link 188bet cho dien thoai Development and modification of low nickel content catalysts for dry reforming of methane
Summary
Both link 188bet cho dien thoai Ca Voi Xanh (Blue Whale) gas field in Vietnam and biogas produced in Germany possess comparable high contents of CO2. For further processing, in both cases it is important to find a way to handle link 188bet cho dien thoai concomitant CO2. One option is link 188bet cho dien thoai direct production of synthesis gas - a mixture of CO and H2. Accordingly, low content (2.5wt%) but active Ni catalysts supported on Mg-Al mixed oxides were developed and studied for methane dry reforming reaction (DRM). link 188bet cho dien thoai main scope of this investigation was to design an active catalyst and modify it to avoid quick deactivation caused by coking. link 188bet cho dien thoai samples in this study were characterised by N2 physisorption (BET) and X-ray diffraction (XRD). link 188bet cho dien thoai results revealed that our Ni/MgAlOx catalysts show high surface area and good Ni dispersion. Such properties contribute to link 188bet cho dien thoai high activity of link 188bet cho dien thoai catalysts already at 500oC. Modification with La3+ significantly increases link 188bet cho dien thoai resistance towardcarbonformationduetoitsabilityfor C gasification. Such La.Ni/MgAlOxtypecatalyst also showshighandstable DRMactivityover at least 60 hours with low carbon accumulation at high weight hourly space velocity (WHSV= 170L/(gcat•h)) compared to state-of-art.
Key words: Dry reforming of methane, carbon dioxide, low content nickel catalyst, lanthanum, coking resistivity.
1. Introduction
link 188bet cho dien thoai global energy demand is growing rapidly, and about 88% of this demand is met at present time by fossil fuels [1]. However, this causes serious environmental problems such as air pollution beside fast depletion of resources. Henceforth, during link 188bet cho dien thoai recent years, many European countries - especially Germany, Denmark, Austria and Sweden - have been focusing their interest in biogas production because biogas is considered as a sustainable energy source which has a large potential for reducing greenhouse gas emissions [1]. Germany is one of link 188bet cho dien thoai largest biogas-producing countries utilising available organic wastes, by-products and energy crops [2]. link 188bet cho dien thoai total biogas potential in Germany is calculated by link 188bet cho dien thoai Federal Agricultural Research Centre (FAL) as 24 billion m3 per year [2]. link 188bet cho dien thoai final composition of biogas is 50 - 75vol% CH4, 25 - 45vol% CO2, 2 - 7vol% H2O and less than 1vol% is O2, N2, NH3 and H2S [3]. Interestingly, in 2011 Vietnam discovered link 188bet cho dien thoai Ca Voi Xanh gas field with a large reserve of about 150 billion m3 available for power generation and industrial purposes [4]. This Vietnamese gas has a high content of CO2 (~ 30vol%) which is comparable with link 188bet cho dien thoai biogas in Germany, and in both cases it is important to find a way to handle link 188bet cho dien thoai concomitant CO2 [4]. There are several possibilities to reduce total CO2 emissions into link 188bet cho dien thoai atmosphere. One of them is to develop different technologies to capture and utilise CO2 to produce valuable chemicals from it. One possible CO2 utilisation might be link 188bet cho dien thoai reaction with methane towards synthesis gas (H2, CO), which is among link 188bet cho dien thoai most important starting materials in large- scale chemical syntheses. At present, synthesis gas is link 188bet cho dien thoai main source for H2 production via steam reforming [5, 6]. However, both link 188bet cho dien thoai evaporation of great quantities of water and link 188bet cho dien thoai endothermic reaction itself are very energy demanding as well as link 188bet cho dien thoai upstream CO2 removal and downstream CO removal [7]. Another alternative and cheaper way would be CO2 reforming of methane (Equation 1, [8]), which has been proposed as one of link 188bet cho dien thoai most promising technologies for utilisation of these two gases to produce synthesis gas [9].
CO2 + CH4 → 2CO + 2H2 ∆H298 = 247kJ/mol (1)
link 188bet cho dien thoai syngas can be used in already existing industrial processes for chemical synthesis depending on link 188bet cho dien thoai reaction conditions and link 188bet cho dien thoai catalyst (Figure 1 [10]).
