Valorization of a profusely available alternate resource, biomass and in particular its 3-D intricate component lignin into low molecular weight aromatic products are used as platform chemicals and fuel additives, and developing a low temperature catalytic process is imperative in preserving atom efficiency. Ionic liquids, due to their unique properties, offer an advantage to develop such methods under milder conditions. Herein, we show use of –SO3H functionalized imidazolium based various recyclable Brønsted acidic ionic liquids (BAILs) in catalytic quantity under ambient pressure at 120 °C for depolymerization of lignin (60000 g mol?1) into THF soluble products with high efficiency (78% yield, 95% ± 5% mass balance). The decoding of this efficiency by 1D and 2D (HSQC/NOSEY) NMR, FT-IR and RAMAN studies exemplify that the –OH group(s) interact with the electron deficient BAIL cation. The mechanistic insights unraveled in this study open a plethora of opportunities to design catalysts for developing efficient processes.

Development of competent method for the conversion of hemicelluloses, a lignocellulosic component into sugars is essential. In a one-pot method, at 160 °C and in water media, Brönsted acidic ionic liquid (BAIL) catalyzed hydrolysis of hardwood hemicellulose gives 87% yield for C5 sugars (xylose+arabinose). The efficiency of the ILs and acid strength both followed the similar trend, [C3SO3HMIM][HSO4] > [C3SO3HMIM][PTS] > [C3SO3HMIM][Cl] > [BMIM][Cl]. The ion-dipole type interaction present between BAIL and substrate which is proposed by 1H NMR study is suggested to help in achieving better activity with BAIL compared to mineral acid, H2SO4.

Promoter effect of Sn in the PtSn/g-Al2O3 (AL) and PtSn/C bimetallic catalysts is studied for the conversion of variety of substrates such as, C5 sugars (xylose, arabinose), C6 sugars (glucose, fructose, galactose), hemicelluloses (xylan, arabinogalactan), inulin and agricultural wastes (bagasse, rice husk, wheat straw) into sugar alcohols (sorbitol, mannitol, xylitol, arabitol, galactitol). In all the reactions, PtSn/AL showed enhanced yields of sugar alcohols by 1.5 to 3 times than Pt/AL. Compared to C, AL supported bimetallic catalysts showed prominent enhancement in the yields of sugar alcohols. Bimetallic catalysts characterized by X-ray diffraction study revealed the stability of catalyst and absence of alloy formation thereby indicating that Pt and Sn are present as individual particles in PtSn/AL. The TEM analysis also confirmed stability of the catalysts and XPS study disclosed formation of electron deficient Sn species which helps in polarizing carbonyl bond to achieve enhanced hydrogenation activity.

Considering the growing importance of furfural, it is necessary to develop a robust solid acid catalyst for the conversion of xylan (hemicelluloses) into furfural in a one-pot method. In this study, the effects of various components (Si, Al and P) and crystallization time on the preparation of acidic materials and their properties are extensively described. It is seen that with the increase in crystallization time (0, 48, 96, 176 h) during the synthesis of silicoaluminophosphate (SAPO-44), materials morphology changes from amorphous to AFI to CHA and the best activity for furfural synthesis (82%) from xylan was observed with SAPO-44 catalyst aged for 176 h. Since, acidity in the SAPO-44 is guided by the incorporation of Si in the ALPO framework, study on Si molar concentration (0.8, 1.0, 1.2, 1.4 and 1.6) in SAPO-44 was carried out and it was observed that SAPO-44 having 1.0 mole of Si content is the best catalyst for the xylan/xylose conversion to furfural. Catalyst structure-activity correlation is thoroughly defined with the help of several physico-chemical properties.

