Uniform Mesoporous Silicoaluminophosphate Derived by Vapor

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Uniform Mesoporous Silicoaluminophosphate Derived by Vapor Phase Treatment: Its Catalytic and Kinetic Studies in Hydro-Isomerization of 1-Octene Arvind Kumar Singh, Kishore Kondamudi, Rekha Yadav, Sreedevi Upadhyayula, and Ayyamperumal Sakthivel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp509421j • Publication Date (Web): 06 Nov 2014 Downloaded from http://pubs.acs.org on November 10, 2014

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The Journal of Physical Chemistry

Uniform Mesoporous Silicoaluminophosphate Derived by Vapor Phase Treatment: Its Catalytic and Kinetic Studies in Hydroisomerization of 1-Octene

Arvind Kumar Singh,a Kishore Kondamudi, b Rekha Yadav,a Sreedevi Upadhyayula,b Ayyamperumal Sakthivel*a a

Inorganic Materials and Catalysis Laboratory, Department of Chemistry University of Delhi, Delhi 110 007 b Department of Chemical Engineering, Indian Institute of Technology Delhi, HauzKhas, New Delhi 110016 Email: [email protected] / [email protected] Phone: +91-8527103259 / +91-9811891257

ABSTRACT:

Ordered mesoporous silicoaluminophosphate (MESO-SAPO-34) assembled from

microporous SAPO-34 precursor was stabilized by post synthesis vapor phase treatment. The resultant MESO-SAPO-34 possesses uniform mesoporosity, high surface area, pore volume, and strong acidity. The resultant materials were studied as catalysts for the hydroisomerization of 1-octene and showed excellent conversion, the maximum branched isomer selectivity of about 52% was achieved at low WHSV. Kinetic models based on Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism were developed to describe the hydroisomerization of 1-octene and the kinetics and adsorption parameters were estimated. A non-linear regression based on modified Levenberg-Marquardt algorithm was used to estimate the parameters and the model values were found to compare well with the experimental 1

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values. Parameters obtained from the LHHW model show that the conversion of 1-octene to other linear octenes requires low activation energy, whereas the conversion of linear octene to branched octene demands the highest activation energy. The enthalpy and entropy of adsorption obtained from Arrhenius plots were found to be consistent with thermodynamics.

Keywords: MESO-SAPO-34, vapor phase treatment, hydroisomerization, Langmuir-HinshelwoodHougen-Watson, kinetic modeling.

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INTRODUCTION The branched isomers of lighter hydrocarbons (C5-C8) have higher octane numbers than their corresponding linear alkanes/alkenes and are consequently valuable additives to the gasoline pool.1-3 Zeolite and zeolite-like microporous molecular sieve materials and other solid-acid catalysts have proved to be promising catalysts for linear hydrocarbon conversion in the petrochemical industry.4 Heterogeneous catalysis has advantages over homogeneous catalysis, since, it can overcome the catalyst recyclability problem associated with the later. In this regard, it is worth mentioning that aluminophosphate (AlPO) molecular sieves are structurally analogous to zeolites, have better framework flexibility with structural diversity, and are potential catalysts for isomerization, alkylation, and disproportionation reactions.5-9 An enormous number of aluminophosphate molecular sieve based materials have been synthesized over the past few decades.1-5 Uneven substitution of aluminium and phosphorous, via replacement by heteroatoms such as silicon and other transition metal ions, introduces extreme structural diversity in aluminophosphate materials.5 Substitution of silicon in the aluminophosphate framework (SAPO) produces materials with unique combinations of channel size and variable acidity.1-13SAPOs with mild acidity shows, enhanced selectivity and reduced deactivation rates is several organic transformations such as oligomerization of lighter olefins, dehydrocyclization, hydroisomerization, xylene isomerization, and the methanol-to-olefins process.1-13 Compared to conventional microporous SAPOs, hierarchical mesoporous SAPOs and nano sized SAPOs have advantages of shorter channels, higher surface area, and a larger number of exposed active sites that exhibit excellent catalytic activity and improve the resistance to coke deposition.12-17 Although mesoporous silicoaluminophosphates and aluminosilicates are also promising catalysts, their structural instability limits their applications. Several researchers have adopted various in situ and ex situ synthesis techniques to develop thermally and mechanically strong mesoporous materials.18-27 Within the domain of aluminosilicate mesophase 3



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materials, surfactants employed in “bottom up” approaches were extensively explored to prepare meso/micro-composite materials by utilizing microporous molecular sieve seeds.25,26 These amendments enhanced the thermal/mechanical stability and also provided excellent catalytic activity in several organic transformations such as esterification28, cracking,29 acylation,30 and synthesis of α-tocopherol.31 These established landmarks in the field of aluminosilicate mesoporous materials motivated researchers to focus on the synthesis of framework materials similar to silicoaluminophosphate.4-11,32,33 Numerous methods like the liquid crystal templating (LCT)34 mechanism, cooperative self-assembly,35,36 acid-base pair,37 sol-gel,38 and solvent-free39 synthetic routes have been used to synthesize mesoporous AlPO. Nevertheless, the lack of long-range order, owing to imperfect crystallization and poor condensation of inorganic moieties like Al, P and Si around micelle aggregates of the surfactant makes the AlPO-based mesophase lamellar. The mesophase possesses a number of poorly connected AlO4-, Al(H2O)63+, and PO43- ions, which are prone to collapse during calcination. In this regard, we have recently established a synthetic route for the preparation of different hierarchical MESO-SAPO-n, by utilizing microporous SAPO-n precursors.40-42 While the resultant short-range-ordered MESO-SAPO-n displayed promising activity for hydroisomerization of olefins, the development of long-range-ordered mesoporous SAPO is the subject of present investigation. It is also expected to provide better branched isomer selectivity for the hydroisomerization of hydrocarbons, since the molecular sieve maintains the long-range order as it remains in the channel for a long period. Vapor phase treatment of dry-gel silicates and aluminosilicate molecular sieves, in a hot aqueous environment with an aluminium source, helps to stabilize mesoporous silicates with long range order.43-45 In addition, the dry-gel conversion method helps to achieve better structure orientation and stability due to the high crystallinity of the nucleus.46,47 Herein, we report post-synthesis treatment of mesoporous silicoaluminiophosphate-34 (MESO-SAPO-34) assembled from microporous SAPO-34 precursor, by dry-gel conversion in the presence of tetraethylorthosilicates (TEOS) and water by vapor phase treatment(VPT). To the best of our knowledge, 4


