Molecular Modeling and Experimental Validation

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Nitrosamine Formation in Amine-Based CO2 Capture in the Absence of NO2: Molecular Modeling and Experimental Validation Huancong Shi, Teeradet Supap, Raphael O. Idem, Don Gelowitz, Colin Campbell, and Max Ball Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 5, 2017

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Environmental Science & Technology

Nitrosamine Formation in Amine-Based CO2 Capture in the Absence of NO2: Molecular Modeling and Experimental Validation Huancong Shia, Teeradet Supapb,*, Raphael Idemb,*, Don Gelowitzc, Colin Campbellc, Max Ballc a

Department of Environmental Science & Engineering, University of Shanghai for Science &

Technology, Shanghai, China b

Clean Energy Technologies Research Institute (CETRI), Faculty of Engineering and Applied

Science, University of Regina, Regina, Saskatchewan, Canada S4S 0A2 c

Saskatchewan Power Corporation (SaskPower), 2025 Victoria Avenue, Regina, Saskatchewan,

Canada S4P 0S1

Keywords: Nitrosamine, CO2 Capture, Molecular modeling

ACS Paragon Plus Environment


1

Abstract

2

A computational chemistry approach was used to elucidate and verify the different

3

nitrosamine formation mechanisms and pathways. These included nitrosamine formation under

4

acid or basic environments in the presence of NO, O2, SO2 and CO2 without NO2. The results

5

clearly showed that nitrosamine could be formed without NO2 via 2 different types of

6

mechanisms, namely, addition and elimination forming N-N bond before proton transfer and

7

proton transfer before N-N bond formation, respectively. The essence of these mechanisms

8

identified in this work was that two reaction steps were required to complete both reaction

9

mechanisms with different nitrosating agents. Two steps were both necessary neither of which

10

could be neglected, if the nitrosamine formation reaction was to be completed. Computational

11

simulation performed on the reactant, intermediate, transition state and product for each set of

12

reactions also validated the proposed mechanisms. Experiment also detected nitrosamine from

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the reaction of diethylamine and NO, SO2, O2 and CO2 in both liquid and gas phase. Thus, NO2

14

is not necessary for nitrosamine formation to occur in the CO2 capture system.

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INTRODUCTION

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Nitrosamines formed as a result of nitrogen oxides (NOx) reaction with amines (specifically

24

secondary amines) constitute one of the concerns in most carbon dioxide (CO2) plants. So far,

25

experimental research to verify the mechanism for nitrosamine formation has involved different

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amines such as monoethanolamine (MEA)1-4, diethanolamine (DEA)4, 2-methyl-2-amino-1-

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propanol (AMP)1-2, methyldiethanolamine (MDEA)1,2,4, piperazine (PZ)1-5, morpholine (MOR)6-

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7

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nitric oxide (NO) and nitrogen dioxide (NO2), as well as dinitrogen trioxide (N2O3) and

30

dinitrogen tetroxide (N2O4)1,6 in the gas phase, the latter two of which are the products of NO2

31

reacting with NO and NO2, respectively1,2.

, and DMA8-9. Besides amines, the focus is also given to type of nitrosating agents such as

32

Amines are categorized into primary (RNH2), secondary (R1R2NH), and tertiary amines

33

(R1R2R3N) of which primary and tertiary amines cannot generate stable nitrosamine with

34

nitrosating agents.10 Only secondary amines such as PZ1-5 and MOR6-7 can generate stable

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nitrosamine with nitrosating agents. Thus, this was used as the basis for selecting secondary

36

amines but of 15 different types for the present study. The classic reaction schemes for

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nitrosamine formation under acidic conditions can be represented in Eq (1) and (2)11. Besides

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NO+, nitrosamine formation involving agents of the form NO-Y in aqueous solution is possible

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where Y can be a variety of anions including thiocyanate (SCN-) and halides (e.g. I-, Br-, and

40

Cl-). Reaction of amine and NO-Y in Eq (2) consists of forward and reverse reactions given

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respectively as Eq (2.1) and Eq (2.2). However, with a shortage of proton (H+) in an amine-

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based CO2 capture environment, reaction shown in Eq (1) will not likely occur. Thus, the

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nitrosation reactions involving Eq (1) and (2) normally presented in the literature11, might not be



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applicable to explain nitrosamine formation in such an alkaline environment in the CO2 capture

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process.

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HNO2 + Y- + H+

Y-NO + H2O

(1)

(2)

47

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Since nitrosamine formation in post-combustion amine-based CO2 capture occurs under

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basic conditions, this aspect of the work will focus on NO2 and NO-X types of nitrosating.1-13

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These nitrosating agents can be categorized into two groups: NO2 and NO-X. Such

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categorization is reasonable based on the molecular structure and origins of the nitrosating

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agents. The NO2 possesses the typical structure as “O=N=O” with two N=O double bonds. The

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NO-X (e.g. NO+, NOCl and N2O3) has a similar structure as “O=N-X”, where the X dissociates

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from the molecule via N-X bond breakage during the nitrosation reaction. It must be noted that

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this NO-X was the type of nitrosating agent being focused on for mechanism development in this

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study.

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The specific nitrosating agent NO2 reaction with secondary amine has been studied since

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1976 where the reaction mechanism followed an amino radical pathway.14 Other nitrosation of

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amines with NO2 can be completed with the aid of HCOH11. NO2 also forms N2O4 which upon

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hydrolysis, generate various ionic products shown in Eq (3) and (4);

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2NO2 N2O4

(3)

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N2O4 + 2OH- NO2- + NO3- + H2O

(4)


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The hydrolysis products of N2O4 are NO2- and NO3- which the former can be generated also

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from N2O3 hydrolysis and other sources. NO3- can either be generated from NO2 dissolution in

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basic solution shown in Eq (4) or directly from a soluble salt such as sodium nitrate (NaNO3).

