Hydrogenation Kinetics of 2, 2-Dimethylol-1-butanal to

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Hydrogenation Kinetics of 2,2-Dimethylol-1-butanal to Trimethylolpropane over a Supported Nickel Catalyst Tiina-Kaisa Rantakyla1 ,† Tapio Salmi,*,† Jeannette Aumo,† Pa1 ivi Ma1 ki-Arvela,† Rainer Sjo1 holm,‡ Tapio Ollonqvist,§ Juhani Va1 yrynen,§ and Lars Peter Lindfors| Laboratory of Industrial Chemistry, Process Chemistry Group, and Laboratory of Organic Chemistry, A° bo Akademi, FIN-20500 Turku/A° bo, Finland, Laboratory of Surface Science, Department of Applied Physics, University of Turku, FIN-20014 Turku, Finland, and Technology Centre, Neste Chemicals, Box 310, FIN-06101 Porvoo, Finland

The hydrogenation kinetics of 2,2-dimethylol-1-butanal (TMP-aldol) over a supported nickel catalyst was determined with experiments carried out in a batchwise operating autoclave at 50-90 °C and 40-80 bar hydrogen. The reaction mixture was analyzed with gas and liquid chromatography. It was found that TMP-aldol can be hydrogenated with a 100% selectivity to the corresponding triol, trimethylolpropane. The effects of the catalyst activation procedure and the formaldehyde concentration on the hydrogenation kinetics were studied. The hydrogenation experiments revealed that catalyst reduction at a high temperature (400 °C) under hydrogen flow was favorable for the catalyst performance. The reason was a more effective reduction of nickel oxides which was confirmed with thermogravimetry and X-ray photoelectron spectroscopy. The presence of formaldehyde had a considerable retarding effect on the aldol hydrogenation kinetics: the hydrogenation rate was low until all of the formaldehyde was hydrogenated to methanol. The retarding effect was more prominent at higher temperatures than at lower temperatures, which indicates that formaldehyde forms oligomers on the catalyst surface as the temperature increases. A kinetic model was proposed for the aldol hydrogenation. The model includes adsorption, desorption, and surface reaction steps as well as the inhibitory effect of formaldehyde on the aldol hydrogenation kinetics. The model was able to describe the experimentally recorded hydrogenation kinetics of TMP-aldol in the presence and in the absence of formaldehyde. Introduction Polyols are used as raw materials in the production of lubricants, plastics, surface coatings, and synthetic resins. The production of polyols can be carried out by the reaction of formaldehyde with an aliphatic aldehyde in the presence of a strongly alkaline catalyst, for instance sodium or calcium hydroxide (Ullmann, 1999). The drawback of the classical synthesis route through the Cannizzaro reaction is the formation of stoichiometric amounts of sodium formate. Because formate is difficult to remove from the product mixture and its production volume exceeds the market demand, the conventional production technology is considered to be inefficient and uneconomical. The appearance of formate can be prevented by applying a two-step synthesis route, in which the first step consists of aldolization with a weakly basic catalyst and the second step of catalytic hydrogenation of the aldol. Typical weak bases used as aldolization catalysts are Ba(OH)2 (Conant and Tuttle, 1941), tertiary amines (Immel et al., 1978; Watanabe et al., 1997), and anion-exchange resins (Astle and Zaslowsky, 1952; Austerweil and Pallaud, 1953). By the use of these weak bases, the reaction is stopped at the aldolization step and a suitable raw material is produced for catalytic hydrogenation. Several catalysts are † Laboratory of Industrial Chemistry, Process Chemistry Group, A° bo Akademi. ‡Laboratory of Organic Chemistry, A ° bo Akademi. § University of Turku. | Neste Chemicals.

mentioned in the literature for aldol hydrogenation, such as supported nickel and copper catalysts (Yoneoka et al., 1995; Ninomiya et al., 1990; Immel et al., 1978). According to the two-step synthesis route, trimethylolpropane (TMP) could be prepared by aldolization of butyraldehyde with formaldehyde to 2,2-dimethylol-1butanal (aldol) over a solid catalyst and hydrogenation of the aldol to trimethylolpropane (TMP). Previous results of our group (Serra-Holm et al., 2000) have shown that the aldolization step can be carried out with a high conversion and selectivity when anion-exchange resins are used as solid catalysts. The present paper concerns the catalytic hydrogenation of 2,2-dimethylol1-butanal to trimethylolpropane over a supported nickel catalyst.