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this is the first report on the stabilization of mesoporous silicoaluminophosphate materials by the VPT route. The pictorial representation of VPT process is depicted in Scheme 1. The stabilized highly ordered mesoporous silicoaluminophosphate (MESO-SAPO-34) materials were studied for vapor phase hydroisomerization of 1-octene. The influences of various kinetic parameters, such as catalyst particle size, space velocity, temperature, and mass transfer properties were systematically studied. A kinetic model, based on the Langmuir-Hinshelwood-Hougen-Watson (LHHW) method, was proposed to describe hydroisomerization of 1-octene on stable mesoporous silicoaluminophosphate (MESO-SAPO-34).

EXPERIMENTAL The materials employed in the synthesis were pseudoboehmite (76% Al2O3; ACE, India), fumed silica (Aerosil-200; Aldrich), orthophosphoric acid (85%, Merck), morpholine (99%, Thomas Baker), cetyltrimethylammonium

bromide

(99%;

Spectrochem),

25

wt.%

aqueous

solution

of

tetramethylammonium hydroxide (TMAOH), and 40 wt.% aqueous solution of tetrapropylammonium hydroxide (TPAOH, Tritech chemical).

Synthesis of microporous SAPO-34 Microporous SAPO-34 was synthesized by a reported procedure

40

with a molar gel composition of 1.0

Al2O3 : 1.06 P2O5 : 1.08 SiO2 : 2.09 R : 66H2O (where R stands for morpholine). Solution A was prepared by mixing a calculated quantity of phosphoric acid and pseudoboehmite in distilled water, followed by stirring for 8 hrs to obtain a uniform mixture. Solution B was obtained by mixing fumed silica and morpholine in distilled water with continuous stirring for 8 hrs. Solution B was added drop wise to solution A with continuous stirring and the final mixture was aged at room temperature for 24 hrs. The

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resultant gel was subjected to crystallization at 200 oC for different durations in a Teflon-lined stainless steel autoclave under autogenous pressure.

Synthesis of mesoporous SAPO-34 (MESO-SAPO-34) Synthesis of mesoporous SAPO-34 (MESO-SAPO-34) using preformed microporous SAPO-34 precursors was carried out by modifying the previously reported procedure.40 This was accomplished in two steps: (i) The surfactant solution was prepared by mixing an appropriate quantity of cetyltrimethylammonium bromide in distilled water under constant stirring. (ii) Preformed microporous SAPO-34 precursor (obtained via the above-mentioned procedure with a crystallization time of 5 hrs) and a calculated amount of 40 wt.% TPAOH solution were introduced to the surfactant solution. The resultant gel was homogenized under constant stirring followed by the addition of calculated quantities of TMAOH and water. This gel was aged for 24 hrs under constant stirring, and then allowed to crystallize at 70 oC for 48 hrs. The resulting samples were washed thoroughly with distilled water and ethanol, and dried before calcination at 550 oC for 6 hrs in an air oven. The final composition of MESO-SAPO-34 was 1.0 Al2O3 : 1.06 P2O5 : 1.08 SiO2 : 2.09 R : 0.57 CTAB : 2.8 TMAOH: 1.0 TPAOH : 300 H2O.

Post synthesis stabilization by vapor phase treatment As-synthesized mesoporous aluminophosphate framework materials possessing strong interaction with surfactant species and the presence of uncondensed alumina and phosphate species results in collapse of structure upon calcination. In this regard, analogues mesoporous silicates materials were stabilized by post synthesis treatment of mesoporous silicate materials by aluminium isopropoxide in the presence of steam not only stabilizes the structure but also results in the incorporation of aluminium species into the silicate framework. We believe that the introduction of an organo silane [such as tetraethylorthosilicate (TEOS)] in the presence of steam or hydroxyl, could facilitate tethering of uncondensed alumina and 6


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phosphate species, and also improve incorporation of silicate species into the framework. This could potentially improve the stability of silicoaluminophosphate mesoporous molecular sieves.43-45 Thus; we introduced TEOS by vapor phase treatment in the presence of steam and acid/base conditions. In a typical vapor phase treatment process, the dried and finely crushed as-synthesized MESO-SAPO-34 sample was transferred into a Teflon cup and placed in a Teflon-lined autoclave. A calculated amount of TEOS and different condensing media [H2O/H3PO4/TMAOH/ acetic acid (AcOH)] were placed at the bottom of the autoclave.47 The post synthesis stabilization of MESO-SAPO-34 was carried out at 140 oC for 14 hrs. The stabilized materials were calcined at 550 oC for 6 hrs in air. The MESO-SAPO-34 stabilized by VPT with TEOS, in presence of water/phosphoric acid/tetramethyl ammonium hydroxide/acetic acid are represented as M-S-34-VPT, M-S-34-H3PO4, M-S-34-TMAOH, and M-S-34-AcOH respectively.

