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This mini-review provides an overview of the current state of acid site control in zeolite catalysts, including methods of synthesis, advanced characterization, and measured effects of acid properties (speciation, concentration, proximity, siting, and spatial distribution) on a variety of commercially-relevant reactions. The diversity of aluminum species is described with respect to their location at specific sites in zeolite crystals as well as mesoscopic gradients in elemental composition that give rise to zoned or core-shell architectures. Challenges in the identification of acid siting are highlighted within the context of trial-and-error synthesis methods for a range of aluminosilicate frameworks, which hinder a priori design of catalysts, and limitations in techniques to suitably characterize active sites. Emphasis is also placed on knowledge gaps in zeolite catalysis wherein broad development of structure-performance relationships relies on future advancement of synthesis and analytical methods in parallel with atomistic modeling.

Conventional methods to prepare hierarchical zeolites depend upon the use of organic structure‐directing agents and often require multiple synthesis steps with limited product yield and Brønsted acid concentration. Here, it is shown that the use of MEL‐ or MFI‐type zeolites as crystalline seeds induces the spontaneous formation of self‐pillared pentasil zeolites, thus avoiding the use of any organic or branching template for the crystallization of these hierarchical structures. The mechanism of formation is evaluated by time‐resolved electron microscopy to provide evidence for the heterogeneous nucleation and growth of sequentially branched nanosheets from amorphous precursors. The resulting hierarchical zeolites have large external surface area and high percentages of external acid sites, which markedly improves their catalytic performance in the Friedel–Crafts alkylation and methanol to hydrocarbons reactions. These findings highlight a facile, commercially viable synthesis method to reduce mass‐transport limitations and improve the performance of zeolite catalysts. J.D.R. acknowledges support primarily from the U.S. Department of Energy Office of Basic Energy Sciences (Award DE-SC0014468). J.D.R. acknowledges additional funding from the Welch Foundation (Award E-1794). N.L and J.G.M thank the European Commission for funding through the H2020-MSCARISE-2019 program (Ref. ZEOBIOCHEM – 872102) and the Spanish MINECO and AEI/FEDER, UE through the project ref. RTI2018-099504-B-C21. N.L. also acknowledges the University of Alicante support (UATALENTO17-05).

A large number of inorganic and organic materials have been postulated to grow via nonclassical pathways involving the assembly of amorphous precursors that serve as growth units for crystallization. Elucidating the assembly, microstructure, and mechanism(s) by which precursors are involved in processes of nucleation and growth is nontrivial because of the lack of available techniques that are capable of monitoring amorphous-to-crystal transformations with sufficient spatiotemporal resolution. Here we propose an elemental mapping method employing field emission scanning electron microscopy with energy dispersive X-ray spectroscopy (FE-SEM-EDX) as a versatile tool to characterize morphological and compositional properties of precursors during the crystallization of five zeolites (MER, MFI, MOR, TON, and LTL) that are used in commercial applications that include (but are not limited to) catalysis, ion exchange, separations, drug delivery, and adsorption. Time-resolved ex situ elemental mapping images of extracted solids during several stages of zeolite crystallization reveal that a sufficient disparity in the chemical composition (e.g., alkali metal content) between amorphous precursors and crystals can be exploited for (i) tracking the early stages of zeolite crystallization, (ii) locating where it happens, and (iii) resolving residual amorphous material among crystalline domains. In this study, we also show that FE-SEM-EDX elemental mapping is a highly sensitive technique to detect the onset of nucleation, the completion of crystallization, and the existence of impurities. Given the ubiquitous presence of amorphous precursors during nonclassical crystallization, we expect that elemental mapping may prove valuable for understanding and tracking the growth of other zeolites, minerals, and related materials. J.D.R. acknowledges funding from the Welch Foundation (Award E-1794). J.G.M. and N.L. acknowledge funding from the Spanish Ministry of Science and Innovation (RTI2018-099504-B-C21). N.L. acknowledges funding from the University of Alicante (UATALENTO17-05).

