177
Secondly, the separation between oxidation and reduction may be difficult
because of the existence of the oxidized intermediate compounds derived from CO 2
reduction cycles. Many attempts have been done to suppress side reactions. For
instance, Yang et al. (2011) mechanistically studied the hydrocarbon formation
under Ti-SBA-15-photocatalyzed CO 2 reduction, indicating that CO and HCHO are
key intermediates in this process, while HCOH was found very reactive over the
catalytic system. In addition, oxidation of hydrocarbon products to CO 2 or CO is the
backward reaction that should also be considered. This work proposed the evaluation of hydrocarbon oxidation for designing new photocatalytic systems.
Consequently, minimizing the hydrocarbon oxidation can be the key to suppressing
the undesirable reactions.
6.4 Other Approaches for Carbon Dioxide Conversion
to Formaldehyde
Formaldehyde is an important and basic C 1 source for industrial utilization. It is
mainly used to produce polymers and other daily life commodities such as paints/
inks, pharmaceuticals, resins, adhesives, and cosmetics. As a crucial industrial
chemical, formaldehyde is also produced using various synthesis routes apart from
the aforementioned CO 2 photoreduction. The worldwide chemical industries currently generate greater than 20 million metric tons of formaldehyde per annum via
methanol oxidation method as seen in Formox process of Fig. 6.7 (Bontemps et al.
2014). About 35% of the total methanol industrially generated from synthesis gas is
employed to fulfill the rising demand for formaldehyde (Heim et al. 2017). However,
formaldehyde generation from this traditional oxidative methanol approach is an
energy-intensive procedure since it requires syngas feedstock produced from significantly endothermic reforming processes and involves other energy-consuming
purification and compression steps (Heim et al. 2017). Thus, the direct formaldehyde synthesis through CO 2 hydrogenation implementing homogeneous and heterogeneous catalysts has emerged as an interesting alternative because it could avoid
the dependence on methanol production and convert CO 2 greenhouse gas to essential chemicals.
Lee et al. (2001) employed different heterogeneous Pt-promoted Cu/SiO 2 catalysts for CO 2 hydrogenation to HCHO at 150 °C and 600 kPa for 2 h. Formaldehyde
could not be detected in the case of using Cu/SiO 2 catalyst, whereas Pt-promoted
specimen with the optimum Pt/Cu ratio of about 0.03 was able to selectively hydrogenate CO 2 to HCHO and reduced methanol formation. Additionally, formaldehyde
formation rate was improved from 0.21 × 10
−4
to 0.87 × 10
−4
mol min
−1
g cat
−1
with
rising H 2 /CO 2 ratio from 3 to 20. The Fourier transform infrared spectrum of used
Pt-Cu/SiO 2 catalyst did not detect the presence of bidentate formate, copper formate, and methanol but observed the existence of adsorbed formaldehyde on the
catalyst surface. This observation could suggest that platinum promotion facilitated
the conversion of intermediate formate to formaldehyde.
6 Conversion of Carbon Dioxide into Formaldehyde
Secondly, the separation between oxidation and reduction may be difficult
because of the existence of the oxidized intermediate compounds derived from CO 2
reduction cycles. Many attempts have been done to suppress side reactions. For
instance, Yang et al. (2011) mechanistically studied the hydrocarbon formation
under Ti-SBA-15-photocatalyzed CO 2 reduction, indicating that CO and HCHO are
key intermediates in this process, while HCOH was found very reactive over the
catalytic system. In addition, oxidation of hydrocarbon products to CO 2 or CO is the
backward reaction that should also be considered. This work proposed the evaluation of hydrocarbon oxidation for designing new photocatalytic systems.
Consequently, minimizing the hydrocarbon oxidation can be the key to suppressing
the undesirable reactions.
6.4 Other Approaches for Carbon Dioxide Conversion
to Formaldehyde
Formaldehyde is an important and basic C 1 source for industrial utilization. It is
mainly used to produce polymers and other daily life commodities such as paints/
inks, pharmaceuticals, resins, adhesives, and cosmetics. As a crucial industrial
chemical, formaldehyde is also produced using various synthesis routes apart from
the aforementioned CO 2 photoreduction. The worldwide chemical industries currently generate greater than 20 million metric tons of formaldehyde per annum via
methanol oxidation method as seen in Formox process of Fig. 6.7 (Bontemps et al.
2014). About 35% of the total methanol industrially generated from synthesis gas is
employed to fulfill the rising demand for formaldehyde (Heim et al. 2017). However,
formaldehyde generation from this traditional oxidative methanol approach is an
energy-intensive procedure since it requires syngas feedstock produced from significantly endothermic reforming processes and involves other energy-consuming
purification and compression steps (Heim et al. 2017). Thus, the direct formaldehyde synthesis through CO 2 hydrogenation implementing homogeneous and heterogeneous catalysts has emerged as an interesting alternative because it could avoid
the dependence on methanol production and convert CO 2 greenhouse gas to essential chemicals.
Lee et al. (2001) employed different heterogeneous Pt-promoted Cu/SiO 2 catalysts for CO 2 hydrogenation to HCHO at 150 °C and 600 kPa for 2 h. Formaldehyde
could not be detected in the case of using Cu/SiO 2 catalyst, whereas Pt-promoted
specimen with the optimum Pt/Cu ratio of about 0.03 was able to selectively hydrogenate CO 2 to HCHO and reduced methanol formation. Additionally, formaldehyde
formation rate was improved from 0.21 × 10
−4
to 0.87 × 10
−4
mol min
−1
g cat
−1
with
rising H 2 /CO 2 ratio from 3 to 20. The Fourier transform infrared spectrum of used
Pt-Cu/SiO 2 catalyst did not detect the presence of bidentate formate, copper formate, and methanol but observed the existence of adsorbed formaldehyde on the
catalyst surface. This observation could suggest that platinum promotion facilitated
the conversion of intermediate formate to formaldehyde.
6 Conversion of Carbon Dioxide into Formaldehyde