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Alkaline-Etched NiMgAl Trimetallic Oxides Supported KMoS-Based Catalysts for Boosting Higher Alcohols Selectivity in CO Hydrogenation Jiaxi Yong, Xuebin Luan, Xiaoping Dai, Xin Zhang, Yang Yang, Huihui Zhao, Meilin Cui, Ziteng Ren, Fei Nie, and Xingliang Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01267 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Materials & Interfaces

Alkaline-Etched NiMgAl Trimetallic Oxides Supported KMoS-Based Catalysts for Boosting Higher Alcohols Selectivity in CO Hydrogenation

Jiaxi Yong, Xuebin Luan, Xiaoping Dai,* Xin Zhang, Yang Yang, Huihui Zhao, Meilin Cui, Ziteng Ren, Fei Nie, Xingliang Huang

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China

* Corresponding author. Prof X. P. Dai: State Key Laboratory of Heavy Oil Processing China University of Petroleum, Beijing 102249, PR China Tel.: +86 10 89734979; Fax: +86 10 89734979. E–mail address: [email protected]

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ABSTRACT: The acidity/alkalinity and structural properties of NiMgAl trimetallic oxides (MMO) can be effectively modulated by the alkaline-etching process with various etching time, which

are

further

used

as

support

to

prepare

KMoS-based

catalysts

through

cetyltrimethylammonium bromide (CTAB) encapsulated Mo-precursor strategy. The enriched surface anion groups in alkaline-etched MMO affect the textural properties, metal-support interaction and sulfidation degree of the as-synthesized KMoS-based catalysts. As a result, KMoS-based catalysts using alkaline-etched MMO as supports effectively enhance the reducibility and dispersion of Mo species, which exert a positive influence on higher alcohols synthesis (HAS) performance in CO hydrogenation. A proper balance between acidity/ alkalinity and structural properties in K, Mo/MMO-x catalysts can significantly enhance the alcohol selectivity in HAS from 55 to 65% (carbon selectivity). The formation of C2+ alcohols can be boosted by adol condensation with optimal acidic-basic properties via suppressing the acidity and increasing amount of basic sites. The alkaline-etching process also significantly improves the space time yield of C2+ alcohols over unit mass of molybdenum. KEYWORDS: Higher alcohols; synthesis gas; mixed trimetallic oxides (MMO); alkaline-etching process; alkalinity/acidity; KMoS/MMO catalyst.

2


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1. Introduction Higher alcohols, especially ethanol, propanol and butanol, can be applied in many fields from chemistry to energy, such as solvent, chemical intermediate, fuel additives and alternate fuels. It has drawn much attention due to their environmental friendliness and the fluctuation of crude oil price. Higher alcohols are traditionally obtained through fermentation of biomass, sugars, and petroleum-based route of hydration of alkenes. Nowadays, synthesis gas (syngas, H2 and CO) can be obtained from coal, shell gas, natural gas and biomass. Direct conversion of syngas to higher alcohols is a promising way for low cost and high efficiency in higher alcohols synthesis (HAS).1-4 There are mainly four kinds of heterogeneous HAS catalysts, namely, modified Cu-based methanol synthesis catalysts, modified Fe/Co-based Fischer-Tropsch catalysts, Rh-based catalysts and Mo-based catalysts.4-8 Among them, Mo-based catalysts are promising catalysts for its low cost, coking resistance and unique sulfur resistant ability.3,4 Researches have revealed that Co, Ni and K additives in Mo-based catalysts can tune the kinetics and reaction pathway in HAS to promote the alcohol selectivity by enhancing the C-C formation and chain growth ability.9-11 Recent studies mainly focus on the impacts of preparation methods or supports on the size and structure of active sites (MoS2). The microemulsion method and hydrothermal method were testified to be able to modulate the dispersion and morphology parameters of Mo active species, and thus were helpful for promoting HAS performance.12-14 Supported molybdenum-based catalysts are benefited from the high surface area, hydrogen active properties, and metal-support interactions of the support materials.15-23 SiO2, Al2O3, MgO, mixed metal oxide, activate carbon and carbon nanotube are the most extensively studied supports for Mo-based catalysts. Among them, mixed metal oxides supported catalysts exhibit superior selectivity towards alcohols and higher alcohols.21,22,24-26 Our group combined CTAB-encapsulated Mo precursors and hydrothermal method 3



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to tune the interaction between Mo species and mixed metal oxide support to optimize the size and stacking degree of MoS2, and successfully promoted the CO conversion and higher alcohols yield.27 Morrill et al.

21,22,24-26

systematically investigated the roles of mixed metal oxides in

molybdenum-based catalysts, and suggested that the aldol condensation and dehydration of alcohols along with CO insertion are responsible for the high selectivity of higher alcohols, and the intrinsically basic properties of mixed metal oxides may boost alcohol-forming reactions and lead to higher alcohol. Thus, alkalinity is crucial for C-O and C-C bond forming reactions in HAS.10 Hiroko et al.

21

compared the effects of MgO, Al2O3 and mixed metal oxides as supports in Mo-based

catalysts, and found that their basicity may has connection with the alcohol distribution and alcohol coupling reaction. The effects of alkali metal promoters were extensively studied in HAS, where K2CO3 loaded with physical-mixing method was found with the best HAS performance for supported Mo-based catalyst.1,3,10,16,28,29 It was also proposed that the excess acidity can lead to dehydration of alcohols, and suppress alcohols selectivity.18,22,25,30 Toyoda et al.

