Ind. Eng. Chem. Res. 1997, 36, 4427-4429
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Porous H-ZSM-5 Zeolite Tube as a Novel Application of Catalyst for the Synthesis of Ethanol by Hydration of Ethylene Tatsumi Ishihara,* Junji Matsuo, Masami Ito,† Hiroyasu Nishiguchi, and Yusaku Takita Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita 870-11, Japan
Gas-phase hydration of ethylene over a porous H-ZSM-5 tube was investigated. Yield of C2H5OH is drastically increased by the porous H-ZSM-5 tube as a catalytic filter which has a function of catalyst and separator. The combination of the porous H-ZSM-5 tube with the powder H-ZSM-5 catalysts is effective for the synthesis of ethanol at high yield. Introduction Ethanol is an important compound for various industrial and food processes. At present, ethanol is industrially produced by the hydration of ethylene with supported phosphoric acid catalysts. However, extremely high reaction pressure is required for the production of C2H5OH due to the restriction in chemical equilibrium. In addition, corrosion of equipment by the eluted or vaporized phosphoric acid sometimes causes serious problems on the reactor system. The hydration of olefins is a typical acid-catalyzed reaction. It is reported that a solid acid of zeolite, in particular, ferrierite (Iwamoto et al., 1986) and ZSM-5 (Eguchi et al., 1986), is active for this reaction. However, the yield of C2H5OH on zeolite catalysts is still low due to the low equilibrium conversion. Furthermore, a dealumination of zeolite resulting in deactivation occurs under severe hydrothermal conditions such as the hydration of ethylene. On the other hand, the combination of the separation system with a catalytic reaction is highly interesting for promoting deviations from equilibrium control. For example, the steam reforming reaction is greatly accelerated by applying a Pd membrane which selectively removes hydrogen from the reaction system (Uemiya et al., 1991). However, all these attempts are based on the concept that the synthesis of the desired product is followed by the separation. On the other hand, simultaneous synthesis and separation is also an interesting subject since a further significant accelerating effect is expected. These systems having catalysis and separation are called a catalytic filter. However, the number of reports on the catalytic filter is quite small and, furthermore, studies on the catalytic filters are limited to a diesel particulate trap (Neeft et al., 1996; Montanaro et al., 1995). However, the diesel particulate trap is not designed to promote deviations from the equilibrium control. Therefore, there is no report concerning a catalytic filter which accelerates the formation of a desired compound deviated from the equilibrium control. There are some attempts reported on the use of zeolite films for the separation of gases on the basis of a molecular-sized pore (Yan et al., 1995; Matsukata et al., 1994). However, all these attempts are aimed only at separation, with no reports on the catalytic filter. We report here that the porous zeolite * Author to whom correspondence should be addressed. Fax: +81-975-54-7979. E-mail:
[email protected]. † Present address: Research and Development Center, Oita University, Dannoharu 700, Oita 870-11, Japan. S0888-5885(97)00233-9 CCC: $14.00
tube as a catalytic filter is highly effective for increasing the yield of ethanol in the gas-phase hydration of ethylene. Experimental Section H+-exchanged ZSM-5 (SiO2/Al2O3 ) 20; Tosoh) was prepared by a conventional ion-exchange method with 1 N HCl followed by calcining at 673 K in air for 2 h. The porous zeolite tube of which the outer and inner diameters are 12 and 6 mm, respectively, was obtained with an isostatic press at 2000 kg/cm2. The porosity of the obtained H-ZSM-5 tube is about 40%. The catalytic reaction was carried out with a double tubular reactor in which the porous zeolite tube is used for an inner tube. The reactor is schematically shown in Figure 1. The porous zeolite tube (about 3 cm in length) was connected to a Pyrex glass tube (6 mm φ) with epoxy adhesive. Sweep Ar and an equimolar mixture of C2H4 and H2O were fed into the outer and inner sides of the porous zeolite tube at flow rates of 50 and 30 cm3 min-1, respectively. The contact time of the reactant in this condition corresponds to W/F ) 18.8 g catalyst‚h/mol, where W and F are the catalyst weight containing the porous zeolite tube and the flow rate of reactant, respectively. In the case of the combination of the powder H-ZSM-5 with the porous tube, H-ZSM-5 particles of 16-32 mesh size were set outside of the zeolite tube with a quartz wool as shown in Figure 1. The products were analyzed with gas chromatography. Results and Discussion Figure 2 shows the pore size distribution of used porous zeolite tubes. The pore size of the zeolite tube used distributed in a narrow range around 0.2 µm. The average pore size and pore volume are 0.233 µm and 0.402 cm3/g, respectively. When the mean free path of gaseous molecules is considerably greater than the pore diameter, the diffusion of gases through the narrow channels of a porous solid becomes Knudsen flow (Coulson et al., 1973). In the region of Knudsen flow, the effective diffusivity of gases is proportional to M-1/2 where M is the molecular weight of permeated gas (Coulson et al., 1973). Consequently, the gaseous molecules can be separated with a porous solid having a pore size of a few micrometers. Parts a and b of Figure 3 show the permeation rate of H2O, C2H4, and C2H5OH in the porous zeolite tube as a function of temperature and M-1/2, respectively. Clearly, the permeation rate is larger in the order C2H5OH < C2H4 < H2O over all temperatures examined. Further© 1997 American Chemical Society
4428 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997
Figure 3. Permeation rate of H2O, C2H4, and C2H5OH of the used porous H-ZSM-5 tube as a function of temperature (a) and M-1/2 at 423 K (b). (The pressure difference is 0.2 kgf/cm-2.)
