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Anhydrous formaldehyde by sodium catalysis
Applied Catalysis A: General 213 (2001) 203–215
Anhydrous formaldehyde by sodium catalysis Steffen Ruf∗ , Alexander May, Gerhard Emig1 Lehrstuhl für Technische Chemie I, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany Received 23 August 2000; received in revised form 5 December 2000; accepted 6 December 2000
Abstract Sodium containing catalysts are convenient catalysts for the pure dehydrogenation of methanol to anhydrous formaldehyde. Elemental sodium catalyses the reaction in a homogeneous vapour phase reaction at mild reaction conditions with high yields. The variation of several process parameters revealed the potential of the new way supplying the catalyst. Additional experiments gave insights into the reaction mechanism showing sodium to act as chain carrier in the free radical reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Formaldehyde; Dehydrogenation; Thermodynamics; Sodium; Sodium methanolate
1. Introduction Anhydrous formaldehyde is required for the production of polyoxymethylenes. It is commonly produced by a two-step process forming hydrous formaldehyde solutions by the silver catalyst process and the subsequent removal of water. Unfortunately, the latter step is expensive and determines the formaldehyde price. An alternative one-step process yielding anhydrous formaldehyde directly, seems desirable. Methanol can be converted to anhydrous formaldehyde and hydrogen without catalyst at high temperatures. Realising residence times shorter than 1 s without any catalyst in a tube reactor, at 973 K reaction temperature no methanol conversion takes place. The maximum formaldehyde yield of less than 20% is found at 1173 K where the methanol conver∗ Corresponding author. Present address: Roche AG, Building 340, 4334 Sisseln, Switzerland. E-mail addresses: [email protected] (S. Ruf), [email protected] (G. Emig). 1 Co-corresponding author.
sion reaches approx. 45% with 43% formaldehyde selectivity. The low selectivity can be explained by the fact — which was revealed by kinetic modelling with a reaction scheme of 13 elementary reactions [8] — that hydrogen atoms act as chain carriers. Hydrogen can react with both methanol (forming formaldehyde) and formaldehyde (forming the main by-product carbon monoxide). The higher the temperature the larger the methanol conversion and, unfortunately, the lower the formaldehyde selectivity. Therefore, the poor yield demands the application of a catalyst. Several catalysts were examined but only sodium containing catalysts showed remarkable activity for the endothermic dehydrogenation of methanol to anhydrous formaldehyde. Former investigations revealed especially sodium aluminate and sodium carbonate as appropriate catalysts [1,2]. Since the separation of methanol and formaldehyde is as difficult as the removal of water from the aqueous formaldehyde solution, total methanol conversion must be realised for economic reasons. Otherwise the benefit of saving the costly separation of formalde-
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hyde and water is lost and the one-step process has no chance of being established. Considering this fact, the aluminate catalyst is preferred because the formaldehyde selectivity exceeds 60% at total methanol conversion while the selectivity of the carbonate catalysts decline to zero under these conditions. Unfortunately, the aluminate catalyst requires an 1173 K reaction temperature. The main by-product is carbon monoxide for both catalysts. Recent investigations elucidate the mode of catalyst action. Sodium aluminate is decomposed at elevated temperatures (above 1173 K) emitting sodium in elemental form into the vapour phase [3]. An additional experiment revealed sodium as active species within the vapour phase. Methanol can also be converted to anhydrous formaldehyde by the action of evaporated elemental sodium at a 973 K reaction temperature in a homogeneously catalysed vapour phase reaction. The formaldehyde selectivity can be increased to 70% at total methanol conversion [4]. The necessary high temperature of 1173 K for converting methanol with sodium aluminate is only required for the catalyst decomposition. The experiments with evaporated sodium prove that for methanol conversion, a temperature of 973 K is sufficient using an appropriate catalyst supply. At 1173 K, the less selective uncatalysed reaction is superimposed to the catalysed reaction lowering formaldehyde selectivity. Since the evaluation of the reaction mechanism is difficult using spectroscopy techniques, several reaction engineering experiments were performed to elucidate the reaction mechanism while simultaneously finding a more simple way of dosing the catalyst.
2. Thermodynamics and catalysts The direct production of anhydrous formaldehyde might be an interesting alternative to the industrially applied silver catalyst process. Thermodynamic calculations show that formaldehyde is not stable at higher temperatures decomposing into hydrogen and carbon monoxide. For achieving high formaldehyde yields an appropriate catalyst is essential. After discussing the thermodynamics of the system, a short overview of more or less active catalysts which were discussed in literature during the last 40 years is given.
Table 1 Possible reactions in the system with the components methanol, formaldehyde, carbon monoxide, hydrogen, methane, carbon dioxide and coke Reaction
HR,973 K (kJ/mol)
No.
CH3 OHCH2 O + H2 CH2 OCO + H2 CH2 OC + H2 O 2COC + CO2 C + 2H2 CH4 CH3 OHCO + 2H2 2CH3 OHCH4 + H2 O + CH2 O CH3 OHC + H2 O + H2 2CH2 OCO + CH3 OH 2CH2 OCH4 + CO2 CO + H2 C + H2 O CO + H2 OCO2 + H2
+92.7 +12.4 −123.4 −170.5 −89.6 −105.1 −27.6 −30.7 −80.3 −235.7 −135.8 −34.7
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
2.1. Thermodynamic calculations The oxidation of methanol to formaldehyde and water is a strongly exothermic process. It can be considered as a set of two consecutive reactions: firstly, the endothermic dehydrogenatiom reaction of methanol to formaldehyde (HR,973 K = 92.7 kJ/mol) and secondly, the strongly exothermic hydrogen–oxygen reaction (HR,973 K = −247.8 kJ/mol) resulting in a high overall reaction enthalpy of −155.1 kJ/mol. In case of the pure dehydrogenation of methanol (in absence of oxygen), several side reactions have to be considered beside the main reaction, the decomposition of methanol to formaldehyde and hydrogen (Table 1). These side reactions explain the formation of the by-products carbon monoxide, carbon dioxide, methane, and coke. Having a look at the distribution of observed experimentally determined products, the decomposition of formaldehyde to carbon monoxide and hydrogen appears as a special reaction of importance. The reaction of methanol to formaldehyde (HR,973 K = 92.7 kJ/mol) is more endothermic than the decomposition of formaldehyde. All other decomposition reactions of formaldehyde are exothermic. The calculated thermodynamic constants in the temperature range from 373 to 1273 K are between 103 and 1043 , showing clearly that the equilibrium is on the product side. Apart from the main reaction, only reaction (7) in Table 1 can explain the formation of formaldehyde, however, yielding an aqueous product.
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Fig. 1. Equilibrium composition in the extended system of important components.
Calculations in the comprehensive system (methanol, formaldehyde, carbon monoxide, carbon dioxide, carbon, methane and hydrogen) using Aspen Plus® have been done by giving these components defining a feed stream of 100 kmol/h of methanol. Fig. 1 shows the molar flows of the single components. From 400 to 1300 K almost all methanol is decomposed. Only in the range of 900 K traces of formaldehyde are predicted. At lower temperatures, the formation of water and methane is dominant, while at higher temperatures the formation of hydrogen and carbon monoxide is preferred. Thermodynamic calculations predict the formation of formaldehyde only in traces, while methanol should be decomposed almost completely. Carbon monoxide, methane and hydrogen should be the products to be formed preferentially. In contrast to these calculations, the experimentally detected main species are methanol, formaldehyde, carbon monoxide and hydrogen. Both methane and carbon dioxide are detected only in traces. Evidently, under technical conditions some reactions do not proceed under thermodynamic but under kinetic control suppressing on the one hand, the formation of the thermodynamically predicted products and enabling on the other hand, the production of formaldehyde. Therefore, the calculation for the reduced system has been repeated twice, based first on reactions (1)
and (2) and second merely on reaction (1). Fig. 2 depicts the degree of methanol conversion and the formaldehyde yield. Above 470 K, the methanol conversion is total, while only small traces of formaldehyde (max. 60 ppm) are predicted. Neglecting reaction (2) leads to a different equilibrium composition. At 673 K, only 20% of methanol was converted, at 873 K already 87% and at 1073 K more than 99%. Considering carbon monoxide in the thermodynamic calculations shows methanol to be decomposed completely at temperatures higher than 473 K and formaldehyde not to be stable decomposing almost totally to carbon monoxide and hydrogen. Restricting the species on methanol, formaldehyde and hydrogen shows methanol to be stable up to higher temperatures. A successful technical realisation requires on the one hand an appropriate catalyst, which is highly active for the main reaction without influencing the side reactions. On the other hand, the reaction must proceed under kinetic control before the thermodynamic equilibrium is established [5]. 2.2. Catalysts for the pure dehydrogenation of methanol Catalysts for the pure dehydrogenation of methanol have been looked for since 1960s. First investiga-
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Fig. 2. Methanol conversion and formaldehyde yield according to equilibrium calculations.
