P-Chloride-Free Synthesis of Phosphoric Esters: Microwave-Assisted Esterification of Alkyl- and Dialkyl Phosphoric Ester-Acids Obtained from Phosphorus Pentoxide

One-Step Esterification of Phosphoric, Phosphonic and Phosphinic Acids with Organosilicates: Phosphorus Chemical Recycling of Sewage Waste

Global concerns regarding the depletion and strategic importance of phosphorus resources have increased demand for the recovery and recycling. However, waste-derived phosphorus compounds, primarily as chemically inert phosphoric acid or its salts, present a challenge to their direct conversion into high-value chemicals. We aimed to develop an innovative technology that utilizes the large quantities of sewage waste, bypasses the use of white phosphorus, and enables esterification of phosphoric acid to produce widely applicable phosphate triesters. Tetraalkyl orthosilicates emerged as highly effective reagents for the direct triple esterification of 85 % phosphoric acid, as well as the esterification of organophosphinic and phosphonic acids. Furthermore, we achieved esterification of recovered phosphoric acid with tetraalkyl orthosilicate, thus pioneering a recycling pathway from sewage waste to valuable phosphorus chemicals. Experimental and theoretical investigations revealed a novel mechanism, wherein tetraalkyl orthosilicates facilitate multimolecular aggregation to achieve alkyl transfer from tetraalkylorthosilicate to phosphoric acid via multiple proton shuttling.

A Study on the Direct Esterification of Monoalkylphosphates and Dialkylphosphates; The Conversion of the Latter Species to Trialkylphosphates by Alkylating Esterification

The microwave (MW)-assisted direct esterification of certain P-acids is a green method. Quantum chemical calculations revealed that the activation enthalpy (ΔH#) for the exothermic monoalkylphosphate → dialkylphosphate transformation was on the average 156.6 kJ mol−1, while ΔH# for the dialkylphosphate → trialkylphosphate conversion was somewhat higher, 171.2 kJ mol−1, and the energetics of the elemental steps of this esterification was less favorable. The direct monoesterification may be performed on MW irradiation in the presence of a suitable ionic liquid additive. However, the second step, with the less favorable energetics as a whole, could not be promoted by MWs. Hence, dialkylphosphates had to be converted to triesters by another method that was alkylation. In this way, it was also possible to synthesize triesters with different alkyl groups. Eventually a green, P-chloride free MW-promoted two-step method was elaborated for the synthesis of phosphate triesters.