Catalyst Characterization. A commercial Ni/SiO2 catalyst was used in this work. The cylindrical catalyst particles were crushed and sieved to particle sizes of 45-150 µm. Before systematic kinetic experiments, the catalyst activation procedure was investigated by means of thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and in situ hydrogenation experiments. TGA experiments were performed at 170, 300,

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and 400 °C. XPS (Perkin-Elmer PHI 5400 ESCA) was used to analyze the chemical state of the catalyst surface. Formaldehyde Separation. Because the aldol solution typically contains formaldehyde, a catalyst poison, most of it was removed before the catalytic hydrogenation. Because formaldehyde oligomerization is promoted by high concentrations, steam distillation was applied (Walker, 1964; Kirk-Othmer, 1994). A straightforward way to carry out steam distillation is simply to put a mixture of the organic compound and water in a rectification device and carry out an ordinary distillation using water addition (Harwood and Moody, 1989). The separation of formaldehyde was performed under an atmospheric pressure and at a temperature of 100 °C. The water feeding was adjusted in order to maintain a constant liquid volume in the distillation flask. After the distillation, the residue solution was evaporated in order to remove the water which was added before the distillation. Kinetic Experiments. When the kinetics of the catalytic hydrogenation was investigated, 12 experiments were performed under different conditions, varying the temperature and the pressure between 50 and 90 °C and 40 and 80 bar. The hydrogenation studies were carried out in a stirred semibatch reactor (total volume of 1 dm3), to which hydrogen was fed continuously to maintain the pressure constant during the experiment. The pressure, the temperature, and the agitation velocity were controlled and registered during the experiment with a data acquisition program. The catalyst (5 g) was placed in the reactor and activated in situ by hydrogen reduction (hydrogen flow of 0.75 dm3/min) at 400 °C and at 2 bar for 1 h before the each experiment. A hydrogenation mixture was prepared by stirring a 0.15 dm3 formaldehyde-separated aldol solution into 0.15 dm3 methanol (>99.8% J. T. Baker). The reaction mixture was saturated with hydrogen for 10-15 min in order to remove dissolved oxygen. The solution was injected into the reactor, and temperature and pressure values were adjusted to desired values. Liquid samples were taken at 5-15 min intervals in the beginning of the reaction and thereafter with a longer interval. The duration of experiments varied from 2 to 7 h depending on conditions. Samples were analyzed by a high-performance liquid chromatograph (Hewlett-Packard 1100), equipped with LiChrosorb RP-18 column and diode array and refractive index detectors, and by a gas chromatograph (Hewlett-Packard 6890), equipped with a 30 m long DBWAX capillary column and a flame-ionization detector. Three different methods were applied for analyzing the compounds: a derivatization method (Lipari and Swarin, 1982) was used for the quantitative determination of aldehydes, aldols, and unsaturated aldehydes, a straightforward method was developed for measuring the contents of triols and organic acids, and a gas chromatographic method was utilized for measuring the alcohol contents. Catalyst Characterization and Activation Results. The thermogravimetric experiments showed that the catalyst reduced most efficiently at 400 °C and the degree of reduction exceeded 10%, whereas the degree of reduction was only 6% at 170 °C. By use of XPS, the oxidation states of the elements present at the solid surface can be determined. The XPS studies also

Figure 1. Effect of pressure on the hydrogenation of aldol at (a) 90, (b) 80, and (c) 65 °C.

