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Catalytic Valorization of Furfural Under Methane Environment Aiguo Wang, Danielle Austin, Hui Qian, Hongbo Zeng, and Hua Song ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01257 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Sustainable Chemistry & Engineering

Catalytic Valorization of Furfural Under Methane Environment Aiguo Wang1, Danielle Austin1, Hui Qian2, Hongbo Zeng3, Hua Song1*

1

Department of Chemical and Petroleum Engineering, University of Calgary 2500 University Drive, NW, Calgary, Alberta T2N 1N4, Canada 2

National Institute for Nanotechnology, National Research Council Edmonton, Alberta, T6G 2M9, Canada

3

Department of Chemical and Materials Engineering, University of Alberta 9211-116 Street NW, Edmonton, Alberta T6G 1H9, Canada

*Corresponding author. Fax: +1 (403) 284-4852; Tel: +1 (403) 220-3792; E-mail: [email protected]

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Abstract The technical feasibility of developing a novel process for coupling bio-derived furans (furfural) with methane was investigated at mild reaction conditions (400 oC and 2.5 MPa) over modified zeolite catalysts. Monoaromatics are the main liquid products with high yield, and the oxygen of furfural is removed in the form of CO and CO2. Compared to other zeolite-supported metal catalysts developed in this study, Zn-Ga/ZSM5 shows better catalytic performance on furfural conversion in terms of a higher yield of aromatics, particularly BTX, slightly lower gas yield and higher methane conversion. The presence of methane provides carbon atoms which are effectively utilized in the formation of aromatics, and further significantly enhances the yield of aromatics under the facilitation of the charged catalyst. Direct evidence of methane participation in the formation of aromatic products is witnessed by liquid 1H, 2H and 13C NMR investigations, which reveal that methane tends to be incorporated into both the attached methyl groups and aromatic ring, preferentially the benzene ring and the meta position of aromatics. Comprehensive catalyst characterizations, including XRD, DRIFT, NH3-TPD, TEM and XPS, demonstrate that higher yield of aromatics from furfural conversion might be attributed to better dispersion of Zn and Ga species on the zeolite, abundant acidic sites with an appropriate ratio of weak acid sites to strong acid sites, as well as the stable oxidation state of metal species (Zn2+ and Ga3+). The reaction pathway involved in the furfural conversion to aromatics under methane environment is reasonably hypothesized based on pseudo-situ investigations. The results reported in this work could provide more insights into the catalytic chemistry of bio-derived furans upgrading and more cost-effective utilization of abundant natural gas and biomass.

Keywords: Furfural, Methane, ZSM-5, Catalyst, Aromatics


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Introduction Diminishing fossil fuel resources accompanied by increasing environmental concerns motivate the development of alternative ways to produce chemicals and fuels from abundantly available, renewable carbon-neutral resources. Catalytic transformation of biomass into high value chemicals and biofuels is expected to partially replace traditional petroleum-based routes and address these issues 1, which has attracted much attention. Furanic compounds such as furfural, furan, and 5-hydroxymethylfurfural, are important platform molecules involved in the process of biomass conversion.2 Among them, furfural has the highest research potential due to its high reactivity and versatility 3-5. As a raw material with many functional groups associated, Furfural is considered as a desirable precursor for the production of valuable chemicals. Furfural is typically produced from xylose or xylan (large quantity in lignocellulosic biomass6) over acid catalysts through hydrolysis followed by dehydration, but also is one of the oxygenated components present in bio-oil.5,

7

In addition to the necessity of removing the

oxygenated compounds to improve the bio-oil quality, excess functionality contained in furfural is disadvantageous for its direct use as fuels and chemicals. Much research effort has been devoted to controlling the functionality in the final product. The various catalytic routes for the furfural conversion towards biofuels, fuel additives and chemicals are critically reviewed.4,

8

Generally, one strategy is to remove the carbonyl group by hydrogenation to produce a range of C4 and C5 molecules 9, including 2-methylfuran5,

7, 10-13

, furfural alcohol

9, 14

, furan15-16, and

tetrahydrofuran17. Another important approach is to lengthen the carbon chain through C-C coupling reactions to obtain liquid hydrocarbons with high energy density, such as aromatics 2, 1823

, or C10 - C15 alkanes

24-25

. Besides, some attempts have been made to convert furfural into

valuable oxygenated chemicals like cyclopentanone26, cyclopentanol27, benzofuran28, 1,5-



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pentanediol29-30, and other useful bio-products31. Furfural upgrading involves hydrogenation, rearrangement, and C-C coupling, which are usually achieved under the facilitation of hydrogen or a H-donor such as formic acid, 2-propanol, or acidic ionic liquids 6. However, these hydrogen sources are not naturally available. Low selectivity of targeted products and subsequent separation are the big issues to be addressed during the furfural conversion to valuable bioproducts. The issues involved in the catalytic conversion of bio-derived furans for producing desired aromatics are low aromatics yield (< 30 wt.%

18-19, 22

) and significant coking without

other carbon sources. Such low liquid yield is due to the high content (~33.3 wt.%) of oxygen in furfural. Theoretically, if the oxygen contained in furfural is all removed as pure CO, or CO2, the gas yield would be 58.3 wt.% and 45.8 wt.%, respectively, which inevitably leads to low yield of aromatics. The co-feeding of olefin (ethylene

20

and propylene

21

) or acrolein

23

is proposed to

improve the liquid yield and reduce coke formation. Unfortunately, these carbon sources are expensive and not readily available. It would be very appealing to maximize the aromatics yield by coupling bio-derived furans with a cheaper carbon source like methane. Methane (CH4), the major component of natural gas, is inexpensive and naturally available. Once methane is activated in the process, it would provide protons for oxygen removal to improve bio-oil quality, and carbon atoms used in the formation of aromatics to improve the yield of the desirable liquid products. However, the coupling conversion of furfural and methane to aromatics is a quite challenging chemical transformation due to the chemical inertness of methane. The C-H bond of methane is very stable, and requires as high as 439 kJ/mol for C-H dissociation. Fortunately, extensive research has demonstrated that methane can be more readily activated and participate the reactions when co-fed with higher hydrocarbons 32-34 or oxygenated hydrocarbons 35-37. The promotion effect of methane on improving the oil quality and enhancing


