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Low-temperature methanol synthesis in catalytic systems composed of nickel compounds and alkali alkoxides in liquid phases
Applied Catalysis A: General 180 (1999) 217±225
Low-temperature methanol synthesis in catalytic systems composed of nickel compounds and alkali alkoxides in liquid phases Seiichi Ohyama1 Central Research Institute of Electric Power Industry, 2-11-1 Iwado-kita, Komae-shi, Tokyo 201-8511, Japan Received 12 June 1998; received in revised form 30 September 1998; accepted 3 October 1998
Abstract Catalysts prepared from NaH, tert-amyl alcohol and nickel acetate were tested for CO hydrogenation in organic solvents in the range of 353±433 K and 1.0±5.0 MPa. Methanol was produced selectively under the studied conditions. Higher temperatures and higher pressures enhanced methanol productivity; a maximum space±time yield of 0.95 kg MeOH lÿ1 hÿ1, higher than that of the conventional methanol production process, was obtained at 433 K and 5.0 MPa. The addition of methanol to the catalyst did not signi®cantly affect the product yields, but such addition did eliminate the induction period that was observed during the run in the absence of methanol addition to the catalyst. This suggested that methanol promoted the formation of a catalytically active species and/or that a certain amount of methanol was required to run a catalytic reaction cycle smoothly, in which methanol would be a reactant. The catalysts exhibited a catalytic decline over a short period due to the consumption of the alkoxide component. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Low-temperature methanol synthesis; Nickel catalyst; Alkali alkoxide; Methanol productivity
1. Introduction Methanol has attracted a great deal of public attention as one of the important alternative fuels to oil. For instance, the electric power industry is considering the introduction of methanol to gas turbines or fuel cells. Furthermore, coproduction of methanol and electricity in an integrated gasi®cation combined-cycle power plant (IGCC) is proposed for operating IGCCs at levels corresponding to peak demand [1]. To employ methanol as a power plant fuel, a more ef®cient and more economical methanol production process, which can reduce methanol cost, is required. 1 Tel.: +81-3-3480-2111; fax: +81-3-3480-1942; e-mail: [email protected]
Methanol synthesis from carbon monoxide and 0 hydrogen is highly exothermic H298 ÿ90:97 kJ molÿ1 and thermodynamically favorable at lower temperatures. In view of the process, it is important to remove the heat of reaction ef®ciently in order to keep the temperature of the catalyst bed constant. Methanol is industrially produced in the gas±solid phase process using Cu/ZnO-based catalysts. In the industrial process, per-pass conversions of the feed gas are forced to be lowered to 10±20% and a large amount of unreacted gas is recycled [2] because of the dif®culty in removing the heat of reaction in the gas±solid phase process. Consequently, if a catalyst highly active at low temperatures is available and the heat of reaction is removed ef®-
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S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
ciently, methanol production with a high per-pass conversion can be achieved, leading to reduction of methanol cost. With regard to the removal of the heat, reaction in a liquid medium is one of the solutions, due to the large heat capacity of the liquid. In recent years, methanol synthesis in the liquid phase at low temperatures of around 373 K has been proposed [3±14] and has received considerable attention, since it has the potential to overcome the aforementioned problems that arise in the conventional methanol processes. The technology can be classi®ed into two types: processes using a nickel compound with alkali alkoxide [3±7], and those using copper chromite with alkoxide [8±14]. Although they are reported to show excellent activity at temperatures much lower than the operating temperatures in the conventional methanol processes, information on their catalytic activity and behavior is sparse in the literature. In particular, activity data on the Ni compound/ alkali alkoxide catalyst system is scarce. The catalysts for the low-temperature methanol synthesis were described in the patents [15]. They reported three types of catalytic systems composed of sodium hydride, alkyl alcohol and metal acetate (Ni, Pd and Co). The patents presented the performance of the Ni catalyst. However, no kinetic data of the Pd and the Co catalysts were provided. Also, with regard to the Ni catalysts, the catalytic activity was investigated over a very limited range of a few reaction variables. The catalytic data provided in the patents were not enough to evaluate the possibility of the catalytic systems. Thus, systematic study on the effect of reaction variables is required. In the present study, the author examined the performance of the catalysts reported in the patents [15]. Furthermore, the catalytic properties of the nickel compound with alkali alkoxide were investigated in detail, and its potential for use in an industrial process was evaluated. 2. Experimental 2.1. Catalyst The catalysts for the low-temperature methanol synthesis were prepared according to the Brookhaven National Laboratory patents [15]. The patents list
three types of catalysts, the metal component for each of which is selected from among Ni, Pd and Co. Sixty mmol of sodium hydride, 52 mmol of tert-amyl alcohol and 10 mmol of metal acetate were employed as standard starting materials. The prepared catalyst was referred to as NaH/tert-amyl alcohol/M(CH3COO)2, where M denotes Ni, Pd or Co. Triethylene glycol dimethyl ether (triglyme; Aldrich) with a volume of 100 cm3 was employed as a solvent. Tetrahydrofuran (THF) and toluene were also employed as solvents. As for the Ni catalysts, the amount of nickel acetate was varied in the range of 2.5±30 mmol. Catalysts in which potassium methoxide was employed in place of sodium hydride and tert-amyl alcohol were also prepared. Potassium methoxide was in the form of 30% CH3OK solution in methanol, 10 cm3 of which contained 40 mmol of CH3OK and 210 mmol of methanol, or in the form of 40 mmol of CH3OK powder (Aldrich). The reagents described in this section were obtained from Wako Pure Chemical Industries, unless otherwise stated. 2.2. Reaction All experiments were carried out in batch operation in a 465 cm3 magnetically stirred autoclave with SUS 316 [16]. A mixture of carbon monoxide and hydrogen with a stoichiometric ratio for methanol synthesis (H2/ CO2) was employed as a reactant. Before starting the run, trapped air in the reactor was purged with a feed gas of 0.5 MPa and vented three times. Then the reactor was pressurized with the feed gas up to the desired pressure at ambient temperature (or at reaction temperatures). The reactor was heated to the reaction temperature using an electric furnace, and the time when the temperature of the reactor reached the desired reaction temperature was taken to be the start of the reaction. The time courses of the pressure and the temperature were monitored during the course of the run. The experiments were carried out under temperatures ranging from 353 to 433 K and initial pressures ranging from 1.0 to 5.0 MPa. After the run, the reactor was rapidly cooled to ambient temperature. The components of the gas phase and the liquid phase were withdrawn from the reactor and analyzed by gas chromatography with a ¯ame ionization detector for organic compounds and a thermal conductivity detector for CO, CO2 and H2. The carbon monoxide con-
S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
version and the selectivity to methanol were evaluated on the carbon basis. CO conversion was obtained as (products formed at the end of the reaction)/(CO admitted to the reactor). The approximate space±time yield (STY) was determined from the amount of methanol formed and the total volume of the catalyst system (typically ca. 100 cm3, including the solvent), assuming that the whole of the reaction proceeds at the initial rate of pressure decrease during the run. 3. Results and discussion 3.1. Mass transfer effect in the experimental system First, the mass transfer effect of the reactants in the reaction system on methanol productivity was investigated. The experiments were conducted under the conditions of 373 K and 5.0 MPa at different agitation rates using the NaH/tert-amyl alcohol/Ni(CH3COO)2 catalysts. Methanol was predominantly produced with the catalysts. Fig. 1 shows the in¯uence of agitation rate upon the catalytic performance. Similar CO conversions were obtained at each agitation rate. However, the methanol productivity (STY) increased with the agitation rate and levelled off over 1000 rpm. Thus, agitation rates of over 1000 rpm were suf®cient to eliminate mass transfer limitations. Hence, all experiments were conducted at 1000 rpm.
