scholarly journals Probing Substrate/Catalyst Effects Using QSPR Analysis on Friedel-Crafts Acylation Reactions over Hierarchical BEA Zeolites

Molecules ◽  
2020 ◽  
Vol 25 (23) ◽  
pp. 5682
Author(s):  
Ruben Elvas-Leitão ◽  
Filomena Martins ◽  
Leonor Borbinha ◽  
Catarina Marranita ◽  
Angela Martins ◽  
...  
Keyword(s):  
Active Sites ◽  
Heterogeneous Catalysts ◽  
Heterogeneous Media ◽  
Kinetic Modelling ◽  
Equilibrium Constants ◽  
Molecular Size ◽  
Reaction Rates ◽  
Large Set ◽  
Structure Property ◽  
Sorption Equilibrium

Attempts to optimize heterogeneous catalysis often lack quantitative comparative analysis. The use of kinetic modelling leads to rate (k) and relative sorption equilibrium constants (K), which can be further rationalized using Quantitative Structure-Property Relationships (QSPR) based on Multiple Linear Regressions (MLR). Friedel-Crafts acylation using commercial and hierarchical BEA zeolites as heterogeneous catalysts, acetic anhydride as the acylating agent, and a set of seven substrates with different sizes and chemical functionalities were herein studied. Catalytic results were correlated with the physicochemical properties of substrates and catalysts. From this analysis, a robust set of equations was obtained allowing inferences about the dominant factors governing the processes. Not entirely surprising, the rate and sorption equilibrium constants were found to be explained in part by common factors but of opposite signs: higher and stronger adsorption forces increase reaction rates, but they also make the zeolite active sites less accessible to new reactant molecules. The most relevant parameters are related to the substrates’ molecular size, which can be associated with different reaction steps, namely accessibility to micropores, diffusion capacity, and polarizability of molecules. The relatively large set of substrates used here reinforces previous findings and brings further insights into the factors that hamper/speed up Friedel-Crafts reactions in heterogeneous media.

What catalysis needs from the electron microscopist

1988 ◽  
Vol 46 ◽  
pp. 694-695
Author(s):  
Alexis T. Bell
Keyword(s):  
Active Sites ◽  
Heterogeneous Catalysts ◽  
Catalyst Deactivation ◽  
Surface Composition ◽  
Internal Surface ◽  
Active Phase ◽  
Atomic Scale ◽  
Metal Catalyst ◽  
Internal Surface Area ◽  
Porous Support

Heterogeneous catalysts, used in industry for the production of fuels and chemicals, are microporous solids characterized by a high internal surface area. The catalyticly active sites may occur at the surface of the bulk solid or of small crystallites deposited on a porous support. An example of the former case would be a zeolite, and of the latter, a supported metal catalyst. Since the activity and selectivity of a catalyst are known to be a function of surface composition and structure, it is highly desirable to characterize catalyst surfaces with atomic scale resolution. Where the active phase is dispersed on a support, it is also important to know the dispersion of the deposited phase, as well as its structural and compositional uniformity, the latter characteristics being particularly important in the case of multicomponent catalysts. Knowledge of the pore size and shape is also important, since these can influence the transport of reactants and products through a catalyst and the dynamics of catalyst deactivation.

scholarly journals Phosphorene–Fullerene Nanostructures: A First-Principles Study

2021 ◽  
Author(s):  
Diego Cortes-Arriagada ◽  
Daniela E. Ortega
Keyword(s):  
Active Sites ◽  
Short Range ◽  
Density Functional ◽  
Dipole Moments ◽  
Molecular Size ◽  
Bandgap Engineering ◽  
Donor Acceptor ◽  
The Stability ◽  
Strong Repulsion ◽  
Intermolecular Distances