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The nitration mechanism of amine and NO3- can be catalyzed by CO2 to generate nitrosamine

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which a simulation work to demonstrate this process has already been reported.15-17 However,

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this mechanism requires experimental verification in order to ascertain whether it will generate

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nitrosamines or nitramines.

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The second group of nitrosating agents constituting the research focus of this aspect of our

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work are NO (i.e. NO-X). Most of these agents, specifically NO-X can be generated from NO in

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the gas phase dissolving in H2O in the presence of O2. The majority of the components of NOx

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(100-300 ppmv) in flue gas is NO (~95%) with NO2 being ~5%.6 NO2 can be mostly removed in

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flue gas preconditioning process before entering the amine scrubbing plant compared unlike NO

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which is approximately 10 times less soluble. Despite the fact that the amount of NO2 (0-5ppm)

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is very little compared to NO (0-100ppm) in the CO2 capture plant system, the formation of

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nitrosamine has been reported, in some cases with clear “absence” of NO2.1,6 Based on these

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studies, different feed component ratios of NO/NO2 provided a strong experimental evidence that

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nitrosamine could be generated with NO alone in the presence of O2. Although this finding has

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triggered concern of NO being able to generate nitrosamine, at the moment, the mechanism of

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nitrosamine formation with NO is not fully understood.

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The present study used data of 15 secondary amines, and their corresponding nitrosamines

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with full details are given in Table S1. These amines were used to develop a possible mechanism

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of nitrosamine formation specifically from the reaction with NO, O2, CO2 and SO2. This

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involved the use of molecular modeling and computational simulation approach to formulating



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the mechanism. The possibility of formation of nitrosamines with NO alone in the presence of O2

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required elucidation of the mechanism so as to help formulate possible inhibition methods. The

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hypothesis used in simulation work was that the common structure of the nitrosating agent was

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of the form NO-X (considered as derivative products with NO) such as NO, HNO2, NO+,

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NOCl(Br), N2O3, NO2-CO2-, NO2-SO2-, and NO2-SO3-. These nitrosating agents were also

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commonly formed as products of NO dissolution in water in the presence of O216,18 of which

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some of the reactions were also reported in the literature.6 The experimental validation was also

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carried out to qualitatively validate the proposed mechanisms derived from the simulation work,

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using 2 kmol/m3 diethylamine (DEA) and NO, SO2, and O2 reacting at the desorber temperature

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of 393 K without/with lean and rich CO2 loading. Detection and identification of NDEA in bulk

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liquid and off-gas were used to confirm and validate the proposed mechanisms.

97

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COMPUTATIONAL STUDY AND MOLECULAR MODELING THEORY

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The computational study and molecular modeling of nitrosamine formation used secondary

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amine (R1R2NH) with NO derivatives (i.e. NO-X; where X is HNO2, NO+, NOCl(Br), N2O3,

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NO2-CO2-, NO2-SO2-, and NO2-SO3-). The process involved the calculation of reaction energies

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using simulation among the 15 secondary amines to detect whether the nitrosation reactions

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could occur spontaneously (i.e. ∆G < 0). DMA [(CH3)2NH] was selected as the standard

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secondary amine for NDMA [(CH3)2NNO] formation mechanism analysis in terms of the

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reaction scheme, reaction pathway, and potential energy surface (PES) diagram. This mechanism

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would be applicable and able to apply to all the 15 secondary amines.


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The general reaction scheme given in Eq (5) presented nitrosamine formation in the amine

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based CO2 scrubbing process under basic conditions. Although there was the similarity of this

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reaction to that of reaction reported in a literature6, NO+ could not be easily generated in a

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neutral or basic solution.

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DMA + NO-X NDMA + HX

(5)

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Under actual CO2 capture plant, 4 major nitrosating agents consisting of N2O3, NO2-CO2-,

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NO2-SO2-, NO2-SO3- can be observed which represent the major research interests of the current

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study instead of NO+ or NOCl formed under acidic condition6. However, NO+ and NOCl were

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still included in the simulation to indicate the applications of our new mechanisms in acidic,

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neutral and basic conditions. Computational simulation works were generated towards forming

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appropriate mechanism under the CO2 capture condition. The theory level, energy barrier

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calculation, molecular structure optimization, and transition states investigations are very

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practical for a detailed study of reaction mechanism. A previous work attempted to illustrate the

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role of different agents, especially CO2 in catalyzing or inhibiting nitrosamine formation.10

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Specifically, for the DMA + NO2- system, CO2 worked as a catalyst when combined with NO2-,

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but took an inhibiting role to react with amine forming carbamate ions. Though the

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computational calculations were also done for this system to obtain the transition state, reactant,

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and product with energies and molecular structure, this work still lacked the detailed evolution of

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the reaction pathways with PES energy diagram and presented only one-step nitrosation process.

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Our preliminary simulation results however, indicated that the nitrosation process actually took

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at least 2 steps with two different possibilities, directing to two reaction pathways. Based on

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these preliminary results, our simulation research on the mechanism of nitrosamine formation

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included 3 important tasks which were to study the similarity of NO-X agents and the typical



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bond stretch, propose complex mechanism of nitrosamine formation with two steps and two

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possible reaction pathways, and generate the PES diagram of both pathways with relative

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molecular structures as well as the energies of reactants, products, intermediates and Transition

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states (TS). These simulation results were then used to discover the similarity of nitrosating

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agent NO-X as well as develop a detailed mechanism and an accurate PES diagram which would

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be used to indicate the effective method to control the nitrosamine formation.

