1,4-Butanediol Selective Dehydration to 3-Butene-1-ol over Ca–Zr–Sn Composite Oxide Catalysts

Abstract

Ca–Zr–Sn composite oxides catalysts for 1,4-butanediol (BDO) selective dehydration to 3-butene-1-ol (BTO) are synthesized by impregnation and co-precipitation in the present work. The results show that Ca–Zr–Sn catalysts prepared from co-precipitation by using NaOH-Na₂CO₃ mixing alkali solution as precipitant exhibit an excellent catalytic property for BDO dehydration to BTO. For instance, Ca–Zr–Sn oxide with Ca/Zr and Sn/Zr molar ratio of 0.68 and 0.28 calcined at 650 °C gives a BDO conversion and BTO selectivity of about 97% and 82%, respectively, and exhibits no deactivation during 1000 h scale-up experimental testing. X-ray diffraction results indicate that catalytic active centers for BDO dehydration to BTO are from Ca_0.15Zr_0.85O crystal phase. NH₃- and CO₂-temperature programmed desorption prove that the surface of obtained catalysts can provide a large amount of acid and base sites simultaneously. FT-IR spectra of pyridine-adsorbed samples show that acid sites on the surface of Ca–Zr–Sn oxide catalyst mainly exist in a state of Lewis acid, which activates terminal -OH groups of BDO molecule through complexing. The activated -OH interacts with β-H activated on base sites O²⁻ anions relative to Ca species, thereby the CH₂=CH- bonds are produced through dehydration to form BTO.

1. Introduction

1,4-butanediol (BDO) containing two hydroxyl groups is an important platform chemical and has been widely applied in the production of tetrahydrofuran (THF), γ-butyrolactone (GBL), polybutylene terephthalate (PBT), polyurethane (PU), and so on [1,2,3,4]. Recently, BDO has been found to be converted to 3-butene-1-ol (BTO) through selective dehydration over metal oxide catalysts such as CeO₂ [5] and ZrO₂ [6,7,8]. BTO is a valuable intermediate due to the presence of a reactive double bond and hydroxyl group; it can be used in the synthesis of heterocyclic drugs such as multidrug resistant compounds, anti-HIV drugs, and antiproliferative agents [9,10,11,12]. Therefore, BDO catalytic selective dehydration to BTO has attracted increasing attention with regards to exploiting a high value-added downstream derivative of BDO [13,14,15,16].

Over solid acid catalysts such as SiO₂-Al₂O₃, Al₂O₃, cation exchange resin, and heteropoly acids [5,17,18], two terminal -OH groups in BDO can be activated simultaneously, and thereby the cyclo-dehydration to THF easily takes place. Therefore, some metal oxides containing acid–base sites such as rare earth oxides (REO) [19,20] and ZrO₂ [8] are selected for catalytic BDO selective dehydration of BTO. However, due to the fact that acid–base properties of these mono-component metal oxides are too poor, these catalysts give a low BDO conversion and BTO selectivity. Some researchers have introduced alkali metal Na or alkali earth metal Ca into ZrO₂ by impregnation in order to improve its catalytic properties in BDO dehydration to BTO [8,13]. It has been found that the selectivity towards BTO can be improved to over 70 mol%. For instance, CaO with a strong basicity and ZrO₂ with more base sites are simultaneously loaded on m-ZrO₂ as a support by using impregnation; CaO-ZrO₂ catalyst with acid–base sites generated by Ca-O-Zr hetero-linkage has been obtained after calcination at above 800 °C. Over the obtained catalyst, the BTO yield in BDO dehydration reaction reaches up to about 83% [3]. It has been thought that the acid sites interact with the terminal hydroxyl groups of BDO, while the basic sites activate the β-H during reaction process.

In the present work, ZrO₂ and CaO are selected to act as important active species providing acid and base centers, respectively. Sn is introduced as the promoter to synthesize Ca–Zr–Sn composite oxides with an excellent acid–base property by using co-precipitation. Meanwhile, the effect of the introduced Ca and Sn amount on the base–base and catalytic property of Ca–Zr–Sn composite oxides in BDO selective dehydration to BTO are also investigated. In addition, combined with XRD, N₂-adsorption/desorption, NH₃-TPD and CO₂-TPD results, the in-situ DRIFTS and pyridine adsorption IR techniques are introduced in order to explore the dehydration mechanism of BDO to BTO.

