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Ethane oxydehydrogenation over TiP2O7-supported NiO catalysts
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Ethane oxydehydrogenation over TiP2O7-supported NiO catalysts Ştefan-Bogdan Ivana, Ioana Fecheteb, Florica Papac, Ioan-Cezar Marcua,d,* a Laboratory of Chemical Technology and Catalysis, Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, 4-12, Blv. Regina Elisabeta, 030018, Bucharest, Romania b International Center for CVD Innovation-Nogent, Pôle Technologique Sud Champagne, Université de Troyes-Antenne de Nogent, 26, rue Lavoisier, 52800, Nogent, France c “Ilie Murgulescu” Institute of Physical Chemistry of the Romanian Academy, 202 Spl. Independentei, 060021, Bucharest, Romania d Research Center for Catalysts and Catalytic Processes, Faculty of Chemistry, University of Bucharest, 4-12, Blv. Regina Elisabeta, 030018, Bucharest, Romania
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxydehydrogenation Ethane Ethylene Nickel oxide catalyst Titanium pyrophosphate
Titanium pyrophosphate-supported NiO catalysts with different Ni loadings in the range from 10 to 26 wt. % were prepared by wet impregnation, characterized by nitrogen adsorption at −196 °C, ICP-OES, XRD, CO2-TPD and H2-TPR techniques and tested in ethane oxydehydrogenation (ODH) in the temperature range from 350 to 450 °C. With increasing the Ni loading, the surface area of the catalysts decreased while the NiO particle size, the basicity and the reducibility increased strongly influencing their catalytic performance. Increasing the Ni loading boosted ethane conversion that reached 36 and 50 % at 425 and 450 °C, respectively, for the highest nickel loading catalyst. Ethylene was the main reaction product for all the catalysts. The ethylene selectivity at isoconversion increases with increasing the Ni loading. The stability of the catalysts was studied at a temperature as high as their calcination temperature, i.e. 450 °C, and at lower temperature.
1. Introduction Ethane oxydehydrogenation (ODH) is an attractive alternative to conventional ethylene production via steam cracking, several review papers being dedicated to this topic in the last years [1–4]. NiO-based systems are among the most active and selective catalysts for lowtemperature ethane ODH to ethylene [1]. The good catalytic performance of NiO in the low-temperature ethane ODH was first reported by Ducarme and Martin [5]. Schuurman et al. [6] showed that the ethylene oxidation activity of NiO is lower than its ethane oxidation activity, thus explaining its interesting catalytic behavior in ethane ODH and demonstrating the potential of NiO-based systems in this reaction. Therefore, several studies focusing on ethane ODH over both unsupported [7–23] and supported [12,24–31] NiO catalysts have been published until now. Among the unsupported Ni-containing oxides, much attention has been paid to the study of Nb-doped NiO [9–11,14,32] which, with a conversion of ca. 66 % and a selectivity for ethylene of 68 % at 400 °C for the optimal catalyst composition, i.e. Ni0.85Nb0.15O, emerged as one of the most active and selective catalysts for low temperature ethane ODH. However, this catalyst has the disadvantage that it deactivates over time on stream [11]. On the other hand, Heracleous et al. [33] studied unpromoted and promoted alumina-supported NiO catalysts in ethane ODH and showed ⁎
that increasing the Ni loading from 8 to 24 wt. % increased the ethane conversion without significantly affecting the ethylene selectivity. The same trend was observed in other studies [31,34,35], clearly suggesting that NiO is a promising catalyst that can be not only very active, but also very selective if it is dispersed on an appropriate support. Indeed, Nakamura et al. [36] tested various supports for NiO, clearly showing that its catalytic performance in ethane ODH strongly depends on the nature of the support. Recently, it has been shown that NiO supported on a Ti-containing Si-pillared porous clay heterostructure has a significantly higher performance in ethane ODH than NiO supported on the Ti-free porous clay heterostructure counterpart, with selectivities to ethylene close to those obtained with the most promising Nb-promoted NiO catalyst [34]. To the best of our knowledge, titanium pyrophosphate has never been used as a support for NiO catalyst. Therefore, in this work ethane ODH over NiO supported on high surface area TiP2O7 was studied. Notably, TiP2O7 was previously shown to be a highly active and selective catalyst for n-butane oxydehydrogenation [37,38], which makes it an even more interesting support material for NiO. Additionally, titanium pyrophosphate was shown to have acidic character [38], being thus expected to enhance the ethylene selectivity of NiO catalyst as demonstrated recently [39]. Indeed, it has been shown that the selectivity to ethylene increases with increasing the surface acidity of the metal oxide-promoted NiO catalyst.
Corresponding author. E-mail address: [email protected] (I.-C. Marcu).
https://doi.org/10.1016/j.cattod.2020.02.005 Received 14 November 2019; Received in revised form 9 January 2020; Accepted 6 February 2020 0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Ştefan-Bogdan Ivan, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.02.005
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2. Experimental
area of the recorded peaks. The calibration of the TCD signal was performed by injecting a known quantity of hydrogen (typically 50 μL) in the carrier gas (Ar). The experimentally obtained peak surface (mV s) was thus converted in micromoles of hydrogen.
2.1. Catalysts preparation First, a high surface area titanium pyrophosphate support (further noted TiP) has been synthesized according to a method described elsewhere [37,38] and based on the reaction between a solution containing 580 ml of water, 40 ml of lactic acid (85 %), 5 ml of NH3 (28 %) and 25.6 g of TiCl4 and a solution of phosphoric acid (106 g, 30 %) added drop-wise. The gel obtained was separated by centrifugation and dried between 80 and 200 °C with a gradual increase of temperature over 6 h. The resulted solid was calcined under air at 300, 450 and 600 °C for 6 h at each temperature. For the synthesis of the supported catalysts with five different Ni loadings, i.e. 10, 15, 18, 22 and 26 wt. %, the wet impregnation method has been used. Thus, 2.5 g of powdered TiP support were added to 120 ml of alcoholic solution (ethanol, 96 %) containing the desired quantity of nickel nitrate and oxalic acid in a 1:1 mol ratio. The resulted suspension was kept at ca. 60 °C under stirring until it became highly viscous. This was dried over night at 80 °C and then calcined at 450 °C for 5 h with a heating rate of 2 °C min−1. The supported catalysts were labeled as Ni(x)TiP with x = 10, 15, 18, 22 and 26, respectively. For comparison, pure NiO was prepared by precipitating a Ni(NO3)2 aqueous solution with NaOH, as described elsewhere [40]. The precipitate obtained was separated by centrifugation, washed, dried at 80 °C overnight and then calcined for 5 h in air at 450 °C.
2.3. Catalytic test Catalytic oxydehydrogenation of ethane over the Ni(x)TiP catalysts was performed in a fixed bed quartz tube down-flow reactor operated at atmospheric pressure in the temperature range from 350 to 450 °C (from 400 to 650 °C for the bare TiP support). The internal diameter of the reactor tube was 18 mm. The catalyst (0.7 g) was supported on quartz wool. The axial temperature profile was measured using an electronic thermometer placed in a thermowell centered in the catalyst bed. The reactor temperature was controlled using a chromel–alumel thermocouple attached to the reactor exterior. The dead volumes on both ends of the catalyst bed were filled with quartz chips to minimize potential gas-phase reactions at higher reaction temperatures. The gas mixture consisted of ethane and air. To determine the catalytic activity as a function of temperature, the W/F ratio and oxygen-to-ethane molar ratio were kept constant at 0.54 g s cm−3 and 1, respectively. To obtain different ethane conversions at 400 °C, the W/F ratio was varied from 0.18 to 1.09 g s cm−3, while the oxygen-to-ethane molar ratio was maintained at 1. Finally, to study the effect of the oxygen-to-ethane molar ratio on the catalytic performance, it was varied from 0.5 to 3 at a constant W/F ratio of 0.54 g s cm−3. Before each activity test, the reactor was heated to the desired temperature while the reactants flowed through it. The system was allowed to equilibrate for approximately 0.5 h at the reaction temperature before the first product analysis was performed. To compare the performances of the catalysts in the above-mentioned reaction conditions, each run was conducted over a period of ca. 2 h until two successive product analyses were identical. The stability tests were conducted over longer periods of time, product analyses being done at different intervals. The reaction products were analyzed using a Clarus 500 gas chromatograph equipped with a thermal conductivity detector and two packed columns in series (6 ft Hayesep and 10 ft molecular sieve 5 Å). For quantitative analysis calibration curves were obtained. Ethylene and CO2 were the only products detected under these reaction conditions. The ethane conversion and products selectivities were expressed in mol% on a carbon atom basis. The carbon balances of all the runs were satisfactory; the error margin was within ± 2 %.
