Co-Electrospun VTiOx Hollow Nanofibers for ... - ACS Publications

Subscriber access provided by GUILFORD COLLEGE

Article

Co-Electrospun VTiOx Hollow Nanofibers for Selective Oxidation of Methanol to High Value Chemicals Chunlei Zhang, Ping Wu, Guojuan Liu, Zhigao Zhu, and Gaofeng Zeng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01094 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44 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

ACS Applied Nano Materials

Co-Electrospun VTiOx Hollow Nanofibers for Selective Oxidation of Methanol to High Value Chemicals Chunlei Zhang,† Ping Wu,† Guojuan Liu, † Zhigao Zhu, †, ‡ Gaofeng Zeng*† , § † CAS

Key Laboratory of Low-carbon Conversion Science and Engineering, Shanghai

Advanced Research Institute, Chinese Academy of Sciences, 100 Haike Road, Shanghai 201210, China. ‡ State

Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

Technology, 92 West Dazi Street, Harbin 150090, China § School

of Chemical Sciences, University of Chinese Academy of Sciences, 19A

Yuquan Road, Beijing 100049, China

*Corresponding Author: Tel/Fax: +86 21 20608002 Email: [email protected]

ACS Paragon Plus Environment

ACS Applied Nano Materials 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

ABSTRACT Direct oxidation of methanol to high value oxygenated chemicals on V2O5/TiO2 based catalysts is a green alternative to the commercial methods. But the V-Ti based catalysts are challenged by the relatively low reactivity. Herein, we demonstrated a facile and practicable strategy to directly fabricate hollow structure VOx/TiO2 nanofibers (VTO-HNF) through the co-electrospinning and calcination method. The effects of V/Ti ratio on the structure, the surface chemistry and methanol oxidation performance were investigated in detail. VTO-HNFs with appropriate V/Ti ratio have hierarchical structure, large surface area, uniform nanoparticles, narrowed grain sizes, and high dispersions of vanadium. Moreover, both the oxidation properties and the acidities of the catalyst were enhanced due to the high content of surface V4+. The VTO-HNF catalysts featured excellent activities and reliable stability for the selective oxidation of methanol to methyl formate (MF) and dimethoxymethane (DMM). The MF yield reached 90% at 150 oC, which is comparable with the noble metal based catalysts. In-situ NAP-XPS investigations suggested that the variable valence of vanadium was involved in the oxidation of methanol. KEYWORDS. Hollow nanofibrous catalyst; Electrospinning; Vanadium oxide - titanium oxide; Methanol oxidation; Methyl formate


Page 2 of 44

Page 3 of 44 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


1. INTRODUCTION Selective oxidation of methanol to valuable oxygenated chemicals is a promising route to expand the methanol utilization in the upcoming “liquid sunshine economy”.1-2 Among the products of methanol oxidation, the higher carbon chemicals like methyl formate (MF) and dimethoxymethane (DMM) are more attractive than formaldehyde and formic acid due to their high-value and versatility in the cosmetics, pharmaceuticals, paints, and rubber industries.3 Comparing with the toxic reagents and corrosive catalysts involved industrial synthesis for MF and DMM, in addition, direct oxidation of methanol on heterogeneous catalysts represents a simple, non-corrosive, and economical process.2 V2O5/TiO2 catalyst is a promising candidate owing to its better availability than noble metal based catalysts and its relatively higher reactivity in comparison with other nonprecious catalysts. 1, 4-9 The structure of V2O5/TiO2 based catalysts, involving particle size, vanadium dispersion and mass diffusion pathways, is considered as the critical factor to impact the reactivity in methanol oxidation.10 The catalyst with small particle size would provide large surface area and more active sites, which is beneficial to the dispersion of vanadium and suppress the formation of crystal V2O5.2, 10-14 Various methods like wet impregnation, sol–gel, rapid combustion and co-precipitation have been proposed to enhance the structure and reactivity of catalysts. 2, 10, 15 However, it is still highly desired to promote the catalysts properties with uniform and narrowed particle size, high dispersion of active sites and low mass transfer resistance through a simple,



