Sorption-enhanced steam reforming of toluene using multifunctional perovskite phase transition sorbents in a chemical looping scheme

Sorption-enhanced steam reforming (SESR) of toluene (SESRT) using catalytic CO2 sorbents is a promising route to convert the aromatic tar byproducts formed in lignocellulosic biomass gasification into hydrogen (H2) or H2-rich syngas. Commonly used sorbents such as CaO are effective in capturing CO2 initially but are prone to lose their sorption capacity over repeated cycles due to sintering at high temperatures. Herein, we present a demonstration of SESRT using A- and B-site doped Sr1−x A’ x Fe1−y B’ y O3−δ (A’ = Ba, Ca; B’ = Co) perovskites in a chemical looping scheme. We found that surface impregnation of 5–10 mol% Ni on the perovskite was effective in improving toluene conversion. However, upon cycling, the impregnated Ni tends to migrate into the bulk and lose activity. This prompted the adoption of a dual bed configuration using a pre-bed of NiO/γ–Al2O3 catalyst upstream of the sorbent. A comparison is made between isothermal operation and a more traditional temperature-swing mode, where for the latter, an average sorption capacity of ∼38% was witnessed over five SESR cycles with H2-rich product syngas evidenced by a ratio of H2: CO x > 4.0. XRD analysis of fresh and cycled samples of Sr0.25Ba0.75Fe0.375Co0.625O3-δ reveal that this material is an effective phase transition sorbent—capable of cyclically capturing and releasing CO2 without irreversible phase changes occurring.


Introduction
Efficient and economic thermochemical conversion of woody biomass wastes (e.g. agricultural and forestry residues) to syngas via gasification will play an important role in diminishing our dependence on fossil fuels. However, state-of-the-art biomass gasification still faces significant technical challenges. One persistent obstacle in the gasification of lignin-containing biomass is the unwanted production of tertiary tars (e.g. methylnaphthalene, toluene, indene, and phenol), which can adversely impact process equipment, lower the product heating value, and deactivate the catalyst through coking [1][2][3]. In the past few decades, many researchers have attempted to address the tar issue by using the approach of catalytic steam reforming where toluene is most frequently used as a model tar compound. In this process, toluene is steam-reformed at temperatures between 600 • C-900 • C in the presence of a catalyst, which typically contains Ni, to produce syngas (equation (1)) concurrent with the water gas shift reaction (equation (2)) [4][5][6][7][8][9]. Steam reforming combined with partial oxidation (equation (3)) can improve the reaction thermodynamics but is typically kinetically limited Besides tar formation, the traditional gasification process produces high amounts of CO 2 , resulting in a low H 2 to CO + CO 2 ratio (H 2 : CO x ) of the product gas. Thus, the reforming of toluene may be further improved with sorption enhancement by which a sorbent captures CO 2 in-situ to drive H 2 formation and thus increase the H 2 : CO x ratio of the product gas. Sorbent design is often a challenging task as many of the more thermodynamically favorable sorbents (e.g. CaO) have low Tamman temperatures in their carbonate forms and sinter at the high temperatures needed for reforming and decarbonation [10][11][12]. Such sintering at reaction conditions leads to an observed loss of sorption capacity over repeated cycles. Additionally, materials that are thermodynamically favorable for carbonation at low temperatures will necessarily require a large thermal swing (100 • C-150 • C) for the subsequent decarbonation step. Therefore, it is necessary to develop sorbents that are resistant to irreversible phase transitions and recyclable repeatedly.