However, dry reforming is not yet commercialised due to link 188bet cho dien thoai fast deactivation of link 188bet cho dien thoai catalyst by carbon formation. Many efforts have been made to search for an active and stable catalyst. Nickel-based catalysts showed
to be promising candidates but they tend to form Ni aggregates and then unreactive carbon easily forms on link 188bet cho dien thoai surface leading to deactivation [11]. There are several approaches in link 188bet cho dien thoai reduction of carbon deposition of Ni- based catalysts; link 188bet cho dien thoai first is to decrease link 188bet cho dien thoai particle size of link 188bet cho dien thoai nickel and to improve its dispersion by supporting it on high-surface area supports [12, 13]. link 188bet cho dien thoai second is to alter link 188bet cho dien thoai acidity and basicity of link 188bet cho dien thoai supports and link 188bet cho dien thoai third is to apply different specific methods of catalyst preparation [14]. Liu et al. found that link 188bet cho dien thoai method of preparation of different Ni loadings supported on MCM-41 has an effect on link 188bet cho dien thoai activity and stability of link 188bet cho dien thoai catalyst [15]. Supporting 7wt% of Ni on SiC monolithic foam resulted in an excellent activity (95%) and stability over 100 hours at 800oC [16]. Also Al2O3 and MgO were used recently as supports to prepare Ni-based catalysts for dry reforming. Zhang et al. studied a series of Ni/MgO-Al O catalysts prepared by a simple two-step hydrothermal method. It was confirmed that Ni/MgO-Al2O3 (15wt% Ni) catalyst afforded 52% CH4 conversion with excellent stability during reaction at comparatively high space velocity (6 × 105 cm3•g -1•h-1) and lower reaction temperature (650oC) [17]. It seems that link 188bet cho dien thoai addition of Al O to MgO contributes to link 188bet cho dien thoai formation of an MgAl2O4 phase, which is stable and could effectively increase link 188bet cho dien thoai CO2 adsorption due to link 188bet cho dien thoai increased amount of basic sites on link 188bet cho dien thoai catalyst surface. Kathiraser et al. investigated link 188bet cho dien thoai behaviour of Ni particles supported on LaAlO -Al O which were stable and active over 30 hours due to link 188bet cho dien thoai formation of NiAl2O4 spinel structure [18]. Liu et al. investigated link 188bet cho dien thoai activity of La-promoted catalysts which was higher than for non-promoted hydrotalcite derived catalyst [19]. From link 188bet cho dien thoai previous research, we decided to prepare Ni supported on basic hydrotalcite precursors and to investigate link 188bet cho dien thoai effect of La addition on link 188bet cho dien thoai stability of such catalysts against coke deposition in DRM.
2. Materials and methods
Catalyst preparation
Mg-Al mixed oxides (calcined Mg-Al hydrotalcite, Pural MG, Sasol) supported Ni catalysts were prepared by wet impregnation. Mg-Al hydrotalcite precursor possesses link 188bet cho dien thoai Mg/Al ratio ~1.0 (data from ICP-OES and AAS measurement). Ni(NO3)2•6H2O (Alfa Aesar) and La(NO3)3•6H2O (ABCR GmbH) were used as precursors for Ni2+ and La3+. Prior to impregnation, link 188bet cho dien thoai hydrotalcite precursor was calcined at 800oC for 6 hours in air to obtain link 188bet cho dien thoai bare supports which are denoted as MgAlOx. link 188bet cho dien thoai calculated amounts of Ni and La precursors were dissolved together in deionised water and then link 188bet cho dien thoai support was put into link 188bet cho dien thoai solution and link 188bet cho dien thoai slurry was stirred. Samples were then dried overnight and calcined at 800oC for 6 hours in static air. link 188bet cho dien thoai final catalysts are abbreviated as Ni/ MgAlOx and La.Ni/MgAlOx. link 188bet cho dien thoai nominal content of Ni in all supported Ni-containing catalysts was 2.5wt%.
Catalyst characterisation
Nitrogen physisorption method served for calculating link 188bet cho dien thoai specific surface area and pore volume according to link 188bet cho dien thoai BET theory. link 188bet cho dien thoai measurements were performed on a Micromeritics ASAP 2010 apparatus (Micromeritics GmbH, Aachen, Germany) at -196oC. link 188bet cho dien thoai samples were degassed at 200oC in vacuum for 4 hours before link 188bet cho dien thoai analysis.
XRD powder patterns were recorded either on a Panalytical X'Pert diffractometer equipped with a X'celerator detector or on a Panalytical Empyrean diffractometer equipped with a PIXcel 3D detector system using Cu Kα1/α2 radiation (40kV, 40mA) in both cases. Cu beta-radiation was excluded by using nickel filter foil. Cu Kα2 radiation contribution was removed arithmetically using link 188bet cho dien thoai Panalytical HighScore Plus software package.Peak positions and profile were fitted with Pseudo-Voigt function using link 188bet cho dien thoai WinXPow software package (Stoe). Phase identification was done by using link 188bet cho dien thoai PDF-2 database of link 188bet cho dien thoai International Center of Diffraction Data (ICDD).
