Zeolite-promoted hydrolysis of cellulose in ionic liquid, insight into the mutual behavior of zeolite, cellulose and ionic liquid

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Abstract

In this paper, a variety of H-type zeolites were demonstrated as efficient catalysts for hydrolysis of cellulose in ionic liquid (IL) to afford high yield of glucose under mild condition without a pretreatment process. By supplying water gradually as the reaction progresses, HY with the acid amount of 11.1 mol% promoted the highest glucose yield of 50.0% at 130 °C, and the yield of the three main products (glucose, cellobiose and 5-hydroxymethylfurfural) was up to 97.2%. It was found that the pore size, the acid amount, the water amount and the timing of water addition are critical factors for high hydrolysis efficiency. XRD characterization suggested that the framework structure of zeolite was particularly stable in 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), while the cell parameter were enlarged due to the dilatation effect of [BMIm]Cl. Elemental analysis of the IL-treated HY demonstrated that IL entered the channel of HY. As such, part of the intra-channel space of HY was occupied, making the average pore diameter, pore volume and the BET surface area of IL-treated HY all decreased, which were demonstrated by physisorption characterization. FT-IR through pyridine adsorption showed that H+ cations generated in-situ from the Brönsted acid sites of zeolite were the key active species for the good hydrolysis performance, while the Lewis acid sites did not exhibit high activity. By simple calcination, the recovered HY showed excellent reusability. Finally, the proposed reaction pathway and recycle of the catalyst were presented.

Highlights

▸ H-type zeolites exhibited high activity for hydrolysis of cellulose in ionic liquid. ▸ Hydrolysis efficiency strongly depends on the pore size, the acid amount, the water amount and the timing of water addition. ▸ H+ generated in-situ from the Brönsted acid sites of zeolite was the key active species. ▸ The structure properties of zeolite in ionic liquid that unlike its usual state were well identified. ▸ After simple calcination of the catalyst, the activity was fully recovered.

Introduction

Studies on the conversion of cellulose into biofuels and chemicals have been one of the most intensive pursuits worldwide [1], [2], [3], [4]. Among various cellulose conversion processes, hydrolysis of cellulose to glucose is virtually an essential step in most practical cellulosic chemical or biofuel production [5]. However, cellulose is a highly crystalline polymer of d-anhydroglucopyranose unites jointed together in long chains by β-1,4-glycosidic bonds, the tight hydrogen-bonding network and van der Waals interactions greatly stabilize cellulose, making it indiscerptible in most solvents and thus notoriously resistant to hydrolysis [6], [7].

Pioneer studies by Rogers and co-workers [8] showed that imidazolium-based ionic liquids (ILs) are a kind of powerful solvents that could dissolve cellulose. It thus opens an avenue for the conversion of cellulose in a homogeneous circumstance since it leaves the cellulose chains more accessible to chemical transformations. Enlightened by this discovery, we firstly reported the hydrolysis behavior of cellulose in ionic liquids in the presence of mineral acids [9], [10]. It was interesting to find that catalytic amounts of acid were sufficient to drive the hydrolysis reaction, while the traditional acid hydrolysis of cellulose in water was inefficient and cost-intensive. Through progressive addition of water during the course of cellulose hydrolysis, Raines [11] and Bell and Dee [12] further enhanced the hydrolysis efficiency. As the recovery of mineral acids is often difficult, we [13] and other groups [14], [15], [16], [17], [18], [19], [20], [21] recently developed solid acids catalyzed hydrolysis of cellulose in water. However, because both cellulose and solid acids are typically not soluble in water, solid acids are generally much less effective than liquid acids. To achieve a high conversion of cellulose, the amount of solid acid used is commonly equal to [14], [15], and even greater [13], [15], [16], [17], [18], [19], [20] than the amount of cellulose. Moreover, this process usually proceeds under harsh hydrothermal conditions with ball-milling pretreated cellulose [13], [16], [20]. To avoid the disadvantages and improve the efficiency, Schüth and co-workers [22], [23] first reported the hydrolysis of cellulose over acid resin Amberlyst 15DRY in ionic liquid. In their catalytic system, cellulose undergoes selective depolymerization to yield exclusively cellulose oligomers (cellooligomers). In despite of these progresses, the above work focuses on the production of cellooligomers, and more efficient process that favors β-1,4-glycosidic bonds cracking to afford high yield of glucose is to be developed. Furthermore, due to the unique ionic environment of ILs, the behavior of solid acid in ILs might be different from that exhibited in water, and the mutual interactions of solid acid, cellulose, as well as ionic liquids remain unclear yet.

In this paper, we focus on performing the depolymerization of cellulose over zeolites, a kind of solid acids containing both Lewis and Brönsted acid sites, in a controlled manner to afford high glucose yield in IL. By using XRD, physisorption, FT-IR, NH3-TPD characterizations and element analysis, we provide new insights into the behavior of zeolite for cellulose hydrolysis in IL. A hydrolysis mechanism over zeolite in IL is proposed based on the catalytic results and the characterization. A catalyst recycle strategy is presented as well.

Section snippets

Materials

Microcrystalline cellulose (extra pure, average particle size 90 μm) is purchased from Acros organics. HY (Si/Al = 4), HBeta (Si/Al = 25), HZSM-5 (Si/Al = 140) and SAPO-34 (Si/P/Al = 0.4/1/1) zeolites are purchased from Nankai University, China. [BMIm]Cl was prepared according to our previous study [9], the water content was determined as (0.0975 ± 0.0005)% through Karl–Fischer titration. All other chemicals were supplied by local suppliers and used without further purification.

Characterization methods

Elemental analysis of ionic

Preliminary study on zeolites promoted cellulose hydrolysis in IL

Table 1 summarizes the catalytic performances of various H-type zeolites, namely HY, HBeta, HZSM-5 and SAPO-34, for cellulose hydrolysis reaction in IL 1-butyl-3-methylimidazolium chloride ([BMIm]Cl). As a reference reaction, cellulose hydrolysis in water using HY as a catalyst was first tested. It was found that owing to the insolubility of cellulose and solid acid, the solid–solid interaction of cellulose and the catalyst was routinely inefficient, only trace amounts of products were detected

Conclusions

In summary, the combination of a variety of zeolites with IL were demonstrated as efficient systems for hydrolysis of cellulose under mild condition without a pretreatment process. It was found that the pore size, the acid amount, the water amount and the timing of water addition are critical factors for high hydrolysis efficiency. By using XRD, physisorption, FT-IR, NH3-TPD characterizations and element analysis, we provide new insights into the behavior of zeolite for cellulose hydrolysis in

Acknowledgements

Supports from the National Natural Science Foundation of China (No. 21003121, 21176235), 973 program of China (2009CB226102) and Dr Start-up Fundation of DICP (S201107) are acknowledged.

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