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Effect of Nb as dopant of hydrotalcite catalysts during the ethanol condensation to optimize n-Butanol production

The present study optimizes Cu-supported catalysts on hydrotalcite for the synthesis of n-butanol whose interest is in its use as biofuel, due to its superior performance in comparison with other short-chain alcohols. The catalytic performance was improved by adding Nb to the catalytic formulation. In this way, several Cu-Nb catalysts supported on hydrotalcites (Mg/Al) with different Nb loadings were synthesized by incipient wetness impregnation and characterized by H2−temperature-programmed reduction, X-ray diffraction, N2 physisorption, CO2− and NH3−thermo-programmed desorption and X-ray photoelectron spectroscopy. It was shown that the incorporation of Nb promoted a suitable balance between the acid-basic and redox sites, modulating the Cu0/Cu+ dehydrogenation/hydrogenation properties for the n-BuOH production. The synthesis of multiple value-added products (n-BuOH, acetaldehyde, ethyl acetate, and 1-hexanol) was achieved in both the liquid− (at 180 ºC) and gas−phase (150−350 ºC), where it was demonstrated that both the n-BuOH selectivity and ethanol conversion improved with the Nb addition. The performance of the catalysts under time on stream and their recycling is also described.

Sulfured NiMo catalysts on TiO2-Nb2O5 nanocomposites for efficient hydrodesulfurization performance

The implementation of increasingly stringent environmental regulations has increased the demand for more active catalysts in hydrotreating. This study proposes the synthesis, characterization, and evaluation of bimetallic NiMo catalysts supported on TiO2-Nb2O5 nanocomposites for dibenzothiophene hydrodesulfurization. Samples containing 3, 5, and 10 wt% Nb2O5 were synthesized to examine the influence of Nb content on their catalytic activity and selectivity. Supports and oxidized catalysts were characterized using various techniques, including SEM-EDS, N2 physisorption, powder XRD, UV–vis DRS, Raman spectroscopy, and H2-TPR, while sulfured catalysts were further characterized by XPS and HRTEM. NiMo/TiNb3 and NiMo/TiNb5 catalysts displayed propene and diisopropyl ether as the main products in the 2-propanol dehydration reaction. A correlation was observed between hydrogenating capabilities in the HDS and dehydration tests with the Nb loading, suggesting that an optimal combination of Lewis/Brønsted acid sites enhances the BP/CHB ratio. All Nb-containing samples exhibited superior activity compared to their Nb-free counterparts, as indicated by the higher values of the pseudo-first-order rate constants (kHDS), which were higher for the NiMo/TiNb3 catalyst. These results were also associated with the respective textural characteristics, metal dispersion, and species distribution.

Effects of impregnation sequence on the NH3-SCR activity and hydrothermal stability of a Ce-Nb/SnO2 catalyst

Hydrothermal stability is crucial for the practical application of deNOx catalyst on diesel vehicles, for the selective catalytic reduction of NOx with NH3 (NH3-SCR). SnO2-based materials possess superior hydrothermal stability, which is attractive for the development of NH3-SCR catalyst. In this work, a series of Ce-Nb/SnO2 catalysts, with Ce and Nb loading on SnO2 support, were prepared by impregnation method. It was found that, the NH3-SCR activities and hydrothermal stabilities of the Ce-Nb/SnO2 catalysts significantly varied with the impregnation sequences, and the Ce-Nb(f)/SnO2 catalyst that firstly impregnated Nb and then impregnated Ce exhibited the best performance. The characterization results revealed that Ce-Nb(f)/SnO2 possessed appropriate acidity and redox capability. Furthermore, the strong synergistic effect between Nb and Sn species stabilized the structure and maintained the dispersion of acid sites. This study may provide a new understanding for the effect of impregnation sequence on activity and hydrothermal stability and a new environmental-friendly NH3-SCR catalyst with potential applications for NOx removal from diesel and hydrogen-fueled engines.

