Reactive Mechanosynthesis of Urea Ionic Cocrystal Fertilizer Materials

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Reactive mechanosynthesis of urea ionic cocrystal fertilizer materials from abundant low solubility magnesium and calcium containing minerals Kenneth Honer, Carlos Pico, and Jonas Baltrusaitis ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03766 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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ACS Sustainable Chemistry & Engineering

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, PA 18015, USA Abstract Urea is a predominantly used nitrogen fertilizer, but it 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 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.

*Corresponding Author: [email protected] (email); +1-610-758-6836 (phone) Keywords: calcium; magnesium; minerals; fertilizers; nitrogen; urea; mechanochemistry; cocrystal; pXRD

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Introduction: Urea, CO(NH2)2, has been the most prominent nitrogen fertilizer making up ~60% of global nitrogen fertilizer use.1 Since the process to synthesize ammonia (NH3), a reactant used to make urea, remains energy intensive and uses up to 1 % of the global energy and ~4 % of natural gas,2–4 it is critical that the urea nitrogen applied to soils is fixated in plants and not released in the form of gaseous NH3 or otherwise lost to the environment.5 Unfortunately, only about 50% of the nitrogen fertilizer applied is absorbed by the crops.6 At the forefront of variety of solutions proposed to improve low sustainability of urea use, farming with rocks and minerals7 is emerging due to the wide availability of raw materials, their low costs and minor environmental impact. It does not necessitate use of complex synthetic chemicals such as urease inhibitors8,9 while also delivering secondary nutrients (Ca, Mg, S) to the plants. Importantly, ionic urea compounds can potentially exhibit controlled release of primary nutrients (N, P, K). For instance, widely available minerals such as KCl and ZnSO4 have been shown to reduce NH3 losses and improve overall nitrogen uptake efficiency when compacted with urea. The decrease in N losses were about 10-20 %.10,11 Although this is a significant improvement from traditional urea fertilizer, an adduct that incorporates both inorganic rocks and nitrogen could yield the benefits of both compounds: key element nutrition combined with slow-release properties. Accordingly, Honer et al. recently showed that urea ionic cocrystals with calcium and magnesium salts can form via mechanochemical synthesis and that the ionic cocrystal, CaSO4⋅4CO(NH2)2, resulted in significant NH3 emission decrease.12 Conceptually, plant available high solubility magnesium salt precursors for urea ionic cocrystals, such as MgSO4, comprise a very small portion (few percent) of the world’s total magnesium mineral reserves.13 In stark contrast, there is a reported estimate of about 65 million tons of low solubility magnesium minerals, such as MgCO3 available in the US with Mg(OH)2 of 3 million tons.14

The United States Geological Survey estimates worldwide resources of

magnesite close to 12 billion tons. Similarly, calcium minerals (lime, quicklime and limestone) are in excess of billions of metric tons.15 Previous attempts to synthesize fertilizer materials using mechanochemical methods mostly relied on the milling of the corresponding precursors, such as urea and calcium sulfate.16–18 While the latter material was particularly targeted to utilize large amounts of phosphorus fertilizer production gypsum deposits (phosphogypsum),19,20 2 ACS Paragon Plus Environment

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virtually no other work was performed on other relevant magnesium and calcium salt-urea ionic cocrystals. While phosphogypsum waste is generated at 100-280 million tons per year21 and presents a large gypsum source to produce urea cocrystals, it contains enhanced natural radiation22 and heavy metals and is a subject to the environmental regulations,23 thus other abundant calcium mineral sources need to be utilized.

These are typically of very low

solubility,24 so reactive sources of inorganic anion need to be used. Reactive mechanochemistry to make fertilizer materials using abundant magnesium or calcium minerals (oxides, hydroxides and carbonates) has seldom been utilized. KMgPO4 was prepared by milling KH2PO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min at mill rotational speeds of 500-600 rpm.25 The same authors also synthesized MgNH4PO4 by milling NH4H2PO4 and Mg(OH)2 at a molar ratio of 1:1 for 120 min at mill rotational speeds of 300-700 rpm. CaO was utilized by milling of KOH, SiO2 and CaO mixtures to obtain slow release K-Si-Ca-O fertilizers of amorphous phase.26

Typically, however, already soluble magnesium (and

potassium) compounds, such as KH2PO4, NH4H2PO4, have been mechanochemically combined with kaolinite25,27,28 or alumina29 to yield slow release fertilizers. Notably, these works did not focus on creating crystalline urea containing fertilizer materials that have the potential to decrease nitrogen losses. In the present work, we explored a reactive milling route utilizing abundant magnesium or calcium minerals (periclase (MgO), brucite (Mg(OH)2), magnesite (MgCO3), lime (CaO), hydrated lime (Ca(OH)2) and calcite (CaCO3)) and solid urea inorganic acid adducts (urea nitrate,30 urea phosphate31 and urea sulfate32) as a source of the reactive urea and inorganic anions to obtain enhanced nitrogen management fertilizer materials.

