysis. ...
The potential of ammonium lignosulfonate (ALS) decolorization and degradation in aqueous solution was studied in a heterogeneous system using hydrogen peroxide and a Cu (II)-chelating ion exchanger. This was based on acrylic copolymers functionalized with N,N dimethylamino propylamine (DMAPA) as a catalyst. In order to optimize the efficiency of the system, the influence of such process parameters like H(2)O(2) concentration, pH, contact time, temperature, ALS concentration and catalyst amount were evaluated. The apparent rate constant of decolorization calculated from the absorbance data indicates that the process profiles follow pseudo-first order kinetics. Lignosulfonate degradation was furthermore studied by FTIR spectroscopy, thermogravimetric analysis and determination in phenolic compounds. The catalyst stability and reusability have also been investigated. Our experimental results clearly indicate that, under optimum conditions, the ammonium lignosulfonate solutions exhibit a total bleaching associated with degradation and significant mineralization to CO(2).
In view of their vast global usage in both consumer products and industrial processes, environmental emission and fates of siloxanes have become concerned issue. This review summarized the research progress, especially in the last decade, on production/consumption data, toxicities, analysis methods, environmental distribution, migration and degradation/transformation of both dimethylsiloxanes and modified siloxanes in atmospheric, aquatic and terrestrial compartments from various areas (especially in China). In spite of their fast degradation (hydrolysis and hydroxylation, etc) in various matrices (except sediment), dimethylsiloxane oligomers have been found in various environmental matrices from many countries due to their constant usage and emission. Moreover, recent literatures have paid attention to behaviors of dimethylsiloxanes in industrial areas, e.g., their higher residual levels compared with residential areas and unique transformed products (such as halogenated products) arose from special industrial production scenarios. Meanwhile, although most prior studies focused on dimethylsiloxanes, identification of modified-siloxanes with other functional groups in environment have been beginning to attract the attention of scientists. Furthermore, related literatures indicated that compared with dimethylsiloxanes, both halogenated-dimethylsiloxanes and modified methylsiloxanes (phenylsiloxanes and trifluoropropylsiloxanes) could have stronger persistence due to their weaker volatilization and degradation, especially in terrestrial matrices.
Hydrogen peroxide was directly produced from oxygen and formic acid, catalysed by a hetero-dinuclear Ir-Ni complex with two adjacent sites, at ambient temperature. Synergistic catalysis derived from the hetero-dinuclear Ir and Ni centres was demonstrated by comparing its activity to those of the component mononuclear Ir and Ni complexes. A reaction intermediate of Ir-hydrido was detected by UV-vis, ESI-TOF-MS, and 1H NMR spectroscopies. It was revealed that the Ir moiety serves as an active species of Ir-hydrido, reacting with oxygen to afford an Ir-hydroperoxide species through O2 insertion, which is the rate-determining step for H2O2 production. Meanwhile, the Ni moiety promotes H2O2 formation by activating solvents as proton sources. We also found that H2O2 production is strongly affected by the solvent dielectric constants (DE); the highest H2O2 concentration was obtained in ethylene glycol with a moderate DE. The catalytic mechanism of H2O2 production by the Ir-Ni complex was discussed, based on kinetic analysis, isotope labelling experiments, and theoretical DFT calculations.
Agar that has been extracted via traditional alkaline pretreatment always appears yellowish because it contains pigments. This characteristic adversely affects the food and biotechnological applications of agar. In this work, agar was bleached with hydrogen peroxide. The whiteness of high-whiteness agar reached 74.4%, which was 32% higher than that of raw agar (56.48%). The high-whiteness agar bleached with hydrogen peroxide exhibited several other excellent qualities over raw agar. These characteristics included, low sulfate content (0.67%), low ash content (1.01%), and high transparency (64.4%). In particular, the transparency of the high-whiteness agar was 13% higher than that of raw agar (56.8%). By contrast, the gel strengths of the two agars did not significantly differ. The structures of raw and high-whiteness agar were characterized by using thermogravimetric (TG)–differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). TG–DSC analysis indicated that the high-whiteness agar had better thermal stability, hygroscopicity, and water-holding capacity than raw agar. SEM images showed that the surface of the high-whiteness agar had imperfections or fissures that were caused by the oxidative degradation of hydrogen peroxide. The diversified application of the high-whiteness agar was explored on the basis of the promotion of agar quality. Results indicated that the visual appearance of the jelly prepared with the high-whiteness agar was better than that of the jelly prepared with raw agar. Moreover, the high-whiteness agar medium was easier to observe and use for colony counting than biochemical medium.
