(771g) Ni-Mg/Al2O3 and Ni-La/Al2O3 Catalysts for Steam Reforming of Biomass Tar

Introduction

Hydrogen generation through a renewable route is highly desirable. The production of renewable hydrogen can be performed through biomass gasification, followed by treatment of the resulting syngas. The presence of heavier and lighter oxygenated organic compounds, commonly referred to as tars, causes several drawbacks in the syngas processing and thus the abatement of these species is necessary.

Catalytic steam reforming seems to be the most promising industrial strategy for tar abatement after biomass gasification. Ni-Al₂O₃ based catalysts are active for this reaction, but due to their deactivation by sulphur contaminants they have been reported as non-applicable below a reaction temperature limit. The sulphur content in syngas produced by gasification processes ranges between 20 and 200 ppm v_H2S/v. On the other hand, reaction temperature for the tar abatement may depend from the overall gasification plant configuration.

It is well known that alkali and alkali earth cations moderate the surface acidity of alumina and can also induce surface basicity, thus besides acting as structural stabilizers of the support, they might allow limiting catalyst coking. However, several studies suggest that these components, namely magnesium, may also increase the catalytic activity of Ni/Al₂O₃, e.g. for ethanol steam reforming and for methane reforming, Ni/MgAl₂O₄acting as excellent tar steam reforming catalysts.

In literature it is reported that Lanthanum is able to increase nickel dispersion and also has got a beneficial effect increasing sulphur resistance of the catalysts. In addition,  the presence of lanthanide oxides enhances the stability of Ni-based catalysts, since these oxides are known promoters of carbon removal from metallic surfaces.

In this communication we present our results on the study of steam reforming of an ethanol-phenol mixture, used as a model for biomass tar, over Ni-Al₂O₃ based catalysts. The effect of catalysts doping with basic elements, such as lanthanum and magnesium, on the catalytic activity and the sulphur deactivation behaviour is also reported.

Experimental

Catalysts preparation

The nickel-based catalysts examined here, were synthesized via wet impregnation of the desired nickel weight amount onto commercial alumina (Siralox 5-170, Sasol, 170 m²/g); Ni16 designates the catalyst containing 16% w/w of Ni. The magnesium containing catalyst (Ni16Mg, with 20% of MgO w_MgO/w_Al2O3) was prepared by sequential wet impregnation of Siralox 5/170 support with magnesium nitrate and nickel nitrate, both steps followed by drying and calcination. For these catalysts after each impregnation, drying at 363K for 8 hours and calcination at 973K for 5 hours have been performed.

Lanthanum containing catalysts were prepared through incipient wetness impregnation of commercial alumina, previously stabilized. Three different amounts of La₂O₃ were added: 5%, 20% and 80% w_La2O3/w_Al2O3. The samples were first calcined, and then nickel addition was done by the same impregnation technique. The final catalyst calcination was conducted from room temperature to 1023 K with a ramping of 2°C/min and a 5-hour hold at the final temperature. The NiO loading for all the catalyst prepared was 20% (w/w), corresponding to 16% Ni loading (w/w), because this amount has been shown from previous studies to give a good compromise between catalytic activity and sulphur deactivation resistance.

Characterization

The materials were characterized for BET, XRD, XPS, H2-TPR, IR, and IR with adsorption of probe molecules. The catalytic activity and a part of the material characterization on Ni-La/Al₂O₃catalysts it is now in progress while Ni16 and Mg-doped catalyst were fully characterized.

