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Mordenite (MOR-type zeolite) is a widely used catalyst, in particular for (hydro-) isomerization and alkylation reactions in the petrochemical industry. However, having a one-dimensional micropore system, this material is susceptible to diffusion limitations and deactivation. To circumvent this problem, typically additional (meso)porosity is created by applying dealumination and/or steaming processes. The detailed description of the dealumination process is of crucial importance to understand how mordenite can be modified into an efficient catalyst. In this work, we present for the first time a simulation model to describe the influence of the dealumination process on the structural properties of mordenite. Using kinetic Monte Carlo simulations, dealumination is described as a multiple-step process consisting of the removal of the framework Al as well as the self-healing of silanol nests by Si atoms. The simulation results are in very good agreement with experimental results from 29Si NMR, XRD, and N2 and Ar physisorption. In particular, the simulations confirm the enlargement of the micropores and the creation of mesopores during dealumination.

Several commercially available samples consisting of agglomerated small mordenite crystallites with low Si/Al ratios (5.7–10 at/at) have been treated in aqueous NaOH solution. It was found that the porosity can be enhanced when the sodium hydroxide solution is sufficiently concentrated. Treatment in 1 M NaOH for 15 min resulted in inter-crystalline porosity and the mesopore volume was increased from 0.01 to 0.21 cm3 g−1 together with an increased external surface area from 36 to 85 m2 g−1. The micropore volume and crystallinity were preserved after the treatment. Both H-MOR and Na-MOR mordenite agglomerates have been successfully treated: the Na-MOR requires a longer contact time to obtain similar porosity. By carefully choosing the alkaline concentration and contact time, intra-crystalline mesoporosity can be obtained for mordenite with Si/Al ratios as low as 10. Catalytic tests with proton mordenite showed that alkaline treatment leads to more than one order of magnitude of activity gain in the liquid-phase alkylation of benzene with propene to form cumene, while selectivity is preserved. These results demonstrate that alkaline treatment also on high-aluminum content mordenites is an effective tool to enhance accessibility and thereby its catalytic performance.

Zeolites and related zeolite-analog materials have found major industrial applications as catalysts and adsorbents, especially in refinery and petrochemical processes where zeolite catalysts exhibit unique shape selectivity endowed by their crystalline yet highly porous structures with well-defined channels of molecular dimensions [1–5]. However, as one side effect of their microporous structure, zeolite catalysts often suffer from restricted diffusion of guest species [6, 7]. Mass transport to and from the active sites inside the micropores (known as configurational diffusion) is much slower than that of molecular and Knudsen diffusion, which has led to lower catalyst utilization, and sometimes fast deactivation due to coke formation. Different approaches have been proposed to alleviate the diffusion limitation and enhance the accessibility of the internal sites [8–21]. One strategy is to synthesize novel zeolitic structures with a larger pore size. Various large-pore zeolites and zeolite analogs have been obtained, for example, VPI-5 [22], UTD-1 [23], SSZ-53 and SSZ-59 [24], ITQ-15 [25], ITQ-21 [26], ITQ-33 [27], and ITQ-37 [28]. However, despite the fact that a considerable amount of knowledge has been gained on the formation mechanisms of zeolites and numerous types of theoretical structures have been proposed for predicative synthesis, synthesis by design of novel zeolite structure is still a challenging topic, and most new structures are discovered by trial-and-error processes.

The main goal of the work described in this thesis is to obtain (meso-) porous zeolites by post synthesis treatment for improved catalytic performance, with a focus on mordenite and cumene synthesis. The zeolite samples consist of small crystallites in the range between 20 and 200 nm that have agglomerated into larger particles of ~0.5 to 2.0 m. It is shown that alkaline or the combination of acid and alkaline treatment results in porous mordenite, with intra- as well as inter-crystalline mesopores present. Electron tomography reveals that mostly intra-crystalline mesopores are formed that are in contact with the external surface, thereby providing fast mass transfer to and from the active sites. Catalytic tests for the production of cumene shows that the activities of these porous mordenites are close to the activity of a commercial catalyst based on zeolite Beta, with reduced selectivity towards the formation of undesirable n-propylbenzene. Alkaline treatment on various template containing zeolites, resulted in purely intercrystalline mesopores and increased external surface areas, independent of the Si/Al ratio. The added porosity was obtained without changing the Brnsted acid sites because of the protection of the template molecules. This enables us to decouple acidity and accessibility of zeolites.

