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Article

Comparative Study on Ethanol-Based Oxygenate Synthesis via Syngas over Monometallic Rh Catalysts Supported on Different Zr-MOFs

by
Ruiqi Yu
1,
Xiangjiang Duan
2,
Xuanwang Yu
2,
Xiang Zheng
3,*,
Haifang Mao
1 and
Jun Yu
1,*
1
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China
2
Zhejiang Haizhou Pharmaceutical Co., Ltd., Taizhou 317000, China
3
School of Perfume and Aroma Technology, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(9), 566; https://doi.org/10.3390/catal14090566
Submission received: 25 July 2024 / Revised: 13 August 2024 / Accepted: 22 August 2024 / Published: 27 August 2024
(This article belongs to the Special Issue Latest Advances and Prospects of Nanomaterials for Catalysis and Energy Storage)
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Abstract

:
Three types of Zr-based metal–organic frameworks (Zr-MOFs) were employed as supports to prepare monometallic Rh catalysts by the impregnation method. The effects of the structural properties of Zr-MOFs on their supported monometallic Rh catalysts for syngas conversion were investigated. The results showed that, compared to catalysts with Rh@MOF-808 and Rh@UiO-66, Rh@UiO-67 had higher CO conversion and C2+ oxygenate selectivity. The state of the Rh site is affected by the different structure of the Zr-MOFs, which is responsible for the difference in catalytic performance. The relatively higher Rh dispersion on the UiO-67 support boosted its CO adsorption ability, and Rh@UiO-67 having the best C2+ oxygenate selectivity was mainly attributed to it having the highest Rh+/Rh0 ratio.

1. Introduction

Ethanol has garnered global attention as a viable clean energy alternative and is considered the most promising candidate for displacing fossil fuels [1]. Ethanol production from syngas, which can be derived from coal, natural gas, or biomass, is considered the most promising production route [2,3,4]. So far, modified methanol synthesis catalysts [5], Mo-based catalysts [6,7], modified Fischer–Tropsch synthesis catalysts [8,9], and Rh-based catalysts [10,11] have been widely reported.
The Rh-based catalyst has attracted much attention because of its high ethanol selectivity [12,13,14,15]. Due to the sensitivity of the CO hydrogenation reaction to the structure and state of the Rh active center, the abilities for CO adsorption and C-C coupling can be improved by the regulation of the state of the Rh active center assisting with supports and additives [16,17,18,19,20,21,22,23]. Generally, traditional metal oxides are used to support Rh catalysts, and the nature of the support can optimize the catalyst structure and enhance the interaction between Rh and the support [24,25,26,27]. However, the relatively small specific surface areas and uneven pore structures of metal oxides are not conducive to the dispersion of metals. Conversely, the particle size and dispersibility of Rh can be effectively controlled by the ordered porous materials, such as carbon nanotubes and molecular sieves [28,29]. Unfortunately, the weak interaction between these carriers and the supported metal cannot effectively adjust the electronic valence state of Rh active sites [30,31,32,33].
The Rh-M bimetallic catalyst has also attracted much attention due to its high ethanol selectivity. Rh-Cu, Rh-Fe, and Rh-Mo bimetallic catalysts present better performances than Rh catalysts [34,35,36]. Unfortunately, the structure of bimetallic catalysts and the interaction between Rh and M during the reaction are not well understood.
As a new type of micro/mesoporous material, metal–organic frameworks (MOFs) have attracted extensive attention in the field of heterogeneous catalysis in recent years. MOFs are a class of crystal materials with periodic multi-dimensional network structure generated through the self-assembly of metal ions and organic ligands, which have the characteristics of high pore volume and adjustable pore surface modification, and can effectively fix the active metals in the uniform caves. At the same time, the metal nodes in MOFs have similar characteristics to traditional metal oxides, and their tunable surface properties can be used to control the structure and valence state of the supported metals [37,38].
Herein, we chose three kinds of Zr-MOFs (UiO-66, UiO-67, and MOF-808) as hosts for encapsulated Rh metal, and the effects of the structural properties of Zr-MOFs on their supported Rh catalysts for syngas conversion were investigated. The physicochemical properties and activities of the Rh catalysts are comprehensively discussed.