2. Results and Discussion

2.1. Selection of Preparation Method

BDO catalytic dehydration can produce two possible products including THF and BTO through cyclo-dehydration and loss of terminal hydroxyl group (see Scheme 1), respectively. From BDO catalytic dehydration of four obtained catalysts (Figure 1), it is found that BDO conversion and BTO selectivity over both Ca_1.2ZrSn_0.4-OH and Ca_1.2ZrSn_0.4-M give higher values than those over Ca_1.2Sn_0.4/ZrO₂, showing that Ca–Zr–Sn composite metal oxides from co-precipitation exhibit more excellent catalytic property in BDO selective dehydration to BTO than that obtained by impregnation. Simultaneously, Ca–Zr–Sn composite oxide obtained by using NaOH-Na₂CO₃ as a precipitant is also discovered to exhibit higher BDO conversion and BTO yield than that prepared by NaOH precipitation. Therefore, co-precipitation by using NaOH-Na₂CO₃ mixing alkali solution as a precipitant should be the best method for preparing Ca–Zr–Sn composite oxide with an excellent catalytic property for BDO selective dehydration to BTO. Meanwhile, the composition of metallic elements in the Ca–Zr–Sn composite oxide catalyst is also discovered to play an important role on its catalytic property for BTO production.

XRD pattern of catalysts (Figure 2) from three methods show that the supported catalyst Ca_1.2Sn_0.4/ZrO₂ gives obvious characteristic diffraction peaks attributed to monoclinic ZrO₂ [13] phase (pdf No. 74-1200) at 2θ of 17.6°, 24.2°, 28.4°, 34.4°, 41.1°, 49.5°, and 50.4°, respectively. Meanwhile, some weak diffraction peaks attributed to perovskite compounds such as CaZrO₃ (pdf No. 35-790) and CaSnO₃ (pdf No. 24-1074) at 2θ of 22.5°, 31.7°, 45.3°, and 55.6°, and even Ca_0.15Zr_0.85O (pdf No.: 26-341) species at 2θ of 30.3° and 65.9° are also discovered, indicating that the introduced Ca species can react with support ZrO₂ to form CaZrO₃ and Ca_0.15Zr_0.85O species, and also interacts with supported Sn to CaSnO₃ during catalyst calcination after impregnation. For the Ca_1.2ZrSn_0.4-OH sample made from co-precipitation using NaOH as the precipitating agent, no diffraction peaks for CaSnO₃ and ZrO₂ phase are detected. However, intensities of those peaks attributed to the Ca_0.15Zr_0.85O crystal become stronger, along with some weak CaZrO₃ diffraction signals. Meanwhile, Ca_1.2ZrSn_0.4-M prepared by using NaOH-Na₂CO₃ mixing alkali solution as precipitant gives some diffraction peaks attributed to CaCO₃ phase (pdf No. 85-1108) at 2θ of 23.1°, 29.5°, 36.0°, 39.6°, 43.3°, 47.7°, and 48.7°, respectively. These illustrate that the formed hydroxides of Ca, Zr, and Sn in catalyst precursors prepared by using NaOH or NaOH-Na₂CO₃ mixing alkali precipitant can interact to form Ca_0.15Zr_0.85O and CaZrO₃ phase during calcination at 650 °C in air. However, the CaCO₃ formed from mixing alkali precipitant is difficultly decomposed.

Their catalytic properties in BDO dehydration to BTO (Figure 1) indicate that both BDO conversion and BTO selectivity are generally found to increase as the following catalyst order: Ca_1.2Sn_0.4/ZrO₂ < Ca_1.2ZrSn_0.4-OH < Ca_1.2ZrSn_0.4-M. Combined with XRD results, it is not difficult to deduce that the main catalytic active phase should be Ca_0.15Zr_0.85O for BDO selective dehydration to BTO. Meanwhile, the Ca_0.8ZrSn_0.4-M sample is found in Figure 1 to exhibit higher BDO conversion and BTO selectivity than Ca_1.2ZrSn_0.4-M, showing the content of Ca in Ca–Zr–Sn composite metal oxide catalyst plays an important role on BDO selective dehydration. As is well known, during catalysts preparation, Ca ions can convert hydroxides and carbonates of calcium when NaOH-Na₂CO₃ mixed alkali solution is used as a precipitant. Two precursors will interact with Zr species to the Ca_0.15Zr_0.85O crystal phase after calcination. From Figure 2, it is found that Ca_1.2ZrSn_0.4-M consists of CaCO₃ besides Ca_0.15Zr_0.85O, indicating that the formed CaCO₃ from excess Ca during precipitation is stable, and difficultly decomposed or converted to Ca–Zr composite oxides. This will exhibit a negative effect on the distribution of acid–base sites, resulting in a low catalytic activity in BDO selective dehydration. Therefore, the two key factors for the formation of the Ca_0.15Zr_0.85O crystal phase including calcination temperature and Ca/Zr molar ratio should be further investigated in the following work.

