Dinesh Gupta*,
Chandrakant Mukesh
and
Kamal K. Pant*
Chemical Engineering, Indian Institute of Technology Delhi, New Delhi 110 016, India. E-mail: kkpant@chemical.iitd.ac.in; dineshguptagkp@gmail.com
First published on 2nd January 2020
The strong interaction of higher transition metal oxides with inorganic non-metals can be promising for generating highly acidic three-dimensional materials by design. A comprehensive controlled acidity of heteropolyacid-like catalyst and interpretation of the microstructure and mechanism of the formation of a versatile heterogeneous solid acid catalyst, HPW4Mo10Ox has been heterogenized by biomass-derived cystine as organic linkers to control the acidity of as-synthesized materials, which have greater acidity and complexity in separation from the reaction mixture. The new and unique results obtained in catalysis done in biphasic reaction. Cystine binds to the surface of HPW4Mo10Ox, and the topotactic transition occurred, change the morphology and lattice parameter. We described here a sustainable transformation of highly acidic (0.84 mmol g−1) heteropoly acid (HPW4Mo10Ox) to cystine anchored on the active surface of the heteropoly acid and controlled the acidity (0.63 mmol g−1) and heterogenized the materials. As synthesized materials have been showing that for the direct formation of alkyl levulinate and furanics intermediate from carbohydrates. HPW4Mo10Ox and HPW4Mo10Ox-Cys, act as acidic catalyst, and catalyse the mono- and disaccharides that are dissolved in primary and secondary alcohols to alkyl levulinate (AL) and alkyl methylfurfural at 170 °C under microwave irradiation with glucose as the substrate, AL yield reaches 62% with 84.95% selectivity. The catalyst can be easily recovered by filtration and minimum five times reused after calcination without any substantial change in the product selectivity. The analytical analysis of as-synthesis materials done by NH3-TPD, BET, XRD, FESEM, TEM, HRTEM, FTIR, ATR, TGA, DTA to stabilized the morphology and acidity controlled mechanism.
Fossil-based petroleum products are currently major, gigantic players in chemicals industry for the synthesis of chemicals, fuels, and materials. However, pose foremost concerns in long smoothly future utilization due to day-by-day depletion of limited fossil resources and increasing energy demands, create huge pressure on fossil-based chemicals and fuels market.10 The search for alternative and sustainable energy is of crucial importance with the ever-growing population with increasing energy demands and environmental concerns, together with the limited reservoir of fossil fuels reserves.11 This is rise issued to the utilization of other sustainable and renewable substrate as an alternative to fossil fuels and chemicals. In this regards, lignocellulosic biomass, attract much more attention as an alternative to fossil fuels and chemicals. Direct utilization of lignocellulosic biomass has hurdles of its over-functionality and excess of oxygen. In this regards chemist and engineered designed environmentally sound reaction-design, cost-effective, and energy saving and atom efficiency chemical processes.12 Design of such type of process, catalyst always lined center-point to minimized the hazardous waste, increased the atom-efficiency and selectivity of product in an energy-efficient manner. Given these parameters, research endeavors directed towards utilization of metal salt as catalysts,13,14 but non-separation and hazardous impact on the environment is a major hurdle to effective utilization in the industry.
