Formation Mechanism of the First Carbon–Carbon Bond and the First Olefin in the Methanol Conversion into Hydrocarbons

Abstract

The elementary reactions leading to the formation of the first carbon-carbon bond during early stages of the zeolite-catalyzed methanol conversion into hydrocarbons were identified by combining kinetics, spectroscopy, and DFT calculations. The first intermediates containing a C-C bond are acetic acid and methyl acetate, which are formed through carbonylation of methanol or dimethyl ether even in presence of water. A series of acid-catalyzed reactions including acetylation, decarboxylation, aldol condensation, and cracking convert those intermediates into a mixture of surface bounded hydrocarbons, the hydrocarbon pool, as well as into the first olefin leaving the catalyst. This carbonylation based mechanism has an energy barrier of 80 kJ mol(-1) for the formation of the first C-C bond, in line with a broad range of experiments, and significantly lower than the barriers associated with earlier proposed mechanisms.

Initial Carbon–Carbon Bond Formation during the Early Stages of the Methanol‐to‐Olefin Process Proven by Zeolite‐Trapped Acetate and Methyl Acetate

“…This is an indication of exchange between anomeric DMM structures on as ub-second timescale (trans, trans and gauche, gauche,F igure 2d). [42,44] Although the plausible existence and/or influence of DMM during MTO was already proposed by Kubelkova et al, [24] Chang et al [26] and Lercher et al, [20] these results provide the first spectroscopic evidence that DMM is indeed formed during MTO. Theacetal signal at 93 ppm also has across-peak in the CH spectrum at 4.7 ppm, but lacks ac orrelation with other Csignals,h ence identified as methanediol, as the hydrolyzed product of DMM (Figure 2e).…”

“…[41] In the carbonyl region of the 13 C-13 Cspectra, clear crosspeaks to methyl carbon atoms were observed at both long and short mixing times (Figure 3), indicative for acetate species. [20] While the signal at 180.5 ppm shows no additional cross-peaks,i n accordance with surface-bound acetate,the signal at 178.5 ppm has aclear cross-peak with a 13 Csignal at 55.1 ppm at longer mixing times.T his 13 Cs ignal correlates with aHsignal at 3.82 ppm and has an additional very weak signal with the methyl signal at 22.3 ppm at longer C À Cmixing times.This crosspeak pattern is typical for methyl acetate (Figure 3a). Similarly,inthe 2D 1 H- 13 CssNMR spectra, the Csignl at 173 ppm correlates with aHsignal at 8-8.5 ppm, which is compatible with surface formate (Figure 3b).…”

“…[9] Interestingly,Hunger and co-workers showed already in 2006 that traces of organic impurities neither have any significant influence on product distribution, nor do they govern the formation of HCP species. [10] Since then, the research groups of Hunger, [10][11][12][13][14][15] Kondo, [16] Fan, [17,18] CopØret and Sautet, [19] and Lercher [20] have provided both experimental and theoretical evidences in support of the direct mechanism during the initial stages of the MTO process. Thee arlier proposed direct MTO routes include the a) oxonium ylide,b )carbene,a nd c) methane-formaldehyde mechanisms.…”

“…[19] Their experimental and theoretically verified proposal involves the CÀCb ond formation over Al 2 O 3 through the CÀHa ctivation of methane in an aluminooxonium (AlO = CH 2 + )/methane adduct, which has conceptual resemblance with the "methane-formaldehyde" mechanism proposed by Hutchings et al, [23] Kubelkova and co-workers [24] and Hirao and co-workers. [25] Independently from each other, the research groups of CopØret and Sautet [19] and Lercher et al [20] postulated the involvement of formate and acetate species bound to the zeolite surface during the first CÀCbond formation at the early stages of the MTOr eaction. This surface formate species could lead to the formation of dimethoxymethane (DMM, CH 3 OCH 2 OCH 3 )d uring the course of the MTO reaction, as suggested by the research groups of Kubelkova, [24] and Chang.…”

“…[26] Within this context it is important to mention that Lercher and co-workers very recently postulated that methyl acetate (CH 3 CO 2 CH 3 )i st he very first C À Cb ond-containing intermediate (derived from surface-bound acetate species) during the MTO reaction over ZSM-5. [20] Despite these encouraging results,n od irect evidence has been presented for the existence of DMM as well as surface-bound acetate and methyl acetate species during the zeolite-catalyzed MTOr eaction.…”