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3.2. Comparison of the Ni catalyst with other patent catalysts The performance of NaH/tert-amyl alcohol/ M(CH3COO)2 (MNi, Pd or Co) was examined at 373 K, as summarized in Table 1. Among the prepared catalysts, only the Ni catalyst showed peculiarly high activity for the selective formation of methanol. The Pd catalyst exhibited high selectivity to methanol with low conversion. In contrast, the Co catalyst showed higher conversions than Pd, but the selectivity to methanol was much lower than that of Ni or Pd. A large amount of acetone was produced over the Co catalyst. Since the patents [15] offered no catalytic data of the Pd and the Co catalysts, this is the ®rst example that shows both catalytic systems have little activity for the methanol production. The Ni catalyst before the reaction was a yellowish green slurry. After the reaction at 373 K, the external appearance of the catalyst changed to a brown or red slurry. The catalysts after reaction at temperatures up to 433 K had the same external appearance. The reaction at temperatures above 433 K, however, transformed the catalyst into a transparent colorless solution and a black ®ne precipitate. Further, a catalyst once subjected to temperatures above 433 K failed to show any catalytic activity in subsequent runs. Therefore, we concluded that the appropriate operation temperature for this catalyst has a maximum of 433 K. The Ni catalyst showed a superior performance for the production of methanol. The catalytic behavior with NaH/tert-amyl alcohol/Ni(CH3COO)2 was examined in the following sections. 3.3. Pressure profile in the course of the run with NaH/tert-amyl alcohol/Ni(CH3COO)2
Fig. 1. Effect of agitation rate on conversion and space±time yield. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K; initial pressure 5.0 MPa (at ambient temperature); reaction time1 h.
Fig. 2 shows an example of the time courses of temperature and pressure changes during the run using the NaH/tert-amyl alcohol/Ni(CH3COO)2 catalyst. In this run, the reactor was charged with the synthesis gas up to 5.0 MPa at ambient temperature and then heated to 373 K. The reactor pressure during the heating process increased with the temperature due to batch operation. An induction period was observed for some time after the temperature reached 373 K, during which the pressure remained almost constant. Then the pressure decreased rapidly due to gas consumption
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S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
Table 1 Catalytic performance with NaH/tert-amyl alcohol/M(CH3COO)2 Catalyst (metal) Time (h)
Conversion (%) Selectivity (%)
Ni Pd Co
68.9 0.3 4.4
1 1 1
94.7 97.7 2.6
Product yieldsa (mmol) MeOH
MF
DME
CO2
CH4
ACTN
172.0 0.7 0.3
4.8 n.d. n.d.
Trace Trace n.d.
Trace n.d. n.d.
n.d.b Trace Trace
n.d. n.d. 3.5
Catalyst: NaH/tert-amyl alcohol/M(CH3COO)2 (MNi, Pd, Co). Solvent: triglyme (100 cm3). Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature). a MeOH: methanol; MF: methyl formate; DME: dimethyl ether; ACTN: acetone. b n.d.not detected.
Hence, we concluded that the reaction proceeded catalytically. 3.4. Effect of reaction variables on methanol productivity
Fig. 2. Time courses of temperature and pressure during the run. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: reaction temperature373 K; initial pressure 5.0 MPa (at ambient temperature).
ascribed to methanol synthesis. The formation of methanol was 172 mmol after the reaction for 1 h, which was over 10 times the amount of nickel acetate and three times that of NaH or tert-amyl alcohol.
3.4.1. Effect of solvents Table 2 illustrates the catalytic performance with NaH/tert-amyl alcohol/Ni(CH3COO)2 dispersed in several solvents. The activity of the Ni catalyst was largely dependent on the solvent; the catalyst showed low conversion and selectivity in toluene, whereas in THF and triglyme, 50±70% CO conversion and 95% selectivity were achieved. Thus, polar solvents were preferable for the production of methanol. The polar solvents might stabilize ionic species, e.g., catalytically active species or intermediates, which are responsible for the methanol formation. A small amount of methyl formate (MF) and dimethyl ether (DME) was also produced in THF and triglyme. MF and DME are considered to be formed by carbonylation of methanol (Eq. (1)) and
Table 2 Catalytic performance with NaH/tert-amyl alcohol/Ni(CH3COO)2 in various solvents Solvent
Time (h)
Conversion (%)
Selectivity (%)
Product yieldsa (mmol) MeOH
MF
DME
CO2
CH4
Triglyme THF Toluene
1 3 3
68.9 46.2 0.3
94.7 95.7 68.8
172.0 108.0 0.5
4.8 2.4 n.d.
Trace Trace n.d.
Trace n.d. 0.2
n.d.b Trace n.d.
Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent100 cm3. Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature). a MeOH: methanol; MF: methyl formate; DME: dimethyl ether; ACTN: acetone. b n.d.not detected.