Recently heteropoly acid (HPA) materials attract more attention as solid acid catalysts possess and show strong Brønsted acidity.15 Studies at molecular level show that it have unique physicochemical properties, with their structural mobility and multi-functionality nature, these make it an ideal catalyst for the biofuels industry.16 Due to their higher acidity, and tunable acidic properties these are classified as a super acid17 because of its strong Brønsted acidity it showed vast application to acid catalysed reactions, HPA has attracted much interest. However, a key challenge that is often encountered is stabilization of HPA in the reaction medium and its higher activity. It is reported that the initial heat of ammonia adsorption was about 200 kJ mol−1 for H3PW12O40, which is much higher than HZSM-5 and SiO2–Al2O3 catalyst.17 HPA functionalized with surface ligands can control the physical as well as chemical properties of the resulting materials and tuned the acidity, heterogeneity, composition, and size of the material. Moreover, the ligand interactions between active catalytic site can be used to control the acidity and interparticle distance and lattice symmetry.18,19 Another strategy is to anchor active metal center on a support with a stronger metal-support interaction. Previously, several many organmetallic20 as well as nanomaterials were loaded with HPA and used as an anchored homogeneous catalyst for selective hydrogenation.21 Many nano-particles are modified with HPA and used as an active catalyst for organic synthesis. TiO2 was modified with polyoxotungstates and used as photo-catalyst for dye degradation.22 Zhang et al. used phosphomolybdic acid for stabilizing and synthesis of a platinum single-atom catalyst and used as hydrogenation catalyst.23 Furthermore, by selecting and manipulating the surface properties of HPA, it would be possible to control the size and shape of the resulting heterogeneous materials. The potential for controlling the acidity by cystine a biomass fraction, in combination with the heterogenization of HPA materials, increases the “green” credentials of the catalytic reaction process, with higher selectivity, conversion, and yield of desired product. The catalyst recovery being proposed advantages over homogeneous highly active HPA catalyst. In this research work, we investigated the biomass-based cystine as effective linkers and binders for the controlling the acidity and preparation of heterogeneous HPA based materials for the selective conversion of alkyl levulinate and other furanics intermediate.
The effectiveness of group 6 metal chlorides, CrCl3-mediated microwave irradiation of cellulose and glucose, in IL as solvent explored by many previous articles and claimed to be energy-efficient and cost-effective for the conversion of biomass into platform chemicals and fuels additive.24 Brønsted acidic ionic liquids were used for HMF production from fructose.25 But, the hazardous nature of CrCl3 is a most significant issue to utilization as a catalyst. Chan et al. explored lower group-6 metal chlorides (WCl6) with anionic liquid in the biphasic solvent system at very low temperature (room temperature to 50 °C) to HMF synthesis.26 Recently molybdenum incorporated catalyst effectively explored for biomass conversion to value-added chemicals and fuels.27,28 However, many possible inorganic materials and a metal oxide that consist of three or more elements are still mislaid from previously available research articles. A few of these materials composition are mislaid because they have not been prepared yet, whereas others are mislaid because of the inconsistent into competing phases.29 Oluwafemi et al.30, used L-cysteine ethyl ester hydrochloride as a capping agent for the synthesis of colloidal CdSe nanoparticles. Mntungwa31 et al. investigated the effect of capping agent (L-cysteine ethyl ester hydrochloride) on the size, structure, and morphology of the as-synthesized nanoparticles. Rahme32 et al. reported that synthesis of AuNPs in the presence of L-cysteine methyl ester hydrochloride as a capping agent and stored materials for 12 months no significant changes observed and reported high stability of the colloidal solution. These above findings indicate that the interesting properties of cysteine can be successfully applied toward capping agent for heterogenization of materials. The several hetrogeneuous and homogeneous catalyst were reported for the preparation of AL, AMF and their derivatives from cellulose or biomass derived building block molecules.33–35 Magnetically recyclable carbonaceous solid acid was used for syntheis of 5-ethoxymethylfurfural (EMF) from carbohydrates.36 Zhang et al. were used aluminum-based mixed-acid to intensify the yield of EMF.37 ALs fined application in different sectors of the chemical industry, like biofuels additives, bio-solvents, an additive for odorous, plasticizing agent and as building blocks for various chemical transformations.4,6,9,38 AMF not only decreases the boiling point but also preserving an increased the calorific value.39 Levulinic acid (LA), methyl levulinate (ML), ethyl levulinate (EL), propyl levulinate (PL) and butyl levulinate (BL) have a potentially versatile building block for the synthesis of several other chemicals and excellent fuel additives properties.40 As reported, EMF is an excellent fuel additive for diesel with high energy density (8.7 kW h L−1), similar to gasoline (8.8 kW h L−1), diesel (9.7 kW h L−1) and higher than ethanol (6.1 kW h L−1).41 Lew et al. reported similar energy density of EMF 30.3 MJ L−1 (8.42 kW h L−1) and other fuels like gasoline 31.1 MJ L−1 (8.64 kW h L−1), diesel 33.6 MJ L−1 (9.34 kW h L−1) and ethanol 23.5 MJ L−1 (6.53).42 In the NREL technical report mentioned the energy density of EL 24.8 MJ kg−1 and butyl levulinate is 27.1 MJ kg−1, which is show vast potential of this biomass-derived oxygenates to use as alternative fuels.43 Currently, there were several methods of the preparation of EMF and EL reported. It is undoubted that the etherification of the hydroxyl group in HMF or furfuryl alcohol with ethanol is the effective route. However, the economical, selective and practically large-scale process of EMF or AMF from HMF is limited due to the high production and separation cost of HMF. The motivation of this work is to explore the composition of mislaid materials composition and controlling the acidity of prepared materials by using a renewable binder. This preparation of materials in the field of catalysis-synthetic solid acid chemistry is also important because of the vast potential to controlled the selectivity of desired products and easily separately by reaction mixture.
