Catalytic cracking is an important process used in petroleum refineries to convert the heavy hydrocarbons in the crude oils into more valuable products including gasoline, olefins and other chemicals. The process mainly involves the breakdown of long carbon chains into simpler organics. Being more energy efficient and environmentally friendly, catalytic cracking is gradually substituting the conventional thermal cracking process. As the most significant products, light olefins are indispensable industrial raw materials in the production of polymers and alkylbenzenes. The most broadly used catalysts for catalytic cracking in industry is zeolite due to its acidic nature and other advantages like low cost, wide variety, and high mechanical strength.
Owing to its intrinsic micropores, zeolite catalyst suffers from slow diffusion and restricted mass transfer of the reactants and products, which greatly affect their activity, selectivity and lifetime. Consequently, much has been published regarding zeolites with the focus mainly being how to better improve their performances in heterogeneous reactions. With recent developments of 3D printing technology, the possibility of fabricating monolithic catalyst with desirable structures and porosities has become a reality.
3D printing manufacturing has attracted growing interests for material synthesis applied in various fields because of its rapid accomplishment, cost effectiveness, approach facilities and structure controllability. In this context, a group of researchers from the Missouri University of Science and Technology: Dr. Xin Li, Dr. Wenbin Li, Professor Fateme Rezaei and led by Professor Ali Rownaghi developed a facile and efficient method for the fabrication of 3D-printed HZSM-5 and HY monoliths with macro-meso-micro-porosity as the heterogeneous catalysts for n-hexane cracking reaction. For the first time, the group employed 3D printing technique to fabricate structured zeolite catalysts and applied them to the heterogeneous reactions. Their interest in the subject matter was motivated by the unexploited advantages offered by 3D-printed structures that could be fine-tuned to improve the catalytic cracking process. Their work is currently published in Chemical Engineering Journal.
The authors reported that the 3D-printed HZSM-5 zeolite monolith exhibited more stable activity in n-hexane cracking and higher selectivity towards light olefins than its powder counterpart. It is due to the hierarchical pores, which improved the diffusion, and consequently reduced the coke formation within the pores of the catalyst. The monolith catalyst retained 99% of the activity within the long-term time on stream, whereas the powder catalyst dropped below 95%. In the investigated reaction conditions, the selectivity towards light olefins over HZSM-5 zeolite monolith could achieve as high as 53.0%, while over HY zeolite monolith was 57.9%, much higher than previous reports.
To improve the catalytic performance, the researchers coated the monoliths surface with a layer of SAPO-34 (CHA framework). As additional active component and molecular sieve, the SAPO-34 layer’s synergetic effects on the 3D-printed monoliths varied with reaction temperature and zeolite type. The presence of SAPO-34 on the HZSM-5 zeolite monolith increased the light olefin selectivity and decreased BTX selectivity at 600 °C whereas a reverse trend was observed at 650 °C. On HY zeolite monoliths, SAPO-34 growth dramatically increased the aromatics selectivity compared to the uncoated sample. The highest aromatics selectivity reached up to 27.5%
In summary, Xin Li and colleagues presented a pioneering, feasible and advantageous approach to fabricate monolithic zeolite catalysts with tunable structure and porosity using 3D printing technology. Remarkably, the novel material exhibited improved performance in catalytic cracking of n-cracking, a model compound for heavy crude oil, especially in the production of valuable light olefins. The lifetime of the catalyst was dramatically extended and the product distribution was demonstrated controllable if zeolite type, reaction condition and surface coating were properly selected.
Follow-ups: Subsequent research works by the same group on the effects of metal dopants on the 3D-printed zeolite catalysts, application of the 3D-printed zeolite monoliths in methanol-to-olefin (MTO) process, and investigation of the reaction mechanism over the 3D-printed zeolite catalysts are also recently published, as listed in the references.
Xin Li, Wenbin Li, Fateme Rezaei, Ali Rownaghi. Catalytic cracking of n-hexane for producing light olefins on 3D-printed monoliths of MFI and FAU zeolites. Chemical Engineering Journal, volume 333 (2018) page 545–553.Go To Chemical Engineering Journal
Xin Li, Abdo-Alslam Alwakwak, Fateme Rezaei, Ali Rownaghi. Synthesis of Cr, Cu, Ni, and Y-doped 3D-printed ZSM-5 monoliths and their catalytic performance for n-hexane Cracking. ACS Applied Energy Materials, volume 1 (2018) issue 6 page 2740-2748.Go To ACS Applied Energy Materials
Xin Li, Fateme Rezaei, Ali Rownaghi. Methanol-to-olefin conversion on 3D-printed ZSM-5 monolith catalysts: Effects of metal doping, mesoporosity and acid strength. Microporous and Mesoporous Materials, volume 276 (2019) page 1-12.Go To Microporous and Mesoporous Materials
Xin Li, Fateme Rezaei, Ali Rownaghi. 3D-printed zeolite monoliths with hierarchical porosity for selective methanol to light olefin reaction. Reaction Chemistry & Engineering, volume 3 (2018) issue 5 page 733-746.Go To Reaction Chemistry & Engineering