Catalysis 2024 Issue

neutralisation of the acid sites by alkali and earth alkaline metals can be better mitigated using a low-sodium content catalyst, such as in-situ manufactured catalysts. Vanadium passivation technology (for example, Valor) might also be needed to minimise the affinity that vanadium might have with alkali metals (such as Na and K) for better activity maintenance. Oxygen-containing molecules present in biogenic feed- stocks will also induce the optimisation of the catalytic sites to manage the deoxygenation reactions that are inevitable through FCC reactions. These reactions typically include dehydration (oxygen lost as H₂O), decarbonylation (oxygen lost as H₂O and CO), and decarboxylation (oxygen lost as CO₂). If these deoxygenation pathways are uncontrolled, this can result in higher coke make and lower biogenic carbon recovery. The FCC catalysts must be fine-tuned to minimise biogenic coke formation and maximise biogenic carbon recovery in the valuable products while minimising hydrogen loss from products (for example, retaining the H/C ratio). For opportunistic feedstocks, technologies increasing the diffusion and conversion of large molecules for bottoms upgrading while producing less coke and dry gas will be required. Higher meso-macro porosity and better pore con- nectivity between the matrix and the zeolite will help con- vert these large molecules. Enhanced nickel and vanadium passivation technologies will help produce less coke and dry gas while enhancing activity maintenance to produce more valuable products. Improved bottoms cracking activ - ity and selectivity to coke is achieved by optimising matrix properties:  Optimal acidity to maintain bottoms cracking while min- imising coke selectivity.  Optimal surface area to provide enough active sites.  Sufficient pore size distribution to ensure accessibility to catalyst surface. A Andrea Battiston, Global Business Advisor, andrea.bat- tiston@ketjen.com, Jaap Bergwerff, Business Development Director Renewables, jaap.bergwerff@ketjen.com, Stefano Melis, Global Hydroprocessing Specialist, stefano.melis@ ketjen.com, Ketjen The energy transition is compelling refiners to process more renewable and recyclable feedstocks like vegetable oil and waste plastic oils (WPOs) for production of transportation fuels and chemicals. These feedstocks present new chal- lenges for the hydrotreating catalyst systems, necessitat- ing enhanced formulations and new ways to apply them in commercial practice. These challenges can be summarised into three main types, each demanding a distinct approach and solution. Inorganic impurities Firstly, the new feedstocks can contain inorganic impurities not present in fossil feedstocks or in different concentration and molecular forms. Removal of the impurities by means of reaction and deposition in the guard catalyst section is required to prevent contamination and deactivation of the main catalyst. The case of phosphorus (P) and metals

trapping is the most common challenge and illustrates how catalyst systems are being improved. P-containing mol- ecules present in fossil-spent lube streams are generally highly reactive. In contrast, the phospholipids prominent in animal fats, for instance, are highly reactive and bulky. As a result, the guard bed catalyst must provide the right balance between its active sites’ accessibility, pore volume storage capacity, and active phase activity. In this way, the maximum amount of phosphorous and metals can be reacted and trapped in the whole catalyst pore volume and not just in the proximity of its external surface. In WPOs phosphorous is present as the remnants of P-containing flame retardants alongside a broad range of, sometimes exotic, elements and metals that one would not find in any other feedstock. The guard catalyst, in this case, needs to be tailored to trap all these elements. Note that for waste plastics hydrotreating, there are large dif- ferences in the pretreatment and the trapping strategy depending on the source of the plastics, be it olefins or aromatics. Oxygenates A second challenge arises from the presence of oxygenates in non-fossil feedstocks, necessitating their removal to meet final product specifications. Once again, the challenges related to removing oxygen depend on the type of feed- stock. Triglycerides contained in vegetable oil and animal fats are readily converted over hydroprocessing catalysts, but the pathway for their decomposition into paraffins can significantly affect the process’s effectiveness. Depending on reaction conditions and catalyst compo- sition, oxygen can be removed via hydrodeoxygenation (HDO), producing water, or via decarbonylation and/or decarboxylation, releasing CO and CO₂, respectively. A high selectivity towards the HDO pathway is generally desired as it maximises the hydrocarbon product yield and, where applicable, prevents downstream catalyst poisoning by CO. For example, when renewable feedstock is co-processed with fossil fuel, CO inhibits the hydrogenolysis reaction pathway to remove suphur, impacting the performance of hydrotreaters loaded with CoMo catalyst, which are typi- cally those operating at low hydrogen pressure. For HDO, selection of the active metals in hydrotreating catalyst formulation is key to balancing the hydrogenolysis and hydrogenation functions. In addition, HDO catalysts need to be accessible to large molecules (triglycerides) and capable of tolerating metal slip from the metal trapping lay- ers above the reaction zone, which can occur later in the operating cycle, so excellent pore accessibility remains a key property throughout the cycle. This explains why HDO catalysts also require an open pore structure. Note that in bio-oils obtained by liquefaction of biomass sources such as lignocellulose, oxygen concen- trations are very significant. The extremely high reactivity of some of the oxygenates can result in stability and han- dling issues, so a stabilisation step at low temperature with a catalyst with a specific composition is applied prior to regular hydrotreating.

Catalysis 2024