Decarboxylation of stearic acid over Ni/MOR catalysts

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Research ArticleReceived: 29 July 2019 Revised: 4 September 2019 Accepted article published: 10 September 2019 Published online in Wiley Online Library: 14 October 2019

(wileyonlinelibrary.com) DOI 10.1002/jctb.6211

Decarboxylation of stearic acid overNi/MOR catalystsJames M Crawford,a Sarah F Zaccarine,b Nolan C Kovach,b

Courtney S Smoljan,aJolie Lucero,aBrian G Trewyn,b,cSvitlana Pylypenkob

and Moises A Carreona*

AbstractBACKGROUND: Oils derived from plants, animal fats, and algae contain both saturated and unsaturated fatty acids. These fattyacids can be converted into liquid fuels and chemicals in the presence of active solid catalysts.

RESULTS: Nickel-based catalysts were supported on mordenite via ion exchange synthesis and evaluated for the deoxygenationof stearic acid to diesel fuels. By tuning the synthesis pH, loadings of over 20 wt% Ni were obtained. Catalysts synthesizedat pH 8.5 displayed the highest Ni loading and the highest activity for the decarboxylation/decarbonylation of stearic acidunder inert nitrogen gas atmospheres, yielding 47% heptadecane. Characterization included scanning transmission electronmicroscopy-energy-dispersive spectroscopy (STEM-EDS), X-ray di raction (XRD), field emission scanning electron microscopyff(FE-SEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), N2physisorption and thermogravimetric analysis(TGA), providing new insights into the recyclability of the catalyst. The observed loss of catalytic activity upon recycling wasattributed to the agglomeration of Ni nanoparticles and the accumulation of carbonaceous coke.

CONCLUSION: This work demonstrates that Ni-based catalysts supported on mordenite zeolite can e ectively convert stearicffacid into heptadecane. Yields to heptadecane were as high as 47%. Mechanistically, the reaction proceeds by decarboxylationand decarbonylation pathways.© 2019 Society of Chemical Industry

Supporting information may be found in the online version of this article.

Keywords: lipid biomass; fatty acids; decarboxylation; stearic acid; heptadecane

INTRODUCTIONAs global greenhouse gas emissions enter a criticalregime, thesearch for low-energy alternatives intensifies.Potential benefitsfrom biofuels include a reduction in carbon emissions as high as70% compared with fossilfuels1 and increased nationalenergysecurity.2 For over 30 years,researchershave developed thethermochemical conversion of oily plants and algae to trans-portation fuels and chemicals. The field originally focused onutilizing sulfided3 and noble metal4–7 catalysts in hydrogen-richatmospheres, yielding both high conversions and high selectivityto alkane products by hydrodeoxygenation (HDO). Unfortunately,sulfided catalysts have poor stability in the presence of water,which is largely unavoidable in biomass feedstocks,8 and noblemetal catalysts are extremely expensive.9 Therefore,low noblemetal loadings were explored under similar conditions,yieldingpromising results.10–14 Simultaneously, researchers began to studythe effect of different gas atmospheres, seeking to remove hydro-gen from the reaction, proposing decarboxylation (DCO)as analternative pathway.15 Currently,researchers are focusing on theuse of non-noble metals under hydrogen-depleted atmospheresfor the selective production of diesel-range alkanes.16–18

Nickel is highly active for the deoxygenation of fatty acids,earth abundant, and relatively inexpensive,making it a highly

suitable catalyst for the abovementioned reaction. The use ofnickel for deoxygenation of fatty acids to liquid fuel hydrocarbonshas been documented. For instance, Miao et al.19 studied Ni/ZrO2catalysts under inert gas with the addition of water as the hydro-gen donor. They observed a yield of 64 and 47% to paraffin foroleic acid and stearic acid,respectively.Santillan-Jimenez et al.20

studied Ni/C for the deoxygenation of tristearin and stearic acidunder a nitrogen atmosphere and observed 81 and 19% conver-sion, respectively.Selectivity for heptadecane was 75 and 50%,respectively.Santillan-Jimenez et al.21 also studied Ni – Al layereddouble hydroxide catalysts for the same reaction,giving paraf-fin yields of 43 and 53% for tristearin and stearic acid,respec-tively. Wu et al.9 studied the conversion of stearic acid over nickel

