938 Catal. Sci. Technol., 2013, 3, 938--941 This journal is c The Royal Society of Chemistry 2013 Cite this: Catal. Sci. Technol., 2013, 3, 938 Heterogeneously catalysed production of isosorbide tert-butyl ethers† Marcus Rose,* Katharina Thenert, Rebecca Pfu ¨tzenreuter and Regina Palkovits* Isosorbide tert-butyl ethers (ITBE) have been produced by the acidic ion exchange resin-catalysed etherification of biogenic iso- sorbide with tert-butanol as well as with isobutene, resulting in high boiling liquids. Lignocellulose is considered the most reliable and sustainable of all renewable biogenic resources. All kinds of biowaste, even wood and straw residues, can be utilized. Thus, no valuable soil for the production of edibles has to be occupied and additionally, no pesticides and fertilizers are necessary for its production. Isosorbide is one of the most promising platform chemicals that can be derived from cellulosic biomass. 1 Nowa- days it is produced from starch. But efficient methods for the direct production from cellulose in one pot reactions are also under investigation. 2 Isosorbide bears great potential for further transformation into valuable products. Especially polymers, solvents, fuels and other consumables are of great interest. 1,3 Nevertheless, the further conversion of isosorbide is yet not well explored for utilization on the industrial scale. Especially the selective etherification of the free hydroxyl groups of isosorbide with aliphatic substituents is aspired. Ether compounds such as methyl-, ethyl- as well as glycerol tert-butyl ethers (MTBE, ETBE, GTBE) are commercially used or in the case of GTBE considered fuel additives to improve the burning behavior as well as increase the octane number. 4 In the case of bio-ethanol or bio-glycerol, the ratio of renewables in ETBE and fully converted GTBE is 30 and 22 wt%, respectively. In contrast, isosorbide tert-butyl ethers (ITBE) provide a significantly higher renewable ratio of 41 wt%. In a similar manner, they could also be of interest, e.g., as biogenic fuel additives or as high boiling solvents. Additionally, the precursor isosorbide can be derived from cellulose as 2 nd generation biomass, i.e., non-edible biomass. Isosorbide dimethyl ether is already commercialized and considered a viable solvent for pharmaceutical applications. 5 Also its use as a diesel substitute or additive is under discussion. 6 So far, most of the research has dealt with the etherification using the Williamson method using alkyl halides as reactants. 7 By applying microwave heating and phase transfer catalysis an efficient synth- esis of aliphatic long chain as well as aromatic ethers with yields >90% within a few minutes has been reported. 8 But the production of stoichiometric amounts of halides as by-products renders this method not suitable for large scale production. Recently, Tundo et al. reported the successful synthesis of isosorbide dimethyl ether using dimethyl carbonate as a solvent and a reactant homo- geneously catalysed by bases such as sodium methanolate. 9 An alternative route for the synthesis of O-C 8 mono- and diethers by telomerization of 1,3-butadiene with isosorbide, giving up to 100% conversion and up to 60% yields of the diether, was published by Lai et al. 10 Unfortunately, due to the reaction path several isomers of the resulting aliphatic chains have been observed instead of a single product. Additionally, this method cannot be applied for the production of short chain ethers (C o8 ). Herein we report for the first time the efficient etherification of isosorbide with tert-butanol and isobutene heterogeneously catalysed by acidic ion exchange resins (Scheme 1). The advan- tages of these routes are clearly the avoidance of by-products except the formation of water from the alcohol condensation reaction. Furthermore, the use of solid catalysts simplifies their recycling. In the ideal case even implementation of a continuous process is facilitated. In all experiments three different isosorbide tert-butyl ether derivatives have been observed: two different monoethers (ME1, ME2) due to the different configuration of the hydroxyl groups and the diether (DE). Separation of these substances by column chro- matography using polar silica gel as the stationary phase exhibited elution occurring in accordance with the polarity of the molecules: DE ( R f = 0.88), ME1 ( R f = 0.46), ME2 ( R f = 0.26), isosorbide ( R f = 0.14). ME1 and DE are colourless liquids at 20 1C while ME2 is a colourless solid like the substrate isosorbide. This difference in polarity and melting point of the two monoethers can be Lehrstuhl fu ¨r Nanostrukturierte Katalysatoren, Institut fu ¨r Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany. E-mail: [email protected]; Fax: +49 241 8022291; Tel: +49 241 8026466 † Electronic supplementary information (ESI) available: Experimental details, NMR, MS, IR data. See DOI: 10.1039/c3cy20752h Received 6th November 2012, Accepted 16th January 2013 DOI: 10.1039/c3cy20752h www.rsc.org/catalysis Catalysis Science & Technology COMMUNICATION Downloaded by University of New Hampshire on 11 March 2013 Published on 17 January 2013 on http://pubs.rsc.org | doi:10.1039/C3CY20752H View Article Online View Journal | View Issue
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938 Catal. Sci. Technol., 2013, 3, 938--941 This journal is c The Royal Society of Chemistry 2013
Cite this: Catal. Sci. Technol.,2013,3, 938
Heterogeneously catalysed production of isosorbidetert-butyl ethers†
Marcus Rose,* Katharina Thenert, Rebecca Pfutzenreuter and Regina Palkovits*
Isosorbide tert-butyl ethers (ITBE) have been produced by the
acidic ion exchange resin-catalysed etherification of biogenic iso-
sorbide with tert-butanol as well as with isobutene, resulting in
high boiling liquids.