2,2’-Dipyridylamine (dpa) and related compounds belong to the family of polydentate nitrogen ligands. More than a century has passed since its first report but new complexes and applications have been...
In this paper, a CuS/graphene oxide (GO)/multiwalled carbon nanotubes (MWCNTs) nanocomposite was firstly prepared for ultrasensitive electrochemical non-enzymatic H2O2 detection in the alkaline environment. The morphology and microstructure of CuS/GO/MWCNTs were systematically characterized, in which the novel hybrid architectures of nanoparticles, nanotubes and nanosheets are obtained. As a result, the as-fabricated electrochemical sensor exhibits an excellent H2O2 amperometric response with a wide linear ranging from 0.45 mM to 60 mM, low detection limit of 0.6 μM and ultra-high sensitivity of 386 μA•mM-1•cm-2. Besides, the long-term stability and outstanding reproducibility were also evaluated, suggesting the promising potential for the determination of H2O2 in practical applications.
Simulated experiments indicated that chlorinated volatile methylsiloxanes, detected by Q-TOF GC/MS, could be generated in a pulp-bleaching process, where poly(dimethylsiloxane)s fluids with volatile methylsiloxanes as impurities and molecular chlorine were used as a defoamer and bleaching agent, respectively. In the producing processes of one papermaking factory, the mean total concentrations of monochlorinated D4, D5, and D6, i.e., D3D(CH2Cl), D4D(CH2Cl), and D5D(CH2Cl), were 0.0430–287 μg/L in aqueous samples, while they were 0.0329–270 μg/g in solid samples. In the coupled papermaking-wastewater treatment processes, D3D(CH2Cl), D4D(CH2Cl), and D5D(CH2Cl) were detected in all water (0.113–8.68 μg/L) and solid samples (0.888–26.2 μg/g), with solid–water partition values (468–3982 L/kg) 1.08–4.82 times higher than those of their corresponding nonchlorinated analogs. The removing efficiencies of D3D(CH2Cl)–D5D(CH2Cl) in the whole wastewater treatment processes were 77.1–81.6%, and sorption to sludge (35.7–74.1%) and removal in the primary clarifier (7.19–32.5%) had major contributions to their total removal. Elimination experiments showed that 1) hydrolysis half-lives of D3D(CH2Cl)–D5D(CH2Cl) (0.9–346 h) in the primary clarifier (pH = 7.8–9.2) were 2.16–3.60 times shorter than those of their nonchlorinated analogs; 2) D3D(CH2Cl)–D5D(CH2Cl) were hardly degraded in oxic sludge treatment process, and their volatilization half-lives (7.38–21.1 h) in oxic sludge were 1.21–1.50 times longer than those of their nonchlorinated analogs.
Hardwood residues of yellow-poplar (Liriodendron tulipifera) and northern red oak (Querus rubra) were pretreated with two alkaline solutions: 1) ammonium hydroxide and sodium hydroxide (ASO) and 2) hydrogen peroxide/ammonium hydroxide and sodium hydroxide (PASO) mixtures. After pretreatment, particulates of yellow-poplar, northern red oak, and pulp were enzymatically hydrolyzed with cellulases (Accellerase 1000 [Genencor International, Rochester, NY]) for comparisons of glucose production. ASO pretreatment increased the lignin solubility of yellow-poplar and northern red oak by an average of 8.6%. PASO pretreatment performed better than ASO pretreatment with respect to lignin decrease in the pretreated wood. Sugar yield after enzymatic hydrolysis ranged from 70-81 mg/mL for PASO-pretreated yellow-poplar and northern red oak residues, respectively, and 50-62 mg/mL for ASO-pretreated yellow-poplar and northern red oak.
An extensive review of 110 references reflecting the state of the art in the bleaching of pulp with hydrogen peroxide is presented.