tions used silver and silver–copper catalysts, which require the presence of oxygen for higher conversions. In absence of oxygen, the dehydrogenation reaction proceeds very slowly. Attempts of admixing oxidic substances (e.g. SiO2 , mica, copper oxide) were not successful. Adding selenium, tellurium or sulphur were not satisfying because of the volatility of these components. Oxidic catalysts were quickly reduced by methanol, while metallic catalysts require elevated temperatures for high methanol conversions. Typical by-products are carbon monoxide, methane, formic acid, methyl ester, carbon dioxide and water. Metallic or oxidic zinc used in molten form or as alloy, is deactivating fast. Zinc oxide is more active than metallic zinc; unfortunately the oxide is reduced by methanol. Other transition metals showed also no industrially acceptable results [1,6]. Sodium containing substances, e.g. Na2 CO3 , NaMoO3 or NaAlO2 , have been found to be promising catalysts for the pure dehydrogenation. Elemental sodium seems to be the active species. With sodium aluminate as catalyst, formaldehyde yields up to 65% were realised in a catalytic tube wall reactor at 1173 K [2]. Rising the methanol fraction in the feed above 10%, deactivation is observed. Sodium carbonate has the same disadvantage. Additionally, the formaldehyde selectivity drops to 50%, and zero when the degree of methanol conversion is 75 and 100%, respectively.
3. Experimental set-up The reaction was carried out in a corundum tube reactor (12 mm × 8 mm × 1000 mm) with a reaction zone of 450 mm centred in the axis of the tube. The products were analysed by on line gas chromatography. Details on reactor and on analysis are given in [7]. For catalysing the pure dehydrogenation of methanol to formaldehyde, small amounts of sodium as catalyst were supplied by leading a nitrogen stream over a flat crucible loaded with elemental sodium [7]. Since sodium has a strong tendency to passivate by the formation of sodium oxide and sodium hydroxide due to small traces of oxygen and water in the nitrogen stream, the crucible as sodium reservoir was not practicable for systematic experimental investigations. The adjustment of continuous and constant sodium flow in the reactor was not possible. Therefore, a new way of supplying sodium that was not effected by passivation, had to be found. A tank containing sodium made a continuous and reproducible dosing of the catalyst sodium possible (Fig. 3). Sodium is supplied in a stainless steel case that is inserted again in a cylindrical tank of stainless steel. The heating of the sodium tank to its maximum temperature of 723 K is performed by a heating collar. Nitrogen was fed by a long dip tube leading into the liquid sodium and by a second shorter tube
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Fig. 3. Tank for evaporating sodium.
leading into the vapour phase above the sodium melt. The first nitrogen stream provided the mixing and the aeration of the liquid sodium — both prevents the passivation of sodium, while the latter ensured an inert atmosphere above the sodium before heating. The vaporised sodium was transported to the reaction zone by the nitrogen stream sent through the sodium tank. The reactant methanol was diluted with a second nitrogen stream. The mixture with 10–30 mol% methanol was fed directly in the reaction zone through a smaller separate tube (6 mm × 4 mm) in order to prevent a contact of the vaporised sodium and methanol before the reaction zone. For the sodium catalysed dehydrogenation of methanol to formaldehyde, the influence of the residence time and both reactor and sodium tank temperature was investigated. The residence time was varied only by changing the volume flow of the methanol/nitrogen mixture. The nitrogen flow through the sodium tank and, thereby, the sodium flow was kept constant. So the ratio of sodium to methanol decreased with increasing flow velocity. The amount of sodium in the reaction zone was varied only by changing the temperature of the
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tank. The gas velocity also influenced the laminar temperature profile in the reaction zone. At high flow velocities, the volume in which the gas reached wall temperature became smaller. Consequently, increasing the gas flow resulted not only in shorter residence times but also in smaller volumes with maximum temperature. The exact determination of the amount of sodium consumed was not possible with the two different sources described above. On the one hand, providing sodium in the crucible suffered from forming sodium hydroxide and/or sodium oxide. Both substances adulterate the weighing, making an exact calculation of sodium consumption impossible. On the other hand, an on line weighing is impossible because of the need to fix the tank strongly; weighing the case before and after the experiment causes the same problems as using the crucible. For this purpose, a crucible containing sodium was placed in the tube between the sodium tank and the tee. The tank was used to purify the nitrogen at 473 K from oxygen and water. At 473 K tank temperature, the partial pressure above the melt is too low to transport sodium to the reaction zone, while the liquid sodium purifies the nitrogen stream by removing moisture and oxygen. The purified nitrogen transports the vaporised sodium of the crucible to the reactor. Passivation of the sodium in the crucible was not observed so that the consumption of sodium could be exactly determined by weighing out for calculating the sodium flow. To obtain more information on the reaction mechanism of the sodium catalysed reaction, two additional experiments were performed. The intention was to get an indication on the intermediates formed during the reaction between methanol and sodium in the gas phase. At first, methanol as reactant was substituted by methane and contacted with vaporised sodium in the reaction zone up to a reactor temperature of 1223 K, secondly elemental sodium as catalyst was replaced by sodium methanolate. For the latter a 30 wt.% sodium methanolate solution (Merck) was diluted with the reactant methanol to 0.05%. The mixture was supplied to the reaction zone by a special designed dosing apparatus (Fig. 4). Attaching the dosing equipment on the top of the corundum tube and changing the direction of the gas flow, the drops were accelerated by both the gravity force and the gas flow.
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4. Results For the dehydrogenation of methanol to formaldehyde with elemental sodium as vaporised catalyst the influence of the operating parameters reactor temperature (923–1123 K), temperature of the sodium tank (573–723 K), mole fraction of methanol in the feed (10–30 mol%) and residence time (gas velocity was varied between 0.75 and 1.5 m/s) on the degree of methanol conversion, selectivity and yield of formaldehyde was investigated. The reaction engineering parameters such as degree of methanol conversion, selectivity and yield of formaldehyde were calculated as follows: degree of methanol conversion: XCH3 OH =
n˙ CH3 OH,in − n˙ CH3 OH,out 100% n˙ CH3 OH,in
selectivity to formaldehyde: SCH2 O =
n˙ CH2 O,out 100% n˙ CH3 OH,in − n˙ CH3 OH,out
yield of formaldehyde: YCH2 O =
n˙ CH2 O,out 100% n˙ CH3 OH,in
Fig. 4. Experimental set-up for dosing diluted sodium methanolate.
4.1. Influence of operating parameters
The methanolic sodium methanolate solution was fed from the top of the apparatus through a tee in a capillary tube (3 mm × 2 mm) that leads into the reaction zone. A thermocouple in the centre of the capillary allowed the measurement of the temperature at the drip off-point. Temperatures over 330 K indicated crystallisation of sodium methanolate and so the danger of clogging the capillary tube. Through a tube (6 mm × 4 mm) also leading in the reaction zone as well and surrounding the capillary, nitrogen was sent (line 1). A second nitrogen stream was fed into the annular gap between the tube surrounding the capillary and the corundum tube of the reactor (line 2). The double thermal isolation avoided the vaporisation of methanol to prevent the crystallisation of sodium methanolate.