Characterization Vibrational spectra were recorded at room temperature on a Perkin-Elmer 2000-FTIR, in the range 400– 4000 cm-1 using KBr pellets. Powder X-ray diffraction (XRD) was performed [18 KW XRD Rigaku (2500V), Japan] with Cu-Kα radiation (λ = 1.54184 Å) to determine the bulk crystalline phases of the materials. The diffraction patterns were recorded in the 2θ range 0.5–30°, with a scan speed and step size of 0.5°/min and 0.02°, respectively. The morphology and size of the materials were investigated by scanning electron microscopy [(SEM) EVO / MA15 Zeiss operated at 10-20 kV] and transmission electron microscopy [(TEM),Phillips Technai G230, operated at 300 kV]. Nitrogen adsorption/desorption isotherms were recorded on an automatic surface area and porosity analyzer (Micromeritics ASAP 2020, USA). The analysis was carried out at -196oC causing samples already degassed at 300oC for 12-14 hrs under 0.1333 Pascal pressure. The BET surface area was calculated in the relative pressure range 0.050.3, over the adsorption branch of the isotherm. Various other textural properties such as DFT surface area, pore volume (BJH, DFT, HK), and pore size distribution (BJH with Fass correction, calculated from 7



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the desorption branch) were elucidated from the isotherm data. Solid-state NMR experiments were carried out on a Bruker AVANCE 400 wide bore spectrometer equipped with a superconducting magnet with a field of 7.1 T using a 4 mm double resonance magic angle spinning (MAS) probe operating at resonating frequencies of 79.4, 104.26 and 161.9 MHz for 29Si, 27Al and 31P respectively. The samples were packed in 4 mm zirconia rotors and subjected to a spinning speed of 10 kHz: single pulse experiment with pulse duration of 4.5 µs and a relaxation delay time of 6 s were used for recording all 29

Si,

27

Al, and

31

P MAS NMR patterns. All chemical shift values are expressed with respect to 2,2,

dimethyl-2 silapentane-5-sulfonate sodium salt (DSS)for the29Si nucleus, 85% H3PO4 for the 31P nucleus, and 0.1 M aqueous solution of Al(NO3)3 for the27Al nucleus. Acidity of the samples were qualitatively measured by pyridine FT-IR spectra collected on a Thermo Scientific Nicolet 6700 FTIR single beam spectrometer using a liquid-nitrogen-cooled MCT detector. Pyridine vapor adsorption studies were carried out in a Harrick Scientific HVC-DR2 reaction chamber with a detachable ZnSe window dome, mounted inside a Harrick DRA-2 Praying Mantis diffusereflectance accessory designed to minimize parasite specular reflectance. About 100 mg of sample (10% of material inKBr) was placed in the sample cup and pre-activated at 3500C for 6 hrs. For pyridine adsorption, N2 gas was passed through a pyridine saturator. A partial pressure of 30 mm Hg of pyridine was maintained in the saturator. For pyridine adsorption, the temperature of sample cup was maintained at 100 oC for 1 hr. After pyridine adsorption, the sample was heated to 150 oC and flushed with ultra-high pure N2for 1 hr to ensure that physically adsorbed pyridine was recovered completely. The sample was cooled to 25 oC and the spectrum was collected using KBr as the background. Further, the sample was degassed at a desired temperature and spectra collected. The quantitative analysis of Brönsted and Lewis acidity was performed by the deconvolution method using a Gaussian function.

Hydroisomerization of 1-octene on M-S-34-VPT 8


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The catalytic activity of M-S-34-VPT (MESO-SAPO-34 stabilized in the presence of water) was investigated in vapor phase hydroisomerization of 1-octene using a fixed-bed reactor (Lab India Scientific Instrument, India). The carrier gas flow was controlled by using MFC (mass flow controller), and liquid feed was introduced into the reactor by a liquid injection pump (Inkarp Instruments Pvt Ltd, model no. SP-22 series).Experiments were carried out by using 1 g of catalyst (M-S-34-VPT) at different reaction temperatures (325-475oC), WHSV (20, 16, 12 and 8 h-1)and mesh size (0.1 and 0.3 mm) by maintaining the flow rate of the carrier gas (H2) at 25 mL/min. The products were analyzed in a HP-5 capillary column (30m, HP-5) using gas chromatography equipped with flame ionizing detector FID (Agilent 7890A Series). The reaction was also studied over conventional microporous SAPO-34 and MESO-SAPO-34 under identical conditions and compared with results of the present investigation.

RESULTS AND DISCUSSION Fourier transform -infrared spectra of all the samples showed (not reproduced here) vibration bands around 1084 and 730 cm-1, characteristic of the silicoaluminophosphate framework stretching. Additional vibration bands observed around 640 and 526 cm-1 were assigned to T-O-T vibrational frequencies of structural building units (SBUs) such as double six-membered ring (D6R) and single fourmembered ring (S4R).40 This supports the claim that MESO-SAPO-34, assembled from microporous precursor SAPO-34, has microporous precursor units on the wall of the mesoporous framework.40 The powder XRD patterns of the as-prepared and calcined vapor-phase-treated samples are shown in Figure 1. All the samples show clear reflections around 2θ of 1.5–2.3° which correspond to the (1 0 0) plane of an ordered hexagonal mesoporous structure. M-S-34 showed a reflection at 2θ ~2.2°, whereas in M-S34-VPT, this peak shifted towards lower 2θ of 2.1°due to the incorporation of larger tetravalent silicon ions (Si4+, 0.04 nm) in the framework position of smaller penta-valent phosphorous sites (P5+, 0.03 nm).48,49 This shift observed in as-synthesized samples is not predominant owing to the presence of 9



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surfactant in the channels could hinder the incorporation of Si4+ into the framework. Low-angle powder XRD patterns of the calcined mesoporous SAPOs showed relatively broad reflections with 2θ values of 2.1°, typical of hierarchical MCM-41-type (MCM = mobile crystalline material) hexagonal mesoporous structures. The XRD patterns of the calcined, vapor-phase-stabilized samples obtained in acidic (H3PO4 and AcOH) and basic (TMAOH) conditions reveal that the structure collapses due to the hydrolysis of aluminophosphate and silicate species during VPT. However, the sample stabilized in the presence of TEOS and steam showed a well-resolved prominent peak at a 2θ of 0.82° which is typical of the (1 0 0) plane in hexagonal mesoporous structures. The water vapor produced during calcination hydrolyzes the TEOS and tethers the uncondensed site present in as-synthesized MESO-SAPO-34, resulting in uniform mesoporosity, which in turn resulted in the shift in 2θ value. The broadening of the XRD reflection of the parent sample is because of short range order in the mesoporous material.40,42 However, theM-S-34-VPT sample showed less broadening in the XRD pattern, indicating that tethering with the help of TEOS resulted in well-resolved intense peaks, characteristic of hexagonal mesoporous structure. From the XRD studies, it reveals that in MESO-SAPO-34, the framework T-O-T species are present in uncondensed forms, which result from a lack of interconnection of T-O-T groups. The addition of TEOS with the help of steam during vapor phase treatment assists in the tethering of these uncondensed species to get uniform stable mesoporous SAPO-34 materials. The SEM images shown in Figure 2(a) revealed the presence of a uniform hexagonal morphology with structures 2–3 μm in length. It was deduced from the images that the edges of these arrays possess surface roughness. The presence of heterogeneity and unevenness along the edges of mesoporous arrays are due to the presence of SAPO-34 secondary building unit precursors present along the walls of mesoporous channels. The TEM image of M-S-34-VPT displayed in Figure 2(b) shows the presence of

