Catalysis Letters (2026) 156:192 https://doi.org/10.1007/s10562-026-05415-3

Mateo Moreno Giraldo mateo.moreno1@udea.edu.co Aída Luz Villa aida.villa@udea.edu.co

1

Environmental Catalysis Research Group, Department of Chemical Engineering, Faculty of Engineering, Universidad de Antioquia UdeA, Medellín, Colombia Abstract The mud provided by the process of purification of water contains metals such as aluminum, silicon and iron which have significant potential for catalytic applications. Sludge after thermal (LS600) and chemical treatment with HCl (LS600- HCl) and subsequently with iron (LS600-HCl-Fe7%) were used as catalysts in the transformation of α-pinene and limo­ nene. The thermal treatment removed organic matter and concentrated the metal oxides, while the HCl treatment improved the acidity that was enhanced with the addition of iron. The LS600-HCl catalyst favored the isomerization of α-pinene with 85% conversion and 39% selectivity towards camphene, while over LS600-HCl-Fe7% catalyst, complete conversion of limonene was reached with 44% selectivity towards p-cymene. The treated materials showed increases in acidic sites, with Lewis acidity associated with metals such as Al and Fe. LS600-HCl and LS600-HCl-Fe7% materials had surface areas of 103 and 84 m²/g, respectively, dominated by mesopores, with slight morphological changes but greater roughness. Mössbauer analysis of the LS600-HCl-Fe7% material showed 36% magnetite and maghemite, and 64% paramagnetic phases such as hematite or goethite.

Received: 11 March 2026 / Accepted: 15 May 2026 © The Author(s) 2026 Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts Mateo Moreno Giraldo1

· Aída Luz Villa1

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192

Page 2 of 21 M. M. Giraldo, A. L. Villa Abbreviations A Anortithe Ai Initial GC area Al Albite An Amphibole Ap GC area of reaction products As GC area of limonene At GC area at reaction time B Brønsted acid sites

BET

Brunauer–Emmett–Teller

BJH

Barrett–Joyner–Halenda C α-quartz

CLIN

Clinoptilolite dp Pore diameter

DRIFTS

Diffuse reflectance infrared Fourier transform Spectroscopy

FID

Flame ionization detector FTIR-Py Fourier transform infrared spectros­ copy of adsorbed pyridine GC-MS Gas chromatography–mass spectrometry H Hematite

IUPAC

International union of pure and applied chemistry L Lewis acid sites LS Sludge from a water treatment plant LS600 Thermally treated sludge LS600-HCl Sludge treated thermally and with HCl LS600-HCl-Fe7% Sludge treated thermally, with HCl and Fe LS600-HCl-R Sludge treated thermally and with reused HCl M Magnetite Mh Maghemite

NH3-TPD

Ammonia temperature-programmed desorption

SBL

Sludge‑based support of the metallic active phase

SEM

Scanning electron microscopy Sj Selectivity for the product of interest

TCD

Thermal Conductivity Detector

TGA

Thermogravimetric analysis Vp Total pore volume X Conversion of substrate

XRD

X-ray diffraction

XRF

X-ray fluorescence Graphical Abstractinline-fig-Figa Keywords  Heterogeneous catalysts · α-pinene · Camphene · Limonene · p-cymene

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192

Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts

1  Introduction

With continuous population growth, the amount of waste generated during water purification processes also increases [1]. In the coagulation-flocculation step, coagulants such as aluminum sulfate or ferric salts destabilize colloidal mat­ ter and generate suspended solids that are subsequently removed by sedimentation and filtration, producing drink­ ing water treatment sludge [2, 3]. This sludge contains large amounts of water as well as organic and inorganic compo­ nents that pose environmental and public-health concerns. Current disposal routes (landfilling, discharge, agricultural application, incineration) have significant operational costs and environmental impacts [4, 5]. A promising alternative valorization route is to use the inorganic fraction of the sludge, which is rich in Al, Si, Ti, and Fe, as a precursor for heterogeneous catalysts applicable to a variety of reactions, including the transformation of terpenes into higher valueadded products [6–8].

α-Pinene and limonene, major components of turpentine and citrus essential oils, respectively, can be isomerized and/or dehydrogenated over acid and bifunctional catalysts to yield camphene (used as a precursor for resins, flavors, fragrances, pharmaceuticals, and fuels) and p-cymene (used as an intermediate in the synthesis of fine chemicals, tere­ phthalic acid, thymol, carvacrol, and as a ligand in metath­ esis catalysts) [8, 16]. Both reactions proceed through carbocationic intermediates that form upon protonation of the C = C bond by Brønsted acid sites and rearrange to the target products. For limonene to p-cymene, an additional dehydrogenation step is required, which is typically pro­ moted by metal or metal-oxide sites.

A wide range of heterogeneous catalysts has been reported for these transformations. For α-pinene isom­ erization to camphene, active systems include acid-modi­ fied natural zeolites (clinoptilolite, mordenite, ferrierite), TiO₂-based materials, mesoporous Ti-SBA-15, molybde­ num-titanium mixed oxides, biomass-derived activated carbons, montmorillonites, and sulfonated carbon acids [6–15]. Reported camphene selectivities typically fall in the 27–69% range at substrate conversions of 40–100%, under temperatures between 70 and 180 °C. A quantitative com­ parison of the most relevant systems is given in Table 1. For limonene to p-cymene, effective catalysts include acidactivated mordenite and montmorillonite, supported noble metals (Pd, Pt, Ni) on alumina, sepiolite modified with Fe or Mn under microwave irradiation, mixed silica–alumina oxides, Ti-MCM-41, supported heteropolyacids, and CdO/ SiO₂ systems [16–23]. Reported p-cymene selectivities span 45–100% at 140–250 °C, as summarized in Table 2. In gen­ eral, high selectivities are achieved either under severe con­ ditions (T ≥ 200 °C, gas phase) or with expensive synthetic supports (mesoporous silicates, noble metals on alumina).

Against this background, there is a clear opportunity to investigate low-cost, waste-derived precursors as alterna­ tives to conventional catalysts. In this work, drinking water treatment sludge from a purification plant located in west­ ern Medellín (Colombia) was characterized and subjected Table 1  Literature reports on the synthesis of camphene from α-pinene Catalyst (g) α-Pinene (mmol) Solvent (mL) T (°C) t (h) Conversion

(%)

Camphene selectivity (%) References

CLIN 0.1 (2.00)

140.0

NR

70

0.06

100

50

[8] 0.7Fe-CLa (0.30)

13.0

NR

155

8

94

66

[6] H-Z1 (0.25)

31.0

NR

75

3

40

51

[9] TiO2 (0.60)

629.0

NR

155

0.67

100

69

[7] Ti-SBA-15 (0.60)

29.0

NR

180

7

100

27

[10] NT-TiO2 (0.05)

0.25

Cyclohexane (3)

90

1

21

50

[11] AcMo (0.05)

0.25

Cyclohexane (3)

90

1

NR

48

MoO3 (0.05)

0.25

Cyclohexane (3)

90

1

NR

29

AcMo/P-25 (0.05)

0.25

Cyclohexane (3)

90

1

21

41

TiO2-acetic acid (0.8)

628.0

NR

155

1.75

100

68

[12] AC_SH_H3PO4 (0.15)

18.0

NR

160

3

50

57

[13] AC_OP_H3PO4 (0.15)

18.0

NR

160

3

47

53

AC_SCG_H3PO4 (0.15)

18.0

NR

160

3

13

55

Al (0.5)

0.59

NR

140

6

35

52

[14] AC400−3-P1.5-SO3H180−8 (0.05)

22.0

NR

110

10

99

51

[15] Nitrogen atmosphere NR not reported, CLIN 0.1 Clinoptilolite treated with 0.1 M H2SO4, 0.7Fe−CL Clinoptilolite treated with 0.7 ppm iron, H−Z1 de−aluminated ferrierite−type zeolite, Ti−SBA−15 SBA−15 material treated with titanium, NT−TiO2 titanium nanotubes, AcMo molybdic acid, AcMo/P−25 titanium−based molybdic acid, AC_SH activated carbon from sunflower husks, AC_OP activated carbon from orange peels, AC_SCG activated carbon from ground coffee, Al 0.5 30% montmorillonite, 0.5% of Al, AC400−3−P1.5−SO3H180−8 0.05 carbon−based acid by sulfonating acti­ vated carbon derived from eucalyptus wood waste

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Page 4 of 21 M. M. Giraldo, A. L. Villa to thermal and chemical treatments (calcination, HCl treat­ ment, and Fe impregnation), and the resulting materials were evaluated as catalysts for the isomerization of α-pinene to camphene and the isomerization/dehydrogenation of limo­ nene to p-cymene. The novelty of the study lies in using an industrial waste stream rather than a synthetic precursor as a source of active catalytic materials, and in showing that modest but meaningful selectivities to the target products can be obtained under mild conditions.

