kèo nhà cái 188bet Development and modification of low nickel content catalysts for dry reforming of methane

Summary

Both kèo nhà cái 188bet 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 kèo nhà cái 188bet concomitant CO2. One option is kèo nhà cái 188bet 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). kèo nhà cái 188bet main scope of this investigation was to design an active catalyst and modify it to avoid quick deactivation caused by coking. kèo nhà cái 188bet samples in this study were characterised by N2 physisorption (BET) and X-ray diffraction (XRD). kèo nhà cái 188bet results revealed that our Ni/MgAlOx catalysts show high surface area and good Ni dispersion. Such properties contribute to kèo nhà cái 188bet high activity of kèo nhà cái 188bet catalysts already at 500oC. Modification with La3+ significantly increases kèo nhà cái 188bet 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

kèo nhà cái 188bet 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 kèo nhà cái 188bet 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 kèo nhà cái 188bet largest biogas-producing countries utilising available organic wastes, by-products and energy crops [2]. kèo nhà cái 188bet total biogas potential in Germany is calculated by kèo nhà cái 188bet Federal Agricultural Research Centre (FAL) as 24 billion m3 per year [2]. kèo nhà cái 188bet 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 kèo nhà cái 188bet 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 kèo nhà cái 188bet biogas in Germany, and in both cases it is important to find a way to handle kèo nhà cái 188bet concomitant CO2 [4]. There are several possibilities to reduce total CO2 emissions into kèo nhà cái 188bet 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 kèo nhà cái 188bet reaction with methane towards synthesis gas (H2, CO), which is among kèo nhà cái 188bet most important starting materials in large- scale chemical syntheses. At present, synthesis gas is kèo nhà cái 188bet main source for H2 production via steam reforming [5, 6]. However, both kèo nhà cái 188bet evaporation of great quantities of water and kèo nhà cái 188bet endothermic reaction itself are very energy demanding as well as kèo nhà cái 188bet 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 kèo nhà cái 188bet most promising technologies for utilisation of these two gases to produce synthesis gas [9].

CO2 + CH4 → 2CO + 2H2 ∆H298 = 247kJ/mol (1)

kèo nhà cái 188bet syngas can be used in already existing industrial processes for chemical synthesis depending on kèo nhà cái 188bet reaction conditions and kèo nhà cái 188bet catalyst (Figure 1 [10]).

However, dry reforming is not yet commercialised due to kèo nhà cái 188bet fast deactivation of kèo nhà cái 188bet 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 kèo nhà cái 188bet surface leading to deactivation [11]. There are several approaches in kèo nhà cái 188bet reduction of carbon deposition of Ni- based catalysts; kèo nhà cái 188bet first is to decrease kèo nhà cái 188bet particle size of kèo nhà cái 188bet nickel and to improve its dispersion by supporting it on high-surface area supports [12, 13]. kèo nhà cái 188bet second is to alter kèo nhà cái 188bet acidity and basicity of kèo nhà cái 188bet supports and kèo nhà cái 188bet third is to apply different specific methods of catalyst preparation [14]. Liu et al. found that kèo nhà cái 188bet method of preparation of different Ni loadings supported on MCM-41 has an effect on kèo nhà cái 188bet activity and stability of kèo nhà cái 188bet 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 kèo nhà cái 188bet addition of Al O to MgO contributes to kèo nhà cái 188bet formation of an MgAl2O4 phase, which is stable and could effectively increase kèo nhà cái 188bet CO2 adsorption due to kèo nhà cái 188bet increased amount of basic sites on kèo nhà cái 188bet catalyst surface. Kathiraser et al. investigated kèo nhà cái 188bet behaviour of Ni particles supported on LaAlO -Al O which were stable and active over 30 hours due to kèo nhà cái 188bet formation of NiAl2O4 spinel structure [18]. Liu et al. investigated kèo nhà cái 188bet activity of La-promoted catalysts which was higher than for non-promoted hydrotalcite derived catalyst [19]. From kèo nhà cái 188bet previous research, we decided to prepare Ni supported on basic hydrotalcite precursors and to investigate kèo nhà cái 188bet effect of La addition on kèo nhà cái 188bet 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 kèo nhà cái 188bet 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, kèo nhà cái 188bet hydrotalcite precursor was calcined at 800oC for 6 hours in air to obtain kèo nhà cái 188bet bare supports which are denoted as MgAlOx. kèo nhà cái 188bet calculated amounts of Ni and La precursors were dissolved together in deionised water and then kèo nhà cái 188bet support was put into kèo nhà cái 188bet solution and kèo nhà cái 188bet slurry was stirred. Samples were then dried overnight and calcined at 800oC for 6 hours in static air. kèo nhà cái 188bet final catalysts are abbreviated as Ni/ MgAlOx and La.Ni/MgAlOx. kèo nhà cái 188bet nominal content of Ni in all supported Ni-containing catalysts was 2.5wt%.