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the yield of aromatics has been experimentally observed in our previous work.36-39 Additionally, thermodynamic analysis shows that the co-aromatization of furfural and methane is viable, given that benzene (C6H6) is the target product, CO and H2 are the gas products. The possible reaction for co-aromatization of furfural (C5H4O2) and methane (CH4) can be simplified to: C5H4O2(g) + 3CH4(g) ⇋ C6H6(g) + 2CO(g) + 5H2(g) and the Gibbs free energy change (∆rG) of this reaction is calculated to be negative (-14.8 kcal/mol) at 400 ºC, which suggests that this novel approach is thermodynamically feasible. Herein, a series of zeolite-supported metal catalysts were prepared to investigate the feasibility of co-aromatization between furfural (as a model compound of bio-derived furans) and methane. In this work, the desired products from furfural conversion are aromatics. Based on what are reported in our previous researches and open publications,36-37 zeolite (ZSM-5) is the preferred support due to its specially ordered pore size and structure for higher selectivity of aromatics. The metal species, Zn, Ga, Ag, Pt, Pd, Ir, were selected to be doped on the zeolite since these metals are reported to be beneficial for the reactions involved in co-aromatization of furfural and methane such as deoxygenation, hydrogenation, and methane activation.10, 24, 29-30, 37, 40-41

The Zn-Ga/ZSM5 catalyst showed excellent performance in terms of BTX (benzene, toluene

and xylene) selectivity and liquid yield under mild conditions (400 ºC and 2.5 MPa). Since BTX are important petrochemical materials with a wide range of industrial applications, they are the preferred liquid products in this study. Methane incorporation into the aromatic products was witnessed by using isotope labeled methane molecules (13CH4 and CD4). The evolution of furfural conversion under CH4 environment is studied through a pseudo-situ experiment. A reasonable mechanism for the coupling conversion of furfural and methane is developed. Comprehensive characterization including x-ray photoelectron spectroscopy (XPS), powder X-



ray diffraction (XRD) analysis, transmission electron microscopy (TEM), diffuse reflectance infrared transform spectroscopy (DRIFT), and ammonia temperature programmed desorption (NH3-TPD), were conducted to reveal the link between the properties of catalyst and its catalytic performance on the furfural conversion. Our mechanistic understanding of this novel process will give more insight into the catalytic upgrading of bio-derived furans and the design of more effective and less costly catalysts for the biomass utilization. Experimental Catalyst synthesis Various zeolite-supported metal catalysts (Zn/ZSM5, Ga/ZSM5, Ag/ZSM5, Pt/ZSM5, Ir/ZSM5, Pd/ZSM5, Zn-Ga/ZSM5) were synthesized by incipient wetness impregnation. The ammonium ZSM5 zeolite with molar SiO2/Al2O3 ratio of 280 (CBV 28014) and specific surface area of 400 m2·g-1 was purchased from Zeolyst and calcined at 600 ºC for 5 h in air to form HZSM5 for further use. Zn/ZSM5, Ag/ZSM5, Ga/ZSM5, Pt/ZSM5, Pd/ZSM5, Ir/ZSM5 and ZnGa/ZSM5 were all synthesized by incipient wetness impregnation of HZSM5. The loading of Zn species was 5 wt.%, and the loading of the other metal species, such as Ag, Ga, Pt, Pd, and Ir, was 1 wt.%. The precursors for acquiring the active materials were Zn(NO3)2·6H2O (99%, Alfa Aesar), AgNO3 (99.0+%, Sigma-Aldrich), Ga(NO3)3·xH2O (99.9%, Alfa Aesar), Pt(NH3)4(NO3)2 (≥50.0% Pt basis, Sigma-Aldrich), Pd(NO3)2·2H2O (~40% Pd basis, Sigma-Aldrich), and IrCl3 ( ≥ 62% Ir, Alfa Aesar). Incipient wetness impregnation involved initially dissolving the corresponding amounts of precursors in 10.0 g deionized water, followed by dropwise impregnation on 8.0 g HZSM5 support until pore saturation. The obtained wet powder was dried in the oven at 92 ºC overnight, followed by calcination at 550 ºC with a heating rate of


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10 °C/min and a holding time of 3 h in ambient air. The resulting catalysts were then stored properly for later use. Catalyst characterization The liquid products were analyzed using Gas Chromatography-Mass Spectrometer (GCMS: PerkinElmer GC Claus 680 and MS Clarus SQ 8T) to determine the composition. A Paraffins-Olefins-Naphthenes-Aromatics (PONA) column (Agilent HP-PONA) was equipped on the GC-MS. The GC temperature programming is described as follows: initially holding at 35 ºC for 15 min, then ramp to 70 ºC at 1.5 ºC/min, continue to heat up to 150 ºC at the rate of 3 ºC /min and hold for 30 min. After that, the temperature increases to 250 ºC at 3 ºC /min and hold for 2 min. The composition of gas products was determined using a four-channel Micro-GC (490, Agilent) equipped with thermal conductivity detectors. After the completion of a reaction, the reactor was cooled down to room temperature, then connected with the Micro GC. The first channel was equipped with a 10 m molecular sieve 5A column, which can precisely analyze H2, O2, N2, CH4, and CO using Ar as the carrier gas. The second channel was installed with a 10 m PPU column for the analysis of CO2, C2H2, C2H4, and C2H6 using He as the carrier gas. The third channel and fourth channels were charged with a 10 m alumina column and one 8 m CP-Sil 5CB column, respectively, which are used for the determination of the composition of C3- C6 and C3= - C5= (“=” denotes alkenes) in the gas products. Upon the composition measured by micro GC, the mass (moles) of each component can be calculated using the ideal gas law. The Transmission Electron Microscopy spectra were acquired on a Philips Tecnai TF-20 TEM instrument operated at 200 kV. The sample was first dispersed in ethanol with sonication for about 10 min, and then supported on honey carbon on a 200 mesh Cu grid before the TEM 7 ACS Paragon Plus Environment