∗ Correspondence to:MA Carreon,Department of Chemicaland BiologicalEngineering, Colorado School of Mines, Golden, CO, 80401, USA.E-mail: [email protected]

a Department of Chemical and Biological Engineering, Colorado School of Mines,Golden, CO, USA

b Department of Chemistry, Colorado School of Mines, Golden, CO, USA

c Materials Science Program, Colorado School of Mines, Golden, CO, USA

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103Decarboxylation of stearic acid www.soci.org

catalysts supported on activated carbon and found the optimalnickel loading to be 20 wt%.These authors showed nearly com-plete conversion and a selectivity to heptadecane of around 80%.Yang et al.22 utilized nickel-containing metal organic frameworkslinked to a zeolite support to study the DCO of oleic acid andfound a conversion of 92% and a heptadecane yield of up to 77%under CO2 atmospheres. Song et al.23 explored the importance ofsynthesis methods by comparing incipient wetness impregnatedNi/HBEA catalysts with ion-exchanged catalysts.For the HDO ofstearic acid, they showed that the ion-exchanged Ni/HBEA with aconversion of 97% outperformed the impregnated catalyst givinga conversion of only 33%.

The use of zeolites as catalytic supports for deoxygenation offatty acids has been explored by our group and independentresearch groups and has yielded promising results as a bifunctionalcatalyst support providing enhanced metal dispersion, higher cat-alytic conversion and improved stability.10,12–14,24–27In the currentstudy, zeolite mordenite (MOR) was selected as the catalytic sup-port material for Ni due to its robust catalytic properties in indus-trial settings, high surface areas, and tunable acidity.28,29MOR is ahigh-silica-content zeolite with a pore diameter of 6.7 Å.In addi-tion, mesoporosity can be introduced into the framework of MORthrough facile thermal treatments,promoting interparticle diffu-sion and activity.30 Herein we report a systematic study of the ionexchange synthesis effects on the loading of Ni on mordenite, thecatalytic properties of Ni/MOR for the deoxygenation of stearicacid under inert atmospheres, and the recyclability of the resultantcatalysts.

EXPERIMENTALMaterialsHexane (99%, SupraSolv,Sigma-Aldrich,St. Louis, MO, USA)and stearic acid (95% ACS,Sigma-Aldrich) were used for deoxy-genation experiments. N,O-Bis(trimethylsilyl)triflouro-acetamide(BTSFA,99% ACS,Sigma-Aldrich)and n-tetradecane (99% ACS,Sigma-Aldrich)were employed for gas chromatography-massspectrometry (GC-MS)analysis. Nickel(II) nitrate hexahydrate(98%,Sigma-Aldrich),NH4-mordenite (Zeolyst,Kansas City,KS,USA)and 28% ammonia solution (Sigma-Aldrich)were used inthe synthesis of Ni/MOR.Hydrofluoric acid (HF,48% AR,Macron)and methylene chloride (DCM, ACS, Fischer) were required for theanalysis of soluble coke compounds. Hydrochloric acid (HCl, 37%ACS,Sigma-Aldrich),nitric acid (HNO3, ACS,Sigma-Aldrich) andHF were used for sample preparation prior to inductively coupledplasma atomic emission spectroscopy (ICP-AES).Nitrogen (N2)and hydrogen (H2) gases utilized in the catalytic reaction and inthe reduction step respectively were of UHP grade.