S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
dehydration of methanol (Eq. (2)), i.e., CH3 OH CO ! HCOOCH3
(1)
and 2CH3 OH ! CH3 OCH3 H2 O
(2)
respectively. These are the reactions where produced methanol is a reactant. Hence, they are favorable in the presence of a large amount of methanol. During the preparation of the catalyst, tert-amyl alcohol would react with sodium hydride to yield sodium tert-amyl alkoxide. Thus, the catalyst precursors are presumed to be transformed during the preparation into a mixture of nickel acetate and sodium tert-amyl alkoxide. This speculation is strengthened by the fact that the catalysts prepared from nickel acetate and potassium methoxide showed a similar performance under the same conditions. Alkali alkoxides are well known to catalyze the carbonylation of methanol to MF [19,20]. Hence, the compounds formed during the preparation should promote the formation of methyl formate. 3.4.2. Effect of nickel precursor Various nickel salts were employed in place of nickel acetate for the preparation of the catalysts. The performance of the catalysts is summarized in Table 3. Although no methanol was produced with the catalysts employing nickel metal or nickel nitrate, some divalent compounds of nickel, such as sulfate and chloride were also effective for the production of methanol. They required longer periods to initiate the reaction than Ni(CH3COO)2 and showed slightly lower methanol productivity than that of Ni(CH3COO)2. The fact that other nickel salts in addition
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to acetate are effective for the formation of methanol suggests that nickel salts are converted to another form that can function as a catalyst. The catalysts prepared from NiSO4 or NiCl2, however, exhibited no catalytic activity in subsequent experiments. The reason for this difference was not identi®ed. 3.4.3. Effect of temperature and initial pressure The dependence of the STY on the temperature and the initial pressure over the NaH/tert-amyl alcohol/ Ni(CH3COO)2 catalyst is shown in Figs. 3 and 4, respectively. Under the conditions studied, methanol was formed with 40±80% of CO conversion and 93± 100% of selectivity. Trace amounts of MF, DME and CO2 were produced as by-products. Increasing temperatures and pressures enhanced methanol produc-
Fig. 3. Effect of temperature on space±time yield. Catalyst: NaH/ tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: initial pressure5.0 MPa (at ambient temperature).
Table 3 Effect of nickel precursors on catalytic performance Precursor
Ni(CH3COO)2 NiSO4 NiCl2 Ni(NO3)2 Ni metal
Time (h) Conversion (%) Selectivity (%)
1 2 2 3 3
68.9 52.0 64.1 0.1 ±
94.7 99.7 99.7 100 ±
STY (kg lÿ1 hÿ1)
Product yields (mmol) MeOH
MF
DME
CO2
172.0 125.8 157.5 0.2 n.d.
4.8 0.2 0.3 n.d. n.d.
Trace Trace Trace n.d. n.d.
Trace n.d. n.d. Trace n.d.
0.168 0.098 0.100 0.000 ±
Catalyst: NaH/tert-amyl alcohol/nickel salt. Solvent: triglyme (100 cm3). Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature).
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S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
Fig. 4. Effect of initial pressure on space±time yield. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K, 433 K.
Fig. 5. Effect of Ni concentration on space±time yield. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2 (Ni2.5±30 mmol). Solvent: triglyme. Reaction conditions: temperature433 K; initial pressure5.0 MPa (at ambient temperature).
tivity; a maximum STY was obtained at 433 K and 5.0 MPa. The reactions at lower temperatures and lower initial pressures tend to show more distinct induction periods in the course of the run. No pronounced induction periods were observed in runs at temperatures over 373 K. 3.4.4. Effect of nickel concentration in the catalyst Fig. 5 shows the effect of Ni concentration on the STY at 433 K and 5.0 MPa. The STY increased slightly with increasing Ni concentration in the catalyst; a STY of 0.95 kg lÿ1 hÿ1 was obtained at 284 mmol lÿ1 of the Ni concentration, which was the maximum STY in this study. 3.4.5. Effect of methanol concentration The effect of methanol concentration in the catalyst was investigated. After various amounts (1±8 cm3) of methanol were added to the catalyst solution, the reaction was carried out at 373 K and 5.0 MPa. Fig. 6 illustrates the effect of methanol on the catalytic performance, i.e., CO conversion, selectivity and STY. Considerable CO conversions were obtained at any methanol concentration, and methanol was produced selectively, independent of the methanol concentration. A small amount of methanol rather enhanced methanol productivity, and then the STY decreased
Fig. 6. Effect of methanol addition on catalytic performance. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K; initial pressure 5.0 MPa (at ambient temperature); reaction time1 h.
slightly at higher methanol concentrations. Therefore, no constraints from chemical equilibrium or inhibition of the reaction due to the addition of methanol were noticed in this study. As mentioned previously, an induction period was observed in the case of NaH/tert-amyl alcohol/ Ni(CH3COO)2 without the addition of methanol. In contrast, no induction period was recognized in the
S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
case of methanol-added NaH/tert-amyl alcohol/ Ni(CH3COO)2. This suggests that methanol is involved in the formation of catalytically active species and promotes the transformation of the catalyst precursors into active species, or it suggests that an additional amount of methanol is required to run some kind of catalytic reaction cycle smoothly, in which methanol is a reactant. For example, assuming that the nickel compound promotes hydrogenation of alkyl formate to alcohols, the following reaction cycle would be feasible [17,18]: ROH CO ! ROCHO
(3)
ROCHO 2H2 ! CH3 OH ROH
(4)
where R represents an alkyl group. Alkali alkoxides are highly effective catalysts for carbonylation of alcohols to alkyl formates [19,20]. Hence, if there is an enough amount of methanol or alcohol at the beginning of the experiments to initiate reaction (3) at a considerable reaction rate, the overall reaction would also proceed ef®ciently. On the other hand, if there is not an enough amount of alcohol in the reactor, it would take a longer period until it shows a distinct pressure decrease during the run. Thus, the induction period observed over the catalysts without the methanol addition could be interpreted as a time when a suf®cient amount of methanol is accumulated in the reactor to run reaction (3) at an appreciable rate. 3.4.6. Effect of CO2 in feed gas Table 4 indicates the effect of CO2 in the feed gas on the catalytic performance. The catalysts were inactive for the feed containing 2% CO2. Alkali alkoxides are reported to react with CO2 to be converted into catalytically inactive compounds [11,12,19±22]. The consumption of alkoxide by CO2 would be the
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reason why the catalyst displayed no activity towards the gas containing CO2. 3.5. Decline of catalytic activity in the multiple charging run Fig. 7 shows time courses of the temperature and the pressure during the multiple charging experiment. It should be noted that no obvious induction period was observed after the heating process, which differs from Fig. 2. This is due to the fact that 373 K is the temperature above which no induction period was observed and below which one was. Even when the pressure decrease became small, the pressure started to decrease again upon recharging the feed gas. The pressure decrease after recharging became smaller with the number of recharges, and no remarkable pressure decrease was observed after
Fig. 7. Time courses of temperature and pressure in multiple charging experiment. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature); recharge of syngas6.0 MPa at 373 K. (#) Point of syngas recharge.
Table 4 Effect of CO2 in feed gas on catalytic performance Feed gas ratio H2/CO/CO2
Time (h)
Conversion (%)
Selectivity (%)
Product yields (mmol) MeOH
MF
DME
CO2
67/33/0 67/31/2
1 3
68.9 0.2
94.7 100
172.0 0.4
4.8 n.d.
Trace n.d.
Trace n.d.
Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme (100 cm3). Reaction conditions: temperature373 K; initial pressure 5.0 MPa (at ambient temperature).