When the temperature of the sodium tank exceeds 523 K, the partial pressure of sodium and the amount of sodium transported to the reaction zone, respectively, are high enough to catalyse the reaction of methanol to formaldehyde. Figs. 5 and 6 show the influence of the sodium tank temperature at a constant flow velocity of 1 m/s for the mole fractions of 10–30% methanol in the feed and for different reactor temperatures. The degree of methanol conversion rises with increasing temperature of the sodium tank and with increasing reactor temperature while the selectivity to formaldehyde decreases in both cases. Carbon monoxide and hydrogen are formed as by-products. Since the decrease of the selectivity to formaldehyde is much smaller than the increase of the degree of conversion, the yield of formaldehyde increases with rising temperature of the sodium tank and with rising temperature of the reactor. With 10–30% methanol in the feed stream at 1023 K
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Fig. 5. Variation of the temperature of the sodium tank (10% methanol in the feed).
Fig. 6. variation of the temperature of the sodium tank (30% methanol in the feed).
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Fig. 7. Variation of the gas flow velocity (10% methanol in the feed; temperature of the sodium tank 623 K).
reactor temperature and at 723 K for the sodium tank, the yield of formaldehyde reaches a maximum in each case of 72 and 50%, respectively. The influence of the gas velocity and the residence time, respectively, is depicted in Fig. 7. Lowering the gas flow leads to an increase of the degree of methanol conversion while the selectivity to formaldehyde is lowered only slightly. For 973 K reactor temperature, the reduction of the gas velocity from 1.5 to 1.0 m/s results in an increase of the degree of conversion from 15 to 25%, a further reduction to 0.75 m/s nearly doubles the degree of conversion. For 1073 K reactor temperature, a decrease of the gas flow from 1.5 to 1.0 m/s results in doubling of the degree of conversion. Similar to the reactor temperature and the temperature of the sodium tank the residence time has only little influence on the selectivity to formaldehyde. Therefore, a higher yield of formaldehyde can be obtained by lowering the gas velocity. The variation of the gas velocity affected the molar ratio of sodium to methanol and the laminar temperature profile in the reactor. Lower gas velocities resulted in increasing sodium to methanol ratios and larger volumes in the reaction zone where the maximum temperature was reached; both effects promoted the increase of the degree of conversion and the yield, additionally.
The catalytic activity of sodium is expressed by the turnover number (TON). The turnover number represents the molar ratio of the consumption of methanol to sodium: TON =
n˙ methanol n˙ sodium
(13)
Values between 500 and 700 for the turnover number were determined, which means that 500–700 methanol molecules were converted by one sodium atom [4]. Since these turnover numbers are probably not accurate due to passivation of the sodium, the sodium tank was calibrated to determine more precise values. For clearly defined reaction conditions (reactor temperature 1073 K; gas velocity 1.0 m/s, 10% mole fraction of methanol in the feed) the temperature of the crucible was adjusted the way that a certain degree of methanol conversion (e.g. 25%) was obtained. Under these conditions, the sodium flow from the crucible into the reaction zone is the same as with the sodium tank for the same degree of conversion. Table 2 shows the experimentally determined sodium flows for 25–40% degree of methanol conversion and the corresponding calculated turnover numbers. At a gas velocity of 1 m/s and 10 mol% methanol in the feed, a degree of methanol conversion of 25% corresponds to a converted methanol molar flow of 202.0 mmol/h
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Table 2 Amount of consumed sodium and turnover numbers Temperature (K) Reactor
Sodium tank
973 973
623 673
Degree of conversion (%)
Sodium flow (mg/h)
Turnover number
25 40
9 15
457 466
and a sodium flow of about 9 mg/h (0.396 mmol/h). Dividing the converted methanol molar flow through the sodium mole flow results in a turnover number of approximately 500. For these calculations it was assumed that for sodium to react as single atoms not as small clusters. Whether clusters or atoms are catalytically active could not be determined at this point (experiments on the reaction mechanism indicate atoms to be active see Chapter 5.1). Considering the accuracy of the measurements, the turnover numbers were equal for a temperature of 623–673 K of the sodium tank for the above mentioned reaction conditions. The more sodium is supplied to the reaction zone, the higher the degree of conversion whereby the dependency on the provided amount of sodium is linear in a certain range for the degree of conversion. 4.2. Investigations of the reaction mechanism Elucidating the reaction mechanism of the sodium catalysed reaction is hardly possible by only analysing the composition of the product stream as many intermediates are not accessible and cannot be detected by gas chromatography. Spectroscopic methods like laser-induced fluorescence or Raman-spectroscopy are in-situ analysis techniques that enable the identification of intermediates as radicals or atoms in the gas phase. Unfortunately, in the case of methanol dehydrogenation, these methods are limited to ideal conditions adapted on each species. For a chemical reaction, a mixture of radicals is obtained which leads to interferences of the specific absorption bands. In addition, very low concentrations of the radicals complicate the identification seriously [9]. Hence two additional reaction engineering experiments were performed in order to reveal the reaction mechanism of the sodium catalysed dehydrogenation of methanol.
First, methanol was substituted by methane, secondly, sodium methanolate was supplied instead of sodium. The experiments should reveal which product is formed during the reaction of methanol and sodium clarifying the role of sodium in the catalytic process. The feed of methane should reveal whether sodium reacts with the hydrogen atoms of methanol to sodium hydride (NaH) and hydroxy-methyl radicals (CH2 OH• ) as assumed by Sauer and Emig [2] or whether sodium and methanol form sodium methanolate as in the liquid phase. Even at 1223 K reactor temperature, no conversion of methane was observed. Since the hydrogen atoms of methane are inert towards elemental sodium a reaction of sodium with the hydrogen atoms of the methyl group in methanol is also unlikely. We conclude that the formation of sodium hydride and hydroxy-methyl is improbable. As the reaction of sodium and methanol to sodium methanolate occurs also in the liquid phase, the same reaction is likely to take place in the gas phase. Supplying methane instead of methanol together with sodium must not be considered as a final proof for the formation of sodium methanolate but as a strong indication. The substitution of sodium by sodium methanolate in methanolic solution should reveal whether a sodium compound, which is supplied homogeneously, with an oxidation state of sodium different from zero, catalyses the dehydrogenation. Further should be examined whether sodium methanolate catalyses the dehydrogenation as well as elemental sodium or whether sodium methanolate is inert. Sodium methanolate was found to catalyse the pure dehydrogenation of methanol to formaldehyde. The turnover numbers obtained with sodium methanolate gave values between 450 and 800 at 973 K which is in the same range as for sodium. Under comparable reaction conditions, similar degrees of conversion of methanol and selectivities to formaldehyde as for
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elemental sodium were achieved. This indicates that sodium methanolate decomposes to elemental sodium and methoxy radicals (• OCH3 ). An alternative for dosing methanolic sodium methanolate represents the feed of dissolved sodium hydroxide in methanol [10]. Methanol and sodium hydroxide are in equilibrium with sodium methanolate and water. At ambient conditions and high methanol excess, the equilibrium is completely on the product side. Increasing the temperature, the equilibrium shifts towards the reactants: at 908 K only 1% of sodium hydroxide form methanolate. Under comparable conditions, similar degrees of methanol conversion and selectivities to formaldehyde as with sodium methanolate were achieved. 5. Discussion 5.1. Aspects of reaction mechanism Vaporised elemental sodium catalyses homogeneously the pure dehydrogenation of methanol to formaldehyde in the gas phase. The experiments with methane instead of methanol and with sodium methanolate instead of sodium gave insights into the reaction mechanism. Finding no reaction between methane and sodium even at high temperatures (1223 K) supported the assumption that sodium reacts with the hydroxyl-group of methanol and not with the hydrogen atoms of the methyl-group (14). CH3 OH + Na CH2 OH• + NaH
(14)
During the reaction of sodium with methanol to sodium methanolate hydrogen atoms are generated as by-product. CH3 OH + Na NaOCH3 + H•
(15)
Hydrogen atoms were found to act as chain carrier for the uncatalysed homogeneous dehydrogenation of methanol [4]. Hence, two ways how sodium could act are possible: first, it could trigger the uncatalysed reaction by producing hydrogen atoms, second a cyclic formation and decomposition of sodium methanolate could occur. The first reaction pathway — sodium as generator for hydrogen atoms, is unlikely since kinetic simulations showed that hydrogen atoms do not react
Fig. 8. Reaction cycle of the sodium catalysed dehydrogenation of methanol.