Ni/MOR-x synthesis by ion exchangeFor the synthesis of Ni/MOR,first 5 g of NH4-mordenite was cal-cined at 735∘C with a ramp rate of 10 ∘C min−1 for 6 h to pro-mote mesoporosity29 and give the protonated zeolite, denoted asH-MOR.Next, an aqueous solution of nickelnitrate hexahydrate(0.5 mol L–1 , 50 mL) was stirred untilhomogeneous.Then 2 g ofH-MOR was added to the nickel nitrate solution and stirred untilhomogeneous, followed by 5 min of sonication. Under continuousstirring, 3% NH3 solution was added dropwise to the mixture untilreaching the selected synthesis pH (x=6.5, 7.5, 8.5, 9.5). On reach-ing the desired pH,the solution was sealed with a paraffin waxfilm and stirred for 3 h at room temperature.The resultant solu-tion was vacuum filtered,rinsed three times with distilled water,

dried at 100∘C overnight, and calcined at 400∘C with a ramp rateof 1∘C min−1 for 4 h. Finally, samples were reduced at 460∘C with aramp rate of 1∘C min−1 for 5 h under pure H2 flow at 150 mL min−1

to give Ni/MOR-x. The selected reduction temperature was guidedby temperature-programmed reduction (TPR) studies showing anonset of reduction occurring around 460∘C (Fig. S1 in File S1).

Deoxygenation experimentsIn a typical deoxygenation experiment, 250 mg of catalyst,1000 mg of stearic acid and 10 mL of hexane were loaded intoa 100 mL batch reactor (Model4560,Parr,Moline, IL, USA).Thereactor was flushed with N2 at 10 bar and discharged five times toremove stagnant air. Then the reactor was sealed and pressurizedto 10 bar N2, stirred at 600 rpm and heated to 300∘C at 5∘C min−1,at which point the reaction time of 3 h was started. The pressureat 300∘C was approximately 38 bar. The reaction was determinedto be kinetically limited with no mass transfer limitations (TableS1 in File S1). After 3 h, the heater was removed from the reactorto allow ambient cooling, and stirring was stopped. Once cooledto room temperature, the pressure was released from the reactor,and liquid samples were collected for product analysis.The cata-lyst was rinsed three times with methanoland three times withhexane and either taken for thermogravimetric analysis (TGA),giving spent Ni/MOR-x,or exposed to calcination and reduction,giving recycled Ni/MOR-x.The recycled catalyst was exposed tothe same characterization as the fresh catalyst and an additionalreaction to test the stability. For time-dependent studies,a diptube with alternating valves was used to pull∼500μL samples atdifferent times.

Catalyst characterizationFresh and recycled catalyst samples were characterizedbefore and after the deoxygenation experiments.Surface area(Brunauer–Emmett–Teller(BET)method), pore volume (t-plotmethod) and pore size distributions (Barrett – Joyner – Halenda(BJH) desorption method) were calculated from the N2 physisorp-tion isotherms (ASAP 2020,Micromeritics,Norcross,GA, USA).Field emission scanning electron microscopy (FE-SEM)imagesof the catalyst morphology were collected using an acceleratingvoltage of 5 kV (ISM-7000F, JEOL Peabody, MA, USA). Nickel crystalsize and chemicalcomposition/distribution were inspected andquantified by scanning transmission electron microscopy (STEM)images and energy-dispersive spectroscopy (EDS) maps collectedby dispersing sample powders onto carbon film 300-mesh coppergrids and imaging using an FEITalos (Hillsboro,OR,USA) F200Xoperated at 200 kV equipped with a Bruker (Billerica,MA, USA)XFlash SDD detector.Crystallite size was estimated by powderX-ray diffraction (XRD) (Kristalloflex800,Siemens,Orlando, FL,USA;25 mA,30 kV,Cu K𝛼radiation) using the Scherrer equation.Nickel loading and SiO2/Al2O3 ratios were calculated using induc-tively coupled plasma-atomic emission spectroscopy (ICP-AES).For ICP-AES,5 mg samples were digested in 150μL of HF (48%)and 300μL of aqua regia (HCl/HNO3 3:1,v/v), diluted to a totalvolume of 5 mL with 5% (w/v) HCl. Once digestion was complete,samples were filtered through a 0.45μm filter and analyzed byEPA Method 200.7 (Perkin Elmer, Waltham, MA, USA, ICP-AES).