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S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
the second recharge of the syngas. Besides, the catalytic activity in subsequent experiments after this run was negligible. We have already con®rmed that the amount of methanol formed in the multiple charging run did not affect the methanol productivity, as discussed in Section 3.4.5. Therefore, it is due to the deactivation of the catalyst that little pressure decrease was observed after the second recharge. The catalytic deactivation over such a short period is a serious problem when the application of the catalyst to industrial processes is considered. As several researchers have pointed out [11,12,19± 23], alkoxides are highly reactive towards CO2 and H2O. They react with CO2 and H2O in ppm orders to form catalytically inactive compounds as follows [19,20±22]. CH3 OK CO2 ! CH3 OCOOK
(5)
CH3 OK H2 O ! CH3 OH KOH
(6)
KOH CO ! HCOOK
(7)
Thus, the catalyst system is sensitive to trace amounts of CO2 and H2O, which can be formed during
the reaction as by-products. In fact, a trace of CO2 was often detected as a product in this study. Although water was not analyzed, it was probably formed during the reaction, because DME that is produced through dehydration of methanol was repeatedly detected. Fig. 8 illustrates the recovery of the catalytic performance with several catalysts employing Ni(CH3COO)2. The catalysts were subjected to several subsequent runs or a multiple charging run and were forced to be deactivated. The deactivation of the catalysts was con®rmed in the subsequent runs, as shown in the second column of each catalyst in the ®gure. The performance of the deactivated catalyst was restored upon adding alkoxide component to the deactivated catalyst. Thus, the consumption of alkoxides is a major factor for the deactivation of this catalyst system. 3.6. Evaluation of NaH/tert-amyl alcohol/ Ni(CH3COO)2 as an industrial catalyst In the industrial methanol production process, copper-based catalysts (CuO/ZnO/Al2O3 or CuO/ZnO/
Fig. 8. Recovery of methanol productivity by addition of alkoxide. Reaction conditions: temperature373±433 K; initial pressure5.0 MPa. Solvent: triglyme. Methanol yield represents increments of methanol before and after each run.
S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
Cr2O3) are employed under the conditions of temperatures of 503±573 K, pressures of 5±20 MPa, and space velocity of 10 000±40 000 hÿ1 [24]. The STY in the ICI process, a typical conventional methanol process, is reported to be 0.5±0.77 kg lÿ1 hÿ1 [24]. Since the NaH/tert-amyl alcohol/Ni(CH3COO)2 catalysts displayed a higher STY (0.95 kg lÿ1 hÿ1) under mild conditions than conventional methanol catalysts, it should be possible, using this catalyst, to eliminate recycling facilities for unconverted gas, which can reduce the production cost of methanol. However, the catalyst showed no catalytic activity towards feed gases containing a small amount of CO2. Therefore, in order to apply the catalytic systems to industrial processes, especially IGCC plants, CO2 in the feed gas should be thoroughly removed. 4. Conclusions The low-temperature methanol synthesis in the liquid phase was carried out using the catalyst, NaH/tert-amyl alcohol/Ni(CH3COO)2. It was highly effective for the selective production of methanol at temperatures lower than the operating temperatures in industrial methanol processes. The maximum STY of 0.95 kg MeOH lÿ1 hÿ1, higher than that in the conventional methanol process, was obtained at 433 K and 5.0 MPa. The addition of methanol to the catalyst did not signi®cantly affect the product yields. However, it shortened the induction period observed during the run, suggesting that methanol promotes the formation of catalytically active species and/or a certain amount of methanol is required to run a reaction cycle. The catalysts were deactivated in a short period, which leads to a serious problem in the application of the catalytic systems to the industrial processes. It was con®rmed that the consumption of alkoxides is a major factor for the deactivation of the catalyst system.
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Acknowledgements The author expresses his sincere thanks to Professor N. Takezawa at Hokkaido University for his kind revision of the manuscript. References [1] A.A. Deane, D.A. Huber, J. DeRosa, Coproduction of methanol and electricity, EPRI Report AP-3749, 1984. [2] E. Supp, Hydrocarbon Processing, March 1981, p. 71; July 1984, p. 34C. [3] D. Mahajan, R.S. Sapienza, W.A. Slegeir, T.E. O'Hare, US Patent 4 992 480 (1991). [4] H. Nakamura, K. Saeki, M. Tanaka, Jpn. Patent 88/51129 (1988). [5] M. Marchionna, M. Lami, F. Ancillotti, R. Ricci, US Patent 5 032 618 (1991). [6] S.T. Sie, E. Drent, W.W. Jager, US Patent 4 812 433 (1989). [7] M. Marchionna, L. Basini, A. Aragno, M. Lami, F. Ancillotti, J. Mol. Catal. 75 (1992) 147. [8] O.T. Onsager, Jpn. Patent 87/500867 (1987); 91/12048 (1991). Ê . Sùrum, O.T. Onsager, Eight Int. Congr. Catal. 2 (1984) 233. [9] P.A [10] D.M. Monti, M.A. Kohler, M.S. Wainwright, D.L. Trimm, N.W. Cant, Appl. Catal. A 22 (1986) 123. [11] V.M. Palekar, H. Jung, J.W. Tierney, I. Wender, Appl. Catal. A 102 (1993) 13. [12] V.M. Palekar, J.W. Tierney, I. Wender, Appl. Catal. A 103 (1993) 105. [13] R.J. Gormley, V.U.S. Rao, Y. Soong, E. Micheli, Appl. Catal. A 87 (1992) 81. [14] D.L. Trimm, M.S. Wainwright, Catal. Today 6 (1990) 261. [15] R.S. Sapienza, W.A. Slegeir, T.E. O'Hare, D. Mahajan, US Patent 4 614 749 (1986); 4 619 946 (1986); 4 623 634 (1986). [16] S. Ohyama, React. Kinet. Catal. Lett. 61 (1997) 331. [17] J.A. Christiansen, US Patent 1 302 011 (1919). [18] D.L. Trimm, M.S. Wainwright, Catal. Today 6 (1990) 261. [19] S.P. Tonner, D.L. Trimm, M.S. Wainwright, N.W. Cant, J. Mol. Catal. 18 (1983) 215. [20] R.J. Gormley, A.M. Giusti, S. Rossini, V.U.S. Rao, Ninth Int. Congr. Catal. 2 (1988) 553. [21] Z. Liu, J.W. Tierney, Y.T. Shah, I. Wender, Fuel Process. Tech. 18 (1988) 185. [22] Z. Liu, J.W. Tierney, Y.T. Shah, I. Wender, Fuel Process. Tech. 23 (1989) 149. [23] S.J. Choi, J.S. Lee, Y.G. Kim, J. Mol. Catal. 85 (1993) L109. [24] R.G. Hermann, K. Klier, G.W. Simmons, B.P. Finn, J.B. Bulko, T.P. Kobylinski, J. Catal. 56 (1979) 407.
