362
6.4 Carbonaceous Materials
Carbonaceous material is one of the important classes of solid acid catalysts derived
from the partial pyrolysis process followed by sulfonation of the carbon-based
materials at specified temperature. These types of materials have advantages like
high catalyst dispersion ability, high thermal stability, high adsorption capacity,
reusability, and easy separation from the reaction mixture [69, 111–116]. Such kind
of solid acid catalyst has the ability to hold various protonic sites and shows very
good catalyst activity in the various organic reactions like esterification, hydration,
and hydrolysis [117]. Carbonaceous materials can be derived by stepwise procedure, pyrolysis, sulfonation, and carbonization [118]. In the second step, sulfonation is carried out by means of mineral acids like sulfuric acid of the carbon materials
[119–121], which is further subjected to hydrothermal carbonization process to get
the catalytic material [122–124]. Li et al. have synthesized such kind of carbonaceous catalyst using loofah sponge as renewable biomass source and applied for the
esterification of ethanol to produce ethyl levulinate. The conversion of LA is
obtained up to 91% using this catalyst with satisfactory reusability [112].
Yang et al. [55] have evaluated the catalytic performance of various solid acid
catalysts and compared with the carbonaceous solid acid catalysts. Zeolites
H-ZSM-5 and H-BEA and resins Amberlyst-15 and Nafion-212 were compared
with carbonaceous solid acid catalysts such as GC400 (glucose-derived amorphous
carbon) and AC400 (commercial grade activated carbon). It was observed that under
optimized reaction conditions, i.e., LA/n-butanol ratio of 1:5, 10% catalyst, 4-h
reaction time, and 100 °C temperature, GC400 displayed a much higher catalytic
efficiency toward esterification among the studied catalysts (sulfonated active carbon, HZSM-5, Nafion-212). The authors concluded that the high acid (–SO 3 H) density and acid strength along with weak acid sites linked on the surface of catalysts
(due to –COOH and phenolic –OH) are responsible factors behind the higher catalytic activity of GC400. This fact is found in accordance with the reported literature
wherein, the authors (Fernandes et al.) have performed comparative catalytic
screening of catalysts [zeolites (ZSM-5, Beta, USY, Mordenite, and MCM-22)
(SnO 2 , ZrO 2 , Nb 2 O 5 , TiO 2 )] for LA esterification reaction. They observed lower
activity of zeolite than sulphated oxides, which is attributed to the formation of
transition state inside the channel structure rather than acidity of microporous zeolite. Author has mentioned that the same reason is responsible for the activity gap
between H-BEA and HZSM-5 i.e. the gap is due to different pore structure [74].
6.5 Solid-Supported Reagents
Solid-supported reagents comprise polymeric/non-polymeric solid supports (alumina, silica, zeolite, etc.) on which reagents (in dissolved form) are supported. In
such cases, problems of removal of the products may arise, but if the chemical
K. C. Maheria et al.
6.4 Carbonaceous Materials
Carbonaceous material is one of the important classes of solid acid catalysts derived
from the partial pyrolysis process followed by sulfonation of the carbon-based
materials at specified temperature. These types of materials have advantages like
high catalyst dispersion ability, high thermal stability, high adsorption capacity,
reusability, and easy separation from the reaction mixture [69, 111–116]. Such kind
of solid acid catalyst has the ability to hold various protonic sites and shows very
good catalyst activity in the various organic reactions like esterification, hydration,
and hydrolysis [117]. Carbonaceous materials can be derived by stepwise procedure, pyrolysis, sulfonation, and carbonization [118]. In the second step, sulfonation is carried out by means of mineral acids like sulfuric acid of the carbon materials
[119–121], which is further subjected to hydrothermal carbonization process to get
the catalytic material [122–124]. Li et al. have synthesized such kind of carbonaceous catalyst using loofah sponge as renewable biomass source and applied for the
esterification of ethanol to produce ethyl levulinate. The conversion of LA is
obtained up to 91% using this catalyst with satisfactory reusability [112].
Yang et al. [55] have evaluated the catalytic performance of various solid acid
catalysts and compared with the carbonaceous solid acid catalysts. Zeolites
H-ZSM-5 and H-BEA and resins Amberlyst-15 and Nafion-212 were compared
with carbonaceous solid acid catalysts such as GC400 (glucose-derived amorphous
carbon) and AC400 (commercial grade activated carbon). It was observed that under
optimized reaction conditions, i.e., LA/n-butanol ratio of 1:5, 10% catalyst, 4-h
reaction time, and 100 °C temperature, GC400 displayed a much higher catalytic
efficiency toward esterification among the studied catalysts (sulfonated active carbon, HZSM-5, Nafion-212). The authors concluded that the high acid (–SO 3 H) density and acid strength along with weak acid sites linked on the surface of catalysts
(due to –COOH and phenolic –OH) are responsible factors behind the higher catalytic activity of GC400. This fact is found in accordance with the reported literature
wherein, the authors (Fernandes et al.) have performed comparative catalytic
screening of catalysts [zeolites (ZSM-5, Beta, USY, Mordenite, and MCM-22)
(SnO 2 , ZrO 2 , Nb 2 O 5 , TiO 2 )] for LA esterification reaction. They observed lower
activity of zeolite than sulphated oxides, which is attributed to the formation of
transition state inside the channel structure rather than acidity of microporous zeolite. Author has mentioned that the same reason is responsible for the activity gap
between H-BEA and HZSM-5 i.e. the gap is due to different pore structure [74].
6.5 Solid-Supported Reagents
Solid-supported reagents comprise polymeric/non-polymeric solid supports (alumina, silica, zeolite, etc.) on which reagents (in dissolved form) are supported. In
such cases, problems of removal of the products may arise, but if the chemical
K. C. Maheria et al.