Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso Conversion of methanol to dimethyl ether over silicoaluminophosphates: Isolated acid sites and the influence of silicon islands. A DFT-ONIOM study Anibal Sierraalta a,∗ , Rafael Añez a , David S. Coll b , Paola Alejos a a Laboratorio de Química Física y Catálisis Computacional, Centro de Química, Instituto Venezolano de Investigaciones Científicas, Apartado 21827, Caracas, 1020-A, Venezuela b Laboratorio de Físico Química Teórica de Materiales, Instituto Venezolano de Investigaciones Científicas, Venezuela ARTICLEINFO Keywords: ONIOM SAPO ωB97X-D DME NH 3 adsorption ABSTRACT ONIOM(DFT:PM3) calculations were analyzed here in order to examine if, and how the isolated Brønsted acid sites,thesiliconislandsandtheporediameteraffecttheactivationenergiesinthedimethyletherformationfrom methanol for a series of silicoaluminophosphate (SAPO) molecular sieves, SAPO-5, SAPO-11, SAPO-34 and SAPO-41. Three density functionals were employed: B3LYP, ωB97X and ωB97X-D. The results revealed that the CH3OH adsorption energy increases with the type of functional used, according to the trend B3LYP < ωB97X < ωB97X-D. In general, the height of the transition state barriers depends on the acid site type.Inthecaseoftheformationofmethoxyspecies,theenergybarriersarelowerinthesiliconislandsthanin theisolatedSisites.Onthecontrary,forthedimethyletherformationfrommethoxyspecies,theenergybarriers are higher at silicon islands than at isolated sites. 1. Introduction Methanol dehydration, by acid catalysts like zeolites or silicoalu- minophosphates,hasreceivedinthelastyearsagrowingattentionfrom scientist communities mainly because, depending on the catalyst and the reaction conditions, different products can be obtained such as ethers, olefins, hydrocarbons, etc. Processes where the methanol de- hydration is the important step such as methanol-to-olefin (MTO), methanol-to-hydrocarbon (MTH), methanol-to-gasoline (MTG), etc. are nowadays investigated extensively. Among these processes the trans- formation of methanol to dimethyl ether (DME) has received attention not only for the importance of DME per se but because it is one of the first steps in the MTO and MTH reactions [1–5]. A variety of silicoaluminophosphate (SAPO) molecular sieves have been used in the processes of methanol dehydration for example SAPO- 34[3,4,7–9],SAPO-5[9–11],SAPO-11[4,9,10],SAPO-41[4,9,10],etc. Depending on the work conditions such as temperature, pressure, flow, etc., the reaction of methanol with these molecular sieves gives rise to different types of products in different percentages such as DME, ethane, propene, propylene, C5–C7 aliphatics, aromatics, etc. In the reaction of conversion of methanol to DME over acidic catalysts, two moleculesofmethanolareconsumedandonewatermoleculeandDME are produced (eq (1)) 2CH 3 OH→CH 3 OCH 3 +H 2 O (1) Two possible pathways, one dissociative and another associative, havebeenproposedintheliteraturetoexplainthisreaction[7,12–18]. In the zeolites as well as in SAPOs both pathways are catalyzed by Brønsted acid sites, located at the O atom bridging an Al atom and a Si atom of the structure. In the dissociative path, one methanol molecule reacts with the acid site forming a surface methoxy group and a water molecule as shown in Scheme 1. Once the surface methoxy group is formed, the reaction continues with the attack of a second methanol molecule to this group, which leads to the formation of DME. On the contrary, in the associative path thereisnotsurfacemethoxygroupformationandtheformationofDME and water occurs after the simultaneous co-adsorption of two methanol molecules at the Brønsted acid site. Experimental and computational studies have been published in the literature; these studies have attempted to reveal the preferred me- chanism of methanol dehydration to DME. The earliest theoretical studies were done by Blaszkowski and van Santen [15,16], using a 3T cluster and the local density approximation (LDA). From their results the authors concluded that the associative pathway is the preferred over the dissociative and over a third reaction path which is actually a mixing between the associative and the dissociative. Waroquier and col [19].usinga5TclusterandB3LYPapproachreportedthattheintrinsic https://doi.org/10.1016/j.micromeso.2019.109732 Received 14 May 2019; Received in revised form 15 September 2019; Accepted 16 September 2019 ∗ Corresponding author. E-mail address: asierral@gmail.com (A. Sierraalta). Microporous and Mesoporous Materials 292 (2020) 109732 Available online 18 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved. T