Lignocellulose is considered the most reliable and sustainableof all renewable biogenic resources. All kinds of biowaste, evenwood and straw residues, can be utilized. Thus, no valuablesoil for the production of edibles has to be occupied andadditionally, no pesticides and fertilizers are necessary for itsproduction. Isosorbide is one of the most promising platformchemicals that can be derived from cellulosic biomass.1 Nowa-days it is produced from starch. But efficient methods for thedirect production from cellulose in one pot reactions are alsounder investigation.2 Isosorbide bears great potential for furthertransformation into valuable products. Especially polymers,solvents, fuels and other consumables are of great interest.1,3
Nevertheless, the further conversion of isosorbide is yet not wellexplored for utilization on the industrial scale. Especially theselective etherification of the free hydroxyl groups of isosorbidewith aliphatic substituents is aspired. Ether compounds such asmethyl-, ethyl- as well as glycerol tert-butyl ethers (MTBE, ETBE,GTBE) are commercially used or in the case of GTBE consideredfuel additives to improve the burning behavior as well as increasethe octane number.4 In the case of bio-ethanol or bio-glycerol, theratio of renewables in ETBE and fully converted GTBE is 30 and22 wt%, respectively. In contrast, isosorbide tert-butyl ethers(ITBE) provide a significantly higher renewable ratio of 41 wt%.In a similar manner, they could also be of interest, e.g., asbiogenic fuel additives or as high boiling solvents. Additionally,the precursor isosorbide can be derived from cellulose as 2nd
generation biomass, i.e., non-edible biomass.
Isosorbide dimethyl ether is already commercialized andconsidered a viable solvent for pharmaceutical applications.5
Also its use as a diesel substitute or additive is under discussion.6 Sofar, most of the research has dealt with the etherification using theWilliamson method using alkyl halides as reactants.7 By applyingmicrowave heating and phase transfer catalysis an efficient synth-esis of aliphatic long chain as well as aromatic ethers with yields>90% within a few minutes has been reported.8 But the productionof stoichiometric amounts of halides as by-products renders thismethod not suitable for large scale production. Recently, Tundoet al. reported the successful synthesis of isosorbide dimethyl etherusing dimethyl carbonate as a solvent and a reactant homo-geneously catalysed by bases such as sodium methanolate.9 Analternative route for the synthesis of O-C8 mono- and diethers bytelomerization of 1,3-butadiene with isosorbide, giving up to 100%conversion and up to 60% yields of the diether, was published byLai et al.10 Unfortunately, due to the reaction path several isomers ofthe resulting aliphatic chains have been observed instead of a singleproduct. Additionally, this method cannot be applied for theproduction of short chain ethers (Co8).
Herein we report for the first time the efficient etherificationof isosorbide with tert-butanol and isobutene heterogeneouslycatalysed by acidic ion exchange resins (Scheme 1). The advan-tages of these routes are clearly the avoidance of by-productsexcept the formation of water from the alcohol condensationreaction. Furthermore, the use of solid catalysts simplifies theirrecycling. In the ideal case even implementation of a continuousprocess is facilitated.
In all experiments three different isosorbide tert-butyl etherderivatives have been observed: two different monoethers (ME1,ME2) due to the different configuration of the hydroxyl groups andthe diether (DE). Separation of these substances by column chro-matography using polar silica gel as the stationary phase exhibitedelution occurring in accordance with the polarity of the molecules:DE (Rf = 0.88), ME1 (Rf = 0.46), ME2 (Rf = 0.26), isosorbide (Rf = 0.14).
