Equilibrium
conditions of methane hydrate formation in the lumens
of natural clay nanotubes were analyzed. The water adsorbed by the
pristine nanotubes is capable to form methane hydrate in the confined
hydrophilic inner pores of 10–100 nm (surface chemistry of
the inner lumens is Al2O3, and external tube’s
surface is SiO2). From 17.5 wt % of water adsorbed by the
clay, 12 wt % was involved in methane hydrate formation (conversion
≈ 70%). The crystal structure of the hydrate inside the nanoconfined
spaces of halloysite did not change as compared with bulk systems.
The formation of methane hydrate occurs during cooling at 0–6
°C simultaneously over the whole clay sample, indicating the
catalytic activity of halloysite surface. This formulation slows down
decomposition of the hydrate confined in the pores at atmospheric
pressure at temperatures below 0 °C. The water is retained in
the inner clay pores over the formation and decomposition of methane
hydrate. We also modified halloysite nanotubes with mesoporous silica
MCM-41 (similar silica gel and sand are routinely used for gas hydrate
formation) increasing the ratio of SiO2 to Al2O3 to compare methane hydrate formation in these chemically
different pores. This allowed us to decrease substantially pore dimensions
in the hybrid system (to 2–3 nm). The fraction of methane hydrate
stable within the temperature range from −18 to 10 °C
in this smaller pore hybrid system was 8 times less as compared to
unmodified halloysite. The very small pores of the halloysite/MCM-41
system allowed formation of hydrate only at a temperatures significantly
less than −18 °C. Natural halloysite clay nanotubes were
suggested as efficient solid containers for methane hydrates encasing.
Halloysite is cheap and scalable up to thousands of tons; it is a
mesomaterial capable of methane storage in clathrate hydrates with
water-based green chemistry processing. Suggested nanoclay-based hydrate
technology is also prospective for gas separation.
This work presents results on CO2 hydrogenation to dimethyl ether (DME) over bifunctional catalysts consisting of In2O3, supported on natural clay halloysite nanotubes (HNT), and HNT modified with Al-MCM-41 silica arrays. The catalysts were characterized by TEM, STEM, EDX-mapping, NH3-TPD, XRD, low-temperature nitrogen adsorption, TPO, and H2-TPR techniques. Catalytic properties of In2O3/HNT and In2O3/Al-MCM-41/HNT in the CO2 hydrogenation to DME were investigated in a fixed-bed continuous flow stainless steel reactor at 10–40 atm, in the temperature range of 200–300°C, at GHSV = 12,000 h−1 and molar ratio of H2:CO2 = 3:1. The best catalyst for CO2 hydrogenation was In2O3/Al-MCM-41/HNT that provided DME production rate 0.15 gDME·(gcat·h)−1 with DME selectivity 53% and at 40 bar, GHSV = 12,000 h−1, and T = 250°C. It was shown that In2O3/Al-MCM-41/HNT exhibited stable operation for at least 40 h on stream.
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