It is imperative to develop an efficient and environmentally benign pathway to valorize profusely available lignin, a component of non-edible lignocellulosic materials into value-added aromatic monomers, which can be used as fuel additives and platform chemicals. To convert lignin, earlier studies used mineral bases (NaOH, CsOH) or supported metal catalysts (Pt, Ru, Pd, Ni on C, SiO2, Al2O3 etc.) under hydrogen atmosphere but these methods face several drawbacks such as corrosion, difficulty in catalyst recovery, sintering of metals, loss of activity etc. Here we show that under inert atmosphere various solid acid catalysts can efficiently convert six different types of lignins in to value-added aromatic monomers. Particularly, SiO2, Al2O3 catalyst gave exceptionally high yields of ca. 60 % for organic solvent soluble extracted products with 95±10 % mass balance in the depolymerization of Dealkaline lignin, Bagasse lignin, ORG and EORG lignins at 250 oC within 30 min. The GC, GC-MS, HPLC, LC-MS, and GPC analysis of organic soluble extracted products confirmed the formation of aromatic monomers with ca. 90 % selectivity. In the products, confirmation of retention of aromatic nature as present in lignin and appearance of several functional groups is done by FT-IR, 1H and 13C NMR studies. Further, isolation of major products by column chromatography was carried out to obtain aromatic monomers in pure form and their further characterization by NMR is presented. Detailed characterization of six different types of lignins obtained from various sources helped in substantiating the catalytic results obtained in these reactions. Meticulous study on fresh and spent catalysts revealed that the amorphous catalysts are preferred to obtain reproducible catalytic results.

Supported Pt catalysts are synthesized, characterized and are used in the liquid phase air oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA). Under the optimum reaction conditions of a stepwise increase in the reaction temperature (75 and 140°C for 12 h each), we achieved as high as 96% FDCA yield in presence of 1 bar oxygen pressure over Pt/g-Al2O3. It is shown that as the oxygen pressure increases (1–10 bar), the FDCA yields decrease, since at higher partial pressure of oxygen, overoxidation reactions of substrate and product(s) are possible. It is interesting to note that even with air as an oxidant, we obtained similar yields of FDCA as that with oxygen. Moreover, the effects of base (weak or strong), its concentration (equimolar or excess) were studied in detail. It is important to increase the reaction temperature in a stepwise manner to achieve higher yields of FDCA since at higher temperatures HMF undergoes self-degradation and thus the yields of FDCA decrease. The self-degradation of HMF is also proved by undertaking the reaction under nitrogen environment. The study on the effect of substrate/catalyst ratio is done to improve up on the economics of overall process. The effect of supports (reducible and non-reducible) and their oxygen storage capacity is discussed and is proposed to be one of the factors to change the course of reaction. Furthermore, we have shown that FDCA formed in the reaction can be successfully isolated (91%, isolated yield) in the pure form and its purity is confirmed by NMR, melting point, and elemental analysis. The catalysts were characterized with X-ray powder diffraction, transmission electron microscopy and inductively coupled plasma-optical emission spectroscopy techniques.

Development of stable and recyclable solid acid catalysts in the efficient valorisation of hemicellulose to yield C5 sugars and furfural is vital to boost the prospects of using lignocelluloses for the chemicals synthesis. Using SAPO-44 catalyst an environmentally benign process of furfural synthesis from diversified real substrates (without any treatment & the need for separation of its components) is shown. In an efficient one-pot methodology, at 443 K and in the presence of biphasic solvent system, selective conversions of hemicelluloses from raw biomass (bagasse, rice husk & wheat straw) to extraordinarily high yields of furfural of about 93% were attainable. Under similar reaction conditions, 82% yield of furfural is also achievable directly from isolated hemicellulose within 10 h. Hydrophilic SAPO-44 having higher hydrothermal stability showed similar activity for all the substrates minimum up to 8 times in recycle runs. Various physico-chemical characterizations (XRD, TGA-DTG, TPD-NH3, solid state NMR) of fresh and spent catalysts edify on the SAPO-44 stability.

Depolymerization of lignin to produce value-added aromatic monomers has attracted a lot of attention since these monomers can potentially be used as fuel additives (octane enhancers) and in turn can improve the prospects of cellulosic ethanol technology to become economically feasible. Here we show that solid acid catalysts could efficiently convert lignin into value added aromatic monomers with 60% yields and ca. 95% mass balance when depolymerization reactions were carried out at 250°C for 30–120 minutes. We found that the reaction parameters have a remarkable effect on improving the yields.

SiO2 and ZrO2 supported W, Mo and Ga oxide catalysts were prepared by sol-gel and wet-impregnation method. The 10wt% WO3/SiO2 (sol-gel) catalyst showed 61±2% furfural yield from hemicellulose in one-pot fashion at least in 8 cycles. Based on XRD, TPD, Hammett acidity characterizations catalyst structure-activity correlation was drawn.

