Solvent-Free Aldol Condensation of Cyclopentanone with Natural Clay-Based Catalysts: Origin of Activity & Selectivity

Abstract

The conversion of biomass resources into high-value fuels and chemicals using thermochemical methods has become an attractive method of energy utilization. In this study, natural minerals were used as raw materials; the acidic sites were introduced by ball-milling modification, and the aldol condensation reaction of the biomass-based cyclopentanone molecule was carried out under solvent-free conditions. It was found that the SO₃H-APG catalyst—with strong medium-based sites when the -SO₃H loading was 4 mmol/g—exhibited excellent acid–base co-activation effects and a significant catalytic effect in the cyclopentanone condensation reaction. The optimization of the reaction conditions showed that the conversion of cyclopentanone reached 85.53% at the reaction temperature of 150 °C and reaction time of 4 h. The selectivity of the dimer and trimer was 69.04% and 28.41%, respectively. The investigation of the cyclopentanone condensation mechanism and kinetic analysis showed that the acid–base presence of an acid–base bifunctional catalyst was important to facilitate the condensation reaction. This research route is in line with the concept of sustainable green production and also provides a promising pathway for catalyst design and the synthesis of long-chain hydrocarbons.

1. Introduction

The utilization of renewable biomass resources to produce biofuels and high-value chemicals has become an important measure to solve the problem of resource consumption [1,2,3,4,5,6], in addition to having great significance in the implementation of the strategic tasks of carbon peaking and carbon neutrality. The thermochemical transformation of biomass resources with abundant sources can generate important chemicals such as acids, aldehydes, ketones, and furans, which are widely used in pharmaceutical, fragrance, cosmetics, and high-density fuel fields, among others [7,8,9,10,11,12]. Cyclopentanone has been extensively studied due to its ability to achieve carbon chain growth through self-condensation and cross-condensation reactions [13,14]. Dimer 2-cyclopentylidene-cyclopentan-1-one (C10)—generated under the catalysis of an acid or base—is a kind of crucial chemical intermediate, which can be used not only as a fragrance additive in everyday chemical products and foods [15], but also as a precursor of high-density fuel oils in aerospace engineering and rockets [16].

The conversion of biomass platform compounds into multi-carbon intermediates is achieved by C–C coupling reactions to growing carbon chains, and is then followed by hydrodeoxygenation to obtain liquid alkane fuels with a high energy density [17,18]. At present, aldol condensation is the most widely used and promising method [19,20]. Aldol condensation reaction is an effective C–C bonding reaction, and some short carbonyl compounds (e.g., ketones, aldehydes) derived from the biomass are particularly suitable for C–C coupling, upgrading and making catalytic upgrading easier for use on fuels and chemicals [21,22,23]. The cyclopentanone can easily undergo an aldol condensation reaction to generate α- and β-unsaturated dimer ketones, which can be generated by hydrodeoxygenation to produce combustible molecules [24]. The dimer (C10) and the trimer (C15) are both valuable chemicals, and the latter is also routinely used as a precursor to diesel-grade products via hydrogenation [23]. This study focused on dimmer yield under solvent-free conditions to achieve the acid–base catalytic conversion of cost-efficient catalysts.

Since the steric hindrance of cyclopentanone makes its self-aldol condensation reaction difficult to achieve, it is necessary to add a catalyst with high catalytic activity to promote the reaction [25]. At present, in addition to the use of homogeneous catalysts such as KOH and NaOH, many efforts have been made by scholars around the world to develop active multiphase catalysts [26,27,28,29]. The use of acid–base bifunctional catalysts has achieved good results in the aldol condensation reaction of cyclopentanone. The Ni-Mg-Al-O/AC bifunctional catalyst was prepared for cyclopentanone aldol condensation and hydrodeoxygenation reaction to obtain high-performance jet fuel [30]. In addition, acid–base bifunctional catalysts with different acid strengths were used by fixing the basic amino groups and changing the type of acid groups, and it has been found that weak acid groups significantly improved the aldol condensation reaction activity [31]. Using aldol condensation over a modified chitosan catalyst and subsequent hydrodeoxygenation, Q. Zhang’s team created long-chain alkanes. A high level of C15 production is the consequence of the complementary effects of the amino and NaOH [32]. A. Amarasekara tested the aldol dimerization of levulinic acid over solid acid catalysts—including Amberlyst-15, SiO₂-SO₃H, Carbon-SO₃H, and H₃PW₁₂O₄₀—at 110–130 °C, and also discussed the possibility of the reuse of catalysts [33]. The main advantage of acid–base bifunctional catalysts was their capability to have their so-called synergistic catalysis be tailored, which relies on the synergistic effect of the acid–base sites to facilitate the hydroxyl aldol condensation reaction. Through the aldol dimerization of levulinic acid, M. Paniagua produced bio-jet fuel precursors without solvents on sulfonic SBA-15 Silicas [34]. Thus, cost-effective acid–base solid catalysts would be designed and fabricated, and solvent-free condensation would be a better approach for the dimerization of cyclopentanone.

