Heterogeneous Metal-Free Hydrogenation over Defect-Laden Hexagonal Boron Nitride

Catalytic
hydrogenation is an important process used for the production
of everything from foods to fuels. Current heterogeneous implementations
of this process utilize metals as the active species. Until recently,
catalytic heterogeneous hydrogenation over a metal-free solid was
unknown; implementation of such a system would eliminate the health,
environmental, and economic concerns associated with metal-based catalysts.
Here, we report
good hydrogenation rates and yields for a metal-free heterogeneous
hydrogenation catalyst as well as its unique hydrogenation mechanism.
Catalytic hydrogenation of olefins was achieved over defect-laden h-BN (dh-BN) in a reactor designed to maximize
the defects in h-BN sheets. Good yields (>90%)
and
turnover frequencies (6 × 10–5–4 ×
10–3) were obtained for the hydrogenation of propene,
cyclohexene, 1,1-diphenylethene, (E)- and (Z)-1,2-diphenylethene, octadecene, and benzylideneacetophenone.
Temperature-programmed desorption of ethene over processed h-BN indicates the formation of a highly defective structure.
Solid-state NMR (SSNMR) measurements of dh-BN with
high and low propene surface coverages show four different binding
modes. The introduction of defects into h-BN creates
regions of electronic deficiency and excess. Density functional theory
calculations show that both the alkene and hydrogen-bond order are
reduced over four specific defects: boron substitution for nitrogen
(BN), vacancies (VB and VN), and
Stone–Wales defects. SSNMR and binding-energy calculations
show that VN are most likely the catalytically active sites.
This work shows that catalytic sites can be introduced into a material
previously thought to be catalytically inactive through the production
of defects.

催化加氢是一类至关重要的工业过程，广泛应用于食品、燃料等诸多品类的生产。当前该过程的多相催化实施路径均以金属作为活性组分。直至近年，无金属固体表面的多相催化加氢反应仍未被发现；若能实现这类催化体系，便可彻底消除金属基催化剂带来的健康、环境与经济隐患。
本文报道了一款无金属多相加氢催化剂的优异加氢速率与收率，及其独特的加氢反应机制。
本研究通过专为最大化六方氮化硼片层缺陷密度而设计的反应器，在富含缺陷的六方氮化硼（defect-laden h-BN，dh-BN）表面实现了烯烃的催化加氢。针对丙烯、环己烯、1,1-二苯乙烯、(E)-与(Z)-1,2-二苯乙烯、十八烯以及亚苄基苯乙酮的加氢反应，该催化剂均实现了高于90%的收率，周转频率介于6×10^-5至4×10^-3之间。
对经处理的六方氮化硼开展的乙烯程序升温脱附实验表明，其表面形成了高度缺陷化的结构。
针对不同丙烯表面覆盖度的富含缺陷六方氮化硼开展的固体核磁共振（Solid-state NMR，SSNMR）表征结果显示，其表面存在四种不同的烯烃结合模式。
在六方氮化硼中引入缺陷，可形成电子亏缺与电子富余的区域。密度泛函理论（Density functional theory，DFT）计算结果表明，在四种特定缺陷——硼取代氮缺陷（BN）、硼空位（VB）与氮空位（VN）以及斯通-威尔士缺陷——表面，烯烃与氢的键级均发生了降低。
固体核磁共振表征与结合能计算结果均表明，氮空位（VN）极有可能是该催化反应的活性位点。
本研究证实，可通过引入缺陷的方式，在以往被认为不具备催化活性的材料中构建催化位点。