Article pubs.acs.org/IECR
Pilot Plant Scale Synthesis of CNS: Influence of the Operating Conditions Vicente Jiménez,* Alfredo Muñoz, Paula Sánchez, José Luis Valverde, and Amaya Romero Facultad de Ciencias Química, Departamento de Ingeniería Química, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain ABSTRACT: The present work was an in-depth study related to synthesis of carbon nanospheres (CNSs) at different scales (lab and pilot) with the end goal to economize the production of these materials on a large scale. Synthesis of large amounts of CNSs relies on the careful control of the operating conditions such as space velocity (helium flow rate), hydrocarbon (benzene) content in feed stream, and synthesis time. The alteration of these variables caused important changes in both the yield and properties of the obtained materials. In general, characterization results of the synthesized CNSs demonstrated that they showed low BET surface area and pore volume values typical of spherical geometrical bodies, good thermal stability, and good crystallinity. Normally, CNSs are presented as conglomerates as consequence of the accretion via the carbon atoms at the edge of the “curling” graphitic flakes. Finally, results demonstrated a successful scale up, obtaining a CNSs yield at pilot scale considerably superior (factor of 3.9) to that obtained at laboratory scale.
1. INTRODUCTION Carbon materials are found in a variety of forms such as graphite, diamond, fullerenes, carbon nanofibers (CNFs), and carbon nanotubes (CNTs). Of these five, the nanostructured carbon materials have sparked an increasing interest for chemists, physicists, and materials scientists worldwide.1 Because of their unique structural, mechanical, and electronic properties, there are numerous envisioned applications of nanostructured carbons including electrode materials for lithium ion batteries, gas storage media, catalyst supports, adsorbents, and electronic devices.2−6 However, a targeted production of carbon nanospheres (CNSs) is only now starting to achieve a significant research activity. Their high surface chemical activity provided by the unclosed graphitic layers, which provides reactive “dangling bonds”,7 makes them suitable materials for catalysis and adsorption processes. Moreover, CNSs have also encountered application as lubricants, polymer additives, in energy storage, and as precursors for diamond film synthesis.8,9 There have been many attempts in the synthesis of CNSs including the carbon arc technique,10,11 ultrasonic treatment,12 high-energy electron-beam irradiation,13 thermal treatment of carbonaceous materials,14 chemical vapor deposition (CVD),15 reduction of carbonate or supercritical carbon dioxide,16,17 thermal treatment of pure carbon soot,18 ultradispersed diamond powders (2−6 nm),19 plasma torch process,20 selfgenerated template approach,21 and hydrothermal reaction.22 A catalytic route, often termed chemical vapor decomposition, has emerged as a lower cost option which exhibits a greater degree of control and a more feasible scale-up.23 Several works report the growth of CNSs by a catalyst-assisted chemical depositon route. Kang and Wang reported the catalytic synthesis of CNSs via the mixed-valent oxide decomposition of natural gas at 1100 °C.7,24,25 Serp et al. reported the decomposition of methane, C5H12, and C2H2 over a range of Ni and Fe catalysts at 700− 1100 °C.8 On the other hand, direct pyrolysis (CVD in the absence of a catalyst) of hydrocarbons has been reported as an © 2012 American Chemical Society
effective means of producing CNSs. Qian et al. obtained CNSs from toluene at temperatures greater than 1000 °C26 and Jin et al. reported the direct synthesis of carbon spheres form the pyrolysis of a range of hydrocarbons, including styrene, toluene, benzene, hexane, cyclohexane, and ethane in the temperature range 800−1200 °C.27 The main problem associated with CNSs is that the lack of sufficient amount of material limits the development of more practical applications. For this reason it is of great importance to develop a low-cost and high-productivity method to synthesize CNSs, which would open novel applications for these materials. Thus, in this work, we report a study of the controlled growth of CNSs, using benzene as carbon source, and a CVD method in the absence of a catalyst. Optimization of the growth conditions is targeted in order to obtain the maximum CNS yield, while controlling the CNS structure and diameter.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Carbon Nanospheres (CNSs). Carbon nanospheres were grown at atmospheric pressure in a fixed-bed reactor that consisted of a quartz tube of 9 cm diameter and 100 cm length located in a horizontal electric furnace (JH Hornos) with an effective heating zone of 80 cm. Thermocouple type K was used for monitoring the temperature of the bed.1 The benzene was supplied by a HPLC pump,28−32 using He as inert carrier gas. Helium flow rate was controlled by mass controllers (Brooks Instruments, model 5850). The influence of flow rate of helium, flow rate of benzene, and time of synthesis were studied with regard to the CNS yield and physicochemical properties of carbon materials. After decomposition Received: Revised: Accepted: Published: 6745