Hydrodeoxygenation (HDO) is an important process to remove oxygen from lignin derived phenolic monomers to obtain chemicals which can be used as fuel or fuel additives. A systematic study is performed to check the effects of supports (acidic, neutral, basic) and noble metals (Pd, Pt, Ru) on HDO of phenol, guaicol and eugenol. When the combinations of metals and supports were evaluated under the similar reaction conditions, it was found that the metals supported on highly acidic support (SiO2-Al2O3) show complete hydrogenation products with a possibility of C-O bond cleavage to achieve a real HDO activity. While on a mildly acidic support (g-Al2O3), complicated product distribution is achieved, neutral (C) and basic (HT) supports gave restricted hydrogenation activity but yielded the products with very high selectivity. Based on the results, reaction pathways are suggested and deliberated. Catalysts showed reproducible activity in recycle runs. Catalysts were characterized by various (XRD, TEM, TPD, ICP-OES) techniques to establish catalyst activity-property relationship.

Biomass derived sugar alcohols (xylitol, arabitol) find numerous uses in the food, oral and pharmaceutical industries. Their direct synthesis from poly-saccharides however still remains an immense challenge. In this study, we demonstrate in detail, the effects of metals, supports and promoters in enhancing the yields of sugar alcohols from mono- and poly-saccharides. We undertook, synthesis of bimetallic catalysts, M-M’/S (M, Metal=Pt, Ru; M’, Promoter=Sn, Ga, Fe; S, support=g-Al2O3 (AL), SiO2-Al2O3 (SA), Carbon (C)) with varying metal loadings (Pt/Ru=2, 3.5wt%; Sn=0.22, 0.43, 0.87, 1.5, 3.5wt%; Ga/Fe=0.25wt%) by co-impregnation method. The catalytic activities of these catalysts were evaluated in the synthesis of sugar alcohols from xylose (mono-saccharide) and hemicellulose (xylan, poly-saccharide) at 130-190oC. Among all bimetallic catalysts, Pt(3.5)Sn(0.43)/AL catalyst (50%) showed 2.8 times improvement in the sugar alcohols yield compared to monometallic Pt(3.5)/AL catalyst (18%). Similarly, in xylose reaction also 2.4 times enhancement in sugar alcohols yield over Pt(3.5)Sn(0.43)/AL (79%) was observed as against 33% yield obtained with Pt(3.5)/AL. By conducting several experiments it is confirmed that the residual Cl- remained on the catalyst even after calcinations and reductions carried out at 400oC, does not play any role in catalysis. The stability of the Pt(3.5)Sn(0.43)/AL catalyst confirmed by XRD and ICP analysis was responsible in achieving reproducible activity in at least 5 consecutive runs. Formation of electron deficient Sn confirmed by XPS analysis helped polarizing carbonyl group which in turn enhanced the sugar alcohols yields. Formation of PtSn and Pt3Sn species was observed when Sn loading is more than 0.87%.

Synthesis of 5-hydroxymethylfurfural (5-HMF) from biomass derived mono- and poly-saccharides is gaining importance because of its usefulness in the preparation of important chemicals. In our work, we have synthesized several silicoaluminophosphate (SAPO) catalysts, and have shown that in the absence of any other pH modifying reagents, those are active in converting mono- and poly-saccharides into 5-HMF under biphasic reaction condition at 175°C. Particularly, SAPO-44 catalyst showed the best activity in the conversion of fructose to yield 78% 5-HMF with 88% selectivity. On the contrary, all other catalysts showed lower yields (H-MOR: 63%, SAPO-5: 32%, 2DCT: 60%). Over SAPO-44, good yields for 5-HMF were observed when glucose (67%), maltose (57%), cellobiose (56%) and starch (68%) were used as substrates. Recycle study carried out with SAPO-44 catalyst in the fructose conversion reaction showed marginal decrease in the activity up to 3rd run and then afterwards constant activity was observed up to 5th run (1st: 78%, 2nd: 71%, 3rd: 66%, 4th: 65%, 5th: 65%). Catalyst characterizations revealed that SAPO catalysts have higher hydrophilic nature than H-MOR (Si/Al = 10) and hence it is postulated that this property may help in achieving better results. Further studies on the catalyst characterizations revealed that SAPO-44 undergoes modifications in its structure. However, ICP-OES data suggests that Al and/or P are not leached out in the solution indicating that change in local environment around elements is possible. The influence of acid amount, type of acid site etc. on the catalytic activity is discussed and found out that strong acid sites are required to boost the 5-HMF yields.

