Green Synthesis of 3-Hydroxybutyraldehyde from Acetaldehyde Catalyzed by La-Ca-Modified MgO/Al₂O₃

Abstract

3-hydroxybutyraldehyde (3-HBA) is mainly employed to synthesize 1,3-BDO (1,3-butanediol), which is one of the most important components in cosmetics moisturizers. In this study, a series of composite oxide catalysts were prepared by bringing alkaline earth metal Ca and rare earth metal La to the composite oxide MgO/Al₂O₃, which were made through the co-precipitation method. These catalysts were applied in the synthesis of 3-HBA through acetaldehyde (AcH) condensation. The structure, texture, and acidic properties of these catalysts were characterized using various characterization methods, and the effects of catalyst composition, reaction temperature, reaction pressure, and residence time on the conversion of AcH were investigated as well. The results showed that the introduction of Ca and La weakened the acidic property and enhanced the basic property, which favored the AcH condensation to synthesize 3-HBA. At a temperature of 20 °C, pressure of 200 kPa, and residence time of 70 min, 0.5%La-2.3%Ca-2MgO/Al₂O₃ exhibited a better catalytic activity, and the conversion of AcH reached 95.89%. The selectivity and yield of 3-HBA were 92.08% and 88.30%, respectively. The stability test indicated that the high AcH conversion could be maintained for 5 h.

1. Introduction

1,3-butanediol (1, 3-BDO) is one of the important components of cosmetics moisturizers. Not only does it have minor skin irritation but it also has good bacteriostatic action. Being one of the indispensable raw materials in the cosmetics industry, it is being widely used in the production of cosmetic water, ointment, toothpaste, and other daily chemicals [1,2,3].

The main synthesis methods of 1, 3-BDO include the biological method, isobutylene-formaldehyde method, propylene-formaldehyde condensation hydrolysis method, acetaldehyde (AcH) condensation hydrogenation method, acrolein-2,2-dimethyl-1,3-propanediol method, and laser irradiation ethanol method [4,5,6,7,8,9]. Considering the factors such as raw material source, product yield, process difficulty, operation cost, and fixed investment, the AcH condensation hydrogenation method is most suitable for industrialization. The specific process goes as follows: under the action of an alkaline catalyst, AcH is first condensed to 3-hydroxybutyraldehyde (3-HBA) and then hydrogenated to 1,3-BDO. Aldol condensation is an important C–C-bond-forming reaction in organic chemistry, and 3-HBA could be synthesized via the aldol condensation of AcH. However, the aldol condensation reaction was often performed in a homogeneous base (NaOH, KOH, or Ca(OH)₂)-catalyzed mixture, which can lead to reactor corrosion and the production of salt wastes [10,11,12,13]. The large number of wastewater and waste salts formed in the process are hostile to the environment and cause additional treatment burden [10,11,12,13]. Nevertheless, it is not easy to control the reaction process of AcH condensation. If the catalyst and process conditions are unfitting, a large number of by-products such as 2-butenal (crotonaldehyde), and dimers and trimers of AcH will emerge, which significantly reduces the selectivity and yield of 3-HBA and makes it difficult to separate and purify the products [14]. Some researchers have also adopted the multi-phase catalytic method of AcH condensation, but the reaction temperature is relatively high (mostly above 100 °C), resulting in the continuous reaction of 3-HBA of the unstable structure, producing 2-butenal [10,13,14,15,16]. At the same time, the purification of 3-HBA among nonrecyclable catalysts and unreacted AcH is also a tough challenge [12,17]. More recently, it was found that basic ionic liquids were active for the aldol condensation reaction, which can achieve a higher total yield (75%) than NaOH (69%), while it is time-consuming and expensive [18]. Solid anion exchange resins were also tested in the aldol condensation of AcH, but the yield of 3-HBA was quite low over acrylic resin (5.5%) and microporous styrene resin (2.5%) [19]. It is of great importance to find a stable and efficient solid base catalyst for the synthesis of 3-HBA from AcH. Some researchers found that when different types of alkali are loaded on Al₂O₃, strong alkali sites can be generated after calcination, which shows good catalytic performance [14,15,16]. Furthermore, as 1,3-BDO is currently all imported, it is urgent to localize the production of 1,3-BDO. Therefore, it is of great significance to develop solid catalysts with stable performance and high selectivity and it is of great value to optimize the syntheses process of 3-HBA-1,3-BDO in order to make it environmentally friendly and inexpensive.