Catalytic sorbents investigated for sorption enhancement typically take the form of an active metal (e.g. Ni) supported on a CaO-based sorbent [13]. As previously mentioned, these CaO-based sorbents are prone to sintering, which has motivated researchers to attempt stabilizing the support with an inert such as Al 2 O 3 as well as trying different materials zirconates [14] and silicates [15]. Perovskite oxides (denoted as ABO 3−δ where A and B are typically group II and transition metal cations, respectively) are a highly tunable, structurally stable, and versatile class of mixed metal oxides that have been successfully demonstrated in various chemical looping applications such as air separation [16][17][18], energy storage [19], and heterogeneous catalysis [20,21]. In a typical chemical looping scheme, perovskite oxides donate their lattice oxygen during the reduction step and are replenished with oxygen from the air during a subsequent regeneration step [22]. The tunability of perovskites via A-and B-site cation doping generates a wide range of reaction possibilities. In the context of sorption enhancement, these materials can serve the dual-functional purpose of oxidatively reforming the carbonaceous feed [23,24], while also using their A-site alkaline earth metals to capture co-produced CO 2 [25][26][27] (equation (4)) as illustrated in figure 1. In the regeneration step, oxygen oxidizes the reduced metal oxides, an exothermic reaction that helps drive the endothermic release of CO 2 from the carbonates (equation (5)) We have previously reported the novel use of A-and B-site doped perovskites as 'phase transition sorbents' (PTSs), specifically using Sr 1−x Ca x Fe 0.9 Ni 0.1 O 3−δ (where x = 0.3, 0.4, 0.5, or 0.6) for the sorption-enhanced steam reforming (SESR) of glycerol in a chemical looping scheme [28]. We showed that Sr 0.5 Ca 0.5 Fe 0.9 Ni 0.1 O 3−δ maintained ∼35% CO 2 sorption capacity for >35 cycles. For these reactions, a sizeable thermal swing was needed to fully decarbonate the PTS under 11% O 2 /Ar between the PTS reduction/carbonation step (at 570 • C) and the regeneration/decarbonation step (at 850 • C). However, most lignin biomass gasification processes take place at higher reaction temperatures (⩾700 • C), and an isothermal operation is always preferable for fluidized bed reactors due to a smaller energy loss and a simpler system [29,30]. In order to explore biomass conversion under a more realistic condition, an isothermal demonstration of the PTS materials is warranted. Additionally, at a fixed steam-to-carbon ratio (S/C) of 1.0, toluene steam reforming is significantly more thermodynamically demanding [9] (∆H • 298 K = +124.3 kJ·mol C −1 ) than glycerol reforming [31] (∆H • 298 K = +42.7 kJ·mol C −1 ). This high reforming endothermicity, and therefore steep temperature requirement, makes isothermal SESR of toluene (SESRT) a challenge since carbonation reactions are exothermic [32].
In this study, we extend the use of A-and B-site doped perovskite oxides to SESRT under isothermal reaction conditions, both as stand-alone multifunctional catalytic sorbents as well as in a sequential packed bed configuration downstream of a commercial Ni-based catalyst (Ni/γ-Al 2 O 3 , Alfa Aesar). Herein, we report the SESRT performance of perovskite PTSs through (a) direct impregnation of 5-10 mol% Ni on the perovskite; (b) a sequential bed configuration with NiO/γ-Al 2 O 3 upstream of Sr 0.25 Ba 0.75 Fe 0.375 Co 0.625 O 3−δ (SBFC-2635). While direct impregnation succeeds in increasing the initial activity for toluene conversion, it Figure 1. A schematic of the proposed chemical looping SESR process using Ni impregnated on a perovskite whereby (a) Ni catalyzes the reaction between toluene, steam, and oxygen provided by the perovskite lattice to form a mixture of syngas and CO2, which (b) further reduces the perovskite and carbonates it, forming ACO3. In the regeneration step, (c) oxygen is flowed in to decarbonate the formed ACO3 and (d) replenish the depleted lattice sites. The reformed perovskite is then ready for a new cycle. This scheme can also be realized in a sequential bed configuration where the active Ni catalyst is physically separate from the perovskite PTS as shown in (e).
was not a long-term solution due to undesirable Ni migration into the bulk as revealed by XRD analysis and Rietveld refinement. The sequential bed option, on the other hand, offered stable performance. While SESR was achieved isothermally for this material, the best results occurred when a 100 • C thermal swing was deployed, resulting in an average sorption capacity of ∼38% and H 2 :CO x > 4.0 across five cycles.