Pt impregnated on g-Al2O3 (acidic support) and hydrotalcite (basic support) catalysts were synthesized, characterized and used in the oxidation and hydrogenation reactions of C5 and C6 sugars. In the absence of homogeneous base, 83% yield for gluconic acid; an oxidation product of glucose can be achieved over Pt/hydrotalcite (HT) catalyst at 50°C under atmospheric oxygen pressure. Similarly, 57% yield for xylonic acid, an oxidation product of xylose is also possible over Pt/HT catalyst. Hydrogenation of glucose conducted using Pt/g-Al2O3 + HT catalytic system showed 68% sugar alcohols (sorbitol + mannitol) formation. The 82% yield for C5 sugar alcohols (xylitol + arabitol) was obtained by subjecting xylose to hydrogenation over Pt/g-Al2O3 + HT at 60°C. UV analysis helped to establish the fact that under alkaline conditions sugars prefer to remain in open chain form in the solution and thus exposes single -CHO group which further undergoes oxidation and hydrogenation reactions to yield acids and alcohols.

Development of stable, reusable, and water-tolerant solid acid catalysts in the conversion of polysaccharides to give value-added chemicals is vital because catalysts are prone to undergo morphological changes during the reactions. With the anticipation that silicoaluminophosphate (SAPO) catalysts will have higher hydrothermal stability, those were synthesized, characterized, and employed in a one-pot conversion of hemicellulose. SAPO-44 catalyst at 170°C within 8 h could give 63% furfural yield with 88% mass balance and showed similar activity up to at least 8 catalytic cycles. The morphological studies revealed that SAPO catalysts having hydrophilic characteristics are stable under reaction conditions.

We present a solid-acid catalyzed one-pot method for the selective conversion of solid hemicellulose without its separation from other lignocellulosic components, such as cellulose and lignin. The reactions were carried out in aqueous and biphasic media to yield xylose, arabinose, and furfural. To overcome the drawbacks posed by mineral acid methods in converting hemicelllulose, we used heterogeneous catalysts that work at neutral pH. In a batch reactor, these heterogeneous catalysts, such as solid acids (zeolites, clays, metal oxides etc.), resulted in >90% conversion of hemicellulose. It has been shown that the selectivity for the products can be tuned by changing the reaction conditions, for example, a reaction carried out in water at 170°C for 1h with HBeta (Si/Al=19) and HUSY (Si/Al=15) catalysts gave yields of 62 and 56% for xylose and arabinose, respectively. With increased reaction time (6h) and in presence of only water, HUSY resulted in yields of 30% xylose+arabinose and 18% furfural. However, in a biphasic reaction system (water+p-xylene, 170°C, 6h) yields of 56% furfural with 17% xylose+arabinose could be achieved. It was shown that with the addition of organic solvent the furfural yield could be increased from 18 to 56%. Under optimized reaction conditions, >90% carbon balance was observed. The study revealed that catalysts were recyclable with a 20% drop in activity for each subsequent run. It was observed that temperature, pressure, reaction time, substrate to catalyst ratio, solvent, and so forth had an effect on product formation. The catalysts were characterized by means of X-ray diffraction, temperature-programmed desorption of NH3, inductively coupled plasma spectroscopy, elemental analysis, and solid-state NMR (29Si, 27Al) spectroscopy techniques.

Pt or Ru catalysts supported on inorganic oxides show high activity for the conversion of cellulose into sugar alcohols (mainly sorbitol) in water under hydrogen pressure; furthermore, the catalysts can be recycled over repeated runs. Glucose is produced in low yields, thus suggesting that the Pt (or Ru)/oxide works as a bifunctional catalyst for the hydrolysis of cellulose and subsequent reduction to sugar alcohols.