Ammonia temperature-programmed desorption (NH3-TPD)experiments were performed using a Micromeritics AutoChemII 2920 chemisorption analyzer fitted with a thermalconductiv-ity detector (TCD). Samples were placed in flow-through glassU-tubes packed atop quartz wool. Approximately 50 mg samples

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were used for each experiment. The catalyst underwent twopretreatment phases consisting offlowing 10% H2/Ar over thesample followed by heating from room temperature (RT) to550∘C at 10∘C min−1 ramp. After returning to RT, the samplewas pulse dosed with 10% NH3/Ar until the signal peak areabecame constant. TPD was completed by heating the NH3-dosedcatalyst to 550∘C at 10∘C min−1 ramp,with 1 s−1 data collection.Temperature programmed reduction (TPR) experiments were alsoconducted using a Micromeritics AutoChem II 2920. Pretreatmentfor TPR runs included flowing 10% H2/Ar over the catalyst andheating to 800 ∘C at 10∘C min−1 ramp. After cooling to RT,theTPR experiment was performed by heating from RT to 550∘C at10∘C min−1 ramp, with 1 s−1 data acquisition.

Liquid product analysisAfter the reactor had cooled to RT, a representative liquid samplewas weighed into a 2 mL glass GC vialand 5μL of tetradecane(internal standard),50μL of BTSFA (silylating agent) and 1000μLof hexane were added.The vial was capped and heated to 60∘Cfor 1 h to allow complete silylation of the sample.Then 0.2μL ofthe sample was injected (Agilent, Santa Clara, CA, USA, AS 7683B)into a GC, (Agilent GC 6980N)-MS,Agilent MSD 5973N)systemand separated on an HP-5 MS column (30 m×250μm ×0.25μm)utilizing the following oven program: 5∘C min−1 ramp rate from 40to 250∘C, then hold for 10 min.

Coke analysisCarbonaceous coke compounds were analyzed in the spent andrecycled-spent Ni/MOR-8.5 catalyst to understand the changes indeactivating products.The spent catalyst was collected immedi-ately after the reaction following the methanol and hexane rins-ing steps. Thermogravimetric analysis (TGA) of the spent catalystprovided an estimate of the weight of combustible species.ForTGA, 10 mg of sample, previously dried in air for 1 h at 100∘C, wasloaded under 50 mL min−1 air and 50 mL min−1 N2 and heated witha ramp rate of 10∘C min−1. The coke was estimated as the weightloss between 250 and 750∘C. Soluble coke composition was ana-lyzed by completely digesting the catalyst in HF and conductinga liquid – liquid extraction with DCM.31,32 Briefly,15 mg of spentcatalyst was completely digested in 1 mL of 48% HF for 20 min.Next, 5 mL of DCM was added and mixed vigorously for 2 min.Finally, the solution was allowed 1 h to separate into two phases.The organic phase was sampled for analysis by GC-MS with thefollowing oven program: 5∘C min−1 ramp rate from 40 to 300∘C,then hold for 10 min. Owing to the complex nature of carbona-ceous coke compounds, only a chromatographic area percent wasreported, as no standards were available for the evaluation of themolar composition.

RESULTS AND DISCUSSIONCharacterization of Ni/MOR-xCatalysts were first investigated by XRD.All samples exhibitedhigh crystallinity,including the characteristic MOR phase (JCPDS#29-1257) (Fig. 1).A clear trend in the increasing intensity of NiOpeaks (JCPDS #47-1049) was evident as the synthesis pH increasedfrom 6.5 to 8.5. For Ni/MOR-9.5, the prominence of the NiO peaksis significantly less than for Ni/MOR-8.5.This observation is wellcorrelated with the bulk Ni loading measured by ICP-AES (Table 1).This trend is expected, as the pH has a strong effect on the mobilityof Ni species in solution.The isoelectric point (IEP)of NiO has

Figure 1. XRD patterns of fresh (a) H-MOR, (b) Ni/MOR-6.5, (c) Ni/MOR-7.5,(d) Ni/MOR-8.5, (e) Ni/MOR-8.5 recycled and (f ) Ni/MOR-9.5.