provided information about the elemental surface composition. Two different activation temperatures were compared, 400 and 170 °C, and XPS studies confirmed that the nickel oxide peak is smaller for the sample reduced at higher temperature. The catalyst activation temperatures were also investigated by in situ hydrogenation experiments using samples reduced by hydrogen at 170 and 400 °C for 1 h. The actual hydrogenation experiments were performed at 80 °C and 80 bar. A complete conversion was achieved in all experiments, but the hydrogenation velocity was most rapid over the catalyst activated at the highest temperature. Therefore, the activation procedure for systematic kinetic experiments was fixed to 400 °C and 1 h of hydrogen flow. Qualitative Kinetics. On the basis of 12 kinetic experiments, the effects of pressure, temperature, and formaldehyde concentration were screened. A complete conversion was achieved in every experiment, with a selectivity varying between 90 and 100%. The selectivity (S) and conversion (X) were defined as follows: S ) triol formed/aldol reacted and X ) aldol reacted/aldol in the beginning. The pressure effect on hydrogenation of aldol is illustrated in Figure 1a-c. There was practically no difference in hydrogenation rates at 70 and 80 bar, whereas the reaction progress was clearly slower at 40 bar. The reaction progresses at lower temperature (65 °C) were almost similar for the highest pressures (6080 bar).

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Reaction Mechanisms and Rate Equations. Some general principles are applied to the consideration of hydrogenation mechanisms. Aldol and hydrogen molecules are assumed to be absorbed on the catalyst surface prior to the hydrogenation. The carbonyl group of aldol interacts with the catalyst surface and the adsorption of hydrogen is dissociative (Selwood, 1962; Smeds et al., 1996). However, transient kinetic studies indicate that hydrogen preserves its molecular identity during hydrogenation (Mirodatos et al., 1987). Thus, we remain with a mechanism where adsorbed aldol reacts with a pair of dissociatively adsorbed hydrogen, giving the reaction product, a triol, which is desorbed from the surface. The surface mechanism of aldol can thus be described as follows:

Figure 2. Effect of temperature on the hydrogenation of aldol at (a) 80, (b) 70, and (c) 40 bar.

At high temperatures the reaction rate was slower in the beginning, and a kinetic shoulder was observed. This effect was independent of the pressure. A possible explanation for this observation could be formaldehyde oligomerization, which inhibits the hydrogenation of aldol by occupying active sites on the catalyst surface. As soon as formaldehyde has been hydrogenated to methanol, the sites are activated and the hydrogenation of aldol is enhanced. The overall reaction rate was, however, still faster at higher temperatures. No kinetic shoulder appeared at lower temperatures. The temperature effect is illustrated in Figure 2. The product formation is illustrated in Figure 3. The figure reveals that the formaldehyde hydrogenation is favored by high temperatures while the product distribution is virtually independent of hydrogen pressure. Usually the molar ratio of formaldehyde and aldol in the hydrogenation mixture was nFA:nA ) 0.35. In one experiment carried out at 90 °C and 70 bar, a considerably lower molar ratio of formaldehyde to aldol was used (nFA:nA ) 0.07). The hydrogenation rate of aldol was 4 times faster with the lower formaldehyde concentration, and no kinetic shoulder was observed (Figure 4). This supports the hypothesis that formaldehyde molecules oligomerize and occupy active sites on the catalyst surface and thereby inhibit the hydrogenation of aldol molecules. The qualitative kinetic results are interpreted with reaction mechanisms and rate equations which are derived in the next section.

A + / H A/

(1)

H2 + 2/ H 2H/

(2)

A/ + 2H/ H T/ + 2/

(3)

T/ H T + /

(4)

where A and T denote the aldol and the triol and / is a catalytic surface site. An exactly analogous mechanism can be written for the hydrogenation of formaldehyde to methanol: the aldol in eqs 1-4 is just replaced by formaldehyde. In the derivation of the rate equation, it is assumed that the surface reaction (3) is irreversible and ratedetermining, whereas the adsorption steps of hydrogen and aldol are rapid enough for the quasi-equilibrium hypothesis to be applied. The desorption step of triol is assumed to be irreversible and very rapid (cT* f 0). Consequently, the hydrogenation rates are given by

rj ) kjci*c2H*

(5)

where i denotes TMP-aldol or formaldehyde, H is hydrogen, and * refers to the concentration of an adsorbed species. Quasi-equilibrium approximations can be applied to the adsorption steps (1)-(2), and a total balance gives a relation between the concentrations of the surface species:

ci* + cH* + c* ) cTot

(6)

The rate equations for the hydrogenation of aldol (I) and formaldehyde (II) can be expressed as follows:

rI ) k1cA*c2H* ) k1KAKHc3*

(7)

rII ) k2cFA*c2H* ) k2KFAKHc3*

(8)

When quasi-equilibrium expressions are substituted into the site balance, the concentration of vacant sites is obtained and the final forms of the rate equations become

rI ) rII )

k′cAcH (1 + KAcA + KFAcFA + xKHcH)3 k′′cFAcH (1 + KAcA + KFAcFA + xKHcH)3

(9)

(10)

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Figure 3. Ability of the catalyst to hydrogenate formaldehyde vs aldol at (a) different temperatures at 80 bar and (b) at different pressures at 80 °C.

