Zeolite ZSM-5 modification with phosphates for alkylation

Transcription

1 Zeolite ZSM-5 modification with phosphates for alkylation reactions 3.1. Introduction Vapor phase alkylation and disproportionation of aromatics are important reactions practiced in petrochemical industries [1]. For these reactions, thermodynamic equilibrium mixtures of all the regioisomers are obtained with nonselective catalyst. Selectivity towards para-isomer can only be increased by the application of shape-selective catalyst. The para-isomer being commercially important than the other two for many organic reactions, post synthesis modification is desirable to improve the product selectivity. Zeolites (microporous crystalline aluminosilicates) have been widely used as eco-friendly catalysts, and have successfully replaced corrosive and environmentally unsafe catalysts. They exhibit shape-selectivity for the chemical reactions due to their microporous channels close to the molecular diameter of the products [1] and are applied for selective organic transformations. However, pore modification is essential for substantially improving the shape selectivity towards a particular product. Post synthesis modification has widely been carried out by silanation, selective coking and metal/non-metal oxide impregnation [2 8]. Zeolites have been post synthetically modified by Mobil and other groups [9 20] with different phosphorus compounds to obtain high selectivity for para-isomer. However, in all these reports, catalytic activity decreased substantially after phosphate modification. Initial research by Mobil on post-synthesis modification of zeolites shows that phosphorus incorporates as its oxide in zeolite pores [10, 11]. Catalyst evaluation by alkylation of toluene with methanol showed that phosphate modification imparts a diffusion barrier in the micropores exhibiting increase in para selectivity. However, in previous reports, conversion of toluene

2 Zeolite ZSM-5 modification with phosphates for alkylation reaction decreased substantially after phosphate modification. Mobil researchers in 1980, modified ZSM-5 with trimethylphosphite and proposed a mechanism of interaction of phosphate with zeolite [12]. According to this report, phosphite reacts with bridging hydroxyl group resulting in the formation of Si-O-P bond. Further studies on the interaction of H 3 PO 4 with USY zeolite by Corma et al [13] showed that H 3 PO 4 dealuminates the zeolite and reacts with extra framework aluminum to form aluminum phosphate whereas Si-O-P bond does not form. They also concluded that isomorphous substitution of framework Si with P does not occur during H 3 PO 4 treatment and calcinations steps. Lischke et al. [14] reported that impregnated phosphate species on ZSM-5 elutes with water and hence calcinations step is necessary after impregnation. Blasco et al. [15] found that H 3 PO 4 and NH 4 H 2 PO 4 react with the bridged hydroxyls located in the ZSM-5 channels. Furthermore, no difference has been observed in the properties of P-modified ZSM-5 catalysts prepared by these two different phosphate sources. Liu et al studied the effect of washing on the properties of P-modified ZSM-5. However, their modification with phosphoric acid resulted in substantial decrease of total acidity due to dealumination [21]. p-diethylbenzene (PDEB) is an important chemical used as a desorbent in PAREX process designed by UOP and Axens APC-Eluxyl process. In these process, p-xylene is separated from its regioisomers and used as a raw material for polyester synthesis such as PET and PTA. Ethylation of ethylbenzene over acidic catalysts in vapor phase gives thermodynamic equilibrium mixtures of o, m, p-diethylbenzenes. A high purity PDEB is required for PAREX and APC-Eluxyl process, which can be synthesized by the reduction of p-ethylacetophenone, known to be an expensive multistep process. Hence, it is necessary to design a selective catalyst for ethylation of ethylbenzene which forms only para isomer. Few research groups have reported catalysts like silica coated ZSM-5, ZSM-5 impregnated with lanthanum and cerium oxides, aluminophosphates, phosphotungstic acid supported on aluminophosphates 58

3 Zeolite ZSM-5 modification with phosphates for alkylation reaction AlPO and modified zeolites to selectively synthesize PDEB from ethylation of ethylbenzene with ethanol or ethylene [22-29]. Halgeri and co-workers had extensively worked on silanation of ZSM-5 for the synthesis of PDEB by ethylation of ethylbenzene [27-29]. Unlike other methods, phosphate modification is an easy and inexpensive route for the catalyst modification, which makes it a good prospect for a commercial process. In this chapter, phosphate modification of ZSM-5 is studied in detail. A novel water treatment step was introduced in the catalyst preparation which enhanced greatly the performance of the catalyst. The enhancement of shape selectivity is demonstrated by probe reactions. Ethyl benzene ethylation was studied in great depth Experimental Catalyst preparation modification Zeolite ZSM-5 with different silica to alumina ratios (SAR) were kindly donated by Süd-Chemie, Germany (Clariant) and Süd-Chemie India Pvt Ltd. Sodium form of zeolites were exchanged with ammonium ions by refluxing in 1 M aq. NH4NO3 solution for 4 h, filtered and dried, and this procedure was repeated twice to ensure maximum ion-exchange. The final product was calcined at 550 C for 3 h to get HZSM-5. Further, HZSM-5 was modified post synthetically by wet impregnation method by treatment with phosphorus reagent. In step 1, required amount of NH 4 H 2 PO 4 was dissolved in 22ml water at 60 C to which 5 g of zeolite was added with stirring and evaporated at 90 C. It was then dried at 120 C for 4 h and calcined initially at 330 C for 3 h and then the temperature was increased to 560 C for 10 h. This calcined catalyst powder was cooled to room temperature and stirred in water at 80 C for 12 h, then filtered and dried at 120 C for 4 h (Step 2). In step 3, it was then extruded with silica binder (Ludox AS 40, zeolite : binder = 60 : 40 ), dried in an oven for 4 h at 120 C and calcined at 500 C for 8 h. Thus prepared catalysts were 59