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hierarchical array of pores with pore sizes of around 3 nm. Further, ED pattern of M-S-34 showed spot pattern, which confirmed periodicity and crystalline wall nature of the mesoporous materials. The N2 sorption isotherm of M-S-34 and M-S-34-VPT is shown in Figure 3. Both the samples show combined features of type-I and type-IV isotherms.40 The sharp uptake below a relative pressure (p/po) of 0.1 indicates strong interaction between adsorbate and adsorbent, which corresponds to a microporous nature present on the surface of M-S-34-VPT, and which arose from the SAPO-34 secondary building unit precursor used to assemble the mesopores. The isotherms are also associated with prominent multilayer adsorption between the relative pressures (p/po) of 0.4 and 0.6 attributed to mesopore filling via capillary condensation. It is important to note that the direct calcined sample showed a sigmoidal curve with continuous multilayer adsorption for relative pressures ranging from 0.4 to 0.8; however, after vapor phase treatment in the presence of TEOS and steam, the adsorption curve became hyperbolic in nature with sharp uptake in the p/po range of 0.4–0.6, owing to the uniform mesoporous nature of the M-S-34-VPT sample. Corresponding curves obtained for VPT samples in the presence of acid-base, showed (Fig. S1) a flattening of N2 uptake in the relative pressure range of 0.4– 0.6 is due to considerable collapse in the mesoporous channel. Thus, tethering of the mesoporous channel occurs only under conditions of VPT in presence of TEOS and steam. Further, the sample showed H4 type hysteresis, which indicates the presence of slit-like pores with “ink-bottle” type geometry. The textural properties of materials synthesized by VPT under different conditions are summarized in Table 1. It is clear from the table that samples prepared in the presence of steam using TEOS tethering showed better BET (466 m2 g-1) and DFT (575 m2 g-1) surface areas and high pore volume (0.37 cm3 g-1). From the desorption curves, the uniform BJH pore size distribution (PSD) was calculated to be 3.6 nm (inset in Figure 3), which is characteristic of well-ordered mesoporous materials. Further analysis by DFT and H-K method showed a PSD of 0.8 nm due to intra-crystalline micro-porosity and surface roughness, 11



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which arises from the precursor unit of SAPO-34 used in the synthesis.50,51 The use of acid-base, such as H3PO4/AcOH/TMAOH in the presence of TEOS, resulted in the formation of a dense phase on the external surface and collapse of the meso-structure upon calcination, decreasing the surface area and pore volume. Further, to understand the textural properties and stability of M-S-34-VPT, N2 sorption analysis was carried out at different degassing temperature, as the degassing temperature could reversible or irreversible change in texture properties and composition.52 The results are shown in Fig. S2 and Table S1. The isotherm obtained at different degassing temperature remains ideal. Increase in degassing temperature increases the extent of nitrogen adsorbed, which shows that amount of physisorbed molecules is not complete unless degassing temperature reaches about 300oC. However, no drastic change in BJH pore volume and surface area was evident with further increase in degassing temperature. The isotherm trend supports that the material’s inherent properties remain intact. It is only occluded physisorbed molecule responsible for the deviation in textural properties. The coordination environment of different framework ions (Al3+, P5+, and Si4+) was investigated by solid state MAS NMR studies and the results are shown in Figure 4. The extent of condensation of silica in the mesoporous SAPO framework was studied by 29Si MAS NMR. The M-S-34-VPT sample(Figure 4a) shows three peaks centered around -110, -103, and -90 ppm representing Q4 (Si(0Al)(4Si)), Q3 (Si(1Al/OH)(3Si)), and Q2 (Si(2Al/OH)(2Si)) sites, respectively. The minimum line broadening was observed for the M-S-34VPT sample, supporting the condensation of surface hydroxyl groups. 53,54 The 27Al MAS-NMR spectrum of M-S-34 shows (Fig. 4b)two peaks at ≈53.1 and a broad signal around 5.7 ppm, which are assigned to tetrahedral Al(OP)4 and penta-coordinated aluminium species (Al(OP)x(OH)5-x), respectively.40,42 M-S-34VPT showed two distinct signals centered at 41.4 and –10.3 ppm, attributed to tetrahedral and octahedral Al species, respectively, typical of hexagonal mesoporous aluminophosphates.55 31P MAS NMR of calcined M-S-34 showed a broad spectrum centered around -26 to -8 ppm, which indicated that phosphorous is present in different environments.42 This broad signal changed to a sharp intense peak 12


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centered at ≈25.3 ppm, corresponding to tetrahedral P(OAl)4units, in M-S-34-VPT.56 The observed chemical shift clearly manifests that VPT assists the incorporation of uncondensed (PO4)-3 moieties into the mesoporous T-O-T framework in M-S-34-VPT. The nature and strength of the surface acidity of M-S-34 and M-S-34-VPT samples were investigated by pyridine desorption FT-IR spectra, and the results are depicted in Figure 5. Both the M-S-34 and M-S-34VPT samples had a strong vibrational band around 1530 cm-1 and the peak intensity at elevated desorption temperatures indicates the presence of strong Brönsted acidic sites.40-43 The broad vibrational bands around 1440–1450 cm‒1 correspond to pyridine coordinated to Lewis acid sites, resulting from the presence of coordinately unsaturated framework aluminium moieties. An additional band around 1480–1490 cm-1 is attributed to both pyridine-bound Lewis and Brönsted acid sites. It is evident from the spectra that the vibrational band at 1530 cm-1, for the M-S-34-VPT sample, is quite broad and more intense than those of M-S-34 samples. Moreover, the relative ratio of Brönsted-toLewis acidic sites (B/L ratio) on M-S-34-VPT and M-S-34 was calculated from the area under the peaks (at 1540 and 1440 cm-1). The results show (see Table )that the B/L ratio of MS-34-VPT is about 5.8 and is much higher than that of pure M-S-34, which is about 2.2 at the desorption temperature of 400 °C. Thus, the incorporation of silicate species during vapor phase treatment resulted in strong Brönsted acidic sites on the M-S-34-VPT sample.

