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Optimization of the Zr content in the CuO-ZnO-ZrO/ SAPO-11 catalyst for the selective hydrogenation of CO +CO mixtures in the direct synthesis of dimethyl ether 2

Miguel Sánchez-Contador, Ainara Ateka, Pablo Rodriguez-Vega, Javier Bilbao, and Andres Tomas Aguayo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04345 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Industrial & Engineering Chemistry Research

Optimization of the Zr content in the CuO-ZnOZrO2/SAPO-11 catalyst for the selective hydrogenation of CO+CO2 mixtures in the direct synthesis of dimethyl ether Miguel Sánchez-Contador*, Ainara Ateka, Pablo Rodriguez-Vega, Javier Bilbao, Andrés T. Aguayo Department of Chemical Engineering, University of the Basque Country UPV/EHU, P.O. Box 644, 48080 Bilbao, Spain *Corresponding author. Tel.: +34 94 6015361; Fax: +34 94 6013500. E-mail address: [email protected] (M. Sánchez-Contador) ABSTRACT Zr incorporation in the CuO-ZnO catalyst for methanol synthesis from CO+CO2 mixtures, for its later use in the bifunctional catalyst conformation for dimethyl ether (DME) direct synthesis has been studied. Different Cu/Zn/Zr ratio catalysts were prepared, via co-precipitation method, and characterized regarding to physical, chemical, structural and metallic properties. Specific Cu surface area and dispersion are responsible for boosting the activity of CuO-ZnO based catalysts, which increases when incorporating ZrO2. Based on the kinetic behavior (COx conversion and methanol yield and selectivity) and stability in the methanol synthesis, CZZr1 (Cu:Zn:Zr = 2:1:1) was selected as the most suitable metallic function, with 8.14 % of COx conversion and methanol selectivity over 98 %. A bifunctional catalyst was prepared by physical mixture of CZZr1 with SAPO-11. The bifunctional catalyst

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activity was tested on the DME direct synthesis, showing a good performance providing a high DME yield and selectivity, with a noticeable stability. Keywords: Dimethyl ether; CO2; Syngas; Bifunctional catalyst; SAPO-11; Cu surface


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1. INTRODUCTION Among the numerous routes studied for chemical recycling and utilization of CO2 as a feedstock, its hydrogenation to methanol and/or dimethyl ether (DME) has gained marked attention due to the wide applicability of these oxygenates in the chemical industry or as potential alternative fuels. In particular, the direct synthesis of DME from syngas and CO2 (STD process) is considered a major driving force for CO2 valorization on a large-scale. 1, 2 The STD process involves the following reactions: Methanol synthesis:

CO + 2H2 ↔ CH3OH

(1)

CO2 + 3H2 ↔ CH3OH+ H2O

(2)

Water gas shift (WGS):

CO + H2O ↔ CO2 + H2

(3)

Methanol dehydration to DME:

2CH3OH ↔ CH3OCH3 + H2O

(4)

Undesired secondary reactions of paraffins (mainly methane) formation: CO +3H2 ↔ CH4 + H2O

(5)

The combination of methanol synthesis reactions, Eqs. (1) and (2), and its subsequent dehydration, Eq. (4), in a single reactor, reduces operating costs and is thermodynamically more favored than the methanol formation for feeds with a high CO2/COx ratio, and therefore, facilitates the conversion of CO and CO2.3 Furthermore, the co-feeding of CO2 provides a favorable effect on the reaction heat, both in the methanol and DME synthesis, since the reverse WGS reaction (reverse of Eq. (3)) is promoted, decreasing the exothermicity of the process. Bifunctional catalysts used for the STD process consist of a metallic function for methanol formation and WGS reaction, and an acid function for the selective dehydration of the formed methanol to DME, being the most commonly used: CuOZnO-Al2O3 (CZA) and γ-Al2O3, respectively. For the methanol synthesis Cu0 and Cu+


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have been identified to be the active species, thereby the metallic functions are based on CuO-ZnO, where the addition of ZnO enhances the dispersion of Cu and the interactions between CuO and ZnO lead to higher activity values,

4, 5

with a great

selectivity to DME. A great deal of effort has been focused in the literature on tailoring the catalytic effectiveness of the catalysts used in this process. 6 The addition of M3+ type ions has been widely reported to increase the activity and specific surface area of active Cu.7 In this regard, the addition of ZrO2 leads to the stabilization of the Cuδ+ sites, resulting in high performance catalysts especially for CO2 hydrogenation,8 with higher tolerance to H2O than those prepared with conventional Al2O3,9 and high thermal stability under reducing and oxidizing conditions.10 For the acid function, a moderate acidity is critical for this process, enough for methanol dehydration to DME, but not excessive in order to inhibit secondary paraffin formation mechanisms, Eq. (5).4, 11-13 Therefore, apart from the aforementioned γ-Al2O3, HZSM-5 zeolites, NaHZSM, mordenite, HY ferrierite, or SAPO-n catalysts have been widely studied.11, 1418

Moreover, the formulation of the final bifunctional catalysts requires an excess of

acid function in order to enhance the previously described thermodynamic advantage, ensuring the displacement of the methanol dehydration reaction.19 The direct synthesis of DME from CO2 hydrogenation is considered a multi-site reaction. Overall, it is considered that CO2 is bridge-adsorbed on ZnO and ZrO2 surfaces, and H2 is dissociatively adsorbed on the metallic Cu. Besides, different reaction mechanisms have been settled for the reaction giving way to methanol throughout different intermediates.9,

