ISSN 2070-0504, Catalysis in Industry, 2009, Vol. 1, No. 2, pp. 153–156. © Pleiades Publishing, Ltd., 2009. Original Russian Text © O.N. Baklanova, V.A. Likholobov, M.S. Tsekhanovich, V.Yu. Davydova, O.A. Chirkova, V.A. Drozdov, Yu.V. Surovikin, 2009, published in Kataliz v Promyshlennosti.
DOMESTIC CATALYSTS
Effect of the Particle Size of Globular Nanodisperse Carbon on the Texture and Strength of Molded Sibunit-Type Materials O. N. Baklanova, V. A. Likholobov, M. S. Tsekhanovich, V. Yu. Davydova, O. A. Chirkova, V. A. Drozdov, and Yu. V. Surovikin Institute of Hydrocarbon Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia
Abstract—New large-scale technology can produce porous carbon–carbon composites shaped as 0.5–5 mm grains on the basis of globular nanodisperse carbon (GNC); these composites are referred to as Sibunit. The use of this material as units of more complex shapes, such as rods, tubes, and petals, can help to reduce the hydraulic drag of the sorbent or catalyst bed. The tasks of this study were to manufacture, via extrusion molding, rods on the basis of GNC with various primary particle sizes and to study the effect of GNC particle sizes and heattreatment parameters on the specific surface area of molded Sibunit. The GNC particle size has a decisive effect on the texture parameters. The greatest specific surface area (600–700 m2/g) was obtained for GNC with particle sizes of (15–20) ± 5 nm with the strength retained at a level of 3–5 N/mm2. The results of this work are recommended for use in the development of large-scale technology for manufacturing Sibunit-type supports in various shapes with particle sizes of 2–12 mm and inner channel diameters of 1–6 mm. DOI: 10.1134/S2070050409020111
Catalytic and adsorption processes use carbons with a developed surface which are stable in acids and alkalis [1, 2]; carbon produced by the carbonization of natural vegetable and fossil materials is used most widely. An essential drawback of such materials is the undesirable and hard to remove impurities [3, 4]; in this context, the preparation of high-purity synthetic carbon materials is a challenging task [5]. Several recent studies concern the design of impurity-free carbon materials [6]. One such material is a carbon/carbon composite based on globular nanodisperse carbon (GNC), in other words, technical carbon (TC) or carbon black (CB) that contains at least 99.0% carbon referred to as Sibunit [7]. However, large-scale technology produces Sibunit only as spheres with diameters of 0.5–5 mm. This form limits the use of this material in a number of adsorption and catalytic processes because of the close packing of fine spherical grains in the sorbent bed and the associated high hydraulic drag in the sorption column. This problem can be solved via changing the shape of Sibunit preforms and preparing them as rods, rings, petals, and other shapes with outer diameters of 2–12 mm and inner channel diameters of 1–6 mm. Large-scale technology for the production of grained Sibunit [8] includes two heat-treatment stages: pyrolytic densification or carbonization and the oxidative gasification or activation of carbonized intermediate products. Extrusion technology for the production of molded carbon adsorption/catalytic materials on the basis of GNC differs essentially from the state-of-the-art syn-
thesis of grained Sibunit; it includes some additional stages: the mixing of GNC with a liquid dispersion medium and the dispersion of the carbon paste inside extruder channels under high shear stresses followed by the extrusion of the homogeneous plastic composite through a die of complex shape. Pertinent dispersion media include mixtures of aqueous solutions with polymer dispersions, such as poly(acrylamide), poly(acrylic acid), and poly(vinyl acetate) [9]. Polymers are used as components of the dispersion medium in technology for enhancing the extrusion moldability of disperse blends. In as much as polymers are contained in extrusionmolded preforms, their possible carbonization during heat treatment should be taken into account. This work has the following tasks: to study the extrusion of carbon pastes and optimize the temperature and time parameters of heat treatment to ensure the manufacture of molded carbon preforms with a developed specific surface (300 m2/g) and a strength (σcompr) of at least 3 N/mm2. EXPERIMENTAL Extrusion was carried out on a Leistritz doublescrew extruder. A feed screw fed GNC to the feed zone of the extruder, and a piston pulp fed the dispersion medium. A plastic carbon paste was blended and homogenized inside the channels of the two screws while moving from the feed zone to the molding die. To study the effect of the precursor particle size on the pore texture and strength of molded preforms, we
153
154
BAKLANOVA et al.
