A study of K-promoted MoP–SiO2 catalysts for synthesis gas conversion

Applied Catalysis A: General 378 (2010) 59–68

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A study of K-promoted MoP–SiO2 catalysts for synthesis gas conversion Sharif F. Zaman, Kevin J. Smith * Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC, Canada V6T 1Z3

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 October 2009 Received in revised form 19 January 2010 Accepted 31 January 2010 Available online 11 February 2010

The conversion of synthesis gas to oxygenated hydrocarbons over a series of K-promoted MoP–SiO2 catalysts, is reported. Catalysts with 5, 10, and 15 wt% MoP on SiO2 and promoted with 1 and 5 wt% K, were prepared, characterized and tested at 548 K, 8.27 MPa and a H2:CO = 1. An increase in CO conversion was observed with increased MoP and K loading and the highest oxygenate space time yield of 147.2 mg g cat1 h1 was obtained over the 5 wt% K–15 wt% MoP–SiO2 catalyst. CH4 was produced with high selectivity (29–40 C atom%) over the un-promoted MoP catalysts, but with the addition of K, the CH4 selectivity was suppressed and the C2+ oxygenated product selectivity increased as the surface K/ Mo atom ratio increased, up to a maximum at a K/Mo ratio of 3.2. The highest selectivity towards C2+ oxygenates (76.6 C atom%) and lowest selectivity towards CH4 (9.7 C atom%) occurred on the 5 wt% K– 10 wt% MoP–SiO2 catalyst. The major oxygenates in the product were acetaldehyde, acetone and ethanol. In all cases, catalyst selectivity to methanol was low (<5 C atom%). The product distribution obtained over the K–MoP–SiO2 catalysts was distinct from that reported on other Mo-based catalysts. ß 2010 Elsevier B.V. All rights reserved.

Keywords: Synthesis gas Gas-to-liquids MoP Catalyst Ethanol Acetaldehyde Acetone C2+ oxygenates Alcohols

1. Introduction Concerns about global climate change and the depletion of fossil fuels have increased interest in alternative fuels development, especially in the area of biomass conversion [1]. Biomass can be converted into a wide range of liquid fuels, such as bio-ethanol and bio-butanol. Ethanol is the focus of attention because it can be produced from agricultural feedstock (e.g. corn, sugarcane) by fermentation [2], or by converting lignocellulose (e.g. corn stove, switch grass, wood waste) to ethanol by an enzymatic process [3]. A third, more flexible approach is biomass gasification to syngas (CO, CO2 and H2) followed by catalytic conversion of the syngas to ethanol and other oxygenated products. Most known catalysts for the conversion of syngas to oxygenated hydrocarbon products suffer from poor selectivity. Rh and Mn-promoted Rh catalysts have been reported to show high selectivity towards C2+ oxygenates (the sum of oxygenates with 2 or more carbon atoms, including ethanol, acetaldehyde and acetic acid) from syngas [4–10], but in nearly all cases the total hydrocarbon selectivity is >40 C atom%. High selectivity towards ethanol (34.8 C atom%) was reported on Li/Na containing Mnpromoted Rh/SiO2 catalysts [6]. More recently Hu et al. [7] reported

* Corresponding author. Tel.: +1 604 822 3601; fax: +1 604 822 6003. E-mail address: [email protected] (K.J. Smith). 0926-860X/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.01.046

44.5% selectivity to ethanol over a 6% Rh–1.5% Mn–SiO2 catalyst, but the hydrocarbon selectivity was 48.5%. Egbebi and Spivey [10] reported high selectivity for acetaldehyde on a Rh–Li–Mn/TiO2 catalyst, explaining that ethanol and acetaldehyde share a common surface intermediate resulting in an increase in ethanol selectivity and a decrease in acetaldehyde selectivity with an increase in the feed gas CO/H2 ratio. However, the high selectivity to hydrocarbons, especially CH4, and cost and supply issues associated with Rh, means that there is significant interest in developing alternative catalysts for selective ethanol synthesis from syngas. Modified methanol synthesis (Cu–ZnO) catalysts, modified Fischer-Tropsch (Fe, Ru, Co) catalysts and Mo-based catalysts have all been investigated for syngas conversion. Mobased catalysts generate mainly linear alcohols with relatively high selectivity towards ethanol [2], whereas Cu–ZnO-based catalysts produce mostly branched alcohols [11]. MoS2 has been claimed to give high selectivity (40 C atom%) towards ethanol [12]. Subsequently, Li et al. [13], Iranmahboob et al. [14] and Li et al. [15] worked with alkali doped MoS2 that was also promoted with Rh, Co and Ni. Ethanol selectivity varied from 10 to 30% on a CO2-free basis. Li et al. [16] reported 15–30% ethanol selectivity on a K– MoS2/C catalyst operated at 598 K, 5.1 MPa and a H2:CO ratio of 1:1. Zhang et al. [17] investigated a K–Co–MoS2 catalyst and obtained 12% ethanol selectivity at 573 K, 6 MPa and a H2:CO ratio of 2:1. Recently, Xiang et al. [18,19] reported on a K-doped b-Mo2C modified with Ni, Co and Fe for alcohol synthesis from syngas. High