images were recorded. Energy dispersive spectrometer mapping of Zn and Ga elements on the catalyst were acquired on the Oxford INCA EDS on JEOL2200FS TEM under STEM mode. A 1.5 nm probe was used for x-ray microanalysis along with high angle annular dark field image. The total acquisition (mapping) time is 1000 seconds (about 16 mins) and the site lock is used for correcting position drift. The desired elements (Zn and Ga) are mapped and the sum spectrum is also obtained at same time. The X-ray Diffraction (XRD, Rigaku ULTIMA III X-ray diffractometer) analysis was conducted scanning from 3-60º at 3º step/min with a Cu Kα irradiation generated at a voltage of 40 kV and current of 44 mA. The 1H, 2H and

13

C liquid nuclear magnetic resonance experiments were conducted at 9.4

T (ν0 (1H) = 400.1 MHz; ν0 (2H) = 61.4 MHz and ν0 (13C) = 100.6 MHz) on a BRUKER AVANCE Ⅲ 400 NMR spectrometer with a BBFO probe. 1H NMR chemical shifts were referenced to CDCl3 at 7.24 ppm. A spectral width of 12 kHz and a pulse delay of 2 s were used to acquire 64 scans per spectrum. A spectral width of 2.5 kHz and a pulse delay of 7 s were used to acquire 512 scans per 2H spectrum.

13

C NMR chemical shifts were referenced to CDCl3 at

77.3 ppm. A spectral width of 26 kHz and a pulse delay of 2 s were used to acquire 10,000 scans per spectrum. Acidity measurements were performed on the Chemisorption Analyzer (Finesorb-3010) using Ammonia as the probe. Prior to measurements, around 200 mg of fresh catalyst was loaded into the U-type quartz tube. The sample was firstly heated up to 600 °C with a ramp rate of 20 °C/min and held for 30 min in in 5%O2/He flow with 30 sccm. Then, it was cooled down to 120 °C and conducted the adsorption of ammonia for 30 min under the flow of 25 sccm of 10% NH3/He. After flushing with He for 10 min, the sample subsequently heated to 600 °C with a 8 ACS Paragon Plus Environment

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heating rate of 30 °C/min and a hold of 30 min. A thermal conductivity detector (TCD) determined the amount of desorbed NH3. The surface acidity was examined using Diffuse Reflectance Infrared Fourier Transform spectroscopy upon pyridine adsorption. The experiment was performed on a Thermo-ScientificNicolet iS50 equipped with an environmental chamber and mercury–cadmium–telluride (MCT) detector. A small amount of sample (fresh catalyst) was loaded in the environmental chamber. Before conducting pyridine adsorption, the sample was heated to 500 ºC and held for 15 min in a 30 sccm N2 flow to remove impurities adsorbed on the catalyst surface. After cooling down the room temperature, the background spectra was collected using 512 scans with a resolution of 2 cm-1. Then, the N2 flow was switched to go through the bubbler filled with pyridine and carried the pyridine vapor into the environmental chamber. After the adsorption of pyridine for 20 min, the gas flow was switched back to bypass the bubbler for another 20 min. The spectrum of the sample was then recorded in an absorbance mode using 512 scans with a resolution of 2 cm-1. XPS analysis of the sample was conducted at nanoFAB using Kratos Axis spectrometer with monochromatized Al Kα (hυ = 1486.71 eV) radiation. The spectrometer was calibrated by the binding energy (84.0 eV) of Au 4f7/2 with reference to Fermi level. The pressure of analysis chamber during the experiment was maintained at lower than 5×10-10 Torr. The core-level spectra was collected on a hemispherical electron-energy analyzer working at the pass energy of 20 eV, while the survey spectrum ranging from 0 to 1100 eV was acquired at analyzer pass energy of 160 eV. The binding energy was calibrated by using C 1s peak at 284.8 eV as a reference. A Shirley background was applied to subtract the inelastic background of core-level peaks. Non-linear optimization using the Marquardt Algorithm (Casa XPS) was used to determine the peak model parameters such as peak positions, widths and peak intensities. Peak 9 ACS Paragon Plus Environment


deconvolution of C 1s and O 1s into components related to different chemical bonds was conducted using Casa XPS software. The model peak to describe XPS core-level lines for curve fitting was a product of Gaussian and Lorentzian functions. The water content of liquid products was measured on Karl Fischer (KF) titration (Metrohm 870 Titrino Plus) through averaging the results obtained from at least three independent measurements. Catalytic performance evaluation The furfural conversion was performed on 100 and 300 mL Parr® reactors under a batch mode, respectively. The catalytic performance on furfural conversion over the synthesized catalysts were evaluated at 400 ºC and 2.5 MPa under methane environment. The produced liquid and gas products were analyzed using GC-MS and micro-GC, respectively. Typically, ~1.0 catalyst with ~0.10-0.12 g furfural was loaded into the reactor. The reactor was flushed three time with N2 to purged air out, then pressurized to 10 bar with a reactive gas (i.e., methane or nitrogen). The reactor was heated to the desired temperature (400 ºC) at a rate of 20 ºC/min. After the reaction of 60 min at target temperature, the reactor was cooled down to room temperature and then connected to micro-GC for the analysis of gas products. ~10 g CS2 (GC grade, EMD Chemicals) solvent was added into the reactor to extract the formed liquid products embedded into the charged solid catalyst, and filtered. Similarly, isotopic labeled reactions between furfural and 13CH4/CD4 (99.9% 13C and 99% 2H, Cambridge Isotope Laboratories Inc.) were conducted in a 100 mL Parr® reactor to limit the usage of 13CH4/CD4. Following the same initial procedure, Pseudo-situ experiments were performed with varying reaction times of 10, 30, 60, and 90 mins and different temperatures of 300 ºC (1 s), 350 ºC (1 s) and 400 ºC (1 s). The reactor was cooled down quickly in cold air flow to quench the reaction at the desired 10 ACS Paragon Plus Environment

