Cellulose conversion in the presence of catalysts based on Sn(IV)

Abstract

A systematic study of the dissolution and conversion of cellulose in the presence of Sn(IV) complexes under several reaction conditions was conducted to evaluate their potential as catalysts. Results were compared with those obtained from non-catalyzed reactions, and from reactions using H₂SO₄ as a catalyst. Sn(IV) catalysts are capable of converting cellulose into valuable chemicals with interesting yield and selectivity, without the problems associated with the use of inorganic acids in these processes.

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1. Introduction

The production of chemical supplies from renewable sources for industrial applications is one of the major challenges of this century, and involves concepts of green chemistry and sustainability.^1,2 Many attempts have been then made to partially or totally replace non-renewable sources of feedstocks, and lignocellulosic biomasses may play an important role in this effort.

There are some critical reviews concerning the catalytic conversion of cellulose^3–6 and several studies have shown that cellulose conversion into chemical supplies can be conducted in the presence of catalysts, such as enzymes,^7,8 inorganic acids,^8–11 bases,^12,13 solid acids,^14–17 and superacids.^18,19 Another approach is to perform this conversion under supercritical conditions, with or without catalyst.^20–23 However, these processes present several drawbacks when applied on industrial scales: they display low catalytic selectivity and/or activity; complex product recovery; corrosion of facilities; and significant quantities of waste products.²⁴

The use of catalysts based on metals exhibiting a Lewis acid character in cellulose conversion has attracted the attention of research groups. Such metals have demonstrated potential for this purpose in terms of activity and selectivity, and the possibility of developing clean technologies based on catalytic “green” processes.²⁵

Cellulose conversion over solid acids (HUSY, HY, SiO₂, HZSM-5, SiO₂–Al₂O₃, etc.) was investigated using reaction temperatures between 150 °C and 250 °C for 24 hours. Under these reaction conditions, glucose yields were below 5%. When supported by metal catalysts (based on Ru and Pt) under hydrogenolysis conditions, yields of sugar alcohols (e.g. sorbitol and mannitol) were approximately 20 to 30% in 24 hours.²⁵ The dissolution–conversion of cellulose was also studied in both the absence of catalyst and in the presence of Pt/γ-Al₂O₃, at temperatures of 150 °C to 190 °C and reaction times between 24 and 100 hours under H₂ and He atmospheres. The main products obtained were glucose and 5-HMF (5-hydroxymethylfurfural), and the presence of the solid catalyst increased the cellulose dissolution–conversion and the distribution of final products.²⁶

The efficiency of solid Lewis acids to depolymerize the cellulose in lactic acid was reported and the results point out that solids such as tungstated zirconia (ZrW) and tungstated alumina (AlW) exhibited a remarkable promoting effect on the cellulose depolymerisation (45%) while an unexpected decrease of the proportion of water soluble oligosaccharides/polymers was observed. Yields of 27 mol% and 18.5 mol% in lactic acid were achieved on AlW and ZrW, respectively, and the catalysts showed a good stability and recyclability.²⁷

Another example is the study of conversion of cellulose to polyols, in the presence of Ni based catalysts. Under hydrogenolysis conditions, yields of sorbitol and mannitol of approximately 45 to 7% in 6 hours were obtained using supported bimetallic Ni–Pt catalysts on the modified beta zeolites.²⁸ The use of cheaper carbon nanofibers grown over Ni supported on a γ-Al₂O₃ catalyst for cellulose conversion, showing >50% hexitol yields, was also related.²⁹ Another system based on a 7.5 wt% Ni/carbon nanofibers catalyst lead to 76% yield in hexitols with 69% sorbitol selectivity at 93% conversion of cellulose.³⁰

The employment of Ru catalysts is also reported. The hydrolytic hydrogenation of cellulose in the presence of Ru-loaded zeolites and trace amounts of mineral acid shows excellent yields (>90%) for hexitols.³¹ The commercial heteropoly acids H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ were demonstrated to be very effective acid catalysts in combination with Ru/C to directly produce hexitols from cellulose, with a conversion of 20 wt% microcrystalline cellulose on a hexitol volume productivity of 83 g L⁻¹ h⁻¹ with a hexitol selectivity of 91%.³² Hexitol yields as high as 60% were obtained when the acid catalysts (Cs_2.5PW and Ru/C or Cs_3.5SiW and Ru/C) were employed for cellulose conversion at elevated temperatures in water.³³