2.2. Calcination Temperature

From XRD patterns (Figure 3) of Ca_0.8ZrSn_0.4-M calcined at different calcination temperatures, it can be clearly found that the sample calcined at 450 °C has no obvious diffraction signals, indicating that the formed CaCO₃ and metal composite oxides of Ca, Zr, and Sn mainly exist in the amorphous phase state. When the calcination temperature reaches 650 °C, catalytic active phase Ca_0.15Zr_0.85O crystal species are formed along with CaCO₃. With further increase in calcination temperature, the diffraction peaks characteristic of Ca_0.15Zr_0.85O become weaker, and those attributed to CaCO₃ disappear. In contrast, diffraction signals characteristic of CaZrO₃ or CaSnO₃ become stronger. It shows that calcination temperatures that are too high easily lead to CaCO₃ decomposition and Ca_0.15Zr_0.85O crystal conversion to perovskite CaZrO₃. Meanwhile, the interaction of metal oxides including Ca and Sn to perovskite CaSnO₃ also takes place under a high calcination temperature.

From the catalytic properties of Ca_0.8ZrSn_0.4-M calcined at different temperature (Figure 4), BDO is mainly converted to THF over Ca_0.8ZrSn_0.4-M catalyst calcined at 450 °C. It indicates that cyclodehydration mainly occurs through two terminal -OH groups in BDO over amorphous metallic oxides phase of Ca–Zr–Sn catalysts. When the calcination temperature is increased to 650 °C, the obtained Ca–Zr–Sn composite oxide sample exhibits a good catalytic property in BDO selective dehydration with an increasing BDO conversion and BTO selectivity, and the formation of THF is drastically inhibited. This is perhaps because the right calcination temperature leads to the formation of more Ca_0.15Zr_0.85O crystal phase, which can provide a proper amount of acid and base sites, simultaneously. The activation of terminal -OH group and its adjacent β-H of BDO easily occurs on acid and base sites provided by the Ca_0.15Zr_0.85O crystal phase, respectively. This can result in the formation of C=C through selective dehydration [3]. Moreover, the presence of acid–base sites weakens the simultaneous activation of two terminal -OH groups in the BDO molecule, inhibiting the cyclo-dehydration reaction to THF. Calcination temperatures that are too high give rise to the conversion of Ca_0.15Zr_0.85O crystal to CaZrO₃, which perhaps only provides acid centers, along with a disappearance of base sites [21]. The simultaneous activation of two terminal -OH groups in BDO molecules over acid sites easily take places, resulting in an increase in THF selectivity through cyclo-dehydration reaction.

2.3. Ca Function

Figure 5 depicts XRD patterns of Ca_xZrSn_0.4-M with different Ca/Zr molar ratio prepared from co-precipitation by using Na₂CO₃-NaOH mixing alkali as a precipitant and calcined at 650 °C. It can be clearly discovered that Ca–Zr–Sn composite metal oxide catalysts only contain the Ca_0.15Zr_0.85O crystal phase when the Ca/Zr molar ratio is below 0.52. With the further increase in Ca content, a new crystal phase of CaCO₃ comes forth in samples. When the Ca/Zr molar ratio exceeds 1.2, the formation of perovskite CaZrO₃ besides CaCO₃ is found, perhaps due to the interaction between redundant Ca species and Zr oxide.

The catalytic properties (Figure 6) of Ca_xZrSn_0.4-M catalysts in BDO dehydration clearly indicate that both BDO conversion and BTO selectivity increase, and selectivity of by-products including THF, γ-butyrolactone, 1-butanol, 2-buten-1-ol, 1,3-butadiene, and so on decrease with an increasing Ca content. When the Ca/Zr molar ratio reaches up to 0.68, the obtained Ca–Zr–Sn composite metal oxide catalyst exhibits the highest BDO conversion and BTO selectivity, with values of about 91.7 mol% and 78.9 mol%, respectively. Meanwhile, it also gives the lowest selectivity towards by-products. However, further increase in the Ca content in the metal oxide catalyst makes BDO conversion and BTO selectivity decrease, and gives an increase in selectivity towards by-products.

