ISSN 0023 1584, Kinetics and Catalysis, 2013, Vol. 54, No. 2, pp. 157–167. © Pleiades Publishing, Ltd., 2013. Original Russian Text © D.E. Zavelev, G.M. Zhidomirov, R.A. Kozlovskii, 2013, published in Kinetika i Kataliz, 2013, Vol. 54, No. 2, pp. 166–176.
Quantum Chemical Study of the Mechanism of the Catalytic Oxyethylation of Ethylene Glycol on Phosphorus Doped Titanium Dioxide: The Role of the Surface Phosphoryl and Hydroxyl Groups of the Catalyst D. E. Zaveleva,*, G. M. Zhidomirovb,c, and R. A. Kozlovskiid a Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia b Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia c Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia d Mendeleev University of Chemical Technology of Russia, Moscow, 125047 Russia *e mail: [email protected] Received June 25, 2012
Abstract—DFT calculations of the oxyethylation pathways of monoethylene glycol (MEG) and diethylene glycol (DEG) were performed on a model fragment of phosphorus doped titanium dioxide (anatase). It was shown that the surface hydroxyl group of titanium dioxide, whose proton initiates C–O bond cleavage in the ethylene oxide molecule, plays the key role in the activation of the molecule. At the same time, the phospho ryl group –P(OH)2O activates the reactant molecule R (MEG, DEG, etc.) and carries out the synchronous proton transfer from R to the hydroxyl oxygen atom of titanium dioxide, thus restoring the catalyst structure and closing the catalytic cycle. This restructuring occurs synchronously in one step. The substitution of the catalyst hydroxyl groups by alkoxyl groups can influence oxyethylation occurring via the bimolecular nucleo philic substitution mechanism and can poison the catalyst in some cases. DOI: 10.1134/S002315841302016X
Ethylene oxide (EO) hydration products with dif way of improving the selectivity is by applying a cata ferent degrees of oxyethylation, viz., monoethyelene lyst having a uniform pore distribution. This drasti glycol (MEG), diethylene glycol (DEG), triethylene cally decreases the formation of products having a glycol (TEG), and polyethyelene glycols find wide degree of oxyethylation above the preset value, since industrial application. At present, MEG is prepared the sieving effect takes place during the reaction [6]. both by hydration of EO and by methods unrelated to Phosphorus–titanate oxides obtained by the alkoxo oxyethylation [1]. However, the problem of DEG method [6], which have a highly organized structure preparation by selective oxyethylation of MEG is still and uniform porosity and exhibit molecular sieve topical [2, 3]. In addition, the mono and diaddition properties in alcohols oxyethylation [7–9], can be of EO to MEG can be regarded as a model reaction for used as such catalysts. Like other heterogeneous acid the preparation of a large class of industrial solvents, catalysts of oxyethylation, they allow one to use a such as cellosolves and carbitols [4]. smaller excess of the second reactant (water or alco hol) than in the homogeneous catalytic reactions and, Conducting the reactions under consideration on accordingly, to reduce the cost of product separation homogenous catalysts involves a number of problems. from the reactants and catalyst. However, their main The mosrt serious ones are the insufficient selectivity advantage over the other heterogeneous catalysts is their toward the formation of the EO monoaddition prod relatively high selectivity toward DEG, which is cur uct and high energy consumption in the removal of the rently produced as a by product of EO hydration [10]. homogeneous catalyst from the products and in the separation of the components of the final reaction Earlier [11], we theoretically studied EO hydration mixture. Application of heterogeneous systems elimi on these systems. It was shown that, in the activating nates the problem of catalyst separation from the reac interaction of EO with the catalyst, of great signifi tion products, since the catalyst is immobilized on a cance are the hydroxyl groups on the anatase surface, carrier, and significantly enhances the process selec which are involved in EO molecule activation and in tivity [5]. This allows one to use the second reactant in proton transfer. Doping of the titanium oxide surface smaller excess and to reduce the expenditures on the with phosphorus plays the determining role in proton separation of the reaction mixture. The most effective transfer.
157 158 ZAVELEV et al.