Extrusion direction Resin
10 µm Fig. 1. Micrograph of a carbon preform manufactured by the extrusion molding of a GNC + dispersion medium plastic blend through a 3-mm die. GNC particle size: 50–60 nm. The test sample was dried to constant weight at 100°C.
used GNC with particle sizes of 15–20, 50–60, and 90−100 nm. The dispersion media used were solutions and dispersions of polymers (poly(acrylamide), poly(acrylic acid) poly(vinyl acetate), and dextrin). After molding, carbon extrudates were heat-treated in several stages at 30–800°C in an inert atmosphere to remove water and carbonize the polymers. The temperature–time schedule of the finish stages (carbonization and activation) were analogues to the schedules used in grained Sibunit technology. The exterior, strength, and pore structure of molded preforms were studied as affected by carbonization parameters (temperature, composition, and volume rate of hydrocarbons + inert gas mixtures) and activation parameters (temperature, composition, and volume rate of the activating mixture) on a laboratory flow-through thermal setup with a horizontal reactor in capacity rotated at a fixed speed. The reaction zone volume was 0.5 dm3. The setup was heated with an external electrical heater equipped with a thyristor temperature-stabilization system (±2-K oscillations). At the carbonization stage, propane/butane was used in a mixture with argon; at the activation stage, steam mixed with argon was used. The texture characteristics (surface area and porosity) of molded activated carbons were studied using adsorption measurements on a Sorptomatic-1900 setup (Carlo Erba) and mercury porometry on a Porosimeter2000 instrument. The strength was ascertained as the crushing strength σcr along the generatrix on an MR-9s tester; σbr was calculated from ten replicate tests. RESULTS AND DISCUSSION In the course of extrusion, GNC is blended with the dispersion medium. An intense shear stress causes the dispersion of the carbon paste inside extruder channels
1 µm Fig. 2. Micrograph of a carbon preform manufactured by the extrusion molding of a GNC + PFR plastic blend through a 3-mm die. GNC particle size: 50–60 nm.
and the extrusion of the homogeneous plastic blend through a die of a complex shape. Evidently, the carbon paste can change its texture, canceling out the structural features of the precursor GNC, in particular, the size of aggregates of primary carbon particles. A change in aggregate size is very undesirable because this will considerably complicate control over the porosity formation in activated carbons. Figure 1 displays the micrograph (obtained with a BS-350 Tesla microscope) of a carbon preform extruded through a 3-mm die. The carbon material after extrusion is not homogeneous (Fig. 1); rather, it consists of dense layers, 200– 300 µm thick, lying parallel to the flow direction and separated by interlayers of lower density material. Chains of empty cells in which the dispersion medium, removed upon drying, was evidently accumulated are clearly seen in the micrograph. This illustration implies that the liquid dispersion medium, located at the outer boundaries of GNC aggregates, migrates under shear strains to the outside of carbon aggregates. As a result, the flow of dense GNC layers over a liquid interlayer occurs, that is, the so-called core/shell mode appears. Shear rates inside the core are low, which stabilizes the core and conserves the structural features of the extruded carbon material virtually unchanged. To verify this hypothesis, we prepared plastic GNCbased carbon composites with 50–60 nm particles using phenol–formaldehyde resin (PFR) as the liquid dispersion medium. Extrudates were exposed at 150– 160°C for 30 min. As a result of PFR curing, the liquid component was firmly fixed in the plastic blend. Micrographs of these blends obtained with a BS-350 Tesla microscope are exemplified by Fig. 2. The dispersion medium (in the case at hand, PFR) is located at the outer boundaries of GNC grains, does not penetrate into the inside of grains, and virtually does not alter the CATALYSIS IN INDUSTRY
Vol. 1
No. 2
2009
EFFECT OF THE PARTICLE SIZE OF GLOBULAR NANODISPERSE CARBON
structure of the carbon material. Thus, Fig. 2 verifies the suggestion that the strong shear deformations experienced by precursor GNC in extruder channels and during extrusion through the die virtually do not alter the structure of the precursor GNC. We studied pore-structure formation in molded activated carbons during multistage heat treatment. Pyrolytic densification or carbonization lasted 0.5–0.6 h; carbonized samples were activated in steam for 0.25– 1.0 h at 850–900°C. After carbonization, the percentage densification α (carbonization) was determined: α = 100 ( m fin – m in )/m in . After activation, the percentage weight loss η was ascertained: η = 100 ( m fin – m in )/m fin .
155
SBET, m2/g 120
(a)
80
40
0 σcr, MPa
2
4
6
8
10 t, h (b)
12
Here, min and mfin are the weights of the sample before and after heat treatment, g. Figures 3a and 3b show the variation in texture and strength characteristics of rod-shaped carbon preforms with Douter = 3 ± 0.2 mm during carbonization. The precursor was GNC with diverse particle sizes. The specific surface area of the material decreases considerably in the first 1–3 h of carbonization, while the strength increases (Fig. 3). The greatest specific surface area (74–80 m2/g) with strength levels of 5.0–11 N/mm2 after 3-h carbonization is observed for rods prepared from GNC with particle sizes of (15–20) ± 5 nm. The table displays the variation in specific surface area and strength of activated carbon materials upon the activation of carbonized molded preforms. The table makes it clear that the primary particle size of precursor GNC is the key parameter for the development of the pore structure of activated carbon materials. The highest texture characteristics for rod-shaped molded preforms can be achieved with the use of GNC with primary particle sizes of (15–20) ± 5 nm. In this case, the specific surface area reaches 600–700 m2/g while the strength of the rods remain at a level of 3– 5 N/mm2.