A selective white rot fungus, Ceriporiopsis subvermispora is shown to, degrade lignin without extensive damage to cellulose. This selective ligninolysis reaction is catalyzed by low molecular mass compounds at a site far from the enzymes. At an incipient stage of the wood decay, the fungus catalyzed in situ lipid peroxidation and secreted alkylitaconic acids and ceriporic acids. The extracellular metabolites are thought to attenuate the iron redox reactions, thereby inhibiting the production of a cellulolytic active oxygen species such as hydroxyl radicals. The selective lignolysis by this fungus can be applied to pretreatments of wood for ethanol fermentation, methane fermentation and feedstuff production.
Simple FeCl3-derived iron catalysts are used for the cleavage of β-O-4 linkages in lignin and lignin model compounds. The degradation of the β-O-4 linkages and the resinol structures in both organosolv and kraft lignin was proven by 2D-NMR (HSQC) experiments, and the oxidative depolymerisation of these lignin sources was confirmed by GPC. Key reactive species facilitating this cleavage are methyl radicals generated from H2O2 and DMSO
Transition-metal-containing hydrotalcites (HTc) and V(acac)3 /Cu(NO3 )2 ⋅3 H2 O (acac=acetylacetonate) mixtures were tested for their catalytic activity in the cleavage of the lignin model compound erythro-1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-l,3-propanediol (1) with molecular oxygen as oxidant. Both catalytic systems displayed high activity and good selectivity and afforded veratric acid as the main product. The catalyst behavior was studied by EPR spectroscopy, XRD, and Raman spectroscopy. After the catalysts were established for the model system, lignin depolymerization studies were performed with various organsolv and kraft lignin sources. The oxidative depolymerization and lignin bond cleavage were monitored by gel permeation chromatography (GPC), MALDI MS, and 2D-NMR (HSQC). Irrespective of the lignin pretreatment, both HTc-Cu-V and V(acac)3 /Cu(NO3 )2 ⋅3 H2 O were able to cleave the β-O-4 linkages and the resinol structures to form dimeric and trimeric products.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Pretreatment is currently the most costly single step (up to 20% of the total cost) and can have significant impacts on the performance and cost of all other sub-sequential steps in the biochemical process of cellulosic ethanol production. This study was to pretreat genetically improved poplar to produce cellulosic ethanol. Juvenile hybrid poplar samples were collected from a plantation on West Virginia University Agronomy Farm. The wood samples were debarked, dried, and milled to particles passing a screen with 40-mesh holes. Then the samples were pretreated using environmentally-friendly methods like hydrogen peroxide exposed in sun and conventional heating system at 80ºC. The particles were characterized using optical microscopy and the particle size and its distributions were analyzed. ACCELLERASETM 1000” enzyme was used to further evaluate the pretreated samples for sugar production. The sugars produced were evaluated with high-performance liquid chromatography. The results indicated that the pretreatments slightly decreased lignin content.
Rice straw was cooked with three levels of alkali content (12, 14 and 16%) in cooking liquor by soda-organosolv (ethanol, diethylene glycol, dimethyl formamide) process. The effect of alkali addition on yield, kappa number, brightness and viscosity of organosolv unbleached pulps was studied. The TCF bleachability of these organosolv pulps and kraft pulp was carried out using a simple three stage peroxide bleaching sequence without oxygen pre-bleaching. All tested pulps were bleached under identical conditions during each stage. After full peroxide bleaching sequences of three alkali-organosolv pulps, some similarity in behaviour of brightness and peroxide consumption for each alkali percentage was observed. The final brightness of 63-70% ISO was attained for all tested pulps. The chemical charge required to reach this level of brightness varied for different pulps (despite the equal initial content of the residual lignin), which was directly related to starting brightness values. The high bleached yield for organosolv pulps (approximately, in range of 90-91% o.d.p) indicates to the limited organosolv carbohydrate degradation during peroxide bleaching. The strength properties of bleached organosolv pulps were higher than kraft pulp. No relation was found between improvement in brightness and lignin removal during hydrogen peroxide bleaching.