20-24

Frusteri et al.9 consider that by spillover,

activated H2* species react with activated CO2* species giving rise to the formation of intermediate species, which further evolve to methanol by hydrogenation, which is subsequently dehydrated to DME on the acid sites of the catalyst. Witoon et al.24, 25


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consider that the activated CO2* species give way to bicarbonate species, which react with activated H2* species, and are, subsequently, hydrogenated towards methanol. This work approaches the study of the incorporation of ZrO2 on the properties of the CuO-ZnO-ZrO2 metallic function. The optimum ZrO2 content has been determined based on the results obtained by characterization and quantification of its kinetic behavior (COx (CO+CO2) conversion, oxygenates yield, DME selectivity and stability) in the methanol synthesis under suitable conditions for the direct synthesis of DME, with reactions carried out in a fixed bed isothermal reactor. The methanol synthesis corresponds to the first reactions of the kinetic scheme of the STD process, Eqs. (1) and (2), which are CO and CO2 hydrogenation reactions. Moreover, the performance of the metallic function, selected based on its good kinetic performance, has been further studied in the direct synthesis of DME, using SAPO-11 catalyst as acid function for the preparation of the CuO-ZnO-ZrO2/SAPO-11 bifunctional catalyst. The SAPO-11 has been selected for this purpose due to its behavior in the dehydration of methanol (more selective towards DME than that of HZSM-5 zeolites subjected to acid passivation treatments and other SAPOs, as -18 and -34)26. The high selectivity to DME and stability of the SAPO-11 are related to the restricted acid strength of its acid sites, suitable for minimizing the formation of paraffins and coke. 2. EXPERIMENTAL 2.1. Preparation of the metallic and acid functions, and bifunctional catalyst CuO-ZnO-ZrO2 (CZZr) metallic functions for methanol synthesis with different Zr content, i.e., different Cu:Zn:Zr atomic ratios, have been prepared by a conventional precipitation method based on that proposed by Ereña et al.27-29 for CZA catalysts. In this preparation, an aqueous solution of Cu, Zn and Zr nitrates (1 M) and an aqueous


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solution of Na2CO3 (1 M), were added drop-wise in distilled water at 70 ºC maintaining the solution pH within the 6.8-7.2 range. In the preparation of each metallic function, the suitable nitrate content (acid solution) for obtaining the desired Cu:Zn:Zr atomic ratio has been used, being the studied Zr contents: 0, 0.3, 0.6, 1, 1.5 and 2. The corresponding metallic functions have been named as follows: CZZr0, CZZr0.3, CZZr0.6, CZZr1, CZZr2, respectively. Nevertheless, a Cu:Zn ratio of 2:1 has been pursued in all cases, which has been determined to be the optimum value for Cu:Zn:Al catalysts for the synthesis of methanol.2, 30 The following steps consist of: i) the aging of the metallic function at 70 ºC for 1 h under stirring; ii) filtering and washing of the precipitate to eliminate Na+ ions; iii) drying at room temperature for 12 h, and at 110 ºC for another 12 h and; iv) calcination at 300 ºC for 10 h. SAPO-11 has been used as acid function in the bifunctional catalyst for the dehydration of methanol to DME, regarding its high selectivity to DME and thermal and hydrothermal stability,31 key factors for the direct synthesis of DME.16 SAPO-11 has been prepared by crystallization in a Teflon coated autoclave reactor (Highpreactor BR-300, Berghof) at 195 ºC for 24 h, and has been dried (following the same two-step drying as for the metallic function) and calcined at 575 ºC for 8 h. H3PO4 (Merk), Ludox AS-40 (Aldrich) and Disperal (Sasol) have been used as P, Si and Al sources, respectively, and di-propylamine (Aldrich) as organic template. This acid function (named S-11 hereafter) has been selected based on previously obtained results,32 where its activity and suitability have been assessed both in the methanol dehydration reaction and on the direct DME synthesis process. In these reactions, SAPO-11 resulted more selective and stable than other acid functions, as HZSM-5 zeolites (with different SiO2/Al2O3 ratios and subjected to various acid passivation treatments) and other SAPOs (SAPO-18 and SAPO-34).


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The bifunctional catalyst used for the activity tests of the direct hydrogenation of CO+CO2 to DME has been prepared by physical mixing of both metallic and acid functions, with a mass ratio of 1/2 between both individual functions respectively (Figure S1 in the Supplementary Information). The catalyst has been finely powdered, pelletized, crushed and sieved to the desired particle size (125-500 µm). Prior to the reaction runs, the catalysts have been subjected to a reduction treatment, which consists of exposing them to a H2 stream diluted in N2 (14 h at 200 ºC, with a flow rate of 5 cm3H2 min-1) and, subsequently, to a higher concentrated H2 stream (1.5 h at 250 ºC, with 10 cm3H2 min-1). 2.2. Characterization of the metallic function and bifunctional catalyst N2 adsorption-desorption (Micromeritics ASAP 2010) technique has been used to determine the physical properties (BET surface area, micropore volume and total pore volume) of the individual functions and bifunctional catalyst. The experimental procedure consisted of the following steps: i) degassing of the sample at 150 ºC for 8 h under vacuum (10-3 mmHg) to eliminate possible impurities adsorbed in the catalyst; ii) N2 adsorption-desorption in multiple equilibrium stages at liquid N2 cryogenic temperature until the complete saturation of the sample. The chemical composition of the metallic functions, i.e., Cu:Zn:Zr atomic ratios, have been determined by means of inductively coupled plasma optical emission spectrometry (ICP-OES), in a Perkin Elmer Optima 8300 equipment. The experimental procedure involves the following steps: i) first acid attack of the sample with HNO3 and H2SO4 (1:2 ratio) maintained at 190 ºC for 24 h in a closed teflon container; ii) evaporation of the dissolution until a solid residue is obtained; iii) addition of HNO3 and successive dilutions to a 1:5 106 concentration.


