Reactive Mechanosynthesis of Urea Ionic Cocrystal Fertilizer Materials

Research Article

Cite This: ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 pubs.acs.org/journal/ascecg

Reactive Mechanosynthesis of Urea Ionic Cocrystal Fertilizer Materials from Abundant Low Solubility Magnesium- and Calcium- Containing Minerals Kenneth Honer, Carlos Pico, and Jonas Baltrusaitis* Department of Chemical and Biomolecular Engineering, Lehigh University, B336 Iacocca Hall, 111 Research Drive, Bethlehem, Pennsylvania 18015, United States

ABSTRACT: Urea is a predominantly used nitrogen fertilizer that is unstable in the environment, quickly hydrolyzing and signiﬁcantly contributing to the global nitrogen cycle disbalance. We demonstrate the application of mechanochemistry to conduct the synthesis of magnesium−urea and calcium−urea ionic cocrystals, including their nitrates, sulfates, and phosphates, in high yields by stoichiometric reactions between abundant low solubility minerals such as oxides, carbonates, and hydroxides and solid urea inorganic acids. The resulting materials possess unique properties inherited from the corresponding inorganic reactants that result in urea stabilization with respect to its deliquescence in moist environments. KEYWORDS: Calcium, Magnesium, Minerals, Fertilizers, Nitrogen, Urea, Mechanochemistry, Cocrystal, pXRD

■ INTRODUCTION Conceptually, plants with available high solubility magnesium salt precursors for urea ionic cocrystals, such as MgSO , comprise Urea, CO(NH ) , has been the most prominent nitrogen 4 2 2 a very small portion (a few percent) of the world’s total fertilizer, making up ∼60% of global nitrogen fertilizer use.1 magnesium mineral reserves.13 In stark contrast, there is a Because the process to synthesize ammonia (NH ), a reactant 3 reported estimate of more than 68 million tons of low solubility used to make urea, remains energy intensive and uses up to 1% of 2−4 magnesium minerals such as MgCO3 and Mg(OH)2 available in the global energy and ∼4% of natural gas, it is critical that the 14 ﬁ the United States. The United States Geological Survey urea nitrogen applied to soils is xated in plants and not released estimates worldwide resources of magnesite close to 12 billion in the form of gaseous NH3 or otherwise lost to the Downloaded via LEHIGH UNIV on July 9, 2018 at 20:11:30 (UTC). 5 tons. Similarly, calcium minerals (lime, quicklime, and lime- environment. Unfortunately, only about 50% of the nitrogen 15 6 stone) are in excess of billions of metric tons. Previous attempts fertilizer applied is absorbed by the crops. At the forefront of a to synthesize fertilizer materials using mechanochemical variety of solutions proposed to improve low sustainability of methods mostly relied on the milling of the corresponding 7 − urea use, farming with rocks and minerals is emerging due to the precursors such as urea and calcium sulfate.16 18 While the latter

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. wide availability of raw materials, their low costs, and minor material was particularly targeted to utilize large amounts of environmental impact. It does not necessitate use of complex phosphorus fertilizer production gypsum deposits (phospho- 8,9 synthetic chemicals such as urease inhibitors while also gypsum),19,20 virtually no other work was performed on other delivering secondary nutrients (Ca, Mg, S) to the plants. relevant magnesium−urea and calcium salt−urea ionic cocrys- Importantly, ionic urea compounds can potentially exhibit tals. While phosphogypsum waste is generated at 100−280 controlled release of primary nutrients (N, P, K). For instance, million tons per year21 and presents a large gypsum source to 22 widely available minerals such as KCl and ZnSO4 have been produce urea cocrystals, it contains enhanced natural radiation 23 shown to reduce NH3 losses and improve overall nitrogen uptake and heavy metals and is subject to environmental regulations; efficiency when compacted with urea. The decrease in N losses thus, other abundant calcium mineral sources need to be utilized. were about 10−20%.10,11 Although this is a significant improve- These are typically of very low solubility,24 so reactive sources of ment from traditional urea fertilizer, an adduct that incorporates inorganic anions need to be used. both inorganic rocks and nitrogen could yield the benefits of both Reactive mechanochemistry to make fertilizer materials using compounds: key element nutrition combined with slow-release abundant magnesium or calcium minerals (oxides, hydroxides, properties. Accordingly, Honer et al. recently showed that urea ionic cocrystals with calcium and magnesium salts can form via Received: October 16, 2017 · mechanochemical synthesis and that the ionic cocrystal, CaSO4 Revised: January 10, 2018 fi 12 4CO(NH2)2, resulted in signi cant NH3 emission decrease. Published: February 15, 2018