In this research, the chemistry of peroxide bleaching effect on rice straw pulp has been thoroughly studied both with lignin model compounds and with degradation of carbohydrates. Rice straw was cooked in by soda-(ethanol, diethylene glycol, dimethyl formamide) and kraft type. After three stage peroxide bleaching of organosolv pulps, some similarity in behavior of brightness and peroxide consumption for each alkali percentage has been observed. Final brightness of 63–70% ISO was attained for all tested pulps. The carbohydrates reaction in peroxide delignification results in a loss of viscosity due to attack by hydroxyl radicals to the carbohydrates chains. The yield loss in chemical pulping due to degradation and dissolution of polysaccharides (particularly hemicellulose) is substantial and constitutes a serious drawback of the process. The high obtained bleached yield for organosolv pulps (in the range of 90–91% o.d.p.) point to limited organosolv carbohydrate degradation during peroxide bleaching.
Hydrogen peroxide molecules play a significant role in controlling certain cellular functions, and its excess causes significant damage to biological systems. There is experimental evidence that its decomposition is accelerated above phospholipids membranes surface. We propose a mechanism for decomposition of hydrogen peroxide on amino-phospholipids surface model based in Dmo13/DPT calculations. The model was built using periodic boundary conditions. Each unit cell contained two phospholipids molecules, two hydrogen peroxide molecules, and nine water molecules. In the studied reaction, two hydrogen peroxide molecules react in a bimolecular reaction to yield an oxygen molecule and two waters, The reaction proceeds by two steps. In the first step, an intermediate hydrogen trioxide from two H2O2 molecules is formed. In the second step, this intermediate is cleaved in O-2 and H2O. There are proton exchanges along the interface between water and amine-phospholipids monolayer in all parts of the pathway. A parallel periodic model of pure water was built to compare the free energy variation through the reaction. Our results show that first step, intermediate hydrogen trioxide formation, is the limiting step of the reaction, having a free energy barrier of 8.76 kcal mol(-1) in the amino-phospholipids surface model and 25.56 kcal mol(-1) in the case of pure water model. It could be hypothesized that cell membrane surface environment could enhance this reaction by a neighboring catalyst effect.
The aim of this work was to study the production of cellulosic dissolving-grade pulp, alpha-cellulose, using corn stalk residue as non-wood material and industrial waste water as pulping liquid. Industrial waste water obtained from a Merox unit operating at the Kermanshah Oil Refinery in Iran and corn stalk residue obtained from local agricultural farms were used as raw materials for the experiment. The pre-hydrolysis process was performed on the corn stalk for 30 min at 160 °C in a mini-digester. Subsequently, the corn stalk was subjected to Kraft pulping and to pulping with industrial waste water at 170 °C over a period of 90 min. Upon completion of the bleaching process of each mixture, the quality of the resulting cellulosic dissolving-grade pulps was studied. The laboratory investigation compared the following parameters of importance: influence of active alkali, sulfidity, and dilution ratio of the industrial waste water on pulp properties such as yield, kappa number and degree of polymerization. Under optimum conditions, the pre-hydrolysis/Kraft pulping with 20% active alkali, 25% sulfidity, and HEHP bleaching resulted in acceptable levels of alpha-cellulose content (94.8%), degree of polymerization (279), and ash content (0.75%) for the produced dissolving pulp. The Kraft pulping was compared with the pulping of corn stalk with industrial waste water, which increased the alpha-cellulose content to 97.4%, with a degree of polymerization of 241 and an ash content of 0.96%. Comparison of both experiments indicates that using industrial waste water in the pulping process gives satisfactory results for industrial applications using a non-wood material, yields a quality product with reduced capital investment and operation costs, and considerably helps the environmental preservation of wood-based raw materials.
Biomass is a renewable class of materials of growing interest amongst researchers aiming to achieve global sustainability. This review focuses on the homogeneous catalysis of the oxidation of biomass, in particular starch, cellulose and lignin. Often such catalytic reactions lead to depolymerisation of the material as happens in Nature with for example brown rot fungi. This depolymerisation can be desirable or not, and control in industrial applications is thus important to obtain the desired outcome. The two main oxidants in use are O2 and H2O2 and their use is described as appropriate. Industrial oxidation catalysis is highly significant in the bleaching of cellulose-containing materials due to its high volume application in the paper, pulp and laundry industries. Here, the presence of a ligand on the oxidising metal ion has a significant effect on the catalyst selectivity and stability. In addition to the bleaching of cellulose-containing materials, the oxidation of cellulose, starch, lignin and lignin model compounds are discussed with a focus on generating even more hydrophilic materials which have important applications or materials which may be further modified. Finally developing applications of biomass are described such as new support materials for catalysts, as supports for sensors and nanomaterials for microbial culture.