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COMPUTATIONAL METHOD

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All the calculations were completed with the Gaussian 03 program package.19 The

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nitrosation reactions of NDMA generation was investigated using the B3LYP method20 (Becke’s

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three-parameter nonlocal exchange functional with the correlation functional of Lee, Yang, and

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Parr) with DFT method (density functional theory)21. The basis set was 6-311+G(d,p).22 The

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structures of the reactants, products, intermediates, and transitions states were fully optimized

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with HF/6-311+G(d,p). Based on the optimized geometries with HF/6-311+G(d,p) level, the

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single-point energy of each stationary point was obtained with B3LYP method, labelled as

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B3LYP/6-311+G(d,p) //HF/6-311+G(d,p) with the code “freq”. As the nitrosation reactions took

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place in aqueous solution, the solvent effect of water was also taken into account. On the basis of

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the optimized geometries, the single-point energy calculation was carried out with CPCM

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(conductor-like polarizable continuum model) at the B3LYP/6-311+G(d,p) level.

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EXPERIMENTS FOR MECHANISM VALIDATION

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Equipment and Chemicals


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Concentrated diethylamine (DEA, 99 % reagent grade, Sigma Aldrich, Canada) was used

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to prepare 2 kmol/m3 aqueous solution which standardized 1 N hydrochloric acid was used to

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confirm the exact concentration of DEA by titration to the endpoint of methyl orange indicator.

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A research grade carbon dioxide (CO2) was used for CO2 loaded experiments while simulated

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feed gas used was 100 ppm NO and 200 ppm SO2 (N2 balance) mixture and 100% oxygen (O2).

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Certificate of analysis from Praxair confirmed that NO2 was not present in NO-SO2-N2 mixture.

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The accuracy of this analysis was within ±2% error. All gas cylinders were supplied by Praxair,

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Regina, Canada. Formation of nitrosamine was carried out using a 0.6 L stainless steel reactor

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(model 4560, Parr Instrument Co., Moline, IL) controlled and regulated by a temperature-speed

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controller (Model 4836, Parr Instrument Co., Moline, IL) of ± 0.1 % accuracy. System pressure

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inside of the reactor could also be monitored by the controller using a pressure transducer.

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Gas chromatograph-mass selective detector (GC-MSD, model 6890-5073) supplied by

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Hewlett-Packard, Canada was used for analysis of NDEA. GC-MSD conditions including inlet

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liner, injection volume, and operating conditions can be found in our previous work.22 GC

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capillary column was RTX-5 Amine with a dimension of 30 mm-length x 250 mm-i.d. x 0.25

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mm-film thickness with

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(Chromatographic Specialties, Ontario, Canada). Sample preconcentration based solid phase

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extraction (SPE) was also used prior to GC-MS analysis for a clear resolution of NDEA. Ethyl

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vinyl benzene-divinyl benzene polymer LiChrolut EN 40 SPE cartridge (120 µm particle size of

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200 mg in 3 ml standard tube, Millipore (Canada) Ltd, Ontario, Canada) was used. The

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extraction conditions were adopted directly from the literature24. The GC-MSD detection limit

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for NDEA in this study was determined using NDEA samples made between the range of 1 –

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100 ppm. 10 ppm was found to be the detection limit of the GC-MSD techniques for all samples.

a stationary phase of 5% diphenyl and 95% dimethyl siloxane



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GC-MSD analysis of all samples were done twice with repeatability reported as 15% averaged

176

standard deviation.

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Nitrosamine Reaction Runs

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The mechanism to be validated was based on formation of nitrosamine from the reaction

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of NO with diethylamine in the presence of O2, SO2, and CO2. During the test, if NDEA was

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formed regardless of its concentration. The mechanism was deemed valid. Thus, the experiment

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was only done qualitatively to verify NDEA formation. The experimental set-up is also given in

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Figure S1. The tests were done using a fixed 2 kmol/m3 concentration of diethylamine solution.

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Feed pressure of O2 and mixed NO and SO2 (N2 balance) were fixed respectively at 96 and 193

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kPa. Temperature at 393 K was used for all tests. Only, CO2 loading was varied from 0 to 0.23,

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and 0.64 mol CO2/mol amine. For a typical run, 4.5 x 10-4 m3 of diethylamine solution

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with/without loaded CO2 was transferred into the reactor vessel, stirred at a speed of 500 rpm

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and heated to 393 K. O2 regulated at 96 psi from its cylinder was introduced into the reactor

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followed by 193 kPa of mixed NO and SO2 (N2 balance) from a separate cylinder. This marked

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as reaction time 0 with the total reactor pressure indicated by the sum of water vapor, 96 kPa O2,

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and 193 kPa NO+SO2. The reaction was left to run continuously for 2 weeks during which,

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samples were taken every day for analysis of NDEA. Collection of gas head space above the

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liquid was also done on the last day of the experiment. Gas samples were flushed out of the

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reactor by N2 gas into the impinger tube immersed in an iced bath for 30 minutes. Liquid sample

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(A) and gas sample (B) were sent for SPE and GC-MS analysis to identify NDEA.

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RESULTS AND DISCUSSION

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The analysis of six nitrosating agents (NO—X as NO derivatives)


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The reaction mechanism focused on NDMA for which DMA was selected as the

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standard. Generic form of nitrosating agent used in this study was NO-X in a liquid solution

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which could be various derivatives of NO. Table S2 summarizes NO-X derivatives considered in

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this study. The accurate structure is given as “O=N—X” in the mechanistic study. Based on

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molecular structure simulation, NO-X nitrosating agent was classified into two groups. The first

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group was NO+, NO-Cl, N2O3 (i.e. ON-NO2), the first one of which was generated by HNO2 and

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HCl reaction in strong acidic solutions. NO-Cl and N2O3 were originated in gas phase of power

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plant flue gas NO with Cl2 and NO2 in the gas phase of a power plant flue gas at ppm level

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which then dissolved into the amine solution. The second group was derived from a combination

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of NO2- anion with dissolved CO2, SO2, and SO3 to form NO2-CO2-, NO2-SO2-, and NO2-SO3-,

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respectively. Despite the difference in the abundance of these 6 nitrosating agents, they were

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simulated equally to demonstrate the wide applicability of the developed formation mechanism.

