Chemistry Reference
In-Depth Information
fatty acids with methanol under mild conditions [
75
]. As an example, the catalyst
showed excellent catalytic activity at room temperature and 94 % isolated yield
was obtained for lauric acid. The catalytic activity showed negligible loss after
five recycles. Supermicroporous iron phosphonate with interparticle mesoporosity
found potential in transesterification reactions under solvent-free conditions [
76
].
The electrophilicity of the carbonyl carbon of the reactant ester group was the main
driving force of the reaction. Negatively charged free P-OH in the porous frame-
work could prevent molecules enriched with
ˀ
-electron clouds from entering the
porous channels. Thus, the
ˀ
electrons were responsible for the minimal conver-
sion of ethyl acrylate to the corresponding transesterified product in comparison
with the other target esters. After the fifth cycle, the yield of methylcyanoacetate
decreased slightly to 85.6 %, compared with 88.9 % in the first run, indicating the
stability of the catalytic activity. The catalytic stability after multiple cycling could
be due to the stability of Me-O-P bond, leading to the solid hybrid phosphonate
frameworks and no leaching of active components during the reaction process.
Acid content is one of the key elements to determine catalytic activity and
efficiency. Mesostructured zirconium organophosphonate, possessing a specific
surface area of 702 m
2
g
−
1
and a uniform pore size of 3.6 nm, was synthesized
with the assistance of C
16
TABr using HEDP as coupling molecule [
77
]. The
existence of defective P-OH was confirmed, showing an ion-exchange capac-
ity of 1.65 mmol g
−
1
. The resultant hydroxyethylidene-bridged mesoporous
zirconium phosphonate served as acid catalyst for the synthesis of methyl-2,3-
O
-
isopropylidene-
ʲ
-
D
-ribofuranoside from
D
-ribose, exhibiting high catalytic activity
with rapid reaction rate (a product yield of 35.6 % after reaction at 70 °C for 3 h),
which were comparable to the catalytic performance of liquid HCl (yield of 26.2 %)
or commercial ion-exchange resin (yield of 33.0 %). The excellent catalytic activity
could be attributable to the high surface area of the synthesized mesoporous zirco-
nium phosphonates. The high specific surface area is beneficial for the distribution
of the active sites and for their exposure so that they can readily be attached by the
reactants, and the mesopores aid in the acceleration of mass transfer.
Nonetheless, it was difficult to achieve a high concentration of the desired
metal phosphonates in the conventional synthesis methods, since the condensa-
tion between P-OH and metal ions during the preparation process often results in
the extensive formation of P-O-M (M
=
Ti, Zr, V, Al, etc.) bonds. In order thus to
increase the defective P-OH concentration in metal phosphates and phosphonates
for the improvement of the H
+
exchange capacity, a series of alkyl amines were
used as protecting groups during the condensation process, based on the reversible
reaction between alkyl amines and P-OH groups in the phosphonic bridging mol-
ecules (Fig.
5.14
) [
64
]. The alkyl amines first partially occupied the P-OH sites by
acid-base reactions, followed by the condensation between the added alkoxides and
residual P-OH and P
=
O groups. Extraction with HCl finally released the P-OH
defects of the resultant solids, leading to a high H
+
exchange capacity and acid con-
tent. In the absence of amines, the P/Ti ratio reached a plateau of 1.35-1.51 when the
added P/Ti ratio was larger than 1.75, due to the limit of coordination ability of Ti
4
+
ions with phosphonic acids. In the presence of amines, the P/Ti ratio of obtained
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