Production of 5-HMF and furfural was investigated using microwave-irradiation, employing ionic liquids (1-butyl-3-methylimidazole chloride or 1-butyl-3-methylimidazole bromide) in the presence of CrCl₃. Directly from cellulose, 5-HMF was obtained at 62% yield,³⁴ while yields of up to 50% 5-HMF and 30% furfural were reported via conversion from lignocellulosic biomass.⁷

In our view, additional catalysts must be tested and evaluated to improve the conversion and selectivity of cellulose, especially those that have already been demonstrated to be active in reactions where the substrate is rich in oxygenated functional groups. Sn(IV) complexes, in particular, appear to be an interesting subject for systematic studies.

There are many examples of industrial processes that have benefited from the Lewis acid character of Sn(IV) complexes. Catalysts based on Sn(IV) complexes have been used for esterification, transesterification and polycondensation reactions, to obtain polymers and other chemical supplies.^35–38 In fact, because of the significant activity of these catalytic species, it is possible to envision expansion of their use; for example in cellulose conversion processes. For example, the use of SnCl₄ in an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate), for conversion of glucose, fructose, cellobiose, inulin and starch into 5-HMF, was recently reported. The process was very efficient, especially when the raw material was glucose.³⁹ Another example is the work of Shimizu et al., in which heteropolyacid catalysts (as example: Sn_0.75PW₁₂O₄₀) were employed for cellulose depolymerization with very promising results.¹⁸

A series of silica materials containing either Sn or Ti metal centers were screened for their activity in the glucose isomerization to fructose. The best results were obtained using Sn-Beta (Sn incorporated into the framework of a large-pore zeolite). For example, a product distribution of 46% (wt/wt) glucose, 29% (wt/wt) fructose, and 8% (wt/wt) mannose was obtained after reacting a 45 wt% glucose solution containing a catalytic amount of Sn-Beta (1 : 225 Sn : glucose molar ratio) for 60 min at 383 K.^40,41

Sucrose, glucose and fructose transformation was investigated in the presence of Lewis acidic (Ti-, Sn-, and Zr-Beta), Brønsted acidic (H-Al-Beta), and nonacidic (Si-Beta) catalysts. At elevated temperatures, these sugars are transformed to methyl lactate in yields up to 44%.⁴²

Bifunctional catalysts based on carbon–silica composites (Lewis acid sites were introduced, prior to carbon filling, by grafting the silica surface with Sn(IV), while the surface functional groups like phenols, anhydrides, and carboxylic acids in the carbon framework deliver weak Brønsted acids) are capable of selectively converting sugars (such as trioses and hexoses) into lactic acid and various alkyl lactates with a high reaction rate for triose conversion. The initial TOF (based on Sn) was increased from 41 to 289 h⁻¹, while a quantitative conversion of triose to ethyl lactate in ethanol was achieved.⁴³

Conversion of various pentoses and hexoses into methyl lactate has been demonstrated for the Sn-Beta catalyst. It is found that pentoses are converted to methyl lactate in slightly lower yields (∼40%) than what is obtained for hexoses (∼50%), but higher yields of glycolaldehyde dimethyl acetal are observed for the pentoses.⁴⁴

In the present work, we conducted a systematic study of the dissolution and conversion of cellulose in the presence of Sn(IV) complexes, and using several reaction conditions, with the aim of evaluating the influence of different variables in this process. Our results were compared with those obtained from non-catalyzed reactions, and from reactions using H₂SO₄ as a catalyst.

2. Methods

2.1. Materials

All chemicals were used as received without further purification Butylstannoic acid, (C₄H₉)SnO(OH) – BTA; di-n-butyl-oxo-stannane, (C₄H₉)₂SnO – DBTO; dibutyltin dilaurate, (C₄H₉)₂Sn(C₁₂H₂₃O₂)₂ – DBTDL; and tin(IV) oxide, SnO₂ nanopowder (<100 nm particle size (BET)) were obtained from Sigma-Aldrich. H₂SO₄ was obtained from Merck (analytical grade), and microcrystalline AVICEL™ PH 101 cellulose (mean particle size 50 μm) was purchased from Fluka.