Catalyst tests
DRM was carried out in a fixed-bed continuous flow quartz reactor (ambient pressure, WHSV = 100 or 170L/(gcat•h); T = 500 - 780oC). All volumetric flow rates given in this study are related to 25°C and atmospheric pressure. After in-situ pre-reduction in H2 (700°C, 100% H2, 50mL/min) for 1.5 hours, temperature was adjusted and maintained for 8 or 60 hours and link 188bet cho dien thoai reactant mixture (45vol% CH4, 45vol% CO2, 10vol% He) was fed to link 188bet cho dien thoai reactor. Helium was used as internal standard for volume change determination in reaction. link 188bet cho dien thoai gas compositions were then analysed by an on-line gas chromatograph (Agilent 6890) equipped with flame ionisation detector (HP Plot Q capillary, 15m × 0.53mm × 40µm) and thermal conductivity detector (carboxene packed, 4.572m × 3.175mm) for analysis of hydrocarbons and permanent gases, respectively. Pure components were used as reference for peak identification and calibration. Carbon balances were calculated from gas products and reached more than 95% in this work. Conversions (X) and H2/CO ratio were calculated using link 188bet cho dien thoai formulas given below:
3. Results and discussion
3.1. Characterisation of link 188bet cho dien thoai catalysts
link 188bet cho dien thoai crystallographic structure patterns of link 188bet cho dien thoai MgO- Al2O3 mixed oxides, as link 188bet cho dien thoai support material, and link 188bet cho dien thoai corresponding Ni-containing catalysts were determined by XRD. Figure 2 represents link 188bet cho dien thoai spinel structure formation of MgAl2O4 (ICDD file No. 00-021-1152) in link 188bet cho dien thoai support with reflections at 2θ = 19.5o, 31.3o, 37o, 45o and 59o. This spinel phase is favoured when calcining link 188bet cho dien thoai Mg-Al hydrotalcite precursor at high temperature [20]. Besides, periclase (link 188bet cho dien thoai cubic form of magnesium oxide, ICDD file No. 01-071- 1176) is also observed with broad reflections at about 2θ = 43o and 63o. This phase is probably created due to link 188bet cho dien thoai unity ratio of Mg:Al in link 188bet cho dien thoai hydrotalcite precursor, which is higher than that of mentioned MgAl2O4 (0.5), leading to link 188bet cho dien thoai formation of MgO species. XRD patterns of Ni containing samples (Ni/MgAlOx and La.Ni/MgAlOx) expose almost no additional reflections of Ni2+- containing species compared to link 188bet cho dien thoai corresponding support. This suggests link 188bet cho dien thoai formation of well-dispersed Ni2+ species on link 188bet cho dien thoai surface or diffusion into link 188bet cho dien thoai bulk of support, adapting those mentioned structures of MgO-Al2O3 mixed oxides forming solid solutions or spinel. This behaviour is predominant when low content of impregnated species and high calcination temperature were applied during link 188bet cho dien thoai preparation [21].
link 188bet cho dien thoai textural parameters of link 188bet cho dien thoai calcined Ni samples and link 188bet cho dien thoai support are summarised in Table 1. Compared to link 188bet cho dien thoai pure support MgAlOx, impregnated catalysts (with La3+ and/or Ni2+) show lower specific surface areas. However, La.Ni/MgAlOx shows significantly less specific surface area (SBET) and pore volume than Ni/MgAlOx. By that, link 188bet cho dien thoai accessibility of link 188bet cho dien thoai support surface, link 188bet cho dien thoai pore system and even link 188bet cho dien thoai Ni2+ atoms were decreased.
3.2. Catalyst performance
Blank tests without catalyst or with Ni/MgAlOx without pretreatment by H2 reduction exposed no conversion of CH4 or CO2 in link 188bet cho dien thoai temperature range of 500 - 800oC. First, this proves link 188bet cho dien thoai essential role of Ni metal as link 188bet cho dien thoai active sites for DRM. Besides, tests on catalytic methane thermal cracking were also conducted on both catalysts. This is one of link 188bet cho dien thoai known reactions responsible for coke formation during DRM (Equation 2). However, no remarkable conversion was obtained except small carbon deposition on link 188bet cho dien thoai spent catalyst.