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S.F. Zaman, K.J. Smith / Applied Catalysis A: General 378 (2010) 59–68

selectivity toward hydrocarbons was observed and ethanol selectivity of 9–14 C atom% was reported on these catalysts at 573 K, 8 MPa and a H2:CO ratio of 1:1. The oxygenated products are produced from syngas through a series of step-wise C–C bond forming reactions. Ethanol selectivity is generally low because of the slow kinetics of the first C–C bond formation step and the rapid chain growth of C2 intermediates [11,20,21]. On alkali promoted Cu–ZnO catalysts, the chain growth occurs via a base-catalysed aldol condensation reaction [21], that propagates the chain to form mostly iso-butanol, whereas, on metal catalysts such as Rh, chain growth occurs via CO insertion into surface CHx fragments generated by the dissociative adsorption of CO [22]. Similar mechanisms have been ascribed to Mo catalysts, although more recent DFT studies have shown that CO does not dissociate on MoS2 [23,24]. Recently, MoP catalysts supported on SiO2 and Al2O3 were investigated for hydrodenitrogenetaion (HDN) and hydrodesulfurization (HDS) reactions [25,26]. Oyama [27] suggested that metal phosphides may also have good activity in other hydrogenation reactions, such as synthesis gas conversion to hydrocarbons and alcohols. However, a DFT study of syngas conversion to CH4 and CH3OH over an Mo6P3 cluster model of an MoP catalyst, demonstrated the tendency to selectively produce CH4 from CO + H2 rather than CH3OH [28]. A high adsorption energy of CH3OH on the cluster suggested that it may assist in the production of higher alcohols [28]. Similar results have also been obtained on MoS2, showing that although CO does not dissociate on MoS2, CH4 is preferentially formed via breaking of the C–O bond of a hydroxymethyl surface intermediate [23]. However, it is well known that alkali promoters decrease selectivity to hydrocarbons [12,16] and increase the selectivity to oxygenated products on MoS2 [16], Rh [8] and Cu–ZnO [11,20] catalysts. Recently, Zaman and Smith [29] reported the first results of syngas conversion over an MoP–SiO2 catalyst, showing that K promotion was effective in reducing selectivity to hydrocarbons (CH4) and increasing selectivity to oxygenated products on this catalyst as well. In the present work, the MoP catalysts have been investigated further, with a focus on determining the effect of MoP and K promoter loading on product selectivity. Results of syngas conversion over 5, 10 and 15 wt% MoP, with 0, 1 and 5 wt% K on SiO2 for syngas conversion to alcohols, are reported. 2. Experimental The silica supported MoP catalysts were prepared by temperature-programmed reduction (TPR) following the procedure established by Phillips et al. [30] for MoP–SiO2 catalysts. To prepare the 5, 10 and 15 wt% MoP–SiO2 catalyst, stoichiometric amounts (Mo/ P = 1) of ammonium heptamolybdate (0.69, 1.39 and 2.09 g of (NH4)6Mo7O244H2O, BDH Chemicals, 99%, respectively) and diammonium hydrogen phosphate (0.52, 1.04 and 1.56 g of (NH4)2HPO4, Sigma–Aldrich, 99%, respectively) were dissolved in 15.7 ml of de-ionized water and impregnated drop-wise onto 10 g of the SiO2 support (Sigma–Aldrich, Grade 62, 60–200 mesh, BET area = 330 m2 g1, pore volume = 1.2 cm3 g1) with continuous stirring. The impregnated support was aged at room temperature for 12 h before being dried at 373 K for 12 h and calcined at 773 K for 5 h. The calcined catalyst precursor was subjected to TPR in a H2 (Praxair, 99.99%) flow of 120 cm3 (STP) min1 g1, at a temperature ramp of 1 K min1 to a final temperature of 923 K. The final temperature was held for 2 h. After reduction, the catalyst was cooled to room temperature in He and prior to removal from the reactor, the catalyst was passivated in a flow of 2 vol.% O2 in He for 2 h at room temperature. Preparation of the 1 and 5 wt% K-promoted catalyst was achieved by first impregnating 10 g of the SiO2 support with a

