Heterogeneously catalyzed gas phase reactions are commonly carried out in fixed bed reactors throughout the chemical processing industry ). Slender tubes with only a few particles along the tube diameter are required for highly endothermic and exothermic reactions, such as the partial oxidation of nbutane to maleic anhydride (MA) ), to efficiently supply or remove the heat while keeping the pressure drop at a moderate level. Due to the pronounced effect of the tube wall on the particle arrangement, the small tube-to-particle diameterratio ( Dt /dp < 10) results in an inhomogeneous packed bed morphology ), affecting fluid flow, heat and species mass transport ), as well as chemical reactions ). Since classical pseudo-continuum models are based on simplifying assumptions about the fixed bed geometry and the fluid flow, they can not account for effects caused by the local packed bed structure and may fail to accurately describe such reactors ). In contrast to classical continuum models, particle-resolved computational fluid dynamics (PRCFD) simulations can describe local phenomena, since the spatial domain is fully resolved and hence every single particle is taken into account when solving the governing equations for momentum, mass, energy and species mass ). For the most part, the solution is computed numerically by discretizing the packed bed geometry with three-dimensional computational cells, i.e., the finite volume methods. In only a few studies, the Lattice-Boltzmann method is used, see e.g. . The packed beds are typically generated synthetically employing the discrete element method ) or the rigid body approach ). Most of the PRCFD studies available in literature neglect chemical reaction to investigate the impact of the particle shape ), the Dt /dp ratio ) or inserts, such as heat fins ) and thermowells ) on pressure drop and heat transport. However, the strong interplay between the transport phenomena and the catalytic chemistry requires to implement reaction in the PRCFD simulations in order to analyse the fixed bed reactor to a sufficiently high degree of detail. If pore diffusion inside the catalyst particle is negligible (instantaneous diffusion), the reaction can be implemented as heat and species mass fluxes at the particle surface. Such an approach based on the operator splitting algorithm is proposed by to couple CFD with complex microkinetic reaction models. Since the active sites catalyzing the gas-phase reaction are commonly located inside a porous structured solid and not directly on the particle surface, extended the method of Maestri and Cuoci by taking into account pore diffusion through an effectiveness factor calculated via the Thiele modulus. Nevertheless, the application of effectiveness factors cannot meet the requirements of an ideal multiscale model, as presented in , since the impact of intra-particle concentration gradients on the reaction are not precisely described. Therefore, suggested to fully couple transport and reaction inside the catalyst with the fluid flow by defining the particles as porous media. As pointed out by , the absence of a no-slip boundary condition at the particle surface, when using the porous media approach, results in an artificial convective transport into the solid domain. However, according to the physical and chemical steps in heterogeneous catalysis described in many textbooks, such as Ertl, , species mass is only transported via diffusion inside the porous catalyst. To overcome this issue, implemented user-defined scalars within the commercial CFD code Fluent to model the diffusive species mass transport inside particles. The catalyst is assumed as a pseudo-continuum. Moreover, the fluid flow is coupled with the solid region by iteratively calculating the boundary conditions for energy and species mass transport at the fluid-solid interface. developed a similar method, capable for large microkinetic models, in the open-source CFD framework OpenFoam. The coupling approaches of and are able to correctly describe the interaction between surrounding fluid flow and reaction inside the porous catalyst. However, they are only valid for computational meshes with orthogonal cells at the particle surface, since both methods neglect secondary gradients in the description of the heat and species mass transfer at the fluid-solid interface. For computational efficiency, complex packed bed geometries are usually discretized with unstructured meshes ), which often contain non-orthogonal cells at the particle surface. As pointed out by , the influence of the secondary gradient on the diffusion flux is small if the non-orthogonality is not severe. However, claimed, that for cases with a large change in transport parameters the accurate calculation of the secondary gradient becomes more important. The relevance of considering the gradient in PRCFD simulations of catalytic fixed beds is still an open question. In this work, a conjugated heat (CHT) and mass transfer (CMT) model is presented within the framework of PRCFD to couple the fluid flow with transport and reaction inside a porous catalyst, which takes into account the secondary gradient. Due to the analogy between heat and species mass transport, the definition of the CMT model is derived from the well established CHT model. Furthermore, the presented method is also valid for turbulent flows with y + values of the wall-nearest cell being larger than one, because it applies wall functions to describe the heat and species mass transfer in turbulent flow regimes. This allows, in contrast to the aforementioned works of and , to use coarser meshes for the simulation of turbulent flows, which reduces the overall computational effort. This novel approach is validated using the analytical solution of heat and species mass diffusion through two connected slabs and standard literature correlations for the flow around a sphere. As an illustrative example, PRCFD simulations are presented for a short catalytic fixed bed containing ring shaped particles catalyzing the selective oxidation of n-butane to maleic anhydride. In this industrial relevant process C 4 H 10 reacts with oxygen to MA over a vanadium phosphorus oxide (VPO) catalyst. As side reactions n-butane as well as MA are oxidized to the unwanted by-products CO and CO 2 . In addition, the water formed in the oxidation reaction inhibits the active sites of the catalyst and decreases its activity ). In industry, the highly exothermic reactions are commonly carried out in multitubular fixed bed reactors with a small Dt /dp ratio cooled by molten salt and fed with air as O 2 source mixed with a mole fraction up to 2 % of C 4 H 10 . The reactors are operated at a constant conversion of 80 % to 86 % resulting in MA yields between 57 % and 61 % ). Insufficient heat removal, particle overheating and pore-diffusion limitation of MA lead to a reduced yield for higher conversion due to the more favored formation of CO and CO 2 . Optimization of the catalyst design could partially solve these problems and would allow operation under more optimal conditions. A method for accurate modeling of transport phenomena coupled with kinetics as shown in this work would be beneficial for such an optimization. For the catalytic fixed bed, the impact of the secondary gradients on the simulation results is investigated.