Hydroisomerization of 1-Octene over M-S-34-VPT M-S-34-VPT was evaluated as a catalyst for the hydroisomerization of 1-octene in vapor phase conditions using a fixed-bed flow reactor at various WHSV, temperatures, and catalyst size. Initial studies were carried out by fixing the reaction temperature at 400 °C and varying the WHSV to 20, 16, 12, and 8 hrs-1 respectively. The results are shown in Figure 6. The gradual decrease in 1-octene

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conversion with increasing WHSV is due to the lower contact time of reactant molecule with the active sites of the catalyst. At a low WHSV of 8 hrs-1, the reactant molecules interact with the active sites on the catalyst for the longest time and showed maximum conversion in the range of 90% with selective formation of isomerized products. Importantly, the catalytic activity remains steady over a period of more than 8 hrs. The effect of reaction temperature on hydroisomerization of 1-octene on M-S-34-VPT was studied in the range of 325-475 °C, with WHSV in the range of 8 -20 hrs-1 as shown in Table 2. As the temperature increases from 375 to 475 °C, a significant improvement in 1-octene conversion from 82.2 to 94.6% was observed. Increasing the temperature beyond 475°C does not show any appreciable change in conversion. Notably, the selectivity of linear octene isomers (B) decreases, while that of mono branched (C) olefin isomers increases proportionally with reaction temperature. The formation of bibranched octene isomers (D) remains constant as the reaction temperature increases from 325 to 400°C; however, upon further increase in reaction temperature to 475°C, the selectivity towards bibranched isomers decreases slightly owing to the reversibility of the reaction. In addition, at high temperature (475 °C), trace amounts of cracked products such as pentane, hexane, and methylbutene were identified. The presence of strong Brönsted acidic sites on M-S-34-VPT results in the observed high conversion and branched isomer selectivity. For comparison, reactions carried out under optimum conditions with microporous SAPO-34 and M-S-34, which showed about 71.7 and 79.6 % conversion of 1-octene, respectively. In both cases, the linear isomerized products were obtained as a major product with a selectivity of 70 and 62 %, respectively, and the remaining products were monobranched isomers. Hence, M-S-34-VPT showed better conversion and branched isomer selectivity than the analogous parent samples due to presence of strong Brönsted acidic sites.

Mass Transfer Considerations and Kinetic modeling

14


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Kinetic investigations were carried out to optimize reaction conditions for maximum branched isomer yields. Firstly, the external mass transfer resistance-free region was determined by conducting experiments with constant space time (τ or W/FAO) and catalyst size, and varying the feed rates.57,58 The results are shown in Table 3. By varying the catalyst loading, the conversion of 1-octene remained constant at different space times, which indicates the absence of external diffusional resistance in the range of the feed flow rates chosen. Further experiments were conducted to test the intra-particle diffusional limitations by varying the catalyst particle size and keeping the space time constant.59 The experimental data obtained are presented in Table 4. Once again, the results showed that there was no significant change in 1-octene conversion with different catalyst mesh size, indicating negligible intraparticle mass transfer resistance, in the range of particle sizes studied. The particle sizes employed in the investigation (i.e. 0.1-0.3 mm) falls within the intra-particle diffusion free regime. Kinetic data were collected at different temperatures in the range of 325-475 oC at atmospheric pressure. At each temperature, the space time (τ = W/FA0), was varied by changing the liquid feed rate. In all the runs, 1 g of catalyst (of pellet size 0.1 to 0.3 mm) was used in the presence of hydrogen as a carrier gas at a flow rate of 25 mL/min. Within experimental error, the mass balance was always maintained at 99.1% or above. The variation in 1-octene conversion with W/FA0 shows that the 1-octene conversion increases with space time at all temperatures. Kinetic models were developed based on homogeneous (empirical) and heterogeneous kinetic model on Langmuir-Hinshelwood-Hougen-Watson (LHHW) approach.58,60 It was found experimentally that the reaction was free from the mass transfer resistances. In accordance with product yields, the reaction mechanism can be described by the reaction pathway shown in figure 7. In the kinetic model developed here, 1-octene hydroisomerization to iso-octenes was considered to be first-order with respect to octene. The mechanistic pathway of the reaction is presented in Figure. 7. From the reaction mechanism, a first order rate equation is formulated. As octene has many different 15



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isomers, for the sake of simplicity, the isomers were grouped based on their bond structure as linear (B), mono-branched(C), and di-branched (D) isomers. The rate of formation of these components can be expressed as follows: Rate of consumption of 1-octene (A)

dC A = −( k1 + k3 )C A + k2CB dτ

(1)

Rate of formation of linear isomers of octene (B)

dCB = k1C A − k2CB − k4CB dτ

(2)

Rate of formation of mono isomers of octene (C)

dCC = k3C A + k4CB − k5CC + k6CD dτ

(3)

Rate of formation of di-isomers of octene (D)

dCD = k5CC − k6CD dτ

(4)

Where, C is the concentration of respective components in mol l−1, τ is the space time (W/FA0) in second (s), k is the respective rate constant in s−1. LHHW (Langmuir–Hinshelwood– Hougen–Watson) model58,60 with dual site and single site with different rate controlling steps were checked for a fit with the experimental data. Since the reaction was found to be kinetically controlled, the adsorption and desorption rate controlling steps were discarded. Only surface reaction with a single site mechanism was considered as the rate determining step (RDS).