2.2. Reaction and analytical procedures

All experiments were performed in a 100 mL batch stainless steel reactor, coupled to a manometer, temperature probe and a magnetic stirrer working at 1000 rpm. The reactor was filled with a cellulose suspension (0.48 g of cellulose in 60 mL of de-ionized water), and, for some experiments, with catalyst (2.69 × 10⁻⁵ mol). The system was heated to the desired temperature (either 150 °C or 190 °C) and the reaction was allowed to proceed for a specified time (1, 2, 4 or 8 hours).

At the end of the reaction, the residual solid was recuperated by filtration and dried at 90 °C for 24 h. The dissolution–conversion (cellulose consumption) was calculated using eqn (1), where C_c is the cellulose consumption percentage, m_o is the initial mass of cellulose and m_f is the mass of the residual solid. The error on the cellulose conversion is 5%.

It is important to mention that insoluble products from cellulose hydrolysis and degradation can be present in this residual solid.

To detect and quantify the different products formed at the liquid phase, a fraction of the filtrate was passed through a 0.45 μm Millipore™ filter before injection onto a ProStar 210 HPLC system (Varian) equipped with a MetaCarb 87H column (300 mm × 7.8 mm). HPLC was conducted at 50 °C at a flow rate of 0.70 mL min⁻¹ using acidified water (5 × 10⁻³ mol L⁻¹). Carbon-based yields (%) were calculated as the ratio between the mol of carbon in the product and the mol of carbon in charged cellulose, multiplied by 100. These values were used to calculate the selectivity (%), by eqn (2). The following products were detected: glucose, fructose, cellobiose, 5-HMF, 1.6-anhydroglucose, furfuraldehyde, lactic, formic, acetic and levulinic acids.

3. Results and discussion

The aim of this study was to investigate the dissolution and/or conversion of cellulose in the presence of Sn(IV) molecular catalysts. To the best of our knowledge, no information is available in the literature concerning these reaction processes in the presence of the metal molecular complexes employed here as catalysts.

Controlling cellulose hydrolysis and degradation is a technological challenge, and the following factors significantly affect cellulose conversion: crystallinity, degree of polymerization and access to chemical functionalities present in the macromolecule.⁴⁵ These factors play an important role in the pathways followed during cellulose hydrolysis and degradation, especially when heterogeneous catalysts are employed.

Under reaction conditions in which cellulose hydrolysis and degradation can occur, two main processes must be taken into consideration: (1) structural modification of cellulose (a change from a crystalline to amorphous phase through breakage of hydrogen bonds) and (2) amorphous cellulose dissolution, which enables better access to reactive sites along the cellulose molecule.

In this work, the dissolution–conversion of cellulose was examined using three different approaches: (1) in the absence of catalyst; (2) in the presence of sulfuric acid, the catalyst normally used on an industrial scale;⁵ and (iii) in the presence of Sn(IV) species (BTA, BTO and DBTDL). Reactions were performed at two temperatures (150 °C and 190 °C) and for different reaction times (1, 2, 4 and 8 h). We evaluated cellulose consumption (dissolution and/or conversion) by gravimetric analysis, and the detection and quantification of formed products was made using HPLC.

3.1. Evaluation of cellulose consumption

Cellulose conversion was observed even in the absence of catalyst at 150 °C or 190 °C (Table 1). The conversion can be explained by the fact that water molecules can be considered to be in a subcritical state under the reaction conditions employed in this study. In addition, at the higher temperature, the water molecules are found mostly in the ionic state, which assists the H-bond breakage and dissolution.⁴⁶ Although many studies have not considered the possibility that this process can occur in the absence of catalyst, there are reports in the literature describing hydrothermal cellulose conversion.^26,31,33 Based on these observations, it is possible to consider that in the absence of catalysts, water molecules are involved in the dissolution of cellulose, although we cannot dismiss the possibility that they serve as catalysts for hydrolysis and degradation, acting as a Brønsted acid.^24,46

Table 1 Cellulose consumption (wt%) at different temperatures and reaction times

Temperature/°C	Catalyst	Reaction time/h
		1	2	4	8
150	Without	4.8	—	6.8	6.10
	H2SO4	6.1	9.4	10.9	13.8
	BTA	10.5	9.7	9.7	9.9
	DBTO	7.0	5.9	5.2	7.1
	DBTDL	7.1	7.7	7.3	10.9
190	Without	12	13.1	15	19.6
	H2SO4	18.4	21	27	35.7
	BTA	11.3	14.3	17.3	22.5
	DBTO	13.2	13.4	17.6	25.4
	DBTDL	9.7	12	16.1	24.1

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In the presence of sulfuric acid, at 150 °C or 190 °C and at various reaction times, cellulose conversion yields (wt%) were moderately greater than the conversion yields observed when Sn(IV) catalysts were used. In reactions run without a catalyst, cellulose consumption was, in general, comparable to reactions conducted in the presence of Sn(IV) species.

