S.Y. Peng

Theoretical studies on the properties of acid site in isomorphously substituted ZSM-5

The structure and electronic properties of the Bronsted acid site in B, Al or Ga isomorphously substituted ZSM-5 zeolites were studied by ab initio HF or DFT methods. The dependence of the calculation results on the employed methods, cluster size and basis sets was examined in details. Several measures for determining of the acidity including the proton affinity, the charge on proton, the hydroxyl group vibrational frequencies, γOH, as well as the adsorption energy of NH3 showed that the acidity of the substituted ZSM-5 increases in the sequence: B–ZSM-5<Ga–ZSM-5<Al–ZSM-5, which is in good agreement with experimental results. Studies on the interaction of NH3 with the Bronsted acid site indicated that NH3 becomes protonated in contact with the zeolite cluster and the configurations in which the protonated NH3 interacts with two lattice oxygen atoms are favored energetically. In addition, the calculated adsorption energy of NH3 on Al–ZSM-5 is comparable with the experimental data.

Monte Carlo simulation of alkylation of toluene with alcohols over zeolite catalysts

Abstract A Monte Carlo model for the simulation of alkylation of toluene with alcohols over zeolite catalysts was developed. On the basis of the experimentally observed parameters, including the intracrystalline diffusivity, equilibrium adsorption constant, and intrinsic rate constant, the alkylations of toluene with methanol and ethanol were simulated. The results were in good agreement with those of the experiments. This suggests that the Monte Carlo method is helpful for investigating the nature of shape selectivity in zeolite-catalyzed reactions.

The role of irreversible and reversible spillover hydrogen in reforming reactions

Abstract Two types of spillover hydrogen, reversible and irreversible, have been detected and their roles studied in reforming reactions occuring on supported Pt catalyst at 450°C. It is found that their amount and ratio depend largely on the catalyst preparation. The results of the conversion of hexane illustrate that both types of spillover hydrogen play important roles.

Synthesis and properties of aluminosilicate mesoporous material with adjustable pore structure

Mesoporous aluminosilicate material with different Si/Al ratios was synthesized from tetraethylorthosilicate (TEOS) and aluminum nitrate nonahydrate (ANN) via sol-gel route. CTAB was used as the structure-directing agent to create mesopore within the colloidal particles. The pore structure of the finally obtained samples changed regularly with the altering of Si/Al ratio by carefully controlling of the sol-gel process. Bimodal pore structure was then obtained when the Si/Al ratio reached a suitable range. As a result of the sol-gel synthesis method and the templating function of CTAB, nanometer mesoporous molecular sieves were obtained. Structural properties of the samples were characterized by means of XRD, BET, NMR and HRTEM.

Preparation of nanometer size Cu-Zn/Al2O3 catalyst by phase transfer Part 3. Sol preparation and phase transfer conditions

Publisher Summary The chapter studies the effects of sol preparation and phase-transfer conditions on the particle size of a solid. The main steps affecting the particle size of a product is the sol preparation and the phase transfer for the preparation of nanometer particles Cu–ZnAl 2 O 3 catalysts with cationic surfactant by phase transfer in benzenic solvent. The optimal mole ratio of a surfactant to nitrate is about 2:10. The optimal amount of precipitation agent ammonia is to ensure final pH higher than 8.5. There exists an optimal concentration range of both ammonia and nitrate for obtaining the highest surface area of the product. The effect of nitrate concentration is more marked. The future research for phase-transfer preparation with a cationic surfactant could be the selection of a surfactant and a synergist in a larger range and the study of their compatibility to increase the potency of the surfactant, that is, to more effectively prevent particles aggregation.

Preparation of nanometer size of Cu-Zn/Al2O3 catalyst by phase transfer Part 1: Study of basic preparation conditions

Publisher Summary Several methods have been developed to prepare nanometer particles. The liquid-phase method can be used for the preparation of a catalyst with nanometer size. The main problem in this process is the way in which the aggregation of fine particles can be avoided, especially at the drying step. The key is to reduce surface tension. The chapter discusses the alcoholate method in which alcoholate hydrolysis instead of aqueous solution to make the sol(gel) is used to avoid surface tension, that is, to avoid vapor–liquid interface; the freeze drying method in which the solution is first frozen and then sublimated to remove it; and the supercritical fluid drying method in which the solvent is solved in a supercritical fluid to remove it. Although these methods have successfully reduced or avoided the effect of surface tension, the preparation cost including raw material, special equipment, and time is high. An alternative way to decrease surface and interface tension is using a surfactant that can weaken the aggregation of the fine particles.