31

reported that

Al2O3 supported Mo catalyst calcined at high temperature possesses low acidity, and enhanced higher alcohols selectivity. The strength and amount of basic/acidic sites were decisive factors in several condensation reactions involving C-C bond formation,32,33 but few reference reported the modulation of acidity/alkalinity together and their combined effect on HAS performance in CO hydrogenation.

Hence,

the

further

promotion

of

HAS

performance

over

supported

molybdenum-based catalyst should be highly desirable by tailoring the surface acidity and alkalinity of supports with a facile method. Notably, dealumination of zeolites through acid/base treatment has generally become an effective strategy to modulate structural and acid-base properties,34,35 which demonstrated that the advantages of dealuminated zeolites as supports for nickel- and cobalt-based catalysts, may not only drive to the tailoring of acidity/alkalinity, but also facilitate formation of 4


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peculiar adsorptive sites for anchoring metal precursors and stabilizing active metal sites.36 Motivated by the above discussion, we herein propose a facile method to modulate acidity/alkalinity and structural properties of NiMgAl trimetallic oxides (MMO), which is further employed as the support to fabricate KMoS-based catalysts through cetyltrimethylammonium bromide (CTAB) encapsulated Mo-precursor strategy. The alkali-etched MMO change the surface acidity/alkalinity of support and the as-prepared catalysts, which also affects the textural properties, improves the sulfidation degree and dispersion of active species on the catalyst, and thus results in high HAS activity and alcohol selectivity. 2. Materials and Methods 2.1 Catalysts preparation Ni, Mg, Al mixed trimetallic oxides (MMO) derived from layered double hydroxides was prepared via co-precipitation method. Specifically, an aqueous solution of 1.20 M NaOH and 0.15 M Na2CO3 was dropwise added into a solution containing 0.39 M Mg(NO3)2, 0.18 M Al(NO3)3 and 0.03 M Ni(NO3)2 under vigorous stirring at 65 oC. The resulting gel stirred at 65 oC for 4 h was collected with filtration, washed with deionized water, dried at 120 oC for 12 h to obtain layered double hydroxides (LDH), and then calcined at 500 oC for 2 h to form mixed trimetallic oxides (MMO). Then, the as-prepared materials were immersed into 0.50 M KOH solution for 0, 2, 10, 30 and 60 min under stirring at room temperature, respectively. The treated materials were immediately collected by filtration, washed with excessive deionized water, dried at 120 oC for 12 h and then calcined again at 500 oC for 2 h, which were denoted as MMO-x (x=0, 2, 10, 30, 60). The loading of K and Mo components were similar with our previous work.28 In detail, CTAB solution was dropwise added into a mixed solution with sodium molybdate and HCl under vigorous stirring, then the pH of the obtained solution was further adjusted to 3.0 with 2.0 M HCl. The 5



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as-above solution was stirred for another 2 h, mixed with 1.0 g as-prepared MMO-x, and transferred into a Teflon-lined stainless steel autoclave to heat at 120 oC for 24 h. The materials were collected with centrifuge, washed, dried and calcined at 500 oC for 4 h. Finally, the solids were mixed with K2CO3 by grinding for 15 minutes, and followed by calcination at 450 oC for 2 h. The obtained K, Mo/MMO were denoted as K, Mo/MMO-x (x=0, 2, 10, 30, and 60), respectively. We also investigated the effects of grinding time (5, 10 and 15 min) with K2CO3 on K, Mo/MMO-0. These two samples were denoted as K, Mo/MMO-0-g5 and K, Mo/MMO-0-g10, respectively. For all samples, K and Mo content were kept constant at around 4.8 wt. % and 8.0 wt. %, respectively. 2.2 Material Characterization X-ray diffraction (XRD) was conducted on a German Bruker D8 Advance diffractometer with Cu Kα radiation at 40 kV and 30 mA at a scan rate of 5 º/min. Solid state

27Al

nuclear magnetic

resonance (NMR) analysis was carried out with a Bruker AVANCE III 600 spectrometer operate at 78.2 MHz with a 4 nm resonance probe and a spin rate at 12 kHz. Surface areas and pore parameters were analyzed with N2 adsorption and desorption on a Micromeritics JW-BK222 through Brunauer-Emmett-Teller (BET) method and Barret-Joyner-Halenda (BJH) method, respectively. Element analysis was carried out through inductively coupled plasma-optical mission spectroscopy (ICP-OES) on a Thermo IRIS Intrepid II XSP Inductively Coupled Plasma Atomic Emission Spectrometer. The X-ray photoelectron spectrum (XPS) analysis was performed on a PHI 5000 VersaProbe system with monochromatic Al Kα radiation at 1486.6 eV. Raman spectra were obtained on a Renishaw Raman microscope light source at λ=532 nm and 1 mV for pre-sulfided catalysts. Transmission electron microscopy (TEM) was performed on a JEM-2100 LaB6 high resolution transmission electron microscope, and more than 200 MoS2 slabs for each sample were used to 6


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obtain statistical data about slab length and stack degree. The average length (Laverage), average stack degree (Naverage) and length variance (σlength) for each sample were calculated from the parameters of MoS2 slabs taken into statistical analysis. The σlength was obtained with equation (1). t

( Li  Laverage ) 2

 length  i 1

(1)

t

In which Li stands for the length of single MoS2 slab and t stands for the total number of slabs taken into measurement for each sample. The fraction of Mo atoms locate on the edge sites (Moedge) among all Mo atoms (Mototal) was estimated with hexagon modal of MoS2 with equation (2).16,37 t

