Delignification of wood and kraft pulp with polyoxometalates

2000

In order to elucidate changes occurring in lignin during polyoxometalate delignification of kraft pulp, residual lignins of a series of POM-delignified kraft pulps of decreasing kappa number were isolated and characterized. Oxidative treatment of commercial unbleached kraft pulp was performed using complex POM solutions containing the active (SiVW11O40)5­ anion. For comparison, oxygen delignification of the same kraft pulp was also

The feasibility of using polyoxometalate (POM) salts and oxygen in the commercial bleaching of chemical pulps is herein demonstrated. A clear advantage of polyoxometalates over oxygen alone, hydrogen peroxide or ozone is their inherently high oxidative selectivity for the residual lignin in softwood kraft pulps. Softwood kraft pulps delignified to low kappa numbers using POM salts possess papermaking properties comparable to those obtained using chlorine/chlorine dioxide and alkali. The goal of ongoing research and development efforts is to develop a highly selective, oxygen-based polyoxometalate delignification and bleaching technology compatible with mill closure.

Industrial & Engineering Chemistry Research, 2003

In this study, an industrial softwood kraft pulp sample was subjected to an oxygen-alkali treatment over three consecutive times as part of a broader research effort to identify methods for selectivity improvements and lignin reactivity inefficiencies. Chemical properties of the pulp were determined that included active functional group analysis of the residual and effluent lignins. Further data for the analysis of the chemical reactions were obtained by collecting and identifying the low molecular weight compounds in the effluent liquors. We determined that the residual lignin cannot be removed beyond 75% of the original levels, although the first oxygen delignification reaction can remove nearly half of the original resident kraft residual lignin. The main factor limiting lignin removal during the oxygen delignification process is the penetration of oxygen and oxygen-active species into the lignin matrix along the microfibrils in the pulp. The matrix structure of carbohydrate and the lignin carbohydrate complex constitute the main barriers against oxidative reactions. We have evidence that the hemicelluloses in the pulp are part of the lignin-carbohydrate complexes, and the robust nature of the native cellulose crystal structure in the pulp fiber is partly responsible for the barrier to lignin removal during oxygen-alkali treatment. In fact, xylan-linked lignin is more resistant to oxidative reactions and tends to remain in the pulp, while galactan-linked lignin tends to dissolve during oxygen delignification. The reduction of reactive groups including hydroxyl and carboxyl groups during oxygen delignification is also responsible for the low level of subsequent delignification. We also support the finding that the p-hydroxylphenyl structures and 5,5′-biphenolic units in residual lignin are stable and accumulate during oxygen delignification.

… Conference, Nov. 4-7, …, 1996

CITATIONS 40 READS 40 7 authors, including: Some of the authors of this publication are also working on these related projects: Transforming agricultural residues into digestible, nutritious feeds for livestock. View project Ira A Weinstock Ben-Gurion University of the Negev 110 PUBLICATIONS 2,300 CITATIONS SEE PROFILE

Chemical data pertinent to most-recently developed POM delignification systems will be presented. These data will be used to demonstrate the fundamental basis for the stability, self-buffering properties, versatility and high selectivity of these systems when used in combination with oxygen to convert native or residual lignin in wood or wood-pulp fibers to CO2 and H2O. OVERVIEW. The potential use of polyoxometalates (POMs) as water-soluble yet oxidatively stable analogs of lignin-degrading enzymes in the chlorine-free production of high-quality delignified wood products was first described at the 2nd EWLP in 1992 (Grenoble, France). 1 A more thorough elaboration, including demonstration data, was presented at the 3rd EWLP (Stockholm, 1994). 2 Presentations in subsequent Workshops (4th and 5th EWLPs) concerned further developments and highlighted the potential of POMs for use in achieving mill closure and for replacing sulfur (new pulping processes) 3,4 During this time, numerous obstacles 5 were overcome by continued fundamenta1 6-10 and applied research. 11-12 As a result of these efforts, new POM systems suitable for use in pilot-scale (10 tons/day) demonstration and testing are now available.

Industrial & Engineering Chemistry Research, 1997

Water-soluble salts of polyoxometalate (POM) anions can be used as (net) catalysts for selective, effluent-free oxygen delignification (bleaching) of unbleached kraft wood-pulps. Essential to this process is the use of the POM anions as catalysts for the wet oxidation of the lignin removed from the pulp fibers. In anaerobic bleaching, a 0.5 M solution of α-Na 5 [PV 2 M o 10 O 40 ] (POM ox ) is combined with wood pulp and the mixture heated to 125 °C. This treatment dissolves certain organic compounds (some lignin and polysaccharide fragments) with a combined chemical oxygen demand (COD) of 1500-2000 mg of O 2 /L. Before the solution can be used again for bleaching, the POM anions (POM red ) must be deoxidized and the dissolved organic compounds removed. Here, a second function of the POM anions is realized. They are used under mild conditions (150 °C, 0.7 MPa of O 2 ) to catalyze the wet air oxidation of the dissolved organic compounds. After several cycles of anaerobic bleaching and wet oxidation, a steady-state condition in COD (at ca. 550 mg of O 2 /L) is achieved.

