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
(B
N
), vacancies (V
B
and V
N
), and
Stone–Wales defects. SSNMR and binding-energy calculations
show that V
N
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.
Mechanochemical approaches to chemical synthesis offer the promise of improved yields, new reaction pathways and greener syntheses. Scaling these syntheses is a crucial step toward realizing a commercially viable process. Although much work has been performed on laboratory-scale investigations little has been done to move these approaches toward industrially relevant scales. Moving reactions from shaker-type mills and planetary-type mills to scalable solutions can present a challenge. We have investigated scalability through discrete element models, thermal monitoring and reactor design. We have found that impact forces and macroscopic mixing are important factors in implementing a truly scalable process. These observations have allowed us to scale reactions from a few grams to several hundred grams and we have successfully implemented scalable solutions for the mechanocatalytic conversion of cellulose to value-added compounds and the synthesis of edge functionalized graphene.
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