Moedge Mototal



 (6n  6) i 1

i

(2)

t

 (3ni 2  3ni  1) i 1

Moedge and Mototal stand for the numbers of molybdenum atoms located on the edge sites in all, respectively. ni stands for the number of molybdenum atoms on one edge of a slab which is related to the Li according to equation (3). ni (Å )=

Li  3.2 6.4

(3)

H2-temperature programmed reduction (H2-TPR), CO2-temperature programmed desorption (CO2-TPD) and NH3-temperature programmed desorption (NH3-TPD) was conducted with a homemade equipment with TCD probe. For all three analysis, a pretreatment with pure Ar flow at 40 mL/min at 500 oC for 30 min will be conducted. The H2-TPR reduction procedure was performed with 6 vol% H2 in Ar (40 mL/min), and the temperature was ramped from 50 to 800 oC with heating rate of 10 oC/min. The quantitative correlation factor for peak area and H2 consumption was calibrated with the reduction of standard CuO/Al2O3. For CO2-TPD, around 45 mg samples were firstly go through adsorption process at 50 oC, in 40 mL/min 90 vol% CO2/N2 for 1 h. Desorption procedure was carried out from 50 to 700 oC with heating rate of 10 oC/min under 40 mL/min Ar 7



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flow. NH3-TPD was carried out in similar process with CO2-TPD, except for the amount of catalysts were 100 mg, the adsorption gas source was 5 vol% NH3/Ar, and the desorption temperature was ranged from 50 to 600 oC. 2.3 Higher alcohol synthesis (HAS) in CO hydrogenation 0.6 g catalysts and 1.8 g quartz sand (40-70 mesh) was mixed without grind and loaded in an internal diameter 11 mm tubular fixed bed reactor. Presulfurization was carried out at 400 oC for 2 h in 10 vol.% H2S/H2. The HAS test was performed at 350 oC, 5.0 MPa with a GHSV of 3000 mL/g·cat/h. The feed gas is synthesis gas containing 45 vol% CO, 45 vol% H2, and 10 vol% N2 as internal standard. The reactants and gas phase product was determined by an on-line SP-7890 gas chromatograph with flame ionization detector (FID) and thermal conductivity detector (TCD), and with three columns (TDX-1, GDX-509 and Porapak Q). Alcohols and oxygenates were collected with a cold trap filled with isopentyl alcohol as absorbent, and analyzed off-line by a SP-6890 gas chromatograph equipped with a FID detector (DB-WAX). The CO conversion and carbon-based selectivity of hydrocarbons (CHs), alcohols (COHs), CO2 and other oxygenates was obtained after internal normalization. The experiments were repeated at least three times to check reproducibility and confirm that the results obtained were within the experimental error of ±2.5%. CO conversion was calculated as the mole percentage of carbon monoxide converted to products: XCO (%)=

moles of COin  moles of COout 100% moles of COin

(4)

Carbon selectivity was the moles of carbon in a given product to the total moles of converted carbon: Carbon selectivity (%)=

moles of a given product  number of carbons in products 100% moles of COin -moles of COout

(5)

The carbon selectivity (Ni) for i in liquid products is calculated with the following equation. Ni=Vi*ρ*ni/Mi

(6) 8


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Where Vi stands for the volume of product i, ρ stands for the density of i, ni stands for the carbon number of one molecule of i, Mi stands for the molar weight of i. 3. Results and discussion 3.1 XRD, 27Al NMR and N2 adsorption/desorption characterization The XRD patterns of LDH, MMO-x supports and K, Mo/MMO-x were shown in Fig. 1. The diffraction peaks for the LDH precursors fit well with the characteristic peaks of magnesium aluminum hydroxide hydrate (PDF. 35-0964). Meanwhile, nickle may exist as nickle aluminum oxide hydrate (PDF. 22-0452), which possesses very similar structure and XRD diffraction peaks with magnesium aluminum hydroxide. However, only peaks at 2θ=36, 44 and 64 o, assigned to NiO (PDF. 44-1159), MgO (PDF. 45-0946) and Ni-Mg-O (PDF. 24-0712), can be observed for MMO-x supports after calcination at 500 oC.22,25,38 The absence of XRD peaks for Al2O3 may due to the formation of Mg-(Al)-O solid solution where Al was dispersed throughout MgO.39-41 XRD patterns of oxidic catalysts were demonstrated in Fig. 1B, which exhibit very similar peaks to MMO-x supports. No apparent peaks of Mo components can be identified, suggesting that Mo may be existed in very small particles and well dispersed due to the CTAB encapsulated method.27 27Al

NMR has been conducted to characterize the state of aluminum and its interaction with

other contents in catalyst. Fig. 2A illustrates

27Al

NMR spectra of K, Mo/MMO-0 and K,

Mo/MMO-2. Two apparent peaks centered at around 11 ppm and 77 ppm can be assigned to octahedral coordinated and tetrahedral coordinated Al species,42 respectively, which is in accordance with the reported mixed magnesium aluminum oxides.42-46 The fraction of different kind of coordinated Al species can be measured with the intensity of corresponding peaks.44,45 The ratios of tetrahedral and octahedral coordinated Al species in KMo/MMO-0 and KMo/MMO-2 are 13:25 (0.52) and 11:20 (0.55), respectively. The slightly increase of tetrahedral coordinated Al atom may 9



