Thermal Dealumination Research Articles

Tuning Al sites in Y-zeolite for selective production of ϒ-valerolactone from levulinic acid

Converting biomass derived levulinic acid (LA) to γ-valerolactone (GVL) through catalytic transfer hydrogenation, using Lewis acidic catalysts and alcohols as hydrogen donors, is an important route for biomass valorization. Zeolites containing Lewis acidic metal ions have been widely studied for catalytic transfer hydrogenation of LA. On the other hand, zeolites themselves have Al-Lewis acid sites, inherent to their aluminosilicate framework, and these sites can be generated and tuned through simple post-synthesis methods. The present study shows that properly tuned Al sites in Y zeolites can catalyze LA to GVL conversion, and the key requirement for this conversion is creating more penta-coordinated Al sites in the catalyst. In order to establish this, a series of thermally-treated (500–800 °C) and steam-treated (500 and 700 °C) Y zeolites were prepared, characterized by X-ray diffraction, N2 adsorption, temperature programmed desorption of NH3, 27Al MAS NMR spectroscopy and pyridine adsorption – FTIR spectroscopy and then tested for LA to GVL conversion. Thermal dealumination of NH4Y zeolite at 700 °C (TY700) gave the highest percentage (21%) of penta-coordinated Al and it was found to be more selective (∼94%) in forming GVL than other catalysts studied under the optimized conditions. The turnover frequency of formation GVL, (TOFGVL) from isopropyl levulinate (IPL) on TY700 was three times higher than that of TY500 (NH4Y thermally treated at 500 °C). During the reaction, acid sites of the zeolite catalyze esterification of LA with isopropyl alcohol and hence LA is readily converted to isopropyl levulinate and then Meerwein–Ponndorf–Verley (MPV) reduction of IPL leads to the formation of isopropyl 4-hydroxy pentanoate, which undergoes lactonization to form GVL. Based on the experimental results, a plausible reaction mechanism involving the penta-coordinated Al Lewis acid sites and -Al-OH group was proposed.

The effect of pre-coking and regeneration on the activity and stability of Zn/ZSM-5 in aromatization of 2-methyl-2-butene

A new method to modify the acidity of zeolite was attempted to improve the activity and stability of Zn/ZSM-5 in aromatization of 2-methyl-2-butene. Pre-coking and regeneration in the presence of O 2 at 773 K generated a large exothermic heat in a short time, which resulted in the thermal dealumination of framework aluminum that acted as a Brønsted acid site. The treatment had a profound effect on the acid properties of the catalyst as probed by 27Al MAS NMR, temperature-programmed desorption of ammonia, and pyridine FTIR. The results showed that the concentration of oxygen in regeneration step had critical effects on the acidity and stability of Zn/ZSM-5. Thus improvement of lifetime of Zn/ZSM-5 in aromatization of 2-methyl-2-butene was achieved without deteriorating activity and selectivity by the pre-coking and controlled regeneration by oxygen.

A new way to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization

A thermal dealumination method was applied to modify HZSM-5 zeolites, and the Mo/HZSM-5 catalyst pre-dealuminated in N2 stream exhibited rather high catalytic activity and stability in the methane dehydroaromatization reaction (MDA). 29Si NMR, FT-IR and TPO measurements show that the thermal treatment of the HZSM-5 in inert atmosphere induced partial removal of tetrahedral coordinated Al from the zeolite lattices leading to elimination of the original excess strong Brönsted acid sites (known as responsible for the coke formation), and thus significantly promoted the coke-resistance of the Mo/HZSM-5 catalyst.

