Prominent role of mesopore surface area and external acid sites for the synthesis of polyoxymethylene dimethyl ethers on a hierarchical H-ZSM-5 zeolite

Significance 

Advances in applied chemistry have shown that alumino-silicate materials, such as zeolites, are attractive for a wide range of applications ranging from catalytic cracking to water softening. These materials are structured in a way that they provide ordered, uniform channels of molecular dimensions. Unfortunately, these very small channels often induce mass transport limitations, thereby lowering the effective active zeolite volume. As such, their performance as catalysts for reactions involving bulky molecules, particularly in the liquid phase is highly reduced. Global trends towards clean energy sources have inspired the production of polyoxymethylene dimethyl ethers (OME); a promising, new type of biofuel that could replace diesel. However, production of OME industrially is quite challenging due to the current expensive and energy-intensive synthesis routes. Worse off, the reaction mechanism of OME synthesis from trioxane (TRI) and OME1 still remains elusive. As such, the potential applicability of zeolites in the synthesis of OME ought to be investigated.

Recently, researchers led by Prof. Oliver Kröcher from the Swiss Federal Institute of Technology in Lausanne (EPFL) presented a study where they focused on addressing the outstanding questions regarding the synthesis of OME. In particular, their goal was to, first; address the effects of mass-transfer limitation on the performance of the catalyst used, and secondly, investigate on the role prominence of external sites compared to internal sites in the catalytic reaction. All factors considered, their purpose was to investigate on the overall qualitative effect of mass transfer in a H-ZSM-5 zeolite for OME synthesis. Their work is currently published in the research journal, Catalysis Science & Technology.

In brief, two treatments were applied to the parent H-ZSM-5 zeolite. First, introduction of mesoporosity was achieved through desilication by alkaline treatment, leading to a hierarchical zeolite. Second, passivation of external Brønsted acid sites was carried out. The resultant materials were then characterized by various techniques such as elemental analysis using inductively coupled plasma optical emission spectrometry. X-ray diffraction patterns were also recorded, after which magic angle spinning solid-state nuclear magnetic resonance (MAS-NMR) spectra were collected. Lastly, reaction network and kinetic model for OME synthesis were developed.

The authors found out that access to active sites in H-ZSM-5 micropores was of high importance since large OME molecules experienced internal diffusion limitation in the zeolite’s micropores. This was a direct indication that the inner volumes of the untreated zeolite were less accessible compared to hierarchical zeolites that have advantageous diffusion properties due to the presence of mesopores. Additionally, a trade-off was observed between mesopores insertion and acidity; a too intense alkaline treatment was detrimental to Brønsted acidity which is essential for OME synthesis.

Implementation of the OME technology requires the development of efficient catalysts, which necessitate understanding of fundamental aspects, such as how the textural properties of the catalyst can impact its performance” said Prof. Dr. Oliver Kröcher the principal investigator in a statement to Advances in Engineering.

In summary, EPFL scientists demonstrated that controlled insertion of intra-crystalline mesopores in H-ZSM-5 led to the formation of a hierarchical material that exhibited superior catalytic performance for the synthesis of OME. Generally, by optimizing alkaline treatment and consequent acid wash of H-ZSM-5, the researchers were able to achieve a two-fold increase in the initial reaction rate and a 10% increase in selectivity towards OME with 3 to 5 oxymethylene units (OME3–5), which are the more desirable products. Altogether, for one to tailor optimum zeolite catalyst post-treatments and design other similar materials for OME synthesis, it is highly important they consider both the accessibility of active sites and the prominent role that external acid sites play.

Reference

Christophe J. Baranowski, Ali M. Bahmanpour, Florent Héroguel, Jeremy S. Luterbacher and Oliver Kröcher. Prominent role of mesopore surface area and external acid sites for the synthesis of polyoxymethylene dimethyl ethers (OME) on a hierarchical H-ZSM-5 zeolite. Catalysis Science & Technology, Issue 2, 2019.

Go To Catalysis Science & Technology

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