Article pubs.acs.org/IECR
Synthesis of a Monolithic Carbon-Based Acid Catalyst with a Honeycomb Structure for Flow Reaction Systems Kazuhiro Murakami, Yoshitaka Satoh, Isao Ogino,* and Shin R. Mukai* Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, N13W8, Kita-Ku, Sapporo, Hokkaido 060-8628, Japan S Supporting Information *
ABSTRACT: A carbon-based monolithic acid catalyst with a honeycomb structure (CMHC) was synthesized via carbonization of a resorcinol-formaldehyde resin and subsequent sulfonation using sulfuric acid. The CMHC has nearly straight flow-through macropores (e.g., 47−78 μm) formed by the directional freezing of the resin precursor. These macropores are surrounded by thin carbon walls of approximately 6-μm thickness that are micro- and mesoporous as characterized by nitrogen adsorption experiments. The straight macropores enable a low hydraulic resistance to a liquid flow as demonstrated by >1000-fold less pressure drops of water flowing through the CMHC than those calculated for a packed bed of spherical particles with the same diameter as the approximate wall thickness of the CMHC. The CMHC shows a stable catalytic activity in the liquid-phase esterification of acetic acid with ethanol at 333 K in a flow reaction system for 50 h of operation. These results show prospective features of the CMHC for applications in flow reaction systems.
1. INTRODUCTION Monolithic materials consisting of hierarchical pore structures have received considerable attention for various applications such as catalysts,1−5 separation media,6−10 and energy storage devices11−13 because of facile mass transfer of reactant(s) and product(s). Micro- and mesopores in these materials exhibit unique functions at the molecular level, while macropores enable fast mass transfer of substrates by convection. Furthermore, when short diffusion path lengths in micro- and mesopores are coupled with macropores, they facilitate mass transfer considerably. This property is particularly important in the catalytic applications of these materials for relatively fast reactions. The diffusion path lengths in porous materials can be reduced by reducing their size. However, smaller sized materials tend to cause higher hydraulic resistances (larger pressure drops). Therefore, we aimed at the synthesis of a monolithic catalyst that has (1) straight macropores that only causes minimal pressure drops and (2) short micro- and mesopores that are embedded in the monolith walls. To accomplish this goal, we used the ice-templating method that can form a honeycomb-type structure consisting of straight macropores of a few tens of micrometers in diameter using various hydrogels such as silica gel and polymers.5,14−17 We chose a resorcinol-formaldehyde (RF) resin18 as the hydrogel because an RF gel (predried RF resin) can be synthesized in an aqueous solution that allows the application of the icetemplating method.15,16 Furthermore, by carbonization and subsequent surface functionalization by sulfonation,12 an RF resin can be converted to a carbon-based acid catalyst. Carbonbased acid catalysts synthesized using various methods have demonstrated prospective catalytic properties in a number of reactions.19−30 In addition, carbonization of an RF resin forms micro- and/or mesopores within it, and the pore structure of the resultant carbon may be tailored by adjusting synthesis conditions. We used liquid-phase esterification of acetic acid © 2013 American Chemical Society
with ethanol as a test reaction and conducted catalysis experiments in a flow reaction system to evaluate the potentials of the synthesized catalyst. Our data show that the monolithic carbon acid catalyst (Carbon MicroHoneycomb Catalyst, CMHC) enables >1000-fold less pressure drops as compared with a packed bed filled with particles having the same diameter as the approximate thickness of honeycomb walls (similar diffusion path lengths in micro- and mesopores). Furthermore, the catalyst shows a stable reactivity in the liquid-phase esterification reaction of acetic acid with ethanol at 333 K for an extended period of time.
2. EXPERIMENTAL SECTION 2.1. Materials. Resorcinol (99.0%), formaldehyde aqueous solution (36.0 wt % aqueous solution stabilized by methanol), sodium carbonate (99.8%), and sulfuric acid (95% min.) were used as received from Wako Pure Chemical Industries. 2.2. Synthesis of Monolith-Shaped Resorcinol-Formaldehyde (RF) Resins. Synthesis of monolithic carbon was prepared according to the literature15,16 with a slight modification. Typically, resorcinol, formaldehyde, and sodium carbonate in water with the composition of resorcinol (R):formaldehyde (F):water (W):sodium carbonate (C) molar ratio of 1:2:61:0.02 was mixed in a 20 mL glass vial. Glass tube molds (50 mm × 8 mm i.d.) sealed at one end were fixed in a vertical position, filled with the mixture, and subsequently heated at 303 K for 20 h to allow the polymerization reaction to proceed. After washing with Special Issue: NASCRE 3 Received: Revised: Accepted: Published: 15372
February 28, 2013 May 1, 2013 May 2, 2013 May 15, 2013 dx.doi.org/10.1021/ie400656x | Ind. Eng. Chem. Res. 2013, 52, 15372−15376
Industrial & Engineering Chemistry Research
Article
capillary column (Shinwa Chemical Industries Ltd., HR-1) and a FID detector.