controllable and green preparation method. Constructing a catalyst with a hierarchical structure, which contains well-organized macro- / meso- and micro-pores, is an efficient approach to provide a large surface area as well as unobstructed diffusion pathway.16-20 It can be achieved by electrospinning technology. Electrospinning is a facile and powerful pre-moulding method to fabricate large-size fibrous materials, which takes great advantages of high homogeneity, facile processing and easy to scale-up.21-24 The electrospun nanofibers have properties of high accessible hierarchical porous structure, large-sized appearance, high surface area, huge aspect ratio and easy functionalization, which are beneficial to mass transfer, the active sites distribution and adsorption capacity.25 The nanofibers prepared by electrospinning have been widely employed in the pollutants adsorption, waste-water treatments and electrode materials.21, 26-27 In addition, the nanofibrous materials also displayed good flexibility in the catalysis applications of NH3 selective catalytic reduction 28,

VOC oxidation29, and photo- / electro-catalysis. 30 Therefore, it is highly attractive to

improve the V-Ti oxide based catalysts for methanol oxidation by the electrospinning method. In this work, we designed and demonstrated a facile strategy to directly fabricate the hollow structure VOx/TiO2 nanofiber (VTO-HNF) catalysts, comprised of hierarchical structure, large and accessible surface area, uniform and narrowed nanoparticles, highly dispersed vanadium oxide and appropriate acidity, through co-electrospinning and


Page 4 of 44

Page 5 of 44 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


calcination. The resultant VTO-HNF catalysts exhibited excellent activity and reliable stability for the selective oxidation of methanol to MF and / or DMM. The methanol oxidation on VTO-HNF was also investigated by in-situ NAP-XPS. 2. EXPERIMENTAL SECTION

2.1 Materials Polyvinylpyrrolidone (PVP, Mw=1,300,000), ethanol (99.0 wt.%), acetic acid (36.0 wt.%), N, N-dimethylformamide (DMF, 99.5 wt.%), tetrabutyl titanate (TNBT, 99.0 wt.%) and vanadyl acetylacetonate (VO(acac)2, 98.0 wt.%) were obtained from Aladdin Chemical Reagent Co. Ltd., China and used as received without further purification. The gas mixture (9.97 vol.% O2 balanced with N2) was obtained from Shanghai Weichuang Gas Co.

2.2 Nanofibrous catalyst preparation The VOx/TiO2 nanofibers with different V/Ti molar ratios were prepared by the combination of co-electrospinning and calcination.25, 27 In a typical synthesis, 7.5 g TNBT and 1.5 g VO(acac)2 (V/Ti molar ratio is 1:4) were dissolved in a mixed solution containing 15 mL ethanol, 20 mL acetic acid and 130 mL DMF and then kept stirring for 20 minutes. Subsequently, 21.5 g PVP was added into the mixture solution to obtain a homogeneous gelatine sol under the help of stirring. The electrospinning process was performed at a fixed voltage of 20 kV and an



injection rate of 1.0 mL h-1 with a work distance of 20 cm between the needle tip and the round aluminium foil collector. The chamber temperature and relative humidity during electrospinning were 23 ± 2 oC and 42 ± 3%. The as-electrospun nanofibers were dried at 60 oC in vacuum for 3 h and then calcined at 400 oC for 6 h in air with a heating rate of 2 oC min-1 to remove the organic chemicals. The VOx/TiO2 nanofibers with different V/Ti molar ratios (i.e. 1:5 and 1:3) were prepared by the same method but adjusting the amount of VO(acac)2. The resultant VOx/TiO2 hollow nanofibers were denoted as VTO-HNF-x, where x represents the V/Ti ratio i.e. 1/5, 1/4 and 1/3 in this work.

