Synthetic methods are reported to prepare chabazite (CHA) zeolites (SSZ-13) at fixed elemental composition but with different arrangements of their framework Al atoms (Al–O(−Si–O) x –Al) between ...paired (x = 1, 2) and isolated (x ≥ 3) configurations. SSZ-13 zeolites (Si/Al = 15–30) that crystallized in hydroxide media and in the presence of only N,N,N-trimethyl-1-admantylammonium cations (TMAda+) contained only isolated framework Al, evident in their inability to exchange divalent Cu2+ or Co2+ cations that titrate paired Al sites. Addition of Na+ to the zeolite crystallization medium, with total cationic charge ((Na+ + TMAda+)/Al) and other synthesis parameters held constant, crystallized SSZ-13 zeolites at a fixed Si/Al ratio but with paired Al densities that increased linearly with the Na+ content incorporated into the crystalline solids. Crystallization media containing equimolar amounts of Na+ and TMAda+, but varying Al content, crystallized SSZ-13 at different compositions (Si/Al = 5, 15, 25) with the paired Al density expected if Al atoms were distributed randomly subject to Löwenstein’s rule. The relative ratio of organic (TMAda+) and inorganic (Na+) cations in the crystallization medium, which determines its cationic charge density, systematically influences the arrangement of isolated and paired Al, which determines the anionic charge density in the zeolite framework. The synthetic strategy of using mixtures of organic and inorganic structure-directing agents extends concepts of charge density mismatch in zeolite crystallization, which have been exploited previously to discover routes to new framework topologies and compositions, to systematically control the atomic arrangement in a given zeolite at fixed composition and, in doing so, provide new methods to introduce structural and catalytic diversity.
The relationships among the macroscopic compositional parameters of a Cu-exchanged SSZ-13 zeolite catalyst, the types and numbers of Cu active sites, and activity for the selective catalytic ...reduction (SCR) of NO x with NH3 are established through experimental interrogation and computational analysis of materials across the catalyst composition space. Density functional theory, stochastic models, and experimental characterizations demonstrate that within the synthesis protocols applied here and across Si:Al ratios, the volumetric density of six-membered-rings (6MR) containing two Al (2Al sites) is consistent with a random Al siting in the SSZ-13 lattice subject to Löwenstein’s rule. Further, exchanged CuII ions first populate these 2Al sites before populating remaining unpaired, or 1Al, sites as CuIIOH. These sites are distinguished and enumerated ex situ through vibrational and X-ray absorption spectroscopies (XAS) and chemical titrations. In situ and operando XAS follow Cu oxidation state and coordination environment as a function of environmental conditions including low-temperature (473 K) SCR catalysis and are rationalized through first-principles thermodynamics and ab initio molecular dynamics. Experiment and theory together reveal that the Cu sites respond sensitively to exposure conditions, and in particular that Cu species are solvated and mobilized by NH3 under SCR conditions. While Cu sites are spectroscopically and chemically distinct away from these conditions, they exhibit similar turnover rates, apparent activation energies and apparent reaction orders at the SCR conditions, even on zeolite frameworks other than SSZ13.
Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of nitrogen oxides (NOₓ) with ammonia (NH₃), but the low-temperature rate dependence on copper (Cu) ...volumetric density is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calculations to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O₂)–mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the volume that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.
Conspectus Copper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NO x ) via selective catalytic reduction (SCR) reactions ...with ammonia as the reductant. The discovery of these materials in the early 2010s enabled a step-change improvement in diesel emissions aftertreatment technology. Key advantages of Cu-CHA zeolites over prior materials include their effectiveness at the lower temperatures characteristic of diesel exhaust, their durability under high-temperature hydrothermal conditions, and their resistance to poisoning from residual hydrocarbons present in exhaust. Fundamental catalysis research has since uncovered mechanistic and kinetic features that underpin the ability of Cu-CHA to selectively reduce NO x under strongly oxidizing conditions and to achieve improved NO x conversion relative to other zeolite frameworks, particularly at low exhaust temperatures and with ammonia instead of other reductants. One critical mechanistic feature is the NH3 solvation of exchanged Cu ions at low temperatures (<523 K) to create cationic Cu–amine coordination complexes that are ionically tethered to anionic Al framework sites. This ionic tethering confers regulated mobility that facilitates interconversion between mononuclear and binuclear Cu complexes, which is necessary to propagate SCR through a Cu2+/Cu+ redox cycle during catalytic turnover. This dynamic catalytic mechanism, wherein single and dual metal sites interconvert to mediate different half-reactions of the redox cycle, combines features canonically associated with homogeneous and heterogeneous reaction mechanisms. In this Account, we describe how a unified experimental and theoretical interrogation of Cu-CHA catalysts in operando provided quantitative evidence of regulated Cu ion mobility and its role in the SCR mechanism. This approach relied on new synthetic methods to prepare model Cu-CHA zeolites with varied active-site structures and spatial densities in order to verify that the kinetic and mechanistic models describe the catalytic behavior of a family of materials of diverse composition, and on new computational approaches to capture the active-site structure and dynamics under conditions representative of catalysis. Ex situ interrogation revealed that the Cu structure depends on the conditions for the zeolite synthesis, which influence the framework Al substitution patterns, and that statistical and electronic structure models can enumerate Cu site populations for a known Al distribution. This recognition unifies seemingly disparate spectroscopic observations and inferences regarding Cu ion structure and responses to different external conditions. SCR rates depend strongly on the Cu spatial density and zeolite composition in kinetic regimes where Cu+ oxidation with O2 becomes rate-limiting, as occurs at lower temperatures and under fuel-rich conditions. Transient experiments, ab initio molecular dynamics simulations, and statistical models relate these sensitivities to the mobility constraints imposed by the CHA framework on NH3-solvated Cu ions, which regulate the pore volume accessible to these ions and their ability to pair and complete the catalytic cycle. This highlights the key characteristics of the CHA framework that enable superior performance under low-temperature SCR reaction conditions. This work illustrates the power of precise control over a catalytic material, simultaneous kinetic and spectroscopic interrogation over a wide range of reaction conditions, and computational strategies tailored to capture those reaction conditions to reveal in microscopic detail the mechanistic features of a complex and widely practiced catalysis. In doing so, it highlights the key role of ion mobility in catalysis and thus potentially a more general phenomenon of reactant solvation and active site mobilization in reactions catalyzed by exchanged metal ions in zeolites.