White-rot fungi are the most active lignin degrading organisms and thus, they play a key role in the carbon cycle on earth. Some species are known to cause heavy damage to wood construction and building materials, requiring that this damage be prevented by wood preservatives. Chemical products to increase wood durability currently in use are very effective but in addition, a strong demand to develop new products with less environmental impact cannot be overlooked. Biotechnological processes have been successfully implemented in the pulp and paper industry during the last decade. In the past, developments were driven mainly by environmental considerations. In the future however, the main driving force for research and development will be reductions in manufacturing costs using new, low investment delignification processes. The application of whiterot fungi, or their ligninolytic systems, is one option for this. Understanding the microbial mechanisms leading to wood- and especially lignin degradation is a prerequisite for understanding both the development of new wood preservatives as well as wood biotechnology processes.
Laccase is a multicopper oxidase which contains four coppers, one type 1, one type 2, and a coupled binuclear type 3 pair, the type 2 and type 3 copper centers together forming a trinuclear copper cluster. The reaction of reduced type 1 mercury derivative of laccase (T1HgLc) with dioxygen produces an oxygen intermediate which has now been studied in detail. Isotope ratio mass spectrometry (IRMS) has shown that both oxygen atoms of O{sub 2} are bound in the intermediate. EPR and SQUID magnetic suseptibility studies have shown that the intermediate is diamagnetic. The results combined with X-ray absorption edge data indicate that the intermediate contains a bound peroxide and that the two electrons have derived from the type 3 center which is antiferromagnetically coupled. EXAFS data show that there is no short Cu-oxo bond in the intermediate and that there is a new bridging interaction in the intermediate, with two coppers being separated by 3.4 A, that is not present in the resting enzyme. Circular dichroism (CD) and magnetic circular dichroism (MCD) studies in the ligand field region confirm that the two type 3 coppers are oxidized and antiferromagnetically coupled and that the type 2 copper is reduced. In addition, the charge transfer (CT) absorption spectrum of the intermediate supports a {mu}-1, 1 hydroperoxide description based on a comparison to Cu(II)-peroxo model spectra. 67 refs., 15 figs., 2 tabs.
A binuclear [Mn(III)Mn(IV)(mu-O)(2)(mu-CH3COO)L](ClO4(-))(2) complex with L=1,2 Bis-(4,7-dimethyl-1.4,7-triazacyclonon-1-yl)-ethane, described as a selective catalyst in hydrogen peroxide bleaching of softwood pulps, was tested in hardwood kraft pulp bleaching. The catalyst application gave rise to a higher consumption of peroxide which resulted in higher pulp brightness. The delignification improvement caused by the catalyst was shown to be much lower compared to catalysed peroxide bleaching of softwood kraft pulp. In contrast to the results of softwood pulp bleaching no selectivity improvements could be found when using the catalyst in bleaching of eucalyptus kraft pulp.
Reactions of copper(II)-polyamine-N-polycarboxylate complexes such as CuII(edta) (edta: ethylenediamenetetraacetic acid) and CuII(dtpa) (dtpa: diethylenetriaminepentaacetic acid) with hydrogen peroxide (H2O2) were investigated spectroscopically in the presence or absence of biological reductants. In the absence of biological reductants such as L-cysteine, N-acetyl-L-cysteine, L-ascorbic acid, NADH and glutathione, no reaction occurred between CuII(edta) and H2O2; but, in the presence of these biological reductants, CuII(edta) was reduced to CuI and then the subsequent redox reaction (Fenton-type reaction) between CuI and H2O2 occurred to yield hydroxyl radicals (·.OH). From these results, it is concluded that copper(II)-polyamine-N-polycarboxylate complexes cannot directly be reduced to CuI by H2O2, because the redox potentials of CU2+ ions towards H2O2 have been changed by ligation with these polyamine-N-polycarboxylate ligands.