2. Results and Discussion

2.1. Structure and Texture Characterization of Supports

The XRD patterns of the MOFs (Figure 1a) exhibited corresponding characteristic diffraction peaks for the MOFs, confirming their well-defined structures [39,40,41]. The thermogravimetric analysis (Figure 1b) showed the well thermal stabilities of the MOFs, which, in turn, were reflected by their framework collapse mainly occurring above 450 °C. The results of N2 sorption (Figure 1c) showed the typical I shape isotherms, and the pore sizes of the MOFs were mainly distributed in the ranges of 0.6–1.2 nm and 1.5–2.5 nm, indicating their microporous structure [39,40,41]. The specific surface areas of UiO-66, UiO-67, and MOF-808 were 1179.1, 1057.0, and 662.4 m2/g, respectively (Table 1). The TEM images (Figure 1d–f) showed that these MOF materials all exhibited regular octahedral shapes, while the crystal structures of UiO-66 and UiO-67 were notably more compact and uniform than that of MOF-808.

2.2. Texture and Structure Characterization of Catalysts

The XRD patterns of the corresponding catalysts (Figure 2a) showed that Rh@UiO-66 and Rh@UiO-67 remained the crystals of UiO-66 and UiO-67. Conversely, the characteristic diffraction peaks of MOF-808 vanished after the loading of Rh, while the diffraction peaks at 2θ of about 41.1° and 47.8° ascribed to Rh (111) and (200) were observed, suggesting the poor structural integrity of MOF-808 and agglomerated Rh species in its caves [42]. The N2 sorption isotherms of the catalysts (Figure 2b) changed into type IV with H4 hysteresis loops, indicating the appearance of mesopores. Meanwhile, after the loading of Rh, part of the micropores in these MOFs disappeared (Figure 2c), accompanied by a decrease in the specific surface areas (Table 1), consistent with the blocking of the pores by Rh nanoparticles. Figure 2(d1–f4) show the TEM images and EDX elemental distributions of catalysts. The Rh@UiO-66 catalyst maintained the three-dimensional frame structure of UiO-66, while the morphologies of Rh@UiO-67 and Rh@MOF-808 became irregular. The EDX element mappings further indicated that Rh species were evenly distributed in the three-dimensional frames of UiO-66 and UiO-67, but severely agglomerated Rh species appeared in Rh@MOF-808 due to the poor structural integrity of MOF-808, which was consistent with the result of XRD.

2.3. Catalyst Performances

Table 2 shows the catalytic performances of the catalysts for syngas conversion at 300 °C and 3 MPa. The CO conversions of the catalysts decreased as follows: Rh@UiO-67 > Rh@UiO-66 > Rh@MOF-808. With respect to selectivity, the catalysts all exhibited similar selectivities for CO2 and CH4. However, compared with Rh@MOF-808, the formation of C2+ hydrocarbons (C2+ H) was restrained by the Rh catalysts supported on UiO-67 and UiO-66, while the selectivity of C2+ oxygenates increased. Combined with the high CO conversion rate and C2+ oxygenate selectivity, Rh@UiO-67 reached the highest C2+ oxygenate productivity of 136.8 mol/molRh·h.

