Two series of bimetallic Ni-Co catalysts and corresponding monometallic catalysts with ca. 20 wt% metal loading were evaluated in hydrodeoxygenation (HDO) of phenol as a model compound for bio-oil. The bimetallic catalysts outperformed the corresponding monometallic catalyst in terms of conversion and cyclohexane selectivity. This could be attributed to the formation of Ni-Co alloy, which caused a decrease in metal particle size and stabilized Ni active sites in the near surface region. The balanced combination of formed Ni-Co alloy with acidity from supports allowed performing all individual steps in the reaction network toward desired products at high rate. Consequently, the two best-performing catalysts were tested in HDO of wood based bio-oil, showing that the bimetallic catalyst 10Ni10Co/HZSM-5 was more effective than 20Ni/HZSM-5 in terms of degree of deoxygenation and upgraded bio-oil yield. These findings might open an opportunity for development of a novel cheap but effective catalyst for a key step in the process chain from biomass to renewable liquid fuels.
Energy and environmental issues are two common concerns of modern society. Worldwide energy consumption increased rapidly from 10.6 to 12.9 billion tons of oil equivalent during the last decade. Fossil fuels based energy resources, including coal, gas and oil, supplied the vast majority (86%) of the total world energy demand in 2014 [
Nevertheless, the use of biofuels is still under debate owing to the competitive use as food and edible oils. Therefore, the focus of research nowadays shifts to the use of waste biomass thanks to its abundant availability, sustainability and CO2-neutrality. Therefrom derived fuels are discussed as 2nd generation biofuels. However, the biggest challenges for the large-scale implementation of renewable fuels from biomass are its low volumetric and energy densities, pointing to high costs for harvesting and transportation. In the light of that, liquefaction and fast pyrolysis appear as promising technologies, which transform biomass into a liquid so-called bio-oil with high energy density compared to original biomass [
However, the biggest drawback of bio-oil is its quality being far away from conventional liquid fuels due to high oxygenates and water contents, originating from the nature of biomass [
The high oxygenates content in bio-oil obtained from FP process (35 - 40 wt% on dry basis) causes some undesired features such as immiscibility with conventional fuels, high viscosity, low volatility, corrosivity and instability during long-time storage. In addition, the water content accounts to 15 - 30 wt%, bound as an emulsion, and it is not easy to separate by conventional methods. Therefore, it is necessary to lower the oxygen content to upgrade the quality of bio-oil and ultimately to make it suitable as a fuel component. For this purpose, hydrodeoxygenation (HDO), which is performed under elevated hydrogen pressure at moderate or high temperature (200˚C - 450˚C) in presence of a heterogeneous catalyst, has been intensively studied.
Variety of catalysts has been developed and tested for HDO of bio-oil and related model compounds, as recently reviewed in detail [
Non-noble monometallic catalysts (e.g. supported Ni, Fe, and Cu catalysts) have already been investigated in HDO studies [
It should be noted that the combination of metallic and acidic sites in suitable catalysts seems to be essential for HDO reaction to catalyze all the elementary steps in the preferred deoxygenation pathways. In addition, the catalyst material should be resistant to coking and heteroatoms in bio-oil in order to increase the catalyst lifetime. For this purpose, we have recently studied bimetallic Ni-Co and Ni-Cu-containing catalysts using HZSM-5 as carrier for aqueous phase HDO of phenol as a model compound for bio-oil to overcome the discussed drawbacks [
Two series of monometallic and bimetallic catalysts with ca. 20 wt% metal loading were prepared by incipient wetness impregnation and co-impregnation methods, respectively. A detailed description of catalyst preparation procedures is available elsewhere [
The physico-chemical methods used for catalyst characterization are presented in the following short summary. Detailed information is also described in our previous study [
Nitrogen physisorption measurements were performed on a Micromeritics ASAP 2010 apparatus at −196˚C. Prior to analysis, the calcined solids were degassed at 200˚C in vacuum for 4 h.
