Crystalline nanoporous materials are one of the most important families of complex functional material. Many questions pertaining to the molecular assembly mechanism of the framework of these ...materials remain unanswered. Only recently has it become possible to answer definitively some of these questions by observation of growing nanoscopic surface features on metal organic frameworks (MOFs) through use of in situ atomic force microscopy (AFM). Here we reveal that a growth process of a MOF, zeolitic imidazolate framework ZIF-8, occurs through the nucleation and spreading of successive metastable unenclosed substeps to eventually form stable surface steps of the enclosed framework structure and that this process is reliant on the presence of nonframework species to bridge the developing pores during growth. The experiments also enable identification of some of the fundamental units in the growth process and the stable crystal surface plane. The former findings will be applicable to numerous nanoporous materials and support efforts to synthesize and design new frameworks and to control the crystal properties of these materials.
Understanding and predicting crystal growth is fundamental to the control of functionality in modern materials. Despite investigations for more than one hundred years, it is only recently that the ...molecular intricacies of these processes have been revealed by scanning probe microscopy. To organize and understand this large amount of new information, new rules for crystal growth need to be developed and tested. However, because of the complexity and variety of different crystal systems, attempts to understand crystal growth in detail have so far relied on developing models that are usually applicable to only one system. Such models cannot be used to achieve the wide scope of understanding that is required to create a unified model across crystal types and crystal structures. Here we describe a general approach to understanding and, in theory, predicting the growth of a wide range of crystal types, including the incorporation of defect structures, by simultaneous molecular-scale simulation of crystal habit and surface topology using a unified kinetic three-dimensional partition model. This entails dividing the structure into 'natural tiles' or Voronoi polyhedra that are metastable and, consequently, temporally persistent. As such, these units are then suitable for re-construction of the crystal via a Monte Carlo algorithm. We demonstrate our approach by predicting the crystal growth of a diverse set of crystal types, including zeolites, metal-organic frameworks, calcite, urea and l-cystine.
Crystal growth of the metal–organic framework MOF‐5 was studied by atomic force microscopy (AFM) for the first time. Growth under low supersaturation conditions was found to occur by a ...two‐dimensional or spiral crystal growth mechanism. Observation of developing nuclei during the former reveals growth occurs through a process of nucleation and spreading of metastable and stable sub‐layers revealing that MOFs may be considered as dense phase structures in terms of crystal growth, even though they contain sub‐layers consisting of ordered framework and disordered non‐framework components. These results also support the notion this may be a general mechanism of surface crystal growth at low supersaturation applicable to crystalline nanoporous materials. The crystal growth mechanism at the atomistic level was also seen to vary as a function of the growth solution Zn/H2bdc ratio producing square terraces with steps parallel to the direction or rhombus‐shaped terraces with steps parallel to the direction when the Zn/H2bdc ratio was >1 or about 1, respectively. The change in relative growth rates can be explained in terms of changes in the solution species concentrations and their influence on growth at different terrace growth sites. These results were successfully applied to the growth of as‐synthesized cube‐shaped crystals to increase expression of the {111} faces and to grow octahedral crystals of suitable quality to image using AFM. This modulator‐free route to control the crystal morphology of MOF‐5 crystals should be applicable to a wide variety of MOFs to achieve the desired morphological control for performance enhancement in applications.
Growth changes: Crystal growth mechanisms of the metal–organic framework MOF‐5 were studied by atomic force microscopy and observed to occur through a process of nucleation and spreading of metastable and stable sub‐layers, revealing that MOFs may be considered as dense phase structures in terms of crystal growth. Crystal growth rates were found to depend on the framework metal ion/organic linker ratio (see figure).
Nanoporous metal organic frameworks (MOFs) form one of the newest families of crystalline nanoporous material that is receiving worldwide attention. Successful use of MOFs for application requires ...not only development of new materials but also a need to control their crystal properties such as size, morphology, and defect concentration. An understanding of the crystal growth processes is necessary in order to aid development of routes to control such properties of the crystallites. In this Perspective article we aim to provide a short overview of the current work and understanding concerning the nucleation and growth processes of nanoporous MOFs and how this work may be expanded upon to further our comprehension of this subject. We also focus heavily on
in situ
studies that provide real time information on the developing materials and generally provide the most conclusive findings on the processes under investigation.
