Large-pore Zeolite Beta Research Articles

Coupled Hydrocarbon Desorption in Zeolite Beta-Containing Monolithic Catalyst

The 3-D, large-pore zeolite beta (BEA) is effective in trapping large hydrocarbons (HCs). We conducted temperature-programmed desorption (TPD) experiments of a BEA-containing monolithic catalyst prestored with various mixtures of propylene, hexane, and toluene. The addition of propylene to a hexane + toluene mixture results in an unusual interaction during TPD. A rather small concentration of propylene increases the desorption temperature of the hexane from a weakly held into a stronger bound component. A variation in the propylene concentration leads to the nonmonotonic dependence of a high temperature hexane desorption peak. The data suggest that the blockage of hexane desorption by propylene depends on the uptake time. The findings complicate the analysis and design of HC traps used in applications such as vehicle aftertreatment systems.

Adsorption and Separation of Aromatic Amino Acids from Aqueous Solutions Using Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are investigated for the adsorption of aromatic amino acids l-phenylalanine (l-Phe), l-tryptophan (l-Trp), and l-tyrosine (l-Tyr) from aqueous solutions. After screening a range of water-stable MOFs, the hydrophobic Zr-MOF MIL-140C emerged as the best performing material, exhibiting uptakes of 15 wt % for l-Trp and 20 wt % for l-Phe. These uptakes are 5-10 wt % higher than those of large-pore zeolites Beta and Y. Both single-compound and competitive adsorption isotherms for l-Phe and l-Trp were experimentally obtained at the natural pH of these amino acid mixtures (pH 6.5-7) without additional pH modification. We find that the hydrophobic nature of MIL-140C and the capacity of l-Trp to form hydrogen bonds favor the uptake of l-Trp with its larger indole moiety compared to the smaller phenyl side group of l-Phe. On the basis of literature and vibrational analysis, observations of hydrogen-bonded l-Trp within the MIL-140C framework are evidenced by red- and blue-shifted -NH vibrations (3400 cm-1) in Fourier transform infrared spectroscopy, which were attributed to types N-Hl-Trp···πMIL-140C and N-Hl-Trp···OMIL-140C, respectively. MIL-140C is shown to be recycled at least three times for both aromatic amino acids without any loss of adsorption capacity, separation performance, or crystallinity. Desorption of aromatic amino acids proceeds easily in aqueous ethanol. Substantial coadsorption of negatively charged amino acids l-glutamate and l-aspartate (l-Glu and l-Asp) was observed from a model solution for wheat straw protein hydrolysate at pH 4.3. On the basis of these results, we conclude that MIL-140C is an interesting material for the recovery of essential aromatic amino acids l-Tyr, l-Phe, and l-Trp and of l-Glu and l-Asp from waste protein hydrolysates.

Zeolite supported palladium catalysts for hydroalkylation of phenolic model compounds

Bifunctional palladium-based catalysts with zeolite supports were investigated in phenol hydroalkylation, representing high-potential model reaction for transformation of lignocellulose-derived compounds to automotive and jet fuels. Conversion of phenol correlated with the amount of accessible acid sites and textural properties of the solid acid support: under the same reaction conditions and at the same Pd loading. The highest phenol conversions (99–100%) were obtained for large-pore zeolite Beta and mesopore-containing MCM-36/56 materials. The selectivity towards the targeted cyclohexylcyclohexane on different supports decreased in the following order Beta (57%) > MCM-36 (42%) ≈ MCM-56 (37%) > MCM-22 (20%), while the contribution of undesired reaction termination through formation of non-reactive cyclohexane was more pronounced for the expanded MCM-36/56 based catalysts. Substrate consumption after 60 min over the least active MCM-22-based catalysts increased with increasing Pd content as follows 27% (0.3 wt % Pd), 80% (0.5 wt % Pd) and 98% (0.7 wt % Pd). High activity of Pd/Beta catalysts was not vitiated even by sintering effects being the most pronounced for Beta as a support.

