The influence of zeolite surface-aluminum species on the deactivation of CuZnAl/zeolite hybrid catalysts for the direct DME synthesis

Abstract

ElsevierGarcía Martinez Feliu, A. (2014). The influence of zeolite surface-aluminum species on the deactivationof CuZnAl/zeolite hybrid catalysts for the direct DME synthesis. Catalysis Today. 227:144-153. doi:10.1016Today. 227:144-153. doi:10. /j.cattod.2013 AbstractIn this work, an original approach for preparing Cu-ZnO+H-ZSM-5 (Si/Al=15, denoted as Z5) hybrid catalysts displaying enhanced stability during the direct DME synthesis from syngas is presented. The adopted preparation strategy was based on the effective confinement of the copper catalyst within the pores of an SBA-15 silica carrier (d pore = 7.0 nm) prior to mixing with the acid zeolite. In order to maximize the Cu-ZnO interface area (where active Cu 0 sites are likely located) the walls of the SBA-15 silica were coated with a near-monolayer of ZnO (17 wt% Zn) prior the incorporation of copper (in concentrations of 10, 15, and 20 wt%) by the so-called ammonia-driving deposition-precipitation (ADP) method. Copper nanoparticles sizing 5-6 nm after H 2 -reduction (HRTEM) were effectively confined inside the SBA-15 mesopores (as supported by HAADF-STEM) for Cu loadings of up to 15 wt%. Higher Cu loadings (20 wt%) lead to large, XRD-visible, CuO nanoparticles residing out of the SBA-15 pores. The confined catalyst loaded with 15 wt% Cu (15Cu/Zn@S15) displayed the highest Cu 0 surface area (determined by N 2 O-RFC) and methanol synthesis activity. Then, hybrid catalysts based on this methanol synthesis function (15Cu/Zn@S15+Z5) were prepared in a 15Cu/Zn@S15:Z5 mass ratio of 2:1 by two methods: a) grinding the mixture of powders prior to pelletizing (method G), and b) mixing the pre-pelletized components (method M). Equivalent hybrids comprising a conventional coprecipitated Cu-ZnO-Al 2 O 3 (CZA) catalyst were also prepared for comparative purposes (CZA+Z5). The catalysts were evaluated for direct DME synthesis (533 K, 4.0 MPa) in runs lasting ca. 24 h. It was found that the confined 15Cu/Zn@S15+Z5 hybrids deactivated at a much lower rate than those based on CZA regardless the method of preparation. In turn, while conventional CZA+Z5 catalysts prepared by grinding (G) experienced a higher deactivation than that obtained by mixing (M) due to the contribution of detrimental interactions between the copper and zeolite components in the former, no differences in the deactivation rate were observed for the 15Cu/Zn@S15+Z5 hybrids prepared by grinding and mixing. The improved stability of these hybrid catalysts is accounted for by the inhibition of both detrimental interactions (by avoiding the direct contact of copper active sites and the zeolite surface) and extensive metal sintering (by limiting the extent of Cu 0 crystal growth) induced by the effective confinement of the Cu-based catalyst within the SBA-15 pores.

Direct dimethyl ether synthesis from synthesis gas: The influence of methanol dehydration on methanol synthesis reaction

“…This is valid for both metallic catalysts and their physical mixtures with all the three solid acid catalysts under the applied operating conditions. This is in agreement with the experimental results showing that interactions between the two components of the catalyst requires a proximity [12] which is not met in the hybrids prepared by physically mixing the pre-pelletized components [8,9,13]. In the absence of poisons from the applied ultra-pure premixed synthesis gases, as well as the absence of iron and nickel carbonyl formation [28] within the tubings and the reactor (as confirmed by XPS analysis of the spent catalysts, not shown), and the lack of any meaningful differences among the stabilities of the CZA, alone and in the hybrid catalysts, sintering of copper crystallites is probably the main cause of the catalyst deactivation.…”

“…However, such deactivation was associated with a considerable Fischer-Tropsch activity and paraffin formation, which is not common for typical Cu-based methanol synthesis catalysts. In addition, as mentioned earlier, detrimental interactions between metallic and acid components of the hybrid catalyst [9][10][11][12] as well as methanol dehydration (by)products (water [16] and hydrocarbons [31]) are reported to be influential on methanol synthesis catalyst deactivation during the direct DME synthesis.…”

“…The same group also found a correlation between the amount of the extra framework aluminum (EFAL) species on the external surface of the zeolite and the deactivation of the Cu-based methanol synthesis catalyst during the direct DME synthesis over the hybrid catalyst prepared by grinding [9,10]. They hypothesized that EFAL species may migrate onto the Cu-based catalyst through a water assisted surface diffusion mechanism and modify the interaction between ZnO x and Cu, causing progressive deactivation of the active copper sites [9]. Ordomsky et al reported that, in a hybrid catalyst prepared by kneading, the hydroxyl groups on the zeolite outer surface assist copper sintering and migration into the zeolite pores, followed by Cu ion exchange with the zeolite protons leading to deactivation of both the metallic and the acid functions of the catalyst [11].…”

“…García-Trenco et al reported detrimental interactions between Cu/ZnO/Al 2 O 3 and HZSM-5 in the hybrid catalysts prepared by slurry or grinding methods, leading to a dramatic loss of the available Brønsted acid sites through partial exchange of zeolite protons by Cu 2+ and Zn 2+ , and blockage of the zeolite micropores by metallic catalyst particles [8]. The same group also found a correlation between the amount of the extra framework aluminum (EFAL) species on the external surface of the zeolite and the deactivation of the Cu-based methanol synthesis catalyst during the direct DME synthesis over the hybrid catalyst prepared by grinding [9,10]. They hypothesized that EFAL species may migrate onto the Cu-based catalyst through a water assisted surface diffusion mechanism and modify the interaction between ZnO x and Cu, causing progressive deactivation of the active copper sites [9].…”

“…Peng et al attributed the catalyst deactivation during slurry-phase direct DME synthesis to a detrimental interaction between the methanol synthesis catalyst and γ-alumina and hypothesized the migration of Cu-and Zn-contating species onto the acid catalyst as the likely mechanism [12]. Such adverse interactions between the metallic and the acid functions of the hybrid catalysts require an intimate solid-state contact between the two components, and hence, are highly dependent on the preparation method of the hybrid [8,9,12,13]. There have been several efforts to minimize these detrimental interactions in the hybrid catalysts with a high degree of interdispersion between their two components [11,14].…”