In this study, a series of MgO/Al₂O₃ composite acid–base solid catalysts are prepared through the coprecipitation method. Ca and La ions are introduced to modify the composite oxides. The catalytic activity is evaluated by a self-made tubular reactor. The catalytic performance of the modified MgO/Al₂O₃ solid catalyst is compared with that of the liquid catalyst (NaOH solution) to investigate the catalytic performance and lifespan of the modified MgO/Al₂O₃ for the synthesis of 3-HBA in AcH condensation.

2. Experimental

2.1. Preparation of Composite Oxide Catalysts

NaOH(≥99%), Mg(NO₃)₂·6H₂O(≥99%), Al(NO₃)₃·9H₂O(≥99%), Ca(NO₃)₂·4H₂O(≥99%), La(NO₃)₃·6H₂O(≥95%), and NH₃·H₂O(28%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. CH₃CHO(≥30%) and CH₃CH₂OH(≥95%) were supplied by Jilin Petrochemical Company, Ltd., PetroChina. N₂(≥99%) and NH₃(≥99%) were purchased from Tianjin Ganda Gas Co., Ltd. and 46th Institute of China Electronics Technology Group Corporation. All the above chemicals were used as received without further purification.

Al(NO₃)₃ 9H₂O and Mg(NO₃)₂ 6H₂O were dissolved in deionized water to prepare a 1:1~4:1 (MgO:Al₂O₃) molar ratio of a magnesium nitrate and aluminum nitrate mixed solution. While the mixed solution was being stirred, an appropriate amount of ammonia was dropped into it, the pH was kept constant at 9~10, and then the reflux started. After that, the solution was heated to 100 ± 1 °C for 3~4 h. Afterward, the solution was cooled for 1–2 h and filtered, and the cake obtained was washed with abundant deionized water until pH 7. The filter cake was then dried in a vacuum oven at 80 °C for 24 h. After the filter cake was calcined at 500 °C for 3–6 h, the MgO/Al₂O₃ catalyst was obtained. The prepared MgO/Al₂O₃ was crushed and sieved, and the particles of 20~40 mesh size were selected. At room temperature, the particles were immersed in a certain concentration of La(NO₃)₃ or Ca(NO₃)₂ solution for 4~8 h in equal volume. After filtration, the filter cake was dried in a vacuum oven at 80 °C for 24 h and then calcined at 500 °C in an air atmosphere for 2 h to obtain Ca-MgO/Al₂O₃ or La-Ca-MgO/Al₂O₃.

2.2. Characterization of Composite Oxide Catalysts

The specific surface area and the pore size of the catalysts were measured by physical and chemical adsorption apparatus (BET, Autosorb-1-C, Anton Paar QuantaTec Inc.; Boynton Beach, FL, USA). The elemental composition was verified by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Agilent 725, Agilent Technologies Inc.; Santa Clara, CA, USA). The crystallinity of samples was characterized by an X-ray diffractometer (XRD, Advance D8, Bruker Corporation; Billerica, MA, USA). Temperature-programmed desorption (TPD) was carried out using CO₂/NH₃ as probe molecules and was used to estimate the number and the intensity of acidity and alkalinity of samples by a chemical adsorption instrument (AutoChemII 2920, Micromeritics Inc., Norcross, GA, USA). Fourier-transform infrared spectroscopy (FTIR, AVATAR 360, Thermo Fisher Scientific Co., Ltd.; Waltham, MA, USA) was used to characterize the composites by pyridine adsorption infrared spectroscopy.