Current understanding of the crystal growth of nanoporous metal organic frameworks obtained primarily from real time studies is presented.
Zeolites play a crucial part in acid-base heterogeneous catalysis. Fundamental insight into their internal architecture is of great importance for understanding their structure-function ...relationships. Here, we report on a new approach correlating confocal fluorescence microscopy with focused ion beam-electron backscatter diffraction, transmission electron microscopy lamelling and diffraction, atomic force microscopy and X-ray photoelectron spectroscopy to study a wide range of coffin-shaped MFI-type zeolite crystals differing in their morphology and chemical composition. This powerful combination demonstrates a unified view on the morphology-dependent MFI-type intergrowth structures and provides evidence for the presence and nature of internal and outer-surface barriers for molecular diffusion. It has been found that internal-surface barriers originate not only from a 90 degrees mismatch in structure and pore alignment but also from small angle differences of 0.5 degrees-2 degrees for particular crystal morphologies. Furthermore, outer-surface barriers seem to be composed of a silicalite outer crust with a thickness varying from 10 to 200 nm.
This study introduces a new framework to quantify the wettability of powdered carbonate rock from existing correlations between zeta potential and contact angle. The new framework has the potential ...to be faster and cheaper than conventional approaches and could increase confidence in surface wetting quantification, since the results are insensitive to the inherent heterogeneity of rock surfaces. The obtained results from experiments were used to develop a set of equations for determining the carbonate rock contact angle from streaming potential data. The equations were validated for the evaluation of changes in the wettability of carbonate rock using different stearic acid oily solutions. The contact angles calculated from the proposed equations were then compared with measured values on the calcite surface. The results show that the proposed framework was able to quantify the wettability of carbonate rock with an acceptable range of error of about 4%–14%.
Crystal growth of the metal–organic framework, MOF-5, using basic zinc benzoate, Zn 4 O(O 2 CC 6 H 5 ) 6 , was studied in real time using atomic force microscopy. The two-dimensional nuclei involved ...in layer growth were found to form by a two-step process whereby 1,4-benzenedicarboxylate units first attach to the MOF-5 surface followed by addition of a layer of Zn species and connecting 1,4-benzenedicarboxylate units. No evidence of a growth mechanism involving nucleophilic substitution of a benzoate group from an intact Zn 4 O(O 2 CC 6 H 5 ) 6 molecule by a surface attached 1,4-benzenedicarboxylate unit was found. This indicates that the Zn 4 O(O 2 CC 6 H 5 ) 6 molecules undergo a degree of dissociation before incorporation into the MOF-5 framework. The Zn 4 O(O 2 CC 6 H 5 ) 6 -containing growth solutions were found to influence the relative growth rates along different crystallographic directions and to lead to a faster nucleation rate under certain conditions when compared to growth solutions containing simpler zinc salts. This suggests a degree of remnant association of the zinc species derived from the Zn 4 O(O 2 CC 6 H 5 ) 6 cluster during crystal growth under these conditions.
The dissolution rates of calcite, aragonite, and ground clam, cockle, and mussel shells were measured at 25 °C as a function of reactive fluid saturation state. All experiments were performed in ...mixed-flow reactors using a pH-4 HCl inlet solution. Reactive solution pH ranged from 5.1 to 9.8, and the chemical affinity of the dissolving carbonates ranged from 0 to 47 kJ/mol in the experiments. BET surface area-normalized dissolution rates for calcite are of the same order of magnitude as those of aragonite. In contrast, geometric surface area-normalized calcite dissolution rates are ∼30% lower than corresponding aragonite rates.