2  Experimental Section

2.1  Materials

Sludge supplied by Empresas Públicas de Medellín (EPM) was used, obtained from the tailings of a water purifica­ tion plant (LS) located in the west of the city of Medellín, Antioquia [2]. α-Pinene (98%, Sigma-Aldrich 98%), cyclo­ hexane (99.5%, Mallinckrodt Chemical), (R)(+) limonene (97% Sigma-Aldrich), hydrochloric acid (37%, Merck), and iron (III) nitrate nonahydrate (Fe(NO3)3.9H2O, Merck) were used without further purification treatments.

2.2  Catalyst Synthesis

2.2.1  Inorganic Fraction of LS

The LS samples were dried at 120 °C for 12 h and then calcined at 600 °C for 5 h at a heating rate of 10 °C/min to obtain the inorganic fraction of the sludge. This material was then sieved using a 200-mesh sieve and designated as LS600.

2.2.2  Treatment of LS600 with HCl

LS600 was suspended in a 2 M HCl solution at a solid-toliquid ratio of 1:10 (solid (g): HCl solution (mL)) and stirred at 300 rpm at room temperature for 30 min. The mixture was then placed in an autoclave at 100 °C for 4 h; after the hydrothermal treatment, the suspension was vacuum‑fil­ tered and washed until reaching a pH of ~ 7. Finally, the solid was dried in an oven at 120 °C for 3 h and calcined at

400 °C for 2 h at a heating rate of 5 °C/min. The material

was then sieved using a 200‑mesh sieve, and the resulting solid was designated LS600‑HCl.

The acid treatment conditions (2 M HCl, 100 °C, 4 h, solid/liquid ratio 1:10) were selected based on a prelimi­ nary screening in which LS600 was also treated with 2 M H₂SO₄ under comparable hydrothermal conditions (120 °C,

3 h, same washing and calcination protocol). LS600-HCl

yielded higher α-pinene conversion (see Table S1) and a Table 2  Literature reports on the synthesis of p-cymene from limonene Catalyst (g) Limonene (mmol) Solvent (mL) T (°C) t (h) Conversion

(%)

p-Cymene selectivity (%) Reference Technosa-S2 (1.00)

113

Tetraethylene glycol dimethyl ether (15)

140

7.0

NR

60

[17] Pd/Al2O3 NR Ethanol, 2-propanol*

350

0.01

100

45

[18] Sepiolita-Fe** (0.50)

31

NR

110

0.3

100

88

[19] Sepiolita-Mn** (0.50)

31

NR

110

0.3

100

77

M2-9 h (1.00)

113

Tetraethylene glycol dimethyl ether (15)

140

9.0

100

75

[20]

SIRAL 40**

(0.50)

31

NR

165

0.3

100

100

[16] Ti-MCM-41

(0.75)

37

NR

170

24.0

96

46

[21] 20%CdO/SiO2*** (0.20) NR NR

250

4.0

100

100

[23] NR not reported *Supercritical alcohols Technosa−S2: natural mordenite treated with H2SO4. M2–9 h: natural montmorillonite treated with HCl and with a reaction time of 9 h. SIRAL 40: mixed oxide of alumina and silica with 40% silica. **Use of microwaves. ***Gas phase

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192

Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts higher concentration of strong Brønsted acid sites than LS600-H₂SO₄ (see Sect.  3.1). The hydrothermal step at

100 °C for 4 h was adopted to promote deeper dealumina­

tion of the tetrahedral Al sites in the anorthite framework and to induce structural reorganization of the silicate phase, generating mesoporosity.

2.2.3  Impregnation of LS600-HCl with Fe

LS600‑HCl (1 g) was added to 10 mL of an aqueous suspen­ sion containing 0.507 g of Fe(NO₃)₃·9 H₂O. After stirring the suspension for 1 h at room temperature, it was dried at

100 °C for 5 h and then calcined at 500 °C for 2 h at a heat­

ing rate of 3 °C/min. The resulting solid was designated as LS600‑HCl‑Fe7%.

The 7 wt% Fe loading was selected based on a prelimi­ nary screening of LS600-HCl-based catalysts impregnated with Fe, Ni, and Zn at 3 and 7 wt%, evaluated under iden­ tical conditions in the limonene transformation reaction (3

mL limonene, 300  mg catalyst, 165  °C, 24  h, 750  rpm).

Among the six screened metal-loaded catalysts, LS600- HCl-Fe7% gave the highest selectivity to p-cymene (42%) at complete limonene conversion. The screening results are summarized in Table S2. Blank tests with commercial hematite (Fe₂O₃) and with a silica-supported iron catalyst (Silica-HCl-Fe20%) showed no conversion under these conditions, confirming that the combination of the sludgederived acid sites, mesoporous structure, and Fe³⁺ species rather than iron oxide alone is responsible for the catalytic activity of LS600-HCl-Fe7%.

2.3  Characterization of Catalysts

The materials were characterized using TGA, XRF, XRD, and SEM-EDS. Textural properties were evaluated using nitrogen adsorption isotherms, and acidity was analyzed by FTIR-Pyridine and TPD-NH₃. Additionally, the Fe phases present in the Fe-impregnated solid were identified by Möss­ bauer spectroscopy. Thermogravimetric analysis (TGA) of LS was performed on a TGA Q500 V20.13 Build 39 instru­ ment, heating the sample (30.97 mg) from room temperature to 900 °C at a rate of 10 °C/min under a nitrogen flow of 40 mL/min. The chemical composition of LS600, LS600-HCl, and LS600-HCl-Fe7% was determined by X-ray fluores­ cence (XRF) using a Thermo ARL Optim’X WDXRF spec­ trometer with Uniquant (semi-quantitative) software. X-ray diffraction (XRD) analyses to identify the crystalline phases present in LS600, LS600-HCl, and LS600-HCl-Fe7% were carried out using a Malvern-PANalytical Empyrean 2012 diffractometer equipped with a Pixel 3D detector and a Cu Kα radiation source (λ = 1.541874 Å) operated at 45 kV and

40 mA; data were collected with a step size of 0.05° and a

counting time of 50 s per step.

The morphology of the LS600, LS600‑HCl, and LS600‑HCl‑Fe7% samples was examined by scanning elec­ tron microscopy (SEM) using a DENTON VACUUM Desk IV sputter coater and a JSM‑6490LV microscope. The sam­ ples were coated with a thin gold layer, mounted on graph­ ite tape, and imaged at an accelerating voltage of 11.3 kV. Elemental analysis was carried out using an INCA PentaF­ ETx3 (Oxford Instruments) EDX microprobe. The porous properties of LS600‑HCl and LS600‑HCl‑Fe7% were deter­ mined by N₂ adsorption isotherms using a Micromeritics ASAP 2020 PLUS sorptometer. Prior to analysis, the sam­ ples were degassed at 350 °C for 4 h under high vacuum. Nitrogen adsorption–desorption isotherms were collected within a relative pressure range of 0.1–0.998 P/P₀. The spe­ cific surface area and pore characteristics were calculated using the BET (surface area), Horváth–Kawazoe, and Bar­ rett–Joyner–Halenda method (BJH) (pore volume and pore diameter) methods.

Ammonia Temperature-Programmed Desorption analy­ sis NH₃‑TPD was performed using a Micromeritics Auto­ Chem 2920 instrument. The LS600, LS600‑HCl, and LS600‑HCl‑Fe7% samples were treated under a helium flow (50 mL/min), heated to 350 °C at 10 °C/min, and held at this temperature for 30 min. After cooling to 50 °C, the samples were saturated with 0.3% NH₃/He (50 mL/min) for 90 min. The non‑chemisorbed ammonia was then removed by purg­ ing with helium (50 mL/min) at 50 °C for 45 min. Finally, desorption was carried out by heating the samples to 600 °C at 10 °C/min under a helium flow of 50 mL/min. The Ther­ mal Conductivity Detector (TCD) signal was previously calibrated using ammonia mixtures of known composition. IR analysis of pyridine adsorption was performed using a Frontier FTIR spectrometer (PerkinElmer, Spectrum 65) equipped with a high‑resolution MCT (Mercury Cadmium Telluride) detector and a diffuse reflectance cell (DRIFT). Spectra were collected at a resolution of 4  cm⁻¹ in the 1750–1350  cm⁻¹ region, averaging 40 scans. The sample was heated to 400 °C (10 °C·min⁻¹) under a helium flow, and background spectra were recorded every 10 °C. After cooling to 40 °C, pyridine adsorption was carried out for

30  min, followed by helium purging for an additional

30  min to remove physisorbed pyridine. The desorption

stage was then conducted by heating the sample to 400 °C at 10 °C·min⁻¹, collecting spectra every 10 °C. Finally, the difference between pyridine‑adsorbed spectra and the cor­ responding blanks was calculated, and the Kubelka–Munk transformation was applied to quantify the areas of the relevant bands. This analysis was performed for LS600, LS600‑HCl, and LS600‑HCl‑Fe7%.