•This review describes an integrated and multidisciplinary approach for the “early” diagnosis of Alzheimer's disease (AD).•An overview of epidemiology, genetic risk factors, and different biomarkers ...of AD is provided.•Latest findings suggest EEG rhythms analysis as a valid screening tool to predict AD conversion.
Alzheimer’s disease (AD) is the most common neurodegenerative disease among the elderly with a progressive decline in cognitive function significantly affecting quality of life. Both the prevalence and emotional and financial burdens of AD on patients, their families, and society are predicted to grow significantly in the near future, due to a prolongation of the lifespan. Several lines of evidence suggest that modifications of risk-enhancing life styles and initiation of pharmacological and non-pharmacological treatments in the early stage of disease, although not able to modify its course, helps to maintain personal autonomy in daily activities and significantly reduces the total costs of disease management. Moreover, many clinical trials with potentially disease-modifying drugs are devoted to prodromal stages of AD. Thus, the identification of markers of conversion from prodromal form to clinically AD may be crucial for developing strategies of early interventions. The current available markers, including volumetric magnetic resonance imaging (MRI), positron emission tomography (PET), and cerebral spinal fluid (CSF) analysis are expensive, poorly available in community health facilities, and relatively invasive. Taking into account its low cost, widespread availability and non-invasiveness, electroencephalography (EEG) would represent a candidate for tracking the prodromal phases of cognitive decline in routine clinical settings eventually in combination with other markers. In this scenario, the present paper provides an overview of epidemiology, genetic risk factors, neuropsychological, fluid and neuroimaging biomarkers in AD and describes the potential role of EEG in AD investigation, trying in particular to point out whether advanced analysis of EEG rhythms exploring brain function has sufficient specificity/sensitivity/accuracy for the early diagnosis of AD.
•There are a variety of technologies valuable for exploring human brain connectivity.•The main aspects of anatomical, functional and effective connectivity are described.•A multimodality approach can ...be useful to evaluate the human brain connectome.
The goal of this paper is to examine existing methods to study the “Human Brain Connectome” with a specific focus on the neurophysiological ones. In recent years, a new approach has been developed to evaluate the anatomical and functional organization of the human brain: the aim of this promising multimodality effort is to identify and classify neuronal networks with a number of neurobiologically meaningful and easily computable measures to create its connectome. By defining anatomical and functional connections of brain regions on the same map through an integrated approach, comprising both modern neurophysiological and neuroimaging (i.e. flow/metabolic) brain-mapping techniques, network analysis becomes a powerful tool for exploring structural–functional connectivity mechanisms and for revealing etiological relationships that link connectivity abnormalities to neuropsychiatric disorders. Following a recent IFCN-endorsed meeting, a panel of international experts was selected to produce this current state-of-art document, which covers the available knowledge on anatomical and functional connectivity, including the most commonly used structural and functional MRI, EEG, MEG and non-invasive brain stimulation techniques and measures of local and global brain connectivity.
Repetitive transcranial magnetic stimulation (rTMS) of human motor cortex can produce long-lasting changes in the excitability of excitatory and inhibitory neuronal networks. The effects of rTMS ...depend critically on stimulus frequency. The aim of our present study was to compare the effects of different rTMS protocols. We compared the aftereffects of 6 different rTMS protocols paired associative stimulation at interstimulus intervals of 25 (PAS(25)) and 10 ms (PAS(10)); theta burst stimulation delivered as continuous (cTBS) or intermittent delivery pattern (iTBS); 1- and 5-Hz rTMS on the excitability of stimulated and contralateral motor cortex in 10 healthy subjects. A pronounced increase of cortical excitability, evaluated by measuring the amplitude of motor evoked potentials (MEPs), was produced by iTBS (+56%) and PAS(25) (+45%). Five-hertz rTMS did not produce a significant increase of MEPs. A pronounced decrease of cortical excitability was produced by PAS(10) (-31%), cTBS (-29%), and 1-Hz rTMS (-20%). Short-interval intracortical inhibition was suppressed by PAS(10). Cortical silent period duration was increased by 1-Hz stimulation. No significant effect was observed in the contralateral hemisphere. Head-to-head comparison of the different protocols enabled us to identify the most effective paradigms for modulating the excitatory and inhibitory circuits activated by TMS.
