Technological approaches which enable the effective utilization of CO2 for manufacturing value-added chemicals and fuels can help to solve environmental problems derived from large CO2 emissions ...associated with the use of fossil fuels. One of the most interesting products that can be synthesized from CO2 is methanol, since it is an industrial commodity used in several chemical products and also an efficient transportation fuel. In this review, we highlight the recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to methanol. The main efforts focused on the improvement of conventional Cu/ZnO based catalysts and the development of new catalytic systems targeting the specific needs for CO2 to methanol reactions (unfavourable thermodynamics, production of high amount of water and high methanol selectivity under high or full CO2 conversion). Major studies on the development of active and selective catalysts based on thermodynamics, mechanisms, nano-synthesis and catalyst design (active phase, promoters, supports, etc.) are highlighted in this review. Finally, a summary concerning future perspectives on the research and development of efficient heterogeneous catalysts for methanol synthesis from CO2 will be presented.
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•Dehydrogenation of methylcyclohexane on partially reduced Pt/MoO2-SiO2 catalysts.•High active phase dispersion and acidity with lowest coke formation: best activity.•High Mo content ...causes the formation of Pt(core)-Mo(shell) structure: low activity.
A series of bifunctional Pt/Mo(x)-SiO2 catalysts were investigated in the context of the hydrogen storage using the methylcyclohexane (MCH)-toluene-hydrogen cycle. The performance of partially reduced catalysts for hydrogen production was evaluated in the MCH dehydrogenation reaction carried out in a fixed-bed flow reactor at 673K and total hydrogen pressure of 2.2MPa. The catalysts with different acidity and Mo loading (4.1–12.7wt.%) were prepared by impregnation of the calcined Mo(x)-SiO2 samples with Pt precursor. The oxide catalyst precursors were characterized by chemical analysis (ICP-AES), N2 physisorption at 77K, X-ray diffraction (XRD), Raman spectroscopy and temperature-programmed reduction (H2-TPR) techniques whereas the partially reduced samples were characterized by temperature-programmed adsorption of ammonia (TPD-NH3), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The catalyst having optimized Mo loading (8.0wt.%) was the most active and its best catalytic performance was attributed to the high dispersion of MoO2 and Pt° phases and its lowest deactivation by coke formation. The activity drop of the catalysts having larger Mo loading (10.6 and 12.7wt.%) was linked with formation of MoOx-Pt core-shell nanoparticles, as confirmed by HRTEM.
The influence of aging of precipitates on the physical and catalytic properties of a copper/zinc oxide-aluminium (Cu/ZnO-Al) catalyst with an optimized composition (low Al concentration, Cu/Zn/Al = ...68/29/3) prepared using co-precipitation has been investigated in detail. The change in the structure of precipitates with aging (from amorphous zincian georgeite to crystalline zincian malachite) strongly influences the micro- and nano-structure (Cu and ZnO crystallite size, exposed copper surface area, Cu-ZnO interactions and stability of ZnO) of the final Cu/ZnO-Al catalysts obtained after calcination and reduction of the precipitates. The results of catalytic activity in methanol synthesis from syngas show the higher intrinsic activity of the catalysts derived from aged zincian malachite precipitates as consequence of the increase in the exposed copper surface area and the Cu-ZnO contacts. The stability of catalysts under the reaction conditions was also improved in the catalysts derived from precipitates aged after crystallization of malachite. The catalyst derived from the precipitate removed close to the point of crystallization of malachite shows very poor activity in the methanol synthesis as consequence of its segregated large Cu crystallites in low contact with ZnO derived from the absence of carbonate retention after calcination of the precipitate and the presence of sodium species after conventional washing which favour the strong sintering and crystallization of Cu during reduction.
The catalysts derived from precipitates aged after crystallization of zincian malachite show higher activity and stability
Ni catalysts supported on different carriers like δ,θ-Al2O3, MgAl2O4, SiO2–Al2O3 and ZrO2–Al2O3 were prepared. The solids were characterized by chemical analysis, N2 adsorption–desorption isotherms, ...X-ray powder diffraction, UV–vis diffuse reflectance spectroscopy, temperature-programmed reduction, high-resolution transmission electron microscopy and temperature-programmed oxidation. The catalytic properties of the samples were evaluated in the reaction of reforming of methane with CO2 at 923 K. It was shown that this kind of support greatly affects the structure and catalytic performance of the catalysts. Ni catalyst supported on MgAl2O4 showed the highest activity and stability due to the presence of small well dispersed Ni particles with size of 5.1 nm. It was shown that the lowest activity of Ni catalyst supported on SiO2–Al2O3 oxide was caused by the agglomeration of nickel particles and formation of filamentous carbon under reaction conditions detected by the high resolution transmission electron microscopy.
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► Catalytic properties of Ni catalysts in dry methane reforming are studied. ► A strong interaction between NiO and MgAl2O4 exists. ► The best activity of Ni/MgAl catalyst is due to the smallest Ni size of 5.1 nm.
CeO
2–ZrO
2 oxides with different CeO
2 content (1–12
wt%) were prepared by the impregnation method. The effects of CeO
2 content, calcination temperature and interaction between the components on ...the structure and reductive properties of CeO
2–ZrO
2 oxides were evaluated by using different techniques such as XRD, DRS, FTIR, Raman, XPS, TGA and TPR. Spectroscopic data showed that the tetragonal phase of zirconia is preserved in all CeO
2–ZrO
2 oxides, although they retain a high number of defect sites caused by a strong interaction between zirconia and cerium oxide species. An enrichment of the zirconia surface with a fluorite structure of CeO
2 is observed for the CeO
2–ZrO
2 oxide with the highest CeO
2 content (12
wt%). The samples subjected to consecutive reduction–oxidation cycles at different temperatures showed good redox properties related to the increase of oxygen mobility. It was concluded that these CeO
2–ZrO
2 oxide systems, displaying high surface and good thermal stability, are similar to chemically mixed oxides due to the strong interaction between the zirconia carrier and deposited ceria.