2.2. Catalysts characterization Nitrogen adsorption isotherms were recorded at liquid nitrogen temperature on a Micromeritics ASAP 2020 surface area and porosity analyzer. Before analysis, the samples were outgassed under vacuum at 200 °C overnight. The nickel content of the prepared Ni(x)TiP samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Varian Liberty 110 Series ICP-OES spectrometer using a five-points calibration curve. Samples were digested using aqua regia. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-7000 diffractometer using Cu Kα radiation (λ =1.5418 Å, 40 kV, 40 mA) at a scanning speed of 2° min−1 over the 2θ range of 5-70°. The NiO particle size was calculated with the Scherrer’s formula applied to the most intense diffraction line observed in the XRD patterns at ca. 43.3° 2-theta. The basic site distribution on the surface of the catalysts was measured with the pre-adsorbed CO2 as probing gas via temperature-programmed desorption of CO2 (CO2-TPD) in a CHEMBET 3000Quantachrome Instruments apparatus equipped with a thermal conductivity detector (TCD). Typically, 50 mg of sample was loaded into a quartz tube, and then 50 ml min−1 of CO2 gas flow was introduced for CO2 adsorption for 60 min. Subsequently, the sample was purged with a 70 ml min−1 Ar stream to remove the physically adsorbed CO2 from the sample surface. After the baseline was stable, the desorption was performed in the temperature range from 50 to 750 °C with a heating rate of 10 °C min−1. The amount of CO2 desorbed was estimated from the area of the recorded patterns. The calibration of the TCD signal was performed by injecting a known quantity of carbon dioxide (typically 50 μL) in the carrier gas (Ar). The temperature-programmed reduction with hydrogen (H2-TPR) measurements were performed using the same CHEMBET 3000Quantachrome apparatus equipped with a thermal conductivity detector. Fresh calcined samples (40 mg) were heated up to 800 °C at the constant rate of 10 °C min−1 in the stream of the 5 vol. % H2/Ar reduction gas and a flow rate of 70 ml min−1. A silica gel water trap was interposed between the analyzed sample and the thermal conductivity detector (TCD) in order to ensure a good stability and sensitivity of the detection system. The hydrogen consumption was estimated from the
3. Results and discussion 3.1. Catalysts characterization The textural properties of bulk NiO, TiP support and TiP-supported NiO catalysts are presented in Table 1. Deposition of NiO on titanium pyrophosphate caused a gradual decrease in the specific surface area from 107.4 m2 g−1 for the bare TiP support to 53.2 m2 g−1 for the highest Ni-loading Ni(26)TiP catalyst, the latter value being higher than that corresponding to bulk NiO. All the materials exhibit type IV nitrogen adsorption/desorption isotherms, according to the BrunauerDeming-Deming-Teller (BDDT) classification [41], with a H3 type hysteresis loop characteristic of materials with interparticle mesoporosity (Fig. 1). These results are consistent with the average pore sizes obtained from desorption branches of the isotherms by the BarrettJoyner-Halenda (BJH) method (Table 1). Relatively large pore distributions of the Ni(x)TiP catalysts are observed (Fig. S1), with the average pore size increasing with the Ni content. The pore volume decreases with increasing the Ni content in the Ni(x)TiP series, but remains larger than that of the bare TiP support (Table 1). Changes in textural parameters upon nickel impregnation have already been reported in the literature [33,36]. They are likely due to nickel oxide species entering the pores of the support. Notably, minor changes in the 2
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Table 1 The physico-chemical characteristics of the catalysts. Sample
Ni content (wt. %)
BET surface area (m2 g-1)
Pore volume (cm3 g-1)
Average pore size (nm)
NiO particle size (nm)
TiPa Ni(10)TiP Ni(15)TiP Ni(18)TiP Ni(22)TiP Ni(26)TiP NiOb Ni(26)TiP usedc
– 9.7 14.7 18.1 20.9 25.5 – n.d.d
107.4 96.6 90.1 74.6 65.0 53.2 35.1 43.5
0.33 0.55 0.46 0.51 0.43 0.44 0.49 0.42
n.d.d 16.9 14.5 20.9 19.1 24.8 35.4 22.8
– 4.4 5.7 6.1 6.2 6.8 21.1 6.8
a b c d
Data from Refs. [37,38]. Data from Ref. [40]. After the stability test at 450 °C. Not determined.
Fig. 2. XRD patterns of the TiP support and Ni(x)TiP catalysts (♦ – TiP2O7, # – NiO).
at high Ni loading and is significantly lower compared to that of NiO particles supported on other support materials at similar loadings [24,33,42]. Notably, no significant differences between the XRD patterns of Ni(26)TiP catalyst before catalysis and after the long-term stability test at 450 °C (see below) can be observed, the used Ni(26)TiP catalyst showing the same NiO particle size as the fresh sample (Table 1). CO2-TPD profiles of the Ni(x)TiP catalysts were obtained in order to examine their basic properties (Fig. 3). The characteristic peaks corresponding to different desorption temperatures are indicative of the strength of the basic sites, with the area below the profile curves being directly proportional to the amount of basicity in the materials [43], which for the Ni(x)TiP catalysts is shown in Table 2. As a general trend, except for Ni(26)TiP, only 2 peaks appeared for all the catalysts, one at 370−440 °C, corresponding to medium-strength basic sites, and other at 440−560 °C, corresponding to strong basic sites. For Ni(26)TiP two additional peaks can be observed, one below 370 °C and other above 560 °C, which are attributed to weak and very strong basic sites, respectively. The weak basic sites are associated with superficial HO– groups forming bicarbonate species with CO2, the medium-strength basic sites account for acid-base Lewis pairs with formation of bidentate carbonate, while the strong and very strong basic sites are attributed to low-coordinated O2– species leading to monodentate carbonate [44]. It has been shown that TiP support is an acidic material [38], therefore all the basicity of the Ni(x)TiP catalysts comes from NiO particles. Indeed, the total number of basic sites increased linearly with increasing the Ni content in the Ni(x)TiP catalysts (Fig. 4), confirming that the supported NiO particles are responsible for the observed basicity. It is noteworthy that the basicity of the Ni(26)TiP after the long-term stability test at 450 °C (see below) strongly decreases (Fig. 3), the amount of basic sites passing from 850.7 μmol g−1 for the fresh catalyst to only 56.9 μmol g−1 for the used system (Table 2). The reducibility of the Ni(x)TiP catalysts was investigated by H2TPR experiments. The obtained TPR profiles are displayed in Fig. 5 and the corresponding hydrogen consumption values are reported in
Fig. 1. Nitrogen adsorption/desorption isotherms at −196 °C of the TiP support and Ni(x)TiP catalysts.
textural properties of the used Ni(26)TiP catalyst after the long-term stability test at 450 °C (see below) were observed (Table 1). The nickel content in the TiP-supported NiO catalysts, determined by ICP-OES, is tabulated in Table 1. It can be observed that the experimental Ni content is very close to the theoretical values for all the samples. Investigation of the crystalline phases present in the catalysts was performed by X-ray diffraction analysis, the obtained diffractograms being presented in Fig. 2. All the samples exhibited low-intensity diffraction lines in the 20-30° 2-theta range characteristic of the TiP2O7 support (PDF 38–1468). As the NiO is deposited on the TiP support and the Ni loading increases, diffraction lines at 2θ of ca. 37.1, 43.3 and 63.2° originating from NiO phase (PDF 47–1049) begin to appear, becoming more intense and sharper, indicating larger nickel oxide particles with higher crystallinity. Indeed, the NiO particle size (Table 1) continuously increases from 4.4 nm for Ni(10)TiP to 6.8 nm for Ni (26)TiP. Bulk NiO is a well-crystallized pure phase, as shown elsewhere [40], and has the greatest particle size. It is worth noting that the size of NiO particles supported on titanium pyrophosphate remains low even 3
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Fig. 4. Total density of basic sites versus Ni loading in the Ni(x)TiP catalysts.