been evaluated by previous researchers to be pH 8,33 8.134 and8.56.35 On comparing the𝜁-potential of NiO35 with the synthesispH of our catalysts,one would expect the highest Ni loadingnearest the IEP, following Ni/MOR-8.5> Ni/MOR-9.5> Ni/MOR-7.5> Ni/MOR-6.5,which was the observed trend.The loading of Niin the Ni/MOR-8.5 catalyst was 21.8 wt%,which is very close tothe optimal 20 wt% established by previous researchers for thedeoxygenation of fatty acids.9,20 Particle size was evaluated by theScherrer equation (Table 1), with the smallest crystallite size in theNi/MOR-7.5 sample and the largest in the Ni/MOR-8.5 sample. Thesize of NiO particles could not be evaluated in the Ni/MOR-6.5sample as the characteristic peaks lacked sufficient intensity. Theparticle sizes from the Scherrer equation were considered to berelatively accurate because of the reasonable agreement with theTEM particle size of the fresh catalyst.

The distribution of elements present in the the catalyst was elu-cidated by STEM-EDS. Figure 2 suggests that the synthesized cata-lyst exhibited a homogeneous distribution of Ni species across thezeolite support.No major particle agglomeration was observedin the analysis of the fresh Ni/MOR-8.5.An important observa-tion from the STEM-EDS analysis is the migration ofAl speciesto the zeolite surface. In conventional zeolite structures,SiO2and Al2O3 form a tetrahedrally coordinated framework. In oursamples,an accumulation of Al at the surface was observed inthe EDS Si+ Al overlay (Fig. 2).This suggests mobility of the Alspecies within the zeolite framework. Steam-and heat-treatedzeolites have been shown to form extra-framework aluminumspecies (EFAl)28,36 which are mobile in the framework. 37,38 Thepresence of EFAlhas been attributed to the high-temperaturecalcination step used to obtain H-MOR from NH4-MOR resultingin the perturbation of some framework Al.28 This result was inagreement with the bulk ICP-AES analysis of the SiO2/Al2O3 ratio(Table 1) showing mild dealumination from the parent NH4-MOto H-MOR. The acidity, measured by NH3-TPD,indicated thatthe high-temperature calcination to obtain H-MOR resulted ina major reduction in the MOR acidity. This could be attributedto the removal of framework Al and the promotion of EFAlin

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Table 1.Textural properties of studied catalysts

SSA (m2 g−1) Pore volume (cm3 g−1) DP (nm)

Sample SiO2/Al2O3a

Acidityb

(mmol g−1)Ni loadinga

(wt%) SBETc Sext

d Vtote Vmicro

d Vmesof XRDg TEMh

NH4-MOR 20 – i 0 465 47 0.25 0.15 0.1 – –H-MOR 20.7 0.03 0 566 108 0.32 0.16 0.16 – –Ni/MOR-6.5 21.8 0.636 0.4 504 150 0.31 0.12 0.19 – –Ni/MOR-7.5 22.4 0.997 8.3 452 116 0.33 0.12 0.21 4.14±0.5 –Ni/MOR-8.5 23.3 0.765 21.8 358 150 0.34 0.07 0.27 6.11±0.2 6.42±0.9j

Recycled Ni/MOR-8.5 23.7 – 21.7 347 181 0.39 0.08 0.31 9.96±1.2 14.8±15.4k

Ni/MOR-9.5 21.1 0.912 15 425 163 0.36 0.08 0.28 4.88±1.4 –

a From ICP-AES.b From NH3-TPD.c Specific surface area (SSA) SBETcalculated by BET method.d External surface area Sext and micropore volume Vmicro calculated by t-plot method.e Total pore volume Vtot calculated from quantity adsorbed at P/P0 =0.975.f Mesopore volume Vmeso=Vtot −Vmicro.g Approximate particle size DP estimated from Scherrer equation.h Approximate particle size DP estimated by representative images from TEM.i Blank entries (—) indicate that data were not obtained for these samples.j Mean±standard deviation calculated from region in File S1, Fig. S2a.k Mean±standard deviation calculated from regions in File S1, Figs S2b and S2c.

agreement with observations from STEM-EDS and ICP-AES.Afterthe introduction of Ni from the ion exchange synthesis, the acid-ity is increased. The highest acidity was noted in Ni/MOR-7.5 with0.997 mmol g−1.

