Rational design and preparation of hierarchical monoliths through 3D printing for syngas methanation

Generally, monoliths have better heat and mass transfer properties and thus are potentially favorable to situations such as highly exothermal reactions. The key advantage of 3D-printed catalysts as compared to the that of the conventionally prepared catalysts is the design of tortuous channels that modulate the transport properties in hierarchical monoliths rather than that of simple, straight or uncontrollable channels. In this study, for the first time, cylinder, tetrahedron, and tetrakaidecahedron periodic structures were modulated via 3D printing and successfully used as a hard template to prepare phenol–formaldehyde-based hierarchical monoliths for CO methanation. The reaction results show that a 3D monolith with a 1.00 mm diameter simple straight channel has a high catalytic performance at similar loadings and compositions of active components. The channel structure can precisely be modulated by 3D printing, and the macro-channels and meso-channels are well connected. More importantly, through modulating the tortuosity of macro-channels, the Ni–Al2O3/C monolith with a 1.25 mm diameter tetrahedral channel shows an excellent CH4 yield as well as a prominent decrease in the temperature gradient and pressure at 24 000 h−1 GHSV as compared to other structures of monolithic catalysts. High mass and heat transport along with reaction activity efficiently facilitate the improvement of catalytic performance. The successful synthesis of these fascinating materials paves the way to explore the application of more complicated hierarchical monoliths for separation and reaction processes particularly for industrial catalyst design.