Acidic zeolite MTBE synthesis

The first in situ MAS NMR investigation of the synthesis of MTBE on acidic zeolites was performed by Mildner et al. (228) under batch reaction conditions. In this investigation, the temperature-jump MAS NMR technique (stop-and-go experiment, see Section III.A) was applied to characterize the reaction dynamics under non-equilibrium conditions on a boron-modified pentasil zeolite ( si/... 

By characterizing various zeolite catalysts under the same reaction conditions, the authors found weaker MAS NMR signals of alkoxy species for the less active zeolites HY and HZSM-5 than for the more active zeolite H-beta (250). This observation suggests that the alkoxy species observed under steady-state conditions act as reactive surface species in the MTBE synthesis from isobutylene and methanol on acidic zeolite catalysts. 

In this work, the triflic acid modified Y-zeolite catalyst has been investigated for the atmospheric synthesis of MTBE and ETBE. In particular, the apparent activation energy for MTBE was determined, and this value is compared with those reported in the literature [1,6]. In addition, for both syntheses, the product selectivities are reported as functions of the contact time at the temperature where the catalyst activity is the highest. The catalyst stability for the MTBE synthesis was also examined. 

A large number of papers have been published on the process modeling and optimization of the etherification process. More details could be found in a handbook. The most important aspect of process improvement is catalyst improvement because the Amberlyst ion-exchange resin used in the MTBE synthesis has an upper thermal stability limit of less than 100°C and there is a need to develop other acidic catalysts with higher thermal stability. Some of the recent papers have described the use of zeolites. 

Conventional MTBE synthesis is carried out over acidic ion-exchange catalysts. The feasibility of the use of shape-selective zeolite catalysts (e.g., ZSM-5 and ZSM-l l) for MTBE synthesis has been studied [54], These zeolite catalysts are thermally stable at high temperatures, give no acid effluent, and are less sensitive to the methanol-to-isobutene ratio. The high selectivity of the zeolite catalysts is attributed to its well-defined geometry of channel structures... 

The first in situ MAS NMR investigation of the synthesis of MTBE on acidic zeolites was performed by Mildner et al. (228) under batch reaction conditions. In this investigation, the temperature-jump MAS NMR technique (stop-and-go experiment, see Section III.A) was applied to characterize the reaction dynamics under non-equilibrium conditions on a boron-modified pentasil zeolite ( si/ Mg = 80). The catalyst was calcined in a glass insert, which was sealed after the loading with MTBE. H MAS NMR spectra were recorded during the heating period of 100 s. Then the laser power was switched off and the temperature of the samples fell back to room temperature within about 60s. During the stop period of 1 h, when the reaction state was frozen, a C MAS NMR spectrum was recorded. By repetition of the stop-and-go periods for several times, the complete reaction could be measured by both H and MAS NMR spectroscopy. 

MTBE is used on a large scale as an octane number boosting additive in unleaded gasoline. Sulfonic acid resins are applied as efficient catalysts for the industrial production of MTBE from methanol and isobutylene (222). Since 1987, investigations of the synthesis of MTBE with reactants in the gas phase have been performed with zeolites HY (223-225), H-Beta (226), HZSM-5 (224,225), and H-BZSM-5 (227) as catalysts. 

The reaction of mono- and poly-alcohols catalyzed by solid acids has been widely investigated. An important application is the synthesis of five membered cyclic ethers starting from di- or triols. Several authors described such cyclisation reactions, starting from 1,2,4-butanetriol (clay) [1], 1,2,5-pentatriol (pentasile, mordenite, erionite) [2]. Linear ethers like dimethyl ether are formed from methanol (modified aluminosilicate, zeolites) [3,4] or MTBE from methanol and i-butene (zeolite, resin) [5,6] The yields of the desired products are often quite high, e g over 90 % in the case of 1,2,4-butanetriol to 3-hydroxy-tetrahydrofiiran and about 60 % in the case of dimethyl ether. The reactions are either carried out in the presence of water as slurry process [1,2] at 150 - 200 °C or at temperatures > 300 °C in the gas phase with a fixed bed catalyst [2-4]... 

Vapor phase synthesis of MTBE over zeolite Beta is very efficient. For example. Beta zeolite is three times more active than Amberlyst-15 for MTBE vapor phase synthesis at 50°C. The better catalytic performance of H-Beta was verified in liquid phase. The external surface area, the amount of bridging AlOHSi, and silanol groups are important zeolite parameters for the ether synthesis. The reaction occurs on bridging AlOHSi acid sites. The highest yields are reached for low SiOH/AlOHSi ratios where methanol clusters bonded to silanol groups allow accessibility of isobutene to the active AlOHSi groups. 

Olah et al. reported the triflic acid-catalyzed isobutene-iso-butylene alkylation, modified with trifluoroacetic acid (TEA) or water. They found that the best alkylation conditions were at an acid strength of about Ho = —10.7, giving a calculated research octane number (RON) of 89.1 (TfOH/TFA) and 91.3 (TfOH/HzO). Triflic acid-modified zeolites can be used for the gas phase synthesis of methyl ferf-butyl ether (MTBE), and the mechanism of activity enhancement by triflic acid modification appears to be related to the formation of extra-lattice Al rather than the direct presence of triflic acid. A thermally stable solid catalyst prepared from amorphous silica gel and triflic acid has also been reported. The obtained material was found to be an active catalyst in the alkylation of isobutylene with w-butenes to yield high-octane gasoline components. A similar study has been carried out with triflic acid-functionalized mesoporous Zr-TMS catalysts. Triflic acid-catalyzed carbonylation, direct coupling reactions, and formylation of toluene have also been reported. Triflic acid also promotes transalkylation and adaman-tylation of arenes in ionic liquids. Triflic acid-mediated reactions of methylenecyclopropanes with nitriles have also been investigated to provide [3 + 2] cycloaddition products as well as Ritter products. Triflic acid also catalyzes cyclization of unsaturated alcohols to cyclic ethers. ... 

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