An essential aspect of oxyethylation on heteroge Since it was shown earlier that the P(V) containing neous catalysts is the immobilization, on the catalyst samples are superior in catalytic properties to the surface, of functional groups of compounds having a P(III) based catalysts, hereafter we will consider pre labile hydrogen atom. In the case of alcohols oxyethy cisely this structure. We also simulated the replace lations, these may be alkoxyl groups. Methanol and ment of the hydroxyl groups of both titanium and ethanol chemisorbed on anatase were shown to form phosphorus with OR groups, keeping in mind that surface esters [12, 13]; i.e., alkoxyl groups appear on ROH is a substrate with a labile hydrogen atom that the surface. It was discovered that alcohols can par can undergo oxyethylation. For example, if the sub tially displace the water adsorbed on the anatase sur strate is MEG, then R = –CH –CH –OH. face [14]. The adsorption capacity of anatase is the 2 2 lower the longer the hydrocarbon chain of the alcohol. After geometry optimization, we optimized the Some cases in which the oxyethylation catalyst structure of the catalyst coordination complex with obtained by the alkoxo method already contains a sto the EO and substrate molecules and calculated the ichiometric amount of alkoxyl groups were described transition states in the elementary reaction steps. To in [5]. It is also known that both ethanol and EO itself verify the calculated structures, we calculated har can act as donors of alkoxyl groups. This is exemplified monic vibration frequencies (the transition state is by the catalytic cycle of oxyethylation of fatty acids on characterized by a single imaginary frequency, which aluminosulfate catalysts [15, 16]. Probably, a portion of the reactants remained adsorbed on the catalyst. corresponds to the vibrations involving atoms between which the bond forms or breaks in the given elemen In the present work, we performed a comparative tary reaction) and additionally checked the corre analysis of the hydration of EO and the oxyethylation of MEG, DEG, and several other compounds con spondence of these states to the reactants and products taining a labile hydrogen atom on phosphorus–titan by descending from the found saddle point to the reac ate catalysts. It is assumed that the reaction proceeds tant and product valleys along the reaction coordinate. via the SN2 mechanism described in [11]. The analysis Water, methanol, ethanol, MEG, and DEG were was performed both taking into account and disre considered as substrates containing a labile hydrogen garding the effect of the compound to be oxyethylated atom. To perform comparative calculations, meth on the catalyst properties. We also studied theoreti anethiol, trifluoromethanol, and 2,2,2 trifluoroetha cally the adsorption of monoalcohols and glycols onto the surface of the phosphorus–titanate oxides. nol were examined. The structures of water, methanol, ethanol, methanethiol, trifluoromethanol, and 2,2,2 trifluoroethanol are such that they cannot have con COMPUTATIONAL DETAILS formational isomers. Calculations were performed via the Firefly pro Free MEG is known to have 10 conformers [30]. gram (version 7.1.G) [17] in terms of density func MEG adsorbed on the catalyst surface can apparently tional theory (DFT) [18] using a hybrid functional have 27 conformers, some of them forming different with Becke’s gradient corrections [19, 20], the Lee– adsorption structures (both monodentate and biden Yang–Parr correlation functional (B3LYP) [21], and the 6 31G** basis set [22–25]. Atomic charges and tate ones). Let us designate the MEG conformers as populations were calculated by Hirshfeld’s method follows: the first and third letter symbols designate the [26] using the NBO program (version 5.0) [27] and by H–O–C–C dihedral angles, the second symbol desig Mulliken’s method [27] using the Firefly program nates the O–C–C–O dihedral angle, the symbol t (version 7.1.G) [17]. The calculated data were visual designates the trans positions of the substituents (the ized using the ChemCraft program [29]. dihedral angle is close to 180°), and the symbols g+ and In the calculations on the oxyethylation mecha g– designate their gauche position (the dihedral angle nism, the model cluster considered in our previous is close to 60° and –60°, respectively). In the case of work [11] was taken as the basis. In this cluster, the MEG adsorbed in the monodentate mode, the first titanium atom is linked with the phosphorus atom letter in the conformer designation refers to the dihe through an oxygen bridge: dral angle formed by the hydroxyl group that is involved in the adsorption of the molecule. If the O O MEG molecule is adsorbed in the bidentate mode, the
O first letter in the conformer designation corresponds to P O Ti the dihedral angle formed by the hydroxyl group O adsorbed on the phosphoryl oxygen atom. O O Both theoretical and experimental studies [30⎯ 35] . provided evidence that the g–g+t conformer (1а)
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 QUANTUM CHEMICAL STUDY OF THE MECHANISM 159
It is known that the relative stabilities of conform ers can change due to both solvation in aqueous solu 2.279 tion [40] and adsorption onto the catalyst surface. O1 As the model conformer, we will consider con O2 – + – + –59.62° former 1b with the g g ttg g structure:
