Catalytic gasification of phenol in supercritical water with Ru/graphitized carbon black

Abstract

Phenol is a long-lived intermediate, and it is ubiquitous in the supercritical water gasification (SCWG) processes of various organic materials such as biomass, lignite and sewage sludge. In this work, ruthenium supported on graphitic carbon black (Ru/GCB) was prepared for catalytic gasification of phenol in supercritical water. It exhibited excellent catalytic performance towards phenol gasification, a nearly complete gasification of phenol was achieved at a moderate temperature of 500 °C with an 1 g g⁻¹ catalyst loading. Kinetic study indicated the activation energy and the frequency factor for phenol gasification were 154.0 ± 27.2 kJ mol⁻¹ and 23.2 ± 4.5, respectively. Furthermore, catalyst recycling experiments were conducted to evaluate the stability of Ru/GCB catalyst in the SCWG. X-ray diffraction, Raman spectrum, N₂ adsorption, X-ray photoelectron spectroscopy and transmission electron microscopy were performed to characterize the fresh and used catalyst in order to investigate possible changes of catalyst after the recycling, the results indicated that the catalyst was found to be stable for phenol gasification under the experimental conditions.

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1. Introduction

Gasification of organic materials in supercritical water (SCW) (T_c = 374 °C, P_c = 22.1 MPa) is regarded as a potential technology to produce gaseous fuels (e.g., H₂, CH₄), especially for those with high moisture content, such as wet biomass,^1,2 sludge^3,4 and lignite.^5–7 SCW is miscible with organic compounds and gases, which allows for a high reaction rate due to the elimination of mass and heat transfer resistance.⁸ In addition, SCW serves as both a solvent and a reactant during the gasification process, thus the need for drying feedstock can be avoided. The supercritical water gasification (SCWG) of organics is an endothermic reaction, elevated temperature is required to achieve a desirable conversion, which inevitably leads to severe operation conditions. Therefore, an appropriate catalyst is needed to decrease the reaction temperature and enhance the yields of target products.

The alkali catalysts, such as KOH, NaOH, Ca(OH)₂ and K₂CO₃ are usually used in SCWG. These alkaline catalysts can catalyze the gasification, reforming and the water–gas shift reactions.^5,7,9 However, the recycle of alkali is difficult.¹⁰ Moreover, their corrosivity brings adverse impact on the service life of the equipment. To overcome the disadvantages, metal catalysts such nickel (Ni)-based and Ru-based catalysts also have been widely employed in SCWG as heterogeneous catalysts because they exhibit high catalytic activity and can also be recycled easily.^6,9,11–14 Among various metal species, Ru shows the best catalytic activity in SCWG.¹⁵

The structural stability of catalyst support also plays a critical role in SCWG because the catalyst stability problems are widely observed in hot compressed water. Al₂O₃, Al₂O₃–SiO₂, CeO₂ and CeO₂–ZrO₂ were typically reported as good candidates for supporting materials. However, the catalyst properties could be changed due to the phase change such materials undergone in SCW.^6,16,17 Even though some ceramics such as SiC, Si₃N₄, and BN could tolerate extreme conditions, they also show poor capability to sustain corrosion in SCW.^18,19 Activated carbon (AC) is reported to be a relatively stable material in SCW, however its high surface area mainly results from micropores, which disfavors the mass transfer, especially for large molecules. What is more, coke formation in SCWG can block the entrances of micropores in AC and thereby deactivate the catalyst.^20,21 The issues mentioned above indicate that the development of stable materials is of significant importance for SCWG.

Richard et al.¹⁸ found that graphite and glassy carbon are very stable in SCW under the experimental conditions of 25 MPa, 550 °C or 350 °C. Devlieger²¹ also reported that carbon nanotubes (CNTs) are a promising stable catalyst support material for the production of hydrogen by reforming of ethylene glycol and acetic acid in SCW. It is believed that the carbon materials such as graphite and CNTs with crystalline nature could be very stable in supercritical water. Hence, it is beneficial to exploit the use of highly graphitized carbon as catalyst support for SCWG. Graphitized carbon black (GCB), which has a high degree of graphitization, has been widely used as catalyst supports in fuel cells due to its high chemical inertness, high surface area and well mesopore structure.^22–24 However, to our best knowledge, there are very few researches reported on the GCB as a catalyst support material for SCWG.