A semiempirical modification is obtained by replacing the coverage of formaldehyde (ΘFA) by its mole fraction in the liquid. Because the total concentration remains virtually constant, we use xFA ) cFA/cL, where cL is the total concentration of liquid. The final forms of the rate expressions thus become

rI )

rII ) Figure 4. Effect of formaldehyde concentration on the hydrogenation of aldol at 90 °C and 70 bar.

where k′ and k′′ are lumped kinetic parameters: k′ ) k1KAKHc3TOT and k′′ ) k2KFAKHc3TOT. According to the experimental data, formaldehyde has a considerable retarding effect on the hydrogenation rate at high temperatures. Therefore, the rate equations (9) and (10) are modified to account for the inhibition. It is a well-known fact that adsorption enthalpies are influenced by the presence of existing adsorbates on the catalyst surface. The absolute value of the adsorption enthalpy decreases with increasing coverage even in the case of single-component adsorption. This phenomenon causes the so-called induced nonuniformity of the catalyst surface (Temkin, 1979). In the present case we assume the simplest dependence of adsorption enthalpy on the surface coverage of formaldehyde. The adsorption enthalpy decreases linearly with increasing coverage, as originally proposed by Temkin (1979).

-∆HA ) -∆H0A - aΘFA

(11)

The adsorption equilibrium constant is assumed to obey the law of van’t Hoff. Provided that an analogous treatment can be applied on the adsorption of hydrogen, the lumped rate parameters become (-E′-a′ΘFA)/RT

k′ ) k′0e

(12)

k′′ ) k′′0e(-E′′-b′ΘFA)/RT

(13)

The lumped parameters in the above expressions are explained in the Notation section. Equations 12 and 13 clearly demonstrate how the apparent rate constants diminish with increasing surface coverage of formaldehyde.

k′0e(-E′+R′f)/RTcAcH

(14)

D3 k′′0e(-E′′+R′′f)/RTcFAcH

(15)

D3

where

D ) 1 + KAcA + KFAcFA + xKHcH

and

f)

cFA cL

The rate equations are combined with the mass balances of the components in order to determine the values of the kinetic parameters. Physical Properties. The concentration of dissolved hydrogen in the reaction mixture (cH) appears in the rate equations for hydrogenation. The hydrogen concentration was estimated from the hydrogen solubilities in water and methanol. The solubility expressions scaled to 1 bar of H2 are provided by Fogg and Gerrard (1991):

ln x/H,W ) -125.939 + 5528.45/(T/K) + 16.8893 ln(T/K) (16) ln x/H,MeOH ) -7.3644 - 408.38/(T/K)

(17)

The solubility in the mixture was calculated as a molar average. The concentration of dissolved hydrogen (cH) at an arbitrary pressure (pH) is obtained from

cH ) x/H(pH/bar)cL

(18)

Reactor Model and Parameter Estimation. The mass balances for the organic components in the batch reactor are written in the following form:

dci ) riφB dt

(19)

where ci is the concentration expressed in mol/kg and φB ) mcat/mL where mL is the mass of the liquid. The stoichiometry gives the relation between hydrogenation rates (rI and rII) and the generation rates of the

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Figure 5. Concentrations cA0 - cA vs cT - cT0 in hydrogenation experiments.

