Ind. Eng. Chem. Res. 2001, 40, 5889-5893
5889
Effects of Alkali Metal Oxide Additives on Cu/SiO2 Catalyst in the Dehydrogenation of Ethanol Yao-Jen Tu and Yu-Wen Chen* Department of Chemical Engineering, National Central University, Chungli 32054, Taiwan
A series of Cu/SiO2 catalysts with various alkali metal oxide additives was investigated to elucidate the effects of additives on the catalytic properties of copper catalysts in the dehydrogenation of ethanol. The catalysts were prepared by the impregnation method and were characterized by X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption of CO2, and H2-N2O titration. The dehydrogenation reaction was carried out in a continuous-flow reactor at 300 °C under atmospheric pressure. The catalytic properties of the copper catalysts were strongly affected by the additives. The addition of alkali metal oxide increased the concentration of weak basic sites of the catalyst. The increase in turnover frequencies of the alkali-metal-oxide-modified catalysts was mainly due to the promotional effects of weak basic sites on Cu catalysts. KCu/SiO2 catalyst was very stable in the reaction process. The stabilities of NaCu/SiO2 and RbCu/SiO2 catalysts were similar to that of the unmodified catalyst. The decay of the catalysts during reaction was caused by sintering. The kinetics for deactivation can be described by a concentration-independent second-order expression. Introduction For many years, acetaldehyde has been one of the most important intermediate aliphatic chemicals serving as a raw material for the production of acetic acid, acetic anhydride, ethyl acetate, butyraldehyde, n-butanol, Pentaerythritol, and many other products. Copper-based catalysts have been found to be good catalysts for the dehydrogenation of alcohols.1-10 However, their activities have been found to decrease within a few hours. Thus, attention has been focused on the influence of promoters on catalytic properties for the dehydrogenation of alcohols.8,11-14 Some inert substances that inhibit the sintering of copper have been used as textural promoters.11-14 The dehydrogenation activity of the catalysts was also found to be influenced by the acid-base properties of the metal oxide combined with copper.2,9 The dehydrogenation activity can be improved by the incorporation of amphoteric or basic metal oxide into the catalysts.2 Chromium has been extensively used as a promoter of copper catalyst.11-14 However, the use of chromium is prohibited because of environmental issues. In a previous paper,15 we reported the effects of alkaline earth oxide additives on copper catalysts. Although the dehydrogenation of alcohols over copperbased catalysts has been studied previously, detailed research into the effects of alkali metal oxide additives on the catalytic properties of copper catalysts has not yet been reported. In the present work, a series of copper-based catalysts with alkali metal oxide additives was investigated to elucidate the effects of additives on the catalytic properties of the copper catalysts. The phase compositions of catalysts were determined by X-ray diffraction (XRD). The variations of the particle sizes and surface areas of copper were determined by H2-N2O titration. The basicities of the catalysts were determined by temperature-programmed desorption (TPD) of CO2. Temperature-programmed reduction * Author to whom correspondence should be addressed. Fax: 886-3-425 2296. E-mail:
[email protected].
