An efficient nanodiamond-based monolithic foam catalyst prepared by a facile thermal impregnation strategy for direct dehydrogenation of ethylbenzene to styrene

Abstract

Nanodiamond (ND) has long been recognized as an effective carbocatalyst for synthesizing styrene via direct dehydrogenation (DDH). However, the induced drastic pressure drop of its powder form limits its industrial application in heterogeneous catalytic process. In this work, we report a facile hexamethylenetetramine nitrate (HN)-assisted thermal impregnation (HNTI) strategy for fabricating a novel nanodiamond-based monolithic foam (ND/CNT-SiC-ms-HN) catalyst through a two-step approach: One is to soak the carbon nanotube-modified SiC foam (CNT-SiC) with the slurry composed of HN, KCl, LiCl, and dispersed ND, and the other is to heat the slurry-soaked CNT-SiC (ND-HN-KCl-LiCl/CNT-SiC) in N2 atmosphere at 750 °C. The as-synthesized ND/CNT–SiC-ms-HN monolithic foam features the enriched surface kenotic CO by promoted ND dispersion and O-doping, abundant stuctural defects, and improved nucleophilicity by N-doping, originating from the promoted ND dispersion by thermal impregnation (TI) in KCl-LiCl molten salt (MS) and the presence of HN in the annealing process. As a result, the ND/CNT–SiC-ms-HN monolithic foam catalyst by HNTI strategy exhibits 1.5 folds higher steady-state styrene rate (5.49 mmol g−1 h−1) associated with 98.4% of styrene selectivity compared to the ND-based monolithic foam catalyst (ND/CNT-SiC). Moreover, the ND/CNT–SiC-ms-HN monolithic foam shows excellent long-term stability for the direct dehydrogenation of ethylbenzene to styrene. This work also comes up with a novel way of preparing other highly-dispersed nanocarbons-based monolithic foam catalysts with promising catalytic performance for diverse transformations.