Intermediate Acid Sites Research Articles

High propylene selectivity in methanol conversion over metal (Sm, Y, and Gd) modified HZSM-5 catalysts in the methanol to propylene process

A protonated form of Zeolite Socony Mobil-5 (H-ZSM-5) catalyst (with Si/Al = 200) was synthesized through a hydrothermal method from various sources of silica, including Ludox, silicic acid, and tetraethyl orthosilicate. We also investigated the effect of loading the catalyst with yttrium (Y), samarium (Sm), and gadolinium (Gd), respectively, on the acidic properties of the catalyst. We compared the catalytic performance of the catalysts with different loadings in methanol to hydrocarbon conversion. The catalysts were characterized using X-ray diffraction analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, surface area analysis measurements, thermogravimetric analysis, and temperature-programmed desorption. Among the metal-loaded Ludox-based H-ZSM-5 catalysts (LHZs), the Sm/LHZ catalyst had the best performance with 68.8 % propylene selectivity due to its high amount of weak and intermediate acid sites at 480 °C, WHSV = 1 h−1 in a methanol to propylene conversion. However, the Gd-LHZ catalyst significantly increased the selectivity toward ethane and propane during the conversion.

Oxidative dehydrogenation of ethane over M1 MoVNbTeO catalysts modified by the addition of Nd, Mn, Ga or Ge

Modification by Nd, Mn, Ga, and Ge substantially affect both physicochemical and catalytic properties of MoVNbTeO oxides in the oxidative dehydrogenation of ethane (ODE). The introduction of modifying additives changes the content of the M1 phase, surface acidity, and surface composition. It does not have an influence on the state of Mo, V, Nb, and Te on catalyst surface, but it induces surface enrichment (Ga, Mn) or depletion (Ge, Nd) by the modifying element. All of these additives have positive effects on catalytic properties at the M/Mo (M = modifying element) ratio of 0.0075 ÷ 0.025. They exhibit an increase of activity and prevent ethylene over-oxidation. The relationship between the rate constant of total ethane transformation and the M1 content confirms that this phase is active in ODE. The correlation between ethylene selectivity and the relative number of intermediate acid sites is revealed, indicating the role of surface acidity on ethylene over-oxidation.

Effect of boron addition on the MoO 3 /CeO 2 –Al 2 O 3 catalyst in the sulfur-resistant methanation

The effect of boron on the performance of MoO3/CeO2–Al2O3 catalysts, which were prepared with impregnation method, was investigated. The catalysts were characterized with N2 adsorption–desorption, XRD, H2-TPR, and NH3-TPD, and were tested in sulfur-resistant methanation. The results indicated that the MoO3/CeO2–Al2O3 catalysts modified by boron showed higher catalytic performance in sulfur-resistant methanation. The CO conversion increased from 47% to 62% with 0.5 wt% boron content. When the content of boron was under 0.5 wt%, the results suggested there was an increase in the amorphous form of MoO3 caused by the generation of weak and intermediate acid sites, which had weakened the interaction between the active components and supports. While, the catalyst added 2.0 wt% boron showed the strong acid sites and the largest crystalline size resulting in the uneven distribution of ceria.

Effect of phosphorus on acidity and catalytic performance of Al-MCM-41 for the selective conversion of ethyl mercaptan to ethylene and H2S

A series of phosphorus modified Al-MCM-41 molecular sieves with various P/Al ratios were prepared via impregnating method and tested in the selective transformation of ethyl mercaptan into ethylene and hydrogen sulfide. The resulting samples were characterized by a combination of X-ray diffraction, N2 adsorption, ICP-OES, NH3 -TPD, 27Al and 31P MAS NMR spectroscopy. The experimental findings demonstrated that P modification can effectively increase the concentration as well as the strength of weak acid sites. Intermediate acid sites were gradually decreased with phosphorus addition, and the product distribution was found to vary with the acidity of the zeolites. Phosphorus content has strong influence on catalytic stability and ethylene selectivity. Upon P treatment, the amount of coke drastically decreased and the catalyst lifetime prolonged. The notable results could be useful for developing a viable process for removing ethyl mercaptan from natural gas.

