Experimental and Theoretical Study of the OH-Initiated Degradation of Piperazine under Simulated Atmospheric Conditions.

Trevor Ingham,Dwayne E Heard,Tomas Mikoviny,Graham A Boustead,Wen Tan,Liang Zhu,Armin Wisthaler,Barbara D’Anna,Jacqueline F Hamilton,Claus J Nielsen,Alexander D Brennan,Naomi J Farren,Simen Antonsen,Yngve Stenstrøm

doi:10.1021/acs.jpca.0c10223

Abstract

The OH-initiated photo-oxidation of piperazine and 1-nitropiperazine as well as the photolysis of 1-nitrosopiperazine were investigated in a large atmospheric simulation chamber. The rate coefficient for the reaction of piperazine with OH radicals was determined by the relative rate method to be kOH-piperazine = (2.8 ± 0.6) × 10–10 cm3 molecule–1 s–1 at 307 ± 2 K and 1014 ± 2 hPa. Product studies showed the piperazine + OH reaction to proceed both via C–H and N–H abstraction, resulting in the formation of 1,2,3,6-tetrahydropyrazine as the major product and in 1-nitropiperazine and 1-nitrosopiperazine as minor products. The branching in the piperazinyl radical reactions with NO, NO2, and O2 was obtained from 1-nitrosopiperazine photolysis experiments and employed analyses of the 1-nitropiperazine and 1-nitrosopiperazine temporal profiles observed during piperazine photo-oxidation. The derived initial branching between N–H and C–H abstraction by OH radicals, kN–H/(kN–H + kC–H), was 0.18 ± 0.04. All experiments were accompanied by substantial aerosol formation that was initiated by the reaction of piperazine with nitric acid. Both primary and secondary photo-oxidation products including 1-nitropiperazine and 1,4-dinitropiperazine were detected in the aerosol particles formed. Corroborating atmospheric photo-oxidation schemes for piperazine and 1-nitropiperazine were derived from M06-2X/aug-cc-pVTZ quantum chemistry calculations and master equation modeling of the pivotal reaction steps. The atmospheric chemistry of piperazine is evaluated, and a validated chemical mechanism for implementation in dispersion models is presented.

Highlights

Piperazine (1,4-diazacyclohexane, PZ) is among the amines considered for use in large-scale Carbon Capture (CC) to reduce CO2 emissions from industrial point sources.[1]
Health recommends that the total amount of nitrosamines and nitramines in the atmosphere should be below 0.3 ng m−3 in air and below 40 ng dm[3] in drinking water for a risk level of 10−5.4 Such low detection levels are currently virtually impossible to monitor with today’s technology, and it is imperative to acquire quantitative information on the degradation pathways for the relevant amines under atmospheric conditions and to implement this information in reliable chemical models for dispersion calculations
The conformational equilibrium will consist of around 55% eq-eq, 42% eq-ax, and 3% ax-ax at 298 K. This issue was not considered in the previous theoretical studies of the reaction, and a detailed theoretical account of the kinetics and of the branching between C−H and N−H abstraction in the initial step is far from trivial and considered outside the scope of the present work

Summary

Introduction

Piperazine (1,4-diazacyclohexane, PZ) is among the amines considered for use in large-scale Carbon Capture (CC) to reduce CO2 emissions from industrial point sources.[1] A 40 wt % amine solution with PZ and 2-amino-2-methyl-1-propanol in a 1:2 M ratio was recently suggested as the new benchmark solvent for CO2 capture technology.[2]. Measurements at the Technology Centre Mongstad (TCM; Norway) have established that at times it can be difficult to avoid ppm-level emissions of amines and their process degradation products to the environment during operation of a large-scale capture plant[3] the concern being that carcinogenic nitrosamines and nitramines are either directly emitted or formed in the subsequent atmospheric photo-oxidation of the fugitive amines.[4] The Norwegian Institute for Public.

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Conclusion