with methanol at temperatures below 973 K even at high concentrations of hydrogen atoms [8]. At these conditions the recombination of hydrogen atoms to molecular hydrogen is much faster than the chain propagation. In addition, with hydrogen atoms as chain carrier, the high selectivities to formaldehyde (S > 75%) for the sodium catalysed dehydrogenation would be inexplicable since typical values for the uncatalysed reaction are smaller. The supply of methanolic sodium methanolate solution showed clearly the catalytic activity and the decomposition of sodium methanolate under reaction conditions. This result proves that the role of sodium is not only of generating hydrogen-atoms, otherwise sodium methanolate would be inert and there would be no methanol conversion. Achieving nearly the same turnover numbers with sodium methanolate as with elemental sodium indicates atomic sodium to be the active component not sodium clusters. If methanol would be converted on sodium clusters, the turnover numbers of elemental sodium would be smaller than for sodium methanolate. Sodium methanolate decomposes to atomic sodium and methoxy radicals. The sodium set free reacts again with methanol to sodium methanolate and closes the catalytic cycle (Fig. 8). The catalytic cycle can be initiated by supplying sodium as catalyst or sodium methanolate as intermediate. The methoxy radicals decompose to the desired product formaldehyde: • OCH
3
CH2 O + H•
(16)
From reaction (15) and (16) hydrogen atoms are formed. They can react with methanol, the
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Fig. 9. Calculated turnover numbers (gas velocity 1 m/s).
methoxy-radical and unfortunately with formaldehyde to carbon monoxide: CH2 O + H• CHO• + H2
(17)
CHO• + H• CO + H•
(18)
2H• → H2
(19)
The appearance of hydrogen atoms and the reaction with formaldehyde gives an explanation for selectivities to formaldehyde lower than 100%. 5.2. Reaction engineering aspects The calibration of the sodium tank allows the calculation of turnover numbers for other reaction conditions varying the reactor temperature or the mole fraction of methanol in the feed. The dependency of the turnover number on the reactor temperature and the methanol content in the feed is shown in Fig. 9. Rising the reactor temperature from 923 to 1123 K, the turnover number increases almost eight times from 211 to 1770 (gas flow 1 m/s; 10 mol% in the feed), assuming complete conversion of methanol, the extrapolated turnover number yields in 1880. An increasing mole fraction of methanol in the feed also results in rising turnover numbers. A dependency on the temperature of the sodium tank cannot be seen from Fig. 9. For 623 and 673 K, the increase of the turnover number with the mole fraction of methanol is nearly identical.
Tripling the mole fraction of methanol, the turnover numbers increase from 456 to 814 (623 K) and from 466 to 797 (673 K), respectively. Since the selectivity to formaldehyde is not influenced disadvantageously by high mole fractions of methanol, it is desirable for reasons of an optimum catalyst efficiency to aim at high mole fractions of methanol in the feed. Furthermore, the reduced content of inert gas results in saving costs for heating and cooling inert gas without effecting the efficiency of the process. The high selectivities to formaldehyde over 70% even at high degrees of methanol conversion are remarkable. These high selectivities are based on the selective reaction of sodium with methanol while sodium is not likely to react with formaldehyde. Beside the product formaldehyde, the reaction between methanol and sodium (15) leads to the formation of hydrogen atoms. In contrast to sodium atoms, hydrogen atoms react with both methanol and formaldehyde. The first reaction produces formaldehyde again while in the latter carbon monoxide and hydrogen are formed (17–19). With increasing degree of methanol conversion, the selectivity to formaldehyde drops. The partial pressure of methanol falls for increasing degrees of methanol conversion, accordingly the partial pressure of the product formaldehyde increases in parallel. Therefore, the consecutive reaction between formaldehyde and hydrogen atoms becomes faster than the formation of formaldehyde by methanol since the probability for a collision of hydrogen atoms
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Fig. 10. Yield vs. conversion — classification for process parameters.
and formaldehyde molecules grows with increasing methanol conversion. In consequence, the selectivity to formaldehyde decreases. Fig. 10 shows the yield of formaldehyde plotted versus the degree of methanol conversion, classified for the four variable process parameters reactor temperature, gas velocity, mole fraction of methanol in the feed and temperature of the sodium tank. The diagonal line represents a formaldehyde selectivity of 100%, the dotted lines 85 and 70%, respectively. As it can be seen, the way of increasing the degree of conversion does not affect the selectivity. None of the four operating parameters change seems to have a predominating
influence on the selectivity. The decrease of the selectivity is only a phenomenon of increasing degrees of methanol conversion. For degrees of conversion lower than 40%, the selectivity exceeds 85% in every case, also for higher degrees of conversion, the selectivity does not fall below 70%.
6. Conclusions As thermodynamic calculations turned out, anhydrous formaldehyde is not stable over a wide temperature range. Working under kinetic instead of
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thermodynamic control enables the production of formaldehyde. Without catalyst, only poor yields of approx. 20% can be achieved. The application of a sodium containing species or even elemental sodium as catalysts increases the formaldehyde yield to 70%. Vaporised elemental sodium catalyses homogeneously the pure dehydrogenation of methanol to formaldehyde with a high selectivity. The influence of reaction temperature, mole fraction of methanol, residence time and amount of sodium was investigated. With increasing degree of methanol conversion only a slight decrease of the formaldehyde selectivity was observed. Even at high degrees of methanol conversion selectivities over 70% were obtained. Experiments on the reaction mechanism indicated that methanol and elemental sodium form sodium methanolate, which decomposes under reaction conditions to atomic sodium and via methoxy radicals to formaldehyde and hydrogen atoms. The sodium set free reacts again with methanol and closes the catalytic cycle. The catalytic cycle can either be initiated by supplying sodium or the intermediate sodium methanolate. The hydrogen atoms react in contrast to sodium with both methanol and formaldehyde. The reaction with the latter leads to the formation of carbon monoxide as by-product. Recovering the catalyst is an interesting question and a major problem to be solved for an industrial production. To give an answer, further investigations
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have to be carried out. Especially the nature of the sodium species, which we actually suppose to be sodium methanolate, must be examined in subsequent experiments. Then an appropriate recovering can be developed. But this requires some efforts not be underestimated, an interesting task for the future we have in mind.
Acknowledgements The work was supported by the Bundesministerium für Bildung und Forschung (BMBF) under the number 03D0012B9. References [1] S. Su, P. Zaza, A. Renken, Chem. Eng. Technol. 17 (1994) 34. [2] J. Sauer, G. Emig, Chem. Eng. Technol. 18 (1995) 284. [3] M. Bender, Thesis, VDI Verlag, 1998. [4] St. Ruf, G. Emig, J. Mol. Catal. A 146 (2000) 271. [5] J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, 1997. [6] V.I. Atroshchenko, I.P. Kushnarenko, Int. Chem. Eng. 4 (1964) 581. [7] St. Ruf, G. Emig, Appl. Catal. A 161 (1997) L19. [8] St. Ruf, Die Natrium-katalysierte Dehydrierung von Methanol zu wasserfreiem Formaldehyd, Shaker Verlag, Aachen, 1998. [9] Department of Technical Thermodynamics, University of Erlangen-Nuremberg, Internal Communication, 1997. [10] DE 198,22,598 A1 (1999).