H10 C2 O3 C1 2.405 H8 2.405 62.71° C4 , O2 –62.71° 1а O1 C2 C3 has the lowest energy (in the gas phase) and the energy difference between the least and most stable conform C1 113.62° ers is no larger than 5 kcal/mol (depending on the . investigation method). In aqueous solution [35, 36] or liquid anhydrous MEG [37], other conformers can be 1b the most stable, both the gauche and trans forms of MEG being present in the liquid phase [38]. However, it is logical to assume that, under the conditions of heterogeneous catalysis at a pore size of about 8–10 Å RESULTS AND DISCUSSION [6–8], both water and the other molecules exert the Formation of Pre Reaction Complexes in the Presence minimum effect. For this reason, below we will con of Substrate Molecules sider the g–g+t conformer of MEG and will perform comparative calculations for several other conformers. Earlier [11], we simulated EO hydration without Moreover, the conditions under which the simulated considering the effect of the medium. It is unlikely that reaction is conducted (temperature of 115°C [6]) are this effect itself can be significant under the accepted such that the energy barrier between conformers can conditions; however, one should take into account readily be overcome due to the thermal motion of the that, when the substrate (ethanol or water) is in large molecules. molar excess, its molecules can adsorb onto active sites of the catalyst. According to the calculation per As for DEG, there had been no publications deal formed, structure 2a can form: ing with all DEG conformers to the date this article was written. Moreover, in view of the fairly large num ber of these conformers, such a study would go beyond O O the scope of this work. On the other hand, there are many works in which a conformation analysis of oligo and polyethylene glycols and their ethers is performed (see, e.g., [39–42]). For example, in the helical struc O O ture of polyethylene oxide, the dihedral angles are τ ° ′ τ ° ′ O (OCCO) = 64 58 and (CCOC) = 188 15 [39]. Ti O Several DEG conformers were studied in [40], and it P was found that some of them, specifically, the con O formers differing from one another only in conforma O tion with respect to the C–C bonds and being in the O , trans conformation with respect to the C–O bonds differ in energy by no more than 0.5 kcal/mol. The 2а “global” trans conformer is 0.03 kcal/mol highert on which corresponds to the local potential energy the energy scale than the conformer having the lowest minimum in the case of adsorption of two mole energy. A study of the dimethylcarbitol conformers led cules. The adsorption energy is –37.6 kcal/mol. For Gejji et al [41] to conclude that the global trans con comparison, the calculated energy of combined former (i.e., the molecule with substituents in trans adsorption of one water molecule and one EO mol positions at all C–C and C–O bonds) has the lowest ecule on the same cluster calculated by the same energy, the energy difference and the barrier between method is –34.4 kcal/mol [11]. Note that, among the the trans and gauche conformers with respect to the three hydroxyl groups at the titanium atom in the clus terminal C–O bonds are 1.3 and 2.0 kcal/mol, respec ter, only one is the true hydroxyl group, while the two tively, and the barrier to rotation about the C–C bond others simulate the anatase crystal lattice. Therefore, is lower than 2.3 kcal/mol. it would be incorrect to consider the structures where
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 160 ZAVELEV et al. the EO and substrate molecules are adsorbed on more the energy of adsorption of two MEG molecules is than one titanium hydroxyl group. equal –48.8 kcal/mol and the energy of replacement It should be noted that the above values, as well as of MEG with an EO molecule is only –0.3 kcal/mol. the values presented below, cannot be regarded as ther For the adsorption of two DEG molecules, these ener mochemically precise ones and cannot be compared gies are –49.3 and 26.8 kcal/mol, respectively. with experimental data, since they depend on the choice of computational method. However, a semi Due to the large number of possible conformers of quantative intercomparison of such values is quite pos the adsorption complex formed by two MEG mole sible. cules on the starting cluster, it would be unreasonable Taking into account the reaction conditions, one to consider all variants. can assume that the potential barrier to the substitu Obviously, under the conditions of real MEG oxy tion of an EO molecule for one of the adsorbed water molecules, which is 3.2 kcal/mol in this case, can ethylation, of great significance is the energy of easily be overcome due to the thermal motion of the replacement of two adsorbed MEG molecules with molecules. EO and DEG with the formation of the pre reaction Note that we failed to find the local minimum cor DEG oxyethylation complex. The energy of this responding to the adsorption of the water and EO mol replacement is –7.1 kcal/mol. (For comparison, the ecules onto the titanium hydroxyl group (the second energy of replacement with one water molecule and water molecule is adsorbed by the phosphoryl oxygen one EO molecule is –5.7 kcal/mol and, as was shown atom); i.e., water forces out the EO molecule. above, the energy of replacement of one MEG mole In the case of methanol oxyethylation, a cule with EO is –0.3 kcal/mol.) structure analogous to structure 2a forms as well. The adsorption energy of two methanol molecules is The calculated data for all of the compounds hav ⎯36.5 kcal/mol, and the energy of replacement of ing a labile hydrogen atom are given in Table 1. methanol with EO is 4.2 kcal/mol. Almost the same The data obtained suggest that, except for the values were obtained for the reaction involving etha adsorption of two DEG molecules (and, probably, the nol: –36.4 kcal/mol and 4.2 kcal/mol, respectively. oxyethylation products of monoalcohols), the molec As for MEG oxyethylation, for complex 2b, ular adsorption of the substrate does not hinder the formation of pre reaction complexes and oxyethyla O tion.