Experimental Mechanochemistry and crystal structure testing.

Mechanochemical treatment of solid

reactant powders was applied as it provides a solvent free, scalable and sustainable route of solid-solid transformations.33–35

During the mechanical initiation of chemical reactions an

activated state is created due to the changes in solid structure followed by the gradual relaxation to the equilibrium state (composition).36,37 Mechanochemistry has been shown to be successful in transforming poorly soluble minerals, such as metal oxides, into the preparation of metalorganic frameworks.38 Notably, while mechanochemistry is considered solvent free, very small 3 ACS Paragon Plus Environment


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amounts of added liquid H2O were used in some of the present experiments as they can dramatically accelerate, and even enable, mechanochemical reactions between solids.34 In a typical procedure, a total of 200 mg to 400 mg sample of Ca or Mg precursor (oxide, hydroxide or carbonate), urea acid cocrystal (urea nitrate, sulfate or phosphate) and urea mixture with the corresponding molar ratios was loaded into a 15 mL stainless steel jar together with three individual 8 mm stainless steel balls and grounded for up to 10 mins at 26 Hz in a Retsch MM300 mixer mill. Crystalline nature of all reactants and products was confirmed using powder X-ray diffraction (Empyrean, PANalytical B.V.). All magnesium and calcium precursors as well as urea sulfate and urea phosphate were obtained from Sigma-Aldrich or Fischer Scientific and were of reagent or similar grade. Urea nitrate was synthesized using stoichiometric amounts of urea and nitric acid in aqueous solutions at 10 oC.39 Hygroscopicity measurements. To qualitative assess the ability of the formed ionic cocrystal material to adsorb water and deliquesce, synthesized samples were exposed to moist air at 23 oC and 100 % relative humidity (RH) in a closed static environment and their physical state was monitored for 4 days.

Results and discussion Mg and Ca-urea cocrystal reactive mechanochemical synthesis. Mechanochemical synthesis of urea ionic cocrystals was attempted to obtain crystalline compounds of CaSO4⋅4CO(NH2)2, Ca(H2PO4)2⋅4CO(NH2)2,

Ca(NO3)2⋅4CO(NH2)2,

Mg(H2PO4)2⋅4CO(NH2)2 and Mg(NO3)2⋅4CO(NH2)2⋅2H2O. synthesized them

MgSO4⋅6CO(NH2)2⋅0.5H2O, Previous work successfully

from the corresponding salts, e.g. CaSO4⋅2H2O and CO(NH2)2,

mechanochemically.12 In the current work, reactive mechanochemistry can proceed via a tandem reaction of the corresponding oxide (hydroxide, carbonate) with the acid component of the solid urea acid cocrystal followed by the transformation of the intermediate formed into the final ionic cocrystal form via (1) through (3) as shown in the example of CaSO4⋅4CO(NH2)2 formation CaO+3CO(NH2)2+CO(NH2)2⋅H2SO4→ CaSO4⋅4CO(NH2)2+2H2O

(1),

Ca(OH)2+3CO(NH2)2+CO(NH2)2⋅H2SO4→ CaSO4⋅4CO(NH2)2+2H2O

(2),

CaCO3+3CO(NH2)2+CO(NH2)2⋅H2SO4→ CaSO4⋅4CO(NH2)2+H2O+CO2

(3).

In all instances stoichiometric amounts of urea acid were added to obtain the corresponding magnesium or calcium salt with the remaining amount of neat urea to yield the desired 4 ACS Paragon Plus Environment