(TPR) was used to obtain information about copperadditive interactions. The dehydrogenation of ethanol was used to determine the activities and stabilities of the copper-based catalysts. The deactivation kinetics on copper catalysts was also studied. Experimental Section Catalyst Preparation. A series of silica-supported copper catalysts with various alkali metal oxides (MCu/ SiO2, M) Na, K, or Rb; M/Cu molar ratio of 1/10; Cu/ SiO2 weight ratio of 14/86) was prepared. The catalysts were prepared by incipient wetness coimpregnation of silica gel (G-57, Davison Chemicals Co., 300 m2/g) with aqueous solutions of Cu and sodium, potassium, or rubidium nitrate. The catalysts were dried at 100 °C overnight and calcined at 400 °C for 4 h before use. XRD. X-ray powder diffraction (XRD) patterns were obtained using a Siemens D-500 diffractometer employing nickel-filtered Cu KR radiation. The X-ray tube was operated at 40 kV and 25 mA. Samples were run as pellets mounted on a glass slide. Spectra were scanned from 10 to 60° at a rate of 0.4°/min (in 2θ). The crystallite sizes were calculated from XRD line broadening analysis using Bragg’s equation D ) K/(b cos θ), where K, the particle shape factor, was taken as 0.89. TPR. The TPR experiments were carried out using a quartz U-tube reactor. The catalyst was loaded on a sintered quartz disk in the reactor with gas flowing upward; the sintered disk served as a good preheater. The catalyst usually formed a bed of less than 6 mm in thickness. The H2 in argon carrier gas was further purified by passing it through an Oxisorb cartridge (Alltech) and a 5A molecular sieve trap to remove trace amounts of oxygen and water. This gas stream was then split, with one path leading to the reference arm of a thermal conductivity detector (TCD) and the other passing through the reactor before going to the detector. Any reaction products generated during TPR were trapped by a dry ice trap located between the reactor and the TCD. The TCD signal was calibrated by
10.1021/ie010272q CCC: $20.00 © 2001 American Chemical Society Published on Web 11/07/2001
5890
Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001
injecting a pure sample using a Valco sampling valve of known volume. A plug of several millimeters of glass wool was added to each end of the reactor to avoid entrance and exit effects, but there was no dilution of the catalysts. Before the TPR experiments were started, 40 mg of catalyst was dried in flowing argon at 100 °C for 1 h. H2 (10%) in argon (40 mL/min) was used as a reducing gas at a constant flow rate. The temperature rise in the TPR experiments was from ambient to 800 °C at a rate of 10 °C/min. TPD of CO2. In a typical experiment, the sample was weighed and then placed in a U-shaped quartz microreactor. First, the catalyst was reduced in a stream of 10% H2 in argon at 300 °C for 2 h. The temperature was then dropped to 100 °C. CO2 gas was flowed at 60 mL/min for 1 h. The catalyst was then brought to room temperature slowly by cooling from 100 °C in CO2 flow. The TPD experiments were carried out using a linear temperature program rise from room temperature to 800 °C in argon at a rate of 10 °C/min. Desorbed CO2 was detected with a Shimadzu gas chromatograph (model GC-8A) equipped with a thermal conductivity detector (TCD). The TCD signals were calibrated by injecting CO2 using a Valco six-port sampling valve of knowing volume. Cu Surface Area. The surface area of metallic copper was determined by reaction of copper with nitrous oxide following the method of Bond and Namijo,5 which includes two reduction processes. The samples were reduced at 300 °C. The temperature was then kept at 60 °C. Pure N2O was allowed to flow over the sample for 1 h at 60 mL/min. It is believed that the surface metallic copper is oxidized to Cu2O under these conditions. H2-N2O titration was carried out under a linear temperature program rise from room temperature to 800 °C at a rate of 10 °C/min.
Cu2O(s) + H2 f 2Cu(s) + H2O(g)
(1)
The amount of H2 consumed was detected with a thermal conductivity detector (TCD). The TCD signals were calibrated by injecting H2 using a Valco six-port sampling valve of known volume. The copper metal surface area of the catalyst can be determined from the amount of H2 consumed. Catalytic Reaction. The dehydrogenation reaction of ethanol was carried out in a continuous U-shaped quartz microreactor. Forty milligrans of fresh catalyst was placed on a layer of quartz wool. The catalyst was reduced under the flow of a 10% H2/Ar mixture at 300 °C. A saturator containing 95% ethanol and 5% water (an azeotrope) was kept at a constant temperature of 22 °C. Nitrogen was used as the carrier gas at a constant flow rate of 60 mL/min. The experiments were carried out at 300 °C under atmospheric pressure. To prevent possible condensation of reactant and products, all connection gas lines and valves were wrapped with heating tape. The reaction product was analyzed by a Shimadzu GC-8A gas chromatograph with a TCD. The column was 3 m long and packed with Porapak QS sorbent. Almost no side reactions were observed, and the selectivity of acetaldehyde was greater than 99%. The turnover frequency (TOF) of the catalyst was calculated on the basis of the initial activity and the metallic copper surface area of the fresh sample.
Figure 1. TPD profiles of CO2 on the catalysts.
