Thermoregulated Phase-Separable Property Of Rhodium Nanoparticle Catalyst And Its Applications

Through investigation on the solubility properties of ionic liquids ([CH3(OCH2CH2)nN+Et3][CH3SO3-], n=12,16,22,42,113; abbreviated as ILPEG550, ILPEG750, ILPEG1000, ILPEG1900and ILPEG5000, respectively) in the mixture of toluene and n-heptane, the critical solution temperature (CST) of ILPEG was found for the first time. Based on the CST property of ILPEG, thermoregulated phase-separable catalysis (TPSC) of Rh nanoparticle catalyst was proposed. The general principle of TPSC is as follows:before reaction, at room temperature (T<CST), the lower ILPEG phase containing Rh nanoparticle catalyst is immiscible with the upper organic phase containing the substrate. When heated to reaction temperature (T> CST), the ILPEG-stabilized Rh nanoparticle catalyst becomes miscible with the organic phase. The whole system is homogeneous and the reaction proceeds smoothly. After reaction, on cooling to room temperature (T<CST), the ILPEG-stabilized Rh nanoparticle catalyst precipitates from the organic phase. Thus, biphasic system forms again. By simple phase separation, the catalyst can be separated from the product.In this paper, the thermoregulated phase-separable Rh nanoparticle catalyst stabilized by ILPEG1000in ILPEG1000/toluene/n-heptane biphasic system was used to catalyzed the selective hydrogenation of1,5-cyclooctadiene (1,5-COD) and α,β-unsaturated aldehydes or ketones, and the hydroaminomethylation of higher olefins. The Rh nanoparticle catalyst exhibited highly catalytic activity and good recyclability. For the selective hydrogenation of1,5-COD, under the conditions of T=60℃, PH2=1.0MPa,t=20min and1,5-COD/Rh=2000(molar ratio), the conversion of1,5-COD and the selectivity for cyclooctene (COE) were99%and90%, respectively, and the TOF was up to5346h-1. The catalyst could be reused for ten times without evident loss of activity and selectivity. The leaching of Rh was under the minimum detectable amount (5μg/L). For the selective hydrogenation of cinnamic aldehyde, under the conditions of T=60℃, PH2=1.0MPa,t=2.0h, cinnamic aldehyde/Rh=500(molar ratio), the conversion of cinnamic aldehyde and the selectivity for hydrocinnamic aldehyde were99%and100%, respectively. The catalyst could be reused for ten times without evident loss of activity and selectivity. For the hydroaminomethylation of1-octene, under the conditions of T=120℃, P=6MPa(CO/H2=1),t=12h,1-octene/Rh=1000(molar ratio), the conversion of1-octene and the selectivity for amines were100%and88%, respectively. Moreover, the catalyst could be used for five runs without any change in conversion of1-octene, and the selectivity for amines remained up to84%.Moreover, we also found that ILmo ([(CH3OCH2CH2OCH2CH2)(n-C8H17OCH2CH2OCH2CH2)N+(CH3)2][CH3SO3-]) possessed the CST property in cyclohexane. The thermoregulated phase-separable Rh nanoparticle catalyst stabilized by IL,no in this novel ILmo/cyclohexane biphasic system was used to catalyze the hydroformylation of higher olefins. For the hydroformylation of1-octene, under the conditions of T=120℃, P=5MPa (CO/H2=1), t=4h,1-octene/Rh=1000(molar ratio), the conversion of1-octene and the yield of aldehydes were100%and99%, respectively. The catalytic activity remained unchanged after the catalyst was reused for five times.Based on the CST properties of ionic liquids in organic solvent, the methodology of TPSC of Rh nanoparticles provides a new approach for recovery and recycling of soluble transition-metal nanoparticle catalysts, especially for noble transition-metal nanoparticle catalysts.