4 Zeolite ZSM-5 modification with phosphates for alkylation reaction designated as XPZSM-5W where X is percentage of P (as phosphate) added during impregnation and W is the water treatment after P impregnation and calcination. Another set of catalysts were prepared by two-step addition of phosphates. For this, the procedure followed was similar to the previously mentioned one except that phosphate impregnation was repeated once after Step 2. Catalyst prepared were 1+4PZSM-5W, 2+3PZSM-5W, 3+2PZSM-5W (a+bpzsm-5w, where a = % P added during first step, b = % P added during second step) Catalyst characterization Catalyst characterization was carried out by various techniques viz., X-ray diffraction studies, FTIR spectra, nitrogen sorption, temperature programmed desorption (TPD) of NH 3, Point of zero charge (PZC), solid state nuclear magnetic resonance with magic angle spinning (MAS NMR) of 31 P and 27 Al nuclei and adsorption studies. Phosphorus content in the modified catalyst was estimated by colorimetry. Morphology of the catalysts was studied by FESEM. Detailed procedure followed for characterization is discussed in Chapter 2. Coke was estimated by calcination of spent catalyst in air to get constant weight (till the coke was completely removed). Difference in weight before and after calcinations is considered as amount of coke formed during reaction. It was normalized to hundred and expressed in terms of percentage coke formed Catalyst activity study Alkylation of toluene with methanol was carried out in a vapor phase continuous down-flow quartz reactor. Quartz reactor was connected to a pre-heater and condenser maintained at 2 C with the help of cryostat. About 2 g of extruded, calcined catalyst of 3-6 mm length and 1.5 mm ϕ was taken in the quartz reactor. Internal diameter of the reactor was 14 mm and ratio of the reactor diameter to catalyst diameter was always in the range of 9-10 as recommended in the literature 60

5 Zeolite ZSM-5 modification with phosphates for alkylation reaction [30]. Catalyst bed was spread evenly with ceramic inert beads on the top. Crushed and calcined beads of mesh size were used to fill the dead volume of reactor of about 10 cm of length and a thermocouple placed near the catalyst bed to monitor the temperature. In a typical procedure, catalyst was activated in-situ at 500 C for 1 h before the reaction and reaction temperature was set at 480 C. A mixture of toluene and methanol was taken in 4:1 mole ratio and fed to the pre-heater at a required rate through syringe pump. Hydrogen as carrier gas was fed through a calibrated rotameter. Gaseous product stream was condensed and collected in gas liquid separator maintained at 2-5 C. Liquid and gas products were analyzed (off-line) through GC using HP-Innowax capillary column; 60 m x 0.25 µm x 0.25 mm and porapak Q packed column; 4 m x 200 mm respectively. The components were identified and quantified with standards, and confirmed by GCMS. Toluene conversion (wt %) = X 100 para-xylene selectivity (wt %) = X 100 Total xylenes (wt %) = X 100 xp = wt of para- xylene in product stream xm = wt of meta-xylene in product stream xo = wt of ortho-xylene in product stream The ethylbenzene alkylation with ethanol, ethylbenzene disproportionation reactions and competitive reactions of ethylbenzene and m-xylene were carried out with the same set-up under the conditions mentioned in the respective tables and figures. GC analyses of the products were carried out as mentioned for toluene alkylation. Conversion and selectivities were calculated similar to the formulae given for toluene alkylation as above. 61

6 Zeolite ZSM-5 modification with phosphates for alkylation reaction 3.3. Results and discussion Catalyst characterization XRD patterns of unmodified HZSM-5 matched well with all phosphate modified samples. This showed the retention of structural integrity of ZSM-5 after phosphate modification followed by water treatment (Figure 3.1, Figure 3.2A, Figure 3.3). It is further confirmed by the IR spectrum that the bands corresponding to pentasil chain (at cm -1 ) and pentasil unit (at cm -1 ) did not shift positions as compared with zeolite without modification (Figure 3.2B, Figure 3.4., Figure 3.5., Figure 3.6.). 3PZSM-5W (38) Intensity (a.u.) 3PZSM-5W (84) 3PZSM-5W (187) 3PZSM-5W (272) 3PZSM-5W (408) o Figure 3.1. XRD of 3PZSM-5W with different SAR Surface area of PZSM-5 after phosphate modification decreased (Table 3.1.) from 525 to 340 m 2 /g which could be due to the impregnated phosphate groups occupied both in external and micropore areas. Surface area was completely restored after 1% P loading and subsequent water treatment whereas at higher loadings, surface area decreased by ~30 %. Pore volume also decreased a little after phosphate modification. However, surface area increased after water treatment from 340 to

7 Zeolite ZSM-5 modification with phosphates for alkylation reaction m 2 /g for 5PZSM-5W due to the removal of water soluble phosphates. Pore volume after modification decreased and partially restored after water treatment. This further indicates the removal of ionic phosphates after water treatment. A B Intensity (a. u.) 1PZSM-5W 3PZSM-5W 5PZSM-5W 7PZSM-5W Transmitance (a. u.) Pentasil chain ZSM-5 (SAR 187) 1PZSM-5W 3PZSM-5W 5PZSM-5W 7PZSM-5W 9PZSM-5W Pentasil unit ( o ) wave number (cm -1 ) Figure 3.2. (A) XRD of PZSM-5W, (B) FTIR spectra of PZSM-5W SAR PZSM-5W Intensity (a. u.) SAR 240 SAR 272 Intensity (a. u.) 240 5PZSM-5W 272 5PZSM-5W SAR PZSM-5W o 2 o Figure 3.3. XRD of HZSM-5 and PZSM-5 with different particle sizes 63

8 A B C Transmittance (a.u.) Pentasil chain ZSM-5 (SAR 38) 3PZSM-5W (SAR 38) Pentasil unit Transmittance (a.u.) Pentasil chain ZSM-5 (SAR 84) 3PZSM-5W (SAR 84) Pentasil unit Transmittance (a.u.) Pentasil chain ZSM-5 (SAR 187) 3PZSM-5W (SAR187) Pentasil unit wavenumber cm wavenumber cm wavenumber cm -1 D E Transmittance (a.u.) Pentasil chain ZSM-5 (SAR 272) 3PZSM-5W (SAR 272) Pentasil unit Transmittance (a.u.) Pentasil chain ZSM-5 (SAR 408) 3PZSM-5W (SAR 408) Pentasil unit wavenumber cm wavenumber cm -1 Figure 3.4. FTIR of phosphate modified ZSM-5 with different SAR A) SAR 38, B) SAR 84, C) SAR 187, D) SAR 287, E) SAR (408)

