Highly active Brønsted acidic silicon phosphate catalyst for direct conversion of glucose to levulinic acid in MIBK–water biphasic system
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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.
KeywordsBrønsted acid Solid catalyst Glucose Levulinic acid 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.
2.1 Chemicals used
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.
2.2 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.
2.3 Catalyst characterization
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.
2.4 Catalyst activity
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:
GI = initial moles glucose used
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.
3 Results and discussion
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.
Distribution of acidity of plane FS and SixPO4 catalysts
BET surface area (m2/g)
Distribution of acidity (mmol of NH3 desorbed/g of 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.
3.2 Catalytic performance
Catalyst screening for glucose conversion to LA
Levulinic acid (mol%)
Furfural total product (mol%)
Total product (mol%)
3.3 Optimization of process parameters
Further optimization of process parameter was carried out over Si30PO4 catalyst to achieve maximum LA yield over Brønsted acidic catalyst.
3.3.1 Influence of catalyst loading
3.3.2 Effect of MIBK to water ratio
3.3.3 Effect of temperature
3.3.4 Kinetics study
3.3.5 Effect of glucose concentration
3.3.6 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.
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.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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