PTS preparation
The perovskite-based PTS particles were synthesized via the modified Pechini method. First, an aqueous solution containing appropriate ratios of the desired metal cations was prepared by dissolving their corresponding nitrates at room temperature. The solution was then heated to 40 • C before citric acid was added to form a metallic citrate. After undergoing further heating to 80 • C, ethylene glycol was introduced at a 3:2 molar ratio of ethylene glycol to citric acid to esterify the metallic citrate into a polymeric resin. After 4-5 h of isothermal mixing, the newly formed gel was then placed in an oven at 130 • C for 16 h before subsequent calcination in air for 10 h at 1000 • C in a muffle furnace with a 3 • C min −1 ramping rate. The resulting solids were then ground with a mortar and pestle and sieved to particle diameters within 180-250 µm for the experiments and <180 µm for the x-ray diffraction (XRD) characterization. Herein, Sr 1−x A' x Fe 1−y B'O 3−δ represents the Sr molar fraction of x in the A-site and the Fe molar fraction of y in the B-site.

Catalyst preparation
A simple wetting procedure was used to introduce nickel (Ni), the metal necessary to catalyze the reforming reaction, to the surface of the perovskite support. The desired molar amount of nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O) was dissolved in filtered water along with the perovskite support powers and mixed at room temperature for 30 min. The resulting mixture was then dried in an oven at 90 • C for 5 h prior to 8 h calcination at 800 • C in a muffle furnace with a 3 • C min −1 ramping rate. The calcined particles were pelletized and sieved to obtain the correct particle size.

Thermogravimetric analysis
Thermogravimetric analyzers (TA Instruments SDT Q600) were used to quickly assess sorption capacity in a simplified gas environment at varying temperatures. For these experiments, ∼50 mg of PTS particles (180-250 µm) were loaded into an alumina crucible cell within the TA furnace and then heated to a desired temperature with a ramping rate of 20 • C min −1 under Ar. The gas flow rates were controlled using Alicat MFCs and achieved reaction conditions of 20% H 2 /10% CO 2 / 70% Ar for the reduction/carbonation step and 20% O 2 /80% Ar for the regeneration/decarbonation step. The total gas flow rate was set to 200 SCCM. Figure S1(a) in the supplementary material file illustrates a typical TGA profile of an isothermal cycle.

XRD and structure refinement
Bulk phase identifications of fresh and reactor-cycled samples were conducted using ex-situ XRD characterization. For this task, a Rigaku SmartLab x-ray diffractometer was utilized (Bragg-Brentano type) with Cu Kα (λ = 0.1542 nm) radiation operating at 40 kV and 44 mA. A scanning range of 10-60 • (2θ) with a step size of 0.1 • holding for 3.5 s at each step was used to generate XRD patterns.
The XRD patterns were refined using the Rietveld program General Structure Analysis System II (GSAS-II) [33] to quantify phase percentages, site occupancy fractions, and mean crystallite sizes. Refined parameters were scale factor, specimen displacement, background, phase fraction, crystallite size, macrostrain, lattice parameters, and occupancy fractions. The background was modeled using the Chebyshev-1 model with six parameters. For model assessment, the R-factors (R F and R F 2 ) are defined in equations (6) and (7) [34]: where F O,hkl and F C,hkl are the observed and calculated structure factors, respectively.

X-ray photoelectron spectroscopy and SEM-EDS
X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of surface Ni for the impregnated samples. Sample powders were pressed onto carbon tape and outgassed at 10 −5 Torr overnight before being introduced into the ultrahigh-vacuum chamber for scanning. A PHOIBOS 150 hemispherical energy analyzer (SPECS GmbH) equipped with a non-monochromatic Mg Kα excitation source (1254 eV) was used to collect the spectra. CasaXPS program (Casa Software Ltd) was used for subsequent data analysis. The binding energy was calibrated to a C 1 s line at 284.8 eV.
Surface morphology and near-surface elemental distribution were assessed via field emission scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) using a Hitachi SU3900 SEM with Oxford Ultim Max 40 EDS in variable pressure mode. A pressure of 70 Pa was used, and the SEM images were taken in backscatter mode at 20 kV.

