Diffusion of Toluene and Ethylene in ZSM-12 zeolite: Experimental and numerical study
Abstract
The Na+ and Ag+ forms of ZSM-12 zeolite microporous were used as effective adsorbents for hydrocarbons trap. Diffusion of a single adsorbed component including ethylene as a light molecule or toluene a heavier one inside a narrow one-dimensional channel was studied. Experimental desorption profiles for these two gases desorbing from both Ag-ZSM-12 and Na-ZSM-12 were obtained at very high heating rates comparable with the one observed during the cold start of a combustion engine. Numerical simulation based on Fick’s law and a transient diffusion boundary value yielded a good fitting of these experimental results. For both ZSM-12 forms considered in this study, diffusion activation energies Eω and EΞ for toluene are much higher than those for ethylene. The values of these parameters are higher for the Ag+ exchanged zeolite than for Na-ZSM-12 and related to the presence of strong Lewis acid sites in the former material. In addition each activation energy for Na-ZSM-12 and Ag-ZSM-12 decreases at increasing heating rate. This was explained by thermal vibrations of the zeolite lattice which allow higher diffusivity at high temperature.
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Introduction
Faced with increasingly strict emission standards, automobile and catalyst manufacturers have investigated a variety of approaches to reduce cold-start emissions. Attention has been focused on the start-up phase, when 70-80% of the hydrocarbons generated (HCs) by the combustion engine are emitted [1]. Cold-start is referring to the short time period of 12 min, after engine ignition and before the three-way catalyst reaches its light-off temperature which is around 200°C. Zeolites have been found to be the preferred molecular sieves and the best candidates for this application due to their stability under a variety of conditions and their affinity to adsorb HCs. In literature, different types of zeolites have been proposed and tested as adsorbents for HC emissions [2]–[11]. It has been reported that ZSM-12, which possesses one dimensional channels with 12-member ring pores, performs better in the envisaged application of hydrocarbon trap compared to more typical threedimensional zeolites [9]–[14]. With the growing use of one dimensional zeolites in separation and reaction processes, the need to better understand the diffusion and transport properties of molecules within the channels of these materials is important not only from a fundamental, but also from a practical point of view. The mechanism used to describe the simultaneous passage of ethylene (light molecules) and toluene (heavy molecules) inside narrow one dimensional channels was designated as the single-file diffusion[15]. This phenomenon has become a topic of current research interest. Heavy aromatic molecules are strongly adsorbed in the zeolite, therefore they need a higher temperature in order to desorb from channels. On the other hand, light hydrocarbon molecules are more weakly adsorbed and easily desorb from channels at lower temperature. Thus in simultaneous diffusion, the desorption of lighter molecules such as ethylene from one dimensional narrow channels may be retarded by co-adsorbed heavier ones such as toluene.
Despite the large number of theoretical studies published on the single-file diffusion over the years, experimental and numerical single component loading (small molecules or aromatic molecules) are still rare. It is important to understand the transport mechanism of these molecules within the pores of zeolites in order to design more efficient applications for these materials. The aim of this paper is to provide a comparison between experimental and simulation results for each single kind of gas after developing a model for diffusion in zeolite pores.
Conclusion
The diffusion of a single adsorbed component, ethylene or toluene, inside the microporous ZSM-12 zeolite was studied experimentally and numerically. The Na+ and Ag+ forms of ZSM-12 zeolite were synthesized and characterized. The thermodesorption process was conducted at three different heating rates (3, 5, and 9 °C/s), selected to be similar to the muffler heating rate during the cold-start process.
Experimentally, it was verified that the detected amounts of both ethylene and toluene were essentially unaffected by the heating rate. In contrast to ethylene adsorbed on Ag-ZSM-12, which desorbed as two distinct peaks, only one desorption peak was observed for toluene. Moreover, the amount of ethylene adsorbed on ZSM-12 was low compared to that of toluene, due to the smaller size of the ethylene molecule relative to the pore diameter of the zeolite adsorbent.
Numerically, by using a transient diffusion boundary value problem coupled with Fick’s law, it was possible to explain all qualitative features of the temperature-dependent desorption profiles observed experimentally. In this model, a coefficient C was included in the diffusion coefficient ω(t) that has units of m2/s. Consequently, the term ρ(t, x), which describes the probability that a molecule occupies a site at a distance x within the zeolite channels at time t, is dimensionless.
For the two solids considered in this study, the activation energies Ew and Eχ for toluene were much higher than those for ethylene, owing to the larger molecular size of toluene. The values of these parameters were also higher for the Ag+-exchanged zeolite than for Na-ZSM-12, due to the presence of strong Lewis acid sites in the former material. In addition, each activation energy for both Na-ZSM-12 and Ag-ZSM-12 decreased with increasing heating rate. This behavior was attributed to lattice thermal vibrations, which allow higher diffusivity at elevated temperatures.