Niobium oxide-phosphorylated carbon xerogel composites as solid acid catalysts for cascade conversion of glucose to 5-hydroxymethylfurfural (HMF) in pure water

The cascade conversion of glucose to a key platform commodity, 5-hydroxymethylfurfural (HMF), was studied as a sustainable method for producing platform molecules from biomass. Bifunctional catalysts were prepared by the deposition/precipitation of Nb2O5 (containing NbO4 Lewis and Nb-OH Brønsted acid sites) onto the surface of phosphorylated carbon xerogels (CX_P) with Brønsted acid sites (BA) in the form of oxygen and phosphorus functionalities. The number of Lewis acid (LA) sites in the composites was tuned by varying the Nb loading (0–60%), and Brønsted acidity was modified by thermal (post)treatments. Catalytic tests carried out in pure water revealed that the LA sites played an important role in selective glucose conversion, enhancing the isomerization of glucose to fructose, whereas BA sites, in the absence of LA sites, catalyzed the degradation of glucose to humin. For bifunctional catalysts, the glucose conversion was proportional to the total Brønsted acidity of the catalysts. Moreover, the glucose conversion to fructose increased with higher Nb loadings, while the selectivity to HMF decreased, resulting in similar HMF yields obtained on the composites. The drop in selectivity to HMF was attributed to the degradation of HMF to humin catalyzed by high concentrations of BA sites. After the removal of strong BA sites originating from Nb2O5, the selectivity to HMF significantly increased, leading to the maximum HMF yield of 31% obtained by a composite containing 60% Nb2O5 and treated at 300 °C in air. Thus, a catalyst with poor Brønsted acidity of Nb2O5 origin (Nb-OH) was shown to be preferable to achieve moderate HMF yield without humin formation in an environmentally friendly and cost-effective process. It was also shown that the type and amount of BA sites from the carbon component affected the yield of HMF. BA in the form of oxygen functionalities increased the selectivity to HMF, whereas larger concentrations of P-OH showed the opposite effect.

Modification of the V2O5-WO3/TiO2 catalyst with Nb to reduce its activity for SO2 oxidation during the selective catalytic reduction of NOx

A series of Nb-modified V2O5-WO3/TiO2 catalysts were prepared by the impregnation method and the effect of Nb loading on their SO2 oxidation activity during the selective catalytic reduction of NOx was investigated. The results indicate that the Nb2O5-V2O5-WO3/TiO2 catalyst with a Nb2O5 loading of 2% exhibits the lowest SO2 conversion of 0.6% for oxidation at 350 °C, whereas the conversion of NOx is still above 95%. The catalysts were characterized by TGA, BET, XRD, H2-TPR, CO2-TPD, XPS and in-situ DRIFTS. The results illustrate that the influence of Nb modification on the crystal structure of V2O5-WO3/TiO2 catalyst is rather insignificant; however, the surface area of the Nb2O5-V2O5-WO3/TiO2 catalyst decreases slightly after the modification with Nb, conducing to a decrease of SO2 adsorption on the catalyst. Meanwhile, the content of oxygen adsorbed on the catalyst surface decreases considerably upon the Nb modification, suggesting a weakened redox performance, which is beneficial to reducing the oxidation of SO2. The in-situ DRIFTS results illustrate that the content of the intermediate VOSO4 product on the catalyst surface decreases over the Nb-modified Nb2O5-V2O5-WO3/TiO2 catalyst, leading to a decrease of SO3 production.

Glucose Conversion into 5-Hydroxymethylfurfural over Niobium Oxides Supported on Natural Rubber-Derived Carbon/Silica Nanocomposite

5-Hydroxymethylfurfural (HMF) is one of the most important lignocellulosic biomass-derived platform molecules for production of renewable fuel additives, liquid hydrocarbon fuels, and value-added chemicals. The present work developed niobium oxides (Nb2O5) supported on mesoporous carbon/silica nanocomposite (MCS), as novel solid base catalyst for synthesis of HMF via one-pot glucose conversion in a biphasic solvent. The MCS material was prepared via carbonization using natural rubber dispersed in hexagonal mesoporous silica (HMS) as a precursor. The Nb2O5 supported on MCS (Nb/MCS) catalyst with an niobium (Nb) loading amount of 10 wt.% (10-Nb/MCS) was characterized by high dispersion, and so tiny crystallites of Nb2O5, on the MCS surface, good textural properties, and the presence of Bronsted and Lewis acid sites with weak-to-medium strength. By varying the Nb loading amount, the crystallite size of Nb2O5 and molar ratio of Bronsted/Lewis acidity could be tuned. When compared to the pure silica HMS-supported Nb catalyst, the Nb/MCS material showed a superior glucose conversion and HMF yield. The highest HMF yield of 57.5% was achieved at 93.2% glucose conversion when using 10-Nb/MCS as catalyst (5 wt.% loading with respect to the mass of glucose) at 190 °C for 1 h. Furthermore, 10-Nb/MCS had excellent catalytic stability, being reused in the reaction for five consecutive cycles during which both the glucose conversion and HMF yield were insignificantly changed. Its superior performance was ascribed to the suitable ratio of Brønsted/Lewis acid sites, and the hydrophobic properties generated from the carbon moieties dispersed in the MCS nanocomposite.