2.3 Methanol oxidation The methanol oxidation was carried out in a tubular fixed-bed stainless steel reactor (inner diameter 8 mm) under atmospheric pressure.20 1.0 g nanofibrous catalyst was packed in the center of the reactor. The catalysts were firstly treated in 10 vol.% O2 – 90 vol.% N2 at reaction temperature for 0.5 h. Then the liquid methanol was fed into the reactor through a vaporizer at 120 oC by a constant flow pump (Elite P230II). The reaction products were analyzed by two series-connected on-line gas chromatographies (Shimazu GC−2014) with thermal conductivity detector (TCD). Methanol, DMM, MF, formaldehyde, dimethyl ether and water were analyzed with a Porapak T column, while the oxygen, nitrogen, CO2 and CO contents were analyzed with a Porapak N column and a MS-13X column. The gas lines between reactor and GC were kept at 120 oC. The


Page 6 of 44

Page 7 of 44 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


product selectivity, Si, was calculated on the molar of the carbon molar base: Si (%) =

ni/Σni × 100%, where i represents product CH3OCH2OCH3, HCOOCH3, HCHO, CH3OCH3, CO, CO2, ni is the carbon atom molar of the specific product i.

2.4 Characterizations The catalyst structure was determined by X-ray powder diffraction (XRD, Rigaku Ultima IV) using Cu K radiation (= 0.15406 nm, 40 kV, 40 mA). The morphology of the catalysts was characterized by transmission electron microscope (TEM, JEM-2100, 200 kV) and scanning electron microscope (SEM, Zeiss SUPRA 55 SAPPHIRE, 2-20 kV). The surface areas and pore structure of the samples were derived from N2 sorption carried out on an automatic micropore physisorption analyzer (TriStar II 3020). Raman spectroscopy measurements were performed using a Renishaw Raman spectrometer using a 12.5 mW laser source at an excitation wavelength of 532 nm. The compositions of the samples were analyzed by X-ray fluorescence (XRF, Rigaku ZSX Primus II). The sub-surface elemental distribution of catalyst was measured by an energy-dispersive spectroscopy (EDS, Oxford Instrument) attached onto the scanning electron microscope. The near-surface chemical information about materials was analyzed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Al Kα radiation, 1486.6 eV, 12 kV, 3 mA). XPS peak positions were calibrated with the help of the C 1s peak at 284.8 eV. A Micromeritics AutoChem II 2920 apparatus, equipped with a thermal conductivity detector (TCD), was used for hydrogen temperature- programmed reduction (H2-TPR)



analysis. The temperature- programmed desorption of ammonia (NH3-TPD) was performed in a fix-bed reactor. The in-situ near atmosphere pressure XPS (NAP-XPS) measurements were carried out on the spectrometer equipped with PHOIBOS semi spherical electron energy analysis, focusing (spot size ~300 μm) monochromatized (Al Kα) X-ray light source and IQE-11A ion gun and infrared laser heater produced by SPECS. Before exposed to the reaction atmosphere, the catalyst was pretreated under 0.2 mbar N2 at 200 oC for 1.0 h. Then the signals of the sample under ultra-high vacuum (UHV) were recorded. After that, 0.4 mbar CH3OH was introduced into the in-situ NAPXPS reaction cell and the temperature was controlled from 30 to 170 oC. During the mixture test, O2 with various partial pressure of 0.1, 0.3, and 0.6 mbar was introduced into the reaction cell at 160 oC. 3. RESULTS AND DISCUSSION

3.1 Structure and texture of catalysts The fabrication procedure of the nanofibrous catalysts is shown in Figure 1A. PVP / VO(acac)2 / TNBT as-electrospun nanofibers were fabricated by the co-electrospinning of V and Ti sources and then the VOx/TiO2 hollow nanofibers (VTO-HNF) were obtained by calcinating the as-electrospun nanofibers in air at 400 °C. The resultant VTO-HNFs were denoted as VTO-HNF-x, where x represents the V/Ti ratio in the preparation recipes i.e. 1/5, 1/4 and 1/3, respectively. The morphologies of nanofibers before and after calcination were monitored by a scanning electron microscope (SEM) and a