С С Hydration of Ethylene Oxide and Oxyethylation O O of Alcohols on the Phosphorus–Titanate Catalysts O O P O O Ti Let us consider oxyethylation proceeding by the С synchronous mechanism [11]. In the case of MEG O O O oxyethylation, the pre reaction complex (3a), transi tion state (3b), and post reaction complex (3c) are as 2b follows:
H9 H14 H12 H11 O8 H11 H12 C2 H7 C4 C3 H14 C3 C2 O10 C1 H15 H13 H8 C1 H11 H7 C4 H10 O1 H13 O9 C2 O8 H6 H12 C3 C1 O3 O8 O5 O9 H10 H15 H6 C4 H13 H10 Ti1 H14 O5 P1 O6 O2 O7 O3 O10 O5 . H3 O6 O6 O4 O1 H15 P1 O4 Ti1 O1 O3 O7 O4 H3 Ti1 O7
O2
3а 3b 3c
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 QUANTUM CHEMICAL STUDY OF THE MECHANISM 161 c a 9.9 14.0 26.7 26.6 26.8 43.1 28.0 25.0 energy Apparent activation activation 17.1 30.5 28.1 24.9 22.5 24.8 22.5 22.4 24.3 22.3 energy Activation a c 0.7 3.2 4.2 4.2 18.2 ment strate oxide –1.3 –7.1 –8.3 Energy energy parameters molecule kcal/mol of the sub of replace by ethylene plex –32.3 –35.2 –24.3 –34.4 –32.3 –32.3 Adsorp gy for the pre reac tion com tion ener –16.0 –49.3 –35.9 13.5 –37.6 –36.5 –36.4 –33.0 –34.0 on the Energy catalyst substrate molecules of adsorp tion of two tion of two b 1.550 1.495 1.857 1.979 1.563 1.505 orbital Atomic Atomic in HOMO in coefficient the labile hydrogen atom and oxyethylation atom the labile hydrogen –0.610 –0.494 –0.520 –0.046 –0.532 –0.534 Mulliken charge, [e] [e] –0.060 –0.752 –0.742 –0.948 –0.750 –0.758 charge, Hirshfeld 0.610 0.046 0.532 0.534 0.494 0.520 and the number between between tral atom Difference population in the neu of electrons the Mulliken d the oxygen or sulfur atom adjacent to d the oxygen or sulfur atom ts for the two hydroxyl oxygen atoms in HOMO are equal. in HOMO oxygen atoms hydroxyl ts for the two atom 0.931 0.726 0.021 0.735 0.743 0.735 aining two DEG molecules on the catalyst cluster. aining two and the number between between ntaining two DEG molecules on the catalyst cluster. ntaining two Difference population of electrons the Hirshfeld in the neutral units 0.171 0.180 0.179 0.170 0.206 0.133 rigidity, rigidity, Hartree Molecular units –0.298 –0.292 –0.265 –0.264 –0.259 0.160 0.747 0.545 –0.763 –0.545 1.486 –28.8 –31.2 –0.3 –0.268 0.161 0.741 0.537 –0.756 –0.537 0.746 –0.358 –0.237 energy, energy, Hartree HOMO H 2 O) 2 ) OH + 2 g ) CH OH Characteristics of the substrate molecules an 2 2 – 2 t) ttg OH Substrate OH 5 SH + + OH CH 3 3 3 3 g g O H 2 – – 2 Values measured versus the adsorption complex cont the adsorption complex measured versus Values Due to the symmetry of the conformer, the coefficien Due to the symmetry of conformer, Values measured versus the adsorption complex co the adsorption complex measured versus Values CH CH HO(CH CF (g CF (g C HO(CH H Table 1. Table a b c
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 162 ZAVELEV et al.