solution of potassium nitrate (KNO3, BDH Chemicals, 99.97%), prepared by dissolving the required amount of KNO3 (0.26 and 1.29 g, respectively) in 12 ml of de-ionized water for incipient wetness impregnation. After aging at room temperature for 12 h, the impregnated SiO2 was dried at 373 K for 12 h followed by calcination at 773 K for 5 h. Subsequently, the K–SiO2 support was impregnated with (NH4)6Mo7O244H2O and (NH4)2HPO4, dried, calcined and reduced as before. The composition of a selected number of the catalysts was confirmed by ICP-AES analysis conducted by Cantest Laboratories, Vancouver, Canada. The catalyst BET surface areas were measured using a Micromeritics FlowSorb II 2300 analyser. About 0.1 g of the catalyst was degassed at 473 K for 2 h and the measurement was made using 30% N2 and 70% He. Temperature-programmed reduction (TPR) experiments were carried out using a Micromeritics AutoChem 2920 apparatus. The sample (160–180 mg) was placed in a quartz U-tube and reduced in a 50 cm3(STP) min1 flow of 9.5% H2 in Ar gas mixture. The temperature was increased at a ramp rate of 5 K min1 to a final temperature of 1023 K, which was maintained for 4 h. Prior to the analyses, the temperatureprogrammed reduction of a reference material (silver oxide) was done at the same conditions to calibrate the TPR peak area to the volume of H2 consumed. The CO uptake was measured by pulsed chemisorption also using a Micromeritics Autochem II 2920 unit. The passivated catalyst was pretreated to remove the passivation layer by passing 50 ml(STP)/min of 10% H2/Ar while heating from 313 to 923 K at a rate of 5 K/min, and maintaining the final temperature for 1 h. The 10% H2/Ar flow was then switched to He (50 ml(STP)/min) at 923 K for 1 h in order to remove the adsorbed species. The reactor was subsequently cooled to 298 K, and 0.5 ml pulses of CO were injected into a flow of He (50 ml(STP)/min) and the CO uptake was measured using a TCD. CO pulses were repeatedly injected until no further CO uptake was observed after consecutive injections. Energy dispersive X-ray analysis (EDX) was performed using a Hitachi S-3000N electron microscope operated with a 20 kV electron beam acceleration voltage. Samples were mounted on carbon planchettes with carbon paste. Gold was sputtered onto the catalyst to ensure sufficient conductivity. At least 10 different particles of the catalyst were analysed by EDX and the average of these are reported herein. A Leybold Max200 X-ray photoelectron spectrometer with an Al Ka photon source was used for the XPS analysis of the reduced catalysts after passivation. Exposure of the samples to ambient atmosphere was minimized by transferring the samples from the reactor to the spectrometer either in vacuum or under N2. No further treatment of the catalysts was done in the XPS chamber. All XPS spectra were corrected to the C 1s peak with a binding energy (BE) of 284.6 eV. Catalyst activities were measured in a laboratory fixed-bed microreactor (o.d. = 9.53 mm and i.d. = 6.35 mm, copper lined stainless steel tube). The catalyst particles (average diameter = 150 mm) were placed on a packed bed of quartz wool inside the reactor and held in place by a bed of quartz beads. The catalyst bed depth was 5–7 cm and calculation showed that this configuration ensured plug-flow through the reactor. The experimental setup is shown in Fig. 1. For all experiments, synthesis gas (H2:CO = 1) was reacted at 548 K, a pressure of 8.27 MPa (1200 psi) and a gas-hourly space velocity (GHSV) = 3960 h1. A high temperature back pressure regulator was used to control the reactor pressure. The reactor was operated at a low CO conversion (1–13% CO) to ensure isothermal operation. Calculation of the internal and external heat and mass transfer rates, and application of the Mears and Weisz-Prater criteria, confirmed that at the conditions of the experiments, the reactor operated free of significant heat and mass transfer resistances. The reaction products were analyzed using an in-line gas chromatograph

S.F. Zaman, K.J. Smith / Applied Catalysis A: General 378 (2010) 59–68

Fig. 1. Experimental setup for syngas conversion to oxygenates.

(GC). Light gases (CO, CO2 and C1–C4 hydrocarbons) were separated using a 5 m temperature-programmed Porapak Q 80/100 packed column and quantified with a thermal conductivity detector. The alcohols, aldehydes, ketones, carboxylic acids and C5+ hydrocarbons were separated using a 30 m temperature-programmed ECTM-wax capillary column (i.d. = 0.53 mm and film thickness 1.20 mm) and quantified using a flame ionization detector. GC/MS analysis was also completed periodically on the liquid product collected at the reactor exit to confirm the identity of the reactor products. All the catalysts were evaluated over a period of at least 60 h of continuous reaction. The syngas conversion, product selectivity, activity of the catalysts and space time yield (STY) of the products were calculated according to the following equations: P Fout  ni Ci product %Conversion ¼  100% F in  Cin % Selectivity ¼

Activity ¼

STY ¼

ni Ci product P  100% ni Ci

CO Conversion  Inlet molar flow of CO Wt: of catalyst

Activity  Selectivity of product i  Molecular wt: product i Carbon number product i

where ni is the carbon number of component i and Ci is the mole fraction of component i. 3. Results After calcination of the catalyst precursors, it was assumed that the Mo and P species were completely oxidized and present on the support as Mo6+ and P5+, respectively. The TPR profiles for the 10 wt% MoP–SiO2 and the 1 and 5 wt% K–10 wt% MoP–SiO2 catalyst precursors are shown in Fig. 2. The TPR profile of the 10 wt% MoP–SiO2 catalyst precursor was very similar to that reported by Zuzaniuk et al. [25] with two peak maxima at 727 and 938 K and a characteristic low temperature shoulder at about 700 K. The low temperature peaks were assigned to the reduction of Mo6+ species to Mo4+ [31], while the large peak at 938 K was assumed to result from the overlapping of several peaks, corresponding to the reduction of Mo4+ to Mo0 and of P5+ to P0. For the 1 wt% K–10 wt% MoP catalyst, the lower peak shifts to a