At a reaction temperature of 150 °C, the catalytic behavior of the three species of Sn(IV) catalysts tested was difficult to establish. Under these reaction conditions, low yields were obtained (between 4.8 and 13.8%), even with 8 hour reaction times, indicating that 150 °C is not a sufficient reaction temperature for this process. However, at 190 °C, significant amounts of cellulose consumption was observed, considering the short reaction time, and yields observed for Sn(IV) based catalysts were similar.

3.2. Evaluation of product formation

At 150 °C, product formation was insignificant, in agreement with data from the literature, since higher temperatures are normally employed in this process.^47,48 When Sn(IV) catalysts were used, only a small amount of glucose was detected at 8 h for each catalyst. At this temperature, it is possible that only a small degree of cellulose dissolution occurs (as evidenced by the lower cellulose consumption yields obtained, Table 1), and the metal catalysts are either not active or have low interactions with the reaction medium.

Table 2 presents product formation results from reactions conducted at 190 °C, which were detected as described in the experimental methods section.

Table 2 Soluble products identified from cellulose conversion at 190 °C (selectivity (%))

Catalyst	Reaction time/h	Glucose (%)	Fructose (%)	Cellobiose (%)	5-HMF (%)	1.6-Anhydroglucose (%)	Lactic acid (%)	Formic acid (%)	Acetic acid (%)	Levulinic acid (%)	Furfuraldehyde (%)
nd = not detected.
Without	1	4.6	nd	2.9	0.5	0.4	nd	nd	nd	nd	nd
	2	7.8	3.2	0.0	2.6	1.1	0.6	nd	nd	nd	nd
	4	11.9	3.9	4.3	5.2	1.3	1.3	nd	nd	nd	nd
	8	12.6	2.1	0.0	7.3	2.6	3.8	0.7	0.5	nd	nd
H2SO4	1	42.6	nd	1.2	2.3	0.5	nd	nd	nd	nd	nd
	2	42.4	6.6	1.0	4.3	0.9	0.4	nd	nd	nd	nd
	4	35.3	5.7	0.6	6.6	1.3	1.1	1.8	0.5	1.2	1.1
	8	21.6	nd	0.4	7.5	1.1	1.3	1.6	0.2	2.7	0.5
BTA	1	3.7	1.2	3.7	1.1	6.2	2.2	1.5	0.7	nd	nd
	2	4.7	1.3	3.8	1.2	6.4	2.8	0.7	0.6	nd	nd
	4	6.3	1.3	2.0	2.1	7.5	2.9	1.3	1.4	nd	nd
	8	9.3	2.0	0.7	1.5	6.7	7.4	1.2	1.1	nd	nd
DBTO	1	2.3	0.8	2.7	0.5	3.7	2.0	nd	nd	nd	nd
	2	5.2	1.1	4.1	2.0	5.3	3.0	0.7	0.7	nd	nd
	4	3.5	0.9	1.6	1.6	6.0	8.8	1.7	1.4	nd	nd
	8	2.4	0.6	0.7	2.1	5.7	10.6	1.6	1.5	nd	nd
DBTDL	1	3.0	1.1	3.2	1.0	3.7	1.7	nd	nd	nd	nd
	2	5.1	1.2	4.6	3.5	5.6	3.4	nd	0.8	nd	nd
	4	4.7	1.1	2.4	2.8	6.9	7.6	1.8	1.5	nd	nd
	8	4.3	1.0	0.9	2.6	6.4	11.3	1.9	2.0	nd	nd

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Under these conditions and in the absence of catalyst, we observed mostly formation of glucose and low amounts of fructose, cellobiose, 5-HMF, 1.6-anhydroglucose and organic acids (lactic, formic and acetic). In the presence of H₂SO₄, there is apparent selectivity for glucose formation and we also observed the formation of low amounts of fructose, cellobiose, 5-HMF, 1.6-anhydroglucose, organic acids and furfuraldehyde. It is important to mention that only in the presence of H₂SO₄ the formation of levulinic acid and furfuraldehyde was observed.

