1,721,020 research outputs found
A temporal analysis of products (TAP) study of C2-C4 alkene reactions with a well-defined pool of methylating species on ZSM-22 zeolite
Get PDFReactions between alkenes and methanol or dimethyl ether (DME) on zeolite catalysts are involved in industrial processes that are highly relevant for the transition to renewable carbon sources, such as the Methanol-To-Hydrocarbons (MTH) process. In MTH chemistry, alkene methylation increases the length of product carbon chains, and its relative rate with respect to other reactions largely controls the overall selectivity. Experimental studies of alkene methylation present a considerable challenge because they are typically accompanied by cracking, hydrogen transfer, and aromatization reactions. Herein, the pulse-response Temporal Analysis of Products (TAP) methodology and complementary FTIR measurements were employed to isolate a well-defined population of surface species, consistent with Surface Methoxy Species (SMS) on Bronsted acid sites that are reactive in alkene methylation on a ZSM-22 (TON) zeolite. Their coverage was determined by TAP titration to be ca. 5% of the total amount of Brønsted acid sties, which was also indirectly suggested by FTIR data. C2-C4 alkenes were quantitatively reacted with SMS to precisely measure the intrinsic kinetic parameters of isolated alkene methylation steps. The rate constants increased and the activation energies decreased as functions of the carbon number (EC2H4 = 52 kJ/mol > EC3H6 = 32 kJ/mol > Ec-C4H8 = 16 kJ/mol). However, the rate constant for iso-butene was comparable to propene, despite its activation energy (Ei-C4H8 = 19 kJ/mol) being much lower than propene's. This effect is in agreement with the increased steric hindrance predicted by DFT for isobutene adsorption and methylation in TON zeolites. Our results considerably extend previously available TAP data on alkene methylation reactions and furthermore validate ab initio models of these crucial steps in the complex MTH chemistry on acidic zeolites
Catalyst sites and active species in the early stages of MTO conversion over cobalt AlPO-18 followed by IR spectroscopy
Get PDFThe isomorphic insertion of cobalt(ii) confers strong acidity to the AlPO-18 zeotype. This makes it a suitable catalyst for the acid catalysed conversion of methanol to olefins (MTO). However, due to the tendency of Co(ii) to be oxidised into Co(iii) or to acquire distorted tetrahedral coordination, the concentration of strong Brønsted acidic sites (BAS) in CoAPO-18 is highly susceptible to catalyst activation conditions. The use of vibrational and electron spectroscopy coupled with the use of probe molecules sheds light on the conditions under which the concentration of active sites can be maximized, limiting the formation of species of different nature. The MTO reaction was followed by in situ FT-IR spectroscopy, revealing that the population of strong BAS profoundly affects the methanol conversion in the very early stages of the process. Indeed, spectroscopic evidence of the consumption of CO (produced in situ from the decomposition of methanol itself) was obtained, when the concentration and availability of BAS are optimized. This gives support to the occurrence of Koch carbonylation of surface methoxides, at the early stages of MTO
Co-catalyst free ethene dimerization over Zr-based metal-organic framework (UiO-67) functionalized with Ni and bipyridine
Get PDFNi functionalized metal organic frameworks (MOF) are promising heterogeneous ethene dimerization catalysts. Activities comparable to or higher than Ni-aluminosilicates have been reported in literature. However, unlike the Ni-aluminosilicates, those Ni-MOFs require a large excess of co-catalyst to initiate the dimerization process and some catalysts generate polymers which lead to catalyst deactivation. Herein, we report a series of Ni(II) and 2,2′-bipyridine-5,5′-dicarboxylate (bpy) functionalized UiO-67 MOF that catalyze the ethene dimerization reaction co-catalyst free. The catalysts were active for ethene dimerization (up to 850 mg butene gcat−1 h−1) after activation at 300 °C in 10 % O2 for 360 min and subsequent exposure to flowing ethene (P(ethene) =26 bar, 250 °C) for 240 min. The catalysts yielded up to 6 % conversion with 99 % selectivity to linear 1- and 2-butenes, which formed in non-equilibrated ratios. Overall, the test data indicate that all three linear butenes are formed on a single active site, in accordance with the Cossee-Arlman mechanism. Ex situ XAS and CO FT-IR spectroscopy studies point towards Ni monomers or, plausibly, low-nuclearity Ni-multimers, docked at bpy linkers with Ni-Ni distances > 4 Å, as the main active site for the ethene dimerization reaction