Hybrid materials formed by carbon fullerenes and layered materials have emerged due to their advantages for several technological applications, and phosphorene arises as a promising two-dimensional semiconductor for C60 adsorption. However, the properties of phosphorenefullerene hybrids remain mainly unexplored. In this work, we employed density functional theory to obtain structures, adsorption energies, electronic/optical properties, binding (AIM, NBO), and energy decomposition analyses (ALMO-EDA) of nanostructures formed by phosphorene and fullerenes (C24 to C70). We find fullerenes form covalent and non-covalent complexes with phosphorene depending on the molecular size, showing remarkable stability even in solution. Two classes of covalent complexes arise by cycloaddition-like reactions: the first class, where short-range effects (charge-transfer and polarization) determines the stability; and the second one, where short-range effects decay to avoid steric repulsion, and balanced longrange forces (electrostatics and dispersion) favors the stability. Otherwise, high-size fullerenes (C50 to C70) only form non-covalent complexes due to strong repulsion at shorter intermolecular distances and lack of dissociation barriers. In terms of electronic properties, fullerenes act as mild p-dopants for phosphorene, increasing its polar character and ability to acquire induced dipole moments (polarizability). Also, small energy-bandgap fullerenes (<0.8 eV) largely increase the phosphorene metallic character. We also note fullerenes retain their donor/acceptor properties upon adsorption, acting as active sites for orbital-controlled interactions and maximizing the phosphorene light absorbance at the UV-Vis region. Finally, we strongly believe our study will inspire future experimental/theoretical studies focused on phosphorene-fullerene uses for storage, anode materials, sensing, phosphorene bandgap engineering, and optoelectronics.<br>

scholarly journals Size Dependent CO2 Reduction Activity of Ag Nanoparticle Electrocatalysts

2021 ◽  
Author(s):  
Xingyi Deng ◽  
Dominic Alfonso ◽  
Thuy-Duong Nguyen-Phan ◽  
Douglas Kauffman
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Particle Diameter ◽  
Small Diameter ◽  
Co2 Reduction ◽  
High Population ◽  
Structure Property ◽  
Size Dependent ◽  
Catalyst Utilization ◽  
Ag Catalysts

Abstract Coinage metals (Au, Cu and Ag) are state-of-the-art electrocatalysts for the CO2 reduction reaction (CO2RR). Size-dependent CO2RR activity of Au and Cu has been studied, and increased H2 evolution reaction (HER) activity is expected for small catalyst particles with high population of undercoordinated corner sites. A similar consensus is still lacking for Ag catalysts because the ligands and stabilizers typically used to control particle synthesis can block specific active sites and mask inherent structure-property trends. This knowledge gap is problematic because increased performance and catalyst utilization are still needed to improve economic viability. We combined density functional theory, microkinetic modeling, and experiment to demonstrate a strong size-dependence for pristine Ag particles in the sub-10 nm range. Small diameter particles with a high population of Ag edge sites were predicted to favor HER, whereas CO2RR selectivity increased towards that of bulk Ag for larger diameter particles as the population of Ag(100) surface sites grew. Experimental results validated these predictions and we identified an optimal particle diameter of 8-10 nm that balanced selectivity and activity. Particles below this diameter suffered from poor selectivity, while larger particles demonstrated bulk-like activity and reduced catalyst utilization. These results demonstrate the size-dependent CO2RR activity of pristine Ag catalysts and will help guide future development efforts.

scholarly journals Identification of different oxygen species in oxide nanostructures with 17O solid-state NMR spectroscopy

2015 ◽  
Vol 1 (1) ◽  
pp. e1400133 ◽  
Author(s):  
Meng Wang ◽  
Xin-Ping Wu ◽  
Sujuan Zheng ◽  
Li Zhao ◽  
Lei Li ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Strong Dependence ◽  
Critical Role ◽  
Oxygen Species ◽  
Structure Property ◽  
Diverse Range ◽  
Oxygen Ions ◽  
Surface Sites ◽  
Nanostructured Oxides