From the N₂ adsorption/desorption results (see

Table 1. Textural and acid–base properties of Ca_xZrSn_0.4-M catalysts calcined at 650 °C.

Ca/Zr Molar Ratio	SBET (m2·g−1)	VBJH (cm3·g−1)	Dpore (nm)	Acid Amount (μmol·g−1) a			Base Amount (μmol·g−1) a
				Weak	Medium	Strong	Weak	Medium	Strong
0.4	3.77	0.07	23.51	187.2	-	-	294.3	-	-
0.52	11.13	0.11	20.16	210.5	-	-	313.2	-	-
0.6	29.73	0.25	16.85	321.3	238.5	-	342.1	284.4	-
0.68	49.23	0.26	10.21	845.3	934.7	253.5	367.8	224.9	1718.3
0.8	57.24	0.32	9.44	795.6	999.5	163.1	381.2	325.7	4306.6
1.2	69.24	0.28	7.97	58.6	210.8	357.7	923.2	203.3	5040.6

^a The amounts of acid and base sites on catalysts are derived from NH₃-TPD and CO₂-TPD analysis, which are expressed as the molar number of NH₃ or CO₂ molecules adsorbed per gram of catalyst.

), the presence of Ca is found to make Ca–Zr–Sn composite oxides exhibit a bigger surface area and pore volume, which can improve the contact chance of reactant BDO with catalytic sites over catalysts. CO₂-TPD results (Figure S1 in Supplementary Materials (SI)) show that the obtained Ca–Zr–Sn composite oxides mainly give three CO₂-desorption peaks at about 78 °C, 490 °C, and 610 °C, which are attributed to weak, medium-strong, and strong base sites, respectively. From Table 1 and Figure S1, it is clearly found that the amount of weak base sites over Ca–Zr–Sn composite oxide catalysts increases with the increasing Ca content. When the Ca/Zr molar ratio is below 0.52, the obtained metal oxide catalysts do not provide medium–strong and strong base sites, which are formed and increase in number with the increasing Ca content when Ca/Zr > 0.6. From XRD results (Figure 5), when Ca/Zr < 0.6, Ca–Zr–Sn composite oxides (Ca_xZrSn_0.4-M) only contain a Ca_0.15Zr_0.85O crystal phase with a Ca/Zr molar ratio of about 0.176. This indicates that the redundant Ca species perhaps incorporate into Sn oxides to form amorphous CaSnO₃. Both Ca_0.15Zr_0.85O crystals and amorphous CaSnO₃ species only provide weak base sites. When Ca content further increases, amorphous CaO species, which can provide medium–strong and strong base sites, are produced besides Ca_0.15Zr_0.85O crystal and amorphous CaSnO₃ over the surface of catalysts. The presence of more Ca species is helpful to the formation of perovskite CaZrO₃ and also produces undecomposable CaCO₃ during co-precipitation. Therefore, it can be concluded that Ca_0.15Zr_0.85O crystal mainly provides weak base sites, and the coexistence of Ca_0.15Zr_0.85O crystal and amorphous CaO species is helpful to the formation of more medium–strong and strong base sites.

NH₃-TPD results (Figure S2 in SI) of Ca_xZrSn_0.4-M with a different Ca content are found to also exhibit a similar trend to CO₂-TPD results. It shows that catalytic active species Ca_0.15Zr_0.85O crystal phase mainly provide weak acid sites. The introduction of more Ca can result in the distortion of the Ca_0.15Zr_0.85O crystal phase, which will provide more and more medium–strong and strong acid sites. The simultaneous presence of acid and base sites with medium–strong and strong intensity makes Ca–Zr–Sn composite oxides exhibit an excellent catalytic property towards BDO selective dehydration to BTO. It indicates that BDO selective dehydration to BTO is achieved through acid–base catalysis.