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The study of the reducibility of the metallic functions has been performed by temperature programmed reduction (TPR) using a Micromeritics Autochem 2920 equipment, in order to determine the suitable temperature to ensure the complete reduction of CuO in the metallic functions to Cu0. In this analysis, a sample of approximately 50 mg was loaded into a U-shaped quartz reactor, and subjected to the following analysis steps: i) sweeping with He to eliminate possible impurities at 200 ºC for 2 h; ii) stabilization of the sample at room temperature (15 min) in a diluted H2 stream (10 % of H2 diluted in Ar); iii) heating of the sample with a temperature ramp of 2 ºC min-1 up to 260 ºC (maintained constant for 2 h) for the complete reduction in order to obtain Cu0 species. The metallic surface areas have been determined (for the metallic functions and bifunctional catalyst) by N2O selective chemisorption in a Micromeritics Autochem 2920 coupled to a mass spectrometer (Pfeiffer-Vacuum OmniStar). This analysis involves a first reduction step, similar to that described for the analysis of the reducibility of the sample (with a 10 % H2+Ar stream at 260 ºC, for 2 h) followed by: i) adsorption of N2O for the oxidation of the Cu0 species at 60 ºC (N2O diluted in He) in multiple steps (20 injections) until the complete saturation of the sample. The unreacted N2O and formed N2 signals have been recorded in a mass spectrometer, enabling therefore the direct correlation of the latter with the active Cu area. The surface morphology has been characterized by means of two techniques: energy dispersive X-ray spectroscopy (EDX); and scanning electron microscopy (SEM), using a JEOL JSM-7000F instrument equipped with a W filament. X-ray diffraction (XRD), in a Bruker D8 diffractometer operating at 40 kV and 40 mA and using Bragg-Brentano geometry and Cu-Kα radiation (λ = 1.5418 Å), has been used to


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study the structural properties of the different metallic functions synthesized. On the other hand, the XRD vs temperature data has been collected using a Bruker D8 diffractometer operating at 30 kV and 20 mA, equipped with a Cu tube (λ = 1.5418 Å), a Vantec-1 PSD detector, and an Anton Parr HTK2000 high-temperature furnace, and Powder Diffraction File (PDF) has been used as database for the preliminary identification of the initial phases. For this analysis, patterns have been recorded in 2θ steps of 0.033º in the 20 ≤ 2θ ≤ 78 range, counting for 0.4 s per step (total time 13 min). Data sets have been recorded from 30 to 810 ºC each 15 ºC with a heating rate of 0.083 ºC s-1 heating rate. The average size of the crystalline domains (coherently diffracting domains) of the samples has been extracted from the broadening of the signal using the Scherrer equation.33 Total acidity and acid strength distribution of the SAPO-11acid function and the CZZr1/S-11 bifunctional catalyst have been determined by means of combining thermogravimetry and calorimetry of NH3 adsorption at 150 ºC (Setaram TG-DSC 111), coupled online to a mass spectrometer (Balzers Instruments Thermostar). This procedure allows monitoring the heat flow together with the variation of the adsorbed mass. Therefore, the direct correlation of both signals provides the amount of heat released per unit mass of adsorbed base, while the total amount of chemically adsorbed NH3 is the total acidity of the material. 2.3. Reaction equipment and product analysis The automated reaction equipment (PID Eng. & Tech. Microactivity Reference) is provided with a high-pressure fixed-bed isothermal reactor system, described in detail in a previous work.2 The reactor is made of 316 stainless steel, has an internal diameter of 9 mm and 10 cm of effective length and is located inside a stainless steel


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covered ceramic chamber heated by an electric resistance. The equipment can operate up to 700 ºC and 100 atm with a catalyst mass up to 5 g. In order to ensure the isothermality of the bed (avoiding hot spots) and attaining a sufficient bed height under low space time conditions, the catalyst is mixed with an inert (SiC of 0.035 mm average particle size). Reaction products (diluted in a He stream of 25 cm3 min-1) are continuously analyzed (online) in a Varian CP-4900 gas micro-chromatograph provided with three analytical modules, with the following columns: (i) Porapak Q (PPQ, 10 m x 20 µm), for the quantification of CO2, methane, ethane, propane, methanol, DME, water and butanes; (ii) a molecular sieve (MS-5, 10 m x 12 µm), for the separation of H2, CO, O2 and N2; (iii) 5CB (CPSiL, 8 m x 2 µm), for the quantification of the C5-C10 hydrocarbons fraction (if any, formed in insignificant quantity). 2.4. Reaction indices The yield of each i product, Yi, has been calculated as the ratio between its molar flow rate and the molar flow rate of COx (CO+CO2) in the feed:

Yi =

n i ⋅ Fi 0 FCO x

⋅ 100

(6)

where F0COx is the molar flow rate of (CO+CO2) in the feed, in carbon units; ni is the number of carbon atoms of each i product; and Fi, the molar flow rate of the i product in the reactor outlet stream in carbon units. Product selectivity, Si, (in carbon units) has been calculated as the ratio between the molar flow rate of the i compound and the sum of the molar flow rates of the organic compounds (DME, methanol and C1-C3 paraffins) in the reactor outlet stream:


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Si =

n i ⋅ Fi ⋅100 ∑ n i ⋅ Fi i

(7)

The conversion of CO+CO2, XCOx, has been calculated as follows:

X CO x =

0 FCO − FCO x x 0 FCO x

⋅ 100

(8)

3. RESULTS AND DISCUSSION In this section, the effect of the Zr content on the properties of the CuO-ZnO-ZrO2 metallic function has been firstly studied. Secondly, a catalyst screening has been carried out in order to select the most suitable metallic function to be used in the bifunctional catalyst for the direct synthesis of DME. This screening has been made by comparing the results obtained with the different metallic functions as catalyst in the synthesis of methanol (activity, selectivity to DME and stability). In third place, the performance of the bifunctional catalyst conformed with the selected metallic function and SAPO-11 as acid function has been assessed in the direct synthesis of DME. 3.1. Effect of the Zr content in the properties of the metallic function 3.1.1. Physical properties From the N2 adsorption-desorption isotherms of the metallic functions (Figure S2 in the Supplementary Information), where type IV isotherms, with H1 hysteresis, characteristic of mesoporous materials have been observed, the specific surface areas have been determined using the BET method. Pore volume values have been


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calculated with BJH method using the adsorption branch of the isotherm. The physical properties of the metallic functions are summarized in Table S1. It was found that overall, the specific BET surface area showed an increasing trend when increasing the amount of ZrO2 in the metallic function. This fact, has previously been reported by Asthana et al.30 when using MgO as a promoter, where the increase of the surface area is attributed to the effect of the promoter on nanostructuring the CuO domains. SBET value enhanced noteworthy (more than doubling its value), from 61 m2 g-1 (CZZr0) to 144 m2 g-1 (CZZr2), when ZrO2 is incorporated to the metallic function. Comparing the surface areas of CZZr0 and CZZ0.3 metallic functions, the latter containing the smallest amount of ZrO2 studied, an increase of 87 % is observed in the specific BET surface area. Nevertheless, both micropore volume and total pore volume go through a maximum for the CZZr0.3 metallic function, as for higher Zr contents, a selective blocking of both micropores and smaller mesopores occurs. 3.1.2. Chemical and metallic properties ICP-OES measurements have been carried out to evaluate whether the metallic content obtained is close the nominal in order to ensure that the synthesis of the metallic function has been carried out properly. From the analyses the following atomic ratios between Cu:Zn:Zr have been determined: 2:0.66:0 for CZZr0; 2:1.01:0.32 for CZZr0.3; 2:0.91:0.55 for CZZr0.6; 2:0.75:0.89 for CZZr1; 2:0.75:1.21 for CZZr1.5 and 2:0.75:2.04 for CZZr2, which are considered to be in relatively good agreement with the nominal composition values (2:1:X). As observed in the TPR profiles, Figure 1, for the six metallic functions studied a temperature below 250 ºC is sufficient for the complete reduction of the Cu species, what is noticeably lower than that reported for CuO-ZnO-Al2O3 metallic functions ACS Paragon Plus Environment

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(about 300 ºC).2,

30

Although TPR profiles show differences depending on the Zr

content, a main peak corresponding to the reduction of the CuO phase appears at around 150 ºC in all cases. The presence of overlapped secondary peaks is attributed to the sequential reduction of the dispersed of Cu2+ species to Cu+ and Cu0 subsequently.21, 34-36 It has been observed that the incorporation of ZrO2 has a remarkable effect on the reduction of CuO. Thus, with low ZrO2 contents (CZZr0.3 and CZZr0.6 catalysts), peak at the lowest temperature in the TPR profile, attributed to the reduction of highly dispersed CuO,37 is displaced when increasing the ZrO2 content. This effect has been related by Agrell et al.35 to the interaction between CuO and the tetravalent cations of Zr. For higher ZrO2 contents than the composition 2:1:1, a different effect has been observed thus, not increasing the sites dispersion but the increasing the TPR total area, which is attributable to the increasing of bulk CuO, whose reduction temperature is higher to those of the disperse sites. Figure 1 The Cu metallic surface area (per unit mass of metallic function and per unit mass of Cu content) and Cu dispersion values (determined using the technique of selective chemisorption of N2O described in Section 2.2) have been summarized in Table S2. It is noteworthy that the incorporation of ZrO2 enhances Cu dispersion, nearly doubling its value when increasing Zr content from 0 (CZZr0) to 0.6 (CZZr0.6) (from 3.51 % to 6.1 %). Nevertheless, the upgrade of the metallic properties, surface area and Cu dispersion, do not show a linear relation with the Zr content. These parameters increase sharply at relatively low Zr contents and stabilize for higher values. Furthermore, as reported for the physical properties, significant differences have been


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observed in the metallic surface area and Cu dispersion when increasing the Zr content from 0 to 0.6, achieving maximal values of of 39.8 m2 gCu-1 and Cu dispersion of 6.1 % for the CZZr0.6 metallic function. For higher Zr contents, not remarkable differences have been observed. However, it should be taken into account that for Zr contents above 1, the metallic properties tend to an asymptotic and stable trend. 3.1.3. Morphology and structural properties The surface SEM images of CZZr0, CZZr1 and CZZr2 (Figure 2) show that these metallic functions are a combination of small particles, lower than 100 nm, not directly related to the Zr content, which has been confirmed by EDX analysis. Figure 2 The normalized XRD patterns of the metallic functions are shown in Figure 3. The broad diffraction peaks observed in the figure reveal the poor crystallinity of the metallic functions. This fact, together with the small size of the ZrO2 crystals, precludes the detection of signals attributable to Zr and, therefore, the diffractograms only reveal the characteristic peaks of CuO and ZnO. Concerning CuO, two intense peaks at 2θ = 35.7º (overlapping the peak of ZnO) and 39º are observed; while ZnO, shows peaks at 32º and 56.7º. Moreover, other peaks of lower intensity associated with ZnO are also observed at 32.8º and 44º.24, 38-40 The absence of hydroxycarbonate intermediates in these patterns indicates that the synthesis of the metallic functions allows a complete transition towards Cu and Zn oxides.24,