8 4
0
2
4
6
8
10 t, h
Fig. 3. Texture and strength characteristics of molded carbon preforms vs. carbonization time at 850°C. Medium: propane. GNC particle size, nm: , (10–20) ± 5; , (50– 60) ± 10; and , 100 ± 10.
Figure 4 compares the specific surface areas of grained Sibunit manufactured by the large-scale process and molded rods 3 mm in diameter. Both grained and molded samples were prepared from GNC with 50–60 nm particle sizes; the percentage densification of the samples after carbonization was 45–50%. When the weight loss on activation does not exceed 20%, the rods and grains have similar specific surface areas of 100– 150 m2/g (Fig. 4). When the weight loss increases to 30–80%, the specific surface area of the activated
Texture and strength of molded preforms as functions of activation time and GNC particle size Carbonization
Activation
GNC particle size, nm
α, %
σcompr, N/mm2
η, %
SBET, m2/g
σcr, MPa
(90–100) ± 10
139
37
(50–60) ± 10
135–160
55–62
(15–20) ± 5
154–168
54–72
22 45 15 47 25 23
142 213 145 273 719 601
3.5 2.1 5.6 2.9 3.4 5.4
CATALYSIS IN INDUSTRY
Vol. 1
No. 2
2009
156
BAKLANOVA et al.
SBET, m2/g 600
450
300 1 2
150
0
20
40
60
80 η, %
Fig. 4. Specific surface area of (1) grains and (2) molded rods vs. weight loss η during oxidative activation. GNC particle size: 50–60 nm.
grained material is higher than that of the molded rods. Evidently, this is because of the closer packing of GNC particles during the extrusion of the plastic carbon blend through the die. The above results can serve as rationale for largescale technology for the manufacture of Sibunit-type materials in rods, tubes, and other shapes with diameters of 2–12 mm and inner channel diameters of 1– 6 mm. The studies of the effects of technology and compounding variations on manufacture process parameters and the texture and strength of molded Sibunit-type items will continue. These studies are necessary for developing large-scale technology for the extrusion manufacturing of Sibunit; this technology will expand the range of carbon supports with improved texture parameters (Ssp > 600 m2/g). The use of extrusionmolded Sibunit-type supports of diverse shapes will increase the operation efficiency of Sibunit-supported catalysts in liquid-phase processes, for example, the efficiency of palladium catalysts in therephthalic acid hydropurification, rosin disproportionation, furfural decarbonylation, and other processes.
CONCLUSIONS (1) We have studied the flow of plastic carbon blends based on globular nanodisperse carbon and a dispersion medium, which is a polymer solution or dispersion. The dispersion medium in carbon extrudates is mostly located at the outer boundary of GNC aggregates and does not destroy primary aggregates of carbon particles. (2) We have studied carbonization and activation schedules on the texture and strength of the molded material in the form of rods 3 mm in diameter; the GNC particle size is decisive for the texture. The greatest Ssp (600–700 m2/g) is in GNC with particle sizes of (15– 20) ± 5 nm, while the strength of extrudates is 3– 5 N/mm2. (3) The comparison of specific surface areas of grains and rods showed that, for low activation levels, these materials have almost identical Ssp; when the activation level is as high as 50°–80°, molded rods are less porous. (4) Our results on the optimization of the compounding parameters at the extrusion stage and the temperature and time parameters at subsequent heat treatment stages have been recommended for use in developing large-scale technology for the production of Sibunit-type catalyst supports in rods, tubes, and other shapes with diameters of 2–12 mm and inner channel diameters of 1–6 mm. REFERENCES 1. Tennison, S., Appl. Catal., A, 1998, vol. 173, p. 289. 2. Jagiello, I. and Thommes, M., Carbon, 2004, vol. 42, no. 7, p. 1227. 3. Rodriguez-Reinoso, F., in Porosity in Carbons: Characterization and Applications, Patric, I.W., Ed., Edward Arnold: London, 1995, p. 253. 4. Van de Sandt, E.J.A.X., Wiersma, A., Makkee, M., et al., Appl. Catal., A, 1998, vol. 173, p. 161. 5. Bashkova, S., Xianxian Wu, F.B., Armstrong, T.R., and Schwaartz, V., Carbon, 2007, vol. 45, no. 6, p. 1354. 6. Fenelonov, V.B., Poristyi uglerod (Porous Carbon), Novosibirsk: Knizhnoe Izd–vo, 1995. 7. Surovikin, V.F., Plaksin, G.V., Semikolenov, V.A., et al., US Pat. 4978649, 1990. 8. Surovikin, V.F., Surovikin, Yu.V., and Tsekhanovich, M.S., Ross. Khim. Zh., 2007, vol. 51, no. 4, p. 111. 9. Baklanova, O.N., Plaksin, G.V., and Duplyakin, V.K., Ross. Khim. Zh., 2007, vol. 51, no. 4, p. 119.
CATALYSIS IN INDUSTRY
Vol. 1
No. 2
2009
