Insights into the Active Sites and Kinetics of Double-site Catalysts for Semi-hydrogenation under Hydrogen Spillover

A well-defined catalyst with platinum (Pt) and gold (Au) encapsulated in micropore and mesopore of micro-mesoporous zeolite (TMSN), respectively, was designed to investigate the original active sites and kinetics of semi-hydrogenation. Specifically, hydrogen molecules are dissociated on Pt nanoclusters (NCs) to form hydrogen atoms that migrate to the surfaces of TMSN zeolite and Au nanoparticles (NPs). Meanwhile, the Au NPs with inferior H dissociation capability in the mesopore can be served as the detector and controller of hydrogen spillover. The Pt NCs in micropore act as H dissociation sites while both the Au NPs and zeolite surface are identified as the semi-hydrogenation sites. Noteworthy, the Pt-Au/TMSN catalyst with double active sites exhibits higher selectivity and rate constant ratio for semi-hydrogenation than Pt/TMSN, as well as higher turnover frequency (TOF) than Au/MSN. This work creates an effective regulation strategy of hydrogen spillover for improving active sites and kinetics of semi-hydrogenation.

Development and characterization of a new dolomite-based catalyst: application to the photocatalytic degradation of pentachlorophenol

The development of new catalysts from abundant raw materials, generating attractive photocatalytic activity, constitutes a real challenge in the context of sustainable development concerns. In this setting, a dolomite was treated at 800 °C (D800) and then chemically modified by Ca(NO3)2 (CaD800) using a simple procedure. The resulting materials were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS), solid state UV spectroscopy, and used as catalysts of pentachlorophenol (PCP) degradation in aqueous solutions under UV light irradiation. The treatment of dolomite at 800 °C enabled a full decarbonation of CaMg(CO3)2, with formation of CaO, Ca(OH)2, and MgO. Additional CaO was generated after chemical treatment as revealed by EDS analysis; the Ca/Mg ratio increased from 1.29 (D800) to 1.44 for CaD800. This CaO in aqueous medium hydrates by giving Ca(OH)2. CaD800 was found to be the best photocatalyst with a PCP degradation rate of 95% after only 1 h of treatment, for a CaD800/D800 degradation rate constant ratio of 1.58. In this regard, we investigated the Fourier transform infrared spectra of CaD800, PCP, and CaD800 loaded with PCP after degradation. We thus evidenced the involvement of Ca(OH)2 in the PCP degradation process. Catalytic activity was discussed through the contribution of OH radicals and electrodonation.

Formation of secondary organic aerosols from the ozonolysis of dihydrofurans

In this work we report the study of the ozonolysis of 2,5-dihydrofuran and 2,3-dihydrofuran and the reaction conditions leading to the formation of secondary organic aerosols. The reactions have been carried out in a Teflon chamber filled with synthetic air mixtures at atmospheric pressure and room temperature. The ozonolysis only produced particles in the presence of SO2. Rising relative humidity from 0 to 40 % had no effect on the production of secondary organic aerosol in the case of 2,5-dihydrofuran, while it reduced the particle number and particle mass concentrations from the 2,3-dihydrofuran ozonolysis. The water-to-SO2 rate constant ratio for the 2,3-dihydrofuran Criegee intermediate was derived from the secondary organic aerosol (SOA) yields in experiments with different relative humidity values, kH2O/kSO2 = (9.8 ± 3.7) × 10−5. The experimental results show that SO3 may not be the only intermediate involved in the formation or growth of new particles in contrast to the data reported for other Criegee intermediate–SO2 reactions. For the studied reactions, SO2 concentrations remained constant during the experiments, behaving as a catalyst in the production of condensable products. Computational calculations also show that the stabilised Criegee intermediates from the ozonolysis reaction of both 2,5-dihydrofuran and 2,3-dihydrofuran may react with SO2, resulting in the regeneration of SO2 and the formation of low-volatility organic acids.

1999 R.U. Lemieux Award Lecture Adventures with azo-, azoxy-, and hydrazoarenes: from the Wallach to the benzidine rearrangement. Molecular electronics