Fig. 3. CO2-TPD profiles of the Ni(x)TiP catalysts.
Table 3. The small reduction peak observed for all Ni(x)TiP systems under 300 °C is attributed to adsorbed oxygen. This peak shifts to lower temperature with increasing the Ni loading, suggesting an increase of the adsorbed oxygen reactivity. All the Ni(x)TiP catalysts also show broad TPR peaks with several maxima at temperatures higher than 300 °C. They were decomposed using OriginPro 7.5 program in several well-defined Gaussian-shaped components (agreement factor of 0.998). The first components, with maxima in the ranges 415−440 °C and 505−530 °C, respectively, are attributed to the reduction of NiO, while the components observed at higher temperature are attributed to the reduction of Ti4+ in the TiP2O7 support. Indeed, a good agreement between the experimental H2 uptake corresponding to the former peaks and the theoretical hydrogen consumption calculated considering the full reduction of Ni2+ to Ni0, was found (Table 3). Notably, the starting reduction temperature corresponding to Ni2+ reduction decreases from ca. 400 to ca. 300 °C as the nickel loading increases from 10 to 26 wt % (Fig. 5), suggesting an increased reducibility of NiOx species. On the other hand, compared with NiO supported on silica, which is reduced between 200 and 500 °C [45], reduction of NiO supported on TiP2O7 takes place at higher temperatures, i.e. between 350 and ca. 600 °C,
Fig. 5. H2-TPR profiles of the Ni(x)TiP catalysts.
Table 2 Amount of basic sites of different strength of the Ni(x)TiP catalysts derived from TPD of CO2. Catalyst
Ni(10)TiP Ni(15)TiP Ni(18)TiP Ni(22)TiP Ni(26)TiP Ni(26)TiP useda a
Total basicity (μmol CO2 g−1)
CO2 desorbed (μmol g-1) [%] Weak
Medium
Strong
Very Strong
– – – – 48.8 [5.7] –
56.9 [88.4] 61.0 [18.5] 81.2 [22.9] 119.1 [19.2] 145.2 [17.1] 26.1 [45.9]
7.5 [11.6] 269.5 [81.5] 273.5 [77.1] 501.4 [80.8] 475.3 [55.9] 29.3 [51.5]
– – – – 181.4 [21.3] 1.5 [2.6]
After the long-term stability test at 450 °C. 4
64.4 330.5 354.7 620.5 850.7 56.9
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2.0 1.8 1.7 1.7 1.5
Theoretical H2 uptake
c
(mmol H2 g cat−1)
suggesting a strong interaction between NiO and TiP2O7 support [46]. Regarding the reduction of Ti4+ in the TiP2O7 support, it takes place at temperatures higher than ca. 600 °C and, interestingly, the corresponding hydrogen consumption increases with increasing the Ni loading in the Ni(x)TiP series. This is obviously due to the presence of an increased density of metallic nickel particles on the TiP support favoring the molecular H2 dissociation and, thus, the reduction of Ti4+ to Ti3+ or to Ti species with lower valence state (Table 3). Nevertheless, this phenomenon takes place at temperatures significantly higher than the catalytic reaction temperature and, hence, it does not influence the catalytic behavior of the Ni(x)TiP systems.
The effect of the reaction temperature on the ethane conversion and ethylene selectivity in ethane oxydehydrogenation over bulk NiO (data from Ref. [40]) and Ni(x)TiP (x = 15, 18, 22, 26) catalysts and TiP2O7 support is shown in Figs. 6 and S2, respectively. As expected, the ethane conversion increases in all cases with increasing reaction temperature. The most active system is bulk NiO, while the least active one is TiP support, the latter giving ethane conversion only at temperatures higher than 450 °C (Fig. S2). Therefore, responsible for the activity of the Ni (x)TiP catalysts at temperatures lower than 450 °C is NiO dispersed on the surface of the TiP2O7 support, without any contribution of the latter. Their activity increases with increasing the Ni content, Ni(26)TiP reaching at 425 °C a conversion level close to that of bulk NiO. Notably, the Ni(10)TiP catalyst starts to be active at 450 °C, showing a conversion degree of ca. 4 % at this reaction temperature. Regarding the ethylene selectivity, it increases with increasing reaction temperature for bulk NiO as previously observed [6], but remains lower than ca. 40 % in the temperature range studied. For the Ni (x)TiP catalysts, at low Ni loading (x = 15, 18), the ethylene selectivity is not much affected by the reaction temperature, while at high Ni loading (x = 22, 26) it slightly decreases to the benefit of carbon dioxide with increasing reaction temperature, mainly at temperatures higher than 400 °C, but it remains superior to 65 %. The apparent activation energies for ethane transformation over the Ni(x)TiP catalysts were calculated based on the Arrhenius equation using low conversion values (Fig. S3). They are close together in the range 100−122 kJ mol−1 (Table 4), being significantly higher than that corresponding to bulk NiO, i.e. 72 kJ mol−1 [40], in line with the highest activity of the latter. The observed increase of the intrinsic rate of ethane conversion with increasing the Ni content in the Ni(x)TiP series (Table 4) is obviously due to the increase of both the density of the surface sites associated to NiO and their reducibility. The effect of ethane conversion on the ODH selectivity of the three most active Ni(x)TiP catalysts in this series, i.e. Ni(26)TiP, Ni(22)TiP and Ni(18)TiP, were investigated by varying the W/F ratio in the range from 0.18 to 1.09 g s cm−3, at 400 °C using an oxygen-to-ethane molar ratio of 1. The results obtained are shown in Fig. 7. It can be observed that for Ni(26)TiP and Ni(22)TiP catalysts the ODH selectivity decreases slowly with increasing conversion, while for Ni(18)TiP the effect of conversion on the ODH selectivity is more important. Extrapolation to zero conversion by using linear regression gives ODH selectivities smaller than 100 % for all catalysts, indicating that the selectivities to carbon dioxide are non-zero. This suggests that in addition to ethylene, carbon dioxide is a primary product that forms via parallel ethane combustion reactions over the titanium pyrophosphatesupported NiO catalysts. This means that there are non-selective active sites on the surface of NiO particles dispersed on TiP2O7. Nonetheless, a great selectivity improvement compared to bulk NiO can be observed, suggesting that in the TiP2O7-supported NiO catalysts the density of the non-selective active sites is strongly reduced. It is worth noting that the ethylene selectivity at isoconversion increases with increasing Ni content (Fig. 7). Moreover, the ethylene selectivity increases with increasing the catalyst basicity, which is unexpected in view of the results
c
Three peaks observed only for Ni(22)TiP sample. Calculated on the basis of the real Ni loading (wt %) and considering full reduction of Ni2+ to Ni0. Calculated on the basis of the real Ti content (wt %) and considering the reduction of Ti4+ to Ti3+. b
1.9 2.1 2.2 2.4 3.9 ― / 640 / 695 600 / 670 / ― ― / 620 / 700 575 / 655 / 720 ― / 640 / 755 1.7 2.5 3.1 3.5 4.3 1.6 2.4 3.2 3.4 4.5 530 510 500 505 510 / / / / / 440 415 420 430 435 Ni(10)TiP Ni(15)TiP Ni(18)TiP Ni(22)TiP Ni(26)TiP
a
Experimental H2 uptake (mmol H2 g cat−1) T (°C) Experimental H2 uptake (mmol H2 g cat−1) T (°C)
Peak 1 + Peak 2
Temperatures of peak maxima and H2 consumptions Sample
Table 3 H2-TPR data for Ni(x)TiP catalysts.
