DOI:10.1002/cssc.201801067 Full Papers Effect of Alcohol Structure on the Kinetics of Etherification and Dehydration over Tungstated Zirconia Julie Rorrer,[a, b] Suresh Pindi,[a, c] F. Dean Toste,[a, c] and Alexis T. Bell*[a, b] Linear and branched ether molecules have attracted recent in- with varying methylbranch positions indicated that substitut- terest as diesel additives and lubricants that can be produced ed alcohols (28,38)and alcohols with branches on the b- from biomass-derived alcohols. In this study,tungstated zirco- carbon readily undergo dehydration, but alcohols with branch- nia was identified as aselectiveand green solid acid catalyst es at least three carbons away from the -OH group are highly for the direct etherification of primary alcohols in the liquid selectivetoether.Anovel model compound, 4-hexyl-1- phase, achieving ether selectivities of >94 %for C6–C12 linear dodecanol, was synthesized and tested to further demonstrate alcohol coupling at 393 K. The length of linear primary alcohols this structure–activity relationship.Trends in the effects of alco- (C6–C12)was showntohave anegligible effect on apparent ac- hol structure on selectivity were consistentwith previously tivation energies for etherification and dehydration, demon- proposed mechanismsfor etherification and dehydration, and stratingthe possibility to produce both symmetrical and asym- help to define possible pathways to selectively form ethers metricallinear ethers. Reactions over aseries of C6 alcohols from biomass-derived alcohols. Introduction As the adverse effectsofanthropogenic climatechange con- (acetone–butanol–ethanol) fermentation.[12–14] Longer chain tinue to increase, the global need for clean and renewable linear alcohols such as 1-octanoland 1-dodecanolcan be pro- energy is now larger than ever before.One method of lower- duced from the condensation of biomass-derived furan plat- ing net atmosphericCO2 emissions is to replaceconventional form molecules, and the hydrolysis of triglycerides andfatty fossil fuels and petroleum-derived specialty chemicals with re- acids, respectively,[15] and then built up into branched alcohols newable alternatives.[1] Ethers have attractedrecent interest as with higher carbon numbersthrough the Guerbet reaction.[16] renewable fuels and lubricants. Symmetric and asymmetric Some generalapproaches to producing linear and branched al- linear ethers in the C8–C12 range are suitable diesel additives coholsfrom biomass-derived platform molecules are summar- owing to their high cetane numbers.[2–5] Branched ethers such ized in Scheme 1. as methyl tert-butylether can also be added to gasoline be- Direct etherification of biomass-derived alcohols over an cause of their high octanenumbers.[6] Longer chain branched acid catalystisanattractive method of producing ethers be- ethers in the C28–C32 range have low pour points andexcellent cause it does not require hydrogen gas and preciousmetal lubricant properties,[7–9] and could replace poly-alpha-olefin lu- catalysts like reductiveetherification.[17] The competing reac- bricants,which are produced fromfossil reserves.[10,11] tion for alcoholdehydration over an acid catalyst is unimolecu- Biomass-derived alcohols are promising candidates for pro- lar dehydration to form alkenes, which is thermodynamically ducing renewable ethers. Ethanol and butanol can be obtained favored at temperatures above approximately 350 K. Owing to in high yields through fermentation processes such as ABE their high volatility,and the propensitytoform gums, alkenes are not desired in fuel and lubricant blends.[18] In apreviousstudy,tungstated zirconia was identified as a [a] J. Rorrer,Dr. S. Pindi, Prof. F. D. Toste, Prof. A. T. Bell Energy Biosciences Institute selectivecatalyst for the liquid-phaseetherification of University of California 1-dodecanol, with ether selectivities above 94 %atmoderate Berkeley,CA94720(USA) temperatures (393 K).[19] This study suggested that the high se- E-mail:[email protected] lectivity to ether is due to acooperative effect between [b] J. Rorrer,Prof. A. T. Bell Brønsted- and Lewis-acid sites on the surface of the catalyst, Department of Chemical and Biomolecular Engineering University of California which promotes the bi-molecularetherification reaction. This Berkeley,CA94720(USA) researchmotivatesthe current study,which aims to evaluate [c] Dr.S.Pindi, Prof. F. D. Toste the extenttowhich tungstated zirconia can be used to pro- Department of Chemistry duce avariety of symmetrical and asymmetrical ethers, and University of California mixtures of ethers, for fuel and lubricant applications. Berkeley,CA94720(USA) Kinetic studies of alcohol dehydration reactions have tradi- Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under: tionally been centered on gas-phase unimolecular dehydra- https://doi.org/10.1002/cssc.201801067. tion.