Nanoscale intimacy in bifunctional catalysts for selective conversion of hydrocarbons

摘要

The ability to precisely control nanoscale features is increasingly exploited to develop and improve monofunctional catalysts^–. Striking effects might also be expected in the case of bifunctional catalysts, which play an important role in hydrocracking of fossil and renewable hydrocarbon sources to provide high-quality diesel fuel^–. Such bifunctional hydrocracking catalysts contain metal sites and acid sites, and for more than 50 years the so-called ‘intimacy criterion’ has dictated the maximum distance between the two site types beyond which catalytic activity decreases. The lack of synthesis and material characterization methods with nanometer precision has long prevented in-depth exploration of the criterion, which has often been interpreted simply as ‘the closer the better’ for positioning metal and acid sites^–. Here we show for a bifunctional catalyst, comprised of an intimate mixture of zeolite Y and alumina binder and with platinum (Pt) metal controllably deposited^, on either the zeolite or the binder, that close proximity between metal and zeolite acid sites can be detrimental: the selectivity when cracking large hydrocarbon feedstock molecules for high-quality diesel production is optimized with the catalyst that contains Pt on the binder, i.e. with a larger distance between metal and acid sites. Cracking of the large and complex hydrocarbon molecules typically derived from alternative sources such as gas-to-liquid technology, vegetable oil or algal oil^– should thus benefit especially from bifunctional catalysts that avoid locating Pt on the zeolite as the traditionally assumed optimal location. More generally, we anticipate that the ability to spatially organize different active sites at the nanoscale demonstrated here will benefit the further development and optimization of the newly emerging generation of multifunctional catalysts^–.