9 Transmittance (a.u.) Pentasil chain ZSM-5 (SAR 187) 1+4PZSM-5W 2+3PZSM-5W 5PZSM-5W Pentasil unit wavenumber cm -1 Figure 3.5. FTIR of phosphate modified ZSM-5 ( 2-step addition) A B C Tra ns m itta nce (a.u.) P en tasil c hain ZSM-5 (SAR 240) 2PZSM-5W (SAR 240) 5PZSM-5W (SAR 240) P en tasil u nit Tra ns m ittan ce (a.u.) P entasil ch ain ZSM-5 (SAR 250) 2PZSM5W-(SAR 250) 5PZSM-5W (SAR 250) P e ntas il unit T ran sm ittanc e (a.u.) P enta sil cha in ZSM-5 (SAR 272) 2PZSM-5W (SAR 272) 5PZSM-5W (SAR 272) P en tasil u nit wavenumber cm -1 wavenumber cm -1 wavenumber cm -1 Figure 3.6. FTIR of phosphate modified ZSM-5 with different particle sizes

10 Zeolite ZSM-5 modification with phosphates for alkylation reaction TPD of ammonia showed that the stronger acid sites were replaced with weaker acid sites after phosphate modification (Figure 3.7., Figure 3.8.). It is evident that the phosphate salt reacts with the acid sites creating new kinds of acid (active) sites. It is evident from the TPD plot that stronger acid sites decreased after phosphate modification but there was a small change in total acidity (Table 3.2., Table 3.3., Table 3.4.). 3P SAR 38 3P SAR 84 3P SAR 187 3P SAR 272 3P SAR 408 TCD signal (a. u.) Temperature o C Figure 3.7. TPD of ammonia trace and acidity of 3PZSM-5W with different SAR Table 3.1. Surface area and pore volume of PZSM-5 (SAR 187) Catalyst Surface area Pore volume Acidity (m 2 /g) (cc/g) (mmol/g) ZSM PZSM-5W PZSM-5W PZSM PZSM-5W PZSM-5W PZSM-5W

11 Zeolite ZSM-5 modification with phosphates for alkylation reaction Table 3.2. Acidity and P-content of PZSM-5 with different SAR P Acidity HZSM-5 P remaining HZSM-5 After water Acidity added water (SAR) treatment (mmol/g) (%) treatment (mmol/g) (%) P/Al Table 3.3. Acidity, P-content of PZSM-5 with different particle sizes Morphology and average particle size µm Spherical 0.63 Irregular 1.2 Cuboid 1.1# Intergrown cube 3.9 SAR Acidity (mmol/g) P added (wt%) P remaining after water treatment (wt%) Acidity after P modification (mmol/g) P/Al 0 - n/a n/a n/a n/a

12 Zeolite ZSM-5 modification with phosphates for alkylation reaction TCD Signal (a. u.) ZSM-5 SAR mmol/g 1PZSM-5W 0.22 mmol/g 3PZSM-5W 0.33 mmol/g 5PZSM-5W 0.24 mmol/g 6PZSM-5W 0.37 mmol/g 9PZSM-5W 0.31 mmol/g Temperature o C Figure 3.8. TPD of ammonia trace and acidity of phosphate modified ZSM-5. Table 3.4. Stepwise phosphate modification of HZSM-5 (SAR-187) Catalyst P remaining Acidity Step 1 Step 2 after water After water P added P added treatment treatment (%) (%) (wt%) (mmol/g) P/Al HZSM PZSM 5W PZSM-5W PZSM-5W PZSM-5W

13 Chapter 3 Zeolite ZSM-5 modification with phosphates for alkylation reaction Figure 3.9. Variation of PZC with phosphate modification on HZSM-5 (SAR 187) Point of Zero Charge (PZC) measurement gives the information regarding the surface coverage [31]. PZC is the characteristic of the surface of a material. It changes if the surface is covered by another material. Hence, PZC measurement gives the information on the extent of surface coverage of catalyst by the impregnating material. The plot of PZC v/s wt% of phosphate on ZSM-5 shows a break at monolayer coverage. This break was observed at 4% P loading (Figure 3.9.), indicating the completion of monolayer islands formation at this loading. Phosphates added were chemisorbed to form monolayer to multilayer. Better performance of the catalyst can be achieved at monolayer formation because higher loadings result in multilayer of phosphates which may block the pores to create diffusion problems. The % of phosphorus added during preparation was plotted with phosphorus remaining after water treatment (Figure 3.10.). In all the preparations of PZSM-5 catalysts, phosphate to aluminium mole ratio added was > 1. The plot shows that, as the phosphate impregnation increased from 0 to 3%, phosphate content in zeolite after water 69

14 Chapter 3 Zeolite ZSM-5 modification with phosphates for alkylation reaction treatment increased linearly and becomes almost constant at % up to 6% impregnation. This shows the saturation of phosphate monolayer islands inside the pores. At higher phosphate impregnations (> 6%), there is an increase in phosphate content which may be due to the formation of multilayer of phosphates. These monolayer phosphate sites catalyze the reaction and increase the selectivity by imparting additional diffusion barrier to formed isomers. Figure Phosphate retained by zeolite after wash as a function of phosphate added during preparation 31 P MAS NMR of the calcined catalyst showed the formation of orthophosphate monoester, pyrophosphate, polyphosphate and aluminophosphate species (Figure (a, b)). These species of phosphates were formed mainly due to the covalent interaction of phosphate with aluminium acid sites of zeolite. After water treatment, the peak corresponding to orthophosphate monoester was completely removed and pyrophosphate, polyphosphate peak intensities decreased significantly. This shows that orthophosphate interaction with zeolite matrix. However, pyrophosphate, polyphosphate, and 70