2.4. Sn Function

From XRD patterns (Figure 7) of Ca_0.68ZrSn_x-M metal oxides with a different Sn content calcined at 650 °C, the samples are clearly observed to mainly contain the CaCO₃ and Ca_0.15Zr_0.85O phase. However, no obvious diffraction signals assigned to Sn oxides, CaZrO₃, Zr-Sn, or Ca-Sn composite oxides are detected. With the increase in Sn content, the intensity of diffraction signals attributed to CaCO₃ is weakened, and that of those assigned to Ca_0.15Zr_0.85O crystal phase is gradually increased. This is mainly due to the introduced Sn species being converted to amorphous CaSnO₃ through the interaction with Ca [22]. The presence of Sn can also be concluded to promote the formation of Ca_0.15Zr_0.85O and inhibit the further conversion of Ca_0.15Zr_0.85O to CaZrO₃.

The catalytic properties of Ca_0.68ZrSn_x-M with a different Sn content in BDO dehydration (Figure 8) show that both BDO conversion and BTO selectivity first increase and then decrease with an increasing Sn amount, and reach up to the highest values when the Sn/Zr molar ratio is 0.28. According to the above XRD results, the presence of proper Sn leads to the formation of catalytic active sites Ca_0.15Zr_0.85O, and further promotes the Ca_0.15Zr_0.85O crystal phase to provide proper amounts of acid–base sites. From CO₂-TPD and NH₃-TPD results, the increase in the introduced Sn amount into catalysts can greatly increase the amounts of acid–base sites, especially medium–strong and strong acid–base sites over the catalyst surface (see

Table 2. Textural and acid–base properties of Ca_0.68ZrSn_x-M catalysts calcined at 650 °C.

Sn/Zr Molar Ratio	SBET (m2·g−1)	VBJH (cm3·g−1)	Dpore (nm)	Acid Amount (μmol·g−1) a			Basic Amount (μmol·g−1) a
				Weak	Medium	Strong	Weak	Medium	Strong
0.12	24.27	0.21	15.68	354.3	163.5	-	298.4	-	39.8
0.2	49.23	0.25	11.22	1213.4	795.5	-	334.2	-	808.9
0.28	49.59	0.28	10.91	1352.3	813.3	318.8	310.6	120.5	1266.9
0.4	56.07	0.31	10.21	1398.3	934.7	323.5	273.5	124.9	1718.3

^a. The amounts of acid and base sites on catalysts are derived from NH₃-TPD and CO₂-TPD analysis, which are expressed as the molar number of NH₃ or CO₂ molecules adsorbed per gram of catalyst.

). It further shows that the medium–strong and strong acid–base sites provided by the Ca_0.15Zr_0.85O phase act as the catalytic sites for BDO dehydration. From the NH₃-TPD curves (Figure S4 in Supplementary Materials) of Ca_0.68ZrSn_x-M with different Sn amounts, it is clear that the introduction of Sn species makes the NH₃-desorption peaks provided by medium–strong and strong acid sites exhibit stronger signals and shift to a higher temperature, indicating that its increase in acidity.

Combined with catalytic results, BDO dehydration is further concluded to mainly occur on the acid–base sites. The terminal -OH groups in BDO molecules are easily activated by medium–strong and strong acid sites. Meanwhile, the activation of H located on the β-CH₂ group easily carries out on medium–strong and strong base sites. These are helpful to BDO selective dehydration to BTO. However, catalytic active sites with an acidity that is too strong can simultaneously activate two terminal -OH groups of BDO, leading to the increase in cyclo-dehydration to THF. Moreover, the obtained Ca_0.68ZrSn_0.28-M is also found from Figure 9 to exhibit a good catalytic stability with no deactivation in BDO selective to BTO after the 1000 h catalytic life scale-up test. BDO conversion and BTO selectivity can keep about 92% and 82%, respectively.

2.5. Mechanism of BDO Dehydration to BTO

FT-IR spectra of adsorbed pyridine on Ca–Zr–Sn composite oxides with different Ca and Sn contents (Figure 10a,b) only feature the adsorption band at about 1440 cm⁻¹, which is assigned to the contribution of the Lewis acid sites over the catalyst surface [23,24,25]. Meanwhile, it is found that the intensity of the adsorption band decreases with the increasing Ca or Sn content. When the content of Ca or Sn is low, the obtained catalysts mainly contain amorphous Zr oxides, producing more Lewis acid sites. The introduced Ca species can interact with amorphous Zr oxides to form the Ca_0.15Zr_0.85O crystal phase (see above XRD results), which can also be improved by the introduction of Sn species. The produced Lewis acid sites easily activate the terminal -OH groups through complexing with O atom located in -OH of BDO. Moreover, the presence of Ca elements in the Ca_0.15Zr_0.85O crystal phase can produce base sites composing of surface O²⁻ anions [26,27], which can activate H located in β-CH₂ of BDO. The activated terminal -OH groups over acid sites interact with β-H activated by base sites to form BTO, as ascribed in Scheme 2.