Canadian Journal of Chemistry, 2001

Softwood kraft pulp was subjected to a laboratory one-and three-stage oxygen delignification process. Pulp and liquor samples were collected at different stages of the process with particular attention being paid to the early and late stages. A novel residual lignin isolation method extracted about 65% of the oxidized residual lignins at a purity exceeding 90%. Using this methodology coupled to quantitative 31 P NMR, 13 C NMR, 2D heteronuclear (HMQC) NMR spectroscopic and analytical pyrolysis techniques allowed a thorough characterization of the residual and solubilized lignin fractions at the various stages of the process. Our conclusions do not point to a single factor as being responsible for the limits confronting oxygen delignification. Amongst the major factors impeding the effectiveness of oxygen delignification are: (i) the accumulation of relatively inert 5,5′ biphenyl structures originally present in kraft lignin from dibenzodioxocin ring opening reactions; and (ii) the accumulation of considerably less reactive p-hydroxyphenyl structures. Detailed quantitative information was obtained and discussed in relation to the presence and role of these unreactive phenolic fractions on the residual oxidized lignins. As such we arrived at important conclusions as to why and how these structures remain and accumulate on the fiber. In addition, issues related to the profiles of the total phenolic hydroxyl content of the residual lignins and the remaining arylglycerol β-O-4 structures are discussed.

Industrial & Engineering Chemistry Research, 2002

The present study explores the efficiency of delignification achieved during standard oxygen, double-oxygen, and mini-oxygen [(E + O)D kf)0.05 (E + O)] delignification of high-(kappa) 56.2) and low-(kappa) 26.6) lignin-content softwood (SW) kraft pulps in the context of the structural changes occurring in the lignin as measured by nuclear magnetic resonance (NMR) spectroscopy. The relative bleachability of the high-kappa pulps was determined to be superior to that of the lower-kappa pulps during the oxygen bleaching experiments. The general trend of increasing ease of bleachability was double oxygen > oxygen > mini-oxygen for both the low-and highlignin-content pulps. NMR spectroscopy demonstrated that part of the rationale for the higher levels of delignification in the high-kappa pulps was due to higher contents of-O-4 and methoxy lignin functional groups. In addition, the high-kappa pulp contained a lower number of resistant 5,5′-condensed lignin units and diphenylmethane structures. The NMR data also provided strong evidence for the presence of p-hydroxyphenyl units, a relatively unique resistant structure whose elimination was approximately the same for both series of pulps. These new structures might potentially function as end-capping termini that hinder access to bulk lignin clusters, thus limiting the overall efficiency of oxygen delignification.

Pure and Applied Chemistry, 2000

A series of oxygen delignification experiments were performed on two softwood kraft pulps that had differing starting lignin contents. One had an initial kappa of 40 and the other 25, corresponding to lignin contents of 6% and 3.75% by dry mass, respectively. Several chemical process modifications were examined to determine their influence over the delignification selectivity and final pulp viscosity. A 2k factorial format was used to assess the significance of varying the temperature, time, and Mg/Mn ratio during the oxygen delignification of the pulps. It was found that the lower lignin content pulp displayed greater delignification selectivity than the higher lignin content pulp. Kappa numbers, viscosity values, and ICP metals contents were determined and are the basis of discussion for the results obtained.

Ind Eng Chem Res, 1997

Water-soluble salts of polyoxometalate (POM) anions can be used as (net) catalysts for selective, effluent-free oxygen delignification (bleaching) of unbleached kraft wood-pulps. Essential to this process is the use of the POM anions as catalysts for the wet oxidation of the lignin removed from the pulp fibers. In anaerobic bleaching, a 0.5 M solution of R-Na 5 [PV 2 Mo 10 O 40 ] (POM ox) is combined with wood pulp and the mixture heated to 125°C. This treatment dissolves certain organic compounds (some lignin and polysaccharide fragments) with a combined chemical oxygen demand (COD) of 1500-2000 mg of O 2 /L. Before the solution can be used again for bleaching, the POM anions (POM red) must be reoxidized and the dissolved organic compounds removed. Here, a second function of the POM anions is realized. They are used under mild conditions (150°C, 0.7 MPa of O 2) to catalyze the wet air oxidation of the dissolved organic compounds. After several cycles of anaerobic bleaching and wet oxidation, a steady-state condition in COD (at ca. 550 mg of O 2 /L) is achieved.