Effects of niobium addition on active metal and support in Co–CeO2 catalyst for the high temperature water gas shift reaction

Nb promoter was introduced in the Co–CeO2 catalyst to improve its catalytic performance in the high temperature water gas shift (HT-WGS) reaction, which involves the production of hydrogen from waste-derived synthesis gas. Thereafter, the physicochemical properties of the promoter as well as those of the catalyst were investigated using various techniques. Thus, it was confirmed that the Nb promoter affects the key properties related to catalytic performance, including oxygen storage capacity (OSC) and dispersion. To maximize the positive effect of the Nb promoter, we further investigated the effect of the Nb loading amount on the performance of the catalyst. The results obtained showed that the 1.5NbCo catalyst exhibited the highest catalytic performance (XCO = 89% at 450 °C) under very severe conditions (gas hourly space velocity (GHSV) = 637,320 h−1). Additionally, this catalyst exhibited stable activity for 50 h. This its high catalytic performance could be attributed to its high OSC and Co dispersion. Therefore, it is a promising catalyst for hydrogen production via the HT-WGS reaction.

The effect of Niobium oxide as a promoter on -Al2O3 based oxygen carrier for chemical looping combustion

Chemical looping combustion (CLC) is known as a low-cost strategy for the capture of carbon dioxide for fuel combustion. In CLC process, oxygen carriers (OCs) are the cornerstone and play a vital role in defining reaction process. The aim of the present work was to investigate the potential of niobium (Nb) as a promoter on the -Al2O3 and its behavior as an oxygen carrier for chemical looping combustion process. In this work, the Nb loading was varied between 5 – 15 wt. %. Their behavior in CLC was analyzed by TPR, TPO, SEM and TGA. From the TPR results, the reduction temperature for 5 wt.%, 10 wt.% and 15 wt.% Nb loading were 560 ℃, 529 ℃ and 545 ℃, respectively which indicated that reduction reaction occurs around 500 ℃ and above. SEM analysis showed that increasing of Nb loading resulted in some agglomeration and thus lowering the ability of metal oxide to gain and release oxygen. The redox characteristics were carried out using TGA with 5% CH4/N2 was used as the reducing gas, while air was used as oxidizing gas. The highest oxygen transfer capacity was 3.0% which is presented by 5 wt.% of Nb loading. Since the addition of Nb successfully improved the oxygen transport capacity, it can be concluded that Nb is the potential candidate for oxygen carrier in CLC.

Effect of niobium on the activity of Pd/xNb/Ce0.5Zr0.5O2 catalyst for CH4 combustion

A series of Pd/NbOx/Ce0.5Zr0.5O2 catalysts of varying Nb loading were synthesized and tested for the methane combustion. The introduction of Nb (Nb2O5) improved significantly the CH4 combustion activity over the Pd/Ce0.5Zr0.5O2 catalyst. The site activity (TOF, s−1) was found to increase with increasing loading of Nb. The latter enhancement was possibly attributed to a synergetic effect between Pd and Nb2O5. The addition of Nb was found to tune not only Pd dispersion but also the concentration of adsorbed surface oxygen species and the surface acidity of the catalyst. The latter was found to be strongly related to the catalytic activity.