15 Chapter 3 Zeolite ZSM-5 modification with phosphates for alkylation reaction aluminophosphate formed a stable interaction with the zeolite hence it was retained even after water treatment. Similar observation for water treated PZSM-5 was reported by Derewinski et al [32]. 27 Al MAS NMR (Figure (c, d)) spectra confirm that all the Al are tetrahedral in nature in both ZSM-5 (SAR 187) and 5PZSM-5W (SAR 187). This indicates that Al sites are retained in the zeolite framework after phosphate modification and dealumination did not occur. The peak at 53.3 ppm corresponding to framework Al shifted downfield to 56.5 ppm after phosphate treatment. This could be due to phosphate and framework Al interactions as shown in Scheme-1. There was no aggregation or change in morphology for 5PZSM-5W contrary to the observations made by the van der Bij et al where water treatment was not carried out [33]. Morphologies of the catalysts before and after modification were spherical with particle size in the range of µm (Figure (A, B)). Figure P MAS NMR of (a) 5PZSM-5 (without H2O treatment), (b) 5PZSM-5(W); 27 Al MAS NMR of (c) H-ZSM-5, (d) 5PZSM-5(W) 71

16 Zeolite ZSM-5 modification with phosphates for alkylation reaction To understand the effect of acidity on phosphate modification, 3PZSM-5W catalysts with different SAR were prepared and characterized for phosphate content and acidity by TPD of ammonia. For unmodified HZSM-5 catalysts, acidity decreased with increase in SAR as expected (Figure 3.7., Table 3.2.). After P-modification, phosphates per Al site increased with increase in SAR even though phosphate added to the zeolite was constant (3 wt%). It could be attributed to the interaction of phosphates predominantly with Al sites at lower SAR and increase in SAR increases phosphatephosphate interactions forming poly-phosphates. Phosphorus retained on zeolite was almost constant for all SAR except SAR 38. The acidity of ZSM-5 increased for higher SAR of 187, 272, 408 after phosphate treatment. The increase in acidity after modification could be due to generation of new kind of low strength acid sites. In previous reports, ZSM-5 with lower SAR < 100 were used invariably for preparing PZSM-5 and reported decrease in acidity after modification. In present study, decrease in acidity after P-modification is true only for ZSM-5 with low SAR and marginal increase acidity was observed for higher SAR. This change in acidity can be explained with respect to the possible modes of interaction of phosphates with zeolite framework as shown in Scheme-3.1. (adopted from references 12, 15, 34, 35). At lower SAR, possibility of formation of structures C and D is relatively high due to high acid site density. For higher SAR, formation of A and B is possible due to well dispersed acid sites which results in the formation of two acid sites by replacing one acid site and hence acidity increased. Formation of species C, D, and E result in decrease of acid sites. 31P MAS NMR spectra of 5PZSM-5W (SAR 187) suggests that all 5 species can form but relative change in acidity can possibly be attributed to relative abundance of these species over PZSM-5 with different SAR. 72

17 Zeolite ZSM-5 modification with phosphates for alkylation reaction Scheme-3.1. Phosphate interaction with zeolite framework as adopted from A [12], B [34], C[15], D [35], E [15]. During 2-step phosphate addition, phosphates retained were almost similar for all catalysts except 2+3 PZSM-5 which showed maximum phosphorus retention of 0.9 % (0.94 P/Al). (Table 3.4., Figure 3.13.). Some Al sites which left unreacted during first step addition may react with phosphates during second step. However, the reaction of phosphate groups formed during the first step with phosphates added in the second step cannot be ruled out. Acidity of different PZSM-5W prepared with 2-step phosphate additions were in the range 0.20 to 0.28 mmol /g whereas unmodified HZSM-5 showed acidity of 0.22 mmol /g. 73

18 Figure Scanning electron micrographs of (A) HZSM-5(SAR 187), (B) 5PZSM-5(W) (SAR 187), C) HZSM-5(SAR-240), D) HZSM-5(SAR-250), E) HZSM-5(SAR-272), F) HZSM-5(SAR-280).

19 Zeolite ZSM-5 modification with phosphates for alkylation reaction Figure TPD trace of phosphate modified ZSM-5 (SAR 187) An adsorption study was carried out with different probe molecules for PZSM- 5(W) catalysts. From Figure 3.14, it is observed that with increase in P content, adsorption capacity per gram of catalyst reduces for a given molecule. Similarly for a given catalyst, the adsorption capacity diminishes with increase in kinetic diameter of the molecule. It is observed that there is a slight increase in the adsorbed volume with increase in effective average pore width before an expected sharp drop (Figure 3.14A). It is due to an increased diffusivity of the molecule which can happen if the molecular diameter is comparable to that of pore diameter. Such an anomalous dependence of diffusivity on radius of the diffusant is known as Levitation Effect. This phenomenon was studied in detail by Yashonath and Ghorai [36, 37]. The plot Δ Vol Ads /Δ kinetic diameter v/s effective kinetic diameter (Figure 3.14B) shows a peak at 5.86 Å. With increase in P content from 1 to 9 %, the peak maximum remained at 5.86 Å but the peak became sharp (narrow) which indicates that the molecules with kinetic diameter larger than 5.86 Å cannot enter the pores of the catalyst. Comparison of unmodified and modified catalyst in 75

20 Zeolite ZSM-5 modification with phosphates for alkylation reaction this plot shows that the effective pore width of the catalyst decreased after P modification. Narrowing of the peak with increase in P content can be attributed to decrease in pore width upon increase in phosphate incorporation in the pores. Figure Adsorption study with PZSM-5W (SAR 187) catalyst (A) Adsorption of molecules with different kinetic diameters. (B) Derivative of curves in (A) 76

21 Zeolite ZSM-5 modification with phosphates for alkylation reaction Part A : Phosphate modified ZSM-5: Generation of new acid site with pore narrowing Catalyst activity studies In this section generation of new acid sites in ZSM-5 by phosphate modification which improves the shape selectivity by pore narrowing is reported. The direct correlation of pore volume with shape selectivity has been established. Toluene methylation, ethylbenzene ethylation and ethylbenzene disproportionation reactions have been used as the probe reactions. Also, constraint index for unmodified and modified zeolites obtained from competitive reaction of ethylbenzene and meta-xylene was used to measure the extent of pore narrowing. The effect of water treatment of phosphate modified ZSM-5 on its acidity and catalytic property are discussed Toluene alkylation with methanol Vapor phase toluene alkylation with methanol (Scheme 3.2) using solid acid catalyst results in the formation of ortho, meta and para-xylenes. For unmodified HZSM- 5, toluene conversion was 21.7 mol% (Theoretical max. = 25 mol %) and para-xylene selectivity was 30.7 mol% (Table 3.5) after 4 h reaction. Methanol, the limiting substrate, gave 99.9 % conversion. The 5P-ZSM-5 without water treatment showed almost 3-fold increase in the para-selectivity (97 %) but toluene conversion reduced to 8.5%. Water treatment of P-ZSM-5 (Step 2 in preparation) remarkably improved the toluene conversion (14.2 mol%) while retaining high selectivity (95.6 mol%) (Figure 3.15). The catalyst showed marginal deactivation for time on stream of 35 h but selectivity remained same during the study (Figure 3.16). Decrease in the catalytic activity without water treatment could be due the pore blocking by condensed phosphate species. Water treatment after calcination of PZSM-5 (Step 2) removes soluble phosphates present in zeolite channels which exposes the masked active sites resulting in increase of 77