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192

Page 6 of 21 M. M. Giraldo, A. L. Villa For the Mössbauer analysis of LS600‑HCl‑Fe7%, a spec­ trometer operating in constant‑acceleration mode was used, with a velocity range of ± 11 mm/s and a ⁵⁷Co source in a rhodium matrix with an initial activity of 25 mCi. Measure­ ments were carried out in transmission geometry at room temperature (25  °C). Obtained data were processed and adjusted with MOSF least squares fitting software.

2.4  Catalytic Evaluation

2.4.1  Isomerization of α-Pinene

In a typical reaction, the catalyst (25–50 mg) with a par­ ticle size smaller than 75 μm was added to 1 mL of a 0.25 mmol solution of the terpene in the selected solvent (tolu­ ene, cyclohexane, acetonitrile, ethyl acetate, or ethanol) in

2 mL vials. The mixture was magnetically stirred for the

selected reaction time (2–5 h) and temperature (70–90 °C. Reaction products were analyzed using an Agilent 7890 A GC–MS system equipped with an FID detector and an Agi­ lent 5975 C mass spectrometer, employing He as the car­ rier gas and an HP‑5MS capillary column (30 m × 0.25 mm × 0.25 μm). The carrier‑gas linear velocity and flow rate were 30.5 cm s− 1 and 2.214 mL min− 1, respectively. The oven temperature was maintained at 70 °C for 1 min, then increased to 90 °C at 10 °C·min− 1 and held for 0.5 min.

It was subsequently raised to 110  °C at 10  °C·min− 1 for

0.5 min, then to 130 °C at the same rate for 0.5 min, and

finally to 160 °C at 15 °C·min− 1 for 1 min. The injection volume was 1 µL in split mode (25:1), with an injector temperature of 250 °C. Compounds were identified using the NIST 5 database. Substrate conversion and selectivity toward the product of interest were calculated using equa­ tions Eq. 1 and Eq. 2, respectively.

d i sp lay-eq-Eq u 1 

(1)

di s play -e q-Equ2 

(2)

Where X is the conversion of substrate, inline-eq-IEq1 is the selectivity for the product of interest, inline-eq-IEq2 is the initial GC area, inline-eq-IEq3 is the GC area at reaction time t.

2.4.2  Isomerization and Dehydrogenation of Limonene

The isomerization and dehydrogenation of limonene were carried out in a 30 mL round‑bottom glass flask equipped with a reflux condenser and a magnetic stirrer. Limonene (3 mL) and the catalyst (200–400 mg, particle size < 75 μm) were added to the reactor and stirred at 750 rpm on a hot plate at 130–165  °C. Reaction products were analyzed using an Agilent 7890 A GC–MS system equipped with an FID detector and an Agilent 5975 C mass spectrometer, employing He as the carrier gas and an HP‑5MS capil­ lary column (30 m × 0.25 mm × 0.25 μm). The carrier‑gas linear velocity and flow rate were 30.5 cm·s− 1 and 2.214 mL·min− 1, respectively. The oven temperature was main­ tained at 70 °C for 1 min, then increased to 90 °C at 10 °C· min− 1 for 0.5 min, followed by increases to 110 °C (10 °C· min− 1, 0.5 min), 130 °C (10 °C·min− 1, 0.5 min), and 160 °C (15 °C· min− 1, 1 min). Finally, the temperature was raised to 190 °C at 15 °C min− 1 and held for 6.5 min. The injec­ tion volume was 1 µL in split mode (25:1), with an injector temperature of 250 °C. Compounds were identified using the NIST 5 database.

Substrate conversion and selectivity to the product of interest were calculated using equations Eq. 3 and Eq. 4, respectively.

d i s playe q-Equ3 

(3)

di s pl a ye q-Equ4

(4)

Where inline-eq-IEq4 is the conversion of substrate, inline-eq-IEq5 is the selectiv­ ity for the product of interest, inline-eq-IEq6 is the GC area of reaction products and inline-eq-IEq7 is the GC area of limonene.

2.5  Leaching Tests

To evaluate the heterogeneity of the LS600‑HCl and LS600‑HCl‑Fe7% catalysts in the isomerization of α‑pinene and the dehydrogenation of limonene, respectively, the solid catalyst was separated after 10 min of reaction (50 mg of catalyst, 750 rpm, 90 °C) or after 1 h of reaction (200 mg of catalyst, 3 mL of limonene, 750 rpm, 165 °C) using an Advantec syringe filter with a pore size of 0.45 μm. An ali­ quot of the reaction mixture was analyzed by GC–MS. The reaction was then continued using the filtrate, and additional samples were analyzed by GC–MS after 2, 4, 6, and 8 h for the α‑pinene isomerization test, or after 8 and 19 h in the case of the LS600‑HCl‑Fe7% catalyst.

2.6  Reuse Tests

2.6.1  Isomerization of α-Pinene

Reuse tests were performed on the LS600‑HCl catalyst to evaluate its stability. The fresh catalyst was tested under reaction conditions (0.25 mmol α‑pinene, 1 mL cyclohex­ ane, 750 rpm, 90 °C, 50 mg catalyst), and after 5 h it was

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Page 7 of 21

192

Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts separated and washed twice with 1 mL of cyclohexane and

then with ethanol (10  min, 750  rpm, room temperature).

Between washes, the solid was recovered by centrifugation (3000 rpm, 10 min). After the final wash, the solid was dried at 80 °C for 12 h and subsequently tested again under the same reaction conditions. The reused solid was designated LS600‑HCl‑R.

2.6.2  Isomerization and Dehydrogenation of Limonene

To evaluate the stability of the LS600‑HCl‑Fe7% cata­ lyst, reuse tests were performed. Initially, the fresh catalyst was tested under reaction conditions (3 mL of limonene, 750 rpm, 165 °C, 8 h, 200 mg catalyst). After the reaction, the solid was washed twice with 1 mL of ethanol and stirred for 10 min at 750 rpm and room temperature, then recovered by centrifugation (3000 rpm, 10 min). After the final wash, the solid was dried at 80 °C for 12 h and subsequently used in four additional reaction cycles.

3  Results and Discussion

3.1  Physicochemical Properties of the Catalysts

Figure  1 TGA thermogram of LS material. The figure shows the mass loss curves and the derivative of mass loss of the sample as a function of temperature. An initial decrease in weight is observed, mainly associated with the removal of moisture at low temperatures. Subsequently, additional stages of mass loss occur, related to the decompo­ sition of organic compounds present in the sample, includ­ ing low- and high-molecular-weight organic molecules. Thermal degradation events are identified by peaks in the DTG curve, indicating different stages of material decom­ position as the temperature increases.

The chemical composition of LS600, LS600‑HCl, and LS600‑HCl‑Fe7% determined by XRF analysis is pre­ sented in Table S3. Al₂O₃ (33.87% w/w) and SiO₂ (33.32% w/w) are the components present in the highest concentra­ tions in LS600. The elemental percentages of Fe₂O₃, CaO, MgO, Na₂O, TiO₂, K₂O, P₂O₅, and MnO are 7.71% w/w, 1.73% w/w, 1.44% w/w, 1.17% w/w, 1.02% w/w, 0.651% w/w, 0.558% w/w, and 0.161% w/w, respectively. After acid treatment of the LS600 sample, a significant decrease in Al content (down to 8.1% w/w) and an increase in Si con­ centration in LS600‑HCl and LS600‑HCl‑Fe7% (68.4 and 57.24% w/w, respectively) were observed. As expected, the iron concentration in LS600‑HCl‑Fe7% (20.31% w/w) was higher than in the untreated LS600 material. Through Rietveld refinement of the diffractograms, a semi‑quantitative analysis of the phases present in the LS600, LS600‑HCl, and LS600‑HCl‑Fe7% samples were obtained. The results of this analysis are presented in Table 3.