Understanding the basic properties of heteropoly acids and capping of ligands will help in developing new types of solid acid materials with improved properties which should lead to, for example, dehydration, etherification and hydrogenolysis more effectively, more selectively and more efficiently. These should be capable of efficient dehydration and etherification of carbohydrate to biofuels which could help us in our future design of biobased fuels and chemicals at moderate cost and environmentally friendly manner.
(ii) Preparation of HPW4Mo10Ox-Cys: for the synthesis of HPW4Mo10Ox, with a controlled acidity and heterogenization, L-cysteine ethyl ester hydrochloride was used as a binder. In a typical synthesis of HPW4Mo10Ox-Cys, step 1 and step 2 are same as HPW4Mo10Ox only here ammonium molybdate used as a precursor in place of sodium molybdate. In step 2, 1.5 equivalent of L-cysteine ethyl ester hydrochloride dissolved in 20 mL (olyal alcohol and oleic acid 1:
1 v/v) added drop-wise and reaction mixture stirred at room temperature to 240 °C, colour change yellow then 2 mL distil water added, reddish-brown precipitate obtained, 0.5 mL chilled H2SO4 added as precipitating agent, and more precipitate obtained, separated by acetone, filter and wash with water and methanol, dry at 250 °C and labelled as HPW4Mo10Ox-Cys.
The L-cysteine ethyl ester hydrochloride was capped on the HPW4Mo10Ox surface after the synthesis ofHPW4Mo10Ox. As synthesis, catalyst showed 140–170 nm outer cores with 600–700 nm total materials size. As-synthesis material was composed of nano-sized MoOx, WOx, and incorporated with phosphorous and hydrogen determined by various techniques before and after the capping the cysteine at the surface transformation. Characterization of as-synthesized materials from TEM and HRTEM are shown in Fig. 1, offered direct visualization of the morphology of the as synthesis materials, which looks like core–shell type of morphology adopted by materials. The HPW4Mo10Ox possessed reproducibly uniform size and shape materials; the low-magnification TEM (Fig. 1c, 4 μm) image indicates excellent particle uniformity. The electron contrast between the cores and shells in the high-resolution (Fig. 1d, 100 nm) and insect of Fig. 1a and b, confirms the formation of hollow particles. The SEM image is shown in Fig. 1a and 1b (inset) demonstrates that the as-synthesis materials adopted hollow-core–shell morphology and could be prepared. Both the SEM image at a higher magnification (Fig. 1a and b) and TEM image (Fig. 1d) reveal that the hollow-core–shell were uniform in diameter and closed pack in arrangement during the sample preparation. To examine the morphology of as-synthesized materials, without capping agent and after the capping, results show in Fig. 1 and 2. Interestingly particles of two different morphologies with different sizes have been observed. Large hexagonal hollow (3D) with average diameter 0.6–0.7 μm (edge to edge) and thickness of wall ∼150 nm are seen in Fig. 1a. However, the as-synthesized HPW4Mo10Ox-Cys is small in diameter and quasi-rhombus in morphology (Fig. 2). The average particle diameter of these nanoparticles is 25 nm. cystine as capping agent favors the formation of thermodynamically more stable shape, stability in nature.
