Alkane Activation Initiated by Hydride Transfer: Co‐conversion of Propane and Methanol over H‐ZSM‐5 Zeolite

Abstract

Co-conversion of alkane with another reactant over zeolite catalysts has emerged as an ew approach to the longstanding challenge of alkane transformation. With the aid of solid-state NMR spectroscopya nd GC-MS analysis,i tw as found that the co-conversion of propane and methanol can be readily initiated by hydride transfer at temperatures of ! 449 K over the acidic zeoliteH-ZSM-5. The formation of 13 C-labeled methane and singly 13 C-labeled n-butanes in selective labeling experiments provided the first evidence for the initial hydride transfer from propane to surface methoxy intermediates.T he results not only providenew insight into carbocation chemistry of solid acids,b ut also shed light on the low-temperature transformation of alkanes for industrial applications.Alkanes are the main components in natural gas and crude oil. Thec onversion of abundant but inert alkanes into highvalue-added products has been al ong-standing challenge to chemists. [1] Thed irect transformation of alkanes [2] suffers from the inertness of C À H/C À Cb onds and unfavorable thermodynamics which result in high reaction temperature, low product yield/selectivity,and unrealistic cost for industrial applications.Alternatively,the co-conversion of alkanes with reactive hydrocarbons [3] or oxygenates [4] over zeolites has recently been explored to overcome these limitations.M ethanol is ak ey platform chemical produced from various sources and can be readily converted over acidic zeolites through the industrial process. [5] Many efforts have accordingly been attempted to use methanol as the co-reactant [6] for coupling with methane, [6a] ethane, [6b] C 3 -C 4 alkanes, [6c] nbutane, [6d] n-hexane, [6e] and petroleum naphtha. [6f] However, the development of robust catalytic systems for efficient activation and further conversion of alkanes is severely hindered by the lack of mechanistic understanding.I nt his regard, solid-state NMR spectroscopy has been applied as ap owerful tool for monitoring the mechanistic events on heterogeneous catalysts. [7] In addition, GC-MS analysis on the distribution of either 13 Cor 2 Hlabels in the reaction products may offer the complementary information on many mechanistic details.T hrough solid-state magic-angle-spinning (MAS) NMR spectroscopy and GC-MS analysis,w ef ound herein, that upon addition of methanol, the C À Hb ond activation of propane was initiated by hydride transfer to surface methoxy species over zeolite H-ZSM-5 (Scheme 1). Initiated by hydride transfer, even at temperature as low as 449 K, the co-conversion of propane and methanol can be further achieved by secondary reactions,s uch as propene methylation (Scheme 2). This work not only details the longpursued evidence for hydride transfer over zeolite catalysts, but also offers insightful information on practical utilization of alkanes through the co-conversion strategy.To obtain unambiguous evidence for the initial hydride transfer, we chose 13 C-labeled methanol and nonlabeled propane (as the model compound of inert alkane) for ...

Activation of n-pentane while prolonging HZSM-5 catalyst lifetime during its combined reaction with methanol or dimethyl ether

“…S10 in Table S2), ethylene-to-ethane and propylene-to-propane ratios are close to one (Table S3). The activation of the paraffin on a Brønsted acid site is a highly demanding step, and the addition of oxygenates could increase its reaction rate [13]. Under our reaction conditions, n-pentane cracking rate is increased at low concentrations of the oxygenate (OX/nC5 ratio ≤ 0.25, see Figure S1), in good agreement with the observations of paraffin activation made by Chang et al [20].…”

“…The synergies between both reactions were also previously observed for a feed mixture of n-butane/methanol in the temperature range of 500-550 ºC and low space time values [12]. The clearest evidences of C-C paraffinic bond activation using methanol were reported by Yu et al [13] in experiments with propane and labeled carbon reactants. They saturated the HZSM-5 zeolite with labeled methoxy species at very controlled conditions and made them react directly with propane, which confirmed a promoted hydride transfer pathway between both species.…”

“…On the other hand, small amounts of propylene formed by the reaction of the adsorbed methoxy species with gas phase oxygenates would be easily protonated by the Brønsted acid sites forming a propenium ion. In both cases, the carbenium ion formed would initiate the propagation cycle of the cracking mechanism by hydrogen transfer with the n-pentane molecules in the feed, as described by Yu et al [13], as well as possible olefin alkylationcracking reactions. Under the same conditions, DME has been reported to be more reactive than MeOH due to the formation of methoxy intermediates in higher amount [29,39].…”

Characterizing the First and Second 27Al Neighbors of Brønsted and Lewis Acid Protons in Zeolites and the Distribution of 27Al Quadrupolar Couplings by 1H{27Al} Offset REAPDOR

“…By virtue of sample rotation, spin packets move through the resonance position at different time points, and it depends on η Q and the efg tensor orientation in the rotor frame, when this happens and whether a spin has two or four zero crossings per rotor period. 29 Thus, the crystallite orientations are timedependent for a rotating sample, which helps spin packets of different crystallite orientations to move into resonance position for systems with large quadrupolar interactions. During these zero crossings, the on-resonance pulse irradiation can induce a change of the 27 Al spin states, and the success of such population transfers depend on the passage speed.…”

Synergy in the Cocracking under FCC Conditions of a Phenolic Compound in the Bio-oil and a Model Compound for Vacuum Gasoil

“…This explains the higher oxygenate deoxygenation. Yu et al 55 used 13 C MAS NMR spectroscopy to explain propane reactivity in its transformation together with methanol. They proved that hydrogen is transferred from propane to methoxy species on the HZSM-5 zeolite surface to form a carbenium ion.…”