γ al2o3

The CZZ catalyst was prepared by continuous co-precipitation method from metal nitrate solution and sodium bicarbonate at pH 7 using a micro jet mixer. The resulting solution was aged at 313 K for 2 h. The precipitate was filtered, dried at 383 K for 16 h and calcined at 623 K with 3 K min⁻¹ for 4 h. The method was described in detail by Polierer et al.^{46 (link)}A commercial CZA catalyst was used for comparison purposes. Commercial γ-Al₂O₃ (Alfa Aesar) or a ferrierite-type zeolite H-FER 20 (FER) (Zeolyst International) were used as dehydration catalysts. Before use, FER was calcined at 823 K for 4 h in air.
For activity tests all catalyst components were finely powdered, pressed and sieved into sieve fractions of 250–500 μm and then physically mixed with a mass ratio of 1 : 1 resulting in three catalytic systems: CZA/FER, CZZ/γ-Al₂O₃ and CZZ/FER. Since reactions (R1) to (R4) are exothermic, the catalysts were diluted with silicon carbide (SiC, Hausen Mineraliengroβhandel GmbH) with the same grain size in a mass ratio of 1 : 10 in order to minimize hot spot formation and therefore ensure largely isothermal operation.

The chemicals used in the present study were all of analytical grade and supplied by Aldrich, UK. These included aluminium chloride anhydride (AlCl₃, 99%) ammonia solution (35%) and copper oxalate hemihydrate (CuC₂O₄.1/2H₂O, 98%). The γ‐Al₂O₃ (BET = 117 m² g^‐1, pore size = 1.035 nm) was prepared by crushing γ‐Al₂O₃ pellets (Alfa Aesar). The He, H₂ and air gases were purchased from BOC with purity 99.99%.

Samples SA1, SA2 and SA3 originate from a batch of commercial aluminium silicate (Sigma–Aldrich), SA1 being the same crystal that was used for the work by Klar et al. (2017 ▸ ). From the previous study it was known that the composition of these samples corresponds to a vacancy concentration of about 0.4. In order to investigate a range of compositions, another sample labelled QG was prepared by mixing powders of γ-Al₂O₃ (Alfa Aesar) and amorphous SiO₂ (Alfa) in a ratio of 5:2. After pressing the precursors into a pellet, the sample was kept in a flame until melting was observed and then quenched to room temperature. Crystallites that appeared single crystalline and transparent in a visible-light microscope were mounted on a polymer loop attached to a sample-holder pin.

A 1% Pt/γ-Al₂O₃ catalyst with an average
Pt particle size of 1.1 nm (σ = 0.4
nm), measured with transmission electron micrograms, was synthesized
via wet impregnation with a H₂PtCl₆ (Sigma-Aldrich
≥99.9% trace metal basis) precursor and γ-Al₂O₃ (Alfa Aesar 99.97%). This was intentionally made to
possess smaller particles and is referred to as Pt_S/γ-Al₂O₃. For comparison, a commercially obtained 5%
Pt/γ-Al₂O₃ catalyst (Sigma-Aldrich #205974)
with an average Pt particle size of ∼4.6 nm (σ = 1.2
nm) was used. This sample is termed Pt_L/γ-Al₂O₃ given its larger metal particle size. The (Brunauer–Emmett–Teller)
BET surface areas of both catalysts along with the Lewis acidity of
the γ-Al₂O₃ were previously calculated
through N₂ and pyridine adsorption, respectively.^{22 (link)} Catalysts were reduced at 500 °C in 7%
(v/v) H₂/He for 2 h prior to experiments. While this temperature
is sufficient for reducing supported Pt particles, the low concentration
of H₂ was used to limit the extent of sintering.^{22 (link)} A variety of organic reagents suspected of poisoning
Pt were used herein (Table 1).