Alkaline treatment of templated Na-ZSM-5 zeolites with NaOH solutions resulted in the creation of intercrystalline mesopores within the zeolite agglomerates, while preserving the micropore volume and crystallinity. It was found that the zeolite external surface area increased with increasing NaOH concentration and pretreatment time. These alkaline-treated samples were loaded with cobalt and a linear relationship between the number of cobalt oxidic species and the external surface area of the zeolite could be established. This in turn leads to a linear relationship between the ZSM-5 surface area and the amount of methanol produced over Co-ZSM-5 from methane and oxygen at 423 K. Attempts to remove extra framework alumina species by an acid treatment were successful. However, this acid treatment increased, after Co-deposition, the amount of highly dispersed Co2+ inside the ZSM-5 channels, which resulted in a higher selectivity towards formaldehyde. The nature of the charge-compensating cation also determined the cobalt speciation. Co-H-ZSM-5 samples contained more cobalt inside the channels and were more selective towards formaldehyde formation than Co-Na-ZSM-5 samples. (C) 2010 Elsevier Inc. All rights reserved.

Alkaline treatment (desilication) is an effective treatment to increase mesoporosity. However, the concomitant decrease in Si/Al ratio affects the strengths of the acidic sites and hence catalytic activity and selectivity. Therefore instead we subjected template containing zeolites to 1 M NaOH to induce additional porosity. All zeolites tested (ZSM-5, ZSM-12 and Beta) consisted of small crystallites (30-200 nm) that were agglomerated into larger particles between 1 and 5 μm. Mesopore formation occurred by slowly removing outer layers of individual crystallites, thereby preserving the Si/Al ratio as well as the crystallinity. Inter-crystalline mesopores were formed for all zeolites, but the treatment was most effective for zeolites with small crystallites. The external surface area of ZSM-5 was increased from 90 to 149 m2 g―1 and for zeolite Beta from 70 to 158 m2 g―1. The catalytic performance was tested at 413 K for 3 h for etherification of 1,2-propylene glycol with 1-octene to form octyl-ether. For ZSM-5, conversion increased from 1.2% to 5.6% upon alkaline treatment in the presence of a template, while only to 5% for the template-free treated sample with a significantly higher external surface area (205 m2 g―1). The parent zeolite Beta was significantly more active (30% conversion, 88% selectivity), which can be ascribed to its larger pore size. Nevertheless also in this case alkaline treatment in the presence of the template significantly increased the activity to 40% conversion with similar selectivity. As only the mesoporosity was changed upon alkaline treatment it suggests that the etherification of 1,2-propylene glycol with 1-octene is affected by intra-crystalline diffusion. Our work illustrates the possibilities to use alkaline treatment of templated zeolites to decouple accessibility changes from acidity, and to gain further insight in zeolite catalysis.

Two commercially available mordenites, obtained from Zeolyst (Si/Al = 10 at/at) and BASF (Si/Al = 8 at/at), were subjected to post-synthesis treatments. The impact of acid treatment, alkaline treatment (desilication) and a combination of both on porosity, crystallinity and catalysis were studied in detail. It was found that sequential acid and alkaline treatments were most effective to obtain mesoporous mordenite with external surface areas up to 250 m2 g−1. Electron tomography was used to visualize the mesoporosity of a series of sequential acid- and alkaline-treated mordenite samples. Mesopore formation started close to the external surface area and progressed toward the center of the crystallites for higher porosities. Liquid-phase alkylation of benzene with propylene to cumene was chosen to study the catalytic performance of the enhanced accessibility of various mordenite samples. The activity of the most porous mordenite was found to be close to that of a commercial zeolite beta, while selectivity toward the undesired n-propylbenzene was found to be significantly lower for mordenite (∼70 ppm) than for zeolite beta (∼175 ppm). These catalytic data indicate that the acid- plus alkaline-treated mordenite could be a viable catalyst in the cumene process.