Due to depletion of fossil fuel reserves and environmental concerns demand the utilization of other renewable feedstocks such as biomass which is available abundantly in different quantities throughout the world. Catalytic transformation of this biomass or biomass-derived products such as glucose to valuable platform biochemicals viz: levulinic acid (LA) is academically as well as industrially important reaction. In the present work, Silicon phosphates (SixPO4) catalysts viz: Si10PO4, Si20PO4, Si30PO4, and Si40PO4 having solely Brønsted acidic properties were prepared by wet impregnation of H3PO4 (10–40 wt%) on fumed silica followed by calcination at 550 °C for 10 h. The prepared catalysts were characterized by various techniques such as XRD; NH3-TPD; FTIR etc. Formation of silicon phosphate structure was found to depend on % of P loading and its interaction with silica. The acidic property of SixPO4 catalyst was characterized by NH3-TPD and Pyridine-IR, confirmed that catalyst has exclusively strong Brønsted acidity. A high LA yield of 81 mol% over Si30PO4 has achieved at an optimized reaction condition of 140 °C; 10 h in MIBK–water (9:1) biphasic system.
Depletion of fossil fuel reserves and an increase in the formation of global warming and CO2 emissions necessitate to find out alternative sources for fuel, energy, and chemical . Due to this, the demand for clean energy and renewable sources has been increased . Among the various renewable sources, biomass, especially agricultural waste, is the only sustainable source of carbohydrate which can be used for the synthesis of renewable fuel and chemicals . Biomass-derived glucose is the most preferred carbohydrate substrate having C6 sugars for the synthesizes of wide varieties of platform chemicals such as hydroxymethyl furfural (HMF), levulinic acid (LA) and furfural. Glucose is sustainable, abundantly available and low in price can commercially be manufactured by the enzymatic hydrolysis of starch like maize, rice, wheat, cassava etc. These advantages of glucose prompted researchers to convert it into valuable products which have industrial relevance. Amongst various renewable products, levulinic acid (LA) is a platform chemical identified by the US Department of Energy for the synthesis of other biochemicals and biofuel additives . LA can be used directly in the chemical industry for the formation of a chiral reagent, adsorbents, and lubricants. LA can also be used as source material for fuel and fuel additives (Ethyl levulinate, Methyltetrahydro furan, gamma-Valerolactone, Methyl levulinate), pharmaceutical chemicals (DALA, calcium levulinate, succinic acid, tetrapyrrole and ketals) and agricultural products (formic acid, lignins and ethyl formate) etc. Indeed from a market perspective, the production of LA remains attractive . From the current research market LA requirement expected to reach 1,50,000 tonnes per annum (TA) by 2020 from 10,000 TA(2017) with market revenue of $19.65 million . Due to the higher demand of LA, more research is needed to identify potential heterogeneous catalyst over homogenous or enzymatic due to the well-known advantages of the heterogeneous catalyst concerning environmental issues and catalyst reusability. Weingarten et al. reported glucose to LA conversion with 17.3 mol% LA yield at 160 °C for 3 h over solid metal phosphate catalyst . Ramli and its co-workers obtained 62 mol% LA yield over indium trichloride catalyst at 180 °C for 1 h . 64.20 mol% LA yield over Fe-NbP catalyst at 180 °C after 3 h was reported by Liu et al. . Sun et al. in his studies show that 52.60 mol% of LA at 130 °C in 30 min by using Ly0.5H2.5PW catalyst . Sai et al. reported that 83.10 mol% yield of LA at 120 °C in 120 min with ArSO3H-Et-HNS catalyst .
Based on the available reports, it is understood that solid acid catalyst with Lewis and Brønsted acid sites is crucial for the conversion of glucose to LA through glucose isomerization to fructose and then dehydration to HMF and further rehydration of HMF to LA and formic acid (FA) [1,2,3,4,5,6,7,8,9,10]. However, Gang Yan et al. demonstrated through computational studies the new reaction path over only Brønsted acidic catalyst can be used for direct transformation of glucose to LA conversion without formation of fructose and formic acid (FA). In this route, glucose is protonated at other reactive sites than its isomerise site for fructose formation, then this protonated glucose dehydrate to HMF and then rehydrate to direct LA formation via a reaction mechanism that does not involve the intermediate formation of fructose and by-product FA . It is worth mentioning here that there is probably no single experimental evidence on the transformation of glucose into LA over solid Brønsted acidic catalyst .