16


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According to this model, the rates of formation of the five components were given by the following differential equations (Eq. 1 to 4). Rate of consumption of 1-octene (A)

dC A = −( k1 + k3 )θ A + k2θ B dτ

(1)

Rate of formation of linear branched isomers of octene (B)

dCB = k1θ A − k2θ B − k4θ B dτ

(2)

Rate of formation of mono branched isomers of octene (C)

dCC = k3θ A + k4θ B − k5θ C + k6θ D dτ

(3)

Rate of formation of di-branched isomers of octene (D)

dCD = k5θ C − k6θ D dτ

(4)

where, θA, θB, θC, θD,and θV represent the fraction of sites covered by A, B, C, D, and vacant sites, respectively and are given by following set of equations

θA =

K AC A (5) (1 + K AC A + K BCB + KC CC + K DCD )

θB =

K BC B (1 + K AC A + K BCB + KC CC + K DCD )

θC =

KC CC (1 + K AC A + K BCB + KC CC + K DCD )

(6)

(7) 17



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θD =

K DCD (1 + K AC A + K BCB + KC CC + K DCD )

Page 18 of 53

(8)

θ A + θ B + θC + θ D + θV = 1

(9)

The above kinetic model involves six kinetic rate constants (k1 to k6) and four equilibrium adsorption constants KA , KB, KC, and KD. Regression analysis and parameter estimation The kinetic and equilibrium constants of the developed models were estimated using the experimental data at three different temperatures, 325, 400, and 475 oC by non-linear regression algorithm based on modified Levenberg–Marquardt61,62 algorithm in MATLAB. The optimum values of the parameters were estimated by minimizing the objective function given by n

f = ∑ ( x pred ) − ( xexp )  i i  i =1

2

The estimated optimized rate constants/equilibrium constants at different temperatures are shown in Tables 5 and 6 for homogeneous and LHHW-based models respectively. It was observed that with increasing temperature, the value of the rate constants increased as expected, and the estimated values of adsorption equilibrium constants decreased. This is also expected as adsorption is an exothermic process. The concentrations of A, B, C, and D are calculated at various reaction times using the estimated rate constants at different temperatures, and the average relative deviation was calculated for both models. The average relative deviation is more for the homogeneous model than for the LHHW-based model. Therefore, the LHHW-based model explains the kinetics well for this reaction over the prepared catalyst. The concentration profile obtained by the LHHW model at 400 oC along with the experimental data is 18


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shown in Figure8. It is well evident from figure that experimental data well fitted with LHHW model. The Arrhenius plot of ln(ki) vs 1/T was then made using rate constants estimated from the LHHW-based model at different temperatures, as shown in Figure 9. The activation energy and pre-exponential factor were obtained from the slope and intercept of the Arrhenius plot as summarized in Table 7. From Tables 6 and 7, it is clear that the formation of linear isomers (B) is more favorable as the rate constants are high at lower temperatures compared to the formation of B. From the error estimates, we can observe that the LHHW model fits the experimental data reasonably well. This was further confirmed by checking for thermodynamic consistency63 using the Mears–Vannice criteria,64-66 and was studied and summarized in Table 8. To check the Mears-Vannice criteria, the enthalpy and entropy of adsorption were calculated using relations given below

K i ( i = A , B ,C , D ) = e ln( Ki ) =

 ∆Si   ∆H i   R   RT  

e

−∆H io ∆Si + (where i=A,B,C,D) RT R

A plot of ln(Ki) vs 1/T was prepared, with ∆Had/R as the slope and ∆Sad/R as the intercept, as shown in Figure 10, which shows a satisfactory straight line trend. Calculated heats of enthalpy and entropy change are reported in Table 9. It is evident from Rule 1, that adsorption, is invariably exothermic; thus, o the enthalpy of adsorption is negative i.e. −∆H ad > 0 . Rule 2 states that there must be a decrease in

o o entropy after adsorption ∆S ad = Sad − S go and according to Rule 3, a molecule or atom cannot lose

more entropy than it possesses prior to adsorption, thus Sad < Sg .The additional guidelines set by o

o

Everett are also satisfied.67 Thus, the thermodynamic consistency is verified and provides a convenient means to predict the reaction rates.

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In summary, based on the kinetic models, the observed heat of enthalpy and entropy changes, the adsorption is an exothermic process and the formation of linear isomers are more favorable at low temperatures while branched isomers are favored at high temperature. The LHHW kinetic model fits well with the experimental data.

CONCLUSION

Vapor-phase-treated mesoporous silicoaluminophosphate assembled from SAPO-34 secondary building unit precursor (MESO-SAPO-34-VPT) showed uniform mesoporosity. Improved surface area, pore volume, and acidic strength were evident in VPT-treated samples. The material was shown to be a potential catalyst for hydroisomerization of 1-octene with about 50% branched isomer selectivity. The kinetic model based on the LHHW mechanism showed that the reaction is first order with respect to 1octene conversion. Furthermore, the reaction proceeds without any external particle mass transfer or interparticle diffusion limitation. Activation energy calculations showed that the formation of linear isomers is preferred at low temperature, while at higher temperatures, branched isomer selectivity improved, which is in agreement with the experimental data. The mono-branched and bibranched isomers remain in equilibrium over the course of the reaction.

20


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ASSOCIATED CONTENT Supporting Information Fig. S1. N2 sorption isotherms of M-S-34-VPT-TMAOH, M-S-34-AcOH and M-S-34-H3PO4. Fig. S2. N2 sorption isotherm of M-S-34-VPT obtained at different degassing temperature. Table S1 Textural properties of M-S-34-VPT obtained at different degassing temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected] / [email protected]; Phone: +91-8527103259 / +919811891257

ACKNOWLEDGMENTS Authors are thankful to DST (SR/S1/PC-11/2011) and University of Delhi (R & D/2013-14/4155) India, for the financial support. Authors also express their sincere thanks to USIC, University of Delhi, and CSIRCECRI Karaikudi, India for their support on instrumentation facility. AKS and RY are grateful to CSIR and UGC, India, for their SRF.

