Nature of Synergy between Brønsted and Lewis Acid Sites in Sn–Beta Zeolites for Polyoxymethylene Dimethyl Ethers Synthesis

Abstract

The role of Lewis and Brønsted acid sites and their potential synergy remains ambiguous for the production of polyoxymethylene dimethyl ethers (OME), which are suitable as a Diesel substitute. Here, this synergistic effect was investigated by using a series of beta polymorph A (BEA) zeolites with various degrees of Brønsted and Lewis acidity. Lewis acidity was introduced in dealuminated zeolites by Sn grafting in dichloromethane. These sites were only active in paraformaldehyde decomposition, OME growth, and acetalization. The Brønsted acid sites arising from bridging hydroxyl groups were active for all reaction steps, and notably for trioxane ring‐opening and dissociation to formaldehyde (FA), which did not occur on the Lewis acid sites. Presence of both Lewis and Brønsted acid sites led to a four‐fold increase in turnover frequency and a significant decrease of byproduct formation compared with the parent zeolite during OME synthesis from dimethoxymethane and trioxane. The synergistic effect between both types of acid sites is explained by FA insertion on Lewis acid sites leading to OME growth. Interaction between tetrahedral Sn and the carbonyl group of FA resulted in an activated carbonyl bond, which was likely the initial step for insertion of FA into OME.

Continuous Synthesis of Oxymethylene Ether Fuels from Dimethyl Ether in a Heterogeneously Catalyzed Liquid Phase Process

“…As surface area, pore volume and the concentration of acid sites is lower than for the BEA catalyst, this illustrates the strong influence of the nature of acid sites for OME synthesis. H-ZSM-5-80 exhibits Brønsted and Lewis acid sites in a ratio of 1.91 in contrast to 1.14 for H-BEA-25, showing the synergism between both types of acid sites, which has also been observed within other studies on OME synthesis [43,54,55].…”

“…Additionally, the number of Lewis acid sites decreases while the amount of Brønsted acid sites increases, altering the nature of acidity [53]. This is important for OME synthesis in general, as literature studies show that Lewis acid sites are only active in the presence of Brønsted acid sites, indicating synergistic effects [54,55]. Regarding the lower OME selectivity of the K10 Sn catalyst, formation of strong Lewis acid sites by incorporation of SnO 2 nanocrystals is suggested [39].…”

Optimizing zeolite stabilized Pt-Zn catalysts for propane dehydrogenation

“…3(c). Parent H-Beta demonstrates two NH 3 desorption peaks, revealing the presence of both weak and strong acid sites, respectively (not shown here) [37][38][39][40][41]. After the complete dealumination of H-Beta, no desorption peaks could be observed for the resulting Si-Beta sample due to the absence of acid sites [42].…”

Techno-economic assessment and carbon footprint of processes for the large-scale production of oxymethylene dimethyl ethers from carbon dioxide and hydrogen

“…For the synthesis of OME, methyl-capping groups such as methanol (H 3 C-OH, MeOH), methylal (H 3 C-O-(CH 2 O) 1 -CH 3 , OME 1 ), and dimethyl ether (H 3 C-O-CH 3 , DME) need to react over acid catalysts with a source of formaldehyde group such as formalin, paraformaldehyde (HO-(CH 2 O) n -H with n ¼ 8-100, pFA), trioxane (C 3 H 6 O 3 , TRI), and anhydrous formaldehyde (H 2 C-O, FA). The reaction proceeds through an initiation, growth, and termination mechanism, as described by Baranowski et al, 44 Schmitz et al, [45][46][47][48] and Oestreich et al 49 This leads to several simultaneous catalyzed and non-catalyzed reactions and the formation of undesirable side-products such as poly-(oxymethylene)hemiformals (HF), poly-(oxymethylene) glycols (MG), water, and others, as shown in Fig. 1 and extended in the ESI.…”

“…Finally, the understandings of this work highlight the critical process components in the OME value chain for further R&D endeavours. The reaction proceeds through an initiation, growth, and termination mechanism as described by Baranowski et al 44 , Schmitz et al [45][46][47][48] , Oestereich et al. 49 This leads to several simultaneous catalyzed and non-catalyzed reactions and the formation of undesired side-products such as poly-(oxymethylene) hemiformals (HF), poly-(oxymethylene) glycols (MG), water, and others as shown in Figure 1 and extended in the ESI.…”