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composition ionic cocrystal. While the composition of the intermediate products shown in eq. (1) through (3) and the corresponding reaction kinetics can only be hypothesized, the same general dry mechanochemical reaction network should be applicable to other solid reactants, such as urea nitric and phosphoric acid cocrystals, CO(NH2)2⋅H3PO4 and CO(NH2)2⋅HNO3, as well as the corresponding magnesium minerals, MgO, Mg(OH)2 and MgCO3. The resulting pXRD patterns of the parent compounds, e.g. MgO, Mg(OH)2, MgCO3 CaO, Ca(OH)2 and CaCO3 as well as simulated and experimental of 2CO(NH2)2⋅H2SO4, CO(NH2)2⋅H3PO4 and CO(NH2)2⋅HNO3 are shown in Figure 1 left and right, respectively. MgO, CaO, Mg(OH)2, Ca(OH)2 and CaCO3 reactant pXRD patterns exhibit characteristic peaks while magnesium carbonate pattern is more complex. MgCO3 used in these experiments, as shown in Figure 1, exhibits a complex pXRD pattern due to the several hydromagnesite forms, such as Mg5(CO3)4(OH)2·4H2O.40,41 Hydromagnesite (along with few other hydrated products, such as nesquehonite, MgCO3⋅3H2O)42 is the most widespread form of hydrated magnesium carbonate minerals43 and is regarded as a metastable phase of the Mg(OH)2 and CO2 reaction product leading to the thermodynamically stable magnesite, MgCO3. This reaction is important due to the potential of Mg(OH)2 to sequester millions of tons of CO2.44 At room temperature, however, the transformation from hydromagnesite to magnesite is very slow45 and hydromagnesite can be potentially available in large amounts. pXRD pattern shown in Figure 1 right of CO(NH2)2·H3PO4 agreed with the simulated pattern from the crystal structure data46,47 as well as that of synthesized CO(NH2)2·HNO3.30,48 We have not been able to measure pXRD pattern of urea sulfate due to its hygroscopicity. Dalman reported both solid H2SO4· CO(NH2)2 and H2SO4·2CO(NH2)2 precipitated from the ternary ureaH2SO4-H2O mixture at 10 and 25 oC.49 The crystal structure has not been determined until 1999 when Chen et al.32 demonstrated the crystal form of H2SO4·2CO(NH2)2 is that of two uronium ions, CO(NH2)2H+ coordinated to neighboring SO42-. Their crystals were grown in a nitrogen atmosphere and exhibited great solubility in both organic solvents and water.50 The crystal structure of H2SO4·CO(NH2)2 is not known to our knowledge and we will further refer to this compound as urea sulfate. However, as we show in our data, the resulting crystal structure is not very sensitive to the original urea sulfate composition or degree of moisture adsorbed.



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The products resulting from the three-component reactive milling are shown in Figure 2 for magnesium compounds and Figure 3 for calcium compounds. A detailed comparison of every reactant, product, and simulated product’s - obtained from the Cambridge Crystallographic Data Centre (CCDC)51 - XRD pattern of the reported molecular crystals in the literature is shown. The XRD patterns were simulated of the target product crystals using conventional solution based routes of CaSO4⋅4CO(NH2)2,52 Ca(H2PO4)2⋅4CO(NH2)2,53 Ca(NO3)2⋅4CO(NH2)2,54 MgSO4⋅6CO(NH2)2⋅0.5H2O,55 Mg(H2PO4)2⋅4CO(NH2)253 and Mg(NO3)2⋅4CO(NH2)2⋅2H2O.56 Unsurprisingly, the overall urea conversion and the corresponding product distribution was different from those obtained during the mechanohemistry of pure magnesium and calcium salts and urea.12 However, we were able to convert most of our insoluble mineral precursors at the considerable conversion into the same urea ionic cocrystal products. In particular, we observed rather robust conversion of all magnesium minerals into Mg(H2PO4)2⋅4CO(NH2)2. In MgO case for this adduct, trace amounts of secondary unidentified product was formed as observed by the peaks at 2Θ of 15, 31 and 49o. An unidentified adduct was formed when all three magnesium minerals were reacted to yield the corresponding nitrate, which did not match Mg(NO3)2⋅4CO(NH2)2 ⋅2H2O crystalline pattern.56 We hence labeled it as Mg(NO3)2⋅xCO(NH2)2 ⋅yH2O in Figure 2. Series of varying composition magnesium nitrate-urea ionic cocrystals have previously been synthesized using ternary systems of magnesium nitrate, urea and water.57–59 Those include Mg(NO3)2⋅2CO(NH2)2⋅6H2O, Mg(NO3)2⋅2CO(NH2)2⋅4H2O and Mg(NO3)2⋅6urea. In general, owing to high propensity between nitrate and water ions and large amount of the latter these compounds are rather difficult to stabilize or control their solid phase reactions to obtain

the

desired

composition.12

During

the

reaction

with

urea

sulfate,

MgSO4⋅6CO(NH2)2⋅0.5H2O55 crystal structure was obtained for Mg(OH)2 and MgO reaction products but not MgCO3. In the latter case very little urea conversion was obtained. Very high product selectivities were achieved during the corresponding calcium mineral mechanochemistry with the urea acid adducts. As shown in Figure 3, all of the cases, including Ca(OH)2, CaO and CaCO3 agree very well with the simulated XRD patterns of CaSO4⋅4CO(NH2)2,52 Ca(H2PO4)2⋅4CO(NH2)2,53 Ca(NO3)2⋅4CO(NH2)2.54 We propose this to be due to the much lower diversity in inorganic calcium salt - urea ionic cocrystal compositions stemming from their high thermodynamic stability. The existence of Ca(H2PO4)2⋅CO(NH2)260


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was proposed but not quite substantiated.