The author's studies with aromatic azo-, azoxy-, and hydrazo-dye molecules, comprising kinetic and equilibrium investigations, as well as synthesis of novel molecules having photogenic properties, are described under the following highlights: A. Wallach rearrangement and cognate studies with azoxyarenes — (1) Elucidation of the mechanism of the Wallach rearrangement of azoxybenzene through the kinetic observation of a two-proton process which, together with a pKa study, was interpreted on the basis of formation of a deoxygenated, dicationic, symmetrical species as a key, short-lived reaction intermediate. (2) The proposal of a general acid-catalyzed pathway in concentrated sulfuric acid (catalysis by H2SO4 and H3SO+4. (3) Elucidation of the consecutive sulfonations of reaction products of azoxybenzene in the 100% H2SO4 region, and the diprotonation equilibria for p-hydroxyazobenzene, thus shedding light on past reaction pathway and product studies. (4) The observation of a novel reaction pathway for 2,4,6,2',4',6'-hexamethy lazoxybenzene. (5) The observation of a dichotomy of reaction pathways for α- and β-2-phenylazoxynaphthalenes: reaction via the dicationic intermediate and via quinoid intermediate species; comprising two isomeric compounds reacting by different pathways to give the same product. (6) Identification and structure proof of α- and β-isomers observed for the first time in the peracid oxidation of phenylazopyridine. (7) Observation of a rate constant ratio of 22 000 in the rearrangement of these α- and β-isomers, and the proposal of differential barriers for transition states leading to a tricationic intermediate. B. Benzidine rearrangement and cognate studies — (8) Observation of the acid-catalyzed hydroxylation of phenylazopyridine to p-hydroxyphenylazopyridine and the proposal of an SNAr mechanism with formation of an intermediate hydrazo species in the reaction. (9) First study of benzidine type rearrangement-disproportionation of phenylhydrazopyridine in acid media. (10) Proposal of a A [Formula: see text] B [Formula: see text] C [Formula: see text] D type reaction profile for the consecutive hydroxylation[Formula: see text]disproportionation processes of phenylazopyridine in aq H2SO4. (11) Proposal of 10-π and 14π-electron electrocyclic processes in the benzidine type rearrangement-disproportionation of phenylhydrazopyridine. (12) Identification and structural elucidation of a dimer formed from phenylazopyridine as a minor product and proposal of a reaction mechanism. C. Facile acid-catalyzed demethylation via SNAr/A-SE2 mechanisms and studies of tautomerism — (13) Observation of an abnormally facile acid-catalyzed cleavage (demethylation) of 4-methoxyphenylazopyridine via an SNAr mechanism. (14) Observation of two reaction pathways, SNAr and A-SE2, for the consecutive demethylations of 3,4-dimethoxyphenylazopyridine, with rate constant ratio of 7 000:1 favoring the SNAr process. (15) Quantitation of the tautomeric and protonation equilibria of 4-hydroxyphenylazopyridine, produced in (13). D. A new solvent polarity scale, molecular switches, and molecular electronics — (16) Establishment of a π*azo solvent polarity scale based on solvatochromism of a series of azomerocyanine molecules ("Buncel's dye"). (17) Some glimpses are presented of current forays into molecular electronics, as emanating from the above studies: (a) spiropyran (SP) <—> merocyanine (MC) thermo- and photochromic "molecular switch" systems; (b) synthesis and characterization of azo-functionalized star-burst dendrimers with photoswitchable properties and potential applications in optical data storage systems, holographic gratings, and drug delivery systems as host molecules.Key words: Wallach rearrangement, benzidine disproportionation, azoarenes, azoxyarenes, dendrimers, hydrazoarenes, dendrimers, solvatochromism, photochromism, thermochromism, spiropyran-merocyanine molecular switch.

Reaction of Trimethyl-and Dimethyl-Silyl Radicals with Pentamethyldisilane

The rate constant for the H atom abstraction of trimethylsilyl radicals from pentamethyldisilane (k(4)) was measured relative to the trimethylsilyl combination reaction k(3). A value for =(8.53 ± 0.08) · 10-11cm3/2s–1/2 was obtained. For the dimethylsilyl radical, a smaller value for the corresponding rate constant ratio (5.9 ± 0.2) · 10-11 cm3/2 s–1/2 was measured, and this was attributed to a disproportionation reaction between the dimethylsilyl and the pentamethyldisilyl radical leading to dimethylsilylene.

Azide ion trapping and lifetime in aqueous solution of a primary carbenium ion stabilized by a 2-imidazoyl ring

2-Chloromethyl-1-methylimidazole undergoes a pH-dependent aqueous hydrolysis with the neutral substrate being the reactive species, and the imidazole-protonated form (pKa = 5.7) unreactive. Addition of sodium chloride retards the hydrolysis, evidence that there is a free carbenium ion intermediate (the common ion effect). The rate constant ratio Kcl/Kw for the reactions of this cation with the added chloride and with the solvent is 7.4 M−1. Further evidence for a free cation is the observation of the 2-azidomethyl product when the hydrolysis is carried out with sodium azide present, but with no change in the rate constant. The Kaz/Kw ratio as determined by product analysis is 1.1 × 102 M−1 With the assumption that kaz represents a diffusion-controlled reaction and has a value of 7 × 109 M−1 s−1, the rate constant kw for the reaction of the cation with solvent is 6 × 107 s−1. A comparison with azide–water selectivity ratios reported for other cations shows that the imidazole-stabilized primary cation of this study is relatively long-lived. A possible explanation for this is given, in terms of the extensive resonance delocalization of the positive charge in this cation. Keywords: solvolysis, carbenium ion, heterocycle.