22 Zeolite ZSM-5 modification with phosphates for alkylation reaction conversion. This observation is supported by the disappearance of peak corresponding to orthophosphate in 31 P MAS NMR spectra (Figure 3.11). Hence, PZSM-5 with water treatment [PZSM-5 (W)] was taken for other shape-selective transformations. Scheme 2. Toluene alkylation with methanol over zeolite ZSM-5 to form xylenes. It is further investigated to find the reason for increase in shape selectivity with phosphate modification. It is now clear that phosphate groups mostly present inside the pores and act as barrier to the diffusion of molecules. It is observed that pore volume of modified ZSM-5 can be changed by varying phosphorus content. Such phosphate modified ZSM-5 catalysts were screened for toluene methylation under identical reaction conditions. We found an interesting correlation with pore volume and shape selectivity for p-xylene (Figure 3.17.). Figure Effect of water treatment of PZSM-5 on activity and selectivity to p-xylene 78

23 Chapter 3 Zeolite ZSM-5 modification with phosphates for alkylation reaction Figure Time on stream of toluene alkylation over 5PZSM-5(w), Conditions: Temperature = 480 C, WHSV = 4 h-1, Catalyst weight = 2 g, H2 : HC = 2, Toluene : Methanol = 4 : 1 p-xylene selectivity remarkably increased from 30.7 to 97 mol % with decrease in pore volume from 0.30 to 0.24 cc/g. As the pore volume decreases, the formation of bulkier meta and ortho isomers gets suppressed resulting in the selective formation of pxylene. In addition to this, the diffusion barrier created inside the pores also could lead to the selective diffusion of smaller para isomer from the pores. Further decrease in pore volume from 0.20 to 0.18 decreased p-xylene selectivity from 97 % to 87 %. This is possible when the nonselective reaction on the external surface becomes predominant compared with the micropores due to high diffusion limitation caused by pore narrowing. To further investigate the shape selectivity of the modified ZSM-5 catalyst, the reaction was carried out under different conditions using 5PZSM-5(W) as shown in Table 3.6. The toluene conversion decreased from 14.2 to 11.4 % upon reducing the temperature from 480 to 440 C. However, the p-xylene selectivity remained constant at ~ 96 %. 79

24 Chapter 3 Zeolite ZSM-5 modification with phosphates for alkylation reaction Figure Effect of pore volume of PZSM-5 on p-xylene selectivity Increase in space velocity from 4 to 8 h-1 decreased the conversion from 14.2 to 11.0 whereas there was a slight improvement in p-xylene selectivity (by 1 %). These results show that varying parameters like temperature and space velocity did not affect the selectivity of p-xylene. When toluene to methanol mole ratio was changed from 4 : 1 to 1 : 1, toluene conversion increased from 14.2 to 33.8 % whereas p-xylene selectivity remained almost the same. This further confirms that high selectivity observed for 5PZSM-5(W) is only due to the increase in shape selectivity of the catalyst through phosphate modification which remained the same under different reaction conditions. 80

25 Zeolite ZSM-5 modification with phosphates for alkylation reaction Table 3.5. Catalytic activity for toluene alkylation with methanol in vapor phase a Catalyst Distribution (mol%) b Toluene conv p-xylene Total Xylene Selectivity ratio Benzene Toluene PX MX OX (Theo.max 25) mol% selectivity mol% selectivity mol% (S para /S meta ) ZSM PZSM PZSM-5(w) a Conditions: Temperature = 480 C, WHSV = 4 h -1, Catalyst weight = 2 g, H 2 : HC = 2, Toluene : Methanol = 4 : 1, Time = 4 h. Methanol conversion 99.9 % ; b PX, MX and OX are abbreviations for o-, m- and p- xylenes respectively. Table 3.6. Toluene alkylation with methanol in vapor phase under different reaction conditions using 5PZSM-5(w) catalyst Performance Temperature ( C) a WHSV (h -1 ) b Mole ratio c :1 1:1 Toluene conversion (mol%) p-xylene selectivity (mol%) Reaction conditions: catalyst weight = 2 g, H 2 : HC = 2, time = 4 h a WHSV = 4 h -1, Toluene : Methanol = 4 : 1; b Temperature= 480 C, Toluene : Methanol= 4 : 1; c Temperature = 480 C,WHSV = 4 h -1 81

26 Ethylbenzene alkylation with ethanol and ethylbenzene disproportionation Ethylbenzene (EB) alkylation with ethanol gives a mixture of ortho, meta and para-diethylbenzenes (DEB) where p-deb is a desirable product. The molecular dimensions of EB and products are bigger than toluene and xylenes. H-ZSM-5 gave 17 mol% EB conversion with 70 mol% p-deb selectivity (Table 3.7.) with almost complete conversion of ethanol. When the zeolite pores are modified with phosphates (5PZSM- 5(W)), para selectivity enhanced to 98 mol%. Notably, EB conversion also improved to 20% after phosphate modification. This could be due to reduction of ethanol to olefin side reaction with low strength acid sites in PZSM-5 resulting in more ethanol utilization to form DEB compared with unmodified ZSM-5. Activity and selectivity of the catalyst did not change during 12 h of time on stream (Figure 3.18.). In both toluene methylation and EB ethylation, selectivity towards para-isomer increased with increase in P content and undesired meta and ortho-isomers of xylene and diethylbenzene decreased. Selectivity ratio (SR) is a measure of extent of pore modification of zeolite represented by the ratio of para to meta isomers. The results show that for toluene alkylation, SR increased substantially after phosphate modification from 0.6 to 32 (Table 3.5.) whereas for ethylbenzene alkylation, it enhanced from 2.7 to 42.7 (Table 3.7.). Table 3.7. Ethylbenzene alkylation with ethanol in vapor phase a Catalyst Distribution (mol%) b EB conv (Theo.max Benzene EB para meta ortho 25) (mol %) p-deb Total selectivity DEB Sel (mol %) (mol %) Selectivity ratio (S para /S meta ) ZSM PZSM-5(W) a Conditions: Temperature = 380 C, WHSV = 3 h -1, Catalyst weight = 2 g, H 2 : HC = 2, EB : Ethanol = 4 : 1, Time = 4 h. Ethanol conversion 99.9 %; b ortho, meta and para are abbreviations for respective DEBs.

