High Catalytic Performance and Sustainability of Zr Modified Aluminophosphate for Vapor-Phase Selective O-Methylation of Catechol with Methanol

Abstract

Guaiacol produced by O-methylation of catechol with methanol over solid catalysts is a green environmental synthesis route. In order to achieve high catalytic efficiency, it is quite necessary to employ low-cost catalysts with high activity and stability. Herein, series of aluminophosphate catalysts were synthesized by a simple precipitation route modified by Zr. The characterization results indicated that the prepared Al-P-Zr catalysts possessed appropriate weak acid and weak base sites, which were beneficial for the O-methylation of catechol with methanol. Different Zr amount and calcined temperature exerted a significant influence on physicochemical properties of the catalysts and catalytic performance. The Al-P-Zr catalysts containing Zr/Al molar ratio of 0.012 calcined under 400 °C showed the optimal catalytic activity and long-term stability for vapor-phase selective O-methylation of catechol with methanol.

1. Introduction

Guaiacol, also known as 2-Methoxyphenol, is an essential fine chemical intermediate, which is widely used in the synthesis of medicine, spices and dyes [1,2,3]. Due to the increasing attention of green chemistry and sustainable development, lots of endeavors are applied to exploit flexible catalytic process to synthesize guaiacol with eco-friendly reductant taking the place of poisonous and harmful agents [4,5,6,7,8,9,10]. Among the reported synthetic routes, vapor-phase O-methylation of catechol with methanol to produce guaiacol is receiving increasing attention because it is an economically and environmentally friendly route for industrial applications [6,7,8,9,10]. Except for the main product guaiacol, o-dimethyl ether is the main byproduct, and many further alkylation products are generated. The distribution of products (O-/C-methylation ratio) is mainly dependent on reaction parameters and physicochemical properties of the catalysts [7]. As a micro exothermic reaction, local overheating in the reaction process incline to produce carbon deposition and high carbon hydrocarbons [9]. Therefore, it is still a great challenge to develop a highly active and selective catalyst for vapor-phase O-methylation of catechol.

Several heterogeneous catalysts have been reported to be active in vapor-phase O-methylation of catechol and methanol, including pure and doped metal oxides [6,7,8], supported tungsten compound [9] and metal phosphates [10,11,12,13,14,15,16], etc. Amorphous aluminophosphates, which possess both acid and base sites, are considered effective catalysts for vapor-phase O-methylation of catechol with methanol [12,13,14,15,16]. The physicochemical properties of aluminophosphate have been reported to depend on modifying catalyst with various metals, optimizing catalyst compositions, and improving catalyst preparation routes [16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]. Generally, aluminophosphates with specific structure are usually synthesized with various surfactant templates. However, it is difficult to adjust the acidic–basic property for avoiding the structure destroyed [27,28,29,30,31]. Many synthetic methods, such as coated onto scaled zeolite [11], precipitation [12,13,14], citric acid route [15,16], etc., were widely used in preparation of aluminophosphates for vapor-phase O-methylation of catechol with methanol [18]. Nevertheless, most synthetic methods usually employ refluxed processing, additional ligand, and washing with distilled hot water, which leads to complex and/or high-cost preparation processes. Hence, it is still important to develop a simple approach to fabricate aluminophosphates with high activity and selectivity for vapor-phase O-methylation of catechol with methanol. It is commonly acknowledged that various metals, especially transition metal, introduced to aluminophosphates could alter the acid–base property of aluminophosphates [17,18,19,20,21,22,23,24,25,26].

In this paper, zirconium-aluminophosphates were prepared through a co-precipitation method with mixed metal nitrates with ammonium hydroxide. The effects of Zr content and calcined temperature on catalysts’ physical and chemical properties were systematically investigated by XRD, N₂ adsorption, TPD, and IR. Their catalytic behavior for vapor-phase O-methylation of catechol with methanol was intensively investigated to design an effective catalyst for O-methylation.

2. Results and Discussion

2.1. Physicochemical Properties of AlP_1.1Zr_x-T

XRD patterns of the prepared samples are shown in Figure 1. The AlP_1.1-400 sample shows four weak diffraction peaks around 2θ = 20.4°, 21.7°, 23.0°, 35.7°, corresponding to the reflections for AlPO₄ phase. Upon addition of Zr to AlP_1.1-400, the diffraction intensities of AlPO₄ phase disappear. Only a very broad band in the 2θrange of 15–30° corresponding to amorphous AlPO₄ [18,19,27] is observed on the AlP_1.1Zr_x-400 samples. These results demonstrate that the addition of Zr significantly promote the formation of smaller crystalline AlPO₄ particles. No characteristic diffraction peaks are observed associated with Zr species, likely due to highly dispersed Zr species with very small particle sizes below the detection limit of XRD measurements. As the calcined temperature increase from 350 °C to 500 °C, the XRD patterns of the samples in Figure 1b are not remarkably altered, revealing that the effect of calcination temperature on crystalline structure is negligible within the examined temperature range.

