Sn-beta zeolite, in combination with borate salts, is a potential inorganic catalyst for sugars epimerization. We investigate, at molecular level, the catalytic mechanism of glucose epimerization to mannose, using density functional theory. Our calculations suggest that the tetrahedral borate ion forms a complex with glucose and inhibits the competitive isomerization reaction. The Lewis-acidic stannanol group of Sn-beta catalyzes glucose ring opening, which is followed by the silanol group (Bronsted acid site) catalyzed enolization. The epimerization then proceeds via an intramolecular 1,2 carbon shift and is found to be the rate-limiting step with an activation enthalpy of 26.3 kcal/mol. Catalytic activities of different tetravalent metal centers are compared, and Sn is found to be the most active metal. Additionally, it was found that the proximity of silanol group to the stannanol group, within the zeolitic framework, plays a key role in enhancing the catalytic activity of the silanol group. Hence, it is crucial to perform calculations with the entire ring structure of Sn-beta that opens up due to the hydrolysis of Sn-O-Si bridge. (C) 2015 Elsevier Inc. All rights reserved.