In this paper, a new method was proposed for the preparation of acid–base bifunctional catalysts using natural clay minerals modified by ball milling. The mechano-chemical approach conforms to the concept of green chemistry and economic efficiency, and it is suitable for the mass production of inexpensive catalysts [35,36]. Natural attapulgite contained Mg²⁺, Al³⁺ and their oxides, which can be used as basic sites, and the precursor was grafted with -SO₃H group by ball milling to introduce acidic sites. The optimization of reaction conditions for the cyclopentanone aldol condensation was carried out on an electric heating jacket with a reflux device and magnetic stirring, and its catalytic properties were studied in solvent-free conditions. Because the use of organic solvents produced environmental waste and high energy consumption to separate the reaction mixture. Thus, the use of a solvent-free system was more conducive to efficient reactions. The construction of environmentally friendly and efficient acid–base bifunctional catalysts provides a new idea for the “one-pot” preparation of biofuel from biomass-based platform compounds in a solvent-free system.

2. Results and Discussion

2.1. Characterization of the Synthesized SO₃H-APG

The morphology and microstructure of the clay-based catalysts were observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) as shown in Figure 1. It can be observed that the catalyst has a rod-like structure and the crystals are stacked with each other. It could also be clearly seen from the EDX pattern that Si, Al, Mg and Ca elements were distributed in the SO₃H-APG catalyst and the abundant basic sites on APG originated from relatively high Mg content.

The FT-IR spectrum of the SO₃H-APG catalyst was shown in Figure 2a, from which it can be seen that the absorption peaks at 795 cm⁻¹ and 1045 cm⁻¹ were attributed to the stretching vibration of the Si-O-Si bond [37], except for the characteristic peaks of the clay at 455 cm⁻¹ and 520 cm⁻¹ (bending vibration of the Si-O bond), and the peaks at 1065 cm⁻¹ and 1172 cm⁻¹ were attributed to the stretching vibration of the O=S=O bond (-SO₃H) [38], indicating that the -SH has been oxidized to -SO₃H and successfully grafted to the APG surface. The bending vibration absorption band of the hydroxyl group at 1634 cm⁻¹, and the absorption peak at 2943 cm⁻¹ were due to the stretching vibration of the C-H bond in the methylene group. The introducing sulfonic acid group did not destroy the crystal structure of attapulgite as also seen in the XRD spectrum (Figure S2).

The N₂ adsorption–desorption isotherms of the SO₃H-APG catalyst and the pore size distribution curves were displayed in Figure 2b. It can be seen that the catalysts exhibited relatively complete adsorption hysteresis curves, indicating that the samples had typical properties of mesoporous materials, and the pore sizes were mainly concentrated around 3.83 nm (Table S4). The grafting of the sulfonic acid group on the outer surface made the catalyst processing good accessibility to active centers, and the larger specific surface area and pore size would help to expose more acid and base sites, thus facilitating the catalytic conversion of the cyclopentanone condensation.

Clinoptilolite (clin) and kaolin were chosen for grafting SO₃H- group, and tested their performance on cyclopentanone condensation. The acid–base density and intensity of the catalysts were determined using CO₂-TPD and NH₃-TPD measurements, and the results were shown in Figure 2c,d. The CO₂ desorption curve showed that there was a diffraction peak below 150 °C which could be attributed to the weakly basic centers, and the diffraction peak in the range of 150–450 °C which could be attributed to the medium sites. The ammonia desorption curve existed an acidic desorption site at approximately 450 °C, which was assumed to be the strong acidity of the sulfonic acid group (-SO₃H) [39]. The results of the quantitative analysis of CO₂-TPD and NH₃-TPD data were shown in

Table 1. Physiochemical properties of catalysts.