Strong Brønsted homogeneous acid catalysts such as H3PO4, H2SO4, and HCl are often used in biomass hydrolysis to produce LA [12, 13]. However, from an environmental point of view, such a homogenous catalyst is not advisable. If attempts are made to make these acid heterogenized, then it will be the excellent catalytic system for biomass transformation reaction. A metal oxide-containing oxoanions [(PO4)3+, (SO4)2+, (ClO4)1+] present a strong surface acidity and increasingly used as a solid acid catalyst [14,15,16,17]. In the present paper, we report preparation and characterization of silica containing phosphate ions (silicon phosphate). Silica, unlike zirconia and titania, is “inert” support and does not show intrinsic Lewis acidity in normal condition.
In this work, silicon phosphate (SixPO4) catalyst having only Brønsted acidic is used for the direct conversion of glucose to LA in methyl isobutyl ketone (MIBK)—water biphasic system. The study is further extended for optimization of process parameters to maximize LA yield.
d(+)-glucose (99.9%) and fumed silica (99.8%) were purchased from Aldrich. MIBK (99%) and o-phosphoric acid (85%) has purchased from Merck Life science private limited, Mumbai and Qualigens respectively.
Synthesis of catalyst
Silicon phosphate (SixPO4, x = % of H3PO4) catalysts with different phosphorus (P) loading were prepared by wet impregnation of H3PO4 on fumed silica (FS) followed by calcination at 550 °C. In a typical synthesis of 30% phosphorus loading sample, 2.142 g of H3PO4 was added into a 10 ml ethanol to form a phosphorous solution, and then 5 g of fumed silica was added in the above-said solution which forms the slurry. The slurry was then stirred for 30 min, and then ethanol was evaporated from slurry at 50 °C till it dry. The dried powder was further calcined at 550 °C for 5 h designated as Si30PO4. The similar procedure was followed for the preparation of 10, 20 and 40% phosphorus loading and samples were designated as Si10PO4, Si20PO4, and Si40PO4 respectively.
XRD patterns were recorded on X-ray diffractometer (P Analytical PXRD system, Model X-Pert PRO-1712) using Cu K∝ radiation at a scanning rate of 0.083 s−1 in the 2θ ranging from 10° to 90°. The surface area was determined by N2 adsorption (SA 3100 analyzer, Beckman Coulter, CA, USA), using the BET method. The acidity was measured by NH3 TPD using a Micromeritics AutoChem (2910, USA) equipped with a thermal conductivity detector. Before the measurements, the sample was dehydrated at 400 °C in He (30 cm2 min−1) for 1 h. The temperature was then decreased to 50 °C, and NH3 was allowed to adsorb by exposing the sample to a gas stream containing 10% NH3 in He for 1 h. It was then flushed with He for another 1 h. The NH3 desorption was carried out in He flow (30 cm2 min−1) by increasing the temperature up to 600 °C with a heating rate of 10 °C min−1.
The nature of acid sites (Brønsted and/or Lewis) in the catalyst was elucidated by ex situ pyridine-FTIR. After activating a catalyst powder sample at 400 °C for 2 h, it was cool down to room temperature under high vacuum. It was then exposed to pyridine vapours for 2 h. The physisorbed pyridine was driven off by activating at 100 °C for 2 h under high vacuum. The FTIR spectra were recorded on a Shimadzu (Model-820 PC) spectrophotometer under DRIFT (Diffuse Reflectance Infrared Fourier Transform) mode.
