Catalyst In Supercritical Water Research Articles

Study on the detailed reaction pathway and catalytic mechanism of a Ni/ZrO2 catalyst for supercritical water gasification of diesel oil

Diesel oil, as a petroleum product, can be used as a model of oilfield wastewater to be gasified in supercritical water (SCW). This paper is aimed to obtain the gasification characteristics, detailed reaction pathway of diesel oil and the catalytic mechanism of a Ni/ZrO2 catalyst in SCW, providing some guidance for supercritical water gasification (SCWG) of oilfield wastewater. The SCWG of 6.8 wt% diesel experiments were carried out in a quartz reactor system with different temperatures (460, 500, 540 °C), reaction time range of 0–90 min. The carbon gasification efficiency achieved 97.5% at 500 °C with the Ni/ZrO2 catalyst, which was over 3 times higher than 27.6% under non-catalytic condition. The liquid intermediate was analyzed by gas chromatography-mass spectrometry, and naphthalene was the most difficulty intermediate to be gasified in diesel SCWG. A kinetic model was established to describe quantitatively the variation of gas yields with experimental conditions. The production and consumption of gases by SCWG of diesel oil under non-catalytic and catalytic conditions were analyzed in detail, and the mechanism of the Ni/ZrO2 catalyst was proposed. The Ni/ZrO2 catalyst effectively enhanced the steam reforming and pyrolysis reactions as well as inhibited the polymerization and aromatization reaction.

Lignin Depolymerization into Aromatic Monomers Using Supported Metal Catalysts in Supercritical Water

Lignin Depolymerization into Aromatic Monomers Using Supported Metal Catalysts in Supercritical Water

Direct conversion of lignocellulosic biomass into aromatic monomers over supported metal catalysts in supercritical water

With the goal of contributing to the valorization of lignocellulosic biomass, we investigated the conversion of Japanese cedar and bagasse into aromatic monomers over charcoal-supported metal catalysts in supercritical water. Over Ru/C, Japanese cedar was converted into gaseous products such as methane and carbon dioxide. Pd/C, Pt/C, and Rh/C were active for the conversion of Japanese cedar into aromatic monomers, and the monomer yields over these catalysts decreased in the order Pt/C > Rh/C > Pd/C when the conversion reaction was carried out at 673 K for 1 h. Bagasse, an herbaceous biomass, could also be converted into aromatic monomers by reaction over Pt/C, Rh/C, or Pd/C at 673 K for 1 h. Catalyst reusability experiments revealed that the activities of Pt/C and Rh/C for Japanese cedar conversion increased with reuse, up until the third reuse.

Effect of Metal Catalysts on Bond Cleavage Reactions of Lignin Model Compounds in Supercritical Water

Lignin depolymerization into its aromatic monomers is necessary for the valorization of lignocellulosic biomass, but cleavage of the C–O ether bonds and C–C bonds between the monomers is challenging. In this study, we investigated bond cleavage reactions of benzyl phenyl ether, diphenyl ether, and biphenyl, which served as models for lignin compounds with α–O–4, 4–O–5, and 5–5 linkages, respectively, by using various metal catalysts in supercritical water at 673 K and a water density of 0.5 g cm−3 without added hydrogen gas. Benzyl phenyl ether was decomposed without catalyst. We found that palladium, platinum, and rhodium catalysts supported on carbon showed activity for cleavage of the C–O ether bonds in benzyl phenyl ether and diphenyl ether but not for the bonds between the aromatic units of biphenyl. Using anisole as a model compound, we also found that methoxy groups, which are present in lignin containing coniferyl and sinapyl alcohol monomers, were converted into phenol groups or were decomposed. The reaction conditions described herein have great potential utility for the conversion of lignin, which can be decomposed and dissolved in supercritical water, into valuable aromatic compounds.

