P . P . 1-9 ( 2017 ) Delignification of Date Palm Fronds using Modified Organosolv Technique

In this study, modified organic solvent (organosolv) method was applied to remove high lignin content in the date palm fronds (type Al-Zahdi) which was taken from the Iraqi gardens. In modified organosolv, lignocellulosic material is fractionated into its constituents (lignin, cellulose and hemicellulose). In this process, solvent (organic)-water is brought into contact with the lignocellulosic biomass at high temperature, using stainless steel reactor (digester). Therefor; most of hemicellulose will remove from the biomass, while the solid residue (mainly cellulose) can be used in various industrial fields. Three variables were studied in this process: temperature, ratio of ethanol to water and digestion time. Statistical experimental design type Central Composite Design (CCD) has been used to find a mathematical relationship between the variables and the remaining lignin percent as dependent variable. The results obtained in this study were represented by a polynomial mathematical equation of the second degree. The results showed that the best digestion time was (80 minutes), which gave the best percent remaining concentration of lignin (3%) at temperature of 185C and ratio of ethanol: water equal to 50: 50 wt/wt. In order to reduce digesting time, the effect of using different catalysts have been studied such as (NaOH, H2SO4, Ca (OH) 2) at low concentration (0.025, 0.025, 0.05M) respectively. It was found that the best catalyst is sodium hydroxide at concentration (0.025) mol/L which gave the same percent of lignin 3% but with low digestion time about 30 min.


Introduction
Delignification is removal of lignin from woody tissue as by natural enzymatic or industrial chemical processes so that it can be used for applications like making Rayon or making paper [1]. Lignin is an organic substance binding the cells, fibers and vessels which constitute wood and the lignified elements of plants, as in straw. It is the most abundant renewable carbon source on Earth [2]. There are several methods concerning delignification. Kraft process is one of these methods [3], which is capable to remove about 95% of lignin from woody biomass. In this method, the wood chips are digesting at high pressure and elevated temperature in the existence of whit liquor (solution of sodium hydroxide and sodium sulfide). This process is carried out either batch or continuous in a bioreactor that called digester [4]. Another method is oxygen delignification, which is a process between cooking and bleaching sequences, where part of the residual lignin left in pulp after cooking is removed using oxygen and alkali [5]. The targeted reactions are the oxidation of lignin and breaking it down parts which dissolve in alkali, as well as destroying the colored groups in lignin and removal of impurities, such as resin. Oxygen delignification is not so efficient as Kraft method, it removes only 55% of lignin [6]. It is, however, often used as a supplementary tandem process to the more traditional Kraft method. Delignification using microorganisms was also investigated. In production of bioethanol from plant material, removal of lignin using microbial reaction was used simultaneously with conversion of cellulose to bioethanol at low level of temperature [7]. Scientist also try to change the content and structure of lignin by microorganisms in order to either improve its digestibility to animals [7] or to increase the utility of the cell walls for biofuel production paper manufacturing [8]. Another technique of delignification is organosolv process which involves contacting a lignocellulosic feedstock such as chipped wood with an aqueous organic solvent at temperatures ranging from 150 to 220 °C [9]. Solvents used include acetone, methanol, butanol, ethylene glycol, formic acid, and acetic acid. The concentration of solvent in water ranges from 20 to 80% [10] . Organosolv has several advantages when compared to other popular methods such as Kraft or sulfite pulping since it has the ability to obtain relatively high quality lignin [4]. Organosolv solvents are easily recovered by distillation, leading to less water pollution and elimination of the odor usually associated with Kraft pulping [11]. The purpose of the current study is delignification of date palm fronds in order to remove about 95% of its lignin content using modified organosolv technique.

Composition analysis of data palm fronds
The date palm fronds samples were collected from Babylon garden from one kind of date palm (Al-Zahdi) and stored in pre-sterilized bags in refrigerator freeze. The Samples then transferred to the laboratory for testing and before being treated, impurities were removed by washing them with distilled water for several times .After that the samples were shredded using electric cutter and crushed with humor then sieved. Fragments with particle size of (90-270) μm were selected for the steps of analysis. The composition of date palm fronds (leaves and stalk) were determined according to National Renewable Energy Laboratory (NREL) LAPs for compositional analysis [12]. Table (1) shows the composition of fronds (leaves and stalk).

Description of the Reactor
Two liter stainless steel reactor was used in this study with 11 cm in diameter and 21cm length. The reactor was heated from the side through a jacket-type heater, while heated from bottom by magnetic stirrer hot plate as shown in Figure (1). The reactor was provided with a thermocouple equipped with temperature controller to control the temperature inside the reactor. The pressure inside the reactor was indicated using pressure gauge.

Procedure
Lab scale experiments were performed in two liter autoclave batch reactor. The date palm fronds (biomass) were milled to about 0.18mm. A suspension of biomass-water-organic solvent (ethanol) was made (typically, 200 ml solvent per 20 g biomass). The effect of ethanol solvent is to hydrolysed lignin in the sample but has no effect on cellulose [13]. In some experiments, catalyst was added. This suspension was heated to a specific reaction temperature (in the range 150 -200 o C) while being stirred. This suspension then kept at its set point during a certain reaction time (typically, 60 min) and subsequently cooled down. After filtration of the resulting slurry, the solid residue was washed with an identical organic solvent-water mixture and dried at 105 o C. From the filtrate solution, samples were taken for UV analysis.