Well-characterized silicon phosphate catalysts (Si10PO4, Si20PO4, Si30PO4, and Si40PO4) were evaluated for the direct conversion of glucose to LA reaction in biphasic MIBK- water system was carried out in 25 ml SS reactor at 800 RPM. The temperature and stirring speed was maintained with an accuracy of ± 5 °C and ± 10 RPM respectively using Magnetic stirrer (Remi, Magnetic stirrer 5MHL). For a typical reaction, the glucose concentration from 25 to 100 g/L in MIBK: water mixture (MIBK:water volume ratio varies as 10:0, 9:1, 7:3, 5:5 and 0:10) with a catalyst loading of 0.63–2.5 wt % wrt total reaction mass was subjected to thermal treatment at a temperature range of 120–150 °C for different time spans of 5–12 h. After completion of the reaction, the reactor was cooled down to ambient temperature, and the catalyst was separated from the reaction mixture by centrifugation. The liquid product was analyzed by gas chromatography (GC-1000) using a flame ionization detector (FID) and the TR-5 column with the length of 30 m, I.D 0.32 mm with nitrogen as a carrier gas and programmable temperature range of 50–280 °C. To calculate the glucose conversion, reaction samples were also analyzed by high-pressure liquid chromatography (HPLC; Agilent 1260 Infinity II Binary LC System) using a Hi-Plex H+ column and a refractive index detector (G1362A).
Glucose conversion and LA, HMF and Furfural yield is calculated by using below formula
Glucose conversion was calculated by the following method:
Unreacted moles glucose in the aqueous and organic phase was calculated by high-pressure liquid chromatography (HPLC; Agilent 1260 Infinity II Binary LC System) using a Hi-Plex H+ column and a refractive index detector (G1362A). Then conversion was calculated by formula
GI = initial moles glucose used
GUR = Sum of unreacted Glucose mol in the organic and aqueous phase.
La = Sum of Levulinic acid mol in the organic and aqueous phase; HMF = mol of hydroxymethyl furfural formed; FA = mol of furfural formed, and GI = initial moles glucose used.
Results and discussion
Powder X-ray diffraction is carried out to identify the phase purity and phase formed by phosphorus loading on fumed silica (FS). Figure 1 depicts XRD patterns of FS, Si10PO4, Si20PO4, Si30PO4, and Si40PO4. The catalyst FS, Si10PO4 exhibits a wide broad peak at 2θ = 22° which is attributed to amorphous SiO2. New diffraction peaks in the XRD patterns of Si20PO4, Si30PO4, and Si40PO4 appeared at 2θ = 10.96°, 22°,25°, 26°, and 32 °C which are well matching with the characteristics peak of silicon phosphate (JCPDS Pdf 40-457). The silicon phosphate is mostly composed of Si3(PO4)4 and Si(HPO4)2·H2O , and it is formed by the solid–solid reaction between SiO2 and H3PO4 during calcination at 550 °C. The intensity of silicon phosphate peak was observed to be increased as phosphorus loading increases from 20 to 40% which may be attributed to the more interaction between phosphate and silica and more formation of silicon phosphate species. The absence of silicon phosphate peaks in Si10PO4 may be due to lower formation of silicon phosphate species which may be below XRD detection limit.
Direct conversion of glucose to LA is an acid catalysed reaction which involves Brønsted and/or Lewis acidity, its contributions and nature etc. Thus SixPO4 samples were investigated for these properties measurements by NH3-TPD and Py-IR.
NH3 adsorption–desorption technique permits the determination of acid strength and total acidity on the catalyst surface. The NH3-TPD profiles of Si10PO4, Si20PO4, Si30PO4, and Si40PO4 are shown in Fig. 2. TPD profile of FS showed a single desorption peak at 120–550 °C (not shown in figure), while all SixPO4 samples exhibited two desorption peaks at 120–550 °C and 550–800 °C. These two desorption peaks were further deconvoluted into three desorption peaks with maxima at 100–300, 300–500 and 500–800 °C. Desorption peaks at 100–300 °C can be attributed to desorption of NH3 from weak acid sites. The features at 300–500 °C and 500–800 °C are assigned to desorption of NH3 from medium and strong acid sites, respectively. The Desorption peaks assigned to weak and medium acidity was observed to be decreased with increase in phosphorus loading from 10 to 40% on FS. Whereas, strong acidity in the range of 500–800 °C was found to be increased with an increase in phosphorus loading on FS .