2.3. Evaluation of Catalyst Performances

The catalyst performance evaluation for the AcH condensation of composite oxide catalysts (MgO/Al₂O₃, Ca-MgO/Al₂O₃, and La-Ca-MgO/Al₂O₃) as well as a soluble catalyst (NaOH solution) were performed in a tubular reactor with 125 mm o.d. and 300 mm length.

Figure 1 depicts the catalyst performance evaluation device for the AcH condensation of composite oxide catalysts MgO/Al₂O₃, Ca-MgO/Al₂O₃, and La-Ca-MgO/Al₂O₃ in contrast with that of the soluble catalyst (NaOH solution). In a typical run, the reactor was loaded with quartz sand at the bottom and then filled with oxide catalyst (1.0 g, 20~40 mesh). Prior to reaction, the reactor was purged in situ for 1 h in 2000 cm³/min dry N₂. The AcH in feed storage tank 1 was injected into the shell (composite oxide catalyst prefilled here) of tubular reactor 2 through plunger pump 4 at the rate of 1~10 mL/min. In the case of the soluble catalyst, the mixed solution NaOH (10 wt%) and AcH (30 wt%) in feed storage tank 1 was injected into tubular reactor 2 through plunger pump 4 at the rate of 1~10 mL/min. The reaction temperature was controlled to 0~30 °C through refrigerator 6. Adjusting the pressure of product receiving tank 5, the pressure in tubular reactor 2 was controlled. Both the tubular reactor and the product receiving tank were protected by nitrogen while starting up and shutting down. The reactants and products in the tubular reactor were both sampled by sampling valve 14 for subsequent analysis. The samples were identified and quantified by an Agilent Technologies 1260 Infinity II liquid chromatography system-based external calibration method. The chromatographic column type was TSK gel ODS-100S. The mobile phase consisted of 50% methanol and 50% deionized water. The analysis was conducted at the column temperature of 30 °C and injection volume of 20 µL. The running time was set to be 15 min at the flow rate of 0.7 mL/min.

The conversion is calculated based on AcH using Equation (1):

$$\mathrm{Conversion}=\frac{C^0\left(\mathrm{AcH}\right)-C^t\left(\mathrm{AcH}\right)}{C^0\left(\mathrm{AcH}\right)}\times 100\%$$

(1)

where C⁰(AcH) is the AcH concentration in the feed line, and C^t(AcH) is the AcH on centration in the outlet stream at any time t.

The selectivity of objective products 3-HBA is calculated according to Equation (2):

$$\mathrm{Selectivity} \left(3\text{-}\mathrm{HBA}\right)=\frac{2C^t\left(3\text{-}\mathrm{HBA}\right)}{C^0\left(\mathrm{AcH}\right)-C^t\left(\mathrm{AcH}\right)}\times 100\%$$

(2)

where C^t(3-HBA) is the 3-HBA concentration in the outlet stream at any time t.

3. Results and Discussion

First, in the catalyst performance experiment, the catalytic activity of different catalysts was compared to investigate the influence of loading elements and loading amount on the catalytic activity of AcH condensation, hence determining the appropriate loading amount. Subsequently, using ICP-OES, XRD, N₂ adsorption–desorption, BET, NH₃/CO₂-TPD, and pyridine adsorption infrared spectroscopy, the catalyst structure and the interaction between the oxides were characterized to determine the sources of alkali and acid active sites in the catalyst. Thirdly, the effects of reaction temperature, residence time, and reaction pressure on the AcH condensation reaction were investigated. Finally, the lifespan and deactivation reasons of the catalyst La-Ca-MgO/Al₂O₃ were studied and analyzed.

3.1. Characterization of Catalyst Performances

The activity of the NaOH solution and a series of composite oxide catalysts were investigated in the experimental device shown in Figure 1. The initial conditions of the AcH catalytic condensation reaction were as follows: AcH feed concentration 30%, reaction temperature 23 °C, residence time 70 min, and reaction pressure 200 kPa.

