Suming Ye

Spent fluid catalytic cracking (SFCC) catalysts represent a high tonnage refinery waste stream enriched with lanthanum and aluminosilicates, offering potential as a secondary source of rare-earth elements. This study presents a comparative laboratory scale investigation and techno-economic assessment (TEA) of three process (Process 1, Process 2 and Process 3) flowsheets designed to recover valuable products from SFCC, including sodium lanthanum sulphate (NaLa(SO4)2), amorphous silica (SiO2), alumina (Al2O3), a La/SiO2:Al2O3, and mullite. The original process employed external sodium sulphate (Na2SO4) to selectively precipitate salts from metal-rich (LaSO4) leachates in the form of NaLa(SO4)2. Modified flowsheets eliminate the need for external Na2SO4, where in-situ generated sodium bicarbonate (NaHCO3) via base leaching suffice the formation of NaLa(SO4)2. Further intensification in the process steps was achieved by omitting the CO2 precipitation step and co-precipitating acid and base leachates to directly yield a La/SiO2:Al2O3 composite. All recovered products were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM), confirming their purity and phase identity. Techno-economic modelling was performed for a 12,500 t/y SFCC processing facility, incorporating industry-derived cost assumptions, heat recovery strategies, and equipment optimization. Among the three flowsheets, Process 3 demonstrated superior economic and environmental performance, with a capital investment of S$19.9 million, operating cost of S$521/t SFCC, annual revenue of S$999/t SFCC, and a net profit of S$5.97 million/year, achieving payback within four years. Heat integration reduced specific heating demand from 3.74 MW to 3.20 MW and lowered CO2 emissions to 0.371 kg/kg SFCC the lowest among all scenarios. While Process 1 and Process 2 remain viable (net profits of S$3.70-3.94 million; 7–8 year paybacks), Process 3 offers the most promising route for industrial-scale SFCC valorisation. However, further refinement is required to meet stringent specifications for reuse in petroleum refinery catalysts. This study underscores the value of coupling process intensification with heat recovery to unlock both economic and sustainability benefits in secondary rare-earth supply chains.

Considering the dangers and risks posed by climate change, many countries and organizations have pledged to achieve “net-zero emissions” by 2050. The present work introduces a new approach that addresses the growing global carbon emissions issue by integrating CO₂ capture and sequestration through the carbonation of recycled concrete aggregates (RCAs), producing an alternative sand (AS) product. This study explores the capture of low-concentration CO₂ and its suitability for sequestration into RCA. The integration of RCA in the process allows concrete manufacturers to reduce their reliance on mined sand, thereby minimizing its impact on the environment. A techno-economic analysis (TEA) was conducted on the CO₂ absorption and mineralization process to assess its economic viability across various processing scales, from 150 kt CO₂ per year to 1 Mt CO₂ per year. Initial bench-scale experiments show that RCA samples have a carbonation capacity of 10% by mass—these experimental results were used to conduct a TEA using Aspen Plus and an Aspen Process Economic Analyzer (APEA). The TEA results reveal a cost of 11.02 USD/t AS at the smallest scale and 8.02 USD/t AS at the largest scale.

The increasing generation of incineration bottom residues poses environmental risks due to heavy metal leaching, while carbonation is one of the effective methods to immobilize heavy metals. This study introduces a novel approach to mitigate heavy metal leaching from incineration bottom ash (IBA) by accelerated carbonation with ammonium carbonate solution. The effect of ammonium carbonate concentration on carbonation efficiency and inhibition of heavy metal leaching was systematically investigated. X-ray fluorescence was used to analyze the composition of IBA, and thermogravimetric analysis (TGA) and inductively coupled plasma optical emission spectroscopy/mass spectrometry were used to determine carbonation capacity and heavy metal leaching, respectively. Results showed that maximum carbonation capacity was achieved at 8 wt% ammonium carbonate concentration at a solid/liquid ratio of 1:5 for 1 h, but the increase in carbonation efficiency slowed down when the concentration exceeded 4 wt%. Ammonium carbonate accelerated carbonation effectively inhibited heavy metal leaching, particularly copper, with an 89% inhibition rate at 10 wt% ammonium carbonate, while diminished effects were observed with chromiun. The effect of the ammonium carbonate concentration on heavy metal inhibition became less significant above 4 wt%, revealing a nuanced relationship between carbonation and heavy metal leaching inhibition. TGA and X-ray diffraction analyses confirmed the formation of insoluble CaCO3 during carbonation, elucidating neomineralization processes that immobilize trace heavy metals. In addition, the study explored the impact of carbonation on leachate pH, emphasizing the interplay between pH reduction and heavy metal leaching inhibition.