Laccase is a multicopper oxidase which contains four coppers, one type 1, one type 2, and a coupled binuclear type 3 pair, the type 2 and type 3 copper centers together forming a trinuclear copper cluster. The type 1 mercury derivative of laccase (T1Hg Lc) has the type 1 center substituted with a redox-inactive Hg2+ ion and an intact trinuclear copper cluster. Reaction of H2O2 with fully oxidized T1Hg Lc produces a peroxide adduct which has now been studied in detail. Peroxide is shown to bind to the trinuclear cluster with low affinity, producing spectral and geometric features similar to the intermediate formed in the reduction of O2 to H2O which had been shown to have the type 2 copper reduced, the type 3 pair oxidized and antiferromagnetically coupled, and two coppers bridged at a distance of 3.4 Å. The peroxide adduct and the intermediate have similar geometric and electronic features with the type 2 oxidized in the adduct. This center is paramagnetic and has been studied in detail. Peroxide binds to the type 2 center. EPR and ligand field (NiR MCD) coupled with CT (absorption and MCD) data demonstrate that peroxide binds to the type 2 Cu which goes from being 3-coordinate in the resting protein to 4-coordinate in the peroxide adduct. Peroxide also binds to the type 3 site from a comparison of ligand field absorption and CD and the presence of more than one intense O22- → Cu(II) band in the CT absorption spectrum. A bridging interaction between coppers at 3.4 Å is seen from the EXAFS data. Possible geometric structures for the peroxide adduct and intermediate are proposed, with the electronic structural difference between the adduct and the intermediate being related to the type 2 copper being reduced in the latter. This study (i) firmly establishes the role of the type 2 in catalysis, (ii) demonstrates a type 2/type 3 bridging mode of binding that promote further 2e- reduction of peroxide to water, and (iii) provides further support for a peroxide-level intermediate in the catalytic cycle of the multicopper oxidases which involve two 2e- steps in the reduction of O2 to H2O.
Copper-zinc superoxide dismutase (CuZnSOD) catalyzes the disproportionation of superoxide to hydrogen peroxide and dioxygen at diffusion controlled rates, Previous mechanistic studies have focused on the dramatic electrostatic guidance mechanism by which superoxide is drawn into the active site of this enzyme. Another striking bur: less understood feature of this enzyme is its ability to dismutate superoxide over a wide range of pH (5-9.5) without any change in rate-determining step or structural changes at the active site copper. To investigate the explanation for this pH independence, we have redetermined the rate: of superoxide disproportionation, k(cat), catalyzed by the zinc-deficient form (Cu-apoSOD) of the enzyme as a function of pH and have found that it is pH-dependent, in contrast to the native enzyme, even under conditions in which the topper ion does not leave the native copper-binding site, In addition, we have determined the rare of reduction, k(1), of Cu-apoSOD by superoxide and have found that this step of the catalytic cycle is pH-independent. We conclude that the reoxidation rate, k(2), of the catalytic cycle is pH-dependent for Cu-apoSOD. These results have led us to propose that the key role of the zinc and of the hist idyl imidazolate that bridges copper and zinc in CuZnSOD is to aid in the rapid dissociation of the product peroxide.
The efficiency of a low molecular weight delignification system consisting of copper (II) and hydroperoxides in the presence of the copper coordinating compound 4-aminopyridine (4-AP) was evaluated using oxygen bleached kraft pulp (OKP). A kappa number reduction of 30% at pH 11.3 could be achieved by one single copper/4-AP/H2O2 stage. Consecutive treatment with alkaline peroxide resulted in a cumulative kappa number reduction of 52%. When H2O2 was replaced by the lipid hydroperoxide model compound cumene hydroperoxide (CHP), the kappa number reduction by the single copper stage increased to 52%. However, the selectivity of delignification was higher when H2O2 rather than organic hydroperoxides were used. The pH optimum for the selectivity of the process at pH 10 to 11 correlated well with that for the stability of the copper/4-AP complex. Studies that were carried out on 14C-labeled lignin and cellulose model compounds further demonstrated the selectivity of the coordinated copper system for lignin structures. Electron spin resonance (ESR) studies suggested that the presence of the copper coordinating compound suppressed the production of free OH-radicals.
