Cavity-controlled diffusion in 8-membered ring molecular sieve catalysts for shape selective strategy

Highlights

• The jump frequency and jump length were determined by a continuous-time random-walk method.

• Diffusion is quantitatively described as the inter-cavity hopping with the jump frequency and jump length.

• Cavity structure and dimension control the diffusion in cavity-type molecular sieves.

• Cavity-controlled diffusion is one of the essential aspects in shape-selectivity of molecular sieves.

Abstract

Shape selectivity is the most important feature and advantage of molecular sieve catalysis. Diffusion of reactant and product molecules inside the confined environments with channels and cavities is the essential aspect of shape selective catalysis of molecular sieve catalysts. Elucidating the diffusion mechanism of molecules in the confined space of catalysts is of great importance for the efficient utilization of the porous materials in the catalytic reaction as well as the strategy proposal of reaction control in shape-selective catalysis. However, it is still a great challenge to understand and quantify the diffusion behavior at the molecular level for the establishment of the diffusion mechanism within the crystal of microporous materials as so far. In this work, with advanced pulsed field gradient (PFG) NMR technique, self-diffusion coefficients of alkanes (methane, ethane and propane) in three cavity-type molecular sieves (LEV, CHA and RHO) with very close eight-membered ring (8-MR) windows were determined. Furthermore, molecular dynamics (MD) simulations predicted the energy barrier for crossing 8-MR to be overcome. Based on MD simulations and a continuous-time random-walk (CTRW) method, not only the diffusion trajectory of the molecule can be visually displayed, but also the diffusion behavior is quantitatively described as the inter-cavity hopping process with the success extraction of the jump frequency (f) and jump length (L). The substantial role played by cavity structure and dimension in diffusion within cavity-type molecular sieves with the close 8-MR windows were revealed in depth, which would help to reveal the mechanism of cavity-controlled diffusion and propose the shape-selective strategy in the methanol-to-olefins process.

Introduction

The well-defined pores, the high crystallinity and the tuneable acidity have stimulated the growing attention in utilizing zeolite and zeotype materials in chemical industry, especially for catalysis and gas separation [1], [2]. For these processes, understanding the confinement effect imposed by zeolite topology on the guest species is the key issue for shape-selectivity of adsorption, diffusion and reaction on the microporous materials, which is also of great significance for understanding the mechanisms of catalytic reactions and separation processes [3], [4], [5].

The significance of the shape-selectivity and the confinement effect in the catalysis of zeolite and zeotype materials has been substantially confirmed in the methanol-to-olefins (MTO) reaction, the most successful catalytic process for the light olefins production via non-petrochemical routes from abundant resources of natural gas or coal [6], [7], [8], [9], [10]. The application of shape-selective catalyst with cavity structure and eight-membered ring (8-MR) channel, such as H-SAPO-34 with CHA topology in the MTO reaction, gives rise to highly-selective production of ethene and propene and also makes H-SAPO-34 an excellent commercial MTO catalyst. Besides H-SAPO-34, other 8-MR and cavity-type molecular sieves have been also proved to be shape-selective catalysts, and interestingly, the unique product selectivity differs with the cavity type of the catalyst [10], [11], [12], [13].

For the MTO reaction conducted over 8-MR and cavity-type catalysts, on one hand, the cavity geometry of these molecular sieves controls the accommodation and stabilization of the confined critical intermediates of the hydrocarbon pool (HP) mechanism [10], [14], [15], [16], [17], [18], [19], [20], which varies the predominant route of olefin products generation during the efficient reaction stage and catalyst deactivation stage [9], [10], [11], [13], [21]. On the other hand, for the heterogeneous reaction catalyzed by aluminosilicates (zeolites) or silicoaluminophosphates (SAPOs), the topology of the catalyst, including the structure and dimension of the channel and cavity, also greatly influences the mass transport of reactants and products inside the catalysts due to that the reactants need to diffuse into the catalysts and contact with active sites and generated hydrocarbons need to diffuse out of the catalysts as effluent products. 8-MR and cavity structure of zeolites or SAPO molecular sieves work as the diffusion path or micro-reactor for mass transport and reaction. The narrow and restricted 8-MR pore openings of cavity-type molecular sieves limit the diffusion of higher hydrocarbon products, leading to extremely high selectivity toward lower olefins in the MTO process. As a result, for the important MTO process catalyzed by zeolites or SAPOs, the issue of guest molecular diffusion within confined surroundings goes beyond the configuration diffusion from the point of mass transfer, and plays a significant role in the achievement of high reaction activity for reactant conversion and selective generation of the products [22], [23], [24], [25], [26]. The strategy proposal for reaction control, the most critical issue of the important catalytic applications, requires the complete comprehension combining the understanding of the reaction and the diffusion of the reactant and product molecules in these catalysts. The detailed study presented the shape selectivity for ethene, propene and butene production can be varied with the usage of 8-MR SAPO materials with different cavity type [10], and revealed a special cavity-controlled intermediates formation and reaction in the cavity-type catalytic environments [9], [10]. The catalysis and shape selectivity have been interpreted successfully based on the intensive efforts on the reaction mechanism [9], [10], while the knowledge concerning the diffusion behavior in the catalysts of the practical industry process have been still highly desirable in this field.