EO coordinated to the proton of one OH group on the C1–O9 bond. The latter process is accompanied by syn catalyst surface (by the O8–H1 hydrogen bond) is acti chronous proton transfer: the O1–H1 bond breaks, and vated due to this coordination, and the C1–O8 bond the O8–H1 bond forms; the O6–H4 bond breaks, and the breaks simultaneously with the nucleophilic attack of the O1–H4 bond forms; the O9–H10 bond breaks, and the alcohol on the C1 atom resulting in the formation of the O5–H10 bond forms. Thus, the catalytic cycle closes:
H H2 H 2 C 2 H2 C H2 H C C C H C OH 2 H2C OH 2 C H2C OH C O O O O O H2C CH2 H2 O H H H H H H O OH O OH H O O O O O O O O Ti P Ti P Ti P
OH OH OH .
In an earlier work [11], we estimated the activation 18.2 kcal/mol and the apparent activation energy is energy of EO hydration on the given catalytic system 43.1 kcal/mol. (The true activation energy appears to as the difference in energy between the transition state remain unchanged.) and the pre reaction complex, which was found to be The corresponding energy diagrams are shown in 24.8 kcal/mol. In view of the effect of the medium, the Fig. 1; for comparison, the energy diagram of noncat energy minimum must correspond to the adsorption alytic MEG oxyethylation is presented. The calcu complex consisting of two water (or alcohol) mole lated data refer to a mechanism analogous to the cules on the model cluster and to the EO molecule at mechanism of noncatalytic EO hydration considered an infinite distance from this complex, and the energy in [11]. of the water molecule being at an infinite distance It might seem that the apparent activation energy from the transition state should be added to the total of DEG oxyethylation is too high; however, since this energy of the transition state. This activation energy, reaction was experimentally studied in a large excess of which will be referred to as “apparent” energy, is MEG [6], it is reasonable to assume that the catalyst 28.0 kcal/mol. surface can be covered by molecularly adsorbed MEG. In the case of MEG oxyethylation, the activation In this case, the apparent activation energy of DEG energy is 24.9 kcal/mol and the apparent activation oxyethylation should be 9.9 kcal/mol. energy is 22.5 kcal/mol. Note also that the activation The energy parameters of this and other substrates energy measured versus the pre reaction complex are given in Table 1. Note that, when using water without considering the second MEG molecule instead of methanol or ethanol, both the activation depends on the MEG conformation in the pre reac energy and the adsorption energy (its absolute value) tion complex and in the transition state. For the calcu in the pre reaction complex decrease slightly, both lated reaction pathways, the activation energy is 14.2 energies depending very weakly on the alkyl chain to 30.1 kcal/mol. (In some reaction pathways, MEG length of the alcohol. The latter is quite natural, since in the pre reaction complex and in the transition state the alkyl group is not directly involved in the reaction. has the same conformation; in other cases, the pre Taking into account this fact, Improta et al. [16] mod reaction complex transforms into several possible eled the dodecyl radical as propyl. Of course, the alkyl transition states.) At the same time, as was specified radical influences the nucleophilicity of the alcohol above, the difference in energy between different oxygen atom; however, it is obvious that the inductive MEG conformations is unlikely to be greater than effect decays along the chain and the nucleophilicity 5 kcal/mol; therefore, this wide range of activation of the oxygen atom in ethanol differs only slightly from energies is explained by the fact that the pre reaction the nucleophilicity of the same atom in higher alco complex involving some MEG conformations is much hols. more stable due to the bidentate adsorption of MEG. The replacement of methanol with methanethiol If the concentration of DEG in the reaction mix does not result in any significant change in the activa ture is sufficiently high, its molecular adsorption on tion energy measured versus the pre reaction com the catalyst cluster can occur and the energy of plex; however, the absolute value of the adsorption replacement of two adsorbed DEG molecules with energy in the pre reaction complex decreases signifi one MEG molecule and one EO molecule becomes cantly and, consequently, the apparent activation
