Ordered Mesoporous Co₃O₄−Al₂O₃Binary Metal Oxides for CO Hydrogenation to Hydrocarbons: Synergy Effects of Phosphorus Modifier for an Enhanced Catalytic Activity and Stability

Abstract

The synergy effects of phosphorus modifier on highly ordered mesoporous binary metal oxides of Co 3 O 4 À Al 2 O 3 (m-CoAl), prepared by nanocasting method using a hard template of KIT-6, were observed by an enhanced catalytic and structural stability of the m-CoAl during CO hydrogenation to hydrocarbons. The enhanced structural stability of the ordered mesoporous structures on the phosphorous-modified m-CoAl at an optimal amount of phosphorous modifier below 0.3 wt%P (P (3)/m-CoAl) was attributed to the partial formation of thermally stable metal phosphates under a reductive Fischer-Tropsch synthesis (FTS) reaction condition. The positive effects of the phosphorous modifier were originated from the partially formed SiO 2 -like AlPO 4 phases on the outer surfaces of the m-CoAl, as well as from partially formed irreducible and thermally stable spinel-type cobalt aluminates (CoAl 2 O 4 ). The hydrophobic SiO 2 -like tridymite AlPO 4 surfaces on the ordered matrices of the P/m-CoAl also effectively prevented the heavy wax (or coke precursors) depositions. The structural instability of the P/m-CoAl was observed at a higher phosphorous content above 0.5wt%P by preferentially forming the largely segregated mixed metal oxides such as Co 3 O 4 À CoAl 2 O 4 À Co 3 (PO 4 ) 2 through the phase transformations of the surface excess AlPO 4 .

Phosphorus‐Modified Mesoporous Inorganic Materials for Production of Hydrocarbon Fuels and Value‐Added Chemicals

“…[43] Besides, the incorporation of saccharide (sucrose) into Co/PÀ Al 2 O 3 , designated as P(x)S(y), where the notations of P and S are the phosphorus and sucrose species respectively, efficiently increased the dispersion and regulated crystallite sizes and shapes of the Co nanoparticles, which overcome the lowered dispersion caused by phosphorus treatment, and thus an enhanced CO conversion ( Table 2). [38] Although FTS activity on the Co/PÀ Al 2 O 3 catalyst was superior, the steady decreases of FTS activity with time on stream (for instance, Figure 3(A)) was generally observed over phosphorus-modified catalysts, which may be largely attributed to the heavy wax depositions on the active sites or possible formations of volatile phosphorus-containing species [17,25,44] under H 2 -rich FTS reaction conditions. In addition, the applications of the ordered mesoporous metal oxides such as Co 3 O 4 and Fe 2 O 3 for FTS reaction were limited due to their intrinsically unstable metal oxide phases by transforming them into corresponding metallic states, which attributed to the severe disintegrations of the mesoporous structures with rapid catalytic deactivation.…”

“…[9,10] The detailed synthesis methods of the mesoporous materials using those templates were already documented well in the previous review papers, [5,[9][10][11][12] and our research group also prepared the highly ordered mesoporous (mixed) metal oxides and wormhole-like mesoporous metal phosphates through the above mentioned template methods. [13][14][15][16][17] The phosphorus-modified mesoporous materials including metal phosphates are composed of the binding of metal with phosphorus species, which have various applications such as catalysis, metal ions purification in water, gas separation, energy storage and bioceramics as shown in Figure 2. Metal phosphates with high surface area and large pore size have been attracted with special interest due to their relatively low cost, tunable surface properties as well as controllable morphologies.…”

“…For example, the addition of optimal amount of phosphorus (0.1-0.3 wt%) on the highly ordered mesoporous Co 3 O 4 À Al 2 O 3 (in-situ) generated thermally stable hydrophobic AlPO 4 phases on the outermost γ-Al 2 O 3 surfaces, which eventually enhanced CO conversion with higher selectivity to C 5 + hydrocarbons and long-term stability. [17] Similarly, the phosphorus-treated Nb 2 O 5 .nH 2 O [18,19] and TiO 2 [20,21] also displayed an improved yield of furfural or 5-(hydroxymethyl)furfural (HMF) and no decreases in conversion of sugars (xylose or fructose/ glucose) even after their several reuses compared to pure metal oxides.…”

Adjusting Hydrocarbon Distribution on the Stabilized Al‐Modified Mesoporous Co3O4‐Fe2O3 Bimetal Oxides for CO Hydrogenation

“…The Al 2 O 3 promoter can also partially generate the thermally stable spinel-type CoAl 2 O 4 , which possibly acted as a structural stabilizer of the highly ordered mesoporous structures as reported by our previous works through X-ray absorption fine structure (XAFS) analysis. [34][35][36] The presence of the spinel-type CoAl 2 O 4 phases was verified through various characterization tools, [25,34] Specific surface area (S g , m 2 /g), pore volume (P V , cm 3 /g) and average pore diameter (P D , nm) of the fresh m-CoFe are summarized in Table 1. Similar with the N 2 sorption patterns of the pristine KIT-6 (S g = 764 m 2 /g and P D = 5.2 nm) as shown in supplementary Figure S1, N 2 adsorption-desorption patterns (supplementary Figure S2(A)) and their pore size distributions (supplementary Figure S2(B)) on the fresh m-CoFe revealed the characteristic type IV isotherm with H1 hysteresis pattern.…”

“…[24] These phenomena were supported by the observed much larger surface areas on the Al-unmodified m-CoFe-u with the values of 55.3-73.1 m 2 /g and their smaller pore diameters in the range of 7.4-12.7 nm as summarized in supplementary Table S1. [38] According to our previous works on a highly ordered mesoporous monometallic Co 3 O 4 , [25][26][27][28][29] two separate reduction peaks appeared at~370 and 446°C can be assigned to the successive two step reductions (Co 3 O 4 + H 2 !3CoO + H 2 O and 3CoO + 3H 2 !3Co + 3H 2 O), and the middle reduction peak at 402°C can be assigned to the partially aggregated cobalt nanoparticles with its much higher reduction peak at 724°C possibly assigned to the spinel-type CoAl 2 O 4 [25,28,35] In case of the Fe-based metal oxides, hematite Fe 2 O 3 phases can be generally known to be reduced through three step reductions such as α-Fe 2 O 3 !Fe 3 O 4 !FeO!Fe 0 . [39] Those characteristic TPR peaks on the ordered mesoporous m-CoFe bimetal oxides shifted to the much higher temperatures, and the maximum temperature peaks of 656 and 758°C can be separately assigned to the reduction behaviors of Fe 2 O 3 and CoFe 2 O 4 phases to metal nanoparticles.…”