Propane Dehydrogenation on Ga₂O₃-Based Catalysts: Contrasting Performance with Coordination Environment and Acidity of Surface Sites

Abstract

α-Ga2O3, β-Ga2O3, and γ-Ga2O3 as well as the silica-supported catalysts γ-Ga2O3/SiO2, β-Ga2O3/SiO2, and Ga­(NO3)3-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 °C. The coordination environment and acidity of surface sites in stand-alone and SiO2-supported Ga2O3 catalysts were studied using FTIR, 15N dynamic nuclear polarization surface-enhanced NMR spectroscopy (15N DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in γ-Ga2O3 and β-Ga2O3 (the latter obtained from colloidal nanocrystals of γ-Ga2O3 via thermal treatment at 750 °C) are similar, except that β-Ga2O3 contains a larger relative fraction of weak Ga3+ Lewis acid sites. In contrast, α-Ga2O3 features mostly strong Lewis acid sites. This difference in surface sites parallels their difference in catalytic activities: i.e., weak Lewis acid surface sites are more abundant in β-Ga2O3 relative to α-Ga2O3 and γ-Ga2O3 and the increased relative abundance of weak Lewis acidity correlates with a higher initial catalytic activity in PDH, 0.41 > 0.28 > 0.14 mmol C3H6 m–2 (Ga2O3) h–1 at 550 °C, for respectively β-, α-, and γ-Ga2O3 with initial propene selectivities of 86, 83, and 88%. Dispersion of γ-Ga2O3 or β-Ga2O3 on a silica support introduces strong as well as abundant weak Brønsted acidity to the catalysts, lowering the PDH selectivity. The γ-Ga2O3/SiO2 catalyst was slightly more active than β-Ga2O3/SiO2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C3H6 mol Ga–1 h–1 (sel = 76 and 73%, respectively). However, these catalysts deactivated by ca. 55% within 100 min time on stream (TOS) due to coking. In contrast, Ga/SiO2, with mostly tetracoordinated surface Ga sites and abundant, strong Brønsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C3H6 mol Ga–1 h–1 and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model β-Ga2O3 surface show that tetra- and pentacoordinated Ga Lewis acid sites bind pyridine more strongly than tricoordinated Ga sites and a higher relative fraction of strong Lewis acid sites correlates with increased coking. Overall, our results indicate that weakly Lewis acidic, tricoordinated Ga3+ sites are likely driving the superior PDH activity of β-Ga2O3.

A Molecular Analogue of the C−H Activation Intermediate of the Silica‐Supported Ga(III) Single‐Site Propane Dehydrogenation Catalyst: Structure and XANES Signature

“…However, this system undergoes rapid deactivation likely due to the reduction of Ga 2 O 3 . [6][7][8][9][10][11][12] While Ga 2 O 3 -based materials readily deactivate and cannot be used on industrial scales for PDH, the corresponding Gazeotype catalysts are used to convert propane into aromatics in the commercial Cyclar© process; this process is proposed to also involve propane dehydrogenation as a key step on isolated Ga sites before the further conversion of propene to aromatics on acid sites. [13 -16] These unique catalytic properties have led to the synthesis of isolated Ga sites at the surface of non-reducible bulk materials such as silica to generate stable propane dehydrogenation catalysts.…”

“…Ga 2 O 3 is one metal oxide that has received significant interest due to its promising initial activity and selectivity in alkane dehydrogenation. However, this system undergoes rapid deactivation likely due to the reduction of Ga 2 O 3 [6–12] . While Ga 2 O 3 ‐based materials readily deactivate and cannot be used on industrial scales for PDH, the corresponding Ga‐zeotype catalysts are used to convert propane into aromatics in the commercial Cyclar © process; this process is proposed to also involve propane dehydrogenation as a key step on isolated Ga sites before the further conversion of propene to aromatics on acid sites [13–16] .…”

Uncovering selective and active Ga surface sites in gallia–alumina mixed-oxide propane dehydrogenation catalysts by dynamic nuclear polarization surface enhanced NMR spectroscopy

“…(B) A possible coordination geometry for a Ga IV -O-Ga VI surface linkage with the attribution of Lewis acidity strength according to the 15 N chemical shift of bound pyridine probe molecule. 12 SP stands for square pyramidal.…”

“…13 PDH and cracking reactions proceed on different sites in Ga 2 O 3 catalysts since the rate of propene formation and propene selectivity decrease with time on stream (TOS) but the selectivity to cracking products and their rate of formation is stable with TOS. 12 One strategy to control the distribution of bulk Ga IV and Ga VI sites, and thereby presumably also inuence the coordination environment of surface Ga sites, is to exploit (Ga,Al) 2 O 3 spineltype solid solutions as alkane dehydrogenation catalysts. 6,9,14 In these materials, the bulk Ga IV : Ga VI ratio can be varied since Al atoms preferentially occupy octahedral positions in the defect spinel-type structure of Ga 2 O 3 , 15 increasing thereby the relative fraction of Ga IV sites (i.e., the proposed active sites).…”

“…9 The reduced deactivation and increased activity might be related to the higher relative fraction of weak LAS in (Ga,Al) 2 O 3 (as assessed by NH 3 -TPD) and linked to a higher relative density of undercoordinated Ga sites, such as putative Ga III sites discussed above. 9,12 Note that the alkane dehydrogenation activity of various phases of Al 2 O 3 (a-, d-, g-, q-) is generally low and requires a pre-treatment with CO or H 2 at high temperature (600 C) to become considerable. 16,17 Due to their limited activity, Al 2 O 3 catalysts are typically tested in PDH at substantially higher temperatures (600-630 C) relative to Ga 2 O 3 catalysts (550 C).…”