When L-cysteine ethyl ester hydrochloride used as capping agent with oleyl alcohol and oleic acid, ∼28 times size of metal particles reduce as shown in Fig. 2. Similar nanoparticle size was observed as previous report.31 Fig. 2 shows TEM images of as-synthesized HPW4Mo10Ox-Cys particles capped with cysteine. Fig. 2c–e show that cysteine capped HPW4Mo10Ox particles are quasi-rhombus, with an average size 25 nm. The corresponding some high-resolution TEM image (Fig. 2c and d) of the cysteine capped HPW4Mo10Ox nanoparticles, reveals the lock-key like an adaptation of materials. TEM images of the HPW4Mo10Ox-Cys (Fig. 2) easily visible that the metal particles embedded by the capping agent. Electron prove-micro analysis (EPMA) and energy dispersive X-ray analysis (EDX) enabled quantification of the metal content in the as-synthesized materials and the extent of metal leaching during the reactions. X-ray photoelectron spectroscopy was utilized to determine the nature of metal oxidation state within the materials.
The FT-IR spectra of as-synthesis HPW4Mo10Ox and HPW4Mo10Ox-Cys materials were to stabilize the acid strength changes due to the free and binding state of inorganic oxygen-containing anion44 are shown in Fig. 3. As it is well-stabilized the fact that heteropolyanion would be strongly adsorbed into supports.44 In the HPW4Mo10Ox, the broad peak has been investigated may be due to a hydroxyl group (3600–3400 cm−1)45 was assigned, which is missing in the HPW4Mo10Ox-Cys spectrum, indicate that hydroxyl group protected or bind with cysteine moiety. The characteristics peaks of HPA shown at 1082 cm−1 may be due to P–O interaction, metal oxygen bridged at 800–900 cm−1 range may be due to W–O–W46 and Mo–O–Mo asymmetric vibration, associated to the typical Keggin anions. At three decade ago, Nomiya et al.47, reported that immobilized HPA in the 1100–700 cm−1 showed four stretching vibration frequency due to M-terminal oxygen, M–O–M octahedral edge, corner-sharing's and heteroatom (X)-triply bridged oxygen. Khder et al.,48 reported that band at 1081, 982, 889, 797 and 595 cm−1 assigned to the stretching vibrations of P–O, W–Ot, W–Oc–C, W–Oe–W and bending vibration of P–O, respectively. Popa49 et al. reported that stretching vibration at 965 cm−1 came due to Mo–Oc–Mo binding and 595 cm−1 arise for the bending vibration P–O. In literature, well stabilized that the bands in the range of 400–1000 cm−1 are attributed to the stretching and bending vibrations of metal–oxygen characteristics.49 The absorptions band located at 711, 600 and 523 cm−1 are attributed to the Mo–O stretching vibration, reported by Song et al.50 They also reported that band at ∼1000 cm−1 is related to the terminal oxygen atoms of Mo. We are also observed the approximately same result in HPW4Mo10Ox materials, but after capping of cystine minimized the characteristics of HPA peaks. Additionally, peak at 1610 and 1627 cm−1 may be due to CO starching. Similarly as for HPW4Mo10Ox-Cys materials, resulting in the lowering the characteristic FTIR vibrations, and another 896 and 831 cm−1 vibration spectra were observed.
Raman scattering spectroscopy was useful to study the original Keggin structure and deformation in structure due to support, because it is extremely sensitive to the Keggin unit, and the support has no significant interference on the Raman scattering originated from the Keggin structure. Guo et al.51, reported that P–O bonds correspond to stretching vibrations at 1009.5 cm−1, WO bonds (993.9 cm−1) and W–O–W bonds correspond to 912.4 cm−1. The investigation also revealed that shifts of the peak position are possible due to the strong interaction between the Keggin unit and the support. As Fig. 4, show that peak at 989 cm−1 and 900 cm−1 may correspond to W
O bonds W–O–W linkage but peak at near 1010 cm−1 peaks is missing, which indicates that as-synthesized materials don't contain Keggin unit. Devassy et al.52, reported that band at 998 and 974 cm−1could be attributed to W
O symmetric and asymmetric stretching modes. They also claim that broad peak at 893 cm−1 can be assigned due to W–O–W asymmetric stretching mode. Popa et al.49, reported that symmetric and asymmetric vibrations of terminal oxygen (Mo–Ot) and corner shared bridged oxygen Mo–O–Mo are lies at 983, 882, 246 and 154 cm−1. In the literature reported that peaks at the lower range might be due to Mo–O interaction. In current work, Mo also interferes the lattice. Therefore two peaks at 796 cm−1 and 900 cm−1 observed, which is shifted than the characteristics peak of WO3 (802 and 716 cm−1),53 indicate that different bonding mode of W.