Aiming at the deeper insights of confinement effect imposed by 8-MR and cavity-type catalysts on the diffusion of guest hydrocarbons, three cavity-type molecular sieves with very close 8-MR windows and different cavities (SAPO-35 with LEV, SAPO-34 with CHA and DNL-6 with RHO topologies) are elaborately selected, which are expected to present the diffusion behavior in the cavity-type catalysts. The intracrystalline diffusion of guest molecule within 8-MR and cavity-type molecular sieves has been depicted as an inter-cavity hopping process, which is a rare event that requires overcoming the diffusion restriction to pass through the 8-MR windows as revealed by computational studies [27], [28], [29], [30]. It has been noticed that the narrow 8-MR pore openings of zeolites or SAPOs impose very strong steric hindrance for the inter-cavity hopping [27], [29], [30], [31], while the whole chemical environments for guest molecular diffusion, originated from the cavity structure of the molecular sieve has not been interpreted in depth. Revealing the host-guest interactions along the intracrystalline diffusion path and quantifying the confinement effect of the chemical environments with 8-MR and different cavity structure on diffusion behavior would give deep insights of the diffusion mechanism and help to propose the shape selective strategy for relative catalytic reactions.

In this work, we intensively studied the diffusion of hydrocarbon molecules (methane, ethane and propane as the probe molecules) in the confined surroundings of 8-MR and cavity-type SAPO molecular sieves. The applications of spectroscopic approaches [32], [33] and theoretical simulations [5], [34] for diffusion measurements enable fundamental studies of mass transport in confined environments and give more useful insights into the effect of zeolite topology on diffusion from a microscopic point of view. Pulsed field gradient (PFG) NMR, a well-suited tool for probing the diffusion in porous media [32], [35], [36], and molecular dynamics (MD) used for the microscopic description of diffusion mechanism inside microporous materials [5], [24], [37], have been employed to reveal that how the cavity structure controls the molecular diffusion through almost identical 8-MR windows. Complementary and reciprocal results based on the applications of the powerful tools present a very special cavity-controlled diffusion in the cavity-type molecular sieves with 8-MR windows, with the correlation of the diffusion of guest molecules to established cavity-controlled intermediates formation and reaction during the MTO process [9], [10], an overall cavity-controlled shape selective reaction system could be constructed (Scheme 1). On one hand, cavity structure controls the accommodation and stabilization of the confined critical intermediates (see Scheme 1b) and the reaction routes of olefins formation [9], [10]. Larger cavity structure (CHA and RHO) favors the formation of bulky-sized heptmethylbenzenium and pentamethylcyclopenyl cations as the most important intermediates, while the spatial restriction imposed by the smaller cavity (LEV) contributes to the generation of small-sized intermediates (tetramethylbenzenium and trimethylcyclopentadienyl cations) [9], [10]. On the other hand, cavity structure and dimension control the molecular diffusion, especially the reactant diffusion, in turn could significantly influence the reactivity of hydrocarbons (see Scheme 1a). RHO with large-sized lta cavity greatly benefits the mass transport, while LEV with small cavity structure imposes stronger diffusion restriction for hydrocarbons. Therefore, both the confinement effect and diffusional restriction imposed by cavity structure contribute to the generation of desirable products, resulting in different MTO activity and product selectivity (see Scheme 1c) [10]. RHO exhibits the highest activity and butene is the predominant product, while LEV presents the lowest methanol conversion and ethene is the main product. Ethene and propene are formed as the main products in CHA. The cavity-controlled diffusion, critical intermediates formation and product generation explicitly present shape selectivity in the methanol-to-olefins process by the usage of different 8-MR and cavity-type catalysts based on host-guest interactions. The complete comprehension of the reaction mechanism and the diffusion of the reactant and product molecules in these catalysts would guide the proposal of reaction control for shape-selective catalysis.