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temperature and completion time, then the product was extracted as before. After reaction, all the water is removed and all the jackets and loops are water free. The gas products are discharged from the reactor, the mass of total gas is then recorded by noting the weight difference of the reactor (including spent catalyst). The mass of coke is determined by weight loss during the calcination of spent catalyst from 200°C to 600°C. The mass of methane in gas products is calculated using the ideal gas law, the gas composition is determined via analysis using a microGC. The mass of liquid products is obtained by using mass balance for all the reactants involved in the reaction. The gas, coke and liquid yield (wt.%), the yield of products (C mol%), product selectivity (C mol or wt.%) and methane conversion reported here are given by the following equations: Gas yield(wt.%)=

mass of total gas, g-mass of methane after reaction,g ×100% weight of fed furfural,g

Coke yield (wt.%)=

mass of coke over the catalyst,g ×100% weight of fed furfural,g

Liquid yield(wt.%)=

mass of liquid collected afer reaction,g ×100% weight of fed furfural,g

Product yield (C mol%) =

moles of carbon in a product ×100% moles of carbon in furfural

Product selectivity (C mol or wt.%)= Methane conversion(%) = (1-

moles or mass of carbon in a product ×100% moles or mass in identified liquid products

mass of methane in gas products,g )×100% mass of mehane before reaction,g

(1) (2) (3) (4) (5) (6)

Generally, the carbon balance was closed and the error was estimated within 5%. Results and Discussion Catalytic performance evaluation The catalytic effects on the selectivity of liquid products, gas yield and methane conversion over various catalysts are summarized in Table 1. Complete conversion of furfural and relatively 11 ACS Paragon Plus Environment


high gas yield (>50 wt.%) are observed over all engaged catalysts after reaction. The high gas yield is the result of the high oxygen content of furfural as discussed in the introduction section. Since there is only trace amounts of oxygenated compounds and no detectable water in the liquid products after reaction, almost all oxygen contained in furfural is transferred into gas products in the forms of CO and CO2, which are the major gas components detected by Micro-GC. It’s noticed that the loading of precious metals such as Ag, Pt, Ir, Pd, leads to more gas products and favors the formation of heavy or poly- aromatics, which is evidenced by higher gas yield and lower selectivity of BTX in Table 1. However, when the transition metals (Zn and/or Ga) are introduced on the ZSM5 framework, BTX selectivity in liquid products is enhanced and the gas yield is slightly reduced. The highest methane conversion is achieved over Ga/ZSM5. When both Zn and Ga species are added on zeolite support, the selectivity of aromatics, particularly BTX, is further improved with a slight decrease in methane conversion. Lower gas yield and higher methane conversion is potentially beneficial for increasing the yield of liquid products. Therefore, Zn-Ga/ZSM5 catalyst is the preferred catalyst for further investigation. Influence of methane existence The effect of methane on the furfural conversion to aromatics is investigated by performing equivalent runs over HZSM5 and Zn-Ga/ZSM5 under CH4 and N2 environments, and the comparative results are tabulated in Table 2. Methane has insignificant influence on furfural conversion over the bare support (HZSM5) in terms of the yield of liquid and gas products and coke formation. This may be due to the fact that bare support has limited ability to activate methane, which is supported by extremely low methane conversion (less than 0.1%). Compared to the yield of liquid products collected over HZSM5, the aromatics yield is increased significantly over Zn-Ga/ZSM5 under both CH4 and N2 environments. Particularly, the yield of


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BTX is improved more pronouncedly under a CH4 environment. This positive effect on the conversion of furfural towards aromatics may be attributed to higher capability of methane activation over Zn-Ga/ZSM5, which potentially provides more protons or CHx species that facilitate the formation of aromatics, or directly participate the reaction to form more BTX in the liquid products by aromatization with intermediates derived from furfural. Moreover, on comparison of the yield of gas components as given in Table 2, more CO2 with less CO gas products are produced over Zn-Ga/ZSM5 under a methane atmosphere, implying more carbon atoms would be effectively used for the formation of aromatics provided that all oxygen atoms of furfural are removed. Fig. 1 shows the stability test of Zn-Ga/ZSM5 for the furfural conversion to aromatics at 400°C, 2.5 MPa of CH4 and 1 h for each batch cycle. During 5 cycles, complete conversion of furfural was observed for each cycle. The selectivity of aromatics in liquid products maintained a constant 71.4 ± 2.3%, the yields of CO and CO2 were 21.3 ± 1.8% and 9.4 ± 0.8%, respectively; and methane conversion was kept constant at 1.0 ± 0.1%. These results suggest that the catalytic activity of Zn-Ga/ZSM5 on furfural conversion to aromatics is quite stable even after 5 cycles with the addition of fresh furfural after each cycle. Methane incorporation The GC-MS analysis of the formed liquid products extracted from the catalyst has confirmed that a mixture of C6H6 (benzene, 32.1 mol%), C7H8 (toluene, 50.6 mol%), C8H10 (ethylbenzene and xylene, 5.9 mol%), C9H12 (C9 aromatics, 3.1 mol%) and C10H8 (naphthalene, 8.3 mol%) is formed during the furfural conversion under