27 Chapter 3 Table 3.8. Disproportionation of ethylbenzene in vapor phasea Distribution (mol%)b Catalyst EB conv Benzene EB para ZSM PZSM-5(w) meta ortho 0.02 p-deb Total DEB Selectivity selectivity Selectivity ratio (mol%) (mol%) (Spara/Smeta) a Conditions: Temperature = 380 C, WHSV = 3 h-1, Catalyst weight = 2 g, H2 : HC = 2, Time = 4 h. bortho, meta and para are abbreviations for respective DEBs. Figure Time on stream of ethylbenzene alkylation with ethanol over 5PZSM-5(w) Conditions: Temperature = 380 C WHSV = 3 h-1, Catalyst weight = 2 g, H2 : HC = 2, Ethylbenzene : Ethanol = 4:1. 83

28 EB disproportionation involves a single reactant, EB which undergoes dealkylation and realkylation to form ortho, meta and para diethylbenzenes. Xylenes and heavy aromatics are formed as side products in minor quantities. The performance of EB disproportionation with ZSM-5 and 5PZSM-5(W) were compared at low temperature of 380 C (low conversion level) because EB dealkylation to benzene is predominant under robust reaction conditions. This reaction also showed similar trend of increase in para selectivity (79 to 99 mol %) after phosphate modification (Table 3.8.). Decrease in EB conversion from 12.7 to 7.1 mol% could be due to the higher diffusion barrier created by the pore modification. Selectivity ratio substantially increased from 3.9 to 180 after phosphate modification of ZSM Competitive reaction of ethylbenzene and m-xylene Probe reactions such as competitive cracking of n-hexane and 3-methylpentane for the determination of pore size difference in zeolites were developed by Mobil researchers expressed in terms of Constraint Index [38, 39]. This technique is widely used to differentiate small, medium and large pore zeolites. Similar type of competitive reaction was developed by Bhat et. al. [40] wherein ethylbenzene and m-xylene mixture was used in 20 : 80 ratio to monitor the extent of pore narrowing by silica coating. This procedure was applied here to determine the pore narrowing of ZSM-5 by phosphate modification. In this competitive reaction, EB undergoes disproportionation whereas m- xylene isomerizes to ortho and para xylenes. Extent of pore narrowing is expressed in terms of Constraint Index (CI) given by the formula CI = [Log (1-X EB ) / Log (1-X MX )] where X EB and X MX are conversions of EB and m-xylene respectively. The higher the CI, the greater the constraint imposed by the pores to the bigger m-xylene and thus CI represents the extent of pore narrowing. EB and m-xylene competitive reaction was carried out using HZSM-5 and PZSM-5(W) with different phosphate loadings (Figure 84

29 Chapter ). Before the modification, EB and m-xylene conversions were 22.2 % and 13.7 % respectively. With increase in P content upto 5 %, m-xylene conversion decreased gradually from 13.7 to 1.8 % and remained the same at higher loading. On the other hand, EB conversion sustained even after modification with high amount of phosphate (7% P). As the kinetic diameter of m-xylene is close to the pore size of ZSM-5, small change in the pore dimensions decrease the m-xylene conversion. However, EB with smaller kinetic diameter, still can diffuse into the pores easily. Hence EB conversion could sustain the pore modification. CI increased substantially from 1.7 to 12.3 with increase in P loading from 0 to 5 % (Figure 3.19.). This clearly is the evidence for pore narrowing after phosphate modification. With further increase in P loading from 5 to 7 %, CI deceased from 12.3 to Because at higher loadings, pore size becomes close to the kinetic diameter of EB causing constraint to the diffusion of EB also. Further studies to understand the mechanism of shape selective catalysis in presence of phosphates in zeolite pores are under investigation. Figure Competitive reaction of ethylbenzene and m-xylene over PZSM-5(w) Conditions: Temperature = 400 C WHSV = 6 h-1, Catalyst weight = 2 g, H2 : HC = 1, Ethylbenzene : meta-xylene = 20 : 80 85

30 Part B: Phosphate modified ZSM-5 for shape-selective synthesis of p- diethylbenzene Catalyst activity studies Only few studies have been reported on alkylation of ethylbenzene and benzene with ethanol/ethylene over phosphate treated ZSM-5 for selective synthesis of PDEB [11, 18]. In these reports, activities of the catalysts reported were very low and hence high concentrations of alkylating agent were used. These catalysts contained ionic phosphates in the channels, which created diffusion problems. In Part A of results and discussion, that water treatment after phosphate impregnation and calcination of ZSM-5 remarkably enhances the activity while retaining product selectivity with probe reactions viz. toluene methylation, ethylbenzene ethylation and disproportionation, and competitive reaction of m-xylene and ethylbenzene was reported. In the present section, detailed studies on role of particle size of ZSM-5 and acidity of water treated phosphate modified ZSM-5 was carried out for ethylbenzene alkylation with ethanol reaction. Effect of different operating parameters on the conversion and product selectivity also studied in detail. Scheme 3.3. Ethylbenzene alkylation with ethanol over zeolite ZSM-5 to form diethylbenzenes. 86