Figure 1. Powder XRD patterns of (left) magnesium oxide, magnesium hydroxide, magnesium (hydroxy)carbonate, calcium oxide, calcium hydroxide, and calcium carbonate and (right) urea, urea sulfate, urea phosphate, and urea nitrate reactants. Only experimental XRD patters are shown for magnesium can calcium reactants, while simulated patterns are also shown for urea sulfate, urea phosphate, and urea nitrate reactants. No experimental pattern was obtained for urea sulfate. and carbonates) has seldom been utilized. KMgPO4 was corresponding molar ratios was loaded into a 15 mL stainless steel jar together with 3 individual 8 mm stainless steel balls and grounded for up prepared by milling KH2PO4 and Mg(OH)2 at a molar ratio of − 25 to 10 min at 26 Hz in a Retsch MM300 mixer mill. Crystalline nature of 1:1 for 120 min at mill rotational speeds of 500 600 rpm. The fi ff same authors also synthesized MgNH PO by milling all reactants and products was con rmed using powder X-ray di raction 4 4 (pXRD, Empyrean, PANalytical B.V.). All magnesium and calcium NH4H2PO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min − precursors as well as urea sulfate and urea phosphate were obtained from at mill rotational speeds of 300 700 rpm. CaO was utilized by Sigma-Aldrich or Fischer Scientific and were of reagent or similar grade. milling of KOH, SiO2, and CaO mixtures to obtain slow release Urea nitrate was synthesized using stoichiometric amounts of urea and 26 K−Si−Ca−O fertilizers of amorphous phase. Typically, nitric acid in aqueous solutions at 10 °C.39 however, already soluble magnesium (and potassium) com- Hygroscopicity Measurements. To qualitative assess the ability of poundssuchasKH2PO4 and NH4H2PO4 have been the formed ionic cocrystal material to adsorb water and deliquesce, mechanochemically combined with kaolinite25,27,28 or alumina29 synthesized samples were exposed to moist air at 23 °C and 100% to yield slow release fertilizers. Notably, these works did not relative humidity (RH) in a closed static environment and their physical focus on creating crystalline urea-containing fertilizer materials state was monitored for 4 days. that have the potential to decrease nitrogen losses. In the present work, we explored a reactive milling route utilizing abundant ■ RESULTS AND DISCUSSION magnesium or calcium minerals (periclase (MgO), brucite Mg−Urea and Ca−Urea Cocrystal Reactive Mechano- (Mg(OH)2), magnesite (MgCO3), lime (CaO), hydrated lime chemical Synthesis. Mechanochemical synthesis of urea ionic (Ca(OH)2), and calcite (CaCO3)) and solid urea inorganic acid cocrystals was attempted to obtain crystalline compounds of 30 31 32 · · · adducts (urea nitrate, urea phosphate, and urea sulfate )asa CaSO4 4CO(NH2)2, Ca(H2PO4)2 4CO(NH2)2,Ca(NO3)2 · · · source of the reactive urea and inorganic anions to obtain 4CO(NH2)2, MgSO4 6CO(NH2)2 0.5H2O, Mg(H2PO4)2 4CO- · · enhanced nitrogen management fertilizer materials. (NH2)2, and Mg(NO3)2 4CO(NH2)2 2H2O. Previous work successfully synthesized them from the corresponding salts, e.g. · 12 ■ EXPERIMENTAL SECTION CaSO4 2H2O and CO(NH2)2, mechanochemically. In the Mechanochemistry and Crystal Structure Testing. Mechano- current work, reactive mechanochemistry can proceed via a chemical treatment of solid reactant powders was applied as it provides a tandem reaction of the corresponding oxide (hydroxide, solvent free, scalable, and sustainable route of solid−solid trans- carbonate) with the acid component of the solid urea acid − formations.33 35 During the mechanical initiation of chemical reactions, cocrystal followed by the transformation of the intermediate an activated state is created due to the changes in solid structure formed into the final ionic cocrystal form via eqs 1−3, as shown followed by the gradual relaxation to the equilibrium state · in the example of CaSO4 4CO(NH2)2 formation: (composition).36,37 Mechanochemistry has been shown to be successful in transforming poorly soluble minerals such as metal oxides into the CaO++· 3CO(NH ) CO(NH ) H SO preparation of metal−organic frameworks.38 Notably, while mecha- 22 22 2 4 →· + nochemistry is considered solvent-free, very small amounts of added CaSO42 4CO(NH )2 H2 O (1) liquid H2O were used in some of the present experiments as they can dramatically accelerate, and even enable, mechanochemical reactions 34 ++· between solids. In a typical procedure, a total of 200−400 mg samples Ca(OH)22 3CO(NH )22 CO(NH )2 H2 SO4 of Ca or Mg precursor (oxide, hydroxide, or carbonate), urea acid →· + cocrystal (urea nitrate, sulfate, or phosphate) and urea mixture with the CaSO42 4CO(NH )22 2H O (2)