Toluene steam reforming reactor
Toluene steam reforming experiments were carried out in a quartz U-tube (8 mm I.D., 280 mm length) fixed-bed reactor positioned in a vertically mounted Carbolite furnace. The furnace temperature was controlled remotely with PLATINUM Configurator software (Omega Engineering), and the top of the U-tube was covered with ceramic fiber insulation to minimize heat loss. For most experiments, 1.0 g of PTS material was positioned at the bottom of the U-tubed, sandwiched on both sides by first quartz wool and then inert silicon carbide grit. For the sequential bed experiments, 0.3 g of Ni-based steam reforming catalyst (NiO/γ-Al 2 O 3 , ThermoFisher) was placed upstream of the PTS bed. Liquid injections of toluene and distilled water were controlled via two 2.5 ml stainless steel syringes (KD Scientific) mounted on programmable injection platforms (Harvard Apparatus). The volumetric flow rate of toluene was fixed at 6.733 µl min −1 while the flow rate of water was varied from 12-20 µl·min −1 corresponding to S/C ratios of 1.5-2.5. To ensure uniform injections of toluene (Sigma-Aldrich, ⩾99.5%) and water into the reactor, an annular design was utilized whereby toluene was fed through a 1/16 in OD (0.01 in ID) SS capillary tube that itself was inside of a 1/8 in OD SS tube where the water flowed (figure 2). The mixture of gas and fuel was preheating by the inert packing material to desired reactor temperatures (750 • C-900 • C) before reaching the catalyst bed(s). 30 SCCM of Ar was kept flowing into the reactor to act as a sweep/purge gas throughout the entire cycle. Heating tape maintained the outlet products in a vapor phase while the sweep gas carried them into an impinger filled with ∼1.5 ml of dodecane solvent, which was submerged in an ice water bath, to entrain any unconverted toluene and heavy products. The gaseous stream exiting the impinger was then fed to an MKS Cirrus II quadrupole mass spectrometer (MS) for real-time stream composition measurement.
In the SESRT experiments presented in this article, each cycle consists of a 15 min 'reduction' step where toluene is steam reformed and the perovskite PTS is reduced/carbonated, a purge step in pure Ar, and a 'regeneration' step where the PTS is reoxidized/decarbonated in 20 vol% O 2 /Ar followed by a final purge before the cycle repeats. For experiments involving the sequential bed, a pre-reduction step was necessary whereby 5 vol% H 2 /Ar was introduced for 30 min at reaction temperature to reduce NiO to catalytically active metallic Ni.
To evaluate the performance of the PTS materials, the toluene conversion (χ T ), outlet gas concentration of species i during the reduction step on a dry and Ar-free basis (y i,r ), and CO 2 sorption capacity are defined as follows: y i,r = n i,r n CO,r + n CO2,r + n H2 + n CH4 × 100% Sorption Capacity mol where is the observed moles of CO 2 released during the decarbonation step.