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be due to the partial removal of Mg2+ and Al3+,46 which are further verified by slightly decreasing Mg/Ni and Al/Ni ratios in MMO-x as alkali-processing time rises from element analysis (Table S1). The Ni(II) can be chosen as reference due to its low etching amount at 0.5 M KOH solution.46 It is clear that the amount of Mg dropped around 30 % within 2 min of alkali-etching. When the alkali-processing time is prolonged to 1 h, only 8 % more Mg lost. The Al content tends to decrease steadily with the alkali-processing time. The results suggest that the loss of Mg2+ in MMO-x and K,Ni/MMO-x should be mainly attributed to the rehydration of MgO in KOH solutions which happens very quickly after it contacts with the solution, while the loss of Al3+ increases smoothly as increasing alkali-processing time, indicating that the dissolution from alkaline etching was the main reason. N2 adsorption/desorption is applied to investigate the effects of alkaline-etching process on the surface area and textural properties. The isotherms and pore diameter distribution are illustrated in Fig. 2B-C and Table S2. It can be found that the alkaline etching does not change the mesoporous nature of MMO, and oxidic catalysts as type IV isotherms with H1 hysteresis loops are observed for all samples. However, the pore volumes of MMO-x decrease sharply (from 0.875 cm3/g to 0.239 cm3/g) as the alkali-processing time increases from 0 to 60 minutes. The same tendency about pore volume can also be observed on K, Mo/MMO-x (from 0.331 cm3/g to 0.158 cm3/g). The pore distribution in the inset of Fig. 2B-C demonstrates that the pore diameters shift to smaller region compared to MMO-0 and K, Mo/MMO-0. Meanwhile, it can be also observed that the average pore diameters of the materials are gradually lowered, indicating more small pores after alkaline etching. The surface area of MMO-x does not vary significantly from that of MMO-0, while on the contrary, for the K, Mo/MMO-x, all samples after treatment have lower surface area than K, Mo/MMO-0 (134.5 m2/g). The N2 adsorption/desorption suggests that the alkaline-etching process may not 10


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change the mesoporous characteristic of the materials, but it can build the new mesopores by reconstructing some micro-structure pores. Some of the large pores may be collapsed during alkaline etching due to the partial removal of Mg2+ and Al3+. 3.2 Acidic and basic properties by CO2 and NH3-TPD The basic and acidic nature of catalysts play crucial roles in HAS, which were investigated by CO2- and NH3-TPD, respectively. CO2-TPD in Fig. 3A shows two peaks below 700 oC, where the broad peak ranged from 50 to 450 oC is corresponding to the combination of weak and medium basic sites. The symmetric peak centered at around 600 oC should be ascribed to the strong basicity of the MMO-x. The basic sites over K, Mo/MMO-x are mainly attributed to three kinds of active groups, OH- for weak basic sites, M-O pairs (basically Mg2+-O2- pairs) for medium basic sites, and low coordinated oxygen anion O2- for strong basic sites.38,47,48 The strength of basic sites, which is related the peak temperature, shows negligible change, while the amount of basic sites, which is related to the peak area, increases significantly as alkali-processing time rises. A correlation between the peak area below 500 oC and alkali-processing time is established (Fig. 3B). A sharp increase in the peak area within short alkali-processing time (2 min) is observed, and then it rises slowly with the increase of alkali-processing time. Obviously, alkaline etching gradually increases the amount of weak basic sites over the surface with a monotone increasing trend. Thus, the alkalinity of the catalyst can be tunable through controlling the alkali-processing time of the support. CO2 desorption below 200 oC can be attributed to weak basicity caused by OH-.38,47,48 The slight shift towards lower temperature for weak basic sites implies the basic sites may be caused by weaker OH- groups on the surface of K, Mo/MMO-x after alkaline etching. The amount of strong basic sites, caused by low coordinated O2-, demonstrates a volcano shape. It was reported that the substitution of Mg2+ with Al3+ will cause the cation vacancies.48 The partial removal of Mg2+ will lead to Schottky defects, and 11



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make more oxygen anions O2- adjacent to Mg2+, and thus Al3+ cations become coordinatively unsaturated to form more strong basic sites.48 NH3-TPD was performed to investigate the impact of alkaline treatment on the acidic sites. Fig. 3C shows the significant changes caused by alkaline-etching process. The pristine K, Mo/MMO-0 has only one broad peak ranged from 100 to 500 oC, which splits into two small peaks after alkaline etching. The low temperature peak caused by weak acidic sites are induced by H-bond between NH3 and OH- group, while the high temperature desorption peak over 200 oC are caused by medium to strong acidic sites, including Lewis type acid of metal cations.49,50 Obviously, the peak centered at below 200 oC demonstrates a significant decreasing in peak area as increasing alkali-processing time. The peak centered at over 200 oC shows a continuing shift toward higher temperature from 212 oC to 310 oC, indicating the even stronger acidity as alkali etching time increases. The splitting of the broad peak into two peaks may be caused by the sharp reduction of the amount of weak acidic sites and the enhancement of Lewis acidic sites. A correlation between the integrated total peak area and alkali-processing time is also established for NH3-TPD (Fig. 3D), indicating the monotone decrease at alkali-processing time. Though the broad peak splits into two small peaks, and the later one trends to shift to higher temperature, the whole tendency of the amount of acidic sites is decreasing with the increasing alkali-processing time. 3.3 Reducibility and sulfidation degree H2-TPR was conducted to examine the reducibility of K, Mo/MMO-x. Two apparent peaks can be observed for K, Mo/MMO-x in Fig. 4. The peak located in the low temperature can be assigned to the reduction of octahedral molybdenum oxide from Mo6+ to Mo4+, while the peak at high temperature is contributed by the reduction of Mo4+ and tetrahedral MoO3.31,51 The peak temperature is related to the reducibility of catalysts, which is regarded as the indicator of metal-support 12


