Quantum Chemical Study of the Mechanism of the Catalytic
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ISSN 00231584, 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 PhosphorusDoped 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 *email: [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 phosphorusdoped 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 molecularsieve 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 byproduct 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 2,2,2trifluoroetha 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 631G** 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 .