2.4. Surface Chemical States

Figure 3a shows the H2-TPR curves of the supports and their corresponding catalysts. The MOFs showed an obvious peak between 500 and 650 °C. Combined with the result of TG, these peaks can be attributed to the decomposition of the organic ligands in the MOFs. After the loading of Rh metal, the H2-TPR curves of catalysts showed a broad peak in the range of 300−600 °C. Due to the interruption of the linkers in the MOFs after Rh loading and calcination, this broad peak resulted from the combination of the reduction in Rh species and the decomposition of the MOF framework, which created a barrier for accurately judging the reduction temperatures of the Rh species. But, in general, the reduction peak of Rh species over the catalysts shifted to the higher temperature as follows: Rh@UiO-66 ˂ Rh@UiO-67 ˂ Rh@MOF-808, which suggested that the larger Rh particles were formed in the Rh@MOF-808 catalyst and promoted the formation of alkanes [43,44,45].
The chemical states of Rh species on the reduction catalysts are shown in Figure 3b. The Rh 3d5/2 peak was divided into two peaks located at 307.2 eV and 308.6 eV, corresponding to Rh0 and Rh+ species, respectively [45]. The peak areas of Rh0 and Rh+ species over the catalysts were further deconvoluted, and the ratios of Rh+/Rh0 decreased sequentially as follows: Rh@UiO-67 > Rh@UiO-66 > Rh@MOF-808. The high ratio of Rh+/Rh0 is conducive to the synthesis of oxygenates from syngas, which is consistent with the C2+ oxygenate selectivities of the catalysts [46].

2.5. DRIFTS Study

Figure 4a–c illustrate the dynamic changes in adsorbed CO over the catalysts as a function of temperature. CO adsorption occurred in three distinct modes: the symmetric and asymmetric carbonyl stretching of the gem-dicarbonyl Rh+(CO)2 [CO (gdc)] with a doublet at ~2090 and ~2020 cm−1, linear-adsorbed CO [CO (l)] at 2040–2065 cm−1, and bridge-bonded CO [CO (b)] at ~1910 cm−1 [47,48]. The catalysts of Rh@UiO-66 and Rh@MOF-808 exhibited a similar trend in CO adsorption behavior. With the increase in temperature, the symmetric carbonyl stretching of CO (gdc) at ~2090 cm−1 decreased with its intensity, accompanied by the increase in the asymmetric carbonyl stretching of CO (gdc) at ~2020 cm−1. At the same time, the adsorbed species of CO (l) and CO (b) were also observed on the surface of Rh@UiO-66 and Rh@MOF-808, and the intensity of CO (l) increased with the increase in temperature. Conversely, Rh@UiO-67 mainly displayed the bands of CO (gdc), and its intensity increased as a function of temperature. These various behaviors of CO adsorption should be attributed to the different states of Rh sites affected by different properties of MOFs.
Figure 4d compares the infrared spectra of chemisorbed CO over the catalysts at the reaction temperature (300 °C). Rh@UiO67 was mainly occupied by the adsorbed species of CO (gdc). Conversely, all the adsorbed species of CO (gdc), CO (l), and CO (b) were left on Rh@UiO-66 and Rh@MOF-808. In general, CO (l) adsorbs on Rh0 clusters, while CO (gdc) is formed on the Rh+ site considered to be the active site for CO insertion [44]. The peak areas of CO (gdc) and CO (l) over the catalysts were deconvoluted, and the ratios of CO (gdc) versus CO (l) [CO (gdc)/CO (l)] are listed in Table 3. The CO (gdc)/CO (l) (Rh+/Rh0) ratios on the catalysts decreased in the sequence Rh@UiO-67 > Rh@UiO-66 > Rh@MOF-808, which is in line with the results of XPS.

2.6. Stability of the Rh@UiO-67 Catalyst

The stability during operation is one of the key issues in catalyst development; the catalytic performance vs. time onstream for the catalyst with Rh@UiO-67 at 300 °C is shown in Figure 5a. Both CO conversion and C2+ oxygenate selectivity exhibited a good reaction stability during the 100 h catalytic test, suggesting that the active sites in Rh@UiO-67 remained stable. XRD and TEM characterizations of the spent Rh@UiO-67 catalyst after 100 h reaction (Figure 5b–d) show that the crystal of UiO-67 is basically maintained, and no sintering of Rh particles can be seen, which is similar to the state of the as-prepared catalyst.