The effect of CeO
2 content, calcination temperature and interaction between components on the structure and redox properties of CeO
2–ZrO
2 oxides was evaluated. At the highest CeO
2 loading (12
wt%) and after high temperature treatment of 1073
K an enrichment of the zirconia surface with a fluorite structure of CeO
2 is observed in the figure. XRD of CeO
2, ZrO
2 and CeO
2–ZrO
2 oxides with different CeO
2 content after temperature treatment at 823 (A) and 1073
K (B).
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In this work, Co-promoted MoS
2
catalysts supported on Al-modified mesoporous silica (HMS and SBA-16) have been prepared by the decomposition of ammonium tetrathiomolybdate and cobalt nitrate ...precursors in a H
2
S/H
2
gas mixture. The effects of the support morphology and Al incorporation method (one-pot synthesis vs. support’s grafting) on the catalyst activity were investigated in the reaction of hydrodesulfurization (HDS) of dibenzothiophene (DBT) conducted in a high-pressure batch reactor at 350 °C and total H
2
pressure of 3.3 MPa. The physical and chemical characterization of the oxide precursors and spent catalysts was studied by a variety of several techniques (N
2
adsorption–desorption, XRD, SEM, EDS,
27
Al MAS NMR, Raman, TG/DTA and DRS UV–vis). The MoS
2
phase of all catalysts exhibits similar size (about 2.8 nm) and stacking layers (4). Activity tests showed that CoAT/HMS-Al was most active and selective toward direct desulfurization (DDS) reaction pathway among the catalysts studied. Its catalytic response was similar to that of CoMo/γ-Al
2
O
3
catalyst activated by reductive sulfiding. A linear correlation was found between DBT conversion at a reaction time of 5 h and the specific surface area and mean pore diameter of the catalysts. The best DBT HDS activity showed CoAT/HMS-Al catalyst having the best textural properties among the catalysts studied. Modification of SBA-16 with Al
3+
ions by the one-step synthesis method rendered the catalyst more active in the HDS reaction than its modification by grafting.
Graphical Abstract
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•Ru enhanced nickel reducibility which may be related to a hydrogen spillover effect.•HDO catalytic activity was correlated with the concentration of weak and middle surface acid ...sites.•A support effect in activity and selectivity was evidenced, originated by different metal-support interactions.•For Al2O3 and ZrO2 a synergetic effect between Ni and Ru was observed in HDO of phenol.
The bio oil of pyrolysis can be upgraded via hydrodeoxygenation to remove unfavorable characteristics related to high levels of oxygen. Sulfided NiMo and CoMo catalyst have been studied extensively in hydrodeoxygenation. However, significant deactivation may occur, due to the lack of sulfur in the charges or to coke deposition, induced by strong acidic supports. Therefore, development of new catalytic materials with alternative supports and active phases is relevant. In this work, Ni, Ru, and Ni-Ru catalysts were prepared by wet impregnation of Ni(NO3)2·6H2O and Ru(NO)(NO3)3 solutions. Al2O3, TiO2, and ZrO2 were used as supports, evidencing influence on hydrodeoxygenation (HDO) of phenol activity and selectivities. TPR and XPS characterization revealed a promoter effect of ruthenium over Ni reducibility and in some cases, a synergetic effect on catalytic activity in phenol HDO reaction was observed. The metallic ruthenium particles were reduced at low temperatures, and they enhanced reduction of Ni by H2 spillover effect, due to a close contact between metals. Furthermore, TPD-NH3 indicated a possible correlation between catalyst acidity and HDO reaction rate.
Mechanism of Pb(II) ions adsorption on the SBA15-NH2 sorbent’s surface. Display omitted
► Mesoporous SBA-15 silica modified with –NH2 functional group is effective sorbent for Pb(II) ions. ► The ...maximum of Pb(II) ions adsorption (93.0%) occurs at pH 5–6. ► The mechanism of Pb(II) adsorption is through electrostatic of Pb(II) ions interaction with –NH2 groups.
SBA-15 substrate functionalized with –NH2 groups were tested in adsorptive elimination of lead (II) from aqueous solutions. The NH2/SBA-15 adsorbents were prepared by grafting of synthesized SBA-15 substrate with different concentrations of 3-aminopropyltriethoxysilane (APTES) in order to determine the optimum amine group concentration. The largest adsorption capacity showed sorbent prepared using tetraethyl orthosilicate (TEOS)/APTES molar ratio of 3.3. All sorbents were characterized by N2 adsorption–desorption isotherms, powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA–DTG) and X-ray photoelectron spectroscopy (XPS). Regardless of the concentration of amine groups, all synthesized NH2/SBA-15 samples showed higher adsorption capacity for Pb (II) ions than pure SBA-15 material. For the optimized sorbent formulation, experimental data showed that the maximum Pb (II) adsorption occurred in the range of pH 5–6 with adsorption maximum of 93%. The characterization of sorbents after Pb(II) adsorption by UV–vis and XPS spectroscopes point out to a mechanism of lead adsorption through van der Waals electrostatic interaction between Pb(II) ions and –NH2 groups located on the sorbent surface. The mechanism of lead adsorption through the ionic exchange of Pb(II) ions with surface Si–NH3+ groups, as determined by XPS, is precluded because they acts repulsively with approaching lead ions.
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