Theoretical H2 uptake
b
(mmol H2 g cat−1)
Peak 3 + Peak 4 + Peak 5
a
3.2. Catalytic study
5
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Fig. 6. The catalytic performances in ethane oxydehydrogenation of bulk NiO and Ni(x)TiP catalysts. Closed symbols and dotted lines: ethane conversion; open symbols and continuous lines: ethylene selectivity.
desorption from the catalyst surface and, hence, the probability of its oxidation into carbon dioxide is diminished leading to higher ethylene selectivity. On the other hand, the ethylene selectivity in ethane ODH over NiO-based catalysts was shown to be favored by a low density of non-selective electrophilic O– species [8,9,20,32], and to explain the increase of ethylene selectivity with increasing the catalyst acidity, López Nieto et al. [39] proposed that ethylene could block the nonselective sites due to its strong interaction with the acid surface of promoted NiO catalyst. This is not necessarily in contradiction with a faster desorption of ethylene from the basic surface of NiO nanoparticles in Ni(x)TiP catalysts, but rather shows that selectivity is influenced in a complex manner by several factors. Finally, it is worth noting that, with 78 % ethylene selectivity at ca. 10 % ethane conversion, the Ni(26)TiP system is at isoconversion more selective than anatase- and silica-supported NiO catalysts [24], and it shows comparable selectivity with NiO supported on alumina [33] or on a porous clay heterostructure with SiO2-TiO2 pillars [24]. However, it is less selective than a Degusa P25 TiO2-supported NiO catalyst [24]. The effect of the ethane-to-oxygen molar ratio on ethane oxydehydrogenation over the most active Ni(x)TiP catalysts was investigated at 400 °C and a W/F ratio of 0.54 g s cm−3. The results obtained are presented in Fig. 8. It can be observed that for all the catalysts ethane conversion decreases when the ethane-to-oxygen molar ratio is increased from 0.33 to 2. At the same time, the ethylene selectivity increases at the expense of the carbon dioxide selectivity, which is typical for light alkanes ODH reactions. With the best Ni(26)TiP catalyst, for the ethane-to-oxygen molar ratio of 2 an ethylene selectivity of ca. 80 % at an ethane conversion of ca. 13 % was obtained. These results can be explained by the decrease of the amount of available oxygen when the ethane-to-oxygen molar ratio is increased. The stability of the Ni(18)TiP and Ni(26)TiP catalysts has been tested by maintaining them on stream at 400 °C for ca. 900 min (Fig. 9a). No change in conversion or selectivity was noticed for both catalysts suggesting that in these reaction conditions the Ni(x)TiP systems are stable in time. The stability of the Ni(26)TiP catalyst has also been tested by maintaining it on stream at the calcination temperature, i.e. 450 °C, the results obtained being shown in Fig. 9b. It can be observed that during the first 150 min on stream, the ethane conversion decreases from 54 to 46 %, while the ethylene selectivity slightly increases from 63 to 67 %. This clearly suggests that at a reaction temperature as high as the calcination temperature, the Ni(26)TiP catalyst
Table 4 The intrinsic rates and the apparent activation energies of ethane transformation over TiP2O7-supported NiO catalysts. Catalyst
Reaction temperature (oC)
Intrinsic rate (108 mol m−2 s−1)
Activation energy (kJ mol−1)
Ni(15)TiP
400 450 400 450 400 450 400 450
0.3 1.4 1.0 3.4 2.7 8.5 5.2 12.7
120
Ni(18)TiP Ni(22)TiP Ni(26)TiP
107 122 100
Fig. 7. Ethylene selectivity vs. conversion variation in ethane oxydehydrogenation over Ni(x)TiP catalysts (x = 18, 22 and 26) at 400 °C (O2-to-C2H6 molar ratio = 1).
reported by López Nieto et al. [39] showing a parallelism between the ethylene selectivity and the surface acidity of the promoted NiO catalysts. However, taking into consideration that ethylene has an electrondonating character, higher surface basicity is expected to favor its 6
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stream at 450 °C, the conversion continues to decrease reaching a plateau of ca. 18 % after 9 more hours. Similarly, the ethylene selectivity increases up to a plateau corresponding to ca. 75 %. This behavior clearly shows that the catalyst changes under the thermal treatment at temperatures as high as the calcination temperature either under air or reaction mixture. As no structural changes of the Ni(26)TiP catalyst after the long-term stability test at 450 °C have been noticed according to the data reported in Table 1 and Fig. 2, its deactivation should be partially due to the observed decrease of its specific surface area (Table 1). However, the decrease in specific surface area cannot explain alone the observed deactivation and, hence, the surface diffusion of phosphorus from the pyrophosphate support, which contains an excess of surface phosphorous [37,38], onto the supported NiO particles should be taken into consideration. Such a hypothesis is supported by the strong decrease of the basicity of the Ni(26)TiP catalyst observed after the long-term stability test at 450 °C (Table 2). Moreover, it has recently been shown [40] that adding phosphorus to NiO results in a decrease of ethane conversion with an increase in ethylene selectivity at the expense of total oxidation selectivity due to the decrease of the surface density of non-selective active sites of NiO.
Fig. 8. Effect of ethane-to-oxygen molar ratio on the oxydehydrogenation of ethane over Ni(x)TiP catalysts (x = 18, 22 and 26) at 400 °C (W/F =0.54 g s cm−3). Closed symbols: ethane conversion; open symbols: ethylene selectivity.
4. Conclusion Five TiP2O7-supported NiO catalysts with Ni loadings in the range from 10 to 26 wt % have been synthesized by wet impregnation of a high surface area titanium pyrophosphate support. The nickel oxide is evenly distributed over the TiP2O7 surface, with nanometric particle size gradually increasing with increasing Ni loading. All the catalysts have basic character, the density of basic sites linearly increasing with the Ni loading. A strong NiO-support interaction has been evidenced. All the catalysts are active in the low temperature oxydehydrogenation of ethane, the activity increasing with increasing both the Ni loading and the reducibility of the NiO particles. The ethylene selectivity also increases with increasing the Ni loading and the basicity of the catalyst. The best catalyst emerged Ni(26)TiP which has shown a relatively high ethylene selectivity of 70 % at a conversion of about 36 % at 425 °C. It was shown to be stable on stream at temperatures lower than the calcination temperature, i.e. 450 °C. However, at reaction temperatures as high as the calcination temperature, phosphorus migrates from the pyrophosphate support onto NiO particles resulting in a decrease of ethane conversion with an increase of ethylene selectivity over time. CRediT authorship contribution statement Ştefan-Bogdan Ivan: Investigation, Validation, Writing - original draft. Ioana Fechete: Methodology, Investigation. Florica Papa: Investigation, Validation. Ioan-Cezar Marcu: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 9. Effect of the time-on-stream on the catalytic properties of Ni(18)TiP and Ni(26)TiP catalysts at 400 °C (a) and of Ni(26)TiP catalyst at 450 °C (b).