ME1 and DE are colourless liquids at 20 1C while ME2 is acolourless solid like the substrate isosorbide. This difference inpolarity and melting point of the two monoethers can be
Lehrstuhl fur Nanostrukturierte Katalysatoren, Institut fur Technische und
This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol., 2013, 3, 938--941 939
explained by the different configurations of the hydroxyl groups(2-O exo, 5-O endo) in isosorbide that consists of two cis-fused,‘V’-shaped tetrahydrofuran rings. Thus, ME2 exhibits a highermelting point as well as a higher polarity than ME1 since itforms intermolecular hydrogen bonds. On the other hand theendo-configured hydroxyl group in ME1 is known to formintramolecular hydrogen bonds with the oxygen atom of theopposing ring resulting in a lower total polarity as well as a lowermelting point.11 Additionally, ME1 and ME2 were differentiatedby comparison of the chemical shifts of the free hydroxylgroups with the exclusively endo-configured hydroxyl groupsof isomannide. The 1H NMR spectra measured for equallyconcentrated solutions in CDCl3 confirm the free hydroxylgroups with endo configuration for ME1 and exo configurationfor ME2. A comprehensive characterization of the novel ethercompounds was carried out using NMR, MS and IR spectro-scopy (ESI,† S1–S3).
Most promising results for the production of ITBE have beenobtained using isobutene as a reactant. Isobutene is a basiccompound of the chemical industry and has already beenestablished in the production of MTBE/ETBE by etherificationwith the respective alcohols. Industrially used isobutene isnowadays derived from fossil resources such as crude oil. Inrecent years, reaction routes for the one-step production ofisobutene from biogenic platform chemicals such as ethanol12
or n-butanol13 have been reported. Both routes require acidiccatalysts such as ZnxZryOz as mixed oxides for the former andacidic zeolites such as Theta-1 and ZSM-23 for the lattersubstrate. This opens up the possibility for future ITBE produc-tion exclusively based on renewable feedstocks. In contrast toknown processes for the production of MTBE/ETBE from liquidsubstrate alcohols, ITBE production requires a solvent for theconversion of the solid substrate isosorbide. This solvent has tofulfil certain requirements. Primarily, it has to be inert in thereaction, especially with regard to an addition reaction to isobutene.Furthermore, the solubility of isosorbide and gaseous isobu-tene should be high. We also considered solvent-free moltenisosorbide (M.P. 60–63 1C) for the reaction. However, only lowconversions were obtained probably by a limited solubility ofnon-polar isobutene in molten polar isosorbide.
For the etherification of isosorbide with isobutene, we firstused slightly excess amounts of isobutene in an autoclave reac-tion. Therein we observed a strong temperature dependence ofthe reaction using Amberlyst-15 as catalyst and ethyl acetate assolvent (ESI,† S4). Upon increasing the temperature from 20–90 1Cthe yields of isosorbide ether derivatives decreased significantlydue to the increased formation of by-products such as isobutenedimers and trimers as well as isosorbide acetate derivatives bytransesterification with the solvent ethyl acetate. Only negligibleamounts of these by-products are observed at 20 1C.
Thus, we changed the reaction conditions to ambient tempera-ture and pressure (20 1C, 1 atm) by sparging isobutene through asolution of isosorbide in different suitable solvents (Fig. 1). Highestyields of the isosorbide ethers of up to 80% have been observedusing ethyl acetate as well as dimethyl carbonate (DMC). Anexplanation might be the difference in solvent polarity andsolubility of the reactants and products influencing the equili-brium reaction. Further investigation of this matter is in progress.Increasing the reaction time to 6 h for the DMC system at 20 1C, atotal conversion of isosorbide to the respective tert-butyl etherderivatives of 97% has been observed, including a diether yield of55%. Additionally, recyclability of the catalyst, Amberlyst-15, hasbeen investigated for the isobutene route (Fig. 2). No deactivationhas been observed over four cycles.
The etherification of isosorbide with tert-butanol is anequilibrium reaction due to the reversible elimination of water.Thus, the observed total yields of ether derivatives do notexceed 40%. An SN1 mechanism is assumed by protonationof the tert-butanol hydroxyl group and elimination of waterresulting in a tert-butyl-stabilized carbocation and subsequentaddition of the isosorbide hydroxyl group. Applying thesame conditions, no etherification of isosorbide with primaryalcohols such as ethanol and n-butanol could be achieved sincethe carbocation is much less stabilized. Tert-butanol is used inexcess as a substrate and as a solvent. As a side reactiondehydration of tert-butanol to gaseous isobutene has beenobserved but not further quantified. This reaction is well
Scheme 1 Isosorbide mono- and diethers are produced via two different routesusing tert-butanol and isobutene heterogeneously catalyzed by acidic ionexchange resins (aat 20 1C).
