2.1 Synthesis of the catalysts

M-H synthesis of Ni-MCM-41 mesoporous molecular sieves with different Si/Ni ratios (Si/Ni = 80, 50, 10) was performed using a MARS5 (CEM Corp., Matthews, NC, USA) microwave digestion system. This system operates at a maximum power of 1200 W and the power can be varied from 0 to 100% and is controlled by both pressure and temperature to a maximum of 350 psi and 513 K, respectively. A 2.45 GHz microwave frequency was used which is the same as that used in domestic microwave ovens. The syntheses were carried out in double-walled digestion vessels, which have an inner liner and cover, made up of Teflon PFA and an outer strength vessel shell of Ultem polyetherimide. In a typical synthesis, 21.32 g of sodium metasilicate (Na₂SiO₃ 9H₂O, CDH) was dissolved in 60 g of water. The reaction mixture was stirred for 2 h. Meanwhile, cetyltrimethylammonium bromide (5.47 g, CTABr, OTTO Chemie) and nickel nitrate were dissolved in 20 g of distilled water. Then, the resultant solution was added dropwise to the sodium metasilicate solution. The final mixture was stirred for 1 h. The pH of the gel was adjusted by using 2 M sulfuric acid (H₂SO₄; 98% Merck) and it was stirred for another 3 h. Thus obtained gel was allowed to crystallize under M-H conditions at 373 K for 2 h. The crystallized product was filtered off, washed with warm distilled water, dried at 383 K and finally calcined at 813 K in air for 6 h.

2.2 Characterization techniques

The X-ray diffraction (XRD) patterns of samples were recorded with a powder XRD instrument (Rigaku D/max 2500PC) with Cu Kα radiation (λ = 0.15418 nm). It was operated at 40 kV and 50 mA. The experimental conditions correspond to a step width of 0.02° and the scan speed of 1°/min. The diffraction patterns were recorded in the 2θ range of 1–10°. Fourier transform infrared spectra of samples were recorded on a Nexus FT-IR 470 spectrometer made by Nicolet Corporation (USA) with KBr pellet technique. The effective range was from 400 to 4000 cm⁻¹. Specific surface area and pore size were measured by using a NOVA2000e analytical system made by Quantachrome Corporation (USA). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The pore size distribution and pore volume were calculated by the Barrett-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) morphologies of samples were observed on a Philips TEMCNAI-12 with an acceleration voltage of 100–120 kV. The nickel content in the samples was determined by the inductively coupled plasma (ICP) technique (Vista-MAX, Varian). The density and strength of the acid sites of the different Ni-MCM-41 samples were determined by the temperature programmed desorption (TPD) of pyridine. About 100 mg of the materials were evacuated for 3 h at 523 K under vacuum (P < 10⁻⁵ kPa). Thereafter, the samples were cooled to room temperature under dry nitrogen followed by exposure to a stream of pyridine in nitrogen for 30 min. Subsequently, the physisorbed pyridine was removed by heating the sample to 393 K for 2 h in a nitrogen flow. The TPD of pyridine was performed by heating the sample in a nitrogen flow (50 mL/min) from 393 to 873 K with a rate of 10 K/min using a high-resolution thermogravimetric analyzer coupled with a mass spectrometer (SETARAM setsys 16MS). The observed weight loss was used to quantify the number of acid sites assuming that each mole of pyridine corresponds to one mole of protons.

2.3 Catalytic testing

Acetylation of 1,2-dimethoxybenzene with acetic anhydride has been carried out under liquid phase conditions. The liquid phase reaction set up consists of two necked 50 mL round bottom flask duly fitted with a condenser in one end for cooling and another vent is closed with Teflon septum for collecting samples by glass syringe at regular intervals. The whole system was kept in a thermostated oil bath attached with a magnetic stirrer coupled with a heating plate. In a typical reaction, the catalyst was added to a solution of 2.6 g 1,2-dimethoxybenzene (18.9 mmol), 0.4 g acetic anhydride (3.8 mmol) and 50 mL chlorobenzene together with 1 g nitrobenzene as internal standard. Then, the reaction mixture was heated to the required temperature and the samples were collected with regular intervals of time. The collected samples were analyzed periodically by a gas chromatograph (HP-6890) equipped with a FID detector using a DB-5 capillary column. The products were confirmed with GC-MS (HP-5973) analysis.

