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11Journal of Siberian Federal University. Chemistry 3 (2014 7) 316-325
УДК 676.014.361:544.478
Influence of TiO2 State on Oxidative Delignification of Aspen Wood and Wheat Straw
Boris N. Kuznetsova,b *, Natalia V. Garyntsevaa, Alexander V. Levdanskya, Laurent Djakovitchc and Catherine Pinelc
aInstitute of Chemistry and Chemical Technology SB RAS 50-24 Akademgorodok, Krasnoyarsk, 660036, Russia
bSiberian Federal University 79 Svobodny, Krasnoyarsk, 660041, Russia cIRCELYON, 2 avenue Albert Einstein Villeurbanne Cedex, Lyon, F-69626, France
Received 11.07.2014, received in revised form 09.08.2014, accepted 29.08.2014
The influence of preparation methods on structural characteristics of TiO2 catalysts was studied by the XRD, SEM, BET, DTA-DSC, FTIR techniques.
Catalytic activity of obtained titanium dioxide samples was compared in reaction of oxidative delignification of aspen wood and wheat straw by hydrogen peroxide in acetic acid- water medium. The use of titanium dioxide in rutile modification allows to produce a cellulose product with lower content of a residual lignin, as compared to TiO2 in anatase modification. The increase of surface area of the TiO2 catalyst reduces its activity in wood and wheat straw delignification.
Keywords: TiO2 catalyst, oxidative delignification, aspen wood, straw wheat, hydrogen peroxide, cellulose.
© Siberian Federal University. All rights reserved
* Corresponding author E-mail address: inm@icct.ru
Влияние состояния TiO2 на окислительную делигнификацию древесины осины и соломы пшеницы
Б.Н. Кузнецов3'6, Н.В. Гарынцеваа, А.В. Левданский3, Л. Дьяковичв, К. Пинельв
аИнститут химии и химической технологии СО РАН Россия, 660036, Красноярск, Академгородок, 50-24 бСибирский федеральный университет Россия, 660041, Красноярск, пр. Свободный, 79 вIRCELYON 2 avenue Albert Einstein Villeurbanne Cedex, Lyon, F-69626, France
Методами РФА, СЭМ, БЭТ, ДТА-ДСК, ИКС изучено влияние способов получения катализаторов TiO2 на их структурные характеристики. Сопоставлена каталитическая активность полученных образцов диоксида титана в реакции окислительной делигнификации древесины осины и соломы пшеницы пероксидом водорода в среде «уксусная кислота-вода». Использование диоксида титана в модификации рутил позволяет получить целлюлозный продукт с более низким содержанием остаточного лигнина, чем при использовании TiO2 в модификации анатаз. Увеличение удельной поверхности катализатора приводит к уменьшению степени делигнификации древесины осины и соломы пшеницы.
Ключевые слова: катализатор TiO2, окислительная делигнификация, древесина осины, солома пшеницы, пероксид водорода, целлюлоза.
Introduction
The process of lignocellulose biomass delignification in the "hydrogen peroxide-acetic acid-water" mixture is considerably accelerated in the presence of different catalysts [1, 2]. Sulfuric acid can act as a catalyst of wood [3] and wheat straw [4] oxidative delignification processes. But, at the same time, H2SO4 promotes the hydrolysis of polysaccharides decreasing of the yield of cellulose product. Besides, H2SO4 catalyst has such technological disadvantages as toxicity and corrosion activity.
Some complexes on the basis of metals ofvariable valency, for example so-called polyoxometalates, can promote the process of wood delignification by oxygen [5, 6].
Catalytic properties of a number of tungsten and molybdenum compounds (Na2WO 4, Na2MoO4, H3PW12O40, CuSO4, H2SO4, Na2WO4-CuSO4, Na2MoO4-CuSO4, H3PW12O40-CuSO4, H3PW12O40-H2SO4, Na2WO4-H3PW12O40-H2SO4) were studied in the processes of wood acetic acid delignification [7]. However the practical use of these catalysts is complicated by their high cost and complexity of regeneration for a reuse.
The more technologically convenient solid TiO2 catalyst promotes effective removal of a residual lignin from technical cellulose in the presence of such oxidants as hydrogen peroxide and oxygen
[8-9]. Also TiO2 can catalyze an oxidative delignification of wood by H2O2 [10]. Advantages of TiO2 application as delignification catalyst are stipulated by the absence of corrosion activity and toxicity, by its availability, low cost and lack of need for its regeneration. It worth to note, that catalytic properties of TiO2-based materials are sensitive to the method of their producing [11].