Low-temperature methanol synthesis in catalytic systems composed of nickel compounds and alkali alkoxides in liquid phases Seiichi Ohyama1 Central Research Institute of Electric Power Industry, 2-11-1 Iwado-kita, Komae-shi, Tokyo 201-8511, Japan Received 12 June 1998; received in revised form 30 September 1998; accepted 3 October 1998
Abstract Catalysts prepared from NaH, tert-amyl alcohol and nickel acetate were tested for CO hydrogenation in organic solvents in the range of 353±433 K and 1.0±5.0 MPa. Methanol was produced selectively under the studied conditions. Higher temperatures and higher pressures enhanced methanol productivity; a maximum space±time yield of 0.95 kg MeOH lÿ1 hÿ1, higher than that of the conventional methanol production process, was obtained at 433 K and 5.0 MPa. The addition of methanol to the catalyst did not signi®cantly affect the product yields, but such addition did eliminate the induction period that was observed during the run in the absence of methanol addition to the catalyst. This suggested that methanol promoted the formation of a catalytically active species and/or that a certain amount of methanol was required to run a catalytic reaction cycle smoothly, in which methanol would be a reactant. The catalysts exhibited a catalytic decline over a short period due to the consumption of the alkoxide component. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Low-temperature methanol synthesis; Nickel catalyst; Alkali alkoxide; Methanol productivity
1. Introduction Methanol has attracted a great deal of public attention as one of the important alternative fuels to oil. For instance, the electric power industry is considering the introduction of methanol to gas turbines or fuel cells. Furthermore, coproduction of methanol and electricity in an integrated gasi®cation combined-cycle power plant (IGCC) is proposed for operating IGCCs at levels corresponding to peak demand [1]. To employ methanol as a power plant fuel, a more ef®cient and more economical methanol production process, which can reduce methanol cost, is required. 1 Tel.: +81-3-3480-2111; fax: +81-3-3480-1942; e-mail: [email protected]
Methanol synthesis from carbon monoxide and 0 hydrogen is highly exothermic H298 ÿ90:97 kJ molÿ1 and thermodynamically favorable at lower temperatures. In view of the process, it is important to remove the heat of reaction ef®ciently in order to keep the temperature of the catalyst bed constant. Methanol is industrially produced in the gas±solid phase process using Cu/ZnO-based catalysts. In the industrial process, per-pass conversions of the feed gas are forced to be lowered to 10±20% and a large amount of unreacted gas is recycled [2] because of the dif®culty in removing the heat of reaction in the gas±solid phase process. Consequently, if a catalyst highly active at low temperatures is available and the heat of reaction is removed ef®-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00338-X
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ciently, methanol production with a high per-pass conversion can be achieved, leading to reduction of methanol cost. With regard to the removal of the heat, reaction in a liquid medium is one of the solutions, due to the large heat capacity of the liquid. In recent years, methanol synthesis in the liquid phase at low temperatures of around 373 K has been proposed [3±14] and has received considerable attention, since it has the potential to overcome the aforementioned problems that arise in the conventional methanol processes. The technology can be classi®ed into two types: processes using a nickel compound with alkali alkoxide [3±7], and those using copper chromite with alkoxide [8±14]. Although they are reported to show excellent activity at temperatures much lower than the operating temperatures in the conventional methanol processes, information on their catalytic activity and behavior is sparse in the literature. In particular, activity data on the Ni compound/ alkali alkoxide catalyst system is scarce. The catalysts for the low-temperature methanol synthesis were described in the patents [15]. They reported three types of catalytic systems composed of sodium hydride, alkyl alcohol and metal acetate (Ni, Pd and Co). The patents presented the performance of the Ni catalyst. However, no kinetic data of the Pd and the Co catalysts were provided. Also, with regard to the Ni catalysts, the catalytic activity was investigated over a very limited range of a few reaction variables. The catalytic data provided in the patents were not enough to evaluate the possibility of the catalytic systems. Thus, systematic study on the effect of reaction variables is required. In the present study, the author examined the performance of the catalysts reported in the patents [15]. Furthermore, the catalytic properties of the nickel compound with alkali alkoxide were investigated in detail, and its potential for use in an industrial process was evaluated. 2. Experimental 2.1. Catalyst The catalysts for the low-temperature methanol synthesis were prepared according to the Brookhaven National Laboratory patents [15]. The patents list
three types of catalysts, the metal component for each of which is selected from among Ni, Pd and Co. Sixty mmol of sodium hydride, 52 mmol of tert-amyl alcohol and 10 mmol of metal acetate were employed as standard starting materials. The prepared catalyst was referred to as NaH/tert-amyl alcohol/M(CH3COO)2, where M denotes Ni, Pd or Co. Triethylene glycol dimethyl ether (triglyme; Aldrich) with a volume of 100 cm3 was employed as a solvent. Tetrahydrofuran (THF) and toluene were also employed as solvents. As for the Ni catalysts, the amount of nickel acetate was varied in the range of 2.5±30 mmol. Catalysts in which potassium methoxide was employed in place of sodium hydride and tert-amyl alcohol were also prepared. Potassium methoxide was in the form of 30% CH3OK solution in methanol, 10 cm3 of which contained 40 mmol of CH3OK and 210 mmol of methanol, or in the form of 40 mmol of CH3OK powder (Aldrich). The reagents described in this section were obtained from Wako Pure Chemical Industries, unless otherwise stated. 2.2. Reaction All experiments were carried out in batch operation in a 465 cm3 magnetically stirred autoclave with SUS 316 [16]. A mixture of carbon monoxide and hydrogen with a stoichiometric ratio for methanol synthesis (H2/ CO2) was employed as a reactant. Before starting the run, trapped air in the reactor was purged with a feed gas of 0.5 MPa and vented three times. Then the reactor was pressurized with the feed gas up to the desired pressure at ambient temperature (or at reaction temperatures). The reactor was heated to the reaction temperature using an electric furnace, and the time when the temperature of the reactor reached the desired reaction temperature was taken to be the start of the reaction. The time courses of the pressure and the temperature were monitored during the course of the run. The experiments were carried out under temperatures ranging from 353 to 433 K and initial pressures ranging from 1.0 to 5.0 MPa. After the run, the reactor was rapidly cooled to ambient temperature. The components of the gas phase and the liquid phase were withdrawn from the reactor and analyzed by gas chromatography with a ¯ame ionization detector for organic compounds and a thermal conductivity detector for CO, CO2 and H2. The carbon monoxide con-
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version and the selectivity to methanol were evaluated on the carbon basis. CO conversion was obtained as (products formed at the end of the reaction)/(CO admitted to the reactor). The approximate space±time yield (STY) was determined from the amount of methanol formed and the total volume of the catalyst system (typically ca. 100 cm3, including the solvent), assuming that the whole of the reaction proceeds at the initial rate of pressure decrease during the run. 3. Results and discussion 3.1. Mass transfer effect in the experimental system First, the mass transfer effect of the reactants in the reaction system on methanol productivity was investigated. The experiments were conducted under the conditions of 373 K and 5.0 MPa at different agitation rates using the NaH/tert-amyl alcohol/Ni(CH3COO)2 catalysts. Methanol was predominantly produced with the catalysts. Fig. 1 shows the in¯uence of agitation rate upon the catalytic performance. Similar CO conversions were obtained at each agitation rate. However, the methanol productivity (STY) increased with the agitation rate and levelled off over 1000 rpm. Thus, agitation rates of over 1000 rpm were suf®cient to eliminate mass transfer limitations. Hence, all experiments were conducted at 1000 rpm.