61

Fig. 2. TPR profiles of 10% MoP–SiO2, 1% K–10% MoP–SiO2 and 5% K–10% MoP–SiO2 catalysts.

higher value (797 K), whereas the higher temperature peak remains unchanged. For the 5 wt% K–10 wt% MoP–SiO2 catalyst, the peaks also shifted to higher temperature compared to the 10 wt% MoP, with peak maxima at 773 and 976 K. The increase in the reduction temperature peak maxima suggests some interaction between the K and the MoP, that is clearly more pronounced at the higher K loading. The reduction stoichiometry of the calcined precursors may be written as: 2MoO3 P2 O5 þ 11H2 ! 2MoP þ 11H2 O Accordingly, the degree of reduction calculated from Fig. 2 for each of the catalyst precursors was 78, 72 and 73% for the 10 wt% MoP–SiO2 and the 1 wt% and 5 wt% K–10 wt% MoP–SiO2 precursors, respectively. However, as noted in several previous studies of supported metal phosphide catalysts [32–34], the stoichiometry of the reduction reaction is somewhat uncertain. The catalyst precursors may include polyphosphate chains [32,34] and species such as HxPO4(x3) and P3+. In addition, during calcination and reduction, some loss of P may occur through volatile phosphorous species that leave the catalyst surface during the reduction or calcination process, all of which may contribute to a higher actual degree of reduction than that calculated from the data of Fig. 2 [34]. Table 1 summarizes the catalyst characterization data for the MoP–SiO2 and the K-promoted MoP–SiO2 catalysts as a function of the mass loading of MoP and K. The BET areas of the reduced catalysts were significantly below that of the corresponding SiO2 (330 m2/g), 1 wt% K–SiO2 (310 m2/g) and the 5 wt% K–SiO2 (264 m2/g) supports. In nearly all cases, the BET area decreased with increased loading of MoP and K, and the same trends were observed for the used catalysts. The EDX data are reported in Table 1 as the Mo:Si atomic ratio for both the fresh and used catalysts. Because of interactions with the Si peak, the P content of the catalysts was over estimated by EDX analysis and is not reported. However, chemical analysis showed excellent agreement between the nominal and actual compositions of the used catalysts, with an average Mo:P atom ratio of 1.05  0.04 for the catalysts of Table 1. The Mo:Si ratio increased with an increase in MoP loading and the K loading did not change this ratio significantly.

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Table 1 Properties of the reduced and passivated K–MoP–SiO2 catalysts as a function of MoP and K loading. Catalyst

5% MoP 10% MoP 15% MoP 1% K–5% MoP 1% K–10% MoP 1% K–15% MoP 5% K–5% MoP 5% K–10% MoP 5% K–15% MoP

BET surface area (m2 g1)

EDX analysis

XPS analysis

Mo:Si atom ratio

C:Si atom ratio

Mo:P atom ratio

Mo:Si atom ratio

P:Si atom ratio

K:Mo atom ratio

(i)

(ii)

(i)

(ii)

(ii)

(i)

(i)

(i)

(i)

259 263 184 255 235 171 75 64 48

234 181 123 110 60 51 21 6 11

0.018 0.040 0.058 0.018 0.034 0.057 0.021 0.047 0.078

0.013 0.029 0.047 0.019 0.044 0.052 0.020 0.047 0.046

1.2 1.4 2.0 1.8 2.5 1.9 1.1 5.6 10.3

1.09 1.11 1.24 1.31 1.37 1.27 1.64 1.29 1.26

0.013 0.027 0.029 0.015 0.025 0.031 0.017 0.028 0.029

0.012 0.024 0.024 0.012 0.018 0.024 0.011 0.022 0.023

– – – 1.17 0.42 0.31 5.83 3.27 1.95

(i) reduced catalyst before reaction. (ii) after 60 h reaction at 548 K, 8.27 MPa, CO:H2 = 1 and GHSV = 3960 h1.