The present paper describes the influence of preparation methods on structural characteristics of TiO2 and on its catalytic properties in the delignification of aspen wood and wheat straw with the acetic acid - hydrogen peroxide - water mixture.
Experimental
TiO2 preparation
Two methods are usually used for TiO2 obtaining: hydrolysis or sedimentation by alkalis from solutions of chloride or sulfate titanium (IV), with the subsequent washing, drying and heat treatment at a temperature not below 400-500 °C or by thermal hydrolysis of titanium salts solutions [12].
In the present study the synthesis of titanium dioxide was carried out by hydrolysis of ammonia solution of titanium tetrachloride with the subsequent calcination of a titanium (IV) hydroxide according to [13].
The necessary amount of TiCl4 was slowly added to a chemical glass with distilled water and the mixture was boiled within 10 minutes. Then a solution was cooled up to the temperature of 70-80 °C and NH4OH was added up to full sedimentation of Ti(OH)4. Precipitate was filtered on Byukhner's funnel and washed out by hot water, up to disappearance of reaction on chlorine ions. Obtained paste of titanium hydroxide (IV) was then transferred to Petri's dish and dried up in muffler at 110 °C up to the constant weight. The dried-up precipitate was placed to the porcelain boat and calcinated in the muffle furnace at various temperatures within one hour.
For obtaining TiO2 samples in the rutile modifications, the dried-up precipitate was calcinated at 800 °C within one hour, then crushed in an agate mortar, washed out by water up to disappearance of reaction on chlorine ions and calcinated at 1000 °C within 1,5 hours. The yield of titanium dioxide in rutile modification was 98.5-99.5 % weight.
In some cases the additional mechanical activation of titanium dioxide was carried out in AGO-2 planetary mill. The time of activation was varied from 10 to 30 min.
Catalytic delignification of wood and wheat straw
Air dry sawdust (fraction 2-5 mm) of aspen wood (Populus tremula) and wheat straw were used as the initial raw materials. The chemical composition ( % wt. on abs. dry raw material) of aspen wood was: 47.3 cellulose, 22.9 lignin, 23.7 hemicelluloses, 3.7 extractive substances; of wheat straw: 39.2 cellulose, 20.6 lignin, 30.7 hemicelluloses, 5.5 extractive substances.
Delignification of wood sawdust and wheat straw was carried out in a glass reactor of 250 cm3 volume supplied by mechanical stirrer, condenser and thermometer. The following reaction conditions were used: temperature 100 °C, CH3COOH concentration - 25 % wt., H2O2 - 4 % wt., TiO2 catalyst -1 % wt., time - 4 h. In order to reduce the diffusion limitation, the high liquid to solid ratio (15) and intensive agitation (700 rpm) were used.
Such parameter, as the residual lignin content in cellulose product was used to evaluate the delignification activity of TiO2 catalysts. Cellulose product yield was estimated by the weight method
and calculated as follows: yield = (m/mj x 100 %, where m is the mass of abs. dry cellulose product (g), mo - the mass of abs. dry wood or straw.
Analysis of catalysts and products
X-ray diffraction analysis of TiO2 catalysts was carried out with Dron-4 diffractometer using a Cu-Ka source. The mass of samples was 1,0-1,5 mg.
The electron microphotographs were obtained by a SEM TM-1000 HITACHI (Japan).
Specific surface area of the TiO2 catalysts was determined by the BET single-point method using nitrogen adsorption at 77 K at a relative pressure P/P0 = 0.2 with the analyzer "Sorbtometr - M". The mass of analyzed samples was 50-60 mg.
The thermal analysis was carried out with thermal analyzer STA 449 F1 Jupiter (NETZSCH firm) in the range of temperatures 20-1000 °C . The speed of temperature rise was 10 degrees/min. The mass of samples was 2.6 mg.
Infrared spectroscopy analysis (FTIR) of TiO2 catalysts was carried out in transmission mode. The spectra were recorded with Vector-22 spectrometer in the 400-4000 cm-1 wavelength range, using 3 mg sample in KBr matrix. Spectral data were processed by program OPUS/YR (version 2.2).
Standard chemical methods were used for analysis of cellulose products composition [14]. The cellulose content in solid products was defined by Kurschner method, the lignin content - by hydrolysis of the sample with 72 % mas. of sulfuric acid at 20 °C for 2.5 h (Komarov's modification), hemicelluloses content - by McKein and Shoorly method using the hydrolysis by 2 % HCl at 100 °C during 3 h.
Results and discussion
Influence of calcination temperature and of mechanical treatment in AGO-2 planetary mill on a phase composition and specific surface area of obtained TiO2 samples was studied (Table 1).