Figure 1. Schematic view of the tubular reactor.
Figure 2. Pore size distribution and pore volume of the H-ZSM-5 tube measured with a mercury porosimeter.
more, the permeation rate is proportional to M-1/2, as shown in Figure 3b. Therefore, it is obvious that the gas flow in the porous zeolite tube prepared is the Knudsen flow. Although the permeation rate decreased with increasing temperature, the order in the permeation rate which is proportional to M-1/2 does not change over the examined temperature lower than 423 K. Consequently, ethanol can be effectively separated from C2H4 and H2O with the porous zeolite tube. It is also noted that there is no film which can permeate C2H5OH but not C2H4 and H2O. Figure 4 shows the temperature dependence of the yield of C2H5OH over the conventional powder, the porous tube, and the combined catalysts. Since C2H5OC2H5 forms successively from C2H5OH, diethyl ester
Figure 4. Temperature dependence of C2H5OH yield (W/F ) 18.8 g of catalyst‚h/mol; pressure, 1 atm; fed C2H4/H2O ) 1): (4) powder H-ZSM-5, (0) powder H-ZSM-5 + porous Al2O3 tube, (O) porous H-ZSM-5 tube, (b) porous H-ZSM-5 tube + powder H-ZSM-5, (- - -) equilibrium yield.
is considered one of the desired products in the gasphase hydration of C2H4. However, the yield of C2H5OC2H5 was not large under the reaction condition examined (less than 20% in the selectivity and 0.2% in yield). Small amounts of CH3CHO (ca. 3% in selectivity), and hydrocarbon, which is the oligomer of C2H4 (less than 10% in selectivity) are formed as byproducts. Although the formation of oligomer drastically increased with increasing temperature, it is negligibly small at temperatures lower than 500 K. It is clearly shown in Figure 4 that the yield of C2H5OH is higher on the porous zeolite tube than on the powder catalyst in spite of the same contact time. Since most of formed C2H5OH and unreacted C2H4 were detected at the outer and inner sides of the zeolite tube, respectively, separation of C2H5OH from the reactant seems to be effectively performed within the porous zeolite tube, resulting in the acceleration of the formation of C2H5OH. On the other hand, such a positive effect is negligibly small when a porous alumina tube which is just a gas separator based on Knudsen flow is applied. Therefore, the catalytic activity and the separation function are essential for the enhanced yield of ethanol. Increases in the contact time of reactant sometimes enhance the yield of products; however, the yield of C2H5OH and/or C2H5OC2H5 is almost independent of the contact time under the reaction condition in the case of the conventional fixed-bed reactor. This suggests that the reaction under the used condition in Figure 4 is controlled by the equilibrium. The combination of the porous zeolite tube with the powder catalyst leads to further high yield of C2H5OH. This may suggest that the contact time of reactant is not sufficient when only the porous zeolite
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4429
of reaction pressure results in the high yield of C2H5OH. The yield of C2H5OH at 2.0 atm on the combination catalyst reached ca. 2%, which is almost the same yield at 5 atm on the conventional powder catalyst. Therefore, the porous zeolite tube combined with the powder catalyst is a promising method for decreasing the reaction pressure. This study reveals that the porous H-ZSM-5 tube is a promising catalytic filter for the synthesis of ethanol in the gaseous hydration of ethylene. The combination of a H-ZSM-5 porous tube with a powder H-ZSM-5 catalyst is highly active for the synthesis of ethanol. Figure 5. Yield of C2H5OH as a function of reaction pressure (W/F ) 18.8 g of catalyst‚h/mol; sweep Ar, 1 atm; fed C2H4/H2O ) 1).
tube is applied. This is because the reactants mainly permeated through the wall of the catalytic tube in a radial direction. Therefore, the contact time on the tubular reactor is much shorter than that on the conventional fixed-bed reactor provided that the same weight of catalysts was used. Namely, the contact time cannot be expressed simply by a ratio of feed rate to catalyst weight in the case of tubular catalysts. The yield of C2H5OH was ca. 1.2% on the combination catalysts at 463 K, which is twice as large as that of the powder catalyst. Although the deviation from chemical equilibrium is expected on the catalytic filters, the yield of C2H5OH did not exceed but almost corresponded to the theoretical yield of C2H5OH in the closed system which is estimated by thermodynamic calculation (Wagman et al., 1982; Gurvich et al., 1989) in this study. On the other hand, C2H5OH is hardly detected at the inner side of the catalytic filter at each experiment and thus C2H5OH was condensed at the outer side of the catalytic filter. For example, the molar ratio of the outlet gas from the outer side of the zeolite tube was C2H5OH:C2H4:H2O ) 48.5:47.7:3.7 at 463 K on the combination catalyst. Figure 5 shows the pressure dependence of yield of the C2H5OH on the combination and the powder catalysts. Although the number of data points is limited, the yield of C2H5OH seems to increase linearly with increasing reaction pressure. However, the pressure dependence of C2H5OH yield is more significant in the case of the combination catalysts. Since the permeation rate increases, the separation effect in the porous zeolite tube appears to be more significant with increasing pressure difference. Consequently, the positive effect
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Received for review March 21, 1997 Revised manuscript received July 1, 1997 Accepted July 5, 1997X IE9702338
X Abstract published in Advance ACS Abstracts, August 15, 1997.