The XPS data of Table 1 suggest a Mo enriched surface for the reduced catalysts with the Mo:P ratio > 1 in all cases. The K:Mo ratio as measured by XPS was also greater than the nominal K:Mo ratio, indicating that the K was preferentially located near the surface of the catalyst. However, because the K was added to the SiO2 before the MoP, the K:Mo ratio decreased as the MoP loading increased. The XPS narrow scan analysis of the 15 wt% MoP–SiO2 reduced catalysts is compared to that of the 5 wt% K–15 wt% MoP– SiO2 in Fig. 3, and a summary of the analysis of all the reduced catalysts is provided in Table 2. The Mo 3d spectra were deconvoluted using three distinct Mo 3d BEs at 228, 230 and 233 eV (0.5 eV) and a spin orbit splitting of 3.2 eV. The low BE peak (227.3– 228.0 eV) was assigned to MoP [35] whereas the higher BE peaks corresponded to Mo4+ and Mo6+ species. The P 2p spectra of Fig. 3(C and D) were de-convoluted into two peaks with BE 132.8–133.9 eV assigned to phosphate species, and BE 127.2–128.5 eV assigned to phosphide species [30]. The curve fit showed that for the catalysts without K, the Mo 3d BE assigned to MoP was 227.9–228.2 eV, whereas for the catalysts with K the BE was 227.3–227.4 eV, a decrease of more than 0.5 eV (see Table 2). These BE shifts are likely a consequence of an interaction between the Mo and K, with an expected electron transfer from the K to the Mo. The K 2p peak was de-convoluted with a 2p BE of 292.8–293.1 eV and a spin orbit splitting of 2.6 eV [35,36]. On Mo2C, the K 2p BE was reported to be 292.5 eV [35] and for K2CO3 added to a Co–MoS2 catalyst a BE of 293.3 eV was reported [37], whereas for K2CO3, Mills [36] reported the K 2p BE as 292.4 eV with a spin orbit split of 2.8 eV. The CO conversion with time-on-stream for each of the catalysts, measured at 548 K and 8.27 MPa with a H2:CO = 1:1 feed gas at a GHSV = 3960 h1, is plotted in Fig. 4(a–c). Initially, relatively high CO conversions were observed but the conversion declined significantly within the first 20 h and then stabilized. Among the catalysts examined, the 5 wt% K–15 wt% MoP–SiO2 had the highest initial activity. The catalyst activities and selectivities were averaged over a period of up to 60 h after the initial 20 h stabilization period. The average CO conversions over the MoP catalysts are presented in Fig. 5 and show a general trend of increased conversion with increased MoP and K loading, with the 5% K–15% MoP–SiO2 catalyst having the highest average conversion. The average catalyst selectivities (C atom%), reported on a CO2free basis, are summarized in Table 3 and Fig. 6. The data of Table 3 show that the MoP–SiO2 catalysts produced mostly CH4 and other hydrocarbons, and as the MoP loading increased the CH4 selectivity decreased, whereas the higher hydrocarbon selectivity increased. DFT studies of syngas conversion over an Mo6P3 cluster model of the MoP catalyst predicted the preferential formation of CH4 versus

methanol from syngas [28,29], as observed for the un-promoted MoP–SiO2 catalysts reported in Table 3. The selectivity to oxygenated products, especially acetaldehyde and ethanol, decreased as the MoP loading increased. With the addition of K to the MoP–SiO2 catalysts, the selectivity to CO2 increased, whereas the selectivity to CH4 (Fig. 6(a)) and other hydrocarbons was suppressed. The selectivity to C2 oxygenated products (Table 3) also increased as K was added to the MoP–SiO2, except for the 5 wt% MoP–SiO2 catalyst. In this case, however, the CO conversion was low (<1%) compared to that obtained with the K-promoted 5 wt% MoP–SiO2 catalysts and selectivity to oxygenated products is expected to increase at lower conversions. The 5 wt% K–10 wt% MoP–SiO2 catalyst showed the highest selectivity towards C2 oxygenates (39.2 C atom%) and the lowest selectivity to C1–C4 hydrocarbons (23.4 C atom%) with a CH4 selectivity of only 9.7 C atom%. The major oxygenated products obtained over all the catalysts were ethanol, acetaldehyde and acetone. For each level of MoP loading, Fig. 6(c) shows that addition of K generally increased the ethanol selectivity. In the case of acetaldehyde (Fig. 6(b)) and acetone (Fig. 6(d)), however, the selectivity trend was more complex such that for the 10 and 15 wt% MoP–SiO2 catalysts, the addition of K only had a small impact on selectivity to these compounds. The average space time yield (STY) of the oxygenated products is plotted in Fig. 7. These data also show a general trend of increased STY with increased MoP loading, as well as with increased K loading at each level of MoP. The highest total oxygenate STY (147.2 mg g cat1 h1) was obtained for the 5 wt% K–15 wt% MoP–SiO2 catalyst. The C2 oxygenate STY (sum of acetaldehyde, ethanol and acetic acid) for the 5, 10 and 15 wt% MoP–SiO2 catalysts promoted with 5 wt% K, was 14.2, 60.7 and 81.3 mg g cat1 h1, respectively at 548 K. 4. Discussion The catalyst characterization data showed that the catalyst BET area decreased with increased MoP and K loading on the SiO2 support, presumably due to pore blockage by the MoP and the K. The XPS data confirmed the presence of an MoP phase on the K– SiO2 support that was also confirmed by XRD data reported previously [29]. The TPR data provide some indication of an interaction between the MoP and the K, reflected in the TPR peak temperature shifts to higher temperature when K was present. The nature of the K–Mo interaction is shown by the XPS data, with a shift in the Mo 3d BE associated with MoP to lower values by at least 0.5 eV upon addition of K to the MoP–SiO2 catalyst. The shift

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Fig. 3. XPS analysis of reduced MoP catalysts after passivation: (A) Mo 3d of 5% K–15% MoP–SiO2; (B) Mo 3d of 15% MoP–SiO2; (C) P 2p of 5% K–15% MoP–SiO2; (D) P 2p of 15% MoP–SiO2; (E) K 2p and C 1s of 5% K–15% MoP–SiO2 catalyst.