The X-ray diffraction patterns of LS600, LS600-HCl, and LS600-HCl-Fe7% are shown in Fig. 2, where every dif­ fraction peak has been labeled with the corresponding phase code (An = amphibole, A = anorthite, Al = albite, H = hema­ tite, C = α-quartz, M = magnetite, Mh = maghemite; full phase names and ICSD codes are given in Table 3). Rietveld Table 3  Semi-quantitative Rietveld refinement results for the LS600, LS600-HCl, and LS600-HCl-Fe7% materials Compound name Code % w/w LS600 LS600-HCl LS600-HCl-Fe7% Amphibole (Na-Mg)

ICSD

98-016-4666

34

30

30

Anortithe

ICSD

98-000-9330

40

20

19

Albite (heat treated)

ICSD

98-008-7659

11

12

12

Quartz α

ICSD

98-001-6332

7

26

19

Hematite

ICSD

98-001-5840

8

12

12

Magnetite

ICSD

98-008-5806

0

0

6

Maghemite

ICSD

98-004-4517

0

0

2

Semi−quantitative Rietveld refinement Quality−of−fit parameters: R_expected = 2.43, R_profile = 3.00, R_ wp = 3.94, GOF = 2.63 Typical absolute uncertainty for phases present below 5 wt% is ±1–2 wt% Fig. 1  shows the thermogravimetric analysis of the LS sample, which presents a weight loss of 32.55% attributed to water between 50 °C and 100 °C and between 210 °C and 260 °C. Low‑molecular‑weight organic molecules (0.78%), followed by the decomposition of higher‑molecular‑weight organic molecules (3.39%), decompose between 400 °C and 600 °C [1]. The total weight loss of the sample was 57.90%, occurring at temperatures below 600 °C

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Page 8 of 21 M. M. Giraldo, A. L. Villa refinement of the diffractograms provided a semi-quantita­ tive estimate of the phase composition of the three materials (Table 3). The quality-of-fit parameters (R_expected = 2.43; R_profile = 3.00; R_wp = 3.94; GOF = 2.63) meet the accepted criteria for reliable Rietveld analysis (R_wp < 10, GOF < 4). As this analysis is semi-quantitative, the typical absolute uncertainty for phases present at < 5 wt% is ± 1–2 wt% (Figs. 3, 4 and 5).

The main compositional changes introduced by the acid and Fe treatments are: (i) the anorthite content decreases from 40 wt% in LS600 to ~ 20 wt% in both LS600-HCl and LS600-HCl-Fe7%, consistent with the removal of tetrahedral Al from the anorthite framework during the HCl-hydrothermal treatment; (ii) this is accompanied by a relative enrichment of the other silicate phases, most nota­ bly α-quartz (from 7 wt% to 26 wt% in LS600-HCl); (iii) the impregnation of LS600-HCl with Fe generates two new iron mineralogical phases magnetite (~ 6 wt%) and maghemite (~ 2 wt%) in LS600-HCl-Fe7%. The charac­ teristic reflections of magnetite and maghemite (2θ ≈ 30.1° and 35.5°) overlap with the stronger reflections of anorthite (30.48°) and albite (35.38°) in the same region, which lim­ its their direct visual identification in the XRD pattern of LS600-HCl-Fe7%. Their presence is nonetheless unam­ biguously confirmed by Mössbauer spectroscopy (Fig. 6), which resolves the characteristic sextets of magnetite and maghemite at the concentrations present here. The acid strength of the materials (LS600, LS600‑HCl, and LS600‑HCl‑Fe7%) was evaluated by NH₃‑TPD. Two types of acid strength were observed for LS600 and LS600‑HCl‑Fe7%, whereas three types were identified for LS600‑HCl. Acidity strength is classified according to the NH₃ desorption temperature as weak acidity (100–250 °C), medium acidity (250–400 °C), and strong acidity (400–600 °C) [24]. Fig. S1 shows the experimental signals and the corresponding deconvolutions of the peaks for each mate­ rial. The appearance of a new signal is associated with the incorporation of additional H⁺ ions into the material struc­ ture. Moreover, treatment with chlorinated acids results in a slight reduction in the number of weak acid sites and the formation of strong acid sites. Additionally, the incorpora­ tion of Fe markedly increases the total acidity of the mate­ rial, mainly due to the contributions of weak and strong acid sites (Table 4).

The Lewis acid sites associated with the bands at 1596 cm⁻¹ and 1449 cm⁻¹, as well as the combined Brønsted– Lewis band at 1492 cm⁻¹, are identified in all solids. Addi­ tionally, the band assigned to Lewis acidity at 1578 cm⁻¹ appears in all samples except LS600. Finally, a band at 1542 cm⁻¹ is observed in LS600‑HCl and LS600‑HCl‑Fe7%, which is attributed to a Brønsted acid site (Fig. 3). These Brønsted sites may form following hydrochloric acid Fig. 3  FTIR‑Py spectra of the LS600, LS600‑HCl, and LS600‑HCl‑Fe7% materials at 50 °C. The FTIR spectra of adsorbed pyridine (FTIR-Py) from samples LS600, LS600-HCl, and LS600- HCl-Fe7% show characteristic bands associated with Brønsted and Lewis acid sites. The bands located at 1596, 1578, and 1449 cm⁻¹ are attributed to Lewis acid sites (L), while the signal at 1542 cm⁻¹ cor­ responds to Brønsted acid sites (B). The band observed at 1492 cm⁻¹ is associated with combined contributions from Brønsted and Lewis acidity (B + L). The dotted lines indicate the positions of the main bands used for the identification of acid sites

Fig. 2  Minerals identified by XRD in the LS600, LS600-HCl, and LS600-HCl-Fe7% samples. The X-ray diffraction patterns of LS600, LS600-HCl, and LS600-HCl-Fe7% samples show signals associated with anorthite (A) (2θ = 20.92° and 30.48°), α-quartz (C) (2θ = 64.96°), amphibole (An) (2θ = 10.54°, 19.75°, 20.92°, 23.00°, 24.25°, 28.66°, 30.48°, 34.60°, 39.56°, and 50.19°), albite (Al) (2θ = 22.07°, 23.60°, 25.50°, 26.69°, 27.99°, 35.38°, and 41.81°), hematite (H) (2θ = 24.25°, 33.15°, 35.54°, 50.10° and 54.15°), magnetite (M) (2θ = 30.11° and 35.54°) and maghemite (Mh) (2θ = 30.11° and 35.54°). The dotted lines indicate the characteristic positions of the main diffraction peaks identified in the samples

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Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts Fig. 6  Mössbauer spectrum at

25 °C for the LS600‑HCl‑Fe7%

sample. Mössbauer spectrum of sample LS600-HCl-Fe7%. The dots correspond to experimental data and the solid line represents the spectrum fit. The fit includes two sextets associated with mag­ netite phases (Fe³⁺ and Fe²·⁵⁺) and two doublets corresponding to paramagnetic Fe³⁺ species. The different contributions to the fit are shown with colored lines

Fig. 5  SEM micrographs of the LS600 (a), LS600‑HCl (b), and LS600‑HCl‑Fe7% (c) samples. Scanning electron microscopy (SEM) micrographs of samples a LS600, b LS600-HCl, and c LS600-HCl- Fe7%. The images show the surface morphology of the materials before and after acid treatment and Fe impregnation. Scale bar: 50 μm

Fig. 4  Adsorption-desorption isotherm of N2 for the LS600-HCl (a) and LS600-HCl-Fe7% (b) materials. The nitrogen adsorption–desorp­ tion isotherms of samples a LS600-HCl and b LS600-HCl-Fe7% are presented. The curves show the amount of N₂ adsorbed as a function of relative pressure (P/P₀) along the adsorption and desorption branches. The separation between the two branches gives rise to a hysteresis cycle characteristic of the adsorption–desorption process in porous materials

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Page 10 of 21 M. M. Giraldo, A. L. Villa treatment, as the removal of aluminum from the crystalline structure of anorthite generates hydroxyl groups that behave as Brønsted acid sites [25].

The analyzed samples primarily exhibited Lewis acidity, which can be attributed to the presence of aluminum and iron. These metals possess empty or partially filled valence orbitals, enabling them to accept electron pairs from donor molecules or ions. When incorporated into complexes or solid structures, these metals typically exist in positive oxi­ dation states. In these ionic states, Fe and Al exhibit high positive charge densities, which enhances their affinity for electrons donated by neighboring species (such as water molecules or ligands). This interaction polarizes surround­ ing bonds and facilitates electron acceptance, thereby inten­ sifying the Lewis acid character and increasing the overall acidity of the material [11].