31 Alkylation of ethylbenzene with ethanol in a continuous down-flow reactor in vapor phase gives mainly a mixture of o, m, p-diethylbenzenes (hereafter DEB) Scheme 3.3. Minor products such as benzene, toluene, xylene, ethyltoluene, triethylbenzene were also formed during the reaction Effect of silica to alumina ratio of ZSM-5 Silica to alumina ratio (SAR) of zeolite is important for any acid catalyzed reaction as the conversions depend upon the acid site concentration and strength. It is generally observed that, with increase in SAR, amount of acid sites decreases so is the conversion. As the acidity of catalyst decreases, alcohol to olefin side reaction also decreases. This leads to a decrease in coke formation and hence catalyst life increases. Phosphate modification of ZSM-5 not only modifies the acid sites but also narrows the pores. To find the best SAR, 3% P loaded zeolites of different SAR were prepared and studied (Figure 3.20A). With increase in SAR, conversion decreased as expected, but selectivity for para isomer increased progressively from SAR 38 to 407. As added phosphates react with aluminum sites, P/Al increases with increase in SAR with identical 3% P loading. Hence for higher SAR, lower amount of phosphates are sufficient for narrowing of pores and as a result, para selectivity increases with increase in SAR. 3PZSM-5W with SAR 187 showed better performance in terms of EB conversion (22.3 %) with lesser side products (diethylbenzene selectivity of 85 %) and hence this was chosen for further studies Optimization of phosphate content Impregnation of phosphate to ZSM-5 increased the EB conversion as well as selectivity to PDEB (Figure 3.20B). Moreover, weaker acid sites generated in 3PZSM- 5W (NH3-TPD, Figure 3.8) decrease the gaseous product formation from ethanol, 87

32 resulting in increased ethanol utilization towards total DEB. Increase in P from 3 to 5% increased PDEB selectivity from 93 to 98.5 % but conversion was almost the same (~22%) for both the catalysts. Further increase in P > 5 % increased the selectivity for PDEB but decreased the conversion. This could be due to the diffusion limitation imparted by narrowed zeolite pores even to reactant molecules at higher P concentrations. Due to smaller molecular size, ethanol can easily diffuse into pores and reach active sites resulting in gas forming reactions leading to decreased ethanol utilization towards alkylation. Maximum of 99.5 % PDEB selectivity was achieved with 9 % P on ZSM-5 (SAR 187) with 9 % EB conversion. Considering both conversion and selectivity, optimum concentration of P was 5 wt % which gave 23 % conversion with 98.5 % PDEB selectivity in 4 h time on stream Effect of 2-step phosphate modification It is shown that phosphates interact with aluminium sites of ZSM-5 and form phosphate islands with increase in phosphate concentration. Modification of ZSM-5 with phosphate was carried out in 2 steps to understand the phosphate and zeolite interaction (Table 3.4, Table 3.9). For this, total phosphates added in 2 steps were kept constant at 5 wt % (as described in experimental section). Phosphate modified catalysts prepared in two steps and in the single step contained similar amount of phosphates ( %) as well as similar acidity (in the range of mmol/g). This is despite the fact that water treatment was carried out in every step to remove water-soluble phosphates. Notably, activity and selectivity of different catalysts prepared in two steps (with % P addition; 1+4, 2+3, 3+2) were same with 21 % EB conversion with 97 % selectivity whereas for single step addition, conversion was 23 % with 98 % selectivity for PDEB. This shows that, single step addition of phosphates gave slightly better performance than two-step addition. 88

33 Conversion /selectivity (wt %) Conversion /selectivity (wt %) Conv DEBs PDEB SAR EB Conversion Toluene Benzene PDEB selectivity DEBs Ethanol utilization Benzene Selectivity A D 8:1 6:1 4:1 2:1 1:1 Ethylbenzene : EtOH Mole ratio Conversion /selectivity (wt %) Conversion /selectivity (wt %) EB Conversion PDEB Selectivity DEBs Ethanol utilization wt % P loading EB conversion PDEB Selectivity DEBs Benzene Selectivity B WHSV E Conversion /selectivity (wt %) Conversion /selectivity (wt %) EB Conversion Benzene PDEB Selectivity DEBs Ethanol utilization Benzene Selectivity Temperature o C EB conversion PDEB Selectivity DEBs Benzene Selectivity H2/Feed C F Figure Representative data from 4 h of TOS (A) Effect of P modification on ZSM- 5 with different SAR; (B) Optimization of P content on ZSM-5(SAR 187) (Conditions :Temperature = 380 C WHSV = 3 h -1, Catalyst weight = 2 g, H 2 : reactants = 2, Ethylbenzene : Ethanol = 4:1); (C) Effect of temperature (Conditions : WHSV = 3 h -1, Catalyst weight = 2 g, H 2 : reactants = 2, Ethylbenzene : Ethanol = 4:1); (D) Effect of reactant mole ratio (Conditions :Temperature = 380 C WHSV = 3 h -1, Catalyst weight = 2 g, H 2 : reactants = 2); (E) Effect of space velocity (Conditions : Temperature = 380 C, Catalyst weight = 2 g, H 2 : reactants = 2, Ethylbenzene : Ethanol = 4:1); (F) Effect of hydrogen as carrier gas (Conditions :Temperature = 380 C WHSV = 3 h -1, Catalyst weight = 2 g, Ethylbenzene : Ethanol = 4:1) Effect of temperature With increase in temperature from 320 to 400 C, activity of the catalyst increased as expected (Figure 3.20C). EB conversion increased from 7.1 to 25 % whereas PDEB 89