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 QUANTUM CHEMICAL STUDY OF THE MECHANISM 163 energy also decreases considerably. The replacement Formation of Surface Esters of methanol with trifluoromethanol slightly increases the activation energy; however, the apparent activation Note that, under the real reaction conditions, the energy decreases. When replacing ethanol with 2,2,2 catalyst contains some amount of surface alkoxyl trifluoroethanol, the activation energy increases and groups. The calculation shows that the energy of sub the apparent activation energy remains almost stitution of the hydroxyl group for the alkoxyl (meth unchanged. oxyl, ethoxyl) group is –3.0 kcal/mol for the catalyst An analysis of the lengths and orders of the forming model considered. and breaking bonds in the transition states, as well as in the pre reaction and post reaction complexes, It follows from the calculations that the energy of shows that, for the oxyethylation of water, methanol, substitution of the phosphorus hydroxyl group for the ethanol, MEG, methanethiol, trifluoromethanol, and methoxyl group is also –3.0 kcal/mol. Obviously, the 2,2,2 trifluoroethanol, these values are very close. At formation of surface esters involving the phosphorus the same time, for DEG oxyethylation, these values hydroxyl groups should not significantly influence the are somewhat different and the transition state is closer to the reaction products than in the previous reaction parameters, except for creating steric hin cases. However, no qualitative differences were drances for coordination of the alcohol molecule with observed. the phosphoryl oxygen atom. Thus, on passing from water to MEG, the apparent The value of this energy for MEG depends strongly activation energy decreases, and it decreases still more greatly on passing from MEG to DEG; however, in the on the conformation of the surface alkoxyl group and latter case, a sufficiently stable adsorption complex can vary in certain ranges: it is –4.2 kcal/mol for the forms, which hampers subsequent oxyethylation, and g+g+t conformer 4a and 1.4 kcal/mol for the con catalyst deactivation can occur. former 4b.
O
C O C C C
O O O
O Ti P O O Ti O O O O P O O O 4а 4b
O O C C O C C C C O O O O O O O P Ti O Ti O O P O Ti O O O O C C O O O C C
4c O 4d 4e
The energy barrier to this substitution for the latter 60.1 kcal/mol, which makes the process very unlikely. conformer is 8.0 kcal/mol. According to the calcula The energy of substitution of the second phosphorus tions, the energy of substitution of one phosphorus hydroxyl group for the MEG group in structures 4d hydroxyl group for the MEG group in the g+g+t con and 4e is –1.3 and 2.1 kcal/mol, respectively. Obvi formation (4c) is –3.5 kcal/mol. However, in this case, ously, there is a low energy barrier to rotation about the the activation energy of substitution reaches Ti–O bond. This energy for all possible conformers
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 164 ZAVELEV et al.
Transition states
43.1 c 32.7 22.5 24.9
b 24.9 Starting compounds а Reaction products Pre reaction complexes 51.9
Post reaction complexes
Fig. 1. Energy diagrams for the oxyethylation of MEG via the synchronous mechanism: (a) noncatalytic reaction, (b) catalytic reaction in the MEG medium, and (c) catalytic reaction in the DEG medium. The energy values are in kcal/mol. can be determined in principle; however, there is no ied by us earlier. In the previous work [11], we demon strong practical reason to do this: quantum chemical strated that this ethylene oxide hydration pathway calculations have confirmed the formation of surface requires a substantially higher activation energy and, esters with titanium hydroxyl groups. Thus, part of the therefore, is not realized. For this reason, this reaction catalyst’s active sites can contain alkoxyl groups in pathway is not considered in the present work. place of the hydroxyl groups. Note that, after the substitution of the titanium The energy of substitution of the titanium hydroxyl group for an alkoxyl group (the remaining two hydroxyl group for an alkoxyl group in DEG con ⎯ hydroxyl groups simulate the crystal lattice; i.e., they are former 5a is 0.1 kcal/mol, but for the model con not involved in the real catalytic cycle), the titanium former g–g+ttg–g+ (5b) the substitution energy is atom can activate EO only through the direct coordina ⎯ 10.7 kcal/mol. The substitution energy for the phos tion of the latter to the oxygen atom, which is typical of phorus hydroxyl group in model conformer 5c is the pre reaction complex in the two step reaction stud ⎯ 2.2 kcal/mol.