The dissolution behaviour of the biogenic samples depends on their composition and the surface area used to normalize rates. In all cases, measured BET-normalized dissolution rates of shells are approximately one order of magnitude lower than corresponding mineral dissolution rates. In contrast, measured geometric surface area-normalized bivalve dissolution rates are equal to within uncertainty of those of aragonite or calcite. Geometric surface area-normalized dissolution rates (
r
gsa) of both aragonite and crushed clam and cockle shells, which are composed of aragonite, can be described within uncertainty using:
r
gsa
mol
/
cm
2
/
s
=
(
2.69
±
0.5
)
×
10
−
10
(
1
−
Ω
)
0.86
±
0.11
where
Ω stands for the saturation state of the dissolving carbonate. Similarly,
r
gsa of calcite can be described using:
r
gsa
mol
/
cm
2
/
s
=
(
1.82
±
0.2
)
×
10
−
10
(
1
−
Ω
)
1.25
±
0.16
where
r
gsa for mussel shells, which are composed of ∼90% calcite and ∼10% aragonite, are similar to those of calcite, but display a complex variation with chemical affinity due to the presence of two minerals. Consistent with previous studies,
r
gsa is found to be accurately described as a function of saturation index independent of pH at neutral to basic conditions.
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•Faujasite zeolites are active in the esterification of 5-hydroxymethylfurfural (5-HMF) to 5-acetoxymethylfurfural (AcMF).•The Si/Al ratio influences the textural and acid properties ...and also modify the catalytic performance in the reaction.•The maximum 5-HMF conversion of 87.28% was achieved using the zeolite with Si/Al = 5.
We present this study on FAU-type zeolites were prepared varying the Si/Al ratio (4, 5 and 6) and crystallization time (4, 6 and 8 h) to produce a highly pure and homogeneous material with enhanced surface area values. Bimetallic Pd-Ru and Pt-Ru (0.5 wt.% of each metal) were impregnated onto the zeolites matrix by the incipient wetness impregnation method. The materials were characterized by X-ray diffraction (XRD), nitrogen physisorption, Fourier Transform Infrared spectroscopy (FT-IR), Scattering Electronic Microscopy (SEM), Scattering and Transmission Microscopy (STEM), temperature-programmed desorption (TPD), temperature-programmed desorption (TPR) and Inductively Couples Plasma- Mass Spectrometer (ICP-MS). Results indicated that using lower Si/Al ratios favored the catalytic activity. Also, the longest crystallization time had a positive effect on surface area, homogeneous particle size distribution and crystallinity. The catalytic performance in the esterification of 5-hydroxymethylfurfural (5-HMF) to produce 5-acetoxymethylfurfural (AcMF) was investigated. The maximum 5-HMF conversion of 87.28 % was achieved using pure zeolite with relation Si/Al = 5, and 8 h of crystallization. Pd-Ru supported onto same zeolite showed a conversion of 84.22 %. The highest selectivity towards AcMF of 71.29 % with pure zeolite Si/Al = 5 and 8 h of crystallization was achieved, followed by Pd-Ru/FAU with Si/Al = 5 and 8 h of crystallization, achieving 60.42 %. Finally, results shown that the interaction between the properties of zeolitic support and the metallic species, specifically Pd, had a positive effect in the catalytic process the pristine zeolite showed improved catalytic characteristics related to its acid strength.
Atomic force microscopy is used to conduct the first detailed nanoscopic study on the crystal growth of a complex mixed metal/metal–organic framework based on the MOF-5 framework topology. Shells of ...isomorphously substituted Co/Zn-MOF-5 and MOF-5 were epitaxially grown on MOF-5 core crystals at room temperature and low supersaturation to produce complex core–shell–shell structures with a hierarchal mixed metal nature involving mixing at the atomic level in the isomorphously substituted Co/Zn-MOF-5 shell and at the nanometer level through segregation of the Co/Zn-MOF-5 and MOF-5 layers. The presence of cobalt in the growth solutions was found to retard the overall rate of surface growth in comparison to a cobalt-free growth solution and stop growth entirely for a growth solution containing a Zn/Co ratio = 0.6. The presence of cobalt in the growth solutions was also found to affect the relative rates of terrace spreading in different crystallographic directions compared to cobalt-free growth with spreading in the ⟨110⟩ directions decreasing relative to the rate along the ⟨100⟩ directions. The work provides new understanding of the crystal growth of complex mixed metal/metal–organic frameworks and a methodology to prepare these complex forms in a more controlled manner.
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