	CO2 Quantity (cm3/g)		Base Amount (mmol/g)			NH3 Quantity (cm3/g)	Total Acids (mmol/g)
	0–150 °C	150–450 °C	Weak Base	Medium Base	Total Basicity
APG	39.76	85.31	1.81	3.88	5.69	17.63	1.04
SO3H-APG	34.67	83.81	1.58	3.81	5.39	99.60	5.86
SO3H-kaolin	-	-	-	-	-	28.39	1.67
SO3H-clin	-	-	-	-	-	31.78	1.87

, which indicated that the SO₃H-APG catalyst possessed both acid–base properties, with its basic sites mainly concentrated in the medium-base position. Additionally, it also reflected that the acid–base bifunctional catalyst could promote the cyclopentanone condensation reaction efficiently.

The TPD curves were fitted with split peaks, and both Yang [40] and Hu [41,42] methods were used to calculate the activation energy of desorption (DAE) for the acidic and basic sites (Table S4). It can be seen that the desorption temperatures (thermos-desorption temperatures of CO₂ and NH₃ are related to basicity and acidity strength, respectively) corresponding to the medium base site desorption peaks of APG and SO₃H-APG catalysts were 236.14 °C and 295.04 °C, and the corresponding E_d values calculated by Hu are 36.29 kJ/mol and 44.11 kJ/mol, respectively. Meanwhile, the desorption temperature corresponding to the medium-strong acid sites desorption peak of SO₃H-APG catalyst was 438.93 °C, and the activation energy of resolution calculated by Yang’s method was 23.72 kJ/mol. The adsorption performance of the adsorbent depended not only on its porous structure but also on the chemical properties of its surface [43]. the stronger the adsorption of the adsorbent with the catalyst’s surface was, and the more difficult for desorption [44]. The above results indicated that the SO₃H-APG catalyst with a strong medium base and acidic sites played an important role in facilitating the catalytic conversion of cyclopentanone.

The surface areas and pore volumes of these three catalysts were listed in

Table 2. Physical properties of SO₃H-APG, SO₃H-kaolin, and SO₃H-clin catalyst.

	BET Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
SO3H-APG	127.08	0.0312	0.95 (micro)/3.83 (meso)
SO3H-kaolin	117.04	0.1399	3.84
SO3H-clin	110.94	0.2639	3.82

. Typical mesoporous structure remained when the sulfonic acid groups were grafted, which may point to a few amounts of pore blockage.

2.2. Screening of Reaction Conditions

The effects of different reaction parameters on the cyclopentanone conversion and the selectivity of the condensation products (C10, C15) with SO₃H-APG as the catalyst were shown in Figure 3. It can be seen that the conversion of cyclopentanone without the introduction of sulfonic acid groups was 53.21 ± 2.66%, and the yield and selectivity of the target product (dimer and trimer) reached 35.85 ± 1.79% and 67.46 ± 3.37% Figure 3a. The highest conversion of cyclopentanone (79.96 ± 3.99%) was achieved at the -SO₃H loading of 4 mmol/g, and the total yield and selectivity of its condensation products also reached the maximum values of 68.01 ± 3.40% and 85.05 ± 4.25%, respectively. As the loading of sulfonic acid groups continued to increase, the conversion and the selectivity of the condensate decreased, which may be because the excess of -SO₃H groups covered part of the active sites on the catalyst, reducing its specific surface area, pore volume as well as the catalytic activity [45]. Natural clay minerals contain basic sites such as MgO and Al₂O₃, and the introduction of acidic sites can achieve acid–base synergistic catalysis and, therefore, promote cyclopentanone condensation.

The influence of reaction temperature on the condensation performance was investigated in the range of 110–170 °C, as shown in Figure 3b. At the condensation temperature of 150 °C, the conversion of cyclopentanone reached the maximum of 85.53 ± 1.28%, and the yield of the target product was approximately 83.35 ± 4.03% (dimer, 59.05 ± 2.95%; trimer, 24.3 ± 1.08%). However, when the temperature was further increased to 170 °C, the conversion and yield decreased significantly, so 150 °C was selected as the optimized condensation temperature. The selectivity of the dimer and trimer as a function of condensation temperature was shown in Figure 3(b2). With the increase of condensation temperature, the selectivity to the trimer inclined clearly, and the dimer selectivity reached more than 60%. In the production of jet fuel intermediates from furfural and acetone, a similar conclusion was drawn by Huang et al. It was found that higher temperature might promote the degradation of the F₃A₂ intermediate, thus, improving the selectivity of aldol condensation products [46].