Notably, Sn(IV) catalytic species appear to be more selective for the formation of cellobiose, 5-HMF, 1,6-anhydroglucose, lactic, formic and acetic acid. Despite the fact that the cellulose consumption was slightly higher in the presence of these Sn(IV) complexes, compared with processes conducted in the absence of catalyst, and a little lower than those observed for reactions catalyzed using H₂SO₄, the product formation profile was considerably different showing the greater potential of these species as catalysts for cellulose consumption processes and valuable organic chemicals production, even following short reaction times.

Comparing the three tested Sn(IV) catalysts, the amount of cellulose consumption was comparable for the three metallic species employed. In addition, product formation profiles were quite similar. This behavior can suggest that the same kind of active species is responsible for the transformations. To clarify this point, the use of SnO₂ as a catalyst was investigated at 190 °C and a reaction time of 4 hours. The cellulose consumption yield (17.0%) was comparable to the average observed for the Sn(IV) catalysts employed, under the same reaction conditions. Also, the product formation profiles were similar (glucose = 5.3%, fructose = 0.3%, cellobiose = 4.1%, 5-HMF = 4.9, 1,6-anhydroglucose = 11.2, lactic acid = 12.5% and formic acid = 4.8%), suggesting that, under these reaction conditions, oxide or hybrid oxide formation is possible as a hydrolysis product from the Sn(IV) molecular species.⁴⁷

It is important to mention that the exception is the amount of lactic acid observed, at 4 and 8 hours of reaction, which is higher when the DBTDL and DBTO catalysts are employed, in comparison to BTA. In this case, due the presence of the same number and type of ligands (alkyl and carboxylate) in the molecular catalyst structure of DBTDL and DBTO, maybe the same species are formed by hydrolysis of these catalysts, as for example, hybrid oxides.⁴⁸

In the presence of a Brønsted acid and water, several pathways may explain the observed product formations. First, the cellulose polymer chains are broken down into low molecular mass fragments, for example cellobiose, which is considered to be the basic unit of cellulose, and is composed of two glucose molecules.⁴⁹ Glucose formation is due to hydrolysis of the cellulose chain and its fragments.^24,50 1,6-Anhydroglucose is formed by dehydration of glucose between the C-1 and C-6 carbon positions of the molecule.^51,52 Note that glucose can also be converted to fructose by isomerization.⁵¹ 5-HMF can be obtained by dehydration of hexoses, mainly glucose and fructose,^50,53,54 while the resulting 5-HMF can be rehydrated to formic and levulinic acid.⁵⁵ Organic acids are formed through degradation of fructose or glucose, and also via 5-HMF, by a pathway that is not very well defined.⁵⁶ For example, the mechanism involved in the acid lactic formation is still under debate and a formal intramolecular Cannizzaro and a redox mechanism involving Meerwein–Ponndorf–Verley reduction and Oppenauer oxidation have been suggested, in the presence of Brønsted or Lewis acids.^5,17,57 Finally, the mechanism involved in furfuraldehyde formation is under investigation and some reports mention the possibility that this product may be formed via consecutive tautomerisation and retro-aldol reactions.^13,58–60

In the case of Sn(IV) species, it is possible to envision that these different pathways can be followed, although certain reactions are preferred, such as those involved in 1,6-anhydroglucose and organic acids formation. A possible reaction mechanism based on species containing Lewis acid sites (such as Sn(IV) species) could involve coordination of cellulose oxygen species to the metal center (Fig. 1), activating bonds for subsequent reactions, such as hydrolysis and degradation.^7,38

Fig. 1 Schematic illustration of oxygen species activation via acid–base Lewis type interactions.

4. Conclusions

Sn(IV) molecular catalysts are capable of converting cellulose into valuable chemical supplies, exhibiting interesting yields and selectivities, without the problems associated with the use of inorganic acids in these processes. Comparing the three Sn(IV) catalysts tested, the amount of cellulose consumption and product formation profiles were quite similar, suggesting that the same kind of active species is formed in the reaction medium.

Acknowledgements

This work was supported by the National Council for Scientific and Technological Development (CNPq) and Brazilian Federal Agency for the Improvement of Higher Education (CAPES). JBS, FLS, FMRSA, TM and JHB thank CNPq for their grants. MRM and SMPM thank CNPq for research fellowships.

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