1. INTRODUCTION
Since nearly two centuries, extensive use of carbon-based fossil energy sources such as coal, oil and natural gas has rapidly promoted the development of human economy and society.1-4 The widespread use of fossil fuels has led to rising concentrations of CO2 in the atmosphere, which brings about prominent problems such as the destruction of the ecological balance and global warming.5-7 As an easily available source of carbon in nature, the efficient conversion of CO2 into industrial raw materials has attracted the attention of many researchers.6,8 The difficulties of the conversion from CO2 to hydrocarbon cause from that the CO2 is a stable chemical substance demanding high energy to break C-O bonding barrier and the C-C coupling follows the Anderson-Schulz-Flory (ASF) distribution law which limits selectivity of the target products.9
The conversion from CO2 to hydrocarbons is mainly achieved by two ways: one is the reaction process of methanol as an intermediate product, and another is achieved by a modified Fischer-Tropsch (FT) reaction.3,10-13 In a methanol-mediated route, mixtures of CO2 and H2 have been reacted to form methanol intermediates, followed by dehydration to hydrocarbons over a zeolite.14 However, the process suffers from many problems, in particular, high CO by-product selectivity, low catalytic activity and poor stability, which hinder its commercialization.15-17 In terms of FTS route, CO2 is firstly transformed into CO via reverse water-gas shift (RWGS) reaction and then the formed CO is subsequently converted to hydrocarbons.18-20 It has been found that the reactivities of Ni, Ru, Co, Cu and Fe metals are high, and widely used to catalyze the hydrogenation of CO2.7,10,21,22 Especially, Fe-based can in-situ form Fe3O4 and FexCy active phase, synergistically catalyzing RWGS and carbon chain propagation.10,21,23,24 Yet despite, the ASF distribution law is not conducive to attaining a high selectivity of target product in single iron-based catalysts. Generally, CO2 adsorption occurs on basic sites, thus alkali metal such as K and Na are applied as promoter to enhance the adsorption capability and/or activation ability of CO2.22,25-28 The alkali metal can improve the electronic environment of iron, which increases the surface basicity and results in an improvement in CO2adsorption. This method improves the conversion of CO2indeed, but the problem of low liquid hydrocarbon selectivity still remains (less than 55%).26,27,29 Furthermore, the incorporation of second active metal (Co or Cu) also exhibits a facilitating effect by pulling RWGS or chain propagation reaction. The corresponding catalyst also achieves a high selectivity of liquid hydrocarbon or yield.30-32
In addition to the systems mentioned above, coupling Fe-based catalysts and zeolite is an alternative prospective way.22,33-35It is an effective chemical process intensification strategy by coupling multiple consecutive chemical reactions in a vessel/catalyst under similar or identical conditions.10,33,35 ZSM-5 have been employed extensively for the isomerization reactions owing to their unique steric properties such as MFI topology, porosity, and acidity.21,36-38 Wei et al. reported a Na-Fe3O4/HZSM-5 for directly converting CO2 to gasoline-range hydrocarbons. Three types of active sites (Fe3O4, Fe5C2 and acid sites) achieve a synergetic catalytic conversion of CO2 to gasoline.39,40 Noreen et al. designed a dual-bed reaction with SAPO-11 and ZSM-5 coupled individually with the NaFe catalyst, obtaining a high octane gasoline fuel.41Since zeolite catalysts can directly participate in the catalytic reaction process, CO2 hydrogenation process can be tuned by controlling the acidity of zeolite. Brønsted-acid site of the zeolite catalyst is derived from the tri-coordinated Si-OH-Al bridge hydroxyl groups on the skeleton and in the pores. The acid site of the zeolite affects the process of its proton transfer or acceptance of electron pairs, thereby affecting its catalytic activity.42,43In previous study, we found that H-ZSM-5 treated by metal nitrate solutions presents different surface acid properties, and the elimination of strong acids is conducive to the formation of high-carbon hydrocarbons.18,44 Similarly, through the precise regulation of zeolite acidity and pore size, it is regarded as an efficient tool for achieving a promoting effect on the selectivity of the gasoline hydrocarbon product.41,45-47
Despite olefins undergo reactions such as polymerization, isomerization, disproportionation, etc. over the acid site of zeolite, there has been no related report mentioning that how the types of olefins species will impact the selectivity of products on zeolite for CO2hydrogenation. In addition, most of these traditional iron-zeolite composite catalysts are generally prepared by physical mixing or impregnating, which results in uneven distribution of active sites or no preferred order of reactions.48-50 Contrary to a catalyst fabricated by physical mixing, a composite with core-shell structure displays distinct advantages. Constructing a catalyst in which the core catalyst produces different types of alkenes and the zeolite has different acidity (that is, the core-shell micro-environment regulation of the catalyst) has potential heuristic significance for the utilization of CO2.
Herein, based on rotation coating method, the capsule catalysts are fabricated with alkali metal modified spinel-like ZnFe2O4 as the core and an outer encapsulated ZSM-5 (molar ratio ≈ 25-30) as the shell. During the reaction process, olefins are formed on spinel-like ZnFe2O4, then the olefins will migrate on H-ZSM-5 shell proceeding catalytic reforming to high carbon hydrocarbons over acidic sites, in which K-modified ZnFe2O4 have a higher heavy olefins selectivity than Na-modified ZnFe2O4. H-ZSM-5 treated by different ions (K and Ce) exchange exhibits enhanced olefins adsorption capacity, further promoting the formation of gasoline-range hydrocarbons. K-ZSM-5 contributes to the isomerization reaction, while Ce-ZSM-5 promotes the aromatization reaction.