Page 8 of 44

Page 9 of 44 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


transmission electron microscope (TEM), as shown in Figures 1B − D and S1. Before calcination, the as-electrospun nanofibres of all samples have high aspect ratio and random orientation, which intricately constructed the 3D network without obvious aggregation. Uniform nanoparitcles were observed from the surface of nanofibers (Figure 1B inset). With the increase in the V/Ti ratio from 1/5 to 1/3, the diameter of the as-electrospun nanofibers increased from ca. 350 to 600 nm (Figures 1B & S1). The increase in the nanofiber diameter might be attributed to the higher viscosity of the electrospinning solution caused by strong chelating between VO(acac)2 and PVP at a higher salt concentration.21 After calcination, the well-defined fibrous morphology was retained and all of the nanofibers were well separated without obvious agglomeration (Figures 1C & S1). However, the diameters of the nanofibers markedly shrank due to the decomposition of the organic component in the as-electrospun nanofibers (Figure S1). For example, the fiber diameter of VTO-HNF-1/4 decreased from 600 to 220 nm (Figure 1C). Moreover, the nanofiber surface was covered by highly dispersed nanoparticles with uniform size of ca. 10 nm. The particle size is smaller and the size distribution is more homogenous in comparison with that synthesized by common methods like precipitation.2 As the nanofiber cross-section shown, interestingly, a concentric cable like shell-hollow-core structure was formed for the calcianted nanofibers, in which the thickness of shell was ca. 40 nm and the diameter of inner core was ca. 80 nm (Figures 1C & S1). This hollow



structure may be caused by the mismatch of decomposition rates between out surface and inner bulk of as-electrospun nanofiber during calcination. The core-shell hollow structured VTO-HNF was further comfirmed by TEM observation of single fibre, as shown in Figures 1D & S1. In the case of VTO-HNF-1/4, the diameter of the core fiber and the thickness of shell was ca. 80 nm and 20 nm, respectively, with a hollow space of ca. 50 nm, in line with the SEM observations. In addition, the clear boundaries between particles in the TEM images evidenced that both shell and core of the nanofiber were composed by the uniform nanoparticles with dimensions of ca. 10 nm. In addition, TEM results also indicate that the interactions between the nanoparticles and the fibers was compelling because the particles survived the grinding and sonication treatments in the sample preparation. Unlike the common preparation methods, the morphology observations reveal that the electrospinning method endowed the catalyst with hollow fiber structure and highly dispersed nanopartilces, which is predictable to impact the catalytic reactivity through enhancing surface area and intensifing mass transfer. The crystalline structure of the nanofibers were characterized by TEM and X-ray diffraction (XRD). As shown in the high-resolution TEM images, TiO2 exsited as anatase phase (101) with the interplanar spacing of 3.5 Å and rutile phase (110) with a lattice distance of 3.2 Å (Figure 2A & Figure S2), which was further comfirmed by the selected area electron diffraction (SAED) pattern of VTO-HNF-1/4 (Figure 2A inset). No vanadia crystalline structure was observed from the HRTEM image even for the sample with high


Page 10 of 44

Page 11 of 44 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


V/Ti ratio of 1:3. This suggests that the size of vanadia species was extremely small and/or they were highly dispersed on the surface of titanium with the help of electrospinning method. The crystalline structure of the hollow nanofibers were further measured by X-ray powder diffraction (XRD), as shown in Figure 2B. In line with the TEM analysis, all samples exhibited the characteristic diffraction peaks of both rutile and anatase TiO2. Moreover, vanadia crystalline structure was undetectable, confirming that vanadium species existed as amorphism and / or the crystalline size is out of the detection limit of XRD. The phase transform of TiO2 depends on the calcination temperature and it is prefer to forming rutile TiO2 at > 500 °C. 8, 31 However, the rutile phase TiO2 formed at the calcination temperature of 400 oC in this case. It is reasonable that the strong exothermal decomposition of PVP elevated the real temperature of the inner part of aselectrospun nanofibers, which therefore induced phase transition of TiO2 from anatase to rutile phase.2 In addition, the temperature gradient between nanofiber surface and bulk would lead to different decomposition rates of organic chemicals and thus resulted in a hollow structure. With increasing the V/Ti ratio from 1:5 to 1:3, furthermore, the intensities of the rutile TiO2 signals gradually weakened, indicating that the content of vanadia affects the transformation of anatase to rutile. Since the existence of vanadium in nanofibers has been proved by multiple methods in Table 1, on the other hand, it confirms that the electrospinning method promoted the dispersion of the vanadia species



