Scalable top-down synthesis of hierarchical zeolites in biomass conversion

Zeolites are used industrially on a large scale in adsorption, ion exchange and catalysis, based on a plethora of advantageous physicochemical properties, which are combined with a relatively straightforward synthesis using mostly low-value commodity chemicals. In catalysis however, their activity is often hampered by the high amount of micropores present, causing mass diffusion limitations or even accessibility problems. In order to overcome these shortcomings, an additional interconnected mesopore network needs to be introduced. These hierarchical zeolites have already been synthesized in many ways. Most of these synthesis strategies are however not feasible on an industrial scale, due to the high amount of unit operations, long synthesis times, high organics consumption and/or need of costly raw materials. One synthesis strategy that is showing potential on an industrial point of view is the alkaline treatment of commercially available zeolites, due to the simplicity and versatility of this method. However, some alkaline treatments still result in severe amorphisation of the zeolite, difficult filtration after treatment and/or have a high organics consumption.

In this thesis, a new variation of the alkaline treatment is proposed. Herein, an alkaline solution is added to a zeolite suspension rather than the other way around, the latter being the standard alkaline treatment up until now. When the alkaline solution is gradually added to the suspension over the treatment time, the pH of the solution is kept low, since OH−-ions are being consumed with the occurring desilication reaction. This lower pH makes the gradual treatment less severe compared to the standard alkaline treatment, and is probably the reason why a reduction in amorphisation is seen, which lowers the need for a high amount of organic additives. Furthermore, the gradual treatment is demonstrated to universally reduce the filtration problem originating after a standard alkaline treatment on commercial zeolites and zeotypes. This is attributed to the prevention of fragmentation, which seems to be induced by a high pH. In addition, it is shown that an increase of the scale of the alkaline treatment with the solid to liquid ratio has a lesser impact on the porous properties with the gradual synthesis strategy compared to the standard method. This is an interesting result, since the solid to liquid ratio is related to the productivity of reactors dedicated to zeolite modification. The prospect of working at a higher solid to liquid ratio is therefore wanted on an industrial point of view. Furthermore, the gradual synthesis strategy enabled the synthesis of a hierarchical USY with an unprecedented combination of high mesoporosity and Brønsted acidity. These unique characteristics have shown their worth in the isomerisation of alpha-pinene, with an extraordinary high activity. This interesting reaction produces a series of useful products that can be used as flavours and fragrances, in the pharmaceutical industry or as polymer additives. This is an important result in the pursuit to sustainable chemistry, since alpha-pinene is derived from woody biomass and can therefore be regarded as a renewable feedstock.

In yet another variation of the alkaline treatment, the filtration set-up is being refurbished into the reactor performing the post-synthetic modification itself. The treatment solution is then brought into contact with a fixed zeolite bed. In this way, a USY zeolite with a Si/Al-ratio of 30 is successfully modified via a treatment with diethylamine. Doing so, one unit operation is saved out, as previously the treatment and separation were done in separate set-ups. As this is particularly useful in sequential treatments, the hierarchisation of zeolite Y is tested, which needs a pretreatment with an acid before the modification with a base. Also in this case the fixed bed post-synthetic modification is successful. These results show the potential of this alternative synthesis strategy, which still has a lot of possibilities for optimisation and future research.