In the SCWG of various organic materials, phenol was often selected as model compounds because it is important structural features of lignin and it often forms as a major byproduct in SCWG of diverse feeds.^25–28 Phenol has been described as one of the last hurdles to complete gasification of biomass, because it is long-lived and ubiquitous intermediates in SCWG.²⁹ In addition, phenol is also a widely encountered industrial pollutant (oil, painting, pesticides, colouring agents and pharmaceutical industries), which is difficult to eliminate from wastewater.³⁰ Thus, the removal of phenol is considered as an important work and hot issue for both energy conversion and environmental protection.

Therefore, the main purposes of this work are to study the catalytic activity of Ru/GCB for gasification of phenol under variable parameters, and to evaluate its stability in supercritical water.

2. Experimental

2.1 Catalyst preparation and characterization

The graphitized carbon black was supplied by Shanghai Pantian powder material Co. Ltd (Shanghai, China) and used as received. The Ru/GCB catalyst (total Ru loading is 2 wt%) was prepared by impregnating GCB powder into aqueous solutions of RuCl₃. After addition of the metal salts, the catalyst was dried at 105 °C for 12 h, and then the precursor was reduced by H₂ (30 mL min⁻¹) with 3 °C min⁻¹ ramp from ambient temperature to 450 °C, hold 3 h at the final temperature and then self-cooled to ambient temperature to obtain the catalysts.

Powder X-ray diffraction (XRD) patterns of fresh and used catalyst were conducted on a Bruker D8 Advance diffractometer system, with Cu Kα radiation (1.5406 Å) and a graphite monochromator. Raman spectra was recorded on a Raman spectrometer (LabRAM HR Evolution, HORIBA Scientific) with a 532 nm laser. Nitrogen adsorption–desorption isotherms were measured at −196 °C using a Micromeritics Tristar II 3020 analyzer. X-ray photoelectron spectrum (XPS) analysis was done using an ULVAC PHI 5000 Versa Probe-II equipment. The morphology and structure were observed using high resolution transmission electron microscopy (HR-TEM) performed on a FEI Tecnai G² TF30 S-Twin transmission electron microscope.

2.2 Gasification of phenol with Ru/GCB

The process and equipment used in this work have been described in details in our previous work.^6,12 316-Stainless steel mini-batch reactors with internal volume of 10 cm³ were used in this study, the reactors had a Swagelok cap on one end and a HiP high-pressure valve assembly on the other. In a typical experiment, 0.055 g phenol (Sigma-Aldrich) and 0.055 g Ru/GCB catalyst were loaded into each reactor together with 1 mL of freshly deionized water. After repeatedly charging and venting with helium, the reactor was filled to 0.2 MPa (gauge) with helium. Reactions were carried out by placing the reactors vertically in a Techne fluidized sand bath (model SBL-2) maintained at the desired temperature by a Techne TC-8D temperature controller. After desired reaction time, the reactors were removed and cooled to room temperature by a fan for gas phase analysis. After the gas phase was analyzed, some reactors were opened and rinsed five times with 30 mL of deionized water, and then the catalysts were filtered and separated from liquid fractions. The liquid fractions were collected for TOC determination. The rest of reactors were washed twice more with additional 6 mL aliquots of dichloromethane to filter and separate catalyst and liquid fractions. The liquid was transferred to a separatory funnel where the organic and aqueous phase separated. The organic phase was then collected for GC-MS measurement. The used catalysts were dried at 105 °C for 15 h for reuse and evaluation of its stability.

2.3 Analysis method and date interpretation

The gas phase (e.g. H₂, CH₄, CO₂, CO, C₂H₄ and C₂H₆) was analyzed by an Agilent Technologies model 7820A gas chromatograph equipped with a thermal conductivity detector (TCD). A 10 ft × 1/8 in. i.d. stainless steel column, packed with 60 × 80 mesh Carboxen 1000 (Supelco) separated each component in the gaseous mixture. The total organic carbon (TOC) of liquid fraction was determined with a Elementar vario TOC. The liquid fraction was analyzed by a GC-MS (PE, SQ 8T-680) equipped with an Elite-5MS non-polar capillary column to separate the constituents.

