ISSN 20700504, Catalysis in Industry, 2013, Vol. 5, No. 2, pp. 156–163. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.V. Gur’yanov, V.M. Mukhin, A.A. Kurilkin, 2013, published in Kataliz v Promyshlennosti.
DOMESTIC CATALYSTS
Developing AshFree HighStrength Spherical Carbon Catalyst Supports V. V. Gur’yanov, V. M. Mukhin, and A. A. Kurilkin OAO Elektrostal’ Research and Production Association Neorganika, Elektrostal’, Moscow oblast, 144001 Russia Received December 8, 2011
Abstract⎯The possibility of using furfurol for the production of ashfree highstrength active carbons with spheroidal particles as adsorbents and catalyst supports is substantiated. A singlestage process that incorpo rates the resinification of furfurol, the molding of a spherical product, and its hardening while allowing the process cycle time and the cost of equipment to be reduced is developed. Derivatographic, Xray diffraction, mercury porometric, and adsorption studies of the carbonization of the molded spherical product are per formed to characterize the development of the primary and porous structures of carbon residues. Ashfree active carbons with spheroidal particles, a full volume of sorbing micro and mesopores (up to 1.50 cm3/g), and a uniquely high mechanical strength (its abrasion rate is three orders of magnitude lower than that of industrial active carbons) are obtained via the vapor–gas activation of a carbonized product. The obtained active carbons are superior to all known foreign and domestic analogues and are promising for the production of catalysts that operate under severe regimes, i.e., in moving and fluidized beds. Keywords: carbon adsorbent, adsorption, porous structure, furfurol, catalysts, polymerization DOI: 10.1134/S2070050413020062
INTRODUCTION Carbon adsorbents are widely used as catalyst sup ports. However, most industrial active carbons are characterized by low mechanical strength (60–75%), considerable concentrations of ash admixtures (5– 20 wt %), and weakly developed mesoporous struc tures. These factors leave no room for the performance of highly selective catalytic processes, especially under the severe regimes of catalyst operation in moving and fluidized beds. To expand the fields of application of carbon adsorbents as catalyst supports and raise the efficiency of catalytic processes, it is necessary to develop ashfree carbon adsorbents with considerably improved performance characteristics. It therefore seems promising to develop such adsorbents by using such nontraditional raw materials as synthetic monomer and polymer products [1–3], which allow the synthesis of nearly ashfree active car bons with the desired physicomechanical, chemical, and adsorption characteristics. One of the most prom ising synthetic materials is furfurol, a product of the primary processing of pentosancontaining plant raw materials. Owing to the high reactivity and availability of these raw materials, it is one of the most widely rec ognized products for the production of various mono mer and polymer materials on the world market. The largescale production of furfurol in the Russian Fed eration, which has considerable and reproducible sources of plant raw materials, allows its use in devel oping the industrial technology for producing carbon
adsorbents characterized by considerably improved physicomechanical and adsorption properties, relative to the known types of active carbons [4–6]. However, all existing furfurol processing technolo gies yield lowmelting liquid or solid resins, which are further used in the manufacturing of engineering arti cles after compounding with fillers and hardeners. Nevertheless, there are no data on the direct use of fur furol for the manufacturing of hardened engineering articles or, moreover, spheroidal particles. From the above, we may conclude that it is necessary to develop a fundamentally new singlestage process for the resinification of furfurol, the molding of spherical product, and its hardening. STUDYING FEATURES OF THE HARDENING OF FURFUROL IN THE PRESENCE OF ACIDS When furfurol is resinificated in the presence of acids, the solution quickly loses its transparency. However, the mixture remains a fluid. One feature of its hardening is the existence of a sharp boundary between the liquid and jellylike states. The jellifica tion time (τjel) of furfurol is an important parameter for organizing the molding of spherical product. The jellification times of furfurol in the presence of strong muriatic and sulfuric acids (35–40 and 92 wt %, respectively) at room temperature are given in Table 1.
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The presented data indicate that the resinification of furfurol is much more intense in the presence of sul furic acid. At an acid/furfurol volumetric ratio C = 0.1, the jellification time of furfurol in the presence of sul furic acid is thus oneseventh the time in the presence of muriatic acid. This can apparently be explained by the high hygroscopicity of sulfuric acid, which can bond water, a product of furfurol polycondensation. Analysis of the obtained experimental data allows us to establish the following relationships between τjel (min) and the parameter C: τjel = –14 – 274 logCHCl, (1)
157
Table 1. Jellification time of furfurol in the presence of mineral acids Volumetric ratio of components
Jellification time, min
C4H3OCHO
HCl
experimental
calculated via Eq. (1)
100
7
325
302
100
10
240
260
100
25
157
151
100
33.3
118
117
(2)
100
50
62
68
As is clear from Table 1, these data can be used to find the jellification time. Note the appreciable effect temperature has on the jellification time of blends. When the process is per formed at C H2 SO4 = 0.1 and 75°C instead of room tem perature, τjel is reduced from 33 to 1.5 min.