This study introduces an innovative strategy to mitigate global carbon emissions by integrating carbon dioxide (CO₂) absorption and sequestration through the carbonation of recycled concrete aggregate (RCA). This approach not only promotes the recycling of concrete waste but also alleviates the demand for new natural resources, addressing both environmental impact and geographical challenges associated with mining. The integrated process utilizes ammonia to capture flue gas emissions from natural gas combined cycle (NGCC) power plants, employing the captured solution to carbonate RCA for effective CO₂ sequestration and to enhance the quality of RCA. The study comprehensively assesses the process’s viability, considering capture performance, energy penalty, compliance with ammonia emissions standards, and capital costs. A techno-economic analysis (TEA) highlights the potential and economic feasibility of the proposed approach. Bench-scale experiments, conducted at low CO₂ concentrations (3~5%), focused on optimizing the carbonation process. The concentration of carbonated ammonia solution and its ratio to RCA were investigated to enhance the yield of carbonated RCA, resulting in an approximate 10% CO₂ capacity when using a 5% carbonated ammonia solution with a 0.25 ratio. The design of a large-scale plant, with an annual carbonated RCA production capacity of around 150 ktonnes, was formulated, and TEA calculations using Aspen Plus® V14 indicated a cost of approximately SGD 13 per tonne of carbonated product. These findings underscore the promising potential of the proposed process in efficiently reducing carbon emissions while providing economic viability at a larger scale.

Halogenation of pyrrole requires strong electrophilic reagents and often leads to undesired polyhalogenated products. Biocatalytic halogenation is a highly attractive approach given its chemoselectivity and benign reaction conditions. While there are several reports of enzymatic phenol and indole halogenation in organic synthesis, corresponding reports on enzymatic pyrrole halogenation have been lacking. Here we describe the in vitro functional and structural characterization of PrnC, a flavin-dependent halogenase that can act on free-standing pyrroles. Computational modeling and site mutagenesis studies identified three key residues in the catalytic pocket. A moderate resolution map using single-particle cryogenic electron microscopy reveals PrnC to be a dimer. This native PrnC can halogenate a library of structurally diverse pyrrolic heterocycles in a site-selective manner and be applied in the chemoenzymatic synthesis of a chlorinated analog of the agrochemical fungicide Fludioxonil.

Dioxaphosphabicyclo[2.2.2]octanes (L1–L4) have been prepared in a one-pot reaction from tris(hydroxymethyl)phosphine and various α,β-unsaturated ketones. The non-volatile phosphines oxidise very slowly in air. They possess highly upfield ³¹P chemical shifts (−59 to −70 ppm), small cone angles (121–140°) and a similar electronic parameter to PPh₃. Reaction of L1 with [Rh(acac)(CO)₂] gave the complex [Rh(acac)(CO)(L1)] with a ν_(CO) of 1981.5 cm⁻¹, whereas reaction L1 with [Rh(CO)₂Cl]₂ gave [Rh(CO)(L1)₂Cl] with a ν_(CO) of 1979.9 cm⁻¹, remarkably similar to the CO stretching frequencies reported for analogous PPh₃ complexes. The cage phosphines were explored as ligands in rhodium catalysed hydroformylation of 1-octene. All of the ligands gave a linear selectivity to n-nonanal of 68%, regardless of the substituents. However the ligand substituents had a significant effect on the catalyst activity, with increased steric bulk around the coordination environment giving a three-fold increase in aldehyde yield. The phosphines undergo ligand subsitution with [Pd(MeCN)₂Cl₂] forming square planar trans-[Pd(L)₂Cl₂] complexes. Subsequent reduction with hydrazine furnishes homoleptic tetravalent [Pd(L1)₄] which was applied as a catalyst in Suzuki–Miyaura couplings, furnishing the C–C coupled products in moderate yields.

RadH is one of the flavin-dependent halogenases that has previously exhibited promising catalytic activity towards hydroxycoumarin, hydroxyisoquinoline, and phenolic derivatives. Here, we evaluated new functional homologs of RadH and expanded its specificities for the halogenation of non-tryptophan-derived, heterocyclic scaffolds. Our investigation revealed that RadH could effectively halogenate hydroxyquinoline and hydroxybenzothiophene. Assay optimization studies revealed the need to balance the various co-factor concentrations and where a GDHi co-factor recycling system most significantly improves the conversion and efficiency of the reaction. A crystal structure of RadH was also obtained with a resolution of 2.4 Å, and docking studies were conducted to pinpoint the binding and catalytic sites for substrates.