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From Table S2, the first group had the obvious structure of “O=N-X”, where X could be

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H2O, Cl-, and NO2- released from NO-X bond breaking. The second group did not possess NO-X

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in the structure. However, the nitrosating agents within this group could be converted to “NO-X”

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with the addition of heat and a certain geometry/structural change. NO2-CO2-, NO2-SO2 and

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NO2-SO3- could be converted to NO-CO32-, NO-SO32-, and NO-SO42- with heat via pathways

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derived respectively from Eq (6), (7), and (8). Under medium heat (e.g. 20-40 kcal/mol), the

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structure of “O=N-X” can be generated. It does not require the ON--X bond to breakdown

216

completely to start the nitrosation. This amount of energy is enough to support the bond stretch.

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NO2-CO2- ONO-CO2- ON(δ+)----O(δ-)-CO2- ON+ + CO32-

(6)

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NO2-SO2- ONO-SO2- ON(δ+)----O(δ-)-SO2- ON+ + SO32-

(7)

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NO2-SO3- ONO-SO3- ON(δ+)----O(δ-)-SO3- ON+ + SO42-

(8)



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The PES diagrams in gas and liquid phases of NO-Cl, N2O3, NO2-CO2-, NO2-SO2-, and NO2-

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SO3- bond stretch of the 5 agents were plotted respectively in Figure S2 (a), (b), (c), (d), and (e).

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The first agent NO+ + H2O was not shown because the cation NO+ was already the product of

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NO—X bond cleavage obtained after the stretch and X- dissociation. From these plots, it was

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clear that with the NO-X bond stretch, the positive charge on the N atom became bigger while

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the energy of the complex being higher compared with the ground state. This stretch was

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important as a preparation step of nitrosamine formation.

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NO-Cl and N2O3 (shown in Figure S2 (a) and (b)) were simple stretch while the agents

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derived from CO2, SO2, and SO3 (Figure S2(c), (d), and (e)) were a little bit complicated because

229

the stretch could lead to a change in complex’s molecular structure. For the intermediate O=N-

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O-CO2-, when heat was applied, two possible bond stretches could occur namely, the N-O bond

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stretch leading to formation of ON(δ+)----OCO2- nitrosating agent and O-C bond stretch and

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breakdown forming O=N-O- with CO2 releasing into the liquid phase. The same was true for

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NO2-SO2- and NO2-SO3- where the free SO2 and SO3 could react with water under basic

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conditions in the absorber to respectively generate SO32- and SO42-.

235

The proposed mechanism of nitrosamine formation

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The NO-X based nitrosating agents discussed in previous section carried a partially

237

positive charge at its nitrogen (i.e. N(δ+)O) that helped to attract the amine molecule. Such a

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positive charge on NO-X helped it to be drawn towards the N atom of the amine (i.e. R1R2NH),

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carrying a partially negative charge (i.e. R2N(δ-)H). This interaction between NO-X and the

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amine finally generated a chemical bond to form an intermediate of R2HN(δ-)---N(δ+)O---X.


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This step shown in Eq (9), is an important initial step which is followed later by the generation of

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N-NO bond under certain conditions.

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R2N(δ-)H + N(δ+)O-X R2HN----NO------X intermediate

(9)

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DMA was used throughout this section to discuss these mechanistic steps essential for

245

elucidation of its nitrosamine formation mechanism (i.e. NDMA). The approach of DMA

246

initiating the reaction with NO—X to form NDMA and its reaction mechanism were proposed in

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Figure 1 (a) and (b) to denote 2 different types of mechanisms namely, addition and elimination

248

forming N-N bond before proton transfer and proton transfer before N-N bond formation,

249

respectively. Due to the complexity of the formation, two reaction steps were essential based on

250

the two possible reaction mechanisms with different agents. Two steps of NDMA formation

251

consisting of N-NO bond generation and proton transfer (proton released to X-) were both

252

necessary neither of which could be neglected, if the nitrosamine formation reaction was to be

253

completed. This phenomenon was different from prior work15where the two steps were written as

254

one step. This assumption made the previously proposed mechanism15 to be over-simplified

255

which may lead to loss of some important information.

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In details, the two-step reaction also resulted in two possible mechanisms either N-NO bond

257

generation before proton transfer or proton transfer before N-NO bond generation. The former

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mechanism was drawn in Figure 1(a) similar to that of classic reaction pathway. The latter given

259

in Figure 1(b) which has not been identified elsewhere. Based on the type of NO-X nitrosating

260

agent used in this study, 2 groups were given. The nitrosating agents in Group I consisting of

261

NO+ & H2O, NOCl, and N2O3, appeared to follow the first mechanism due to their weak proton

262

affinity of X- (i.e. H2O, Cl-, and NO2-). Thus, the N---NO bond was therefore, likely to be

263

generated first despite the longer bond length of N---NO compared to that in NDMA, followed



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by dissociation of X- from the intermediate that attracted the proton (H+) later on. After the

265

proton transfer, the N-NO bond shrink to complete NDMA formation. The nitrosating agents in

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Group II, NO2-CO2-, NO2-SO2-, NO2-SO3- however, were likely to follow mechanism of Figure

267

1(b) due to a stronger proton affinity of X- (CO32-, SO32-, and SO42-) which contained higher

268

negative charge close to 2. The DMA and nitrosating agents approached each other and

269

generated the intermediate with a specific structure resembling a 6 membered ring which

270

consisted of amine’s N-H and the agent’s --O-C-O--N-. The intermediate was formed firstly by a

271

transfer of the DMA’s proton (H+) which was attracted by the X- of the nitrosating agent. This

272

step was followed immediately by N-NO bond formation. The last N-O bond broke off while

273

HCO3- finally dissociated. The formation started with proton transfer, O-H bond generation and

274

ended with N-NO---O bond cleavage.