Figure 6. Logarithmic test plots ln(cA0/cA) vs ln(cFA0/cFA) from hydrogenation experiments.

components (ri). Before the numerical parameter estimation is commenced, some simple analytical expressions for the product distribution are derived in the sequel. Division of the balances for aldol and triol gives after integration the simple relation

Table 1. Estimated Parametersa

cT - c0T ) c0A - cA

(20)

i.e., the amount of formed triol equals the amount of reacted aldol. For parallel hydrogenation of different aromatic compounds, Wauquier and Jungers (1957) have proposed a division of the balances of the reagents to achieve simple and illustrative relationships. We get for aldol and formaldehyde at a constant temperature

cFA dcFA ) Re-βcFA dcA cA

(21)

Equation 21 is easily solved by separation of variables, development of the exponential term to a Taylor series, and integration within the limits [cFA0, cFA] and [cA0, cA]. The following simple relationship is obtained:

ln

( ) cFA0 cFA

∞

+

∑ i)1

( ( )) ( )

(-1)iβi(cFA0)i (i)(i!)

1-

cFA

cFA0

i

) R ln

cA0 cA

(22)

Equation 22 is very informative: for low formaldehyde concentrations, when the inhibitory effect is negligible, a double-logarithmic plot ln(cFA0/cFA) versus ln(cA0/cA) should give a straight line with the slope R. For higher concentrations of formaldehyde, a curvature is introduced into the plot because of the sum term in eq 22. Test plots representing eqs 20 and 22 are displayed in Figures 5 and 6. As revealed by the figures, the experimental data obey the prediction provided by the kinetic model. The numerical parameter estimation was based on kinetic experiments out of which the temperatures and concentrations were recorded for the compounds. The kinetic and adsorption parameters were estimated by using the simplex Levenberg-Marquardt method (Marquardt, 1963), which with nonlinear regression minimizes the residual sum of squares between the estimated and the experimental concentrations. The ordinary differential equations, which describe the mass balances, were numerically solved during the parameter estimation. The backward difference method was used

a

parameter

estimated value

E′, kJ/mol E′′, kJ/mol R′ R′′ KA, kg/mol KH, kg/mol KFA, kg/mol k′0, mol/(kg min) k′′ 0, mol/(kg min)

42.2 ( 3.5 36.7 ( 5.8 -0.945 × 107 0.409 × 107 1.05 ( 0.16 0.100 × 10-9 b 0.765 × 10-5 b 8.72 ( 1.54 38.4 ( 8.7

Degree of explanation ) 98.6%. b Large standard error.

as a numerical algorithm, suitable for stiff differential equations (Hindmarsh, 1983). The model was treated in MODEST, software for parameter estimation, simulation, and optimization (Haario, 1994). The objective function Q, which was minimized in the nonlinear regression, is given by

Q)

∑t ∑i (cit - cit,exp)2

(23)

where i is the component index and t denotes different experimental times. The estimated parameters were the adsorption constants, the lumped rate constants at the average temperature, and the apparent activation energies. The adsorption constants were assumed to be independent of temperature in order to suppress the number of adjustable parameters in regression. The parameter estimation statistics is summarized in Table 1, and examples of the fit of the model to the data sets are provided in Figure 7. As can be seen from the figures, the proposed model describes the experimental data reasonably well. Conclusions The kinetic experiments showed that supported nickel is a suitable catalyst for the selective hydrogenation of TMP-aldol to the corresponding triol, trimethylolpropane. The catalyst activation temperature had a considerable effect on the hydrogenation activity, which was explained by the presence of metallic nickel on the catalyst surface. The kinetic experiments showed that formaldehyde retarded the hydrogenation kinetics of the aldol (Figure 4). The effect was more prominent at higher experimental temperatures, which was explained by the oligomerization of formaldehyde. We proposed a kinetic model (eqs 14 and 15), which accounts for competitive adsorption of the reagents, surface reactions, and the inhibitory effect of formaldehyde. It turned out that the experimental data were rather well-

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Figure 7. Measured (symbols) and estimated (curves) concentrations of aldol, formaldehyde, and triol.

described with the rate equations based on the kinetic model (Figure 7). Notation a, b ) adsorption parameters ci ) concentration of component i D ) denominator in the kinetic expression E′, E′′ ) lumped activation energies f ) ratio of concentrations ∆H ) enthalpy k ) rate constant

K ) equilibrium constant m ) mass n ) molar amount Q ) objective function r ) rate R ) gas constant S ) selectivity T ) temperature t ) time x ) mole fraction X ) conversion