3. Materials and Method

3.1. Preparation of Catalysts

The MOFs (UiO-66, UiO-67, and MOF-808) were prepared by solvothermal reaction [49,50,51]. Standard synthesis of UiO-66 (UiO-67 or MOF-808), containing 1,4-benzenedicarboxylate (4,4′-biphenyl-dicarboxylate or benzene-1,3,5-tricarboxylic acid) struts with the secondary building units (SBUs) of Zr6(μ3-O)4(μ3-OH)4, was performed by dissolving 1.0 mmol ZrCl4 (99.95+%, J&K) and 1.5 mmol 1,4-benzenedicarboxylic acid (4,4′-biphenyl-dicarboxylic acid or benzene-1,3,5-tricarboxylic acid) (99%, DamasBeta) in 30 mL of N,N-dimethylformamide (DMF, 99%, DamasBeta) with 2 mL concentrated HCl (37%, SCRC) at 25 °C. The mixed solution was transferred into sealed glass bottles and placed in an oven at 80 °C for 10 h, then the crystallization was carried out. After cooling down, the obtained precipitates were washed with DMF and acetone and collected by centrifugation.
An aqueous solution of RhCl3 was prepared by dissolving a certain amount of 3H2O·RhCl3 in the deionized water. Catalysts were prepared by impregnation to the incipient wetness of the prepared Zr-MOFs (1.0 g) with the aqueous solution of RhCl3, aging for 12 h, drying overnight in the oven at 80 °C, then roasting in a muffle furnace at 300 °C for 4 h. The catalysts were named Rh@UiO-66, Rh@UiO-67, and Rh@MOF-808. For all the catalysts, the weight percentage of Rh was 2 wt.%.

3.2. Catalyst Characterization

The thermogravimetric test was carried out using the STA 449-F3 thermal analyzer produced by NETZSCH Company in Selb, Germany. In the analysis, about 15 mg of sample was used, and it was raised from 30 °C to 600 °C in the air atmosphere with a flow rate of 50 mL/min. Powder X-ray diffraction (PXRD) was carried out using the PANalytical X’Pert diffractometer (Malvern Panalytical, Malvern, UK). The scanning range 2θ is from 5° to 80°, and the rate is 6°/min.
The texture characterizations of all the samples were carried out at −196 °C with a Micromeritics ASAP–2020 HD88 adsorption apparatus (Norcross, GA, USA). Prior to measurement, the samples (100 mg) were degassed at 200 °C for 10 h under vacuum conditions.
X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) to determine the chemical valences of the surface elements. Prior to analysis, the samples were reduced in situ at 350 °C for 1 h. The temperature-programmed reduction of H2 (H2-TPR) was performed in a quartz micro-reactor. First, 100 mg of sample was purged by N2 at 300 °C for 30 min with 1.01 × 105 Pa H2 pressure in the pretreatment chamber. All binding energies (BEs) were calibrated relative to the C 1s line (284.6 eV) from adventitious carbon.
The temperature-programmed reduction of H2 (H2-TPR) was performed in a quartz micro-reactor. Firstly, 100 mg of sample was purged by N2 at 300 °C for 30 min. After cooling down to room temperature, the reducing gas of 10 vol% H2/N2 (50 mL/min) was switched on, and then the measurements were carried out from 50 °C to 650 °C with a heating rate of 5 °C/min.
CO adsorption was investigated using a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific) equipped with a diffuse reflectance infrared Fourier transform (DRIFT) cell with CaF2 windows. The powder of samples pressed in the cell was pretreated using a mixed 10 vol% H2/N2 (50 mL/min) at 350 °C for 1 h, followed by He flow (50 mL/min) flushing at 350 °C for 0.5 h. During cooling down to room temperature, a series of background spectra were captured at required temperatures. Following the introduction of 5 vol% CO/He (50 mL/min) into the IR cell, the IR spectra at the specified temperatures were recorded. The spectral resolution was 4 cm−1, and there were 64 scans.