Anhydrous formaldehyde by sodium catalysis Steffen Ruf∗ , Alexander May, Gerhard Emig1 Lehrstuhl für Technische Chemie I, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany Received 23 August 2000; received in revised form 5 December 2000; accepted 6 December 2000
Abstract Sodium containing catalysts are convenient catalysts for the pure dehydrogenation of methanol to anhydrous formaldehyde. Elemental sodium catalyses the reaction in a homogeneous vapour phase reaction at mild reaction conditions with high yields. The variation of several process parameters revealed the potential of the new way supplying the catalyst. Additional experiments gave insights into the reaction mechanism showing sodium to act as chain carrier in the free radical reaction. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Formaldehyde; Dehydrogenation; Thermodynamics; Sodium; Sodium methanolate
1. Introduction Anhydrous formaldehyde is required for the production of polyoxymethylenes. It is commonly produced by a two-step process forming hydrous formaldehyde solutions by the silver catalyst process and the subsequent removal of water. Unfortunately, the latter step is expensive and determines the formaldehyde price. An alternative one-step process yielding anhydrous formaldehyde directly, seems desirable. Methanol can be converted to anhydrous formaldehyde and hydrogen without catalyst at high temperatures. Realising residence times shorter than 1 s without any catalyst in a tube reactor, at 973 K reaction temperature no methanol conversion takes place. The maximum formaldehyde yield of less than 20% is found at 1173 K where the methanol conver∗ Corresponding author. Present address: Roche AG, Building 340, 4334 Sisseln, Switzerland. E-mail addresses: [email protected] (S. Ruf), [email protected] (G. Emig). 1 Co-corresponding author.
sion reaches approx. 45% with 43% formaldehyde selectivity. The low selectivity can be explained by the fact — which was revealed by kinetic modelling with a reaction scheme of 13 elementary reactions [8] — that hydrogen atoms act as chain carriers. Hydrogen can react with both methanol (forming formaldehyde) and formaldehyde (forming the main by-product carbon monoxide). The higher the temperature the larger the methanol conversion and, unfortunately, the lower the formaldehyde selectivity. Therefore, the poor yield demands the application of a catalyst. Several catalysts were examined but only sodium containing catalysts showed remarkable activity for the endothermic dehydrogenation of methanol to anhydrous formaldehyde. Former investigations revealed especially sodium aluminate and sodium carbonate as appropriate catalysts [1,2]. Since the separation of methanol and formaldehyde is as difficult as the removal of water from the aqueous formaldehyde solution, total methanol conversion must be realised for economic reasons. Otherwise the benefit of saving the costly separation of formalde-
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hyde and water is lost and the one-step process has no chance of being established. Considering this fact, the aluminate catalyst is preferred because the formaldehyde selectivity exceeds 60% at total methanol conversion while the selectivity of the carbonate catalysts decline to zero under these conditions. Unfortunately, the aluminate catalyst requires an 1173 K reaction temperature. The main by-product is carbon monoxide for both catalysts. Recent investigations elucidate the mode of catalyst action. Sodium aluminate is decomposed at elevated temperatures (above 1173 K) emitting sodium in elemental form into the vapour phase [3]. An additional experiment revealed sodium as active species within the vapour phase. Methanol can also be converted to anhydrous formaldehyde by the action of evaporated elemental sodium at a 973 K reaction temperature in a homogeneously catalysed vapour phase reaction. The formaldehyde selectivity can be increased to 70% at total methanol conversion [4]. The necessary high temperature of 1173 K for converting methanol with sodium aluminate is only required for the catalyst decomposition. The experiments with evaporated sodium prove that for methanol conversion, a temperature of 973 K is sufficient using an appropriate catalyst supply. At 1173 K, the less selective uncatalysed reaction is superimposed to the catalysed reaction lowering formaldehyde selectivity. Since the evaluation of the reaction mechanism is difficult using spectroscopy techniques, several reaction engineering experiments were performed to elucidate the reaction mechanism while simultaneously finding a more simple way of dosing the catalyst.
2. Thermodynamics and catalysts The direct production of anhydrous formaldehyde might be an interesting alternative to the industrially applied silver catalyst process. Thermodynamic calculations show that formaldehyde is not stable at higher temperatures decomposing into hydrogen and carbon monoxide. For achieving high formaldehyde yields an appropriate catalyst is essential. After discussing the thermodynamics of the system, a short overview of more or less active catalysts which were discussed in literature during the last 40 years is given.
Table 1 Possible reactions in the system with the components methanol, formaldehyde, carbon monoxide, hydrogen, methane, carbon dioxide and coke Reaction
HR,973 K (kJ/mol)
No.
CH3 OHCH2 O + H2 CH2 OCO + H2 CH2 OC + H2 O 2COC + CO2 C + 2H2 CH4 CH3 OHCO + 2H2 2CH3 OHCH4 + H2 O + CH2 O CH3 OHC + H2 O + H2 2CH2 OCO + CH3 OH 2CH2 OCH4 + CO2 CO + H2 C + H2 O CO + H2 OCO2 + H2
+92.7 +12.4 −123.4 −170.5 −89.6 −105.1 −27.6 −30.7 −80.3 −235.7 −135.8 −34.7
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
2.1. Thermodynamic calculations The oxidation of methanol to formaldehyde and water is a strongly exothermic process. It can be considered as a set of two consecutive reactions: firstly, the endothermic dehydrogenatiom reaction of methanol to formaldehyde (HR,973 K = 92.7 kJ/mol) and secondly, the strongly exothermic hydrogen–oxygen reaction (HR,973 K = −247.8 kJ/mol) resulting in a high overall reaction enthalpy of −155.1 kJ/mol. In case of the pure dehydrogenation of methanol (in absence of oxygen), several side reactions have to be considered beside the main reaction, the decomposition of methanol to formaldehyde and hydrogen (Table 1). These side reactions explain the formation of the by-products carbon monoxide, carbon dioxide, methane, and coke. Having a look at the distribution of observed experimentally determined products, the decomposition of formaldehyde to carbon monoxide and hydrogen appears as a special reaction of importance. The reaction of methanol to formaldehyde (HR,973 K = 92.7 kJ/mol) is more endothermic than the decomposition of formaldehyde. All other decomposition reactions of formaldehyde are exothermic. The calculated thermodynamic constants in the temperature range from 373 to 1273 K are between 103 and 1043 , showing clearly that the equilibrium is on the product side. Apart from the main reaction, only reaction (7) in Table 1 can explain the formation of formaldehyde, however, yielding an aqueous product.
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Fig. 1. Equilibrium composition in the extended system of important components.
Calculations in the comprehensive system (methanol, formaldehyde, carbon monoxide, carbon dioxide, carbon, methane and hydrogen) using Aspen Plus® have been done by giving these components defining a feed stream of 100 kmol/h of methanol. Fig. 1 shows the molar flows of the single components. From 400 to 1300 K almost all methanol is decomposed. Only in the range of 900 K traces of formaldehyde are predicted. At lower temperatures, the formation of water and methane is dominant, while at higher temperatures the formation of hydrogen and carbon monoxide is preferred. Thermodynamic calculations predict the formation of formaldehyde only in traces, while methanol should be decomposed almost completely. Carbon monoxide, methane and hydrogen should be the products to be formed preferentially. In contrast to these calculations, the experimentally detected main species are methanol, formaldehyde, carbon monoxide and hydrogen. Both methane and carbon dioxide are detected only in traces. Evidently, under technical conditions some reactions do not proceed under thermodynamic but under kinetic control suppressing on the one hand, the formation of the thermodynamically predicted products and enabling on the other hand, the production of formaldehyde. Therefore, the calculation for the reduced system has been repeated twice, based first on reactions (1)
and (2) and second merely on reaction (1). Fig. 2 depicts the degree of methanol conversion and the formaldehyde yield. Above 470 K, the methanol conversion is total, while only small traces of formaldehyde (max. 60 ppm) are predicted. Neglecting reaction (2) leads to a different equilibrium composition. At 673 K, only 20% of methanol was converted, at 873 K already 87% and at 1073 K more than 99%. Considering carbon monoxide in the thermodynamic calculations shows methanol to be decomposed completely at temperatures higher than 473 K and formaldehyde not to be stable decomposing almost totally to carbon monoxide and hydrogen. Restricting the species on methanol, formaldehyde and hydrogen shows methanol to be stable up to higher temperatures. A successful technical realisation requires on the one hand an appropriate catalyst, which is highly active for the main reaction without influencing the side reactions. On the other hand, the reaction must proceed under kinetic control before the thermodynamic equilibrium is established [5]. 2.2. Catalysts for the pure dehydrogenation of methanol Catalysts for the pure dehydrogenation of methanol have been looked for since 1960s. First investiga-
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Fig. 2. Methanol conversion and formaldehyde yield according to equilibrium calculations.