in BE is a reflection of an interaction between the MoP and the K, and the decrease in BE is a consequence of a transfer of electron density to the Mo from the basic K promoter. Previous studies have shown that K addition to a Mo2C catalyst decreased the MoII 3d5/2 BE by 0.4 eV [35], whereas a decrease of 0.6 eV was observed with K addition to a Co–MoS2 catalyst [37]. In the latter case, the reduction in BE was observed for Mo 3d5/2, S 2p3/2 and Co 2p3/2. Contrary to these results, Martin-Aranda et al. [38] reported no change in the Mo 3d BE of a NiMoO4 catalyst upon addition of KNO3 and after calcination at 873 K. The K 2p BE of 297.3 eV was consistent with the presence of K2O that would be obtained after calcination at this temperature [38]. In the present work, the Mo 3d BE associated with Mo4+ and Mo6+ species were not shifted by the addition of K, consistent with the results of Martin-Aranda et al. [38], although the K 2p BE (292.8–293.1 eV) was consistent with

the lower calcination temperature of the present study (773 K) that would likely yield KOH rather than K2O [39]. The Mo:P atom ratio as determined by XPS was >1 for all catalysts of Table 1, suggesting some Mo enrichment on the surface of the catalyst compared to the bulk composition. The Mo enrichment is likely because of some P loss to the support during calcination and reduction [26], since the bulk chemical analysis showed that the amount of P was within 5% of the nominal catalyst composition. Nonetheless, the XPS data showed that for the un-promoted MoP–SiO2 catalysts, increased MoP loading increased the surface Mo:Si and P:Si ratio. Furthermore, because the K was added to the SiO2 before the addition of the MoP, the K loading did not have a significant impact on the surface Mo:Si and P:Si ratios measured on the K-promoted MoP–SiO2 catalysts. The Mo:Si and P:Si ratio as determined by XPS, are a measure of the MoP dispersion on the

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Table 2 Summary of XPS peak analysis of reduced and passivated K–MoP–SiO2 catalysts. Catalyst

Mo 3d BE

P 2p BE

K 2p BE eV

eV

eV

5% MoP

228.0 229.4 232.0

– 133.9

5% K–5% MoP

227.4 229.9 233.0

127.5 132.9

10% MoP

227.9 229.9 233.0

127.2 133.4

5% K–10% MoP

227.3 229.8 233.0

127.6 132.8

15% MoP

227.9 230.0 233.2

127.4 133.5

5% K–15% MoP

227.4 230.1 233.0

128.5 133.0

293.1

292.8

292.8

Fig. 4. Syngas conversion with time-on-stream over MoP–SiO2 catalysts: (a) 5 wt% MoP, (b) 10 wt% MoP, and (c) 15 wt% MoP. Reaction conditions: temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

Fig. 5. Average CO conversion over K–MoP–SiO2 catalysts as a function of MoP and K content (wt%). Reaction conditions: temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

catalyst. CO chemisorption performed on the 15 wt% MoP–SiO2 catalyst, showed the MoP dispersion to be 3.4%, close to the XPS Mo:Si ratio of 2.9% (Table 1). The XPS data therefore suggest that increased MoP loading increased MoP dispersion whereas the K did not significantly affect the dispersion of the MoP on the SiO2. Consequently, as the MoP loading on the un-promoted MoP–SiO2 catalysts increased, an increase in CO conversion was observed (Table 3), despite a decrease in total surface area. For the Kpromoted MoP–SiO2 catalysts, however, the CO conversion was influenced by the MoP dispersion and the surface K/Mo ratio. Fig. 8 shows the CO conversion as a function of both the K/Mo and Mo/Si ratio as determined by XPS. The response surface shown was obtained using a Kriging correlation to interpolate between the measured data points. The surface plot shows an increase in CO conversion with increased MoP dispersion (Mo:Si ratio) and a maximum in CO conversion at a K/Mo ratio of approximately 2, for all levels of Mo/Si investigated. The maximum in CO conversion as a function of surface K/Mo ratio is a consequence of increased C2+ oxygenate selectivity (see Fig. 9 discussed below) and the corresponding increase in produced water that results in an increase in CO conversion through the water-gas-shift reaction. The CO consumption turn over frequency (TOF) of the 15 wt% MoP–SiO2 catalyst was estimated based on the CO chemisorption measurement made for this catalyst. At 548 K and 8.27 MPa a TOF of 0.024 s1 was calculated. Using XPS data to estimate the MoP dispersion of the most active 1 wt% K–5 wt% MoP–SiO2 catalyst, a value of 0.120 s1 was obtained. Although CO consumption TOF data are not to our knowledge available for MoS2 or Mo2C catalysts, Gao et al. have reported CO consumption and CO chemisorption data for a series of La–V–Rh catalysts [51] and from these data an estimated TOF of 0.019 s1 at 503 K and 0.18 MPa was obtained for the Rh(1.5)–La(2.6)–V(1.5)–SiO2 catalyst. Although the CO conversion was mainly dependant on the MoP dispersion of the catalysts, the C2+ oxygenate product selectivity was dependant on the K/Mo ratio as determined by XPS, and shown in Fig. 9. The decreased CH4 selectivity with increased K/Mo ratio, observed at each level of MoP loading (Fig. 9), is well known in syngas conversion, and is due to the introduction of sites that favor non-dissociative adsorption of CO and C-C chain growth, which enhance the production of higher hydrocarbons and liquid oxygenates [40,41]. Fig. 9 suggests an optimum K/Mo ratio for maximum C2+ oxygenate selectivity. Above a K/Mo ratio of about 3, the C2+ oxygenate selectivity decreased, most likely a consequence