Washing and filtration of the sample after delignification
After the run completed, the treated sample was washed with ethanol /water solution( 80:20 , ethanol: water) and then with distilled water for several times until the color of liquor change from brown color to colorless , then the solution was filtered using vacuum pump as shown in the

Bleaching
Bleaching is a chemical processing carried out on various types of wood pulp to remove the color of the pulp, so that it becomes whiter. The goal of bleaching chemical pulps is to remove essentially all of the residual lignin; hence the process is often referred to as delignification [4]. Chlorine is the basis for the most common bleaching process. Three grams of dried sample was weighted and put into 500 ml flask, then 250ml of sodium chlorite solution (60 g/l sodium chlorite and 60 g/l sodium acetate) was added, the sample was soaked for four days [14]. Then it was washed with distilled water for several times, followed by extraction with 0.15 N NaOH twice and then washed with hypochlorite and distilled water thoroughly. Finally, the sample was freeze-dried. It was note that the color was changed from dark brown to white as show in the Figure (

Lignin Content
The lignin content was determined according to the renewable energy laboratory standard procedure [15], 0.3 g of date palm frond with particle size less than 180 μm was put into a dried test tube and then 3 ml of H2SO4 72% was added to it. The tube was placed in water bath at 30 o C for 1hr then the sample mixed with 84 ml distilled water in glass bottle and placed in autoclave for 1 hr at 125 o C. Then the sample filtered with weighed crucible using vacuum pump. Two types of lignin were obtained, lignin acid soluble and lignin acid insoluble. The weight percent of lignin acid insoluble residue (AIR) can be calculated by eq. (1).

Results and Discussion
Three parameters have been studied (temperature, ethanol/water ratio and time) with respectively ranges (150-220) C o , (20-80) Wt.%, (20-80) min. Seventeen experiments were conducted in this study with parameters values were selected according to the statistical experimental design type (CCD). The experimental values were shown in Table (2).

Mathematical Model
In the experimental work, the values of independent variables, which were temperature, ethanol: water ratio and digesting time, were selected according to Central Composite Design (CCD). Design of experiments (DOE) program using Central response technique has been used to find a mathematical relationship between the three dependent variables and the independent variable which is the remaining lignin percent in the sample. A second -order polynomial correlation was obtained as shown in eq. (6).

Estimation the Coefficients of the Second Order Equation
The coefficients of equation (6) can be determined by using computer software (Design Expert) so equation (6) can be written as follow: Correlation coefficient = 0.9681 Relative standard deviation = 3.6%

Effect of Temperature and Ethanol/Water Ratio on Lignin Content
According to the mathematical model of eq. (6) the effects of temperature and ethanol/water ratio were studied at certain value of time. Figure  (4) shows the contours of lignin percent content at different values of temperature and ethanol/water at constant time 20 min. Minimum value of lignin percent contained was 6.89% which obtained at a lowest temperature of 191 o C and solvent ratio (ethanol: water equal to 50:50 wt/wt). Decreasing the temperature below 191 o C will led to increase the value of lignin% content in the sample. This is because decreasing the temperature will prevent full fractional of lignocellulosic material and then the lignin percent will be increase at temperature below 180 o C.  Although increasing temperature will lead to decrease the lignin content, but it is not recommended to exceed the digestion temperature above 185 o C because many changes will happen to the cellulose structure and destroy it [17].

Effect of using Catalyst
As it has been shown from the above context, the time of digestion is very important parameter in the delignification process and time 80 min gave perfect lignin percent content, but this time is too long therefore; a possibility to improve the organosolv process might be the application of a catalyst. In the literature, H2SO4, NaOH and Ca (OH) 2 are a well-known catalysts for biomass pretreatment processes [9]. Catalysts were used with concentration of 0.025M for each of H2SO4 and NaOH and 0.05M for Ca (OH) 2. The experiments were conducted at 180 o C and ratio of ethanol: water equal to 50:50. Figure (7) shows that the treated sample without catalysts gave 3% lignin content and 80 min digestion time, while all catalysts reduced the digestion time to 30 min, but lignin percent content was minimum (3%) when using NaOH. Other catalysts gave higher lignin content as shown in figure (7). So it can be concluded that among the catalysts used in the experiments sodium hydroxide was the most efficient in reducing time of digestion while keeping the lignin content at minimum value. On the other hand sodium hydroxide has another benefit. It is working on reducing the degree of polymerization of remains sample (mainly cellulose) and this is important in the subsequent steps in the process of synthesis of rayon [18].

Conclusion
Based upon the results obtained so far, there is a possibility to dignify the date palm fronds using modified organosolv. The more effective parameters in the delignification process were: temperature, ethanol/ water ratio and time.
Without catalyst, the optimum conditions for delignification process were: temperature 185 o C, ethanol: water 50:50 and digestion time 80 min. In order to reduce the digestion time, three types of catalysts were tested: H2SO4, NaOH and Ca (OH) 2. Among them, sodium hydroxide (0.025M) was found the best one which reduces the time from 80 min to 30 min.