Abstract
Dimethyl ether synthesis was performed from syngas over hybrid catalysts comprising a Cu/Zn/Al as metallic function and K10-montorillonite (K10) as acidic function. The acidic properties of K10 were tuned by deposition of one monolayer of tungstophosphoric acid (H3PW12O40·23.5 H2O, TPA). The changes in acidity were determined based on pyridine adsorption with FTIR detection. Due to the fact that TPA does not possess Lewis acid sites, deposition of TPA on the K10 surface (TPA-K10) resulted only in increase in Brønsted acid site concentrations and in increase in their strength in comparison with unmodified K10. Since both modified and unmodified K10 exhibited the same Lewis acid site concentrations and their strength, it was possible to investigate solely the impact of Brønsted acidity differences of acidic functions on the hybrid catalysts activity in syngas-to-DME process (STD). Additionally, the effect of TPA supported on K10 was investigated in methanol dehydration to DME under atmospheric pressure. In order to prepare metallic functions which differed in activity in CO hydrogenation to methanol (first step of STD), two synthesis methods were used: co-precipitation method (metallic function CZA) and decomposition of citrate complexes of metals (metallic function CZAcitric). The hybrid catalysts (CZA/K10, CZA/TPA-K10, CZAcitric/K10, CZAcitric/TPA-K10) for STD were prepared by physical mixing of metallic and acidic function in volume ratio equal to 2:1. The impact of K10 substitution with TPA-10 in hybrid catalysts depended on whether STD process was controlled by methanol synthesis step or dehydration methanol step. When K10 possessed adequate acidity to dehydrate methanol formed on metallic function (i.e. CZAcitric), it substitution with TPA-K10 in hybrid catalyst did not improve DME yield. On the other hand, when metallic function (i.e. CZA) exhibited higher methanol activity and methanol dehydration rate was limited by insufficient acidity of K10 then usage of TPA-K10 of higher acidity in hybrid catalyst was found to increase significantly DME yield without light parrafins formation.
Highlights
The diminishing world resources of crude oil and its unstable market prices together with aggravated requirements concerning pollutant emission levels for combustion engines are one of the reasons for seeking a new technology of obtaining clean fuels from renewable energy sources
The acidic properties of K10 can be tuned by tungstophosphoric acid (TPA) deposition on its surface, solely leading to enhancement of Brønsted acidity of K10 while Lewis acidity is unchanged
Both unmodified K10 and modified K10 with TPA can be successfully used as active components of hybrid catalysts in syngas-toDME process (STD) process
Summary
The diminishing world resources of crude oil and its unstable market prices together with aggravated requirements concerning pollutant emission levels for combustion engines are one of the reasons for seeking a new technology of obtaining clean fuels from renewable energy sources. Influence of promoter additions on physicochemical properties of Cu-based components of hybrid catalysts and on their activity in STD process is well known and explained [13,14,15]. In order to investigate STD process under condition when it is limited by methanol synthesis rate or methanol dehydration rate, we synthesized first metallic functions of different activities in methanol synthesis It is well know from the literature [32] that BET surface area, copper surface area and copper dispersion are one of the factors which have impact on the activity of copper based catalysts in the methanol synthesis from syngas. In the presented studies, we report for the first time, the usage of K10-montmorillonite as acidic function of hybrid catalyst for syngas-to-DME process. The influence of K10 modification with tungstophosphoric acid (TPA) on its physicochemical properties (e.i.: acidity, porosity) and on activity in STD process has been studied
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