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3D microscopy-guided assembly of novel hierarchical, multi-pore, multi-function catalysts for clean fuel production

Final Report Summary - 3DMULTICAT (3D microscopy-guided assembly of novel hierarchical, multi-pore, multi-function catalysts for clean fuel production)

Currently, there is a strong reliance on petroleum imports to satisfy Europe´s demand of liquid fuels and platform chemicals. For economic and environmental reasons, a shift in the raw materials base to natural gas and renewable nonedible biomass resources is a priority. Dealing with such non-conventional resources will require a significant decentralization, downscaling and intensification of the chemical processes. A pivotal role in process intensification is reserved for catalysis, i.e. the use of catalysts (to a large majority solid porous materials) to accelerate and steer chemical reactions selectively to the desired product/s. A major emerging intensification strategy proposes the integration of two catalysts within a single reactor, to effect consecutive reactions in a tandem fashion. The "3DMultiCAT" project has concentrated on the generation of fundamental insights, and the development of innovative solid catalysts, for a tandem catalytic process which targets the single-step production of branched hydrocarbons (liquid fuels) from synthesis gas (CO+H2), a gas mixture which can be obtained from natural gas or biomass feedstocks. The process involves the integration of two catalytic reactions in a single reactor, namely the Fischer-Tropsch (FT) synthesis of hydrocarbons and the "in situ" hydroprocessing (isomerization and cracking) of the primary products on a hydrocracking (HC) catalyst.

The major objectives of the project were: (i) to gain fundamental understanding on the catalytic consequences of spatial effects such as relative location and distance between the two integrated catalytic functionalities, at different length scales from the nanometer to the micrometer regimes, and (ii) to achieve a precise diagnosis and control over such spatial features to design advanced solid catalysts capable of overcoming the most important hurdles currently faced by this tandem process, i.e. the difficulties to reconcile a full depletion of heavy wax (solid) products with a limited over-cracking into gaseous hydrocarbons, and the management of the different frequencies of rejuvenation/regeneration treatments required for the two integrated solid catalysts, which deactivate at notably different rates under industrially relevant operation conditions. These objectives have been achieved by a combination of fundamental catalytic studies using model reagent compounds and innovative solid catalyst designs assisted by 3D structural quantification methods at the nano-to-micrometer length scales. Major findings of the project are summarized in the following:

• Unlike under standard hydrocracking conditions (hydrogen atmosphere), the presence of syngas, which mimics the conditions encountered in the tandem process, results in severe poisoning of the metallic (de)hydrogenation function of the hydrocracking catalyst by strong CO adsorption, which causes a notable divergence in the reaction pathway for the two major types of FT primary products: n-paraffins and α-olefins. Our results highlight the significance of the nature of the FT primary products exchanged by the tandem catalysts for the overall process efficiency, thus pointing to it as a major design variable.

• Under relevant reaction conditions, pore mass transport phenomena can notably affect the primary FT product pattern. A research platform has been developed to design supported cobalt-based FT catalysts with bespoke multimodal porosities, which integrates soft- and hard-templating synthesis routes with structural diagnostics obtained, a.o. by tomographic FIB-SEM imaging and quantitative 3D image analysis routines. Research in this area has led to the development of solid catalysts with high specific surface areas, uniformly distributed catalytic metal species and elusive trimodal porosities combining 3 interconnected sets of pores: 1 set of mesopores and two levels of macropores with average diameters in the nanometer and micrometer regimes, respectively. The novel architectures proved efficient to control molecular traffic in the working catalysts and adjust the nature of the primary FT for the tandem catalytic process.

• The novel porosity design provides a lever on the rate of mass transport phenomena, restraining by an order of magnitude the extent of secondary reactions of primary α-olefin FT products before they reach, and react on, the partner HC catalyst. In this project, we have leveraged these findings to disentangle the chemical and spatial (and thermal) “spacings” between the catalysts operating in tandem. Unprecedented efficiencies were unlocked, achieving the sought conciliation of wax hydrocarbons depletion with a 2-fold higher yield to middle distillates (precursors for clean synthetic diesel fuels) and a significantly lower production of the most penalizing gaseous hydrocarbon products.

The results open new perspectives for the design of solid multifunctional catalysts for tandem catalytic processes via a decoupled tailoring of chemical and spatial proximities between integrated solid catalysts. The intensification achieved is expected to improve the efficiency and reduce the environmental impact of processes targeting the valorization of delocalized feedstocks such as associated or shale gas wells and lignocellulosic biomass into fuels and platform chemicals. These insights are of scientific significance in the rapidly emerging field of process intensification by tandem catalysis. The findings have also technological significance, in particular for European companies which focus their business in the field of "compact" gas-to-liquid processes.

In the course of the project, research cooperations have been initiated with research institutions in Germany (Duisburg-Essen University and Max Planck Institut fuer Eisenforschung) and Spain (ICP-CSIC). The results of the project have led to 6 scientific publications (4 already published, 2 currently in preparation) in high-impact scientific journals, and have been presented and discussed at 7 international conferences in the fields of material science, catalysis, microscopy and mathematics.

In summary, the Marie Curie Fellowship has enabled the development of a research platform which brings the design of complex porous materials in the field of catalysis to a next level. The insights generated are relevant to the production of fuels and chemicals in a more sustainable fashion, valorizing resources alternative to petroleum which are currently not exploited or simply wasted (e.g. natural gas flaring). This is a field of strategic significance for the EU, as it relates directly to major societal challenges such as the energy independence, the sustainable exploitation of natural resources and the contribution to environment preservation and climate change mitigation.