Up to now, the member of zeolite
family has expanded to more than
230. However, only little part of them have been reported as catalysts
used in reactions. Discovering potential zeolites for reactions is
significantly important, especially in industrial applications. A
carbonylation zeolite catalyst Al-RUB-41 has special morphology and
channel orientation. The 8-MR channel of Al-RUB-41 is just perpendicular
to its thin sheet, making a very short mass-transfer distance along
8-MR. This specific nature endows Al-RUB-41 with efficient catalytic
ability to dimethyl ether carbonylation reaction with beyond 95% methyl
acetate selectivity. Compared with the most widely accepted carbonylation
zeolite catalysts, Al-RUB-41 behaves a much better catalytic stability
than H-MOR and a greatly enhanced catalytic activity than H-ZSM-35.
A space-confined deactivation mechanism over Al-RUB-41 is proposed.
By erasing the acid sites on outer surface, Al-RUB-41@SiO2 catalyst achieves a long-time and high-efficiency activity without
any deactivation trend.
Reduction process is a key step to fabricate metal-zeolite catalysts in catalytic synthesis. However, because of the strong interaction force, metal oxides in zeolites are very difficult to be reduced. Existing reduction technologies are always energy-intensive, and inevitably cause the agglomeration of metallic particles in metal-zeolite catalysts or destroy zeolite structure in severe cases. Herein, we disclose that zeolites after ion exchange of ammonium have an interesting and unexpected self-reducing feature. It can accurately control the reduction of metal-zeolite catalysts, via in situ ammonia production from ‘ammonia pools’, meanwhile, restrains the growth of the size of metals. Such new and reliable ammonia pool effect is not influenced by topological structures of zeolites, and works well on reducible metals. The ammonia pool effect is ultimately attributed to an atmosphere-confined self-regulation mechanism. This methodology will significantly promote the fabrication for metal-zeolite catalysts, and further facilitate design and development of low-cost and high-activity catalysts.
Deposition of carbonaceous compounds was used to improve the propylene selectivity of ZSM-5 by deactivating some acid sites meanwhile maintaining the high activity for methanol conversion. The carbonaceous species of pre-coked samples before and after MTP reactions were investigated by elementary analysis and thermogravimetric analysis (TGA). The results showed that pre-coke formed at low temperature (250°C) was unstable and easy to transform into polyaromatics species at the high reacting temperature, while combining 5% pre-coking process with 95% steam treatment at high temperature (480°C) was effective in inhibiting the formation of coke deposits and presented a significant improvement in the propylene selectivity.
Nanostructured Cu/ZnO catalysts were prepared by a simple solid-state method, using cheap metal nitrates as raw materials. Special attention was paid to the influence of different chelating agents such as citric acid, formic acid and oxalic acid on the physicochemical properties of Cu/ZnO catalysts, which were characterized by detailed investigations. CuO crystallite size, oxygen vacancies and surface compositions were clearly affected by chelating agent types, leading to different CuO _ ZnO interactions in the calcined catalysts as well as distinct reducibility. Different chelating agents also had significant effects on specific surface area, metallic Cu 0 surface area, and Cu crystallite size, as well as ZnO (002) plane to ZnO (100) plane ratio (I(002)/I(100)) calculated based on the peak intensity of XRD patterns of the reduced samples, so influencing the catalytic activity for low-temperature methanol synthesis from syngas containing CO2. The structure-activity correlations, particularly the relationships between space time yield (STY) of methanol with Cu 0 surface area and I(002)/I(100) ratio, were evaluated. Cu/ZnO catalyst prepared with oxalic acid showed the highest STY value, which was 2.3 and 5.3 times higher that of the catalysts obtained with citric acid and formic acid, respectively. The excellent catalytic performance was due to its greater Cu 0 surface area, higher specific surface area and smaller Cu crystallite size. Stronger interactions between Cu and ZnO in the reduced samples were also favorable for enhancing catalytic activity for low-temperature methanol synthesis.
The content of mononuclear Al (Ala%) changed with its determination time (ta) under different dosages of Ferron (7-iodo-8-hydroxyquinoline-5-sulfonic acid, [Ferron]), and the change of Ala% with [Ferron] at different ta was systematically investigated for the first time. Thus, the most appropriate ta was found with the optimal [Ferron]. Also, the judgment of the platform (flat or level portion) of the complete reaction on the absorption-time curve determined in the hydroxyl polyaluminum solution by Ferron timed spectrophotometry (Ferron assay) was first digitized. The time point (tb) of complete reaction between the medium polyaluminum (Alb) and Ferron reagent depended on the reaction extent, and time could not be used only to judge. Thus, the tb was accurately determined and reduced to half of original, which improved the experiment efficiency significantly. The Ferron assay was completely optimized.
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