Catalyst characterisation

Nitrogen physisorption method served for calculating kèo nhà cái 188bet specific surface area and pore volume according to kèo nhà cái 188bet BET theory. kèo nhà cái 188bet measurements were performed on a Micromeritics ASAP 2010 apparatus (Micromeritics GmbH, Aachen, Germany) at -196oC. kèo nhà cái 188bet samples were degassed at 200oC in vacuum for 4 hours before kèo nhà cái 188bet 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 kèo nhà cái 188bet Panalytical HighScore Plus software package.Peak positions and profile were fitted with Pseudo-Voigt function using kèo nhà cái 188bet WinXPow software package (Stoe). Phase identification was done by using kèo nhà cái 188bet PDF-2 database of kèo nhà cái 188bet 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 kèo nhà cái 188bet reactant mixture (45vol% CH4, 45vol% CO2, 10vol% He) was fed to kèo nhà cái 188bet reactor. Helium was used as internal standard for volume change determination in reaction. kèo nhà cái 188bet 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 kèo nhà cái 188bet formulas given below:

3. Results and discussion

3.1. Characterisation of kèo nhà cái 188bet catalysts

kèo nhà cái 188bet crystallographic structure patterns of kèo nhà cái 188bet MgO- Al2O3 mixed oxides, as kèo nhà cái 188bet support material, and kèo nhà cái 188bet corresponding Ni-containing catalysts were determined by XRD. Figure 2 represents kèo nhà cái 188bet spinel structure formation of MgAl2O4 (ICDD file No. 00-021-1152) in kèo nhà cái 188bet support with reflections at 2θ = 19.5o, 31.3o, 37o, 45o and 59o. This spinel phase is favoured when calcining kèo nhà cái 188bet Mg-Al hydrotalcite precursor at high temperature [20]. Besides, periclase (kèo nhà cái 188bet 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 kèo nhà cái 188bet unity ratio of Mg:Al in kèo nhà cái 188bet hydrotalcite precursor, which is higher than that of mentioned MgAl2O4 (0.5), leading to kèo nhà cái 188bet 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 kèo nhà cái 188bet corresponding support. This suggests kèo nhà cái 188bet formation of well-dispersed Ni2+ species on kèo nhà cái 188bet surface or diffusion into kèo nhà cái 188bet 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 kèo nhà cái 188bet preparation [21].

kèo nhà cái 188bet textural parameters of kèo nhà cái 188bet calcined Ni samples and kèo nhà cái 188bet support are summarised in Table 1. Compared to kèo nhà cái 188bet 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, kèo nhà cái 188bet accessibility of kèo nhà cái 188bet support surface, kèo nhà cái 188bet pore system and even kèo nhà cái 188bet 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 kèo nhà cái 188bet temperature range of 500 - 800oC. First, this proves kèo nhà cái 188bet essential role of Ni metal as kèo nhà cái 188bet active sites for DRM. Besides, tests on catalytic methane thermal cracking were also conducted on both catalysts. This is one of kèo nhà cái 188bet known reactions responsible for coke formation during DRM (Equation 2). However, no remarkable conversion was obtained except small carbon deposition on kèo nhà cái 188bet spent catalyst.

DRM performance tests of pre-reduced Ni/MgAlOx at different temperatures in kèo nhà cái 188bet range of 500 - 780oC were investigated with kèo nhà cái 188bet same feed composition and WHSV 100L/g. Figure 3 discloses that kèo nhà cái 188bet performance regarding kèo nhà cái 188bet 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, kèo nhà cái 188bet DRM reaction could be thermodynamically beneficial above 647oC [8]. Some investigations concluded that kèo nhà cái 188bet catalysts might be active in DRM already at lower (400oC [23] or 450oC [24]). However, therein WHSV was lower, kèo nhà cái 188bet 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 kèo nhà cái 188bet 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 kèo nhà cái 188bet 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 kèo nhà cái 188bet effect of temperature on kèo nhà cái 188bet H2/CO ratio. kèo nhà cái 188bet lower kèo nhà cái 188bet reaction temperature, kèo nhà cái 188bet lower kèo nhà cái 188bet H2/CO ratio and H2 selectivity. Subsequently, H2O forms as side product and kèo nhà cái 188bet relative conversion ratio between CO2 and CH4 increases [22].

Figure 4 shows kèo nhà cái 188bet conversions of CH and CO as well as kèo nhà cái 188bet amount of carbon deposition depending on kèo nhà cái 188bet CO /CH ratio for runs with Ni/MgAlO and La.Ni/MgAlO.

kèo nhà cái 188bet carbon accumulation in DRM was examined on kèo nhà cái 188bet spent catalysts after 8 hours on stream in terms of weight percentage. Typically a CO /CH ratio of 1 is used for DRM,whereas kèo nhà cái 188bet ratio of 0.5 is close to kèo nhà cái 188bet composition of typical biogas as well as kèo nhà cái 188bet 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 kèo nhà cái 188bet DRM reaction between both CH and CO . kèo nhà cái 188bet CO conversion achieves a maximum (70%) at CO2/CH4 = 0.5 as kèo nhà cái 188bet result of excess amount of CH4 shifting DRM more to kèo nhà cái 188bet production side.

