Polymorph Selection by Continuous Precipitation

When the dominant rate-mechanisms within a mixed-suspension mixed-product removal (MSMPR) crystallizer are secondary nucleation and size-independent linear crystal growth, the effluent crystal distribution is guaranteed to exhibit a single polymorphic solid form at steady state. However, multiple solid forms are often simultaneously observed during the continuous precipitation of CaCO₃. Accounting for agglomeration within the population balance reconciles model predictions with experiments. Here, we elucidate the steady state structure and linear stability features of an agglomeration-enabled continuous precipitator model. We demonstrate that one can make rational process design and operation decisions to select the effluent solid form, regardless of its thermodynamic stability. Specifically, we utilize these results to choose process conditions that yield pure, thermodynamically metastable vaterite during CaCO₃ precipitation, based on powder X-ray diffraction, solid-state ⁴³Ca NMR, and scanning electron microscopy. This new design framework enables predictive modeling of CaCO₃ precipitation, but more generally, it is expected to enable rational decision making during the design and operation of other agglomerative precipitation processes for which solid form selection is desired

A General Protocol for Determining the Structures of Molecularly Ordered but Noncrystalline Silicate Frameworks

A general protocol is demonstrated for determining the structures of molecularly ordered but noncrystalline solids, which combines constraints provided by X-ray diffraction (XRD), one- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles quantum chemical calculations. The approach is used to determine the structure(s) of a surfactant-directed layered silicate with short-range order in two dimensions but without long-range periodicity in three-dimensions (3D). The absence of long-range 3D molecular order and corresponding indexable XRD reflections precludes determination of a space group for this layered silicate. Nevertheless, by combining structural constraints obtained from solid-state ²⁹Si NMR analyses, including the types and relative populations of distinct ²⁹Si sites, their respective ²⁹Si–O–²⁹Si connectivities and separation distances, with unit cell parameters (though not space group symmetry) provided by XRD, a comprehensive search of candidate framework structures leads to the identification of a small number of candidate structures that are each compatible with all of the experimental data. Subsequent refinement of the candidate structures using density functional theory calculations allows their evaluation and identification of “best” framework representations, based on their respective lattice energies and quantitative comparisons between experimental and calculated ²⁹Si isotropic chemical shifts and ²<i>J</i>(²⁹Si–O–²⁹Si) scalar couplings. The comprehensive analysis identifies three closely related and topologically equivalent framework configurations that are in close agreement with all experimental and theoretical structural constraints. The subtle differences among such similar structural models embody the complexity of the actual framework(s), which likely contain coexisting or subtle distributions of structural order that are intrinsic to the material

Highly Graphitic Mesoporous Fe,N-Doped Carbon Materials for Oxygen Reduction Electrochemical Catalysts

The synthesis, characterization, and electrocatalytic properties of mesoporous carbon materials doped with nitrogen atoms and iron are reported and compared for the catalyzed reduction of oxygen gas at fuel cell cathodes. Mixtures of common and inexpensive organic precursors, melamine, and formaldehyde were pyrolyzed in the presence of transition-metal salts (e.g., nitrates) within a mesoporous silica template to yield mesoporous carbon materials with greater extents of graphitization than those of others prepared from small-molecule precursors. In particular, Fe,N-doped carbon materials possessed high surface areas (∼800 m²/g) and high electrical conductivities (∼19 S/cm), which make them attractive for electrocatalyst applications. The surface compositions of the mesoporous Fe,N-doped carbon materials were postsynthetically modified by acid washing and followed by high-temperature thermal treatments, which were shown by X-ray photoelectron spectroscopy to favor the formation of graphitic and pyridinic nitrogen moieties. Such surface-modified materials exhibited high electrocatalytic oxygen reduction activities under alkaline conditions, as established by their high onset and half-wave potentials (1.04 and 0.87 V, respectively vs reversible hydrogen electrode) and low Tafel slope (53 mV/decade). These values are superior to many similar transition-metal- and N-doped carbon materials and compare favorably with commercially available precious-metal catalysts, e.g., 20 wt % Pt supported on activated carbon. The analyses indicate that inexpensive mesoporous Fe,N-doped carbon materials are promising alternatives to precious metal-containing catalysts for electrochemical reduction of oxygen in polymer electrolyte fuel cells

Macroscopic Structural Compositions of π‑Conjugated Polymers: Combined Insights from Solid-State NMR and Molecular Dynamics Simulations

Molecular dynamics simulations are combined with solid-state NMR measurements to gain insight into the macroscopic structural composition of the π-conjugated polymer poly­(2,5-bis­(3-tetradecyl-thiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene) (PBTTT). The structural and dynamical properties, as established by the NMR analyses, were used to test the local structure of three constitutient mesophases with (i) crystalline backbones and side chains, (ii) lamellar backbones and disordered side chains, or (iii) amorphous backbones and side chains. The relative compositions of these mesophases were then determined from the deconvolution of the ¹H and ¹³C solid-state NMR spectra and dynamic order parameters. Surprisingly, on the basis of molecular dynamics simulations, the powder composition consisted of only 28% of the completely crystalline mesophase, while 23% was lamellar with disordered side chains and 49% amorphous. The protocol presented in this work is a general approach and can be used for elucidating the relative compositions of mesophases in π-conjugated polymers