Tetrakis (urea) monocalcium phosphate,

Ca(H2PO4)2⋅4CO(NH2)253, on the other hand, has a well-established crystal structure. Similarly, two crystal structures of Ca(NO3)2⋅4CO(NH2)254 and Ca(NO3)2⋅CO(NH2)2⋅3H2O61 were reported. Finally, calcium tetrakis - (urea) sulfate CaSO4·4CO(NH2)2, has a well-known crystal structure of triclinic pseudotetragonal cell with linear chains of CaSO4 and dodecahedral coordination of calcium ions52,62 and has been synthesized from aqueous solutions.63

Notable little or no

crystalline water content in thermodynamically stable polymorphs of calcium salt - urea ionic cocrystals suggests that magnesium salts can form alternative more thermodynamically stable polymorphs or completely different cocrystals with different urea and water amounts in the unit cell, such as in our attempts to synthesize Mg(NO3)2⋅4CO(NH2)2⋅2H2O shown in Figure 2. To support these observations, we attempted to perform the optimization of the mechanosynthesis parameters by varying total loading amount, reactant ratios and milling time, as shown in Table 1. In nearly all situations there was small differences in urea conversion but, with the exception of MgCO3+3CO(NH2)2+CO(NH2)2⋅H2SO4 where the product was unidentified and very little conversion occurred, the same final products always formed. This suggests that we always arrived at the thermodynamically stable case via fast kinetics as manifested by short milling times.

Water effect on urea cocrystal reactive mechanochemical synthesis. Addition of few drops of liquid water to the reacting salts before milling was found to have a very profound effect towards product formation. From the basic stoichiometry provided in eq. (1) through (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 synthesis,64 compiled Figure 4 suggests different solution effects in inorganic salt – urea system. Best conversion was obtained for MgO and CaO only in the presence of water while the corresponding hydroxide and carbonates underwent conversion in most instances with no water needed. This suggests that 7 ACS Paragon Plus Environment


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hydration reaction takes place to yield hydroxides before the corresponding adduct can form. Hence the rate limiting step that governs the reactive kinetics is metal oxide moiety hydration rather than the following magnesium or calcium salt formation and thus the reactive network observed is less dependent on the structure and composition of the urea inorganic acid precursor, as shown in urea sulfate case. This is consistent with a recent observation65 where L-serine– oxalic acid liquid assisted grinding was proposed to take place in the liquid phase at the contact between the solid particles and did not depend on the crystal structures of the initial components.

Hygroscopicity testing. There is little known about the hygroscopity of the resulting materials. Early, Frazier et al.66 isolated the dry, solid Ca(H2PO4)2·4CO(NH2)2 which was nonhygroscopic at relative humidities below about 60%.

Furthermore, Ca(NO3)2·4CO(NH2)2 contains no

crystalline water and is less hygroscopic than Ca(NO3)2·4H2O.67 The latter compound is a major problem with maintaining certain fertilizer stability due to the extreme hygroscopicity. Finally, below 75 % relative humidity, CaSO4·4CO(NH2)2 is dry and free flowing powder. This suggests that it is less hygroscopic than urea itself which becomes wet at around 72 % relative humidity. The effect of different anion is particularly interesting when elucidating stability of these salts in moist

environments.

CaSO4⋅4CO(NH2)2,

We

performed

accelerated

experiments

where

we

exposed

Ca(H2PO4)2⋅4CO(NH2)2 and Ca(NO3)2⋅4CO(NH2)2 to 100 % relative

humidity (~21 mm Hg) at 23 oC. Time resolved images were acquired and shown in Figure 5. It can be seen that CO(NH2)2 deliquesced very quickly and absorbed moisture forming liquid droplets. Similarly and somewhat unexpectedly, Ca(NO3)2·4CO(NH2)2 also deliquesced after 1 day. Both CaSO4⋅4CO(NH2)2 and Ca(H2PO4)2⋅4CO(NH2)2, on the other hand, exhibited a minor amount of moisture formation which was largely surface bound, and did not reach a state similar of the urea after one day of moisture exposure until their respective third day of 100 percent relative humidity exposure. This suggests that ionic cocrystal materials formed inherit stability properties of the inorganic parent compounds as they can stabilize urea from hydrolysis thus, potentially, minimizing NH3 losses, in agreement with recent gas emission measurement experiments.12