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3.2. Comparison of the Ni catalyst with other patent catalysts The performance of NaH/tert-amyl alcohol/ M(CH3COO)2 (MNi, Pd or Co) was examined at 373 K, as summarized in Table 1. Among the prepared catalysts, only the Ni catalyst showed peculiarly high activity for the selective formation of methanol. The Pd catalyst exhibited high selectivity to methanol with low conversion. In contrast, the Co catalyst showed higher conversions than Pd, but the selectivity to methanol was much lower than that of Ni or Pd. A large amount of acetone was produced over the Co catalyst. Since the patents [15] offered no catalytic data of the Pd and the Co catalysts, this is the ®rst example that shows both catalytic systems have little activity for the methanol production. The Ni catalyst before the reaction was a yellowish green slurry. After the reaction at 373 K, the external appearance of the catalyst changed to a brown or red slurry. The catalysts after reaction at temperatures up to 433 K had the same external appearance. The reaction at temperatures above 433 K, however, transformed the catalyst into a transparent colorless solution and a black ®ne precipitate. Further, a catalyst once subjected to temperatures above 433 K failed to show any catalytic activity in subsequent runs. Therefore, we concluded that the appropriate operation temperature for this catalyst has a maximum of 433 K. The Ni catalyst showed a superior performance for the production of methanol. The catalytic behavior with NaH/tert-amyl alcohol/Ni(CH3COO)2 was examined in the following sections. 3.3. Pressure profile in the course of the run with NaH/tert-amyl alcohol/Ni(CH3COO)2
Fig. 1. Effect of agitation rate on conversion and space±time yield. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K; initial pressure 5.0 MPa (at ambient temperature); reaction time1 h.
Fig. 2 shows an example of the time courses of temperature and pressure changes during the run using the NaH/tert-amyl alcohol/Ni(CH3COO)2 catalyst. In this run, the reactor was charged with the synthesis gas up to 5.0 MPa at ambient temperature and then heated to 373 K. The reactor pressure during the heating process increased with the temperature due to batch operation. An induction period was observed for some time after the temperature reached 373 K, during which the pressure remained almost constant. Then the pressure decreased rapidly due to gas consumption
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Table 1 Catalytic performance with NaH/tert-amyl alcohol/M(CH3COO)2 Catalyst (metal) Time (h)
Conversion (%) Selectivity (%)
Ni Pd Co
68.9 0.3 4.4
1 1 1
94.7 97.7 2.6
Product yieldsa (mmol) MeOH
MF
DME
CO2
CH4
ACTN
172.0 0.7 0.3
4.8 n.d. n.d.
Trace Trace n.d.
Trace n.d. n.d.
n.d.b Trace Trace
n.d. n.d. 3.5
Catalyst: NaH/tert-amyl alcohol/M(CH3COO)2 (MNi, Pd, Co). Solvent: triglyme (100 cm3). Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature). a MeOH: methanol; MF: methyl formate; DME: dimethyl ether; ACTN: acetone. b n.d.not detected.
Hence, we concluded that the reaction proceeded catalytically. 3.4. Effect of reaction variables on methanol productivity
Fig. 2. Time courses of temperature and pressure during the run. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: reaction temperature373 K; initial pressure 5.0 MPa (at ambient temperature).
ascribed to methanol synthesis. The formation of methanol was 172 mmol after the reaction for 1 h, which was over 10 times the amount of nickel acetate and three times that of NaH or tert-amyl alcohol.
3.4.1. Effect of solvents Table 2 illustrates the catalytic performance with NaH/tert-amyl alcohol/Ni(CH3COO)2 dispersed in several solvents. The activity of the Ni catalyst was largely dependent on the solvent; the catalyst showed low conversion and selectivity in toluene, whereas in THF and triglyme, 50±70% CO conversion and 95% selectivity were achieved. Thus, polar solvents were preferable for the production of methanol. The polar solvents might stabilize ionic species, e.g., catalytically active species or intermediates, which are responsible for the methanol formation. A small amount of methyl formate (MF) and dimethyl ether (DME) was also produced in THF and triglyme. MF and DME are considered to be formed by carbonylation of methanol (Eq. (1)) and
Table 2 Catalytic performance with NaH/tert-amyl alcohol/Ni(CH3COO)2 in various solvents Solvent
Time (h)
Conversion (%)
Selectivity (%)
Product yieldsa (mmol) MeOH
MF
DME
CO2
CH4
Triglyme THF Toluene
1 3 3
68.9 46.2 0.3
94.7 95.7 68.8
172.0 108.0 0.5
4.8 2.4 n.d.
Trace Trace n.d.
Trace n.d. 0.2
n.d.b Trace n.d.
Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent100 cm3. Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature). a MeOH: methanol; MF: methyl formate; DME: dimethyl ether; ACTN: acetone. b n.d.not detected.
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dehydration of methanol (Eq. (2)), i.e., CH3 OH CO ! HCOOCH3
(1)
and 2CH3 OH ! CH3 OCH3 H2 O
(2)
respectively. These are the reactions where produced methanol is a reactant. Hence, they are favorable in the presence of a large amount of methanol. During the preparation of the catalyst, tert-amyl alcohol would react with sodium hydride to yield sodium tert-amyl alkoxide. Thus, the catalyst precursors are presumed to be transformed during the preparation into a mixture of nickel acetate and sodium tert-amyl alkoxide. This speculation is strengthened by the fact that the catalysts prepared from nickel acetate and potassium methoxide showed a similar performance under the same conditions. Alkali alkoxides are well known to catalyze the carbonylation of methanol to MF [19,20]. Hence, the compounds formed during the preparation should promote the formation of methyl formate. 3.4.2. Effect of nickel precursor Various nickel salts were employed in place of nickel acetate for the preparation of the catalysts. The performance of the catalysts is summarized in Table 3. Although no methanol was produced with the catalysts employing nickel metal or nickel nitrate, some divalent compounds of nickel, such as sulfate and chloride were also effective for the production of methanol. They required longer periods to initiate the reaction than Ni(CH3COO)2 and showed slightly lower methanol productivity than that of Ni(CH3COO)2. The fact that other nickel salts in addition
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to acetate are effective for the formation of methanol suggests that nickel salts are converted to another form that can function as a catalyst. The catalysts prepared from NiSO4 or NiCl2, however, exhibited no catalytic activity in subsequent experiments. The reason for this difference was not identi®ed. 3.4.3. Effect of temperature and initial pressure The dependence of the STY on the temperature and the initial pressure over the NaH/tert-amyl alcohol/ Ni(CH3COO)2 catalyst is shown in Figs. 3 and 4, respectively. Under the conditions studied, methanol was formed with 40±80% of CO conversion and 93± 100% of selectivity. Trace amounts of MF, DME and CO2 were produced as by-products. Increasing temperatures and pressures enhanced methanol produc-
Fig. 3. Effect of temperature on space±time yield. Catalyst: NaH/ tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: initial pressure5.0 MPa (at ambient temperature).
