Green Procedure for Aerobic Oxidation of Benzylic Alcohols with Palladium Supported on Iota-Carrageenan in Ethanol

Abstract

The search for selective heterogeneous catalysts for the aerobic oxidation of alcohols to ketones and aldehydes has drawn much attention in the last decade. To that end, different palladium-based catalysts have been proposed that use various organic and inorganic supports. In addition, supports that originate from a biological and renewable source that is also nontoxic and biodegradable were found to be superior. We heterogenized palladium chloride or acetate complexes with triphenylphosphine trisulfonate on iota-carrageenan xerogel by simple mixing of the complex and the polysaccharide in water. The resulting polysaccharide-catalyst mixture then underwent deep freeze and lyophilization, after which the catalyst was characterized by TEM, XPS and SEM-EDS and tested in aerobic oxidation. The new heterogeneous catalysts were successfully used for the first time in the aerobic oxidation of benzylic alcohols. Moreover, they were easily removed from the reaction mixture and recycled, yielding an increase in activity with each subsequent reuse. As determined by TEM and XPS, the reduction in palladium and the formation of nanoparticles during the reaction in ethanol yielded more active species and, therefore, higher conversion rates. A SEM-EDS analysis indicated that the palladium was thoroughly dispersed in the xerogel catalysts. Moreover, the xerogel catalyst was observed to undergo a structural change during the reaction. To conclude, the new heterogeneous catalyst was prepared by a simple and straightforward method that used a non-toxic, renewable and biodegradable support to yield an active, selective and recyclable heterogeneous system.

The Effect of Alcohol on Palladium Nanoparticles in i-Pd(OAc)2(TPPTS)2 for Aerobic Oxidation of Benzyl Alcohol

“…As noted above, the use of i-Pd(OAc) 2 (TPPTS) 2 xerogel as the heterogeneous catalyst in the aerobic oxidation of benzylic alcohols in ethanol led to a reduction in the palladium complex and the formation of palladium nanoparticles during the reaction [21]. We previously report edthat the ratio of Pd(0)/Pd(II) was increased following 24 h of the reaction, based on XPS analysis, i.e., 46% of the Pd(II), derived from Pd(OAc) 2 , was reduced to its metallic form during 24 h of reaction [21]. Thus, we have already demonstrated that following 24 h of reaction the formation of palladium nanoparticles is enhanced.…”

“…It was thus assumed that the impregnation duration time enhanced/reduced the palladium activity due to two opposing effects: (1) The reduction of palladium and formation of nanoparticles that accelerated the reaction, and (2) The growth of palladium nanoparticles and the formation of aggregates, which reduced the palladium outer surface area and may have decreased its activity. Therefore, our hypothesis was that until 24 h the palladium complex was reduced by the ethanol, and, in turn, that the conversion rate increased due to the formation of Pd(0) and the increase in the surface area of the palladium [21]. However, it appears that after 24 h the nanoparticles were agglomerated and, therefore, the outer surface area of the palladium was reduced, hence the reduction of the activity towards benzyl alcohol oxidation.…”

“…The palladium complexes that were immobilized on iota Carrageenan (i) were the most producible compared to other polysaccharide-based systems. Moreover, these heterogeneous catalysts, designated i-Pd(OAc) 2 (TPPTS) 2 , were also active in the aerobic oxidation of benzylic alcohol in ethanol and were easily separated and recycled without palladium leaching to the reaction mixture [21]. Notably, under reaction conditions in ethanol, the Pd was reduced and formed nanoparticles that, in turn, probably increased its oxidation activity in every re-use.…”

Recyclable Pd‐based polysaccharide catalyst for aerobic oxidation of benzyl alcohol

“…29 Thus, the investigation is initiated with the oxidation of benzyl alcohol (Figure 1) in three representative solvents with different polarities: ethanol, ethyl acetate, and hexane (Table 1). The conversion of the reactions in ethanol has already been studied, 27 and those results were used as comparative reference data to assess the results for the other solvents. At first a simple Pd(OAc) 2 was used as a catalyst without or with the addition of sodium carbonate as cocatalyst (entries 1 and 2, respectively).…”

“…As previously noted, when ethanol was used as the solvent, the palladium complex was reduced and nanoparticles formed. 27 Moreover, the formation of these nanoparticles was suggested as the reason that higher conversion yields were obtained with the recycled catalyst. Thus, the i-Pd(OAc) 2 (TPPTS) 2 also underwent TEM analyses after reactions in hexane and ethanol (Figure 7).…”