Controlling the Synthesis of Metal-Organic Framework UiO-67 by Tuning Its Kinetic Driving Force
No full textThe successful synthesis of metal-organic framework (MOF) compounds relies on an intricate interplay between the components of the synthesis liquor at the given synthesis conditions. Herein, we explore the interdependence of modulator, linker, and solvent amounts in the synthesis of the Zr-based MOF, UiO-67. Our study suggests that control of linker vacancy defects in UiO-67 is feasible by tuning the ratios of these components, and that such control derives from recognizing the kinetic driving forces during MOF crystal growth. Importantly, we show that linker vacancy defects (and modulator molecules occupying linker sites) can be reduced by limiting the solvent amount to maintain a saturated concentration of linker throughout the synthesis. The method enables formation of UiO-67 with an ideal 1:1 ratio between Zr and the 4,4′-biphenyldicarboxylic acid linker, without surplus linker in the mother liquor or additional post-synthetic steps, and reduces the amount of dimethylformamide (DMF) solvent to less than 20% the amount in previously reported procedures
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Synthesis of mesoporous ZSM-5 zeolite encapsulated in an ultrathin protective shell of silicalite-1 for MTH conversion
No full textCoke formation is a major reason in deactivation of acidic zeolite catalysts in industrial processes such as methanol to hydrocarbons conversion. Protecting the surface of acidic zeolite with an inert porous shell can greatly hinder the coke formation on the surface, and hence boost the lifetime of the catalyst. In this work, a solid-state steam-assisted method for synthesis of such optimized protective shell (silicate-1, ~15 nm thickness) is designed. This general and simple protocol can be applied to acidic zeolite catalysts to improve their catalytic lifetime. The silicalite-1 shell is synthesized on mesoporous ZSM-5 zeolite to explore its catalytic activity in methanol to hydrocarbons conversion. XPS and TEM analysis confirm the coverage of mesoporous zeolite crystals by non-acidic shell. In addition, nitrogen physisorption shows the accessibility of mesoporous ZSM-5 via microporous silicalite-1 network. Applying this protective shell increases the lifetime of the catalyst by 100% and its conversion capacity by 130%, in comparison to mesoporous ZSM-5 without the shell. The controlled formation of thin layer of microporous silicalite-1 around mesoporous ZSM-5 crystals (without growth of individual silicalite-1) accounts for enhanced catalytic improvement
On the conversion of CO2 to value added products over composite PdZn and H-ZSM-5 catalysts: Excess Zn over Pd, a compromise or a penalty?
No full textA challenge in converting CO2 into hydrocarbons (HC) via methanol (MeOH) is the gap between the optimal temperature for each step (i.e. ≤250 °C for MeOH and ≥350 °C for HC). The focus of this study is to elucidate the cause of the limitations associated to oxygenate and hydrocarbon formation in combined PdZn and H-ZSM-5 catalysts at 250 to 350 °C. Starting with two different chemical states of Pd and Zn from two preparation approaches (physical mixture and surface organometallic chemistry grafting), operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) studies revealed similar PdZn alloy active phase formed during pretreatment in flowing H2/Inert at 400 °C. The physical mixture performed better than the grafted analogue, with up to 8.8% yield to oxygenates (MeOH and dimethyl ether (DME); MeOH+) at 300 °C, close to the estimated thermodynamic yield (9.0%). The space-time yield (STY) of oxygenates increased from 250 to 300 °C, reaching 8.5 mol(MeOH+) kg(PdZn)-1 h -1. The catalyst performance surpassed other reported yields in similar systems, which activity declined with temperature even below 300 °C. Operando XAS and PXRD experiments further showed that the PdZn phase active for MeOH formation was maintained during testing in the 250-350 °C range. InfraRed (FT-IR) and XAS experiments revealed the poisoning of Brønsted acid sites in H-ZSM-5 by Zn(ii) exchange, thereby rendering it inactive for hydrocarbon formation. Overall, to avoid biasing the hybrid catalyst performance, a careful and balanced choice of the compositional characteristics will be crucial in designing an improved multi-functional catalytic system for CO2 valorisation
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