Theoretical Study on Adsorption of Methanol on Zeolite and Phosphorus Modified Zeolite

Scientific ResearchTheoretical Study on A dsorption of Methanolon Zeolite and Phosphorus Modified ZeoliteLi Renqing'; Li Ting'; Gu Jun'; Liu Chenguang'(1 University of Petroleum, Dongying 257061; 2 Dongxin Oil Production Factory)Abstract: Methanol adsorption in zeolite and phosphorus modified zeolite has been investigated withinthe cluster model framework of quantum chemical calculation. Full optimization and frequency analysis ofall cluster model have been carried out using Gaussian 94 soft package with Hartree- Fock method andB3LYP, B3P86 hybrid methods of density functional theory at 3-21G, 6-31G basis set level for hydrogenatoms and 6-31G+(d) basis set level for the other atoms performed on small cluster model forCH,OH, HAI(OH)SiH(H,PO), H,Al(OH)SiH(H,SiO), H,AI(OH-CH,OH)SiH,(HPO) and HAI(OH-CH,OH)SiH,(H,SiO). The results show that phosphorus grafting in the zeolite framework has modifiedthe chemical environment in the vicinity of the zeolite bridging hydroxyl. Phosphorus modification canenhance the acid strength of zeolite bridging hydroxyl, which was suggested by the lengthening ofzeolite bridging hydroxyl O- H bond and the increasing methanol adsorption energy. This may befavorable to the initial CH OCH, formation in the methanol to gasoline (MTG) process.Key words: methanol adsorption; zeolite; Hartree Fock method; density functional theory1 Introductionmolecular dynamics simulations. Payne et alsl calculated thefree energy barrier for the reaction of methanol in a zeoliteZSM- 5, a member of the pentasil family of high silica zeolites, catalyst by first principles. Cluster models [161 and periodichas a wide application as a catalytic material due to its un- structuresl7]1 have been calculated to investigate the pos-usual properties. It is common practice that zeolite must be sible protonation of small polar molecules in zeolite. Nusterercarefully tuned to the desired catalytic process prior to its et al81 studied the adsorption of methanol in silicon richemployment. Phosphorus-containing compounds were usu- sodalite using the PAW method with periodic boundary con-ally chosen to modify ZSM-5 zeolites to be applied in many ditions including full dynamics. Shah et al19.201 usedcatalytic reactions such as conversion of methanol to gaso- pseudopotentials to calculate methanol in sodalite andline MTG)I4, selective alkylation of toluene with methanol chabazite. Hasse et all21 studied the adsorption of a singleto produce para-xylene [5 +0]. Yuan et al" investigated the methanol molecule in the series of acidic zeolite theta- 1,methanol adsorption properties in heteroatoms substituted ferrierite and ZSM-5 by ab initio molecular simulations basedZSM-5 zeolites by density functional theory. Feng et all21 on density functional theory together with a plane wave ba-employed density functional method to study methanol ad- sis set. To my knowledge, there were no reports about com-sorption in TS-1. Ab initio molecular dynamics simulations parative study on methanol adsorption in zeolite and phos-based on density functional theory together with a plane phorus modified zeolite, which was used as an important cata-wave basis set and Vanderbilt pseudopotential were used to lyst for methanol related reactions. So, the aim of our study isexplore the potential energy surface of a methanol molecule as follows: (1) comparing the changes in structure of zeoliteinteracting with the Bronsted site of chabazitel131. Schwarz et model cluster and phosphorus modified zeolite model cluster;al. [141 took the infinite zeolite structure into full account and (2) investigating the changes of methanol adsorbed complex;presented the adsorption of methanol and water in zeolite by and (3) determining the reaction energy of methanolfirst principle (based on electronic structure calculations) adsorption.