The effect of different reaction times on the condensation performance of cyclopentanone was displayed in Figure 3c. The conversion enhanced obviously with the increase in reaction time at the early stage of the reaction, and reached quickly 80.23 ± 2.40% at 4 h. At this time, the yield and selectivity of the condensation products (dimer and trimer) were 69.81 ± 2.43% and 87.02 ± 4.35%. When the reaction time was extended, the conversion of cyclopentanone remained stable, so the optimal reaction time was chosen to be 4 h.

The influence of the different catalysts on the cyclopentanone conversion and product selectivity were shown in Figure 3d. It was seen that SO₃H-APG exhibited a superior catalytic effect in the cyclopentanone condensation reaction compared to SO₃H-kaolin and SO₃H-clin catalysts. The conversion of cyclopentanone reached 85.53 ± 0.85%, meanwhile, the selectivity of the target products (Figure S1) also attained the highest (dimer, 69.04 ± 3.45%; trimer, 28.41 ± 1.42%). Moreover, tt indicated that the acid–base synergism promoted the efficient condensation of cyclopentanone.

2.3. Response Surface Methodology (RSM) Analysis

Based on the single-factor experiment, the C10 yield was taken as the response value, and the three factors that had a significant effect on the cyclopentanone condensation reaction, -SH loading (X1), reaction temperature (X2) and reaction time (X3), were investigated and analyzed according to the Box–Behnken principle using Design expert V8.0.6 software (Table S2). The response results of the interaction among the factors were shown in Figure 4.

From the analysis, it can be seen that when the -SH loading, and reaction time were unchanged. The yield of C10 showed a trend of increasing and then decreasing with the heightened reaction temperature in response to the sensitivity. When the reaction temperature and -SH loading was fixed, the yield of C10 also showed a tendency to raise and then reduce with the protracted reaction time, which had a large effect. The reaction time and reaction temperature were fixed, and the yield of C10 showed the same trend and corresponding sensitivity as the loading of sulfhydryl groups increased. The contours were closed elliptical and the response surface was convexed, indicating a strong interaction between the factors and a maximum.

The results were analyzed by stepwise regression using the software (Tables S2 and S3), and the optimal reaction conditions were determined as -SH loading 1.65 mL, reaction temperature 150.81 °C, and reaction time 4.11 h. At this time, the yield of C10 was 59.02%. To facilitate the practical operation, the optimal reaction conditions were revised to -SH loading 1.6 mL, reaction temperature 150 °C, and reaction time 4 h. The yield of C10 reached 59.05% under these optimal conditions, which was in good agreement with the predicted value of the model and verified the reliability of the approach.

2.4. Reaction Kinetic Analysis

To investigate the operating reaction mechanisms of the acid–base bifunctional catalyst, the variations of cyclopentanone concentration as the change of time on the different catalysts were measured under water-free conditions. When the surface coverage was low enough to vary with reactant concentration, the intrinsic reaction order could be better individualized at low reactant concentrations [47]. In addition to obtaining kinetic expressions useful for the calculation of activation energy, the kinetic model for the catalytic and non-catalytic conversion of cyclopentanone also provided a clearer knowledge of how the conversion process at the catalysts works. The graphs of ln(C₀/C) against time gave good straight-line plots, and this revealed that the cyclopentanone aldol condensation reaction followed first-order kinetics Figure 5a. Based on this, we fitted the typical Langmuir-Hinshelwood (L-H) model, and the results showed that the R² was all greater than 0.95 Figure 5b, indicating a strong fit, and the detailed derivation process was described in the supplementary material.

The basic steps of the catalytic cyclopentanone via aldol condensation mainly included the adsorption of cyclopentanone, the formation of enol, C–C coupling, dehydration and product desorption [48,49,50], and the detailed mechanism was shown in Equations (1)–(6), where C represented the reactant cyclopentanone, * stood for the available surface active sites, A was the alcohol product, D was the dimer product (C10), and W was water. The attack of the enol on the electrophilic carbon of the other carbonyl group was the rate-limiting step of the reaction [47], which means that the C–C coupling had a certain limiting effect on the cyclopentanone self-condensation.

