Synthesis of MTBE on Acidic Zeolites

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. 

Reversible Conversion oe MTBE on Boron-Modieied Pentasil under Batch 

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. 

Using the above-mentioned experimental approach, Mildner et al. (228) recorded H MAS NMR spectra of the MTBE/B-pentasil system at 373, 389,405, and 421 K. These spectra were obtained with the sample in thermal equilibrium after the 

MTBE Synthesis on Zeolite H-Beta Under CF Conditions 

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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. 

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. 

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. 

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