Table 1, elaborated the percent distribution of weak, medium, strong and total acidity of FS, Si10PO4, Si20PO4, Si30PO4, and Si40PO4. FS (Table 1) contributed minor acidity due to physisorbed or weak acid sites, whereas after incorporation of phosphorus in FS, total acidity of SixPO4 catalyst drastically increased from 0.024 to 0.89 mmol NH3/g. The decrease of weak (0.19–0.10 mmol of NH3/g) and medium (0.37–0 mmol of NH3/g) acidity was observed with increased in phosphorus loading from 10 to 40%. Whereas, strong acid sites was found to be enhanced from 0.3 to 0.56 mmol of NH3/g with increased in phosphorus loading. NH3-TPD studies confirmed that silicon phosphate has strong acidic sites; these strong acidic sites are crucial for the conversion of glucose to LA, which is a highly Brønsted acidic reaction .
Pyridine IR is one of the most important method to identify the type of surface acidity. The bands representing at 1455 and 1545 cm−1 in pyridine-FTIR spectra are assigned to pyridine coordinated to Lewis and Brønsted acid sites, respectively . Si10PO4, Si20PO4, Si30PO4, and Si40PO4 catalyst was characterized by ex situ pyridine-FTIR. The pyridine-FTIR spectra of all the samples are illustrated in Fig. 3. The spectra of all SixPO4 catalyst showed two bands, one at 1490 and another at 1545 cm−1 which are assigned to combination of Brønsted + Lewis and Brønsted acid site respectively. There is no band at 1455 cm−1 which is assigned to Lewis acid site. Absences of Lewis acidic band at 1455 cm−1 confirm that SixPO4 catalyst has only Brønsted acid site. Since there is no Lewis acidity, band at 1490 cm−1 was also assigned to Brønsted acid sites only. The intensities of 1490 and 1545 cm−1 band were found to be increased with increase in P loading from 10 to 30% which confirmed that after loading of P on neutral FS generated only Brønsted acid sites with no contribution of Lewis acid sites. Based Py-IR result, it was concluded that the increase in strong acidity (Fig. 2 and Table 1) might be born by strong Brønsted acid sites. The BET surface area of FS and SixPO4 catalyst is also tabulated in Table 1. BET surface area of SixPO4 catalyst (150–4 m2/g) is much lower than FS (420.5 m2/g). The surface area of SixPO4 catalyst from Si10PO4 to Si40PO4 decreases as the intensity of silicon phosphate peak (Fig. 1), which indicate that the distortion of FS reduces the surface area of silicon phosphate catalyst.
Thus, characterization studies reveal that the catalyst prepared in this work by impregnation of phosphorus on FS followed calcination at 550 °C formed silicon phosphate with generation of strong Brønsted acid sites.
Plane FS and SixPO4 catalyst were evaluated for direct glucose conversion to LA (Table 2). The studies of glucose to LA were carried out at 140 °C for 3 h in the MIBK-water biphasic system at 2.5 wt % (w.r.t. reaction mass) catalyst loading. In all experimentations glucose conversion is in the range of 95–100% except FS. Apart from LA; HMF and Furfural, balance glucose is converted to total reducing sugars (TRS) and partly humins. Plane FS which is having the small contribution of weak/physiosored acidity or neutral did not show any activity as glucose to LA is acid driven reaction. In the case of SixPO4 catalysts, desired product i.e. LA formation was observed to be increased from 5 to 40 mol% with an increase in P loading on FS from 10 to 40 mol%. This increased in LA formation is due to enhancement in strong acidity (crucial for glucose to LA formation) with an increase in P loading (Table 1, Figs. 2, 3). The trend of mol% of LA formation was calculated to be FS(0) < Si10PO4 (5 mol%) < Si20PO4 (30 mol%) < Si30PO4 (38 mol %) < Si40PO4 (40 mol%). The yield of HMF and Furfural were found to be decreased from Si10PO4 to Si40PO4. Increasing LA yield with an increase of P loading may be attributed to the increased of strong Brønsted acidity which helps to increase the rate of glucose dehydration and HMF rehydration.
