Reactive Mechanosynthesis of Urea Ionic Cocrystal Fertilizer Materials
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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 mechanochem- istry 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 © 2018 American Chemical Society 4680 DOI: 10.1021/acssuschemeng.7b03766 ACS Sustainable Chem. Eng. 2018, 6, 4680−4687 ACS Sustainable Chemistry & Engineering Research Article 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