tions used silver and silver–copper catalysts, which require the presence of oxygen for higher conversions. In absence of oxygen, the dehydrogenation reaction proceeds very slowly. Attempts of admixing oxidic substances (e.g. SiO2 , mica, copper oxide) were not successful. Adding selenium, tellurium or sulphur were not satisfying because of the volatility of these components. Oxidic catalysts were quickly reduced by methanol, while metallic catalysts require elevated temperatures for high methanol conversions. Typical by-products are carbon monoxide, methane, formic acid, methyl ester, carbon dioxide and water. Metallic or oxidic zinc used in molten form or as alloy, is deactivating fast. Zinc oxide is more active than metallic zinc; unfortunately the oxide is reduced by methanol. Other transition metals showed also no industrially acceptable results [1,6]. Sodium containing substances, e.g. Na2 CO3 , NaMoO3 or NaAlO2 , have been found to be promising catalysts for the pure dehydrogenation. Elemental sodium seems to be the active species. With sodium aluminate as catalyst, formaldehyde yields up to 65% were realised in a catalytic tube wall reactor at 1173 K [2]. Rising the methanol fraction in the feed above 10%, deactivation is observed. Sodium carbonate has the same disadvantage. Additionally, the formaldehyde selectivity drops to 50%, and zero when the degree of methanol conversion is 75 and 100%, respectively.
3. Experimental set-up The reaction was carried out in a corundum tube reactor (12 mm × 8 mm × 1000 mm) with a reaction zone of 450 mm centred in the axis of the tube. The products were analysed by on line gas chromatography. Details on reactor and on analysis are given in [7]. For catalysing the pure dehydrogenation of methanol to formaldehyde, small amounts of sodium as catalyst were supplied by leading a nitrogen stream over a flat crucible loaded with elemental sodium [7]. Since sodium has a strong tendency to passivate by the formation of sodium oxide and sodium hydroxide due to small traces of oxygen and water in the nitrogen stream, the crucible as sodium reservoir was not practicable for systematic experimental investigations. The adjustment of continuous and constant sodium flow in the reactor was not possible. Therefore, a new way of supplying sodium that was not effected by passivation, had to be found. A tank containing sodium made a continuous and reproducible dosing of the catalyst sodium possible (Fig. 3). Sodium is supplied in a stainless steel case that is inserted again in a cylindrical tank of stainless steel. The heating of the sodium tank to its maximum temperature of 723 K is performed by a heating collar. Nitrogen was fed by a long dip tube leading into the liquid sodium and by a second shorter tube
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Fig. 3. Tank for evaporating sodium.
leading into the vapour phase above the sodium melt. The first nitrogen stream provided the mixing and the aeration of the liquid sodium — both prevents the passivation of sodium, while the latter ensured an inert atmosphere above the sodium before heating. The vaporised sodium was transported to the reaction zone by the nitrogen stream sent through the sodium tank. The reactant methanol was diluted with a second nitrogen stream. The mixture with 10–30 mol% methanol was fed directly in the reaction zone through a smaller separate tube (6 mm × 4 mm) in order to prevent a contact of the vaporised sodium and methanol before the reaction zone. For the sodium catalysed dehydrogenation of methanol to formaldehyde, the influence of the residence time and both reactor and sodium tank temperature was investigated. The residence time was varied only by changing the volume flow of the methanol/nitrogen mixture. The nitrogen flow through the sodium tank and, thereby, the sodium flow was kept constant. So the ratio of sodium to methanol decreased with increasing flow velocity. The amount of sodium in the reaction zone was varied only by changing the temperature of the
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tank. The gas velocity also influenced the laminar temperature profile in the reaction zone. At high flow velocities, the volume in which the gas reached wall temperature became smaller. Consequently, increasing the gas flow resulted not only in shorter residence times but also in smaller volumes with maximum temperature. The exact determination of the amount of sodium consumed was not possible with the two different sources described above. On the one hand, providing sodium in the crucible suffered from forming sodium hydroxide and/or sodium oxide. Both substances adulterate the weighing, making an exact calculation of sodium consumption impossible. On the other hand, an on line weighing is impossible because of the need to fix the tank strongly; weighing the case before and after the experiment causes the same problems as using the crucible. For this purpose, a crucible containing sodium was placed in the tube between the sodium tank and the tee. The tank was used to purify the nitrogen at 473 K from oxygen and water. At 473 K tank temperature, the partial pressure above the melt is too low to transport sodium to the reaction zone, while the liquid sodium purifies the nitrogen stream by removing moisture and oxygen. The purified nitrogen transports the vaporised sodium of the crucible to the reactor. Passivation of the sodium in the crucible was not observed so that the consumption of sodium could be exactly determined by weighing out for calculating the sodium flow. To obtain more information on the reaction mechanism of the sodium catalysed reaction, two additional experiments were performed. The intention was to get an indication on the intermediates formed during the reaction between methanol and sodium in the gas phase. At first, methanol as reactant was substituted by methane and contacted with vaporised sodium in the reaction zone up to a reactor temperature of 1223 K, secondly elemental sodium as catalyst was replaced by sodium methanolate. For the latter a 30 wt.% sodium methanolate solution (Merck) was diluted with the reactant methanol to 0.05%. The mixture was supplied to the reaction zone by a special designed dosing apparatus (Fig. 4). Attaching the dosing equipment on the top of the corundum tube and changing the direction of the gas flow, the drops were accelerated by both the gravity force and the gas flow.
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4. Results For the dehydrogenation of methanol to formaldehyde with elemental sodium as vaporised catalyst the influence of the operating parameters reactor temperature (923–1123 K), temperature of the sodium tank (573–723 K), mole fraction of methanol in the feed (10–30 mol%) and residence time (gas velocity was varied between 0.75 and 1.5 m/s) on the degree of methanol conversion, selectivity and yield of formaldehyde was investigated. The reaction engineering parameters such as degree of methanol conversion, selectivity and yield of formaldehyde were calculated as follows: degree of methanol conversion: XCH3 OH =
n˙ CH3 OH,in − n˙ CH3 OH,out 100% n˙ CH3 OH,in
selectivity to formaldehyde: SCH2 O =
n˙ CH2 O,out 100% n˙ CH3 OH,in − n˙ CH3 OH,out
yield of formaldehyde: YCH2 O =
n˙ CH2 O,out 100% n˙ CH3 OH,in
Fig. 4. Experimental set-up for dosing diluted sodium methanolate.
4.1. Influence of operating parameters
The methanolic sodium methanolate solution was fed from the top of the apparatus through a tee in a capillary tube (3 mm × 2 mm) that leads into the reaction zone. A thermocouple in the centre of the capillary allowed the measurement of the temperature at the drip off-point. Temperatures over 330 K indicated crystallisation of sodium methanolate and so the danger of clogging the capillary tube. Through a tube (6 mm × 4 mm) also leading in the reaction zone as well and surrounding the capillary, nitrogen was sent (line 1). A second nitrogen stream was fed into the annular gap between the tube surrounding the capillary and the corundum tube of the reactor (line 2). The double thermal isolation avoided the vaporisation of methanol to prevent the crystallisation of sodium methanolate.