O O O O C C O C O C P C C O O O O O C O Ti O C O O P C C Ti C O P O O O O O O Ti
O 5a 5b 5c
The calculated data confirm the logical assumption Regarding MEG, it is obvious that, even after the that EO does not coordinate to the carbon atoms or formation of a surface ester, it still has a labile hydro proton of the surface methoxyl group. The methyl gen atom and, consequently, it is still able to undergo protons cannot be regarded as labile, so the surface oxyethylation. methoxyl group itself does not undergo oxyethylation. The energy of substitution of the surface hydroxyl Since the substitution of the hydroxyl group for the groups of titanium and phosphorus for an alkoxyl alkoxyl group is energetically favorable, the formation group is, respectively, 1.0 and 7.8 kcal/mol for trifluo of surface esters in the oxyethylation of alkanols is one romethanol and –3.1 and –0.9 kcal/mol for 2,2,2 tri of the ways leading to catalyst deactivation. fluoroethanol.
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Table 2. Calculated energy parameters of the hydration of ethylene oxide and the oxyethylation of methanol and monoet hylene glycol on the phosphorus–titanate catalyst
Energy Energy of adsorption Adsorption of substitution Apparent Alcohol involved of two energy for of the substrate Activation activation in the formation substrate the pre reac molecule energy energy Substrate of the surface ester molecules tion complex for ethylene with the phosphorus on the catalyst oxide hydroxyl group kcal/mol
– –37.6 –34.4 3.2 24.8 28.0 H2O CH3OH –31.2 –31.0 0.2 24.7 24.9 – + HO(CH2)2OH (g g t) –32.9 –29.2 3.7 24.7 28.4 – –36.5 –32.3 4.2 22.5 26.7 CH3OH CH3OH –32.6 –28.8 3.8 22.3 26.1 – –28.8 –31.2 –0.3 24.9 22.5 – + – + HO(CH2)2OH (g g t) HO(CH2)2OH (g g t) –31.8 –23.4 6.5 15.3 23.8
Effect of Surface Esters on Oxyethylation tion (except for possible steric hindrance). The calcu As was mentioned above, the formation of a surface lated adsorption and activation energies in the cases ester involving the titanium hydroxyl group prevents considered are given in Table 2. Below, we show the this group from participation in the catalytic cycle. pre reaction complex (6a), the transition state (6b), The formation of the surface ester on the phospho and the post reaction complex (6c) in the oxyethyla rus hydroxyl group does not interfere with oxyethyla tion of MEG.
O C C O C O O C C C C C O C C O O O
O O O O O O O O O O P P Ti O O Ti P O O O Ti O O O O C O O C C C O O C C
6a 6b 6c
Figure 2 shows the energy diagrams for the reac Adsorption of Reaction Products in Pores tion. As can be seen, the surface esters have no consid As was established earlier, the mean pore diameter erable effect on the activation parameters of EO of the heterogeneous catalysts examined is 8–10 Å hydration and methanol oxyethylation. In the case of [6 ⎯ 8]. Therefore, the predominant formation of DEG MEG oxyethylation, the situation is somewhat differ from the starting MEG can be explained by the sieve ent: the true activation energy is appreciably lower effect. The molecular size of DEG is fairly large (up to than that for the starting catalyst, and the apparent 7–8 Å across, depending on the conformation) and its activation energy is slightly higher. entry into the pore can be quite hindered; the forma tion of TEG during the reaction will probably result in If both hydroxyl groups at the phosphorus atom stickking of this molecule in the pore (i.e., it will be form surface esters, the above catalytic cycle will not adsorbed in the polydentate mode with a sufficiently take place. high adsorption energy) and in the blocking of the
KINETICS AND CATALYSIS Vol. 54 No. 2 2013 166 ZAVELEV et al.
Transition and surface ester formation taken into account. We did states not reveal any regularities relating these parameters to the electronic characteristics of the substrates having a labile hydrogen atom. It was demonstrated that cata lyst deactivation can occur due to the formation of в
15.3 surface esters and the polydentate adsorption of the 23.8 22.5 б reaction products in pores. Starting The high selectivity of the catalytic system in the compounds oxyethylation of MEG is explained not only by the а 24.9 Reaction molecular sieve effect, but also by the adsorption of Pre reaction complexes products reaction products (such as tri and tetraethylene gly cols) in the catalyst pores, which results in catalyst Post reaction complexes deactivation, but not decreases its selectivity.