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interaction.27,52 The hydrogen consumptions of deconvoluted peaks in H2-TPR were given in Table S3. The center temperature of the first peak do not change significantly as increasing alkali-processing time except for K, Mo/MMO-60. However, center temperature of the second peak shows clear change with the alkali-processing time, which slightly shifts toward low temperature in K, Mo/MMO-2 and K, Mo/MMO-10, indicating the weakened interaction between Mo and MMO. Quantitative analysis in Table S3 suggests that K, Mo/MMO-2 and K, Mo/MMO-10 present higher H2 consumption for the reduction of octahedral MoO3 (0.684 and 0.777 mmol/g·cat) and lower consumption for the reduction of tetrahedral MoO3 and MoO2 (1.33 and 1.22 mmol/g·cat), indicating that they possess more amount of reducible octahedral MoO3 structures, and making them easy to be pre-sulfided in catalyst activation process. High-resolution Mo 3d XPS profiles of sulfided catalysts were shown in Fig. 5. The patterns can be deconvoluted into the characteristic peaks of Mo4+, Mo5+, Mo6+ and their conjugated peaks.16,27,53-55 Peaks at around 228.5 eV, 230.2 eV and 232.2 eV are attributed to Mo 3d5/2 of Mo4+, Mo5+ and Mo6+, respectively. The binding energies are shown in Table 1, where the peak at around 225 eV is assigned to the satellite peak of S 2s. The sulfidation degree of molybdenum components, which is defined as the fraction of Mo4+ among all Mo contents, is crucial for Mo-based catalysts in HAS.27,53,55 The sulfidation degrees obtained from XPS peak area for sulfided K, Mo/MMO-x in Table 1 show the highest value of 57.8% for K, Mo/MMO-2. Obviously, the sulfidation degree is relevant with the reducibility of Mo species, implying the weakened metal-support interaction in K, Mo/MMO-2 after alkaline etching,27,56 which also accords with the H2-TPR results (Fig. 4). 3.4 Electronic microscopy characterization Representative TEM images of the sulfided K, Mo/MMO-0 and K, Mo/MMO-2 were illustrated in Fig. 6A-B and Fig. S1, respectively, with their representative high resolution images 13



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showed in Fig. 6C-D. The black thread-like fringes with inter-lamellar distance of around 0.63 nm are ascribed to the MoS2 (002) basal plane, which is usually adopted as the indicator of the stacking degree of MoS2 crystals.22,25,27 The statistic results of stacking degree and slab length distribution of MoS2 crystals calculated from around 20 pictures and more than 200 slabs for each sample are shown in Fig. 6E-F and Table 2. The results unveil that there are smaller MoS2 slab length with dominant 3-5 nm and more uniform distribution with smaller variance for samples after alkaline-etching process. The lower average slab length and narrow distribution may be attributed to a better dispersion of Mo over the supports, which are favorable to expose more active edge Mo atoms to the reactants.16,52,53,57 The MMO surface with higher OH- density after alkaline-etching process should be helpful to enhance the dispersion of Mo by the electrostatic force between the surface anion group OH- and O2- with the CTA+ shell of Mo precursor. No significant difference in stacking degree of MoS2 is observed, which is also testified by Raman spectra (Fig. S2), where shows similar frequency distance between characteristic peaks E12g and A1g vibration mode of MoS2 for all samples.58 TEM images and statistic results of reacted catalysts were shown in Fig. S3 and Table S4. Comparing with the pristine sulfided catalysts, slightly increased slab length and stacking degree of MoS2 can be observed for all reacted catalysts, which also show slight shifts of slab length and stacking degree distributions. Notably, the reacted catalysts show negligible change in the ratio of Mo edge site. The results have demonstrated the stability of Mo active phase. SEM with Energy-dispersive X-ray spectroscopy element mapping on sulfided K, Mo/MMO-10 in Fig. S4 confirms the well dispersion of Mo throughout the catalyst. 3.5 HAS performance in CO hydrogenation Fig. 7A illustrates the CO conversions over K, Mo/MMO-x, which increases from 8.08 % to 8.73 % on K, Mo/MMO-2 with short time of alkaline etching, but further alkaline etching depresses 14


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CO conversion, which goes through a continuous decline to 7.37 % on K, Mo/MMO-60. Alcohols and hydrocarbons selectivity were shown in Fig. 7B. Alcohols are the dominant products for all catalysts, where the selectivities are higher than 55 %. Notably, the alcohols selectivity of K, Mo/MMO-2 and K, Mo/MMO-10 are 64.9 % and 66.1 %, respectively. Then it goes down to around 61 % for K, Mo/MMO-30 and K, Mo/MMO-60. Hydrocarbons selectivity demonstrates an opposite trend, which exhibits the lowest hydrocarbon selectivities of 8.87 % and 9.31 % on K, Mo/MMO-10 and K, Mo/MMO-2, respectively. Intriguingly, K, Mo/MMO-x after alkaline-etching process shows a higher total alcohol selectivity than pristine K, Mo/MMO-0, and as a result, space time yield of higher alcohols significantly increases to 0.71 g/gMo/h (Table S5). However, methanol selectivity in total alcohols is slightly increased through the depression of the formation of C4 alcohols, and the distribution of alcohols shifts toward the ethanol and propanol in Fig. 7C. The increased methane selectivity in hydrocarbons is observed in Fig. 7D, which stays almost stable as increasing alkali-processing time. The impact of grinding time with K2CO3 on HAS performance was also investigated, as shown in Table S5. When grinding time is raised from 5 min to 15 min, slight increase in CO conversion and C2+ alcohols selectivity can be observed. Wang group group