38-40

Overall, the signal

intensity of the XRD patterns lessens when increasing the Zr content of the sample, disclosing the loose of crystallinity. Figure 3


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Likewise, in order to study the possible Cu sintering, X-ray diffraction patterns have been obtained at different temperatures, between 30 and 810 ºC. The XRD patters vs temperature for CZZr0, CZZr1 and CZZr2 metallic functions have been plotted in Figure 4. Attending to these results, it can be seen that when increasing temperature the characteristic peaks of CuO and ZnO intensify and get narrowed, evidencing the sintering of the metal. In addition, for an overall view of the influence of temperature on the evolution of the estimated CuO crystal size, the Scherrer equation has been used.33 According to this procedure, it has been observed that the higher the Zr content, the increase of the CuO crystals size takes place at a higher temperature, and gives way to smaller crystals. Consequently, Zr favors the thermal resistance of Cu, avoiding the irreversible deactivation by sintering of the metallic function when operating at reaction temperatures commonly used in the STD process (below 300 ºC). For CZZr0 a sintering temperature around 450 ºC has been determined (using the Scherrer equation), while CZZr1 and CZZr2 exhibit a similar sintering temperature, 550 and 600 ºC respectively. Figure 4 3.2. Kinetic behavior of the metallic functions in the methanol synthesis The effect of the Zr content in the metallic function has been studied comparing the catalytic performance of the different functions in the synthesis of methanol from H2+CO+CO2 ternary mixtures. Based on the obtained results, the most suitable function for the preparation of the bifunctional catalyst has been selected. The reaction runs of methanol synthesis have been carried out feeding H2+CO+CO2 mixtures under the following reaction conditions: 275 ºC; 30 bar; space-time, 0.88 gCu h (molC)-1; H2/COx molar ratio in the feed, 3; CO2/COx molar ratio, 0.5


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(corresponding to 50 % CO + 50 % CO2). This operating conditions have been established in previous works as the most suitable for the comparison of catalysts in the direct synthesis of DME when co-feeding CO2 together with syngas.41 Figure 5 shows the COx conversion and product selectivity (methanol and paraffins) at zero time on stream for the different metallic functions. The results at zero time on stream (Figure 5) reveal that COx conversion, and therefore, methanol yield, increase by increasing Zr content in the metallic function. Moreover, an upgrade of 28 % in the COx conversion with the incorporation of Zr should be highlighted comparing the results obtained with CZZr0 and CZZr0.6 metallic functions, and of 37 % when comparing CZZr0 and CZZr1. On the other hand, for Zr contents above 1, the reaction indices follow an asymptotic trend as shown in Figure 5, that is, no enhancement has been observed in any of the studied reaction indices from CZZr1 to CZZr2. On regard to product selectivity, the high selectivity of the process to methanol is remarkable (above 98 % in all cases), whereas not significant differences have been observed for the studied metallic functions, especially from Zr contents higher than 0.6 (Figure 5). Figure 6 relates methanol yield values (at zero time on stream) with the Cu dispersion of the corresponding metallic function (reported in Section 3.1.3). It is noteworthy that the performance of the metallic function is closely related to the metallic properties (Cu dispersion) of the catalyst, as suggested by Frusteri et al.42 and Arena eta al.43 In this regard, as reported by other authors, the relationship between the activity of Cu-ZnO based catalysts modified with MOx (M= Ga, Al, Zr, Cr, Mn…) for CO2 hydrogenation can be considered to follow a linear 47, 48

10, 44-46

or not linear trend

between the conversion of CO2 and Cu surface, whether the metallic properties


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of the catalysts (Cu dispersion or specific Cu surface area) interactions with the support influence the activity. Analyzing the results obtained for the CZZr metallic functions studied in this work (Table S2 and Figure 6), an almost linear correlation between activity (COx conversion and methanol yield and selectivity) and Cu dispersion and specific surface area is observed. From these results, a similar catalytic performance for CZZr0.6, CZZr1, CZZr1.5 and CZZr2 metallic functions is to be expected. Figure 5 Figure 6 In order to achieve a greater accuracy in setting the suitable Cu:Zn:Zr atomic ratio of the metallic function, besides the catalytic performance at zero time on stream, the stability of the individual metallic functions has been investigated by means of runs carried out with a time on stream of 24 h, at 275 ºC, 30 bar, 0.88 gCu h (molC)-1 spacetime, CO2/COx= 0.5 and H2/COx= 3 feeds. The results have been plotted in Figures 79. Figure 7 shows the evolution with time on stream of COx conversion for the metallic functions with different Zr contents. The results reveal there is a significant boost of the COx conversion with the incorporation of Zr, thus, when comparing CZZr0 and CZZr0.6; whereas for higher contents (CZZr1, CZZr1.5 and CZZr2 metallic functions), an almost identical catalytic performance and stability has been observed, which is consistent with the aforementioned hypothesis derived from the results of catalyst characterization, where a similar specific Cu surface and dispersion where determined for these metallic functions. Figure 7