Figure 2 depicts the IR spectra of the prepared samples. The bands at 1134 and 495 cm⁻¹ can be attributed to triply degenerate P-O stretching vibration and triply degenerate O-P-O bending vibration of tetrahedral PO₄³⁻, respectively. The band at 726 cm⁻¹, assigned to the stretching vibration of Al-O in combination with P-O, begins to be weaken with the increasing Zr contents. This may be associated with the amorphous structure of AlPO₄ [17,18,27,31]. These results further indicate the structure of AlPO₄ of AlP_1.1Zr_x-T samples.

The basic physical properties of the AlP_1.1Zr_x-T samples are summarized in

Table 1. Physical properties of AlP_1.1Zr_x-T catalysts.

Catalysts	SBET (m2 g−1)	Vp (cm3·g−1)	Da (nm)
AlP1.1-400	126	0.83	23.5
AlP1.1Zr0.006-400	141	0.50	19.0
AlP1.1Zr0.009-400	163	0.84	17.2
AlP1.1Zr0.012-400	183	0.83	18.8
AlP1.1Zr0.018-400	195	0.68	15.0
AlP1.1Zr0.012-350	183	0.83	19.1
AlP1.1Zr0.012-450	181	0.82	18.8
AlP1.1Zr0.012-500	180	0.82	18.6

. The AlP_1.1-400 had a specific surface area of 126 m²/g, a pore volume of 0.83 cm³·g⁻¹, and a pore size of 23.5 nm. The addition of Zr resulted in an increase of specific surface area and a decrease of pore size. There is a decreasing trend of the pore volume. However, the calcined temperature has no distinct difference on specific surface areas, pore volume, and BJH pore sizes. These results indicate that a certain amount of Zr can improve the S_BET of AlP_1.1 samples, likely due to the variation in the interaction between Al and P by Zr addition.

NH₃-TPD profiles are displayed in Figure 3. In terms of the AlP_1.1-400 sample, the NH₃-TPD profile illustrates two broad NH₃ desorption peaks centered at ca. 140 °C and 175 °C respectively, which are ascribed to two different weak acid centers [15]. In comparison with the AlP_1.1-400, the AlP_1.1Zr_x-400 samples show a dominant NH₃ desorption peak in the range of 80–250 °C, corresponding to unitary weak acid center [13]. The desorption peaks shift to higher NH₃ desorption temperature with the increasing Zr content, indicating that the addition of Zr strengthens the acidity of the catalysts. However, the area of the NH₃ desorption peak has no obvious change, likely due to adding a small quantity of Zr. These results indicate that the incorporation of Zr into the aluminum phosphate could alter the acidic strength without influencing the acidic amount of the samples. As for AlP_1.1Zr_0.012-T samples, the positions of desorption temperature shift to lower temperatures and the area of the desorption peaks in the range of 80–250 °C decrease with the elevating calcined temperature, meaning that increasing calcined temperature can decrease both the acidic strength and the acidic amount of the samples.

Figure 4 presents the Py-IR spectra of the AlP_1.1Zr_x-400 and AlP_1.1Zr_0.012-T samples. All the tested samples exhibit a weak band and two strong bands at 1542 cm⁻¹, 1492 cm⁻¹, and 1448 cm⁻¹, respectively. The weak band at 1542 cm⁻¹ is attributed to the presence of trace amounts of Brönsted acidic sites. The strong band at 1448 cm⁻¹ resulted from a great number of Lewis acidic sites. Notably, the strong absorbance at 1492 cm⁻¹ is the co-adsorption of Brönsted and Lewis acidic sites [15,24,25]. By comparing with the NH₃-TPD results, we can deduce that all the prepared catalysts mainly possess weak Lewis acidic sites. Neither adding Zr to AlP_1.1-400 nor altering calcined temperature change the type of acid center.