Influence of reaction conditions on the catalytic activity of a nickel during the supercritical water gasification of dewatered sewage sludge

Dewatered sewage sludge with different moisture contents was treated with a nickel catalyst in supercritical water in a batch quartz tube micro-reactor at different temperatures and reaction times. The catalytic activity of the nickel wire catalyst was investigated. Because of the gradual deactivation of the catalyst, the hydrogen yield decreased by 8–72% after five catalytic cycles. The temperature has the largest influence on the deactivation rate of the nickel catalyst. The decline rate of the hydrogen yield decreased from 67% to 8% when the temperature was increased from 400 °C to 500 °C. SEM, EDS and TPO were used to characterize the spent catalyst. The results showed that the deactivation of the nickel catalyst is caused by the deposition of char/coke on the catalyst surface. Finally, the relationship between the reaction conditions and catalyst deactivation was investigated from the viewpoint of the organic matter degradation pathway.

Catalyst Oxidation and Dissolution in Supercritical Water

We use thermodynamic models to predict catalyst oxidation and dissolution in supercritical water (SCW) and use experiments to assess the viability of the models for practical SCW reaction systems and provide relative rates for these mechanisms. We examined the oxidation and dissolution of noble and transition metals, metal oxide catalyst supports, and transition metal carbides and nitrides under SCW conditions. The materials were tested in batch reactors at 400 °C for 60 min, and the SCW density was varied from 0 to 0.5 g/mL to observe the influence of the solvent properties on stability. Oxidation and dissolution were determined by comparing the initial catalyst composition and structure with those of the catalysts recovered from the reactors after exposure to the SCW environment. The gas-phase recovered from the reactors was analyzed for H2 produced from oxidation. The aqueous phase was analyzed for metals from dissolution. The ΔGrxn for oxidation and the solubility of the catalysts in SCW at the experi...

Acid catalytic properties of TiO2, Nb2O5, and NbOX/TiO2 in supercritical water

The acid catalytic properties of TiO2, Nb2O5, and NbOX/TiO2 in supercritical water at 400 °C and 25–65 MPa were investigated. Two model reactions were used to elucidate the effects of water properties on catalysis and their variation with the catalysts. The kinetics of both reactions obeyed the Langmuir-Hinshelwood model, and the competitive adsorption of water suppressed these reactions. An increase in the ion product of water with increasing water density promoted these reactions, particularly with TiO2 and Nb2O5. In addition, temperature-programmed desorption suggested that active sites with weaker affinity for water are mainly active in supercritical water, wherein Nb2O5 shows Brønsted acidity, NbOX/TiO2 shows Lewis and Brønsted acidity, and TiO2 tends to show Lewis acidity. The Brønsted acidities of TiO2 and NbOX/TiO2 increased with increasing water density. These findings will allow us to predict the acidities of solid acid catalysts in supercritical water.

Bond cleavage of lignin model compounds into aromatic monomers using supported metal catalysts in supercritical water

More efficient use of lignin carbon is necessary for carbon-efficient utilization of lignocellulosic biomass. Conversion of lignin into valuable aromatic compounds requires the cleavage of C–O ether bonds and C–C bonds between lignin monomer units. The catalytic cleavage of C–O bonds is still challenging, and cleavage of C–C bonds is even more difficult. Here, we report cleavage of the aromatic C–O bonds in lignin model compounds using supported metal catalysts in supercritical water without adding hydrogen gas and without causing hydrogenation of the aromatic rings. The cleavage of the C–C bond in bibenzyl was also achieved with Rh/C as a catalyst. Use of this technique may greatly facilitate the conversion of lignin into valuable aromatic compounds.

Behavior of Cholesterol and Catalysts in Supercritical Water

Cholesterol reacted over 5 wt % Pt/C, 5 wt % Pd/C, and HZSM-5 in supercritical water at 400 °C. The major products with the Pt/C and Pd/C catalysts are cholesterol-derived steroids, polynuclear aromatic compounds, aliphatic hydrocarbons, and gases (H2, CH4, and C2H6), resulting from hydrogenation, dehydrogenation, and cracking reactions. HZSM-5 favored initial dehydration of cholesterol to form cholesta-3,5-diene and then catalyzed further isomerization and cracking reactions. The presence of H2 had little or no effect, except when the catalyst was absent, in which case added H2 led to higher cholestadiene yields and lower cholesterol conversion. Increasing the catalyst loading resulted in higher yields of lower molecular weight products but also significantly lower carbon recoveries. Characterization of the Pt/C and Pd/C catalysts showed carbon support gasification and particle growth after exposure to supercritical water.