Monte Carlo simulation of effects of pore entrance deactivation on reaction performance

The complex isomerization A=B=C over zeolites is simulated using a Monte Carlo method which accounts for the elementary processes of adsorption, diffusion, reaction and desorption. The effects of pore entrance deactivation on the product selectivity and effectiveness are investigated, and the results show that the shape selectivity in zeolites-catalyzed reactions can be enhanced by pore entrance deactivation as the reaction is operated in diffusion-controled region.

New Dynamic Method Approach to The Roles of Reversible and Irreversible Adsorption in Heterogeneous Catalysis

Publisher Summary The catalytic behavior of reversibly adsorbed species, which presents on the catalyst surface under reactant stream and desorbs upon evacuation or purgation, is ambiguous because of the lack of a suitable characterizing method. The reversible adsorption takes an important role in ethylene and acetylene hydrogenation, and the relative reactivity of reversibly and irreversibly adsorbed acetylene determines the selectivity. But little information is now available about the kinetic behavior of respective adsorbed species. Recently, a new dynamic model to elucidate the dynamic behavior of reversible and irreversible species in catalytic reaction was developed. This chapter discusses the results of the application of this new dynamic model to the hydrogenation of acetylene on Pt/Al 2 O 3 catalyst. Based on the theoretical and experimental results, it can be concluded that in acetylene hydrogenation on Pt/Al 2 O 3 , the irreversible acetylene is mainly responsible for ethane formation. The contribution of reversible acetylene, which is abundant on the catalyst surface and is mainly hydrogenated to ethylene, is ∼1.5–2.0 times the irreversible species. From the relative amount of respective species and their reactivity, together with the additional results for acetylene and ethylene competing adsorption, it seems that, in addition to taking part in the hydrogenation, the irreversible acetylene also serves as the modifier of the catalyst surface, while the reversible acetylene acts as a competing adsorption agent for the ethylene intermediate, which results in the high ethylene selectivity of the metal catalyst.

A study of the deactivation and regeneration behaviour and related catalytic properties of modified zeolite catalysts

The coking and oxidative regeneration behaviour of HY-type zeolite catalysts containing additive elements such as Pt, Cu, or Zn were investigated. It was found that Pt or Cu element causes different variations to the cracking activity in n-heptane conversion as compared with parent HY, and that accelerated oxidation of coke was observed in temperature-programmed oxidation (TPO) of coked PtHY and CuHY catalysts. XPS and other techniques provide information concerning the distribution and variations of metallic species after sample calcination, reduction, deactivation and regeneration processes, and it is postulated that electron transfer between these metallic species and coke, or hydrogen, or oxygen molecules is responsible for the experimentally observed behaviour.

On the Role of Reversible and Irreversible Adsorption Hydrogen in the Dehydrogenation and Reforming Reactions

Publisher Summary The conversion of hydrocarbon over Pt catalysts is very important in petroleum processing. Their catalytic performance, particularly product selectivity, depends strongly on the pressure of H2 and its sorption behavior. Thus, much work has been done to investigate the effect of H2 adsorption on hydrocarbon conversion over supported Pt catalysts. However, only irreversible adsorption hydrogen has been considered in almost all conclusions. As per the study described in the chapter, under a real reaction condition, irreversible and reversible adsorption species co-exist on the catalyst surface. The irreversible and reversible species could play different roles in a catalytic reaction. The reversible adsorption species refers to a type of adsorbed species existing on the catalyst surface at reaction temperature when there is its gas phase pressure in the system, and once the pressure is removed by evacuation or purging, it would be completely desorbed, while the irreversible adsorption species cannot be removed from the catalyst surface. Therefore, it is necessary to investigate deeply the roles of the two adsorbed species. This chapter focuses on the roles of irreversible and reversible adsorbed hydrogen in dehydrogenation and reforming reaction over supported Pt catalysts.