CO₂-TPD was used to investigate the basic property of the prepared catalysts. Figure 5 displays the CO₂-TPD patterns of AlP_1.1Zr_x-T samples. The AlP_1.1-400 sample give a broad CO₂ desorption temperature peak centered at 170 °C, suggesting the presence of weak basic sites [15]. When a certain amount of Zr are added to AlP_1.1-400, the AlP_1.1Zr_0.006-400 sample displayed two broad CO₂ desorption peaks centered at 130 °C and 177 °C, respectively, with an appreciable decrease of the CO₂ desorption peak area. With increasing Zr content to x of 0.009, the CO₂ desorption peak centered 130 °C shift to a slightly lower temperature and the CO₂ desorption peak centered 177 °C shift to a higher temperature of ca. 210 °C. Meanwhile, the CO₂ desorption peak area decreased slightly. With further addition of Zr, no significant change occurs in the position and area of CO₂ desorption temperature on AlP_1.1Zr_0.012-400 and AlP_1.1Zr_0.018-400. The above results indicate that Zr addition increases the kinds of weak basic sites, alters the strength of basic sites, and decreases the number of base centers. With regard to the influence of calcination temperature on basic property, the patterns of CO₂-TPD is shown in Figure 5b. With the elevation of calcination temperature, the CO₂ desorption peak centered at 130 °C shifts to higher CO₂ desorption temperature and the CO₂ desorption peak area decrease slightly, indicating that increasing calcined temperature can decrease the basic sites but increase the strength of basic sites of the samples.

2.2. Catalysis Reaction

The catalytic performance of the prepared catalysts was investigated for vapor-phase selective O-methylation of catechol with methanol under optimized reactions. The effect of Zr content on catalytic performance is first investigated and the results are displayed in Figure 6. It can be seen that all the AlP_1.1Zr_x-400 catalysts exhibit high initial activities for the O-methylation of catechol. However, the catalyst stability obviously increases with increasing the Zr content in the range of 0.006–0.012. For example, the catechol conversion of AlP_1.1Zr_0.006-400 sample decline from 83% to 60% within the 12 h reaction time, while the catechol conversion is basically maintained at ca. 87% on the AlP_1.1Zr_0.012-400 sample. With the further increased Zr content, the AlP_1.1Zr_0.018-400 catalyst present poor stability for the O-methylation of catechol. After the reaction was carried out for 12 h, the catechol conversion decreased to 57%. Comparatively, AlP_1.1-400 shows an initial catechol conversion of 65% and 12 h catechol conversion of 54%. For all tested AlP_1.1Zr_x-400 catalysts, guaiacol selectivity is almost ca. 91%. The effect of calcination temperature on catalytic performance of the AlP_1.1Zr_0.012-T catalysts for selective O-methylation of catechol are provided in Figure 7. Catechol conversion increase with increasing calcination temperature from 350 to 400 °C. Further increasing the calcination temperature to 450 °C, the catechol conversion decrease. Guaiacol selectivity had a negligible effect on calcination temperature. The AlP_1.1Zr_0.0126-400 exhibited the optimal catalytic activity. Combined with the characterization of acid–base properties, the appropriate Lewis acidic sites and basic sites on AlP_1.1Zr_0.0126-400 contribute to the good catalytic performance for O-methylation of catechol, which is consistent with the reported in the literature [10,13].

The stability tests were carried out for the selective O-methylation of catechol to guaiacol at 280 °C and LHSV = 0.2 h⁻¹ with a methanol to catechol of 5 over AlP_1.1Zr_0.012-400 catalyst. One hundred–hour stability tests were conducted, and the results are shown in Figure 8. The initial catechol conversion and guaiacol selectivity are ca. 82.7% and 89.7%, respectively. Catechol conversion decrease slightly and guaiacol selectivity increase slightly in the initial reaction time of ca. 25 h, followed by the almost constant catechol conversion and guaiacol selectivity at ca. 80% and 92% respectively, during the subsequent 75 h reaction period. The AlP_1.1Zr_0.012-400 catalyst possess superior long-term stability for O-methylation of catechol. It has been established that carbon deposition is mainly responsible for the deactivation of acid/base catalysts. Thus, TG was employed to investigate the amount of carbon deposition on the spent AlP_1.1Zr_0.012-400 catalyst after 100 h of O-methylation of catechol (denoted as AlP_1.1Zr_0.012-400-S). For the AlP_1.1Zr_0.012-400-S, the deposited amount of carbon was only 44 mg/g_cat. These results demonstrated that the AlP_1.1Zr_0.012-400 catalyst possessed a stable coking resistance.