Table 1. Catalytic performance evaluation of composite oxides on AcH condensation.

NO.	Catalyst	Conversion of AcH/%	Selectivity of 3-HBA/%	Yield of 3-HBA/%
1	20%wt NaOH	95.39	93.45	89.14
2	MgO/Al2O3	64.13	86.42	55.42
3	2MgO/Al2O3	82.61	97.39	80.45
4	3MgO/Al2O3	88.23	85.38	75.33
5	4MgO/Al2O3	92.45	81.25	75.12
6	2.0%Ca-2MgO/Al2O3	84.83	77.43	65.68
7	2.3%Ca-2MgO/Al2O3	92.45	90.14	83.33
8	2.7%Ca-2MgO/Al2O3	88.50	82.92	73.38
9	3.0%Ca-2MgO/Al2O3	89.75	87.35	78.40
10	4.0%Ca-2MgO/Al2O3	90.17	85.42	77.02
11	0.1%La-2.3%Ca-2MgO/Al2O3	92.23	89.89	82.91
12	0.3%La-2.3%Ca-2MgO/Al2O3	94.19	90.43	85.18
13	0.5%La-2.3%Ca-2MgO/Al2O3	95.89	92.08	88.30
14	0.8%La-2.3%Ca-2MgO/Al2O3	92.53	89.10	82.44
15	1.0%La-2.3%Ca-2MgO/Al2O3	92.40	84.10	77.71

Note: 0.5%La-2.3%Ca-2MgO/Al₂O₃ indicates that the molar ratio of MgO to Al₂O₃ is 2:1; the level of La₂O₃ is 0.5 wt%; the level of CaO is 2.3 wt%.

indicates a high catalytic activity of the 20% NaOH solution. The conversion of AcH, and the selectivity and yield of 3-HBA reach 95.39%, 93.45%, and 89.14%, respectively. However, due to the large use of water and alkali solution in the reaction process, dilute H₂SO₄ or HCl had to be introduced for subsequent treatment to neutralize the system in acid and alkali, resulting in a large number of solid wastes Na₂SO₄ or NaCl, and causing great environmental pressure.

Compared with the NaOH solution, the catalytic effect of the MgO/Al₂O₃ catalyst still has a certain gap. In most reactions, the conversion of raw materials and the selectivity and yield of target products have varying degrees of decline. With the increase in MgO content in the composite oxide, the catalytic effect is significantly improved. The conversion of AcH increases from 64.13 to 92.45%, and the yield of 3-HBA increases from 55.12 to 75.42%. Meanwhile, the selectivity of 3-HBA decreases from 97.39 to 81.25%. The decrease in selectivity leads to the increase in by-products, which makes the post-processing and separation of products far more difficult.

The introduction of Ca and La has an appreciable effect on the catalytic activity of the composite oxides. With the increase in Ca and La content, the conversion of raw materials and the selectivity and yield of the product increase first and then decrease. 0.5% La-2.3% Ca-2MgO/Al₂O₃ gives the best catalytic effect and possesses a conversion of 95.89%, somewhat better than those of the traditional catalyst NaOH solution. The selectivity and yield of the product are 92.08% and 88.3%, respectively, which is similar to the catalytic effect of the NaOH solution.

From the experimental results, the introduction of La-Ca makes a significant difference on the selectivity and conversion of the target product in the AcH condensation reaction. With the addition of La-Ca, the basicity of the catalyst can be strengthened and the mild reaction can be tempered, which avoids the continuous reaction of 3-HBA as a result of local overheating and promotes the selectivity of 3-HBA. On the other hand, the application of the tubular reactor in the reaction process is realized by replacing the homogeneous catalyst with a solid catalyst. Compared with the kettle reactor using the traditional homogeneous catalyst, the mass and heat transfer of the reaction system are significantly improved, which is also proven from the increase in raw material conversion.