The titanium hydroxide sample, which has been dried at 110 °C represents mix of amorphous and crystal modification of anatase. This conclusion is confirmed by data of X-ray diffraction analysis (Fig. 1) [15]. This sample has a maximum value of specific surface area - 260 m2 /g (Table 1).
Table 1. Phase composition and specific surface area of obtained TiO2 samples
№ Temperature of calcinating, °C Phase composition Time of treatment in AGO-2, min Surface area, m2/g
1 110 TiO2 (anatase) - 260
2 300 TiO2 (anatase) - 111
3 500 TiO2 (anatase) - 89
4 1000 TiO2 (rutile) - less 1
5 1000 TiO2 (rutile) 10 2
6 1000 TiO2 (rutile) 15 7
7 1000 TiO2 (rutile) 20 6
8 1000 TiO2 (rutile) 30 11
>00
-100
B
■u 300
■Ih 200
Sm
100
0
300 *C qitts
20 30 JO 50 60 70 2fl
Fig. 1. X-ray diffraction pattern of TiO2 samnles, obtnined at different temperatures
The increase of treatment temperature from 100 to 1000 °C intensifies nte dehydrafion processes with transfotmarion of amorphous hydeafed titrniumdioxide into cryrtalline anatase writlt fifllowing transformation of anatase into rutile form of TiO2 and recrystallization of the rutile. The increase of calcination temperntuee reduces the specific rurface area or ti^^niuir^ dioxide.
Arecord^ng; io SEM cli^tai trite crystals oft TiO2 in u rutfe mndification have a spherical form (Fig. 2). Treatment of titrnium dioxide sample in autile modifiaatian in a planetary mill increases the aurface area of samples from 4 to 27 times, depending on the duration of treatment (Table 1). The destruction of spherical particles of TiO2 was observed after mechanical treatment (Fig. 2).
Tne def ree of pertieles grinding ir increaring with a time oa treafment.
X-rnn diffraction pattern of all TiO2 sampler, treeted in a planetary mill, have a similar firm typical ro rutile mo iificadion off titanium dioxide.
According to thermal analysis data, the most high loss of weight (20,64 %) was observed in the case of TiO2 sample, obtained by thermal treatment at U0 °C (Fig. 3).
TiO2 samples calcinated at 300 and 500 °C lose only 8.86 and 8.75 % of weight respectively. Exothermic effect at temperature about 800 °C may be connected with the structural transformation of anatase to rutile.
IR-spectra of TiO2 samples of anatase and rutile modifications were compared (Fig. 4 and 5). The absorption bands corresponding to deformation (1040-1140 cm-1) and stretching (3200-3400 cm-1)
initial 10 min. 15 min.
Fig. 2. SEM images of initial and mechanically treated samples of titanium dioxide in rutile modification (obtained at 1000 °C)
TT* JÏK .ludlu-i
__,_,_,_,_JtÈ
■aaoHcpmmmaiw
lèinOTin .X L., ..
Fig. 3. Thermal behavier ofTi02 sample inanatase ratification obtained at 110 °C
vibrations of Ti-0 -H bonds are more intensive in anatase spec tram than in rutile spectrum. Tliis fact indicats on the higher contents of OH - groups in anatase. The intensity of absorption bands of OH -groups in samples of titanium dioxide in anatase structuran modificauion depends on temperature of heat treatment.
The highest intensity of OH-groups absorption bands is observed for the TiO2 sample treated at 110 °C. The increase of temperature of TiO2 treatment to 300 and 500 rC ronsideiably decreases the intensity of OH-groups absoiption bands.
- n21 -
1 J it w If \
2 J —-' J 1
3
JM» 1QQO 3*0D 2000 11® 1MO iOO
k*V■ i*nv!r% ii i.m-i
Fig. 4. IR - spectra of samples of titanium dioxide in anatase modification, obtained at 110 (1), 300 (2) and 500 "C (3)
3« ja:
^«H^ I
1» ■
Kit 1W
SB n
«HU4KI
1« Ul
Fig. 5. IR - spectra of samples ofOnitial titanium dioxide in rutile modification (1t and after mechad'eal trcatmen)s during; 10 mio. (2), t5 ntin. (3),20 min. (4) and 30 min. (5)
I 3 4 5
Catalytic properties of TiO2 samples in anatase and rutile mo dificctions wete compared in the procecs of rspen wood rnd wheat straw oxidative delignification by H2O2.
High catalytic activiiy oi titanium dioxiae in the oxidation processes witd hydrogon peroxide is explained by its ability to initiate the formation of radicals •OH and •OOH from H2O2 [16]. Being formed radicals participate in reactions of oxidative destruction of lignin [17].