Copper(II) complexes of the general formula [LCu(H2O)4]n+ (where L is a bidentate ligand andn = 1 or 2) activate hydrogen peroxide for the oxidation of quinaldine blue, an oxidation indicator. The copper(II) complexes of tri- and tetradentate ligands are shown to be inactive, as are the bis-complexes of bidentate ligands. The proposed mechanism for peroxide activation involves the formation of a copper(II)-hydroperoxide complex, which then rapidly oxidized the substrate. Comparison of reaction rates with different ligand systems, and different ligand-to-metal ratios, lead to the conclusion that the most active complexes are those in which two equatorial coordination positions are occupied by easily displaced water to form the active catalyst. Rate studies are performed which give an experimental rate law which is first order in copper(II) complex, zero order in substrate, and variable order in peroxide. These kinetics are predicated by the rate law derived from our proposed mechanism. The variable order in peroxide can be explained in terms of Michaelis-Menten-type kinetics, as linear Lineweaver-Burk plots of (rate−1) vs ([O2H−]−1) are obtained from our experimental data. This is consistent with our proposed mechanism, as the derived rate law can be rearranged into the Michaelis-Menton equation.
Thus far, it has not been recognized that copper complexes are able to depolymerize lignin under physiological conditions of white rot decay. However, we have found that both phenolic and non-phenolic synthetic lignins were intensively depolymerized by Cu(II) and lipid hydroperoxide model compounds in the presence of a metabolite of ligninolytic fungi, pyridine at room temperature in aqueous media. Treatment of 14C-labeled oxygen-prebleached kraft pulp (OKP) by the copper-dependent reaction evidenced effectiveness of this reaction for the delignification of kraft pulps. In contrast to the organic peroxide system, Cu(II)/pyr/H2O2 system was much less effective for the lignin depolymerization. However, treatment of unbleached kraft pulp (UKP) by Cu(II)/H2O2 and Cu(II)/pyr/H2O2 systems demonstrated that the damage of cellulose was suppressed by the coordination of pyridine although high brightness gain was obtained independently of the presence of the coordinator. Spin trapping experiments demonstrated that not hydroxyl radical but superoxide anion is involved in the Cu(II)/pyr/H2O2 system. This finding not only introduces a new concept of non-enzymatic lignin biodegradation by wood-degrading fungi but also presents a new strategy for decomposing lignin and lignin-related compounds by copper complexes and peroxide-producing system.
Representative azo, triphenylmethane, heterocyclic and polymeric synthetic dyes have been decolorized by two biological non-ezymatic systems, copper/pyridine/H2O2 and the Fenton reagent. With the former system, intensive decolorization measured after 1 h was obtained with phenol red (89%), tropaeolin 00 (58%), Evans blue (95%), eosin yellowish (84%), and Poly B-411 (92%). The rate of decolorization was not affected by pH in the range of 3-9 and increased with increasing temperature. The use of the radical scavengers thiourea and superoxide dismutase showed that hydroxyl radicals rather than superoxide anions are involved in the reaction. Omission of pyridine led to a substantial decrease in the extent of decolorization (20-50% decolorization). The use of organic peroxide instead of H2O2 resulted in slightly slower decolorization, similar values of decolorization being obtained only after a 2-h incubation. Decolorization of the dyes by the Fenton reagent was also very effective but slower than that obtained with the first system. Except for phenol red and eosin yellowish, (decolorization 8% and 52%, respectively) the dyes were decolorized up to 99% after 1-day incubation.
Although the number of proteins that utilize copper as an essential cofactor is not exceptionally large, the role played by many of these proteins appear central to the success of the organism. Over the last decade, great advances have been made in the understanding of copper proteins that utilize dioxygen in the course of the functionalization of organic substrates. These types of enzymes can be categorized, depending on whether the copper centers involved in substrate functionalization exist alone (mononuclear) or in a complex with other copper ions (bi- or polynuclear). This review is focused on the former category of proteins. One important feature of the mononuclear copper catalysts is the difficulty inorganic chemists have encountered in devising suitable model systems for the generation of biological activity. For this reason, the biological systems have often led the way, providing new paradigms for catalysis that can then be tested in appropriately designed model reactions. 162 refs.
Degradation of lignin model compound with coordinated copper in the presence of peroxide. The International Research Group on wood preservation
- K Koller
- T Watanabe
- K Messner
K. Koller, T. Watanabe, K. Messner, Degradation of lignin model compound with coordinated copper in the presence of peroxide. The International Research Group on wood preservation. IRG/W 98-10282 (1998).