The nitrogen adsorption–desorption isotherms of LS600-HCl and LS600-HCl-Fe7% exhibit Type IV behav­ ior according to IUPAC classification, with an adsorption branch at P/P₀ > 0.5 and a hysteresis loop characteristic of multilayer adsorption and capillary condensation in meso­ pores (Fig. 4). Table 5 summarizes the textural properties and iron content of the two materials. Both samples exhibit a predominance of mesopores, as confirmed by their pore-size distributions (Fig. S2a and Fig. S2b). After Fe impregnation, the BET surface area decreases from 103 m²/g (LS600-HCl) to 84 m²/g (LS600-HCl-Fe7%) and the total pore volume decreases from 0.21 to 0.13 cm³/g, consistent with partial pore occupation by iron species. The iron loading of LS600- HCl-Fe7% is 20 wt% (from XRF), which on a surface-area basis corresponds to 43 µmol_Fe/m². We note that this sur­ face loading does not directly establish a well-dispersed distribution of iron, and that iron dispersion was not quanti­ fied in this study. EDS mapping (Fig. S3) indicates that Fe is distributed across the analyzed regions together with Si and Al. The catalytic performance reported hereafter should therefore be interpreted as reflecting the combined effect of iron content, acid-site distribution, and mesoporous texture, rather than a well-defined Fe dispersion.

The micrographs reveal non-homogeneous surfaces; LS600 exhibits a coarse morphology, whereas LS600-HCl displays increased surface porosity and more prominent protuberances (Fig. 5). Despite the more irregular and unde­ fined texture observed in LS600-HCl-7%, no substantial morphological transformations were identified as a result of the acid treatment or Fe loading.

The Mössbauer spectrum of LS600-HCl-Fe7% was recorded in transmission geometry at room temperature and processed with the MOSF least-squares fitting software to identify the iron-containing phases and to quantify their rel­ ative abundance in the material (Fig. 6). The spectrum was fitted using two sextets and two doublets. The two sextets exhibit hyperfine parameters characteristic of magnetite: one corresponding to tetrahedral sites occupied by Fe3+ ions, and the other to octahedral sites occupied by Fe2.5+ interme­ diate valence ions. At room temperature, this intermediate valence arises from electron hopping between neighboring octahedral sites occupied by Fe3+ and Fe2+ ions. Because the Mössbauer transition occurs on a timescale (10− 8 to 10− 9 s) faster than the relaxation time of this hopping, an average oxidation state is observed. This oxidation state is further corroborated by the isomer shifts (δ) of both sextets. The Fe3+ sextet may also include a contribution from maghemite (γ-Fe2O3), a highly oxidized form of magnetite commonly found in samples exposed to oxygen-rich atmospheres. According to the fitting results, the magnetite-maghemite phases account for 36% of the sample, with 28% attributed to Fe3+ (magnetite/maghemite) and 8% to Fe2.5+ (magne­ tite). Doublets 1 and 2 correspond to paramagnetic phases containing Fe3+. These cannot be uniquely identified by Table 4  Distribution of acid strength in the LS600, LS600-HCl, and LS600-HCl-Fe7% materials Catalyst Weak acidity (100–250 °C) Medium acidity (250–400 °C) Strong acidity (400–600 °C) Total acidity mmolNH3 g−1 catalyst mmolNH3 g−1 catalyst

%

mmolNH3 g−1 catalyst

%

mmolNH3 g−1 catalyst

%

LS600

0.211

73.3

0.00

0

0.075

26.7

285.7

LS600-HCl

0.114

33.8

0.14

40.7

0.086

25.5

337.5

LS600-HCl-Fe7%

0.374

70.8

0.00

0

0.154

29.2

528.2

Table 5  Textural properties and active‑site concentrations of the LS600‑HCl and LS600‑HCl‑Fe7% materials Sample SBET (m2/g) SMesoporous (m2/g) Vp (cm3/g)a Vmesoporous (cm3/g) dp (nm)b Iron content (% w/w) µmolFe/m2 LS600-HCl

103

71

0.21

0.20

2.6

0

0

LS600-HCl-Fe7%

84

68

0.13

0.13

3.8

20

43

Vp total pore volume, dp pore diameter a Calculated with the BJH method b Calculated with the Horvath−Kawazoe method

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Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts room-temperature Mössbauer spectroscopy alone, as their hyperfine parameters are consistent with both superpara­ magnetic goethite (α-FeOOH) and hematite (α-Fe2O3) [26]. However, based on the X-ray diffraction results (Fig. 2), these paramagnetic phases are identified as hematite. Col­ lectively, these phases represent 64% of the sample (12% for Doublet 1 and 52% for Doublet 2).

3.2  Catalytic Production of Camphene from

α-Pinene The influence of various reaction conditions on the synthe­ sis of camphene from α-pinene was evaluated, and catalyst stability was investigated through leaching and reuse tests. Additionally, a statistical analysis of substrate conversion and selectivity toward camphene is presented, alongside a proposed reaction scheme.

3.2.1  Effect of the Solvent

Table 6 summarizes the influence of the solvent on the cata­ lytic isomerization of α‑pinene over LS600‑HCl. No reaction occurred in the presence of polar solvents such as acetoni­ trile, ethyl acetate, or ethanol. Among the nonpolar media evaluated, cyclohexane provided the highest α‑pinene con­ version and the greatest selectivity toward camphene. The superior performance of cyclohexane compared to toluene may be related to differences in how these solvents stabilize the carbocationic intermediate formed upon coordination of the monoterpene’s C = C bond with the acid sites. The aromatic structure of toluene could modify the stabilization environment or compete more strongly for adsorption sites, affecting the strength and nature of the adsorption relative to cyclohexane. Such variations in adsorption behavior may ultimately influence the observed selectivity toward cam­ phene [11].

3.2.2  Effect of Temperature

At 80 °C, the conversion reached 12%, while at 90 °C it increased to 30%. The selectivity toward camphene remained essentially constant at approximately 38% across the evaluated temperature range. This behavior is consistent with the expected increase of the reaction rate constant with temperature (Fig. 7a). Previous studies support this trend; for example, it was reported [10] that varying the tempera­ ture from 20 °C to 200 °C (7 h, 10% Ti‑SBA‑15 relative to α-pinene) led to an increase in α-pinene conversion up to a maximum of 60% at 180 °C, followed by a slight decrease to 55% at 200 °C, attributed to temperatures approaching the boiling point of the reaction mixture. Similarly, using a CLIN 0.1 catalyst (1 h, 7.5 wt% catalyst), α-pinene con­ version increased from 89% to 99% when the temperature was increased from 70 °C to 80 °C. In this case, selectiv­ ity toward camphene (53–55%) and limonene (29–31%) remained nearly unchanged within the 30–80 °C range [8]. Overall, the results demonstrate that temperature sig­ nificantly influences α-pinene conversion, but exerts only a minor effect on product selectivity under the conditions investigated.

3.2.3  Effect of Reaction Time

As the reaction progressed, the selectivity toward limonene increased to 35%, together with the formation of minor isomerization products such as terpinolene (11%) (Fig. 7b). These products are consistent with the predominance of Lewis acid sites in the studied catalyst, which favor skel­ etal rearrangements of monoterpenes. Miądlicki et al. [8] reported a markedly faster reaction using the CLIN 0.1 catalyst, where α-pinene was fully converted within 210 s under their conditions. In their study, product selectivity varied significantly during the first 30–270 s of reaction, but remained unchanged at longer times.

3.2.4  Effect of Catalyst Amount

Increasing the amount of catalyst enhances the availabil­ ity of Brønsted and Lewis acid sites, which in turn pro­ motes α-pinene conversion. (Fig. 7c). Previous studies reported similar behavior; for example, it is reported [10] that α-pinene conversion reached a maximum of 95% at a Ti‑SBA‑15 loading of 15 wt%; further increasing the cata­ lyst content to 20 wt% did not improve conversion but pro­ moted the formation of polymeric by‑products. Likewise, when the concentration of the CLIN 0.1 catalyst was varied Table 6  Effect of the solvent on the catalytic isomerization of α‑pinene over the LS600‑HCl catalyst Solvent Polarity % X % Selectivity Camphene β-Pinene α-Felandrene p-Cymene Limonene 3-Carene Terpinolene Others Acetonitrile Polar

0

0

0

0

0

0

0

0

0

Ethyl acetate Polar

0

0

0

0

0

0

0

0

0

Ethanol Polar

0

0

0

0

0

0

0

0

0

Toluene Non polar

10

27

0

3

2

68

0

0

0

Cyclohexane Non polar

30

38

1

2

2

35

1

2

18

Reaction conditions: 0.25 mmol α−pinene (98%), 1 mL solvent, 750 rpm, 90 °C, 2 h, 25 mg LS600−HCl

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Page 12 of 21 M. M. Giraldo, A. L. Villa between 2.5 and 12.5 wt% at 70 °C, complete conversion was achieved with 10 wt% catalyst [8]. In that study, cam­ phene selectivity remained unchanged for catalyst loadings above 7.5 wt%, although increased isomerization of limo­ nene to α‑ and γ‑terpinene, terpinolene, and p‑cymene was observed.