3.2. Characterization of Catalyst Structure

Figure 2 describes the XRD patterns of composite oxides 2MgO/Al₂O₃, 2.3%Ca-2MgO/Al₂O₃, and 0.5% La-2.3%Ca-2MgO/Al₂O₃. It can be seen that 2MgO/Al₂O₃ shows characteristic bands of Al₂O₃ at 57°. Researchers have pointed out that when the calcination temperature is low, Al₂O₃ generally exists in the form of γ-Al₂O₃, so it is speculated that Al₂O₃ here basically exists in the form of γ-Al₂O₃, and characteristic bands of MgO appear at about 50° and 74°, indicating the formation of MgO crystal [20,21,22]. When Ca is introduced, characteristic bands of CaO appear at about 38°, indicating that calcium oxide has been loaded on the surface of the material [23,24]. When La is introduced again, characteristic bands of -La₂O₃ appear at about 24° and 29° [25]. The introduction of a small amount of La could change the acidity and alkalinity of the material. Combined with the experimental results of Table 1, the modified composite oxide 0.5% La-2.3% Ca-2MgO/Al₂O₃ shows good catalytic activity in the AcH condensation reaction. The conversion of AcH reaches 95.89%. The selectivity of 3-HBA is 92.08% and the yield is 88.30%. All these data demonstrate that the catalyst performs obviously better than that before modification.

Figure 3 represents the N₂ adsorption–desorption isotherms of 2MgO/Al₂O₃, 2.3% Ca-2MgO/Al₂O₃, and 0.5% La-2.3% Ca-2MgO/Al₂O_3. It can be seen from Figure 3 that the three composites all exhibit type IV curves, showing an H1 hysteresis loop at higher pressure. It is analyzed that this phenomenon is caused by the capillary condensation of nitrogen in the pores of the composites, indicating that the prepared composites have an obvious mesoporous structure [26,27]. It can be seen from

Table 2. Structural properties of composite oxides.

Composite Oxides	MgO:Al2O3(ICP)	CaO/wt%(ICP)	La2O3/wt%(ICP)	Specific Surface Area/m2/g	Pore Volume/cm3/g	Pore Size/nm
2MgO/Al2O3	2.11:1	0	0	322.8	0.46	4.9
2.3%Ca-2MgO/Al2O3	2.17:1	2.42	0	285.3	0.45	5.2
0.5%La-2.3%Ca-2MgO/Al2O3	2.09:1	2.39	0.44	280.9	0.44	5.3

that the contents of La, Ca, Mg, and Al match the theoretical ones in most cases. Otherwise, after the introduction of Ca and La ions, the specific surface area of the composites decreases from 322.8 m²/g to about 280.9 m²/g. The pore volume changes little, being maintained at about 0.44 cm³/g. The pore size increases from 4.9 nm to 5.3 nm. According to the experimental results in Table 1, the increase in pore size is favorable to the catalytic condensation of AcH to synthesize 3-HBA. A larger pore size is conducive to the diffusion of 3-HBA from the pores of the composites, which reduces the probability of continuous reaction to 2-butenal, thus improving the selectivity of 3-HBA.

Figure 4 and Figure 5 show the CO₂-TPD and NH₃-TPD spectra of the composite oxides 2MgO/Al₂O₃, 2.3%Ca-2MgO/Al₂O₃, and 0.5%La-2.3%Ca-2MgO/Al₂O₃, respectively. The total basicity/acidity listed in

Table 3. Acid/Base properties of composite oxides.

Composite Oxides	2MgO/Al2O3	2.3%Ca-2MgO/Al2O3	0.5%La-2.3%Ca-2MgO/Al2O3
total alkali content/mmol/g	0.135	0.157	0.916
total acid content/mmol/g	0.574	0.391	0.413
T1(150~250 °C)	0.052	0.038	0.209
T2(250~320 °C)	0.357	0.287	0.031
T3(320~500 °C)	0.165	0.066	0.173

was estimated from the area below the TPD curve due to CO₂/NH₃ desorption by using calibration from a known amount of CO₂/NH₃ decomposition in helium flow (25 mL/min). The CO₂/NH₃-TPD profiles of 0.5%La-2.3%Ca-2MgO/Al₂O₃ were deconvoluted by using the Gaussian multipeak fitting function built into Origin7.5 software.