275

For Figure 1(a), the two steps of nitrosation seem to be separate and occur in sequence

276

followed by one (N-NO) by proton transfer. It can be regarded as a clear two-step mechanism.

277

For Figure 1(b), the proton transfer occurs just before the N-NO generation, and the two steps of

278

nitrosation seem to occur almost at the same time. Sometimes, it can be considered as a one-step

279

mechanism15 but proton transfer has to occur as an initiating step to induce the overall reaction.

280

Finally, the mechanism of nitrosamine formation with amine + NO-X was also developed herein

281

which covered two possibilities. Most nitrosation reaction schemes with NO-Y are written as Eq

282

(1) and (2), similar to Figure 1(a). Some researchers have proposed the TS state structure close to

283

6 atom ring, but these lack a detailed evolution of the reaction process15. The nitrosation

284

mechanism is complex as compared to other reactions, since there are two possible pathways due

285

to different proton affinities of the nitrosating agents X.

286

PES diagram of nitrosamine formation with various nitrosating agents


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Since, the mechanisms were proposed in Figure 1. The reaction pathways needed to be

288

validated with computational simulation. The computational simulations were performed on the

289

reactant, intermediate, the transition state and the product for each set of reactions. For

290

nitrosating agent group I, Figure 2 shows PES diagram of NDMA formation from NO+ and H2O.

291

For this simulation, the DMA + NO-X were calculated in one input file, even with reactant

292

(DMA+NOX) and product (NDMA + HX). Such simulation was necessary in order to be

293

consistent with the intermediate and transitions state calculations. The previous calculation of

294

reaction energies was completed with calculation of each chemical (DMA, NOX, NDMA, HX)

295

separately (single point energy). The NO+-H2O diagram followed the mechanism shown

296

previously in Figure 1(a) with PES shape close to “И”. When the DMA approaches NO-X, the

297

energy of both reactants decreases and reaches the intermediate at a stable state. Then, the

298

reaction proceeds to proton transfer with heat through the transition state with an increase in the

299

energy to reach the product state by releasing heat. The overall process goes through “up-down-

300

up-down” steps. The TS state is the most difficult one, which requires energy on the peak of И.

301

The same trend was observed for the rest of nitrosating agents from Group I shown also in

302

Figure S3 (a) and (b), respectively for NOCl and N2O3.

303

PES diagram of NDMA formation generated from group II nitrosating agents was

304

different from those of group I. For example, ONO-SO2- followed Figure 1(b) mechanism.

305

However, the shape of PES of ONO-SO2- shown in Figure 3, was close to “M”, which contained

306

two peaks. The first peak was needed because it required heat for ONO-SO2- and DMA to

307

approach and reach the proper position of Intermediate 3. The energy of the intermediate 3

308

decreased to the middle of the “M”, and stabilized to a local minimum or saddle point. DMA and



Page 16 of 35

309

NO2-SO2- were placed at proper structures of the intermediate, with the 6 atom ring being

310

generated and ready for the proton transfer. Then, the proton transfer occurred during the

311

transition state, and required more energy. This was the second large peak of “M”. The NDMA

312

was generated after proton transfer with H-X generated with heat release. ONO-SO3- agent

313

behaved similarly as shown in Figure S4. The overall process of ONO-SO2- and ONO-SO3- goes

314

through “down-up-down-up-down” steps. The typical structures of TS state and intermediates

315

are the 6 atom ring consisting of “N-H--- O-C(S)-O--N-”. ONO-CO2- also followed a similar

316

mechanism of NDMA formation, shown in Figure 1(b), which the shape of PES was one big

317

peak with TS state of proton transfer (Figure S5).

318

The similarity of the PES diagrams of Figure 2 and S3 (a)-(b) and Figure 3, S4 and S5

319

validated the proposed mechanisms of Figure 1(a) and (b), respectively. The nitrosation

320

mechanism of NO-X has two different reaction pathways due to different agents. For NO+,

321

NOCl, N2O3, the X- is not strong enough to attract H first. The bond approach occurs before

322

proton transfer. For Figure 1 (b), it requires small energy to reach the proper intermediate

323

structure (6-atom ring), thus requiring large energy for proton transfer. As long as the proton is

324

transferred from N to O, the N-NO bond can be generated right away.

325

Application of the mechanism into experiments in CO2 capture plant

326

Finally, the mechanism and PES diagram of amine + NO-X are proposed. The list of

327

Nitrosamine formation study based on nitrosating agents has been combined with the mechanism

328

related to NO2 generated. The two major groups of agents NO2, NO-X (NO) are listed with

329

various mechanisms collected (in Table S3). The researchers can identify the nitrosating agents

330

first, and then sort out the corresponding mechanism. Based on power plant operations, most


Page 17 of 35


331

NO2 is removed in the gas preconditioning process before the CO2 capture plant. The major

332

component of NOx of the flue gas is NO instead of NO2. The NO can react with O2 (typically

333

contains 6% concentration) in the flue gas in aqueous solution to generate HNO2 which can

334

convert to NO2- in basic conditions. This occurrence has been performed by other

335

researchers3,6,25 With the abundant amount of CO2 in the desorber, the secondary amine

336

(R1R2NH) reacts with NO2-CO2- via the mechanism represented in Figure 1(b) and 2(a) to

337

generate nitrosamine and bicarbonate HCO3-. This mechanism can also explain why even though

338

the NO2 from the flue gas is removed before entering into the amine CO2 capture plant,

339

nitrosamine is still formed in trace amounts1,6. The major source of nitrosating agent is NO in the

340

gas which is oxidized with O2 and converted to NO2-CO2- in the liquid phase. CO2 acts as a

341

catalyst for nitrosamine formation if the agent NO2-CO2- is generated.