changes on stream. To rule out a SMSI-like (strong metal-support interaction) effect, the catalyst was thermally treated at 450 °C under air for ca. 4 h, before feeding again the reaction mixture into the reactor (Fig. 9b). As after the thermal treatment under air the initial catalytic performance was not established, a SMSI-like effect can unambiguously be ruled out. Moreover, during the thermal treatment under air the catalyst further changed. Indeed, ethane conversion decreases from 46 % before to 28 % after the treatment in air, while the ethylene selectivity increases from 67 to 70 %. Maintaining further the catalyst on
Acknowledgments The authors are grateful to Dr. Sorin Avramescu from the University of Bucharest for performing the ICP-OES measurements. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2020.02.005. 7
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Ethane oxydehydrogenation over TiP2O7-supported NiO catalysts Ştefan-Bogdan Ivana, Ioana Fecheteb, Florica Papac, Ioan-Cezar Marcua,d,* a Laboratory of Chemical Technology and Catalysis, Department of Organic Chemistry, Biochemistry and Catalysis, Faculty of Chemistry, University of Bucharest, 4-12, Blv. Regina Elisabeta, 030018, Bucharest, Romania b International Center for CVD Innovation-Nogent, Pôle Technologique Sud Champagne, Université de Troyes-Antenne de Nogent, 26, rue Lavoisier, 52800, Nogent, France c “Ilie Murgulescu” Institute of Physical Chemistry of the Romanian Academy, 202 Spl. Independentei, 060021, Bucharest, Romania d Research Center for Catalysts and Catalytic Processes, Faculty of Chemistry, University of Bucharest, 4-12, Blv. Regina Elisabeta, 030018, Bucharest, Romania
A R T I C LE I N FO
A B S T R A C T
Keywords: Oxydehydrogenation Ethane Ethylene Nickel oxide catalyst Titanium pyrophosphate
Titanium pyrophosphate-supported NiO catalysts with different Ni loadings in the range from 10 to 26 wt. % were prepared by wet impregnation, characterized by nitrogen adsorption at −196 °C, ICP-OES, XRD, CO2-TPD and H2-TPR techniques and tested in ethane oxydehydrogenation (ODH) in the temperature range from 350 to 450 °C. With increasing the Ni loading, the surface area of the catalysts decreased while the NiO particle size, the basicity and the reducibility increased strongly influencing their catalytic performance. Increasing the Ni loading boosted ethane conversion that reached 36 and 50 % at 425 and 450 °C, respectively, for the highest nickel loading catalyst. Ethylene was the main reaction product for all the catalysts. The ethylene selectivity at isoconversion increases with increasing the Ni loading. The stability of the catalysts was studied at a temperature as high as their calcination temperature, i.e. 450 °C, and at lower temperature.
1. Introduction Ethane oxydehydrogenation (ODH) is an attractive alternative to conventional ethylene production via steam cracking, several review papers being dedicated to this topic in the last years [1–4]. NiO-based systems are among the most active and selective catalysts for lowtemperature ethane ODH to ethylene [1]. The good catalytic performance of NiO in the low-temperature ethane ODH was first reported by Ducarme and Martin [5]. Schuurman et al. [6] showed that the ethylene oxidation activity of NiO is lower than its ethane oxidation activity, thus explaining its interesting catalytic behavior in ethane ODH and demonstrating the potential of NiO-based systems in this reaction. Therefore, several studies focusing on ethane ODH over both unsupported [7–23] and supported [12,24–31] NiO catalysts have been published until now. Among the unsupported Ni-containing oxides, much attention has been paid to the study of Nb-doped NiO [9–11,14,32] which, with a conversion of ca. 66 % and a selectivity for ethylene of 68 % at 400 °C for the optimal catalyst composition, i.e. Ni0.85Nb0.15O, emerged as one of the most active and selective catalysts for low temperature ethane ODH. However, this catalyst has the disadvantage that it deactivates over time on stream [11]. On the other hand, Heracleous et al. [33] studied unpromoted and promoted alumina-supported NiO catalysts in ethane ODH and showed ⁎
that increasing the Ni loading from 8 to 24 wt. % increased the ethane conversion without significantly affecting the ethylene selectivity. The same trend was observed in other studies [31,34,35], clearly suggesting that NiO is a promising catalyst that can be not only very active, but also very selective if it is dispersed on an appropriate support. Indeed, Nakamura et al. [36] tested various supports for NiO, clearly showing that its catalytic performance in ethane ODH strongly depends on the nature of the support. Recently, it has been shown that NiO supported on a Ti-containing Si-pillared porous clay heterostructure has a significantly higher performance in ethane ODH than NiO supported on the Ti-free porous clay heterostructure counterpart, with selectivities to ethylene close to those obtained with the most promising Nb-promoted NiO catalyst [34]. To the best of our knowledge, titanium pyrophosphate has never been used as a support for NiO catalyst. Therefore, in this work ethane ODH over NiO supported on high surface area TiP2O7 was studied. Notably, TiP2O7 was previously shown to be a highly active and selective catalyst for n-butane oxydehydrogenation [37,38], which makes it an even more interesting support material for NiO. Additionally, titanium pyrophosphate was shown to have acidic character [38], being thus expected to enhance the ethylene selectivity of NiO catalyst as demonstrated recently [39]. Indeed, it has been shown that the selectivity to ethylene increases with increasing the surface acidity of the metal oxide-promoted NiO catalyst.
Corresponding author. E-mail address: [email protected] (I.-C. Marcu).
https://doi.org/10.1016/j.cattod.2020.02.005 Received 14 November 2019; Received in revised form 9 January 2020; Accepted 6 February 2020 0920-5861/ © 2020 Elsevier B.V. All rights reserved.
Please cite this article as: Ştefan-Bogdan Ivan, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.02.005
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2. Experimental
area of the recorded peaks. The calibration of the TCD signal was performed by injecting a known quantity of hydrogen (typically 50 μL) in the carrier gas (Ar). The experimentally obtained peak surface (mV s) was thus converted in micromoles of hydrogen.
2.1. Catalysts preparation First, a high surface area titanium pyrophosphate support (further noted TiP) has been synthesized according to a method described elsewhere [37,38] and based on the reaction between a solution containing 580 ml of water, 40 ml of lactic acid (85 %), 5 ml of NH3 (28 %) and 25.6 g of TiCl4 and a solution of phosphoric acid (106 g, 30 %) added drop-wise. The gel obtained was separated by centrifugation and dried between 80 and 200 °C with a gradual increase of temperature over 6 h. The resulted solid was calcined under air at 300, 450 and 600 °C for 6 h at each temperature. For the synthesis of the supported catalysts with five different Ni loadings, i.e. 10, 15, 18, 22 and 26 wt. %, the wet impregnation method has been used. Thus, 2.5 g of powdered TiP support were added to 120 ml of alcoholic solution (ethanol, 96 %) containing the desired quantity of nickel nitrate and oxalic acid in a 1:1 mol ratio. The resulted suspension was kept at ca. 60 °C under stirring until it became highly viscous. This was dried over night at 80 °C and then calcined at 450 °C for 5 h with a heating rate of 2 °C min−1. The supported catalysts were labeled as Ni(x)TiP with x = 10, 15, 18, 22 and 26, respectively. For comparison, pure NiO was prepared by precipitating a Ni(NO3)2 aqueous solution with NaOH, as described elsewhere [40]. The precipitate obtained was separated by centrifugation, washed, dried at 80 °C overnight and then calcined for 5 h in air at 450 °C.
2.3. Catalytic test Catalytic oxydehydrogenation of ethane over the Ni(x)TiP catalysts was performed in a fixed bed quartz tube down-flow reactor operated at atmospheric pressure in the temperature range from 350 to 450 °C (from 400 to 650 °C for the bare TiP support). The internal diameter of the reactor tube was 18 mm. The catalyst (0.7 g) was supported on quartz wool. The axial temperature profile was measured using an electronic thermometer placed in a thermowell centered in the catalyst bed. The reactor temperature was controlled using a chromel–alumel thermocouple attached to the reactor exterior. The dead volumes on both ends of the catalyst bed were filled with quartz chips to minimize potential gas-phase reactions at higher reaction temperatures. The gas mixture consisted of ethane and air. To determine the catalytic activity as a function of temperature, the W/F ratio and oxygen-to-ethane molar ratio were kept constant at 0.54 g s cm−3 and 1, respectively. To obtain different ethane conversions at 400 °C, the W/F ratio was varied from 0.18 to 1.09 g s cm−3, while the oxygen-to-ethane molar ratio was maintained at 1. Finally, to study the effect of the oxygen-to-ethane molar ratio on the catalytic performance, it was varied from 0.5 to 3 at a constant W/F ratio of 0.54 g s cm−3. Before each activity test, the reactor was heated to the desired temperature while the reactants flowed through it. The system was allowed to equilibrate for approximately 0.5 h at the reaction temperature before the first product analysis was performed. To compare the performances of the catalysts in the above-mentioned reaction conditions, each run was conducted over a period of ca. 2 h until two successive product analyses were identical. The stability tests were conducted over longer periods of time, product analyses being done at different intervals. The reaction products were analyzed using a Clarus 500 gas chromatograph equipped with a thermal conductivity detector and two packed columns in series (6 ft Hayesep and 10 ft molecular sieve 5 Å). For quantitative analysis calibration curves were obtained. Ethylene and CO2 were the only products detected under these reaction conditions. The ethane conversion and products selectivities were expressed in mol% on a carbon atom basis. The carbon balances of all the runs were satisfactory; the error margin was within ± 2 %.
