Acidity modification of ZSM-5 for enhanced production of light olefins from CO2
Graphical abstract
Introduction
The increasing global CO2 levels has led to a massive thrust in research on both Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU). It has been posited that, in terms of volume, the contribution of CCU will be significantly less as compared to CCS for avoiding CO2 emissions and achieving the “2 degree scenario” (2DS) goals [1]. However, what cannot be denied is that the immediate economic potential of CCU far outweighs that of CCS especially considering the fact that large scale capital investment is required in case of the latter [2]. While it is clear that CCS does need to be implemented in order to realistically achieve the 2DS goals, the reuse of stored CO2 (i.e. CCU) can become beneficial if consumption of fossil fuels is greatly reduced over the next century and the stored CO2 becomes the chief feedstock for carbon-based chemicals.
Currently, light olefins (i.e. ethylene, propylene and butylene) and aromatics (especially Benzene, Toluene, Ethylbenzene and Xylene; BTEX) are the most demanded carbon based chemicals being produced worldwide due to their wide variety of applications. Thus, in order for CCU to be implemented realistically on a large scale, these high value products need to be targeted in order to ensure economic sustainability for the whole process.
To that end, CO2 hydrogenation has emerged as a potential route for effective implementation of CCU. Standalone catalysts have given rise to products like methanol [3], [4], [5], [6] and olefinic and paraffinic hydrocarbons [7], [8], [9]. However, they suffer from a number of drawbacks including high CO/CH4 selectivity and wide product distributions. In the latter case, low yields of target products would lead to prohibitive separation costs and thus wide product distributions are undesirable. Furthermore, direct synthesis of aromatics from CO2 via a standalone catalyst is probably impossible given the current state of the art. Nevertheless, this is a work in progress with new and improved catalysts being discovered continuously – be it for methanol synthesis [10], [11] or olefin synthesis [12], [13] from CO2.
In this context, multifunctional catalysts can offer a way forward. It has been seen that the combination of metal/metal oxide and zeolitic catalysts offers intriguing product yields for CO2 hydrogenation. These include olefins [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], paraffins [18], [30], [31], [32], [33] and aromatics [14], [31], [32], [34], [35], [36], [37], [38], [39]. Among olefins, the target of interest are usually light olefins due to their industrial utility. Similarly, among paraffins, the target of interest has been iso-paraffins due to their potential for use as high-octane fuel. However, most of the attention has been focused on targeting either light olefins or aromatics as desired products due to their inherently higher value as compared to paraffins. A number of excellent reviews have also been published which highlight the various products obtained via hydrogenation of CO2 [40], [41], [42], [43], [44]. It has been well established that the multifunctional thermocatalytic hydrogenation of CO2 can proceed through two alternative routes depending on the catalyst used – i) the MeOH route (i.e. hydrocarbon synthesis with methanol/DME as an intermediate) and ii) the RWGS route (hydrocarbon synthesis via a modified Fischer Tropsch mechanism) [45]. It has generally been observed that the RWGS route is much more productive in terms of hydrocarbon yields as compared to the MeOH route. However, having high selectivity to target products like light olefins and aromatics remains a challenge for this process.
Thus, in this work, a potential approach to increase light olefin selectivity has been explored. This has been achieved by means of modifying the acidity of the ZSM-5 zeolite by incorporating Ca into it via incipient wetness impregnation. Combining this modified zeolite together with a Fischer-Tropsch Fe-K catalyst previously reported by us [13] results in an overall selectivity to C2–C4 olefins of approximately 38% at CO2 conversions close to 50% (both on a carbon mole basis) and low CO and CH4 selectivities. Additionally, this catalyst combination is quite stable for approximately 40 h TOS with no significant deactivation observed within this timeframe (Fig. S5). Considering thermocatalytic multifunctional hydrogenation of CO2, this is among the highest yield of light olefins reported to date (Fig. S1 and Table S1) at investigated conditions (30 bars and CO2:H2 of 1:3).
Section snippets
Chemicals
Iron oxide (Fe2O3, Aldrich), Potassium superoxide (KO2, Aldrich), ZSM-5 (SiO2/Al2O3 = 26, ACS Materials) and Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, Alfa Aesar) were used as received.
Catalyst preparation
The Fe-K catalyst was prepared as described previously [13]. Briefly, the Fe2O3 and KO2 were mortar mixed keeping a Fe:K molar ratio of 2. The resultant mixture was heated up to 100 °C for 12 h prior to the catalytic measurements.
The Ca-ZSM5 was obtained as follows. ZSM-5 was initially dried under vacuum at
Results & discussion
As we observed previously [14], combining an Fe catalyst and a zeolite can give distinct product distributions for CO2 hydrogenation as compared to the Fe catalyst alone depending on the zeolite used. Thus, while we saw previously that combination of the Fe-K catalyst and mordenite zeolite gives higher proportion of light olefins as compared to the standalone Fe-K catalyst (~34% selectivity in the former case and ~30% selectivity in the latter case) [14], we see that the combination of the Fe-K
Conclusion
Acidity modification of zeolite ZSM-5 with alkaline earth metal Ca leads to higher selectivity of light olefins when combined with an Fe catalyst for CO2 hydrogenation. As revealed by NMR spectroscopy, the presence of Ca seems to enhance the incorporation of CO (via a ketene like intermediate) onto the zeolite giving rise to surface acetate species which give rise to higher ethylene/propylene/butylene production as compared to the standalone Fe catalyst (which by itself produces a respectable
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
Funding for this work was provided by King Abdullah University of Science and Technology (KAUST). We would like to thank Dr. Jullian Vittenet for carrying out ICP measurements for all the samples.
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2022, Chem CatalysisCitation Excerpt :However, the organic part of the reaction mechanism remains heavily controversial: the surface-carbide mechanism is the oldest and most accepted FTS mechanism to date,1,11 while oxygenate (both surface-enol1,11–13 and CO-insertion)1,14,15 mechanisms cannot be ignored either. Given the recent renewed interest in the CO2-derived FTS process in the context of carbon management,4,6,16–20 we have now attempted to decode the reaction mechanism of the (syngas-derived) FTS process to deliver a more tailor-made hydrocarbon range of products. In this work, we have relied upon thermally catalyzed cascade-style bi-/multi-functional catalytic approaches, comprising potassium superoxide-doped iron oxide (Fe2O3@KO2) along with acidic zeolites.