ABSTRACT
In this work we investigate the synthesis of alanine from lactic acid, a biobased platform chemical, using ammonia as a nitrogen source and Ru/zeolite catalysts. We report a high alanine selectivity when using Ru/BEA of 80-93%. Reaction side products were identified as ethanol, propionic acid or propanamide and the reaction mechanism was investigated. We further optimised reaction conditions resulting in turn over numbers five times higher than previously reported and could reduce Ru leaching by 30-40%. However, leaching and catalyst stability remains a concern. Furthermore, we critically analyse the benefits of Ru/zeolites versus their stability under the basic, high temperature reaction conditions.
Subject(s)
Zeolites , Alanine , Amination , Catalysis , Lactic AcidABSTRACT
Interzeolite conversion, a synthesis technique for several zeolite frameworks, has recently yielded a large amount of high-performing catalytic zeolites. Yet, the mechanisms behind the success of interzeolite conversion remain unknown. Conventionally, small oligomers with structural similarity between the parent and daughter zeolites have been proposed, despite the fact these have never been observed experimentally. Moreover, recent synthesis examples contradict the theory that structural similarity between the parent and daughter zeolites enhances interzeolite conversion. In this perspective it is proposed that heteroatoms, such as aluminium, are key players in the processes that determine the successful conversion of the parent zeolite. The role of Al during parent dissolution, and all consecutive stages of crystallization, are discussed by revising a vast body of literature. By better understanding the role of Al during interzeolite conversions, it is possible to elucidate some generic features and to propose some synthetic guidelines for making advantageous catalytic zeolites. The latter analysis was also expanded to the interconversion of zeotype materials where heteroatoms such as tin are present.
ABSTRACT
The first selective oxidation of methane to methanol is reported herein for zinc-exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM-5 both form zinc formate and methanol. Selective methane activation to form [Zn-CH3 ]+ species was confirmed by 13 C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM-5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the 13 C MAS NMR spectrum, resulting from two distinct [Zn-CH3 ]+ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn-CH3 ]+ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure-function correlations for methane conversion over zinc-exchanged zeolites.
ABSTRACT
The direct conversion of methane to methanol has been an active area of research for over a century, though a viable industrial process is yet to be realised. However, in the last three decades substantial progress has been made in the field through homogeneous and heterogeneous approaches. This perspective article explores the latest advances in the field of direct methane to methanol conversion by zeolites containing extra-framework d-block metals, focussing on first row, d-block metals. The article highlights the similarities and differences in the nature and formation of the active site, the mechanism of methane activation as well as mode of functionalisation, and where appropriate draws on understanding gained from theoretical studies. From the insight obtained into the different roles of the extra-framework metal and zeolite framework we propose new areas of research which the authors believe will be of benefit to the field.