and supressed the growth of crystalline V2O5. Figure 2C showed the N2 sorptions isotherm of hollow nanofibers. All samples show a steep increase at P/P0 around 0, which is due to the filling of micropores.19, 32 In addition, the samples exhibited type IV isotherms with clear H3 hysteresis loops in the P/P0 >0.45 region owing to capillary condensation in the mesopores.33 The corresponding pore size of the samples were concentrated at 4 - 25 nm (Figure 2D). The average pore size of the nanofiber samples slightly decreased from 8.9 nm for VTO-HNF-1/3 to 6.9 nm for VTOHNF-1/5 (Table 1). Combined with the hollow spacing and open porous texture, the nanofibers possessed micro-meso-macro hierarchical pore structure, which is beneficial to fast mass diffusion in the catalytic reaction. With decreasing vanadium content in catalysts, the Brunauer–Emmett–Teller (BET) surface area of the catalysts increased remarkably from 52.7 m2g-1 for VTO-HNF-1/3 to 71.5 m2g-1 for VTO-HNF-1/4 and futher reached 79.4 m2g-1 for VTO-HNF-1/5 (Table 1).

3.2 Chemical properties of hollow nanofibers The elemental composition of the bulk and the surface of nanofibers were measured by X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectrometer (EDS). The bulk V/Ti ratio of catalysts decided by XRF were 0.33 for VTO-HNF-1/3, 0.24 for VTO-HNF-1/4 and 0.21 for VTO-HNF-1/5, respecitvely, which are highly close to the chemical ratios in the synthesis recipes i.e. 0.33, 0.25 and 0.21 (Table 1). This suggests that the target elements were completely retained with the


Page 12 of 44

Page 13 of 44 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


electrospinning method. It is well known that the reangent ultilizations normally are quite low for the nanoscaled VOx/TiO2 catalyst preparation by the co-precipitron methods34, leading to high waste emmision, low product yeild and high cost. In contrast, the electrospinning method not only leads to a good economical with complete conversion of V and Ti but also exhibits a more accurate control capacity on the composition than the common catalyst preparation methods. These merits are convenient for quality control and scale-up in real practices. The out surface V/Ti ratios of the nanofibers measured by XPS were much higher than those in the bulk, revealing that vanadia was enriched on the nanofiber surfaces (Table 1). The composition of the nanofiber cores determined by EDS displayed similar V/Ti ratios to that of bulk (Table 1), suggesting the homogeneity of shell and inner core. The state of vanadium oxide and titanium oxide was analyzed by Raman spectroscopy (Figure 3A). The bands at 403, 523 and 639 cm−1 for all samples were attributed to Ti−O groups of anatase TiO2 while the two peaks at 445 and 610 cm−1 were assigned to rutile TiO2,35 in line with the XRD observations. The bands at 282, 301, 478, 697 and 995 cm−1 were assigned to crystalline V2O5.6 The intensity of the characteristic peaks of V2O5 gradually weakened with the decrease of V/Ti molar ratio from 1:3 to 1:5 due to the low vanadium content in the catalyst suppressing the formation of V2O5. At the same time, the band around 1026 cm−1 was ascribed to the two-dimensional polymeric net-work of octahedral sharing corners and/or edges (oligomeric VOx). 36 The intensity of oligomeric



VOx signal increased with the decrease of V/Ti ratio, implying low content of vanadium in catalyst prefer to form more oligomeric VOx. The surface chemistry of the catalysts was measured by XPS (Figure 3B). The XPS V2p3/2 spectra of all samples were composed by two peaks at the binding energies of 517.3 and 516.1 eV, which can be assigned to V5+ and V4+, respectively. 6 This reveals that vanadium existed as oligomeric VOx (1.5

Co-Electrospun VTiOx Hollow Nanofibers for ... - ACS Publications

Recommend Documents