Results and Discussion XRD. The phase compositions of the calcined and reduced catalysts were examined by XRD. The calcined catalysts exhibited only the CuO phase, and the reduced catalysts exhibited only the metallic Cu phase. This shows that the alkali metal oxide additive has no influence on the phase composition of copper. In addition, the copper oxide in the calcined catalysts was easily reduced to the metallic phase by the flow of 10% H2 in argon at 300 °C. The XRD results did not show any peaks of alkali metal oxides in the samples, indicating that their concentrations were too small to be detected or that they were amorphous. TPD of CO2. The TPD profiles of CO2 on various catalysts are shown in Figure 1. The Cu/SiO2 catalyst has a desorption peak at 200 °C. The desorption peak at 200 °C is attributed to weak basic sites on the Cu/ SiO2 catalyst.15 CO2 adsorbed on copper only at lower temperature. Hass and Pritchard (1990)21 reported that CO2 is not adsorbed on polycrystalline copper at 25 °C. If this is true, CO2 should not adsorb on unmodified copper in this study. It has been reported16,17 that silica is a weak acid and base material, but the presence of impurities such as Na2O and CaO might alter its behavior. The silica support was tested to check its basicity. A desorption peak at nearly 200 °C was observed. The weak basic sites of the Cu/SiO2 catalyst result from the basic properties of the silica support, rather than the copper metal. All of the alkali-metaloxide-modified catalysts exhibited only a desorption peak at around 200 °C, as shown in Figure 1, indicating that only weak basic sites exist on these catalyst surfaces. The area of the desorption peak represents the concentration of basic sites on the catalyst surface. The amounts of CO2 desorbed from the catalysts are shown in Figure 2. The amount of CO2 desorbed from Cu/SiO2 catalyst is 18.4 µmol of CO2/g. The amount sof CO2 desorbed from the alkali-metal-oxide-modified catalysts are greater than that desorbed from the unmodified catalyst. The addition of alkali metal oxide can increase the concentration of basic sites on the surface of the catalyst. The amount of CO2 desorbed from the catalyst was in the order KCu/SiO2 > RbCu/SiO2 > NaCu/SiO2 > Cu/SiO2.
Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5891 Table 1. Surface Areas and Particle Sizes of Copper Metal on the Catalysts Cu/SiO2 NaCu/SiO2 KCu/SiO2 RbCu/SiO2 S0a [m2/(g of Cu)] dCu,0b (nm) S1c [m2/(g of Cu)] dCu,1d (nm) S1/S0
51.4 13.1 23.1 29.1 0.45
24.1 27.9 12.0 56.1 0.49
47.5 14.2 38.6 17.4 0.81
49.2 13.7 24.2 27.8 0.49
a S ) surface area of copper metal in the fresh catalyst. b d 0 Cu,0 ) particle size of copper metal in the fresh catalyst. c S1 ) surface area of copper metal in the spent catalyst after reaction for 4 h. d d Cu,1 ) particle size of copper metal in the spent catalyst after reaction
and RbCu/SiO2 catalysts were 24.1, 47.5, and 49.2 m2/ (g of Cu), respectively. The copper surface areas of the fresh alkali-metal-oxide-modified catalysts were lower than that of the unmodified one. The ratio of S1/S0 exhibits the extent of decrease of the surface area of the copper metal after reaction. The S1/S0 values are less than unity for all of the catalysts. This indicates that sintering occurred during the reaction process for all of the catalysts. The KCu/SiO2 catalyst had the best stability. The S1/S0 values of the NaCu/SiO2 and RbCu/ SiO2 catalysts were slightly greater than that of the unmodified one. The average volume/surface diameter ratio can be determined as
Figure 2. Amounts of CO2 desorbed from the catalysts.
D (nm)) 6 × 1011/SF
where S is the specific surface area (m2/g) and F is the density (g/cm3). For metallic copper, FCu ) 8.92 × 106 g/cm3. Thus, the average particle size of Cu (dCu) can be calculated as5
Figure 3. TPR profiles of the catalysts.
