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GENERAL RESEARCH Characterization of Fiber Carboxylic Acid Development during One-Stage Oxygen Delignification Dongcheng Zhang, Xin-Sheng Chai, Qingxi Hou, and Arthur Ragauskas* Institute of Paper Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332
This study examines the changes in fiber carboxylate content during a series of one-stage oxygen delignification experiments using a southern United States pine kraft pulp. The carboxylic acid content of the oxygen-delignified pulps before and after holocellulose pulping was determined to establish the distribution of acid groups in lignin and pulp carbohydrates. The carboxylic acid content in residual lignin were also examined and compared with the holocellulose results in an attempt to decouple the responses from residual lignin and carbohydrate. It was found that oxygen-delignified kraft pulps exhibited an initial 4%-13% increase in total fiber carboxylic acid content in the first 10-30 min, followed by a steady state or slight decrease. In contrast, the holocellulose of oxygen-delignified pulps exhibited an initial 6%-8% decrease in carboxylic acid content in the first 10 min, followed by a small increase and then a slow decline. The residual lignin isolated from these pulps showed an initial 37% carboxylic acid content rise in the first 10 min, followed by a rapid drop and then a further slow increase at 100 °C with 2.5% NaOH and 800 kPa of O2. The characteristic kappa numbers for the maximum carboxylic acid content were observed in the range of 19-24 for the oxygen-delignified fibers and the corresponding holocellulose, and ∼25 for the residual lignin of oxygen-delignified pulps under experimental conditions. Overall, acid groups in the residual lignin significantly affect the total fiber carboxylic acid content in the initial phase, whereas acid groups in the carbohydrate fraction control the total fiber carboxylic acid behavior in the remaining phases. The optimal conditions for obtaining a pulp with high carboxylic acid content, under the conditions studied, were observed to be 100 °C with 2.5% NaOH and 800 kPa of oxygen. Introduction Oxygen delignification (O) is widely used for lignin removal before bleaching and has become one of the dominant post-kraft pulping delignification technologies for both element chlorine-free (ECF) and totally chlorinefree (TCF) operations.1 In the past fifty years, the development and implementation of oxygen delignification technology has been achieved because of the contribution from discoveries in lignin/carbohydrate chemistry, improved reaction selectivity, energy, environmental concerns, and the desire for higher pulp yields.2 Recently, the interests in this area have focused on extending the limit of delignification,3,4 oxygen mass transfer in gas/liquid interface,5,6 and fiber properties improvement. The fiber charge of chemical pulps is an important chemical property that can affect the binding of metal ions to pulps, fiber swelling, water removal during wet pressing, the rate of refining, the adsorption of retention aids, and the strength and optical properties of the resultant papersheets.7,8 Carboxylic acid groups in cellulosic fibers are the main functional groups responsible for surface and bulk charge of kraft pulps. Therefore, the introduction of carboxylic acid groups into * To whom correspondence should be addressed. Tel.: 404894-9701. Fax: 404-894-4778. E-mail: Arthur.ragauskas@ ipst.gatech.edu.
fiber by direct chemical and enzymatic modification of pulp, such as grafting or the addition of additives has been extensively studied.9-19 In contrast, the parameters that are important to maximizing the fiber charge in modern pulping and bleaching operations have only recently begun to be examined.20-22 As reported recently, oxygen delignification can enhance or slightly diminish fiber acid group content for softwood (SW) kraft pulps.22-24 An increase in the fiber charge after an O-stage can be attributed, in part, to oxidative depolymerization reactions with lignin.25,26 Simultaneously, the reactive oxygen species in an O-stage have been shown to yield aldonic acids27 that can either enhance the overall fiber charge or contribute to pulp yield loss and a reduction in fiber charge. Therefore, changes in fiber charge after oxygen delignification are anticipated to be dependent on process conditions, and this has not been fully investigated. In addition, the kinetic changes in fiber carboxylic acid content and its distribution between residual lignin and the carbohydrate fraction during oxygen delignification remain ill-defined and require further investigation. This study examines the changes in fiber charge during oxygen delignification of a SW kraft brownstock pulp and establishes the extent to which this charge is associated with lignin and pulp carbohydrates.
