Cascade Conversion of Biomass Platform Chemicals with Multifunctional Zeolitic Materials

Abstract

With supply beginning to outweigh demand, bioethanol is predicted to become a major platform chemical within the coming decades. Zeolites are speculated to be robust, versatile and efficient catalysts for the conversion of biomass derived platform molecules through cascade reactions. This project aims to understand and improve multifunctional zeolite catalysts for cascade conversions by exploring the ability to control the nature and location of disparate catalytic sites.

In Chapter 4, zinc oxide supported on alkali cation-exchanged mordenite (ZnO/M–MOR) prepared by a simple wetness impregnation method, is shown to be a selective and stable catalyst for the direct dehydrogenation of ethanol to acetaldehyde at 400 °C under continuous flow conditions. Through variation of the ZnO loading and the zeolite counter-cation (Na, K, Rb, Cs), an optimum catalyst material was identified, ZnO/Rb–MOR loaded at 3.5 wt% Zn. Detailed analysis of the optimized system reveals excellent selectivity and stability beyond 120 h time on stream, resulting in an average acetaldehyde productivity of 16 mmol gcat−1 h−1 and overall acetaldehyde selectivity of 90% whilst operating at an ethanol conversion level of 40 %.

In Chapter 5, the synthesis and characterization of a series of heteroatomically substituted M4+–MFI type materials (where M = Si, Sn, Ti, Zr or Hf) alongside their catalytic activity for the transformation of ethanol to 1,3–butadiene with and without ZnO doping is reported. The optimum material tested, ZnO/Zr–MFI, produced 3.7 mmol gcat−1 h−1 of 1,3–butadiene at 80% ethanol conversion at the onset of reaction. A long-term stability test revealed that a significant change in product distribution from 1,3–butadiene to acetaldehyde is observed at increasing time on stream, suggesting deactivation of tetrahedral Zr centres. This deactivation is investigated and found to be largely resultant from catalyst coking and it was proven possible for the catalyst to be regenerated by thermal treatment. Additionally, optimization of ZnO/Zr–MFI by variation of hydrothermal synthesis conditions was able to improve 1,3–butadiene productivity to 4.5 mmol gcat−1 h−1.

In Chapter 6, the synthesis and characterization of heteroatomically substituted M2+–MFI type materials (where M = Mg, Zn) alongside their catalytic activity for the transformation of ethanol to 1,3-butadiene with and without ZnO doping is reported. Zn–MFI is found to be able to catalyse the complete cascade reaction without additional doping of ZnO, achieving a 10% selectivity to 1,3–butadiene at 50% ethanol conversion. It is predicted that framework Zn–O bonds in open tetrahedral sites can mimic the ethanol dehydrogenation activity of bulk ZnO. Additional doping of ZnO onto Zn–MFI is shown to increase 1,3–butadiene selectivity to 17% at 70% ethanol conversion. Doping of excess ZnO as a dedicated dehydrogenation site is also seen to reduce ethylene selectivity by up to 50%. This work lays the foundations for further investigation into the Zn–MFI system for ethanol conversion by hydrothermal synthesis variations and extended optimisation.

In Chapter 7, an attempt to marry the beneficial features of the MOR framework with Lewis acidic framework substitution is undertaken with a target of direct production of 1,3–butadiene from ethanol. Further, the effect of the tetrahedral position in which the framework is substituted with a Lewis acidic metal centre is explored. Analysis of MOR materials that were dealuminated to various extents by 133Cs NMR spectroscopy would appear to demonstrate that aluminium in the T1 position is first to be removed followed by all other positions in equal proportions. Catalytic testing of the dealuminated materials in ZnO/Rb–deAl–MOR form for reaction of ethanol showed increased Brønsted acidic activity, consistent with the presence of acidic silanol nests resulting from dealumination. Sn atoms were successfully grafted into the newly formed silanol nests by reaction with SnCl4 as evidenced by solid-state 119Sn NMR spectroscopy. Catalytic testing of ZnO/Rb–SnAl–MOR materials revealed retention of Brønsted acidic activity but achieved no notable productivity of 1,3–butadiene from ethanol.