Carbon gasification efficiency (CGE, %), gas yield mmol g⁻¹) and molar fraction of product gas (%) are used to evaluate the gasification of phenol. The expressions are defined as follows (eqn (1)–(3)):

3. Results and discussion

3.1 Catalytic activity of Ru/GCB

SCWG of phenol was conducted under the conditions of 400–500 °C, 3–32 min, 5.2 wt% loading of phenol and a fixed 1 g g⁻¹ Ru/GCB loading with a constant water density of 0.1 g cm⁻³. Fig. 1 shows the gas yields of CO₂, CH₄ and H₂ under various conditions. A trace amount of CO and C₂H₆ were also detected, but their yields were negligible and not listed. As shown in Fig. 1, the yields of CO₂ and CH₄ increased with time at all temperature employed. At 400 °C, CO₂ was the major gaseous product while CH₄ was the major gaseous product at 450 °C and 500 °C. H₂ reached its maximum yield in the initial time at 450 °C and 500 °C, and then leveled off. In contrast, at a relatively low temperature of 400 °C, the yields of H₂ increased to about 9.62 mmol g⁻¹ in the initial 9 min and then notably decreased to 5.55 mmol g⁻¹. The yields of CH₄ to the contrary increased from 0.42 to 7.37 mmol g⁻¹ with increasing time from 3 to 32 min. The phenomenon maybe caused by the methanation of CO₂ (CO₂ + 4H₂ → CH₄ + 2H₂O), which is an exothermic reaction and it became more dominant at lower temperature.²⁰ Thus, the yield of CH₄ increased at the expense of H₂ production. Compared with time, the effect of temperature on gasification is more significant. As can be seen in Fig. 1, the reaction equilibrium can be accelerated at higher temperatures. The equilibrium of gaseous compositions needs about 17 min at 450 °C and about 9 min at 500 °C, while at 400 °C, the equilibrium was not observed even at 32 min.

Fig. 1 The effect of time and temperature on yields of gaseous products (5.2 wt% phenol, 0.1 g cm⁻³ water density, 1 g g⁻¹ Ru/GCB).

The effect of time and temperature on molar fraction of various gaseous products is shown in Fig. 2. The molar fraction of CO₂ changed slightly with time at all temperature examined, while the molar fraction of CH₄ increased with the decreasing of H₂. The tendency is very similar to our previous study on SCWG of phenol with Ru/CeO₂ catalyst.¹⁷ The molar fractions of valuable gases (H₂ and CH₄) were also calculated to further evaluate the effect of temperature on energy recovery (ER), the sums of molar fraction of CH₄ and H₂ are respective about 41.1%, 61.7% and 62.6% at 400 °C, 450 °C, 500 °C and 32 min, indicating that increasing temperature is in favor of enhancement of energy recovery. These results are consistent with the conclusion in the study of SCWG of alga,¹ in which the ER increased from about 10–15% to nearly 60% with increasing temperate from 450 °C to 500 °C. The observation is mainly caused by that the higher temperature is, the more hydrogen atoms are released from water. In this work, the hydrogen gasification efficiency (HGE, defined as the ratio of total number of hydrogen atom in gaseous product to that in phenol added into reactor) are respective about 82.6%, 219.1% and 268.4% at 400 °C, 450 °C, 500 °C and 32 min in this work. Savage et al.²⁶ also discovered that more than 70% of the hydrogen atoms in the product gases come from the supercritical water at the equilibrium of phenol SCWG. Only 1.2% of the HGE is obtained in the non-catalytic gasification of phenol under same conditions, indicating that the Ru/GCB catalyst greatly improved the release of hydrogen atom in SCWG.

Fig. 2 The effect of time and temperature on molar fraction of gaseous products (5.2 wt% phenol, 0.1 g cm⁻³ water density, 1 g g⁻¹ Ru/GCB).

Fig. 3 shows the CGE obtained under different time and temperature. Clearly, the CGE increased with time and temperature. It is noted that the CGE reached to essentially 100% at 500 °C, indicating that phenol was likely completely gasified by adding catalyst. In order to further confirm the complete gasification of phenol, the TOC in liquid fractions was determined by a TOC analyzer. The value of TOC obtained at 500 °C and 32 min was 22.49 mg L⁻¹, which represents negligible carbon (0.67 mg) was in the liquid fraction. The result strongly suggest that nearly complete gasification of phenol was achieved at a moderate temperature of 500 °C. It's worth mentioning that under the same conditions but without any additive, the CGE was only 0.65%. Similar gasification results were also obtained if only GCB was used as catalyst, indicating that GCB do not show catalytic activity for SCWG of phenol. The CGE was about 80% when Ru/CeO₂ used as catalyst at similar conditions in our previous study, thus, the Ru/GCB catalyst exhibits superior activity for SCWG of phenol.