The optimization of Nb loading amount over Cu–Nb–CeO2 catalysts for hydrogen production via the low-temperature water gas shift reaction

The effect of Nb promotion over a Cu–CeO2 catalyst was investigated in the low-temperature water gas shift reaction. The Nb loading amount was systematically varied from 0 to 5 wt% for the Cu–Nb–CeO2 catalyst, and the 1 wt% Nb promoted Cu–Nb–CeO2 catalyst exhibited the highest catalytic performance even at extremely high GHSV of 72,152 h−1. The catalysts were characterized through various techniques such as Brunauer-Emmet-Teller measurements, X-ray diffraction, N2O-chemisorption, H2-temperature programmed reduction, X-ray photoelectron spectroscopy, and transmission electron microscopy. It was found that the superior performance of the 1 wt% Nb promoted Cu–Nb–CeO2 catalyst was due to its enhanced reducibility, high BET surface area, small metallic Cu crystallite size, and high number of oxygen vacancies.

110th Anniversary: Near-Total Epoxidation Selectivity and Hydrogen Peroxide Utilization with Nb-EISA Catalysts for Propylene Epoxidation

The Nb-EISA catalyst with relatively low Nb loadings (∼2 wt %) shows exceptional propylene epoxidation performance with H2O2 as oxidant at 30–40 °C, 5–9 bar propylene pressure with nearly total pro...

Selective hydrogenolysis of glycerol to 1,2-propanediol on Nb-modified Pd−Zr−Al catalysts

Here, the Nb-modified Pd−Zr−Al catalysts with different Nb loadings were prepared and evaluated for glycerol hydrogenolysis to 1,2-propanediol. Characterization studies revealed that the addition of Nb to Pd−Zr−Al catalyst not only favored the reduction of PdO species, but also enhanced the acid strength and Pd dispersion on catalyst surface, which led to increased catalytic activity for glycerol hydrogenolysis. The optimal 8Nb/Pd−Zr−Al catalyst obtained 69.2% glycerol conversion and 84.5% 1,2-propanediol selectivity with a stable activity and a good reusability at 200 °C and 3.5 MPa H2, attributable to high Pd dispersion and strong interaction between the Pd and Nb species.

Oxidative dehydrogenation of ethane on diluted or promoted nickel oxide catalysts: Influence of the promoter/diluter

Ti- and Nb- containing NiO catalysts have been synthesized by two different preparation methods: i) by precipitation (Me-Ni-O oxides, Me = Nb or Ti), in order to prepare promoted NiO catalysts; and ii) by wet impregnation on TiO2 or NbOx supports, in order to prepare diluted/supported NiO catalysts. The catalysts have been also characterized and tested in the oxidative dehydrogenation of ethane. The catalytic performance of Ti- and Nb-promoted catalysts strongly depends on the composition, although in both cases the optimal one is found at similar Ti or Nb loadings (ca. 90 wt% NiO), showing similar ethylene selectivity in the ODH of ethane (ca. 90% at 10–20% ethane conversion). However, in the case of diluted catalysts, the catalytic behavior of Ti- and Nb-containing catalysts is drastically different. Then, over TiO2-diluted NiO catalyst, the highest selectivity to ethylene (ca. 90% selectivity) is achieved at NiO loading of 20 wt.%. However, over Nb2O5-diluted NiO catalyst, selectivity to ethylene was lower than 70%. A discussion on the characteristics of selective catalysts is done. In this case, the best catalysts must present a low concentration of free NiO and TiO2 or Nb2O5 phases, maximizing the Ni-O-Ti or Ni-O-Nb interaction. Interestingly, this takes place at different NiO loading depending on the preparation method and the nature of promoted/diluter. The low selectivity to ethylene achieved by NiO diluted with Nb2O5 has been related to the low interaction of NiO with the surface of Nb2O5, which hinders the elimination of unselective electrophilic O species.

Investigation of the role of Nb on Pd−Zr−Zn catalyst in methanol steam reforming for hydrogen production

