Generation, Characterization, and Transformations of Unsaturated Carbenium Ions in Zeolites†

This review is a summary of the work done and potential opportunities for inexpensive and easily accessible base catalysts, such as alkaline earth metal oxides and hydroxides, as well as alkali metals and oxides supported on alkaline earth metal oxides Preparation methods of these materials, as well as characterization of basic sites are reported An extensive review of their catalytic applications for a variety of organic transformations including isomerization, carbon–carbon and carbon–oxygen bond formation, and hydrogen transfer reactions is presented

Optimization of Alkaline Earth Metal Oxide and Hydroxide Catalysts for Base-Catalyzed Reactions

Structural, compositional and acidic characteristics of nanosized amorphous or partially crystalline ZSM-5 zeolite-based materials

The Bronsted acid-catalyzed gas-phase dehydration of 1-propanol (0.075–4 kPa) was studied on zeolite H-MFI (Si/Al = 26, containing minimal amounts of extra framework Al moieties) in the absence and presence of co-fed water (0–2.5 kPa) at 413–443 K. It is shown that propene can be formed from monomeric and dimeric adsorbed 1-propanol. The stronger adsorption of 1-propanol relative to water indicates that the reduced dehydration rates in the presence of water are not a consequence of the competitive adsorption between 1-propanol and water. Instead, the deleterious effect is related to the different extents of stabilization of adsorbed intermediates and the relevant elimination/substitution transition states by water. Water stabilizes the adsorbed 1-propanol monomer significantly more than the elimination transition state, leading to a higher activation barrier and a greater entropy gain for the rate-limiting step, which eventually leads to propene. In a similar manner, an excess of 1-propanol stabilizes the...

Dehydration Pathways of 1-Propanol on HZSM-5 in the Presence and Absence of Water.

Cerium-containing mesoporous materials have been synthesized by hydrothermal method and characterized by IR, DR-UV–vis and DRIFT spectroscopy, XRD and N2 adsorption methods. It was established that the d100 and unit-cell (a0) parameter increase with the increase of cerium content up to 2% in SBA-15 and then tend to remain the same. According to DR-UV–vis spectroscopic data, an agglomeration of cerium atoms was observed in the form of fine CeO2 crystallites. Important factors affecting the catalytic activity of Ce-SBA-15, namely the effect of cerium content, the state of cerium ions, the state of silanol groups on the surface of Ce-SBA-15, and stability of the catalyst have been studied in the cyclohexanol and cyclohexene oxidation with hydrogen peroxide.

Ce-silica mesoporous SBA-15-type materials for oxidative catalysis : Synthesis, characterization, and catalytic application

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.

Eugenii A. Paukshtis

Papers

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

Mechanistic Studies of the Catalytic Dehydration of Isobutyl Alcohol on NaH-ZSM-5.