The assembly and structural evolution of amorphous precursors during zeolite crystallization is an important area of interest owing to their putative roles in the nucleation and growth of aluminosilicate microporous materials. Precursors range in complexity from oligomeric molecules and colloidal particles to gels comprised of heterogeneous silica and alumina domains. The physical state of precursors in most zeolite syntheses is generally not well understood; however, it is evident that the physicochemical properties of precursors depend on a wide range of conditions that include (but are not limited to) the selection of reagents, the composition of growth mixtures, the methods of preparation, and the use of inorganic and/or organic structure-directing agents. The fact that precursors evolve in size, shape, and/or microstructure during the course of nucleation and potentially throughout crystallization leads to questions pertaining to their mode of action in the formation of zeolites. This also highlights...

Crystal engineering relies upon the ability to predictively control intermolecular interactions during the assembly of crystalline materials in a manner that leads to a desired (and predetermined) set of properties. Economics, scalability, and ease of design must be leveraged with techniques that manipulate the thermodynamics and kinetics of crystal nucleation and growth. It is often challenging to exact simultaneous control over multiple physicochemical properties, such as crystal size, habit, chirality, polymorph, and composition. Engineered materials often rely upon postsynthesis (top-down) processes to introduce properties that would otherwise be challenging to attain through direct (bottom-up) approaches. We discuss the application of crystal engineering to heterogeneous catalysts with a focus on four general themes: ( a) tailored nanocrystal size, ( b) controlled environments surrounding active sites, ( c) tuned morphology with well-defined facets, and ( d) hierarchical materials with disparate pore size and active site distributions. We focus on nonporous materials, including metals and metal oxides, and two classes of porous materials: zeolites and metal organic frameworks. We review novel synthesis methods involving synergistic experimental and computational design approaches, the challenges facing catalyst development, and opportunities for future advancement in crystal engineering.

Zeolite crystallization occurs in the presence of inorganic and/or organic structure-directing agents (SDAs) that facilitate the formation of microporous crystals with various pore geometries. One of the most common zeolites is ZSM-5 (MFI type), which is used as a catalyst and sorbent in a wide range of industrial applications. One of the challenges with ZSM-5 synthesis is to identify new and inexpensive organic SDAs that can tailor the physicochemical properties of the final product. The most frequently used SDA in ZSM-5 synthesis is tetrapropylammonium (TPA); however, recent studies have shown that the surfactant cetyltrimethylammonium (CTA) can be used as an alternative SDA. Here, we examine the effects of dual structure-directing agents using CTA and TPA as organic SDAs in combination with a variety of alkali metals as inorganic SDAs. Our findings reveal that the selection of SDA combinations has a significant impact on the kinetics of ZSM-5 crystallization, as well as the properties of the resulting crystals. Notably, we show that TPA/Na and CTA/K are optimal combinations of SDAs that can markedly alter the size, morphology, and aluminum distribution in ZSM-5. Using a combination of experiments and molecular modeling, we explore the use of CTA as an alternative organic SDA for zeolite MFI and show that we can achieve smaller crystals (ca. 600 nm) in similar time (

Application of ultrasound in crystallization has showed improved process characteristics. Although several attempts have been made in the past to study the sono-crystallization kinetics, only nucleation and crystal growth were considered, neglecting breakage and agglomeration of crystals. In this study, an attempt is made for the estimation of the kinetic parameters of all the phenomena occurring simultaneously during sono-crystallization. For this, both conventional and ultrasonic crystallization of K2SO4-water system has been reported. Sono-crystallization experiments were carried out using ultrasonic horn operating at 20 kHz frequency. Reduction in the induction time, reduction in metastable zone width (MSZW), narrowing of crystal size distribution (CSD) were the key observations of sono-crystallization experiments. Population balance equations (PBE) were used to model the crystallization system and the various kinetic parameters have been estimated. The kinetic parameters obtained for conventional crystallization and sonocrystallization were compared. The estimated parameters suggest an increase in nucleation and breakage rate during sono-crystallization. Growth rates were observed to be of the same order of magnitude for both conventional and sonocrystallization. While agglomeration during sono-crystallization was found to be negligible.