Impact of Zeolite Beta on Hydrocarbon Trapping and Light-Off Behavior on Pt/Pd/BEA/Al2O3 Monolith Catalysts

The impact of large-pore zeolite beta (BEA) on the performance of model monolithic Pt/Pd/Al2O3 diesel oxidation catalyst is investigated in terms of hydrocarbon (HC) storage capacity, light-off, and oxidation. Dodecane (C12) is employed as the model HC to elucidate BEA loading impact on catalyst performance during temperature-programmed oxidation of CO and C12 and mixtures in the absence of feed water. A C12 pre-storage experiment protocol quantifies storage at room temperature, the effect of C12 storage on light-off temperature, and the extent of release and conversion of C12. The temperature ramp experiments indicate a negligible impact of the zeolite on the individual reactant (CO and C12) light-offs. Co-feed experiments (HC + CO simultaneous feed) reveal light-off inhibition by CO and C12 in addition to that by pre-stored C12. This trend is consistent for all zeolite loading levels. During both co-feed and C12 pre-storage experiments, an intermediate zeolite loading results in a decreased CO light-off temperature when compared to the zeolite-free catalyst. This beneficial effect is attributed to the zeolite providing alternative storage sites for the HC, thereby mitigating the inhibition on the CO light-off by freeing up precious metal sites for CO oxidation. However, the highest zeolite loading catalyst has an increased CO light-off temperature in C12 pre-storage experiments compared to the catalyst with intermediate loading. This trend reversal is attributed to the kinetic and/or transport inhibitory effect of stored C12 which overcomes the enhancement at lower loadings. The data indicate that the highest zeolite loading yields the highest percentage of stored C12 converted to CO2, a finding relevant for HC trap catalyst design. The study findings suggest a need to optimize the zeolite loading for the desired operating conditions to achieve desired light-off and HC trapping and conversion. The impact of feed water will be investigated in future work to evaluate the generality of these findings.

Acylation of p-xylene over zeolites

Zeolites differing in structure and acidity were tested in liquid phase acylation of p-xylene. Hexanoyl chloride, propionic anhydride and isobutyric anhydride were used as acylating agents. The highest conversions of acylating agent were achieved over large pore zeolites USY (66.3%) and Beta (58.2%). It was found that acylation of p-xylene proceeds only over large pore zeolites Beta and USY. Selectivities to monoacylated p-xylene obtained over USY zeolite decreased in the order: propionic anhydride 78.0% > hexanoyl chloride 67.2% > isobutyric anhydride 33.1%. Diacylated product was formed over zeolite USY with all acylating agents tested but only with hexanoyl chloride over zeolite Beta. It was found that the optimum Si/Al ratio of zeolite Beta for p-xylene acylation with propionic anhydride is 25, while for isobutyric anhydride is 19. Conversion of isobutyric anhydride decreased with increasing isobutyric anhydride concentration and increased with increasing amount of catalyst.

Acylation of anisole with long-chain carboxylic acids over wide pore zeolites

The acylation of anisole with long-chain carboxylic acids (hexanoic, octanoic and decanoic) has been studied over three large pore zeolites—beta (BEA), faujasite (FAU) and mordenite (MOR). The acylation of anisole with the long chain acids produced mainly the ketone (4-methoxy phenyl alkyl ketone) and small amounts of the ester (phenyl alkanoate). The results revealed the reaction to be influenced by the type of zeolite and its Si/Al ratio (acidity) besides the chain length (carbon number) of the carboxylic acid. In the acylation of anisole with hexanoic acid, the activity of the zeolites increased with dealumination as it led to the generation of mesopores that resulted in a decrease in diffusion resistance of the zeolites. The reactivity of the acids in the acylation reaction was found to decrease with increase in the carbon number. The experimental data have been fitted into a pseudo first order kinetic model. Acylation of anisole with long-chain carboxylic acids (hexanoic, octanoic and decanoic) was studied over three large pore zeolites—beta, faujasite and mordenite. Acylation produced mainly 4-methoxy phenyl alkyl ketone and small amounts of phenyl alkanoate. The reaction was influenced by the zeolite type, Si/Al ratio and the chain length of the acid. The reaction followed a pseudo first order kinetics.

On the role of zeolite structure and acidity in toluene acylation with isobutyric acid derivatives

Zeolite catalysts of various structures and type and concentration of acid sites were investigated in a liquid phase acylation of toluene with derivatives of isobutyric acid. The highest conversions of isobutyryl chloride or isobutyric anhydride were obtained over three-dimensional large-pore zeolite Beta and zeolite Y possessing medium concentration of acid sites. In general, the conversion of acylating agents increases with increasing pore size diameter and dimensionality of the channel system. Large-pore zeolites with three-dimensional channel system enable easier desorption and transport of bulky isopropyltolyl ketone. The activity of the zeolites used can be also controlled by tailoring their hydrophobic/hydrophilic properties. The higher initial rate of toluene acylation with isobutyric anhydride compared to isobutyryl chloride was found. However, only one possible acylating group of isobutyric anhydride is used for acylation reaction.