2.2. Catalysts characterization Nitrogen adsorption isotherms were recorded at liquid nitrogen temperature on a Micromeritics ASAP 2020 surface area and porosity analyzer. Before analysis, the samples were outgassed under vacuum at 200 °C overnight. The nickel content of the prepared Ni(x)TiP samples was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Varian Liberty 110 Series ICP-OES spectrometer using a five-points calibration curve. Samples were digested using aqua regia. Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-7000 diffractometer using Cu Kα radiation (λ =1.5418 Å, 40 kV, 40 mA) at a scanning speed of 2° min−1 over the 2θ range of 5-70°. The NiO particle size was calculated with the Scherrer’s formula applied to the most intense diffraction line observed in the XRD patterns at ca. 43.3° 2-theta. The basic site distribution on the surface of the catalysts was measured with the pre-adsorbed CO2 as probing gas via temperature-programmed desorption of CO2 (CO2-TPD) in a CHEMBET 3000Quantachrome Instruments apparatus equipped with a thermal conductivity detector (TCD). Typically, 50 mg of sample was loaded into a quartz tube, and then 50 ml min−1 of CO2 gas flow was introduced for CO2 adsorption for 60 min. Subsequently, the sample was purged with a 70 ml min−1 Ar stream to remove the physically adsorbed CO2 from the sample surface. After the baseline was stable, the desorption was performed in the temperature range from 50 to 750 °C with a heating rate of 10 °C min−1. The amount of CO2 desorbed was estimated from the area of the recorded patterns. The calibration of the TCD signal was performed by injecting a known quantity of carbon dioxide (typically 50 μL) in the carrier gas (Ar). The temperature-programmed reduction with hydrogen (H2-TPR) measurements were performed using the same CHEMBET 3000Quantachrome apparatus equipped with a thermal conductivity detector. Fresh calcined samples (40 mg) were heated up to 800 °C at the constant rate of 10 °C min−1 in the stream of the 5 vol. % H2/Ar reduction gas and a flow rate of 70 ml min−1. A silica gel water trap was interposed between the analyzed sample and the thermal conductivity detector (TCD) in order to ensure a good stability and sensitivity of the detection system. The hydrogen consumption was estimated from the
3. Results and discussion 3.1. Catalysts characterization The textural properties of bulk NiO, TiP support and TiP-supported NiO catalysts are presented in Table 1. Deposition of NiO on titanium pyrophosphate caused a gradual decrease in the specific surface area from 107.4 m2 g−1 for the bare TiP support to 53.2 m2 g−1 for the highest Ni-loading Ni(26)TiP catalyst, the latter value being higher than that corresponding to bulk NiO. All the materials exhibit type IV nitrogen adsorption/desorption isotherms, according to the BrunauerDeming-Deming-Teller (BDDT) classification [41], with a H3 type hysteresis loop characteristic of materials with interparticle mesoporosity (Fig. 1). These results are consistent with the average pore sizes obtained from desorption branches of the isotherms by the BarrettJoyner-Halenda (BJH) method (Table 1). Relatively large pore distributions of the Ni(x)TiP catalysts are observed (Fig. S1), with the average pore size increasing with the Ni content. The pore volume decreases with increasing the Ni content in the Ni(x)TiP series, but remains larger than that of the bare TiP support (Table 1). Changes in textural parameters upon nickel impregnation have already been reported in the literature [33,36]. They are likely due to nickel oxide species entering the pores of the support. Notably, minor changes in the 2
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Table 1 The physico-chemical characteristics of the catalysts. Sample
Ni content (wt. %)
BET surface area (m2 g-1)
Pore volume (cm3 g-1)
Average pore size (nm)
NiO particle size (nm)
TiPa Ni(10)TiP Ni(15)TiP Ni(18)TiP Ni(22)TiP Ni(26)TiP NiOb Ni(26)TiP usedc
– 9.7 14.7 18.1 20.9 25.5 – n.d.d
107.4 96.6 90.1 74.6 65.0 53.2 35.1 43.5
0.33 0.55 0.46 0.51 0.43 0.44 0.49 0.42
n.d.d 16.9 14.5 20.9 19.1 24.8 35.4 22.8
– 4.4 5.7 6.1 6.2 6.8 21.1 6.8
a b c d
Data from Refs. [37,38]. Data from Ref. [40]. After the stability test at 450 °C. Not determined.
Fig. 2. XRD patterns of the TiP support and Ni(x)TiP catalysts (♦ – TiP2O7, # – NiO).
at high Ni loading and is significantly lower compared to that of NiO particles supported on other support materials at similar loadings [24,33,42]. Notably, no significant differences between the XRD patterns of Ni(26)TiP catalyst before catalysis and after the long-term stability test at 450 °C (see below) can be observed, the used Ni(26)TiP catalyst showing the same NiO particle size as the fresh sample (Table 1). CO2-TPD profiles of the Ni(x)TiP catalysts were obtained in order to examine their basic properties (Fig. 3). The characteristic peaks corresponding to different desorption temperatures are indicative of the strength of the basic sites, with the area below the profile curves being directly proportional to the amount of basicity in the materials [43], which for the Ni(x)TiP catalysts is shown in Table 2. As a general trend, except for Ni(26)TiP, only 2 peaks appeared for all the catalysts, one at 370−440 °C, corresponding to medium-strength basic sites, and other at 440−560 °C, corresponding to strong basic sites. For Ni(26)TiP two additional peaks can be observed, one below 370 °C and other above 560 °C, which are attributed to weak and very strong basic sites, respectively. The weak basic sites are associated with superficial HO– groups forming bicarbonate species with CO2, the medium-strength basic sites account for acid-base Lewis pairs with formation of bidentate carbonate, while the strong and very strong basic sites are attributed to low-coordinated O2– species leading to monodentate carbonate [44]. It has been shown that TiP support is an acidic material [38], therefore all the basicity of the Ni(x)TiP catalysts comes from NiO particles. Indeed, the total number of basic sites increased linearly with increasing the Ni content in the Ni(x)TiP catalysts (Fig. 4), confirming that the supported NiO particles are responsible for the observed basicity. It is noteworthy that the basicity of the Ni(26)TiP after the long-term stability test at 450 °C (see below) strongly decreases (Fig. 3), the amount of basic sites passing from 850.7 μmol g−1 for the fresh catalyst to only 56.9 μmol g−1 for the used system (Table 2). The reducibility of the Ni(x)TiP catalysts was investigated by H2TPR experiments. The obtained TPR profiles are displayed in Fig. 5 and the corresponding hydrogen consumption values are reported in
Fig. 1. Nitrogen adsorption/desorption isotherms at −196 °C of the TiP support and Ni(x)TiP catalysts.
textural properties of the used Ni(26)TiP catalyst after the long-term stability test at 450 °C (see below) were observed (Table 1). The nickel content in the TiP-supported NiO catalysts, determined by ICP-OES, is tabulated in Table 1. It can be observed that the experimental Ni content is very close to the theoretical values for all the samples. Investigation of the crystalline phases present in the catalysts was performed by X-ray diffraction analysis, the obtained diffractograms being presented in Fig. 2. All the samples exhibited low-intensity diffraction lines in the 20-30° 2-theta range characteristic of the TiP2O7 support (PDF 38–1468). As the NiO is deposited on the TiP support and the Ni loading increases, diffraction lines at 2θ of ca. 37.1, 43.3 and 63.2° originating from NiO phase (PDF 47–1049) begin to appear, becoming more intense and sharper, indicating larger nickel oxide particles with higher crystallinity. Indeed, the NiO particle size (Table 1) continuously increases from 4.4 nm for Ni(10)TiP to 6.8 nm for Ni (26)TiP. Bulk NiO is a well-crystallized pure phase, as shown elsewhere [40], and has the greatest particle size. It is worth noting that the size of NiO particles supported on titanium pyrophosphate remains low even 3
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Fig. 4. Total density of basic sites versus Ni loading in the Ni(x)TiP catalysts.