The surface area and micropore volumes from N2 physisorption(Table 1) tended to decrease as the Ni loading increased, indicat-ing that Ni was forming complexes inside or at the opening ofthe micropores of the MOR (Fig. S3 in File S1).Table 1 also high-lights the importance of the calcination of NH4-MOR to obtainH-MOR,which leads to increased pore volumes and higher sur-face areas. Interestingly, the total pore volume appears to increasewith higher synthesis pH, indicating the formation of voids in thesamples.This is supported qualitatively by the FE-SEM imagesshowing that the roughness of resultant catalysts increases as pHincreases, leading to higher porosity (Fig. 3). Also, the abundanceof mesopores around 43 Å tended to increase as the synthesis pHincreased (Fig. S4 in File S1),which could promote more interac-tions between the catalyst and the stearic acid.

Deoxygenation resultsTable 2 showsthe liquid product distribution for the deoxy-genation of stearic acid over Ni/MOR-x catalysts.Catalytic dataindicated that Ni/MOR-8.5 was the highest-performing catalystwith a maximum conversion of 54% and a heptadecane (C17) selec-tivity as high as 87% (Table 2, entry 4). To confirm this result, freshNi/MOR-8.5 was tested in an independent reaction, giving a similarperformance (entry 5). The performance of Ni/MOR-8.5 was moni-tored during the reaction to understand the evolution of productsin the deoxygenation of stearic acid (Fig. 4). When conducting thetime-resolved sampling trials,some experimentalerror may bepossible,as the removalof products can affect the equilibriumof the reaction owing to Le Chatelier’s principle.Consumptionof stearic acid began during the heating phase and continuedat a fairly constant rate throughout the reaction. Intermediateproducts detected in the reaction included nonadecanone andheptadecene.The concentration of these products was relativelysmall as compared with the formation of heptadecane.DCO,

which involves the removal of the carboxylate group from theCn fatty acid as CO2, leaving Cn−1 products, was hypothesizedto be the dominant pathway. Another likely reaction pathwaywas decarbonylation (DCN) giving unsaturated products via theremoval of CO,evidenced by the formation of heptadecene.39–41

The formation of heptadecane over octadecane indicated thatHDO was not a competing pathway with DCO and DCN.10,12,13

When comparing the performance of the Ni/MOR-x catalysts,aclear correlation between the loading of Niand the conversionexists.As the loading of Ni increased,the conversion increased.Also, as the porosity of the Ni/MOR-x catalysts increased,theconversion increased.Overall,selectivity was moderate to highfor all catalysts,but conversion was limited by the loading ofNi. In the catalysts Ni/MOR-7.5,Ni/MOR-8.5 and Ni/MOR-9.5,theformation of some benzene indicates that cracking pathwayswere competing with DCO.We hypothesize that the high acidityin these catalysts promoted some cracking product competition.

The improved catalytic performance of the Ni/MOR-8.5 catalystwas attributed to high surface area, porosity, and optimal Ni load-ing. Specifically, mesoporosity was hypothesized to alleviate masstransfer limitations encountered by bulky molecules in the pores.Previous studies have demonstrated that mesoporosity can pro-mote high activity in the deoxygenation of fatty acids.42,43 Theloading of Ni was around 20 wt% ,which was previously reportedto be the optimal loading for deoxygenation activity.9,20 Finally,the dispersion of Ni was shown to be very uniform across the sup-port, without agglomerates. In principle, high dispersion leads tomore active centers and therefore higher activity for the deoxy-genation of stearic acid.The catalyst did exhibit some selectiv-ity losses to the formation of primarily heptadecenes,crackedparaffin pentadecane,aromatic dodecylbenzene and alcoholic2-nonadecanone.These products are characteristic ofthe DCOand DCN pathways.15,44,45

Catalyst recyclabilityThe recycled Ni/MOR-8.5 catalyst displayed a decay in catalyticactivity, suffering a 24% decrease in conversion.Typically,loss


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Figure 2. STEM/EDS chemical composition of fresh Ni/MOR-8.5 exhibiting a highly dispersed layer of Ni across the support.