It is known that alkali metal oxides are strong bases. However, the strengths of the basic sites on the alkalimetal-oxide-modified catalysts were the same as the weak basic sites on the unmodified catalyst in this study. It has been reported16,17 that the basicity of K2O is different for different calcination temperatures. The basicity of an alkali metal oxide is expected to be influenced by calcination temperature. The calcination temperature of 400 °C in this study resulted in only weakly basic sites on the catalysts. TPR. The TPR profiles of the catalysts are shown in Figure 3. The TPR profile of each catalyst exhibited only one reduction peak. This peak is attributed to the reaction5
CuO + H2 f Cu + H2O
(3)
(2)
The TPR profile for CuORb2O/SiO2 is broad. The possibility of two overlapping peaks can be excluded because Rb2O cannot be reduced at this temperature. The peak maximum for the Cu/SiO2 catalyst was at 265 °C, and the peak maximum for the modified catalysts was at 300 °C. This result indicates that the alkali metal oxide additives decreased the reactivity of the copper oxide in the reduction process. The TPR results did not show any reduction peak for the alkali metal oxides, indicating that the alkali metal oxide was not reduced in the reduction process at temperatures below 800 °C. Cu Surface Area. The surface areas of copper metals in the fresh (S0) and the used (S1) catalysts are listed in Table 1. The Cu/SiO2 catalyst had an S0 value of 51.4 m2/(g of Cu). The S0 values of the NaCu/SiO2, KCu/SiO2,
dCu(nm) ) 6.73 × 1011/SCu
(4)
The dCu values of the catalysts are listed in Table 1. The particle size of copper metal in the catalyst showed the reverse trend of surface area, as expected. Catalytic Activity. The dehydrogenation of ethanol to acetaldehyde and hydrogen is an endothermic reaction. The reaction selectivities of acetaldehyde on all catalysts were nearly 100% in this study. Thus, only the activity is discussed in this section. The reaction conversions of the catalysts versus time on stream at 300 °C are shown in Figure 4. The initial conversions of the KCu/SiO2 and RbCu/SiO2 catalysts were greater than that of the Cu/SiO2 catalyst, whereas the NaCu/SiO2 catalyst had a lower conversion. The lower initial conversion of the NaCu/SiO2 catalyst resulted from the lower copper surface area in the fresh catalyst, as shown in Table 1. The turnover frequency is calculated on the basis of the initial activity and the metallic copper surface area of the fresh catalyst. The turnover frequencies (TOFs) of the modified catalysts were higher than that of the unmodified one, as shown in Table 2. The higher TOFs of the modified catalysts resulted from more of the weak basic sites being on the catalyst. This indicates that the active sites of the catalysts are either the basic sites alone or the structural promotional effects of the weak basic sites on the copper catalysts. The silica support alone and Na2O/SiO2, K2O/SiO2, and Rb2O/SiO2 in the absence of copper were tested for the dehydrogenation reaction under the same conditions (300 °C). The conversions were less than 0.2%. Alkaline
5892
Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001
Figure 4. Reaction conversions vs time on stream. Reaction conditions: 300 °C, F/W ) 0.238 mol of ethanol/(g of catalyst h). Table 2. TOFs and Deactivation Rate Constants of the Catalysts (s-1
TOF kd (h-1)
site-1)
Cu/SiO2
NaCu/SiO2
KCu/SiO2
RbCu/SiO2
0.218 0.30
0.349 0.31
0.287 0.07
0.252 0.28
metal oxide catalysts are active for alcohol dehydrogenation as reported by several authors,20 but the activation energy (29.3 kcal/mol) is much higher than that on a copper catalyst (12.8 kcal/mol). Therefore, the conversions on the silica-supported alkali metal oxides were low at the 300 °C temperature used in this study. The results indicate that the weak basic sites play the role of promotion, rather than being active sites. The increase in the TOFs of the alkali-metal-oxide-modified catalysts was mainly from the promotional effects of the weak basic sites on Cu metal, rather than the dehydrogenation activities of the weak basic sites themselves. The alkali-metal-oxide-modified catalysts have more weak basic sites, as shown in Figure 2. The higher TOFs of the alkali-metal-oxide-promoted catalysts result from more weak basic sites being on the catalysts. The amount of CO2 desorption from the NaCu/SiO2 catalyst was smaller than those from the KCu/SiO2 and RbCu/ SiO2 catalysts, as shown in Figure 2. However, the TOF of the NaCu/SiO2 catalyst was greater than those of the other modified catalysts. Ai2 reported that the dehydrogenation activity of methanol to methyl formate increases markedly with the addition of a small amount of alkali metal oxide. However, the activities of the catalysts were only stable below 180 °C. In this study, the KCu/SiO2 catalyst was stable even at 300 °C. Catalyst Deactivation. It has been reported that the decay of copper catalyst in ethanol dehydrogenation is mainly caused by sintering,13-15,18-19 rather than by coking, at reaction temperatures below 300 °C. In this study, negligible amounts of coke were present in the catalysts used. It is known that deactivation by sintering usually follows concentration-independent secondorder kinetics.15 Thus
(1 - a)/a ) kdt
(5)
where a is the normalized activity, i.e., reaction rate at time t divided by the reaction rate at t ) 0, and kd is the rate constant for deactivation. Plots of (1 - a)/a
Figure 5. Test for second-order deactivation.