34 selectivity remained almost the same (~98 %) at all temperatures. However, total DEB selectivity increased from 90.9 to 93.4 % as the temperature increased from 320 to 360 C and it decreased to 84.9 % with further increase in temperature to 380 C. As the temperature was increased from 380 to 400 C, EB conversion improved by 2 % however, DEB selectivity decreased due to higher dealkylation to form benzene. Ethanol utilization towards DEB increased as the temperature was increased upto 380 C and then remained constant. Hence, 380 C was optimum temperature with good conversion (23 %), high PDEB selectivity (98.5 %) and ethanol utilization towards DEB (69.64 %). Table 3.9. Stepwise phosphate modification of HZSM-5 (SAR-187) Catalyst Step 1 P added (%) Step 2 P added (%) *EB Conversion (wt%) *Ethanol Conversion (wt %) *PDEB Selectivity (wt%) HZSM PZSM 5W PZSM-5W PZSM-5W PZSM-5W * Conditions :Temperature = 380 C WHSV = 3 h -1, Catalyst weight = 2 g, H 2 : reactants = 2, Ethylbenzene : Ethanol = 4:1, Time = 4 h Effect of reactants mole ratio Effect of mole ratio was studied at 380 C, WHSV of 3 h -1 and H 2 / reactants mole ratio of 2 (Figure 3.20D). Limiting reactant, ethanol conversion was invariably > 99 % in all the experiments. With decrease in EB to ethanol mole ratio from 8:1 to 1:1, the conversion of EB increased from 17.2 to 33.5 %. With increase in concentration of alkylating agent, total DEB increased from 72.6 to 90.8 % instead of heavy aromatics. This could be due to the restricted pore size of PZSM-5, which avoids the formation of 90

35 bulkier products inside the channels. Even though the conversion of EB increased with increase in alkylating agent, it did not increase proportionately with respect to increase in ethanol concentration in the feed because more amount of ethanol converted into gaseous products at higher ethanol concentrations (Table 3.10.). Due to this, ethanol utilization towards DEB decreased substantially from 77.3 to 29.6 % with decrease in EB : Ethanol from 8:1 to 1:1. Optimum mole ratio was found to be 4:1, considering good conversion ( 22.8 %) and high ethanol utilization (69.6 %). Table Gaseous product distribution for different mole ratios of feed Ethylbenzene: Ethanol mole ratio 2:1 8:1 Compounds in effluent gas x (10-3 mol/h) x (10-3 mol/h) Methane Ethylene Ethane Propylene + Propane C4-hydrocarbons Conditions :Temperature = 380 C WHSV = 3 h -1, Catalyst weight = 2 g, H 2 : reactants = Effect of space velocity Weight hourly space velocity (WHSV) gives a correlation between throughput and contact time. With increase in WHSV from 1 to 10 h -1, conversion progressively decreased from 23.9 to 12.5% (Figure 3.20E) due to decrease in effective contact time of reactants with the catalyst. At lower WHSV, the side product formation was higher whereas at higher WHSV side products decreased with improvement in total DEB formation. At WHSV = 1 h -1, conversion was 23.9 % and DEB selectivity was 77 %, whereas at WHSV = 3 h -1 conversion decreased to 22.8 % but total DEB increased considerably (85.3 %). With further increase in WHSV from 3 to 10 h -1, conversion of 91

36 EB decreased progressively whereas DEB selectivity improved only a little. Considering both conversion and selectivities for DEB and PDEB, WHSV of 3 h -1 was chosen as the optimum space velocity Effect of hydrogen as carrier gas Effect of carrier gas H 2 on the catalyst performance was studied by varying H 2 to reactants mole ratio (Figure 3.20F). Without carrier gas (H 2 /reactant = 0), dehydration of ethanol predominated over the alkylation forming high amount of gaseous side products. Due to this, the conversion without carrier gas was low (16.7 %) with total DEB of 90 %. With the introduction of hydrogen as carrier gas (H 2 : Reactants = 1), conversion increased to 21 % whereas total DEB decreased to 84.0 %. With further increase in hydrogen flow of H 2 : Reactants = 2, there was marginal variation in conversion and DEB (22.8 % and 85.3 % respectively). With further increase in the hydrogen flow (H 2 : Reactant = 4), conversion and DEB selectivity decreased considerably to 19.8 % and 82.5 % respectively. Selectivity towards PDEB was constant (~98 %) at different carrier gas concentrations in the feed and for further studies were carried out with H 2 : Reactant = 2. It is important to note that PDEB selectivity in the mixture of DEB was very high and remained almost constant (98-99 %) under various reaction conditions such as temperature, reactants mole ratio, space velocity and carrier gas concentration in the feed. This suggests that high selectivity achieved for PDEB is independent of operating parameters and truly due to shape selectivity obtained by phosphate modification of ZSM-5 catalyst Effect of particle size and morphology Study on the effect of zeolite particle size is important as it gives information on the mechanism of shape selectivity of the catalyst [38]. Particle size has a direct 92

37 correlation with selectivity to para isomer formed during aromatic substitution reactions. For a catalyst with larger particle size, the selectivity of para-isomer is greater than that of selectivity in thermodynamic equilibrium mixture of isomers. It could be observed from optimization of reaction parameters that PDEB selectivity is not affected by the reactions conditions and conversion levels of EB because the selectivity for PDEB is the property of shape selectivity of phosphate modified ZSM-5. Hence the difference in conversion levels obtained for different particle sizes could be ignored while correlating particle size with PDEB selectivity. Silica extrudates of three different particle sizes of ZSM-5 with similar SAR (similar acidity) were studied for ethylation of ethylbenzene under the same reaction conditions (Table 3.11., Figure 3.12.). Unmodified HZSM-5 catalysts with different particle sizes were compared to understand the size effect. It is observed that as the average particle size increased from 0.3 to 1.2 µm, EB conversion decreased from 35.6 to 16.7 % whereas PDEB selectivity enhanced from 38.6 to 66.1 %. With particle size of 1.1 µm and cuboid morphology did not change the conversion and selectivity appreciably (16.3 % conversion and 62.7 % selectivity). For ZSM-5 with large crystals (Avg. particle size = 3.9 µm), conversion increased to 25.5 % whereas there was a marginal decrease in PDEB selectivity (61.4 %). For zeolite with smaller particle size, product can diffuse easily without much diffusion barrier and hence the activity of the catalyst was high. It is found that benzene formation due to dealkylation of EB was higher for unmodified ZSM-5 with small and large crystals. Dealkylation decreased a little after phosphate modification but benzene still remained as major side product. At lower concentration of phosphates (2% P), PDEB selectivity increased with increase in particle size. At higher concentrations (5 % P), small particle size (0.63 µm) attained 88.3% PDEB selectivity and it increased to 91.5% for 1.1 µm with cuboid shape. Para isomer selectivity was maximum for 1.2 µm with irregular shape and 3.9 µm size crystals (~98%). Difference in para selectivity for 1.1 µm size crystals could be due to their 93