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Accordingly, in Figure 8, the evolution of methanol and paraffin selectivity has been depicted only for CZZr0, CZZr1 and CZZr2 metallic functions for a more clear comparison of the results; since CZZr0.6 results do not differ significantly from those of CZZr1, and CZZr0.3 gives way to intermediate results between CZZr0 and CZZr1 (closer to those obtained with the former). As observed at zero time on stream, high methanol selectivity values have been registered in all cases, and have remained almost constant with time on stream. Despite a slight improvement is observed with increasing Zr content, it hardly represents an increase of 0.7 in the selectivity to methanol from CZZr0 to CZZr2. On regard to paraffin formation, even if very low values of paraffin selectivity have been registered for all cases, it is limited by the incorporation of Zr to the metallic function, as a reduction on 50 % can be observed in Figure 8. Figure 8 When comparing the effect of time on stream on methanol and paraffin yields for different metallic functions (Figure 9), once more, the results indicate that the highest methanol yield is obtained for CZZr1, CZZr1.5 and CZZr2, being the registered values similar for the three catalysts. The evolution of these reaction indices with time on stream, together with the low and stable paraffin yield observed (lower than 0.1 % in all cases), evidences a small deactivation of the metallic function. This effect is more significant in the first stage of the reaction, and attenuates gradually, what is presumably attributed to the deactivation by coke deposition, which takes place in parallel to the CO+CO2 hydrogenation reactions as reported for different CuO-ZnO based catalysts.49-51 Figure 9 ACS Paragon Plus Environment

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On the basis of the results obtained from the activity testing runs (COx conversion, product yield and selectivity, and stability of the catalyst), together with the results obtained from the characterization, it has been determined that Cu metallic surface and dispersion is the key point to be considered for selecting the most appropriate metallic function for CO+CO2 hydrogenation. Hence, bearing in mind there is a critical difference for the metallic functions with a Zr content below 0.6, and the stable trend of those with a Zr content above 1, CZZr1 has been selected as the most suitable metallic function. This Zr content provides the CuO-ZnO catalyst the aforementioned upgrade, and ensures the catalytic performance is not conditioned by slight differences in the Cu:Zn:Zr ratio as it may occur for contents of 0.6 (as it can be observed in Figure 6), where a vague difference in the Zr content would lead to a sharp lessening of the COx conversion and methanol yield. In view of this, CZZr1 has been used in the succeeding sections. 3.3. Kinetic behavior of the bifunctional catalyst in the direct synthesis of DME In order to verify the suitability of the selected metallic function (CZZr1) in the STD process, a bifunctional catalyst (CZZr1/S-11) has been prepared by physical mixture of the CZZr1 metallic function with SAPO-11 acid function, following the procedure described in Section 2.1. It should be mentioned that prior to the configuration of the bifunctional catalyst, as a preliminary study, the good performance of the SAPO-11 in the dehydration of methanol to DME has been verified by runs carried out feeding pure methanol under the reaction conditions described in Section 2.3, obtaining DME yield and selectivity values over 85 % and 98 % respectively (Supplementary Information, Figures S3-S4).32 Table S3 shows the physical properties of CZZr1, SAPO-11 and CZZr1/S-11 bifunctional catalyst (Figure


19


Page 20 of 47

S5 in the Supplementary Information). It is observed that the value of the specific surface area of the acid function (S-11), 155 m2g-1, is higher than that of the metallic functions (Table S2) and similar to that reported by Yoo et al.

16

Furthermore, it

should be noted that the physical properties of the bifunctional catalyst are close to the average of the individual metallic (CZZr1) and acid (S-11) functions mixed with a mass ratio of 1/2. The deviation is due to the partial blocking of the micropores produced as a result of the preparation of the bifunctional catalyst by physical mixing. Table S4 shows the acid properties of SAPO-11 acid function and CZZr1/S-11 bifunctional catalyst. The total acidity of SAPO-11 is 0.208 mmolNH3·g-1, and the unique peak registered at 287 ºC in the NH3-TPD gives way to a uniform acid strength (89 kJ·molNH3-1), fulfilling therefore, the pursued requirements of a suitable acid function for methanol dehydration to DME. In the bifunctional catalyst (CZZr1/S-11) the total acidity is lower, 0.168 mmolNH3·g-1. However, the effect over the acid strength (85 kJ·molNH3-1) is little and is also little the displacement of the NH3-TPD peak to a lower temperature (250 ºC) (results not shown). Table S5 gathers the metallic properties of CZZr1 metallic function and CZZr1/S-11 bifunctional catalyst. When the metallic function is combined with the acid function to conform the bifunctional catalyst, the dilution of the metallic particles in the acid particles results in a greater accessibility of H2 to the metallic surface and therefore, higher metallic surface, 44.4 m2gCu-1, and Cu dispersion, 6.8 %, is achieved. The activity of the bifunctional catalyst has been assessed under similar reaction conditions to those used in Section 3.2 for the synthesis of methanol, that is: 275 ºC; 30 bar; space time, 2.5 gcat. h (molC)-1 (corresponding to 0.88 gCu h (molC)-1); CO2/COx molar ratio, 0.5; H2/COx molar ratio, 3. The results at zero time on stream have been