100
66
50
47
C4H3OCHO
H2SO4
experimental
calculated via Eq. (2)
100
2.5
290
284
100
4
109
113
100
6
60
60
100
10
33
29
100
12
23
22
100
18
10
11.5
2
–2
τ jel = –19 + 48 [ log ( C H2 SO4 × 10 ) ] .
EFFECT OF COPOLYMERIZATION ON THE RATE OF FURFUROL HARDENING The resinification and hardening of furfurol are considerably accelerated if some organic compounds that are able to react with the monomer during copo lymerization are introduced into the blend in addition to acids. Active components of the resinification and hardening of furfurol in an acidic medium are various monomer and polymer products, e.g., acetone, phe nol, propyl aldehyde, diethylene glycol, phurphuryl alcohol, formaldehyde, different thermoactive syn thetic resins (phenolic, epoxy, wood chemical, and coaltar resins), and mixtures of the above com pounds). The data given in Table 2 indicate the high activity of the introduced compounds. At C H2 SO4 = 0.1, even small amounts of acetone and epoxy resin (C = 0.02) reduce the jellification time from 33 to 6 and 2.1 min, respectively (see Table 1). Owing to the low concentrations of introduced compounds, we cannot assert that copolymerization alone consider ably reduces the value of τjel. It seems likely that the interaction of furfurol with the abovementioned addi tives catalyzes the principal reaction of furfurol homopolymerization. At the high rates of jellification (τjel < 10 min) in the initial furfurol hardening stage, we are dealing with a deep process that leads to the formation of a rigid spa tially crosslinked resite structure, i.e., to the harden ing of blends upon heating and the release of reaction product vapors. The ultimate hardening time of blends is 70–140% of their jellification time. Hence, introducing various active compounds into furfurol and raising the process temperature allows us to attain very short jellification times, indicating the CATALYSIS IN INDUSTRY
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possibility of organizing a singlestage process for the molding and hardening of spherical product. PRODUCING SPHERICAL CARBON ADSORBENT A process flowsheet for the production of an active carbon (furfurol spherical adsorbent, PSA) is shown in Fig. 1. The product is molded at a temperature of 100– 110°C in a tube reactor that is 1.5 m tall and filled with mineral oil. Furfurol, sulfuric acid, and active organic additives are combined in a mixer whose volume is such that the time a mixture remains in it does not exceed the time of its jellification. The blend, partially resinified in the mixer, is fed by gravity into a distribu tor through whose nozzles it issues in the form of jets. The jets enter a layer of hot oil in which they are bro ken down into drops of the desired size, depending on the viscosity of oil. The complete resinification and thermohardening of the product in the reactor takes 15–18 s. The product is continuously fed from the reactor into a collector (omitted in the flowsheet) and, after filling the collector, into a centrifuge. Once separated from the oil, the product consists of sphe roidal grains 1–3 mm in diameter (dominant frac tion, 2–3 mm) that are then subjected to thermal treatment. The thermal treatment of the molded product takes place in a rotary electrical furnace in a carbon dioxide
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Table 2. Jellification time of furfurol in the presence of acetone, ED20 epoxy resin, and sulfuric acid Volumetric ratio of components Jellification time, τjel, min C4H3OCHO
acetone
H2SO4
100
1
10
100
2
100 100
Volumetric ratio of components
Jellification time, τjel, min
C4H3OCHO
ED20
H2SO4
8.1
100
1
10
6.9
10
6
100
2
10
2.1
3
10
5.1
100
3
10
4
10
4
100
4.6
flow at a temperature raised gradually from 150 to 820°C at a rate of 10°C/min. The thermally treated product is activated with a mixture of water vapor and carbon dioxide (ratio, 3 : 1), also in a rotary electrical furnace at temperatures of 850–870°C. The gaseous and vapor products formed during carbonization and activation are exhausted into the atmosphere after they pass through an afterburning furnace. The acti vated carbon is then filtered to remove dust and coarse inclusions and subjected to magnetic separation. In a PBSU 40/10 drum magnetic separator, the product is purified of metallic scales of carbonization and activa tion furnace retorts. The obtained carbon adsorbent (PSA) consists of spheroidal grains with diameters of 0.7–2.5 mm and a smooth, shiny, nondusting sur face.