Catalytic upgrading of pretreated algal bio-oil over zeolite catalysts in supercritical water

We report the catalytic hydrothermal upgrading of pretreated algal bio-oil. The reaction was performed at 400°C for 240min with the addition of 6MPa H2 and 10wt.% zeolite catalyst in supercritical water (ρH2O=0.025g/cm3). Nine zeolites (Hβ, HZSM-5 (SiO2/Al2O3=25:1), HZSM-5 (SiO2/Al2O3=50:1), HZSM-5 (SiO2/Al2O3=170:1), HY (5% Na2O), HY (0.8% Na2O), SAPO-11, MCM-41 (50% Si), and MCM-41 (100% Si) were screened to investigate their effects on the yields of the product fraction and the properties (e.g., elemental composition and heating value) of the upgraded bio-oil. The catalyst type affected the yields of the product fractions: SAPO-11 produced the lowest upgraded bio-oil yield of 42.4wt.%, and MCM-41 provided the highest yield of 54.5wt.%. Compared with non-catalytic upgrading reactions, all of the zeolites promoted the denitrogenation, deoxygenation, and desulfurization of the pretreated bio-oil due to the presence of acid sites. HY (5% Na2O), HY (0.8% Na2O), and HZSM-5 (SiO2/Al2O3=25:1) showed the highest activity toward denitrogenation, deoxygenation, and desulfurization, respectively. The upgraded bio-oil mainly consisted of hydrocarbons, accounting for 80% in total and as high as 95.6% of the fraction below 400°C.

Influence of NaOH and Ni catalysts on hydrogen production from the supercritical water gasification of dewatered sewage sludge

Dewatered sewage sludge was treated with NaOH additive and Ni catalyst in supercritical water in a high-pressure autoclave to examine the effects of separate and combined NaOH additive and Ni catalyst on hydrogen generation. The effects of Ni/NaOH ratio on hydrogen production were also investigated to identify possible catalytic mechanism and interactions. NaOH and Ni, separately or in combination, improved the hydrogen production and hydrogen gasification efficiency. The addition of NaOH additive not only promoted the water–gas shift reaction, but also favored H2 generation of Ni catalyst by capturing CO2. The hydrogen yield of combined catalysts with different Ni/NaOH ratios was higher than the theoretical sum of hydrogen yield from the mixture by 10–33%. The largest hydrogen yield, of 4.8 mol per kilogram of organic matter, which was almost five times as much as without catalyst, was achieved with the addition of 3.33 wt% Ni and 1.67 wt% NaOH. The combined NaOH additive and Ni catalyst also improved the gasification of several other dewatered sewage sludges, increasing the hydrogen yield by four to twelve times that seen without catalyst. Combined NaOH additive and Ni catalyst are effective in dewatered sewage sludge gasification at low temperature.

On-Stream Regeneration of a Sulfur-Poisoned Ruthenium-Carbon Catalyst Under Hydrothermal Gasification Conditions

Catalytic processes that employ Ru catalysts in supercritical water are capable of converting organics, such as wood waste or biosolids, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, Ru catalysts are prone to S poisoning and are quickly deactivated. As S is ubiquitous in raw biomass and technologies to remove S from hydrothermal biomass feeds are lacking, regeneration protocols that efficiently reactivate S-poisoned catalysts are required to realize efficient conversion processes and long catalyst lifetimes. In this work, we developed a method to remove S from a S-poisoned Ru catalyst under hydrothermal conditions through an oxidative treatment in the aqueous phase. By using in situ X-ray absorption spectroscopy under the reaction conditions, we show that Ru is oxidized by dilute H2O2 at low temperatures, which leads to the removal of adsorbed S species from the catalyst surface. By optimizing the regeneration conditions, it was possible to prevent oxidation of the catalyst carbon support, as revealed by ex situ TEM. This treatment led to a reactivation of the Ru catalyst with a significant increase in carbon-to-gas conversion and methane selectivity.