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Table 3 Results of synthesis gas conversion to oxygenates on K–MoP–SiO2 catalysts. Reaction conditions: temperature 548 K, pressure 8.27 MPa, H2:CO 1:1 and GHSV 3960 h1. Catalyst

XCO (C atom%)

SCO2 (C atom%)

5% MoP 10%MoP 15% MoP 1% K–5% MoP 1% K–10% MoP 1% K–15% MoP 5% K–5% MoP 5% K–10% MoP 5% K–15% MoP

0.85 2.8 4.4 3.2 6.3 5.7 2.8 8.4 12.4

24.4 24.1 20.3 42.0 36.0 40.0 45.4 44.9 46.4

C atom selectivity (%) HC

CH4

AcH

Acetone

Methanol

Ethanol

C2 oxy

C3+oxy

54.9 57.3 59.7 45.2 48.9 45.8 45.3 23.4 31.1

40.0 32.5 28.9 21.1 21.7 23.3 14.6 9.7 13.0

24.7 19.1 17.3 17.7 21.2 21.5 14.1 20.5 19.8

3.3 13.8 11.5 17.7 12.6 13.8 14.3 14.1 15.7

1.7 0.5 4.9 1.0 1.9 3.3 0.0 4.1 1.1

7.8 4.4 3.3 7.6 9.8 8.6 11.6 15.3 14.8

33.5 24.0 20.9 26.3 28.6 30.7 27.3 39.2 36.1

9.9 18.3 14.5 27.4 20.7 20.2 27.4 33.3 31.6

Total oxygenate STY (g kg cat1 h1) 6.8 20.4 31.5 29.3 55.9 53.9 26.5 112.9 147.2

SCO2 = Selectivity of CO2; AcH = Acetaldehyde; C2 oxy is the sum of acetaldehyde, ethanol and acetic acid; C3+oxy is the sum of propanol (<8 C atom%), butanol (<3 C atom%), acetone, acetic acid, propionic acid (<2 C atom%), rest (<8 C atom %, lump of higher carboxylic acids, aldehydes, and esters); NA = data not available; STY = space time yield.

of the fact that both basic sites (K) and metal sites (MoP) are required for C2+ oxygenate synthesis and an optimum number of each is required at the surface for maximum selectivity to C2+ oxygenates. The MoP catalysts showed several distinct characteristics in product selectivity compared to MoS2 [12,14,42], b-Mo2C [18,35,47], Mo2O3 [43] and Mo [44] alcohol synthesis catalysts. Table 4 compares the results of synthesis gas conversion over the 5 wt% K–10 wt% MoP–SiO2 catalyst of the present study, and Mo catalysts reported in the literature. The major liquid oxygenated products obtained on the catalysts of Table 4 were linear alcohols, although Muramatsu et al. [45] reported small amounts of

acetaldehyde and acetone over the K–Mo/SiO2 catalysts. Furthermore, in most cases, methanol was produced with selectivity >10 C atom% and ethanol had the highest or second highest selectivity among the higher alcohols. The alcohol distribution followed the Anderson–Schulz–Flory chain growth distribution. For MoP, however, a very low selectivity (0.5–4.9 C atom%) to methanol was observed for all the catalysts studied. DFT studies have shown that CH3OH adsorbs more strongly on MoP than MoS2, Mo2C or Cu [28,46] and consequently, on MoP the CH3OH is likely available for further reaction and C-addition to yield C2+ products. The major oxygenated products of the reaction were acetaldehyde, ethanol and acetone, suggesting that the hydrogenation kinetics of

Fig. 6. Selectivity (C atom%) of major products of syngas conversion over K–MoP–SiO2 catalysts; (a) methane, (b) acetaldehyde, (c) ethanol, and (d) acetone. Reaction conditions: temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

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Fig. 7. Space time yield (STY) of major products of syngas conversion over K–MoP–SiO2 catalysts; (a) total oxygenates, (b) acetaldehyde, (c) ethanol, and (d) acetone. Reaction conditions: temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

Fig. 8. Averaged CO conversion as a function Mo/Si and K/Mo ratio as measured by XPS. (^) 0 wt% K; (*) 1 wt% K; (~) 5 wt% K. Reaction conditions: temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

Fig. 9. Methane and C2+ oxygenate selectivity over MoP catalyst as a function of K:Mo surface atom ratio. (&, &) 5 wt% MoP; (*, *) 10 wt% MoP; (~, ~) 15 wt% MoP. Reaction conditions: temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

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Table 4 Comparison of product selectivities from synthesis gas conversion over Mo-based catalysts. Catalyst

Temperature (K)

Pressure (MPa)

H2/CO ratio

XCO (C atom%)

SCO2 (C atom%)

C atom selectivity (%)

Refs.