Fig. 3 The effect of time and temperature on the carbon gasification efficiency (5.2 wt% phenol, 0.1 g cm⁻³ water density, 1 g g⁻¹ Ru/GCB).

To further clarify the effect of Ru/GCB on the gasification, the liquid fractions obtained at 400 °C, 450 °C and 32 min were analyzed by GC-MS. In the non-catalytic gasification of phenol, twenty different compounds (mainly polycyclic aromatic hydrocarbons and benzofuran compounds) were identified as liquid intermediates.²⁷ The phenols such as p-cresols, catechol, 2,6-dimethoxy phenol and 3,4-dimethyl phenol were also detected in the SCWG of phenol with K₂CO₃ as catalyst.²⁵ Our previous study showed that ruthenium catalyst effectively suppress the formation of dimers and polycyclic aromatic hydrocarbons (PAHs), and only cyclohexanol and cyclohexanone were detected as main intermediates at 500 °C and 30 min.¹⁷ Huelsman et al.²⁷ implicated dibenzofuran and other phenolic dimers as precursor molecules for char formation pathways. The char formation pathways will inhibit the gasification of phenol. In this case, there are only two chromatographic peaks (shown in Fig. 4) were observed and they were identified as cyclohexanol and phenol, suggesting char formation was likely suppressed because that most of intermediates were inhibited or swiftly gasified when Ru/GCB was used as catalyst, and thus a desired gasification efficiency was achieved.

Fig. 4 Chromatogram of liquid phase obtained from SCWG of phenol at 32 min, (a) 400 °C and (b) 450 °C.

3.2 Kinetic study

Many reactions (including reforming, hydrogenation, deoxygenation, etc.) can be involved in the SCWG of various organic feeds. It is difficult to learn properties of each reaction in this process. And thus, the overall reaction rate have been investigated to evaluate the gasification process in many previous studies.^6,31 In addition, pseudo-first-order reaction kinetic model have often been used to describe the gasification process of various organic feeds. Therefore, the overall reaction rate in terms of CGE and first order kinetics were employed to study the SCWG of phenol in this work. The rate γ can be written aswhere t is the reaction time (min) and k is the reaction rate constant (min⁻¹). Furthermore, integral form of eqn (4) can be derived as following:

−ln[1 − CGE] = k × t (5)

The scatters of CGE shown in Fig. 3 was used to fit the rate coefficient k in eqn (5). The kinetic plots are shown in Fig. 5. The correlation coefficients R² are greater than 0.93, indicating that it is possible to use the pseudo-first-order reaction model to investigate the kinetic information on SCWG of phenol in the present case. The rate coefficient k can be described by the Arrhenius equation as follow:

where A, E_a, R and T are the pre-exponential factor, activation energy, universal gas constant (8.314 J mol⁻¹ K⁻¹) and temperature (in kelvin), respectively. And therefore, the activation energy of 153.98 ± 27.2 kJ mol⁻¹ was calculated at the temperature region of 400 °C to 500 °C. Furthermore, the experimental CGE was compared with the values calculated by the model to check the accuracy of model, and the results are shown in Fig. 6. The results show a good fit of the model to the experimental date, indicating the gasification of phenol can be well described with the kinetic model. A kinetic model for SCWG of phenol with Ru/CeO₂ was also developed in our previous work,¹⁷ the k obtained at 500 °C was 0.67, which is close to that obtained in this work. However, the E_a in this work is larger than that (84.24 ± 22 kJ mol⁻¹) obtained in our previous study. We believe that the different E_a is largely caused by the different model adopted. In previous study, the rate of phenol conversion was contributed by two reaction rates, one was the formation rate for gaseous product, and the other was the formation rate for char.¹⁷ In this work, however, only the overall reaction rate for carbon gasification was used to describe the gasification of phenol in supercritical water, the char formation and the interreaction between intermediates were not taken into account.

Fig. 5 First-order plot for gasification of phenol in SCW with Ru/GCB catalyst.

Fig. 6 The comparison between model and experimental carbon gasification efficiency.