When the temperature of the sodium tank exceeds 523 K, the partial pressure of sodium and the amount of sodium transported to the reaction zone, respectively, are high enough to catalyse the reaction of methanol to formaldehyde. Figs. 5 and 6 show the influence of the sodium tank temperature at a constant flow velocity of 1 m/s for the mole fractions of 10–30% methanol in the feed and for different reactor temperatures. The degree of methanol conversion rises with increasing temperature of the sodium tank and with increasing reactor temperature while the selectivity to formaldehyde decreases in both cases. Carbon monoxide and hydrogen are formed as by-products. Since the decrease of the selectivity to formaldehyde is much smaller than the increase of the degree of conversion, the yield of formaldehyde increases with rising temperature of the sodium tank and with rising temperature of the reactor. With 10–30% methanol in the feed stream at 1023 K
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Fig. 5. Variation of the temperature of the sodium tank (10% methanol in the feed).
Fig. 6. variation of the temperature of the sodium tank (30% methanol in the feed).
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Fig. 7. Variation of the gas flow velocity (10% methanol in the feed; temperature of the sodium tank 623 K).
reactor temperature and at 723 K for the sodium tank, the yield of formaldehyde reaches a maximum in each case of 72 and 50%, respectively. The influence of the gas velocity and the residence time, respectively, is depicted in Fig. 7. Lowering the gas flow leads to an increase of the degree of methanol conversion while the selectivity to formaldehyde is lowered only slightly. For 973 K reactor temperature, the reduction of the gas velocity from 1.5 to 1.0 m/s results in an increase of the degree of conversion from 15 to 25%, a further reduction to 0.75 m/s nearly doubles the degree of conversion. For 1073 K reactor temperature, a decrease of the gas flow from 1.5 to 1.0 m/s results in doubling of the degree of conversion. Similar to the reactor temperature and the temperature of the sodium tank the residence time has only little influence on the selectivity to formaldehyde. Therefore, a higher yield of formaldehyde can be obtained by lowering the gas velocity. The variation of the gas velocity affected the molar ratio of sodium to methanol and the laminar temperature profile in the reactor. Lower gas velocities resulted in increasing sodium to methanol ratios and larger volumes in the reaction zone where the maximum temperature was reached; both effects promoted the increase of the degree of conversion and the yield, additionally.
The catalytic activity of sodium is expressed by the turnover number (TON). The turnover number represents the molar ratio of the consumption of methanol to sodium: TON =
n˙ methanol n˙ sodium
(13)
Values between 500 and 700 for the turnover number were determined, which means that 500–700 methanol molecules were converted by one sodium atom [4]. Since these turnover numbers are probably not accurate due to passivation of the sodium, the sodium tank was calibrated to determine more precise values. For clearly defined reaction conditions (reactor temperature 1073 K; gas velocity 1.0 m/s, 10% mole fraction of methanol in the feed) the temperature of the crucible was adjusted the way that a certain degree of methanol conversion (e.g. 25%) was obtained. Under these conditions, the sodium flow from the crucible into the reaction zone is the same as with the sodium tank for the same degree of conversion. Table 2 shows the experimentally determined sodium flows for 25–40% degree of methanol conversion and the corresponding calculated turnover numbers. At a gas velocity of 1 m/s and 10 mol% methanol in the feed, a degree of methanol conversion of 25% corresponds to a converted methanol molar flow of 202.0 mmol/h
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Table 2 Amount of consumed sodium and turnover numbers Temperature (K) Reactor
Sodium tank
973 973
623 673
Degree of conversion (%)
Sodium flow (mg/h)
Turnover number
25 40
9 15
457 466
and a sodium flow of about 9 mg/h (0.396 mmol/h). Dividing the converted methanol molar flow through the sodium mole flow results in a turnover number of approximately 500. For these calculations it was assumed that for sodium to react as single atoms not as small clusters. Whether clusters or atoms are catalytically active could not be determined at this point (experiments on the reaction mechanism indicate atoms to be active see Chapter 5.1). Considering the accuracy of the measurements, the turnover numbers were equal for a temperature of 623–673 K of the sodium tank for the above mentioned reaction conditions. The more sodium is supplied to the reaction zone, the higher the degree of conversion whereby the dependency on the provided amount of sodium is linear in a certain range for the degree of conversion. 4.2. Investigations of the reaction mechanism Elucidating the reaction mechanism of the sodium catalysed reaction is hardly possible by only analysing the composition of the product stream as many intermediates are not accessible and cannot be detected by gas chromatography. Spectroscopic methods like laser-induced fluorescence or Raman-spectroscopy are in-situ analysis techniques that enable the identification of intermediates as radicals or atoms in the gas phase. Unfortunately, in the case of methanol dehydrogenation, these methods are limited to ideal conditions adapted on each species. For a chemical reaction, a mixture of radicals is obtained which leads to interferences of the specific absorption bands. In addition, very low concentrations of the radicals complicate the identification seriously [9]. Hence two additional reaction engineering experiments were performed in order to reveal the reaction mechanism of the sodium catalysed dehydrogenation of methanol.
First, methanol was substituted by methane, secondly, sodium methanolate was supplied instead of sodium. The experiments should reveal which product is formed during the reaction of methanol and sodium clarifying the role of sodium in the catalytic process. The feed of methane should reveal whether sodium reacts with the hydrogen atoms of methanol to sodium hydride (NaH) and hydroxy-methyl radicals (CH2 OH• ) as assumed by Sauer and Emig [2] or whether sodium and methanol form sodium methanolate as in the liquid phase. Even at 1223 K reactor temperature, no conversion of methane was observed. Since the hydrogen atoms of methane are inert towards elemental sodium a reaction of sodium with the hydrogen atoms of the methyl group in methanol is also unlikely. We conclude that the formation of sodium hydride and hydroxy-methyl is improbable. As the reaction of sodium and methanol to sodium methanolate occurs also in the liquid phase, the same reaction is likely to take place in the gas phase. Supplying methane instead of methanol together with sodium must not be considered as a final proof for the formation of sodium methanolate but as a strong indication. The substitution of sodium by sodium methanolate in methanolic solution should reveal whether a sodium compound, which is supplied homogeneously, with an oxidation state of sodium different from zero, catalyses the dehydrogenation. Further should be examined whether sodium methanolate catalyses the dehydrogenation as well as elemental sodium or whether sodium methanolate is inert. Sodium methanolate was found to catalyse the pure dehydrogenation of methanol to formaldehyde. The turnover numbers obtained with sodium methanolate gave values between 450 and 800 at 973 K which is in the same range as for sodium. Under comparable reaction conditions, similar degrees of conversion of methanol and selectivities to formaldehyde as for
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elemental sodium were achieved. This indicates that sodium methanolate decomposes to elemental sodium and methoxy radicals (• OCH3 ). An alternative for dosing methanolic sodium methanolate represents the feed of dissolved sodium hydroxide in methanol [10]. Methanol and sodium hydroxide are in equilibrium with sodium methanolate and water. At ambient conditions and high methanol excess, the equilibrium is completely on the product side. Increasing the temperature, the equilibrium shifts towards the reactants: at 908 K only 1% of sodium hydroxide form methanolate. Under comparable conditions, similar degrees of methanol conversion and selectivities to formaldehyde as with sodium methanolate were achieved. 5. Discussion 5.1. Aspects of reaction mechanism Vaporised elemental sodium catalyses homogeneously the pure dehydrogenation of methanol to formaldehyde in the gas phase. The experiments with methane instead of methanol and with sodium methanolate instead of sodium gave insights into the reaction mechanism. Finding no reaction between methane and sodium even at high temperatures (1223 K) supported the assumption that sodium reacts with the hydroxyl-group of methanol and not with the hydrogen atoms of the methyl-group (14). CH3 OH + Na CH2 OH• + NaH
(14)
During the reaction of sodium with methanol to sodium methanolate hydrogen atoms are generated as by-product. CH3 OH + Na NaOCH3 + H•
(15)
Hydrogen atoms were found to act as chain carrier for the uncatalysed homogeneous dehydrogenation of methanol [4]. Hence, two ways how sodium could act are possible: first, it could trigger the uncatalysed reaction by producing hydrogen atoms, second a cyclic formation and decomposition of sodium methanolate could occur. The first reaction pathway — sodium as generator for hydrogen atoms, is unlikely since kinetic simulations showed that hydrogen atoms do not react
Fig. 8. Reaction cycle of the sodium catalysed dehydrogenation of methanol.