3. Materials and Methods

3.1. Material Preparation

Zr modified aluminophosphate were prepared by a precipitation method, as noted previously [32]. A typical synthesis, stoichiometric aluminum nitrate, zirconium nitrate and ammonium dihydrogen phosphate were dissolved in 100 mL of deionized water. Then, a 28% ammonia solution was dropped into the mixed aqueous solution with vigorous magnetic stirring until the mixture completely precipitated. At this time, the pH value of the precipitate was ca. 10. Then, the obtained precipitate dried at 100 °C overnight. After that, the dried solid was calcined at a certain temperature for 5 h in air. According to the pre-experiment results, P/Al molar ratio is fixed at 1.1. The prepared materials were denoted as AlP_1.1Zr_x-T, where x represented Zr/Al molar ratio, and T represented the calcination temperature.

3.2. Material Characterization

XRD was performed with a D/Max-2550 diffractometer (Rigaku, Japan) using Cu Ka radiation (40 kV, 200 mA). FT-IR spectra were recorded on a Tensor 27 spectrometer. Py-IR were acquired on a Nicolet iS50. Self-supporting wafers of pure catalysts were putted in the in-situ infrared cell, heated at 300 °C for 60 min, and then cooled to room temperature in vacuum of 10⁻⁴ mmHg. Pyridine was then exposed to the pre-treated samples at room temperature and was evacuated at 300 °C. BET surface areas of the catalysts were measured by N₂ adsorption–desorption using a Micromeritics ASAP 2020 Sorptometer (Micromeritics Instrument Corp., Norcross, GA, USA) at liquid nitrogen temperature (−196 °C). Before the measurement, each sample was degassed to eliminate volatile adsorbents on the surface at 300 °C for 6 h. TPD was carried out using NH₃ or CO₂ as probe molecules on homemade equipment to observe the acid and base properties of the catalysts. In a standard procedure, TPD measurements were carried out with 0.1 g of catalyst, placed in a quartz reactor and first pretreated in an Ar stream (40 mL min⁻¹) at 300 °C for 0.5 h to remove moisture and other absorbed impurities. The catalyst was cooled down to 50 °C and then exposed to a certain amount of ammonia or carbon dioxide for 0.5 h. After that, the system was swept with flowing Ar stream for 1 h. The catalyst was raised to 450 °C at a heating rate of 10 °C min⁻¹ in Ar stream (40 mL min⁻¹). The amount of the desorbed NH₃ or CO₂ was measured with an online thermal conductivity detector. The identification of reaction products was analyzed with a gas chromatography-mass spectrometer (GC–MS) (Sunny Hengping Instrument, Shanghai, China). The column was an Agilent DB-5MS capillary column (cross-linked poly 5% diphenyl/95% dimethylsiloxane), 30 m × 0.25 mm (i.d.) × 0.25 μm film thickness. The oven temperature curve started at 130 °C and then rose to 250 °C at a rate of 10 °C/min. The injection port and transfer line temperatures were both set at 250 °C. The ion source temperature was set at 230 °C. The amount of carbon deposition on the used samples was determined with a STA 4449 F3 thermogravimetric analyzer (Netzsch, Germany). The used catalysts were pre-treated at 50 °C for 30 min and then heated up to 800 °C with a rate of 10°C min⁻¹ in an air flow of 30 mL min⁻¹.

3.3. Catalytic Test

The selective O-methylation of catechol to guaiacol was conducted in a vertical continuous-flow fixed-bed reactor (inner diameter of 8 mm and a length of 800 mm) at atmospheric pressure. Prior to the reaction, the catalyst (40–60 mesh) was first disposed in situ at 280 °C for 0.5 h in nitrogen. The reactant of the pre-mixed catechol–methanol mixture was controlled using a syringe pump. The products were analyzed with a gas chromatograph equipped with a capillary column and were identified using known standards and GC–MS.

4. Conclusions

AlP_1.1Zr_x-T catalysts were successfully prepared through a simple process of precipitation method without organic surfactants and used for selective O-methylation of catechol to guaiacol. AlPO₄ species, mainly in the form of smaller crystalline AlPO₄ particles, produced weak acidic and basic sites. Both Zr addition and calcination temperature had an influence on the strength and amount of the acidic and basic sites. AlP_1.1Zr_0.012-400 catalyst exhibited optimal catalytic properties with a superior guaiacol selectivity and catechol conversion. The stability test showed that AlP_1.1Zr_0.012-400 retained long-term stability without deterioration during the 100 h reaction period.