60

59

and Bao

proposed that the mixing state will affect the reaction path route of bifunctional catalyst

where better mixing favor those reactions which require synergy of two active centers. The more intimate contact between K and Mo will be helpful for the formation of K-Mo-S phase, and facilitate the synergy of dual active sites for HAS, and thus provides better performance on K, Mo/MMO-0 with grinding time of 15 min. 3.6 The structure-performance relationship on K, Mo/MMO-x in HAS The schematic structure-performance relationship on K, Mo/MMO-x in HAS was shown in Fig. 8. The acidity/alkalinity of catalysts play an important role in alcohol formation, which can affect the 15



Page 16 of 38

activity and alcohol selectivity. The balance of acidity and alkalinity is vital for HAS. CO hydrogenation over single Mo-based catalysts supported on acidic support like aluminum are most likely to produce hydrocarbons rather than alcohols. As dissociated CO on Mo over acidic sites will directly hydrogenated to hydrocarbons, and go through further chain growth process.16,29 In HAS reaction, the strong acidic sites can also boost alcohol dehydration, and thus decrease the alcohol selectivity.18,49 It was reported that Mo catalyst supported on Al2O3 support with lower acidity shows the decreasing CO conversion, and promoted alcohol selectivity.31 The suppressed acidity of K, Mo/MMO-0 and K, Mo/MMO-10 can promote the oxidative hydrogen addition, and thus leads to the enhancement in alcohol selectivity.16 However, the continuing strengthened Lewis acidity unfortunately promoted the hydrogenation activity and the dehydration of higher alcohols, resulting lower the alcohol selectivity on K, Mo/MMO-30 and K, Mo/MMO-60 with longer etching time.18,22 Basic sites are necessary for C-C and C-O bond forming reactions in HAS, and the amount of basic site in the catalyst is another factor that influences the performance.10,18,49 Mo phases on the alkali sites are responsible for the formation of alcohols.10,16,22,49 The increasing alkalinity can also be observed on catalysts with high K content, which suppresses the hydrogenation activity of Mo-based catalysts, and changes the products from alkanes to alcohols with low CO conversion and high alcohol selectivity.28 Weakly absorbed H* over basic sites is easy to desorb, which can facilitate the oxidative hydrogenation of CO to form alcohols while it may not be good for conversion.16,61 The increasing alkalinity and decreasing acidity of catalysts are able to tune the hydrogenation activity of Mo by depressing the C-O bond cleavage during CO hydrogenation.59 Generally, alkali promoters are needed for Mo-based catalyst in HAS,10 but excessive surface alkalinity may not be positive to the higher alcohol production. Thus, the proper balance of alkalinity and acidity are beneficial to the high alcohol selectivity of K, Mo/MMO-2 and K, Mo/MMO-10. 16


Page 17 of 38 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


The alcohol coupling reaction may take the major responsibility in alcohol distribution and the high selectivity of ethanol and propanol. Some studies about methanol and ethanol co-feed testify that alcohol coupling reaction and aldol condensation will promote higher alcohols selectivity for Mo-based catalyst.22,26,62 High C2+ alcohol selectivity can be observed in this work and other HAS research over MMO supported Mo-based catalysts.22,24-27 As illustrated in Fig. 7C and Fig. S5, chain growth probabilities (α) for hydrocarbon products are kept unchanged, but the alcohols distribution deviate from ASF distribution, and show different chain growth probabilities. This is because of the alcohol coupling reaction, aldol condensation and dehydration of methanol, which are favored over MMO supported molybdenum catalysts.22,24-27 When magnesium is partially removed during alkaline-etching process, the above mentioned reactions are weakened, and thus a higher selectivity towards methanol can be obtained for K, Mo/MMO-2, K, Mo/MMO-10 and K, Mo/MMO-30. Meanwhile, the reduced amount of acidity can facilitate the aldol condensation reactions, which is beneficial to the formation of higher alcohols, leading to the increase of the fraction of C2+ alcohol on K, Mo/MMO-10 to K, Mo/MMO-60.54 The weak metal-support interaction between Mo and MMO, and higher amount of octahedral Mo species make Mo on K, Mo/MMO-2 and K, Mo/MMO-10 easier to be sulfurized and to form more MoS2. The CO conversion shows the same trend with the sulfidation degree. The better dispersion of MoS2 species and shortened slab length after alkaline etching make more molybdenum active sites available, lead to higher conversion on K, Mo/MMO-2 and K, Mo/MMO-10.16,27,52,53 However, the successively weakened acidity and enhanced alkalinity of catalysts can also depress the hydrogenation activity of catalyst, and lead to the continuous decline in CO conversion from K, Mo/MMO-2 to K, Mo/MMO-60.18,28,31,49 It is worth to be noted that the decrease in the surface area, pore volume and average pore diameter after alkaline-etching process can also affect the catalytic 17