342

On the other hand, this same mechanism can be used to control nitrosamine formation. The

343

typical method is either to scavenge nitrosating agent with inhibitors, or to increase CO2 loading

344

to generate carbamate to reduce the presence of free amine6. For mechanism in Figure 1(a), as

345

the energy of TS is comparable to or lower than the free energy of the reactant, the nitrosation

346

reaction can occur spontaneously. This method is applicable to inhibit the agents. For the

347

mechanism in Figure 1(b), the proper intermediate 3 with 6 atom ring is the local minimum. The

348

agent NO-X has a strong proton affinity of X-, an inhibitor which can neutralize the agents are

349

preferred. If proton transfer cannot be completed for both pathways, nitrosation can be

350

prevented. It is not clear if the proposed mechanisms would find its application in non-aqueous

351

solvent for carbon capture. This subject will be for a future study.

352

Experimental Validation of Nitrosamine Formation Mechanism



353

Page 18 of 35

Liquid phase formation of NDEA

354

Run without CO2 was initially carried out using the reaction conditions previously explained

355

in Nitrosamine reaction runs, to check for NDEA formation from DEA in the presence of only

356

NO, O2, and SO2. 1 day sample was first collected and processed through the SPE technique

357

(also given in section 3.2) to clean up and preconcentrate the sample, thus increase a change of

358

GC-MS detection of NDEA. Figure S6 (a) and (b) confirmed formation of NDEA after only

359

1day reaction against standard NDEA. Further analysis using mass spectrometer of the 2 peaks

360

confirmed their identify as NDEA shown by mass spectra patterns with a percent match quality

361

over 90% as shown in Figure S7 (a) and (b). It must be noted that standard injection of

362

nitrosamine was done on a different day from that of 1 day sample. In addition, prior to analysis,

363

baking of GC column, inlet, and MSD was also carried out periodically, thus eliminating risk of

364

previous run’s carry-over. Positive NDEA peak in 1 day sample also indicated its concentration

365

being higher than 10 ppm detection limit. 0 day sample of 2 kmol/m3 DEA freshly prepared for

366

the reaction was also used as a blank run, also given in Figure S8, neither showed positive

367

NDEA peak (only flat baseline detected) nor MS spectrum match at 4.4 min. Therefore, with all

368

these evidence, it was confirmed that peak found at 4.4 min in 1 day sample, as well as other

369

samples from later days, was not caused by a drift of GC baseline but in fact, the actual peak of

370

NDEA generated as a reaction product of DEA, NO, O2, and SO2 with or without CO2.

371

372

Samples collected on day 2 to 5 were also analyzed for NDEA which their GC

373

chromatograms were also plotted together in Figure S8 with 1 day sample. It is clear that without

374

NO2, NDEA could be generated by only NO, SO2, and O2. Although, quantitative analysis was


Page 19 of 35


375

not carried out. Comparison of sample collected daily clearly showed an increasing trend of peak

376

size of NDEA as the reaction proceeded forward. This can actually confirm that NDEA could be

377

formed in liquid phase even if NO, O2, and SO2 were only present in the reaction system. The

378

same phenomenon should also occur with those secondary amines commonly used in CO2

379

capture process including piperazine (PZ) and its derivatives and diethanolamine.

380

CO2 was also used to evaluate its effect on formation of NDEA from 2 kmol/m3 DEA. In

381

addition to similar concentration of NO, SO2, and O2 used in previous experiment, lean and rich

382

loading conditions set at 0.23 and 0.64 mol CO2/mol amine were also used, respectively. Figure

383

4 shows formation of NDEA in liquid phase at various reaction days under 0.23 CO2 loading

384

condition. The Figure shows the extent of NDEA formation with lean CO2 loading. Runs without

385

CO2 were also given in the same Figure for comparison. It is evident from GC analysis that

386

NDEA was still detected even with lean CO2 condition. Rich CO2 condition (i.e. 0.64 mol

387

CO2/mole amine) also generated NDEA in liquid phase as shown in Figure S9. It should be

388

noted that CO2 however, showed a mild effect in inhibiting amine degradation which also led to

389

a decrease in the extent of NDEA formation. This effect has been observed already in many

390

degradation works previously done by our research group and possibly, could be used to help

391

control nitrosamine formation while the capture plant being operated.

392

Gas Phase formation of NDEA

393

Due to NDEA being a volatile nitrosamine, analysis of gas phase samples was also

394

carried. Gas samples of runs without and with CO2 loading of 0.23 and 0.64 mol CO2 per mol

395

amine were also collected on the last day of the experiment and analyzed by GC-MS similar to

396

the liquid samples discussed in the previous section. Figure 5 shows overlays of gas sample



Page 20 of 35

397

chromatograms of 0.23 and 0.64 CO2 loading with that of run without CO2 included in the same

398

Figure for a clear comparison. NDEA was clearly detected in both run with CO2. In comparison

399

to run without CO2, NDEA peak size of lean and rich CO2 loadings was much smaller. This

400

trend was in a good agreement with liquid sample analysis previously discussed that CO2 loading

401

if controlled properly, could help suppress formation of NDEA during the reaction. Also

402

observed in the same Figure, runs involving CO2 also generally helped reduce formation other

403

degradation products. The results from gas sample analysis also confirmed the existence of

404

NDEA derived from the reaction of NO, O2, SO2, and CO2.

