Catalytic Co-Homologation of Alkanes and Dimethyl Ether and Promotion by Adamantane As a Hydride Transfer Co-Catalyst Dante A
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DOI: 10.1002/cctc.201000383 Catalytic Co-Homologation of Alkanes and Dimethyl Ether and Promotion by Adamantane as a Hydride Transfer Co-Catalyst Dante A. Simonetti, John H. Ahn, and Enrique Iglesia*[a] We provide kinetic and isotopic evidence for the co-homologa- and larger carbenium ion formation energies, which deter- tion of linear and branched alkanes with dimethyl ether (DME) mines hydride-transfer rates to a given acceptor molecule. to form larger branched alkanes and isobutane on H-BEA zeo- Adamantane increased the hydride-transfer rates to bound alk- lites, and for the role of adamantane as a hydride transfer co- oxides from branched alkanes, and even from unreactive linear catalyst that allows the activation of CÀH bonds in alkanes at alkanes, while also increasing their extent of incorporation into low temperatures (<500 K) on Brønsted acid sites. Branched DME homologation pathways; adamantane acts as a reversible alkanes (isobutane, isopentane, and 2,3-dimethylbutane) pres- hydrogen donor that mediates dehydrogenation of alkanes at ent in equimolar mixtures with DME form the corresponding low temperatures on acid sites. The co-homologation of alka- alkenes via hydride transfer to bound alkoxides (formed in nes with DME avoids the need for carbon rejection in the form DME homologation steps) and subsequent deprotonation; of arenes to satisfy the hydrogen balance in the DME conver- these alkenes, derived from the added alkanes, are then me- sion to alkanes, provides a robust strategy for increasing the thylated to lengthen their chain by using DME-derived C1 spe- chain length and extent of branching in light alkanes through 13 cies, as shown by the isotopologues formed in reactions of C- the selective addition of C1 species, and mitigates the forma- DME with 12C-alkanes. Linear alkanes are much less reactive tion of unsaturated by-products ubiquitous in the homologa- than branched alkanes, because of their stronger CÀH bonds tion of DME or methanol on Brønsted acids. 1. Introduction Competitive reactions between 13C-labeled dimethyl ether (13C-DME) and unlabeled alkenes have been used to examine the mechanistic details of chain growth and termination pathways leading to the se- lective formation of triptane and isobutane by means of the homologation of DME or methanol (Scheme 1).[1] These studies have also provided spe- cific and quantitative evidence for the ubiquitous role of carbenium-ion stability on the relative rates of competing reactions catalyzed by solid acids. Homologation of C1 reactants occurs at modest temperatures (400–500 K) through Brønsted acid cat- alyzed pathways mediated by carbenium ion transi- [2–6] tion states in homogeneous ZnI2 and InI3 liquid- phase systems and on acidic zeolites (ZSM-5, USY, [1,7] BEA), amorphous SiO2ÀAl2O3, and supported [8] Keggin heteropolyacids (5 wt% H3PW12O40/SiO2). The remarkable selectivity to isobutane and triptane Scheme 1. The chain growth sequence for co-homologation of dimethyl ether and alka- reflects a preference for methylation at positions that nes with adamantane co-catalyst. The asterisk indicates the position of alkoxide attach- preserve hydrocarbon chains with four-carbon back- ment to the surface. “CH3” represents surface methylating species derived from dimethyl ether. Adamantane is shown as a dehydrogenation-hydrogenation co-catalyst. bones; the resulting alkoxides do not readily restruc- ture via skeletal isomerization or b-scission, but ter- minate reversibly by means of deprotonation to form alkenes and also through essentially irreversible hydride-trans- [a] Prof. D. A. Simonetti, J. H. Ahn, Dr. E. Iglesia fer reactions to form alkanes. Department of Chemical Engineering, University of California at Berkeley Berkeley, CA 94720 (USA) Alkoxides can desorb as alkenes, which can undergo subse- Fax: (+ 1)510-642-4778 quent methylation by DME-derived intermediates, isomeriza- E-mail: [email protected] 704 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2011, 3, 704 – 718 Catalytic Co-Homologation of Alkanes and Dimethyl Ether tion, or b-scission at rates that depend strongly on the degree transfer steps prevalent in acid-catalyzed C1 homologation re- of substitution at carbon atoms along the backbone.[1,3,6] De- actions, as well as the effects of adamantane on the rate and sorption as alkanes through hydride transfer is essentially irre- selectivity of alkane-DME co-homologation. This study provides versible, because the strong alkane CÀH bonds formed are rel- experimental evidence for the role of alkanes as H-donors and atively unreactive at the temperatures required for DME homo- for the reactivity of the resulting alkenes in chain growth. Fur- logation.