13

CH4 at 400 ºC. The molecular ion

region of the mass-spectra of major products (C6H6, C7H8, C8H10, and C10H8) collected from 13Clabeled and regular methane are shown in Fig. 2. Preliminary mass spectrometry analysis 13 ACS Paragon Plus Environment


confirms the presence of singly (13C1) and doubly (13C2) labeled aromatic molecules (Fig. 2b), which provides a complement to the evidence of methane incorporation into aromatic products. More convincing evidence of methane participation in the furfural conversion is witnessed from the liquid 13C NMR spectra in Fig. 3 and semi-quantitative analysis in Table 3. It’s clearly noted that the 13C signals collected from liquid products produced from the furfural and 13CH4 run are noticeably enhanced when comparing with that from the non-isotopic labeled counterparts, which strongly supports the involvement of methane in the furfural conversion. Four signals in the region of 120-140 ppm (in Fig. 3b) are corresponding to the resonances of aromatic ring carbons, which are further assigned to different carbon sites of the aromatic products according to their characteristic

13

C chemical shifts

42-43

. The tiny peak at 137.44 ppm is assigned to the

phenyl carbon directly bonded with alkyl substituent groups, while the signals at 129.03 and 125.41 ppm are attributed to the ortho- and para- carbon positions of the alkyl substituent in the aromatic rings, respectively. The minimal increase in these peak intensities suggests that methane might not favor these substituent positions. The peak at 128.32 ppm with the highest intensity among them is associated with carbons of benzene and the meta carbon sites in the substituted aromatic rings. There are two aliphatic resonances appearing at 30.15 and 21.72 ppm, which are derived from the benzylic carbon. The relatively high peak intensity at aromatic carbon sites (128.32 ppm) as well as the significant increase (>2 times) in the signal of the benzylic carbon (21.72 ppm) imply that methane tends to be incorporated into both the side chain, particularly at the benzylic position, and aromatic ring, preferentially in benzene ring and the meta position of substituted aromatics. In addition, the evolution of hydrogen atoms in methane was investigated using deuteriumenriched methane (CD4). The 1H (Fig. 4a) and 2H NMR (Fig. 4b) spectra of the liquid products


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under a CD4 environment are acquired and compared with those of the non-deuterium-enriched products. These peaks appearing in Fig. 4a are categorized into three groups and the peak area ratios with respect to CDCl3 are presented in Table 4. The intensity of signals due to the protons on the phenyl rings (7.0-8.0 ppm) and benzylic hydrogens (2.0-3.0 ppm) are significantly reduced, while there is no significant change on the alkyl hydrogens (0.0-2.0 ppm). These observations demonstrate that the hydrogen atoms of methane favor the aromatic and benzylic hydrogen sites over the alkyl group hydrogen sites, which is highly consistent with observations in the 2H NMR spectra (Fig. 4b) that there are two strong NMR peaks appearing in the aromatic (7.0- 8.0 ppm) and benzylic (2.0-3.0 ppm) hydrogen regions, and small peaks seen in the alkyl (0-2.0 ppm) hydrogen region. This compliments with the conclusion from the analysis of

13

C

NMR spectra (Fig. 3) that methane favors both aromatic ring and benzylic carbon sites. Reaction pathway investigation The reaction pathway involved in the co-aromatization of furfural and methane was further investigated in detail by quenching the reaction at different temperatures (300 ºC, 350 ºC, and 400 ºC) of the initial stage and terminating the reaction with various reaction times (10, 30, 60, and 90 min) at 400 °C under a CH4 environment. The liquid and gas products collected from these trials were quantified by GC-MS and Micro-GC, respectively. As shown in Fig. 5 and 6, initially, furfural is converted into alkylfuran via decarbonylation followed by alkylation benzofuran through the Diels-Alder (DA) reaction over Bronsted acid sites

28

15

and

. Alkyl furan and

benzofuran are the dominant liquid products observed from the trials at lower reaction temperatures. As the temperature increases, furfural and alkyl furan are gradually consumed while benzofuran initially increases and then gradually decreases. This is because alkyl furan can undergo the DA reaction, which leads to more benzofuran initially. As the reaction proceeds,



benzofuran is further converted into aromatics or naphthalene, 19 which results in more aromatics accompanied by less benzofuran in the final products as observed in Fig. 5. Meanwhile, alkyl furan could react with hydrocarbons to form aromatics or naphthalene, which may be the main route for the formation of aromatics at the initial stage. C1-C5 hydrocarbon gas products are probably generated in the ring-opening of abundant alkyl furan or furfural, which is observed at the reaction temperatures below 400 °C in Fig. 6. Once the temperature reaches 400 °C, furfural and alkyl furan become limited, and these intermediate hydrocarbons disappear and are ultimately converted into aromatics or naphthalene. After the reaction of 60 min at 400 ºC, the distribution of liquid products becomes steady and the maximum methane conversion is achieved. Decrease in methane conversion after 1 h of reaction at 400 ºC may be due to the demethylation of branched aromatics such as toluene, xylene and methyl naphthalene, which removes the methyl group in the form of methane. CO and CO2 are the dominant gas products with trace amount of H2 at 400 °C as shown in Fig. 6. The formation of CO is due to the decarbonylation of furfural. CO may also be released during the ring-opening and deoxygenation of alkyl furan or benzofuran. The formation of CO2 might be involved in the secondary reactions of carbon monoxide such as Boudouard reaction and water-gas shift reaction (WGSR), which consumes the water that is produced in the DA reaction of furfural or alkyl furan. Hydrogen is probably formed during aromatization. Based on these observations, a reasonable reaction pathway for the conversion of furfural to aromatics under methane environment is hypothesized in Fig. 7, which requires further verification in our future mechanistic study. Catalyst characterization The XRD patterns of these synthesized catalysts are shown in Fig. 8a. Typical peaks of ZSM5 are obviously detected in all the catalysts, implying that the doped metals species are