10.1021/ie050489a CCC: $30.25 © 2005 American Chemical Society Published on Web 10/29/2005
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Figure 1. Total fiber carboxylic acid content development at different initial alkalinities (NaOH ) 1.5%, 2.5%, and 3.5%; temperature ) 100 °C and O2 pressure ) 800 kPa): (a) total fiber carboxylic acid content versus time and (b) total fiber carboxylic acid content versus kappa number.
Figure 2. Total carboxylic acid content development in oxygen-delignified pulps at different temperatures (85, 100, 115 °C) and oxygen pressures (640, 800, 960 kPa): (a) total fiber carboxylic acid content versus time at 2.5% NaOH and a O2 pressure of 800 kPa; (b) total fiber carboxylic acid content versus kappa number at 2.5% NaOH and a O2 pressure of 800 kPa; (c) total fiber carboxylic acid content versus time at 2.5% NaOH and a temperature of 100 °C; and (d) total fiber carboxylic acid content versus kappa number at 2.5% NaOH and a temperature of 100 °C.
Experimental Section Materials. A commercial southern United States pine kraft pulp with a kappa number of 32.5 was used for oxygen delignification studies. All other chemicals and solvents were commercially purchased and used as received, with the exception of 1,4-dioxane, which was freshly distilled over NaBH4 prior to use.
Oxygen Delignification. All one-stage oxygen delignification experiments were conducted in a 1-L inclined rotary-stirred Parr reactor that was filled with 30.00 oven-dry (OD) grams of pulp at 10% consistency. MgSO4 was charged to the reactor so that the Mg/Mn molar ratio in the pulp was kept at (30-33):1, to offset the detrimental effects of Mn2+.28 The varied experi-
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Figure 3. Carboxylic acid content profile in residual lignin during oxygen delignification (conditions: 2.5% NaOH, 800 kPa of O2, 100 °C, 0-60 min): (a) total fiber carboxylic acid content versus time and (b) total fiber carboxylic acid content versus kappa number.
mental parameters include NaOH content (1.5%, 2.5%, and 3.5%), temperature (85, 100, and 115 °C), oxygen pressure (640, 800, and 960 kPa), and reaction time (10, 20, 30, 45, 60, and 80 min). The oxygen-delignified pulps were washed and air-dried or stored at 2 °C prior to analysis. Holopulping. The oxygen-delignified pulp was holocellulose-pulped (holopulp), following the procedure that is outlined below. In brief, pulp samples (2.00 g) were dispersed into 75.00 mL of deionized water, treated with 0.50 mL of glacial acetic acid and 0.60 g of NaClO2. The resulting mixture was warmed to 75 °C. After stirring for 1 h, additional glacial acetic acid (0.50 mL) and NaClO2 (0.60 g) were added and the reaction was continued for another 1 h. This process was repeated for a total of 3 h. The treated pulp was then cooled to 0 °C, filtered, washed using deionized water, and air-dried for further determination. Analytical Methods. The metal-ion concentration in the pulp samples was analyzed using inductively coupled plasma-emission spectroscopy (ICP-ES), following literature methods.29,30 The carboxylic acid content in samples was determined using headspace gas chromatography (HSGC),31 and the content of hexenuronic acid (HexA) in pulps was determined using a spectroscopic method.32 Residual lignin was isolated from the pulp, using a mild acid hydrolysis procedure that has been described in the literature.33 In addition, TAPPI standard methods34 were used to determine the pulp kappa number (T236 cm-85), pulp brightness (T452 om-92), pulp viscosity (T230 om-94), and paper sheet tensile strength (T404 om-87). Fiber length was measured using a fiber quality analyzer.35 The standard deviation at 95% confidential level was used for the experimental error evaluation for all of these measurements. Results and Discussion Profiling Carboxylic Acid Content in OxygenDelignified Fiber. A series of oxygen delignification experiments were conducted at varied reaction times, oxygen pressures, caustic charges, and temperatures. The development of total carboxylic acid content in oxygen-delignified pulps is summarized in Figures 1 and 2.