Beckmann rearrangement of cyclohexanone oxime over Al-MCM-41 and P modified Al-MCM-41 molecular sieves

Abstract P modified Al-MCM-41 molecular sieves were prepared via impregnating method and applied to vapor-phase Beckmann rearrangement reaction. The catalysts were characterized by XRD, N2 adsorption, ICP-OES, NH3-TPD and 27Al and 31P MAS NMR spectroscopy. NH3-TPD results reveal that P modification of Al-MCM-41 can increase the number as well as the strength of weak acid sites, whereas the introduction of P decreases the number of intermediate acid sites. 27Al and 31P MAS NMR results indicate that formation of Si-O-P-O-Al seems to be favored in P modified Al-MCM-41 catalysts. Overall, these results indicate that P modified Al-MCM-41 catalysts can improve their selectivity for caprolactam. In addition, more abound weak acid sites with certain acid strength are favorable to Beckmann rearrangement.

Aromatization of Methane Over Mo-Fe/ZSM-5 Catalysts

Two groups of samples were studied for the aromatization of methane over Mo-Fe/ZSM-5 catalysts. The first group contains fixed loading (5 wt%) and variable Mo/Fe ratio. The second group was prepared with fixed Mo/Fe ratio and variable loading. The samples were characterized by TGA, S BET, XRD, NH3-TPD and TPO analysis. NH3-TPD results indicate that the strength of strong acid sites increases when Mo/Fe ratio decreases for samples with fixed loading. The amount of strong sites decreases and new intermediate acid sites appear on samples containing both Mo and Fe. XRD results show the presence of iron molybdate for samples impregnated with both Mo and Fe. The catalytic properties of these samples were related not only to the amount and strength of acid sites but also to the iron molybdate phase.

Liquid-phase alkylation of phenol with t-butanol over various catalysts derived from MWW-type precursors

Three types of MWW-based catalysts (MCM-22, MCM-36 and ITQ-2) were studied in the liquid-phase alkylation of phenol by tert-butanol (TBA). Ammonia adsorption calorimetry was applied to investigate the acid properties of zeolites with different framework topology and crystallinity. It was established that acid properties of MCM-22 family depend mainly of the solid Al-content (the Si/Al ratio varied from 9.1 to 46.0). Delamination of MCM-22 precursor (MCM-22(P)) leading to ITQ-2 results in the decrease of the total acidity and the increase of the intermediate acid sites concentration, while the pillaring process yielding MCM-36 affected mainly the total concentration of acid sites, the acid strength distribution being similar to the corresponding MCM-22 zeolite. The catalytic activity and selectivity of zeolite-based catalysts are discussed. The most active catalyst for the studied reaction was MCM-22 B (Si/Al ~ 15). The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-MR pores in the structure of this zeolite, no enhanced selectivity to p- tert-butyl phenol ( p-TBP) is observed. This has been rationalized by the assumption that most of reactions occurring in this system take place on acid sites at or close to the external surface. This assumption is also supported by the similar catalytic selectivity of the MWW-derived structures (viz. MCM-36 and ITQ-2), which expose a higher concentration of external pockets. However, it is noticeable the high catalytic activity of MCM-22 zeolites for the liquid-phase alkylation of phenol by TBA that is comparable to the gas-phase activity of large pore zeolites like as HY or HM. To cite this article: E. Dumitriu et al., C. R. Chimie 8 (2005).

A comparative study of supported TiO2 catalysts and activity in ester exchange between dimethyl oxalate and phenol

Various supported TiO2 catalysts, TiO2/SiO2, TiO2/Al2O3 and TiO2/MgO, were prepared by impregnation method and their activities in the transesterification of dimethyl oxalate (DMO) with phenol were tested. To further investigate the relationship between the catalyst performances and carrier properties, a series of characterization methods, such as BET specific surface area measurement, temperature-programmed desorption (NH3-TPD, CO2-TPD), and FT-IR analysis of adsorbed pyridine were utilized. The results indicated that the carrier had an important effect on the selectivities to products. The activities of supported TiO2 catalysts were improved significantly when TiO2 was supported on high surface area carriers. The weak acid sites were responsible for the formation of methyl phenyl oxalate (MPO) and diphenyl oxalate (DPO), while the intermediate acid sites and base sites were favorable for the formation of byproduct, anisole (AN).