Fig. 3. CO2-TPD profiles of the Ni(x)TiP catalysts.
Table 3. The small reduction peak observed for all Ni(x)TiP systems under 300 °C is attributed to adsorbed oxygen. This peak shifts to lower temperature with increasing the Ni loading, suggesting an increase of the adsorbed oxygen reactivity. All the Ni(x)TiP catalysts also show broad TPR peaks with several maxima at temperatures higher than 300 °C. They were decomposed using OriginPro 7.5 program in several well-defined Gaussian-shaped components (agreement factor of 0.998). The first components, with maxima in the ranges 415−440 °C and 505−530 °C, respectively, are attributed to the reduction of NiO, while the components observed at higher temperature are attributed to the reduction of Ti4+ in the TiP2O7 support. Indeed, a good agreement between the experimental H2 uptake corresponding to the former peaks and the theoretical hydrogen consumption calculated considering the full reduction of Ni2+ to Ni0, was found (Table 3). Notably, the starting reduction temperature corresponding to Ni2+ reduction decreases from ca. 400 to ca. 300 °C as the nickel loading increases from 10 to 26 wt % (Fig. 5), suggesting an increased reducibility of NiOx species. On the other hand, compared with NiO supported on silica, which is reduced between 200 and 500 °C [45], reduction of NiO supported on TiP2O7 takes place at higher temperatures, i.e. between 350 and ca. 600 °C,
Fig. 5. H2-TPR profiles of the Ni(x)TiP catalysts.
Table 2 Amount of basic sites of different strength of the Ni(x)TiP catalysts derived from TPD of CO2. Catalyst
Ni(10)TiP Ni(15)TiP Ni(18)TiP Ni(22)TiP Ni(26)TiP Ni(26)TiP useda a
Total basicity (μmol CO2 g−1)
CO2 desorbed (μmol g-1) [%] Weak
Medium
Strong
Very Strong
– – – – 48.8 [5.7] –
56.9 [88.4] 61.0 [18.5] 81.2 [22.9] 119.1 [19.2] 145.2 [17.1] 26.1 [45.9]
7.5 [11.6] 269.5 [81.5] 273.5 [77.1] 501.4 [80.8] 475.3 [55.9] 29.3 [51.5]
– – – – 181.4 [21.3] 1.5 [2.6]
After the long-term stability test at 450 °C. 4
64.4 330.5 354.7 620.5 850.7 56.9
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2.0 1.8 1.7 1.7 1.5
Theoretical H2 uptake
c
(mmol H2 g cat−1)
suggesting a strong interaction between NiO and TiP2O7 support [46]. Regarding the reduction of Ti4+ in the TiP2O7 support, it takes place at temperatures higher than ca. 600 °C and, interestingly, the corresponding hydrogen consumption increases with increasing the Ni loading in the Ni(x)TiP series. This is obviously due to the presence of an increased density of metallic nickel particles on the TiP support favoring the molecular H2 dissociation and, thus, the reduction of Ti4+ to Ti3+ or to Ti species with lower valence state (Table 3). Nevertheless, this phenomenon takes place at temperatures significantly higher than the catalytic reaction temperature and, hence, it does not influence the catalytic behavior of the Ni(x)TiP systems.
The effect of the reaction temperature on the ethane conversion and ethylene selectivity in ethane oxydehydrogenation over bulk NiO (data from Ref. [40]) and Ni(x)TiP (x = 15, 18, 22, 26) catalysts and TiP2O7 support is shown in Figs. 6 and S2, respectively. As expected, the ethane conversion increases in all cases with increasing reaction temperature. The most active system is bulk NiO, while the least active one is TiP support, the latter giving ethane conversion only at temperatures higher than 450 °C (Fig. S2). Therefore, responsible for the activity of the Ni (x)TiP catalysts at temperatures lower than 450 °C is NiO dispersed on the surface of the TiP2O7 support, without any contribution of the latter. Their activity increases with increasing the Ni content, Ni(26)TiP reaching at 425 °C a conversion level close to that of bulk NiO. Notably, the Ni(10)TiP catalyst starts to be active at 450 °C, showing a conversion degree of ca. 4 % at this reaction temperature. Regarding the ethylene selectivity, it increases with increasing reaction temperature for bulk NiO as previously observed [6], but remains lower than ca. 40 % in the temperature range studied. For the Ni (x)TiP catalysts, at low Ni loading (x = 15, 18), the ethylene selectivity is not much affected by the reaction temperature, while at high Ni loading (x = 22, 26) it slightly decreases to the benefit of carbon dioxide with increasing reaction temperature, mainly at temperatures higher than 400 °C, but it remains superior to 65 %. The apparent activation energies for ethane transformation over the Ni(x)TiP catalysts were calculated based on the Arrhenius equation using low conversion values (Fig. S3). They are close together in the range 100−122 kJ mol−1 (Table 4), being significantly higher than that corresponding to bulk NiO, i.e. 72 kJ mol−1 [40], in line with the highest activity of the latter. The observed increase of the intrinsic rate of ethane conversion with increasing the Ni content in the Ni(x)TiP series (Table 4) is obviously due to the increase of both the density of the surface sites associated to NiO and their reducibility. The effect of ethane conversion on the ODH selectivity of the three most active Ni(x)TiP catalysts in this series, i.e. Ni(26)TiP, Ni(22)TiP and Ni(18)TiP, were investigated by varying the W/F ratio in the range from 0.18 to 1.09 g s cm−3, at 400 °C using an oxygen-to-ethane molar ratio of 1. The results obtained are shown in Fig. 7. It can be observed that for Ni(26)TiP and Ni(22)TiP catalysts the ODH selectivity decreases slowly with increasing conversion, while for Ni(18)TiP the effect of conversion on the ODH selectivity is more important. Extrapolation to zero conversion by using linear regression gives ODH selectivities smaller than 100 % for all catalysts, indicating that the selectivities to carbon dioxide are non-zero. This suggests that in addition to ethylene, carbon dioxide is a primary product that forms via parallel ethane combustion reactions over the titanium pyrophosphatesupported NiO catalysts. This means that there are non-selective active sites on the surface of NiO particles dispersed on TiP2O7. Nonetheless, a great selectivity improvement compared to bulk NiO can be observed, suggesting that in the TiP2O7-supported NiO catalysts the density of the non-selective active sites is strongly reduced. It is worth noting that the ethylene selectivity at isoconversion increases with increasing Ni content (Fig. 7). Moreover, the ethylene selectivity increases with increasing the catalyst basicity, which is unexpected in view of the results
c
Three peaks observed only for Ni(22)TiP sample. Calculated on the basis of the real Ni loading (wt %) and considering full reduction of Ni2+ to Ni0. Calculated on the basis of the real Ti content (wt %) and considering the reduction of Ti4+ to Ti3+. b
1.9 2.1 2.2 2.4 3.9 ― / 640 / 695 600 / 670 / ― ― / 620 / 700 575 / 655 / 720 ― / 640 / 755 1.7 2.5 3.1 3.5 4.3 1.6 2.4 3.2 3.4 4.5 530 510 500 505 510 / / / / / 440 415 420 430 435 Ni(10)TiP Ni(15)TiP Ni(18)TiP Ni(22)TiP Ni(26)TiP
a
Experimental H2 uptake (mmol H2 g cat−1) T (°C) Experimental H2 uptake (mmol H2 g cat−1) T (°C)
Peak 1 + Peak 2
Temperatures of peak maxima and H2 consumptions Sample
Table 3 H2-TPR data for Ni(x)TiP catalysts.