中国煤化工MHCNMH G.China Petroleum Processing and Petrochemical TechnologyNo.1, March 20042 Cluster Models and ComputationalAll calculations have been performed using HP method andMethodselectron correlation considered B3LYP, B3P86 hybrid meth-ods of density functional theory in Gaussian 94 soft-packageFour cluster models considered in this study were H,Al(OH) and 3-21G, 6-31G basis sets for hydrogen atoms as well as 6-SiH,(H,PO) (denoted as AI-Si-P), HAI(OH)SiH,(H,SiO)31G+(d) for the other atoms taking into account the polariza-(denoted asAl-Si- Si), HAI(OH-CH,OH)SiH,(HPO)(denoted tion functions and diffuse functions because of their atomicas Al-Si-P-CH,OH) and HAI(OH-CH,OH)SiH,(H,SiO) properties.(denoted as Al-Si-Si-CH,OH). The cluster models shown inFigure 1 were terminated by hydrogen atoms.Geometry optimization calculations have been carried out toa minimum degree for CH,OH, Al-Si-P and Al- Si Si as well asIn this study, the molecular systems are zeolite cluster and adsorption complexesof Al-Si-P-CH,OHand Al-Si-Si-CH,OH.phosphorus modified zeolitic cluster interacting with The frequency analyses were also performed using secondmethanol. The Al-Si-Si and Al-Si-P have been used to repre- derivatives in order to check the stationary point that exhib-sent the acidic zeolite and phosphorus modified acidic zeolite. ited none of imaginary frequency to a minimum extent.The main consequence of the use of cluster models is theabsence of any cavity-dependent effect. The cluster model The strategy for calculating the interaction energy is a simpleapproach has well known limitations, but on the other hand,one - the energy of the molecular complex (Emole ) was com-because of finite computer resources and because of the size pared to the sum of the energy of the individual monomers.of the systerms, the approximation was invoked. It was pointed The interaction energy of zeolite as well as phosphorus modi-out that especially the concentration of adsorbed molecules fied zeolite and methanol can be calculated using the followsensitively depended on a match of molecular and ing equation:microporous size, our study on interaction between metha- E=E(Al-Si-P)+ Ep(Al-Si-P)+ E (CH,OH)+Ep(CH,OH)-nol and cluster models were free from pore size constraints [(Al-Si-P-CH,OH)+E.(Al-Si-P-CH,OH)]due to the small dimensions of methanol.It is well known that calculating the interaction energy in thismanner results in a spuriously low value. This is an“error"H181091522that has been introduced into the calculation known as thebasis set superposition error (BSSE). What has occurred isg?)而that different basis sets have been used to study one system, .HUresulting in a non-physical lowering in the energy. In our明o咖明study, the corrections were not made for BSSE because wefocused on the change trends of methanol adsorption en-A1-Si-PAl-Si-P-CH,OHergy between methanol and zeolite as well as phosphorusmodified zeolite.心14@3 Results and Discussionon4心U器”The fully optimized geometries of cluster models at Hartree-0n2Fock level and density functional level for all systems con-sidered were presented in Table 1, Table 3 and Table 5.AI-Si-Si-CH,OHAl-Si-SiFigure 1 depicted the initial structure for AI-Si-P-CH,OH, Al-Si-P, AI-Si-Si-CH,OH and Al-Si-Si clusters. There are someFigure 1 Schematic representation ofsimilar and different characteristics among them. The opti-cluster modelsmized structure中国煤化工ubstitute, andMYHCNMHG14.Scientific Researchmethanol interacts with the zeolite framework through two drogen bond is formed by an oxygen atom of the zeolite frame-hydrogen bonds. One is through the bridging hydroxyl spe- work and the methanol proton with an 0- H bond distancecies of the zeolite cluster model and the oxygen of methanol of 0.16- 0.18 nm. Because methanol interacts with zeolite andwith the 0- H hydrogen bond length of 0.14 0.16 nm as phosphorus modified zeolite, O; H bond is elongated, whilethe major hydrogen bond interaction. The other weaker hy- Al- 0, and O,- Sig bond distances contract, which is con-Table 1 The partial optimized parameters for four cluster models by Hartree-Fock method at6-31G level for hydrogen atoms and 6-31G+(d) for the other atomsBond parameterAl-Si-P-CH,OHAl-Si-PAl-Si-Si-CH.OHAl-Si-SiBond length (A)Al 021.9321.9461.9281.940O2- Si。1.7171.7461.722O,H,1.0050.9700.9980.967O,H40.9621.5251.548H-01.8061.807| Bond angle, (°)A1,-02 Si。119.675114.943125.041122.063Al,-O2H,118.295124.646116.368121 568Si。