Ab initio molecular orbital calculations of the protonation of propylene and isobutene by acidic hydroxyl groups of isomorphously substituted zeolites

Abstract

Ab initio molecular-orbital calculations using the 3-21 G basis set are performed to study the protonation reaction of propylene and isobutene by zeolite bridged hydroxyls ZOH, which are simulated by a cluster model which consists of two Si tetrahedra and one A1 tetrahedron. We have extended this study to include clusters with different compositions by introducing B and Ga in TI1' positions. The calculations show that in all reactions the adsorption of the olefin molecule on the acidic OH group takes place, leading to a stable r-complex with a structure similar to those of the isolated olefin and clusters constituents. The r-complex is transformed into a zeolite-alkoxide of covalent characteristics. The surface alkoxides are the most stable structures with reaction energies in the range of -1 5 to -1 7 kcal/mol. These reactions take place through transition states whose organic fragment has a geometry and charge distribution resembling those of the 2-propyl and tert-butyl classical carbenium ions for zeolite-propylene and -isobutene, respectively. The obtained activation energies (30-40 kcal/mol) are of the same order of magnitude in all the reactions considered. The bifunctional mechanism of the reactions is rather complicated and implies a concerted process involving the proton transfer from the zeolite toward a carbon of the olefin double bond, and the simultaneous C-O bond formation at the adjacent oxygen on the zeolite structure. The reaction mechanism and the properties of the transition states are practically the same, irrespective of the framework TI1' atom and the increase in propyleneisobutene branching. It is then proposed that the geometric conformation of transition state can be more determinant than the chemical composition of the zeolite. In this respect, the flexibility of the zeolite structure plays an important role. IntroductionThe use of zeolites as acidic catalysts has raised interest in the structure and properties of their active sites.t-s The sources of Brbnsted acidity in zeolites are bridged hydroxyls which arise from the presence of A1 or more generally TI1' atoms replacing Si in their structure (see Scheme I).Its is generally accepted that interaction of olefins with strong acid hydroxyls in zeolites results in the formation of adsorbed carbenium ions and their evolution can explain the mechanism of the reactions catalyzed by solid acids such as cracking, isomerization, and alkylation.Ig6 This mechanism is apparently indistinguishable from that proposed on homogeneous acid catalysts, since in the formal mechanisms, the solid acid catalysts are not considered explicitly, and therefore their influence on the formation and reactivity of the carbocations is not considered.Kazansky et al.'-I2 have shown that this traditional point of view is too simplified. Their ab initio quantum chemical calculation of ethylene interaction with zeolite OH groups, simulated by the simplest cluster HO(H)AI(OH)2OH, and the semiempirical study of the electronic structure and geometry of different surface alko...

Quantumchemical study of the isobutane cracking on zeolites

“…In accordance with earlier calculations performed for ethene [14,31] and for propene and isobutene [15], the reaction starts with the formation of the w-complex. This elementary step does not require any activation energy and is exothermic at the MP2/6-31 + + G* *//6-31G* level (with ZPE corrections) by 7.4 kcal/mol.…”

“…This is close to the heat effects of 7.3-11.3 kcal/mol obtained from quantumchemical calculations in Ref. [15]. The transformation of the w-complex into a covalently bonded surface tert-butyl fragment occurs through a transition state TS-I of Fig.…”

“…This is slightly less than the values of 8.2-11 kcal/mol calculated in Ref. [15] with different basis sets. Only minor changes in both cluster and isobutene geometry take place when the w-complex is formed.…”

“…This is somewhat less than the value of 30.1 kcal/mol obtained from quantumchemical calculations in Ref. [15]. The difference is most likely accounted for different basis sets and cluster dimensions used in Ref.…”

Positron emission imaging in catalysis

“…Promoting platinum acts as an alkane dehydrogenation and an alkene hydrogenation catalyst. The zeolitic protons protonate the olefins, which then, as predicted by theory [49,50], remain strongly adsorbed (enthalpy of protonation 180+20 kJ/mol).…”