20

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plotted in Figure 10, where high COx conversion values and oxygenate yield can be observed. Among the oxygenates, a selectivity to DME over 80 % is to be highlighted, with a low paraffin formation, below 1 %, which is even lower than that obtained in the synthesis of methanol with CZZr1 (Figures 5 and 8). These results evidence the good kinetic performance of the bifunctional catalyst, where on the one hand, the metallic function is active for the hydrogenation of CO and CO2; and on the other hand, the acid function has appropriate properties for the dehydration of the formed methanol, restricting any activation of the hydrocarbon pool mechanism51 and therefore lessening paraffin formation.52 Figure 10 As for the metallic functions, the stability of the bifunctional catalyst has also been assessed by runs of 24 h time on stream. Figure 11 displays the evolution with time on stream of the yield and selectivity of the reaction products (DME, methanol and paraffins) at the same operating conditions as in Figure 10. Attending to the DME yield profile and to the selectivity of methanol and DME, it is evident that deactivation affects in a higher extent to the activity of the metallic function, since the dehydration capacity of the SAPO-11 remains almost unaltered (almost constant values of DME and methanol selectivity are observed throughout the whole experiment) and DME yield diminishes with time on stream. This trend is in accordance with the widespread hypothesis of deactivation by coke formation in parallel with the rate limiting step of the STD process, that is, methanol formation.49, 50

Figure 11 4. CONCLUSIONS ACS Paragon Plus Environment

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The synthesis of CZZr metallic functions with different Cu:Zn:Zr ratios, thus varying the amount of Zr as a promoter from 0 to 2, and their characterization have been reported. Furthermore, their catalytic activities on the synthesis of methanol (first step of direct DME synthesis with a bifunctional catalyst) from H2+CO+CO2 mixtures have been studied, under the suitable conditions for the direct DME synthesis. The characterization of the metallic functions has evidenced that the specific Cu surface area and Cu dispersion are the critical properties for enhancing the activity of CuO-ZnO based catalysts, which improves when incorporating Zr. Overall, BET surface area, Cu dispersion and catalyst activity increase with increasing Zr up to a content of 0.6, and follow an asymptotic trend for higher amounts of Zr, thus, the results obtained with CZZr0.6, CZZr1 and CZZr2 are very similar. Based on the catalytic performance, CZZr1 has been selected as the optimal metallic function. The corresponding Zr content (2:1:1) ensures a high activity, methanol selectivity and stability (avoiding Cu sintering). The kinetic performance results obtained with the bifunctional catalyst (prepared by physical mixture of CZZr1 and SAPO-11 in a mass ratio of 1/2) are encouraging for the utilization of CZZr1/S-11 in the direct DME synthesis process from H2+CO+CO2 feeds, obtaining high DME yield and selectivity with almost negligible paraffin formation. Acknowledgements This work has been carried out with the financial support of the Ministry of Economy and Competitiveness of the Spanish Government (CTQ2013-46173-R and CTQ2016-77812-R), the ERDF funds and the Basque Government (Project IT74813). Miguel Sánchez-Contador and Ainara Ateka are grateful for the Ph.D. grants


22

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from the Department of Education, University and Research of the Basque Government (PRE_2013_1_841 and BFI09.69, respectively). Pablo Rodriguez-Vega is grateful for the Ph.D. grant from the Ministry of Economy and Competitiveness of the Spanish Government (BES-2014-069892).


23


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NOMENCLATURE CZZrX

CuO-ZnO-ZrO2 metallic function, with X content of Zr in the Cu:Zn:Zr (2:1:X) atomic ratio.

DME

Dimethyl ether.

F0COx

Molar flow rate of (CO+CO2) in the feed, molC h-1. Eqs. (6) and (8).

Fi

Molar flow rate of i component in the reactor outlet stream, molC h-1. Eqs. (1) - (3).

ni

Number of carbon atoms in the i product. Eqs. (6) and (7).

S-11

SAPO-11 acid function.

SBET

BET specific surface area, m2 g-1.

SCu, S’Cu

Cu specific surface area, m2 gCu-1 and m2 gcat.-1, respectively.

Vm, Vp

Micropore volume and total pore volume, respectively, cm3 g-1.

XCOx

Conversion of CO + CO2, %. Eq. (8).

Yi, Si

Yield and selectivity of i component, respectively, %. Eqs. (6) and (7) respectively.


24

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FIGURE CAPTIONS Figure 1. TPR profiles of the metallic functions. Figure 2. SEM images and EDX analyses of CZZr0 (a), CZZr1 (b) and CZZr2 (c) metallic functions. Figure 3. XRD pattern of CZZr0, CZZr1 and CZZr2 metallic functions. Figure 4. XRD patterns vs temperature for CZZr0, CZZr1 and CZZr2 metallic functions. Figure 5. Effect of the Zr content in the metallic function on COx conversion and product selectivity, in the synthesis of methanol. Reaction conditions: 275 ºC; 30 bar; space-time, 0.88 gCu h (molC)-1; H2/COx molar ratio, 3; CO2/COx molar ratio, 0.5. Figure 6. Effect of the Zr content in the metallic function on Cu dispersion and methanol yield. Reaction conditions: 275 ºC; 30 bar; space-time, 0.88 gCu h (molC)-1; H2/COx molar ratio, 3; CO2/COx molar ratio, 0.5. Figure 7. Evolution with time on stream of COx conversion in methanol synthesis, for CZZr0, CZZr0.3, CZZr0.6, CZZr1, CZZr1.5 and CZZr2 metallic functions. Reaction conditions: Reaction conditions: 275 ºC; 30 bar; space-time, 0.88 gCu h (molC)-1; H2/COx molar ratio, 3; CO2/COx molar ratio, 0.5. Figure 8. Evolution with time on stream of methanol and paraffin selectivity, in the synthesis of methanol for CZZr0, CZZr1 and CZZr2 metallic functions. Reaction conditions: Reaction conditions: 275 ºC; 30 bar; space-time, 0.88 gCu h (molC)-1; H2/COx molar ratio, 3; CO2/COx molar ratio, 0.5.