-O, H,121.914120.411118.484Table 2 The heat of formation (HF) and zero point energy (ZPE) as well as ZPE corrected energyby Hartree-Fock methodtemAI-Si-PCH,OHHF (au)-1365.3468725-1250.3215-1314.3270485-1199.307051-114.9881655ZPE (au)0.1552260.0975770.1667010.1091020.054870Corrected energy ( au)-1365.191646-1250.223948-1314.160347-1199.197949-114.9332955Adsorption energy,kJ/mol90.3276.41Table 3 The partial optimized parameters for four cluster models by B3LYP method at 3-21G levelfor hydrogen atoms and 6- 31G+(d) for the other atomsItemAI-Si-Si-CH.OHAl,-O,1.8961.8911.9351.941O2-Si1.7661.7331.764O2H,1.0571.0161.0551.0030, H1.0001.4151.414H2- 01.6481.636Bond angle, (°)A1,-O2 Si102.116101.304117.734Al,-0,-H,132.021137.90322.079129.729Sig 0,-H,125.803120.750中国煤化工15.531YHCNMHG15.China Petroleum Processing and Petrochemical TechnologyNo.1, March 2004Table 4 The heat of formation (HF) and zero point energy (ZPE) as well as ZPE corrected energyby B3LYP method at 3-21G level for hydrogen atoms and 6-31G+(d) for the other atomsItemAl-Si-P-CH,OHAl-Si-PA-Si-Si-CH.OHAl-Si-SiCH,OH| HF (au)-1369.7605886-1254.053749-1318.6684314 1202.9624333 -115.6718202|ZPE(au)0.14452636730.09120077850.1551384590.1019700.051219I corrected energy( au)-1369.616062-1253.962548-1318.513293-1202.860463 -1 15.6206012| adsorption energy, kJ/ mol86.4184.61Table 5 The partial optimized parameters for four cluster models by B3LYP method at 6-31G levelfor hydrogen atoms and 6- 31G+(d) for the other atomsAI-Si-Si-CH.OHA-Si-Si| Bond length (A)A1,-O21.9001.8931.9361.943O2-Si,1.7651.8001.734O2-H,1.0571.0141.0541.003O,H141.000.9990,-H,1.4161.414H 0151.6521.638Bond angle, (°)Al,-O2- Sig102.667101.378118.382115.363Al,-O2 H,131.743137.892121.633129.223Si.O,H,125.516120.689119.863115.377Table 6 The heat of formation (HF) and zero point energy (ZPE) as well as ZPE corrected energyby B3LYP method at 6-31G level for hydrogen atoms and 6-31G+(d) for the other atomsAI-Si- Si-CHOH|HF, au-1369.7839796-1254.0691146-1318.6942913 -1202.9802714 -115.6795018ZPE, au0.1441830.0910960.1549420.1018780.051072Corrected energy, au-1369.639796-1253.978018-1318.539349-1202.878393-1 15.6284298| Adsorption energy, kJ/mol87.5sistent with the bond order conservation principle and the bond parameters in terms of bond lengths and bond anglesGutmann' s rule [2, a lengthening of the bridging hydroxyl are significantly different among various optimizationBronsted O,- H, bond, a shortening of Al,- O, and a methods, especially the Hartree- Fock method and densitylengthening of Sig- 0, bond. The other major changes of functional method, because the latter takes the electron cor-structure parameters are Al,- 02 Si, Al,-O2 H, and Si,- relation into consideration.O2 H, bond angles. Phosphorus grafting in the zeolite frame-work makes the Al,- 02- Si. bond angle lessen, while the The Mulliken population ananlysis displayed that theinteraction of methanol with zeolite bridging hydroxyl resulted Mulliken atomic charges associated with methanol interac-inthe Al,- 0,- Si, and Si, 0, H, bond angles enlarged tion are basis set dependent. However, comparison of atomicand Al,- 0,- H, bond angle lessened for both zeolite and charges change trends with related structure will certainlyphosphorus modified zeolite. This may be attributed to the give us a meaningfulvalne. Bv analvzing the charges on Al,lengthening of O2- H, bond distance. It was also noted that O, H and Si, at(中国煤化工ocalelectronic .YHCNMHG16.Scientific Researchstructure of the methanol adsorbed super-molecules is con- phorus modified zeolite were stabilized by two hydrogensistent with the bonding properties of the zcolite cluster and bonds, one of which was the weaker hydrogen bond and thethe methanol molecule. The O- H bonds in both methanol other was the stronger hydrogen bond. Modification of zeo-and zeolite cluster were polarized causing increased charges lite by phosphorus makes the modified zeolite more acidicat H, and O, atoms because of the formation of hydrogen than the parent zeolite, as can be seen via the lengthening ofbonds between zeolite cluster model and methanol.bridging hydroxyl Bronsted OH and the increasing methanoladsorption energy. The trends for bond length changes areTable 2, Table 4 and Table 6 show the energy of the methanol in accordance with the bond order conservation principleadsorption on the zeolite cluster model and phosphorus modi- and Gutmaan' s rule. The Al.