405

AUTHOR INFORMATION

406

Corresponding Authors

407

*Phone: (306) 337-2468; Fax: (306) 585-4855; E-mail: [email protected]

408

*Phone: (306) 585-4470; Fax: (306 585-4855; Email: [email protected]

409 410

ACKNOWLEDGEMENTS

411

The authors would like to acknowledge Clean Energy Technologies Research Institute

412

(CETRI), The University of Regina for research facilities used to carry out the entire work. The

413

financial support provided by Innovation Saskatchewan (Government of Saskatchewan) and

414

Saskatchewan Power Corporation (SaskPower) are also greatly acknowledged.

415

SUPPORTING INFORMATION

416

Table S1 summarizes amines and their corresponding nitrosamines used in this study. Table S2

417

and S3 summarize major nitrosating agents and nitrosation reaction mechanisms with different

418

agents. Figure S1 show the experimental set up of nitrosamine formation experiment. Table S3


Page 21 of 35


419

summarizes nitrosation reaction mechanisms with different agents: NO2 and NO-X. Figure S2

420

(a) – (e) shows the PES diagram of nitrosating agents in both gas and liquid conditions for NOCl,

421

N2O3, CO2, SO2, and SO3, respectively. Figure S3(a)-(b) and S4 show the PES diagram for the

422

NDMA formation with NOCl, and N2O3 and NO-SO3- while S5 is that of CO2. Figure S6 (a) –

423

(b) are GC chromatograms of reaction NDEA without and with NDEA standard. Figure S7 (a) -

424

(b) show mass spectrum of sample and standard NDEA. Figure S8 shows NDEA formation from

425

various reaction days. Figure S9 shows nitrosamine formation at rich loading condition.

426

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427

(1)

Dai, N., Shah, A.D., Hu, L., Plewa, M.J., McKague, B., Mitch, W.A. 2012.

428

“Measurement of nitrosamine and nitramine formation from NOx reactions with amines

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Dai, N., Mitch, W.A., 2013. “Influence of Amine Structural Characteristics on N‑

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Fine, N.A., Nielsen, P.T.; Rochelle, G.T., 2014a. “Decomposition of Nitrosamines in

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CO2 Capture by Aqueous Piperazine or Monoethanolamine” Environ. Sci. Technol. 48,

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Fine, N.A, Goldman, M.J., Rochelle, G.T., 2014b. “Nitrosamine Formation in Amine Scrubbing at Desorber Temperatures” Environ. Sci. Technol. 48, 8777−8783.



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Nitramine Formation in Postcombustion CO2 capture Systems” Environ. Sci. Technol.

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Chandan, P. A.; Remias, J. E.; Neathery, J. K.; Liu, KL., 2013. “Morpholine nitrosation

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Padhye, L., Wang, P., Karanfil, T., Huang, C.H., 2010. “Unexpected role of Activated

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Douglass, M.L., Kabacoff. B.L., Anderson, G.A., Cheng, M.C., 1978. “The chemistry of nitrosamine formation, inhibition and destruction.” J. Soc. Cosmet. Chem. 29, 581-606.

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Ignaro, L.J., Fukuto, J. M., Griscavage, J.M., Rogers, N. E. 1993. “Oxidation of the nitric

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formed nitric oxide from L-arginine” Proc. Natl. Acad. Sci. 90, 8103-8107.

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Frisch, M. J., Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,

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J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;

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Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;

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Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,

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J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J.

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E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;

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Stratmann, R. E.; Yazyev, O.;Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;

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V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.

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D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;

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Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;

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Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;

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Methods 25. Supplementary Functions for Gaussian Basis Sets” J. Chem. Phys. 80,

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Supap, T., Idem, R., Tontiwachwuthikul, P., Saiwan, C., 2006. “Analysis of

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Jurado-Sánchez, B., Ballesteros, E., Gallego M., 2009. “Comparison of Several Solid-

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Reynolds, A. J.; Verheyen, T. V.; Adeloju, S.B.; Meuleman, E.; Feron, P., 2012. “Towards commercial scale postcombustion capture of CO2 with monoethanolamine


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solvent: key considerations for solvent management and envirionmental impacts”

506

Environ. Sci. Technol. 46, 3643-3654.

507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527



Page 26 of 35

528

List of Figures

529

Figure 1 Proposed mechanism of nitrosamine formation for NOCl, N2O3, ONOCO2-, ONOSO2-;

530

(a) Addition before proton transfer and (b) Proton transfer before bond formation.

531

Figure 2 The PES diagram for the NDMA formation of DMA + NO+ and H2O

532

Figure 3 The PES diagram for the NDMA formation of DMA + NO2-SO2-

533

Figure 4 Nitrosamine formation in liquid sample (2kmol/m3 DEA, 193 kPa 100 ppm NO + 200

534

ppm SO2 and 96 kPa 100% O2, 393 K): with 0.23 mol CO2/mole amine and without CO2 and

535

Figure 5 NDEA in gas sample with 0.23 and 0.64 CO2 loading and without CO2

536

TOC/Abstract Art

537

538

539

540

541

542

543

544

545


Page 27 of 35


546

Supporting Information

547

Table S1 Summary of amines and their corresponding nitrosamines used in this study

548

Table S2 The major six nitrosating agents of NOX in two groupsa

549

Table S3 The summary of nitrosation reaction mechanisms with different agents: NO2 and NO-X

550

551

Figure S1 Experiment set-up for nitrosamine test

552

Figure S2 The PES diagram of nitrosating agents in both gas and liquid conditions. (a) NOCl to