Theoretical H2 uptake
b
(mmol H2 g cat−1)
Peak 3 + Peak 4 + Peak 5
a
3.2. Catalytic study
5
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Fig. 6. The catalytic performances in ethane oxydehydrogenation of bulk NiO and Ni(x)TiP catalysts. Closed symbols and dotted lines: ethane conversion; open symbols and continuous lines: ethylene selectivity.
desorption from the catalyst surface and, hence, the probability of its oxidation into carbon dioxide is diminished leading to higher ethylene selectivity. On the other hand, the ethylene selectivity in ethane ODH over NiO-based catalysts was shown to be favored by a low density of non-selective electrophilic O– species [8,9,20,32], and to explain the increase of ethylene selectivity with increasing the catalyst acidity, López Nieto et al. [39] proposed that ethylene could block the nonselective sites due to its strong interaction with the acid surface of promoted NiO catalyst. This is not necessarily in contradiction with a faster desorption of ethylene from the basic surface of NiO nanoparticles in Ni(x)TiP catalysts, but rather shows that selectivity is influenced in a complex manner by several factors. Finally, it is worth noting that, with 78 % ethylene selectivity at ca. 10 % ethane conversion, the Ni(26)TiP system is at isoconversion more selective than anatase- and silica-supported NiO catalysts [24], and it shows comparable selectivity with NiO supported on alumina [33] or on a porous clay heterostructure with SiO2-TiO2 pillars [24]. However, it is less selective than a Degusa P25 TiO2-supported NiO catalyst [24]. The effect of the ethane-to-oxygen molar ratio on ethane oxydehydrogenation over the most active Ni(x)TiP catalysts was investigated at 400 °C and a W/F ratio of 0.54 g s cm−3. The results obtained are presented in Fig. 8. It can be observed that for all the catalysts ethane conversion decreases when the ethane-to-oxygen molar ratio is increased from 0.33 to 2. At the same time, the ethylene selectivity increases at the expense of the carbon dioxide selectivity, which is typical for light alkanes ODH reactions. With the best Ni(26)TiP catalyst, for the ethane-to-oxygen molar ratio of 2 an ethylene selectivity of ca. 80 % at an ethane conversion of ca. 13 % was obtained. These results can be explained by the decrease of the amount of available oxygen when the ethane-to-oxygen molar ratio is increased. The stability of the Ni(18)TiP and Ni(26)TiP catalysts has been tested by maintaining them on stream at 400 °C for ca. 900 min (Fig. 9a). No change in conversion or selectivity was noticed for both catalysts suggesting that in these reaction conditions the Ni(x)TiP systems are stable in time. The stability of the Ni(26)TiP catalyst has also been tested by maintaining it on stream at the calcination temperature, i.e. 450 °C, the results obtained being shown in Fig. 9b. It can be observed that during the first 150 min on stream, the ethane conversion decreases from 54 to 46 %, while the ethylene selectivity slightly increases from 63 to 67 %. This clearly suggests that at a reaction temperature as high as the calcination temperature, the Ni(26)TiP catalyst
Table 4 The intrinsic rates and the apparent activation energies of ethane transformation over TiP2O7-supported NiO catalysts. Catalyst
Reaction temperature (oC)
Intrinsic rate (108 mol m−2 s−1)
Activation energy (kJ mol−1)
Ni(15)TiP
400 450 400 450 400 450 400 450
0.3 1.4 1.0 3.4 2.7 8.5 5.2 12.7
120
Ni(18)TiP Ni(22)TiP Ni(26)TiP
107 122 100
Fig. 7. Ethylene selectivity vs. conversion variation in ethane oxydehydrogenation over Ni(x)TiP catalysts (x = 18, 22 and 26) at 400 °C (O2-to-C2H6 molar ratio = 1).
reported by López Nieto et al. [39] showing a parallelism between the ethylene selectivity and the surface acidity of the promoted NiO catalysts. However, taking into consideration that ethylene has an electrondonating character, higher surface basicity is expected to favor its 6
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stream at 450 °C, the conversion continues to decrease reaching a plateau of ca. 18 % after 9 more hours. Similarly, the ethylene selectivity increases up to a plateau corresponding to ca. 75 %. This behavior clearly shows that the catalyst changes under the thermal treatment at temperatures as high as the calcination temperature either under air or reaction mixture. As no structural changes of the Ni(26)TiP catalyst after the long-term stability test at 450 °C have been noticed according to the data reported in Table 1 and Fig. 2, its deactivation should be partially due to the observed decrease of its specific surface area (Table 1). However, the decrease in specific surface area cannot explain alone the observed deactivation and, hence, the surface diffusion of phosphorus from the pyrophosphate support, which contains an excess of surface phosphorous [37,38], onto the supported NiO particles should be taken into consideration. Such a hypothesis is supported by the strong decrease of the basicity of the Ni(26)TiP catalyst observed after the long-term stability test at 450 °C (Table 2). Moreover, it has recently been shown [40] that adding phosphorus to NiO results in a decrease of ethane conversion with an increase in ethylene selectivity at the expense of total oxidation selectivity due to the decrease of the surface density of non-selective active sites of NiO.
Fig. 8. Effect of ethane-to-oxygen molar ratio on the oxydehydrogenation of ethane over Ni(x)TiP catalysts (x = 18, 22 and 26) at 400 °C (W/F =0.54 g s cm−3). Closed symbols: ethane conversion; open symbols: ethylene selectivity.
4. Conclusion Five TiP2O7-supported NiO catalysts with Ni loadings in the range from 10 to 26 wt % have been synthesized by wet impregnation of a high surface area titanium pyrophosphate support. The nickel oxide is evenly distributed over the TiP2O7 surface, with nanometric particle size gradually increasing with increasing Ni loading. All the catalysts have basic character, the density of basic sites linearly increasing with the Ni loading. A strong NiO-support interaction has been evidenced. All the catalysts are active in the low temperature oxydehydrogenation of ethane, the activity increasing with increasing both the Ni loading and the reducibility of the NiO particles. The ethylene selectivity also increases with increasing the Ni loading and the basicity of the catalyst. The best catalyst emerged Ni(26)TiP which has shown a relatively high ethylene selectivity of 70 % at a conversion of about 36 % at 425 °C. It was shown to be stable on stream at temperatures lower than the calcination temperature, i.e. 450 °C. However, at reaction temperatures as high as the calcination temperature, phosphorus migrates from the pyrophosphate support onto NiO particles resulting in a decrease of ethane conversion with an increase of ethylene selectivity over time. CRediT authorship contribution statement Ştefan-Bogdan Ivan: Investigation, Validation, Writing - original draft. Ioana Fechete: Methodology, Investigation. Florica Papa: Investigation, Validation. Ioan-Cezar Marcu: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 9. Effect of the time-on-stream on the catalytic properties of Ni(18)TiP and Ni(26)TiP catalysts at 400 °C (a) and of Ni(26)TiP catalyst at 450 °C (b).
changes on stream. To rule out a SMSI-like (strong metal-support interaction) effect, the catalyst was thermally treated at 450 °C under air for ca. 4 h, before feeding again the reaction mixture into the reactor (Fig. 9b). As after the thermal treatment under air the initial catalytic performance was not established, a SMSI-like effect can unambiguously be ruled out. Moreover, during the thermal treatment under air the catalyst further changed. Indeed, ethane conversion decreases from 46 % before to 28 % after the treatment in air, while the ethylene selectivity increases from 67 to 70 %. Maintaining further the catalyst on
Acknowledgments The authors are grateful to Dr. Sorin Avramescu from the University of Bucharest for performing the ICP-OES measurements. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2020.02.005. 7
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