L. Huang et al.
Molecular Catalysis 506 (2021) 111535
selectivity is often insufficient, typically with severe selectivity drops
observed for terminal alkynes at high conversions or after full conver-
sion. In particular, with respect to the Lindlar catalyst, the use of toxic Pb
also poses serious problems as its leaching contaminates the alkene
products.
Experimental section
Materials
Na2PdCl4 was obtained Shanghai Tuosi Chemical Co. Ltd. (China). A
Pd@CaCO3 catalyst, with Pd nanoparticles (5 wt%) supported on
CaCO3, was synthesized according to a literature procedure [17]. Two
optimized metallic sulfide-based solid-phase ligands (SPL8-4 and SPL8-6
Another strategy involves the modification of Pd nanocatalysts with
soluble organic modifiers/ligands containing coordinating atoms (N, S,
or P). Organic ligands can reversibly bind to the Pd active sites, modify
the electronic state of active sites, and alter the relative adsorption
strength of alkyne/alkene. With the binding ability of ligands often in-
termediate between alkyne and alkene (weaker than alkyne but stronger
than alkene), their binding can effectively block alkene adsorption, thus
suppressing over-hydrogenation [1,2]. Many organic ligands in both
small molecules [4,14–22] and macromolecules [20,23] have been re-
ported to effectively improve the alkene selectivity. However, their use
poses serious issues for practical applications. Used often at excessive
quantities [4], the soluble organic ligands are undesired contaminants to
the alkene products; their thorough removal from the products adds
significant extra costs. Meanwhile, permanent modification of the Pd
catalysts with these ligands is impossible due to their reversible binding.
As such, re-modification of the recycled catalysts with a fresh dosage of
ligands is needed for replenishment in a following cycle of reaction,
which is inconvenient for practical applications. To address the issues
with soluble ligands, one alternative approach is to covalently immo-
bilize the organic ligand on an inorganic support, which requires so-
phisticated synthesis [24,25]. For example, silica with covalently
tethered polyethyleneimine (PEI) has been used to support Pd nano-
catalysts, where PEI serves as immobilized ligand for the Pd active sites
to suppress over-hydrogenation [25]. Though effective in the
semi-hydrogenation of diphenylacetylene as an internal alkyne, its
performance towards phenylacetylene as a terminal alkyne is still rather
restricted with serious over-hydrogenation observed even well before
the full conversion of the substrate.
with the average particle size of around 5 and 17 μm, respectively) were
provided by Zhejiang Superior Technology Corporation (China). Two
commercial Lindlar catalysts (LC1 and LC2, both with 5 wt% of Pd on
CaCO3, poisoned with lead at different contents) were purchased from
Shaanxi Rock New Materials Co. Ltd. (China) and Aladdin Co. Ltd.
(China), respectively. All alkyne substrates, including 2-methyl-3-buty-
n-2-ol (98 %), phenylacetylene (97 %), 4-ethynylbenzonitrile (≥99.7
%), 1-ethynylcyclohexene (>98 %), 1-heptyne (97 %), diphenylacety-
lene (99 %), 1-phenyl-1-propyne(>98 %), 4-octyne (99 %), and 1,
4-butynl-diol (98 %), were obtained from Aladdin Co. Ltd. (China)
and were used directly. Other regents, including methanol, n-hexane
(anhydrous, 99 %), tetrahydrofuran (THF) and ethanol (anhydrous,
≥99.7 %) were purchased from Sinopharm Chemical Reagent Co. Ltd.
and were used as received.
General procedure of alkyne hydrogenation
Liquid phase alkyne hydrogenation reactions were conducted in a
rubber septum-sealed 50/100 ml flask immersed in a water bath main-
tained at prescribed temperatures. The flask, containing prescribed
amounts of Pd catalyst (Pd@CaCO3 or Lindlar catalysts), solid-phase
ligand, alkyne substrate, and solvent, was firstly evacuated under vac-
uum and then filled with H2 to start the hydrogenation reaction. A
hydrogen balloon was attached to the reaction flask to maintain the
hydrogen atmosphere and vigorous stirring was applied to avoid mass
transfer limitations. To monitor the reaction kinetics, the reaction so-
lution was withdrawn at prescribed times. The sampled solution was
Tackling the issues with existing organic ligands, a solid-phase ligand
technology based on inexpensive inorganic metal sulfides has recently
been developed and commercialized by Zhejiang Superior Technology
Corporation (China) for the highly selective liquid-phase alkene semi-
hydrogenation [26]. Under common reaction conditions for
liquid-phase semi-hydrogenation, the solid phase ligands, as stable
inorganic powders, are insoluble or have only extremely low solubility
in the organic reaction systems, but highly efficient in markedly
enhancing alkene selectivity at high alkyne conversions by suppressing
over-hydrogenation. With the unique solid-phase feature, the
solid-phase ligand, together with the heterogeneous Pd nanocatalyst,
can be conveniently separated from the reaction solution by simple fil-
tration/sedimentation/centrifugation. This advantageously facilitates
easy yet efficient recycling and reuse of the catalyst/ligand system
without contamination of the reaction products by ligands. To verify the
outstanding performance of the solid-phase ligand technology, we
report herein a systematic study on the semi-hydrogenation of a broad
range of alkyne substrates, including both internal and terminal alkynes,
with the use of a supported Pd nanocatalyst (Pd@CaCO3) modified with
the proprietary solid-phase ligands. The reusability of the catalys-
t/ligand system has also been investigated over 10 cycles of recycle/r-
euse. The underlying mechanism of the solid-phase ligands in enhancing
alkene selectivity has also been studied. This work confirms the strong
promise of this solid-phase ligand technology in industrial applications
for reusable, highly active, highly selective liquid-phase semi--
hydrogenation of alkynes.
diluted with methanol, filtered through a syringe filter (0.22 μm PTFE),
and then analyzed with gas chromatography (GC). GC analysis was
performed on a HP GC-5890II instrument equipped with a flame ioni-
zation detector. Depending on the alkyne substrate, the GC columns
with different polarities were used, including a supelcowax®10 capil-
lary column (30 m × 0.2 mm ×0.2
μ
m), Agilent J&W DB-1 capillary
m), and Agilent J&W DB-5 capillary
m). Alkene selectivity was calculated
column (30 m ×0.32 mm ×0.3
column (50 m ×0.2 mm ×0.33
μ
μ
from the GC results as the percentage of alkenes produced among all the
products. For internal olefin products, the molar fraction of E-olefin, E/
(Z + E), was also calculated from the GC results.
Results and discussion
A Pd nanocatalyst (Pd@CaCO3) with Pd nanoparticles (5 wt%)
supported on CaCO3 microparticles (average size: 16 μm) is used in this
study for the semi-hydrogenation of a broad range of alkyne substrates
as shown below. Two inexpensive proprietary solid-phase ligands (SPL8-
4 and SPL8-6) in the form of fine powders, developed and commer-
cialized by Zhejiang Superior Technology Inc., are employed in this
study to demonstrate the efficiency of the solid-phase ligand technology.
Both solid-phase ligands are air- and moisture-stable. The latter has a
smaller average particle size (about 5 vs. 17
μm) than the former. The
particle size is expected to affect the performance given their solid-phase
2