Catalytic experiments were made at earlier established optimum delignification parameters [18]: temperature 100 °C, H2O2 - 4 % wt., CH3COOH - 25 % wt., TiO2 - 1 % wt., liquid/solid ratio - 4, time - 4 h. Results of the accomplished studies are presented in Tables 2 and 3.
The obtained data show that the use of titanium dioxide catalyst in rutile modification gives cellulose product with lower content of residual lignin, as compared to TiO2 in anatase modification.
Samples of titanium dioxide in anatase modification which were obtain at temperatures 110 °C, 300 °C and 500 °C have a rather high surface area (260, 111 and 89 m2/g). The reduced catalytic
Table 2. Influence of TiO2 catalysts and conditions of aspen wood delignification on the yield and composition of cellulose products
Sample number TiO2 modification TiO2 treatments Yield of cellulose product, % wt.* Composition of cellulose product, % wt.**
Cellulose Lignin Hemicelluloses
1 Anatase 110 °C 74.5 79.4 12.2 8.1
2 Anatase 300 °C 68.6 77.6 9.8 12.3
3 Anatase 500 °C 66.3 86.3 7.8 5.6
4 Rutile 1000 °C 50.8 91.8 1.6 6.3
5 Sample 4 ArO-2, 10 min. 53.2 89.4 3.8 6.5
6 Sample 4 ArO-2, 15 min. 58.4 87.7 4.2 7.8
7 Sample 4 ArO-2, 20 min. 56.7 88.0 3.9 7.8
8 Sample 4 ArO-2, 30 min. 65.8 85.9 5.6 8.2
* on abs.dry wood
** on abs.dry cellulose product
Table 3. Influence of TiO2 catalysts and conditions of wheat straw delignification on the yield and composition of cellulose products
Sample number TiO2 modification TiO2 treatments Yield of cellulose product, % wt.* Composition of cellulose product, % wt.**
Cellulose Lignin Hemicelluloses
1 Anatase 110 °C 72.2 71.4 14.8 9.8
2 Anatase 300 °C 68.4 75.5 10.2 10.3
3 Anatase 500 °C 66.3 76.8 8.4 10.8
4 Rutile 1000 °C 54.0 81.0 4.2 10.8
5 Sample 4 ArO-2, 10 min. 54.8 80.5 5.2 10.3
6 Sample 4 ArO-2, 15 min. 62.9 78.2 6.4 11.4
7 Sample 4 ArO-2, 20 min. 64.6 77.4 6.8 11.8
8 Sample 4 ArO-2, 30 min. 65.7 78.1 7.1 10.8
* on abs.dry straw
** on abs.dry cellulose product
activity of these samples in oxidative delignification of aspen wood and wheat straw may be explaned by two reasons.
Probably, the developed porosity of anatase samples reduces their catalytic activity in delignification processes owing to strengthening of diffusion limitations inside pores. The heat treatment of titanium dioxide at 1000 °C promotes its transformation to rutil modification, reduces its surface area and increases the size of pores. The latter results in the reduction of diffusion limitation inside pores and increases a catalytic activity of TiO2 in delignification processes.
Besides that, a higher concentration of hydrogen groups on the surface of anatase modification of TiO2 as compared to its rutile modification can prevent the formation from H2O2 the radical species (•OH and •OOH) active in destruction of lignin.
Conclusion
The influence of thermal and mechanical treatments of titanium dioxide on its structural characteristics and catalytic properties in oxidative delignification of aspen wood and wheat straw was studied.
It was determined that the increase of temperature treatment from 110 to 1000 °C results in the reduction a specific surface area of TiO2 from 260 m2 /g to less 1 m2 /g and promotes the transition of anatase modification to rutile modification.
Mechanical treatment of titanium dioxide in the form of rutile using planetary mill AGO-2 increases its specific surface area from 4 to 27 times, depending on the duration of treatment.
It was found, that the best catalytic activity in aspen wood and wheat straw delignification possesses has a titanium dioxide in rutile modification with a low surface area, obtained at 1000 °C.
The use of this catalyst made it possible to obtain from aspen wood and wheat straw the cellulose products with a high yield and with the low content of a residual lignin (1.6 and 4.2 % wt. respectively).
Acknowledgment
The reported study was partially supported by RFBR (research project No 12-03-93117) and by Ministry of Education and Science RF (project RFMEFI60714X003). This work is part of the GDRI "Catalytic biomass valorization" between France and Russia.
In this work devices of the Krasnoyarsk regional center of collective using of the Siberian Branch of the Russian Academy of Science were used.
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