Fig. 2. Energy diagrams of the oxyethylation of MEG via REFERENCES the synchronous mechanism: (a) noncatalytic reaction, 1. Weissermel, K. and Arpe, H. J., Industrial Organic (b) catalytic reaction in the MEG medium on the catalyst Chemistry, Weinheim: Wiley VCH, 1997, 3rd ed. cluster containing no alkoxyl groups, and (c) catalytic reaction in the MEG medium on the catalyst cluster con 2. Chinn, H. and Kumamoto, T., Chemical Economics taining an alkoxyl group at the phosphorus atom. The Handbook [2010]. http://www.sriconsulting.com/CEH/ energy values are in kcal/mol. Public/Reports/652.4000/. Cited June 17, 2012. 3. US Patent 7164048, 2007. access of other molecules to the active sites. Since oxy 4. Dyment, O.N., Kazanskii, K.S., and Miroshnikov, A.M., Glikoli i drugie proizvodnye okisei etilena i propilena ethylation is irreversible, the formation of tri and tet (Glycols and Other Derivatives of Ethylene and Propy raethylene glycols in the catalyst pores can be one of lene Oxides), Moscow: Khimiya, 1976. the causes of the irreversible deactivation of the cata 5. Bialowas, E. and Szymanowski, J., Ind. Eng. Chem. lyst during the experiment. Res., 2004, vol. 43, p. 6267. In addition to undergoing molecular adsorption, 6. Kozlovskii, R.A., Yushchenko, V.V., Kitaev, L.E., DEG can be involved in the formation of surface Bukhtenko, O.V., Voloshchuk, A.M., Vasil’eva, L.N., esters. As was mentioned above, this process is ther and Tsodikov, M.V., Russ. Chem. Bull., 2002, vol. 51, modynamically favorable; therefore, some portion of no. 6, p. 967. the resulting DEG immediately forms surface esters. 7. Tsodikov, M.V., Bukhtenko, O.V., Slivinskii, E.V., Slas It is reasonable to assume that the adsorption of tikhina, L.N., Voloshchuk, A.M., Kriventsov, V.V., and TEG (both molecular adsorption and adsorption with Kitaev, L.E., Russ. Chem. Bull., 2000, vol. 49, no. 11, the formation of surface esters) can make the pores p. 1803. impassable (due to the large chain size), but is thermo 8. Tsodikov, M.V., Slivinskii, E.V., Yushchenko, V.V., dynamically very favorable. Kitaev, L.E., Kriventsov, V.V., Kochubei, D.I., and Teleshev, A.T., Russ. Chem. Bull., 2000, vol. 49, no. 12, Thus, the adsorption of DEG and, to a still greater p. 2003. extent, the adsorption of glycols with higher degree of 9. Kozlovskiy, R.A., Shvets, V.F., Koustov, A.V., oxyethylation are among the causes of the deactivation Kitaev, L.E., Yushchenko, V.V., Kriventsov, V.V., of the catalysts. Kochubey, D.I., and Tsodikov, M.V., Chem. Sustainable Dev., 2003, vol. 11, p. 123. CONCLUSIONS 10. Khandal, R.K., Kaushik, S., Seshadri, G., and Khan dal, D., Handbook of Detergents, Part F: Production, In the present work, we carried out a theoretical Boca Raton, Fla.: CRC, 2009, p. 491. study of alcohols adsorption (both molecular adsorp 11. Zavelev, D.E., Tsodikov, M.V., Zhidomirov, G.M., and tion and adsorption with the formation of surface Kozlovskii, R.A., Kinet. Catal., 2011, vol. 52, no. 5, esters) onto the phosphorus–titanate catalyst and its p. 659. effect on the oxyethylation reaction. It was shown 12. Davydov, A.A. and Shepot’ko, M.L., Theor. Exp. that, under the real reaction conditions, the processes Chem., 1988, vol. 24, no. 6, p. 676. occurring on the porous catalyst include both molec 13. Shepot’ko, M.L. and Davydov, A.A., Theor. Exp. ular adsorption of alcohols onto all active surface sites Chem., 1991, vol. 27, no. 2, p. 210. and the formation of surface esters. In some cases, this 14. Carrizosa, I. and Munuera, G., J. Catal., 1977, vol. 49, results in catalyst deactivation. The activation param p. 174. eters of oxyethylation of water, glycols, and some other 15. Di Serio, M., Iengo, P., Gobetto, R., Bruni, S., and compounds containing a labile hydrogen atom were Santacesaria, E., J. Mol. Catal. A: Chem., 1996, compared, including with the effects of the medium vol. 112, p. 235.
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