Figure 3. FE-SEM images of (a) H-MOR,(b) Ni/MOR-6.5,(c) Ni/MOR-7.5,(d) Ni/MOR-8.5,(e) Ni/MOR-8.5 recycled and (f ) Ni/MOR-9.5 showing higherroughness at higher synthesis pH.



Table 2.Liquid product distribution for deoxygenation of stearic acid over Ni/MOR-x catalystsa

Product distribution (mol%)

Entry Catalyst XTOTb(%) Benzenes C17 C17-ene Other Yield C17 (%) TOFc(h−1)

1 H-MOR 12 0 90 0 10 11 –2 Ni/MOR-6.5 16 0 93 0 7 15 5.33 Ni/MOR-7.5 20 1 87 4 7 17 3.24 Ni/MOR-8.5 54 1 87 8 7 47 3.35 Ni/MOR-8.5 52 3 77 5 15 40 3.26 Recycled Ni/MOR-8.5 16 0 90 8 2 14 1.07 Ni/MOR-9.5 30 1 92 1 5 28 2.7

a Reaction conditions: catalyst/SA=250:1000 (mg), P=38 bar N2, T=300∘C,𝜔 =600 rpm, t=3 h.b Conversion.c Turnover frequency=g SA consumed g−1 Ni h−1, where g Ni=g catalyst·Ni loading (ICP).

Figure 4. Liquid product distribution of () stearic acid, () heptadecane,( ) heptadecenes,( ) benzenes and ( ) other products during a typicalreaction with Ni/MOR-8.5. Error bars represent the range about the averageof two data points. Reaction conditions:1000 mg stearic acid,250 mgNi/MOR-8.5, P=38 bar N2, T=300∘C,𝜔 =600 rpm, t=3 h.

in catalytic activity can be attributed to metallic leaching, lossof surface area,or accumulation of carbonaceous coke com-pounds. When comparing the fresh and recycled catalyst, theloading of Ni is very similar, changing from 21.8 to 21.7 wt%,indicating minimal leaching during reaction. The surface areawas also surprisingly similar,incurring only a 3% decrease.Car-bonaceous coke was analyzed by TGA and GC-MS.The spentNi/MOR-8.5 catalyst exhibited ∼20 wt% coke accumulationcompared with the recycled-spent Ni/MOR-8.5 with a high∼60 wt% coke (Fig. S5 in File S1).The composition of the cokecompounds was divided between oxygenated compounds andhydrocarbons for the fresh catalyst,whereas the recycled cata-lyst exhibited primarily oxygenated coke compounds (Table S2in File S1).

To further understand the loss in activity for the recycled cat-alyst,a comparison of the crystallinity and the morphology wasrequired. One obvious difference in the catalyst was the change incrystal structure, evidenced by the shift from NiO to Ni (Fig. 1). Thischange indicated that the reaction and regeneration of the cat-alyst via calcination and reduction resulted in metallic Ni speciesrather than the expected NiO. This observation was confirmed bySTEM-EDS (Fig. 5),where Ni was observed with no strong corre-lation with O species. Additionally, the recycled catalyst exhibited

metal particle agglomeration, which was not observed in the initialmorphology of the fresh catalyst with a homogeneous distributionof Ni species. Figure 6(a) illustrates the homogeneous distributionof particles in the Ni/MOR-8.5 sample compared with the largeagglomeration in the recycled catalyst (Figs 6(b)and 6(c)).Thisagglomeration was also evidenced by the particle size analyzed byXRD showing a shift from∼6 nm in the fresh catalyst to∼10 nmin the recycled catalyst. Clearly, the recycled catalyst experiencedboth drastic accumulation of carbonaceous coke and agglomera-tion of metal species due to the regeneration of the catalyst.Wehypothesize that the deoxygenation experiments resulted in NiOparticle instability and that, on recycling, the particles were moreprone to reduction and agglomeration, resulting in the observedmetallic Ni species.