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Greek Letters R ) lumped parameter, eq 21 β ) lumped adsorption parameter, eq 21 R′, R′′ ) lumped adsorption parameters, eqs 13 and 14 Θ ) fractional coverage φB ) catalyst mass-to-liquid mass Subscripts and Superscripts 0 ) initial or reference state I ) aldol hydrogenation II ) formaldehyde hydrogenation cat ) catalyst exp ) experimental value i ) component index, general index j ) reaction index L ) liquid / ) adsorption site Abbreviations A ) aldol FA ) formaldehyde H ) hydrogen MeOH ) methanol T ) triol

Literature Cited (1) Astle, M. J.; Zaslowsky, J. A. Aldol Condensation. Ind. Eng. Chem. 1952, 44 (No. 12). (2) Austerweil, G. V.; Pallaud, R. Les echangeurs d’anions dans les aldolisations, cetolisations et leurs reactions consecutives. Bull. Soc. Chim. Fr. 1953. (3) Conant, J. B.; Tuttle, N. Org. Synth. 1941, I. (4) Fogg, P. G. T.; Gerrard, W. Solubility of Gases in Liquids; John Wiley & Sons: New York, 1991. (5) Haario, H. MODESTsUser’s Guide; Profmath Oy: 1994. (6) Harwood, L. M.; Moody, C. J. Experimental Organic Chemistry; Blackwell Scientific Publications: Malden, MA, 1989. (7) Hindmarsh, A. C. ODEPACK, A Systematized Collection of ODE Solvers. In Scientific Computing; Stepleman, R. S., et al., Eds.; IMACS/North-Holland Publishing Co.: Amsterdam, The Netherlands, 1983.

(8) Immel, O.; Schwartz, H.-H.; Weissel, O.; Krimm, H. Process for the preparation of trimethylolalkanes. U.S. Patent 4 122 290, 1978. (9) Kirk-Othmer. Encyclopedia of Chemical Technology 11; John Wiley & Sons: New York, 1994. (10) Lipari, F.; Swarin, S. J. Determination of formaldehyde and other aldehydes in automobile exhaust with an improved 2,4dinitrophenylhydrazine method. J. Chromatogr. 1982, 247. (11) Marquardt, D. W. An algorithm for least squares estimation of nonlinear parameters. J. Soc. Ind. Appl. Math. 1963, 11. (12) Mirodatos, C. Steady-state and isotopic transient kinetics of benzene hydrogenation on nickel catalysts. J. Catal. 1987, 105. (13) Ninomiya, T.; Furuta, T.; Kita, S.; Fujii, Y. Process for producing neopentyl glycol. U.S. Patent 4 933 473, 1990. (14) Selwood, P. W. Adsorption and Collective Paramagnetism; Academic Press: New York, 1962. (15) Serra-Holm, V.; Salmi, T.; Multama¨ki, J.; Reinik, J.; Ma¨kiArvela, P.; Sjo¨holm, R..; Lindfors, L. P. Aldolization of butyraldehyde with formaldehyde over a commercial anion-exchange resins kinetics and selectivity aspects. Appl. Catal. A 2000, in press. (16) Smeds, S.; Salmi, T.; Lindfors, L. P.; Krause, O. Chemisorption and TPD studies of hydrogen on Ni/Al2O3. Appl. Catal. A 1996, 144. (17) Temkin, M. I. The kinetics of some industrial heterogeneous catalytic reactions. Adv. Catal. 1979, 28. (18) Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Electronic Release; Wiley-VCH: New York, 1999. (19) Walker, J. F. Formaldehyde; Reinhold Publishing Corp.: New York, 1964. (20) Watanabe, T.; Ikebe, T.; Iwamoto, A. Process for producing ditrimethylolpropane. EP 0 799 815, 1997. (21) Wauquier, J.-P.; Jungers, J. C. La cinetique quantitative en catalyse heteroge`ne. L’influence du milieu sur l’activite´ et la selectivite´ du catalyseur. Memoires Presente´ s a` la Societe´ Chimique; 1957; No. 228. (22) Yoneoka, M.; Watanabe, K.; Matsuda, G. Process for producing neopentyl glycol. U.S. Patent 5 395 989, 1995.

Received for review October 12, 1999 Revised manuscript received April 3, 2000 Accepted April 25, 2000 IE990745H