Heterogeneous cyclization of sorbitol to isosorbide catalyzed by a novel basic porous polymer-supported ionic liquid

Highlights

• A facile method for basic porous polymer-supported ionic liquid was demonstrated.

• Basic ILs were successfully supported on the internal surface of the mesoporous polymer by covalent bond.

• Efficient and heterogeneous synthesis of isosorbide from sorbitol and DMC under base condition was achieved for the first time.

• The catalyst exhibited excellent catalytic activity and recyclability.

Abstract

In this study, heterogeneous cyclization of sorbitol to isosorbide under basic condition was realized for the first time with a novel porous polymer-supported ionic liquid as catalyst. These polymer-supported ILs were synthesized through the suspension polymerization of 4-vinylbenzyl chloride and divinylbenzene, followed by a quaternization reaction. As compared to those of non-porous, the porous polymers had high specific surface area and large number of active sites. Consequently, they exhibited excellent catalytic activity in the cyclization of sorbitol with dimethylcarbonate (DMC) to isosorbide. As a result, a high conversion of sorbitol (99%) was achieved with 83% yield of isosorbide under optimized conditions. Importantly, the catalysts could be easily separated by decantation and reused for five times without obvious loss of catalytic activity.

Introduction

Isosorbide is a versatile platform molecule with wide applications as a monomer or building block for biogenic polymer, functional materials, pharmaceutical molecules, and new solvents [[1], [2], [3], [4], [5], [6], [7], [8]]. Various homogeneous catalysts have been developed to promote transformation of sorbitol to isosorbide, and this reaction is standing as the final step of industrial process for isosorbide from starch [[9], [10], [11], [12], [13]]. In this regard, the direct acid catalysis has dominated the field, which involves two stepwise protonation of the regioselective hydroxyl group, and each followed by cyclization with the carbon atom [14,15]. Recently, much effort has been made towards addressing the catalyst recyclability by grafting the reactive Brφnsted acid into the skeleton of ionic liquids (ILs) [16]. However, serious drawbacks of environmental pollution arising from significant handling risk, corrosive nature of the catalyst and stability issues of the products are presented under this protocol. In comparison with these homogeneous catalysts, heterogeneous catalysts are industrially preferred for ease of separation and reuse. Therefore, solid acids with various structures are chosen to promote the transformation from sorbitol to isosorbide [[17], [18], [19], [20], [21], [22], [23]]. However, the catalytic system is still lack of practical value because of the requirement for high reaction temperature, long reaction time, abundant catalyst loading, reduced pressure condition and limited selectivity of product as well. Many studies have confirmed that neither homogeneous acid catalysts nor solid acids are ideal choices for achieving the green production of isosorbide. Alternatively, the less explored base catalysis in the presence of stoichiometric amount of dimethyl carbonate functioning as carboxymethylating agent for hydroxyl group provides an attractive choice since it can be conducted under mild conditions with high efficiency [24]. Recently, such a conversion was achieved with a high yield in the presence of catalytic amounts of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) [25]. To the best of our knowledge, the application of basic heterogenous system for synthesis of isosorbide has not been reported yet.

As one type of the potential and powerful supports, polymers have several advantages, such as easy preparation, structural tunability and fast recovery. The polymer-supported catalysts have been widely used in heterogeneous transition metal catalysis [[26], [27], [28], [29]] and organocatalysis [30,31]. The polymers with uniform size and controlled pore structures enable themselves to be used to develop fixed-bed flow reactors for continuous flow processes [32,33]. On the other hand, supported ILs which achieve the advantages of combining the ILs and heterogenous catalysis, have attracted growing interest [[34], [35], [36]]. For instance, supported acidic ILs have been employed as catalysts in the bulk and fine chemical industries [37,38]. Among these, the polymer supported acidic IL catalyst exhibits high selectivity in the dehydration of sorbitol to 1,4-anhydro-D-sorbitol [39]. Meanwhile, supported basic ILs show good catalytic activity for base-catalyzed reactions. Examples include Knoevenagel reaction [40], Henry reaction [32], transesterification reaction [41,42] and cascade reaction [43]. In most cases, the active sites of the supported basic IL catalysts are basic anions, such as hydroxy, acetate or bicarbonate anions. The anions are introduced into the catalyst through an ion-exchange process. However, deactivation of the catalyst often occurs during the reaction imposed by the slow exchange of the catalytic anion with other less reactive anions [32].

We speculated that the nitrogen tricyclic group is more stable and efficient than the basic anion as an activator for DMC. A novel porous polymer supported IL with nitrogen tricyclic group was designed as heterogenous catalyst for the conversion of sorbitol with DMC. Compared with traditional basic anion type supported basic IL, this catalyst will be much more stable. Furthermore, the porous structure will ensure higher degree of exposure of active sites to the reactants.

In this article, we demonstrate a facile two-step synthetic method to prepare polymer-supported basic IL catalyst and the first heterogenous cyclization of sorbitol with DMC under basic condition. In the prepared catalyst, the catalytic sites were connected to catalyst matrix through covalent bond rather than basic anions such as hydroxyl. In addition, the catalyst with mesopores had much higher loading amount of active ILs moieties than nonporous one. By launching this novel basic polymer-supported IL catalyst, isosorbide could be obtained with 83% yield at 140 °C. The catalyst could be reused for five times with 7% decrease in yield of isosorbide.