The X-ray photoelectron spectroscopy was used to determine the chemical bonding, the valence state of W, Mo and surface composition of as-synthesized HPW4Mo10Ox materials. The XPS survey spectrum of materials was shown in the Fig. 5a. The survey spectrum shows peaks for W, Mo, C, O, S and P. The Mo 3d core level spectrum show 3d3/2 and Mo 3d5/2 binding energy for as-synthesis materials is observed at 231.65 eV and 234.76 eV with spin–orbit separation of 3.11 eV and Mo d3/2/Mo5/2 ratio of 0.73, as shown in Fig. 5b, which is lesser than the typical characteristics peak of Mo6+and indicate the reduced form of Mo.54 The XPS high-resolution spectra of W 4f core levels for the as-synthesized materials shown in Fig. 5c. The binding energy for W 4f7/2 and W 4f5/2 levels are observed at 34.42 eV and 36.59 eV respectively, with spin–orbit separation of 2.17 eV and the W 4f5/2/W 4f7/2 ratio of 0.814. The energy position of the doublet is lesser than the W6+ reported in the literature55 and best matched with W5+, and indicted that W is attached with hydrogen.55 The XPS high-resolution spectra of P 2p core levels show the binding energy 132.80 eV in Fig. 5d, indicated that P–O linkage in the as-synthesized materials.55 Combined with FTIR, XRD, and TEM analysis, it is clear that the Mo ions as the doping were successfully incorporated into the crystal lattice of WO3 and maybe W–Mo boding formed, because bonding energy of W and Mo had a trend to lower, suggesting the presence of W5+ and Mo5+.56 Shpak et al., reported binding energy for W 4f7/2 and W 4f5/2 levels of tungsten atoms for W5+-sates of oxide (34.8 eV), well matched with our as-synthesized catalyst.57 The narrow scan of as-synthesis materials shows the O 1s core level located at 530.42 eV, which was different than the pure WO3 (529.4 eV)56 trends to different nature of W–O bonding. Shpak et al., also reported that binding energy of O 1s (530.6 eV) correspond to O1-levels of oxygen atoms O2− in the synthesized materials57 and may be the composition written as Wx5+W1−x6+O3−x.Li et al.58, reported that peaks at 231.7 eV and 234 eV belongs to Mo5+ oxidation state, which is close to (231.65 and 234.65 eV) our as-synthesis material.
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Fig. 5 XPS survey spectrum (a), 3d core-level spectrum of molybdenum (b), C 1s core level spectrum of carbon in the synthesized material (c), P 2p core level spectrum of phosphorous (d). |
Acidity analysis of as-synthesis HPW4Mo10Ox, and HPW4Mo10Ox-Cys material at 750 °C, shows much higher total acidity (0.84 mmol g−1 and 0.63 mmol g−1, respectively) from temperature programmed desorption of ammonia (TPDA). Which is much higher than the alumina-supported catalyst (0.34 mmol g−1) and silica (0.07 mmol g−1).59 Alhanash et al.,60 reported acidity strength on the basis of TPDA peaks at different temperature range. They mentioned that absorption peaks below 300 °C correspond to weak acidity, above 300 °C, moderate acidity and the peaks at more than 500 °C due to the stronger acidity of materials. As-synthesis materials (HPW4Mo10Ox), show a strong peak at lower acidity range (Fig. 6) and HPW4Mo10Ox-Cys show a strong peak at higher temperature range. On the basis of calibrations data, total adsorption of NH3 for every peak find-out by instrument and acidity for each peak calculated. HPW4Mo10Ox, show three peaks at 203, 394 and 753 °C and contributed 0.79, 0.046 and 0.0075 mmol g−1 respectively. HPW4Mo10Ox-Cys show four peaks at 340, 224, 454, 659 °C and contributed 0.15, 0.037, 0.15 and 0.29 mmol g−1 respectively. As graph (Fig. 6) and calculation of acidity trend indicate that after using the capping agent, decreased (0.84 to 0.63 mmol g−1) the total acidity and most significantly, change the trend of acidity pattern. Without the capping agent as-synthesized poly-metallic acidic catalyst show strong peaks and acidity strength (0.79 mmol g−1) at a lower temperature (203 °C) range, while after the capping, strong strength (0.29 mmol g−1) show at 659 °C. The change of material acidity nature provides to design the more selective synthesis of a desired product.