[20–22] Experimental and computational studies of gas- ChemSusChem 2018, 11,3104 –3111 3104 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Papers Scheme1.Generalapproach to producinglinear and branched alcohols from biomass-derived platform molecules. phase unimolecular dehydrationofalcohols over g-Al2O3 have ed zirconia is thermally stable, does not deactivate or swell in supported the theory that increasing the substitution of the al- the reactant media, and has been shown to be an effective cohol increases the stability of the intermediate in the rate- catalystfor etherification of alcohols. Thus, this paper will com- limiting step for dehydration,resulting in adecrease in activa- prehensively study the effects of linear alcohol length, carbon tion barrier for dehydration with increasingsubstitution.[20,23] chain branches, andpositions of carbon chain branches on the Althoughstudying the effect of alcohol structure in the gas kinetics of etherification and dehydration reactions over tung- phase where unimolecular dehydration dominates has provid- stated zirconia in the liquid phase, with the aim of understand- ed some insight into the relationship between unimolecular ing the relationship between ether selectivity and alcohol dehydration rates and substitution, the kinetics of etherifica- structure. tion must also be explored to understand the relationship be- tween alcohol structure and selectivity to ether.Some efforts have been made to investigate the catalytic conversion of al- Results and Discussion cohols to both symmetrical and asymmetricalethers at moder- Catalyst characterization ate temperatures where etherification dominates. Various acidic resins and zeoliteshave been shown to be effective in Tungstated zirconia (WOx/ZrO2,12.6 wt%W)was characterized the liquid-phase etherification of 1-pentanol, 1-hexanol, and 1- with X-ray diffraction, Ramanspectroscopy,and diffuse reflec- octanol to produce symmetrical ethers.[24–27] Tejero et al. have tance infrared Fouriertransform spectroscopy (DRIFTS) FTIR demonstrated that it is also possible to produce asymmetrical with adsorbed pyridine to determine the structure and confirm ethers like ethyl-hexyl ether and ethyl-octyl ether through the presence of Brønsted- and Lewis-acid sites. From the XRD cross-coupling of 1-hexanol or 1-octanol with ethanol or dieth- pattern in Figure 1a,the peaks at 2q angles of 30, 35, 50, and ylcarbonate.[5,28] 598 (yellow) indicatethe presence of tetragonal zirconia,[30–32] But, to our knowledge,the literature is lacking acomprehen- and the peaks between 23–258 (pink) represent the presence sive study of the kinetics of liquid-phaseetherification reac- of bulk tungsten oxide.[33] From the Raman spectra in Fig- 1 tions over alarge range of long-chain 1-alcohols (C6–C12), and ure 1b,the bands at 274, 715, and 807 cmÀ (pink) indicate the amore thorough investigation into the effect of branched al- W Ostretchingofbulk tungsten oxide,[34] and the band at À 1 cohols on the formation of symmetrical and asymmetrical 1020 cmÀ (yellow) is indicative of polymerictungsten oxide [35] ethers in the lubricant range (C12–C36). Although acidic resins terminated by atungsten oxygen double bond. The DRIFTS such as Amberlyst 70 and Nafion NR-50 have been shown to FTIR spectra of adsorbed pyridine shown in Figure 1c demon- be effective for etherification of linear alcohols in the liquid stratesthat both Brønsted- and Lewis-acid sites are present on phase, measuring accurate rate data is difficult owing to the the surface of the catalyst, as evident from bandsofadsorbed 1 1 swelling of these resins in the solvent, whichcauses changes pyridine at 1540 cmÀ (blue) and 1444 cmÀ (green), respective- in the number of accessible active sites over time.[29] Tungstat- ly.Additional catalystcharacterization is summarized in Table 1. ChemSusChem 2018, 11,3104 –3111 www.chemsuschem.org 3105 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Papers For all of the linear alcohols studied, the selectivity to ether decreaseswith increasing temperature. However, the carbon chain length does not appear to have any significant effect on the selectivity.Nel and De Klerk observed that ether selectivity of C6–C12 linear alcohols decreased with increasing carbon chain length for gas-phase reactions over h-alumina at 523 K.[2] The authors claimed that their system was limited by external mass transfer,which could explain why they observed ade- crease in selectivity for longer chain alcohols. The apparent activation energies for etherification and dehy- dration of linear alcohols in the liquid phase were obtainedby