3.1 Characterization of the samples

The small angle XRD patterns of Ni-MCM-41 (Si/Ni = 80, 50, 10) synthesized by the microwave irradiation method are shown in Fig. 1. The Ni-MCM-41 (80) sample gives a very strong (100) peak followed by (110) and (200) lower intensity peaks. All three distinct Bragg reflections at low angles were indexed on a hexagonal lattice. However, when the nickel content ratio increased from 50 to 10, the intensities of the long range ordered peaks were gradually reduced. However, compared with the pattern of the typical MCM-41 mesoporous molecular sieve [23], it can be noted that the d₁₀₀ spacing in Ni-MCM-41 samples is significantly larger than MCM-41 (for both as-synthesized and calcined samples) shows the incorporation of Ni in the MCM-41 structure. The nickel content in Ni-MCM-41 is increased with Ni content in the synthesis gel (Table 1). The porosity of the samples was evaluated by N₂ adsorption isotherms. The specific surface areas and pore size distributions and pore volumes calculated by BET and BJH methods are summarized in Table 1. However, combined with the surface area data (689m²/g) listed in Table 1 and the XRD data (Fig. 1), the sample Ni-MCM-41 (10) still has a partial mesoporous structure, but the ordering is poor. Fig. 2 presents the pore size distribution curves of the three calcined samples Ni-MCM-41 synthesized by microwave irradiation method. As shown in Fig. 2, narrow and sharp peaks can be observed in an pore size range of ca. 2–3 nm for the samples Ni-MCM-41 (80) and Ni-MCM-41 (50), and the intensity of the peak also is strong as compared with that of the sample Ni-MCM-41 (10), indicating that the two samples have uniform pore size distribution as well. Additionally, as the nickel content in the sample increased, the intensity of the pore size distribution peak became weak, which reflects that the increase of nickel ions incorporated into the silica framework of MCM-41 mesoporous molecular sieve caused the partiel distortion of the mesoporous framework, resulting in the irregular pore size distribution and the poor mesoporous ordering. From Table 1, we can conclude that the specific surface area and pore volume of the resulting samples gradually decreased as the nickel content increased, and the pore size is in the range of 2.5–2.9 nm. Combined with the results of XRD, it is reasonable to conclude that the mesoporous ordering of the sample Ni-MCM-41 synthesized by the microwave irradiation method gradually decreased with the increase of the nickel content incorporated into the mesoporous framework. Fig. 3 presents the FT-IR spectra of the synthesized Ni-MCM-41 (50) sample before and after calcination at 813 K in air for 6 h. As shown in Fig. 3, the band at 3500 cm⁻¹ is the characteristic band of the water adsorbed on the sample surface. The bands at 1620–1640 cm⁻¹ are aroused by the flexion vibration of the OH bond [24]. The band at 1080 cm⁻¹ is from the asymmetric extension vibration of Si-O-Si. The band about 810 cm⁻¹ is due to the corresponding symmetric vibration of Si-O-Si bond, while the band at 460 cm⁻¹ is assigned to rocking vibration of the Si-O-Si bond. The bands at 2921, 2850 and 1480 cm⁻¹ are the characteristic bands of the surfactant alkyl chains. After the sample Ni-MCM-41 (50) was calcined at 813 K in air for 6 h, the bands at 2921, 2850 and 1480 cm⁻¹ disappeared, certifying that the template had been effectively removed. The TEM images of the samples Ni-MCM-41 (80) and Ni-MCM-41 (50) are shown in Fig. 4. It is observed that the two samples exhibited the hexagonal arrays structure, indicating that the two samples synthesized under microwave irradiation conditions process the mesoporous framework. However, the mesoporous ordering of the sample Ni-MCM-41 (50) is poor as compared with that of the sample Ni-MCM-41 (80). Additionally, it can also be observed that there are not the particles and/or clusters containing nickel species on the surface of the two samples in Fig. 4. Furthermore, the acid site distribution and acid amounts of Ni-MCM-41 synthesized under microwave irradiation condition possess were determined using TPD of pyridine and the data are collected in Table 2. Weak (423 and 633 K), moderate (633–743 K) and strong (> 743 K) acid sites are found in all samples. The weak acid sites are attributed to surface hydroxyl groups and the medium and the strong acid sites originate probably from the incorporation of nickel atoms into the MCM-41 walls. It is interesting to note that the number of weak acid sites decreases with decreasing Si/Ni ratio. However, the amount of medium and strong acid sites decreases with increasing Si/Ni ratio. It should be noted that the total number of acid sites (medium and strong acid sites) of Ni-MCM-41 (10) prepared by M-H is higher than that of Ni-MCM-41 (50) and Ni-MCM-41 (80).

3.2 Catalytic activity

3.2.2 The effect of reaction temperature in the acetylation of 1,2-dimethoxybenzene with acetic anhydride

The effect of reaction temperature on the conversion of acetic anhydride in the acetylation of 1,2-dimethoxybenzene over Ni-MCM-41 (M-H) (10) at different reaction time is shown in Table 4. As expected, an increase in the reaction temperature had a favorable effect on the conversion of the acetic anhydride. The conversion of acetic anhydride increases with increasing the reaction temperature. However, the selectivity to 3,4-dimethoxyacetophenone was not changed with increasing the reaction temperature. The selectivity of the product was almost 100% for all the reaction temperature studied. It must be noted that the rate of the reaction at lower temperature is rather slow. On the other hand, at the reaction temperature more than 333 K, the rate of the reaction is very fast. The apparent rate constant of the reaction at different reaction temperature was calculated using the pseudo-first order rate law:

Log1/1−x=ka/2.303t−t0

where k_a is the apparent first order rate constant, x the fractional conversion of acetic anhydride, t the reaction time and t₀ the induction period corresponding to the time required for reaching equilibrium temperature. A plot of log [1/1–x] as a function of the time gives a linear plot over a large range of acetic anhydride conversions (Fig. 5). As expected, the apparent rate constant for the acetylation reaction was increased from 13.0 × 10⁻³ min⁻¹ to 96.3 × 10⁻³ min⁻¹ with increasing the reaction temperature from 313 to 353 K. The activation energy for the Ni-MCM-41 (10) calculated from an Arrhenius plot, was found to be 27.9 kJ mol⁻¹. From these results, it is found that the reaction temperature of 333 K is the best temperature for Ni-MCM-41 (10) and is preferred for the subsequent catalytic studies.

Table 4

Catalytic activities of Ni-MCM-41 microwave-hydrothermal (M-H) (10) at different temperatures: 313, 333 and 353 K, stoichiometric ratio 1,2DMB/AC = 5, catalyst weight = 0.1 g.

Temperature (K)	Timea (min)	Selectivity to 3,4-dimethoxyacetophenone (%)	Apparent rate constant ka (× 103min-1)
313	209.0	100.0	13.0
333	96.2	100.0	42.1
353	42.3	100.0	96.3

^a Time required for complete conversion of acetic anhydride.