4681 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 ACS Sustainable Chemistry & Engineering Research Article

− · · · Figure 2. Powder XRD patterns of magnesium urea ionic cocrystal products, e.g. MgSO4 6CO(NH2)2 0.5H2O, Mg(H2PO4)2 4CO(NH2)2, and · · “•” Mg(NO3)2 4CO(NH2)2 2H2O. Peaks due to the trace unreacted urea are noted with .

++· complex. MgCO used in these experiments, as shown in Figure CaCO32 3CO(NH )22 CO(NH )2 H2 SO4 3 1, exhibits a complex pXRD pattern due to the several →· ++ · 40,41 CaSO42 4CO(NH )2 H22 O CO (3) hydromagnesite forms such as Mg5(CO3)4(OH)2 4H2O. Hydromagnesite (along with few other hydrated products, such · 42 In all instances, stoichiometric amounts of urea acid were added as nesquehonite, MgCO3 3H2O) is the most widespread form to obtain the corresponding magnesium or calcium salt with the of hydrated magnesium carbonate mineral43 and is regarded as a remaining amount of neat urea to yield the desired composition metastable phase of the Mg(OH)2 and CO2 reaction product, ionic cocrystal. While the composition of the intermediate leading to the thermodynamically stable magnesite, MgCO . − 3 products shown in eqs 1 3 and the corresponding reaction This reaction is important due to the potential of Mg(OH)2 to 44 kinetics can only be hypothesized, the same general dry sequester millions of tons of CO2. At room temperature, mechanochemical reaction network should be applicable to however, the transformation from hydromagnesite to magnesite other solid reactants such as urea nitric and phosphoric acid is very slow,45 and hydromagnesite can be potentially available in · · cocrystals CO(NH2)2 H3PO4 and CO(NH2)2 HNO3 as well as large amounts. · the corresponding magnesium minerals MgO, Mg(OH)2, and pXRD pattern shown in Figure 1 right of CO(NH2)2 H3PO4 MgCO3. The resulting pXRD patterns of the parent compounds, agreed with the simulated pattern from the crystal structure 46,47 · 30,48 e.g. MgO, Mg(OH)2, MgCO3 CaO, Ca(OH)2, and CaCO3 as data as well as that of synthesized CO(NH2)2 HNO3. · well as simulated and experimental compounds of 2CO(NH2)2 We were unable to measure pXRD pattern of urea sulfate due to · · · H2SO4, CO(NH2)2 H3PO4, and CO(NH2)2 HNO3 are shown in its hygroscopicity. Dalman reported both solid H2SO4 CO- · Figure 1 left and right, respectively. MgO, CaO, Mg(OH)2, (NH2)2 and H2SO4 2CO(NH2)2 precipitated from the ternary − − ° 49 Ca(OH)2, and CaCO3 reactant pXRD patterns exhibit character- urea H2SO4 H2O mixture at 10 and 25 C. The crystal istic peaks, while the magnesium carbonate pattern is more structure was not determined until 1999 when Chen et al.32