Comparison with state-of-the-artTable 3 shows a comparison of the current study withstate-of-the-art Ni-based catalysts for the deoxygenation ofmodel compound stearic acid to heptadecane.Previous studieshave reported other important model compounds, includingsoybean oil, linoleic acid, oleic acid, methyl oleate and fattyacid methyl esters.46–53 For the fresh Ni/MOR-8.5 catalyst,goodcatalytic performance was observed underan N2 atmosphere.The DCO pathway yielded up to 47% heptadecane (Table 3,entry 11). This result is comparable to the yield of Ni/AC (entry10).40 The major difference is that our system exhibited highselectivity and lower conversions.A number of entries in Table 3were conducted in microbatch reactors where control of thegas atmosphere was not possible owing to the design of thereactor system (entries 3 – 10).For those studies, direct com-parison with our current work is not possible as experimentalconditions are not the same. Compared with the supportedNi/ZrO2 catalysts reported by Peng et al.55 (entry 1), our yieldsare significantly lower. These authors utilized the HDO path-way with P =40 bar H2, T =260∘C and t =8 h. Long batchreaction times of 8 h would be more challenging for scalabil-ity than shorter reaction times. To our best knowledge, theNi/MOR catalysts presented here display the highestreportedheptadecane yields from the deoxygenation of stearic acid underinert gas environments in a batch reactor. In addition, the ionexchange synthesis utilized in this study is potentially amenableto scale-up owing to the low temperatures, lack of template mate-rials,short exchange times,and overall simplicity as compared


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Figure 5. STEM/EDS chemical composition of recycled Ni/MOR-8.5 catalyst exhibiting agglomeration of Ni into heterogeneous clusters. The Ni appearsto be primarily metallic as it is found on the surface without correlation to the zeolite support or to oxygen.

Figure 6. The distribution of particles was (a) homogeneous in the fresh Ni/MOR-8.5 catalyst compared with (b, c) heterogeneous in the recycled catalystat various scales.

with hydrothermal, templating, and co-precipitation synthesisconditions.54

CONCLUSIONSNickel-based catalystssupported on mordenite zeolite weresynthesized via ion exchange.By modifying the synthesis pH,

tunable control of the metal loading provided a well-dispersedNi-based catalyst. The fresh catalyst exhibited a unique NiOmorphology with a homogeneous distribution of Ni species andboth high surface area and porosity. The catalyst was used inthe conversion of stearic acid for the production of diesel-rangealkane heptadecane.Yields to heptadecane were as high as 47%following the DCO/DCN pathway. Detailed characterization of the



Table 3.State-of-the-art Ni-based catalysts for conversion of stearic acid to heptadecane

Entry Reference Catalyst Synthesisa Gas atmosphere Major product XTOT(%) Yield (%)

1 55 Ni/ZrO2 IW H2 C17 100 962 16 NiMo/Al2O3-𝛽 IW N2 C17 53 133 9 Ni/SiO2 CP b C17 37 204 9 Ni/C IW b C17 60 425 9 Ni/ZrO2 CP b C17 54 436 40 Ni/C IW b C17 100 507 40 Ni/Al2O3 IW b C17 64 168 40 Ni/TiO2 IW b C17 95 179 40 Ni/ZrO2 IW b C17 71 2110 40 Ni/AC IW b C17 99 5011 This work Ni/MOR-8.5 IE N2 C17 54 47

a IW, incipient wetness impregnation; CP, co-precipitation; IE, ion exchange synthesis.b Microbatch reactor experiments without gas atmosphere control.

fresh and recycled Ni/MOR-8.5 catalyst yielded new insights intothe regeneration of ion-exchanged catalysts.By comparing theSTEM/EDS results of the fresh and recycled catalyst, a clear shift inmorphology from NiO to agglomerated metallic Niwas a majorcause of activity loss.

ACKNOWLEDGEMENTMA Carreon thanks the NationalScience Foundation NSF-CBETAward # 1705675 for supporting this work financially.

Supporting InformationSupporting information may be found in the online version of thisarticle.

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Decarboxylation of stearic acid over Ni/MOR catalysts