3.3. Activity Test

The catalyst (0.3 g, 40–60 mesh) was evenly mixed with 0.5 g quartz sand and filled into the reaction tube. The reaction tube was fixed in the reactor, while the catalyst was located in the constant-temperature zone. In the reaction process, the catalyst was firstly reduced in the mixture of 10 vol% H2/N2 (50 mL/min) at 350 °C for 2 h, then decreased to the reaction temperature. Then, the gas was switched to the reaction gas (molar ratio of H2/CO = 2, total flow rate = 50 mL/min), and the gas pressure was increased to 3.0 MPa. The products were analyzed online by gas chromatography (Fuli 9720, Chongqing, China). The main products were CO2, hydrocarbons, and oxygen-containing compounds. We used the HP-PLOT/Q column with an FID detector to detect hydrocarbons and oxygenates, and the TDX-01 column with a TCD detector to detect permanent gasses. The CO conversion was calculated based on the fraction of CO that formed carbon-containing products, and the selectivity of a certain product was calculated based on carbon efficiency, as reported previously [49,52].

4. Conclusions

Three types of Zr-MOFs (UiO-66, UiO-67, and MOF-808) with octahedral morphology were prepared by the solvothermal method. The effect of the structural properties of Zr-MOFs on their supported Rh catalysts for syngas conversion were investigated. Compared with the catalysts with Rh@UiO-66 and Rh@MOF-808, Rh@UiO-67 exhibited a superior CO conversion and C2+ oxygenate selectivity and reached the highest C2+ oxygenate productivity of 136.8 mol/molRh·h.
The relatively weak thermal stability of MOF-808 leads to structural deterioration in the catalyst preparation procedure, resulting in Rh agglomeration within the pores of the carrier. Compared with Rh@MOF-808, the catalysts with Rh@UiO-67 and Rh@UiO-66 had good thermal stability, and the Rh species were evenly distributed within the pores. These highly dispersed Rh active sites favored CO adsorption, resulting in increased CO conversion. In addition, XPS and IR spectroscopy showed that Rh@UiO-67 had the highest Rh+/Rh0 ratio, which encouraged the CO insertion reaction and contributed to its superior C2+ oxygenation selectivity.

Author Contributions

Methodology, Investigation, Writing—original draft, R.Y.; visualization, supervision, X.D. and X.Y.; validation, Visualization, Data curation, X.Z.; resources, H.M.; Writing—review and editing, Project administration, Funding acquisition, J.Y. All authors have read and agreed to the published version of the manuscript.

Funding

The researchers wish to thank the Shanghai Local Capacity Building Project (23010504600) for funding this research.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

