Controllable Fabrication And Catalytic Activity Of B-Oriented HZSM-5Coatings

Structured catalysis structured catalysts and reactors have been becomingsignificant process intensification technologies in gas–solid and gas–liquid–solidreactions due to their unique advantages of low pressure-drop, mass-transfer ratio andhigh thermal conductivity. This paper focuses on the controllable fabrication ofb-oriented HZSM-5zeolite-typed structured catalysts and studies their catalyticperformance. The study includes the effects of Al3+,the inter-surface propertiesbetween crystal nucleus and substrate on the growth of b-oriented HZSM-5monolayer, and the suppression growth of a-oriented twin crystal during thefabrication of b-oriented HZSM-5multilayer. Then further examine of their catalyticperformance using a model reaction of catalytic cracking of n-dodecane (550oC,4Mpa) was carried out to study the effects of orientation on catalytic reaction.The controllable fabrication of b-oriented HZSM-5monolayer is facing a greatchallenge due to that the presence of Al3+in the synthesis mixtures plays severaleffects on zeolite crystallization which makes it more difficult to control the crystalorientation and coating continuity. The effect of Al3+on zeolite nucleation and theinteraction between the crystal nucleus and substrate are two key points affecting thegrowth of b-oriented HZSM-5coating. The former was investigated by changing theAl content and hydrolysis condition and the latter by adjusting the properties of thesubstrate with different oxides sol-gel layer. It was found that high Al content led toinhibition of zeolite crystallization and reduction of nucleus amount, which resulted indecrease of continuity on zeolite coating and Si/Al=100is suitable for b-orientedHZSM-5monolayer. TiO2layer was found to more favorable for controllablefabrication of b-oriented HZSM-5catalytic coating. This TiO2interlayer plays threecrucial roles, including:(1) providing a smooth surface for growing b-orientedHZSM-5crystals.(2) Promoting the attraction forces between the support and zeolitecrystals.(3) Providing a high density of hydroxyl groups on the surface of supports.The quality of the b-oriented HZSM-5monolayer can be improved by adjusting theconcentration and the type of Ti-OH on the TiO2sol-gel layer.Direct growth on b-oriented HZSM-5monolayer gave a bilayer with manyamounts of a-oriented twin crystals on the top layer. Considering the positive effects of Ti-OH in controlling the b-oriented crystal orientation, two methods to introduceTi-OH on HZSM-5monolayer for the preparation of multilayer b-oriented HZSM-5coatings were developed. First, TiO2coating was prepared with sol-gel method usingdifferent mass ratio of ethanol to titanium dioxide solution, and then the b-orientedbilayers were synthesized. However, we found the additional mass-transfer resistancebrought by the TiO2coating when the coating thickness was higher to achieve the fullcoverage and higher orientation. To solve this problem, Ti-OH groups were thenintroduced by tetra n-butyl ortho-titanate (TBOT)-modified HZSM-5monolayer.The performance of b-oriented HZSM-5zeolite coating was tested using thecatalytic cracking of supercritical n-dodecane as a model reaction. The catalyticperformance of the HZSM-5coating was affected by zeolite orientation，zeolite massloading, zeolite layers and the crystal size. It was found that multilayer b-orientedHZSM-5coating with small crystal size exhibited high catalytic reactivity and greatstability.