C+*↔C* Adsorption of cyclopentanone

(1)

C*↔[C]* Formation of enol

(2)

[C]*+C*↔A*+* Surface reaction (C–C coupling)

(3)

A*+*↔D*+W* Dehydration

(4)

D*↔D+* Desorption of dimer

(5)

W*↔W+* Desorption of water

(6)

2.5. The Mechanism of Acid–Base Bifunctional Catalyst Action

Given that the SO₃H-APG catalyst showed high catalytic activity in the cyclopentanone aldol condensation reaction, it is evident that the acid and base centers have an important role in this reaction, and therefore the reaction mechanism of the acid–base bifunctional catalyst in this condensation reaction was investigated. APG is a mineral aluminosilicate with about 2:1 (mol ratio) Mg to Al in its framework, which made it has abundant accessible base sites and active for base-catalytic condensation. Trace amount of Fe, Ni cations in APG would promote H₂O adsorption and dissociation. As shown in Figure 6, the α-H on the carbonyl oxygen atom side of the cyclopentanone was deprotonated by the catalytic action of the basic site on the SO₃H-APG catalyst to form a carbon-negative ion, which was then added to the carbonyl carbon atom of another cyclopentanone molecule to form a β-hydroxy aldehyde after deprotonation. Finally, the dimer was prepared by dehydration under heating conditions using hydroxy aldehyde as the raw material. The α-H on the other side reacted with the carbonyl carbon atom of another cyclopentanone molecule in the nucleophilic addition reaction, and after dehydration, the target product (trimer) was formed.

Under the catalytic effect of the acidic sites, the enolized structure of cyclopentanone and the carbon-positive ion structure proceeds to an addition reaction. Followed by dehydration to form an enone structure, which was unstable, and the electron cloud was shifted toward the hydroxyl group to form a carbon-positive ion after protonation, and the carbon-positive ion was dehydrogenated to form a stable dimer. Accordingly, the acid–base sites cooperated to promote the continuity of the cyclopentanone condensation reaction, which was also consistent with the analytical results of CO₂-TPD as well as NH₃-TPD. Additionally, it was indicated as an exothermal adsorption of cyclopentanone. Increasing in the adsorbate electron density will facilitate its activation and further condensation (Figure S5).

As for the mechanism that was proposed by the Wang group [51] for efficient C–C coupling of acetone by separated dual sites, carbocation derived on the acid site was combined with carbanion on the base site at a reasonably high rate (1020 mg·g_CAT⁻¹·h⁻¹). The enhanced cyclopentanone yield on the SO₃H-APG catalyst can illustrate the promotion effect of the ion pair (positive-negative) interaction on spatially isolated acid and base sites that worked synergistically.

2.6. Catalyst Stability and Recycling

These mineral-based catalysts were easily recycled at the end of the reaction and could be recycled after filtering, washing, and drying. The changes during the conversion of cyclopentanone and the C10 and C15 selectivity of the SO₃H-APG catalyst after five cycles were demonstrated in Figure 7a. As can be seen from the figure, the overall conversion and selectivity of the catalysts remained stable with the increase in the number of cycles, and the reduced activity may be caused by the deactivation of some active centers of the catalysts.

To explore the reusability of the catalyst, the catalyst after recycling for five times was subjected to FT-IR analysis, and the results were shown in Figure 7b. The results showed that the FT-IR spectra of the recycled SO₃H-APG catalyst revealed bands that were similar to the fresh catalyst, indicating that the catalyst was stable and still had a significant amount of acid–base centers after participating in the reaction.

3. Materials and Method

3.1. Materials and Equipment

APG, SiO₂ (purity 85-90%) were purchased from Changzhou DingBang Mineral Products Technology, Huaian, China. 3-mercaptopropyl trimethoxysilane (MPTMS, purity 95%) were purchased from Aladdin Reagent (Shanghai) Co., Ltd., Shanghai, China. H₂O₂ were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Ball mill was purchased from Zhuo’s Instrument Co., Ltd., Shanghai, China. All chemicals were used without further purification.

3.2. Preparation of SO₃H-APG Catalyst

The synthesis of the SO₃H-APG catalyst was derived from a previous study [52]. Firstly, 2 g attapulgite, 1.6 mL 3-mercaptopropyl trimethoxysilane (MPTMS, 95%), 10 mL water were added to a 50 mL resin tank (containing agate medium pellets of 5–9 mm in diameter), and reacted using a planetary ball mill at 300 rpm for 2.5 h to obtain the APG-SH. Then, the prepared APG-SH was functioned by consecutive H₂O₂ (30 wt%) oxidation (stirred at room temperature for 12 h), washed with distilled water after oxidation, and dried at 80 °C to obtain the SO₃H-APG catalyst (Figure 8).

Other natural aluminosilicates, kaolin, and clinoptilolite (clin) are also abundant mineral clays that have porous structures and acid sites that are benign for cyclopentanone condensation. Which made them the references of APG clay. SO₃H-kaolin and SO₃H-clin samples were synthesized following a similar procedure of SO₃H-APG preparation for reference.