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highly dispersed on the support or in the channel of the zeolite support, and cause insignificant distortion of the framework. Since the platinum has a higher absorption coefficient for the XRD, the principle diffraction peaks at 2θ = 40° and 46° attributed to Pt species (platinum oxide and platinum) were detected on the Pt/ZSM5 catalyst.44 As shown in Fig. 8b, it’s worth noting that the intensity of the peaks originating from (101) and (020) crystal planes of ZSM5 (JCPDS file No. 79-2401) are significantly reduced under N2 environment while well maintained under CH4 environment when compared with that from fresh Zn-Ga/ZSM5 catalyst. This is probably because Zn and/or Ga species might migrate away from these two planes on which they are originally deposited upon reaction, increasing the crystallinity under CH4 environment. On the contrary, the doped metal ions agglomerate to some degree under N2 environment, which leads to the reduction of the crystallinity. Namely, methane may facilitate the dispersion of metal species during the reaction. The surface acidity of various catalysts employed in this work is investigated by using pyridine as the probe and recorded in terms of DRIFT spectra in Fig. 9. The peaks at 1440 cm-1 are attributed to Lewis acid sites

45-46

, while the signals at 1533, 1538, 1542 and 1558 cm-1 are

due to Bronsted acid sites upon pyridine adsorption. The peaks at 1485 cm-1 are assigned to coordinately bonded pyridine (pyridine and pyridinium ion) due to the Lewis and Bronsted acid sites.47 The broad peaks around 1600 cm-1 (at 1588 and 1604 cm-1) are assigned to the hydrogenbonded pyridine48 closely associated to Bronsted acid sites, and some small peaks near 1619 cm-1 are due to pyridinium H-bonded with pyridine, which are assigned to Lewis acid sites.47-49 The tiny peaks at 1641cm-1 due to pyridinium H-bonded with pyridine and the ZSM-5 framework correspond to the Bronsted acid sites.45 Spectra of all catalysts display the characteristic peaks from pyridine molecules coordinated at Lewis sites and pyridine protonated at Bronsted sites,



with similar distribution. However, these acid sites have distinctive acidic properties as shown in NH3-TPD curves in Fig. 10 and Table 5. The peaks in the low temperature region (< 300 ºC) are due to the desorption of NH3 molecule from the weak acidic sites, and the peaks located in the range of 300-400 ºC are indicative of the presence of sites with medium acidity,50 while the peaks at high temperature (> 400 ºC) probably originate from the NH3 molecule desorbed from strong surface acidic sites.51-52 The loading of Zn on the zeolite support significantly increases the total acidic site densities (by 61%), especially weak acidic sites. The broad desorption response of weak acid sites for Zn-containing catalysts suggests the presence of different Zn species.22 Although the total acidic sites are reduced due to the addition of Ga, the content of weak acid sites increases greatly. All weak acidic sites almost disappear after introducing precious metal species, Ag, Pt, Pd, Ir, on HZSM5, instead the content of moderate acidic sites is increased significantly. It is notable that both Zn/ZSM5 and Zn-Ga/ZSM5 show a similar ratio of weak to strong acidic sites, but the acidic strength of Zn-Ga/ZSM5 is weaker than that of Zn/ZSM5 due to the co-loading of Ga. Combing the catalytic performance over these catalysts listed in Table 1, it is speculated that an abundance of acidic sites and an appropriate proportion of sites with weak acidity might facilitate the formation of desired BTX products during furfural conversion. As shown in Fig. 11, the loaded metal species on the fresh catalyst are uniformly distributed over the zeolite support, which is further evidenced by EDS mapping of Zn (Fig. 11d) and Ga (Fig.11g) elements. Based on the TEM images in Fig. 11a-c, the average sizes of metal species on the fresh catalyst and spent catalysts from CH4 and N2 runs are estimated to be 5.7 (±0.2), 3.6 (±0.1), and 8.5 (±0.3) nm, respectively. The metal particles become smaller in the presence of methane but larger under N2 environment, compared to the fresh one. Besides, it’s


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also observed that these particles are more dispersed under methane (Fig. 11b) while agglomerating to a certain extent after the reaction under N2 atmosphere (Fig. 11c). This distinct change in the particle size and dispersion demonstrates that methane is beneficial for high and uniform dispersion of metal species throughout the surface. Comparing the spatial distributions and local enrichments of Zn and Ga over the fresh catalyst, the content of Zn (Fig. 11e) and Ga (Fig.11h) is less on the surface of catalyst collected from the CH4 run. However, the Zn and Ga species on the spent catalyst surface collected from the N2 trial (Fig. 11 f and i) is significantly enriched compared to those of the fresh catalyst. These observations coincide with the change of particle size and dispersion observed by the TEM investigation, which strongly support that the existence of methane has a positive effect on the metal distribution during the reaction and thereby contributes to the better catalytic performance on the furfural conversion. The surface chemical environment of fresh and spent catalysts are explored using XPS to conduct specific scans at Zn 2p, Ga 2p, C 1s and O 1s regions, and their corresponding XPS spectra are shown in Fig.12. As shown in Fig.12a, the Zn 2p region has two spin-orbit peaks appearing at 1022.75 and 1045.83 eV, which are attributed to Zn 2p3/2 and Zn 2p1/2 of ZnO, respectively.53-55 The spin energy separation between them (23.08 eV) is in line with the reported data (∆= 23 eV)53. These results are indicative of the presence of ZnO species (+2 oxidation state) on the catalysts. The peak centered at 1118.34 eV in Fig.12b is attributed to Ga3+ ions bound to O,56

