from first principles PP-I-1

PP-III-105Oligomerization of Ethylene over Ga + /ZSM-5 Zeolite from DiffuseReflectance IR and Quantum-Chemical DataSokolova N.A., Kuz’min I.V., Subbotina I.R., Zhidomirov G.M.N.D. Zelinskii Institute of Organic Chemistry RAS, Moscow, Russiasubbot@ioc.ac.ruDiffuse reflectance IR spectroscopy and quantum-chemical calculations were used to studyadsorption and subsequent ethylene transformations on gallium cations in gallium-modifiedZSM-5 zeolite. Modification of HZSM-5 zeolites with gallium cations was carried out viaanchoring of trimethyl gallium (TMG) or by incipient wetness impregnation with a solutionof gallium nitrate. Both ways of preparation result in similar quantitative substitution of allacidic protons by Ga + ions. DRIFTS measurements of ethylene adsorption on both samples ofGa + /ZSM-5 at 300 K show the similar results of decreasing the intensity of the bands at 3091and 2990 cm -1 related to adsorbed ethylene and increasing the intensity of the bands at 2860and 2937 cm -1 related to growing oligomer chains. These bands could not be removed byevacuation with heating up to 773 K. This data indicate on ethylene oligomerization ongallium cations Ga + without participation acidic protons even at room temperature. The rate ofethylene oligomerization on Ga + /ZSM-5 was some times smaller than in the case of HZSMsamples in the same conditions.Quantum-chemical calculations of adsorption and subsequent ethylene transformations onGa + exchanged cation of zeolite lattice were carried out using density functional theory with aB3LYP functional in the 6-31G basis set and the GAUSSIAN 03 program. Three kinds ofethylene activation on gallium cation were analyzed: activations which accompanied bycleavage of С-Н or С=С double bond and weak ethylene adsorption. It is found that activationof one ethylene molecule connected with cleavage of С-Н or С=С double bond required highactivation energy (~ 60 kcal/mol). On the other side the synchronous connection of twoethylene molecules with formation 5-membered heterocycle of Ga-C 4 H 8 can occur withsmaller activation energy (~ 20 kcal/mol). However it was found that removal of thesestructures from zeolite surface is energetically complicated. Process of hydrogenation of aheterocycle with formation of butane molecule was also considered and it was estimatedactivation barriers of this process (~ 30 kcal/mol) were estimated.Thus, one expects that Ga cations may also be involved in the secondary reactions of olefinsformed upon alkane dehydrogenation over Ga/ZSM-5 zeolite during process of aromatizationof light alkanes.265