Catalytic reaction conditions

The total flow rate fed to the tubular quartz flow reactor was 40 Nml/min, with the following composition: 4.1% (v/v) of ethanol, 2% (v/v) of phenol, 54.6% (v/v) of water and 39.3% (v/v) of helium as carrier gas. The reactor was filled up for every test with 440 mg of sieved silica glass (60-70 mesh) and 44.1 mg of catalyst. Products analysis was performed with a gas-chromatograph Agilent 4890 equipped with a Varian capillary column “Molsieve 5A/Porabond Q Tandem” and TCD and FID detectors in series. Between them a Nickel Catalyst Tube was employed to reduce CO to CH₄. The sampling of the gases was made by injection, using a gas-tight with a nominal volume of 0.25*10^-6 m³. A sampling injection point was also available at the end of the preheating zone to analyse the reagents and determine the possible decomposition/change in the feed. Precise identification of the compounds in the products stream was also performed on GC/MS (ThermoScientific). Catalytic experiments were performed both in rising and in reducing reaction temperature (773K, 873 K, 973 K, 1023 K and reverse) in order to reveal if there a conditioning effect was affecting the catalytic performance.

Sulphur poisoning was examined adding tetrahydrotiophene (C₄H₈S, THT) in the feed. In one series of experiments, performed at 973 K, 210 ppm of THT were continuously fed with the reactants on stream for up to six hours. In a second series of experiments performed at the same temperature, two sequential pulses of THT (first 0.011 mol_S/mol_Ni and after 0.033 mol_S/mol_Ni) and a subsequent hold in the sulphur-free stream were performed.

Results

The experiments performed with sulphur free feed showed that pure alumina catalyses the conversion of ethanol to ethylene with traces of hydrogen exchange reaction to give ethane and acetaldehyde. Alkylation of phenol mainly to o-ethylphenol is observed. Experiments using increasing reaction temperature show that Ni containing catalysts catalyse the low temperature alkylation of phenol more than the pure alumina does. Complete steam reforming of both reactants to CO and CO₂ with traces of methane is obtained at 973 K for Ni5 and Ni16, while for Ni39 the steam reforming is almost complete at already 873 K. Experiments in decreasing reaction temperature revealed a catalyst conditioning effect that occured during high temperature operation, resulting in higher catalytic activity to steam reforming at lower temperature. Over Ni16 catalyst, in the increasing temperature experiment, ethanol shows an almost complete conversion already at 773 K while phenol is only barely converted. The main products are: alkylation products, ethylene, acetaldehyde and dialkylates. No activity in steam reforming was present. At 873 K phenol conversion remains around 25% while ethanol conversion is 86%. There is a little selectivity toward CO and CO₂ but the main products remain ethylene, mono-alkylate products, acetaldehyde and ethane. At 973 K and 1023 K we have complete activity to steam reforming, and CO and CO₂ as main products both in the increasing temperature experiment and in the decreasing one, accompanied by the complete conversion of both reactants and a hydrogen yield ranging around 80-82%.

For the magnesium containing catalyst (Ni16Mg), in the increasing temperature experiment, the main product at 773 K is acetaldehyde, followed by C8 alkylated product, ethylene, CO₂ and acetone. By increasing the temperature at 873 K ethanol conversion is complete while phenol one decreases to 28%. Acetaldehyde and ethylene together with CO₂, CO, mono-alkylates, methane, acetone and benzene are the detected products. At 973 K complete steam reforming of both reactants occurs to CO₂ (70% selectivity) and CO, with a hydrogen yield of 88%. By further increase of the reaction temperature to 1023K we have as expected complete steam reforming but with an increase of CO selectivity to 39% being CO₂ one 61%. The same activity and selectivity observed for the increasing temperature experiment is found at 1023 K and 973 K. A completely different behavior is observed at 873 K, where we have complete activity towards the steam reforming reaction and CO₂ is found as the main product with a selectivity of 74%, together with CO and methane. The hydrogen yield obtained in this case is 84%. At the lower temperature (773 K) almost complete conversion of ethanol and 44% conversion of phenol are observed. A different product distribution is found in this case: the main product  is CO₂together with ethylene, monoalkylates, CO and acetaldehyde.

Conclusions

The tests performed on Ni16Mg show that this catalyst it is more active in steam reforming reaction than the catalyst without magnesium but it is also more prone to sulphur deactivation than Ni16. The performances of Lanthanum containing catalysts are now in progress, showing interesting preliminary results.