Special Issue Editorial: Catalytic Conversion of Carbonaceous Materials to Fuels and Chemicals

Conversion of syngas or CO₂ greenhouse gas derived from various carbon-containing materials including coal, natural gas, biomass, waste plastics and biogas, or power plant, and petroleum is paramount to ensure global energy security, and recycle carbon in the earth and atmosphere and reach the global goal of carbon neutrality by 2050 [1]. This is especially true when cheaper renewable electrolysis hydrogen (green H₂) is available in the near future [2]. Consequently, the special issue titled “Catalytic conversion of carbonaceous materials to fuels and chemicals” was created in the open access MDPI journal, Reactions, by the end of 2020.

Researchers across nine countries (USA, Canada, China, South Africa, Norway, Paraguay, Brazil, Japan, and India) contributed thirteen papers in this special issue. The research topics cover entire XTL (X = coal, biomass, natural gas, and waste biogas and power plant emitted CO₂) process, spanning the technologies of gas purification, Fischer–Tropsch synthesis (FTS) reaction and product upgrading. For example, among the contributions, two papers investigated sulfur-removal technologies for gas cleanup, six papers explored Fischer–Tropsch Synthesis (FTS, CO/CO₂ hydrogenation) over various iron and cobalt catalysts, and two papers studied upgrading of heavy hydrocarbons to liquid fuels. Moreover, one paper explored another catalytic approach (depolymerization/dehydrogenation in ionic liquid) for valorizing cellulose to bio-chemicals over zeolite catalysts. Two reviews are also included commenting on the particle size effect of Fe, Co, and Ru on FTS performance and the catalyst development for the syngas to light olefin (FTO).

Two research groups from Canada and Norway reported novel sulfur-removal technology and optimized conditions for the gas cleanup process. Professor Dalai group of the University of Saskatchewan studied a low-cost sulfur clean technology, i.e., oxidative desulfurization (ODS), to remove sulfur in the tire pyrolysis oil [3]. Effective Ti-incorporated mesoporous supports with 20 wt% loaded heteropoly molybdic acid catalysts (20% HPMo/Ti-Al₂O₃ and 20% HPMo/Ti-TUD-1) have been developed. Based on ANOVA statistically analyzing results, it was found that oxidant/sulfur and catalyst/oil ratios played a crucial role on the ODS of tire pyrolysis oil relative to reaction temperature. A pseudo-first-order kinetics over HPMo/Ti-TUD-1 was proposed. Professor Blekkan group of the Norwegian University of Science and Technology reported an effective sulfur sober 15 Mn 8 Mo that was able to reduce the residual H₂S concentration in the biomass-derived syngas to below 1 ppm under high temperatures of 400–1000 °C [4]. Theoretical sulfur adsorption equilibrium and effects of various parameters including temperature, space velocity, H₂S concentration in the feed gas, and steam content on effluent concentrations of sulfur compounds, i.e., H₂S and SO₂ in a syngas mixture over the sulfur sober were systematically explored. Low temperature (500–600 °C), low space velocity (30,000 mL gas/gsorbent h), low H₂S concentration (<2000 ppm), and a dry feed were proposed for achieving a low residual concentration of sulfur compounds.

In this special issue, the researchers of six different groups studied FTS, which is a key reaction to convert syngas and CO₂ to hydrocarbons. Professor Yao of the University of South Africa studied the CO₂ hydrogenation to fuels and chemicals using single process and serial dual reactors over Cu (200–350 °C) and Co catalysts (200 °C) [5]. Use of series dual reactors significantly enhanced CO₂ activity (24% to 36%) and improved the selectivity of C₅₊ hydrocarbons (1% to 45%). This is an important study on the CO₂ utilization which supported Yao et al. proposing a multi-reactor system (tri-reactors in series), in which a high-temperature Fe catalyst reactor (300–350 °C) was suggested to add for producing both fuels and chemicals [5]. Professor Dadybujor’s group of the West Virginia University and Professor Luo’s group of the Beijing Institute of Petrochemical Technology studied cheap carbon supported iron catalysts for the FTS [6,7]. Important contributions have been made. Prof Dadyburjor group carefully studied a series of FeMoCuK catalyst based on a comprehensive experiment design (Fe, 0–32%, Mo, 0–12%, Cu, 0–1.6% and K, 0–1.8%) [6]. An active carbon supported iron catalyst with an optimized composition was developed (16% Fe/6% Mo/0.9% K/0.8% Cu); on the contrary, Professor Luo’s group focused on studying the effects of various carbon supports (carbon nanotube, activated carbon, graphene oxide, reduced graphene oxide, and carbon nanofiber) on the performance of 15% cobalt catalyst [7]. Through extensive experimental work on catalyst characterizations and catalyst testing, carbon nanofiber-supported cobalt catalyst stood out, which obtained 18.7% CO conversion and 94.7% C₅₊ selectivity at 210 °C, 2.5 MPa, H₂/CO = 2 and 5 NL/g-cat/h. A unique FTS study over Pt-Co/SiO₂ catalyst prepared with alternative cobalt precursors was reported by Professor Jacobs and his group of the University of Texas at San Aton Antonio [8]. Two low-cost cobalt catalyst precursors (acetate and chloride) were investigated along with varying thermal treatments. Advanced synchrotron technique was used to explore the interaction between Co and SiO₂. The study generated high active Pt-Co/Acetate catalyst (47% CO conversion and 66%C₅₊ at 220 °C, 2.0 MPa, and H₂/CO = 2). The study suggested a promising cobalt acetate precursor and provided a potential approach to lowering cost of Co catalyst. Professor Zhou’s group of the national energy center for coal to liquids, Synfuels China Technology Co., Ltd., experimentally studied a mechanism of FTS over an industrial Fe-based catalyst [9]. By developing an elegant approach of co-feeding aldehyde, alcohol, and alkene with syngas, two parallel chain propagation pathways, i.e., the surface carbide mechanism and the CO insertion mechanism with CH_x and CO as monomers over reduced iron have been evidenced. This is an important discovery (different from the conventional single carbide, enol, or CO insertion mechanism), which makes it possible to develop more accurate kinetics of FTS in the future. Professor Ribeiro’s groups of the FACEN/UNA in Paraguay and the Universidade Federal Fluminense in Brazil employed advanced in-situ synchrotron X-ray powder diffraction (SXRPD) technique and explored the effect of the ionic charge and valence level energy of Ca and K promoters on Fe catalyst performance [10]. The intrinsic reason of the performance difference between the Ba and K promoted iron catalysts was reviewed (H₂ adsorption on Fe° surface sites was not depressed as effectively by Ba²⁺ as K⁺).