Table 1. Hexenuronic Acids (HexA) in Oxygen-Delignified Pulp and Corresponding Holopulpsa HexA Content (µmol/g) time (min)
O-pulpb
holopulpc
0 10 20 30 45 60 average
28.7 27.8
3.5 2.9 2.8 2.2 2.0 3.5 2.8
28.0 28.2 28.2
a Conditions: NaOH, 2.5%; temperature, 100 °C; and O pres2 sure, 800 kPa. b Oxygen-delignified pulp, 0.38% standard deviac tion. Holocellulose made from oxygen-delignified pulp, 0.64% standard deviation.
Figure 1a shows that two distinct phases exist for the total fiber carboxylic acid content profile in oxygendelignified pulps: an initial 4%-13% increase in the first 10-30 min, followed by a slow decrease phase under different initial alkalinity. Figure 1b shows that the maximum total fiber carboxylic acid content occurs at a kappa number of 21-24 and further delignification is obviously not beneficial to the total fiber acid increase. The same profile for total fiber carboxylic acid content was also observed in oxygen-delignified pulps at different temperatures and oxygen pressures, as shown in Figure 2. Similarly, the maximum total fiber carboxylic acid content was observed at a kappa number of 19-24 under different reaction temperatures (Figure 2b) and varied oxygen pressure (Figure 2d). Previous oxygen delignification studies2 have shown that residual lignin exhibits a rapid initial degradation, followed by a slower delignification phase. The oxidized residual lignin in the pulp will contribute to fiber charge of oxygen-delignified pulps. Likely, cellulosic and hemicellulose aldolic acids formed during oxygen delignification are also a source of fiber charge for oxygendelignified pulps. Therefore, the total fiber carboxylic acid content in oxygen-delignified pulps results from the formation of carboxylic acids in different chemical components of the fiber. The two-phase development for total fiber carboxylic acid of oxygen-delignified pulps should be ascribed to different acid groups from these chemical components in each phase. As shown in the literature,2 the degree of delignification increases with alkalinity and/or reaction tem-
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Figure 4. Carboxylic acid in holocellulose of oxygen-delignified pulps: (a) variance in alkalinity (NaOH ) 1.5%, 2.5%, or 3.5%) for 100 °C and 800 kPa of O2, (b) variance in temperature (85, 100, 115 °C) for 2.5% NaOH and 800 kPa of O2, (c) variance in O2 pressure (640, 800, 960 kPa) for 2.5% NaOH and 100 °C.
perature. However, Figures 1 and 2 show that higher NaOH content (3.5%) and higher temperature (115 °C) did not result in higher total fiber carboxylic acid content in oxygen-delignified pulps. In addition, oxygen charge shows no significant effect on fiber carboxylic acid development under the experimental conditions studied. The optimal fiber carboxylic acid content for the conditions studied was obtained at 100 °C with 2.5% NaOH and 800 kPa of oxygen. Chemical Component Contributions to Total Carboxylic Acid Content of Oxygen-Delignified Pulps. The source of total acid groups in bulk pulp
generally consists of three parts: residual lignin (RL), carbohydrate, and extractives. For oxygen-delignified pulps, the contribution from extractives can be considered to be negligible. Therefore, the carboxylic acid in residual lignin and carbohydrate are two major contributors to total fiber acid content in oxygen-delignified pulps. In unbleached kraft pulps, HexA are an important contributor to fiber charge. However, few HexA remain in oxygen-delignified pulps after holopulping, because of the presence of the ClO2 species (see Table 1). The absolute amounts of HexA present in SW and hardwood
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(HW) kraft pulps have been reported to vary substantially, depending on several parameters, including wood furnish and pulping parameters.36 For ECF bleaching protocols, it is well-established that chlorine dioxide readily reacts with HexA, and these pulps typically contain only small amounts of these unsaturated sugars (