X-ray powder diffraction (XRD) measurements used for the phase composition study were performed by using a theta/theta diffractometer (X’Pert Pro from Panalytical, Almelo) with CuKa radiation (l = 0.15418 nm, 40 kV, 40 mA) and a X’Celerator RTMS Detector. Crystallite size was calculated by the Scherrer equation.
Acidic properties were determined by pyridine IR measurements in transmission mode on a Bruker Tensor 27 spectrometer. The pre-reduced catalysts were pretreated in 5% H2/He at 400˚C for 10 min. After cooling to RT and evacuation, the pyridine adsorption was performed until saturation. Then, the reaction cell was evacuated to remove physisorbed pyridine and finally the desorption of pyridine was followed by heating the sample in vacuum up to 300˚C and recording spectra every 50 K.
HDO of phenol as a model compound was carried out in an autoclave (Parr Instruments, 25 ml).Typically, phenol (0.5 g), H2O (10 g), and catalyst (0.025 g) were placed into the reactor. Argon was used to remove air and then the autoclave was pressurized with H2 to 50 bar at RT. The start time was recorded when the temperature reached 250˚C and then stirring speed was set to 650 rpm. After cooling to RT, the product gas was analyzed by gas chromatography (GC, HP 5890) online from autoclave. The liquid products (organic and aqueous phase) were analyzed by another GC (Shimadzu 17A) with autosampler. The internal standards mesitylene and 1,4-dioxane were used for quantification of organic and aqueous phases, respectively. Conversion and selectivity were calculated based on the following equations:
The carbon balances were calculated from the detected products and reached more than 90% in this work. Missing carbon was due to mostly work-up procedure (e.g. extraction step), deposits on the surface of catalysts and some unknown minor peaks in chromatograms.
HDO of bio-oil was performed in another autoclave (Parr Instruments, 100 ml). Typically, bio-oil and catalyst were placed into the autoclave. The reactor was flushed with N2 to remove air and subsequently pressurized to 60 bar H2 at RT. The reactor was heated to desired temperature and the starting time (t = 0) was recorded as the stirring speed was set to 650 rpm. After completing the reaction, the gas phase was analyzed by a GC (Agilent 7890). Dichloromethane (DCM) was used to extract the organic phase and the resulting solution was evaporated to remove DCM and finally an upgraded bio-oil (UBO) was obtained. The amounts of UBO, aqueous phase, gas phase and char were determined experimentally and the product yields were calculated on wet basis by following equation.
Water content and pH of the original bio-oil were measured by a Karl-Fischer-titration (MKS-520 Mettler Toledo) and an UB-10 pH/mV/Temp (Denver Instrument), respectively. Elemental compositions (C, H, N) of the parent bio-oil and UBO were carried out on a CHN/CHNS Vario Macro analyzer according to ASTM D5291-10. Subsequently, the dry elemental compositions were calculated and then the oxygen content was calculated by difference. Degree of deoxygenation (DOD) and the higher heating value (HHV) using Dulong’s formula were calculated by the following equations.
The coke deposited on spent catalyst was determined by a CHN analyzer (LECO-CS600).
The catalyst samples have been denoted as xNiyCo/Z, where Z is the name of support and x and y are the contents (wt%) of nickel and cobalt, respectively.