Initial PTS screening
Prior to catalyst screening, a sensitivity analysis was conducted in ASPEN Plus to determine the equilibrium product compositions of toluene steam reforming using an RGibbs reactor. The predicted equilibrium gas phase mole fractions (y i ) and H 2 : CO x ratios of the reduction step across three temperatures (750, 800, and 850 • C) and varying S/C ratios are presented in figures 3(a) and (b). Across all conditions modeled, the equilibrium toluene conversion was predicted to be 100%, suggesting that the low conversion of toluene in biomass gasification is limited by kinetics rather than thermodynamics. Coke formation was not anticipated by the model at any of the conditions screened. These sensitivity analysis forecasts also reveal that equilibrium product gas composition, and therefore H 2 : CO x ratio, is more sensitive to the S/C ratio than to temperature. Conversion data from reactor blank experiments conducted with inert packing material (SiC) highlight both the inherent kinetic limitations of toluene conversion as well as its sensitivity to the S/C ratio. The results of these experiments (figure S2 in the supplementary materials file) display a minuscule toluene  conversion over a 30-minute injection period, less than 1% for S/C ratios of 0.5 and 1.5, with an increase to values between 1.5%-3.5% for an S/C ratio of 2.5 across all three temperatures. The witnessed formation of CH 4 and CO 2 during the injection period followed by the release of CO and CO 2 in the 20% O 2 regeneration step suggests that the coke formation via toluene cracking was the primary reaction. Clearly, even within thermodynamically favorable conditions, the lack of any catalyst produces negligible performance. In the initial catalytic sorbent screening stage, SrFeO 3−δ was selected both for its thermodynamically favorable reducibility [16] in low p O2 environments as well as the observed ability of its brownmillerite phase (SrFeO 2.5 ) to carbonate to SrCO 3 at temperatures below ∼850 • C [25,35]. A-site doping of the perovskite with alkaline earth metals was conducted in order to further facilitate carbonate formation. To draw a comparison between the effects of either dopant, two A-site doped SrFeO 3-δ materials (Sr 0.4 A' 0.6 FeO 3−δ (A' = Ba, Ca)) were tested across three temperatures (750 • C, 800 • C, and 850 • C) and at a fixed S/C ratio of 1.5. At 750 • C, Sr 0.6 Ca 0.4 FeO 3−δ (SCF-64) displayed toluene conversions comparable to the SiC blank with conversions averaging ∼3.7% across three cycles. Sr 0.6 Ba 0.4 FeO 3−δ (SBF-64) was introduced at 800 • C where its measured conversion was 9.7%, and SCF-64 witnessed an improved conversion to 16.4%. However, the H 2 : CO x ratio at these conditions were low at 1.1 for SCF-64 and 0.57 for SBF-64. These poor H 2 : CO x values suggest that little, if any, sorption enhancement was occurring. Previous TGA screening experiments had ruled out the use of Ca-doped SrFeO 3−δ materials as viable sorbents above 800 • C since no carbonation had been witnessed. Therefore, only SBF-64 was subjected to toluene cycling at 850 • C where its conversion and  1). Further modification of the sorbent was needed to enhance activity.

Effects of introducing ni to the perovskite surface
To further enhance the activity for toluene reforming, Ni was impregnated to the perovskite surface. A number of previous studies showed that supported nickel catalysts were highly effective for toluene reforming [4,6,8,9,23,24,36]. However, sorption enhancement was not attempted in these studies. Impregnating the surface of the perovskite sorbents with 5-10 mol% Ni significantly improved toluene conversion. This corresponds well to the presence of surface Ni detected by XPS ( figure S3). For example, 10Ni/SCF-64 demonstrated a more than threefold increase in conversion from 16.4% to 59.0% at 800 • C, S/C = 1.5. A more modest improvement was witnessed with 10Ni/SBF-64 at the same condition with a conversion increase from 9.7% to 28%. Additionally, the Ni-impregnated samples demonstrated better H 2 :CO x ratios during the reduction step with 10Ni/SCF-64 showing an increase from 1.1 to 1.5. When pre-treated with H 2 , the H 2 :CO x ratio of 10Ni/SCF-64 increased further still to 1.8. Interestingly, the MS profiles for 10Ni/SCF-64 show differing behavior during the reduction step. As can be seen in figure 4, the sample without H 2 pre-reduction (figure 4(a)) first produces CO 2 before the onset of H 2 and CO. The reason for this likely has to do with the oxidation state of Ni. Pre-reduction with H 2 will reduce any NiO on the surface to metallic Ni, which will in turn attenuate the oxidation of CO by NiO as well as unselective hydrocarbon combustion [37]. It is also possible that pre-reducing the sorbent partially reduces