From DRIFTS of BDO adsorbed on the Ca_0.68ZrSn_0.28-M catalyst, two strong adsorption bands at about 3692 and 3702 cm⁻¹ (see Figure 11a) are observed for samples after adsorbing BDO at 40 °C. They are assigned to the stretching vibration of intermediate species from the -OH groups of BDO activated on Lewis acid sites of the catalyst surface [16,28,29,30]. When adsorption temperature is increased to 300 or 380 °C, the intensity of both adsorption bands weakens. This is perhaps due to the occurrence of dehydration to BTO through the interaction between -OH and adjacent β-H at a high temperature. There are some weak adsorption bands at 3734, 3739, and 3750 cm⁻¹, respectively. These can be assigned to the stretching vibration of the isolated -OH groups of the formed BTO [28,29]. Moreover, when adsorption temperature reaches up to above 300 °C, two adsorption bands at about 2852 and 2920 cm⁻¹ (Figure 11b) assigned to the C–H asymmetrical and symmetrical stretching vibration of α-CH₂ and β-CH₂ of adsorbed BDO [16] disappear, and new weak bands appear, including those attributed to the C-H asymmetrical and symmetrical stretching vibration of -CH=CH₂ (3006 and 3029 cm⁻¹ in Figure 11b) and -CH₂- (2870 cm⁻¹ in Figure 11b) of the formed BTO [16]. More importantly, an obvious positive adsorption band also appears and becomes stronger at 873 cm⁻¹ (Figure 11c) with an increasing adsorption temperature, which is assigned to the rocking vibration H-C located on -HC=CH₂ olefinic bond surface [30]. All evidences indicates that BDO selective dehydration takes place through the interaction of terminal -OH groups and their adjacent β-H to olefinic compounds.

3. Materials and Methods

3.1. Catalysts Preparation

Ca–Zr–Sn composite oxide catalysts were prepared by using co-precipitation in the present work. All used chemical regents including calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O), basic zirconium carbonate (Zr(OH)₂CO₃), tin (IV), and chloride pentahydrate (SnCl₄·5H₂O) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd., Shanghai, China. Basic zirconium carbonate was first converted to zirconium nitrate salt by using 63% concentrated nitric acid and diluted by an amount of distilled water. Ca(NO₃)₂·4H₂O and SnCl₄·5H₂O were then dissolved successively in the zirconium salt solution obtained above. The total concentration of metallic ions was controlled to 1 mol·L⁻¹. Under drastic stirring, the metallic ions solution and mixed alkali solution containing 0.2 mol·L⁻¹ Na₂CO₃ and 1 mol·L⁻¹ NaOH as precipitant were simultaneously added dropwise into a 5 L reactor containing 200 mL water for co-precipitation. The temperature and pH value of the co-precipitation slurry solution were controlled to 70 °C and 8.5, respectively. After finishing, the slurry solution was aged at 70 °C for 12 h and then filtered and washed with distilled water. The obtained filtered cash was dried at 60 °C and calcined at a desired temperature in air for 6 h. The obtained Ca–Zr–Sn composite oxide catalysts were denoted as Ca_xZrSn_y-M, wherein x and y represented the molar ratio of Ca and Sn relative to Zr, respectively. For comparison, another Ca–Zr–Sn composite oxide denoted as Ca_1.2ZrSn_0.4-OH was prepared by using 1 mol·L⁻¹ NaOH aqueous solution as precipitant according to the same method. Additionally, the supported catalyst Ca_1.2-Sn_0.4/ZrO₂ was prepared by incipient wetness impregnation as following: 50 g ZrO₂ are impregnated by using about 60 mL aqueous solution containing 0.488 mol Ca(NO₃)₂ and 0.163 mol SnCl₄. The obtained sample was dried at 110 °C and then calcined at 650 °C for 6 h in air. The support ZrO₂ was obtained from the decomposition of basic zirconium carbonate through calcination at 500 °C in air.