The treatment of zeolites with surfactants in alkaline media is an effective and versatile technique to impart intracrystalline well-defined mesoporosity in these materials. In this study, the dynamics of surface reconstruction that occurs during the treatment of USY zeolite by surfactant-templating was monitored in situ by atomic force microscopy. The development of surfactant-templated mesoporosity and the concurrent healing of defects that are characteristic of steamed zeolites occur in less than 1 h at room temperature, which emphasizes the low energy barriers needed to reorganize the crystalline structure of this zeolite. This transformation was also followed by X-ray diffraction, N2 adsorption, and transmission electron microscopy analysis of ultramicrotomed samples to confirm that the rapid formation of surfactant-templated mesoporosity and the reconstruction of the zeolite crystals occur not only on the surface of the zeolite but also homogeneously throughout the whole zeolite. This process involves a significant and rapid breaking and re-formation of bonds; however, the zeolite does not dissolve during this process as solid recovery at any given time of the treatment is approximately 100% and the concentration of soluble Si or Al species in the liquid is negligible. Parametric analysis revealed that excessive NaOH leads to the partial transformation of zeolite into an amorphous mesoporous solid, while insufficient quantity of base and/or treatment time can lead to an incomplete mesostructuring of the zeolite, which highlights the importance of judiciously selecting the treatment conditions for every given zeolite. N.L. acknowledges funding from the University of Alicante, through the “Programa de retención y captación de talento” (ref. UATALENTO17-05). J.D.R. acknowledges funding from the Welch Foundation (Award E-1794).

A mathematical model is proposed that could predict the kinetic parameters for adsorption of divalent heavy metal ions (lead, copper and zinc) onto low-cost adsorbents such as pyrophyllite and rock phosphate using experimental data. The experiments were conducted with the initial concentrations of metal ions ranging from 10mg/L to 100mg/L. The mathematical model is based on the application of the Redlich-Peterson isotherm to mass transfer across the film surrounding the adsorbent. The developed non-linear sorption kinetic (NSK) mathematical model was solved using numerical integration by the trapezoidal method in Microsoft Excel along with the SOLVER function to obtain the best simulated values of the Redlich-Peterson constants A, B, r, the order of reaction n, and the film transfer coefficient α. Dissolution followed by precipitation was found to be the most probable mechanism responsible for heavy metal ion uptake by rock phosphate, while for pyrophyllite physical adsorption was governing mechanism at low concentrations (

Chemical reaction probabilities, defined as the number of silicon atoms removed per incident fluorine atom, have been investigated in mixtures of NF3 and SF6 plasmas in an inductively-coupled plasma reactor. Fluorine atom densities were measured by optical emission actinometry, and isotropic etching rates were measured by the degree of undercutting of SiO2-masked silicon by cross-sectional scanning electron microscopy. In addition, atomic force microscopy was used to examine surface morphology of etched Si surfaces. The F atom reaction probabilities derived from isotropic etching rates indicate an ∼30-fold higher reaction probability in SF6 plasmas compared with values in NF3 plasmas. Surfaces etched in SF6 plasmas were much smoother than those etched in NF3 plasmas. The addition of only 10% SF6 to an NF3 plasma produced a much higher reaction probability (∼5-fold) than in a pure NF3 plasma. This surprising enhancement of reaction probabilities for F with Si in SF6 plasmas is further investigated, based on the mechanism of adsorbed sulfur acting as a catalyst to greatly enhance the etching rate of Si. Dilute sulfur solutions in isopropyl alcohol were allowed to evaporate on the masked Si samples, depositing sulfur in relatively high concentrations near mask edges in ∼2 μm diameter periodic “strings of beads.” The sulfur-dosed sample etched several times faster at the center of each bead than a sample not exposed to sulfur that was placed side by side. The catalytic effect of sulfur is ascribed to an enhanced F sticking coefficient and/or decreased desorption rate on a surface covered with sulfur.Chemical reaction probabilities, defined as the number of silicon atoms removed per incident fluorine atom, have been investigated in mixtures of NF3 and SF6 plasmas in an inductively-coupled plasma reactor. Fluorine atom densities were measured by optical emission actinometry, and isotropic etching rates were measured by the degree of undercutting of SiO2-masked silicon by cross-sectional scanning electron microscopy. In addition, atomic force microscopy was used to examine surface morphology of etched Si surfaces. The F atom reaction probabilities derived from isotropic etching rates indicate an ∼30-fold higher reaction probability in SF6 plasmas compared with values in NF3 plasmas. Surfaces etched in SF6 plasmas were much smoother than those etched in NF3 plasmas. The addition of only 10% SF6 to an NF3 plasma produced a much higher reaction probability (∼5-fold) than in a pure NF3 plasma. This surprising enhancement of reaction probabilities for F with Si in SF6 plasmas is further investigated, based o...