the perovskite and removes unselective lattice oxygen, diminishing the potential for overoxidation. This is supported by the higher observed H 2 : CO x ratios of the H 2 -treated, bare perovskite samples (i.e. those lacking surface Ni) presented in table 1. Additionally, CO 2 spikes at the onset of the reduction step were not witnessed for those bare perovskite samples not pre-reduced in H 2 , which further substantiates the role of NiO for nonselective oxidation. As for evidence of sorption enhancement, CO 2 release during the isothermal regeneration/decarbonation step was not detected by the MS, however, XRD analysis of a cycled 5Ni/SCF-64 sample ( figure 5(b)) revealed that SrCO 3 had formed and encompassed 20.9 wt% of the final material. CaCO 3 was not detected in the XRD patterns of the cycled sample as the typical experimental CO 2 concentrations were much smaller than the equilibrium pressure of CO 2 from CaCO3 decomposition at 800 • C (∼0.25 atm) [38]. While surface impregnation of Ni improved conversion and H 2 composition, the Ni did not remain confined to the surface as evidenced by the Rietveld refined XRD scans presented in figures 5(a) and (b). Rietveld refinement conducted in GSAS-II confirmed that the fresh sample was 84.7 wt% Sr 0.4 Ca 0.4 FeO 2.875 , 12.2 wt% SrFeO 2.5 , 0.9% CaO, and 2.1 wt% NiO represented by two phases: 1.2% cubic, Fm-3m and ∼1% rhombohedral, R-3m corresponding to ∼4.7 mol% Ni. The majority SCF-64 phase is a tetragonal phase perovskite (I4/mmm). The cycled sample, by comparison, shows greater phase complexity. A complete list of phases identified for the cycled material is presented in decreasing wt% in table 2 (PFD identification numbers are provided in table S1 in the supplementary materials file). Two main conclusions can be drawn from the results in table 2: (i) the isothermal regeneration/decarbonation at 800 • C was unable to fully reproduce the original perovskite phase and completely decarbonate the formed SrCO 3 ; and (ii) the Ni impregnated on the surface had migrated into the bulk of the mixed metal oxide phases. This indicates that surface impregnation of the perovskite PTS with Ni cannot be sustained while phase transitions are occurring, as there are many routes for Ni migration. Therefore, Ni impregnation on (Sr/Ca)FeO 3 would not lead to a stable, multifunctional catalytic sorbent for sorption enhanced toluene reforming.

Sequential bed operation
In light of the inability of the Ni-impregnated SCF-64 and SBF-64 materials to achieve stable, repeatable SESR with high activity for toluene conversion, three components of the system were altered: (i) instead of impregnating the Ni onto the surface of the perovskite, a sequential bed configuration was adopted where 0.3 g of commercial steam reforming catalyst (Ni/γ-Al 2 O 3 ) was placed upstream of the 1.0 g of PTS material; (ii) the B-site of the perovskite PTS was doped with Co to attempt to increase the extent of reduction; and (iii) the S/C ratio was increased from 1.5-2.5 to minimize the coking that is known to occur when commercial Ni-based steam reforming catalysts are used. Additionally, Ca was abandoned as an A-site dopant due to its poor performance as a carbonate-forming metal at high temperatures. As a point of comparison, 0.3 g of H 2 pre-treated Ni/γ-Al 2 O 3 was tested by itself at 800 • C and S/C = 2.5 where a toluene conversion of 95% was witnessed and an H 2 : CO x = 1.9. For all the sequential bed experiments, a 5 vol% H 2 pre-treatment was used prior to the reduction step to reduce any NiO formed from the previous regeneration/decarbonation step. Sr 0.25 Ba 0.75 Fe 0.375 Co 0.625 O 3−δ (SBFC-2635) was identified as a promising CO 2 sorbent from TGA screening, which indicated that the material could achieve a sorption capacity as high as ∼35% and with an average of 31.5% over 10 repeated cycles in a reducing/carbonating environment of 20 vol% H 2 and 10 vol% CO 2 at 850 • C ( figure S1(b)). To evaluate its performance in the reactor, SBFC-2635 was first subjected to three steam reforming cycles without the Ni catalyst pre-bed upstream. Without the Ni catalyst, the SBFC-2635 perovskite was still able to release lattice oxygen for steam reforming and generate syngas as well as initially capture some of the co-produced CO 2 as shown in figure 6(a), but once the sorbent saturation point had been reached, it proceeded to produce only H 2 and CO 2 . The perovskite sorbent successfully demonstrated sorption enhancement, but it was much more modest than the TGA results with a sorption capacity of 6.6% for the first cycle and a conversion and H 2 : CO x ratio of 52% and 1.9, respectively. Furthermore, after the first cycle, the sorption capacity and toluene conversion decreased to 3.9% and 38.4%, respectively, by cycle three.