1
2
<1
7
2.5
STUDYING THE PRIMARY AND POROUS STRUCTURES AND ADSORPTION PROPERTIES OF CARBONIZED AND ACTIVATED FURFUROL SPHERICAL ADSORBENTS The obtained carbon adsorbents were studied by derivatography, Xray diffraction, and adsorption and mercury porometry. Differential thermal analysis (DTA) of the carbon adsorbents was performed on a MOM OD103 deri vatograph in a nitrogen atmosphere at a heating rate of 5 K/min. The Xray diffraction spectra of powder pressed in quartz cells were recorded on a DRON5 diffractome ter using filtered FeKα radiation. The Xray density ρ was calculated as described in [7]. The interplanar dis
3
4
CO2 5
7 8 H2O
6 9
10
Fig. 1. Process flowsheet for the production of active carbon (PSA): (1), (2), and (3) furfurol, acid, and oil storage tanks, (4) mixer, (5) reactor, (6) centrifuge, (7) carbonization furnace, (8) activation furnace, (9) screening device, (10) containers. CATALYSIS IN INDUSTRY
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DEVELOPING ASHFREE HIGHSTRENGTH SPHERICAL CARBON Δm/mres ΔT, K–1 28
159
Adsorption isotherms of benzene vapors were obtained at 293 K on a highvacuum sorption system with spring quartz microscales with a sensitivity of nearly 20 μg at loads of up to 0.2 g.
ω, %
24 20
100
16
80
12
60
8
40
4
20
0 200
300
500
400
700
600
900
800
1000 T, °C
1100 T, K
Fig. 2. Integral and differential mass loss curves (Δm/Δmres ΔT) for the thermodestruction of products of (䊊) spherical and (䊉) crushed furfurol hardening (ω is the carbon residue yield).
tance d002 was determined according to the Wulf– Bragg equation, and the average height of stacks Lc and their diameter La were calculated using the Sherer– Selyakov formulas [8]. The parameters of the porous structure were deter mined on a PA3M porometer.
Carbonized Spherical Carbon Adsorbents The derivatograms of the mass losses Δm/Δmres ΔT (K–1) versus the carbonization temperature of ther mally hardened samples are plotted in Fig. 2. It can be seen that the derivatograms of spherical product and a crushed sample obtained via block polymerization and the hardening of furfurol virtually coincide. In the dif ferential mass loss curves, we observe the most pro nounced peaks at 200°C and the highest peaks at 450– 500°C. Both products are also characterized by the same carbon residue yields ω at 950°C. However, ther mal treatment of a spherical sample yields a much denser product whose bulk density is 670–700 g/dm3, while the bulk density of the crushed product is 440– 460 g/dm3. This is due to considerable differences between the furfurol thermohardening conditions. The crushed product was obtained by hardening the blend (200 mL) near room temperature. In the case of liquid molding, the volume of a single drop heated to 100°C for several seconds is only ~4 × 10–3 cm3. Such considerable distinctions between the hardening pro cesses (and probably the hydrostatic compression of a drop due to the surface tension of the spherical menis cus) lead to the formation of a more defectfree prod uct that is 1.5 times denser. The elemental composition of carbon residues obtained during the carbonization of molded spherical product and the results from their Xray diffraction analysis are given in Table 3. The results from Xray diffraction analysis indicate that the samples subjected to thermal treatment below 400°C remained Xray amorphous. Stacks of carbon
Table 3. Changes in the primary structure parameters and elemental composition of carbon residues during the carbonization of molded spherical product Carbonization temperature, °C Parameters 150
d 002 ⎫ ⎪ Lc ⎬ nm La ⎪⎭
300
550
700
850
Xray amorphous
0.391
0.386
0.385
» » » »
1.20
1.09
1.07
2.97
3.55
3.77
2.0
2.07
2.10
23
37
40
ρ, g/cm % of ordered carbon 3
C⎫ ⎪ O⎪ ⎬% H⎪ S ⎪⎭
400
68.8
74.6
83.1
92.8
96.2
98.1
24.7
20.1
12.0
2.5
1.4
0.3
5.7
4.7
4.4
4.2
2.0
1.3
1.1
0.6
0.5
0.5
0.4
0.3
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Table 4. Change in the porous structure parameters of molded spherical product during carbonization Volume of pores (cm3/g) within a range of equivalent radii (nm) of 5–10
10–25
25–50
50–100
>100
Mercury porometric volume, VHg, cm3/g
0.00
0.01
0.11
0.02
0.00
0.14
320
0.00
0.04
0.18
0.02
0.00
0.24
370
0.01
0.06
0.24
0.02
0.00
0.33
420
0.00
0.09
0.26
0.03
0.00
0.38
550
0.01
0.07
0.27
0.02
0.00
0.37
700
0.01
0.07
0.25
0.02
0.00
0.35
870
0.02
0.03
0.29
0.02
0.00
0.36
Carbonization temperature, °C 200
networks were formed when the temperature was raised from 400 to 550°C, and the sample mass and oxygen content were sharply reduced due to the destruction of furan rings with the formation of cycli cally polymerized carbon. A further rise in tempera ture was accompanied by structural ordering, mani fested as a drop in the values of d002 and Lc and an increase in the density of carbon, the amount of car bon ordered into stacks, and the diameter of stacks. The results from our mercury porometric studies of the development of meso and macroporous struc tures during the carbonization of molded spherical product are listed in Table 4. The data indicate that the samples have virtually no pores of more than 50 and