553

NO+, Cl- and (b) N2O3 to NO+, NO2- in gas and liquid phase; (c) NO2--CO2 to NO+, CO32- , (d)

554

NO2--SO2 to NO+, SO32- and (e) NO2---SO3 to NO+, SO42- in gas and liquid phase

555

Figure S3 The PES diagram for the NDMA formation with 3 nitrosating agents; (a) DMA +

556

NOCl and (b) DMA + NO-NO2 (N2O3)

557

Figure S4 The PES diagram for the NDMA formation of DMA + NO2-SO3-

558

Figure S5 The PES diagram for the NDMA formation of DMA + NO2-CO2

559

Figure S6 GC chromatogram of 1 day degraded DEA sample (2 kmol/m3 DEA, 193 kPa 100

560

ppm NO + 200 ppm SO2 and 96 kPa 100% O2, 393 K); (a) NDEA peak of 1 day degraded DEA

561

sample and (b) Overlay GC chromatograms of 1 day sample NDEA and Standard NDEA

562

Figure S7 Mass spectrum of NDEA: (a) NDEA in reaction sample and (b) NDEA in standard

563

sample



Page 28 of 35

564

Figure S8 Detection of NDEA from various reaction days (2 kmol/m3 DEA, 193 kPa 100 ppm

565

NO + 200 ppm SO2 and 96 kPa 100% O2, 393 K)

566

Figure S9 Nitrosamine formation at 0.64 mol CO2/mol amine in liquid sample at various

567

reaction days (2 kmol/m3 DEA, 193 kPa 100 ppm NO + 200 ppm SO2 and 96 kPa 100% O2, 393

568

K)

569

570

571

572


Page 29 of 35


Step 0:

O

N

O

X

−

δ+ N

δ X

N-X stretch

Step 1:

Addition and Elimination O

H 3C

Ν

N H 3C

Step 2:

X

H 3C

Ν

N H

H 3C

H

X

H 3C

Ν

N H

X

Charge Tranfer O H 3C

O H 3C

Ν

N

X

H

H 3C

Step 3:

O

O

H 3C

Ν

N

H 3C

H

X

Release Proton (H+) O H 3C

N

Ν

+

HX

H3C

(a) (X = Cl-, NO2-)

Figure 1 Proposed mechanism of nitrosamine formation for NOCl, N2O3, ONOCO2-, ONOSO2-; (a) Addition before proton transfer and (b) Proton transfer before bond formation.



Step 0:

-

N

O

−

δ+ N

O

OCO2

Page 30 of 35

δ OCO2

N-X stretch

Step 1:

Addition and Elimination for NDMA with NO2-CO2

N H 3C

Ν

-

H 3C O

O

Ν

N

+

H

O

H 3C

O

H 3C

O

H 3C

O

H

H 3C

C

O

H O

C

O

O

-

Ν

N

O

C

-

O

Im 1 and Transition State

Start O

H 3C

Ν

N H 3C

O

H O

-

Proton transfer before N-N formation

C O

(b) Anion can be NO- CO32- , NO-SO32-, NO-SO42-

Figure 1 Proposed mechanism of nitrosamine formation for NOCl, N2O3, ONOCO2-, ONOSO2-; (a) Addition before proton transfer and (b) Proton transfer before bond formation.


Page 31 of 35


A)

(R N-N = 1.83 Å) (R N-H = 1.05 Å)

(R O-H = 1.69 Å)

Reactant 1

Intermediate 2

(R N-N = 1.44 Å) (R N-H = 1.64 Å) (R O-H = 0.97 Å)

Transition State 3

Product 4

Figure 2 The PES diagram for the NDMA formation of DMA + NO+ and H2O



Page 32 of 35

(R N-N = 2.42 Å)

(R N-N = 1.87 Å)

(R N-H = 1.01 Å)

(R N-H = 1.66 Å)

(R N-O = 1.31 Å)

(R O-H = 2.09 Å)

(R N-O = 2.06 Å)

(R O-H = 1.10 Å)

(R N-N = 2.11 Å)

(R S-O = 2.21 Å)

(R S-O = 1.56 Å)

(R N-H = 1.03 Å)

(R S=O = 1.48 Å)

(R S-O = 1.63 Å)

(R N-O = 1.88 Å)

(R O-H = 1.76 Å) (R S-O = 1.62 Å)

(R S=O = 1.54 Å)

Start 1

Intermediate 3

Intermediate 2

O

H 3C

Ν

N H3C

O

H O

S O

Product 5 Transition State 4

The 6-atom ring of Im 3 and TS 4

Figure 3 The PES diagram for the NDMA formation of DMA + NO2-SO2-



GC response

Page 33 of 35

NDEA formed without CO2 NDEA formed with CO2

4.20

4.30

4.40

Retention time (min)

Figure 4 Nitrosamine formation in liquid sample (2 kmol/m3 DEA, 193 kPa 100 ppm NO + 200 ppm SO2 and 96 kPa 100% O2, 393 K): with 0.23 mol CO2/mole amine and without CO2



Page 34 of 35

Abundance TIC: Nitrosamine032.D\data.ms TIC: Nitrosamine041.D\data.ms TIC: Nitrosamine048.D\data.ms

300000

GC response

280000 260000

Without CO2

240000 220000

0.64 CO2 loading

200000 180000

0.23 CO2 loading

160000 140000 120000 100000 80000

4.20

4.28

4.36

4.16 4.18 4.20 4.22 4.24 4.26 4.28 4.30 4.32 4.34 4.36 4.38 4.40 Time-->

Retention time (min)

Figure 5 NDEA in gas sample with 0.23 and 0.64 CO2 loading and without CO2


Page 35 of 35


TOC/Abstract Art


Molecular Modeling and Experimental Validation

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