According to the study of Garbarino [28], there are at least three kinds of acidic sites on the surface of Al₂O₃, namely, weak acidic sites at 150 °C~250 °C, medium-strong acidic sites at 250~320 °C, and strong acidic sites at 320~500 °C. Comprehensive analysis of Figure 5 and Table 3 finds that the introduction of Ca ions significantly reduces the number of strong acid sites on the surface of 2MgO/Al₂O₃, while the introduction of La ions slightly increases the number of strong acid sites on the surface of 2.3%Ca-2MgO/Al₂O₃. Bao et al. studied the aldol condensation of methyl acetate with formaldehyde over a Ba-La/Al₂O₃ catalyst. They found that a decrease in surface acidity not only inhibited the formation of coke but also improved the catalytic activity [29,30,31]. The experimental data listed in Table 1 also show that the decreases in surface acidity probably has negative effects on selectivity, but it has a significant promoting effect on improving AcH conversion and 3-HBA yield.

In addition, the broad peaks can be deconvoluted into a desorption of CO₂ on the weak (peak below 300 °C), medium (peak between 300 °C and 450 °C), and strong basic sites (peak above 450 °C) [32,33]. As shown in Figure 4, on the surfaces of 0.5% La-2.3% Ca-2MgO/Al₂O₃, there exists several weakly alkaline sites corresponding to a desorption temperature below 300 °C. After the introduction of Ca ions, the number of alkaline sites increases on the surface of 2MgO/Al₂O₃. When La ions are introduced, not only do alkaline sites on the surface of 2MgO/Al₂O₃ composites increase in number, but the strength of alkaline sites is also enhanced. Combined with Figure 2 and Figure 4 and Figure 5, it is shown that the introduction of Ca and La changes the surface properties of 2MgO/Al₂O₃. After the introduction of Ca, the number of acid sites on the surface of 2.3%Ca-2MgO/Al₂O₃ decreases slightly while the total alkali contents increase significantly. When La is introduced into 2.3% Ca-2MgO/Al₂O₃, the total amount of alkali increases greatly and the total amount of acid slightly increases. The final effect is reflected in the experimental results of the AcH catalytic condensation reaction given in Table 1. The use of the 0.5% La-2.3% Ca-2MgO/Al₂O₃ catalyst improves the conversion, which shows that the suitable surface of the catalyst is beneficial to improve the conversion of raw materials.

Figure 6 elaborates the pyridine adsorption infrared characterization of composite oxides 2MgO/Al₂O₃, 2.3% Ca-2MgO/Al₂O₃, and 0.5% La-2.3% Ca-2MgO/Al₂O₃. It is used to evaluate the strength and types of acid sites of the composite oxides.

It can be seen from Figure 6 that the spectra of pyridine adsorbed on composite oxides 2MgO/Al₂O₃, 2.3% Ca-2MgO/Al₂O₃, and 0.5% La-2.3% Ca-2MgO/Al₂O₃ displaying bands around 1450 cm⁻¹, 1533 cm⁻¹, and 1630 cm⁻¹ are assigned to pyridine coordinated to Lewis acid sites [34,35,36]. In Figure 6, no characteristic bands of the interaction between the Bronsted acid site and pyridine ring are found at 1545 cm⁻¹, indicating that the surface of the prepared composite oxides is mainly a Lewis acid site [37,38]. With the introduction of Ca-La ions, it is obvious that the characteristic bands at 1533 cm⁻¹ is weakened, which reduces the number of Lewis acid sites on the surface of 2MgO/Al₂O₃. This is consistent with the results shown in Table 3 and Figure 5. This may be the result of the interaction between the oxide formed by Ca-La and the acid sites on the surface of 2MgO/Al₂O₃ or the coverage of the acid sites on the surface of 2MgO/Al₂O₃. In accordance with the experimental results of Table 1, the reduction in Lewis acid sites helps to improve the conversion of AcH, as well as the selectivity and yield of the 3-HBA.