57

which is slightly higher than the reported binding energy (1117.15 eV) of the Ga 2p3/2

state.58 This peak is rather symmetric, implying the chemical state of Ga species on the catalyst surface is uniform. The increased binding energy is probably related to the strong covalent bonding characteristic between the highly dispersed Ga species (GaO)+ and the framework of ZSM5 zeolite.59 The asymmetric peaks shown in the O 1s region of Fig. 12c are further



deconvoluted into two components. The major contribution of oxygen at 532.63 eV matches the characteristic of singly bonded oxygen in SiO2 abundant in ZSM5 support,60-61 and the minor shoulder peak at lower binding energy (530.74 eV) is assigned to M-O-M bonds (M: metal) of metal oxides (ZnO, Ga2O3). Compared to those on the fresh catalyst, it is clearly noted that the intensities of peaks assigned to Zn and Ga species are slightly reduced under CH4 environment while significantly increased under N2 environment, which highly coincides with the observation in O 1s region that the component for metal oxides is a little lower (from 7.35% to 7.22%) under methane environment and higher (from 7.35% to 8.39%) under nitrogen environment. This interesting phenomenon can be explained by the possibility that Zn and Ga species might readily migrate from external surface into the interior of zeolite porous structure in the presence of methane, while these metal species probably agglomerate to form the big particles on the catalyst surface under N2 atmosphere. This explanation is quite reasonable and evidenced by the observations in the XRD pattern in Fig. 8b, the TEM images and EDS mapping of Zn and Ga elements in Fig. 11. The migration of metal species from exterior surface into inner pores might contribute to its better catalytic performance under methane. Enhanced concentration of active metals inside the inner pores may be beneficial for methane activation and furfural conversion within zeolite channels, providing more intermediate moieties for the subsequent reaction, which would lead to higher yield of liquid products, especially BTX, as observed in Table 2. The C 1s XPS spectra of the catalysts in Fig. 12d is deconvoluted into three components, which are assigned to carbon atoms in different functional groups: C-C at 284.80 eV, C-O at 285.91 eV, and C=O at 288.85 eV. 62 63 64 The peak at 284.80 eV could be due to the existence of adventitious carbon such as coke. Three components are present on the fresh catalyst due to atmospheric contamination.65 The concentration of the component at 284.80 eV compared to that


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of fresh catalyst increases after reaction under both CH4 and N2 environments, suggesting coke formation during the furfural conversion. The content of the component at 284.80 eV from the methane run is slightly higher than that from its N2 counterpart, thereby the modification of the catalyst to reduce the coke formation would be a critical future work. The peaks at 285.91 and 288.85 eV are probably associated with the oxygen-containing hydrocarbon intermediates derived from the furfural conversion. Compared to the nitrogen run, less intermediates are absorbed on the catalyst surface under methane environment, implying more intermediates are transferred into the final products and desorbed from the catalyst surface. Namely, methane atmosphere may enhance the liquid yield. Conclusion The present work demonstrates the feasibility of the coupling conversion of furfural and methane to aromatics over Zn-Ga modified zeolite catalysts at relatively mild conditions. The introduction of metal species (Zn and Ga) would promote the activation of methane and slightly reduce the gas yield. In addition to promoting the dispersion of metal species, co-feeding methane can significantly improve the yield of the desired aromatic products, BTX. Methane incorporation into the furfural conversion towards aromatics is evidenced by

13

C, 1H, and 2H

NMR spectroscopy. The results of isotopic labeling experiments suggest that methane preferentially incorporate into the phenyl ring and benzylic carbon sites of the formed products during the reaction. Comprehensive characterizations show that the good catalytic performance of Zn-Ga/ZSM-5 on furfural conversion may be ascribed to high dispersion of metal species, the proper ratio of weak to strong acidic sites, and the relatively stable chemical state. Our mechanistic investigation of this novel process will provide valuable insights into the mechanism



of furfural upgrading, and open perspectives for valorization of biomass-derived furans with cheaper alternatives.

Acknowledgements We gratefully acknowledge the financial support from Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant Program (RGPIN/04385-2014).


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Tables Table 1. The selectivity of liquid products (C mol%), gas yield (wt.%), and methane conversion (%) collected over various supported metal catalysts

Zn/ ZSM5

Ga/ ZSM5

Ag/ ZSM5

Pt/ ZSM5

Ir/ ZSM5

Pd/ ZSM5

Zn-Ga/ ZSM5

BTX

61.1

60.2

54.8

57.7

55.2

52.7

68.4

C9 aromatics

9.1

9.7

11.1

10.0

11.7

11.1

8.0

C10 aromatics

2.5

1.5

1.5

1.8

1.6

2.4

1.8

Naphthalene

23.2

25.3

25.2

23.2

25.6

28.4

17.6

Benzofuran

2.8

1.9

5.6

5.7

4.5

4.0

3.7

Others

1.3

1.3

1.8

1.6

1.5

1.4

0.6

Gas yield (wt.%)

52.8

55.9

57.1

56.9

56.8

57.0

53.3

Methane conversion (%)

0.5

1.3

1.3

0.9

0.8

0.3

1.0

Catalysts

Selectivity (C mol%)

BTX: benzene, toluene, xylene (tiny proportion of ethylbenzene); C9 and C10 aromatics refer to monoaromatic with 9 and 10 carbons; Naphthalene: naphthalene and alkyl naphthalene; Benzofuran: benzofuran and alkyl benzofuran; Others: phenol and trace amounts of oxygenated compounds. Reaction conditions: 0.10~0.12 g of furfural with 1.0 g catalyst, 400 °C, 2.5 MPa, 60min.



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Table 2. The catalytic performance over HZSM5 and Zn-Ga/ZSM5 catalysts under N2 and CH4 environments

Zn-Ga/ZSM5

HZSM5

CH4

N2

CH4

N2

The yield of liquid products (C mol%) BTX

52.7

38.6

32.8

30.5

C9 aromatics

6.2

4.2

6.8

5.1

C10 aromatics

1.4

0.8

1.0

0.8

Naphthalene

13.6

8.7

13.6

13.0

Benzofuran

2.8

1.7

1.7

1.2

Others

0.5

0.3

0.8

0.6

The yield of gas products (C mol%) CO

19.5

27.5

30.6

30.3

CO2

10.5

6.5

4.9

4.9

Coke yield (wt.%)

12.8

11.7

10.8

13.6

Methane conversion (%)

1.0

N/A

0.1

N/A

Reaction conditions: 0.10~0.12 g of furfural with 1.0 g catalyst, CH4 and N2 environments, 400 °C, 2.5 Mpa, 60min. BTX: benzene, toluene, xylene (tiny proportion of ethylbenzene); C9 and C10 aromatics refer to monoaromatic with 9 and 10 carbons; Naphthalene: naphthalene and alkyl naphthalene; Benzofuran: benzofuran and alkyl benzofuran; Others: phenol and trace amounts of oxygenated compounds.