versus t for the catalysts are shown in Figure 5. Straight lines pass through the origin for all catalysts; thus, a second-order deactivation kinetics that is concentrationindependent applied in this study. The kd values of the catalysts are listed in Table 2. The lower the kd value, the more stable the catalyst. As shown in Table 2, the KCu/SiO2 catalyst was more stable than the unmodified one, and the NaCu/SiO2 and RbCu/SiO2 had stabilities similar to that of unmodified one. Alkali metal oxides have higher melting points than copper metal. It has been reported11 that the better stability of Cr-Cu/SiO2 catalyst results from the higher melting point of Cr2O3 particles acting as spacers between crystallites of copper, which help to prevent the sintering of copper throughout the period of reaction. This might well explain the results for the KCu/SiO2 catalyst. However, it cannot explain the results for NaCu/SiO2 and RbCu/ SiO2. At this stage, we cannot draw any conclusions. However, the activity and stability of KCu/SiO2 are similar to those of the CrCu/SiO2 catalyst.14 Conclusion This study of the effects of alkali metal oxides on copper catalysts in dehydrogenation of ethanol leads to the following conclusions: 1. The calcined catalysts exhibit only the CuO phase, and the reduced catalysts exhibit only the metallic copper phase. The XRD results did not show any peaks of alkali metal oxides, indicating that their concentrations were either too small to be detected or that they were in the amorphous phase. 2. For the TPD of CO2, each catalyst had a desorption peak at 200 °C. The amounts of CO2 desorbed from the alkali-metal-oxide-modified catalysts were greater than the amount desorbed from the unmodified one. The addition of alkali metal oxide can increase the concentration of weak basic sites on the surface of the catalyst. The amounts of CO2 desorbed from the catalyst were in the order KCu/SiO2 > RbCu/SiO2 > NaCu/SiO2 > Cu/ SiO2. 3. The turnover frequencies of the catalysts in dehydrogenation of ethanol were in the order: NaCu/SiO2 > KCu/SiO2 > RbCu/SiO2 > Cu/SiO2. 4. The higher TOFs of the alkali-metal-oxide-modified catalysts resulted from the promotional effects of the weak basic sites on the copper metal. 5. The stabilities of the catalysts were in the order KCu/SiO2 > Cu/SiO2 ) NaCu/SiO2 ) RbCu/SiO2.