4682 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 ACS Sustainable Chemistry & Engineering Research Article

− · · · Figure 3. Powder XRD patterns of calcium urea ionic cocrystal products, e.g. CaSO4 4CO(NH2)2, Ca(H2PO4)2 4CO(NH2)2, and Ca(NO3)2 “•” “▲” 4CO(NH2)2. Peaks due to the trace unreacted urea are noted with , while those due to the unreacted Ca(OH)2 are noted with . · ff demonstrated the crystal form of H2SO4 2CO(NH2)2 is that of distribution were di erent from those obtained during the + two uronium ions, CO(NH2)2H coordinated to neighboring mechanohemistry of pure magnesium and calcium salts and 2− 12 SO4 . Their crystals were grown in a nitrogen atmosphere and urea. However, we were able to convert most of our insoluble exhibited great solubility in both organic solvents and water.50 mineral precursors at the considerable conversion into the same · The crystal structure of H2SO4 CO(NH2)2 is not known to our urea ionic cocrystal products. In particular, we observed rather · knowledge, and we will further refer to this compound as urea robust conversion of all magnesium minerals into Mg(H2PO4)2 sulfate. However, as we show in our data, the resulting crystal 4CO(NH2)2. In MgO case for this adduct, trace amounts of structure is not very sensitive to the original urea sulfate secondary unidentified product was formed as observed by the composition or degree of moisture adsorbed. peaks at 2Θ of 15, 31, and 49°. An unidentified adduct was The products resulting from the three-component reactive formed when all three magnesium minerals were reacted to yield · milling are shown in Figure 2 for magnesium compounds and the corresponding nitrate, which did not match the Mg(NO3)2 · 56 Figure 3 for calcium compounds. A detailed comparison of every 4CO(NH2)2 2H2O crystalline pattern. We hence labeled it as ’ · · reactant, product, and simulated product s (obtained from the Mg(NO3)2 xCO(NH2)2 yH2OinFigure 2. A series of varying Cambridge Crystallographic Data Centre (CCDC)51) XRD composition magnesium nitrate−urea ionic cocrystals was pattern of the reported molecular crystals in the literature is previously synthesized using ternary systems of magnesium 57−59 · shown. The XRD patterns were simulated of the target product nitrate, urea, and water. Those include Mg(NO3)2 2CO- · · · · · crystals using conventional solution-based routes of CaSO4 (NH2)2 6H2O, Mg(NO3)2 2CO(NH2)2 4H2O, and Mg(NO3)2 52 · 53 · 4CO(NH2)2, Ca(H2PO4)2 4CO(NH2)2, Ca(NO3)2 4CO- 6CO(NH2)2. In general, owing to high propensity between 54 · · 55 · (NH2)2, MgSO4 6CO(NH2)2 0.5H2O, Mg(H2PO4)2 4CO- nitrate and water ions and large amount of the latter, these 53 · · 56 ffi (NH2)2, and Mg(NO3)2 4CO(NH2)2 2H2O. Unsurpris- compounds are rather di cult to stabilize or control their solid ingly, the overall urea conversion and the corresponding product phase reactions to obtain the desired composition.12 During the