A wide range of etherification, dehydration and rehydration reactions can be efficiently catalyzed by these materials, which can be designed to the synthesis of control acidity as well as high degrees of reaction selectivity of the desired product. The elemental analysis of as synthesis materials was performed with the help of electron probe micro-analyser (EPMA) with LaB6 gun as shown in Tables S1 and S2.† Tables S1 and S2† show the 10-point analysis of as-synthesis materials with ranges minimum-to maximum 0.49% to 1.79 mol% of phosphorous, 4.04 to 22.33 mol% of oxygen, 0 to 1.49 mol% of nitrogen, 8.84 to 35.6 mol% of molybdenum, 6.66 to 28.02 mol% of tungsten in HPW4Mo10Ox-Cys catalyst. As-synthesis HPW4Mo10Ox catalyst show minimum-to maximum 1.13% to 1.47 mol% of phosphorous, 10.76 to 25.68 mol% of oxygen, 22.7 to 29.7 mol% of molybdenum, 28.02 to 37.20 mol% of tungsten. The estimated value of the metal higher and in regular pattern in HPW4Mo10Ox compare to HPW4Mo10Ox-Cys.
Fig. 7 shows the powder-X-ray diffraction patterns of as-synthesized materials with the capping agent and without the capping agent. As far as the as-synthesized materials with capping agent (HPW4Mo10Ox-Cys), show broad characteristics peaks of metal oxide indicate that nano-range particles are formed. Without capping agent materials (HPW4Mo10Ox) show that various intensive and sharp peaks at corresponding 2θ value and respective d-spacing of the lattice fringes was found to be, 10.91 (0.81 nm), 15.0 (0.59 nm), 18.88 (0.47 nm), 21.85 (0.41 nm), 24.35 (0.36 nm), 26.79 (0.33 nm), 30.93 (0.29 nm), 32.79 (0.27 nm), 36.35 (0.25 nm), 39.58 (0.22 nm), 42.77 (0.21 nm), 44.20 (0.20 nm), 47.02 (0.193 nm), 48.45 (0.19 nm), 51.05 (1.79 nm), 52.33 (0.175 nm), 54.88 (0.17 nm), 56.10 (0.16 nm), 58.49 (0.16 nm), 63.11 (0.15 nm), and 65.39 (0.14 nm). In literature peak at 23.12 (002), 23.58 (020) and 24.38 (200) correspond to WO3 and 20.48 (111), 23.58 (020) for the WO3·H2O. As XRD pattern of HPW4Mo10Ox-Cys indicate that the broadening of XRD peaks distinctly reflects the nanocrystalline nature of the materials which is more pronounced in the case of HPW4Mo10Ox-Cys materials. In our synthesis method, an excess of cystine, oleic acid and oley alcohol was used to ensure stabilization and restricting the size of the particles in the nano range. The XRD pattern of as-synthesis HPW4Mo10Ox-Cys show reflections at 2θ with respective d-spacing of the lattice fringes was found to be at 22.14 (0.40 nm), 26.45 (0.34 nm), 31.89 (0.28 nm), 37.45 (0.24 nm), 53.97 (0.17 nm), 60.93 (0.15 nm), 63.29 (0.15 nm), 67.14 (0.14 nm).
To further calculate the crystallite sizes of as-synthesized materials by Scherrer equation, which indicate that peak width is inversely proportional to crystallite size. In heterogenized materials HPW4Mo10Ox-Cys, show broad reflection peaks and indicate the nano-range. As per calculation its show 10 to 30 nm size of metal nanoparticles. As an obtained result was found to be in good agreement with the size evaluated from TEM and prove that topotactic conversion can be achieved by capping agent and also reducing the particle size. The generation of nano-range particles through the process of topotactic intercalation changes the arrangement of particles that can enhance the activity and selectivity of furanics products. We believe that nano-range particles assembly and controlling the acidity are these two advances promise to synthesis and modification of heteropoly anion with tunable physical properties create new facet in catalysis.
Fig. 8 depicts the thermogravimetric curves of the HPW4Mo10Ox and HPW4Mo10Ox-Cys materials. The HPW4Mo10Ox show 24.39 weight%, mass loss whereas HPW4Mo10Ox-Cys only 5.2% weight loss from room temperature to 600 °C, result indicate that HPW4Mo10Ox-Cys, show higher thermal stability in compared to without capping agent. At initial weight loss were attributed to the loss of physically-adsorbed and chemically bonded water.