Fig. 1 Yields of the isosorbide ether derivatives at 20 1C using Amberlyst-15 ascatalyst and sparging isobutene as a reactant through the solution (THF –tetrahydrofuran, EA – ethyl acetate, DMC – dimethyl carbonate).
940 Catal. Sci. Technol., 2013, 3, 938--941 This journal is c The Royal Society of Chemistry 2013
known and has been described in the scientific and the patentliterature repeatedly.14
However, we investigated different parameters in the etherifica-tion reaction with tert-butanol at ambient pressure. Upon increasingthe temperature up to 70 1C, the yields of the isosorbide etherderivatives increased significantly (ESI,† S5). Lower yields underreflux conditions (83 1C) might be attributed to the dehydrationreaction of tert-butanol to isobutene as a competing, and attemperatures above 70 1C preferred, process. In an open systemthis reaction can be considered irreversible due to evaporation ofisobutene as reported in the literature before.15 Thus, this effect hasnot been investigated further. If the reaction is carried out in anautoclave at 100 1C, significant formation of isobutene has beenobserved. Nevertheless, total yields of ITBEs do not exceed 33% asexpected in the presence of isobutene and water. This indicates thereversibility of the ether formation in a chemical equilibrium withthe de-/rehydration of tert-butanol and isobutene.
Different acidic ion exchange resins have been tested (ESI,†S6). The Dowex G26-H catalyst exhibits almost no activity. Thiscan be attributed to low accessibility due to the gel-type structureand presumably low degree of swelling in the respective solvent.In comparison, the Amberlyst materials are macroreticularresins with an open and thus accessible pore structure indepen-dent of the solvent. The catalysts Amberlyst-15, -36, -40 and-70 show total yields of ether derivatives of 30, 35, 15 and 14%,respectively. These values correspond to the different amounts ofsulfonic acid groups of 4.7, 5.4, (unspecified) and 2.5 eq kg�1,respectively.16 In accordance, the influence of the amount ofcatalyst has also been shown (ESI,† S7). With an increasing massratio of Amberlyst-15/isosorbide from 0.05 to 0.2, an increase inthe conversion of isosorbide is observed. Further increase doesnot affect the isosorbide conversion significantly. Further experi-ments showed the recyclability of the catalysts (ESI,† S8). Oneload of Amberlyst-15 has been used in four cycles exhibiting onlyslight variation in the catalytic performance. For a detailed studyof the reaction path and kinetics the initial water content and itsdevelopment over time should be taken into account workingunder an inert atmosphere.
The kinetic data obtained at 70 1C using Amberlyst-15 (ESI,†S9) show that the chemical equilibrium is reached after approxi-mately four hours yielding 19% ME1, 13% ME2 and 2% DE. Thepreferential formation of ME1 over ME2 in all experiments mightbe explained by sterical effects providing a lower accessibility ofthe endo hydroxyl group for voluminous reactants.
Conclusions
Summarizing, we reported for the first time the efficient productionof isosorbide tert-butyl ether (ITBE) compounds heterogeneouslycatalysed by commercially available acidic ion exchange resins.We investigated the addition reaction to isobutene as well asthe condensation reaction of isosorbide with tert-butanol.Especially the former reaction poses great potential for anefficient production process under mild reaction conditionsand the option to start exclusively from renewable platformchemicals. In fact, no energy input in the form of heating isnecessary for a high isosorbide conversion and good yields ofthe different ether derivatives. Especially the products beingliquid at room temperature pose great potential, e.g., forapplications as high boiling solvents or additives for diesel orkerosene fuels although the key properties of the compoundsstill have to be investigated. Our future work will focus on thereaction network analysis and kinetic investigations as well asfurther optimization of the reaction and approach continuousprocessing for an efficient scale-up of isosorbide ether production.
Acknowledgements
We would like to thank Hannelore Eschmann, Julia Wurlitzerand Elke Biener for the support regarding the GC analysis. TheRobert Bosch Foundation within the Robert Bosch Fellowshipfor sustainable utilization of renewable natural resources andthe ‘‘Fond der Chemischen Industrie’’ (FCI) are gratefullyacknowledged for the financial support. This work was alsoperformed as part of the Cluster of Excellence ‘‘Tailor-MadeFuels from Biomass’’ (TMFB) funded by the Excellence Initia-tive of the German federal and state governments to promotescience and research at German universities.
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