Methanol steam reforming is regarded as a very promising process to generate H2 suitable for fuel cells. Typically, the Pd-based catalysts can catalyze efficiently methanol steam reforming for hydrogen production. But their high selectivity to CO, a byproduct of methanol reforming reaction, severely limits their potential application. In this work, a series of Nb-modified Pd−Zr−Zn catalysts with different Nb loadings were prepared to study their catalytic activities with more focus on the role of Nb on Pd−Zr−Zn catalyst for methanol steam reforming. The prepared catalysts were fully analyzed by using various characterization techniques, for example, ICP, BET, SEM, XRD, H2-TPR, NH3-TPD, HRTEM, CO chemisorption, XPS, and Raman. The experimental results showed that an increase in Nb loading for the Nb-modified Pd−Zr−Zn catalysts led to a decrease of the methanol conversion and H2 production rate. This was probably due to the decrease in the amount of oxygen vacancies on the catalyst surface. However, introduction of Nb into Pd−Zr−Zn catalyst increased the acid strength on the catalytic surface. The aldehyde species derived from methanol decomposition were readily transformed to HCOOH, thus yielding high selectivity to CO2 for the Nb-modified Pd−Zr−Zn catalysts. Significantly, the addition of Nb to Pd−Zr−Zn catalyst facilitated the incorporation of Pd into the ZnO lattices, which led to the formation of Pd−Zn alloy. Consequently, the Nb-modified Pd−Zr−Zn catalysts exhibited significantly lower CO selectivity and production rate than the Pd−Zr−Zn catalyst. From the results, this work offers a new way to the rational design of selective methanol steam reforming catalysts to decrease the formation of byproduct CO.

Niobium-doped TiO2 solid acid catalysts: Strengthened interfacial polarization, amplified microwave heating and enhanced energy efficiency of hydroxymethylfurfural production

TiO2 solid acid catalysts with different niobium (Nb) doping ratios (8, 16, and 24 mol%) were synthesized for microwave-assisted catalytic conversion of fructose to hydroxymethylfurfural (HMF). Nb doping has been demonstrated effective to enhance the quantity of acid sites and increase the acid strength. At optimal Nb loading of 16 mol%, it was found that the reaction rate constant and energy efficiency were significantly enhanced by 32 and 49%, respectively. The enhancement mechanism of Nb-doping was investigated by both experiment and density functional theory (DFT) calculation. It was revealed that Nb doping promoted the electron transfer from the bulk catalyst to surface sulfonic groups, resulting in enhanced acidity and polarity of sulfonic groups. The electron transfer could strengthen the sulfonic group-water interaction at catalyst surface and created a unique polarized interface region where heat generation and the catalytic reaction can be coupled for enhanced energy efficiency of such reactions.

Remarkable epoxidation activity of neat and carbonized niobium silicates prepared by evaporation-induced self-assembly

A series of mesoporous Nb-silicates with varying Nb loading from 2.2 to 22.0 wt% were synthesized by the Evaporation-Induced Self-Assembly (EISA) method using Pluronic P123 as mesopore forming agent. High dispersion of NbOx species was achieved up to a Nb loading of ∼22 wt% without any formation of bulk crystalline Nb2O5. Small angle X-ray scattering, TEM and N2 physisorption confirm mesoporosity in these materials with short-range ordering. The BET surface area and pore volume range from 779 to 978 m2/g and 0.76–1.21 cm3/g, decreasing at higher Nb loadings. Isolated NbO4 and oligomeric NbOx species were evident from diffuse reflectance UV-Vis spectra. For the epoxidation of cyclohexene with H2O2, the Nb-EISA catalysts generally perform better in terms of activity, epoxidation selectivity and H2O2 utilization compared to other mesoporous niobium silicates. Further, a significant improvement in the epoxide selectivity was achieved with Nb-EISA samples calcined in N2 during synthesis such that a thin C layer is left behind. The C layer may induce pore hydrophobicity that prevents the ring-opening reaction of the primary epoxide product.

Effects of tunable acidity and basicity of Nb‐KIT‐6 catalysts on ethanol conversion: Experiments and kinetic modeling

The relative amounts of acidic and basic sites in niobium‐containing ordered mesoporous silicates (Nb‐KIT‐6) are tunable with metal loading. The acidity and basicity change from 0.11 to 0.34 mmol NH3‐equiv./g cat. and 0.17 to 0.31 mmol CO2‐equiv./g cat., respectively, as the Nb loading is increased from 1.5 to 10.9%. This work harnesses this unique feature to better understand acidity and basicity effects on ethanol conversion activity and product selectivity. It is shown that the yields of ethylene and acetaldehyde, the dominant products, are tuned with Nb loading. Catalyst characterization results and designed kinetic experiments provide evidence of acid‐base pairs involved in the formation of various products. A macrosite kinetic model based on these observations and published mechanistic pathways fits the data for formation of the major products (ethylene, acetaldehyde, diethyl ether, and ethane) remarkably well. These results provide guidance for the rational design of bifunctional mesoporous materials exhibiting tunable acidity and basicity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2888–2899, 2017

Stable Metal-Organic Framework-Supported Niobium Catalysts.

