Kirill I. Zamaraev

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

The dehydration reaction of tert-butyl alcohol, selectively labelled with13C in CH3 or C-O groups (t-BuOH[2−13C2] andt−BuOH[1-13C]), as well as selectively deuterated in methyl groups (t-BuOH[2-2H9]), was studied on H-ZSM-5 zeolite simultaneously with13 C CP/MAS and2H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2−13C1] andt-BuOH[1−13C] give rise to identical13C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the “side” reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.

Possible Impact of Heterogeneous Photocatalysis on the Global Chemistry of the Earth's Atmosphere

Abstract Photochemistry is recognized to be important for various physicochemical processes in the atmosphere, such as formation of the ozone layer and smogs, degradation of waste substances, etc. [1]. However, up to the present the emphasis in atmospheric photochemistry has been mainly on the study of photochemical reactions that occur with molecules directly excited by absorption of light quanta. However, the major components and impurities of the earths atmosphere (such as nitrogen, oxygen, water, carbon dioxide, methane, methane halides, etc.) are totally transparent to most solar radiation. Electronically excited states of these molecules are formed only upon absorption of vacuum ultraviolet light quanta with energy hv ≥ 5 eV (i.e., with wavelength λ ≤ 200 nm). Only a small portion of the energy of solar light is found in this spectral region. In other words, most of the energy of the solar flux cannot participate in such direct photochemical reactions.

Mechanistic studies of the catalytic dehydration of isobutyl alcohol on NaH-ZSM-5

Using in situ FTIR and GC kinetic studies, we have examined the mechanism of dehydration of isobutyl alcohol to butene on well characterized ZSM-5 zeolite (number of active sites determined by various methods). Dehydration takes place on Bronsted-acid sites. The kinetics of water evolution from butanol is followed by in situ FTIR and both the rate constant and the activation energy of water evolution are estimated [k2=k02 exp (–E2/RT), where k02= 2 × 109 s–1 and E2= 19 ± 3 kcal mol–1]. At low temperatures (45–70 °C), elimination of water is accompanied by simultaneous formation of isobutyl ether (which at the given temperatures is adsorbed and desorbed with difficulty, but is able to form inside the channels). At higher temperatures (125 °C), there is a shift in the equilibria of various reaction steps, resulting in the formation of butene. This butene may desorb into the gas phase with traces of ether (in conditions of excess alcohol, flow GC experiments) or form oligomers which remain adsorbed in the zeolite (no excess of alcohol, static IR experiments). The measured rate constant and activation energy [k4=k04 exp(–E4/RT), where k04= 3 × 1014 s–1 and E4= 32 ± 2 kcal mol–1] for butene formation are effective values, containing contributions from several reaction steps, which explains the rather high value of E4.

Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5

It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits carbenium ion properties. Namely: (1) According to2H NMR, the proton of the acidic ≡Al-OH-Si≡ group of the zeolite is transferred into the CH2= group of the octene-1 molecule. (2) According to13C NMR the13C label inserted into the terminal CH2= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formation of the linear rather than branched carbeniumion.

Modelling of the prebiotic synthesis of oligopeptides: silicate catalysts help to overcome the critical stage.

On the basis of experimental studies of the initial stages of glycine oligomerization in aqueous suspension of zeolite and kaolinite catalysts, a model is suggested for the prebiotic synthesis of oligopeptides from α-amino acids. The formation of linear dipeptides by hydrolysis of one amide bond in the cyclic piperazinedione intermediate (formed from glycine spontaneously) is found to be the critical stage of the reaction. This stage is base catalyzed and its rate increases when pH of the medium goes up. The linear glycyl-glycine yield rises under effect of hydroxyl anions generated from different sources including insoluble silicates and soluble sodium bicarbonate. During prebiotic evolution silicates capable of cation-exchange can serve as local sources of the hydroxyl anions which dramatically accelerate formation of linear dipeptides from cyclic ones. Oligopeptides of higher molecular weight are then easily formed from the linear dipeptides at neutral pH, even in the absence of catalysts or sources of energy (e.g. such as light). The described catalytic synthesis could occur in the proximity of submarine hydrothermal vents.