less than 10 nm in size. The main volume of large mesopores is distributed within a narrow range of equivalent radii of 25–50 nm and formed predomi nantly during the early thermal treatment stage, at temperatures below 420°C. The adsorption isotherms of benzene vapors on carbonized and activated spherical carbon adsorbents are shown in Figs. 3 and 4. The isotherms in Fig. 4 reflect two groups that differ considerably from each other. The isotherms obtained for the Xray amor phous samples belong to the first group. Their distinc tive feature is low adsorption values at low pressures and a rise in the curves in the region of polymolecular adsorption. The second group is composed of adsorp
Q, mmol/g 2.5 1123 K 973 K
2.0
823 K 1.5
693 K
1.0
593 K 473 K 373 K
0.5
0
015
0.30
045
0.60
0.75
0.90 P/Ps
Fig. 3. Adsorption isotherms of benzene vapors on molded spherical product samples subjected to thermal treatment at different temperatures (P/Ps is the relative pressure of benzene vapors). CATALYSIS IN INDUSTRY
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161
a, mmol/g
15
4
3
10
2
5 1
0
0.2
0.4
0.6
0.8
1.0 P/Ps
Fig. 4. Adsorption isotherms of benzene vapors on spherical carbon adsorbents (PSAs) activated to bulk densities (Δ) of (1) 605, (2) 425, (3) 343, and (4) 309 g/dm3 (a is adsorption; P/Ps, the relative pressure of benzene vapors).
tion isotherms obtained during the carbonization of product at 550–850°C and characterized, as above, by the presence of cyclically polymerized carbon net works. These are typical adsorption isotherms obtained for microporous adsorbents with the sharp rise typical of their curves at low pressures. The micro and mesoporous structural characteris tics obtained for carbonized spherical product from processing the adsorption isotherms via the δmethod [9, 10] are given in Table 5. From the above data it follows that the sample sub jected to thermal treatment at 100–420°C contained no micropores. The basic volume of molecularsized pores formed within the narrow temperature range of 420–550°C, when stacks of carbon networks were formed in the course of structural aromatization. CATALYSIS IN INDUSTRY
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Activated Spherical Carbon Adsorbents (Carbons) The adsorption isotherms of benzene on a number of activated adsorbents (Fig. 4) indicate that the car bons contain both micropores and large mesopores that can be filled at high gauge pressures. As can be seen from Table 6, the carbons contain no macropores, and the entire volume of pores is formed of sorbing micro and mesopores only when the total porosity is raised to 1.50 cm3/g. It is noteworthy that the growth in the volume of mesopores from 0.37 to 0.70 cm3/g during activation was not due either to an increase in their size or the formation of new mesopo res, since their volumetric fraction remained nearly the same at 0.22–0.24 cm3/cm3 layer. The mesoporous structure parameters are thus governed by the charac
162
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Table 5. Change in the textural characteristics of the molded product in the process of carbonization Carbonization temperature, °C
Volume of micropores, cm3/g
Surface area of mesopores, m2/g
100
0.00
70
200
0.01
70
320
0.00
100
420
0.01
100
550
0.14
40
700
0.15
31
850
0.16
34
micropores of up to 0.80 cm3/g. Such high values lie at the limit of the accepted method for calculating abra sion strength. We therefore modified the method with regard to PSAs and increased the time of grinding in a rod mill from 3 min to 30 h. The strength of PSA carbon with VΣ = 1.22 cm3/g was 98%, thereby corresponding to abrasion rate coefficient K = 1.1 × 10–3% min–1. Active AG and SKTtype carbons with strengths of 80% formed 20% of the dust produced during 3min grinding, corresponding to K = 6.67 % min–1. Hence, the abrasion rate of PSA carbons was more than three orders of magnitude lower than the abrasion rate of industrial active carbons. PSAs also have high crushing strength properties. Upon activation, the grain destroying force Fp changes from 220 to 190 N, thus being 17 and 40 times higher than the Fp values for AG3 and AG5 carbon grains, respectively. The results from the considered method are of a relative nature, so it was of interest to determine such physicomechanical carbon material characteristics as the ultimate compression strength σc. A rough esti mate yields values of σc = 740–40 MPa, which are 3– 7 times higher than the ultimate compression strength of such materials as granite, quartzite, composite epoxy material, and electrocarbons and are compara ble to the σc of cast iron (400–1000 MPa) [14, 15].