with methanol at temperatures below 973 K even at high concentrations of hydrogen atoms [8]. At these conditions the recombination of hydrogen atoms to molecular hydrogen is much faster than the chain propagation. In addition, with hydrogen atoms as chain carrier, the high selectivities to formaldehyde (S > 75%) for the sodium catalysed dehydrogenation would be inexplicable since typical values for the uncatalysed reaction are smaller. The supply of methanolic sodium methanolate solution showed clearly the catalytic activity and the decomposition of sodium methanolate under reaction conditions. This result proves that the role of sodium is not only of generating hydrogen-atoms, otherwise sodium methanolate would be inert and there would be no methanol conversion. Achieving nearly the same turnover numbers with sodium methanolate as with elemental sodium indicates atomic sodium to be the active component not sodium clusters. If methanol would be converted on sodium clusters, the turnover numbers of elemental sodium would be smaller than for sodium methanolate. Sodium methanolate decomposes to atomic sodium and methoxy radicals. The sodium set free reacts again with methanol to sodium methanolate and closes the catalytic cycle (Fig. 8). The catalytic cycle can be initiated by supplying sodium as catalyst or sodium methanolate as intermediate. The methoxy radicals decompose to the desired product formaldehyde: • OCH
3
CH2 O + H•
(16)
From reaction (15) and (16) hydrogen atoms are formed. They can react with methanol, the
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Fig. 9. Calculated turnover numbers (gas velocity 1 m/s).
methoxy-radical and unfortunately with formaldehyde to carbon monoxide: CH2 O + H• CHO• + H2
(17)
CHO• + H• CO + H•
(18)
2H• → H2
(19)
The appearance of hydrogen atoms and the reaction with formaldehyde gives an explanation for selectivities to formaldehyde lower than 100%. 5.2. Reaction engineering aspects The calibration of the sodium tank allows the calculation of turnover numbers for other reaction conditions varying the reactor temperature or the mole fraction of methanol in the feed. The dependency of the turnover number on the reactor temperature and the methanol content in the feed is shown in Fig. 9. Rising the reactor temperature from 923 to 1123 K, the turnover number increases almost eight times from 211 to 1770 (gas flow 1 m/s; 10 mol% in the feed), assuming complete conversion of methanol, the extrapolated turnover number yields in 1880. An increasing mole fraction of methanol in the feed also results in rising turnover numbers. A dependency on the temperature of the sodium tank cannot be seen from Fig. 9. For 623 and 673 K, the increase of the turnover number with the mole fraction of methanol is nearly identical.
Tripling the mole fraction of methanol, the turnover numbers increase from 456 to 814 (623 K) and from 466 to 797 (673 K), respectively. Since the selectivity to formaldehyde is not influenced disadvantageously by high mole fractions of methanol, it is desirable for reasons of an optimum catalyst efficiency to aim at high mole fractions of methanol in the feed. Furthermore, the reduced content of inert gas results in saving costs for heating and cooling inert gas without effecting the efficiency of the process. The high selectivities to formaldehyde over 70% even at high degrees of methanol conversion are remarkable. These high selectivities are based on the selective reaction of sodium with methanol while sodium is not likely to react with formaldehyde. Beside the product formaldehyde, the reaction between methanol and sodium (15) leads to the formation of hydrogen atoms. In contrast to sodium atoms, hydrogen atoms react with both methanol and formaldehyde. The first reaction produces formaldehyde again while in the latter carbon monoxide and hydrogen are formed (17–19). With increasing degree of methanol conversion, the selectivity to formaldehyde drops. The partial pressure of methanol falls for increasing degrees of methanol conversion, accordingly the partial pressure of the product formaldehyde increases in parallel. Therefore, the consecutive reaction between formaldehyde and hydrogen atoms becomes faster than the formation of formaldehyde by methanol since the probability for a collision of hydrogen atoms
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Fig. 10. Yield vs. conversion — classification for process parameters.
and formaldehyde molecules grows with increasing methanol conversion. In consequence, the selectivity to formaldehyde decreases. Fig. 10 shows the yield of formaldehyde plotted versus the degree of methanol conversion, classified for the four variable process parameters reactor temperature, gas velocity, mole fraction of methanol in the feed and temperature of the sodium tank. The diagonal line represents a formaldehyde selectivity of 100%, the dotted lines 85 and 70%, respectively. As it can be seen, the way of increasing the degree of conversion does not affect the selectivity. None of the four operating parameters change seems to have a predominating
influence on the selectivity. The decrease of the selectivity is only a phenomenon of increasing degrees of methanol conversion. For degrees of conversion lower than 40%, the selectivity exceeds 85% in every case, also for higher degrees of conversion, the selectivity does not fall below 70%.
6. Conclusions As thermodynamic calculations turned out, anhydrous formaldehyde is not stable over a wide temperature range. Working under kinetic instead of
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thermodynamic control enables the production of formaldehyde. Without catalyst, only poor yields of approx. 20% can be achieved. The application of a sodium containing species or even elemental sodium as catalysts increases the formaldehyde yield to 70%. Vaporised elemental sodium catalyses homogeneously the pure dehydrogenation of methanol to formaldehyde with a high selectivity. The influence of reaction temperature, mole fraction of methanol, residence time and amount of sodium was investigated. With increasing degree of methanol conversion only a slight decrease of the formaldehyde selectivity was observed. Even at high degrees of methanol conversion selectivities over 70% were obtained. Experiments on the reaction mechanism indicated that methanol and elemental sodium form sodium methanolate, which decomposes under reaction conditions to atomic sodium and via methoxy radicals to formaldehyde and hydrogen atoms. The sodium set free reacts again with methanol and closes the catalytic cycle. The catalytic cycle can either be initiated by supplying sodium or the intermediate sodium methanolate. The hydrogen atoms react in contrast to sodium with both methanol and formaldehyde. The reaction with the latter leads to the formation of carbon monoxide as by-product. Recovering the catalyst is an interesting question and a major problem to be solved for an industrial production. To give an answer, further investigations
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have to be carried out. Especially the nature of the sodium species, which we actually suppose to be sodium methanolate, must be examined in subsequent experiments. Then an appropriate recovering can be developed. But this requires some efforts not be underestimated, an interesting task for the future we have in mind.
Acknowledgements The work was supported by the Bundesministerium für Bildung und Forschung (BMBF) under the number 03D0012B9. References [1] S. Su, P. Zaza, A. Renken, Chem. Eng. Technol. 17 (1994) 34. [2] J. Sauer, G. Emig, Chem. Eng. Technol. 18 (1995) 284. [3] M. Bender, Thesis, VDI Verlag, 1998. [4] St. Ruf, G. Emig, J. Mol. Catal. A 146 (2000) 271. [5] J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, 1997. [6] V.I. Atroshchenko, I.P. Kushnarenko, Int. Chem. Eng. 4 (1964) 581. [7] St. Ruf, G. Emig, Appl. Catal. A 161 (1997) L19. [8] St. Ruf, Die Natrium-katalysierte Dehydrierung von Methanol zu wasserfreiem Formaldehyd, Shaker Verlag, Aachen, 1998. [9] Department of Technical Thermodynamics, University of Erlangen-Nuremberg, Internal Communication, 1997. [10] DE 198,22,598 A1 (1999).