3.3. The Effect of Process Condition on Catalyst Performances

The catalytic performance of 0.5% La-2.3% Ca-2MgO/Al₂O₃ in the catalytic condensation of AcH to 3-HBA is studied by a single-factor experiment on the catalyst activity characterization device shown in Figure 1. The effects of reaction temperature, residence time, and reaction pressure on AcH conversion, as well as selectivity and yield of 3-HBA, are investigated.

3.3.1. The Effect of Reaction Temperature on Catalyst Performances

When the feed concentration of AcH is 30%, the residence time is 70 min, and the reaction pressure is 200 kPa, the effect of temperature on the synthesis of 3-HBA by AcH condensation is investigated. The experimental results are shown in Figure 7.

It can be seen from Figure 7 that under the action of the 0.5% La-2.3% Ca-2MgO/Al₂O₃ catalyst, the conversion of AcH and the yield of 3-HBA are very low at 5 °C, indicating that most AcH does not participate in the reaction. With the increase in temperature, the conversion of AcH and the yield of 3-HBA increase remarkably, and the selectivity of 3-HBA decreases slightly. When the reaction temperature reaches 20 °C, the conversion of AcH increases to 95.89%, and the selectivity and yield of 3-HBA are 92.08% and 88.30%, respectively. The selectivity and yield of 3-HBA decrease with the increase in reaction temperature, although the conversion of AcH continues to grow, manifesting that the reaction temperature is too high. Under the action of 0.5% La-2.3% Ca-2MgO/Al₂O₃, part of the synthesized 3-HBA continues to react to form 2-butenal or tripolymerized AcH. Thereupon, although the conversion of AcH remains high, the selectivity of the target product decreases, which results in poor yield.

3.3.2. The Effect of Residence Time on Catalyst Performances

When the feed concentration of AcH is 30%, the reaction temperature is 20 °C, and the reaction pressure is 200 kPa, the effect of reaction residence time on the synthesis of 3-HBA by AcH condensation is investigated.

It can be seen from Figure 8 that under the action of the 0.5% La-2.3% Ca-2MgO/Al₂O₃ catalyst, when the residence time increases from 40 min to 120 min, the conversion of AcH increases from 83.63 to 97.33%, indicating that most AcH is involved in the reaction. The selectivity and yield of 3-HBA first increase and then decrease. When the residence time increases from 40 min to 70 min, the selectivity and yield of 3-HBA increase from 66.44% and 55.56% to 92.08% and 88.30%, respectively. With the increase in residence time, the selectivity and yield decrease to 79.07% and 76.96%, respectively. Analyzing the reason, it is found that at the reaction temperature of 20 °C, a longer residence time leads to the continuous reaction of 3-HBA. Considering the conversion of AcH as well as selectivity and yield of 3-HBA, the suitable residence time is 70 min.

3.3.3. The Effect of Reaction Pressure on Catalyst Performances

When the concentration of AcH is 30%, the reaction temperature is 20 °C, and the residence time is 70 min, the effect of reaction pressure on the synthesis of 3-HBA by AcH condensation is investigated.

It can be seen from Figure 9 that when 0.5% La-2.3% Ca-2MgO/Al₂O₃ is used as the catalyst for the AcH condensation reaction, the conversion and the selectivity are relatively low at 100 kPa. This may be due to the fact that the boiling point of AcH is 20.8 °C under this pressure condition, and the reaction temperature is 20 °C. Due to the local overheating in the liquid phase, some AcH is vaporized and then enters into the gas phase. The decrease in the concentration of AcH in the liquid phase significantly reduces the conversion and yield, which also indicates that the presence of AcH gas in the reactor has no positive effect on the catalytic reaction. The contact between reactants is mainly liquid–solid contact. Both the boiling point of AcH and the concentration of AcH in the liquid phase increase with the increase in the reaction pressure. The conversion of AcH and yield also increase noticeably with the increase in reaction pressure. When the reaction pressure continues to increase, the conversion of AcH and the selectivity of 3-HBA hardly change. In order to maintain the reaction system in a liquid phase, the reaction pressure is selected as 200 kPa.