Introducing the Ni catalyst pre-bed substantially improved sorption performance, as seen by the comparison of MS profiles in figures 6(a) and (b). These product gas profile differences are likely owed to the greater extent of reduction of the PTS induced by the flow of reforming products from the pre-bed as shown schematically in figure 1(e). Essentially, the Ni catalyst pre-bred reforms the incoming toluene to a mixture of mostly syngas and CO 2 (due to the higher S/C ratio) and some CO. These product gases then pass over the PTS bed (represented as ABO 3−δ where δ = [0, 0.5]) which had been slightly pre-reduced during the H 2 pre-treatment step. The partially reduced PTS is now primed to capture CO 2 produced by the Ni bed as well as oxidize any CO produced. The SESR performance results of the sequential bed configuration operated either isothermally at 850 • C or with a thermal swing from 850 • C to 950 • C during the regeneration/decarbonation step are compared in figure 7(a). Liquid samples taken from the impinger downstream of the reactor and analyzed with GC   confirmed high toluene conversion, averaging at ∼96% across 5 cycles for both operation conditions, which is consistent with the conversions measured for the Ni catalyst by itself. However, the calculated sorption capacities for the thermal swing mode were ∼10%-20% higher than for the isothermal mode, despite having similar conversions. This indicates that isothermal decarbonation at 850 • C is insufficient to fully decarbonate the PTS, and as a result, the sorption capacity steadily decreases after cycle 2. When a 100 • C thermal swing is used to decarbonate the PTS, the performance stabilizes after cycle 3 and the observed H 2 :CO x ratios remain >4.0. Additionally, XRD analysis of a fresh vs. 5 × thermal swing-cycled SBFC-2635 sample ( figure 7(b)) confirmed that no carbonate species remained after the decarbonation and that the sample performed excellently as a PTS-returning completely to its original phase.

Conclusion
The application of A-and B-site doped SrFeO 3−δ perovskite oxide based PTSs was demonstrated for SESRT in a chemical looping scheme. The effects of sorbent compositions, reactor arrangements, and temperature swings between the sorption and desorption steps were investigated. Within the operating temperature range of interest, toluene reforming was determined to be kinetically limited using the unmodified sorbents. Two strategies, i.e. direct Ni impregnation and use of a Ni/ γ-Al 2 O 3 pre reforming bed, were investigated to enhance toluene activation and conversion. Impregnation of Ni on SCF was found to be effective to enhance the initial reforming activity. However, Ni migration into the oxide phase was observed based on Rietveld refinment of XRD patterns, leading to substantial deactivation of Ni/SCF. On the other hand, Sr 0.25 Ba 0.75 Fe 0.375 Co 0.625 O 3−δ (SBFC-2635) in the sequential bed demonstrated successful isothermal SESRT with an average sorption capacity of ∼25% across 5 cycles, but a steady decrease in sorption capacity was observed. By employing a 100 • C thermal swing between the reduction/carbonation and regeneration/decarbonation steps, the highest obtained sorption capacity was witnessed in the first cycle at 47%, after which the values plateaued to ∼38%. Additionally, the H 2 : CO x values across all 5 cycles were consistently >4.0 further confirming the efficacy of the sorption enhancement approach. To summarize, cyclic SESRT was most effective in a sequential bed configuration using a thermal swing, and SBFC-2635 sorbent displayed complete phase recyclability. This research carries positive implications for biomass conversion, particularly in scenarios where tar buildup poses challenges. Our study illustrates that high conversions can be reached at standard gasification temperatures with concurrent CO 2 sorption. Future research in this area should shift focus from fixed-bed reactor demonstrations to fluidized bed reactors where the Ni-based catalyst and the PTS are mixed in the reducer. Meaningful scaleup is warranted to assess the long-term feasibility of this sorption-enhancement strategy.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).