Gang Yang et al. reported  a comprehensive computational study to validate a route for Brønsted acid catalysed direct glucose transformation to LA by dehydration of glucose. This direct reaction mechanism is attributed to the regioselectivity of the initial glucose protonation steps without formation of fructose. The detailed reaction mechanism is presented (Scheme 1). Scheme 1 reveals that LA formation can take place via the protonated dehydration process at O2, O3 sites. O5 site protonation is more relevant when fructose is used. Other sites i.e. O1, leading to the low formation of LA and more formation of carbonaceous compound and humins. There is dehydration and intramolecular H transfer between O2–3 and O3–4 which directly leads to the removal of formic acid and formation of stable carbonaceous species, which can directly convert to LA. Thus, direct glucose conversion to LA is more favorable at O2 and O3 sites. In this study, SixPO4 (silicon phosphate) is a highly Brønsted acidic catalyst was used for the direct conversion of glucose to LA in MIBK: Water biphasic system.
Optimization of process parameters
Further optimization of process parameter was carried out over Si30PO4 catalyst to achieve maximum LA yield over Brønsted acidic catalyst.
Influence of catalyst loading
The effect of Si30PO4 catalyst loading on the yield of LA, HMF, and Furfural was carried out from 0.5 up to 2.5 wt% with respect to the reaction mass. The results are depicted in Fig. 4. It can be seen that, as catalyst loading increases from 0.5 up to 2.5 wt %, the LA yield was observed to be increased exponentially. The highest LA yield of 54% was obtained at the catalyst loading of 2 wt %. The increase in LA formation with increase in catalyst loading is due to the availability of sufficient Brønsted acid sites, which enhances the protonation of glucose at O2 and O3 position, increases glucose dehydration to HMF and subsequently rehydration to LA. However, above 2 wt % catalyst loading, LA yield was found to be decreased. At higher catalyst loading, the presence of more acidic sites leads to the condensation and hydrolysis which is the favorable condition for the humins formation and lowering of LA formation [6, 21]. At the higher catalyst loading of 2.5 wt%, there is also poor external diffusion of reactants/solvents in the catalyst pores which reduces the desired activity. Thus, 2 wt % catalyst loading of Si30PO4 seems to be the optimum.
Effect of MIBK to water ratio
Figure 5 reveals that when 100% water is used as a reaction medium, the glucose is highly soluble and subsequently converted into TRS (Total Reducing Sugar), however conversion of these TRS to organic compounds such as 5-HMF and its further transformation to LA is nil (Fig. 5). It is confirmed that in the presence of only water, the stability of 5-HMF is very low and it immediately decomposed into polymeric compounds such as humins. The present work observed the highest formation of humins with water. This also tends to no formation of LA. Whereas in a biphasic system of MIBK and water, MIBK helps to extract organic compound and suppress the oligomeric formation [20, 22, 23]. Thus, in this work the optimum composition of MIBK and water is evaluated with MIBK: Water ratio of 0:10, 5:5, 7:3, 9:1 and 10:0. As MIBK: water ratio increased up to 9:1, LA yield was observed to be reached at 54% with the minimal formation of 5-HMF and Furfural. In the case of 100% MIBK without water system, LA was observed to be decreased marginally to 51%. In biphasic MIBK: water system of 9:1 ratio and pure MIBK, strong Brønsted acidic sites of catalyst highly interacted with ketone group of adsorbed MIBK by the formation of hydrogen bond . This strong interaction of MIBK displaces intermediate product from the acid sites into the reaction medium with extraction of an intermediate product like 5-HMF by MIBK prevent the decomposition of 5-HMF to humins, which increases the rate of 5-HMF rehydration to LA .
Effect of temperature
Temperature is a crucial parameter which affects the reaction rate and yield of LA. Si30PO4 catalyst was evaluated at 120, 140 and 160 °C at identical pre-optimized reaction conditions (Fig. 6). As illustrated in Fig. 6, LA yield increases marginally from 55 to 57 mol% with an increase in temperature from 120 to 140 °C. Above 140 °C, there is a more formation of Furfural which is attributed to increased rate of 5-HMF conversion to Furfural by loss of formaldehyde at higher temperature.