deionized water, each of the resorcinol-formaldehyde hydrogels prepared was taken out of the glass mold and loaded in a propylene tube (13 mm i.d. × 125 mm). The propylene tubes containing the hydrogels were dipped into a liquid nitrogen bath at a rate of 60 mm/h during which ice crystals form directionally within the gels. After the propylene tubes containing the frozen gels were taken out of the liquid nitrogen bath, the gels were immersed in 10-times their volume of tertbutyl alcohol (t-BuOH) and were treated for 1 day to exchange the water included in their structure with t-BuOH. This washing process was repeated three times. In some experiments, the frozen gels were immersed in 20 mL of 1 N HCl aq. in glass vials at room temperature for 1−14 days before washing with t-BuOH. The materials that had been washed with tBuOH were freeze-dried at 263 K, cut into small pieces using a razor blade, and carbonized as described in the next section. 2.3. Carbonization of RF Resins. Monolithic resorcinolformaldehyde hydrogels prepared as in the preceding section were carbonized at 673−1073 K for 4 h in a tubular reactor in a nitrogen flow of 100 mL/min. 2.4. Sulfonation of Carbonized RF Resins. A carbonized RF resin was sulfonated using sulfuric acid at 353 K for 10 h and subsequently at 423 K for 5 h. The resultant material is designated as CMHC. 2.5. Measurements of Hydraulic Resistance of CMHC. The cylinder-shaped CMHC was cladded with a heatshrinkable PFA tube, ended by two glass tubes. The PFA tube was held vertically, and water was passed through CMHC from its bottom. Pressure-differences between the inlet and the outlet were measured using a pressure gauge (COPAL Electronics, PZ-200). 2.6. Characterization. Morphology of the synthesized materials was characterized by a scanning electron microscope (JEOL Japan Inc., JSM-5410). Nitrogen adsorption experiments were performed on an adsorption apparatus (Belsorp Japan, BELSORP-mini II) at liquid nitrogen temperature. After each of the carbonized materials was pretreated in a glass cell at 523 K for 4 h in a flow of dry nitrogen, adsorption measurements were conducted at 77 K with the equilibrium time of 300 s at each data point. Apparent surface areas were calculated by the Braunner-Emmett-Teller (BET) method. Micropore and total pore volumes were estimated adsorbed volumes at P/P0 = 0.05 and 0.98, respectively. Mesopore size distributions were calculated by applying the Dollimore-Heal method to the adsorption isotherms. A Powder X-ray pattern was collected on a JEOL JDX-8020 using a CuKα source with a step site of 0.02°. Solid-state 13C CP MAS NMR spectra were recorded on a Bruker MSL-300 at a Lamor frequency of 75.4 MHz, a spinning rate of 8.0 kHz, and a contact time of 1 ms. Titration of acid sites was performed using the standard backtitration method using NaCl and NaOH. 2.7. Esterification Reaction. Catalysis of the CMHC was tested using liquid-phase esterification of acetic acid with ethanol at 333 K in a flow reaction system. The CMHC was sealed in a heat-shrinkable PFA tube whose ends were connected to Teflon tubes. The CMHC in the tube was held vertically in a thermostat bath at 333 K, and an equimolar mixture of ethanol and acetic acid was passed through the inlet Teflon tube and fed upward from the bottom of the CMHC at a liquid-hourly space velocity of 1.2 h−1. A timer was started as the first drop of products came out of the CMHC. The effluent from the CMHC was sampled and analyzed with a gas chromatography (Shimadzu, GC-17A) equipped with a
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of CMHC and Precursor RF Resins and Carbonized RF Resins. Synthesized CMHC has a cylinder shape approximately 6 mm in diameter and 18 mm in length (a photograph of CMHC is shown in the inset in Figure 1). CMHC has nearly straight
Figure 1. SEM image characterizing the CHMC. The inset shows a photograph of CMHC.
macropores running along the cylinder axis of the monolith. The diameter of the macropores ranged from 47 to 78 μm as shown by the SEM data in Figure 1. These macropores are formed using ice rods as the template when a precursor RF gel is frozen directionally. When RF resins prepared without HCl treatment were carbonized by pyrolyzation in a nitrogen flow at various temperatures, micropores developed within them at temperatures greater than 773 K as shown by the nitrogen gas adsorption experiments (entries 1−3 in Table 1, see Figure S1 in the Supporting Information for the corresponding adsorption isotherms). When HCl treatment was added after the directional freezing of RF resins, nitrogen gas adsorption data characterizing the corresponding materials carbonized at 673 or 773 K show type IV isotherms (Figure S2 in the Supporting Information), indicating the presence of mesopores in these materials. Analysis of the adsorption data by the Dollimore-Heal method shows that the HCl treatment for 1−7 days resulted in increased mesopores centered at approximately 2.4 nm (Figure S3 in the Supporting Information). Longer periods of HCl treatment tend to result in the formation larger mesopores as shown by the shifts of the pore size distributions to larger radii (Figure S3 in the Supporting Information). These changes are presumably caused by growth of RF resin particles by the acid treatment via partial dissolution and precipitation of RF particles. The materials treated with HCl aq. have greater micropore volumes and much higher total pore volumes than the material synthesized without HCl treatment (entries 4−8 vs entries 1 and 2 in Table 1). Additional sulfonation of the material treated with HCl aq. and carbonized at 673 K to synthesize a CMHC led to slight increases in micropore and total pore volumes (entry 9 in Table 1 and Figure 2) while retaining its honeycomb structure (Figure 1). 15373
dx.doi.org/10.1021/ie400656x | Ind. Eng. Chem. Res. 2013, 52, 15372−15376
Industrial & Engineering Chemistry Research
Article
Table 1. Results from N2 Adsorption Experiments for Carbonized RF Resins and CMHC
a
entry
HCl treatmenta
carbonized temp [K]
sulfonation
BET surface area [m2/g]
micropore volumeb [cm3/g]
total pore volumec [cm3/g]
1 2 3 4 5 6 7 8 9
none none none 1 day 4 days 7 days 14 days 4 days 4 days
673 773 1073 773 773 773 773 673 673
none none none none none none none none yes
6 401 537 740 710 750 780 619 640