Kinetic studies of catalytic dehydration of tert-butanol on zeolite NaH-ZSM-5

Abstract Using a combination of FTIR spectroscopy and GC kinetic methods, we have studied in detail the adsorption and dehydration reaction of tert -butanol ( t -BuOH) on H-ZSM-5. From FTIR studies of the kinetics of t -BuOH adsorption, we estimate the diffusion coefficient of t -BuOH in the zeolite channels as 5 × 10 −11 cm 2 s −1 at 23°C. The course of the dehydration reaction with time (in the temperature interval 23–60°C) is followed by simultaneous growth of a peak for adsorbed water, changes in the stretching and deformation vibrations for the adsorbed organic, and changes in the nature of the H-bonded species. These changes show that dehydration is accompanied by rapid isomerization and oligomerization of the butene product (the rates of isomerization and oligomerization substantially exceed the rate of dehydration), making it impossible to identify the intermediate for the dehydration reaction (be it carbonium ion or alkoxy species). Under reaction conditions the oligomers (most likely linear C8 species) remain adsorbed on the active sites inside the zeolite channels, resulting in rapid deactivation of the sites. When the sites within the zeolite pores are completely deactivated, the dehydration reaction proceeds on sites located at the external surface of the zeolite crystallites. The latter sites are deactivated much more slowly than the former ones under our reaction conditions.

Giant palladium clusters: synthesis and characterization

A series of palladium clusters containing from four to several hundred Pd atoms in the metal skeleton has been prepared and characterized with respect to structure and chemical properties, including catalytic activity. For smaller clusters good agreement was observed. between single-crystal X-ray and EXAFS structural data. Giant clusters approximating to Pd561L60(OCOCH3)180(L = phen, bipy) and Pd561phen60O60X60(X = PF6, ClO4, BF4, CF3CO2), were characterized with TEM, SAXS, EXAFS, NMR and magnetic succeptibility data. These clusters contain a closepacked metal core and ligands L and X that are located at the periphery of a cluster. They are very soluble in water and polar organic solvents and can be considered as a bridge between low molecular clusters and particles of colloidal metals. Giant Pd clusters were found to be active homogeneous catalysts for various organic reactions, e.g. oxidative acetoxylation of alkenes and alkylarenes; oxidation of alkenes, formic acid and alcohols; dehydration of alcohols and formation of acetals. The kinetics and mechanism of the homogeneous oxidation of alkenes and HCO2H in solutions of giant clusters were elucidated.

13C CP/MAS NMR study of isobutyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of stable isobutyl silyl ether intermediate

Dehydration of isobutyl alcohol selectively labelled with a13C nucleus in the CH2 group (i-BuOH[1−13C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using13C CP/MAS NMR. The formation of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positions in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formation of carbenium ion as the reaction intermediate.

13C solid state NMR evidence for the existence of isobutyl carbenium ion in the reaction of isobutyl alcohol dehydration in H-ZSM-5 zeolite

Using two-dimensionalJ-resolved and CP/MAS13C NMR, the pathway for the transfer of the13C label from the CH2 group of isobutyl alcohol into the hydrocarbon skeleton of butene oligomers has been elucidated in the course of isobutyl alcohol dehydration inside H-ZSM-5 zeolite. First, the label is transferred selectively into the CH2 group of the isobutyl silyl ether reaction intermediate (IBSE), and then into the CH and CH3 groups of the isobutyl fragment (-CH2CH(CH3)2) of IBSE and/or butene oligomers. Finally, it is scrambled over the carbon skeleton of the oligomers. The obtained data suggest that isobutyl carbenium ion is formed as a reaction intermediate or transition state during the transformation of isobutyl silyl ether into butene oligomers.

Dehydration of n-butanol on HNa-ZSM-5

In the present study, we aim, by means of kinetic studies, to clarify the mechanism of the dehydration of n-butanol on HNa-ZSM-5 (65% H + , 35% Na + ) and the role played by n-dibutyl ether in this reaction