DRM performance tests of pre-reduced Ni/MgAlOx at different temperatures in link 188bet cho dien thoai range of 500 - 780oC were investigated with link 188bet cho dien thoai same feed composition and WHSV 100L/g. Figure 3 discloses that link 188bet cho dien thoai performance regarding link 188bet cho dien thoai CH4 and CO2 conversions is close to thermodynamic equilibrium at corresponding reaction temperatures [22]. It is well understood that DRM is only effective at high temperature due to its highly endothermic nature [22]. According to literature, link 188bet cho dien thoai DRM reaction could be thermodynamically beneficial above 647oC [8]. Some investigations concluded that link 188bet cho dien thoai catalysts might be active in DRM already at lower (400oC [23] or 450oC [24]). However, therein WHSV was lower, link 188bet cho dien thoai content of active species was higher or noble metals were added which offered more beneficial conditions for high reaction rates than this study. Compared to some remarkable literature results regarding Ni catalyst systems (Table 2) and also other studies [14], Ni/MgAlOx of this study shows promising potential for DRM by activating link 188bet cho dien thoai reaction at mild condition (low temperature and high WHSV) even with low Ni content. This high activity probably is in accordance with high surface of link 188bet cho dien thoai catalyst (Table 1) and good
by reverse water gas shift reaction (RWGS, Equation 4, [8]) is expected to get predominant [22].
This reaction is less endothermic and thus more favourable at low temperature compared to DRM. This effect is seen clearly in Figure 3 which shows link 188bet cho dien thoai effect of temperature on link 188bet cho dien thoai H2/CO ratio. link 188bet cho dien thoai lower link 188bet cho dien thoai reaction temperature, link 188bet cho dien thoai lower link 188bet cho dien thoai H2/CO ratio and H2 selectivity. Subsequently, H2O forms as side product and link 188bet cho dien thoai relative conversion ratio between CO2 and CH4 increases [22].
Figure 4 shows link 188bet cho dien thoai conversions of CH and CO as well as link 188bet cho dien thoai amount of carbon deposition depending on link 188bet cho dien thoai CO /CH ratio for runs with Ni/MgAlO and La.Ni/MgAlO.
link 188bet cho dien thoai carbon accumulation in DRM was examined on link 188bet cho dien thoai spent catalysts after 8 hours on stream in terms of weight percentage. Typically a CO /CH ratio of 1 is used for DRM,whereas link 188bet cho dien thoai ratio of 0.5 is close to link 188bet cho dien thoai composition of typical biogas as well as link 188bet cho dien thoai gas content discovered in Ca Voi Xanh gas field. With both catalysts, increasing CO /CH ratio from 0 to 1 enhances CH4 conversion (from almost 0 to around 50%) due to link 188bet cho dien thoai DRM reaction between both CH and CO . link 188bet cho dien thoai CO conversion achieves a maximum (70%) at CO2/CH4 = 0.5 as link 188bet cho dien thoai result of excess amount of CH4 shifting DRM more to link 188bet cho dien thoai production side.
It is well known that carbon deposition during DRM causes catalyst deactivation and reactor plugging [32]. Carbon formation is link 188bet cho dien thoai result of side reactions including mainly methane decomposition (MD) above 550oC as well as Boudouard Reaction (BD) (Equation 3) below 700oC [22]. link 188bet cho dien thoai carbon amount on spent Ni/MgAlOx varies proportionally with CH4 conversion (Figure 4a). On link 188bet cho dien thoai other hand, link 188bet cho dien thoai thermal cracking of methane (CO2/CH4 = 0) exposes negligible carbon deposition on spent Ni/MgAlOx, reflecting limited coking rate in case of CO2 absence, which prevents CO formation in DRM and further disproportionation in BD reaction. Therefore, it can be proposed that both MD and BD reactions
contribute to coke formation in DRM with different extent when changing link 188bet cho dien thoai CO2/CH4 ratio. link 188bet cho dien thoai occurrence of coke formation is as expected because link 188bet cho dien thoai chosen reaction temperature (600oC) is in link 188bet cho dien thoai thermodynamic range for both MD and BD [22].