teristics of the initial carbonized material, and the increase in the volume of micropores per unit mass upon activation was due only to the gasification of car bon as the microporous structure developed from a layer measuring 0.18 to 0.28 cm3/cm3 (0.29– 0.80 cm3/g) due to the linear size of micropores grow ing from 1.21 to 1.75 nm. The thermally treated product was activated at temperatures of 850–870°C for 6 h to obtain samples with different porous structures. As might be expected, the use of furfurol (a syn thetic monomer) as the initial raw material allowed us to obtain nearly ashfree active carbons. However, the most characteristic feature of the PSAs was their uniquely high mechanical strength. The abrasion strength thus did not drop below 98% and was close to 100% for most of the active carbons from activation until the development of a total porosity of up to 1.50 cm3/g and a volume of
CONCLUSIONS As a result of studying the hardening of furfurol in the presence of acids and active organic monomer compatible additives, we developed a new technique for the liquid molding of spherical product by combin ing three traditionally separate stages: (1) the resinifi cation of furfurol via polycondensation and polymer
Table 6. Characteristics of activated spherical carbon adsorbents (PSAs) Volume Total volume Size Bulk density, of pores (V ), of micropores of micropores 3 Σ 3 (Vmicro), cm /g g/dm (H), nm cm3/g (cm3/cm3)
Volume Volume Ash Abrasion of mesopores of macropores content, % strength, % (Vmacro), (Vmeso), cm3/g (GOST (GOST 3 3 12596) 16188) cm3/g (cm /cm )
605
0.66
0.29 (0.18)
1.21
0.37 (0.22)
0.0
0.03
99.1
574
0.76
0.34 (0.20)
1.28
0.42 (0.24)
0.0
0.01
99.0
509
0.92
0.45 (0.23)
1.45
0.47 (0.24)
0.0
0.03
99.7
470
1.01
0.51 (0.24)
1.50
0.50 (0.23)
0.0
0.06
436
1.09
0.56 (0.24)
1.59
0.53 (0.23)
0.0
0.05
98.8
405
1.22
0.66 (0.27)
1.63
0.56 (0.23)
0.0
0.05
99.5
345
1.50
0.80 (0.28)
1.75
0.70 (0.24)
0.0
0.08
98.0
100
2/3
Note: The size of slit micropores (H) is calculated as H = 10.8/E 0 , where E0 is the characteristic adsorption energy (kJ/mol) corre sponding to the adsorption isotherm equation in the theory of the volumetric filling of micropores [11, 12]. CATALYSIS IN INDUSTRY
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DEVELOPING ASHFREE HIGHSTRENGTH SPHERICAL CARBON
ization; (2) the molding of spherical product; and (3) its hardening. We optimized the parameters of the carbonization and activation of molded product to obtained an ash free uniquely highstrength carbon adsorbent, ensur ing its promise as a catalyst support for such severe operating regimes as moving and fluidized beds. REFERENCES 1. Burushkina, T.N., Zh. Ross. Khim. Ova im. D.I. Men deleeva, 1995, vol. 39, no. 6, p. 122. 2. Kryazhev, Yu. G., Abstract of Papers, Materialy XII vse rossiiskogo simpoziuma s uchastiem inostrannykh uchenykh “Aktual’nye problemy teorii adsorptsii, poris tosti i adsorptsionnoi selektivnosti” (Proc. of XIIth All Russia Symposium with the Participation of Foreign Scientists “Urgent Problems of the Theory of Adsorp tion, Porosity, and Adsorption Selectivity”), Moscow, 2008, p. 69. 3. Kartel’, N.T., in Adsorbtsiya, adsorbenty i adsorbtsion nye protsessy v nanoporistykh materialakh (Adsorption, Adsorbents, and Adsorption Processes in Nanoporous Materials), Tsivadze, A.Yu., Ed., Moscow: Granitsa, 2011, p. 381. 4. RF Patent 2026813, 1993. 5. RF Patent 2257343, 2003.
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Translated by E. Glushachenkova