Authors Xiangjiang Duan, Xuanwang Yu were employed by the company Zhejiang Haizhou Pharmaceutical Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Catalysts 14 00566 g001
Figure 1. XRD patterns (a), thermogravimetric curve (b), and N2 sorption and pore size distribution (c) of the MOFs, and TEM images of UiO-66, UiO-67, and MOF-808 (d–f).
Figure 1. XRD patterns (a), thermogravimetric curve (b), and N2 sorption and pore size distribution (c) of the MOFs, and TEM images of UiO-66, UiO-67, and MOF-808 (d–f).
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Catalysts 14 00566 g002aCatalysts 14 00566 g002b
Figure 2. XRD patterns (a), N2 sorption (b), and pore size distribution (c) of the catalysts, and TEM, HR-TEM, and EDX elemental distributions of Rh@UiO-66 (d1–d4), Rh@UiO-67 (e1–e4), and Rh@MOF-808 (f1–f4).
Figure 2. XRD patterns (a), N2 sorption (b), and pore size distribution (c) of the catalysts, and TEM, HR-TEM, and EDX elemental distributions of Rh@UiO-66 (d1–d4), Rh@UiO-67 (e1–e4), and Rh@MOF-808 (f1–f4).
Catalysts 14 00566 g002aCatalysts 14 00566 g002b
Catalysts 14 00566 g003
Figure 3. H2-TPR profiles (a), XPS spectra of Rh 3d (b) over the catalysts.
Figure 3. H2-TPR profiles (a), XPS spectra of Rh 3d (b) over the catalysts.
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Figure 4. The infrared spectra of chemisorbed CO over Rh@UiO-66 (a), Rh@UiO-67 (b), and Rh@MOF-808 (c) as a function of temperature, and the comparison of infrared spectra of chemisorbed CO over the catalysts at 300 °C (d).
Figure 4. The infrared spectra of chemisorbed CO over Rh@UiO-66 (a), Rh@UiO-67 (b), and Rh@MOF-808 (c) as a function of temperature, and the comparison of infrared spectra of chemisorbed CO over the catalysts at 300 °C (d).
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Figure 5. Catalytic performance vs. time onstream for Rh@UiO-67 (reaction conditions: T = 300 °C, P = 3 MPa, H2/CO = 2, flow rate = 50 mL/min) (a); XRD pattern (b); and TEM (c,d) of spent Rh@UiO-67.
Figure 5. Catalytic performance vs. time onstream for Rh@UiO-67 (reaction conditions: T = 300 °C, P = 3 MPa, H2/CO = 2, flow rate = 50 mL/min) (a); XRD pattern (b); and TEM (c,d) of spent Rh@UiO-67.
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Table 1. BET surface area and pore volume of different samples.
Table 1. BET surface area and pore volume of different samples.
SampleSurface Area (m2·g−1)Pore Volume (cm3·g−1)
TotalMicroporeExternalTotalMicroporeMesopore
UiO-661179.11004.7174.40.690.510.18
Rh@UiO-66903.9622.8281.10.530.320.21
UiO-671057.0912.4144.60.610.460.15
Rh@UiO-67805.7573.6232.10.540.300.24
MOF-808662.4455.4107.00.330.230.10
Rh@MOF-808145.243.4101.80.150.020.13
Table 2. CO hydrogenation performance with the catalysts.
Table 2. CO hydrogenation performance with the catalysts.
CatalystCO
Conv.
(%)		Selectivity (%)C2+ Oxy
Productivity
(mol/molRh·h)
CO2CH4MeOHHACEtOHC2+ H aC2+ Oxy b
Rh@UiO-6628.22.022.29.619.231.116.050.288.7
Rh@UiO-6736.51.920.24.917.532.114.558.5136.8
Rh@MOF-80818.02.323.89.517.723.519.944.551.9
Reaction conditions: T = 300 °C, P = 3 MPa, catalyst = 0.3 g, and flow rate = 50 mL/min (H2/CO = 2), data taken after 13 h when steady state reached. Experimental error: ±5%. a C2+ H denotes hydrocarbons containing two and more carbon atoms. b C2+ oxy denotes oxygenates containing two and more carbon atoms.
Table 3. Content and proportion of different Rh species on the catalyst surface.
Table 3. Content and proportion of different Rh species on the catalyst surface.
Catalysta Rh+a Rh0Rh+/Rh0b CO (gdc)b CO (l)CO (gdc)/CO (l)
Rh@UiO-662978.34659.80.646.813.092.24
Rh@UiO-672530.83711.90.686.762.502.62
Rh@MOF-8081556.16529.10.244.753.611.32
a Peak areas of Rh+ and Rh0 are derived from XPS peak data. b Peak areas of CO (gdc) and CO (l) are derived from IR spectra.
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Yu, R.; Duan, X.; Yu, X.; Zheng, X.; Mao, H.; Yu, J. Comparative Study on Ethanol-Based Oxygenate Synthesis via Syngas over Monometallic Rh Catalysts Supported on Different Zr-MOFs. Catalysts 2024, 14, 566. https://doi.org/10.3390/catal14090566

AMA Style

Yu R, Duan X, Yu X, Zheng X, Mao H, Yu J. Comparative Study on Ethanol-Based Oxygenate Synthesis via Syngas over Monometallic Rh Catalysts Supported on Different Zr-MOFs. Catalysts. 2024; 14(9):566. https://doi.org/10.3390/catal14090566

Chicago/Turabian Style

Yu, Ruiqi, Xiangjiang Duan, Xuanwang Yu, Xiang Zheng, Haifang Mao, and Jun Yu. 2024. "Comparative Study on Ethanol-Based Oxygenate Synthesis via Syngas over Monometallic Rh Catalysts Supported on Different Zr-MOFs" Catalysts 14, no. 9: 566. https://doi.org/10.3390/catal14090566

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