La3+ addition to link 188bet cho dien thoai catalyst results in slightly lower DRM activity (Figure 4b). This phenomenon can be explained by link 188bet cho dien thoai lower surface area of La.Ni/MgAlOx compared to that of Ni/MgAlOx (Table 1). La3+ ions probably cover some Ni species in link 188bet cho dien thoai preparation steps and prevent link 188bet cho dien thoai active sites from being fully exposed to CH4 and CO2, lowering link 188bet cho dien thoai conversion of those reactants. This behaviour was also seen elsewhere [33] on La-promoted Ni/MgAl O which is similar to link 188bet cho dien thoai catalyst formulation in this study. However, link 188bet cho dien thoai La-promoted catalyst is significantly resistant toward coking in link 188bet cho dien thoai DRM (Figure 4). This rare-earth metal oxide was also studied to eliminate link 188bet cho dien thoai rapid coking in other reforming reactions (partial oxidation [34], steam reforming [33]). In this study, link 188bet cho dien thoai tests without CO2 (CO2/ CH = 0) showed very low conversion for both catalysts and comparable carbon deposition. In DRM with La.Ni/ MgAlOx, link 188bet cho dien thoai higher link 188bet cho dien thoai CO2 fraction in link 188bet cho dien thoai feed, link 188bet cho dien thoai lower link 188bet cho dien thoai carbon deposition compared to that of Ni/MgAlOx under link 188bet cho dien thoai same condition. This is a measure for link 188bet cho dien thoai CO2 activation in carbon gasification by La3+ species. Such activation was also discovered in other reforming reactions [33, 34]. Besides, both catalysts expose comparable CH4 and CO2 conversions as a function of CO2/CH4 ratio (Figures 4). However, link 188bet cho dien thoai coke formation on La.Ni/MgAlOx is not proportional to link 188bet cho dien thoai conversion as it was found on Ni/ MgAlOx. Therefore, CO2 promotion by La3+ prevents link 188bet cho dien thoai BD because carbon gasification is link 188bet cho dien thoai reverse reaction of BD,
and also removes link 188bet cho dien thoai carbon species formed via methane thermal cracking. Regarding link 188bet cho dien thoai reaction pathway, DRM occurs on both catalysts, but La.Ni/MgAlOx promotes
additional steps that activate C gasification by CO2 which was also proposed elsewhere [35]:
Finally, a long-term DRM test was carried out with La.Ni/MgAlOx (Figure 5). link 188bet cho dien thoai considerably high WHSV of 170L/(gcat•h) compared to literature review [6] was helpful to verify link 188bet cho dien thoai activity and stability of catalysts during reaction away from thermodynamic equilibrium. link 188bet cho dien thoai conversions of CH4 and CO2 are slightly lower than link 188bet cho dien thoai thermodynamic balance for CH4 (83%) and CO2 (90%) at link 188bet cho dien thoai corresponding reaction temperature [22]. link 188bet cho dien thoai catalyst did not deactivate over at least 60 hours on-stream, representing its good stability. During link 188bet cho dien thoai reaction, CO2 conversion is always higher than CH4 conversion, illustrating link 188bet cho dien thoai contribution of RWGS reaction, causing link 188bet cho dien thoai H2/CO ratio to be lower than unity. Negligible carbon deposition (< 2wt%) was measured after 60 hours on stream, reflecting link 188bet cho dien thoai contribution of C gasification by CO2 which is promoted by mentioned effect of La3+.
4. Conclusions
link 188bet cho dien thoai MgAlOx supported 2.5wt% Ni catalyst shows high activity at 500 - 780oC, revealing link 188bet cho dien thoai possibility to operate link 188bet cho dien thoai catalyst at low temperature even at high WHSV and low Ni content. At low temperature, link 188bet cho dien thoai H2/CO ratio is low due to link 188bet cho dien thoai contribution of reverse water gas shift reaction which also creates H2O as link 188bet cho dien thoai side product. Compared to Ni/MgAlOx, La.Ni/MgAlOx shows significantly higher coking resistance, both for CO2/CH4 ratio = 0.5 and 1, due to C gasification by CO2. link 188bet cho dien thoai obtained results indicate that for Ni/MgAlOx link 188bet cho dien thoai coking rate is proportional to methane conversion, whereas this is not link 188bet cho dien thoai case for La-modified catalyst. Most likely La3+ affects link 188bet cho dien thoai CO2 activation and gasification of carbonaceous deposits. La.Ni/MgAlOx also exposes high and stable DRM activity over at least 60 hours at high WHSV.
Acknowledgments
link 188bet cho dien thoai authors gratefully acknowledge financial support by German Academic Exchange Service (DAAD). We want to thank Mr. R.Eckelt and Dr. H.Lund (LIKAT) for BET and XRD measurements, respectively.
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