122
The importance of the formation of a mixed copper–zinc hydroxycarbonate is
related in the publications. However, the composition and phase which are beneficial for the activity of the final catalyst are controversial. For some authors, the
malachite, (CuZn) 2 (CO 3 ) 2 (OH) 2 , is the phase to be synthesized in the precursors
(Kim et al. 2018; Li and Inui 1996; Lee et al. 2013), whereas other studies claim that
aurichalcite, (Cu, Zn) 5 (CO 3 ) 2 (OH) 6 , is the optimal phase (Fujita et al. 1998).
Farahni and co-workers (2014) studied the coprecipitation of Cu–ZnO methanol
catalysts. They showed that the aging time and temperature influenced the hydroxycarbonate crystallization and particle size, having a direct effect on the copper surface of the reduced catalyst and, as a consequence, of the space time yield of
methanol. As an example, at 50 bar and 230 °C, a space time yield of around
100 g MeOH kg cat h
−1
is obtained for the catalyst prepared by aging the precursor at
40 °C, whereas it was of 550 g MeOH kg cat h
−1
after aging at 60 °C, when the precursors
became organized in crystalline structure, and around 450 g MeOH kg cat h
−1
after aging
at 80 °C, due to their growth to larger particles. These results are in agreement with
those of Baltes and co-workers (Baltes et al. 2008) who reported an optimal aging
temperature of 70 °C for the synthesis of Cu–ZnO–Al 2 O 3 catalysts.
Jeong and co-workers (2016) studied the influence of pH in the range 4–10 in the
synthesis of Cu/ZnO materials and claimed that the higher pH, the higher the catalytic activity to methanol. Lee and co-workers (s2013) observed that high pH leads
to increase the particle size and is detrimental to the specific surface area. For
Behrens and co-workers (2011), the optimal pH is the one which permits complete
precipitation of Zn-based species, but not higher.
The washing procedure of the precipitate is an important step in case of precipitation with sodium carbonates. Sun and co-workers (2002) clearly established the
negative effect of surface residual sodium on the catalytic performances for methanol synthesis, mainly on the carbon dioxide conversion. This was confirmed by
Prieto and co-workers (2013) who demonstrated that specific surface area and copper dispersion increased with the number of successive washing steps.
Some contradictory results about the effect of the synthesis parameters on the
catalytic activity may be explained by the bad local control inherent to the coprecipitation in batch mode. Recently, Angelo and co-workers (2016) developed a continuous flow coprecipitation setup, based on three co-axial capillaries, which allows
a much better local control of the synthesis parameters of methanol catalysts, which
leads to materials with enhanced local homogeneity and improved catalytic performances. L’Hospital and co-workers (L’Hospital 2018, 2019) went further in the
development and the knowledge of this technique and prepared materials with even
higher performances: record methanol productivity of 1135 g methanol kg cat
−1
h
−1
was
achieved. It is demonstrated to be due by the formation of a real ternary copper–
zinc–zirconium precursor in the process of continuous coprecipitation.
Other preparation methods are reported for methanol catalysts. Among them we
can cite inverse coprecipitation under ultrasounds (Arena et al. 2007), microwaveassisted method (Cai et al. 2015), urea combustion (Guo et al. 2009), sol-gel (Köppel
et al. 1998; Angelo et al. 2015), and impregnation (Choi et al. 2001; Yu et al. 2012).
D. P. Minh et al.
The importance of the formation of a mixed copper–zinc hydroxycarbonate is
related in the publications. However, the composition and phase which are beneficial for the activity of the final catalyst are controversial. For some authors, the
malachite, (CuZn) 2 (CO 3 ) 2 (OH) 2 , is the phase to be synthesized in the precursors
(Kim et al. 2018; Li and Inui 1996; Lee et al. 2013), whereas other studies claim that
aurichalcite, (Cu, Zn) 5 (CO 3 ) 2 (OH) 6 , is the optimal phase (Fujita et al. 1998).
Farahni and co-workers (2014) studied the coprecipitation of Cu–ZnO methanol
catalysts. They showed that the aging time and temperature influenced the hydroxycarbonate crystallization and particle size, having a direct effect on the copper surface of the reduced catalyst and, as a consequence, of the space time yield of
methanol. As an example, at 50 bar and 230 °C, a space time yield of around
100 g MeOH kg cat h
−1
is obtained for the catalyst prepared by aging the precursor at
40 °C, whereas it was of 550 g MeOH kg cat h
−1
after aging at 60 °C, when the precursors
became organized in crystalline structure, and around 450 g MeOH kg cat h
−1
after aging
at 80 °C, due to their growth to larger particles. These results are in agreement with
those of Baltes and co-workers (Baltes et al. 2008) who reported an optimal aging
temperature of 70 °C for the synthesis of Cu–ZnO–Al 2 O 3 catalysts.
Jeong and co-workers (2016) studied the influence of pH in the range 4–10 in the
synthesis of Cu/ZnO materials and claimed that the higher pH, the higher the catalytic activity to methanol. Lee and co-workers (s2013) observed that high pH leads
to increase the particle size and is detrimental to the specific surface area. For
Behrens and co-workers (2011), the optimal pH is the one which permits complete
precipitation of Zn-based species, but not higher.
The washing procedure of the precipitate is an important step in case of precipitation with sodium carbonates. Sun and co-workers (2002) clearly established the
negative effect of surface residual sodium on the catalytic performances for methanol synthesis, mainly on the carbon dioxide conversion. This was confirmed by
Prieto and co-workers (2013) who demonstrated that specific surface area and copper dispersion increased with the number of successive washing steps.
Some contradictory results about the effect of the synthesis parameters on the
catalytic activity may be explained by the bad local control inherent to the coprecipitation in batch mode. Recently, Angelo and co-workers (2016) developed a continuous flow coprecipitation setup, based on three co-axial capillaries, which allows
a much better local control of the synthesis parameters of methanol catalysts, which
leads to materials with enhanced local homogeneity and improved catalytic performances. L’Hospital and co-workers (L’Hospital 2018, 2019) went further in the
development and the knowledge of this technique and prepared materials with even
higher performances: record methanol productivity of 1135 g methanol kg cat
−1
h
−1
was
achieved. It is demonstrated to be due by the formation of a real ternary copper–
zinc–zirconium precursor in the process of continuous coprecipitation.
Other preparation methods are reported for methanol catalysts. Among them we
can cite inverse coprecipitation under ultrasounds (Arena et al. 2007), microwaveassisted method (Cai et al. 2015), urea combustion (Guo et al. 2009), sol-gel (Köppel
et al. 1998; Angelo et al. 2015), and impregnation (Choi et al. 2001; Yu et al. 2012).
D. P. Minh et al.