3.3. Catalyst Characterization

The FT-IR spectra of the catalysts were recorded on Nicolet 6700 (Thermo Electron Corporation, Waltham, MA, USA) in the range from 400 to 4000 cm⁻¹. The N₂ adsorption–desorption isotherms were collected with a BSD-PM2 instrument, and the pore size distribution curves were determined using the NLDFT mathematical model. The high-resolution morphology was observed under the SEM microscope (Gemini SEM 300, ZEISS, Jena, Germany) and TEM microscope (JEM-2100, JEOL Ltd., Tokyo, Japan) to characterize the catalyst. The temperature-programmed desorption tests, NH₃-TPD and CO₂-TPD, were performed with AutoChem II 2920 (Micromeritics Instrument Corp., Norcross, GA, USA). Catalyst samples were pretreated at He (300 °C, 60 min) and exposed to ammonia at 200 °C until saturation and then stopped heating. After the chamber was cooled down to room temperature, the NH₃-TPD experiments were performed between 50 °C and 700 °C with a desorption rate of 10 °C/min. The CO₂-TPD process was carried out in the same way, with the catalyst adsorbed to saturation with CO₂.

3.4. Aldol Condensation of Cyclopentanone

The aldol condensation reaction of cyclopentanone (analytic purity) was carried out on a digital thermostatic heating sleeve with magnetic stirring. Under solvent-free conditions, a quantitative amount of catalyst and cyclopentanone (catalyst/cyclopentanone mass ratio, 0.15) were added to a round-bottom 100 mL glass flask and mixed well. The electric heating sleeve was preheated to the specified temperature and then stirred at 200 rpm under reflux. After the reaction, the flask was cooled to room temperature, and the condensation samples were transferred and processed in a high-speed centrifuge. The liquid products were diluted with CH₂Cl₂ (chromatographic purity) into the volumetric flask for analysis, and repeated experiments were conducted after the solid catalyst was washed and dried.

Using Design expert V8.0.6 software to fit the data in Table S2, the regression equation S1 was obtained. Analysis of variance was performed on the above regression equation and the results are tabulated in Table S3.

Filtration was used to recover post-reaction catalysts, which were then cleaned with ethanol under ultrasonic conditions for 1h and dried overnight before being reused.

3.5. Products Analysis

The reaction products were analyzed by gas chromatography-mass spectrometry (GC-MS, Agilent 7890-5975C, Santa Clara, CA, USA), equipped with an Agilent 7683B auto-injector, an HP-5 capillary column and a Flame Ionization Detector (FID), and gas chromatography (Agilent 6890, HP-5 capillary column, 30.0 m × 250 mm × 0.25 mm) was used for quantitative analysis. The conversion of cyclopentanone, the selectivity of C10, and the yield were calculated according to the following Equations (7)–(9).

$$\mathrm{Conversion}=(1-\frac{\mathrm{Moles}\mathrm{of}\mathrm{cyclopentanone}}{\mathrm{Moles}\mathrm{of}\mathrm{cyclopentanone}\mathrm{loaded}\mathrm{initially}})\times 100\%$$

(7)

$$\mathrm{Selectivity}\mathrm{of}\mathrm{C}10=\frac{\mathrm{Moles}\mathrm{of}\mathrm{cyclopentanone}\mathrm{to}\mathrm{C}10}{\mathrm{Moles}\mathrm{of}\mathrm{cyclopentanone}\mathrm{converted}}\times 100\%$$

(8)

$$\mathrm{Yield}\mathrm{of}\mathrm{C}10=\mathrm{selectivity}\times \mathrm{conversion}$$

(9)

4. Conclusions

A variety of mineral-based catalysts were synthesized by a simple and eco-friendly milling method and used in the cyclopentanone aldol condensation reaction under solvent-free conditions. Among them, the SO₃H-APG catalysts with large specific surfaces and abundant acid–base sites showed good catalytic effects. The optimization of the reaction conditions and the response surface analysis revealed that the conversion of cyclopentanone reached a maximum of 85.53% when the -SO₃H loading of 4 mmol/g, reaction temperature of 150 °C, and reaction time of 4 h. The selectivity and yield of the dimer and trimer were also maximized at 69.04% and 28.41%, respectively. In addition, the SO₃H-APG catalyst has shown excellent stability in long-term reaction and recycling runs.