Structure-Directing Roles and Interactions of Fluoride and Organocations with Siliceous Zeolite Frameworks

Interactions of fluoride anions and organocations with crystalline silicate frameworks are shown to depend subtly on the architectures of the organic species, which significantly influence the crystalline structures that result. One- and two-dimensional (2D) ¹H, ¹⁹F, and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy measurements establish distinct intermolecular interactions among F^– anions, imidazolium structure-directing agents (SDA⁺), and crystalline silicate frameworks for as-synthesized siliceous zeolites ITW and MTT. Different types and positions of hydrophobic alkyl ligands on the imidazolium SDA⁺ species under otherwise identical zeolite synthesis compositions and conditions lead to significantly different interactions between the F^– and SDA⁺ ions and the respective silicate frameworks. For as-synthesized zeolite ITW, F^– anions are established to reside in the double-four-ring (D4R) cages and interact strongly and selectively with D4R silicate framework sites, as manifested by their strong ¹⁹F–²⁹Si dipolar couplings. By comparison, for as-synthesized zeolite MTT, F^– anions reside within the 10-ring channels and interact relatively weakly with the silicate framework as ion pairs with the SDA⁺ ions. Such differences manifest the importance of interactions between the imidazolium and F^– ions, which account for their structure-directing influences on the topologies of the resulting silicate frameworks. Furthermore, 2D ²⁹Si{²⁹Si} double-quantum NMR measurements establish ²⁹Si–O–²⁹Si site connectivities within the as-synthesized zeolites ITW and MTT that, in conjunction with synchrotron X-ray diffraction analyses, establish insights on complicated order and disorder within their framework structures

Particulate inorganic (PIC) and organic carbon (POC) content per cell.

<p><i>E</i>. <i>huxleyi</i> strains CCMP3266 (A & C) and CCMP371 (B & D) cultured under multiple temperature conditions during exponential and stationary growth phases. Thick black line in boxplot represents median value for each experimental treatment.</p

Intraspecific Differences in Biogeochemical Responses to Thermal Change in the Coccolithophore <i>Emiliania huxleyi</i>

<div><p>The species concept in marine phytoplankton is defined based on genomic, morphological, and functional properties. Reports of intraspecific diversity are widespread across major phytoplankton groups but the impacts of this variation on ecological and biogeochemical processes are often overlooked. Intraspecific diversity is well known within coccolithophores, which play an important role in the marine carbon cycle via production of particulate inorganic carbon. In this study, we investigated strain-specific responses to temperature in terms of morphology, carbon production, and carbonate mineralogy using a combination of microscopy, elemental analysis, flow cytometry, and nuclear magnetic resonance. Two strains of the cosmopolitan coccolithophore <i>E</i>. <i>huxleyi</i> isolated from different regions (subtropical, CCMP371; temperate, CCMP3266) were cultured under a range of temperature conditions (10°C, 15°C, and 20°C) using batch cultures and sampled during both exponential and stationary growth. Results for both strains showed that growth rates decreased at lower temperatures while coccosphere size increased. Between 15°C and 20°C, both strains produced similar amounts of total carbon, but differed in allocation of that carbon between particulate inorganic carbon (PIC) and particulate organic carbon (POC), though temperature effects were not detected. Between 10°C and 20°C, temperature effects on daily production of PIC and POC, as well as the cellular quota of POC were detected in CCMP3266. Strain-specific differences in coccolith shedding rates were found during exponential growth. In addition, daily shedding rates were negatively related to temperature in CCMP371 but not in CCMP3266. Despite differences in rates of particulate inorganic carbon production, both strains were found to produce coccoliths composed entirely of pure calcite, as established by solid-state ¹³C and ⁴³Ca NMR and X-ray diffraction measurements. These results highlight the limitations of the species concept and the need for a trait-based system to better quantify diversity within marine phytoplankton communities.</p></div

Solid-state ¹³C NMR.

<p>1D single-pulse ¹³C MAS NMR spectra of <i>E</i>. <i>huxleyi</i> strains (A) CCMP371 and (B) CCMP3266 acquired at 11.7 T, 25°C, and 10 kHz MAS. The asterisks indicate a frequency artifact corresponding to the ¹³C excitation pulse used in the experiment.</p

Coccosphere morphological parameters of <i>Emiliania huxleyi</i> strains CCMP371 and CCMP3266.

<p>Coccosphere morphological parameters of <i>Emiliania huxleyi</i> strains CCMP371 and CCMP3266.</p

Daily production of particulate inorganic (PIC) and organic carbon (POC) content.

<p><i>E</i>. <i>huxleyi</i> strains CCMP3266 (A & C) and CCMP371 (B & D) cultured under multiple temperature conditions during exponential growth. Thick black line in boxplot represents median value for each experimental treatment.</p