The XRD patterns of the HZSM-5 and HBeta supported pre-reduced catalysts were discussed elsewhere [
Samplesa | Surface area (m2/g) | Crystallite size (nm) | Acidity (μmol/g) | ||
---|---|---|---|---|---|
Brønsted | Lewis | Total | |||
20Ni/ZrO2 | 29 | n/d | n/d | n/d | n/d |
10Ni10Co/ZrO2 | 40 | 29.8 | 0 | 44 | 44 |
20Ni/HY | 540 | n/d | n/d | n/d | n/d |
10Ni10Co/HY | 568 | 23.3 | 259 | 505 | 765 |
20Ni/HBetab | 443 | n/d | 187 | 870 | 1.057 |
10Ni10Co/HBetab | 443 | 19.0 | 137 | 984 | 1.121 |
20Ni/HZSM-5c | 281 | 29.7 | 243 | 345 | 588 |
10Ni10Co/HZSM-5c | 281 | 15.1 | 162 | 509 | 672 |
an/d: not determined, bAcidity data from [
mission electron microscopy (TEM) studies. This effect was observed also for the other bimetallic catalysts used in this study. The acidities of the pre-reduced catalysts are presented in
In previous study, we observed that bimetallic Ni-Co/HZSM-5 outperformed monometallic catalysts and Ni-Cu/ HZSM-5 series in terms of phenol conversion, cyclohexane selectivity and coke deposition [
Phenol HDO leads to oxygenated (cyclohexanol, cyclohexanone) and deoxygenated products (cyclohexane, cyclohexene, benzene, and methylcyclopentane (MCP)). The reaction pathways are depicted in
Regarding the bimetallic catalysts, the phenol conversion reached 100% for 10Ni10Co/ZrO2 despite its comparatively low BET surface area, whereas the values for HZSM-5, HBeta and HY supported samples reached 100%, 85% and 36%, respectively. On the other side, also the related product distributions differed significantly from each other. Selectivity toward deoxygenated products reached ca. 10% for 10Ni10Co/ZrO2, whereas almost 100% were achieved with 10Ni10Co/HZSM-5. Intermediate values of 84% and 30% were obtained for 10Ni10Co/HBeta and 10Ni10Co/HY, respectively.
The high yields toward hydrocarbon on 10Ni10Co/HZSM-5 can be attributed to the additional high density of Brønsted acid sites introduced by the support (
products deviate from the above presented order of Brønsted acid site densities of the supports. With second ranking 10Ni10Co/HY, the selectivity toward hydrocarbon reached only 30%, though this catalyst also possessed high acid density. The comparatively low phenol conversion as well as the poor HDO selectivity can be assigned to the collapse of zeolite Y structure [
Based on above described experiments combined with results from HDO of the observed intermediates as individual feeds as shown in previous study [
The proposed pathways are supported by findings of Zhang et al. [
The bio-oil used in this study was produced by fast pyrolysis of wood (PYTEC GmbH) by the hot rotating disk technique. Typically, bio-oil represents a complex mixture of hundreds of chemical compounds.
heavy fuel oil. With phenol as a model compound as shown above, the classical parameters conversion and selectivity were used for catalyst evaluation. However, these parameters are not accessible for the evaluation of bio-oil HDO due to the complexity of the original feed and because the exact molecular composition is unknown. Instead, easily accessible sum parameters like the product yields per weight or DOD value (specific change in oxygen content) together with a van Krevelen plot, which displays the O: C and H: C molar ratios as a measure of fuel quality, are common tools for catalyst evaluation in bio-oil HDO.