Studies on the effects of temperature and catalyst load­ ing indicate an interplay between these variables: higher temperatures reduce the amount of catalyst required to achieve high α-pinene conversions, whereas increasing the catalyst loading allows comparable conversions to be reached at lower temperatures. Under all evaluated condi­ tions, camphene selectivity remained essentially constant at approximately 38%, likely due to the combined action of Brønsted and Lewis acid sites, which are both necessary for the rearrangement of α-pinene [11]. Because LS600‑HCl contains a relatively low concentration of Brønsted acid sites, these are rapidly saturated by the camphene‑forming pathway, while the remaining Lewis acid sites promote competing isomerization reactions. Similar behavior has been reported for Ti‑SBA‑15, where camphene selectivity remained nearly constant at around 27% despite variations in reaction parameters [10].

Fig. 7  Main effects on the conversion of α-pinene and the selectivity of isomerization products. Conversion of α-pinene and selectivity toward the main reaction products on the LS600-HCl catalyst as a function of a temperature, b reaction time, and c catalyst amount. The bars corre­ spond to the conversion of α-pinene and selectivity towards camphene, β-pinene, α-phellandrene, p-cymene, limonene, 3-carene, terpinolene, and other products. Reaction conditions: 0.25 mmol α-pinene, 1 mL cyclohexane, 90 °C (b and c), 25 mg catalyst (a and b), 5 h (a and c), and 750 rpm

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Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts

3.2.5  Statistical Analysis of the Effect of Reaction

Conditions The main-effects plot (Fig.  8) provides an exploratory, screening-level assessment of the relative influence of the operational variables on α-pinene conversion over LS600- HCl within the tested range. Within this range, increasing the catalyst amount leads to the most pronounced rise in conversion, consistent with the greater availability of acid sites; the effect of temperature is moderate; and the effect of reaction time is relatively weak. Camphene selectivity remained essentially constant at ~ 38% across all tested con­ ditions. We emphasize that a two-level factorial design cap­ tures trends but does not permit the identification of optima or non-linear responses. A detailed response-surface study over a wider range of catalyst loadings and temperatures is recommended for process optimization, and would also help to identify the onset of mass-transport limitations expected at higher catalyst-to-substrate ratios.

3.2.6  Leaching and Reuse Tests

The conversion remained essentially unchanged at 3.7% after the catalyst was removed from the reaction mixture at

10 min, indicating that no active species were leached into

the liquid phase. In contrast, when the catalyst remained in the system, the conversion reached 85% after 5 h of reaction (Fig. 9a). These results confirm that the catalytic activity arises exclusively from the solid phase and that LS600‑HCl operates as a truly heterogeneous catalyst. As shown in Fig.  9b, α-pinene conversion decreased progressively with reuse cycles, from 83% over the fresh catalyst to 28% after the fourth cycle, whereas camphene selectivity remained essentially constant at around 38% throughout the stability test. This pattern loss of activity without a change in the product distribution suggests that deactivation does not involve a chemical modification of the acid sites of the catalyst, but rather a reduction in the num­ ber of sites accessible to the substrate.

The FTIR-Py spectra of fresh LS600-HCl and reused LS600-HCl-R (recovered after the fourth cycle), recorded

at 50  °C, are shown in Fig.  10. Both spectra display the

characteristic bands of pyridine coordinated to Lewis sites (1449 and 1596  cm⁻¹), pyridine protonated on Brønsted sites (1542 cm⁻¹), and the combined band associated with both site types (1492 cm⁻¹). A rigorous quantitative compar­ ison of these spectra is not appropriate in this case, since the absolute band intensities depend on experimental variables Fig. 9  Leaching test for LS600-HCl (a). Reuse test for LS600-HCl (b). a Leaching test to evaluate the heterogeneous nature of the LS600- HCl catalyst. The catalyst was removed from the reaction mixture after

10 min and the reaction was allowed to continue under the same condi­

tions. b Reuse of the LS600-HCl catalyst in the conversion of α-pinene for four consecutive cycles. Reaction conditions: 0.25 mmol α-pinene,

1 mL cyclohexane, 90 °C, 750 rpm, and 50 mg catalyst

Fig. 8  Graph of main effects for the conversion-isomerization of α-pinene. Prepared using Minitab® software. Graph of main effects for the conversion of α-pinene. The graph shows the effect of reaction variables on average conversion: A amount of catalyst in mg, B tem­ perature in °C, and C reaction time in h

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Page 14 of 21 M. M. Giraldo, A. L. Villa that were not strictly controlled during acquisition, in par­ ticular the ambient humidity and the amount of solid loaded into the DRIFT cell, which was not identical for both sam­ ples. The comparison between the two samples is therefore restricted to a qualitative interpretation. From this qualitative perspective, the relevant observa­ tion is that the full set of bands associated with adsorbed pyr­ idine Brønsted at 1542 cm⁻¹, Lewis at 1449 and 1596 cm⁻¹, and the combined band at 1492  cm⁻¹ remains present in the spectrum of the reused catalyst. This indicates that the chemical nature of the acid sites is preserved after the reac­ tion cycles, and that the decrease in conversion cannot be attributed to a structural loss or to an irreversible chemical transformation of the Brønsted or Lewis sites of the catalyst. A more consistent explanation, in agreement with the full set of experimental observations, is the gradual blocking of the active sites by heavy organic species formed during the reaction and retained on the surface and within the meso­ pores of the catalyst. This interpretation is supported by sev­ eral converging observations. First, the material exhibits a moderate surface area (103 m²/g) dominated by mesopores with an average diameter of 2.6 nm (Table 5), a size range that is particularly susceptible to blocking by oligomeric and polymeric species derived from terpenes. Second, α-pinene isomerization proceeds through pinyl and bornyl carboca­ tion intermediates, which are well-known precursors of par­ allel oligomerization reactions in the presence of acid sites [8, 17]. Third, the chromatograms of the reaction mixture systematically show heavy products grouped as “others” in the selectivity distributions (Table 6), consistent with the formation of oligomeric species that may remain adsorbed in the catalyst pores between reaction cycles. Finally, this physical-blocking interpretation is fully consistent with the preservation of camphene selectivity across the stability test, since an approximately uniform deposition of organic species on the surface reduces the number of accessible sites without altering the relative proportion of Brønsted and Lewis sites or modifying the reaction pathways that govern the product distribution.

3.2.7  Proposed Reaction Scheme

A reaction scheme is proposed in Fig. 11 based on catalyst characterization, catalytic performance, literature reports [6, 8, 10], and the reaction products identified in this study (Fig. 7c, Fig. S4). α‑Pinene is a bicyclic monoterpene whose endocyclic double bond and strained bicyclo[3.1.1] heptane framework make it highly susceptible to acid‑cat­ alyzed isomerization (Fig. 11). The scheme begins with the interaction of α‑pinene with the Brønsted acid sites of the LS600‑HCl catalyst, promoting the opening of the cyclo­ propane ring and generating a highly unstable pinyl‑type carbocation. This intermediate can rearrange to form a pinylcarbonium ion [8], which subsequently reorganizes to produce a bornylcarbonium ion; the latter undergoes depro­ tonation to yield camphene [6].

From the same carbocationic intermediate, alternative rearrangement pathways lead to the formation of other observed products. The formation of 3‑carene proceeds through a skeletal rearrangement in which the cyclic frame­ work is reorganized and reclosed to produce the charac­ teristic bicyclic structure of this monoterpene. Similarly, deprotonation of the carbocation followed by double‑bond rearrangement leads to limonene, characterized by a six‑membered ring bearing one endocyclic and one exocy­ clic double bond. Further isomerization of limonene affords α‑phellandrene and terpinolene, while consecutive dehydro­ genation (loss of 2 H⁺) results in the formation of p‑cymene [8, 10].

3.3  Catalytic Production of p-Cymene from

Limonene The influence of different reaction conditions on the synthe­ sis of p‑cymene from limonene was systematically evalu­ ated, and the stability of the catalyst was assessed through leaching and reuse tests. In addition, a statistical analysis of substrate conversion and p‑cymene selectivity was per­ formed to identify the most influential operational variables.

Fig. 10  FTIR-Py spectra LS600-HCl-R and LS600-HCl at 50  °C.

FTIR spectra of adsorbed pyridine (FTIR-Py) from fresh and reused LS600-HCl samples recorded at 50 °C. The bands located at 1596, 1578, and 1449 cm⁻¹ are attributed to Lewis acid (L) sites, while the band at 1542 cm⁻¹ corresponds to Brønsted acid (B) sites. The signal observed at 1492 cm⁻¹ is associated with combined contributions from Brønsted and Lewis acidity (B + L)

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Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts Finally, a reaction‑route scheme is proposed based on experimental observations, catalyst characterization, and literature reports.