Nanostructured oxides find multiple uses in a diverse range of applications including catalysis, energy storage, and environmental management, their higher surface areas, and, in some cases, electronic properties resulting in different physical properties from their bulk counterparts. Developing structure-property relations for these materials requires a determination of surface and subsurface structure. Although microscopy plays a critical role owing to the fact that the volumes sampled by such techniques may not be representative of the whole sample, complementary characterization methods are urgently required. We develop a simple nuclear magnetic resonance (NMR) strategy to detect the first few layers of a nanomaterial, demonstrating the approach with technologically relevant ceria nanoparticles. We show that the 17O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency 17O chemical shifts being observed for the lower coordinated surface sites. H217O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. 17O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature. Density functional theory calculations confirm the assignments and reveal a strong dependence of chemical shift on the nature of the surface. These results open up new strategies for characterizing nanostructured oxides and their applications.

scholarly journals New Insights about Enzyme Evolution from Large Scale Studies of Sequence and Structure Relationships

2014 ◽  
Vol 289 (44) ◽  
pp. 30221-30228 ◽  
Author(s):  
Shoshana D. Brown ◽  
Patricia C. Babbitt
Keyword(s):  
Structure Function ◽  
Active Sites ◽  
Large Scale ◽  
Common Ancestor ◽  
Enzyme Evolution ◽  
Large Set ◽  
Substrate Specificities ◽  
Functionally Diverse

Understanding how enzymes have evolved offers clues about their structure-function relationships and mechanisms. Here, we describe evolution of functionally diverse enzyme superfamilies, each representing a large set of sequences that evolved from a common ancestor and that retain conserved features of their structures and active sites. Using several examples, we describe the different structural strategies nature has used to evolve new reaction and substrate specificities in each unique superfamily. The results provide insight about enzyme evolution that is not easily obtained from studies of one or only a few enzymes.

Crystal Engineering for Catalysis

2018 ◽  
Vol 9 (1) ◽  
pp. 283-309 ◽  
Author(s):  
Jeffrey D. Rimer ◽  
Aseem Chawla ◽  
Thuy T. Le
Keyword(s):  
Crystal Engineering ◽  
Active Sites ◽  
Heterogeneous Catalysts ◽  
Computational Design ◽  
Crystal Nucleation ◽  
Synthesis Methods ◽  
Crystalline Materials ◽  
Simultaneous Control ◽  
Metal Organic ◽  
Controlled Environments

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.

Coexistence of three liquid phases in atmospheric aerosol particles

2021 ◽  
Author(s):  
Fabian Mahrt ◽  
Yuanzhou Huang ◽  
Shaun Xu ◽  
Manabu Shiraiwa ◽  
Andreas Zuend ◽  
...  
Keyword(s):  
Air Quality ◽  
Kinetic Modelling ◽  
Aerosol Particles ◽  
Nile Red ◽  
Cloud Droplet ◽  
Organic Aerosol ◽  
Reaction Rates ◽  
Phase Behaviour ◽  
Radiative Properties ◽  
Liquid Phases

&lt;p&gt;Aerosol particles are ubiquitous in the atmosphere and play an important role for air quality and Earth&amp;#8217;s climate. Primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA) constitute a significant mass fraction of these particles. POA, SOA, and SIA can become internally mixed within the same particle though different processes such as coagulation, gas&amp;#8211;particle partitioning. To predict the role of these internally mixed particles in climate and air quality information on their phase behaviour is needed, i.e. information on the number and type of phases present within these particles. As an example, a particle with a single homogeneous liquid phase can have different radiative properties, reaction rates, uptake kinetics, and potential to change cloud microphysical properties by activating into a cloud droplet, compared to a particle with multiple liquid or solid phases.&lt;/p&gt;&lt;p&gt;In the current study we used Nile red, a solvatochromic dye, and fluorescence microscopy in order to determine the phase behaviour of POA+SOA+SIA particles. Squalane was used as a proxy of POA, ammonium sulfate was used as SIA and 1 of 23 different oxidized organic molecules were used as proxies of SOA. We demonstrate that three liquid phases often coexist within individual particles. We find that the phase behaviour strongly depends on the oxygen-to-carbon ratio of the SOA proxies. Experiments with SOA generated by dark ozonolysis of &amp;#945;-pinene in an environmental chamber are consistent with these observations. We also used thermodynamic and kinetic modelling to investigate the atmospheric implications of our experimental results.&lt;/p&gt;

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