3.4. Catalyst Lifespan and Regeneration

The lifespan of the 0.5% La-2.3% Ca-2MgO/Al₂O₃ catalyst is investigated under the conditions of 30% AcH feed concentration, 20 °C reaction temperature, 70 min residence time, and 200 kPa reaction pressure.

It can be seen from Figure 10 that the catalytic activity of 0.5% La-2.3% Ca-2MgO/Al₂O₃ is relatively high within 5 h. The conversion of AcH is above 95%, and the selectivity and yield of 3-HBA are also maintained at above 90% and 85%, respectively. With the increase in the operation time of the device, the selectivity and yield of 3-HBA decrease, but the conversion of AcH remains above 90%. This may be due to the long diffusion time of 3-HBA due to carbon deposition in the pores of the composite at this time, resulting in continuous reaction to produce 2-butenal. Some researchers also studied the causes of catalytic deactivation in a relatively low-temperature reaction. They found that the formation of carbon deposits led to the catalytic deactivation. The used catalyst contained a significant amount of carbon (14.9 wt%), while the precursor only contained 2 wt% [39,40]. In addition, the presence of impurities in the reactants or the formation of inactive metal hydroxides by by-product H₂O may probably reduce the activity of the catalyst.

With the increase in operation time, the conversion of AcH and the selectivity and yield of 3-HBA reduce significantly. It is possible that the reactants cover most of the catalyst surface with alkaline sites, preventing the diffusion of AcH to the catalyst surface, thus making some AcH dimers or even trimers. At 20 h, the yield of the product reduces to less than 30%.

Based on previous studies, the deactivated catalyst was regenerated (500 °C; 2 h) and put into the reaction system again. The results are shown in Figure 10. Compared with the fresh catalyst, the catalytic activity of the regenerated catalyst does not show a significant downward trend, and no irreversible deactivation such as structural damage and loss of active components is found. The catalyst could be burned completely at 500 °C for 2 h to recover the catalytic activity well, which could realize the continuous operation of the catalytic condensation reaction.

4. Conclusions

(1)

A series of composite oxides 2MgO/Al₂O₃ are prepared by the co-precipitation method. Ca and La ions are introduced and applied to the catalytic condensation of AcH to synthesize 3-HBA. The catalytic activity of the composite materials is significantly improved. The catalyst performance of 0.5% La-2.3% Ca-2MgO/Al₂O₃ is better than those of other composite oxides used in the study. Under the conditions of an AcH content of 30%, reaction temperature of 23 °C, reaction time of 70 min, and reaction pressure of 200 kPa, the conversion of AcH reaches 95.89%, the selectivity of 3-HBA is 92.08%, and the yield is 88.30%.

(2)

The characterization of process conditions shows that reaction temperature has a significant effect on the selectivity of the product due to the fact that the aldol condensation of AcH is an exothermic process and the transformation of AcH is unfavorable at higher temperature.

(3)

The characterization results show that after the introduction of Ca and La ions into the composite oxide 2MgO/Al₂O₃, the acid and alkaline sites on the surface are significantly improved and optimized, but the surface area of the material decreases and the pore size increases. It is beneficial to the reaction from the perspective of the catalytic condensation of AcH to 3-HBA.

(4)

The investigation on the catalytic lifespan of 0.5% La-2.3% Ca-2MgO/Al₂O₃ shows that within 5 h, the conversion of AcH and the selectivity and yield of 3-HBA are high, and the catalytic activity recovers well after regeneration. Under the same process conditions, the catalytic performance of the composite oxide is equivalent to that of the traditional liquid alkaline catalyst, and it is environmentally friendly. Therefore, it is feasible and has broad prospects in industrial application to use this catalyst for the catalytic condensation of AcH to synthesize 3-HBA.