Two contributions dealt with upgrading of heavy hydrocarbons to liquid fuels. Professor Liu’s group of the National Institute of Advanced Industrial Science and Technology (AIST), Japan studied hydrocracking of polyethylene to Jet Fuel Range fuel over Pt and Al doubly modified MCM-48 bi-functional catalyst [11]. 1%Pt/Al/MCM48 and 1%Pt/Al-MCM48 mesoporous catalysts with relatively high acid strength (H-Y > Al/MCM48 > Al-MCM48 > MCM48) were evident, and obtained jet fuel selectivity as high as 77–85.9% at 300 °C and 4 MPa, while the Pt/HY catalyst showing the highest acid strength produced mainly C₁−C₈ light hydrocarbons. The effect of Pt loading (0–2%) and reaction temperature (200–400 °C) on the yield of various products were studied and optimized. Ma et al. of the University of Kentucky studied hydrocracking of octacosane and cobalt Fischer–Tropsch wax over nonsulfided NiMo and Pt-Based Catalysts [12]. Authors discovered an effective N₂ activation approach for activating NiMo/Al catalyst, which pushed catalyst activity increasing to 75% at 380 °C and 3.5 MPa from 8% that was obtained after normal H₂ or H₂S activation. The N₂ activation was much less pronounced on the active NiMo/SiAl catalyst. Higher number of Brønsted acid sites (BAS) on the catalysts was found to be responsible for the high activity on the catalysts. The study also reported that the nonsulfided NiMo/Al and Pt/Al catalysts, and the NiMo/Si-Al catalyst predominantly converted wax to diesel (sel. 50–70%) and gasoline range (sel. > 50%) hydrocarbons, respectively, which added more value of the contribution to the liquid fuel industry.

The contribution of valorization of microcrystalline cellulose over two heterogeneous protonated zeolite catalysts, i.e., HZSM-5 and Cr/HZSM-5 was reported by Professor Jain and Kassaye at Indian Institute of Technology, Delhi and Alabama State University [13]. High yields of 75% and 53% were obtained in the depolymerization of Cellulose to monomeric glucose and dehydrating fructose to 5-HMF, respectively. The benefit from using the ionic liquid is to improve the rate by reducing the reaction activation energy.

Two review contributions are included in this special issue as well. One review by Professors Ma and Dalai highlighted the effects of structure and particle size of Fe, Co, and Ru catalysts on Fischer–Tropsch Synthesis (FTS) [14,15]. The complicated crystal structure effect including phase type, for example, various iron carbides (Fe₇C₃, χ-Fe₅C₂, θ-Fe₃C, ε-Fe₂C), fcc and hcp cobalt and fcc and hcp ruthenium, phase shape, and crystal facets, were discussed and reviewed [14]. Two particle size regions spitted at 7–8 nm were pointed out for all metals, and the useful intrinsic activity values of all metals were estimated, i.e., TOF 0.046–0.20 s⁻¹ for Fe catalysts (10 nm) at 260–300 °C, 0.1 s⁻¹ and 0.4 s⁻¹ (the highest) for Co and Ru catalysts (7–70 nm) at 220 °C, respectively. Another paper by Professor Dalai group provided a comprehensive review of catalyst development for the conversion of syngas to light olefins, which are the most important building blocks in chemical industry [15]. Larger pore size of Fe-based catalysts and smaller crystal size of Co active metal under high temperatures were suggested to increase light olefin production. The techno-economic analysis and lifecycle assessment (LCA) of conventional FTS process plus using waste biogas biomass CO₂ materials and renewable green H₂ (renewable electrolytic hydrogen) were also carefully reviewed and discussed, demonstrating the economic and environmental feasibility of the FTO/FTS under governmental subsidy and tax concession or exemption in FTS plants.

Finally, the guest editors would like to thank all the authors and reviewers who showed extraordinary support to make this special issue possible. Special thanks go to the managing editor Annie Zhou and other editorial staff who devoted times to reach out to a large number of colleagues in the area and helped in editing this special issue. Thank you all for the contribution of your time, knowledge, experience, and passion in forming this special issue, which, we believe, will guide present and future researchers working in catalytic conversion of carbonaceous materials to fuels and chemicals.