Ind. Eng. Chem. Res., Vol. 40, No. 25, 2001 5893
6. Potassium oxide additive is an excellent promoter for copper catalyst in terms of activity and stability. It is comparable to chromium in its effects. Acknowledgment This research was supported by the National Science Council of the Republic of China. Literature Cited (1) Church, J. M.; Joshi, H. K. Dehydrogenation of Methanol over Copper Catalysts. Ind. Eng. Chem. 1951, 43, 1804. (2) Ai, M. Dehydrogenation of Methanol to Methyl Formate over Copper-Based Catalysts. Appl. Catal. 1984, 11, 259-270. (3) Balandin, A. A. The Nature of Active Centers and the Kinetics of Catalytic Dehydrogenation. Adv. Catal. 1958, 10, 96129. (4) Benitez, S.; Caro, B. Experimental Study of Selective Dehydrogenation of Ethanol to Acetaldehyde in a Differential Catalytic Reactor. Bol. Soc. Quim. Peru 1996, 62, 24-29. (5) Bond, G. C.; Namijo, S. N. An Improved Procedure for Estimating the Metal Surface Area of Supported Copper Catalysts. J. Catal. 1989, 118, 507-510. (6) Chhabra, M. S.; Naidu, S. R. Ethanol Dehydrogenation CatalystssA Developmental Approach. Chem. Ind. Dig. 1996, 9, 118-122. (7) Chung, M. J.; Moon, D. J.; Kim, H. S.; Park, K. Y.; Ihm, S. K.; Higher Oxygenate Formation from Ethanol over Cu/ZnO Catalysts: Synergism and Reaction Mechanism. J. Mol. Catal. A: Chem. 1996, 113, 507-515. (8) Gil, A.; Ruiz, P.; Delmon, B. Effect of Support and Added Oxides on the Bistability Observed in the Oxidative Dehydrogenation of 2-Proponal over Copper Supported Catalysts. Catal. Today 1996, 32, 185-191. (9) Guerrero-Ruiz, A.; Rodriguez-Ramos, I.; Fierro, J. L. G. Dehydrogenation of Methanol to Methyl Formate over Supported Copper Catalysts. Appl. Catal. 1991, 72, 119-137. (10) Han, G.; Li, L. Study on Supported Cu-Base Catalyst for the Dehydrogenation of Methanol to Formaldehyde. Huagong Yejin 1997, 18, 38-42.
(11) Prasad, Y. S.; Padalia, B. D.; Raman, S. K. Role of Chromia in Copper Catalysts for Dehydrogenation of Methanol. J. Chem. Technol. Biotechnol. 1985, 35, 15-20. (12) Tonner, S. P.; Trimm, D. L.; Wainwright, M. S.; Cant, N. W. Dehydrogenation of Methanol to Methyl Formate over Copper Catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 384-388. (13) Tu, Y. J.; Chen, Y. W.; Li, C. Characterization of Unsupported Copper-Chromium Catalysts for Ethanol Dehydrogenation. J. Mol. Catal. 1994, 89, 179-190. (14) Tu, Y. J.; Li, C.; Chen, Y. W. Effect of Chromium Promoter on Copper Catalysts in Ethanol Dehydrogenation. J. Chem. Technol. Biotechnol. 1994, 59, 141-147. (15) Tu, Y. J.; Chen, Y. W. Effects of Alkaline Earth Oxide Additives on Silica-supported Copper Catalysts in Ethanol Dehydrogenation. Ind. Eng. Chem. Res. 1998, 37, 2618-2622. (16) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases, Their Catalytic Properties; Kodansha Ltd.: Tokyo, 1989; pp 317-322. (17) Hattori, H. Catalysis by Basic Metal Oxides. Mater. Chem. Phys. 1988, 18, 533-560. (18) Marchi, A. J.; Fierro, J. L. G.; Santamaria, J.; Monzon, A. Dehydrogenation of Isopropyl Alcohol on a Cu/SiO2 Catalyst: A Study of the Activity Evolution and Reactivation of the Catalyst. Appl. Catal. A: Gen. 1996, 142, 375-386. (19) Jung, K. D.; Joo, O. S.; Han, S. H.; Chung, I. J. Deactivation of Cu/ZnO Catalyst During Dehydrogenation of Methanol. Catal. Lett. 1995, 35, 303-311. (20) McCaffrey, E. F.; Micka, T. A.; Ross, R. A. Kinetic Studies of the Catalytic Activity of Alkaline-Earth Oxides in 2-Proponal Decomposition. J. Phys. Chem. 1972, 76, 3372-3376. (21) Hass, T.; Pritchard, J. Adsorption of Carbon Dioxide on Polycrystalline Copper. J. Chem. Soc., Farad. Trans. 1 1990, 86, 1889-1892.
Received for review March 27, 2001 Revised manuscript received September 11, 2001 Accepted September 11, 2001 IE010272Q