The molecular structure and cationic charge density of organic and inorganic structure-directing agents (SDAs) influence the siting and arrangement of Al substituted in zeolite frameworks. Yet, ...developing robust synthesis–structure relations for MFI zeolites is difficult because of the complexities inherent to its low-symmetry framework (12 unique tetrahedral sites), which generates a large combinatorial space of Al–Al site pairs to exhaustively model by density functional theory (DFT) and quantify by experiment. Here, we develop an experimental protocol to reproducibly quantify Co2+-titratable Al–Al site pairs in MFI with saturation uptakes validated by corroborating spectroscopic and cation site balance data. Using tetrapropylammonium (TPA+) as the sole SDA, MFI zeolites were crystallized with varying Al contents (Si/Al = 37–185; 0.52–2.52 Al per unit cell) within a composition range consistent with charge density mismatch theory and the occlusion of one TPA+ per channel intersection with fractions of paired Al (0.0–0.34) that increased with bulk Al content. DFT calculations performed using a 96 T-site MFI unit cell containing either an isolated Al site (all 96 configurations) or various Al–Al site pairs (1773 out of 13 680 total configurations), charge balanced by one or two TPA+, respectively, reveal the dominant influence of electrostatic interactions between the cationic N of TPA+ and the anionic lattice charge on Al siting energies. Together with DFT calculations of Co2+ exchange energies at Al–Al site pairs, theory predicts that two TPA+ cations confined within adjacent channel intersections can form many Al–Al site pair ensembles that are Co2+-titratable, rationalizing the considerable presence of paired Al sites in MFI samples crystallized using only TPA+. The use of TPA+ and Na+ as co-SDAs in the synthesis gel, while varying the Na+/TPA+ ratio (0–5) at a constant SDA/Al ratio ((TPA+ + Na+)/Al = 30), crystallized MFI with a similar bulk Al content (Si/Al ≈ 50) but varying fractions of Al in pairs (0.12–0.44). Separate crystallization experiments performed using charge-neutral organic SDAs, either pentaerythritol or a mixture of 1,4-diazabicyclo2.2.2octane and methylamine, together with Na+ to compensate for framework Al, crystallized MFI at similar bulk Al content (Si/Al ≈ 50) but with lower fractions of Al in pairs (<0.14). Among MFI samples crystallized with an organic SDA and Na+ as a co-SDA, the number of paired Al sites formed generally increased with the co-occluded Na+ content on the zeolite, a synthesis–structure relation that resembles our prior observations on CHA zeolites. The combined theoretical and experimental approach used here provides a microscopic model to define and quantify Al–Al site pairs in MFI, which can be adapted to do so for other framework topologies. These findings highlight how such Al siting models can be exercised to quantitatively characterize zeolite materials to develop synthetic strategies that can predictably vary their framework Al arrangements and catalytic and adsorption properties in turn, as shown here for samples of essentially constant bulk composition by exploiting mixtures of organic and inorganic SDAs during hydrothermal crystallization.
Highlights • This review is an up-date document on basic aspects of non-invasive stimulation of brain, spinal cord and nerve roots. • The main physiological, theoretical and methodological features ...of transcranial magnetic stimulation (TMS) are described. • Instructions for practical use of non-invasive stimulation in clinical applications and research are provided.
The aim of this study was to determine the optimal regimen for the management of pain following total knee arthroplasty (TKA) by comparing the outcomes and cost-effectiveness of different protocols ...implemented at a large, urban, academic medical centre.
Between September 2013 and September 2015, we used a series of modifications to our standard regimen for the management of pain after TKA. In May 2014, there was a department-wide transition from protocols focused on femoral nerve blocks (FNB) to periarticular injections of liposomal bupivacaine. In February 2015, patient-controlled analgesia (PCA) was removed from the protocol while continuing liposomal bupivacaine injections. Quality measures and hospital costs were compared between the three protocols.
The cohort being treated with PCA-less liposomal bupivacaine injections had a significantly higher percentage of patients who were discharged to their home (p = 0.010) and a significantly shorter length of stay (p < 0.001). Patient-reported Hospital Consumer Assessment of Healthcare Providers and Systems (HCAHPS) scores relating to pain being "well-controlled" and "overall pain management" also favoured this cohort (p = 0.214 and p = 0.463, respectively), in which cost was significantly lower compared with the other two cohorts (p = 0.005).
The replacement of FNBs injections and the removal of PCAs, both of which are known to be associated with high rates of adverse outcomes, and the addition of liposomal bupivacaine periarticular injections to a multimodal pain regimen, led to improvements in many quality measures, HCAHPS pain scores, and cost-effectiveness. Cite this article:
2018;100-B(1 Supple A):55-61.
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