166
the biomass to facilitate hydrolysis and dehydration to produce hydrocarbons,
N-containing compounds, oxygenates, gases, and solids. They improve biocrude oil
yield and reduce char production. However, homogeneous catalysts do not easily
facilitate the generation of “drop-in” fuels or the use of homogenous catalysts during the upgrading of biocrude oil from the liquefaction of microalgae and tend to
produce hydrocarbons unfit for fuel use (i.e., gasoline, jet fuel, diesel) [58, 61, 98].
Na 2 CO 3 , Ca 3 (PO 4 ) 2 , and NiO were studied by Jena et al. in the HTL of Spirulina
platensis and upgrading of its associated biocrude oil [44]. HTL conditions were
300–350 °C and 25 wt% algae concentration. Na 2 CO 3 produced the highest yield of
biocrude oil at 59 wt%, compared to the other two catalysts which lowered yield.
Na 2 CO 3 is the most widely used homogeneous catalyst, as confirmed in this study
and many other studies [25, 61, 72, 73, 113, 114]. Na 2 CO 3 as a catalyst has numerous advantages. First, it increases the biocrude oil’s C5 to C18 hydrocarbons by
45 wt% as well as stimulates benzene, toluene, ethylbenzene, and xylene production
[115]. Second, its effectiveness is due to its high reactivity in the formation of
CH 3 COONa, an organic salt instrumental in decarbonylation, dehydration, and
hydroxylation reactions [78]. Third, it increases HHV and H and C contents of the
biocrude oil while it simultaneously lowers the oil’s O, N, and S contents [19, 116–
118]. However, it also has negative effects. First, it is sensitive to loading amount
which directly raises solution pH and cost associated with recovery and reuse [119].
Second, the use of Na 2 CO 3 saponifies the lipids and forms sodium aliphatate, a solid
residue [19, 117, 120]. Third, it reacts with the organic acids produced via decarboxylation [19]. Thus, Na 2 CO 3 lowers organic acid concentration and increases CO 2
fraction in the gas phase.
Alkali catalysts in general are sensitive to temperature. For example, using
Na 2 CO 3 at 240 °C results in effective depolymerization of proteins to N- and
O-containing heteroatoms than using Na 2 CO 3 at 280 °C [120]. In a similar manner,
Na 2 CO 3 produces higher biocrude oil yields at 300 °C than at 350 °C [77]. Alkali
catalysts such as KOH facilitates the production of additional 5–10 wt% biocrude
oil compared to the non-catalytic reaction, as exhibited in an HTL experiment using
Cyanidioschyzon merolae at 180–300 °C for 30 min with 10 wt% loading [121].
One of the drawbacks in using alkali catalysts is they have limited effect to intermediate reactions necessary to generate fuels with high content of hydrocarbons. These
reactions include isomerization, decarboxylation of organic acids, and aromatization [78]. They are also nonrecyclable, and their disposal at the end of the liquefaction process contribute to environmental pollution.
Organic acids such as CH 3 COOH and HCOOH are homogeneous catalysts
responsible in the increase of gas fraction and decrease in biocrude oil viscosity.
CH 3 COOH and HCOOH raise the gas fraction of the HTL process by 16–22 wt%
and 30 wt%, respectively [72]. Interestingly, the S content of biocrude oil is doubled
by using organic acid catalysts compared to alkali catalysts [72]. Comparatively,
catalytic activity of homogeneous catalysts as a function of biocrude oil yield is
Na 2 CO 3 > CH 3 COOH > KOH > HCOOH [72]. Collectively, homogeneous catalysts
still do not gain industrial acceptance as catalyst of choice in mediating HTL microalgae reactions despite their apparent benefits. The reasons stemmed from their
E. P. Resurreccion and S. Kumar
the biomass to facilitate hydrolysis and dehydration to produce hydrocarbons,
N-containing compounds, oxygenates, gases, and solids. They improve biocrude oil
yield and reduce char production. However, homogeneous catalysts do not easily
facilitate the generation of “drop-in” fuels or the use of homogenous catalysts during the upgrading of biocrude oil from the liquefaction of microalgae and tend to
produce hydrocarbons unfit for fuel use (i.e., gasoline, jet fuel, diesel) [58, 61, 98].
Na 2 CO 3 , Ca 3 (PO 4 ) 2 , and NiO were studied by Jena et al. in the HTL of Spirulina
platensis and upgrading of its associated biocrude oil [44]. HTL conditions were
300–350 °C and 25 wt% algae concentration. Na 2 CO 3 produced the highest yield of
biocrude oil at 59 wt%, compared to the other two catalysts which lowered yield.
Na 2 CO 3 is the most widely used homogeneous catalyst, as confirmed in this study
and many other studies [25, 61, 72, 73, 113, 114]. Na 2 CO 3 as a catalyst has numerous advantages. First, it increases the biocrude oil’s C5 to C18 hydrocarbons by
45 wt% as well as stimulates benzene, toluene, ethylbenzene, and xylene production
[115]. Second, its effectiveness is due to its high reactivity in the formation of
CH 3 COONa, an organic salt instrumental in decarbonylation, dehydration, and
hydroxylation reactions [78]. Third, it increases HHV and H and C contents of the
biocrude oil while it simultaneously lowers the oil’s O, N, and S contents [19, 116–
118]. However, it also has negative effects. First, it is sensitive to loading amount
which directly raises solution pH and cost associated with recovery and reuse [119].
Second, the use of Na 2 CO 3 saponifies the lipids and forms sodium aliphatate, a solid
residue [19, 117, 120]. Third, it reacts with the organic acids produced via decarboxylation [19]. Thus, Na 2 CO 3 lowers organic acid concentration and increases CO 2
fraction in the gas phase.
Alkali catalysts in general are sensitive to temperature. For example, using
Na 2 CO 3 at 240 °C results in effective depolymerization of proteins to N- and
O-containing heteroatoms than using Na 2 CO 3 at 280 °C [120]. In a similar manner,
Na 2 CO 3 produces higher biocrude oil yields at 300 °C than at 350 °C [77]. Alkali
catalysts such as KOH facilitates the production of additional 5–10 wt% biocrude
oil compared to the non-catalytic reaction, as exhibited in an HTL experiment using
Cyanidioschyzon merolae at 180–300 °C for 30 min with 10 wt% loading [121].
One of the drawbacks in using alkali catalysts is they have limited effect to intermediate reactions necessary to generate fuels with high content of hydrocarbons. These
reactions include isomerization, decarboxylation of organic acids, and aromatization [78]. They are also nonrecyclable, and their disposal at the end of the liquefaction process contribute to environmental pollution.
Organic acids such as CH 3 COOH and HCOOH are homogeneous catalysts
responsible in the increase of gas fraction and decrease in biocrude oil viscosity.
CH 3 COOH and HCOOH raise the gas fraction of the HTL process by 16–22 wt%
and 30 wt%, respectively [72]. Interestingly, the S content of biocrude oil is doubled
by using organic acid catalysts compared to alkali catalysts [72]. Comparatively,
catalytic activity of homogeneous catalysts as a function of biocrude oil yield is
Na 2 CO 3 > CH 3 COOH > KOH > HCOOH [72]. Collectively, homogeneous catalysts
still do not gain industrial acceptance as catalyst of choice in mediating HTL microalgae reactions despite their apparent benefits. The reasons stemmed from their
E. P. Resurreccion and S. Kumar