It is well known that carbon deposition during DRM causes catalyst deactivation and reactor plugging [32]. Carbon formation is kèo nhà cái 188bet result of side reactions including mainly methane decomposition (MD) above 550oC as well as Boudouard Reaction (BD) (Equation 3) below 700oC [22]. kèo nhà cái 188bet carbon amount on spent Ni/MgAlOx varies proportionally with CH4 conversion (Figure 4a). On kèo nhà cái 188bet other hand, kèo nhà cái 188bet 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 kèo nhà cái 188bet CO2/CH4 ratio. kèo nhà cái 188bet occurrence of coke formation is as expected because kèo nhà cái 188bet chosen reaction temperature (600oC) is in kèo nhà cái 188bet thermodynamic range for both MD and BD [22].

La3+ addition to kèo nhà cái 188bet catalyst results in slightly lower DRM activity (Figure 4b). This phenomenon can be explained by kèo nhà cái 188bet lower surface area of La.Ni/MgAlOx compared to that of Ni/MgAlOx (Table 1). La3+ ions probably cover some Ni species in kèo nhà cái 188bet preparation steps and prevent kèo nhà cái 188bet active sites from being fully exposed to CH4 and CO2, lowering kèo nhà cái 188bet conversion of those reactants. This behaviour was also seen elsewhere [33] on La-promoted Ni/MgAl O which is similar to kèo nhà cái 188bet catalyst formulation in this study. However, kèo nhà cái 188bet La-promoted catalyst is significantly resistant toward coking in kèo nhà cái 188bet DRM (Figure 4). This rare-earth metal oxide was also studied to eliminate kèo nhà cái 188bet rapid coking in other reforming reactions (partial oxidation [34], steam reforming [33]). In this study, kèo nhà cái 188bet tests without CO2 (CO2/ CH = 0) showed very low conversion for both catalysts and comparable carbon deposition. In DRM with La.Ni/ MgAlOx, kèo nhà cái 188bet higher kèo nhà cái 188bet CO2 fraction in kèo nhà cái 188bet feed, kèo nhà cái 188bet lower kèo nhà cái 188bet carbon deposition compared to that of Ni/MgAlOx under kèo nhà cái 188bet same condition. This is a measure for kèo nhà cái 188bet 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, kèo nhà cái 188bet coke formation on La.Ni/MgAlOx is not proportional to kèo nhà cái 188bet conversion as it was found on Ni/ MgAlOx. Therefore, CO2 promotion by La3+ prevents kèo nhà cái 188bet BD because carbon gasification is kèo nhà cái 188bet reverse reaction of BD,

and also removes kèo nhà cái 188bet carbon species formed via methane thermal cracking. Regarding kèo nhà cái 188bet 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). kèo nhà cái 188bet considerably high WHSV of 170L/(gcat•h) compared to literature review [6] was helpful to verify kèo nhà cái 188bet activity and stability of catalysts during reaction away from thermodynamic equilibrium. kèo nhà cái 188bet conversions of CH4 and CO2 are slightly lower than kèo nhà cái 188bet thermodynamic balance for CH4 (83%) and CO2 (90%) at kèo nhà cái 188bet corresponding reaction temperature [22]. kèo nhà cái 188bet catalyst did not deactivate over at least 60 hours on-stream, representing its good stability. During kèo nhà cái 188bet reaction, CO2 conversion is always higher than CH4 conversion, illustrating kèo nhà cái 188bet contribution of RWGS reaction, causing kèo nhà cái 188bet H2/CO ratio to be lower than unity. Negligible carbon deposition (< 2wt%) was measured after 60 hours on stream, reflecting kèo nhà cái 188bet contribution of C gasification by CO2 which is promoted by mentioned effect of La3+.

4. Conclusions

kèo nhà cái 188bet MgAlOx supported 2.5wt% Ni catalyst shows high activity at 500 - 780oC, revealing kèo nhà cái 188bet possibility to operate kèo nhà cái 188bet catalyst at low temperature even at high WHSV and low Ni content. At low temperature, kèo nhà cái 188bet H2/CO ratio is low due to kèo nhà cái 188bet contribution of reverse water gas shift reaction which also creates H2O as kèo nhà cái 188bet 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. kèo nhà cái 188bet obtained results indicate that for Ni/MgAlOx kèo nhà cái 188bet coking rate is proportional to methane conversion, whereas this is not kèo nhà cái 188bet case for La-modified catalyst. Most likely La3+ affects kèo nhà cái 188bet 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

kèo nhà cái 188bet 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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