3.2. Catalysts Characterization

N₂-adsorption/desorption of catalysts were carried out on an SSA-4200 aperture analysis device (Beijing Builder Instrument Co. Ltd., Beijing, China) at −196 °C. The specific surface area of the sample was calculated by the BET method according to N₂-adsorption isotherm. Pore volume and average pore diameter were calculated by BJH method according to desorption isotherm. X-ray diffraction patterns (XRD) were recorded on a DX-2700BH (Dandong Haoyuan Instrument Co., Ltd., Dandong, China) using Cu Kα radiation (λ = 0.154056 nm) to detect the crystal structure of the samples. The tube voltage and current were 40 kV and 30 mA, respectively.

Temperature-programmed desorption of NH₃ and CO₂ profiles were collected on a Builder PCA-1200 chemisorption apparatus (Beijing Builder Instrument Co. Ltd., Beijing, China) and were measured to estimate the acidity and basicity of the catalysts, respectively. 100 mg sample was preheated at 650 °C for 1 h in a 30 mL·min⁻¹ He flow. During adsorption, the adsorption gas used was 30 mL·min⁻¹ 5%NH₃-95%N₂ mixed gas and pure CO₂ for NH₃ and CO₂-TPD, respectively. The adsorption temperature and time were room temperature and 0.5 h, respectively. For desorption, the sample was heated from room temperature to 650 °C at a heating rate of 10 °C·min⁻¹ under a 30 mL·min⁻¹ He flow. The desorbed molecules were monitored by a thermal conductivity detector (TCD).

In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of catalysts were performed on a Nicolet iS50 spectrometer (Thermo Scientific, Waltham, MA, USA) equipped with a diffuse IR reflectance accessory, which consists of a heat chamber, DRIFT cell with KBr window, and an MCT/A detector. All spectra were collected in Kubelka-Munk units and were averaged over 32 scans at 4 cm⁻¹ resolution at a desired temperature. In a typical process, a sample pellet was placed in a DRIFTS cell, heated to 500 °C in a 30 mL·min⁻¹ Ar flow, and kept for 1 h. Then, the temperature was controlled to a desired value under Ar flow. After stabilization, the background spectrum was collected. BDO was introduced by bubbling with a 30 mL·min⁻¹ Ar flow into sample cell, and the spectra were recorded every 30 s at the corresponding temperature, simultaneously. The FT-IR spectra of pyridine-adsorbed catalysts was collected according to the similar method at room temperature.

3.3. Catalytic Test

BDO dehydration was carried out in a fixed-bed continuous reactor under atmospheric pressure with a 60 mL·min⁻¹ N₂ as balance gas at 380 °C and BDO LHSV 2 h⁻¹. For a typical procedure, 15 g catalysts were sandwiched by quartz sand in a stainless tube reactor (i.d. 2 cm) and pretreated for 1 h at 380 °C in a 60 mL·min⁻¹ N₂ flow. The introduction of BDO with a liquid flow of 30 mL·min⁻¹ was through pumping by a HPLC pump. The reacted mixture was collected by cooling through a water condenser. For the scale-up experimental, catalyst weight, BDO liquid flow, and balance gas N₂ flow were 300 g, 600 mL·h⁻¹, and 600 mL·min⁻¹, respectively. The un-converted BDO, target product BTO, by-products including tetrahydrofuran THF, and others in the collected mixture were analyzed by gas chromatography (GC) equipped with a DB-5 capillary column (30 m × 0.32 mm × 0.5 μm) and flame ionization detector (FID). BDO conversion, selectivity towards target product BTO, and by-products were calculated according to GC analysis results of the reaction mixture.

4. Conclusions

Co-precipitation by using NaOH-Na₂CO₃ mixing alkali solution as precipitant is the best methods for preparation of Ca–Zr–Sn composite oxide catalysts for BDO selective dehydration to BTO. The Ca_0.15Zr_0.85O crystal phase Ca–Zr–Sn composite oxide is proved to be a catalytic active species for BTO production. The introduction of Sn is helpful to the formation of the Ca_0.15Zr_0.85O crystal phase from the solid reaction between amorphous CaCO₃ and Zr hydroxides during calcination. Overall, 650 °C is an optimum calcination temperature for the formation of Ca_0.15Zr_0.85O, which can be further converted to CaZrO₃ at a higher temperature. The catalytic active species Ca_0.15Zr_0.85O can simultaneous provide acid and base sites, which activate the terminal -OH and its adjacent β-H of BDO molecules, respectively. In situ IR results show that the activation of terminal -OH group mainly occurs on the Lewis acid sites. β-H activation takes place on the base sites composing of O²⁻ anions provided by Ca species.