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

: 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. x -400 samples. These results demonstrate that the addition of Zr signiﬁcantly promote the formation of smaller crystalline AlPO 4 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.


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.
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.

Physicochemical Properties of AlP1.1Zrx-T
XRD patterns of the prepared samples are shown in Figure 1. The AlP1.1-400 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 AlP1.1-400, 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 AlP1.1Zrx-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 PO 4 3− , 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 AlPO 4 [17,18,27,31]. These results further indicate the structure of AlPO 4 of AlP 1.1 Zr x -T samples. cm −1 can be attributed to triply degenerate P-O stretching vibration and triply degenerate O-P-O bending vibration of tetrahedral PO4 3− , 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 AlP1.1Zrx-T samples. The basic physical properties of the AlP1.1Zrx-T samples are summarized in Table 1. The AlP1.1-400 had a specific surface area of 126 m 2 /g, a pore volume of 0.83 cm 3 ·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 SBET of AlP1.1 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 AlP1.1-400 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 AlP1.1-400, the AlP1.1Zrx-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 The basic physical properties of the AlP 1.1 Zr x -T samples are summarized in Table 1. The AlP 1.1 -400 had a specific surface area of 126 m 2 /g, a pore volume of 0.83 cm 3 ·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. NH 3 -TPD profiles are displayed in Figure 3. In terms of the AlP 1.1 -400 sample, the NH 3 -TPD profile illustrates two broad NH 3 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 1.1 Zr x -400 samples show a dominant NH 3 desorption peak in the range of 80-250 • C, corresponding to unitary weak acid center [13]. The desorption peaks shift to higher NH 3 desorption temperature with the increasing Zr content, indicating that the addition of Zr strengthens the acidity of the catalysts. However, the area of the NH 3 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 1.1 Zr 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. alter the acidic strength without influencing the acidic amount of the samples. As for AlP1.1Zr0.012-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 AlP1.1Zrx-400 and AlP1.1Zr0.012-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 Brӧnsted 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 Brӧnsted 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 AlP1.1-400 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 AlP1.1Zrx-T samples. The AlP1.1-400 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 AlP1.1-400, the AlP1.1Zr0.006-400 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    [15,24,25]. By comparing with the NH 3 -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.
alter the acidic strength without influencing the acidic amount of the samples. As for AlP1.1Zr0.012-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 AlP1.1Zrx-400 and AlP1.1Zr0.012-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 Brӧnsted 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 Brӧnsted 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 AlP1.1-400 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 AlP1.1Zrx-T samples. The AlP1.1-400 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 AlP1.1-400, the AlP1.1Zr0.006-400 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  CO 2 -TPD was used to investigate the basic property of the prepared catalysts. Figure 5 displays the CO 2 -TPD patterns of AlP 1.1 Zr x -T samples. The AlP 1.1 -400 sample give a broad CO 2 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 1.1 Zr 0.006 -400 sample displayed two broad CO 2 desorption peaks centered at 130 • C and 177 • C, respectively, with an appreciable decrease of the CO 2 desorption peak area. With increasing Zr content to x of 0.009, the CO 2 desorption peak centered 130 • C shift to a slightly lower temperature and the CO 2 desorption peak centered 177 • C shift to a higher temperature of ca. 210 • C. Meanwhile, the CO 2 desorption peak area decreased slightly. With further addition of Zr, no significant change occurs in the position and area of CO 2 desorption temperature on AlP 1.1 Zr 0.012 -400 and AlP 1.1 Zr 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 CO 2 -TPD is shown in Figure 5b. With the elevation of calcination temperature, the CO 2 desorption peak centered at 130 • C shifts to higher CO 2 desorption temperature and the CO 2 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. 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 AlP1.1Zr0.012-400 and AlP1.1Zr0.018-400. 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.

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 AlP1.1Zrx-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 AlP1.1Zr0.006-400 sample decline from 83% to 60% within the 12 h reaction time, while the catechol conversion is basically maintained at ca. 87% on the AlP1.1Zr0.012-400 sample. With the further increased Zr content, the AlP1.1Zr0.018-400 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, AlP1.1-400 shows an initial catechol conversion of 65% and 12 h catechol conversion of 54%. For all tested AlP1.1Zrx-400 catalysts, guaiacol selectivity is almost ca. 91%. The effect of calcination temperature on catalytic performance of the AlP1.1Zr0.012-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 AlP1.1Zr0.0126-400 exhibited the optimal catalytic activity. Combined with the characterization of acid-base properties, the appropriate Lewis acidic sites and basic sites on AlP1.1Zr0.0126-400 contribute to the good catalytic performance for O-methylation of catechol, which is consistent with the reported in the literature [10,13].

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 1.1 Zr 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 1.1 Zr 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 1.1 Zr 0.012 -400 sample. With the further increased Zr content, the AlP 1.1 Zr 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 1.1 Zr x -400 catalysts, guaiacol selectivity is almost ca. 91%. The effect of calcination temperature on catalytic performance of the AlP 1.1 Zr 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 1.1 Zr 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 1.1 Zr 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 AlP1.1Zr0.012-400 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 AlP1.1Zr0.012-400 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 AlP1.1Zr0.012-400 catalyst after 100 h of O-methylation of catechol (denoted as AlP1.1Zr0.012-400-S). For the AlP1.1Zr0.012-400-S, the deposited amount of carbon was only 44 mg/gcat. These results demonstrated that the AlP1.1Zr0.012-400 catalyst possessed a stable coking resistance. 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 AlP1.1Zr0.012-400 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 AlP1.1Zr0.012-400 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 AlP1.1Zr0.012-400 catalyst after 100 h of O-methylation of catechol (denoted as AlP1.1Zr0.012-400-S). For the AlP1.1Zr0.012-400-S, the deposited amount of carbon was only 44 mg/gcat. These results demonstrated that the AlP1.1Zr0.012-400 catalyst possessed a stable coking resistance. 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 1.1 Zr 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 1.1 Zr 0.012 -400 catalyst possess superior long-term stability for Omethylation 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.

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 AlP1.1Zrx-T, where x represented Zr/Al molar ratio, and T represented the calcination temperature.

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 ×

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 1.1 Zr x -T, where x represented Zr/Al molar ratio, and T represented the calcination temperature.

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 N 2 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 NH 3 or CO 2 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 NH 3 or CO 2 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 .

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.

Conclusions
AlP 1.1 Zr 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. AlPO 4 species, mainly in the form of smaller crystalline AlPO 4 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 1.1 Zr 0.012 -400 catalyst exhibited optimal catalytic properties with a superior guaiacol selectivity and catechol conversion. The stability test showed that AlP 1.1 Zr 0.012 -400 retained long-term stability without deterioration during the 100 h reaction period.