Page 18 of 38

performances. The continuous decrease of CO conversion from K, Mo/MMO-2 to K, Mo/MMO-60 and the decrease of alcohol selectivity from K, Mo/MMO-10 to K, Mo/MMO-60 may be partly due to the smaller pore structure and the smaller surface area, along with the change in the alkalinity and acidity. Hence, the proper balance of acidity and alkalinity over K, Mo/MMO-2 and K, Mo/MMO-10 exhibits promotion in both CO conversion and alcohol selectivity, which is also attributed to the proper textural properties, better dispersion, reducibility and high sulfidation degree. 4. Conclusions A facile process by the combination of alkaline-etched process of MMO support and CTAB-encapsulated precursors strategy is proposed to prepare KMoS-based catalysts for HAS in CO hydrogenation. The basicity/acidity and structural properties of MMO and as-prepared KMoS-based catalysts can be tuned by controlling alkali-etching time, which further exert influence on the strengthened weak basicity, modified reducibility and better dispersion of Mo species. The formation of C2+ alcohols is boosted by suppressing acidity and increasing amount of basic sites. The optimal K, Mo/MMO-2 with the highest sulfidation degree, proper balance of acidity and alkalinity, and higher ratio of octahedral Mo species promotes the HAS activity, higher alcohol selectivity, and shows the highest yield for higher alcohols. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. ICP-OES analysis, textural parameters from N2 adsorption/desorption, H2 consumption in TPR, representative TEM images of after sulfurization and CO hydrogenation reaction, Raman spectra, statistical results of morphology parameters and fraction of Mo atoms, SEM-EDS element mapping, , ASF distributions chain growth probability, and HAS performance (PDF). 18


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AUTHOR INFORMATION Corresponding Author E–mail: [email protected] Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgments The authors acknowledge the financial supports from the NSFC (Nos. 21576288 and U1662104). Reference [1] Luk, H. T.; Mondelli, C.; Ferre, D. C.; Stewart, J. A.; Perez-Ramirez, J. Status and Prospects in Higher Alcohols Synthesis from Syngas. Chem. Soc. Rev. 2017, 46, 1358-1426. [2] Yue, H.; Ma, X.; Gong, J. An Alternative Synthetic Approach for Efficient Catalytic Conversion of Syngas to Ethanol. Acc. Chem. Res. 2014, 47, 1483-1492. [3] Zaman, S.; Smith, K. J. A Review of Molybdenum Catalysts for Synthesis Gas Conversion to Alcohols: Catalysts, Mechanisms and Kinetics. Catal. Rev. 2012, 54, 41-132. [4] Xiao, K.; Bao, Z. H.; Qi, X. Z.; Wang, X. X.; Zhong, L. S.; Fang, K. G.; Lin, M. G.; Sun, Y. H. Advances in Bifunctional Catalysis for Higher Alcohol Synthesis from Syngas. Chin. J. Catal. 2013, 34, 116-129. [5] Choi, Y.; Liu, P. Mechanism of Ethanol Synthesis from Syngas on Rh(111). J. Am. Chem. Soc. 2009, 131, 13054-13061. 19



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[62]Claure, M. T.; Lee, L. C.; Goh, J. W.; Gelbaum, L. T.; Agrawal, P. K.; Jones, C. W. Assessing C3-C4 Alcohol Synthesis Pathways over a MgAl Oxide Supported K/MoS2 Catalyst via 13C

2-Ethanol

and 13C2-Ethylene Co-Feeds. J. Mol. Catal. A: Chem. 2016, 423, 224-232.

27



Figures

Fig. 1. XRD patterns of (A) LDH, MMO-x, and (B) K, Mo/MMO-x.

28


Page 28 of 38

A

B

Octahedral Al

Absorbed Quantity (cm3/g)

Tetrahedral Al

Intensity (a. u.)

K, Mo/MMO-2

K, Mo/MMO-0 100

50

0

600 0.07 0.05

400 300

C

MMO-0

MMO-0 MMO-2 MMO-10 MMO-30 MMO-60

0.06

500

0.04 0.02

MMO-2 MMO-10

0.01

MMO-30

0.03

0.00

200

1

4

7

10 13 16 19 22 25

Pore Diameter (nm)

MMO-60

100 0

-50

0.0

0.2

Chemical Shift (ppm)

0.4

0.6

0.8

1.0

1.2

Relative Pressure

250 0.03

200

150

100

K, Mo/MMO-0 K, Mo/MMO-2 K, Mo/MMO-10 K, Mo/MMO-30 K, Mo/MMO-60

0.02

dV/dr

3

Absorbed Quantity (cm /g)

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


dV/dr

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0.01

0.00

1

3

5

7

9 11 13 15 17 19 21

Pore Diameters (nm)

50


0

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure

Fig. 2. (A) 27Al NMR spectra of K, Mo/MMO-0 and K, Mo/MMO-2, the isotherms and pore distribution of (B) MMO-x, (C) K, Mo/MMO-x.

29



CO2 Desorption (a. u.)

K,Mo/MMO-60 K,Mo/MMO-30 K,Mo/MMO-10 K,Mo/MMO-2 K,Mo/MMO-0

B

1100

CO2 Desorption peak area

A

1000 900 800 700 600

50

150

250

350

450

550

0

650

10

Temperature (C)

20

30

40

50

60

50

60

Etching Time (min) K, Mo/MMO-0 K, Mo/MMO-2 K, Mo/MMO-10 K, Mo/MMO-30 K, Mo/MMO-60

D 1800

NH3 Desorption peak area

C NH3 desorption (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

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1600 1400 1200 1000 800

100

200

300

400

500

0



10

20

30

40

Etching time (min)

Temperature ( C)

Fig. 3. (A) CO2-TPD, (B) integrated peak area vs. alkaline-etching time of CO2-TPD, (C) NH3-TPD results and (D) integrated peak area vs. alkaline-etching time of CO2-TPD of K, Mo/MMO-x.