4683 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 ACS Sustainable Chemistry & Engineering Research Article · · 55 reaction with urea sulfate, MgSO4 6CO(NH2)2 0.5H2O crystal Table 1. Selected Results of Optimization for the Magnesium structure was obtained for Mg(OH)2 and MgO reaction and Calcium Mineral Conversion into Urea Adducts Using · products but not MgCO3. In the latter case, very little urea Urea (CO(NH2)2) and Urea Inorganic Acids (CO(NH2)2 · · conversion was obtained. H2SO4, CO(NH2)2 H3PO4, and CO(NH2)2 HNO3) Very high product selectivities were achieved during the loading milling time corresponding calcium mineral mechanochemistry with the urea entry reactantsa amount (mg)b (min) acid adducts. As shown in Figure 3, all of the cases, including 1 Ca(OH)2 + 3CO(NH2)2 + 200 10 Ca(OH)2, CaO and CaCO3 agree very well with the simulated CO(NH ) ·H SO · 52 · 2 2 2 4 XRD patterns of CaSO4 4CO(NH2)2, Ca(H2PO4)2 4CO- 2 Ca(OH) + 2CO(NH ) + 400 10 53 · 54 2 · 2 2 (NH2)2, Ca(NO3)2 4CO(NH2)2. We propose this to be 2CO(NH2)2 HNO3 due to the much lower diversity in inorganic calcium salt - urea 3 Ca(OH)2 + 2CO(NH2)2 + 400 7 · ionic cocrystal compositions stemming from their high 2CO(NH2)2 H3PO4 · 4 Mg(OH)2 + 3CO(NH2)2 + 200 10 thermodynamic stability. The existence of Ca(H2PO4)2 CO- · 60 CO(NH2)2 H2SO4 (NH2)2 was proposed but not quite substantiated. Tetrakis 5 Mg(OH) + 2CO(NH ) + 400 10 · 53 2 · 2 2 (urea) monocalcium phosphate, Ca(H2PO4)2 4CO(NH2)2, on 2CO(NH2)2 HNO3 the other hand, has a well-established crystal structure. Similarly, 6 Mg(OH)2 + 2CO(NH2)2 + 400 10 · 54 2CO(NH ) ·H PO two crystal structures of Ca(NO3)2 4CO(NH2)2 and Ca- 2 2 3 4 · · 61 7 CaO + 3CO(NH ) + CO(NH ) · 200 0.5 (NO3)2 CO(NH2)2 3H2O were reported. Finally, calcium 2 2 2 2 · H2SO4 tetrakis (urea) sulfate CaSO4 4CO(NH2)2 has a well-known · 8 CaO + 2CO(NH2)2 + 2CO(NH2)2 400 5 crystal structure of triclinic pseudotetragonal cell with linear HNO3 chains of CaSO4 and dodecahedral coordination of calcium 9 CaO + 2CO(NH ) + 2CO(NH ) · 400 0.5 52,62 63 2 2 2 2 ions and was synthesized from aqueous solutions. Notable H3PO4 · little or no crystalline water content in thermodynamically stable 10 MgO + 3CO(NH2)2 + CO(NH2)2 200 0.5 polymorphs of calcium salt - urea ionic cocrystals suggests that H2SO4 · magnesium salts can form alternative more thermodynamically 11 MgO + 2CO(NH2)2 + 2CO(NH2)2 400 3.5 HNO3 stable polymorphs or completely different cocrystals with · ff 12 MgO + 2CO(NH2)2 + 2CO(NH2)2 400 5 di erent urea and water amounts in the unit cell, such as in H3PO4 · · · our attempts to synthesize Mg(NO3)2 4CO(NH2)2 2H2O 13 CaCO3 + 3CO(NH2)2 + CO(NH2)2 200 0.5 shown in Figure 2. To support these observations, we attempted H2SO4 · to perform the optimization of the mechanosynthesis parameters 14 CaCO3+2CO(NH2)2+2CO(NH2)2 400 5 HNO3 by varying total loading amount, reactant ratios, and milling time, · 15 CaCO3 + 2CO(NH2)2 + 2CO(NH2)2 400 1 as shown in Table 1. In nearly all situations, there were small H PO ff 3 4 di erences in urea conversion but, with the exception of 16 MgCO + 3CO(NH ) + CO(NH ) · 200 2 · 3 2 2 2 2 MgCO3+3CO(NH2)2+CO(NH2)2 H2SO4 where the product H2SO4 was unidentified and very little conversion occurred, the same 17 MgCO3 + 2CO(NH2)2 + 400 0.5 · final products always formed. This suggests that we always 2CO(NH2)2 HNO3 18 MgCO + 2CO(NH ) + 400 1 arrived at the thermodynamically stable case via fast kinetics as 3 · 2 2 2CO(NH2)2 H3PO4 manifested by short milling times. a b Water Effect on Urea Cocrystal Reactive Mechano- Molar ratios. Total sample loading in the mill. chemical Synthesis. The addition of a few drops of liquid water to the reacting salts before milling was found to have a very profound effect toward product formation. From the basic stoichiometry provided in eqs 1−3, it is apparent that water is formed during the reaction of inorganic acid component with the · · basic metal mineral. However, only MgSO4 6CO(NH2)2 · · 0.5H2O and Mg(NO3)2 4CO(NH2)2 2H2O require water to form ionic urea crystal hydrates. In the rest of the cases, water formed is likely to become physisorbed or evaporate during milling. More importantly, as shown in Figure 4, most of the reactions required additional water for the products to form even when the final ionic cocrystals had no crystalline water in their structure. While liquid assisted grinding (LAG) has previously been shown to be very effective for organic cocrystal hydrate 64 synthesis, compiled Figure 4 suggests different solution effects Figure 4. Tabulated reactivity of Ca and Mg minerals toward ionic urea in inorganic salt−urea system. Best conversion was obtained for nitrate, sulfate, and phosphate cocrystal formation as a function of the MgO and CaO only in the presence of water, while the water added to facilitate reactive milling. corresponding hydroxide and carbonates underwent conversion in most instances with no water needed. This suggests that structure and composition of the urea inorganic acid precursor, hydration reaction takes place to yield hydroxides before the as shown in the urea sulfate case. This is consistent with a recent 65 corresponding adduct can form. Hence, the rate limiting step that observation where L-serine−oxalic acid liquid assisted grinding governs the reactive kinetics is metal oxide moiety hydration was proposed to take place in the liquid phase at the contact rather than the following magnesium or calcium salt formation, between the solid particles and did not depend on the crystal and thus, the reactive network observed is less dependent on the structures of the initial components.

