Marina A. Makarova

Brønsted acid strength in US-Y: FTIR study of CO adsorption

Proton affinities of Bronsted hydroxy groups in H-forms of zeolites and SAPOs are determined by an FTIR study of the low-temperature adsorption of carbon monoxide. This method reveals the range of heterogeneity of Bronsted hydroxy groups associated with unmodified zeolites where only one band interacting with carbon monoxide is evident in the infrared spectrum. Three types of acid hydroxy groups associated with different IR bands are detected in a modified zeolite (US-Y), (i) strongly acidic [3599 cm–1, proton affinity (Epa)= 1112 kJ mol–1], (ii) medium acidic (3627 cm–1, Epa= 1142 kJ mol–1) and (iii) weakly acidic (ca. 3740 and 3675 cm–1, Epa= 1192 ± 29 kJ mol–1).

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.

Brønsted acid sites in zeolites. FTIR study of molecular hydrogen as a probe for acidity testing

Hydrogen is used as a probe molecule for characterisation of Bronsted acidity in zeolites. Hydrogen adsorption is monitored volumetrically (physisorption of hydrogen) and by FTIR (OH⋯H2 complex formation), simultaneously. The physisorption of hydrogen is a function of the pore size of a zeolite whereas the OH⋯H2 interaction reflects the strength of the acid sites. Hydrogen complexes observed by FTIR are characterised by four parameters which depend upon acid strength: shifts in the position and the increase in absorbance of the hydroxy and the H2 bands. For the zeolites studied the behaviour of these parameters is consistent and shows the following order of acidity: SAPO-37 <H-Y <H-EMT <H-ZSM-5 <H-MOR. Hydrogen is also compared with carbon monoxide, another probe widely used for acidity determination.

Brönsted acidity in US-Y zeolites

FTIR analysis of the hydroxyl region in a series of US-Y zeolites was used for semiquantification of the enhanced acidity, involving Bronsted hydroxyls in both super and β cages, which correlates with the catalytic activity in n-hexane cracking.

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

Influence of pore confinement on the catalytic dehydration of isobutyl alcohol on H-ZSM-5

The dehydration of isobutyl alcohol has been used as a test to probe the differences in catalytic behaviour between an (acidic) amorphous aluminosilicate and samples of H-ZSM-5 of various crystallite size but of similar acidity ([H+]≈ 3 × 1020 g–1) as the amorphous aluminosilicate. Dehydration rates are shown to be independent of crystallite size (in the range 0.5–20 µm), but rates of oligomerization of the product butene are higher for the large crystallites.

Mechanistic study of sec-butyl alcohol dehydration on zeolite H-ZSM-5 and amorphous aluminosilicate

The dehydration of sec-butyl alcohol has been studied by in situ FTIR and gas-chromatographic (GC) kinetic methods in the range 60–140 °C on zeolite H-ZSM-5 and amorphous aluminosilicate (AAS) samples with a well characterized number and strength of Bronsted acid sited. Under flow conditions (GC kinetic studies), the reaction yields butenes [but-1-ene, (Z)- and (E)-but-2-ene] and water, with an activation energy of 40 ± 1 kcal mol–1 determined form, steady-state data. Under non-steady-state conditions, the so-called ‘stop effect’ is observed: namely, an increase in the rate of butene evolution (as compared with that at steady state) when the flow of alcohol into the reactor is halted. The course of dehydration on H-ZSM-5 in a static IR cell was followed by the appearance and growth of a peak for adsorbed water (water deformation peak at 1640 cm–1). The rate constant determined from the kinetics of water formation in the FTIR experiments (1.1 × 10–3 s–1 at 70 °C) is found to be 400 times as high as the rate constant calculated from GC steady-state kinetic data. All these anomalous phenomena observed under flow conditions (the low rate of reaction, the high activation energy and the ‘stop effect’) can be explained by the slowing down of dehydration under these conditions as a result of the reverse reaction, i.e. the hydration of the product butene with product water. When the zeolite pores are free from physically adsorbed reactants (in the FTIR experiments or during the ‘stop effect’), the extent of the reverse reaction decreases and the rate of butene formation increases. On AAS, which has acid sites of similar strength, but which has a much more open surface (average pore diameter ca. 50 A compared with 5.5 A for ZSM-5), similar effects are observed, but they are much less pronounced. This probably arises from the lower reactant concentration in the AAS at steady state and hence, a lower concentration of water in the vicinity of the active sites.

Limitation in the application of pyridine for quantitative studies of brönsted acidity in relatively aluminous zeolites

Abstract The titration of Bronsted acid sites in H-Y ( Si Al = 2.7 ) and H-EMT ( Si Al = 4.0 ) with pyridine, monitored by Fourier transform infrared (FTIR) techniques has demonstrated that the amount of PyH+ ions (the IR band at 1545 cm−1) formed at saturation is the same and does not depend on the Si Al ratio within the range considered, although the HF band (3650 cm−1) is fully perturbed in both cases. The integrated extinction coefficient, ePyH+, has an effective value (1.1 cm μmol−1 at 150°C and 1.6 cm μmol−1 at 350°C) reflecting three processes simultaneously: (1) the interaction of HF hydroxyl groups with Py leading to PyH+ ion formation; (2) the perturbation of HF hydroxyl groups by PyH+ ions as OH··PyH+; (3) the perturbation of LF hydroxyl groups by Py resulting in hydrogen-bonded O··H··Py complexes. Py sorption cannot be used for quantification of Bronsted hydroxyls in faujasitic zeolites with Si Al since only a limited amount of PyH+ ions per supercage can be generated.

A combined MAS nuclear magnetic resonance spectroscopy, in situ FT infrared spectroscopy and catalytic study of the conversion of allyl alcohol over zeolite catalysts

A combined study of allyl alcohol conversion over zeolite catalysts using catalytic measurements in a flow microreactor, in situ FTIR and MAS NMR spectroscopy is reported. Rate constants for the conversion in the flow reactor and the static in situ reactor used in the FTIR studies are in broad agreement, emphasising the viability of the experimental approach. In the flow microreactor allyl alcohol conversion over the zeolite catalyst is shown to form diallyl ether, hydrocarbons and acrolein. The in situ study successfully models the formation of diallyl ether and hydrocarbon as initial reaction products, but unfortunately acrolein is found to be rapidly converted to hydrocarbons under the condition used in the in situ cells. The studies are combined to provide a model for the reaction which involves two parallel pathways for the formation of the hydrocarbons and acrolein.

Intrinsic and enhanced broensted acidity in zeolites

Publisher Summary Acid zeolite catalysts are widely used commercially for hydrocarbon transformations. Consequently, there is continued discussion concerning both the measurement and interpretation of acidity in solid acids, and also the basis for enhanced acidity in zeolites. Several investigations suggest that in high-silica zeolites (Si/AI>7) acid sites tend to be isolated, have similar if not identical acid strength and function independently. Other reports emphasize co-operative features suggesting that neutralization of a few framework Broensted sites can decrease the acidity of the remainder. In hydrothermally treated zeolites enhanced acidity is associated with the presence of dislodged aluminium species in a manner not clearly defined. Recently we experimentally modelled sites having enhanced acidity by adding Lewis acids to H-forms of zeolites to provide some clarification of Lewis–Broensted acid interactions in solid acids. This chapter (1) extends theoretical modeling of the Broensted sites with enhanced acidity and (2) examines the effect of neutralizing framework Broensted sites on intrinsic acidity using both experimental methods and theoretical calculations.