The optimum batch-operating HDO conditions (5 g of bio-oil, 25 g of H2O, 1 g of catalyst, p = 60 bar H2 at RT, T = 300˚C, t = 4 h) were applied in this study based on our previous investigation. The obtained product is furthermore denoted as upgraded bio-oil (UBO). The yields for the various product phases and the DOD are shown in
Obviously, the UBO yield obtained with 10Ni10Co/HZSM-5 was slightly higher than that obtained with 20Ni/HZSM-5, whereas the fraction of products in the aqueous phase was comparable. This is in line with the higher percentage of gas phase which was produced with monometallic catalyst (roughly 20 wt%) than with bimetallic catalyst (15 wt%). This can be attributed to the higher number of Brønsted acid sites in monometallic catalyst compared to bimetallic catalyst (see
In addition to the higher UBO yield, the bimetallic catalyst also showed a higher activity for oxygen elimination than the monometallic catalyst as reflected by the DOD value. As the number of acid sites is less for the bimetallic catalyst, this must be a particular feature of the metal sites. The dispersion of Ni active sites is higher, and additionally smaller crystallites were formed by substituting Ni with Co (see
Properties | Bio-oil | Heavy fuel oila | |
---|---|---|---|
Wet basis | H2O (wt%) | 32.6 | 0.1 |
pH | 3.2 | - | |
Dry basis, wt% | C | 55.3 | 85 |
H | 6.9 | 11 | |
N | 0.3 | 0.3 | |
O | 37.4 | 1.0 | |
HHV (MJ/kg) | 21.9 | 40 |
aData obtained from reference [
Samplesa | Yield (wt%) | Elemental composition (dry basis) | DOD | HHV | Coke deposits | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
UBO | AQ | Gas | Char | Loss | C | H | N | O | (%) | (MJ/kg) | (wt%) | |
Bio-oil | - | - | - | - | - | 55.3 | 6.9 | 0.3 | 37.4 | - | 21.9 | |
20Ni/HZSM-5 | 32.3 | 35.6 | 20.4 | 2.5 | 9.2 | 62.6 | 8.9 | 0.23 | 28.2 | 25 | 29.0 | 6.5 |
10Ni10Co/HZSM-5 | 36.7 | 35.3 | 14.6 | 2.0 | 11.3 | 66.4 | 10.6 | 0.26 | 22.8 | 39 | 33.6 | 4.8 |
aUBO = upgraded bio-oil, AQ = aqueous phase, Gas = gas phase, DOD = degree of deoxygenation, HHV = higher heating value.
with 10Ni10Co/HZSM-5 was much higher than that of conventional heavy fuel oil and comparable to those of gasoline and diesel, whereas the O: C ratio was relatively higher, as expected. As a result, a direct use of such HDO product as a fuel component still is no option because of residual O: C ratio; however, it might be suited as a co-feed for standard refinery unit (e.g. fluid catalytic cracking, hydrotreating).
The highest oil yield on wet basis (37 wt%) together with highest DOD (39%) were achieved with the bimetallic catalyst (10Ni10Co/HZSM-5) under the investigated conditions.
This study presents results from aqueous phase HDO over various supported transition metal-containing catalysts with phenol as a model compound for screening purpose. The supports determine the surface area of the catalyst and introduce acidity into the catalyst, which is necessary to perform the dehydration steps in the reaction network. It must be pronounced that the hydrothermal stability of support plays an important role in catalyst performance, too. Among investigated supports, HZSM-5 was the best due to its high acidity in combination with hydrothermal stability in aqueous environment. In addition, various HDO tests were carried out using a bio-oil obtained from flash pyrolysis of wood and the same catalysts in batch mode.
The tests with HZSM-5 supported catalysts revealed that bimetallic 10Ni10Co/HZSM-5 exhibited significantly higher efficiency compared to monometallic 20Ni/HZSM-5 in terms of degree of deoxygenation and UBO yield. The substitution of Co with Ni leads to formation of Ni-Co alloy with better stabilized Ni domains found in better dispersed metal crystallites on the catalyst surface. Compared to monometallic catalyst, the balance is slightly shifted towards more effective metal sites, whereas the acid sites are slightly less, and this gives a better balance of the metal and acid sites in the bimetallic catalyst. These conditions not only catalyzed all individual reactions toward desired products but also decreased the amount of coke deposits on the surface of catalyst and improved catalyst performance. This finding might offer a new catalyst formulation with cheap materials but effective for HDO of bio-oil.
The authors want to thank Dr. M. Schneider, Dr. U. Bentrup and Ms. A. Lehmann (all at LIKAT) for analytical support. Financial support by the Vietnam Oil and Gas Group (Petrovietnam) and LIKAT is gratefully acknowledged.
Thuan MinhHuynh,UdoArmbruster,Luong HuuNguyen,Duc AnhNguyen,AndreasMartin, (2015) Hydrodeoxygenation of Bio-Oil on Bimetallic Catalysts: From Model Compound to Real Feed. Journal of Sustainable Bioenergy Systems,05,151-160. doi: 10.4236/jsbs.2015.54014