4684 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 ACS Sustainable Chemistry & Engineering Research Article

Hygroscopicity Testing. There is little known about the exhibit improved stability in moist air. Future work will focus on hygroscopicity of the resulting materials. Previously, Frazier et identifying reactive intermediates involved in this reactive 66 · al. isolated the dry, solid Ca(H2PO4)2 4CO(NH2)2, which was mechanochemical transformations of low solubility magnesium nonhygroscopic at relative humidities below about 60%. and calcium salts to urea ionic cocrystals using spectroscopic · 69 Furthermore, Ca(NO3)2 4CO(NH2)2 contains no crystalline methods and measuring the corresponding reaction kinetics. · 67 water and is less hygroscopic than Ca(NO3)2 4H2O. The latter compound is a major problem with maintaining certain fertilizer ■ AUTHOR INFORMATION stability due to the extreme hygroscopicity. Finally, below 75% Corresponding Author · fl relative humidity, CaSO4 4CO(NH2)2 is a dry and free- owing *E-mail: [email protected]; Phone: 1-610-758-6836. powder. This suggests that it is less hygroscopic than urea itself, ORCID which becomes wet at around 72% relative humidity. The effect of different anion is particularly interesting when elucidating Jonas Baltrusaitis: 0000-0001-5634-955X stability of these salts in moist environments. We performed Notes · fi accelerated experiments where we exposed CaSO4 4CO(NH2)2, The authors declare no competing nancial interest. · · Ca(H2PO4)2 4CO(NH2)2, and Ca(NO3)2 4CO(NH2)2 to 100% relative humidity (∼21 mmHg) at 23 °C. Time resolved images ■ ACKNOWLEDGMENTS were acquired and are shown in Figure 5. It can be seen that This material is based upon work supported by the National Science Foundation under Grant CHE 1710120. ■ REFERENCES (1) Prud’homme, M.. Global Fertilizer Supply and Trade: 2016−2017. In IFA Strategic Forum, Dubai; International Fertilizer Association (IFA): Dubai, UAE, 2016. (2) Patil, B. S.; Wang, Q.; Hessel, V.; Lang, J. Plasma N2-fixation: 1900−2014. Catal. 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4685 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 ACS Sustainable Chemistry & Engineering Research Article

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4687 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687