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NOTATION A0

Apparent Arrhenius frequency factor

CA0

Concentration of A (kg mol m-3)

Ea

Apparent activation energy (kJ mol-1)

f

Optimization function value

FA0

Feed flow rate (kg mol h-1) Standard enthalpy of adsorption (kJ mol-1)

ki

Rate constants of ith component (S-1)

Ki,ad

Equilibrium adsorption constants ( ) Difference in standard entropy between adsorbed component to the same component

in gas phase at (101.325 kPa) (kJ mol-1 K-1) R

Universal gas constant (8.314 J mol-1 K-1 ) Standard total entropy in gas phase (kJ mol-1 K-1)

T

Temperature (oC)

TOS

Time on stream (hrs)

W

Weight of catalyst (kg)

WHSV

Weight hourly space velocity (kgmol kg-1 h-1)

Xi

Fractional conversion of ith component

τ

Space time (S)

θi

Fractional adsorption of ith species on catalyst surface

ν

Wave number (cm-1)

Subscripts A

n-octene

B

linear iso-octene

C

mono-branched octene

D

di-branched octene

i

species number (A, B, C or D)

22


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52. Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquérol, J.; Siemieniewska, T. Reporting Physisorption Data For Gas/Solid Systems With Special Reference to the Determination of Surface Area and Porosity. Pure & Appl. Chem. 1985, 57, 603-619.

53. Tan, J.; Liu, Z.; Bao, X.; Liu, X.; Han, X.; He, C.; Zhai, R. Crystallization and Si Incorporation Mechanisms of SAPO-34. Microporous Mesoporous Mater. 2002, 53, 97–108.

54. Martins, G. A. V.; Berlier, G.; Coluccia, S.; Pastore, H. O.; Superti, G. B.; Gatti G.; Marchese, L. Revisiting the Nature of the Acidity in Chabazite-Related Silicoaluminophosphates: Combined FTIR and 29Si MAS NMR Study J. Phys. Chem. C, 2007, 111, 330-339.

55. Feng, P.; Xia, Y.; Feng, J.; Bu, X.; Stucky, G. D. Synthesis and Characterization of Mesostructured Aluminophosphates Using the Fluoride Route. Chem. Commun. 1997, 949-950.

56. Conesa, T. D.; Mokaya, R.; Campelo, J.M.; and Romero, A. A. Synthesis and Characterization of Novel Mesoporous Aluminosilicate MCM-41 Containing Aluminophosphate Building Units. Chem. Commun. 2006, 1839-1841.

57. Mears, D. E. Tests for Transport Limitations in Experimental Catalytic Reactors. Ind. Eng. Chem. Process Des. Dev. 1971, 10, 541–547.

58. Fogler, H. S. Elements of chemical reaction engineering. Prentice-Hall International, Inc., New Jersey, US,1986.

59. Madon, R. J.; Boudart,M. Experimental Criterion for the Absence of Artifacts in the Measurement of Rates of Heterogeneous Catalytic Reactions. Ind. Eng. Chem. Fundamen., 1982, 21, 438–447.

60. Smith, J.M. Chemical Engineering Kinetics. 3d edition, McGraw-Hill, New York, United States, 1981. 61. Marquardt, D. W. An Algorithm for Least-Squares Estimation of Nonlinear Parameters. J. Soc. Ind. & Appl. Math. 1963, 11, 431–441.

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62. Fan, J. A Modified Levenberg-Marquardt Algorithm for Singular System of Nonlinear Equations. J. Comput. Math., 2003, 21, 625-636.

63. Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces. John Wiley & Sons: USA.; 1996. 64. Boudart, M. Two-Step Catalytic Reactions, AlChE J. 1972, 18, 465-478. 65. Vannice, M. A. Kinetics of Catalytic Reactions. Springer US, USA.; 2005. 66. Vannice, M.A.; Hyun, S.H.; Kalpakci, B.; Liauh, W.C. Entropies of Adsorption in Heterogeneous Catalytic Reactions. J. Catal. 1979,56, 358-362.

67. Everett, D. H. The Thermodynamics of Adsorption. Part II.-Thermodynamics of Monolayers on Solids. Trans. Faraday Soc. 1950, 46, 942-957.

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Scheme Caption

Scheme 1 Pictorial representation of M-S-34-VPT preparation by VPT process.

Figure Caption

Figure 1. Powder XRD pattern of as-synthesized and calcined MESO-SAPO-34 obtained by VPT process. Figure 2. (a) SEM and (b) TEM images of M-S-34-VPT. Figure 3. N2 sorption isotherms of M-S-34 and M-S-34-VPT (inset shows BJH pore size distribution derived from desorption curves). Figure 4. (A)29Si(B)27Al and (C) 31PMAS-NMR spectra of(a) calcinedM-S-34 and (b) M-S-34-VPT. Figure 5. Pyridine desorption spectra of M-S-34(left) and M-S-34-VPT (right). Figure 6. Effect of WHSV on 1-octene isomerization at 400 oC over M-S-34-VPT. Figure 7. Possible reaction pathway for the hydroisomerization of 1-octene. Figure 8. Experimental data vs model prediction at 400ºC. Figure 9. Arrhenius behaviors of kinetic and rate constants. Figure 10. Arrhenius behaviors of the adsorption equilibrium constants.

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Table Captions

Table 1. Textural data of parent MESO-SAPO-34 and various VPT-treated materials. Table 2. Effect of temperature on octene isomerization over M-S-34-VPT† Table 3. Effect of external mass transfer resistances on conversion of 1-octene over the catalyst. † Table 4. Effect of intra-particle diffusion on conversion of 1-octene.† Table 5.Kinetic rate constants calculated from the homogeneous model. Table 6. Estimated kinetic and equilibrium adsorption constants. Table 7. Estimated Activation energy (E) and pre-exponential factors (A). Table 8. Criteria to evaluate adsorption equilibrium constants obtained as fitting parameters in a LHHW rate expression63-65 Table 9. Enthalpy and entropy change during adsorption (calculated from the adsorption constants)

32


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Scheme 1 : Arvind Kumar Singh et al

33



(1 0 0 )

As Synthesized

Calcined

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

M-S-34-AcOH M-S-34-H3PO4 M-S-34-TMAOH

2

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3

4

5

M-S-34-AcOH M-S-34-H3PO4 (1 0 0 ) M-S-34-TMAOH

M-S-34-VPT

M-S-34-VPT

M-S-34

M-S-34

6

2θ θ (degree)

7

8

9

10

1

2

3

4

5

6

2θ θ (degree)

Figure 1: Arvind Kumar Singh et.al.