3.3 Stability of Ru/GCB catalyst

The stability of catalyst is important in terms of industrial application. Therefore, recycling experiments were conducted for used catalyst to examine the stability of catalysts under the condition of 500 °C, 32 min and 1 g g⁻¹ catalyst loading with a water density of 0.1 g cm⁻³. The CGE and gas yields are given in Fig. 7. As can be seen, no dramatic deactivation was observed and the CGE was maintained at a higher level than 93% in the 7 consecutive runs. In addition, obvious fluctuation of the yield of H₂ did not occur in the recycling experiments. The yields of CO₂ and CH₄ dropped a little. These results indicate that Ru/GCB exhibited sufficient stability in SCWG of phenol. The stability of Ru supported on CeO₂ was previous studied in gasification of phenol,¹⁷ in which the CH₄ decreased from about 27.7 to 22.2 mmol g⁻¹ and CO₂ decreased from about 22.4 to 21.2 mmol g⁻¹ in the second use. Masayuki et al.³² also evaluated the stability of supported ruthenium catalysts in the SCWG of lignin. They found that the gas yield decreased from 50 C% to 20 C% in the second run due to the decrease of surface area of Ru/C catalyst. The gasification activity for Ru/Al₂O₃ was also rapidly lost in the initial stage of reaction because of the change in its structure.

Fig. 7 Recycling experiments of Ru/GCB (500 °C, 32 min, 5.2 wt% phenol, 1 g g⁻¹ catalyst loading and 0.1 g cm⁻³ water density).

In order to further clarify the stability of Ru/GCB catalyst, the fresh and spent catalysts (after 7 runs) were characterized by Raman spectra, N₂ adsorption, XRD, XPS and HR-TEM. Raman spectroscopy is commonly used to study crystallinity of carbon nano-materials²⁰ and therefore we further used this technique to study the stability of GCB support. The Raman spectra of fresh and spent catalyst are shown in Fig. 8. The most prominent peak (1584 cm⁻¹) is attributed to the G-band, which is characteristic of defect-free sp² hybridized carbon systems.³³ Another obviously peak located at 1346 cm⁻¹ is attributed to the defect induced D band of sp² carbon systems. The I_D/I_G intensity ratio between the disorder-induced D-band and the Raman allowed G-band is generally used to check the degree of graphitization.²¹ In this case, the I_D/I_G intensity ratio were 1.03 and 1.02 for fresh and used catalyst, respectively. The results confirm that the crystallinity of the GCB support were not destroyed under the experimental condition. In order to further verify the stability of GCB support in the SCWG, the N₂ physical adsorption of the pristine GCB, fresh and used Ru/GCB catalyst were also performed, the N₂ adsorption–desorption isotherms of the samples are shown in Fig. 9. According to Fig. 9, one can find that the N₂ adsorption–desorption isotherms of pristine support, fresh and used catalyst are exactly alike, indicating there is no observable structural change of the support. Moreover, the surface areas and average pore widths of the samples (seen in Table 1) are also very similar. The results discussed so far show that GCB is a very stable catalyst support material for SCWG.

Fig. 8 Raman spectra of the fresh and used Ru/GCB.

Fig. 9 N₂ adsorption–desorption isotherm of the pristine GCB, fresh and used Ru/GCB.

Table 1 Crystallinity of fresh and used Ru/GCB catalysts (XRD. FWHM); disorder in the carbon system (Raman spectrum. ratio I_D/I_G); surface area and average pore width of fresh and used Ru/GCB

Sample	FWHM GCB (002)	ID/IG	BTE (m^2 g^−1)	Average pore width (BJH adsorption, nm)
Pristine GCB	6.48	—	112.8	28.1
Fresh Ru/GCB	5.85	1.03	108.5	27.9
Used Ru·GCB	6.34	1.02	99.6	29.2

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XRD patterns of the fresh and used catalysts are shown in Fig. 10. Typical diffraction peaks of the graphite carbon were observed from the fresh and used catalysts. The diffraction line corresponding to the (002) plane originates from the hexagonal sp² hybridized carbon network and the line was further used to study the changes of the crystallinity of the GCB by comparing full-width at half maximum (FWHM, a change in FWHM is indicative of structural changes in the grain size and hence a change in crystallinity).²⁰ The FWHM values of the pristine GCB, fresh and used Ru/GCB are shown in Table 1. The little change indicates that the crystallinity of the GCB support was not affected in the recycling. For the Ru species, there is no marked diffraction peak attributed to Ru species detected over the fresh catalyst, due to high dispersion of Ru on GCB support. But for the used catalyst, peak at 35.1° can be observed, which is attributed to RuO₂ (101).³⁴ Moreover, the peaks at 38.38°, 42.15° and 44°, characteristic for the metal Ru (PDF# File 06-0663), became stronger than that of fresh catalyst. It can be inferred that the aggregation of highly dispersed metal particles likely occurred during the cycle use. The fine change may contribute to the slow decrease of the yields of CO₂ and CH₄ (shown in Fig. 7).