The preparation of nanosized zeolites is critical for applications where mass‐transport limitations within microporous networks hinder their performance. Often the ability to generate ultrasmall zeolite crystals is dependent upon the use of expensive organics with limited commercial relevance. Herein, we report the generation of zeolite L crystals with uniform sizes less than 30 nm using a facile, organic‐free method. Time‐resolved analysis of precursor assembly and evolution during nonclassical crystallization highlights key differences among silicon sources. Our findings reveal that a homogenous dispersion of potassium ions throughout silicate precursors leads to the formation of a metastable nonporous phase, which undergoes an intercrystalline transformation to zeolite L. The generation of highly interdispersed alkali‐silicate precursors is seemingly critical to enhancing the rate of nucleation and facilitating the formation of ultrasmall crystal. J.D.R. acknowledges support primarily from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0014468. Additional support was provided by the Welch Foundation (Award E-1794). N.L. acknowledges support from the University of Alicante under the project GRE15-07.

Application of ultrasound in crystallization has showed improved process characteristics. Although several attempts have been made in the past to study the sono-crystallization kinetics, only nucleation and crystal growth were considered, neglecting breakage and agglomeration of crystals. In this study, an attempt is made for the estimation of the kinetic parameters of all the phenomena occurring simultaneously during sono-crystallization. For this, both conventional and ultrasonic crystallization of K

In current study isolates of two native microalgae species were screened on the basis of growth kinetics and lipid accumulation potential. On the basis of data obtained on growth parameters and lipid accumulation, it is concluded that Scenedesmus dimorphus has better potential as biofuel feedstock. Two of the isolates of Scenedesmus dimorphus performed better than other isolates with respect to important growth parameters with lipid content of ~30% of dry biomass. Scenedesmus dimorphus was found to be more suitable as biodiesel feedstock candidate on the basis of cumulative occurrence of five important biodiesel fatty acids, relative occurrence of SFA (53.04%), MUFA (23.81%) and PUFA (19.69%), and more importantly that of oleic acid in its total lipids. The morphological observations using light and Scanning Electron Microscope and molecular characterization using amplified 18S rRNA gene sequences of microalgae species under study were also performed. Amplified 18S rRNA gene fragments of the microalgae species were sequenced, annotated at the NCBI website and phylogenetic analysis was done. We have published eight 18S rRNA gene sequences of microalgae species in NCBI GenBank.

Early sex determination of dioecious plants is essential for successful genetic improvement and commercial cultivation. Male and female seabuckthorn plants are morphologically similar and cannot be distinguished prior to 3–4 years of growth, i.e., at the time of flowering. India has three species of seabuckthorn namely Hippophae rhamnoides, Hippophae salicifolia and Hippophae tibetana. Applicability of sex linked SCAR markers HrX1 and HrX2 developed for H. rhamnoides were tested in H. salicifolia and H. tibetana using H. rhamnoides as control. HrX1 produced female specific amplification in both H. salicifolia and H. tibetana while HrX2 did not amplify except for H. rhamnoides. Sequence based analysis of amplified SCAR marker products HrX1, HsX1 and HtX1 has shown homology with known plant acyl CoA ligases.

Seabuckthorn is an economically important dioecious plant in which mechanism of sex determination is unknown. The study was conducted to identify seabuckthorn homologous genes involved in floral development which may have role in sex determination. Forty four putative Genes involved in sex determination (GISD) reported in model plants were shortlisted from literature survey, and twenty nine seabuckthorn homologous sequences were identified from available seabuckthorn genomic resources. Of these, 21 genes were found to differentially express in either male or female flower bud stages. HrCRY2 was significantly expressed in female flower buds only while HrCO had significant expression in male flowers only. Among the three male and female floral development stages (FDS), male stage II had significant expression of most of the GISD. Information on these sex-specific expressed genes will help in elucidating sex determination mechanism in seabuckthorn.