3.3.1  Effect of the Reaction Temperature

The conversion of limonene remained constant at 100% at both tested temperatures (130 and 165 °C). However, p-cymene selectivity increased from 32% at 130 °C to 44% at 165 °C (Fig. 12a). This trend suggests that higher temperatures favor the disproportionation of p-ment-3-ene (4%) and enhance the dehydrogenation pathway leading to p-cymene, an endothermic process. Similar temperature dependent behavior has been reported in the literature. For example, Retajczyk [21] showed that after 24 h of reaction over a Ti-MCM-41 catalyst, limonene conversion increased from 92% to 96% and p-cymene selectivity rose from 7% to 46% when the temperature was increased from 160 °C to

170 °C. In another study [23], p-cymene selectivity reached

92% at 200 °C and 100% at 250 °C using a 20% CdO/SiO₂ catalyst.

3.3.2  Effect of Reaction Time

Figure 12b shows that increasing the reaction time from 8 to 24 h leads to an increase in conversion from 70% to 100%, while p-cymene selectivity rises from 20% to 30%. Lon­ ger reaction times also result in higher selectivities toward secondary products, namely 15% for p-ment-3-ene, 5% for cis-ment-8-ene, and 31% for polymeric species. This behav­ ior can be attributed to the occurrence of consecutive reac­ tions involving both limonene and its primary isomerization products. The observed increase in limonene conversion and in p-cymene selectivity is consistent with reports in the literature; for instance, using a Ti-MCM-41 catalyst, con­ version values up to 96% and p-cymene selectivity of 46% were obtained after extending the reaction time to 24 h [21].

3.3.3  Effect of the Amount of Catalyst

Figure 12c shows that limonene conversion remains con­ stant at 100% for both tested catalyst loadings (200 and

400 mg). In contrast, p-cymene selectivity decreases from

44% to 30% when the catalyst amount is increased from

200 mg to 400 mg. This decline in selectivity is likely asso­

ciated with the enhanced contribution of side reactions at Fig. 11  General scheme of the isomerization reaction of α-pinene with the LS600-HCl catalyst. Based on [8]. Proposed scheme for the acid isomerization of α-pinene on the LS600-HCl catalyst, show­ ing the main reaction pathways to camphene, 3-carene, limonene, α-phellandrene, terpinolene, and p-cymene

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Page 16 of 21 M. M. Giraldo, A. L. Villa higher catalyst loadings. Based on these results, the reaction conditions that maximize both limonene conversion and p-cymene selectivity are 24 h of reaction time, 165 °C, and 200 mg of catalyst.

The observation that increasing the catalyst loading from

200 to 400 mg does not improve p-cymene selectivity but

increases the contribution of side reactions (polymers, 31%; p-menth-3-ene, 15%) is consistent with the expected onset of diffusional limitations and enhanced secondary chemistry at high catalyst-to-substrate ratios in a liquid-phase batch system. This behavior also supports the choice of 200 mg as the reference loading for the subsequent studies.

3.3.4  Statistical Analysis of the Effect of Reaction

Conditions Figure 13a shows the main-effects plot for limonene con­ version, from which it can be concluded that temperature, Fig. 13  Graphs of main effects. Elaboration using Minitab® software. Main effect graphs for reaction conditions. a Effect of reaction vari­ ables on limonene conversion. b Effect of reaction variables on selec­ tivity towards p-cymene. The factors evaluated correspond to: (A) temperature (°C), (B) amount of catalyst (mg), and (C) reaction time (h)

Fig. 12  Main effects on limo­ nene conversion and selectivity of isomerization and dehydro­ genation products. Limonene conversion and selectivity toward the main reaction products on the LS600-HCl-Fe7% catalyst as a function of a reaction tem­ perature, b reaction time, and c catalyst amount. The bars repre­ sent limonene conversion and selectivity towards p-cymene, terpinolene, p-menth-3-ene, α-terpinene, cis-menth-8-ene, polymeric species, and other products. Reaction conditions:

3 mL limonene, 200 mg catalyst

(a and b), 165 °C (b and c),

750 rpm, and 24 h (a and c)

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Page 17 of 21

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Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts catalyst loading, and reaction time all exert a positive influ­ ence on conversion. Among these variables, reaction time has the strongest effect, whereas temperature exhibits only a minor impact within the evaluated range. Conversion increases with catalyst loading up to 400 mg, beyond which no further changes were detected.

From Fig. 13b, it can be concluded that all three factors have a significant impact on p‑cymene selectivity. Both temperature and reaction time exert a positive effect, with reaction time being the most influential variable. In contrast, catalyst loading displays a maximum in its effect on selec­ tivity, likely due to the promotion of competing reaction pathways at higher catalyst amounts.

3.3.5  Leaching and Reuse Tests

The conversion remained essentially unchanged at 10% after the catalyst was removed from the reaction mixture at

1 h, indicating that no active species were leached into the

liquid phase (Fig. 14a). In contrast, when the catalyst was left in the system, the conversion reached 100% after 24 h of reaction. These findings demonstrate that the catalytic activ­ ity originates exclusively from the solid phase and confirm that LS600-HCl-Fe7% operates as a truly heterogeneous catalyst.

Reuse tests of the LS600-HCl-Fe7% catalyst show that p-cymene selectivity remains nearly constant at approxi­ mately 24% across all cycles (Fig.  14b), indicating that the catalyst maintains good chemical stability with respect to product distribution. In contrast, limonene conversion remains stable at ~ 68% up to the second reuse, after which an 18% decrease is observed, dropping from 68% to 56% over the last two reaction cycles. Overall, these results demonstrate that the catalyst exhibits satisfactory stability in terms of both limonene conversion and selectivity toward p-cymene, despite some loss of conversion upon successive reuse.

3.3.6  Scheme and Proposed Reaction Route

Based on catalyst characterization, catalytic tests, literature reports [20, 27], and the reaction products identified in this work (Fig. 12c, Fig. S7), a general reaction scheme for the isomerization and dehydrogenation of limonene is proposed (Fig. 15). In most cases, p‑cymene formation from limo­ nene begins with the isomerization of limonene into other terpene intermediates, which occurs through migration of the exocyclic double bond into the ring (pathway I). These intermediates subsequently undergo dehydrogenation of the carbocyclic ring to yield p‑cymene. Additionally, dispro­ portionation of limonene (pathway II) and of terpene inter­ mediates such as terpinolene, α‑terpinene, p‑ment‑3‑ene, and cis‑ment‑8‑ene also contributes to p‑cymene forma­ tion through subsequent dehydrogenation. Large quantities of high‑molecular‑weight “polymeric” products are also formed (pathway III), arising from polymerization reactions of limonene and its intermediates. Isomerization, dispropor­ tionation, and polymerization occur on the acidic sites of the catalyst via carbenium‑ion mechanisms. In contrast, the dehydrogenation step taking place on metal sites is endo­ thermic, requiring relatively high temperatures to achieve acceptable yields of p‑cymene. Because these pathways are all promoted by acidic sites, competitive and simultaneous reactions proceed across the three routes [27, 28]. For the formation of p-cymene, a bifunctional cata­ lyst containing both acid and metal sites is required. The Fig. 14  Leaching test for LS600-HCl-Fe7% (a). LS600-HCl-Fe7% reuse test (b). a Leaching test performed to evaluate the heterogeneous nature of the LS600-HCl-Fe7% catalyst. The catalyst was removed from the reaction mixture after 1 h and the reaction was allowed to continue under the same conditions. b Reuse of the LS600-HCl- Fe7% catalyst in the conversion of limonene and selectivity towards p-cymene during consecutive reaction cycles. Reaction conditions: a 3 mL of limonene, 200 mg of catalyst, 165 °C, and 750 rpm; b 3 mL of limonene, 300 mg of catalyst, 165 °C, 750 rpm, and 8 h

1 3

192

Page 18 of 21 M. M. Giraldo, A. L. Villa LS600-HCl-Fe7% catalyst fulfills these criteria, as it exhib­ its strong Lewis acidity and a total acidity of 528.2 mmol NH3 g− 1. In addition, it contains an active metallic phase in the form of hematite (Fe2O3), which provides Fe3+ sites. Based on these characteristics, the reaction pathway for the dehydrogenation of α-terpinene to p-cymene shown in Fig.

