IR study of adsorption and reaction of methylbutynol on the surface of pure and modified MgO catalysts: probing the catalyst surface basicity

FT infrared spectroscopy (FTIR spectroscopy) has been used to study the ads orption and reactivity of 2-methyl-3-butyn-2-ol (MBOH) on the Surfaces of p ure and Cs+ - and Ba+ -modified MEG. It has been found that MBOH is adsorbe d via two different mechanisms. Dissociative adsorption at acid-base (M-O) pair-sites with the creation of new H-bonded surface OHs groups is the most favorable adsorption mode in the case of Cs/MgO, This is attributed to the strong Lewis basic sites generated on the catalyst surface upon impregnati on with Cs+ cations which facilitate abstraction of hydrogen from MBOH, lea ding to the formation of alcoholate species. On the other hand, the Ba/MgO catalyst adsorbs MBOH preferentially via interaction with surface hydroxyl groups. Both these adsorption modes are operative on the surface of pure Mg O. The acetylenic group is also involved in the adsorption of MBOH. The aci dic acetylenic hydrogen interacts with the Lewis basic sites, whereas the L ewis acid sites interact preferably with the acetylenic pi-electron system. With regard to surface reactivity,the catalysts are active towards the deco mposition of MBOH to acetone and acetylene, revealing their basic propertie s. The Cs/MgO catalyst exhibits higher activity than MgO, which appears to be more active than catalysts impregnated with Ba2+ ion. The formation of B aCO3 las detected by X-ray diffractometry (XRD) on the MgO surface may be r esponsible for this lower activity. It has been found that the reaction pro duct acetone is polymerized rapidly and adsorbed strongly on the surfaces o f Ba/MgO, In conclusion, the basicity of this series of catalysts can be ra nked as follows: Cs/MgO > MgO > Ba/MgO (C) 2000 Elsevier Science B.V. All rights reserved.