January 28, 2012 March 29, 2012 April 28, 2012 April 28, 2012 dx.doi.org/10.1021/ie300254w | Ind. Eng. Chem. Res. 2012, 51, 6745−6752
Industrial & Engineering Chemistry Research
Article
Table 1. Reaction Conditions and Physico-Chemical Characteristics for CNSs Synthesis at Pilot Scale helium flow rate (mL/min)
benzene content (%)
100 300 800 1300
0.3 0.3 0.3 0.3
100 100 100 100 500 500 500 500
reaction time (h)
CNSs production (g)
CNSs yield (gc/gbenzene)
BET surface area (m2/g)
1 1 1 1
13.0 20.4 25.3 33.6
82.3 42.6 20.0 16.0
4.95 5.28 5.54 6.21
5.6 6.5 7.8 9.4
× × × ×
0.3 1.4 2.4 4.0
1 1 1 1
13.0 13.6 25.0 32.4
82.3 19.1 19.8 15.4
4.95 4.75 4.58 4.05
5.6 3.5 1.9 1.1
0.06 0.06 0.06 0.06
0.5 1 4 6
3.2 7.5 25.4 43.5
10.1 47.5 40.2 45.9
6.45 3.98 2.42 2.38
8.8 7.1 6.8 6.1
yield to CNSs(g CNSs/g benzene) = m CNSs/mbenzene*100
2.2. Characterization of Carbon Materials. Surface area/ porosity measurements were carried out using a QUANTACHROME (model QUADRA SORB) sorptometer apparatus with N2 at 77 K as the sorbate. The samples were outgassed at 453 K under vacuum (6.6 × 10−9 bar) for 16 h prior to analysis; specific surface areas were determined by the multipoint BET method and specific total pore volumes were evaluated from N2 uptake at a relative pressure (P/P0) = 0.99. XRD analyses were carried out on a Philips X’Pert instrument using nickel-filtered Cu−Kα radiation; the samples were scanned at a rate of 0.02° step−1 over the range 5° ≤ 2θ ≤ 90° (scan time = 2 s step−1). This technique was used to evaluate the graphitic nature of the carbon materials.33−40 Average diameter and morphology of the different CNSs were probed by transmission electron microscopy (TEM) using a Philips Tecnai 20T, operated at an acceleration voltage of 200 keV. Suitable specimens were prepared by ultrasonic dispersion in acetone with a drop of the resultant suspension evaporated onto a holey carbon supported grid. The average diameter was measured by counting ∼200 CNSs on the TEM images. Mean CNS diameter is quoted in this paper as number average diameter (d̅n):
∑i nidi ni
Ta max. weight loss (°C)
Lc002 (Å)
d002 (Å)
10−2 10−2 10−2 10−2
13.58 13.22 12.48 10.18
3.45 3.49 3.53 3.73
585 564 546 490
× × × ×
10−2 10−2 10−2 10−2
13.58 13.67 14.70 15.02
3.45 3.44 3.44 3.43
585 562. 629 670
× × × ×
10−2 10−2 10−3 10−3
10.22 13.95 14.98 15.70
3.56 3.52 3.51 3.49
524 544 800 813
of CNSs, the carbon production and the CNSs yield after 1 h of reaction were measured (Table 1). At the same time, CNS production and CNS yield have been shown as a function of helium flow rate in Figure 1a). It is possible to see that CNSs production increased and CNS yield decreased with increasing the He flow rate (space velocity). Thus, with an He flow rate increase, the carbon source (benzene) passed through the reactor more quickly decreasing the residence time. Therefore, benzene had insufficient time to decompose and as a consequence the CNSs yield decreased (this fact could be checked by observing the condenser located at the exit of the reactor, where the unreacted benzene was accumulated mainly when the highest He flow was used). N2 adsorption−desorption isotherms at 77 K of the synthesized CNSs are show in Figure 2a. Isotherms can be attributed to a profile type IV with a hysteresis loop characteristic of mesoporous materials (IUPAC classification). All samples were characterized by low N2 adsorption−desorption volumes and thus, low surface area and pore volume values which is in agreement with the data described in the literature for CNSs42 and is a direct consequence of its spherical morphology, i.e., the sphere is the geometrical body with the lowest exposed surface area per unit volume.43 As observed in Table 1, the pore volume and BET surface area values increased with increasing the helium flow rate. The pore size distribution of the CNSs synthetized at different He flow rates is presented in Figure 3a). It could be observed that the volume associated with small pores (