Knoevenagel condensation over β and Y zeolites in liquid phase under solvent free conditions

Liquid phase Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was carried out over Hβ, HY, thermally dealuminated Hβ and Zn 2+ exchanged Hβ and HY at 140 °C under solvent-free conditions. The materials were characterized by X-ray diffraction, BET surface area analysis, 27Al-magic angle spinning-nuclear magnetic resonance ( 27A1-MAS-NMR), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and atomic absorption spectrometry (AAS). 27A1-MAS-NMR revealed the generation of extraframe work aluminium upon thermal dealumination of Hβ. The amount of zinc present in Znβ and ZnY was estimated by AAS. The hydroxyl vibrational regions of Hβ and HY zeolites were identified by in situ DRIFT. Desorption studies were performed for pyridine-adsorbed Znβ at 100, 200, 300 and 400 °C and the spectrum was recorded at each temperature. Znβ was found to be a good catalyst for Knoevenagel condensation. The generation of Lewis acid sites upon thermal dealumination of Hβ zeolite is not favourable for the formation of Knoevenagel product. The various active methylene group compounds, such as diethyl malonate, malononitrile, methyl acetoacetate, ethyl acetoacetate and acetylacetone, were chosen to react with benzaldehyde/acetophenone. Ethyl cayanoacetate was found to be a good active methylene compound. The yield of Knoevenagel product with acetophenone was less than with benzaldehyde, which is in agreement with the general conclusion that aldehydes are more reactive than ketones.

Degree of crystallinity of dealuminated offretites determined by X-ray diffraction and by a new method based on nitrogen adsorption

Samples obtained from thermal dealumination (in the presence and absence of water vapour) of three offretite zeolites with, respectively, 4.4, 3.4, and 2.9 aluminium atoms per unit cell, were investigated by i.r. spectroscopy. X-ray diffraction, and nitrogen adsorption. The percentage of crystallinity was then evaluated by two different methods: the standard XRD technique and a new method based on nitrogen adsorption data analysed through the α s -method. In this case the volume of the ultramicropores (width less than 0.7 nm) was considered. Differences between the two estimates, as well as the loss of crystallinity during dealumination, which is expressed by the representation of both crystallinity evaluations, C XRD or C ADS , versus ν 1 (the frequency of the stretching asymmetric T-O band), were analysed for self-steamed and steamed samples. The results obtained agree within 20%, and often better. For self-steamed samples the crystallinity evaluated from XRD patterns is always smaller than that estimated from adsorption data, whereas for steamed samples the opposite situation is generally observed.

Microporous characteristics of HY, H-ZSM-5 and H-mordenite dealuminated by calcination

The modifications of the microporous volume of zeolites resulting from thermal dealumination have been examined by measuring the changes in the unit cell parameters as well as the N 2 and n-pentane adsorption isotherms. For the cubic HY and orthorhombic H-ZSM-5 zeolites, the differential decrease of the microporous volume per non-framework aluminum (NFAl) can be predicted by assuming NFAl is concentrated in pores available to N 2. For the orthorhombic H-mordenite, NFAl is homogeneously spread in all pores, available or not to N 2. The Gurvitch rule (volume of adsorbed N 2 = volume of adsorbed pentane) is respected in HY and H-ZSM-5, but not in H-mordenite, irrespective of the degree of dealumination.

Double rotation and variable field 27Al n.m.r. study of dealuminated Y zeolites

27Al MAS n.m.r. spectroscopy has been applied to dealuminated Y zeolites at two separate fields. The spectra clearly show that the nonframework aluminum species associated with a peak observed at 30 ppm are different in samples that had different hydrothermal treatments. In samples that were treated with a single thermal dealumination, the peak at 30 ppm is caused by a tetrahedrally coordinated aluminum species with a sufficiently large quadrupole coupling to cause significant second-order shifts. The resonance at 30 ppm in a sample that was dealuminated using two thermal treatments did not exhibit large second-order shifts. These results were confirmed by double-rotation (DOR) n.m.r., which averages the anisotropy of the second-order quadrupolar interactions. The DOR spectra clearly show the existence of two tetrahedral species at 59 (framework) and 47 (nonframework) ppm in the sample treated with a single dealumination.

Effect of thermal treatments on shape selectivity and acidity of ZSM-5 type zeolite catalysts

Thermally treated H-ZSM-5 samples were investigated as catalysts of toluene disproportionation and xylene isomerization and they were characterized by i.r. and TPD techniques. Thermal pretreatment enhanced selectivity of p-xylene formation and decreased activity. These effects can be partially reversed by subsequent treatment of the sample with HCl. It is suggested that extra lattice aluminium, originating from thermal dealumination, forms diffusion barriers in the pores, which are responsible for the catalytic effects and that HCl treatment partially dissolves these barriers.