S. no | Substrate | Catalyst | Time (min) | Conversion (%) | Yielda (%) | Product distribution selectivity | |||
---|---|---|---|---|---|---|---|---|---|
HMF | EMF | EL | Othersb | ||||||
a Isolated mol% yield.b Other = LA, formylated HMF, FA. AHPW4Mo10Ox, B= HPW4Mo10Ox-Cys, other reaction condition: catalyst = 20 mg, temperature = 170 °C, solvent (6![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
|||||||||
1 | Glucose | A | 10 | 84 | 56 | 22.45 | 42.68 | 12.82 | 6.10 |
2 | Glucose | A | 20 | 97 | 78 | 8.95 | 26.25 | 51.67 | 10.13 |
3 | Glucose | A | 30 | ∼100 | 73 | 0 | 2.40 | 84.95 | 11.60 |
4 | Glucose | B | 10 | 56 | 31 | 35.95 | 15.25 | 3.48 | 1.32 |
5 | Glucose | B | 20 | 92 | 76 | 50.12 | 26.76 | 7.10 | 7.20 |
6 | Fructose | A | 20 | ∼100 | 84 | 0 | 1.2 | 87.86 | 10.83 |
7 | Fructose | B | 20 | ∼98 | 89 | 54.65 | 28.80 | 7.40 | 7.15 |
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Fig. 9 Dehydration and etherification of glucose at different catalyst loading, other reaction condition: glucose 100 mg, temperature 170 °C, time 20 min. |
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Fig. 11 Reusability study of HPW4Mo10Ox-Cys catalyst for glucose dehydration and yields of products. Other reaction condition: glucose = 100 mg, catalyst = 20 mg, at 170 °C for 20 min. |
HMF has two functional group, carbonyl, and alcohol, and therefor in carbonyl group (n to π and π to π*) transition are possible and in alcohol (n to σ*) transition possible. UV absorption spectra for HMF show two transition to π* and π to π*. Characteristic absorption peaks of HMF (286 nm) after etherification as shown in Fig. 12. The spectra of pure HMF, and after etherification were easily distinguished by UV-visible. Results indicate that the absorption due to π to π* transition decrease (286 nm) along with increasing n to π transition intensity (225 nm) (Fig. 12, 2 h). As the reaction time increases beyond this initial period, the UV spectra begin to show an increase in absorbance at wavelengths lower than 230 nm, along with a decrease in absorbance observed for wavelengths above 284 nm and therefore no appearance of alcohol peak. Fig. 12 shows a spectrum of HMF at zero h and 2 h, in view of the spectra where increase in absorbance can be seen as deepness of the valley increased.
Compound | Lower heating value (MJ L−1) | Boiling point (°C) | RON |
---|---|---|---|
a MTHF = 2-methyltetrahydrofuran, DMF = 2,5-dimethylfuran, MF = 2-methylfuran, EL = ethyl levulinate, BL = butyl levulinate, MP = methyl pentanoate, GVL = γ-valerolactone. | |||
Gasoline | 30–33 | 27–225 | 88–98 |
Ethanol | 21.4 | 78 | 109 |
n-Propanol | 24.7 | 97.2 | 104 |
Iso-propanol | 24.1 | 82.3 | 106 |
n-Butanol | 26.9 | 117.7 | 98 |
Iso-butanol | 26.6 | 107.9 | 105 |
n-Pentanol | 28.5 | 137.8 | 78 |
MTHF | 28.2 | 78 | 86 |
DMF | 30.1 | 94 | 119 |
MF | 27.6 | 64.7 | 103 |
EL | 24.8 | 206 | 110 |
BL | 27.1 | 237.5 | 98 |
MP | 25.9 | 126 | 105 |
GVL | 26.2 | 218–220 | 100 |
Footnote |
† Electronic supplementary information (ESI) available: EPMA elemental analysis data of as-synthesis materials, TEM-EDX analysis data, SEM, TEM images at different magnification, and NMR spectra of products. See DOI: 10.1039/c9ra03300a |
This journal is © The Royal Society of Chemistry 2020 |