Gold-Based Catalysts - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 148

products can be formed came from a more recent study by Prati and co-
workers, who used carbon-supported palladium (1% Pd/C) under acidic
conditions (pH 5). 46 Even though the activity was low (5% conversion of
glycerol after 2 h), it is worth mentioning that C 1 compounds were the
principal products, as a consequence of C-C bond cleavage induced by
hydrogen peroxide, a known by-product under these reaction conditions. 46
Platinum has also been employed for selectively oxidizing glycerol
under acidic or basic conditions. A study in the mid-1990s examined the
catalytic activity of carbon-supported platinum catalysts at pH 6 and 11.
The catalytic activity was significantly lower in acidic media than under basic
conditions and even the overall yield of the identified products (glyceric acid,
dihydroxyacetone and tartronic acid) was favored by higher pH, leading to
35% under acidic and 70% under basic conditions. As in the case of pal-
ladium, the principal product was glyceric acid but with lower yield (from 70
to 55%) while the dihydroxyacetone yield increased from 8 to 12%. 91,93-95
More recent studies using carbon-supported platinum under acidic media
were reported by Liang et al. 96 and Prati and co-workers. 46,76 Whereas two
decades ago low activity was observed for this kind of catalyst at low pH,
Liang et al. claimed a 50% conversion and Prati and co-workers even 78%.
Owing to the different experimental conditions, a clear cross-evaluation of
such results is impossible, but both groups reported an important selectivity
towards C1 compounds, namely carbon dioxide and formic acid. An older
earlier paper from by Hutchings and co-workers showed that passing from
acidic to basic conditions doubled the glycerol conversion by on increasing
the NaOH/- to-glycerol molar ratio in the range 0-2 with 5 wt% Pt/C at 60 1C
(conversion from 27 to 56% after 3 h of reaction). At the same time, the
selectivity towards glyceric acid and glyceraldehyde increased from 28 to
69% and from 0 to 30%, respectively, whereas oxalic acid was detected only
under acidic conditions. Interestingly, the nature of metal hydroxides was
found to markedly affect the catalytic performance following this trend re-
lated to conversion: NaOH (63%) 4 CsOH (52%) 4 LiOH (49%) 4 RbOH
(37%) 4KOH (35%). 92
Coming back to gold after this short excursion on platinum and pal-
ladium, research on the selective oxidation of glycerol with gold catalysis is
still in progress, as the most recent papers witness. 97,98 Efforts are being
made to improve mainly the experimental conditions and catalyst design,
even searching for novel supporting materials, such as hydrotalcite, 97 or
boosting more traditional supports, such as alumina, 98 with a particular
interest in bimetallic formulations as the only ones allowing alkali-free
conditions.

d n 4 r 4 n g | 3

.

5.3.2 Allyl Alcohol Oxidation
The interest of the scientific community in 3-hydroxypropionic acid (3-HP) is
related to the manufacture of high-performance polymers where 3-HP is an