30


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


TCD Signals (a. u.)

Page 31 of 38

K,Mo/MMO-60 K,Mo/MMO-30 K,Mo/MMO-10 K,Mo/MMO-2 K,Mo/MMO-0 100

200

300

400

500

600

700

Temperature (C)

Fig. 4. H2-TPR patterns of K, Mo/MMO-x.

31


800


A

Mo4+

B

Mo6+

Mo4+

Mo5+

Intensity (a. u.)

Intensity (a. u.)

Mo5+ S 2s

224

226

228

230

232

234

236

224

226

S 2s

224

226

228

230

232

232

234

236

234

Mo6+

Mo5+ S 2s

Intensity (a. u.)

Mo

230

Mo4+

D

Mo6+

5+

228

Binding Energy (eV)

Mo4+

C

Mo6+

S 2s

Binding Energy (eV)

Intensity (a. u.)

236

224

226

Binding Energy (eV)

228

230

232

234

236

Binding Energy (eV) 4+

Mo

E

Mo6+ Mo5+

S 2s

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

Page 32 of 38

224

226

228

230

232

234

236

Binding Energy (eV)

Fig. 5. Mo3d XPS spectra of pre-sulfided (A) K, Mo/MMO-0, (B) K, Mo/MMO-2, (C) K, Mo/MMO-10, (D) K, Mo/MMO-30 and (E) K, Mo/MMO-60.

32


Page 33 of 38

A

B Double Layers

Distroted Double Layers

Double Layers

Quadruple Layers

Single Layer

Triple Layers

C

D Triple Layers 0.63nm Double Layers 0.63nm Double Layers

Triple Layers

5 nm E

5 nm

0.5

F


0.4

0.3

0.5


0.4

Frequency

Frequency

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


0.2

0.3

0.2

0.1

0.1

0.0

0.0 1

3

5

7

9

1

11

2

3

4

5

6

Stack degree

Length (nm)

Fig. 6. Representative TEM images of (A) K, Mo/MMO-0 and (B) K, Mo/MMO-2 after sulfurization, high resolution TEM images of (C) K, Mo/MMO-0 and (D) K, Mo/MMO-2, statistical distributions of (E) MoS2 slab length and (F) stacking degree.

33



A

B

9.0

Selectivity (C%)

CO Conversion (C%)

15 12

8.5

8.0

7.5

7.0

9

CHs

6 66 63 60

COHs

57 54

6.5

K, M

C

o /M

MO

-0 K, M

o /M

MO

-2 K, M

M o /M

O-1

0

K, M

M o /M

O-3

0

K, M

M o /M

O-6

0

K, M

D

1.0 0.9

C4OH

0.7

Hydrocarbon Distribution

0.8

Alcohol Distribution

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 38

C3OH

0.6 0.5 0.4 0.3

C2OH

0.2 0.1

C1OH

0.0

0 0 0 -2 -0 O-1 O-3 O-6 MO MO o/M o/M /MM /MM /MM o o o M M , , M M M K K K, K, K,

1.0 0.9

o /M

MO

-0 K, M

o /M

-2 -1 0 -3 0 -6 0 MO MO MO MO o /M o /M o /M M M M , , , K K K

C4H C3H

0.8

C2H 0.7 0.6

C1H

0.5 0.4

0 0 0 O-0 O-2 O-1 O-3 O-6 /MM /MM MM MM /MM o o / / o o o M M M K, K, K, K, M K, M

Fig. 7. (A) CO conversion, (B) selectivity towards hydrocarbons and alcohols, (C) alcohol products distribution, and (D) hydrocarbon products distribution of K, Mo/MMO-x.

34


Page 35 of 38 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


Fig. 8. The schematic structure-performance relationship on K, Mo/MMO-x in HAS

35



Page 36 of 38

Tables

Table 1. Binding Energies and sulphidation degree of presulfided K, Mo/MMO-x. Binding Energies for Mo3d5/2 (eV) Sulphidation degree Mo4+

Mo5+

Mo6+

K, Mo/MMO-0

228.4

230.2

232.1

54.2 %

K, Mo/MMO-2

228.6

230.4

232.4

57.8 %

K, Mo/MMO-10

228.5

230.2

232.2

56.0 %

K, Mo/MMO-30

228.3

230.2

232.1

53.7 %

K, Mo/MMO-60

228.5

230.2

232.2

53.4 %

36


Page 37 of 38 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


Table 2. Statistical results of morphology parameters and fraction of Mo atoms on the edge sites of K, Mo/MMO-x. Laverage (nm)

σlength

Naverage

Fraction of Moedge

K, Mo/MMO-0

5.48

4.35

2.20

0.192

K, Mo/MMO-2

4.87

4.05

2.04

0.216

K, Mo/MMO-10

4.95

4.02

2.17

0.207

K, Mo/MMO-30

4.82

3.91

2.05

0.218

K, Mo/MMO-60

5.21

3.78

2.17

0.208

37



Graphical Abstract

1


Page 38 of 38

Alkaline-Etched NiMgAl Trimetallic Oxides ... - ACS Publications

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