Copper-Modified Zeolites and Silica for Conversion of Methane to Methanol

Abstract

Powder materials containing copper ions supported on ZSM-5 (Cu-Zeolite Socony Mobil-5) and SSZ-13 (Cu-Standard Oil synthesised zeolite-13), and predominantly CuO nanoparticles on amorphous SiO 2 were synthesised, characterised, wash-coated onto ceramic monoliths and, for the first time, compared as catalysts for direct conversion of methane to methanol (DCMM) at ambient pressure (1 atm) using O 2 , N 2 O and NO as oxidants. Methanol production was monitored and quantified using Fourier transform infrared spectroscopy. Methanol is formed over all monolith samples, though the formation is considerably higher for the copper-exchanged zeolites. Hence, copper ions are the main active sites for DCMM. The minor amount of methanol produced over the Cu/SiO 2 sample, however, suggests that zeolites are not the sole substrate that can host those active copper sites but also silica. Further, we present the first ambient pressure in situ infrared spectroscopic measurements revealing the formation and consumption of surface methoxy species, which are considered to be key intermediates in the DCMM reaction.

Methane adsorption and methanol desorption of copper modified boron silicate

“…The accumulation of methoxy groups on copper sites indicates that the copper species in the BS sample are capable of catalysing the oxidation of methane. Unlike Cuzeolites, where methoxy groups are observed on the framework structure and zeolite defects, 6,10 no methoxy groups on boron or silicon sites can be observed for the Cu-BS sample during methane oxidation. This can be explained by the weaker acidity of the boron sites compared to the aluminium sites.…”

Approaches for Selective Oxidation of Methane to Methanol

“…Metal oxides, such as MoO 3 [76,77] [79], SiO 2 [80], and CeO 2 [81][82][83], are reported to activate methane in DMTM. Oxide surfaces are often considered to be "living", as in a breathing motion, as a result of the Mars-van Krevelen mechanism.…”

Desorption products during linear heating of copper zeolites with pre-adsorbed methanol

“…The preparation of the Cu-ZSM-5, Cu-SSZ-13 and Cu/SiO 2 samples has been described in detail previously. 22,34 Briefly, the Cu-ZSM-5 and Cu-SSZ-13 samples were prepared using aqueous ion-exchange in which each parent zeolite (H-ZSM-5, Si/Al = 13.5; or H-SSZ-13, Si/Al = 10) was mixed with an aqueous solution of Cu(NO 3 ) 2 (Sigma-Aldrich, ACS reagent, 0.1 M, 100 ml g À1 zeolite) for 24 h. The suspension was filtered and dried at 120 1C overnight to give the powder sample. The Cu/SiO 2 sample was prepared using incipient wetness impregnation where an aqueous solution of Cu(NO 3 ) 2 Á5H 2 O (Sigma-Aldrich, ACS reagent, 0.29 M, 1.66 ml g À1 silica) was slowly added to the silica (Akzo Nobel, Kromasil, 200 Å, 5 mm).…”

“…The basic characteristics of the powder samples have been reported in our previous study. 34 Shortly, the X-ray diffraction (XRD) results show that the zeolite framework structures of the Cu-zeolite samples are well-preserved during the Cu ion-exchange process. The Cu species in the Cu-ZSM-5 and Cu-SSZ-13 samples are mainly isolated Cu ions and clusters according to ex situ X-ray absorption spectroscopy analysis, whereas the Cu/SiO 2 sample (predominantly) consists of CuO nanoparticles with an average size of 28.3 nm as calculated from the XRD pattern.…”

“…The wash-coating procedure is described elsewhere. 34 Approximately 0.16 g of each powder sample was coated on the monoliths using water dispersible boehmites (Sasol, Disperal P2) as the binder.…”