Liquid-Phase Synthesis of Isoprene from Methyl tert-Butyl Ether and Formalin Using Keggin-Type Heteropolyacids

Abstract

A liquid-phase single-stage synthesis of isoprene from methyl tert-butyl ether (MTBE) and formalin using Keggin-type heteropolyacids, phosphotungstic acid (PTA), phosphomolybdic acid (PMA), and silicotungstic acid (STA) was performed in a batch reactor. The effects of catalyst concentration, reaction temperature, MTBE/formaldehyde ratio, and solvent type on isoprene yield were evaluated. At moderate reaction conditions, the two-phase organic−aqueous system with cyclohexane addition and high MTBE/formaldehyde ratio enhanced the isoprene formation. When the catalyst concentration was increased, the formaldehyde conversion and turnover frequency increased, but isoprene selectivity slightly decreased. At high temperature or low MTBE/formaldehyde ratio, isoprene yield decreased because of the formation of 3,4-dihydro-4-methyl-2H-pyran. When the catalyst activities are compared, the STA catalyst exhibited formaldehyde conversion, isoprene yield, and selectivity that were slightly higher than those of the PTA and PMA catalysts. Among the Prins reaction products formed, 4,4-dimethyl-1,3-dioxane was the key precursor of isoprene formation in the two-phase reaction system.

Acid-catalyzed chemoselective C- and O- prenylation of cyclic 1,3-diketones

“…Unfortunately, the yield of 3a gradually decreased after each cycle. The polymerization of isoprene could not be avoided in the presence of the strong acid at high temperatures [53][54][55][56][57]. Thus, after several runs under the optimized conditions, the accumulated coke on the surface of Nafion led to its diminished catalytic activity.…”

Synthesis of 1,3-Butadiene and Its 2-Substituted Monomers for Synthetic Rubbers

“…Moreover, additional studies on the synthesis of isoprene by alternative routes have been reported. Songsiri et al [72,73] synthesized isoprene using heteropolyacids as catalysts in a liquid-phase system under moderate conditions in which carbon deposits can be inhibited. Methyl tertiary-butyl ether (MTBE) was used as the source of isobutene.…”

Synthesis of 1,3-Diols from Isobutene and HCHO via Prins Condensation-Hydrolysis Using CeO2 Catalysts: Effects of Crystal Plane and Oxygen Vacancy

“…The total selectivities of 4,4-dimethyl-1,3-dioxane (3) and 3-methyl-1,3-butanediol (4) are above 98% throughout the temperature range (60-150 °C) investigated. No dehydration products (such as 3-methyl-butenol isomers and isoprene) [22,34] derived from 3-methyl-1,3-butanediol, which are usually obtained via vapor-phase dehydration at high temperatures (≥300 °C), were observed. In our catalytic system, the reactions were conducted at liquid phase (Reaction temperature ≤ 150 °C) and in water, which did not favor the dehydration of 3-methyl-1,3-butanediol.…”

“…In the synthesis route, both the Prins condensation and hydrolysis reaction require acid catalysts [3,15,16]. Various solid acid catalysts, such as metal oxide catalysts [17], zeolite catalysts [18], phosphate catalysts [15,19,20], and heteropolyacids catalysts [21,22], have been developed for the two-step reaction (Prins condensation and hydrolysis reaction). Alternatively, a single-stage 3-methyl-1,3-butanediol synthesis from isobutene and HCHO in water is one interesting route to be investigated, and a water-tolerant acid catalyst is critical.…”