- 0,- Si。 bond angle broaden-fied zeolite cluster model. It is regarded that the larger the ing of methanol complex adsorbed on zeolite may be ascribedadsorption energy, the stronger the interaction between to the lengthening of O2- H, bond distance. It can be con-methanol and zeolite acid site. It can be seen that the zero- cluded that phosphorus modification of zeolite frameworkpoint corrected methanol adsorption energy of phosphorus make the framework chemical environment change and canmodified zeolite is more than that of zeolite. The stronger affecet the framework interaction with methanol molecule. Phos-interaction between methanol and phosphorus modified zeo- phorus modification can not only stabilize the framework alu-lite may be assigned to the higher electronegativity of phos- minum and enhance the sinuosity of the zeolite channel (26),phorus grafing to the zeolite framework resulting in an en- but can also change the interaction between zeolite frame-hancement of the zeolite framework geometric mean electrone- work bridging hydroxyl and methanol, which may facilitategativity (231. In addition, the calculated adsorption energy of the initial process of methanol to gasoline (MTG).methanol (90.32kJ/mol) on phosphorus modified zeolite ismuch higher than that of zeolite (76.4 1 kJ/mol) without con-Acknowledgementssidering the electron correlation of Hartree- Fock method, the The calculations reported here were performed at Computerdifference of the calculated adsorption energy of methanol Center of College of Chemistry and Chemical Engineering,between zeolite and phosphorus modified zeolite is smaller University of Petroleum. This work was supported by thewhile considering the electron correlation of density func- Doctor’ s Foundation of University of Petroleum.tional method. 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Chem Phys Lett,Microporous Mesoporous Materials, 2001, 48: 375- 3811998, 283: 402- 408[16] Bates S, Dwyer JJ Mol Struct (Theochem), 1994, 306: [26]Li Renqing . The Ph. D disertation of Nankai University,57- 652003,6[17] Gale J D, Catlow C R A, CarruthersJ R. Chem Phys Lett,Shanghai to Construct Isocyanate Uhit forPr oducti on Expansi onIn November 2003 Shanghai Huayi (Group) Company，time 160 kt/a TDI and the feedstock nitric acid andSINOPEC Shanghai Gaoqiao Branch Company, Shanghai dinitrotoluene, and the production units will be jointly con-Chloroalkali Company Ltd. and their foreign partners- BASF structed by BASF, Shanghai Huayi (Group) Company andand Huntsman Company declared the planned productionGaoqiao Petrochemical Company. The Huntsman Polyure-capacity optimization of the integrated isocyanate unit in thane Shanghai Company Ltd. is also a plant for refining ofShanghai Chemical Industry Area, intending to expand the MDI, and its units will be constructed jointly by HuntsmanMDI capacity from 160 kt/a to 240 kt/a and the TDI capacity and Shanghai Chloroalkali Company Ltd. These companiesfrom 130 kt/a to 160 kt/a. The construction of the projected is are intending to provide the Bayer Company with the rawexpected to commence by the end of 2003 with the invest-MDI and TDI products they manufacture.ment in the project totaling around one billion US dollars.According to the project plan Shanghai Lianheng Isocyan- contracts for these units have been awarded to Fluor Com-ate Company Ltd. will be responsible for production of 240 pany of USA, Dalin Engineering Complex of Korea and Tai-kt/a MDI, aniline feedstock and nitrobenzene, while the units wan Zhongding Engineering Company respectively.will be jointly constructed by BASF, Huntsman, ShanghaiChloroalkali Company Ltd. and Gaoqiao Petrochemical It is projected that the polyurethane market in China will growCompany. Shanghai BASF Polyurethane Company Ltd. is 10% a year within 12 years in the future, and will become theresponsible for refining of MDI while providing in the mean- largest polyureth;中国煤化I)15.MYHCNMH G48.