Direct synthesis of DME from syngas on hybrid CuZnAl/ZSM-5 catalysts: New insights into the role of zeolite acidity

Abstract

A commercial HZSM-5 zeolite (Si/Al=16) was submitted to mild acid treatment and we have found that different conclusions regarding the effect of zeolite acidity may be reached depending on the specific method used for preparing the hybrid catalysts when the overall STD process becomes controlled by the methanol synthesis rate on the Cubased catalyst (i.e. using hybrids with a CZA:zeolite mass ratio of 2:1, that is, with an "excess" of acid sites). Thus, for hybrids prepared by mixing the pre-pelletized components, the same CO conversion and product selectivity (with values approaching those predicted by the thermodynamic equilibrium at the studied conditions) with no signs of deactivation during at least 50 h on stream was attained irrespective of the zeolite acidity. By contrast, significant differences in catalyst stability were found for hybrids prepared by grinding the component powders prior to pelletizing, pointing towards the occurrence of detrimental interactions between the CZA and zeolite components during the grinding preparation stage. Moreover, the different TOS behaviors observed between these hybrids depending on the particular zeolite used suggested that the kind and extent of such detrimental interactions should be tightly related to the acidity and/or chemical composition of the ZSM-5 zeolite.

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.…”

“…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].…”

Activity and deactivation studies for direct dimethyl ether synthesis using CuO–ZnO–Al2O3 with NH4ZSM-5, HZSM-5 or γ-Al2O3

“…The changes of mol% DME, mol% MeOH and selectivities of the main products as function of time are represented in Figure (6) and Figure (7), respectively. It is clear that DME and methanol follow the same trend, as shown in Figure ( 6). DME production and MeOH production at 260 °C decreased from 10.5 % to 8.8 % and from 0.68 % to 0.58 %, respectively, which means that the admixed catalyst deactivated and lost around 16.2 % of its initial activity in the case of DME and 14.7 % of its initial activity in case of methanol.…”

“…Furthermore the fact that the relative rates of the main reactions change considerably with temperature suggests that this ratio is also strongly related to the operating conditions of the reactor. Recently, García-Trenco et al [6] studied the effect of the preparation method on the stability of the catalyst and they found that the catalyst prepared by mixing the pre-pelletized components showed high stability with no signs of deactivation over 50 h. under operation are discussed. Within this manuscript we will focus on using a moderate pressure i.e.…”

“…Catalysts for the STD process are thus usually composed of a methanol synthesis catalyst and a solid acid catalyst for methanol dehydration, for which the most common solid acids are γ-Al 2 O 3 and HZSM-5 [3][4][5][6]. Copper based catalysts are considered to be the standard for methanol synthesis (metal function) with the most common being a CuO/ZnO/Al 2 O 3 (CZA) catalyst used under typical conditions of 220-280 °C and 50 to 100 bar [7].…”