34


7

8

9

10

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35


Page 36 of 53

B 50 cm3/g

A

dV/dlog(D) Pore Volume (cm3 /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume Adsorbed ( cm 3 /g)


A : M-S-34 B : M-S-34-VPT 0.0

0.2

B A 3

6

9

12

15

18

21

24

27

30

Pore diametre (nm)

0.4

0.6

0.8

Relative Pressure ( p/po )


36


1.0

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Figure 4: Arvind Kumar Singh et.al. 37



B

B L+B

o

500 C

Intensity (a.u.)

L+B

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 53

L

400 oC o

300 C

L o

500 C o

400 C o

300 C o

200 C o

200 C 1400

1450

1500

Wavenumber (cm-1)

1550

1400

1450

1500

-1

Wavenumber (cm )


38


1550

Page 39 of 53

90 85

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

8.2 12 16 20.2

75 70 65 60

0

1

2

3

4

5

6

Time on stream (h) Figure 6: Arvind Kumar Singh et.al.

39


7

8


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40


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41



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1.Textural data of parent MESO-SAPO-34 and various VPT-treated materials.

Samples name

#

Surface Area (m²/g)

BJH Pore Volume

Average desorption pore diameter (nm)

Brönsted /Lewis ratio#

BET

DFT

(cm3/g)

BJH

H-K

DFT

M-S-34- VPT

466

575

0.37

3.6

0.86

0.87

5.4

M-S-34

457

586

0.35

3.6

0.86

0.87

2.2

M-S-34-TMAOH

292

433

0.43

3.9

0.88

0.5

-

M-S-34-AcOH

330

702

0.47

5.0

0.89

0.5

-

M-S-34-H3PO4

372

544

0.34

3.9

1.6

1.2

-

o

calculated from the peak area of pyridine desorbed at 400 C

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Effect of temperature on octene isomerization over M-S-34-VPT† WHSV (hrs-1) at different temperatures (a, b, c)

Product yield (%) 8.2a

20a

8.2b

20b

8.2c

20c

1-Octene conversion

82.2

60.5

88.8

66.8

94.6

71.2

Linear isomer selectivity (SB)

56.7

72.7

47.6

65.2

41.9

58.2

Mono branched selectivity (SC)

17.1

24.6

25.8

23.1

34.1

20.1

Bibranched selectivity (SD)

26.2

3.7

26.6

12.3

24.0

21.7

†

o

o

o

Reaction conditions: Temperature a = 325 C, b = 400 C, c = 475 C; Catalyst size = 0.3 mm; Catalyst loading 1 g and TOS=4h

45



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Page 46 of 53

Table 3.Effect of external mass transfer resistances on conversion of 1-octene over the catalyst. † FAo /W (hrs‒1)

†

% conversion of 1-octene E

F

8.2

88.9

89.0

12.0

81.1

80.9

16.2

68.6

68.7

20.2

62.3

62.2

o

Reaction conditions: Temperature 400 C; Catalyst size = 0.3 mm, Catalyst loading (E) 1 g of catalyst and (F) 2 g of catalyst, TOS = 4hrs.

46


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Effect of intra-particle diffusion on conversion of 1-octene.† % conversion of 1-octene at various WHSV (h-1)

Particle size

†

8.2

12.0

16.2

0.1 mm

88.8

81.2

68.5

0.3 mm

88.9

81.1

68.6

o

Reaction conditions: Temperature = 400 C; TOS = 4 hrs, Catalyst loading = 1 g.

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Page 48 of 53

Table 5.Kinetic rate constants calculated from the homogeneous model. Kinetic rate constants (S-1)

T (ºC) k1

k2

k3

k4

k5

k6

475

1.69

0.93

0.53

0.98

7.018

4.057

400

0.85

0.47

0.47

0.883

3.14

1.61

325

0.54

0.19

0.39

0.83

2.17

0.86

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 6.Estimated kinetic and equilibrium adsorption constants. Kinetic rate constants (S-1)

T

Adsorption coefficients

(oC )

k1

k2

k3

k4

k5

k6

KA

KB

KC

KD

475

3.73

3.90

2.78

2.75

3.06

1.99

0.41

0.26

2.89

2.78

400

1.01

0.92

0.57

0.82

0.95

0.72

1.49

1.03

6.32

6.70

325

0.39

0.26

0.17

0.23

0.34

0.31

3.20

3.22

8.88

8.59

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Table 7.Estimated Activation energy (E) and pre-exponential factors (A) Activation Energy (kJ/mole)

Pre-exponential factor

k1

55.2 ± 8.8

23668.49

k2

66.7 ± 7.0

162818.28

k3

68.3 ± 10.2

143250.48

k4

61 ± 3.1

48380.40

k5

54.4 ± 5.4

17818.10

k6

45.5 ± 5.6

2783.96

Rate constant (S-1)

(S-1)

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Table 8. Criteria to evaluate adsorption equilibrium constants obtained as fitting parameters in a LHHW rate expression63-65 Rule 1:

o −∆H ad > 0 (Q ad > 0)

Rule 2& 3:

o 0 < Sad ≤ S go

Guidelines:

o 10 ≤ ∆Sado ≤ 12 − 0.0014 ( ∆H ad )

o o Where S go is the standard entropy of the gas (1 atm), S go , S ad , H ad are in cal/mole

51



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Page 52 of 53

Table 9. Enthalpy and entropy change during adsorption (calculated from the adsorption constants) Equilibrium adsorption constants

*

(kJ/mole)

(J/mole-K)

(J/mole-K)*

KA

50.4

73.6

500.1

KB

61.7

92.8

701.8

KC

27.3

26.8

796.4

KD

27.2

26.7

716.2

calculated from HSC chemistry 7.0 for model compounds.

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GRAPHICAL ABSTRACT

53


Uniform Mesoporous Silicoaluminophosphate Derived by Vapor

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