[9,10] The reversal of the chain termination steps that thermore, this study provides mechanistic insights into the form alkanes becomes more facile for molecules with higher high reactivity of tertiary alkanes in hydride-transfer reactions degrees of backbone substitution, which undergo hydride and the effects of adamantane on chain termination via hy- transfer with surface alkoxides in steps with lower activation dride transfer to bound alkoxides and on its microscopic re- barriers that reflect weaker tertiary CÀH bonds. Chains that verse, the incorporation of alkanes into DME homologation grow beyond triptyl species undergo facile isomerization and pathways. This study also demonstrates a route for upgrading b-scission reactions, which form fragments that ultimately light alkanes to higher value alkanes with specific isomeric desorb as isobutene or isobutane.[1,6,7] These mechanistic fea- structures and molecular weight distributions via addition of tures reflect a preference for elementary steps involving cat- DME-derived C1 species using adamantane to control hydride- ionic transition states with lower free energies, because of transfer rates. either enthalpic or entropic stabilization (as shown previous- [9,11–13] ly ); consequently, the conclusions reached apply in gen- 2. Results and Discussion eral to reactions catalyzed by Brønsted acids that are mediated by charged transition states. Co-reaction of DME (40 kPa) individually with each of several Chain termination by means of hydride transfer from preva- alkanes (40 kPa; n-butane, isobutane, isopentane, or 2,3-dime- lent gas phase hydrocarbons (alkanes and alkenes) occurs at thylbutane) were used to probe the effects of added alkanes rates that reflect the stability of the carbenium ions derived and adamantane on homologation selectivities. The competi- from the alkoxide hydrogen acceptor and the energy required tive reactions of 13C-DME and these unlabeled alkanes, in the to cleave the CÀH bond in the H-donor molecule.[2–6,14, 15] presence and absence of small amounts of adamantane Therefore, tertiary alkoxides that form tertiary carbenium-ion (1 kPa), were used to probe the mechanism of alkane incorpo- transition states undergo hydride transfer much faster than ration into the DME homologation paths. Additionally, the ef- secondary or primary alkoxides. Alkenes with tertiary allylic CÀ fects of adamantane as a hydride transfer co-catalyst were ex- H bonds (the weakest among the prevalent hydrocarbons[16,17]) amined for DME/alkane co-homologation. We address first the are the most reactive H-donors and form highly unsaturated al- effects of adamantane on DME homologation rates and selec- kenyl species upon hydride transfer. These alkenyl species tivities in the absence of added alkanes and then examine its desorb as conjugated dienes with even weaker CÀH bonds, effects on homologation of alkane-DME co-reactants. which can undergo subsequent hydride transfer and dehydro- genation to form trienes. Trienes then convert to arenes, such 2.1. Effect of adamantane on chain termination during DME as hexamethyl benzene (HMB) by means of Diels–Alder reac- homologation tions to provide the carbon-rejection mechanism required to satisfy the stoichiometry of DME conversion to alkanes[1,3,6,7] The addition of adamantane (1 kPa) in reactions of DME [Eq. (1)]: (70 kPa) with propene (2 kPa to eliminate induction periods[3,7]) decreased selectivities to C4 and C7 hydrocarbons (Figure 1a), ½3 n þ 12CH3OCH3 ! 6CnH2nþ2þ2C6ðCH3Þ6ðHMBÞ increased those to C and C hydrocarbons (Figure 1a), and de- ð1Þ 5 6 þ½3 n þ 12H2O creased alkene-to-alkane ratios (by about 3 and > 10 for the C4 and C5-C6 fractions, respectively; Table 1). Inlet and outlet ada- Alkanes can also re-enter chain growth pathways by acting mantane mole fractions were similar (0.01), indicating that as H-donors; the rates of these reactions increase in the pres- adamantane was not consumed during DME homologation on ence of a hydride transfer co-catalyst (e.g., adamantane), as H-BEA. shown previously for co-homologation of DME and alkanes in The selectivities to 2,3-dimethylbutyl isomers (2,3-dimethyl- [5] liquid ZnI2 systems and for acid-catalyzed alkane isomeriza- butane and 2,3-dimethylbutenes; -tyl suffix is used here and tion and alkylation reactions.[18–24] Thus, co-homologation of al- elsewhere to denote alkanes and alkenes with a given back- kanes and DME forms larger alkanes via routes that satisfy stoi- bone structure) within C6 hydrocarbons (Table 1) decreased chiometry without the need for carbon rejection as arenes slightly (39 to 36%), whereas the methyl-pentyl isomer selec- [Eq. (2)]: tivity concurrently increased (56 to 60%). Upon addition of hi hi adamantane to DME reactants the triptyl selectivities within C7 m À n m À n CH OCH þ C H ! C H þ H O; m > n decreased (71 to 54%), whereas dimethyl-pentyl and iso- 2 3 3 n 2nþ2 m 2mþ2 2 2 heptyl selectivities concurrently increased (24 to 39 %). These ð2Þ increases in selectivity to the C6 and C7 isomers, which do not lie along the path to triptane (with adamantane addition to We explore in this study the introduction of alkanes into DME), indicate that adamantane causes these chains to desorb DME chain growth pathways through the reverse of hydride- as alkanes (through hydride transfer) before they are selective- ChemCatChem 2011, 3, 704 – 718 2011 Wiley-VCH Verlag GmbH & Co.