Table 3 Effect of nickel precursors on catalytic performance Precursor
Ni(CH3COO)2 NiSO4 NiCl2 Ni(NO3)2 Ni metal
Time (h) Conversion (%) Selectivity (%)
1 2 2 3 3
68.9 52.0 64.1 0.1 ±
94.7 99.7 99.7 100 ±
STY (kg lÿ1 hÿ1)
Product yields (mmol) MeOH
MF
DME
CO2
172.0 125.8 157.5 0.2 n.d.
4.8 0.2 0.3 n.d. n.d.
Trace Trace Trace n.d. n.d.
Trace n.d. n.d. Trace n.d.
0.168 0.098 0.100 0.000 ±
Catalyst: NaH/tert-amyl alcohol/nickel salt. Solvent: triglyme (100 cm3). Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature).
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Fig. 4. Effect of initial pressure on space±time yield. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K, 433 K.
Fig. 5. Effect of Ni concentration on space±time yield. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2 (Ni2.5±30 mmol). Solvent: triglyme. Reaction conditions: temperature433 K; initial pressure5.0 MPa (at ambient temperature).
tivity; a maximum STY was obtained at 433 K and 5.0 MPa. The reactions at lower temperatures and lower initial pressures tend to show more distinct induction periods in the course of the run. No pronounced induction periods were observed in runs at temperatures over 373 K. 3.4.4. Effect of nickel concentration in the catalyst Fig. 5 shows the effect of Ni concentration on the STY at 433 K and 5.0 MPa. The STY increased slightly with increasing Ni concentration in the catalyst; a STY of 0.95 kg lÿ1 hÿ1 was obtained at 284 mmol lÿ1 of the Ni concentration, which was the maximum STY in this study. 3.4.5. Effect of methanol concentration The effect of methanol concentration in the catalyst was investigated. After various amounts (1±8 cm3) of methanol were added to the catalyst solution, the reaction was carried out at 373 K and 5.0 MPa. Fig. 6 illustrates the effect of methanol on the catalytic performance, i.e., CO conversion, selectivity and STY. Considerable CO conversions were obtained at any methanol concentration, and methanol was produced selectively, independent of the methanol concentration. A small amount of methanol rather enhanced methanol productivity, and then the STY decreased
Fig. 6. Effect of methanol addition on catalytic performance. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K; initial pressure 5.0 MPa (at ambient temperature); reaction time1 h.
slightly at higher methanol concentrations. Therefore, no constraints from chemical equilibrium or inhibition of the reaction due to the addition of methanol were noticed in this study. As mentioned previously, an induction period was observed in the case of NaH/tert-amyl alcohol/ Ni(CH3COO)2 without the addition of methanol. In contrast, no induction period was recognized in the
S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
case of methanol-added NaH/tert-amyl alcohol/ Ni(CH3COO)2. This suggests that methanol is involved in the formation of catalytically active species and promotes the transformation of the catalyst precursors into active species, or it suggests that an additional amount of methanol is required to run some kind of catalytic reaction cycle smoothly, in which methanol is a reactant. For example, assuming that the nickel compound promotes hydrogenation of alkyl formate to alcohols, the following reaction cycle would be feasible [17,18]: ROH CO ! ROCHO
(3)
ROCHO 2H2 ! CH3 OH ROH
(4)
where R represents an alkyl group. Alkali alkoxides are highly effective catalysts for carbonylation of alcohols to alkyl formates [19,20]. Hence, if there is an enough amount of methanol or alcohol at the beginning of the experiments to initiate reaction (3) at a considerable reaction rate, the overall reaction would also proceed ef®ciently. On the other hand, if there is not an enough amount of alcohol in the reactor, it would take a longer period until it shows a distinct pressure decrease during the run. Thus, the induction period observed over the catalysts without the methanol addition could be interpreted as a time when a suf®cient amount of methanol is accumulated in the reactor to run reaction (3) at an appreciable rate. 3.4.6. Effect of CO2 in feed gas Table 4 indicates the effect of CO2 in the feed gas on the catalytic performance. The catalysts were inactive for the feed containing 2% CO2. Alkali alkoxides are reported to react with CO2 to be converted into catalytically inactive compounds [11,12,19±22]. The consumption of alkoxide by CO2 would be the
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reason why the catalyst displayed no activity towards the gas containing CO2. 3.5. Decline of catalytic activity in the multiple charging run Fig. 7 shows time courses of the temperature and the pressure during the multiple charging experiment. It should be noted that no obvious induction period was observed after the heating process, which differs from Fig. 2. This is due to the fact that 373 K is the temperature above which no induction period was observed and below which one was. Even when the pressure decrease became small, the pressure started to decrease again upon recharging the feed gas. The pressure decrease after recharging became smaller with the number of recharges, and no remarkable pressure decrease was observed after
Fig. 7. Time courses of temperature and pressure in multiple charging experiment. Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme. Reaction conditions: temperature373 K; initial pressure5.0 MPa (at ambient temperature); recharge of syngas6.0 MPa at 373 K. (#) Point of syngas recharge.
Table 4 Effect of CO2 in feed gas on catalytic performance Feed gas ratio H2/CO/CO2
Time (h)
Conversion (%)
Selectivity (%)
Product yields (mmol) MeOH
MF
DME
CO2
67/33/0 67/31/2
1 3
68.9 0.2
94.7 100
172.0 0.4
4.8 n.d.
Trace n.d.
Trace n.d.
Catalyst: NaH/tert-amyl alcohol/Ni(CH3COO)2. Solvent: triglyme (100 cm3). Reaction conditions: temperature373 K; initial pressure 5.0 MPa (at ambient temperature).