Effects of water density on acid-catalytic properties of TiO2 and WO3/TiO2 in supercritical water

The effects of water density on the acid-catalytic properties of TiO2 and WO3/TiO2 catalysts in supercritical water at 400°C were investigated by using the kinetic analysis of the dehydration reaction of glycerol. The reaction selectivity of TiO2 and WO3/TiO2 catalysts and the apparent-reaction orders for water indicated that the acid-catalytic properties of these two catalysts show different dependence on water density. In the reaction using TiO2, the contribution of Lewis acid sites in TiO2 was large at a low water density, while the contribution of Brönsted acid sites in TiO2 increased with increasing water density. On the other hand, the reaction using WO3/TiO2 was mainly catalyzed by Brönsted acid sites in WO3/TiO2 even at a low water density, and the nature of Lewis/Brönsted acid sites in WO3/TiO2 was not influenced by the water density.

Hydrogen production from 2-propanol over Pt/Al2O3 and Ru/Al2O3 catalysts in supercritical water

One of the alternative energy sources to fossil fuels is the use of hydrogen as an energy carrier, which provides zero emission of pollutants and high-energy efficiency when used in fuel cells, hydrogen internal combustion engines (HICE) or hydrogen-blend gaseous fueled internal combustion engines (HBICE). The gasification of organics in supercritical water is a promising method for the direct production of hydrogen at high pressures, with very short reaction times. In this study, hydrogen production from 2-propanol over Pt/Al2O3 and Ru/Al2O3 catalysts was investigated in supercritical water. To investigate the influences on hydrogen production, the experiments were carried out in the temperature range of 400–550°C and in the reaction time range of 10–30s, under a pressure of 25MPa. In addition, different 2-propanol concentrations and reaction pressures were tested in order to comprehend the effects on the gasification yield and hydrogen production. It was found that Pt/Al2O3 catalyst was much more selective and effective for hydrogen production when compared to Ru/Al2O3. During the catalytic gasification of a 0.5M solution of 2-propanol, a hydrogen content up to 96mol% for a gasification yield of 5L/L feed was obtained.

Oxidative Degradation of Lurgi Coal-Gasification Wastewater with Mn2O3, Co2O3, and CuO Catalysts in Supercritical Water

Lurgi coal-gasification wastewater was degraded in supercritical water using Mn2O3, Co2O3, and CuO as catalysts. The experiments were performed in a batch reactor at temperatures of 380–460 °C and oxygen ratios of 1.5–3.5. The results involved evaluation of TOC and NH3–N removal efficiencies; detection of the main products in the effluent; XRD, SEM, and BET analyses of the catalysts; and detection of metal ions leached from the catalysts. Maximum TOC and NH3–N removals were found with Co2O3 catalyst at 460 °C and OR = 3.5. The effluent quality could meet class-I criteria of the Integrated Wastewater Discharge Standard (GB 8978-1996). The catalytic effects on pollutant removal were in the order Co2O3 > Mn2O3 > CuO. The major phase of Mn2O3 transformed into MnO2 with a decreasing BET surface area at 460 °C and an oxygen ratio of 3.5. Serious Cu-ion leaching occurred during the process and intensified with increasing temperature.

Gasification of Organosolv-lignin Over Charcoal Supported Noble Metal Salt Catalysts in Supercritical Water

Charcoal supported metal salt catalysts showed activities for the lignin gasification at 673 K, especially the catalysts without chloride anion showed the complete gasification. The order of activity for the gasification among the metal species was following: ruthenium > rhodium > platinum > palladium ~ nickel. X-ray absorption fine structure analysis revealed that small metal particles were formed on the charcoal support after the gasification for the supported ruthenium, rhodium, and platinum salt catalysts without chloride anion; on the other hand, larger metal particles were formed in the presence of chloride anion. Lower activities and deactivation for supported metal salt catalysts having the chloride anion were caused by poisoning by chloride anions and the smaller number of metal sites.

Evidence of Scrambling over Ruthenium‐based Catalysts in Supercritical‐water Gasification

