Theoretical Study Of The Selective Oxidation Of Propylene On MoO₃/Bi₂O₃Catalyst

Selective heterogeneous oxidation catalysis is of vital importance to the well being of society, since it produces about one quarter of the most important industrial organic chemicals and intermediates used in the manufacture of industrial products and consumer goods. Oxidation of propylene to acrolein by bismuth molybdate catalyst has been applied in industry over50years. Although much attention has been focused on this reaction, there still remain quite a number of problems to be tackled.Experiments reveal that both activity and selectivity of the reaction that propylene converts to acrolein over the complex compound Bi2O3/MoO3can exceed90%. But this reaction can hardly take place over MoO3surface. When the reactant of propylene is replaced by these could produce allyl easily, such allyl iodide, the acrolein becomes the main product again. The most abundant product is1,5-hexadiene, which is the dimerization of allyl, while propylene was over Bi2O3surface. Based on these facts, the first dehydrogenation reaction is associated with bismuth and the second dehydrogenation is on the molybdenum site in the most popular accepted mechanism. The computation results give some different conclusions. Based on the cluster model (Mo3O9, Bi4O6, Bi4O7), the second dehydrogenation reaction of propylene has a larger barrier than the first one over the molybdenum oxide; not Bi(â…¢) but Bi(â…¤) oxide can catalyze the first dehydrogenation reaction. Another computation on the Bi2Mo3O12(010) surface pointed out that all the dehydrogenation reactions take place on molybdenum centres. The role of bismuth is to provide the requisite structural and electronic environment at the active site.To clarify these conflicts, density functional theory is used to investigate the properties and reaction mechanisms of propylene over MoO3and Bi2O3surfaces. The primary investigation contents and conclusions are summarized as follows.At first, some properties especially the Raman spectra of MoO3are investigated. In experiment, the peaks at about996,820and668cm-1are always used to distinguish the three types of oxygen:terminal (Ot), asymmetric (Oa) and symmetric (Os). Our calculations pointed out that the peak at about820cm-1may also be attributed to the vibration of symmetric oxygen (Os). The peaks that should be attributed to the asymmetric oxygen should be at about899and723cm-1. Single hydrogen atom is favorably bound on the asymmetric oxygen in MoO3(010) surface. While two hydrogen atoms are bound on surface, the terminal oxygen becomes the favorable position. The averaging average binding energy of two H atoms sharing one terminal oxygen atom is larger than that for one H atom on the terminal site. The apparently controversial viewpoints about the stability ordering of oxygen point defect is rationalized, in oxidized or inert situations, the ordering is determined by the removal of oxygen atoms whereas in H2atmosphere it is controlled by water molecule desorption.The adsorption of other species, such as propylene, allyl and acrolein, are also invstigated on the MoO3(010) surface. From the most stable chemical adsorbed propylene, which is called di-a bonded propylene, it is found hard to convert to the corresponding allyl due to the large barrier. On other forms of bound propylene the first dehydrogenation reaction can be relative easy but they are restrained by the small binding energies. The first dehydrogenation reactions are always endothermic reactions but further dehydrogenation reactions from the allyl are exothermic. The barriers for the second dehydrogenation reactions are very low. This result is consistent with the experimental facts but in conflict with the results of the cluster model. Including the hepta-ring mechanism (also called as (5+2) mechanism), the penta-ring mechanism (also called as (3+2) mechanism) may also play important role in the dehydrogenation process. The point defect is found to make the first dehydrogenation reaction feasible.To correlate the reactive barriers and the reaction energies, the traditional Bronsted-Evans-Polanyi (BEP) relation is used. It works well in the first dehydrogenation reactions but fail in the second dehydrogenation reactions. Based on the idea of Marcus equation, a distance corrected BEP or Marcus relation is suggested. This relation can work well both in the first and second dehydrogenation reactions.According to the Mars-van Krevelen mechanism, the reactants are oxidized by the lattice oxygen of catalyst and the reduced catalyst is replenished by the gas di-oxygen. To make clear the latter process on MoO3(010) surface, the formation energy of two or more vacancies is first investigated. When two or more terminal vacancies are along a (asymmetric oxygen) direction, the average formation energy decreases gradually. But when the vacancies are along b (symmetric oxygen) direction, the average formation energy increases gradually. These different trends would further lead to the different characters in surface phase diagram. The intersection point of line defect and point defect along a direction is above the surface energy line of perfect surface. This point along b direction is below the energy line of the perfect surface. This conclusion can be extended to many AxBy systems. When the formation energies increase gradually, point and line defects appear gradually. While the formation energies decrease gradually, only the line defect appears.Next, the properties and reactions on another catalytic component, Bi2O3, are investigated. Since theoretical and experimental characterizations of BiaO3surfaces are lacking, the stability of terminations and surfaces are studied at first. Ten different terminations along [100] direction which has both polar and non-polar terminations due to alternating stacking of Bi layers and O layers. Our calculated surface free energies show that the stoichiometric symmetric terminations are most stable at both high and low oxygen pressures, followed by the T2O/T40terminations at low/high oxygen pressures. In the low Miller index planes, the (010) plane is the most stable whereas the (110) plane is the least stable. There is a nice linear relationship between the surface density of broken short Bi-O bonds and the surface energy before relaxation.The dehydrogenation reactions on various Bi2O3surfaces are then investigated. On the most stable (010) surface and the most stable termination of (100) T2Bi(â…¡), the dehydrogenation is thermodynamic forbidden. The reaction on the high miller index surface (211) has the smallest barrier and it may be one of the main ways to dehydrogenate on Bi2O3surface.At last, the role of elementary reaction step number in the catalysis reactions is analyzed based on the consecutive reaction mechanism. It is found that the total reaction rate is invariant when any two reaction rate constants of elementary step exchange. A term1/N is introduced into the apparent pre-exponential factor and activation energy. This lead to an optimum step exists for many catalysis reactions. Comparison with different steps reaction, one transformation temperature may be found, which is same as the compensation effect. Further analysis demonstrates that some catalysts may become inhibitor under different temperatures.