16 is proposed. It is plausible that the isomerization of limo­

nene into other terpene intermediates occurs on the acidic sites located within the mesopores of the catalyst. Initially, the exocyclic double bond of limonene is protonated to form a primary carbenium ion, followed by proton shifts that stabilize the intermediate as tertiary carbenium ions, from which terpinolene and α-terpinene are generated [21] (Fig. 15, route I). Starting from α-terpinene, the dehydrogenation step is proposed to proceed through abstraction of an allylic hydrogen by an oxo-Fe(III) site, generating a surface-bound intermediate. Subsequent removal of a second hydrogen atom leads to the formation of p-cymene coordinated to the Fe(III) center through a π-interaction. Final desorption of p-cymene and release of H2 closes the catalytic cycle (Fig. 16). This reaction pathway aligns with dehydrogenation mechanisms of hydrocarbons on metal oxide surfaces pre­ viously reported in the literature [23].

Fig. 16  Reaction pathway for the dehydrogenation step leading to p‑cymene formation, based on [23]. Proposed scheme of the α-terpinene dehydrogenation mechanism for the formation of p-cymene on the LS600-HCl-Fe7% catalyst. The process occurs on Fe(III) sites of the supported iron oxide and involves the sequential abstraction of hydro­ gen and the subsequent formation and desorption of p-cymene. SBL corresponds to the sludge-based support, whose composition is mainly represented by the LS600-HCl material

Fig. 15  General reaction scheme for the isomerization and dehydro­ genation of limonene, based on [20, 27]. General reaction scheme proposed for the isomerization and dehydrogenation of limonene to p-cymene. The formation of p-cymene can proceed via: (I) isomeriza­ tion of limonene to terpenic intermediates followed by dehydrogena­ tion; (II) disproportionation reactions that generate intermediates such as p-ment-3-ene and cis-ment-8-ene, which are subsequently dehydro­ genated to p-cymene; and (III) polymerization reactions that lead to high molecular weight products

1 3

Page 19 of 21

192

Transformation of α-Pinene and Limonene Over Water Treatment Sludge-Derived Catalysts A consolidated comparison of the catalytic performance of LS600-HCl and LS600-HCl-Fe7% with representative literature benchmarks is presented in Table 7. For α-pinene isomerization to camphene, LS600-HCl achieves 85% con­ version and 38% selectivity at 90 °C with only 50 mg of catalyst, whereas TiO₂ [7] reaches 100% conversion and 69% selectivity but requires 155 °C and 600 mg of catalyst. For the limonene to p-cymene transformation, LS600-HCl- Fe7% achieves complete conversion and 44% selectivity at 165 °C with 200 mg of catalyst under neat conditions, whereas 20% CdO/SiO₂ [23] reaches 100% p-cymene selectivity but only under gas-phase conditions at 250 °C. Although our sludge-derived catalysts do not match the highest selectivities reported in the literature, they oper­ ate under significantly milder thermal conditions and with lower catalyst loadings, and are obtained from a low-cost, widely available waste stream underscoring their potential as sustainable catalytic alternatives.

4  Conclusions

Calcined and chemically treated sludges from a drinking water treatment plant proved to be viable precursors for the preparation of heterogeneous catalysts applicable to the transformation of monoterpenes. Acidification with HCl, particularly when combined with hydrothermal pretreat­ ment, enhanced the catalytic properties of the sludge by generating Brønsted acid sites and reducing the aluminum content of the material, which in turn favored the relative enrichment of silica and the persistence of crystalline phases such as hematite that contribute to the catalytic behavior. Metal impregnation with iron further modified the cata­ lytic profile by increasing the contribution of Lewis acid sites and generating magnetite and maghemite phases. LS600-HCl exhibited appreciable selectivity toward cam­ phene in the isomerization of α-pinene, attributable to the presence of Brønsted acid sites of moderate strength neces­ sary to initiate the reaction. In contrast, LS600-HCl-Fe7% was selective toward p-cymene in the isomerization and dehydrogenation of limonene, consistent with its enhanced Lewis acidity and the presence of Fe³⁺ species capable of promoting the dehydrogenation step.

The reaction pathways proposed for both substrates indi­ cate that p-cymene formation depends on the combined action of acidic and metallic sites, whereas excessive acidity favors the formation of high-molecular-weight species. Sta­ bility tests confirmed the heterogeneous nature of both cata­ lysts and their capacity to be reused, with a gradual decrease in conversion but with stable selectivity toward the main products across cycles. Compared with several benchmark systems reported in the literature (Table  7), LS600-HCl- Fe7% reached complete limonene conversion and moder­ ate p-cymene selectivity under significantly milder thermal conditions and with lower catalyst loadings, although with lower selectivity than the most optimized synthetic catalysts. The influence of the reaction variables was assessed through two complementary statistical strategies: a full fac­ torial design was applied to the isomerization of α-pinene over LS600-HCl in order to identify the main effects of catalyst amount, temperature, and reaction time on conver­ sion within the studied range; whereas a central composite design was used for the transformation of limonene over LS600-HCl-Fe7% to model the response surfaces of con­ version and p-cymene selectivity as a function of the same variables. These analyses provided a consistent description of the relative influence of the operating variables on the catalytic performance of each material.

Several limitations of the present study should be acknowledged in order to delimit the scope of these conclu­ sions. First, iron dispersion on the catalyst surface was not directly measured, so the catalytic behavior of LS600-HCl- Fe7% should be interpreted as resulting from the combined effect of iron content, acid-site distribution, and mesopo­ rous texture, rather than from a well-defined dispersion of the metal. Second, the comparison of the FTIR-Py spectra between fresh and reused catalyst is restricted to a qualitative interpretation, since the absolute band intensities depend on experimental variables (sample loading, ambient humidity) Table 7  Comparison of the catalytic performance of LS600-HCl and LS600-HCl-Fe7% with representative literature benchmarks for α-pinene isomerization to camphene and limonene transformation to p-cymene Reagent Product Catalyst T (°C) t (h) Mass (mg) Solvent Conversion

(%)

Selectivity (%) Reference α-Pinene Camphene TiO₂

155

0.67

600

None

100

69

[7] α-Pinene Camphene

CLIN 0.1

70

0.06

2000

None

100

50

[8] α-Pinene Camphene Ti-SBA-15

180

7

600

None

100

27

[10] α-Pinene Camphene LS600-HCl

90

5

50

Cyclohexane

85

38

This work Limonene p-cymene 20% CdO/SiO₂

250

4

200

Gas phase

100

100

[23] Limonene p-cymene Ti-MCM-41

170

24

750

None

96

46

[21] Limonene p-cymene

SIRAL 40

165

0.3

500

Microwave

100

100

[16] Limonene p-cymene LS600-HCl-Fe7%

165

24

200

None

100

44

This work

1 3

192

Page 20 of 21 M. M. Giraldo, A. L. Villa that were not strictly controlled across measurements; this prevents a quantitative assessment of acid-site populations in the spent material, although the qualitative observation that the full set of pyridine bands is preserved supports the interpretation of deactivation by physical blocking of active sites rather than by chemical modification of the acid sites. Finally, the statistical analyses (factorial and central com­ posite designs) describe the catalytic response within the operational ranges explored in this work and should not be extrapolated outside those ranges.

Overall, the combination of thermal and chemical treat­ ments applied to the inorganic fraction of drinking water treatment sludge is shown here to be an effective strategy to obtain catalytically active materials for the transformation of α-pinene and limonene under mild operating conditions, and illustrates a potential valorization route for this indus­ trial waste stream.

Supplementary Information  The online version contains supplementary material available at ​h​t​t​p​s​:​/​/​d​o​i​.​o​r​g​/​1​0​.​1​0​0​7​/​s​1​0​5​6​2​-​0​ 2​6​-​0​5​4​1​5​-​3​.​ Acknowledgements  The authors gratefully acknowledge to Univer­ sidad de Antioquia (UdeA) for providing financial support for this research and EPM for supplying the sludge. M.M.G. thanks scholar­ ship as Instructor Student from UdeA.

Author Contributions  All authors reviewed the manuscript. Concep­ tualization and formal analysis: Mateo Moreno Giraldo and Aída Luz Villa. Methodology, Validation, Investigation, and Writing – original draft by Mateo Moreno Giraldo. Writing – review & editing, Supervi­ sion, Project administration, Funding acquisition: Aída Luz Villa. All authors read and approved the final manuscript. Funding  Open Access funding provided by Colombia Consortium. The authors gratefully acknowledge to Universidad de Antioquia (UdeA) for providing financial support for this research. M.M.G. thanks scholarship as Instructor Student from UdeA. Data Availability  The authors declare that the data supporting the find­ ings of this study are available within the paper and its Supplementary Information files.

Declarations Competing Interests  The authors declare no competing interests. Open Access  This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit ​h​t​t​p​:​/​/​c​r​e​a​t​i​v​e​c​o​m​m​o​n​s​.​o​ r​g​/​l​i​c​e​n​s​e​s​/​b​y​/​4​.​0​/​.​ References

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