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Table 3. 13C liquids NMR peak area ratio with respect to CDCl3 of the products from furfural conversion under 13CH4 and CH4 environment

Chemical shift/ppm

Peaks assignment

137.44

13

CH4

CH4

Phenyl carbon directly bonded with alkyl substituent groups

0.008

0.009

129.03

Ortho positions of the substituted phenyl ring

0.064

0.054

128.32

Benzene ring /Meta positions of the alkyl substituted phenyl ring

0.210

0.140

125.41

Para positions of the substituted phenyl ring

0.035

0.019

0.055

0.024

30.15, 21.72

alkyl

alkyl

Benzylic carbon



Page 26 of 45

Table 4. 1H liquids NMR peak area ratio with respect to CDCl3 of the products from the

The type of proton

Chemical Shifts/ppm

Furfural + CH4

Furfural + CD4

aromatic

7.0-8.0

5.57

1.53

benzylic

2.0-3.0

2.26

0.51

Alkyl

0.0-2.0

4.86

4.89

reactions between furfural and CH4/CD4


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Table 5. Characterization of acid sites of various supported metal catalysts by NH3-TPD Catalysts HZSM5 Ga/ZSM5 Ag/ZSM5 Zn/ZSM5 Pt/ZSM5 Pd/ZSM5 Ir/ZSM5

Zn-Ga/ZSM5

Acid site type

Peak position (ºC)

Acid site content (%)

WA a SA a WA SA MA a SA WA SA MA SA

251 424 251 424 322 389 b 233, 279 378 b 305 425

37.2 62.8 52.5 47.5 41.6 58.4 57.5 42.5 47.7 52.3

MA WA MA

340 252 320

100.0 6.5 64.3

SA

454

29.2

WA SA

229, 275 376 b

57.7 42.3

a

Total sites ratio c 1.00 0.63 0.56 1.61 0.51 0.13 1.10

1.47

WA, MA, and SA refer to weak, medium and strong acidity. The desorption peak of sites with strong acidity shifts to around 380 ºC due to the loading of Ag and Zn species.22 c Total sites ratio is NH3-TPD peak ratio with respect to HZSM5, and calculated based on the area of the deconvoluted peaks. b



Figures

Figure 1. Stability test of Zn-Ga/ZSM5 for the furfural conversion to aromatics under a methane environment. reaction conditions: mass ratio of furfural to catalyst is 0.11~0.12, 2.5 MPa methane at 400 °C. the errors of each conversion are estimated to be within ±3%.


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Figure 2. Mass spectra of major products (C6H6, C7H8, C8H10, and C10H8): (a) with the natural abundance of 13C and (b) formed form 13CH4 and furfural over Zn-Ga/ZSM5 (400 ºC, 60min) with the estimated isotopic composition (mol%).



Figure 3. The

13

C liquids NMR spectra (a) and its zoom-in views in 120-140 ppm and 12-36

ppm chemical shift regions (b) of the liquid products collected from the reactions between furfural and 13CH4 /CH4. The peak at 77.3 ppm in (a) belongs to the internal reference CDCl3.


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Figure 4. The 1H NMR spectra (a) and 2H NMR spectra (b) of the liquid products collected from the reaction between furfural and CH4/CD4. The peak at 7.24 ppm was attributed to the internal reference CDCl3.



Figure 5. The selectivity of liquid products collected in the furfural conversion into aromatics at 400 ºC under a CH4 environment. Alkyl furan: furan and alkyl furan; Aromatics: C6~C10 monoaromatics and small amount of indene derivatives; Naphthalene: naphthalene and alkyl naphthalene; Benzofuran: benzofuran and alkyl benzofuran. reaction conditions: 0.10~0.12 g of furfural with 1.0 g catalyst, 400 °C, 2.5 Mpa, CH4 environment.


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Figure 6. The yields of gas products and methane conversion collected in the furfural conversion into aromatics at 400 ºC under a CH4 environment.



Figure 7. Proposed reaction network for the furfural conversion to aromatics under methane environment over Zn-Ga/ZSM5 at 400 °C.


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Figure 8. Powder XRD patterns of (a) various fresh catalysts synthesized by incipient wetness impregnation and (b) fresh HZSM5 and Zn-Ga/ZSM5, as well as spent Zn-Ga/ZSM5 catalysts collected under CH4/N2 environments.



Figure 9. DRIFT spectra collected over various fresh catalysts upon pyridine adsorption.


Page 36 of 45

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Figure 10. NH3-TPD profiles and Gaussian deconvoluted peaks of various fresh catalysts.



Figure 11. The TEM images of fresh Zn-Ga/ZSM5 catalyst (a), spent Zn-Ga/ZSM5 catalyst collected under environments of CH4 (b) and N2 (c). EDS mapping analysis of Zn for fresh ZnGa/ZSM5 catalyst (d), spent Zn-Ga/ZSM5 catalyst collected under environments of CH4 (e) and N2 (f). EDS mapping analysis of Ga for fresh Zn-Ga/ZSM5 catalyst (g), spent Zn-Ga/ZSM5 catalyst collected under environments of CH4 (h) and N2 (i).


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Figure 12. Zn 2p (a), Ga 2p (b), O1s (c) and C1s (d) XPS spectra of fresh Zn-Ga/ZSM5 (black line), spent Zn-Ga/ZSM5 collected from CH4 (red line) and N2 (blue line) runs. C1s and O1s XPS spectra are fitted with Lorentzian-Gaussian peaks of 30:70 ratio. Dotted line represents the fitting curve, solid line represents the experimental data. The fitted peak positions and concentration are indicated on the spectra. 39 ACS Paragon Plus Environment


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Enhanced BTX yield by coupling bio-derived furfural and methane over supported Zn-Ga catalyst


Catalytic Valorization of Furfural Under Methane ... - ACS Publications

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