Conclusions and sustainability implications


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This work provides for novel mechanochemical route to yield urea ionic cocrystals from the alkaline (Mg or Ca) low solubility minerals and urea inorganic acid cocrystals. These ionic cocrystals have otherwise been synthesized through solution based methods necessitating large volume solution handling, crystallization and evaporation.52–56,68

The resulting sulfates,

phosphates and nitrates improve urea nitrogen management and decrease its hydrolysis rate. Additionally, the resulting fertilizer materials can be seen as a further alternative to urea acid fertilizers which have been previously synthesized on an industrial scale but suffer from high hygroscopicity. The ionic cocrystals of urea made in this work contain additional nutrients, such as magnesium, calcium, sulfur or phosphorus and were shown to exhibit improved stability in moist air. Future work will focus on identifying reactive intermediates involved in this reactive mechanochemical transformations of low solubility magnesium and calcium salts to urea ionic cocrystals using spectroscopic methods69 and measuring the corresponding reaction kinetics.

Acknowledgments This material is based upon work supported by the National Science Foundation under Grant No. CHE 1710120.



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Table 1. Selected results of optimization for the magnesium and calcium mineral conversion into urea adducts using urea (CO(NH2)2) and urea inorganic acids (CO(NH2)2⋅H2SO4, CO(NH2)2⋅H3PO4 and CO(NH2)2⋅HNO3) Entry

1

Ca(OH)2+3CO(NH2)2+CO(NH2)2⋅H2SO4

Loading amount (mg)b 200

2

Ca(OH)2+2CO(NH2)2+2CO(NH2)2⋅HNO3

400

10

3

Ca(OH)2+2CO(NH2)2+2CO(NH2)2⋅H3PO4

400

7

4

Mg(OH)2+3CO(NH2)2+CO(NH2)2⋅H2SO4

200

10

5

Mg(OH)2+2CO(NH2)2+2CO(NH2)2⋅HNO3

400

10

6

Mg(OH)2+2CO(NH2)2+2CO(NH2)2⋅H3PO4

400

10

7

CaO+3CO(NH2)2+CO(NH2)2⋅H2SO4

200

0.5

8

CaO+2CO(NH2)2+2CO(NH2)2⋅HNO3

400

5

9

CaO+2CO(NH2)2+2CO(NH2)2⋅H3PO4

400

0.5

10

MgO+3CO(NH2)2+CO(NH2)2⋅H2SO4

200

0.5

11

MgO+2CO(NH2)2+2CO(NH2)2⋅HNO3

400

3.5

12

MgO+2CO(NH2)2+2CO(NH2)2⋅H3PO4

400

5

13

CaCO3+3CO(NH2)2+CO(NH2)2⋅H2SO4

200

0.5

14

CaCO3+2CO(NH2)2+2CO(NH2)2⋅HNO3

400

5

15

CaCO3+2CO(NH2)2+2CO(NH2)2⋅H3PO4

400

1

16

MgCO3+3CO(NH2)2+CO(NH2)2⋅H2SO4

200

2

17

MgCO3+2CO(NH2)2+2CO(NH2)2⋅HNO3

400

0.5

18

MgCO3+2CO(NH2)2+2CO(NH2)2⋅H3PO4

400

1

Reactantsa

a. Molar ratios b. Total sample loading in the mill


Milling time (min) 10

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Figures 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 also simulated patterns are shown for urea sulfate, urea phosphate and urea nitrate reactants. No experimental pattern was obtained for urea sulfate. 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 ‘•’. 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 with ‘’. Figure 4. Tabulated reactivity of Ca and Mg minerals towards ionic urea nitrate, sulfate and phosphate cocrystal formation as a function of the water added to facilitate reactive milling. Figure 5. Time resolved optical images of urea, CaSO4⋅4CO(NH2)2, Ca(H2PO4)2⋅4CO(NH2)2, and Ca(NO3)2⋅4CO(NH2)2 exposed to saturated water vapor at 23 oC.



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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TOC. Mechanochemical treatment of insoluble Mg and Ca mineral oxides, carbonates and hydroxides with urea acids and urea results in facile synthesis of soluble urea fertilizer ionic cocrystals with enhanced nitrogen management properties.


Reactive Mechanosynthesis of Urea Ionic Cocrystal Fertilizer Materials

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