Another crucial process parameter is reaction time, as this is a complex reaction system involves isomerisation, dehydration, re-hydration steps. In this reaction system rehydration of 5-HMF to LA is a slow step as compared to other steps . Figure 7, reveals that, as the reaction time increases up to 10 h, the glucose to LA formation increased substantially up to 81%, which is probably the highest value over reported by all possible routes. Above 10 h, there is a possibility that the active centres of the catalyst may be covered by humins and other polymeric compounds increase which reduces the LA activity and restricted to 5-HMF. This was also evidence that after 12 h, the catalyst was completely black.
Effect of glucose concentration
The concentration of glucose in the reaction medium is also an important parameter to make the process economical. Higher the glucose concentration better process economics, as productivity will be more. Figure 8 presented the data generated on the Si30PO4 catalyst as a function of glucose concentration. The present study confirmed that Si30PO4 catalyst could tolerate glucose concentration up to 75 g/L, which is probably the highest so far (reported 10 g/L) . Higher the glucose concentration lower the LA formation due to layering of glucose on catalyst active surface as well as glucose can decompose into humins and other polymeric compounds which deactivate the catalyst. Si30PO4 can tolerate up to 75 g/L glucose probably due to the higher surface area of FS and optimum distribution of P on the external and internal surface of FS makes more active sites available for reaction.
Reusability of catalyst
The reusability of Si30PO4 was studied for three cycles using identical experimental conditions of glucose-75 g/L, catalyst loading: 2 wt%, MIBK:water-9:1, temp.-140 °C for 10 h, RPM-800. After completion of each reaction, the catalyst was filtered and used without any post-treatment. The yield of LA was observed to be stable up to second cycles. After the second cycles, the yield of LA decreases significantly from 75 mol% (LA yield of 2nd cycle) to 35 mol%. Powder XRD and NH3-TPD characterized used Si30PO4 catalysts after the third cycle. Characterization studies exhibited the decreased intensity of silicon phosphate phase and total acidity (0.32 mmol NH3/g) than parent catalyst (0.66 mmol NH3/g). These changes in physico chemical properties of used Si30PO4 are ascribed to the deposition of humins and other carbon materials. Therefore, Four-time used catalyst was regenerated by solvent washing followed by recalcined at 500 °C for 5 h. The regenerated catalyst was again re-evaluated for the reaction at identical reaction conditions. The regenerated catalyst exhibited improved LA yield (53 mol%) than the third cycle, but much lower than the fresh catalyst. The lower catalytic activity of regenerated catalyst may be attributed to the change of structural feature and change in acidic properties of the catalyst.
Si10PO4, Si20PO4, Si30PO4, and Si40PO4 catalyst having silicon phosphate topology with strong Brønsted acidity have been synthesized by wet impregnation of H3PO4 on FS followed by calcination at 550 °C for 10 h. Synthesized catalysts were fully characterized by different characterization techniques such as powder XRD, N2 adsorption–desorption, NH3-TPD, and Py-IR. Characterization of catalysts confirmed that after incorporation of phosphorous (P) on FS leads to formation of silicon phosphate phase with strong Brønsted acidity. These 10–40SixPO4 Brønsted acid catalysts having silicon phosphate phase were evaluated for direct conversion of glucose to levulinic acid (Platform Chemical) in one step, which is an important reaction in the area of utilization of biomass resources to valuable products follows green chemistry principles with “Waste to Wealth” concept. Amongst studied, Si30PO4 catalyst was found to be highly active with 81 mol% LA yield, which is probably the highest LA yield. Moreover, the present catalyst Si30PO4 catalyst can handle more glucose concentration (75 g/L) than reported (10 g/L). These findings make the process industrially relevance and academically attractive.
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Authors would like to acknowledge the Department of Biotechnology, Govt. of India Project No.: BT/PR12277/PBD/26/434/2014 and CSIR Mission Mode Project: Catalysis for Sustainable Development for Funding.
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Bokade, V., Moondra, H. & Niphadkar, P. Highly active Brønsted acidic silicon phosphate catalyst for direct conversion of glucose to levulinic acid in MIBK–water biphasic system. SN Appl. Sci. 2, 51 (2020). https://doi.org/10.1007/s42452-019-1827-z
- Brønsted acid
- Solid catalyst
- Levulinic acid
- Biphasic system