25


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Figure 9. Evolution with time on stream of methanol and paraffin yields for CZZr0, CZZr0.3, CZZr0.6, CZZr1, CZZr1.5 and CZZr2 metallic functions. Figure 10.

COx conversion and product yield (a) and selectivity (b), in the direct DME synthesis, at zero time on stream for CZZr1/S-11 bifunctional catalyst.

Reaction

conditions:

275 ºC;

30

bar;

space

time,

2.5 gcat. h (molC)-1; CO2/COx molar ratio, 0.5; H2/COx molar ratio, 3. Figure 11.

Evolution with time on stream of product yield (a) and selectivity (b), in the direct DME synthesis over a CZZr1/S-11 catalyst. Reaction conditions: 275 ºC; 30 bar; space time, 2.5 gcat. h (molC)-1; CO2/COx molar ratio, 0.5; H2/COx molar ratio, 3.


26

Page 27 of 47

FIGURES

CZZr2

CZZr1.5 TCD Signal/gCu (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CZZr1

CZZr0.6

CZZr0.3

CZZr0 50

100

150

200

250

Temperature (ºC)

Figure 1.


27


2000

a)

Cu Zn

a.u.

1500 1000 C

O

500 0 0

1

2

3

4

3

4

3

4

keV 2000

b)

Cu Zn 1500 Zr

a.u.

O 1000 500 C 0 0

1

2

keV 2000

c)

1500

a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 47

Cu Zn O

1000

Zr

C

500 0 0

1

2

keV

Figure 2.


28

Page 29 of 47

CuO CuO CuO + ZnO

ZnO

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZnO

CZZr0

CZZr1 CZZr2

0

20

40

60

2θ

Figure 3.


29


Page 30 of 47

Figure 4.


30

Page 31 of 47

SMethanol

100

SParaffins

XCOx

10

8 90 6 80

4

2

20 0

XCOx (%)

SMethanol, SParaffins (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0.3

0.6

1

1.5

2

0

Zr content (Cu2:Zn1:ZrX) Figure 5.


31


9.0

Cu dispersion, YMethanol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 47

7.5 6.0 4.5 3.0 1

YMethanol Cu dispersion

0

0

0.3

0.6

1

1.5

2

Zr content (Cu2:Zn1:ZrX) Figure 6.


32

Page 33 of 47

10 CZZr2

CZZr1

8

XCOx

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

CZZr1.5

CZZr0.6

4

CZZr0 CZZr0.3 CZZr0.6 CZZr1 CZZr1.5 CZZr2

2 0 0

240

CZZr0

480

720

960

CZZr0.3

1200

1440

time on stream (min)

Figure 7.


33

100

10

98

8

SMethanol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 47

96

Methanol Paraffins

6 CZZr0 CZZr1 CZZr2

94 92

0

4

SParaffins (%)


2

0

240

480

720

960

1200

0 1440


Figure 8.


34

Page 35 of 47

CZZr0 CZZr0.3

CZZr0.6

CZZr1 CZZr1.5

CZZr2

Methanol Paraffins

10

0.6

0.4 6 4 0.2

YParaffins (%)

8

YMethanol (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 0

0

240

480

720

960

1200

0.0 1440


Figure 9.


35


a)

Page 36 of 47

b)

Figure 10.


36

8 7 6 5 4 3 2 0.4 0.2 0.0

a) DME Methanol Paraffins

0

240

480

720

960

1200

1440

time on stream (min) 100 b)

Si (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yi (%)

Page 37 of 47

75 DME Methanol Paraffins

50 25 2 1 0

0

240

480

720

960

1200

1440

time on stream (min) Figure 11.


37


Page 38 of 47

Supporting Information Evolution of COx conversion and DME, methanol and paraffins yield with the mass relation between metallic and acid functions (Figure S1), N2 adsorptiondesorption isotherms of the metallic functions, CZZr0, CZZr0.3, CZZr0.6, CZZr1, CZZr1.5 and CZZr2 (Figure S2), effect of the Zr content in metallic function of CuOZnO-ZrO2 physical properties (Table S1), metallic surface area and dispersion of the synthesized metallic functions of CuO-ZnO-ZrO2 (Table S2), DME yield and selectivity obtained in the methanol dehydration reaction over SAPO-11 (Figure S3), evolution with time on stream of DME and HC yields in the methanol dehydration reaction over SAPO-11 (Figure S4), N2 adsorption-desorption isotherms of CZZr1, S11 and the bifunctional catalyst CZZr1/S-11 (Figure S5), physical properties of the CZZr1 metallic function, S-11 acid function and CZZr1/S-11 bifunctional catalyst. (Table S3), acid properties of the acid function (SAPO-11) and bifunctional catalyst (CZZr1/S-11) (Table S4), and metallic properties of the CZZr1 metallic function and CZZr1/S-11 bifunctional catalyst (Table S5).


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For Table of Contents Only

CuO-ZnO-ZrO2/SAPO-11

H2 CO CO2

XCOx (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu dispersion (%)

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DME MeOH HC

Zr content (Cu2:Zn1:ZrX)


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Optimization of the Zr Content in the CuO-ZnO ... - ACS Publications

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