Developing structurally well-defined, supported oxide catalysts remains a significant challenge. Here, we report the grafting of Nb(V) oxide sites onto the nodes of the Zr-based metal organic framework (MOF) NU-1000 as a stable, well-defined catalyst support. Nb(V) oxide was deposited with loadings up to 1.6 mmol/g via two postsynthetic methods: atomic layer deposition in a MOF, and solution-phase grafting in a MOF. Difference envelope density measurements indicated that the two synthetic methods resulted in different local structures of the Nb(V) ions within NU-1000. Despite their high Nb(V) loadings, which were equivalent to >60% surface coverage, nearly all Nb(V) sites of the MOF-supported catalysts were active sites for alkene epoxidation, as confirmed by phenylphosphonic acid titration. The MOF-supported catalysts were more selective than the control Nb-ZrO2 catalyst for cyclohexene epoxidation with aqueous H2O2 and were far more active on a gravimetric basis.

CeO2 morphology-dependent NbOx –CeO2 interaction, structure and catalytic performance of NbOx/CeO2 catalysts in oxidative dehydrogenation of propane

A series of NbOx/CeO2 catalysts were synthesized employing CeO2 cubes (c-CeO2), CeO2 rods calcined at 500°C (r-CeO2-500) and CeO2 rods calcined at 700°C (r-CeO2-700) as the supports, and their structures and catalytic performances in the oxidative dehydrogenation of propane reaction were studied. Strong CeO2 morphology-dependent NbOx –CeO2 interaction, structure and catalytic performance of NbOx/CeO2 catalysts were observed. The supported NbOx species evolve with the increasing Nb loading from the monomeric Nb species to the polymeric Nb species and the CeNbO4 species. The monomeric Nb5+ species interacting with oxygen vacancy/Ce3+ on CeO2 can be reduced to form the monomeric Nb4+ species. The formed monomeric Nb species follows the order of NbOx/r-CeO2–500>NbOx/c-CeO2>NbOx/r-CeO2-700 while the formed monomeric Nb4+ species is most abundant on the c-CeO2 support. The loading of NbOx suppresses the surface reduction of c-CeO2 and r-CeO2-500 but promotes the surface reduction of r-CeO2-700. R-CeO2-500 and r-CeO2-700 are more catalytic active than c-CeO2 in the oxidative dehydrogenation of propane reaction. The loading of NbOx slightly enhances the catalytic activity of c-CeO2 but decreases the catalytic activity of r-CeO2-700. A loading of 0.6 Nb/nm2 much enhances the catalytic activity of r-CeO2-500 at 200°C, but further increasing the Nb loading results in the decrease of the C3H8 conversion. These results add a solid example to vindicate the morphology engineering strategy to modify the structure and catalytic performance of CeO2-based catalysts.

Comparative Study of Nb-Incorporated Cubic Mesoporous Silicates as Epoxidation Catalysts

Niobium in varying amounts was successfully incorporated into SBA-16 material via a one-pot direct synthesis technique. Complementary small angle X-ray spectroscopy (SAXS), wide-angle X-ray diffraction (XRD), N2 sorption isotherms, and high resolution transmission electron microscopy (HR-TEM) imaging results confirm the structural features and integrity of the prepared materials. The Nb-SBA-16 materials possess high surface areas (829–1060 m2/g) and pore volumes (0.69–0.82 cm3/g) that decrease with increasing niobium content. Niobium exists mostly as tetrahedral NbO4 and oligomeric NbO4 species. NH3-TPD results reveal that Nb insertion imparts Lewis acidity to the SBA-16 supports, with the acidity increasing with Nb loading. The Nb-SBA-16 materials show good cyclohexene epoxidation activity with H2O2 as oxidant, with the epoxide selectivity decreasing at higher Nb loadings due to ring-opening and other side reactions of the epoxide, attributed to increased acidity. The Nb-SBA-16 materials generally show s...