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the second recharge of the syngas. Besides, the catalytic activity in subsequent experiments after this run was negligible. We have already con®rmed that the amount of methanol formed in the multiple charging run did not affect the methanol productivity, as discussed in Section 3.4.5. Therefore, it is due to the deactivation of the catalyst that little pressure decrease was observed after the second recharge. The catalytic deactivation over such a short period is a serious problem when the application of the catalyst to industrial processes is considered. As several researchers have pointed out [11,12,19± 23], alkoxides are highly reactive towards CO2 and H2O. They react with CO2 and H2O in ppm orders to form catalytically inactive compounds as follows [19,20±22]. CH3 OK CO2 ! CH3 OCOOK
(5)
CH3 OK H2 O ! CH3 OH KOH
(6)
KOH CO ! HCOOK
(7)
Thus, the catalyst system is sensitive to trace amounts of CO2 and H2O, which can be formed during
the reaction as by-products. In fact, a trace of CO2 was often detected as a product in this study. Although water was not analyzed, it was probably formed during the reaction, because DME that is produced through dehydration of methanol was repeatedly detected. Fig. 8 illustrates the recovery of the catalytic performance with several catalysts employing Ni(CH3COO)2. The catalysts were subjected to several subsequent runs or a multiple charging run and were forced to be deactivated. The deactivation of the catalysts was con®rmed in the subsequent runs, as shown in the second column of each catalyst in the ®gure. The performance of the deactivated catalyst was restored upon adding alkoxide component to the deactivated catalyst. Thus, the consumption of alkoxides is a major factor for the deactivation of this catalyst system. 3.6. Evaluation of NaH/tert-amyl alcohol/ Ni(CH3COO)2 as an industrial catalyst In the industrial methanol production process, copper-based catalysts (CuO/ZnO/Al2O3 or CuO/ZnO/
Fig. 8. Recovery of methanol productivity by addition of alkoxide. Reaction conditions: temperature373±433 K; initial pressure5.0 MPa. Solvent: triglyme. Methanol yield represents increments of methanol before and after each run.
S. Ohyama / Applied Catalysis A: General 180 (1999) 217±225
Cr2O3) are employed under the conditions of temperatures of 503±573 K, pressures of 5±20 MPa, and space velocity of 10 000±40 000 hÿ1 [24]. The STY in the ICI process, a typical conventional methanol process, is reported to be 0.5±0.77 kg lÿ1 hÿ1 [24]. Since the NaH/tert-amyl alcohol/Ni(CH3COO)2 catalysts displayed a higher STY (0.95 kg lÿ1 hÿ1) under mild conditions than conventional methanol catalysts, it should be possible, using this catalyst, to eliminate recycling facilities for unconverted gas, which can reduce the production cost of methanol. However, the catalyst showed no catalytic activity towards feed gases containing a small amount of CO2. Therefore, in order to apply the catalytic systems to industrial processes, especially IGCC plants, CO2 in the feed gas should be thoroughly removed. 4. Conclusions The low-temperature methanol synthesis in the liquid phase was carried out using the catalyst, NaH/tert-amyl alcohol/Ni(CH3COO)2. It was highly effective for the selective production of methanol at temperatures lower than the operating temperatures in industrial methanol processes. The maximum STY of 0.95 kg MeOH lÿ1 hÿ1, higher than that in the conventional methanol process, was obtained at 433 K and 5.0 MPa. The addition of methanol to the catalyst did not signi®cantly affect the product yields. However, it shortened the induction period observed during the run, suggesting that methanol promotes the formation of catalytically active species and/or a certain amount of methanol is required to run a reaction cycle. The catalysts were deactivated in a short period, which leads to a serious problem in the application of the catalytic systems to the industrial processes. It was con®rmed that the consumption of alkoxides is a major factor for the deactivation of the catalyst system.
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Acknowledgements The author expresses his sincere thanks to Professor N. Takezawa at Hokkaido University for his kind revision of the manuscript. References [1] A.A. Deane, D.A. Huber, J. DeRosa, Coproduction of methanol and electricity, EPRI Report AP-3749, 1984. [2] E. Supp, Hydrocarbon Processing, March 1981, p. 71; July 1984, p. 34C. [3] D. Mahajan, R.S. Sapienza, W.A. Slegeir, T.E. O'Hare, US Patent 4 992 480 (1991). [4] H. Nakamura, K. Saeki, M. Tanaka, Jpn. Patent 88/51129 (1988). [5] M. Marchionna, M. Lami, F. Ancillotti, R. Ricci, US Patent 5 032 618 (1991). [6] S.T. Sie, E. Drent, W.W. Jager, US Patent 4 812 433 (1989). [7] M. Marchionna, L. Basini, A. Aragno, M. Lami, F. Ancillotti, J. Mol. Catal. 75 (1992) 147. [8] O.T. Onsager, Jpn. Patent 87/500867 (1987); 91/12048 (1991). Ê . Sùrum, O.T. Onsager, Eight Int. Congr. Catal. 2 (1984) 233. [9] P.A [10] D.M. Monti, M.A. Kohler, M.S. Wainwright, D.L. Trimm, N.W. Cant, Appl. Catal. A 22 (1986) 123. [11] V.M. Palekar, H. Jung, J.W. Tierney, I. Wender, Appl. Catal. A 102 (1993) 13. [12] V.M. Palekar, J.W. Tierney, I. Wender, Appl. Catal. A 103 (1993) 105. [13] R.J. Gormley, V.U.S. Rao, Y. Soong, E. Micheli, Appl. Catal. A 87 (1992) 81. [14] D.L. Trimm, M.S. Wainwright, Catal. Today 6 (1990) 261. [15] R.S. Sapienza, W.A. Slegeir, T.E. O'Hare, D. Mahajan, US Patent 4 614 749 (1986); 4 619 946 (1986); 4 623 634 (1986). [16] S. Ohyama, React. Kinet. Catal. Lett. 61 (1997) 331. [17] J.A. Christiansen, US Patent 1 302 011 (1919). [18] D.L. Trimm, M.S. Wainwright, Catal. Today 6 (1990) 261. [19] S.P. Tonner, D.L. Trimm, M.S. Wainwright, N.W. Cant, J. Mol. Catal. 18 (1983) 215. [20] R.J. Gormley, A.M. Giusti, S. Rossini, V.U.S. Rao, Ninth Int. Congr. Catal. 2 (1988) 553. [21] Z. Liu, J.W. Tierney, Y.T. Shah, I. Wender, Fuel Process. Tech. 18 (1988) 185. [22] Z. Liu, J.W. Tierney, Y.T. Shah, I. Wender, Fuel Process. Tech. 23 (1989) 149. [23] S.J. Choi, J.S. Lee, Y.G. Kim, J. Mol. Catal. 85 (1993) L109. [24] R.G. Hermann, K. Klier, G.W. Simmons, B.P. Finn, J.B. Bulko, T.P. Kobylinski, J. Catal. 56 (1979) 407.