Fig. 10 XRD patterns of fresh and used Ru/GCB catalysts.

XPS characterizations of fresh and spent catalysts are shown in Fig. 11. Peaks corresponded to carbon, oxygen and ruthenium were presented in the survey scan (Fig. 11a). The main peaks observed in Fig. 11a at around 285 eV and 532 eV could be assigned to C 1s and O 1s, respectively. There is no obvious change of the characteristic peaks between the fresh and used catalyst. The chemical states of Ru were studied and shown in Fig. 11b, the observed main peak in XPS spectrum of catalysts at 284.4 eV could be assigned to Ru 3d_3/2 in Ru/GCB, which was overlapped with C 1s of the carbon atoms of the GCB support. The weak peak at about 280.3 eV could be assigned due to Ru 3d_5/2 photoelectron line.³⁵ It is noted that the overlap of the Ru 3d peaks and C 1s around 285 eV makes it difficult to analyze this range for ruthenium correctly (Fig. 11b). Therefore, the XPS of Ru 3p region of the catalysts have also been performed, as shown in Fig. 11c. Moreover, the peaks of Ru 3p_3/2 were deconvolved to study the change of Ru in detail, as can be seen in Fig. 11d. Through the deconvolution of XPS spectrum of Ru 3p_3/2 profiles, there are three major Ru species locating at 461.9 eV, 463.2 eV and 466.6 eV were obtained, and corresponded to metallic Ru, RuO₂, and hydrous amorphous RuO₂·xH₂O species, respectively.³⁶ The concentration of metal Ru were 83.0% and 79.4% for the fresh and used catalyst, respectively. Moreover, the oxygen species in the fresh and used catalyst were clarified according to the O 1s peaks of XPS spectra, shown in Fig. 11e. The concentrations of surface lattice oxygen and hydroxyl groups/carbonate were very similar. These results strongly support that the chemical state of Ru remains stable after used, which is consistent with our previous studies.^6,17

Fig. 11 XPS spectra of fresh and used Ru/GCB catalyst (a) survey scan, (b) Ru 3d peaks, (c) Ru 3p region peaks, (d) Ru 3p_3/2 peaks, (e) O 1s peaks.

Fig. 12 TEM of the fresh and used Ru/GCB (a) and (c) fresh Ru/GCB, (d) and (f) used Ru/GCB; size distribution of metal loaded on fresh (b) and used (e) catalyst.

Fig. 12 shows the TEM micrographs and metal particle size distribution of fresh and used catalyst. For the size distribution of catalyst particles loaded, more than 100 particles in the TEM micrographs were counted. As can be seen, the metal particles loaded are well dispersed on the surface of the GCB support. For the fresh catalyst, the average size of metal particles is about 2 nm and the range of particle size are between 1 and 3 nm. However, for the used catalyst, the mean diameter increases to about 3.41 nm (in the range of 1–6 nm), suggesting that metal particles aggregated into large particles in the recycling of catalyst. The observation is consistent with the results obtained by XRD. The aggregation of metal particle will lead to some reduction of catalyst activity, therefore the aggregation may help to explain the CGE decreased from 100% to about 93% in the 7 consecutive runs in this work.

4. Conclusions

In this work, the ruthenium supported on graphitic carbon black was prepared and used as a catalyst for gasification of phenol in supercritical water. The results indicated that the catalyst showed excellent catalytic performance. 5.2 wt% of phenol can be gasified completely at a moderate temperature 500 °C with 1 g g⁻¹ catalyst loading. The kinetic study of gasification by pseudo-first-order reaction model suggested the activation energy E_a was 154.0 ± 27.2 kJ mol⁻¹. The characterization of the fresh and used catalyst showed that the GCB support is very stable in supercritical water. However, in the recycling, a mild aggregation of Ru particles occurred, which may contribute to slight loss of catalytic activity for SCWG of phenol. Even though, recycling experiments confirmed that the catalytic activity of Ru/GCB is relatively stable for SCWG of phenol, the CGE can be maintained at a higher level than 93% in 7 consecutive runs.

Acknowledgements

This work was supported by the High Technology Talent Introduction Project of Yunnan in China (Project No. 2010CI110), Science and Technology Major Project of Yunnan Province (Project No. 2012ZB002), National Natural Science Foundation of China (21307049 and U1137603) and Yunnan Provincial Fund project (KKSY201222073).

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