molecules Review Recent Advances in Solvents for the Dissolution, Shaping and Derivatization of Cellulose: Quaternary Ammonium Electrolytes and their Solutions in Water and Molecular Solvents Marc Kostag 1 ID , Kerstin Jedvert 2 ID , Christian Achtel 3, Thomas Heinze 3 and Omar A. El Seoud 1,* ID 1 Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes 748, 05508-000 São Paulo, SP, Brazil; [email protected] 2 Bio-based Fibres, Swerea IVF, P.O. Box 104, SE-431 22 Mölndal, Sweden; [email protected] 3 Centre of Excellence for Polysaccharide Research, Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University of Jena, Humboldtstraße 10, 07743 Jena, Germany; [email protected] (C.A.); [email protected] (T.H.) * Correspondence: [email protected]; Tel.: +55-11-3091-3874 Received: 15 December 2017; Accepted: 14 February 2018; Published: 25 February 2018 Abstract: There is a sustained interest in developing solvents for physically dissolving cellulose, i.e., without covalent bond formation. The use of ionic liquids, ILs, has generated much interest because of their structural versatility that results in efficiency as cellulose solvents. Despite some limitations, imidazole-based ILs have received most of the scientific community’s attention. The objective of the present review is to show the advantages of using quaternary ammonium electrolytes, QAEs, including salts of super bases, as solvents for cellulose dissolution, shaping, and derivatization, and as a result, increase the interest in further investigation of these important solvents. QAEs share with ILs structural versatility; many are liquids at room temperature or are soluble in water and molecular solvents (MSs), in particular dimethyl sulfoxide. In this review we first give a historical background on the use of QAEs in cellulose chemistry, and then discuss the common, relatively simple strategies for their synthesis. We discuss the mechanism of cellulose dissolution by QAEs, neat or as solutions in MSs and water, with emphasis on the relevance to cellulose dissolution efficiency of the charge and structure of the cation and. We then discuss the use of cellulose solutions in these solvents for its derivatization under homogeneous and heterogeneous conditions. The products of interest are cellulose esters and ethers; our emphasis is on the role of solvent and possible side reactions. The final part is concerned with the use of cellulose dopes in these solvents for its shaping as fibers, a field with potential commercial application. Keywords: quaternary ammonium electrolytes; molecular solvents; super bases; cellulose dissolution mechanism; cellulose derivatization; cellulose shaping; biomass conversion 1. Introduction The demand for cellulosic fibers—natural and man-made—is increasing continually because of world population growth. Cotton production, however will not meet this increased demand. Consequently, a rational strategy to close this “cellulosic fiber gap” is to increase the production of man-made cellulosic fibers, following the principles of green chemistry [1–3]. Industrially, fibers, e.g., from wood cellulose, are obtained from Viscose (cellulose xanthate in alkali solution) by extrusion in acid bath to produce the fiber Rayon [4–6], or by regeneration of cellulose solutions in N-methylmorpholine N-oxide (NMMO) hydrate in an aqueous bath to produce the fiber Lyocell [7–11]. Molecules 2018, 23, 511; doi:10.3390/molecules23030511 www.mdpi.com/journal/molecules MoleculesMolecules2018 2018, 23,, 22 511, x 2 of2 39 of 38 [7–11]. The limitations of both solvents and processes, coupled with the increased emphasis on Thesustainability limitations of have both prompted solvents andthe processes,search for coupledalternative with greener the increased substitutes. emphasis This necessity on sustainability is the haveimpetus prompted for the the continued search forinterest alternative in developing greener new substitutes. cellulose solvents, This necessity despite isthe the existence impetus of for theseveral continued ones interestin commercial in developing use [12–16]. new Imidazole-based cellulose solvents, ILs have despite gained the importance existence in of the several areas of ones in commercialcellulose dissolution, use [12– 16processing]. Imidazole-based and derivatization, ILs have resulting gained importancein a large number in the of areas publications, of cellulose dissolution,including processingreview articles and [17–46]. derivatization, Some quaternary resulting ammonium in a large numberelectrolytes of publications,(QAEs) have melting including reviewpoints articles below [10017– 46°C]. and Some can, quaternary therefore, be ammonium classified as electrolytes a subclass of (QAEs) ionic liquids have (ILs). melting In the points present below 100review,◦C and we can, cover therefore, QAEs that be classifieddo not bear as heterocyc a subclasslic rings, of ionic e.g., liquids tetraalkylammonium (ILs). In the present halides. review, We wealso cover discuss QAEs deep that eutectic do not bearsolvents heterocyclic (DES), and rings, a special e.g., tetraalkylammoniumclass of heterocyclic derivatives, halides. We namely, also discuss the deepsalts eutectic obtained solvents by neutralization (DES), and aof special heterocyclic class of superbases. heterocyclic After derivatives, giving a namely,short account the salts on obtained their byhistory, neutralization we dwell of heterocyclicon recent advances superbases. in their After use as giving solvents a short for cellulose account dissolution, on their history, derivatization, we dwell on recentand advancesshaping. in their use as solvents for cellulose dissolution, derivatization, and shaping. The use of aqueous solutions of cellulose in urea, urethane, guanidine and their derivatives was The use of aqueous solutions of cellulose in urea, urethane, guanidine and their derivatives was already reported in 1924 [47]. Alkali-containing [48,49] and alkali-free aqueous quaternary already reported in 1924 [47]. Alkali-containing [48,49] and alkali-free aqueous quaternary ammonium ammonium hydroxides were reported as cellulose solvents as well (Figure 1, top) [50]. hydroxides were reported as cellulose solvents as well (Figure1, top) [ 50]. Benzyl-substituted Benzyl-substituted quaternary ammonium hydroxides were found to be more efficient cellulose quaternary ammonium hydroxides were found to be more efficient cellulose solvents [51,52], leading to solvents [51,52], leading to the discovery of the well-known electrolytes Triton B and Triton F the(Figure discovery 1, bottom) of the [53,54]. well-known Nevertheless, electrolytes these cell Tritonulose Bsolvents and Triton did not F have (Figure technical1, bottom) importance, [ 53,54 ]. Nevertheless,mainly due these to their cellulose high water solvents content, did not and have use technical of expensive importance, silver oxide mainly in duetheir to synthesis their high (for water content,halide/hydroxide and use of expensiveanion exchange). silver oxide in their synthesis (for halide/hydroxide anion exchange). FigureFigure 1. Top:1. Top: from from Lilienfeld’s Lilienfeld’s 1924 1924 patentedpatented electrolytes for for cellulose cellulose dissolution dissolution [48,49]; [48,49 bottom:]; bottom: the first commercially available ammonium hydroxides for cellulose dissolution, known as Triton B the first commercially available ammonium hydroxides for cellulose dissolution, known as Triton B (left) and Triton F (right) [51]. (left) and Triton F (right) [51]. The inorganic compound ammonium thiocyanate in combination with ammonia was also reportedThe inorganic as a cellulose compound solvent ammonium quite early [55]. thiocyanate It was shown in combination that several with freeze-thawing ammonia wascycles also reportedenhanced as acellulose cellulose dissolution solvent quite[56,57]. early Because [55]. wa Itter was and shown ammonia that are several not suitable freeze-thawing for obtaining cycles enhancedsome cellulose cellulose derivatives, dissolution and [56 inorganic,57]. Because electrolyt wateres andhave ammonialimited solubility are not suitablein organic for solvents, obtaining someorganic cellulose ammonium derivatives, salts became and inorganic the logical electrolytes candidates have to focus limited on. solubility Hence, in in 1986 organic the binary solvents, organicmixture ammonium of tetraethylammonium salts became the chloride logical (N candidates2222Cl) in DMSO to focus was on. reported Hence, to in dissolve 1986 the up binary to 12 mixturewt% of tetraethylammoniumcellulose at 100 °C [58]. chloride Pyridine (N and2222 DMFCl) inwere DMSO suitable was MSs reported as well. to This dissolve solvent up system to 12 wt (QAE/MS) % cellulose at 100is the◦C[ organic58]. Pyridine counterpart and of DMF the solvent were suitable LiCl/DMAc MSs that as well.was introduced This solvent a few system years (QAE/MS)earlier [59]. is the organicIntroduction counterpart of of imidazole-based the solvent LiCl/DMAc ILs as cellulose that was solvents introduced [17], increased a few years noticeably earlier the [59 ].effort to developIntroduction new solvents of imidazole-based for cellulose. The ILs structural as cellulose vers solventsatility of ILs [17 ],is increasedshared by simple noticeably QAEs the that effort are to used alone, or as solutions in water and MSs. In the present review, we focus on QAE-based develop new solvents for cellulose. The structural versatility of ILs is shared by