Curse and Blessing–The Role of Water in the Homogeneously Ru-Catalyzed Epoxidation of Technical Grade Methyl Oleate

Abstract

A homogeneous Ru-catalyst was investigated toward its potential recycling in methyl oleate epoxidation under mild conditions using hydrogen peroxide as a green oxidant. The literature-known catalyst complex, in situ formed from ruthenium acetylacetonate and dipicolinic acid, offers excellent selectivity toward epoxidation of technical grade (91.5%) methyl oleate. The resulting product methyl 9,10-epoxy stearate exhibits versatile applications. It is used industrially as a biobased epoxide for plasticizers and lubricants, or it can be further processed into polyurethanes. In this work, we first report a concept for homogeneous catalyst recycling, including extraction of the epoxide into an organic phase. Particular emphasis was on the effect of water, as it would accumulate due to recycling of the aqueous catalyst phase and replenishment of hydrogen peroxide prior to each recycling run. Consequently, mass transfer limitations needed to be addressed. By a combined extraction-distillation technique, we report the first sufficient homogeneous catalyst recycling strategy for Ru-catalyzed epoxidation of methyl oleate. Removal of water and separation of epoxide allowed for a ∑TON of 1243 after seven recycling runs with 97% selectivity toward epoxide and a TTON of 1384 after 13 runs, respectively, while catalyst leaching into the product phase was negligibly small. Moreover, pervaporation using a poly(vinyl alcohol) membrane was investigated as a promising technique for both removal of water and retention of the catalyst showing very promising results.

Progressing theCrystalWay to Sustainability: Strategy for Developing an Integrated Recycling Process of Homogeneous Catalysts by Selective Product Crystallization

“…The desired, linear diester offers potential for the synthesis of valuable polymers and is therefore required in high purity. Because of the 100% atom efficient reaction, no stoichiometric byproduct is formed, which would complicate the separation and recycling process upon accumulation (see Table , entry 3), as discussed in our work on epoxidation . Avoiding any auxiliary, selective product crystallization is considered as a promising method to recycle the catalyst and reach our target of increased catalyst productivity and high product purity in this model reaction.…”

Catalytic Synthesis of Methyl 9,10‐dihydroxystearate from Technical Feedstocks in Continuous Flow via Epoxidation and Hydrolysis

“…As shown in our previous work on the epoxidation of methyl oleate, acetonitrile (ACN) is an efficient solvent for the Ru-catalyzed epoxidation of methyl oleate 1. [30] However, tert-butanol had already been investigated in both epoxidation [31] of methyl oleate and hydrolysis of methyl 9,10epoxystearate. [17] The solubility of the diol 3 is much higher in tert-butanol than in ACN (250 g L −1 vs 20 g L −1 at a temperature of 25 °C).…”

“…Thus, regarding subsequent product isolation via crystallization, ACN as solvent is beneficial. We carried out one batch experiment based on the previously described epoxidation of methyl oleate 1 , [ 30 ] followed by the addition of water to reach an ACN:water ratio of 3:1 as previously described in tert ‐butanol. [ 17 ] Thereby, assuming quantitative conversion of 1 and the epoxide 2 , the maximum concentration of the diol 3 in the reaction solution is 62 g L −1 .…”

From tandem to catalysis – organic solvent nanofiltration for catalyst separation in the homogeneously W-catalyzed oxidative cleavage of renewable methyl 9,10-dihydroxystearate

“…12,24,26,27,37 Furthermore, we chose acetonitrile, since it was described as a suitable co-solvent in the W-catalyzed epoxidation of cyclohexene 38 and in the Rucatalyzed epoxidation of methyl oleate. 39,40 The catalyst loading was initially set to 5 mol% since we expect higher conversion at higher catalyst loading. The temperature was set to 100 °C and reactions were performed in glass pressure tubes, allowing for such high temperature without evaporation/reflux of solvent.…”