AbstractCatalytic processes that employ Ru catalysts in supercritical water have been shown to be capable of converting organics, such as wood waste, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, the exact roles of the catalyst and the descriptors that would enable the search for other catalysts with high conversions have not been determined. In the current work, we use electronic structure calculations coupled with batch experiments to understand the interaction of methane (CH4) and water (H2O) with a common catalyst material, ruthenium, to understand the final steps of the methanation reaction. The calculations predict that when CH4 and H2O react with the Ru surface, the species will undergo rapid scrambling; interchanging most of the hydrogens with the surface before escaping as CH4 and H2O once again. We conducted experiments using CH4 as a feedstock in supercritical D2O (deuterated water) in the presence of a commercially available carbon‐supported Ru catalyst, and found this mechanism to be confirmed: nearly all reacted CH4 was converted to the fully substituted CD4 or the 3/4‐substituted CHD3 isotopomers, with less significant production of the 1/4‐ or 1/2‐substituted species CH3D and CH2D2. The experiment was repeated with an in‐house impregnated RuO2‐on‐carbon catalyst, with similar results. Although other criteria such as the ability to cleave CC and CO bonds and resistance to poisoning will also prove important, this study suggests that a characteristic of an effective catalyst for supercritical water gasification to methane is its ability to promote rapid equilibria through scrambling mechanisms.

Gasification of Sugarcane Bagasse over Supported Ruthenium Catalysts in Supercritical Water

Gasification of sugarcane bagasse on activated-carbon- and titania-supported ruthenium (Ru/C and Ru/TiO2) catalysts in supercritical water was studied. Sugarcane bagasse was completely gasified to methane, carbon dioxide, and hydrogen over Ru/C and Ru/TiO2 catalysts at 673 K. The carbon yield of the gas products from sugarcane bagasse, cellulose, and lignin over the Ru/TiO2 catalyst at a water density of 0.33 g cm–3 for 15 min was 50.3, 74.4, and 31.1 C %, respectively. In addition, it was observed that an increase in the water density enhanced the initial carbon yield of the gas products for 15 min. The recyclability of the Ru/C catalyst with complete gasification could make it a sustainable process.

Hydrogen production by supercritical water gasification of glucose with Ni/CeO2/Al2O3: Effect of Ce loading

Ni-based SCWG catalysts attract more attention for its high activity and relatively low cost. However, carbon deposition, which is one of the serious problems, reduces the activity of Ni-based catalysts. Ni/CeO2/Al2O3 catalysts with different Ce loading were prepared by a wet impregnation method. The performance of catalyst in supercritical water was tested in an autoclave reactor at 673K, 24.5MPa. The catalysts before and after reaction were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET specific surface area measurements and thermo-gravimetric analyses (TGA). The effects of Ce loading in catalysts on glucose gasification were studied. The results showed that hydrogen yield and hydrogen selectivity increased sharply with addition of Ni/CeO2/Al2O3 catalysts. When the Ce loading content was 8.46wt.%, the maximum H2 yield and H2 selectivity were obtained. The carbon deposition and coking will lead to the deactivation of the catalysts. Based on the thermo-gravimetric analyses, the oxidant kinetic data of carbon deposited on the used catalysts with air was obtained. CeO2 in the Ni/CeO2/Al2O3 catalyst plays an important role in inhibiting carbon deposition by increasing the Ni dispersion and reacting with deposited carbon.

Oleic acid gasification over supported metal catalysts in supercritical water: Hydrogen production and product distribution

Oleic acid was examined as a model compound for lipids, which was gasified in supercritical water (SCW) using a batch reactor from 400 to 500 °C at 28 MPa. The influence of operating temperature and several commercial catalysts on the gasification efficiency, hydrogen gas yield, and residual liquid product quality was examined and discussed. The main gaseous components measured were carbon dioxide (CO 2), hydrogen (H 2), methane (CH 4), and traces of carbon monoxide (CO). The residual liquid after reaction was characterized by analyzing the chemical oxygen demand (COD), total organic carbon (TOC), volatile fatty acids (VFAs), and the long chain fatty acids (LCFAs), namely, palmitic, myristic, stearic, linoleic, and oleic acids. The results showed that an increase of temperature coupled with the use of catalyst enhanced the gas yield dramatically. The H 2 yield was 15 mol/mol oleic acid converted using both the pelletized Ru/Al 2O 3 and powder Ni/Silica–alumina catalysts which gave 4 times higher than the equilibrium yield. The COD reduction efficiency ranged from 31% at 400 °C without catalyst to 96 % at 500 °C in the presence of Ni/Silica–alumina catalyst. The composition of residual liquid products was studied using gas chromatography/mass spectrometry (GC–MS), with a generalized reaction pathway for oleic acid decomposition in SCW reported.