HC

CH4

Methanol

Ethanol

C2+oxy

5% K–10% MoP–SiO2 K–10% Mo–SiO2 K–b-Mo2C K–a-Mo2C K2CO3–MoS2 K–Co–MoS2/C

548 573 573 573 573 603

8.3 1.6 8.0 7.9 8.2 5.0

1 1 1 1 1 2

8.4 12.9 23.4 14.4 29.4 11.7

44.9 4.9 NA NA 27.4 NA

23.4 49.5 47.4 53.4 32.8 58.1

9.7 22.0 NA NA 31.2 NA

4.1 17.0 18.8 21.4 47.1 18.7

15.3 21.0 22.0 15.0 16.5 13.2

63 30 34 25 20 23

This work [44] [48] [47] [49] [50]

SCO2 = selectivity of CO2; C2+oxy is the sum of all oxygenates with carbon number 2 or greater; NA = data not available.

oxygenated intermediates on the MoP surface was slower than that which occurs on MoS2 and Mo2C. The product distribution over the MoP catalysts is similar to that reported on Rh catalysts, where high selectivities to ethanol, acetaldehyde and acetic acid have also been observed [4,7]. Although the reaction conditions, in particular the H2/CO ratio and temperature, vary among the different studies, the data of Table 4 are reported at similar levels of CO conversion at the optimized conditions of each study. Under these conditions, the K– MoP catalysts had the lowest selectivity to hydrocarbons among K–Mo [44], K–a-Mo2C [47], K–b-Mo2C [48], K–MoS2 [49] and K– Co–MoS2 [14], although the former catalyst was examined with a H2:CO ratio of 2:1. Importantly, the K–MoP catalyst also had the highest C2 oxygenate selectivity among all of the Mo-based catalysts. Comparing the nine catalysts of the present study, the 5 wt% K– 10 wt% MoP showed the highest selectivity for liquid oxygenates (76.5 C atom%), especially towards C2 oxygenates (39.2 C atom% for the sum of acetaldehyde, ethanol and acetic acid). Although this catalyst had moderate selectivity towards ethanol (15.3 C atom%), it also showed very low selectivity towards CH4 (<10 C atom%), which is typically not the case for other Mo-based catalysts such as MoS2 and Mo2C [14,15,18,35,42]. 4.1. CO2 formation via the water–gas-shift reaction CO2 was formed with high selectivity in all cases over the MoP catalysts due to the fact that Mo is an active catalyst for the water– gas-shift (WGS) reaction. CO2 selectivity was lower on the un-

promoted catalysts (Table 3) compared to those doped with K. The increased CO2 selectivity was most likely a consequence of the increase in liquid oxygenated products and the corresponding increase in produced water that was rapidly converted to CO2 over the K-doped MoP catalysts. Although CO2 was produced at the expense of CO, this would reduce the cost of water removal from the oxygenated products. 4.2. Characterization of used catalysts The properties of the catalysts measured after 60 h reaction at 548 K, 8.27 MPa with a H2:CO = 1:1 synthesis gas feed and a GHSV of 3960 h1 are presented in Tables 1 and 2. The data of Table 1 show that the surface area of the used catalysts decreased significantly compared to the reduced catalysts. The change in BET area as a function of MoP and K loading followed the same trends noted for the reduced catalysts. The C:Si ratio of the used catalysts, determined by EDX analysis, showed evidence of significant carbon deposition as the likely cause of the decreased catalyst surface areas and reduced CO conversion observed in the initial 20 h of operation shown in Fig. 4. The EDX data also show that there was an increase in carbon deposition with increased K content of the catalysts. Analysis of the C 1s peak of the used catalysts did not show any evidence of K2CO3 (C 1s BE 288.4 eV [36]) present on the catalyst (Fig. 10), suggesting that the carbon deposit arose from the heavier products or carbon generated by the Boudouard reaction. 5. Conclusions Results presented herein show that MoP catalysts have a distinct product distribution compared to other Mo-based catalysts for syngas conversion. Methane dominated the products from synthesis gas conversion over MoP–SiO2 catalysts. Addition of K increased selectivity towards liquid oxygenates and decreased methane selectivity. The maximum liquid oxygenate selectivity (76.5 C atom%) was obtained over the 5 wt% K–10 wt% MoP–SiO2 catalyst. The major liquid oxygenated products were acetaldehyde, acetone and ethanol with a low selectivity to methanol. Acknowledgement Financial support from the Natural Science and Engineering Research Council (NSERC) of Canada is gratefully acknowledged. References [1] [2] [3] [4] [5]

Fig. 10. XPS analysis of carbon C 1s over different loadings of MoP catalyst after 60 h reaction at temperature, 548 K; pressure, 8.27 MPa; H2:CO, 1:1; GHSV, 3960 h1.

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